DEVELOPMENTS IN SEDIMENTOLOGY 38
EOLIAN SEDIMENTS AND PROCESSES
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DEVELOPMENTS IN SEDIMENTOLOGY 38
EOLIAN SEDIMENTS AND PROCESSES
FURTHER TITLES IN THIS SERIES VOLUMES 1,2,3,5,8 and 9 are out of print
4 F.G. T I C K E L L THE TECHNIQUES O F SEDIMENTARY MINERALOGY 6 L. V A N D E R P L A S THE IDENTIFICATION O F DETRITAL FELDSPARS I S. D Z U L Y N S K I and E.K. W A L T O N SEDIMENTARY FEATURES O F FLYSCH AND GREYWACKES 10 P.McL.D. DUFF, A. H A L L A M and E.K. W A L T O N CYCLIC SEDIMENTATION 11 C.C. R E E V E S Jr. INTRODUCTION TO PALEOLIMNOLOGY 12 R.G.C. B A T H U R S T CARBONATE SEDIMENTS AND THEIR DIAGENESIS 13 A.A. M A N T E N SILURIAN REEFS O F GOTLAND 14 K. W. G L E N N I E DESERT SEDIMENTARY ENVIRONMENTS 15 C.E. W E A V E R and L.D. P O L L A R D THE CH’EMISTRY OF CLAY MINERALS 16 H.H. R I E K E III and G. V. C H I L I N G A R I A N COMPACTION O F ARGILLACEOUS SEDIMENTS 11 M.D. P I C A R D and L.R. HIGH Jr. SEDIMENTARY STRUCTURES O F EPHEMERAL STREAMS 18 G.V. C H I L I N G A R I A N and K.H. W O L F , Editors COMPACTION O F COARSE-GRAINED SEDIMENTS 19 W. S C H W A R Z A C H E R SEDIMENTATION MODELS AND QUANTITATIVE STRATIGRAPHY 20 M.R. W A L T E R , Editor STROMATOLITES 21 B. V E L D E CLAYS AND CLAY MINERALS IN NATURAL AND SYNTHETIC SYSTEMS 22 C.E. W E A V E R and K.C. BECK MIOCENE O F THE SOUTHEASTERN UNITED STATES 23 B.C. H E E Z E N , Editor INFLUENCE O F ABYSSAL CIRCULATION ON SEDIMENTARY ACCUMULATIONS IN SPACE AND TIME 24 R.E. GRIM and G U V E N BENTONITES 25A G. L A R S E N and G. V. C H I L I N G A R , Editors DIAGENESIS I N SEDIMENTS AND SEDIMENTARY ROCKS, I 26 T. SUDO and S. S H I M O D A , Editors CLAYS AND CLAY MINERALS O F JAPAN 21 M.M. M O R T L A N D and V.C. F A R M E R , Editors INTERNATIONAL CLAY CONFERENCE 1918 28 A. N I S S E N B A U M , Editor HYPERSALINE BRINES AND EVAPORITIC ENVIRONMENTS 29 P. T U R N E R CONTINENTAL R E D BEDS 30 J.R.L. A L L E N SEDIMENTARY STRUCTURES 31 T . SUDO, S . S H I M O D A , H. Y O T S U M O T O and S. A I T A ELECTRON MICROGRAPHS OF CLAY MINERALS 32 C.A. N I T T R O U E R , Editor SEDIMENTARY DYNAMICS O F CONTINENTAL SHELVES 33 G.N. B A T U R I N PHOSPHORITES ON THE SEA FLOOR 34 J.J. F R I P I A T , Editor ADVANCED TECHNIQUES FOR CLAY MINERAL ANALYSIS 35 H. V A N OLPHEN and F. V E N I A L E , Editors INTERNATIONAL CLAY CONFERENCE 1981 36 A. IIJIMA, J.R. HEIN and R . S I E V E R , Editors SILICEOUS DEPOSITS IN THE PACIFIC REGION 31 A. S I N G E R and E. G A L A N , Editors PALYGORSKITE AND SEPIOLITE: OCCURRENCES, GENESIS AND USES
DEVELOPMENTS IN SEDIMENTOLOGY 38
EOLIAN SEDIMENTS AND PROCESSES Edited by
M.E. BROOKFIELD and T.S. AHLBRANDT Department of Land Resource Science, Ontario Agricultural College, University of Guelph, Ont. N1G 2W1 (Canada) 1376 S. Perry Pk. Road, Sedalia, CO 80135 (U.S.A.)
ELSEVIER Amsterdam
-
Oxford
- New
York
- Tokyo
1983
ELSEVIER SCIENCE PUBLISHERS B.V. 1 Molenwerf P.O. Box 211, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, N Y 10017
I . i h i . ~ r \ 01
< snyrr\r
C.11dlogilig In P u h l i i a l i o n 1)dt.I
Main entry under title: Eolian sediments and processes. (Developments in sedimento5oa ; 38) Bibliography: p. 1. Wind erosion--Congresses. 2. Sediments (Geoloa) --Congresses. I. Rrookfield, M. F. (Michael E.) 11. Ahlbrandt, Thomas S. 111. Series.
W597.E55 1983 551.3'7 ISBN 0-444-42233-1
83-14081
ISBN 0-444-42233-1(Vol. 38) ISBN 0-444-41238-7 (Series) 0 Elsevier Science Publishers B.V., 1983
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 330, 1000 AH Amsterdam, The Netherlands Printed in The Netherlands
V
PREFACE
This volume i s the r e s u l t of a three day symposium, consisting of f i f t y two papers, on "Eolian sediments and processes" presented a t the 11th International Association of Sedimentologists Congress held in Hamilton, Ontario, in August 1982. Many of the papers included in t h i s volume were given o r a l l y a t t h a t sumposium; however, we have included a few o t h e r s t o broaden or f i l l gaps in our coverage. The symposium and volume were organized t o include the e n t i r e spectrum of e o l i a n i n v e s t i g a t i o n s , ranqing from the niicroscopic level to regional svntheses and f i n a l l y t o ancient eolian deposits and t h e i r i n t e r p r e t a t i o n . Thus, we have included a very diverse g r o u p o f papers in one volume, with the following aims. F i r s t l y , t o bring to your a t t e n t i o n aspects of eolian sediments a n d processes which you may n o t normally consider, b u t which could be illuminating in your own i n v e s t i g a t i o n s : secondly, t o summarize the present s t a t e of eolian research.
No one author can now hope t o keep a b r e a s t of a l l c u r r e n t developiiients in t h i s rapidly expanding f i e l d , y e t summaries a r e p e r i o d i c a l l y needed. Ile r e g r e t n o t being a b l e t o include papers o n important areas of economic research, such as the d e f i n i t i o n of i s o l a t e d eolian r e s e r v o i r s using liigli resolution seismic d a t a . Such material i s generally confidential and will n o t be iiiade a v a i l a b l e until i t s proprietary value i s diminished. We wish t o dedicate t h i s volume t o E.D. McKee, in recognition of h i s pioneering and continuing research on eolian sediments a n d processes. Many of t h e , s t u d i e s reported here a r e an outgrowth of his e f f o r t s and were helped by his encouragement. We would l i k e to thank most of the authors f o r t h e i r careful preparation and modification of t h e i r manuscripts. \le have not standardized s p e l l i n g , so b o t h North American and European was accepted. Lastly, we hope you enjoy reading t h i s voluiiie.
Hichael E. Brookfield Thomas S. Ahlbrandt
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VII
CONTENTS
Preface
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction 1. McKee, E . D .
Eolian sand bodies o f the world ......................
. . v 1
The sediment
2. Binda, P.L. 3. 4.
5. 6.
On t h e skewness o f some eolian sands from Saudi Arabia ......................................... Schenk, C . J . Textural and s t r u c t u r a l c h a r a c t e r i s t i c s of some experimentally formed eolian s t r a t a . . ................ Smalley, I . J . and Smalley, V . Loess material and loess deposits: formation, d i s t r i b u t i o n and consequences ............. Derbyshire, E. Oriqin and c h a r a c t e r i s t i c s o f some Chinese l o e s s :...... a t two l o c a t i o n s in China ..................... Ruhe, R . V . Clay minerals in t h i n l o e s s , Ohio r i v e r basin,
27 41
51 69
................................................
91
S a l t a t i o n threshold and deposition r a t e modellinq ............................................
103
U.S.A Processes
7 . Iversen, J.D. 8. Gerety, K.M.
and Slingerland, R . Nature of the s a l t a t i n g population in wind tunnel experiments with heterogeneous size-density sands ........................... 9. Greeley, R . , Williams, S.H. and Marshall, J.R. Velocities of windblown p a r t i c l e s in s a l t a t i o n : preliminary
115
laboratory and f i e l d measurements ....................
133
10. Stapor, F.W. J r . , Play, J.P. and Barwis, J . Eolian shape-sorting and aerodynamic t r a c t i o n equivalence in t h e coastal dunes o f Hout Bay, Republic o f South Africa .......... 11. Wasson, R.J.
149
Dune sediment type, sand colour, sediment provenance and hydrology i n t h e Strzelecki-Simpson dunefield, Australia ............................................ Early post-depositional modification of aeolian dune sands ...........................................
197
13. Whitney, PI.
Eolian f e a t u r e s shaped by aerodynamic and v o r t i c i t y processes ............................................
223
14. Tsoar, H.
Wind tunnel modelling o f echo and climbing dunes .....
24 7
12. Pye, K.
165
Recent eolian
VIII
15. Lancaster, N.
C o n t r o l s of dune morphology i n t h e Namib sand sea...
16. Brown, R.A.
The f l o w i n the P l a n e t a r y Boundary Layer
17. Hyde, R . and Iiasson, R.J.
............
261 291
R a d i a t i v e and m e t e o r o l o g i c a l c o n t r o l
on t h e movement o f sand a t Lake Munqo, New South
....................................
Wales, A u s t r a l i a 18. Hesp, P.
311
Morphodynamics o f i n c i p i e n t foredunes i n New South Wales, A u s t r a l i a ....................................
19. Warren, A. and Knott, P.
325
Desert dunes: a s h o r t r e v i e w o f needs
i n d e s e r t dune research and a r e c e n t study o f m i c r o m e t e o r o l o q i c a l d u n e - i n i t i a t i o n mechanisms ........... 20. Flainguet, P I . and Chemin, f1.-C.
343
Sand seas o f t h e Sahara and Sahel:
an e x p l a n a t i o n o f t h e i r t h i c k n e s s and sand dune t y p e by t h e sand budget p r i n c i p l e 21. Anton, D.
........................
353
Flodern e o l i a n d e p o s i t s o f t h e Eastern Province o f Saudi Arabia ........................................
22. Ahlbrandt, T.S.,
Swinehart, J.B.
and flaroney, D.G.
365
The dynamic
Holocene dune f i e l d s o f t h e Great P l a i n s and Rocky Mountain Basins, U.S.A
..............................
379
Ancient e o l i a n 23. Rubin, D.M.
and Hunter, R.E.
Reconstructing bedform assemblaqes
from compound cross-bedding 24. Hunter, R.E.
and Rubin, D.M.
.........................
407
I n t e r p r e t i n g c y c l i c crossbedding,
w i t h an example from t h e Navajo Sandstone ...........
25. Ruegg, G.H.J.
P e r i q l a c i a l e o l i a n e v e n l y laminated sandy d e p o s i t s i n t h e l a t e P l e i s t o c e n e o f N.W.
Europe, a f a c i e s
unrecorded i n modern sedimentoloqical handbooks.. 26. Ross, G.N.
429
...
455
Bigbear Erg: a P r o t e r o z o i c intermontane e o l i a n sand sea i n t h e Hornby Bay Group, Northwest T e r r i t o r i e s , Canada ..............................................
27. Glennie, K.W.
N o r t h Sea area.............................. 28. Steele, R.P.
........
521
L o n g i t u d i n a l draa i n t h e Permian Yellow Sands o f n o r t h - e a s t England
29. Blakey, R.C.
433
Lower Permian Rotliegend d e s e r t sedimentation i n t h e
and Middleton, L.T.
..................................
543
Permian shore1 i n e e o l i a n complex
i n c e n t r a l Arizona: dune changes i n response t o c y c l i c sea-level changes
30. Mader, D.
...................................
551
A e o l i a n sands t e r m i n a t i n g an e v o l u t i o n o f f l u v i a l d e p o s i t i o n a l environment i n M i d d l e Buntsandstein (Lower T r i a s s i c ) o f t h e E i f e l , Federal Republic Germany
.............................................
583
IX
31. Middleton, L.T. and Blakey, R . C .
Processes and c o n t r o l s on the
intertonguing o f t h e Kayenta and Navajo Formations, 32. Marzolf, J.E.
northern Arizona: eolian-fluvial i n t e r a c t i o n s ....... Changing wind and hydrologic regimes during deposition
613
of t h e Navajo and Aztec Sandstones, J u r a s s i c ( ? ) , southwestern United S t a t e s ..........................
635
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EOLIAN
SAND
BODIES OF THE WORLD
EDWIN
D. MCKEF:
U.S. G e o l o g i c a l Survey, P.O. Box 2 5 0 4 6 , D e n v e r F e d e r a l C e n t e r , Denver, C0 80225
I PITRODIICTI Ow M i n d causes t h e a c c u m u l a t i o n o f sand and t h e m i g r a t i o n o f sand b o d i e s i n many parts o f t h e world,
where i t f o r m s dunes,
A limited
sand s h e e t s and t o n g u e s .
niimber of b a s i c o r s i m p l e dune t y p e s i s r e c o g n i z e d ( F i g . l), b u t a g r e a t number o f varieties,
q e n e r a l l y r e f e r r e d t o as compound o r complex f o r m s , e x i s t .
The f a c t o r s
t h a t , c r e a t e and c o n t r o l t h e s e sand b o d i e s a r e p r i m a r i l y t h e s t r e n g t h and d i r e c t i o n of
wind,
t h e a v a i l a b l e supply o f
f a c t o r s as t o p o g r a p h y ( F i q . vpqptation (Figs.
sand,
and,
t o a lesser extent,
such p h y s i c a l
2 ) and w a t e r b o d i e s ( F i g s . 3, 4 ) and t h e p r e s e n c e o f
5, 6 ) .
F i q . 1. S i m p l e dune t y p e s r e s u l t i n g f r o m u n i d i r e c t i o n a l w i n d s , a r r a n g e d i n normal sequence downwind w i t h d e c r e a s i n g sand s u p p l y o r w i n d v e l o c i t y , and s t a b i l i z a t i o n . A l t h o u g h advances i n t h e s t u d y o f dune sands and o f dune s y s t e m s have been made i n recent years, remain t o images extreme,
many a s p e c t s a r e s t i l l p o o r l y u n d e r s t o o d and numerous problems
be c l e a r l y delineated.
from s a t e l l i t e s
hundreds
Recent of
miles
investigations away
and
have i n v o l v e d L a n d s a t
included,
at
the
opposite
t h e p e t r o g r a p h i c e x a m i n a t i o n o f i n d i v i d u a l sand g r a i n s and t h e a n a l y s i s
o f s t r a t i f i c a t i o n and f a b r i c t h r o u g h l a t e x p e e l s .
The p r i m a r y o b j e c t i v e o f t h e
p r e s e n t d i s c u s s i o n i s t o s u g g e s t p r o b l e m s i n v a r i o u s c a t e g o r i e s t h a t need f u r t h e r investigation. composition,
These c a t e g o r i e s i n v o l v e a l l o f t h e p r i n c i p a l a t t r i b u t e s o f dunes:
texture, structure,
and f a b r i c ; a l s o c o n s i d e r e d a r e t h e t w o p r i n c i p a l
components o f m o s t dune systems, t h a t i s , dune and i n t e r d u n e d e p o s i t s .
2
Sangre d~ C r i s t o “ a n w f o r r s trail Dunes, C o l o r a d c , U.S.A.
F i g . 2.
s a n d ( f o r e g r o u n d ) a t Great Sand
F i g . 3. S a l t polygons i n sebkha i n t e r d u n e , s o u t h o f Dhahran, Saudi A r a b i a ; photo by T. S. A h l b r a n d t .
Fig. 4.
Fog o v e r dunes in Namib Desert, Soutrivier, South West Africa.
Fig. 5. Stabilization of sand by vegetation, dune coppice north o f Sandwich Harbor, South West Africa.
a
Fiq. 6. Veqetation a n d rock b a r r i e r s t h a t r e t a r d dune migration along Kuiseb River, Namib Desert, South West Af r i ca; photo by E. Tad Nichols.
Fig. 7. Large s c a l e , high angle f o r e s e t s i n Ple istoc e ne e o l i a n i t e s , Mallorca, Balearic I s l a n d s; t a b u l a r planar s e t ; photo by M. Esteban.
5 COMPOSITION
The dominant m i n e r a l however,
r e p r e s e n t e d i n dune sands,
i n some r e g i o n s c a l c i u m c a r b o n a t e ( F i g .
forms most o f t h e sand ( F i g . 8 ) . and
other
impurities
occur
in
is,
7 ) and,
o f course, i n others,
quartz; gypsum
P a r t i c l e s o f volcanic rock, f e l d s p a r grains various
proportions
in
some
dune
fields.
L o c a l l y , as w i t h v o l c a n i c r o c k fragments a t G r e a t Sand Dunes, Colorado, such e x o t i c g r a i n s may c o n s t i t u t e more t h a n h a l f o f t h e sand.
Aerial F i g . 8. Mexico, U.S.A.
view o f gypsum dunes,
White Sands N a t i o n a l Monument,
New
A r e c e n t r e v i e w o f c a r b o n a t e e o l i a n i t e s (McKee and Ward, i n p r e s s ) has shown 26 p r i n c i p a l l o c a l i t i e s around t h e w o r l d , m a i n l y i n t h e e q u a t o r i a l b e l t , where 9).
Sand t h a t forms e o l i a n
1 imestones c o n s i s t s l a r g e l y o f o o l i t e s , f o r a m i n i f e r s ,
dunes o f c a r b o n a t e sand a r e w e l l developed ( F i g .
o r s h e l l fragments, and
commonly i s i n d i s t i n g u i s h a b l e i n c o m p o s i t i o n f r o m beach o r b a r sands nearby. This s i m i l a r i t y
i n composition could explain,
i n part,
why r e l a t i v e l y few
c a r b o n a t e e o l i a n i t e s have been r e c o g n i z e d i n t h e e a r l y g e o l o g i c r e c o r d .
6
F i g . 9. Distribution hemispheres.
of
carbonate
eolianites
in
eastern
and
western
TEXTURE D e s p i t e t h e f a c t t h a t many analyses o f sand g r a i n s have been made i n dune f i e l d s throughout t h e world,
v a r i o u s a s p e c t s of t h e d i s t r i b u t i o n p a t t e r n s o f
sandsize and s o r t i n g do n o t seem t o be f u l l y e s t a b l i s h e d o r u n d e r s t o o d except on a l o c a l b a s i s .
Evidence t h a t a m a j o r p a r t o f a l l dune sands a r e o f f i n e
g r a i n s i z e (1/8-1/4
mm) and a r e m o d e r a t e l y w e l l s o r t e d seems t o be e s t a b l i s h e d
( A h l b r a n d t , 1979, p. 24).
L o c a l l y , however, sand on dune f l a n k s i s f o u n d t o be
c o a r s e r on t h e average and more p o o r l y s o r t e d t h a n t h a t on t h e c r e s t s , based on a comparison o f analyses f r o m dunes o f A u s t r a l i a , South West A f r i c a , Sand Dunes, Colorado (E. D. McKee unpub. d a t a ) .
and Great
These d a t a suggest t h e need
7
f o r g r e a t e r uniformity i n sampling, e s p e c i a l l y as regards s p e c i f i c locations on a dune and d i f f e r e n t types and s i z e s of dunes in order t o make possible worldwide comparisons. Analyses regarding grain s i z e and s o r t i n g d i s t r i b u t i o n a t Great Sand Dunes (Fig. 10) i n d i c a t e t h a t t h e higher dunes (>30 m ) contain much more fine-grained sand and much l e s s medium-grained sand than do t h e lower dunes, and, i n a d d i t i o n , they a r e d e f i n i t e l y b e t t e r sorted. In t h i s p a r t i c u l a r dune f i e l d , t h e coarsest and most poorly sorted sand i s on t h e windward slope, b u t how t h i s f e a t u r e compares w i t h t h a t of o t h e r dune f i e l d s i s n o t known. I n any event, much more information i n t h e form of analyses from selected regions t o show d i f f e r e n c e s r e s u l t i n g from o r responsible f o r dune type and dune s i z e a r e needed f o r a b e t t e r understanding of processes and forms. Information on t h e r e l a t i o n s between local wind movements, on and around individual dunes, and t h e r e s u l t i n g t e x t u r a l p a t t e r n s of t h e s e dunes, a l s o i s rare.
COARSE COARSE
MEDIUM
FINE
MEDIUM
FINE
VERY FINE
VERY FINE
COARSE
MOD. WELL SORTED
WELL SORTED
MEDIUM
FINE
VERY FINE
MOD, MODERATELY SORTED WELL SORTED
Fig. 10. Low barchanoid ridge dune, Great Sand Dunes, Colorado, showing d i s t r i b u t i o n of grain size and s o r t i n g along p r o f i l e .
U.S.A.,
8
STRUCTURE Processes o f sand t r a n s p o r t by w i n d C r o s s - s t r a t i f i c a t i o n i n e o l i a n deposits i s t h e product o f several d i s t i n c t processes o f t r a n s p o r t (Hunter, 1981; Kocurek and D o t t , 1981).
These processes
11) t h a t i n c l u d e s v a r i o u s amounts
are (1) s a l t a t i o n o r r i p p l e m i g r a t i o n (Fig.
o f s u r f a c e creep, ( 2 ) g r a i n f a l l o r a d r o p p i n g o f g r a i n s f r o m sand i n suspension (Fig.
12),
and
(3)
avalanching
13,
(Figs.
14)
either
b y mass
movement
(slumping) o r by g r a i n f l o w a g e which i n v o l v e s t h e d o w n h i l l r o l l i n g o r s l i d i n g o f discrete particles.
Experiments made i n a w i n d t u n n e l b y C h r i s t o p h e r J .
Schenk show t h a t t h e s e t h r e e processes f o r m t y p e s o f s t r a t i f i c a t i o n g e n e r a l l y s i m i l a r i n c h a r a c t e r and, i n many examples,
d i f f i c u l t t o d i s t i n g u i s h one f r o m
A few f e a t u r e s , however, a r e d i s t i n c t i v e and, where t h e s e o c c u r i n a
another.
deposit, are diagnostic o f a p a r t i c u l a r o r i g i n . C r i t e r i a f o r recognizing eolian f a b r i c Useful i n d i c a t o r s o f d e p o s i t i o n a l process a r e t h e v a r i o u s forms o f deformational
s t r u c t u r e such as t h r u s t s and flames,
c h a r a c t e r i s t i c o f avalanching;
t h e y commonly f u r n i s h evidence o f t h e degree o f c o h e s i o n i n sand a t t h e t i m e o f deposition.
The b e v e l l e d bottoms o f f o r e s e t s a r e p r e s e r v e d i n some c l i m b i n g
r i p p l e s t r u c t u r e s ; t h e y n o t o n l y i n d i c a t e t h a t sand movement was by s a l t a t i o n , b u t a l s o h e l p t o d e t e r m i n e o r i e n t a t i o n o f t h e r i p p l e d s u r f a c e on t h e dune.
A
t h i r d f e a t u r e p r e s e r v e d i n some e o l i a n c r o s s - s t r a t a c o n s i s t s o f sand f l o w t o e s ( F i g . 1 2 ) , composed o f r e l a t i v e l y c o a r s e sand tongues a t t h e base o f f o r e s e t s i n a m a t r i x o f f i n e r sand d e p o s i t e d as g r a i n f a l l . Many more s t u d i e s need t o b e made on methods o f d i f f e r e n t i a t i n g between f a b r i c s o f t h e t h r e e main t y p e s o f e o l i a n d e p o s i t s , and on comparisons w i t h s t r a t a f r o m o t h e r environments. Basic s t r u c t u r a l types By f a r
t h e commonest t y p e s
tabular-planar
o r wedge-planar
of
s t r u c t u r e i n most
cross-strata.
sand seas a r e e i t h e r
These t y p e s
are t h e
normal
p r o d u c t o f l e e - s i d e a v a l a n c h i n g on dunes; t h e y a r e m o s t l y medium t o l a r g e s c a l e and t h e y d i p a t r e l a t i v e l y h i g h angles.
The f a c t o r s r e s p o n s i b l e f o r each of
t h e s e t y p e s a r e n o t y e t understood, n o r i s i t c l e a r why some dune forms e x h i b i t a g r e a t e r percentage o f t a b u l a r - p l a n a r t h a n wedge-planar s t r a t a .
A
scarcity
of
trough
structures
i n most
e o l i a n deposits
is
generally
recognized; however, some e x c e p t i o n s occur, e s p e c i a l l y i n c o a s t a l dunes.
Large
t r o u g h s t r u c t u r e s f o r m i n g f e s t o o n s ( F i g . 1 5 ) , have been r e c o r d e d f r o m c a r b o n a t e eolianites
i n t h e saddles between " s p i l l - o v e r
1967; MacKenzie, 1964).
lobes"
o f dune r i d g e s
(Ball,
9
F i g . 11. C o n t a c t between a v a l a n c h e s a n d ( b o t t o m ) and r i p p l e - p r o d u c e d s t r a t a ( t o p ) ; l e e b a s e of l a r g e dune; G r e a t Sand Dunes, C o l o r a d o , U.S.A.; p h o t o by n. C . S c h n a b e l .
Fig. 12. Sand-flow t o e s a t b a s e o f a v a l a n c h e d e p o s i t s ( d a r k ) , w i t h n e a r l y s t r u c t u r e l e s s s a n d ( l i g h t ) , p r o b a b l y f r o m g r a i n f a l l o u t , between wedges; G r e a t Sand Dunes, C o l o r a d o , U.S.A.; p h o t o by D. C. S c h n a b e l .
10
F i q . 13. L.enses i n s t r a t a f o r m e d by slump sand masses; s e c t i o n normal t o w i n d p h o t o by d i r e c t i o n on l e e s l o p e o f dune; G r e a t Sand Dunes, C o l o r a d o , U.S.A.; n. C. Schnabel.
F i g . 14. Laminae f r o m a v a l a n c h i n g ( g r a i n f l o w ) o f d r y sand i n w h i c h e a c h s i z e g r a d e was dyed a p a r t i c u l a r c o l o r t o d e t e r m i n e sorting fabric; laboratory experiment.
11
F i q . 15. Bahamas.
Trough cross-bedding Photo by R. Redfern.
i n e o l i a n carbonates;
New P r o v i d e n c e I s l a n d ,
A q u a n t i t a t i v e s t u d y o f t h e p r o p o r t i o n s o f each b a s i c dune s t r u c t u r e t h a t A comparison
occurs i n various d e p o s i t i o n a l environments should be rewarding.
o f t h e c r o s s - s t r a t a t y p e s i n dunes o f d i f f e r e n t c o m p o s i t i o n , d i f f e r e n t f o r m and different
wind
regime
probably
d i s t i n g u i s h i n q between v a r i e t i e s c o n t r o l l i n g genesis. little
or
establish
dune
and
for
additional
criteria
for
recognizing the features
Because t h e t r o u g h s t r u c t u r e i s t h e p r o d u c t o f r e l a t i v e l y
understood
absence
would of
processes
general
and
proportion
is
comparatively
probably
is
an
uncommon,
its
important
indicator
presence, of
d e p o s i t i o n a l environment i n general. D e f o r m a t io n a l S t r u c t u r e s Penecontemporaneous faults
and f o l d s
(Fig.
r o t a t e d blocks (Fig. serve
as
useful
indicators therefore
deformational
structures
including
17),
16), f l a m e s ( F i g .
tools
i n determining genesis.
p r o v i d e i n f o r m a t i o n on t h e
important,
they
(Fig.
types
of
18),
and
1 9 ) a r e s u f f i c i e n t l y numerous i n m o s t e o l i a n d e p o s i t s t o These s t r u c t u r e s a r e good
o f t h e nature o f t h e stress--whether
deformation.
various
break aparts
record the
degree
part
of
of
cohesion
compression o r tension--and
a sand body i n v o l v e d . represented a t
More
the time o f
Thus, t h e y p e r m i t f a i r l y r e l i a b l e d i s t i n c t i o n s t o h e made between
sand bodies t h a t ,
when d e p o s i t e d ,
were
(1) s a t u r a t e d ,
(2) dry,
s u r f a c e ( c r u s t ) , o r ( 4 ) w e t t e d t h r o u g h o u t ( w a t e r drawn o f f ) .
(3) wetted
12
16. S t a i r - s t e p f o l d i n middle p a r t of a v a l a n c h e s l o p e , w e t t e d sand i n dune ; Denver Sediment a r y S t r u c t u r e s Labor a t o r y
Fig.
.
Fig. 17. Deformational s t r u c t u r e s i n a v a l a n c h e s t r a t a , Flame f o l d s i n d r y sand n e a r b a s e o f s l o p e ; Denver Sedimentary S t r u c t u r e s Laboratory.
13
F i g . 18. B r e a k - a p a r t l a m i n a e i n l o w e r p a r t o f s l i p f a c e o f dune w i t h w e t c r u s t ; Denver S e d i m e n t a r y S t r u c t u r e s L a b o r a t o r y .
F i g . 19. R o t a t e d b l o c k s i n dry sand, Sedimentary S t r u c t u r e s Laboratory.
near top o f avalanche slope;
Denver
14 Nine
or
more
principal
types
of
deformational
structure
have
been
recognized, b o t h i n experiments (McKee, Douglass, and R i t t e n h o u s e , 1971) and i n t h e f i e l d (McKee and B i g a r e l l a , 1972).
Some o f t h e s e forms a r e t y p i c a l o f more
t h a n one c o n d i t i o n o f cohesion, b u t o t h e r s seem t o be u n i q u e t o a p a r t i c u l a r c a t e g o r y o f m o i s t u r e c o n t e n t , and t h e r e f o r e a r e e s p e c i a l l y i m p o r t a n t i n d i c a t o r s o f genesis.
L i k e assemblages o f f o s s i l s , a s s o c i a t e d groups o f s t r u c t u r e s a r e
more i m p o r t a n t t h a n i s an i n d i v i d u a l sediment i n which t h e y were formed.
f o r demonstrating t h e nature o f t h e
Further investigations i n t h i s f i e l d w i t h
a d d i t i o n a l c o n f i r m a t i o n o f s t r u c t u r a l i n t e r p r e t a t i o n s o f f e r good o p p o r t u n i t i e s f o r advancing o u r knowledge o f dune sands. D u n e - l i k e s t r u c t u r e s i n o t h e r environments Among t h e
various
features t h a t
commonly a r e a t t r i b u t e d t o t h e e o l i a n
d e p o s i t i o n o f sand a r e t h e c o n s i d e r a b l e l e n g t h and h i g h - a n g l e d i p o f many cross-strata.
S u f f i c i e n t o b s e r v a t i o n s have been r e c o r d e d t o e s t a b l i s h beyond
reasonable doubt t h a t t h e l e e s i d e o r s l i p f a c e o f most modern dunes stands a t 30 degrees o r s l i g h t l y more and t h a t t h e l e n g t h o f t h e f o r e s e t s developed on i t
may be 8 o r 10 meters and o f t e n much more.
Furthermore,
the resulting
s t r u c t u r e s o f these deposits are dominantly o f planar type--either
wedge o r
tabu1 a r . The s i z e and a t t i t u d e o f dune s t r u c t u r e s ,
however, a r e n o t b y themselves
s u f f i c i e n t t o p o s i t i v e l y i d e n t i f y t h e e o l i a n o r i g i n o f a sand d e p o s i t .
There
a r e many e x c e p t i o n s t o t h e normal, r e s u l t i n g f r o m v a r i a b i l i t y i n t h e process o f development. and
G r a i n s i z e , cohesion o f g r a i n s , s t r e n g t h and v a r i a b i l i t y o f w i n d
numerous
other
factors
d e p o s i t i o n a l product.
are
responsible
for
great
differences
i n the
Subsequent processes such as compaction and d i a g e n e s i s
a l s o must b e c o n s i d e r e d (Walker and Harms, 1972). I n t h e r e c o g n i t i o n o f e o l i a n d e p o s i t s and i n t h e i n t e r p r e t a t i o n o f v a r i o u s dune t y p e s ,
i t i s n o t enough t o be f a m i l i a r w i t h wind-formed c h a r a c t e r i s t i c s
alone, such as s t r u c t u r e s , t e x t u r e s and f a b r i c .
A l s o i m p o r t a n t i s t o know how
t h e s e f e a t u r e s compare w i t h s i m i l a r ones i n f l u v i a l and m a r i n e environments. F o r example, e x t e n s i v e , r a t h e r p y r e sand d e p o s i t s a r e known t o o c c u r i n p a r t s o f t h e N o r t h Sea ( H o u b o l t , 1968; McCave, 1971) and o f f t h e New England c o a s t (Jordan,
1962;
Swift,
1975) where t h e c r e s t s o f v e r y l a r g e sand waves w i t h
asymmetrical
form
are
indicated
similarities,
t h e marine l i n e a r
on
sand
bottom ridges
profiles. probably d i f f e r
s u b a e r i a l dunes as d i s c u s s e d b y Walker and M i d d l e t o n (1977).
Despite greatly
these from
They show t h a t
t h e exaggerated v e r t i c a l s c a l e o f t h e N o r t h Sea p r o f i l e s g i v e s a d i s t o r t e d p i c t u r e o f t h e steep-side,
s l o p e a n g l e which
erroneous i d e a o f t h e f o r e s e t l e n g t h s .
i s v e r y l o w and l e a d s t o an
Because l i t t l e i n f o r m a t i o n i s a v a i l a b l e
15 on t h e n a t u r e o f t h e i r characteristics,
stratification,
on t h e i r m i n o r s t r u c t u r e s o r g r a i n
o r on t h e i r r e l a t i o n s t o a s s o c i a t e d sediments o f o t h e r t y p e s
o r t o faunas, v a l i d comparisons can n o t be made y e t betweeen t h e s e marine sands and t h o s e o f e o l i a n o r i g i n .
A
second environment,
comparison,
g r e a t l y i n need o f i n v e s t i g a t i o n f o r purposes o f
i s t h e l a r g e r i v e r system.
Recent o b s e r v a t i o n s on t h e Orinoco
R i v e r i n Venezuela b y C a r l N o r d i n o f t h e U.S. that
sand
deposits
kilometers,
reach
across
the width
G e o l o g i c a l Survey, have shown of
the
and e x t e n d f o r many k i l o m e t e r s downstream.
under w a t e r d u r i n g f l o o d stage,
river,
more t h a n t w o
T h i s sand, which i s
i s exposed as a s e r i e s o f t r a n s v e r s e b a r s
Trenches made b y N o r d i n show l a r g e - s c a l e t a b u l a r -
d u r i n g l o w w a t e r (Fig. 20).
p l a n a r s t r u c t u r e s a t moderate t o h i g h angles t h a t d i p d o w n r i v e r (Fig. Details
f r o m measurements on l e n g t h s ,
angles,
21).
and t h i c k n e s s e s a r e n o t y e t
a v a i l a b l e , b u t t h e importance o f a c a r e f u l analyses o f t h e s e d e p o s i t s s h o u l d be apparent. Shingle structures Lack o f
i n t e r d u n e s r e s u l t s f r o m one dune c l i m b i n g o r m i g r a t i n g up t h e
windward s l o p e o f t h e one ahead.
I n most cases,
t h i s s i t u a t i o n i s brought
about b y t h e presence o f a b a r r i e r such as a r o c k r i d g e o r mountainous t e r r a n e t h a t slows down t h e f o r w a r d movement o f t h e sand masses and causes one dune t o p i l e up on another. Colorado,
U.S.A.,
Such a s i t u a t i o n i s i l l u s t r a t e d b y Great Sand Dunes i n
where t h e Sangre de C r i s t o Range ( F i g . 2) e f f e c t i v e l y b l o c k s
a g e n e r a l e a s t e r l y movement o f t h e dunes. strike
can
be
seen
to
have
Sand r i d g e s w i t h l a r g e l y n o r t h - s o u t h
relatively
short
distances
between c r e s t s .
I n d i v i d u a l dune r i d g e s a r e n o t s e p a r a t e d b y t h e n e a r - h o r i z o n t a l interdunes,
surfaces o f
b u t a r e advancing up g e n t l e windward s l o p e s o f t h e n e x t dune
downwind l i k e o v e r l a p p i n g s h i n g l e s (Fig. 22). Not a l l s h i n g l e s t r u c t u r e s i n t r a n s v e r s e dune f i e l d s a r e caused by mountain barriers. ridges
I n t h e Namib D e s e r t o f South West A f r i c a , f o r i n s t a n c e , l a r g e dune
bordering
the Atlantic
coast
s o u t h o f t h e Kuiseb R i v e r
(Fig.
23)
i l l u s t r a t e dunes s t a c k e d c l o s e t o g e t h e r w i t h o u t f l a t - l y i n g i n t e r d u n e s between; one dune c l i m b s o n t o t h e next.
Here, however, no p h y s i c a l b a r r i e r can be c i t e d
t o e x p l a i n a s l o w i n g down o f t h e n o r t h e a s t w a r d advance o f sand r i d g e s .
One
p o s s i b l e e x p l a n a t i o n i s t h e n o r t h e a s t e r l y w i n t e r w i n d (Berg Wind) t h a t opposes t h e normal w i n d o f f t h e South A t l a n t i c .
Thus,
i n t h i s desert c o n f l i c t i n g
seasonal winds may cause a s h i n g l e development n o t f a r i n l a n d f r o m t h e coast.
16
D e p o s i t s of sand waves a c r o s s t h e O r i n o c o R i v e r i n V e n e z u e l a , f o r m e d F i g . 20. d u r i n g f l o o d s t a g e , p h o t o by C a r l N o r d i n .
T r e n c h showing l a r g e s c a l e f o r e s e t s - i n sand wave d e p o s i t s o f O r i n o c o , F i g . 21. Venezuela; p h o t o by C a r l Mordin.
17
Fig. 22. Great S a n d Dunes, Colorado, U.S.A., view from soutn showing shingle s t r u c t u r e (overlapping d u n es ) ; photo by D. C. Schnabel.
Fig. 23. Overlapping t r a n s v e r s e dunes ( n o n interdune) near Sandwich Bay, South West A f r i c a ; photo by E. Tad Nichols. S t r u c t u r a l evidence of s h i n g l e s t r u c t u r e i s recorded in s t r a t i f i c a t i o n as seen i n trenches located a t t h e base of any t r an sve rse dune t h a t i s climbing t h e g e n tly sloping windward f ace of a preceding dune (Fig. 2 4 ) .
This s t r u c t u r e nor-
mally c o n s i s t s of st e ep l y dipping f o r e s e t s of an e a r l i e r dune, bevelled t o a lowangle s u r f a c e d i p p i n g windward and commonly covered with a veneer of s a n d marking t h e base of t h e new dune.
This i n turn may be p a r t l y covered by high-angle
s t r a t a , dipping downwind and representing avalanching de posits o f t h e new dune.
18
Fig. 24. Low-angle windward-side s t r a t a o v e r l y i n g b e v e l l e d s u r f a c e o f e a r l i e r high a n g l e f o r e s e t s i n s h i n g l e s t r u c t u r e , Great Sand Dunes, Colorado, U.S.A.; photo by D. C. Schnabel.
F i g . 25. I n t e r d u n e a t b a s e o f l a r g e l i n e a r dune, n o r t h e a s t p a r t , Rub a1 K h a l i ; photo by T. S . Ahlbrandt.
19 MORPHOLOGY
Interdunes The s u b j e c t o f i n t e r d u n e s and t h e i r importance as i n t e g r a l p a r t s o f dune systems has been l a r g e l y n e g l e c t e d u n t i l r e c e n t y e a r s
(Kocurek,
1981).
In
modern dune f i e l d s , i n t e r d u n e s o c c u r as e s s e n t i a l l y f l a t s u r f a c e s between dune r i d g e s ( F i g . 25) o r dune mounds; i n a n c i e n t r o c k s t h e y f o r m e i t h e r t h i n , p l a n a r s t r a t a o f mud and s i l t o r a r e r e p r e s e n t e d b y bedding p l a n e s , s e p a r a t i n g s e t s o f cross-strata deposition,
(Hunter,
1981).
Because t h e y a r e formed n o t p r i m a r i l y by wind
b u t r a t h e r as a r e s u l t o f ponding,
o t h e r m i s c e l l a n e o u s processes,
deflation,
p l a n t growth and
o p e r a t i n g between t i m e s o f dune b u i l d u p , t h e y
a r e d i s t i n c t i v e i n s e v e r a l ways (as d i s c u s s e d below) and commonly a r e i n marked c o n t r a s t w i t h t h e e o l i a n sands t h a t e n c l o s e them. C h a r a c t e r i s t i c f e a t u r e s o f i n t e r d u n e d e p o s i t s i n c l u d e adhesion s t r u c t u r e s (Fig.
26),
evaporite
crusts
i r r e g u l a r bedding s u r f a c e s .
(Fig.
3),
carbon
residues
from
plants,
and
T h e i r r e l a t i o n s h i p t o l a g o o n a l sediments, a l l u v i a l
f a n s , and o t h e r d i s t i n c t i v e n o n - e o l i a n d e p o s i t s suggests t h a t t h e y should have numerous
criteria for
recognition,
but
apparently they are s t i l l
commonly
unrecognized among s t r a t a o f p r o b a b l e e o l i a n genesis i n a n c i e n t f o r m a t i o n s . I n v e s t i g a t i o n s o f t h e c h a r a c t e r i s t i c s and s t r a t i g r a p h i c r e l a t i o n s o f i n t e r d u n e deposits
s h o u l d t h e r e f o r e be h i g h l y rewarding,
and,
although t h e interdunes
s e p a r a t i n g dunes o f u n i d i r e c t i o n a l winds a r e f a i r l y w e l l understood (McKee and Moiola,
1975),
virtually
no good d e s c r i p t i o n s
are a v a i l a b l e f o r interdunes
between c l u s t e r s o f s t a r dunes ( F i g . 27) o r rows o f l i n e a r dunes (Fig. 28). O r i g i n o f 1 in e a r dunes The o r i g i n o f l i n e a r dunes, a l s o r e f e r r e d t o as s e i f o r l o n g i t u d i n a l dunes, has l o n g been a m a t t e r o f much c o n t r o v e r s y . p a r a l l e l sand r i d g e s t h a t have two s l i p f a c e s , ridge, d i p p i n g i n opposite directions. l i n e a r dunes o f v a r i o u s sand seas. A f r i c a c h a r a c t e r i s t i c a l l y form zig-zag,
These dunes a r e d e f i n e d as l o n g
i.e.,
one on each s i d e o f t h e
Some m a j o r d i f f e r e n c e s i n f o r m o c c u r i n Those o f t h e L i b y a n D e s e r t o f n o r t h e r n r a t h e r than s t r a i g h t l i n e ridges (Fig.
28); t h o s e o f A u s t r a l i a ' s Simpson D e s e r t commonly develop branches known as Yjunctions
(Fig.
29).
I n t h e Namib Desert o f South West A f r i c a , many l i n e a r
dunes a r e complex w i t h s t a r dunes o r o t h e r t y p e s a l o n g t h e i r c r e s t s (Fig. 30).
P o s s i b l y because o f t h e s e v a r i a t i o n s i n form,
genesis i s r e p r e s e n t e d b y dunes o f t h e l i n e a r c l a s s .
more t h a n one t y p e o f
20
26. A d h e s i o n s t r u c t u r e s i n i n t e r d u n e p l a y a between b a r c h a n s , l d a l v i s Ray, South West A f r i c a .
Fig.
south of
S t a r dune c l u s t e r w i t h i r r e g u l a r l y spaced i n t e r d u n e s ; Sahara, N o r t h F i g . 27. A f r i c a ; a i r p h o t o b y H. T. U . Smith.
21
F i g . 28. Z i g - z a g t r e n d s o f l i n e a r ( s i e f ) dunes n o r t h o f Sebha O a s i s , L i b y a ; a e r i a l view.
Fig. 29. Y - j u n c t u r e i n l i n e a r dune, p h o t o b y J . F. S c h r e i b e r , Jr.
G r e a t Sandy D e s e r t , A u s t r a l i a ;
aerial
22
F i g . 30. Complex l i n e a r and s t a r dunes, southwest o f Gobabeb, Namib D e s e r t , South West A f r i c a ; p h o t o by E . Tad Nichols. P r i n c i p a l hypotheses o f genesis t h a t have been proposed f o r l i n e a r dunes a r e r e f e r r e d t o as
(a) d e f l a t i o n ,
(b)
helical
roll,
( c ) dune m o d i f i c a t i o n and
e v o l u t i o n , ( d ) l e e - s i d e accumulation, and ( e ) b i d i r e c t i o n a l w i n d (McKee, 1982, p.
25).
Arguments f o r and a g a i n s t t h e s e v a r i o u s hypotheses a r e numerous and
combinations o f two o r more e x p l a n a t i o n s seem l i k e l y t o a p p l y i n some sand seas.
Many a d d i t i o n a l f a c t s need t o be g a t h e r e d t h r o u g h d e t a i l e d s t u d i e s ,
especially o f
structures,
and wind d a t a ,
b e f o r e more p o s i t i v e c o n c l u s i o n s
r e g a r d i n g t h e o r i g i n o f dunes o f t h i s c l a s s can be reached. Selective preservation The m i g r a t i o n o f sand b o d i e s i n any dune f i e l d n o r m a l l y i s a process o f essentially
simultaneous
erosion
d e p o s i t i o n on t h e o t h e r ( l e e ) .
on
one
S e t s of
side
of
a
dune
(windward)
d i p p i n g l a m i n a e may, t h e r e f o r e ,
and be
p a r t l y , o r even e n t i r e l y , removed d u r i n g a subsequent b e v e l l i n g o f t h e windward s u r f a c e as a dune advances.
T h i s process u l t i m a t e l y r e s u l t s i n a s e l e c t i v e
p r e s e r v a t i o n w i t h b u r i a l o f o n l y t h e l o w e r o r basal p a r t o f an o r i g i n a l dune d e p o s i t (Kocurek and D o t t , 1981; Rubin and Hunter, 1982). Through t r u n c a t i o n o f t h e upper p a r t s o f f o r e s e t s t h a t were o r i g i n a l l y d e p o s i t e d on a dune s l i p f a c e as seen a t White Sands, New Mexico ( F i g . 3 1 ) , t h e amount o f sand removed f r o m t h e s u r f a c e and t r a n s p o r t e d downwind may v a r y
23 considerably.
Probably
the
sand
supply
available
and
the
strength
and
A t one extreme, these
c o n s i s t e n c y o f t h e wind moving i t a r e major c o n t r o l s .
f a c t o r s may be so weak t h a t a c t u a l t h i n windward-slope d e p o s i t s , d i p p i n g upwind
a t low a n g l e s , may be p r e s e r v e d a s i n t r e n c h e s of t r a n s v e r s e dunes a t Great ( F i q . 11) and i n t h e Permian Coconino Sandstone of Grand Canyon, Arizona. A t t h e o t h e r e x t r e m e , t h e wind may be s o s t r o n g t h a t i t
Sand Dunes, Colorado, U.S.A.
removes a l l sand i n a given p l a c e down t o t h e u n d e r l y i n g i n t e r d u n e . dunes
commonly
removed.
have
a
considerable
percentage
of
the
original
Normal deposit
C l e a r l y , t h i s s u b j e c t d e s e r v e s much more c a r e f u l s t u d y t h a n has been
g i v e n , f o r o n l y t h e s t r u c t u r e s i n t h e lower p a r t of a s l i p f a c e a r e p r e s e r v e d i n t h e q e o l o q i c r e c o r d by t h i s s e l e c t i v e p r o c e s s .
Windward s u r f a c e of barchanoid r i d g e showing b e v e l l e d t o p s o f Fiq. 31. f o r e s e t s as dune m i q r a t e s , White Sands, New Mexico, I1.S.A. RevPrsing dunes R e v e r s i n q dunes ( F i g . 32) probably a r e f a r more common t h a n i s g e n e r a l l y recoqnized,
although
relatively
s t r u c t u r e have been made.
few
detailed
studies
of
their
form
and
E a r l i e s t r e c o r d s of such dunes d a t e back almost a
c e n t u r y t o t h e o b s e r v a t i o n s of Hedin (1896) and Cornish (1897) and t h e y have been examined s i n c e t h a t t i m e i n a number o f d i f f e r e n t sand s e a s .
They a r e
formed under c l i m a t i c regimes i n both humid and a r i d a r e a s , and i n c o l d lowa l t i t u d e and c o l d h i q h - a l t i t u d e
a r e a s , a p p a r e n t l y l a r g e l y a s a f u n c t i o n of
c o n t r a s t i n q wind d i r e c t i o n s (McKee, 1979, p. 106).
24
F i g . 32. R e v e r s i n g dunes o f t r a n s v e r s e t y p e , Santa C a t a r i n a , B r a z i l .
f r o m s o u t h n e a r c o a s t a t Loga,
A l t h o u g h t h e b a s i c f e a t u r e s o f r e v e r s i n g dune development a r e r e c o r d e d , many d e t a i l s c o n c e r n i n g s t r u c t u r e p a t t e r n , r a t e o f movement, r e l a t i o n s t o i n t e r d u n e s and o t h e r a s p e c t s have n o t been c l e a r l y d e f i n e d .
R e v e r s i n g dunes a r e f o r m e d on
t r a n s v e r s e dune r i d g e s i n G r e a t Sand Dunes, C o l o r a d o , U.S.A., l i n e a r dune r i d g e s i n t h e Namib D e s e r t , caused by winds,
S o u t h West A f r i c a .
b u t t h e y o c c u r on They seem t o be
c o n t r o l l e d by m o u n t a i n b a r r i e r s i n some r e g i o n s ,
open d e s e r t t e r r a i n elsewhere.
but are i n
C h a r a c t e r i s t i c p a t t e r n s o f c r o s s - b e d d i n g and
r e l a t i o n s t o i n t e r d u n e development a r e o t h e r f e a t u r e s n e e d i n g i n v e s t i g a t i o n . REFERENCES A h l b r a n d t , T.S., 1979. T e x t u r a l p a r a m e t e r s o f e o l i a n d e p o s i t s , C h a p t e r B, I n Edwin D. McKee ( E d i t o r ) , A S t u d y o f G l o b a l Sand Seas. U.S. Geol. S u r v e y P r o f . Paper 1052, pp. 21-51. B a l l , M.M., 1967. C a r b o n a t e sand b o d i e s o f F l o r i d a and t h e Bahamas. Jour. Sed. P e t r o l o g y , 37: 556-591. C o r n i s h , Vaughan, 1879. On t h e f o r m a t i o n o f sand dunes. Geog. Jour., 9: 278309. H e d i n , Sven, 1896. A j o u r n e y t h r o u g h t h e Takla-Makan D e s e r t Chinese Turkistan. Geog. Jour., 8: 264-278. 1968. Recent s e d i m e n t s i n t h e s o u t h e r n b i g h t o f t h e N o r t h H o u b o l t , J.J.H.C., Sea. G e o l o g i e e n Mijnbouw, 47: 245-273. 1981. S t r a t i f i c a t i o n s t y l e s i n e o l i a n sandstones: Some H u n t e r , R.E., P e n n s y l v a n i a n t o J u r a s s i c examples f r o m t h e w e s t e r n i n t e r i o r U.S.A. SOC. Econ. P a l e o n t o l o g i s t s and M i n e r a l o g i s t s Spec. Pub. 31: pp. 315-329. J o r d a n , G.F., 1962. L a r g e s u b m a r i n e sand waves. S c i e n c e , 136: 839-848.
25 Kocurek, G., 1981. S i g n i f i c a n c e o f i n t e r d u n e d e p o s i t s and bounding s u r f a c e s i n e o l i a n dune sands. Sedimentology, 28: 753-780. Jr., 1981. D i s t i n c t i o n and uses o f s t r a t i f i c a t i o n Kocurek, G. and D o t t , R.H., types i n t h e i n t e r p r e t a t i o n o f e o l i a n deposits. Jour. Sed. P e t r o l o g y , 51: 579-595. MacKenzie, F.T., 1964. Bermuda P l e i s t o c e n e e o l i a n i t e s and paleowinds. Sedimentology, 3: 52-64. McCave, I.N., 1971. Sand waves i n t h e N o r t h Sea o f f t h e coast o f Holland. Marine Geology, 10: 199-225. 1979. A s t u d y o f g l o b a l sand seas. U.S. Geol. Survey Prof. Paper McKee, E.D., 1052, 429 pp. McKee, E.D., 1982. Sedimentary s t r u c t u r e s i n dunes o f t h e Namib Desert, South West A f r i c a . Geol. SOC. America, Spec. Paper 188, 64 pp. McKee, E.D. and B i g a r e l l a , J.J., 1972. Deformational structures i n B r a z i l i a n c o a s t a l dunes. Jour. Sed. P e t r o l o g y , 42: 670-681. Douglass, J.R. and Rittenhouse, Suzanne, 1971. Deformation o f McKee, E.D., l e e - s i d e laminae i n e o l i a n dunes. Geol. SOC. America B u l l . , 82: 359-378. McKee, E.D. and Moiola, R.J., 1975. Geometry and growth o f t h e White Sands U.S. Geol. Survey Jour. Research, 3: 59-66. dune f i e l d , New Mexico. and Ward, W.C., ( i n press). Carbonate dunes and e o l i a n McKee, E.D. limestones. Am. Assoc. Petroleum G e o l o g i s t s Mem. Rubin, D.M. and Hunter, Ralph, 1982. Bedform c l i m b i n g i n t h e o r y and nature. Sedimentology, 29: 121-138. S w i f t , D.J.P., 1975. T i d a l sand r i d g e s and s h o a l - r e t r e a t massifs. Marine Geology, 18: 105-134. and M i d d l e t o n , G.V., 1977. F a c i e s Models 9, E o l i a n Sands. Walker, R.G. Geoscience Canada, 4: 182-190. 1972. E o l i a n o r i g i n of f l a g s t o n e beds, Lyons Walker, T.R. and Harms, J.C., Sandstone (Permian), t y p e area, Boulder County, Colorado, In Environments o f sandstone, carbonate, and e v a p o r i t e d e p o s i t i o n . Mtn. G e o l o g i s t , 9: 279-288.
This Page Intentionally Left Blank
27
ON THE SKEWNESS OF SOPIE EOLIAN SANDS FROM SAUDI A R A B I A P I E R L. BINDA:
Geology Department, U n i v e r s i t y o f Regina, Regina, Saskatchewan, Canada S4S OA2
INTRODUCTION
P o s i t i v e skewness o f g r a i n - s i z e d i s t r i b u t i o n s ,
s. curve departing from
n o r m a l i t y i n h a v i n g a t a i l i n t h e f i n e f r a c t i o n , i s o f t e n quoted as a t y p i c a l characteristic o f eolian transport.
T h i s seems t o h o l d t r u e whether one uses
the s t a t i s t i c a l l y more p r e c i s e measure o f t h e t h i r d moment o r a g r a p h i c a l l y derived approximation o f i t . I n t h e l a t e f i f t i e s and e a r l y s i x t i e s , i n s p i t e o f some c o n t r o v e r s i a l r e s u l t s , the b a l a n c e o f e v i d e n c e showed t h a t skewness can be used i n d i s c r i m i n a t i n g dune from beach sand (Mason and F o l k , 1958; Shepard and Young, 1961; Friedman, 1961; Folk, 1962).
Data on i n l a n d , d e s e r t , dunes seemed t o c o n f i r m , a l b e i t w i t h some
r e s e r v a t i o n s , t h a t p o s i t i v e skewness i s c h a r a c t e r i s t i c o f wind-blown sand. Sieve a n a l y s i s o f e o l i a n sand f r o m areas around Jeddah, p a r t l y c a r r i e d o u t as sedimentology e x e r c i s e s w i t h s t u d e n t s of K i n g A b d u l a z i z U n i v e r s i t y , r e v e a l e d t h a t , a l t h o u g h most g r a i n - s i z e d i s t r i b u t i o n s a r e i n f a c t f i n e skewed, a few coarse ( n e g a t i v e ) skewed sands do o c c u r ( s e e a l s o Binda, i n p r e s s ) . I n o r d e r t o e x p l a i n t h e anomalous d i s t r i b u t i o n s , p a i r s o f c r e s t - a n d - t r o u g h samples o f e o l i a n r i p p l e s were analyzed, as i t was suspected t h a t m i x i n g o f sand from t h e two " m i c r o - e n v i ronments" under a regime o f changing wind d i r e c t i o n was the cause. BACKGROUND
ON STATISTICAL PARAMETERS OF EOLIAN
SANDS
A comprehensive r e v i e w by F o l k (1971) shows t h a t i n i n l a n d dunes t h e mean g r a i n s i z e i s g e n e r a l l y i n t h e 2 - 3 o range and s t a n d a r d d e v i a t i o n between about . 2 5 and .70
0.
A l t h o u g h i n many cases d i f f e r e n c e s a r e n o t s t a t i s t i c a l l y s i g n i -
f i c a n t , c r e s t s a r e g e n e r a l l y b e t t e r s o r t e d than f l a n k s , and l e e w a r d f l a n k s may be b e t t e r s o r t e d t h a n windward ones.
Skewness i s g e n e r a l l y p o s i t i v e w i t h o n l y
a few sands s l i g h t l y n e g a t i v e l y skewed. L o n g i t u d i n a l dunes o f t h e Simpson D e s e r t o f A u s t r a l i a ( F o l k , 1971) have c r e s t s coarser t h a n f l a n k s .
Standard d e v i a t i o n may be as h i g h as 1.0
b e t t e r s o r t e d than f l a n k s .
0,
w i t h crests
Over 92% o f t h e l o n g i t u d i n a l dunes samples analyzed
by F o l k a r e p o s i t i v e l y skewed, w i t h c r e s t s more p o s i t i v e l y skewed t h a n f l a n k s , and windward s l i g h t l y more skewed t h a n l e e w a r d f l a n k .
28 I n t e r p a r a m e t r i c r e l a t i o n s h i p s o f sands from t h e Simpson D e s e r t do n o t r e v e a l any p a r t i c u l a r p a t t e r n , e x c e p t f o r p l o t s o f mean s i z e versus skewness: c o a r s e r t h e mean, t h e more p o s i t i v e t h e skewness, t a i l o f f i n e material.
G. t h e
the
more pronounced t h e
T h i s r e l a t i o n s h i p i s e x p l a i n e d by F o l k n o t as a m i x i n g
of p o p u l a t i o n s ; t h e p o s i t i v e t a i l i s always p r e s e n t , b u t i t becomes more e v i d e n t as t h e p r i m a r y mode s h i f t s t o c o a r s e r s i z e s . F o l k (1968 and 1971) shows t h a t t h e b e s t method o f s t u d y i n g e o l i a n sand s i z e s
i s t h e c o n s t r u c t i o n o f f r e q u e n c y c u r v e s which r e v e a l c h a r a c t e r i s t i c modes. B r i e f l y , wind has a tendency t o p i c k up from t h e d e s e r t f l o o r g r a i n s h a v i n g d i a meters around 2.5
@
(0.177 mm).
Thus, dune sands w i l l be unimodal w i t h a
pronounced mode around t h i s value, whereas d e f l a t i o n areas ( r e g ) , from which t h e 2.5
0
m a t e r i a l i s removed, a r e c h a r a c t e r i z e d by bimodal o r polymodal f r e q u e n c y
d i s t r i b u t i o n s w i t h an obvious s a d d l e i n correspondence w i t h t h i s v a l u e .
The
a n a l y s i s o f modes i n f r e q u e n c y d i s t r i b u t i o n curves has been s u c c e s s f u l l y employed by Binda (1972) and B i n d a and H i l d r e d (1973) f o r t h e r e c o g n i t i o n o f e o l i a n o r i g i n o f some sub-Recent " K a l a h a r i Sands" o f Zambia.
However, SkoEek and S a a d a l l a h
(1972) r e p o r t e d t h a t 12 o u t o f 32 dune sands from t h e Southern D e s e r t o f I r a q have bimodal g r a i n - s i z e frequency d i s t r i b u t i o n s .
They e x p l a i n t h e b i m o d a l i t y as
due t o m i x i n g o f a suspension, d u s t , p o p u l a t i o n w i t h a s a l t a t i o n , sand, p o p u l a t i o n . A h l b r a n d t (1979), i n h i s e x t e n s i v e l i s t i n g o f s t a t i s t i c a l parameters o f i n l a n d dunes f r o m v a r i o u s l o c a l i t i e s , has a few e n t r i e s c l e a r l y l a b e l e d : ward, c r e s t , s l i p f a c e .
barchan, wind-
From these few d a t a i t appears t h a t i n barchans t h e mean
g r a i n s i z e i s n o t dependent upon t h e p o s i t i o n i n t h e dune, b u t perhaps upon t h e a v a i l a b l e m a t e r i a l and wind v e l o c i t y
--
i t varies w i t h l o c a l i t y .
Crests tend t o
be b e t t e r s o r t e d t h a n windward and l e e (s1i p f a c e ) , and a1 1 ( 1 3 ) samples have p o s i t i v e skewness.
A h l b r a n d t comments on skewness b e i n g dependent on mean g r a i n
s i z e , a l t h o u g h t h i s i s more e v i d e n t l y d i s p l a y e d i n h i s f i g u r e 21 ( p l o t o f mean versus skewness f o r i n t e r d u n e m a t e r i a l ) than i n f i g u r e 20 (mean versus skewness f o r i n l a n d dunes), t h e l a t t e r showing a r a t h e r " f l a t " p a t t e r n . R e l e v a n t t o t h e p r e s e n t s t u d y i s a l i s t i n g o f parameters o f r i p p l e t r o u g h s ( 6 samples) and c r e s t s ( 3 samples) f r o m t h e G r e a t Sand Dunes N a t i o n a l Monument of Colorado ( A h l b r a n d t , 1979, p . 4 6 ) . than t h e t r o u g h s .
A l l t h e c r e s t s have c o a r s e r mean d i a m e t e r
Two c r e s t s have p o s i t i v e skewness, one n e g a t i v e , whereas a l l
b u t one t r o u g h have n e g a t i v e skewness.
A c a u t i o n a r y comment on t h e use o f skewness as an i n d i c a t o r o f e o l i a n o r i g i n o f sand i s expressed by B i g a r e l l a (1972, p. 1 4 ) :
" p o s i t i v e skewness, c o n s i d e r e d
by some an i n d i c a t o r o f dune environment, i s open t o q u e s t i o n because n e g a t i v e skewness has a l s o been r e c o r d e d i n dune sand."
29
I
I
I
I
I
I
26'
I
I /1 \
EDGE OF PRECAMBR/AN SHIELD
'r
-
\ 24'
\
c
1
AR
I
MADINAH
-
I
I
I
-\
NAFUD RUMHAT
\
*
\
\
WAD/ KHUL E/S
JEDDAH
I 36O
Figure 1.
I
I 38'
I
I 40'
I 42O
I
I 440
I
I
I
46O
L o c a t i o n map.
LOCALITIES ( F i g . 1) The samples analyzed f o r t h i s s t u d y a r e from: (NR),
Wadi K h u l e i s ( K ) , Nafud Rumhat
and Jeddah ( E a s t and S o u t h ) .
The area o f Wadi K h u l e i s l i e s i n t h e Red Sea c o a s t a l p l a i n a p p r o x i m a t e l y 80 km t o t h e n o r t h e a s t o f Jeddah, about 5 km w e s t o f K h u l e i s v i l l a g e , a t a p p r o x i m a t e l y 22O10' N and 39O20'
E,
and i s bordered t o t h e e a s t and s o u t h e a s t by Precambrian
metavolcanics and T e r t i a r y b a s a l t r e s p e c t i v e l y .
I n some p a r t s o f t h e b a s i n dunes
a r e continuous, i n o t h e r p a r t s t h e y a r e s c a t t e r e d on a d e f l a t e d g r a v e l l y s u r f a c e . Dunes a r e m a i n l y o f t h e barchan type, up t o 3 . 5 m h i g h , 45 m l o n g and 30 m wide, but g e n e r a l l y s m a l l e r ( F i g . 2 ) .
They have a g e n t l e n o r t h w e s t e r l y windward s l o p e
( 3 O t o 8O) and a s t e e p s o u t h e a s t e r l y avalanche face (30° t o 3 2 O ) .
Owing t o
crossing o f p r e v a i l i n g n o r t h w e s t e r l y winds w i t h seasonal n o r t h - n o r t h e a s t e r l i e s , some dunes a t t h e s o u t h e r n edge o f t h e b a s i n a c q u i r e a l o n g i t u d i n a l shape (Zaidi, i n press).
30
Figure 2 .
Siiia11 barchan dunes and d e f l a t e d paveillent a t Wadi Khuleis.
Sairiples NR were c o l l e c t e d a t t h e s o u t h e r n edge of Nafud R u i n h a t , a t approximately
23'30'
N and 43030' E .
Nafud R u i n h a t i s a n o r t h w e s t - t r e n d i n g barchan f i e l d a few
t e n s of k m long and a few kin wide.
The wind i s g e n e r a l l y from the n o r t h , t h e r e -
f o r e t h e s t e e p avalanche s l o p e s f a c e t o t h e s o u t h . 11
iii
long and 2
The dune which was sampled i s
high and a p p e a r s t o be t y p i c a l o f t h e dunes i n t h e a r e a .
rii
At
v a r i a n c e with t h e barchans of Wadi K h u l e i s , t h e ones i n Nafud R u m h a t have an almost smooth s u r f a c e , devoid of n o t i c e a b l e r i p p l e s . E a s t Jeddah samples were c o l l e c t e d i n Jeddah froiii e o l i a n s h e e t s forming a t h i n veneer on t o p of Precambrian o u t c r c p s , e a s t of t h e r i n g - r o a d t h a t c o n n e c t s Plakkah and Madinah r o a d s .
U n t i l a few y e a r s a g o , barchan dunes were a l s o p r e s e n t i n the
a r e a , b u t they have s i n c e been bulldozed t o ):lake p l a c e f o r urban development.
In
t h i s a r e a , p a i r s of samples were c o l l e c t e d from c r e s t s and t r o u g h s of u n i d i r e c t i o n a l e o l i a n r i p p l e s 2 t o 5 cm i n h e i g h t and 15 t o 30 cni i n wavelength ( F i g . 3 ) . The f o u r South Jeddah samples were c o l l e c t e d a p p r o x i m a t e l y 20 kni s o u t h o f J e d d a h , only 200
in1
from the s h o r e l i n e .
They r e p r e s e n t c r e s t s and t r o u g h s o f
u n i d i r e c t i o n a l e o l i a n r i p p l e s formed by wind reworking o f r a i s e d marine t e r r a c e s .
A t t h i s l o c a l i t y , wind r i p p l e s a r e o f c o n s i d e r a b l e magnitude, r e a c h i n g 15 cm i n h e i g h t and 1 m i n wavelength.
31
Figure 3.
Eolian ripples froni East Jeddah.
Figure 4.
E o l i a n ripples a t r i g h t a n g l e s on a dune s u r f a c e a t Wadi Khuleis
32 METHODS Samples o f d u n e s f r o m Wadi K h u l e i s were c o l l e c t e d w i t h a g l a s s j a r , f r o m an a r e a a p p r o x i m a t e l y 20 x 20 cm and t o a d e p t h o f 5 - 10 cm.
As i s apparent f r o m
f i g u r e 4, s h i f t i n g winds produce r i p p l e s a t r i g h t angles t o each o t h e r , t h e r e f o r e t h e samples c o n t a i n b o t h c r e s t and t r o u g h sand.
The samples were q u a r t e r e d and
a p p r o x i m a t e l y 100 grams o f m a t e r i a l was s i e v e d . For t h e d e t a i l e d d i s c r i m i n a t i o n o f r i p p l e c r e s t s and t r o u g h , a c c u r a t e sampling was achieved i n E a s t and South Jeddah and i n Wadi K h u l e i s by t a k i n g s m a l l e r amounts (60
-
70 g) o f sand f r o m t h e s u r f a c e o f c r e s t s and t r o u g h s .
For t h e
t r o u g h s a m e t a l t a b l e spoon proved t o be an adequate t o o l , whereas f o r t h e c r e s t s a f l a t p l a s t i c t o y scoop was used. Samples were s i e v e d a t '5
intervals.
Cumulative percentages were p l o t t e d on
a r i t h m e t i c graph paper and percentages were o b t a i n e d g r a p h i c a l l y .
Statistical
parameters were c a l c u l a t e d a c c o r d i n g t o t h e formulae o f F o l k and Ward (1957). Frequency d i s t r i b u t i o n curves were c o n s t r u c t e d f r o m t h e c u m u l a t i v e curves w i t h t h e t a n g e n t method d e s c r i b e d by F o l k ( 1 9 7 4 ) . RESULTS AND D I S C U S S I O N Ranges and averages o f s t a t i s t i c a l parameters a r e shown i n Table 1.
A full
l i s t i n g o f t h e parameters i s g i v e n i n Binda ( i n p r e s s ) . Dunes _ _ Sands f r o m Nafud Rumhat a r e g e n e r a l l y c o a r s e r g r a i n e d t h a n t h e ones f r o m Wadi Khuleis.
The a r i t h m e t i c mean o f t h e mean d i a m e t e r i s 1.88 0 i n Nafud Rumhat and
2.52 i n Wadi K h u l e i s .
These mean d i a m e t e r s conform w i t h p u b l i s h e d r e s u l t s on
wind-blown sands f r o m o t h e r p a r t s o f t h e w o r l d (Friedman, 1961; F o l k , 1971; A h l b r a n d t , 1979).
I t i s n o t p o s s i b l e t o d i s c r i m i n a t e among v a r i o u s p a r t s o f t h e
dunes on t h e b a s i s o f mean d i a m e t e r .
A l l t h e frequency d i s t r i b u t i o n s , e x c e p t one
( c r e s t ) f r o m Wadi K h u l e i s , a r e unimodal. Nafud RumhZt, and between 2 and 3
0
Modes a r e g e n e r a l l y l e s s t h a n 2
@
in
i n Wadi K h u l e i s , t h e l a t t e r c o n f o r m i n g more
c l o s e l y t o t h e "quanta" model o f F o l k (1971). Sands f r o m Nafud Rumhat a r e g e n e r a l l y b e t t e r s o r t e d t h a n t h e ones f r o m Wadi Khuleis.
I n t h e l a t t e r l o c a l i t y , ranges o f s t a n d a r d d e v i a t i o n o f v a r i o u s p a r t s
of t h e dunes show a c o n s i d e r a b l e o v e r l a p .
I n Nafud Rumhat t h e two c r e s t samples
show b e t t e r s o r t i n g t h a n t h e o t h e r s , b u t t h e s m a l l number o f samples does n o t warrant generalizations.
Skewness i s p o s i t i v e i n a l l Nafud Rumhat samples,
v a r i a b l e a t Wadi K h u l e i s , where, however, leeward samples t a k e n f r o m t h e avalanche f a c e t e n d t o be more symmetrical t h a n o t h e r s ( F i g . 5 ) . The h i g h e r v a l u e s o f k u r t o s i s (more l e p t o k u r t i c c u r v e s ) o c c u r i n b o t h l o c a l i t i e s on t h e c r e s t s ; windward sands t e n d t o have h i g h e r k u r t o s i s t h a n leeward ones, i n agreement w i t h r e s u l t s r e p o r t e d by F o l k (1971).
Table I Range a n d A r i t h m e t i c Mean o f S t a t i s t i c a l P a r a m e t e r s of E o l i a n Sands from Nafud
@ Mean (Mz)
0 S t a n d a r d Deviation(GI)
Sample (No)
Mean
Range
Mean
1.88
0.43 - 0 . 7 1
0.57
2.82
2.52
0.52
-
0.85
0.67
2.67
2.47
0.58 - 0.85
0.72
Range
RumhBt, Wadi K h u l e i s , and Jeddah.
Skewness (SkI ) Range
K u r t o s i s (IQ Nean
Range
M e an
1.1.4 - 2 . 1 0
1.53
Dunes Nafud Rumhat ( 7 )
1.53
-
2.12
-
M.40
M.32
-0.10
-
M.21
M .05
0.69
-0.10
- +0.19
iil.08
0.79 - 1.00
0.87
H.09
-
1.18 0.90
Wadi Khuleis ( 1 5 )
2.21
Wadi Khuleis windward ( 7 )
2.21
Wadi Khuleis crest ( 4 )
2.26 - 2.82
2.45
0.52 - 0.83
0.64
-0.17
- M.21
N .07
0.87 - 1 . 1 8
0.99
Wadi K h u l e i s Lee ( 4 )
2.57
-
2.77
2.69
0.55
0.78
0.62
-0.05
-
-0.01
0.69
-
0.92
0,84
1 . 7 1 - 2.68
2.29
0.43 - 0.69
0.55
-0.21 - M . 3 0
a.08
0.75
-
1.19
1.01
H.13
0.75 - 1.19
1.02
-
H.01
Ripples
East Jeddah ( 2 2 ) E a s t Jeddah crests (11)
1 . 7 1 - 2.68
2.12
0.43 - 0 . 6 4
0.54
-0.05
- a.30
E a s t Jeddah Troughs (11)
2.27
-
2.47
0.40
-
0.56
-0.08
-
2.68
0.69
-
M.18 w . 0 4 M . 4 5 H 10
.
0.87
-
1.18
1.01
0.70
-
2.68
1.27
Wadi K h u l e i s ( 6 )
1.38
2.71
2.24
0.45
0.64
0.60
-0.30
Wadi K h u l e i s crests ( 3 )
1.38 - 2.41
1.62
0.45 - 0.62
0.55
N.12 - M.45
iil.25
0.96 - 2.68
1.68
Wadi K h u l e i s t r o u g h s ( 3 )
2.26
2.71
2.56
0.57
0.64
0.60
-0.30
-
-0.06
0.70
-
0.88
0.86
T o t a l R i p p l e Crests ( 1 4 )
1.38
-
2.68
2.08
0.43 - 0.67
0.54
-0.05
- +0.45
+O. 15
0.75 - 2.68
1.16
T o t a l R i p p l e Troughs ( 1 4 )
2.26 - 2 . 7 1
2,49
0.40 - 0.69
0.57
-0.30
- M.22
w.02
0.70 - 1,18 0 . 9 8
S . Jeddah* r i p p l e crests ( 2 ) S . Jeddah* r i p p l e t r o u g h s ( 2 )
-0.19
1.84
-
-
M.22
- 0.14
0.89 - 1.25
+0.51 - M . 6 2
-
0.85
-0.21
2.14
-
1.31
-
-0.14
1 . 4 1 - 2.31 0.97
-
1.05
*Not computed w i t h e o l i a n r i p p l e s s i n c e t h e y are " i n s i t u " wind-reworked m a r i n e s a n d s . W W
WADI KHULEIS DUNES
WINDWARD
CREST
Figure 5 .
Variations i n frequency d i s t r i b u t i o n curves of dune samples from Wadi Khuleis.
35
The most obvious d i f f e r e n c e between t h e g r ai n s i z e d i s t r i b u t i o n s in the two l o c a l i t i e s where dunes have been sampled i s t h a t the curves of Wadi Khuleis a r e heterogeneous, whereas the ones from Nafud Rumhat a r e f a i r l y s i m i l a r t o each o th e r . As i t was suspected t h a t the heterogenity of the Wadi Khuleis samples was a consequence of the s h i f t i n g i n wind d i r e c t i o n in the near-coastal area of Wadi Khuleis, and of various admixtures of r i p p l e c r e s t and trough sand, i t was decided t o do a d e t a i l e d a n a l y s i s of sand from c r e s t s and troughs of e olia n ripples. Ripples Three types o f r i p p l e s have been sampled f o r t h i s study.
The East Jeddah
samples r e p r e s e n t r i p p l e s t h a t form on s h eet s of wind-blown sand climbing on h i l l s of Precambrian bedrock.
The Wadi Khuleis samples re pre se nt r i p p l e s t h a t
form on dunes i n a shifting-wind realm, b u t a few samples were c a r e f u l l y c olle c te d from places where i t was p o s s i b l e t o do so without mixing c r e s t sand with trough sands.
South Jeddah samples a r e from a marine t e r r a c e o r ra ise d beach on which
wind has produced r i p p l e s .
Therefore, t h e l a t t e r a r e n o t , s t r i c t l y speaking,
e o lia n sands; probably not much s a l t a t i o n t r a n s p o r t i s involved here because of the coarse nature of t h e sand, r o l l i n g being the most probable mechanism. I n a l l si x t e e n p a i r s of r i p p l ed sands analysed, c r e s t samples a r e c oa rse r grained than troughs.
This observation had already been made by Bagnold (1941). I n ten p a i r s , c r e s t s a r e b e t t e r s o r t e d , t h e contrary being t r u e f o r the othe r
six.
Kurtosis i s higher i n troughs i n nine cases and lower in seven.
The d e t a i l e d study of wind-generated r i p p l e s sheds some l i g h t on skewness of eolian sands, t h e parameter which some authors deem c r i t i c a l t o the i d e n t i f i c a tion of e o l i a n t r a n s p o r t .
Not a l l e o l i a n sands have positive skewness; negative
skewness i s common in the troughs of e o l i a n r i p p l e s . Excluding t h e f o ur South Jeddah samples, eo l i a n r i p p l e c r e s t s range from - 0 . 0 5 t o + 0.45 i n skewness, averaging + 0.15; twelve samples have p o s i t i v e skewness, two negative.
Ripple troughs range from - 0.30 t o + 0.22, averaging + 0.01; e i g h t
samples have p o s i t i v e skewness, s i x negative.
I n f i g u r e 6 a r e shown ranges and
mean skewness of dunes and of r i p p l e s . Including the f o u r South Jeddah samples, i n eleven of the sixte e n p a i r s , c r e s t s have more p o s i t i v e ( o r l e s s n eg at i v e) skewness than troughs, in four p a i r s the opposite i s t r u e , and i n one case skewness i s i d e n t i c a l i n c r e s t and trough I n f i g u r e 7 a r e shown t y p i cal frequency d i s t r i b u t i o n curves of c r e s t and trough sands.
In F o l k ' s (1974) terminology, c r e s t samples a r e thus d i s t r i b u t e d :
3
strongly f i n e skewed, 8 f i n e skewed, 5 near symmetrical, whereas trough samples are:
4 f i n e skewed, 9 near symmetrical, 3 coarse skewed.
These r e s u l t s agree
with d a t a from the Great Sand Dunes National Monument l i s t e d i n Ahlbrandt (1979
36
N.
RUMHZT DUNE
( 7 ) - -I
W.KHULElS DUNES (15) 1-1
1
RIPPLE CRESTS . (14)
RIPPLE TROUGHS (14) 1 -
1
1
I
I
I
-.2
-.4
I
0
+.2
I +.4
SKEWNESS (Sk, )
1-1
Figure 6.
RANGE
MEAN
( 7 1 SAMPLES
Ranges and mean skewness o f 50 e o l i a n sands f r o m Saudi A r a b i a
I t i s perhaps p o s s i b l e t o i n f e r from t h i s s t u d y t h a t , p o s i t i v e skewness b e i n g
normal f o r wind-blown sand, t h e n e g a t i v e skewnesses encountered i n r i p p l e t r o u g h s i s b r o u g h t a b o u t e i t h e r by r o l l i n g o f a few coarse g r a i n s down f r o m t h e c r e s t s , o r by c o a r s e g r a i n s t h a t t h e w i n d i s n o t capable o f p u s h i n g up t o t h e c r e s t s and thus l a g behind i n the troughs.
The f o r m e r e x p l a n a t i o n i s more l i k e l y , a t l e a s t
f o r t h e s t r i c t l y e o l i a n samples where c r e s t s d i s p l a y c o a r s e r low p e r c e n t i l e s .
CONCLUSIONS G r a i n - s i z e f r e q u e n c y - d i s t r i b u t i o n curves o f dune sands f r o m Nafud Rumhat and Wadi K h u l e i s a r e a l m o s t a l l unimodal ( 2 1 o u t o f 22) w i t h modes between 1.5 and 2 0 , and 2 and 3 0 , r e s p e c t i v e l y . There i s a c e r t a i n amount o f o v e r l a p i n t h e v a l u e s o f s t a t i s t i c a l parameters f r o m windward, c r e s t , and l e e o f t h e dunes; t h u s i t would be d i f f i c u l t t o a s s i g n an unknown sample t o a s p e c i f i c dune sub-environment.
However, i n t h r e e dunes
37
/ I 2
I
I
I': I \ I
I
I
2
3
C.
\
3
A.
W . KHULEIS
B.
JEDDAH EAST
c. JEDDAH
---
I -I Figure 7 .
I 0
I I
I
I
2
3
4
SOUTH
RIPPLE CREST RIPPLE TROUGH
Frequency d i s t r i b u t i o n curves of selected p a i r s of r i p p l e samples showing t h a t c r e s t s are coarser grained and more p o s i t i v e l y skewed than troughs.
o u t of f o u r , sand from the c r e s t i s b e t t e r sorted and has higher kurtosis than e i t h e r windward o r leeward sand. An i n t e r e s t i n g conclusion can be reached with regard t o skewness, a parameter which i s often c i t e d as being c r i t i c a l t o the i d e n t i f i c a t i o n o f the eolian o r i g i n of a sand. I n Nafud R u m h a t , where the dune surface i s almost smooth, a l l samples are p o s i t i v e l y skewed. I n Wadi Khuleis, where the dunes' surface i s r i p p l e d , and where the wind d i r e c t i o n i s not constant, some samples a r e p o s i t i v e l y skewed, others a r e negatively skewed, regardless o f t h e i r position on the dune. I n general, frequency d i s t r i b u t i o n s of dune sands from Wadi Khuleis a r e more heterogeneous than the ones from Nafud R u m h a t .
38
+.e 0
X
x
X
+2
X
x
X
0
A
0
H
Y
cn Y
+.I
+
0.
0
0 0
+
X
0 - t
+
0
0
0
02%
.+ .+
v,
t n o
0
W
z 3
+
tr
0
+
0
-.I
0
+
+
+
O
+.
.
W Y
+
tn
-. 3
0
I
I
I
1.5
DUNES
Figure 8.
I
1
I
I
I
I
I
I
I
2.o MEAN ( M t ) x N.RUMH~~T
+ W . KHULEIS
I
I
I
2.5
R I PPLES
o CREST 0
TROUGH
Pl o t of mean diameter versus skewness of 50 e olia n sands from Saudi Arabia.
The a n a l y s i s of s i x t e e n p a i r s of r i p p l e c r e s t and t r o u g h samples from Wadi Khuleis and from two Jeddah l o c a l i t i e s shows t h a t most c r e s t sands a r e p o s i t i v e l y skewed, whereas n e g at i v el y skewed o r symmetrical d i s t r i b u t i o n s a r e common i n troughs.
Also, f o r a l l e o l i a n samples analyzed i n t h i s study the re seems t o be
a c o r r e l a t i o n between mean diameter and skewness:
the c oa rse r t h e mean, the
more pronounced the f i n e t a i l ( F i g . 8 ) . I t would be beyond t h e scope of t h i s paper t o embark on a discussion of the
dynamics of r i p p l e formation. However, from the data presented here i t stands t o reason t h a t a s o r t i n g process takes place i n the formation of e o l i a n r i p p l e s which se p a r a t e s more p o s i t i v e l y skewed sand on t he c r e s t from symmetrical t o n e g a ti v e l y skewed sand i n t h e troughs.
T h u s t h e v a r i a t i o n i n skewness among t h e Wadi Khuleis samples may be explained by the mixing of r i p p l e c r e s t and r i p p l e trough sands, brought about by changing
39 It may be i n f e r r e d t h a t , i f a sample has a predominance o f c r e s t m a t e r i a l , i t w i l l wind d i r e c t i o n , and hence the d i f f i c u l t y o f c o l l e c t i n g homogeneous samples. be p o s i t i v e l y skewed.
I f , on t h e o t h e r hand, t r o u g h m a t e r i a l i s predominant,
t h e r e s u l t i n g g r a i n - s i z e d i s t r i b u t i o n w i l l t e n d t o be e i t h e r symmetrical o r n e g a t i v e l y skewed. ACKNOWLEDGMENTS
I wish t o thank t h e s t u d e n t s o f t h e sedimentology c l a s s a t t h e F a c u l t y of Earth S c i e n c e s , King Abdulaziz U n i v e r s i t y , Jeddah, who c o l l e c t e d and s i e v e d some of t h e samples; S a i d Zaidi who f i r s t i n d i c a t e d t o me t h e Wadi Khuleis dune f i e l d ;
Drs. S a i d Oinara and Mohammed Askari who accompanied me t o Wadi Khuleis. D. M. J . Kent and R . Rahmani c r i t i c a l l y r e a d the m a n u s c r i p t .
Drs.
REFERENCES A h l b r a n d t , T . S . , 1979. T e x t u r a l p a r a m e t e r s o f e o l i a n d e p o s i t s . I n : E . O . McKee ( E d i t o r ) , A Study o f Global Sand S e a s . U.S.G.S. P r o f . Paper 1052, p p . 21-51. Bagnold, R . A . , 1941. The P h y s i c s of Blown Sands and Sand Dunes. Methuen & Co. L t d . , London, 265 p p . B i g a r e l l a , J . J . , 1972. E o l i a n environments: t h e i r c h a r a c t e r i s t i c s , r e c o g n i t i o n , and i m p o r t a n c e . I n : J . K . Rigby and W . K . Hamblin ( E d i t o r s ) , Recognition of Ancient Sedimentary Environments. SOC. Econ. P a l e o n t . Mineral. S p e c i a l P u b l i c a t i o n 1 6 , p p . 12-62. Binda. P . L . . 1972. On the sedimentoloqv o f some cover sands from Zambia. R.C.M. Report GR 44, K a l u l u s h i , Zambia. Binda, P . L . ( i n p r e s s ) . G r a i n - s i z e s t u d y o f some e o l i a n sands from Saudi Arabia. B u l l . Fac. E a r t h S c i . , K . A . U . , Jeddah. Binda, P . L . and H i l d r e d , P . R . , 1973. Bimodal g r a i n - s i z e d i s t r i b u t i o n s of some K a l a h a r i - t y p e s a n d s from Zambia. Sedimentary Geology, 10: 233-237. Folk, R . L . , 1962. Of skewnesses and s a n d s . J o u r . Sed. P e t r o l o g y , 32: 145-146. Folk, R . L . , 1968. Bimodal supermature s a n d s t o n e s : p r o d u c t o f the d e s e r t f l o o r . I n t . Geol. C o n g r . , 2 3 r d , Prague, P r o c . S e c t . 8, p p . 9-32. Folk, R . L . , 1971. L o n g i t u d i n a l dunes of the n o r t h w e s t e r n edge of t h e Simpson D e s e r t , Northern T e r r i t o r y , A u s t r a l i a , 1. Geomorphology and g r a i n s i z e r e l a t i o n s h i p s . Sedimentology, 16: 5-54. F o l k , R . L . , 1974. P e t r o l o g y o f Sedimentary Rocks. Hemphill, A u s t i n , Texas, 182 p p . Folk, R . L . and Ward, W . C . , 1957. Brazos r i v e r b a r - a s t u d y i n the s i g n i f i c a n c e of g r a i n - s i z e p a r a m e t e r s . J o u r . Sed. P e t r o l o g y , 27: 3-27. Friedman, G . M . , 1961. D i s t i n c t i o n between dune, beach, and r i v e r sands from t h e i r t e x t u r a l c h a r a c t e r i s t i c s . J o u r . Sed. P e t r o l o g y , 31: 514-529. Mason, C . C . and Folk, R . L . , 1958. D i f f e r e n t i a t i o n o f beach, dune, and e o l i a n f l a t environments by s i z e a n a l y s i s , Mustang I s l a n d , Texas. J o u r . Sed. P e t r o l o g y , 28: 211-226. S h e p a r d , F . P . and Young, R . , 1961. D i s t i n g u i s h i n g between beach and dune s a n d s . J o u r . Sed. P e t r o l o g y , 31: 196-214. SkoEek, V. and S a a d a l l a h , A . A . , 1972. G r a i n - s i z e d i s t r i b u t i o n , c a r b o n a t e c o n t e n t and heavy m i n e r a l s i n e o l i a n s a n d s , Southern D e s e r t , I r a q . Sedimentary Geology, 8 : 29-46. Z a i d i , S.M.S. ( i n press). Geomorphology o f Wadi Khulays a r e a . F a c u l t y E a r t h S c i . , K . A . U . , Research S e r i e s , J e d d a h , Saudi A r a b i a . _I
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41
.mn
TEXTIJRAL
STRUCTURAL CHARACTERISTICS
FORMED EOLIAN
OF SOME EXPERIMENTALLY
STRATA CHRISTOPHER J . SCHEFIK, U.S. G e o l o g i c a l Survey, P.O. Denver F e d e r a l Center, Denver, CO 80225
Box 25046,
INTRODUCTION
The purpose o f t h i s paper i s t o compare t h e t e x t u r a l and s t r u c t u r a l f e a t u r e s of t h e t h r e e main t y p e s o f e o l i a n s t r a t a based on a s e r i e s o f d e p o s i t s formed i n a wind tunnel.
The b e l i e f h e r e i s t h a t t h e d e p o s i t i o n a l processes c o n t r o l
t e x t u r a l and s t r u c t u r a l c h a r a c t e r i s t i c s o f e o l i a n sediments and, consequently, t h a t t h e s e c h a r a c t e r i s t i c s a r e u s e f u l i n d i s t i n g u i s h i n g between t h e t h r e e main t y p e s of s t r a t i f i c a t i o n i n s m a l l samples, such as core. The main t y p e s o f e o l i a n s t r a t i f i c a t i o n ,
e l u c i d a t e d by H u n t e r (1977 a, b )
f r o m r e c e n t d e p o s i t s a l o n g t h e Texas c o a s t i n c l u d e avalanche, s e v e r a l forms o f e o l i a n r i p p l e s t r a t i f i c a t i o n . Hunter
(1981)
i l l u s t r a t e d how t h e s e t y p e s
grainfall,
and
Kocurek and D o t t (1981) and
o f s t r a t a can be i d e n t i f i e d on
outcrop. The d i f f e r e n t t y p e s o f s t r a t a , as i n h e r e n t i n t h e g e n e t i c c l a s s i f i c a t i o n o f Hunter
(1977a),
reflect
the
processes
of
development o f each t y p e o f s t r a t i f i c a t i o n .
deposition
responsible
for
the
Sand i s d e p o s i t e d by w i n d t h r o u g h
a t l e a s t t w o p r i m a r y processes--by g r a i n f a l l and by a c c u m u l a t i o n d u r i n g r i p p l e migration--leading
t o t h e p r o d u c t i o n o f g r a i n f a l l and r i p p l e s t r a t i f i c a t i o n .
G r a i n f a l l d e p o s i t i o n w i l l o c c u r on a s l o p i n g s u r f a c e u n t i l t h e a n g l e o f i n i t i a l y i e l d i s reached.
A t t h i s p o i n t an avalanche t a k e s p l a c e ,
f o r m a t i o n o f avalanche s t r a t i f i c a t i o n .
resulting i n the
Each o f t h e s e processes r e s u l t s i n
g r a i n arrangements t h a t can be u s e f u l i n i d e n t i f y i n g t h e t y p e o f s t r a t a . METHODS
OF STUDY
Each t y p e o f e o l i a n s t r a t i f i c a t i o n was formed i n a w i n d t u n n e l .
I n t h e view
of t h e t u n n e l shown i n F i g u r e 1, t h e w i n d t r a v e l s f r o m r i g h t t o l e f t a l o n g t h e h o r i z o n t a l f l u m e s e c t i o n (A), r e s u l t i n g i n t h e f o r m a t i o n o f w i n d r i p p l e s t r a t a , t h e n proceeds o v e r t h e s i m u l a t e d f o r e s e t s l o p e (B), where t h e sand i s d e p o s i t e d as g r a i n f a l l i n t h e zone o f f l o w s e p a r a t i o n . qrainfall,
t h e slope eventually
avalanche downslope.
As more sand i s d e p o s i t e d as
becomes u n s t a b l e ,
and t h e g r a i n f a l l s t r a t a
42
F i q . 1.
Photoqraph o f t h e wind t u n n e l .
A f t e r each e x p e r i m e n t , l a t e x p e e l s were p r e p a r e d f r o m a l l o f t h e s t r a t i f i c a t i o n types
i n a direction
parallel
samples c u t f r o m t h e p e e l s . point-countinq technique.
t o t h e wind.
T h i n s e c t i o n s were made f r o m
P o r o s i t y was d e t e r m i n e d i n t h i n s e c t i o n u s i n g a
Care was t a k e n t o a v o i d t h e v a r i o u s e r r o r s t h a t a r e
p o s s i b l e w i t h t h i s method ( H a l l e y , 1 9 7 8 ) . The sand used i n a l l o f t h e e x p e r i m e n t s was a n a t u r a l l y q r a d e d e o l i a n dune sand;
i t was f i n e q r a i n e d and w e l l s o r t e d and h a d p o s i t i v e skewness. R ESVLT? Ripple s t r a t i f i c a t i o n
Commonly, d u r i n q e o l i a n r i p p l e r n i q r a t i o n , s a l t a t i o n bombardment, r i p p l e upwind
and t h e
( F i q 2).
remainder
Deposits
subcritically climbinq translatent each r i p p l e p r e s e r v e d rates
of
deposition,
depends and r a t e s
i s buried by the
f o r m e d by t h i s
process
next
factors,
succeeding
h a v e been t e r m e d
s t r a t a b y H u n t e r (1977a).
on s e v e r a l of
p a r t o f each r i p p l e i s removed by
The amount of
including wind velocity,
r i p p l e migration.
This
process
is
not
r e s t r i c t e d t o h o r i z o n t a l surfaces b u t can occur across slopes approaching t h e a n q l e o f i n i t i a l y i e l d ( F r y b e r q e r and Schenk, 1981).
43
F i g . 2. R e l i e f l a t e x peel i l l u s t r a t i n g a coset o f e x p e r i m e n t a l l y formed e o l i a n ripple stratification. Incomplete preservation o f t h e e n t i r e r i p p l e leads t o Finer grained layers exhibit s t r a t a t h a t a p p e a r p a r a l l e l o r n e a r l y so. positive relief. Arrows b r a c k e t a r i p p l e s t r a t u m . Low a n g l e i l l u m i n a t i o n causes shadows i n c o a r s e r g r a i n e d , r e c e s s e d a r e a s . S c a l e i n cm. R i p p l e s t r a t i f i c a t i o n o f t h i s t y p e i s c h a r a c t e r i z e d by t h i n , parallel
strata
s e p a r a t e d by p l a n a r b o u n d i n q s u r f a c e s
(Fig.
essentially
2).
Peels o f
r i p p l e s t r a t a i l l u s t r a t e w h a t a p p e a r s t o b e a n i n t e r c a l a t i o n o f c o a r s e r and f i n e r qrained layers. peels.
a r e r a r e l y observed. sortinq
The f i n e r q r a i n e d l a y e r s e x h i b i t p o s i t i v e r e l i e f i n t h e
Althouqh t h e s e d e p o s i t s a r e formed d u r i n g r i p p l e m i g r a t i o n ,
foresets
The l a c k o f f o r e s e t s i s e x p l a i n e d p a r t l y by t h e d e g r e e of
P x h i b i t e d b y t h e sand d e p o s i t e d o v e r t h e b r i n k
o f each r i p p l e and
p a r t l y by t h e f a c t t h a t sand does n o t a v a l a n c h e down t h e l e e s l o p e s o f e o l i a n ripples. Ourinq
ripple
miqration,
motion by t h e s a l t a t i o n
the
surface-creep
bombardment
(Bagnold,
population
1941).
is
maintained
in
The g r a i n s m o v i n q as
s u r f a c e c r e e p a r e d r i v e n o v e r t h e r i p p l e b r i n k and c o l l e c t on t h e l e e s l o p e .
A
c e r t a i n amount o f s a l t a t i n q sand i s d e p o s i t e d i n t h e t r o u g h o f each r i p p l e , and t h i s sand i s q e n e r a l l y f i n e r q r a i n e d t h a n t h e s u r f a c e - c r e e p m a t e r i a l d e p o s i t e d on t h e l e e s l o p e .
As t h e r i p p l e migrates, t h e c o a r s e r l e e s l o p e d e p o s i t s cover
t h e f i n e q r a i n s i n t h e t r o u q h , r e s u l t i n q i n a s t r a t u m t h a t i s i n v e r s e l y graded.
44
The inverse grading of r i p p l e s t r a t i f i c a t i o n i s best viewed i n t h i n section (Fig. 3 ) . The contacts between r i p p l e s t r a t a a r e generally s h a r p ; f i n e r grains a t t h e base of one stratum r e s t on t h e coarser grains of t h e stratum below. A t r a i n of miqratinq r i p p l e s can produce a coset of inversely graded s t r a t a . No other eolian depositional process appears t o produce t h i s inverse grading so repeti t i vely.
Fiq. 3. Photomicrograph of r i p p l e s t r a t a . Note t h e inverse grading a n d s h a r p boundaries between s t r a t a . Primary porosity in t h e coarser-grained p a r t of each stratum comnonly i s higher than t h e finer-grained p a r t . Black i s pore space. Photographed under plane polarized l i g h t , usinq a red f i l t e r . Bar s c a l e i s 3 mm long. Primary porosity in experimentally formed r i p p l e s t r a t i f i c a t i o n averaged 3 9 X , t h e lowest of t h e t h r e e main types of s t r a t a . Measurements have shown t h a t t h e porosity can he higher in t h e coarser grained p a r t of each inversely qraded stratum. Ripple s t r a t a a r e commonly preserved as crosswind-formed deposits a t t h e base of t h e high-angle s t r a t a in a dune, a n d some a r e
I n t h i s position, r i p p l e s t r a t a commonly form t h e tangential bedding frequently observed in e o l i a n deposits. i n t e r c a l a t e d with avalanche s t r a t a .
Grainfall s t r a t i f i c a t i o n The t e x t u r e of g r a i n f a l l s t r a t i f i c a t i o n depends d i r e c t l y on t h e velocity h i s t o r y of t h e wind because i t i s formed i n zones of flow separation. As t h e velocity increases, t h e s a l t a t i o n population becomes coarser grained, a n d t h e sand subsequently deposited i n a zone of flow separation a l s o becomes coarser. In addition t h e thickness o f a g r a i n f a l l stratum i s dependent on the duration of t h e wind event. Because of t h e s e dependencies, g r a i n f a l l
45 stratificdtion i s d i f f i c u l t t o characterize texturally.
S e t s of y r a i n f a l l
s t r a t a t h i n and becoiiie f i n e r yrained downslope (Fig. 4 ) .
F i g . 4.. G r a i n f a l l s t r a t a produced by a s e r i e s of wind gusts. Finer yrained 1 ayers e x h i b i t p o s i t i v e r e l i e f i n t h i s l a t e x peel. Contacts between s t r a t a a r e Thickness of adjacent y r a i n f a l l s t r a t a genera l l y n o t s h a r p , b u t gradational. Bar s c a l e i s 2 cm l o n y . can be extremely v a r i ab l e, because of wind gusting. A t h i n se c t i o n view of g r a i n f a l l s t r a t i f i c a t i o n i l l u s t r a t e s t h e yra da tiona l,
r a t h e r than sharp c on t act s ex h i b i t ed by a coset of experimentally formed s t r a t a (Fig. 5 ) . each
of
lowered.
The s t r a t a i l l u s t r a t e d i n Figure 5 were formed during wind gusts, in which,
t h e wind was
gradually
r ai s e d
t o a high p o i n t and then
The arrows point t o co ar s er grained la ye rs deposited during t h e
higher winds. The t e x t u r a l c o n t r a s t can be weak o r strong, depending on t h e d if f e r e n c e between t h e wind v el o ci t y maxima and t h e threshold ve loc ity necessary t o i n i t i a t e sand movement.
46
Fig. 5. Photomicrograph of e x p e r i m e n t a l l y formed g r a i n f a l l s t r a t a . Note t h e q r a d a t i o n a l a s p e c t of t h e s t r a t i f i c a t i o n and t h e t e x t u r a l c o n t r a s t s produced by a s e r i e s of wind qrlsts. Arrows p o i n t t o c o a r s e r - g r a i n e d l a y e r s . Black i s pore space. Photoqraphed under p l a n e p o l a r i z e d l i g h t , u s i n q a red f i l t e r . Rar s c a l e i s .5 cm lonq. Primary p o r o s i t y of t h e g r a i n f a l l
s t r a t a was v a r i a b l e b u t g e n e r a l l y was
i n t e r m e d i a t e between t h e r i p p l e and a v a l a n c h e p o r o s i t i e s , with an a v e r a q e of 4 3%.
Avalanche s t r a t i f i c a t i o n Avalanche s t r a t i f i c a t i o n
i s produced
by t h e r e d e p o s i t i o n
w a i n f a l l d e p o s i t s by slumpinq or sandflow. q e n e r a l l y heqan a s a slump h l o c k , which sandflows a s i t proceeded downslope.
of p r e - e x i s t i n q
Durinq t h e e x p e r i m e n t s , a v a l a n c h e s d i s p e r s e d wholly o r i n p a r t
into
C e r t a i n minor d e f o r m a t i o n a l s t r u c t u r e s ,
such as t h r u s t s , f l a m e s , and f o l d s , a r e c o n s i d e r e d c h a r a c t e r i s t i c of d r y sand a v a l a n c h i n q (McKee e t a l . , 1971). T h r u s t - t y p e s t r u c t u r e s formed d u r i n q a n a v a l a n c h e can be observed i n p e e l s hetween
the
structurps deposits.
boundinq are
formed
surfaces as
a
of slump
a n avalanche s t r a t u m ( F i q . 6 ) . block
overrides
pre-existing
These avalanche
47
F i r r . 6. T h r u s t s t r u c t u r e s f o r m e d d u r i n q s l r r m p i n q on t h e s i m u l a t e d f o r e s e t slope. Thpse s t r u c t u r e s a r e commonly f o r m e d a b o u t m i d - s l o p e above t h e a r e a where slumps d e q e n e r a t e t o s a n d f l o w s . Arrows p o i n t t o t h r u s t s t r u c t u r e s . Dormslope i s t o t h e r i q h t . S c a l e i n cm.
F i q . 7. R e l i e f p e e l i l l u s t r a t i n g t h e t e x t u r a l c o n t r a s t between e x p e r i m e n t a l l y former! s a n d f l o w s t r a t a ( n e q a t i v e r e l i e f i n p e e l ) and f i n e r q r a i n e d g r a i n f a l l strata. N o t e t h a t t h e a v a l a n c h e s t r a t a do n o t f o r m a t a n g e n t i a l c o n t a c t a t t h e base of t h e f o r e s e t s l o n e . S c a l e i n crn.
48
Fiq. 8. Photomicroqraph of a n a v a l a n c h e s t r a t u m formed by sandflowage. F i n e r q r a i n s a r e c o n c e n t r a t e d a l o n q the base of each f l o w ( a r r o w ) and g i v e t h e s t r a t um a weak i n v e r s e grading. S t r a t a d i p t o the r i g h t . Black i s pore Photoqraphed under p l a n e p o l a r i z e d l i q h t , u s i n g a r e d f i l t e r . Bar space. s c a l e i s 1 mm lonq. O n t h e lower h a l f of t h e s i m u l a t e d f o r e s e t s l o p e , sandflow r a t h e r t h a n slump
p r o c e s s e s predominate. During s a n d f l o w s , a l l of t h e g r a i n s are i n motion r e l a t i v e t o one a n o t h e r and d e f o r m a t i o n a l s t r u c t u r e s a r e not produced. In a d d i t i o n , d u r i n g sandflow, t h e f i n e g r a i n s end up e i t h e r a t o r n e a r t h e b a s e of t h e flow (Bagnold, 1954; Middleton, 1970) and a r e e i t h e r l e f t behind on t h e p l a n e o f s h e a r i n g o r a r e i n c o r p o r a t e d i n t o t h e b a s e of t h e a v a l a n c h e s t r a t u m when t h e f l o w movement c e a s e s .
49 T h i s process l e a d s t o s t r a t a t h a t a r e g e n e r a l l y w e l l s o r t e d and c o a r s e r downslope.
Some avalanche s t r a t a e x h i b i t
e f f e c t d i m i n i s h e s downslope.
a weak i n v e r s e g r a d i n g ,
but t h i s
Because t h e c o a r s e r g r a i n s t r a v e l a l o n g t h e edges
and t o p o f t h e flow,
t h e y r e a c h t h e base o f t h e s l o p e f i r s t ,
c o a r s e n i n g downslope.
A t t h e base, t h e avalanche s t r a t a wedge o u t i n t o e i t h e r
grainfall
or
ripple stratification.
There
leading t o a
i s a marked t e x t u r a l c o n t r a s t
between t h e c o a r s e r g r a i n e d s a n d f l o w " t o e s " and t h e f i n e r g r a i n e d g r a i n f a l l o r r i p p l e s t r a t i f i c a t i o n ( F i g . 7). I n t h i n s e c t i o n , t h e i n v e r s e g r a d i n g can be observed i n an avalanche s t r a t u m upslope f r o m t h e s a n d f l o w t o e s , r i p p l e s t r a t a (Fig.
8).
b u t i t i s n o t as pronounced t h e r e as i n t h e
I n g e n e r a l , avalanche s t r a t a a r e w e l l s o r t e d because
o f t h e removal o f t h e f i n e r g r a i n s . thick
Each s t r a t u m commonly i s a few c e n t i m e t e r s
and i s s e p a r a t e d f r o m a d j a c e n t
s t r a t a by sharp c o n t a c t s .
Avalanche
d e p o s i t s were c h a r a c t e r i z e d by t h e l o o s e s t p a c k i n g and t h e h i g h e s t p o r o s i t y (avg. 474,) o f t h e t h r e e t y p e s o f s t r a t a . CONCLUSIONS The processes o f d e p o s i t i o n i m p a r t c h a r a c t e r i s t i c t e x t u r a l and s t r u c t u r a l f e a t u r e s t o each o f t h e main t y p e s o f e o l i a n s t r a t i f i c a t i o n .
These f e a t u r e s ,
such as n a t u r e o f c o n t a c t s ,
and presence o r
grading,
thicknesses o f s t r a t a ,
absence o f s m a l l - s c a l e d e f o r m a t i o n a l s t r u c t u r e s ,
are useful i n discriminating
between r i p p l e and avalanche s t r a t i f i c a t i o n i n o u t c r o p and c o r e samples. REFERENCES Bagnold, R.A., 1941. The p h y s i c s o f blown sand and d e s e r t dunes. Plethuen, London, 265 pp. Bagnold, R.A., 1954. Experiments on a g r a v i t y - f r e e d i s p e r s i o n o f l a r g e s o l i d spheres i n a Newtonian f l u i d under shear. Proc. R. SOC. London, 225A: 49-63. 1981. Wind s e d i m e n t a t i o n t u n n e l experiments on F r y b e r g e r , S.G. and Schenk, C.J., t h e o r i g i n s o f e o l i a n s t r a t a . Sedimentology, 28: 805-821. H a l l e y , R.B., 1978. E s t i m a t i n g p o r e cement volume i n t h i n s e c t i o n . Jour. Sediment. P e t r o l . , 48: 642-650. Hunter, R.E., 1977a. B a s i c t y p e s o f s t r a t i f i c a t i o n i n small e o l i a n dunes. Sedirnentology, 24: 361-387. Hunter, R.E., 1977b. Terminology o f c r o s s - s t r a t i f i e d sedimentary l a y e r s and c l i m b i n g r i p p l e s t r u c t u r e s . J o u r . Sediment. P e t r o l . , 47: 697-706. Hunter, R.E., 1981. S t r a t i f i c a t i o n s t y l e s i n e o l i a n sandstones: some Pennsylvanian t o J u r a s s i c examples f r o m t h e Western I n t e r i o r , U.S.A. SOC. Econ. P a l e o n t . M i n e r a l . Spec. Publ . , 31: 315-329. IKocurek, G. and D o t t , R.H., 1981. D i s t i n c t i o n and uses o f s t r a t i f i c a t i o n t y p e s i n t h e i n t e r p r e t a t i o n o f e o l i a n sand. J o u r . Sediment. P e t r o l . , 51: 579-595. McKee, E.D., Douglass, J.R. and R i t t e n h o u s e , S., 1971. D e f o r m a t i o n o f l e e s i d e l a m i n a e i n e o l i a n dunes. Geol. SOC. Amer. B u l l . , 82: 359-378. 1970. Experimental s t u d i e s r e l a t e d t o t h e problems o f f l y s c h M i d d l e t o n , G.V., s e d i m e n t a t i o n . Geol. Assoc. Canada Spec. Publ., 7: 253-272.
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51
LOESS MATERIAL ARD LOESS DEPOSITS:
FORMATION, DISTRIBUTION AND CONSEQUENCES
I A N J . SMALLEY and VALERIE SMALLEY:
Dept. o f E a r t h Sciences, U n i v e r s i t y o f
W a t e r l o o , W a t e r l o o , O n t a r i o , Canada N2L 3G1
INTRODUCTION:
THE FOUR STAGES
Loess i s d e p o s i t e d b y w i n d :
t o t h i s i t owes i t s s p e c i a l c h a r a c t e r i s t i c s ,
i . e . t h e n a r r o w s i z e r a n g e i n t h e c o a r s e s i l t r e g i o n , t h e open s t r u c t u r e , t h e t e n d e n c y t o c o l l a p s e when l o a d e d and w e t t e d , and t h e m a n t l i n g o f t h e landscape.
Loess r e f l e c t s t h e a e o l i a n i n f l u e n c e m o s t s t r o n g l y and i t s
l a r g e s c a l e d e p o s i t s a r e among t h e m o s t s p e c t a c u l a r f e a t u r e s o f a e o l i a n geology.
T h e r e i s a p r a c t i c a l a s p e c t t o l o e s s as w e l l , i t f o r m s t h e p a r e n t
m a t e r i a l f o r some o f t h e w o r l d ' s m o s t p r o d u c t i v e a g r i c u l t u r a l s o i l s (Chesworth 1982). The s t u d y o f l o e s s as a d e p o s i t has p r o v i d e d one h u n d r e d and f i f t y y e a r s of c o n t r o v e r s y and e x c i t e m e n t b u t t h e t i m e has come when we s h o u l d f o c u s our a t t e n t i o n on t o l o e s s m a t e r i a l i n a l l i t s forms, r a t h e r than j u s t t h e most s p e c i a l o c c u r r e n c e s .
For a simple b a s i c l o e s s d e p o s i t t o be formed
four major events u s u a l l y occur: has t o b e formed; posited;
(%
20-60 w i )
3 ) , and de-
4 ) , t h e n , u s u a l l y , a f e w p o s t - d e p o s i t i o n a l changes o c c u r and we
have w h a t we c a l l l o e s s .
as:
l ) , the loess-size material
2 ) , t h i s m a t e r i a l has t o be t r a n s p o r t e d ;
l ) ,make;
2), move;
I n s h o r t h a n d t e r m s t h e sequence can be d e s c r i b e d
3), place;
and 4 ) , change.
T h i s f o u r e v e n t sequence h a s been d i s c u s s e d e l s e w h e r e ( S m a l l e y 1966, 1978a) and i t has been p o i n t e d o u t t h a t t h e w i d e r a n g e o f ' l o e s s t h e o r i e s ' o c c u r r e d because i n v e s t i g a t o r s t e n d e d t o c o n c e n t r a t e t h e i r a t t e n t i o n o n one e v e n t , and t o n e g l e c t t h e r e s t .
O b v i o u s l y many l o e s s d e p o s i t s a r e f o r m e d a f t e r
a c o n s i d e r a b l y more complex h i s t o r y , b u t t h e r e a r e a s i g n i f i c a n t number o f m a j o r d e p o s i t s w h i c h f o r m e d a f t e r a v e r y modest e v e n t sequence.
llanecki
and D o m i n i k ( 1 9 7 2 ) have d e s c r i b e d a sequence o f e v e n t s f o r t h e Z w i e r z y n i e c loess i n Poland;
t h i s i s a f a i r l y simple d e p o s i t , p o s s i b l y what Smalley I,
and K r i n s l e y ( 1 9 8 1 ) w i s h e d t o c a l l ' U r l o s s ' .
C u t t s ( 1 9 7 3 ) has d e s c r i b e d
a sequence f o r M a r t i a n ' l o e s s ' , w h i c h has been d i s c u s s e d and d e v e l o p e d b y Smalley and K r i n s l e y ( 1 9 7 9 ) i n an a t t e m p t t o compare a e o l i a n d e p o s i t s on E a r t h and Mars.
The l o n g e s t e v e n t sequence has been p r o p o s e d f o r t h e T a s h k e n t
l o e s s ( S m a l l e y 1980) and as t h i s n i n e e v e n t sequence has a c r i t i c a l b e a r i n g on t h e d e s e r t l o e s s p r o b l e m i t w i l l be d i s c u s s e d l a t e r , i n t h e d e s e r t l o e s s
52 section.
An e l e v e n e v e n t sequence w i l l be p r o p o s e d f o r t h e l o e s s compr s i n g
t h e N o r t h China p l a i n . The q u e s t i o n t h a t a r i s e s , and f o r m s t h e m a j o r w h a t s t a g e do we i d e n t i f y l o e s s ? after the f i r s t
PO
n t o f t h i s paper i s
at
I s i t reasonable t o i d e n t i f y loess material
'make' e v e n t has o c c u r r e d r a t h e r t h a n w a i t i n g t o a p p l y t h e
d e s i g n a t i o n u n t i l a f t e r t h e d e p o s i t has f o r m e d .
T h i s a p p r o a c h may be neces-
s a r y ifwe a r e t o g i v e p r o p e r a t t e n t i o n t o t h e l o e s s a t t h e p e r i p h e r y o f t h e main event r e g i o n s .
I t i s no s u r p r i s e t h a t some o f t h e most s i g n i f i c a n t
p a p e r s i n w h i c h t h i s new a p p r o a c h becomes a p p a r e n t c o n c e r n l o e s s i n B r i t a i n a t t h e end o f t h e n o r t h European l o e s s band.
-
F o r example C a t t e t a1 ( 1 9 7 1 )
s t u d i e d t h e c o n t r i b u t i o n o f l o e s s m a t e r i a l t o t h e s o i l s o f N o r f o l k and were a b l e t o show t h a t i t was a m a j o r c o n s t i t u e n t , even t h o u g h t h e d e p o s i t i o n a l c h a r a c t e r i s t i c s were l a c k i n g .
T h e r e were enough m a t e r i a l c h a r a c t e r i s t i c s
and s u p p o r t i n g e n v i r o n m e n t a l knowledge t o a l l o w them t o s t a t e c o n f i d e n t l y t h a t t h e l o e s s , w h i c h had l o s t some c h a r a c t e r i s t i c s b y w e a t h e r i n g and o t h e r s o i l - f o r m i n g p r o c e s s e s , was t h e m a i n c o n s t i t u e n t o f t h e c o v e r l o a m . Thus we have l o e s s as a c o n s t i t u e n t as w e l l as l o e s s as a d e p o s i t , and i n p r a c t i c a l a n d economic t e r m s l o e s s as a c o n s t i t u e n t may be as i m p o r t a n t as l o e s s as a d e p o s i t . identified.
Two a s p e c t s o f l o e s s as a c o n s t i t u e n t need t o be
What C a t t e t a1 ( 1 9 7 1 ) d e s c r i b e d f o r N o r f o l k was a t h i n ( 7 0 cm)
l o e s s d e p o s i t w h i c h had been i n c o r p o r a t e d i n t o t h e l o c a l c o v e r l o a m
-
an
a c t u a l a e o l i a n e x i s t e d , w h i c h had s u b s e q u e n t l y l o s t i t s d e p o s i t i o n a l characteristics.
It seems r e a s o n a b l e t o i d e n t i f y a n o t h e r t y p e o f a d m i x t u r e
o r c o n s t i t u e n t system, one i n w h i c h occurred.
only
t h e i n i t i a l 'make' e v e n t has
Here we a r e s e e k i n g t o i d e n t i f y l o e s s m a t e r i a l w h i c h has n o t
reached ' d e p o s i t ' s t a t u s b u t which e x i s t s i n s u f f i c i e n t c o n c e n t r a t i o n s i n s o i l systems t o c o n t r i b u t e good s t r u c t u r e and n u t r i e n t s t a t u s .
There w i l l
e x i s t s o i l s w h i c h owe t h e i r h i g h a g r i c u l t u r a l v a l u e t o s l o e s s c o n s t i t u e n t w h i c h has o n l y e x i s t e d as l o e s s ' m a t e r i a l ' .
T h i s phenomenon w i l l be r e -
s t r i c t e d t o l o e s s produced by g l a c i a l a c t i o n . I n t h e e a r l y p a p e r s i n w h i c h e v e n t sequences f o r l o e s s d e p o s i t s were i d e n t i f i e d ( S m a l l e y 1966, S m a l l e y and V i t a - F i n z i , f o r t h e d e f i n a b l e e v e n t s was p r o p o s e d .
1968) a s i m p l e n o m e n c l a t u r e
The 'make' e v e n t s , t h o s e a s s o c i a t e d
w i t h t h e p r o d u c t i o n o f t h e b a s i c l o e s s m a t e r i a l were d e s i g n a t e d P e v e n t s
(P f o r p r o v e n a n c e ) ;
t h o s e a s s o c i a t e d w i t h t r a n s p o r t a t i o n were l a b e l l e d T
e v e n t s and t h o s e w i t h d e p o s i t i o n were e v e n t s b o t h had a
D
designation.
D events.
The ' p l a c e ' and ' c h a n g e '
I t i s j u s t p o s s i b l e t h a t we w i l l be a b l e
t o r e c o g n i z e n a t u r a l s y s t e m s i n w h i c h t h e number o f s i g n i f i c a n t l o e s s f o r m i n g e v e n t s may have any t o t a l r a n g i n g f r o m one t o t e n o r e l e v e n o r more; shows some s u g g e s t e d l i m i t s .
table 1
53 TABLE 1 Some o u t l i n e e v e n t sequences;
f o u r examples chosen t o i l l u s t r a t e
the possible variations i n complexity o f loess-deposit-forming s y s tems Loess
E v e n t Sequence
Chinese l o e s s : N o r t h C h i n a P l a i n Loess n e a r T a s h k e n t Loess i n N o r f o l k , E n g l a n d Loess m a t e r i a l . N. Canada
P1, T1, 01, T2, T3, D2, T4, D3, 04, T5, 05 P1, T1, D1, T2, T3, D2, T4, D3, D4 P1, T1, D1, D2 P1
THE FORMATION OF LOESS MATERIAL Smalley (1966) proposed t h a t g l a c i a l g r i n d i n g p r o v i d e d l o e s s m a t e r i a l f o r the bulk o f the world's loess deposits; p r o v i d e d by d i r e c t g l a c i a l a c t i o n . and V i t a - F i n z i
i.e.
t h e 'make' e v e n t was u s u a l l y
T h i s a s s e r t i o n was r e p e a t e d by S m a l l e y
( 1 9 6 8 ) and s u p p o r t e d b y B o u l t o n ( 1 9 7 8 ) who w r o t e :
" V i t a - F i n z i and S m a l l e y ( 1 9 7 0 ) have a r g u e d t h a t a m a j o r p r o p o r t i o n o f t h e
I w o u l d s u p p o r t them i n t h i s , and
w o r l d ' s s i l t i s produced by g l a c i e r s .
go f u r t h e r t o s u g g e s t t h a t m o s t o f t h i s i s p r o d u c e d i n t h e b a s a l zone o f t r a c t i o n which I b e l i e v e t o be a u n i q u e l y g l a c i a l environment i n which l a r g e forces a t non-inertial
s h e a r c o n t a c t s p r o d u c e f i n e - g r a i n e d wear p r o d u c t s . "
( B o u l t o n 1978, p. 796) R e c e n t l y a l t e r n a t i v e s o u r c e s have been e x p l o r e d and some s u g g e s t e d ' m a k e ' mechanisms s h o u l d be d i s c u s s e d h e r e .
The a r c h e t y p a l l o e s s p a r t i c l e can
be e n v i s a g e d as a 30 um q u a r t z p a r t i c l e and t h e make e v e n t has t o p r o d u c e l a r g e q u a n t i t i e s o f these;
t h i s i s t h e m a j o r make q u e s t i o n - how a r e t h e
predominant q u a r t z o a r t i c l e s produced?
Since t h e Smalley (1966) g l a c i a l
g r i n d i n g p r o p o s a l was made i t has been shown, l a r q e l y by Moss and cow o r k e r s (Moss and Green 1 9 7 5 ) b u t a l s o b y R i e z e b o s and Van d e r I i a a l s ( 1 9 7 4 ) , t h a t q u a r t z i n i g n e o u s r o c k s c o n t a i n s many d e f e c t s and f r a c t u r e s - w h i c h make i t r a t h e r weaker t h a n was o r i g i n a l l y e n v i s a g e d .
I n t h e Smalley (1966)
s c e n a r i o t h e o n l y n a t u r a l e n e r g y s o u r c e o f s u f f i c i e n t m a g n i t u d e t o Droduce l a r g e s c a l e f r a c t u r i n g o f s t r o n g q u a r t z was g l a c i a l a c t i o n .
We see now
t h a t t h e q u a r t z i s n o t s o s t r o n q as was o r i g i n a l l y t h o u g h t and t h u s , a l t h o u g h g l a c i a l g r i n d i n g s t a y s as e f f i c i e n t as i t e v e r was, o t h e r n a t u r a l f o r c e s may c r o s s t h e c r i t i c a l e n e r g y t h r e s h o l d and be a b l e t o p r o d u c e s i g n i f i c a n t q u a r t z breakage.
A c l e a r d i s t i n c t i o n s h o u l d be made between ' s i g n i f i c a n t ' b r e a k a g e w h i c h can o r o d u c e l a r g e amounts o f l o e s s m a t e r i a l , and 'some' b r e a k a g e w h i c h m i g h t F r o d u c e p a r t i c l e s on a s m a l l s c a l e .
G l a c i e r s produce s i g n i f i c a n t
b r e a k a g e and c a n a c c o u n t s a t i s f a c t o r i l y f o r t h e p a r t i c l e s u p p l y t o t h e N o r t h American and n o r t h European d e p o s i t s .
A l t e r n a t i v e p a r t i c l e producing
54 mechanisms t e n d t o a p D l y t o o t h e r o f t h e w o r l d ' s d e o o s i t s . t i c u l a r approaches s h o u l d be d i s c u s s e d ;
Three par-
t h o s e o f W h a l l e y e t a1 ( 1 9 8 2 ) ,
Nahon and T r o m p n t t e ( 1 9 8 2 ) and Goudie e t a1 ( 1 9 7 9 ) .
A l l look towards
l o e s s p r o d u c t i o n away f r o m g l a c i a l r e g i o n s , and p r o p o s e p a r t i c l e p r o duction alternatives t o glacial grinding. W h a l l e y e t a1 ( 1 9 8 2 ) f a v o u r sand g r a i n i m p a c t as t h e f o r m a t i o n iiiechanisrii. They c o l l e c t e d t h e p r o d u c t s o f sand a b r a s i o n e x p e r i m e n t s and o b s e r v e d t h e presence o f loess-sized p a r t i c l e s .
They c o n c l u d e d t h a t an a e o l i a n
a b r a s i o n a l o r i g i n f o r some l o e s s i c m a t e r i a l seemed p o s s i b l e , and i n p a r t i c u l a r t h a t t h e Gobi d e s e r t c o u l d s u p p l y m a t e r i a l f o r t h e Chinese loess.
T h e i r abrasion experinients produced s i g n i f i c a n t l y d i f f e r e n t re-
s u l t s f r o m t l i o s e o f Kuenen ( 1 9 6 0 ) who f o u n d t h a t l o e s s - s i z e d m a t e r i a l was n o t produced.
Nahon and T r o m p e t t e ( 1 9 8 2 ) s u g g e s t e d t h a t c h e m i c a l w e a t h e r i n g ,
p a r t i c u l a r l y i n t r o p i c a l a r e a s , i s an a c t i v e a g e n t o f s i l t f o r m a t i o n . Ideak i i i i n e r a l s d i s a p p e a r b y i n c o n g r u e n t d i s s o l u t i o n and q u a r t z i s f r a g m e n t e d by p a r t i a l c h e m i c a l d i s s o l u t i o n .
The s o r t i n g o f t h e d e t r i t a l f r a c t i o n ,
e s s e n t i a l l y s i l t , may be i n i t i a t e d i n s o i l s by t h e p e d o g e n e t i c s e p a r a t i o n o f c l a y and t h e p r o g r e s s i v e d i s s o l u t i o n o f t h e cement.
I n t h i s way i m -
p o r t a n t c o n c e n t r a t i o n s o f s i l t on t h e s u r f a c e a r e o b t a i n e d by s e l e c t i v e r e 1 a t i ve accumul a t io n .
The Goudie e t a1 ( 1 9 7 9 ) mechanism i s a l s o t r o p i c a l
and c h e m i c a l - s a l t we t h e r i n g by s o d i u m s u l p h a t e
-
and t h e i r e x p e r i m e n t s
d i d p r o d u c e sand g r a i n b r e a k a g e and i n d i c a t e a p o s s i b l e s o u r c e o f some loess-sized particles G o u d i e e t a1 ( 1 9 7 9 )
Pye and S p e r l i n g , 1 9 8 3 ) . i n t h e i n t r o d u c t i o n t o t h e i r p a p e r , c i t e d as
f r e q u e n t l y f a v o u r e d p a r t i c l e p r o d u c t i o n mechanisms g l a c i a l g r i n d i n g ( S m a l l e y 1 9 6 6 ) , i n s o l a t i o n w e a t h e r i n g , f r o s t a c t i o n ( Z e u n e r 1949) and s p a l l i n g d u r i n g w i n d t r a n s p o r t ( S m a l l e y and V i t a - F i n z i 1 9 6 8 ) . second o f t h e s e , f r o s t w e a t h e r i n g , f o r glacial grinding.
The
now l o o k s l i k e an e f f i c i e n t complement
The o r i g i n a l o b s e r v a t i o n s b y Zeuner ( 1 9 4 9 ) have
been s u p p o r t e d b y B r o c k i e ' s ( 1 9 7 3 ) i n v e s t i g a t i o n s i n F!ew Z e a l a n d and i t appears t h a t c o l d w e a t h e r i n g p r o c e s s e s m u s t now be c o n s i d e r e d as a m a j o r method f o r p r o d u c i n g l o e s s m a t e r i a l .
I n f a c t c o l d m o u n t a i n s o u r c e s can
be seen as s u p p l y i n g m a t e r i a l f o r w h a t were t h o u g h t o f as ' d e s e r t ' l o e s s d e p o s i t s , and a s i m p l e d i s t i n c t i o n o f l o e s s i n t o e i t h e r ' i c e - s h e e t ' o r ' m o u n t a i n ' may c o v e r t h e i i i a j o r o c c u r r e n c e s ( S m a l l e y 1 9 7 8 b ) . The p a r t i c l e p r o d u c t i o n p r o b l e m needs more i n v e s t i g a t i o n .
The e a r l i e r
g e n e r a l i z a t i o n s a r e becoming more r e f i n e d and a r e b e i n g i n c o r p o r a t e d i n t o comprehensive e v e n t sequence models f o r t h e f o r m a t i o n o f s p e c i f i c d e p o s i t s , b u t more knowledge o f t h e a c t u a l p a r t i c l e f o r m i n g mechanism i s r e q u i r e d , and t h e r e a r e o t h e r h i g h e n e r g y e n v i r o n m e n t s t h a t s h o u l d be i n v e s t i g a t e d .
55 Palmer (1982) has shown t h a t high energy r i v e r s a s s o c i a t e d with deformable rocks can produce l o e s s - s i z e m a t e r i a l and i t s e e m l i k e l y t h a t in New Zealand, a country o f s t e e p r i v e r s , a r e a s o n a b l e p r o p o r t i o n o f l o e s s m a t e r i a l cotlies from t h i s s o u r c e . B u t f r o s t w e a t h e r i n g must c e r t a i n l y be seen a s a iiiajor rnechanisni f o r s u p p l y i n g l o e s s m a t e r i a l . Experimental i n v e s t i g a t i o n s c a r r i e d o u t o v e r t h e l a s t f i f t e e n y e a r s a t t h e CPlRS Centre de Georiiorphologie in Caen have r e c e n t l y been reviewed and suiiimarized ( L a u t r i d o u and Ozouf 1982) a n d i t i s c l e a r t h a t m a t e r i a l o f t h e right s i z e and iiiineralogy can be pro-
duced, i n t h e r e q u i r e d amounts, by f r o s t a c t i o n . SIZE LIMITS AN0 DEFINITIONS S i z e has always been a d e f i n i n g c h a r a c t e r i s t i c of l o e s s , but i f we a r e going t o eiiiphasise t h e m a t e r i a l c h a r a c t e r i s t i c s , t h e l i m i t s need t o be more c a r e f u l l y d e f i n e d .
A major problem has a r i s e n with t h e use o f t h e
term ' s i l t ' , and i n p a r t i c u l a r w i t h t h e way i n which s i l t i s simply used as a s i z e terrn t o f i l l t h e s p a c e between c l a y and sand.
Both c l a y a n d sand
do have c e r t a i n g e o l o g i c a l p r o c e s s e s r e f l e c t e d i n t h e i r c h a r a c t e r i s t i c s i z e s b u t t h e s i l t term l a c k s t h i s j u s t i f i c a t i o n and i t has a very v a r i a b l e range i n t h e v a r i o u s e x t a n t s i z e s c a l e s .
For example i n t h e ISSS ( I n t e r -
n a t i o n a l S o c i e t y o f S o i l S c i e n c e ) system s i l t i s between 2 um and 20 Wni and l o e s s a t 20-60 uin would be c l a s s i f i e d a s a f i n e s a n d .
B u t a 4 uri
p a r t i c l e which would be s i l t on the ISSS system i s c l a y on t h e U.S. of S o i l s s y s t e m .
Bureau
The c h o i c e o f a s i z e range f o r l o e s s m a t e r i a l has t o be a somewhat a r b i t r a r y p r o c e s s - b u t n o t i n the same way t h a t naming a l l p a r t i c l e s i z e classes i s ;
a f t e r a l l we do have a l o t o f accumulated s i z e a n a l y s i s d a t a .
We propose t h a t t h e 20-60
pm
range could be c a l l e d l o e s s - s i z e .
Browzin
(1981) has r e c e n t l y argued f o r a 10-50 urn ' l o e s s - s i z e ' range and t h e r e a r e c e r t a i n h i s t o r i c a l a s p e c t s which can be c i t e d i n s u p p o r t o f t h i s p a r t i c u l a r r a n g e . Russell ( 1 9 4 4 ) , who made c o n s i d e r a b l e e f f o r t s t o produce
a r i g o r o u s d e f i n i t i o n o f l o e s s , p l a c e d h i s s i z e l i m i t s a t 10 and 50 urn; and i f i t ( l o e s s ) i s t o be t h o u g h t o f a s a s i l t y d e p o s i t i t i s c o n v e n i e n t
t h a t t h e s i l t r a n g e s o f t h e U.S. Department o f A g r i c u l t u r e and t h e U.S. Bureau o f S o i l s have upper bounds a t 50 urn. The b e s t reason f o r p u t t i n g t h e lower bound a t 20 um ( r a t h e r than a t 10 um) i s t o o b t a i n t h e maximum d i s t a n c e from t h e s i z e range i n which f i n e a e r o s o l i c d u s t i s found, s a y < l o um. A range o f 20-60 urn p r o v i d e s a b e t t e r s e p a r a t i o n from o t i i e r , m a t e r i a l s than a 10-50 pm range. Manecki e t a1 (198C)) favoured t h e 20-60 urn. r a n g e , which t h e y c a l l e d t h e ' l o e s s i a l ' f r a c t i o n and t h i s seems a range d e s e r v i n g o f some s u p p o r t , a l t h o u g h i t i s f o r ' t a c t i c a l ' r a t h e r than s c i e n t i f i c r e a s o n s t h a t i t might be favoured o v e r t h e 10-50 urn
56
range. The Smalley a n d Vita-Finzi (1968) d e f i n i t i o n of loe ss was: 'Loess i s a c l a s t i c deposit which c o n s i s t s predominantly of quartz part i c l e s 20-50 urn i n diameter a n d which occurs as wind l a i d s h e e t s . '
his
has l a s t e d f a i r l y well and i s s t i l l an acceptable b r i e f d e f i n i t i o n .
Siiia 1 1 ey
and Vita-Finzi (1968, p . 766) quoted a d e f i n i t i o n by Pe ttijohn (1957 p . 377-8) a n d i t i s timely here t o quote a more recent d e f i n i t i o n from a l a t e r e dition o f t h e same work;
t h i s i s l o es s defined within a s t r i c t l y sediniento ogical
framework (PettL john 1975, p . 290-1 ) : "Loess i s an unconsolidated porous s i l t , commonly buff in colour ( l o c a l l y gray, yellow, brown o r r e d ) , ch ar act er i zed by i t s lack of s t r a t i f i c a t i o n and remarkable a b i l i t y t o stand in a v e r t i c a l s lope .
I t conimonly shows a crude columnar s t r u c t u r e . I t i s generally highly calcareous a n d e f f e r v e sc e s in weak aci d . Loess i s e s s e n t i a l l y a s i l t . " THE DESERT LOESS PROBLEM The d e s e r t l o e ss problem begins witJi Obruchev (1911). He f i r s t proposed, on th e b a s i s of f i e l d observations made in the Central Asian d e s e r t s , t h a t t h e r e e x i s t e d a s e p a r a t e , d ef i n ab l e ' h o t ' l o es s which was produced in h o t (sandy) d e s e r t s . The l o es s regions of Central Asia, and those of China, do e x i s t next t o d e s e r t s a n d t h i s appeared t o give support t o the d e s e r t lo e s s hypothesis. Butler (1956) however, a f t e r a careful consideration of the A u s t r a l i a n s i t u a t i o n suggested t h a t "considering the vast areas o f d e s e r t s i n t h e world, and o u r r e l a t i v e ignorance of ' h o t ' l o e s s , the l a t t e r may be more hypothetical t h a n real . " The Butler suggestion provoked Sniallcy and Vita-Finzi (1968) i n t o a survey o f d es er t l oe ss a n d they concluded t h a t th e r e does not e x i s t a s p e c i f i c and e f f i c i e n t mechanism which can produce lo e s s p a r t i c l e s i n h o t d e s e r t environments. Fine p a r t i c l e s , usually l e s s t h a n 10 i n diameter, a r e produced i n d e s e r t s b u t these a r e c a rrie d long d is t a n c e s i n suspension and do n o t form l o es s d eposits. The g r e a t bulk of S a h a r a n d u s t i s of t h i s type a n d i s car r i ed o u t over the A t l a n t i c - the Sahara i s not a producer of l o es s p a r t i c l e s in s i g n i f i c a n t amounts - as Penck (1930 observed long ago. Recently Gouc'ie e t a1 (1979) have proposed p a r t i c l e formation by s a l t weathering a n d lihalley e t a1 (1392) have produced some loe ss-siz e d material by sand grain impacts. The most convincing s t u d i e s of d e s e r t loess f o r mation have been those by Yaalon and coworkers in the '!egev ( e . g . Yaalon and Dan 1974). They have been ab l e t o show t h a t some loe ss p a r t i c l e s a r e produced i n t h e Sinai d e s e r t and c a r r i e d up t o form de posits and contribut i o n s t o s o i l s in t h e Negev a n d o t h e r p a r t s o f I s r a e l . I t i s i n t e r e s t i n g t o note t h a t the p a r t i c l e s a r e formed i n t h e rocky p a r t s of the Sinai de se rt
57
by d i r e c t w e a t h e r i n g o f exposed rocks and t h a t t h e o t h e r p a r t s of t h e d e s e r t do n o t produce l o e s s p a r t i c l e s .
w i l l produce ments;
I t thus seeins l i k e l y t h a t v a r i o u s p r o c e s s e s
modest amounts o f l o e s s s i z e d p a r t i c l e s i n hot d e s e r t e n v i r o n -
t h e q u e s t i o n remains - can t h e s e p r o c e s s e s produce enouqh m a t e r i a l
t o form a
III~J>~ deposit?
Wlialley e t a1 (1982) s u g g e s t e d t h a t t h e impact
mechanism n i g h t c o n t r i b u t e s u b s t a n t i a l amounts o f iiiaterial to t h e Chinese loess d e p o s i t s ;
t h e p a r t i c l e formation p r o c e s s e s o c c u r r i n g i n t h e Gobi
d e s e r t and being e s s e n t i a l l y t h e same a s t h o s e d e s c r i b e d by Sinalley a n d
The most t e l l i n g argument a g a i n s t t h e impact p r o c e s s being a major producer o f l o e s s m a t e r i a l i s t h e l a c k of t h e r e l e v a n t 20-60 Ilm p a r t i c l - s i n t h e p r o d u c t s of t h e P!orth A f r i c a n d e s e r t s . Major Vita-Finzi ( 1 9 6 8 ) .
s t u d i e s on Saharan d u s t have been c a r r i e d o u t r e c e n t l y (Morale8 1979) and t h e r e i s no i n d i c a t i o n t h a t t h e major d u s t p r o d u c t has a mode s i z e o t h e r
t h a n in t h e expected <10
pm
range (Junge 1979, p . 5 5 ) , and t h e d i s t r i b u -
tion curve f a l l s o f f very s h a r p l y towards t h e l a r g e r s i z e s . Smalley and K r i n s l e y ( 1 9 7 8 ) reviewed the d e s e r t l o e s s problem, w i t h some emphasis on t h e C e n t r a l Asian r e g i o n s , and concluded t h a t cold w e a t h e r i n g processes could s t i l l be used t o e x p l a i n t h e f o r m a t i o n o f t h e w o r l d ' s major l o e s s d e p o s i t s .
Smalley (1980) produced an e v e n t sequence f o r t h e
Tashkent l o e s s which a l l o w s a c o l d o r i g i n i n nearby mountains and i n e i g h t s i g n i f i c a n t e v e n t s can produce a ' d e s e r t ' l o e s s d e p o s i t .
This sequence
will be c o n s i d e r e d h e r e , and extended t o cover t h e i n l a n d l o e s s in North China, and t h e r e d e p o s i t e d m a t e r i a l on t h e North China p l a i n . The C e n t r a l Asian and Chinese l o e s s d e p o s i t s a r e t h e major ' d e s e r t ' l o e s s o c c u r r e n c e s and a s a t i s f a c t o r y e x p l a n a t i o n o f t h e i r f o r m a t i o n w i l l s o l v e t h e d e s e r t l o e s s problem ( i n i t s b a s i c f o r m ) .
The Tashkent l o e s s ( s e e n a s an i d e a l
d e s e r t l o e s s ) could be formed by t h e f o l l o w i n g e v e n t sequence (based on Small ey 1980) : P1 Rock d e t r i t u s i s produced i n t h e Tien Shan mountains by c o l d w e a t h e r i n g p r o c e s s e s ; g l a c i a l a c t i o n niakes a c o n t r i b u t i o n . Much m a t e r i a l i s produced d u r i n g Quaternary c o l d phases b u t t h e mountains s t a y c o l d enough in p o s t - g l a c i a l t i m e s f o r p r o d u c t i o n t o c o n t i n u e a t a reduced r a t e .
T1 Crushed rock m a t e r i a l , w i t h a range of p a r t i c l e s i z e , i s c a r r i e d down t h e mountains by m e l t w a t e r and o t h e r f l u v i a l a g e n c i e s . 01 A imixed d e p o s i t i s formed in t h e f o o t h i l l s r e g i o n ;
some s o r t i n g may have
o c c u r r e d b u t mixed d e p o s i t s s h o u l d predominate. T2 Some complex t r a n s p o r t a t i o n a c t i v i t y h e r e ;
some p a r t i c l e s may be r a i s e d
by the wind from t h e mixed d e p o s i t and c a r r i e d away i n s u s p e n s i o n b u t t h e major a c t i v i t y i s probably ( i n terms o f e v e n t u a l l o e s s d e p o s i t s ) s t i l l f l u v i a l and i n v o l v e s the i n t r o d u c t i o n o f t h e f i n e r p a r t i c l e s (sand s i z e
and smaller) i n t o t h e major r i v e r s . T3 Trailsport out i n t o t h e d e s e r t regions by ( i n p a r t i c u l a r ) t h e Amu-Da.r'ya and the Syr-Dar'ya. These r i v e r s a r e powerful t r a n s p o r t e r s of suspended m a t e r i a l , i n f d c t Suslov (1961, p . 4 7 2 ) claimed t h a t "There i s no o t h e r r i v e r in t h e world t h a t c a r r i e s as much suspension material as does t h e Amu-Dar'ya."
D2 Relatively well s o r t e d deposits a r e formed, on floodplains out in the d e s e r t region. The p a r t i c l e s i z e i s in the loess range; Suslov recorded 64.3% of these p a r t i c l e s in the .4mu-Dar'ya alluvium. These a r e the raw material f o r d e s e r t l o e s s , and i t i s t h e r i v e r s which place them in t h e d e s e r t s e t t i n g , as 02 alluvium. T4 Although t h e sorted deposits have a r e l a t i v e l y high s t a b i l i t y t h e i r d e s e r t s i t u a t i o n exposes them t o erosion by sand grain impact a n d t h i s i n j e c t s l o e s s sized p a r t i c l e s i n t o t h e atmosphere (Smalley 1970) and they a r e transported in suspension - f o r a r e l a t i v e l y s h o r t distance. D3 Deposition a t t h e d e s e r t f r i n g e s . The formation of deposits with loess sized p a r t i c l e s and open s t r u c t u r e s ; most material delivered by t h e T4 stage b u t some d i r e c t from Dl/T2. D4 Post-depositional changes: t h e D3 deposit i s more s t a b l e than t h e 02 deposit because i t i s l e s s exposed t o d e s e r t processes. Some lime and clay minerals may ?.ccumulate, these will s t a b i l i z e t h e l o e s s s t r u c t u r e and produce t h e c l a s s i c l o e s s composition. Thus nine stages can be i d e n t i f i e d in t h e complex formation process leading t o t h e Tashkent l o e s s , they a r e a l s o shown in Figure 1 . I t i s aeolian deposit b u t several o t h e r equally c r i t i c a l events must occur, and although D4 i s mechanically s t a b l e with respect t o natural forces i t will be disturbed by a g r i c u l t u r a l operations and p a r t i c l e s will be moved by t h e wind. As Suslov (1961, p. 464) remarked: " I n summer, c i t i e s of Central Asia may be recognized, even from f a r o f f , by t h e thick shrouds of l o e s s d u s t . " A s i m i l a r sequence of events could operate f o r t h e Chinese l o e s s . Stage T3 would be the t r a n s p o r t a t i o n of loess material by t h e Yellow River out i n t o t h e d e s e r t regions.
The Yellow River, l i k e t h e Amu-Dar'ya i s a very
considerable t r a n s p o r t e r o f material in suspension. Stages D3 and D4 would r e f e r t o t h e formation of t h e c l a s s i c inland primary l o e s s of Kansu, Shensi and Shansi.
I f we add on two more stages we have an event sequence leadino
t o t h e formation of t h e North China plain. T5 Eroded l o e s s material i s c a r r i e d by t h e Yellow River. Over t h e years t h e course of the Yellow River has varied considerably and material has been c a r r i e d both north and south of Shantung Island.
59
F i g . 1 . Simple event sequencc tiiaqran f o r t h e forma-tion o f a l o e s s cieposi.: a t t h e mountain/desert t r a n s i t i o n reqion. This i s a very much idealised view o f t h e Central Asian situa-tion. External symbols indicate: P I Tien %:an mountains, D d e s e r t , S Aral Sea. Tizio r i v e r s ( t h e Amu- and Syr-Dar'ya) f l m from PI t h r o u g h D t o S . The sequence o f P, T and D events leads t o the formation of a l o e s s deposi-t ( D 3 ) a t the desere fringe.
60
D5 The deposited loess material forms t h e riorth China plain and Shantung Island i s transformed i n t o Shantunq yminsula (Tinq 1965). These new lands t o t h e e a s t allow the e a r l y Chinese t o move from t h e i r si'ies in the primary loess country and find fresh f e r t i l e s o i l s (Snalley 1968). A recent study by Derbyshire (1983) of t h e loess a t Jiuzhoutai i l l u s t r a - t e s some major points of t h e event sequence. He concluded t h a t the p a r t i c l e shape, s i z e and f a b r i c of t h e loess were c o n s i s t e n t with o r i q i n by def l a t i o n o f s i l t s from wadis, fans and d e s e r t plains t o t h e noreh a n d northwest of Lanzhou. This ' o r i g i n ' r e l a t e s t o stage T4- - t h e major aeolian transporCation event which leads t o D3 and D4 t h e main inland deposition events. However many s i g n i f i c a n t events have occurred before T4 takes place. The de?osi t s We o f f e r no hard and f a s t d i s t i n c t i o n s , no simple d i v i s i o n i n t o major and minor d e p o s i t s , or even i n t o l o e s s a n d not-loess. The most l o e s s i c of l o e s s d e y o s i t s i s t h e Chinese loess - recently celebrated by Wang and Zhang (1980): a l l t h e c l a s s i c nrocesses have operated - and a l l t h e and deposit f a c t o r s a r e v i s i b l e . A.L the other end of t h e s c a l e must systems i n which l o e s s material forms a modest admixture t o a s o i l without even an aeolian event t o emplace t h e material - t h e basic P1
material be t h e possibly
loess found on t h e f r i n g e o f ice-shee-t deposits. Table 2 l i s t s l o e s s deposits and attemnts t o i n d i c a t e t h e i r type - j u s t a simple dis-tinction i n t o i c e s h e e t s (IS-material l a r g e l y produced by g l a c i a l grinding) and inountain (material produced by f r o s t weathering). Regions and r i v e r s a r e indicated,
TABLE 2 Loess deposits in various p a r t s of t h e world - type a n d source P!ame ( r e f e r e n c e )
Central North American
North European Band China, inland
type
IS
IS Hounta i n
China, North China Plain (Ting 1965, Smalley, t h i s paper) New Zealand, North Island
Ploun t a i n
New Zealand, S o u t h I s .
Mountain
Mountain ( 5 tephric)
source region; r i v e r s involved
confidence
northern g l a c i e r s Mississippi and Missouri r i v e r s northern g l a c i e r s
* * *
Himalayan a n d Tibetan uplands Hwang Ho (T3) as China inland b u t Hwang Ho twice (T3 and T5) Tararua Mts. Ruahine Mts. Southern A1 ps
-
* * * * * * *
* **
61
Name ( r e f e r e n c e ) New Zealand, South Island Europe, Rhinel and Smalley, Krinsley & Vita-Finzi 1973 Europe, East-Central Smalley & Leach 1978
type Mou n t a i n Mount a i n
Voun t a i n + some IS
Soviet Central Asia Smalley 1980
Mountain
Kashmir
Mountain
England, S.E. Lill & Smalley 1978 Argentina and level o f confidence:
IS
Mountain
source region; r i v e r s involved Southern A1 ps Rakaia River European A1 ps Rhine Alps anti nor-thern g l a c i e r s Danube and Tisza Tien S h a n Plts. Amu-Dar ' y a , Syr-Dar 'ya H imal aya s northern g l a c i e r s Andes
confidence
** *
* * * * *
* * *
from 3 s t a r s f o r a high level of confidence t o
1 s t a r f o r l i t t l e . This tioes n o t mean t h a t t h e one s t a r estimates a r e necessarily doubtful, j u s t t h a t t h e supporting data a r e scarce. Three deposit regions have dominated loess discussions: China, North
America and Europe. A n o u t l i n e sequence of events f o r the Chinese loess seems t o be appearing; Wen e t a1 (1981) have suggested t h a t the r a r e e a r t h element d i s t r i b u t i o n p a t t e r n s in l o e s s i n d i c a t e t h a t the loess material was transported from i t s provenance ( P ) region by moving water, i n t o the sedimentary system and then transported by wind t o where i t i s now d i s t r i buted. That f i t s in well with t h e event sequence described in t h i s paper. Wide ranging discussion s t i l l continues on the Chinese l o e s s . The North American l o e s s i s e s s e n t i a l l y a g l a c i a l material - i t s o r i g i n s a r e d i r e c t l y linked t o t h e northern ice-sheets, and of t h e t h r e e deposits t h i s would seem to be the l e a s t c o n t r o v e r s i a l . The northern band of European loess i s a l s o equally g l a c i a l - close t o the Smalley 1966 i d e a l . The loess in Poland for example i s c l a s s i c ice-sheet l o e s s , b u t the l o e s s f u r t h e r south - in t h e Danube v a l l e y - s t i l l needs i n v e s t i g a t i o n and explanation. Smalley a n d Leach (1978) have suggested some possible formation and d i s t r i b u t i o n evenlls b u t t h e region i s complex and much more investigation of loess d i s t r i b u t i o n
in i t s e n t i r e t y i s required. LOESS AS A CONSTITUENT
Bruce (1978) has mapped loess material in t h e South Island of New Zealand w i t h t h r e e l e v e l s of r e s o l u t i o n . He shows areas of s o i l s in which loess predominates, areas in which i t i s a major c o n s t i t u e n t , and areas in which i t i s a minor c o n s t i t u e n t . This simple mapping s t y l e has produced a very e f f e c t i v e and useful map with uses in land development p r o j e c t s and t h e
-
62
current operations f o r reclaiming opencast coal s i t e s . (Loess i s a very e f f e c t i v e material f o r opencast coal s i t e reclamation - see Drake and R i r i e 1981). Of course in t h e Bruce map t h e t h r e e divisions grade i n t o each other, b u t they give a t r u e r p i c t u r e o f loess in t h e South Island t h a n a ' d e p o s i t ' map in which a l i n e would be drawn r o u n d the ' l o e s s predominates' area and an a r t i f i c i a l d i s t i n c t i o n from other loess-affected areas produced. The ' l o e s s as a c o n s t i t u e n t ' idea can be traced back t o t h e pioneer work by Perrin (1956) on t h e Chalk Heath s o i l s in southern England. Perrin e t a1 (1974) developed t h i s work and produced a map of t h e d i s t r i b u t i o n of l a t e Pleistocene aeolian deposits in eastern and southern England. I n t h i s case, and in t h e Norfolk s o i l s considered by Catt e i a1 ( 1 9 7 1 ) i t was proposed t h a t a t h i n deposit o f aeolian l o e s s was formed which was then i n corporated i n t o t h e s o i l - t h e ' d e p o s i t ' character being l o s t - b u t having once been possessed; t h e material had gone t h r o u g h a deposi-L stage ( s e e Table 1 where t h e Norfolk event sequence i s given a s P1, T 1 , D1 deposit formed, D2 material incorporated iilto s o i l ) . The d e l i c a t e question on t h e t o p i c of ' l o e s s a s a c o n s t i t u e n t ' i s should we recognize t h e l o e s s inaterial which has not gone t h r o u g h t h e aeolian T event? I s i t useful o r s i g n i f i c a n t enough t o r e q u i r e recognition? The question a r i s e s f i r s t in regions of ice-sheet loess b u t may be s i g n i f i c a n t i n many p a r t s of t h e world. The ice-sheet maltes t h e 20-60 um loess material acd some of i t subsequentlbl g e t s transnorted bj/ t h e wind t o form l o e s s d e p o s i t s . Much of i t goes t o form s o i l s of high a g r i c u l t u r a l vroduct i v i t y - and i t i s t h e g r e a t value o f the l o e s s component which tempts one t o recognize i t a t a s e a r l y a stage as possible. In Canada t h e r e a r e moves t o increase food production by a s i g n i f i c a n t a m o u n t , and t h i s required t h e recognition and u t i l i z a t i o n o f more good s o i l - s o i l with a l o e s s c o n s t i t u e n t . Chesworth's (1982) map of major croplands shows t h e reiar!:able coincidence of l o e s s with d e s i r a b l e s o i l s ; one of the major non-loess regions shown i s t h e band across north India - which Chesworth indicates as alluvium. This i s i n f a c t l o e s s material; a l l across India and i n t o Bangladesh t h e t h e major r i v e r s carry l o e s s material made by f r o s t weathering in the Himalayan highlands. The events lead t o a l l u v i a l s i l t deposition - material from t h e southern Himalayas i s (was) c a r r i e d by t h e Ganges in t h e same way t h a t material from t h e Tien Shan i s c a r r i e d by t h e Amu-Dar'ya - b u t t h e Ganges does not pass through a d e s e r t region, and deposition stops a t t h e D2 s t a g e . This gives t h e broad b e l t of a l l u v i a l s o i l s across north India one of t h e world's major croplands - and forms the nation of Bangladesh. The s t a t e of Iowa i s l a r g e l y covered by l o e s s deposits - b u t t h e nation o f Bangladesh i s l a r g e l y made o f l o e s s material.
63 Loess i n Canada In the 1971 B i b l i o g r a p h y o f Canadian S o i l S c i e n c e (from 1803 on! t h e r e hiere f o u r e n t r i e s under ' l o e s s ' i n t h e index (Atkinson 1 9 7 1 ) .
This
s u g q e s t s t h a t t h e r e i s e i t h e r n o t iliuch l o e s s i n Canada, o r t h a t Canadian r e s e a r c h e r s a r e n o t much i n t e r e s t e d i n l o e s s , o r both of t h e s e . A c t u a l l y idhile l o e s s was r e c o q n i z e d s o l e l y a s a d i s c e r n a b l e d e p o s i t of r e a s o n a b l e s i z e i t was not s u r p r i s i n q t h a t t h e r e \was l i t t l e Canadian a c t i v i t y . now we s e e , i n p a r t i c u l a r
n B r i t a i n ( P e r r i n 1956, P e r r i n e t a1 1974,
C a t t e t a1 1971, C a t t 1977
1978, L i l l and Sirialley 1978, Eden 1980,
But
Burrin 1981, Lee 1979, e t c ) t h a t major and very s i q n i f i c a n t a c t i v i t y can occur i n a r e q i o n of t h i n and a p p a r e n t l y i n s i q n i f i c a n t l o e s s - and t h a t t h i s tiiodest l o e s s can be a c o n s i d e r a b l e n a t i o n a l r e s o u r c e and a s e t t i n g f o r s c i e n t i f i c a l l y useful i n v e s t i g a t i o n s . Dumanski e t a1 (1930) have s t u d i e d t h e l o e s s - d e r i v e d s o i l s i n Hinton, Alberta.
These p r o v i d e a r e c o r d of s o i l f o r m a t i o n f o r t h e e n t i r e p o s t -
g l a c i a l p e r i o d , and s u g g e s t t h a t i n t h e Canadian west t h e r e could be many loess resources.
Loess has been r e c o q n i z e d i n t h e n o r t h ;
R u t t e r e t a1
(1978) r e p o r t e d t h a t t h i n l o e s s d e p o s i t s o c c u r widely on f l a t o r g e n t l y s l o p i n g s u r f a c e s i n t h e g l a c i a t e d p a r t s of c e n t r a l Yukon, commonly e x t e n d i n q as d i s c r e t e , r e c o g n i z a b l e l a y e r s t o 300 111 o r iliore above t h e f l o o r s o f major valleys.
Loess i s s i m i l a r l y d i s p o s e d i n t h e u n g l a c i a t e d a r e a a d j a c e n t t o
the Yukon River and iiiajor t r i b u t a r i e s such a s P e l l y , S t e w a r t , Klondike a n d Ih!hite R i v e r s t h a t c a r r i e d q l a c i a l imeltwater beyond i c e f r o n t a l p o s i t i o n s duri ng s u c c e s s i v e g l a c i a t i o n s . There a r e t r a c e s o f l o e s s r e p o r t e d i n s o u t h e r n O n t a r i o - a r e g i o n where i t inight be e x p e c t e d .
Chapman and Putnairi (1951) mentioned a t h i n l a y e r
o f l o e s s ( a b o u t 40 cm) c o v e r i n q t h e drunilins i n t h e Woodstock d r u m l i n f i e l d .
This l o o k s very l i k e the E n g l i s h s i t u a t i o n with t h i n d e p o s i t s o f i c e - s h e e t l o e s s e x i s t i n g n e a r the p e r i p h e r y o f known l a r g e i c e s h e e t s . The Woodstock drumlin l o e s s i s u n l i k e l y t o e x i s t i n complete i s o l a t i o n and i t seeiiis reasonable t o assume t h a t most ( a l l ? ) o f s o u t h e r n O n t a r i o was once subj e c t e d t o a l o e s s f a l l o f some d e g r e e o r o t h e r . I t i s s a i d t h a t a person standing on t o p of t h e CN Tower i n Toronto can s e e most of the f i r s t g r a d e a q r i c u l t u r a l l a n d i n Canada - t h i s i s probably an e x a g g e r a t i o n b u t t h e r e i s no doubt o f t h e p r o d u c t i v i t y o f t h e s o u t h e r n O n t a r i o c r o p l a n d s . This i s p o s s i b l y due t o a c o n s i d e r a b l e adiiiixture o f l o e s s m a t e r i a l g i v i n g n u t r i e n t supply and good s t r u c t u r e .
A 40 cin l o e s s cover would p r o v i d e
ample m a t e r i a l f o r a qood s o i l , and of c o u r s e l o e s s i s d e l i v e r e d where i t s needed
-
on t o p .
64
This i s why l o e s s leads t o such good s o i l s , a n d why the Chesworth coincidence i s so e x a c t , and y et i t i s obvious t h a t a va st amount of loe ss material i s n o t needed t o produce a qood s o i l .
Highly productive s o i l s
in southern England have only benefited from a modest l o e s s f a l l . c r i t i c a l question i s
-
A
does t h a t material have t o be delivered by loe ss
f a l l or could i t be incorporated i n some o t h er way? The North China p l a i n , which has been discussed a t some lenqth in t h i s paper, demonstrates t h a t l o es s material turned i n t o a
lo e s s f a l l i s n o t ab s o l u t el y necessary: productive a l l u v i a l s o i l .
I n Canada o u r l o es s material i s as f a r from
being in a t r u e l o e s s d ep o s i t as t h a t forming the North China p l a i n , b u t i t s on the o t h e r s i d e o f t h e main l o es s d ep o s it events. O u r one event loess i s made of t h e same material as t h e eleven event Chinese l o e s s , b u t does i t have s i m i l a r a g r i c u l t u r a l advantages? This however i s material for speculation r a t h e r than reasoned s c i e n t i f i c disc ourse ; i f concentratio n s o f P1 l o e s s material can be found - then a whole new chapter of the lo e ss s t o r y can commence. CONCLUSIONS A N D PROPOSALS 1 . "Ohne Frost, kein L B s s " - Carl T r o l l ' s famous statement i s s t i l l essent i a l l y true.
Cold conditions produce l o e s s - i t i s a product of the
cold phases of t h e Quaternary. 2 . Two major mechanisms produce
most of
t h e world' s l o e s s ;
1 ) g l a c i a l g r i n d i n g , a n d 2 ) f r o s t weathering.
the se a r e
Other mechanisms
do
produce l o e s s material b u t they produce i n lim ite d amounts or supply r e l a t i v e l y small d ep o s i t s ( r e l a t i v e t o say the Chinese l o e s s ) . 3. The two mechanisms suggest t h a t we demarcate two major types of l o e s s :
1 ) i c e - s h e e t l oes s
-
as s o ci at ed with major continental g l a c i a t i o n s a n d
produced by g l a c i a l g r i n d i n g , and 2 ) mountain loe ss
-
produced by f r o s t
weathering and, by geomorphological circumstance, providing the material f o r major ' d e s e r t ' l o e s s d ep o s i t s (China a n d Central A sia ). 4 . From a p r a c t i c a l ( p a r t i c u l a r l y a g r i c u l t u r a l ) point of view loe ss i s so important t h a t we propose t o i d e n t i f y t h e material even when i t i s not incorporated i n t o a d ep o s i t . l o e s s s i z e - 10-50
pm
The problem i s t o s e t a c h a r a c t e r i s t i c
has been much used and i s possibly the best choice,
b u t we a l s o propose 20-60 vm - l a r g e l y t o make a c l e a r d i s t i n c t i o n be-
tween l o e ss and t h e f i n e r a e r o s o l i c d u s t (such a s can a c t u a l l y be produced i n hot sandy d e s e r t s , <10
p ~ ) .
5 . The use of phi u n i t s shoud be discouraged, b u t i t has t o be admitted t h a t +4d t o +62 makes a r a t h e r neat l o e s s s i z e range. A loe ss mode
a t 5d has been reported by Derbyshire (1983) from China and Eden (1980) from Essex
-
a remarkable s i m i l a r i t y f o r two such widely separated and
65 mechanistically differenct deposits. 6. Loess as a constituent can be mapped, as Bruce (1978) has demonstrated so elegantly in New Zealand, and this sort o f activity could be very worthwhile. It raises again however the problem of loess recognition. The 'loess fraction' will be determined by size so that dispersed deposit material will not be distinguishable from P1-only material. 7. P1 loess should be recognized - but strict terminological controls must be exercised. If P1 loess is recognized then the great alluvial soils o f north India and Bangladesh, and the Nile delta, can be seen to consist of loess material. Thus virtually all of the major croplands of the world are formed on loess or are composed of loess material or have a significant loess constituent. Loess is a major resource naterial, more should be sought and that which is used should be used carefully, with an eye to the future. Soil material which blew into p:ace can blow away again. '5. "Loess is certainly one of the most remarkable accumulations of Pleis'.occiie age" James Geikie's statement is still absolutely true. His use of the word 'accumulation' serves to remind us that loess accumulates in many places and in many ways; aeolian deposition is of prime importance, and loess is a major aeolian sediment, but we must keep in view all the other factors and mechanisms involved in what is often a very complex process. Acknowledgements We thank the New Zealand Soil Bureau for providing 1) the opportunity to associate with some o f the world's southernmost loess and 2) peace and seclusion for thinking and writing. We thank Chris Fordham, Peter Martini and Peter Callander for their comments on earlier drafts of this paper. IJS thanks the Eoyal Society (of London) for grant support. Lesley Sinclair produced the camera-ready copy. REFERENCES Atkinson, H.J., 1971. A Bibliography o f Canadian Soil Science. Research Branch, Canada Dept. Agriculture publ. 1452, 303 p. Boulton, G.S., 1978. Boulder shapes and grain-size distributions o f debris as indicators of transport paths through a glacier and till genesis. Sedimentology 25, 773-799. 9 n .Experimental frost-shattering. Proc. 7th New Zealand Brockie, W.J., 1 Geog. Conf., New Zealand Geographical Society, Hamilton 1972, Conf. Ser. 7, 177-186 Eroizin, B.S., 1981. Rheological definitions of loess subsidence. Proc. 10th Int. Conf. Soil Mechanics and Foundation Eng. Stockholm 1, 05-68. Bruce, J.G., 1978. Loess content of soils, South Island, New Zezland (map, scale 1: 1,000,000to accompany Otago Catchment Board publication no. 4)
66
B u r r i n , P.J., 1981. Loess i n t h e Weald. P r o c . G e o l . Assoc. 92, 8 7 - 9 2 . B u t l e r , B.E., 1956. Parna - a n a e o l i a n c l a y . A u s t r a l i a n J . S c i e n c e 145-1 51 C a t t , J.A., 1977. Loess and c o v e r s a n d s . I n : B r i t i s h Q u a t e r n a r y S t u d i e s Recent Advances, e d . F.W. S h o t t o n . O.U.P. O x f o r d . C a t t , J.A., 1978. The c o n t r i b u t i o n o f l o e s s t o s o i l s i n l o w l a n d B r i t a i n . I n : The E f f e c t o f Man on t h e Landscape o f t h e Lowland Zone. ed. S. L i m b r e y and J.S. Evans. C o u n c i l B r i t . A r c h a e o l . Rept. no. 21. C a t t , J.A., C o r b e t t , W.M., Hodge, C.A.H., M a d g e t t , P . A . , T a t l e r , i' and N e i r , A.H., 1971. Loess i n t h e s o i l s o f n o r t h N o r f o l k . J. S o i l S c i . 22, 444-452. ChapTan, L . J . and Putnam, D.F., 1951. The p h y s i o g r a p h y o f s o u t h e r n O n t a r i o . Toronto, U n i v e r s i t y o f Toronto Press f o r O n t a r i o Research Foundation. Chesworth, W . , 1982. L a t e Cenozoic q e o l o g y and t h e second o l d e s t p r o f e s s i o n Geoscience Canada 2, 54-61. C u t t s , J.A., 1973. N a t u r e and o r i q i n o f l a y e r e d d e p o s i t s o f t h e M a r t i a n p o l a r r e g i o n s . J . Geophys. Res. &?, 4231-4249. D e r b y s h i r e , E . , 1983. The l o e s s a t J i u z h o u t a i , P e o p l e s R e p u b l i c o f China: a n o t e . Loess L e t t e r n o . 9, t c he F u b l i s h e d . Drake, L.D. and R i r i e , G.T., 1981. P. l o w - c o s t iiiethod o f r e c l a i m i n g s t r i p mined l a n d i n Iowa t o a g r i c i i l t u r c . E n v i r o n . Geol. 3, 267-279. Duirianski, J., Pawluk, S . , V u c e t i c i ? , C.G. and L i n d s a y , J.D., 1980. Pedo g e n e s i s and t e p h r o c h r o n o l o g y o f ! ( l e s s d e r i v e d s o i l s , H i n t o n , A l b e r t a . 5L-3.7. Canadian J . E a r t h S c i . Eden, D.N., 1980. The l o e s s cf - 3 - e a s t Essex, E n g l a n d . B o r e a s 2, 165-1 77. Goudie, A . S . , Cooke, R.U. and ?w::ikamp, J.C., 1979. The f o r m a t i o n o f s i l t froin q u a r t z dune sand b y s z : - t - w a t h e r i n g p r o c e s s e s i n d e s e r t s . J. A r i d E n v i r o n m e n t s 2, 105-112. Junge, C . , 1979. The i m p o r t a n c e o f m i n e r a l d u s t a s a n a t m o s p h e r i c c o n s t i t 14, John u e n t . I n : Saharan D u s t . ed. C. M o r a l e s . Scope r e p o r t -~ !li1e y , Chi c hes t e r . . Kuenen, Ph.H., 1960. E x p e r i m e n t a l a b r a s i o n 4: e o l i a n a c t i o n . J. G e o l o q y 68, 427-449. L a u F i d o u , J.P. and Ozouf, J.C., 1982. E x p e r i m e n t a l f r o s t s h a t t e r i n g : 15 y e a r s o f r e s e a r c h a t t h e C e n t r e d e Geomorphologie d u CNRS. P r o g . P h y s i c a l Geog. 6, 215-232. L i l l , G.O. and S m a l l e y , I . J . , 1978. D i s t r i b u t i o n o f l o e s s i n B r i t a i n . Proc G e o l . Assoc. 57-65. Lee, M.P., 1979. Loess f r o m t h e P l e i s t o c e n e o f t h e ! d i r r a l P e n i n s u l a , M e r s e y s i d e . P r o c . G e o l . Assoc. 21-26. Manecki, A . and D o m i n i k , J., 1972. Remarks o n t h e m i n e r a l c o m p o s i t i o n and g e n e s i s o f l o e s s . B u l l . d e 1 ' A c a d . P o l o n . des S c i e n c e s : S e r i e des S c i e n c e s de la T e r r e 20, 241-248. Manecki, A., M u s z y n s k i , M. and Wrzak, J., 1980. F i n e - q r a i n e d d e p o s i t s f r o m t h e b o t t o m o f B r o g g i g l a c i e r and i t s f o r e l a n d . Prace Mineralogiczne 64. 27-46. M o r n e s , C., 1979. Saharan D u s t . Scope r e p o r t 14, John W i l e y , C h i c h e s t e r , 297 p . Moss, A.J. and Green, P . , 1975. Sand and s i l t g r a i n s : p r e d e t e r m i n a t i o n o f t h e i r f o r m a t i o n and p r o p e r t i e s b y m i c r o f r a c t u r e s i n q u a r t z . J. G e o l . SOC. A u s t r a l i a 22, 485-495. Nahon, D. and T r o m p e t t e , R., 1982. O r i g i n o f s i l t s t o n e s : g l a c i a l g r i n d i n g 25-35. versus weathering. Sedimentology Obruchev, V.A., 1911. The q u e s t i o n o f t h e o r i g i n o f l o e s s - i n d e f e n s e o f t h e a e o l i a n hypothesis. I z v e s t i y a Tomskogo T e k h n o l o g i c h e s k o g o I n s t i t u t a 23, p a r t 3 ( i n Russian; 2nd p a r t o f a t w o p a r t p a p e r - t h e f i r s t p a r t p u b l i s h e d i n 1909).
18,
~
c,
89,
90,
29,
67 Palmer, A.S., 1532. S t r a t i g r a p h y and s e l e c t e d p r o p e r t i e s o f l o e s s i n W a i r a r a p a , i:w Z e a l a n d . Ph.D. t h e s i s . V i c t o r i a U n i v e r s i t y , l , l e l l i n q t o n , New Z e a l a n d . Penck, A . , 15311. C e n t r a l A s i a . G e o g r a p h i c a l J . 76, 477-487. 1956. N a t u r e o f ' C h a l k H e a t h ' SOTS. P e r r i n , R.M.S., Nature 31-32. Perrin, R.M.S., D a v i e s , H. and Fysh, M.D., 1974. D i s t r i b u t i o n o f l a t e P l e i s t o c e n e a e o l i a n d e p o s i t s i n e a s t e r n and s o u t h e r n England. N a t u r e 248, 320-323. P e t t i j o h n , T . J . , 1957. S e d i m e n t a r y Rocks, 2nd ed. H a r p e r , New York, 718 p. P e t t i j o h n , F . J . , 1975. S e d i m e n t a r y Rocks, 3 r d ed. H a r p e r and Row, New York, 628 p . Pye, K. and S p e r l i n g , C.H.B., 1983. E x p e r i i i i e n t a l i n v e s t i g a t i o n o f s i l t formation by s l a t i c breakage processes: t h e e f f e c t o f temperature, m o i s t u r e and s - l t on q u a r t z dune sand and g r a n i t e l e g o l i t h . S e d i m e n t o l o g y 30, 49-62. R i e z b o s , P . A . and Van d e r Llaals, L . , 1974. S i l t - s i z e d q u a r t z p a r t i c l e s : a p r o p o s e d s o u r c e . S e d i m e n t . G e o l . 12, 279-285. P u s s e l l , R.J., 1944. Lower M i s s i s s i p p i - V a l l e y l o e s s . G e o l . SOC. Amer. B u l l . 55, 1-40. E u t t f r , N.W., F o s c o l o s , A.E. and Hughes, O . L . , 1978. C l i m a t i c t r e n d s d u r i n g t h e Q u a t e r n a r y i n c e n t r a l Yukon based upon p e d o l o g i c a l and qeomorphol o g i c a l evidence. I n : Q u a t e r n a r y S o i l s . ed. b1.C. Mahaney, Geo A b s t r a c t s , N o r w i c h . 508 p . Smalley, I . J . , 1966. The p r o p e r t i e s o f g l a c i a l l o e s s and t h e f o r m a t i o n o f l o e s s d e p o s i t s . J . Sediment. P e t r o l . 36, 669-676. Smalley, I . J . , 1968. The l o e s s d e p o s i t s ZYd N e o l i t h i c c u l t u r e o f n o r t h e r n China. Man. ( n . s . ) 3, 224-241. Srnalley, I . J . , 1970. Cohesion o f s o i l p a r t i c l e s and t h e i n t r i n s i c r e s i s t a n c e o f s i m p l e s o i l systems t o w i n d e r o s i o n . J . S o i l S c i . 154-161. Smalley, I . J . , 1978a. Doluchaev and t h e R u s s i a n a p p r o a c h t o t h e s t u d y o f l o e s s . P r o c . Leeds P h i l . and L i t . SOC., S c i . S e c t i o n LO-, 231-243. Smalley, I . J . , 1978b. The New Z e a l a n d l o e s s and t h e m a j o r c a t e g o r i e s o f loess classification. S e a r t h 9 , 281-282. Smalley, I . J . , 1980. The f o r m a t i o n o f l o e s s i n a t e r i a l s and l o e s s d e p o s i t s : some o b s e r v a t i o n s o n t h e T a s h k e n t l o e s s . Geophys. u. G e o l . Geophys. Vertlff d . KMU L e i p z i g 2, 247-257. Smalley, I . J . and K r i n s l e y , D.H., 1978. Loess d e p o s i t s a s s o c i a t e d w i t h d e s e r t s . Catena 5, 53-66. Smalley, I . J . and K r i n s l e y , D.H., 1979. E o l i a n s e d i m e n t a t i o n o n E a r t h and Mars: some c o m p a r i s o n s . Icarus 276-288. Smalley, I . J . and K r i n s l e y , D.H., 1981. The U r l l l s s c o n c e p t and c h a n g i n g ideas o f loess formation. New Z e a l a n d S o i l News 29, 57-59. Smalley, I . J . and Leach, J.A., 1978. The o r i g i n a n d 7 i s t r i b u t i o n o f t h e l o e s s i n t h e Danube b a s i n and a s s o c i a t e d r e g i o n s o f E a s t - C e n t r a l E u r o p e a r e v i e w . S e d i m e n t . G e o l . 21, 1-26. Smalley, I . J . and V i t a - F i n z i , 1968. The f o r m a t i o n o f f i n e p a r t i c l e s i n sandy d e s e r t s and t h e n a t u r e o f ' d e s e r t ' l o e s s . J . Sediment. P e t r o l . 38, 766-774. Smamey, I . J . , K r i n s l e y , D.H. and V i t a - F i n z i , C . , 1973. O b s e r v a t i o n s o n t h e K a i s e r s t u h l l o e s s . G e o l . Mag. 110, 29-36. Suslov, S.P., 1961. P h y s i c a l G e o g r a p h y f A s i a t i c R u s s i a . Freeman, San F r a n c i sco. T i n g , !d.S., 1965. The g e o m o r p h o l o g y o f t h e N o r t h China P l a i n and h i s t o r y o f t h e e a r l y C h i n e s e . B u l l . I n s t . E t h n o l o g y , Acad. S i n i c a ( T a i w a n ) 20, 155-162. V i t a - F i n z i , C. and S m a l l e y , I . J . , 1970. O r i g i n o f q u a r t z s i l t : comments o n a n o t e b y Ph.H. Kuenen. J . Sediment. P e t r o l . 40, 1367-1369. !Jang Yong-yan and Zhang Zong-hu, 1930. Loess i n m i n a . S h a s n x i P e o p l e s A r t P u b l i s l i i n g House.
2,
1,
40,
r,
68 Wen Qizhong, Yu Suhua, G u Xiongfei and Lei J i a n q u a n , 1951. A p r e l i m i n a r y n v e s t i g a t i o n o f r a r e e a r t h e l e m e n t s ( R E E ) i n l o e s s . Geocl-iir,iica, f o r 981, no. 6 , 151-157. lhlha l e y , W . B . , M a r s h a l l , J . R . and Smith, B . J . , 1982. The o r i g i n of d e s e r t o e s s : some e x p e r i m e n t a l o b s e r v a t i o n s . Nature 300, 433-435. Yaalon, D . H . and Dan, J . , 1974. Accumulation and z t r i b u t i o n of l o e s s d e r i v e d d e p o s i t s i n t h e s e m i - d e s e r t and d e s e r t f r i n g e a r e a s of I s r a e l . Z e i t . Geomorph. N . F. Suppl . 8, 91 -105. Zeuner, F . E . , 1949. F r o s t s o i l s o n Mount Kenya, and the r e l a t i o n o f f r o s t s o i l s t o a e o l i a n d e p o s i t s . J . S o i l S c i . 1,20-30.
69
O R I G I N AND CHARACTERISTICS OF SOME CHINESE LOESS AT TldO LOCATIONS I N CHINA EDWARD DERBYSHIRE:
S o i l s Research L a b o r a t o r y , U n i v e r s i t y o f Keele,
U.K.
INTRODUCTION The Loess P l a t e a u o f Shanxi, Shaanxi, Gansu and n e i g h b o u r i n g p r o v i n c e s o f c e n t r a l China has a s t r o n g c l a i m t o be regarded as t h e w o r l d ' s t y p e r e g i o n O f t h e t o t a l area m a n t l e d by l o e s s o f f o r the l o e s s f o r m a t i o n ( F i g . 1 ) . 2 631,000 km (Wang and Zhang 1980) o v e r h a l f l i e s i n t h e Loess P l a t e a u
( L i u e t a l . 1964).
I t e x t e n d s eastwards i n t o Henan and e a s t e r n Hebei, and
becomes c o a r s e r towards t h e n o r t h - w e s t where i t grades i n t o t h e sandy l o e s s , blown sand, and gobi o f I n n e r M o n g o l i a . region i s negative:
The m o i s t u r e budget i n t h i s l a r g e
mean annual p r e c i p i t a t i o n i s i n t h e range 250-500 mm
but annual p o t e n t i a l e v a p o t r a n s p i r a t i o n exceeds 1000 mm.
Forty per cent o f
the annual p r e c i p i t a t i o n may f a l l i n a s i n g l e s t o r m and up t o 70 p e r c e n t o f the annual t o t a l i s c o n c e n t r a t e d i n J u l y , August and September.
Lying i n
the h e a r t o f a huge c o n t i n e n t a t a l t i t u d e s o f between 1000 and 2000m above sea l e v e l , t h e w i n t e r s a r e severe w i t h o n l y a t h i n snow cover f o r p a r t o f t h e season.
Mean m o n t h l y temperatures below O°C o c c u r i n a t l e a s t t h r e e months.
Leeward o f t h e Loess P l a t e a u , mantles o f a e o l i a n s i l t a t l e a s t l m t h i c k can be found a t l e a s t as f a r s o u t h as 29'
4 d N ( D e r b y s h i r e 1982).
Thicknesses
exceed l O O m o v e r much o f t h e p l a t e a u , b u t t h e g r e a t e s t known i s i n t h e v i c i n i t y
o f t h e c i t y o f Lanzhou, Gansu P r o v i n c e , on t h e d r y , w e s t e r n margins o f t h e Loess P l a t e a u .
On t h e e a s t e r n s i d e , g r e a t t h i c k n e s s e s o f l o e s s - d e r i v e d a l l u v -
ium a r e found i n t h e f l o o d p l a i n s and t e r r a c e s o f t h e Hwanq He ( Y e l l o w , R i v e r ) : i t reaches o v e r 1200m i n t h i c k n e s s i n t h e Wei He graben c l o s e t o t h e c i t y o f Xian ( X i a and L i 1980) and forms t h e huge d e l t a i c p l a i n o f Henan, Shandong, Anhui and J i a n g s u .
T r a n s l o c a t i o n o f l o e s s on s l o p e s by t h e processes o f r a i n -
beat and s l u r r y f l o w i s widespread i n China and g i v e s r i s e t o a mixed f a c i e s with d i s t i n c t i v e properties. The l o e s s i s g e o l o g i c a l l y v e r y young.
The lower, o r Wucheng l o e s s (named
From t h e t y p e p r o f i l e i n t h e L i u s h u V a l l e y , blucheng County, Shanxi) i s 121m t h i c k and r e s t s on Neogene c l a y s , sands and g r a v e l s i n which Hipparion remains occur.
I n t h e Luochuan s e c t i o n ( S h a a n x i ) , t h e Wucheng l o e s s shows r e v e r s e d
p o l a r i t y w i t h t h e J a r a m i l l o normal e v e n t 20m above t h e base (Hang, Yue, blu, Chen and Dun 1980).
T h i s a l s o shows about 14m above t h e base o f t h e lhquanshan
s e c t i o n i n t h e s o u t h e r n suburbs o f Lanzhou, from which i t i s i n f e r r e d (blang and Yue 1982) t h a t ago.
l o e s s d i d n o t b e g i n t o accumulate a t Lanzhou u n t i l about 1.2m y r
The succeeding L i s h i s h l o e s s i s s e p a r a t e d f r o m t h e Wucheng b y t h e Brunhes-
70 ~~
Loess Plateau
r. .. 1
Lanzhou loess
Thick, terraced
Fig.
.1
L o c a t i o n map o f t h e Loess Plateau and s i t e s s t u d i e d .
/-
I/l
I
71
Matuyma t r a n s i t i o n ( A n , blang and Li 1977) which i s 105m above the base of t h e Lanzhou l o e s s a t J i u z h o u t a i ( D e r b y s h i r e i n p r e s s ( a ) ) .
The Malan Loess of
Upper P l e i s t o c e n e age l i e s unconformably on t h e L i s h i h and has a maximum t h i c k ness of 34m a t Lanzhou.
Drapes of Holocene l o e s s up t o 5m t h i c k a r e known and
l o e s s c o n t i n u e s t o be d e p o s i t e d from t h e a i r a t r a t e s o f s e v e r a l mm/yr (Derbyshire i n press ( b ) ) . This c h a p t e r w i l l examine t h e v a r i a t i o n i n some p r o p e r t i e s o f l o e s s and l o e s s d e r i v e d s i l t a r i s i n g from d i f f e r e n c e s i n a g e , d e p o s i t i o n a l p r o c e s s , w e a t h e r i n g and overburden a t two c o n t r a s t i n g s i t e s n e a r t h e e a s t e r n and western margins of t h e Loess P l a t e a u :
Heng Xian, Henan P r o v i n c e , and J i u z h o u t a i i n e a s t e r n Gansu
(Fig. 1 ) THE TER R AC E LOESS AT PIENG XIAN
.(he t e r r a c e s of the Hwang He r i s e above t h e f l o o d p l a i n t o a h e i r h t of a b o u t lOOm Samples t a k e n 3m below t h e s u r f a c e of t h e uppermost ( f o u r t h ) near Meng Xian. t e r r a c e and 15m below t h e second t e r r a c e were a n a l y s e d t o d e t e r m i n e t h e i r p a r t i c l e s i z e , f a b r i c , c l a y mineralogy and c o n s o l i d a t i o n c h a r a c t e r i s t i c s . l o e s s i s very p o o r l y s o r t e d on the Folk and Nard (1957) s c a l e
(x = 2 . 6 )
The
and
s i g n i f i c a n t l y d i f f e r e n t from t h e s l i g h t l y sandy l o e s s i c alluvium o f t h e p r e s e n t Hwang He f l o o d p l a i n which has a s o r t i n g c o e f f i c i e n t of 1 . 3 4 (Derbysiiire 1 9 7 8 ) . The t e r r a c e l o e s s i s f i n e r and b e t t e r s o r t e d than t h e a l l u v i u m and t h e Shanxi l o e s s t o windward ( K u n g and Chiang 1 9 7 7 ) , a t r e n d o f general v a l i d i t y recognised i n s e v e r a l a u t h o r i t a t i v e Chinese t e s t s ( e . q .
Liu e t a l . 1 9 6 6 ) .
All a r e
s t r o n g l y p o s i t i v e l y skewed. The c l a y g r a d e p e r c e n t a g e of t h e samples from t e r r a c e two ( 2 4 p e r c e n t ) i s h a l f a s g r e a t a g a i n a s t h a t i n t h e f o u r t h t e r r a c e When immersed samples and o v e r t w i c e t h e a v e r a g e f o r t h e w e s t e r n Shanxi l o e s s . i n d i s t i l l e d w a t e r , the l c e s s from t h e f o u r t h t e r r a c e s l a k e d more r e a d i l y than t h e more c l a y - r i c h t e r r a c e two m a t e r i a l and a f t e r one m i n u t e ' s immersion showed l i t t l e suspended m a t t e r i n c o n t r a s t t o t h e muddy s u s p e n s i o n produccd i n t h e same time by t h e second t e r r a c e sample. Both t h e Meng Xian t e r r a c e s i l t s d i s p l a y t h e i n s t a n t a n e o u s c o l l a p s e known a s h y d r o c o n s o l i d a t i o n when f l o o d e d under a l o a d .
The d e t a i l e d response i s
d i f f e r e n t i n each c a s e , however, a s a r e s u l t o f d i f f e r e n c e s i n c e r t a i n p r o p e r t i e s . The i n i t i a l v o i d s r a t i o of t h e f o u r t h t e r r a c e m a t e r i a l ( 0 . 8 8 7 ) i s s i g n i f i c a n t l y h i g h e r than t h a t o f t h e t e r r a c e two s i l t ( 0 . 8 0 9 ) a s i s the p l a s t i c i t y index ( 1 0
per c e n t and 17 per c e n t r e s p e c t i v e l y ) .
The rebound c u r v e s f o r samples con-
s o l i d a t e d i n the d r y s t a t e show an a l m o s t 50 p e r c e n t g r e a t e r r e d u c t i o n i n voids r a t i o i n t h e t e r r a c e f o u r sample compared t o t h a t from the second t e r r a c e ( F i g . 2 ) . 2 Flooding i n t h e oedometer a t 640kN/m produced a c o l l a p s e r a t i o ( R f a c t o r ) of 1 0 . 1 % i n the c a s e o f t e r r a c e 4 , b u t o n l y 6 . 8 % f o r t e r r a c e two s i l t .
Both
73
3. X-ray d i f f r a c t i o n o f < 20fim f r a c t i o n of l o e s s from Meng Xian (Cu K X r a d i a t i o n ) . a , b , c: unheated, heated and glycolated samples from t e r r a c e 4 . d , e , f : unheated, heated and glycolated samples from t e r r a c e 2 .
Fig.
form frequent b u t t r e s s e s between s i l t g r a i n s .
Oriented sampling f a i l e d t o
d e t e c t a s i g n i f i c a n t preferred o r i e n t a t i o n ( c f . Matalucci e t a l . 1970), although a decrease in mean d i p and increased f a b r i c anisotropy occurred following hydroconsolidation (Fig. 5 ) . I n c o n t r a s t t o t h e shearing of clay buttresses which occurs when l o e s s i s loaded in t h e dry condition, consolidation when the sample i s flooded r e s u l t s in dispersion of the clay grade aggregations ( ' f l o c s ' ) produced by c y c l i c drying a t mean f i e l d moisture contents i n the range 2-15 per cent. A f u r t h e r r e s u l t of flooding under load i s increased adhesion on s i l t grain surfaces. The c l a y and f i n e s i l t b u t t r e s s e s and bridges, being a product of c y c l i c drying following varying degrees of p a r t i a l s a t u r a t i o n , a r e not
74
Fig. face.
4.
Meng X i a n l o e s s f r o m t e r r a c e 4 .
S c a l e b a r i s 40,Kni
d i r e c t l y the product o f sedimentation from the a i r .
long.
Vertical
The random, open f a b r i c
o f t h e s i l t ' s k e l e t o n ' and t h e p r e s e n c e o f s i l t - s i z e d a g g r e g a t e s o f c l a y , on t h e o t h e r hand, 2 r f v e r . y
s i m i l a r t o t h a t found i n t h e e x t e n s i v e Malan l o e s s (Fig.14)
w h i c h i s w i d e l y r e g a r d e d i n C h i n a a s an a e o l i a n s i l t (liang, Teng and Yue 1 9 8 2 ) . A l t h o u g h t h e g r e a t e r age o f t h e second t e r r a c e l o e s s has p r o d u c e d a n i n c r e a s e i n c l a y p e r c e n t a g e , t h e m i c r o f a b r i c o f t h e two t e r r a c e s i l t s i s e s s e n t i a l l y s i m i l a r when due a l l o w a n c e i s made f o r t h e d i f f e r e n c e i n o v e r b u r d e n and i s c o n s i s t e n t with aeolian deposition.
I t seems p r o b a b l e , t h e r e f o r e ,
s i l t s are wind-deposited,
t h e f i n e r mean g r a i n s i z e b e i n g a f u n c t i o n o f age (see
t h a t b o t h Meng X i a n
below). THE LANZHOU LOESS To t h e n o r t h w e s t o f t h e c i t y o f t.anzhou, t h e u p p e r s l o p e s o f t h e m o u n t a i n J i u z h o u t a i (2067m) a r e made u p o f some 335111 o f l o e s s , r e s t i n g o n i m b r i c a t e d g r a v e l s o n t h e l e v e l o f t h e f o u r t h t e r r a c e o f t h e Hwang He.
A b o u t 1On1 o f
f i n e l y bedded and l a m i n a t e d s i l t s o f a l l u v i a l t y p e a r e succeeded b y l O l m of o f L i s h i h l o e s s and u p t o 34111 o f l o e s s o f M a l a n age. Wucheng l o e s s , 204111
This
75
Fig. 5 . Ivleng Xian t e r r a c e 4 l o e s s a f t e r h y d r o - c o n s o l i d a t i o n a t 640 K l i / i ~2i S c a l e bar i s 1OOfini l o n g . Vertical face. a r e a c o n t a i n s t h e t h i c k e s t l o e s s s u c c e s s i o n known anywhere i n t h e world (Figs. 6 , 7 ) . The Malan l o e s s i s c l e a r l y c o a r s e r (mean 5.28: c o a r s e s i l t ) than t h e L i s h i h and Wucheng below i t , and c o a r s e r a t bleng Xian.
than t h e Malan l o e s s on t h e f o u r t h t e r r a c e
All a r e v e r y p o o r l y s o r t e d but t h e s o r t i n g o f t h e Malan l o e s s
i s b e t t e r than both o f t h e o l d e r l o e s s e s and two d i s t i n c t p o p u l a t i o n s a r e
suggested both a t J i u z h o u t a i and Meng Xian ( F i g . 8 ) .
The s i l t s k e l e t o n of t h e
Lanzhou l o e s s i s predominantly of q u a r t z with f e l d s p a r and mica a s i m p o r t a n t subsidiary minerals. The heavy mineral s u i t e , p r i n c i p a l l y touriiialine, m a g n e t i t e , e p i d o t e , hornblende and b i o t i t e , a v e r a g e 4 per c e n t .
Total carbon-
a t e s v a r y from 8 t o 1 9 per c e n t , and s u l p h a t e s a r e o f roughly equal importance. The mineralogy of the f r a c t i o n f i n e r than 2 ~ n ii s dominated by q u a r t z , with f e l d s p a r , c a l c i t e and i l l i t e l h y d r o m i c a a s a n c i l l a r y m i n e r a l s .
There i s a
mixed l a y e r assemblage around 1 4 1 i n some samples but t h i s d o e s n o t a p p e a r t o be s n i e c t i t e i n t y p e . A1 though n i o n t n i o r i l l o n i t e has r e c e n t l y been r e p o r t e d
76
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I
207f
6 . The environs of eas t er n Lanzhou in t h e Hwang He gorge. Fig. l i n e r e f e r s t o Fig.7.
The p r o f i l e
from t h e Luochuan type s i t e (Han 1982) i t has s t i l l t o be confirmed a t Lanzhou. The X-ray diffractograms o f t h e Jiuzhoutai succession a r e s t r i k i n g l y c onsiste nt ( F i g . 9 ) t h e r e being no s i g n i f i c a n t d i f f e r e n c e in c la y grade mineralogy from Wucheng t o Flalan. This accords with t h e view t h a t the l o e s s was derived from a d r y , a l k a l i n e environment with only very weak e luvia tion (Wang, Wu and Yue 1978).
The pale coloursof t h e palaeosol s and weathering horizons a r e c onsiste nt
with a g e n e r a l l y d ry climate in t h i s region over t h e past 1 . 2 m.yr.
Even in the
more obvious palaeosol p r o f i l e s , organic co n t ents a r e very low, nowhere reaching 1 per cent (Table 1)
.
metres 1 100;
JIUZHOUTfiI
- SEI
2000-
1900-
1800-
I70 0-
16nw
I
. .. .... .
A L L U V I U M Two textural components uppetloess. lower gravel
-
w v ouanG
. - -.
.
MA L A N LOESS
LlSHl
FORM*TION
. . ..
.. --
LOESS
HE KOU F O R M A T I O N
(Reddish sandy co n g l o m e r a te i
[Conglomerate) SPRING
F i g . 7 . Cross s e c t i o n o f t h e succession a t Lanzhou. Frm an unpublished drawing k i n d l y s u p p l i e d by Zhang Linyuan.
78
MALAN
t-
z
-
0
LlSHlH
A
WUCHENG
@
MENGXIAN
W
u
LL
LL
L
W
0
u
zd,t A '0
(3
z -
t-
I Y 0 Ln
I
+3
I
+4
I
+5
I
+6
I
+7
M E A N GRAIN SIZE U N I T S
Fig. 8 . Co-plot of mean grain s i z e v.sorting c o e f f i c i e n t f o r t h e loesses from Meng Xian and Jiuzhoutai. The f a b r i c of t h e Lanzhou l o e s s i s one of single-grain, loosely-packed type with a s i g n i f i c a n t number o f edge t o f a c e contacts, clay-grade p a r t i c l e s occurring a s coatings, c l u s t e r s , b u t t r e s s e s and bridges between the elements of the coarse t o medium s i l t ' s k e l e t o n ' . This f a b r i c v a r i e s t o some degree with differences in overburden, weathering history and age, b u t the components a r e These a r e a framework of generally angular, clean s i l t g r a i n s of consistent. q u a r t z , f e l d s p a r and mica d i t h s i l t - s i z e d f l o c s of clay grade material occurring in the l a r g e r voids and i n b u t t r e s s form. Wetting and drying following deposit i o n of the l o e s s induces f l o c c u l a t i o n o f f i n e s e s p e c i a l l y with the increase in c a t i o n i c concentration during decrease in porewater content (Grim 1953). Thus clay aggregations a r e drawn by porewater menisci t o the pore margins producing t h e b u t t r e s s e s , bridges and clay adhesions t o be seen in l o e s s of a l l ages. The d i s c r e t e s i l t - s i z e d bundles o f c l a y s , however, a r e probably the product of d e f l a t i o n of f l o c s from d e s e r t pans and ephemeral stream courses t o be found over an area of 1 million ltm2 or more to the north and west of Lanzhou. This explains the f a c t t h a t laboratory breakdown of l o e s s produces a n a r t i f i c i a l l y high sorting c o e f f i c i e n t : l o e s s i s moderately well sorted in dynamic terms.
79
A3, , LI,
....1.;:1._._.;_...-
, 39
37
35
33
31
29
27
25
23
21 -7
19
17
15
13
11
9
7
5
3'26
Fig. 9. X-ray d i f f r a c t o g r a r n s o f i 2 0 , w r n f r a c t i o n o f l o e s s from J i u z h o u t a i ( C u K o c r a d i a t i o n ) . Malan - t o p ; L i s h i h - m i d d l e two; Wucheng - lower two.
80
TABLE 1 Total carbonates, organic matter content and s p e c i f i c g r a v i t y for selected
Chinese l o e s s samples
Meng Xian Meng Xian Lanzhou: Lanz hou : Lanzhou: Lanzhou: Lanzhou: Lanzhou:
terrace terrace Flalan Ma1 an Lishih Lishih Wucheng Wucheng
4 2
( B horizon) ( A horizon) ( C horizon)
Total Carbonates
Organic Matter
%
%
9.25 9.34 11.59 11.19 11.67 15.37 8.76 16.03
0.41 0.23 0.44 0.39 0.44 0.38 0.80 0.44
Specific Gravity 2.58
2.53
-
Increase in compaction proportional t o overburden occurs by ' d r y ' i n t e r granular shearing, c l a y b u t t r e s s e s becoming disrupted b u t not dispersed. Voids r a t i o s decrease with a g e i n the Jiuzhoutai p r o f i l e from l e s s than 0.6 i n t h e Wucheng t o over 0.9 i n some of t h e Malan samples. A f u r t h e r cause of reduced voids r a t i o and increased strength i s the presence of cement. The Lanzhou l o e s s has lower mean carbonate contents than a r e found in the ' c l a s s i c ' loess of Shaanxi and Shanxi: much of i t i s dispersed, as i s the sulphate. Loess ' d o l l s ' a r e r a r e , although small ones of b o t h CaC03 and CaS04 can be found. Siliceous cements a r e important in the older l o e s s e s , iron being a n important a n c i l l a r y mineral a s revealed by electron probe analysis. I n the Wucheng l o e s s , the open f a b r i c i s maintained with only minor cementation a t grain contacts (Fig. 10) b u t in the lower B and C horizons of t h e palaeosols, s i l i c a occurs in inter-grain cements (Fig. 11) and a s pore l i n i n g s and overgrowths on s i l t-sized quartz and f e l d s p a r g r a i n s (Fig. 1 2 ) s i g n i f i c a n t l y reducing the voids r a t i o s . Clay grade p a r t i c l e s a r e held by s i l i c a p r e c i p i t a t i o n a s coatings, b u t t r e s s e s and d e l i c a t e bridge s t r u c t u r e s in the horizons of palaeosols in both bducheng and Lishih l o e s s e s (Fig. 1 3 ) . The Malan l o e s s has a coarser s i l t ' s k e l e t o n ' , and high t o very high voids r a t i o s . Adhesion of clay-grade p l a t e l e t s i s q u i t e common b u t random, t h e r e being some extremely d e l i c a t e bridging structures (Fig. 1 4 ) . Overgrowths and widespread cementation do not occur in the loess of bialan age, b u t both s i l i c a re-deposition (Wang, Teng and Yue 1982) and iiodular zones of carbonate and sulphate do occur. llalan l o e s s a t an a l t i t u d e of 1810m some 1.7 km south-east o f t h e summit of Jiuzhoutai contains well developed cemented zones r e s u l t i n g in an extremely compact f a b r i c (Fig. 15) made u p of complex intergrowths of c a l c i t e and gypsum in pores. During the infrequent r a i n f a l l events when moisture contents in the Lanzhou l o e s s approach s a t u r a t i o n l e v e l s , i t s u f f e r s hydroconsol idation and c a t a s t r o phic s l u r r y flows occur. This disperses t h e clay f l o c s , destroys the clay
81
Fig. 10. Open f a b r i c of t h e Wucheng age l o e s s from Jiuzhoutai a t the level of the s i x t h t e r r a c e of t h e Hwang He. Scale bar i s 4)tm long. Vertical f a c e , showing bridge of f i n e s i l t and cl ay p a r t i c l e s .
buttresses, reduces t h e voids r a t i o s a n d i n cr ease s t h e anisotropy of the f a b r i c . Re-deposition by c o l l u v i a l and a l l u v i a l processes has produced these changes in considerable volumes of sediment in t h e Loess Plateau.
Loessic colluvium a n d
alluvium d i sp l a y d i s t i n c t i v e sedimentary p r o p ertie s such a s lamination and s i z e sorting (Fig. 1 6 ) and, in t h e absence of such i n d i c a t o r s , a d i s t i n c t i v e microfabric. The l o e s s i c colluvium found i n g u l l y systems south-east of Lanzhou consists of t h i n beds and laminae of s i l t interbedded with sand dipping a t 15O and grading upslope i n t o i n t e r c a l a t e d s i l t beds and sandy cobble gra ve ls. The laminated s i l t s , which a r e derived from Malan l o e s s , have coarse s i l t and f i n e sand g r a i n s coated with dispersed clays and f i n e s i l t s t h r o u g h o u t (Fig. 1 7 ) . Characteristic a o l i an f e a t u r e s such a s s i l t - s i z e d f l o c s , f r a g i l e gra in dispositions, f i n e bridges and high voids r a t i o s a r e absent ( c f . Wang, Teng and Yue Recognition of c h a r a c t e r i s t i c microfahric types may provide, with care1982). ful use, a means of d i s cr i mi n at i n g between l o es s a n d l o e s s i c colluvium and loessic alluvium in massive s i l t s .
82
Fig. 1 1 . Cemented a g g r e g a t e s e n c l o s i n g s i l t g r a i n from C horizon of a c a l a e o s o l of Wucheng age from Jicirnoutai (itwan9 lie s i x t h t e r r a c e l e v e l ) Vertical face. S c a l e bar i s 10fifii l o n g .
THE ORIGIli OF TtIE S I L T The s u r f a c e forni and t e x t u r e of s i l t g r a i n s from both ileng Xian and Jiuzhoutai a r e dominated by p l a n a r f r a c t u r e s u r f a c e s with a n g u l a r , o f t e n s h a r p edges and corners.
C o n c a v i t i e s tend t o be broad and form t h e l o c u s of s i l i c a r e -
precipitation, especially in the older loesses. Sharp, conchoidal f r a c t u r e s u r f a c e s a r e e x t r e m e l y r a r e and edge damage i s o n l y o c c a s i o n a l and minor and u n l i k e l y t o be confused with t h e edge-crushing and H e r t z i a n c r a c k s so common on sands and c o a r s e s i l t s i n s u b g l a c i a l t i l l s , f o r exanlple ( D e r b y s h i r e in p r e s s ( b ) ) Recent work has suggested t h a t weathering p r o c e s s e s may be an i m p o r t a n t s o u r c e of s i l t - s i z e d d e b r i s . The i n h e r e n t p l a n a r m i c r o f r a c t u r e s i n q u a r t z sand g r a i n s produced by s t r e s s g r a d i e n t s d u r i n g metamorphism and during c o n t r a c t i o n c o o l i n g from t h e molten c o n d i t i o n (Snialley 1974) niay p r o v i d e an avenue f o r mechanical breakage (Riezebos and van d e r Waals 1 9 7 4 ) .
This may r e s u l t from
f r e e z e - t h a w s t r e s s ( D y l i k and Klatka 1 9 5 2 ) , f l u v i a l t r a n s p o r t impact (Whalley 1 9 7 9 ) , a e o l i a n impact (Srnalley and V i t a - F i n z i 1 9 6 8 ) , and d i s s o l u t i o n i n seasonal
Fig.
12.
S i l i c a overgrowths (to17 l e f t and bottom r i g h t ) on f e l u s p a r s i l t s r a i n s i n t h e C horizon o f the liuctieng l o e s s a t J i u d i o u t a i . S c a l e bar i s 4,im l o n g . Vertical face.
Fir.
13.
Gelicate s i l t o f aeolian
around ? r a i n (E horizon) lOlrni l o n g .
Fig.
14.
C l e a n a n g u l a r s i l t g r a i n s i n randoni, openwork f a b r i c t y p i c a l o f M a l a n loess from Jiuzhoutai. Scale bar i s 20Lrm l o n g . V e r t i c a l f a c e .
Fig.
15.
Detail o f from Jiuzhoutai. from cor.iplex gypsum between 20,1m 1 ong.
a5
Fig. 16. Bedded a l l u v i a l s i l t d e r i v e d from l o e s s r e s t i n g upon the i m b r i c a t e d gravels a t t h e f o u r t h t e r r a c e l e v e l of t h e Hwang He. J i u z h o u t a i mountain,
Lanz hou .
86
F i g . 17. Sand and c o a r s e s i l t g r a i n s w i t h d i s p e r s e d f i n e s i l t and c l a y p a r t i c l e s a d h e r i n g t o m o s t s u r f a c e s i n t h e l o e s s - d e r i v e d c o l l u v i a l s i l t f r o m Z h i gou men, 14 Km. s o u t h - e a s t o f Lanzhou. S c a l e b a r i s 4 0 um l o n g . t r o p i c a l c l i n i a t e s d u r i n g t h e d e g r a d a t i o n o f f e r r i c r e t e s (Nahon and T r o n i p e t t e 1982).
S a l t weathering i s an i m p o r t a n t producer o f s i l t i n i n t e r t r o p i c a l
d e s e r t p l a i n s (Cooke 1 9 8 1 ) , p o l a r d e s e r t s ( P r e b b l e 1 9 6 7 ) and a r i d m o u n t a i n s (Goudie i n p r e s s ) .
Quartz g r a i n s saturated w i t h sulphates i n experimental
c o n d i t i o n s p r o d u c e a n g u l a r s i l t - s i z e p a r t i c l e s (Goudie, C o o k and Doornkarnp
979).
Scanning e l e c t r o n m i c r o s c o p e e x a m i n a t i o n o f t h e p r o d u c t s o f t h e s e siniul a t e d s a l t w e a t h e r i n g p r o c e s s e s r e v e a l s c o n c h o i d a l and s t e p p e d f r a c t u r e s u r f a c e s o n some g r a i n s which match v e r y c l o s e l y those h i t h e r t o regarded a s s p e c i f i c t o g l a c i a l crushing (Derbyshire i n press (b) ) .
C o n c e n t r a t e d s a l t w e a t h e r i n g i s coninion i n
t h e a r r o y o s , a l l u v i a l f a n s and p l a y a s o f H i g h A s i a .
The combined a c t i o n of
s a l t e n r i c h m e n t and c o n t r a c t i o n c r a c k i n g o w i n g t o d e s i c c a t i o n i n w e t t i n g and d r y i n g c y c l e s a l s o g i v e s r i s e t o s i l t - s i z e d b u n d l e s o f c l a y - g r a d e g r a i n s (Yaalon and G a i n o r 1 9 7 9 ) w h i c h c a n be nioved b y d e f l a t i o n a t w i n d t h e s h o l d v e l o c i t i e s c o n s i d e r a b l y l o w e r t h a n t h o s e needed t o e n t r a i n t h e c o n s t i t u e n t f i n e s ( G i l l e t t e e t a l . 1980).
The s e v e r i t y o f t h e w i n t e r i n H i g h A s i a , c h a r a c t e r i z e d b y months
of f r e e z i n g t e m p e r a t u r e s and o n l y a t h i n and p a t c h y snow c o v e r , p r o b a b l y p r o v i d e s s i g n i f i c a n t c o n t r i b u t i o n s o f f r o s t - s h a t t e r e d s i l t s a1 so.
The 4km
h i g h Q i n g h a i - X i z a n g P l a t e a u ( T i b e t ) i s a u n i q u e m i d - l a t i t u d e p o l a r d e s e r t and
87 i t may be s i g n i f i c a n t t h a t some p e r i g l a c i a l i n v o l u t i o n s i n t h i s r e g i o n a r e a t
l e a s t 1.7 m.yr. o l d , a n t e d a t i n g t h e b e g i n n i n g o f l o e s s accunimulation a t Lanzhou by 100,000 yr and perhaps by as much as 500,000 y r ( C u i no d a t e ) .
A recent
r e v i e w o f t h e f o r m and s u r f a c e t e x t u r e o f t h e Chinese l o e s s by Wang, Teng and Yue (1982) emphasizes d e s e r t weathering and breakage by f r e e z i n g . Recent advances i n knowledge o f P l e i s t o c e n e i c e e x t e n t s i n China have l e d t o severe r e v i s i o n o f t h e widespread g l a c i a t i o n shown i n t h e map o f Sun and Yang (1961).
There appears t o be l i t t l e o r no evidence o f P l e i s t o c e n e g l a c i a t i o n i n
south-east China s o u t h of t h e Chang J i a n g (Yangtze R i v e r ) i n c l u d i n g t h e t y p e r e g i o n f o r t h e t r a d i t i o n a l Chinese g l a c i a l s t r a t i g r a p h y i n t h e Lushan, J i a n g x i Province ( D e r b y s h i r e 1982).
Derivation
of g l a c i a l l y - c r u s h e d s i l t from t h e
south-east can be d i s c o u n t e d , and t h i s accords w i t h t h e m i n e r a l o g i c a l , r a r e earth, and source r o c k e v i d e n c e o f d e r i v a t i o n f r o m a n o r t h - w e s t e r l y source ( L i u and Chang 1964; of f r o s t - s h a t t e r e d
Zhang 1964;
Wen e t a l . 1981).
A l t h o u g h l i m i t e d amounts
s i l t s may w e l l have been d e r i v e d f r o m t h e Q i n g h a i - X i z a n g
Plateau, g l a c i a l i c e d i s t r i b u t i o n i s now known t o have been v e r y l i m i t e d d u r i n g the Pleistocene.
The g l a c i e r s were s t r o n g l y t o p o g r a p h i c a l l y c o n t r o l l e d and
d e n d r i t i c and palmate i n form w i t h a few small i c e caps b u t no i c e sheets (Cui 1980, 1981;
Zheng and L i 1981).
G l a c i a t i o n became p r o g r e s s i v e l y c o n t i n e n t a l
and l o w a c t i v i t y i n s t y l e as t h e P l e i s t o c e n e progressed and, as t h e P l a t e a u u p l i f t a c c e l e r a t e d t o r e a c h 4 kr;i i n l e s s t h a n 3m.yr ( L i e t a l . 1979), t h e p e r i g l a c i a t i o n became more severe.
Loess d i d n o t b e g i n t o accumulate i n t h e Loess
Plateau u n t i l t h e t i m e of t h e Xixabangma G l a c i a t i o n ( i n t h e l a t t e r p a r t o f t h e Lower P l e i s t o c e n e ) when t h e mean a l t i t u d e o f t h e P l a t e a u reached 2000m. P r o g r e s s i v e d e c l i n e i n t h e advected m o i s t a i r from t h e Bay o f Bengal gave r i s e t o p r o g r e s s i v e d e s i c c a t i o n of High A s i a .
By t h e Upper P l e i s t o c e n e , t h e d e s e r t s
were w e l l o v e r an o r d e r of magnitude more e x t e n s i v e t h a n t h e g l a c i e r s .
Moreover,
t h e expanded d e s e r t s were t o t h e n o r t h and west, windward o f t h e Loess Plateau, w h i l e t h e l i m i t e d a r e a s o f g l a c i a l i c e l a y t o t h e south-west. CONCLUSIONS The f o l l o w i n g c o n c l u s i o n s a r e c o n s i s t e n t w i t h t h e sedimentary p r o p e r t i e s o f t h e l o e s s and l o e s s - d e r i v e d d e p o s i t s i n t h e two c o n t r a s t i n g and w i d e l y separated s i t e s a t Meng X i a n and Lanzhou and w i t h t h e work r e c e n t l y p u b l i s h e d and c u r r e n t l y being pursued by Chinese s c i e n t i s t s . 1 . The p a r t i c l e s i z e , f a b r i c , c l a y m i n e r a l o g y , cementation and g e o t e c h n i c a l behaviour of l o e s s show s i g n i f i c a n t d i f f e r e n c e s w i t h age and overburden i n a d e p o s i t a l l o f which i s p r o b a b l y l e s s t h a n 1.2 m.yr o l d .
88 2. The m i c r o f a b r i c o f t r u e a e o l i a n s i l t ( g e n e t i c a l l y termed l o e s s ) i s q u i t e
d i s t i n c t i v e and, a l t h o u g h preserved i n e s s e n t i a l s even under s u b s t a n t i a l o v e r burdens, i s d r a s t i c a l l y a1 t e r e d by t r a n s l o c a t i o n and r e - d e p o s i t i o n i n s l u r r i e s and streams.
F a b r i c s t u d y t h u s p r o v i d e s a means o f d i s t i n g u i s h i n g l o e s s f r o m
massive d e p o s i t s o f l o e s s i c c o l l u v i u m and l o e s s i c a l l u v i u m . 3. The s i l t s a t Meng Xian and a l l b u t t h e lowermost 1Om o f t h e huge
Jiuzhoutai section are aeolian i n o r i g i n .
T r a n s l o c a t i o n by s l u r r y f l o w i s
common i n t h e Loess P l a t e a u and g i v e s r i s e t o d i s t i n c t i v e f a c i e s f r o m massive s i l t s t o s i l t y d i a m i c t s containing cobbles. 4. The compaction o f l o e s s i n t h e d r y s t a t e appears t o be by s h e a r i n g
c o n t a c t s and l o c a l d i s r u p t i o n o f c l a y and f i n e s i l t b u t t r e s s e s w i t h o u t d i s p e r s ion.
H y d r o c o n s o l i d a t i o n r e s u l t s i n some d i s p e r s i o n o f t h e f i n e r grades and
v a r y i n g degrees o f adhesion t o t h e s i l t s k e l e t o n .
C l a y b r i d g e s do n o t s u r v i v e
t h i s process.
5. The p a l a e o s o l s and w e a t h e r i n g h o r i z o n s i n t h e l o e s s a t J i u z h o u t a i , Lanzhou, show w e l l developed cementation a t c o n t a c t s and some overgrowths i n c l u d i n g amorphous s i l i c a .
I n t h e Malan l o e s s gypsum i s o f comparable importance t o
c a l c i t e , both o f which a r e dispersed w i t h o n l y l i m i t e d nodule layers.
Cementa-
t i o n i s a source o f reduced v o i d s r a t i o s and enhanced s t r e n g t h s i n t h e L i s h i h and Wucheng l o e s s e s .
Unloading j o i n t s a r e b e t t e r developed i n t h e s e l o e s s e s
t h a n i n t h e Upper P l e i s t o c e n e Malan l o e s s . 6. The p a r t i c l e shape, s i z e and f a b r i c o f t h e l o e s s a t Meng X i a n and
J i u z h o u t a i a r e c o n s i s t e n t w i t h a source i n t h e wadis, f a n s and p l a y a s t o t h e n o r t h and west o f t h e Loess P l a t e a u .
The p a r t i c l e p r o p e r t i e s , i n c l u d i n g damaged
s i l i c a o v e r g r o w t h s and a d h e r i n g p a r t i c l e s o f c l a y - g r a d e q u a r t z a r e c o n s i s t e n t w i t h an o r i g i n as d e s e r t d u s t ( c f . F o l k 1978;
Whalley and Smith 1981). T r i a s s i c f e l d s p a t h i c sandstones and s h a l e s and t h e Neogene r e d sandstones underl y i n g t h e l o e s s on t h e n o r t h s i d e of J i u z h o u t a i a r e r i c h i n s u l p h a t e s .
7. The basal l o e s s a t Lanzhou was a response t o t h e i n c r e a s i n g d e s i c c a t i o n T h i s began i n t h e Lower P l e i s t o c e n e w i t h t h e i n i t i a t i o n o f r a p i d u p l i f t of Q i n g h a i - X i z a n g and t h e Himalaya which exceeded 3.5 im i n 2 m. yr and of High A s i a .
c o n t i n u e s a t a comparable r a t e .
Loess a c c u m u l a t i o n h e r e i s t h u s a concomitant
o f g l a c i a t i o n and p e r i g l a c i a t i o n i n Q i n g h a i - X i z a n g r a t h e r t h a n a r e s u l t o f i t .
ACKNOWLEDGEMENTS
I am pleased t o acknowledge t h e o p p o r t u n i t y p r o v i d e d for t h e s t u d y o f t h e l o e s s o f China by Academia S i n i c a and t h e U n i v e r s i t y o f Lanzhou. t h a n k s a r e due t o Shi Yafeng,
Special L i J i j u n , Xu Shuying, Zhang L i n y u a n and Chon
X i l o n g and o t h e r f r i e n d s and c o l l e a g u e s t o o numerous t o mention.
89
REFERCNCES An Zhisheng, Wang Junda and L i Huamei, 1977. Palaeomagnetic r e s e a r c h o f t h e Luochuan l o e s s s e c t i o n . Geochimica, 12, 239-249 ( i n Chinese). Cooke, R.U., 1981. S a l t weathering i n d e s e r t s . Proc. G e o l o g i s t s ' A s s o c i a t i o n , 92, 1-16. Cui Z h i - J i u , 1980. E s s e n t i a l f e a t u r e s o f t h e development o f g l a c i a l landforms on Q i n g h a i - X i z a n g P l a t e a u . Acta Geographica S i n i c a , 35, 137-148. Cui Z h i - J i u , 1981. P e r i g l a c i a l l a n d f o r m s and t h e i r r e g i o n a l c h a r a c t e r i s t i c s on Q i n g h a i - X i z a n g P l a t e a u . I n G e o l o g i c a l and E c o l o g i c a l S t u d i e s o f QinghaiXizang Plateau, Proc. Symp. q i n g h a i - X i z a n g ( T i b e t ) Plateau, pp. 1777-1787, Science Press, B e i j i n g . Cui Z h i - J i u . , no d a t e . P e r i g l a c i a l phenomena and environmental r e c o n s t r u c t i o n on Q i n g h a i - X i z a n g ( T i b e t ) P l a t e a u . S c i e n t i f i c Papers on Geology f o r I n t e r tiydrogeology, e n g i n e e r i n g geology, Q u a t e r n a r y Geology, n a t i o n a l Exchange 5 and yeomorphology. P u b l i s h i n g House o f Geology, B e i j i n g . pp. 109-122 ( i n Chinese). Denisov, N.G., 1951. S e t t l e m e n t p r o p e r t i e s o f l o e s s i a l s o i l s . T r a n s l a t e d by Karpoff, K.P. and H.J. Gibbs, U.S. Bureau o f Reclamation, Denver, Colorado. Derbyshire, E . , 1978. The m i d d l e Hwang Ho l o e s s l a n d s . Geographical J o u r n a l , 144, 191-194. Derbyshire, E., 1982. The Lushan dilemma: P l e i s t o c e n e g l a c i a t i o n s o u t h o f t h e Chang J i a n g . J o u r n a l o f G l a c i o l o g y and Cryopedology, 4, 4, 1-26 ( i n Chinese). Derbyshire, E . , i n p r e s s ( a ) On t h e morphology, sediments and o r i g i n o f t h e Loess P l a t e a u o f c e n t r a l China. I n Scoging, H. and Gardner, R., Large S c a l e Landforms. OUP Derbyshire, E., i n p r e s s ( b ) Granulometry and f a b r i c o f t h e l o e s s a t J i u z h o u t a i , Lanzhou, P e o p l e ' s R e p u b l i c o f China D y l i k , J. and K l a t k a , T., 1952. Recherches n i c r o s c o p i q u e s s u r l a d ' e s i n t e g r a t i o n p e r i g l a c i a i r e . B u l l . SOC. S c i . L e t t r e s , Kodz, 3, 1-12. Feda, J . , 1966. S t r u c t u r a l s t a b i l i t y o f s u b s i d e n t l o e s s s o i l s from Praha D e j v i c e . E n g i n e e r i n g Geology, 3, 201-219. Folk, R.L., 1978. A n g u l a r i t y and S i l i c a c o a t i n g s o f Simpson D e s e r t sandgrains, N o r t h e r n T e r r i t o r y , A u s t r a l i a . J o u r n a l o f Sedimentary P e t r o l o g y , 48, 611-624. 1957. Brazos R i v e r b a r . A s t u d y i n t h e s i g n i f i c a n c e Folk, R.L. and Ward, W.C., of g r a i n s i z e parameters. J o u r n a l o f Sedimentary P e t r o l o g y , 27, 3-27.
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Adams, J., Endo, A., and Smith, D. 1980. T h r e s h o l d v e l o c i t i e s G i l l e t t e , D.A., f o r i n p u t o f s o i l p a r t i c l e s i n t o t h e a i r by d e s e r t s o i l s . J. Geophysical Research, 85, 5621 -5630. i n press. S a l t e f f l o r e s c e n c e s and s a l t weathering i n t h e Hunza Goudie, A.S., V a l l e y , Karakoram Mountains, P a k i s t a n . Proceedings o f t h e I n t e r n a t i o n a l Karakoram P r o j e c t , Vol. 2. Goudie, A.S., Cooke, R.U., and Doornkamp, J.C., 1979. The f o r m a t i o n o f s i l t from q u a r t z dune sand by s a l t w e a t h e r i n g processes i n d e s e r t s . J . A r i d Environments, 2, 105-112. G r i m , R.E., 1953. C l a y m i n e r a l o g y . McGraw-Hill , New York. Han Jia-mao, 1982. A p r e l i m i n a r y s t u d y on t h e c l a y m i n e r a l o g y o f t h e l o e s s a t Luochuan s e c t i o n . I n L i u Dongsheng ( e d . ) , Q u a t e r n a r y Geology and Environment i n China. B e i j i n g , China Ocean Press, 67-72. Kung, S.Y. and Chiang, T.C., 1977. S o i l e r o s i o n and i t s c o n t r o l i n small g u l l y watersheds i n t h e r o l l i n g l o e s s a r e a i n t h e m i d d l e reaches o f t h e Y e l l o w River. B e i j i n g . L i J i j u n , Wen Shixuan, Zhang Qingsong, liang Fubao, Zheng Benxing and L i Bing Yuan 1979. A d i s c u s s i o n on t h e p e r i o d , a m p l i t u d e and t y p e o f t h e u p l i f t of t h e Q i n g h a i - X i z a n g P l a t e a u . S c i e n t i a S i n i c a , 22, 1314-1328. L i u Dong Sheng and Chang Tsung-Hu, 1964. The 'Huangtu' ( l o e s s ) o f China. R e p o r t s of t h e s i x t h INQUA Congress, 4, 503-524. L i u Dong Sheng e t a l . , 1964. Loess on t h e m i d d l e reaches of t h e Y e l l o w R i v e r . Science Press, B e i j i n g , 234 pp. ( i n Chinese).
90 L i u Dong sheng e t a l . , 1966. Composition and t e x t u r e o f t h e l o e s s . Science Press B e i j i n g , 132 pp. ( i n Chinese). M a t a l u c c i , R.V., Abel-Hardy, M., Shelton, J.W., 1970. I n f l u e n c e o f m i c r o s t r u c t u r e o f l o e s s on t r i a x i a l shear s t r e n g t h . Eng. Geol. ( E l s e v i e r ) 4 pp. 341-351. Nahon, D., and Trompette, R. , 1982. O r i g i n o f S i l t s t o n e s : g l a c i a l g r i n d i n g v e r s u s weathering. Sedimentology, 29, 25-35. Prebble, M.M., 1967. Cavernous weathering i n t h e T a y l o r d r y v a l l e y , V i c t o r i a Land, A n t a r c t i c a . Nature, 216 1194-1195. Riezebos, P.A. and Van d e r Waals, L., 1974. S i l t - s i z e d q u a r t z p a r t i c l e s : a proposed source. Sedimentary Geology, 12,279-285. Srralley, I . J . , 1974. Fragmentation o f g r a n i t i c q u a r t z i n w a t e r . D i s c u s s i o n Sedimentology, 21, 633-635. and V i t a - F i n z i , C., 1968. The f o r m a t i o n o f f i n e p a r t i c l e s i n sandy Smalley, I.J. d e s e r t s and t h e n a t u r e of d e s e r t l o e s s . J. Sedimentary P e t r o l o g y , 38, 766774 Sun T i e n - c h i n g and Yang Huan-jen, 1961 . The G r e a t I c e Age g l a c i a t i o n i n China. ( i n Chinese). - Acta Geologica S i n i c a , 41, 234-244 hang Yong-yan, Teng Zhi-hang and Yue Le-ping 1982. M i c r o t e x t u r e s o f l o e s s i n China. I n Q u a t e r n a r y Geology and Environment i n China (ed. L i u Tung-sheng), 43-48 (China Ocean Press, B e i j i n g ) . Wang Yong-yan, Wu Zhai Bo and Yue Leping, 1978. C o n s t i t u e n t m a t e r i a l s and s t r u c t u r e s o f l o e s s i n Lanzhou and t h e d a t e o f i t s f o r m a t i o n . Northwest U n i v e r s i t y B u l l e t i n ( S c i e n c e E d i t i o n ) , 3, 1-27, ( i n Chinese). Wang Yong-yan and Yue Leping, 1982. Palaeomagnetic s t r a t i g r a p h y o f l o e s s i n China. I n L i u Dongsheng (ed.), Q u a t e r n a r y Geology and Environment of China, B e i j i n g , Ocean Press, 42 ( a b s t r a c t ) . Wang Yong-yan, Yue Leping, Wu Zhai Bao, Chen X i L i n g and Dun Wen, 1980. On t h e s t r a t i g r a p h i c problems o f t h e l o e s s on t h e p l a t e a u n o r t h o f Weihe r i v e r , Shaanxi P r o v i n c e i n accordance w i t h t h e palaeomagnetic d a t a . G e o l o g i c a l Review, 26, 141-147 ( i n Chinese). Wang Yong-yan and Zhang Zhong-Hu, 1980. Loess i n China.Shaanxi P e o p l e ' s A r t P u b l i s h i n g House, Xian, n.p. Wen Qizhong, Yu Suhua Gu X i o n g f e i and L e i Jianquan, 1981. A p r e l i m i n a r y i n v e s t i g a t i o n o f r a r e e a r t h elements (RRE) i n l o e s s . Geochimica, 6, 151 -1 57. U h a l l e y , W.B., 1979. Q u a r t z s i l t p r o d u c t i o n and sand g r a i n s u r f a c e t e x t u r e s f r o m f l u v i a l and g l a c i a l environments. Scanning E l e c t r o n Flicroscopy, 1, 547-554. I l h a l l e y , N.B. and Smith, B.J., 1981. M i n e r a l c o n t e n t o f Harmattan d u s t f r o m n o r t h e r n N i g e r i a examined by scanning e l e c t r o n microscopy. J o u r n a l o f A r i d Environments, 4, 21-29. Xia Yuei and L i Ping-Yeng, 1980. Four stages i n t h e development o f Q u a t e r n a r y s e d i m e n t a t i o n around Xian. Geographical J o u r n a l ( P h y s i c a l ) , 10, 52-70 ( i n Chinese). Yaalon, D.H. and Gainor, E., 1979. E a s t M e d i t e r r a n e a n t r a j e c t o r i e s o f d u s t c a r r y i n g storms f r o m t h e Sahara and t h e S i n a i . I n Morales, C . ( e d . ) Saharan Dust, pp. 197-196, W i l e y , New York. Zhang Zong-Hu, 1964. C h a r a c t e r i s t i c s o f t h e m i c r o s t r u c t u r e and t e x t u r e of t h e l o e s s t y p e s o f China. A c t a Geologica S i n i c a , 44, 357-369. Zheng Ben-Xing and L i J i j u n , 1981. Q u a t e r n a r y g l a c i a t i o n o f t h e Q u i n g h a i Xizang Plateau. Proc. Symp. Q u i n g h a i - X i z a n g ( T i b e t ) Plateau, pp. 16311640. Science Press, B e i j i n g .
91
CLAY MINERALS IN THIN LOESS, OHIO R I V E R BASIN, U.S.A.
ROBERT V . RUHE, Indiana Un i v er s i t y , Bloomington, Indiana, 47405, U.S.A. INTRODUCTION The Ohio River basin occupies about 261,500 km2 above t h e mouth of t h e Wabash River in southwestern Indiana ( F i g . 1 ) a n d i s emplaced eastward across the geologic s t r u c t u r e , t h e Cincinnati a r c h .
Eastward from the mouth of the Wabash
River t o near C i n c i n n at i , Ohio, successively o l d e r bedrock i s crossed from Pennsylvanian c l a s t i c rocks a n d coal measures, t o Mississippian carbonate rocks, Devonian s h a l e s , S i l u r i a n limestones, and Ordovician carbonate and c l a s t i c rocks.
Farther eastward t h e rock sequence i s reversed.
Within t h i s major drainage basin t h e Wabash and Ohio Rivers a r e confluent streams.
The Wabash drainage basin includes 87,300 km2 and 91% of i t i s covered
by g la c i a l d r i f t .
Wisconsin g l a c i a l d r i f t occupies 47,400 km2, and I l l i n o i a n
glacial d r i f t covers 32,375 km2.
The major v al l e ys in t h i s drainage basin a r e
dominated by g l a c i a l outwash and alluvium derived from g l a c i a l d r i f t .
Both
sediments provided source material f o r l o e s s .
I n c o n t r a s t , t h e Ohio drainage basin excluding the Wabash ba sin, has about 46,900 km2 of Wisconsin g l a c i a l d r i f t and only 13,400 km2 of I l l i n o i a n g l a c i a l d r i f t . The major p a r t o f t h e Ohio basin (>200,000km2 o r 77%) i s nongla c ia l,
92 so t h a t s e d i m e n t d e r i v e d f r o m b e d r o c k o r s o i l s f o r m e d i n b e d r o c k p r o v i d e d a l l u viurn t o t h e O h i o R i v e r v a l l e y .
C o n s e q u e n t l y , t h e s o u r c e m a t e r i a l f o r l o e s s was
composed o f n o n g l a c i a l a l l u v i u n i as w e l l a s g l a c i a l o u t w a s h .
The s o i l s i n t h e
n o n g l d c i a l r e g i o n a r e I n c e p t i s o l s , A l f i s o l s , and U l t i s o l s ( S o i l S u r v e y S t a f f ,
1975).
I n c e p t i s o l s a r e weakly weathered s o i l s w i t h few d i a g n o s t i c pedologic
features.
A l f i s o l s a r e moderately weathered s o i l s w i t h high-base s t a t u s t h a t
formed under f o r e s t .
U l t i s o l s a r e s t r o n g l y weathered s o i l s w i t h low-base s t a t u s
t h a t f o r m e d u n d e r f o r e s t on g e n e r a l l y g e o l o g i c a l l y o l d e r l a n d s c a p e s .
These
s o i l s o c c u p y a l l t h e t r i b u t a r y d r a i n a g e b a s i n s t h a t d r a i n t o t h e O h i o R i v e r from t h e Green R i v e r i n w e s t e r n K e n t u c k y , e a s t w a r d t h r o u g h West V i r g i n i a , and n o r t h ward i n c l u d i n g t h e A l l e g h e n y R i v e r i n w e s t e r n P e n n s y l v a n i a .
These s o i l s a l s o
o c c u r i n s o u t h e r n p a r t s o f w a t e r s h e d s a c r o s s O h i o and s o u t h e r n I n d i a n a . Wisconsin l o e s s covers a l a r g e p a r t o f t h e uplands i n t h e Ohio drainage b a s i n and i s >5 in t h i c k j u s t e a s t o f t h e Wabash R i v e r v a l l e y i n s o u t h w e s t e r n I n d i a n a and a l s o w e s t e r n K e n t u c k y .
E a s t w a r d i n I n d i a n a and K e n t u c k y and i n
Ohio, t h e l o e s s g e n e r a l l y i s 1 t o 2
m thick.
I n t h e Wisconsin d r i f t r e g i o n the
l o e s s b u r i e s g l a c i a l t i l l o r s t r a t i f i e d d r i f t w i t h o u t an i n t e r v e n i n g p a l e o s o l . On u p l a n d summits i n t h e I l l i n o i a n d r i f t and n o n g l a c i a l r e g i o n s , t h e l o e s s b u r i e s p a l e o s o l s f o r m e d i n t i l l o r s t r a t i f i e d d r i f t o r m a t e r i a l s d e r i v e d froin them o r p a l e o s o l s f o r m e d i n b e d r o c k o r i n i a t e r i a l s d e r i v e d from i t .
A p r o b l e m a r i s e s i n t h e t h i n - l o e s s r e g i o n when c l a y m i n e r a l s a r e used as i n d i c a t o r s o f s o u r c e s o f l o e s s o r as i n d i c a t o r s o f t h e d i s t r i b u t i o n of t h e s e d i ment w i t h i n t h e l o e s s - d i s p e r s i o n system.
Where l o e s s i s t h i n , s o i l s have formed
t h r o u g h o u t t h e l o e s s and merge downward w i t h t h e b u r i e d s o i l s f o r m e d i n t h e substrata.
T h i s phenomenon i s t e r m e d s o i l weldiizc; (Ruhe and O l s o n , 1 9 8 0 a ) .
To
f u r t h e r c o m p l i c a t e t h e d u a l s o i l - s t r a t i g r a p h i c system, a f r a g i p a n (Grossman and C a r l i s l e , 1969) i s formed i n t h e l o w e r p a r t o f t h e l o e s s j u s t above t h e b u r i e d s o i l (Ruhe and O l s o n , 1 9 8 0 a ) .
The l o e s s - d e r i v e d s o i l m o r p h o l o g i c a l l y a p p e a r s t o
be s t r o n g l y d e v e l o p e d and a l s o a p p e a r s t o be s t r o n g l y w e a t h e r e d . a r e v e r y s t r o n g l y a c i d i c w i t h pH v a l u e s r a n g i n g f r o m 3 t o 5.3. then,
The s u b s o i l s The p r o b l e m ,
i s w h a t c l a y m i n e r a l s r e p r e s e n t o r i g i n a l components o f t h e l o e s s s e d i m e n t ,
o r what c l a y m i n e r a l s a r e a l t e r e d products o f s o i l weathering. attacked i n t h i s study.
T h i s problem i s
I n t h i c k l o e s s t h e p r o b l e m c a n be a v o i d e d by s t u d y i n g
t h e c l a y m i n e r a l s i n t h e l o e s s - d e r i v e d s o i l s and t h e u n d e r l y i n g l o e s s p a r e n t m a t e r i a l i n c l u d i n g r e l a t i v e l y unweathered calcareous l o e s s .
METHODS S o i l c o r e s were e x t r a c t e d i n t h e f i e l d w i t h a t r u c k - m o u n t e d h y d r a u l i c s o i l c o r i n g machine.
A l l s i t e s were l o c a t e d o n u p l a n d summits t h a t were e s s e n t i a l l y
f l a t o r w i t h l o w - s l o p e g r a d i e n t so t h a t p o s t - l o e s s e r o s i o n w o u l d b e m i n i m a l . c o r e s c o m p l e t e l y p e n e t r a t e d t h e l o e s s and l o e s s - d e r i v e d s o i l and e x t e n d e d
All
93 downward i n t o the s u b s t r a t u m p a l e o s o l s and where p o s s i b l e i n t o t h e p a r e n t m a t e r i a l s of t h e b u r i e d s o i l s .
The p e d o l o g i c , l i t h o l o g i c , and s t r a t i g r a p h i c niorphologies
of each c o r e were d e s c r i b e d and measured and s e r v e d a s t h e b a s i s f o r sampling for laboratory s t u d i e s .
F i f t y - s i x s i t e s were c o r e d :
Kentucky, and 8 i n Ohio ( F i g . 1 ) .
31 i n I n d i a n a , 17 i n
The number of samples p e r c o r e v a r i e d from
1 5 t o 33 and averaged a b o u t 20. Each sample of each c o r e was a n a l y z e d i n the l a b o r a t o r y f o r p h y s i c a l , chemic a l , and m i n e r a l o g i c p r o p e r t i e s u s i n g methods of t h e U. S . National S o i l Survey Laboratory ( S o i l Survey S t a f f , 1 9 7 2 ) . P a r t i c l e s i z e was determined by t h e pipet method, and s i z e s e p a r a t e s were sand (>62pm), c o a r s e s i l t (62-16pm), f i n e s i l t (16-2pm), and c l a y (<2pm). Fine c l a y ( < O . Z p m ) was s e p a r a t e d by c e n t r i f u g e technique f o r s e l e c t e d sarnpl e s . Chemical a n a l y s e s i n c l u d e d : ( 1 ) pH i n a 1 : l s o i l - w a t e r p a s t e , ( 2 ) e x t r a c t able bases (Ca, Mg, Na, K ) by NH,OAC e x t r a c t i o n followed by a t o m i c - a b s o r p t i o n measurement, ( 3 ) c a t i o n - e x c h a n g e c a p a c i t y ( C E C ) by NH,OAC method, ( 4 ) base s a t uration c a l c u l a t e d by I: e x t r a c t a b l e bases/CEC x 1 0 0 , ( 5 ) e x t r a c t a b l e Fe and Mn by d i t h i o n i t e e x t r a c t i o n followed by a t o m i c - a b s o r p t i o n a n a l y s i s , and ( 6 ) o r g a n i c carbon by a c i d - d i c h r o m a t e d i g e s t i o n . Clay mineralogy of a l l samples was analyzed by X-ray p o w d e r - d i f f r a c t i o n t e c h nique u s i n g o r i e n t e d g l a s s s l i d e s w i t h C U K E r a d i a t i o n with a scanning r a t e of 2"2e/minute from 3" t o 30'2e.
D u p l i c a t e s l i d e s of t h e 2um f r a c t i o n were
prepared f o l l o w i n g d i s p e r s i o n w i t h sodium rnetaphosphate. before and a f t e r h e a t i n g f o r 1 5 m i n u t e s a t 450°C.
One s l i d e was X-rayed
The o t h e r s l i d e was g l y c o -
lated ( e t h y l e n e - g l y c o l v a p o r i z a t i o n i n a d e s s i c a t o r f o r 48 h o u r s ) and then
On s e l e c t e d samples a t h i r d s l i d e was X-rayed f o l l o w i n g h e a t i n g a t In a d d i t i o n , the <0.2pm f r a c t i o n s o f s e l e c t e d samples were s u b j e c t e d t o the same procedures. Clay m i n e r a l s were i d e n t i f i e d by d i f f r a c t i o n peaks a s f o l l o w s : K a o l i n i t e , ( K ) , 7 . 2 A ; i l l i t e ( I ) , 1 0 i ; expandable m i n e r a l s ( X ) , 14 t o 1 7 A f o l l o w i n g g l y c o 0 lation with v e r m i c u l i t e (X14 o r V ) a t 14A and m o n t m o r i l l o n i t e (X17 o r El) a t 17A. Very l i t t l e c h l o r i t e was i d e n t i f i e d i n a l l samples a s the 14A peak c o l l a p s e d t o 0 0 10A f o l l o w i n g h e a t i n g a t 450°C. A 14A peak should p e r s i s t i f c h l o r i t e was p r e s e n t . 0 The 7.2A peak d i s a p p e a r e d f o l l o w i n g h e a t i n g a t 600°C i n d i c a t i n g t h a t k a o l i n i t e was t h e dominant m i n e r a l . The K and Mg s a t u r a t i o n s were used t o i d e n t i f y i l l i t e and vermiculite-montmorillonite i n p o o r l y c r y s t a l l i z e d o r mixed l a t t i c e m i n e r a l s . Q u a n t i t a t i v e e s t i m a t e s o f t h e c l a y m i n e r a l s i n each sample were made by u s i n g the method o f a d d i t i o n s of p r o p o r t i o n s of s o i l c l a y t o a c o n s t a n t s t a n d a r d (Ruhe and Olson, 1979; B r i n d l e y , 1981) and a s improved ( R u h e and Matney, 1 9 8 0 ) . Another problem i s now e v i d e n t . With more than 50 s o i l p r o f i l e s , numerous sample h o r i z o n s per p r o f i l e , and numerous a n a l y s e s p e r sample, t h e t o t a l d a t a X-rayed.
600°C.
S e l e c t e d samples were a l s o X-rayed f o l l o w i n g K and Mg s a t u r a t i o n .
0
94
a r e voluminous. Consequently, s t a t i s t i c a l technique must be used t o s a t i s f a c t o r i l y analyze t h e d a t a .
WEATHERING IN THIN LOESS Another problem i s involved in t h e t h i n l o e s s . The lower p a r t of t h e loess i s sandy, and t h e sand content i n cr eas es downward t o a b o u t the amount contained in th e buried s o i l r eg ar d l es s of g l a c i a l t i l l o r bedrock substratum.
A textural
d i s c o n t i n u i t y e x i s t s i n t h e material i n which t h e loess-derived s o i l formed. The fragipan g e n e r a l ly occurs a t a n d beneath the t e x t u r a l change t o sandy loess. The sand c o n t e n t i n t h e t h i n loess has been a t t r i b u t e d t o source inaterial derived from t h e paleosol landscape and i t s s u b s t r a t a a n d the mixing of t h i s material i n t h e lower p a r t of t h e Wisconsin l o es s (Ruhe a n d Olson, 1980b). A new research study i s c u r r e n t l y d i r e c t e d toward t h e n at u r e a n d o r i g i n o f the sandy phase of the loess.
An example of t h e problems involved in s o i l development in t h i n loe ss i s the Fig. 2 ) , a Typic Fragiudalf (Soil Survey S t a f f , 1975),
Hosmer S i l t Loam ( O H - 1 , near C i n c i n n a t i , Ohio.
Hosmer S i l t Loam This s o i l i s formed in 1 . 8 m of \,!isconsin l o ess t h a t burie s t h e B horizon of a paleosol formed in g l a c i a l t i l l .
The s o i l has a t e x t u r a l B horizon ( F i g . 2 :
K 0%
I 40
0%
40
X
0%
40
0 a r ts
80
1';
Bare saturation
xLil X I7
100
b
1
2
3
PH 4
5
6
7
OHIO- I
Fig. 2 . P a r t i c l e s i z e , cl ay mineralogy, and chemistry f o r Hosrner S i l t Loam, Soil horizons a r e l a bel ed , and sample horizons a r e numbered 1 t o 15. See t e x t f o r o t h e r symbols. X14 and X17 a r e parts o r percent of X .
95 B21, 822 h o r i z o n s ; sample h o r i z o n s 3 - 5 ) above t h e f r a g i p a n (BX1, 2BX2; 6 - 7 ) . c l a y c o n t e n t i s g r e a t e s t i n t h e BX1 h o r i z o n .
and
Sand c o n t e n t g r e a t l y i n c r e a s e s i n
the 2BX2 h o r i z o n and i r r e g u l a r l y i n c r e a s e s downward t o t h e B21b h o r i z o n o f t h e buried s o i l .
Sample h o r i z o n s 6 t h r o u g h 1 0 a r e t h e sandy l o e s s , and h o r i z o n s
1 through 5 a r e t h e s i l t y l o e s s .
The f r a g i p a n i s f o r m e d j u s t above and i n t h e
upper p a r t o f t h e v e r y sandy zone. E x c l u d i n g n e a r - s u r f a c e h o r i z o n e f f e c t s caused by a g r i c u l t u r a l l i m i n g s u c h as h i g h Ca, base s a t u r a t i o n , and pH, t h e c h e m i c a l p r o p e r t i e s o f t h e p r o f i l e a r e independent of t h e l i t h o l o g i c s t r a t i f i c a t i o n ( F i g . 2 ) .
E x t r a c t a b l e Ca and Mg
and base s a t u r a t i o n d e c r e a s e t o minimum v a l u e s a c r o s s t h e t e x t u r a l c o n t a c t and then i n c r e a s e w i t h d e p t h .
The s u b s o i l i s v e r y a c i d i c w i t h pH v a l u e s of 4 . 3 t o
4.5 and base s a t u r a t i o n s o f
20 t o 30% i n t h e B, BX, and C h o r i z o n s .
T h i s v e r y a c i d i c s o i l e n v i r o n m e n t a p p a r e n t l y has l i t t l e e f f e c t on t h e c l a y m i n e r a l s (2um f r a c t i o n ) w h i c h a r e d i s t i n c t l y s t r a t i f i e d w i t h marked changes i n c o n t e n t s of k a o l i n i t e ( K ) , loess c o n t a c t ( F i g . 2).
i l l i t e ( I ) , and e x p a n d a b l e m i n e r a l s ( X ) a t t h e sandy
W i t h i n t h e X f r a c t i o n t h e changes i n v e r m i c u l i t e (X14)
a n d m o n t m o r i l l o n i t e (X17) a r e i n d e p e n d e n t o f t h e t e x t u r a l s t r a t i f i c a t i o n .
The
X14 f r a c t i o n d e c r e a s e s and t h e n i n c r e a s e s , and t h e X17 f r a c t i o n i n c r e a s e s and then d e c r e a s e s i n t o t h e BX2, B X , and C h o r i z o n s ( F i g . 2 ) .
I n a c i d i c s o i l environments mica ( i l l i t e ) can be a l t e r e d t o v e r m i c u l i t e b y weathering, and v e r m i c u l i t e c a n w e a t h e r p r o g r e s s i v e l y t o m o n t m o r i l l o n i t e ( J a c k s o n , 1964).
The a l t e r a t i o n o f m i c a t o v e r m i c u l i t e depends o n l e a c h i n g of p o t a s s i u m
from t h e m i c a l a t t i c e , and where s u c h a l t e r a t i o n has o c c u r r e d , t h e r a t i o o f v e r m i c u l i t e t o i l l i t e d e c r e a s e s downward f r o m t h e s o i l s u r f a c e ( D o u g l a s , 1 9 7 7 ) .
In
the 2pm c l a y f r a c t i o n of t h e Hosmer S i l t Loam, t h e V / I r a t i o p r o g r e s s i v e l y i n creases f r o m 0.68 i n h o r i z o n 1 t o 1 . 5 9 i n h o r i z o n 4, d e c r e a s e s t o 1 . 2 6 i n h o r i z o n 6, a b r u p t l y i n c r e a s e s t o 3 . 0 a c r o s s t h e sandy c o n t a c t i n t o h o r i z o n 7, and c o n t i n u e s t o i n c r e a s e t o 5 . 9 i n h o r i z o n 9.
S t r a t i f i c a t i o n i s again evident,
but t h e r e i s l i t t l e e v i d e n c e f o r a l t e r a t i o n o f i l l i t e t o v e r m i c u l i t e .
The a l t e r -
a t i o n a p p e a r s t o be f r o m v e r m i c u l i t e t o m o n t m o r i l l o n i t e as shown by t h e p r o g r e s s i v e changes downward i n t h e p r o f i l e i n s t r a t i g r a p h i c a l l y a l i g n e d X - r a y d i f f r a c t i o n p a t t e r n s ( F i g . 3 ) and a s q u a n t i f i e d ( F i g . 2 ) . The t e x t u r e p r o f i l e i n t h e Hosmer S i l t Loam i s d i r e c t l y d e p e n d e n t upon t h e amount o f f i n e (<0.2um) c l a y ( F i g . 4 ) .
T h i s f a c t i s emphasized by t h e f i t o f
h i g h e r d e g r e e p o l y n o m i a l s i n d e p t h d i s t r i b u t i o n s o f t o t a l c l a y , c o a r s e c l a y , and fine clay.
W i t h i n t h e f i n e - c l a y f r a c t i o n , t h e e x p a n d a b l e m i n e r a l s ( X ) a r e domi-
nant, and m o n t m o r i l l o n i t e , i n t u r n , i s d o m i n a n t w i t h i n t h e X f r a c t i o n ( F i g . 4 ) . Clay-mineral s t r a t i f i c a t i o n i s n o t e v i d e n t i n t h e f i n e - c l a y f r a c t i o n across t h e sandy l o e s s c o n t a c t and shows t h e p e d o g e n i c n a t u r e o f t h e s e c l a y s .
There i s no
s i g n i f i c a n t d i f f e r e n c e between t h e d e p t h d i s t r i b u t i o n s o f i l l i t e i n t h e t o t a l c l a y
96
-I--p
6G
4
h
Fig. 3 . S t r a t i g r a p h i c a l l y aligned X-ray d i f f r a c t i o n p a t t e r n s of f i r s t o r d e r peaks o f c l a y iiiinerals in Hosiiier S i l t Loaiii i n sample h o r i z o n s 1 t o 9 . Sainple h o r i z o n s a r e l a b e l e d , and G i s g l y c o l a t e d , and H i s hea&ed a t 450°C. Note biiiiodal peaks a t 14A t o 1 7 A and iiiethod of s e p a r a t i o n shown by v e r t i c a l l i n e s . Note p r o g r e s s i v e changes in peak a r e a s under 1 4 8 and 178 iiiodes.
q
7 h I
7%
I
10 14 17 I
7g
1
10
,
14 17
3nd f i n e - c l a y f r a c t i o n s . The V/I r a t i o s i n the f i n e - c l a y p r o f i l e a r e a s n o n d i a g n o s t i c a s i n t h e t o t a l - c l a y f r a c t i o n . The main g r a p h i c c o r r e l a t i o n s ( F i g . 4 ) a r e t h e c l a y and m o n t m o r i l l o n i t e d e p t h d i s t r i b u t i o n s and enhancement The X-ray d i f f r a c t i o n p a t t e r n s
of montlllorillonite a s v e r m i c u l i t e i s d e p l e t e d .
of t o t a l and f i n e c l a y of samples o f t h e €321 and 2BX2 h o r i z o n s v e r i f y t h e s e c o n c l u s i o n s ( F i g . 5 ) , and t h e y a p p l y i n g e n e r a l t o t h e o t h e r s o i l s s t u d i e d .
The amounts of K , I , and X i n t h e sandy l o e s s and s u b j a c e n t b u r i e d s o i l s a r e very s i m i l a r , b u t t h e y d i f f e r from the composition i n the uppermost s i l t y l o e s s This g e n e r a l i z a t i o n a l s o a p p l i e s t o t h e o t h e r t h i n l o e s s - d e r i v e d s o i l s . e n t l y c l a y has been mixed i n the sandy l o e s s a s well a s s a n d .
Appar-
97 Clay - % 0 10 20
I-%
K- % 30 0
10
20
30
I F
I=;p Fig. 4. Depth d i s t r i b u t i o n s of
1 4 ~ e I ' I
'
I
'
I
'
I
'
4 I
F i g . 5 . Comparison o f g l y c o l a t e d X-ray d i f f r a c t i o n p a t t e r n s of t o t a l c l a y and f i n e c l a y i n two h o r i z o n s of Hosnier S i l t Loam. Decimals show t h e r e l a t i v e a r e a s b e n e a t h o t h e f i K s t - o r d e r peaks. Note t h e s h i f t from 14A t o 17A between t o t a l and fine clay.
I
78
I 10
I 14
I
17
S o i l s i n T h i n Loess The o t h e r s o i l p r o f i l e s i n the Ohio b a s i n were i n d i v i d u a l l y s u b j e c t e d t o t h e same a n a l y s i s a s the Hosmer S i l t Loam. The voluminous p h y s i c a l , c h e m i c a l , and mineralogic d a t a were a b s t r a c t e d f o r e a c h s o i l p r o f i l e by m u l t i p l e r e g r e s s i o n .
98
analysis sample horizon by sample horizon from the surface to depth in the profile. As the clay content, 2 or 0.2um, indicates the pedogenic profile the depth distributions of the clay minerals were related as:
I(%)
+ a3 X ( % ) + a,,
M(% of X)
The coefficient of determination ( R 2 ) for the overall relation and the various paired correlation coefficients (r) within the relation were calculated. In the majority of profiles, the niontmorillonite distribution relates best to the clay distribution and dominantly influences the overall ( R 2 ) relation (Fig. 6). In 90% of the 56 profiles the R 2 value is 0.6 or more, and in 70% of
Fig. 6. Summary of R2 and r values o f multiple regressions for all soil profiles formed in thin loess in the Ohio River drainage basin. Regressions are clay (C) to clay minerals and montmorillonite (M) to chemistry. See text.
99
theprofiles the r value of M to clay is 0.6 or more. The relations of K, I , and X to clay are generally poor and alike. In 30% of the profiles the r values of these minerals to clay are 0.6 or more. This bulk evidence again supports the pedogenic production of montmorillonite from vermiculite within the X fraction, but little production of vermiculite within the X fraction from illite. If montmorillonite relates to the pedogenic indicator clay, montmorillonite should be the clay-mineral indicator of pedogenesis. Does the montmorillonite relate to the chemistry of these soils? Multiple regression was applied to the depth distributions sample horizon by sample horizon in the form: M ( % of X ) = a. + a,Ca
+ a,Mg
f
a,BS + a,pH
where extractable Ca and Mg are me/100g, base saturation ( B S ) is Z, and pH is the 1:l soil-water value. In 80% o f the profiles the R7 value is 0.7 or more and extractable Mg dominates the overall relation (Fig. 6). In 60% of the profiles the r value of Mg to M is 0.6 or more. The relations of Ca, BS, and pH to M are generally poor and similar. In 30 to 40% of the profiles the r values are 0.6 or more. Magnesium is a dominant component in vermiculite and is less abundant in montmorillonite (Grim, 1968). The magnesium cation occupies the interlayer region in the vermiculite structure (Walker, 1961). In these thin loess-derived soils as vermiculite alters to montmorillonite, the interlayer Mg may be made available for exchange (extraction) so that the Mg relates to the altered product , mon tmori 1 1 oni te . In summary o f weathering in these soils in thin loess, the main clay-mineral alteration is vermiculite to montmorillonite with little evidence for weathering of illite or kaolinite. The main alteration is in one mineral group termed expandabZe minern2s ( X ) . Consequently, the K, I , and X components in the thin loess are primarily sedimentologic features and can be used to identify loess sources. Although vermiculite is altered to montmorillonite, specific amounts of these species vary from profile to profile and by groups of profiles from area to area. When properly grouped these minerals also relate to different sources. CLAY MINERALS AND LOESS SOURCES In the profiles in thin-loess weighted means of K, I , and X were calculated for the silty loess, the lower sandy loess, and the substrata paleosols in the till and bedrock terrains. As previously noted in the Hosmer Silt Loam, the clay-mineral compositions of the sandy loess and paleosols are very similar, and this generalization applies to all thin-loess profiles. The weighted means change up-valley and broadly relate to the distribution of bedrock in the Ohio
100 River drainage basin.
This r e l a t i o n i s most d i s t i n c t near t h e c r e s t of the
Cincinnati a r c h in southeastern Indiana, southwestern Ohio a n d a dja c e nt p a r t s of Kentucky ( F i g . 1 ) .
I n t h a t area t h e clay-mineral s u i t e s in the sandy loe ss
average 8% K, 14 t o 17% I , and 62 t o 75% X, and the average in the subjacent paleosols a r e 3 t o 1 1 % K , 14 t o 22% I , and 65 t o 73% X.
These r e l a t i o n s furthe r
s u b s t a n t i a t e t h e p r i o r conclusion (Ruhe a n d Olson, 1980b) of the riiixing of overlying Wisconsin l o e s s .
I n t h e lower Ohio River drainage basin above a n d below the m o u t h of the Wabash River i n Indiana a n d Kentucky ( F i g . l ) , t h e K , I , a n d X compositions a r e more haphazard among t h e t h r e e s t r a t i g r a p h i c u n i t s .
The Wabash River drainage basin
i s dominated by g l a c i a l d r i f t , a n d l o e s s i a l sediment derived from t h e d r i f t has
a r e l a t e d clay-mineral composition t h a t d i f f e r s from t h e upper Ohio basin loess (Ruhe and Olson, 1980b). Weighted means of t h e s i l t y l o es s i n t h i n and t h i c k loe ss show d i s t i n c t groups when p l o t t e d on a t r i a n g u l a r composition g r a p h (Fig. 7 ) . I n the upper Ohio basin p r o f i l e s from Indiana, Kentucky, and Ohio have 13 t o 25% K, 19 t o 38% I , and 35 t o 64% X . I n t h e lower p a r t of t h e basin under Wabash Valley in f lu e n c e , p r o f i l e s from Indiana and Kentucky have 10 t o 22% K , 42 t o 62% I , and 27 t o 45% X .
I n a new a n d cu r r en t research p r o j e c t , t h e loe ss of the lower
-K
Fig. 7 . Clay-mineral compositions of s i l t y l o e s s where K i s k a o l i n i t e , I i s i l l i t e , and X i s expandable c l a y minerals. Each point i s the weighted mean comp o s i tio n of one p r o f i l e . Grab samples ( x ) a r e from Iowa and Nebraska.
101
Mississippi River region is under study. Below the mouth of the Ohio River in western Kentucky and southward in western Tennessee and Mississippi the loess contains 4 to 13% K, 20 to 45% I , and 46 to 76% X . This lower valley loess has a compositional trend toward loess grab samples from Iowa and Nebraska where K i s 3 to 8%, I is 1 1 to 25X, and X is 71 to 87%. Vermiculite-montmorillonite relations are more distinct (Fig. 8). In the upper Ohio River basin vermiculite, 21 to 52%, dominates montmorillonite, 2 to 29%. In the lower part o f the basin under Wabash Valley influence, composition shifts toward montmorillonite, 8 to 34%, and vermiculite, 6 to 24%. In the lower Mississippi Valley region montmorillonite, 34 to 75%, dominates vermiculite 2 to 14%, and the trend i s again toward the western grab samples where montmorillonite is 65 to 81%,and vermiculite is 5 to 12%. CONCLUSIONS The effects of pedogenic weathering of clay minerals in thin Wisconsin loess in the Ohio River basin are not severe enough to negate use of the clay minerals as sedimentologic indicators of loess sources. The clay-mineral suites are excellent indicators of sources in the upper and lower Ohio River basin and in the lower Mississippi valley region.
Upper Ohio
0
I
00
V 20-
O
Lower M ississippi
A A A
10-
0
Lower Ohio :
70 0
0
X
o
A
A
a
0
o
0
o
o
x
0
0 oo I
I
I
I
x
;
o o o
oooo I
* x
1
I
I
Fig. 8. Vermiculite ( V ) and montmorillonite ( M ) compositions of silty loess. Each point i s the weighted mean composition of one profile. Grab samples (x) are from Iowa and Nebraska.
102 ACKNOWLEDGMENT T h i s s t u d y was s u p p o r t e d i n p a r t u n d e r U. S . N a t i o n a l S c i e n c e F o u n d a t i o n G r a n t s EAR-79-1 121 6 and EAR-81 -1 3071 . REFERENCES
B r i n d l e y , G . N . , 1980. Q u a n t i t a t i v e X - r a y m i n e r a l a n a l y s i s o f c l a y s . I n : G. W . B r i n d l e y and G. Brown ( E d i t o r s ) , C r y s t a l S t r u c t u r e s o f C l a y M i n e r a l s and T h e i r X - r a y I d e n t i f i c a t i o n . M i n e r a l o g i c a l S o c i e t y London Monograph 5, pp. 411-438. Douglas, L . A . , 1977. V e r m i c u l i t e s . I n : J . B. D i x o n and J . B. !deed ( E d i t o r s ) , M i n e r a l s i n S o i l E n v i r o n m e n t s . S o i l S c i e n c e S o c i e t y A m e r i c a , Madison, Wisc o n s i n , pp. 259-292. G r i m , R. E . , 1968. C l a y M i n e r a l o g y . M c G r a w - H i l l , I n c . , New Y o r k , 596 p p . Grossman, R. 6 . and C a r l i s l e , F. G . , 1969. F r a g i p a n s o i l s o f t h e e a s t e r n U n i t e d S t a t e s . Advances i n Agronomy 21: 237-279. J a c k s o n , J . L . , 1964. Chemical c o m p o s i t i o n o f s o i l s . I n : F. E. B e a r ( E d i t o r ) , C h e m i s t r y of t h e S o i l . Van N o s t r a n d R e i n h o l d Co., New Y o r k , pp. 71-141. Ruhe, R. V. and Matney, E. A . , 1980. C l a y M i n e r a l o g y o f S e l e c t e d Sediments and S o i l s a t t h e Savannah R i v e r P l a n t , A i k e n , S o u t h C a r o l i n a . Savannah R i v e r L a b o r a t o r y DP-MS-80-119, T e c h n i c a l I n f o r m a t i o n S e r v i c e , Oak R i d g e , Tennessee, 48 PP. Ruhe, R. V . and O l s o n , C . G . , 1979. E s t i m a t e o f c l a y - m i n e r a l c o n t e n t : A d d i t i o n s of p r o p o r t i o n s o f s o i l c l a y t o c o n s t a n t s t a n d a r d . C l a y s and C l a y M i n e r a l s 27: 322-326. Ruhe, R . V . and O l s o n , C . G., 1980a. S o i l W e l d i n g . S o i l S c i e n c e 130: 132-139. Ruhe, R . V . and O l s o n , C . G., 1980b. C l a y - m i n e r a l i n d i c a t o r s o f g l a c i a l and Geon o n g l a c i a l sources o f Wisconsinan loesses i n southern Indiana, U.S.A. derma 2 4 : 283-297. S o i l S u r v e y S t a f f , 1972. S o i l S u r v e y L a b o r a t o r y Methods and P r o c e d u r e s f o r C o l l e c t i n g S o i l Samples. U. S. D e p t . A g r i c u l t u r e , S o i l C o n s e r v a t i o n S e r v i c e , S o i l S u r v e y I n v e s t i g a t i o n s R e p t . 1 , U. S. G o v t . P r i n t i n g O f f i c e , Washington, 0 . C . , 63 pp. S o i l S u r v e y S t a f f , 1975. S o i l Taxonomy--A B a s i c System o f S o i l C l a s s i f i c a t i o n f o r M a k i n g and I n t e r p r e t i n g S o i l S u r v e y s . U. S . D e p t . A g r i c u l t u r e Handbook 436, U. S. G o v t . P r i n t i n g O f f i c e , Washington, D . C . , 754 pp. W a l k e r , G. F . , 1961. V e r m i c u l i t e m i n e r a l s . I n : G . Brown ( E d i t o r ) , The X - r a y I d e n t i f i c a t i o n and C r y s t a l S t r u c t u r e s o f C l a y M i n e r a l s . M i n e r a l o g i c a l S o c i e t y London, p p . 279-324.
103
SALTATION THRESHOLD AND DEPOSITION RATE MODELING JAMES D. IVERSEN Department o f Aerospace Engineering; Iowa S t a t e U n i v e r s i t y ; Awes. Iowa
L I S T OF SYMBOLS B a g n o l d ' s t h r e s h o l d c o e f f i c i e n t , A = uAt (p/ppgDp) 1/2
A A1
C o e f f i c i e n t (Eq. 18) Drift c r o s s - s e c t i o n area
AC
Ad
DP e
D r i f t p l a n area P a r t i c l e diameter Coefficient o f restitution
9 h
Gravitational acceleration
k
Von Karman's c o n s t a n t
K
C o e f f i c i e n t (Eq. 18)
Reference o r c h a r a c t e r i s t i c h e i g h t
L
Reference o r c h a r a c t e r i s t i c l o n g i t u d i n a l dimension ( l e n g t h o r w i d t h )
Lm
Model v a l u e o f L
L
S t a b i l i t y l e n g t h (see I v e r s e n e t a l . , 1973)
a
Any l o n g i t u d i n a l dimension
P
Pressure
q
Mass t r a n s p o r t p e r u n i t t r a n s v e r s e w i d t h p e r u n i t t i m e Mass d e f l a t e d o r d e p o s i t e d p e r u n i t t r a n s v e r s e w i d t h p e r u n i t t i m e
'd R
Correlation coefficient T h r e s h o l d Reynolds number, u , ~ D ~ / v
R*t t Time U
Wind speed
u*
F r i c t i o n speed i n s a l t a t i o n
u*O
F r i c t i o n speed (square r o o t o f t h e r a t i o o f s u r f a c e shear s t r e s s t o a i r density)
T h r e s h o l d f r i c t i o n speed U*t P a r t i c l e t e r m i n a l speed UF D e p o s i t e d o r d e f 1a t e d v o l ume V V e r t i c a l h e i g h t above s u r f a c e
Z
S u r f a c e roughness h e i g h t (see Eq. 1)
zO
'om Z'
0
Model v a l u e o f zo S u r f a c e roughness h e i g h t i n s a l t a t i o n
q
Any v e r t i c a l dimension
v
Kinematic a i r v i s c o s i t y
p
A i r density
pp
P a r t i c l e density.
INTRODUCTION The e n v i r o n m e n t a l w i n d t u n n e l has p r o v e n a v a l u a b l e t o o l f o r i n v e s t i g a t i n g t h e b a s i c p h y s i c s o f a e o l i a n processes and modeling some o f t h e s e processes a t small scale.
T h i s paper r e v i e w s t h e methods and r e q u i r e m e n t s f o r s m a l l s c a l e
modeling and a l s o t h e s a l t a t i o n t h r e s h o l d experiments w i t h which t h e a u t h o r has been i n v o l v e d . Several w i n d t u n n e l s have been c o n s t r u c t e d and used f o r t h e purpose o f research i n t o t h e b a s i c p h y s i c s o f b l o w i n g sand and s o i l . a r e t h o s e o f Bagnold (1941),
C h e p i l (1945),
has been documented by B e l l y (1964), a l . (1982), Greeley e t a l . (1981), (1981), White and Schulz (1977),
Most n o t a b l e o f t h e e a r l y e f f o r t s
and Z i n g g (1953).
More r e c e n t work
F r y b e r g e r and Schenk (1964), G i l l e t t e e t
L y l e s and Krauss (1971),
and W i l l i a m s (1964).
N i c k l i n g and Ecclestone
T h i s volume i n c l u d e s work
by G e r e t y and S l i n g e r l a n d (1982), G r e e l e y (1982), Tsoar (1982) and Walker and Southard (1982). SIMILITUDE PARAMETERS The s t r u c t u r e o f t h e t u r b u l e n t p l a n e t a r y boundary l a y e r has been s t u d i e d extensively.
Jensen (1958) and o t h e r s have observed t h a t t h e l o w e r p o r t i o n o f
t h e p l a n e t a r y boundary l a y e r ( i n a n e u t r a l l y s t r a t i f i e d atmosphere) f o l l o w s t h e l o g a r i t h m i c law
as l o n g as t h e v a l u e o f t h e roughness Reynolds number 30
Z ~ U * ~ / Vi
s 70 o r g r e a t e r .
The parameter u* i s t h e f r i c t i o n speed (square r o o t o f shear s t r e s s d i v i d e d by a i r d e n s i t y ) , and k i s von Karman's c o n s t a n t ( k E 0.4).
Jensen showed t h a t f o r
s i m u l a t i o n o f t h e atmospheric boundary l a y e r , t h e roughness parameter i n t h e model s h o u l d be t h e same as t h a t i n t h e atmosphere, i . e . , zO
m - Lm
zo
i n which L = a c h a r a c t e r i s t i c length.
Most i n v e s t i g a t o r s have r e l i e d on a l o n g
w i n d - t u n n e l t e s t s e c t i o n t o s i m u l a t e t h e t u r b u l e n t boundary l a y e r p r o f i l e as a n a t u r a l l y t h i c k , f u l l y t u r b u l e n t boundary l a y e r . I f s i m p l e dimensional a n a l y s i s i s used t o group t h e i m p o r t a n t v a r i a b l e s , t h e
f o l l o w i n g l i s t o f s i m i l i t u d e parameters, u s e f u l i n t h e d e s c r i p t i o n o f s a l t a t i o n phenomena, can be w r i t t e n :
-
(1) Dp/L
P a r t i c l e diameter
(2)
u(h)/Uf
Reference t o p a r t i c l e t e r m i n a l speed r a t i o
(3)
[u(h)]*/gL
Froude number
length r a t i o
105 (4)
e
Coefficient o f r e s t i t u t i o n
(5)
2/L, q/h
Topographic geometric s i m i l a r i t y
(6)
zo/L
Roughness s i m i l i t u d e
(7)
h/L
Reference h e i g h t r a t i o
(8)
z/L*
S t a b i l i t y parameter
(9)
UF/U*t
Particle property similitude
(10)
P a r t i c l e f r i c t i o n Reynold's number
U*tDp/V
(11) u(h) L/u
Flow Reynold's number
(12)
u*/u*t
F r i c t i o n speed r a t i o
(13)
p/pP
Density r a t i o
(14) u(h)t/L Time s c a l e Ifa t r u e model i s t o be a t t a i n e d , i t i s necessary t h a t a l l o f t h e dimensionless parameters i n t h e above l i s t have t h e same value i n t h e small-scale model as they do f o r t h e f u l l - s c a l e phenomenon t o be modeled.
However, i t i s impossible
t o accomplish t h i s , as can be i l l u s t r a t e d simply by c o n s i d e r i n g t h e f i r s t and t h i r d parameters.
For a small value o f c h a r a c t e r i s t i c l e n g t h L , t h e p a r t i c l e
diameter D would have t o be s o small f o r t h e f i r s t parameter t h a t a l l p a r t i c l e s P would become suspended by t u r b u l e n c e upon becoming airborne. For parameter 3 t o be s a t i s f i e d , t h e wind speed u(h) would be below t h r e s h o l d speed and no p a r t i c l e motion would take place.
The author has attempted t o overcome t h i s problem by
varying t h e values o f v a r i a b l e s as much as p o s s i b l e by changing p a r t i c l e density, p a r t i c l e diameter, wind speed, and model s c a l e as much as p r a c t i c a b l e , and by grouping t h e dimensionless parameters by t h e o r e t i c a l means i n order t o reduce the number o f v a r i a b l e s .
TRANSPORT RATE SIMILITUDE The Mariner 9 s p a c e c r a f t t h a t began o r b i t i n g t h e p l a n e t Mars i n 1971 revealed the presence o f many c r a t e r s on t h e surface which possess a e o l i a n streaks extending i n t h e leeward d i r e c t i o n .
Many o f these streaks a r e probably caused by d e f l a t i o n
o f small p a r t i c l e s r e s u l t i n g from increased shear s t r e s s i n t h e c r a t e r wake.
Experimental c o r r e l a t i o n o f gross e r o s i o n a l and d e p o s i t i o n a l f e a t u r e s near model c r a t e r s i n a wind tunnel was obtained ( I v e r s e n e t a l . , 1975) by basing a s i m i l i t u d e on r a t e o f mass movement.
The t r a n s p o r t r a t e s i m i l i t u d e i s based on
the t h e o r e t i c a l p a r t i c l e mass t r a n s p o r t r a t e .
Bagnold (1941) d e r i v e d t h e expres-
s i o n f o r mass t r a n s p o r t r a t e f o r sand i n s a l t a t i o n as
q
3 - Pu*/g
(3)
Mass o f sand p e r u n i t l a t e r a l dimension p e r u n i t time = q; p = t h e a i r d e n s i t y ; and u* = t h e surface f r i c t i o n speed i n s a l t a t i o n .
A m o d i f i c a t i o n o f t h i s equation, which worked w e l l f o r a systematic s e r i e s o f raised-rim c r a t e r streak simulations, i s
106
The r a t e a t which a h o r i z o n t a l area i s covered o r d e f l a t e d by a d r i f t i n g m a t e r i a l can be expressed as
S i m i l a r l y , t h e volume r a t e i s
and t h e cross-sectional
area r a t e (perpendicular t o wind d i r e c t i o n ) i s
dAc - qd dt Pp
(7)
where qd i s t h e r a t e o f d e p o s i t i o n o r d e f l a t i o n . I f these equations are combined w i t h t h e equation f o r mass r a t e o f movement,
Eq. (4), the f o l l o w i n g equations r e s u l t :
This equation provides a b a s i s f o r a n a l y s i s o f s a l t a t i o n s i m u l a t i o n by combining forms o f s i x s i m i l i t u d e parameters p r e v i o u s l y l i s t e d (parameters 3, 5, 12, 13 and 14). EFFECT OF FROUDE NUMBER AND DENSITY R A T I O The e q u i v a l e n t roughness h e i g h t i n s a l t a t i o n , z,; mined.
has n o t g e n e r a l l y been deter2 However, Owen (1964) shows t h a t i t i s p r o p o r t i o n a l t o u,/g, i.e.,
I f t h e roughness h e i g h t - c h a r a c t e r i s t i c l e n g t h r a t i o zb/h i s important, then t h e
Froude number u:/gh
i s a l s o i m p o r t a n t , according t o Eq. ( 9 ) .
Consider a d e p o s i t i o n s i t u a t i o n i n which d e p o s i t i o n i s governed by a topographic f e a t u r e o f h e i g h t h and l a t e r a l w i d t h f a c i n g escarpment).
L (such as a r o c k o r leeward- o r windward-
The v o l u m e t r i c r a t e a t which m a t e r i a l i s deposited should
be p r o p o r t i o n a l t o t h e p r o d u c t of wind speed ( o r f r i c t i o n speed), v o l u m e t r i c c o n c e n t r a t i o n and f r o n t a l area.
The maximum mass c o n c e n t r a t i o n p o s s i b l e i n s a l t a -
t i o n i s u s u a l l y considered t o be t h e f l u i d d e n s i t y p, so t h a t t h e v o l u m e t r i c c o n c e n t r a t i o n i s p/p
P'
Thus,
107 dV/dt
- (p/pp)hLu,
(10)
and t h e mass d e p o s i t i o n r a t e would be qdL = p d V / d t P
- phLu,
(11)
From Eq. ( 3 ) , t h e r a t i o o f d e p o s i t i o n r a t e t o t o t a l mass f l o w r a t e i s thus qd/q
- phLu,/(pu,L/g)3
= gh/u,
2
or
so t h a t a dimensionless d e p o s i t i o n r a t e would be expected t o decrease w i t h increase
i n Froude number. Experiments w i t h d i f f e r e n t values o f p a r t i c l e d e n s i t y have shown t h e d e p o s i t i o n r a t e t o be a f u n c t i o n o f d e n s i t y r a t i o as w e l l (Iversen, 1980a).
I f the p a r t i c l e
concentration w i t h i n t h e d e p o s i t i o n r e g i o n i s volume l i m i t e d r a t h e r than mass l i m i t e d , then t h e d e p o s i t i o n r a t e becomes
A s e r i e s o f wind tunnel experiments was performed i n a manner s i m i l a r t o the model c r a t e r experiments t o determine t h e s n o w - d r i f t d e p o s i t i o n r a t e associated w i t h an i n t e r s t a t e highway grade separation s t r u c t u r e (Iversen, 1980a).
Values
o f f r i c t i o n speed were estimated from t h e wind speed measurements u s i n g Owen’s
(1964) equation f o r v e l o c i t y p r o f i l e .
The d a t a f o r mass d e p o s i t i o n r a t e from
ten experiments were c u r v e - f i t by l i n e a r r e g r e s s i o n t o compare Eqs. (12) and
(13).
The r e s u l t s a r e as f o l l o w s : -0.49
qdg/pu2 = 1.4(ui/gh)
,
R = 0.74
(14)
It i s c l e a r t h a t t h e c o r r e l a t i o n ( R = c o r r e l a t i o n c o e f f i c i e n t ) w i t h densimetric
Froude number (Eq. 15) i s b e t t e r than w i t h t h e o r d i n a r y Froude number (Eq. 14). However, t h e dimensionless depos t i o n r a t e i s n o t as s t r o n g a f u n c t i o n o f Froude number o r densimetric Froude number as p r e d i c t e d by Eqs. (12) o r (13).
By
m u l t i p l e regression, t r e a t i n g t h e Froude number and d e n s i t y r a t i o separately, the b e s t f i t was found t o be qdg/pu,(u,2
-
u,~)
= 0.0696(p/pp)-2’5(u$/gh)
-7/9
,
R = 0.97
108 The f o r m o f t h e b e s t f i t t o any s e t o f d a t a i s undoubtedly a s t r o n g f u n c t i o n o f t h e geometry o f t h e boundary l a y e r o b s t r u c t i o n .
S i n c e o n l y a v e r y few e x p e r i -
ments o f t h i s t y p e have y e t been performed, much work remains t o e s t a b l i s h t h e mass t r a n s p o r t s i m i l i t u d e on a f i r m e r f o u n d a t i o n . t h e d a t a as shown i n F i g . 1.
E q u a t i o n (16) i s used i n p l o t t i n g
F u l l - s c a l e v a l u e s o f t h e d e n s i t y r a t i o - F r o u d e number
parameter a r e approached a t t h e l e f t s i d e o f t h e f i g u r e .
A
I0
I
I
0.01
0.02
3.99 - MODEL 2
I 0.03
DENSITY RATIO-FROUOE NUMBER PARAMETER ( p/pP)’I5(
u:/~h)~/’
D e p o s i t i o n r a t e versus d e n s i t y r a t i o - F r o u d e number parameter.
THE SIMILITUDE FUNCTION The mass t r a n s p o r t f u n c t i o n , Eq. (8), p r o v i d e s a bas s f o r a n a l y s i s o f snowdrifting simulation.
Thus, c o n s i d e r i n g parameters 3, 5, 6, 7 , 9
11, 12, 13, 1 4 and Eq.
(a),
t h e s i m i l i t u d e problem reduces t o
E q u a t i o n (17) a l l o w s f o r t h e p o s s i b i l i t y o f v e r t i c a l g e o m e t r i c d i s t o r t i o n and uses J e n s e n ’ s c r i t e r i o n f o r normal roughness modeling.
It does n o t a l l o w f o r
t h e s a t i s f a c t i o n o f many o f t h e o r i g i n a l d i m e n s i o n l e s s terms. model i s s t i l l h i g h l y d i s t o r t e d .
Thus, t h e s a l t a t i o n
By v a r y i n g p a r t i c l e s i z e and d e n s i t y , model
speed, s c a l e , and p o s s i b l y v e r t i c a l d i s t o r t i o n , a range i n degree o f d i s t o r t i o n can r e s u l t so t h a t f u l l - s c a l e p r e d i c t i o n s can be made.
The e f f e c t s o f d i s t o r t i o n
become p a r t i c u l a r l y s e r i o u s , however, i f s m a l l - s c a l e f e a t u r e s such as s u r f a c e r i p p l e s become l a r g e enough t o obscure o r i n t e r f e r e w i t h g r o s s d r i f t geometry
109 a s s o c i a t e d w i t h i m p o r t a n t t o p o g r a p h i c f e a t u r e s o f t h e model.
Examples o f model
r e s u l t s and f u r t h e r d i s c u s s i o n o f t h e - s i m i l i t u d e parameters may be found i n Greeley
e t a l . (1974),
I v e r s e n (1979,
and Jensen (1981),
1980a,b, 1981), I v e r s e n and G r e e l e y (1978),
Iversen
I v e r s e n e t a l . (1973, 1975, 1976a).
WIND TUNNEL THRESHOLD EXPERIMENTS Bagnold (1941) d e r i v e d an e m p i r i c a l c u r v e f r o m t h e r e s u l t s o f h i s w i n d t u n n e l
1/2
experiments f o r t h e d i m e n s i o n l e s s t h r e s h o l d f r i c t i o n speed A = u,t(p/ppgDp)
= u D /V by d e t e r m i n i n g t h e f r i c t i o n *t P speed a t t h r e s h o l d f o r sand p a r t i c l e s o f v a r i o u s average diameters.
as a f u n c t i o n o f f r i c t i o n Reynolds number Rkt
Succeeding experiments f o r p a r t i c l e s o f d i f f e r e n t d e n s i t y ( I v e r s e n e t a l . , 1976b,c) suggested t h a t t h e t h r e s h o l d c o e f f i c i e n t A i s a f u n c t i o n o f p a r t i c l e diameter independent of Reynolds number Rxt. e t al.,
The MARSWIT w i n d t u n n e l (Greeley
1976, 1980, 1981) was b u i l t f o r t h e purpose o f s t u d y i n g p a r t i c l e m o t i o n
a t atmospheric p r e s s u r e f r o m one atmosphere down t o 5 mb, a p p r o p r i a t e f o r Mars. A t y p i c a l s e t o f d a t a i s shown i n F i g . 2, where t h e t h r e s h o l d parameter A i s p l o t t e d as a f u n c t i o n o f p a r t i c l e f r i c t i o n Reynolds number.
These d a t a i l l u s t r a t e
0
.80
0
-
7
I
0
.70
RUN - 4-12-78.2 GAS - A I R
?
X
v
PART - D I A .
-
153.75 urn
2185 5 p p / p
5
370,100
4
CL
2 W
1.60
32 0
d
1.50 580 Pa
t Ln w
pp
=
2650 kg/m3
5
P
5
2
101300 Pa
EQUATION 19
1.40
0
1.30 1x l o-2
I
I
0.03
1
I
0.05
I
I
I
I
1x10-’
I
1
PARTICLE F R I C T I O N REYNOLDS NUMBER R, F i g . 2.
I
0.3
I
0.5
I
I
I
I
1 .o
t
T h r e s h o l d parameter A as a f u n c t i o n of p a r t i c l e Reynolds number R,,t
from
I v e r s e n and White (1982). The d a t a (MARSWIT, Greeley e t a l . , 1980) a r e f o r a c o n s t a n t v a l u e o f p a r t i c l e d i a m e t e r and d e n s i t y (154 pm sand) b u t w i t h d i f f e r i n g v a l u e s o f f l u i d d e n s i t y . The c u r v e i s Eq. 19.
110 t h e i s o l a t e d e f f e c t of Reynolds number, s i n c e t h e y a r e p l o t t e d f o r a s i n g l e v a l u e o f p a r t i c l e d i a m e t e r (154 pm).
The experiments were r u n by measuring t h r e s h o l d
As t h e p r e s s u r e i s i n c r e a s e d ,
speed f o r s t e p w i s e i n c r e a s e s i n ambient p r e s s u r e . t h e t h r e s h o l d Reynolds number Rkt The i n c r e a s e i n Rkt
i n c r e a s e s , c a u s i n g c o r r e s p o n d i n g changes i n A.
i s due t o a g r e a t e r r e d u c t i o n i n v i s c o s i t y
11
than i n threshold
f r i c t i o n speed ukt as ambient p r e s s u r e i n c r e a s e s . The t h r e s h o l d speed d a t a were a n a l y z e d i n t h e f o l l o w i n g manner ( I v e r s e n and White, 1 9 8 2 ) .
Curves were p l o t t e d , as i n F i g . 2, f o r each o f 2 1 t e s t r u n s (8
p a r t i c l e diameters:
37 t o 673 pm; two f l u i d s :
p a r t i c l e densities:
1100 and 2650 kg/m3).
a i r and carbon d i o x i d e ; and 2
I t was f i r s t assumed t h a t t h e f o r m
of t h e t h r e s h o l d e q u a t i o n i s
1/2
w h i c h assumes t h a t t h e i n t e r p a r t i c l e f o r c e I t o t h e exponent 3-n.
i s p r o p o r t i o n a l t o p a r t i c l e diameter P Values of A were i n t e r p o l a t e d f r o m t h e e x p e r i m e n t a l curves
f o r c o n s t a n t v a l u e s o f Reynolds number RAt be c o n s t a n t .
f o r which t h e p r o d u c t A1 f ( R k t )
F i g u r e 3 i l l u s t r a t e s t h e s e c r o s s - p l o t s f o r 5 v a l u e s o f Rkt.
t h e t h r e s h o l d parameter A were a f u n c t i o n of Rkt
would If
alone, as some i n v e s t i g a t o r s
have assumed, t h e n t h e c u r v e s i n F i g . 3 would have t o be h o r i z o n t a l . The r a p i d 2 i n c r e a s e i n A as d i a m e t e r decreases below 80 pm demonstrates t h e p o w e r f u l e f f e c t o f cohesive forces f o r small p a r t i c l e s . t h e n f o r c o n s t a n t Rxt, becomes l a r g e .
I f t h e exponent n i n Eq. 18 i s p o s i t i v e ,
A s h o u l d approach an a s y m p o t o t i c l i m i t as p a r t i c l e diameter
T h i s seems t o be t h e case.
L i n e a r r e g r e s s i o n a n a l y s i s was used
t o determine b e s t f i t s f o r t h e c o e f f i c i e n t K and exponent n.
= 0.006 g cm0 ' 5 s - ~ and n = 2 . 5 .
The r e s u l t s a r e K
The s e m i - e m p i r i c a l e q u a t i o n s o b t a i n e d f r o m t h e
e n t i r e s e t o f data are
2 ' 5 ) 1'2/(1 + 2 . 5 RAt) 1/2 f o r 0.03 5 Rt,
A = 0.129
5 0.3
(1 + O.O06/p gD 2/5) 'I2/ ( 1 . 9 2 8 RAt
f o r 0 . 3 5 Rkt
P
-
1) 1 / 2
P
6 10
2. f o r 1 0 6 RAt
0.092
')
"$1
-
-0.0617(R
0.0858e
I
For p a r t i c l e s between 50 and 600 pn i n s i z e , Eq. 19 g i v e s e s s e n t i a l l y t h e Same r e s u l t as t h o s e o f I v e r s e n e t a l . ( 1 9 7 6 ~ ) .
111 CONCLUSIONS
S t a r t i n g w i t h Bagnold's c l a s s i c work, t h e environmental wind tunnel has proven essential f o r a i d i n understanding a e o l i a n phenomena.
Wind tunnel t e s t i n g o f
snow o r sand d r i f t i n g phenomena has been q u i t e u s e f u l i n the study o f basic physics I f q u a n t i t a t i v e r e s u l t s are desired
and the design o f d r i f t - c o n t r o l measures.
f o r small-scale models i n order t o make f u l l - s c a l e p r e d i c t i o n s , o r i f q u a l i t a t i v e r e s u l t s are t o be r e p r e s e n t a t i v e i n d e t a i l , then c l o s e a t t e n t i o n must be p a i d t o
0.:
EQ. 19 1 2
3
4 5
N
a
", 0.2
GAS -
P.iR
c02 c02 c02
R"t
~~(kg/m~)
1100
0.05
1 1 00
0.2
1 100
0.3
11 00
0.5 2.0
2650
L
x
2 4 a
n
1
L 0 W co
p:
I k-
0.1
0.c
I
30
I
1
50
1
1
1
80 100
200
300
PARTICLE DIAMETER ( S V ) Fig. 3.
Threshold parameter A2 as a f u n c t i o n o f p a r t i c l e diameter D
from Iverson P' and White (1982). The data (MARSWIT, Greeley e t a l . , 1980) show c l e a r l y t h a t A 2 i s n o t a f u n c t i o n o f Reynolds number alone. The curves a r e Eq. 19
the p r i n c i p l e s o f s i m i l i t u d e as o u t l i n e d i n t h i s paper.
The complexity o f t h e
s i m i l i t u d e problem f o r a e o l i a n processes creates t h e necessity f o r d i s t o r t e d models.
The use o f t h e t r a n s p o r t r a t e s i m i l i t u d e i s necessary t o determine t h e
e f f e c t s o f d i s t o r t i o n , and thus make f u l l - s c a l e e x t r a p o l a t i o n s from model measurements. Determination o f p a r t i c l e t h r e s h o l d speeds i s one example o f t h e basic type o f physics experiment which has proven u s e f u l .
Threshold experiments a t low a i r
d e n s i t y have given new i n s i g h t i n t o t h e separate r o l e s o f Reynolds number and
112 small p a r t i c l e cohesion on t h e v a l u e s o f t h r e s h o l d speeds.
Much p r o g r e s s has
been made f r o m w i n d t u n n e l t e s t i n g b o t h i n b a s i c p h y s i c s and i n s m a l l - s c a l e modeli n g , b u t t h e r e i s s t i l l much more t o be l e a r n e d i n b o t h areas. REFERENCES Bagnold, R. A.
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E a r t h and
I A S Congress, H a m i l t o n , O n t a r i o .
Greeley, R . ,
J . D. I v e r s e n , J. B. P o l l a c k , N. Udovich, and B. R. White.
Wind t u n n e l s t u d i e s of m a r t i a n e o l i a n processes.
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B. R. White, J. B. P o l l a c k , J. 0. I v e r s e n , and R. Leach.
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C o n s i d e r a t i o n s and s i m u l a t i o n s i n d e s e r t dust:
c h a r a c t e r i s t i c s , and e f f e c t on Mars.
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G e o l o g i c a l S o c i e t y o f America, S p e c i a l
Paper 186, 101-121. I v e r s e n , J. 0.
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D r i f t i n g snow s i m i l i t u d e .
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Proceedings, American S o c i e t y o f C i v i l E n g i n e e r i n g , 105, HY6, 737-753. I v e r s e n , J. D.
D r i f t i n g snow s i m i l i t u d e - - t r a n s p o r t r a t e and roughness
J o u r n a l o f G l a c i o l o g y , 26, 393-403.
modeling. I v e r s e n , J. 0. controls.
1980b.
Wind t u n n e l modeling o f snow fences and n a t u r a l s n o w d r i f t
3 7 t h E a s t e r n Snow Conference, Peterborough, O n t a r i o , 106-124.
I v e r s e n , J. D. drifts.
1980a.
1981.
Comparison o f w i n d t u n n e l model and f u l l - s c a l e snow fence
J o u r n a l o f Wind E n g i n e e r i n g and I n d u s t r i a l Aerodynamics, 8, 231-249.
113 Iversen, J. D. and R. Greeley.
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Atmospheric and w i n d t u n n e l experiments
o f t h e Amboy c r a t e r sand-covered l a v a f l o w .
Iowa S t a t e U n i v e r s i t y Report,
ISU-ERI-78235. Iversen, J. D. and V. Jensen.
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Wind t r a n s p o r t a t i o n o f d u s t f r o m c o a l p i l e s .
S k i b s t e k n i s k L a b o r a t o r i u m Lyngby, Denmark, SL r e p o r t 81054. Iversen, J. D. and B. R. White. Venus.
Sedimentology,
Iversen, J. D . ,
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S a l t a t i o n t h r e s h o l d on E a r t h , Mars, and
29, 111-119.
R. Greeley, J. B. P o l l a c k , and B. R. White.
1973.
o f m a r t i a n e o l i a n phenomena i n t h e atmospheric w i n d t u n n e l .
Simulation
Space S i m u l a t i o n ,
NASA Spec. P u b l . SP-36, 191-213. Iversen, J. D . ,
R. G r e e l e y , B. R. White, and J. B. P o l l a c k .
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R. Greeley, B. R. White, and J. B. P o l l a c k .
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Eolian erosion
I c a r u s , 26, 321-331. 1976a.
The e f f e c t
of v e r t i c a l d i s t o r t i o n i n t h e m o d e l i n g o f s e d i m e n t a t i o n phenomena: c r a t e r wake s t r e a k s . Iversen, J. D . ,
J. Geophys. Res., 81, 4846-4856.
J. B. P o l l a c k , R. Greeley, and B. R. White.
t h r e s h o l d on Mars:
Jensen, M.
1958.
Saltation
I c a r u s , 29, 381-393.
R. G r e e l e y , and J. B. P o l l a c k .
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1976b.
The e f f e c t o f i n t e r p a r t i c l e f o r c e , s u r f a c e roughness, and
low atmospheric d e n s i t y . Iversen, J. D . ,
Martian
1976c.
Windblown d u s t on E a r t h ,
J. Atmos. S c i . , 33, 2425-2429. The model-law f o r phenomena i n n a t u r a l wind.
I n g e n i d r e n , 2,
121-128. Lyles, L. and R. K. Krauss.
1971.
T h r e s h o l d v e l o c i t i e s and i n i t i a l p a r t i c l e
m o t i o n as i n f l u e n c e d by a i r t u r b u l e n c e .
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14, 563-566. N i c k l i n g , W. G. and M. E c c l e s t o n e .
1981.
t h r e s h o l d shear v e l o c i t y o f f i n e sand. Owen, P. R.
1964.
The e f f e c t s o f s o l u b l e s a l t s on t h e Sedimentology, 28, 505-510.
S a l t a t i o n o f uniform grains i n a i r .
Journal o f F l u i d
Mechanics, 20, P t . 2, 225-242. Tsoar,
H.
1982.
S i m u l a t i o n o f echo and c l i m b i n g dunes i n t h e w i n d t u n n e l .
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Congress, H a m i l t o n , O n t a r i o . Walker, J. D. and J. B. Southard.
1982.
Experimental s t u d y o f w i n d r i p p l e s .
I A S Congress, H a m i l t o n , O n t a r i o . White, 8. R. and J. C. Schulz.
1977.
Magnus e f f e c t on s a l t a t i o n .
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F l u i d Mechanics, 81, 497-512. Williams, G.
1964.
Some aspects o f t h e e o l i a n s a l t a t i o n l o a d .
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This Page Intentionally Left Blank
115
NATURE O F THE SALTATING POPULATION I N WIND TUNNEL EXPERIMENTS WITH HETEROGENEOUS SIZE-DENSITY SANDS KATHLEEN M. GERETY a n d RUDY SLINGERLAND, The P e n n s y l v a n i a S t a t e U n i v e r s i t y , Department o f Geosciences, U n i v e r s i t y P a r k , PA 16801 L i s t o f Symbols
A = p r o p o r t i o n a l i t y c o n s t a n t i n Bagnold's impact t h r e s h o l d e q u a t i o n and Greeley e t a l ' s . f i u i d t h r e s h o l d equation. C i = g r a i n c o n c e n t r a t i o n s o f each s i z e - d e n s i t y sub-population i n t h e s a l t a t i o n layer, i n weight percent D = g r a i n s i z e ( b a x i s ) , i n mm H i = mean h e i g h t o f jump o f e a c h s u b - p o p u l a t i o n , i n cm k, = r o u g h n e s s h e i g h t , i n cm Qs = v e r t i c a l l y i n t e g r a t e d mass f l u x o f sand, i n g/cm2-sec Uy = t i m e - a v e r a g e d f l u i d v e l o c i t y a t h e i g h t y o f f t h e bed, i n cm/sec U* = f r i c t i o n v e l o c i t y , i n cm/sec U*t= t h r e s h o l d f r i c t i o n v e l o c i t y i n cm/sec y = d i s t a n c e o f f t h e bed, i n cm p = f l u i d d e n s i t y i n gm/cm3 pb = mass c o n c e n t r a t i o n o f sand, i n gm/cm3 a = g r a i n d e n s i t y i n gm/cm3 INTRODUCTION An u l t i m a t e g o a l o f a e o l i a n s e d i m e n t t r a n s p o r t s t u d i e s i s t o p r e d i c t t h e s i z e d i s t r i b u t i o n s and d e n s i t i e s o f sand g r a i n s t h a t w i l l be e n t r a i n e d t o g e t h e r , transported together, and deposited t o g e t h e r i n atmospheric f l o w s o f v a r y i n g strengths.
N o t a b l e advances t o w a r d s t h i s e n d h a v e been made i n w i n d - t u n n e l
s t u d i e s b y B a g n o l d (1941,
1973), Z i n g g (1953),
C h e p i l (1945),
Greeley e t al.
(1974, t h i s volume), a n d I v e r s e n (1973, t h i s volume) b u t t o make t h e p r o b l e m t r a c t a b l e t h e s e s t u d i e s r e s o r t e d t o homogeneous s i z e - d e n s i t y sands. Our o b j e c t i v e h e r e i s t o d e t e r m i n e e x p e r i m e n t a l l y t h e s i z e d i s t r i b u t i o n s a n d mass f l u x e s o f t h r e e d i f f e r e n t d e n s i t y sands t r a v e l i n g t o g e t h e r i n s a l t a t i o n a t v a r y i n g h e i g h t s above a m o b i l e s a n d bed. force balances
The r e s u l t s g i v e i n f o r m a t i o n o n t h e
on g r a i n s u n d e r g o i n g s a l t a t i o n a n d hence p r o v i d e a t e s t o f t h e
v a r i o u s s a l t a t i o n models.
They a l s o p r o v i d e a means o f t e s t i n g a p r o c e s s
c l a s s i f i c a t i o n o f a e o l i a n b e d d i n g t y p e s ( H u n t e r , 1977).
Finally, they help
e x p l a i n t h e a p p a r e n t s e t t l i n g - v e l o c i t y r e l a t i o n s h i p s o f l i g h t a n d heavy m i n e r a l s seen i n a n c i e n t w i n d - l a i d sands. METHODS The i m p o r t a n t i n d e p e n d e n t v a r i a b l e s a r e f l u i d d e n s i t y ( p ) , (a),
grain size
(D), f r i c t i o n v e l o c i t y (U*),
grain density
and roughness h e i g h t (ks).
The
116 d e p e n d e n t v a r i a b l e s o f i n t e r e s t a r e t h e t o t a l mass f l u x o f s e d i m e n t ( O s ) , g r a i n c o n c e n t r a t i o n o f each s i z e - d e n s i t y s u b - p o p u l a t i o n i n t h e s a l t a t i o n l a y e r ( C i ) , t h e mean h e i g h t o f jump o f e a c h s u b - p o p u l a t i o n ( H i ) ,
and t h e s i z e and
d i s c h a r g e r a t i o s o f d i f f e r e n t - d e n s i t y g r a i n s i n t h e s a l t a t i n g p o p u l a t i o n (Dh/Dl; OSh/OS1). As p r e s e n t e d b y I v e r s e n ( t h i s v o l u m e ) , i t g e n e r a l l y i s n o t p o s s i b l e t o s a t i s f y a l l t h e dimensionless terms necessary t o s i m u l a t e t h e mechanics o f s a l t a t i o n i n a wind tunnel.
D i s t o r t i o n c a n b e m i n i m i z e d , however, b y
a p p r o p r i a t e c h o i c e s o f model s c a l e . These c o n s i d e r a t i o n s a n d o u r s p e c i f i c o b j e c t i v e s impose t h e f o l l o w i n g c o n d i t i o n s on t h e experimental design: 1.
A t l e a s t t h r e e d i f f e r e n t d e n s i t y g r a i n s m u s t b e a v a i l a b l e i n t h e same s i z e
r a n g e a n d w i t h t h e same mean s i z e . 2.
G r a i n d e n s i t y f o r each t y p e must be w e l l - d e f i n e d and v a r i a b l e w i t h i n v e r y
n a r r o w l i m i t s (e.g.,
tourmaline i s unacceptable).
3.
G r a i n shapes m u s t be s i m i l a r , a n d m u s t n o t b e b l a d e d o r p l a t y .
4.
The p o p u l a t i o n o f g r a i n s o f e a c h d e n s i t y m u s t b e a p p r o x i m a t e l y l o g n o r m a l l y
distributed, 5.
t o simulate natural conditions.
F l o w c o n d i t i o n s m u s t b e s p e c i f i e d b y t h e f r i c t i o n v e l o c i t y , U*,
average v e l o c i t y .
n o t by an
The r a n g e o f c o n d i t i o n s m u s t i n c l u d e a f r i c t i o n v e l o c i t y
c l o s e t o t h r e s h o l d and i n t h e r i p p l e and p l a n e bed f i e l d s , and t h e f l o w must c o n s i s t o f a f u l ly-developed t u r b u l e n t boundary l a y e r . 6.
The bed m u s t b e m o b i l e a n d r o u g h , w e l l - m i x e d ,
and sand must be f e d i n t o t h e
t u n n e l d u r i n g t r a n s p o r t measurements t o e n s u r e c o n t i n u e d a v a i l a b i l i t y o f a l l t h e s i z e - d e n s i t y s u b - p o p u l a t i o n s p r e s e n t i n t h e s t o c k sand. To s a t i s f y t h e s e r e q u i r e m e n t s , a s t o c k sand o f c r u s h e d q u a r t z , g a r n e t , and o l i v i n e was p r e p a r e d i n w h i c h t h e heavy m i n e r a l s h a d t h e same s i z e r a n g e a s t h e q u a r t z and c o m p a r a b l e means.
The i n d i v i d u a l d i s t r i b u t i o n s , shown i n
F i g u r e 1 a s f r e q u e n c y b y number a g a i n s t s i z e , a r e m o d e r a t e l y w e l l - s o r t e d ,
but
t h e q u a r t z a n d o l i v i n e d i s t r i b u t i o n s a r e p l a t y k u r t i c a n d q u a r t z i s skewed. T o t a l heavy m i n e r a l c o n t e n t i n t h e s t o c k s a n d was 4 2 p e r c e n t b y w e i g h t ( 3 3 p e r c e n t by volume), w i t h 2 3 w e i g h t p e r c e n t g a r n e t and 1 9 w e i g h t p e r c e n t o l i v i n e ( g a r n e t and o l i v i n e e a c h 16.6
p e r c e n t b y volume).
o f t h e g a r n e t a n d o f t h e o l i v i n e w e r e 4.3
Measured d e n s i t i e s
g/cm3 a n d 3.35 g/cm3,
respectively.
G r a i n s had i n t e r m e d i a t e s p h e r i c i t y , and were a n g u l a r t o subangular as e s t i m a t e d v i s u a l l y f r o m P o w e r ' s diagram. The s a n d was t r a n s p o r t e d i n a s u c t i o n - t y p e w i n d t u n n e l t e n m e t e r s l o n g w i t h a c r o s s - s e c t i o n 35 cm w i d e a n d 5 0 cm h i g h .
Adjustments t o an a i r i n l e t
i n t h e r o o f o f t h e t u n n e l c o n t r o l l e d t h e f r e e stream wind v e l o c i t y by changing t h e pressure drop over t h e tunnel length.
W i r e mesh i n s e r t s a t b o t h ends o f
t h e t u n n e l s t r a i g h t e n e d a i r t u r b u l e n c e a n d damped s e c o n d a r y c i r c u l a t i o n . C a l i b r a t i o n t e s t s i n d i c a t e d t h a t a t u r b u l e n t boundary l a y e r f u l l y developed t o
117 a t h i c k n e s s of 13 cm was p r e s e n t i n t h e t e s t s e c t i o n , 8 ni f r o m t h e t u n n e l e n t r a n c e , and v e l o c i t y c r o s s - p r o f i l e s were s y m m e t r i c a l a b o u t t h e t u n n e l centerline.
Time-averaged v e l o c i t y p r o f i l e s , d e t e r m i n e d f r o m p o i n t v e l o c i t i e s
measured by a s i n g l e p i t o t t u b e d u r i n g sand t r a n s p o r t ( f i g .
2), a l l f o l l o w t h e
l o g a r i t h m i c l a w above 2 cm:
Uy
=
5.75
(1 1
U* l o g ( y / k s )
where Uy i s t h e t i m e - a v e r a g e d v e l o c i t y a t h e i g h t y. The l o g a r i t h m i c l a w c a n a l s o be w r i t t e n as: Uy = U* 5.75
l o g (y) -U*
5.75
log (ks)
(2) from w h i c h U* c a n be o b t a i n e d as t h e s l o p e o f t h e b e s t - f i t l i n e between Uy and
-
P
d=246 s2= 0 5 fi,=O41 b2= 2 35
I
I -
-I
10
20
30
,
40
-
GARNET
d = 2 3 $9 s 2 = 031
$ = - 0 067 b2= 3 2
10
w
-
LI
20
30
40
-
OLIVINE
d=255$0
10 -
5 -
9 10
05
025
GRAIN
0125
00625
mm
SIZE
Calculated Fig. 1. S i z e - f r e q u e n c y h i s t o g r a m s of_ s t o c k sand u s e d i n e x p e r i m e n t s . moments o f t h e d i s t r i b u t i o n s a r e : d = mean, s2 = sample v a r i a n c e , E i = G a u s s i a n d i s t r i b u t i o n h a s 5 1 = 0.0 and e s t i m a t o r o f skewness, b2 = k u r t o s i s . bp = 3.0.
118 t h e q u a n t i t y 5.75
log(y).
Correlation coefficients for the best-fit lines t o
t h e d a t a were a l l b e t t e r t h a n 0.9. F o u r e x p e r i m e n t s were c o n d u c t e d a t
U* v a l u e s o f 20, 40, 52, and 110 cm/sec i n
w h i c h t h e s t o c k sand was f e d i n t o t h e t u n n e l c o n t i n u o u s l y d u r i n g each r u n and s a l t a t e d over a f u l l y mobile,
r o u g h bed composed o f t h e same sand.
The bed was
r e p l e n i s h e d w i t h s t o c k sand, homogenized, and l e v e l e d between r u n s , and t h e r a t e o f s e d i m e n t f e e d was r o u g h l y a d j u s t e d t o t h e r a t e o f s e d i m e n t d i s c h a r g e by o b s e r v i n g t h e amount o f e r o s i o n o r d e p o s i t i o n on t h e bed. were sampled a t f i v e l o c a t i o n s f r o m 0.5
Grains i n t r a n s p o r t
t o 5 cm above t h e bed i n a v e r t i c a l
p r o f i l e i n t h e plane o f t h e tunnel c e n t e r l i n e , near t h e s i t e o f t h e v e l o c i t y profiles.
The s a m p l i n g d e v i c e was an L-shaped s u c t i o n t u b e , made f r o m g l a s s
t u b i n g w i t h an i n s i d e d i a m e t e r of 4 mm, t h a t was i n s e r t e d t h r o u g h t h e t u n n e l Each sample was weighed and s e p a r a t e d
r o o f f o r minimum d i s t u r b a n c e of t h e f l o w .
i n t o heavy and l i g h t m i n e r a l f r a c t i o n s by s t a n d a r d f l o t a t i o n t e c h n i q u e s u s i n g bromoform.
The b axes o f a p p r o x i m a t e l y 300 g r a i n s o f each m i n e r a l i n each
sample were measured m i c r o s c o p i c a l l y , u s i n g a d i g i t i z i n g t a b l e . i n p u t t o NORMSTAT, a FORTRAN program w r i t t e n by J. C.
These d a t a were
Griffiths, for calculation
o f s i z e f r e q u e n c y d i s t r i b u t i o n s and s t a t i s t i c s . The r e s u l t i n g d a t a m a t r i x ( T a b l e 1) c o n t a i n s Os and mean g r a i n s i z e i n t r a n s p o r t as a f u n c t i o n o f f r i c t i o n v e l o c i t y , e l e v a t i o n above t h e bed, and g r a i n density.
0
5
D O
1'-
U
A
A A A
A A
.*
m 'e B
..
A
0 0
.
A.
0
oA
I
I
2
3
4
1
i
5
6
VELOCITY
7
8
9
10
(miset)
F i g . 2. Wind v e l o c i t y p r o f i l e s measured d u r i n g s a l t a t i o n . U* v a l u e s were 0 = 40 cm/sec, A = 52 o = 20 cm/sec, c a l c u l a t e d from r e g r e s s i o n l i n e s t o d a t a . Arrows mark h e i g h t s a t w h i c h samples o f s a l t a t i n g g r a i n s cm/sec, 0 = llOcm/sec. were t a k e n .
TABLE 1
Q = q u a r t z , G = g a r n e t , OL = o l i v i n e
Summary o f experimental r e s u l t s . = 20 cm/sec
40 cm/sec
52 cm/sec
0.218
0.332
0.334
0.232
0.335
0.24
5.1 10-2
6.3
1.06-2 10
1.22-1 10
4.2 10-2
2.95 9.7 1.62 1010-1 x
0.161
0.216
0.196
0.175
0.204
0.197
1.43
4.27
6.43 10-4
2.82 10-
4.9
0.254
0.232
0.220 3.66
4.4
110 cmlsec
0.225 9.66
d.291 6.74-1
10
0.232 1.06-1 10
0.236 1.47-1
10
0.215
0.176
0.237
0.238
0.229
2.8
1.77
1.81
7.6
6.6
1o-I
0.173
0.175
0.155
0.198
0.185
0.172
3.213
0.191
0.155
0.21
0.207
0.174
. 0
2.2
5.75
1.2
1.5
2.4
3.1
9.1
6.8
7.7
8.4
1.1 IO-~
0.163
0.178
0.162 1 0 . 1 7 9
0.190
0.165
~
0.190
~~
0.162 10.187
I no samples
I
0.152
0.139
no samples
rain size, mm
.
sec
0.16510.185
I
0.194
no samples
I
0.16419.19
0.176
no
samples
0.17
10.185
0.208
0.178
0.171
0.201
0.160
1.5 IO-~
1.3
1.4 10;~
120 RESULTS AND INTERPRETATION
1.
Sizes o f quartz, garnet, and o l i v i n e i n s a l t a t i o n
(i) v a r i a t i o n w i t h h e i g h t above t h e bed The mean g r a i n s i z e s o f t h e s u c t i o n - p r o b e samples a r e p r e s e n t e d i n f i g u r e
3 as a f u n c t i o n o f h e i g h t above t h e bed, g r a i n d e n s i t y and f r i c t i o n v e l o c i t y . F o r a l l v a l u e s o f U* and g r a i n d e n s i t y t h e c o a r s e s t g r a i n s a r e t r a v e l i n g c l o s e t o t h e bed ( a t o r b e l o w 1 cm), w i t h increasing height.
and t h e r e i s a g e n e r a l d e c r e a s e i n mean s i z e
A p a i r e d t - t e s t shows t h a t t h e s i z e d e c r e a s e f o r
q u a r t z i s s t a t i s t i c a l l y s i g n i f i c a n t between each p a i r o f e l e v a t i o n s f r o m 0.5
1 1 / 1 1 1 1 / ) 1 1 1 02
01
02
03
03
01
D (rnrn)
D(rnrn)
03
D (rnml
01
02
03
D (mrn)
F i g . 3. V a r i a t i o n i n mean g r a i n d i a m e t e r i n t r a n s p o r t , D, w i t h h e i g h t above t h e bed, y, f o r e x p e r i m e n t s a t d i f f e r e n t f r i c t i o n v e l o c i t i e s . = quartz, A = garnet, o = o l i v i n e .
121 t o 5 cm f o r U* o f 20 and 110 cm/sec. s i g n i f i c a n t s i z e d e c r e a s e f r o m 0.5
A t I h o f 40 and 52 cm/sec,
there i s a
t o 1 cm and f r o m 1 t o 2 cm, b u t s i z e s
above 2 cm a r e n o t s i g n i f i c a n t l y s m a l l e r .
G a r n e t shows s t a t i s t i c a l l y
s i g n i f i c a n t s i z e d e c r e a s e s w i t h each i n c r e m e n t o f h e i g h t f o r U* o f 20 and 52 cm/sec,
b u t f o r U* o f 40 cm/sec t h e s i g n i f i c a n t d e c r e a s e o c c u r s i n t h e
i n t e r v a l 0.5
t o 2 cm, w i t h no s i g n i f i c a n t change i n s i z e above 2 cm.
shows a c o n s i s t e n t d e c r e a s e i n s i z e w i t h h e i g h t f o r IJ* o f 40 cm/sec. of 52 cm/sec t h e r e i s a s i g n i f i c a n t d e c r e a s e f r o m 0.5 s i g n i f i c a n t d e c r e a s e f r o m 2 t o 5 cm.
F o r [I*
s i g n i f i c a n t d e c r e a s e i n s i z e between 0.5 For U* o f 110 cm/sec,
Olivine F o r U*
t o 2 cm, and a
o f 20 cm/sec o l i v i n e shows a
and 1 cm, b u t no change above 1 cm.
t h e r e i s s i g n i f i c a n t d e c r e a s e f r o m 0.5
t o 2 cm, t h e n a
small change t o 7 cm. F o r a l l t h r e e m i n e r a l s , t h e mean g r a i n s i z e t r a v e l i n g a t 0.5
cm above t h e
bed i s c o a r s e r t h a n t h e mean s i z e o f t h e a v a i l a b l e p o p u l a t i o n , and t h e means o f samples c o l l e c t e d a t 3 cm and a t 5 cm a r e f i n e r t h a n t h e mean s i z e o f t h e s t o c k sand.
A t l o w v a l u e s o f U* samples o f a l l t h r e e m i n e r a l s a t 1 cm and o f
q u a r t z a t 2 cm have mean s i z e s e q u a l t o o r l e s s t h a n t h e mean s i z e o f t h e s t o c k sand.
A t h i g h U* samples a t t h e same h e i g h t s a r e c o a r s e r t h a n t h e mean
s i z e o f t h e s t o c k sand. The o b s e r v e d g r a i n s i z e v a r i a t i o n w i t h h e i g h t above t h e bed i s i n agreement w i t h t h e o b s e r v a t i o n s o f W i l l i a m s (1964) and o f C h i u (1972) f o r t h e s a l t a t i o n o f homogeneous q u a r t z sands and c r u s h e d q u a r t z i t e s .
I n Williams'
e x p e r i m e n t s , o v e r a r a n g e o f U* f r o m 30 t o 135 cm/sec, s a l t a t i n g p o p u l a t i o n t r a p p e d a t 0.5 s t o c k sand.
1964)
t h e mean s i z e o f t h e
cm i s c o a r s e r t h a n t h e mean s i z e o f h i s
Means o f t h e samples a t s u c c e s s i v e h e i g h t s a r e sma l e r t h a n t h a t a t
0.5 cm, and a r e e i t h e r s i m i l a r t o each o t h e r o r show a g r a d u a l d e c r e a s e w i t h height.
C h i u (1972) d i d n o t sample s e d i m e n t i n t r a n s p o r t b e l o w 1 cm above t h e
bed, b u t i n most o f h i s e x p e r i m e n t s , w i t h a r a n g e o f U* f r o m 53 t o 135 cm/sec, t h e samples a t 1 cm a r e c o a r s e r t h a n t h e mean s i z e o f t h e s t o c k sand. W i l l i a m s ' (1964) d a t a ,
In
p a r t o f t h e c o n t r a s t i n s i z e between t h e samples c l o s e t o
t h e bed and samples a t h i g h e r e l e v a t i o n s i s due t o t h e f a c t t h a t t h e 0.5
cm
sample i n c l u d e d a l l g r a i n s m o v i n g b e l o w a h e i g h t o f 1 cm, i n c l u d i n g t h e c r e e p population.
I n o u r e x p e r i m e n t s t h e 0.5
cm samples a t U* o f 20 and 40 cm/sec
i n c l u d e c r e e p o n l y when r i p p l e c r e s t s passed t h r o u g h t h e s a m p l i n g space d u r i n g the sampling time.
These samples a l s o i n c l u d e t h e g r a i n s w i t h t h e l o w e s t
saltation trajectories.
W i t h o u t s e p a r a t e d a t a on t h e s i z e d i s t r i b u t i o n o f t h e
c r e e p p o p u l a t i o n i t i s i m p o s s i b l e t o e v a l u a t e t o what e x t e n t t h e mean g r a i n s i z e s o f t h e 0.5 cm samples a r e b i a s e d by t h e p r e s e n c e o f c r e e p i n g g r a i n s .
For
t h i s r e a s o n t h e s e samples a r e n o t i n c l u d e d i n t h e subsequent d i s c u s s i o n o f t h e r a t i o s o f heavy t o l i g h t m i n e r a l g r a i n s i z e s i n s a l t a t i o n . I n summary,
s i z e s f o r each m i n e r a l d e c r e a s e r a p i d l y w i t h h e i g h t above t h e
122 bed up t o 2 cm, b u t a r e more o r l e s s c o n s t a n t o r decrease s l i g h t l y w i t h h e i g h t above 2 cm.
T h i s p a t t e r n o f s i z e v a r i a t i o n w i t h h e i g h t has been observed i n
t h e s a l t a t i o n o f s i n g l e - d e n s i t y sands, and has been i n t e r p r e t e d t o mean t h a t jump h e i g h t i s an i n v e r s e f u n c t i o n o f g r a i n s i z e . ( i i ) variation i n size with f r i c t i o n velocity The t r e n d i n g r a i n s i z e w i t h i n c r e a s i n g f r i c t i o n v e l o c i t y ( f i g . 4 w i t h e l e v a t i o n above t h e bed, and i s s i m i l a r f o r q u a r t z and garnet.
varies Sizes of
b o t h m i n e r a l s t r a v e l i n g a t 1 cm above t h e bed i n c r e a s e w i t h each i n c ement of
QUARTZ
-E
30-
E
v
w 25-
N
7
GARNET
v)
z 20-
a W I
15 -
I
1
I
I
OLIVINE
I
6
I
I
20
40
60
80
U,
I
100
(cm/sec)
F i g . 4. V a r i a t i o n i n mean g r a i n d i a m e t e r i n s a l t a t i o n , h e i g h t s above t h e bed; o = 0.5 cm, O = 1 cm, A = 2 cm, 0 - 7 cm.
D, w i t h U* f o r d i f f e r e n t I)
= 3 cm,
0
= 5 cm,
123 U* o v c r t h e w h o l e e x p e r i m e n t a l
r i p p l e r a n g e (U*
range.
20 t o 52 crn/sec)
=
U* t o 110 cm/sec.
A t 2 cm, q u a r t z i n c r e a s e s t h r o u g h t h e
and t h e n s t a y s c o n s t a n t w i t h i n c r e a s e i n
G a r n e t samples a t 2 cm show i n s i g n i f i c a n t s i z e i n c r e a s e s
w i t h i n c r e a s i n g U* u n t i l t h e i n c r e m e n t frorn U* o f 52 cm/sec t o U* of 110 A t 3 and 5 cm e l e v a t i o n s , s i z e s o f b o t h q u a r t z and g a r n e t i n c r e a s e
cm/sec.
U* o f 20 t o U* o f 40 cm/sec, b u t q u a r t z s t a y s c o n s t a n t i n s i z e from (I* o f 40 t o U* o f 110 cm/sec whereas g a r n e t c o n t i n u e s t o i n c r e a s e i n s i z e ,
froin
although n o t s t e a d i l y .
I n general,
l o w i n t h e s a l t a t i o n c l o u d ( < 2 cm h e i g h t )
g r a i n s i z e s i n c r e a s e w i t h i n c r e a s i n g U* f r o m n e a r - t h r e s h o l d c o n d i t i o n s t o plane bed s t a g e .
Higher i n t h e s a l t a t i o n cloud, sizes increases s i g n i f i c a n t l y
through p a r t o r a l l o f t h e r i p p l e range, b u t do n o t i n c r e a s e f u r t h e r a t h i g h e r The U* v a l u e a t w h i c h a g r a i n r e a c h e s i t s maximum jump h e i g h t seems t o be
U*.
higher f o r c o a r s e r g r a i n s .
There i s n o e v i d e n c e t o s u p p o r t a s t r o n g
dependence o f jump h e i g h t on g r a i n d e n s i t y .
This conclusion supports,
in a
general way, t h e p r e d i c t i o n s o f W h i t e (1975).
2.
S i z e r a t i o s o f heavy and l i g h t m i n e r a l s t r a v e l i n g t o g e t h e r (i)
garnet-quartz
ratios
The e x p e r i m e n t a l d a t a p l o t i n a c l o u d a b o u t an e q u a l - s i z e l i n e when p l o t t e d as mean s i z e g a r n e t a g a i n s t mean s i z e q u a r t z ( f i g .
5).
There i s n o
apparent s y s t e m a t i c d i s p o s i t i o n o f t h e p o i n t s g r o u p e d by e l e v a t i o n o r by U*, except t h e g e n e r a l t r e n d o f i n c r e a s i n g s i z e f o r b o t h g a r n e t and q u a r t z w i t h i n c r e a s i n g U*.
DG
= 0.75
A r e g r e s s i o n l i n e t h r o u g h t h e d a t a has t h e e q u a t i o n :
DQ + 0.53
(3) where DG i s t h e mean d i a m e t e r o f g a r n e t and DQ i s t h e mean d i a m e t e r o f q u a r t z . The r e g r e s s i o n c o r r e l a t i o n c o e f f i c i e n t ,
a t t h e 95% l e v e l .
and t h e y - i n t e r c e p t i s n o n - z e r o , line.
r = 0.77,
i s statistically significant
The s l o p e o f t h e l i n e i s s i g n i f i c a n t l y d i f f e r e n t f r o m 1.0,
so t h e l i n e i s d i f f e r e n t f r o m t h e e q u a l - s i z e
Our i n i t i a l i n t e r p r e t a t i o n ( G e r e t y and S l i n g e r l a n d , 1982) was t h a t t h i s
r a t i o of s i z e s r e p r e s e n t e d s a l t a t i o n e q u i v a l e n t s i z e s o f g a r n e t and q u a r t z , which we e x p r e s s e d as:
DG/DQ
= ( UQ/ UG) 1/3. (4) We no l o n g e r b e l i e v e t h a t t h i s i n t e r p r e t a t i o n i s j u s t i f i e d by t h e d a t a ,
because t h e r e i s an i n s u f f i c i e n t r a n g e o f v a l u e s o f q u a r t z and g a r n e t s i z e s . t - t e s t s o f t h e means show t h a t f e w e r t h a n h a l f o f t h e p a i r s have means w h i c h
are s t a t i s t i c a l l y s i g n i f i c a n t l y d i f f e r e n t . and t h e mean s i z e r a t i o i s 1.01,
o f 1.08.
The r a n g e i n DG/DQ i s 0.9
t o 1.18,
compared w i t h a s i z e r a t i o i n t h e s t o c k sand
We c o n c l u d e , t h e r e f o r e , t h a t e q u a l - s i z e d g a r n e t and q u a r t z
on t h e
average, t r a v e l t o g e t h e r i n s a l t a t i o n when t h e y a r e a v a i l a b l e i n t h e same s i z e range. ( i i ) olivine-quartz size ratios The mean s i z e s o f o l i v i n e and q u a r t z t r a v e l i n g t o g e t h e r i n s a l t a t on a l s o
124 p l o t i n a f a i r l y close cluster o f points (fig.
t - t e s t s o f t h e means
6).
show t h a t q u a r t z i s l a r g e r t h a n o l i v i n e i n most o f t h e samples.
A regression
l i n e t h r o u g h t h e s e d a t a has t h e e q u a t i o n : DOL = 0.8
DQ + 0.02
(5) which i s s t a t i s t i c a l l y
w i t h t h e r e g r e s s i o n c o r r e l a t i o n c o e f f i c i e n t r = 0.75 significant.
The mean s i z e r a t i o o f o l i v i n e t o q u a r t z i n s a l t a t i o n i s 0.89,
compared w i t h a s i z e r a t i o o f 0.95
i n t h e s t o c k sand.
The a p p a r e n t
c o n c l u s i o n , t h a t o l i v i n e and q u a r t z s i z e s i n s a l t a t i o n a r e n o t e q u a l , w o u l d n o t be e x p e c t e d because t h e d e n s i t y c o n t r a s t between o l i v i n e and q u a r t z i s l e s s t h a n t h a t between g a r n e t and q u a r t z , y e t e q u a l - s i z e d g a r n e t s and q u a r t z are i n transport together.
I f t h e p r o c e s s c a n and does s e l e c t e q u a l - s i z e d
g a r n e t s and q u a r t z f o r t r a n s p o r t , t h e n i t s h o u l d be a b l e t o s e l e c t e q u a l - s i z e o l i v i n e and q u a r t z .
The f a c t t h a t i t does n o t i s i n t e r p r e t e d t o mean t h a t
o l i v i n e i s i n s u f f i c i e n t l y a v a i l a b l e i n coarser sizes.
A l t h o u g h t h e mean s i z e s
o f q u a r t z and o l i v i n e i n t h e s t o c k sand a r e e q u a l , and t h e r a n g e o f s i z e s a v a i l a b l e i s e q u a l , t h e mode o f t h e o l i v i n e s i z e d i s t r i b u t i o n i s c l o s e r t o 0.125
mm t h a n i t i s t o t h e mean, 0.195
mm.
Much more f i n e o l i v i n e i s
a v a i l a b l e t h a n c o a r s e o l i v i n e , w h i c h a p p a r e n t l y i n f l u e n c e s t h e mean s i z e transported.
T h a t t h i s i s a r e a s o n a b l e i n t e r p r e t a t i o n i s s u p p o r t e d by
B a g n o l d ' s o b s e r v a t i o n (1941) t h a t an i r r e g u l a r i t y i n t h e c o a r s e o r t h e f i n e f r a c t i o n o f a s t o c k sand p e r s i s t s i n t h e s i z e d i s t r i b u t i o n o f t h e c o a r s e o r fine fraction,
respectively, o f t h e transported population.
As a r e s u l t , we
b e l i e v e t h a t t h e garnet-quartz r e l a t i o n s h i p s a r e b e t t e r i n d i c a t o r s o f process s o r t i n g t h a n t h e o l i v i n e q u a r t z r e l a t i o n s h i p s are.
03.
W
z
LE J
E
0w 02-
r\l
cn
/
Z
,/
01
02
03
MEAN SIZE QUARTZ (mrn)
F i g . 5. Mean s i z e s o f g a r n e t and quartz i n saltation together f o r a l l samples a t and above 1 cm above t h e bed.
01
02
03
MEAN SIZE QUARTZ (rnrn)
F i g . 6. Mean s i z e s o f o l i v i n e and q u a r t z i n s a l t a t i o n t o g e t h e r f o r a l l samples a t and above 1 cm above t h e bed.
125 3.
Rates o f t r a n s p o r t i n s a l t a t i o n The mass f l u x i n s a l t a t i o n d e c r e a s e s e x p o n e n t i a l l y w i t h h e i g h t above t h e
bed f o r a l l t h e e x p e r i m e n t s ( f i g . w i t h (I*.
7 ) , and t h e i n t e g r a t e d mass f l u x i n c r e a s e s
These o b s e r v a t i o n s a r e c o n s i s t e n t w i t h p r e v i o u s work f o r homogeneous
sands ( C h i u , 1972; W i l l i a m s , s o r t i n g ( W i l l i a m s , 1964).
1964; Z i n g g , 1953) and f o r q u a r t z sands w i t h p o o r
Most o f t h e t r a n s p o r t ( f r o m 60 t o 9 0 1 ) o c c u r s
w i t h i n 2 cni o f t h e bed s u r f a c e . The w e i g h t p e r c e n t g a r n e t i n t h e s a l t a t i n g p o p u l a t i o n ( f i g . 8 ) d e c r e a s e s w i t h h e i g h t above t h e bed and i n c r e a s e s w i t h \I* i n t h e r i p p l e r a n g e ((I* t o 52 cm/sec) a t any one h e i g h t .
o f 110 cm/sec f o r samples a t 1 cm and a t 5 cm above t h e bed.
cm and a t 2 cm d e c r e a s e i n w e i g h t p e r c e n t g a r n e t a t h i g h U*, cannot y e t e x p l a i n .
= 20
The i n c r e a s e c o n t i n u e s up t o a (I* Samples a t 0.5 an o b s e r v a t i o n we
V a r i a t i o n i n w e i g h t p e r c e n t t o t a l heavy m i n e r a l s f o l l o w s
the same p a t t e r n . The f o r m o f t h e f u n c t i o n o f w e i g h t p e r c e n t g a r n e t v e r s u s U* m i g h t be e x p l a i n e d by t h e i n c r e a s e i n s a n d - l o a d i n g i n t h e s a l t a t i o n l a y e r .
Following
Owen (1964) and Maegley ( 1 9 7 4 ) , t h e l o a d i n g r a t i o , t h e r a t i o o f mass c o n c e n t r a t i o n o f sand,
pb, t o f l u i d d e n s i t y , p , i s r e l a t e d t o t h e r a t i o o f
threshold f r i c t i o n v e l o c i t y (U*t) t o e x i s t i n g f r i c t i o n v e l o c i t y ,
as:
Pb/P 1- ( U * t 2 / \ h 2 ) (6) and cannot exceed 1. S a t u r a t i o n o f t h e s a l t a t i o n l a y e r , d e f i n e d by a l o a d i n g
r a t i o o f 0.9,
o c c u r s when U* i s a p p r o x i m a t e l y t h r e e t i m e s t h e t h r e s h o l d v a l u e .
Far t h e sand i n t h e s e e x p e r i m e n t s , s a t u r a t i o n i s r e a c h e d a t a U* o f a b o u t 60 cm/sec,
c o i n c i d i n g w i t h t h e t r a n s i t i o n t o p l a n e bed t r a n s p o r t s t a g e .
reasonable, t h e r e f o r e ,
i n v a r i a n t w i t h i n c r e a s i n g Ilk above a v a l u e o f a b o u t 60 cm/sec, supply i s adequate.
It i s
t h a t t h e w e i g h t p e r c e n t g a r n e t i n t r a n s p o r t s h o u l d be provided that
An i m p o r t a n t consequence o f t h e s a t u r a t i o n h y p o t h e s i s i s
t h a t we e x p e c t more v a r i a t i o n i n t h e s i z e - d e n s i t y c o m p o s i t i o n o f t h e s a l t a t i o n p o p u l a t i o n f o r r i p p l e s t a g e t r a n s p o r t t h e n f o r p l a n e bed s t a g e . 4.
Summary o f e x p e r i m e n t a l r e s u l t s ( 1 ) Equal s i z e s o f q u a r t z and g a r n e t a r e t r a n s p o r t e d t o g e t h e r a t each
height up t o 7 cm i n s a l t a t i o n , o v e r a w i d e r a n g e o f U*,
i f they are available
i n t h e same s i z e r a n g e and i n a p p r o x i m a t e l y t h e same s i z e d i s t r i b u t i o n .
This
i s t h e most s i g n i f i c a n t r e s u l t o f t h e e x p e r i m e n t s . ( 2 ) S i z e s o f g a r n e t and o f q u a r t z d e c r e a s e , a p p a r e n t l y c o n c o r d a n t l y , w i t h height above t h e bed, and i n g e n e r a l i n c r e a s e w i t h i n c r e a s i n g U*,
but not
uniformly.
( 3 ) Weight p e r c e n t g a r n e t i n t r a n s p o r t d e c r e a s e s w i t h h e i g h t above t h e bed.
T o t a l w e i g h t p e r c e n t g a r n e t i n s a l t a t i o n i n c r e a s e s r a p i d l y w i t h U*
through t h e r i p p l e r a n g e (U* = 20 cm/sec t o U* = 52 cm/sec) and a t y = l cm increases s l o w l y w i t h i n c r e a s i n g
U* up t o p l a n e b e d c o n d i t i o n s .
The t o t a l
126
7
\
- 6
E
o c
2 I
lo-‘
10-2
Q F i g . 7. V a r i a t i o n i n mass f l u x , o = 20 cm/se:’b= f o r d i f f e r e n t U:,
10‘
( g/cm2- sec) 2 i n g/cm -sec, w i t h h e i g h t above t h e bed, y, 40 cm/sec, A = 52 cm/sec, = 110 cm/sec.
U,
(cm/sec)
F i g . 8. V a r i a t i o n i n w e i g h t p e r c e n t g a r n e t i n t r a n s p o r t w i t h U, and h e i g h t above t h e bed o = 0.5 cm, O = 1 cm, A = 2 cm, .= 3 cm, 0 = 5 cm, O = 7 cm.
weight p e r c e n t t r a n s p o r t e d does n o t exceed t h e amount a v a i l a b l e i n t h e s t o c k sand.
( 4 ) Jump h e i g h t f o r sands i n h e t e r o g e n e o u s m i x t u r e s appears t o be a f u n c t i o n o f g r a i n s i z e and o f U*, b u t a p p a r e n t l y i s n o t a s t r o n g f u n c t i o n o f g r a i n d e n s i t y , a t l e a s t f o r t h e r a n g e o f h e i g h t s sampled. ( 5 ) The s i z e d i s t r i b u t i o n o f t h e s o u r c e m a t e r i a l s i g n i f i c a n t l y i n f l u e n c e s the s i z e d i s t r i b u t i o n o f t h e s a l t a t i o n p o p u l a t i o n , a t l e a s t f o r s h o r t transport distances. DISCUSSION
Can c u r r e n t t h e o r i e s o f s e d i m e n t t r a n s p o r t , d e r i v e d f o r homogeneous sands, e x p l a i n t h e s e r e s u l t s f o r h e t e r o g e n e o u s sands? types:
( 1 ) B a g n o l d ' s t h e o r y ( B a g n o l d 1941,
The t h e o r i e s a r e of t w o
1956, 1973) and o t h e r s ( T s u c h i y a ,
1970, R e i z e s , 1978) w h i c h assume t h a t b a l l i s t i c p r o c e s s e s i n s a l t a t i o n a r e most i m p o r t a n t i n c o n t r o l l i n g e n t r a i n m e n t and t r a n s p o r t .
( 2 ) T h e o r i e s w h i c h assume t h a t f l u i d l i f t and d r a g a r e t h e f o r c e s c o n t r o l l i n g e n t r a i n m e n t and t r a n s p o r t .
These i n c l u d e m o d i f i c a t i o n s o f E i n s t e i n ' s 1950
s t o c h a s t i c b e d l o a d f u n c t i o n ( C h i u , 1972; K a d i b , 1965) w i t h new c o n s t a n t s t o p r e d i c t mass t r a n s p o r t o f u n i f o r m q u a r t z i n w i n d and w i t h c o r r e c t i o n f a c t o r s t o account f o r g r a i n i m p a c t by s a l t a t i o n .
O t h e r models i n t h i s c l a s s a r e l e s s
concerned w i t h p r e d i c t i n g mass f l u x , and u s e n u m e r i c a l models based on t h e e q u a t i o n s o f m o t i o n t o e x p l a i n t r a j e c t o r y h e i g h t s as a f u n c t i o n o f g r a i n s i z e , g r a i n d e n s i t y , U*,
and l i f t , l a r g e l y f o r a p p l i c a t i o n t o s t u d i e s o f m a r t i a n
sandstorms ( I v e r s e n e t a l . , Schul z,
1.
1973; White,
1975; W h i t e e t al.,
1976; W h i t e and
1977).
B a l l i s t i c models o f s a l t a t i o n
As an example o f t h i s c l a s s , c o n s i d e r B a g n o l d ' s b e d l o a d t r a n s p o r t t h e o r y , developed f o r u n i f o r m spheres, and based i n p a r t on e x p e r i m e n t s w i t h q u a r t z sands and a l s o on some a s s u m p t i o n s a b o u t p a t h shapes and g r a i n v e l o c i t i e s t h a t have n o t been s u p p o r t e d e x p e r i m e n t a l l y .
A f u n d a m e n t a l p r e m i s e i s t h a t once
s a l t a t i o n has begun, v i r t u a l l y a l l momentum t r a n s f e r f r o m t h e f r e e s t r e a m w i n d t o t h e s t a t i c bed i s a c c o m p l i s h e d b y c o l l i s i o n o f t h e s a l t a t i n g g r a i n s w i t h the g r a i n s i n t h e bed.
The n e c e s s a r y consequence i s t h a t t h e f l u i d s h e a r
s t r e s s a t t h e bed i s r e d u c e d b e l o w t h e f l u i d t h r e s h o l d , and g r a i n s a r e e n t r a i n e d o n l y by g r a i n i m p a c t , n o t by f l u i d shear. C o n s i d e r o u r o b s e r v a t i o n t h a t e q u a l s i z e s o f g a r n e t and q u a r t z travel t o g e t h e r i n s a l t a t i o n . e n t r a i n m e n t by i m p a c t ?
Can t h i s be e x p l a i n e d as a s i m p l e f u n c t i o n o f
Bagnold's impact t h r e s h o l d equation i s :
128 where t h e v a l u e o f A was e m p i r i c a l l y d e t e r m i n e d t o he 0.08 ( R a g n o l d 1941).
ti*
=
20 cm/sec,
mm, and t h e mean o f t h e t r u n c a t e d i n i t i a l q u a r t z d i s t r i b u t i o n ( F i g .
b e 0.175 mm. was 0.i8 mm,
At
t h e c o a r s e s t q u a r t z g r a i n s w h i c h s h o u l d he e n t r a i n e d a r e 0.29 The o b s e r v e d
1) w o u l d
mean s i z e o f t o t a l q u a r t z i n s a l t a t i o n a t t h i s (I*
and t h e c o a r s e s t 107, ( b y number) were 0.33 nim.
Thus,
i f the
s i z e s are t r a n s p o r t e d a t a r a t e d i r e c t l y p r o p o r t i o n a l t o t h e i r frequency i n t h e s o u r c e sand, t h e n t h e d i s t r i b u t i o n c a p t u r e d c o u l d he s i m p l y t h e entrainment-truncated i n i t i a l distribution. a t U* = 20 cm/sec,
The c o a r s e s t g a r n e t i n s a l t a t i o n
f r o m e q u a t i o n 7, s h o u l d he 0.18 mm, and t h e inean o f t h e
t r u n c a t e d i n i t i a l g a r n e t d i s t r i b u t i o n w o u l d he 0.14 inm.
The o b s e r v e d mean
g a r n e t s i z e , however, was 0.18 min, and t h e c o a r s e s t 10% were 0.26 mm. Contrary t o quartz,
e i t h e r the garnet sizes a r e n ' t transported a t a r a t e
p r o p o r t i o n a l t o t h e i r frequency i n t h e source, o r Ragnold's impact t h r e s h o l d e q u a t i o n i s wrong, o r b o t h .
A t UX = 40 cin/sec t h e i m p a c t f o r i n u l a p r e d i c t s
t h a t a l l s i z e s o f q u a r t z up t o 1.17 rnm s h o u l d be e n t r a i n e d .
Rut t h e c o a r s e s t
f r a c t i o n o f b o t h q u a r t z and g a r n e t i n t h e s t o c k d i s t r i b u t i o n s a r e n o t s i g n i f i c a n t l y represented i n t h e observed s a l t a t i o n population.
This suggests
t h a t t h e i m p a c t t h r e s h o l d e q u a t i o n c a n n o t be a p p l i e d t o t h e s a l t a t i o n o f h e t e r o g e n e o u s s i z e - d e n s i t y sands. The i m p a c t t h r e s h o l d i s d e f i n e d as t h e (I* j u s t s u f f i c i e n t t o m a i n t a i n s a l t a t i o n o f u n i f o r m sands u n d e r g o i n g i m p a c t hy i d e n t i c a l g r a i n s , w h i c h t r a v e l i n t h e " c h a r a c t e r i s t i c path."
T h i s i s i n a p p l i c a b l e t o t h e case o f
h e t e r o g e n e o u s s i z e - d e n s i t y sands f o r s e v e r a l reasons.
Because t h e g r a i n s a r e
n o t a l l t h e same s i z e , t h e y have d i f f e r e n t e x p o s u r e on t h e s u r f a c e , and s m a l l g r a i n s can h i d e between l a r g e r ones. a r e unequal f o r d i f f e r e n t s i z e s . possible.
As a r e s u l t , t h e p r o b a b i l i t i e s o f i m p a c t
A l s o , more t h a n one t y p e o f c o l l i s i o n i s
A g r a i n o f a s p e c i f i e d s i z e and d e n s i t y m i g h t he s t r u c k by a g r a i n
l a r g e r , s m a l l e r , o r e q u a l i n s i z e , and o f t h e same o r o f d i f f e r e n t d e n s i t y . It m i g h t be e x p e c t e d t h a t g r a i n s l a r g e r t h a n t h e mean a r e more l i k e l y t o be
h i t by g r a i n s e q u a l l y o r more m a s s i v e t h a n t h e m s e l v e s t h a n t h e y w o u l d be i n u n i f o r m sands.
T h i s would t e n d t o r a i s e t h e i r t h r e s h o l d f r i c t i o n v e l o c i t y i n
t h e m i x t u r e o v e r what i s p r e d i c t e d by B a g n o l d ' s i m p a c t law.
I n contrast,
g r a i n s s m a l l e r t h a n t h e mean a r e , p e r h a p s , l e s s l i k e l y t o be s t r u c k t h a n t h e y w o u l d be i n u n i f o r m sand, b u t when s t r u c k a r e more l i k e l y t o he h i t by g r a i n s more m a s s i v e t h a n t h e m s e l v e s and, t h e r e f o r e , more l i k e l y t o be e n t r a i n e d . T h i s suggests t h a t t h e t h r e s h o l d f r i c t i o n v e l o c i t y f o r these g r a i n s i s lower i n t h e h e t e r o g e n e o u s sand t h a n i n a u n i f o r m sand.
Similarly, quartz grains i n
t h e m i x t u r e a r e l i k e l y t o have l o w e r t h a n p r e d i c t e d U * t because o f t h e p r e s e n c e o f g a r n e t g r a i n s o f t h e same s i z e s , b u t g a r n e t s w o u l d t e n d t o have h i g h e r U * t because most c o l l i s i o n s w i l l be w i t h q u a r t z .
It a l s o f o l l o w s f r o m
129 t h i s a n a l y s i s t h a t Bagnol d ' s " c h a r a c t e r i s t i c p a t h " i s an u n r e a l is t i c a p p r o x i m a t i o n o f g r a i n b e h a v i o r i n h e t e r o g e n e o u s s i z e - d e n s i t y sands. What s h o u l d a r a t i o n a l i m p a c t t h r e s h o l d f o r m u l a be based on?
We c o n c l u d e
t h a t t h e t h r e s h o l d o f e n t r a i n m e n t by i m p a c t f o r i n d i v i d u a l s i z e - d e n s i t y components o f a h e t e r o g e n e o u s sand must depend on t h e m a g n i t u d e - f r e q u e n c y distribution o f collisions.
T h i s i n t u r n depends on t h e bed s t a t e and on t h e
saltation trajectories o f the d i f f e r e n t size-density grains i n transport. Unfortunately these s a l t a t i o n t r a j e c t o r i e s are not presently predictable. example,
For
T s u c h i y a (1970) used a b a l l i s t i c model t o p r e d i c t p a t h shapes f o r
uniform g r a i n s i n steady s a l t a t i o n over a f i x e d rough surface. g r a i n d e n s i t y and U*,
For constant
p r e d i c t e d jump h e i g h t i s h i g h e r f o r s m a l l e r g r a i n s , b u t
t h e d i f f e r e n c e between s m a l l and l a r g e g r a i n s d e c r e a s e s w i t h i n c r e a s i n g (I*. For a c o n s t a n t s i z e and U*, than l i g h t e r ones.
d e n s e r g r a i n s have l o w e r p r e d i c t e d jump h e i g h t s
But t h e model p r e d i c t s 0.15
mm and s m a l l e r g a r n e t w o u l d
have t h e same jump h e i g h t as 0.3 mm and l a r g e r q u a r t z . i n l i g h t o f o u r e x p e r i m e n t a l d a t a ( f i g . 5).
This i s unreasonable
A t p r e s e n t no b a l l i s t i c t h e o r y o f
entrainment i s adequate t o e x p l a i n t h e behavior o f a heterogeneous s a l t a t i o n population.
And w i t h o u t an a d e q u a t e e n t r a i n m e n t f u n c t i o n i t a l s o i s n o t
p o s s i b l e t o e x p l a i n t h e o b s e r v a t i o n s as a s i m p l e f u n c t i o n o f d i f f e r e n t i a l transport rates.
2.
F l u i d - l i f t models o f s a l t a t i o n Can t h e o b s e r v a t i o n o f e q u a l s i z e s o f q u a r t z and g a r n e t i n s a l t a t i o n be
e x p l a i n e d as a f u n c t i o n o f an i m p a c t t h r e s h o l d f o r m u l a w h i c h i n c l u d e s f l u i d lift?
Because none o f t h e t h r e s h o l d f o r m u l a e c o n s i d e r t h e combined e f f e c t s o f
impact and f l u i d l i f t , t h i s i s p r e s e n t l y i m p o s s i b l e t o answer.
The most
recent v e r s i o n o f t h e f l u i d t h r e s h o l d e q u a t i o n ( G r e e l e y e t a l . ,
1974) a c c o u n t s
f o r complex dependence o f t h e c o n s t a n t A, o f e q u a t i o n 7, on R*,
b u t does n o t
apply t o e n t r a i n m e n t u n d e r i m p a c t .
K a d i b (1965) r e c o g n i z e d t h a t t h e e f f e c t o f
impact w o u l d be t o i n c r e a s e t h e l i f t c o e f f i c i e n t , t h u s i n c r e a s i n g t r a n s p o r t r a t e s , b u t he based h i s c o r r e c t i o n f a c t o r on B a g n o l d ' s a s s u m p t i o n s a b o u t t h e c h a r a c t e r i s t i c p a t h i n s a l t a t i o n , and d e t e r m i n e d i t s v a l u e e m p i r i c a l l y o n l y
for u n i f o r m q u a r t z sands.
Without a v a l i d f u n c t i o n a l expression o f t h e
correction f a c t o r f o r s a l t a t i o n impact,
h i s t r a n s p o r t e q u a t i o n s c a n n o t be
extended t o t h e t r a n s p o r t o f h e t e r o g e n e o u s s i z e - d e n s i t y sands.
3.
Geologic i m p l i c a t i o n s A p a r t f r o m t h e d a t a p r e s e n t e d i n t h i s p a p e r , t h e o n l y a v a i l a b l e d a t a on
sizes o f heavy and l i g h t m i n e r a l s i n a e o l i a n sands a r e f i e l d measurements i n c o a s t a l dunes ( v o n Engel h a r d t , 1940; Hand, 1964; B a r n d o r f f - N i e l s e n e t al.,
1982), and i n i n f e r r e d a n c i e n t dunes ( S t e i d t m a n n and Haywood, 1982).
Their
p r i n c i p a l r e a s o n f o r m e a s u r i n g t h e s i z e s was t o e s t a b l i s h a c r i t e r i o n f o r d i s t i n g u i s h i n g w a t e r - l a i d f r o m w i n d - l a i d sands based on t h e s i m p l e h y p o t h e s i s
130 t h a t s i z e s e n t r a i n e d , t r a n s p o r t e d and d e p o s i t e d t o g e t h e r s h o u l d be s e t t l i n g equivalents i n air.
I n l i g h t o f H u n t e r ' s (1977) work, s e t t l i n g e q u i v a l e n t
s i z e s s h o u l d be e x p e c t e d i n g r a i n f a l l l a m i n a e , b u t d i s p e r s i v e e q u i v a l e n t s i z e s ( S a l l e n g e r , 1979) s h o u l d o c c u r i n s a n d f l o w s t r a t a , and i n t r a n s l a t e n t s t r a t a t h e s i z e s s h o u l d be t h o s e o f heavy and l i g h t m i n e r a l s t r a n s p o r t e d t o g e t h e r i n s a l t a t i o n and c r e e p .
A l l o f t h i s assumes t h a t an a d e q u a t e r a n g e o f heavy
mineral sizes i s available.
Our e x p e r i m e n t s s u g g e s t t h a t t h e c o a r s e s t heavy
minerals a v a i l a b l e w i l l t r a v e l w i t h equal-sized o r coarser quartz grains. s i z e r a t i o o f h e a v i e s and l i g h t s i n l a m i n a e , t h e r e f o r e ,
The
w i l l depend on t h e
mean s i z e o f q u a r t z , t h e r a n g e of heavy s i z e s a v a i l a b l e , and t h e t y p e o f lamination.
CONC LUS I ON S Wind t u n n e l e x p e r i m e n s on t h e s a l t a i o n o f h e t e r o g e n e o u s s i z e - d e n s i t y sands, i n w h i c h g a r n e t , q u a r t z , and o l i v i n e had t h e same mean s i z e and s i m i l a r f r e q u e n c y d i s t r i b u t i o n s , o v e r a range o f t r a n s p o r t s t a g e , show t h a t :
Ilk
f r o m n e a r - t h r e s h o l d t o p l a n e bed
s i z e s o f q u a r t z and g a r n e t i n s a l t a t i o n a r e e q u a l
a t h e i g h t s f r o m 1 cm t o 5 cm above t h e bed, w e i g h t p e r c e n t g a r n e t d e c r e a s e s w i t h h e i g h t above t h e bed, and t o t a l w e i g h t p e r c e n t g a r n e t i n c r e a s e s w i t h i n c r e a s i n g U*.
The s i z e s o f heavy and l i g h t m i n e r a l s t r a v e l i n g t o g e t h e r a t a
p a r t i c u l a r h e i g h t i n s a l t a t i o n must be a f u n c t i o n o f t h e p r o b a b i l i t y o f entrainment t i m e s t h e p r o b a b i l i t y o f t r a n s p o r t a t equal r a t e s times t h e p r o b a b i l i t y o f sampling a s i z e a t a p a r t i c u l a r p o i n t i n i t s t r a j e c t o r y .
None
o f t h e c u r r e n t m o d e l s o f sediment e n t r a i n m e n t and t r a n s p o r t i n s a l t a t i o n , d e r i v e d f o r u n i f o r m sands, can p r e d i c t t h e s e p r o b a b i 1 it i e s f o r h e t e r o g e n e o u s sands.
The g e o l o g i c i m p l i c a t i o n s a r e t h a t ( 1 ) d i s t i n c t i o n o f w a t e r - l a i d f r o m
w i n d - l a i d sands c a n n o t be based on a p r e m i s e t h a t s i z e s o f h e a v i e s and l i g h t s s h o u l d be s e t t l i n g e q u i v a l e n t s , and ( 2 ) t r a n s l a t e n t s t r a t a i n a e o l i a n sands s h o u l d show e q u a l - s i z e d h e a v i e s and l i g h t s i f t h e y a r e a v a i l a b l e i n t h e source. ACKNOWLEDGEMENTS T h i s r e s e a r c h f o r m s p a r t o f K . G e r e t y ' s Ph.D.
thesis,
supervised by R. Slinger-
l a n d , n a r t l y s u p p o r t e d by; t h e G e o l o g i c a l S o c i e t y o f A m e r i c a P e n r o s e Fund, t h e P. D. K r y n i n e Fund (Geosc. Dept.,
Penn. S t a t e U n i v . ) and a G r a d u a t e School F e l l o w -
s h i p . The C o l l e g e of E a r t h a n d M i n e r a l S c i e n c e a n d t h e D e p t . o f Geology, D i c k i n s o n C o l l e g e p r o v i d e d c o m p u t e r f u n d s and l a b o r a t o r y space r e s p e c t i v e l y . Sharon Gabel, H e n r y Hanson and James J e n n i n g s Howard h e l p e d c o l l e c t t h e d a t a . REFERENCES Bagnold, R. A., 1 9 4 1 ( r e p r i n t e d 1954). The P h y s i c s o f Blown Sand and D e s e r t Dunes. Chapman & H a l l , London, 265 pp.
131 Proc. Roy. Ragnold, R. A., 1956. The f l o w o c o h e s i o n l e s s g r a i n s i n f l u i d s . SOC. London A 249:235-297. Bagnold, R. A., 1973. The n a t u r e o f s a l t a t i o n and o f "bed l o a d " t r a n s p o r t i n water. Proc. Roy. SOC. London A 332:473-504. B a r n d o r f f - N i e l s e n , 0., O a l s g a a r d , K., H a l g r e e n , C., Kuhlman, H., M o l l e r , J. T. and Schou, G., 1982. V a r i a t i o i n p a r t i c l e s i z e d i s t r i b u t i o n o v e r a s m a l l dune. S e d i m e n t o l o q--v 29:53-65. Chepil, W. S., 1945. Dynamics o f w i n d e r o s i o n : 11. I n i t i a t i o n o f s o i l movement. S o i l S c i e n c e 60:397-411. Chiu, T. Y., 1972. Sand T r a n s p o r t by Wind. Ilniv. o f F l o r i d a , nept. Coastal ! I Oceanographic E n g i n e e r i n g , Tech. Rept. #1, 53 pp. E i n s t e i n , H. A., 1950. The Bed-load F u n c t i o n f o r Sediment T r a n s p o r t a t i o n i n Open Channel Flows. U.S. n e p t . A g r i c . , Tech. R u l l . 1026, 68 pp. E n g e l h a r d t , W. von, 1940. D i e U n t e r s c h e i d u n g wasser- und w i n d s o r t i e r t e r Sande a u f Grund d e r K o r n g r o s s e n v e r t e i l u n g i n h r e r l e i c h t e n und schweren Gemengteile. Chemie d e r E r d e 12:445-465. "Saltation equivalence" i n Gerety, K. M. and S l i n g e r l a n d , R. L., 1982. a e o l i a n sand t r a n s p o r t . (abs.) 1 1 t h c o n g r e s s , I A S , p. 673. Greeley, R., I v e r s e n , J. D., P o l l a c k , J . B., Udovich, N. and White, B. R., 1974. Wind t u n n e l s t u d i e s o f M a r t i a n e o l i a n p r o c e s s e s . Proc. Roy. SOC. London A 341 :331-360. Greeley, R., W i l l i a m s , S. H. and M a r s h a l l , J. R., 1983. V e l o c i t i e s o f Windblown p a r t i c l e s i n S a l t a t i o n . T h i s volume. Hand, H. M., 1967. D i f f e r e n t i a t i o n o f beach and dune sands u s i n g s e t t l i n g v e l o c i t i e s o f l i g h t and heavy m i n e r a l s . J o u r . Sed. Pet. 37:514-521. Hunter, R., 1977. B a s i c t y p e s o f s t r a t i f i c a t i o n i n s m a l l e o l i a n dunes. S e d i m e n t o l o g y 24:361-387. Iversen, J. D., 1982. S a l t a t i o n t h r e s h o l d and d e p o s i t i o n r a t e m o d e l i n g (abs.) I A S , 1 1 t h Congress, p. 62. I v e r s e n , J. D., G r e e l e y , R., P o l l a c k , J. R. and White, R. R., 1973. S i m u l a t i o n o f M a r t i a n e o l i a n phenomena i n t h e a t m o s p h e r i c w i n d t u n n e l . NASA Spec. Pub. Sp. 336, p. 191-213. A F u n c t i o n o f Sand Movement by Wind. Ilniv. C a l i f . Kadib, A. A., 1965. B e r k e l e y , H y d r a u l i c E n g i n e e r i n g L a b o r a t o r y , T e c h n i c a l R e p o r t HEL-2-12, 9 1 PP. Maegley, W. J., 1974. Prediction o f Physical Characteristics of Martian Sandstorms. M a r t i n - M a r i e t t a Corp., Denver, V i k i n g E n g i n e e r i n g R e p o r t VER-281, 3 1 pp. Owen, P. R., 1964. S a l t a t i o n o f u n i f o r m g r a i n s i n a i r . Jour. F l u i d Mechanics 20:225-242. 1978. Numerical study o f continuous s a l t a t i o n . Am. SOC. C i v i l Reizes, J. A., Eng., J o u r . Hyd. n i v . , io4:1305-1321. S a l l e n g e r , A. H., 1979. I n v e r s e g r a d i n g and h y d r a u l i c e q u i v a l e n c e i n grain-flow deposits. Jour. Sed. Pet. 49:553-562. Steidtmann, J. R. and Haywood, H. C., 1982. S e t t i n g v e l o c i t i e s o f q u a r t z and t o u r m a l i n e i n e o l i a n sandstone s t r a t a . J o u r . Sed. Pet. 52:395-400. Tsuchiya, Y., 1970. S u c c e s s i v e s a l t a t i o n o f a sand g r a i n by wind. Proc. 1 2 t h Conf. on C o a s t a l E n g i n e e r i n g , v o l . 1, p. 1417-1427. S a l t a t i o n i n T e r r e s t r i a l and M a r t i a n Atmospheres. Ph.n. White, 6 . R., 1975. T h e s i s , Iowa S t a t e Ilniv., Ames, Iowa, 200 pp. White, 5. R., G r e e l e y , R., I v e r s e n , J. 0. and P o l l a c k , J. R., 1976. Estimated g r a i n s a l t a t i o n i n a m a r t i a n atmosphere. J o u r . Geophys. Res. 81: 5643-5650. 1977. Magnus e f f e c t i n s a l t a t i o n . Jour. White, 6 . R. and S c h u l z , J. C., F l u i d Mech. 81:497-512. Some a s p e c t s o f t h e e o l i a n s a l t a t i o n l o a d . Williams, 6. P., 1964. Sediment01 ogy 3: 257-287. Zingg, A. W., 1953, Wind t u n n e l s t u d i e s o f t h e movement o f s e d i m e n t a r y m a t e r i a l . P r o c e e d i n g s o f t h e 5 t h H y d r a u l i c Conference. B u l l . 24, pp. 111-135, Univ. o f Iowa S t u d i e s i n Eng., Iowa City.
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133
VELOCITIES
OF WINDBLOWN
PARTICLES I N
SALTATION:
PRELIMINARY LABORATORY
AND
FIELD MEASUREMENTS
R. GREELEY: Dept. o f Geology, A r i z o n a S t a t e U n i v e r s i t y , Tempe, AZ
85287
S.H.
WILLIAMS: Dept. o f Geology, Arizona S t a t e U n i v e r s i t y , Tempe, AZ
J.R.
MARSHALL: NASA-Ames Research Center, M o f f e t t F i e l d , CA
g
85287
94035
gravitational acceleration
urn f r e e s t r e a m wind speed
u,
wind f r i c t i o n speed
u,.~
wind t h r e s h o l d f r i c t i o n speed
x
h o r i z o n t a l d i s t a n c e on wind t u n n e l t e s t p l a t e
E
roughness h e i g h t
1.0
INTRODUCTION The speeds o f g r a i n s c a r r i e d by t h e wind has been a t o p i c o f i n t e r e s t s i n c e
Bagnold's (1941)
p i o n e e r i n g work on
t h e physics
o f windblown p a r t i c l e s .
i n t e r e s t i n t h e s u b j e c t developed f r o m r e s e a r c h on t e r r e s t r i a l t r i a l a e o l i a n processes ( T a b l e l ) , e s p e c i a l l y sion on
Mars (Greeley e t al.,
tion i s c r i t i c a l
1982).
Our
and e x t r a t e r r e s -
i n r e g a r d t o r a t e s of
wind a b r a -
D e t e r m i n i n g p a r t i c l e speed d u r i n g s a l t a 1) t h e r a t e o f a b r a s i o n of s u r f a c e
i n t h e understanding o f :
m a t e r i a l s by i m p a c t i n g p a r t i c l e s , 2) t h e r a t e and n a t u r e o f p a r t i c l e comminution i n t h e s a l t a t i o n c l o u d , 3) t h e r e l a t i o n s h i p between s a l t a t i o n p a r t i c l e wind speed and i t s s i g n i f i c a n c e f o r sediment r e d i s t r i b u t i o n , and 4) of saltating
p a r t i c l e s i n general.
We have
i n v e s t i g a t e d p a r t i c l e speed under
simulated m a r t i a n , t e r r e s t r i a l , and Venusian e n v i r o n m e n t a l c o n d i t i o n s mine
i t s relationship t o
t h e key
parameters o f atmospheric
diameter, and wind speed (Wil'liams and Greeley, results.
I n addition,
planets which e x p e r i e n c e a e o l i a n processes:
t o deter-
density, p a r t i c l e
1983).
T h i s r e p o r t p r e s e n t s t h e methods used t o determine p a r t i c l e gives p r e l i m i n a r y
f l u x and
t h e physics
we compare
v e l o c i t i e s and
r e s u l t s among t h e
Earth, Mars, and Venus.
three
134 TABLE 1.
IMPORTANT PARAMETERS OF THE AEOLIAN ENVIRONMENTS OF EARTH, MARS, AND VENUS EARTH
VENUS
Atmospheric Composition
96%
COP
N2
S u r f a c e Pressure
3.5%
MARS
N2
77%
c02
02
21%
N2
2.7%
Ar
1.6%
H20
1%
Ar
0.9%
90 bar
95%
1 bar
7 mb
-288
-218
~
Temperature ( K e l v i n )
-730 2.2 cm/s
Hinimum t h r e s h o l d winc, r r i c t i o n speed f o r p a r t i c l e e n t r a i n m e n t ( f r o m I v e r s e n e t a l . , 1976).
\J,
1.2
250 cm/s
20.5 cm/s
APPROACH
The approach
used i n t h i s
study involved
the analysis o f
windblown par-
t i c l e s i n wind t u n n e l s u s i n g an i n s t r u m e n t t o measure d i r e c t l y t h e speed of particles,
supplemented by high-speed
second) o f g r a i n s i n f l i g h t . tained i n
a field
motion p i c t u r e s
I n addition,
experiment
(up t o
10,000
frames per
high-speed m o t i o n p i c t u r e s were ob-
t o determine
particle velocities
in
a natural
aeol ian e n v i ronment. Three atmospheric b o u n d a r y - l a y e r wind t u n n e l s were used i n t h e experiments, a l l o f which a r e a p a r t o f d i n a t e d by A r i z o n a
t h e NASA P l a n e t a r y Geology A e o l i a n
State University
(Greeley e t
al.,
Consortium coor-
1977; 1981).
l o c a t e d a t NASA's Ames Research Center, M o f f e t t F i e l d , C a l i f o r n i a : Surface ___ lating
Wind Tunnel --
(MARSWIT; Fig.
l),
m a r t i a n atmosphere c o m p o s i t i o n
t e r r e s t r i a l gravity,
a 13 m long, o p e n - c i r c u i t (Coil) and
system o p e r a t i n g a t 1 g a t a CO2 p r e s s u r e
o f 35 b a r and an
s u r f a c e o f Venus).
University,
The t h i r d
i s an open c i r c u i t , 20
b o u n d a r y - l a y e r t u n n e l (Fig. T e s t s were
run i n
aeolian a c t i v i t y i n
all
mb) at
2), a c l o s e d - c i r c u i t ambient temperature
o f 220 C ( p r o d u c i n g t h e same f l u i d d e n s i t y as t h e -90 b a r , 730 K near t h e
system simu-
surface pressures (-7
and t h e --Venus Wind Tunnel (VWT; Fig.
Two are
t h e Martian
C02 atmosphere
wind t u n n e l , l o c a t e d a t Arizona
m l o n g by 1.0 m2
State
c r o s s - s e c t i o n atmospheric
3) o p e r a t i n g a t ambient t e r r e s t r i a l c o n d i t i o n s . t h r e e wind
t u n n e l s under
optimal
t h e p l a n e t a r y environment o f concern.
s e l e c t e d f o r a n a l y s i s i n c l u d e d t h o s e most e a s i l y
c o n d i t i o n s for
The p a r t i c l e
moved by t h e wind (Fig.
sizes 4) as
135
Fig. 1. P h o t o g r a p h o f t h e M a r t i a n S u r f a c e Wind Tunnel a t NASA-Ames Research Center. The o p e n - c i r c u i t a t m o s p h e r i c b o u n d a r y - l a y e r t u n n e l i s 13 m l o n g and has a 1.1 m2 t e s t s e c t i o n l o c a t e d 5 m f r o m t h e e n t r a n c e . It i s o p e r a t e d i n a l o w pressure chamber ( b a c k g r o u n d ) i n a C O P atmosphere a t -7 mb t o s i m u l a t e m a r t i a n conditions. w e l l as common dune sand particle
bed
s i z e s (-300
covered t h e
urn) on E a r t h .
e n t i r e bed
of
the test
I n most
experiments, t h e
s e c t i o n and
there
was an
unlimited supply o f p a r t i c l e s a v a i l a b l e f o r entrainment.
2.0
WIND TUNNEL MEASUREMENTS
L a b o r a t o r y e x p e r i m e n t s have t h e a d v a n t a g e t h a t c o n d i t i o n s can
be m o n i t o r e d
and c l o s e l y c o n t r o l l e d , t h u s r e d u c i n g t h e u n c e r t a i n t i e s i n i n t e r p r e t a t i o n o f t h e results
and a l l o w i n g a w i d e r a n g e
o f c o n d i t i o n s t o be t e s t e d .
e x t r a t e r r e s t r i a l s t u d i e s , l a b o r a t o r y s i m u l a t i o n s p r o v i d e an
I n t h e case of
extremely important
data base f o r a n a l y z i n g t h e l i m i t e d r e s u l t s o b t a i n e d i n s i t u f r o m s p a c e c r a f t . P a r t i c l e s u s e d i n t h e w i n d t u n n e l e x p e r i m e n t s were o b t a i n e d f r o m commercial sources
and f o r
t h e m a r t i a n and
w e l l - s o r t e d samples. Walnut-shell
Venusian runs
Q u a r t z sands
p a r t i c l e s were
were crushed and s i e v e d t o
were used
used i n obtain the
walnut s h e l l s (-1.1 g/cm3) a l l o w s
were f u r t h e r s i e v e d i n E a r t h and
the martian
simulations;
sizes of interest.
a partial
scaling f o r the
t o obtain
Venus s i m u l a t i o n s . these p a r t i c l e s
The l o w
d e n s i t y of
low g r a v i t a t i o n a l
136
F i g . 2. View o f t h e Venus Wind Tunnel a t NASA-Ames Research C e n t e r . This closed-circuit a t m o s p h e r i c b o u n d a r y - l a y e r w i n d t u n n e l has a 1 m l o n g by (3.14)(10-L)m2 t e s t s e c t i o n and o p e r a t e s a t a m b i e n t t e m p e r a t u r e w i t h carbon d i o x i d e a t 35 b a r t o s i m u l a t e V e n u s i a n c o n d i t i o n s . field
o f Mars,
appropriate
p r e v i o u s l y (Greeley e t al., The w i n d
f o r experiments
v e l o c i t y measured i n
t h e wind
speed, urn, f o u n d above t h e b o u n d a r y l a y e r . t e r r e s t r i a l and m a r t i a n c a s e s a l l o w wind f r i c t i o n
c o n d u c t e d on
Earth
as d i s c u s s e d
1981; 1982).
speed, u,(after
t u n n e l s was
the
f r e e s t r e a m wind
Boundary-layer p r o f i l e s taken i n the
c o n v e r s i o n o f urn t o t h e
Bagnold, 1941).
c o r r e s p o n d i n g wind
F o r t h e VWT t e s t s , u,
was d e t e r -
m i n e d from urn by u s i n g t h e e q u a t i o n o f S c h l i c h t i n g ( 1 9 6 8 ) :
where x
=
d i s t a n c e down t h e t e s t p l a t e and
p a r t i c l e diameter).
Boundary l a y e r
E
=
roughness h e i g h t ( e s s e n t i a l l y the
studies are
v a r i o u s s u r f a c e roughness v a l u e s used f o r t h e VWT. p l i c a t i o n a r i s e s when sand
i s set i n motion; t h e
t r a n s f e r o f momentum f r o m t h e w i n d t o t h e r e p o r t e d here a r e approximate.
grains
c u r r e n t l y underway t o
assess
It m u s t be n o t e d t h a t a com-
u,/urn
r a t i o changes due
n motion, thus t h e
to a
u, v a l u e s
137
Fig. 3. View o f t e s t s e c t i o n o f t h e one atmosphere, o p e n - c i r c u i t w i n d t u n n e l i n t h e P l a n e t a r y Geology L a b o r a t o r y a t A r i z o n a S t a t e U n i v e r s i t y . The t u n n e l i s 19 m long and has a 1.1 m2 c r o s s s e c t i o n . Shown h e r e i s t h e e x p e r i m e n t a l s e t - u p t o f i l m windblown p a r t i c l e s i n f l i g h t a g a i n s t a c m - g r i d b a c k d r o p ( i n t e s t s e c t i o n ) using a Hycam Model I 1 m o t i o n p i c t u r e camera ( a t r i g h t ) .
2.1
PARTICLE VELOCIMETER Figure
5 i s a diagram o f
experiments.
t h e p a r t i c l e v e l o c i m e t e r used i n
The d e s i g n o f t h i s d e v i c e i s based
Forest S e r v i c e ( S c h m i d t , 1977) t o ticles.
measure t h e
the laboratory
on a system used by
v e l o c i t y o f windblown
t h e U.S. snow p a r -
The v e l o c i m e t e r u t i l i z e s a l i g h t s o u r c e w h i c h p r o j e c t s a beam a c r o s s a
s m a l l gap t o t w o p h o t o t r a n s i s t o r s .
As a w i n d b l o w n g r a i n passes t h r o u g h t h e gap,
i t i n t e r r u p t s t h e beam and c a s t s a
shadow on t h e f i r s t d e t e c t o r
w h i c h responds
p u l s e ; i t t h e n shadows t h e
second d e t e c t o r
by g e n e r a t i n g a p o s i t i v e e l e c t r i c a l which g e n e r a t e s a n e g a t i v e p u l s e .
for
These s i g n a l s
l a t e r a n a l y s e s w i t h an o s c i l l o s c o p e .
a r e r e c o r d e d on m a g n e t i c t a p e
The v e l o c i t y
i s determined from t h e
distance t r a v e l e d (known f r o m t h e geometry o f t h e windows) and f r o m t h e travel
(determined from t h e
signal displayed
on t h e o s c i l l o s c o p e ) .
signals r e p r e s e n t i n g g r a i n s e i t h e r p a s s i n g t h r o u g h t h e gap a t o b l i q u e i n clusters are Approximately
i d e n t i f i e d by
150 s i g n a l s
velocity d i s t r i b u t i o n s .
were
asymmetric o r analyzed f o r
distorted each r u n
time o f Spurious angles o r
oscilloscope tracings. to
determine p a r t i c l e
138
Fig. 4. T h r e s h o l d f r i c t i o n speed (m/s) as a f u n c t i o n o f p a r t i c l e d i a m e t e r f o r Earth, Mars, and Venus; speed r e q u i r e d f o r p a r t i c l e m o t i o n i s i n v e r s e l y proport i o n a l t o t h e atmospheric d e n s i t y ( T a b l e 1); n o t e t h a t t h e optimum g r a i n s i z e ( i d e n t i f i e d on each curve) f o r p a r t i c l e threshold i s about t h e same i n a l l three planetary environments; ( a f t e r I v e r s e n and White, 1982).
1
(I)
-> \
E
-
t
115
k
0 0 0l.e 8 O A W
>
z
-
0
I-
0
EARTH 0.2
75
a L
0.10
P A
-
0 I u)
0.02 -
w 0.03 a I I-
75
0
. 2030
10
0
50
100
1 200
5 500 1
0
P A R T I C L E D I A M E T E R ((lm)
above the
To determine t h e range o f p a r t i c l e v e l o c i t i e s a t a s i n g l e h e i g h t s u r f a c e , v e l o c i t y measurements were grouped plotted
as a h i s t o g r a m expressed as
s p e c i f i e d v e l o c i t y range. percentage o f
i n even
i n c r e m e n t s o f one
t h e percentage
f r e e s t r e a m wind speed.
This l a t t e r
function i s
o r d e r s o f magnitude.
As shown
range.
has demonstrated t h e a c c e l e r a t i o n o f
saltation Figure 6
i n Figure
path, and we c o n s i d e r t h e
experiment
t o r e f l e c t grains a l s o shows
in a
V e l o c i t i e s a r e g i v e n b o t h as an a c t u a l speed and as a useful i n nor-
m a l i z i n g d a t a f o r i n t e r p l a n e t a r y comparisons i n which w i n d speeds may White (1979)
m/s and
o f grains t r a v e l l i n g
6, p a r t i c l e v e l o c i t i e s
wide range
sampled i n
t h a t some g r a i n s
d i f f e r by
have a
i n v e l o c i t i e s measured
different parts o f
exceed t h e
wide
p a r t i c l e s along the i n our
t h e i r trajectory.
f r e e s t r e a m wind
a t t r i b u t e t h i s observation t o very high accelerations r e s u l t i n g from
speed.
We
rebound o f
s a l t a t i n g g r a i n s f r o m t h e s u r f a c e and from i n t e r g r a i n c o l l i s i o n s . Tests t h e surface. speed w i t h
were r u n t o d e t e r m i n e p a r t i c l e
As shown
i n Figure
7, t h e r e i s
height, r e f l e c t i n g , f i r s t ,
t h r o u g h t h e boundary
layer,
speed as a f u n c t i o n o f a general
t h e increase
and second,
t h e longer
h e i g h t above
increase i n p a r t i c l e
i n wind speed particle
w i t h height
trajectory
--
139
" . . ,
PHOTOTRANSISTORS
-
Fig. 5. Diagram o f t h e p a r t i c l e v e l o c i me t e r used t o measure t h e speed o f windblown g r a i n s ( f r o m Greeley e t al., 1982)
p a r t i c l e p a t h l e n g t h i n c r e a s e s w i t h h e i g h t and a f f o r d s a l o n g e r t i m e e r a t i o n by t h e wind.
f o r accel-
A s i m i l a r r e l a t i o n s h i p was found i n experiments s i m u l a t i n g
the m a r t i a n environment (Fig. 8).
The p a r t i c l e v e l o c i m e t e r does n o t o p e r a t e i n
the Venus Wind Tunnel, and no measurements were made w i t h t h i s technique. F i g u r e 9 shows t h e e f f e c t at
a s i n g l e height.
o f wind v e l o c i t y on p a r t i c l e
Two p a r t i c l e
martian atmospheric
conditions.
f r e q u e n t l y measured
speed).
v e l o c i t i e s t h a n l a r g e r ones. wind and t h e
smaller
s i z e s (350 urn and 92 P a r t i c l e speeds
I n general,
v e l o c i t y , measured
urn) were t e s t e d under
a r e modal
smaller p a r t i c l e s
values ( t h e travel a t
most higher
We a t t r i b u t e t h i s t o a b e t t e r c o u p l i n g between t h e
p a r t i c l e s owing
t o their
larger
cross-sectional
area-
to-mass r a t i o . Figure
9 a l s o shows t h a t p a r t i c l e
markably c o n s t a n t t h r o u g h a wide may shed some l i g h t on known t h a t Kawamura,
the characteristics of particle flux.
increases i n 1951; and
v e l o c i t y a t a given height
wind
others).
remains r e -
range o f f r e e s t r e a m wind speeds. speed cause However,
increases i n it
has n o t
This r e s u l t
It has l o n g been
flux
(Bagnold, 1941;
been known
whether
the
increase i n f l u x r e s u l t e d p r i m a r i l y from h i g h e r p a r t i c l e v e l o c i t i e s o r p r i m a r i l y from i n c r e a s e s i n t h e number o f g r a i n s i n t r a n s p o r t , o r both.
F i g u r e 9 suggests
t h a t i n c r e a s e s i n f l u x may be t h e r e s u l t o f more g r a i n s i n t r a n s p o r t r a t h e r t h a n o f higher p a r t i c l e v e l o c i t i e s .
2.2
HIGH SPEED MOTION PICTURES
A Hycam Model I 1 16 mm camera (Fig.
o f windblown
g r a i n s a t f r a m i n g r a t e s up
were photographed a g a i n s t a back-drop ysis
of the f i l m .
The f i e l d
3) was used t o o b t a i n t o 10,000
frames p e r second.
w i t h a cm-scale g r i d t o
o f view had t o be
motion p i c t u r e s Grains
f a c i l i t a t e anal-
s u f f i c i e n t l y small
t o permit
i n d i v i d u a l g r a i n s t o be r e s o l v e d and y e t l a r g e enough t o c h a r a c t e r i z e t h e f l i g h t path o f t h e g r a i n s
and t o d e t e r m i n e t h e i r v e l o c i t i e s .
analyzed (-600 pm), t h e maximum u s e f u l f i e l d o f view
For t h e
largest grains
was about 6 cm h i g h
by 10
140
3 5 0 p m QUARTZ 16.1cm HEIGHT
~ ~ ' 1 0 . 2m2/ s
PARTICLE VELOCITY (m/roc) I
1
0
20
60
40
80
I
100
120
% O F FREESTREAM WIND S P E E D
Fig. 6. D i s t r i b u t i o n o f p a r t i c l e v e l o c i t i e s a t a h e i g h t o f 16.1 cm above t h e s u r f a c e f o r 350 um d i a m e t e r q u a r t z g r a i n s s u b j e c t e d t o a f r e e s t r e a m windspeed of -10.22 m/s i n a s i m u l a t i o n o f E a r t h (u, = 0.5 m/s). The v e l o c i t y d i s t r i b u t i o n i s t y p i c a l f o r b o t h wind t u n n e l and f i e l d experiments and w i l l be r e p r e s e n t e d i n subsequent f i g u r e s as a one-standard d e v i a t i o n v e l o c i t y range around a mean p a r t i c l e velocity. cm wide.
Thus f a r i n o u r i n v e s t i g a t i o n o n l y a l i m i t e d number o f runs have been
made i n which h i g h speed m o t i o n p i c t u r e s were o b t a i n e d , a l l a t E a r t h
(wind t u n -
n e l and f i e l d c o n d i t i o n s ) and Venus c o n d i t i o n s , b u t n o t m a r t i a n c o n d i t i o n s . Data were
obtained from
processed
L a f a y e t t e Analyzer o n t o a 40" by 60" GTCO 11/45 computer. i n t o the
The g r i d
image
via a
x-y d i g i t i z i n g t a b l e l i n k e d t o
a PDP
f i l m s by
p o s i t i o n , f r a m i n g r a t e , and wind
program and t h e p a r t i c l e p o s i t i o n s
f i l m sequence.
projecting the
were p l o t t e d
speed were e n t e r e d f o r each frame
in a
The reduced d a t a i n c l u d e v e l o c i t i e s i n t h e f o l l o w i n g c a t e g o r i e s :
1) particles i n r i s i n g trajectories,
2) p a r t i c l e s i n f l a t t r a j e c t o r i e s ,
t i c l e s i n f a l l i n g t r a j e c t o r i e s and 4) combined t r a j e c t o r i e s ;
3) p a r -
p a r t i c l e speeds are
g i v e n i n m/s and as a percentage o f f r e e s t r e a m wind speed (urn).
141
25 0-
E
0
20
W
0 4: LL
a
15
3
fn W
> 0
m
EARTH CASE
10
3 5 0 ptn Q U A R T Z P A R T I C L E S
4
v
I-
r
?!
urnr 1 0 . 5 r n / s
5
W
r I
I
I
I
1
I
I
MEAN PARTICLE VELOCITY ( m / s ) 1
I
I
0 20 40 60 80 100 M E A N P A R T I C L E V E L O C I T Y (%urn)
Fig. 7. P a r t i c l e v e l o c i t i e s f o r 350 um q u a r t z g r a i n s measured a t f o u r h e i g h t s above t h e s u r f a c e i n a wind t u n n e l a t 1 b a r p r e s s u r e and a f r e e s t r e a m wind speed o f 10.5 m/s (u, = 0.5 m/s). Figure
10 shows
typical
results for
speeds i n c r e a s e w i t h h e i g h t above with the
p a r t i c l e velocimeter, although
f i l m i s limited.
E a r t h cases;
t h e surface
in
general, p a r t i c l e
similar t o the
t h e range
r e s u l t s obtained
i n the heights
analyzed by
A t t h i s t i m e , i d e n t i c a l experiments have n o t been r u n t o com-
pare r e s u l t s f r o m t h e v e l o c i m e t e r w i t h t h o s e f o r t h e m o t i o n p i c t u r e s . shows
r e s u l t s o b t a i n e d i n t h e Venus
tures.
Wind Tunnel
f r o m analyses o f
Although t h e a b s o l u t e v e l o c i t i e s a r e v e r y low
(a r e f l e c t i o n of t h e low
wind speeds on Venus), n o t e t h a t t h e p a r t i c l e s a c h i e v e n e a r l y t h e as f r e e s t r e a m
wind speed,
in
marked c o n t r a s t
t o Mars
F i g u r e 11 motion p i c -
where
same v e l o c i t y
p a r t i c l e s seldom
reach f r e e s t r e a m v e l o c i t i e s . I n a d d i t i o n , p a r t i c l e v e l o c i t i e s on r i s i n g , f l a t , and were assessed (Fig. Earth increase
12).
throughout
falling trajectories
As p r e d i c t e d by White (1979), p a r t i c l e v e l o c i t i e s the saltation
b e f o r e impact w i t h t h e s u r f a c e ,
t r a j e c t o r y , reaching
a
on
maximum j u s t
142
25 A
E 0
20
W
0
U
u. U 3
15
ln W
> 0
m U
MARS CASE
10
3 5 0 p m SHELL PARTICLES
-
c
u
I
a -
6 5 m/ s
5
W
I I
(
I
1
I
1
0
I
1
I
1 35
I
30 20 25 15 10 MEAN PARTICLE VELOCITY (m/s)
5
I 50
I
I
10 20 30 40 M E A N P A R T I C L E V E L O C I T Y (%Urn)
Fia. 8. P a r t i c l e v e l o c i t i e s f o r f o u r d i f f e r e n t h e i a h t s above t h e s u r f a c e for 356 w a l n u t s h e l l p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m wind speed o f 65 m/s (u, = 3.4 m/s) i n a low atmospheric p r e s s u r e (6.6 mb) t o s i m u l a t e t h e martian e n v i ronment. 3.0
FIELD STUDIES Although l a b o r a t o r y s i m u l a t i o n s e n a b l e a e o l i a n processes t o be i n v e s t i g a t e d
under c o n t r o l l e d c o n d i t i o n s , q u e s t i o n s the results
as a p p l i e d t o n a t u r a l c o n d i t i o n s .
t i o n s , a f i e l d experiment was particles.
inevitably arise
conducted t o
I n order
v a l i d i t y of
as t o t h e
t o address such ques-
obtain v e l o c i t y data
f o r saltating
The experiment was conducted 3 November 1981 a t Waddell Creek State
Beach i n C a l i f o r n i a , about 90
km s o u t h o f San Francisco.
The beach
i s one o f
t h e w i n d i e s t on t h e c o a s t and i s f a i r l y wide, e n a b l i n g a good s a l t a t i o n c l o u d t o develop across
t h e dry
p a r t o f the
beach b e f o r e
r e a c h i n g t h e area
where the
f i l m i n g t o o k place. Using t h e same i n s t r u m e n t s as ( a t a h e i g h t o f 1.0 m above Greeley e t al., taneously.
t h e surface), p a r t i c l e f l u x (using
1982), and h i g h
The p a r t i c l e
wind v e l o c i t y
employed i n t h e w i n d t u n n e l s ,
speed m o t i o n p i c t u r e s were a l l
c o l l e c t o r s , see o b t a i n e d simul-
v e l o c i m e t e r m a l f u n c t i o n e d and d a t a were
w i t h i t i n t h e f i e l d experiment.
n o t obtained
143
92vm
350um 130
120
Siia
\
E
10(
k
0 0 A
9c
W
> n
z
3 z a
W
8C
7C
a
+ W W
6(
U LL
5c 1
I
5
10
1
1
I
1
20
25
30
35
I
15
PARTICLE VELOCITY ( Y O D E , m / a )
Fig. 9. S t a t i s t i c a l modes o f v e l o c i t i e s as a f u n c t i o n o f f r e e s t r e a m wind speed a t a h e i g h t o f 7.1 cm above t h e s u r f a c e f o r 350 urn and 92 urn s h e l l p a r t i c l e s i n a m a r t i a n s i m u l a t i o n ; p a r t i c l e v e l o c i t y remains c o n s t a n t o v e r a wide range of freestream wind speeds. Four s u c c e s s f u l r u n s were completed, hold.
i n t h e wind t u n n e l runs. is
a l l a t wind speeds j u s t
The m o t i o n p i c t u r e s were analyzed f o l l o w i n g t h e
somewhat d i f f e r e n t
above t h r e s -
same procedures as used
F i g u r e 13 shows t h e r e s u l t s ; a l t h o u g h t h e f r o m t h o s e used
i n the
wind t u n n e l runs,
grain size the particle
v e l o c i t y d i s t r i b u t i o n s a r e w i t h i n t h e range expected.
4.0
SUMMARY AND CONCLUSIONS
Velocities o f speed,
windblown
h e i g h t above t h e
p a r t i c l e s were
ground, and
d e t e r m i n e d as
p a r t i c l e diameter f o r
functions
of wind
various conditions
s i m u l a t i n g Earth, Mars, and Venus i n e n v i r o n m e n t a l wind t u n n e l s .
S i m i l a r data,
although o f l i m i t e d range, were o b t a i n e d f r o m a f i e l d experiment
f o r comparison
w i t h t h e wind t u n n e l r e s u l t s s i m u l a t i n g t h e t e r r e s t r i a l environment.
144
-
-
EARTH CASE
E
-
ASU WIND TUNNEL
0
W
0
a Y
U 3 UJ W
> 0
m
a I-
I
-
Q W
I
I
'0
I
I
I
I
I
J
1
5 6 7 3 4 2 MEAN PARTICLE VELOCITY (m/s)
1 I
I
I
I
I
1
I
70
80
I
20 30 40 50 60 MEAN PARTICLE VELOClTY
10
8
(%Urn)
F i g . 10. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s e s of h i g h speed m o t i o n p i c t u r e s f o r 400 q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m w i n d speed of 10.5 m/s (u, = 0.55 m / s ) i n t h e 1.0 b a r a t m o s p h e r i c w i n d t u n n e l a t A r i z o n a S t a t e Un iv e r s i t y
.
I n g e n e r a l , t h e r e s u l t s show
that particles
travel a t higher
speeds w i t h
i n c r e a s e d h e i g h t above t h e ground, and t h a t s m a l l e r p a r t i c l e s t r a v e l f a s t e r t h a n l a r g e r ones. not increase
However, f o r a g i v e n h e i g h t above t h e ground, p a r t i c l e speed does with higher
t h i s i s with reference t o
f r e e s t r e a m w i n d speeds.
modal v a l u e s
and
I t must
be remembered t h a t
n o t t o maximum v a l u e s ; a
few p a r t i -
c l e s do i n c r e a s e i n v e l o c i t y w i t h f r e e s t r e a m w i n d v e l o c i t i e s . Comparisons o f r e s u l t s f o r
E a r t h , Mars,
and Venus r e v e a l
some r e m a r k a b l e
differences.
As shown i n F i g u r e 14, most p a r t i c l e s a c h i e v e speeds n e a r l y equal
t o freestream
w i n d speed o n Venus, b u t
seldom a c h i e v e
Mars; E a r t h c a s e s a r e o f i n t e r m e d i a t e v a l u e s . ences
i n a t m o s p h e r i c d e n s i t y and t o
p l a n e t a r y e n v i r o n m e n t s ( T a b l e 1). V e n u s i a n atmosphere t h a n on and f o r t h e (just
t h e t h r e s h o l d w i n d speeds among
speeds), t h e
speed on the three
P a r t i c l e s a r e more e a s i l y moved i n t h e dense
Mars; c o n s e q u e n t l y , t h r e s h o l d speeds a r e
r a n g e o f w i n d speeds i n
above t h r e s h o l d
h a l f t h e wind
This i s attributed t o the d i f f e r -
w h i c h most grains
movement i s presumed
need n o t
be m o v i n g
very
v e r y low, t o occur fast to
145
-
3
-
VENUS CASE
E
VENUS WIND TUNNEL
0 Y
5 0 0 - 6 0 0 p m QUARTZ PARTICLES
W
0
__t_
ucO = 3.621111s
U
u . 2 K 3 v)
W
> 0
m
-
u 1
-
I-
-
r
-
0
-
W
I I
0
I
1
,
-
I
I
1
1
I
I
I
I
I
I
I
1
60
l
1
1
I
4.0 I
I
I
l
3.5
2.5 3.0 MEAN PARTICLE VELOCITY (m/s)
0
80 90 70 MEAN PARTICLE VELOCITY (%urn)
100
Fig. 11. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f h i q h speed m o t i o n p i c t u r e s f o r 500 t o 600 pm q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m w i n d speed o f 3.62 m/s (u, = 0.2 m/s) a t 30 b a r s a t m o s p h e r i c p r e s s u r e i n t h e Venus Wind Tunnel. achieve
100% o f t h e w i n d speed.
C o n v e r s e l y , p a r t i c l e s on Mars must a c c e l e r a t e
very r a p i d l y t o a c h i e v e t h e speed o f t h e h i g h w i n d s r e q u i r e d f o r despite
the f a c t t h a t s a l t a t i o n path
lengths are
l o n g on Mars
most g r a i n s f a l l t o t h e s u r f a c e b e f o r e a c h i e v i n g even 50-60% o f
t h r e s h o l d , and ( W h i t e , 1979), freestream wind
speed. F u t u r e e x p e r i m e n t s w i l l i n v o l v e e x p a n s i o n o f t h e d a t a base t o a w i d e r r a n g e o f wind-tunnel
and f i e l d c o n d i t i o n s and f o r a w i d e r r a n g e o f p a r t i c l e d i a m e t e r s ,
wind speeds, and measurements f o r
v a r i o u s h e i g h t s above t h e s u r f a c e .
tion, a v e l o c i m e t e r i s under development t h a t w i l l be f i e l d - p o r t a b l e obtain
a l a r g e r number
of velocity
measurements t h a n a r e
I n addii n order t o
presently available
from h i g h - s p e e d m o t i o n p i c t u r e s . ACKNOWLEDGEMENTS A l l a s p e c t s o f t h i s work
were s u p p o r t e d
by t h e P l a n e t a r y
Geology O f f i c e ,
N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n . We t h a n k t h e f o l l o w i n g i n d i v i d u a l s f o r t h e i r c o n t r i b u t i o n t o D.
B a l l f o r p h o t o g r a p h i c s u p p o r t , G.
Kuhl f o r d a t a r e d u c t i o n , and
Beardmore,
P.
Parkes, K.
t h i s study:
Malone,
and D.
R. Leach f o r a s s i s t a n c e i n o b t a i n i n g t h e w i n d t u n -
146 n e l d a t a and development o f t h e p a r t i c l e v e l o c i m e t e r . i n the i n i t i a l
arrangements f o r
Alan P e t e r f r e u n d a s s i s t e a
t h e f i e l d experiment.
R. Leach, M.
We thank
M a l i n , and K. Gerety f o r h e l p f u l comments on t h e manuscript. 60
f0
501
DESCENDING
0
6
4
2
8
10
PARTICLE VELOCITY (m/s) I
0
I
10
, 20
I
30
I
I
40
50
I
60
PARTICLE VELOCITY
70
80
I
I
90
100
(%Urn)
Fig. 12. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f high-speed m o t i o n p i c t u r e s f o r 500 t o 600 vm q u a r t z p a r t i c l e s s u b j e c t e d t o a f r e e s t r e a m wind speed of 10.5 m/s (u, = 0.5 m / s ) a t one b a r atmospheric pressure, comparing v e l o c i t i e s f o r g r a i n s on t h e r i s i n g , h o r i z o n t a l , and descending p a r t s o f t h e i r t r a j e c We p r e s e n t l y have no e x p l a n a t i o n f o r t h e bimodal v e l o c i t y d i s t r i b u t i o n tories. o f g r a i n s on t h e h o r i z o n t a l p a r t o f t h e t r a j e c t o r y .
147
-Eo
-
12
4
-
EARTH CASE FIELD EXPERIMENT WADDELL BEACH, CA
W
0
-
10
8
LL
-
Dp
__t_
= 300pm
a a m
6
W
> 0
m 4
W
I
4
2 i 1
0
3
2
5
4
MEAN PARTICLE VELOCITY (m/S) I
I
1
1
I
I
I
I
I
0
10
20
30
40
50
60
70
80
M E AN P A R T I C L E V E L 0 C I T Y ( % uoo)
Fig. 13. P a r t i c l e v e l o c i t i e s o b t a i n e d f r o m a n a l y s i s o f h i g h speed m o t i o n p i c t u r e s f o r n a t u r a l beach sands o f -300 um d i a m e t e r s u b j e c t e d t o a wind speed o f -6 m/s measured a t a h e i g h t o f 1 m above t h e s u r f a c e a t Waddell Creek S t a t e Beach, C a l i f o r n i a .
148
-E 0
30
25
w
0 U
;2 0 3 v)
w 15
> 0
10 I-
I Q
i i 5 r
0
I
I
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
100
P A R T I C L E V E L O C I T Y (MODE,%u,)
F i g . 14. Comparison o f v e l o c i t i e s on E a r t h , Mars, and Venus. Two s i z e s o f p a r t i c l e s , each s u b j e c t e d t o r e l a t i v e l y l o w and h i g h w i n d speeds, were t e s t e d under E a r t h and Mars c o n d i t i o n s ; one s i z e p a r t i c l e was t e s t e d a t a l o w v e l o c i t y under Venus c o n d i t i o n s . I n g e n e r a l , g r a i n s a c h i e v e a much h i g h e r v e l o c i t y i n r e l a t i o n t o t h e w i n d speed u n d e r V e n u s i a n c o n d i t i o n s t h a n u n d e r m a r t i a n c o n d i t i o n s , and g r a i n s u n d e r t e r r e s t r i a l c o n d i t i o n s have i n t e r m e d i a t e v e l o c i t i e s . REFERENCES Bagnold, R.A., 1941. The P h y s i c s o f Blown Sand and D e s e r t Dunes: Methuen, London, 265 pp. G r e e l e y , R., R.N. Leach, S.H. W i l l i a m s , B.R. White, J.B. P o l l a c k , D.H. K r i n s l e y , a n d J.R. M a r s h a l l , 1982. R a t e o f Wind A b r a s i o n on Mars: J. _ Geophy. _ _ _ - Res., 87, pp. 10,009-10,024. G r e e l F , R., B.R. White, J.B. P o l l a c k , J.D. I v e r s e n , and R.N. Leach, 1977. Dust NASA T e c h n i c a l Memo. TM s t o r m s on Mars: C o n s i d e r a t i o n s and S i m u l a t i o n s : 78423, pp. 29. G r e e l e y , R., B.R. W h i t e , J.B. P o l l a c k , J.D. I v e r s e n , and R.N. Leach, 1981. Dust s t o r m s on Mars: C o n s i d e r a t i o n s and S i m u l a t i o n s : I n Desert Dust: Origin, C h a r a c t e r i s t i c s , and E f f e c t on Man. Geol. SOC. her.--1 T r o y Pewe, pp. l O T i 2 i 7 ' - - I v e r s e n , J.E., R. G r e e l e y , and J.B. P o l l a c k , 1976. Windblown d u s t on Earth, Mars, and Venus: J. Atmos. S c i . , 2, 2425-2429. I v e r s e n , J.D. and B . R T W W 1 9 8 2 . S a l t a t i o n t h r e s h o l d on E a r t h , Mars, and Venus: S e d i m e n t o l o g y , 29, pp. 111-119. S t u d y of sand movement by w i n d : Phy. S c i . Res. I n s t . , Kawamura, R., 1951. Tokyo Univ., 5, 95-112, ( o r i g i n a l i n Japanese; NASA t r a n s l a t i o n ) . S c h l i c h t i n g , H., 1368. Boundary-Layer Theory: M c G r a w - H i l l Book Company, New York, pp. 748. Schmidt, R.A. 1977. A System T h a t Measures B l o w i n g Snow: U.S. Dept. Agric. Res. Paper, RM -194, 80 pp. WhiteXR-79. S o i l T r a n s p o r t by Winds on Mars: J. Geophy. Res., 84, pp. 4643-4651. Williams, S.H. and R. G r e e l e y , 1983. F l u x o f w i n d b l o w n p a r t i c l e s on Venus: Preliminary laboratory results: NASA Rept. P l a n e t a r y Geology, ( a b s t r a c t , i n press).
149
EOLIAN SHAPE-SORTING AND AERODYNAMIC TRACTION EQUIVALENCE I N THE COASTAL DUNES OF HOUT BAY, REPUBLIC OF SOUTH AFRICA FRANK W. STAPOR, JR.l,
0. Box 12559, Charleson, S. C. 29412
MRR1,P.
JAMES P. MAY, Dept. of Chemistry and Geology, The C i t a d e l , C h a r l e s t o n , S. C. 29409 JOHN BARWIS2, Geology Department, U n i v e r s i t y o f South C a r o l i n a , Columbia, S.C.
INTRODUCTION This study i n v e s t i g a t e s e o l i a n shape-sorting,
aerodynamic t r a c t i o n equiva-
lence, and t h e i n t e r a c t i o n between s u r f a c e c r e e p and s a l t a t i o n i n a c t i v e c o a s t a l dunes near Hout Bay, R e p u b l i c o f South A f r i c a . experience a
These dunes have a p o i n t source,
m a r k e d l y u n i d i r e c t i o n a l wind regime, and a r e composed o f a 50/50
mixture o f q u a r t z and b i o c l a s t i c c a l c i t e sand. observed i n these c a l c i t e g r a i n s .
No d i a g e n e t i c m o d i f i c a t i o n s were
These c a l c i t e g r a i n s were d e r i v e d f r o m
gastropod s h e l l s and a r e more p r i s m a t i c t h a n plate-shaped.
No m i n e r a l o g i c
laminations were observed, u n l i k e t h e more common s i t u a t i o n i n c o a s t a l dunes i n which t h e c a l c i t e g r a i n s a r e plate-shaped and congregated i n t o d i s t i n c t laminae. This sand c o m p o s i t i o n i s i d e a l f o r examining e o l i a n shape-sorting,
given the
s i m i l a r d e n s i t i e s o f q u a r t z and c a l c i t e and t h e 50/50 m i x t u r e t h a t p r o b a b l y precludes g r a i n a v a i l a b i l i t y problems.
O n l y s t o s s s u r f a c e s were sampled i n o r d e r
t o maximize d e p o s i t i o n f r o m e o l i a n t r a c t i o n , s u r f a c e creep and s a l t a t i o n . faces,
s i t e s o f g r a i n f l o w and g r a i n f a l l d e p o s i t i o n ,
Slip
were avoided as were
interdune r e g i o n s . The r e s u l t s o f t h i s s t u d y i n d i c a t e t h a t e o l i a n t r a c t i o n t r a n s p o r t p r e f e r e n t i a l l y s e l e c t s l e s s s p h e r i c a l g r a i n s and t h i s s h a p e - s o r t i n g determines aerodynamic t r a c t i o n e q u i v a l e n c e .
There appears t o be m i n i m a l r a t h e r t h a n c o n t i n u a l
exchange between t h e c r e e p and s a l t a t i o n components d u r i n g s t o s s deposition;
surface
t h e creep component a t any g i v e n t i m e was p r e v i o u s l y p a r t o f t h e
s a l t a t i o n cloud.
-lExxon P r o d u c t i o n Research Co., % h e l l O i l Co.,
P.O.
P.O.
Box 2189, Houston, Texas 77001
Box 527, Houston, Texas 77001
29208
150
0
YLTERS
-
SAMPLE LOCATION
+"4 MAJOR DUNE SLIPFACE
-_- SADDLE CREST ---
F i g . 1: Hout Bay dune f i e l d , with sample l o c a t i o n s . Note n o r t h w e s t e r l y o r i e n t a t i o n o f t r a n s p o r t a x i s , as i n d i c a t e d by f i e l d and s l i p f a c e o r i e n t a t i o n s . Dune s l i p f a c e o r i e n t a t i o n s and t h e o v e r a l l p a r a b o l i c geometry o f t h e f i e l d make i t c l e a r t h a t d e f l a t i o n o f t h e p o c k e t beach p r o v i d e s t h e o n l y source o f t h i s e o l i a n sand.
As a s m a l l c l o s e d system, t h e dune f i e l d t h u s p r o v i d e s an unusual
and i d e a l n a t u r a l l a b o r a t o r y t o s t u d y t h e t e x t u r a l and m i n e r a l o g i c a l e f f e c t s of e o l ian t r a n s p o r t .
151 GEOLOGIC SETTING Hout Bay i s a s o u t h - f a c i n g c r e s c e n t i c embayment a l o n g t h e west f l a n k o f t h e Cape o f Good Hope, South A f r i c a , about 18 km southwest o f Cape Town ( F i g u r e 1). The c o a s t l i n e o f t h e e n t i r e Cape, i n c l u d i n g Hout Bay, c o n s i s t s o f s t e e p - c l i f f e d exposures o f t h e Precambrian Cape G r a n i t e .
High peaks above t h e s e c o a s t a l
c l i f f s , which i n p l a c e s r i s e t o o v e r 1,100 m, a r e h e l d up b y Lower P a l e o z o i c sediments o f t h e Table M o u n t a i n Group.
These r o c k s a r e made up almost e n t i r e l y
o f quartz a r e n i t e s , w i t h m i n o r amounts o f r e d shale. The n o r t h e r n s h o r e l i n e o f Hout Bay i s a t w o - k i l o m e t e r - l o n g pocket beach. sediment comprises
fine-
t o medium-grai ned q u a r t z sand mixed w i t h v a r y i ng
amounts o f b i o c l a s t i c carbonate, l a r g e l y modern g a s t r o p o d d e t r i t u s . component has o n l y two p o s s i b l e sources. beach t o t h e e a s t and west.
Beach
The q u a r t z
One i s t h e e r o d i n g g r a n i t e f l a n k i n g t h e
The o t h e r i s i n t e r m i t t e n t streams d r a i n i n g t h e q u a r t z
arenites t h a t cap t h e peaks s u r r o u n d i n g t h e bay. Dune F i e l d Geometry and Source Extending n o r t h - n o r t h w e s t f r o m t h e p o c k e t beach i s a narrow, 3.5 fairway o f t r a n s v e r s e t o b a r c h a n - l i k e dunes ( F i g u r e 1).
km-long
These dunes c l i m b a
t h r e e - k i l o m e t e r - l o n g ramp t o t h e saddle between L i o n s Head and Suther Peaks, a point a p p r o x i m a t e l y 125 m above mean sea l e v e l ( m s l ) .
Northwest f r o m t h i s saddle
the f i e l d s l o p e s down t o a p a r a b o l i c t e r m i n u s j u s t above Sandy Bay ( F i g u r e 2). The geometry o f t h e f i e l d resembles t h e p a r a b o l i c dunes d e s c r i b e d b y McKee (1979), and r e s u l t s f r o m t h r e e f a c t o r s .
F i r s t , dune f i e l d l o c a t i o n and w i d t h a r e
constrained by s u r r o u n d i n g bedrock topography.
Second, t h e beach i s r e l a t i v e l y
small and p r o v i d e s what amounts t o a p o i n t source o f sand.
T h i r d , dominant winds
are s o u t h e a s t e r l y ( F i g u r e 3 ) and a r e f u n n e l e d northward through t h e e n t r a n c e t o
Hout Bay, c r e a t i n g a w i n d - t u n n e l e f f e c t t h a t r e s u l t s i n a "blowout" o f beach sand. Although t h e wind r o s e i n F i g u r e 3 shows two m a j o r modes, n o r t h w e s t e r l i e s may have l i t t l e e f f e c t o n t h e Hout Bay dune f i e l d .
Most o f t h e f i e l d , because i t l i e s
south o f t h e saddle p o i n t , may be t o p o g r a p h i c a l l y s h i e l d e d f r o m a l l winds b u t those w i t h a s i g n i f i c a n t s o u t h e r l y component.
I n addition,
b d i t e r r a n e a n - t y p e c l i m a t e w i t h a w i n t e r r a i n y season.
t h e Cape h a s a
Strong northwesterl i e s
we u s u a l l y accompanied by r a i n , which should d r a m a t i c a l l y decrease e o l i a n transport.
Consequently,
even i n t h e v i c i n i t y o f t h e saddle,
m a i n NW-oriented y e a r round.
dune s l i p f a c e s
N e v e r t h e l e s s , some b r i n k p o i n t s do show s m a l l
k v e r s a l s l i k e t h o s e d e s c r i b e d by H u n t e r e t a l . ( i n p r e s s ) , as shown i n F i g u r e 4.
152
F i g . 2: O b l i q u e a e r i a l p h o t o g r a p h o f t h e H o u t Bay dune f i e l d . View l o o k i n g s o u t h a c r o s s Sandy Bay, w i t h H o u t Bay i n d i s t a n c e . N o t e s a d d l e p o i n t and t e r m i n u s o f a c t i v e dune i n f o r e g r o u n d .
JANUARY (SUMMER. DRY)
JULY
(WINTER. WET)
CAPE TOWN, R.S.A
F i g . 3: Wind d a t a f o r Cape Town (D. F. M a l a n A i r p o r t ) f o r p e r i o d 1956-1970. Note dominance o f s o u t h e r l y mode ( d r y summer w i n d s ) . S t r o n g e s t n o r t h w e s t e r l i e s o c c u r d u r i n g w i n t e r r a i n s ( J u l y through October). The number i n t h e c e n t e r i s t h e p e r c e n t a g e o f c a l m c o n d i t i o n s ; each a r c r e p r e s e n t s 5 p e r c e n t . Dune F i e l d O r i g i n U n d e r l y i n g t h e m o b i l e dunes j u s t above t h e modern beach i s a s e m i - c o n s o l i d a t e d sequence of P l e i s t o c e n e s h o r e l i n e d e p o s i t s .
T h i s sequence c o n s i s t s o f a r a i s e d
c o b b l e beach, t h e t o p o f w h i c h l i e s a t + 4 m above m s l and r a n g e s up t o 9 m above m s l e l s e w h e r e i n H o u t Bay (Buchanan, 1 9 7 7 ) .
T h i s p e r c h e d beach i s o v e r l a i n b y 4
153
Fig. 4: V e r t i c a l a e r i a l p h o t o o f t h e H o u t Bay dune f i e l d j u s t sout,h o f t h e s a d d l e point. N o t e r e v e r s e d b r i n k p o i n t ( a r r o w ) o n l a r g e dune a t c e n t e r . B a r i s 100 m long and i s o r i e n t e d N-S.
m o f massive, s h e l l - and a r t i f a c t - b e a r i n g sand which i s i n t u r n capped b y 6+ m o f c r o s s - s t r a t i f i e d sand ( I n s k e e p ,
1975). A w h o l e s h e l l I4C d a t e o f 47,100 y e a r s
was r e p o r t e d b y I n s k e e p (1975) f o r a bed 1 m b e l o w t h e t o p o f t h e m a s s i v e u n i t .
I f one c a n presume t h i s d a t e t o be i n f i n i t e o r "dead" w i t h r e s p e c t t o I4C ( s e e Fairbridge,
1971), t h e r a i s e d beach was most 1 i k e l y d e p o s i t e d d u r i n g t h e Sangarnon
(Eemian) 120,000-year
h i g h s t a n d , as p r o p o s e d b y B a r w i s and T a n k a r d ( i n p r e s s ) f o r
a very s i m i l a r s u c c e s s i o n 27 km t o t h e e a s t ,
i n F a l s e Bay.
The dunes t h e r e f o r e p o s t d a t e t h e l a s t i n t e r g l a c i a l .
L i k e those b o r d e r i n g False
Bay, t h e y p r o b a b l y were i n i t i a t e d d u r i n g t h e W i s c o n s i n a n l o w s t a n d ( B a r w i s and Tankard,
i n p r e s s ) and l i k e l y c o u l d h a v e r e m a i n e d a c t i v e e v e r s i n c e . SAMPLING PROCEDURE
Sand samples were c o l l e c t e d a t t h e s i t e s shown i n F i g u r e
1
u s i n g a 15-cm-long
(2.5 cm d i a . ) p l e x i g l a s s c y l i n d e r w h i c h was pushed v e r t i c a l l y i n t o t h e bed.
The
e n t i r e c o n t e n t s o f t h e c y l i n d e r were t h e n r e t a i n e d , i n c l u d i n g p e b b l e s and l a r g e molluscan f r a g m e n t s ,
w h i c h were l a t e r removed i n t h e l a b o r a t o r y .
A l l samples
were t a k e n f r o m t h e s t o s s s u r f a c e s o f a c t i v e dunes t h a t r a n g e d i n c r e s t s p a c i n g
154 from a few t e n s o f meters t o more t h a n 200 m.
No s l i p f a c e samples were taken, nor
were any samples t a k e n f r o m d e e p l y e r o d i n g areas as evidenced b y l o c a l topography and g r a v e l d e f l a t i o n lags. water,
and d r i e d .
The b u l k samples were s p l i t , washed i n d e - i o n i z e d
Two s u b - s p l i t s were t h e n t a k e n o f each sample:
(1)
untreated f r a c t i o n
(2)
q u a r t z f r a c t i o n ( t r e a t e d w i t h HC1 t o remove c a l c i u m carbonate).
Weight p e r c e n t c a l c i u m carbonate was d e t e r m i n e d f r o m t h i s t r e a t m e n t f o r each sample.
Organics, h e a v y m i n e r a l s ,
and o t h e r contaminants were n o t p r e s e n t i n
s u f f i c i e n t abundance t o w a r r a n t f u r t h e r t r e a t m e n t . G R A I N CHARACTERISTICS The b i o c l a s t i c c a l c i t e g r a i n s showed no e v i d e n c e o f d i a g e n e t i c m o d i f i c a t i o n s when viewed under a b i n o c u l a r microscope. bladed.
T h e i r shape i s p r i s m a t i c r a t h e r t h a n
T h e i r m i n e r a l o g y - d e n s i t y was determined b y f l o t a t i o n i n a bromoform
s o l u t i o n c a l i b r a t e d t o f l o a t c a l c i t e and q u a r t z b u t t o a l l o w a r a g o n i t e t o s i n k . DETERMINATION OF SETTLING SPEED DISTRIBUTIONS The u n t r e a t e d f r a c t i o n and t h e q u a r t z f r a c t i o n were analyzed f o r s e t t l i n g speed d i s t r i b u t i o n i n The C i t a d e l Sediment A n a l y z e r (CITSA). t y p e a n a l y z e r p a t t e r n e d a f t e r t h a t o f Gibbs (1974). and t h e f a l l d i s t a n c e i s 120 cm.
CITSA i s a s e t t l i n g tube-
The t u b e i s 16 cm i n diameter
The p a r t i c l e s s e t t l e o n t o a pan suspended from
a Cahn E l e c t r o b a l a n c e (Model RTL),
t h e o u t p u t f r o m which c o n s i s t s o f a t i m e
d i s t r i b u t i o n o f accumulated weight,
which i s c o n v e r t e d t o a s e t t l i n g speed
distribution.
The ouput i s f e d t o a s t r i p - c h a r t r e c o r d e r f o r analog d i s p l a y and
through a n a n a l o g - t o - d i g i t a l c o n v e r t e r t o a TRS-80 microcomputer f o r s t a t i s t i c a l t r e a t m e n t o f t h e data. A l l s e t t l i n g speeds were c o r r e c t e d f o r t e m p e r a t u r e v a r i a t i o n t o a standard
v a l u e o f 2OoC.
The t e m p e r a t u r e - c o r r e c t e d s e t t l i n g speed was t h e n c o n v e r t e d t o a
d i m e n s i o n l e s s parameter ( C h i ) b y t h e f o l l o w i n g t r a n s f o r m : x (Chi) = -log2(s/so)
where s i s t h e t e m p e r a t u r e - c o r r e c t e d s e t t l i n g speed i n m/s and so i s standard s e t t l i n g speed o f 1 m/s. (1981).
The use o f t h e Chi parameter i s t r e a t e d i n d e t a i l by May
S e t t l i n g speeds o f t h e q u a r t z f r a c t i o n were d i r e c t l y measured.
Calcite
s e t t l i n g speeds were c a l c u l a t e d b y s u b t r a c t i n g t h e q u a r t z f r a c t i o n v a l u e s from those o f t h e u n t r e a t e d f r a c t i o n . The use o f a w a t e r - f i l l e d s e t t l i n g t u b e t o analyze q u a r t z and c a l c i t e e o l i a n sands was e v a l u a t e d b y t h e f o l l o w i n g t e s t .
Assuming t h a t two p a r t i c l e s , one o f
q u a r t z and one o f c a l c i t e , a r e d e p o s i t e d i n aerodynamic s e t t l i n g e q u i l i b r i u m ,
155 then t h e i r t e r m i n a l s e t t l i n g v e l o c i t i e s a r e i d e n t i c a l .
The d i a m e t e r s o f t h e i r
corresponding nominal spheres can be computed u s i n g t h e f o r m u l a s o f Rubey (1933), Gibbs
g d. (1971),
and Reed
g
z. (1975).
Table 1 shows t h e computed nominal
diameters f o r s e v e r a l t e r m i n a l v e l o c i t i e s i n a i r .
-e t_a l .
The Gibbs _ et _ a l . and t h e Reed
r e s u l t s a r e s i m i l a r , as t h e y were d e r i v e d f r o m t h e same e x p e r i m e n t a l data.
The Rubey v a l u e s a r e s i m i l a r , though s l i g h t l y l e s s f o r slow v e l o c i t i e s ( s m a l l p a r t i c l e s i z e ) and d i v e r g e t o h i g h e r v a l u e s f o r t h e h i g h e r v e l o c i t i e s .
A l l three
formulas c o n s i s t e n t l y p r e d i c t t h a t t h e q u a r t z d i a m e t e r s h o u l d be 1-2% g r e a t e r than t h e a e r o d y n a m i c a l l y e q u i v a l e n t c a l c i t e .
Next, t h e s e q u a r t z and c a l c i t e
computed nominal d i a m e t e r s were c o n v e r t e d back t o a s e t t l i n g v e l o c i t y i n w a t e r using t h e same t h r e e formulas.
Again, t h e Gibbs
ec. and t h e Reed gal. values
are i n c l o s e agreement ( w i t h i n 1-2%), whereas t h e Rubey v a l u e s p r e d i c t slower values on t h e slow end and f a s t e r v a l u e s on t h e f a s t end.
A l l three formulas
p r e d i c t t h a t q u a r t z and c a l c i t e p a r t i c l e s i n aerodynamic s e t t l i n g e q u i v a l e n c e should s e t t l e with d i f f e r e n t v e l o c i t i e s i n water, w i t h t h e q u a r t z v e l o c i t y b e i n g about 1%slower.
T h i s v a l u e i s a p p r o x i m a t e l y t h e same magnitude as t h e e r r o r i n
the s e t t l i n g t u b e apparatus.
I t i s concluded t h a t a w a t e r - f i l l e d s e t t l i n g t u b e
i s s u i t a b l e f o r e v a l u a t i n g q u a r t z and c a l c i t e aerodynamic s e t t l i n g equivalence. And t h i s e q u i v a l e n c e s h o u l d be i n d i c a t e d b y and c a l c i t e s e t t l i n g speeds.
"0 s i g n i f i c a n t d i f f e r e n c e s i n q u a r t z
Significant
d i f f e r e n c e s would suggest g r a i n
a v a i l a b i l i t y problems and/or t h e o v e r p r i n t o f t h e t r a n s p o r t i n g process. RESULTS AND DISCUSSION The q u a r t z and c a l c i t e f r a c t i o n s o f t h e s e Hout Bay e o l i a n sands sampled f r o m stoss s u r f a c e s a r e n o t i n aerodynamic s e t t l i n g e q u i l i b r i u m . speeds average 8% slower.
D i f f e r e n c e s i n mean
2% (95% Confidence I n t e r v a l ) w i t h t h e c a l c i t e f r a c t i o n s b e i n g
D i f f e r e n c e s i n s o r t i n g average 22%
fractions being the b e t t e r sorted.
6% (95% C.I.)
w it h t h e c a l c i t e
Table 2 p r e s e n t s t h e mean, s o r t i n g , skewness,
and k u r t o s i s moment measures which d e s c r i b e i n d i v i d u a l s e t t l i n g speed d i s t r i b u tions.
Mean s e t t l i n g speed v e r s u s s o r t i n g v a l u e s f o r q u a r t z and c a l c i t e
f r a c t i o n s of each sample a r e p l o t t e d i n F i g u r e 5.
As these samples a r e v e r y n e a r l y 50/50 m i x t u r e s of q u a r t z and c a l c i t e , mean weight p e r c e n t c a l c i t e i s 47% & 3% (95% C.I.), most l i k e l y m i n i m a l .
a nest o f q u a r t e r p h i screens.
D i f f e r e n c e s i n q u a r t z and c a l c i t e s i e v e -
determined mean g r a i n s i z e average 13% sands, see Table 3.
problems o f g r a i n a v a i l a b i l i t y a r e
To f u r t h e r examine t h i s problem t h e samples were s i e v e d i n
:6% (95% C.I.)
f o r t h e Hout Bay e o l i a n
However, Ludwick and Henderson (1968) show t h a t s i e v i n g c a n
underestimate t h e modal i n t e r m e d i a t e d i a m e t e r o f low s p h e r i c i t y g r a i n s b y 10% t o
20%. Hence t h e r e appears t o be no s h o r t a g e o f s u i t a b l e c a l c i t e g r a i n s .
156 Table 1: Gibbs,
S e t t l i n g speeds o f q u a r t z and c a l c i t e g r a i n s c a l c u l a t e d by medns o f the
z. (1971),
Reed,
g
a. (1975),
and Rubey (1933) formulae.
d e n s i t y of 0.0012 and a v i s c o s i t y of 0.00018. v i s c o s i t y of 0.01. Vai r ( m/ sec 1
dqtz. (mm)
A i r has a
Water has a d e n s i t y o f 1 and a
Q u a r t z has a d e n s i t y o f 2.65 and c a l c i t e 2.72 dcal. (mm)
% diff.
% diff
Vqtz/water ( m/set)
Vcal/water
1.;4 1.2 1.4 2.0 1.2 1.2 1.3 1.5 1.4 X=1.5
0.00075 0.00158 0.0350 0.00816 0.0192 0.0318 0.0449 0.0585 0.0997
0.00076 0.00161 0.00355 0.00818 0.0195 0.0322 0.0455 0.0590 0.101
1.3 1.9 1.4 0.2 1.6 1.3 1.3 0.9 1.3 x=1.2
1.4 1.2 1.3 2.0 1.8 2.5 1.6 1.6 1.5 X=1.6
0.000745 0.00157 0.00347 0.00807 0.00191 0.0315 0.0446 0.0581 0.0992
0.000755 0.0016 0.00353 0.00808 0.00192 0.0313 0.0449 0.0585 0.100
1.3 1.9 1.7 0.1 0.7 0.6 0.7 0.7 0.8 x.0.9
1.4 1.3 1.4 1.4 2.0 2.2 2.3 2.3 2.7 x=1.9
0.000708 0.00143 0.00299 0.00665 0.0171 0.0311 0.0465 0.0620 0.106
0.000717 0.00145 0.00303 0.00674 0.0171 0.0312 0.0468 0.0625 0.107
1.3 1.4 1.3 1.4 0.0 0.3 0.6 0.8
(m/sec)
A c c o r d i n g t o Gibbs e_t _ a l . (1971)
0.0625 0.125 0.25 0.50 1.0 1.5 2.0 2.5 4.0
0.0291 0.0426 0.0646 0.103 0.174 0.246 0.320 0.397 0.643
0.0287 0.0421 0.0637 0.101 0.172 0.243 0.316 0.391 0.634
A c c o r d i n g t o Reed et a l . (1975)
0.0625 0.125 0.25 0.50 1.0 1.5 2.0 2.5 4.0
0.029 0.0425 0.0642 0.102 0.172 0.242 0.314 0.388 0.625
0.0286 0.042 0.0634 0.1 0.169 0.236 0.309 0.382 0.616
A c c o r d i n g t o Rubey (1933)
0.0625 0.125 0.25 0.50 1.0 1.5 2.0 2.5 4.0
0.0281 0.0400 0.0581 0.0881 0.152 0.236 0.349 0.496 1.15
0.0277 0.0395 0.0573 0.0869 0.149 0.231 0.341 0.485 1.12
0.9 -
X.o.9'
157 Table 2:
S t a t i s t i c a l moment measures which d e s c r i b e t h e s e t t l i n g speed d i s t r i -
butions o f t h e q u a r t z and c a l c i t e f r a c t i o n s .
on s t o s s s u r f a c e s o f t h e Hout Bay dunes. 0.00 i s t h e skewness (Skew.) Sample
Weight %
These e o l i a n sands were sampled
Mean and s o r t i n g a r e i n Chi u n i t s .
and k u r t o s i s ( K u r ) v a l u e o f a log-normal d i s t r i b u t i o n Quartz Fraction
Calcite
Mean
Sorting
Skew.
2 3
45 46
4.74 4.68
.58 .66
-0.18 -0.32
4
45
4.74
.54
0.03
6
52
4.42
.51
0.26
7
53
4.47
.56
8
50
4.62
.52
Calcite Fraction Kur.
Mean
Sorting
Skew.
Kur.
0.42
.41
0.45
5.40 5.27
.34
0.07 0.15
-0.21 -0.10
-0.35
4.78
.39
-0.10
-0.71
0.62
4.80
.47
0.32
0.50
0.10
-0.07
4.76
.46
0.14
-0.21
0.03
0.68
5.13
.39
0.34
-0.31 0.60
9
56
4.44
.53
-0.04
0.72
5.04
.37
0.38
11
54
4.86
.43
-0.04
0.92
5.27
.34
0.33
0.60
12
55
5.04
.44
-0.20
1.92
5.38
.36
0.31
0.06
13 15
46
4.61
.69
-0.13
-0.12
4.88
.48
0.20
0.33
36
4.01
.87
0.23
-1.07
4.88
.72
-0.02
-1.17
16
36
4.94
.49
-0.02
-0.04
5.12
.76
-0.52
0.44
17
45
4.34
.62
0.05
0.08
5.00
.59
-0.33
0.43
18
54
4.72
.46
0.09
0.47
5.13
.39
0.31
1.01
21
46
5.26
.37
-0.05
0.57
5.55
.31
0.31
0.24
24
43
4.94
.35
0.10
0.93
5.26
.37
0.25
0.41
0.20
0.44
5.19
.33
0.36
-0.31
25
46
4.94
.44
26
49
4.93
.37
0.31
1.23
5.33
.36
0.38
0.37
27
51
5.05
.38
0.29
0.70
5.40
.33
0.38
1.29
2a
39
5.16
.40
0.16
0.17
5.32
.32
0.22
0.91
Thus, t h e r e appears t o be a s u b t l e , b u t s i g n i f i c a n t , s h a p e - r e l a t e d r e d u c t i o n i n s e t t l i n g - s p e e d between t h e s e q u a r t z and c a l c i t e f r a c t i o n s . The n a t u r e o f any p r e f e r e n t i a l s p h e r i c i t y s e l e c t i o n d u r i n g e o l i a n t r a c t i o n t r a n s p o r t has been a r a t t e r o f some c o n t r o v e r s y :
s t u d i e s have i n d i c a t e d 1) low s p h e r i c i t y g r a i n s
Selected (Free, 1911; Mattox, 1955; W i nkelmolen, 1971; Stapor, 1973; Veenstra,
1982), 2) h i g h s p h e r i c i t y g r a i n s s e l e c t e d (MacCarthy,
1935;
MacCarthy and
Ihrddle, 1938; Shepard and Young, 1961) and 3) a wind v e l o c i t y dependance i n which
low s p h e r i c i t y p a r t i c l e s a r e e n t r a i n e d b y l o w e r winds and h i g h e r s p h e r i c i t y p a r t i c l e s b y h i g h e r winds ( W i l l i a m s , 1964).
The r e s u l t s o f t h i s s t u d y i n d i c a t e
158
[IUARTZ rn CALCITE
4.0 (FASTER)
4.5
5.5
5.0
(SLOWER)
MEAN SETTLING SPEED (CHI1
F i g . 5: Mean s e t t l i n g speed versus s o r t i n g f o r q u a r t z and c a l c i t e f r a c t i o n s o f e o l i a n sands. These samples a r e from s t o s s surfaces o f t h e Hout Bay dune f i e l d . Mean s e t t l i n g speeds and s o r t i n g values are i n Chi u n i t s .
that
grains o f
lower s p h e r i c i t y
t r a c t i o n transport.
are p r e f e r e n t i a l l y
I n a d d i t i o n , these d i f f e r e n c e s
selected d u r i n g eolian
i n mean s e t t l i n g - s p e e d and
s o r t i n g d e f i n e t h e aerodynamic t r a c t i o n e q u i v a l e n c e between q u a r t z and c a l c i t e d e p o s i t e d on s t o s s s u r f a c e s . The mean s e t t l i n g speed of b o t h q u a r t z and c a l c i t e f r a c t i o n s decreases downtransport.
These g r a d i e n t s ,
a l t h o u g h s i g n i f i c a n t a t t h e 10% l e v e l , are r a t h e r
g e n t l e and account f o r o n l y 21% and 18% of t h e t o t a l v a r i a n c e i n q u a r t z and c a l c i t e mean s e t t l i n g speed, r e s p e c t i v e l y .
These percentages are unexpectedly
l o w g i v e n t h e r e l a t i v e l y s h o r t and narrow t r a n s p o r t path, t h e p o i n t source, and t h e u n i f o r m i t y o f t h e dominant s o u t h e r l y summer winds.
I t may be t h a t the
n o r t h w e s t e r l y w i n t e r winds, a1 though accompanied by r a i n , a r e indeed e f f e c t i v e i n sand t r a n s p o r t .
P l o t s of q u a r t z and c a l c i t e mean s e t t l i n g speed versus
d i s t a n c e a r e shown i n F i g u r e s 6 and 7.
159 Table 3:
R e s u l t s o f t h e s i e v e analyses.
Mean and s o r t i n g a r e i n p h i
units.
0.00 i s t h e skewness (Skew.) and k u r t o s i s ( K u r . ) v a l u e o f a l o g normal d i s t r i b u t i o n . These samples were s i e v e d i n a nest o f q u a r t e r
p h i screens f o r t h i r t y minutes.
Q u a r t z and c a l c i t e means have an
average d i f f e r e n c e of 13% + 6% (95% C. I . ) w i t h t h e c a l c i t e b e i n g smaller Quartz Fraction Sample
Calcite Fraction
Mean
Sorting
Skew.
Kur.
Mean
Skew.
Kur.
02
1.80
.56
0.17
1.13
1.37
1.89 1.80 1.61
* 57 -48
-.44 +. 12
-.34 -.lo
2.72 0.73
.45
+. 24
1.91 0.69 1.92
.56 .47 .44
-.25
03 04 06
2.06 2.15 2.05
.51
+. 10 -.13 -.05 +.03 -.15 -.09 +.42 -.44
0.87 1.25
.49 .55
0.97
1.63 1.73 1.71
1.83 1.82
+.14
07 08 09 11
1.94 1.89 2.10
.54 .47 .40
-.05 -.lo
2.17 1.91
.35 .58
0.53
1.63 2.05
.80 .47
0.72 1.78 2.07 1.57 0.01 1.46 -.70
+.01
0.42
1.83
.57
+.01
+. 18
1.91 1.43
1.98 2.42
.43 .34
+.02
-.02
+.08
0.47
4.12
2.09
.34
+.13
0.47
2.06 2.69 4.73 0.99
2.18
.42
2.16
.39
-.09 -.27
0.15 1.77
2.26 2.28
.39 .32
-.21 -.14
1.28 0.07
12 13 15
16 17 18 21 24 25 26 27 28
1.97 2.06 1.70 1.01 1.76 1.52 1.87 2.30 2.00 1.99 2.00 2.12 2.21
.54 .49 .40 .42 .65 .76 .76 .61 .41 .34 .33
+.03 +. 29
.37 .32
+.40 -.43
.36 .36
+. 13 +. 21
1.69 2.69 3.46 0.15 0.03
Sorting
-.02
+.13 +.lo -.33 -.09 -.30
1.01 0.50
I n d i v i d u a l q u a r t z and c a l c i t e s e t t l i n g - s p e e d d i s t r i b u t i o n s were d i s s e c t e d i n t o log-normal components through t h e use o f t h e ROKE program o f C l a r k (1977).
The
basic s t a t i s t i c a l d i s t r i b u t i o n o f monomineralic sand g r a i n s e x p e r i e n c i n g a p a r t i c u l a r mode o f t r a n s p o r t / d e p o s i t i o n d i s t r i b u t i o n s have been advocated,
i s assumed t o be log-normal.
f o r example,
the hyperbolic
Other
(Barndorff-
N i e l s e n etc.,1982), however, i t i s beyond t h e scope o f t h i s s t u d y t o c r i t i c a l l y discuss t h e m e r i t s o f v a r i o u s d i s t r i b u t i o n s .
The ROKE program employs a
n o n l i n e a r l e a s t - s q u a r e s a l g o r i t h m t o determine t h e mean, s o r t i n g , and p r o p o r t i o n o f two o r more components whose c o m b i n a t i o n b e s t e s t i m a t e s t h e g i v e n d i s t r i b u tion.
160
0
0
5.0 -
Y
RUARTI
0
E] 4.0
0
F i g . 6: Q u a r t z f r a c t i o n mean s e t t l i n g speed p l o t t e d a g a i n s t d i s t a n c e n o r t h from t h e southernmost sample. The s t r a i g h t l i n e i s a r e g r e s s i o n l i n e , s l o p e i s s i g n i f i c a n t a t t h e 10% l e v e l , which accounts f o r 21% o f t h e t o t a l v a r i a n c e inmean s e t t l i n g speed: mean s e t t l i n g speed = 4.52 + O.O0015y, y = d i s t a n c e i n meters n o r t h o f s o u t h e r r m o s t sample.
P r i o r t o d i s s e c t i o n , a one component ROKE a n a l y s i s was made o f each q u a r t z and c a l c i t e f r a c t i o n t o d e t e r m i n e j u s t how w e l l a log-normal d i s t r i b u t i o n d e s c r i b e d each measured s e t t l ing-speed d i s t r i b u t i o n .
Root-mean-square
i n d i c a t e t h e goodness o f t h e log-normal d e s c r i p t i o n .
(RMS) d e v i a t i o n s
Next a two-component ROKE
a n a l y s i s was made and t h e e x t r a c t e d log-normal components accepted i f 1) t h e RMS d e v i a t i o n s decrease t o a t l e a s t one t h i r d o f t h e one component v a l u e and 2) t h e l e s s e r component makes up a t l e a s t 10% o f t h e m i x t u r e . one-component and t h e accepted two-component Tables 4 and 5.
The r e s u l t s o f a l l t h e
ROKE analyses a r e p r e s e n t e d i n
A m i x t u r e o f two log-normal components b e t t e r d e s c r i b e s t h e
measured d i s t r i b u t i o n i n 50% o f t h e q u a r t z and 70% of t h e c a l c i t e f r a c t i o n s .
A
s i n g l e log-normal component b e t t e r d e s c r i b e s t h e o t h e r q u a r t z f r a c t i o n s and 15% of t h e c a l c i t e .
A m i x t u r e o f t h r e e log-normal components b e s t d e s c r i b e s t h e
remaining c a l c i t e f r a c t i o n s . W i t h i n t h e two-component m i x t u r e s , d i f f e r e n c e s i n mean s e t t l i n g speed and s o r t i n g average 8% 2 1.5% (95% C.I.) calcite. 5%
2
and 70% 2 30% (95% C.I.)
respectively for
Q u a r t z f r a c t i o n d i f f e r e n c e s i n mean s e t t l i n g speed and s o r t i n g averge
3% (95% C.I.)
and 90%
2
30% (95% C.I.)
respectively.
Thus, each m i n e r a l
161 Table 4:
R e s u l t s o f t h e one- and two-component ROKE analyses o f t h e q u a r t z
fractions.
Mean and s o r t i n g a r e i n Chi u n i t s .
RMS stands f o r root-mean-square
deviations o f t h e ROKE-predicted m i x t u r e f r o m t h e observed d i s t r i b u t i o n Quartz Sample
2 3 4 6 7 8 9 11 12 13 15 16 17 18 21 24 25 26 27 28
ONE-COMPONENT Mean Sorting
4.77 4.72 4.75 4.40 4.46 4.63 4.45 4.87 5.05 4.64 3.97 4.96 4.35 4.72 5.27 4.94 4.93 4.92 5.03 5.16
.57 .64 .56 .49 .57 .50 .50 .40 .42 .71 .96 .50 .62 .45 .37 .34 .43 .35 .36 .41
RMS
.00774 .0155 .00411 .0134 .00827 .0106 .00909 .00706 .0113 .00752 .0467 .00744 .00719 .00554 .00953 .00607 .0148 .0114 .0118 .0120
Mean1
TWO-COMPONENT Sorting1 %I
Mean2
RMS
Sorting2
4.30
.38
71
4.77
.7 1
.00303
4.71 4.44 4.88
.69 .67 .54
52 58 55
4.56 4.45 4.86
.31 .29 .26
.00284 .00207 .00227
3.38
.33
60
4.99
.45
.00331
4.69
.40
87
4.95
.83
.00166
5.00 4.79
.39 .29
67 58
4.84 5.19
.21 .56
.00204 .00386
4.93 5.05
.28 .31
76 70
5.43 5.50
.43 .52
.0018 .00379
Table 5: R e s u l t s o f t h e one- and two-component ROKE analyses o f t h e c a l c i t e f r a c t i o n f r a c t i o n s . Mean and s o r t i n g a r e i n Chi u n i t s . RMS stands f o r root-mean-square d e v i a t i o n s o f t h e ROKE-predicted m i x t u r e f r o m t h e observed m i x t u r e Calcite Sample
ONE-COMPONENT Mean Sorting
RMS
Mean1
TWO-COMPONENT Sorting1 %I
Mean2
Sorting2
RMS ~~
2 3 4 6 7 8 9
11 12
13 16 17 18 21 24 25 26 27 28
5.40 5.26 4.79 4.77 4.75 5.11 5.02 5.25 5.36 4.86 4.89 5.12 5.05 5.10 5.53 5.25 5.17 5.31 5.37 5.31
.39 .34 .40 .44 .47 .38 .36 .32 .36 -46 .80 .61 .50 .37 .30 .35 .32 .35 .31 .29
.0107 .00512 .00642 .01420 .00719 .0132 .0150 .0188 .0142 .00685 .0288 .0536 -0337 .0127 .0155 .0129 .0140 .0155 .0172 .00805
5.43
.49
69
5.34
.19
.00392
4.65 5.01 4.91 4.83 5.15 5.20
.31 .42 .24 .22 .24 .24
60 54 50 53 67 60
5.03 4.46 5.34 5.27 5.51 5.65
.57 .33 .38 .36 .38 .36
.00275 .00231 .00482 .00485 .00182 .00383
5.00 5.40 5.33 5.03 5.17 5.26 5.26
.28 .17 .37 .21 .24 .21 .22
71 52 76 61 66 72 63
5.44 5.72 5.04 5.44 5.64 5.76 5.42
.43 .34 .13 .34 .37 .33 .41
.00184 .00230 .00360 .00262 .00353 .00200 .00143
162
.
m
.
5.5
..
=.
CALCITE
F i g . 7: C a l c i t e f r a c t i o n mean s e t t l i n g speed p l o t t e d a g a i n s t d i s t a n c e north from t h e southerrmost sample. The s t r a i g h t l i n e i s a r e g r e s s i o n l i n e , slope i s s i g n f i c a n t a t t h e 10% l e v e l , which accounts f o r 18% o f t h e t o t a l variance inmean s e t t l i n g speed: mean s e t t l i n g speed = 4.98 + O.OOOly, y = d i s t a n c e i n meters n o r t h o f southerrmost sample. f r a c t i o n i s composed of a f a s t e r and b e t t e r s o r t e d component and a g e n e r a l l y slower and more p o o r l y s o r t e d one. deposited
from
creep and
These two components may r e p r e s e n t grains
s a l t a t i o n e o l i a n transport,
respectively.
The
interchange of g r a i n s between s a l t a t i o n and surface creep may not be continuous as i n d i c a t e d b y Bagnold (1941). Although these hypothesized creep and s a l t a t i o n components p l o t i n d i f f e r e n t p o r t i o n s o f a s o r t i n g versus mean s e t t l i n g - s p e e d
diagram (Fig.
8),
average
d i f f e r e n c e s i n q u a r t z and c a l c i t e mean s e t t l i n g speed and s o r t i n g are i n d i s tinguishable.
W i t h i n t h e creep components q u a r t z and c a l c i t e f r a c t i o n s have
average d i f f e r e n c e s of 6%
T 1%(95% C.I.)
speed and s o r t i n g r e s p e c t i v e l y .
and 25%
f e r e n c e s i n mean s e t t l i n g - s p e e d and s o r t i n g o f 9% (95% C.I.)
respectively.
b e t t e r sorted.
7% (95% C.I.)
i n mean s e t t l i n g
The s a l t a t i o n components have average d i f -
T 4% (95% C.I.)
and 31%
5%
I n both cases t h e c a l c i t e f r a c t i o n i s t h e slower and
T h i s suggests t h a t t h e processes which produced these two
components are s i m i l a r , a t l e a s t i n regards t o q u a r t z / c a l c i t e d e n s i t y and shape contrasts:
an unexpected r e s u l t g i v e n t h a t g r a i n s i n s u r f a c e creep g e t t h e i r
momentum from t h e impact o f s a l t a t i n g g r a i n s w h i l e t h e l a t t e r g e t t h e i r momentum
163 SALTATION 9 QUARTZ - CALCITE
(FASTER)
\
4.5
CREEP QUARTZ =CALCITE
5.0
5.5
MEAN SETTLING SPEED (CHI)
(SLOWER)
Fig. 8: A p l o t o f mean s e t t l i n g speed versus s o r t i n g f o r q u a r t z / c a l c i t e f r a c t i o n s i n t e r p r e t e d t o be creep and s a l t a t i o n components. Only those samples f o r which the two-component ROKE a n a l y s i s was accepted f o r both mineral f r a c t i o n s are shown i n t h i s diagram. Note t h e two d i s t i n c t , non-overlapping c l u s t e r s . d i r e c t l y from the wind (Bagnold, 1941). However, we are d e a l i n g w i t h d e p o s i t s produced by a sequential r e d u c t i o n i n wind v e l o c i t y . Thus, creep components previously may w e l l have been p a r t o f the s a l t a t i o n cloud. CONCLUSIONS The r e s u l t s o f t h i s p r e l i m i n a r y study i n d i c a t e t h a t e o l i a n t r a c t i o n t r a n s p o r t p r e f e r e n t i a l l y s e l e c t s g r a i n s o f lower s p h e r i c i t y and t h i s s e l e c t i o n d e f i n e s aerodynamic t r a c t i o n e q u i l i b r i u m .
I n a d d i t i o n , the s t a t i s t i c a l d i s s e c t i o n of
settling-speed d i s t r i b u t i o n s suggests t h a t both creep and s a l t a t i o n components
are present deposition,
i n these
stoss
surface deposits.
This
o r c o n d i t i o n s o f f a l l i n g wind v e l o c i t y ,
implies t h a t during
t h e creep and s a l t a t i o n
components are not c o n t i n u o u s l y interchanged. ACKNOWLEDGEMENTS The h e l p o f Mrs. Mary Joe C l i s e , MRRI, i n s i e v i n g these samples i s g r a t e f u l l y acknowledged.
Dr. Graham Gash, MRRI, a s s i s t e d i n the computer p r o g r a m i n g and
data management. Dr. Peter McCabe and Dr. Ken Stanley, both o f Exxon P r o d u c t i o n Research Co., reviewed t h i s paper and made c r i t i c a l comnents which s i g n i f i c a n t l y
164 improved t h e o r i g i n a l manuscript. REFERENCES Bagnold, R. A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen and Co., 265 pp. Barndorff-Nielsen, 0.; Dalsgaard, K.; Halgreen, C.; Kuhlman, H.; M o l l e r , J. T.; and G. Schou, 1982. V a r i a t i o n i n p a r t i c l e s i z e d i s t r i b u t i o n over a small dune. Sedimentology, 29: 53-66. Barwis, J. H., and A. J. Tankard, i n press, P l e i s t o c e n e s h o r e l i n e d e p o s i t i o n and sea-level h i s t o r y a t d w a r t k l i p , South A f r i c a . Jour. Sed. Pet. Buchanan, W. F., 1977, Rescue d i g a t a Late Stone Age Cave, Hout Bay, Cape Province. Unpub. Archaeol. A d d i t i o n a l Proj., Univ. Cape Town, 13 pp. Clark, I . , 1977. ROKE, a computer program f o r n o n l i n e a r least-squares decompcs i t i o n o f mixtures o f distributions. Computers and Geosciences, 3,(2). F a i r b r i d g e , R. W., 1971. Quaternary s h o r e l i n e problems a t INQUA, 1969. Q u a t e r n a r i a , 15: 1-17. Free, E. E., 1911. The movement o f s o i l m a t e r i a l by t h e wind. B u l l . 68, Bureau o f S o i l s , U.S. Dept. o f A g r i c u l t u r e , 37 pp. Gibbs, R. J., 1974. A s e t t l i n g tube system f o r sand-sized analysis. Jour. Sed. Pet. 44: 583-588. Gibbs, R. J., Matthews, M. D., and D. A. Link, 1971. The r e l a t i o n s h i p between sphere s i z e and s e t t l i n g v e l o c i t y . Jour. Sed. Pet., 41: 7-18. Hunter, R. E., Richmond, B. M., and T. R. Alpha, i n press. Storm-controlled o b l i q u e dunes o f the Oregon coast. B u l l . Geol. SOC. Am. Inskeep, R. R., 1976. A note on t h e Melkbos and Hout Bay r a i s e d beaches and t h e M i d d l e Stone Age. S. A f r . Archaeol. B u l l . , 31: 26-28. Ludwick, J. C. and P. L. Henderson, 1968. P a r t i c l e shape and inference o f s i z e from s i e v i n g . Sedimentology, 11: 197-235. MacCarthy, G. R., 1935. E o l i a n sands: a comparison. Am. Jour. Sci., 30: 81-95. MacCarthy, G. R. and 3. W. Huddle, 1938. Shape-sorting o f sand g r a i n s by wind a c t i o n . Am. Jour. Sci., 35: 64-73. Mattox, R. B., 1955. E o l i a n shape s o r t i n g . Jour. Sed. Pet., 25: 111114. a proposed standard parameter f o r s e t t l i n g tube May, J. P., 1981. CHI ( X ) : a n a l y s i s o f sediments. Jour. Sed. Pet., 51: 607-610. McKee, E. D., ed., 1979. A Study o f Global Sand Seas. U. S. Geol. Surv., P r o f . Paper 1052, 429 pp. Reed, W. E . , LeFever, R., and G. J. Moir, 1975, D e p o s i t i o n a l environment i n t e r p r e t a t i o n from s e t t l i n g v e l o c i t y ( p s i ) d i s t r i b u t i o n s . B u l l . Geol. SOC. Am., 86: 1321-1328. Rubey, W. W., 1933. S e t t l i n g v e l o c i t i e s o f gravel, sand, and s i l t p a r t i c l e s . Am. Jour. Science, 25: 325-338. Shepard, F. P. and R. Young, 1961. D i s t i n g u i s h i n g between beach and dune sands. Jour. Sed. Pet., 31: 196-214. Stapor, F. W., 1973. Heavy m i n e r a l c o n c e n t r a t i n g processes and density/shape s i z e e q u i l i b r i a i n t h e marine and c o a s t a l dune sands o f t h e Apalachicola, F l o r i d a , region. Jour. Sed. Pet., 43: 396-407. Veenstra, H. J., 1982. Size, shape and o r i g i n o f sands o f t h e East F r i s i a n I s l a n d s ( N o r t h Sea, Germany). Geol. Mijnbouw, 61: 141-146. Williams, G., 1964. Some aspects o f the e o l i a n s a l t a t i o n load. Sedimentology, 3: 257-287. Winkelmolen, A. M., 1971. R o l l a b i l i t y , a f u n c t i o n a l shape p r o p e r t y o f grains. Jour. Sed. Pet., 41: 703-714.
165
DUNE SEDIMENT TYPES, SAND COLOUR, SEDIMENT PROVENANCE
AND HYDROLOGY I N THE
STRZELECKI-SIMPSON DUNEFIELD, AUSTRALIA R.J. WASSON*:
D e p t . o f B i o g e o g r a p h y and Geomorphology, A u s t r a l i a n Y a t i o n a l
U n i v e r s i t y , C a n b e r r a , A.C.T.,
Australia
INTRODUCTION The l a r g e S t r z e l e c k i - S i m p s o n d u n e f i e l d o f A u s t r a l i a ( F i g . 1) c o n s i s t s p r i n c i p a l l y o f l o n g i t u d i n a l dunes o f v a r i o u s k i n d s .
One o f t h e iiiost s t r i k i n g
r e g i o n a l c h a r a c t e r i s t i c s o f t h e s e dunes i s t h e i r c o l o u r ( F i g . 2 ) .
The b r i g h t
red-brown dunes o f t h e n o r t h e r n Simpson d u n e f i e l d a r e o f t e n c i t e d as b e i n g t y p i c a l o f t h i s d u n e f i e l d , b u t t h e r e a r e a l s o e x t e n s i v e a r e a s o f p a l e brown, orange brown, and w h i t e dunes.
The z o n a t i o n o f t h e s e v a r i o u s l y c o l o u r e d sands
has been i n t e r p r e t e d as t h e r e s u l t o f p r o g r e s s i v e r e d d e n i n g and a g e i n g o f sand g r a i n c o a t i n g s downwind f r o m s o u r c e s o f sand ( W o p f n e r and T w i d a l e , 1 9 6 7 ) . There a r e v a r i o u s l i n e s o f e v i d e n c e w h i c h s u g g e s t t h a t t h i s h y p o t h e s i s i s inadequate.
I n t h i s p a p e r s e d i m e n t s and m o r p h o l o g y w i t h i n each of t h e m a j o r
c o l o u r zones a r e examined, and t h e p r o c e s s e s o f s e d i m e n t m o b i l i z a t i o n f o r dune c o n s t r u c t i o n examined.
I n t h e l i g h t o f t h i s examination t h e hypothesis o f
downwind a g e i n g o f dune sand i s c r i t i c a l l y t e s t e d and f o u n d t o b e i n a d e q u a t e . SPATIAL DISTRIBUTION OF DUNE SEDIMENT TYPES Colour, i r o n c o n t e n t and m i n e r a l o g y ( f o r methods see Append'ix) Two d o m i n a n t g r o u p s o f c o l o u r s o c c u r i n t h e d u n e f i e l d ( F i g . 2 ) : brown (10YR7/4 t o 5YR6/8; (5YR6/8 t o 2.5YR4/8).
Japanese S t a n d a r d S o i l C o l o u r C h a r t ) ;
1, p a l e 2, r e d - b r o w n
N e a r t h e b i g l a k e s , w h i c h f o r m an a r c f r o m Lake Frome t o
Lake E y r e ( F i g s . 1 and 2), t h e dunes a r e p a l e c o l o u r e d .
These p a l e dunes
extend o n l y a s h o r t d i s t a n c e f r o m L a k e s Frome and C a l l a b o n n a , e x t e n d some 300 km f r o m Lake B l a n c h e , and w e l l n o r t h o f Lake Eyre.
Downwind o f t h e a r e a o f
pale dunes e a s t o f Lake Frome, a t r a n s i t i o n zone a b o u t 10-20 km o c c u r s between pale and red-brown dunes.
F u r t h e r n o r t h S t r z e l e c k i Creek f o r m s t h e b o u n d a r y
between p a l e and r e d - b r o w n dunes, a n d t h e b o u n d a r y i s as w i d e as t h e c r e e k ' s immediate f l o o d p l a i n i n some p l a c e s ;
t h a t i s , 1-2 km.
I n t h e Simpson dune-
f i e l d , t h e b o u n d a r y i s g r a d a t i o n a l , once a g a i n o v e r 10-20 km.
Apart from these
obvious b o u n d a r i e s , t h e r e a r e c o l o u r g r a d i e n t s w i t h i n t h e a r e a s o f p a l e dunes.
*
P r e s e n t a d d r e s s : C S I R O , D i v i s i o n o f W a t e r and Land Resources, P.O. Canberra City, A.C.T. 2601, A u s t r a l i a .
Box 1666,
166
Fig. 1. Simpson a n d Strzelecki dunefields, and catchments of streams entering the dunefields.
For example, sands close t o Kallakoopah Creek a r e very pale (10YR7/4) and rapidly darken t o 7.5YR7/4 in a distance of about 25 km downwind. Representative sample points a r e shown on Figure 2. Systematic sampling has been c a r r i e d out on t h r e e t r a v e r s e s within t h e dunefield (Fig. 2 ) . Traverse 1 from Moolawatana Bore t o Hawker Gate i s 140 km long and i s p a r a l l e l t o the dune trend. Each sample was taken from t h e c r e s t s of longitudinal dunes, and cons i s t s of the upper 10 cm a t each point. On Traverse 1 the sands a r e pale near Lake Frome a n d t h e i r hues redden from about 65 km downwind of Moolawatana Bore (Fig. 3 ) . Colour values show no systematic trend. Total f e l d s p a r was determined by s t a i n i n g polished sections with sodium c o b a l t i n i t r i t e a f t e r etching with hydrofluoric acid. Feldspar q u a n t i t i e s show a systematic decline from Lake Frome, b u t maintain low values from 65 km onwards. Heavy mineral content i s higher in the yellower sands near Lake Frome than in the redder sands f u r t h e r e a s t . The change in colour, t o t a l f e l d s p a r content, and heavy mineral content of c r e s t a l sands occurs a t about the same place (Fig. 3 ) .
167
Fig. 2. Dune sand c o l o u r s i n t h e Simpson and S t r z e l e c k i d u n e f i e l d s . C o l o u r s recorded a t p o i n t l o c a t i o n s a r e c o n s i d e r e d r e p r e s e n t a t i v e o f l a r g e a r e a s a r o u n d each. The c o l o u r b o u n d a r i e s a r e b a s e d on g r o u n d t r a v e r s e s , c o l o u r a e r i a l photographs, and s a t e l l i t e images. T r a v e r s e 2 ( F i g . 4 ) i s 174 km l o n g and c u t s a c r o s s t h e t r e n d o f t h e l o n g i t u d i n a l dunes ( F i g . 2).
S t a r t i n g a t t h e e a s t e r n end, t h e dunes a r e
red-brown ( d o m i n a n t hue i s 2 . 5 Y R ) Creek i s r e a c h e d a t 130 km. creek.
becoming d o m i n a n t l y 5 Y R u n t i l t h e S t r z e l e c k i
P a l e dunes ( d o m i n a n t l y 7.5YR/6)
o c c u r w e s t of t h e
A l t h o u g h t h e sample number i s s m a l l , t o t a l f e l d s p a r c o n t e n t seems t o be
168
lower in the red sands ( t h e eastern p a r t o f Traverse 2 , known as T1) t h a n in the pale sands ( t h e T2 p a r t o f Traverse 2 ) . Furthermore, the percentage of heavy minerals i s a l s o lower in the red sands t h a n in the pale sands.
H u e '5
(YR) 2.5
I
5-
Heavy minerals 0.2-
-
I
I 1
I
.
. .
*
I
.I
I
t
I
4
I
I
I
I
. . . . . . .
I.
I.
Traverse 1, Strzelecki dunefield.
Fig. 3.
Colour Value
I
. . . . .
Value 6 -
%
I
I
I
.
*
. . .
0
.
I
Location shown on Fig. 2 .
.. ... .... . ..... ... .... .... ., I. I I . , I
0..
0.0
4
0.
0.
0
0.
L
I
. O %
L
"
'
Feldspar 2
O
(East)
Fig. 4.
"
" 20
. 40
60
80
,
I
.
.
0 minerals02
O
I
*
.
4
a
.I
. . . 100
I
.I. 120
%
*. 140
Kilometres
Traverse 2 , Strzelecki dunefield.
Location shown on Fig. 2.
.
..
#
160 (West)
169 Traverse 3 (Fig. 5 ) i s 122 km l o n g , runs p a r a l l e l t o t h e longitudinal dunes, and s t a r t s a t the ' b i g bend' of Kallakoopah Creek in the Simpson Desert. The dune sands a r e pale near the creek, as noted e a r l i e r , becoming red-brown about 80 km downwind.
Colour values decline from south t o north. Heavy mineral content i s generally lower in the pale sands t h a n in the red sands, the reverse t o the pattern found on Traverse 2 .
Hut
I
Fig. 5.
Traverse 3 , Simpson dunefield.
Location shown on Fig. 2.
Traverse 2 was selected t o examine t h e petrographic a n d chemical differences between pale ( T 2 ) a n d red-brown ( T l ) dune sands, in a n attempt t o explain the colour differences. The following r e s u l t s were obtained: 1. The f r a c t i o n of 250 pm t o 1000 pm in diameter was separated and the grain coatings rubbed off by rubber p e s t l e in a mortar. Standard X-ray d i f fraction ( X R D ) analyses of 19 samples (13 from T1 and 6 from T 2 ) of the :oatings revealed mixtures of k a o l i n i t e , i l l i t e , and mixed-layer i l l i t e iontrnori 11 oni t e , with kaol i n i t e dominating. The only d i s c e r n i b l e difference ay in the absence of montmorillonite in some T2 samples. 2 . Chemically scrubbed and ground separations of heavy minerals from 6 iamples on T 1 and 8 samples on T2 were examined by XRD. The dominant minerals ire ilmenite, magnetite, zircon, and r u t i l e . Half t h e samples have s l i g h t l y
170 more z i r c o n t h a n m a g n e t i t e , w h i l e t h e o t h e r h a l f d i s p l a y a d e c l i n e f r o m ilmenite t o r u t i l e .
There i s no d i f f e r e n c e between T1 and T2 i n terms o f
abundant heavy m i n e r a l composition.
Four samples f r o m T1 and 3 f r o m T2 were
examined by XRD more e x t e n s i v e l y , and a l l samples c o n t a i n anatase, s p i n e l , pyroxene, hornblende, t o u r m a l i n e , topaz, g a r n e t , and a p a t i t e , w h i l e a l l samples f r o m b o t h p a r t s o f T r a v e r s e 2 c o n t a i n monazite, sphene, and a u g i t e .
3.
E n e r g y - d i s p e r s i o n X-ray (EDAX) mapping o f s e c t i o n s c u t t h r o u g h g r a i n
c o a t i n g s on two samples each f r o m T I and T2 showed random d i s t r i b u t i o n s o f t h e elements Fe, A l , S i , T i , and K.
The sample a n a l y s i s was c a r r i e d o u t on a l l
samples f r o m T r a v e r s e 2, and t h e same r e s u l t found.
From these analyses i t i s
n o t p o s s i b l e t o conclude whether t h e Fe occurs as c r y s t a l l i t e s between t h e c l a y s , o r occurs p r i n c i p a l l y w i t h i n t h e l a t t i c e s o f t h e c l a y m i n e r a l s .
4. Coatings were rubbed o f f sub-samples o f 250-1000 um g r a i n s f r o m 8 samples f r o m T1 and 6 samples f r o m T2, and t o t a l carbon determined. mean C = 81
?
40 ppm, and f o r T2 mean C = 87 i 48 ppm.
F o r T1
There i s no d i f f e r e n c e
between t h e samples, a l t h o u g h i t must be s a i d t h a t r u b b i n g abrades t h e g r a i n s u r f a c e s s o t h a t d i f f e r e n c e s between c o a t i n g s c o u l d be masked by t h e removal technique.
5.
I r o n as Fe203 was determined f o r b u l k samples ( w i t h heavy m i n e r a l s g r a i n s f o r 13 samples f r o m T1 and 6 samples f r o m T2
removed) o f 250-1000 ( T a b l e 1).
The ranges o f values c a l c u l a t e d by t a k i n g 2a about t h e means do n o t
o v e r l a p , s o t h e sample means a r e d i f f e r e n t . T1 samples t h a n i n t h e T2 samples.
That i s , t h e r e i s more i r o n i n t h e
Examination o f t h e q u a r t z g r a i n s f r o m these
samples i n t h i n s e c t i o n d i d n o t r e v e a l obvious d i f f e r e n c e s i n i m p u r i t i e s ,
it
SO
i s concluded t h a t t h e d i f f e r e n c e s d e t e c t e d i n Fe% r e f l e c t d i f f e r e n c e s i n t h e i r o n content o f t h e g r a i n coatings.
6.
Haematite was r e c o r d e d by XRD analyses i n s e v e r a l samples f r o m b o t h T1
and T2, b u t g e n e r a l l y c r y s t a l l i n e i r o n m i n e r a l s were n o t d e t e c t e d . l i k e l y t o be i n t h e Fe+3 f o r m (Walker, 1979).
A l l iron is
T h i s p r o p o s i t i o n was t e s t e d f o r NO
t h r e e samples from each o f T1 and T2 by d e t e r m i n i n g t o t a l Fe and t h e n Fet3. Fe”
was d e t e c t e d by d i f f e r e n c e . 7.
Examination o f t h i n s e c t i o n s f r o m T1 and T2 shows t h a t g r a i n c o a t i n g s
a r e g e n e r a l l y t h i c k e r i n T1 samples.
Maximum t h i c k n e s s i n T1 g r a i n s i s about
35 urn, w h i l e on T2 t h e y a r e about 10-15 urn. c o a t i n g s i n T1 i s g r e a t e r t h a n f o r T2.
T h i s suggests t h a t t h e volume of
To t e s t t h i s p r o p o s i t i o n c o a t i n g s were
rubbed o f f and t h e i r w e i g h t determined as a p r o p o r t i o n o f t h e b u l k 250-1000 sub-sample f r o m T1 samples and T2 samples. 0.75% and f o r T2 0.38
k
0.17%.
F o r T1 t h e mean v a l u e i s 0.88
*
The range o f v a l u e s f o r 2a about t h e means
o v e r l a p s , so t h e samples a r e n o t d i f f e r e n t .
T h i s r e s u l t may have been caused
by t h e i n a c c u r a c y o f t h e r u b b i n g method, o r i t may be r e a l .
Sonic removal o f
c o a t i n g s ( c f . Walker, 1979) i s o b v i o u s l y a more e f f i c i e n t method.
171 The s i g n i f i c a n t r e s u l t t o emerge f r o m t h e s e analyses i s t h a t t h e redness o f the dune sands i s a f u n c t i o n o f t h e i r o n c o n t e n t o f t h e g r a i n c o a t i n g s .
This
r e s u l t i s d e r i v e d o n l y f r o m T r a v e r s e 1, and so i t was t h o u g h t d e s i r a b l e t o t e s t i t elsewhere.
The same a n a l y s i s was c a r r i e d o u t on T r a v e r s e 3 ( T a b l e l ) , and
i t i s c l e a r t h a t t h e r e d sands have more i r o n t h a n t h e p a l e sands.
TABLE 1 Loca 1 i t y
%Fe
(x
?
s)l
Traverse 1 : T1
0.26 t 0.06
T2 : pale red Curdlawidny : p a l e red
0.18 t 0.03 0.14 ? 0.06 0.41 ? 0.04 0.34 t 0.04 0.56 i 0.04
Traverse 3
2
Coat m i n e r a l ogy2
K a o l i n i t e , i l l i t e : mont., haemati t e K a o l i n i t e , i l l i t e : mont., Haematite, k a o l i n i t e , i l l i t e Haematite, k a o l i n i t e , i l l l i t e Haematite, k a o l i n i t e , i l l i t e Haematite, k a o l i n i t e , i l l i t e
Sample no. (n 1 13 6 3 3 8 8
Mean t s t a n d a r d d e v i a t i o n , as w e i g h t % b a s i s on 250-1000 fraction. Determined by X-ray d i f f r a c t i o n on c o a t i n g s rubbed o f f 250-1000 fraction. Another t e s t was c a r r i e d o u t i n t h e P a r a k y l i a d u n e f i e l d o f South A u s t r a l i a , j u s t west o f Lake Torrens, i n t h e v i c i n i t y o f Curdlawidny Lagoon ( F i g . 6 ) . lake i s bounded by t h r e e r i d g e s which p a r a l l e l t h e l a k e ' s e a s t e r n shore.
The These
ridges a r e l u n e t t e - l i k e and a t l e a s t t h e upper 1-2 m c o n s i s t s o f a e o l i a n sand.
-dune
crest
Fig. 6. Map of dunes downwind o f t h e s o u t h e r n p a r t o f Curdlawidny Lagoon. Sample p o i n t 1 i s i n pale-brown dunes, and 2 i s i n red-brown dunes.
172 L o n g i t u d i n a l dunes e x t e n d downwind f r o m t h e s e t r a n v e r s e r i d g e s .
The sands o f
t h e t r a n s v e r s e r i d g e s and a d j a c e n t l o n g i t u d i n a l dunes a r e p a l e (2,5YR6/6), w h i l e t h e l o n g i t u d i n a l dunes f u r t h e r downwind, and n o t a t t a c h e d t o t h e t r a n s v e r s e r i d g e s , a r e r e d - b r o w n (10YR5/5). F o u r samples were t a k e n f r o m t h e c r e s t s o f one p a l e and one red-brown l o n g i t u d i n a l dune a t C u r d l a w i d n y ( F i g . 6 ) , and each sample was s p l i t .
The Fe
c o n t e n t o f t h e p a l e sand and r e d sand i s d i f f e r e n t u s i n g t h e 2 0 c r i t e r i o n ( T a b l e l ) , t h a t i s , r e d sands c o n t a i n more Fe t h a n p a l e sands.
As n o t e d e a r l i e r , d i f f e r e n c e s i n c o l o u r o c c u r w i t h i n a r e a s d e s i g n a t e d as p a l e o r red-brown.
On T r a v e r s e 3, sand hue r e d d e n s t o w a r d s t h e n o r t h , a t r e n d
which i s p a r a l l e l e d by decreasing c o l o u r value (Fig. 5).
Total i r o n content i s
n o t o n l y h i g h e r i n t h e r e d d e r sands b u t t h e r e i s a t r e n d o f i n c r e a s i n g i r o n content towards t h e north.
That i s , i r o n i s p o s i t i v e l y c o r r e l a t e d w i t h b o t h
c o l o u r and d i s t a n c e f r o m K a l l a k o o p a h Creek.
On T r a v e r s e 1, hue y e l l o w s w i t h i n
T 1 a b o u t 40 km f r o m t h e e a s t e r n end, and t h e n y e l l o w s a g a i n a t S t r z e l e c k i Creek i n t o T2 ( F i g . 4 ) .
These s t e p - l i k e changes i n hue a r e p a r a l l e l e d b y s t e p - l i k e
increases i n c o l o u r value.
T o t a l i r o n c o n t e n t a l s o p a r a l l e l s t h e s e changes i n
c o l o u r , so t h a t i r o n p e r c e n t a g e i s h i g h e s t where hue i s m o s t r e d , t h e n a t 40 km i r o n c o n t e n t f a l l s as hue y e l l o w s , and i r o n c o n t e n t i s l o w e s t where hue i s l e a s t red.
N o t o n l y a r e c o l o u r and i r o n c o n t e n t c o r r e l a t e d between m a j o r zones
o f d i f f e r e n t c o l o u r , b u t s u b t l e changes i n c o l o u r and i r o n c o n t e n t p a r a l l e l each o t h e r w i t h i n each zone. The r e s u l t s p r e s e n t e d i n F i g u r e s 3, 4, and 5 i n d i c a t e t h a t t h e dune sands a r e f a i r l y homogeneous i n t h e i r h e a v y m i n e r a l c o n t e n t , b u t a s i g n i f i c a n t d i f f e r e n c e o c c u r s between T r a v e r s e 1 and 2 i n f e l d s p a r c o n t e n t .
This order o f
m a g n i t u d e d i f f e r e n c e i n f e l d s p a r c o n t e n t between t h e p a l e sands o f t h e t w o traverses i s hard t o i n t e r p r e t i n isolation, but i s l i k e l y t o r e f l e c t a difference
i n t h e s e d i m e n t s f r o m w h i c h t h e dune sands were d e r i v e d .
Grain-size Samples were t a k e n f r o m f l a t p o r t i o n s o f dune c r e s t s o n T r a v e r s e 1 and s i e v e d a t 0.25 fl i n t e r v a l s w i t h o u t t r e a t m e n t o t h e r t h a n oven d r y i n g . t h e g r a p h i c mean
Values o f
(MZ) and i n c l u s i v e g r a p h i c s t a n d a r d d e v i a t i o n (Q,) were c a l -
c u l a t e d u s i n g t h e methods o f F o l k ( 1 9 6 8 ) .
A t t h e 90% c o n f i d e n c e l e v e l , v a l u e s
o f MZ f o r T2 c a n be a p p r o x i m a t e d b y t h e normal d i s t r i b u t i o n , b u t T 1 i s b i m o d a l .
As a consequence, MZ v a l u e s f o r T 1 and T2 were compared u s i n g t h e nonp a r a m e t r i c Mann-Whitney U t e s t .
T h i s t e s t showed t h a t a t p <0.00003 t h e
samples o f MZ f r o m T 1 ( n = 4 3 ) and T2 ( n = 1 9 ) a r e n o t f r o m t h e same p o p u l a t i o n , and m o s t of T2 i s f i n e r - g r a i n e d t h a n m o s t o f T1.
U s i n g t h e same t e s t t h e r e i s
173 no s i g n i f i c a n t d i f f e r e n c e between t h e f i r s t p a r t o f T 1 ( 0 - 4 6 km) and t h e second p a r t (46-130 km) f o r w h i c h c o l o u r and heavy m i n e r a l c o n t e n t d i f f e r e d . A t t h e 90% c o n f i d e n c e l e v e l , v a l u e s o f Q,
by t h e normal d i s t r i b u t i o n . between t h e means o f
Using a t - t e s t ,
f o r T 1 and T2 c a n b e a p p r o x i m a t e d t h e r e i s no s i g n i f i c a n t d i f f e r e n c e
Q, f o r t h e t w o p a r t s o f T r a v e r s e 2 ( c f . F i g . 7 ) .
Samples w e r e c o l l e c t e d f r o m t h e Simpson d u n e f i e l d f o r p u r p o s e s o t h e r t h a n t h e c o m p a r i s o n o f sands o f d i f f e r e n t c o l o u r .
Therefore, a formal s t a t i s t i c a l
a n a l y s i s l i k e t h a t f o r T 1 and T2 c a n n o t b e c a r r i e d o u t i n t h e Simpson, b u t some comparisons c a n b e made.
Figure 7 i s a bivariate
sands f r o m t h e S i m p s o n - S t r z e l e c k i D u n e f i e l d .
M Z /Q, d i a g r a m f o r c r e s t a l
There i s v e r y l i t t l e d i f f e r e n c e
i n t h e r a n g e o f Q, f o r t h e t h r e e f i e l d s o f p o i n t s , and t h e r e i s no sound e v i d e n c e o f a r e l a t i o n s h i p between Q
and M I Z' As a l r e a d y seen, t h e b u l k o f T 1 i s c o a r s e r t h a n t h e b u l k o f T2.
The s p r e a d
o f d a t a f r o m t h e Simpson d u n e f i e l d shows t h a t t h e b u l k o f t h e s e d a t a o v e r l a p with both Strzelecki fields.
However, t h e f i n e s t c r e s t a l sands i n t h e e n t i r e
S i m p s o n - S t r z e l e c k i D u n e f i e l d o c c u r i n t h e Simpson n e a r K a l l a k o o p a h Creek, n e a r Lake Eyre, and o n t h e f l o o d p l a i n o f t h e D i a m a n t i n a R i v e r n e a r B i r d s v i l l e . A p a r t f r o m one Simpson d u n e f i e l d sample (MZ=1.54),
t h e c o a r s e s t dunes sampled
o c c u r a t t h e e a s t e r n end o f T1.
I t i s e v i d e n t from F i g . 7 t h a t t h e r e i s no a b s o l u t e s e p a r a t i o n o f c r e s t a l sands a c c o r d i n g t o g r a i n - s i z e and l o c a t i o n i n t h e d u n e f i e l d .
However, t h e r e i s
a t e n d e n c y f o r f i n e sands t o o c c u r n e a r t h e modern c h a n n e l s o f Cooper Creek (TZ), t h e D i a m a n t i n a r i v e r , and K a l l a k o o p a h Creek. a l l pale coloured.
These f i n e dune sands a r e
The c o a r s e r c r e s t a l sands t e n d t o o c c u r away f r o m t h e
modern c h a n n e l s and a r e g e n e r a l l y red-brown.
O f course t h e r e i s a l a r g e area
o f o v e r l a p where b o t h p a l e and r e d - b r o w n sands have s i m i l a r
MZ v a l u e s .
I f a l l t h e sands o f t h e Simpson d u n e f i e l d , f o r example, were d e r i v e d f r o m t h e c h a n n e l s n e a r L a k e E y r e , t h e n t h e t e n d e n c y t o c o a r s e n i n g downwind i s impossible t o e x p l a i n w i t h o u t l o c a l c o n t r i b u t i o n s during migration o f t h e dunes.
A b e t t e r explanation o f t h e grain-size data l i e s i n t h e granulometry o f
t h e s e d i m e n t s w h i c h l i e c l o s e t o t h e sampled dunes.
I n t h e area o f Kallakoopah
Creek t h e a l l u v i u m l y i n g b e n e a t h tk dunes and i n t e r d u n a l s w a l e s i s d o m i n a n t l y f i n e sand, whereas t h e a l l u v i u m a t t h e n o r t h e r n end o f T r a v e r s e 3 i s a m i x t u r e o f c o a r s e and f i n e sand.
F u r t h e r m o r e , i t w i l l b e a r g u e d b e l o w t h a t t h e dunes
themselves p r o v i d e l i t t l e e v i d e n c e o f l o n g - d i s t a n c e t r a n s p o r t o f sand b y w i n d ,
so t h e dunes have been d e r i v e d f r o m a g r e a t many l o c a l s o u r c e s o f sand. Clay P e l l e t s E x a m i n a t i o n o f t h i n s e c t i o n s o f c r e s t a l sands f r o m T 1 and T2 shows t h a t c l a y p e l l e t s ( F i g . 8 ) o c c u r i n t h e p a l e sands o f T2 b u t n o t i n t h e r e d - b r o w n sands o f T1.
174
The p e l l e t s c o n s i s t of sand-size aggregates of c l a y , s i l t a n d very f i n e sand s i z e p a r t i c l e s , which form p a r t o f the creep and s a l t a t i o n load of aeolian t r a n s p o r t . I n Australia clay p e l l e t s have been described by Bowler (1973), Butler (1974), Rhodes (1980), Dare-Edwards (1979; 1982) from both inland a n d coastal s i t e s . I n the i n t e r i o r sand-size p e l l e t s p r i n c i p a l l y form l u n e t t e s , t h a t i s , lunate transverse dunes on t h e downwind margins o f lakes.
-j
T1
n =44
T2
n :20
SimDson
I
0
L
0.2
0.6
0.4 0 1
0.8
J
1.0
(44
Fig. 7 . Mean grain-size ( M Z ) versus s o r t i n g ( Q , ) f o r c r e s t a l sands from longitudinal dunes in t h e Simpson a n d Strzelecki dunefields.
175
F i g . 8a. P h o t o m i c r o g r a p h o f q u a r t z sand and f i n e - s a n d s i z e c l a y p e l l e t s f r o m t h e L a s t G l a c i a l Sand i n M1. P l a i n l i g h t .
F i g . 8b.
As i n 8a b u t u n d e r p o l a r i z e d l i g h t .
176 The s a n d - s i z e c l a y p e l l e t s a r e g e n e r a l l y sub-rounded t o s u b - a n g i r l a r , sub-spherical
(cf.
F i g . 9 and 1 0 ) .
and
The c l a y f r a c t i o n o c c u r s as plasma w i t h a
f l e c k e d e x t i n c t i o n p a t t e r n under p o l a r i s e d l i g h t .
I n a f e w cases c l a y o c c u r s
as o p t i c a l l y o r i e n t e d c o a t i n g s ( c u t a n s ) on s i l t - s i z e p a r t i c l e s w i t h i n t h e pellets.
M i c r i t i c c a l c i t e o c c u r s w i t h i n t h e plasma on r a r e o c c a s i o n s .
F i g . 9. S c a n n i n g e l e c t r o n m i c r o g r a p h o f a modern c l a y p e l l e t f r o m Lake E y r e . Sample s u p p l i e d b y J . D u l h u n t y . I n t h i n s e c t i o n s o f c r e s t a l sands f r o m T2, c l a y p e l l e t s c o n s t i t u t e u s u a l l y l e s s t h a n 5'4 (number f r e q u e n c y ) and a l w a y s l e s s t h a n 12%.
Clay p e l l e t s a l s o
o c c u r w i t h i n t h e p a l e sands o f t h e s o u t h e r n 8 0 km o f T r a v e r s e s 1 and 3 , t h e t r a n s v e r s e dunes on t h e downwind edges o f Lakes Frome, B l a n c h e and E y r e , i n t h e c r e s t s o f l o n g i t u d i n a l dunes t o t h e e a s t o f Lake E y r e and on t h e f l o o d p l a i n of t h e Diamantina R i v e r near B i r d s v i l l e .
S e v e r a l dunes were sampled t o t h e n o r t h
o f Cooper Creek i n t h e p a l e dune p a r t o f t h e S t r z e l e c k i d u n e f i e l d , and once a g a i n c l a y p e l l e t s were f o u n d i n t h i n s e c t i o n s . T h i n s e c t i o n e v i d e n c e d e m o n s t r a t e s t h a t t h e c r e s t s of t h e p a l e dunes c o n t a i n c l a y p e l l e t s , u s u a l l y i n q u a n t i t i e s l e s s t h a n 5%.
Samples examined f r o m t h e
r e d - b r o w n dunes on T1 ( T r a v e r s e 2 ) , T r a v e r s e s 1 and 3 , f r o m t h e n o r t h w e s t e r n and s o u t h w e s t e r n Simpson d u n e f i e l d n e a r Andado and M o k a r i b o r e r e s p e c t i v e l y , show t h a t c l a y p e l l e t s do n o t o c c u r i n t h e s e sands.
C l a y f l a k e s (Dare-Edwards,
1982) o c c u r i n b o t h r e d - b r o w n and p a l e dunes, c o n s i s t i n g o f p a r a l l e l l a m i n a e o f w e l l - o r i e n t e d c l a y ( c f . Wopfner and T w i d a l e , 1967, p . 1 3 2 ) .
177
Fig. 10a.
Photomicrograph of modern p e l l e t s from Lake Eyre.
Fig. l o b .
A s f o r 10a b u t under p o l a r i z e d l i g h t .
Plain l i g h t .
178 The i n t e r i o r s o f b o t h p a l e and red-brown dunes have been examined i n d e t a i l a t a few l o c a t i o n s , b u t one s i t e near Moomba camp i n t h e S t r z e l e c k i serves t o i l l u s t r a t e t h e main c h a r a c t e r i s t i c s o f t h e p a l e dunes.
A t s i t e M 1 ( F i g s . 11
and 12) t h e p e r i o d i c a l l y m o b i l e cap t o t h e dune has accumulated on t h e western f l a n k as w e l l as on t h e c r e s t o f t h e dune.
T h i s cap (some 200 cm t h i c k ,
F i g . 12) o v e r l i e s , w i t h a sharp boundary, tough sandy sediment which a l t e r n a t e s w i t h s o f t e r sands u n t i l a t about 530 cm a n o t h e r sharp break occurs. sediment below t h i s break i s r e d d e r between 530 and 640 cm (7.5YR7/6)
The than t h a t
above and, a t about 820 cm, c o n t a i n s pedogenic c a l c i t e nodules and r o o t pseudomorphs o f l i k e l y pedogenic o r i g i n .
The reddened sand i n t h e upper p a r t
o f t h i s u n i t , and t h e carbonate nodules, a r e evidence o f weak pedogenesis.
The
sharp break a t 530 cm i s i n t e r p r e t a t e d , t h e r e f o r e , as a diastem.
West
East
Moomba Ck
Fig.l2+
,
.
.
.
O
0Mobile cap
40 m
Last Glacial sand
Coarse sandy alluvium
F i g . 11. C r o s s - s e c t i o n o f a l o n g i t u d i n a l dune j u s t e a s t o f Moomba Camp, S t r z e l e c k i d u n e f i e l d . T h i s i s t h e s i t e of M 1 ( F i g s . 12 and 1 7 ) , l o c a t e d i n a p i t dug on t h e western f l a n k o f t h e dune. Twigs and l e a f d e b r i s b u r i e d a t ca. 200 cm gave a 14C age* o f 116% modern (ANU-2199), which i s a d a t e o f l a t e 1950s. O t h e r m o b i l e caps t o dunes i n t h e Moomba area c o n t a i n backed blades, stone a r t i f a c t s which were produced d u r i n g t h e l a s t 4000 y r s .
T h e r e f o r e , some caps c o u l d be s e v e r a l thousand y e a r s o l d .
The t o u g h e r sand between 210 and 530 cm i s c o r r e l a t e d w i t h t h e l a s t m a j o r phase o f f i n e - g r a i n e d a l l u v i a t i o n by Cooper Creek.
14C dates on t i n y c h a r c o a l p a r -
t i c l e s p l a c e t h i s phase o f a l l u v i a t i o n and, by c o r r e l a t i o n and d a t i n g , a e o l i a n a c t i v i t y between 13,000 and 23,000 y r s . B.P.
1 4 C d a t e s , on c h a r c o a l b u r i e d
i n t h e t o p o f t h i s same dune sand ( s t r a t i g r a p h i c a l l y e q u i v a l e n t t o ca. 210300 cm i n M1;
*
F i g . 1 2 ) , a r e 13850+190 (ANU-2278) and 13150k830 (ANU-2279).
A l l 14C d a t e s i n t h i s paper a r e u n c a l i b r a t e d .
179
a
%
Thin
(+
SIC1
sections
05 Oi LLU
a
pellets
0
12
U
7
0
9 O E
0
____ 0
P
a
P
m
i
a a
a
0
6
0
11
0
62
-o
28
0
24
0
a a -
0
a
P
0
a
0
: E
a
I
a a
0
a
0
C m
-0
I
m
a
0
a
a
a
a a
a a
a
m
m
a
19
o
0
50
0
33
0
39
0
5
a
4
3 +
0
a
E
0
I a
-
a
0
0
a
0 1
m
a
a
a
0
0
m
a
nodules
Fig. 12. S t r a t i g r a p h i c d i a g r a m o f m o s t e a s t e r l y p a r t o f t h e M 1 p i t . l o c a t i o n see F i g . 11.
For
180 T h i s c o n f i r m s t h e c o r r e l a t i o n , t h e b a s i s f o r w h i c h w i l l be e x p l a i n e d b e l o w . The a e o l i a n sand between 210 and 530 cm i n M 1 i s t h e r e f o r e o f L a s t G l a c i a l age, and was d e p o s i t e d a t t h e peak o f t h a t g l a c i a l p e r i o d ( c f . S a r n t h e i n , 1 9 7 8 ) . The a e o l i a n sand b e l o w 530 cm i s o b v i o u s l y v e r y much o l d e r t h a n 23,000 y e a r s , as evidenced by t h e d i a s t e m discussed e a r l i e r . T u r n i n g now t o t h e s e d i m e n t s o c c u r r i n g i n M1, i t i s c l e a r t h a t t h e tnacroscopic features observable i n t h e f i e l d c o r r e l a t e w i t h microscopically determined c h a r a c t e r i s t i c s .
The number f r e q u e n c y o f c l a y p e l l e t s has been
d e t e r m i n e d i n t h i n s c , - t i o n s b y p o i n t c o u n t i n g 400 g r a i n s i n each s l i d e . g r a i n - s i z e c h a r a c t e r i s t i c s w e r e d e t e r m i n e d b y d r y s i e v i n g a t 0.25
0
a f t e r g e n t l e d i s a g g r e g a t i o n w i t h a r u b b e r p e s t l e and oven d r y i n g .
The
intervals
F i v e samples
were t r e a t e d b y u l t r a s o n i c d i s p e r s i o n ( s e e a p p e n d i x ) and t h e samples d r y s i e v e d t o 4 0 and t h e n p i p e t t e d .
The r e s u l t s a r e s e t o u t i n T a b l e 2.
TABLE 2 Sampl e no.
M1/3 M1/13 M1/23 M1/33 M1/45
No d i s p e r s i o n
%14 0
Q,
MZ
2.32 2.15 2.17 2.32 2.01
0.37 0.46 0.42 0.51 0.58
0.01 0.02 0.04 1.20 0.8
U1 t r a s o n i c a l l y d i s p e r s e d Q, 70<4 0 4<9 0 MZ
2.45 2.3 2.52 2.46 2.25
0.6 0.74 4.02 3.33 1.06
6.07 5.99 13.72 11.60 10.42
Pellets
1.78
10 8 60 17 42
0
8.46 6.03 2.67
The samples w h i c h r e c e i v e d no d i s p e r s i o n y i e l d e d v e r y l i t t l e m a t e r i a l <4
0.
U l t r a s o n i c d i s p e r s i o n y i e l d e d b o t h s i l t and c l a y ( T a b l e 2 ) i n p r o p o r t i o n s ( y ) w h i c h a r e r e l a t e d t o t h e number f r e q u e n c y o f c l a y p e l l e t s ( x ) b y t h e l i n e a r e q u a t i o n y = 6.15 + 0.12x,
f o r w h i c h r2 = 0.68 and n
=
5.
The s h i f t i n t h e
l o c a t i o n o f t h e cumulative frequency d i s t r i b u t i o n by u l t r a s o n i c dispersion i s i l l u s t r a t e d i n F i g . 13.
QI
T h i s r e s u l t s i n changes t o c a l c u l a t e d v a l u e s o f MZ and
( T a b l e 2 ) , t h a t i s , t h e samples a p p a r e n t l y become f i n e r and more p o o r l y
s o r t e d as f i n e - g r a i n e d t a i l s a p p e a r on t h e f r e q u e n c y d i s t r i b u t i o n s .
These
r e s u l t s confirm observations i n t h i n section t h a t the pel l e t s contain q u a n t i t i e s o f c l a y and s i l t .
These f i n e s c a n n o t be d e t e c t e d i n t h e f i e l d ,
u n l e s s t h e s e d i m e n t i s hand t e x t u r e d w i t h w a t e r .
The m a t e r i a l s a r e s u b p l a s t i c ,
t h a t i s , on p h y s i c a l w o r k i n g t h e y become h e a v i e r t e x t u r e d ( B u t l e r , 1956; Dare-Edwards,
1982), a s a r e s u l t o f t h e d i s a g g r e g a t i o n o f t h e p e l l e t s .
The u l t r a s o n i c t r e a t m e n t d e s c r i b e d above does n o t r e l e a s e a l l o f t h e f i n e s from t h e c l a y p e l l e t s .
Dare-Edwards ( p e r s . comni.) exposed l u n e t t e p e l l e t s t o
much l o n g e r u l t r a s o n i c t r e a t m e n t b u t c o u l d n o t c o m p l e t e l y d i s p e r s e t h e p e l l e t s .
181
_---
100-
/-
80 SAMPLE
s
-- No -
60-
Ultrasonically dispersed
I I
E 40-
0 1
2
MI123 dispersion
3
4
+
J
I
5
6
7
8
Fig. 13. Cumulative frequency grain-size distribution for sample 23 from 230 cm at M1 (Fig. 12), showing the effects o f ultrasonic dispersion. Note that the ordinate is arithmetic and the phi scale is used for the abscissa. Therefore, it is likely that the quantities of material <4 ,0 in Table 2 do not represent the total quantity o f fines in the pellets. The results of the dispersion experiment show that a unique grain-size distribution for these sands is not attainable. However, for reasons discussed below, it seems that dry sieving yields a distribution closest to that of the sediments when they were being transported and deposited. This distribution is the most useful for the purposes of this paper. Dry sieving obviously abrades clay pellets, especially those which have begun to breakdown after deposition. However, the MZ values in Fig. 12 generally accord with qualitative observations of thin sections. That is, the finest samples, on the basis of MZ values, contain the largest quantities of small clay pellets. The quantity of pellets in crestal sands is generally < 5 % , therefore the grain-size analyses reported on the various traverses are unlikely to have been significantly affected by the presence of pellets. At M1 (Fig. 12) the hardness of the sand, as qualitatively judged by resistance to a spade, is correlated with the quantity of clay pellets. The modern cap is soft and loose, and contains <12% pellets; thereby falling into Dare-Edwards' (1982) sandy clay dune facies in which pellets make up a maximum o f 15-30% of the sediment. The remainder consists o f individual mineral grains of quartz, feldspar and other minerals. The Last Glacial sediment (210-530 cm) contains both hard and soft units, and the former have more pellets than the latter. Both fall into Dare-Edwards' clay dune facies.
182 V a l u e s o f MZ i n t h e h a r d u n i t s a r e more v a r i e d t h a n i n t h e s o f t u n i t s ( F i g . 1 2 ) , p o s s i b l y because o f d i f f e r e n t i a l breakdown o f p e l l e t s d u r i n g sieving.
However, t h i n s e c t i o n s show t h a t t h e r e i s more v a r i a t i o n i n t h e h a r d
u n i t s , w i t h dramatic v a r i a t i o n s i n t h e s i z e o f p e l l e t s . I n t h e modern cap o f l o o s e sand, t h e p e l l e t s a r e d o m i n a n t l y o f f i n e sand s i z e (2.0 t o 3.0
0), a r e
coatings (cutans).
sub-rounded,
and a f e w have v e r y t h i n o r i e n t e d c l a y
I n t h e L a s t G l a c i a l Sand t h e p e l l e t s i n t h e h a r d u n i t s a r e
commonly o f v e r y f i n e sand s i z e ( 3 . 0 t o 4.0
a),
b o t h sub-rounded and sub-
a n g u l a r , w i t h some sub-rounded f i n e sand s i z e p e l l e t s .
I n the soft units the
p e l l e t s a r e more commonly o f f i n e sand s i z e . The c l a y p e l l e t s i n t h e modern c a p o f sand a r e d i s c r e t e f e a t u r e s , w h i l e i n t h e L a s t G l a c i a l Sand t h e s m a l l p e l l e t s a r e sometimes f u s e d , and t h e r e i s a c o n s i d e r a b l e q u a n t i t y o f c l a y ' s m e a r e d ' between q u a r t z g r a i n s .
This clay i s
o f t e n w e l l o r i e n t e d b u t p e l l e t b o u n d a r i e s can b e seen i n some c a s e s .
I n the
o l d e s t u n i t i n M 1 ( b e l o w 530 cm) t h e r e a r e v e r y f e w d i s c r e t e c l a y p e l l e t s ( F i g . 1 2 ) and most c l a y o c c u r s as l i n i n g s on v o i d s and as o r i e n t e d domains between q u a r t z and f e l d s p a r g r a i n s . The same sequence o f p r o g r e s s i v e dest r u c t i o n o f c l a y p e l l e t s has been o b s e r v e d a t s i t e M3, some 11 km n o r t h w e s t o f Moomba camp, and a t L a r k ' s p i t a b o u t 10 km s o u t h o f Moomba.
Furthermore, t h e
same c o r r e l a t i o n between h a r d n e s s and p e l l e t c o n t e n t seen a t M 1 has been observed a t these two s i t e s . The breakdown o f c l a y p e l l e t s o b v i o u s l y o c c u r s a f t e r d e p o s i t i o n .
While the
d i f f e r e n c e s i n c l a y p e l l e t m o r p h o l o g y between t h e t h r e e m a j o r u n i t s r e c o g n i z e d a t M 1 a r e r e a d i l y o b s e r v a b l e , some t h i n s e c t i o n s w i t h i n t h e L a s t G l a c i a l h a r d sands show fusion.
i t t l e e v i d e n c e o f p e l l e t c o l l a p s e , w h i l e o t h e r s show e v i d e n c e o f
Da e-Edwards ( 1 9 8 2 ) made s i m i l a r o b s e r v a t i o n s i n l u n e t t e s , a r g u i n g
t h a t t h e co l a p s e o f p e l l e t s o c c u r s b y w e t t i n g and d r y i n g c y c l e s t h e a m p l i t u d e o f w h i c h i s d e t e r m i n e d b y t h e p e r m e a b i l i t y o f i n d i v i d u a l beds.
The h a r d sand
u n i t s a t M 1 a r e c l e a r l y l e s s permeable t h a n t h e s o f t sand u n i t s , and s o t h e p e l l e t s s h o u l d n o t be s t r e s s e d as o f t e n .
Dare-Edwards a l s o showed t h a t p e l l e t s
a r e b r o k e n down b y c a r b o n a t e c r y s t a l l i s a t i o n ,
b u t t h i s process i s r e l a t i v e l y
u n i m p o r t a n t i n t h e w e a k l y c a l c a r e o u s L a s t G l a c i a l dune sands o f t h e S t r z e l e c k i dunefield.
The d e g r e e o f p e l l e t breakdown i s n o t , t h e r e f o r e , a s i m p l e f u n c t i o n
o f deDth b e l o w t h e s u r f a c e .
Dune Types and Geornorphic S e t t i n g
As a l r e a d y seen, dune sand c o l o u r c o r r e l a t e s w i t h i r o n c o n t e n t i n sand g r a i n c o a t i n g s , and w i t h t h e p r e s e n c e and absence o f c l a y p e l l e t s .
The m o r p h o l o g y o f
p a l e c o l o u r e d dunes i s a l s o d i f f e r e n t f r o m t h e m o r p h o l o g y o f r e d - b r o w n dunes.
183 The red-brown dunes a r e a l m o s t everywhere s i m p l e l o n g i t u d i n a l f e a t u r e s which r e s t d i r e c t l y on e i t h e r a l l u v i u m o r bedrock and g r a v e l ( g i b b e r ) .
Over
distances o f t e n s o f k i l o m e t r e s t h e i n t e r d u n a l swales l i e a t about t h e same The dunes j o i n by means o f y - j u n c t i o n s (Mabbutt and
e l e v a t i o n ( F i g . 14a).
S u l l i v a n , 1968), and u s u a l l y b e g i n d i r e c t l y f r o m t h e i n t e r d u n a l s u r f a c e s (Fig. 1 5 ) .
I n some cases t h e y e x t e n d downwind f r o m t r a n s v e r s e dunes which
themselves l i e downwind o f e i t h e r l a k e s o r streams ( c f . Twidale, 1972; 1976).
Wasson,
Carbonate s e g r e g a t i o n s beneath t h e s u r f a c e o c c u r i n zones t h a t a r e
often up t o 1 m t h i c k , c o n s i s t o f nodules and r o o t pseudomorphs, and a p p r o x i mately p a r a l l e l t h e s u r f a c e i n c r o s s - p r o f i l e .
These s e g r e g a t i o n s a r e i n t e r -
preted as h a v i n g formed n e a r t h e ground s u r f a c e i n t h e zone o f dense p l a n t r o o t growth by pedogenic l e a c h i n g and r e - p r e c i p i t a t i o n .
They a r e t h e r e f o r e c a l -
careous p a l a e o s o l s ( c f . Churchward, 1963). The presence o f t h e s e p a l a e o s o l s shows t h a t t h e dunes a c c r e t e d v e r t i c a l l y , and a u g e r i n g w i t h i n dunes and e x a m i n a t i o n o f n a t u r a l g u l i e s has shown t h a t each u n i t o f sand between p a l a e o s o l s has an e q u i v a l e n t a t h e downwind end.
m (a.s.l.1
0.7
1.4
2.1
26
35
42
1
2
3
4
5
6
7
8 km
120
Fig. 14. C r o s s - p r o f i l e o f l o n g i t u d i n a l dunes i n t h e Simpson d u n e f i e l d . a, l o w , c l o s e l y spaced red-brown dunes. b, h i g h , w i d e l y spaced red-brown dunes. c, pale-brown dunes on a s w e l l o f sand. A l l p r o f i l e were drawn f r o m l e v e l l i n g data f o r s e i s m i c s h o t l i n e s , s u p p l i e d by D e l h i A u s t r a l i a Pty. L t d . Note t h e different v e r t i c a l exaggerations.
184
F i g . 15. Map o f r e d - b r o w n l o n g i t u d i n a l dunes f r o m t h e n o r t h e r n Simpson dunef i e l d s h o w i n g t h a t t h e dunes b e g i n f r o m t h e i n t e r d u n a l s w a l e s , r a t h e r t h a n from t r a n s v e r s e dunes o r r i v e r s . T h a t i s , each phase of a e o l i a n a c t i v i t y f o u n d e x p r e s s i o n b o t h i n v e r t i c a l a c c r e t i o n and downwind e x t e n s i o n .
The u p w i n d ends o f l o n g i t u d i n a l dunes a r e
o f t e n e r o d e d P x p o y i n g t h e maximum number o f p a l a e o s o l s w i t h i n t h e dune.
It
seems l i k e l y t h a t each t i m e a e o l i a n a c t i v i t y began, t h e p r e - e x i s t i n g dune sands were d e f l a t e d t o c o n t r i b u t e t o t h e new u n i t . The p a l e - b r o w n l o n g i t u d i n a l dunes a r e o f t e n s i m p l e f e a t u r e s w i t h i n t e r v e n i n g s w a l e s l y i n g a t a b o u t t h e same e l e v a t i o n ( F i g . 1 4 b ) .
However, more commonly
t h e p a l e l o n g i t u d i n a l dunes o c c u r i n g r o u p s , each l o n g i t u d i n a l f e a t u r e l y i n g on a l a r g e s w e l l of sand ( F i g . 1 4 c ) .
D r i l l i n g w i t h i n t h e s e s w e l l s has
d e m o n s t r a t e d t h a t t h e y c o n s i s t of a e o l i a n sand, and e x p o s u r e s w i t h i n t h e swales between t h e l o n g i t u d i n a l s show t h a t a e o l i a n a c c u m u l a t i o n has o c c u r r e d i n a l l p a r t s o f t h e s e compound f e a t u r e s (McKee, 1979).
B o t h t h e dune and t h e swale
sands c o n t a i n c l a y p e l l e t s , and c a l c a r e o u s p a l a e o s o l s . The s w e l l s commonly e x t e n d downwind, a l o n g w i t h t h e l o n g i t u d i n a l dunes, from t r a n s v e r s e dunes ( F i g . 1 6 ) w h i c h , i n t h e S t r z e l e c k i d u n e f i e l d , a r e a d j a c e n t t o f l o o d f l a t s of Cooper Creek and i t s v a r i o u s d i s t r i b u t a r i e s , and i n t h e Simpson d u n e f i e l d a r e downwind o f pans, s a l i n a s and some r i v e r c h a n n e l s .
I n both
185
dunefields the pale-brown compound dunes l i e in the topographically lowest areas, near modern streams. I n the Simpson t h i s low-lying area appears t o be the r e s u l t of t e c t o n i c movement (Krieg and Callen, 1980). I n the Strzelecki dunefield, Early T e r t i a r y Eyre Formation rocks crop o u t in the red dunes e a s t o f Strzelecki Creek a n d occur in d r i l l holes about 200 m below the surface t o the west of t h e creek, suggesting t h a t a flexure occurs along the creek. The pale-brown dunes in the S t r z e l e c k i a l s o seem t o l i e in a n area of subsidence.
B
I I
Linear dunes Transverse dune foreslopes
I-1
Waterhole
Palaeochannel
Modern channel
Spot height tm)
-
0
5krn
GN
t
Fig. 16. Dune forms a n d channel arrangements in the area j u s t n o r t h o f Moomba Camp, Strzelecki dunefield. The dated Late Pleistocene palaeochannels a r e shown in t h e southern p a r t of the map.
186 The r e d - b r o w n dunes, b y c o n t r a s t , a r e u n d e r l a i n by t h i n ( 1 - 2 m) s h e e t s o f sandy a l l u v i u m i n t h e S t r z e l e c k i d u n e f i e l d , and t h i s s e d i m e n t i s u n d e r l a i n by bedrock.
The remnants o f l o n g - d e f u n c t m e a n d e r i n g c h a n n e l s a p p e a r i n s w a l e s ,
and a r e o v e r l a i n b y dunes.
I n t e r d u n a l pans a r e a l i g n e d t r a n s v e r s e t o t h e
l o n g i t u d i n a l dunes e a s t o f S t r z e l e c k i Creek ( L o f f l e r and S u l l i v a n , 1979) and t h e s e pans p r o b a b l y r e p r e s e n t t h e remnants o f an enormous l a k e .
The
d e p o s i t i o n a l e q u i v a l e n t o f t h e l a k e i s p r o b a b l y t h e Mid-Miocene Etadunna F o r m a t i o n w h i c h l i e s a t t h e s u r f a c e i n v a r i o u s p l a c e s e a s t and s o u t h e a s t o f S t r z e l e c k i Creek, and a b o u t 150 m b e l o w t h e s u r f a c e t o t h e w e s t o f t h e f l e x u r e p o s t u l a t e d above (Wasson, i n p r e s s ) . The r e d - b r o w n dunes o f t h e n o r t h e r n Simpson d u n e f i e l d a r e u n d e r l a i n b y up t o 100 m o f sandy a l l u v i u m n e a r t h e n o r t h e r n end o f T r a v e r s e 3 ( F i g . 2 ) (Masson,
i n press).
T h i s t h i c k n e s s d e c r e a s e s t o w a r d s t h e n o r t h u n t i l e q u i v a l e n t s o f the
Etadunna F o r m a t i o n c r o p o u t i n t h e s w a l e s n e a r t h e r i v e r s d r a i n i n g f o r m t h e north.
These r i v e r s debouch between t h e dunes, b u t t h e r e i s no e v i d e n c e t h a t
t h e y r e a c h e d t h e a r e a o f p a l e dunes d u r i n g a t l e a s t t h e m o s t r e c e n t phases o f dune a c c u m u l a t i o n . I t i s c l e a r t h a t t h e c o n t r a s t i n s e d i m e n t s between t h e p a l e - b r o w n and r e d - -
brown dunes i s a l s o r e f l e c t e d i n t h e i r g e o m o r p h i c s e t t i n g .
The p a l e dunes l i e
i n a r e a s w h i c h have r e c e i v e d a l l u v i a l s e d i m e n t s b y means o f t h e l a r g e r i v e r s These r i v e r s
(Cooper Creek, D i a m a n t i n a R i v e r ) and t h e i r i m m e d i a t e a n c e s t o r s .
d r a i n t h e J u r a s s i c and C r e t a c e o u s r o c k s o f s o u t h w e s t e r n Q u e e n s l a n d ;
rocks
w h i c h a r e commonly f i n e - g r a i n e d s a n d s t o n e s , and mudstones. The r e d - d u n e s i n t h e S t r z e l e c k i d u n e f i e l d l i e o n a t e c t o n i c a l l y s t a b l e base o f C r e t a c e o u s s a n d s t o n e , E a r l y T e r t i a r y sands, and M i d - T e r t i a r y d o l o m i t i c clays.
The s t r e a m s t h a t p r e s e n t l y e n t e r t h e a r e a , and t h e i r a n c e s t o r s , d r a i n
C r e t a c e o u s r o c k s o f t h e G r e y Range t o t h e e a s t ( F i g . 1).
I n t h e south, east
o f L a k e Frome, c h a n n e l s t h a t f o r m e r l y e n t e r e d t h e d u n e f i e l d d r a i n e d t h e c r y s t a l l i n e r o c k s o f t h e s o u t h e r n B a r r i e r r a n g e and O l a r y A r c h , and t h e a r e n a c e o u s r o c k s o f t h e n o r t h e r n B a r r i e r Ranges.
I n t h e Simpson d u n e f i e l d , the
r e d - b r o w n dunes o v e r l i e a l l u v i u m t h a t was p r o b a b l y d e r i v e d f r o m t h e n o r t h , by a n c e s t o r s o f t h e H a l e , Hay and P l e n t y R i v e r s .
The m i n e r a l o g y o f t h e s e sands
s u g g e s t s a p a r t i a l s o u r c e i n t h e Archaean A r u n t a Complex ( C a r r o l l , 1944; Y e a t e s , 1 9 7 1 ) , i n a d d i t i o n t o M e s o z o i c s a n d s t o n e s and mudstones o f t h e Eromanga Basin.
I n summary, i t seems t h a t t h e r e d - b r o w n dunes have been d e r i v e d f r o m a l l u v i u m , some l a c u s t r i n e s e d i m e n t , and t h e breakdown o f b e d r o c k .
It i s l i k e l y
t h a t t h e b u l k o f t h e s e a e o l i a n sands o r i g i n a t e d as a l l u v i u m w h i c h was d e r i v e d f r o m r o c k t y p e s somewhat ( b u t n o t c o m p l e t e l y ) d i f f e r e n t f r o m t h o s e w h i c h s u p p l i e d s e d i m e n t s f o r Cooper Creek and t h e D i a m a n t i n a R i v e r .
There i s
187 evidence t h a t t h e dunes o f d i f f e r e n t c o l o u r were d e r i v e d f r o m s e d i m e n t s o f d i f f e r i n g provenance.
T h i s h y p o t h e s i s s t a n d s i n marked c o n t r a s t w i t h t h e much
discussed v i e w t h a t , i n t h e Simpson D e s e r t , dunes r e d d e n w i t h d i s t a n c e o f t r a n s p o r t downwind;
t h a t i s , t h e y r e d d e n w i t h t i m e ( W o p f n e r and T w i d a l e , 1967,
p.132;
Breed and Breed, 1979).
F o l k , 1976;
The p r o g r e s s i v e r e d d e n i n g o f
dunes away f r o m t h e i r p o s t u l a t e d s o u r c e s i s a common theme i n t h e s e d i m e n t o l o g y o f d e s e r t d u n e f i e l d s ( W a l k e r , 1979;
G a r d n e r and Pye, 1 9 8 1 ) .
I n the following
s e c t i o n o f t h i s p a p e r t h e adequacy o f t h i s h y p o t h e s i s , as a n e x p l a n a t i o n o f t h e results presented e a r l i e r , w i l l be t e s t e d . There a r e t w o k e y e l e m e n t s i n t h i s t e s t .
F i r s t l y , an e x p l a n a t i o n o f t h e
o r i g i n o f c l a y p e l l e t s and, s e c o n d l y , a n e x a m i n a t i o n o f b o t h h o r i z o n t a l and v e r t i c a l sand c o l o u r p a t t e r n s .
CLAY PELLET FORMATION Environments i n w h i c h c l a y p e l l e t s a r e b e i n g f o r m e d u n d e r p r e s e n t c o n d i t i o n s have been d e s c r i b e d i n Texas b y P r i c e ( 1 9 6 3 ) , i n n o r t h A f r i c a b y B o u l a i n e (1956), i n w e s t A f r i c a b y T r i c a r t ( 1 9 5 4 ) and i n A u s t r a l i a b y T e l l e r (1982).
et.
B o w l e r ( 1 9 7 3 ) summarized t h e common f e a t u r e s n e c e s s a r y f o r p e l l e t
formation i n t h e s e c o a s t a l l a g o o n s / m u d f l a t s and i n l a n d p a n s / l a k e s .
Seasonally
high w a t e r t a b l e s a r e r e q u i r e d i n t h e s o u r c e a r e a s , i n w h i c h f l u c t u a t i n g h i g h l y saline groundwater a l l o w s c r y s t a l l i s a t i o n o f s a l t s ( e f f l o r e s c e n c e ) w i t h i n t h e surface muds on t h e edges o f s a l t marshes, l a k e s and s a l i n e pans as e v a p o r a t i o n occurs f r o m t h e c a p i l l a r y f r i n g e ( e v a p o r a t i v e pumping).
The s a l t s m e c h a n i c a l l y
p e l l e t i s e t h e muds and w i n d t r a n s p o r t s t h e s a n d - s i z e p e l l e t s i n t o a d j a c e n t dunes,
The f i n e r p e l l e t s a r e b l o w n away as s u s p e n s i o n l o a d d u s t t o a c c u m u l a t e
a s parna ( B u t l e r , 1 9 7 4 ) . Teller
gal. (1982)
have d e s c r i b e d a modern example o f c l a y p e l l e t f o r -
mation, and a c c u m u l a t i o n o f a l o w c l a y l u n e t t e a t Pup Lagoon, a d j a c e n t t o Lake Tyrell i n northwestern V i c t o r i a .
J. Dulhunty provided t h e present author w i t h
a sample o f ' f l u f f y ' mud f r o m n e a r t h e n o r t h e r n s h o r e o f Lake E y r e N o r t h , where
the Warburton R i v e r e n t e r s t h e l a k e .
T h i s sample c o n s i s t s o f s a n d - s i z e c l a y
p e l l e t s ( F i g s . 9 and 10) w h i c h a r e b e i n g p r o d u c e d i n s m a l l q u a n t i t i e s u n d e r present c o n d i t i o n s . I n a d d i t i o n t o t h e s e c o n d i t i o n s , s t r o n g w i n d s a p p e a r t o b e n e c e s s a r y t o move pellets which e a s i l y adhere t o each o t h e r by hygroscopic a t t r a c t i o n .
Once o n a
dune, r a i n f a l l o r dew w i l l n o r m a l l y cause p e l l e t s t o a d h e r e t o one a n o t h e r . This p r o c e s s does n o t a l l o w h i g h - a n g l e b e d d i n g t o d e v e l o p , so t h a t one c h a r a c t e r i s t i c o f c l a y l u n e t t e s i s t h e i r f l a t t o low-angle bedding (Bowler,
1973).
The l o n g i t u d i n a l dunes o f t h e S t r z e l e c k i d u n e f i e l d , however, show some
high-angle b e d d i n g i n sandy c l a y u n i t s c o n t a i n i n g l e s s t h a n 30% p e l l e t s , w i t h
188
avalanche foreset laminae at 30" (Fig. 17). The clay dune facies at M1 have dips of 10-15" although some are nearly flat bedded. The ability of moderately pellet-rich sand to form avalanche laminae suggests that winds were strong and the atmosphere dry. These conditions apply especially to the Last Glacial unit.
Interbedded ripple-form I laminae and mud drapes
WeSl
Fig. 17. .Cross-stratification exposed in the east-west face of the pit at M1. For location see Fig. 8. In the Strrelecki dunefield the conditions required for clay pellet formation were met between about 23,000 and 12,000 yrs. B.P. The floodflats between the groups of dunes in the Moomba/Cooper Creek area (Fig. 16) were accumulating with muddy sand brought down Cooper Creek. Prior to this period of mud deposition, the ancestral Cooper was carrying a dominantly sandy load in large amplitude meanders, the remnants o f which can still be seen between the dunes (Fig. 16). A radiocarbon date of 22,300 f 1780 yrs. B.P. (ANU-2659) was determined on charcoal, at a site 13 km northwest of Moomba, for the transition from sand deposition to muddy sand deposition. A t the same site, the floodflat ceased to accumulate about 12,460 f 160 yrs. B.P. (ANU-2747). On the palaeochannel in Fig. 13, charcoal at the base of the muddy sand veneer, overlying the old point bar, dates to 12,020 f 150 yrs. B.P. (ANU-2862). The surface of this, the lowest part of the floodflat, is still accumulating small quantities of sediment during major floods. A more detailed account o f these dated sites is given by Wasson (in press). As seen earlier, the Last Glacial phase of dune construction ended about 13,000 yrs. B.P., approximately at the time when the deposition of the muddy veneer on the floodflats was rapidly contracting. Clay pellets can only be produced from a sediment containing mud, so the beginning of clay pellet dune
189 c o n s t r u c t i o n i s t h o u g h t t o have begun when t h e a n c e s t r a l Cooper Creek was changing f r o m a s e r i e s o f m e a n d e r i n g v i g o r o u s s t r e a m s t o a l e s s s i n u o u s m i x e d load d i s t r i b u t a r y s y s t e m a b o u t 23,000 y r s . B . P .
T h i s p e r i o d o f dune con-
s t r u c t i o n i s a p p r o x i m a t e l y t h e same as t h a t r e c o r d e d i n The M a l l e e o f New S o u t h Wales and V i c t o r i a b y B o w l e r ( 1 9 7 8 ) , and i n o t h e r m i d - l a t i t u d e d e s e r t s (Sarnthein, 1978). I n t h i s model, t h e c l a y p e l l e t s and q u a r t z o s e sands w h i c h f o r m t h e dunes o f the S t r z e l e c k i d u n e f i e l d were d e r i v e d b y d e f l a t i o n f r o m c o n t e m p o r a n e o u s l y deposited s e d i m e n t s on t h e f l o o d f l a t s .
The s a l i n e g r o u n d w a t e r was m a i n t a i n e d
by s u r f a c e f l o w , b u t s e a s o n a l a n d / o r l o n g e r - t e r m f l u c t u a t i o n s o f t h e w a t e r table p r o d u c e d t h e n e c e s s a r y c o n d i t i o n s f o r p e l l e t i s a t i o n .
The s a l i n e w a t e r
with m o n t m o r i l l o n i t i c c l a y s may have a i d e d p e l l e t i s a t i o n b y means o f f l o c c u lation.
S t r o n g w i n d s c a r r i e d o u t t h e d e f l a t i o n , and t h e s a l i n i s e d f l o o d f l a t s
were n o t e n c o u r a g i n g t o v e g e t a t i o n . These c o n d i t i o n s o b v i o u s l y n o l o n g e r p r e v a i l .
Surface waters a r e f r e s h ,
supporting a C o n c h o s t r a c a n f a u n a i n i n t e r d u n a l s w a l e s (De D e c c k e r , p e r s .
comm.).
The modern s t a t i c w a t e r t a b l e l i e s a b o u t 40 m b e l o w t h e s u r f a c e i n t h e
Moomba a r e a , and t h e g r o u n d w a t e r i s h i g h l y s a l i n e .
The L a t e H o l o c e n e sand
which caps most p a l e dunes i n t h e S t r z e l e c k i d u n e f i e l d i s l a r g e l y r e w o r k e d f r o m the o l d e r p a r t s o f t h e dunes.
This i s demonstrated by t h e l a c k o f small c l a y
p e l l e t s , and t h e g e n e r a l roundness o f t h e p e l l e t s i n t h e m o b i l e caps.
The
upwind ends o f t h e s e dunes a r e commonly d e f l a t e d , t h e sand m o v i n g down-dune. The contemporaneous d e p o s i t i o n o f a l l u v i u m and f o r m a t i o n of dunes, i n t h e S t r z e l e c k i d u n e f i e l d w e s t o f S t r z e l e c k i Creek, may a l s o have o c c u r r e d i n t h e Simpson d u n e f i e l d n e a r t h e modern c h a n n e l s .
But i t i s clear that the controls
on d e f l a t i o n and dune f o r m a t i o n n o r t h o f K a l l a k o o p a h Creek, i n t h e v i c i n i t y o f
the pans, were r a t h e r d i f f e r e n t .
The pans more t h a n 20 km n o r t h o f t h e
Kallakoopah have n o t been c o n n e c t e d t o t h e c r e e k d u r i n g a t l e a s t t h e l a s t period o f dune c o n s t r u c t i o n , f o r t h e r e i s n o e v i d e n c e o f e i t h e r y o u n g a l l u v i u m or palaeochannels between t h e dunes.
The pans a r e c u t i n sandy a l l u v i u m w h i c h
l i e s i m m e d i a t e l y b e n e a t h t h e dunes and o v e r l i e s l a c u s t r i n e and a l l u v i a l sediments.
E x p o s u r e a r o u n d t h e pans a r e o f t e n good, and s t r a t i g r a p h i c
relationships can be observed. The pans have v e r y s m a l l c a t c h m e n t s , and t h e y a r e o b v i o u s l y c r e a t e d and enlarged b y d e f l a t i o n . formation,
To p r o d u c e t h e n e c e s s a r y c o n d i t i o n s f o r c l a y p e l l e t
i t must b e presumed t h a t t h i s d e f l a t i o n was g r o u n d w a t e r c o n t r o l l e d .
Oisplacive gypsum c r y s t a l s w i t h i n dune sand h i g h above t h e pans p r o v i d e s
A t present t h e s t a t i c groundwater t a b l e i s some 20 m b e l o w t h e pans i n t h e a r e a 50-100 km n o r t h o f K a l l a k o o p a h Creek o n
evidence o f g r o u n d w a t e r f l u c t u a t i o n s .
Traverse 3 and d e f l a t i o n f r o m t h e pans i s v e r y l i m i t e d .
190 The absence o f c l a y p e l l e t s i n t h e r e d - b r o w n dunes and t h e g e o m o r p h i c s e t t i n g o f t b e s e dunes shows t h a t t h e c o n d i t i o n s f o r p a l e dune f o r m a t i o n and r e d dune f o r m a t i o n were v e r y d i f f e r e n t .
The m o b i l i z a t i o n o f sand f o r t h e p a l e
dunes i n v o l v e d g r o u n d w a t e r , w h i l e m o b i l i z a t i o n f o r f o r m a t i o n o f t h e r e d dunes presumably r e q u i r e d l o w values o f t h e aerodynamic roughness l e n g t h , f r e q u e n t d r y i n g o f t h e s e d i m e n t s b e i n g d e f l a t e d , a n d / o r enhanced w i n d i n e s s (Ash and Wasson, i n p r e s s ) .
The r e d brown dunes were f o r m e d b y d e f l a t i o n o f s e d i m e n t
l y i n g i n s l i g h t l y e l e v a t e d a r e a s where g r o u n d w a t e r p r o b a b l y d i d n o t r e a c h t h e i n t e r d u n a l swales.
As n o t e d e a r l i e r , t h e r e d - b r o w n dunes commonly b e g i n f r o m
t h e s u r f a c e u p o n w h i c h t h e y r e s t , r a t h e r t h a n e x t e n d i n g downwind f r o m t r a n s v e r s e dunes.
T h e r e f o r e , t h e s o u r c e o f t h e sands i s c l o s e t o t h e dunes, n o t
hundreds o f k i l o m e t r e s upwind. The age o f t h e p a l e dunes o f t h e S t r z e l e c k i d u n e f i e l d i s r e a s o n a b l y w e l l known, a t l e a s t a t a f e w s i t e s , b u t t h e r e i s no c o m p a r a b l e c h r o n o l o g i c a l control
on t h e r e d - b r o w n dunes.
The n o n - c a l c a r e o u s caps o f w e l l - b e d d e d r e d -
brown sand c o n t a i n b a c k e d b l a d e s , s m a l l s t o n e t o o l s made o n l y d u r i n g t h e l a s t 4,000 y r s . ( P . Hughes, p e r s . comm.).
B e n e a t h t h i s p e r i o d i c a l l y m o b i l e cap
t h e r e i s a c a l c a r e o u s p a l a e o s o l w h i c h gave an a p p a r e n t I 4 C age o f 15,900 t 320 y r s . B.P.
(ANU-2280) a t a s i t e i n f a r n o r t h w e s t e r n N.S.W.
( F i g . 1).
I t seems
t h a t t h e m o b i l e c a p i s o f L a t e H o l o c e n e age w h i l e t h e 16,000 y r p a l a e o s o l t o p s t h e L a s t G l a c i a l sand.
T h e r e a r e p r e - L a s t G l a c i a l u n i t s as w e l l .
This i s a
s p e c u l a t i v e c h r o n o l o g y and r e q u i r e s e x t e n s i v e t e s t i n g . W h i l e i t i s n o t c l e a r i f t h e r e d - b r o w n and p a l e - b r o w n dunes a r e o f t h e same age, i t i s l i k e l y t h a t t h e enhanced w i n d i n e s s p o s t u l a t e d f o r c l a y p e l l e t movement a l s o m o b i l i s e d sand i n t h e a r e a s o f r e d dunes (Ash and Wasson, i n press).
I t i s a r e a s o n a b l e w o r k i n g h y p o t h e s i s t h a t t h e dunes a r e o f t h e same
age b u t t h e p r o c e s s e s o f t h e i r f o r m a t i o n a r e v e r y d i f f e r e n t . The s i m p l e scheme o f p r o g r e s s i v e a g e i n g o f dune sand away f r o m s o u r c e a r e a s does n o t t a k e a c c o u n t o f t h e v e r y d i f f e r e n t p r o c e s s e s o p e r a t i n g i n a r e a s o f d i f f e r e n t sand c o l o u r .
F u r t h e r m o r e , i t does n o t t a k e a c c o u n t o f m u l t i p l e
s o u r c e s f o r dune sands, b u t p o s t u l a t e s t h a t a l l dune sand o r i g i n a t e d n e a r Lake E y r e i n t h e Simpson d u n e f i e l d , f o r example.
As a l r e a d y seen, t h e g r a i n - s i z e
data suggest t h a t t h i s p o s t u l a t e i s i n c o r r e c t .
COLOUR PATTERNS The e x p l a n a t i o n o f t h e s p a t i a l ( o r h o r i z o n t a l ) p a t t e r n s o f sand c o l o u r ( F i g . Z ) , as t h e r e s u l t o f p r o g r e s s i v e r e d d e n i n g downwind, f a i l s d r a m a t i c a l l y i n one i m p o r t a n t a r e a .
The r e d - b r o w n dunes t o t h e e a s t o f S t r z e l e c k i Creek are
a d j a c e n t t o p a l e dunes on, and t o t h e n o r t h o f , T r a v e r s e 2.
It i s impossible
t o e x p l a i n t h i s b o u n d a r y as t h e r e s u l t o f d i f f e r e n t r a t e s o f r e d d e n i n g , away
191 from source a r e a s , o n e i t h e r s i d e o f t h e c r e e k , because t h e p a l e dunes t o t h e west o f t h e c r e e k do n o t r e d d e n downwind, The v e r t i c a l p a t t e r n s o f c o l o u r i n red-brown sand, d e t e r m i n e d b y a u g e r i n g , and seen a t a number o f s e c t i o n s seen t h r o u g h dunes, show u n e q u i v o c a l l y t h a t the dunes a r e r e d t h r o u g h o u t .
The c o a t i n g s on t h e g r a i n s c o n s i s t o f i r o n and
clay l y i n g i n h o l l o w s o r p i t s on t h e g r a i n s , p r o t e c t e d f r o m a b r a s i o n ( c f . F o l k ,
1976).
The dune sands a p p e a r t o have been d e r i v e d f r o m a ' p r e - r e d d e n e d '
sediment, j u s t l i k e t h a t w h i c h l i e s b e n e a t h b o t h t h e dunes and i n t e r d u n a l swales.
Pedogenesis w i t h i n a l l u v i u m , and s e d i m e n t s o f o t h e r o r i g i n s , has
coated t h e g r a i n s w i t h i r o n and c l a y , and t h e s e c o a t i n g s have been a b r a d e d during a e o l i a n t r a n s p o r t .
I f t h e dunes had c o n s i s t e d o f p a l e brown sands w h i c h
were reddened i n s i t u , t h e n t h e r e d d e n i n g s h o u l d d e c r e a s e b e l o w t h e s u r f a c e . This i s p r e c i s e l y t h e p a t t e r n t h a t i s f o u n d i n t h e p a l e dunes o f b o t h d u n e f i e l d s , where t h e o l d e s t p a l a e o s o l s a r e s l i g h t l y r u b i f i e d ( 7 . 5 Y R 7 / 4 ) . Some o f t h e s e o b j e c t i o n s t o t h e downwind r e d d e n i n g h y p o t h e s i s p o s s i b l y c a n be c o u n t e r e d b y s u g g e s t i n g t h a t t h e s e dunes t u r n o v e r w i t h t i m e t h e r e b y producing u n i f o r m l y r e d sand f r o m t o p t o b o t t o m .
However, t h e e v i d e n c e o f
palaeosols s e p a r a t i n g b o d i e s o f sand i n v e r t i c a l e x p o s u r e s s u g g e s t s t h a t , w h i l e some downwind movement o b v i o u s l y o c c u r s , t h e dunes do n o t m i g r a t e v e r y f a r during each i n t e r v a l o f a e o l i a n a c t i v i t y . v e r t i c a l g r o w t h o f t h e s e dunes.
T h e r e i s abundant e v i d e n c e f o r t h e
I t must be c o n c l u d e d t h a t a s p e c t s o f t h e
colour p a t t e r n s d o n o t s u p p o r t t h e downwind r e d d e n i n g h y p o t h e s i s , b u t a c t u a l l y counter t h e h y p o t h e s i s . How t h e n can we e x p l a i n t h e p a t t e r n o f sand c o l o u r s a t C u r d l a w i d n y Lagoon (Fig. 6 ) and o t h e r c a s e s o f a s i m i l a r k i n d n o t documented i n t h i s p a p e r ?
Sand
t h a t e n t e r s l a k e s , e i t h e r b y w i n d o r w a t e r t r a n s p o r t a t i o n , t e n d s t o l o s e some o f i t s i r o n l c l a y c o a t i n g d u r i n g wave t r a n s p o r t .
T h i s i s c l e a r l y seen a t
Curdlawidny w h e r e r e d - b r o w n dunes s u p p l y s a n d t o t h e l a k e on t h e w e s t e r n s i d e but t h e dunes o n t h e e a s t e r n (downwind) s h o r e a r e p a l e .
It i s proposed t h a t
f u r t h e r downwind t h e p a l e sands m i x w i t h r e d - b r o w n sands d e r i v e d f r o m o l d e r material which surround t h e lake.
T h i s m i x i n g o f sands o f d i f f e r e n t s o u r c e s
(on a meso t o m i c r o s c a l e ) can b e seen e a s i l y i n t h e f i e l d w i t h a hand l e n s . As a g e n e r a l p r o p o s i t i o n i t i s s u g g e s t e d t h a t downwind r e d d e n i n g o f dunes may r e f l e c t downwind r e d d e n i n g ( a n d a g e i n g ) o f t h e s o u r c e s o f dune sands.
In
the S i m p s o n - S t r z e l e c k i d u n e f i e l d t h e r e d s e d i m e n t s , f r o m w h i c h r e d dunes have been d e r i v e d , a r e p r o b a b l y o l d e r t h a n t h e p a l e a l l u v i u m f r o m w h i c h t h e p a l e dunes have f o r m e d .
T h i s p r o p o s i t i o n may a l s o a p p l y t o t h e w e l l documented
downwind r e d d e n i n g i n w e s t e r n L i b y a ( W a l k e r , 1 9 7 9 ) .
192 CONCLUSION One o f t h e m o s t o b v i o u s f e a t u r e s o f t h e dunes o f t h e S i m p s o n - S t r z e l e c k i dunefield i s t h e i r colour.
Dune c o l o u r s h a v e been mapped and t h e sands and
dunes examined w i t h i n each o f t h e m a j o r c o l o u r r e g i o n s .
T h r e e t r a v e r s e s were
l a i d o u t t o c r o s s examples o f b o u n d a r i e s between t h e s e r e g i o n s , and a s p e c t s o f m i n e r a l o g y , i r o n c o n t e n t and g r a i n - s i z e examined.
The b o u n d a r i e s a c t u a l l y
c o n s i s t o f t r a n s i t i o n a l zones, b u t t h e s e zones a r e q u i t e n a r r o w i n some c a s e s . T h e r e f o r e t w o p r i m a r y c o l o u r s can be d i s t i n g u i s h e d :
p a l e brown and red-brown.
I n t w o o u t o f t h r e e t r a v e r s e s , t h e r e d - b r o w n dunes have l o w e r heavy m i n e r a l I n both
c o n t e n t s t h a n t h e p a l e brown dunes, t h e e x c e p t i o n b e i n g T r a v e r s e 3.
T r a v e r s e 1 and 2 t o t a l f e l d s p a r c o n t e n t i s l o w e r i n t h e r e d - b r o w n t h a n i n t h e p a l e brown dunes.
I n b o t h T r a v e r s e 2 and 3 t o t a l i r o n c o n t e n t i s h i g h e r i n t h e
r e d - b r o w n dunes t h a n i n t h e p a l e brown dunes.
O f these three variables, only
i r o n c o n t e n t i s l i k e l y t o a f f e c t sand c o l o u r , and so a n o t h e r a r e a was sampled n e a r C u r d l a w i d n y Lagoon i n t h e P a r a k y l i a d u n e f i e l d .
Total i r o n content i s
a g a i n h i g h e r i n t h e r e d - b r o w n dunes. While i t appears t h a t t h e c o l o u r d i f f e r e n c e s can be accounted f o r b y differences i n i r o n content, importance.
t h e t h i c k n e s s o f g r a i n c o a t i n g s i s a l s o o f some
That i s , c o a t i n g s a r e t h i c k e s t on t h e reddest g r a i n s .
The o t h e r
c h a r a c t e r i s t i c s n o t e d o n t h e t r a v e r s e s ( f e l d s p a r and h e a v y m i n e r a l c o n t e n t ) s i m p l y r e f l e c t d i f f e r e n c e s i n t h e source sediments.
This interpretation i s
s u p p o r t e d b y t h e q u a n t i t a t i v e d i f f e r e n c e s i n m i n e r a l o g y between a r e a s o f d i f f e r i n g sand c o l o u r . There i s no c l e a r s e p a r a t i o n on an M Q
Z I
parts o f the dunefields.
d i a g r a m o f c r e s t a l sands i n v a r i o u s
However, t h e r e i s a t e n d e n c y f o r t h e f i n e s t ( a n d
p a l e s t ) sands t o o c c u r n e a r t h e modern c h a n n e l s o f Cooper Creek, t h e D i a m a n t i n a R i v e r , and K a l l a k o o p a h Creek. red-brown,
The c o a r s e r c r e s t a l sands a r e g e n e r a l l y
and l i e away f r o m t h e s e c h a n n e l s .
T h i s t r e n d cannot be e a s i l y
i n t e r p r e t e d b y t h e n o t i o n t h a t a l l o f t h e dune sand o r i g i n a t e d i n t h e modern c h a n n e l s. E x a m i n a t i o n o f b o t h p a l e brown and r e d - b r o w n dune sand i n t h i n s e c t i o n has d e m o n s t r a t e d q u i t e c l e a r l y t h a t t h e f o r m e r c o n t a i n o f t e n l a r g e q u a n t i t i e s of c l a y p e l l e t s w h i l e t h e l a t t e r c o n t a i n none. w i t h those found i n c l a y lunettes. p e l l e t formation,
The c l a y p e l l e t s a r e i d e n t i c a l
By r e f e r e n c e t o modern examples o f c l a y
i t has been p o s s i b l e t o s p e c i f y t h e c o n d i t i o n s w h i c h p r e -
v a i l e d between 23,000 and 13,000 y r s ago when t h e y w e r e a c t i v e l y b e i n g produced.
S a l i n e g r o u n d w a t e r , w h i c h i s now t e n s o f m e t r e s b e l o w t h e s u r f a c e ,
i n t e r s e c t e d t h e surface possibly seasonally.
Drying allowed s a l t s t o
c r y s t a l l i z e i n t h e sandy muds, s u p p l i e d i n t h e c a s e o f t h e S t r z e l e c k i d u n e f i e l d b y Cooper Creek, t h e r e b y p e l l e t i s i n g t h e s e d i m e n t s .
I n t h e Simpson d u n e f i e l d
193 old alluvium was p e l l e t i s e d by groundwater f uctuation, b u t contemporaneous alluvial deposition and d e f l a t i o n was probab y limited t o the floodplains of Kallakoopah Creek a n d t h e Diamantina River. Falling water-tables, reduced a l l u v i a t i o n , a n d possibly decreased windiness around 13,000 stopped pale dune construction. Unfortunately the age of the red-brown dunes i s not well enough known t o say i f they became s t a b l e a t the same time. A Late Holocene phase of re-working appears t o have affected both pale and red-brown dunes. The formation of t h e pale dunes required p a r t i c u l a r hydrologic conditions, while the red-brown dunes could have been formed under any climatic conditions which limited vegetation cover and maintained strong winds. The differences in geomorphic s e t t i n g of the two groups of dunes shows t h a t there i s no simple gradation from one t o the other. This in turn suggests t h a t explanation of the downwind reddening of dunes, seen e s p e c i a l l y in the Simpson dunefield, i s n o t simply a function of ageing of the iron minerals in grain coatings. The progressive ageing hypothesis a l s o does not account f o r the l i k e l y multiple sources of sediment f o r dune construction. The hypothesis a l s o f a i l s t o account f o r e i t h e r complete reddening of red-brown dunes, or t h e boundary between pale a n d red-brown dunes e i t h e r side of the NNE-SSW trending Strzelecki Creek. I t i s concluded t h a t reddening of dune sand in the Simpson-Strzelecki dunefield largely occurred p r i o r t o dune construction. The red-brown dunes a r e b u i l t of pre-reddened sediments while the pale dunes a r e b u i l t of pale sediments. Both bodies of sediment were derived from d i f f e r e n t sources, so t h a t dune colour ultimately r e f l e c t s source sediment provenance. This paper gives the f i r s t account of a vary l a r g e f i e l d (some 100,000 kmL) of longitudinal d e s e r t dunes rich in clay p e l l e t s . The palaeohydrological conditions proposed t o explain t h e i r formation appears t o be r a t h e r d i f f e r e n t from the hot waterless environment normally associated with d e s e r t dune formation. The presence o f water in swales with a seasonally warm t o hot atmosphere combines environmental f a c t o r s no longer found in Australia.
The
h i g h groundwater level was probably t h e r e s u l t of a long period of positive water balance preceding 25,000 y r s B.P. (Bowler, 1978) r a t h e r t h a n a s t a t e in equilibrium with conditions a t the peak of the Last Glacial. These
relationships show t h a t a simple uniformitarian model f o r the i d e n t i f i c a t i o n of aeolian deposits in the rock record i s l i k e l y t o be unsuccessful. A more fruitful approach i s t o i n v e s t i g a t e t h e evolution of 'modern' sediments through
at l e a s t t h e Late Quaternary.
194 ACKNOWLEDGMENTS
I n t h e l a b o r a t o r y Mr Jim Caldwell performed a n a l y s e s w i t h s k i l l , w h i l e in t h e f i e l d K . Q u a y l e and M. Campion provided c o n s i d e r a b l e a s s i s t a n c e . J.N. J e n n i n g s and R . W . Galloway read t h e m a n u s c r i p t and made h e l p f u l comments. F i n a l l y , J.M. Bowler c o n t r i b u t e d t o the i d e a s i n t h i s paper d u r i n g many conversations. FIEFE R E N C E S
Cish, J.E. and Wasson, R.J., in p r e s s . Vegetation and sand m o b i l i t y i n t h e A u s t r a l i a n d e s e r t d u n e f i e l d . Z e i t . Geomorphol. N . F . , Suppl. Bd. E o u l a i n e , J . , 1956. Les l u n e t t e s des b a s s e s p l a i n e s o r a n a i s e s ; f o r m a t i o n e o l i e n n e a r g i l e u s e s l i e s a l ' e x t e n s i o n des s o l s s a l i n s ; La Sebkha de Ben Ziane; l a d e p r e s s i o n de C h a n t r i t : Proc. Conf. I n t . Assoc. Quaternary Res., 4 t h , pp.143-150. Eowler, J.M., 1973. Clay dunes: t h e i r o c c u r r e n c e , formation and e n v i r o n mental s i g n i f i c a n c e . E a r t h - S c i e n c e Reviews, 9: 315-338. Bowler, J.M., 1978. G l a c i a l age e v e n t s a t high and low l a t i t u d e s : a s o u t h e r n hemisphere p e r s p e c t i v e . I n : E . M . v a n Zinderen Bakker ( E d i t o r ) , A n t a r c t i c G l a c i a l H i s t o r y and World Palaeoenvironrnents. A . A . Balkema, Rotterdam, pp.149-172. F r e e d , C.S. and Breed, W.J., 1979. Dunes and o t h e r windforms of c e n t r a l A u s t r a l i a ( a n d a comparison with l i n e a r dunes on t h e Moenkopi P l a t e a u , A r i z o n a ) . I n : Farouk El-Baz and D.M. Warner ( E d i t o r s ) , Apollo-Soyuz T e s t P r o j e c t Summary S c i e n c e R e p o r t , V . 2 , Earth O b s e r v a t i o n s and Photography. U.S. National A e r o n a u t i c s and Space A d m i n i s t r a t i o n SP-412, pp.319-358. B u t l e r , B . E . , 1956. P a r n a , an a e o l i a n c l a y . Aust. J . S c i . , 18: 145-151. E u t l e r , B . E . , 1974. A c o n t r i b u t i o n towards t h e b e t t e r s p e c i f i c a t i o n of parna a n d some o t h e r a e o l i a n c l a y s i n A u s t r a l i a . Z e i t . Geomorphol. N . F . , Suppl. Bd., 20: 106-116. C a r r o l l , D . , 1944. The Simpson D e s e r t E x p e d i t i o n , 1939. S c i e n t i f i c r e p o r t s : No. 2 , Geology - d e s e r t s a n d s . T r a n s . R . S O C . S . Aust., 68: 49-59. Churchward, H . M . , 1963. S o i l s t u d i e s a t Swan H i l l , V i c t o r i a , A u s t r a l i a , IV: Ground-surface h i s t o r y and i t s e x p r e s s i o n i n t h e a r r a y of s o i l s . Aust. J . S o i l Res., 1: 242-255. Dare-Edwards, A . J . , 1979. L a t e Q u a t e r n a r y s o i l s on c l a y dunes of t h e Willandra Lakes, New South Wales. PhD t h e s i s , Aust. Nat. U n i v . , Canberra. Dare-Edwards, A . J . , 1982. Clay p e l l e t s of c l a y dunes: t y p e s , mineralogy, o r i g i n and e f f e c t o f p e d o g e n e s i s . In: R.J. Wasson ( E d i t o r ) , Q u a t e r n a r y Dust Mantles o f China, New Zealand and A u s t r a l i a . Aust. N a t . Univ. Folk, R . L . , 1968. P e t r o l o g y of Sedimentary Rocks. A u s t i n , Texas. H e m p h i l l ' s 170pp. Folk, R . L . , 1976. Reddening of d e s e r t s a n d s : Simpson D e s e r t , N.T., A u s t r a l i a . J . Sediment. P e t r o l . , 46: 604-615. Gardner, R . and Pye, K . , 1981. N a t u r e , o r i g i n and palaeoenvironmental s i g n i f i c a n c e o f red c o a s t a l and d e s e r t dune s a n d s . Prog. Phys. Geogr. 5 : 514-534. Krieg, G.W. and C a l l e n , R . A . , 1980. Geological o b s e r v a t i o n s i n the playa r e g i o n of t h e Simpson D e s e r t , South A u s t r a l i a . Dept. Mines and Energy .. South A u s t r a l i a , Report Book No. 80/68. L o f f l e r , E . and S u l l i v a n , M . E . , 1979. Lake D i e r i r e s u r r e c t e d : an i n t e r p r e t a t i o n u s i n g s a t e l l i t e imagery. Z e i t . Geomorphol., 23: 233-242. Mabbutt, J.A. and S u l l i v a n , M.E., 1968. The f o r m a t i o n of l o n g i t u d i n a l dunes: e v i d e n c e from t h e Simpson D e s e r t . Aust. Geogr., 10: 483-487. McKee, E . D . ( E d . ) , 1979. A Study o f Global Sand S e a s . U.S. Geol. S u r v . , P r o f . P a p . 1052, 42913.
195 P r i c e , W.A., 1963. Physicochemical and environmental f a c t o r s i n c l a y dune genesis. J. Sediment. P e t r o l . , 33: 766-778. Rhodes, E.G., 1980. Modes o f Holocene c o a s t a l p r o g r a d a t i o n , G u l f o f C a r p e n t a r i a . PhD t h e s i s , Aust. Nat. U n i v . Canberra. Sarnthein, M., 1978. Sand d e s e r t s d u r i n g g l a c i a l maximum and c l i m a t i c optimum. Nature, 272: 43-44. T e l l e r , J.T., Bowler, J.M. and Macumber, P.G., 1982. Modern s e d i m e n t a t i o n and hydrology i n Lake T y r r e l l , V i c t o r i a . J. Geol. SOC. A u s t . , v . 29, p.159-176. T r i c a r t , J., 1954. I n f l u e n c e des s o l s s a l e s s u r l a d e f l a t i o n e o l i e n n e en basse M a u r i t a n i e e t dans l a D e l t a du Senegal. Rev. Geomorphol. Dyn., 5 : 124-132. Twidale, C.R., 1972. E v o l u t i o n o f sand dunes i n t h e Simpson D e s e r t , c e n t r a l A u s t r a l i a . I n s t . B r i t i s h Geographers T r a n s a c t i o n , Publ. No. 56, pp.77-109. Walker, T.R., 1979. Red c o l o r i n dune sand. I n : E.D. McKee ( E d i t o r ) , A Study of Global Sand Seas. U.S. Geol. Surv., P r o f . Pap. 1052, pp.61-82. 1976. Holocene a e o l i a n landforms i n t h e Belarabon area, S.W. o f Wasson, R.J., J . and Proc. R. SOC. N.S.W., 109: 91-101. Cobar, N.S.W. i n press. The C a i n o z o i c h i s t o r y o f t h e S t r z e l e c k i and Simpson Wasson, R.J., d u n e f i e l d s ( A u s t r a l i a ) , and t h e o r i g i n o f t h e d e s e r t dunes. Z . Geomorph. Suppl , Bd. Wopfner, H. and Twidale, C.R., 1967. Geomorphological h i s t o r y o f t h e Lake Eyre Basin. I n : J.N. Jennings and J.A. Mabbutt ( E d i t o r s ) , Landform S t u d i e s from A u s t r a l i a and New Guinea. A u s t . Nat. Univ. Press, Canberra, Aust., pp. 118-143. Yeates, A.N., 1971. S h a l l o w s t r a t i g r a p h i c d r i l l i n g , western Eromanga B a s i n and Alcoota sheet area, N o r t h e r n T e r r i t o r y , 1971. Bureau o f M i n e r a l Resources, Geology and Geophysics, Record 1971/120. Appendix
ANALYTICAL METHODS.
1. Standard c l a y - s i z e m i n e r a i i d e n t i f i g a t i o n by XRD was c a r r i e d o u t u s i n g CUKU r a d i a t i o n , h e a t i n g a t 450 C and 550 C, and g l y c o l a t i o n . 2. Sand c o l o u r s were determined on samples b r o u g h t from t h e dunes t o t h e l a b oratory, n o t i n t h e f i e l d . C o l o u r s were t h e n determined under u n i f o r m l i g h t i n g on a i r - d r y samples. S i z i n g was n o t done b e f o r e c o l o u r d e t e r m i n a t i o n . The Standard Japanese C o l o u r Code was used.
3. Heavy m i n e r a l s were separated u s i n g bromoform, and t h e s e p a r a t e s ground f o r XRD a n a l y s i s . 4. T o t a l i r o n was d e t e r m i n e d c o l o r i m e t r i c a l l y a f t e r n i o i s t e n i n g w i t h 6N HC1 and complexing w i t h KCNS. F e r r i c i r o n was determined i n t h e same way, w i t h t h e a d d i t ion o f a s t e p i n v o l v i n g a p r o t e c t i v e c o v e r o f e t h y l e t h e r f o r 24 hours. F e r r o u s i r o n can be c a l c u l a t e d as t h e d i f f e r e n c e between t o t a l i r o n and f e r r i c i r o n .
5. CaC03 was determined by v o l u m e t r i c c a l c i m e t e r , iiieasuring C02 volume. 6. Carbon i n g r a i n c o a t i n g s was d e t e r m i n e d by p h y s i c a l removal o f t h e c o a t i n g s , then o x i d a t i o n i n a f u r n a c e and d i r e c t iiieasureiiient o f CO a f t e r s c r u b b i n g t o remove s u l p h u r , o x i d e s o f n i t r o g e n , e t c . T h i s method a l l h s d e t e r m i n a t i o n o f as l i t t l e as 1 0 ppm. carbon.
7 . G r a i n s f o r EDAX were mounted i n a r a l d i t e t h e n ground and f i n i s h e d w i t h 0.5
pm
diamond l a p .
8. U l t r a s o n i c d i s p e r s i o n was performed by a probe r u n a t 20,000 c y c l e s / m i n u t e f o r 10 m i n u t e s i n a m i x t u r e o f 100 m l d i s t i l l e d water, 5 m l o f 5% c a u s t i c soda and 10 m l o f 10% t r i - p o l y - p h o s p h a t e .
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197
EARLY POST-DEPOSITIONAL MODIFICATION OF AEOLIAN DUNE SANDS
K. PYE, Department o f E a r t h Sciences, Cambridge U n i v e r s i t y , Downing S t r e e t , Cambridge, CB2 3EQ, England
1.
INTRODUCTIOri
The p r i m a r y sedimentary c h a r a c t e r i s t i c s o f a e o l i a n dune sands may be m o d i f i e d s i g n i f i c a n t l y w i t h i n a s h o r t t i m e o f d e p o s i t i o n by s u r f a c e - r e l a t e d processes.
An
understanding o f these processes and t h e i r e f f e c t s i s i m p o r t a n t f o r c o r r e c t i n t e r p r e t a t i o n o f t h e environment o f f o r m a t i o n and d i a g e n e t i c h i s t o r y o f sand bodies i n the g e o l o g i c a l r e c o r d , and f o r t h e r e s o u r c e and e n g i n e e r i n g e v a l u a t i o n o f Quaternary dune d e p o s i t s .
Because o f t h e i r h i g h i n i t i a l p o r o s i t y and p e r m e a b i l i t y ,
dune sands a r e p o t e n t i a l l y i m p o r t a n t a q u i f e r s and hydrocarbon r e s e r v o i r s .
Pro-
cesses which m i g h t a f f e c t these p r o p e r t i e s , e i t h e r l o c a l l y o r r e g i o n a l l y , a r e therefore o f m a j o r i n t e r e s t i n o i l and gas e x p l o r a t i o n (Coneybeare, 1976; Glennie e t a1 ., 1970).
Weathering, l e a c h i n g and a u t h i g e n e s i s i n dune sands can
a l s o , under c e r t a i n circumstances, l e a d t o t h e f o r m a t i o n o f e c o n o m i c a l l y v a l u a b l e o r e s . High grade s i l i c a g l a s s sands, f o r example, a r e sometimes formed by p r o longed w e a t h e r i n g under c o n d i t i o n s o f s t r o n g l e a c h i n g . Although t h e r e a r e many p u b l i s h e d s t u d i e s which aim t o i n t e r p r e t and d e s c r i b e
t h e d i a g e n e t i c h i s t o r i e s o f a n c i e n t a e o l i a n sandstones, t h e r e have been few det a i l e d i n v e s t i g a t i o n s of e a r l y d i a g e n e s i s i n Q u a t e r n a r y dune sands w i t h t h e except'on o f carbonate c o a s t a l a e o l i a n i t e s . The l i t e r a t u r e which does e x i s t i s scattered i n t h e j o u r n a l s o f sedimentology, s o i l science, geomorphology, e n g i n e e r i n g geology and Q u a t e r n a r y s t u d i e s . T h i s paper t h e r e f o r e a t t e m p t s t o p r o v i d e a
summary o f e a r l y p o s t - d e p o s i t i o n a l a1 t e r a t i o n processes, u s i n g as i l l u s t r a t i o n s examples drawn f r o m t h e a u t h o r ' s own r e c e n t work i n A u s t r a l i a and N o r t h America. I n t h i s discussion e a r l y post-depositional m o d i f i c a t i o n i s considered t o i n clude t h e e f f e c t s o f processes which a p e r a t e f r o m t h e t i m e a sediment i s i n i t i a l l y deposited by w i n d u n t i l i t becomes b u r i e d t o a depth o f a few t e n s o f metres. The d e f i n i t i o n t h u s corresponds w i t h t h e ' s y n d i a g e n e s i s ' o f F a i r b r i d g e (1967). Most
of the Q u a t e r n a r y dune sand
b o d i e s d i s c u s s e d have n o t been b u r i e d by l a t e r s e d i -
ments ( e x c e p t by younger dune sands i n some c a s e s ) , and have been exposed t o sub-aerial processes f o r t i m e p e r i o d s r a n g i n g up t o s e v e r a l hundred thousand years. Such processes i n c l u d e slumping and compaction, b i o t u r b a t i o n , mechanical and chemical weathering, a d d i t i o n o f a l l o c h t h o n o u s sediment components, p r o f i l e differentiation,
and f o r m a t i o n o f a u t h i g e n i c pigments and cements. The e f f e c t s
of these processes a r e r e f l e c t e d i n g r a i n s i z e d i s t r i b u t i o n s , m i n e r a l o g y , s u r f a c e textures, sediment f a b r i c and sedimentary s t r u c t u r e s .
198 F a c t o r s which i n f l u e n c e t h e n a t u r e and r a t e o f e a r l y p o s t - d e p o s i t i o n a l
modifi-
c a t i o n i n c l u d e c l i m a t e , i n i t i a l t e x t u r e and m i n e r a l o g y o f t h e dune sand, morphol o g y and p h y s i o g r a p h i c s e t t i n g o f t h e d e p o s i t , t h e c h e m i s t r y o f groundwaters, and t h e n a t u r e o f fauna and f l o r a . The most i m p o r t a n t s i n g l e f a c t o r i s r a i n f a l l amount, s i n c e t h i s c o n t r o l s r a t e o f m i n e r a l w e a t h e r i n g and l e a c h i n g , v e g e t a t i o n cover, and i n t e r s t i t i a l geochemical c o n d i t i o n s i n t h e s u b - s u r f a c e sands. Other t h i n g s b e i n g e q u a l , dune sands i n humid areas a r e l i k e l y t o be m o d i f i e d much more r a p i d l y t h a n i n d e s e r t s . Although some a l t e r a t i o n can and does t a k e p l a c e w i t h i n l a r g e a c t i v e l y moving dunes, t h e e f f e c t s become e s p e c i a l l y pronounced a f t e r s t a b l i i z a t i o n and e s t a b l i s h m e n t o f a p a r t i a l o r complete v e g e t a t i o n cover.
2.
MODIFICATION BY RAINFALL AliD R U N N I N G WATER Dune s l o p e s a r e a f f e c t e d by r a i n s p l a s h , s u r f a c e wash, s o i l creep and s m a l l -
s c a l e slumping phenomena. Even on a c t i v e dunes, p e r i o d i c s m a l l - s c a l e slumping and flowage o f wet sand i s a common phenomenon ( F i g . 1 ) which l e a d s t o t h e f o r m a t i o n o f s y n g e n e t i c c o n t o r t e d b e d d i n g , s m a l l - s c a l e f a u l t s , and b r e c c i a t e d sands (McKee and B i g a r e l l a , 1972; B i g a r e l l a , 1975a). I n areas o f h i g h r a i n f a l l , dunes a r e
F i g . 1 . S m a l l - s c a l e slump f e a t u r e formed ( a p p r o x . 80cm h i g h ) by f l o w a g e o f sand a f t e r heavy r a i n , S i l t c o o s on t h e c o a s t o f Oregon.
199 r a p i d l y m o d i f i e d a f t e r s t a b i l i z a t i o n by s l o p e processes which t r a n s p o r t sand from dune c r e s t s i n t o i n t e r - d u n e d e p r e s s i o n s . L a t e P l e i s t o c e n e c o a s t a l dunes which now show subdued rounded r e l i e f due t o such m o d i f i c a t i o n o c c u r e x t e n s i v e l y i n e a s t e r n A u s t r a l i a and have been d e s c r i b e d by Ward (1977) and by Pye ( i n press
a, i n press b ) . Where dunes r e s t a g a i n s t h i g h e r ground, slopewash may t r a n s p o r t pebbles and f i n e r m a t e r i a l o u t o n t o t h e dune s u r f a c e ( F i g . 2 ) . Subsequent e r o s i o n
Fig.2. Pebble bed i n t e r b e d d e d w i t h a e o l i a n sands and s u r f a c e wash d e p o s i t s near the summit o f a l a r g e dune, N o r t h Queensland. The pebbles have been t r a n s p o r t e d by surface wash f r o m an area o f bedrock h i g h ground a g a i n s t which t h e dune r e s t s . (Pebbles approx. 2-3cm i n d i a m e t e r ) . by r a i n s p l a s h nay l e a v e t h e pebbles perched on 5 - lOcm h i g h p e d e s t a l s o f p r o -
t e c t e d sand ( F i g . 3 ) . Such s t r u c t u r e s may be p r e s e r v e d i f t h e i r f o r m a t i o n i s f o l l o w e d by renewed a e o l i a n s e d i m e n t a t i o n .
200
F i g . 3 . Pebble-capped sand p e d e s t a l s (10-15cm h i g h ) f o r m e d b y r a i n s p l a s h e r o s i o n o n t h e s u r f a c e o f a p a r t i a l l y w e a t h e r e d dune, N o r t h Q u e e n s l a n d . E s p e c i a l l y i n o l d e r w e a t h e r e d dune sands, w h i c h c o n t a i n a c e r t a i n amount o f s i l t and c l a y , g u l l y i n g commonly o c c u r s ( e . g . T a l b o t and W i l l i a m s , 1 9 7 7 ) . Eroded sand t r a n s p o r t e d o u t t h r o u g h t h e g u l l i e s i s t y p i c a l l y d e p o s i t e d as s m a l l - s c a l e , l o w - a n g l e a l l u v i a l f a n s a t t h e dune b a s e . B i g a r e l l a ( 1 9 7 5 a ) has s u g g e s t e d t h e term ' d i s s i p a t i o n s t r u c t u r e s ' f o r t h e v a r i e t y o f sedimentary s t r u c t u r e s formed by dune d e g r a d a t i o n p r o c e s s e s .
3.
COMPACT1 ON The i n i t i a l p o r o s i t i e s o f f r e s h l y d e p o s i t e d sands, i n c l u d i n g dune sands,
t y p i c a l l y r a n g e f r o m a b o u t 25% t o 65% d e p e n d i n g on t h e s e d i m e n t s o r t i n g , p a c k i n g a r r a n g e m e n t a n d p a r t i c l e shape ( P r y o r , 1971; R i t t e n h o u s e , 1971; B e a r d and Weyl,
1973). I n i t i a l p a c k i n g arrangement and v o i d s r a t i o s t r o n g l y i n f l u e n c e t h e comp r e s s i b i l i t y o f a sand d e p o s i t . I n a e o l i a n d e p o s i t s , p a c k i n g arrangement and degree o f sediment s o r t i n g show c o n s i d e r a b l e v a r i a b i l i t y , depending on t h e mode
o f d e p o s i t i o n o f p a r t i c u l a r laminae; t h a t i s , whether t h e y were d e p o s i t e d as i m pact r i p p l e s , by g r a i n f a l l , o r as avalanche d e p o s i t s . G r a i n f a l l d e p o s i t s i n p a r t i c u l a r o f t e n d i s p l a y a l o o s e p a c k i n g arrangement. Several mechanisms may cause rearrangement o f t h e g r a i n s t o f o r m t i g h t e r p a c k i n g . These i n c l u d e earthquake shock, overburden pressure, m o i s t u r e s a t u r a t i o n , and s e l e c t i v e w e a t h e r i n g . Overburden pressures i n s u r f a c e sediments a r e r e l a t i v e l y small i n comparison w i t h those experienced d u r i n g deep b u r i a l ( A l l e n and C h i l i n g a r i a n , 1975), b u t nevertheless may be s u f f i c i e n t t o cause some g r a i n r o t a t i o n by s l i d i n g a l o n g f l a t s u r faces, compression and p l a s t i c f l o w o f micas, c l a y s and o t h e r s o f t m i n e r a l s . Very a n g u l a r and i r r e g u l a r l y shaped p a r t i c l e s o f b r i t t l e m a t e r i a l s , such as c a r bonate s k e l e t a l fragments, may a l s o e x p e r i e n c e some s h a t t e r i n g o f g r a i n c o r n e r s . The s i l t and c l a y - s i z e p a r t i c l e s so produced f i l l i n t h e p o r e spaces between l a r g e r g r a i n s , t h e r e b y r e d u c i n g t h e p o r o s i t y . I n g e n e r a l , however, most o c c l u s i o n
of pore space occurs i n n e a r - s u r f a c e sands as a r e s u l t o f s e t t l i n g d u r i n g weatheri n g and p r e c i p i t a t i o n o f a u t h i g e n i c m i n e r a l s .
4.
BIOTURBATION B i o t u r b a t i o n i n a e o l i a n d e p o s i t s may be accomplished by t h r e e main groups o f
organisms: ( 1 ) v e r t e b r a t e animals, ( 2 ) i n s e c t s , and ( 3 ) p l a n t s . Among t h e more important v e r t e b r a t e s a r e r a b b i t s , snakes and b i r d s w i t h a b u r r o w i n g h a b i t , a l though many o t h e r genera a r e l o c a l l y i m p o r t a n t . Amongst i n s e c t s , sand wasps, beetles, s p i d e r s , a n t s and t e r m i t e s a r e o f m a j o r importance i n a e o l i a n d e p o s i t s . Ahlbrandt e t a l . (1978) have d e s c r i b e d t h e o c c u r r e n c e and n a t u r e o f t h e burrows and o t h e r f e a t u r e s formed. Sand wasp burrows, f o r example, may be up t o 40cm long and a r e n o r m a l l y f o u n d a t an o b l i q u e a n g l e t o t h e p r i m a r y bedding s t r a t i f i c a t i o n on dune s l o p e s o f moderate a n g l e . Growth o f p l a n t r o o t s , l i k e t h e a c t i v i t i e s o f b u r r o w i n g organisms, has t h e e f f e c t o f d e s t r o y i n g p r i m a r y s t r u c t u r e s and c a u s i n g m i x i n g o f sediment w i t h i n o r i g i n a l g r a i n s i z e laminae (e.g. B i g a r e l l a , 1975a). The r o o t s o f some dune annuals and p e r e n n i a l s may e x t e n d t o depths o f 3
- 5m
(Amos, 1959; Ranwell, 1972)
and t r e e r o o t s t o lorn o r more. The sands n e x t t o r o o t s may become p r e f e r e n t i a l l y cemented by c a l c i u m carbonate, gypsum,
s i l i c a o r i r o n o x i d e s as s o i l m o i s t u r e
i s absorbed by t h e p l a n t . On d e a t h t h e r o o t ( o r g a n i c m a t t e r ) may become e r l t i r e l y oxidized,
l e a v i n g a h o l l o w r o o t c a s t which may l a t e r become i n f i l l e d w i t h un-
c o n s o l i d a t e d sand f r o m above o r by a u t h i g e n i c p r e c i p i t a t e s . Such f e a t u r e s a r e g e n e r a l l y known as ' r h i z o c o n c r e t i o n s ' Evamy, 1968).
( K i n d l e , 1923) o r ' d i k a k a ' ( G l e n n i e and
202 5.
A D D I T I O N OF ALLOCHTHONOUS SEDIMENT F i n e a l l o c h t h o n o u s sediment may be added p o s t - d e p o s i t i o n a l l y t o a dune sand
body by s u r f a c e wash, by l a t e r a l l y m i g r a t i n g groundwater, and as a e o l i a n d u s t . I n most areas t h e l a t t e r i s by f a r t h e most i m p o r t a n t . The r a t e s , n a t u r e and consequences o f d u s t d e p o s i t i o n i n d i f f e r e n t p a r t s o f t h e w o r l d have r e c e n t l y been reviewed by Goudie (1978) and by Pewe ( 1 9 8 2 ) . Dust storms a r e common i n and around a l l o f t h e m a j o r d e s e r t s . Such areas p r o v i d e a source o f s i l t and c l a y through w e a t h e r i n g o f bedrock, wind a b r a s i o n , and by d e f l a t i o n o f a l l u v i a l s u r faces and d r i e d - u p l a k e beds. I n some areas, i n c l u d i n g I s r a e l , t h e s o u t h - c e n t r a l U n i t e d S t a t e s , S o v i e t C e n t r a l A s i a and I r a q , p r e s e n t r a t e s o f d u s t d e p o s i t i o n a r e e s t i m a t e d t o exceed 100mm/1000 y r (Goudie, 1978). Rates o f d e p o s i t i o n were a l m o s t c e r t a i n l y much h i g h e r a t t i m e s d u r i n g t h e l a t e P l e i s t o c e n e when c o n d i t i o n s a r e known t o have been b o t h w i n d i e r and d r i e r ( 6 o w l e r , 1978). The c o m p o s i t i o n o f d u s t v a r i e s f r o m r e g i o n t o r e g i o n depending on t h e n a t u r e of t h e source area, b u t common c o n s t i t u e n t s a r e q u a r t z , f e l d s p a r s ,
micas, c l a y
m i n e r a l s , carbonates and b i o l i t h s (diatoms and p h y t o l i t h s ) . Yaalon and Ginzbourg (1966) r e p o r t e d up t o 48% CaC03 i n Negev d u s t , and Kukal and S a a d a l l a h (1973) r e p o r t e d 66
-
69% carbonate i n d u s t f r o m I r a q . The mean s i z e o f a e o l i a n d u s t g e n e r -
a l l y l i e s i n t h e range 20 - 4 5 m . The p o s s i b l e consequences o f d u s t d e p o s i t i o n f o r s o i l development and diagenes i s have been n o t e d by Syers e t a l . (1969), Yaalon and Ganor (1973) and Sidhu (1977). Carbonate c o n t a i n e d i n d u s t i s c o n s i d e r e d by some a u t h o r s t o be a m a j o r f a c t o r l e a d i n g t o t h e f o r m a t i o n o f c a l c r e t e s i n a r i d r e g i o n s (Goudie, 1973; i n press b ) . W i n d - t r a n s p o r t e d gypsum has s i m i l a r l y been suggested as t h e main source
o f cement i n some gypsum c r u s t s (Coque, 1955; Uatson, 1979, i n press a ) . I n n o r t h w e s t Mexico, Walker e t a l . (1978) c o n s i d e r e d a e o l i a n d u s t t o be a m a j o r source o f c l a y m i n e r a l s and i r o n o x i d e s f o u n d i n Cenozoic a l l u v i a l r e d beds. M a l k e r (1979) a l s o b e l i e v e d t h a t c l a y m i n e r a l s f o u n d i n reddened dune sands i n L i b y a r e p r e s e n t e d d u s t d e f l a t e d f r o m areas o f weathered bedrock and subsequently i n f i l t r a t e d i n t o t h e dune sands by r a i n w a t e r . A s i m i l a r o r i g i n o f c l a y c o a t i n g s on dune sand g r a i n s i n t h e Simpson D e s e r t o f A u s t r a l i a was porposed by F o l k (1969). However, i t i s d i f f i c u l t t o d i f f e r e n t i a t e a l l o c h t h o n o u s c l a y components f r o m a u t h i g e n i c c l a y s where t h e m i n e r a l o g i c a l c o m p o s i t i o n o f t h e two i s s i m i l a r , o r if t h e a1 lochthonous c l a y s themselves have undergone s i g n i f i c a n t p o s t - d e p o s i t i o n a l
a1 t e r -
ation. 6.
WEATHERING AND LEACHING
6.1.
Chemical l i e a t h e r i n g
M i n e r a l s d i f f e r i n t h e i r degree o f s t a b i l i t y under e a r t h s u r f a c e c o n d i t i o n s and hence t h e r a t e s a t which t h e y break down t o more s t a b l e p r o d u c t s under g i v e n c o n d i t i o n s . F o r example, s i l i c a t e s w i t h r e l a t i v e l y few s t r o n g S i - 0 bonds, such as
2 03 pyroxenes and amphiboles, break down much more r a p i d l y t h a n m i n e r a l s w i t h r e l a t i v e l y l a r g e numbers o f S i - 0 bonds such as q u a r t z (Loughnan, 1969; C a r r o l l , 1970). Absolute r a t e s o f m i n e r a l d i s s o l u t i o n a r e c o n t r o l l e d p a r t l y by t h e r a t e s o f chemical r e a c t i o n s a t m i n e r a l s u r f a c e s (Berner, 1978), and p a r t l y by t h e r a t e a t which weathering p r o d u c t s a r e f l u s h e d o u t o f t h e sediment system (McClelland, 1950). I n v e r y a r i d areas r a i n f a l l i s s u f f i c i e n t l y l o w t o p r e v e n t t o t a l l e a c h i n g of soluble s a l t s and carbonates f r o m t h e sediment column, and c o n s e q u e n t l y t h e y accumulate as d i s c r e t e c r y s t a l s , nodules o r more c o n t i n u o u s cemented l a y e r s . Under such c o n d i t i o n s m o i s t u r e a v a i l a b i l i t y f o r h y d r o l y s i s and o t h e r w e a t h e r i n g r e a c tions i s s t r i c t l y l i m i t e d and r a t e s o f m i n e r a l w e a t h e r i n g a r e slow. With i n c r e a s ing p r e c i p i t a t i o n , s a l t s and carbonates a r e p r o g r e s s i v e l y leached f r o m t h e system and feldspars,
micas and u n s t a b l e heavy m i n e r a l s b e g i n t o break down more r a p i d l y
t o form c l a y s and o t h e r a u t h i g e n i c p r o d u c t s . Under v e r y humid c o n d i t i o n s , f e l d -
spars and u n s t a b l e heavy m i n e r a l s a r e d e s t r o y e d e x t r e m e l y r a p i d l y , and n o r m a l l y r e s i s t a n t g r a i n s o f q u a r t z , k a o l i n i t e , z i r c o n and i l m e n i t e may be a t t a c k e d . Williams and Yaalon (1977) found t h a t e x p e r i m e n t a l l e a c h i n g by h o t and c o l d water i n a f r e e - d r a i n i n g S o x h l e t column a r e capable o f c a u s i n g s i g n i f i c a n t a l t e r a t i o n o f heavy m i n e r a l s ( m a i n l y hornblende) c o n t a i n e d i n dune sand w i t h i n a p e r i o d of three months. Leaching caused o b s e r v a b l e d i f f e r e n c e s i n t h e s u r f a c e t e x t u r e s of the hornblende g r a i n s and l e d t o l o s s o f Na, Ca, Flg, K and A1 i o n s i n s o l u t i o n .
Fe released by l e a c h i n g was p r e c i p i t a t e d w i t h i n t h e sediment column as a t h i n oxide s t a i n i n g on t h e q u a r t z sand g r a i n s . Walker (1979) observed p i t t i n g o f a u g i t e and hornblende g r a i n s i n L i b y a n dune sands which he a t t r i b u t e d t o _ i n_ s i_ t u d i s s o l u t i o n . The g r a i n s showed 'cockscombl i k e ' t e r m i n a t i o n s s i m i l a r t o t h o s e d e s c r i b e d i n weathered a l l u v i a l d e p o s i t s from Baja C a l i f o r n i a by Walker e t a l . ( 1 9 7 8 ) . Walker (1979) a l s o found a p r o g r e s s i v e increase i n degree o f a l t e r a t i o n o f b o t h a u g i t e and hornblende w i t h d i s t a n c e t h e dunes had t r a v e l l e d f r o m t h e i r a p p a r e n t source ( i . e . w i t h age). C l o s e t o t h e source, a u g i t e , t h e more u n s t a b l e o f t h e two s i l i c a t e s , was f o u n d t o be o n l y weakl y etched w h i l e hornblende was u n a f f e c t e d . A t t h e downwind end o f t h e sand sea, where the sands were o l d e s t , hornblende was found t o be n o t i c e a b l y e t c h e d .
I n southeast I n d i a , an a r e a which e x p e r i e n c e s a s e m i - a r i d t r o p i c a l c l i m a t e (mean annual r a i n f a l 1 600mm), Gardner ( 1 981 ) concluded f r o m m i n e r a l frequency studies o f modern and weathered c o a s t a l dune sands t h a t f e l d s p a r s and almandine garnet a r e d e s t r o y e d by w e a t h e r i n g t o f o r m a u t h i g e n i c haematite, k a o l i n i t e , and i l l i t e . Pyroxenes and amphiboles were i n i t i a l l y p r e s e n t o n l y i n v e r y l o w concent r a t i o n s i n these sands. Radiocarbon d a t i n g o f u n d e r l y i n g d e p o s i t s i n d i c a t e d t h a t the weathered dune sands a r e l e s s t h a n 25,000 C-14 y e a r s o l d . B r o a d l y s i m i l a r deposits a r e found i n S r i Lanka and p a r t s o f N a t a l where comparable w e a t h e r i n g h i s t o r i e s a r e envisaged (Gardner, i n press a ) . More severe w e a t h e r i n g o f dune sands has o c c u r r e d under s e a s o n a l l y humid t r o -
2 04 p i c a 1 c o n d i t i o n s i n n o r t h e a s t A u s t r a l i a (mean a n n u a l r a i n f a l l 1800mm). The charact e r i s t i c s o f t h e p o d z o l i c p r o f i l e s d e v e l o p e d i n t h e s e dunes have been d e s c r i b e d b y Pye (1981, 1982a, i n p r e s s b ) . Any f e l d s p a r s and u n s t a b l e heavy m i n e r a l s which may have been p r e s e n t i n t h e sands i n i t i a l l y have s i n c e been t o t a l l y d e s t r o y e d , l e a v i n g a r e s i d u a l p r o d u c t r i c h i n q u a r t z a n d r e s i s t a n t heavy m i n e r a l s such as z i r c o n , r u t i l e , i l m e n i t e , t o u r m a l i n e and a n d a l u s i t e . A u t h i g e n i c k a o l i n i t e , g i b b s i t e , f e r r i c o x i d e s and o r g a n i c m a t t e r have a c c u m u l a t e d i n t h e i l l u v i a l B h o r i zons o f t h e w e a t h e r i n g p r o f i l e s ( F i g . 4 ) . SEM e x a m i n a t i o n o f t h e s u r f a c e t e x t u r e s
F i g . 4 . A 60m h i g h w e a t h e r e d q u a r t z dune sand sequence n e a r Cape B e d f o r d , N o r t h Queensland, A u s t r a l i a .
205
o f z i r c o n , i l m e n i t e and r u t i l e g r a i n s suggests t h a t these m i n e r a l s t o o have experienced severe d i s s o l u t i o n , p a r t i c u l a r l y i n t h e A1 and A 2 h o r i z o n s (Pye, i n press b ) . Only t o u r m a l i n e , s t a u r o l i t e and a n d a l u s i t e seem t o be l i t t l e a f f e c t e d by the p r e v a i l i n g c o n d i t i o n s . Q u a r t z g r a i n s show c l e a r evidence o f p r e f e r e n t i a l s o l u t i o n a l o n g l i n e a r m i c r o f r a c t u r e s and i n t e r - c r y s t a l l i t e boundaries i n p o l y c r y s t a l l i n e g r a i n s . I n some i n s t a n c e s such s e l e c t i v e d i s s o l u t i o n has r e s u l t e d i n t o t a l g r a i n f r a g m e n t a t i o n and t h e f o r m a t i o n o f q u a r t z s i l t (Pye, i n press c ) . S i m i l a r breakdown o f q u a r t z i n dune sands was observed i n s o u t h e r n Queensland by L i t t l e e t a l . ( 1 9 7 8 ) . The e x t r e m e l y r a p i d r a t e o f m i n e r a l w e a t h e r i n g i n t h e Queensland c o a s t a l dune complexes i s a l m o s t c e r t a i n l y enhanced by o r g a n i c a c i d s released by t h e a c i d o p h y l l o u s
dune v e g e t a t i o n ( e . g . Pye and Jackes, 1981). The
e f f e c t i v e n e s s o f such a c i d s has p r e v i o u s l y been demonstrated i n a number o f e x p e r i mental s t u d i e s (Baker, 1973; Dumon, 1976). I n c a r b o n a t e - r i c h dune sands, such as a r e commonly found on a r i d zone coasts, weathering o f s i l i c a t e s and o t h e r non-carbonate m i n e r a l s may be a r r e s t e d by e a r l y d i a g e n e t i c cementation. Carbonate cement, n o r m a l l y d e r i v e d f r o m t h e r a p i d d i s s o l u t i o n o f d e t r i t a l s k e l e t a l fragments, i s d e p o s i t e d a t p o i n t s o f g r a i n c o n t a c t and i n i n t e r g r a n u l a r p o r e spaces, t h e r e b y impeding t h e movement o f vadose w a t e r . F o r t h i s reason P l e i s t o c e n e a e o l i a n i t e s o f t e n c o n t a i n m i n e r a l assemblages which a r e more immature t h a n m i g h t be expected. For example, Gardner (1981) found t h a t a e o l i a n i t e s which u n d e r l i e t h e r e d ' t e r i ' dune sands i n s o u t h e a s t I n d i a show v e r y l i t t l e evidence o f d e p l e t i o n o f f e l d s p a r s , g a r n e t and opaque heavy m i n e r a l s when compared w i t h t h e modern dune sands i n t h e a r e a . The d i s s o l u t i o n o f allochems i n c a r b o n a t e sands has been t h e s u b j e c t o f much i n v e s t i g a t i o n (Friedman, 1964; Gavish and Friedman, 1969; Land, 1967; Ward, 1975; Bathurst, 1971; Buchbinder and Friedman, 1980) and l i m i t a t i o n s o f space do n o t a l l o w a f u l l r e v i e w h e r e . I n s i m p l e terms, however, allochems o f marine o r i g i n which a r e composed o f m e t a s t a b l e a r a g o n i t e , high-Mg c a l c i t e o r m i x t u r e s o f t h e two, a r e g r a d u a l l y a l t e r e d under s u b a e r i a l c o n d i t i o n s t o low-Mg c a l c i t e . A number of d i f f e r e n t processes may be i n v o l v e d , o f t e n s i m u l t a n e o u s l y . F i r s t , a r a g o n i t e , and p o s s i b l y high-Mg c a l c i t e , may r e c r y s t a l l i z e i n s i t u t o f o r m low-Wg c a l c i t e (Schlanger, 1964; B a t h u r s t , 1971). Second, high-Mg allochems may be a l t e r e d t o low-llg c a l c i t e by s e l e c t i v e l e a c h i n g o f Mg2+ f r o m t h e c r y s t a l s t r u c t u r e . T h i s m i ; happen if t h e i n t e r s t i t i a l p o r e w a t e r i s s a t u r a t e d w i t h r e s p e c t t o c a l c i u m
b u t n o t t o magnesium c a r b o n a t e . The FlgC03 which i s l o s t may be, b u t i s n o t necess a r i l y , r e p l a c e d by c a l c i t e . I n e i t h e r case t h e o r i g i n a l t e x t u r e o f t h e allochem i s normally preserved. Third, i f pore water s o l u t i o n s are n o t saturated w i t h respect t o c a l c i t e b o t h Mg2+ and Cap+ may be removed, l e a v i n g a v o i d mould.
6.2
Mechanical Weathering R e l a t i v e l y l i t t l e i s y e t known a b o u t t h e e f f e c t s o f mechanical Weathering p r o -
206 cesses on dune sands i n n a t u r e . L a b o r a t o r y e x p e r i m e n t a l work has shown t h a t sodium s u l p h a t e w e a t h e r i n g i s capable o f c a u s i n g mechanical f r a c t u r e o f dune sand g r a i n s under s i r i u l a t e d h o t d e s e r t c l i m a t i c c o n d i t i o n s (Goudie e t a l . ,
1979; Pye
and S p e r l i n g , 1983). Pye and S p e r l i n g (1983) have demonstrated t h a t f e l d s p a r and mica g r a i n s a r e much more s u s c e p t i b l e t o s a l t damage t h a n q u a r t z g r a i n s , and t h a t q u a r t z which has e x p e r i e n c e d r e l a t i v e l y l i t t l e t r a n s p o r t and s o r t i n g i s more susc e p t i b l e t h a n mature q u a r t z which has e x p e r i e n c e d s e v e r a l phases o f f l u v i a l , mari n e and a e o l i a n r e w o r k i n g . Large sand g r a i n s a r e a l s o more l i k e l y t o f r a c t u r e t h a n f i n e sand g r a i n s , p r o b a b l y because t h e y c o n t a i n a g r e a t e r number o f m i c r o f r a c t u r e s and l i n e s o f c r y s t a l l o g r a p h i c weakness. The a c t i o n o f temperature and h u m i d i t y v a r i a t i o n s t y p i c a l o f t h o s e o c c u r r i n g i n n a t u r a l warm d e s e r t s was found by Pye and S p e r l i n g t o be i n e f f e c t i v e i n c a u s i n g p a r t i c l e breakage, as was diurnal w e t t i n g and d r y i n g . I n n a t u r e t h e e f f e c t i v e n e s s o f s a l t w e a t h e r i n g on dune sands i s l i k e l y t o depend on ( a ) t h e a v a i l a b i l i t y o f s u i t a b l e s a l t s , and ( b ) t h e t e x t u r a l and composit i o n a l m a t u r i t y o f t h e dune sands. D e s e r t dunes composed o f r e l a t i v e l y immature sand, f o r example such as o c c u r w i t h i n many s m a l l i n t e r - m o n t a n e b a s i n s i n t h e southwestern U n i t e d S t a t e s , may s u f f e r m a j o r s a l t damage i f s a l t i s p r e s e n t i n groundwater s o l u t i o n s o r i s blown o n t o t h e dunes f r o m nearby s a l i n e f l a t s . Rates o f s a l t d e p o s i t i o n f r o m f o g i n c o a s t a l d e s e r t s a r e a l s o known t o be h i g h , and may a l l o w s a l t w e a t h e r i n g and s a l t c r u s t f o r m a t i o n t o t a k e p l a c e (Watson, i n press a, i n press b; Goudie, i n press a ) . S i l t and c l a y l e n s e s which o c c u r w i t h i n i n t e r dune depressions i n many d e s e r t areas may r e p r e s e n t m a t e r i a l l a r g e l y formed by s a l t w e a t h e r i n g and c o n c e n t r a t e d by p e r i o d i c s t o r m r u n o f f . Experimental d a t a (Pye, u n p u b l i s h e d ) i n d i c a t e t h a t t h e mechanical e f f e c t o f f r o s t a c t i o n i s s i m i l a r t o t h a t o f s a l t c r y s t a l l i z a t i o n i n causing g r a i n fracture and s i l t f o r m a t i o n . F e l d s p a r s and micas a r e a g a i n more s u s c e p t i b l e t h a n q u a r t z on account o f t h e i r w e l l - d e v e l o p e d cleavage and common p a r t i a l chemcial a l t e r a t i o n . However, no d a t a a r e p r e s e n t l y a v a i l a b l e c o n c e r n i n g t h e importance o f f r o s t weatheri n g i n dune sands. 7.
TRANSLOCATION AIiD PROFILE DIFFERENTIATION Weathering p r o d u c t s and a l l o c h t h o n o u s m a t e r i a l d e p o s i t e d on dune s u r f a c e s are
commonly t r a n s l o c a t e d down t h e sediment column by vadose water, l e a d i n g t o prof i l e d i f f e r e n t i a t i o n . T r a n s l o c a t i o n ( e l u v i a t i o n ) may o c c u r as a s i m p l e mechanical process due t o r a i n w a t e r f l u s h i n g ( W r i g h t and Foss, 1968). F i n e s i l t and c l a y - s i z e p a r t i c l e s a r e s m a l l enough t o be washed through t h e i n t e r g r a n u l a r pore spaces i n w e l l - s o r t e d sands. C o n c e n t r a t i o n s o f m e c h a n i c a l l y - e l u v i a t e d s i l t a r e commonly found a t t h e base o f t h e A2 h o r i z o n i n p o d z o l i c w e a t h e r i n g p r o f i l e s (Nornberg, 1980; Pye, 1982 b ) . More o f t e n , however, e l u v i a t i o n i n v o l v e s chemical as w e l l as mechanical processes. Organic a c i d s r e l e a s e d by t h e decay o f v e g e t a t i o n f o r m com-
2 07 plexes w i t h c a t i o n s r e l e a s e d b y m i n e r a l w e a t h e r i n g which a r e t h e n t r a n s p o r t e d i n s o l u t i o n o r c o l l o i d a l suspension ( S c h n i t z e r and Kahn, 1972; Mohr e t a l . ,
1972).
The downward movement o f e l u v i a t e d m a t e r i a l may be i n t e r r u p t e d by a change i n e i t h e r p h y s i c a l o r chemical c o n d i t i o n s (De Coninck, 1980). P h y s i c a l f a c t o r s which encourage d e p o s i t i o n a r e d r y i n g o u t o f t h e p r o f i l e , a r e d u c t i o n i n sediment p e r m e a b i l i t y , o r i n t e r c e p t i o n by t h e w a t e r t a b l e . Chemical f a c t o r s i n c l u d e changes i n pH o r Eh and changes i n t h e r e l a t i v e c o n c e n t r a t i o n s o f d i f f e r e n t i o n i c species and o r g a n i c m a t t e r . The e f f e c t i v e n e s s o f t r a n s l o c a t i o n processes i s c o n t r o l l e d by drainage c o n d i t i o n s ( r e f l e c t i n g topography,
sand s i z e and s o r t i n g c h a r a c t e r i s t i c s ) ,
r a i n f a l l amount and i n t e n s i t y , and t h e c h a r a c t e r o f o r g a n i c a c i d s r e l e a s e d b y dune v e g e t a t i o n . Rapid e l u v i a t i o n i s f a v o u r e d b y deep, w e l l - d r a i n e d sands, v e r y high r a i n f a l l and a c i d o p h y l l o u s v e g e t a t i o n . I n areas r e c e i v i n g more t h a n a p p r o x i mately 1200mn of r a i n f a l l p e r annum, marked p o d z o l i c p r o f i l e s , c h a r a c t e r i s e d by
a bleached A h o r i z o n , a r e l i k e l y t o f o r m w i t h i n a few hundreds t o a few thousand years, depending i n p a r t on such f a c t o r s as temperature, sand m i n e r a l o g y and d r a i nage c o n d i t i o n s . I n l e s s humid areas bleached s u r f a c e h o r i z o n s develop o n l y r a r e l y and o v e r much l o n g e r p e r i o d s . Most w e a t h e r i n g p r o f i l e s formed i n dunes i n such
areas a r e l a t o s o l i c (e.g. De A l w i s and P l u t h , 1976a, 1976b; Gardner, 1981; i n press a ) . L a t o s o l p r o f i l e s a r e n o r m a l l y reddened t h r o u g h o u t t h e A and B h o r i z o n s by f e r r i c o x i d e s , a l t h o u g h t h e r e may be some p r e f e r e n t i a l a c c u m u l a t i o n o f c l a y and i r o n i n t h e B h o r i z o n . The geopetal arrangement o f c l a y m i n e r a l s as cutans around q u a r t z and o t h e r d e t r i t a l g r a i n s o f t e n p r o v i d e s evidence t h a t some v e r t i c a l t r a n s l o c a t i o n o f c l a y s has o c c u r r e d . 7.2
I n f i l t r a t i o n structures I n f i l t r a t i o n s t r u c t u r e s a r e t h i n , d a r k , o f t e n d i s c o n t i n u o u s wavy bands which
r e s u l t from the concentration o f v e r t i c a l l y i n f i l t r a t e d f i n e m a t e r i a l . Deposition o f i n f i l t r a t e d f i n e m a t e r i a l o f t e n occurs p r e f e r e n t i a l l y a l o n g p r i m a r y f i n e g r a i n s i z e laminae o r a l o n g secondary d i s s i p a t i o n s t r u c t u r e s such as shear planes assoc i a t e d w i t h slumps and sand f l o w s . Indeed, A h l b r a n d t and F r y b e r g e r (1980) use t h e term ' d i s s i p a t i o n s t r u c t u r e ' f o r f e a t u r e s formed by i n f i l t r a t i o n , a l t h o u g h i n i t s o r i g i n a l sense as used b y B i g a r e l l a ( 1 975a, 1975b) t h e t e r m in c l udes c o n t o r t e d bedding and r e l a t e d s t r u c t u r e s which a r e formed b y a range o f p h y s i c a l dune deg r a d a t i o n processes. I n some cases, however, c o n t o r t e d s i l t and c l a y - r i c h laminae bear l i t t l e o r no r e l a t i o n s h i p t o e i t h e r o r i g i n a l bedding laminae o r secondary d e f o r m a t i o n a l s t r u c t u r e s . I n t h e Nebraska S a n d h i l l s , A h l b r a n d t and F r y b e r g e r (1980) suggested t h a t d e p o s i t i o n o f f i n e s has o c c u r r e d a t t h e i r r e g u l a r boundary between s e a s o n a l l y f r o z e n abd u n f r o z e n sands. However, s i m i l a r i r r e g u l a r deposit i o n i s found i n t h e c o a s t a l dunes o f n o r t h e a s t A u s t r a l i a which a r e never f r o z e n . D e p o s i t i o n o f s i l t h e r e may s i m p l y b e r e l a t e d t o t h e l i m i t o f p e n e t r a t i o n o f t h e
2 08 w e t t i n g f r o n t a f t e r h e a v y r a i n f a l l . I n s t a n c e s have been o b s e r v e d i n t h i s a r e a where i n f i l t r a t i o n s t r u c t u r e s a r e s u p e r i m p o s e d b o t h on p r i m a r y c r o s s - b e d d i n g and s e c o n d a r y c o n t o r t e d b e d d i n g . Once f i n e s have been d e p o s i t e d a t a p a r t i c u l a r l e v e l , t h e reduced p e r m e a b i l i t y w i l l tend ferential
t o f a v o u r f u r t h e r a c c u m u l a t i o n and perhaps pre-
c e m e n t a t i o n by i r o n o x i d e s o r o t h e r m i n e r a l s ( F i g . 5 ) .
F i g . 5 . I r o n o x i d e cemented l a y e r ( a r r o w e d ) a t i n t e r f a c e between i r o n - s t a i n e d a r g i l l a c e o u s sand ( d a r k ) and u n d e r l y i n g w h i t e sands, Cape B e d f o r d , N o r t h Queensl a n d . Note t h a t t h e iron-cemented s t r u c t u r e bears l i t t l e r e l a t i o n t o primary g r a i n s i z e l a m i n a e . P i t c u r e w i d t h a p p r o x . 20cm.
8.
A U TH I GENES I S Ad I D C E I.1E NTAT I 0 N
An i m p o r t a n t f e a t u r e o f e a r l y d i a g e n e s i s i n dune sands i s p r e c i p i t a t i o n of aut h i g e n i c m i n e r a l s f r o m i n t e r s t i t i a l s o l u t i o n s , sometimes l e a d i n g t o c e m e n t a t i o n o f t h e s e d i m e n t s . The n a t u r e o f t h e m i n e r a l s p r e c i p i t a t e d i s a f u n c t i o n o f t h e p o r e w a t e r c h e m i s t r y . C l a y m i n e r a l s , i r o n o x i d e s and h y d r o x i d e s , c a l c i t e , gypsum and s i l i c a a r e t h e m o s t common a u t h i g e n i c p r o d u c t s . 0.1
Clay minerals Formation o f a u t h i g e n i c c l a y s o f t e n begins e a r l y i n the d i a g e n e t i c h i s t o r y o f
dune d e p o s i t s , a l t h o u g h , as has p r e v i o u s l y been n o t e d , some o f t h e a c c u m u l a t e d c l a y may r e p r e s e n t a l l o c h t h o n o u s m a t e r i a l . I n d e s e r t e n v i r o n m e n t s , where K, Na
209 and o t h e r c a t i o n s r e l e a s e d by m i n e r a l w e a t h e r i n g a r e n o t c o m p l e t e l y leached away, i l l i t e and mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e a r e t h e most common a u t h i g e n i c c l a y minerals found i n n e a r - s u r f a c e sediments (Walker, 1976; Walker e t a1
.,
1978).
P a l y g o r s k i t e may a l s o f o r m a u t h i g e n i c a l l y i n such environments (Walker, 1979). Where l e a c h i n g o f c a t i o n s i s more e f f e c t i v e , k a o l i n i t e becomes more abundant r e l a t i v e t o i l l i t e and mixed l a y e r c l a y s . I n t h e weathered dunes o f s o u t h e r n I n d i a , f o r example, k a o l i n i t e i s t h e dominant a u t h i g e n i c c l a y m i n e r a l w i t h i l l i t e a secondary b u t s i g n i f i c a n t component (Gardner, 1981). Under t h e most severe l e a c h i n g conditions, such as a r e f o u n d i n t r o p i c a l n o r t h e a s t A u s t r a l i a , k a o l i n i t e i s metastable and weathered dune sands may c o n t a i n s u b s t a n t i a l amounts o f g i b b s i t e (Pye, i n press b ) .
8.2
I r o n o x i d e s and h y d r o x i d e s The n a t u r e and o r i g i n o f r e d c o l o u r a t i o n ( i r o n o x i d e p i g m e n t a t i o n ) i n dune
sands has p r o b a b l y r e c e i v e d more a t t e n t i o n t h a n any o t h e r aspect o f t h e i r e a r l y diagenesis. The q u e s t i o n o f dune r e d d e n i n g has r e c e n t l y been reviewed by Gardner and Pye ( 1 9 8 1 ) . A l t h o u g h some r e d dune sand bodies have undoubtedly been formed by l o c a l r e w o r k i n g o f r e d a l l u v i a l o r o t h e r d e p o s i t s (Glennie, 1970; F o l k , 1976), i n many i n s t a n c e s r e d d e n i n g has o c c u r r e d p o s t - d e p o s i t i o n a l l y (Setlow, 1978; Gardner, 1981; Pye, 1981). A u t h i g e n i c i r o n m i n e r a l s t y p i c a l l y o c c u r i n Q u a t e r n a r y dune sands as d i s s e m i n a t e d f i n e c r y s t a l s (<0.5pm d i a m e t e r ) which may adhere d i r e c t l y t o q u a r t z and o t h e r d e t r i t a l g r a i n s b u t more commonly a r e h e l d on t h e s u r face o r edges o f c l a y m i n e r a l s . G o e t h i t e (aFeOOH),
which i s y e l l o w i s h brown i n
colour, i s t h e most common a u t h i g e n i c i r o n m i n e r a l formed under o x i d i z i n g c o n d i tions, b u t t r u e r e d c o l o u r a t i o n i s n o r m a l l y dependent on t h e presence o f haemat i t e (aFe 0 ) o r i t s h y d r a t e d p r e c u r s o r s . Formation o f h a e m a t i t e i s a time-depen-
2 3
dent and r e l a t i v e l y slow process under e a r t h s u r f a c e c o n d i t i o n s ( N o r r i s , 1969; Walker, 1967). I t i s f a v o u r e d b y h i g h temperatures and low i n t e r s t i t i a l w a t e r a c t i v i t y f o r a t l e a s t p a r t o f t h e y e a r (Schmalz, 1968). Under s e a s o n a l l y humid t r o p i c a l c o n d i t i o n s i n N o r t h Queensland, which may be c l o s e t o t h e optimum, f i n e l y d i v i d e d h a e m a t i t e and a s s o c i a t e d i r o n h y d r o x i d e s have i m p a r t e d b r i g h t r e d
c o l o u r a t i o n t o s t a b i l i z e d dune sands which have been r a d i o c a r b o n d a t e d as <7,500 years o l d (Pye, 1981). Surface dune sands r a r e l y become c o m p l e t e l y cemented by i r o n o x i d e s , b u t p a r t i c u l a r h o r i z o n s , s t r u c t u r e s and laminae may become p r e f e r e n t i a l l y cemented w h i l e the remainder o f t h e sand column i s o n l y pigmented. Such i r o n oxide-cemented l a y e r s have been termed ' p e t r o f e r r i c l a y e r s ' by Pye ( i n press d ) . Cementation o f t e n
occurs p r e f e r e n t i a l l y i n t h e f i n e s t and l e a s t w e l l - s o r t e d laminae which have low p o r o s i t y and p e r m e a b i l i t y . and i n d i s s i p a t i o n s t r u c t u r e s which have been e n r i c h e d post-depositionally w i t h i n f i l t r a t e d f i n e m a t e r i a l . I n northeast Australia,
petro-
f e r r i c l a y e r s a t t a i n a maximum t h i c k n e s s o f a b o u t 25cm b u t a r e more t y p i c a l l y 5 -
210
lOcm t h i c k . I n t e r g r a n u l a r pore s p a c e s a r e completely i n f i l l e d by g r a n u l a r o r f i brous i r o n o x i d e cement ( F i g . 6). I n d u r a t i o n of t h e o x i d e s i s probably accompli-
F i g . 6. Scanning e l e c t r o n micrograph of a q u a r t z g r a i n cemented by i r o n oxide in a p e t r o f e r r i c l a y e r from North Q u e e n s l a n d . shed by i r r e v e r s i b l e d r y i n g d u r i n g p e r i o d s of w a t e r t a b l e l o w e r i n g . I n o t h e r p a r t s of the world i r o n o x i d e c e m e n t a t i o n o c c u r s only w i t h i n c o n c r e t i o n s lOcm 1 metre i n d i a m e t e r , w h i l e the s u r r o u n d i n g sands a r e poor i n i r o n and f r i a b l e (Fig. 7). 8.3
Gypsum and h a l i t e
Glennie (1970, p.131) noted t h a t gypsum cementation may o c c u r i n a r i d a r e a s where dunes form a b a r r i e r p r e v e n t i n g the flow of w a t e r from a wadi o r sabkha t o the s e a . Evaporation o f s a l i n e w a t e r s w i t h i n t h e zone o f groundwater c a p i l l a r y r i s e may thus l e a d t o t h e f o r m a t i o n o f a s u b - p a r a l l e l s u b s u r f a c e cemented l a y e r . Such gypsum-cemented q u a r t z dunes o f p r o b a b l e P l i o c e n e age o c c u r a d j a c e n t t o Sebkha Matti on the P e r s i a n Gulf ( G l e n n i e , 1 9 7 0 ) . Gypsum a n d / o r h a l i t e cementation of s u r f a c e sand l a y e r s may a l s o o c c u r where s a l t s a r e d e f l a t e d from s a l i n e lakes
211 and mud f l a t s and then r e - d e p o s i t e d by wind on marginal d u n e f i e l d s . The development of gypsum and h a l i t e c r u s t s i n dune and o t h e r sediments has r e c e n t l y been
reviewed by Watson ( i n p r e s s a , i n p r e s s b ) .
Fig. 7. C o n c r e t i o n s weakly cemented by i r o n o x i d e s i n o t h e r w i s e uncemented w h i t e q u a r t z dune s a n d s , s o u t h e r n Oregon. In some examples o n l y the t h i n rim of t h e concretion i s impregnated with i r o n . The l a r g e c o n c r e t i o n i n the c e n t r e of t h e photograph has a max. d i a m e t e r o f a p p r o x . 1.5m. 8.4
Silica Waugh (1970) s u g g e s t e d t h a t s i l i c a c e m e n t a t i o n , i n the form of o p t i c a l l y con-
tinuous q u a r t z o v e r g r o w t h s , was penecontemporaneous w i t h d e p o s i t i o n of the aeol i a n P e n r i t h Sandstone (Lower Permian) o f Northwest England. Waugh b e l i e v e d t h e source o f cement t o be q u a r t z d u s t g e n e r a t e d by a e o l i a n g r a i n a b r a s i o n which was subsequently d i s s o l v e d i n s a l i n e dew and then p r e c i p i t a t e d by e v a p o r a t i o n . Kuene and Perdok (1 962) t h o u g h t t h a t t h e s u r f a c e s o f q u a r t z sand g r a i n s themselves mi g h i
be d i r e c t l y d i s s o l v e d under a l k a l i n e d e s e r t c o n d i t i o n s and t h e s i l i c a precipitate again on e v a p o r a t i o n , g i v i n g the g r a i n s an o v e r a l l ' f r o s t e d ' a p p e a r a n c e . Folk (1978) proposed t h a t opal p h y t o l i t h s r e l e a s e d by d e s e r t v e g e t a t i o n might p r o v i d e
212 a f u r t h e r source o f r e a d i l y - s o l u b l e s i l i c a . F o l k observed t h a t modern dune sand g r a i n s f r o m t h e Sirnpson D e s e r t o f A u s t r a l i a a r e c h a r a c t e r i z e d b y a ' g r e a s y ' surfact o f secondary s i l i c a f o r which he gave t h e name ' t u r t l e - s k i n c o a t ' . However, Folk d i d n o t r e p o r t any i n s t a n c e s o f o p t i c a l l y - c o n t i n u o u s overgrowths o r i n t e r g r a n u l a r cementation. Indeed, a l t h o u g h s i l c r e t e s a r e widespread i n many Q u a t e r n a r y s e d i mentary f o r m a t i o n s , t h e r e a r e few r e p o r t s o f t h e i r development i n a e o l i a n sands (Summerfield,
i n p r e s s ) . W i l l i a m s o n (1957) observed w e l l - r o u n d e d a e o l i a n q u a r t z
g r a i n s cemented by o p t i c a l l y c o n t i n o u s q u a r t z r i m s i n t h e n o r t h e r n p a r t o f West e r n A u s t r a l i a , b u t t h e age and p r e c i s e s t r a t i g r a p h i c r e l a t i o n s h i p s o f t h e cemented layers are not clear. A l t h o u g h t h e o r e t i c a l and e x p e r i m e n t a l s t u d i e s suggest t h a t t h e s o l u b i l i t y of s i l i c a i s h i g h e s t under a l k a l i n e c o n d i t i o n s such as o c c u r i n d e s e r t s ( S i e v e r , 1962; Krauskopf, 1956; Morey e t a l . ,
1962), t h e r e i s ample f i e l d evidence of
s i l i c a d i s s o l u t i o n and r e p r e c i p i t a t i o n a t low pH, p a r t i c u l a r l y where o r g a n i c a c i d s a r e abundant (Van d e r Waals, 1967; Crook, 1968; C l e a r y and Connonly, 1971). I n N o r t h Queensland p o d z o l i c w e a t h e r i n g p r o f i l e s , q u a r t z g r a i n s i n t h e l o w e r part of t h e A2 h o r i z o n s a r e sometimes weakly cemented t o g e t h e r by secondary s i l i c a a t p o i n t c o n t a c t s ( F i g . 8; Pye, i n p r e s s c ) , b u t no l a y e r s a r e s u f f i c i e n t l y indurated t o be termed s i l c r e t e s .
8.5
Calcium carbonate Dune sands cemented by c a l c i u m carbonate o c c u r w i d e l y i n a r i d and s e m i - a r i d
p a r t s o f t h e w o r l d , p a r t i c u l a r l y i n c o a s t a l areas where t h e r e a r e l a r g e accumul a t i o n s o f b i o g e n i c sands. (see Gardner, i n p r e s s b f o r a r e v i e w ) . The cement i n v i r t u a l l y a l l cases i s low-Mg c a l c i t e , b u t t h e t e x t u r e , f a b r i c and mode o f cementation d i f f e r
c o n s i d e r a b l y . Dunes which a r e cemented t h r o u g h o u t o r i n
l a r g e p a r t by c a l c i t e a r e g e n e r a l l y r e f e r r e d t o as ' a e o l i a n i t e s ' . T h i s t e r m was o r i g i n a l l y proposed by Sayles (1931) t o d e s c r i b e ' a l l c o n s o l i d a t e d sedimentary r o c k s which have been d e p o s i t e d by t h e w i n d ' , b u t most l a t e r workers have r e s t r i c t e d i t s usage t o d e s c r i b e a e o l i a n sands which have been cemented b y c a l c i t e under e a r t h s u r f a c e c o n d i t i o n s . Degree and r a t e o f c a l c i t e c e m e n t a t i o n i n most cases r e f l e c t s two f a c t o r s , t h e i n i t i a l carbonate c o n t e n t o f t h e sands, and t h e amount o f w a t e r p a s s i n g t h r o u g h t h e sand column. L o c a l l y , however, v e g e t a t i o n may p l a y a c e n t r a l r o l e i n c o n t r o l l i n g c a l c i t e p r e c i p i t a t i o n v i a the r a t e o f evapo-transpirative water l o s s (Sernenuik and Heagher:
1981). I n areas o f I s r a e l which r e c e i v e 300-600mm o f r a i n -
f a l l p e r annum, Yaalon (1967) found t h a t a ninimum o f 8 - 10% i n i t i a l carbonate c o n t e n t i s r e q u i r e d f o r c e m e n t a t i o n t o proceed. T h i s f i g u r e i n c r e a s e s i n regions w i t h h i g h e r r a i n f a l l . I n v e r y humid areas where r a t e s o f carbonate l e a c h i n g are h i g h i t i s unusual t o f i n d a e o l i a n i t e s even where t h e i n i t i a l c a r b o n a t e content i s more than 60% (Roberts e t a1 . 1973).
213
Fig. 8. Scanning e l e c t r o n m i c r o g r a p h showing s e c o n d a r y s i l i c a p r e c i p i t a t i o n on the surface o f a q u a r t z dune sand g r a i n f r o m t h e A 2 h o r i z o n o f a p o d z o l s o i l p r o f i l e , North Q u e e n s l a n d . Yaalon (1967) c o n s i d e r e d t h a t c e m e n t a t i o n p r o c e e d s f r o m t h e t o p o f a dune down-
ward, as c a l c i u m c a r b o n a t e d e r i v e d f r o m d i s s o l u t i o n o f s k e l e t a l f r a g m e n t s b y vadose water i s r e p r e c i p i t a t e d a t p o i n t s o f g r a i n c o n t a c t and i n a d j a c e n t p o r e spaces. The cemented l a y e r s i n I s r a e l i a e o l i a n i t e s were t h u s i n t e r p r e t e d b y Yaalon
2 14 as Bca o r BCCa s o i l h o r i z o n s . I n many cases t h e l a r g e l y d e c a l c i f i e d A h o r i z o n sands have been s t r i p p e d away, a l l o w i n g c a l c r e t i z a t i o n o f t h e s u b a e r i a l l y exposed cemented B h o r i z o n sands (see Goudie, i n press b
f o r a discussion o f calcrete
f e a t u r e s and genesis ) . A l t e r n a t i v e l y , Land (1970) has argued t h a t vadose cementat i o n i n Bermuda a e o l i a n i t e s i s v e r y slow, l e a d i n g o n l y t o t h e f o r m a t i o n o f f i n e c a l c i t e s p a r around g r a i n c o n t a c t s , whereas p h r e a t i c zone c e m e n t a t i o n i s e x t r e mely r a p i d and l e a d s t o t h e f o r m a t i o n o f c o a r s e - g r a i n e d p o r e - f i l l i n g c a l c i t e spar. Cementation o f s u b s u r f a c e sands i n t h e zone o f c a p i l l a r y r i s e j u s t above t h e w a t e r t a b l e has a l s o been d e s c r i b e d i n Holocene dune sands f r o m Western A u s t r a l i a by Semeniuk and Neagher ( 1 9 8 1 ) . I n Q u i n t a n a Roo, Mexico, Ward (1973, 1975) observed t h a t many P l e i s t o c e n e a e o l i a n i t e s a r e l e s s w e l l cemented t h a t t h e i r Holocene c o u n t e r p a r t s . T h i s was a t t r i b u t e d by Ward t o t h e v e r y e a r l y development o f a c a l c r e t e cap on t h e l a t e P l e i s t o c e n e dune sands under c l i m a t i c c o n d i t i o n s more a r i d t h a n t h e p r e s e n t . This c a l c r e t e l a y e r a c t e d as an aquaclude, p r e v e n t i n g i n f i l t r a t i o n o f m e t e o r i c water and s e v e r e l y r e t a r d i n g t h e r a t e o f vadose d i a g e n e s i s . More t y p i c a l l y however, i n most o t h e r p a r t s o f t h e w o r l d development o f c a l c r e t e h o r i z o n s i n a e o l i a n i t e sequences i s c o n s i d e r e d t o have o c c u r r e d a f t e r i n i t i a l l i t h i f i c a t i o n o f t h e dune sands (e.g. Read, 1974). Reported r a t e s o f d i a g e n e s i s i n carbonate dune sands v a r y c o n s i d e r a b l y . I n I s r a e l Gavish and Friedman (1969) r e p o r t e d t o t a l l o s s o f high-Mg c a l c i t e w i t h i n 10,000 y e a r s and t o t a l l o s s o f a r a g o n i t e w i t h i n 50,000 years, w i t h concomitant n e o - f o r m a t i o n o f low-Mg c a l c i t e and complete o c c l u s i o n o f pore space w i t h i n 80100,000 y e a r s . Reeckman and G i l l (1981), on t h e o t h e r hand, e s t i m a t e i t takes 100,000 y e a r s f o r t h e t o t a l disappearance o f high-Flg c a l c i t e and 600,000 years f o r d e s t r u c t i o n o f aragonite i n t h e coastal a e o l i a n i t e s o f southern Victoria, A u s t r a l i a . A c c o r d i n g t o t h e s e a u t h o r s , complete i n f i l l i n g o f p o r e spaces by lowFlg c a l c i t e cement t a k e s a p p r o x i m a t e l y 1 m i l l i o n y e a r s . 8.6
Humate 'Humate' i s a c o l l e c t i v e t e r m f o r t h e dark-brown t o b l a c k g e l - l i k e humic sub-
stances formed as a r e s u l t o f t h e decomposition o f o r g a n i c m a t t e r i n s o i l s and sediments ( Swanson and Palacas, 1 9 6 5 ) . Humate i s t r a n s l o c a t e d by vadose water i n t o t h e s u b s o i l where i t may be d e p o s i t e d o r l o s t i n t o streams and l a k e s . Subsurface accumulations o f humate o c c u r most commonly i n s t r o n g l y leached q u a r t z sand bodies and a r e a l a t e stage f e a t u r e o f p r o g r e s s i v e p o d z o l i z a t i o n (Ward e t a l . , 1979; Thompson, 1981; Pye, 1982a, i n p r e s s b ) . When sands c o n t a i n i n g humate are a l l o w e d t o d r y o u t , t h e humic m a t e r i a l may harden i r r e v e r s i b l y t o f o r m an i n t e r g r a n u l a r cement. Humate-cemented sediments have been termed ' h u m i c r e t e s ' by anal o g y w i t h c a l c r e t e s and s i l c r e t e s (Pye, 1982a). Humicretes a r e widespread i n t h e o l d e r c o a s t a l dune complexes o f e a s t e r n A u s t r a l i a where t h e y have p r e v i o u s l y beer,
215 referred t o as ' s a n d r o c k ' ,
'coffee-rock',
'woolloomooloo' o r 'waterloo r o c k '
(McGarity, 1956; C o a l d r a k e , 1955; Ward e t a1 ., 1 9 7 9 ) . O t h e r o c c u r r e n c e s i n dune sands and r e l a t e d c o a s t a l s e d i m e n t s have been r e p o r t e d f r o m t h e s o u t h and s o u t h eastern p a r t o f N o r t h A m e r i c a ( K u r z , 1942; P r i c e , 1962; Swanson and Palacas, 1965; Thorn, 1967; Du B a r e t a1 ., 1 9 7 4 ) and f r o m s o u t h - e a s t e r n B r a z i l ( B i g a r e l l a , 1 9 7 5 a ) .
A t y p i c a l c o a s t a l e x p o s u r e o f h u n i i c r e t e i n S o u t h e r n Q u e e n s l a n d i s shown F i g .
I).
Fig. 9. Boulders o f humate-cemented sand exposed o n a beach f o r e s h o r e b y wave e r o sion o f l a t e P l e i s t o c e n e q u a r t z dunes, S o u t h e r n Q u e e n s l a n d . M o s t o f t h e b o u l d e r s are 1-1.5111 i n d i a m e t e r .
9.
POST-CEMENTATION WEATHERING AND SOLUTION Post-cementation w e a t h e r i n g and s o l u t i o n m o s t o f t e n a f f e c t s a e o l i a n i t e s a l t h o u g t
other types o f cemented a e o l i a n d e p o s i t may be s i m i l a r l y m o d i f i e d . P r o g r e s s i v e l e a ching o f a e o l i a n i t e , e s p e c i a l l y i f a c c e n t u a t e d b y a t r e n d t o w a r d s w e t t e r c l i m a t i c conditions, e v e n t u a l l y l e a d s t o t h e f o r m a t i o n o f a d e c a l c i f i e d s u r f i c i a l l a y e r r i c l
in quartz, o t h e r r e s i s t a n t d e t r i t a l m i n e r a l s , a l l o c h t h o n o u s d u s t components, b i o -
216 l i t h s , and a u t h i g e n i c w e a t h e r i n g p r o d u c t s such as c l a y s and i r o n o x i d e s . The bound a r y between d e c a l c i f i e d and c a l c i t e - c e m e n t e d sands i s o f t e n a b r u p t and i r r e g u l a r . P r o g r e s s i v e pedogenesi s o f aeol i a n i t e sandstone has been d e s c r i b e d by Amiel (1975) w h i l e d e t a i l e d accounts o f t h e s u r f i c i a l r e d s o i l s o v e r l y i n g a e o l i a n i t e i n Bermuda have been p u b l i s h e d by Ruhe e t a l . (1961) and B l a c k b u r n and T a y l o r ( 1 9 6 9 ) . Solut i o n o f t e n occurs p r e f e r e n t i a l l y along v e r t i c a l j o i n t s , leading t o the formation o f s o l u t i o n p i p e s and o t h e r k a r s t i c phenomena (Day, 1928). S t r i p p i n g o f t h e overl y i n g r e s i d u a l d e p o s i t s by a e o l i a n o r f l u v i a l a c t i o n may expose t h e k a r s t i f i e d a e o l i a n i t e s u r f a c e ( e . 9 . Coetzee, 1975a, 1975b). Where low-Mg c a l c i t e - c e m e n t e d a e o l i a n i t e s become p a r t l y drowned by a r i s e i n r e l a t i v e sea l e v e l , o r a r e o t h e r w i s e a f f e c t e d by s a l i n e groundwater,
slow 'regres-
s i v e d i a g e n e s i s ' may o c c u r , i n v o l v i n g p a r t i a l replacement by high-Mg forms o f calcite 10
( M i i l l e r and T i e t z , 1975). CONCLUSIONS
T h i s r e v i e w has shown t h a t a e o l i a n dune sands can e x p e r i e n c e s u b s t a n t i a l modif i c a t i o n w i t h i n a few thousand y e a r s o f d e p o s i t i o n . Some a l t e r a t i o n processes are s t r i c t l y s y n - d e p o s i t i o n a l i n t h a t t h e y o c c u r w i t h i n o n l y a few y e a r s o f sedimentat i o n . A l t e r a t i o n i s a time-dependent process, b u t o n l y r a r e l y can degree o f weathe r i n g o r d i a g e n e s i s be used as a means o f age c o r r e l a t i o n between d e p o s i t s i n d i f f e r e n t a r e a s . T h i s i s because r a t e s o f m i n e r a l w e a t h e r i n g , l e a c h i n g , and authigenesis r e f l e c t d i f f e r e n c e s i n c l i m a t e , v e g e t a t i o n , topography, d r a i n a g e condit i o n s , sand m i n e r a l o g y , and t h e n a t u r e and s i g n i f i c a n c e o f a1 lochthonous i n p u t s . G e n e r a l l y speaking, p o s t - d e p o s i t i o n a l m o d i f i c a t i o n l e a d s t o t h e f o l l o w i n g changes i n t h e p h y s i c a l p r o p e r t i e s o f dune sands; r e d u c t i o n i n mean g r a i n s i z e , poore r sediment s o r t i n g , r e d u c t i o n i n v o i d s r a t i o , p o r o s i t y and p e r m e a b i l i t y , and i n creased i n t e r p a r t i c l e c o h e s i o n . Many o f t h e s e changes r e f l e c t i n c r e a s e d c l a y cont e n t due t o w e a t h e r i n g o f d e t r i t a l f e l d s p a r s and heavy m i n e r a l s and t o i n f i l t r a t e d a e o l i a n d u s t . I n f a v o u r a b l e circumstances p r e c i p i t a t i o n o f a u t h i g e n i c m i n e r a l s o t h e r t h a n c l a y s can l e a d t o i n t e r g r a n u l a r cementation. D u r i n g w e a t h e r i n g and c e m e n t a t i o n a e o l i a n t r a n s p o r t t e x t u r e s on q u a r t z g r a i n s a r e o f t e n obscured and o b l i t e r a t e d by s i l i c a s o l u t i o n and d e p o s i t i o n , such t h a t s u r f a c e f e a t u r e s c a n n o t b e used as d i a g n o s t i c i n d i c a t o r s o f mode o f d e p o s i t i o n i n most weathered sands. P r i m a r y b e d d i n g s t r u c t u r e s u s u a l l y become p a r t l y o r w h o l l y o b l i t e r a t e d i n the upper few metres o f dune sands t h r o u g h b i o t u r b a t i o n , w e a t h e r i n g and t r a n s l o c a t i o The p r i m a r y f e a t u r e s may b e r e p l a c e d o r o v e r p r i n t e d b y secondary s t r u c t u r e s such as d i s s i p a t i o n s t r u c t u r e s , burrows, r o o t c a s t s , nodules and c o n c r e t i o n s . 11.
ACKNOWLEDGEMENTS The r e s e a r c h i n c o r p o r a t e d as p a r t o f t h i s r e v i e w was s u p p o r t e d by t h e U.K.
217 Natural Environmental Research C o u n c i l and by t h e Royal S o c i e t y . I thank O r . Andrew Goudie, P e t e r F r i e n d and Mike B r o o k f i e l d f o r comments on an e a r l i e r v e r s i o n o f t h e manuscript. REFEREllCES
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220 Sayles, R.W., 1931. Bermuda d u r i n g t h e I c e Age. Proc. Am. Acad. A r t s . S c i . , 66: 381-468. 1964. P e t r o l o g y o f t h e l i m e s t o n e s o f Guam. U.S.G.S. P r o f . Paper, Schlanger, S.O., 403D: 1-52. Schmalz, R.F., 1968. F o r m a t i o n o f r e d beds i n modern and a n c i e n t d e s e r t s . Discuss i o n . Geol. SOC. Am. B u l l . , 79: 277-280. S c h n i t z e r , M. and Khan, S.U., 1972. Humic Substances i n t h e Environment. Dekker, New York. Setlow, L.W., 1978. Age d e t e r m i n a t i o n o f reddened c o a s t a l dunes b y scanning e l e c t r o n microscopy. I n : Whalley, W.B. ( E d i t o r ) , Scanning E l e c t r o n Microscopy i n t h e Study o f Sediments. Geoabstracts, Norwich, pp. 283-306. Semeniuk, V. and Meagher, T.D., 1981. C a l c r e t e i n Q u a t e r n a r y c o a s t a l dunes i n southwestern A u s t r a l i a : a c a p i l l a r y r i s e phenomenon a s s o c i a t e d w i t h p l a n t s . J. Sed. P e t r o l . , 51: 47-68. Sidhu. P.S., 1977. A e o l i a n a d d i t i o n s t o t h e s o i l s o f n o r t h w e s t I n d i a . Pedoloqie, 27; 323-336. Siever, R., 1962: S i l i c a s o l u b i l i t y O°C - 200°C and t h e d i a g e n e s i s o f s i l i c e o u s sediments. J. Geol.. 70: 127-150. Summerfield, M.A., i n p r e s s . S i l c r e t e . I n : Goudie, A.S. and Pye, K. ( E d i t o r s ) , Chemical Sediments and Geomorphology. Academic Press, London. Swanson, V.E. and Palacas, J.G., 1965. Humate i n c o a s t a l sands o f n o r t h w e s t F l o r i d a . U.S.G.S. B u l l . , 1214B.: Bl-B29. Syers, J.K., Jackson, M.L. B e r k h e i s e r , V.E., C l a y t o n , R.N., and Wex, R.W., 1969. E o l i a n sediment i n f l u e n c e on pedogenesis d u r i n g t h e Q u a t e r n a r y . S o i l S c i ., 107: 421 -427. (1978) E r o s i o n o f f i x e d dunes i n t h e Sahel, Centra T a l b o t , M.R. and W i l l i a m s , M.A.J. N i g e r . E a r t h S u r f . Processes, 3: 107-114. Thom. B.G., 1967. Humate i n c o a s t a l geomorphology. Coastal Stud. B u l l . , 1 : 15-17. Thompson, C.H., 1981. Podzol chronosequences on c o a s t a l dunes i n e a s t e r n A u s t r a l i a . Nature, 291: 59-61. Van d e r Waals, L., 1967. M o r p h o l o g i c a l phenomena on q u a r t z g r a i n s i n unconsolidated sands, due t o m i g r a t i o n o f q u a r t z n e a r t h e e a r t h ' s s u r f a c e . Meded. Neth. Geol. S t i c h t . N.S., 18: 47-51. Walker, T.R., 1967. F o r m a t i o n o f r e d beds i n modern and a n c i e n t d e s e r t s . Geol. SOC. Am. B u l l . , 78: 353-368. Walker, T.R., 1976. D i a g e n e t i c o r i g i n of c o n t i n e n t a l r e d beds. I n : F a l k e , H . (Editor: The C o n t i n e n t a l Permian i n C e n t r a l , West and South Europe. R e i d e l , Dordrecht, pp. 240-282. Walker, T.R., 1979. Red c o l o r i n dune sand. I n : McKee, E.D. ( E d i t o r ) , A Study of P r o f . Paper, 1052: 62-81. Global Sand Seas. U.S.G.S. Walker, T.R., Waugh, B. and Crone, A.J., 1978. D i a g e n e s i s i n f i r s t c y c l e d e s e r t a l l u v i u m o f Cenozoic age, southwestern U n i t e d S t a t e s and n o r t h w e s t e r n Mexico. Geol. SOC. Am. B u l l . , 89: 19-32. Ward, W.C., 1973. I n f l u e n c e o f c l i m a t e on t h e e a r l y d i a g e n e s i s o f carbonate eolianitt Geology, 1 : 171-174. 1975. P e t r o l o g y and d i a g e n e s i s o f c a r b o n a t e e o l i a n i t e s o f northeastern Ward, W.C., Yucatan Peninsula, Mexico. I n Wantland, K.F. and Pusey, W.C. I11 ( E d i t o r s ) B e l i z e S h e l f - Carbonate Sediments, C l a s t i c Sediments and Ecology. Am. A S S . P e t r o l . Geol , pp. 500-571. Ward, W.T., 1977. Sand movement on F r a s e r I s l a n d . U n i v . Queensland Occ. Papers i n Anthropology, 8: 113-126. Ward, W.T., L i t t l e , I . P . and Thompson, C.H., 1979. S t r a t i g r a p h y o f two sandrocks a t Rainbow Beach, Queensland, and some n o t e s on t h e i r c o m p o s i t i o n . Palaeogeog. P a l a e o c l i m a t o l . Palaeoecol.. 26: 305-316. Uatson, A., 1979. Gypsum c r u s t s i n d e s e r t s . J . A r i d . Environmen S , 2: 3-20. Ilatson, A., i n press a. Gypsum C r u s t s . I n : Goudie, A.S. and Pye K. ( E d i t o r s ) , Chemical Sediments and Geomorphology. Academic Press, London Ldatson, A., i n press b. H a l i t e C r u s t s . I n : Goudie, A.S. and Pye K. ( E d i t o r s ) , Chemical sediments and Geomorphology. Academic Press, London Waugh, B., 1970. P e t r o l o g y , provenance and s i l i c a d i a g e n e s i s i n t h e P e n r i t h Sandstone (Lower Permian) o f n o r t h w e s t England. J . Sed. P e t r o l . , 40: 1226-1240.
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Villiams, C . and Yaalon, D.H., 1977. An e x p e r i m e n t a l i n v e s t i g a t i o n of reddening i n dune s a n d . Geoderma, 1 7 : 181-191. Williamson, W.O., 1957. S i l i c i f i e d s e d i m e n t a r y rocks i n A u s t r a l i a . Am. J . S c i . , 255: 23-42. Vriqht, U . R . and Foss, J.E. 1968. Hovement o f s i l t - s i z e d p a r t i c l e s i n sand columns. Froc. S o i l . S c i . An., 32: 446-448. Yaalon. D.H.. 1967. F a c t o r s a f f e c t i n a t h e l i t h i f i c a t i o n o f a e o l i a n i t e and i n t e r p r e t a t i o n - o f i t s environmental s i G n i f i c a n c e i n t h e c o a s t a l p l a i n of I s r a e l . J . Sed. P e t r o l . , 37: 1189-1199. Yaalon, D . H . , and Ginzbourg, D., 1966. Sedimentary c h a r a c t e r i s t i c s and c l i m a t i c a n a l y s i s of e a s t e r l y d u s t s t o r m s i n the liegev ( I s r a e l ) . Sedimentology, 6 : 315-332. Yaalon, D.H. and Ganor, E . , 1973. The i n f l u e n c e of d u s t on s o i l s i n t h e Q u a t e r n a r y . S o i l S c i . , 116: 146-155.
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EOLIAN FEATURES SHAPED BY AERODYIdAMIC AND VnRTICITY PROCESSES MARION I . WHITNEY: C e n t r a l M i c h i g a n U n i v e r s i t y , Mount P l e a s a n t , M i c h i g a n 48859, U.S.A.
INTRODUCTION E o l i a n f e a t u r e s have a v a r i e t y o f shapes because numerous f a c t o r s a r e involved i n t h e i r f o r m a t i o n . One i m p o r t a n t f a c t o r i s wind regime; t h e s i m p l e r the wind regime t h e more l i k e l v a r e s t r e a m l i n e d forms t o develop. Thus i n a monodirectional regime, yardangs a r e common, b u t so a r e c o n i c a l h i l l s . Hence, o t h e r f a c t o r s t h a n w i n d d i r e c t i o n a r e s i g n i f i c a n t i n d e t e r m i n i n g e o l i a n
forms, one o f t h e most i m p o r t a n t o f which i s response o f t h e wind t o t h e shape
o f the f e a t u r e on which i t impinges, i n o t h e r words t h e aerodynamic system t h a t i s developed under p r e s s u r e c o n t r o l . T h i s i s n o t j u s t a d i v i d i n g and c l o s u r e o f streamlines b u t i n v o l v e s t h e development o f a complex system o f s u b s i d i a r y f l o w along t h e i n t e r f a c e between a i r and r o c k . Most o f t h e d a t a was o r i g i n a l l y o b t a i n e d by f l o w t e s t i n g and wind b l a s t experiments t h a t produced e r o s i o n , and by comparing r e s u l t s o f t h e s e t e s t s w i t h d e t a i l s o f e r o s i o n on o r i e n t a t e d f l u t e d and p i t t e d v e n t i f a c t s c o l l e c t e d f r o m multiwind regimes (Whitney, 1978). Snow d r i f t s , however, o f f e r a s i m p l e v i s u a l means o f d e m o n s t r a t i n g a c t u a l f l o w p a t t e r n s and t h e i r a t t e n d a n t e r o s i o n f o r comparison w i t h s t r e a m l i n e d yardangs, sand d r i f t s and v e n t i f a c t s . Snow f l a k e s o u t l i n e f l o w l i n e s and v o r t i c e s , t h u s y i e l d i n g s u p p o r t i n g d a t a and i n s e v e r a l important r e s p e c t s a d d i t i o n a l d a t a a p p l i c a b l e t o e r o s i o n o f r o c k f e a t u r e s . Subsequently, t h e s e o b s e r v a t i o n s have been checked i n t h e m a j o r m o n o d i r e c t i o n a l wind regime o f t h e western d e s e r t o f Egypt. DEFINITIONS F l u t e s a r e grooves w i t h e s s e n t i a l l y symmetrical c r o s s s e c t i o n s a l o n g w h i c h the wind t r a n s p o r t s v o r t e x t r a i n s and t o o l s o f e r o s i o n . P r i m a r y f l u t e s t r e n d i n the d i r e c t i o n s o f p r i n c i p a l f l o w l i n e s : secondary f l u t e s t r e n d i n t h e d i r e c t i o n o f minor i n t e r f a c i a l f l o w p a t t e r n s , commonly t r a n s v e r s e t o t h e p r i n c i p a l f l o w lines.
Keels a r e
s h a r p r i d g e s between f a c e s o r f l u t e s and r e s u l t from s i m u l t a n -
eous offsweep f r o m t h e two c o n t i g u o u s s u r f a c e s . A r e e n t r a n t i s a n e g a t i v e t o p o graphic f e a t u r e such as a c o n c a v i t y , a p i t o r a groove. A s a l i e n t i s a p o s i t i v e topographic f e a t u r e such as a r i d g e . A v o r t e x p i t i s i n i t i a l l y round b u t may become o t h e r shapes where n e g a t i v e f l o w c r o s s e s i t o r where i t j o i n s w i t h o t h e r p i t s . A yardang i s a w i n d eroded r i d g e , commonly s t r e a m l i n e d : i t o f t e n t a p e r s
224 leeward b u t may o r may n o t t a p e r t o windward. Z a s t r u g i a r e snow f e a t u r e s o f aerodynamic o r i g i n , d e e p l y u n d e r c u t t o windward and h a v i n g e i t h e r a f l a t t e n e d o r a k e e l e d d e c l i n i n g t o p . N e g a t i v e f l o w i s a r e v e r s a l o f wind f l o w a l o n g an i n t e r f a c e , caused by l o w p r e s s u r e a t some p o i n t i n t h e aerodynamic system.
A v e n t u r i i s an a c c e l e r a t e d f l o w , u s u a l l y due t o c o n t r i c t i o n . THE S H A P I N G OF A WEDGE-SHAPED SNOW DRIFT The wedge-shaped snow d r i f t , f a s h i o n e d by c o n c u r r e n t d e p o s i t i o n and e r o s i o n , i s one o f t h e most b e a u t i f u l shapes i n n a t u r e ( f i g . 1 ). Forminq i n winds t h a t s t r i p snow f r o m f i e l d s , t h e s e d r i f t s develop by t h e m i l l i o n o b l i q u e l y across d i t c h e s , g u t t e r s and snow banks. These d r i f t s o f t e n form as r e s u l t a n t s between t h e main wind and t h a t o f a d e f l e c t e d f l o w . Once formed, however, t h e i r shapes t h e n g u i d e t h e l o c a l f l o w , which i n t u r n qoverns subsequent m i n o r changes.
F i g . 1. A
B
-
Wedge-shaped snow d r i f t i n a d i t c h showing offsweep t r e n d s on keel. Wedge-shaped snow d r i f t on a snow bank, showing s l o p e change o f keel l i n e .
225 Unless t h e v f o r m a t o p a r i d q e ( f i q . lA), most d r i f t s have no prominent head zones on t h e i r windward ends, as do yardangs, b u t s i m p l y e x t e n d o u t from t h e i r sources o f snow, d i v i d i n g t h e s t r e a m l i n e s around them. Thus t h i s t y p e have t h e i r p r i n c i p a l f l o w l i n e s above them, one on e i t h e r s i d e o f t h e c r e s t l i n e ( f i g . 2A). Throuqh i n t e r v e n i n q t r o u g h s between d r i f t s , wind sweeps downward i n t o t h e d i t c h , e n q i r d l i n g bases o f t h e d r i f t s w i t h v e n t u r i s which b r i n g t h e lower s t r e a m l i n e s t o c l o s u r e i n t h e l e e o f t h e d r i f t . Some d r i f t s have i n c l i n e d c r e s t l i n e s , b u t on o t h e r s a c o n s i d e r a b l e p o r t i o n of the c r e s t l i n e i s h o r i z o n t a l , becoming i n c l i n e d o n l y where t h e f e a t u r e narrows leeward i n t h e c l o s u r e o f s t r e a m l i n e s o v e r t h e c r e s t . Closure b r i n q s these f l o w l i n e s c l o s e r t o t h e i n t e r f a c e , and t h e y begin t o c r e a t e coarse f l u t i n g on s i d e s and a c r o s s t h e l e e k e e l ( f i g . 2 ) . Whereas off-sweep o f t h e p o s i t i v e f l o w l i n e s dominates on t h e l e e d e c l i v i t y i n t h e s e forms, secondary
flow transverse t o the streamlines operates a t t h e i n t e r f a c e i n t h e horizontal sector, i n i t i a l l y c r e a t i n g small wedqe-shaped f l u t e s whose s i d e s converqe upwards t o c l o s e d f l u t e heads. Flow l i n e s t h r o u g h t h e s e f l u t e s c a r r y v i s i b l e t r a v e l l i n g v o r t i c e s which t r a n s p o r t snow i n t o t h e o v e r r i d i n g f l o w l i n e s , l i f t i n g o v e r t h e f l u t e heads. ldhere t h e s e secondary f l o w l i n e s b e g i n t o d e f l e c t leeward,
the secondary f l u t e s a l s o b e g i n t o bend leeward ( f i q . 1 A ) . The f l u t e head i s breached and widens a l o n q t h e c r e s t l i n e . I n waning phases, v o r t i c e s which have already escaped d u r i n g q u s t i n g phases, and moved leeward i n t o t h e crowded t o p
f l o w l i n e s , r e v e r s e and move back down a l o n g t h e f l a n k s t h r o u g h t h e secondary f l u t e s . The snow may be c a r r i e d t o t h e bottom o f t h e f l a n k and d e p o s i t e d , o r a new g u s t i n g phase may be i n i t i a t e d b e f o r e t h i s happens and t h e snow-laden v o r t i c e s a g a i n c r o s s t h e c r e s t l i n e and escape downwind. Thus, i t i s p o s s i b l e t o see p r i m a r y and secondary f l o w moving s i m u l t a n e o u s l y a t d i f f e r e n t l e v e l s and
in d i f f e r e n t d i r e c t i o n s a c r o s s a s i n g l e l o c u s . The f l u t i n g t h a t t h e s e f l o w l i n e s create can be l i k e n e d t o p r i m a r y and secondary f l u t i n g on v e n t i f a c t s and y a r d angs. The w i d e n i n g o f t h e secondary f l u t e s a l o n g t h e c r e s t l i n e has i t s c o u n t e r part i n c o a r s e f l u t i n g on v e n t i f a c t s , such as t h e prominent expansions a t t h e top ends o f f l u t e s on t h e b o u l d e r v e n t i f a c t s a t Garnet Ridge, C a l i f o r n i a . The offsweep and onsweep a t t h e c r e s t l i n e d u r i n g q u s t i n g and waning stages serves t o hone a f i n e k e e l . But i f t h e wind e s c a p i n q f r o m t h e two s i d e s o f t h e d r i f t
i s n o t i n phase, t h e n t h e k e e l may sometimes become a z i g z a g l i n e ( f i q . 2 A ) . One way o f a t t a i n i n g a sharp keel i s f o r t h e wind t o impinge somewhat o b l i q u e l y t o t h e l o n g a x i s o f t h e f e a t u r e , so t h a t offsweep i s d e f l e c t e d downwind a l o n g the keel. T h i s p u l l s a t r a i l i n g offsweep f r o m t h e o p p o s i t e s i d e o f t h e k e e l and may a l s o p u l l up n e g a t i v e f l o w from t h e l o w e r l e e s e c t o r o f t h e o p p o s i t e s i d e which i s s t r o n g enough t o c r e a t e f l u t e s , v o r t e x p i t s o r basal m a r g i n c a v i t a t i o n .
A l l o f t h e s e a r e a l s o common on v e n t i f a c t s and yardanqs. On snow d r i f t s e r o s i o n
i s l a r g e l y by l i f t : on r o c k f e a t u r e s i t i s accomplished p r i n c i p a l l y by t h e f i n e r
226
>3-
->
_-- +
FIELD
F i g . 2.
A
MAIN STREAMLINES NEGATIVE FLOW SECONDARY FLOW
\-
-
\DITCH
P l a n v i e w o f a wedge-shaped snow d r i f t i n a d i t c h , showing t h e
dispersal o f flow patterns.
B
-
P r o f i l e v i e w o f a wedge-shaped snow d r i f t i n a d i t c h , showing
dispersal patterns.
227 suspensates, t r a n s p o r t e d by s m a l l - s c a l e v o r t i c e s . Repeated d e l i c a t e b r u s h i n g o f surfaces i n l o w p r e s s u r e v o r t i c i t y and t h e p u l s a t i n g wind p a t t e r n account f o r f i n e shaping, b u r n i s h e d s u r f a c e s and s c u l p t u r a l d e t a i l s such as f l u t e s , v o r t e x p i t s and k n i f e - e d g e d k e e l s , l i k e t h o s e developed on v e n t i f a c t s i n A n t a r c t i c a by s i l t - s i z e d i c e c r y s t a l s (Whitney and S p l e t t s t o e s s e r ,
1982). Also, t h e snow d r i f t
demonstrates t h a t f i n e k e e l s f o r m by simultaneous offsweep f r o m b o t h s i d e s . I f t h e e n t i r e c r e s t l i n e o f t h e snow d r i f t i s i n c l i n e d , and t h e r e i s no appr-
e c i a b l e e l e v a t i o n a t i t s head zone t o c r e a t e s t r o n g n e g a t i v e f l o w back t o windward, t h e n c l o s u r e o f t h e s t r e a m l i n e s may o c c u r a l l a l o n g t h e c r e s t , f o r m i n g leeward t r e n d i n g f l u t e s down t h e c r e s t l i n e . I f , however, t h e d r i f t i s e l e v a t e d i n t h e head zone, some wind impact on t h i s head w i l l d e f l e c t upwards and escape, lowering i n p r e s s u r e and a t t r a c t i n g a s t r o n g n e g a t i v e f l o w up t h e c r e s t . The p r i n c i p a l s t r e a m l i n e s w i l l d i v i d e around t h e head zone and move leeward; b u t negative f l o w , r i s i n g o v e r t h e t o p a t t h e i n t e r f a c e , dominates t h e s c u l p t u r a l p a t t e r n , d e f l e c t i n g t h e f l u t i n g upwind a l o n g t h e c r e s t l i n e , as on sand stream-
ers i n t h e l e e o f v e g e t a t i o n . Flow l i n e s i n t h e a n g l e around t h e basal m a r g i n o f a snow d r i f t a r e i n t h e nature o f v h t u r i s , t h a t i s , a c c e l e r a t e d , c a r r y i n g l o w p r e s s u r e t h a t p u l l s secondary f l o w down l a t e r a l s u r f a c e s o f t h e d r i f t , and t h u s p r o d u c i n g a second
s e t o f secondary f l u t e s ( f i g . 2B). T h i s s e t may remain s h o r t and w i d e l y s e p a r a t ed from t h e s e t r e l a t e d t o t h e t o p f l o w l i n e s , o r i t may e l o n g a t e upwards and either j o i n t h e t o p s e t o r i n t e r d i g i t a t e w i t h it. T h i s i s a l s o a p p l i c a b l e t o dunes and f l u t e d v e n t i f a c t s . Where stream1 i n e s dominate t h e e r o s i v e process, secondary f l u t e s may be absent, o r t h e y may be p r e s e n t as segments t h a t t r a n s e c t l a r q e r f l u t e s eroded by t h e s t r e a m l i n e s . Hence, i n a r e a s o f snow and b r i s k winds, wedge-shaped d r i f t s can a f f o r d visual evidence o f processes and e r o s i o n a l r e s u l t s o f t h e complex d i s p e r s a l o f an aerodynamic system. Thus we i n f e r t h a t what i s produced i n an i n s t a n t a n e o u s event on a snow d r i f t may a l s o be produced on c o n s o l i d a t e d m a t e r i a l s , i n a l i k e manner o v e r an extended p e r i o d o f t i m e , by m i l l i o n s o f r e p e t i t i o n s o f p a t t e r n s
o f f l o w a l o n g t h e same t r e n d s . THE SAND DRIFT OR STREAMER The sand d r i f t has much i n common w i t h wedge-shaped snow d r i f t s i n form, patterns o f w i n d d i s p e r s a l and consequent s c u l p t u r a l d e t a i l . Large streamers, developed i n t h e l e e o f v e g e t a t i o n and l o g s , on Padre I s l a n d Texas, e x h i b i t : separated s e t s o f f l u t i n g , r e l a t e d t o t o p and b o t t o m f l o w l i n e s ; i n t e r d i g i t a t i o n
as i n t h e midground; and l e e w a r d d e f l e c t i o n , as a t t h e r i g h t a l o n g t h e t o p margin. Most small streamers t o t h e l e e o f v e g e t a t i o n have s t r o n g l y r e f l e x e d f l u t i n g along t h e k e e l l i n e ( f i g . 3 B ) . T h i s i s due t o t h e f a c t t h a t t h e p l a n t s t o
228
F i g . 3.
A
- P r o f i l e view o f a wedge-shaped sand d r i f t , showing a s e t o f l a t e r a l f l u t i n g r e l a t e d t o t h e t o p f l o w l i n e , a second s e t r e l a t e d t o a basal f l o w l i n e and i n t e r d i g i t a t i o n o f t h e two s e t s . ldind d i r e c t i o n from t h e l e f t . Padre I s l a n d , Texas.
B - R e f l e x e d f l u t i n g c o n v e r g i n g t o a k e e l l i n e i n t h e l e e o f a sand d r i f t i n t h e Kharga Oasis, Egypt. N e g a t i v e f l o w l i n e s j o i n b e f o r e entering the positive flow. windward a c c e n t u a t e t h e u p d r a f t and a l s o cause t h e h i q h e s t p a r t o f t h e d e p o s i t t o f o r m a t t h e windward margin. The u p d r a f t a t t r a c t s n e g a t i v e f l o w u p t h e keel l i n e w h i c h f o r c e s t h e p o s i t i v e f l o w s t i l l h i g h e r . T h i s causes t h e n e g a t i v e f l o w l i n e s t o a t t e n u a t e and converge upward and windward towards t h e k e e l l i n e , formi n q f l u t e s i n t h e i r wake. As t h e p o s i t i v e f l o w bends down t o l e e w a r d o f t h e head zone, p e r i o d i c a l l y t h e a t t e n u a t e d n e g a t i v e f l o w l i n e s f r o m o p p o s i t e s i d e s escape
229 across t h e k e e l l i n e and j o i n w i t h each o t h e r b e f o r e e n t e r i n q t h e d e c l i n i n q p o s i t i v e flow. F i q u r e 4 shows a v e r y s m a l l e r o s i o n a l sand f e a t u r e and diaqram o f t h e f l o w p a t t e r n around i t . U n l i k e t h e streamer i n t h e l e e o f t h e bush, t h i s f e a t u r e had very l i t t l e e l e v a t i o n o r b u l k a t i t s windward end. Hence, t h e r e was l i t t l e t o c r e a t e an u p d r a f t . The wind impinqed o h l i a u e l y on t h e narrow end and t h e s t r e a n l i n e s d i v i d e d around i t w i t h t h e s t r o n q e r f l o w s w i n g i n g c o u n t e r c l o c k w i s e . Those s t r e a m l i n e s c l o s e s t t o t h e f e a t u r e c l o s e d downwind a l o n q a n e a r l y h o r i z o n t a l keel. H e l i c a l scores and s l i q h t depressions i n t h e f l o w t r e n d s i n d i c a t e v e r t i c a l axis v o r t i c e s i n t h e f l o w l i n e s . A n e i g h b o u r i n g sand f e a t u r e o f b a r c h a n o i d f o r m ( f i q . 5 ) a t t a i n e d enough breadth and b u l k i n t h e windward zone t o spread t h e s t r e a m l i n e s c o n s i d e r a b l y more t h a n was t h e case i n t h e k e e l e d f e a t u r e . Furthermore, t h e b u i l d - u p i n height was g r a d u a l f r o m t h e windward m a r q i n t o t h e mid-zone. Thus, when wind swept up t h i s s l o p e a t a l o w a n g l e , t h e r e was no h i q h u p d r a f t and n e g a t i v e f l o w , therefore,
was n o t drawn h i q h above t h e s u r f a c e and g r e a t l y a t t e n u a t e d , b u t
j o i n e d t h e p o s i t i v e f l o w d i r e c t l y , f o r m i n q t h e sharp t r a n s v e r s e k e e l and t h e concave l e e slope, i n s t e a d o f an a x i a l k e e l as was t h e case w i t h t h e streamer i n t h e l e e o f a bush. These two sand s c u l p t u r e s were about a m e t r e a p a r t on a i l i c h i q a n sand dune. There was n o t h i n q i n t h e environment t o i n d i c a t e why one s h o u l d develop as a streamlined f o r m and t h e o t h e r s h o u l d n o t . Hence, t h e e x p l a n a t i o n must l i e i n t h e d i s t r i b u t i o n o f t h e masses so as t o c r e a t e two d i f f e r e n t aerodynamic systems
as t h e wind d i s p e r s e d around them; t h a t i s , d i f f e r e n t d i s t r i b u t i o n o f t h e wind impact, f l o w p a t t e r n s and offsweep p a t t e r n s . Each o f t h e s e t h r e e aspects o f t h e aerodynamic system e x e r t s m a j o r shaping c o n t r o l . Thus, when t h e system i s understood, a g i v e n f e a t u r e has i n t e r p r e t a t i v e value; t h a t i s , i t i s i n d i c a t i v e o f a p a t t e r n o f f l o w , For i n s t a n c e , t h e o r i e n t a t i o n o f t h e k e e l o f t h e streamer ( F i g .
38) and i t s r e f l e x e d f l u t e s r e p r e s e n t t h e escape o f convergent l i n e s o f n e g a t i v e flow p r i o r t o j u n c t u r e w i t h t h e p o s i t i v e f l o w . B u t t h e t r a n s v e r s e k e e l o f f i g u r e
5A i n d i c a t e s d i r e c t j u n c t u r e o f p o s i t i v e and n e g a t i v e f l o w . I n r e p o r t i n g t h e barchan dune s t u d y (Whitney, 1978, p. 15-17),
I n o t e d t h a t t h e k e e l s have been
formed between o p p o s i t e l y moving p o s i t i v e and n e g a t i v e f l o w , i n a case where t r a n s p o r t e d m a t e r i a l s o u t l i n e d t h e f u l l p a t t e r n o f f l o w on b o t h s i d e s o f t h e
c r e s t and converqed a g a i n s t t h e k e e l i n o f f s w e e p and j u n c t u r e . Mainguet (1978) showed t h a t wind impinqed l a r q e l y o b l i q u e l y t o t h e axes o f the g r e a t s e i f dunes o f t h e Sahara. I suggested t o h e r i n 1979 t h a t , i n t h i s case; 1 ) t h e r e had t o be n e g a t i v e f l o w on t h e s i d e s o p p o s i t e t h e w i n d impact,
2 ) t h e w i d t h s o f l i n e s o f i m p a c t i n g w i n d f l o w a r e p r o b a b l y governed by t h e diameters o f t h e v o r t i c e s t h e y c a r r y , and 3) v e l o c i t y i s p r o b a b l y l o w e r i n t h e
230
MAIN STREAMLINES
-+ NEGATIVE FLOW - _ _ + SECONDARY FLOW 71-
N+
YJ
.d
?$
HELICAL SCORE
-/
F i g . 4.
A - S t r e a m l i n e d sand s c u l p t u r e : t o p o f S i l v e r Lake Dunes, Oceana County, M i c h i g a n . About n a t u r a l s i z e . Wind f r o m t h e l e f t . B - Diagram o f t h e f l o w p a t t e r n s on and around t h e sand s c u l p t u r e shown i n f i g . 4A.
231
I.
’
>R--+
-+
(k%
MAIN STREAMLINE NEGATIVE FLOW SECONDARY FLOW HELICAL SCORE
Fig. 5. Barchanoid sand s c u l p t u r e , t o p o f S i l v e r Lake Dunes, Oceana County, Michigan. A - photograph. B - Flow p a t t e r n s . Both n a t u r a l s i z e . Wind from t h e l e f t .
232 i n t e r z o n e s of t h e v o r t i c e s t h a n a t t h e c e n t r e s o f t h e f l o w t r e n d s , t h u s causing f l u c t u a t i n q f o r c e o f t h e o b l i q u e impact on one f l a n k and a f l u c t u a t i n q deqree of n e q a t i v e f l o w on t h e o p p o s i t e f l a n k . ldhere t h e n e q a t i v e f l o w i s s t r o n q e s t , i t would erode t h e l e e f l a n k and t h u s bend t h e keel toward t h e i m p a c t i n g wind. A s e r i e s o f f l o w l i n e s i m p i n g i n g o b l i q u e l y on one f l a n k o f a l i n e a r dune would t h e n produce a sinuous c r e s t . Mainquet (1982, p e r s . comm.) showed me a e r i a l photographs d e m o n s t r a t i n g t h i s h y p o t h e s i s , i n which n e g a t i v e f l o w n e a r t h e tapered l e e s e c t o r s o f l o n g s e i f dunes had segmented t h e dunes, w i t h e s s e n t i a l l y p a r a l l e l l i n e s c u t t i n g o b l i q u e l y a c r o s s t h e dune c r e s t , f i r s t making a number o f s a l i e n t s and f i n a l l y a t e r m i n a l c h a i n o f s e v e r a l s h o r t dunes. The p a r a l l e l i s m o f t h e l i n e s o f n e q a t i v e f l o w showed c l e a r l y t h a t a s i n g l e wind d i r e c t i o n and i t s opposing n e g a t i v e f l o w must have been r e s p o n s i b l e f o r t h e segmentation. The o t h e r concept f o r p r o d u c t i o n o f s i n u o s i t y o f t h e s e dunes i n v o l v e s a l t e r n a t i n g wind d i r e c t i o n s . I n t h i s case, t h e l i n e s o f n e q a t i v e f l o w would n o t be p a r a l l e l u n l e s s t h e two winds were b l o w i n g i n o p p o s i t e d i r e c t i o n s . There i s s t i l l another p o s s i b i l i t y t h a t i s r e l a t e d t o t h e i n t e r d i g i t a t i o n along t h e crest l i n e . I f t h e r i s i n g columns o f secondary f l o w a r e n o t i n phase o r t h e offsweep o f t h e main s t r e a m l i n e s i s n o t symmetrical t o t h e k e e l , t h m i n t e r d i q i t a t i o n and u l t i m a t e l y s i n u o s i t y would r e s u l t . The o r i g i n o f s i n u o s i t y i n s e i f dunes has l o n q been argued f r o m t h e s t a n d p o i n t o f t h e w i n d reqime, b u t p r o b a b l y n o t from t h e p a t t e r n s o f c r e s t a l offsweeps. Since offsweep i s so v e r y i m p o r t a n t i n t h e shaping o f snow d r i f t s and r o c k f e a t u r e s ,
i t mus: a l s o be i m p o r t a n t i n dune
development. J u s t beyond t h e end o f t h e runway a t C a i r o A i r p o r t , Eqypt, I noted a number o f segmented s e i f dunes. I n t h i s area o f Egypt, where winds a r e v a r i a b l e , a s t u d y m i q h t s u p p o r t an a l t e r n a t i n q wind e x p l a n a t i o n . S i n q l e causes may n o t be a p p l i c a b l e , f o r t h e a p p l i c a t i o n o f aerodynamics t o a e o l i a n problems shows t h a t f a c t o r s n o t p r e v i o u s l y c o n s i d e r e d a r e o f q r e a t importance. Shape d i f f e r e n c es a l o n e i n t r o d u c e g r e a t v e r s a t i l i t y i n e n t i r e f l o w systems. THE STREAMLINED YARDANG Yardangs commonly o c c u r as s e t s o f p a r a l l e l r i d g e s i n l a r g e and o f t e n v e r y l o n g yardang f i e l d s i n zones o f h y p e r a r i d i t y t h a t a r e c o n t r o l l e d by wind d i r e c t i o n ; some f i e l d s b e i n g many t e n s o f k i l o m e t r e s l o n g . Yardangs a r e u s u a l l y a t l e a s t t h r e e t i m e s as l o n q as wide (McCauley e t a l . ,
1977). I n d i v i d u a l s range
from f e a t u r e s a m e t r e o r so l o n g , t o g r e a t s t r e a m l i n e d h i l l s s e v e r a l k i l o m e t r e s i n length. The sandstone yardang,shown
i n p r o f i l e on f i g u r e 6, i s about 1.7 metres h i g h
and 4.4 metres l o n q . I t i s l o c a t e d i n t h e u n i d i r e c t i o n a l wind regime o f southwestern Egypt, a r e g i o n o f such h y p e r a r i d i t y t h a t r a i n f a l l s o n l y a few t i m e s each c e n t u r y . For i n s t a n c e , i n 1978, t h e r e s e a r c h group t h a t v i s i t e d an abandon-
233 ed caiiip o f R. A. B a g n o l d , d a t i n g f r o m 1939, f o u n d t h a t t i r e t r a c k s l e f t by h i s v e h i c l e s were s t i l l f r e s h .
/ R
MONODIRETIONAL
0
SHALLOW VORTEX P I T S
F i q . 6. A - Small y a r d a n g and v e n t i f a c t s n e a r B l a c k H i l l , h y p e r a r i d s o u t h w e s t e r n
d e s e r t o f E q y p t . S t r e a m l i n e d f o r m due t o u n i d i r e c t i o n a l w i n d f r o n t h e r i q h t ( n o r t h ) . ( P h o t o b y J . F. PlcCauley, w i t h p e r m i s s i o n , c o u r t e s y o f C a r o l S. B r e e d ) . B - I n f e r r e d wind f l o w p a t t e r n s around t h e s t r e a m l i n e d yardang. The n o r t h w a r d f a c i n q c o n v e x w i n d i m p a c t s u r f a c e a t t h e r i g h t end of t h i s yardang i s e s s e n t i a l l y v e r t i c a l , t h o u q h v e r y s l i g h t l y u n d e r c u t i n t h e l o w e r p a r t . Above t h e v e r t i c a l s e c t i o n o n t h e w i n d w a r d s i d e , t h e r e i s a f l a t t e n e d ,
2 34
i n c l i n e d facet,
a f e a t u r e n o t commonly found on yardanqs. Breed (pers.comm.,
1982) r e p o r t e d t h a t t h e whole c r e s t a l zone was f l a t t e n e d , b u t t h a t n e i g h b o u r i n g yardangs were c r e s t a l l y keeled. From t h e apex, t h e c r e s t l i n e drops away leeward i n t h r e e stages, s t e e p e n i n q i n t h e m i d - s e c t i o n , w i t h a s a l i e n t a n g l e above and a r e e n t r a n t a n g l e below i t s s h o r t m i d s e c t i o n . A sand streamer covers t h e termi n a l s e c t i o n o f t h e low, t a p e r e d l e e end. The f i g u r e d s u r f a c e ( l e f t s i d e ) i s concave i n i t s m i d zone and bears s e v e r a l l a r q e , s h a l l o w p i t s a t t h e t o p o f t h e c o n c a v i t y ; and, above t h e p i t s , s e v e r a l l o n g v e r t i c a l l y o r i e n t a t e d f l u t e s whose bounding r i d g e s t e n d t o converqe towards t h e c r e s t l i n e . S h o r t , v e r t i c a l l y o r i e n t a t e d f l u t e s o c c u r a l o n g t h e b o t t o m m a r g i n o f t h e f a c e . Longer v e r t i c a l l y o r i e n t a t e d f l u t e s o c c u r i n t h e l o w e r p a r t o f t h e head zone a l o n q w i t h s e v e r a l wide, s h a l l o w v o r t e x p i t s on t h e l o w e r and m i d s e c t i o n s o f t h e impact s u r f a c e . The head zone i s s l i q h t l y w i d e r t h a n t h e main body, making a d i s t i n c t s u r f a c e o f d i s c o n t i n u i t y between t h e head zone and l e f t s i d e t h a t i s o r i e n t a t e d o b l i q u e l y f r o m about t h e m i d d l e o f t h e basal m a r g i n up t o t h e base o f t h e f l a t f a c e t ,
c a u s i n g t h i s s i d e o f t h e head zone t o appear t r i a n g u l a r i n t h e i l l u s t r a t i o n . One o f t h e s h a r p e r c o l o u r s l i d e s o f t h i s yardang showed s e t s o f f l u t e s on t h e f l a t f a c e t , separated by a p i t t e d d e p r e s s i o n a c r o s s t h e l o w e r p a r t o f t h e f a c e t ( f i g . 68).
Windward o f t h e yardang, t h e r e i s a s h o r t s h a l l o w moat bordered by a d e p o s i t o f sand, on which Breed i s s t a n d i n g i n f i q u r e 6A. P r i o r t o t h i s photograph, t h e d e p o s i t formed a f l u t e d c o r n u c o p i a w i t h a concave f a c e m o d i f i e d by a c e n t r a l r i d q e ( f i g . 6B). Windward o f t h i s e l e v a t e d d e p o s i t i s a l a r q e f l a t a u r e o l e , t h e c e n t r e o f w h i c h i s t h i n l y covered w i t h rebounded sand. The a d j a c e n t d e s e r t f l o o r i s covered w i t h g r a n u l e s , b u t i n t h e d a r k e r b o r d e r o f t h e a u r e o l e , t h e g r a n u l e c o v e r seems t o t h i c k e n . T h i s b o r d e r zone marks t h e b e g i n n i n g o f t h e d i v i d i n g o f t h e s t r e a m l i n e s i n t h e i n i t i a l p a r t o f t h i s p a r t i c u l a r aerodynamic system. F u l l c l o s u r e o f t h i s system i s l i k e l y t o be some d i s t a n c e t o t h e l e f t o f t h e i l l u s t r a t i o n . I n t h e Kharga a r e a o f Egypt, i n t h e l e e o f yardangs o f a b o u t t h i s same s i z e , t h e qround i s swept f r e e o f g r a n u l e s f o r about 20 metres. Hence, t h e aerodynamic system c r e a t e d by a s a l i e n t f e a t u r e i s c o n s i d e r a b l y l o n q e r t h a n t h e feature i t s e l f . G r a i n impact, u p d r a f t , and n e g a t i v e f l o w r e t u r n i n g f r o m leeward, share i n c r e a t i n g t h e steep windward f a c e : b u t g r a i n impact has l i t t l e e f f e c t elsewhere
on t h i s yardang s i n c e t h e head zone s h i e l d s t h e r e s t o f t h e s u r f a c e , and f l o w l i n e s and small s c a l e n o r m a l - a x i s v o r t i c e s b r i n g g r a i n s and f i n e r suspensates t o s u r f a c e c o n t a c t s i n a p e r s i s t e n t b r u s h i n g mode a l o n g s p e c i f i c t r e n d s o r p r e s s u r e g r a d i e n t s o r i e n t a t e d t o l o c i o f l o w p r e s s u r e b o t h on t h e head zone and over t h e r e s t o f t h e feature.
235 The c l o s u r e o f t h e s t r e a m l i n e s o v e r t h e c r e s t l i n e i s r e s p o n s i b l e f o r t h e development of s h o r t u i d e f l u t e s a l o n a t h e m a r q i n o f t h e m i d and l o w e r p a r t o f the c r e s t l i n e and f o r a t t r a c t i n q l o n g secondary f l o w l i n e s near t h e apex, where d e t a i l e d photographs r e v e a l e d l o n q f l u t e s and sharp r i d q e s c o n v e r q i n g upwards a q a i n s t t h e c r e s t l i n e . Much o f t h e e r o s i o n o f t h e l e f t f a c e may be caused by n e g a t i v e flow, moving t o windward and upward; by secondary f l o w moving toward basal and t o p f l o w l i n e s ; and by more o r l e s s s t a t i c c e n t r e s o f v o r t i c i t y . A l s o , s u b s i d i a r y f l o w i s p a r t i a l l y r e s p o n s i b l e f o r e r o d i n q t h e t o p f a c e t , as evidenced by t h e f l u t i n g . R i s i n g n e g a t i v e f l o w and d e c l i n i n g p o s i t i v e f l o w share i n erodi n q t h e l o n q f l a t t e n e d c r e s t a l s u r f a c e . From t h e r e e n t r a n t a n g l e on t h e c r e s t a l l i n e leeward, some o f t h e s t r e a m l i n e s a l s o become i n t e r f a c i a l i n escaping o v e r the d e c l i n i n g l e e s e c t i o n . F o l l o w i n g impact, l o w p r e s s u r e develops on t h e windward s u r f a c e and b r i n q s a g e n e r a l p a t t e r n o f n e g a t i v e f l o w back t o windward f r o m t h e zone i n t h e l e e o f t h e d e c l i n i n q c r e s t l i n e . L i k e w i s e , u p d r a f t a t t h e base
o f t h e f l a t f a c e t creates low pressure,brinqing l i n e s o f negative f l o w both downward on t h i s f a c e t and a l s o o b l i q u e l y upward o v e r t h e broad c e n t r a l zone o f the l e f t face. I n t e r s e c t i o n o f t h e s e m a j o r l i n e s o f n e g a t i v e f l o w c r o s s i n g t h i s f a c e undoubt e d l y p l a y s a r o l e i n c r e a t i n g t h e v o r t i c i t y t h a t causes t h e l e f t f a c e t o become p i t t e d and concave. Another f a c t o r , however, i s most l i k e l y i n v o l v e d . I n t h e n o r t h e r n hemisphere i r i p i n g i n q winds t e n d t o d e f l e c t s l i q h t l y more c o u n t e r c l o c k wise t h a n c l o c k w i s e around o b j e c t s , commonly c a u s i n q s t r o n q e r u n d e r c u t t i n g o f yardangs and v e n t i f a c t s t o t h e r i g h t t h a n t o t h e l e f t o f t h e i r impact zones. When t h e s t r o n q e r d e f l e c t i o n r i s e s o v e r t h e n o r m a l l y convex windward end o f t h e r i g h t face, i t c r o s s e s t h e c r e s t l i n e o b l i q u e l y . T h i s b r i n g s a p a t t e r n o f n e q a t i v e f l o w o b l i q u e l y up t h e l e f t f a c e f r o m t h e l e f t s i d e o f t h e l e e end t o j o i n w i t h t h e o v e r r i d i n g c r e s t a l offsweep. T h i s n e g a t i v e f l o w produces f l u t e s , v o r t e x p i t s and c o n c a v i t i e s on t h e l e f t f a c e s o f v e n t i f a c t s and yardangs. On t h e l i m e stone yardangs o f llaqub Asyot Plateau, e a s t o f Kharga, Eqypt, i t erodes huge basal m a r g i n a l c a v i t a t i o n s on t h e s o u t h e a s t s i d e s o f t h e l e e t e r m i n i . T h i s i s a most s t r i k i n g example o f t h e e x p r e s s i o n o f t h e o b l i q u e l i n e o f concavo-convex-
i t y . Also, most of t h e l a k e bed yardangs t h a t I saw were e i t h e r s t e e p l y convex
or v e r t i c a l on t h e r i g h t and e i t h e r concave o r g e n t l y i n c l i n e d on t h e l e f t . The axes o f t h e yardangs seemed t o a l i g n p r e t t y w e l l w i t h t h e wind d i r e c t i o n . Hence, b o t h t h e b a s i c wind d i r e c t i o n and t h e d e f l e c t e d wind p a t t e r n r e q i s t e r e r o s i o n p a t t e r n s on t h e s e f e a t u r e s . Flainguet (1978) c l a i m s t h a t t h i s d e x t r a l d e f l e c t i o n a l i g n s l i n e a r dunes o b l i q u e t o t h e wind, b u t a p p a r e n t l y on yardangs and v e n t i f a c t s , i t i s s u b o r d i n a t e t o t h e b a s i c wind d i r e c t i o n . The moat on t h e windward end ( f i g . 6A) i s s h o r t and s h a l l o w and may n o t have a c e n t r a l k e e l e d spetum. The septum i s a common f e a t u r e o f moats where n e g a t i v e
236 f l o w l i n e s meet and r i s e a l o n g t h e impact face. Long, deep moats o f t e n have a k e e l e d septum 1 m e t r e h i q h o r more a t t h e windward base, d i v i d i n g t h e moat i n t o r i g h t and l e f t t r o u g h s . Yardangs o f t h e Kharga a r e a a r e composed o f u n c o n s o l i d a t e d s i l t s d e p o s i t e d i n ponds d u r i n g N e o l i t h i c and Roman t i m e s . D e s p i t e t h e weakness of t h e s e m a t e r i a l s , septa a r e n o t o n l y h i g h b u t o c c a s i o n a l l y e x t e n d up t o t h e apex of t h e yardangs as a windward k e e l . When I s a t down i n t h e shaded r i g h t t r o u g h o f one o f t h e s e l a k e bed yardangs, I n o t e d t h a t warm incoming wiqd was s t r i k i n g me on t h e r i g h t , b u t t h a t c o o l n e g a t i v e f l o w was s t r i k i n g my l e f t arm. Hedin (1903) s t a t e d t h a t around t h e Lop Nor yardanqs o f t h e T a k l a Makhan d e s e r t t h e r e was no s h e l t e r f r o m t h e b i t i n g c o l d winds, and t h a t i n f u r r o w s 4 metres o r more deep w i n d v e l o c i t y seemed even h i g h e r t h a n eisewhere around t h e yardangs ( c i t e d i n McCauley e t a l . ,
1977, p. 1 5 ) . Hence, i t i s c l e a r t h a t w i t h i n
f l u t e s and moats, v e n t u r i s develop t h a t must a c c e l e r a t e e r o s i o n a l o n q t h e l i n e of channeled f l o w . Hedin (1903) a l s o mentioned t l i e f i e r c e eddy e f f e c t f r o m b o t h s i d e s o f t h e l e e end o f a yardang. The same q e n e r a l e f f e c t , b u t t o a m i l d e r deqree, r e s u l t s from t h e m e e t i n q o f t h e l i n e s o f n e g a t i v e f l o w r e t u r n i n g t o t h e windward end o f t h e yardang; b u t here, on some shapes, t h e r e nay be a steep face up which t o d e f l e c t . The m e e t i n q o f t h e s e l i n e s o f n e g a t i v e f l o w above a keeled septum a t t h e windward end e s t a b l i s h e s a p a t t e r n o f simultaneous offsweep o f t h e n e g a t i v e f l o w t h a t c o n t i n u e s up t h e m i d s e c t i o n o f t h e windward f a c e t o f i n a l escape a t t h e apex, c r e a t i n g a windward k e e l between t h e c o n v e r g i n g f l o w l i n e s and a t t h e same t i m e a f f o r d i n g some degree o f p r o t e c t i o n o f t h e windward s u r f a c e f r o m s a n d b l a s t . These k e e l s a l s o g i v e us t h e c l u e t o t h e o r i g i n o f windward k e e l s on t h e g r e a t d o u b l y t e r m i n a t e d yardangs o f t h e p l a t e a u e a s t o f Kharga. I t i s i m p o r t a n t t o n o t e t h a t around t h e l a k e bed yardangs o f t h e Kharga Oasis, t h e r e i s no s h o r t a g e o f sand. The yardangs a r e among, and o f t e n o v e r r i d d e n by, barchan dunes
-
b u t s t i l l m a i n t a i n steep windward f a c e s .
When e v e n t u a l l y a d i v i d i n g septum i n t h e moat o f a l a k e bed yardang i s d e s t r oyed, t h e two l i n e s o f n e g a t i v e f l o w c r e a t e s t r o n g v o r t i c i t v i n t h e i r m e e t i n g t h a t u n d e r c u t s t h e windward end o f t h e yardang, commonly somewhat more t o r i g h t t h a n t o l e f t d u r i n g t h e i n i t i a l stages; b u t , i n t i m e , t h e e n t i r e windward end becomes d e e p l y undermined, p r o d u c i n g a p r o f i l e s i m i l a r t o t h a t o f i c e z a s t r u g i (fig.
7 ) . FlcCauley e t a l . (1979, p. 8224) f i g u r e a group o f small q u a r t z i t e
yardangs on which t h e head zones had become so g r e a t l y u n d e r c u t t h a t t h e y had t o p p l e d . Because o f t h e complex p r e s s u r e r e l a t i o n s h i p s w i t h i n a v o r t e x , v o r a c i o u s e r o s i o n can be o c c u r i n g i n t h e h i g h e r p r e s s u r e p e r i m e t e r w h i l e t h e l o w p r e s s u r e i n t h e c e n t r a l zone may a c t u a l l y r e t a r d e r o s i o n by removing t o o l s . Hence, s a l i e n t s may remain a t p i t c e n t r e s , and t a l l columns may s t a n d on d e s e r t f l o o r s . I n t h e u n d e r c u t zones of t h e l a k e bed yardangs o f t h e Kharga area, v o r t i c i t y has sometimes p r e s e r v e d remarkable p r o t r u s i o n s j u t t i n g t o windward, c a l l e d f i n g e r s , and composed o f d e l i c a t e l y c o h e r e n t p a r t i c l e s . These f i n g e r s a r e o f t e n
237
Fiq. 7.
P r o f i l e view o f l a k e bed yardang w i t h t h e f o r m o f an i c e z a s t r u g i , where n e g a t i v e f l o w has u n d e r c u t t h e windward s i d e and v o r t i c i t y has preserved f i n g e r s extendinq t o windmrd. Kharqa, Eqypt.
no more t h a n a few m i l l i m e t r e s wide, b u t many c e n t i m e t r e s lonq,and a r e s t r u c t u r e s t h a t can n o t w i t h s t a n d t h e m e r e s t t o u c h o f a human hand, y e t a r e o r i e n t a t e d i n t o the wind ( f i g . 7 ) . Seeing these f e a t u r e s on a windward s u r f a c e o f h i g h l y f r i a b l e m a t e r i a l , and l i k e w i s e t h e thousands o f d e l i c a t e f i n g e r s p r o t r u d i n q t o windward on l i m e s t o n e v e n t i f a c t s o f t h e r e g i o n around Kharga, causes one t o wonder what the r o l e o f s a n d b l a s t i n g r e a l l y i s . The presence o f f i n g e r s must s u r e l y denote r e t a r d a t i o n o f e r o s i o n a t c e n t r e s o f v o r t i c i t y , p r o b a b l y a l o n g l i n e s where negative f l o w j o i n s p o s i t i v e f l o w . L i t h o l o g y and t i m e a r e two o t h e r i m p o r t a n t f a c t o r s i n shaping yardangs. The f r i a b l e m a t e r i a l s and t h e coarse g r a i n e d r o c k s i n t h e m o n o d i r e c t i o n a l wind r e q ime, whether sandstone o r weak l a k e beds, t e n d t o erode w i t h v e r t i c a l windward
faces. Many o f t h e l a r g e Nubian Sandstone yardangs s o u t h o f t h e Abu T a r t o u r Plateau i n t h e c e n t r a l western d e s e r t o f Egypt, a r e steep a t b o t h ends and may be h i g h e r t o l e e w a r d t h a n t o windward because o f a r e g i o n a l n o r t h w a r d d i p , t h a t
i s t o windward, and a r a t h e r r e s i s t e n t bed t h a t commonly forms a n o r t h w a r d d e c l i n i n g cap. On t h e Naqub Asyot P l a t e a u , e a s t o f Kharga, t h e e x c e e d i n g l y f i n e - g r a i n ed Eocene Limestone has been c a r v e d i n t o thousands o f d o u b l y t e r m i n a t e d , o r b o a t shaped, yardangs. These yardangs have been f o r m i n g f o r many thousands o f y e a r s . Some have steep windward f a c e s , b u t many have more g e n t l y i n c l i n e d k e e l e d and f l u t e d windward f a c e s . One t y p e i s h i q h e r t o t h e l e e t h a n t o windward and i s almost i n v a r i a b l y d e e p l y u n d e r c u t on t h e l e e s i d e , more t o t h e southeast than the southwest l e e s i d e . T h i s f o r m may have v a s t numbers o f v o r t e x p i t s on t h e l e f t s i d e s , b u t none on t h e r i g h t s i d e s . B o t h s i d e s , however, commonly bear
2 38 remarkable coarse, h i q h l y b u r n i s h e d f l u t e s ( f i o . 8 ) .
F i g . 8. Several small l i m e s t o n e yardanqs w i t h l a r g e basal c o n c a v i t i e s i n t h e i r l e e , some b e a r i n g f l u t e s and v o r t e x p i t s . N o n o d i r e c t i o n a l wind from t h e n o r t h ( r i g h t s i d e ) . Naqub Asyot P l a t e a u , e a s t o f Kharqa, Eqypt. Since t h e yardang i s p a r t o f t h e bedrock, i t does n o t have t h e p o t e n t i a l f o r s h i f t i n g around, a s does a v e n t i f a c t . Since most g e o l o g i s t s c o n s i d e r t h a t t h e l o w f a c e s on v e n t i f a c t s r e p r e s e n t t h e windward s i d e s and t h a t i f t h e s e l o w faces a r e on t h e l e e , t h e n t h e v e n t i f a c t has been r e o r i e n t a t e d , t h e small yardang ( f i g . 6A) i s s i g n i f i c a n t t o t h e s t u d y o f v e n t i f a c t development, p a r t i c u l a r l y i n having a l o w l e e and h i g h steep windward f a c e - f o r so do most o f t h e v e n t i f a c t s associated w i t h it. PATTERNS OF DEVELOPMENT OF BRAZILNUT AND TRIQUETROUS VEIITIFACTS Whereas yardangs a r e c o n s i d e r e d t o have l o n g axes a l i g n e d w i t h t h e wind, t h i s i s n o t always t r u e o f s t r e a m l i n e d v e n t i f a c t s . While, i n g e n e r a l , v e n t i f a c t s remain s t a t i o n a r y t h r o u g h o u t much o f t h e i r h i s t o r y , t h e r e a r e cases where t h e y move, e s p e c i a l l y when reduced i n s i z e . I f t h e l o n q a x i s o f a r o c k i s t r a n s v e r s e t o t h e wind, offsweep w i l l be l a t e r a l , and offsweep k e e l s w i l l t h e n be r i g h t and l e f t i n s t e a d o f windward and l e e . H o s t v e n t i f a c t s f o r m i n b i d i r e c t i o n a l and m u l t i d i r e c t i o n a l wind regimes: t h o s e t h a t f o r m i n m o n o d i r e c t i o n a l regimes a r e s i m i l a r , but d r e i kanter are rare. B r a z i l n u t forms a r e s i m i l a r t o t r i q u e t r o u s forms b u t t e n d t o be b u l k i e r a t the windward end and t o have f o u r o r more f a c e s i n s t e a d o f t h e t h r e e t h a t c h a r a c t e r i z e t h e t r i q u e t r o u s forms. The v e n t i f a c t s used i n t h e s t u d y o f t h e s e two general forms range i n age f r o m Precambrian (Gowganda T i l l i t e ) t o P l e i s t o c e n e and modern
239 v e n t i f a c t s . The N i c h i g a n v e n t i f a c t s were l a r g e l y f r o m l a g g r a v e l s i n t h e i n t e r dune c o r r i d o r s a l o n g Lake M i c h i g a n , and f r o m m o r a i n a l g r a v e l s about 150 metres above Lake M i c h i g a n on S l e e p i n g Bear Moraine, Leelanau County, Michigan. T h i s l a t t e r area has a m u l t i d i r e c t i o n a l wind regime, b u t w i t h l o c a l l y dominant n o r t h west and southsouthwest winds, as i n t h e c l i f f c r e s t s e t t i n q i n which t h e B r a z i l nut ( f i g . 9A) developed. From t h e s t a n d p o i n t o f t h e a x i a l l i n e o f t h e v e n t i f a c t , both o f t h e s e winds impinged o b l i q u e l y on t h e v e n t i f a c t . Hence, t h e f i n e keel i s the r e s u l t a n t o f t h e s e two wind d i r e c t i o n s and t h e i r d e f l e c t i o n a l o n g i t ( f i g .
9B). The d e f l e c t i o n o f t h e k e e l a t t h e l e f t i s due t o n e g a t i v e f l o w up t h e smoothe r surface i n response t o t h e n o r t h w e s t wind r i s i n q o v e r t h e apex. The p r o f i l e
view i s t h a t o f t h e z a s t r u q a form, w i t h a v e r y l a r g e smooth u n d e r c u t f a c e t on t h e windward end. T h i s f a c e t i s s e p a r a t e d f r o m t h e f l a t base under t h e t a p e r e d end by a sharp margin, e x t e n d i n g a c r o s s t h e u n d e r s i d e normal t o t h e c r e s t a l k e e l . Three s h a l l o w , smooth-surfaced round v o r t e x p i t s o c c u r on t h e t a p e r e d base i n t h e offsweep zone. Symmetry among v e n t i f a c t s i s e x t r e m e l y r a r e , even i n m o n o d i r e c t i o n a l regimes. One reason f o r t h i s i s t h a t t h e o r i g i n a l forms a r e r a r e l y symmetrical. Also, i n m u l t i d i r e c t i o n a l wind regimes, one wind m o d i f i e s t h e work o f another. I n e i t h e r m o n o d i r e c t i o n a l o r m u l t i d i r e c t i o n a l wind regimes, dominance o f a d e x t r a l o r s i n i s t r a l wind d e f l e c t i o n t o one s i d e o f t h e o t h e r o f t h e impact f a c e c r e a t e s a l i n e o f p o s i t i v e f l o w opposed by n e g a t i v e f l o w , c r o s s i n g t h e c r e s t o b l i q u e l y and causing c o n v e x i t y t o develop t o windward and c o n c a v i t y t o l e e o f t h e c r e s t . Despite t h e s e s e v e r a l i n f l u e n c e s , B r a z i l n u t and t r i q u e t r o u s forms o c c a s i o n a l l y develop w i t h a f a i r degree o f symmetry. The t r i q u e t r o u s f o r m has two s i d e s and a f l a t t e n e d basal s u r f a c e . I n t h e l a g gravels, f o r m e r beach pebbles, i n t h e dune c o r r i d o r s a l o n g Lake Michigan, e v e r y stage i n t h e h i s t o r y o f t r i q u e t r o u s forms i s r e p r e s e n t e d . F i g u r e 10A and 106 represent i n t e r m e d i a t e stages between a beach p e b b l e and a t r i q u e t r o u s f o r m ( f i g . 1OC): however, i n t h e l a t t e r case t h e winds were o p p o s i t e l y o r i e n t a t e d . The t r i q u e t r o u s f o r m develops i n s i t u , a f a r d i f f e r e n t h i s t o r y t h a n e n v i s i o n e d
f o r i t by o t h e r g e o l o g i s t s . The l a g g r a v e l s a f f o r d t h e f u l l spectrum o f v e n t i f a c t i o n f r o m beach pebbles t o t a t t e r e d w i n d - s h a t t e r e d fragments, i n a v a s t v a r i e t y
o f l i t h o l o g i e s , and a l s o t h e g r e a t advantage o f knowinq something o f t h e o r i g i n a l form o f t h e r o c k p r i o r t o v e n t i f a c t i o n . t i e n c e a t r i q u e t r o u s form i s l i k e l y t o s t a r t as a p l a t y beach p e b b l e w i t h convex s u r f a c e s and steep rounded s i d e s t h a t p e r m i t a submarginal f l o w ( a v e n t u r i ) t o develop around i t s base t h a t p u l l s i n secondary f l o w f r o m t h e s i d e s above and f r o m t h e p e r i p h e r a l base, f l u t i n q t h e sides and i n i t i a t i n g t h e f l a t t e n i n g o f t h e p e r i p h e r y o f t h e base. Since t h e s i d e s are steep, w i n d c r e a t e s an u p d r a f t . \{hen t h i s a i r f a l l s on t h e rounded t o p i t c r e a t e s v o r t i c e s t h a t e v e n t u a l l y erode many p i t s
-
u s u a l l y broad and shallow,
though o c c a s i o n a l l y f u n n e l shaped. A t t h e base, i n t h e l e e o f t h e h i g h e r v e l o c i t y
240
MAIN STREAMLINES NEGATIVE FLOW SECONDARY FLOW
F i g . 9. B r a z i l n u t v e n t i f a c t e r o d e d i n a m u l t i d i r e c t i o n a l w i n d r e g i m e , o f w h i c h t h e n o r t h w e s t wind i s t h e dominant o f t h e two p r i n c i p a l wind d i r e c t i o n s . S l e e p i n g B e a r t l o r a i n e , L e e l a n a u C o u n t y , M i c h i g a n . V e n t i f a c t i s 8.4 cm. long. A - surface view. B - diagram o f f l o w patterns. wind, n e q a t i v e f l o w b e g i n s t o work i t s way u n d e r t h e v e n t i f a c t , c r e a t i n q v o r t i c i t y and e r o d i n g a l e e b a s a l c o n c a v i t y . As t h e v e n t i f a c t i o n p r o c e s s c o n t i n u e s ,
t h i s c o n c a v i t y m i g r a t e s t o w i n d u a r d . The v o r t e x p i t s on t o p expand and c r o w d each o t h e r u n t i l r i m s become hexagonal i n shape. S i d e s b e g i n t o f l a t t e n u n d e r t h e combined i n f l u e n c e o f t h e s u b m a r g i n a l v e n t u r i a n d t h e u p d r a f t and l a t e r a l d e f l e c t i o n : t h e s e a c t i o n s b e g i n t o c r e a t e a s h a r p m a r q i n between s i d e s and t o p which p e r m i t a c c e l e r a t e d f l o w i n t h e l e e w a r d d i r e c t i o n and a t t e n u a t i o n o f t h e o f f s w e e p .
“!s t h e l o w p r e s s u r e a i r escapes, i t p u l l s i n t e r f a c i a l f l o w s f r o m b o t h t h e s i d e
241 and t h e t o p a c r o s s t h e s h a r p m a r g i n . T h i s n o t o n l y c o m p l e t e s t h e f l a t t e n i n g o f t h e t o p s u r f a c e b u t c r e a t e s f l u t i n g . F l u t i n g has a b e t t e r chance o f s u r v i v a l on t h e t o p s u r f a c e t h a n on t h e s i d e s , b u t away f r o m l o c i o f s t r o n g g r a i n i m p a c t , l a t e r a l s u r f a c e s a r e o f t e n w e l l f l u t e d . Many o f t h e t o p s u r f a c e p i t s a r e c o n v e r ted t o f l u t e s , b u t those t h a t remain o f t e n share r i m s w i t h f l u t e s . Both rin type and s u r f a c e c h a r a c t e r o f t h e p i t s a n d f l u t e s a r e s i m i l a r . B o t h f e a t u r e s b e a r numerous m i c r o s c o p i c h e l i c a l and r a d i a l s c o r e s . I t was t h e d i s c o v e r y o f t h e s e scores t h a t showed t h e common o r i g i n , b y v o r t i c i t y , o f f l u t e s and p i t s and i n i t iated t h e s t u d i e s t h a t demonstrated t h e aerodynamic e r o s i o n o f v e n t i f a c t s . I n t h e f l a t t e n i n g o f t h e t o p and s i d e s , t h e n a r r o w i n o p r o c e s s b e q i n s , b e v e l i n g t h e f l a t tened s i d e s and c r e a t i n g t h e o f f s w e e p k e e l s . O f f s w e e p f r o m one s i d e o f a k e e l p u l l s a t r a i l i n g offsweep from t h e opposite side, sharpening t h e terminal keel ( f i q . 10A). The v e n t i f a c t i s now a t a n i n t e r m e d i a t e s t a q e between a beach p e b b l e and t h e t r i q u e t r o u s f o r m ( f i q . lOB)., t h a t i s , a t t h e f l a t t o p p e d s t a g e . I n t h e next p e r i o d o f i t s d e v e l o p m e n t , t h e f l a t t e n i n g o f t h e b a s a l s u r f a c e p r o g r e s s e s inward, d e s t r o y i n g a l l o r m o s t o f t h e b a s a l c o n c a v i t y . When t h e base i s f l a t , t h e b a s a l m a r g i n becomes a t h i n s h a r p l i n e , and t h e b a s a l f l o w now o p e r a t e s above t h e m a r g i n , a t t a c k i n g and d e s t r o y i n g t h e r e m a i n i n q f l u t e s and m a k i n g t h e s i d e s concavo-convex.
I n t h e narrowing process, t h e terminal keels a r e lengthening
u n t i l t h e y j o i n a s a s i n g l e c r e s t a l k e e l ( f i g . l O C ) , and e v e r y s t a g e o f t h i s process i s r e p r e s e n t e d i n t h e l a g g r a v e l s . The l a s t v e n t i f a c t , however, a t t a i n e d i t s shape i n a c h a n n e l where o p p o s i t e w i n d s i m p i n g e d o n t h e t e r m i n i .
F i g . 10. A - f l o w a r o u n d a f l a t - t o p p e d v e n t i f a c t , w i t h emphasis on n o r t h w e s t w a r d flow. B - Top v i e w o f f l a t - t o p p e d v e n t i f a c t , p r o t o t y p e o f t r i q u e t r o u s v e n t i f a c t , o r i e n t a t e d w i t h l o n g a x i s as r e s u l t a n t o f two wind d i r e c t i o n s . C - T r i q u e t r o u s v e n t i f a c t , e r o d e d i n a b i d i r e c t i o n a l w i n d system where t h e t w o o p p o s i t e w i n d s i m p i n g e d on t h e t e r m i n i . L e n g t h 3.2 cm.
242
The t r i q u e t r o u s form miqht seem an ideal shape t o gradually reduce in s i z e , b u t i t i s by no neans t h e end s t a q e , as some have considered. Because t h i s form usually lacks symmetry, concavity may increase on one f l a n k , s h i f t i n q t h e keel l a t e r a l l y u n t i l i t disappears, and t h e f l a t t e n i n o process s e t s in aqain. Appare n t l y , however, basal marginal o u t l i n e s of t h e t r i q u e t r o u s form a r e more likely t o be retained than keels. A t some l o c i on the llaqub Asyot Plateau, Eqypt, the qround i s paved with t h i n , f l a t wafers of limestone, of fusiform and other shapes, no more than about one centimetre long. There a r e , of course, millions of v e n t i f a c t forms. The best known a r e the rare named forms, souqht a f t e r by c o l l e c t o r s , and commonly used t o i l l u s t r a t e ventif a c t s . The t r i q u e t r o u s form i s a c t u a l l y q u i t e r a r e , and takes a long time t o develop. Hence, when i t occurs in f l i n t , a s i t does on t h e fiaqub Asyot Plateau, i t qives us a perspective on t h e lonq period o f a r i d i t y in t h i s part of the world. I n l e s s a r i d regions, f l i n t v e n t i f a c t s a r e so r a r e t h a t I had never seen one: however, t h e Eqyptian d e s e r t i s paved with them. I f wind can shape touqh f l i n t t o a streamlined form, and c r e a t e vortex p i t s and f l u t e s o n t h e i r surfaces, then we do not have t o c a l l upon weatherinq, s o l u t i o n , o r even streams, t o explain erosion o f g r a n i t e , q u a r t z i t e and o t h e r hard rocks in hyperarid environments such a s central Eqypt, where modern stream channels a r e absent, and ancient ones a r e progressively o b l i t e r a t e d by t h e wind t h e deeper t h e d e s e r t i s penetrated. Wind i s capable of finding and exploitinq t h e minutest surface irregularity, For instance, in southwest Eqypt, o r t h o q u a r t z i t e with s i l i c e o u s cement has been converted t o v e n t i f a c t s of spongy, l a c e - l i k e appearance; t h a t i s , bearinq thousa n d s of p i t s and c a v i t i e s where v o r t i c i t y has removed cement, worked inwards and eroded from t h e inside out by loosening sand g r a i n s , which then tumble in the voids t o enlarge t h e inner c a v i t i e s (McCauley e t a l . , 1979). THE NON-STREAMLINED FORMS PRODUCED PRIMARILY BY VORTICITY. This category of wind eroded f e a t u r e s includes vortex p i t s a n d numerous types of s a l i e n t s such a s knobs, domes, p i l l a r s , chimneys, odd-shaped hoodoo forms and conical h i l l s where t h e vortex i s s t a t i o n a r y o r recurrent a t the sane locus and often very l a r q e . Conical h i l l s i l l u s t r a t e t h i s method of erosion. They occur by t h e thousands in Eqypt, e s p e c i a l l y in Cretaceous sandstone, and a r e especially numerous in t h e Abu Simel area along t h e Nile v a l l e y , and in the central western d e s e r t south of t h e Abu Tartour Plateau in association with yardanqs. Saddles form in t h e c r e s t s of t h e yardangs, due t o offsweep and v o r t i c i t y and other facto r s . The intervening c r e s t s then become centres of v o r t i c i t y . The upward escaping a i r r e t a r d s erosion a t the top by removing t o o l s of erosion. This often preserves a small steep-sided cap or tapered cone, b u t the higher pressure a i r , engirdling the lower s e c t i o n , rapidly carves the yardang i n t o a s e r i e s of aligned conical h i l l s . The slopes of these h i l l s become covered with enormous t a l u s blocks
243
because t h e cyclinq wind removes t h e weaker beds, undermininq more massive beds above. Mass wasting then hastens t h e er o s i v e process and continues t o work on the f a l l e n blocks a s t h e wind p en et r at es alonq t he more f r i a b l e l a y e r s . I n time, the h i l l disappears. C i r cu l ar a r e a s bearing a few fraqments of sandstone sometime mark t h e l o c a t i o n of a former h i l l . Ile found in such remnants, and a t the bases of conical h i l l s l i k e t h a t in f i q u r e 1 1 A , blocks somwhat comparable t o McCauley
e t a l ' s (1979) q u a r t z i t e v e n t i f a c t s in t h a t erosion had proqresses f a r back into the rocks: b u t in t h i s cas e, i t \vas alonq beddino planes and did not involve the formation of many p i t s and p er f o r at i o n s . The marqins of the remaining beds, were however, incised i n t o wedqe-shaped f i n g e r s o r i e n t a t e d t o windward ( f i q . 1 1 B ) .
Fig. 11. A - Conical h i l l shaped by v o r t i c i t y and mass wasting. Central western d e s e r t of Egypt. B - Wind erosion along bedding p l an es , c ontributic q t o mass wasting. Central western d e s e r t of Eqypt.
244
COllCLlJS 1Ol.I.S I n windv r e q i o n s , a r i d o r o t h e r w i s e , a l l f e a t u r e s a r e s u b j e c t t o aerodynamic and v o r t i c i t y f o r c e s . I n an aerod.vnamic system, t h e main s t r e a m l i n e s d i v i d e t o windward o f a f e a t u r e , f l o w around i t and c l o s e somewhere 1teLiard. T h i s o f t e n permits l i t t l e contact o f the p r i n c i p a l f l o w p a t t e r n s w i t h the surface o f the f e a t u r e u n l e s s i t s l e e s e c t i o n i s t a p e r e d so as t o i n i t i a t e t h e c l o s u r e o f the s t r e a m l i n e s on t h e f e a t u r e i t s e l f and b r i n o t h e p o s i t i v e f l o w i n t o i n t i m a t e conta c t w i t h t h e s u r f a c e o f t h e l e e s e c t o r o f t h e f e a t u r e . It i s f o r t h i s reason t h a t e r o s i o n i s o f t e n a c c e l e r a t e d i n t h e l e e o f a f e a t u r e and t h a t i n a s i m p l e monod i r e c t i o n a l wind regime o r o p p o s i t e l y b l o w i n g b i d i r e c t i o n a l regime, s t r e a m l i n e d forms develop. I n most o t h e r s e c t o r s around t h e f e a t u r e , e r o s i o n o c c u r s a l o n q l i n e s o f subsi d i a r y f l o w where p r e s s u r e q r a d i e n t s , t r a n s p o r t i n q v o r t i c i t y and susDensates form r e g u l a r p a t t e r n s o f i n t e r f a c i a l f l o w t o some p o i n t o f l o w p r e s s u r e wind-escape such as a m a j o r f l o w l i n e , a k e e l , a v o r t e x p i t o r t h e impact s u r f a c e i t s e l f . Hence, much of t h e shaping and s c u l p t u r i n g o f f l u t e s , p i t s , k e e l s and o t h e r s t r u c t u r e s i s accomplished by t h e s u b s i d i a r y f l o w systems. Thus an aerodynamic system i s f a r more complex t h a n many i n v e s t i q a t o r s have r e a l i z e d and v a r i e s w i t h e v e r y a l t e r a t i o n i n shape and d e t a i l . T h e r e f o r e , i n a m u l t i d i r e c t i o n a l wind, a d i f f e r e n t aerodynamic system r e s u l t s on any one f e a t u r e w i t h e v e r y wind s h i f t , because t h e r e l a t i o n s h i p o f wind d i r e c t i o n t o f e a t u r e o r i e n t a t i o n i s a l t e r e d and
so a l t e r s t h e d i v i s i o n o f t h e s t r e a m l i n e s and t h e s u b s i d i a r y f l o w p a t t e r n s . T h i s amounts t o a shape change. I n a m o n o d i r e c t i o n a l wind t h e f u l l c o n c e n t r a t i o n o f f o r c e s i s m a i n t a i n e d i n a s i n g l e system o f f l o w t r e n d s , a l t e r i n g o n l y g r a d u a l l y as e r o s i o n proceeds. One p a t t e r n common i n a m o n o d i r e c t i o n a l system occurs when t h e c r e s t l i n e o f a f e a t u r e erodes i r r e g u l a r l y , t h u s c a u s i n q l o c i o f s t a t i o n a r y v o r t i c i t y t o develop and c r e a t e c e n t r e s o f v o r t i c i t y around s a l i e n t s . T h i s acticn t o g e t h e r w i t h undermining and mass w a s t i n g produces c o n i c a l h i l l s t h a t a r e so o f t e n a s s o c i a t e d w i t h s t r e a m l i n e d forms. I f aerodynamics and v o r t i c i t y can produce a yardang, t h e s e processes must
a l s o have some f u n c t i o n i n shaping dunes.
AC KNOWLEDGEMEIITS I w i s h t o t h a n k J . F . McCauley, Carol S. Breed, M.J.
G r o l i e r and Bahay I s s i w i
f o r a r r a n g i n g and a s s i s t i n g my s t u d y o f w i n d e r o s i o n i n Egypt, C . Breed f o r f u r n i s h i n g yardang i l l u s t r a t i o n s and R . V . r e a d i n g and c r i t i c i z i n g t h e m a n u s c r i p t .
D i e t r i c h and (1.F. S p l e t t s t o e s s e r f o r
245 REFERENCES
Hedin, Sven, 1903. C e n t r a l A s i a and T i b e t , v o l . 1 81 2. C h a r l e s S c r i b n e r & Sons, Mew York, 608 pp. McCauley, J.F., e t a l . , 1977. Yardangs o f Peru and o t h e r d e s e r t r e q i o n s . U.S. Geol. Survey, I n t e r a q e n c y r e p o r t , A s t r o g e o l o q y , no. 81. McCauley, J.F., e t a l . , 1979. P i t t e d and f l u t e d r o c k s i n t h e western d e s e r t o f Egypt: V i k i n q conparisons. J o u r . Geophys. Res., 84: 8222-8232. Mainguet, M.M., 1978. L ' E r q de F a c h i - B i l m a . C . R . R . S . , Mem. e t Doc., Nnuv. Ser., 18: 1-184. Whitney, M . I . , 1978. The r o l e o f v o r t i c i t y i n d e v e l o p i n g l i n e a t i o n by wind e r o s i o n . Geol. SOC. Amer. B u l l . , 89: 1-18. Whitney, M.I. and S p l e t t s t o e s s e r , J.F., 1982. V e n t i f a c t s and t h e i r f o r m a t i o n : Darwin H o u n t a i n s , A n t a r c t i c a . I n : D.H. Yaalon ( E d i t o r ) , A r i d i c s o i l s and geomorphic processes. Proc. I n t e r n a t . Conf. I n t e r n a t . SOC. S o i l Sci., Jerusalem, I s r a e l . Z e i t s c h r . Geomorph. Catena Suppl., 1 : 175-194.
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247
WIND TUNNEL MODELING OF ECHO AND CLIMBING DUliES H A I H TSOHR, Department o f Geography, Ben G u r i o n U n i v e r s i t y o f t h e liegev, Beer
Sheva 84 120, P.O.Box 653, I s r a e l . L i s t o f symbols Az A m p l i f i c a t i o n f a c t o r = "2/U1 d
Distance f r o m t h e c l i f f ( l e n g t h )
g
A c c e l e r a t i o n due t o g r a v i t y ( l e n g t h / t i m e 2 )
h
Height o f t h e c l i f f ( l e n g t h )
H
Height o f t h e dune ( l e n g t h )
L
C h a r a c t e r i s t i c h o r i z o n t a l dimension ( l e n g t h )
Re* Roughness Reynolds number
S
Point o f stagnation
Sh Height o f p o i n t o f s t a g n a t i o n ( l e n g t h )
U
Wind v e l o c i t y ( l e n q t h / t i m e )
U1 U n d i s t u r b e d w i n d v e l o c i t y ( l e n g t h / t i m e )
U2 D i s t u r b e d w i n d v e l o c i t y ( l e n g t h / t i m e ) U*
F r i c t i o n v e l o c i t y (length/time)
z0 1
Mean s i z e ( P h i )
+
Roughness l e n g t h o f t h e ground ( l e n g t h )
v+ Standard d e v i a t i o n ( P h i )
Sk$ Skewness ( P h i ) K
$
Kurtosis ( P h i )
v
2 Kinematic v i s c o c i t y ( l e n g t h / t i m e )
a
I n c l i n a t i o n o f t h e c l i f f (degrees) INTRODUCTION E o l i a n sand dunes o r i g i n a t e as a c c r e t i o n s o f sand on e x i s t i n g sand patches,
and may develop i n d e p e n d e n t l y o f f i x e d s u r f a c e f e a t u r e s (Bagnold, 1941). A p a r t
from these s e l f - a c c u m u l a t e d dunes, sand accumulations a r e f o u n d i n f r o n t o f , o r behind, t o p o g r a p h i c o b s t a c l e s such as c l i f f s , shrubs, b o u l d e r s , e t c . (Bagnold,
1941; Cooke and Warren, 1973; Elabbutt, 1977). The f o r m o f such dunes ensues f r o m t h e a c c e l e r a t i o n o f t h e w i n d around t h e o b s t r u c t i o n , t h e d e c e l e r a t i o n i n f r o n t
of, o r behind, i t , and t h e d e f o r m a t i o n o f t h e w i n d d i r e c t i o n around i t . According t o t h e i r l o c a t i o n r e l a t i v e t o t h e o b s t r u c t i o n s , t h e s e dunes can be divided i n t o l e e w a r d and windward a c c u m u l a t i o n s (Cooke and Warren, 1973; Mabbutt,
248 1977
.
I n t h e l e e of the o b s t acl e, sand accumulates t o form l e e dunes ( e . g . S m i t h ,
1963 f i g . 6 ; Smith, 1978, f i g s . 5-2a and 5 - 2 b ) . The c h a r a c t e r i s t i c flow-field t h a t causes l e e f e a t u r e s has been revealed by Greeley e t a l .
1 9 7 4 a ) in wind t u n n e l
s i m u la t i o n s . Wind tunnel and f i e l d work t o determine the a i r f
ON
Dast shrubs shows
s i m i l z r l e e f e a t u r e s (Hesp, 1981). Windward accumulations depend upon t h e slope of the obstac e . They can be subdivided i n t o echo dunes - s i n g l e sand ridges formed p a r a l l e l t o v e r t i c a l cl ffs (Clos-Arceduc, 1 9 6 9 ) , and f e a t u r e l e s s dunes t h a t climb ge ntle slopes (Evans 1962) k n o w n as climbing dunes ( f i g s . 1 and 2 ) .
-
Fig. 1. Echo dunes i n f r o n t of a cucsta c l i f f a t Paiute Point, northern Arizona. Note t h a t t h e s i d e s of t h e echo dune develop i n t o climbing dunes on t h e g e n t l e s l o p e s of t h e g u l l i e s .
Fig. 2 . Climbing dune on a g e n t l e slope in t h e Ploenkopi Pla te a u, northern Arizona.
249
Fig. 3. An echo dune i n f r o n t o f an abandoned r a i l w a y s t a t i o n , n o r t h e r n S i n a i . T h i s paper p r e s e n t s d a t a c o n c e r n i n g t h e f o r m a t i o n o f echo and c l i m b i n g dunes gathered d u r i n g w i n d t u n n e l t e s t s made on s e v e r a l s i m u l a t e d c l i f f s w i t h d i f f e r e n t i n c l i n a t i o n s . Three t y p e s o f measurement were t a k e n : f i r s t , modeling o f echo and c l i m b i n g dunes i n f r o n t o f s i m u l a t e d c l i f f s ; second, t r a c i n g o f eddies and wind turbulence i n f r o n t o f t h e s i m u l a t e d c l i f f s and o v e r t h e echo dune models, through a bubble g e n e r a t o r ; t h i r d , measurements o f w i n d v e l o c i t y i n f r o n t o f t h e s i m u l a t e d c l i f f s and o v e r t h e echo dune models, w i t h a h o t - w i r e anemometer. Echo and c l i m b i n g dunes a r e f o u n d i n f r o n t o f d e s e r t c l i f f s ( f i g s . 1 and 2 ) , coastal c l i f f s (Bowman, 1981, f i g . 2e) and a r t i f i c i a l c o n s t r u c t i o n s i n areas o f s h i f t i n g sands ( f i g . 3 ) . P r e v i o u s s t u d i e s o f echo and c l i m b i n g dunes Deposits caused d i r e c t l y by f i x e d o b s t r u c t i o n s a r e d e s c r i b e d i n Bagnold (1941) as sand shadows. A c c o r d i n g t o Bagnold (1941), sand g r a i n s t h a t s t r i k e a v e r t i c a l
obstacle rebound o f f i t and come t o r e s t i n t h e s t a g n a n t a i r b e f o r e t h e o b s t a c l e , thus f o r m i n g a heap. More sand s l i d e s down t h e s l o p e s and j o i n s sand streams passing a l o n g t h e s i d e s o f t h e o b s t a c l e . A c c o r d i n g t o Clos-Arceduc (1969) and Embleton e t a1
.
(1979), echo dunes a r e generated by a s t a n d i n g v o r t e x o f a i r
created beneath t h e f l o w up and o v e r an escarpment. D e s c r i p t i o n s o f c l i m b i n g dunes are g i v e n by Smith (1954), Evans (1962) and G o l d s m i t h (1978). M o d e l i n g o f dunes and c l i f f s i n t h e w i n d t u n n e l The p r e s e n t s t u d y was conducted a t an open c i r c u i t w i n d t u n n e l w i t h a w i d t h
o f 1 metre, a h e i g h t o f 0.82 metres, and a l e n g t h o f 12.5 metres. Rigorous r e s t r i c t i o n s govern t h e i n t r o d u c t i o n o f sand i n t o t h e w i n d t u n n e l . Studies u s i n g sand have h i t h e r t o o n l y examined sand t r a n s p o r t and r i p p l e s (Bagnold,
250 1941; Ford, 1957; Seppala and Linde, 19781, w i n d s t r e a k s ( G r e e l e y e t a l . ,
1974a,b)
and e r o s i o n of s o i l and dune sands ( C h e p i l and Woodruff, 1963; G i l l e t t e , 1978; L o g i e , 1981). Model s t u d y i n a w i n d t u n n e l r e q u i r e s two t y p e s o f s i m i l a r i t y : 1 ) t h e model and t h e p r o t o t y p e s h o u l d be g e o m e t r i c a l l y s i m i l a r ; 2 ) t h e model and t h e prototype flows s h o u l d be k i n e m a t i c a l l y s i m i l a r (dynamic s i m i l a r i t y ) . A l l l i n e a r dimensions of wooden models a r e r e l a t e d t o t h e c o r r e s p o n d i n g dimensions o f t h e p r o t o t y p e by a c o n s t a n t s c a l e f a c t o r o f about 50. To e s t a b l i s h t h e c o n d i t i o n s f o r dynamic s i m i l a r i t y , a l l t h e f o r c e s r e l e v a n t t o t h e f l o w must be c o n s i d e r e d . T h i s can be achieved w i t h t h e Buckingham P i Theorem ( M i r o n e r , 1979), i n which a l l t h e i m p o r t a n t v a r i a b l e parameters a r e matching factor between t h e model and t h e p r o t o t y p e f l o w s .
It i s i m p o s s i b l e t o r e a c h a s a t i s f a c t o r y dynamic s i m i l a r i t y o f a l l parameters 2 ( I v e r s e n , 1979). Froude number (U / g L ) i s t h e most p o p u l a r parameter as appropriate f o r s i m i l i t u d e (Iversen,
1980). The f u l l scale-model w i n d speed r a t i o would be
j u s t t h e square r o o t o f t h e l e n g t h r a t i o , which i s n o t a r e a l i s t i c v a l u e . Therefore t h e r a t e o f a c c u m u l a t i o n o f sand i n t h e dune i s n o t c o r r e c t l y modeled and t h e f i n a l shape o f t h e dune i s p r o b a b l y somewhat d i f f e r e n t f r o m f u l l s c a l e ( I v e r s e n , 1982, p e r s o n a l communication). The two main l i m i t a t i o n s t o accuracy a r e g r a i n s i z e and wind v e l o c i t y . Dynamic s i m i l a r i t y would r e q u i r e m i n u t e sand g r a i n s t h a t would n o t s a l t a t e and v e l o c i t i e s t h a t would be below t h e t h r e s h o l d o f a l l g r a i n s i z e s . The c l o s e s t p o s s i b l e approxi m a t i o n can be a c h i e v e d by u s i n g t h e s m a l l e s t a v a i l a b l e g r a i n s i z e t h a t s a l t a t e s , and v e l o c i t i e s t h a t approach t h e t h r e s h o l d . The f o u r moment s t a t i s t i c s (McBride, 1971) o f t h e sand used h e r e were:
x4
= 3 . 2 ; 0 $ = 0.58;
Sk
= 0.4;
K
,+
T h i s sand i s f i n e r b y l . O , + t h a n n a t u r a l d e s e r t dune sand.
$
= 4.07.
S t r i c t m a t c h i n g o f Reynolds number i s n o t r e a l l y necessary f o r dynamic s i m i l a r i t , (Howard e t a l . ,
1977). When t h e t u r b u l e n t f l o w i s c o m p l e t e l y developed, t h e specifii
numerical v a l u e o f t h e Reynolds number i s u n i m p o r t a n t as l o n g as we have: Re* = U*z0/v
less than 3
(Sundaram e t a1
. , 1972)
(1)
The o n l y way t o g e t a f u l l y t u r b u l e n t f l o w i s by i n c r e a s i n g zo. T h i s was done by i n t r o d u c i n g a rough s u r f a c e b e f o r e t h e t e s t s e c t i o n i n t h e t u n n e l . RESULTS AND DISCUSSION The a c c u m u l a t i o n o f sand i n f r o n t o f a c l i f f The f o r m a t i o n o f echo dunes was m o n i t o r e d i n f r o n t o f a v e r t i c a l l o w board ( s i m u l a t e d c l i f f ) l o c a t e d a c r o s s t h e t u n n e l and spanning t h e f u l l w i d t h o f t h e t u n n e l ( f i g . 41. The source sand was i n s e r t e d 1.6 metres upwind f r o m t h e simulated
251 c l i f f . The wind v e l o c i t y as measured t h r o u g h a p i t o t t u b e i n t h e m i d d l e o f t h e tunnel was between 6 and 9 metres/second ( j u s t above t h e t h r e s h o l d speed o f t h e sand.).In a l l cases, sand tended t o accumulate a t a c e r t a i n d i s t a n c e f r o m t h e
c l i f f ( f i g . 4).
Fig. 4. A s i m u l a t e d v e r t i c a l c l i f f across t h e wind t u n n e l . N o t i c e t h a t t h e sand accumulates up t o a c e r t a i n d i s t a n c e i n f r o n t o f t h e c l i f f . Measurements o f t h i s d i s t a n c e ( d ) a t d i f f e r e n t c l i f f h e i g h t s ( h ) show t h a t d u r i n g t h e f i r s t s t a g e o f sand a c c u m u l a t i o n i n f r o n t o f t h e s i m u l a t e d c l i f f we do n o t
g e t any sand a t d / h l e s s t h a n 0 . 3 ( t a b l e 1 , f i g . 5 , I
-
111).
TABLE 1 Results o f measurements o f t h e w i d t h ( d ) o f t h e a r e a f r e e o f sand i n f r o n t o f simulated c l i f f s h a v i n g d i f f e r e n t lieights.(h)
h
d
(cm. 1 1.90
(cm. 1 0.60
3.90
1.30
0.32
5.70
1.90
0.33 0.33
7.60
2.50
0.33
9.55
3.70
12.00
4.50
0.39 0.38
252
F i g . 5. Cross s e c t i o n s t h a t r e p r e s e n t s i x stages i n t h e f o r m a t i o n and development o f an echo dune i n t h e w i n d t u n n e l . h = s i m u l a t e d c l i f f h e i g h t ; d = d i s t a n c e f r o m t h e c l i f f ; t = t i m e i n hours f r o m t h e b e g i n n i n g o f the test, When more sand i s added, an echo dune i s g r a d u a l l y c r e a t e d w i t h a c r e s t between 0.5 d / h t o 0.6 d/h ( f i g . 5, I 1 1
-
V I ) . As t h e dune becomes h i g h e r , sand covers
t h e a r e a where d / h i s l e s s t h a n 0.3 ( f i g . 5, I V
-
V I ) , b u t t h e c r e s t l i n e remains
a t 0.5 d/h. When t h e s i m u l a t e d c l i f f i s i n c l i n e d , t h e r a t e s d/h o f t h e accumulation i n f r o n t o f t h e c l i f f i s changed. T a b l e 2 g i v e s t h e v a l u e s o f accumulation, d/h, i n various slope i n c l i n a t i o n s , a
.
The c o r r e l a t i o n between t h e d/h values and those
o f o( i s v e r y good ( r = 0 . 9 8 8 ) . It seems t h a t s l o p e s o f 50 degrees and l e s s bring a b o u t c l i m b i n g dunes, w h i l e s t e e p e r i n c l i n a t i o n s e f f e c t accumulations a t some value o f d/h i n f r o n t o f t h e c l i f f . TABLE 2 The r a t e s o f d/h o f sand a c c u m u l a t i o n i n f r o n t o f i n c l i n e d s l o p e s i n a degrees
d/ h
(degraees)
0
50
0.06
55
0.11
60
0.29
75
0.37
90
253 Long t e r m m o n i t o r i n g o f t h e a c c u m u l a t i o n o f sand i n f r o n t o f 60 degree c l i f f showed t h a t a f t e r 28 hours o f wind f l o w , j u s t above t h e t h r e s h o l d , sand accumulated
up t o d / h v a l u e s equal t o 0.04. A f t e r 52 hours t h e sand s t a r t e d t o c l i m b t h e c l i f f and turned i n t o a c l i m b i n g dune. r l o n i t o r i n g o f a 75 degree c l i f f showed t h a t accumulation a f t e r 24 hours o f wind f l o w was up t o d/h = 0.1,
and t h e c r e s t l i n e
was a t 0.375 d / h . A t t h i s stage t h e sand accumulation had a morphology o f an echo dune ( f i g . 6 ) .
I
I
t=24hr
1
1.5 1:5
.O .o
Fig. 6. A p r o f i l e o f an echo dune, formed i n f r o n t o f a s i m u l a t e d 75 degree slope a f t e r 24 hours o f wind r u n . h = s i m u l a t e d c l i f f h e i g h t , d = d i s t a n c e f r o m the c l i f f .
Measurements o f eddies and w i n d t u r b u l e n c e i n f r o n t o f t h e s i m u l a t e d c l i f f and o v e r t h e echo dune models A bubble g e n e r a t o r was used t o t r a c e t h e eddies and t h e wind t u r b u l e n c e i n f r o n t of t h e s i m u l a t e d c l i f f and on t h e l e e s i d e o f t h e echo dune models. The generator e m i t t e d bubbles made o f helium, a i r and d e t e r g e n t f r o m a p i p e p l a c e d 10 c e n t i m e t r e s above t h e t u n n e l f l o o r , a t a d i s t a n c e o f 1.5 metres f r o m t h e model s. F i g u r e 7 shows t h e t r a c k s o f t h e bubbles w i t h v a r i o u s models. F i g u r e 7A,B shows t h e p a t t e r n o f f l o w i n f r o n t o f t h e s i m u l a t e d c l i f f . When t h e a i r f l o w approaches t h e c l i f f p e r p e n d i c u l a r l y , i t slows down w i t h a consequent b u i l d - u p o f pressure a g a i n s t t h a t f a c e (Eaton, 1981). As a r e s u l t t h e s t r e a m l i n e s s t a r t t o separate f r o m t h e f l o o r a t d/h = 1.0 t o 2.0.
Some o f t h e s t r e a m l i n e s r i s e and
flow o v e r t h e c l i f f , w h i l e o t h e r s make a l o o p and c r e a t e a r e v e r s e - f l o w eddy.
The p o i n t on t h e c l i f f a t which t h e s t r e a m l i n e s d i v i d e i n t o f l o w t h a t t r a v e r s e s the c l i f f and f l o w t h a t c r e a t e s t h e r e v e r s e - f l o w eddy, i s c a l l e d t h e p o i n t o f stagnation
(s).
The h e i g h t o f t h e p o i n t o f s t a g n a t i o n (Sh) i n one case was 0.72h and i n t h e other 0.63h ( f i g . 7A,B). S i m i l a r r e s u l t s were a c h i e v e d by Pande e t a1.(1980) i n f r o n t o f a fence, where t h e upstream s e p a r a t i o n l e n g t h was 1.22h and t h e p o i n t
o f s t a g n a t i o n was a t a h e i g h t o f 0.72h.
A
2
C
r-
2
I
1.5
1 d/h .5
F i g . 7. Tracks o f bubbles i n *Front o f and o v e r d i f f e r e n t models: A,B cliff;
C
-
75 degree slope; D
-
60 degree slope; E
-
0
-
vertical
45 degree slope;
F
-
echo dune model, H/h = 0.18 [ H = dune h e i g h t , h = c l i f f h e i g h t ) ;
G
-
echo dune model, H/h = 0.324;
H
-
echo dune model, H/h = 0.6.
S = s t a g n a t i o n p o i n t , Sh = h e i g h t o f S.
TABLE 3 The r e l a t i v e h e i g h t o f t h e s t a g n a t i o n p o i n t (Sh) i n d i f f e r e n t i n c l i n a t i o n s ( a ) o f the c l i f f a (degrees)
Sh
90
0.63
75
0.44
60
0.27
255 The d e p o s i t i o n o f sand a t d / h g r e a t e r t h a n 0.3 ( t a b l e 1) was a r e s u l t o f t h e reverse f l o w . A t t h a t d i s t a n c e t h e r e was an encounter between two o p p o s i t e d i r e c t ions, which r e s u l t e d i n a l o w e r i n g o f t h e wind v e l o c i t y and caused d e p o s i t i o n . When i n c l i n e d c l i f f s were modeled ( f i g . 7C,O & E ) t h e r e v e r s e f l o w eddy became smaller. Slopes o f 60 and 45 degrees c r e a t e d a small o r i n d i s c e r n i b l e l o o p and the p o i n t o f s t a g n a t i o n was lowered a c c o r d i n g l y . There i s a v e r y good c o r r e l a t i o n ( r = 0.99) between t h e h e i g h t o f t h e p o i n t
o f stagnation
(37)
and t h e i n c l i n a t i o n ( a ) o f t h e c l i f f ( t a b l e 3 ) . The c o r r e l a t -
ion e q u a t i o n i s :
- 0.453
Sh = 12 x
(2)
When Sh = 0, t h e n a i s 38 degrees a c c o r d i n g t o e q u a t i o n 2. T h i s means t h a t t h e r e
w i l l be no s t a g n a t i o n p o i n t and no s e p a r a t i o n i n f r o n t o f slopes whose i n c l i n a t i o n s are l e s s than 38 degrees. T a b l e 2 shows t h a t sand tends t o c l i m b a 50 degree slope, and t h a t when i t g a t h e r s i n some q u a n t i t y i n f r o n t o f a 60 degree s l o p e i t a l s o s t a r t s t o c l i m b . When t h e r e v e r s e - f l o w eddy i s v e r y small case of a 60 degree s l o p e ( f i g . 7D)
-
-
as i n t h e
sand a c c u m u l a t i n g a t t h e base o f t h e c l i f f
causes a r e d u c t i o n o f t h e v i r t u a l s l o p e which i t e v e n t u a l l y begins t o ascend. When echo dune models b u i l t o f wood were p l a c e d i n f r o n t o f t h e s i m u l a t e d c l i f f w i t h shape and p o s i t i o n s i m i l a r t o t h o s e m o d e l l e d w i t h sand ( f i g .
5), t h e bubbles showed c o n s t a n t and p r o m i n e n t r e v e r s e - f l o w eddies between t h e dune and t h e c l i f f ( f i g . 7F,G,H).
I t seems t h a t t h e p r e s s u r e f i e l d b u i l d - u p on t h e
dune i n c r e a s e d t h e r e v e r s e f l o w i n f r o n t o f t h e c l i f f . The s t a g n a t i o n p o i n t moved up t h e c l i f f , as t h e dune model h e i g h t r e l a t i v e t o t h e c l i f f increased. Measurements o f wind v e l o c i t y i n f r o n t o f c l i f f s and o v e r echo dune models Wind v e l o c i t y
measurements were c a r r i e d o u t by a h o t w i r e anemometer, whose
probe was s e n s i t i v e t o t h e l o n g i t u d i n a l v e l o c i t y component o f t h e f l o w . The anemometer probe was p l a c e d on t h e wind t u n n e l f l o o r o r t h e model s u r f a c e , so t h a t the h o t w i r e was 6 m i l l i m e t r e s above t h e s u r f a c e . I n o r d e r t o compare t h e r e s u l t s
o f t h e measurements t a k e n on d i f f e r e n t s i z e s o f model, t h e d i s t u r b e d wind v e l o c i t y o v e r and n e a r t h e models ( U 2 ) , was r e l a t e d t o t h e u n d i s t u r b e d wind v e l o c i t y ( U l ) . The r a t i o between t h e two, U2/U1,
i s c a l l e d t h e speed-up r a t i o o r t h e a m p l i f i c a t i o n
f a c t o r , Az (Bowen and L i n d l e y , 1977) Figure
8A shows t h e a m p l i f i c a t i o n f a c t o r , Az, i n f r o n t o f a s i m u l a t e d c l i f f .
Az s t a r t s t o d r o p a t a d i s t a n c e o f d / h = 3.3 as a r e s u l t o f t h e b u i l d - u p of pressure a g a i n s t t h e c l i f f , and reaches a minimum a t d / h = 0.74, opposite f l o w d i r e c t i o n s meet ( s e e f i g . imum a t d/h
=
where t h e two
7A,B). From t h e r e Az i n c r e a s e s t o a max-
0.275 and t h e n decreases again. T h i s i s t h e area where t h e r e v e r s e
256
B
A
1 .5 0 L
F i g . 8. V a r i a t i o n o f the a m p l i f i c a t i o n f a c t o r (Az) i n f r o n t o f simulated c l i f f s and slopes: A - v e r t i c a l c l i f f ; B - 75 degree slope; C - 60 degree slope; 0 - 50 degree slope; E - 45 degree slope. h = simulated c l i f f height, d = d i s t a n c e from t h e c l i f f o r slope base. flow eddy e x i s t s ( f i g . 7A,B).
A slope i n c l i n e d a t 75 degrees ( f i g . 88) shows t h e
same p a t t e r n , but t h e minimum and maximum p o i n t s s h i f t toward t h e c l i f f ( d/h = 0.5 and 0.125,
r e s p e c t i v e l y ). T h i s i s a r e s u l t o f t h e l o w e r i n g o f t h e height o f
t h e stagnation p o i n t and t h e r e d u c t i o n i n diameter o f t h e reverse-flow eddy (fig.
7 C ) . An i n c l i n e d c l i f f o f 60 degrees does n o t show a prominent r i s e i n Az
c l o s e t o t h e c l i f f ( f i g . 8C), as i s a l s o t r u e f o r a c l i f f i n c l i n e d a t 50 degrees ( f i g . 8D).
A slope i n c l i n e d a t 45 degrees has no p o i n t o f maximum Az i n f r o n t o f
t h e c l i f f ( f i g . 8E). The p o i n t o f minimal Az i s on t h e base o f t h e c l i f f . Measurements o f Az over echo dune models t h a t a r e i n t h e same p o s i t i o n as the models o f f i g u r e 7 (F,G,H)
show a decrease i n Az toward t h e windward base of the
dune model, and then an increase toward t h e c r e s t ( f i g . 9A,B,C).
I n t h e lee,
between t h e dune model and t h e simulated c l i f f t h e r e i s a sharp abatement i n AZ, a subsequent increase, and f i n a l l y another drop toward t h e c l i f f . This pattern can be explained by reference t o f i g u r e 7 (F,G,H).
The r a t e o f increase i n
AZ
on t h e windward slope of t h e modelled dune depends on t h e r e l a t i v e s i z e o f the dune. The increase i n Az on t h e l e e s i d e i s a r e s u l t o f t h e prominent reversef l o w eddy. When t h e echo dune i s small r e l a t i v e t o t h e h e i g h t o f t h e c l i f f , t h e magnitude o f t h e reverse-flow i s h i g h e r than t h e increased wind on t h e dune crest ( f i g . 9A). A t t h i s stage t h e dune develops ( f i g . 5 ) .
257
Az
20
15
3.5
3 2.5 2
gh
1.5
1
.5
0
Fig. 9. V a r i a t i o n o f t h e a m p l i f i c a t i o n f a c t o r (Az) o v e r echo dune models. A : H/h = 0.18; B: H/h = 0.324; C : H/h = 0.6. (H = dune h e i g h t , h = c l i f f height). Where t h e dune has accumulated t o t h e p o i n t t h a t t h e magnitude o f t h e wind on the c r e s t has become h i g h e r t h a n t h e r e v e r s e - f l o w ( f i g . 9B), i t ceases t o grow. It seems t h a t t h e echo dune comes t o a s t e a d y s t a t e when t h e h e i g h t o f t h e dune
(H) i s a b o u t 0.3 t o 0.4h. A t t h i s h e i g h t t h e a i r f l o w on t h e windward s i d e o f t h e dune equals t h e r e v e r s e - f l o w on t h e l e e s i d e and t h e s t a g n a t i o n p o i n t ( S ) i s c l o s e t o t h e r i m o f t h e c l i f f ( f i g . 7G,H), w h i c h means a l o w e r magnitude r e v e r s e - f l o w eddy. The s i t u a t i o n p r e s e n t e d on f i g u r e 9C does n o t e x i s t i n r e a l i t y . Sand t h a t was added t o an echo dune i n a s t e a d y s t a t e c o n d i t i o n would move t o t h e t r o u g h between t h e dune and t h e c l i f f and be c a r r i e d a l o n g i t by a l o n g r o l l e r v o r t e x . T h i s s i t u a t i o n was observed a t P a i u t e P o i n t i n n o r t h e r n A r i z o n a ( f i g . 1). Here t h e s i d e s o f t h e echo dunes d e v e l o p i n t o c l i m b i n g dunes on t h e gentle s l o p e s o f t h e small g u l l i e s . Sand t h a t moves l a t e r a l l y a l o n g t h e t r o u g h between t h e echo dune and t h e c l i f f e n t e r s t h e c l i i i i b i n g dune and i s pushed up slope t o t h e p l a t e a u ( f i g . 1). SUMMARY AND CONCLUSIONS According t o t h e r e s u l t s o b t a i n e d i n t h e w i n d t u n n e l , t h e f o l l o w i n g s t a g e s f o r t h e development o f echo dunes i n f r o n t o f a c l i f f were i d e n t i f i e d .
1) When wind e n c o u n t e r s a v e r t i c a l c l i f f i t s v e l o c i t y b e g i n s t o d r o p a t a d i s t a n c e o f about d / h = 3, and a t t a i n s minimum v e l o c i t y a t d / h = 0.75.
Closer t o t h e c l i f f
2 58 t h e v e l o c i t y i n c r e a s e s and t h e d i r e c t i o n r e v e r s e s as a r e s u l t o f a reverse-eddy flow. 2 ) Sand d e p o s i t s s t a r t a t a d i s t a n c e o f d / h = 2.0 t o 0.4.
I n no case was sand
f o u n d between t h e c l i f f ( d / h = 0) and d / h = 0.3,
a t the s t a r t o f deposition.
3 ) When sand p i l e s up i t c r e a t e s an "echo-dune".
I n t h e l e e o f t h i s dune a separ-
a t i o n eddy develops which has t h e same phase as t h e r e v e r s e - f l o w eddy. The reverse. f l o w i s s t r e n g t h e n e d and a t t a i n s h i g h e r v e l o c i t i e s t h a n t h e wind above t h e dune c r e s t . T h i s b r i n g s a b o u t a r a p i d i n c r e a s e i n t h e h e i g h t o f t h e dune. 4 ) As t h e dune g a t h e r s h e i g h t , t h e magnitude o f t h e wind f l o w above t h e dune c r e s t i n c r e a s e s . A t a h e i g h t o f about 0.3 t o 0.4h t h e v e l o c i t y o v e r t h e c r e s t e q u a l s t h e r e v e r s e - f l o w v e l o c i t y . The dune a t t h i s stage i s i n a s t e a d y s t a t e . The same amount o f sand t h a t j o i n s t h e dune l e a v e s i t , t h r o u g h a r o l l e r v o r t e x movement a l o n g t h e t r o u g h between t h e dune and t h e c l i f f .
5 ) When t h e c l i f f i s i n c l i n e d , t h e s i z e o f t h e r e v e r s e - f l o w eddy decreases. Slopes of g r e a t e r t h a n 55 degrees have small r e v e r s e - f l o w eddies which have no e f f e c t on t h e sand, which t h e n t e n d s t o accumulate a t t h e base o f t h e s l o p e and t o climb i t . T h i s i s t h e f o r m a t i o n o f a " c l i m b i n g dune".
ACKNOWLEDGEMENTS T h i s work was conducted a t t h e P l a n e t a r y Geology L a b o r a t o r y o f t h e Department of Geology, A r i z o n a S t a t e U n i v e r s i t y . The a u t h o r i s d e e p l y i n d e b t e d t o Professor Ronald G r e e l e y f o r h i s h e l p , a d v i c e and s u g g e s t i o n s d u r i n g t h e s i m u l a t i o n proceedu r e . Comments and c r i t i c a l r e v i e w by P r o f e s s o r James D. I v e r s e n and D r . Andrew Warren a r e g r a t e f u l l y acknowledged. The work was s u p p o r t e d by t h e P l a n e t a r y Geology O f f i c e , N.A.S.A.,
t h r o u g h t h e Mars Data A n a l y s i s Program.
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CONTROLS OF DUNE HORi'HOLOGY 11.1 THE NAI.113 SAND SEA
N . LANCASTER, Desert E c o l o g i c a l Research U n i t , I l a l v i s Bay, S.W.A./Namibia.
IN'ROOUCTION
The advent o f s a t e l l i t e and o t h e r remote sensing imagery o f d e s e r t areas has shown t h e g r e a t v a r i e t y o f dune morphology i n sand seas, b u t a t t h e same t i m e et _ a1 (1979) has f a c i l i t a t e d t h e i r mapping, as IlcKee and Breed (1976) and 3reed _ have demonstrated. Host i n v e s t i g a t i o n s o f dune morphology have been l a r g e l y concerned w i t h t h e typology o r gross morphology o f dunes and t h e i r s p a t i a l d i s t r i b u t i o n (e.g. 1958, Holm 1960, Mabbutt 1968, Mainguet 1976, 1978, Besler 1980).
Plonod
A potentially
valuable area i s t h e study o f dune morphometry, p a r t i c u l a r l y t h e l e n g t h , h e i g h t and spacing o f dunes, b u t t h i s has r e c e i v e d l i t t l e a t t e n t i o n (Wilson 1972, Breed and Grow 1979, Lancaster 1981a).
However, t h e r e have been few s t u d i e s o f t h e
factors which c o n t r o l t h e morphology and morphometry of dunes w i t h i n a sand seaSuch s t u d i e s may make an important c o n t r i b u t i o n t o our understanding o f t h e factors which c o n t r o l t h e shape and s i z e o f dunes and t h e ways i n which e o l i a n sand bodies accumulate. The a i m o f t h i s paper
is t o d e s c r i b e t h e morphology and morphometry o f dunes
i n t h e Namib sand sea and t o r e l a t e t h e s p a t i a l v a r i a t i o n i n these p r o p e r t i e s t o changes i n t h e g r a i n s i z e and s o r t i n g c h a r a c t e r i s t i c s o f t h e sands and i n wind regimes.
I n t h i s paper, data obtained from remote sensing imagery i s
combined w i t h f i e l d observations and measurements o f dune morphometry and sediments a t 26 s i t e s throughout t h e sand sea (Fig. 1). C h a r a c t e r i s t i c s o f dunes i n t h e Namib sand sea
2 . It s t r e t c h e s
The Namib sand sea occupies an "rea o f approximately 34 000 km
f o r over 300 km between L u d e r i t z (26's)
and t h e Kuiseb r i v e r (23°-23030'S) on
t h e A t l a n t i c c o a s t o f southern A f r i c a .
Dunes a r e found over i t s 100
-
120 km
width from sea l e v e l t o t h e v i c i n i t y o f t h e 1 000 m c o n t o u r a t t h e base o f t h e Great Escarpment, which borders t h e e a s t e r n s i d e o f t h e sand sea.
The v a r i e t y
o f dune forms i n such a small area has a t t r a c t e d t h e a t t e n t i o n o f previous
262
I
\
0
50 kin
Fig. 1. Location of sites at which dunes were sampled and morphometric measurements made.
investigators, particularly Barnard (1973) who divided the northern part o f the sand sea i n t o three zones: a coastal belt o i transverse dunes, an interior zone of "longitudinal" dunes, which he believed were formed by deflation and an eastern zone of complex "multicyclic" dunes. Besler (1980)produced a complex map of dune types in the sand sea as part of a study of the origins of the sand sea. VORPHOLOGY Three major dune types occur in the Namib sand sea. Their distribution is
DUNE
263
F i g . 2. The Namib Sand Sea: mosiac of LANDSAT images. 22232-08083; 22232-08090;22232-08092;22287-08131;22237-08133.
shown by the satellite image in Fig. 2. ard the map based upon LANDSAT imagery, supplemented by air photo interpretation, in Fig. 3. The proportion of dunes of different types i s given in Table 1 . Terminology employed follows McKee (1579).
2 64
’So
239
2LC
25 Sylvia Hill
26
-
0
20 km
T
Fig. 3. Dune types in the Namib Sand Sea. Map compiled from LANDSAT imagery, supplemented by air photo interpretation. 1 Complex linear dunes 2 Compound linear dunes a Straight dunes b Anastomosing and reticulate patterns 3 Simple linear dunes 4 a Barchans b Simple transverse and barchanoid ridges c Compound transverse dunes 5 Star dunes and chains of star dunes 6 Rolling dunes without slip faces 7 Sand sheets.
265 TAELE 1 Relative importance o f dune types i n t h e Namib sand sea Dune t .v. w
Area covered ( % o f sand sea) 74.04
Linear ComDlex Compound S t r a ig h t Anastomosing and r e t i c u l a t e patterns Simple Transverse and barchanoid Barchans Simple Compound S t a r dunes and chains o f s t a r dunes Low r o l l i n g dunes w i t h o u t s l i p faces Sand sheets
36 -89 35.02 19.56 15.46 2.13 13.32 0.02 3.48 9 -82 9.31 1.54 3.33
Transverse and barchanoid dunes o f simple and compound form a l i g n e d normal t o They a l s o
SSUto SW winds occur i n a s t r i p up t o 20 km wide along t h e coast.
occur l o c a l l y f u r t h e r i n l a n d where dune p a t t e r n s a r e d i s t u r b e d , f o r example west o f t h e Tsondab F l a t s and a t Sossus V l e i . areas o f t r a n s v e r s e and c r e s c e n t i c dunes. extends from E l i z a b e t h 8ay t o S y l v i a H i l l .
There a r e two major c o a s t a l
The southern area o f these dunes It s t a r t s as a t r a i n o f
i r r e g u l a r l y spaced barchans and barchanoid r i d g e s extending from t h e beaches of E l i z a b e t h and Chamais Bays.
North o f L u d e r i t z , an e x t e n s i v e area o f simple
transverse and barchanoid r i d g e s fans o u t i n t o t h e main sand sea, w h i l s t along the coast t h e r e i s a zone, up t o 10 km wide, o f compound t r a n s v e r s e dunes which extends t o S y l v i a H i l l .
The n o r t h e r n group o f t r a n s v e r s e dunes begins on t h e
coast southeast o f Fleob Bay and continues t o t h e n o r t h e r n boundary o f t h e sand sea a t t h e Kuiseb r i v e r .
Most dunes i n t h i s area a r e l a r g e compound forms
c o n s i s t i n g o f a main t r a n s v e r s e r i d g e w i t h small barchanoid r i d g e s on t h e i r crests and upper southwest slopes.
Between Meob and Conception Bays t h e r e
are areas o f barchans and small t r a n s v e r s e and barchanoid r i d g e s .
Some o f
these j o i n t h e main area o f compound t r a n s v e r s e dunes from t h e west. Linear dunes on N-S t o NW-SE alignments a r e t h e dominant dune form i n t h e Nanib sand sea.
Complex l i n e a r dunes w i t h a sinuous sharp c r e s t l i n e , s t a r
form peaks a t i n t e r v a l s and barchanoid r i d g e s on t h e i r e a s t e r n f l a n k s a r e widely developed i n n o r t h e r n and c e n t r a l p a r t s o f t h e sand sea.
I n western
areas small simple l i n e a r dunes commonly cross t h e c o r r i d o r s between t h e major dunes.
I n southern p a r t s o f t h e sand sea most l i n e a r dunes a r e o f compound
form and c o n s i s t of 2
a broad rounded s w e l l .
-
5 p a r a l l e l o r converging small r i d g e s running along Along t h e e a s t e r n margins o f t h e sand sea compound
266
and simple linear dunes, largely fixed by vegetation, occur in anastomosing and reticulate patterns. Transitional to these dunes are broad linear ridges with a retjculate pattern of small dune crests on their summits. Dunes of star form are found in three areas along the eastern margins of the sand sea: north of Tsondab Vlei, around Sossus Vlei and in three groups along the eastern edge of the southern part o f the sand sea. Few of these dunes have a true stellate form. Many consist of a characteristic narrow, relatively short, steep sided ridge with a preferred crestal orientation, usually NW-SE and curving arms on alignments roughly perpendicular to this. Frequently dunes of star type are found in chains (c.f. the compound forms of Breed and Grow 1979) and are transitional to complex linear dunes. Within the sand sea, sand covered areas without significant dune development are rare. On the southern margins o f the sand sea an extensive gently rolling sand plain extends for 20 km north of the Koichab river. South and southwest of the Uri Hauchab mountains there is an area of rolling dunes without slip faces up to 30 m high, comparable with the Mrey6 of Monod (1958). Throughout the sand sea but particularly in southern areas low rolling dunes without slip faces (c.f. the "zibar" of Holm, 1960) occur in the interdune corridors between complex and compound linear dunes, frequently on a trend normal to that of the 1 inear dunes. DUNE MORPHOMETRY Transverse dunes Spacing o f transverse dunes in the Namib sand sea is variable (Fig. 4) and ranges from 100 to 1 400 m with a mean o f 610 m. Most transverse dunes have spacings between 200 and 1 000 m. Those with spacings of 200 - 300 m or less are generally simple forms and tend to be low (5 - 10 m high) transverse or barchanoid ridges. More widely spaced dunes are invariably of compound form with the main dunes having a spacing of 600 - 1 000 m, rather less than those described as compound crescentic by Breed and Grow (1979),but comparable with simple transverse dunes in the United Arab Emirates, A1 Jiwa and Takla Makan. On the upper parts Of their stoss slopes small (2 - 5 m high) barchanoid and crescentic dunes with spacings of 50 - 80 m are frequently developed.
Rolling dunes without slip faces in the area south of the Uri Hauchab mountains (site XVI) have a similar morphometry to that of compound transverse dunes.
267
These dunes have spacings of 1 000 - 1 100 rn with a height o f 20 - 30 m. On their stoss slopes and rounded crests are small rounded undulations similar to the "zjbsr" o f Holm (1960) with a spacing of 170 - 180 rn.
:."I r-!Tm P 1993 rn
4,
Linear
!
0 ' 1200
1800 O
ZLOO
W
1800
ZLOO
rn
rn
X 1L32 rn
10 600
1200
::m!h
." 301
0
0
P610 rn
.1
600
Transverse
1200
rn
Dune spacing
Fig. 4. Spacings of transverse, linear and star dunes in the Namib sand sea. There is a systematic variation in dune spacing within the area of compound transverse dunes. This is particularly striking in the northern group o f dunes. Dune spacing increases northwards from 700 to 1 000 - 1 100 m in the area northeast o f Conception Bay, then decreases progressively to 300 - 400 m north of Sandwich Harbour. Using data from sample sites only, Fig. 5a shows that there is a close relationship between transverse dune height and spacing (r = 0.75, significant at the 0.05 level). Similarly close relationships have been established by Lancaster (1982b) for simple and compound transverse and barchanoid dunes in the Skeleton Coast dunefield on the northern Namib coast. Linear dunes Aspects of the rnorphometry of linear dunes in the Namib sand sea have been discussed by Lancaster (1981 ). Their morphometric characteristics are shown
2 68
0
0
so0 Dune spoclng r m l
1000
x
01
.
800
1000
3001
I
’
complex
..
y~OllX.1L3L r =
2500
1500 2000 Dune spacing Iml
x
0-72
2800
c
n;11
X
300
!5@0
1000
2000
2500
Dune SSOL8ng Irnl
Fig. 5. R e l a t i o n s h i p s between dune h e i g h t and spacing: A Transverse dunes B Linear dunes C S t a r dunes. i n Fig. 6.
Dune spacings range from 1 200 t o 2 800 m, w i t h a mean o f 1 993 m
and most l i n e a r dunes a r e 1 800
-
2 200 m a p a r t .
I n a d d i t i o n , barchanoid
r i d g e s w i t h a mean spacing o f 87 m, commonly j o i n i n g t h e main dune c r e s t i n an en echelon manner, occur on t h e e a s t e r n f l a n k s o f many complex l i n e a r dunes, p a r t i c u l a r l y i n western and c e n t r a l areas o f t h e n o r t h e r n p a r t o f t h e sand sea. Spacing o f t h e m u l t i p l e r i d g e s o f compound l i n e a r dunes averages 160 m, o r approximately one t e n t h t h a t o f t h e average spacing o f t h e main dunes
( 1 680 m).
of 838 m.
Widths o f t h e l i n e a r dunes a r e commonly 600 Most complex l i n e a r dunes a r e 80
-
-
1 000 m w i t h a mean
150 m h i g h , w h i l s t compound
269
-
.)
7
25.
0,
Dune
.
height
0
600
1200 m
Dune w d l h
? 1332
i 1993
L 50!
Dune i p o c l n g
Dune FpocNng
Fig. 6. Morphometric characteristics o f linear and star dunes in the Namib sand sea. linear dunes are between 30 - 50 m in height. In terms o f their size and spacing complex and compound linear dunes in the Namib sand sea are comparable with similar dunes in the Rub a1 Khali and southern Sahara. The height and spacing o f linear dunes in the Namib sand sea varies systematically, as described by Lancaster (1982~). Such dunes are highest at 150 - 170 m in the central parts o f the sand sea and become progressively smaller towards its margins. Throughout the southern parts o f the sand sea linear dunes are less than 50 m high. The spacing o f this type o f dune varies in a similar way, with the most widely spaced dunes occuring between the Tsondab and Tsauchab valleys (Fig. 8a and 8b). This apparent relationship between linear duie height and spacing is born out by Fig. 5b, which shows that, as with transverse dunes, the height and spacing
270
of linear dunes is closely correlated (r = 0.77, significant at the 0.05 level) Similar, but less clear, relationships (Fig. 7a) occur between the width and spaciq of linear dunes (r = 0.54) confirming the data of Breed and Grow (1979)
I : 0 28X r.051
.
.
251 75
A
X complex Compound
.
0 1 800
1000
1500 O""P
I
:0 - 2 7 X
t
2000 Irnl
2506
2800
spac8ng
252.10
B
7.068 n
i
11
Fig. 7. Relationships between dune width and spacing: Linear dunes B Star dunes. A
Star dunes -~ Spacings o f star and related chains of star dunes widely, from 600 to 2 600 m (Fig. 6). Most dunes of spacings between 1 000 and 1 800 m , with a mean of 1 dunes are between 400 and 1 000 m and average 651 m.
in the Namib sand sea vary this type however have 332 m. Widths of star Star dunes of the type
271
Found in the Namib are thus narrower and more closely spaced than the comlex 1 inear dunes.
I
A
I
15'
Fig. 8. Spatial variation in dune height ( A ) and dune spacing ( 6 ) in the Nanib sand sea. The largest star dunes occur in the vicinity of Sossus Vlei, where they may reach heights o f 200 - 300 m. Star dunes in the southern parts of the sand sea are generally 80 - 100 m high, and relatively closely spaced ( + 1 000 rn). Mean star dune height for the sand sea is 145 m, or rather greater than that of linear dunes. In many areas there are small barchanoid or reversing dunes, with spacings of 90 - 120 m between the star dunes. As with other dune types described above, star dune height and spacing are closely correlated (r = 0.72, significant at the 0.05 level) Fig. 5c.
272
Although Breed and Grow (1979) could find no statistically significant relationship between star dune width and spacing for their world-wide sample of these dunes, such a relationship can be established for the Namib star dunes (Fig. 7b) (r = 0.68, significant at the 0.05 level). The pattern of dune alignments I n many areas of the Namib sand sea the pattern of dune alignments consists of a major or dominant trend, together with one or two subsidiary elements. From place to place, the relative importance of these trends differs. Thus a major trend in one area may be continued as a subsidiary trend elsewhere, or a minor trend i s accentuated to dominate in another area, as Fig. 9 illustrates. The trend of the crests of most compound and simple transverse dunes i s generally 300 - 320°, with NE facing slip faces, tending to swing round slightly inland (Fig. 9e). In the Conception - Meob area simple transverse and barchan dunes have slip faces to the N or NNE and redge trends of 240 280'. Especially along the inland margin of their distribution, maiy compound transverse dunes exhibit prominent N-S oriented linear elements which cross from one dune to another (Fig. 9f). These may be considered incipient linear dunes in many cases. The major dune trend in the sand sea is that of the linear dunes, which have a HNhJ-SSE to NNE-SSU alignment in its central and northern areas (Fiq. 9a). I n the southern parts of the sand sea compound linear dunes occur on NW-SE alignments The trend of fixed anastomosing and reticulate linear dunes along the eastern margins of the sand sea is strongly affected by topography in many places, but elsewhere is generally WNW-ESE to NW-SE. Nithin the complex linear dune landscape two subsidiary trends occur. Crest trends of east flank barchanoid dunes are 320 - 330' (Fig. 9c), with NNE to NE oriented slip faces in western areas, swinging round eaztwards to 340 - 355O, with slip faces facing east. This appears to continue the trend of the transverse dunes along the coast. Corridor crossing dunes follow a consistent alignment of 060' or WSW-ENE in western parts of the sand sea from Sossus Vlei northwards, and locally in the southern part of the sand sea (Fig. 9b). The trend of the major ridges of most star dunes follows the pattern of the linear dunes as Fig. 9a illustrates. Subsidiary trends of star dune arms are LISU-ENE following that of the crossing dunes (Fig. 9b), whilst barchanoid and reversing dunes amongst the star dunes continue the trend of east flank barchanoid dunes elsewhere.
273
Fio. 9. Dune alignments in the Namib sand sea: A
B
C 0 E F
Linear dunes and major ridges of star dunes Corridor crossing dunes and secondary ridges of star dunes East flank and related barchanoid dunes in linear and star dune landscapes: bar indicates ridge trend; tick, dominant slip face orientation Low rolling dunes without slip faces in interdune corridors Transverse dunes: bar indicates crest trend; tick, major slip face orientation Oblique linear elements in transverse dune areas.
274
The pattern of dune morphology in the sand sea Considering all dune types, the pattern of dune morphology and morphometry, particu’3rly dune height and spacing, varies in a systematic way throughout the s?nd sea. The largest and thus the most widely spaced dunes are found in the central and some northern parts of the sand sea, with progressively smsller dunes towards the margins. With the exception of small areas of star dunes, most dunes in the southern parts of the sand sea are less than 50 m high and are generally less than 1 700 m apart. A line drawn from the coast south of Meob Bay through the Uri Hauchab mountains effectively divides the Namib sand sea into two parts. The southern section is characterised by low, mostly compound, linear dunes, with wide areas of rolling dunes without slip faces and a narrow zone of compound transverse dunes along the coast. The northern and central section is dominated by large, complex linear dunes, locally grading into chains of star dunes and changing eastwards to lower reticulate and anastomosing compound linear dunes, largely fixed by vegetation. Along the coast is a belt of moderately sized compound transverse dunes. CONTROLS OF DUNE MORPHOLOGY
The morphology of desert dunes is principally a product of the interaction between the sand surface and the wind. This interaction is modified by the growth of the dunes which project into the airflow and deflect it in their vicinity . Thus two major influences on dune morphology may be recognised: the character of the dune sands, particularly their grain size and sorting characteristics, and the character of the wind regime. Further, as sand seas are the product of an ongoing process of sediment accumulation, the effects of this are also important. An additional factor in the eastern part of the Namib sand sea is the presence of a partial vegetation cover on the dunes. Many features of the reticulate and anastomosing simple and compound linear dunes in these areas can be related to development of blowouts and parabolic dunes during years o f low rainfall, which are revegetated after occasional heavy rains (e.g. 1976-1978) The influence of grain size and sorting The evidence for relationships between dunes of different types arid sand with different grain sizes and sorting characteristics is confusing. In the Algerian Sahara, Bellair (1953) found that barchan and transverse dunes were composed o f well sorted unimodal sands but complex linear and star dunes were formed of bi
275
or trimodal sands. However, Alimen et_a1 (1958) and Capot-Rey and Gremion (1964) could find no consistent relationship and pointed to the complex patterns of grain size and sorting on larger dunes. McKee (1966) found that there was a progressive decrease in grain size and increase i n sorting from dunes of dome type, through barchans and transverse dunes to parabolic dunes at Llhite Sands, New Mexico. Similarly, i n the Sudan, Warren (1970) put forward evidence to indicate that sands from undulating sand sheets were coarser and less well sorted than those from transverse dunes, which were in turn coarser and less well sorted than those from linear dunes. A number 0-f studies have noted the relationship between coarse bi or multi modal sands and low rolling dunes without slip faces or "zibar" (Capot-Rey 1947, McKee and Tibbitts 1964, Warren 1972, Tsoar 1978, Lancaster 198213). Sands from adjacent linear or transverse dunes are also consistently finer and better sorted than those of low rolling dunes. Further, Warren (1972) noted that, whilst the low rolling crests were transverse to the resultant sand flow direction, the linear dunes were on oblique alignments.
There is also some evidence to suggest that the grain size characteristics of the sand may control the spacing of dunes, especially those of transverse form. Wilson (1972) suggested that, for each class of his bedform hierarchy dune spacing was proportional to the size of the coarse fraction o f the sand of which it was composed. Such a hypothesis was confirmed by Lancaster (1982b) who -Found a close relationship between the grain size of the coarse 5th percentile and the spacing of simple and compound transverse dunes i n the northern Namib.
In the Namib sand sea, it is possible to demonstrate that dunes of different types are associated with sands of different characters. But how far this is a Genetic relationship i s uncertain. The closest relationship to emerge is that between areas of low rolling dunes without slip faces and coarse, poorly sorted, multimodal or bimodal sands with mean grain sizes between 1.71 and 2.07 phi and a phi standard deviation between 0.87 - 1.45. Such dunes occur widely in the southern parts of the Namib satid sea, and i n western coastal areas northeast o f Conception Bay. They also occur i n the interdune corridors between linear dunes in northern and central parts of the sand sea. Although most of these dunes are spaced between 80 and 150 m apart, with an amplitude o f 1 - 4 IV they may be up to 30 m high and 1 100 m apart in some areas (e.g. site XVI). The absence of slip faces, even
276
in large forms, may be attributed to the long grain paths of coarse sands when in saltation, an observation suggested also by the widespread association o f mega ripples with these dunes. In all areas sand from low rolling dunes is consistently coarser and less well sorted than that of adjacent linear dunes. The alignments of low rolling dunes without slip faces (Fig. 9d) show them to be transverse to south, and locally south southwest, winds in northern and central parts of the sand sea and to southeasterly winds in southern areas. Although winds from these directions are less frequent than south southwest or southwest winds, they are usually the strongest winds from this sector, and thus the only ones capable of moving coarse sands to any extent.
A 0 0
0 Low rolling dunes
0 0
0
04 1.5
20
x
2-5
Transverse dunes Linear dunes Star dunes
3.0
Mean
Lo
01
b ul Y
Mean
Fig. 10. Grain size and sorting character of sands from different dune types (phi units). Mean values from crest sands at sample sites in Fig. 1.
277
Apart from the above there are few consistent relationships between sand grain size and sorting characteristics and dune types, in the Namib sand sea, as Fi;. 10 indicates. Crestal sands from dunes of star type tend to be the best sorted of all dune sands. Sands from some areas of transverse dunes, at sites IX, XXI and XXII are clearly coarser and less well sorted than sands from linear dunes, but those from site XXIII are very similar to those of compound linear dunes in the southern parts of the sand sea. Sands from compound transverse dunes at site XI are intermediate in composition, but much less well sorted than nearby linear dunes at site 111. There thus seems to be some evidence for a progressive increase i n sorting and a fining of sands from transverse through linear to star dunes. This may be the result of the pattern of sand movements on different dune types. On transverse dunes this i s unidirectional, with sands being buried on avalanche slopes, to be recycled later as these deposits are re-exposed by the advance of the dune. Crestal areas of linear dunes are reversed seasonally and undergo more frequent resorting. On star dunes the sands are exposed to multi directional winds and constant resorting as Folk (1971) predicted. However, the apparent change i n the grain size and sorting character of different dune types in the Namib may equally well be explained by their position relative to sand transport paths, with transverse dunes located i n near source upwind areas and linear and star dunes i n downwind areas. It might be expected that different elements o f a complex pattern of dunes would be composed of sands with different grain sizes. Grain size, by controlling the threshold velocity for sand movement, controls the effective wind regime and thus may strongly influence dune alignments. However, i n most areas o f the Namib sand sea this i s not the case. Figure 1 1 indicates that there i s a progressive change i n grain size and sorting from the interdune and plinths to the crestal areas of the linear dunes. Sands from east flank barchanoid and crossing dunes are very similar i n composition, being slightly less well sorted and coarser than adjacent crest sands, but the differences are n o t significant i n terms of threshold velocities and the movement of sand by the wind. Locally, greater differences between elements of the dune landscape do exist. At Sossus Vlei small barchanoid and reversing dunes on the vlei surface are composed of grey brown sands which are coarser and less well sorted (average mean grain size = 2.06, average standard deviation = 0.49) than adjacent large complex star dunes (F= 2.38, oI = 0 . 2 7 ) .
ID EPWP
X
1.5
EF S C
20
2.5
Mean Fig. 1 1 . Grain size and sorting variations in the linear dune landscape ( p h i u n i t s ) . Mean values from s i t e s in Fig. 1 . ID Interdunes EP,WP East and west plinths X Corridor crossing dunes EF East flank barchanoid dunes S Slip faces C Crests.
IDP
RD
T
T
N X
Mean
Grain size ana sorting differences between dune types a t s i t e Ix (phi u n i t s ) . Mean values from each dune type. ID Interdunes Low rolling dunes without s l i p faces RD T Transverse and barchanoid c r e s t s P Plinths o f linear dunes X WSW - ENE linear dune c r e s t s N N - S linear dune c r e s t s .
F i g . 12.
279
Northwest of the Tsondab Flats (site IX), the N-S linear dunes are disrupted as they cross former river courses and a complex pattern occurs, with transverse and barchanoid dunes aligned transverse to southwest winds, and linear dunes on WSW-ENE and N-S alignments. Between them are areas of low rolling dunes. Differences between the grain size and sorting character of these elements of tt:e dune landscape lend support to the hypothesis of a progressive improvement in sorting from low rolling dunes through transverse to linear and star dunes. Figure 12 shows that clear differences in grain size and sorting exist between adjacent dune types. Thus low rolling dunes without slip faces are significantly coarser and less well sorted (Ks = 2.04 2.27, zI = 0.87 - 0.96) than transverse and barchanoid ridges, which have average mean grain sizes between 2.31 and 2.36 phi and an average st.andard deviation of 0.55 - 0.60. Low,simple linear dunes on WSW-ENE alignments (similar to corridor crossing dunes elsewhere) are finer still ( 6 = 2.46) and much better sorted (oI = 0.35) than transverse dunes but similar in composition to crestal sands of the N-S linear dunes. Grain size may have an important influence on dune spacing, as suggested by Wilson (1972) who argued that dune spacing was a function of the windspeed required to move the coarse sand on the dune surface, which had the effect of "protecting" finer sands from movement. In the Namib sand sea, the size of this fraction is approximated by the grain size of the 5th percentile of surface sands from dune crests. In the northern group of compound transverse dunes, spacing changes from south to north, increasing from 700 - 800 m in the south to a maximum of 1 100 - 1 200 m in central parts of this strip of dunes and then declining again steadily to 300 - 400 m at the Kuiseb river. Although grain size was sampled at widely spaced intervals, it appears to follow a similar pattern, with coarse sands being associated with the most widely spaced dunes. For the sand sea as a whole, a clear relationship between the grain size of the 5th percentile of sand from dune crests and the spacing o f transverse and related dunes can be demonstrated, as F i g . 13 shows (r = -0.88, significant at the 0.05 level). This is perhaps the most important effect of grain size on dune morphology. However, as Lancaster (1981 ) has shown, no such relationship can be established for linear or star dunes, and their spacing shows no relationship to any aspect of their grain size character. If anything, large, widely spaced dunes of these types tend to be composed of finer sand than smaller dunes. The above suggests that, although the spacing of all dunes may be aerodynamically determined, different factors affect the spacing of linear and
280
transverse dunes, because of their different relationship to the wind. Thus in areas of transverse dunes, winds are primarily from one narrow directional sector (Fryberger and Dean, 1979) and the spacing between these dunes can be reasonab!y compared to the width of the zone in which the wind i s disturbed as it passes over the dune. The size of this zone will differ with dune height and also with the effective wind velocity, which is in turn controlled by grain size. Howard _ et _ a1 (1978)have further suggested that the equilibrium size o f transverse and barchanoid dunes may result from the merging of dunes o f different sizes and rates of movement. Dunes of coarser sand will probably move more slowly than those of fine sand, because of the higher effective wind velocities required. They will normally trap more rapidly moving dunes and hence tend to grow in size. Thus dunes of coarse sand will be larger, and more widely spaced than those of finer sands.
i5001
y
= 2 07- 0 O O l X
r - -088 n = 1L
\
- 1000-E m
:: 2 500.
X
0
0
10
05 Groin
20
15
size of 51h percentile
25
lphi un115I
Fig. 13. Relationship between grain size of coarse 5th percentile o f crest sands and spacing of transverse and barchanoid dunes.
The overall tendency for linear dunes is to extend at a small angle to the most persistent sand moving winds (Tsoar, 1978). Because they extend parallel to each other, the rate of movement of one dune will not affect the adjacent dunes and their grain size character will be immaterial in controlling spacing and aerodynamic effects may be paramount. Although Tsoar (1978) suggested that the spacing of linear dunes was 1 1 - 15 times their height, or the distance at which the disturbance of the wind by the dune should have ceased, formative winds blow obliquely to the dune and thus the separation
281
between dunes possible that winds than do normslly been
along the line of the wind i s much greater. However it is wide obstructions, like dunes, may have a greater effect upon narrow ones, such as hedges and lines of trees, which have considered by micrometeorologists.
The influence of wind regimes Compared to many desert areas, a relatively large amount of information on winds i s available for the Namib. Data for the central Namib are summarised et _ a1 (in press). Additional data for stations A,B and E i n fig. 14 by Lancaster _ was obtained from information in Ward (1983). Major spatial variations in available wind energy and directional variability occur in the Namib. In northern areas, and apparently throughout the sand sea, the frequency and strength o f winds from south to southwest directions decreases from the coast to inland areas. Additionally the most frequently occurring wind in this sector changes direction from south or south southwest on the coast to southwest in central areas and west southwest to west on the eastern edge of the sand sea. During the winter months, the eastern Namib in particular i s affected by light to moderate easterly to northeasterly winds corresponding to a katabatic flow from the escarpment zone bordering the desert. From time to time, when regional pressbre gradients are normal to the coast strong to very strong northeast to east "berg winds" blow. These, too, most frequently affect eastern areas. The nature of wind regime changes from north to south are less well known. Descriptions in Royal Navy and South African Air Force (1944) and from unpublished data on file at Gobabeb indicate that the Luderitz area experiences a very high energy wind regime, dominated by south and south southeast winds. Data from Koichab Pan and Aus (Royal Navy and South African Air Force, 1944) indicate a decrease in wind energy inland, but a much windier climate than in northern areas. Winds at Aus are from three main directions, SW, SE and NW, whilst at Koichab Pan, SW and SE sand movements apparently dominate. Resultant sand movement i s thus towards the north. Coastal areas of the Namib are characterised by a relatively high energy, narrow unimodal wind regime. Near Walvis Bay 80% of sandflow is from the SSW (Fig. 14a). Similar, but higher energy, wind regimes occur at Kolmanskop near Luderitz, where 92% of annual sandflow is from SSW and SSE. Thus, as in
282
many other sand seas (Fryberger and Dean, 1 9 7 9 ) , such a wind regime gives rise to transverse and barchanoid dunes. Barchans occur in the southern areas o f higher wind energy.
) wind roses
Fig. 14. Sand movements i n the northern parts of the Namib sand sea. Resultant sandflows in tonnes m-’yr-’Inland, available wind energy drops sharply and the areas of linear dunes in the central and northern parts o f the sand sea are characterised by a low to moderate energy wind regime. Towards the coast northeast to east winds are infrequent and the wind regime i s wide unimodal (Fig. 14b), with 80% of the sand movement from SE to SW directions. In the centre of the area of linear dunes (Fig. 14c and 14d), the annual wind regime is bimodal, with a major
283
sobtherly mode ( S t o SW) accounting f o r 60 - 80% of t h e annual sand movement and a minor N N E t o E mode giving r i s e t o 15 - 20% of annual sandflow.
Locally,
as F i a . l k shows, a t h i r d mode, N N W t o N, appears and i s responsible f o r 6 - 8% of annual sandflow. Seasonally t h e importance of t h e s e modes changes considerably (Fig. 15). The southerly mode p e r s i s t s a l l y e a r , b u t accounts for only 12 - 15% of sandflow i n May t o August and peaks a t 70 - 80% in September t o November. Sand movements from N N E - E dominate in winter months when they average 50 - 60% of monthly sandflows, b u t a r e r a r e in other months. The northerly mode i s most important in November t o February when i t may account f o r 12 - 15% of sandflow.
Jan
April
July
Oct
\
1/25 L
Fiq. 15. Seasonal changes i n sand movements in t h e n o r t h e x p a r t s o f t h e Namib sand sea. Data f o r s t a t i o n s B , C and F of Fig. 14. Resultant sandflows in tonnes m-lyr-’.
284
In the central and northern parts of the sand sea alignments of linear dunes are N-S to NNE-SSW, yet the major sand moving winds are from the south to south southwest and resultant sandflow directions are at angles of 20 - 40' to the dunes. Similar observations led Besler (1980)to conclude that the linear dunes were a product of late Glacial wind regimes when southerly winds may have dominated, and that they were being modified slightly under present conditions. Coi?troversy has surrounded the relationship of linear dunes to wind directions. Process studies by Tsoar (1978) and a review of the available evidence by Lancaster (1982a) have facilitated the development of a general model for linear dune formation. This proposes that linear dunes form in a bimodal wind regime where the modes are less than 180' apart. Linear dunes are not necessarily aligned with the resultant direction of sand movement, but frequently form at a small angle (20 - 40') to the most persistent sand moving winds. These winds are diverted to move sand parallel to the dune where they cross the crest and are instrumental in extending the dune. The contribution of winds from different directions to the overall morphology of the dune is thus related to the angle at which they cross its crestline and to their persistence. In the central and northern parts of the Namib sand sea, persistent SII - SSlf winds cross linear dune crests at optimal angles for dune extension and are thus responsible for the overall trend of the dunes. The effect of NE - E winds i s mainly to reverse the slip face position and to create what are effectively reversing dunes on those parts of the sinuous crestline which are transverse to SSW and NNE winds. Because sand stays i n those parts of the dune, such areas tend to grow upward, and with the contribution of northerly winds in summer, form star peaks. This model i s supported by studies of internal structures by McKee (1982). Such a model can also explain the coexistence of different dune alignments in a single area. Elements of the pattern at angles of 20 - 40' to the persistent winds will be selected and progressively extended. These may be equivalent to the left and right hand oblique elements of Cooke and Warren (1973). In central and northern parts of the Namib sand sea two linear elements occur on N-S and WSW-ENE alignments. The major wind is oblique to them, but transverse to east flank barchanoid elements (Fig. 16). Of the linear oblique elements the N-S is dominant, partly because it lies upwind and thus traps most sand. The corridor crossing dunes are at an angle of 50' to dominant sand movements and thus consequently experience less extension and more deposition. Their form is also emphasised by north and north northwest
285
winds, giving a small reversing ridge.
Linear dunes
East flank
/
Most pers Ist en t sand moving winds
Fie. 16. Relationships between wind direction and linear dune alignment. Towards the east of the sand sea, sand flows from the southerly sector swing round to SW and I.ISI.l (Fig. 14e and 14f). The alignments of linear dunes and their east flank barchanoid dunes i n the central parts of the area follow the pattern, as illustrated by Fig. 9c. The frequency and sand moving effect of east and northeast winds increases eastwards and may even become the dominant sand movement sector. Thus 10 km east of Gobabeb ( F i a . 14e) 64% of sandflow i s from SU and SSW and 15% from NNE to ESE. Close to the eastern margin of the sand sea (Fig. 14f), 53% of sandflow is from NE to E and 35% from l*ISW to WW. Major sandflows are therefore directly opposed in direction and amount, and Under these conditions dune extension winds cross the dunes at close to 90'. will be minimal and the dunes will become effectively reversing types. In such a situation sand stays on the dune, which does not extend, but grows upwards by deposition. Many of the star form dunes along the eastern edge of the Namib are of this type. Yet not all dunes in the eastern Namib are of star form and most are simple and compound linear dunes. Many of the easterly winds are relatively light and the effects of topography are required to concentrate the winds and increase their velocity, as suggested by McKee (1982). Hence the
286
association o f the groups of star and reversing dunes with prominent valleys extending from the escarpment zone. The largest of these valleys i s that o f the Tsauchsb river leading to Sossus Vlei, which also has the largest concentration and maximum size of star dunes. In the southern Namib, winter winds from the northwest also play a part, and in a trimodal wind regime (Fryberger and Dean, 1979) take on a more truly stellate form. Effects of the sand accumulation process Regional changes in wind regimes over the Namib result in a decrease in wind energy and a parallel increase in its directional variability from south to north and west to east. Studies of sand seas elsewhere (Wilson 1971, Mainguet 1978, Fryberger and Ahlbrandt 1979) indicate that such changes are a major influence on the locus of sand accumulations. Thus many sand seas are located in areas of low total or resultant wind energy. This appears to be the case in the Namib, where sand i s moved from higher energy wind regimes with little directional variability in southern and western coastal areas, to accumulate in central and northern areas of the sand sea which have lower energy wind regimes and opposed wind directions. In these areas, the largest dunes occur, representing the zone o f maximum sand accumulation, whilst most dunes in the southern sand sea are relatively small. As has been suggested elsewhere (Mainguet and Callot 1974, Lancaster 1982c) the pattern of dune size and spacing in a sand sea i s a reflection of the ratio between wind energy and sand availability. Areas of low dunes are found where wind energy is great relative to sand supply, and in the case of linear dunes also when most winds blow at a small angle to the dune. Large linear and star dunes will occur where sand i s abundant, but total or resultant wind energy is low, and winds at large angles to the dunes frequent. It is also probable that dunes of simple and compound varieties will predominate where net sand transport dominates and complex and star dunes in zones of net sand accumulation. A similar pattern was observed by blainguet (1978)for the Erg Fachi-Bilma in Niger. One effect of the movement o f sand into zones of net sand accumulation will be to concentrate coarse sands in upwind areas. Coarse sands will be moved by the wind less frequently and then mostly as surface creep or traction load. They will thus move more slowly than fine sand which will be transported more rapidly in saltation by a greater frequency of winds. Consequently, dune types such as low rolling dunes without slip faces will be most common in upwind areas. Such a situation appears to apply in the Namib sand sea.
287
COIiCLUS IONS Spatial variability i n wind regimes can account for most of the observed pattern of dune types in the Namib sand sea. Narrow unimodal wind regimes in coast31 areas produce transverse and barchanoid dunes. Wide unimodal to bimodal regimes are common and are responsible for the formation and maintenance of the linear dunes which dominate the sand sea. Such dunes extend at a small angle (20 - 40') to the most persistent sand moving winds (SSW -SW). Where winds at high angles to the dunes are frequent, deposition rather than dune extension occurs and dunes grow in size. On the eastern margins of the sand sea, topographically induced funnelling of easterly katabatic winds creates complex reversing and star dunes. In such areas, sand, once on the dune, rarely leaves it and dunes attain large sizes. Regional changes i n wind regimes result in the accumulation of sand in central and northern areas of the sand sea, which are characterised by large, widely spaced dunes of complex morphology. Most southern areas are zones o f active sand transport and possess relatively low simple or compound dune forms. The major effect of sand grain size and sorting on dune morphology is its control o f the spacing of transverse and barchanoid dunes. Throughout the sand sea, but particularly in southern areas, coarse bimodal or multimodal sands are associated with subdued or low rolling dunes, generally without slip faces. Elsewhere, although sands with different grain size and sorting characters are associated with different dune types, this is frequently a product of different source areas and of the nature of the sorting process on dunes of differing morphologies. In general, therefore, the character o f the wind regime plays a major role in determining the morphology of dunes in the Namib sand sea. This suggests that further study of winds in sand seas, at varying scales, is of paramount importance in seeking to explain the form and movement of desert dunes. ACKNOWLEDGEMENTS I thank the C.S.I.R. and Transvaal Museum for assistance and the Directorate of Nature Conservation, S.W.A.A. for facilities and permission to work i n the Namib Desert Park. Consolidated Diamond Mines (S.W.A.) are also thanked for permission to investigate dunes in southern parts of the sand sea. E.D. McKee, Andrew Goudie and John Rogers reviewed this paper. I much appreciate their comments and suggestions for its improvement. I thank J.D. Ward o f t h e Kuiseb Environmental P r o j e c t f o r access t o h i s u n p u b l i s h e d wind d a t a .
288
REFERENCES
Alimen, N.H., Buron, M. and Chavaillon, J . , 1958. CaractPres granulometriques s d ' e r g s d u Sahara nord-occidental. Academie des Sciences, de ~ u ~ l o u edunes P a r i s , C.R. 247: 1753-1761. Barna-d, N.S., 1973. Duinformasies in d i e Sentrale Namib. Tegnikon, Des 1973. 5 : 2-13.
B e l l a i r , P.,1953. Sables desertiques e t morphologie eolienne. I n : Proceedings 1 9 t h International Geological Congress, Algiers, 1952. Vol 7 : 113-118. Besler, H., 1980. Die Dunen-Namib: Entstehung u n d Dynamik eines Ergs. S t u t t g a r t e r Geographische Studien. 96: 241 pp. Breed, C.S., Fryberger, S.G., Andrews, S.C., Mc Cauley, C., Lennartz, F. Gebel, D. and Horstman, K., 1979. Regional s t u d i e s of sand seas using Landsat (ERTS) imagery. In: E.D. McKee ( E d i t o r ) , A S t u d y of Global Sand Seas. United S t a t e s Geological Survey, Professional Paper 1052: 305-398. Breed, C.S. and Grow, T., 1979. Morphology and d i s t r i b u t i o n of dunes in sand seas observed by remote sensing. I n : E.D. McKee ( E d i t o r ) , A Study of Global Sand Seas. United S t a t e s Geological Survey Professional Paper 1052: 253-304. Capot-Rey, R., 1947. L'Edeyen de Mourzouk. Travaux I n s t i t u t e de Recherches Sahariennes. 4: 67-109. Capot-Rey, R. and Gremion, M., 1964. Remarques sur quelques sables Sahariennes. Travaux I n s t i t u t e de Recherches Sahariennes. 23: 153-163. Cooke, R . U . and Ilarren, A., 1973. Geomorphology in Deserts. Batsford. Folk, R.L., 1971. Longitudinal dunes of t h e northwestern edge of t h e Simpson Desert, Northern T e r r i t o r y , Australia. 1 : Geomorphology and grain s i z e r e l a t i o n s h i p s . Sedimentology. 16: 5-54. Fryberger, S.G. and Ahlbrandt, T.S., 1979. Mechanisms f o r t h e formation of eolian sand seas. Z e i t s c h r i f t f u r Geomorphologie, N . F . 23: 440-460. Fryberger, S.G. and Dean, G., 1979. Dune forms and w i n d regime. In: E.D. McKee ( E d i t o r ) , A S t u d y of Global Sand Seas. United S t a t e s Geological Survey Professional Paper 1052: 137-170. Holm, D.A., 1960. Desert geomorphology in t h e Arabian Peninsula. Science, 132: 1369-1379.
Howard, A . D . , Morton, J.B., Gad-el Hak, M. a n d Pierce, D., 1978. Sand t r a n s p o r t model of barchan dune equilibrium. Sedimentology, 25: 307-338. Lancaster, J . , Lancaster, N . and Seely, M.K., Climate of t h e c e n t r a l Namib Desert. Madoqua, ( i n p r e s s ) . Lancaster, N., 1981. Aspects of t h e morphometry of l i n e a r dunes of t h e Namib Desert. South African Journal o f Science, 77: 366-368. Lancaster, N . , 1982a. Linear Dunes. Progress in Physical Geography, 6 : 475-504. Lancaster, N., 1982b. Dunes on t h e Skeleton Coast, S.W.A./Namibia: Geomorphology a n d grain s i z e r e l a t i o n s h i p s . E a r t h Surface Processes and Landforms, 7: 575-537. Lancaster, N . , 1982c. Spatial v a r i a t i o n s in l i n e a r dune morphology and sediments in t h e Namib Sand Sea. In: Proceedings 5th SASQUA Conference, Paleoecology of Africa, 15 ( i n p r e s s ) . blcKee, E.D., 1966. Structure of dunes a t Ilhite Sands National llonument, New Mexico ( a n d a comparison w i t h s t r u c t u r e s of dunes from o t h e r selected a r e a s ) . Sedimentology, 1 : 1-69. McKee, E.D., 1979. Introduction t o a s t u d y of global sand s e a s . I n : E.D. McKee ( E d i t o r ) , A S t u d y of Global Sand Seas. United S t a t e s Geological Survey Professional Paper 1052: 3-19. HcKee, E.D., 1982. Sedimentary s t r u c t u r e s in dunes of t h e Namib Desert, South West Africa. Geological Society of America, Special Paper 188: 64 pp. McKee, E . D . and Breed, C.S., 1976. Sand Seas of t h e Norld. United S t a t e s Geological Survey Professional Paper 929: 81-88. McKee, E . D . and T i b b i t t s , G.C., 1964. Primary s t r u c t u r e s of a s e i f dune and assbciated deposits in Libya. Journal of Sedimentary Petrology, 34: 5-17.
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Mabbutt, J.A., 1968. Aeolian landforms in central Australia. Australian Geographical Studies, 6 : 139-150. blainguet, rl., 1976. Propositions pour une nouvelle classification des edifices s ? b l e v x boliens, d'apre's les imape des satellites Landsat 1 , Gemini, Noaa 3. Zeitschrift fur Geomorphologie, N.F. 20: 3 , 275-296. Mainguet, M., 1978. L'erg de Fachi-Bilma (Tchad - Niger). Memoires et Documents C.N.R.S. 18: 184 pp. Mainguet, M. and Callot, Y., 1974. Air photo study of typology and interrelations between the texture and structure o f dune patterns i n the Fachi-Bilma erg, Sahara. Zeitschrift fur Geomorphologie, Supplement Bd 20: 62-63. Monod, Th., 1953. Majabat Al-koubra. Memoires de 1 ' Instuit Francais D'AfriqueNoire. 52: 406 pp. Royal Navy and South African Air Force, 1944. Weather on the coasts o f southern Africa: Vol 2, Pt 1: The west coast of Africa from the River Congo to Olifants River. 1-61. Tsoar, H., 1978. The dynamics of longitudinal dunes. Final Technical Report, European R2search Office, United States Army. 171 ppUard, J.D., 1983. Sand dynamics along the Kuiseb River. In: Huntley, B.J.(Editor) Kuiseb Environmental Project: the development of a monitoring base1 ine. South African National Scientific Programs, Report 68. Varren, A., 1970. Dune trends and their implications in central Sudan. Zeitschrift fur Geomorphologie, Supplement Bd 10: 154-180. llarren, A., 1972. Observations on dunes and bimodal sands in the Tenere Desert. Sedimentology, 19: 37-44. Wilson, I.G., 1971. Desert sandflow basins and a model for the development of ergs. Geographical Journal, 137: 180-197. Nilson, I.G., 1972. Aeolian Bedforms - their development and origins. Sedimentology, 19: 173-210.
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291
THE FLOW IN THE PLANETARY BOUNDARY LAYER R.A.
BRDUN: P o l a r S c i e n c e C e n t e r , U n i v e r s i t y o f Washington, 4057 R o o s e v e l t Way N.E.,
S e a t t l e , l l l a s h i n g t o n 98105, U . S . A .
OVERVIEW The p u r p o s e o f t h i s p a p e r i s t o g i v e some b a s i c i n f o r m a t i o n a b o u t t h e f l o w i n t h e P l a n e t a r y Boundary L a y e r (PBL). I t i s d e s i g n e d t o i n t r o d u c e t h e f l o w f e a t u r e s which a r e l i k e l y t o b e s i g n i f i c a n t t o t h e s e d i m e n t o l o q i s t s . c o n c e n t r a t e d o n t h e a v e r a g e , o r mean f l o w . flow analysis i s presented.
Therefore i t i s
However, a n o v e r a l l p i c t u r e o f PBL
T h i s i s t o p r o v i d e an h i s t o r i c a l p e r s p e c t i v e , t h e
problems f a c e d , t h e a s s u m p t i o n s made and t h e t r i u m p h s needed i n o r d e r t o f u r n i s ' l
a f i n a l , r a t h e r sirriple, p i c t u r e o f t h e PBL f l o w .
T h i s knowledae s h o u l d h e l p
geophysical f l u i d d y n a m i c i s t s t o i n f e r c h a r a c t e r i s t i c f l o w c o n d i t i o n s from sedimentary d e p o s i t c h a r a c t e r i s t i c s .
I t should also serve t o e x p l a i n t h e nature
of d e p o s i t s based o n t h e PBL f l o w c h a r a c t e r i s t i c s .
A t t h e minimum, i t s h o u l d
a l e r t anyone c o n c e r n e d w i t h phenomena a t a f l u i d b o u n d a r y w h i c h has a s c a l e w h i c h i s s i g n i f i c a n t w i t h r e s p e c t t o t h e boundary l a y e r depth t o t h e f a c t t h a t secondary flows may b e i m p o r t a n t . L i k e t h e w e l l - k n o w n e x p r e s s i o n o f t h e f o r e s t n o t seen f o r t h e t r e e s , i n v e s t i g a t i o n s o f t h e f l o w i n t h e PBL f r e q u e n t l y a r e p r e o c c u p i e d w i t h t h e t r e e s , i n t h i s case r e p r e s e n t e d b y t u r b u l e n t e d d i e s .
E l a b o r a t e i n s t r u m e n t a t i o n and t e c h n i q u e s
have been d e v e l o p e d f o r m e a s u r i n g , c o u n t i n g , c a t e g o r i z i n g and a v e r a q i n g t h e t u r b u l e n c e i n v a r i o u s ways.
T h i s i s a h i g h l y i n s t r u c t i v e and a v e n e r a b l e p r o c e s s
f o r b u i l d i n g a l a r g e s c a l e s o l u t i o n from fundamental b u i l d i n g b l o c k s .
However,
t h e r e a r e two p r o b l e m s w i t h t h i s p r o c e s s . F i r s t , t h e b u i l d i n g blocks, t h e t u r b u l e n t eddies, a r e s t i l l d i f f i c u l t t o p i n down w i t h s p e c i f i c measurements, o r w i t h a a e n e r a l d e f i n i t i o n . mixing motion o f molecules,
U n l i k e t h e random,
t h e t u r b u l e n t e d d i e s a r e d e p e n d e n t o n t h e mean f l o w ,
and t e n d t o m i n g l e b e f o r e m i x i n g w i t h v a r i o u s d e g r e e s o f e f f i c i e n c y , a g a i n depending o n t h e mean f l o w c h a r a c t e r i s t i c s .
I n i t i a l l y , small scale eddies i n t e r -
a c t t o f o r m a mean f l o w , w h i c h i s t h e n u n s t a b l e t o i n f i n i t e s i m a l p e r t u r b a t i o n s , and l a r g e s c a l e e d d i e s d e v e l o p .
I n some c a s e s t h e s e e d d i e s r e a c h a n e q u i l i b r i u m
and r e s u l t i n a s e c o n d a r y f l o w .
These o r g a n i z e d e d d i e s become p a r t o f ( a n d
a l t e r ) t h e mean f l o w s t a t e .
I t i s d i f f i c u l t t o measure t h e l a r g e e d d i e s ( w h i c h
have d i m e n s i o n s o f k i l o m e t e r s i n t h e a t m o s p h e r e ) .
The s e c o n d a r y f l o w i s a f i n i t e
p e r t u r b a t i o n , w i t h m a g n i t u d e s a r o u n d 10% o f t h e mean f l o w . d i f f e r e n t f r o m t h e random t u r b u l e n t e d d y v e l o c i t i e s .
This i s n o t very
Careful averaging i s
292 necessary t o d i s t i n g u i s h v a r i o u s s c a l e t u r b u l e n c e eddies from t h e organized secondary f l o w .
P u r s u i n g t h e f o r e s t a n a l o q y , one t y p e o f t r e e b u i l d s a f o r e s t ,
i n which another type o f f r e e f l o u r i s h e s , q i v i n g r i s e t o another type o f forest. T h e r e e x i s t s a good i l l u s t r a t i o n o f t h e p o s i t i o n o f t h e s c i e n t i s t t r y i n q t o tie i s l i k e a b l i n d e x p e r i m e n t e r a t t e m p t i n o t o
measure g e o p h y s i c a l t u r b u l e n c e .
determine t h e v e h i c u l a r t r a f f i c c h a r a c t e r i s t i c s on a r o a d which c a r r i e s bicycles H i s i n s t r u m e n t s c o u n t o n l y w h e e l s , and he knows t h e t o t a l number o f
and c a r s .
v e h i c l e s and p e o p l e t r a n s p o r t e d . 2.9 w h e e l s / v e h i c l e ,
He i s l i k e l y t o come up w i t h a n a v e r a q e of
and w i l l p r o b a b l y d e v e l o p a model w i t h a v e h i c l e w i t h t h r e e
w h e e l s , one o f w h i c h d o e s n ' t t o u c h g r o u n d p a r t o f t h e t i m e ( a m o t o r c y c l e w i t h sidecar).
An a v e r a g e number ( s a y , 1 . 7 ) o f p e o p l e c o u l d b e p l a c e d i n t h e v e h i c l e ,
and t h e a v e r a g e f l o w m i g h t be s u c c e s s f u l l y p a r a m e t e r i z e d - - f o r t h e s e s p e c i f i c conditions.
When t h e c o n d i t i o n s change, e.g.,
r a i n i n h i b i t i n g c y c l i n n , o r an
OPEC gas embargo r e s t r i c t i n g sedan d r i v i n g , t h e p a r a m e t e r i z a t i o n m u s t change. F u r t h e r m o r e , d r a s t i c changes i n t h e model a r e n e c e s s a r y when t h e s t a t i s t i c s change, such as changed d r i v i n g h a b i t s f o r a 2.3 w h e e l / v e h i c l e a v e r a n e , o r a new v e h i c l e ( " o f t h e second k i n d " ) c o n c e p t f o r a 3 . 2 w h e e l / v e h i c l e a v e r a g e .
The
s c i e n t i s t i s u s u a l l y up t o p r o d u c i n g i m a g i n a t i v e r e v i s i o n s , b u t t h e model g e n e r a l l y becomes cumbersome.
The p r o b l e m , o f c o u r s e , i s t h a t t h e p a r a m e t e r i z a -
t i o n i s based o n a f a u l t y u n d e r s t a n d i n g o f t h e b a s i c e l e m e n t s o f t r a n s p o r t a t i o n . The o t h e r p r o b l e m i s a l a c k o f a good p i c t u r e o f t h e f o r e s t
=
t h e mean f l o w .
I n g e o p h y s i c a l f l u i d dynamics, t h e s c a l e s a r e l a r q e , a n d i n s t r u m e n t s and sample
volumes a r e s m a l l .
F o r i n s t a n c e , t h e l a r g e , two-km r o l l s w h i c h I w i l l d i s c u s s ,
may t a k e 20 m i n u t e s t o h o u r s t o p a s s o v e r a g i v e n p o i n t . a l o n g r e c o r d and a s t e a d y s t a t e f i e l d i s r e q u i r e d .
To d e t e c t t h e s e eddies,
C o n s e q u e n t l y , t h e y were not
g e n e r a l l y r e c o g n i z e d a s a n i n t e g r a l p a r t o f t h e mean PBL f l o w u n t i l r e c e n t l y . E a r l i e s t observations were o f seagulls, which soared i n c i r c u l a r p a t t e r n s f o r l o w w i n d s ( c o n v e c t i o n a l o n e ) and l i n e a r p a t t e r n s p a r a l l e l t o t h e mean w i n d f o r moderate winds.
G l i d e r p i l o t s learned t o recognize t h e l i n e a r u p d r a f t regions D i s t a n c e r e c o r d s w e r e s e t by
b y f o l l o w i n g t h e c l o u d s w h i c h accompanied them.
f l y i n g a l o n g t h e mean w i n d u n d e r t h e c l o u d " s t r e e t s " .
They a l s o u n d o u b t e d l y
became aware o f t h e d o w n d r a f t s e x i s t i n g h a l f w a y between a d j a c e n t s t r e e t s . The w e a l t h o f s a t e l l i t e p i c t u r e s s h o w i n g u b i q u i t o u s c l o u d s t r e e t s and l i n e a r c h a r a c t e r i s t i c s o f many l a r g e s c a l e f l o w s e s t a b l i s h e d t h e i m p o r t a n c e o f these flow patterns.
However, measurements o f t h e windspeeds and d i r e c t i o n s o f both
t h e r o l l f l o w a n d t h e l a r g e s c a l e mean f l o w has been a t t e m p t e d o n l y i n t h e past decade.
C u r r e n t l y , t h e r e e x i s t many q u a l i t a t i v e i n d i c a t i o n s o f t h e s e steady-state
s e c o n d a r y f l o w e d d i e s embedded i n t h e PBL---so
many t h a t we m u s t assume t h a t they
a r e a n i n t r i n s i c p a r t o f t h e mean f l o w s o l u t i o n . ments have been v e r y l i m i t e d .
However, q u a n t i t a t i v e measure-
293 There a r e d i v e r s e methods a v a i l a b l e f o r d e s c r i b i n g t h e f l o w i n t h e PBL.
They
range i n c o m p l e x i t y from f i n i t e d i f f e r e n c i n g t h e complete e q u a t i o n s t o t h e s i m p l e a n a l y t i c s o l u t i o n s o f f e r e d by f i r s t o r d e r c l o s u r e ( K - t h e o r y ) .
The model w h i c h
I
am advocating s t a r t s w i t h t h e Navier-Stokes e q u a t i o n s , and accents t h e Boussinesq eddy v i s c o s i t y assumption f o r t h e s m a l l s c a l e eddies o n l y .
A f i n i t e perturbation
i s c a l c u l a t e d u s i n g energy c r i t e r i a f o r e q u i l i b r i u m , and t h i s produces l a r g e s c a l e coherent s t r u c t u r e s embedded w i t h i n t h e boundary l a y e r as p a r t o f t h e mean f l o w .
I b e l i e v e t h a t we a r e now a t t h e p o i n t where we can g i v e a f a i r l y good p i c t u r e Of
the mean PBL f l o w t o g e t h e r w i t h i t s secondary f l o w , and t h e v a r i a b i l i t y o f
both w i t h changing c o n d i t i o n s .
I n t h i s p i c t u r e , I b e l i e v e there are explanations
f o r v a r i o u s s e d i m e n t a t i o n phenomena. I t i s a p p a r e n t t h a t t h e wave forms o c c u r r i n g i n g e o p h y s i c a l f l u i d dynamics a r e s i m i l a r t o those f o u n d i n sedimentary d e p o s i t s .
The s e i f sand dunes 100
meters h i g h , hundreds o f km l o n q , a r e b u t one v e r y s t r i k i n g example o f t h i s f a c t . The i n i t i a l s a t e l l i t e p i c t u r e w h i c h i n s p i r e d me (and o t h e r s ) t o s t u d y t h e two dimensional waves w i t h c r e s t s and t r o u g h s n e a r l y p a r a l l e l t o t h e mean f l o w i n the atmosphere was t h a t o f c l o u d s t r e e t s shown i n f i g u r e 1 .
However, a l s o a v a i l -
able a t t h a t t i m e were Gemini photos of t h e s e i f sand dunes as seen i n f i g u r e 2. The t a n t a l i z i n g s i m i l a r i t y i n s c a l e s and wavelenqths s t r o n g l y suqqested atmospheric f o r c i n g .
However, when I f i r s t p r e s e n t e d t h i s idea, some a t m o s p h e r i c
p h y s i c i s t s were i n c r e d u l o u s t h a t a p e r t u r b a t i o n f l o w i n t h e PBL c o u l d move such mountains o f sand. be s t r o n g evidence.
However, t h e remarkable c o i n c i d e n c e of s c a l e s seems t o me t o A paper by Hanna (1969) documented these s i m i l a r i t i e s .
As
y e t , I have n o t seen a q u a n t i t a t i v e a n a l y s i s f r o m t h e s e d i m e n t o l o q i s t ' s p o i n t o f view y i e l d i n g t i m e s c a l e s . I n t h e remainder o f t h e paper
I w i l l t r y t o p r o v i d e a summary o f o u r under-
standing o f t h e PBL f l o w i n c l u d i n g t h e b a s i s f o r development o f secondary f l o w s , which a r e s i g n i f i c a n t f o r sedimentary d e p o s i t s on c e r t a i n s c a l e s . d i s c u s s t h e c o n d i t i o n s i n w h i c h v a r i o u s f l o w s develop.
Then I w i l l
For f u r t h e r d i s c u s s i o n ,
see Brown (1980). HISTORY
The g e n e r a l e q u a t i o n s o f f l u i d f l o w i n a continuum, t h e Navier-Stokes equat i o n s , were a v a i l a b l e around 1880.
Immediately, Boussinesq o f f e r e d t h e h y p o t h e s i s
t h a t c o m p l e t e l y t u r b u l e n t f l o w , as i n a c o n v e c t i v e PBL, c o u l d be t r e a t e d assuming the momentum and h e a t t r a n s p o r t were accomplished by t h e s m a l l t u r b u l e n t eddies i n a s i m i l a r f a s h i o n t o t h e m o l e c u l a r m o t i o n which u n d e r l i e s d i f f u s i o n .
I n 1894,
i n h i s two-year d r i f t a c r o s s t h e A r c t i c Ocean, Nansen observed t h a t t h e pack i c e m o t i o n was a b o u t 45" t o t h e r i g h t o f t h e s u r f a c e wind v e c t o r . assigned r e s p o n s i b i l i t y t o t h e r o t a t i n g frame o f r e f e r e n c e .
He c o r r e c t l y I n 1904, Ekman
i n c l u d e d t h e v i r t u a l C o r i o l i s f o r c e i n t h e Navier-Stokes e q u a t i o n s and found an
2 94
F i g u r e 1 . A p o l l o photoqraph o f t h e Georqia c o a s t , 4 A o r i l 1968, frori 125 miles. The c l o u d s t r e e t s a r e over t h e l a n d , a l i g n e d approxiiiiately with the mean wind >which i s due n o r t h . The a r e a p i c t u r e d i s about 100 km s q u a r e , t h e cloud rows a r e 2-3 km a p a r t a t about 1 km h e i q h t . North i s a t t h e t o D .
295
Figure 2 . Gemini ohotoqraph o f s e i f sand dunes. The a r e a o i c t u r e d i s about 100 km s q u a r e and t h e dune rows a r e s e p a r a t e d by a b o u t 2 k i l o m e t e r s . Mean wind f l o w i n t h e a r e a i s p a r a l l e l t o t h e dunes.
296
exact solution f o r the balance between C o r i o l i s , viscous and pressure gradient f o r c e s . This i s the Ekman PBL s o l u t i o n , which p r e d i c t s a velocity p r o f i l e in the' form of a logarithmic s p i r a l (Ekman, 1905). A sketch i s shown i n f i q u r e 3. Looking down, the surface flow i s 45" counterclockwise from the f r e e stream flow. A p l o t of the locus of the velocity vectors i s called a hodoqraph, shown in the
x-y plane i n f i g u r e 3. Each level of flow represents the flow down the pressure gradient, retarded by f r i c t i o n and viewed from a ( d i f f e r e n t ) non-inertial frame of reference. This i s a unique solution of the flavier-Stokes equations f o r three f o r c e s , which a r e l e f t in the boundary layer l i n i t equations. I t i s so eleqant t h a t i t i s taught i n every oceanography and atmospheric science beqinninq dynamics c l a s s . This i s d e s p i t e the f a c t t h a t a qenuine Ekman s p i r a l has nossibly never been observed. I say possibly because one may have been found i n the high Arctic Ocean under the pack i c e i n a 1960's expedition.
Projection of Wind Vectors on S u r f a c e is Hodograph
of E k m a n Spiral (height in meters)
Figure 3. A sketch of PEL wind v e l o c i t i e s above the e a r t h . the vectors on t h e surface forms a hodograph.
The projection of
297
A SKETCH O F THE ANALYTIC UNDERSTANDING OF MEAN PBL FLOW no /
observational t
I
2-1 ayer oa t c hed so 1 u t i on
S t u a r t --+ Fa1 1e r Lilly's Instability of the Ekman s o l u tion
3 o u s s i n e sq ' s vi scosi t y
c
I
1880
Benard ' s convection cell s 1900
1920
1940
Modified Ekman solution with secondary flow s %abl i za t i on
1960
Ekman la ve r -obse rva tion in A rc tic Ocean
1980
2000
YEAR
Figure 4. A sketch of progress i n the a n a l y t i c understanding of the average PBL flow s i n c e c i r c a 1880.
I can move quickly t h r o u q h t h e h i s t o r y by usinq the schematic of f i q u r e 4. Now, I would never show t h i s hiqhly personalized view of the history of PBL u n d w standing t o a group of boundary l ay er s c i e n t i s t s . This i s because I have l e f t o f f many small v e r t i c a l jumps i n understandinq.
This re pre se nts how well I , a s a
qeophysical f l u i d dynamicist, would have understood the nean flow in the PBL havinq read the l i t e r a t u r e u p t o any qiven d a t e . Takinq o f f from t h e base of the HavierStokes equations, the f i r s t j u m p was the eddy v i s c o s i t y hyoothesis.
Benard's
experiments showed r eq u l ar convection cel I s , and Rayleiqh provided the mathematical solution. This was a good beginning, and s a t e l l i t e photos reveal such c e l l s on geophysical s c a l e s ( f i g u r e 5 ) . Continuing o n the schematic, note t h a t I have made 100% understanding a t somewhat more than present knowledge with due modesty, and an eye t o f u r t h e r fundinq. This i s a l s o done with t h e recognition t h a t PBL s c i e n t i s t s of Ekman a n d Ta ylor' s day probably f e l t t h a t a near 100% understanding had been achieved with the Ekman/ Taylor s o l u t i o n s .
Figure 6 shows linked Ekman s p i r a l s and the correspondinq hodo-
graphs i n t h e ocean-atmosphere i n t e r f a c e .
Another Ekman s p i r a l would occur a t the
ocean bottom.
However, t h er e was steady erosion i n the confidence in Ekman's solution a s experiments continued t o f a i l t o f i n d an Ekman s p i r a l . I n the 193Os, Rossby and ilontqomery added t h e r ecen t l y developed Prandtl loqarithmic la ye r t o the bottom of t h e Ekman l ay er f o r a two-layer s o l u t i o n .
This resolved the problerr
298
F i g u r e 5 . NOAA 4 p h o t o q r a p h o f c l o u d s i n t h e B e r i n g Sea. The a r e a i s a p p r o x i m a t e l y 1600 km s q u a r e ; c l o u d s t r e e t s a r e s e F a r a t e d b y 2 - 5 km and t h e downstream c e l l s a r e a b o u t 20-50 km i n d i a m e t e r . The f l o w i s f r o m t h e u p p e r r i q h t (NE) t o t h e l o w e r l e f t . I t changes f r o m a s t r o n g f l o w i n t h e c l o u d s t r e e t r e g i o n t o l o w sFeeds i n t h e c e l l u l a r r e g i o n . b e t w e e n t h e m a t h e m a t i c a l Ekman s o l u t i o n and t h e o b s e r v a t i o n a l l y e s t a b l i s h e d o a r a l l e l flow loqarithniic v e l o c i t y p r o f i l e i n the l a y e r near the surface.
However,
l a c k o f o b s e r v a t i o n a l a q r e e m e n t c a s t i n c r e a s i n g d o u b t on t h e s o l u t i o n s f o r t h e PBL flow.
I n t h e 19605, t h e Ekman m a t h e m a t i c a l s o l u t i o n was f o u n d t o be u n s t a b l e t o
infinitesimal perturbations.
S i n c e t h e PBL i s r i f e w i t h i n f i n i t e s i m a l perturbations,
and t h e y qrow e x n o n e n t i a l l y , t h i s s i g n a l l e d t h e end of a b e l i e f t h a t Ekrnan's solut i o n c o u l d be exoected t o s a t i s f a c t o r i l y r e p r e s e n t boundary f l o w .
299
e of reference
velocity-30cm/sec
Figure 6 . Sketch of linked Ekman s p i r a l s in t h e ocean and atmosphere. Note the s c a l e change from atmosphere t o ocean. Many meteorologists assumed t h a t t h e f a u l t l a y in the eddy-viscosity assumption, Progress was made in understanding the turbulence. However understandinq, or p r e d i c t a b i l i t y , and the attempts t o d i r e c t l y account f o r the turbulence came t o the f r o n t .
of the mean flow reached a new low. I n c i d e n t a l l y , the Arctic observation of t h e Ekman s p i r a l was compatible with the mathematics, since the Reynolds number of the flow was very low, below the c r i t i c a l f o r i n s t a b i l i t i e s . Usual flows in the ocean or atmosphere a r e a t Reynolds numbers q r e a t l y in excess of the c r i t i c a l .
300
rileanwhile, t h e r e were many i n d i c a t i o n s i n t h e ocean and t h e atmosphere t h a t p e r s i s t e n t , s t e a d y - s t a t e wave s o l u t i o n s e x i s t e d i n t h e mean f l o w .
Based on obser-
v a t i o n s o f g l i d e r p i l o t s and s e a g u l l s , K u e t t n e r d i s c u s s e d t h e presence o f r o l l v o r t i c e s i n t h e atmosphere, and Langmuir, i n v e s t i g a t i n q windrows i n t h e ocean, proposed and measured r o l l v o r t i c e s i n t h e w a t e r boundary l a y e r . I n 1970, a s o l u t i o n was found wherein t h e i n s t a b i l i t y qrew o n l y t o a f i n i t e p e r t u r b a t i o n v a l u e and t h e n came t o e q u i l i b r i u m .
There i s now a s t a b l e , steady-
s t a t e s o l u t i o n t o t h e N a v i e r - S t o k e s e q u a t i o n s f o r t h e PBL which i n c l u d e s an embedded secondary f l o w o f h o r i z o n t a l v o r t i c e s w i t h a l t e r n a t i n q c i r c u l a t i o n and o r i e n t e d a l o n g t h e mean f l o w d i r e c t i o n o f f l o w .
Furthermore, t h e presence o f t h e s e vortices
e x p l a i n s why o b s e r v a t i o n s o f t h e f l o w which q e n e r a l l y were o f t o o s h o r t d u r a t i o n t o i n c l u d e more t h a n a p o r t i o n o f a r o l l , had i n c o n s i s t e n t hodoqraphs. shown i n f i q u r e s 7 and 8.
This f a c t i s
F i q u r e 7 shows a schematic o f t h e r o l l c i r c u l a t i o n i n
t h e Ekman l a y e r and a hodoqraph o f t h e mean f l o w .
F i g u r e 8 shows t h e hodograph
which would r e s u l t i f a q u i c k ( s e v e r a l minutes i n t h e atmosphere) a s c e n t b a l l o o n were used t o measure t h e v e r t i c a l v e l o c i t y p r o f i l e a t v a r i o u s s t a t i o n s i n t h e r o l l . The dashed l i n e s i n f i g u r e 4 r e p r e s e n t t h e assumed l e v e l o f u n d e r s t a n d i n g o f many members of t h e meteorology community, who e i t h e r d i d n ' t know o f , o r questioned t h e v a l i d i t y o f t h e o b s e r v a t i o n s and t h e i n s t a b i l i t y t h e o r y . i s s o n i c e t h a t i t was h a r d t o q i v e up.
A l s o , Ekman's solution
T h i s p e r s i s t e n c e was j u s t i f i e d somewhat
when t h e q u a s i - n o n l i n e a r s o l u t i o n i n d i c a t e d t h a t t h e Ekman s o l u t i o n was s t i l l valid if one added a f i n i t e p e r t u r b a t i o n secondary f l o w t o t h e eddy-laminar f l o w solution. Much o f PBL modeling r e q u i r e s o n l y an average v a l u e f o r t h e momentum, h e a t o r moisture f l u x f o r a l a r g e scale region.
For i n s t a n c e , t h e g r i d D o i n t s on an atmos-
p h e r i c g e n e r a l c i r c u l a t i o n model a r e 400 km a p a r t .
I t was p o s s i b l e t o determine an
eddy v i s c o s i t y c o e f f i c i e n t t o p a r a m e t e r i z e t h e a d v e c t i v e e f f e c t o f t h e l a r q e eddies. However, t h e b a s i c d i f f u s i o n e q u a t i o n i s Q = K dS/dz, where Q i s t h e d i f f u s e d quant i t y , and S t h e p o t e n t i a l f u n c t i o n , e.q.,
or
9=
heat, S = temperature.
when Q i s momentum f l u x , S i s v e l o c i t y ;
The l i m i t s r e q u i r e d by t h e fundamental theorem o f
CalCUlUS t o d e f i n e t h e d e r i v a t i v e must n o t a f f e c t t h e random c h a r a c t e r o f t h e eddies which a r e modeled i n t h e eddy c o e f f i c i e n t K.
T h i s means t h e r e must be a l o t o f them
l e f t i n t h e t h i n l a y e r where t h e average d e r i v a t i v e i s d e f i n e d ( t h e continuum hypothesis).
T h i s r e q u i r e s an eddy d e f i n i t i o n l i k e t h e b l i n d t r a n s p o r t m o d e l e r ' s
v e h i c l e of t h e second k i n d .
S t i l l , f o r areas l a r g e enouqh t o c o n t a i n many r o l l s ,
a h y p o t h e t i c a l v a r i a b l e K - p r o f i l e can work r e a s o n a b l y w e l l i n m o d e l i n q s p e c i f i c average v a l u e s . necessary.
However, i n f i n i t e , and d i s c o n t i n u o u s
K d i s t r i b u t i o n s were sometimes
301
Typical Secondary Flow in the Planetary Boundary Layer (Modified Ekman Layer) Mean Wind Hodograuh
F i g u r e 7. S k e t c h o f s e c o n d a r y r o l l f l o w w i t h a mean w i n d hodograph. The a l t e r n a t i n g r o t a t i o n o f t h e v o r t i c e s p r o d u c e s c l o u d bands i n enhanced v e r t i c a l v e l o c i t y r e g i o n s . The t u r n i n g o f t h e w i n d f r o m f r e e s t r e a m ( G e o s t r o p h i c ) t o t h e s u r f a c e t h r o u q h a n g l e E i s shown.
Z/8=1
STAT1ON A 1
-o.o#oo
-0.OkGG5k-
-0.050 -0.025
v/u, SHORT TERM VELOCITY
HODOGRAPHS IN THE PEL
F i g u r e 8. Hodographs s h o w i n g t h e i n s t a n t a n e o u s v e l o c i t y p r o f i l e s a t s t a t i o n s A, B, and C i n f i g u r e 7. 6 i s t h e Ekman s c a l e h e i q h t ; PBL h e i q h t i s a n p r o x i m a t e l y ~ 6 .
302 P o l l u t i o n n o d e l e r s u s i n g t h - l a r g e s c a l e a v e r a q e d Ks, may have f o u n d t h e i r averane p o l l u t a n t v a l u e s w e r e adequa-t?, b i i t a n o t h e r c r i t e r i a ,
t h e maximun c o n c e n t r a t i o n s ,
were o f t e n o b s e r v e d t o b e muc:i g r e a t e r t h a n t h i s a v e r a q e . c o n p a r a b l e t o t h e dent11 o f t h e l a y e r . t i o n o f t h e secondary f l o i r s .
T h i s o c c u r r e d o n a scale
These phenomena a r e e x p l a i n e d b v a considera-
The r o l l s cause a l t e r n a t i n q l i n e s o f a s c e n d i n n and
d e s c e n d i n g n o t i o n , a n d c o r r e s p o n d i n g l i n e s o f c o n v e r q e n c e a n d d i v e r q e n c e a t t h e to? and b o t t o m as seen i n f i g u r e 9 .
These l i n e s o f c o n c e n t r a t i o n have been o b s e r v e d
i n moisture, insects, e l e c t r i c f i e l d s ,
t u r b u l e n c e and d u s t .
SCJW i n v e s t i o a t i o n s o f t h e t r a n s F o r t o f s e d i m e n t have been c o n c e r n e d w i t h l a w
s c a l e a v e r a q e s w h i l e o t h e r s have been c o n c e r n e d w i t h sria11 s c a l e v a r i a t i o n s , eacli w i t h an a p p r o p r i a t e PBL f l o i i s o l u t i o n . t h i n q about the other.
O f c o u r s e , each s c a l e f l o u c a n t e l l son’?-
For i n s t a n c e , i n t h e atmos?here, the s i m l l s c a l e c l o u d
s t r e e t s a r e r o u t i n e l y u s e d t o d e t e r m i n e t h e d i r e c t i o n o f t h e mean f l o w ( n a r a l l e l t o the s t r e e t s ) .
L i k e w i s e c e r t a i n p a t t e r n s o f s e d i n e n t a t i o n t e l l s o m e t 9 i n g about
t h e p r e v a i l i n q mean f l o w d i r e c t i o n and n i a q n i t u d e .
Soiw u n d e r s t a n d i n ? o f t h e theory
f o r secondary flow w i l l a i d i n t h i s e v a l u a t i o n .
F i g u r e 9. S k e t c h o f S e c o n d a r y f l o w v o r t i c e s s h o w i n q c o n v e r g e n c e / d i v e i - g e n c e p a t t e r n s , d e n s i t y o f a n y o a s s i v e m a t e r i a l and mean f l o w a v e r a g i n g volume.
303 Secondary Flow and Naves Eknan's solution was f o r the three force balance, P /p - f V - K U z z = 0 X
P /p
Y
f U
f
-
K Uzz= 0
where P i s pressure, p i s d e n s i t y , f i s the Coriolis parameter equal t o twice the local r o t a t i o n of the e a r t h , U a n d V a r e the x and y components of the velocity, K i s eddy v i s c o s i t y , a n d s u b s c r i p t s x , y a n d z denote p a r t i a l d i f f e r e n t i a t i o n .
1.2
-
I
E = 20"
,'
1.0 -
I
F .---.
Q'OO
L
8.2
8.2
0.8 -
is=i I
0
s= I
I
3
20.6 -
I
I I
8
I
\ \ \ \\
\
0
-0.4
-0.2
v/
0.0
ug
0.2
-0.4
-0.2
0.0
02
v / ug
Figure 10. Hodographs of mean flow solutions from ( 1 ) (dashed) and ( 2 ) ( s o l i d ) . E i s t h e angle between the freestream flow (Ug) end the r o l l a x i s ; 6 i s the Ekman s c a l e height = (ZK/f)k. The l e f t p r o f i l e i s f o r s t a b l e layer s t r a t i f i c a t i o n , the r i g h t i s f o r unstable s t r a t i f i c a t i o n . The solution i s a s p i r a l o r a hodoqraph, shown in f i g u r e 10 as the dashed l i n e s . llhen the Reynolds number f o r the flow (based o n the heiqht, H), 'ie = U H / K exceeds a c e r t a i n , small value, the i n e r t i a l force i s l a r q e with respect t o the viscous f o r c e .
The f l u i d cannot s u s t a i n the shearino and develops a more e f f i c i e n t momentum f l u x ( s t r e s s ) mode; i n s t a b i l i t y waves a r i s e . These qrow u n t i l the\( destroy o r rearranqe the flow s o l u t i o n . This can be shown by considerino the t o t a l velocity
304 as c o n s i s t i n g o f a n a v e r a g e f l o w w h i c h i s t h e h o r i z o n t a l a v e r a g e o v e r t h e e n t i r e domain; p l u s a f i n i t e p e r t u r b a t i o n v e l o c i t y w h i c h has t h e f o r m o f t h e i n s t a b i l i t y waves and a z e r o mean o n l a r g e h o r i z o n t a l s c a l e s ; p l u s t h e p e r t u r b a t i o n component w h i c h r e D r e s e n t s t h e random, s m a l l s c a l e t u r b u l e n c e ,
-
U = U,(mean)
-
+ u p ( z e r o mean waves) + u- ' ( t u r b u 1 e n c e ) . x
The s m a l l s c a l e p e r t u r b E t i o n s , c o s i t y t e r m , K dU/dz.
(u',
v',
w'),
a r e r e p r e s e n t e d i n t h e eddy v i s -
The ( u p , v 2 , w 2 ) t e r m s a r e a l l o w e d t o grow t o a f i n i t e
p e r t u r b a t i o n and f o r m t h e c o h e r e n t e d d i e s o f t h e s e c o n d a r y f l o w .
The shape i s
determined from t h e i n s t a b i l i t j / a n a l y s i s o f t h e f l o w p r o f i l e determined from the solution t o (1).
The m a g n i t u d e i s d e t e r m i n e d f r o m t h e e n e r g y e q u a t i o n f o r t h e
t r a n s f e r o f e n e r q y between t h e mean f l o w and t h e p e r t u r b a t i o n .
When t h e mean pllis
f i n i t e p e r t u r b a t i o n t e r m s a r e s u b s t i t u t e d i n t o ( 1 ) and a h o r i z o n t a l a v e r a g e i s __ t a k e n , one a d d i t i o n a l t e r m , w 2 v 2 i s n o n z e r o , Px/p
- f V - K Uzz = 0
P/p
+ f U - K V Z z = G
Y
Re
The s o l u t i o n t o t h i s inhomogeneous e q u a t i o n f o r t h e mean f l o w c a n be w r i t t e n as s i m p l y t h e sum o f t h e Ekman s o l u t i o n p l u s a mean component due t o t h e secondary flow,
_Urn
-
= UE
f i 7 u r e 10.
+ -U p .
The m o d i f i e d hodographs a r e shown b y t h e s o l i d l i n e s i n
These a r e t h e new s o l u t i o n s f o r t h e a v e r a q e o v e r t h e e n t i r e domain.
If a n a v e r a g e i s t a k e n w h i c h i s s h o r t w i t h r e s p e c t t o t h e s e c o n d a r y f l o w wavelength, o r of i n s u f f i c i e n t time f o r several wavelenqths t o advect p a s t t h e s i t e , then the
a v e r a g e w o u l d n ' t b e z e r o , and hodographs l i k e t h o s e o f f i g u r e 8 m i g h t
be o b s e r v e d , d e p e n d i n g o n t h e s t r a t i f i c a t i o n o f t h e l a y e r and where i n t h e a l t e r n a t i ng c o n v e r g e n c e / d i v e r q e n c e zones t h e a v e r a q e was t a k e n . I n Ekman's s i m p l e s t s o l u t i o n , a c o n s t a n t K was assumed.
T h i s i s c r i t i c i z e d on
t h e a r g u m e n t t h a t K m u s t go t o z e r o a t t h e b o t t o m , where t h e e d d i e s m u s t v a n i s h ( n o room), and t h e t o p , where t h e z e r o eddy v i s c o s i t y s o l u t i o n w o r k s f i n e .
However,
t h e t w o - l a y e r s o l u t i o n adds t h e l o g a r i t h m i c v e l o c i t y w h i c h c o r r e s p o n d s t o a l i n e a r l y increasing K proportional t o
z a t t h e b o t t o m ; and t h e s t r e s s t e r m , K dU/dz, qoes
z e r o w i t h dU/dz a t t h e t o p , r e n d e r i n g t h e v a l u e o f K m o o t t h e r e .
to
Another c r i t i c i s m
o f K-theory i s t h a t a d i f f u s i o n c o e f f i c i e n t cannot represent t h e advection o f e d d i e s whose s c a l e i s t h a t o f t h e PBL d e p t h .
The s e c o n d a r y f l o w s o l u t i o n e x p l i c i t l y
d e s c r i b e s t h e s e e d d i e s as p a r t o f t h e mean f l o w and p a r a m e t e r i z e s o n l y t h e s m a l l scale eddies w i t h the d i f f u s i o n c o e f f i c i e n t . One o f t h e e f f e c t s o f t h e s e c o n d a r y f l o w i n t h e mean i s t h a t t h e s u r f a c e f l o w direction i s closer t o the freestream direction. from t h e s u r f a c e t o t h e f r e e s t r e a m .
That i s , t h e f l o w t u r n s l e s s
The Eknan l a y e r t e n d e n c y t o have f l o w speeds
305 h i g h e r t h a n t h e f r e e s t r e a m i s enhanced i n v a r y i n q d e o r e e , d e p e n d i n q uoon l a y e r H h i l e t h e magnitude o f t h e secondary f l o w i t s e l f i s simply a
stratification.
f i n i t e p e r t u r b a t i o n o n t h e mean f l o w , t h e m o d i f i c a t i o n t o t h e mean f l o w a t any l a y e r c a n be l a r q e , d e p e n d i n q o n l a y e r s t r a t i f i c a t i o n .
T h i s r e s u l t s , amonq o t h e r
t h i n g s , i n a mean v e l o c i t y i n t h e u p p e r p a r t o f t h e PBL w h i c h may be s u b s t a n t i a l l y greater than t h e f r e e stream v e l o c i t y .
A l s o , m i x i n g i s q r e a t e r and more momentum,
heat o r o t h e r passive m a t e r i a l s can be t r a n s p o r t e d v e r t i c a l l y .
The s o l u t i o n a t a
p o i n t depends upon t h e s t e a d y - s t a t e f r e e - s t r e a m f l o w , s u r f a c e rouqhness and l a y e r stratification.
Y . km
r
1
I
I
I
1
'0
'
I
Y . km
F i g u r e 11. ( w h e r e $,
L a t e r a l c r o s s - s e c t i o n of s e c o n d a r y f l o w . =
VP,
-$y = w p ) ,
The s t r e a m l i n e f u n c t i o n , @
and t h e l a t e r a l s e c o n d a r y f l o w v e l o c i t y , v2/Ug,
a r e shown.
A c r o s s - s e c t i o n o f t h e s e c o n d a r y f l o w i s shown i n f i g u r e 11.
Under t h e assump-
t i o n s o f t h e s o l u t i o n , t h e r e i s no change i n t h e d i r e c t i o n normal t o t h e s e s e c t i o n s , o r n e a r l y i n t h e d i r e c t i o n o f t h e mean f l o w . t h e l a t e r a l component of t h e mean f l o w .
The t i l t t o t h e v o r t i c e s i s caused b y
T h i s p i c t u r e changes f o r d i f f e r e n t l a y e r
306 stratificstions.
As t h e b a s i c f l o w b o u n d a r y c o n d i t i o n s change, a p o i n t - b y - p o i n t
change i n t h e e q u i l i b r i u m s o l u t i o n w i t h s e c o n d a r y f l o w c o u l d b e f o u n d .
Because o f
t h e enhanced m i x i n g t h e a d j u s t m e n t t i m e i s f a i r l y s h o r t , a b o u t o n e - h a l f
h o u r i n the
atmosphere, compared t o 12-24 h o u r s i f i t depended o n d i f f u s i v e m i x i n g a l o n e . an a v e r a g e w i n d , t h i s t r a n s l a t e s t o a h o r i z o n t a l d i s t a n c e o f a b o u t 20 km.
For
One
s h o u l d a l w a y s remember t h a t i n p r a c t i c e , t h e e n t i r e p r o c e s s i s u s u a l l y s t r o n g l y i n f l u e n c e d , if n o t d r i v e n , by c o n v e c t i v e e n e r g y .
However, f o r r e a s o n s based on
t h e b a s i c n a t u r e o f t h e i n s t a b i l i t i e s , I have a r g u e d t h a t t h e c h a r a c t e r i s t i c wavel e n g t h o f t h e secondary f l o w i s q e n e r a l l y o f t h e o r d e r o f t w i c e t h e l a y e r depth. C o n v e c t i v e e n e r g y a l w a y s pushes t h e f l o w t o w a r d more p a r a l l e l f l o w ( w i t h l e s s t u r n i n g ) , and e n e r g i z e s t h e s e c o n d a r y f l o w . T h e r e i s a n o t h e r p o s s i b l e f a c t o r i n f l u e n c i n g t h e shape o f t h e r o l l s .
This
o c c u r s when t h e s e c o n d a r y f l o w i s c o n f i n e d t o a n imposed b o u n d a r y l a y e r d e p t h . The m o s t common c a s e i n t h e atmosphere o c c u r s when a c a p p i n g i n v e r s i o n e x i s t s dlle
to s y n o p t i c s c a l e i n f l u e n c e s .
A s t a t i o n a r y h i g h p r e s s u r e r e g i o n o r a downslope
( k a t a b a t i c ) w i n d m i g h t cause t h i s c o n d i t i o n .
A c o m p l e t e a n a l y s i s h a s n ' t been done,
however, p r e l i m i n a r y r e s u l t s i n d i c a t e t h a t a l i d f o r c e s l o n g e r w a v e l e n q t h c i r c u l a tions.
SOME APPLICATIONS The s e i f sand dunes have a l r e a d y been m e n t i o n e d , b u t m e r i t more a t t e n t i o n .
These
a r e l o n g i t u d i n a l dunes, 20 t o 60 m e t e r s h i g h , e x t e n d i n g u p t o hundreds o f km i n l e n g t h h a v i n g w a v e l e n g t h s f r o m 1 t o 8 km, b u t t y p i c a l l y a b o u t 2 km.
A qood summary
o f t h e i r o c c u r e n c e i n t h e w o r l d ' s d e s e r t s was done b y Hanna ( 1 9 6 9 ) .
He f o u n d t h a t
t h e i r o r i e n t a t i o n was i n v a r i a b l y i n t h e d i r e c t i o n o f t h e p r e v a i l i n g w i n d s .
Cases
e x i s t where dune a l i g n m e n t t u r n s i n a c c o r d a n c e w i t h t h e l o c a l p r e v a i l i n g w i n d s . O t h e r cases show one s e t o f l o n g i t u d i n a l dunes o v e r l y i n g a n o t h e r , s u g g e s t i n q d i f f e r e n t w i n d and c l i m a t e c o n d i t i o n s d u r i n g d i f f e r e n t b u i l d i ng p e r i o d s .
The sand
t r i n s p o r t depends o n t h e cube o f t h e w i n d v e l o c i t y , and s t o r m e p i s o d e s may o c c u r i n d i r e c t i o n s o t h e r than p r e v a i l i n g winds. Evidence o f t h i s can a l s o be found i n superimposed l o n g i t u d i n a l dunes.
B a r c h a n dunes c a n o c c u r i n r e g i o n s where t h e
p r e v a i l i n g w i n d d i r e c t i o n changes i n d i f f e r e n t p e r i o d s .
They c o u l d r e s u l t f r o m
t h e s u p e r - p o s i t i o n o f d i f f e r e n t v o r t e x d i r e c t i o n s , o r f r o m some o t h e r dynamic, c o n v e c t i v e o r t o p o g r a p h i c feedback mechanism.
The Yardangs, s m a l l s c a l e l o n g i -
t u d i n a l dunes w i t h k n i f e - e d g e r i d g e s and U-shaped v a l l e y s , a r e c o m p a t i b l e w i t h vortex flow distributions. I f l o n g i t u d i n a l a t m o s p h e r i c v o r t i c e s do c a u s e dunes, t h e n t h e d i f f e r e n t wave-
l e n g t h s would c o r r e l a t e t o d i f f e r e n t c h a r a c t e r i s t i c boundary l a y e r s (and c l i m a t i c conditions).
There i s a p a u c i t y o f d a t a i n t h e d e s e r t r e g i o n s t o i n v e s t i g a t e this
correlation.
N e v e r t h e l e s s , t h e o r e t i c a l w a v e l e n g t h s f o r e x p e c t e d a t m o s p h e r i c vortices
f a l l p r e c i s e l y i n t h e r a n g e o f t h e l o n g i t u d i n a l dune w a v e l e n g t h s .
There i s a problem in t h a t theory p r ed i ct s l a t e r a l advection of the r o l l s . I f they c o n s t a n t l y moved sideways, t h e r o l l s would n o t be a s l i k e l y t o build lonqit u d i n a l dunes, or even s t a t i o n a r y cloud s t r e e t s .
However, there a r e qood q u a n t i t a t i v e arguments which make p l au s i b l e a fixed r o l l loc a tion. One i s the tendency of convective energy t o favor zero phase speeds ( n o l a t e r a l movement). Another i s the p o s i t i v e feedback furnished by t h e d i f f e r e n c e i n heatinq c h a r a c t e r i s t i c s between the dunes a n d the troughs, which enhances convection alonq the dune c r e s t s . The study of p e r i g l a c i a l winds i n conjunction with loe ss and g l a c i a l debris deposits involves the i n t e r a c t i o n between PBL flow c h a r a c t e r i s t i c s , climate a n d sedimentation p a t t e r n s . The p e r s i s t e n t , k at ab at ic winds a ssoc ia te d with the g la c ie r s and the d e p o s i t ch ar act er can give evidence of g l a c i a l presence, e xte nt a n d d u r a t i o n . The high d en s i t y of the cold a i r can move gra ins more e f f e c t i v e l y .
I n f a c t , s a l t a t i o n and d r i f t forming processes i n t h e snow can be s i m i l a r t o t h a t i n d u s t a n d sand, providing information on a s h o r t e r time s c a l e . Longitudinal d r i f t s orie nte d with the wind form quickly i n a snow storm.
This i s e v i d e n t i n c u r r e n t An t ar ct i c s t u d i e s .
F in a l l y , t h e r e has been much i n t e r e s t i n the d i f f e r e n t aspects of aeolian processes on Mars and Venus.
O n Mars, longitudinal clouds frequently occur as seen in f i g u r e 1 2 , and barchan a n d t r an s v er s e dune systems have been observed. SUMMARY I have presented the flow s o l u t i o n s f o r t h e geophysical PBL.
I t a pplie s to any
atmosphere, o r l a r g e body of water. Although a r o t a t i n q frame of reference produces the turning i n t h e PBL, turning from any cause w ill produce i n s t a b i l i t y waves above a c r i t i c a l Reynolds number.
T h e o r i e n t a t i o n of these waves with
respect t o the mean flow c h a r a c t e r i s t i c s i s shown in f i g u r e 13.
These waves
apparently o f t e n come t o a n equilibrium a t a f i n i t e value, producing a secondary flow. A knowledge of the nature of these flows w ill ?resumably aid the sediment o l o g i s t ' s study of d ep o s i t i o n . I n reading f o r t h i s paper, I have noted t h a t this i s only a s t e p i n an enormously complex problem, so I ' l l t r y t o capsulize the r e s u l t s . The most e a s i l y observed parameters a r e the wavelength and the angle between the wave a x i s and the mean flow.
Roughly, i n PBL flow, the pe rturba tion wave-
lengtli equals twice the depth of the l a y e r of the flow influenced by the bottom (and thereby influencing s ed i men t at i o n ) . Longitudinal pa tte rns a r e frequently associated with t h e i n f l e c t i o n p o i n t i n s t a b i l i t y , which occurs due to the turninq of the mean flow.
Flow v e l o c i t i e s must be above a moderate minimum value.
Con-
vection i n the presence of a moderate mean flow a l s o produces longitudinal waves of t h i s wavelength.
These waves a r e r e l a t i v e l y long, corresponding t o the deep
layers containing the i n s t a b i l i t y .
I f conditions such a s l a r g e s c a l e atmosnheric
subsidence o r k a t a b a t i c winds produce a capping inversion on the PBL flow, then
308
F i g u r e 12. Cumulus c l o u d s o n Mars, t a k e n f r o m t h e V a r s O r b i t e r . The area i s at 1 2 " S , 90"W. The c e l l s a r e a p p r o x i m a t e l y 6 km i n d i a m e t e r , e l o n q a t e d i n t h e w i n d d i r e c t i o n . The i n d i c a t e d a l i g n m e n t i s o f r o l l s 8 - 1 0 kni a p a r t w i t h clouds a t a b o u t 2 . 5 km h e i g h t . T h i s s e p a r a t i o n / h e i g h t r a t i o o f 3 . 2 - 4 i s i n aqreement w i t h t h e r o l l theory.
309
(a)
j_______ Y
convergence/ divergence /44
, wave direction / ' '
Figure 13. Sketch o f v e l o c i t y shear and c o r r e s p o n d i n g i n s t a b i l i t y waves. ( a ) i l l u s t r a t e s t h e i n f l e c t i o n p o i n t i n e v i t a b l y a s s o c i a t e d w i t h a speed chanse (Vt-Vo) between two l a y e r s o f d e p t h , h. The r e s u l t i n g wave c r e s t s ( i n t h e lower p a r t o f t h e f i g u r e ) a r e normal t o t h e f l o w d i r e c t i o n ( $ = 0 ) . These ( c ) shows t h e would be a s s o c i a t e d w i t h r i p p l e s , b i l l o w s o r t r a n s v e r s e dunes. I n t h i s case t h e case o f p u r e t u r n i n g between l a y e r s ( V t - V o = 0, E = 3 0 " ) . i n e v i t a b l e i n f l e c t i o n p o i n t i n s t a b i l i t y produces waves w i t h c r e s t s p a r a l l e l t o t h e mean f l o w i n t h e l a y e r . I n ( b ) , a mixed case i s shown. wavelengths can be l o n g e r t h a n t w i c e t h e d e p t h o f t h e l a y e r .
There a r e a l s o s p e c i a l
flow c o n d i t i o n s ( s h a r p t u r n i n q i n t h e l a y e r near t h e s u r f a c e f o r i n s t a n c e ) which lead t o l o n g i t u d i n a l p a t t e r n s on t h e s u r f a c e w i t h s c a l e s o f 10s o f m e t e r s . When t h e f l o w i s p e r p e n d i c u l a r t o t h e wave c r e s t s , t h e dynamic i n f l e c t i o n p o i n t i n s t a b i l i t y i s t h e l i k e l y source o f enerqy. called Kelvin-Helmholz i n s t a b i l i t y . c o r r e s p o n d i n g l y s h o r t e r wavelengths.
T h i s i n s t a b i l i t y can o c c u r i n t h i n l a y e r s w i t h B i l l o w s i n c l o u d s , r i p p l e s i n sand and t r a -
verse sand dunes may be d i r e c t r e s u l t s .
L a t e r a l wave p a t t e r n s a r e f r e q u e n t l y
observed superimposed on l o n g i t u d i n a l ones. l a t e r a l waves.
T h i s case o f p u r e speed shear i s o f t e n
No e q u i l i b r i u m t h e o r y e x i s t s f o r these
S t r o n g i n t e r a c t i o n w i t h t h e s u r f a c e topoqraphy i s p o s s i b l e when
the roughness h e i g h t s a r e s i q n i f i c a n t compared t o t h e t h i n l a y e r depth.
These flow
s i t u a t i o n s go beyond t h e s i m p l e a n a l y t i c s o l u t i o n s which I have discussed. J u s t as m e t e o r o l o g i s t s use t h e l o w - l e v e l c l o u d s t r e e t s t o i n f e r t h e wind d i r e c tions f o r t h e i r a n a l y s i s , t h e s e d i m e n t a t i o n c h a r a c t e r i s t i c s can be used t o p r o v i d e useful c l i m a t i c i n f o r m a t i o n i n r e g i o n s and t i m e s where o t h e r d a t a a r e scarce.
This
i s t h e case f o r sparse measurement r e g i o n s such as t h e d e s e r t s and t h e ocean f l o o r s , ancient f l o w s and f l o w on o t h e r p l a n e t s f o r i n s t a n c e .
A knowledge o f t h e i n s t a -
b i l i t y waves i s most h e l p f u l i n u n d e r s t a n d i n q t h e complex b u t o b v i o u s l y o r o a n i z e d patterns which o c c u r i n g e o p h y s i c a l f l o w s and t h e i r boundaries.
310 References Brown, R . A . , 1980. L o n g i t u d i n a l i n s t a b i l i t i e s and secondary flows i n t h e planetar 653-697. boundary l a y e r : a review. Rev. o f Geophys. and Space P h y s i c s , z(3), Ekman, \!. W . , 1905. O n the i n f l u e n c e o f the e a r t h ' s r o t a t i o n on ocean c u r r e n t s Arkiv. Math. A s t r o . F y s i k . , z(11). 1-53. Hanna, S . R . , 1969. The f o r m a t i o n of l o n q i t u d i n a l sand dunes by l a r q e h e l i c a l e d d i e s i n the atmosphere. J . Appl. M e t e o r o l . , 8 ( 6 ) , 874-880. Rossby, C . G . and R . B . Hontqomery, 1935. The l a y e r s o f f r i c t i o n a l i n f l u e n c e in wind and ocean c u r r e n t s . HIT P a p e r s , 3 ( 3 ) , 101 P O .
311
RADIATIVE AND METEOROLOGICAL CONTROL ON THE MOVEMENT OF SAND AT LAKE MUNGO, N.S.W., AUSTRALIA R. HYDE, School o f Earth Sciences, Macquarie U n i v e r s i t y , Sydney, N.S.W., Australia
R.J. WASSON*, Department o f Biogeography and Geomorphology, The A u s t r a l i a n National U n i v e r s i t y , Canberra, A.C.T., A u s t r a l i a
INTRODUCTION Most studies o f a e o l i a n t r a n s p o r t have been p r i m a r i l y concerned w i t h the movement o f d r y sand and t h e o r i e n t a t i o n o f dunes t o the p r e v a i l i n g sand-moving winds (Bagnold, 1941; Fryberger, 1979; Warren, 1979; Wilson, 1972). This preoccupation w i t h d r y sand may be an acceptable p r o p o s i t i o n f o r dunes i n a completely a r i d environment where t h e subsurface l a y e r s are dry, b u t i n other s i t u a t i o n s t h i s approach does n o t take i n t o account t h e presence o f a wet core o f sand below the surface and t h e p o t e n t i a l c o n t r o l t h i s could e x e r t on the
movement o f sand.
I n wet-core dunes i t i s u s u a l l y o n l y the f i r s t few tens o f
centimetres, a t most, t h a t a r e d r y and r e a d i l y mobile.
Therefore, i n strong
winds, t h e surface d r y l a y e r may be r a p i d l y removed, and t h e p o t e n t i a l decrease i n sand transpor.t, once t h e wet core i s exposed, must be considered i n any complete a n a l y s i s o f sand t r a n s p o r t . Wet-core dunes are n o t unique t o A u s t r a l i a , and t h e i r existence has been noted elsewhere by several workers (e.g. Sharp, 1966;
Tsoar, 1978).
Nor do we
be1 ieve t h a t wet cores are a t r a n s i e n t phenomenon o c c u r r i n g o n l y a f t e r periods of rain.
Rather, they probably represent the normal subsurface c o n d i t i o n o f
dunes i n areas outside hyper-arid environments, and t h e i r dynamic i m p l i c a t i o n s need t o be investigated. The increase i n the threshold v e l o c i t y r e q u i r e d t o move wet sand has long been recognized (e.g.
Chepil , 1956;
Woodruff and Siddoway, 1965).
However,
what has n o t been considered i s the r o l e o f s o l a r r a d i a t i o n i n the d r y i n g o f subsurface layers, and t h e p o t e n t i a l c o n t r o l t h a t t h i s could have on the amount
o f dry sand a v a i l a b l e f o r subsequent t r a n s p o r t by t h e wind. I t i s n o t t h e i n t e n t i o n o f t h i s paper t o examine i n d e t a i l the complex r a d i a t i v e and energy exchanges i n v o l v e d i n t h e d r y i n g o f near-surface sand. Rather i s i t an attempt t o discuss t h e i n t e r n a l s t r a t i f i c a t i o n and moisture content o f a l i n e a r dune i n a semi-arid environment i n terms o f the meteorol o g i c a l and shortwave r a d i a t i v e conditions during the previous twelve months. *present address: CSIRO D i v i s i o n o f Water and Land Resources, Canberra, A.C.T.
312
In this way we hope to draw attention to the need to take into account factors other than just the wind velocity when considering the dynamics of sand transport and dune mobilization. The dunefield to be discussed in this paper is situated at Lake Mungo in the far west of New South Wales, Australia (see Fig. 1). Lake Mungo is one of the
Fig. 1. Map of southeastern Australia showing mean annual rainfall isohyets and location of Mildura and Lake Mungo. Willandra Lakes, and the structure of the lunettes surrounding the lakes is well documented (Bowler, 1971; 1978). The present-day environment is one where the clay and quartz sand lunettes around Lake Mungo are being eroded and old lunette deposits (which are >14 000 yrs B.P.: Bowler, 1976) are now capped by a sheet of highly mobile, unvegetated, aeolian sand currently moving eastwards onto the surrounding plain. The quartz sand-sheet is surmounted by linear dunes forming in the lee of residuals of lunettes being eroded (see Fig. 2). The axes o f the linear dunes are oriented approximately west-east and, in the area being investigated, this is at right-angles to the edge of the lake.
31 3
I+
I
Large s l i p face S _ Small slip face
__
_ - . - Dune
crest - line
Margins of d u n e
of lunette material @ Residual A old
- O - O
- O
5km
I km
70 m
B
c
Fig. 2. a ) Lake Mungo. b ) Sand-sheet overriding lunette on eastern side of Lake Mungo. c ) Linear dune forming in lee of remnant knob on eroding lunette. Our i n t e r e s t in Lake Mungo s t a r t e d in July 1978 when a two-metre trench was dug across the t o p of one linear dune on the sand-sheet (see Fig. 2 ) . Details of the s t r a t i f i c a t i o n and internal moisture content of t h i s dune are given in Figure 3. Of major i n t e r e s t t o us were the evidence of a recent reversal in orientation of the internal s t r a t i f i c a t i o n of the dune near i t s c r e s t , and the existence of an extensive layer of dry sand below the surface of the northern flank of the dune. Although extensive digging was carried out on the southern
314
I
A
14 Distance From cenlle Llne (metres)
Fig. 3. North-south transect across linear dune a t Lake Mungo, showing g r a v i metric moisture content, internal s t r a t i f i c a t i o n and position of buried dry layer i n July 1978. side, no equivalent subsurface dry layer could be found. The only dry sand that could be seen on the southern flank was a shallow layer a few centimetres deep just below the surface, which significantly was not present on the opposite side of t h e dune. I t i s t h i s reversal in dune-crest orientation and the disposition of dry and moist layers t h a t we will now attempt t o explain. THE METEOROLOGICAL AND CLIMATIC ENVIRONMENT AT LAKE MUNGO In common with many semi-arid regions of the world, the area around Lake Mungo i s not well endowed with reliable long-term meteorological records. The only measurement routinely made in the area i s r a i n f a l l , which i s of prime i n t e r e s t t o pastoral i n t e r e s t s i n the region, b u t these data are often irregular. Reliable, long-term records are available from Mildura, approximately 100 km southwest of Lake Mungo (see Fig. 11, where the Australian Bureau of Meteorology maintains a f u l l weather-station w i t h both surface- and upper-air measurements. The station a l s o records half-hourly integrated values of global shortwave radiation. Rai nfal 1 The annual average r a i n f a l l f o r Mildura is 250 m and i s s e l a t i v e l y evenly distributed throughout the year (see Fig. 4a). However, i n the 12 months preceding July 1978 the pattern of r a i n f a l l was e r r a t i c (see Fig. 4a), with
315
A
S
O
N
D
J
F
M
A
4 0 r rnrn M i I dura
i
0,
L
aug
,
I
M
J
J
(b)
I
I
j an
,
”
J
. L , . . ,b
L jul
Fig. 4. a ) Monthly average rainfall (30-yr record) and actual rainfall (hatched) , August 1977-July 1978; Mildura. b ) Daily rainfall August 1977July 1978; Mildura.
nwnthly t o t a l s generally l e s s than the average, except f o r June and July 1978 when approximately double the average rainfall was recorded. Rainfall amounts on individual days during the 12-month period are shown in Figure 4b. Wind speed and direction Three-hourly wind-speed and -direction s t a t i s t i c s from Mildura were used t o obtain a measurement of potential sand-moving winds a t Lake Mungo. These were calculated u s i n g the method described by Fryberger (1979) and a value of 6 m s - l a t 10 metres f o r the threshold wind speed. The annual d r i f t potentials from each direction, along w i t h the resultant d r i f t potential ( R D P ) , are shown i n Figure 5 , and monthly d r i f t potentials a r e given i n Figure 6. Using Fryberger’s (1979) c l a s s i f i c a t i o n , the wind environment i s wide unimodal and low energy, since the total d r i f t potential is only 162. The data i n Figure 5 show t h a t
316
the RDP vector i s within 10" of the axes of the linear dunes a t Lake Mungo. Monthly d r i f t potentials i n Figure 6 show marked seasonal changes in both potentials and the direction o f the resultant d r i f t potential f o r each month. Between January and June d r i f t potentials were small, b u t increased rapidly in July and maintained t h e i r strength and direction into December. Since the axes of the dunes are approximately west-east, the monthly resultant d r i f t potentials show a seasonal change i n the across-dune component of the RDP, being towards the south between April and November, and towards the north during the remainder of the year.
F i g . 5. Annual d r i f t potential a t Mildura and axial orientation of linear dune a t Lake Mungo.
ja n
0
xc)i
feb
mar
X L ju n
oct
Fig. 6.
-
nov
0
5
0
10
dec
10 DRIFT POTENTIAL
20 RESULTANT DRIFT POTENTIAL
Monthly potentials and resultant d r i f t potentials a t Mildura.
317 Wind speeds and d i r e c t i o n s f o r the period A u g u s t 1977 t o July 1978 inclusive were a l s o examined t o see whether wind conditions d u r i n g t h i s period were similar t o those given by t h e long-term wind p o t e n t i a l s plotted i n Figure 6. Instead of constructing monthly sand r o s e s , t h e 1977/78 wind-drift p o t e n t i a l s in Figure 7 have been resolved i n t o across- and along-dune components. For these c a l c u l a t i o n s a s l i g h t l y simpler approach was adopted and the l i n e s in Figure 7 a r e values of Vz(V-Vt), where V i s the measured wind speed and V t the threshold speed of 6 ms-1. The data i n Figure 7 show overall consistency with the longer-term s t a t i s t i c s i n Figure 6 with a seasonally changing across-dune component superimposed upon a dominant along-dune p o t e n t i a l ; between August 1977 and t h e beginning of October potential across-dune t r a n s p o r t was predominantly towards the south, and towards t h e north between October and the end of March. Subsequent along- and across-dune p o t e n t i a l s were extremely low, but by July t h e across-dune potential towards the south had reappeared.
r
4000 t o w a r d s n o r t h I
4000 L t o w a r d s s o u t h 1
I
I
aug
I
I
I
I
j an
I
I
I
I
I
1
jul
4000 t o w a r d s e a s t
F
I
t
4808 t o w a r d s w e s t
Fig. 7. Daily across- and along-dune wind p o t e n t i a l s Y 2 (V-V,), A u g u s t 1977July 1978; Mildura.
318 Radiation The final parameter t o be discussed i s shortwave radiation from the Sun. We realize t h a t any detailed discussion of the radiative and energy balance over a dune should also deal w i t h the components of incoming and outgoing longwave radiation, net radiation (available radiation) , the flux of heat into the sand, and the t r a n s f e r of sensible and l a t e n t heat from the dune into the atmosphere. None of these fluxes were measured during the period being studied and they are almost impossible t o estimate. However, we believe t h a t the amount of solar radiation potentially available f o r absorption by the dune will be an indication of the amount of energy available f o r e i t h e r drying the surface of the wet core i f i t became exposed d u r i n g periods of strong winds, or t o increase the depth of the surface dry layer when wind speeds were low. Shortwave radiation incident a t the outside of the atmosphere can be calculated quite e a s i l y using established trigonometric expressions (e.g. Sellars, 1965; Kondrat'ev, 1969; Paltridge and P l a t t , 1976), which take into account seasonal and diurnal changes i n the Sun's zenith distance and the eccentricity of the Earth's o r b i t around the Sun. The amount of radiation received a t the surface i s , however, l e s s than t h a t received a t the outside of the atmosphere, because of absorption, scattering and reflection w i t h i n the atmosphere. In addition, a proportion of the shortwave radiation incident a t the surface i s reflected back t o space. Finally, radiation incident upon sloping surfaces has t o be dealt w i t h separately, taking into account the slope of the surface and i t s orientation r e l a t i v e t o the Sun. Since we were primarily interested i n calculating the amount of s o l a r radiation available f o r absorption by the dune, i t was necessary t o calculate the transmissivity of the atmosphere, using data from Mildura, and t o determine the albedo ( r e f l e c t i v i t y ) of the sand a t Lake Mungo. An average daily value of atmospheric transmissivi t y was determined by comparing measured values of global shortwave radiation ( d i r e c t plus diffuse) a t Mildura with daily t o t a l s calculated for the outside of the atmosphere a t the same l a t i t u d e (34O14'S). Although some s l i g h t seasonality was apparent, an average value of 0.71 has been used in subsequent calculations. To determine the albedo of the sand a t Lake Mungo, incoming and reflected shortwave radiation were measured above a f l a t t i s h section of the sand-sheet d u r i n g September and December 1981, using Kipp solarimeters and a Middleton pyrano-albedometer. These r e s u l t s are shown i n Figure 8. Paltridge and Platt (1976) showed t h a t most surfaces displayed a Fresnel type r e f l e c t i v i t y curve which could be expressed as a ( e ) = al + ( l - a l ) exp C-k(90-e)l
where e i s the solar elevation, al i s the albedo of the surface a t elevations
319
Sun's
01
1
I
1
30
Fig. 8.
I
I
elevation I
I
60
I
90
Change of albedo w i t h Sun's elevation over sand a t Lake Mungo.
above 50" and k i s a constant which, l i k e a, will vary between surfaces. In Figure 8 the curved l i n e was plotted using the expression given above with a1 0.46 and k = 0.1, and i t is c l e a r t h a t t h i s equation adequately describes the relationship between solar elevation and albedo a t Lake Mungo. To calculate the radiation incident upon each flank of the dune i t isnecessary t o know the angle of the slope ( i ) and the azimuth of the normal of the slope w i t h respect t o south ( a l ) (see Fig. 9 ) . W i t h this information, plus data on the change i n the Sun's azimuth and zenith distance ( 0 ) throughout the day, the equations given by Sellars (1965) can be used to calculate the zenith distance (01) of the Sun r e l a t i v e t o each flank throughout the day. Daily t o t a l s of radiation can then be calculated, taking into account the average transmissivity of the atmosphere. For convenience, a value of 214" was used f o r the slope of
320
Dune
4
South
Fig. 9. Schematic diagram showing orientation o f the linear dune at Lake Mungo to the Sun. (Symbols explained in the text).
4000
r
10's o f kilojoules/sq.m
NORTH SLOPE
0 j an
jul
Fig. 10. Daily totals o f global radiation August 1977-July 1978; Mildura. The curves show daily totals o f solar radiation potentially absorbed on each side o f the dune.
321
both flanks of the dune a t Lake Mungo (shown by the dashed l i n e i n Figure 3). Since the dunes a r e oriented approximately west-east, the azimuth of the normal o f l h e slopes with respect t o south was taken as zero (a1 = 0 ) . These calculations were carried out f o r each day between August 1977 and July 1978 and the results are given in Figure 10. Also shown are the daily totals of global radiation received a t Mildura d u r i n g the same period. The envelope containing these data is the maximum global solar radiation that could have been received a t Mildura during the year. These measurements take into account the transmissivity of the atmosphere and the depletion of solar radiation on cloudy days, b u t are not values of absorbed radiation, since they do not take into account solar radiation reflected away from the surface. The Mildura data show the marked seasonal change i n incoming solar radiation that occurs a t t h i s l a t i t u d e , w i t h l e s s than a third of the sumner daily t o t a l s being received by a horizontal surface in winter. The two solid lines in Figure 10 are the daily t o t a l s of shortwave radiation potentially absorbed by each flank of the dune throughout the 12-month period. Whereas the radiation potentially available f o r absorption by the north flank does not change much throughout the year, radiation available on the south flank is affected by both the adverse slope of the dune and h i g h values of albedo d u r i n g the winter months. This results in very small daily t o t a l s being absorbed i n May, June and July. A t the height of sumner, solar radiation on the southern flank exceeds that on the northern side o f the dune, an apparent anomaly, b u t actually caused by the Sun rising and s e t t i n g south of the axis of the dune during the summer halfyear.
DISCUSSION The exact sequence of events a t Lake Mungo prior t o the trench being dug during July 1978 cannot be determined since no observations of any kind had previously been made over the dune. Despite t h i s , i t i s possible t o r e l a t e the features revealed by the trench w i t h meteorological and radiative conditions d u r i n g the preceding few months. The existence of a thick, subsurface, dry layer on the northern flank of the dune, and i t s absence on the opposite side, suggest that northward sand transport across the dune had been occurring i n the preceding months. The only period when t h i s was possible was between October 1977 and the end of March 1978, when both the along- and across-dune wind potentials were high (see Fig. 7 ) . This coincided w i t h a period when rainfall was low and generally below the average (see F i g s . 4a, 4b), and was also the time of the year when large amounts o f solar radiation would have been absorbed by each side of the dune (see Fig. 10). These factors are thought t o have combined t o produce a continuing supply o f dry sand f o r transport across the dune t o i t s northern flank. In the
322 following months both along- and across-dune wind potentials were low and there would have been l i t t l e , i f any, transport of sand on the dune. During May, June and July there were a number of days with small amounts of r a i n , dampening the surface sand (see Fig. 4b). Figure 7 shows t h a t , d u r i n g July, an acrossdune component of wind potential towards the south has reappeared, and evidence for the actual transport of sand back across the dune can be seen in the s t r a t i fication near the c r e s t of the dune (see Fig. 3 ) . This occurred a t a time when the flanks of the dune were probably damp, t h u s impeding the transport of sand along and across the dune. Figure 3 also shows the existence of a shallow layer of dry sand very close t o the surface on the southern flank of the dune, although there was no sign of a corresponding near-surface layer on the northern flank. Since a t this time of the year the amount of solar radiation available for absorption by the south-facing side of the dune was low (see Fig. 6 1 , sand must have been transported from the opposite side of the dune, following efficient u t i l i z a t i o n of solar radiation i n drying surface sand on the northern flank. Figure 9 shows t h a t i n June and July the northern side of the dune can potentially absorb almost as much radiation as i t d i d d u r i n g the summer months because of i t s favourable orientation t o the Sun. Therefore, on sunny days d u r i n g l a t e June and July, near-surface sand on the northern flank would have been dried and subsequently transported across the dune when wind conditions were favourable. Since the moisture content of the sand i n the reverse s t r a t i fied c r e s t of the dune was moist (see Fig. 3 ) , there must have been a continuing sequence of radiative drying, transport and rewetting of sand d u r i n g t h i s winter period. The shallow, dry layer of sand on the southern flank of the dune can be regarded only as a transient feature and the damp sand above i t is evidence t h a t this layer would probably disappear d u r i n g the next rain or drizzle. CONCLUSIONS In this paper we have attempted t o explain the internal s t r a t i f i c a t i o n and moisture content of a l i n e a r dune a t Lake Mungo i n terms of wind speed and direction, r a i n f a l l and solar radiation d u r i n g the preceding 12 months. The sequence of meteorological events i n the year preceding our observations correl a t e s w i t h the features observed i n the dune i n July 1978 and i l l u s t r a t e s the need f o r factors other than just wind velocity t o be considered when discussing the dynamics of sand dunes. Of special importance is the role of shortwave radiation from the Sun i n the drying of wet sand exposed a t the surface of dunes and the degree t o which an exposed wet core limits potential sand transport along o r across a dune.
323 REFERENCES Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen, London, 265 pp. Bowler, J.M., 1971. Pleistocene salinities and climatic change: evidence from lakes and lunettes in S.E. Australia. In: D.J. Mulvaney and J. Golsen (Editors), Aboriginal Man Environments in Australia. Australian National University Press, Canberra, pp. 46-63. Bowler, J.M., 1976. Recent developments in reconstructing late quaternary environments in Australia. In: R.L. Kirk and A.G. Morne (Editors), The Origin of the Australians. Human Biology Series No. 6. Australian Institute of Aboriginal Studies, Canberra, pp.55-57. Bowler, J.M. and Magee, J.W., 1978. Geomorphology of the Mallee Region in semi-arid northern Victoria and western New South Wales. Proc. Roy. SOC. Vic., 90, 1: 5-26. Chepil, W.S., 1956. Influence of moisture on erodibility of soil by wind. Soil Sci. SOC. Am. Proc., 20: 288-292. Fryberger, S.G., 1979. Dune forms and wind regime. In: E.D. McKee (Editor), A Study of Global Sand Seas. U.S. Geological Survey Professional Paper 1052. Washington, pp. 137-170. Kondrat'ev, K.I., 1969. Radiation in the Atmosphere. Academic Press, New York, 912 pp. Paltridge, G.W. and Platt, C.M.R., 1976. Radiative Processes in Meteorology and Climatology. Elsevier Scientific Publishing Co., New York, 318 pp. Sellars, W.D., 1965. Physical Climatology. The University of Chicago Press, Chicago, 272 pp. Sharp, R.P., 1966. Kelso dunes, Mohave Desert, California. Geol. SOC. Am. Bull. , 77: 1045-1074. Tsoar, H. , 1978. The dynamics of longitudinal dunes. Final Technical Report, European Research Office, U.S. Army, London, 171 pp. Warren, A., 1979. Aeolian processes. In: C. Embleton and J. Thornes (Editors), Processes in Geomorphology. Edward Arnold, p p . 3 2 5 - 3 5 1 . Wilson, I.G., 1972. Aeolian bedforms, their development and or.igins. Sedimentology, 19: 173-210. Woodruff, N.P. and Siddoway, F.H., 1965. A wind erosion equation. Proc. Soil Sci. SOC. Am., 29: 602-608.
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325
MORPHODYNAI.1ICS OF INCIPIENT 1 OHEDUNES IN NL W SOUTH \\ALES, AUSTRALIA PATRICK HESP D i v i s i o n o f R e s o u r c e E1driagemc.n Depdr t m e n t o f Aqr i c u l t i r e 3 a r r a h R o a d , S o u t h P e r t h , W.A. 6 1 5 1
INTRODUCTION F o r e d u n e s are t h e f o r e m o s t v e g e t a t e d s a n d d u n e s o c c u r r i n y on t h e b d c k s h o r e zone of sandy beaches.
They a r e g e n e r a l l y f o r m e d p a r a l l e l t o t h e c o a s t .
a u t h o r d i s t i n g u i s h e s two b a s i c t y p e s o f f o r e d u n e s : established foredunes.
The
i n c i p i e n t f o r e d u n e s and
I n c i p i e n t f o r e d u n e s dre t h e i n i t i a l f o r e d u n e f o r m e d by
t h e t r a p p i n g o f sand w i t h i n pioneer v e g e t a t i o n s p e c i e s .
Where t h e s e d u n e s a r e
c o l o n i s e d by a g r o u p o f ' w o o d y ' v e g e t a t i o n s p e c i e s ( e . q . mat a n d t u f t e d p l a n t s ) , they are termed e s t d b l i s h r d f o r e d u n e s . I n r e c e n t y e a r s w h i l s t t h e r e h a s b e e n a s i g n i f i c a n t i n c r e a s e i n r e s e a r c h on t h e d y n a m i c s o f u n v e g e t a t e d c o a s t a l a n d d e s e r t d u n e s a n d r e l a t e d phenomena ( e g . Walker a n d M a t s u k u r a , 1 9 7 9 ; Howard e t a l . , 1 9 7 8 ; G r e e l e y e t a l . , 1 9 8 0 ) , t h e r r has n o t b e e n a s i g n i f i c a n t p a r a l l e l i n c r e a s e i n r e s e a r c h on v e g e t a t e d coastal dunes, e s p e c i a l l y f o r e d u n e s .
I n p a r t i c u l a r , i f one is interested i n derivinq
e x p l a n a t i o n s o f f o r e d u n e m o r p h o l o g y , t h e s e m i n a l w o r k of O l s o n ( 1 9 5 8 a , b ) s t i l l provides t h e most d e t a i l e d d i s c u s s i o n a v a i l a b l e on dynamic p r o c e s s e s o p e r a t i n g on f o r e d u n e s .
T h i s p a p e r t h e n , i s c o n c e r n e d w i t h d e t a i l i n g a s p e c t s o f t h e morpho-
d y n a m i c s of i n c i p i e n t f o r e d u n e s , a s t h e s e r e p r e s e n t t h e i n i t i a l s t a r t i n g p o i n t i n foredune formation.
I n d e t a i l , t h e f o l l o w i n g is an attempt t o d e r i v e l i n k -
ages between e c o l o g i c , aerodynamic and t r a n s p o r t p r o c e s s e s , and observed dune morphologies.
It c o n c e n t r a t e s o n i n c i p i e n t f o r e d u n e s formed by s a n d d e p o s i t i o n
w i t h i n l a t e r a l l y e x t e n s i v e ( a l o n g s h o r e ) c o l o n i e s of e i t h e r p l a n t s e e d l i n q s o r rhiromeslstolons.
I t i s n o t c o n c e r n e d w i t h i n c i p i e n t f o r e d u n e s i n i t i a t e d by
s a n d d e p o s i t i o n w i t h i n d i s c r e t e p l a n t s o r g r o u p s of p l a n t s
ds
d e s c r i b e d by
S a l i s b u r y ( 1 9 5 2 ) , Land ( 1 9 6 4 ) , G o d f r e y d n d G o d f r e y ( 1 9 7 6 ) , Woodhouse ( 1 9 7 8 ) a n d Hesp ( 1 9 8 1 ) . EXPERIMENTAL PROCEDURES Much o f t h e t o p o g r a p h i c a n d v e g e t a t i o n s u r v e y d a t a p r e s e n t e d b e l o w were o b t a i n e d from t h r e e - d i m e n s i o n a l ,
permanent survey p l o t s l o c a t e d a t v a r i o u s foredune-beach
s i t e s i n F e n s e m b a y m e n t , M y a l l L a k e s r e g i o n , N e w S o u t h Wales (see Thorn e t a l . , 1981, a n d S h o r t a n d H e s p , 1 9 8 2 , f o r a w i d e r d i s c u s s i o n of t h i s l o c a t i o n ) .
In thc
p l o t s , t h e c h a n g e i n s u r f a c e e l e v a t i o n o n e a c h c o r n e r of e v e r y o n e metre s q u , i r c
326 was surveyed w i t h a l e v e l , and numbers o r percent cover o f each p l a n t species, i n each square metre were counted o r estimated.
Monthly, and occasional d a i l y
surveys were conducted over a t h r e e year p e r i o d .
P l o t dimensions were e i t h e r 10
o r 20 m alongshore, and 30 m wide. Aerodynamic s t u d i e s were undertaken u t i l i s i n g a maximum o f twelve, low i n e r t a , Rimco m i n i a t u r e cup anemometers (cup diameter 32 mm, s t a l l i n g speed 0 . 1 ms-l; see Bradley, 1969).
The anemometers were mounted on 12 mm diameter, aluminium
masts, and on t h e surface.
Instantaneous sand t r a n s p o r t observations were made
v i s u a l l y , and by use o f slow motion f i l m .
Mean, r e l a t i v e sand t r a n s p o r t r a t e s
were a l s o determined using Leatherman (1978) sand t r a p s . Observations o f i n t e r n a l s t r u c t u r e were made i n hand-dug trenches u t i l i s i n g techniques described by B i g a r e l l a ( 1 9 7 2 ) , i n c l u d i n g n a i l and s t r i n g g r i d s and photography.
BIOLOGICAL PROCESSES I n New South Wales, i n c i p i e n t foredunes are predominantly i n i t i a t e d by the growth of S p i n i f e x h i r s u t u s .
Spinifex hirsutus L a b i l l (Hairy Spinifex; family:
Gramineae) i s a p e r e n n i a l n a t i v e grass commonly about 25 t o 30 cm high, w i t h s t o u t rhizomes and s t o l o n s which can extend f o r s e v e r a l metres.
Male p l a n t s
have two-flowered s p i k e l e t s , arranged i n spikes which are c l u s t e r e d i n groups of 4, 5 o r 6. g l o b u l a r head.
Female p l a n t s have numerous s p i k e l e t s arranged i n a l a r g e , dense, I t f l o w e r s d u r i n g spring-summer
occurs by t h r e e mechanisms:
(Beadle e t a l . ,
1972).
Growth
germination o f seeds, shoot p r o d u c t i o n , and l a t e r a l
spread o f rhizomes and s t o l o n s . Seeds ( u s u a l l y caryopses i n s p i k e l e t s ) are e i t h e r wind blown from landward source areas, or wave t r a n s p o r t e d t o t h e beach and backshore.
I n New South Wales,
w e s t e r l y o f f s h o r e winds, which occur throughout autumn and w i n t e r , appear primarily r e s p o n s i b l e f o r t h e t r a n s p o r t o f seeds onto t h e beach.
Here they a r e incorporated
i n t o the beach sediments by t h e a c t i o n o f swash and a e o l i a n sand t r a n s p o r t . Maximum germination occurs i n s p r i n g , p a r t i c u l a r l y e a r l y October through November, on t h e upper p o r t i o n o f t h e backshore zone a t t h e l i m i t o f s p r i n g t i d e swash. Swash deposited seeds may germinate as a d i s c r e t e group from, o r as an attached group t o a p r i o r S p i n i f e x vegetated foredune.
The s e e d l i n g zone may be o f vary-
i n g width, d e n s i t y and d i s t r i b u t i o n . The beach may a l s o be vegetated by t h e seaward growth o f S p i n i f e x rhizomes and stolons.
A t i n t e r v a l s o f 10 t o 15 cm nodes a r e formed from which r o o t s and semi-
d i s c r e t e p l a n t s emerge. (Hesp, 1982).
S t o l o n growth r a t e s range from 0.4 t o 1.5 m per month
Shoot growth responds t o b o t h a temperature regime, a t t a i n i n g a
maximum growth p e r i o d i n summer, and t o a c c r e t i o n r a t e .
327 AERODYNAMICS OF- SPINIFEX SURFACES A s soon a s S p i n i f e x p l a n t s c o l o n i s e t h e b a c k s h o r e , t h e y i n c r e a s e t h e aerodynamic roughness of t h e s u r f a c e and i n t e r a c t w i t h t h e a i r f l o w i n g o v e r t h e s u r f a c e .
In
an a t t e m p t t o d e t e r m i n e t h e n a t u r e o f i n t e r a c t i o n s between S p i n i f e x d e n s i t y and d i s t r i b u t i o n , f l o w dynamics and s a n d t r a n s p o r t , two q u e s t i o n s a p p e a r e d t o be of i n i t i a l importance:
1) what was t h e s t r u c t u r e o f wind f l o w a c r o s s a r e l a t i v e l y
homogeneous v e g e t a t e d s u r f a c e , and 2 )
what e f f e c t d i d l a t e r a l v a r i a t i o n s i n
p l a n t d e n s i t y and d i s t r i b u t i o n have on t h e f l o w s t r u c t u r e ? F i g u r e 1 i l l u s t r a t e s v e l o c i t y p r o f i l e s measured a c r o s s a r e l a t i v e l y homogeneous, The p r o f i l e s a r e e x p r e s s e d a s a
Spinifex dominated, s m a l l i n c i p i e n t foredune.
r a t i o o f a mean v e l o c i t y measured a t 4 m h e i g h t , upwind on t h e u n v e g e t a t e d beach. Table 1 l i s t s p r o f i l e s t a t i s t i c s , d e r i v e d from a l e a s t s q u a r e s r e g r e s s i o n a n a l y s i s (e.g. Reitsma, 1978), f o r p r o f i l e s one t o f o u r .
Correlations for profile f i t
are a l s o given. F i g u r e 1 i l l u s t r a t e s t h a t t h e lower p o r t i o n o f t h e p r o f i l e s d i s p l a y a r e l a t i v e l y uniform t e n d e n c y t o d e c r e a s e i n n e a r - s u r f a c e v e l o c i t y w i t h i n c r e a s i n g f e t c h downwind o f t h e change i n r o u g h n e s s ( i . e .
beach t o v e g e t a t i o n ) .
T h i s downwind
d e c r e a s e i n n e a r - s u r f a c e v e l o c i t y i s accompanied by a downwind i n c r e a s e i n relative shear velocity.
I n a d d i t i o n , r e l a t i v e roughness lengths i n c r e a s e t o
p r o f i l e 3 , f i v e metres downwind o f t h e l e a d i n g e d g e ( T a b l e 1; c f . Kutzbach, 1961; Bradley, 1968).
Note t h a t v e g e t a t i o n h e i g h t r e m a i n s c o n s t a n t a p p r o x i m a t e l y
one h a l f of a metre downwind of t h e l e a d i n g e d g e . Mean v e l o c i t y i n c r e a s e s i n t h e u p p e r p a r t of t h e p r o f i l e s one t o f o u r d u e t o There is a l s o a
lower p r o f i l e d r a g and f l o w c o m p r e s s i o n o v e r t h e f o r e d u n e .
d r a m a t i c r e d u c t i o n i n v e l o c i t y , and f o r m a t i o n of a s e p a r a t i o n e n v e l o p e i n t h e l e e of t h e dune c r e s t . types of r i d g e s (e.g.
This t y p e o f flow s t r u c t u r e has been observed f o r v a r i o u s Taylor and Gent, 1974).
The e f f e c t o f l a t e r a l v a r i a t i o n s i n p l a n t d e n s i t y o n f l o w s t r u c t u r e a r e now b r i e f l y examined.
I m p l i c a t i o n s of t h e aerodynamic d a t a f o r sand t r a n s p o r t i n
Spinifex a r e discussed i n t h e following section. W h i l s t n a t u r a l p l a n t p o p u l a t i o n s may be o c c a s i o n a l l y r e l a t i v e l y homogeneous,
t h e y more commonly d i s p l a y s p a t i a l v a r i a b i l i t y i n d e n s i t y and d i s t r i b u t i o n .
In
o r d e r t o a s s e s s t h e n a t u r e of f l o w s t r u c t u r e i n c a n o p i e s o f v a r y i n g d e n s i t y , mean wind v e l o c i t y p r o f i l e s were measured s i m u l t a n e o u s l y w i t h i n a d j a c e n t low and high d e n s i t y S p i n i f e x h i r s u t u s p o p u l a t i o n s . both.
Figure 2 depicts t h e r e s u l t s .
F e t c h morphology was i d e n t i c a l f o r
The lower d e n s i t y p r o f i l e i l l u s t r a t e s t h e
g r e a t e r f l o w p e n e t r a t i o n of t h e v e g e t a t i o n , and o c c u r r e n c e o f h i g h e r v e l o c i t i e s n e a r t h e s u r f a c e , compared t o t h e h i g h e r d e n s i t y p r o f i l e .
Above t h e v e g e t a t i o n
canopy, t h e p r o f i l e s r e v e r s e a s f l o w a c c e l e r a t e s o v e r t h e h i g h e r d e n s i t y vegetation.
R e l a t i v e roughness l e n g t h s , c a l c u l a t e d using a l l s i x v e l o c i t i e s on
each p r o f i l e ,
were 3 . 6 cm and 8 . 4 cm f o r t h e
low and h i g h
density
328
/-. 3 05 \ I0
I
0
2
3
4
5 6 Disiance fin)
7
8
9
I0
!I
F i g . 1. Mean w i n d v e l o c i t y p r o f i l e s e x p r e s s e d a s a p e r c e n t a g e o f t h e u n d i s t u r b e d , upwind f l o w a t 4 m h e i g h t , m e a s u r e d o v e r a s m a l l , a s y m m e t r i c i n c i p i e n t f o r e d u n e , Hawks Nest. The d a s h e d l i n e i n d i c a t e s t h e t o p o f t h e 25 cm h i g h S p i n i f e x h i r s u t u s c o v e r . P r o f i l e l o c a t i o n s a r e i n d i c a t e d by d o t s ( s u r f a c e ) a n d n u m b e r s (lower) on t h e dune. profiles respectively.
The d a t a show t h a t a s p l a n t c o v e r o r d e n s i t y v a r i e s
l a t e r a l l y , t h e w i t h i n - c a n o p y flow s t r u c t u r e a l s o v a r i e s l a t e r a l l y . S i m i l a r l y , mean v e l o c i t y p r o f i l e s m e a s u r e d p r o g r e s s i v e l y downwind a c r o s s s u r f a c e s on which p l a n t d e n s i t y v a r i e s d i s p l a y a l i k e flow s t r u c t u r e t o t h a t described above(Hesp, 1982).
T h a t i s , Lhe
p l a n t d e n s i t y and d i s t r i b u t i o n .
f l o w a d j u s t s r a p i d l y t o local changes i n
T h e s e o b s e r v a t i o n s c o n c u r w i t h t h e work of
M u l h e a r n a n d F i n n i g a r i ( 1 9 7 8 ) a n d R a u p a c h e t a l . (1980) who d e m o n s t r a t e t h a t s p a t i a l v a r i a t i o n i n f l o w v e l o c i t y i s a common f e a t u r e of f l o w s o v e r s u r f a c e s where roughness is s p a t i a l l y v a r i a b l e . TABLE 1.
Profile
A i r - p l a n t i n t e r a c t i o n d a t a f o r p r o f i l e s 1 t o 4 of F i g u r e 1.
Shear Velocity u,(m/s)
Roughness l e n g t h Ln (cm)
.10
0.11
.16 .38 .50
0.43 8.17 8.73
Correlation co-efficient .96 .99 .93 .98
329
90. 70.
50.
30.
15.
4-
Fig. 2 . Mean wind v e l o c i t y p r o f i l e s measured s i m u l t a n e o u s l y a c r o s s a h i g h d e n s i t y ( b r o k e n l i n e ) and moderate-low d e n s i t y ( c o n t i n u o u s l i n e ) S p i n i f e x vegetated f e t c h . SAND TRANSPORT
Once t h r e s h o l d v e l o c i t i e s a r e r e a c h e d , s a n d g r a i n s a r e i n c o r p o r a t e d i n t o t h e wind f l o w , a n d i n t e r a c t w i t h t h e v e g e t a t i o n .
I n o r d e r t o e l u c i d a t e t h e sand
t r a n s p o r t p r o c e s s e s which t h e a u t h o r b e l i e v e s o p e r a t e w i t h i n v e g e t a t e d s u r f a c e s , two t y p e s i t u a t i o n s dre d i s c u s s e d below.
The f i r s t c o n s i d e r s a n i d e a l s i t u a t i o n
where t h e s u r f a c e is i n i t i a l l y f l a t and c o v e r e d w i t h a c o n t i n u o u s , homogeneous, high d e n s i t y c o v e r of S p i n i f e x ; t h e s e c o n d d e a l s w i t h a s i t u a t i o n where t h e vegetation density is s p a t i a l l y variable. S i t u a t i o n 1:
High d e n s i t y S p i n i f e x
I t is assumed t h a t a e o l i a n s a n d t r a n s p o r t i s o c c u r r i n g on t h e upwind, unvegetated beach.
As t h e s a n d f l o w meets t h e l e a d i n g e d g e of t h e S p i n i f e x
vegetation, observations i n d i c a t e t h a t s e v e r a l events occur i n combination.
For
a g i v e n m o d e r a t e wind v e l o c i t y ( s a y 5-7 m s e c - l a t z = 1 0 cm) r i p p l e f o r m a t i o n c e a s e s , a n d t h e r e i s a r a p i d d e c e l e r a t i o n o f f l o w v e l o c i t i e s n e a r t h e bed i n t h e l e a d i n g e d g e r e g i o n ( f i g u r e 1; see a l s o Townsend, 1966; Sadeh e t a l . , 1971; Kotoda, 1 9 7 9 ) .
L i m i t e d f i e l d e x p e r i m e n t s c o n d u c t e d u t i l i s i n g Leatherman ( 1 9 7 8 )
330 s a n d t r a p s s i t e d a c r o s s t h e l e a d i n g e d g e r e g i o n , show t h a t t h e r a t e o f s a n d t r a n s p o r t d e c r e a s e s downwind a s n e a r - s u r f a c e f l o w v e l o c i t y d e c r e a s e s (Hesp, 1982). A n a l y s i s of slow-motion f i l m s a n d t o p o g r a p h i c m i c r o - s u r v e y s i n d i c a t e t h a t sand t r a n s p o r t a p p r o a c h e s a minimum r a t e , a t some p o i n t downwind o f t h e l e a d i n g edge of t h e vegetation.
I n a d e n s e c a n o p y , o b s e r v a t i o n s show t h a t when s a n d g r a i n s
s a l t a t e i n t o t h e v e g e t a t i o n , t h e y may d i s l o d g e o t h e r g r a i n s , b u t d i s l o d g e d g r a i n s c a n n o t move f a r b e c a u s e o f t h e p r e s e n c e o f v e g e t a t i o n .
In addition, the
downwind r e d u c t i o n i n t h r e s h o l d v e l o c i t y n e g a t e s t h e o p p o r t u n i t y f o r f u r t h e r s a l t a t i o n t o take place ( f i g . 1 ) .
Thus, f o r a g i v e n windspeed t h e r e a p p e a r s t o
b e a l i m i t t o t h e w i d t h , o r f e t c h o v e r which upwind i n d u c e d s a l t a t i o n w i l l occur i n dense vegetation. The c r e e p l o a d moves a c r o s s t h e l e a d i n g e d g e u n d e r t h e canopy.
A t , or v e r y
n e a r t h e s u r f a c e , Sadeh e t a l . ( 1 9 7 1 ) h a v e shown t h a t t h e p r e s e n c e o f stems e x e r t a d r a g o n t h e flow.
I i n f e r t h a t t h i s a d d i t i o n a l d r a g , p l u s t h e downwind
r e d u c t i o n i n s a l t a t i o n bombardment, would l e a d t o a p r o g r e s s i v e downwind d e crease i n t r a c t i o n load within t h e vegetation. S i t u a t i o n 2:
Variable density Spinifex
O b s e r v a t i o n s i n d i c a t e t h a t lower d e n s i t y v e g e t a t e d s u r f a c e s d i f f e r i n s e v e r a l r e s p e c t s from t h e h i g h d e n s i t y s u r f a c e ( S i t u a t i o n 1 ) d i s c u s s e d a b o v e .
Firstly,
s a n d t r a n s p o r t i n d u c e d upwind o f t h e v e g e t a t i o n , may t r a v e l f u r t h e r i n t o t h e vegetation a s near-surface v e l o c i t i e s a r e higher than i n t h e higher density case (Fig. 2).
S e c o n d l y , i n c o m i n g g r a i n s may i n d u c e f u r t h e r t r a n s p o r t t o t a k e
p l a c e w i t h i n t h e v e g e t a t i o n by s a l t a t i o n c o l l i s i o n s .
T h i r d l y , my o b s e r v a t i o n s
i n d i c a t e t h a t c r e e p a n d s a l t a t i o n a p p e a r t o b e i n d u c e d by l i f t and d r a g f o r c e s o p e r a t i n g w i t h i n t h e more open r e g i o n s i n v e g e t a t i o n , i n d e p e n d e n t l y o f upwind induced s a l t a t i o n e j e c t i o n s (Fig. 2 ) .
These o b s e r v a t i o n s a c c o r d w i t h Kuhlrnan's
(1957, 1959) o b s e r v a t i o n s of sand t r a n s p o r t on s u r f a c e s e x h i b i t i n g a v a r i a b l e vegetation cover. DUNE MORPHOLOGY The d i s c u s s i o n above s u g g e s t s t h a t a t a g i v e n p o i n t downwind o f t h e l e a d i n g edge i n d e n s e v e g e t a t i o n , s a n d t r a n s p o r t r e a c h e s a minimum.
Upwind i n d u c e d
s a l t a t i n g g r a i n s have a l l been t r a p p e d , a n d t h e r e i s l i t t l e , i f a n y , w i t h i n canopy i n d u c e d s a n d t r a n s p o r t .
The e f f e c t o f a downwind d e c r e a s e i n s a n d
t r a n s p o r t i s t o produce a t h i n lamina o f sand of a given width, t h e dimensions of which are a p p a r e n t l y d e p e n d e n t o n p l a n t d e n s i t y and wind v e l o c i t y .
If the
wind v e l o c i t y r e m a i n s r e l a t i v e l y c o n s t a n t f o r a p e r i o d of time, c o n t i n u o u s d e p o s i t i o n of s a n d o v e r a r e l a t i v e l y f i n i t e d i s t a n c e p r o d u c e s a series of l a m i n a e
of s i m i l a r w i d t h , a n d r e s u l t s i n t h e f o r m a t i o n o f a wedge- or t r i a n g u l a r - s h a p e d dune.
The windward s l o p e o f t h e dune i s l o n g and low, t h e l e e w a r d s l o p e s h o r t
331 and s t e e p .
The d u n e i s t r i a n g u l a r i n s h a p e b e c a u s e t h e r e i s a downwind i n c r e a s e
i n t h e r a t e of s a n d d e p o s i t i o n .
Although v e l o c i t i e s may a b r u p t l y d e c r e a s e j u s t
w i t h i n t h e l e a d i n g e d g e of v e g e t a t i o n , sand g r a i n s do n o t f a l l v e r t i c a l l y t o t h e bed, b u t f o l l o w t r a j e c t o r i e s s u s t a i n e d by upwind i n d u c e d momentum.
This r e s u l t s
i n d e p o s i t i o n b e i n g lowest a t t h e l e a d i n g e d g e , and i n c r e a s i n g l y g r e a t e r downwind of t h e l e a d i n g e d g e .
A dune shape such a s t h a t depicted i n f i g u r e 1
results. Where t h e v e g e t a t i o n d e n s i t y i s e i t h e r low, or t h e d u n e s u r f a c e e x h i b i t s l a r g e s p a t i a l v a r i a t i o n s i n d e n s i t y ( a s i n s i t u a t i o n 2 above), t r i a n g u l a r dunes a r e not a s common.
R a t h e r , l o n g e r , a s y m m e t r i c , convex d u n e s a r e formed.
This appears
t o be due t o t h e i m p o r t a n c e o f l o c a l s a n d t r a n s p o r t i n a r e a s o f low d e n s i t y ,
o r v a r i a b l e d i s t r i b u t i o n s , w i t h i n t h e canopy. I t i s s u g g e s t e d a b o v e t h a t f o r a g i v e n wind v e l o c i t y
,
t h e f e t c h (or w i d t h ) o v e r
which most a e o l i a n s a n d d e p o s i t i o n o c c u r s i s d e t e r m i n e d by p l a n t d e n s i t y .
If
t h e change i n e l e v a t i o n o v e r time o f a series o f a d j a c e n t p o i n t s a l o n g an i n c i p i e n t f o r e d u n e s t o s s f a c e i s m o n i t o r e d , and t h e p l a n t d e n s i t y a r o u n d t h o s e p o i n t s is a s s e s s e d , t h e c h a n g e i n e l e v a t i o n s h o u l d t h e n b e a f u n c t i o n of p l a n t d e n s i t y and wind v e l o c i t y .
T h a t i s , t h e e l e v a t i o n a t o n e p o i n t on t h e f o r e d u n e
s t o s s f a c e s h o u l d b e h i g h e r where upwind p l a n t d e n s i t y is h i g h ( s i n c e s a n d i s supposedly t r a p p e d s o o n e r w i t h i n t h e s e l o c a t i o n s ) , t h a n where p l a n t d e n s i t y i s low. F i g u r e 3a i l l u s t r a t e s two p l o t s o f numbers o f S p i n i f e x ( d e n s i t y ) and s a n d a c c r e t i o n , f o r e a c h v e g e t a t e d , s q u a r e metre closest t o t h e beach on e a c h a d j a c e n t l i n e w i t h i n a c o n t i n u o u s l y m o n i t o r e d s i t e ( s e e s e c t i o n 2 f o r d e t a i l s ) . The s o l i d l i n e and crosses i l l u s t r a t e a v e r a g e a c c r e t i o n (m 3 p e r s q u a r e m e t r e ) and a v e r a g e S p i n i f e x numbers f o r t h e p e r i o d November 18, 1978 t o 3 a n u a r y 2 0 , 1979 ( 3 s u r v e y s , 64 day a v e r a g e ) .
The b r o k e n l i n e and c i r c l e s i l l u s t r a t e a v e r a g e a c c r e t i o n f o r
t h e same p e r i o d b u t p l a n t numbers a s a t 3 a n u a r y 2 0 , 1 9 7 9 f o r t h e same l o c a t i o n s . Correlation c o e f f i c i e n t s a r e i n d i c a t e d , and a r e s i g n i f i c a n t a t t h e 95 p e r c e n t l e v e l . Although c o r r e l a t i o n s a r e low, t h e l i n e s i n d i c a t e t h a t t h e r e e x i s t s a r e a s o n a b l e r e l a t i o n s h i p between p l a n t d e n s i t y and s a n d a c c r e t i o n i n t h e s e l e c t e d l o c a t i o n . F i g u r e 3 b i l l u s t r a t e s t h e m o r p h o h g i c r e s u l t where s a n d h a s been t r a n s p o r t e d a c r o s s two a d j a c e n t z o n e s of v a r y i n g v e g e t a t i o n d e n s i t y .
The d a t a is d e r i v e d
from s u r v e y s and p l a n t c o u n t s i n t h e same p l o t s i t e a s t h a t n o t e d above.
The
f i g u r e shows t h e numbers of S p i n i f e x p l a n t s c o u n t e d w i t h i n e a c h s q u a r e metre along two o n e metre wide l i n e s , 5 metres a p a r t o n March
2 4 , 1979.
I t may be
seen t h a t t h e r e are many more S p i n i f e x p l a n t s p e r s q u a r e metre on l i n e 5 compared t o l i n e 10, and t h a t t h e h i g h e s t d e n s i t y z o n e of S p i n i f e x p l a n t s p e r s q u a r e
metre o n l i n e 5 is a l s o l o c a t e d f u r t h e r s e a w a r d s t h a n l i n e 10. of t h e i n c i p i e n t f o r e d u n e i s a l s o shown f o r l i n e 5 and l i n e 10.
The morphology The f o r m o f t h e
dune a t l i n e 5 is h i g h e r , s h o r t e r a n d more a s y m m e t r i c t h a n t h e dune form a t l i n e 10.
332 A l a g e f f e c t on sand d e p o s i t i o n may a l s o be observed on l i n e 5.
F i g u r e 4 i l l u s t r a t e s two p o r t i o n s o f an i n c i p i e n t foredune which was i n i t i a t e d by s e e d l i n g g e r m i n a t i o n d u r i n g October ( s p r i n g ) , 1978.
The area shown i n f i q u r e
4c and d i n i t i a l l y d i s p l a y e d a p p r o x i m a t e l y t w i c e t h e d e n s i t y o f t h e a d j a c e n t (200 m a p a r t ) a r e a i l l u s t r a t e d i n f i g u r e 4a and b.
The h i g h e r d e n s i t y i n c i p i e n t
foredune p o r t i o n has a narrower b a s a l w i d t h , and i s h i q h e r t h a n t h e l o w e r density f oredune zone.
7 /x
x
o x
0 0
25
X 50
75
100
125
I
1
150
175
Distance seawards (m)
Spinfex numbers (density)
F i g . 3a. R e l a t i o n s h i p between S p i n i f e x h i r s u t u s d e n s i t y and sand a c c r e t i o n f o r each 24-25 square metre l o c a t i o n on a l l P l o t l i n e s (1-10) f o r 20.1.79, and f o r t h e p e r i o d 18.11.78 - 20.1.79. F i g . 3b. Morphology of l i n e s 5 and 10, and S p i n i f e x numbers w i t h i n t h e 20-30 sq.m. zone (down t h e dune) a t 24.3.79. A s h o r t e r , asymmetric r i d g e forms i n t h e h i g h e r d e n s i t y ( l i n e 5 ) zone.
Swales Swales a r e concave t r o u g h s w h i c h o f t e n
occur i n t h e l e e o f foredunes.
The
d a t a p r e s e n t e d i n f i g u r e s 3 and 4 show t h a t where v e g e t a t i o n d e n s i t y i s moderate t o h i g h , t h e i n c i p i e n t foredune t r a p s and r e t a i n s t h e l a r g e s t volume o f incoming a e o l i a n sand.
Swales a r e t h u s commonly ' f o r m e d ' as m i n i m u m d e p o s i t i o n zones, as
suggested by S a l i s b u r y ( 1 9 5 2 ) , Ranwell (1972) and Quinn ( 1 9 7 7 ) .
Swale d e p t h
depends on t h e d e n s i t y o f p l a n t s on t h e foredune t o seaward, and wind v e l o c i t y . Swale d e p t h (and w i d t h ) w i l l be l e s s where v e g e t a t i o n d e n s i t y i s l o w o r wind v e l o c i t i e s are
h i g h , as more sand w i l l be t r a n s p o r t e d i n t o t h e swale zone.
Swale w i d t h t h e n depends on one o f t h e f o l l o w i n g :
the i n i t i a l location of the
333 Spinifex seedliny zone with respect t o
d
landward foredune (see s e c t i o n 3) ;
the d e n s i t y o f thc seawdrd v e g e t a t i o n r o n e ( a s a b o v e ) ; t h c width o f scawdrd
rhiLome g r o w t h ; m d w i n d v e l o c i t y (ds a b o v e ) .
F i g . 4. E v o l u t i o n o f t w o p o r t i o n s of d n i n c i p i e n t f o r e d u r i r f o r m e d by S p i n i f e x s e e d l i n g c o l o n i s d t i o n o f t h e b a c k s h o r e d u r i n g O c t o b e r , 1978. ( d ) and ( b ) i l l u s t r d t e t h c l o w - m o d e r d t e d e n s i t y p o r t i o n ( 2 4 . 3 . 7 9 drid 26.4.80), arid ( c ) d n d ( d ) t l i c h i g h d e n s i t y p o r t i o n (samf' d d t e s ) . T t i r f u r t h e r s e d w a r d a new S p i n i f e x s e e d l i n g z o n e , the w i d r r p o t r n t i d l swdle
width.
The l o c d t i o n o f t h e s e e d l i n g z o n e i s d e t e r m i n e d by t h e i r i t r r a c t i o r i o f
spring t i d e heighL and bedch morphology, and/or t h e rate o f beach p r o q r a d d t i o n . The w i d e r t h e b e a c h , o n a v e r d y e , t h e f u r t h e r s e d w a r d w i l l t h e s e e d l i n g Lone b e l o c a t e d , dnd h e n c e t h e w i d e r t h e swdle b e t w e e n dn o l d e r d u n e and t h e s e e d l i n g zorle.
R a p i d b e a c h p r o g r d d a t i o n w i l l medn t h a t t i m e - d i s c r e t e s e r d l i n q Lories mdy
p o t e n t i a l l y be w i d e l y s e p a r a t e d .
S i m i l a r comments a p p l y t o areas experiericiricj
r a p i d s e a w a r d c o l o n i s a t i o n by S p i n i f e x r h i r o m r s .
334 E f f e c t o f v a r i a t i o n s i n wind v e l o c i t y V a r i a t i o n s i n wind v e l o c i t y i n d u c e v a r i a t i o n s i n t h e volume o f s a n d t r a n s p o r t e d , t h e aerodynamic behaviour o f t h e v e g e t a t i o n , and dune morphology.
As w i n d v e l -
o c i t i e s i n c r e a s e o v e r v e g e t a t i o n , t h e c a n o p y i s p e n e t r a t e d more e f f e c t i v e l y ( e s p e c i a l l y d u r i n g g u s t s ) , a n d s h e a r stresses a r e o r d e r s of m a g n i t u d e t h a n when wind s p e e d s a r e low ( F i n n i g a n , 1 9 7 9 ) .
greater
Furthermore, with increasing
w i n d v e l o c i t y , t h e v e g e t a t i o n i s f o r c e d closer t o t h e g r o u n d ( s t r e a m l i n e d ) , a n d roughness l e n g t h s ( z ) are lowered (Sellers, 1965;
Thom, 1 9 7 1 , 1 9 7 2 ) .
Upwind
o f t h e v e g e t a t i o n , t h e s a l t a t i o n ' c o l u m n ' i s h i g h e r , g r a i n t r a j e c t o r i e s are l o n g e r , a n d t h e v o l u m e o f s a n d i n t r a n s p o r t i s much g r e a t e r ( W i l l i a m s , 1 9 6 4 ) . The p o t e n t i a l f o r s a n d t r a n s p o r t ( i n c l u d i n g e r o s i o n ) i s a l s o much g r e a t e r w i t h i n the vegetation. of
I n a d d i t i o n , i f sand d e p o s i t i o n o c c u r s o v e r a l o n q enough period
time, o r i f t h e r a t e o f s d n d t r a n s p o r t i s h i g h , v e g e t a t i o n may b e c o m p l e t e l y
b u r i e d , a n d i n c o m i n g s a n d may t h e n b y p a s s t h e o r i g i n a l d e p o s i t i o n z o n e ( i . e . z0 w i l l a p p r o a c h z e r o ) .
S h o r t term m o n i t o r i n g of s i n g l e storm e v e n t s , a n d l o n g term s u r v e y i n q o f i n c i p i e n t foredune formation (Hesp, 1982) i l l u s t r a t e s t h a t for a given S p i n i f e x d e n s i t y , sand t r a n s p o r t and d e p o s i t i o n w i l l t a k e p l a c e over a l o n g e r d i s t a n c e d u r i n g h i g h w i n d v e l o c i t i e s t h a n d u r i n g low t o m o d e r a t e w i n d v e l o c i t i e s . O b s e r v a t i o n s i n d i c a t e t h a t w h e r e t h e v e g e t a t i o n i s f a i r l y d e n s e , most s a n d i s d e r i v e d f r o m t h e b e a c h , u p w i n d of t h e v e g e t a t e d z o n e .
However, w h e r e t h e
v e g e t a t i o n d e n s i t y i s low t o m o d e r a t e , more s a n d i s t r a n s p o r t e d f r o m w i t h i n t h e v e g e t a t i o n a s wind v e l o c i t i e s i n c r e a s e .
Velocity p r o f i l e s adjust very rapidly
i n response t o local v a r i a t i o n s i n v e g e t a t i o n d e n s i t y (Fig. 2; and Bradley, 1968). By i m p l i c a t i o n , s a n d t r a n s p o r t w o u l d a l s o o c c u r more r e a d i l y a s v e g e t a t i o n d e n s i t y d e c r e a s e s , a n d t h u s w i t h i n - c a n o p y t r a n s p o r t becomes i n c r e a s i n g l y i m p o r t a n t a s velocities increase.
I n a d d i t i o n , it is l i k e l y t h a t a p o r t i o n of t h e flow a t
t h e l e a d i n g e d g e o f t h e v e g e t a t i o n moves u n d e r t h e c a n o p y .
D u r i n g h i g h wind
v e l o c i t i e s S a d e h e t a 1 . ( 1 9 7 1 ) n o t e t h a t j e t t i n g commonly o c c u r s a r o u n d e a c h p l a n t stem.
T h i s i n c r e a s e s t h e a v e r a g e v e l o c i t y n e a r t h e b e d , and would p r e -
sumably i n c r e a s e t h e p o t e n t i a l f o r sand t r a n s p o r t w i t h i n v e g e t a t i o n . t i g u r e 5 i l l u s t r a t e s two p h o t o g r a p h s , t a k e n o n e month a p a r t , o f a p o r t i o n o f
a c o n t i n u o u s l y m o n i t o r e d f o r e d u n e p l o t (same drea a s F i g . 4 a , b ) .
Sand d e -
p o s i t i o n and e r o s i o n h a s o c c u r r e d across a wide zone i n t h e h i g h e r v e l o c i t y e v e n t ( 5 b ) , and dune h e i g h t is lowered w h i l s t dune l e n g t h is i n c r e a s e d . E f f e c t of v a r i a t i o n s i n p l a n t growth p a t t e r n s Where t h e i n i t i a l l o n g s h o r e d i s t r i b u t i o n o f S p i n i f e x s e e d l i n g s i s c o n t i n u o u s ,
lateral v a r i a t i o n s i n density (alongshore) produce foredune morphologic variations. T h e i n c i p i e n t f o r e d u n e w i l l b e h i g h e r a n d o f s h o r t e r b a s a l w i d t h i n areas o f
335 h i g h e r d e n s i t y , a n d lower a n d l o n g e r i n a r e a s o f lower d e n s i t y . produces a l o n g s h o r r 1958b).
This variation
lee s l o p e and c r e s t a l s i n u o s i t y (see f i g . 4 d , and O l s o n ,
When r h i z o m e l s t o l o n c o l o n i s a t i o n i s t h e d o m i n a n t p r o c e s s , s p a t i a l
v d r i a t i o n i n s u r f a c e c o v e r mdy b e p r o n o u n c e d .
F o r e x a m p l e , s i r r q l c o r sevcrdl
r h i z o m e s may a l t e r n a t e w i t h u n v e g e t a t e d z o n e s a l o n g s h o r e , a n d c r e s t a l s i n u o s i t y
i s a common f e a t u r e o f t h e i n c i p i e n t f o r e d u n e .
F i g . 5. T e m p o r a l v a r i a t i o n of a n i n c i p i e n t f o r e d u n e f o l l o w i n g d p e r i o d o f low-moderate wind v e l o c i t i e s ( a ; 2 6 . 8 . 7 9 ) , a n d a p e r i o d o f h i g h wind v e l o c i t i e s ( b ; 24.9.79).
The mode of b e a c h v e g e t a t i v e c o l o n i s a t i o n , a n d s p a t i a l v d r i d t i o r r s i n s h o o t and r h i z o m e growth s t r u c t u r e a n d l o c a t i o n p r o d u c e v a r i a t i o n s i n i n c i p i e n t f o r e dune morphology.
T h r e e major i n c i p i e n t f o r e d u n e m o r p h o l o g i e s may b e r e c o q n i s e d :
ramps, terraces and r i d g e s .
Ramp i n c i p i e n t f o r e d u n e s d e v e l o p p r i m a r i l y o n
i n h e r i t e d s e a w a r d s l o p i n g s u r f a c e s c o l o n i s e d by S p i n i f e x r h i z o m e s o r s e e d l i n g s . The ramp m o r p h o l o g y i s m a i n t a i n e d w h e r e s e a w a r d r h i z o m e g r o w t h k e e p s p d c e
with
sand d e p o s i t i o n , o r w h e r e s e e d l i n g d e n s i t y is low a n d s a n d d e p o s i t i o n o c c u r s throughout t h e vegetated zone allowing in-place build-up o f t h e sloping surface.
Terrace i n c i p i e n t f o r e d u n e s d e v e l o p w h e r e ( a ) S p i n i f e x s e e d l i n y s p r o p a g a t e on f l a t beach ( i n h e r i t e d morphology); or (b) where t h e v e g e t a t i o n
d
d e n s i t y i s low
t o m o d e r d t e , o r d i s t r i b u t i o n s a r e i r r e g u l a r , s a n d i s more a b l y a n d more e v e n l y t r a n s p o r t e d through t h e whole v e g e t a t i o n zone.
Terraces may b e e x t e n s i v e
where b e a c h p r o g r a d a t i o n i s r a p i d , a n d t h e r a t e o f s e a w a r d S p i n i f e x rhizome and s h o o t c o l o n i s a t i o n i s h i g h .
Ridge i n c i p i e n t foredunes i n i t i a l l y develop
b e d c u s e o f p r e f e r e n t i a l d e p o s i t i o n o f s a n d w i t h i n t h e f i r s t f e w metres o f
d
high d e n s i t y S p i n i f e x zone. T e m p o r a l v a r i a t i o n s i n g r o w t h p a t t e r n s i n d u c e f u r t h e r m o r p h o l o g i c charrge. s h o o t s and r h i z o m e s p r o d u c e d w i t h i n e x i s t i n g c o m m u n i t i e s c o l o n i s e t h e seaward u n v e g e t a t e d b e a c h , p r o d u c i n g a s e a w a r d d i s p l a c e m e n t o f t h e major z o n e o f s a n d d e p o s i t i o n and a r e s u l t i n g c h a n g e i n t h e i n c i p i e n t f o r e d u n e morpholoyy (see
New
336 e . g . O l s o n , 1958b, p l a t e 1 D ; McKenzie, 1 9 5 8 ) .
For e x a m p l e , ramp f o r e d u n e mor-
p h o l o g i e s may d e v e l o p t h r o u g h an e v o l u t i o n a r y c o n t i n u u m from t e r r a c e s t o r i d g e s where r h i z o m e s c o l o n i s e seaward s u r f a c e s , t r a p p i n g s a n d t o i n i t i a l l y form a t e r r a c e , and t h e n a r i d g e a s s e a w a r d growth i s i n h i b i t e d by t i d a l i n u n d a t i o n , or a s density increases. SEDIMENTARY STRUCTURES I n t h e f o l l o w i n g , two e x c a v a t i o n s a r e i l l u s t r a t e d t o i n d i c a t e t h e t y p i c a l i n t e r n a l s t r u c t u r e o f i n c i p i e n t f o r e d u n e s formed i n S p i n i f e x h i r s u t u s .
The
t e r m i n o l o g y u s e d i s t h a t of McKee and Wier ( 1 9 5 3 ) . F i g u r e 6 i l l u s t r a t e s a t r e n c h e x c a v a t e d i n a s m a l l i n c i p i e n t f o r e d u n e approxi m a t e l y SO metres west of a c o n t i n u o u s l y m o n i t o r e d p l o t .
I n t h i s a r e a , a low
d e n s i t y zone o f S p i n i f e x s e e l i n g s p r o p a g a t e d on t h e b a c k s h o r e .
Aeolian sand
d e p o s i t i o n o c c u r r e d t h r o u g h o u t , and beyond t h e S p i n i f e x z o n e . The lowermost s e t ( A i n f i g . 6 d ) i s bounded a t t h e t o p by a l i n e o f d r i f t m a t e r i a l , m o s t l y wood and s e e d f r a g m e n t s ( s e e b o t t o m f i g . 60).
T h i s was d e -
p o s i t e d by s p r i n g t i d e s w a s h , a n d t h e S p i n i f e x h i r s u t u s growing above o r i g i n a t e d from s e e d s d e p o s i t e d w i t h i n t h i s l i t t e r .
T h i s lowermost s e t o f
s t r a t a i s a s i m p l e t a b u l a r s e t c o m p r i s i n g e x t r e m e l y l o w - a n g l e , medium-scale ( l i k e l y t o be l a r g e - s c d l e ; i . e . l e n g t h g r e a t e r t h a n 6 m), e v e n , p a r a l l e l , swash deposited laminae.
Laminae i n t h e f a c e a t t h e r e a r o f t h e t r e n c h ( n o r m a l t o
p r e v a i l i n g wind) a r e h o r i z o n t a l . a t a n g l e s o f less t h a n
4O,
The rest ( t r e n c h f r o n t and p a r a l l e l f a c e ) d i p
and most commonly a t 1' t o .'2
Near t h e m i d d l e o f
t h e t r e n c h t h e d i r e c t i o n o f d i p c h a n g e s from a seaward t o a landward o n e . The s e c o n d s e t ( B i n f i g . 6 d ) i s a s m a l l , s i m p l e l e n t i c u l a r set c o m p r i s i n g medium-scale, l o w - a n g l e c r o s s - l a m i n a e which a r e e v e n , and g e n e r a l l y p a r a l l e l . The wavy s t r a t a i n t h e b a c k s e t (windward) z o n e ( f i g . 6 c ) p r o b a b l y r e s u l t from This set (B) r e p r e s e n t s t h e i n i t i a l a c c r e t i o n u n i t w i t h i n
human f o o t p r i n t s .
the Spinifex seedlings. The sets ( C , D , E) l y i n g above s e t 6, a p p e a r t o be s i m p l e ( n o n - e r o s i o n a l l e n t i c u l a r sets. other.
B d c k s e t bounding s u r f a c e s a r e p r e d o m i n a n t l y p a r a l l e l t o e a c h
The sets r e s u l t from minimal b a c k s e t d e p o s i t i o n and maximum t o p s e t and
foreset deposition.
Wind v e l o c i t i e s were p r o b a b l y h i g h e r on a v e r a g e t h a n i n t h e
d e p o s i t i o n a l e v e n t ( s ) f o r m i n g s e t B. d i p s of less t h a n 13',
A l l c r o s s - s t r a t a i n s e t s C , D and E had
and most were i n t h e r a n g e S o t o 9".
t h e uppermost p o r t i o n o f s e t E ( i . e . c r o s s - s t r a t a t o be recognised.
The s a n d c o m p r i s i n g
E i i i was u n f o r t u n a t e l y t o o d r y f o r t h e
The c u r v a t u r e and s h a p e o f t h e E i i u n i t , which
p a r a l l e l s t h e f i r s t v i s i b l e lamina i n t h e backset s t r a t a , i n d i c a t e s t h a t t h i s u n i t is p a r t of t h e E l e n t i c u l a r set.
S e t shape i n d i c a t e s t h a t i n d i v i d u a l
l a m i n a e were t h i c k e s t i n t h e c e n t r e ( d u n e c r e s t ) , and t h i n n e s t a t t h e windward and l e e w a r d e n d s .
Metres seaward
W W U
338 The c r o s s - s t r a t a i n d i c a t e t h e d e v e l o p m e n t from an i n i t i a l low v e l o c i t y micror i d g e f o r m a t i o n , t h r o u g h a p e r i o d o f m o d e r a t e a n d h i g h e r v e l o c i t y t e r r a c e forma t i o n , t o a r i d g e capping.
S t r a t a comprising t h e ' t e r r a c e ' sets continue
landwards ( f i g . 6 a , b) i n t o a n o l d e r vegetated i n c i p i e n t foredune zone.
Aeolian
a c c r e t i o n a l s t r a t a d e p o s i t e d o n t h e upwind b e a c h a r e c o n t i n u o u s w i t h t h e ' t e r r a c e ' strata.
Ridge d e v e l o p m e n t ( u n i t E i i ) p r i n c i p a l l y o c c u r r e d a s t h e d e n s i t y of T h i s p r o c e s s was a i d e d , i n p a r t by t h e o p e r a t i o n
t h e local vegetation increased.
of wind v e l o c i t y e v e n t s which were more c o n s t a n t , and of less v e l o c i t y t h a n p r e v i o u s e v e n t s which formed t h e ' t e r r a c e ' s t r a t a . B a c k s e t and f o r e s e t c r o s s - s t r a t a a r e c o n t i n u o u s a c r o s s t h e whole i n c i p i e n t foredune.
B i g a r e l l a e t a l . (1969) a l s o found t h i s t y p e of continuous s t r a t i f i c a t i o
i n a " s a n d r i d g e dune" ( a f o r e d u n e ) .
McKee ( 1 9 7 9 , p . 9 7 ) s t a t e s t h a t " t h e
abundance o f s a n d s u p p l y and t h e s l o w r a t e of movement ( b e c a u s e o f i n t e r n a l dampness) a r e r e s p o n s i b l e f o r t h i s f e a t u r e " .
I n t h e trench i l l u s t r a t e d i n
f i g u r e 6 , c o n t i n u o u s c r o s s - s t r a t a a r e formed by c o n t e m p o r a n e o u s d e p o s i t i o n a c r o s s t h e whole dune s u r f a c e . Figure 7 i l l u s t r a t e s a trench vegetated i n c i p i e n t foredune.
excavated i n a l a r g e r , o l d e r , moderately
The l o w e s t t a b u l a r s e t (1 on f i g . 7 ) c o n s i s t s
o f v e r y low a n g l e , p a r a l l e l c r o s s - l a m i n a e i n which d i p s d o n o t e x c e e d 5'.
The
lower-most p o r t i o n o f t h i s s e t i s i n t e r p r e t e d on t h e b a s i s of h e i g h t above MSL a s b a c k s h o r e swash l a m i n a e which g r a d e upwards i n t o a e o l i a n l a m i n a e .
s e t h a s b e e n swash e r o d e d on t h e s e a w a r d e d g e ( s e t 2 ) .
This
F i n e heavy-mineral
l a m i n a e were p r e s e n t w i t h i n t h e p l a n a r wedge-shaped s e c o n d s e t . The t h i r d set i s a s i m p l e , l e n t i c u l a r set c o m p r i s i n g c o n v e x , g e n e r a l l y T h i s s e t r e p r e s e n t s i n i t i a l r i d g e formatior
p a r a l l e l , continuous cross-laminae. within Spinifex.
Above t h e t h i r d s e t , t h e r e i s a t l e a s t o n e s e t o f l e n t i c u l a r ,
convex, continuous cross-laminae.
The l o w e r bounding s u r f a c e o f t h e s e t ( s ) i s
p l a n a r , a t l e a s t i n t h e upwind s e c t i o n . downwind o f t h e r i d g e crest o f set 3 . (mid-way t o c r e s t ) , t o 11'
14'
t o 6'
(downwind e n d ) .
Deposition has predominatly occurred The c r o s s - l a m i n a e d i p from 6'
( j u s t upwind o f c r e s t ) , t o 1 0'
upwind t o
( l e e of crest),
Cross-laminae a r e continuous a c r o s s t h e crest, j o i n i n g
t h e l o w e r s e t ( 3 ) t a n g e n t a l l y a t low a n g l e s downwind.
I n s p e c t i o n of t h e i n i t i a l
convex set ( 3 ) and uppermost s e t ( s ) , shows t h a t t h e c r o s s - l a m i n a e t h i c k e n landw a r d s i n t o t h e c e n t e r o f t h e s e t ( t h e most c o n v e x , r i d g e p o r t i o n ) , and t h e n e i t h e r d i m i n i s h i n s i z e downwind, o r t e r m i n a t e .
This depositional process
r e s u l t s i n ridge formation. I n summary, s i m p l e and p l a n a r , t a b u l a r and l e n t i c u l a r s e t s of l o w - a n g l e , continuous cross-laminae dominate t h e s e i n c i p i e n t foredunes. d i p a t a n g l e s o f l e s s t h a n 15'.
Most c r o s s - s t r a t a
Goldsmith (1973, 1978) h a s a l s o s t r e s s e d t h e
abundance o f l o w - a n g l e c r o s s - b e d d i n g i n f o r e d u n e s .
339
F i g . 7 . I n t e r n a l s e d i m e n t a r y s t r u c t u r e s of a n i n c i p i e n t f o r e d u n e . T r e n c h d e p t h i s 0.95 cm ( c r e s t t o b a s e ) , a n d l e n g t h i s 5 m. Lower s e t s a r e i d e n t i f i e d (see t e x t ) , a n d t h e a r r o w i n d i c a t e s t h e c r e s t s u r f a c e . Wind f l o w s f r o m l o w e r right t o upper left.
CONCLUSION I n c i p i e n t f o r e d u n e s formed w i t h i n l a t e r a l l y c o n t i n u o u s S p i n i f e x p o p u l a t i o n s r e s u l t from a complex i n t e r a c t i o n between b i o l o g i c , aerodynamic, and geomorphic processes.
T h i s p a p e r h a s o n l y b r i e f l y d i s c u s s e d some o f t h e major f a c t o r s
i n v o l v e d , a n d many a d d i t i o n a l f a c t o r s ( e . g . wave e r o s i o n ; p r e s e n c e o f o t h e r p l a n t s p e c i e s w i t h i n S p i n i f e x z o n e s ) dct t o i n t r o d u c e V a r i a t i o n s i n f i n a l d u n e morphology a n d i n t e r n a l s t r u c t u r e .
However, s e v e r a l g e n e r a l p o i n t s may b e made
concerning i n c i p i e n t foredune e v o l u t i o n : ( i ) The n e a r - s u r f a c e f l o w s t r u c t u r e w i t h i n S p i n i f e x v e g e t a t i o n p r i n c i p a l l y
varies dccording t o density/distribution variations.
When s a n d i s t r a n s p o r t e d
w i t h i n t h e f l o w , s e d i m e n t a t i o n p a t t e r n s a r e a l s o i n p a r t d e t e r m i n e d by p l a n t cover.
Thus f o r e d u n e h e i g h t i n c r e a s e s and b a s a l w i d t h d e c r e a s e s as p l a n t
density increases.
F o r e d u n e s t e n d t o b e more a s y m m e t r i c , w e d g e - s h a p e d f o r m s
as p l a n t d e n s i t y i n c r e a s e s .
L o n g s h o r e v a r i a t i o n s i n crestal and lee s l o p e
s i n u o s i t y o f dunes is a p r o d u c t of v a r i a t i o n s i n p l a n t d e n s i t y and d i s t r i b u t i o n . ( i i ) As w i n d v e l o c i t y i n c r e a s e s , P o r e d u n e h e i g h t d e c r e a s e s a n d d u n e l e n g t h
increases for a given vegetation d e n s i t y .
34 0 ( i i i ) S w a l e s a r e o f t e n formed
ds
non- t o l i m i t e d - d e p o s i t i o n a l z o n e s .
( i v ) S e d i m e n t a r y s t r u c t u r e s a r e dominated by l o w - a n y l e c r o s s b e d s . and b a c k s e t s a r e c o n t i n u o u s , and s l i p f a c e s a r e g e n e r a l l y a b s e n t .
Topsets
For a g i v e n
wind v e l o c i t y , s h o r t e r , s t e e p e r wedge- o r t r i d n g u l d r - s h d p e d s e t s of c r o s s l a m i n a e a r e formed i n h i g h d e n s i t y S p i n i f e x z o n e s .
Where t h e d e n s i t y o f S p i n i f e x
i s low t o m o d e r a t e , l o n g e r , l o w e r more convex s e t s o c c u r .
Individudl cross-
lamina a r e t h i c k e r i n t h e higher d e n s i t y case than i n t h e lower d e n s i t y c a s e . F o r a g i v e n v e g e t a t i o n d e n s i t y , ttre h i g h e r t h e a v e r a g e wind v e l o c i t y , t h e l o n g e r and l o w e r t h e s e t s , and t h e g r e a t e r t h e d e g r e e of b a c k s e t l o w - a n g l e truncation.
A s a v e r a g e v e l o c i t y i n c r e a s e s , i t i s e x p e c t e d t h e r e would be
t r e n d from s i m p l e t o p l a n a r l e n t i c u l a r s e t s .
d
These c o n c l u s i o n s i n d i c a t e t h a t
a modification i s r e q u i r e d t o Yaalori's (1975) suggestion t h a t a r e l a t i o n s h i p between p l a n t d e n s i t y and l o w - a n g l e c r o s s - b e d d i n g might be e x p e c t e d . angle (less than 2 0')
c r o s s - b e d s do d o m i n a t e w e l l v e g e t a t e d s u r f a c e s .
LowHowever,
a s p l a n t d e n s i t y i n c r e a s e s t h e s c a l e o f c r o s s - b e d d i n g d e c r e a s e s and t h e s l o p e increases. ACKNOWLEDGEMENTS T h i s s t u d y formed p a r t o f a d o c t o r a l r e s e a r c h programme f u n d e d by the Department of Geography, U n i v e r s i t y o f S y d n e y , and t h e S o i l C o n s e r v a t i o n S e r v i c e of N e w S o u t h Wales. Frank Burrows k i n d l y l o a n e d t h e a u t h o r h i s anemometers, R o b e r t Hyde p r o v i d e d g u i d d n c e i n i n i t i a l d a t a c o l l e c t i o n a t t e m p t s , B j o r n Kjerfve a s s i s t e d i n d a t a a n d l y s i s , and Alex C o l v i n , L i n d a Huzzey and C a r o l Luddington s u r v e y e d d u n e s u n d e r a l l c o n d i t i o n s . A d d i t i o n a l d a t a were c o l l e c t e d w i t h ttre s u p p o r t o f ttre W e s t e r n A u s t r a l i a n Department of A g r i c u l t u r e . G l e n n i s Fdrrow kindly typed t h e manuscript. R t t ERENCES
B e a d l e , N . C . W . , O.D. E v a n s , R . C . C d r o l i n , and M . D . T i n d a l e , 1 9 7 2 . F l o r a o f t h e Sydney R e g i o n . A . H . a n d A.W. Heed, 724 pp. B i g a r e l l a , 3.3., 1972. Eolian environments: Their c h a r a c t e r i s t i c s , recognition, and i m p o r t a n c e . I n : R i g b y , 3 . K . and W . K . Hamblin ( e d ' s ) , R e c o g n i t i o n of A n c i e n t S e d i m e n t a r y E n v i r o n m e n t s . S.t .P.M. Spec.Pub. No. 16: 12-62. B i g a r e l l a , 3 . 3 . , R . N . B e c k e r , and G . M . D u a r t e , 1969. C o a s t a l dune s t r u c t u r e s from P a r a n a ( B r a z i l ) . Mar. G e o l . , 7: 5 - 5 5 . B r a d l e y , E.F., 1968. A m i c r o m c t e o r o l o g i c a l s t u d y o f v e l o c i t y p r o f i l e s and s u r f a c e d r a g i n t h e r e g i o n m o d i f i e d by a change i n s u r f a c e r o u g h n e s s . Q . 3 . Roy. Met. S O C . , 9 4 : 361-379. B r a d l e y , E . F . , 1969. A s m a l l , s e n s i t i v e anemometer s y s t e m f o r a g r i c u l t u r a l m e t e o r o l o g y . Agr. Net., 6 : 185-193. F i n n i g a n , 3 . 3 . , 1979. T u r b u l e n c e i n waving wheat IT. S t r u c t u r e of momentum t r a n s f e r . B o u n d a r y - l a y e r Met., 1 6 : 213-236. G o l d s m i t h , V . , 1 9 7 3 . I n t e r n a l geometry and o r i g i n of V e g e t a t e d c o a s t a l sand d u n e s . 3. S e d . P e t . , 4 3 ( 4 ) : 1128-1142. G o l d s m i t h , V . , 1978. C o a s t a l Dunes. I n : R . A . D a v i s J r . ( e d . ) , C o a s t a l S e d i mentary E n v i r o n m e n t s . S p r i n g e r - V e r l a g , C h p t . 4 : 171-230. G o d f r e y , P . 3 . and M.M. G o d f r e y , 1976. B a r r i e r l s l a n d Ecology o f Cape Lookout N a t i o n a l S e a s h o r e and V i c i n i t y , N o r t h C a r o l i n a . N a t . P a r k S e r . S c i . Monograph S e r . No. 9 , 1 6 0 pp.
341 Greeley, R . , R . Leach, B. White, 3. Iversen and J . P o l l a c k , 1980. Threshold windspeeds f o r sand on Mars: Wind t u n n e l s i m u l a t i o n s . Geophys, Res. L e t t e r s , L 7 ( 2 ) : 121-124. hesp, P . A . , 1981. The f o r m d t i o n o f shadow dunes. J. Sed. P e t . , 5 1 ( 1 ) : 101-111. Hesp, P . A . , 1982. Dynamics and Morphology of Foredunes i n South E a s t A u s t r a l i a . Ph.D. D i s s e r t a t i o n , U n i v . Sydney. Howard, A . D . , 3 . 0 . Morton, M . Gad-€1-Hak, and D . B . P i e r c e , 1978. Sand t r a n s p o r t model of barChdn dune e q u i l i b r i u m . Sedimentology, 25: 307-338. Kotoda, K . , 1979. Wind p r o f i l e and aerodynamic p a r a m e t e r s above and w i t h i n a p l a n t canopy. A n n . R e p . I n s t . C e o s c i . , Univ. Tsukuba, No. 5 : 11-14. Kuhlman, H . , 1957. S a n d f l u g t og K l i t d a n n e l s e . Geograf. T i d s s k r i f t , 56: 1-19. Kuhlman, H. 1959. Q u a n t i t a t i v e measurements of a e o l i a n sand t r a n s p o r t . Geograf. T i d s s k r i f t , 57: 51-74. Kutzbach, J . C . , 1961. I n v e s t i g a t i o n s of t h e m o d i f i c a t i o n of wind p r o f i l e s by a r t i f i c i a l l y c o n t r o l l e d s u r f a c e roughness. Ann. Rep. No. Dd-36-039-SC80282: 71-113. D e p t . M e t e o r o l . U n i v . Wisconsin, Madison, Wisconsin. Land, L.S., 1964. E o l i a n c r o s s - b e d d i n g i n t h e beach dune environment, S a p e l o I s l a n d , Georgia. J. Sed. P e t . , 3 4 ( 2 ) : 389-394. Leatherman, S . P . , 1978. A new a e o l i a n sand t r a p d e s i g n . Sedimentology, 25: 303-306. McKee, E . D . , 1979. Sedimentary s t r u c t u r e s i n dunes. I n McKee, E . D . ( e d . ) , A Study of Global Sand S e a s . Geol. Surv. P r o f . Paper 1052: Chpt. E: 83-134. McKee, E . D . and G.W. Weir, 1953. Terminology f o r s t r a t i f i c a t i o n and c r o s s s t r a t i f i c a t i o n i n s e d i m e n t a r y r o c k s . Geol. Soc. A m e r . B u l l . , 64: 381-390. McKenzie, P . , 1958. T h e development of sand beach r i d g e s . Aust. 3 . S c i . , 20: 213-14. Mulhearn, P . 3 . and 3 . 3 . F i n n i a a n , 1978. T u r b u l e n t flow o v e r a very rouqh, random s u r f a c e . Boundary-layer Met,, 15: 109-32. Olson, 3 . S . , 1958a. Lake Michigan dune development. 1. Wind-velocity p r o f i l e s . 3 . G e o l . , 66: 254-63. Olson, 3 . S . , 1958b. Lake Michigan dune development. 2 . P l a n t s a s a g e n t s and t o o l s i n geomorphology. 3. G e o l . , 66: 345-51. Q u i n n , C . M . , 1977. Sand Dunes. Formation, E r o s i o n and Management. An Foras F o r b a r t h a , D u b l i n , 92 pp. Ranwell, D.S., 1972. Ecology of S a l t Marshes and Sand Dunes. Chapman and H a l l , London, 258 pp. Raupach, M . R . , A S . Thorn, and I . Edwards, 1980. A wind-tunnel s t u d y of t u r b u l e n t flow c l o s e t o r e g u l a r l y a r r a y e d rough s u r f a c e s . Boundary-Layer Met., 18: 373-97. Reitsma, T . 1978. W i n d - p r o f i l e Measurements i n d Maize Crop. A g r i c . Res. Rept. 882, ISBN 90 220 0684 0 ; A g r i c . Pub. Doc. C e n t r e , Waqeningen, 103 p p . Sadeti, W . Z . , 3 . E . Cermak, and T . Kawatani, 1971. Flow over high roughness e l e m e n t s . Boundary-Layer Met., 1: 321-44. S a l i s b u r y , E . , 1952. Downs and Dunes. T h e i r P l a n t L i f e and Environment. 6 . B e l l and Sons L t d , London. S e l l e r s , W . D . , 1965. P h y s i c a l C l i m a t o l o g y . Univ. of Chicago P r e s s . S h o r t , A . D . and P.A. Hesp, 1982. Wave, beach and dune i n t e r a c t i o n s i n s o u t h e a s t e r n A u s t r a l i a . Mar. G e o l . , 48: 259-284. Taylor, P.A. and P.R. Gent, 1974. A model of a t m o s p h e r i c boundary-layer flow above an i s o l a t e d two-dimensional ' h i l l ' ; an example of flow above ' g e n t l e t o p o g r a p h y ' . Boundary-Layer Met. 7 : 349-362. Thom, A.S., 1971. Momentum a b s o r p t i o n by v e g e t a t i o n . Q . 3 . Roy. Met. SoC., 97: 414-428. Thom, A.S., 1972. Momentum, mass and h e a t exchange of v e g e t a t i o n . Q . 3 . Roy. Met. S a c . , 98: 124-134. Thom, B.G., G.M. Bowman, and P.S. Roy, 1981. L a t e Q u a t e r n a r y e v o l u t i o n of c o a s t a l sand b a r r i e r s , P o r t Stephens-Myall Lakes a r e a , c e n t r a l NSW, A u s t r a l i a . Q u a t . Res. 15: 345-364.
34 2 Townsend, A.A., 1966. T h e f l o w i n a t u r b u l e n t boundary l a y e r a f t e r a change i n s u r f a c e roughness. 3 . F l u i d Mech., 26: 255-266. Walker, H.3. and Y . Matsukura, 1979. Barchans and h a r c h a n - l i k e dunes a s develope i n two c o n t r a s t i n g a r e a s w i t h r e s t r i c t e d s o u r c e r e g i o n s . Ann. Rep. I n s t . G e o s c i . , Univ. Tsukuba, No. 5 : 43-46. W i l l i a m s , G . , 1964. Some a s p e c t s of t h e e o l i a n s a l t a t i o n l o a d . Sedimentology, 3 : 257-207. Woodhouse, W . W . 3 r . , 1978. Dune B u i l d i n g and S t a b i l i s a t i o n w i t h V e g e t a t i o n . U.S. Army, Corps o f E n g r s . , Spec. Rept. No. 3 , 112 pp. Yadlon, D . H . , 1975. D i s c u s s i o n o f " I n t e r n a l geometry of v e g e t a t e d c o a s t a l sand dunes". 3 . Sed. P e t . , 45: 359.
343
DESERT DUNES: A SHORT R E V I E W OF NEEDS I N DESERT DUNE RESEARCH AND A RECENT STUDY
OF MICROMETEOROLOGICAL DUNE-INITIATION MECHANISMS A. WARREN and P. KNOTT: Department o f Geography, U n i v e r s i t y C o l l e g e London, Gower S t . ,
London W C l E 6BT, Enqland.
I n t h e l a s t decade t h e r e have been v e r y c o n s i d e r a b l e a d d i t i o n s i n o u r underIt i s t r u e t h a t a t the 'steady' scale -
standing o f t h e morohology o f d e s e r t dunes.
the s c a l e o f t h e movement o f sand - t h e r e have been few fundamental advances. Although we do n o t have b o t h more r i g o r o u s and more p r a c t i c a l formulae f o r r e l a t i n g sand movement t h e wind v e l o c i t y ( L e t t a u & L e t t a u , i n press and Hsu, 1973), and we have a l s o had o u r p e r c e p t i o n o f t e r r e s t r i a l processes sharpened by debates about wocesses on o t h e r p l a n e t s ( e . g . Sagan & Bagnold, 1975, Greeley, 1979), we have seen a s l o w e r t h a n usual advance i n fundamental s t u d i e s o f moving sand.
A t the 'graded' scale
-
t h a t o f a dune
-
we a r e now b e g i n n i n g t o have p r e s e n t -
able models o f t h e processes o f dune movement (Howard e t a1
.,
1977) and we a l s o
have c a r e f u l s t u d i e s o f dune s u r f a c e processes by Tsoar (1978) and K n o t t (1979). A t t h e ' c y c l i c ' s c a l e - t h a t o f whole sand seas, and f o l l o w i n g Schumm and
L i c h t y ' s (1964) argument, t h a t o f m i l l e n n i a
-
we now have enormously improved
knowledge of g l o b a l and even p l a n e t a r y e r g s (Mainguet & C a l l o t , 1978, McKee, 1979, Breed e t a l . ,
1979, Tsoar e t a l . ,
1979).
For d e s e r t dunes, as f o r a l l geomorphology, advances come l a r g e l y f r o m r e d u c t i o n s i n t h e s c a l e of e n q u i r y : most o f t h e v a r i a t i o n i n l a r g e s c a l e ' c y c l i c ' p a t t e r n s i s understandable, u l t i m a t e l y , o n l y i f graded s c a l e processes a r e e x p l a i n e d and most of these, i n t u r n , a r e u n d e r s t a n d a b l e o n l y i f t h e r e a r e reasonable models a t t h e 'steady' scale.
O f course, each s c a l e a l s o has elements t h a t can o n l y be under-
stood a t t h a t s c a l e , and, moreover, we must acceDt ' b l a c k boxes' o f D o o r l y understood concepts t o r e a c h any k i n d o f e x p l a n a t i o n o f g r o s s p a t t e r n s .
Desoite t h i s
unavoidable r e d u c t i o n , we want t o argue t h a t t h e s c a l e o f s t u d y t h a t w i l l y i e l d and i s now y i e l d i n g t h e g r e a t e s t advance i s t h e ' g r a d e d ' s c a l e o f t h e s i n g l e dune: the o r o p e r s t u d y o f dunes i s t h e dune. There a r e f o u r reasons.
F i r s t , we b e l i e v e t h a t we have models a t t h e steady
scale t h a t a r e s u f f i c i e n t f o r s t u d y a t t h e graded s c a l e : Howard e t a l . (1977) Tsoar (1978) and K n o t t (unpubl
.
1979) a l l made use o f a range o f stead;(
scale
models b u t found t h a t t h e i r d i f f e r e n c e s were n o t c r i t i c a l .
Second, i t i s a t t h i s scale t h a t s t u d i e s o f subsqueous dunes a r e y i e l d i n g t h e g r e a t e s t advances (e.g.
A l l e n , 1976).
T h i r d , we have a b a t t e r y o f t e c h n i q u e s and now models f o r t h i s s c a l e
344 o f s t u d y ( K n o t t & Idarren, 1980, Howard e t a l ,
1976).
F o u r t h , a number o f c r i t i c a l
o u e s t i o n s a r e s t i l l o u t s t a n d i n g a t t h e c y c l i c s c a l e t h a t can o n l v be r e s o l v e d a t t h e graded s c a l e .
Ide w i l l t a k e o n l y one examnle o f t h e s e , b e f o r e d e s c r i b i n n the
F i q . 1 . S i n a i : b a r c h a n s and s l o u k s w i t h d i f f e r i n q t r e n d s i n t h e l e e o f a n escarn rnent. N o r t h i s t o t h e t o p o f t h e n i c t u r e . The o r i n c i n l e w i n d i s f r o m s l i o h t l y w e s t o f n o r t h . S c a l e 1:50,000 a p p r o x .
345 outcome of some new work. The r e l a t i o n of the trend of l i n e a r dunes t o the ambient wind regiqe i s s t i l l unresolved. I t i s t r u e t h a t Fryberger and Dean's (1979) broad survey of l i n e a r dunes found t h a t t h e i r alignment was o f t e n closely associated with the r e s u l t a n t of broad unimodal p a t t e r n s , b u t Muriel Brookfield (1970) found p e r s i s t e n t deviations of the two in Central A u s t r a l i a , ldarren (1972) and Plainguet & C a l l a t (1978) a l s o noted c o n s i s t e n t discrepancies in the Fachi-Blima Erg, and Warren (1976a) seoorted a notable mismatch i n the Pleistocene Nebraska Sand H i l l s . Yet, another examole, s t r i k i n g l y s i m i l a r t o P l a t e VI i n Mainguet and C a l l o t ' s study comes fr,om Sinai (Fig.1) where the barchans and slouk show d i s t i n c t trends. Are these discrepancies explicable with Wilson's (1972) hypothesis of 'oblique d u n e s ' , with Wipperman's (1969) ideas about i n s t a b i l i t y of vortices i n the Ekman boundary layer, or with Warren's (1976b) theory about the roughness f a c t o r in the Ekman s p i r a l ? ( i t s e l f a t variance with the ' C o r e o l i s ' hypothesis of Maingy~et & C a l l o t , 1978.) A resolution of these arguments i s c r i t i c a l t o i n t e r p r e t a t i o n s of ancient dune patterns ( e . 9 . Warren, 1970, Fryberger, 1980). B u t the resolution can only come from s t u d i e s a t the graded s c a l e of the flow around individual dunes.' An example: Dune I n i t i a t i n g Processes
A fundamental process in the formation of dunes i s t h e i r i n i t i a t i o n . We have moved away, i n general, from the idea of ,chance obstacles, l i k e camels o r nebkha, t o the idea t h a t dunes can be i n i t t a t e d , somehow, by converging o r diverging flow (Wilson, 1972). B u t how does t h i s begin i n t h e atmosphere? One of us (Knott) studied t h i s process in the v i c i n i t y of I n Salah in Algeria ( K n o t t , 1979) where t h e r e i s a strong unimodal wind regime ( F i g . 2 ) associated with barchans in the small Erg Sidi Moussa. I
Knott used t h r e e principal methods of observation. I n the Eulerian mode he tracked f i r s t p i l o t balloons q t 15 second i n t e r v a l s with two t h e o d o l i t e s 500 m a p a r t , and second tetroons released a t a predetermined constant density surface; he a l s o used tetroon t r i a d s t o study convergence a n d divergence. I n the Lagrantian mode he used cup and strain-guage anemoters on booms on a 10 metre mast a t 1 , 2 , 4 a n d 8 metres above the surface. His f i r s t discovery of relevance-here was t h a t on 15 out of h i s 16 days of observation a temoerature inversion developed overnight from about 2200 hrs t o 0800 hrs a f t e r which t h e r e was a raDid change t o a d i a b a t i c and then suoeradiabatic conditions. The inversion tended t o deeoen continuously from about 10 t o about 200 m u n t i l 0700 hrs and i n t e n s i f i e d from 0530 o r 0630 h r s . A t i t s most intense i t was 3.O0C/1O0 m . ( J u l y 1 7 , 1976) ( F i g . 3 ) . His second relevant discovery, not s u r p r i s i n g i n view of o t h e r s t u d i e s of inversions (Offen & Kline, 1975), was t h a t a j e t developed near the inversion where the wind was deroupled from surface f r i c t i o n . I t was d i s t i n c t between 0000 and 0630 h r s . For example, on July ZZnd, 1976 a t 0630 hrs t h e r e was a wind
34 6 o f 18.5 m sec-'
a t 200 m w h i l e a t 10 m t h e wind was o n l y 4.9 m s e c - l .
Wind shear
was commonly 7 m s e c - l / 1 0 0 m . ( F i g . 4 ) . The t h i r d r e l e v a n t d i s c o v e r y ( a l s o n o t s u r p r i s i n g i n view o f t h e e a r l i e r work) was about t h e d i s p e r s i o n o f t h e j e t i n t h e e a r l y morning: i n t e n s e ' b u r s t s ' o f energy were f i r s t r e l e a s e d t o t h e s u r f a c e i n ' w o u n d j e t s ' . from t h e f l i g h t o f t e t r o o n s . t e t r o o n s were grounded.
These were discovered
Some o f t h e downward j e t s were so i n t e n s e t h a t
Others m e r e l y p r o j e c t e d t h e t e t r o o n downward and then
allowed i t t o r i s e again (Fig.5).
Waves i n t h e j e t were f a i r l y s t a b l e b e f o r e s u n r i s e when t h e h e i g h t v a r i a t i o n s o f t e t r o o n s were o f l o w a m o l i t u d e and s h o r t
F i g . 2 . I n Salah and t h e Erg S i d i Moussa. L o c a t i o n o f e x p e r i m e n t a l s i t e s . The l o w e r d i a g r a T shows t h e y e a r l y p a t t e r n o f sand movement and i t s r e s u l t a n t above threshold. t h e 6 m sec
34 7 wavelength, b u t t h e y p r o g r e s s e d t o h i g h a m p l i t u d e and l o n g wavelengths b y about
1200 h r s .
In t h e e a r l y morning v e r t i c a l v e l o c i t i e s seldom exceeded
but by 1000 h r s t h e y were o c c a s i o n a l l y
2 2
-f
2 m sec-l,
m sec-l.
The e x i s t e n c e o f some process such as ground j e t s was c o n f i r m e d by two f u r t h e r observations.
F i r s t , t h e o b s e r v a t i o n s o f t h e t e t r o o n t r i a d s showed t h e r e t o be
divergence a t l o w l e v e l s ( F i g . 6 ) .
Second t h e Lagrangian s t r a i n quage measurements
showed v e r y e r r a t i c t u r b u l e n c e i n t h e morning w i t h d i s t i n c t ' b u r s t s ' o f h i g h e r i n t e n s i t y ( h i g h u"
and h i g h
I s o l a t i n g these by eye r e v e a l e d t h a t t h e
U").'
f r a c t i o n o f t h e t i m e o c c u n i e t by b u r s t s was 0 . 3 and t h a t t h e y D e r s i s t e d f o r 3-4 minutes ( F i g . 7 ) .
In summary, t h e m e t e o r o l o g i c a l measurements showed: ( 1 ) Rapid c o o l i n g of t h e s u r f a c e a f t e r sunset: an i n v e r s i o n aDpeared i n w h i c h buoyant and mechanical m i x i n g a r e suopressed below t h e i n v e r s i o n .
a ) Temperature Profiles
3001
, { ,
200
2
100 50 10
pr
0430 ".. , $400.0400
32
,
36
34
b ) Velocity Profiles
,
,
,
,
3a 40 ("C1
42
Temperature
300-
P?
4 6 8 10 12 14 16 18 20
i t
0500i
/
Wind Speed (m/sec)
PP
200-
150~ ~ 2 5 3 0
100-
5010-
cf
,. Q
200
300
P
/b600
150 100 50 10 4 6 8 10 12 14 16 18 20
150 loo-
0630
22nd July, 1976 I
Fig.3. Temperature and v e l o c i t y p r o f i l e s , 22nd J u l y 1976. Records from p i l o t b a l l o o n a s c e n t s .
Times a r e shown.
348 ( 2 ) A j e t formed above the inversion ( a t c 200 m ) and i n t e n s i f i e d towards
sunrise (Fig.8 ( 3 ) The j e t then became unstable f i r s t in i n t e n s i f y i n g 2-0 waves and then in 3-D v o r t i c e s and f i n a l l y
( 4 ) Ground j e t s appeared a t about 1100 hrs disnersing the inversion by midda:,
Time-height Section of the Wind Field (m.sec:’)
10 Time
Fig.4. The wind f i e l d 22nd July 1976.
Data from tetroon t r i a d s .
We can hypothesise t h a t the t r a n s f e r of horizontal momentum from high t o low a l t i t u d e s could be the mechanism whereby the s t a t i c threshold of sand movement ( V , t ) i s f i r s t surpassed in the e a r l y morning. This would be a l o c a l i s e d process moving s a n d downwind t o accumulate as t h e beginnings of a new dune. Once one transverse dune h a d grown by trapping sand in the c l a s s i c Bagnoldian manner, K n o t t (1979) hypothesised from o t h e r findings (Fig. 9 ) t h a t another dune could not grow in the turbulent zone downwind: i t could only grow a t a c e r t a i n distance. A regularly spaced groun of transverse dunes would form i n t h i s way.
Answers t o some of the f a s c i n a t i n g questions raised by regional dune surveys, a n d t o some of the problems of i n t e r p r e t i n g ancient ergs can only come from s t u d i e s such as these a t the dune s c a l e . Symbols
.___
u
mean wind velocity
u ’ deviation of actual from mean wind v e l o c i t y .
V,t
threshold shear v e l o c i t y .
34 9
350
Vertical Profile of Divergence
0000 0900
1300
1900
3007
-E 200I
E 150-
.-0,
I”
100-
50-
Fig.6.
Divergence p a t t e r n s 1 1 t h August 1976.
Measurements w i t h t e t r o o n t r i a d s .
Velocity Traces at 10m height 22nd July, 1976 -
0930
31 TIME ( G M T )
TIME ( G M T )
F i g . 7 . V e l o c i t y t r a c e s a t 10 m. by eye.
S t r a i n - g u a g e measurements.
Bursts i d e n t i f i e d
351
Time Variation of Turbulence Intensity-22nd July,1976
0.3
-
0.2-
0.1-
0
06 08 10 12 14 16 18 20 Time(GMT) Fig.8.
1
Tiiiie v a r i a t i o n o f t u r b u l e n c e i n t e n s i t v .
F i o . 9 . Hynothesis o f f l o w round a b a r c h a n , u s i n n measurecients a t n o i n t shown.
352 REFERENCES A l l e n , J . R . L . , 1976. Computational models f o r dune t i m e - l a g : g e n e r a l i d e a s , d i f f i c u l t i e s and e a r l y r e s u l t s . Sedimentary G e o l . , 15: 1-53. Breed, C . S . , G r o l i e r , M . J . and McCauley, J . F . , 1979. Morohology and d i s t r i b u t i o n of common ' s a n d ' dunes on Mars: comparison w i t h E a r t h . J . Geoohys. Res., 8 4 : 81 83-9204. B r o o k f i e l d , M u r i e l , 1970. Dune t r e n d s and wind reqime i n c e n t r a l A u s t r a l i a . S e i t z . f b r Geomorph. S u p p l . 10: 121-53. F r y b e r g e r , S . G . , 1980. Dune forms and wind regime, M a u r i t a n i a , '*lestA f r i c a : i m p l i c a t i o n s f o r p a s t c l i m a t e . I n : 11. S a r n t h e i n , S c i b o l d , E . and Rognon, P . ( E d i t o r s ) , Sahara and Surrounding S e a s , Balkema, Rotterdam, 79-96. F r y b e r g e r , S . G . and Dean, G . , 1979. Dune forms and wind regime. I n : E . D . McKee ( E d i t o r ) , A Study of Global Sand S e a s , U.S. Geol. Survey P r o f . Paner 1052, 137-1 70. G r e e l e y , R . , 1979. S i l t - c l a y a g g r e g a t e s on Mars: a model f o r one f o r m a t i o n of ' s a n d ' . J . Geophys. R e s . , 8 4 : 6248-6254. Howard, A . D . , Morton, J . B . and Gad-el-Hak, PI., 1977. S i m u l a t i o n model of e r o s i o n and d e p o s i t i o n on a barchan dune. R e p t . NASA, CR-2838 C o n t r a c t NGR-47-005172, 82 p p . Howard, A . D . , Morton, J . B . , Gad-el-Hak, PI. & P i e r c e , D . B . , 1978. Sand t r a n s p o r t model o f barchan dune e q u i l i b r i u m . Sedimentology, 25: 307-338. Hsu, S.A., 1973. Computing e o l i a n sand t r a n s p o r t from s h e a r v e l o c i t y measurements. J . G e o l . , 81: 739-743. Knott, P . , 1979. The s t r u c t u r e and p a t t e r n of dune-forming winds. Unpubl. P h . D . T h e s i s , Univ. o f London, 2 v o l s . , 403 and 249 p p . Knott, P . and Narren, A . , 1980. Aeolian P r o c e s s e s . I n : A. Goudie, e t a l . ( E d i t o r ) , Geomorphological Techniques, George A l l e n & U n w i n , London, 226-246. L e t t a u , Katharina and Hans, unpub. Experimental and m i c r o m e t e r o l o g i c a l f i e l d s t u d i e s o f dune m i g r a t i o n . Chap.IX i n Univ. o f !Jisconsin Report on t h e C l i m a t e o f Coastal P e r u . Mai?guet, M . and C a l l o t , Y . , 1978. L'Erg de Fachi-Bilma ( T c h a d - N i g e r ) . C o n t r i b u t i o n a l a c o n n a i s s a n c e de l a dynamique des e r g s e t d e s dunes des zones a r i d e s chaudes. Me'm. e t Documents, 18: S e r v . de documentation e t de c a r t o g . geog. Ed. CNRS, P a r i s , 184 PD. McKee, E . D . ( E d i t o r ) , 1979. A s t u d y of g l o b a l sand s e a s . U . S . Geol. Survey Prof. Paper 1052: 429 p p . Dffen, G . R . and K l i n e , S . J . , 1975. A proposed model of t h e b u r s t i n g p r o c e s s i n t u r b u l e n t boundary 1a y e r s . J . F1 u i d Mech . , 70: 209-228. Sagan, C . and Bagnold, R . A . , 1975. F l u i d t r a n s o o r t on e a r t h and a e o l i a n t r a n s p o r t on Mars. I c a r u s , 26: 209-218. Schumm, S.A. and L i c h t y , R . W . , 1964. Time, s p a c e and c a u s a l i t y i n geomorphology. Amer. J . S c i . 263, 2: 110-119. T s o a r , H., 1978. The dynamics o f l o n g i t u d i n a l dunes. Dept. of Geog. Ben Gurion U n i v . B e ' e r Sheva, I s r a e l , U.S. Army Grant no. DA-ERD-76-9-072, 171 p p . T s o a r , H . , G r e e l e y , R . and P e t e r f r e u n d , A . R . , 1979. Mars: the North P o l a r Sand Sea and r e l a t e d wind p a t t e r n s . J . Geophys. Res., 84: 8167-8180. Warren, A . , 1970. Dune t r e n d s and t h e i r i m p l i c a t i o n s i n the c e n t r a l Sudan. Z e i t s c h r . f b r Geomorphologic Suppl. 1 0 , 154-179. I l a r r e n , A . , 1976a. Morphology and Sediments o f the Nebraska Sand H i l l s i n r e l a t i o n t o P l e i s t o c e n e Winds and the development o f a e o l i a n bedforms. J . G e o l . , 84: 685-700. Warren, A . , 1976b. Dune t r e n d and the Ekman s p i r a l . Nature ( L o n d . ) , 259: 653-654. Idilson, I . G . , 1972. Aeolian bedforms - t h e i r development and o r i g i n . Sedimentology, 19: 173-210. Wipperman, F., 1969 The o r i e n t a t i o n o f v o r t i c e s due t o i n s t a b i l i t y o f t h e Ekmanboundary 1a y e r . B e i t r a g e z u r Physik d e r Atmosph2re, 42: 225-244.
353
SAND SEAS OF THE SAHARA AND SAHEL: AN EXPLANATION OF THEIR THICKNESS AND SAND DUNE TYPE BY THE SAND BUDGET PRINCIPLE M. MAINGUET and f1.-C.
CHEPIIN, L a b o r a t o i r e de Geoqraphie Physique Zonale,
U n i v e r s i t e de Reims, Reims, France TNTRWJCTION U n t i l now, t h e d e f i n i t i o n o f sand seas has been d e s c r i p t i v e and has taken account o n l y o f t h e sand d e p o s i t as t h e s t a t i c component (Capot-Key, 1970, PlcKee, 1979) w h i l e n e g l e c t i n g t h e dynamic component r e p r e s e n t e d by t h e sandc a r r y i n q winds which i r a v e r s e t h e sand seas. The two dynamic a e o l i a n parameters, t r a n s p o r t a t i o n and d e p o s i t i o n , combine t o determine t h e sand sea sand budqet. The i n t e r a c t i o n between sand i m p o r t and sand e x p o r t w i t h i n t h e sand sea determines t h e t h i c k n e s s o f t h e sand c o v e r and t h e t y p e o f dunes formed. A f t e r d e f i n i n g t h e i d e a o f sand budqet, and dunes formed by e r o s i o n and depo s i t i o n , t h e s e d e f i n i t i o n s w i l l be used i n a model which i s a p p l i e d t o t h e Sahara system. The r e s u l t s w i l l e x p l a i n and demonstrate t h e reasons f o r t h e r a r i ' o f sand i n t h e h y p e r a r i d Sahara and t h e v a r i a t i o n s i n t h e t h i c k n e s s o f t h e sand covers. THE SAIJD SEA SAND BUDGET Mainguet and C a l l o t (1978) have demonstrated by means o f t h e p r i n c i p l e o f loac s u b s t i t u t i o n f o r l a r g e systems ( B r o o k f i e l d , 1970, Hainguet e t a l . ,
1980, Wilson,
1973), u s i n g t h e "Grand Erg de Fachi B i l m a " as an example, t h a t t h e same winds a r e a b l e t o d e p o s i t and remove t h e sand. When t h e i n p u t o f sand i s g r e a t e r t h a n t h e o u t p u t , t h e sand budget i s p o s i t i v i and t h e sand sea t h i c k n e s ; i t s s u r f a c e i s moulded i n t o dunes o f d e p o s i t i o n c o a l e s c i n g barchans, barchan c h a i n s o r t r a n s v e r s e c h a i n s . T h i s behaviour can be seen i n t h e s a h e l i a n sand sea o f N i g e r where t h e sand c o v e r , moulded i n t o l a r g e and small sand h i l l s , reaches a t h i c k n e s s o f 60 metres. The sand budget i s p o s i t i v e , even though t h e r e a r e l o c a l l y conspicuous d e f l a t i o n s t r e a k s i n d i c a t i n g t h e b e g i n n i n g o f a n e t l o s s o f sand. When t h e o u t p u t o f sand i s g r e a t e r t h a n t h e i n p u t , t h e sand budget i s n e g a t i v e and t h e sand sea t h i n s ; d e f l a t i o n c o r r i d o r s w i t h l e v e l l e d s u r f a c e s appear, which a r e dunes o f e r o s i o n , o r sand seas o f t h e sand r i d g e t y p e . The b e s t examples o f t h i s t y p e o f sand sea, a l m o s t f r e e o f sand, a r e t h e " E r g s I g u i d i and Chech".
3 54 There, t h e c o r r i d o r s a r e t e n t i m e s as wide as t h e dunes w h i c h s e p a r a t e them. A c o n t i n u a l n e g a t i v e sand budget can l e a d t o t h e disappearance o f sand r i d g e s and t h i s o c c u r s when t h e d e f l a t i o n c o r r i d o r s merge. Under t h e s e c o n d i t i o n s , t h e sand sea i s reduced t o a l a y e r o f c o a r s e sand, which i s a winnowed residue,as t h e o l d d e f l a t i o n c o r r i d o r s t e n d t o c u t down i n t o bedrock. A good example o f t h i s i s t h e n o r t h e a s t e r n p a r t o f t h e Tenere. APPLICATION OF THESE I D E A S TO THE SAHARAN SYSTEM U n t i l now, i t was t h o u g h t t h a t t h i c k a e o l i a n sand d e p o s i t s were an i n d i c a t i o n
of a r i d i t y (Capot-Rey,
1970, Monod, 1958, T r i c a r t and C a i l l e u x , 1964). The obser-
v a t i o n o f s c a r c i t y o f sand i n t h e Sahara i s c o v e r e d w i t h sand sheets
-
-
where o n l y 20 p e r c e n t o f t h e surface
i s i n c o n t r a s t t o t h e c o n t i n u o u s GO m e t r e t h i c k
sand c o v e r o f t h e s o u t h e r n s a h e l i a n margins. These f e a t u r e s l e d us t o c a t e g o r i c a l l y r e j e c t t h e commonly accepted concept t h a t t h e sand c o v e r t h i c k n e s s i n d i c a t e s , i n a r i d r e g i o n s , a r i d i t y i n p a s t t i m e s (Hainguet, 1982). An a t t e m p t w i l l be made t o demonstrate t h a t t h e h e t e r o q e n e i t y o f sand accumu l a t i o n i n t h e Sahara and i t s m a r g i n s must be c o n s i d e r e d i n i t s e n t i r e t y f o r t h e best palaeoclimatic i n t e r p r e t a t i o n o f aeolian deposits. The two Saharan margins, t h e n o r t h e r n subhumid and t h e s o u t h e r n s a h e l i a n , do n o t e x h i b i t t h e same b e h a v i o u r i n r e g a r d t o a e o l i a n phenomena. These behavioural d i f f e r e n c e s r e s u l t , i n s p i t e o f a NE-SW u n i t y i n wind c i r c u l a t i o n , f r o m a palaeod i f f e r e n c e : i n t h e n o r t h Sahara Q u a t e r n a r y ; i n t h e s o u t h Sahara
-
more permanent humid c o n d i t i o n s d u r i n g t h e more a r i d c o n d i t i o n s . I n s u p p o r t o f t h i s ,
Dresch (1982) demonstrated t h a t , d u r i n g t h e Q u a t e r n a r y , t h e n o r t h e r n m a r g i n o f t h e Sahara d i d n o t have an a r i d o r h y p e r a r i d c l i m a t e , and t h a t t h e Maghreb and t h e A t l a s Mountains s t a y e d s e m i a r i d w i t h a f u n c t i o n i n g h y d r o g r a p h i c network whose widespread ephemeral streams p r o v i d e d sand and s i l t t o be m o b i l i z e d by t h e wind. The l a r g e Western and E a s t e r n sand seas, which w i l l be c a l l e d "Mediterranean Sand Seas", s t i l l b e n e f i t f r o m t h i s supply, as i s t e s t i f i e d by t h e i r dune types ( f i g . l ) , The bouquets o f l i n e a r dunes and t h e barchan c h a i n s which f o r m t h e n o r t h e r n t w o t h i r d s o f t h e "Great Western Erg", a s w e l l as t h e barchan c h a i n s and s t a r dunes o f t h e n o r t h e r n two t h i r d s o f t h e "Great E a s t e r n t r g " , i n d i c a t e sand i n p u t i s g r e a t e r t h a n sand o u t p u t . I n t h e s o u t h e r n Sahara ( f i g . 2), t h e o p p o s i t e s i t u a t i o n p r e v a i l s . The area s u p p l y i n g sand f r o m upwind i s t h e Sahara i t s e l f , where t h e p e r e n n i a l l y a r i d c l i m a t e reduce t h e e f f e c t i v e n e s s o f t h e source. A l a r q e p a r t o f t h e sand e x p o r t ed f r o m t h e Sahara i s t r a p p e d i n t h e Sahel r e g i o n , w h i l e some i s blown i n t o t h e A t l a n t i c Ocean. Under t h e p r e s e n t c l i m a t i c d e t e r i o r a t i o n , t h e e x p o r t f r o m t h e Sahel r e g i o n i s i n c r e a s i n g because o f a d e c r e a s i n g e f f e c t i v e n e s s o f t h e f r a g i l e
355 v e g e t a t i o n cover.
+ .South A t l a s Mediterranean ergs
3-- ;I.
High group n o r t h o f t h e c e n t r a l Sahara
3. H i g h group : T a s s i l i , Haggar, Tademait, l i m i t t h e passage o f sand between t h e e x p o r t i n g m e d i t . e r g s and t h e h y p e r a r i d Sahara.
--
H y p e r a r i d Sahara
2. Sand accumulations o f t h e mediterranean ergs 2a. N o r t h e r n area : p o s i t i v e sand budget 2b. Southern area : n e g a t i v e sand budget
4. Very d e f i c i e n t h y p e r a r i d s e c t o r .
FIG. 1. SOURCE AND SAND DEPOSITS : NORTH SAHARA MODEL
H y p e r a r i d Sahara
. A r i d Sahara
121
1-2. D e f i c i e n t areas p r o v i d i n g sand
++
.Sahel i a n e r g s
,3. Area o f e f f e c t i v e a c c u m u l a t i o n i n t h e vegetated saharo-sahelian and s a h e l i a n a r e a s
.Sahel i a n - s o u d a n i a n zone
41
4. E x p o r t area below t h e Sahel
.FIG. 2. SOURCE AND SAND DEPOSITS : SOUTH SAHARA MODEL
356 VERIFICATION OF THE SAHARAN MODEL OF SAND DISTRIBUTION A.Sand d i s t r i b u t i o n i n t h e Sahara and on i t s s a h e l i a n m a r g i n s From Meteosat s a t e l l i t e images, i t i s g e n e r a l l y p o s s i b l e t o a n a l y s e t h e d i s t r i b u t i o n of sand a c c o r d i n g t o ground r e f l e c t a n c e ( f i g . 3 ) . One must, however, be c a r e f u l s i n c e t h e d i f f e r e n c e s i n r e f l e c t a n c e can i n d i c a t e : a ) d i f f e r e n c e s i n t h i c k n e s s o f t h e sand c o v e r ; b ) v a r i o u s degrees o f i n t e n s i t y o f a e o l i a n a c t i v i t y ; c ) d i f f e r e n c e s i n t h e d e n s i t y and n a t u r e o f t h e v e g e t a t i o n . F o r example, t h e n o r t h e a s t e r n p a r t o f t h e Tenere, where t h e sand c o v e r i s t h i n o r l o c a l l y none x i s t a n t , i s more r e f l e c t i v e t h a n t h e v e r y t h i c k Haoussa sand sea. The reason i s t h a t t h e n o r t h e a s t e r n Tenere i s b a r r e n and t r a v e r s e d by v e r y a c t i v e sand-laden winds, w h i l e t h e Haoussa sand sea i s v e g e t a t e d and i n f l u e n c e d by winds w i t h very l i t t l e s a n d - c a r r y i n g c a p a c i t y . Thus, s a t e l l i t e imaqes a l o n e a r e i n s u f f i c i e n t f o r p r o p e r i n t e r p r e t a t i o n and a d d i t i o n a l knowledge must be drawn on. ( i ) The l a c k o f sand i n t h e c e n t r a l Sahara Again, based on t h e s a t e l l i t e images, t h e h y p e r a r i d Sahara shows few sand seas w i t h dunes. O n l y 2 p e r c e n t o f i t s a r e a has t h i c k d e p o s i t s which a r e : t h e c e n t r e o f t h e "Mourzouk Erg", t h e " B r u s s e t and B r e a r d Ergs", and a band t h a t s t r e t c h e s f r o m t h e southwestern f o o t o f T i b e s t i , n o r t h o f T e r m i t , t h r o u g h t h e "Bilma Erg" and t h e s o u t h e r n Tenere. These t h i c k d e p o s i t s a l l have some l o c a l e x p l a n a t i o n : p r o t e c t i o n a g a i n s t sand e x p o r t by topography i n t h e case o f t h e "Mourzouk Erg"; t h e e f f e c t o f t h e o b s t a c l e o f t h e A 9 r f o r t h e " B r u s s e t and B r e a r d Ergs"; and t h e c o n v e r g e n t e f f e c t s downwind o f T i b e s t i f o r t h e B i l m a Tenere band. On t h e o t h e r hand, a l m o s t s a n d - f r e e seas, where t h e sand dunes a r e separated by l a r g e i n t e r d u n e s , occupy
15 p e r c e n t o f t h e Sahara. These i n d i c a t e a v e r y
n e g a t i v e sand budget: examples a r e t h e G r e a t Sand Sea, t h e Calansho Sand Sea, t h e Rebiana Sand Sea, t h e "Erg Chech" and t h e "Erg I g u i d i " ( f i g s . 4,5). ( i i ) The s o u t h e r n p a r t o f t h e Sahara (medium sand c o v e r ) T h i s r e g i o n has a t h i c k e r sand c o v e r t h a n t h e c e n t r a l Sahara. The sand sea topography c o n s i s t s o f r e c e n t sand r i d g e s which a r e l a r g e r t h a n t h e i n t e r d u n e c o r r i d o r s between them, i n d i c a t i n g a l e s s n e g a t i v e sand budget t h a n t h e c e n t r a l Saharan sand seas. T h i s s o u t h e r n p a r t o f t h e Sahara i s i n t e r m e d i a t e i n c h a r a c t e r between t h e c e n t r a l Sahara, d e f i c i e n t i n sand, and t h e f i e l d s o f d e n s e l y vegetated dunes o f t h e Sahel: i t forms a b e l t c o n t a i n i n g p o c k e t s whose l e n g t h reaches f r o m 3O0E.to 26OW. These p o c k e t s a r e formed by t h e m a s s i f s o f Ennedi, A f r , Adrar o f I f o g h a s , A d r a r o f C h i n g u e t t i and Tagant. The sand seas o f t h i s b e l t a r e , from e a s t t o west: t h e Sudan ghoz, e a s t o f Ennedi; t h e Chad sand sea, between t h e Mourdi d e p r e s s i o n and Lake Chad; t h e s o u t h e r n p a r t o f t h e B i l m a Erg and t h e Tenere; t h e sand seas o f T a l a k and Azaouak a t M a l i ; t h e M a k t e i r , t h e I j a f e n e ,
E
L aJ '3
2
357
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.r LL
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m L W
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..
3 58
F i g . 4. G r o w t h i n " v e n t i l a t i o n " o f an e r g w i t h e n l a r q e m e n t o f i n t e r d u n e c o r r i d o r s . T h i s L a n d s a t 1 i n a q e o f 1 6 t h tdoveriber, 1972 ( s c a l e a b o u t 1:1,000,000) c o v e r s t h e n o r t h e r n p a r t o f t h e " E r g Chech". T h i s e r q , f o r m e d e n t i r e l y o f sand r i d q e s has a n e g a t i v e sand b u d q e t . On t h e image, t h e r a t i o o f r i d g e w i d t h s (Cd) t o i n t e r d u n e c o r r i d o r w i d t h s ( C l ) decreases from west t o east. I n s e c t i o n A , t h e e m p t y i n g r a t i o ( E = C l / C d ) i s f r o m 1 t o 0.5; i n s e c t i o n B, f r o m 0 . 4 t o 0.2. T h i s d e c r e a s e i n t h e r a t i o c o n f i r m s t h e i n c r e a s e i n sand e x p o r t i n t h e e r g f r o m w e s t t o e a s t , a s we p a s s away f r o m t h e s h e l t e r i n g ,?nd w i n d shadow a r e a o f t h e m a s s i f t o t h e n o r t h w e s t . t h e Ouarane, t h e Amoukrouz, t h e A z e f a l a n d t h e A k c h a r i n M a u r i t a n i a . ( i i i ) The t h i c k l y c o v e r e d s o u t h e r n s a h e l i a n r e g i o n o f t h e Sahara These r e g i o n s c o n s i s t o f b a r c h a n c h a i n s , t r a n s v e r s e r i d g e s and h i l l s o f sand
359
F i g . 5. S a n d r i d g e s and i n t e r d u n e c o r r i d o r s on a n a e r i a l p h o t o q r a p h o f " E r q Chech". T h i s a e r i a l p h o t o q r a p h ( s c a l e 1:50,000) shows t h e s a n d r i d n e s and i n t e r d u n e c o r r i d o r s o f f i g u r e 4 i n more d e t a i l . It s h o m : t h e a s y m m e t r i c f o r m o f t h e r i d g e s , more r e c t i l i n e a r o n t h e e a s t t h a n o n t h e w e s t , and t h e i r r e q u l a r s u r f a c e morphology. The r i d q e s c o n s i s t o f e x t r e m e l y s i n u o u s l i n e a r dunes, t e r m i n a t i n g a r o u n d d e p r e s s i o n s c a l l e d " q h o r r a f a s " . The i n t e r d u n e c o r r i d o r s a r e d e s t i t u t e o f sand, b u t r o u g h n e v e r t h e l e s s . The s t r i k i n g s c a r c i t y o f sand i n t h e sand sea i s shown b e t t e r t h a n on t h e s a t e l l i t e image. The l o n g i t u d i n a l sand r i d g e s have a maximum w i d t h o f 500 m e t r e s , w h i l e t h e i n t e r d u n e a r e a s r e a c h a w i d t h o f 3500 m e t r e s . and t h e i r sand seas have a p o s i t i v e sand b u d g e t . The b e s t examples a r e : t h e llreyye, t h e Aoukar, t h e M a u r i t a n i a n p a r t o f t h e A f t o u t - e s - S a h e l i
sand seas and
t h e Haoussa sand sea (Grove, 1 9 5 8 ) . The l a s t , w h i c h i s v e r y t h i c k , i s p a r t i a l l y i n t e r r u p t e d upwind
o f t h e A d e r D o u t c h i f o r l a c k o f a sand s u p p l y : downwind t h e
edifices
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361 B. Reasons f o r t h e s c a r c i t y o f sand i n t h e Sahara
( i ) A d e f i c i e n t supply
1) Lack o f l o c a l supply. The Sahara i s an o l d d e s e r t , p r o b a b l y d a t i n q from t h e Miocene. Sarntheim (1978) demonstrated t h a t t h e f i r s t a e o l i a n sands o f Saharan o r i g i n were d e p o s i t e d i n t h e A t l a n t i c Ocean d u r i n g t h e Oliqocene. F o r t h e Miocene, seasonal a r i d i t y i s documented by Jaeger (1975), and r e a l a r i d i t y a t t h e end o f t h e Miocene by Maley (1980). The p r e v a i l i n g a r i d i t y l i m i t s t h e supply o f sediment, s i n c e t h e p o t e n t i a l sand sources a r e a l l used up. The s o i l cover has been eroded: f l u v i a t i l e d e p o s i t s have had t h e i r s a n d - s i l t - c l a y f r a c t i o n s winnowed away: l i b e r a t i o n o f sand by c o r r a s i o n i s h i n d e r e d by d e s e r t v a r n i s h which s t r e n q t h e n s t h e r o c k . The s c a r c i t y o f sand i n t h e L i b y a n hamadas, between t h e G r e a t E a s t e r n Erq and t h e Calansho Sand Sea, can be e x p l a i n e d by t h e n a t u r e o f t h e s e c a l c a r e o u s hamadas l a c k i n g i n q u a r t z .
2 ) Shortage o f e x t e r n a l supply. The p r e s e n t l y s e m i - a r i d r e g i o n s o f t h e Maghreb have known l e s s a r i d p e r i o d s , w i t h t h e f o r m a t i o n o f ephemeral stream d e p o s i t s , as Dresch (1982) has c l e a r l y shown. However, t h i s sediment source can n o t be t r a n s p o r t e d by t h e w i n d a c r o s s t h e mountainous b a r r i e r s o f t h e n o r t h e r n Sahara: t h e winds a r r i v e a t t h e n o r t h e r n e r g s f r e e o f sand. ( i i ) Dynamic causes f o r t h e absence o f d e p o s i t s
1) The s h e l t e r e f f e c t . The topography o f t h e a r i d and h y p e r a r i d Sahara i s n o t f a v o u r a b l e f o r d e p o s i t i o n because o f b a r r i e r s . The f u s e d p l a t e a u x o f T a d e m a i t - T a s s i l i - H o g g a r i s a s u f f i c i e n t b a r r i e r t o sand movement t o f o r m t h e T a n z e r o u f t and i t s e x t e n s i o n s t o t h e n o r t h and south, where absence o f sand i s t h e r u l e ( f i g . 7 ) . L i k e w i s e , t h e Sahara Saraoui i s sand f r e e downwind o f t h e n o r t h Sahara A t l a s c h a i n . 2 ) Topographic i n c r e a s e i n wind v e l o c i t y . The h i g h e r t h e wind v e l o c i t y , t h e g r e a t e r t h e c a p a c i t y f o r sand t r a n s p o r t a t i o n . Topographic o b s t a c l e s produce wind a c c e l e r a t i o n e f f e c t s which can be f u r t h e r i n c r e a s e d when t h e s e o b s t a c l e s form c o r r i d o r s . Whenever t h e wind v e l o c i t y i n c r e a s e s , t h e wind power increases, as i n d i c a t e d by s m a l l e r d i s t a n c e s between t h e s t r e a m l i n e s . llhere t h e w i n d v e l o c i t i e s a r e h i g h , sand t r a n s p o r t i s t h e main a e o l i a n process. Where t h e w i n d v e l o c i t i e s a r e l o w , t h e sand c a r r y i n g c a p a c i t y decreases and d e p o s i t i o n occurs. Both o f t h e s e e f f e c t s a r e produced by n a t u r a l as w e l l as by man-made o b s t a c l e s , such as v i l l a g e s , oases, e t c . ( i i i ) The i n c r e a s e i n sand e x p o r t under p r e s e n t dynamic c o n d i t i o n s Over t h e l a s t t w e n t y y e a r s , t h e a r i d i t y has i n c r e a s e d t h e e f f i c i e n c y o f sand t r a n s p o r t by t h e winds. The s p e c i a l topography o f t h e r e g i o n has i n c r e a s e d t h e e x p o r t o f sand f r o m t h e Sahara and a c c e l e r a t e d t h e process o f emptying o f sand from t h e c e n t r a l Sahara ergs, w h i l e moving t h e e x p o r t l i m i t i n g l i n e towards t h e south. As a r e s u l t , t h e s a h e l i a n zone i s becoming an e x p o r t s i t e , w i t h t h e l o c a l
362
m , . R o c k massif responsable f o r shelter effect
El
131*Clouds
n
.She1 t e r w i t h sand t r a n s p o r t
Limit of efficient 'shel t e r ='.Perfect
.Imperfect shel t e r
shel t e r
0.Sand v e i l =.Sand
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P I G . 7. SHELTER EFFECT OF THE COMBINED TASSILI N'AJJER-HOGGAR-TADEMAIT PLATEAUX FROM 1:7.000.000 METEOSAT SATELLITE IMAGE
363 development of d e f l a t i o n s t r i a t i o n s . A l l q r a i n s i z e s l e s s than 400 microns are removed by t h e wind, as i s observed i n Niger. North of Lake Chad, where t h e dunes a r e t r a n s v e r s e chains, these l o n q i t u d i n a l s t r i a t i o n s a r e superimposed, g i v i n g a chequered p a t t e r n t o t h e sand sea surface. I n t h e t h i c k sand sheets of Niger, p a r t i c u l a r l y i n t h e vegetated Haoussa erq, west o f Lake Chad, t h e sand l o s e s cohesion and d e f l a t i o n s t r i a t i o n s appear as soon as t h e v e q e t a t i o n cover i s degraded. S a t e l l i t e images can be used t o c o n f i r m these s t r i a t i o n s up t o 10' N., v e r i f y i n g t h a t t h i s area i s t e n d i n q towards a negative sand budget. CONCLUSIONS The d i f f e r e n c e i n behaviour between t h e h y p e r a r i d r e q i o n s o f t h e Sahara, dominated by surfaces underqoing s t r o n g d e f l a t i o n , hence areas o f e x p o r t o f sediment, and t h e semi-arid margins, which a r e surfaces o f d e c c e l e r a t i o n , hence
o f d e p o s i t i o n and accumulation, i s a f a c t which can n o t be neglected i n a e o l i a n sedimentology. It e x p l a i n s n o t o n l y t h e p r o f u s i o n o f a e o l i a n deposits i n t h e semi-arid regions, b u t a l s o t h e p r o f u s i o n o f loess, whose a e o l i a n o r i g i n i n deserts i s u s u a l l y accepted. REF E RENC ES
B r o o k f i e l d , M., 1970. Dune t r e n d s and wind regime i n c e n t r a l A u s t r a l i a . Z. Geomorph. Suppl., 10: 121-158. Capot-Rey, R., 1970. Remarques sur l e s ergs du Sahara. Ann. Gebqr. P a r i s , 431: 2-19. Dresch, J . , 1982. Sur l a semi-aridit; du Maghreb au P l i o - Q u a t e r n a i r e . B u l l . Assoc. Ge'ogr France, 483-4 : 42-45. Grove, A.T., 1958. The a n c i e n t e r g o f Hausaland and s i m i l a r formations on t h e south s i d e o f t h e Sahara. Geogr. Jour., 124: 528-533. Jaeger, J.J., 1975. Les rongeurs du Mioc'ene moyen e t s u p g r i e u r du Maqhreb. Th'ese Sci. M o n t p e l l i e r , fasc. 1, 164pp. Maley, J . , 1980. Les changements c l i m a t i q u e s de l a f i n du T e r t i a r e en A f r i q u e : l e u r consgquence s u r l ' a p p a r i t i o n du Sahara e t sa ve'q6tation. I n : A.J. W i l l i a m s and H. Faure ( e d i t o r s ) , The Sahara and t h e N i l e . A.A. Balkema, Rotterdam, pp. 63-86. Mainguet, M . , 1982a. Les dunes d ' 6 r o s i o n : s i g n i f i c a t i o n morphodynamique e t c l i m a t i q u e de l e u r e x i s t e n c e . Wllrzb. geogr. Arb., 56: 79-92. Mainguet, M., 1982b. L ' g p a i s s e u r des d6pats sableux g o l i e n s e s t - e l l e un i n d i c a t e u r d ' a r i d i t e ? L ' a r i d i t 6 sahareinne. B u l l . Assoc. Ge'ogr. France, 483-4: 64-67. Mainguet, M. and C a l l o t , Y., 1978. L ' e r g de Fachi Bilma (Tchad-Niger). C o n t r i b u t i o n 'a l a connaissance de l a dynamique des ergs e t des dunes des zones a r i d e s chaudes. Mem. e t Doc. , C.N.R.S. Paris, 19, 184 pp. Mainguet, M., Cossus, L. and Chapelle, A.M., 1980. U t i l i s a t i o n des imaqes M6te'os a t pour p r k i s e r l e s t r a j e c t o i r e s eoliennes au s o l , au Sahara e t sur l e s marges sah6liennes. SOC. Photogramm. e t T e l g d i c t . , 78: 1-12. McKee, E.D.(editor), 1979. A s t u d y o f Global Sand Seas. U.S. Geol. Surv., Prof. Paper, 1052, 429 pp. Monod, Th., 1958. Majabat-al-Koubra. C o n t r i b u t i o n 'a 1 'Ctude de "1 'empty q u a r t e r " ouest-saharien. M6m. I.F.A.N., Dakar, 406 pp, 81 p l a t e s . Sarntheim, M., 1978. Sand d e s e r t s d u r i n g g l a c i a l maximum and c l i m a t i c minimum. Nature, 272: 868-890. T r i c a r t , J . and C a i l l e u x , A., 1964. Le model6 des re'gions s'eches. Les Cours de l a Sorbonne, C.D.U., Paris, 2 v o l s . , 129 pp. and 179 pp. (paper t r a n s l a t e d by M.E. B r o o k f i e l d )
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365
PIODERH EOLIAN DEPOSITS OF THE EASTERli P R O V I N C E
OF SAUDI A R A B I A
DANILO ANTON: Sand Research Program, U n i v e r s i t y o f Petroleum and M i n e r a l s , Dharan, Saudi A r a b i a . REGIONAL SETTING AND GEOLOGICAL EVOLUTION The E a s t e r n P r o v i n c e o f Saudi A r a b i a o c c u p i e s t h e e a s t e r n t h i r d o f t h e c o u n t r y , e x t e n d i n g a b o u t 1200 Km. f r o m South Yemen and Oman i n t h e south, t o I r a q and Kuwait i n t h e n o r t h , w i t h an a r e a o f n e a r l y 0.5 Km.
2
(fiq. 1).
The s o u t h e r n p a r t o f t h e p r o v i n c e i n c l u d e s t h e empty sand f i e l d s o f t h e e a s t e r n R u b ' a l K h a l i . The c e n t r a l and n o r t h e r n p a r t s a l o n g t h e A r a b i a n G u l f a r e more v a r i e d , i n c l u d i n g t h e J a f u r a h and Dahna sand f i e l d s , a l l u v i a l v a l l e y s and fans and an e x t e n s i v e sedimentary p l a t e a u . I n t h i s a r e a a r e l o c a t e d most o f t h e towns and t h e b u l k o f economic a c t i v i t y .
I
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Ocean
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366 The area i s u n d e r l a i n by s h e l f sedimentary r o c k s which d i p eastwards o f f t h e Arabian s h i e l d . D u r i n q t h e T e r t i a r y , t h e western edge o f t h e s h i e l d r o s e and t h e e a s t e r n p a r t subsided. I n t h e i n t e r i o r ( c e n t r a l ) homocl i n e , a r e a s which underwent g r e a t e r subsidence accumulated t h i c k e r sedimentary sequences f o r m i n g t h e main sedimentary basins; t h e Rub'al K h a l i , S i r h a n - T u r a y f ,
Northern Arabian
G u l f , and Dibba b a s i n s . The f i r s t w e l l - d e f i n e d T e r t i a r y c o n t i n e n t a l d e p o s i t s a r e o f t h e ? e a r l y Pliocene Iiadrukh Formation which u n d e r l i e s , w i t h d e p o s i t i o n a l c o n t i n u i t y , t h e Pliddle Miocene Dam Formation. The Hadrukh F o r m a t i o n i s m a r i n e a l o n g t h e e a s t e r n edge, near t h e G u l f , and c o n t i n e n t a l elsewhere. I t c o n s i s t s m o s t l y o f m a r l y sandstones, sandy c l a y s and sandy l i m e s t o n e s w i t h c l a y and c h e r t and l e s s f r e q u e n t l y gypsum, s u q q e s t i n q a l l u v i a l and l a c u s t r i n e environments o f a s e m i - a r i d c l i m a t e . Some e o l i a n a c t i v i t y may have occured because, thouqh no e o l i a n s t r u c t u r e s a r e found a t o u t c r o p , P.L. V i n c e n t ( p e r s . comm.,
1982) found f r o s t e d , well-rounded e o l i a n
sand g r a i n s i n s e v e r a l l a y e r s i n and above t h e Hadrukh Formation s u q q e s t i n q a r e l a t e d e o l i a n source. Younqer l a c u s t r i n e and a l l u v i a l d e p o s i t s o f t h e H o f u f Formation o v e r l i e t h e Dam Formation. The t y p e s e c t i o n o f t h i s u n i t (Powers e t al.,
1966) s t a r t s w i t h a basal 20 m e t r e
t h i c k conglomerate i n c l u d i n q b o u l d e r s
and pebbles o f l i m e s t o n e i n a q u a r t z m a t r i x , i n d i c a t i n q an a l l u v i a l environment under s e m i - a r i d c o n d i t i o n s . Above t h i s a r e t h i c k a r g i l l a c e o u s sandstones. A second c l e a r l y i d e n t i f i e d a l l u v i a l e p i s o d e i s seen i n t h e upper p a r t o f t h e type s e c t i o n where a l i m e s t o n e p e b b l e conglomerate i s exposed. Only m i n o r e o l i a n sediments o c c u r i n t h e H o f u f Formation, s u g q e s t i n q t h a t t h e r e was l i t t l e e o l i a n a c t i v i t y d u r i n q i t s d e p o s i t i o n i n Plio-Pliocene t i m e s . Larqe a l l u v i a l fans were formed i n Pliocene-Ouaternary t i m e s and p r o v i d e d a source o f m a t e r i a l f o r l a t e r r e w o r k i n q by t h e wind. The two p r i n c i p a l f a n s which reached t h e e a s t e r n p a r t o f t h e p e n i n s u l a a r e Cladi Dawasir and Wadi Sahba. The Dawasir western f a n formed i m m e d i a t e l y downstream o f t h e qorqe near S u l a y y i l . Eastwards, a l l u v i a l s e d i m e n t a t i o n t o o k p l a c e i n a f l a t wide area down t o t h e p r e s e n t sabkha M a t t i i n t h e U n i t e d Arab E m i r a t e s c o a s t l i n e o f t h e G u l f . Wadi Sahba a l l u v i a l fan e x t e n d s e a s t o f t h e Haradh r e g i o n , beyond t h e l a s t h i l l s o f t h e e a s t e r n s e d i m e n t a r y p l a t e a u , d i p p i n g eastwards under t h e e o l i a n d e p o s i t s o f t h e J a f u r a h sand f i e l d s . The e o l i a n d e p o s i t s i n t h e E a s t e r n P r o v i n c e ( J a f u r a h , Dahna and Rub'al K h a l i ) a r e always f o u n d o v e r l y i n q t h e s e a l l u v i a l sediments.
2. 2.1
EOLIAN SAliD FIELDS AND DEPOSITS J a f u r a h sand f i e l d s The J a f u r a h sand f i e l d s a r e l o c a t e d between 27'
49'
and 24'
South, e a s t o f t h e
30' East m e r i d i a n , i n t h e c o a s t a l l o w l a n d s below t h e 200 m e t r e c o n t o u r l i n e
a l o n g t h e A r a b i a n G u l f l i t t o r a l . They a r e a n a r r o w band i n t h e n o r t h t h a t widens southward t o merge w i t h t h e Rub'al K h a l i sand f i e l d s .
367 I n s p i t e o f a r e l a t i v e l y h i g h humidity, associated w i t h the p r o x i m i t y o f the Gulf, and a r a i n f a l l o f n e a r l y 80 mm./year on t h e n o r t h e r n edge, t h e morphodynamics o f t h e J a f u r a h a r e o f a r i d t y p e . Due t o l o w t o p o g r a p h i c p o s i t i o n , s u r f i c i a l a q u i f e r s a r e o f t e n v e r y s h a l l o w . There has been a l o n g h i s t o r y o f m i q r a t i o n o f water, t h r o u q h a r t e s i a n , n o n - a r t e s i a n sources and w e l l s , f r o m deep and s h a l l o w a q u i f e r s , r a i s i n g s t i l l f u r t h e r t h e w a t e r l e v e l , and l o c a l l y improvi n g t h e q u a l i t y o f t h e s u r f i c i a l groundwater. R e c e n t l y t h i s t r e n d has been s u b s t a n t i a l l y i n c r e a s e d by new a g r i c u l t u r a l developments and e x t e n s i v e d r i l l i n g c o n n e c t i n g up o t h e r w i s e i s o l a t e d a q u i f e r s . A l s o , t h e p r o x i m i t y o f t h e G u l f i n c r e a s e s t h e average h u m i d i t y , d e c r e a s i n g e v a p o r o t r a n s p i r a t i o n . As a r e s u l t , t h e v e g e t a t i o n may be q u i t e dense i n some areas, w i t h s h a l l o w
r e l a t i v e l y sweet s u r f i c i a l qroundwater. I n o t h e r areas, t h e presence o f a s h a l l o w s a l i n e s u r f i c i a l a q u i f e r o r t h e e x c e s s i v e d e p t h o f t h e water l e v e l may p r e v e n t p l a n t growth. I n t h e f i r s t case, t h e f i n a l r e s u l t i s a sabkha p l a i n ; i n t h e second, a f i e l d o f a c t i v e dunes. Thus, i n t h e J a f u r a h area, a r i d c o n d i t i o n s produce dune f i e l d s o n l y i n a p p r o p r i a t e a r e a s . Dune f i e l d s a r e l o c a l and t h e s i z e o f e o l i a n f e a t u r e s s m a l l . Due t o a n e a r l y c o n s t a n t n o r t h e r l y wind and f r e q u e n t l y h i g h wind speeds, barchans and barchanoid r i d g e s a r e t h e commonest d e p o s i t i o n a l forms. I s o l a t e d barchans t r a v e r s e t h e f l a t sabkha s u r f a c e s o r merge i n t o barchan dune f i e l d s . On t h e sabkhas themselves, dunes o f t e n have t h e i r basal l a y e r s preserved, a s m o i s t u r e cominq f r o m t h e s h a l l o w w a t e r t a b l e l e a d s t o s a l t c e m e n t a t i o n which p r e v e n t s d e f l a t i o n below a c e r t a i n l e v e l : w h i l e t h e t o p s a r e l o s t due t o i n c r e a s e d wind v e l o c i t y a s s o c i a t -
ed w i t h decreased s u r f a c e rouqhness. I n t h e dune f i e l d s , t h e dunes remain a c t i v e . I n f o r m a t i o n f r o m a n e t w o r k o f a z i m u t h a l sand t r a p s i n n o r t h J a f u r a h has shown t h a t sand i s m a i n l y t r a n s p o r t e d f r o m t h e N.N.W.
Sand samples were t a k e n f r o m
v a r i o u s geomorphological s i t e s i n t h e sand f i e l d s . S o r t i n g v a r i e s between 1.15 and 2.0, w i t h g r a i n s i z e Hd v a l u e s r a n g i n g f r o m 0.12 t o 0.4 mm. T y p i c a l g r a i n s i z e d i s t r i b u t i o n s a r e shown on f i g u r e s 2 and 3. Q u a r t z c o n t e n t v a r i e s between 90 and 99 p e r c e n t , w i t h l o w e r v a l u e s i n t h e s m a l l e r f r a c t i o n s . The c o l o u r o f t h e sand i s n o r m a l l y around 1 0 YR 7/3 on t h e N u n s e l l c h a r t ( v e r y p a l e brown near l i g h t g r a y ) , p r o b a b l y due t o t h e f a c t t h a t few y e l l o w q r a i n s and no i r o n c o a t i n g s a r e found. Rounded g r a i n s f o r m between 20 and 30 p e r c e n t o f t h e t o t a l , b u t a n g u l a r q r a i n s may f o r m between 1 0 and 30 p e r c e n t o f some samples. 2.2
The Dahna sand f i e l d s The Dahna sand f i e l d s a r e formed o f a l o n g and narrow s t r i p o f e o l i a n sand
from 20'
3 0 ' t o 28'
3 0 ' N o r t h . The w i d t h o f t h e s t r i p does n o t exceed 40 Kin. b u t
i t s l e n g t h reaches more t h a n 1100 Km. The Dahna sand f i e l d s a r e m o s t l y composed o f s e v e r a l r e l a t i v e l y p a r a l l e l
3 68
2.0 mm
0.5
1.0
0.3
0.2
F i g . 2. R e p r e s e n t a t i v e g r a i n s i z e s f o r N o r t h J a f u r a . Number 13 Hd = 0.30 mm.,
So = 1.27.
Number 1 6
-
0.06
0.1
-
0.03
barchan:
p a r a b o l i c dune: Pld = 0.285 mm.,
So = 2.52.
r i d g e s i n c l u d i n g complex dune systems. I n Landsat imaqery t h e p a t t e r n i s n o t always w e l l - d e f i n e d .
I n t h e south, N 20 t o Pi o r i e n t a t e d s t r i p s o c c u r : t h e i r
spacinq v a r i e s between 0.8 and 1.2 Km. and t h e i r l e n g t h s ranqe from 1 0 t o 20 h. F u r t h e r n o r t h , t h e s t r i p s a r e n o t c l e a r l y observed, b u t two d i r e c t i o n s can be d e f i n e d ( N 80 and
N
1 7 0 ) . I n t h e n o r t h e r n s e c t o r , d i r e c t i o n s change from N 100
t o N 120: spacing i s much g r e a t e r ( 3 t o 9 K m . ) and l e n g t h s range from 1 0 t o 100 Km. The f a c t t h a t t h e Dahna sand f i e l d s occupy a r e l a t i v e l y l o w t o p o q r a p h i c p o s i t i o n and a p p r o x i m a t e l y f o l l o w t h e c u r v e d q e o l o q i c a l s t r u c t u r e s , sugqests a q e n e s i s r e l a t e d t o nearby sand sources i n t h e c o n t i g u o u s watersheds. I n t h e area west o f Kurays, f i v e s t r i p s w i t h i n t e r d u n e v e g e t a t e d sand sheets a r e observed. E o l i a n a c t i v i t y i s more o r l e s s r e s t r i c t e d t o t h e dune r i d g e s . Beneath t h e r i d q e s , two p a l a e o s o l s developed i n e o l i a n sands a r e i n t e r e s t i n q evidence o f t h e e x i s t e n c e o f a t l e a s t two humid p e r i o d s i n t e r r u p t i n g t h e e o l i a n
369 dynamics in t h e region ( f i g . 3 ) .
TOP OF THE EXPOS RE
3.3
3.0
MODERN
EOLIAN - - SAND
5
2.1
DUNES 1
(ACTIVE 2 .d (WELL DEFINED STEEPLY 2.1
DIPPING " LAMINAE, DUNAR MORPHOUIQY 1
77-77-7
c
SOIL ( S I L T Y
SAND W I T R SOME CLAY)
LW
18
CaC03 tubes 1.5
(colcified roots
-
EOLIAN SAND (WELL DEFINED
I2
WELL
,DIPPING STEEPLY ,DIPPING
LAMINAE, LAMINAE, ROOTS ROOTS CANALS CANALS
PRESERVED)
0.9
SANDY SOIL ( SILTY-CLAYEY
SA
0.E
EOLIAN SAND WITH SOME ALLUVIAL REWORKING 0.3
I
(VARIABLE
m . 0.0
DIP, MINOR OCCURENCES OF COARSE GRAINED LAYERS)
4
BOTTON OF THE P I T
Fiq. 3. Exposure in p i t , 15 Km. west of Khurays in Dahna sand f i e l d s . I n 1 , 3, and 5 , typical e o l i a n s t r u c t u r e s a r e observed: c r ~ s s b e d d i n q ,s l i g h t l y i r r e g u l a r parallel lamination (dikaka type) and s t e e p l y dipping bedding. The Dahna e o l i a n sediments have well-defined c h a r a c t e r i s t i c s . Grain s i z e Md values range from 0.16 t o 0.45 mm. and So from 1.1 t o .8. Typical cumulative curves a r e shown in f i g u r e s 4 and 5 . The sands a r e strongly reddish (Plunsell 7.5 YR 6/8, fal'ling in the reddish-yellow zones) and quartz represents about 99 per cent of the t o t a l samples with mainly subrounded yellow grains and iron
370 c o a t i n g s i n a p p r o x i m a t e l y 75 t o 8 0 p e r c e n t o f t h e cases.
F i g . 4. R e p r e s e n t a t i v e g r a i n s i z e s f o r J a f u r a h ( n o s . 3 and 4 ) and Dahna (no. 2) dune f i e l d s . No. 3 N o r t h Jafurah,Abuhadriyah hiqhway, shadow f i n e sand: Md = 0.14 mm., So = 1.22. No. 4 - N o r t h Jafurah,Abu H a d r i y a h hiqhway, e o l i a n sand, open a r e a b e h e e n mounds: Pld = 0.315, So = 1.89. No. 2 - Dahna sand f i e l d , f i n e sand i n shadow o f small mound 9 Km. west o f Khurays: Md = 0.188, So = 1.17.
-
Along t h e Dahna l o w c o r r i d o r , small and medium s i z e d a l l u v i a l f a n s a r e seen, i n which r e d d i s h s o i l s have developed. These a l l u v i a l fans, o f p r o b a b l e P l e i s t ocene age, seem t o have been t h e source f o r t h e i r o n c o a t e d g r a i n s i n t h e younge r f o r m a t i o n s , s u p p o r t i n g t h e i d e a t h a t t h e sand f i e l d s o f Dahna a r e m a i n l y c o n t r o l l e d by l o c a l and r e g i o n a l g e o l o q i c a l s t r u c t u r e and qeomorpholoqy, and only t o a l e s s e r e x t e n t by c l i m a t e . 2.3
The R u b ' a l K h a l i The R u b ' a l K h a l i i s t h e l a r q e s t s i n g l e r e g i o n o f a r i d i t y . More t h a n 550,000
Km.'
i n t h e s o u t h e r n h a l f of t h e c o u n t r y (about 50 p e r c e n t o f t h e a r e a i n the
E a s t e r n P r o v i n c e s ) a r e covered w i t h more o r l e s s c o n t i n u o u s e o l i a n accumulations, r a r e l y i n t e r r u p t e d by eroded remnants o f o l d e r r e l i e f ( m a i n l y near t h e edges),
371 100
80
SILT
8 .-c>
60
0)
0
z
0
40
20
Fig. 5. R e p r e s e n t a t i v e g r a i n s i z e s f o r Dahna sand f i e l d s . No. 1 0 - o l d e r e o l i a n sand f r o m t r e n c h , d e p t h 2.0 t o 2.5 m e t r e s : Hd = 0.199.mn. So,= 1.39. 1'0. 20 - t o p o f barchanoid r i d g e , 17 Km. west o f Khurays: Md = 0.184 mm., so = 1.12.
o r by uncovered g r a v e l pavements o r sabkha f l a t s i n some i n t e r d u n e areas. P r e s e n t r a i n f a l l i n t h e r e g i o n seems t o be l e s s t h a n 50 mm.,
although accur-
a t e m e t e o r o l o g i c a l i n f o r m a t i o n i s n o t a v a i l a b l e o v e r most o f t h e r e g i o n . When i t r a i n s , most o f t h e w a t e r j u s t humects t h e s u r f a c e g r a i n s , p r o m p t l y e v a p o r a t i n g o r , d u r i n g heavy r a i n s , i n f i l t r a t e s t h r o u g h t h e t h i c k sandy c o v e r t o t h e water t a b l e i n a r e a s w i t h s h a l l o w s u r f i c i a l a q u i f e r s . Very r a r e l y , a small amount r u n s down t h e s l o p e s of t h e dune r i d g e s , and s o r t i n q o f g r a i n s o c c u r s on micro-pedime n t s s l o p i n g toward t h e i n t e r d u n e d e p r e s s i o n s . No c o n c e n t r a t e d r u n o f f o c c u r s anywhere i n t h e r e g i o n and no aqueous qeomorphic f e a t u r e s can be seen. N e v e r t h e l e s s , t h e R u b ' a l K h a l i i s r e a l l y a complex a r e a w i t h a l o n g h i s t o r y o f c l i m a t i c and geomorphological changes. Two main f a c t o r s have i n f l u e n c e d t h e e v o l u t i o n o f t h e landscape i n t h e r e g i o n . I n t h e f i r s t place, t h e amount o f r a i n f a l l and, a s s o c i a t e d w i t h i t t h e d e n s i t y and permanence o f v e g e t a t i o n c o v e r
372 and secondly, t h e changes o f wind d i r e c t i o n . Changes i n r a i n f a l l have c o n t r o l l e d t h e presence o r absence o f f l u v i a l f l o w , t h e r i s e o r d r o p i n l a k e l e v e l s , s o i l f o r m a t i o n o r e r o s i o n , s a l t a c c u m u l a t i o n and e o l i a n r e w o r k i n q o f e x i s t i n g sand. Chanqes i n wind d i r e c t i o n have m a i n l y a f f e c t e d dune morphology: o l d r i d q e s a r e being r e b u i l t i n a new a l i q n m e n t , complex dunes a r e b e i n g formed, and sand sources changing t h e i r l o c a t i o n . Two main dune systems occur. The o l d e s t one o v e r l i e s t h e P l i o - P l e i s t o c e n e a l l u v i a l s u r f a c e s , and l o c a l l y l a t e P l e i s t o c e n e l a c u s t r i n e d e p o s i t s . These o l d dunes developed a p a l e o s o l d u r i n q e a r l y Holocene t i m e s and were covered by new dunes i n a l a t e r stage. The p a l e o s o l s on t h e dunes a r e composed o f r o o t and stem e n c r u s t a t i o n s (McClure, 1978) and seem t o have been contemporary w i t h t h e younger l a k e s i n t h e i n t e r d u n e d e p r e s s i o n s , w h i c h a r e now covered by modern e o l i a n deposits. S e l e c t e d samples were o b t a i n e d f r o m t h e w e s t e r n and south-western areas. G r a i n s i z e v a r i e s c o n s i d e r a b l y depending on geomorphic p o s i t i o n o f t h e samples. T y p i c a l dune sand Pld v a l u e s range f r o m 0.15 t o 0.4 mm. w i t h good s o r t i n g on h i q h dune r i d g e s (So = 1.1 t o 1.4) and w i t h poor s o r t i n g near d e f l a t i o n zones (So up t o 2.6).
R e p r e s e n t a t i v e c u m u l a t i v e c u r v e s a r e shown on f i g u r e 6. Q u a r t z
i s o f t e n more t h a n 97 p e r c e n t o f t h e samples, w i t h f e l s p a r v a r y i n q f r o m 0.5 t o 2 p e r c e n t and m a f i c m i n e r a l s b e i n g l e s s t h a n 3 p e r c e n t . G r a i n s a r e subrounded
and subanqular, w i t h rounded g r a i n s f o r m i n g a b o u t 25 p e r c e n t o f t h e samples. G r a i n c o l o u r ranqes f r o m y e l l o w t o l i g h t y e l l o w , w i t h d u l l s u r f a c e s and remnants o f i r o n c o a t i n g s i n about 10 t o 20 p e r c e n t o f t h e g r a i n s . The qeneral c o l o u r o f t h e sand v a r i e s between 5 YR 7/6 t o 7/8 and 7.5 YR 7 / 6 t o 7/8 o f t h e Munsell colour charts, corresponding t o reddish-yellow colours.
3. 3.1
QUATERNARY EVOLUTION Background I d e n t i f i c a t i o n o f t h e p r e s e n t , qeomorphic systems r e q u i r e s o n l y f i e l d observ-
a t i o n and m e t e o r o l o g i c a l d a t a . A c t i v e wadis and dune f i e l d s , presence o f g u l l i e s o r r a v i n e s , and o b s e r v a t i o n o f t h e v e g e t a t i o n c o v e r , complemented w i t h d a t a on r a i n f a l l , wind d i r e c t i o n and speed, h u m i d i t y and temperature, can p r o v i d e a good u n d e r s t a n d i n g o f t h e c u r r e n t qeomorphic system. I d e n t i f i c a t i o n o f o l d e r systems r e q u i r e s a d i f f e r e n t approach; sedimentary accumulations, l a n d f o r m s and paleopedogenic processes must be d e s c r i b e d , a n a l y s e d and i n t e g r a t e d i n order t o o b t a i n t h e necessary knowledge t o r e c o n s t r u c t t h e paleoenvironment. Taking i n t o account t h e s e elements, s i x main c l i m a t i c phases a r e proposed for t h e l a t e P l i o c e n e t o Q u a t e r n a r y i n t h e p r e s e n t s t u d y . D u r i n q each phase, t h e d i s t r i b u t i o n o f qeomorphic systems i n t h e r e g i o n changed. D u r i n g 'humid' phases, a r i d systems became s e m i - a r i d systems and s e m i - a r i d systems became semi-humid.
373 100
8 C -SAND-
6C (D
.->
c
0 3
5
0 4c
2(
-
1.0
mm
0.5
0.3
0.2
0.1
0.06
0.03
Fig. 6 . Representative qrain s i z e analyses f o r t h e northwest Rub'al Khali. 110. 5 - slope of s t a r dune: Md = 0.332 nm., So = 1.13. No. 6 - eolian sand on top o f l a r g e dune: Md = 0.201 rnm., So = 1.12. During the ' a r i d ' phases, the opposite phenomenon occured; semi-humid systems became semi-arid, and semi-arid became a r i d . Eolian dynamics and c o r r e l a t i v e sediments a r e a r e s u l t of these c l i m a t i c o s c i l l a t i o n s . Arid phases correspond with periods of sand i n s t a b i l i t y - d e f l a t i o n , ,formation of dunes, e t c . and humid phases a r e c o r r e l a t i v e with a l l u v i a l sedimentation
along wadis, s o i l development o n t h e slopes, and dune immobilization. 3.2
The l a t e Pliocene - e a r l y Pleistocene humid phase
No s i g n i f i c a n t eolian a c t i v i t y seems t o have occured durinq l a t e Pliocenee a r l y Pleistocene times. On t h e contrary, a v a i l a b l e evidence shows the existence
of a r e l a t i v e l y long humid period s t a r t i n q sometime in the l a t e Pliocene and ending in the e a r l y Pleistocene. Widespread gravel deposits (associated with present o r old drainaqe systems), red s o i l s in the western mountains o f t h e Arabian Peninsula, weathering on shield
374 rocks and b a s a l t , a l l u v i a l f a n s and t e r r a c e s , seem to confirm the existence of t h i s humid phase during the l a t e Cenozoic (Anton, 1980; Hoetzl e t a l . , 1978). Two b a s a l t s gave K/Ar ages of 1.1 0.3 Ha. and 3.5 f 0.3 Ma. The gravels
*
were therefore accumulated a f t e r t h i s . Due to sedimentoloqical and qeomorphologi c a l s i m i l a r i t i e s i t i s reasonable t o assume t h a t o t h e r qravelly accumulations and deep weathering in the peninsula a l s o occured during the same period. In t h e Eastern Province, i n the Iiadi Sahba basin, the valleys a r e f i l l e d with a basal coarse g r a v e l l y deposit underlying a f i n e r formation which seems to correspond with t h i s ancient humid period. Downstream of A1 Kharj, a single valley i s observed running eastwards through t h e Dahna eolian c o r r i d o r and down t o the Jafurah sand f i e l d s . Along t h i s v a l l e y , t e r r a c e s composed of gravelly d e p o s i t s , and o f t e n covered with well-defined c a l c i c r u s t s , a r e encountered. These t e r r a c e s a r e normally dissected along the a x i s of t h e valley and t o a l e s s e r degree alonq the edges, next to bedrock outcrops. The elevation of the t e r r a c e s reaches a maximum in the high gradient section west of Haradh s t a t i o n , and decreases in height eastwards, with t h e i r f l a t tops merging w i t h t h e surface of an extensive a l l u v i a l plain ( a c t u a l l y a t r u e alluvial f a n ) with probably s i m i l a r age. The fan i s p a r t i a l l y covered with eolian sand and has been dissected by a l a t e r channel; b u t can s t i l l be e a s i l y traced on Landsat images and observed in the f i e l d . Extensive gravel pavements, consisting mainly of quartz pebbles, a r e observed in many places. No eolian deposits have been i d e n t i f i e d in t h i s f a n , which seems t o be one of t h e main sources f o r the younger eolian sand f i e l d s . The Middle Pleistocene a r i d phase Several l i n e s of evidence show t h a t the Middle Pleistocene was not a s humid a s t h e e a r l y Pleistocene, and t h a t t h i s trend continued u n t i l l a t e Pleistocene times. In the west of the country, t h e younger b a s a l t s of t h e tlarrat a r e n o t strongly weathered and no deep valleys have been c u t into them (Hoetzl e t a l . ,
3.3
1978), which can be i n t e r p r e t e d a s due t o t h e absence of long humid periods during the r e s t of t h e Quaternary.
The d e p o s i t s t h a t may correspond with t h i s Pleistocene sub-epoch a r e f i n e and r i c h in calcium carbonate o r gypsum, suqgesting a semi-arid t o a r i d climate with l i t t l e runoff and limited f l u v i a l flow. Even though some eolian sediments belonq in t h i s period, a r i d i t y does not seem t o have reached the threshold needed f o r widespread eolian a c t i v i t y . A moderately dense xerophytic vegetation cover would have been adequate t o produce t h i s type of environment. Thic period corresponds with the accumulation o f t h e s i l t y deposits which f i l l almost completely t h e valleys in the Arabian s h i e l d area (Anton, 1980). I t i s possible t h a t p a r t of t h i s material could be reworked l o e s s derived from d e f l a t i o n of t h e more a r i d regions of t h e central and eastern p a r t s of the
375 peninsula, r e t a i n e d by v e g e t a t i o n i n t h e more humid western r e g i o n . 3.4
The l a t e P l e i s t o c e n e humid phase
A l a t e P l e i s t o c e n e humid phase was d e f i n e d i n t h e A r a b i a n s h i e l d by Anton (1980) based on a t h i n l a y e r o f c o a r s e sediment c o v e r i n g t h e M i d d l e P l e i s t o c e n e s i l t s . The d i s s e c t i o n o f Wadi Sahba and llladi B a t i n a l l u v i a l f a n s seems t o be also r e l a t e d t o t h i s l a t e Pleistocene increase i n humidity. I n t h e Rub'al Khal i, presumably c o r r e l a t i v e l a c u s t r i n e d e p o s i t s , d a t e d between 36,031)
and 17,000 y e a r s B.P. were n o t e d by PlcClure (1978).No s i g n i f i c -
a n t e o l i a n a c t i v i t y seems t o have o c c u r e d d u r i n g t h i s stage. 3.5
The l a t e P l e i s t o c e n e
-
e a r l y Holocene a r i d phase
I t i s q e n e r a l l y accepted t h a t , d u r i n g l a t e P l e i s t o c e n e times, a marked i n c r e a s e i n a r i d i t y occured i n t h e A r a b i a n and Saharan d e s e r t s ( H o e t z l e t a l . , 1978; Alayne and Grove, 1979; Butzer, 1980; Anton, 1980). Dune systems s t a r t e d t o f o r m i n t h e v a s t expanses o f t h e R u b ' a l K h a l i and Nafud, and t o a l e s s e r e x t e n t i n J a f u r a h and Dahna. F l u v i a l f l o w decreased t o a minimum. No a l l u v i a l accumulations occured, w i t h t h e p r o b a b l e e x c e p t i o n o f e v a p o r i t e s and some f i n e l o c a l d e p o s i t s . Rub'al K h a l i l a c u s t r i n e sediments, s t r a t i g r a p h i c a l l y l o c a t e d above and below t h e s e e o l i a n d e p o s i t s , were d a t e d between 17,000 and 19,000 y e a r s B.P.
(McClure, 1978). No d a t e s a r e a v a i l a b l e f o r t h e E a s t e r n Province, b u t
a number o f exposures o f e o l i a n sands have been i d e n t i f i e d i n t h e J a f u r a h r e g i o n which appear t o c o r r e s p o n d w i t h t h i s phase. I n t h e Dahna area, o l d e r e o l i a n sands u n d e r l y i n ? two p a l e o s o l s have been observed, which seem t o be c o r r e l a t i v e w i t h t h i s phase ( f i g . 3 ) .
A c o r r e l a t i v e a r i d p e r i o d was i d e n t i f i e d i n t h e N i l e v a l l e y
-
dune i n v a s i o n s
o f t h e f l o o d p l a i n s ( B u t z e r , 1980); i n t h e A f a r and E t h i o p i a n r i f t l a k e s from 17,000 t o 12,000 y e a r s B.P.
(Gasse e t a l . ,
-
1980); and i n most o f t h e Sahar-
i a n and S a h e l i a n l a k e s (Alayne and Grove, 1979). 3.6
The e a r l y Holocene humid phase I n l a t e r times, s h a l l o w l a k e s a g a i n formed i n t h e R u b ' a l K h a l i . These a r e
dated between 9,000 and 6,000 y e a r s B.P.
(McClure, 1978). S o i l s developed on
t h e Dahna sand f i e l d s and c o a r s e a l l u v i a l d e p o s i t s accumulated i n Wadi Sahba and Wadi B a t i n . T h i s suggests a marked i n c r e a s e i n h u m i d i t y c o i n c i d i n g w i t h t h e b e g i n n i n g o f t h e Holocene a b o u t 12,000 t o 10,000 y e a r s ago. I n t h e A r a b i a n s h i e l d , t h e g r a v e l t e r r a c e s were d i s s e c t e d and t h i n c o a r s e d e p o s i t s formed i n t h e wadi v a l l e y s (Anton, 1980). I n t h e A r a b i a n p e n i n s u l a , Hoetzl e t a1.(1978)
proposed an e a r l y Holocene humid phase e x t e n d i n g f r o m 9,000
t o 4,500 y e a r s B.P.
Based on carbon 1 4 d a t e s and r e l a t e d geomorphological data,
376 t h e y a l s o proposed a s h o r t a r i d episode from 8,000 t o 7,000 y e a r s B.P.,
corresp-
o n d i n g t o a " N e o l i t h i c P l u v i a l " . Gasse e t a1.(1980) proposed a l o w e r l i m i t o f 12,000 y e a r s B.P.
f o r t h i s humid t h r o u q h o u t E t h i o p i a . However, f o r t h e more a r i d
Lake Abha b a s i n i n C e n t r a l A f a r t h e y c o n s i d e r t h a t t h e main r i s e i n w a t e r l e v e l happened l a t e r , ar'3und 10,000 y e a r s B.P.
I n t h e E q y p t i a n Sahara, ldendorf and
Hassan (1980) proposed a humid phase s t a r t i n q a b o u t 10,000 y e a r s B.P. w i t h a s h o r t a r i d phase f r o m 7,600 t o 7,400 y e a r s B.P.,
and a l a s t humid phase a b o u t
6,500 y e a r s aqo. I n t h e Dahna sand f i e l d , t h e e o l i a n non-weathered h o r i z o n s seem t o correspond t o d r i e r phases and t h e two p a l e o s o l s t o s u c c e s s i v e humid p e r i o d s ( f i q . 3 ) . The upper p a l e o s o l i s covered by a c t i v e e o l i a n d e p o s i t s which appear t o correspond t o t h e l a t e Holocene a r i d phase.
3.7
The l a t e Holocene a r i d phase The s h a l l o w l a k e s o f t h e R u b ' a l K h a l i s t a r t e d t o d r y up about 6,000 y e a r s B.P.
(McClure, 1 9 7 8 ) . Evidence i n t h e J a f u r a h r e g i o n show t h a t t h e e x t e n s i o n o f t h e dune f i e l d s t o o k p l a c e v e r y q u i c k l y . Present-day m i q r a t i n q barchans on sabkhas a r e g e n e r a l l y a few k i l o m e t r e s ( l e s s t h a n 1 0 Km.) downwind o f i d e n t i f i a b l e source a r e a s . Assuming p r e s e n t m i g r a t i o n r a t e s o f about 1 0 m e t r e s / y e a r , t h e o l d e s t dunes a r e l e s s t h a n 1,000 y e a r s o l d . I f m i g r a t i o n r a t e s were f o r m e r l y slower, the a c t u a l aqe m i g h t be i n c r e a s e d t o 2,000 t o 3,000 y e a r s . Some a u t h o r s (e.g N i c h o l s o n e t a1 ., 1980; Gasse e t a l . ,
1980) sugqest t h a t a s l i q h t l y w e t t e r
episode f r o m 2,500 t o 1,000 y e a r s B.P. may have i n t e r r u p t e d t h i s a r i d phase. I f t h i s i s c o n f i r m e d f o r t h e E a s t e r n Provinces r e g i o n , i t seems l i k e l y t h a t
dune m i g r a t i o n had stopped due t o v e q e t a t i o n s t a b i l i z a t i o n d u r i n g some c e n t u r i e s i n t h e second and t h r i d m i l l e n i a b e f o r e t h e p r e s e n t . Even supposing t h a t dunes remained s t a b l e f o r a thousand y e a r s , and a l l o w i n g s e v e r a l c e n t u r i e s f o r dune formation,
i t i s u n l i k e l y t h a t t h e J a f u r a h dunes a r e o l d e r t h a n 4,000 y e a r s B.P.
Thus, t h e p r e s e n t e o l i a n system o f t h e J a f u r a h sand f i e l d s i s o n l y a r e c e n t development r e l a t e d t o g r a z i n g and o t h e r human a c t i v i t i e s d u r i n g t h e l a s t t h r e e t o f o u r m i l l e n i a . G r a z i n q a c t i v i t i e s were expanded around f o u r thousand y e a r s ago when t h e camel was d o m e s t i c a t e d (Doe, 1971) and i n c o r p o r a t e d i n t o herds, t h u s c o n s i d e r a b l y e x t e n d i n g t h e g r a z i n g areas. I n t h e J a f u r a h , t h e o r i g i n o f the main dunes i s r e l a t e d t o d e g r e d a t i o n o f t h e v e g e t a t i o n c o v e r i n upwind a r e a s . 4.
CONCLUSION The c l i m a t e o f t h e E a s t e r n P r o v i n c e o f Saudi A r a b i a has been e v o l v i n g f r o m
humid and s e m i - a r i d d u r i n q t h e e a r l y T e r t i a r y , t o a r i d and s e m i - a r i d i n Quaterna r y times. D u r i n g t h i s e v o l u t i o n , p l a n t communities have more o r l e s s s u c c e s s f u l l y adapted t o t h e i n c r e a s i n g d r o u g h t and have k e p t a r e l a t i v e l y dense c o v e r on e x i s t i n q
377 sandy s o i l s , a l l o w i n q sand movement o n l y d u r i n q t h e most severe a r i d episodes. Amonq t h e i d e n t i f i a b l e a r i d i t y peaks, o n l y t h e t h r e e most r e c e n t ones have produced widespread e o l i a n e f f e c t s on sandy areas. A v a i l a b l e d a t e s f o r t h r e e peaks a r e : f r o m 17,000 t o 10,000 y e a r s B.P.;
-
from 6,000
5,500 y e a r s B.P.
f r o m 9,000 t o 7,000 y e a r s B.P.;
and
t o t h e present.
The l a s t peak i s composed o f two stages: a f i r s t one i n w h i c h g e n e r a l a r i d i t y developed q r a d u a l l y , p r o b a b l y r e s p o n d i n q t o p l a n e t a r y c l i m a t i c chanqes; and a second one i n which t h e t r e n d t o i n c r e a s i n g a r i d i t y a c c e l e r a t e d , a p p a r e n t l y due t o human a c t i v i t y i n v o l v i n q o v e r q r a z i n q and b u r n i n q o f v e g e t a t i o n i n t h e d r i e s t areas and d e p r e d a t o r y a q r i c u l t u r e i n l e s s a r i d reg,ions. The i n f e r r e d h i s t o r y i s summarized i n t a b l e 1 below.
CORRELATION OF CLIMATIC PHASES ' GEOMORPHOLOGICAL EVOLUTION A N 0 GEOLOGICAL DYNAMICS
TABLE 1 _ .
1011
.................
I
IoluiWYERED BY DUNES
.............
S"*LLOW IDOL
SfMIIRID
I ----+-DUNES
LA16 PLISITOEINE SEMIARID
9
EOLIAN
.............................
'2.9.
I
.....................
WILS WYERfO 8" DUNES
......................
WlLSON DUNPS
SOIL EROSION
378
REFEREllCES Alayne, S.F. and Grove, A.T., 1979. Global maps o f l a k e l e v e l f l u c t u a t i o n s since 30,000 B.P. Q u a t e r n a r y Res., 12: 83-118. Al-Sayari, S.S. and ZBti, J.G., 1978. Quaternary Period i n Saudi Arabia. Springer Verlag, Wien, 334 pp. Anton, D., 1980. C l i m a t i c i n f l u e n c e i n t h e Cenozoic e v o l u t i o n o f t h e Arabicn S h i e l d south. Abs. 26th I n t e r n a t . Geol. Conqr., Paris, 1980. Breed, C.S. e t a l . , 1979. Reqional s t u d i e s o f sand seas u s i n g Landsat (E9TS) imagery. I n : McKee, E.D. ( E d i t o r ) , A study o f q l o b a l sand seas. U.S. Geol. Surv. Prof. Paper, 1052: 305-397. Butzer, K.W., 1980. P l e i s t o c e n e h i s t o r y o f t h e N i l e v a l l e y i n Eqypt and Lower Nubia. I n : Williams, H.A.J. and Faure, H. ( E d i t o r s ) , The Sahara and t h e Nile, A.A. Bal kema, Rotterdam, pp.253-280. Doe, B., 1971. Southern Arabia. NcGraw H i l l , N.Y., p. 267. Gasse, F. e t a l . , 1980. Quaternary h i s t o r y o f t h e A f a r and E t h i o p i a n r i f t lakes. I n : I l i l l i a m s , I1.A.J. and Faure, H. ( E d i t o r s ) , The Sahara and t h e N i l e , fi.A. Bal kema, Rotterdam, pp. 361-400. Hoetzl, H. e t a1 ., 1978. Wadi A r Rimah: Wadi Ad Dawasir and i t s h i n t e r l a n d . I n : Al-Sayari, S.S. and Z B t l , J.G. ( E d i t o r s ) , Quaternary Period i n Saudi Arabia. Springer-Verlaq, M e n , pp, 173-193 and 226-251. HcClure, H.A., 1978. A r Rub'al K h a l i . I n : Al-Sayari, S.S. and Faure, J.G. ( E d i t . ) Quaternary Period i n Saudi Arabia. Springer-Verlaq, IJien, pp. 252-263. Nicholson, S.E., 1980. Saharan c l i m a t e s i n h i s t o r i c times. I n : Williams, I1.A.J. and Faure, H. ( E d i t o r s ) , The Sahara and t h e N i l e , A.A. Balkema, Rotterdam, pp. 173-200. Patterson, R.J. and Kinsman, D.J.oI., 1981. H y d r o l o q i c a l framework o f a sabkha along t h e Arabian G u l f . Amer. Ass. P e t r o l . Geol. B u l l . , 65: 1457-1475. 1966. Geology o f t h e Arabian Peninsula: Sedimentary geology Powers, R.W. e t a1 o f Saudi Arabia. U.S. Geol. Surv. P r o f . Paper, 560-D: 1-147. Wendorf, F. and Hassan, F.A., 1980. Holocene ecoloqy and p r e h i s t o r y i n t h e Egyptian Sahara. I n : Williams, M.A.J. and Faure, H. ( E d i t o r s ) , The Sahara and t h e N i l e , A.A. Balkema, Rotterdam, pp. 407-420.
.,
379
THE DYNAMIC HOLOCENE DUNE FIELDS OF THE GREAT PLAINS AND ROCKY MOUNTAIN BASINS, U.S.A.
THOMAS S. AHLBRANDT, P e t r o s t r a t C o n s u l t a n t s , 9600 E . Arapahoe Rd. S u i t e 250, Englewood, Colorado 80112 (USA) JAMES B. SWINEHART, C o n s e r v a t i o n and Survey D i v i s i o n , I A N R , U n i v e r s i t y o f Nebraska, L i n c o l n , Nebraska 68588-0517 (USA) D A V I D G. MARONEY, Route 2, Ladoga, I n d i a n a 47954 (USA) INTRODUCTION The dune f i e l d s i n t h e n o r t h e r n Great P l a i n s and Rocky Mountain b a s i n s ( F i g . 1) t r a d i t i o n a l l y have been t h o u g h t t o have formed d u r i n g t h e l a t e P l e i s t o c e n e . These c o n c l u s i o n s were based upon q u a l i t a t i v e geomorphic a n a l y s e s such as c o r r e l a t i o n s w i t h l o e s s o r t e r r a c e f i l l sequences o r upon i n f e r e n c e s c o n c e r n i n g s t r o n g k a t a b a t i c winds r e l a t e d t o g l a c i a l p e r i o d s .
However, t h e dune f i e l d s we have s t u d i e d
p r o v i d e no d i r e c t e v i d e n c e o f dune f o r m a t i o n d u r i n g t h e P l e i s t o c e n e a l t h o u g h we a n t i c i p a t e t h a t some l a t e P l e i s t o c e n e e o l i a n a c t i v i t y e v e n t u a l l y may be documented. Our evidence f r o m w e l l - d a t e d l o c a l s t r a t i g r a p h i c sequences, combined w i t h an abundance o f i n d i r e c t evidence, s u p p o r t s a m u l t i p h a s e h i s t o r y o f e o l i a n a c t i v i t y i n t h e s e dune f i e l d s d u r i n g t h e Holocene. Several d i s c i p l i n e s must be c a l l e d on t o e x p l a i n a d e q u a t e l y t h e l a t e P l e i s t o c e n e (Wisconsin) and Holocene h i s t o r y p e r t i n e n t t o t h e dune f i e l d s shown i n F i g u r e 1. These dune f i e l d s and t h e i r m a r g i n a l a r e a s have been occupied by p a l e o - I n d i a n o r younger c u l t u r e s u n t i l q u i t e r e c e n t l y .
A r c h e o l o g i s t s have e s t a b l i s h e d a
s t r a t i g r a p h i c s u c c e s s i o n i n t h e Holocene o f t h i s r e g i o n by u s i n g a r t i f a c t s as "index f o s s i l s ' ' (Frison e t a l . , d a t i n g o f t h e dune f i e l d s .
1974) f a c i l i t a t i n g g r e a t l y t h e r e l a t i v e age
P l e i s t o c e n e and Holocene v e r t e b r a t e f a u n a l successions
p r o v i d e a n o t h e r method o f r e l a t i v e d a t i n g i n t h i s area (Anderson, 1974; Wilson, 1974).
As d e s c r i b e d by Wilson (1974), t h e Holocene succession o f b i s o n i s p a r -
t i c u l a r l y important.
Moreover, some i n f o r m a t i o n on r e g i o n a l c l i m a t e can be
o b t a i n e d f r o m t h e s e successions.
P a l y n o l o g i s t s a l s o have p r o v i d e d a r e l a t i v e
s t r a t i g r a p h i c f l o r a l s u c c e s s i o n i n t h e Wisconsin and Holocene, and t h i s i n f o r m a t i o n has been used t o r e c o n s t r u c t t h e e v o l u t i o n of r e g i o n a l c l i m a t i c p a t t e r n s . The r e c o r d o f Holocene g l a c i a l expansion i n t h e Rocky Mountains p r o v i d e s a d d i t i o n a l c l u e s t o r e g i o n a l Holocene c l i m a t e s . l a r g e measure on r a d i o c a r b o n dates.
However, o u r arguments depend i n
We b e l i e v e t h e s e s e v e r a l d i s c i p l i n e s and
techniques r e i n f o r c e o u r h y p o t h e s i s o f s i g n i f i c a n t e o l i a n a c t i v i t y i n t h e Holocene. We conclude t h a t t h e Holocene has been a v e r y dynamic p e r i o d p u n c t u a t e d by p e r i o d s o f r a p i d e o l i a n s e d i m e n t a t i o n and a b r u p t e n v i r o n m e n t a l changes. We w i l l a t t e m p t t o draw t o g e t h e r v a r i o u s d i s c i p l i n e s i n d i s c u s s i n g t h e d i f f e r <
38 0
I
----WYOMING
II
-I
,
I I I t
I-
Killpecker
"\\
Ferris Dunes
I
I
I '--- _ _-R[
I
0 "
N o Dunes rth P
i
I
\ I
\
p
j
__-_---\
I
I I
I I
i
G r e a l Sand Dunes
I
DUNE FIELD
*
a
PRIMARY DIRECTION OF S A N D T R A N S P O R T
0
I00 M I L E S
I
0
J
100 K I L O M E T E R S
Figure 1. G e n e r a l i z e d d i s t r i b u t i o n and primary d i r e c t i o n of sand t r a n s p o r t of l a t e P l e i s t o c e n e and Holocene e o l i a n sands i n Kansas, Nebraska, South Dakota, Colorado, and Wyoming. Dune f i e l d s c o n s i d e r e d i n t h i s s t u d y a r e named. phases o f e o l i h n a c t i v i t y and w i l l f o c u s i n d e t a i l on t h e f o l l o w i n g dune f i e l d s : K i l l p e c k e r Dunes, Wyoming; t h e North Park Dunes and G r e a t Sand Dunes, Colorado and the Nebraska Sand H i l l s .
Other dune f i e l d s i n t h e a r e a t h a t have been studied
i n c l u d e t h e Casper Dunes, Wyoming ( A l b a n e s e , 1 9 7 4 ) , the F e r r i s Dunes, Wyoming ( G a y l o r d , 1 9 7 9 ) , dunes i n n o r t h e a s t e r n Colorado and s o u t h w e s t e r n Kansas (Muhs and Madole, 1980) and dunes i n s o u t h w e s t e r n and c e n t r a l Kansas (Smith, 1940; Frye and Leonard, 1 9 5 2 ) . Most o f these dune f i e l d s r e s u l t e d from winds blowing
Thlrs, dome, b a r c h a n , bardhanoid r i d g e , t r a n s v e r s e , p a r a b o l i c ' a n d blowout t y p e s a r e dominant i n most. En echelon barchan dunes p r e s e r v e d i n a presumed t r a n s i t i ' o n a l s t a g e t o l i n e a r dunes o c c u r i n t h e
p r i n c i p a l l y from one d i r e c t i o n .
Nebraska Sand Hi'lls ( A h l b r a n d t and F r y b e r g e r , 1 9 8 0 ) . G r e a t Sand Dunes c o n t a i n s t h e most compl'ex. dune form$' a d d , a s d i s c u s s e d by Andrews ( 1 9 8 1 ) , t h e r e v e r s i n g dunes and s t a r dunes r e f l e c t ?opograpt?ic b a r r i e r s and n o t changes i n c l i m a t e . I
I
381 Most o f t h e dune f i e l d s shown o n F i g u r e 1 have been s t a b i l i z e d .
Portions o f
K i l l p e c k e r , F e r r i s , N o r t h P a r k , and G r e a t Sand Dunes a r e p r e s e n t l y a c t i v e and a l l a r e s u r r o u n d e d b y a l a r g e r e g i o n o f d o r m a n t dunes. WISCONSIN AND HOL0,CENE ENVIRONMENTS We w i l l r e v i e w b r i e f l y some s a l i e n t f e a t u r e s o f t h e l a t e P l e i s t o c e n e ( W i s c o n s i n ) and Holocene i n t h e g e n e r a l a r e a o f t h i s s t u d y ( F i g . 2 ) . Late Wisconsin g l a c i a t i o n ,
i n t h e f o r m o f a c o n t i n e n t a l i c e s h e e t and as a l p i n e
g l a c i e r s i n t h e Rocky M o u n t a i n s ( F l i n t , ,1971), was a s s o c i a t e d w i t h a c o n s i d e r a b l y cooler c l i m a t e than t h e present.
Late Wisconsin p e r i g l a c i a l features (patterned
ground, p e r m a f r o s t sand-wedge r e l i c s a n d / o r i c e wedge c a s t s ) i n Wyoming have been documented by Flears ( 1 9 8 1 ) .
T h e i r l o c a t i o n s a r e shown i n Fi,gure 2 .
Fauna and f l o r a o f t h e l a t e W i s c o n s i n a l s o p r o v i d e e v i d e n c e o f c o l d e r o r a t l e a s t considerably c o o l e r c o n d i t i o n s than a t present i n t h e , n o r t h e r n Great Plains
DUNE
‘ f
P E R I G L A C I A L WEDGES
FIELD
W
‘c;tE
~
~
S P R U C E ( B 0 RE A L ) F O R E S T l ~ ~
E O R E O M O N TA N E
*
FAUNAS
-
100 M I L E S
0
0
100 K I L O M E T E R S
w
RANGIFER TARANDUS L O C A L I T Y
F i g u r e 2. L a t e P l e i s t o c e n e c o o l o r c o l d c l i m a t e i n d i c a t o r s i n n o r t h - c e n t r a l G r e a t P l a i n s . S i t e l o c a l i t i e s : Ma, M a d e l i a ; WO, West Lake O k o b o j i ; Rb, Rosebud; C R , C r e i g h t o n Road (Kx-128); L i , L i t c h f i e l d (Sin-102); Sc, S c h a f f e r t S i t e ( F T - 1 0 7 ) ; RW, Red W i l l o w (RW-101); MA, M u s k o t a h / A r r i n g t o n ; Ts, T r a p s h o o t ; BS, Boney S p r i n g .
382 dnd Rocky l l o u n t a i n b a s i n s .
The L i t c h f i e l d ( C o r n e r , 1982) and t h e Trapshoot
( S t e w a r t , 1978) s i t e s ( F i g . 2 ) have produced boreomontane faunas t h a t i n c l u d e Thomomys t a l p o i d e s ( n o r t h e r n p o c k e t gopher), Clethrionomys g a p p e r i (red-backed v o l e ) and Phenacomys i n t e r m e d i u s (montane h e a t h e r v o l e ) .
Corner (1977 and pers.
comm., 1982) r e p o r t e d l a t e Wisconsin occurrences o f R a n g i f e r tarandus (woodland c a r i b o u ) from Nebraska ( F i g . 2 ) and n o t e d t h a t t h e h a b i t a t o f t h e modern caribou i s i n a l p i n e t u n d r a and b o r e a l c o n i f e r o u s - f o r e s t e d r e g i o n s .
I n a d d i t i o n t h e Red
W i l l o w s i t e i n s o u t h e r n Nebraska (RW, F i g . 2 ) has produced t h e southernmost occurrence i n N o r t h America of Ovibos moschatus (muskox).
Anderson (1974) concluded
t h a t b o r e a l mammals expanded t h e i r ranges away f r o m t h e g l a c i a t e d Rocky Mountains d u r i n g t h e l a t e P l e i s t o c e n e i n Wyoming.
Hoffman and Jones (1970) p r e s e n t e d evidenc
t h a t a b o r e a l woodland b i o t a o r a c o l d l o e s s steppe and t u n d r a occupied Nebraska, Wyoming, and South Dakota d u r i n g t h e l a t e Wisconsin. S i m i l a r l y , t h e l a t e Wisconsin f l o r a l evidence suggests t h a t b o r e a l spruce f o r e s t s c o v e r e d much o f t h e c e n t r a l and n o r t h e r n G r e a t P l a i n s ( F i g . 2 ) j u s t p r i o r t o r e t r e a t o f t h e Wisconsin g l a c i e r s .
As summarized by Bradbury (1980), evidence
f o r s p r u c e f o r e s t s e x i s t s a t M a d e l i a i n s o u t h e r n Minnesota (Jelgersma, 1962); a t Rosebud i n s o u t h e r n South Dakota (Watts and W r i g h t , 1966); a t Muskotah/Arrington i n n o r t h e a s t e r n Kansas ( G r i g e r , 1973); a t Boney S p r i n g i n western M i s s o u r i (King and L i n d s a y , 1976); and a t West Lake Okoboji i n n o r t h w e s t e r n Iowa (Van Zant, 1976). W r i g h t (1970, p. 158) concluded t h a t a b o r e a l spruce f o r e s t dominated t h e landscape t h r o u g h o u t most o f t h e e a s t e r n edge o f t h e C e n t r a l P l a i n s f r o m about 24,000 t o a b o u t 12,000 y e a r s ago.
U n f o r t u n a t e l y no d a t a on f l o r a f r o m l a t e Wisconsin
s i t e s i n Nebraska o r Kansas west o f t h e M u s k o t a h / A r r i n g t o n area have been published
J. D. Steward ( p e r s . comm., 1982) r e p o r t e d t h a t a p r e l i m i n a r y p o l l e n p r o f i l e i n l a t e Wisconsin l o e s s n e a r t h e Trapshoot s i t e ( F i g . 2 ) c o n t a i n e d o n l y a small percentage o f a r b o r e a l p o l l e n c o n s i s t i n g e n t i r e l y o f p i n e .
He suggested t h a t the
combined f l o r a l and f a u n a l e v i d e n c e argued f o r an open p i n e p a r k l a n d environment i n n o r t h - c e n t r a l Kansas d u r i n g t h e l a t e Wisconsin.
W r i g h t (1970, p. 164) hypothe-
s i z e d t h a t a " t r e e l e s s landscape developed i n t h e w e s t e r n t w o - t h i r d s o f Nebraska" d u r i n g t h e Wisconsin and t h a t a d e c l i n e i n p e r i g l a c i a l winds and s t a b i l i z a t i o n o f t h e Sand H i l l s a l l o w e d t h e s p r u c e f o r e s t t o spread r a p i d l y westward across t h e Sand H i l l s and appear a t t h e Rosebud s i t e ( F i g . 2 ) some 13,000 y r s . B.P.
These
hypotheses i m p l y t h a t dunes o f t h e Nebraska S a n d h i l l s ( F i g . 1 ) were formed under p e r i g l a c i a l c o n d i t i o n s d u r i n g Wisconsin g l a c i a t i o n .
However, as we w i l l attempt
t o show i n t h i s r e p o r t , s i g n i f i c a n t sand dune f o r m a t i o n i n t h e Nebraska Sand H i l l s d u r i n g t h e l a t e Wisconsin has n o t been documented.
A t t h e end o f t h e P l e i s t o c e n e (10,000-12,000
y r s . B.P.)
c o n t i n e n t a l i c e began
r e t r e a t i n g n o r t h e a s t w a r d i n response t o a r a p i d c l i m a t i c change (Bryson e t a l . , 1969).
E x t i n c t i o n o f many P l e i s t o c e n e mammals o c c u r r e d w i t h b i s o n and man
d o m i n a t i n g t h e new environment (Wilson, 1974; F r i s o n e t a l . ,
1974).
Concurrently,
383 t h e f l o r a changed f r o m a b o r e a l asseiiiblage t o a m o s a i c o f p i n e and s t e p p e v e g e t a t i o n i n t h e C e n t r a l G r e a t P l a i n s and t o a t e m p e r a t e d e c i d u o u s f o r e s t i n t h e e a s t e r n p a r t o f t h e P l a i n s ( W r i g h t , 1976; B r a d b u r y , 1 9 8 0 ) . P r i m a r i l y o n t h e b a s i s o f p o l l e n a n a l y s e s , t h e H o l o c e n e c l i m a t e sequence t r a d i t i o n a l l y has been v i e w e d as a r a t h e r smooth and g e n t l e c u r v e o f warming a n d / o r d r y i n g w i t h a c u l m i n a t i o n i n t h e mid-Holocene o f t h e Hypsithermal ( A l t i t h e r m a l ) i n t e r v a l (Wright, trend.
1970 and 1976) and f o l l o w e d b y a g r a d u a l c o o l i n g and m o i s t e r
However, B r y s o n e t a l . ( 1 9 7 0 ) p r o p o s e d a n e p i s o d i c model w i t h more r a p i d
and s t e p l i k e c l i m a t i c changes b o u n d i n g a s e r i e s o f d i f f e r e n t c l i m a t i c e p i s o d e s w i t h i n t h e Holocene.
M o s t a u t h o r s , Haynes ( 1 9 6 5 ) among them, r e s t r i c t t h e m i d -
Holocene A1 t i t h e r m a l o r H y p s i t h e r n i a l t o a n i n t e r v a l o f f r o m p e r h a p s 7,500 y r s .
B.P.
u n t i l 5,000 y r s . B.P.
W r i g h t (1976) reviewed t h e h i s t o r y o f t h i s concept
and o f i t s i n d e f i n i t e t i m e b o u n d a r i e s . A l t i t h e r m a l ( c a . 7,000-6,500
Benedict (1979) p o s t u l a t e d a two-drought
B.P. a n d 6,000-5,000
B.P.) u s i n g human p o p u l a t i o n
f l u c t u a t i o n s i n w e s t e r n N o r t h A m e r i c a as h i s b a s i s .
T h i s mid-Holocene a r i d episode
was marked b y a p r o n o u n c e d d w a r f i n g o f b i s o n ( W i l s o n , 1974) and, a c c o r d i n g t o W r i g h t (1970, p . 1 7 0 ) f l o r a l d a t a ( m o s t l y p r a i r i e v e g e t a t i o n ) a l s o r e f l e c t a x e r i c environment.
A p p a r e n t l y , c o n d i t i o n s m o d e r a t e d a b o u t 4,000 y r s . B.P.,
v e g e t a t i o n became e s t a b l i s h e d i n t h e G r e a t P l a i n s ( B r a d b u r y , 1 9 8 0 ) .
when modern Moderation
o f t h e mid-Holocene A l t i t h e r i n a l i n t e r v a l p o s s i b l y r e f l e c t e d t h e onset o f Neoglacia t i o n i n t h e Rocky M o u n t a i n s ( B e n e d i c t , 1 9 7 3 ) . HOLOCENE EOLIAN ACTIVITY--ROCKY MOUNTAIN BASINS Phase I ( E a r l y H o l o c e n e ) P l o d e r a t i o n o f t h e l a t e P l e i s t o c e n e c l i m a t e i n t h e Rocky M o u n t a i n b a s i n s was r e f l e c t e d b y t h e o n s e t o f t h e f i r s t p e r i o d o f dune f i e l d d e v e l o p m e n t d u r i n g t h e e a r l y Holocene ( F i g . 3 A ) .
Age d e t e r m i n a t i o n f o r t h e Phase I a c t i v i t y comes f r o m
a r c h e o l o g i c and t r e n c h s i t e s i n d o r m a n t dunes w i t h i n t h e K i l l p e c k e r and Casper dune f i e l d s i n Wyoming where e o l i a n sands o v e r l i e l a t e P l e i s t o c e n e a l l u v i a l f i l l s , and f r o m p r e l i n i i n a r y w o r k o n t h e G r e a t Sand Dunes i n C o l o r a d o .
The F i n l e y a r c h e o -
l o g i c s i t e o n t h e w e s t e r n m a r g i n o f t h e K i l l p e c k e r Dunes has been i n v e s t i g a t e d s e v e r a l t i m e s s i n c e 1941. by Howard and Hack ( 1 9 4 3 ) .
I t was f i r s t d e s c r i b e d by Howard e t a l .
(1941), then
S u b s e q u e n t l y many a s p e c t s o f t h e s i t e have been s t u d i e d
i n d e t a i l by p a l y n o l o g i s t s , v e r t e b r a t e p a l e o n t o l o g i s t s , a r c h e o l o g i s t s , and g e o l o g i s t (Hoss e t a l . ,
1951).
I t was t r e n c h e d a g a i n i n t h e e a r l y 1 9 7 0 ' s t o r e e v a l u a t e
c o n c e p t s and o b t a i n r a d i o c a r b o n d a t e s ( T a b l e 1 and A h l b r a n d t ,
1974).
The s t r a t i -
g r a p h i c s u c c e s s i o n t h e r e i s shown i n F i g u r e 4 . P a l e o - I n d i a n s u s e d Phase I p a r a b o l i c o r b l o w o u t dunes i n b o t h t h e K i l l p e c k e r and t h e Casper dune f i e l d s as n a t u r a l c o r r a l s f o r b i s o n k i l l s ( A h l b r a n d t , 1974; Albanese, 1 9 7 4 ) .
A l b a n e s e ( 1 9 7 4 ) d e s c r i b e d s k e l e t a l r e n i a i n s o f 74 b i s o n ( B i s o n
a n t i q u u s ) a l i g n e d i n a " w i n d r o w " a l o n g t h e a x i s o f a n o l d " b l o w o u t " dune.
All
384
f
>
ROCKY M 0 UNTA IN BASINS
c
L
3
a
A
I
9
>
c -
I II
> c
0
a W
Z 3
a
B THOUSAND Y E A R S 6 . P t
- - -Holocene
>
F i g u r e 3 . H o l o c e n e e o l i a n a c t i v i t y i n ( A ) Rocky Plountain b a s i n s o f C o l o r a d o and Wyoming and in (B) Nebraska Sand H i l l s .
TABLE 1.
RADIOCARBON DATES FROM THE KILLPECKER DUNE FIELD, WYOMING
Site
Legal l o c a t i o n
Depth below s u r f a c e (cm)
Radiocarbon age (yrs.BP) and l a b . no.
M a t e r i a l dated Wood (sage) emerging f r o m windward s l o p e o f dune
Sage Wood
SEL,SEL,sec. 15, T. 24N, R. 104W.
0- 10
2202 90 ( 1 - 6 4 8 7 ) l
B i s o n Sand
SEL,SWL,sec.30, T.24N,R. 103W.
30.5
7755 90 (1-6320)1
Upper Sand
Center sec. 1 T.23N,R. 105W.
91
28202 95 (I-8319)~
M a r l and r o o t t u b u l e s i n Upper Sand
Finley
SWk,SW%,sec. 19, T.24N,R.l05W.
122
58455115 (1-6486)2
Marl and r o o t t u b u l e s above M i d d l e Sand
B i s o n bone and charcoal i n Upper Sand
lWood and charcoal samples p r e t r e a t e d t o remove carbonates and humic a c i d s - Teledyne Labs, Westwood, N.J. 2 T r e a t e d w i t h a c i d s o l u t i o n u n t i l 62%, by w e i g h t , o f r o o t - t u b e samples removed, i n n e r 38% then d a t e d - Teledyne Labs, Westwood, N.J.
386 o t h e r s k e l e t a l remains recovered a t t h e s i t e a r e o f animals l i k e those p r e s e n t l y l i v i n g i n t h e area.
R a d i o c a r b o n d a t e s o f 9,830+350 y r s . B.P.
(RL-125) o n c h a r -
c o a l and 10,670+170 y r s . B.P. (RL-208) o n bone i n d i c a t e a n age o f a b o u t 10,000 y r s . B.P. a t t h e Casper s i t e ( A l b a n e s e , 1 9 7 4 ) .
Eolian deposits a t t h i s s i t e overlie
an a l l u v i a l fill c o n s i d e r e d t o b e l a t e P l e i s t o c e n e age b y A l b a n e s e and W i l s o n (1974, p. 1 3 ) . The F i n l e y s i t e a l s o c o n t a i n e d numerous b i s o n bones t h a t o r i g i n a l l y were i d e n t i -
Bison
f i e d as B i s o n o c c i d e n t a l i s b u t a c c o r d i n g t o W i l s o n ( 1 9 7 4 ) c l e a r l y r e s e m b l e a n t i q u u s and a r e H o l o c e n e f o r m s .
The e a r l i e s t c u l t u r a l components f o u n d i n p l a c e
i n t h e K i l l p e c k e r Dunes was a t t h e F i n l e y s i t e ( F i g u r e 4 ) and i n c l u d e s Eden and S c o t t s b l u f f p o i n t s o f t h e p a l e o - I n d i a n Cody Complex a s s o c i a t e d w i t h f r a g m e n t s o f d e c a l c i f i e d b i s o n bone.
F r i s o n e t a l . ( 1 9 7 4 ) e s t a b l i s h e d a t y p o l o g i c a l sequence
o f a r t i f a c t s i n t h e H o l o c e n e o f Wyoming.
U t i l i z i n g t h i s concept o f stratigraphic
a r c h e o l o g y , where numerous s i t e s i n Wyoming i n t e g r a t e s p e c i f i c a r t i f a c t s d i r e c t l y w i t h a d j a c e n t r a d i o c a r b o n d a t e s as shown b y F r i s o n e t a l . ( 1 9 7 4 ) , we c a n conclude t h a t t h e Eden a n d S c o t t s b l u f f p o i n t s a t t h e F i n l e y s i t e t y p o l o g i c a l l y d a t e t o a p e r i o d between 7,000 and 10,000 y r s . B.P.
A r a d i o c a r b o n d a t e o f 5,845k115 y r s .
B.P. was o b t a i n e d f r o m m a r l j u s t above a h o r i z o n c o n t a i n i n g t h e s e a r t i f a c t s n o r t h o f t h e F i n l e y s i t e ( F i g . 4 and T a b l e 1 ) .
F r i s o n ( p e r s . comm.,
1982) r e p o r t e d
t h a t paleo-man a r t i f a c t s were f o u n d i n p l a c e i n t h e d o r m a n t dunes a t G r e a t Sand Dunes, C o l o r a d o . I t i s n o t e w o r t h y t h a t f o s s i l b i s o n r e m a i n s a t t h e s e k i l l s i t e s were w h o l l y of
e a r l y H o l o c e n e age a n d y i e l d e d no e x t i n c t P l e i s t o c e n e t a x a .
The p a l e o - I n d i a n
c u l t u r e s r e s p o n s i b l e f o r t h e k i l l s i t e s a l s o w e r e e a r l y H o l o c e n e as c o n f i r m e d by s t r a t i g r a p h i c , a r c h e o l o g i c and r a d i o c a r b o n d a t i n g m e t h o d s . The Lower, M i d d l e , a n d Upper Sands a t K i l l p e c k e r ( d o r m a n t dune s t r a t i g r a p h y , F i g . 4 ) may r e f l e c t l a t e P l e i s t o c e n e a n d H o l o c e n e c l i m a t i c changes.
The Lower
Sand i s s t r u c t u r e l e s s and d a t a f r o m t r e n c h e s a n d b o r e h o l e s i n d i c a t e d i t i s r e s t r i c t e d t o t h e w e s t e r n ( u p w i n d ) m a r g i n o f t h e dune f i e l d . o n g r a v e l s o f t h e Upper F a r s o n a l l u v i a l f i l l .
The Lower Sand r e s t s
The t e r r a c e a s s o c i a t e d w i t h t h i s
f i l l was c o n s i d e r e d t o b e a l a t e P l e i s t o c e n e t e r r a c e ( W i s c o n s i n - P i n e d a l e ) and Moss (1955, p. 6 4 4 ) .
by Holmes
The Lower Sand a p p e a r s t o have been d e p o s i t e d i n a
r e d u c i n g e n v i r o n m e n t as e v i d e n c e d b y i t s p a l e g r e e n c o l o r , w h i c h may r e f l e c t a colder climate i n the l a t e Pleistocene. f o r t h e Lower Sand i s n o t known.
However, t h e d e p o s i t i o n a l e n v i r o n m e n t
The M i d d l e Sand c o n t a i n s t h e o l d e s t d a t e d material
a t K i l l p e c k e r and r e c o r d s Phase I dune a c t i v i t y .
I t i s s e p a r a t e d f r o m t h e Lower
Sand b y a p a l e o s o l and a zone o f c a l c i u m c a r b o n a t e n o d u l e s . Phase I 1 ( M i d d l e H o l o c e n e ) A b r i e f p l u v i a l p e r i o d a p p e a r s t o have o c c u r r e d between Phase I and Phase I 1 activity.
As u s e d h e r e , p l u v i a l i n d i c a t e s a p e r i o d o f g r e a t e r e f f e c t i v e m o i s t u r e
387 and may b e due t o d e c r e a s e d e v a p o r a t i o n , i n c r e a s e d p r e c i p i t a t i o n , o r b o t h .
Phase
I 1 dune a c t i v i t y seems t o c o i n c i d e w i t h t h e commencement o f t h e a r i d A l t i t h e r m a l p e r i o d a b o u t 7,500 y r s . B.P.
As d i s c u s s e d p r e v i o u s l y , t h e A l t i t h e r m a l i s a s s o c i -
a t e d w i t h s e v e r e a r i d i t y s u f f i c i e n t t o c a u s e d w a r f i n g o f b i s o n , m i g r a t i o n o f human c u l t u r e s and b i s o n and t r a n s i t i o n o f f l o r a t o a p r a i r i e v e g e t a t i o n ( g r a s s e s ) a t t h e expense o f e a r l i e r t h e r m o p h i l o u s f o r e s t s o f d e c i d u o u s t r e e s and p i n e w h i c h d o m i n a t e d t h e e n v i r o n m e n t u n t i l a b o u t 8,000 y r s . B.P.
The x e r i c e n v i r o n m e n t d u r i n g
t h e A l t i t h e r m a l p e r m i t t e d m a j o r dune sand movement f u r t h e r i n t e n s i f i e d by i n c r e a s e d number and d u r a t i o n o f summer d r o u g h t s d u r i n g t h e A l t i t h e r m a l as n o t e d by W r i g h t (1968). Phase I 1 a c t i v i t y seems t o have been t i e d more c l o s e l y t o t h e A l t i t h e r m a l i n t h e i n t e r m o n t a n e dune f i e l d s , t h a n o n t h e p l a i n s and i t d o m i n a t e s b o t h t h e K i l l p e c k e r and F e r r i s dunes i n Wyoming.
A m a j o r h i a t u s o f p e r h a p s 2,500 y r s . d u r a t i o n
o c c u r s between t h e M i d d l e Sand and Upper Sand o n t h e u p w i n d end o f t h e K i l l p e c k e r dunes.
As d i s c u s s e d b y A h l b r a n d t ( 1 9 7 4 ) , t h i s m a j o r h i a t u s p r e d a t e s a b a s a l c a l -
C r e t e i n t h e Upper Sand d a t e d t o 5,845+115 y r s . B . P . a n d p o s t d a t e s M i d d l e Sand a r t i f a c t s t y p o l o g i c a l l y c o r r e l a t i v e t o 7,000 t o 10,000 y r s . B.P. a t t h e F i n l e y s i t e and shown i n c o l u m n a r f o r m i n F i g u r e 4.
The m a j o r e r o s i o n a l d i a s t e m on t h e
upwind end o f t h e K i l l p e c k e r Dunes m u s t be r e f l e c t e d b y m a j o r downwind d e p o s i t i o n d u r i n g t h i s phase.
The F e r r i s Dunes a r e a downwind ( e a s t e r l y ) e x t e n s i o n o f t h e
e a s t e r l y t a i l o f t h e K i l l p e c k e r Dunes.
As shown i n F i g u r e 4 , p a s t and p r e s e n t
sand t r a n s p o r t i n t h e K i l l p e c k e r Dunes i s f r o m w e s t t o e a s t a l o n g w h a t Kolm ( 1 9 7 4 ) c a l l s t h e Wyoming w i n d c o r r i d o r . and C14
Gaylord (1979) s t a t e s t h a t " t h e s t r a t i g r a p h y
d a t e s i n d i c a t e t h a t m o s t o f t h e F e r r i s dune d e p o s i t i o n o c c u r r e d u n d e r r e l -
a t i v e l y d r y c l i m a t i c c o n d i t i o n s between 7,660 and 6,460 y r s . B.P."
He s t a t e s
f u r t h e r t h a t e o l i a n d e p o s i t i o n y o u n g e r t h a n 6,460 y r s . B.P. has a g r e a t e r number and f r e q u e n c y o f i n t e r d u n a l pond l a y e r s s u g g e s t i n g t h a t t h e c l i m a t e has i n g e n e r a l ( w i t h m i n o r f l u c t u a t i o n s ) been l e s s a r i d s i n c e t h i s m i d - H o l o c e n e t i m e . Phase I11 ( L a t e H o l o c e n e ) The t h i r d phase o f H o l o c e n e e o l i a n a c t i v i t y i n i n t e r m o n t a n e b a s i n s p o s t d a t e s t h e T r i p l e Lakes N e o g l a c i a l advance (5,000-3,000 (Benedict,
1973).
y r s . B . P . ) i n t h e Rocky Plountains
M o d e r a t i o n o f t h e x e r i c A l t i t h e r m a l p e r i o d i s marked by m a r l s
and r o o t t u b u l e s i n t h e K i l l p e c k e r Dunes Upper Sand w h i c h i s d a t e d a t 2,820595 y r s . B.P.
(Table 1).
However, s u c h m a r l s a r e l a t e r a l l y d i s c o n t i n u o u s w i t h i n a
dune f i e l d and we m u s t r e l y o n o n l y t h o s e dune f i e l d s t h a t came i n t o e x i s t e n c e d u r i n g Phase 111 t o g e t a more a c c u r a t e d e t e r m i n a t i o n o f t h e o n s e t o f Phase 111. The N o r t h P a r k Dunes i n C o l o r a d o o r i g i n a t e d a t t h i s t i m e .
As n o t e d i n F i g u r e 5
and T a b l e 2, a l l u v i a l m a t e r i a l exposed a l o n g N o r t h Sand Creek and E a s t Sand Creek contains radiocarbon d a t a b l e m a t e r i a l i n t h e form o f peats which predate t h e d o r m a n t dunes.
Dune movement a f t e r 2,000 y r s . B.P. b u t b e f o r e 1,000 y r s . B.P.
388 ______ 0'55'
.
106O15
106910'
106'05'
Tllh TlOh
TlOl
Figure 5. Map of North Park dune f i e l d showing a c t i v e and dormant eolian sand a n d l o c a t i o n o f radiocarbon dated samples. I n s e r t shows s t r a t i g r a p h y a t sampled s i t e s . Map modified from Ahlbrandt and Andrews (1978, Fig. 2 ) . i s demonstrated both by radiocarbon dates of material beneath the dunes a n d by material buried within the dormant dunes. The East Sand Creek s e c t i o n contains a t h i c k ( 9 1 cm) peat bog with well preserved twigs dated 2,100+200 y r s . B . P . a t a depth of 1,006 cm below the surface.
Two radiocarbon dates were made on material
from t h e North Sand Creek section (Fig. 5 ) ; a 2,830+200 y r s . B.P. date f o r a 15 cm peat horizon 686 cm below the s u r f a c e and a 1,070+200 y r s . B . P . date f o r a very t h i n ( 2 . 5 cm) peat l a y e r 610 cm below the surface. Possibly the young date f o r the t h i n peat l a y e r may r e f l e c t contamination, s i n c e a radiocarbon date on
389 TABLE 2.
RADIOCARBON DATES FROM NORTH PARK DUNES, COLORADO Depth below s u r f a c e (cm)
Radiocarbon age (yrs.BP) and l a b . no.
1006
2110t200 (W-3656)I
Peat Bog ( 9 1 cm t h i c k ) w i t h s t i c k s i n place (predune a l l u v i u m )
NEk,NEk,sec.l2, T. 10N, R. 79W.
686
2830+200 (W-3655)I
Peat ( 1 5 cm t h i c k ) (predune a1 1 u v i urn)
N o r t h Sand Creek
NEk,NEk,sec.lZ, T.lON,R.79W.
6 10
1070+200 (W-3644)'
T h i n p e a t (2.5 cm t h i c k ) (predune a1 1u v i um)
N o r t h Sand
SWk,NWk,sec.6, T.lON,R.78W.
0- 10
1250+200 (W-3653)'
Charcoal and wood f r o m t r e e stump emerging f r o m barchan dune
Site
Legal l o c a t i o n
East Sand Creek
NW'*,NWk,sec. 12, T.9N,R. 78W.
N o r t h Sand Creek
Hills
'Meyer Rubin,
Material dated
U.S. G e o l o g i c a l Survey Radiocarbon L a b o r a t o r y , Reston, Va.
c h a r c o a l and wood from a t r e e b u r i e d b y d o r m a n t dunes was 1,250i-200 y r s . B.P. (Table 2 ) .
Dune movement between 2,000 and 1,000 y r s . B.P. r e f l e c t s t h e fitkt
p u l s e o f Phase 111.
A second p u l s e o f Phase I 1 1 seems t o have o c c u r r e d between
500 a n d 1 , 0 0 0 y r s . B.P. i n t h e K i l l p e c k e r Dunes, where dune sand o v e r l i e s bone and c h a r c o a l d a t e d a t 7 5 5 i 9 0 y r s . B.P.
( T a b l e 1 ) . T h i s second p u l s e of Phase I 1 1
a c t i v i t y a p p e a r s t o p o s t d a t e B e n e d i c t ' s ( 1 9 7 3 ) Audubon ( N e o g l a c i a l ) advance (1,850-
950 y r s . B . P . ) . Phase I V ( R e c e n t ) As we s t a t e d p r e v i o u s l y , s e v e r a l dunes f i e l d s - i n c l u d i n g t h e K i l l p e c k e r , F e r r i s , N o r t h P a r k , and G r e a t Sand Dunes, c o n t a i n a r e a s o f p r e s e n t l y a c t i v e dunes.
How-
e v e r , t h e a c t i v e dune a r e a s a r e s u r r o u n d e d b y much l a r g e r a r e a s o f d o r m a n t dunes, as i l l u s t r a t e d f o r K i l l p e c k e r Dunes i n F i g u r e 4 and N o r t h P a r k Dunes i n F i g u r e 5 . The o n l y e v i d e n c e f o r t h e o n s e t o f Phase I V i s t h e d a t e o f 22Oi90 y r s . B.P. f o r dead sage ( A r t e m i s i a ) e m e r g i n g f r o m t h e t r a i l i n g edge o f a c t i v e dunes ( F i g . 4 and T h i s d a t e i n d i c a t e s t h a t Phase I V a c t i v i t y e x t e n d s back a t l e a s t two
T a b l e 1).
hundred y e a r s . HOLOCENE EOLIAN ACTIVITY--NEBRASKA SAND H I L L S The Sand H i l l s o f c e n t r a l Nebraska ( F i g . 1 ) have a n a r e a o f a p p r o x i m a t e l y
2
57,000 km a n d c o n s t i t u t e t h e l a r g e s t sand sea i n t h e Western Hemisphere (Smith, 1965, p . 5 5 7 ) . (e.g.
A v a r i e t y o f c h r o n o l o g i e s f o r dune a c t i v i t y have been p r o p o s e d
Lugn, 1935, p . 161; S m i t h , 1965, p . 573; Reed a n d Dreeszen, 1965, p. 4 ) .
These s h a r e a common theme, t h a t t h e f o r m a t i o n o f t h e l a r g e - s c a l e dune f o r m s comp r i s i n g m o s t o f t h e Sand H i l l s o c c u r r e d d u r i n g t h e l a t e P l e i s t o c e n e .
Another e l e -
ment common t o t h e s e c h r o n o l o g i e s i s t h e l a c k o f d i r e c t e v i d e n c e f r o m w e l l - d a t e d
'
390 l o c a l i t i e s documenting the s t r a t i g r a p h i c r e l a t i o n s h i p s between eolian sand and o l d e r o r younger deposits. For example, Lugn (1935, p . 161) c o r r e l a t e d the majori t y of the Nebraska Sand H i l l s with l a t e Wisconsin Peoria loess occurring e a s t and south of the sand sea. This c o r r e l a t i o n was based primarily on a reported decrease in sand content and thickness of the loess with increasing distance from the margin of the Sand H i l l s . Lugn (1968, p . 161) a l s o reported t h a t the loess and dune sand a r e "somewhat interbedded" in the loess-Sand H i l l s t r a n s i t i o n zone. However, our f i e l d work and t h a t of Ahlbrandt a n d Fryberger (1980) could not subs t a n t i a t e any interbedding. Ahlbrandt a n d Fryberger (1980, p . 2 2 ) concluded t h a t the dune sand unconformably o v e r l i e s t h e loess a t the southeast edge of the Sand H i l l s . They i n t e r p r e t e d sands interbedded in the loess sequence t o have been deposited in a f l u v i a l , n o t e o l i a n , environment. I n a study of the e n t i r e Nebraska Sand H i l l s , Smith (1965) discerned two main periods of eolian a c t i v i t y . The f i r s t was provisionally thought t o have occurred in the e a r l y Wisconsin and formed the l a r g e transverse dunes ( S e r i e s I dunes). He suggested t h a t a s i g n i f i c a n t s h i f t in wind regime occurred in the l a t e Wisconsin and t h a t a l e s s intense period of eolian a c t i v i t y produced smaller longitudinal dunes (Series 1 1 ) , which a r e superimposed on the o l d e r and l a r g e r dunes in many places. Smith based h i s conclusion of a change in wind regime on external dune morphology. Ahlbrandt and Fryberger (1980) examined internal s t r u c t u r e s in dunes c l a s s i f i e d by Smith as longitudinal ( S e r i e s 11) dunes a n d found them t o be transverse dunes. They suggested t h a t only one major episode of dune formation, dominated by a s i n g l e wind regime, was necessary t o generate the sand sea. Warren (1976) a l s o argued t h a t b o t h large- a n d small-scale dunes could have been generated under one wind regime, although he accepted a l a t e Pleistocene age f o r the Sand H i l l s . Smith (1965) believed t h a t post-glacial e o l i a n a c t i v i t y was imi ted t o s u p e r f i c i a l modification of the o l d e r dunes. Previous s t u d i e s by Sears (1961) and Watts a n d Wright (1966) have reported radiocarbon dates of 5,040k95 and 12,600?160 y r s . B . P . ( F i g . 6 ) f o r organ c material recovered from i s o l a t e d cores beneath Sand H i l l s lakes and interdune v a l l e y s . A core taken a t Krause Lake (Fig. 6 ) yielded a date of 12,080i380 y r s . B.P. a t 275280 cm and 3,140t-187 y r s . B . P . a t 120-130 cm (Odgen a n d Hay, 1965, p . 168). I n a d d i t i o n , Stuvier (1969, p . 578) reported a date of 8,950k160 y r s . B.P. on material cored between 1,472-1,482 cm below Swan Lake (Fig. 6 ) . However, a t none of these s i t e s was the s t r a t i g r a p h i c r e l a t i o n s h i p of the dated horizon t o nearby dune sand documented. Before such s i t e s can provide unequivocal data f o r e s t a b l i s h i n g an eolian chronology, t h a t r e l a t i o n s h i p must be e s t a b l i s h e d . This will require a s e r i e s of cores or t e s t holes extending from the interdunes well i n t o the dunes themselves. Until such work i s done, radiocarbon-dated sediments cannot be assumed t o postdate the surrounding dunes simply by t h e i r geographic position or by continuity of sediment within the cores. We conclude, as did F l i n t (1976, p . 525),
391
0
20
40 ,
60KlLOMETERS
1
F i g u r e 6. Map s h o w i n g r a d i o c a r b o n ages, i n y r s . B . P . , o f samples f r o m t h e Nebraska Sand H i l l s . I f more t h a n one sample was d a t e d a t a g i v e n l o c a t i o n o n l y t h e y o u n g e s t age i s shown. S i t e s I - V I I a r e new l o c a l i t i e s d e s c r i b e d i n t h i s p a p e r . Dates shown i n b r a c k e t s a r e f r o m c o r e s i n i n t e r d u n e p o s i t i o n s and t h e i r s t r a t i g r a p h i c r e l a t i o n s h i p t o t h e dune sand has n o t been e s t a b l i s h e d . t h a t a r e a s o n a b l y c o r r e c t c h r o n o l o g y o f e o l i a n a c t i v i t y i n t h e Nebraska Sand H i l l s had n o t been e s t a b l i s h e d b y t h i s e a r l i e r w o r k . I n t h i s p a r t o f o u r s t u d y we w i l l a t t e m p t t o document t h e s t r a t i g r a p h i c p o s i t i o n o f r a d i o c a r b o n d a t e d o r g a n i c - r i c h h o r i z o n s w i t h r e s p e c t t o e o l i a n sand.
With
t h e s e w e l l - d a t e d l o c a l s t r a t i g r a p h i c sequences and a r e i n t e r p r e t a t i o n o f p r e v i o u s d a t e s , we hope t o make some p r o g r e s s t o w a r d s a more f i r m l y based c h r o n o l o g y o f e o l i a n a c t i v i t y i n t h e Nebraska Sand H i l l s such as t h a t shown i n F i g u r e 3B. E a r l y Holocene E a r l y H o l o c e n e e o l i a n a c t i v i t y , i f i t o c c u r r e d a t a l l , i n t h e Sand H i l l s i s p o o r l y documented.
S i t e V I ( F i g s . 6 and 7 ) a t t h e head o f W h i t e t a i l Creek i n t h e
s o u t h w e s t e r n p a r t o f t h e Sand H i l l s y i e l d e d a r a d i o c a r b o n d a t e o f 9,930+140 y r s .
B.P.
( T a b l e 3 ) f o r t h e b a s a l 3 cm o f a 2 . 1 m t h i c k o r g a n i c - r i c h sandy s i l t .
This
u n i t c o n t a i n s some p l a n t r e m a i n s a l o n g w i t h s c a t t e r e d g a s t r o p o d s and p e l e c y p o d s . R i c h a r d M a d o l e o f t h e USGS i n Denver, C o l o r a d o u s e d a m o d i f i e d K i h l t r e a t m e n t ( K i h l , 1975) t o e x t r a c t t h e o r g a n i c c a r b o n f r o m t h i s sample w h i c h c o n t a i n e d some modern r o o t l e t s .
Up t o 6 m o f e o l i a n sand d i r e c t l y o v e r l i e s t h e o r g a n i c - r i c h u n i t
and b a r c h a n dunes u p t o 40 m h i g h o c c u r w i t h i n 0 . 4 km o f t h e d a t e d h o r i z o n ( F i g . 7 ) . I t i s p o s s i b l e t h a t e o l i a n a c t i v i t y , c o r r e l a t i n g w i t h Phase I a c t i v i t y d e s c r i b e d
above f o r Wyoming, commenced s h o r t l y a f t e r d e p o s i t i o n o f t h e o r g a n i c - r i c h u n i t . However, w i t h o u t a d d i t i o n a l s t r a t i g r a p h i c i n f o r m a t i o n and r a d i o c a r b o n d a t i n g , l a t e
392
1020
5 150-f400 8410fllO 1000
980
f 980 a cn J""
-049
I Dismal River Ranch
(A)
II: Dismal River Ronch ( 8 )
IUWarner Bridge 1080
v,
n
W b-
1060
w
860
880
840
860
820
840
z W 0
3
1040
e
1020
l-
1
P Natick
l P C o l l i e r Ranch
1
a
1100
41: Whitetail Creek
AGE
Alluvial sond ond sllt containing organic-rich
1080
1060
Pliocene ( 7 )
Alluvial sond, silt and grovel 0 5 KILOMETER
YU Snake River
V e r t i c a l e xo g g e ro t io n x18
F i g u r e 7 . G e n e r a l i z e d g e o l o g i c s e c t i o n s a t Sand H i l l s s i t e s I - V I I . A l l u v i a l sand and s i l t u n i t c o n t a i n i n g o r g a n i c - r i c h zones may, i n p a r t , be o l d e r t h a n Holocene. Refer t o Table 3 f o r d e t a i l s on s p e c i f i c s i t e s . Wisconsin o r e a r l y Holocene a c t i v i t y i n t h i s area must remain s p e c u l a t i v e . W r i g h t , J r . ( p e r s . comm., y r s . B.P.
H. E .
1982) s u g g e s t e d t h a t t h e r a d i o c a r b o n d a t e o f 8,950+150
(Ogden and Hay, 1965, p . 1 6 8 ) f o r o r g a n i c - r i c h sand a t t h e base o f t h e
14 m c o r e f r o m Swan Lake ( F i g . 6 ) p o s t d a t e d e o l i a n a c t i v i t y o n t h e s o u t h w e s t e r n edge o f t h e Sand H i l l s .
The c o r e was composed e n t i r e l y o f homogeneous, f i n e - g r a i n e d
a l g a l s e d i m e n t h a v i n g no i n t e r b e d s o f sand.
I s o l a t e d b a r c h a n dunes o c c u r i n t h e
v i c i n i t y o f Swan Lake and may have f o r m e d p r i o r t o 8,950 y r s . B . P . Holocene phase o f e o l i a n a c t i v i t y .
However,
i n an e a r l y
i t i s a l s o p o s s i b l e t h a t t h e barchan
393 TABLE 3.
NEW RADIOCARBON DATES FROM THE NEBRASKA SAND HILLS
Site
Legal l o c a t i o n
I-Dismal R i v e r Ranch ( A )
SWL,SE%, sec. 35, T. 2IN, R. 33W.
11-Dismal R i v e r Ranch ( B )
SEh,NEL,sec.30, T.22N, R. 32W.
I II-Warner Bridge NWL, NEL, sec. 2 2 ,
Depth below e o l i a n sand (cm) 40- 45 40- 45 45- 55 345-360 10- 17 30- 45
0- 40
Radiocarbon age (yrs.BP) and l a b . no.
3000+400 3560T 70 3810T 80 4900T500
(NWU-83)4 (BETA-5429) (W-4900)5 (NWU-82)4
5150+400 (NWU-84): 8410+llO (W-4926) 3600+400 (NWU-85)4
1
Thickness of overlying eolian sand (m)
44
40
1
T.21N,R.28W. I V - C o l l i e r Ranch
7220+1202 (1-9839)
14
410-420 530-590
3110+ 80 (W-4923)5 5040z 802 (DIC-2075)
13
199-202
9930+140 ( W-4904)
SWk,NWk,sec.23, T.21N,R. 26W.
80-100
SW%,NEL,sec.21, T.23N,R. 27W.
VI-Whitetail Creek
SEL, SE4, sec. 26, T. 16N,R.38W.
VII-Snake R i v e r
SEL,SWk,sec. T.30N,R.38W.
V- Nat ic k
18,
0- 10
8605 553 (DIC-2074)
6 8
' A l l dates made on o r g a n i c - r i c h sands o r s i l t s except DIC-2075, which was on wood %amp1 e r e c e i v e d h o t NaOH treatment 3Average o f two determinations 940+55 (no NaOH treatment) and 780+60 ( t r e a t e d w i t h NaOH) 4George Coleman, Nebraska Wesleyan-Radiocarbon Laboratory IiMeyer Rubin, U.S. Geological Survey Radiocarbon Laboratory, Reston, Va.
dunes could have m i g r a t e d o v e r the Swan Lake s i t e w i t h o u t l e a v i n g any r e c o r d a s i s t h e c a s e i n d e f l a t i o n a r y i n t e r d u n e s ( A h l b r a n d t and F r y b e r g e r , 1981, p . 2 9 8 ) . Middle Holocene The p r e s e n c e o f a mid-Holocene warm a n d / o r a r i d p e r i o d ( A l t i t h e r m a l o r Hypsit h e r m a l ) l a s t i n g some 2,500 t o 3,000 y r s . has been well documented i n t h e Central and Northern P l a i n s ( W r i g h t , 1970 and 1976) and t h e Rocky Mountains ( B e n e d i c t , 1973). I n western Iowa, B e t t i s (1982) p r e s e n t e d e v i d e n c e o f a major e r o s i o n a l unconformity i n small v a l l e y a l l u v i a l f i l l s spanning a p p r o x i m a t e l y 8,000 - 3,400
2
y r s . B . P . Grigal e t a l . (1976) documented a 580 km dune f i e l d i n n o r t h - c e n t r a l Minnesota t h a t was a c t i v e between a b o u t 8 , 0 0 0 and 5,000 y e a r s ago. In s p i t e of
t h i s e v i d e n c e f o r a major warming and probably a r i d p e r i o d a c r o s s t h e p l a i n s and u n l i k e t h e Wyoming dune f i e l d s , we have no d i r e c t l y d a t e d mid-Holocene e o l i a n a c t i v i t y i n the Nebraska Sand H i l l s . A 4 . 8 m c o r e o b t a i n e d by S e a r s (1961) from Hackb e r r y Lake ( F i g . 6 ) c o u l d be i n t e r p r e t e d t o p o s t d a t e e o l i a n a c t i v i t y i n t h e a r e a .
He o b t a i n e d a r a d i o c a r b o n d a t e o f 5 , 0 4 0 i 9 5 y r s . B . P . on brown ooze ( g y t t j a ) a t a d e p t h of 4 . 5 m and p r o b a b l y n e a r t h e beginning of l a c u s t r i n e d e p o s i t i o n . The p o l l e n p r o f i l e from t h i s c o r e i n d i c a t e s a f o r e s t maximum ( h a l f of t h e f o r e s t p o l l e n i s
3 94 p i n e ) between d e p t h s o f 4 and 4 . 8 m e t e r s .
A g a i n , we h a v e n o s a t i s f a c t o r y means
t o e v a l u a t e t h e s t r a t i g r a p h i c r e l a t i o n s h i p o f t h e s u r r o u n d i n g dune sand t o t h e d a t e d sequence i n t h i s c o r e . B r i c e (1964, p . D5) d a t e d a p e a t a t 8,400+250 y r s . B.P. exposed a l o n g t h e North Loup R i v e r ( F i g . 6 ) .
The p e a t bed i s a b o u t 1 . 5 m e t e r s b e l o w t h e t o p o f a t e r r a c e
w h i c h i s o v e r l a i n b y u p t o 7 m e t e r s o f dune s a n d .
B r i c e (1964, p . D9) a l s o n o t e d
t h a t dune sand m a n t l e s P e o r i a Loess a l o n g t h e n o r t h s i d e o f t h e N o r t h Loup R i v e r and t h a t t h e l o e s s i s " r e m a r k a b l y f r e e o f i n t e r b e d d e d dune s a n d . " dence l e a v e s open t h e p o s s i b i l i t y o f m i d - H o l o c e n e e o l i a n a c t i v i t y .
Brice's eviHowever, t h i s
a r e a i s o n l y a b o u t 40 km e a s t o f t h e D i s m a l - M i d d l e Loup r e g i o n where we have good evidence f o r o n l y late-Holocene a c t i v i t y (see below).
T h e r e f o r e , we c o n s i d e r i t
more l i k e l y t h a t l i t t l e , i f a n y , H o l o c e n e e o l i a n d e p o s i t i o n o c c u r r e d i n t h e N o r t h Loup a r e a p r i o r t o 3,000 y r s . B.P. We f e e l c e r t a i n t h a t , s o o n e r o r l a t e r , r a d i o c a r b o n d a t a b l e m a t e r i a l o r archeol o g i c e v i d e n c e w i l l a l l o w d o c u m e n t a t i o n o f e o l i a n a c t i v i t y d u r i n g t h e mid-Holocene. U n t i l t h e n , we m u s t r e l y o n i n d i r e c t e v i d e n c e f r o m s u r r o u n d i n g a r e a s t o p o s t u l a t e a m a j o r p e r i o d o f dune f o r m a t i o n d u r i n g t h e m i d - H o l o c e n e i n t h e Nebraska Sand H i l l s ( F i g . 38).
V e r y l i t t l e l a t e P l e i s t o c e n e - H o l o c e n e s e d i m e n t s u n d e r l i e t h e dune
sand o u t s i d e o f t h e g e n e r a l a r e a o f t h e D i s m a l , M i d d l e and N o r t h Loup r i v e r s .
In
t h e n o r t h e r n and n o r t h w e s t e r n p o r t i o n s o f t h e sand sea t h e dune sand g e n e r a l l y r e s t s d i r e c t l y o n t h e O g a l l a l a Group ( M i o c e n e ) s e d i m e n t s .
Therefore, obtaining
u s e f u l l i m i t s o n e o l i a n a c t i v i t y t h e r e may p r o v e t o b e q u i t e d i f f i c u l t .
The most
p r o d u c t i v e f u t u r e r e s e a r c h may l i e i n d e t a i l e d d r i l l i n g and c o r i n g o f h o l e s on t r a n s e c t s t h a t c r o s s i n t e r d u n e s and dunes. L a t e Holocene The b e s t documented phase o f e o l i a n a c t i v i t y o c c u r s between a p p r o x i m a t e l y 3,000 and 1,500 y r s . B.P.
i n t h e a r e a s o f t h e Dismal a n d M i d d l e Loup r i v e r s ( F i g s . 7
and 8 ) i n t h e e a s t - c e n t r a l p a r t o f t h e Sand H i l l s . (Pliocene?) alluvial-lacustrine v a l l e y - f i l l
A 60-70
m t h i c k l a t e Cenozoic
sequence t r e n d i n g s o u t h w e s t t o n o r t h -
e a s t i s p r e s e n t i n t h i s a r e a ( F i g . 9 ) b e n e a t h t h e dune sand (Maroney, 1 9 7 8 ) .
This
sequence i s o v e r l a i n by H o l o c e n e f l u v i a l c h a n n e l - f o r m sand b o d i e s t h a t a r e as t h i c k as 15 m and t h a t l o c a l l y c o n t a i n beds o f l a t e r a l l y d i s c o n t i n u o u s , o r g a n i c r i c h sand and s i l t up t o 3 m t h i c k .
The means o f t e n r a d i o c a r b o n d a t e s o b t a i n e d
o n m a t e r i a l f r o m t h e s e o r g a n i c - r i c h u n i t s a t f i v e s i t e s r a n g e f r o m 8,410 t o 3,000 y r s . B.P.
(Table 3 ) .
The s i t e s were chosen b y S w i n e h a r t a n d Maroney because
( e x c e p t a t S i t e 111) s i g n i f i c a n t t h i c k n e s s e s ( u p t o 40 m) o f e o l i a n sand o v e r l i e the dated horizons.
No w e a t h e r i n g p r o f i l e o r o t h e r f e a t u r e i n d i c a t i n g a h i a t u s
a t t h e a l l u v i a l - d u n e sand c o n t a c t e x i s t s a t any o f t h e f i v e s i t e s .
Moreover, t h e
dune sand and a l l u v i a l s e d i m e n t s a p p a r e n t l y a r e n o t contemporaneous f a c i e s because no i n t e r b e d d i n g o f t h e t w o o r g r a d a t i o n f r o m one t o t h e o t h e r i s e v i d e n t .
We
Figure 8. Part o f Landsat iiriage ( E - 1 5 2 9 - 1 6 5 3 0 - 7 ) o f t h e i l i d d l e Loup-Dismal River a r e a in t h e ilehraska Sand H i l l s . Image taken o n J a n . 3 , 1 9 7 4 . Recent snow f a l l coiribined with low w i n t e r sun a n g l e a c c e n t u a t e s t o p o g r a p h i c f e a t u r e s . Priiiiary dune t y p e s o c c u r r i n g w i t h i n t h e S a n d H i l l s a r e i n d i c a t e d along with l o c a t i o n o f g e o l o g i c s e c t i o n A - A ' .
w W ul
396
WEST
EASl AGE
SITE A
SITE
I
;I
\ 71 K
I 'olocene
tm
UNITS
A'
Dune sand
r I080
I Alluvial
sond a n d s i l t contalnlnq local o r q o m c - r i c h horlzons
P l i o c e n e ( 7 ) j Alluvial % I t , sand, and g r a v e l Miocene
1
1 1 I
t
Oqollalo Group I990
1
Dismal River p r o f i l e /
I
~
-.-
0-
---a10
20 Km
10 I
F i g u r e 9 . S c h e m a t i c c r o s s - s e c t i o n A - A ' a l o n g t h e Disiiial R i v e r s h o w i n g l o c a t i o n s o f r a d i o c a r b o n sample s i t e s a n d a n a d d i t i o n a l 18 measured s e c t i o n s . T e s t h o l e 33-B-71 p r o j e c t e d f r o i i i a p p r o x i m a t e l y 5 km n o r t h o f t h e D i s m a l R i v e r . c o n s i d e r t h e dune sand t o kfave formed i n a d i f f e r e n t and l a t e r e n v i r o n m e n t , c e r t a i n l y one w i t h c o n s i d e r a b l y l e s s e f f e c t i v e p r e c i p i t a t i o n .
F u r t h e r m o r e , we p o s t u -
l a t e t h a t t h e c l i m a t i c a n d o t h e r changes n e c e s s a r y t o a l l o w e o l i a n s e d i i i i e n t a t i o r l t o r e p l a c e a l l u v i a l s e d i m e n t a t i o n t o o k p l a c e o v e r t h e same p e r i o d o f t i m e a c r o s s much o f t h e c e n t r a l a n d e a s t e r n Sand H i l l s b u t t h e b e g i n n i n g o f e o l i a n Sedimentation was n o t n e c e s s a r i l y s y n c h r o n o u s t h r o u g h o u t t h e e n t i r e a r e a .
S i m i l a r l y , t h e youngest
a l l u v i a l s e d i m e n t s a t a n y s i t e w o u l d n o t n e c e s s a r i l y r e c o r d t h e end o f a l l u v i a l d e p o s i t i o n everywhere.
Based o n t h e s e a r g u m e n t s , we c o n c l u d e t h a t an e n v i r o n m e n t
f a v o r a b l e f o r e o l i a n s e d i m e n t a t i o n began i n t h i s a r e a soiiietiiiie a r o u n d 3,000 y r s .
B.P.
Now we w i l l e x a m i n e t h e i n d i v i d u a l s i t e s i n more d e t a i l .
A t s i t e I ( F i g . 10A) t h e S o u t h F o r k o f t h e D i s m a l R i v e r has u n d e r c u t i t s n o r t h bank a n d p r o d u c e d an e x p o s u r e 70 m h i g h . a b l y o n a 40 cm t h i c k , l i g h t - g r a y ,
A t t h i s s i t e , e o l i a n sand r e s t s conforin-
f i n e - g r a i n e d a l l u v i a l sand which i s u n d e r l a i n
by a 35 cm t h i c k , v e r y d a r k g r a y o r g a n i c - r i c h ,
diatomaceous s i l t y sand.
This or-
g a n i c - r i c h sand has some s m a l l r o d e n t ( ? ) b u r r o w s i n t h e u p p e r 15 cm a n d has an i r r e g u l a r upper c o n t a c t . dated (Table 3 ) .
T h r e e samples o f t h i s o r g a n i c - r i c h sand were r a d i o c a r b o n
A sample c o l l e c t e d b y t w o o f us ( S w i n e h a r t and M a r o n e y ) i n 1977
f r o i n t h e t o p 5 cm o f t h e o r g a n i c - r i c h s a n d y i e l d e d a d a t e o f 3,000~400 y r s . B.P.
(NWU-83). tion.
The sample c o n t a i n e d a s m a l l amount ( ~ 1 2 )o f modern r o o t l e t c o n t a m i n a -
I n 1980, S w i n e h a r t and R i c h a r d M a d o l e o f t h e U . S . G e o l o g i c a l S u r v e y c o l -
l e c t e d a sample f r o m a p p r o x i m a t e l y 5-15 cm b e l o w t h e t o p o f t h e o r g a n i c - r i c h sand. T h i s sample a l s o c o n t a i n e d a s m a l l amount o f modern r o o t l e t s .
Madole used a
[ m o d i f i e d K i h l t r e a t m e n t ( K i h l , 1975) t o c o n c e n t r a t e t h e o r g a n i c s i n t h e sample
397
F i g u r e 10. R a d i o c a r b o n sample s i t e s I , 1 1 , I V , and V s h o w i n g t h e c o n t a c t between dune sand and a l l u v i a l s a n d . R a d i o c a r b o n d a t e s shown i n y r s . B . P . A, S i t e I on t h e s o u t h f o r k o f t h e Disriial R i v e r . A p p r o x i i i i a t e l y 2 5 111 o f e o l i a n sand i s exposed above t h e a l l u v i a l sand. P h o t o g r a p h t a k e n i n A p r i l , 1980. B . S i t e I 1 on t h e s o u t h f o r k o f t h e D i s m a l R i v e r . A p p r o x i i i i a t e l y 20 in o f dune sand exposed above a l l u v i a l s a n d . P h o t o g r a p h t a k e n i n May 1978. C , S i t e I V a l o n g t h e Dismal R i v e r s o u t h o f H a l s e y F o r e s t . Maxiriiurii h e i g h t o f e x p o s u r e i s 25 111. P h o t o g r a p h t a k e n i n Hay 1978. D. S i t e V a l o n g t h e M i d d l e Loup R i v e r . Combined t h i c k n e s s o f a l l u v i a l s s n d , s i l t , and p e b b l y sand a n d o r g a n i c r i c h s i l t s and p e a t l e n s e s above r i v e r l e v e l i s 6 m. Two s o u t h - s o u t h e a s t d i p p i n g t a b u l a r - p l a n a r e o l i a n c r o s s bed sets a r e v i s i b l e . L o w e s t s e t a p p r o x i m a t e l y 6 in t h i c k . P h o t o g r a p h t a k e n i n June 1980.
398 and a d a t e o f 3,810k80 y r s . B.P.
(W-4900) was o b t a i n e d by Fleyer R u b i n o f t h e USGS
R a d i o c a r b o n Lab i n Reston, V i r g i n i a .
F i n a l l y i n 1982, S w i n e h a r t r e s a m p l e d t h e
t o p 5 cm and c a r e f u l l y e l i m i n a t e d as much o f t h e s m a l l r e s i d u a l amount o f modern r o o t l e t s a s p o s s i b l e f r o m t h e sample.
I t was s u b m i t t e d t o a t h i r d l a b ( B e t a Ana-
l y t i c I n c . ) and a d a t e o f 3,560t70 y r s . B.P.
( B e t a - 5 4 2 9 ) was o b t a i n e d .
Given
t h e l a r g e e r r o r a s s o c i a t e d w i t h sample NWU-83 ( T a b l e 3 ) , we c o n c l u d e t h a t a d a t e o f a b o u t 3,400 y r s . B.P.
r e p r e s e n t s a good e s t i m a t e o f t h e t e r m i n a t i o n o f a l l u v i a l
sedimentation a t t h i s s i t e . I n a d d i t i o n t o t h e above t h r e e r a d i o c a r b o n d a t e s , a sample o f o r g a n i c - r i c h sand from 345 t o 360 cm b e l o w t h e base o f t h e dune s a n d y i e l d e d a s t r a t i g r a p h i c a l l y c o n s i s t e n t age o f 4,900+500 y r s . B.P. ( T a b l e 3 ) .
It i s o f i n t e r e s t t o note that
e s s e n t i a l l y n o o r i g i n a l s e d i m e n t a r y s t r u c t u r e s were o b s e r v e d i n t h e 25 m e t e r s o f dune sand e x p o s e d above t h e a l l u v i a l sand ( F i g . 10A).
I n s t e a d , t h e s e c t i o n con-
t a i n s a s e r i e s o f b r o w n i s h g r a y (10YR6/2) t o brown (10YR6/3) d i s c o n t i n u o u s , wavy bands between 1 and 10 cm t h i c k and spaced 10 t o 25 cm a p a r t .
These bands have
a h i g h e r s i l t p l u s c l a y c o n t e n t t h a n t h e l i g h t e r c o l o r e d sand and a r e s l i g h t l y indurated.
S i m i l a r s t r u c t u r e s w e r e d e s c r i b e d b y A h l b r a n d t and F r y b e r g e r (1980,
p. 1 3 ) and t h e y f o u n d them t o o c c u r t h r o u g h o u t t h e S a n d h i l l s .
They c o n c l u d e d
t h a t t h i s t y p e o f d i s s i p a t i o n s t r u c t u r e was p r o d u c e d b y a f r e e z e - t h a w mechanism. R e g a r d l e s s o f how t h e s e s t r u c t u r e s f o r m , i t m u s t b e a v e r y r a p i d p r o c e s s f o r over 100 bands have f o r m e d i n t h e e o l i a n sand a t s i t e I i n l e s s t h a n p r o b a b l y 2,000 y e a r s ( a l l o w i n g 1,500 y r s f o r a c c u m u l a t i o n o f dune s a n d ) .
We b e l i e v e i t was
s t r u c t u r e s s i m i l a r t o t h e s e and d e s c r i b e d by S m i t h (1968, p . 3 2 ) as "a more weathe r e d and somewhat i n d u r a t e d dune sand, w i t h a d i s t i n c t i v e b r o w n i s h s t r e a k i n g o r mottling,"
t h a t he i n t e r p r e t e d t o r e p r e s e n t "an o l d e r s o i l zone . . . p r o v i s i o n a l l y
assigned t o t h e e a r l i e r p a r t o f t h e l a s t g l a c i a l stage."
C l e a r l y , a t s i t e I,
these d i s s i o a t i o n s t r u c t u r e s a r e n o t t h e product o f a s o i l forming process. S i t e I 1 a l s o i s s i t u a t e d a l o n g t h e S o u t h F o r k o f t h e Dismal R i v e r , ( F i g s . 7 and 8 ) a n d has a s i m i l a r s t r a t i g r a p h y t o s i t e I. B o t h r a d i o c a r b o n samples f r o m t h i s s i t e ( T a b l e 3 ) were d r i e d and s i e v e d t h r o u g h a #230 ( 6 2 ~ 1 )s i e v e and a l l v i s i b l e modern r o o t l e t s were p i c k e d f r o m t h e samples p r i o r t o d a t i n g .
Though a l l u v i a l
d e p o s i t i o n may have ended a t p e r h a p s 4,500 y r s . B.P. a t t h i s s i t e , i t does n o t n e c e s s a r i l y f o l l o w t h a t e o l i a n d e p o s i t i o n began i m m e d i a t e l y a f t e r t h i s t i m e .
At
s i t e I, o n l y 4 km n o r t h e a s t , a l l u v i a l d e p o s i t i o n c o n t i n u e d u n t i l a b o u t 3,400 y r s . B.P.
Reed and D r e e s z e n (1965, p . 6 5 ) f i r s t d e s c r i b e d t h e s e o r g a n i c - r i c h sands
f r o m e x p o s u r e s o n t h e Dismal R i v e r 4 km e a s t o f s i t e 1 1 .
It i s interesting t o
n o t e t h a t t h e y c o n s i d e r e d t h i s h o r i z o n c o r r e l a t i v e e i t h e r w i t h t h e G i l m a n Canyon F o r m a t i o n ( a b o u t 30,000 y r s o l d ) o r w i t h t h e Sangamon s o i l ( p r e - W i s c o n s i n ) and r e p o r t e d i t t o b e o v e r l a i n by 43 m o f " m e d i a l W i s c o n s i n P e o r i a Dune Sand." Our p r i m a r y c o n c e r n i n i n t e r p r e t i n g t h e s t r a t i g r a p h y exposed a l o n g t h e Dismal R i v e r was t h e p o s s i b i l i t y t h a t t h e o r g a n i c - r i c h sands, r a t h e r t h a n b e i n g p a r t o f
~.
.
~
399 an a l l u v i a l f i l l c o n f i n e d t o an o l d e r and somewhat broader Dismal R i v e r .
Site
I11 ( F i g s . 7 and 8), where t h e d a t e d h o r i z o n m i g h t be i n t e r p r e t e d t o be p a r t o f a postdune a l l u v i a l f i l l , c o u l d be used t o s u p p o r t such a h y p o t h e s i s .
I f so, then
one c o u l d argue t h a t t h e dune sand o v e r l y i n g s i t e s I and I 1 r e p r e s e n t s a m i n o r , l a t e Holocene r e a c t i v a t i o n ( i . e . a " s k i n e f f e c t " ) .
However, as F i g u r e 8 shows,
t h e South Fork o f t h e Dismal R i v e r between s i t e s I and I 1 c u t s across t h e a x i a l t r e n d o f l a r g e - s c a l e dunes and i n d i c a t e s t h a t t h e r i v e r and any f i l l s a s s o c i a t e d w i t h i t p o s t d a t e t h e dunes.
I n a d d i t i o n , a t e s t h o l e (33-8-71) d r i l l e d i n 1971
by t h e C o n s e r v a t i o n and Survey D i v i s i o n o f t h e U n i v e r s i t y o f Nebraska 5 km n o r t h o f s i t e I 1 ( F i g . 8 ) p e n e t r a t e d a s i m i l a r s t r a t i g r a p h y t o t h a t exposed a l o n g t h e Dismal R i v e r ( F i g . 9 ) .
A t t h e t e s t h o l e s i t e 22 m o f dune sand i s u n d e r l a i n by
a t l e a s t 4 m o f g r a y sand w i t h i n t e r b e d d e d s i l t s and p e a t .
Unfortunately, n o t
enough o r g a n i c m a t t e r c o u l d be c o l l e c t e d t o o b t a i n a r a d i o c a r b o n d a t e .
Thus, we
b e l i e v e t h e a v a i l a b l e e v i d e n c e i n d i c a t e s t h a t t h e o r g a n i c - r i c h sand u n i t predates t h e f o r m a t i o n o f l a r g e - s c a l e dunes and t h e e s t a b l i s h m e n t o f t h e Dismal R i v e r . S i t e s 111, I V , and V ( F i g s . 7, 8, and l O C , D) a r e a t t h e s o u t h e a s t e r n edge o f t h e a r e a o f l a r g e - s c a l e dunes and where p a r a b o l i c dunes, p r o b a b l y t h e S e r i e s I 1 dunes o f Smith (1965), have been superimposed on l a r g e - s c a l e dunes.
We b e l i e v e
t h a t s i t e V p r e s e n t s i n d i s p u t a b l e evidence f o r s i g n i f i c a n t l a t e Holocene e o l i a n activity.
The M i d d l e Loup R i v e r i s a c t i v e l y c u t t i n g a near v e r t i c a l exposure
here and two t a b u l a r p l a n a r c r o s s bed s e t s w i t h s l i p - f a c e d e p o s i t s d i p p i n g up t o
24'
r e s t on an a l l u v i a l sequence ( F i g . 10D).
A l o c a l c o n c e n t r a t i o n o f t w i g s near
t h e base o f t h e exposure y i e l d e d a r a d i o c a r b o n d a t e o f 5,040+80 y r s B.P.
(DIC-
2075) and an 8 cm t h i c k o r g a n i c - r i c h s i l t , w i t h no v i s i b l e modern r o o t l e t contami n a t i o n , gave a d a t e o f 3,110+80 y r s B.P.
(W-4923) ( T a b l e 3 ) .
A 4 m thickness
o f f i n e t o coarse, t a b u l a r cross-bedded sand w i t h some i n t e r b e d d e d s i l t y sand s e p a r a t e t h e d a t e d o r g a n i c - r i c h s i l t f r o m t h e e o l i a n sand. d a t e o f a b o u t 3,000 y r s B.P.
We would suggest a
f o r t h e c e s s a t i o n o f a l l u v i a l d e p o s i t i o n and t h e
beginning o f e o l i a n deposition. Some rough e s t i m a t e s as t o t h e t e r m i n a t i o n o f t h i s l a t e Holocene e o l i a n a c t i v i t y can be made f r o m a r c h e o l o g i c evidence.
The Kelso s i t e ( F i g . 6 ) d e s c r i b e d by
K i v e t t (1970) as a Woodland complex (a r i p a r i a n - b a s e d c u l t u r e ) s i t e was dated a t
1,150+200 y r s B.P.on c h a r c o a l .
K i v e t t ' s (1970) c h a r a c t e r i z a t i o n o f Woodland
peoples i n d i c a t e s t h e y would n o t have occupied a r e g i o n t h a t was a r i d enough f o r l a r g e - s c a l e dunes t o form.
I n a d d i t i o n , Sears (1961) p o l l e n p r o f i l e f o r Hackberry
Lake ( F i g . 6 ) shows an i n c r e a s e i n sedge and a q u a t i c p o l l e n and t r e e p o l l e n somet i m e a f t e r 1,200 y r s B.P.
T h e r e f o r e , t h e l a t e Holocene phase o f a c t i v i t y i n t h e
Nebraska Sand H i l l s p r o b a b l y ended around 1,500 y r s B.P. Muhs and Madole (1980) i n d i c a t e d t h a t dunes a l o n g t h e South P l a t t e R i v e r i n Colorado and a l o n g t h e Arkansas and Cimarron R i v e r s i n Kansas have s i m i l a r s o i l p r o f i l e s t o t h e V a l e n t i n e s o i l s ( E l d e r , 1969, p. 15) developed on dunes i n t h e
400 a r e a o f t h e Cismal and M i d d l e Loup r i v e r s .
However, s i n c e t h e V a l e n t i n e s o i l s
e x i s t i n t h o s e p a r t s o f t h e Nebraska Sand H i l l s where e o l i a n a c t i v i t y p o s s i b l y was l i m i t e d t o t h e m i d - H o l o c e n e , s o i l s d a t a p r o b a b l y do n o t p r o v i d e a b a s i s f o r d i f f e r e n t i a t i o n o f s o i l s f o r m e d w i t h i n t h e l a s t 8,000 y r s . The b e s t e v i d e n c e t h a t l a t e H o l o c e n e e o l i a n a c t i v i t y was accompanied by a region. a l l y warmer and more a r i d c l i m a t e comes f r o m t h e C o l o r a d o F r o n t Range.
Benedict
(1973, p . 592) d e s c r i b e d an i m p o r t a n t n o n g l a c i a l i n t e r v a l between a b o u t 3,000 and 1,800 y r s B.P. and s a i d i t was a t i m e o f s i g n i f i c a n t s o i l f o r m a t i o n . . . "perhaps on a par with the 'Altithermal'
..."
However t h e e v i d e n c e f r o m s m a l l d r a i n a g e s i n
w e s t e r n Iowa ( B e t t i s , 1982) i n d i c a t e s a p e r i o d o f s o i l f o r m a t i o n o n l y f r o m about
2,000 t o 1,800 y r s B.P. Very L a t e H o l o c e n e t o R e c e n t Evidence f o r q u i t e r e c e n t e o l i a n a c t i v i t y i s found a t several s c a t t e r e d loc a l i t i e s i n t h e Nebraska Sand H i l l s .
A t s i t e V I I a l o n g t h e Snake R i v e r ( F i g s .
6 and 7, T a b l e 3 ) , a n o r g a n i c - r i c h sand d a t e d a t 860.55 d i r e c t l y o v e r l a i n b y up t o 8 m o f dune sand.
y r s B.P. (DIC-2074) i s
The K e l s o s i t e ( F i g . 6) d e s c r i b e d
above has 3 rn o f dune sand o v e r l y i n g t h e o c c u p a t i o n s i t e d a t e d a t 1,150+200y r s B.P.
B r a d b u r y (1980, p. 33) r e p o r t e d
a
d a t e o f 500i200 y r s B . P . o n o r g a n i c - r i c h
i n t e r d u n e sediments n o t associated w i t h a c u r r e n t interdune basin.
It i s not
s u g g e s t e d t h a t l a r g e - s c a l e dunes f o r m e d a t a n y o f t h e s e l o c a l i t i e s .
R a t h e r , during
t h e l a s t 500 t o 800 y e a r s d r o u g h t c o n d i t i o n s p e r s i s t e d l o n g enough i n p a r t s o f t h e Sand H i l l s t h a t v e g e t a t i o n was r e d u c e d enough f o r e o l i a n a c t i v i t y t o o c c u r a number of t i m e s .
Weakly (1962) p r e s e n t e d d e n d r o c h r o n o l o g i c e v i d e n c e f r o m Ash
H o l l o w , Nebraska, f o r s i x d r o u g h t s , e a c h l a s t i n g more t h a n 15 y e a r s , between 586 and 258 y r s B.P.
Ash H o l l o w i s l o c a t e d a b o u t 40 km e a s t o f s i t e V I ( r i g . 6) j u s t
o u t s i d e o f t h e s o u t h e r n edge o f t h e Sand H i l l s .
A t present, except f o r m a l l
b l o w o u t s , t h e Nebraska Sand H i l l s a r e s t a b i l i z e d and d o r m a n t . LATE PLEISTOCENE VERSUS HOLOCENE SAND DUNE ACTIVITY Most p r e v i o u s c h r o n o l o g i e s f o r t h e dune f i e l d s shown i n F i g u r e 1 ( e . g . Frye and Leonard, 1954; S m i t h , 1965; Reed a n d Dreeszen, 1965) p l a c e d m a j o r dune format i o n i n e i t h e r t h e e a r l y o r l a t e Wisconsin.
H o l o c e n e dune a c t i v i t y g e n e r a l , l y ,
was c o n s i d e r e d t o r e p r e s e n t o n l y m i n o r r e w o r k i n g o f o l d e r dunes.
However, a l l
t h e e v i d e n c e u s e d t o i n d i c a t e a P l e i s t o c e n e age f o r t h e s e dune f i e l d s i s e i t h e r circumstantial o r indirect. I f , as we p o s t u l a t e , t h e l a s t 11,000 y e a r s o r so has seen s i g n i f i c a n t sand dune f o r m a t i o n ,
t h e n why s h o u l d i t be d i f f i c u l t t o f i n d d i r e c t e v i d e n c e o f
Pleistocene episodes o f e o l i a n s a n d , a c t i u i t y ?
Certainly conditions o f sufficient
w i n d s , sand s u p p l y and l a c k o f v e g e t a t i v e c o v e r w o u l d have combined s e v e r a l times d u r i n g t h e P l e i s t o c e n e t o a l l o w s i g n i f i c a n t a c c u m u l a t i o n s o f e o l i a n sand.
Moreover,
401
well-documented P l e i s t o cen e eo l i an a c t i v i t y i s evidenced by widespread loess accumulations i n many areas of t h e Great P l ai n s .
Yet evidence seems lacking f o r
the e x i s t e n c e of major Pleistocene dune f i e l d s , why? One explanation f o r t h e i r absence or nonrecognition involves the e f f e c t of cold climate processes on dune f i e l d s .
A s,tudy of t h e North Park Dunes of Colorado by
A h l b r a n d t a n d Andrews (1978) demonstrated t h a t the r a t e of dune migration in a n
a c t i v e dune f i e l d su b j ect t o seasonal cold climate processes was roughly ten times slower than i n e q u i v al en t wind regimes i n a warm climate. Furthermore, they pointec o u t t h a t t h e presence of a high water t a b l e , e i t h e r frozen or unfmzen, gre a tly reduces sand t r a n s p o r t i n cold climate dunes a n d t h a t i n t e r c a l a t i o n of i c e and snow combined w i t h f r eeze and t h a w cycles cause extensive modification of inte rna l s t r u c t u r e s a n d r e t a r d dune growth because of a frozen dune c ore .
Thus, a major
dune f i e l d forming under a g l a c i a l climate would re quire a very long time t o build
and would e x h i b i t deformed a n d disrupted i n t er n al s t r u c t u r e s . Such s t r u c t u r e s would form n o t only concurrently with deposition b u t a l s o during subsequent cold climate periods ( e i t h e r seasonally o r p e r e n n i a l l y ) .
Of course, dune f i e l d s
formed i n i t i a l l y during i n t e r g l a c i a l periods and subjected l a t e r t o cold climate processes a l s o could be d r amat i cal l y modified as i s apparent in the North Park Dunes ( A h 1 b r a n d t and Andrews, 1978), t h e Ki 1 1 pecker Dunes ( A h 1 b r a n d t , 1975), and the Nebraska Sand H i l l s (Ahlbrandt and Fryberger, 1980). The l a t e Wisconsin pe rig la c ia l sand wedges documented by Mears (1981) i n the intermontane basins of Wyoming c l e a r l y i n d i c a t e t h e synchroneity of cold climates a n d minor e olia n sand tr a n s p o r t where sand i s incorporated irl wedges within o u r area of study (Fig. 3 ) . Late P l e i s t o c e n e sh eet d ep o s i t s of e o l i a n sand o r cover sands disrupted by cold climate processes were described. by Ruegg (1981, and t h i s volume) and Koster (1982) from a n approximately equivalent l a t i t u d e in Europe t o o u r study a re a . 2 Carter (1981, 1982) described a l a r g e (7,000 km ) l a t e Pleistocene sand se a , composed of l i n e a r dunes, on the Alaskan Ar ct i c Coastal Plain and suggested the dune f i e l d was a c t i v e from perhaps 40,000 y r s . B . P . u n t i l 12,000 y r s . B . P .
Locally,
the e o l i a n sand i s more than 20 m t h i ck and occurs in subpa ra lle l ridges u p t o 20 km long a n d 1 km wide.
A l t h o u g h l o c a l l y some l a r g e - s c a l e , high-angle cross-
beds a r e preserved, C ar t er (1981, p . 381) noted t h a t " the la rge dunes have been extensively modified by thermokarst processes making them d i f f i c u l t t o de te c t on the g r o u n d o r from a i r c r a f t . "
He f u r t h e r s t a t e d ( p . 382) " s l i p f a c e s t h a t would
have f a c i l i t a t e d the c l a s s i f i c a t i o n of d u n e types in the sand sea have been obliterated." We conclude from t h e above d i s cu s s i o n t h a t any dune f i e l d formed p r i o r t o about
12,000 y r s . B . P . i n o u r study area ( F i g . 1). should bear the d i s t i n c t i v e signa ture
of cold climate processes.
I f l a t e P l ei s t o cen e e olia n sands e x i s t , they probably
would be represented by sand s h eet d ep o s i t s a n d i f any la rge -sc a le dunes a r e preserved they should be highly modified and probably n o t e a s i l y recognized from
402 t h e i r morphology a l o n e .
A l a t e Wisconsin e o l i a n sequence s h o u l d c o n t a i n some
r e c o r d o f t h e c o e x i s t i n g b o r e a l fauna and f l o r a .
I n addition, the s o i l p r o f i l e
on these o l d e r dunes ought t o be b e t t e r developed t h a n t h e s o i l p r o f i l e on known Holocene dunes (Muhs and Madole, 1980). The b e s t p o s s i b i l i t y f o r pre-Holocene dunes i n t h e s t u d y a r e a appears t o be i n s m a l l areas o f subdued dune topography i n n o r t h e a s t e r n Colorado (Madole, 1981). These dunes have s o i l p r o f i l e s t h a t a r e more s t r o n g l y developed t h a n t h o s e o f dunes o v e r l y i n g r a d i o c a r b o n d a t e d m i d - t o - l a t e Holocene f l u v i a l u n i t s i n t h e Nebraska Sand H i 11s . SIGNIFICANCE OF MAJOR HOLOCENE ACTIVITY We b e l i e v e t h i s paper documents m u l t i p l e phases o f s i g n i i c a n t e o l i a n a c t i v i t y i n t h e G r e a t P l a i n s and Rocky Mountains d u r i n g t h e Holocene ( F i g . 3 ) . w i s h t o i m p l y t h a t e o l i a n d e p o s i t i o n was simultaneous i n a1 i t was n o t .
We do n o t
areas because c l e a r l y
However, t h e concept o f an e s s e n t i a l l y t h r e e p a r t c l i m a t i c s u b d i v i s i o n
of t h e Holocene i n which t h e r e i s a g r a d u a l s h i f t t o warmer and/or d r i e r c l i m a t e d u r i n g t h e mid-Holocene and a g r a d u a l r e t u r n t o a l e s s severe c l i m a t e ( W r i g h t , 1976) comes i n t o q u e s t i o n .
L i k e many o t h e r g e o l o g i c processes ( e . g . g l a c i a l ad-
vance and r e t r e a t ; a l l u v i a l d e g r a d a t i o n vs a g g r a d a t i o n ) t h e f a c t o r s c o n t r o l l i n g e o l i a n sand-dune a c t i v i t y a r e complex and v e r y d i f f i c u l t t o q u a n t i f y .
Very pos-
s i b l y t h e l a t e Holocene l a r g e - s c a l e dune f o r m a t i o n we e n v i s a g e o c c u r r i n g i n t h e s o u t h e a s t e r n p a r t o f t h e Nebraska Sand H i l l s d i d n o t r e p r e s e n t a s i g n i f i c a n t enough d e p a r t u r e f r o m t h e r e g i o n a l c l i m a t e t o a f f e c t t h e a l l u v i a l and f l o r a l record t o t h e degree t h a t o c c u r r e d d u r i n g t h e mid-Holocene.
Yet i t does appear t o be
g e n e r a l l y c o i n c i d e n t w i t h a n o n g l a c i a l i n t e r v a l i n t h e Colorado F r o n t Range ( B e n e d i c t , 1973).
I n a d d i t i o n , we t h i n k t h a t t h e Holocene e o l i a n r e c o r d i n d i c a t e s
some r a t h e r a b r u p t t r a n s i t i o n s f r o m p l u v i a l t o a r i d c o n d i t i o n s .
We t e n d t o agree
w i t h many o f t h e arguments o f Bryson e t a l . (1970), who emphasized a m u l t i p h a s e Holocene c l i m a t i c h i s t o r y w i t h f a i r l y a b r u p t b o u n d a r i e s .
However, we do n o t see
any advantage i n a d o p t i n g t h e B l y t t - S e r n a n d e r c l i m a t i c c l a s s i f i c a t i o n advocated by Bryson e t a l . (1970) a t t h i s t i m e .
Much more d a t a , p a r t i c u l a r l y i n t h e form
o f w e l l - d a t e d l o c a l s t r a t i g r a p h i c sequences and accompanying f l o r a l and f a u n a l evidence i s needed i n o r d e r t o t e s t t h e r e l i a b i l i t y o f such a c l a s s i f i c a t i o n . T h i s paper r a i s e s q u e s t i o n s on t h e r a t e o f development o f dune f i e l d s .
Using
conceptual models o f an upwind sand s u p p l y and p r o g r e s s i v e downwind development o f t h e f i e l d , g e o l o g i s t s have s p e c u l a t e d t h a t l a r g e e o l i a n dune bedforms must take many thousands o f y e a r s t o develop. d i f f e r e n t scenario.
However, t h e Nebraska Sand H i l l s r e f l e c t a
The sand sea developed on an u n c o n s o l i d a t e d o r p o o r l y c o n s o l i -
d a t e d a l l u v i a l sand s u b s t r a t e , t h e r e b y e l i m i n a t i n g t h e need f o r l o n g d i s t a n c e sand transport.
The a r i d c l i m a t e and abundant sand s u p p l y combined t o g i v e r a p i d r i s e
t o a l a r g e dune f i e l d .
As d i s c u s s e d above, l a r g e - s c a l e dunes o v e r l i e l a t e
4 03 Holocene a l l u v i u m i n p a r t s o f t h e Nebraska Sand H i l l s .
C l e a r l y , sand seas have
formed r a t h e r r a p i d l y , if we a c c e p t t h e s t r a t i g r a p h i c and r a d i o c a r b o n evidence from dune f i e l d s o f t h e Great P l a i n s and Rocky Mountain b a s i n s .
The dune f i e l d s
described h e r e p r o v i d e a l o o k a t e o l i a n a c t i v i t y d u r i n g a v e r y s h o r t p e r i o d of time, perhaps 10,000 y e a r s , and r e c o r d a v e r y dynamic and complex h i s t o r y .
A t h i r d p o i n t o f d i s c u s s i o n r e l a t e s t o t h e t y p e o f dunes i n these f i e l d s compared t o o t h e r r e g i o n s o f t h e w o r l d .
L i n e a r dunes a r e p r o b a b l y t h e most common
f o r m i n l a r g e dune f i e l d s , y e t , t h e y a r e v i r t u a l l y n o n - e x i s t e n t ( w i t h t h e except i o n o f en echelon barchan dunes i n t h e Sand H i l l s ) i n t h e dune f i e l d s o f t h e Great P l a i n s and t h e Rocky Mountain b a s i n s .
Does t h i s r e f l e c t t h e s h o r t - l i v e d
development o f t h e s e western U n i t e d S t a t e s dune f i e l d s , t h e i r m i d - l a t i t u d e s e t t i n g o r abundant sand s u p p l y ? F i n a l l y , t h e dynamic e o l i a n a c t i v i t y i n t h e Holocene r e f l e c t s responses t o both p o s t g l a c i a l and a r i d p e r i o d s .
E o l i a n a c t i v i t y has been e a s i l y and r e p e a t -
e d l y s t i m u l a t e d d u r i n g t h e Holocene and s h o u l d be an i n d i c a t i o n o f t h e r a p i d i t y w i t h which a s i g n i f i c a n t e p i s o d e o f e o l i a n a c t i v i t y c o u l d r e t u r n t o t h e Great P l a i n s and Rocky Mountain b a s i n s . ACKNOWLEDGEMENTS Four r a d i o c a r b o n d a t e s f r o m t h e Nebraska Sand H i l l s were k i n d l y p r o v i d e d by Meyer Rubin o f t h e U.S.
G e o l o g i c a l Survey Radiocarbon Lab, Reston, V i r g i n i a .
We
thank R i c h a r d Madole f o r h i s a s s i s t a n c e i n c o l l e c t i n g and p r e p a r i n g these samples and f o r h i s h e l p f u l d i s c u s s i o n s . s t i m u l a t i n g discussions.
R. George Corner and Daniel Fluhs a l s o p r o v i d e d
T h i s paper was improved by t h e r e v i e w s o f Raymond
B e n t a l l , David Loope, M a r v i n C a r l s o n and Robert D i f f e n d a l , J r .
V i n c e n t Dreeszen,
d i r e c t o r o f t h e C o n s e r v a t i o n and Survey D i v i s i o n , p r o v i d e d guidance and s u p p o r t f o r t h e Nebraska Sand H i l l s p o r t i o n o f t h i s s t u d y .
We thank t h e many landowners
who gave p e r m i s s i o n t o work and c o l l e c t on t h e i r p r o p e r t y . REFERENCES Ahlbrandt, T.S., 1974. Dune s t r a t i g r a p h y , archaeology and t h e c h r o n o l o g y o f t h e K i l l p e c k e r dune f i e l d . I n : M. W i l s o n ( E d i t o r ) , A p p l i e d Geology and Archaeology: The Holocene H i s t o r y o f Wyoming. Wyoming Geol. Survey, Rept. I n v e s t . , 10:51-60. Ahlbrandt, T.S., 1975. Comparison o f t e x t u r e s and s t r u c t u r e s t o d i s t i n g u i s h e o l i a n environments, K i l l p e c k e r dune f i e l d , Wyoming. Mountain G e o l o g i s t ,
12:61-73. Ahlbrandt, T.S. and Andrews, Sarah, 1978. D i s t i n c t i v e sedimentary f e a t u r e s o f c o l d - c l i m a t e e o l i a n d e p o s i t s , N o r t h Park, Colorado. Palaeogeography, Palaeoc l i m a t o l o g y , Pal aeoecology , 25:327-351. A h l b r a n d t , T.S. and F r y b e r g e r , S.G., 1980. E o l i a n d e p o s i t s i n t h e Nebraska Sand H i l l s . I n : G e o l o g i c and P a l e o e c o l o g i c S t u d i e s o f t h e Nebraska Sand H i l l s . U.S. Geol. Survey P r o f . Paper, 112O:l-24. A h l b r a n d t , T.S. and F r y b e r g e r , S.G., 1981. Sedimentary f e a t u r e s and s i g n i f i c a n c e o f i n t e r d u n e d e p o s i t s . I n : F.G. E t h r i d g e and R.M. F l o r e s ( E d i t o r s ) , Recent and A n c i e n t Nonmarine D e p o s i t i o n a l Environments. SOC. Economic P a l e o n t o l o g i s t s and M i n e r a l o g i s t s S p e c i a l Pub., 31:293-314.
4 04
Albanese, J . P . , 1974. Geology o f t h e Casper a r c h a e o l o g i c a l s i t e , Natrona County, Wyoming. I n : M . Wilson ( E d i t o r ) , Applied Geology and Archaeology: The Holocene H i s t o r y of Wyoming. Wyoming Geol. Survey Rept. I n v e s t . , 10:46-50. Albanese, J. P. and Wilson, M . , 1974. P r e l i m i n a r y d e s c r i p t i o n o f the t e r r a c e s of t h e North P l a t t e R i v e r a t Casper, Wyoming. I n : M . Wilson ( E d i t o r ) , Applied Geology and Archaeology: The Holocene H i s t o r y o f Wyoming. Wyoming Geol. Survey, Rept. I n v e s t . , 10:8-18. Anderson, E l a i n e , 1974. A survey o f t h e l a t e P l e i s t o c e n e and Holocene mammal fauna o f Wyoming. I n : M . Wilson ( E d i t o r ) , Applied Geology and Archaeology: The Holocene H i s t o r y of Wyoming. Wyoming Geol. Survey, Rept. I n v e s t . , 10:79-87. Andrews, S . G . , 1981. Sedimentology o f G r e a t Sand Dunes, Colorado. I n : F.G. Ethridge and R.M. F l o r e s ( E d i t o r s ) , Non-marine D e p o s i t i o n a l Environments: Models f o r E x p l o r a t i o n . SOC. Economic P a l e o n t o l o g y and Mineralogy S p e c i a l P u b l . , 31:270291. B e n e d i c t , J . B . , 1973. Chronology o f c i r q u e g l a c i a t i o n , Colorado F r o n t Range. Q u a t e r n a r y Research, 3:584-599. B e n e d i c t , J . B . , 1979. G e t t i n g away from i t a l l : A s t u d y o f man, mountains, and the two-drought A l t i t h e r m a l . Southwestern Lore, 4 5 , no. 3 : l - 1 2 . B e t t i s 111, E . A . , 1982. Geochronology o f l a t e Wisconsinan and Holocene alluvium i n t h e Flissouri d r a i n a g e of wpstern Iowa (USA) [ a b s . ] . I n : A b s t r a c t s Eleventh I n t ' l . Congress on Sedimentology, I n t ' l . Assoc. Sedimentology, p . 136. Bradbury, J . P . , 1980. Late Quaternary v e g e t a t i o n h i s t o r y of the c e n t r a l Great P l a i n s and i t s r e l a t i o n s h i p t o e o l i a n p r o c e s s e s i n t h e Nebraska Sand H i l l s . U.S. Geol. Survey P r o f . P a p e r , 1120C:25-36. B r i c e , J . C . , 1964. Channel p a t t e r n s and t e r r a c e s of the Loup r i v e r s i n Nebraska. U.S. Geol. Survey P r o f . P a p e r , 422D, 41 p p . Bryson, R . A . , B a e r r e i s , D . A . , and Wendland, W . M . , 1970. The c h a r a c t e r o f l a t e g l a c i a l and p o s t - g l a c i a l c l i m a t i c changes. I n : W. D o r t , J r . and J . K . J o n e s , J r . ( E d i t o r s ) , P l e i s t o c e n e and Recent Environments o f the C e n t r a l G r e a t Plains, Univ. o f Kansas, Dept. Geology S p e c i a l Pub., 3:53-74. Bryson, R . A . , Wendland, W . M . , Ives, J.D., and Andrews, J . T . , 1969. Radiocarbon i s o c h r o n e s on t h e d i s i n t e g r a t i o n of t h e L a u r e n t i d e i c e s h e e t . A r c t i c and Alpine Research, 1 : l - 1 4 . C a r t e r , L. David, 1981. A P l e i s t o c e n e sand s e a on t h e Alaskan A r c t i c Coastal P l a i n . S c i e n c e , 211:381-383. C a r t e r , L . David, 1982. Late Wisconsin d e s e r t i f i c a t i o n i n n o r t h e r n Alaska [ a b s . ] . I n : Geol. SOC. America A b s t r a c t s w i t h Programs, 1982 A n n . Mtg , p . 461. Corner, R . G . , 1977. A l a t e P l e i s t o c e n e - H o l o c e n e v e r t e b r a t e fauna from Red Willow County, Nebraska. Nebraska Acad. S c i e n c e T r a n s . , 4:77-93. Corner, R . G . , 1982. A l a t e P l e i s t o c e n e f a u n a from L i t c h f i e l d , Sherman County, Nebraska [ a b s . ] . Nebraska Acad. S c i e n c e , P r o c . , 92nd A n n . Mtg., p . 46. E l d e r , J . A . , 1969. S o i l s o f Nebraska. C o n s e r v a t i o n and Survey Div., Univ. of Nebraska, Resource Report 2 , 60 p p . F l i n t , R . F . , 1971. G l a c i a l and Quaternary Geology. New York, John Wiley and Sans, I n c . , 892 P P . F l i n t , R . F . , 1976. P h y s i c a l e v i d e n c e o f Quaternary c l i m a t i c change. Qua t e m a ry Research. 6 ~ 5 1 9 - 5 2 8 . F r i s o n , G . C . , Wilson, M . , and Wilson, D . J . , 1974. The Holocene s t r a t i g r a p h i c a r c h a e o l o g y of Wyoming: an i n t r o d u c t i o n ; I n : M . Wilson ( E d i t o r ) , Applied Geology an-d Archaeology: The Holocene H i s t o r y of Wyoming, Wyoming Geol. Survey, Rept. I n v e s t . , 10:108-127. Frye, J . C . and Leonard, A . B . , 1952. P l e i s t o c e n e Geology of Kansas. Kansas Geol. Survey, B u l l . 99, 230 p p . Gaylord, R . D . , 1979. Holocene c l i m a t i c changes r e c o r d e d i n t h e F e r r i s Dune Field, s o u t h e r n Wyoming [ a b s . ] . Geol. SOC. America A b s t r a c t s with Programs, 11, n o . 6 , 263. G r i g a l , D . F . , Severson, R . C . , and G o l t z , G . E . , 1976. Evidence of e o l i a n a c t i v i t y i n n o r t h - c e n t r a b M i n n e s o t a 8,000 t o 5,000 y r s ago. Geol. SOC. America R u l l . , 87:1251-1254. . GrGger, J . , 1973. S t u d i e s on the l a t e Q u a t e r n a r y v e g e t a t i o n h i s t o r y o f n o r t h e a s t e r n Kansas: Geol. SOC. America B u l l . , 84:239-250.
405 Haynes, C . V . , J r . , 1965. G e o c h r o n o l o g y o f l a t e Q u a t e r n a r y a l l u v i u m . I n : R. M o r r i son and H.E. W r i g h t , J r . ( E d i t o r s ) , Means o f C o r r e l a t i o n o f Q u a t e r n a r y Succ e s s i o n s . U n i v . U t a h P r e s s , S a l t Lake City, Utah, p p . 591-631. Hoffman, R . S . a n d Jones, J.K., J r . , 1970. I n f l u e n c e o f l a t e - g l a c i a l and p o s t g l a c i a l e v e n t s o n t h e d i s t r i b u t i o n o f r e c e n t mammals on t h e N o r t h e r n G r e a t P l a i n s . I n : W. P o r t , J r . and J . K . Jones, J r . ( E d i t o r s ) , P l e i s t o c e n e and Recent E n v i r o n m e n t s o f t h e C e n t r a l G r e a t P l a i n s , U n i v . Kansas, D e p t . Geology S p e c i a l Pub., 31355-394. Holmes, C.W. and Moss, J.H., 1955. P l e i s t o c e n e g e o l o g y o f t h e s o u t h w e s t e r n Wind R i v e r M o u n t a i n s , Wyoming. G e o l . SOC. A m e r i c a B u l l . , 66:629-654. Howard, E . B . , S a t t e r t h w a i t e , L., J r . , and Bache, C . , 1941. P r e l i m i n a r y r e p o r t on a b u r i e d Yuma s i t e i n Wyoming. America A n t i q u i t y , 7:70-74. Howard, E.B. and Hack, J . T . , 1943. The F i n l e y S i t e . A m e r i c a n A n t i q u i t y , 8:224-241. Jelgersma, S . , 1962. A l a t e - g a l c i a l p o l l e n d i a g r a m f r o m M a d e l i a , s o u t h - c e n t r a l M i n n e s o t a . Am. J o u r . S c i . , 260, no. 7:522-529. K i h l , R., 1975. P h y s i c a l p r e p a r a t i o n o f o r g a n i c m a t t e r samples f o r 14C d a t i n g . I n : J . T . Andrews, R a d i o c a r b o n d a t e l i s t I 1 f r o m Cumberland P e n i n s u l a , B a f f i n I s l a n d , N.W.T. Canada. A r c t i c and A l p i n e Research, 7:90-91. K i n g , J.E. and L i n d s a y , E.H., 1976. L a t e Q u a t e r n a r y b i q t i c r e c o r d s f r o m s p r i n g d e p o s i t s i n w e s t e r n M i s s o u r i . I n : P r e h i s t o r i c Man and H i s E n v i r o n m e n t s , A Case S t u d y i n t h e Ozark H i g h l a n d : Academic P r e s s , New York, p p . 63-78. K i v e t t , M.F., 1970. E a r l y c e r a m i c e n v i r o n m e n t a l a d a p t a t i o n s . I n : W. D o r t , J r . and J.K. Jones, J r . ( E d i t o r s ) , P l e i s t o c e n e and Recent E n v i r o n m e n t s o f t h e C e n t r a l G r e a t P l a i n s , U n i v . o f Kansas, D e p t . Geology S p e c i a l Pub., 3:93-102. Kolm, K.E., 1974, ERTS MSS i m a g e r y a p p l i e d t o mapping o f sand dunes i n Wyoming. I n : M. W i l s o n ( E d i t o r ) , A p p l i e d G e o l o g y and A r c h a e o l o g y : The Holocene H i s t o r y o f Wyoming. Wyoming G e o l . S u r v e y , R e p t . I n v e s t . 10:34-39. K o s t e r , E . A . , 1982. T e r m i n o l o g y and 1 i t h o s t r a t i g r a p h i c d i v i s i o n o f ( s u r f i c i a l ) sandy e o l i a n d e p o s i t s i n t h e N e t h e r l a n d s : An e v a l u a t i o n . G e o l o g i c en M i j n b o u w , 61:121-129. Lugn, A . L . , 1935. The P l e i s t o c e n e g e o l o g y o f Nebraska. Nebraska G e o l . Survey B u l l . 10, 2nd S e r i e s , 223 pp. Lugn, A.L., 1968. The o r i g i n o f l o e s s e s and t h e i r r e l a t i o n t o t h e G r e a t P l a i n s i n N o r t h A m e r i c a . I n : C . S c h u l t z and J.C. F r y e ( E d i t o r s ) , Loess and R e l a t e d E o l i a n D e p o s i t s o f t h e World, U n i v . o f Nebraska P r e s s , L i n c o l n , Neb., pp.
139-182. Madole, R . F . , 1981. G r e a t P l a i n s e o l i a n p r o c e s s e s . I n : G e o l o g i c a l Research 1982, U.S. G e o l o g i c a l S u r v e y P r o f . Paper, 1275:178. Maroney, D . G . , 1978. A s t r a t i g r a p h i c and p a l e o e c o l o g i c s t u d y o f some L a t e Cenoz o i c s e d i m e n t s i n t h e c e n t r a l Sand H i l l s P r o v i n c e o f Nebraska. D i s s e r t a t i o n , U n i v . Nebraska, L i n c o l n , Nebraska, 181 pp. Mears, B.M., J r . 1981. P e r i g l a c i a l wedges and t h e l a t e P l e i s t o c e n e e n v i r o n m e n t o f Wyoming's i n t e r m o n t a n e b a s i n s . Q u a t e r n a r y Research, 15:171-198. Moss, J.H. and o t h e r s , 1951. E a r l y Man i n Eden V a l l e y . U n i v . P e n n s y l v a n i a Museum, Museum Monographs, 6, 124 pp. Muhs, D . R . and Madole, R.F., 1980. S o i l - g e o m o r p h i c e v i d e n c e f o r w i d e s p r e a d movement o f dune sand o n t h e c e n t r a l G r e a t P l a i n s d u r i n g t h e A l t i t h e r m a l [ a b s . ] . T h i r t y - E i g h t h P l a i n s C o n f e r e n c e Program and A b s t r a c t s : 3 3 . Ogden 111, J.G. and Hay, R.J., 1965. O h i o Wesleyan U n i v e r s i t y n a t u r a l r a d i o c a r b o n measurements 11. R a d i o c a r b o n , 7:166-173. Reed, E . C . and Dreeszen, V . H . , 1965. R e v i s i o n o f t h e c l a s s i f i c a t i o n o f t h e P l e i s t o cene d e p o s i t s o f Nebraska. Nebraska G e o l . S u r v e y B u l l . 23, 65 pp. Ruegg, G e r a r d , 1981. S e d i m e n t a r y f e a t u r e s and g r a i n s i z e o f g l a c i o - f l u v i a l and p e r i g l a c i a l P l e i s t o c e n e d e p o s i t s i n t h e N e t h e r l a n d s and a d j a c e n t p a r t s o f w e s t e r n Germany. V e r h . n a t u r w i s s Ver. Hamburq, 24:133-154. Sears, P.B., 1961.. A p o l l e n p r o f i l e f r o m t h e g r a s s l a n d p r o v i n c e . S c i e n c e , 134:
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406 S m i t h , H . T . U . , 1968. Nebraska dunes compared w i t h t h o s e o f North A f r i c a and o t h e r r e g i o n s . In: C . B . S c h u l t z and J.C. Frye ( E d i t o r s ) , Loess and Related Eolian D e p o s i t s of the World. Univ. o f Nebraska Press, L i n c o l n , Nebraska, p p . 29-47. S t e w a r t , J.D., 1978. Mammals o f the T r a p s h o o t l o c a l f a u n a , l a t e P l e i s t o c e n e of Rooks County, Kansas [ a b s . ] , Nebraska Acad. S c i e n c e 88th Ann. Htg, p p . 45-46. S t u v i e r , Minze, 1969. Yale n a t u r a l r a d i o c a r b o n measurements IX. Radiocarbon, 11~545-658. Van Z a n t , Kent, 1976. Late- and p o s t - g l a c i a l v e g e t a t i o n a l h i s t o r y of n o r t h e r n Iowa. P h . D . d i s s e r t a t i o n , Univ. Iowa, Iowa C i t y , 197 pp. Warren, Andrew, 1976. Morphology and sediments o f the Nebraska Sand H i l l s i n r e l a t i o n t o P l e i s t o c e n e winds and the development of a e o l i a n bed forms. J o u r . Geology, 84:685-700. Watts, W.A. and Wright, H . E . , J r . , 1966. Late-Wisconsin p o l l e n and seed a n a l y s i s from the Nebraska Sand H i l l s . Ecology, 47:202-210. Wilson, Michael, 1974. H i s t o r y of t h e b i s o n i n Wyoming, with p a r t i c u l a r r e f e r e n c e t o e a r l y Holocene forms; i n Applied Geology and Archaeology. I n : M . Wilson ( E d i t o r ) , The Holocene H i s t o r y o f Wyoming. Wyoming Geol. Survey Rept. I n v e s t . , 10: 91-99. Weakly, H . E . , 1962. H i s t o r y o f d r o u g h t i n Nebraska. J o u r . of S o i l and Water C o n s e r v . , 17:271-274. Wright, H . E . , J r . , 1968. H i s t o r y o f the P r a i r i e P e n i n s u l a . I n : The Quaternary of I l l i n o i s , Spec. Publ. U n i v . I l l i n o i s C o l l . A g r i c . , 14:78-88. Wright, H . E . , J r . , 1970. V e g e t a t i o n a l h i s t o r y o f the C e n t r a l P l a i n s . I n : W. Dort, J r . and J.K. J o n e s , J r . , ( E d i t o r s ) , P l e i s t o c e n e and Recent Environments of the C e n t r a l G r e a t P l a i n s . Univ. Kansas Dept. Geology Spec. Pub., 3:157-172. Wright, H . E . , J r . , 1976. The dynamic n a t u r e o f Holocene v e g e t a t i o n . Q u a t . Research, 6: 581-586.
407
RECONSTRUCTING BEDFORM ASSEMBLAGES FRCM C@lPOUND CROSSBEDDI NG D M. RUBIN & RALPH E. HUNTER U.S. Geological Survey, Menlo Park, California 94025, USA
INTRODUCTION During t h e l a s t two decades, sedimentologists have observed t h a t small bedforms a r e commonly superimposed on l a r g e bedforms, a n d they have r e a l i z e d
t h a t many compl icated c o s e t s of crossbeds i n eolian and subaqueous sandstones can be r e a d i l y explained by t h e migration of small bedforms on t h e l e e surfaces of o t h e r l a r g e bedforms (Beutner, Flueckinger, and Gard, 1967; Allen, 1968, 1980; Banks, 1973; Brookfield, 1977; Jones, 1979; Kocurek, 1981). The purposes of t h e present
paper a r e :
( 1 ) t o present techniques t h a t can be used t o
reconstruct t h e geometry of bedforms t h a t deposited compound crossbeds, (2) t o i l l u s t r a t e t h e s t r u c t u r e s produced by several common bedform assemblages, and (3) t o show how t h e s e reconstructions can be used t o i d e n t i f y deposits of ob 1 i que and 1 ong i t u d i nal b e d f o n s .
Compound Crossbedding A c r o s s - s t r a t i f i e d bed i s defined t o be simple i f i t does not contain
internal erosion s u r f a c e s (Fig. 1 A ) .
I n c o n t r a s t , where a c r o s s - s t r a t i f i e d bed
does contain erosional surfaces (Fig. l B ) , t h e s t r u c t u r e of t h e c o s e t has been defined a s f u r i o u s or double crossbedding crossbedding (Harms e t a l . , 1975).
(Reiche,
1938)
or
compound
The c r i t i c a l f a c t o r t h a t determines whether t h e crossbedding deposited by a bedform i s simple o r compound i s t h e absence o r c c u r r e n c e of i n t e r m i t t e n t erosion o n t h e l e e slope.
Where no erosion or-.urs, beds deposited on t h e l e e
slope a r e not t r u n c a t e d , bounding surfaces a r e not generated within t h e s e t , crossbedding i s simple. I n c o n t r a s t , where p a r t s of a l e e slope
and
occasionally undergo erosion, l a y e r s on t h e l e e slope a r e truncated, bounding surfaces a r e produced within t h e compound.
s e t , and t h e r e s u l t i n g crossbedding
is
Two kinds of processes can cause erosion on a l e e slope: ( 1 ) flow changes, such a s s h i f t i n g winds o r reversing c u r r e n t s , t h a t change bedform morphology a n d produce r e l a t i v e l y synchronous bounding surfaces t h a t a r e c a l l e d r e a c t i v a t i o n s u r f a c e s , and ( 2 ) local erosion t h a t may occur continuously ( b u t
a t s h i f t i n g locations) in t h e troughs o r on t h e stoss slopes of small bedforms t h a t migrate across t h e l e e slope of a l a r g e r bedform.
4 08
A
I
Crossbed
Cross-stratified b e d or set of crossbeds
Coset of crossbeds, compound set of crossbeds, or doubly cross-stratified bed Crossbed
\
Cross-stratified crossbed or set of crossbeds F i g . 1. ( A ) S e t o f s i i n p l e c r o s s b e d s ; n o b o u n d i n g s u r f a c e s w i t h i n t h e s e t . (B) Set o f compound c r o s s b e d s ; e r o s i o n a l b o u n d i n g s u r f a c e s w i t h i n t h e s e t . The o r i g i r - and s t r u c t u r e o f compound c r o s s b e d d i n g
produced b y s h i f t i n g
b e d f o r i n s and o t h e r t o p o g r a p h i c f e a t u r e s i s d i s c u s s e d i n t h i s paper.
The o r i g i n
a n d s t r u c t u r e o f compound c r o s s b e d d i n g p r o d u c e d b y f l u c t u a t i n g f l o w ,
and t h e
d i f f i c u l t p r o b l e m o f d i s t i n g u i s h i n g t h e t w o t y p e s o f compound c r o s s b e d d i n g , a r e d i s c u s s e d e l s e w h e r e ( H u n t e r and IRubin, 1 9 8 3 ) . Approach The
primary obstacle a
geometry i s t h a t preserved; surfaces
only
depositional
commonly
enviroment.
s e d i m e n t o l o g i s t faces i n r e c o n s t r u c t i n g bedform
usually only constitute
F o r example,
small
surfaces a
o n l y a small
can
be
small
i n a dune f i e l d ,
s m a l l f r a c t i o n o f t h e s u r f a c e area. depositional,
fragments o f t h e o r i g i n a l bedforms a r e preserved,
fraction
of
and a
depositional
"depositional"
depositional ( l e e ) slopes cover a
O f t h i s f r a c t i o n o f the surface t h a t i s
f r a c t i o n t h a t i s t o p o g r a p h i c a l l y l o w w i l l escape
b e i n g r e w o r k e d b y s u b s e q u e n t dune s t o s s s u r f a c e s a n d t r o u g h s ( R u b i n and H u n t e r ,
1982), a l l o w i n g p r e s e r v a t i o n o f p o s s i b l y o n l y a few p e r c e n t o f t h e o r i g i n a l bedform surface.
I n addition,
where a d u n e ' s l e e s u r f a c e i s c o v e r e d w i t h
s m a l l e r dunes o r o t h e r m i g r a t i n g topographic f e a t u r e s , t h e e r o s i o n a l surfaces o f those
features are
not
presewed either,
a n d t h e r e s u l t i n g d e p o s i t may
r e p r e s e n t l e s s t h a n o n e p e r c e n t o f t h e o r i g i n a l dune s u r f a c e . samples,
From t h e s e t i n y
s e d i m e n t o l o g i s t s a t t e m p t t o r e c o n s t r u c t e n t i r e b e d f o r m s and r e g i o n a l
flow fields.
409 D e s p i t e t h e small sample o f p r e s e r v e d s u r f a c e area, compound crossbedding commonly c o n t a i n s enough i n f o r m a t i o n t o a l l o w d e t a i l e d r e c o n s t r u c t i o n s o f t h e migration direction, the
c r e s t plan, and t r o u g h p r o f i l e o f p r i m a r y bedforms, and
migration direction,
bedforms.
I n addition,
crest
plan,
and
trough
profile o f
superimposed
some c o s e t s c o n t a i n evidence i n d i c a t i n g bedform s i z e
(Hunter, 1981; Rubin and Hunter,
1982) o r m i g r a t i o n r a t e s (Hunter and Rubin,
1983). The key t o r e c o n s t r u c t i n g bedform geometry f r o m compound crossbedding i s visualizing
the
translated planes.
structures
through
space
generated
and
later
when
hypothetical
lee
exposed
i n variably
oriented
I n t h e s i m p l e case where f l o w i s steady,
slopes
outcrop
two f a c t o r s c o n t r o l t h e
geometry o f t h e i n t e r n a l s t r u c t u r e s : ( 1 ) t h e shape o f t h e l e e s u r f a c e , and the
direction
that
the
surface
is
translated.
i n t e r p r e t e d by v i s u a l i z i n g v a r i o u s l e e - s l o p e until
finding
A
specific
deposit
(2) is
shapes and m i g r a t i o n d i r e c t i o n s
t h e c o m b i n a t i o n t h a t most c l o s e l y d u p l i c a t e s
observed i n t h e f i e l d .
are
To i l l u s t r a t e t h i s approach,
the structures
s t r u c t u r e s produced by
several common bedform assemblages a r e d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n s . SUPER IMPOSED BEDFORMS
L a r g e bedforms commonly s u p p o r t smal l e r superimposed bedforms because o f fluctuating flow conditions (Allen,
1978) and because l a r g e bedforms g e n e r a t e
n e a r - s u r f a c e boundary l a y e r s t h a t i n t u r n produce small bedforms even i n steady flows
(Rubin and McCulloch,
slope
of
a
l a r g e bedform
1980).
produces
r e c o n s t r u c t i n g bedform assemblages. scour
M i g r a t i o n o f small bedforms over t h e l e e
i n t o t h e l a r g e bedform,
two
structures
that
are
useful
for
F i r s t , t h e t r o u g h s o f t h e small bedforms
t h e r e b y g e n e r a t i n g bounding s u r f a c e s (A1 l e n ,
1968; B r o o k f i e l d , 1977; Jones, 1979).
Second, t h e l e e slopes o f t h e m i g r a t i n g
superimposed bedforms d e p o s i t crossbeds. The s c o u r i n g o f bounding s u r f a c e s b y t h e small bedforms i m p l i e s t h a t t h e small bedforms were c l imbing a t small a n g l e s ( t y p i c a l l y l e s s t h a n 15") r e l a t i v e t o t h e s u r f a c e o f t h e l a r g e bedform ( F i g . bedforms a r e two-dimensional,
2).
Consequently, where t h e small
t h e r e l a t i v e l y p l a n a r bounding s u r f a c e s t h a t t h e y
scour c a n b e measured t o e s t i m a t e t h e s t r i k e and d i p o f t h e l e e s l o p e o f t h e T h i s a p p r o x i m a t i o n may n o t always be a c c u r a t e where bounding
l a r g e dune.
s u r f a c e s d i p a t a n g l e s approaching 15" o r l e s s . Having approximated t h e t r e n d o f a l a r g e bedform w i t h t h e mean s t r i k e d i r e c t i o n o f bounding s u r f a c e s scoured by s m a l l superimposed bedforms, t h e n e x t s t e p i n r e c o n s t r u c t i n g a b e d f o r m assemblage i s t o d e t e r m i n e t h e t r e n d s o f t h e s m a l l bedforms. t h e bounding
Two k i n d s o f p l a n a r s t r u c t u r e s g e n e r a t e d b y t h e s e bedforms--
surfaces
t h a t t h e y scour and t h e crossbeds
that they
deposit--
410
A
B
s 'ig. 2. Compound crossbedding deposited by downslope-migrating dunes. ( A ) Vertical s e c t i o n showing dune morphology. ( B ) Block d i a g r a m showing inte rna l s t r u c t u r e . Crossbeds a n d bounding s u r f aces t h a t se pa ra te crossbeds d i p i n same d i rec t i on.
i n t e r s e c t a l o n g t he t r o u g h 1 i n es of t h e superimposed bedforins. the line of stereonet,
intersection
of
those two
planes,
Consequently,
conveniently plotte d on a
i n d i c a t e s t h e trend of t h e Superimposed bedforms.
Thus, cross-
s t r a t i f i e d beds deposited by small bedforms migrating on t h e l e e slopes of l a r g e bedforms bedforms.
can
be
used
to
determine t h e
tre nds
of
both
s c a l e s of
This r e co n s t r u ct i o n technique a n d o t h e r s discussed in t h e t e x t a r e
smmarized in Table 1. Of a l l t h e assemblages of bedforms t h a t generate compound crossbedding, the
two-dimensional
cas e
where
r eg u l ar
straight-crested
bedforms
migrate
d i r e c t l y downslope i s t h e e a s i e s t t o v i s u a l i z e ( F i g . 2A; o r Allen, 1968, fig. 5.16b),
although downslope migration may be no more common t h a n oblique or
alongsl ope migration.
Superimposed bedforms t h a t migrate downslope deposit
crossbeds and scour bounding s u r f aces t h a t d i p in t h e same d i r e c t i o n , which i s t h e d i p d i r e c t i o n of t h e l e e slope of t h e l a r g e bedform (Fig. 2B). Another
bedform
assemblage
that
g en era te s
compound
crossbedding
superimposed bedforms t h a t migrate d i r e c t l y alongslope (Fig. 3A). t h i s assemblage a r e r e l a t i v e l y d i f f i c u l t crossbeds deposited by alongslope-migrating
is
Deposits of
t o v i s u a l i z e a c c u r a t e l y , because bedforms d i p obliquely down the
primary l e e s l o p e , r a t h e r t h a n d i r e c t l y alongslope (Fig. 38). For example, the compound crossbedding i n F i g . 3C was deposited by superimposed dunes migrating almost exactly alongslope (toward 290" on a l e e slope dipping toward Z O O " ) .
Lee slope of l a r g e dune
N
S t o s s s l o p e of l a r g e dune
/
I
/
I
w
scoured b v superimposed
/I
deposited by superimposed
B
C
I n t e r s e c t i o n line and t r o u g h s of s u p e r i m p o s e d d u n e s t r e n d t o w a r d 200°
Fig. 3. Compound c r o s s b e d d i n g g e n e r a t e d b y a l o n g s l ope( A ) Diagrammatic s k e t c h m i g r a t i n g s t r a i g h t - c r e s t e d dunes. showing o b l i q u e v i e w o f dune m o r p h o l o g y and i n t e r n a l structure. ( B ) B l o c k d i a g r a m showing i n t e r n a l s t r u c t u r e . ( C ) Example from E n t r a d a Sandstone n e a r Page, A r i z o n a ; p r i m a r y dune m i g r a t e d s o u t h w e s t f r o m r i g h t t o l e f t ( t h e d i r e c t i o n o f bounding surface d i p ) ; superimposed dunes m i g r a t e d n o r t h w e s t (away f r o m v i e w e r ) . (D) S t e r e o n e t p l o t P r i m a r y dune m i g r a t e d i n d i p d i r e c t i o n o f b e d s shown i n C. o f t h e b o u n d i n g s u r f a c e s s c o u r e d by s u p e r i m p o s e d dunes. T r e n d o f s u p e r i m p o s e d dunes i s d e f i n e d b y t h e l i n e o f i n t e r s e c t i o n o f t h e bounding s u r f a c e s and crossbedding t h a t t h e superimposed dunes d e p o s i t e d , a s e x p l a i n e d i n t e x t .
412 However,
t h e crossbeds
deposited by the
downslope ( t o w a r d 245").
superimposed
dunes
dip
obliquely
W i t h o u t p e r f o r m i n g a s t e r o n e t a n a l y s i s ( F i g . 3D), t h e
m i g r a t i o n d i r e c t i o n o f t h e superimposed dunes m i g h t b e i n f e r r e d t o b e i n t h e d i p d i r e c t i o n o f t h e c r o s s b e d s d e p o s i t e d b y t h e s u p e r i m p o s e d dunes, and a 45degree e r r o r would r e s u l t . Where
the
dimensional, (Fig.
small
bedforms
superimposed
w i l l approximately p a r a l l e l
4A)
on
a
lee
slope
are
three-
t h e a x e s o f t h e t r o u g h - s h a p e d b o u n d i n g s u r f a c e s t h a t t h e y scour t h e l e e s l o p e o f t h e l a r g e bedform.
C o n s e q u e n t l y , where t r o u g h a x e s h a v e d i v e r g i n g a z i m u t h s , t h e l e e s l o p e o f t h e l a r g e dune c a n b e a p p r o x i m a t e d b y t h e p l a n e t h a t c o n t a i n s t h e t r o u g h a x e s ( F i g . 4B).
Where t h e t r o u g h a x e s have s i m i l a r a z i m u t h s , t h e l e e s u r f a c e o f t h e l a r g e (1) t h e average
dune i s a p p r o x i m a t e d b y a p l a n e t h a t i s d e f i n e d b y t w o l i n e s : t r o u g h a x i s and ( 2 )
t h e l i n e r e p r e s e n t i n g t h e average t r a c e o f t h e trough
bounding 'surfaces as observed i n a s e c t i o n o b l i q u e o r transverse t o t h e trough a x e s ( F i g . 4C). CRABBING BASE-OF-SLOPE SCOUR PITS AND SCALLOPED CROSSBEDDING Migrating capable o f
superimposed bedforms a r e
causing
the
compound c r o s s b e d d i n g .
local
not t h e o n l y topographic
erosion that
produces bounding
features
s u r f a c e s and
Scour p i t s a n d i n t e r v e n i n g s p u r s t h a t c r a b , o r m i g r a t e
l a t e r a l l y , a l o n g t h e b a s e o f a l e e s l o p e c a n a l s o p r o d u c e l o c a l e r o s i o n and thereby generate bounding surfaces.
For reasons discussed below,
deposits
produced b y t h e s e f e a t u r e s r e q u i r e d i f f e r e n t i n t e r p r e t i v e techniques from those developed f o r superimposed bedforms. As a s i n g l e s c o u r p i t m i g r a t e s w i t h o u t c h a n g i n g shape o r d i r e c t i o n , l e a d i n g s u r f a c e erodes t h e sediment t h a t t h e scour p i t encounters, trailing
surface i s preserved b y deposition.
s i n g l e trough-shaped s e t o f crossbeds.
The r e s u l t i n g
its
and i t s
structure
is a
The t r o u g h a x i s i n d i c a t e s t h e p a t h o f
m i g r a t i o n , w h i c h i s n e a r l y h o r i z o n t a l b e c a u s e t h e s c o u r p i t o c c u r s a t t h e base o f t h e l e e slope,
a n d t h e shape o f t h e c r o s s b e d s i n d i c a t e s t h e shape o f t h e
scour p i t ' s t r a i l i n g s u r f a c e (Fig. r e w o r k i n g b y subsequent s c o u r p i t s ,
5A).
Where t h e t r o u g h - s h a p e d s e t escapes
a s when m i g r a t i o n c e a s e s o r where s c o u r
p i t s a r e w i d e l y spaced a n d d o n o t m i g r a t e o n o v e r l a p p i n g p a t h s , t h e s e t may be preserved r e l a t i v e l y completely (Fig.
I n many c a s e s ,
however,
5B).
trough-shaped
s e t s a r e r e w o r k e d b y subsequent
s c o u r p i t s , e i t h e r a t random o r i n some o r g a n i z e d p a t t e r n .
F o r example, where
a t r a i n o f s c o u r p i t s m i g r a t e s a l o n g t h e b a s e o f a m i g r a t i n g l e e s l o p e , each scour
p i t may s c o u r
preceding
scour
pit.
i n t o p a r t o f t h e trough-shaped The r e s u l t i n g
structure
i s a
set
deposited b y the
sequence o f l a t e r a l l y
t r u n c a t e d t r o u g h - s h a p e d s e t s t h a t become p r o g r e s s i v e l y y o u n g e r i n a c o n s i s t e n t
413
Fig. 4. Compound c r o s s b e d d i n g g e n e r a t e d by smal 1 t h r e e - d i m e n s i o n a l bedforms superimposed on l a r g e r bedforms. A. Example from t h e Cntrada Sandstone near B. Block diagram i l l u s t r a t i n g s t r u c t u r e produced by Page, Arizona. superimposed t h r e e - d i m e n s i o n a l bedforrns t h a t m i g r a t e d i n d i f f e r e n t d i r e c t i o n s . Lee s u r f a c e o f t h e l a r g e bedform i s approximated by t h e p l a n e t h a t c o n t a i n s t h e axes of t h e t r o u g h s t h a t were scoured by t h e small dunes. C. Block diagram i l l u s t r a t i n g s t r u c t u r e produced by superimposed three-dimensional bedfoniis t h a t migrated i n t h e same d i r e c t i o n . Lee s u r f a c e of t h e l a r g e bedform i s approximated by t h e p l a n e t h a t i s d e f i n e d by t w o l i n e s : ( 1 ) t h e a v e r a g e trough a x i s , and ( 2 ) t h e a v e r a g e b o u n d i n g - s u r f a c e t r a c e ( d e t a i l s i n t e x t ) .
414
A
B
F i g . 5. Trough-shaped s e t s o f c r o s s b e d s d e p o s i t e d b y m i g r a t i n g s c o u r p i t . (A) ( B ) Example f r o m t h e N a v a j o Sandstone a t Snow Canyon, Utah; s e t Block diagram. i s a p p r o x i m a t e l y 10 m t h i c k .
415
d i r e c t i o n ( F i g . GA and B ) . t h i s process will
The lower bounding s u r f a c e o f a s e t d e p o s i t e d by
usually appear scallop-shaped,
and we t h e r e f o r e use t h e
d e s c r i p t i v e terms s c a l l o p e d c r o s s b e d d i n g o r s c a l l o p s f o r t h e s e s t r u c t u r e s . S c a l l o p e d c r o s s b e d d i n g c a n a l s o form when f l u c t u a t i n g cause
trough t o
a bedform
a l t e r n a t e l y aggrade and
flow c o n d i t i o n s
scour d u r i n g m i g r a t i o n
(Hunter and Ruhin, 1 9 8 3 ) , b u t t h e two k i n d s of s c a l l o p s c a n b e d i s t i n g u i s h e d by the three-dimensional bedforins
during
I n s c a l l o p s d e p o s i t e d by
geometry o f t h e s t r u c t u r e s .
fluctuating
flow, crossbeds d i p
in t h e
same d i r e c t i o n a s
I n s c a l l o p s d e p o s i t e d by c r a b b i n g s c o u r p i t s , c r o s s b e d d i p
bounding s u r f a c e s .
d i r e c t i o n s d e v i a t e from b o u n d i n g - s u r f a c e d i p d i r e c t i o n s ; t h e d e v i a t i o n i n d i p direction hounding
is
not
constant,
but
s u r f a c e s and g r e a t e s t
i s least
i n beds t h a t
i n beds t h a t
immediately o v e r l y
immediately u n d e r l i e t h e next
bounding s u r f a c e ( h o r i z o n t a l s e c t i o n in Fig. 6B). We have i d e n t i f i e d s c a l l o p e d c r o s s b e d d i n g d e p o s i t e d by c r a b b i n g s c o u r p i t s
i n e o l i a n s a n d s t o n e s on t h e Colorado P l a t e a u ( F i g . 6C, D , and E ) , i n f l u v i a l beds i n t h e Wingate Sandstone ( T r i a s s i c ) i n Utah, i n Brazos River d e p o s i t s , and S t r u c t u r e s t h a t a p p a r e n t l y o r i g i n a t e d by
i n modern dunes o f t h e Oregon Coast.
a
similar
process
have been
r e p o r t e d by
Brookfield
(1979).
S c a l l o p s of
undetermined o r i g i n a r e i l l u s t r a t e d by l4cKee (1966, f i g . 6 , 1979, f i g . 153B), Gregory (1950, f i g . 45a and f ) , and B i g a r e l l a and Salaniuni ( 1 9 6 7 , f i g . 1 1 - 1 ) . D i s t i n g u i s h i n g Crabbing Scour P i t s from Sumperimposed Bedforns Scour
pits
along
t h e base
of
a
l e e s l o p e and
superimposed
resemble each o t h e r and may even g r a d e from one t o t h e o t h e r . distinguishing
their
deposits
is
i m p o r t a n t because
d i f f e r e n t i n t e r p r e t i v e t e c h n i q u e s (Table 1 ) .
t w kinds of f e a t u r e s i s size. be
superimposed
on
s t b s t a n t i a l l y smal l e r .
a
primary
bedfonns However,
their deposits require
One m a j o r d i f f e r e n c e between t h e
In o r d e r f o r a t r a i n o f secondary bedfonns t o bedform,
the
secondary bedforms
must
be
Consequently, superimposed bedfonns t e n d t o d e p o s i t
c r o s s - s t r a t i f i e d b e d s t h a t a r e t h i n compared t o t h e compound set d e p o s i t e d b y t h e primary bedform ( F i g . 2 A ) . Unlike superimposed bedforms, which c a n b e thought o f a s small s u r f i c i a l f e a t u r e s , s c o u r p i t s and t h e r i d g e s o r spurs s e p a r a t i n g them c a n e i t h e r b e small superimposed f e a t u r e s o r c a n b e p a r t o f t h e b a s i c bedform s t r u c t u r e and may b e comparable i n s i z e t o t h e primary bedform.
Scour p i t s may b e a s deep a s
bedforms a r e h i g h , and a s a r e s u l t , t h e s e t s d e p o s i t e d by c r a b b i n g s c o u r p i t s may b e a s t h i c k a s t h e e n t i r e c o s e t o f beds d e p o s i t e d by a primary bedform.
The o t h e r m a j o r d i f f e r e n c e between t h e s e two morphological end members i s t h a t s c o u r p i t s along t h e b a s e o f a l e e s l o p e a r e t o p o g r a p h i c a l l y very low. Consequently, t h e y c a n s c o u r a d i s t i n c t i v e s c a l l o p e d s u r f a c e t h a t f o n s t h e
B
_A_
Horizontal section
Miaration direction of primary dune
-
Level of horizontal B section _ _ _ - in -
& ----.-
m
-Lee
slope of
--,-----
\EcTI
Scourpit migration direction
’
Leading (erosional) surface of scour pit
Trilling (depositinonal) surface of scour pit
Vertica.I s’ections’
F i g . 6. O r i g i n o f s c a l l o p e d c r o s s b e d d i n g . ( A ) S c h e m a t i c c o n t o u r map o f dune t o p o g r a p h y . (B) D i a g r a m showing s t r u c t u r e s d e p o s i t e d b y t h e dune and s c o u r p i t s i n A. (C) Scalloped crossbedding i n a plane t h a t i s approximately p a r a l l e l t o t h e m i g r a t i o n d i r e c t i o n o f t h e primary (D) Snow Canyon dune; N a v a j o Sandstone a t Snow Canyon, Utah. S e t i s a p p r o x i m a t e l y 10 in t h i c k . ( E ) Two s e t s o f scallops i n a plane t h a t i s oblique t o d i r e c t i o n o f scour-pit migration. s c a l l o p s i n N a v a j o Sandstone, Z i o n N a t i o n a l Park. S e t s a r e 5-10 m t h i c k . ( F ) Modern a n a l o g f r o m Oregon c o a s t - - d u n e s have s p u r s a n d s c o u r p i t s t h a t m i g r a t e t o w a r d v i e w e r .
418 lower bounding surface of a coset. the
Because the topograpically low p o s i t i o n o f
scour p i t s g i v e s them a very high
preservation p o t e n t i a l ,
scalloped
crossbedding i s a useful tool f o r i n t e p r e t i n g bedform behavior. Direction of Scour-Pit Migration The d i r e c t i o n of migration of a scour p i t can be determined e i t h e r by measuring the a x i s of t h e trough-shaped set t h a t i t deposited o r by measuring t h e s t r i k e of t h e steeply dipping s i d e s of t h e trough.
The trough a x i s i s more
precise in theory, because i t i s l e s s a f f e c t e d by changes in s i z e undergone by t h e scour p i t during migration.
In many outcrops, however, the s t r i k e of t h e
trough s i d e s can be measured more accurately. Orientation of Lee Slope of Primary Bedform
I n c o n t r a s t t o t h e bounding surfaces scoured by superimposed bedforms, those scoured by scour p i t s crabbing along the base of a l e e slope may not approximate t h e trend of the primary bedform. This d i f f e r e n c e a r i s e s because scour p i t s may not always be s u p e r f i c i a l features. They may, t h e r e f o r e , extensively rework t h e bedform on which they e x i s t . Another technique i s necessary t o determine t h e trend of the primary bedform. Where scour p i t s migrate nearly p a r a l l e l t o t h e l e e slope of t h e primary bedform, t h e p r e f e r e n t i a l l y preserved s i d e s of t h e troughs t h a t they scour trend nearly p a r a l l e l t o t h e l e e slope of t h e primary bedform.
I n contrast,
where scour p i t s migrate i n nearly t h e same d i r e c t i o n as primary bedforms, t h e mean d i p d i r e c t i o n of a l l crossbeds deposited within scour p i t s , r a t h e r t h a n bounding s u r f a c e s , approximates t h e d i p d i r e c t i o n of t h e generalized l e e slope. A t a l l intermediate angles, t h e lee-slope d i p d i r e c t i o n l i e s between the mean d i p d i r e c t i o n of t h e crossbeds and t h a t of t h e i r bounding surfaces (Fig. 7).
For convenience we can approximate t h e original lee-slope d i p d i r e c t i o n
with t h e c e n t r a l value of t h i s range by taking t h e mean of those two q u a n t i t i e s - -t he bounding- surface d i p d i r e c t i o n and t h e c rossbed d i p d i r e c t i o n (Fig. 7 ) . I t can be shown g r a p h i c a l l y t h a t t h i s c a l c u l a t e d o r i e n t a t i o n i s most accurate i f crossbed d i p d i r e c t i o n s a r e averaged a t t h e top of a scallop. Evaluating t h i s approximation with c i r c u l a r scour p i t s having a diameter D, center-to-center
spacings ranging from D t o 10D, and a l l angles of crab t h a t
generate compound crossbedding, we f i n d t h a t t h e mean e r r o r i s 11" and the maximum e r r o r 45"; 80 percent of t h e approximations have a n e r r o r l e s s t h a n 20". Where t h e bounding-surface d i p d i r e c t i o n d e v i a t e s by l e s s t h a n 45" from t h e mean crossbed d i p d i r e c t i o n , a s was t h e case f o r many of t h e s e t s we observed in t h e f i e l d , e r r o r s a r e l e s s (23" maximum, 5" mean).
419 (1) POSITIONS
OF
(3) BOUNDING SURFACE
LEE SLOPE
(2) MIGRATION DIRECTION
GENERATED BY MIGRATING
OF PRIMARY DUNE AT EARLY
LATER TIME
(6)DIP DIRECTION OF PRIMARY LEE SLOPE Fig. 7. H o r i z o n t a l s e c t i o n t h r o u g h beds d e p o s i t e d by a dune w i t h c r a b b i n g scour pits. D i p d i r e c t i o n o f g e n e r a l i z e d l e e s l o p e ( 6 ) l i e s between d i p d i r e c t i o n o f crossbeds w i t h i n s c a l l o p s ( 5 ) and d i p d i r e c t i o n o f bounding s u r f a c e s o f s c a l 1 ops
.
S c o u r - P i t Shape and O r i g i n s o f Bounding Surfaces w i t h i n Trougho r Scallop-Shaped S e t s Crossbeds
iri a
trough-shaped
set
d e p o s i t e d by a m i g r a t i n g
scour
pit
p r e s e r v e t h e stlape o f t h e scour p i t ' s t r a i l i n g s u r f a c e ( A l l e n , 1965, Chapter
5.6).
The shape o f t h e l e a d i n g s u r f a c e c a n o n l y b e i n f e r r e d , because i t i s
e r o s i o n a l and l e a v e s no d e p o s i t . A l l t h e examples d i s c u s s e d above i l l u s t r a t e t r o u g h - o r scallop-shaped s e t s t h a t were d e p o s i t e d b y n e a r l y c i r c u l a r scour p i t s m i g r a t i n g i n s t r a i g h t l i n e s , and c r o s s b e d d i n g w i t h i n such s e t s i s s i m p l e (no bounding s u r f a c e s c o n t a i n e d within
sets).
However,
crossbedding
s b s e t s b y i n t e r n a l bounding surfaces. form b y a t migration,
least
three
processes:
within
s c a l l o p s may b e d i v i d e d i n t o
These i n t e r n a l bounding s u r f a c e s can
(1) changes i n d i r e c t i o n o f s c o u r - p i t
( 2 ) changes i n s c o u r - p i t morphology,
i n c l u d i n g s i z e , shape, o r t h e
l o c a t i o n o f t o p o g r a p h i c f e a t u r e s such a s bedforms w i t h i n scour p i t s , and ( 3 ) m i g r a t i o n o f s c o u r p i t s w i t h c o n v o l u t e d t r a i l i n g surfaces. o p e r a t e s where t h e t r a i l i n g surfaces
that
surfaces)
are
m i g r a t i o n (i.e.
dip
in
the
surface o f a general
separated b y a
scour
migration
pit
direction
s u r f a c e d i p p i n g away
an erosional surface).
T h i s l a s t process
i s so i r r e g u l a r t h a t (i.e.
depositional
from t h e d i r e c t i o n o f
S c a l l o p s c o n t a i n i n g subsets formed by
t h i s process a r e i l l u s t r a t e d i n t h e a n c i e n t example d i s c u s s e d below.
420
RECOGNITION OF LONGITUDINAL AND OBLIQUE DUNES Small bedforms superimposed on a l a r g e oblique or longitudinal bedform can b e expected t o migrate systematically with a component in a preferred alongc r e s t d i r e c t i o n (Beutner,
Fluekinger, and Card,
1967; Jones, 1979; Hunter,
I n c o n t r a s t , on a transverse bedform, superimposed bedforms would n o t
1981).
be expected t o migrate with a preferred along-crest component. longitudinal
Consequently,
o r oblique bedforms can be i d e n t i f i e d by documenting t h a t the
superimposed bedforms migrated with a preferred along-crest component over t h e l a r g e bedforms. EXAMPLES
Ancient The Navajo Sandstone a t Diana's Throne ( e a s t of Zion National Park a t 37"
11' l a t i t u d e , 112" 38' l o n g i t u d e ) , Utah, contains a s e t of scallop-shaped beds t h a t p a r t i c u l a r l y well i l l u s t r a t e s t h e i n t e r p r e t i v e potential of t h e techniques presented above. The c h a r a c t e r i s t i c s of t h e exposed beds a r e r e l a t i v e l y simple t o describe, b u t mentally v i s u a l i z i n g the process whereby those beds were deposited by a s i n g l e complicated dune can be an extremely d i f f i c u l t a n d f r u s t r a t i n g exercise. However, t h i s kind of exercise i s unavoidable i f reconstructing dune morphology i s d e s i r e d , o r i f t h e d e p o s i t s of longitudinal a n d oblique dunes a r e t o be recognized. The example i s a s e t of scallop-shaped beds with t h e following generalized c h a r a c t e r i s t i c s (Fig. 8 A ) : ( 1 ) Trends of s c a l l o p axes and s t r i k e s of s c a l l o p s i d e s a r e between 135" a n d 170".
(2)
Scallops a r e each f i l l e d with two subsets of crossbeds.
Crossbeds i n the
basal subset a r e r e l a t i v e l y planar and d i p e a s t (80"-95").
Crossbeds i n
t h e overlying subset a r e curved and d i p in d i r e c t i o n s ranging from southeast t o southwest. Crossbed d i p d i r e c t i o n s within scallops range from 85" t o 240", indicating t h a t the scallops were generated by scour p i t s migrating i n t o t h e southeast quadrant (Fig. 88). The trough-axis t r e n d s and bounding-surface s t r i k e s i d e n t i f y t h e mean d i r e c t i o n o f migration a s approximately 160'. Using t h e technique developed above, t h e generalized d i p d i r e c t i o n of t h e primary l e e slope along which t h e scour p i t s migrated l i e s between t h e mean d i p d i r e c t i o n o f crossbeds a t the tops of t h e scallops (approximately 225") an!
d i r e c t i o n of bounding s u r f a c e s (250').
t h e mean d i p
For convenience, t h e c e n t r a l value of
t h i s range (235"-240") can b e used t o c h a r a c t e r i z e t h e generalized lee-slope dip direction.
421
By t r i a l and e r r o r , we discovered t h a t a "0"-shaped scour p i t with an internal spur (Fig. 8C), when t r a n s l a t e d toward 160" and observed in a n e a s t west-trending outcrop, would generate a scallop with internal s t r u c t u r e s identical t o those observed. The s t r a i g h t s i d e of t h e "D" generates a subset
o f eastward-dipping planar crossbeds, and the curved portion of t h e t r a i l i n g surface generates a subset of southwest-dipping concave-downwind crossbeds. The bounding surface between t h e two subsets i s scoured by an erosional trough north of a spur projecting i n t o t h e scour p i t ( l i n e TR in Fig. 8C). That t r o u g h plunges eastward, and, when t r a n s l a t e d toward 160", generates an eastward-dipping bounding surface t h a t separates t h e two subsets deposited by t h e t r a i l i n g surface of t h e scour p i t (Fig. 80). The eastern s c a l l o p in t h e s e t i l l u s t r a t e d
in
Fig.
8A contains a n
additional f e a t u r e not found in t h e others--a t h i r d subset of crossbeds ( t h a t dip t o w a r d 15"). T h a t subset i s inferred t o have been deposited by a bedform-possibly another spur projecting into the scour p i t - - t h a t migrated northward within t h e southeast-migrating scour p i t . The reconstruction presented above was a r r i v e d a t by t r i a l and e r r o r , b u t Figures 8C and D v e r i f y t h a t t h e proposed bedforms would accurately generate t h e bedding observed
in t h e f i e l d .
Moreover,
no other mechanism we can
envision i s capable of producing even a remotely s i m i l a r s t r u c t u r e . The assemblage of bedforms a r i s i n g from t h i s reconstruction (Fig. 8E) i s r e l a t i v e l y complex, b u t i t contains no f e a t u r e s t h a t a r e unusual i n modern dune fields. For example, a f a i r l y exact modern analog of such an assemblage of bedforms occurs in a dune f i e l d on t h e Oregon coast. In t h a t f i e l d , small dunes migrate along t h e troughs and lower l e e slopes of t h e l a r g e dunes (Fig. 8F and Hunter, Richmond, and Alpha, in press). Topographic depressions, o r scour p i t s , a r e bounded by t h e two s e t s of dunes; migration of t h e secondary dunes along t h e base of t h e primary l e e slope causes t h e scour p i t s t o crab. Scour p i t s i n t h e modern example (Fig. 8F) even contain spurs l i k e those we envision f o r t h e Navajo Sandstone example. A d i f f e r e n c e between t h e s e modern dunes and the reconstructed
ancient example i s t h a t t h e modern dunes a r e
destroyed and regeneraed a s a r e s u l t of seasonally changing winds. The examples of compound crossbedding presented above (Figs. 2-6 and 8 ) a r e r e l a t i v e l y straightforward because t h e i r formation can be visualized a s r e l a t i v e l y steady processes. We have a1 so encountered some extremely complicated examples t h a t suggest unsteady processes: zigzagging scour p i t s and intervening spurs (Hunter and Rubin, 1983), scour p i t s t h a t moved in nmerous d i r e c t i o n s in apparently disorganized fashion, and one scour p i t t h a t appeared
a s though i t may have temporarily followed a v e r t i c a l l y corkscrewing p a t h .
rossbed dip direction
ounding surface strikes
rough axis trends
S'
.I
t
r
B
160"
16
424 No d e r n
Cocipound crossbedding produced by superimposed b e d f o r m has been found i n modern a s well a s a n c i e n t e o l i a n sands.
One l o c a l i t y where Lie have seen such
f e a t u r e s i s on wind-scoured s t o s s s l o p e s of barchanoid dunes j u s t north o f the
m o u t h of S u t t o n Creek, 11
krii
north of Florence, Oregon.
Ilere, t h e coiiipound
crossbedding has a component of d i p t o the r i g h t (southwest) of t h e primary crossbedding, which d i p s t o t h e s o u t h e a s t ( F i g . 9 ) . Short-term observations o f dune processes suggest crossbedding
in
these
dunes
is
produced
by
differences
that
in
the
d i r e c t i o n s of t h e dunes and of f e a t u r e s superimposed on the dunes. superimposed f e a t u r e s
include
coi8iporind
t h e iiiigration These
t h e plan-fon:i s i n u o s i t i e s o f t h e barchanoitl
dunes, downwind-pointed s p u r s , a n d scour p i t s between t h e spurs ( F i g s . GF, 8F, a n d Cooper, 195S, p.
44-45).
Because t h e barchanoid dunes have a n averdge
trend t h a t i s n o t p e r f e c t l y t r a n s v e r s e t o t h e n o r t h e a s t e r l y simiier winds t h a t
Fig. 9. Wind-scoured, nearly h o r i z o n t a l , lower s t o s s slope of dune near S u t t o n Creek, Oregon. Crossbeds a n d bounding s u r f a c e s d i p mainly t o w a r d t h e t o p of photo ( i n t h e general d i r e c t i o n of dune i n i g r a t i o n ) , b u t several s u b s e t s t h a t were deposited by alongslope-migrating spurs have components of crossbed d i p t o the right.
425 shape them (Cooper, 1958, p. 31-33) t h e i r l e e s l o p e s a r e a f f e c t e d b y lee-eddy winds t h a t have an a l o n g s l o p e component (Hunter, 1981).
Under t h e i n f l u e n c e o f
these l e e eddies, t h e s a l i e n t s and spurs t e n d t o m i g r a t e c r a b w i s e a l o n g t h e l e e slopes. the
Given t h e u n i f o r m i t y o f t h e wind c l i m a t e a l o n g t h e Oregon c o a s t d u r i n g
summers,
this
process
could
easily
produce
the
observed
compound
crossbeddi ng. CONCLUSIONS Much o f t h e c o m p l e x i t y o f c r o s s - s t r a t i f i e d
beds i s n e i t h e r random nor
i n e x p l i c a b l e , b u t c a n b e demonstrated t o r e s u l t f r o m t h e m i g r a t i o n o f s m a l l bedforms on t h e l e e s l o p e s o f l a r g e bedforms. M i g r a t i n g bedforms w i t h l e e s l o p e s t h a t never e x p e r i e n c e e r o s i o n d e p o s i t simp1 e c r o s s - s t r a t i f i e d beds. One process t h a t causes e r o s i o n on l e e s l o p e s ( t h e r e b y p r o d u c i n g compound c r o s s b e d d i n g ) i s t h e m i g r a t i o n o f small superimposed bedforms across t h e l e e s l o p e o f a l a r g e bedform.
I n general, superimposed bedforms d e p o s i t
c r o s s - s t r a t i f i e d beds t h a t a r e t h i n compared t o t h e compound c r o s s s t r a t i f i e d b e d d e p o s i t e d by t h e l a r g e bedform.
The m i g r a t i o n d i r e c t i o n
o f t h e l a r g e bedform i s i n t h e d i p d i r e c t i o n o f bounding s u r f a c e s scoured b y t h e superimposed bedforms,
not necessarily i n the d i p direction o f
crossbeds t h a t t h e superimposed bedforms d e p o s i t .
The mean t r e n d o f t h e
superimposed bedforms i s para1 l e l t o t h e i n t e r s e c t i o n o f t h e crossbeds and bounding s u r f a c e s generated b y t h e superimposed bedforms.
The mean
m i g r a t i o n d i r e c t i o n o f t h e superimposed bedforms i s normal t o t h e i r mean trend. Scour p i t s t h a t m i g r a t e l a t e r a l l y , o r c r a b , a l o n g a l e e s l o p e generate compound crossbeds w i t h scallop-shaped l o w e r bounding surfaces.
The d i p
d i r e c t i o n o f t h e g e n e r a l i z e d l e e s l o p e l i e s between t h e d i p d i r e c t i o n s o f crossbeds and bounding s u r f a c e s generated b y t h e m i g r a t i n g scour p i t s . The m i g r a t i o n d i r e c t i o n o f a scour p i t i s p a r a l l e l t o t h e a x i s o f t h e trough-
o r scallop-shaped
s e t o f crossbeds d e p o s i t e d b y t h e m i g r a t i n g
scour p i t and i s p a r a l l e l t o t h e s t r i k e o f t h e bounding s u r f a c e scoured by t h e m i g r a t i n g scour p i t . The d e p o s i t s o f l o n g i t u d i n a l and o b l i q u e bedforms c a n b e r e c o g n i z e d where small
bedforms m i g r a t e d
systematically
w i t h a component
i s preferred
along-crest d i r e c t i o n . ACKNOWLEDCMENTS Ed C1 i f t o n (USGS) and Michael B r o o k f i e l d ( U n i v e r s i t y o f Guelph) reviewed a n e a r l y v e r s i o n o f t h i s paper.
Jeanne Blank d r a f t e d t h e f i n a l i l l u s t r a t i o n s .
426
TABLE 1 TECHNIQUES FOR INTERPRETING COMPOUND CROSSBEDDING DEPOSITED BY SUPERIMPOSED BEDFORMS AND C R A B B I I G SCOUR PITS
TYPE OF SECOliDARY FEATURE I n t e r p r e t i v e Goal
T\r o - D imen s io na 1 Superimposed Bedf o rms
Crabbing Scour P i t s
D i s t ing u i s h ing between superimposed bedforms and c r a b b i n g scour p i t s
Sets a r e generally t h i n compared to coset ; bounding s u r f a c e s a r e n o t scalloped; sets are r e l a t i v e l y tabu1 ar.
S e t s can b e as t h i c k a s coset; 1ower bounding surface of coset is scalloped; i n horizontal plane crossbeds are curved.
Determining orientation o f secondary f e a t u r e s
Trend i s p a r a l l e l t o l i n e of intersection of c rossbeds a nd bound ing surfaces generated by superimposed bedform.
Not appl i c a b l e -- scour p i t s may b e r e l a t i v e l y equidimensional.
Determining m i g r a t ion d ir e c t ion o f secondary features
Migration direction is normal to the trend d e t e r m i n e d above.
is Migration direction parallel to: (1) t r e n d o f trough o r scallop axis, and ( 2 ) bounding-surface strike.
Determining d i p direction o f primary bedform's 1 ee s l o p e
Dip o f primary l e e slope is approximated b y d i p direction of bounding surfaces deposited by superimposed bedforms.
Dip direction of the generalized l e e slope l i e s between t h e f o l l o w i n g two d ir ec t ion s : (1) mean crossbed d i p direction, and (2) d i p d i r e c t i o n o f bounding s u r f a c e s scoured b y m i g r a t i n g scour p i t .
D e t e r m i n i n g shape o f secondary features
Shape o f l e e s l o p e superimposed bedforms preserved by shape c rossb eds
of is of
T r a i l i n g s u r f a c e o f scour p i t s i s p r e s e r v e d b y shape of crossbeds within s c a l l ops.
Determining c r e s t p l a n shape o f primary bedform
Crest-plan shape is i n f e r r e d f r o m shape o f b o u n d i n g s u r f a c e s scoured b y superimposed bedforms.
Crest-plan shape is i n f e r r e d f r o m s i z e , shape, and spacing o f scour p i t s .
.
427 REFERENCES A l l e n , J. R. L., 1968. C u r r e n t r i p p l e s . N o r t h - H o l l a n d Pub. Co., Amsterdam, 433 PA l l e n , J. R. L., 1978. Polymodal dune assemblages: a n i n t e r p r e t a t i o n i n terms o f dune c r e a t i o n - d e s t r u c t i o n i n p e r i o d i c flows. Sedimentary Geology, 20:
17-28. J. R.
L., 1980. Sand waves: a model o f o r i g i n and i n t e r n a l s t r u c t u r e . Sedimentary Geology, 26: 281-328. Banks, Id. L., 1973. The o r i g i n and s i g n i f i c a n c e o f some down-current d i p p i n g c r o s s - s t r a t i f i e d s e t s : Jour. Sed. P e t r o l o g y , 43: 423-427. Beutner, F. C., F l u e c k i n g e r , L. A., and Gard, T. M. 1967. Bedding geometry i n a P e n n s y l v a n i a n channel sandstone. B u l l . Geol. SOC. America, 78: 911-916. B i g a r e l l a , J. J., and Salamuni, Riad, 1967. Some palaeogeographic and p a l a e o t e c t o n i c f e a t u r e s o f t h e Parana Basin: I n B i g a r e l l a , J. J., Becker, R. D., and P i n t o , I. D. ( E d i t o r s ) , Problems i n B r a z i l i a n Gondwana Geology, Internat. Symposium on t h e Gondwana S t r a t i g r a p h y and P a l e o n t o l o g y , l s t , Mara Del P l a t a , p. 235-301. B r o o k f i e l d , M. E., 1977. The o r i g i n o f bounding s u r f a c e s i n a n c i e n t e o l i a n sandstones. Sediment01 ogy , 24: 303-332. B r o o k f i e l d , M. E., 1979. Anatomy o f a Lower P e r i m i a n a e o l i a n sandstone complex, S o u t h e r n Scotland. S c o t t . J. Geol., 15: 81-96. Cooper, W. S., 1958. Coastal sand dunes o f Oregon and Washington. Geol. SOC. America Mem. 72, 169 p. Gregory, H. E., 1950. Geology and geography o f t h e Z i o n Park r e g i o n , Utah and U. S. Geol. Survey, P r o f . Paper 220, 200 p. Arizona. Harms, J. C., Southard, J. B, Spearing, D. R., and Walker, R. G., 1975. Depositional environments a s i n t e r p r e t e d from p r i m a r y sedimentary s t r u c t u r e s and s t r a t i f i c a t i o n sequences. SOC. Econ. P a l e o n t o l o g i s t s and M i n e r a l o g i s t s , S h o r t Course No. 2, D a l l a s , Texas, 1 6 1 p. Hunter, R. E., 1981. S t r a t i f i c a t i o n s t y 1 es i n e o l i a n sandstones: some Pennsylvanian t o J u r a s s i c examples f r o m t h e western i n t e r i o r U.S.A.: In F. G. E t h r i d g e , and R. M. F l o r e s ( E d i t o r s ) , Recent and a n c i e n t non-marine depositional envirorments: models for exploration SOC. Econ. P a l e o n t o l o g i s t s and M i n e r a l o g i s t s , S p e c i a l P u b l i c a t i o n no. 31, pp. 315Allen,
329. Hunter, R. E., Richnond, B. M., and Alpha, T. R., i n press, Storm-controlled o b l i q u e dunes o f t h e Oregon coast. Geol. SOC. America B u l l . Hunter, R. E., and Rubin, D. M., 1983. I n t e r p r e t i n g c y c l i c crossbedding, w i t h an example from t h e Navajo Sandstone. I n M. E. B r o o k f i e l d and T. S. Ahlbrandt (Editors), Aeolian sediments and processes, E l sevier, Amsterdam Jones, C. M., 1979. T a b u l a r cross-bedding i n Upper Carboniferous f l u v i a l channel sediments i n t h e s o u t h e r n Pennines, England. Sedimentary Geology,
.
24: 85-104. Kocurek, Gary, 1981. S i g n i f i c a n c e o f i n t e r d u n e d e p o s i t s and bounding s u r f a c e s i n aeol.ian dune sands. Sedimentology, 28: 753-780. McKee, E. D., 1966. S t r u c t u r e o f dunes a t White Sands N a t i o n a l Monunent, New Mexico (and a comparison w i t h s t r u c t u r e s o f dunes f r o m o t h e r s e l e c t e d areas). Sedimentology, 7: 3-69. McKee, E. D., 1979. A n c i e n t sandstones c o n s i d e r e d t o b e e o l i a n . I n E. 0. McKee, ( E d i t o r ) , A s t u d y o f q l o b a l sand seas U. S. Geol. Survey P r o f . Paper i 0 5 2 , p. 187-233.Rubin, D. M., and McCulloch, D. S., 1980. S i n g l e and superimposed bedforms: a s y n t h e s i s o f San F r a n c i s c o B a y and f l u m e observations. Sedimentary G o 1 ogy , 26: 207-231. Rubin, D. M., and Hunter, R. E. 1982. Bedform c l i m b i n g i n t h e o r y and nature. Sedimentology, 29: 121-138.
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429
INTERPRETING CYCLIC CROSSBEDDING, WITH A N EXAMPLE FROM THE NAVAJO SANDSTONE RALPH E. HUNTER & DAVID M. RUBIN: U.S. M e n l o P a r k , C a l i f o r n i a 94025, U.S.A.
G e o l o g i c a l Survey
INTRODUCTION The e x i s t e n c e o f c y c l i c c r o s s b e d d i n g and J u r a s s i c )
i n t h e N a v a j o Sandstone ( T r i a s s i c ?
and o t h e r e o l i a n s a n d s t o n e s o f t h e w e s t e r n U n i t e d S t a t e s was
n o t e d b y S t o k e s ( 1 9 6 4 ) , who i n t e r p r e t e d t h e c y c l e s a s a n n u a l l a y e r s p r o d u c e d b y (The t e r m " c y c l e " w i l l b e used h e r e f o r b o t h a
s e a s o n a l l y f l u c t u a t i n g winds.
d e p o s i t and f o r t h e wind v a r i a t i o n s t h a t produced t h e deposit.)
Cyclic cross-
bedding i s manifested as repeated v a r i a t i o n s i n t h e s t r u c t u r e o r t e x t u r e o f crossbeds w i t h i n a set.
Where t h e l a m i n a t i o n w i t h i n e a c h c y c l e i s p a r a l l e l t o
t h e bounding surfaces o f t h e c y c l e ,
as
i n t h e c y c l e s described b y Stokes
(1964), t h e s t r u c t u r e i s h e r e r e f e r r e d t o a s "concordant c y c l i c crossbedding" (fig.
1A).
R e l a t e d c y c l e s d e s c r i b e d h e r e b u t n o t b y S t o k e s (1964) a r e man-
i f e s t e d a s compound c r o s s b e d d i n g ( t e r m i n o l o g y o f Harms a n d o t h e r s , 1975, p. 51-
i n w h i c h t h e c r o s s b e d s composing a
FA\,
s e t a r e separated b y surfaces o f
, s i o n and a r e i n t e r n a l l y c r o s s b e d d e d ( f i g . 1B). I n t h e i n t e r p r e t a t i o n o f s e d i m e n t a r y c y c l e s , one o f t h e most f u n d a m e n t a l q u e s t i o n s t o b e answered i s w h e t h e r t h e c y c l i c i t y i s a l l o c y c l i c
-
that is,
p r o d u c e d b y changes i n t h e t o t a l e n e r g y o r m a t e r i a l i n p u t t o t h e s e d i m e n t a r y system
-
1964).
o r autocyclic
-
that
is,
i n d e p e n d e n t o f s u c h changes (Beerbower,
S t o k e s (1964) r u l e d o u t one t y p e o f a u t o c y c l i c i t y
r e p r e s e n t s a s i n g l e a v a l a n c h e down a dune s l i p f a c e .
-
sider another t y p e o f a u t o c y c l i c i t y
-
t h a t each c y c l e
However, h e d i d n o t con-
t h a t each c y c l e r e p r e s e n t s t h e d e p o s i t o f
a dune t h a t moved o v e r a l a r g e r dune. The p r o b l e m o f d i s t i n g u i s h i n g a l l o c y c l i c a n d a u t o c y c l i c c r o s s b e d d i n g a l s o arises
i n the
compound
study o f t i d a l
and f l u v i a l
compound c r o s s b e d d i n g ,
where t h e
s t r u c t u r e o f t h e crossbedding a r i s e s from t h e presence o f e r o s i o n
surfaces ( l i k e those surfaces"
( C o l l inson,
i n fig.
1B) t h a t have come t o b e c a l l e d " r e a c t i v a t i o n
1970).
These s u r f a c e s c a n b e p r o d u c e d b y a1 1 o c y c l i c
mechanisms s u c h a s t i d a l f l o w r e v e r s a l s (Boersma, 1969; K l e i n , 1970; D a l r y m p l e and o t h e r s , 1975; C l i f t o n , 1979; A l l e n , 198Da, 1980b; V i s s e r , 1980; K o h s i e k a n d Terwindt,
1981;
(Collinson, However,
Terwindt,
1970; J a c k s o n ,
1981)
o r changes
i n f l o w speed and
flow
depth
1976; J o n e s , 1977, 1979; J o n e s a n d McCabe, 1980).
s e v e r a l w o r k e r s have shown t h a t e r o s i o n s u r f a c e s o f n e a r l y i d e n t i c a l
a p p e a r a n c e c a n b e f o r m e d b y t h e a u t o c y c l i c mechanism o f s m a l l e r b e d f o r m s m o v i n g o v e r a l a r g e r one ( A l l e n ,
1968,
1973;
Banks, 1973;
McCabe a n d Jones, 1977).
430
A.
B.
F i g . 1. Schematic c r o s s s e c t i o n s o f t w o k i n d s o f c y c l i c crossbedding. (A) Concordant c y c l i c crossbeds. The t w o p a t t e r n s r e p r e s e n t d i f f e r e n t s t r a t i f i c a t i o n t y p e s o r g r a i n s i z e s ; t h e e s s e n t i a l f e a t u r e i s t h a t a l l t h e laminae ( 6 ) Crossw i t h i n c y c l e s and t h e bounding s u r f a c e s o f c y c l e s a r e p a r a l l e l . b e d d i n g whose c y c l i c appearance i s due t o t h e compound s t r u c t u r e o f t h e c r o s s b e d d i n g ( t h a t i s , due t o t h e d i v i s i o n o f t h e s e t i n t o subsets b y s u r f a c e s o f e r o s i o n , h e r e r e p r e s e n t e d b y s o l i d 1i n e s ) . Even i f e r o s i o n s u r f a c e s a r e n o t formed on t h e l e e s l o p e o f t h e l a r g e r bedform, t h e movement o f s m a l l e r bedforms o v e r a l a r g e r one c a n c r e a t e c y c l i c v a r i a t i o n s i n t h e t e x t u r e and s t r u c t u r a l c h a r a c t e r i s t i c s o f crossbeds w i t h i n a s e t (Smith, 1972), and such c y c l e s resemble t h o s e d e s c r i b e d b y Stokes (1964).
I n both
e o l i a n and t i d a l crossbedding, t h e a b i l i t y t o i d e n t i f y a c y c l i c i t y c o n t r o l l e d b y astronomic
processes would
represent
a
powerful
tool
for
interpreting
bedform dynamics and f l o w c o n d i t i o n s .
A m a j o r aim o f t h i s paper
is to
define c r i t e r i a
f o r distinguishing
crossbedding produced b y f l u c t u a t i o n s i n f l o w c h a r a c t e r ( c y c l e s o f t h i s t y p e
w i l l be c a l l e d "fluctuating-flow cycles")
f r o m c r o s s b e d d i n g produced by t h e
movement o f s m a l l e r bedforms over l a r g e r ones ("superimposed-bedform c y c l e s " ) . U s i n g t h e s e c r i t e r i a , we e v a l u a t e and s u p p o r t Stokes' (1964) i d e n t i f i c a t i o n o f fluctuating-flow
cycles
i n t h e Navajo Sandstone and i n t e r p r e t
compound c r o s s b e d d i n g as f l u c t u a t i n g - f l o w c y c l e s .
some o f t h e
We t h e n p r e s e n t a n a n a l y s i s
t h a t s u p p o r t s Stokes' i n t e r p r e t a t i o n o f annual p e r i o d i c i t y .
I n a n o t h e r paper
( R u b i n and Hunter, t h i s volume) we d e s c r i b e o t h e r c y c l i c compound crossbedding t h a t we i n t e r p r e t t o b e o f t h e superimposed-bedform type. DISTINGUISHING THE TYPES OF CYCLICITY C h a r a c t e r o f F l u c t u a t i n g - F l o w Cycles Cyclic periodic
crossbedding
fluctuations
aqueous e n v i r o r m e n t s ,
of
concordant structure
fluctuating-flow
i n flow direction,
t y p e c a n b e produced by
flow velocity,
flow i n
The c y c l e s may o r may n o t b e bounded b y e r o s i o n a l
I n t h e absence o f any e r o s i o n a l s u r f a c e s ( f i g . and
depth o f
o r o t h e r parameters a f f e c t i n g sediment t r a n s p o r t r a t e s
and d e p o s i t i o n a l mechanisms. surfaces.
the
a r e made e v i d e n t b y v a r i a t i o n s
( f o r example,
i n texture
by a l t e r n a t i o n s o f g r a i n f a l l
s t r a t a , i n t h e t e r m i n o l o g y o f Hunter, 1977a).
l A ) , t h e c y c l e s are or
sedimentary
and g r a i n f l o w cross-
431
A.
B. EXPLANATION
+--
Flow a t time of sketch
-
Erosional or hiatal surface
_ _ _ _ Representative C.
---
depositional surface Former bedform surface, now destroyed
Fig. 2. Schematic c r o s s sections of bedforms showing c y c l i c erosion surfaces ( r e a c t i v a t i o n s u r f a c e s ) produced by f l u c t u a t i n g flow. ( A ) Erosion surfaces on upper l e e slope and depositional wedges a t base of l e e slope produced by reversed flow. L e f t , a t end of buildout phase; r i g h t , a t end of backcutting phase. ( 8 ) Differing degrees of lee-slope erosion by reversed flow. Left, s l i g h t erosion; Right, extensive erosion. Note t h a t extent of erosion in f i g . 1 A i s intermediate between those shown i n f i g . 1 B . ( C ) Erosional scallops a t base of l e e slope, produced by periods of i n t e n s i f i e d c i r c u l a t i o n of t h e l e e eddy. Fluctuating-flow c y c l e s bounded by surfaces of erosion ( f i g . 1 B ) generally imply major changes in flow d i r e c t i o n o r , in aqueous flows, changes in water depth. Erosion surfaces produced by r e v e r s a l s of t h e normal-to-bedform component of flow tend t o be b e s t developed on t h e upper l e e slope and t o become nonerosional h i a t a l surfaces downward.
Erosion on t h e upper l e e slope i s often
accompanied by t h e formation of depositional wedges on t h e lower l e e slope ( f i g . 2A; Hunter and o t h e r s , i n press). Erosion surfaces t h a t a r e r e s t r i c t e d t o t h e upper l e e slope a r e formed during r e l a t i v e l y short periods of reversed flow. If t h e reversed flow continues long enough, t h e l e e slope i s extensively c u t back, and any i n i t i a l depositional wedge a t i t s base i s destroyed ( f i g .
2 B ) . The extent of backcutting i s , of course, influenced by additional f a c t o r s such as t h e strength of t h e reversed flow and t h e bedform s i z e . An erosion s u r f a c e may be produced on t h e lower l e e s i d e of a bedform s o l e l y by an increase in flow v e l o c i t y i f t h e increase causes s i g n i f i c a n t l y
432 m o r e v i g o r o u s b a c k f l o w i n t h e l e e eddy. suggest t h a t such e r o s i o n surfaces,
O b s e r v a t i o n s i n modern e o l i a n dunes
i n c o n t r a s t t o t h o s e produced b y reversed
f l o w , t e n d t o b e most c o n s p i c u o u s a t t h e b a s e o f t h e l e e s l o p e ( f i g . The
character
of
fluctuating-flow
cyclic
d i f f e r i n g p l a n - f o r m c u r v a t u r e o f t h e bedform. without
superimposed bedforms,
strike parallel bedform.
t o one a n o t h e r
all
crossbedding
2C).
varies
with
On a s t r a i g h t - c r e s t e d b e d f o r m
t h e crossbeds
and e r o s i o n s u r f a c e s must
even i f t h e f l o w i s n o t t r a n s v e r s e t o t h e
C y c l i c i t y on such bedforms c a n m a n i f e s t i t s e l f o n l y b y changes i n d i p
a n g l e , commonly a s s o c i a t e d w i t h e r o s i o n s u r f a c e s , a n d b y changes i n t h e t e x t u r e o r s t r a t i f i c a t i o n type. On c u r v e d b e d f o r m s ,
fluctuating-flow
c y c l e s can manifest themselves by
v a r i o u s f e a t u r e s n o t p o s s i b l e on s t r a i g h t - c r e s t e d bedforms.
R e l a t i v e l y small
f l u c t u a t i o n s i n f l o w d i r e c t i o n can cause s i g n i f i c a n t s h i f t s i n t h e l o c i o f maximum d e p o s i t i o n o r ,
i f t h e b e d f o r m i s s t r o n g l y enough c u r v e d ,
c a n cause
a l t e r n a t i n g e r o s i o n and d e p o s i t i o n t h a t a r e o u t o f phase o n d i f f e r e n t p a r t s o f t h e l e e slope.
O b s e r v a t i o n s o n modern e o l i a n dunes i n d i c a t e t h a t e r o s i o n s u r -
f a c e s p r o d u c e d b y s m a l l changes i n f l o w d i r e c t i o n a r e more common o n t h e downcurrent-convex s a l i e n t s o r downcurrent-pointed spurs o f curved bedforms t h a n on t h e downcurrent-concave r e - e n t r a n t s ( f i g s . alternations
3A, 3B; H u n t e r , 1977a, f i g .
9).
The
o f d e p o s i t i o n and e r o s i o n f r o m one s i d e t o t h e o t h e r o f t h e s e
f e a t u r e s c r e a t e d i s t i n c t i v e s t r u c t u r e s , some o f w h i c h r e s e m b l e B a g n o l d ' s (1941) i n f e r r e d s t r u c t u r e o f l o n g i t u d i n a l dunes. We r e f e r t o t h e d i s t i n c t i v e p a t t e r n s v i s i b l e i n t r a n s v e r s e s e c t i o n s ( s e c t i o n s normal t o t h e m i g r a t i o n d i r e c t i o n o f t h e bedform) o r i n h o r i z o n t a l exposures a s "zigzagging e r o s i o n s u r f a c e s " ( f i g .
3A) and " i n t e r l e a v e d e r o s i o n s u r f a c e s " current re-entrants
(fig.
3B).
I n t h e r a r e c o n c a v e down-
i n f i l l e d f r o m t w o s i d e s t h a t meet a b r u p t l y a t a c o r n e r ,
nonerosional zigzags a r e formed ( f i g .
3C; K o c u r e k a n d D o t t , 1981, f i g .
12).
C h a r a c t e r o f Superimposed-Bedform C y c l e s A s s m i n g t h a t b e d f o r m s o f d i f f e r e n t s i z e s c a n b e s t a b l e i n t h e same s t e a d y f l o w ( f o r evidence s u p p o r t i n g t h i s assumption, cyclic
crossbedding
cyclic.
o f the
see R u b i n and M c C u l l o c h , 1980),
superimposed-bedform t y p e c a n b e e n t i r e l y auto-
Each c y c l e o f t h i s t y p e i s t h e d e p o s i t o f a s i n g l e s u p e r i m p o s e d b e d -
f o r m , a n d t h e c y c l i c a p p e a r a n c e a r i s e s f r o m t h e s i m i l a r i t y o f t h e superimposed bedforms. Superimposed-bedform c y c l e s d i f f e r c o n s i d e r a b l y , d e p e n d i n g o n w h e t h e r t h e s u p e r i m p o s e d b e d f o r m s e x i s t o n b o t h t h e s t o s s a n d l e e s l o p e s o f t h e m a i n bedf o r m o r whether t h e y e x i s t o n l y o n t h e s t o s s slope,
m o v i n g up i t u n t i l t h e y
r e a c h t h e c r e s t , whereupon t h e i r sand a v a l a n c h e s down t h e s l i p f a c e o f t h e m a i n bedform.
Where superimposed b e d f o r m s e x i s t o n l y o n t h e s t o s s s l o p e o f t h e m a i n
433
A
B
ALTERNATING FLOW DIRECTIONS (for all diagrams)
C F i g . 3. Schematic horizontal sections showing l a t e r a l l y discontinuous erosion surfaces a n d c y c l e s formed by moderate f l u c t u a t i o n s i n flow d i r e c t i o n over bedAppearance i n s t r i k e c r o s s section i s forms t h a t a r e curved in plan form. similar. Symbols same a s in f i g u r e 1. ( A ) Zigzagging erosion surfaces formed o n downcurrent-pointed spur; r e - e n t r a n t s a r e a t edges of sketch. ( B ) Interleaved erosion surfaces formed on downcurrent-convex sal i e n t ; re-entrants a r e a t edges of sketch. ( C ) Zigzagging l a y e r s f i l l i n g a concave-downcurrent reentrant of a bedform.
bedform, no erosion surfaces a r e produced on t h e l e e slope except possibly on i t s uppermost p a r t (McCabe a n d Jones, 1977). The r e s u l t i n g cycles a r e made evident in longitudinal c r o s s section ( s e c t i o n parallel t o t h e migration d i r e c t i o n of t h e bedform) only by v a r i a t i o n s in t e x t u r e
(Smith, 1972) or
s t r a t i f i c a t i o n type ( f i g . 1A). Superimposed bedforms can migrate over t h e l e e slope of a l a r g e r bedform i f t h a t slope i s r e l a t i v e l y g e n t l e ( a t l e a s t a few degrees l e s s than t h e angle of repose). Under such conditions, t h e superimposed bedforms produce c l imbing t r a n s l a t e n t s t r a t a (terminology of Hunter, 1977b). Generally these s t r a t a a r e bounded by erosion surfaces a n d a r e i n t e r n a l l y crossbedded a s well a s being crossbeds in a l a r g e r s e t ( t h e s e t formed by t h e main bedform)(fig. 1B).
In
t h i s paper we discuss only those special f e a t u r e s of such compound crossbedding t h a t a r e useful
i n distinguishing i t from compound crossbedding produced by
flow f l u c t u a t i o n s .
In a r e l a t e d paper (Rubin and Hunter, t h i s volume) we
discuss i n g r e a t e r d e t a i l t h e compound crossbedding generated by t h e migration of superimposed bedforms. The s t r u c t u r e of compound crossbedding produced by superimposed bedforms i s controlled by such f a c t o r s a s t h e c r e s t length, plan-form curvature, and migration d i r e c t i o n of t h e superimposed bedforms r e l a t i v e t o t h e main bed-
434
PLAN FORM OF PRIMARY BEDFORM
I CURVED
STRAIGHT
F i g . 4. B l o c k d i a g r a m s showing v a r i e t i e s o f c y c l i c c r o s s b e d d i n g p r o d u c e d b y m i g r a t i o n o f r e l a t i v e l y s t r a i g h t c r e s t e d , l o n g c r e s t e d superimposed bedforms down o r a l o n g l e e s l o p e o f a l a r g e r , p r i m a r y b e d f o r m . Symbols same a s i n f i g u r e 1. Three s u r f a c e s a r e shown i n e a c h d i a g r a m : a h o r i z o n t a l s e c t i o n ( a t upper l e f t ) , a c r o s s s e c t i o n t h a t i s l o n g i t u d i n a l w i t h r e s p e c t t o main crossb e d d i n g ( a t l o w e r l e f t ) and t h e l e e s l o p e o f t h e b e d f o r m ( a t r i g h t ) . form.
Superimposed b e d f o r m s c a n r a n g e f r o m r e l a t i v e l y s h o r t c r e s t e d a n d c u r v e d
t o r e l a t i v e l y l o n g c r e s t e d and s t r a i g h t . short-crested,
Compound c r o s s b e d d i n g p r o d u c e d b y
c u r v e d superimposed b e d f o r m s i s c h a r a c t e r i z e d b y 1 e n t i c u l a r i t y
o f t h e s u b s e t s a s seen i n c r o s s s e c t i o n s t h a t a r e t r a n s v e r s e w i t h r e s p e c t t o t h e m i g r a t i o n d i r e c t i o n o f t h e superimposed b e d f o r m s (McCabe and J o n e s , 1977). Compound c r o s s b e d d i n g
formed b y t h e m i g r a t i o n o f superimposed bedforms
a l o n g t h e l e e s l o p e o f a l a r g e r b e d f o r m d i f f e r s c o n s i d e r a b l y f r o m t h a t formed b y t h e m i g r a t i o n o f s u p e r i m p o s e d b e d f o r m s down t h e l e e s l o p e ( f i g . 4).
Where
t h e s u p e r i m p o s e d b e d f o r m s h a v e a component o f movement a l o n g t h e l e e s l o p e o f t h e main bedform,
t h e y form subsets of crossbeds t h a t a r e v i s i b l e i n cross
s e c t i o n s t r a n s v e r s e w i t h r e s p e c t t o t h e m i g r a t i o n d i r e c t i o n o f t h e l a r g e r bedform.
Where t h e s u p e r i m p o s e d b e d f o r m s have a component o f movement u p o r down
t h e l e e s l o p e o f t h e main bedform,
t h e y form s u b s e t s o f c r o s s b e d s t h a t a r e
435 visible i n cross sections longitudinal with respect t o the migration direction o f t h e main l e e slope. The p l a n - f o r m c u r v a t u r e o f t h e m a i n b e d f o r m a l s o a f f e c t s t h e s t r u c t u r e o f compound c r o s s b e d d i n g ( f i g .
4).
The s i t u a t i o n i n w h i c h t h e s u p e r i m p o s e d bed-
forms a r e s t r a i g h t e r t h a n t h e m a i n b e d f o r m ( l o w e r l e f t , f i g . 4) i s uncommon. D i s t in g u i s h i ng C r i t e r i a Criteria
for
distinguishing
c y c l e s a r e l i s t e d i n t a b l e 1.
superimposed-bedform
and
f l u c t u a t i n g - f l ow
Where c y c l i c c r o s s b e d s a r e c o n c o r d a n t and d i p a t
t h e a n g l e o f repose ( f i g . l A ) , t h o s e produced b y f l o w f l u c t u a t i o n s a r e probably indistinguishable i n longitudinal
s e c t i o n from t h o s e produced b y t h e s p i l l i n g
o f superimposed bedforms o v e r t h e b r i n k o f t h e main s l i p f a c e . section o r transverse cross section, guishable.
however,
In horizontal
t h e t w o t y p e s may b e d i s t i n -
As n o t e d b y McCabe and J o n e s ( 1 9 7 7 ) , a c y c l i c c r o s s b e d p r o d u c e d b y
s p i l l o v e r o f a s u p e r i m p o s e d b e d f o r m c a n e x t e n d l a t e r a l l y no f a r t h e r t h a n t h e l a t e r a l e x t e n t o f t h a t b e d f o r m ( a d i s t a n c e g e n e r a l l y much l e s s t h a n t h e c r e s t l e n g t h o f t h e m a i n b e d f o r m ) , whereas a c y c l i c c r o s s b e d o f f l u c t u a t i n g - f l o w t y p e should extend l a t e r a l l y along t h e e n t i r e l e e slope o f t h e bedform (unless t h e l e e s l o p e i s so s t r o n g l y c u r v e d t h a t p a r t s o f i t a r e n o n d e p o s i t i o n a l ) . over,
More-
i f t h e superimposed bedform i s n o t p e r f e c t l y p a r a l l e l t o t h e b r i n k l i n e
o f t h e m a i n s l i p f a c e , t h e s p i l l o v e r w i l l m i g r a t e a l o n g t h e m a i n s l i p f a c e and t h u s c r e a t e compound c r o s s b e d d i n g
s i m i l a r t o t h a t shown i n t h e u p p e r - r i g h t
d i a g r a m o f f i g u r e 4. Where c y c l i c c r o s s b e d s a r e c o n c o r d a n t b u t d i p l e s s s t e e p l y t h a n t h e a n g l e o f r e p o s e ( a n d t h e r e f o r e must have b e e n f o r m e d b y d e p o s i t i o n a l p r o c e s s e s o t h e r t h a n a v a l a n c h i n g ) , t h e y were p r o b a b l y f o r m e d b y f l u c t u a t i n g f l o w .
Superimposed
b e d f o r m s c a n f o r m c o n c o r d a n t c r o s s b e d s o n l y i f t h e b e d f o r m s a r e d e s t r o y e d when t h e y r e a c h t h e l e e s l o p e o f t h e m a i n b e d f o r m , a n d a v a l a n c h i n g down a s l i p f a c e i s p r o b a b l y t h e o n l y a d e q u a t e mechanism o f s u c h d e s t r u c t i o n .
Where t h e l e e
slope o f t h e main bedform d i p s l e s s s t e e p l y t h a n t h e angle o f repose, t h e superimposed b e d f o r m s w o u l d n o t b e i m m e d i a t e l y d e s t r o y e d and w o u l d f o r m compound r a t h e r t h a n c o n c o r d a n t c r o s s b e d d i n g . Compound c y c l i c c r o s s b e d d i n g o f f l u c t u a t i n g - f l o w and s u p e r i m p o s e d - b e d f o r m types can b e s t b e d i s t i n g u i s h e d by t h e l e n g t h s and widths o f i n d i v i d u a l subsets ( r e l a t i v e t o t h e t h i c k n e s s o f t h e i n d i v i d u a l s u b s e t s o r o f t h e e n t i r e compound set).
The l e n g t h s a n d w i d t h s o f t h e s u b s e t s a r e measured i n s e c t i o n s t h a t a r e
l o n g i t u d i n a l and t r a n s v e r s e ,
respectively,
with respect t o the migration di-
r e c t i o n o f t h e f e a t u r e t h a t formed t h e subset crossbedding. Lenticularity o f the individual s b s e t crossbedding
subsets i n a transverse section o f t h e
s u g g e s t s a superimposed-bedform o r i g i n
(McCabe and J o n e s ,
436 TABLE 1 Features u s e f u l f o r d i s t i n g u i s h i n g c y c l i c c r o s s b e d d i n g o f superimposedbedform and f l u c t u a t i n g - f l o w o r i g i n s
Concordant c y c l i c c rossbedd ing
Cycl i c crossbedding o f super imp0 sed-b ed f orm o r i g in
C y c l i c crossbedding o f f 1 uc t ua t ing- f 1ow o r ig in
Complete concordance ( i .e., i n b o t h t r a n s v e r s e and 1ong i t u d i n a l c r o s s s e c t i o n ) uncommon, p o s s i b l y none x i s t e n t where crossbeds d i p a t l e s s than angle o f repose
Complete concordance commonly produced
C y c l e widtha commonly s m a l l
C y c l e widtha g r e a t unless plan-form curvature i s great
Probably not d i s t i n g u i s h a b l e i n longitudinal cross section
Compound c y c l i c c r o s sb edd ing
Cycles whose crossbedding indicates formation b y f e a t u r e m i g r a t i n g along a l a r g e r l e e s l o p e produced readily
Cycles whose crossbedding i n d i c a t e s formation by f e a t u r e m i g r a t i n g along a l a r g e r l e e s l o p e produced o n l y where p l a n - f o r m curvature o f t h a t slope i s g r e a t
C y c l e 1 engtha commonly great
Cycle lengtha g e n e r a l l y smal 1
Cycl e w i d t ha commonly small
Cycle w i d t h a g r e a t unless p l an-form c u r v a t u r e o f l a r g e r l e e slope i s great
Features produced b y r e verse f l o w compatible w i t h f o r m a t i o n b y l e e eddy simultaneously w i t h forward flow
Features produced b y r e v e r s e f l o w may i n d i c a t e o r i g i n a t t i m e s when forward f l o w d i d not occur
aCycle w i d t h and l e n g t h a r e t h e l a t e r a l e x t e n t o f t h e c y c l e i n t r a n s v e r s e and l o n g i t u d i n a l cross sections, respectively. A l o n g i t u d i n a l cross section i s one p a r a l l e l t o t h e m i g r a t i o n d i r e c t i o n o f t h e f e a t u r e t h a t produced t h e crossbedding. 1977).
Such l e n t i c u l a r i t y a r i s e s
i n superimposed-bedform
c y c l e s from the
t h r e e - d i m e n s i o n a l f o r m ( t h a t i s , t h e f i n i t e c r e s t l e n g t h s and i r r e g u l a r trough e l e v a t i o n s ) o f t h e superimposed bedforms.
F l u c t u a t i n g - f l o w c y c l e s c a n b e len-
t i c u l a r o n l y i f t h e bedform undergoes s u b s t a n t i a l changes i n f o r m d u r i n g t h e f 1uc t ua t io ns.
437 Where a n i n d i v i d u a l s u b s e t i s v e r y l o n g r e l a t i v e t o i t s t h i c k n e s s a s seen i n a longitudinal
s e c t i o n o f t h e s u b s e t c r o s s b e d d i n g , a superimposed-bedform
o r i g i n i s suggested.
The l e n g t h o f a s u b s e t p r o d u c e d b y a superimposed b e d f o r m
equals t h e d i s t a n c e across which t h e bedform migrated d u r i n g d e p o s i t i o n (except f o r a n y s h o r t e n i n g due t o e r o s i o n b y f o l l o w i n g b e d f o r m s ) , a n d t h i s d i s t a n c e c a n e a s i l y b e a l a r g e m u l t i p l e o f t h e s u b s e t t h i c k n e s s ( R u b i n and H u n t e r , The f o r m a t i o n o f a f l u c t u a t i n g - f l o w thickness,
on t h e o t h e r
hand,
r e q u i r e s extensive b u i l d o u t o f t h e l e e slope
followed by almost equally extensive backcutting ( r i g h t , required distance o f
fluctuating-flow
l i k e l y t h a n a superimposed-bedform o r i g i n . creases,
fig.
26).
As
the
n e a r l y b a l a n c e d b u i l d o u t and b a c k c u t t i n g becomes much
greater t h a n t h e c y c l e thickness,a
o r i g i n becomes much l e s s
C o n v e r s e l y , a s s u b s e t l e n g t h de-
f l u c t u a t i n g - f l ow a n d superimposed-bedform
e q u a l l y probable.
1982).
cycle t h a t i s very long r e l a t i v e t o i t s
o r i g i n s become more n e a r l y
However, no q u a n t i t a t i v e l i m i t s c a n b e p u t o n t h i s r u l e .
The above r u l e o f s u b s e t l e n g t h i s most e a s i l y a p p l i e d where t h e s u b s e t c r o s s b e d d i n g has a component o f d i p a l o n g t h e p r i m a r y l e e slope.
Where t h e
m a i n b e d f o r n i was s t r a i g h t a n d t h e s u b s e t s a r e v e r y l o n g ( u p p e r r i g h t , f i g . a s u p e r i m p o s e d - b e d f o r m i n t e r p r e t a t i o n i s t h e o n l y p l a u s i b l e one. b e d f o r m becomes more h i g h l y c u r v e d , fluctuating-flow
cycles
3A,
(figs.
4),
As t h e m a i n
t h e s u b s e t l e n g t h t e n d s t o decrease,
and
3B) become more d i f f i c u l t t o d i s t i n g u i s h
f r o m s u p e r i m p o s e d - b e d f o r m c y c l e s ( u p p e r l e f t , f i g . 4).
I n such c a s e s , e v i d e n c e
o f r e v e r s a l s i n t h e a l o n g s l o p e component o f f l o w ( f i g s .
3A, 38) almost c e r -
t a i n l y i n d i c a t e s f l u c t u a t i n g flow. Compound c y c l i c c r o s s b e d d i n g b y g e n e r a t e d t h e m i g r a t i o n o f l o n g - c r e s t e d , s t r a i g h t - c r e s t e d s u p e r i m p o s e d b e d f o r m s d i r e c t l y down t h e l e e s l o p e o f a l a r g e r , s t r a i g h t - c r e s t e d b e d f o r m c a n b e e x t r e m e l y d i f f i c u l t t o d i s t i n g u i s h f r o m compound c y c l i c
crossbedding produced b y f l u c t u a t i n g f l o w
diagram o f f i g .
4 with figs.
2A, 2B).
(Compare l o w e r - r i g h t
The r u l e o f s u b s e t l e n g t h ( i n l o n -
g i t u d i n a l c r o s s section) i s a suggestive guide:
The l o n g e r t h e s u b s e t r e l a t i v e
t o s e t o r subset t h i c k n e s s , t h e more a superimposed-bedform i n t e r p r e t a t i o n i s preferrable t o a fluctuating-flow
interpretation.
E v i d e n c e o f r e v e r s a l s i n t h e f l o w a c r o s s a b e d f o r m c a n b e used i n some cases cles.
to
distinguish
fluctuating-flow
cycles
from
superimposed-bedform c y -
I f t h e r e v e r s e d f l o w c a n b e shown t o h a v e accompanied f o r w a r d m i g r a t i o n
o f t h e b e d f o r m , t h e r e v e r s e d f l o w must have b e e n m e r e l y a l e e - e d d y e f f e c t , and superimposed-bedfom c y c l e s a r e n o t r u l e d out. o f reversed
f l o w c a n b e shown t o
I f , o n t h e o t h e r hand, p e r i o d s
have a l t e r n a t e d w i t h
periods o f forward
m i g r a t i o n o f t h e p r i m a r y b e d f o r m , t h e c y c l e s must b e o f f l u c t u a t i n g - f l o w ( H u n t e r , 1981).
type
438 Superimposed-bedform and f l u c t u a t i n g - f l o w c y c l e s c a n c o e x i s t w i t h i n the
I f t w o o r more d i f f e r e n t s t y l e s o r s c a l e s o f c y c l i c i t y
same s e t o f crossbeds. can b e i d e n t i f i e d , field,
1979).
t h e o r i g i n o f each must b e c o n s i d e r e d s e p a r a t e l y (Brook-
As one example o f c o m p l e x i t y , superimposed bedforms may e x i s t
d u r i n g one p a r t o f a c y c l e o f f l o w f l u c t u a t i o n s b u t n o t d u r i n g a n o t h e r p a r t . CYCLIC CROSSBEDDING I N THE NAVAJO SANDSTONE C h a r a c t e r o f t h e C y c l i c Crossbeddinp General f e a t u r e s .
C y c l i c crossbedding f o r which a f l u c t u a t i n g - f l o w o r i g i n
i s c o n c e i v a b l e i s f a i r l y comnon i n p a r t s o f t h e Navajo Sandstone, e s p e c i a l l y i n and near Z i o n N a t i o n a l appearance, however,is
Park,
southwestern
U t a h (Stokes,
1964).
A cyclic
w e l l developed i n o n l y a small f r a c t i o n o f t h e sets.
A
s e t t h a t does c o n t a i n w e l l developed c y c l e s commonly e x t e n d s f o r a t l e a s t hundreds o f m e t e r s and comprises hundreds o f c y c l e s ( f i g . commonly d o n o t have w e l l developed c y c l i c i t y .
5 ) , b u t a d j a c e n t sets
The reasons f o r such r e s t r i c -
t i o n o f w e l l developed c y c l i c i t y t o a small f r a c t i o n o f t h e s e t s have not y e t b een d i scov ered. Two t y p e s o f c y c l i c crossbedding occur i n t h e Navajo Sandstone: m u t u a l l y concordant compound,
(fig.
lA),
such a s t h e examples d e s c r i b e d b y Stokes (1964), and
i n which t h e crossbeds a r e separated by e r o s i o n s u r f a c e s and are
i n t e r n a l l y crossbedded ( f i g .
1B).
Both t y p e s o f c y c l i c crossbedding occur i n
r e l a t i v e l y t a b u l a r s e t s t h a t a r e g e n e r a l l y 5 t o 15 m t h i c k and average about 10 m i n thickness,
near t h e average f o r a l l s e t s o f crossbeds i n t h e Navajo Sand-
s t o n e o f t h e Z i o n area.
As seen i n h o r i z o n t a l s e c t i o n o r i n t r a n s v e r s e cross
s e c t i o n , t h e t w o t y p e s o f c y c l e s a r e s i m i l a r i n b e i n g l a t e r a l l y e x t e n s i v e (no l a t e r a l p i n c h o u t s were seen) and i n h a v i n g l a m i n a t i o n p a r a l l e l t o t h e bounding The t w o t y p e s o f c y c l e s d i f f e r , however,
surfaces o f t h e cycles. ways d e s c r i b e d below.
i n several
Cycles t h a t a r e i n one way o r a n o t h e r i n t e r m e d i a t e be-
tween t h e t w o main t y p e s a r e d e s c r i b e d subsequently. Concordant c y c l i c crossbedding. crossbeds
are
concave-upward
(in
Almost
all
longitudinal
t a n g e n t i a l t o t h e base o f t h e s e t ( f i g s . 6, 7). o c c u r near t h e t o p o f t h e s e t , approach 27", t h e Navajo Sandstone when p o s t d e p o s i t i o n a l ( H u n t e r , 1981).
o f t h e concordant cyclic
cross
section)
and
nearly
The maximum d i p angles, which
which i s t h e a n g l e o f repose i n compaction i s t a k e n i n t o account
The t h i c k n e s s o f a c y c l e t e n d s t o v a r y p r o p o r t i o n a l l y w i t h the
s i n e o f t h e d i p angle,
so t h a t t h e dune advance p e r c y c l e remains nearly
c o n s t a n t f r o m t h e t o p t o t h e base o f t h e s e t a s t h e c y c l e t h i c k n e s s decreases f r o m t o p t o base. about 0.3
I n c y c l e s o f t h i s t y p e , t h e dune advance p e r c y c l e averages
m.
A l t h o u g h Stokes (1964)
described g r a i n - s i z e v a r i a t i o n s t h a t g i v e a Cyclic
439 Dune advance 1.0m 0 7 100,
Dune advance 0.5 1.0m
,,
Dune advance
0 I2001
0 5
1.0m
100
F i g . 5. Dune advance p e r c y c l e f o r a l o n g s e r i e s o f m u t u a l l y c o n c o r d a n t c y c l i c c r o s s b e d s i n t h e N a v a j o Sandstone a l o n g S t a t e Highway 9, 2.7 km (1.7 m i l e s ) west o f t h e e a s t - e n t r a n c e g a t e h o u s e , Z i o n N a t i o n a l P a r k , Utah. Data r e p r e s e n t c o n d i t i o n s midway b e t w e e n t o p and b a s e o f a n 1 1 - m - t h i c k s e t o f c r o s s b e d s ; some c y c l e s t h i c k e n a t t h e expense o f a d j a c e n t c y c l e s t o w a r d t o p o r b a s e o f s e t . Measurements were made f r o m p h o t o g r a p h s and p r o b a b l y have some e r r o r s due t o i r r e g u l a r i t i e s o f o u t c r o p surface. L a y e r s l a b e l e d "A" a r e a v a l a n c h e ( s a n d flow) c r o s s - s t r a t a t h a t a r e n o t assigned t o a c y c l e ; L a y e r s l a b e l e d "(A)" a r e c y c l e s t h a t c o n t a i n a v a l a n c h e c r o s s - s t r a t a a l o n g l i n e o f measurement. Other a v a l a n c h e c r o s s - s t r a t a o c c u r h i g h e r i n t h e s e t , and a l l a v a l a n c h e c r o s s - s t r a t a p i n c h o u t toward base o f set. Layers l a b e l e d "P" a r e cycles, n o t avalanche cross-strata, t h a t p i n c h o u t toward base o f set. L a y e r s l a b e l e d " R " mark t h e a r r i v a l of a phase o f r e i n v i g o r a t e d d e p o s i t i o n t h a t m i g r a t e d t o w a r d b a s e o f s e t during t h e course o f several cycles. appearance, t h e c o n c o r d a n t c y c l e s a r e m a n i f e s t e d m a i n l y b y r e p e a t e d v a r i a t i o n s i n the type o f stratification. d e s c r i b e d b y H u n t e r (1977a,
Most o f t h e t y p e s o f d r y - e o l i a n s t r a t i f i c a t i o n
1 9 8 1 ) , K o c u r e k a n d D o t t ( 1 9 8 1 ) , and F r y b e r g e r and
Schenk (1981) a r e r e c o g n i z a b l e .
Most commonly,
the cycles are defined by al-
t e r n a t i o n s o f what a r e i n t e r p r e t e d a s g r a i n f a l l d e p o s i t s a n d d e p o s i t s formed b y climbing wind r i p p l e s ( f i g . irregular levels
8).
C y c l e s o f t h i s t y p e a r e i n t e r r u p t e d a t wide,
i n t e r v a l s b y s o l i t a r y s a n d f l o w l a y e r s t h a t p i n c h o u t downward,
where
the
dip
angle
of
the
cyclic
at
c r o s s b e d s was t o o l o w f o r sand
440
CROSSBEDS
Fig. 6. Schematic rilodel of [:lost coi1ii:ion tylie of Concordant c y c l i c crossbedding in t h e llavajo Sandstone i n t h e v i c i n i t y of Zion IJational P a r k . Dashed l i n e s i n i n s e t b o x e s , g r a i n f a l l 1ai:iinae; s o l i d l i n e s , t r d n s l a t e n t s t r a t a foriried by s t o s s - e r o s i o n a l c l iiiibing wind r i p p l e s .
.
Lower p a r t o f a s e t o f c o n c o r d a n t c y c l i c c r o s s b e d s a t same l o c a l i t y a s Fig 7. i n f i g u r e 5. Note t a n g e n t i a l b a s e s o f c r o s s b e d s . Another photograph of t h i s s e t o f c r o s s b e d s i s shown by P e t t i j o h n and P o t t e r ( 1 9 6 4 , P l a t e 3 6 ) . flowage. Grainfall
deposits
in
t h e c o n c o r d a n t c y c l e s a r e c h a r a c t e r i z e d by the
f a i n t n e s s and n e a r l y p e r f e c t p a r a l l e l i s m o f t h e l a m i n a t i o n and by t h e absence o f any r e g u l a r l y r e p e a t e d v a r i a t i o n s i n g r a i n s i z e o r o t h e r c h a r a c t e r i s t i c s
w i t h i n a b u n d l e o f g r a i n f a l l laminae ( t h a t i s , by t h e absence o f any c y c l i c i t y
a t t h e s c a l e of individual laminae). grainfall action 1977a).
The i n t e r p r e t a t i o n o f t h e s e d e p o s i t s a s
d e p o s i t s i s not i n t e n d e d t o r u l e o u t t h e o c c u r r e n c e o f some wind
and
tractional
transport
C1 imbing-wind-ripple
across
the
depositional
surface
(Hunter,
d e p o s i t s in t h e concordant c y c l e s c o n s i s t o f
t r a n s l a t e n t s t r a t a ( H u n t e r , 1977b) formed by wind r i p p l e s c l i m b i n g a t v e r y low
441
F i g . 8. Transverse exposure o f concordant c y c l i c crossbeds i n lower p a r t o f s e t a t same l o c a l i t y a s i n f i g u r e 5. A r r o w s inark c y c l e c o n t a c t s , w h i c h a r e t h e upper c o n t a c t s o f i n t e r v a l s i n which 1 i g h t - w e a t h e r i n g c l i m b i n g - w i n d - r i p p l e d e p o s i t s a r e doiTiinant a n d l o w e r c o n t a c t s o f i n t e r v a l s i n w h i c h d a r k - w e a t h e r i n g g r a i n f a l l d e p o s i t s a r e dominant. N o t e upward i n c r e a s e i n p r o p o r t i o n o f g r a i n f a l l d e p o s i t s and i n t h i c k n e s s o f c y c l e s . a n g l e s ( s u b c r i t i c a l a n g l e s o f H u n t e r , 1977b, o r s t o s s - e r o s i o n a l a n g l e s o f R u b i n and H u n t e r ,
1982).
These d e p o s i t s d i f f e r f r o m t h e a s s o c i a t e d g r a i n f a l l de-
p o s i t s b y t h e g r e a t e r d i s t i n c t n e s s and l e n t i c u l a r i t y o f t h e l a m i n a t i o n , tendency
for
r i p p l e-foreset
and b y t h e p r e s e n c e o f s o w
t h e l a m i n a t i o n t o appear c y c l i c , crosslainination
and
preserved
by a
r i p p l e cross
sections.
The
r i p p l e s and r i p p l e - f o r e s e t c r o s s l a m i n a e i n d i c a t e r i p p l e m i g r a t i o n i n a l o n g s l o p e d i r e c t i o n s (coimonly i n b o t h alongslope d i r e c t i o n s ) . The c o n t a c t s o f t h e c o n c o r d a n t c y c l e s a r e s h a r p s u r f a c e s ,
some o f w h i c h
a r e s l i g h t l y wavy b e c a u s e o f p r e s e r v e d w i n d r i p p l e s t h a t t r e n d n e a r l y p a r a l l e l
t o t h e d i p o f t h e surface. and c l i r n b i n g - w i n d - r i p p l e
G r a i n f a l l deposits form t h e lower p a r t o f a cycle, deposits form t h e
upper
part
(fig.
8);
conimonly a
t r a n s i t i o n zone o f t h i n l y i n t e r b e d d e d b u n d l e s o f g r a i n f a l l and c l i i n b i n g - w i n d r i p p l e d e p o s i t s occurs i n the middle o f a cycle. climbing-wind-ripple crossbedding.
The r a t i o o f g r a i n f a l l t o
d e p o s i t s w i t h i n a g i v e n c y c l e i n c r e a s e s upward a l o n g t h e
The l a m i n a t i o n w i t h i n a c y c l e i s p a r a l l e l o r n e a r l y p a r a l l e l t o
t h e c y c l e contacts. Compound c y c l i c c r o s s b e d d i n g .
The compound-crossbed c y c l e s a r e d e f i n e d
p r i m a r i l y b y t h e erosional bounding surfaces t h a t t r u n c a t e crossbeds o f t h e
442
F i g . 9. Schematic model o f most common t y p e o f compound c y c l i c crossbedding i n t h e N a v a j o Sandstone i n t h e v i c i n i t y o f Z i o n N a t i o n a l P a r k .
F i g . 10. Compound s e t o f c y c l i c c r o s s b e d s i n t h e Lamb P o i n t Tongue o f the Highway 89, 8.2 km (5.1 m i l e s ) n o r t h o f Kanab, N a v a j o Sandstone a l o n g U.S. Utah. Most o f t h e a p p a r e n t c u r v a t u r e o f t h e c r o s s b e d s i s due t o outcrop c u r v a t ure. and t h e d i p a n g l e s o f t h e e r o s i o n surfaces a r e moderate, g e n e r a l l y longitudinal cross section,
most o f t h e i r l e n g t h b u t c u r v e i n t o concave-upward n o n e r o s i o n a l t h e base o f t h e set.
2Oo-25O.
In
t h e e r o s i o n s u r f a c e s a r e n e a r l y s t r a i g h t through s u r f a c e s near
The dune advance p e r c y c l e a v e r a g e s a b o u t 1.5 m.
Compound-crossbed c y c l e s a r e d e f i n e d b y v a r i a t i o n s i n t h e t y p e o f s t r a t i f i c a t i o n as w e l l a s b y t h e presence o f e r o s i o n surfaces.
S a n d f l o w deposits,
easily recognizable i n d i p cross section b y t h e i r toes (fig.
l l ) , a r e much more
abundant i n t h e s e c y c l e s t h a n i n t h e concordant cycles.
The s a n d f l o w deposits
443
Fig. 11. Lower p a r t o f compound s e t o f c y c l i c c r o s s b e d s shown i n f i g u r e 10. Sandflow c r o s s - s t r a t a ( S ) a r e r e l a t i v e l y d a r k . Basal wedges ( m a r k e d b y b r a c k e t s ) and b o t t o m s e t d e p o s i t s contemporaneous w i t h s a n d f l ows ( 5 ) a r e r e l a t i v e l y 1ight. i n t e r f i n g e r downward w i t h more g e n t l y d i p p i n g d e p o s i t s , f o r m e d b y c l i m b i n g w i n d ripples o r b y grainfall,
w h i c h make up b o t t o m s e t d e p o s i t s t h a t r e s t o n t h e
lower b o u n d i n g s u r f a c e o f a c y c l e .
A
d i s t i n c t i v e wedge composed o f c l i m b i n g - w i n d - r i p p l e d e p o s i t s ,
deposits,
or grainfall
deposits t y p i c a l l y occurs
cycle near t h e base o f t h e
set,
filling
the triangular
youngest s a n d f l o w c r o s s b e d o f a g i v e n c y c l e , natively,
the
gently
dipping
youngest s a n d f l o w c r o s s b e d ) , 11).
bottomset
space b e t w e e n t h e
t h e base o f t h e s e t ( o r , a l t e r -
deposits
contemporaneous
with
the
and t h e u p p e r b o u n d i n g s u r f a c e o f a c y c l e ( f i g .
L a m i n a t i o n w i t h i n t h i s wedge ( h e r e c a l l e d a " b a s a l
Hunter and o t h e r s ,
planebed
i n e a c h compound-crossbed
wedge",
following
i n press) tends t o f a n o u t from p o i n t s near t h e updip and
downdip c o r n e r s o f t h e wedge.
The l a m i n a t i o n i s t r u n c a t e d b y t h e e r o s i o n a l
bounding s u r f a c e n e a r t h e u p d i p c o r n e r o f t h e wedge b u t i s c o n c o r d a n t w i t h t h e more g e n t l y d i p p i n g
downdip c o n t i n u a t i o n o f t h i s
same s u r f a c e .
The u p d i p
p i n c h o u t s o f t h e wedges s u g g e s t d e p o s i t i o n b y w i n d s t h a t b l e w u p s l o p e . C y c l i c crossbedding intermediate
o f i n t e r m e d i a t e types.
i n one way o r
Among t h e s e t y p e s a r e ( 1 )
Several types o f c y c l e s a r e
a n o t h e r b e t w e e n t h e t w o t y p e s d e s c r i b e d above. Cycles t h a t a r e concordant b u t t h a t resemble t h e
compound-crossbed c y c l e s i n b e i n g r e l a t i v e l y t h i c k ( d u n e advance o f a b o u t 1 rn
444 per c y c l e ) , containing
sandflow d e p o s i t s a s a
conspicuous c o n s t i t u e n t , a n d
having concave-up c u r v a t u r e o n l y near t h e b a s e o f t h e s e t ; closely
( 2 ) cycles t h a t
resemble t h e c o n c o r d a n t - c r o s s b e d c y c l e s e x c e p t t h a t t h e i r b o u n d i n g
s u r f a c e s t r u n c a t e t h e l a i n i n a t i o n w i t h i n t h e c y c l e s a t siila11 a n g l e s ( l e s s t h a n 5 " ) in d i p c r o s s s e c t i o n ; and ( 3 ) c y c l e s whose bounding s u r f a c e s a r e concordant
and s t r a i g h t i n t h e upper p d r t of t h e s e t b u t g r a d e downward i n t o concaveupward e r o s i o n a l s u r f a c e s t h a t foriii s c a l l o p s a t t h c b a s e of t h e s e t , a s shown in f i g u r e 2C.
These t h r e e interriiediate t y p e s o f c y c l e s a r e l e s s coiiunon t h a n
t h e tvo main t y p e s . i v i d e n c e f o r Fluctuating-Flovi O r i g i n The p a r a l l e l i s i n between t h e l a m i n a t i o n w i t h i n t h e c y c l e s a n d t h e b o u n d i n g s u r f a c e s of t h e c y c l e s , a s seen in t r a n s v e r s e c r o s s s e c t i o n s or i n horizontal s e c t i o n s o f b o t h t y p e s o f c y c l e s ( f i g s . 1 2 , 1 3 ) , does not s u p p o r t a n o r i g i n o f t h e s e c y c l e s by t h e iiiigr3tion o f superiiiiposcd dunes along t h e l e e s l o p e of the [:lain dune ( s u c h a s i n t h e upper diaqrains o f f i g . 4 ) .
If superimposed dunes
ivrere i n v o l v e d , t h e y must h a v e been s t r a i g h t c r e s t e d arid p a r a l l e l t o t h e main dune.
Floreover, t h e a b s e n c e o f cycl:
pinchouts i n t r a n s v e r s e c r o s s section and
in h o r i z o n t a l s e c t i o n i n d i c a t e s thiit superiii~posedd u n e s , i f they were involved,
~ u s thave been long c r e s t e d .
Fig. 12. Exhuned upper c o n t a c t o f a s e t o f c o n c o r d a n t c y c l i c c r o s s b e d s i n t h e llavajo Sandstone along S t a t e Highway 9, 2.4 km (1.5 m i l e s ) west of easte n t r a n c e g a t e h o u s e , Zion National Park. Note p a r a l l e l i s m of c y c l e c o n t a c t s a n d l a m i n a t i o n w i t h i n c y c l e s and a b s e n c e o f l a t e r a l p i n c h o u t s .
445
Fig. 13. Exposure o f a coinpound s e t o f c y c l i c c r o s s b e d s i n t h e Lamb P o i n t Tongue o f t h e Navajo Sandstone along U.S. llighway 89, 7.9 kill (4.9 m i l e s ) north of Kanab, Utah. Note g e n e r a l p a r a l l e l i s m of c y c l e c o n t a c t s ( t h e iriost conspicuous r i b s ) and l a m i n a t i o n w i t h i n c y c l e s a s seen i n t h i s view, which n e a r l y p a r a l l e l s s t r i k e of c r o s s b e d d i n g .
The c o n c o r d a n t c y c l i c c r o s s b e d s c o u l d not have been formed by superimposed dunes t h a t e x i s t e d on t h e l e e s l o p e of t h e main dune, f o r t h e n coiiqiound c r o s s bedding would have been formed. produced by t h e a v a l a n c h i n g o f
Moreover, t h e s e c y c l e s c o u l d not have been sand,
previously contdined
iri superii:iposed
dunes, down t h e s l i p f a c e o f t h e main dune, because t h e c y c l e s were d e p o s i t e d o n gently d i p p i n g p a r t s o f t h e l e e s l o p e by p r o c e s s e s o t h e r t h a n a v a l a n c h i n g . Superimposed dunes c o u l d have formed t h e c o n c o r d a n t c y c l i c c r o s s b e d s only i f the a r r i v a l o f superimposed dunes a t t h e c r e s t o f the main dune c o u l d have a l t e r e d wind p a t t e r n s f a r down t h e l e e s l o p e o f t h e main dune.
Such a l t e r -
a t i o n s would b e c o n c e i v a b l e o n l y i f t h e s i z e o f a superimposed dune were a l a r g e f r a c t i o n o f t h a t o f t h e main dune.
However, t h e dune advance per c y c l e
i s such a small f r a c t i o n o f t h e s e t t h i c k n e s s (and an even s m a l l e r f r a c t i o n o f the o r i g i n a l dune h e i g h t ) t h a t t h e superimposed dunes, i f p r e s e n t , must have been v e r y small
i n comparison with t h e main dune.
A superimposed-bedform
o r i g i n , t h e r e f o r e , seems much l e s s l i k e l y t h a n a f l u c t u a t i n g - f l o w o r i g i n f o r the concordant c y c l i c c r o s s b e d s . Deciding between a superimposed-bedform and a f l u c t u a t i n g - f l o w o r i g i n f o r the compound c y c l i c c r o s s b e d s i s even more d i f f i c u l t t h a n f o r t h e c o n c o r d a n t cyclic crossbeds.
A1 though the amounts o f e r o s i o n r e p r e s e n t e d by t h e bounding
surfaces of t h e c y c l e s a r e c e r t a i n l y not t o o l a r g e t o b e e x p l a i n e d by a l t e r nating
dune b u i l d o u t and b a c k c u t t i n g caused by f l o w r e v e r s a l s ,
the erosion
could be e q u a l l y well e x p l a i n e d by t h e m i g r a t i o n o f superimposed bedforms down the l e e slope.
446 The d e p o s i t i o n a l wedges a t t h e b a s e o f a t y p i c a l compound s e t o f crossbeds o f f e r some e v i d e n c e f o r d e c i d i n g b e t w e e n a superimposed-bedform and a fluctuating flow origin.
Such wedges a r e e a s i l y e x p l a i n e d b y f l u c t u a t i n g flow; the
wedges w o u l d h a v e b e e n d e p o s i t e d a t t h e b a s e o f t h e dune s l o p e d u r i n g periods o f r e v e r s e d f l o w , c o i n c i d e n t w i t h e r o s i o n o f t h e u p p e r l e e slope.
To explain
t h e s e wedges b y t h e m i g r a t i o n o f s u p e r i m p o s e d dunes down t h e l e e slope of the m a i n dune,
o n e m u s t suppose t h a t t h e l e e - e d d y c i r c u l a t i o n a t t h e base of the
l e e s l o p e o f t h e m a i n dune v a r i e d i n s t r e n g t h a s t h e superimposed dunesmig r a t e d downslope.
Such a n i n t e r p r e t a t i o n w o u l d b e r e a s o n a b l e o n l y i f t h e size
o f a s u p e r i m p o s e d dune were a l a r g e f r a c t i o n o f t h a t o f t h e m a i n dune. P e r h a p s t h e most c o n c l u s i v e e v i d e n c e f o r a f l u c t u a t i n g - f l o w o r i g i n o f both t y p e s o f c y c l i c c r o s s b e d d i n g i s f e a t u r e s i n d i c a t i v e o f r e v e r s a l s i n t h e alongs l o p e component o f t h e w i n d d u r i n g each c y c l e . National
Park,
sharp-cornered the
zigzag
dune r e - e n t r a n t
junction
f r o m two s i d e s .
a r e concordant
and a s
c r o s s b e d s exposed e l s e w h e r e i n t h e s e t . n e a r Kanab,
Utah,
The l a y e r s o n e i t h e r side of t h i c k a s t h e concordant c y c l i c
I n a s e t o f compound c y c l i c crossbeds
i s an exposure o f zigzagging erosional
s i m i l a r t o t h o s e shown i n f i g u r e 3A. bedding
I n one s e t o f crossbeds i n Z i o n
z i g z a g g i n g l a y e r s s i m i l a r t o t h o s e shown i n f i g u r e 3C f i l l a
i n t h i s and n e a r b y o u t c r o p s s u g g e s t t h a t t h e
d o w n w i n d - p o i n t e d s a l i e n t o f t h e dune,
s u r f a c e s ( f i g . 14)
The d i r e c t i o n a l p r o p e r t i e s o f t h e crosszigzags
formed on a
where t h e t w o s i d e s o f t h e s a l i e n t faced
F i g . 14. C r o s s b e d d i n g i n z i g z a g g i n g s e t s f o r m e d o n t w o s i d e s o f a downwindp o i n t e d dune s a l i e n t i n Lamb P o i n t Tongue o f N a v a j o Sandstone a l o n g west side o f Kanab Creek, 8.2 km (5.1 m i l e s ) n o r t h o f Kanab, Utah. F i v e zigzag apexes are visible.
447
OTHER LOCAL CROSSBEDS
CROSSBEDS DEPOSITED ON SALIENT
BEDS DEPOSITED ON EASTWARDDIPPING SURFACE OF SALIENT
SOUTHWARD-DIPPING SURFACE OF SALIENT
Fig. 15. R e l a t i o n o f z i g z a g g i n g crossbeds ( a t same l o c a l i t y a s i n f i g . 14) t o l a r g e - s c a l e c r o s s b e d d i n g elsewhere i n t h e v i c i n i t y . The t w o modes o f zigzagg i n g crossbeds a r e i n t e r p r e t e d as d e f i n i n g f a c i n g d i r e c t i o n s o f t w o s i d e s o f a downwind-pointed s p u r , whereas t h e mean o f o t h e r crossbedding i s i n t e r p r e t e d a s a p p r o x i m a t i n g average f a c i n g d i r e c t i o n o f dunes i n t h e v i c i n i t y . about 45" t o e i t h e r s i d e o f t h e average f a c i n g d i r e c t i o n o f t h e dune ( f i g . 15).
The v a r i a t i o n s i n wind d i r e c t i o n must have been g r e a t enough t o cause
d e p o s i t i o n and e r o s i o n t o a l t e r n a t e f r o m one s i d e o f t h i s s a l i e n t t o t h e o t h e r . Even w i t h o u t t h e e v i d e n c e o f a f l u c t u a t i n g a l o n g s l o p e component o f t h e wind,
we g r e a t l y f a v o r a f l u c t u a t i n g - f l o w o r i g i n f o r t h e Navajo c y c l i c c r o s s -
bedding d e s c r i b e d i n t h i s paper.
Too many i m p r o b a b l e occurrences a r e r e q u i r e d
f o r a superimposed-bedform i n t e r p r e t a t i o n .
The superimposed dunes would have
had t o b e p a r a l l e l o r v e r y n e a r l y p a r a l l e l t o t h e m a i n dune, would have had t o be so l o n g c r e s t e d t h a t l a t e r a l p i n c h o u t s o f t h e i r d e p o s i t s would b e r a r e l y i f e v e r seen, and would have had t o have a l a r g e i n f l u e n c e o n wind c o n d i t i o n s o v e r t h e e n t i r e l e e s l o p e o f t h e m a i n dune, even where t h e superimposed dunes were r e l a t i v e l y s m a l l and d i e d o u t soon a f t e r p a s s i n g t h e c r e s t o f t h e m a i n dune. Evidence f o r Annual P e r i o d i c i t y Methods o f A n a l y s i s .
Wind f l u c t u a t i o n s r a n g i n g i n p e r i o d f r o m a day t o a
y e a r c o u l d c o n c e i v a b l y g e n e r a t e c y c l i c crossbeds o f t h e s c a l e observed i n t h e Navajo Sandstone (Stokes, 1964).
F l u c t u a t i o n s o f even l a r g e r p e r i o d a r e con-
c e i v a b l e , o f course, b u t a r e n o t c o n s i d e r e d h e r e because a y e a r l y p e r i o d i c i t y i s l o n g enough t o reduce t h e r e q u i r e d wind speeds t o q u i t e r e a s o n a b l e l e v e l s . Besides d a i l y and y e a r l y a s t r o n o m i c a l l y c o n t r o l l e d c y c l e s ,
t h e most prominent
448 wind f l u c t u a t i o n s i n t h e r a n g e o f p e r i o d s c o n s i d e r e d h e r e a r e t h e i m p e r f e c t l y p e r i o d i c ones a s s o c i a t e d w i t h t h e passage o f m e t e o r o l o g i c f r o n t s , t y p i c a l l y a t i n t e r v a l s o f a few days t o a few weeks.
A c h o i c e between t h e s e wind c y c l e s
must b e made on t h e b a s i s o f ( 1 ) e v a l u a t i o n s o f t h e p r o b a b i l i t y t h a t t h e wind c y c l e s c o u l d b e d i s t i n c t and r e g u l a r enough t o c r e a t e t h e observed s e r i e s o f c y c l i c crossbeds, and ( 2 ) e v a l u a t i o n s o f t h e reasonableness o f t h e r a t e o f dune movement i m p l i e d b y t h e observed dune advance p e r c y c l e and b y a hypothesized frequency o f wind f l u c t u a t i o n s .
The reasonableness o f an imp1 i e d m i g r a t i o n
r a t e c a n b e e v a l u a t e d b y comparisons w i t h observed m i g r a t i o n r a t e s o f modern dunes and b y c a l c u l a t i o n s o f t h e wind v e l o c i t y r e q u i r e d t o cause a g i v e n m i g r a t i o n rate. D i s t i n c t n e s s and R e g u l a r i t y o f Wind Cycles. monly w e l l developed i n c o a s t a l c l u d i n g d e s e r t s ( D u b i e f , 1952).
D a i l y ’ w i n d c y c l e s a r e com-
r e g i o n s b u t a l s o occur i n o t h e r areas, i n The s t r e n g t h o f d a i l y wind c y c l e s a t nmerous
l o c a l i t i e s i n t h e U n i t e d S t a t e s c a n b e e a s i l y e v a l u a t e d from t h e Local C l i m a t o l o g i c a l Data sheets p u b l i s h e d m o n t h l y b y t h e Environmental Data S e r v i c e o f t h e U.S.
Department o f Commerce.
cycles a r e well
These d a t a sheets show t h a t d a i l y sea-breeze
developed i n many c o a s t a l
areas,
e s p e c i a l l y during
smmer
months, and a r e c h a r a c t e r i z e d b y l a r g e v a r i a t i o n s i n wind v e l o c i t y t h r o u g h t h e day.
On i n l a n d a r i d p l a i n s ( s u c h a s Las Vegas, Nevada, and Yuma, A r i z o n a ) , i n
c o n t r a s t , t e n d e n c i e s t o w a r d d a i l y wind c y c l e s a r e n o t so w e l l developed. i n t h e c o a s t a l dunes o f C a l i f o r n i a ,
Oregon,
and s o u t h e r n Texas,
Even
where d a i l y
wind c y c l e s d u r i n g t h e summer a r e as w e l l developed a s anywhere i n t h e United States,
m e t e o r o l o g i c f l u c t u a t i o n s w i t h a p e r i o d i c i t y o f about a week prevent
t h e development o f l o n g s e r i e s o f u n i f o r m d a i l y l a y e r s .
The e x p e c t a b l e ir-
r e g u l a r i t y o f d a i l y wind c y c l e s d e t r a c t s f r o m t h e i r p l a u s i b i l i t y a s a n exp l a n a t i o n f o r t h e Navajo c y c l i c c r o s s b e d d i n g , even though t h e Navajo d e s e r t may have been a d j a c e n t t o a c o a s t ( S t a n l e y and o t h e r s , 1971). Wind f l u c t u a t i o n s h a v i n g an approximate p e r i o d o f a few days t o a few weeks a r e prominent i n most temperate and s u b t r o p i c a l c l i m a t e s .
Considered as
a p o s s i b l e cause o f c y c l i c crossbedding, however, such f l u c t u a t i o n s s u f f e r frm a g r e a t r a n g e i n t h e d u r a t i o n and s t r e n g t h o f i n d i v i d u a l c y c l e s .
Moreover, i n
most p a r t s o f t h e w o r l d such f l u c t u a t i o n s v a r y g r e a t l y from season t o season. Such wind c y c l e s a r e t h e r e f o r e u n l i k e l y t o g e n e r a t e l o n g s e r i e s o f uniform c y c l i c crossbeds. Y e a r l y wind c y c l e s i n many p a r t s o f t h e w o r l d a r e w e l l developed and e x h i b i t a s m a l l e r range i n r e s u l t a n t s a n d - t r a n s p o r t i n g power t h a n do c y c l e s o f shorter period.
O f t h e c y c l e s c o n s i d e r e d here, y e a r l y ones a r e t h e most l i k e l y
t o generate long s e r i e s o f cycles s i m i l a r i n thickness.
44 9 I m p l i e d Rates o f Dune Movement.
I n e v a l u a t i n g t h e reasonableness o f a n
i n t e r p r e t e d r a t e o f dune movement, one must t a k e i n t o account t h e dune s i z e , form,
and o r i e n t a t i o n r e l a t i v e t o t h e wind, because t h e r a t e o f movement i s
c o n t r o l l e d b y t h e s e f a c t o r s as w e l l 1982). dune.
a s b y wind s t r e s s
(Rubin and Hunter,
Given t h e same wind s t r e s s , a l o w dune m i g r a t e s f a s t e r t h a n a h i g h Even t a k i n g i n t o account t h e i n c r e a s e d wind s t r e s s over h i g h e r dunes
caused by t h e g r e a t e r crowding o f f l o w l i n e s , measurements show t h a t l o w dunes m i g r a t e f a s t e r t h a n h i g h ones, a t l e a s t i f t h e dunes a r e o f barchan t y p e (Long and Sharp,
1964;
Hastenrath,
1967).
Dune form a f f e c t s t h e r a t e o f dune m i -
g r a t i o n b y i t s e f f e c t on s a n d - t r a p p i n g e f f i c i e n c y .
A dune whose l e e s l o p e
t r a p s a l l t h e sand t h a t passes t h e dune c r e s t w i l l m i g r a t e f a s t e r t h a n a dune o f low sand-trapping e f f i c i e n c y .
Dune o r i e n t a t i o n r e l a t i v e t o t h e wind d i -
r e c t i o n has a d i r e c t e f f e c t o n dune m i g r a t i o n as w e l l as a n i n d i r e c t one due t o i t s e f f e c t on s a n d - t r a p p i n g e f f i c i e n c y .
Other f a c t o r s r e m a i n i n g c o n s t a n t , a
t r a n s v e r s e dune w i l l m i g r a t e f a s t e r t h a n one t h a t t r e n d s o b l i q u e l y t o t h e wind, and
i t s r a t e o f migration w i l l
1 ong it ud in a l
.
I n considering
decrease
as t h e dune becomes more n e a r l y
t h e m i g r a t i o n r a t e s o f modern dunes,
we
restrict
a t t e n t i o n t o dunes l a r g e enough t o b e comparable t o t h e Navajo dunes.
our The
average h e i g h t o f t h e Navajo dunes must have been a t l e a s t a s g r e a t a s t h e average t h i c k n e s s o f t h e p r e s e r v e d s e t s o f crossbeds, about 10 m.
More l i k e l y ,
t h e Navajo dunes averaged a t l e a s t 33 m i n h e i g h t ( R u b i n and Hunter, 1982). The m i g r a t i o n r a t e s o f l a r g e modern dunes have r a r e l y been measured. Among t h e measured dunes most comparable i n s i z e t o t h e Navajo dunes a r e t h e Algodones dunes o f s o u t h e r n C a l i f o r n i a . and p r o b a b l y t r a n s v e r s e , 1979).
These dunes, which a r e about 60 m h i g h
m i g r a t e a t a r a t e o f about 0.4
m per y e a r
(Sharp,
The r e l a t i o n s between dune h e i g h t and m i g r a t i o n r a t e f o r t h e barchans
o f P e r u ( H a s t e n r a t h , 1967) and s o u t h e a s t e r n C a l i f o r n i a (Long and Sharp, 1964), i f e x t r a p o l a t e d a s power f u n c t i o n s t o dune h e i g h t s o f 30 m, i n d i c a t e m i g r a t i o n
r a t e s o f 1 2 a n d 10 m/yr, r e s p e c t i v e l y . movements o f 0.5
Although we have m o n i t o r e d d a i l y dune
m on Oregon c o a s t a l t r a n s v e r s e dunes 5 m h i g h under t h e i n -
f l u e n c e o f sunmer sea-breeze winds t h a t reached a maximum speed o f about 15 m/s (34 m i l e s / h o u r )
i n t h e afternoon,
than one day a t a time. interpretation f o r crossbeds
such r a t e s a r e seldom m a i n t a i n e d f o r more
Modern dune m i g r a t i o n r a t e s , t h e r e f o r e , f a v o r a y e a r l y
t h e wind c y c l e s t h a t
( a v e r a g e dune advance,
0.3
formed b o t h t h e c o n c o r d a n t c y c l i c
m per cycle)
and t h e compound c y c l i c
crossbeds (average dune advance, 1.5 m p e r c y c l e ) .
In c a l c u l a t i n g a wind speed f r o m t h e amount o f dune movement r e p r e s e n t e d by a c y c l i c crossbed, we assume a r e l a t i o n between wind speed and sand t r a n s p o r t o f t h e t y p e f o r m u l a t e d by Ehgnold (1941),
and i n i t i a l l y we assume a
450 v a l u e s used b y Hunter and o t h e r s ( i n p r e s s ) .
We emphasize t h a t o t h e r published
e q u a t i o n s and v a l u e s o f c o n s t a n t s c a n r e s u l t i n sand t r a n s p o r t r a t e s d i f f e r i n g b y a f a c t o r o f t w o o r more f r o m t h o s e c a l c u l a t e d here.
From t h e r e l a t i o n
between wind speed and t r a n s p o r t r a t e , graphs showing t h e m i g r a t i o n r a t e s of dunes o f g i v e n h e i g h t , structed (fig. The
orientation,
and s a n d - t r a p p i n g e f f i c i e n c y c a n b e con-
16).
p l o t s r e l a t i n g dune m i g r a t i o n r a t e ,
wind
speed,
and dune height
i n d i c a t e t h a t a 10-m-high t r a n s v e r s e dune o f p e r f e c t s a n d - t r a p p i n g e f f i c i e n c y m i g r a t e s a t a r a t e o f 0.3
m/day when t h e wind speed a t a h e i g h t o f 10 m over
t h e dune c r e s t i s 14 m/s (32 m i / h r ) and a t a r a t e o f 1.5 speed i s 23 m/s (52 m i / h r ) .
m/day when t h e wind
Such wind speeds a r e q u i t e h i g h i n comparison w i t h
l o n g - t e r m mean wind speeds measured a t t h e p r e s e n t t i m e anywhere i n t h e world. F o r example, t a b l e s i n t h e s e r i e s W o r l d Survey o f C l i m a t o l o g y ( G e n t i l l i , 1971; G r i f f i t h s , 1972; Lydolph, 1977; Schwerdtfeger, 1976) i n d i c a t e t h a t y e a r l y mean w i n d speeds i n h o t t o temperate d e s e r t c l i m a t e s r a r e l y exceed 6 m/s, even along desert
coasts.
The w i n d i e s t nonpolar a r i d o r s e m i a r i d r e g i o n r e c o r d e d i n t h i s
mls
A
10
20
30
40
so
60 m m r
Wind speed 10m above dune crest
Fig. 16. R e l a t i o n o f c a l c u l a t e d dune m i g r a t i o n r a t e t o dune h e i g h t and wind Sol i d curves, speed f o r t r a n s v e r s e dunes o f p e r f e c t s a n d - t r a p p i n g e f f i c i e n c y . m i g r a t i o n r a t e i n m e t e r s p e r year; dashed c u r v e s , m i g r a t i o n r a t e i n m e t e r s per day.
451 s e r i e s i s Patagonia, where t h e y e a r l y mean wind speed ( a t Comodoro Rivadavia, A r g e n t i n a ) i s 9.0
m/s.
The inadequacy o f modern d e s e r t winds t o cause enough
d a i l y sand t r a n s p o r t t o account f o r c y c l e s o f Navajo s c a l e i s s i m i l a r l y i n d i c a t e d b y t h e c a l c u l a t e d sand t r a n s p o r t r a t e s o f F r y b e r g e r (1979). A d a i l y p e r i o d i c i t y i s even more s t r o n g l y d i s f a v o r e d when one c o n s i d e r s
t h a t ( 1 ) t h e dunes may have been c o n s i d e r a b l y h i g h e r t h a n 10 m,
(2) t h e dunes
may have been o b l i q u e t o t h e main winds ( R u b i n and Hunter, i n p r e s s ) , ( 3 ) t h e wind c y c l e was undoubtedly c h a r a c t e r i z e d b y d i r e c t i o n a l as w e l l a s speed v a r i a b i l i t y , and (4) t h e l e n g t h o f t h e day was s l i g h t l y s h o r t e r d u r i n g T r i a s s i c and J u r a s s i c
time than a t present
(Pannella,
1972;
Roserberg and Runcorn:
A l l t h e s e c o n s i d e r a t i o n s suggest t h a t g r e a t e r wind speeds t h a n were
1975).
c a l c u l a t e d would b e r e q u i r e d t o g i v e t h e & s e r v e d amount o f sand t r a n s p o r t p e r cycle.
Such h i g h wind speeds seem e s p e c i a l l y u n l i k e l y when one c o n s i d e r s t h a t
t h e T r i a s s i c and J u r a s s i c were t i m e s o f l o w p o l e - t o - e q u a t o r
temperature gra-
d i e n t and would n o t b e expected t o have had a s t r o n g atmospheric c i r c u l a t i o n (Donn and Shaw, 1977).
A y e a r l y i n t e r p r e t a t i o n f o r t h e Navajo c y c l i c c r o s s -
bedding i s t h e r e f o r e s t r o n g l y p r e f e r r e d .
I n the yearly interpretation,
the
r e q u i r e d wind speeds a r e q u i t e modest, and t h e sand may n o t have moved d u r i n g many days o f a y e a r ( f i g . 16). CONCL US I O N S
A s e t o f crossbeds may b e composed o f c y c l i c b u n d l e s d e f i n e d by repetitions o f
structural,
textural,
o r mineralogic
features.
The bounding
s u r f a c e s o f a n i n d i v i d u a l c y c l e may b e c o n c o r d a n t w i t h t h e crossbedding w i t h i n t h e c y c l e o r may b e e r o s i o n a l and d i s c o r d a n t , i n which case t h e crossbedding i s compound as w e l l as c y c l i c .
B o t h c o n c o r d a n t and compound c y c l i c crossbedding
can b e produced b y e i t h e r o f t w o processes: eter,
especially flow direction,
f l u c t u a t i o n s i n some f l o w param-
o r t h e m i g r a t i o n o f bedforms o v e r t h e l e e
s l o p e o f a l a r g e r bedform on which t h e y a r e superimposed.
Distinguishing these
t w o p o s s i b l e causes o f c y c l i c c r o s s b e d d i n g i s b y no means simple.
Fluctuating-
f l o w c y c l e s c a n b e s t b e r e c o g n i z e d b y s t r u c t u r a l f e a t u r e s a r i s i n g from r e v e r sal s
i n t h e a1 ongsl ope o r a c r o s s - s l ope component o f flow.
Superimposed-
bedforms c y c l e s c a n b e s t b e r e c o g n i z e d b y f e a t u r e s p r o v i n g t h a t p a r t o f one c y c l e formed contemporaneously w i t h p a r t o f a n o t h e r c y c l e ( t h a t i s , b y e v i d e n c e t h a t t h e c y c l e s a r e c l imbing t r a n s l a t e n t s t r a t a ) present.
I n t h e absence o f such f e a t u r e s ,
,but
such f e a t u r e s a r e seldom
superimposed-bedform
cycles can
comnonly b e r e c o g n i z e d b y f e a t u r e s t h a t a r i s e f r o m t h e superimposed bedforms b e i n g s h o r t e r i n c r e s t l e n g t h t h a n t h e l a r g e r bedform, t r e n d i n g a t a n a n g l e t o t h e l a r g e r bedform through a t l e a s t p a r t o f t h e i r extent, o r p e r s i s t i n g w h i l e m i g r a t i n g f o r c o n s i d e r a b l e d i s t a n c e s o v e r t h e l e e s l o p e o f t h e l a r g e r bedform.
452 By using such c r i t e r i a , we found what we i n t e r p r e t t o be c y c l e s of b o t h fluctuating-flow a n d superimposed-bedform o r i g i n in t h e Navajo Sandstone.
The
c y c l e s of fluctuating-flow o r i g i n include b o t h t h e concordant types f i r s t described by Stokes (1964) a n d compound crossbedding. The superimposed-bedform c y c l e s a r e described in a n accompanying paper (Rubin and Hunter, t h i s volume). From considerations of t h e d i s t i n c t n e s s , r e g u l a r i t y in s t r u c t u r e and thickness, a n d s c a l e of t h e cycles, we strongly support Stokes' (1964) interpretation of t h e c y c l i c i t y a s annual. This i n t e r p r e t a t i o n should eventually prove valuable f o r making more sophisticated i n t e r p r e t a t i o n s of t h e dynamics of the Navajo dunes, b u t t h e problems of why c y c l i c i t y i s well developed only in some s e t s o f crossbeds a n d of why t h e c y c l i c i t y t a k e s a t l e a s t two d i s t i n c t forms need t o be solved before t h e f u l l potential of Stokes' (1964) insight i s realized. REFERENCES Allen, J.R.L., 1968. Current r i p p l e s , t h e i r r e l a t i o n t o p a t t e r n s of water and sediment motion. North-Holland Publishing Co., Amsterdam, 433 pp. Allen, J.R.L., 1973. Features of c r o s s - s t r a t i f i e d u n i t s due t o random and o t h e r changes in bed forms. Sedimentology, 20:189-202. Allen, J.R.L., 1980a. Sand waves: a model of o r i g i n a n d internal structure. Sedimentary Geology, 26:281-328. Allen, J.R.L., 1980b. Sand-wave immobility a n d t h e internal master bedding of sand-wave deposits. Geol. Mag., 117:437-446. 1941. The physics of blown sand a n d d e s e r t dunes. Methuen, Bagnold, R.A., London, 265 pp. Banks, N.L., 1973. The o r i g i n and s i g n i f i c a n c e of some downcurrent-dipping c r o s s - s t r a t i f i e d s e t s . Jour. Sed. Petrology, 43:423-427. Beerbower, J.R., 1964. Cyclothems and c y c l i c depositional mechanisms in In: O.F. Merriam, ( E d i t o r ) Symposim on c y c l i c a l l u v i a l plain sediments. sedimentation. Kansas S t a t e Geol. Survey Bull., 169:31-42. Boersma, J.R., 1969. Internal s t r u c t u r e of some t i d a l mega-ripples on a shoal i n t h e Westerschelde e s t u a r y , t h e Netherlands: report of a preliminary investigation. Geol. en Mijnbouw, 48:409-414. Brookfield, M.E., 1979. Anatomy of a Lower Permian aeolian sandstone complex, southern Scotland. Scottish J. Geol., 15:81-96. Clifton, H.E., 1979. Tidal channel deposits of middle Eocene age, Torrey Pines I n : P.L. Abbott, ( E d i t o r ) , Eocene depositional S t a t e Reserve, California. systems, San Diego, California. P a c i f i c Section of SOC. Econ. Paleont o l o g i s t s Mineralogists, Los Angeles, California, pp. 35-42. Collinson, J.O., 1970. Bedforms of t h e Tana River, Norway. Geog. Annaler, 52Az31-56. Dalrymple, R.W., Knight, R.J., a n d Middleton, G.V., 1975. I n t e r t i d a l sand bars in Cobequid Bay (Bay of Fundy). In: L.E. Cronin, ( E d i t o r ) , Estuarine research. Geology and engineering, 2:293-307. Donn, W.L., and Shaw, D.M., 1977. Model of climate evolution based on cont i n e n t a l d r i f t and polar wandering. Geol. SOC. America Bull., 88:390-396 Dubief, J., 1952. Le vent e t l e deplacement d u sable au Sahara. Inst. Rech. Sahariennes Trav., 8:123-162. Fryberger, S.G., 1979. Dune forms a n d wind regime. I n : E.D. McKee ( E d i t o r ) , A study of global sand seas: U.S. Geol. Survey Prof. Paper, 1052:137-169. Fryberger, S. G., and Schenk,. Christopher, 1981. Wind sedimentation tunnel experiments on t h e o r i g i n s of aeolian s t r a t a : Sedimentology, 28:805-821.
453 C l i m a t e s o f A u s t r a l i a and New Zealand. World G e n t i l l i , J., ( E d i t o r ) , 1971. s u r v e y o f c l i m a t o l o g y , 13, 405 pp. G r i f f i t h s , J. F., ( E d i t o r ) , 1972. Climates o f Africa. World survey o f c l i m a t o l o g y , 10, 604 pp. Southard, J. B., Spearing, D. R., and Walker, R. G., 1975, Harms, J. C., Depositional environments a s i n t e r p r e t e d from primary sedimentary SOC. Econ. P a l e o n t o l o g i s t s s t r u c t u r e s and s t r a t i f i c a t i o n sequences: M i n e r a l o g i s t s S h o r t Course No. 2, L e c t u r e Notes, 1 6 1 pp. The barchans o f t h e Arequipa r e g i o n , s o u t h e r n Peru. H a s t e n r a t h , S. L., 1967. Z. Gemorph., 11:300-331. 1977a. B a s i c t y p e s o f s t r a t i f i c a t i o n i n small e o l i a n dunes. Hunter, R. E., Sediment01 ogy, v. 24, 362-387. Terminology o f c r o s s - s t r a t i f i e d sedimentary l a y e r s and Hunter, R. E., 1977b. c l i m b i n g - r i p p l e s t r u c t u r e s : Jour. Sed. P e t r o l o g y , 47:697-706. Hunter, R. E., 1981. S t r a t i f i c a t i o n s t y 1 es i n e o l i a n sandstones: some Pennsylvanian t o J u r a s s i c examples f r o m t h e western i n t e r i o r U.S.A. In: F. G. E t h r i d g e and R. M. F l o r e s , ( E d i t o r s ) , Recent and a n c i e n t n o m a r i n e depositional env ir o m e n t s : models for exploration SOC. Econ. P a l e o n t o l o g i s t s M i n e r a l o g i s t s Spec. Publ. No. 31, pp. 315-329. Hunter, R. E., Richmond, B. M., and Alpha, T. R., i n press. Storm-controlled o b l i q u e dunes o f t h e Oregon coast. Geol. SOC. America B u l l . Jackson, R. G., 11, 1976. L a r g e s c a l e r i p p l e s o f t h e l o w e r Wabash R i v e r . Sediment01 ogy , 23: 593-623. 1977. E f f e c t s o f v a r y i n g d i s c h a r g e regimes on bed form Jones, C. M., sedimentary s t r u c t u r e s i n modern r i v e r s . Geology, 5:567-570. Jones, C. M., 1979. T a b u l a r cross-bedding i n Upper C a r b o n i f e r o u s f l u v i a l channel sediments i n t h e s o u t h e r n Pennines, England. Sediment. Geol., 2 4: 85-1 04. Erosion surfaces w i t h i n g i a n t f l u v i a l Jones, C. M., and McCabe, P. J., 1980. cross-beds o f t h e c a r b o n i f e r o u s i n n o r t h e r n England. Jour. Sed. P e t r o l o g y , 50: 613-620. K l e i n , G. D., 1970. D e p o s i t i o n a l and d i s p e r s a l dynamics o f i n t e r t i d a l sand bars. Jour. Sed. P e t r o l o g y , 40:1095-1127. H., Jr., 1981. D i s t i n c t i o n s and uses o f Kocurek, Gary, and D o t t , R. s t r a t i f i c a t i o n t y p e s i n t h e i n t e r p r e t a t i o n o f e o l i a n sand. Jour. Sed. P e t r o l o g y , 51: 579-595. Kohsiek, L. ti. M., and T e r w i n d t , J. H. J., 1981. Characteristics o f foreset and t o p s e t b e d d i n g i n m e g a r i p p l e s r e l a t e d t o hydrodynamic c o n d i t i o n s on a n i n t e r t i d a l shoal: I n : S.-0. Nio, R. T. E. Schuttenheim, and T. C. E. v a n Weering, ( E d i t o r s ) , Holocene m a r i n e s e d i m e n t a t i o n i n t h e N o r t h Sea b a s i n . I n t e r n a t l . Assoc. S e d i m e n t o l o g i s t s Spec. Publ. 5, pp. 27-37. Barchan-dune movement i n I m p e r i a l V a l l e y , Long, J. T., and Sharp, R. P., 1964. C a l i f o r n i a . Geol. SOC. h e r i c a B u l l . , 75:149-156. Lydolph, P. E., 1977. C l i m a t e s o f t h e S o v i e t Union. World survey o f c l i m a t o l o g y , 7, 443 pp. 1977, F o r m a t i o n o f r e a c t i v a t i o n s u r f a c e s McCabe, P. J., and Jones, C. M., w i t h i n superimposed d e l t a s and bedforms. Jour. Sed. P e t r o l o g y , 47:707-715. Pannella, G i o r g i o , 1972. P a l e o n t o l o g i c a l evidence o n t h e E a r t h ' s r o t a t i o n a l h i s t o r y s i n c e E a r l y Precambrian. Astrophys. Space Sci., 16:212-237. P e t t i j o h n , F. J., and P o t t e r , P. E., 1964. A t l a s and g l o s s a r y o f p r i m a r y sedimentary s t r u c t u r e s . S p r i n g e r - V e r l a g , New York, 370 pp. Rosenberg, G. D., and Runcorn, S. K., ( E d i t o r s ) , 1975. Growth r h y t h s and t h e h i s t o r y o f t h e E a r t h ' s r o t a t i o n . W i l e y - I n t e r s c i e n c e , New York, 559 pp. Bedform c l i m b i n g i n t h e o r y and nature. Rubin, D. M. and Hunter, R. E., 1982. Sedimentology, 29:121-138. Rubin, D. M. and Hunter, R. E., t h i s volme. R e c o n s t r u c t i n g bedform assemblages f r o m compound crossbedding. Rubin, D. M., and Hunter, R. E., i n press. Why d e p o s i t s o f l o n g i t u d i n a l dunes a r e r a r e l y r e c o g n i z e d i n t h e g e o l o g i c record. ,Nature. S i n g l e and superimposed bedforms: a Rubin, D. M. and McCulloch, 0. S., 1980.
4 54 s y n t h e s i s o f San F r a n c i s c o Bay and f l u m e o b s e r v a t i o n s . Sedimentary Geology, 26:207-231. Schwerdtfeger, Werner, 1976. C1 imates o f C e n t r a l and South America. World survey o f c l i m a t o l o g y , 12, 532 pp. Sharp, R. P., 1979. I n t r a d u n e f l a t s o f t h e Algodones c h a i n , I m p e r i a l V a l l e y , California. Geol. SOC. America Bull., pt. I, 90:908-916. Smith, N. D., 1972. Some s e d i m e n t o l o g i c a l a s p e c t s o f p l a n a r crosss t r a t i f i c a t i o n i n a sandy b r a i d e d r i v e r . Jour. Sed. P e t r o l o g y , 42:624-634. S t a n l e y , K. O., Jordan, W. M., and D o t t , R. H., 1971. New h y p o t h e s i s o f Early J u r a s s i c paleogeography and sediment d i s p e r s a l f o r western U n i t e d States. Am. Assoc. P e t r o l e u n G e o l o g i s t s B u l l . , 55:lO-19. Stokes, W. L., 1964. E o l i a n v a r v i n g i n t h e Colorado Plateau. Jour. Sed. P e t r o l ogy , 34: 429-432. T e r w i n d t , J. H. J., 1981. O r i g i n and sequences o f sedimentary s t r u c t u r e s i n i n s h o r e m e s o t i d a l d e p o s i t s o f t h e N o r t h Sea. In: S.-D. Nio, R. T. E. Schuttenheim, and T. C. E. van Weering, ( E d i t o r s ) , Holocene marine s e d i m e n t a t i o n i n t h e N o r t h Sea b a s i n . I n t e r n a t l . Assoc. Sedimentolcgists Spec. Publ. 5, pp. 4-26. V i s s e r , M. J., 1980. Neap-spring c y c l e s r e f l e c t e d i n Holocene s u b t i d a l larges c a l e bedform d e p o s i t s : a p r e l i m i n a r y note: Geology, 8:543-546.
455
PERISLACIAL EOLIAN E V E N L Y LAMINATED SANDY DEPOSITS IN THE LATE PLEISTOCENE OF N'4 E U R O P E , A F A C I E S U N R E C O R D E D IN MODERN SEDIMENTOLOGICAL HANDBOOKS
G E R A R D H.J. R U E G G ( G e o l o g i c a l Survey of The N e t h e r l a n d s , Box 157,
2000 AD Haarlem, The N e t h e r l a n d s )
INTRODUCTION P u b l i c a t i o n s on sandy e o l i a n d e p o s i t s in t h e world mostly concern dune d e p o s i t s , t h e dominant s e d i m e n t a r y s t r u c t u r e of which i s c r o s s bedding ( t o low-angle bedd i n g ) . Regional sand s h e e t s - n o t a s i n t e r d u n e a r e a s - a r e a much l e s s f r e q u e n t f a c i e s and have been s t u d i e d much l e s s o f t e n . The paragraph t h a t Reineck & Singh (1980) devoted t o sand s h e e t s w i t h o u t pebble l a y e r s runs as f o l l o w s : "There a r e v a s t a r e a s of sand s h e e t s known which a r e devoid o f any kind of p e b b l e s . Such sand s h e e t s a r e made u p of w e l l - s o r t e d e o l i a n sand w i t h well developed h o r i z o n t a l l y laminated s a n d . A combination of r a p i d s e d i m e n t a t i o n , high wind v e l o c i t i e s and f a i r l y uniform g r a i n s i z e o f t h e sand cause d e p o s i t i o n of s h e e t sand with an abundantly developed e v e n l y laminated sand bedding ( c f . Bagnold 1954a; Glennie 1970) 'I.
Hunter (1977) mentions t h e p r o d u c t i o n of "planebed l a m i n a t i o n " by winds of 40 mi l e s p e r hour ( 1 8 m / s e c ) . F r y b e r g e r e t a l . ( 1 9 7 9 ) d e s c r i b e r e c e n t low-angle e o l i a n d e p o s i t s of a sand s h e e t , o c c u r r i n g as a t r a n s i t i o n a l f a c i e s between high-angle e o l i a n dunes and non-eolian d e p o s i t s , n e a r Great Sand Dunes National Monument, Colorado, U.S.A. They l i s t e l e v e n s e d i m e n t a r y f e a t u r e s . Most of t h e s e f e a t u r e s a r e a l s o p r e s e n t i n an e o l i a n s u b f a c i e s under c o n s i d e r a t i o n ( s e e below); g e n e r a l l y , however, f e a t u r e s r e l a t e d t o b i o l o g i c a l a c t i v i t y a r e very s c a r c e in t h i s subf a c i e s in t h e European d e p o s i t s .
The Parsonsburg Sand i n t h e C e n t r a l Delmarva P e n i n s u l a , Maryland and Delaware, d e s c r i b e d by Denny e t a l . ( 1 9 7 9 ) , presumably r e p r e s e n t s a f o s s i l analogue in North America. A r e c e n t d e p o s i t from Banks I s l a n d shown in F i g . 8 of a paper by P i s s a r t e t a l . (1977) i s h i g h l y comparable t o a Dutch p e r i g l a c i a l e o l i a n subf a c i e s ( s u b f a c i e s A , see below), although e v i d e n t l y the Banks I s l a n d d e p o s i t c o n t a i n s abundant o r g a n i c laminae ( f r a g m e n t s of willow s h r u b s ) in c o n t r a s t t o t h e bulk of t h e Dutch d e p o s i t s ; t h e d e p o s i t on Banks I s l a n d seems t o be r e l a t i v e l y s m a l l , and connected with r i v e r v a l l e y s . S i m i l a r ( s u b ) r e c e n t d e p o s i t s seem t o
o c c u r on Cumberland P e n i n s u l a , B a f f i n I s l a n d ( c f . Thompson 1954, c i t e d in Andrews e t a l . 1979 and i n Dyke e t a l . 1 9 8 2 ) .
456
legend L Cross-section Montlerlond
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F i g . 1. Map o f The N e t h e r l a n d s s h o w i n g l o c a t i o n s r e f e r r e d t o i n Legends t o t h e f i g u r e s ( l e f t ) , and map o f NW E u r o p e ( r i g h t ) . REGIONAL SETTING Deposits o f t h i s k i n d d a t i n g from t h e Weichselian stage (equivalent t o the W i s c o n s i n s t a g e ) o c c u r i n n o r t h e r n F r a n c e (Somm6 e t a l . 198O), B e l g i u m ( e . 9 . M a r 6 c h a l & M a a r l e v e l d 1955; de P l o e y 1961; Zagwi j n & Paepe 1968; Vandenberghe & Gul l e n t o p s 1977; Vandenberghe 1981), West Germany (Dewers 1932, 1934; Ducker & M a a r l e v e l d 1957; Ruegg 1 9 8 1 ) , E a s t Germany ( N e u m e i s t e r 1971; see a l s o Eissmann 1 9 8 1 ) , P o l a n d ( e . g . K o z a r s k i e t a l . 1969; Nowaczyk 1976; K o z a r s k i 1980) and p r e s u m a b l y a l s o i n t h e USSR ( F i g . 1 ) . I n t h e e a s t e r n p a r t o f t h i s s a n d - b e l t ,
the
d e p o s i t s a r e p r e d o m i n a n t l y p r e s e n t as dune f i e l d s r e l a t e d t o r i v e r ( p r a d o l i n a ) v a l l e y s (Nowaczyk 1976; K o s t e r 1 9 8 2 ) .
I n The N e t h e r l a n d s , p e r i g l a c i a l d e p o s i t s a r e known f o r a t l e a s t s i x g l a c i a l stages (Fig. 2 ) . Important c o n t r i b u t i o n s concerning t h e occurrence, stratigraphy,
457 d a t i n g , and g e n e s i s o f p e r i g l a c i a l ! d e i c h s e l i a n d e p o s i t s have been p u b l i s h e d by, e . g . van d e r Hammen
(1951), van d e r Hammen h V a a r l e v e l d ( 1 9 5 2 ) , ) 4 a r @ c h a l &
M a a r l e v e l d (1955), D u c k e r 7, r l a a r l e v e l d ( 1 3 5 7 ) , Z a g w i j n ( 1 9 6 1 ) , van den T o o r n ( 1 9 6 7 ) , van d e r Hammen e t a l . ( 1 9 6 7 ) , Z a g w i j n & Paepe ( 1 9 6 8 ) , van d e r Hammen & Wijmstra (eds.) (1971), Bisschops (1973), Zagwijn (1974), Maarleveld (1976), K o l s t r u p & W i j m s t r a ( 1 9 7 7 ) , K o l s t r u p ( 1 9 8 0 ) , Vandenberghe ( 1 9 8 1 ) , Vandenberghe & Krook ( 1 9 8 1 ) and K o s t e r ( 1 9 8 2 ) . T h e r e i s a g e n e r a l agreement a b o u t t h e e o l i a n dominated o r i g i n o f t h e s e d e p o s i t s ,
a t p r e s e n t known as t h e Twente F o r m a t i o n .
The d e p o s i t s i n q u e s t i o n d r a p e an i r r e g u l a r t o p o g r a p h y ( g e n e r a l f i e l d name: c o v e r s a n d s ) ; on a r e g i o n a l s c a l e t h e y f o r m s a n d s h e e t s . The d e p o s i t s o c c u r o v e r l a r g e a r e a s , n o r m a l l y as a b l a n k e t w i t h a t h i c k n e s s o f some d e c i m e t e r s t o some m e t e r s ; t h e y a r e a b s e n t o n l y on e l e v a t e d a r e a s where t h e y may n o t have been dep o s i t e d , and i n c e r t a i n l o w r e g i o n s due t o s u b s e q u e n t e r o s i o n . The l a t e r a l and v e r t i c a l r e l a t i o n s as w e l l as t h e r e l a t i v e abundances o f t h e d i s t i n g u i s h e d sub-
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F i g . 2. L e f t : Q u a t e r n a r y c h r o n o s t r a t i y r a p h y o f The N e t h e r l a n d s s h o w i n g t h e i n t e r v a l s (hatched) i n which p e r i y l a c i a l d e p o s i t s were formed. R i g h t : c h r o n o s t r a t i g r a p h y and c l i m a t i c c u r v e o f t h e W e i c h s e l i a n ( a f t e r van S t a a l d u i n e n e t a l . 1 9 7 9 ) .
458
F i g . 3. C r o s s - s e c t i o n through t h e Y o n t f e r l a n d ice-pushed r i d g e in t h e eastern p a r t of t h e c e n t r a l Netherlands ( a f t e r van de Meene 1 9 7 7 ) . f a c i e s a r e s t r o n g l y dependent on t h e morphology of t h e s u b s o i l , which permits d i v i s i o n of The N e t h e r l a n d s i n t o a n o r t h e r n , a c e n t r a l , and a southern coversand a r e a . Moreover, t h e r e i s a l s o r o u g h l y a general i n c r e a s e of s i l t f r a c t i o n from N N W t o SSE; in t h e southernmost p a r t of t h e c o u n t r y only l o e s s d e p o s i t s occur.
I n a c o n s i d e r a b l e p a r t of t h e n o r t h e r n a r e a a g e n e r a l l y t h i n cover of periglacial Weichselian d e p o s i t s i s s e p a r a t e d from i d e n t i c a l d e p o s i t s of S a a l i a n age ( E i n d h o ven Formation) by a S a a l i a n b a s a l t i l l ; moreover, v a l l e y f i l l s o c c u r , in which s i g n i f i c a n t l o c a l - f l u v i a l d e p o s i t s may be p r e s e n t . I n t h e c e n t r a l p a r t , the subs t r a t u m was p r e v i o u s l y modelled by t h e S a a l i a n i n l a n d i c e and by Eemian interg l a c i a l l e v e l l i n g p r o c e s s e s ( m a i n l y niarine s e d i m e n t a t i o n in t h e former glacial b a s i n s ) ; h e r e , t h e Twente Formation, predominantly r e p r e s e n t e d by e o l i a n deposits, shows s t r o n g v a r i a t i o n of t h e t h i c k n e s s , r a n g i n g from some t e n s of meters in the
459
former g l a c i a l basins t o n i l l o c a l l y on the ice-pushed ridges ( F i g . 3 & Towards the German border, l o c a l - f l u v i a l s h e e t l i k e i n t e r c a l a t i o n s proportionally increase.
I n t h e southern p a r t of the coversand area, the existence of a horst and graben system i s strongly r e f l e c t e d in the extent and thickness of the p e r i g l a c i a l deposits (where the frequency o f the various subfacies i s largely u n k n o w n ) ; in the Central Graben thicknesses up t o 40 meters are known f o r the stacked p e r i g l a c i a l deposits of the l a s t t h r e e g l a c i a l stages ( E l s t e r i a n , Saalian, Weichselian: Nuenen Group) (Fig. 4 ) . More t o the west, f l a t areas with a cover of eolian dep o s i t s are i n t e r s e c t e d by valley f i l l s in which flowing-water sediments are present.
Fig. 4. Variations i n t h e thickness of the Nuenen Group, sheet 51 East ( a f t e r Bisschops 1973) and sheet 51 West ( J . P . Broertjes, Geological Survey, pers. comm.).
I n many areas of t h e country, these deposits are present a t the surface, and have been l i t t l e affected by Holocene processes. Often a so-called "coversand r e l i e f " i s present, with elevations mostly not exceeding 2 t o 3 , and in no case 5, meters in height (Koster 1982) (Fig. 5 ) . SEDIMENTARY (SUE) FACIES The following four subfacies can be distinguished within the p e r i g l a c i a l dep o s i t s of The Netherlands and adjacent areas: (1) eolian subfacies A , ( 2 ) eolian subfacies 6, ( 3 ) l a c u s t r i n e subfacies, and ( 4 ) flowing-water subfacies ( l o c a l
460
F i g . 5 . Ridges of Late Weichselian coversand on t h e s u r f a c e of a former g l a c i a l basin ( G e l d e r s e V a l l e i ) ; 1. b r o o k s , 2 . coversand r i d g e , 3. ice-pushed r i d g e ( a f t e r Maarlevel d 1 9 6 0 ) . d i s c h a r g e ) . The l i t h o l o g i c a l and s t r u c t u r a l c h a r a c t e r i s t i c s w i l l be mentioned b r i e f l y ; t h e photographs supplement t h e d e s c r i p t i o n s . Eolian s u b f a c i e s A ( s e e F i g s . 6-11) Predominantly unimodal w e l l - s o r t e d uniform s a n d s ; e v e n l y l a m i n a t e d , o f t e n with a " f l o w i n g " appearance w i t h t a p e r i n g u n i t s and low-angle c o n t a c t s ; r a r e l y cross-bedded; o c c a s i o n a l l y with laminae composed of g r a n u l e s o r small p e b b l e s ( m o s t l y w i n d - p o l i s h e d , l e s s commonly as v e n t i f a c t s ) ; l o c a l l y c r a c k s and small i n t e r n a l f r o s t wedges, t h i n moss-peat i n t e r c a l a t i o n s and adhesion r i p p l e l a y e r s a r e p r e s e n t ; i n t e r n a l c r y o t u r b a t i c l e v e l s a r e o f t e n l e s s numerous and t h i n n e r than t h o s e a s s o c i a t e d w i t h t h e n e r t s u b f a c i e s ( c f . Maarleveld 1 9 7 6 ) ; b u r i e d accumulations a r e very i n f r e q u e n t ; v a r i o u s low r e l i e f forms (Maarleveld 1960) inay be a s s o c i a t e d . Where c o a r s e sediment was a v a i l a b l e ( e . g . , n e a r ice-pushed r i d g e s ) , laminae o r l e n s e s ( t h e l a t t e r p o s s i b l y r e p r e s e n t i n g degraded g r a n u l e r i p p l e s ; c f . Bagno d 1941, p . 1 5 5 ) c o n t a i n i n g g r a n u l e s o r small p e b b l e s may o c c u r , as well a s l e v e l s
461 w i t h uniformly dispersed gravel-sized p a r t i c l e s ( c f . Bagnold 1941, p . 159). These
phenomena may a l s o be encountered in r e l a t i v e l y coarse eolian subfacies B deposits Note: the deposits depicted by Ahlbrandt & Fryberger (1982) in t h e i r Figs. 18C a n d 18D, show much resemblance t o eolian subfacies A deposits (except f o r the bioturbation t r a c e s , not present in the l a t t e r ) .
Eolian subfacies B ( s e e Figs. 11-20) I n t e r c a l a t e d yellowish a n d greyish evenly laminated sands ( " l a y e r cake" e f f e c t ) ; greyish laminae are more or l e s s s i l t y ; minor grain-size peaks occur the coarse s i I t and/or
in
t h e 75-88
ym
in
f r a c t i o n s ; i n t e r n a l c r y o t u r b a t i c levels
and f r o s t wedges are commonly p r e s e n t . Whaleback-like low-relief forms may consist of t h i s tyue of deposit (de Jong 1975). Locally, the following f e a t u r e s may be found: t h i c k e r s i l t y layers w i t h a loess o r loess-approaching grain s i z e ( m a i n peak
in
the coarse s i l t or
i n
the 75-88ym f r a c t i o n s ) , peat l a y e r s , and adhesion
r i p p l e l a y e r s ; " v e r t i c a l - p l a t y " crack systems ( c f . V i n k & Sevink,
in
van der
Hammen & Wijmstra 1971) may be preserved. Note: the often v i s i b l e c r i n k l y appearance
in
eolian subfacies B deposits ( s e e
Figs. 12-17) i s t h e r e s u l t of contortion by freeze/thaw action on previously e x i s t i n g laminae, and i s not caused by i n f i l t r a t i o n of very finesdeveloping d i s s i p a t i o n s t r u c t u r e s as defined by Ahlbrandt & Fryberger 1980, 1982. Lacustrine subfacies ( s e e Figs. 21-24) Varying composition of d e p o s i t s ; sandy p a r t s are evenly or wave-ripple laminated; s i l t layers composed of material w i t h a loess or l o e s s - l i k e grain s i z e and g y t t j a layers may be i n t e r c a l a t e d ; frost-thaw and f r o s t s t r u c t u r e s occur r a r e l y . Flowing-water subfacies ( s e e Figs. 25-29) Cross-bedded and cross-laminated deposits ( t h e l a t t e r made by among other things climbing r i p p l e s i n d i c a t i n g declining current v e l o c i t i e s ) , represented by p e r s i s t e n t layers as well as by complex c h a n n e l - f i l l s ; cross-bedded cosets locally contain macoscopic p l a n t remains (reworked), e s p e c i a l l y in c h a n n e l - f i l l s . This subfacies i s a t t r i b u t e d t o local discharge systems, as distinguished from the f l u v i a l deposits of t h e trunks of t h e Rhine-Meuse r i v e r system. RELATION OF SUBFACIES IN TIME A N D PLACE
Although the Twente Formation i s eolian-dominated, a t l e a s t in most of the exposures, l a c u s t r i n e a n d flowing-water deposits play an important r o l e l o c a l l y . Grain-size i n v e s t i g a t i o n s have shown t h a t the water-laid deposits are generally reworked eolian sediments. Many authors ( e . g . van der Hammen e t a l . 1967; Vandenberghe 1981) found a c o r r e l a t i o n between f a c i e s a n d subdivision of the g l a c i a l s t a g e . D u r i n g the Early
462
F i g . 6 . E o l i a n s u b f a c i e s A d e p o s i t showing even l a m i n a t i o n , d e f l a t i o n levels and f r o s t c r a c k s ( a g e : Late ! ! e i c h s e l i a n ) ( r o d s c a l e i n cm); Claricurri, Closterineent .
F i g . 7 . Eolian s u b f a c i e s A d e p o s i t s ( a g e : Late I l e i c h s e l i a n ) o v e r l y i n g Saalian t i l l ; e o l i a n accumulation l e v e l l e d by subsequent e o l i a n d e p o s i t i o n ( l e n g t h o f j o i n t e r 3 3 c m ) ; E r i c a , n e a r Crnmen, motorvay c r o s s i n g .
F i g . 8. E o l i a n subfacies A sedivent (I!eicflselian) o v e r l y i n g Saalian sandur deoosits ( r u l e r l e n g t h 2 5 crn); Z e i s t , rnotorway c r o s s i n g .
F i g . 9. D e t a i l o f upuer sediment i n F i g . 8 ( h e i g h t 27 cri).
P
m
W
464
F i g . 11. Eolian s u b f a c i e s A sediment o v e r l y i n g e o l i a n s u h f a c i e s R d e p o s i t s . The former- i s v i r t u a l l y f r e e of s i l t and more s u s c e p t i b l e t o wind a c t i o n ; ldeelde ( B e l g i u m ) , sand p i t .
465
F i g . 1 2 . Eolian s u b f a c i e s R sediments with a f a i n t v e r t i c a l - p l a t y crack system ( a g e : ivliddle W e i c h s e l i a n ) . The h e i g h t o f t h e l a c q u e r p e e l i s 7 7 cm; L o s s e r , Dinkel v a l l e y , o u t c r o p i n concave bank.
466
F i g . 13. E o l i a n s u b f a c i e s 3 sediments ( a g e : ' l i d d l e l i e i c h s e l i a n ) ; t h e lower part i s c r y o t u r h a t i c a l l y deformed a n d tonord by a r e o i o n a l d e f l a t i o n l e v e l (Ceuningen % r a v e l 3ed); t h e unper p a r t c o n t a i n s s o m e i n t e r c a l a t i o n s l a i d down by flowing water- ( h e i y l i t o f l a c q u e r p e e l : 35 c m ) ; L o s s e r , D i n k e l valley, o u t c r o p i n concave bank,
F i a . 1 4 . F o l i a n s u h f a c i e s " sediments ,"iit'i a consrlicLioii5 i r l t r a f o r m a t i o n a l minor c r y o t u r b a t c d a n d s l i c h t l y s l t i m ~ ?l r i v e l ( a n c : ',li;idle L'leichselian). T'ie h e i g h t o f t h e l a c q u e r p e e l i s 35 ci'1 an3 t h e depth of t h e d p n o s i t i s a b o u t 1 2 . 5 ~1 !ielo./ s i i r f a c Q l c w l ; Voorthuizen, sand p i t .
F i ? . 1 5 . Eolian s u b f a c i e s E sediments s h o ' v i n g
i m n y c r y o t u r b a t e d l e v e l s ( a g e : ' l i d d l e I4eiciiseli a n ) . 'Jete s p e c i f i c s u r f a c e p a t t e r n i n f l u e n c e d b y v e r t i c a l - v l a t y crack systems ( r u l e r 33 ciii l o n q ) . ? e D o s i t s i t u a t e d a b o u t 1 5 ni belo:.i s u r f a c e l e v c l ; ' i o o r t l i u i r e n , sand D i t .
e m v
4 68
F i g . 1 7 . Major c r y o t u r b a t i c a l d e f o r m a t i o n s i n mainly e o l i a n s u b f a c i e s E s e d i m e n t s ; Uitwel l i n g e r g a , aquaduct p i t .
c a s t irl c!olian s u b facias r sedimc'nts ( a w o f t l i e d e q o s i t i s S a a l i a n , age of t h e c a s t i s : ! e i c t i s e l i a n ) ( I i e i g i i t o f lacqirer p e e l : ?.$1 m ) ; Sevenuni, S c h a t h e r p s a n d ; l i t . F i g . 19. Ice-:/edqe
470
F i g . 20. V e r t i c a l - n l a t y Lrrick s y i t e n i v i s i b l e iri e o l i a n S u b f a L i P i C ietiiiiients a b o i r t 1 0 . 5 in below s u r f a c e l i ? v i ' l ( a s p : Y i d t l l ~ : ' : I p i i I . i 5 e l i a n ~ ( r i i l e r : 3') c r l o n s ) ; Voorttiuizi'n, s a n d p i t .
471
2 1 . D-noiits o f t h e l a c u s t r i n e ii.i'>facies; s i l t , i and f i n e sands ,:iitii l m t i c u l a r anri w a v y h e d d i n q ( t i e i o l i t : 45 c v ) ; 7irirl1ioven, r a i l v a y - t u n n e l p i t
Fin.
F i g . 2 2 . L a c u s t r i n e d e p o s i t s showing f i n e sands, s i l t s and g y t t j a , i n t e r s e c t e d by ice-wedge c a s t s ( a b o u t R in b e l o w O . D . ; Middle !.!eictiselian); Den t i e l d e r , d o c k yard p i t .
472
F i g . 23. D e p o s i t s of t h e l a c u s t r i n e s u b f a c i e s , with h a l f - s t a t i o n a r y : i ~ v t ? r i p p l e c o s e t s ; a t ttie b a s e t h e r e a r e d e b r i s flow sedirnents )with remnants of t h e o r i g i n a l sediment resembling ttie o v e r l y i n g d e p o i i t s (about, 10 111 b e l o w O.D.; Middle Weictiselian) ( s c a l e on spade s h a f t in drii); Deri l l e l d e r , dockyard lit.
F i g . 2 4 . D e t a i l o f F i g . 2 3 ( j o i n t e r measures 33 crri).
473
F i q . 2 5 . Flowing-'qater deposits intprcalated w i t h eolian subfacies C deposits (aoe: "iddle ' ! e i c h s e l i a n ) ; l a c q u e r o e e l ranoe 4.4Q-5.SQm !)elow s u r f a c e l e v e l ; Voor-ttiuizen, s a n d p i t .
F i g . 2 6 . I n t e r c a l a t i o n of s h e e t l i k e flowino-:vatcr d e 9 o s i t s i n eolian iiibfacies 4 deonsits; Overdinki'l, s a n d o i t..
474
F i q . 2 7 . C h a n n e l - f i l l s i n c i s e d i n a c r y o t u r h a t i c a l l y d e f o r m e d s u p e r n o s i t i o n of p e r i g l a c i a l l a c u s t r i n e d e p o s i t s , Eemian p e a t , and S a a l i a n p e r i g l a c i a l deposits; Cindhoven, r a i l w a y - t u n n e l n i t .
F i g . 28. C h a n n e l - f i l l w i t h a b u n d a n t p l a n t m a t e r i a l l o c a l l y and p e a t lumps a t the base, i n c i s e d i n e o l i a n s u b f a c i e s R d e p o s i t s s i t u a t e d a b o u t 9 m below O.D. (age: M i d d l e W e i c h s e l i a n ) ; Den H e l d e r , d o c k y a r d p i t .
475
F i g . 29. C r o s i - l a m i n a t e d d e p o s i t l a i d down b y f l o w i n g w a t e r \which have f i l l e d up s h a l l o w c h a n n e l - f i l l s ; a b o u t 16 m h e l o w 0.D. ( a g e : E a r l y ' l e i c h s e l i a n ) ( r u l e r i n crn); Den i i e l d e r , d o c k y a r d p i t . and l a t e W e i c h s e l i a n , m a i n l y l o a m - f r e e c o v e r s a n d s were d e p o s i t e d ,
hereas d u r i n g
t h e M i d d l e W e i c h s e l i a n ( P l e n i g l a c i a l ) m a i n l y loamy c o v e r s a n d s were f o r m e d . Van den T o o r n ( 1 9 6 7 ) f o u n d a s i m i l a r s u p e r p o s i t i o n i n t h e graben a r e a ; he d e s c r i b e d an a n a l o g o u s s e t t i n g f o r t h e u n d e r l y i n g S a a l i a n p e r i g l a c i a l d e p o s i s ( E i n d h o v e n F o r m a t i o n ) . M a a r l e v e l d ( 1 9 7 6 ) s u g g e s t e d t h a t t h e " O l d e r Coversands
(Middle
Weichselian; g e n e r a l l y e o l i a n s u b f a c i e s B ) arose under permafrost c o n d i t i o n s and t h e "Younger C o v e r s a n d s " ( L a t e W e i c h s e l i a n ;
generally e o l i a n subfacies A)
were d e p o s i t e d i n t h e absence o f p e r m a f r o s t . W i t h r e s p e c t t o t h e f o u r s u b f a c i e s , i t appears t h a t d e p o s i t s o f s u b f a c i e s A d o m i n a t e d i n t h e E a r l y and L a t e W e i c h s e l -
i a n s e d i m e n t a t i o n , whereas t h e o t h e r t h r e e s u b f a c i e s d o m i n a t e d u n d e r p l e n i g l a c i a l c o n d i t i o n s . The p r e s e n c e o r absence o f p e r m a f r o s t m i g h t a l s o e x p l a i n why s h e e t s o r c h a n n e l s were f o r m e d w i t h i n t h e f l o w i n g - w a t e r s u b f a c i e s . D e p o s i t s o f t h e l a c u s t r i n e and f l o w i n g - w a t e r s u b f a c i e s seem t o be i m p o r t a n t i n t h e southern coversand area, v i z . ,
t h e C e n t r a l Graben a r e a . Here, t h i c k l e n s e s
o f sandy s i l t ( i n c r o s s - s e c t i o n s up t o more t h a n 10 km w i d e and up t o 5 m t h i c k ) o c c u r ( " B r a b a n t s i l t " ) ( v a n den T o o r n 1967; B i s s c h o p s 1 9 7 3 ) , w h i c h a r e c o n s i d e r e d t o be d e p o s i t s f o r m e d by e o l i a n s u p p l i e d m a t e r i a l i n s h a l l o w s h e e t s o f s t a n d i n g water i n areas w i t h blocked r u n - o f f .
476 SEQUENTIONAL TRENDS The f o l l o w i n g gradual s u p e r p o s i t i o n s ( o r p a r t s of them) have been observed:
+
eolian subfacies A eolian subfacies B
eolian subfacies A
t
and
eolian subfacies B
+
.f
(lacustrine subfacies)
1a c u s t r i ne s ubf a c i e s
+
flowing-water subfacies I n v e s t i g a t i o n of a Weichselian v a l l e y f i l l l e d t o a s c h e m a t i c r e p r e s e n t a t i o n of an " i d e a l " p e r i g l a c i a l sequence b u t h e r e w i t h o u t l a c u s t r i n e d e p o s i t s - shown now i n F i g . 30 ( F i g . 2 i n Ruegg 1 9 7 5 ) .
v
H
DESERT PAVEMEN1
G
DRY EOLIbN
F
MOlST EOLIAN
E
WET EOLIAN
D
WATER CURRENT V E L O C l T I LOW TO NIL
c
R&TT*ER F I S T R U N N I N G WATER R IPPLE PHASE LOW ENERGY
6
FAST RUNNING WATER DUNE P W S E LOW ENERGY (OFTEN ASSENT)
&
V E R I FbST RUNNING WbTER HIGH ENERGY
a h k A 7 5L
Fig. 30. Schematic r e p r e s e n t a t i o n of a p e r i g l a c i a l sequence as deduced from d e p o s i t s i n a v a l l e y f i l l n e a r P e e l o , The N e t h e r l a n d s . The lower d i v i s i o n s ( A - D ) were formed under d e c l i n i n g c u r r e n t v e l o c i t y c o n d i t i o n s and t h e upper p a r t ( E - H ) was g e n e r a t e d mainly s u b a e r i a l l y and under i n c r e a s i n g e o l i a n i n f l u e n c e ; l a c u s t r i n e d e p o s i t s a r e n o t p r e s e n t and c r y o t u r b a t i c d e f o r m a t i o n s a r e r a t h e r s c a r c e ( a f t e r Ruegg 1 9 7 5 ) .
G R A I N SIZE F i g . 31 shows t y p i c a l g r a i n - s i z e p o p u l a t i o n s found i n samples of e o l i a n subf a c i e s A and 6. The p a t t e r n of the l a t t e r i s c h a r a c t e r i z e d by t h e o c c u r r e n c e of peaks i n the c o a r s e - s i l t and 7 5 - 8 8 ~ mf r a c t i o n s . As an a s p e c t s of t h e i n v e s t i g a t ion performed f o r t h e c l a s s i f i c a t i o n of Dutch Holocene s o i l s , c a r r i e d o u t by the S o i l Survey I n s t i t u t e ( d e Bakker & S c h e l l i n g 1 9 6 6 ) , a g r a i n - s i z e continuum was found between p e r i g l a c i a l e o l i a n sands and s i l t s ( i n c l u d i n g l o e s s ) ( F i g . 3 2 ) .
477 F i g . 31. R e p r e s e n t a t i v e g r a i n s i z e h i s t o g r a m s f o r e o l i a n subf a c i e s A ( u p p e r t h r e e ) and eolian subfacies B (lower three deposits.
BLA RlCUM
z.
2
&
0‘
u-u
“
2
i
-
s
0-2um
F i g . 32. P r o p o r t i o n s of c l a y , s i l t , and sand in samples from t h e coversand and l o e s s a r e a in The N e t h e r l a n d s ( a f t e r de Bakker & Schelling 1966).
478
TENTATIVE EXPLANATION OF THE EOLIAN SUBFACIES The f o l l o w i n g g e n e r a l remarks d e a l w i t h t h e e o l i a n environment of d e p o s i t i o n . The f i r s t three u n d e r l i n e t h e predominantly n o n - f l u v i a l o r i g i n , and t h e l a s t t h r e e c o n s i d e r t h e e o l i a n s e d i m e n t a t i o n i n more d e t a i l .
1. The more o r l e s s uniform h a b i t u s coupled w i t h the o v e r a l l o c c u r r e n c e and v a r i e g a t e d palaeotopography, as w e l l as t h e kind of s u r f a c e r e l i e f , argue a g a i n s t a f l u v i a l mode of o r i g i n . 2 . The absence of c r o s s bedding and c r o s s l a m i n a t i o n and the dominance of even l a m i n a t i o n , i n combination w i t h t h e f i n e - g r a i n e d l i t h o l o g y , cannot be exp l a i n e d i n terms of f l o w i n g - w a t e r p r o c e s s e s . 3. The d i s t u r b a n c e s c o n s i d e r e d t o be c r y o g e n e t i c a r e g e n e r a l l y u n l i k e disturbances seen i n w a t e r - l a i n n o n - p e r i g l a c i a l d e p o s i t s , as Maarleveld (1976) a l r e a d y concluded f o r " d r o p l i k e r e g u l a r d e f o r m a t i o n s " and G u l l e n t o p s & P a u l i s s e n (1978) f o r a kind of "drop s o i l " . 4. Organic l a y e r s o r r o o t d i s t u r b a n c e s a r e g e n e r a l l y s c a r c e o r a b s e n t , c o n t r a r y t o the s i t u a t i o n i n ( s u b ) r e c e n t e v e n l y laminated d e p o s i t s on Banks I s l a n d ( P i s s a r t e t a l . 1977) and B a f f i n I s l a n d ( e . g . , Dyke e t a l . 1 9 8 2 ) . 5 . On t h e b a s i s of the c r y o t u r b a t i c s t r u c t u r e s , p o l a r d e s e r t c o n d i t i o n s a r e inf e r r e d f o r p a r t of t h e Middle Weichselian ( P l e n i g l a c i a l ) . Twice a complete d i s a p p e a r a n c e of p e r m a f r o s t has been deduced from t h e o c c u r r e n c e of two major d i s t u r b e d l e v e l s ( e . g . , Maarleveld 1 9 7 6 ) . 6 . M e l t i n g - o u t of snow l e a d s t o d i s t u r b a n c e s of v a r i o u s s c a l e s , u p t o complete homogenization ( d i a m i c t i z a t i o n ) , b u t t h i s f e a t u r e i s v i r t u a l l y a b s e n t in the i n v e s t i g a t e d d e p o s i t s . On t h e c o n t r a r y , t h e g e n e r a l l y very s h a r p and cons p i c u o u s l a m i n a t i o n , w i t h laminae i n p l a c e s t r a c e a b l e l a t e r a l l y f o r more t h a n 10 m e t e r s , e x c l u d e s such p r o c e s s e s as m e l t i n g - o u t and a l s o e r o s i o n on a micros c a l e due t o r u n - o f f . The f o l l o w i n g t e n t a t i v e h y p o t h e s i s c o n c e r n i n g t h e d e p o s i t i o n of sandy and s i l t y laminae has been proposed ( c f . Ruegg 1975, 1 9 8 1 ) : pure s a n d laminae : d e p o s i t s from t h e t r a c t i o n c a r p e t on a d r y s e d i m e n t a t i o n surf ace; s i l t y s a n d laminae: a s t h e f o r e g o i n g , b u t s u s p e n s i o n load may s t i c k t o a somet i m e s damp s e d i m e n t a t i o n s u r f a c e ; s i l t layers
: wet d e p o s i t i o n a l s u r f a c e ; t r a c t i o n c a r p e t f i x e d upwind; only
the s u s p e n s i o n load reached t h e s p o t . An i n f l u e n c e of t h e humidity of t h e s e d i m e n t a t i o n s u r f a c e on l o e s s deposition has been d e m o n s t r a t e d by Cegla (1969, 1972; c f . Smalley 1 9 7 5 ) . French ( 1 9 7 6 ) d i s c u s s e d the p o s s i b l e d i f f e r e n c e s between t h e P l e i s t o c e n e perig l a c i a l environment of m i d - l a t i t u d e s and t h a t of p r e s e n t - d a y l a t i t u d e s from a
479 g e o m o r p h i c p o i n t o f v i e w . F o r t h e f o r m e r , he a s s i g n e d i m p o r t a n c e t o t h e r e g i m e o f d i u r n a l s o l a r r a d i a t i o n which l e d t o greater m o b i l i t y o f t h e s u r f i c i a l material He a l s o i n f e r r e d s t r o n g e r w i n d s . The g r e a t i m p o r t a n c e o f w i n d s i s c o n c l u d e d f r o m t h e presumed e x i s t e n c e o f g r e a t e r w i n d g r a d i e n t s due t o t h e s p a t i a l c o n c e n t r a t i o n o f t h e v a r i o u s c l i m a t i c zones. I n a d d i t i o n , a n t i c y c l o n i c c o n d i t i o n s p r e v a i l i n g above t h e e x t e n d e d i c e s h e e t s were p r e s u m a b l y much more s e v e r e . F r e n c h m e n t i o n e d t h e abundance o f s u i t a b l e m a t e r i a l a r i s i n g f r o m t h e i c e - m a r g i n a l p o s i t i o n o f t h e P l e i s t o c e n e p e r i g l a c i a l zone. Because o f t h e c o l d e r c l i m a t e and t h e r o u t e o f t h e w e s t e r l i e s , he supposed a r e l a t i v e l y l o w l e v e l o f p r e c i p i t a t i o n . However, t h i s does n o t a c c o u n t f o r t h e d i f f e r e n c e i n t h e d e g r e e o f P l e i s t o c e n e p e r i g l a c i a l dep o s i t i o n i n N o r t h A m e r i c a ( a s f a r as p u b l i s h e d ) v e r s u s NW Europe. S t r o n g subs i d e n c e and t h e a v a i l a b i l i t y o f an abundance o f s u i t a b l e m a t e r i a l i n an e x t e n s i v e d e l t a i c s e t t i n g i n t h e w e s t e r n p a r t o f t h e l a t t e r r e g i o n m u s t have been i m p o r t a n t contributory factors. CONCLUSIONS I n t h e NW p a r t o f c o n t i n e n t a l Europe, s t r e t c h i n g f r o m n o r t h e r n F r a n c e t o t h e B a l t i c s t a t e s o f t h e USSR, t h e r e i s a l a r g e s a n d - b e l t o f w i n d - d o m i n a t e d d e p o s i t s g e n e r a l l y devoid o f h i g h angle cross-beds formed d u r i n g g l a c i a l stages b u t n o t d i r e c t l y r e l a t e d t o i n l a n d i c e . I n The N e t h e r l a n d s t h e s e d e p o s i t s o r i g i n a t e d i n a t l e a s t s i x g l a c i a l s t a g e s ; s t a c k e d sequences a r e l o c a l l y up t o 40 m e t e r s t h i c k . S i g n i f i c a n t d e p o s i t i o n took p l a c e d u r i n g t h e l a s t g l a c i a l stage; a t present, d e p o s i t s o f t h e l a t t e r s t a g e c o v e r a t l e a s t i n The N e t h e r l a n d s an a r e a o f a p p r o x i m a t e l y 30,000 s q km. A f a c i e s w i t h even l a m i n a t i o n shows s t r o n g l a t e r a l e x t e n s i o n and i s r o u g h l y
concordant w i t h t h e p r e - e x i s t i n g topography ("coversands").
Two s u b f a c i e s have
been r e c o g n i z e d , one w i t h a n d t h e o t h e r w i t h o u t more o r l e s s s i l t y l a m i n a e ; t h e f o r w r i s p a r t o f a c o n t i n u u m o f w h i c h t h e l a t t e r and l o e s s a r e t h e endmembers. The t w o s u b f a c i e s a r e t h o u g h t t o be e o l i a n , r e p r e s e n t i n g e n v i r o n m e n t s w i t h a l t e r n a t i n g w e t and d r y d e p o s i t i o n a l s u r f a c e s o r o n l y d r y d e p o s i t i o n a l s u r f a c e s . Moreo v e r , l o c a l - f l u v i a l and l a c u s t r i n e d e p o s i t i o n a l s o t o o k p l a c e i n t h i s p e r i g l a c i a l environment. The f r e q u e n t o c c u r r e n c e o f t h e s e d e p o s i t s i n The N e t h e r l a n d s and a d j a c e n t a r e a s i s c l e a r l y r e l a t e d t o r e g i o n a l s u b s i d e n c e combined w i t h e x t e n s i v e d e l t a - b u i l d i n g , w h i c h l e d t o an abundance o f s u i t a b l e m a t e r i a l . I n E a s t Germany and P o l a n d , t h e p e r i g l a c i a l deposition i s predominantly r e s t r i c t e d t o broad f l u v i a l v a l l e y s , e s p e c i a l l y t h o s e o f W e i c h s e l i a n p r a d o l i n a s ; t h e r e , dune f i e l d s a r e t h e d o m i n a n t phenomenon. C l i m a t o l o g i c a l l y , t h e d i u r n a l s o l a r r a d i a t i o n p a t t e r n as w e l l as t h e e x i s t e n c e o f s t r o n g m i d - l a t i t u d e w e s t e r l i e s m u s t have been i m p o r t a n t f a c t o r s .
I n t h e s c a r c e l i t e r a t u r e c o n c e r n i n g e v e n l y l a m i n a t e d e o l i a n sand s h e e t s , t h i s
480 f a c i e s i s t h o u g h t t o be r e l a t e d t o t h e a v a i l a b i l i t y o f abundant m a t e r i a l , h i g h w i n d v e l o c i t i e s , and u n i f o r m g r a i n s i z e . ACKNOWLEDGEMEN T S I thank: t h e D i r e c t o r o f t h e Netherlands G e o l o g i c a l Survey f o r p e r m i s s i o n t o
c a r r y o u t t h e i n v e s t i g a t i o n s ; J. Vandenberghe and J. Schwan f o r d i s c u s s i o n s i n t h e f i e l d ; W.H.
Zagwijn,
J.G.
Zandstra, E.A.
K o s t e r , 3. Vandenberghe and T.S.
A h l b r a n d t f o r c r i t i c a l r e a d i n g o f t h e m a n u s c r i p t ; Mrs. A.C.H.M.
Niessen f o r h e l p
w i t h l i t e r a t u r e , e s p e c i a l l y f r o m P o l a n d ; F . W i l l e m s e n f o r some p h o t o g r a p h s , A . K o e r s f o r p r e p a r a t i o n o f t h e d r a w i n g s ; M r s . I. Seeger f o r r e a d i n g t h e E n g l i s h
text,
and Mrs. M . E . I .
Jouini f o r typing.
REFERENCES A h l b r a n d t , T.S. and F r y b e r g e r , S.G., 1980. E o l i a n d e p o s i t s i n t h e N e b r a s k a Sand H i l l s . U.S. G e o l . S u r v e y P r o f . P a p e r 1120A: 1-24. A h l b r a n d t , T.S. and F r y b e r g e r , S.G., 1982. E o l i a n d e p o s i t s . I n : P . A . S c h o l l e and D . R . S p e a r i n g ( e d s . ) , Sandstone D e p o s i t i o n a l E n v i r o n m e n t s . AAPG Mem. 31: 11-47. A l l e n , J.R.L., 1970. P h y s i c a l p r o c e s s e s of s e d i m e n t a t i o n . A l l e n & Unwin, London: 248 p p . Andrews, J . T . , Webber, P.J. and N i c h o l s , H . , 1979. A L a t e Holocene p o l l e n d i a g r a m f r o m P a n g n i r t u n g Pass, B a f f i n I s l a n d , N.W.T., Canada. Rev. P a l a e o b o t . P a l y n o l . , 27: 1 - 2 8 . Bagnold, R . A . , 1941. The p h y s i c s o f b l o w n s a n d and d e s e r t dunes. Yethuen, London: 265 p p . B a k k e r , H. de, en S c h e l l i n g , J., 1966. Systeem van b o d e m c l a s s i f i c a t i e v o o r Nederl a n d . P u d i c , Wageningen: 217 pp. B i s s c h o p s , J.H., 1973. T o e l i c h t i n g b i j de G e o l o g i s c h e K a a r t van N e d e r l a n d 1:50.000. B l a d E i n d h o v e n Oost ( 5 1 0 ) . G e o l . S u r v e y o f The N e t h . , Haarlem: 132 pp. B l a t t , H . , M i d d l e t o n , G . V . and M u r r a y , R . C . , 1972: O r i g i n o f s e d i m e n t a r y r o c k s . P r e n t i c e - H a l l , New J e r s e y : 634 pp. Cegla, J., 1969. I n f l u e n c e o f c a p i l l a r y g r o u n d m o i s t u r e on e o l i a n a c c u m u l a t i o n o f l o e s s . B u l l . Acad. P o l . S c i . G e o l . Geog. S e r . , 1 7 ( 1 ) : 25-27. Cegla, J., 1972. Loess s e d i m e n t a t i o n i n P o l a n d ( i n P o l i s h , w i t h e x t e n s i v e E n g l i s h summary). A c t a U n i v . W r a t i s l a v S t u d . Geogr., 1 7 ( 1 6 8 ) : 53-71. C o l l i n s o n , J.D., 1978. D e s e r t s . I n : H.G. R e a d i n g ( e d . ) , S e d i m e n t a r y e n v i r o n m e n t s and f a c i e s . B l a c k w e l l , London: 80-96. Denny, C.S., Owens, J.P., S i r k i n , L.A. and Meyer, R., 1979. The P a r s o n s b u r g Sand i n t h e C e n t r a l D e l m a r v a P e n i n s u l a , M a r y l a n d and D e l a w a r e . U.S. G e o l . S u r v e y P r o f . P a p e r 1067-B: 1-16. Dewers, F . , 1932. F l o t t s a n d g e b i e t e i n N o r d w e s t d e u t s c h l a n d , e i n B e i t r a g zum L o s s p r o b l e m . Abh. n a t u r w . V e r . Bremen, X X V I I I : 131-204. Dewers, F . , 1934. Probleme d e r F l u g s a n d b i l d u n g i n N o r d w e s t d e u t s c h l a n d . Abh. n a t u r w . Ver. Bremen, X X I X : 324-366. Ducker, A . und M a a r l e v e l d , G.C., 1957. Hoch- und s p a t g l a z i a l e a o l i s c h e Sande i n N o r d w e s t d e u t s c h l a n d und i n den N i e d e r l a n d e n . G e o l . Jb., 73: 215-234. Dyke, A.S., Andrews, J.T. a n d , M i l l e r , G.H., 1982. Q u a t e r n a r y g e o l o g y o f Cumberl a n d P e n i n s u l a , B a f f i n I s l a n d , D i s t r i c t o f F r a n k l i n . G e o l . S u r v e y o f Canada, Mem. 403: 32 p p . Eissmann, L . , 1981. P e r i g l a z i a r e P r o z e s s e und P e r m a f o r s t s t r u k t u r e n aus sechs K a l t z e i t e n des Q u a r t a r s . A l t e n b u r g e r n a t u r w i s s . F o r s c h . , 1: 1-171. F r e n c h , H.M., 1976. The P e r i g l a c i a l E n v i r o n m e n t . Longman, London-New Y o r k : 309 pp. Friedman, G.M. and Sanders, J.E., 1978. P r i n c i p l e s o f s e d i m e n t o l o g y . W i l e y , C h i c h e s t e r : 808 pp. F r y b e r g e r , S.G., A h l b r a n d t , T.S. and Andrews, S., 1979. O r i g i n , s e d i m e n t a r y f e a t -
481 u r e s , and s i g n i f i c a n c e o f l o w - a n g l e e o l i a n " s a n d s h e e t " d e p o s i t s , G r e a t Sand Dunes N a t i o n a l Monument and v i c i n i t y , C o l o r a d o . J o u r . Sed. P e t r . , 49: 733-746. G u l l e n t o p s , F. and P a u l i s s e n , E . , 1978. The d r o p s o i l o f t h e E i s d e n t y p e . B i u l . P e r y g l a c j a l n y , 27: 105-115. Hammen, Th. van d e r , 1951. L a t e - g l a c i a l f l o r a and p e r i g l a c i a l phenomena i n The N e t h e r l a n d s . L e i d s e G e o l . Meded., 17: 71-184. Hammen, Th. van d e r and M a a r l e v e l d , G . C . , 1952. Genesis and d a t i n g o f t h e p e r i g l a c i a l d e p o s i t s a t t h e e a s t e r n f r i n g e o f t h e Veluwe. G e o l . en M i j n b . , (NS) 14: 47-54. Hammen, Th. van d e r , M a a r l e v e l d , G . C . , V o g e l , J.C. and Z a g w i j n , W.H., 1967. S t r a t i graphy, c l i m a t i c s u c c e s s i o n and r a d i o c a r b o n d a t i n g o f t h e L a s t G l a c i a l i n The N e t h e r l a n d s . G e o l . en M i j n b . , 46: 79-95. Hammen, Th. van d e r and W i j m s t r a , T.A. ( e d s . ) , 1971. The Upper Q u a t e r n a r y o f t h e D i n k e l V a l l e y . Meded. R i j k s G e o l . O i e n s t , (NS) 22: 55-214. H u n t e r , R . E . , 1977. B a s i c t y p e s o f s t r a t i f i c a t i o n i n s m a l l e o l i a n dunes. SedimentOlogy, 24: 361-387. Jong, H. de, 1975. M e r k w a a r d i g e d e k z a n d k o p j e s i n de omgeving van Y a r k e l o . Boor en Spade, 1 9 : 79-85. K o l s t r u p , E. and W i j m s t r a , T.A., 1977. A p a l y n o l o g i c a l i n v e s t i g a t i o n o f t h e M o e r s h o o f d , H e n g e l o and Denekamp i n t e r s t a d i a l s i n The N e t h e r l a n d s . G e o l . en M i j n b . , 56: 85-102. K o l s t r u p , E . , 1980. C l i m a t e and s t r a t i g r a p h y i n n o r t h w e s t e r n Europe between 30,000 B.P. and 13,000 B . P . , w i t h s p e c i a l r e f e r e n c e t o The N e t h e r l a n d s . Meded. R i j k s G e o l . O i e n s t , 3 2 ( 1 5 ) : 181-253. K o s t e r , E . A . , 1982. T e r m i n o l o g y and l i t h o s t r a t i g r a p h i c d i v i s i o n o f ( s u r f i c i a l ) sandy e o l i a n d e p o s i t s i n The N e t h e r l a n d s : an e v a l u a t i o n . G e o l . en M i j n b . , 61: 121-129. K o z a r s k i , S . , Nowaczyk, B . , R o t n i c k i , K . and T o b o l s k i , K . , 1969. Problems c o n c e r n i n g t h e e o l i a n phenomena i n W e s t - C e n t r a l P o l a n d w i t h s p e c i a l r e f e r e n c e t o t h e c h r o n o l o g y o f phases o f e o l i a n a c t i v i t y . G e o g r a p h i a P o l o n i c a , 17: 231-248. K o z a r s k i , S., 1980. An o u t l i n e o f V i s t u l i a n s t r a t i g r a p h y and c h r o n o l o g y o f t h e G r e a t P o l a n d L o w l a n d . Q u a t . S t u d i e s i n P o l a n d , 2: 21-35. M a a r l e v e l d , G . C . , 1960. Wind d i r e c t i o n s and c o v e r sands i n The N e t h e r l a n d s . B i u l . P e r y g l a c j a l n y , 8: 49-58. M a a r l e v e l d , G . C . , 1976. P e r i g l a c i a l phenomena and t h e mean a n n u a l t e m p e r a t u r e d u r i n g t h e l a s t g l a c i a l t i m e i n The N e t h e r l a n d s . B i u l . P e r y g l a c j a l n y , 26: 57-78. Mare'chal, R. e t M a a r l e v e l d , G . C . , 1955. L ' e x t e n s i o n des phe'nomgnes p e ' r i g l a c i a i r e s en B e l g i q u e e t aux Pays-Bas. Meded. G e o l . S t i c h t i n g , (NS) 8: 77-86. Meene, E . A . van de, 1977. T o e l i c h t i n g b i j de G e o l o g i s c h e K a a r t van N e d e r l a n d 1: 50.000. B l a d Arnhem O o s t ( 4 0 0 ) . G e o l . S u r v e y o f The N e t h . , Haarlem: 147 p p . N e u m e i s t e r , H., 1971. J u n g p l e i s t o z a n e Decksedimente und B o d e n e n t w i c k l u n g i n d e r Umgebung von L e i p z i g . Z p r d v y , V I I I : 23-72. Nowaczyk, B., 1976. E o l i a n c o v e r sands i n C e n t r a l - W e s t P o l a n d . Q u a e s t i o n e s Geog r a p h i c a e , 3: 57-77. P i s s a r t , A., V i n c e n t , J.-S. e t E d l u n d , S.A., 1977. Oe'pbts e t phe'nomgnes e ' o l i e n s s u r l ' ? l e de Banks, t e r r i t o i r e s du Nord-Ouest, Canada. Can. J. E a r t h S c i . , 14: 2462-2480. P l o e y , J. de, 1961. M o r f o l o g i e e n k w a r t a i r - s t r a t i g r a f i e van de Antwerpse N o o r d e r kempen. A c t a G e o g r a p h i c a L o v a n i e n s i a , 1: 1-130. Reineck, H.-E. and S i n g h , I . B . , 1980. D e p o s i t i o n a l s e d i m e n t a r y e n v i r o n m e n t s . S p r i n g e r V e r l a g , B e r l i n : 549 pp. ( s e c o n d e d . ) . Ruegg, G.H.J., 1975. S e d i m e n t a r y s t r u c t u r e s and d e p o s i t i o n a l e n v i r o n m e n t s o f M i d d l e - and U p p e r - P l e i s t o c e n e g l a c i a l t i m e d e p o s i t s f r o m an e x c a v a t i o n a t P e e l o , n e a r Assen, The N e t h e r l a n d s . Meded. R i j k s G e o l . D i e n s t , ( N S ) 26: 17-24. Ruegg, G.H.J., 1981. S e d i m e n t a r y f e a t u r e s and g r a i n s i z e o f g l a c i o - f l u v i a l and p e r i g l a c i a l d e p o s i t s i n The N e t h e r l a n d s and a d j a c e n t p a r t s o f Western Germany. Verh. n a t u w i s s . V e r . Hamburg, (NF) 2 4 ( 2 ) : 133-154. S e l l e y , R.C., 1978. A n c i e n t s e d i m e n t a r y e n v i r o n m e n t s . Chapman and H a l l L t d . , London 287 p p . ( s e c o n d e d . ) .
482 S m a l l e y , I . J . , 1975. Loess, l i t h o l o g y and g e n e s i s . Benchmark P a p e r s i n Geology, 26. Dowden, H u t c h i n s o n & Ross, I n c . , S t r o u d s b u r g , Penns.: 430 p p . Somme', J., Paepe, R. e t L a u t r i d o u , J.P., 1980. P r i n c i p e s , methodes e t systgme de l a s t r a t i g r a p h i e du Q u a t e r n a i r e dans l e Nord-Ouest de l a F r a n c e e t l a B e l g i q u e . I n : J. C h a l i n e ( e d . ) , P r o b l h e s de s t r a t i g r a p h i e q u a t e r n a i r e en F r a n c e e t dans l e s p a y s l i m i t r o p h e s . Supple'ment au B u l l . de l ' A F E Q , N.S. 1: 148-162. S t a a l d u i n e n , C.J. van, A d r i c h e m B o o g a e r t , H.A. van, B l e s s , M.J.M., D o p p e r t , J.W. Chr., H a r s v e l d t , H.M., M o n t f r a n s , H.M. van, O e l e , E., Wermuth, R . A . and Zagwijn, W.H., 1979. The g e o l o g y o f The N e t h e r l a n d s . Meded. R i j k s G e o l . D i e n s t , 3 2 ( 2 ) : 9-49. Toorn, J.C. van den, 1967. T o e l i c h t i n g b i j de G e o l o g i s c h e K a a r t van N e d e r l a n d 1: 50.000. B l a d V e n l o West ( 5 2 W). G e o l . S u r v e y o f The N e t h . , Haarlem: 163 pp. Vandenberghe, J. and G u l l e n t o p s , F., 1977. C o n t r i b u t i o n t o t h e s t r a t i g r a p h y o f t h e W e i c h s e l p l e n i g l a c i a l i n t h e B e l g i a n c o v e r s a n d a r e a . G e o l . en M i j n b . , 56: 123-128. Vandenberghe, J., 1981. W e i c h s e l i a n s t r a t i g r a p h y i n t h e S o u t h e r n N e t h e r l a n d s and N o r t h e r n B e l g i u m . Q u a t . S t u d i e s i n P o l a n d , 3: 111-118. Vandenberghe, J. and K r o o k , L . , 1981. S t r a t i g r a p h y and g e n e s i s o f P l e i s t o c e n e d e p o s i t s a t A l p h e n ( s o u t h e r n N e t h e r l a n d s ) . G e o l . en M i j n b . , 60: 417-426. W a l k e r , R.G. and M i d d l e t o n , G . V . , 1979. E o l i a n sands. I n : R.G. W a l k e r ( e d . ) , F a c i e s m o d e l s . G e o s c i e n c e Canada, R e p r i n t s e r i e s 1: 33-41. Z a g w i j n , W.H., 1961. V e g e t a t i o n , c l i m a t e and r a d i o c a r b o n d a t i n g s i n t h e L a t e P l e i s t o c e n e o f t h e N e t h e r l a n d s . P a r t I: Eemian and E a r l y W e i c h s e l i a n . Meded. G e o l . S t i c h t i n g , 1 4 : 15-45. Z a g w i j n , W.H. u n d Paepe, R., 1968. D i e S t r a t i g r a p h i e d e r w e i c h s e l z e i t l i c h e n A b l a gerungen d e r N i e d e r l a n d e und B e l g i e n s . E i s r e i t a l t e r u. Gegenwart, 19: 129-146. Z a g w i j n , W.H., 1974. V e g e t a t i o n , c l i m a t e and r a d i o c a r b o n d a t i n g s i n t h e L a t e P l e i s t o c e n e o f The N e t h e r l a n d s . P a r t 11: M i d d l e W e i c h s e l i a n . Meded. R i j k s Geol. D i e n s t , ( N S ) 25: 101-111.
483
BIGBEAR ERG: A PROTEROZOIC INTERMONTANE EOLIAN SAND SEA I N THE HORNBY BAY GROUP, NORTHWEST TERRITORIES, CANADA. GERALD M. ROSS, D e p a r t m e n t o f Geology, C a r l e t o n U n i v e r s i t y , Ottawa, Canada, K1S 586; P u b l i c a t i o n #02-83 o f t h e O t t a w a - C a r l e t o n C e n t r e f o r Geoscience Studies INTRODUCTION The r e l a t i v e l y r e c e n t d e s c r i p t i o n o f s e d i m e n t a r y s t r u c t u r e s formed by p r o cesses i n t r i n s i c t o t h e m i g r a t i o n o f e o l i a n bedforms, i n p a r t i c u l a r t h e small s c a l e e o l i a n s t r a t i f i c a t i o n d e s c r i b e d f r o m modern dunes by H u n t e r (1977a) and t h e bounding s u r f a c e h i e r a r c h y described by B r o o k f i e l d (1977) p r o v i d e potent i a l l y p o w e r f u l t o o l s f o r r e c o g n i z i n g and i n t e r p r e t i n g e o l i a n d e p o s i t s ( s e e Hunter,
1981;
amples). and/or
Kocurek and Oott,
1981;
K o c u r e k 1981a,b,
These s t u d i e s a r e complemented
by work
which
s a t e l l i t e imagery w i t h s u r f a c e m e t e o r o l o g i c a l
mechanisms Wilson, McKee,
f o r excellent
ex-
combines a i r p h o t o
data t o i n f e r causal
f o r t h e f o r m a t i o n o f e o l i a n sand seas ( e r g s ) and bedforms ( c f .
1971, Breed,
1972a;
F r y b e r g e r and Ahlbrandt,
and F r y b e r g e r ,
sophisticated
facies
1979).
models
1979;
Breed e t al.,
1979a;
It i s now p o s s i b l e t o c o n s t r u c t f a i r l y
for
ancient
eolianites
which
take
into
c o n s i d e r a t i o n b o t h l a r g e and s m a l l - s c a l e d e p o s i t i o n a l p r o c e s s e s . The p u r p o s e o f t h i s p a p e r i s t o document an e o l i a n o r i g i n f o r p a r t of a sequence
of
Northwest
Proterozoic
Territories
(Y.5Ga) of
redbeds o f
Canada.
t h e Hornby Bay Group
Recognition
of
the
i n the
eolianites
and
r e c o n s t r u c t i o n o f b e d f o r m s i s based on c o m p a r i s o n o f o b s e r v e d s t r a t i f i c a t i o n w i t h t h a t d e s c r i b e d f r o m modern, and some a n c i e n t , e o l i a n s e d i m e n t s . L a t e r a l changes
i n stratification
styles
within
the eolianites
are interpreted i n
t e r m s o f c h a n g i n g b e d f o r m s w i t h i n t h e b a s i n . T h i s f a c i e s change r e f l e c t s t h e complex i n t e r p l a y o f w i n d regime, and c l i m a c t i c f a c t o r s . for
40
sand s u p p l y ,
a i r flow-bedform i n t e r a c t i o n ,
The r e s u l t a n t b a s i n a l b e d f o r m p r o f i l e s ,
km p e r p e n d i c u l a r
to
depositional
strike,
are
which extend
similar
to
those
d e s c r i b e d f r o m modern a r i d e n v i r o n m e n t sand seas. The
example
from
the
Hornby
Bay
Group
represents,
to
the
author's
knowledge, o n l y t h e f i f t h r e p o r t e d o c c u r r e n c e o f P r e c a m b r i a n e o l i a n s a n d s t o n e (Donaldson, 1981;
1965;
Clemmy,
modification
M e i n s t e r and T i c k e l l ,
1976), of
although
arenites
1981; P e t t i j o h n , P o t t e r ,
there
by e o l i a n and S i e v e r ,
1976; are
processes 1973;
Hyde,
studies
1980; which
(Chaudhuri,
Folk,
1968).
Goode and H a l l , suggest 1977;
textural Ramaekers,
T h i s i s unusual i n
v i e w o f t h e w i d e l y h e l d a s s u m p t i o n t h a t most t e r r e s t r i a l s e d i m e n t a t i o n i n t h e
484 P r e c a m b r i a n ( a n d p r e - D e v o n i a n ) was by b r a i d e d s t r e a m complexes (Schumm, 1967; R u s t , 1979;
C o t t e r , 1 9 7 8 ) , t h e emergent p a r t s o f w h i c h w o u l d be s u s c e p t i b l e
t o eolian deflation.
The cause o f t h e a p p a r e n t l a c k o f P r e c a m b r i a n e o l i a n i t e s
has been d i s c u s s e d i n an e a r l i e r p a p e r (Ross, i n p r e s s ) . TERMINOLOGY T h r e e c l a s s e s o f e o l i a n b e d f o r m s have been d i s t i n g u i s h e d on t h e b a s i s o f wavelength, draas "draa"
amplitude,
(Wilson,
and g r a i n s i z e ;
1972a,b).
(Kocurek,
dunes,
and
Some c o n f u s i o n e x i s t s as t o t h e u s e o f t h e t e r m
1981b,
e o l i a n bedforms w i l l
these are wind r i p p l e s ,
p.754).
For the
purpose o f
be r e f e r r e d t o as d r a a ,
this
paper,
compound
which i m p l i e s only t h a t these
were t h e l a r g e s t b e d f o r m s i n t h e e r g and t h a t t h e y were compound s t r u c t u r e s (i.e.
b e d f o r m s t h a t have one o r more s u p e r p o s e d b e d f o r m s ) .
Use o f t h e t e r m
d r a a i n t h i s p a p e r does n o t i m p l y a n y t h i n g a b o u t b e d f o r m s p a c i n g and a m p l i t u d e ( c f . W i l s o n , 1972a,b). R e c e n t advances i n t h e u n d e r s t a n d i n g o f m i g r a t i o n o f e o l i a n b e d f o r m s has l e d t o r e c o g n i t i o n o f a h i e r a r c h a l arrangement o f bounding surfaces ( s u r f a c e s t h a t bound s e t s o f s t r a t i f i c a t i o n ) i n e o l i a n a r e n i t e s ( B r o o k f i e l d , 1977). The descriptive
hierarchy
connotations. (first-order t h e draa;
of
these bounding
Laterally surfaces)
extensive,
surfaces
nearly
carries
flat-lying
implicit planar
genetic surfaces
r e s u l t f r o m m i g r a t i o n o f t h e l a r g e s t e o l i a n bedform,
s e c o n d - o r d e r s u r f a c e s , w h i c h a r e t r u n c a t e d by f i r s t - o r d e r s u r f a c e s ,
r e p r e s e n t m i g r a t i o n o f s u p e r p o s e d b e d f o r m s ( d u n e s ) o v e r t h e d r a a s u r f a c e and third-order
s u r f a c e s f o r m by d e f l a t i o n and r e a c t i v a t i o n o f dune l e e s l o p e s
(Brookfield,
1977).
In this
paper,
s u r f a c e s t h a t bound t h e l a r g e s t
first-order
bounding surfaces
crossbed s e t s
(megasets)
s u r f a c e s bound i n t r a s e t s w i t h i n t h e megasets.
are the
and s e c o n d - o r d e r
I n t h e example d e s c r i b e d i n
t h i s p a p e r t h e l a c k o f c o n t r o l on t h e l a t e r a l e x t e n t and v e r t i c a l d i s t r i b u t i o n of first-order that
b o u n d i n g s u r f a c e s made i t d i f f i c u l t t o u n a m b i g u o u s l y c o n c l u d e
these surfaces
nomenclature w i l ' l
formed
by
draa
migration.
Thus
the
bounding
surface
be a p p l i e d i n a p u r e l y d e s c r i p t i v e sense.
REGIONAL STRATIGRAPHIC SETTING The Coppermine H o m o c l i n e , intracratonic
basins
that
Thelon
Basin,
developed
f o l l o w i n g c e s s a t i o n o f "Hudsonian"
in
(1900
-
and A t h a b a s c a the
northwest
1750Ga)
Basin
are three
Canadian
Shield
orogenic a c t i v i t y (Fig.
1). They r e c o r d a p e r i o d o f p r o t r a c t e d a n o r o g e n i c s e d i m e n t a t i o n t h a t c o n t i n u e d u n i n t e r r u p t e d f o r more t h a n
.6Ga.
I n general
these basins c o n t a i n a basal
s u c c e s s i o n o f t e r r e s t r i a l t o m a r i n e s i l i c i c l a s t i c s o v e r l a i n by t r a n s g r e s s i v e marine carbonates
(the
so-called
"orthoquartzite
t o carbonate
suite",
cf.
D o t t , 1981; Krumbein and S l o s s , 1 9 6 3 ) . The s t r a t i g r a p h i c s u c c e s s i o n s p r e s e r v e d
485
F i g . 1. Main t e c t o n i c elements o f t h e Northwest Canadian S h i e l d . S t i p p l e d b a s i n s (Thelon, Athabasca, and E l u Basins and Coppermine Homocline (upper l e f t ) ) a r e a p p r o x i m a t e l y a g e - e q u i v a l e n t c r a t o n i c cover sequences. Dashed l i n e s i n C h u r c h i l l P r o v i n c e and Wopmay Orogen a r e s t r u c t u r a l s u b d i v i s i o n s . AA: Athapuscow Aulacogen; KB: K i l o h i g o k Basin; MF: McDonald F a u l t ; BF: B a t h u r s t F a u l t ; GSL: G r e a t S l a v e Lake; GBL: G r e a t Bear Lake; LA: Lake Athabasca; CG: C o r o n a t i o n G u l f ; HB: Hudson's Bay. ( m o d i f i e d f r o m Lewry and S i b b a l d , 1980 and Hoffman 1980). i n t h e s e b a s i n s d i s p l a y no evidence o f r e g i o n a l metamorphism and/or mation. The Coppermine Homocline i s a -10km
v o l c a n i c rocks t h a t r e s t unconformably on Lower P r o t e r o z o i c (1.84 Van Schmuss and Bowring, 1980) and Archean (2.6 1978) basement
(Fig.
defor-
t h i c k sequence of sedimentary and
-
3.15Ga,
-
1.92Ga,
Krough and G i b b i n s ,
1 ) . The Homocline c o n t a i n s two s u c c e s s i v e t e r r e s t r i a l
s i l i c i c l a s t i c t o marine c a r b o n a t e c y c l e s
( t h e Hornby Bay and Dismal Lakes
groups: Donaldson and Baragar, 1973; Kerans e t al.,
1981), o v e r l a i n by a t h i c k
sequence of c o n t i n e n t a l p l a t e a u b a s a l t s (Coppermine R i v e r Group) and s h a l l o w !water carbonates and s i l i c i c l a s t i c s o f t h e Rae Group (Baragar and Donaldson, 1973). These rocks were d e p o s i t e d i n an e n s i a l i c c r a t o n i c d e p r e s s i o n ( i n t e r i o r [ p l a t f o r m b a s i n o f Sleep e t al., c r u s t a l extension.
1980) t h a t formed as t h e r e s u l t o f anorogenic
486 U n t i l 1978, t h e Coppermine H o m o c l i n e had o n l y been p a r t i a l l y mapped by heli c o p t e r s u r v e y a t 1 : 2 5 0 , 0 0 0 ( B a r a y a r a n d D o n a l d s o n , 1 9 7 3 ) ; e a r l i e r work was of a
reconnaissance
H o f f m a n (1978,
nature
(Fraser
et
1952) mapped p a r t o f
H o m o c l i n e as p a r t o f r e y i o n a
al.,
1960;
t h e southern
Cook
and
Aitken,
margin o f
1971).
t h e Coppermine
p r o j e c t t o c o v e r Wopmay Orogen. D e t a i l e d mapping
and s e d i m e n t o l o y i c a n a l y s i s o f t h e Hornby Bay a n d D i s m a l u n d e r t a k e n i n 1978 and camp e t e d i n 1 9 8 1 ( K e r a n s e t a l . , I n 1980, t h e a u t h o r mapped t h e a r e a d i s c u s s e d
ILakes groups was 1981; Ross, 1982).
i n t h i s p a p e r a t a scale of
1 : 50,000. The Hornby Bay Group, t h e b a s a l g r o u p o f t h e H o m o c l i n e , thick.
i s inore than 2 k m
I t r e s t s u n c o n f o r i n a b l y on Lower P r o t e r o z o i c basement (Wopmay Orogen,
H o f f m a n , 1 9 8 0 ) and i s c o n f o r m a b l y t o u n c o n f o r m a b l y o v e r l a i n by s i l i c i c l a s t i c s of
t h e D i s m a l Lakes Group ( F i g .
2 and 3 ) .
I t i s composed o f t h r e e u n i t s of
f l u v i a l t o rnari ne s i 1 i c i c l a s t i c s o v e r 1 a i n by t r a n s g r e s s i v e m a r i n e dolostones and r e g r e s s i v e d e l t a i c t o m a r i n e s i l i c i c l a s t i c s . been s u b d i v i d e d i n t o t h r e e d e p o s i t i o n a l
systems
The b a s a l s i l i c i c l a s t i c s have (Bigbear,
F a u l t R i v e r , and
F i g . 2. G e o l o g y o f a p o r t i o n o f t h e Coppermine H o m o c l i n e . F a u l t symbols have t e e t h on downthrown s i d e (eg. n o r m a l f a u l t s ) . TBP: T e s h i e r p i - B i g t r e e p l a n a t i o n s u r f a c e . L i m e s t o n e p a t t e r n r e p r e s e n t s m a r i n e d o l o s t o n e and s i l i c i c l a s t i c s from u p p e r Hornby Bay Group.
487 Lady Nye systems) based on l i t h o f a c i e s assemblages, i n f e r r e d d e p o s i t i o n a l h i s t o r i e s (Kerans e t a l . , The B i g b e a r system,
p a l e o c u r r e n t t r e n d s , and
1981, f o r d e t a i l s ) .
which c o n t a i n s t h e e o l i a n i t e s ,
i s “1.6km
thick,
and
r e c o r d s i n f i l l o f an i n t e r m o n t a n e b a s i n t h a t had a s o u t h e a s t - d i p p i n g paleoslope.
Prolonged e r o s i o n o f t h e B i g b e a r system source area r e s u l t e d i n f o r -
mation o f a l o w - r e l i e f t h e word)
p l a n a t i o n s u r f a c e ( p e n e p l a i n i n t h e l i t e r a l sense o f
(Teshierpi-Bigtree planation surface;
Fig.
2 ) w i t h an area o f a t
l e a s t 3000 km2. Synsedimentary normal f a u l t a c t i v i t y and d e p o s i t i o n o f i n t r a formational
conglomerates
d u r i n g l a t e r p e r i o d s o f B i g b e a r system d e p o s i t i o n
marked t h e b e g i n n i n g o f c r u s t a l u p l i f t and change f r o m a s o u t h e a s t - d i p p i n g t o southwest-dipping
paleoslope.
T h i s was
a s s o c i a t e d w i t h t h e f o r m a t i o n of
a
s h a l l o w graben a l o n g t h e F a u l t R i v e r ( F i g . 2 ) and i n c i s i o n o f t h e p e n e p l a i n by southwest-flowing
rivers.
Coarse
paraconglomerates,
arenites,
and
pebbly
a r e n i t e s o f t h e F a u l t R i v e r system were d e p o s i t e d by a d r a i n a g e system t h a t Wowed west-southwest
subparallel
conglomerates
cobbles
apparently
contain eroded
during
and
t o the
graben
boulders
paleoslope
of
change
axis.
F a u l t R i v e r system
Bigbear
system
arenites,
and
drainage
pattern
r e o r g a n i z a t i o n . Both t h e B i g b e a r and F a u l t R i v e r systems a r e conformably DISMAL LAKES
LADY NY’E FLUVIAL SYSTEM
FAULT RIVER FLUVIAL SYSTEM
BIGBEAR FLUVIAL SYSTEM BUNN CREEK CONGLOMERATE
meters
<
EOLIAN DEPOSITS
F i g . 3. Regional l i t h o s t r a t i g r a p h y o f t h e Hornby Bay Group. F l u v i a l systems are c o r r e l a t e d u s i n g t h e basal conglomerate (Bunn Creek conglomerate) o f t h e Lady Nye system. Note t h e s t r a t i g r a p h i c p o s i t i o n o f t h e e o l i a n i t e s .
o v e r l a i n by t h e d i s t i n c t i v e b a s a l c o n g l o m e r a t e o f t h e Lady Nye s y s t e m (Bunn Creek c o n g l o m e r a t e ;
3).
Fig.
L a d y Nye s y s t e m c o n g l o m e r a t e s and a r e n i t e s were
d e p o s i t e d by D o n j e k - t y p e and P l a t t e - t y p e ( t e r m i n o l o g y o f M i a l l , 1977) b r a i d e d streams,
respectively.
The s o u t h w e s t - f l o w i n g
r i v e r s i n f i l l e d and b u r i e d t h e
i n c i s e d p e n e p l a i n and f e d an e x t e n s i v e sandy b r a i d p l a i n . C o n t i n u e d s u b s i d e n c e a n d / o r e u s t a t i c sea l e v e l r i s e l e d t o m a r i n e t r a n s g r e s s i o n and f o r m a t i o n o f a southwest-facing carbonate platform. I n t e r t i d a l t o s u b t i d a l m a r i n e s i l i c i c l a s t i c f a c i e s i n u p p e r Lady Nye s y s t e m a r e n i t e s a r e t h e f i r s t e v i d e n c e o f m a r i n e - i n f l u e n c e d d e p o s i t i o n i n t h e Hornby Bay Group ( K e r a n s e t a l . ,
1981; Ross, 1982). T h i s f a c i e s o c c u r s i n r o c k s t h a t
l i e more t h a n 900m s t r a t i g r a p h i c a l l y Bigbear system (Fig.
above t h e
eolian
sandstones
i n the
3 ) . The i n t e r v e n i n g a r e n i t e s were d e p o s i t e d e x c l u s i v e l y
by f l u v i a l p r o c e s s e s . T h i s i s s i g n i f i c a n t i n t h a t i t removes t h e p o s s i b i l i t y t h a t t h e s e d i m e n t a r y s t r u c t u r e s i n t h e B i g b e a r s y s t e m a r e n i t e s f o r m e d as a r e s u l t o f m a r i n e p r o c e s s e s . M i g r a t i o n o f l a r g e - s c a l e m a r i n e sandy bedforms can produce
sets
of
cross-stratified
arenite
d i s t i n g u i s h from e o l i a n arenites (cf.
that
may
prove
difficult
K o c u r e k and D o t t , 1981).
to
I n addition,
t h e s t r a t i g r a p h i c p o s i t i o n o f t h e B i g b e a r system, b e l o w a t h i c k s u c c e s s i o n o f f l u v i a l sedimentary rocks,
and i t s a s s o c i a t e d c r a t o n w a r d - d i r e c t e d
s u g g e s t an i n t e r m o n t a n e r a t h e r t h a n c r a t o n - m a r g i n b a s i n .
paleoslope,
Closed intermontane
b a s i n s a r e c o n s i d e r e d t h e most l i k e l y r e g i o n s f o r t h e f o r m a t i o n and p r e s e r v a t i o n o f an e o l i a n sand sea ( B r e e d e t a l . , and D o t t ,
1 9 8 1 ) and t h u s
one m i g h t
1979; B r e e d and Grow, 1979; K o c u r e k
suspect
a p r i o r i t h a t B i g b e a r system
a r e n i t e s should contain e o l i a n facies. BIGBEAR DEPOSITIONAL SYSTEM The B i g b e a r s y s t e m r e s t s u n c o n f o r m a b l y on v o l c a n i c - p l u t o n i c r o c k s ( G r e a t B e a r B a t h o l i t h ) and m e t a m o r p h i c - p l u t o n i c o f Wopmay Orogen ( F i g .
4).
r o c k s (Hepburn M e t a m o r p h i c Complex)
It i s i n p a r t c o n f o r m a b l y o v e r l a i n by t h e b a s a l
c o n g l o m e r a t e o f t h e Lady Nye s y s t e m and i n p a r t u n c o n f o r m a b l y o v e r l a i n by b a s a l c l a s t i c s o f t h e D i s m a l Lakes Group ( F i g . 5). The B i g b e a r s y s t e m o u t c r o p s as a c o n t i n u o u s s h e e t n o r t h o f t h e Coppermine R i v e r and i n s c a t t e r e d grabens ( Z e p h y r graben,
St.
Germain graben,
B e l l e a u g r a b e n ) and h a l f - g r a b e n s
M o u n t a i n o u t l i e r ) up t o 60km away f r o m t h e m a i n a r e a o f e x p o s u r e ( F i g .
(Mist 4).
S t r a t i g r a p h i c c o r r e l a t i o n between g r a b e n sequences and t h e main o u t c r o p a r e a i n d i c a t e s t h a t t h e B i g b e a r s y s t e m had, a r e a o f a t l e a s t 4D00km2.
a t one t i m e ,
a depositional surface
Outcrop o f t h e Bigbear system i s g e n e r a l l y f a i r t o
e x c e l l e n t a l t h o u g h f r o s t heave and heavy l i c h e n c o v e r makes r e c o g n i t i o n o f s e d i menta ry s t r u c t u r e s 1o c a l 1y d i f f ic u 1t
.
The B i g b e a r s y s t e m has been s u b d i v i d e d i n t o t h r e e s t a g e s o f d e p o s i t i o n based
on
mappable
lithofacies
assemblages
("members")
and
paleocurrent
489
F i g . 4. G e n e r a l g e o l o g y o f t h e B i g b e a r system. G r e a t B e a r B a t h o l i t h , Hepburn M e t a m o r p h i c - P l u t o n i c Complex, and A s i a k T h r u s t B e l t a r e p a r t o f t h e basement (Wopmay Orogen). Muskox U l t r a m a f i c Complex i s y o u n g e r t h a n b o t h t h e Hornby Bay and o v e r l y i n g D i s m a l Lakes groups. P r o m i n e n t n o r t h e a s t - n o r t h w e s t o r i e n t a t i o n o f f a u l t s r e f l e c t s t h e e f f e c t s o f p l a t e c o l l i s i o n i n u n d e r l y i n g Wopmay Orogen; grabens w i t h B i g b e a r s y s t e m 1 i t h o l o g i e s r e c o r d r e a c t i v a t i o n o f t h e s e f a u l t s d u r i n g syn- t o p o s t - H o r n b y Bay Group s e d i m e n t a t i o n . Note t h e l o c a t i o n o f M i s t Mountain o u t l i e r . LN: Lady Nye system.
490
LADY NYE FORMATION
1600 1500
A7
-
1400
Stage 3 A6
Stage 2
A3
A2
Stage 1
F i g . 5. G e n e r a l i z e d s t r a t i g r a p h y o f t h e B i g b e a r system. Scale t o t h e l e f t i s i n meters. a n a l y s i s ( F i g . 5 and 6).
These r e c o r d i n i t i a l b a s i n i n f i l l and f o r m a t i o n o f an
e x t e n s i v e b r a i d p l a i n (Stage l ) , f o r m a t i o n o f an e o l i a n sand sea ( e r g ) (Stage 2),
and basement f a u l t r e a c t i v a t i o n and p a l e o s l o p e change w i t h mixed f l u v i a l
and e o l i a n ( ? ) d e p o s i t i o n (Stage 3).
Stage 2 d e p o s i t s r e c o r d a r e n i t e d e p o s i t i o n
i n an e o l i a n sand sea ( e r g ) s u b j e c t e d t o p e r i o d i c f l u v i a l i n c u r s i o n s along the p r o x i m a l ( w e s t e r n ) margin o f t h e basin. grade
B r a i d p l a i n a r e n i t e s o f upper Stage 1
i n t o t h e o v e r l y i n g f r i a b l e r e d e o l i a n a r e n i t e s o f l o w e r Stage 2.
l o w e r u n i t o f Stage 2 fanglomerates
p i n c h e s o u t towards t h e west-northwest
and a r e n i t e s .
Unfortunately the t r a n s i t i o n
The
into alluvial
from f l u v i a l
into
e o l i a n d e p o s i t s i s unexposed. The upper u n i t (A5) o f e o l i a n i t e s i n Stage 2 i s composed o f r e l a t i v e l y i n d u r a t e d q u a r t z a r e n i t e s ; i n t e r b e d s o f massive f l u v i a l paraconglomerates composed o f l o c a l l y - d e r i ved basement fragments
occur along
t h e western margin o f t h e basin. D E S C R I P T I O N OF EOLIAN ARENITES I n t h e f o l l o w i n g d e s c r i p t i o n s t h e r e a r e few examples where t h e s t r a t i f i c a t i o n i s u n e q u i v o c a l l y e o l i a n i n o r i g i n . However, when c o n s i d e r e d t o g e t h e r w i t h the
regional
depositional
setting of
position of the eolianites, d e p o s i t i o n emerges.
t h e B i g b e a r system and s t r a t i g r a p h i c
a more c o m p e l l i n g argument
i.n f a v o r o f e o l i a n
The most d i s t a l d e p o s i t s o f Stage 2 a r e d e s c r i b e d f i r s t ,
4 91
I RIVERS MENT DNOCKS
TALUS FANS SHALLOW LAKES
$&\
BRAIDPLAIN DUNES
UPtAFT
m
CORONATION
BRAIDPLAIN
QULF
\ \
Fig. 6: D i agrammatic r e c o n s t r u c t i o n o f paleogeography and d e p o s i t i o n a l environments f o r t h e t h r e e stages o f d e p o s i t i o n i n t h e B i g b e a r system. See F i g u r e 1 f o r l o c a t i o n o f S i n i s t e r F a u l t (Stage 3). because t h e s e rocks a r e well-exposed
and p r o v i d e i n s i g h t as t o s e t and coset
geometry d i f f i c u l t t o a s c e r t a i n i n r e g i o n s o f c o m p a r a t i v e l y poor exposure. The most d i s t a l d e p o s i t s o f Stage 2 o c c u r a t t h e base o f a 1.2km t h i c k sequence o f B i g b e a r system a r e n i t e s p r e s e r v e d i n t h e h a l f - g r a b e n o f t h e M i s t Mountain o u t l i e r ,
a l o n g t h e S i n i s t e r F a u l t , “30 km f r o m t h e main area o f expo-
s u r e ( F i g . 4 ) . Lack o f exposure i n t h e i n t e r v e n i n g area and l i t h o f a c i e s d i s s i m i l a r i t y do n o t a l l o w e x a c t s t r a t i g a p h i c c o r r e l a t i o n w i t h t h e main r e g i o n o f exposure.
The sequence w i t h i n t h e M i s t Mountain o u t l i e r i s composed almost
exclusively of
variably
hematitic
mudstones and r a r e p e b b l y l a y e r s .
quartz
a r e n i t e w i t h subordinate t h i n red
The l o w e r 250111o f s e c t i o n c o r r e l a t e s w i t h
Stage 2 d e p o s i t s t o t h e n o r t h w e s t ( u n i t s A4, A 5 ) , whereas t h e r e m i n d e r o f t h e sequence was d e p o s i t e d by f l u v i a l processes coeval w i t h r e g i o n a l normal f a u l t a c t i v i t y (Stage 3).
Stage 1 d e p o s i t s a r e almost e n t i r e l y absent f r o m t h e M i s t
Mountain o u t l i e r and o c c u r o n l y as a t h i n c a l c a r e o u s paraconglomerate.
(<5m)
veneer o f l o c a l l y d e r i v e d
492
F i g . 7. ( a ) T h r e e s t a c k e d megasets ( 1 - 3 ) t y p i c a l o f M i s t M o u n t a i n o u t l i e r a r e n i t e s . T r a n s p o r t on a l l s e t s was t o t h e r i g h t , i n t o t h e p h o t o . S e t 1 i s t h e r i g h t s i d e o f a t r o u g h - s h a p e d s i m p l e s e t . Set 2 i s t a b u l a r i n shape and cont a i n s a t l e a s t f o u r i n t r a s e t s bounded by r e a c t i v a t i o n s u r f a c e s ( R S ) . S e t 3 i s a l a r g e t r o u g h - s h a p e d r w g a s e t . N o t e t h e l o w a n g l e o f c r o s s - s t r a t a and r e a c t i v a t i o n s u r f a c e s r e l a t i v e t o t h e megaset b o u n d a r i e s . Hammer i n c e n t e r f o r s c a l e . ( b ) Trough-shaped ( 2 ) and t a b u l a r - s h a p e d ( 3 ) megasets. N o t e i n t e r b e d d e d t a b u l a r - p l a n a r and p l a n e - b e d d e d i n t r a s e t s i n megaset 3. The t a b u l a r p l a n a r s e t o f c r o s s - s t r a t a was d e p o s i t e d by a s u p e r p o s e d dune; t h e p l a n e - b e d d e d s e t f o r m e d by a c c r e t i o n o f c l i m b i n g w i n d r i p p l e s a n d / o r g r a i n f a l l i n t h e t o e r e g i o n o f t h e r w g a s e t bedform.
493
( c ) D e t a i l o f i n t r a s e t s w i t h i n a megaset. Megaset p a l e o t r a n s p o r t t o t h e r i g h t , i n t o t h e page. The i n t r a s e t s i n megaset 1 c o n s i s t o f t a b u l a r - p l a n a r c r o s s - s t r a t i f i e d s e t s ( T ) and s e t s w i t h p a r a l l e l l a m i n a t i o n ( P ) . I n t r a s e t s m i g r a t e d b o t h p a r a l l e l and o b l i q u e t o megaset b e d f o r m l e e s l o p e . Some i n t r a s e t The b o u n d i n g s u r f a c e s a r e r e a c t i v a t i o n s u r f a c e s w i t h i n t h e megaset ( R S ) . a p p a r e n t h i g h a n g l e t o t h e c r o s s b e d d i n g i s t e c t o n i c . Hammer a t l e f t f o r scale. (d) Small-scale t a b u l a r planar c r o s s - s t r a t i f i e d i n t r a s e t s w i t h i n a megaset. These u n i t s were d e p o s i t e d by g r a i n f l o w ( ? ) on t h e s l i p f a c e s o f m i g r a t i n g s u p e r p o s e d dunes and r e s t on megaset t o e s e t s t r a t a (TS).
4 94
-First-order Bounding Surface -- Second-order Bounding Surface
MEGASET 2
Lamina tion
F i g . 8. Megaset and i n t r a s e t r e l a t i o n s h i p s i n t r o u g h crossbeds f r o m M i s t Mountain o u t l i e r . F i r s t - o r d e r s u r f a c e s mark t h e passage o f t h e l a r g e s t bedforms and bound megasets. Second-order s u r f a c e s bound i n t r a s e t s t h a t mark t h e passage of superposed dunes ( a ) and d e f l a t i o n / r e a c t i v a t i o n of megaset bedform l e e s l o p e s f o l l o w e d by wind r i p p l e m i g r a t i o n ( b ) , r e s p e c t i v e l y . Megaset 1 i s a s i m p l e s e t o f low a n g l e b o t t o m s e t s t r a t a . Megaset 2 t y p i f i e s composite c h a r a c t e r o f t h e t h i c k e r megasets. Downcurrent dimension and geometry o f megasets and f i r s t - o r d e r bounding s u r f a c e s i s i n f e r r e d as i t c o u l d n o t be observed i n outcrop. Stage 2 a r e n i t e s a r e composed e n t i r e l y o f t e x t u r a l l y and c o m p o s i t i o n a l l y mature medium-grained
quartz a r e n i t e w i t h a variegated b r i c k
red t o white,
h e m a t i t i c p i g m e n t a t i o n . The o n l y sedimentary s t r u c t u r e p r e s e n t i n t h e a r e n i t e s i s c r o s s - s t r a t i f i c a t i o n which occurs i n s e t s t h a t d i s p l a y t r o u g h t o t a b u l a r cross-sectional Large-scale
geometries
and a v a r i e t y
of
internal
(1-7m t h i c k ) s e t s o f c r o s s - s t r a t a
structures
(Fig.
8).
a r e t h e most obvious bedding
s t y l e and t h e e n t i r e 250111 t h i c k sequence o f a r e n i t e i s composed e s s e n t i a l l y o f stacked sets
of
cross-strata.
I n cross-sections
perpendicular t o paleoflow
i n d i v i d u a l s e t s have an u n d u l o s e - t a b u l a r t o g e n t l e t r o u g h shape ( F i g .
7a, b).
The s u r f a c e s t h a t bound t h e s e t s ( f i r s t - o r d e r s u r f a c e s ) a r e p l a n a r , s u b p a r a l l e l t o concave-up
surfaces
that
truncate underlying cross-stratification.
bounding s u r f a c e s c o u l d n o t be t r a c e d p a r a l l e l exposure;
Such
t o p a l e o f l o w because o f poor
t h e i r maximum observed l a t e r a l e x t e n t i n s e c t i o n s p e r p e n d i c u l a r t o
p a l e o f 1ow i s “50m. Closer
inspection
of
large-scale
sets
shows
that
most
individual
sets
c o n t a i n numerous smal l e r - s c a l e s e t s and s h o u l d t h e r e f o r e be p r o p e r l y d e s c r i b e d ,as cosets. Because n o t a l l o f t h e l a r g e - s c a l e s e t s c o n t a i n s m a l l e r - s c a l e s e t s (and t h e r e f o r e a r e n o t c o s e t s ) ,
t h e y a r e c o l l e c t i v e l y r e f e r r e d t o as megasets,
and s e t s o f s t r a t a w i t h i n t h e megasets a r e r e f e r r e d t o as i n t r a s e t s ( c f .
Fig.
4 95 The s m a l l e r megasets (1-3m;
8).
Fig.
7b) a r e g e n e r a l l y devoid o f i n t r a s e t s .
They t y p i c a l l y d i s p l a y concave-up c u r v e d p a r a l l e l s t r a t i f i c a t i o n i n s e c t i o n s p e r p e n d i c u l a r t o paleoflow, t h a t i s , t a n g e n t i a l t o t h e lower bounding surface in
sections
cross-strata
parallel
to
paleoflow.
The
maximum
observed
and t h e l o w e r b o u n d i n g s u r f a c e i s 260,
angle
between
a l t h o u g h a n g l e s o f (150
a r e by f a r t h e most common. The t h i c k e r megasets commonly c o n t a i n i n t r a s e t s t h a t a r e s t a c k e d t a b u l a r t o wedge-shaped 7c,d).
sets that
d i p downcurrent
a t angles o f
less
than
loo
(Fig.
The i n t r a s e t s d i s p l a y p l a n a r l a m i n a t i o n t h a t i s p a r a l l e l t o g e n t l y i n -
c l i n e d ((15O) surfaces
t o t h e lower bounding surface.
(second-order
surfaces)
t o the
B o t h l o w e r and u p p e r b o u n d i n g
intrasets
a r e p l a n a r and t r u n c a t e
underlying stratification. Internally, tion,
b o t h megasets and i n t r a s e t s d i s p l a y t h e same s t y l e o f l a m i n a -
consisting o f individual
p l a t y weathering.
beds and c r o s s b e d s a c c e n t u a t e d by f l a g g y t o
I n d i v i d u a l beds a r e composed o f medium-grained we1 1 - s o r t e d
sand and a p p e a r homogenous;
t h e weathering-accented
parting reflects slight
d i f f e r e n c e s i n g r a i n s i z e between a d j a c e n t beds. P a l e o c u r r e n t s were measured f r o m b o t h megasets and i n t r a s e t s by r e c o r d i n g t h e downdip a z i m u t h f o r c r o s s - s t r a t i f i c a t i o n
and, where p o s s i b l e , t h e a z i m u t h
o f t r o u g h a x e s ( F i g . 9 ) . The megaset a z i m u t h s d e f i n e a t i g h t unimodal p a t t e r n (>go% i n one 90° q u a d r a n t ) t h a t i n d i c a t e s t r a n s p o r t t o w a r d t h e s o u t h e a s t . The i n t r a s e t s , however, show s i g n i f i c a n t d e v i a t i o n f r o m t h e mean megaset a z i m u t h . Most measurements f a l l i n t o t h e s o u t h e a s t q u a d r a n t , fall
into the
northeast
quadrant,
resulting
but a significant portion
i n an a c u t e
bimodal
pattern
described
cross-
(Fig.9). Interpretation Reconstruction
of
the
bedform(s)
that
produced
the
s t r a t i f ic a t io n must s a t is f a c t o r i 1y e x p l a i n t h e f o l 1 owi ng :
1. The g e n t l e t r o u g h t o t a b u l a r shape o f t h e megasets; 2. The c o m p o s i t e c h a r a c t e r o f t h e l a r g e r megasets; 3. The p a l e o c u r r e n t p a t t e r n s o f t h e megasets and contained intrasets. The t r o u g h - l i k e
geometry o f t h e megasets s u g g e s t s t h a t t h e b e d f o r m w h i c h
p r o d u c e d t h e s e s t r u c t u r e s had a somewhat s i n u o u s c r e s t ( c f .
Harms,
e t al.,
1975; R e i n e c k a n d S i n g h , 1975).
However, p a r t s o f t h e b e d f o r m must have been
comparatively
(two dimensional)
sets.
straight-crested
Alternatively,
t o deposit
tabular-shaped
t h e v a r i a t i o n i n s e t g e o m e t r y may r e f l e c t t h e v a r i a t i o n
i n t h e e r o s i v e c a p a b i l i t i e s o f t h e f l u i d a t t h e p o i n t o f f l o w r e a t t a c h e m e n t on t h e s t o s s s i d e o f t h e i m m e d i a t e l y d o w n c u r r e n t bedform.
496
IFig. 9. P a l e o c u r r e n t map o f e o l i a n c r o s s - s t r a t a . See F i g u r e 4 f o r l o c a t i o n o f ' M i s t M o u n t a i n o u t l i e r . I n t r a s e t s and megasets a r e p l o t t e d s e p a r a t e l y f o r M i s t IMountain o u t l i e r , b u t a r e combined f o r o t h e r s t a t i o n s . N o t e t h e i n c r e a s e d d i s p e r s i o n and b i m o d a l c h a r a c t e r o f r o s e s f r o m west t o e a s t i n t o t h e b a s i n . The c o m p o s i t e c h a r a c t e r o f t h e t h i c k e r megasets s u g g e s t s t h a t a t l e a s t two o r d e r s o f b e d f o r m s were p r e s e n t ;
a l a r g e r s c a l e b e d f o r m w h i c h was r e s p o n s i b l e
f o r t h e o v e r a l l g e o m e t r y o f t h e megasets, and s m a l l e r s u p e r p o s e d b e d f o r m s t h a t l e d t o intraset deposition. f l o w d i s c h a r g e o r s t a g e (e.g. active
second-order
dunes,
Bedform s u p e r p o s i t i o n r e s u l t s from a
change i n
d e g r a d a t i o n a l f i r s t - o r d e r dunes w i t h superposed cf.
Allen
and C o l l i n s o n ,
1974),
as
a
natural
consequence o f f l o w u n s t e a d i n e s s and n o n u n i f o r m i t y ( A l l e n , 1 9 7 3 ) , as a r e s u l t of b e d f o r m - f l o w
i n t e r a c t i o n ( R u b i n and M c C u l l o c h , 1 9 8 0 ) ,
o r as a consequence
o f t h e amount o f a sand a v a i l a b l e ( W i l s o n , 1971, 1973). The u n i m o d a l p a l e o c u r r e n t p a t t e r n f o r t h e megasets s u g g e s t f o r m a t i o n as a r e s u l t o f m i g r a t i n g t r a n s v e r s e bedforms under c o n d i t i o n s o f u n i d i r e c t i o n a l flow.
The i n t r a s e t s show c o n s i d e r a b l e v a r i a t i o n i n p a l e o c u r r e n t a z i m u t h o r i e n -
t a t i o n , although a l a r g e percentage o f t h e i n t r a s e t bedforms _migrated i n t h e same d i r e c t i o n as t h e megasets. The d o w n c u r r e n t d i p o f t h e i n t r a s e t s i n d i c a t e s t h a t bedforms m i g r a t e d down t h e l e e s l o p e o f t h e megaset bedform.
Intrasets
4 97
/ /
'
SUPERPOSED
\
BEOFORMS
\
F i g . 10. B e d f o r m r e c o n s t r u c t i o n f o r megasets and i n t r a s e t s i n M i s t M o u n t a i n o u t l i e r . ( A ) shows t h e i n f e r r e d m o r p h o l o g y o f t h e megaset bedforms; t h e s u r f a c e s o f t h e s e b e d f o r m s a r e c o v e r e d w i t h s u p e r p o s e d bedforms i l l u s t r a t e d , s c h e m a t i c a l l y i n ( B ) , a d e t a i l o f a megaset b e d f o r m s u r f a c e . A l t h o u g h t h e s u p e r p o s e d dunes a r e drawn w i t h t h e i r c r e s t l i n e s p a r a l l e l t o t h e megaset b e d f o r m c r e s t l i n e , i t i s more p r o b a b l e t h a t t h e y were o b l i q u e as superposed dunes m i g r a t e d down and away f r o m t h e o b s e r v e r ( i n t o t h e page). As superposed b e d f o r m s c l i m b downslope t h e y w i l l p r o d u c e an i n t r a s e t o f t a b u l a r - p l a n a r cross-strata ( t e r m e d dune s l i p f a c e stratification). Para1 l e l - l a m i n a t e d i n t r a s e t s f o r m e d e i t h e r by c l i m b i n g w i n d r i p p l e s a n d / o r g r a i n f a l l d e p o s i t i o n .
t h a t t h a t have p a l e o c u r r e n t a z i m u t h s p e r p e n d i c u l a r t o megaset a z i m u t h s were d e p o s i t e d by b e d f o r m s t h a t m i g r a t e d p a r a l l e l t o t h e megaset b e d f o r m axes. The v e r t i c a l
s t a c k i n g o f b o t h megasets and i n t r a s e t s s u g g e s t s t h a t t h e y
were d e p o s i t e d by t r a i n s o f downwind c l i m b i n g bedforms ( c f . A l l e n , 1970; R u b i n and H u n t e r , 1982). Megasets i n t h e a r e n i t e s o f t h e M i s t M o u n t a i n o u t l i e r a r e inferred
t o have been d e p o s i t e d by s i n u o u s t o r e l a t i v e l y s t r a i g h t - c r e s t e d
large-scale
transverse
bedforms
that
migrated
under
conditions
of
u n i d i r e c t i o n a l f l o w (Fig. 10). The r e c o n s t r u c t e d b e d f o r m s a r e n o t u n i q u e t o any p a r t i c u l a r d e p o s i t i o n a l environment.
Large-scale
composite
sets
of
cross-stratified
p r e s u m a b l y d e p o s i t e d by s u p e r p o s e d b e d f o r m s , may f o r m i n f l u v i a l C o l l i n s o n , 1974;
McCabe and Jones,
1977;
( R u b i n and M c C u l l o c h , 1980; Bouma e t a l . ,
Banks, 1973; Jones,
( A l l e n and
1979), marine,
1977; H o u b o u l t , 1968; McCave, 1 9 7 1 ) ,
o r e o l i a n ( B r o o k f i e l d , 1977, 1979; H u n t e r , Moiola, 1975) environments.
arenite,
1981; K o c u r e k , 1981a,b;
McKee and
4 98 A marine o r i g i n f o r t h e c r o s s - s t r a t i f i e d
a r e n i t e s o f Stage 2 (and the
B i g b e a r system i n g e n e r a l ) i s d i s c o u n t e d on t h e b a s i s o f t h e s t r a t i g r a p h i c p o s i t i o n o f Stage 2 a r e n i t e s (bounded above and below by c o n t i n e n t a l f l u v i a l deposits)
and
the
paleogeographic
intermontane basin).
position
of
the
Bigbear
system
(an
A f l u v i a l o r i g i n i s d i s c o u n t e d on t h e b a s i s o f t h e lack
o f v e r t i c a l c y c l i c i t y i n s t r a t i f i c a t i o n ( c f . M i a l l , 1977, 1978; Rust, 1979), which
is
considered
an
important
point
becauses
suggests
it
relatively
c o n t i n u o u s d e p o s i t i o n by t r a i n s o f down-current c l i m b i n g bedforms r a t h e r than e p i s o d i c bedform m i g r a t i o n t y p i c a l o f b r a i d e d r i v e r systems ( M i a l l , 1977). I n a d d i t i o n , d i s t i n c t i v e s m a l l - s c a l e e o l i a n s t r a t i f i c a t i o n occurs i n c o r r e l a t i v e a r e n i t e s i n other p a r t s o f the basin (Fig. Stage 2 a r e n i t e s bedforms.
llc,d;12).
i n t h e M i s t Mountain o u t l i e r were
In this
light
it i s
I t i s concluded t h a t
deposited
by
eolian
i n t e r e s t i n g t o note the strong s i m i l a r i t y
between t h e megaset geometry d e s c r i b e d h e r e and t h a t found i n t h e T r i a s s i c B o t u c a t u and Sambaiba Sandstones o f B r a z i l , well-documented and Salamuni
( B i g a r e l l a , 1972, p.39,
f o r which an e o l i a n o r i g i n i s
F i g . 21a-f;
Almeida,
1953; B i g a r e l l a
, 1961).
D e p o s i t i o n a l Processes The
megasets
were
formed
by
migration of
sinuous-crested
transverse,
compound dunes ( d r a a s ) t h a t a r e known as a k l 6 (Wilson, 1971; Cooke and Warren, 1973;
Brookfield,
1977).
The
m i g r a t i o n and
deposition
of
the
draas was
accompl i s h e d by t h r e e processes: a ) m i g r a t i o n o f s m a l l e r superposed t r a n s v e r s e dunes ( F i g . l o ) , b ) wind r i p p l e m i g r a t i o n , and c ) g r a i n f a l l i n t h e zone o f flow
s e p a r a t i o n on dune and draa
smaller
megasets
(1-2m),
l e e slopes.
which t y p i c a l l y
lack
Cross-stratification intrasets,
probably
i n the formed
e i t h e r by g r a i n f a l l d e p o s i t i o n o r c l i m b i n g w i n d r i p p l e s o r by a combination o f t h e s e processes.
M i g r a t i o n o f s m a l l e r superposed t r a n s v e r s e dunes down and
a c r o s s a k l e ' l e e s l o p e s a r e recorded by t h e occurrence o f cross-stratified
intrasets.
tabular-planar,
I n t r a s e t s i n which l a m i n a t i o n i s p a r a l l e l t o t h e
l o w e r bounding s u r f a c e ( c f . F i g . 7 c ) b u t s u b p a r a l l e l t o t h e l a m i n a t i o n i n the i m m e d i a t e l y s u b j a c e n t i n t r a s e t , r e c o r d p e r i o d s o f wind r i p p l e and/or g r a i n f a l l d e p o s i t i o n t h a t f o l l o w e d p e r i o d s o f dune l e e s l o p e d e f l a t i o n . Thus some o f the i n t r a s e t s have bounding s u r f a c e s which a r e r e a c t i v a t i o n surfaces, o f McCabe and Jones (1977, p.713)
i n t h e sense
and B r o o k f i e l d (1977). As f o r e s e t l a m i n a t i o n
i s r a r e l y developed i n e o l i a n wind r i p p l e s (Sharp, 1963; Hunter, 1977a) i t may p r o v e d i f f i c u l t t o d i s t i n g u i s h between g r a i n f a l l s t r a t i f i c a t i o n and c l i m b i n g t r a n s l a t e n t s t r a t i f i c a t i o n formed by wind r i p p l e s ( H u n t e r , 1981).
I n view o f
t h e f a c t t h a t c l i m b i n g t r a n s l a t e n t s t r a t a have been i d e n t i f i e d elsewhere i n t h e basin (Fig.
llc,d;12)
and t h a t g r a i n f a l l d e p o s i t s a r e r a r e l y preserved
499 r e l a t i v e t o c l i m b i n g t r a n s l a t e n t s t r a t i f i c a t i o n , i t i s p r o b a b l e t h a t much o f the lamination i n these arenites
formed by t h e m i g r a t i o n o f s u b c r i t i c a l l y
c l i m b i n g w i n d r i p p l e s ( t e r m i n o l o g y o f Hunter, 1977b, 1981).
I n addition, the
l a c k o f p r e s e r v e d bedding-plane s t r u c t u r e s a l s o suggests t h a t much o f t h e t h e s t r a t i f i c a t i o n was formed by wind r i p p l e m i g r a t i o n ( c f . 1981 )
Kocurek and D o t t ,
.
Azimuths o f t h e megasets plunge towards t h e southeast and d e f i n e a unimodal p a t t e r n on a r o s e diagram ( F i g . 9 ) which i s c o n s i s t e n t w i t h o t h e r p a l e o c u r r e n t d a t a from Stage 2 a r e n i t e s .
Therefore the southeast-directed transport o f the
draas p r o b a b l y r e p r e s e n t s r e g i o n a l wind f l o w p a t t e r n s . numerous workers (e.g.
Hunter,
As has been shown by
1981) some t r a n s v e r s e dunes may develop w i t h
c r e s t l i n e s t h a t a r e o b l i q u e t o r e g i o n a l wind c u r r e n t s , must be e x e r c i s e d data.
The
intrasets
bimodal reflect
i n interpreting
southeast-northeast the
and t h u s some c a u t i o n
t h e s i g n i f i c a n c e o f e o l i a n paleocurrent paleocurrents
measured
from
the
i n f l u e n c e o f t h e o r i e n t a t i o n o f t h e draa c r e s t on
secondary wind f l o w p a t t e r n s , a l t h o u g h t h e p o s s i b i 1 i t y o f seasonal c r e s t - p a r a l l e l winds cannot be excluded.
The n o r t h e a s t p a l e o c u r r e n t component o f t h e
i n t r a s e t s represents draa-parallel 1981,
p.326).
Together,
migration o f l e e side eddies (cf.
Hunter,
t h e s e d a t a suggest t h a t t h e r e g i o n a l wind f l o w was
from t h e west-southwest and t h a t draa c r e s t l i n e s were o b l i q u e t o wind f l o w . REGIONAL FACIES PATTERNS Two b a s i n a l
cross-sections
r e s u l t a n t sets o f cross-strata proximal
d e s c r i b e d below show how e o l i a n bedforms and change f r o m d i s t a l f a c i e s ,
outlier,
to
facies,
along
exposure.
Two u n i t s w i t h i n Stage 2
the
northwest
a t M i s t Mountain
margin
( u n i t s A4 and A5) w i l l
of
preserved
be described.
A l t h o u g h t h e t r a n s i t i o n i s n o t exposed b o t h u n i t s a r e i n f e r r e d t o be f a c i e s c o r r e l ' a t i v e w i t h t h e e o l i a n i t e s p r e s e r v e d i n t h e M i s t Mountain o u t l i e r . Lower U n i t (A4) This u n i t
i s composed o f two i n t e r b e d d e d l i t h o l o g i e s ,
a f r i a b l e and a
r e s i s t a n t w e a t h e r i n g a r e n i t e , t h e r e l a t i v e p r o p o r t i o n s o f which change across t h e basin. The r e s i s t a n t a r e n i t e i s a medium-grained q u a r t z a r e n i t e which d i s p l a y s a p a l e p i n k t o mauve p i g m e n t a t i o n . grained than t h e f r i a b l e arenite;
I t i s b e t t e r s o r t e d and c o a r s e r
t h e former c h a r a c t e r i s t i c has a l l o w e d f o r
g r e a t e r p r i m a r y p o r o s i t y and p r e c i p i t a t i o n o f s i l i c a cement.
The r e s i s t a n t
a r e n i t e i s most abundant i n t h e b a s i n a l p o r t i o n o f u n i t A4, where i t composes >60% o f t h e u n i t . T h i s l i t h o l o g y decreases t o <15% towards t h e western l i m i t
of
the unit.
The
friable
arenite
is
medium- t o f i n e - g r a i n e d q u a r t z a r e n i t e .
a moderately
sorted,
hematite-rich,
Abundant i n t e r s t i t i a l h e m a t i t e i n t h i s
a r e n i t e i m p a r t s a d i s t i n c t i v e dark r e d c o l o r and c o n t r i b u t e s t o t h e p l a t y ,
500
F i g . 11. ( a ) I n t e r b e d d e d r e s i s t a n t and f r i a b l e a r e n i t e i n b a s i n a l f a c i e s o f u n i t A4. P o r t i o n s o f t w o (1,2) t a b u l a r megasets, s e p a r a t e d by a f i r s t - o r d e r s u r f a c e , a r e exposed. P a r a l l e l - l a m i n a t e d f r i a b l e a r e n i t e o c c u r s a t t h e base o f i n t r a s e t s o f r e s i s t a n t a r e n i t e . The r e s i s t a n t a r e n i t e d i s p l a y s p a r a l l e l t o low-angle c r o s s - s t r a t i f i c a t i o n w i t h i n c l i n e d t o e s e t l a m i n a t i o n t h a t grades downcurrent i n t o h o r i z o n t a l bottomset lamination. K n i f e i n center o f photo i s 7cm l o n g . ( b ) I n t e r b e d d e d f r i a b l e and r e s i s t a n t a r e n i t e f r o m p r o x i m a l A4 f a c i e s . F i e l d o f v i e w i s 60cm i n v e r t i c a l d i m e n s i o n and i s p a r t o f a megaset i n w h i c h t r a n s p o r t was i n t o t h e p h o t o . L o w - a n g l e w e d g e - p l a n a r t o p a r a l l e l - l a m i n a t e d t a b u l a r r e s i s t a n t a r e n i t e w i t h wedge- t o t a b u l a r - s h a p e d beds o f p a r a l l e l - l a m i n a t e d f r i a b l e arenite. Pencil f o r scale. ( c ) D e t a i l o f s t r a t i f i c a t i o n i n f r i a b l e a r e n i t e bed. T h r e e i n t r a s e t s o f f r i a b l e a r e n i t e ( 1 - 3 ) a r e o v e r l a i n by a bed o f r e s i s t a n t a r e n i t e ( 4 ) . These i n t r a s e t s a r e p a r t o f a megaset w i t h a d i r e c t i o n o f t r a n s p o r t i n t o t h e page. I n d i v i d u a l s t r a t a l u n i t s i n t h e f r i a b l e a r e n i t e represent t h e d e p o s i t o f a s i n g l e s u b c r i t i c a l l y c l i m b i n g wind r i p p l e . C o i n i s V c m . ( d ) S u p e r c r i t i c a l l y c l i m b i n g t r a n s l a t e n t s t r a t a f r o m an i n t r a s e t o f f r i a b l e a r e n i t e . The megaset d i r e c t i o n o f t r a n s p o r t i s i n t o t h e page, whereas t h e r i p p l e s c l i m b e d o b l i q u e l y down t h e megaset b e d f o r m s l i p f a c e ( t o w a r d s t h e l e f t ) . C l i m b i n g r i p p l e s g r a d e i n t o r i p p l e f o r m s t r a t a . Above r i g h t o f c o i n i s u n i t o f w a v e ( ? ) r i p p l e s t h a t have been p r e s e r v e d by g r a i n f a l l deDos it. f r i a b l e character o f the rock. The a r e n i t e s a r e a r r a n g e d i n s t a c k e d s e t s , t a b u l a r t o wedge-shaped g e o m e t r y ( F i g .
50 - 300cm t h i c k ,
t h a t have a
1 3 ) . The a v e r a g e t h i c k n e s s o f t h e s e t s
i s l e s s t h a n t h o s e a t M i s t M o u n t a i n o u t l i e r and c o n t i n u e s t o d e c r e a s e t o w a r d s t h e w e s t e r n ( p r o x i m a l u p w i n d ) m a r g i n o f t h e b a s i n . As i s t h e case i n t h e M i s t
501
F i g . 12. D e t a i l o f s t r a t i f i c a t i o n w i t h i n i n t r a s e t s o f f r i a b l e a r e n i t e . Two i n t r a s e t s ( 1 , Z ) a r e f r o m megaset t h a t has a d i r e c t i o n o f t r a n s p o r t i n t o t h e page. The i n t r a s e t s a r e s e p a r a t e d by a r e a c t i v a t i o n s u r f a c e ( R S ) . The t h i n s t r a t a w i t h the ribbed weathering character are s u b c r i t i c a l l y climbing t r a n s l a t e n t s t r a t a f o r m e d by w i n d r i p p l e m i g r a t i o n . The r e s i s t a n t l a y e r i s r e l a t i v e l y c o a r s e - g r a i n e d c r e e p l o a d d e p o s i t e d by t h e r i p p l e c r e s t whereas t h e r e c e s s i v e l a y e r i s t h e f i n e r - g r a i n e d s a l t a t i o n / s u s p e n s i o n l o a d . The t h i c k e r s t r a t a (A,B) d i s p l a y f o r e s e t c r o s s - l a m i n a t i o n formed by w i n d r i p p l e s t h a t m i g r a t e d t o t h e l e f t . W i d t h o f p h o t o g r a p h i s 10 cm.
5 02
-First-order
TA~ULAR MEGASET
Bounding Surfoce
-- Second-order
Bounding Surface
Lamination
F i g . 13. S c h e m a t i c megaset and i n t r a s e t r e l a t i o n s h i p s i n t a b u l a r rnegasets f r o m u n i t A4. See t e x t f o r d i s c u s s i o n . M o u n t a i n o u t l i e r , t h e s e s e t s commonly have a complex i n t e r n a l a r r a n g e m e n t o f i n t r a s e t s and r e a c t i v a t i o n s u r f a c e s .
P a l e o c u r r e n t measurements a r e s t r o n g l y
unimodal t o t h e southeast (Fig. 9). F i g u r e l l a shows a p o r t i o n o f a s i n g l e c r o s s b e d s e t t h a t i s t r u n c a t e d by a p l a n a r e r o s i o n s u r f a c e ( f i r s t - o r d e r s u r f a c e ) and o v e r l a i n by t h e b a s a l p o r t i o n of a n o t h e r megaset. The p l a n a r c h a r a c t e r o f t h e s e b o u n d i n g s u r f a c e s i s t y p i c a l o f t h i s u n i t b u t because o f t h e i r p o o r e x p o s u r e t h e i r l a t e r a l e x t e n t and v e r t i c a l s p a c i n g c o u l d n o t be d e t e r m i n e d .
The l o w e r megaset i n F i g u r e l l a i s com-
posed l a r g e l y o f d o w n c u r , r e n t - d i p p i n g
t a b u l a r beds o f r e s i s t a n t a r e n i t e t h a t
d i s p l a y even l a m i n a t i o n t h a t i s p a r a l l e l
or inclined (
t o t h e lower
b o u n d i n g s u r f a c e . The f r i a b l e a r e n i t e o c c u r s o n l y as t h i n ( 4 l O c m ) p a r a l l e l l a m i n a t e d beds between c r o s s - s t r a t i f i e d i n t r a s e t s , and has l a m i n a t i o n p a r a l l e l t o t h e lower bounding s u r f a c e (Fig. 13). Although t h e external unchanged
throughout
significantly.
In
the
more
geometry o f t h e s e t s i n t h i s u n i t r e m a i n l a r g e l y outcrop proximal
belt,
their
regions,
internal
intrasets
of
d e c r e a s e t o C15X and o c c u r as t h i n (10-30cm) wedge-shaped to
low-angle
(450)
cross-stratification
o c c u r s i n t a b u l a r t o wedge, p l a n a r t o concave-up i n t r a s e t s a r e even, grained
arenite
sets with parallel
llb).
The f r i a b l e a r e n i t e i n t r a s e t s bounded by
erosion surfaces
recessively-weathering
lower
r e s i s t a n t - w e a t h e r i n g u p p e r zone ( F i g . (second-order)
changes
resistant
and r a r e l y t r o u g h - s h a p e d
p a r a l l e l , 0.5-2cm
(Fig.
character
(Fig.
12).
Laminations w i t h i n these
t h i c k , and c o m p r i s e a r e l a t i v e l y f i n e r zone
and
a
slightly
coarser-grained
l l c , d and 1 2 ) . I n t r a s e t s a r e bounded by
surfaces t h a t t r u n c a t e u n d e r l y i n g laminae a t angles o f 5-loo
5 03 (Fig.
llc).
I n s e v e r a l examples,
within individual strata (cf. i n t h e f r i a b l e arenites,
r i p p l e f o r e s e t c r o s s - l a m i n a t i o n i s preserved
Fig.
l l d and 12). Another s t r a t i f i c a t i o n s t y l e
a l b e i t rare,
i s s u p e r c r i t i c a l l y climbing translatent
s t r a t i f i c a t i o n and r i p p l e f o r m l a m i n a t i o n ( F i g .
lld)
( t e r m i n o l o g y o f Hunter,
1977b). Dune Bedforms The p l a n a r c h a r a c t e r o f f i r s t - o r d e r
bounding s u r f a c e s i n t h i s u n i t i s i n
c o n t r a s t t o t h e commonly c r e s c e n t i c shape observed i n M i s t Mountain o u t l i e r . Analogy
is
drawn
between
the
crescentic
bounding
surfaces
typical
of
subaqueous dunes ( t r o u g h c r o s s b e d s ) and t h e p l a n a r bounding surfaces produced by
sandwave m i g r a t i o n
(Harms
et
al.,
1975).
This
implies
that
the
dune
bedforms r e s p o n s i b l e f o r t h e megaset geometry had s t r a i g h t e r c r e s t l i n e s t h a n t h e akle' draa a t M i s t Mountain o u t l i e r .
I t i s concluded t h a t t h e s t a c k e d
meagsets o f t h i s u n i t formed by c l i m b o f r e l a t i v e l y s t r a i g h t - c r e s t e d The common occurrence
of
i n t r a s e t s suggests
that
dunes.
t h e dunes were compound
bedforms and c o u l d t h e r e f o r e be r e f e r r e d t o as draa,
i n t h e p r e v i o u s l y used
sense o f t h e word. The s t r a t i f i c a t i o n on F i g u r e l l a i s a s c r i b e d t o wind r i p p l e migration,
small
dune
migration,
and
grainfall
micaceous a r e n i t e on t h e draa l e e slope.
deposition
been d e s c r i b e d f r o m t h e Permian o f S c o t l a n d by B r o o k f i e l d Cretaceous
of
Mongolia
by
of
friable,
T h i s l a t t e r t y p e o f d e p o s i t i o n has
Gradzinskii
and
Jerzykiewicz
(1979) and t h e (1974).
The
s t r a t i f i c a t i o n d e s c r i b e d from p r o x i m a l d e p o s i t s o f f r i a b l e a r e n i t e ( F i g . l l b - d and 1 2 ) r e p r e s e n t s d e p o s i t i o n f r o m s u b c r i t i c a l l y c l i m b i n g w i n d r i p p l e s t h a t migrated
across
stratification,
and
down
observed
in
the
draa
modern
lee dunes,
slopes. is
Essentially
produced
by
m i g r a t i o n of wind r i p p l e s a t v a r i o u s angles o f c l i m b (Hunter, suggested t h a t t h e r e s i s t a n t l a y e r s ( F i g . coarser-grained,
12),
the
identical downwind
1977a).
It i s
which a r e b e t t e r s o r t e d and
formed d u r i n g p e r i o d s o f h i g h e r wind energy r e l a t i v e t o t h e
p e r i o d s o f d e p o s i t i o n of t h e f r i a b l e a r e n i t e l a y e r s . B a s i n a l P r o f i l e f o r t h e Lower U n i t F i g u r e 14 shows a schematic r e c o n s t r u c t i o n o f t h e i n f e r r e d e o l i a n bedforms and t h e i r l a t e r a l v a r i a t i o n s and f a c i e s e q u i v a l e n t s d u r i n g d e p o s i t i o n o f u n i t s A 3 , A4,
and t h e M i s t Mountain o u t l i e r .
I n proximal regions, t h e deposits are
dominated by a l l u v i a l fanglomerates and a s s o c i a t e d f a n f r i n g e - a l l u v i a l arenites.
plain
I n t r o d u c t i o n o f wind-borne sediment i n t o t h e b a s i n l e d t o dune nuc-
l e a t i o n and growth o f a c l o s e d b a s i n e r g (sand sea). Dunes t h a t were p r o x i m a l t o aqueous d e p o s i t s were r e l a t i v e l y small s t r a i g h t - c r e s t e d t r a n s v e r s e compound bedforms.
An observed i n c r e a s e i n t o t h e b a s i n ( p a r a l l e l
t o paleoflow) o f
5 04
RIPPLED M)
-40 km
F i g . 14. B a s i n a l f a c i e s r e c o n s t r u c t i o n f r o m w e s t e r n ( p r o x i m a l - u p w i n d ) margin o f t h e b a s i n s o u t h e a s t t o M i s t Mountain o u t l i e r (see F i g u r e 4 ) . U n i t A3 a l l u v i a l f a n and f l u v i a l f a c i e s grade ( ? ) i n t o p r o x i m a l A 4 e o l i a n facies. These i n t u r n grade basinward i n t o t h e compound akle’ draa bedforms a t M i s t Mountain o u t 1 i e r . Bedforms a r e drawn w i t h o u t superposed dune and r i p p l e bedforms. Not t o scale. average s e t t h i c k n e s s i s i n t e r p r e t e d as r e f l e c t i n g an i n c r e a s e i n draa bedform s i z e , a l t h o u g h i t c o n c e i v a b l y c o u l d r e p r e s e n t an i n c r e a s e i n t h e a n g l e o f bedf o r m climb. The s i n u o s i t y o f t h e bedforms i n c r e a s e d as s t r a i g h t - c r e s t e d t r a n s v e r s e draa graded ( ? , t r a n s i t i o n a t M i s t Mountain o u t l i e r .
i s n o t exposed) i n t o t h e o b l i q u e akle’ bedform
The change i n bedform h e i g h t and s i n u o s i t y across
t h e b a s i n r e f l e c t s i n h e r e n t f e a t u r e s o f e o l i a n sand sea dynamics. The apparent i n c r e a s e d bedform s i z e i n t o t h e b a s i n i s t h e response o f t h e bedforms t o changes i n sand s u p p l y and d i f f e r e n c e s i n t i m e r e q u i r e d f o r n u c l e a t i o n o f bedforms o f d i f f e r e n t s i z e . (eg.
Along b a s i n margins,
t h e sand supply would be less
sand s a t u r a t i o n would be l e s s ) t h a n i n i n t e r i o r p o r t i o n s o f t h e basin,
t h u s r e s t r i c t i n g bedform s i z e and a n g l e o f c l i m b . I n a d d i t i o n i t takes longer f o r l a r g e r bedforms t o n u c l e a t e and e q u i l i b r a t e w i t h a i r f l o w (Wilson, 1972a). Therefore,
l a r g e r bedforms would be expected a l o n g d i s t a l p a r t s o f t h e sand
f l o w path. T h i s does, however, r e q u i r e an i n c r e a s e i n sand a v a i l a b i l i t y i n the b a s i n because d e p o s i t i o n w i l l o c c u r o n l y i f t h e c r o s s - s e c t i o n a l
area o f a bed-
f o r m decreases as i t moves downcurrent o r i f bedforms merge (Rubin and Hunter, 1982).
The change i n bedform c r e s t l i n e s i n u o s i t y i s i n t e r p r e t e d
as t h e re-
sponse o f a i r f l o w t o i n t e r a c t i o n w i t h bedforms. The i n c r e a s e i n secondary t u r b u l e n c e and l o n g i t u d i n a l f l o w component, which g i v e s r i s e t o t h e a k l 6 bedform, a r i s e s spontaneously f r o m t h e i n t e r a c t i o n o f t h e a i r f l o w and bedforms (Wilson, 1972a).
5 05 Upper U n i t (A5) The upper u n i t ( u n i t A5 o f F i g u r e 5 ) o f Stage 2 a r e n i t e s i s composed p r e d o m i n a n t l y o f t e x t u r a l l y and c o m p o s i t i o n a l l y mature q u a r t z a r e n i t e w i t h a p a l e red t o pink pigmentation. h e m a t i t e cement
The a r e n i t e s have a w e l l developed q u a r t z - i l l i t e -
and a r e w e l l
indurated.
Thus t h e y weather w i t h a massive
aspect which, combined w i t h heavy l i c h e n c o v e r , makes r e c o g n i t i o n o f sedimenta r y s t r u c t u r e s d i f f i c u l t . T h i n (
r e d mudstones,
lenses o f mudchip-rich
and massive t a b u l a r beds o f basement-derived
paraconglomerate a r e
s u b o r d i n a t e l i t h o l o g i e s , t h e l a t t e r o c c u r r i n g o n l y along t h e western ( p r o x i m a l t o source a r e a ) f r i n g e s o f t h e o u t c r o p area.
The a r e n i t e s occur as s t a c k e d
2-10m t h i c k t a b u l a r bodies t h a t d i s p l a y e i t h e r s e t s and cosets o f low-angle (
cross-stratification
bedding
(plane-bedded
(cross-laminated
facies)
(Fig.
facies)
or
parallel
to
wavy
The plane-bedded f a c i e s w i l l
15a).
be
described f i r s t .
Plane-Bedded F a c i e s Plane-bedded a r e n i t e s occur i n sequences o f 3 t o 4 stacked t a b u l a r beds s e p a r a t e d by s i n g l e i n t e r b e d s o f c r o s s - s t r a t i f i e d
arenite,
o r more r a r e l y , a
massive paraconglomerate bed. I n d i v i d u a l t a b u l a r beds a r e 1-3m t h i c k and sepa r a t e d by t h i n (
commonly
dimensions, of
at
have
least
sedimentary
sand-filled
desiccation
cracks.
When
exhumed
i n three
t h e s e bedding p l a n e s a r e seen t o be remarkably p l a n a r o v e r areas 400m2
(Fig.
structures
15b).
(Fig.
In
15a,c)
contain a variety of structures.
the the
single
well-exposed
apparently
Very even,
plane
example
of
bedded a r e n i t e s
p a r a l l e l t o wavy l a m i n a t i o n i n
beds t h a t a r e 5-40cm t h i c k i s t h e most common s t r a t i f i c a t i o n s t y l e . These beds are
interlayered with thin
climbing r i p p l e s (Fig. lamination,
but
(2-10cm)
beds o f
starved r i p p l e s
s t r o n g Liesegang banding and g r a i n s i z e homogeneity makes
p o s i t i v e i d e n t i f i c a t i o n o f these s t r u c t u r e s d i f f i c u l t . channels,
and in-phase
15c). Some beds appear t o c o n t a i n v e r y i r r e g u l a r wavy
commonly < l m
wide,
S h a l l o w (<20cm) s c o u r
a r e f i l l e d w i t h massive coarser-than-average
a r e n i t e and a n g u l a r r e d mudchips ( F i g .
15c).
C u r r e n t and wave r i p p l e m a r k s
( l i n e a r asymmetric r i p p l e marks) a r e common on bedding p l a n e surfaces, and one example o f
structures
Glennie, 1972; Hunter,
that
resemble
adhesion
ripples
was
observed
(cf.
1980; Kocurek, 1981a; Kocurek and F i e l d e r , 1982) (Fig.
15d). Interpretation The process o f d e p o s i t on f o r t h e s e a r e n i t e s i s d i f f i c u l t t o i n f e r . P e r i o d s o f sand depos t i o n were i n t e r r u p t e d by i n c u r s i o n s o f mud-rich
sheet f l o o d
506
F i g . 1 5 . ( a ) Exposure o f u n i t A5 ( a t y p i c a l because o f t h e l a c k of lichen c o v e r ) . C l i f f f a c e i s composed o f a l o w e r u n i t o f plane-bedded a r e n i t e (PB) o v e r l a i n by t w o t a b u l a r s e t s (CL) w i t h l o w - a n g l e c r o s s - s t r a t i f i c a t i o n and l a m i n a e t h a t a r e t a n g e n t i a l t o t h e l o w e r b o u n d i n g s u r f a c e . C l i f f i s 5m high, hammer i n c e n t e r ( a r r o w ) . ( b ) P l a n e - b e d d e d a r e n i t e f a c i e s o f u n i t A 5 showing t h e “ t a b l e t o p ” w e a t h e r i n g c h a r a c t e r o f t h e f a c i e s . F l a t s u r f a c e s are exhumed b e d d i n g p l a n e s . Recesses between t a b u l a r a r e n i t e beds a r e r e d mudstone l a y e r s . F i e l d o f view is”50m. ( c ) P l a n e - b e d d e d a r e n i t e f a c i e s o f u n i t A5. J u s t above p e n c i l s h a r p e n e r i s s m a l l c h a n n e l f i l l e d w i t h r e d mudchips and medium- t o c o a r s e - g r a i n e d a r e n i t e . A r e n i t e above and below c h a n n e l - f i l l d i s p l a y s p a r a l l e l l a m i n a t i o n , r i p p l e d r i f t l a m i n a t i o n , s t a r v e d r i p p l e s , and wavy p a r a l l e l l a m i n a t i o n . ( d ) B e d d i n g p l a n e s i n plane-bedded a r e n i t e w i t h p o s s i b l e adhesion r i p p l e s t r u c t u r e s .
material (cf.
Bull,
1972, p.
7 2 ) w h i c h l e f t t h i n l a y e r s o f r e d ilud t h a t were
d e s i c c a t e d a f t e r d e p o s i t i o n . The s c o u r - f i l l
l e n s e s and beds o f s t a r v e d and/or
cli1nbiii3 ripples
from
all
represent
deposition
a
shallow
aqueous medium.
A l t h o u g h t h e o r i g i n o f t h e p a r a l l e l t o wavy l a m i n a t e d a r e n i t e s i s ambiguous, t h r e e imain p r o c e s s e s a r e p o s s i b l e . Horizontal
s t r a t i f i c a t i o n o f sand-size
m a t e r i a l by aqueous t r a c t i o n cur-
r e n t s can f o r m d u r i n g b o t h u p p e r and l o w e r f l o w regimes (Harms e t al.,
1975,
among o t h e r s ) . The l a c k o f c u r r e n t l i n e a t i o n on b e d d i n g p l a n e s suggests t h a t i f any o f t h e h o r i z o n t a l s t r a t i f i c a t i o n was formed by t r a c t i o n c u r r e n t s , they
507 must have been w i t h i n t h e l o w e r f l o w regime.
Lower f l o w regime h o r i z o n t a l
s t r a t i f i c a t i o n has been observed t o f o r m i n some m d e r n f l u v i a l s e t t i n g s , such as t h e P l a t t e R i v e r of
Nebraska
(Smith,
l a b o r a t o r y flumes (McBride e t al.,
1971)
and has
been produced i n
1975). T h i s t y p e o f s t r a t i f i c a t i o n forms by
c o m b i n a t i o n o f m i g r a t i o n o f low-amp1 i t u d e r i p p l e s (2-8mm)
( l o w - a m p l i t u d e sand
waves o f Smith, 1971) t h a t do n o t have f o r e s e t s b u t have pronounced l a t e r a l s e g r e g a t i o n o f g r a i n s i z e s , and by a gradual r i s e i n water depth. a s h a l l o w w a t e r d e p t h (<5cm, McBride e t al., t h e formation o f t h i s type o f lamination.
I n addition,
1975; Smith, 1971) i s c r i t i c a l t o I n t h e example d e s c r i b e d f r o m u n i t
A5, a s h a l l o w water d e p t h i s c o n s i s t e n t w i t h t h e occurrence o f t h i n channelf i l l lenses, t h i n beds o f r i p p l e - l a m i n a t e d a r e n i t e , and beds o f d e s i c c a t e d r e d
mudstone. I t i s t h u s p o s s i b l e t h a t much o f t h e h o r i z o n t a l l a m i n a t i o n i n t h e s e rocks
formed d u r i n g p e r i o d s
of
inundation
of
the depositional
s u r f a c e by
s h a l l o w (C15cm) water. U n f o r t u n a t e l y , t h e n o r m a l l y graded laminae d e s c r i b e d by Smith were n o t observed' i n t h e s e a r e n i t e s . A l t e r n a t i v e l y , t h e h o r i z o n t a l l y l a m i n a t e d a r e n i t e s may have been d e p o s i t e d by e o l i a n processes. sand
sheets
may
(Fryberger, e t al.,
S u b c r i t i c a l l y c l i m b i n g w i n d r i p p l e s t h a t m i g r a t e across
deposit
sets
of
horizontal
to
low-angle
laminated
sand
1979). I n a d d i t i o n , h o r i z o n t a l l y bedded and l a m i n a t e d sand
(planebed l a m i n a t i o n o f H u n t e r ,
1977) forms
i n e o l i a n sands when t h e w i n d
v e l o c i t y i s h i g h and r i p p l e f o r m a t i o n i s suppressed ( c f . Glennie, 1970, U b a r i Sand Sheet; Bagnold, 1954).
However,
t h i s t y p e o f l a m i n a t i o n has a low p r e -
s e r v a t i o n p o t e n t i a l because i t commonly develops on dune c r e s t s r a t h e r t h a n i n more p r o t e c t e d f l a n k s
(Hunter,
1981).
McKee and T i b b e t s
(1964)
describe
h o r i z o n t a l l y l a m i n a t e d i n t e r d u n e sands f r o m L i b y a , and numerous a u t h o r s have described h o r i z o n t a l l y 1981a;
Brookfield,
among o t h e r s ) .
laminated a r e n i t e s from interdune
1980;
McKee,
1979a;
facies
(Kocurek,
G r a d z i n s k i i and J e r z y k i e w i c z ,
1974,
I t i s p r o b a b l e t h a t some o f t h e h o r i z o n t a l l y s t r a t i f i e d a r e n i t e
i n u n i t A5 r e p r e s e n t s v e r t i c a l
a g g r a d a t i o n o f sand sheets by c l i m b i n g w i n d
r i p p l e s . Sedimentary s t r u c t u r e s d e s c r i b e d f r o m low-angle "sand s h e e t " d e p o s i t s i n t h e upwind p o r t i o n o f Great Sand Dune N a t i o n a l ( F r y b e r g e r e t al.,
Monument
i n Colorado,
1979), resemble t h o s e o f u n i t A5, and t h u s o f f e r a p o s s i b l e
modern analoque f o r these beds.
The modern d e p o s i t s o c c u r a l o n g t h e upwind
margin o f t h e dune f i e l d , a geographic p o s i t i o n t h a t i s c o n s i s t e n t w i t h t h a t i n f e r r e d f o r s i m i l a r d e p o s i t s i n t h e B i g b e a r erg. The a r e n i t e beds w i t h t h e i r r e g u l a r wavy p a r a l l e l l a m i n a t i o n resemble t h e stratification
produced
in
wet
adhesion r i p p l e s ( H u n t e r , 1980, 1982).
interdune 1981;
sediments
Kocurek,
1981a;
by
the
migration
of
Kocurek and F i e l d e r ,
However, a more d e t a i l e d e x a m i n a t i o n o f t h e s m a l l - s c a l e s t r a t i f i c a t i o n
s t y l e s i n t h i s f a c i e s i s necessary t o v e r i f y i t s genesis.
5 08
F i g . 16. ( a ) P a r t o f megaset i n A 5 c r o s s - l a m i n a t e d f a c i e s t h a t displays low-angle t o e s e t s t r a t i f i c a t i o n t h a t grades downcurrent i n t o horizontal b o t t o m s e t s t r a t a . D i r e c t i o n o f t r a n s p o r t o f megaset i s t o w a r d s t h e r i g h t , a t an o b l i q u e a n g l e i n t o t h e page. ( b ) D e t a i l o f s t r a t i f i c a t i o n w i t h i n megaset w i t h d i r e c t i o n o f t r a n s p o r t i n t o t h e p h o t o . Two i n t r a s e t s a r e p r e s e n t and show a r e s e p a r a t e d by a s e c o n d - o r d e r b o u n d i n g s u r f a c e . The r i b b e d appearance r e f l e c t s g r a i n s i z e d i f f e r e n c e s i m p a r t e d by t h e m i g r a t i o n o f wind ripples ( c f . F i g u r e 1 2 ) w h i c h has been p a r t l y o b s c u r e d by d i a g e n e t i c cementation ( i . e . i t i s p o s s i b l e t o t r a c e r i b b e d , p l a t y l a y e r s i n t o b e t t e r cemented flaggy weathering beds).
509 I n c o n c l u s i o n , t h e p l a n e - b e d d e d a r e n i t e f a c i e s p r o b a b l y formed by a v a r i e t y deposi'.iorl
:if I J ~ * O ~ : ~ ? S ~ , ~:?l ~u ,' .i ' a l
i s ryct>r!iel! 1'2 th,? l?:isi.s o f mliclchip b r e c c i a
and beds w i t h s m a l l - s c a l e t r a c t i o n c u r r e n t s t r u c t u r e s . The t n i n ~iuiI,,~orie,; ,lril i n f e r r e d t o have been d e p o s i t e d d u r i n g t h e w a n i n g s t a g e s o f s h a l l o w s h e e t floods.
The
fluvial
sedimentation
horizontally
stratified
(cf.
Smith,
arenites
1971)
or
:may
represent
vertical
either
shallow
a g g r a d a t i o n on a sand
s h e e t ( c f . G l e n n i e , 1 9 7 0 ) . The s e d i l n e n t a r y s t r u c t i i r e s w i t h i n t h e a r e n i t e s sugg e s t d e p o s i t i o n on a s h a l l o w ,
i n t e r m i t t e n t l y a c t i v e f l u v i a l f l o o d p l a i n (water
d e p t h <15cm when a c t i v e ) w h i c h was s u b j e c t t o i n c u r s i o n s o f d r y e o l i a n sand s h e e t s and o c c a s i o n a l w i n d tnoveinent o f wet sand.
Cross-laminated Facies T a b u l a r a r e n i t e beds t h a t d i s p l a y l a r g e - s c a l e c r o s s - s t r d t i f i c a t i o n 1 5 a ) o c c u r as s o l i t a r y s e t s (0.5
(Figure
- 2m t h i c k ) i n t e r b e d d e d w i t h t h e p l a n e bedded
a r e n i t e s i n p r o x i m a l ( w e s t e r n ) r e g i o n s , b u t forrn t h i c k e r c o s e t s ( u p t o 30m) i n inore d i s t a l
regions.
P r o x i m a l a r e n i t e beds a r e composed o f a s o l i t a r y t a b u l a r
s e t o f c r o s s - s t r a t a t h a t i n t e r s e c t t h e lower bounding siirface o f t h e s e t a t a n g l e s o f 10-ZOO.
The c r o s s - s t r a t a
a r e p l a n a r t o concave-up and a r e commonly
t a n g e n t i a l t o the lower bounding surface.
The l o w e r b o u n d i n g s u r f a c e does n o t
t r u n c a t e t h e u n d e r l y i n g s t r a t a (plane-bedded f a c i e s ) ,
an o b s e r v a t i o n t h a t
is
c r i t i c a l t o understanding t h e genesis o f these crossbed sets. F a r t h e r i n t o the b a s i n , t a b u l a r beds o f c r o s s - s t r a t i f i e d a r e n i t e a r e composed o f 4 - 7 i n d i v i d u a l sets,
3-5m t h i c k ,
t h a t have a t a b u l a r t o t r o u g h - s h a p e d c r o s s s e c t i o n . P a l e o -
c u r r e n t s a r e u n i m o d a l t o t h e s o u t h e a s t . Most o f t h e l a r g e c r o s s b e d s e t s (megas e t s ) c o n t a i n wedge t o t a b u 1 a r - s h a p e d , to
those
described
c o n t a i n even
from Mist
parallel
Mountain
lamination
that
d o w n c u r r e n t - d i p p i n g in t r a s e t s s i m i 1 a r outlier is
(Fig.
parallel
to
16b).
The
intrasets
the
lower
bounding
s u r f a c e ( F i g . 16a). InterDretation An e o l i a n o r i g i n
i s proposed f o r t h e generation o f t h e c r o s s - s t r a t i f i e d
a r e n i t e beds on t h e b a s i s o f a ) t h e l a c k o f s c o u r a l o n g t h e l o w e r b o u n d i n g s u r f a c e o f megasets, b ) t h e l a r g e s i z e o f t h e megasets, and c ) t h e o c c u r r e n c e of intrasets.
I f t h e megasets were
f o r m e d by aqueous
r e q u i r e a s i g n i f i c a n t change i n w a t e r d e p t h ,
bedforms,
they would
i n contrast t o the shallow water
d e p t h i n f e r r e d f o r t h e i n t e r b e d d e d p l a n e bedded f a c i e s .
I f t h i s d i d happen t h e
l o w e r b o u n d i n g s u r f a c e s o f t h e megasets s h o u l d have t r u n c a t e d t h e u n d e r l y i n g s t r a t a , and m u d c h i p s p r o b a b l y w o u l d be common w i t h i n t h e megasets. these features
was o b s e r v e d .
Neither of
Based on t h e t a b u l a r t o g e n t l e t r o u g h - s h a p e o f
t h e megasets, r e l a t i v e l y s t r a i g h t - t o somewhat s i n u o u s - c r e s t e d compound e o l i a n
510
COALESCENT BARCHANOID RIDGES ' A K L I ~ BEDFORM)
SIMPLE CRESCE:NTIC t D U N -F- S
-
LOW AMPL I T U D E SAND WAVES EMERGENT BARS MUDFLATS 4
f
-.v.
V
I
-40 km
F i g . 17. Schematic b a s i n a l r e c o n s t r u c t i o n f r o m western (proximal-upwind) margin o f t h e b a s i n s o u t h e a s t i n t o M i s t Mountain o u t l i e r (see F i g u r e 4). Low-energy f l u v i a l f a c i e s a r e a s s o c i a t e d w i t h s o l i t a r y s i m p l e c r e s c e n t i c dunes and e o l i a n sand sheet f a c i e s . Basinward, s o l i t a r y dunes coalesce and presumably grade i n t o a k l 6 draa a t M i s t Mountain o u t l i e r . S m a l l e r superposed bedforms have n o t been drawn on bedform l e e slopes. Not t o s c a l e .
dunes a r e i n f e r r e d t o have been r e s p o n s i b l e f o r t h e d e p o s i t i o n of t h e megasets ( F i g . 17). I n proximal regions,
s o l i t a r y e o l i a n dunes m i g r a t e d basinwards across an
i n t e r m i t t e n t l y a c t i v e shallow f l u v i a l sheets
(marginal erg facies).
f l o o d p l a i n and a s s o c i a t e d e o l i a n sand
The amount o f sand a v a i l a b l e / t r a p p e d i n the
b a s i n i n c r e a s e d downwind, and caused s o l i t a r y dunes t o coalesce t o f o r m l a r g e r " p a t c h e s " o f dunes. I n t h e most d i s t a l p o r t i o n s o f t h e b a s i n ( M i s t Mountain o u t l i e r ) f l u v i a l and e o l i a n sand sheet d e p o s i t s a r e absent: here t h e u n i t i s composed
entirely
of
eolian
arenite
deposited
by
sinuous-crested
akle'
bedforms. The decrease i n aqueously d e p o s i t e d a r e n i t e f a c i e s i n t o t h e b a s i n i s i n t e r p r e t e d t o be t h e r e s u l t o f an i n c r e a s e i n t h e amount o f sand a v a i l a b l e for
dune
nucleation
and
growth
(i.e.
sandflow
saturation)
p e n e t r a t i o n o f s u r f i c i a l w a t e r r u n o f f i n t h e basin. l o w e r e o l i a n i t e f a c i e s ( u n i t A4),
and decreased
As i s t h e case i n the
t h e change f r o m t a b u l a r t o trough-shaped
megasets p a r a l l e l t o p a l e o f l o w r e f l e c t s an i n c r e a s e i n bedform s i n u o s i t y , a n a t u r a l consequence o f b e d f o r m a i r f l o w i n t e r a c t i o n ( c f .
Wilson,
1972;
Allen,
1968, 1970).
BEDFORM MECHANICS A n a l y s i s o f s t r a t i f i c a t i o n and p a l e o c u r r e n t s has l e d t o t h e c o n c l u s i o n t h a t megaset
bedforms
were
transverse
to
oblique
to
paleoflow.
The
unimodal
511 p a l e o c u r r e n t s t y p i c a l of t h e s e a r e n i t e s a r e a l s o t y p i c a l o f many Phanerozoic eolianites
(see McKee,
1979b) t h e
d e p o s i t i o n by t r a n s v e r s e bedforms.
majority
of
which
Unfortunately
appear t o
represent
t h i s conclusion concerning
bedform o r i e n t a t i o n r e l a t i v e t o paleowind d i r e c t i o n may r e f l e c t a l a c k o f c r i t e r i a f o r r e c o g n i z i n g t h e d e p o s i t s of c l i m b i n g l o n g i t u d i n a l dunes r a t h e r than pers.
unequivocal comm.,
documentation
1983).
of
Nonetheless,
transverse
bedforms
transverse
to
(Rubin and Hunter,
oblique
eolian
bedforms
commonly develop i n " c l o s e d b a s i n " ergs, t o p o g r a p h i c depressions which a c t t o impede sand f l o w and t h e r e b y a c t as e o l i a n depocenters (Breed, e t a l . ,
1979;
F r y b e r g e r and A h l b r a n d t , 1979; Wilson, 1973). I t was mentioned e a r l i e r i n t h i s paper and has been d e t a i l e d elsewhere (Kerans, e t a1
., 1981),
t h a t t h e Bigbear
system was d e p o s i t e d i n a c l o s e d i n t e r m o n t a n e b a s i n , an area which i s i d e a l f o r e o l i a n sand accumulation. The concept of
climbing
large-scale
bedforms
(Rubin and Hunter,
1982;
B r o o k f i e l d , 1977; Kocurek, 1981a,b) h e l p s t o e x p l a i n t h r e e n o t a b l e f e a t u r e s of these a r e n i t e s : a) l a c k o f l a t e r a l l y extensive nearly horizontal t r u n c a t i o n surfaces (cf.
Stokes,
1968;
first-order
bounding s u r f a c e s o f B r o o k f i e l d ,
1977). b ) l a c k o f s a n d f l o w c r o s s - s t r a t a ( c f . Hunter, 1977a, 1981). c)
the
low
angle
of
of
intersection
megaset
cross-stratification
and t h e l o w e r bounding s u r f a c e t o t h e set. Many e o l i a n a r e n i t e s c o n t a i n i n t e r v a l s o f i n t e r d u n e sediments t h a t o v e r l i e nearly
flat-lying
Brookfield,
planar
truncation
surfaces
(first-order
Kocurek, 1981a,b;
G r a d z i n s k i i and J e r z y k i e w i c z ,
of
1974; Adams and P a t t o n , 1974,
among o t h e r s f o r d e s c r i p t i o n s o f i n t e r d u n e f a c i e s ) . here,
surfaces
1977; m u l t i p l e p a r a l l e l t r u n c a t i o n s u r f a c e s o f Stokes, 1968; see
no such i n t e r d u n e sediments were
I n t h e example d e s c r i b e d
recognized although
the
low-angle
s t r a t i f i c a t i o n a t t h e base o f many megasets ( t o e s e t and bottomset d e p o s i t s ) could
be
considered
interdune
sediments.
The
lack
of
a
well-developed
i n t e r d u n e f a c i e s may r e f l e c t e i t h e r a low sand d r i f t s a t u r a t i o n o r t h a t n e a r l y a l l t h e sand e n t e r i n g t h e b a s i n was t r a p p e d on dunes ( c f . Kocurek, 1981a). Modern dunes e x h i b i t a v e r t i c a l z o n a t i o n i n s t r a t i f i c a t i o n s t y l e t h a t i s related t o the steepness)
development
and
morphology
o f t h e bedform s l i p f a c e
(Fig.
18,
(particularly with t h i s paper;
Fig.
respect 10,
to
Hunter,
1981). The a n g l e o f c l i m b o f a dune o r draa i s c r i t i c a l i n d e t e r m i n i n g what p a r t o f t h e l e e - s l o p e p r o f i l e w i l l be p r e s e r v e d (Fig.
18; t h i s paper; Kocurek
and D o t t , 1981). Sand-flow c r o s s - s t r a t a develop o n l y on t h e upper p o r t i o n s of t h e dune s l i p f a c e , whereas t r a c t i o n c u r r e n t d e p o s i t s t e n d t o dominate on t h e lower p o r t i o n s o f t h e s l i p f a c e .
W i t h a low-angle
of
climb
(depending
on
512
SLIPFACE DEPOSITS
0
GRAINFALL and WIND RIPPLE DEPOSITS
F i g . 18. Schematic diagram o f downwind c l i m b i n g two-dimensional bedforms. S l i p f a c e d e p o s i t s a r e sandflow c r o s s - s t r a t a and slumps whereas l o w e r s l i p f a c e ( t o e s e t ) d e p o s i t s a r e composed o f g r a i n f a l l and wind r i p p l e d e p o s i t s . Set boundaries r e f l e c t t h e a n g l e o f bedform c l i m b . With a low a n g l e o f c l i m b only t h e l o w e r s l i p f a c e d e p o s i t s , which t y p i c a l l y have l o w - a n g l e c r o s s - s t r a t i f i c a t i o n , w i l l be preserved ( a f t e r Kocurek and D o t t , 1981; Hunter, 1981).
bedform s c a l e , Hunter, the
t h i s may amount t o o n l y a few t e n t h s o f a degree, Rubin and
1982) o n l y t h e lowermost l e e - s l o p e d e p o s i t s w i l l be preserved.
lack
of
sand-flow
cross-strata,
and
slump
deposits
in
Thus
is
general,
a t t r i b u t e d t o a low-angle o f dune c l i m b , a l t h o u g h i t i s p o s s i b l e t h a t a steep dune s l i p f a c e s i m p l y d i d n o t develop ( s e e F i g u r e 10d, Hunter, 1981). Much o f t h e c r o s s - s t r a t i f i c a t i o n i n t h e megasets i s low-angle i.e.
strata
commonly i n t e r s e c t t h e l o w e r bounding s u r f a c e t o a s e t a t angles (150 and only r a r e l y does t h i s a n g l e approach t h e observed maximum o f 2 6 O ( F i g . 8,13).
This
i s w e l l below t h e t y p i c a l a n g l e o f repose o f 30-340 commonly c i t e d f o r e o l i a n cross-stratification
(McKee,
1979b).
A l t h o u g h compaction w i l l
maximum a n g l e o f repose p r e s e r v e d i n c r o s s - s t r a t a
decrease the
(Rittenhouse,
1972),
it
cannot account f o r t h e p r e v a l e n c e o f low-angle c r o s s - s t r a t i f i c a t i o n observed i n t h e M i s t Mountain o u t l i e r and r e p o r t e d f r o m many examples i n t h e l i t e r a t u r e ( s e e Hunter,
1981).
I n modern dunes low-angle c r o s s - s t r a t i f i c a t i o n
develops i n t h e lowermost p o r t i o n s o f t h e dune l e e - s l o p e t h e dune i s c l i m b i n g a t a s u f f i c i e n t l y low-angle,
typically
( H u n t e r , 1981).
If
t h e n t h e c r o s s - s t r a t a pre-
served w i l l make a low a n g l e w i t h t h e l o w e r set-bounding s u r f a c e ( c f . Kocurek and D o t t , 1981). IMPLICATIONS FOR REGIONAL SANDFLOW AND ERG FORMATION The B i g b e a r e r g developed i n a closed, the
cratonward-directed
facies,
fluvial
i n t e r m o n t a n e basin, as suggested by
paleocurrents
of
Stage 1,
lack
of
marine
and by v i r t u e o f t h e f a c t t h a t t h e B i g b e a r system i s conformably over-
l a i n by t h e Lady Nye f l u v i a l system. As p o i n t e d o u t by Breed e t a l . (1979a,b), c l o s e d b a s i n s a r e most l i k e l y t o accumulate d e p o s i t s o f e o l i a n sand. Paleocurr e n t s measured f r o m t h e e o l i a n i t e s a r e g e n e r a l l y p a r a l l e l t o t h o s e f r o m f l u v -
513 i a l f a c i e s (Stage 1 i n p a r t i c u l a r ) . p a l e o w i n d f l o w was p a r a l l e l
T h i s i s i m p o r t a n t because i t i m p l i e s t h a t
t o regional paleoslope,
and hence e l u c i d a t e s t h e
mechanism r e s p o n s i b l e f o r sand a c c u m u l a t i o n and f o r m a t i o n o f B i g b e a r e r g . Sand seas may be c r e a t e d by any s i t u a t i o n t h a t r e s u l t s i n a downwind dec r e a s e i n sand t r a n s p o r t (e.g. e r t h a n t h e amount
t h e amount o f sand e n t e r i n g t h e b a s i n i s g r e a t -
l e a v i n g ) s u c h as a i r f l o w i n t e r a c t i o n w i t h t o p o g r a p h y o r
changes i n c l i m a t e i n t h e d i r e c t i o n 4 h l h r a n d t , 1979).
of
( F r y b e r g e r and
o f continuous s t r a t i g r a p h i c section, t h a t probably
r e q u i r e d m i l l i o n s o f y e a r s t o form, windflow pattern. interaction
sand d r i f t
C o n s i s t e n t o r i e n t a t i o n o f e o l i a n i t e p a l e o c u r r e n t azimuths i n
t h e Bigbear system for"500m
by
regional
implies a long-lived,
uniform,
regional
T h i s s u g g e s t s t h a t e f f e c t i v e w i n d e n e r g y was n o t d i m i n i s h e d
of
airflow
vectors
of
variable
orientation,
but
rather the
a c c u m u l a t i o n o f t h e B i g b e a r e r g was due m a i n l y t o t o p o g r a p h i c b a r r i e r s i n t h e p a t h o f r e g i o n a l w i n d f l o w and sand t r a n s p o r t . The w i n d s p r o b a b l y were p a r t of a permanent o r semi-permanent
atmospheric
cell
(e.g.
p r e v a i l i n g winds),
t h a n k a t a b a t i c w i n d s f r o m t h e c l a s t i c s o u r c e area.
rather
The main e v i d e n c e f a v o r i n g
t h i s c o n c l u s i o n i s t h a t t h e p a l e o g e o m o r p h o l o g y o f t h e c l a s t i c s o u r c e a r e a was e s s e n t i a l l y a peneplain (Teshierpi-Bigtree peneplain, Fig. create the differences
2 ) which would n o t
i n a i r t e m p e r a t u r e and d e n s i t y t h a t a r e r e q u i r e d t o
g e n e r a t e k a t a b a t i c w i n d s ( B a r r y and C h o r l e y , 1971). As w e s t e r l y winds came o f f the
Teshierpi-Bigtree
peneplain
they
encountered
a
topographic
basin.
T o p o g r a p h i c l e e e f f e c t s on w i n d f l o w cause a downwind d e c r e a s e i n e f f e c t i v e w i n d e n e r g y , a l o s s i n a i r f l o w competency,
and hence, sand a c c u m u l a t i o n . S i m i l a r i l y ,
t h e r e g i o n a r o u n d M i s t M o u n t a i n o u t l i e r a p p e a r s t o have been a p a l e o h i g h d u r i n g Stage 1 deposition,
and t h e r e f o r e may have a c t e d as a t o p o g r a p h i c b a r r i e r t o
t h r o u g h g o i n g s a n d f l o w d u r i n g e r g g r o w t h a n d S t a g e 2 d e p o s i t i o n . Thus t h e main control
o f sand a c c u m u l a t i o n i n t h e B i g b e a r e r g was t h e i n t e r a c t i o n between
regional resultant
sand-transporting b e d f o r m sequences,
prevailing parallel
winds
and
topographic
t o windflow,
barriers.
The
a r e s i m i l a r i n f o r m (and
s c a l e ? ) t o t h o s e a l o n g t h e n o r t h e a s t ( u p w i n d ) m a r g i n o f t h e Cherchen D e s e r t o f n o r t h w e s t China ( c f .
Breed,
e t al.,
1979a,
p.385,
F i g u r e 260;
p.391,
Figure
i n paleocurrent
roses
between
261). It i s
interesting t o note the difference
s t r o n g l y unimodal roses, that typify
basinal
t y p i c a l o f p r o x i m a l e o l i a n i t e s and t h e b i m o d a l r o s e s
eolianites
(Fig.
9).
T h i s may r e f l e c t a change i n t h e
r e l a t i v e a n g l e between dune c r e s t and w i n d f l o w v e c t o r such t h a t b a s i n a l e o l i a n bedforms were more o b l i q u e t o t h e w i n d and, dune
- parallel
deflection
bedform
migration.
One
hence,
possible
c o n t a i n more e v i d e n c e o f explanation
may
involve
o f t h e g e o s t r o p h i c w i n d f l o w due t o changes i n s u b s t r a t e r u g o s i t y
f r o m t h e margin towards t h e i n t e r i o r o f t h e erg,
a phenomenon known as t h e
514 "Ekman S p i r a l " (Warren, 1976). T h i s would i n v o l v e t h e c o u n t e r c l o c k w i s e r o t a t i o n o f wind v e c t o r s as t h e wind e n t e r e d t h e e r g such t h a t b a s i n a l duneforms would be
oblique
to
t h e wind.
Although
this
is
a somewhat
loosely
constrained
h y p o t h e s i s , t w o p o i n t s a r e i n f a v o r : ( a ) t h e s c a l e o f t h e change ( a c r o s s t h e map a r e a ) o f t h e p a l e o c u r r e n t roses and thus,
dune o r i e n t a t i o n ,
i s close t o that
d e s c r i b e d by Warren (1976) and ( b ) t h e i n f e r r e d c o u n t e r c l o c k w i s e r o t a t i o n of t h e wind i s c o n s i s t e n t w i t h a southern l a t i t u d e ,
as suggested by a v a i l a b l e
paleomagnetic d a t a ( I r v i n g and McGlynn, 1979). CONCLUSIONS This
paper
demonstrates
that
r e c o g n i t i o n o f e o l i a n i t e s (Hunter,
recently 1977a,1981;
developed
criteria
for
the
Q r o o k f i e l d , 1977; Kocurek and
D o t t , 1981) can be s u c c e s s f u l l y a p p l i e d t o Precambrian sedimentary rocks as o l d as sand
1.5Ga.
seas
Furthermore,
(McKee,
F r y b e r g e r , e t al.,
1979c;
f a c i e s p a t t e r n s deduced f r o m s t u d i e s o f modern Breed and Grow,
1979;
Breed e t
al.,
1979a,b;
1979) can be used t o i n t e r p r e t t h e f a c i e s p a t t e r n s i n these
rocks. 1 ) The B i g b e a r system developed i n an i n t e r m o n t a n e / i n t r a c r a t o n i c b a s i n a t a t i m e when t h e n o r t h w e s t Canadian S h i e l d was a l a r g e , l o w - r e l i e f , m i d - l a t i t u d e , continental basin,
landmass.
Sand-laden
p r e v a i l i n g winds encountered a t o p o g r a p h i c
d e f i n e d i n p a r t by t h e p r e s e n t o u t c r o p o f t h e B i g b e a r system,
a c t e d t o t r a p sand and l e d t o growth o f t h e B i g b e a r sand sea ( e r g ) . fan,
eolian
sand
sheet,
and
low-energy
fluvial
floodplain
that
Alluvial
environments
developed a l o n g t h e m a r g i n a l , upwind edge o f t h e sand sea. 2 ) The e o l i a n i t e s were r e c o g n i z e d on t h e b a s i s o f : a ) t h e composite c h a r a c t e r o f most o f t h e crossbed sets, typical
o f d e p o s i t i o n by superposed bedforms,
which i s
o f which e o l i a n
dunes a r e an excel l a n t example. b ) t h e recognition o f d i s t i n c t i v e small-scale
sedimentary s t r u c t -
u r e s and s t r a t i f i c a t i o n s t y l e s common i n modern dunes. c ) t h e r e g i o n a l s e t t i n g o f t h e B i g b e a r system i n terms o f s t r a t i g r a p h i c p o s i t i o n and paleotopography.
3 ) R e c o n s t r u c t e d e o l i a n bedforms d i s p l a y a downwind i n c r e a s e i n bedform s i z e and c r e s t l i n e s i n u o s i t y .
T h i s r e f l e c t s t h e i n t e r a c t i o n o f sandflow and
bedforms; t h e bedforms a c t e d as sand t r a p s t h a t impeded t h r o u g h g o i n g sand f l o w ( b e d f o r m s i z e ) and m o d i f i e d w i n d c u r r e n t s ( c r e s t l i n e s i n u o s i t y ) .
E o l i a n bed-
forms were composite i n c h a r a c t e r so t h a t megasets r e f l e c t o v e r a l l bedform c h a r a c t e r whereas i n t r a s e t s r e c o r d t h e d e p o s i t i o n by superposed wind r i p p l e s and s m a l l dunes. Downwind c l i m b o f bedforms e x p l a i n s t h e v e r t i c a l s t a c k i n g o f megasets and p r e s e r v a t i o n o f o n l y t o e s e t and b o t t o m s e t p o r t i o n s o f t h e megaset bedforms.
515 Acknowledgements T h i s p a p e r p r e s e n t s r e s u l t s f r o m a Ph.D. that
h a s been s u p p o r t e d
Council
grant
by
an National
t o my a d v i s o r J.A.
t h e s i s on t h e Hornby Bay Group Science
Donaldson
c r i t i c a l e d i t i n g and commentary o f J.A.
and E n g i n e e r i n g R e s e a r c h
(operating
Donaldson, F.H.A.
grant
A5536).
The
Campbell ( G e o l o g i c a l
S u r v e y o f Canada), T. A h l b r a n d t ( P e t r o s t a t C o n s u l t a n t s ) , D.Rubin and R. H u n t e r (U.S.G.S.,
M e n l o P a r k ) , M.E.
and s i n c e r e l y a p p r e c i a t e d .
B r o o k f i e l d ( U n i v e r s i t y o f G u e l p h ) were i n v a l u a b l e I n a d d i t i o n , D.Rubin,
R.Hunter,
and G. K o c u r e k a r e
t h a n k e d f o r t h e i r c a r e f u l c r i t i c i s m and f o r k e e p i n g t h e a u t h o r a b r e a s t o f hot-off-the-press bedforms.
reprints
and new d e v e l o p m e n t s i n t h e t h e o r y o f c l i m b i n g
T h i s does n o t i m p l y t h a t t h e s e p e o p l e n e c e s s a r i l y c o n c u r w i t h t h e
c o n c l u s i o n s p r e s e n t e d i n t h i s paper.
The l o g i s t i c a s s i s t a n c e o f BP M i n e r a l s
(Canada), who p r o v i d e d h e l i c o p t e r s u p p o r t i n an o t h e r w i s e p o o r l y a c c e s s i b l e region,
i s a l s o appreciated.
F i e l d work was s u p p o r t e d and e x p e d i t e d by t h e
D e p a r t m e n t o f I n d i a n A f f a i r s and N o r t h e r n Development, Y e l l o w k n i f e ,
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K.W. G L E N N I E Shell Internation4lc Pr.troleum Mid., The Hague, The Netherlands INTRODUCTION During the past two decades, o u r knowledge of the d i s t r i b u t i o n and environment of deposition of the Rotliegend has been added t o g r e a t l y by t h e d r i l l i n g of many wells in t h e North Sea area; f i r s t , in t h e southern North Sea where Rotliegend sandstones form the main r e s e r v o i r f o r g a s (e.g. Van Veen, 1975), and l a t e r in the central North Sea where these sandstones ( i n addition t o Zechstein carbonates) a r e o i l reservoirs in two producing f i e l d s , Auk (Brennand and Van Veen, 1975) a n d Argyll (Pennington,
1975). The "Rot1 iegendes" have long been recognised by miners of the Kupferschiefer in Germany and Poland as t h e red beds t h a t underlie the Late Permian Zechstein sequence of N . W . Europe. The Rotliegend sequences o v e r l i e Carboniferous or older s t r a t a . The o r i g i n of t h e Rotliegend sediments, and of t h e i r equivalents in B r i t a i n , as the deposits of an a r i d continental environment, has been advocated f o r many decades ( e . g . Born, 1921). An a n a l y s i s of the l i t h o l o g i e s and inferred depositional environments of the Rotliegend d e s e r t sequence of t h e southern North Sea was given by Glennie (1972) following extensive s t u d i e s of modern d e s e r t sediments (Glennie, 1970), outcrops of Permian s t r a t a , and of cores and wireline logs from wells. Since then, more has become known of t h e Rotliegend sedimentary rocks, especially in the U K p a r t of the central North Sea (Glennie, 1983), as additional well d a t a i s released t o the public each year by t h e U K Department of Energy. Rotliegend deposition t o o k place in t h e environment of a Northern Hemisphere Trade Wind d e s e r t during t h e l a t e r phases of the major Southern Hemisphere g l a c i a t i o n over Gondwana. I intended t o show t h a t there probably was a causal r e l a t i o n s h i p between Rotliegend deposition and polar g l a c i a t i o n , and t h a t , despite probable lowered global a i r temperatures, the marked a r i d i t y of t h e Rotliegend d e s e r t s was the r e s u l t of winds t h a t e i t h e r were stronger o r , more l i k e l y , t h a t blew a t sand-transporting v e l o c i t i e s f o r much longer periods of the year t h a n i s now t h e case. If c o r r e c t , then t h e l a t e Carboniferous-early Permian peak t o the Gondwana g l a c i a t i o n a l s o implies a stronger coeval wind regime outside t h e polar and equatorial areas of the worl d . Rotliegend deposition was b r o u g h t t o an a b r u p t end e a r l y in the Late Permian as the r e s u l t of the rapid flooding of the continental basins by the marine waters of t h e Zechstein Sea ( s e e e.g. Smith, 1970, 1979).
THE ROTLIEGEND BASINS The ' e a r l y ' Permian Rotliegend sedimentary sequences of the North Sea area were
522
EXISTING MAJOR FAULTS
DUNESAND DUNE 8 WADI
\
SABKHA
MORAY FIRTH BASIN
9/20-2 ELGIN UEVELDE
PALAEOWIND DIRECTION
L
FLUVIALTRANSPORT DIRECTION
0
0
PRE-PERMIAN OUTCROP
B
A E
&
DESERT LAKE
-
DUTCH BANK BASIN
MF
/
a Fig. 1
ROTLIEGEND DEPOSITIONAL/EROSIONAL EDGE
DB
D
100 KM
---L--l
Upper Rotliegend Facies and Palaeowind Map o f North Sea Area
49/26-2 DURHAM
523
deposited in t h r e e , e s s e n t i a l l y east-west trending basins (Fig. 1 ) . The l a r g e s t of these b a s i n s , now known as the Southern Permian Basin ( Z i e g l e r , 1978, 1982) extends almost 1500 km from eastern England t o about t h e Polish-Soviet border. The preservedwidth of t h e basin l o c a l l y approaches 400 k m , and sediments accumulated within i t t o a thickness of over 1500 m ( G i l l , 1967). I n the area of the southern North Sea, t h i s sedimentary sequence has been penetrated by many wells and i s now becoming increasingly recognisable a t depth on modern high-resolution seismic l i n e s . The smaller Northern Permian Basin i s separated from i t s southern neighbour by the east-west trending Mid North Sea-Ringkabing-Fyn system of highs and intervening grabens (Central and Horn Grabens). The basin i s now divided i n t o two parts by the NNW-SSE trending northern extension of the Central Graben. I t s eastern l i m i t seems t o coincide with t h e southerly extension of the NNE-SSW trending Oslo Graben, whereas l i t t l e more than 500 km t o the west, the basin character i s l o s t i n the v i c i n i t y of the Forth Approaches. Rotliegend sequences u p t o 600 m thick have been penetrated by the d r i l l . Rotliegend sandstones exceeding 100 m in thickness, have a l s o been penet r a t e d in several wells u p t o 150 km northwards i n t o t h e Viking Graben (Fig. 1 ) . The r e l a t i v e l y small Moray F i r t h Basin i s separated from the Northern Permian Basin by t h e SW-NE trending Grampian S p u r , over which Rotliegend s t r a t a a r e n o t found. Rotliegend sandstones accumulated t o a thickness of 600 m in the centre o f t h e basin. The western margin of t h e basin coincides with the Great G ! w Fault. Although n o t s t r i c t l y Rotliegend, because they a r e not overlain by Zechstein s t r a t a , sedimentary sequences of s i m i l a r age and f a c i e s a l s o occur in a s e r i e s of half grabens t h a t s t r e t c h in an a r c from the south of England, t h r o u g h the west Midlands of England, t h e Cheshire, Manx-Furness and Ulster Basins t o the Minch Basins of N . W . Scotland (Smith e t a l . , 1974). A s h o r t e r b u t p a r a l l e l a r c of small basins curves from the Vale of Eden, t h r o u g h southwest Scotland, t o t h e I s l e o f Arran (Smith e t a l . , 1974; Brookfield, 1978; Lovell, 1983). The development of these half grabens seems t o be matched by s i m i l a r depressions in S.W. Germany and eastern France, and thus r e f l e c t s t h e e a r l y collapse of the Variscan Highlands a n d i t s northern foreland under an E-W tensional regime. Sedimentary sequences of d e s e r t o r i g i n t h a t extend back i n t o the Pennsylvanian have been described from t h e west-central United S t a t e s ( s e e McKee, 1979a). Making allowance f o r t h e l a t e r opening of t h e A t l a n t i c , these American sequences probably formed p a r t of t h e same Northern Hemisphere b e l t of Trade Wind d e s e r t s as the Rotliegend of N . W . Europe ( s e e Fig. 4 ) . ROTLIEGEND FACIES AND DEPOSITIONAL ENVIRONMENTS The Rotliegend i s divided i n t o two d i s t i n c t rock-stratigraphic u n i t s , the Lower and Upper Rotliegend, on t h e presence or absence of volcanic rocks associated with the sedimentary sequence. Although the Lower Rotliegend i s generally overlain by the Upper, t h e l a t t e r may l o c a l l y be coeval with younger p a r t s of t h e Lower Rotliegend in areas where t h e r e was no Early Permian volcanic a c t i v i t y (Fig. 2 ) .
524
................................. ................................ ................................. MUDSTON ES
................................ ...............................
................... ..................
STEPHANIAN WESTPHALIAN
m
a
C B A
3
NAMURIAN HALITE
R = REWORKED
WADI CGLS
F
= FLUlDlSED
& DEFORMED
NODULAR ANHYDRITE
Fig. 2 a)
-
Rotliegend, Rock-stratigraphic Facies Diagram
Lower Rotliegend The range in composition of t h e basic t o intermediate volcanics of the Lower
Rotliegend, and t h e i r d i s t r i b u t i o n adjacent t o known or inferred f a u l t s , suggests t h a t t h e i r o r i g i n was probably r e l a t e d t o the e a r l i e s t tensional movements connected not only with the Permian basins, b u t a l s o with the North Sea graben systems. Katzung (1975) considered t h a t these f a u l t s were i n i t i a t e d during the StephanianAutunian. The Lower Rotliegend i s best developed in northern Germany, where i t forms a continuous sequence u p t o 2000 m thick ( P l e i n , 1978) t h a t i s inferred t o range in from t h e Stephanian i n t o the Autunian, and in t h e area of t h e Oslo Graben-Bamble Trough. Similar rock associations of about t h e same age, b u t covering generally
agl
smaller a r e a s , a r e known in France, S.W. England, S.W. Scotland, a n d in some of the flank areas of the Mid North Sea-RingkBbing-Fyn Highs ( s e e Ziegler, 1982). No volcanic a c t i v i t y o f Saxonian age i s known on continental Europe (Falke, 1976).
525
b)
ii?p?r Rqtl iegend I n t h e Southern Permian B a s i n , t h e Upper R o t l i e g e n d i s made u p of f o u r d i s t i n c t i v e f a c i e s a s s o c i a t i o n s which have been i n t e r p r e t e d a s t h e p r o d u c t s of s e d i m e n t a t i o n i n f l u v i a l ( w a d i ) a e o l i a n , sabkha and l a c u s t r i n e ( d e s e r t l a k e ) environments; the l a t t e r i n c l u d e s d e p o s i t s of bedded h a l i t e . T h e i r main sedimentary c h a r a c t e r s a s recognised i n c o r e s a r e i l l u s t r a t e d by Glennie (1972) and P l e i n ( 1 9 7 8 ) . These f a c i e s a r e d i s t r i b u t e d from s o u t h t o n o r t h a c r o s s the b a s i n i n e s s e n t i a l l y t h e same o r d e r a s
given above, w i t h t h e d e s e r t - l a k e f a c i e s c o i n c i d i n g with t h e main a x i s of subsidence. Only a narrow s t r i p of r a r e l y d r i l l e d R o t l i e g e n d o c c u p i e s t h e a r e a between t h e l a k e sediments and t h e a r e a of t h e h i g h s t o t h e i r n o r t h . On p r e s e n t e v i d e n c e , t h e same s e d i m e n t a r y f a c i e s ( w a d i , a e o l i a n , sabkha, l a c u s t r i n e ) a l s o occur i n t h e Northern Permian and t h e Ploray F i r t h b a s i n s , a l t h o u g h the p r e s e n c e of a bedded l a c u s t r i n e h a l i t e has y e t t o be demonstrdted. I t should be r e a l i s e d , however, t h a t t h e Permian geology of t h e s e two b a s i n s i s s t i l l i n c o m p l e t e l y known; t h e Rotliegend i n t h e deeper p a r t s of t h e b a s i n s i s r a r e l y reached by the d r i l l and, e x c e p t on t h e Moray c o a s t n e a r Elgin ( F i g . l ) , i t i s n o t seen i n o u t c r o p .
Ihe-fluvlal-sesuences
i) of the Southern Permian Basin comprise l o c a l l y conglome r a t i c s a n d s t o n e s t h a t a r e c h a r a c t e r i s e d by t h e o c c u r r e n c e of c u r l e d c l a y f l a k e s , d e n o t i n g s u b a e r i a l d e s i c c a t i o n . Conglomerates form a g r e a t e r p a r t of t h e sequence i n The N e t h e r l a n d s and i n Germany t h a n i n B r i t i s h w a t e r s , and i n the former two l a n d s they a r e generally rich i n volcanic c l a s t s . The f l u v i a l s a n d s t o n e s commonly a l t e r n a t e with well l a m i n a t e d , h o r i z o n t a l l y t o r e l a t i v e l y s t e e p l y i n c l i n e d (20-30") s a n d s t o n e s t h a t a r e i n t e r p r e t e d t o be aeol t a n , and w i t h s t r u c t u r e l e s s s a n d s t o n e s , which a r e t h o u g h t t o have been homogenised by l i q u e f a c t i o n of p r e v i o u s l y d e p o s i t e d sands d u r i n g l o c a l f l u v i a l f l o o d i n g ( c f . S e l l e y , 1 9 6 9 ) . T h u s , t o g e t h e r , t h e s e beds i n d i c a t e t h e ephemeral n a t u r e of the s t r e a m flow w i t h i n t e r v e n i n g p e r i o d s of s u b - a e r i a l e x p o s u r e , d e s i c c a t i o n and wind a c t i v i t y . Some t h i c k e r mud-cracked c l a y s seem t o have had t h e i r c r a c k s i n f i l l e d w i t h sand from above, whereas o t h e r s were r e c o g n i s a b l y i n j e c t e d from below by a s l u r r y of sand and w a t e r t o form s a n d s t o n e dykes ( G l e n n i e , 1970; 1 9 7 2 ) . The f l u v i a l sequences a r e more common a l o n g the s o u t h e r n margin of t h e b a s i n and i n t h e lower p a r t of t h e R o t l i e g e n d ; many s m a l l e r wadi c h a n n e l s probably d i d n o t e x t e n d a s f a r a s t h e d e s e r t l a k e , b u t t e r m i n a t e d i n small i n t e r d u n e sabkhas. I n t e r m i t t e n t l y a l o n g t h e s o u t h e r n margin of t h e b a s i n , major wadi c h a n n e l s c u t through t h e b e l t of dune s a n d s , b u t even h e r e , the f l u v i a l sands commonly a l t e r n a t e with t h o s e of a e o l i a n o r i g i n ( F i g . 1 ) thus emphasising t h e s p o r a d i c n a t u r e of r a i n f a l l i n t h e a r e a . E l l e n b e r g e t a l . (1976) d e s c r i b e l o c a l d e l t a i c s e d i m e n t a t i o n a l o n g t h e s o u t h e r n margin of the d e s e r t l a k e i n E. Germany, and i n Poland, Pokorski (1978) d e s c r i b e d widespread b r a i d e d river c h a n n e l s and ephemeral f l u v i a l a c t i v i t y .
526
The character of wireline l o g s i n d i c a t e s t h a t f l u v i a l sands a l s o occur in sone p a r t s of the Northern Permian and Moray F i r t h basins. The m a i n d i r e c t i o n s of fluvial t r a n s p o r t have n o t y e t been determined, however.
Ihe-aeollan-sesuencer
penetrated in North Sea wells generally comprise s e t s of ii) moderately t o well-laminated sandstones t h a t a r e horizontally bedded a t the base, and upwards become inclined a t angles t h a t increase t o a maximum of 25" or 30". The set i s then generally overlain by horizontally bedded sandstones forming the base of the succeeding s e t ( s e e cores in Glennie, 1972; 1983). Locally, low-angle sandstones nay be overlain by the steeply-dipping laminae of a sequence of prograding sandstones. The well laminated steeply-dipping sandstones a r e i n t e r p r e t e d t o have resulted from deposition by g r a i n f a l l (Hunter, 1977), and the decimetre-thick, poorly t o non-laminated s e t s from sandflow. Both types a r e common in North Sea wells. Ripples a r e r a r e l y well displayed. I n sub-horizontal bedding, however, s l i g h t changes in lamina thickness within the width of a core (10 cm) possibly i n d i c a t e a n o r i g i n by r i p p l e migration in an interdune area as described by Kocurek (1981). I r r e g u l a r adhesion-ripple laminae have been described by Glennie (1972, Fig. 1 4 ) . They a r e possibly more common in areas where t h e r a t e of subsidence matched t h a t of deposit i o n , as l o c a l l y seems t o have been t h e case in t h e Sole P i t Basin (Glennie, Mudd and Nagtegaal , 1978). The s e t s of laminae seen in North Sea cores between d e f l a t i o n surfaces generally range between about one and seven metres in thickness. Locally, however, s e t s have been measured t h a t comprise u p t o 20 m o r so of continuous bedding. Since most of t h e thinner bedding s e t s probably represent only the preserved lowermost portions o f migrating dunes, the very thick s e t s probably represent an unusual event in which most o f t h e lee-slope bedding of a dune i s preserved. This could happen in a sand s e a , f o r instance, where a temporary wadi channel becomes permanently i n f i l l e d by a s i n g l e phase of migrating dune. I t could a l s o be achieved by the i n f i l l of interdune hollows in a growing sand sea such as those i l l u s t r a t e d by Glennie (1970, Fig. 7 2 ) from t h e A1 Liwa, eastern Arabia. For example, t h e Rotliegend sand sea reached a t o t a l thickness of some 300 m in t h e Sole P i t Basin (Glennie and Boegner, 1981). Thus, although we have no idea of how much of the upper p a r t of such l a r g e dunes was removed during l a t e r dune migration, t h e 20 m thick s e t s of bedding can be taken a s a crude approximation t o t h e minimum height of t h e dune on which i t was formed. I n t h i s context, Glennie and Buller (1983) have presented o t h e r evidence suggesting t h a t many Rotliegend transverse dunes of t h e Southern Permian Basin were probably over 50 m high a t the time of the Zechstein transgression. The bedding a t t i t u d e s of t h e dune sands measured on cores and deduced from dipmeter logs of wells in t h e Southern Permian Basin i n d i c a t e t h a t t h e Mid Permian windr blew over t h e area from e s s e n t i a l l y e a s t t o west (Glennie, 1972; 1983; Ellenberg et a l . , 1976; Van Wijhe e t a l . , 1980). A t l e a s t in t h e western half of t h e Northern
527
Permiarl Basin, however, t h e winds blew from the northwest ( F i g . 1 ) . Thus, a centre of barometric high pressure must have been located over the Elid North Sea High (Glennie, in p r e s s ) . A s i m i l a r c e n t r e of high pressure t o the west of Wyoming and North Cclorado has been proposed by Fryberger (1979) t o account f o r southerlyd i r e c t e d f o r e s e t s found in t h e Pennsylvanian-Permian Weber Sandstone. I n t h e Rotliegend sandstones of many North Sea wells, the r e l a t i v e l y narrow spread of bedding a t t i t u d e s indicates t h a t these sands were deposited largely on dunes o f t h e transverse type (Fig. 3 ) . I n N . E . England ( D in Fig. l ) , however, wind strengths seem t o have been t o o strong f o r transverse dunes t o be s t a b l e , and longitudinal ( s e i f ) dunes were formed. Because of t h e i r almost complete preservation beneath the marine Marl S l a t e (Kupferschiefer equivalent) a t the base of the succeeding Zechstein sequence, much o f t h e i r original shapes can be followed in a s e r i e s of l a r g e quarries ( s a n d p i t s ) , mine s h a f t s a n d borings. These dunes a r e aligned along a n ENE-WSW a x i s (N60'E) a n d have been preserved t o a height of u p t o 60 m above t h e Carboniferous surface (Smith a n d Francis, 1967, F i g . 18 a n d PL 15; Glennie a n d Buller, 1983). I n t h e interdune a r e a s , t h e Marl S l a t e generally r e s t s d i r e c t l y on Carboniferous s t r a t a with no Rotliegend sandstone intervening. The thickness of sand v i s i b l y reworked from the dunes by t h e transgressing Zechstein Sea ranges between some 10 cm in w h a t a r e believed t o be former dune-crest l o c a t i o n s , t o a maximum of around 4 m on the dune flanks (Glennie a n d Buller, 1983). Marine faunas with Zechstein a f f i n i t i e s occur in t h e 2-3 cm immediately below t h e Marl S l a t e (Bell e t a l . , 1979). N
N
N
24 DIPS LIEVELDE-I WIND FROM NIOOOE
127 DIPS 49/26-2 WIND FROM Ni05OE
5 4 DIPS COMBINED DIP DATA FROM FIELD HOUSE AND CRIME RIGGS SAND PITS COUNTY DURHAM WIND F'ROM N60°E
Fig. 3 - The poles of dune bedding (derived from dipmeter l o g s ) plotted on Polar Nets (upper hemisphere) f o r three d i f f e r e n t l o c a l i t i e s in the Southern Permian Basin. The d i s t r i b u t i o n of bedding a t t i t u d e s suggests the type of dune of which they form a p a r t , and arrows give deduced wind d i r e c t i o n s : A ) Limited d i s t r i b u t i o n - simple transverse dune C) Most dips concentrated in areas ( b ) on e i t h e r s i d e of t h e dune a x i s - s e i f dune B) Most bedding a t t i t u d e s concentrated in a general down-wind location on the net b u t with some dips almost a t r i g h t angles t o t h e deduced wind d i r e c t i o n , and so suggestive of limited transverse i n s t a b i l i t y and the possible presence of barchan-like horns - barchanoid dune. Note: Bedding a t t i t u d e s in A o r i g i n a l l y published by Van Wijhe e t a l . (1980) as a rose diagram.
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This limited amount of reworking i s in marked c o n t r a s t t o the much g r e a t e r t h i c k ness of reworked sandstone l o c a l l y present in the T r i a s s i c - J u r a s s i c E n t r a d a Sandstone of New Mexico (Tanner, 1970). There, dunes, with a n original r e l i e f of over 45 r i , were truncated and had t h e i r interdune areas i n f i l l e d with water-laid sandstones some 30 m thick in the aquatic environment of ‘Lake T o d i l t o ’ . A t outcrop, the s e i f dunes of Durham a r e n o t red b u t yellow in colour, from which they derive t h e i r name of ‘Yellow Sands’. I n North Sea w e l l s , in Germany a n d in Poland, however, the uppermost sands of t h e Rotliegend sequence a r e commonly grey o r white f o r u p t o some 50 m beneath the Kupferschiefer, a n d a r e referred t o as the Wei s s l iegend. The o r i g i n of these sands has been the subject of dispute over the decades, with b o t h aeolian and marine derivations being a t t r i b u t e d t o them ( s e e references in Pryor, 1 9 7 1 a , b ; and Nemec a n d Porebski, 1 9 7 7 ) . I n b o t h the southern North Sea and in Poland, the Weissliegend dune bedding grades i n t o large-scale soft-sediment deformation s t r u c t u r e s . Similar s t r u c t u r e s occur in the middle of the Hopeman Sandstones (Peacock, 1966) on t h e southern coast of the Moray F i r t h near Elgin (Fig. 1 ) . Here, a zone of deformation divides the dune sequence of the area i n t o two u n i t s : a lower u n i t of southward-dipping dune sands f o r which correlation with t h e Rotliegend i s suggested, a n d an upper u n i t of southwest-dipping dune sands of probable Late Permian or Early T r i a s s i c age (Walker, 1973). The creation of t h e deformation a t the t o p of the lower dune u n i t , a n d a l s o of s i m i l a r s t r u c t u r e s i n t h e Weissliegend of t h e North Sea and Poland, a r e a t t r i b u t e d t o the e f f e c t s of the Zechstein transgression (Glennie and Buller, 1983). This event will be referred t o again. i i i ) Ihe-lacusfrlne-s‘9”’”ce i n the Southern Permian Basin (Fig. 1 a n d 2 ) consists primarily of red-brown mudstone with minor s i l t s t o n e . Several horizons of h a l i t e , with a cumulative thickness in excess of 100 m ( P l e i n , 1978), a r e developed over much of the axial p a r t of the basin. The h a l i t e i s concentrated i n the middle p a r t of t h e
Upper Rotliegend sequence in the axial p a r t of t h e basin in Germany, b u t wedges o u t towards the basin margins so t h a t in U K waters, f o r example, i t i s limited t o the lower p a r t of the s e c t i o n . In northern Germany, where the l a c u s t r i n e sequence reaches a thickness of around 1500 m , t h e h a l i t e s a t t a i n a s u f f i c i e n t thickness t o r e a c t d i a p i r i c a l l y ; the sequence i s then r e f e r r e d t o as t h e Haselgebirge f a c i e s . Carbonates a n d b o t h bedded and nodular anhydrite a r e absent from t h e l a c u s t r i n e f a c i e s . The l a c u s t r i n e s t r a t a seem t o be devoid of f o s s i l s except in the metre or so beneath the Kupferschiefer (Plumhoff, 1966). As these f o s s i l s have strong Zechstein a f f i n i t i e s , they should, perhaps more c o r r e c t l y , a l s o be a t t r i b u t e d t o t h e Zechstein marine transgression (Fa1 ke, 1976). The l a c u s t r i n e environment of deposition of t h i s sequence was deduced from the lack of f o s s i l s , from t h e absence of carbonates, and from the non-marine composition of the evaporites (Glennie, 1972). This i n t e r p r e t a t i o n i s supported by t h e low
529
bromide c o n t e n t of t h e R o t l i e g e n d h a l i t e and by t h e s u l p h u r - i s o t o p e a n a l y s i s of t h e a s s o c i a t e d anhydri tes (Hol s e r , 1 9 7 9 ) .
Hol s e r (1979) s u g g e s t s t h a t because of t h e i r
r e l a t i v e l y g r e a t volume, t h e s e s a l t s must have been d e r i v e d from o l d e r d e p o s i t s of marine o r i q i n , and s u g g e s t s exposed Devonian and E a r l y Permian ha1 i t e s of European USSR a s p o s s i b l e s o u r c e s . The w r i t e r ' s e x p e r i e n c e i n modern d e s e r t s convinces him,
however, t h a t t h e r e q u i r e d volume of h a l i t e can be c o n c e n t r a t e d and p r e c i p i t a t e d from l a k e w a t e r s u p p l i e d by a normal system of s e a s o n a l l y ( ? ) f l o w i n g r i v e r s , provided t h e r a t e of e v a p o r a t i o n i s g r e a t enough and of long d u r a t i o n . As w i l l be shown l a t e r , both t h e s e f a c t o r s were probably o p e r a t i v e i n t h e e a r l y Permian Rotliegend d e s e r t . There a r e no known s e d i m e n t a r y s t r u c t u r e s w i t h i n t h e l a c u s t r i n e sequence t h a t a r e i n d i c a t i v e of s u b - a e r i a l d e s i c c a t i o n o r e r o s i o n , a l t h o u g h such s t r u c t u r e s a r e known i n t h e a n h y d r i t i c mudstones of t h e sabkha f a c i e s , which e n c i r c l e s t h i s d e s e r t l a k e . The l a k e i s t h e r e f o r e b e l i e v e d t o have been a permanent f e a t u r e of t h e Southern
A t i t s f u l l e s t e x t e n t , t h e l a k e must have covered an a r e a of some 1000 km e a s t - w e s t , by u p t o 200 km n o r t h - s o u t h . Permian Basin t h r o u g h o u t t h e e a r l i e r Permian.
The e v i d e n c e of f l u v i a l t r a n s p o r t d i r e c t i o n s i n t h e R o t l i e g e n d sequences of t h e Southern Permian Basin i n d i c a t e s t h a t most of t h e w a t e r s u p p l i e d t o t h e d e s e r t l a k e was d e r i v e d from t h e s o u t h , from t h e Variscan Mountains, with only minor q u a n t i t i e s from o t h e r s o u r c e s .
F l u v i a l a c t i v i t y seems t o have been more i m p o r t a n t in t h e e a s t
than i n t h e w e s t . The s o u r c e s of w a t e r f o r t h e n o r t h e r n two b a s i n s a r e unknown. i v ) As we have s e e n , a_sesuenre_of_Sabkha_deeoslts occupied a broad a r e a e n c i r c l i n g t h e d e s e r t l a k e i n t h e Southern Permian B a s i n , and d e p o s i t s of t h i s t y p e a r e found i n s m a l l e r a r e a s of l o c a l s u b s i d e n c e i n t h e Northern Permian and Moray F i r t h Basins (Deegan and S c u l l , 1975; Smith, 1976, F i g . 1 ) . Using t e t r a p o d f o o t p r i n t s f o r d a t i n g , Haubold and Katzung (1978) i n d i c a t e t h a t p l a y a and sabkha sediments a l r e a d y e x i s t e d i n Germany d u r i n g t h e L a t e Autunian. These s e d i m e n t a r y r o c k s comprise poorly-bedded c l a y s with minor s i l t s and sands t h a t d i s p l a y many f e a t u r e s i n d i c a t i v e of a sabkha ( e . g . d e s i c c a t i o n c r a c k s , sandstone d y k e s , adhesion r i p p l e s , a n h y d r i t e n o d u l e s ) .
These f e a t u r e s c o l l e c t i v e l y i n d i c a t e an
a q u a t i c d e p o s i t i o n a l a r e a t h a t was s u b j e c t t o e x t e n s i v e s u b a e r i a l d e s i c c a t i o n i n an a r i d environment ( G l e n n i e , 1 9 7 0 ) . T h u s t h e sabkha sediments r e p r e s e n t t h e a r e a t h a t was covered by w a t e r o n l y d u r i n g t h e maximum e x t e n s i o n s of t h e d e s e r t l a k e .
Although
t h e r e probably were annual f l u c t u a t i o n s i n t h e e x t e n t of t h e l a k e and a s s o c i a t e d sabkha, t h i s i s d i f f i c u l t t o i l l u s t r a t e with a l i m i t e d number of well b o r i n g s . F l u c t u a t i o n s on a much l o n g e r time s c a l e a r e c l e a r l y a p p a r e n t , however, and a r e i n d i c a t e d s c h e m a t i c a l l y on t h e r o c k - s t r a t i g r a p h i c f a c i e s diagram ( F i g . 2 ) , and i n t h e w e l l - l o g c o r r e l a t i o n s i l l u s t r a t e d by Adrichem Boogaert (1976) f o r t h e Netherlands. Another i m p o r t a n t p o i n t t h a t t h e s e f i g u r e s i l l u s t r a t e i s t h a t h a l i t e p r e c i p i t a t i o n i n t h e Southern Permian Basin seems t o have been coeval w i t h t h e maximum development of the aeolian facies.
T h i s p o i n t w i l l be r e f e r r e d t o a g a i n .
530
HERCYNIAN MOUNTAINS
<
PALAEOWIND DIRECTIONS
/ CALEDONIAN MOUNTAINS PRESUMED OCEANIC PALAEO-TRADE WINDS
DESERT S EDlMENTS ( E. PERMIAN IN EUROPE, PENN.- M. PERMIAN IN
N. AMERICA )
Fig. 4 - P a l a e o - r e l a t i o n s h i p s of R o t l i e g e n d and North American D e s e r t s . P l a t e reconstruction a f t e r Scotese e t a l . (1979): Late Carboniferous. HISTORICAL DEVELOPMENT OF ROTLIEGEND CLIMATE The change from the humid e q u a t o r i a l c o n d i t i o n s under which t h e Carboniferous Coa' Measures were d e p o s i t e d , t o t h e a r i d c l i m a t e of R o t l i e g e n d d e p o s i t i o n , was probably mostly a r e s u l t of t h e northward d r i f t of L a u r a s i a . The R o t l i e g e n d b a s i n s came t o occupy a l a t i t u d i n a l p o s i t i o n n o r t h of the e q u a t o r ( F i g . 4 ) s i m i l a r t o t h a t of the p r e s e n t North African-Arabian d e s e r t s . (During t h e p a s t d e c a d e , t h e p u b l i s h e d palaeo. magnetic l o c a t i o n of NW Europe has v a r i e d by some of 10" of l a t i t u d e - s e e e . g . d i f f e r e n t l a t i t u d e s used by G l e n n i e , 1972 and 1983; t h e l a t i t u d e s i l l u s t r a t e d by S c o t e s e e t a l . , 1979, a r e adopted f o r t h i s p a p e r , and a r e s i m i l a r t o t h o s e used by Glennie, 1972). During t h e L a t e C a r b o n i f e r o u s , t h e newly c r e a t e d Variscan Highlands i n western Europe occupied an e q u a t o r i a l c l i m a t i c n i c h e and, t o j u d g e from c o a l s of Stephanian and Autunian ages i n S a a r l a n d and Bohemia, had a f a i r l y h i g h r a i n f a l l . A timer e l a t e d r e d u c t i o n i n t h e volume of R o t l i e g e n d f l u v i a l sediments i n t h e Southern
531 Permian Basin, and an a s s o c i a t e d spread o f dune sediments, may i n d i c a t e t h a t t h e main sources o f r i v e r w a t e r i n t h e V a r i s c a n Mountains were a l s o moving away f r o m t h e e q u a t o r i a l r a i n be1 t. These c l i m a t i c changes seem t o have been h e r a l d e d i n B r i t a i n and n o r t h Germany by a n o r t h w a r d m i g r a t i n g a l t e r a t i o n w i t h t i m e f r o m g r e y c o a l measures d u r i n g t h e e a r l i e r Westphalian (A + B) t o a "Barren" red-bed f a c i e s o f upper Westphalian C t o Stephanian age. Over much of t h e area, t h e r e was a m a j o r h i a t u s between C a r b o n i f e r o u s and Upper R o t l i e g e n d s t r a t a ( s e e e.g.
Van W i j h e e t a l . ,
1980; F i g . l o ) , so t h a t t h e d e t a i l e d
h i s t o r y o f c l i m a t i c change d u r i n g t h a t t i m e span has t o be deduced f r o m areas beyond t h e d e s e r t where s e d i m e n t a t i o n was c o n t i n u o u s . I n t h e s o u t h e r n Pyrenees, f o r example, Nagtegaal (1969) has shown t h a t t h e r e was a p r o g r e s s i v e change f r o m a humid Westp h a l i a n D c l i m a t e t o one t h a t was s e m i - a r i d ( a l l u v i a l f a n / s t e p p e ) a t about t h e Autunian-Saxonian t i m e boundary. A s i m i l a r c l i m a t i c change has been deduced f r o m t h e l a t e C a r b o n i f e r o u s and e a r l y Permian s t r a t a o f France (e.g.
(Hol ub, 1976).
Feys, 1976) and Bohemia
Hol ub, however, s t r e s s e s t h a t superimposed on t h i s gradual i n c r e a s e
i n a r i d i t y were a l t e r n a t i o n s o f more humid and more a r i d p e r i o d s , w i t h c u l m i n a t i o n s o f a r i d i t y d u r i n g t h e Saxonian (U. R o t l i e g e n d ) and t h e T h u r i n g i a n ( Z e c h s t e i n ) . These two phases o f i n c r e a s e d a r i d i t y seem t o be matched i n t h e Southern Permian B a s i n by t h e two m a j o r p e r i o d s o f e v a p o r i t e p r e c i p i t a t i o n
-
the evaporite5 o f the
R o t l i e g e n d d e s e r t l a k e and o f t h e l a t e r Z e c h s t e i n Sea; and as may be expected, t h e e a r l i e r o f t h e two seems t o c o i n c i d e w i t h maximum dune a c t i v i t y .
The d e p o s i t i o n a l
h i s t o r y o f t h e l a t e r R o t l i e g e n d i s one o f e x t e n s i o n o f t h e d e s e r t l a k e and sabkha f a c i e s a t t h e expense o f t h e dune sands ( F i g . 2 ) . D e p o s i t i o n o f t h e c o n t i n e n t a l R o t l i e g e n d d e s e r t sequence was b r o u g h t a b r u p t l y t o an end by t h e Z e c h s t e i n t r a n s g r e s s i o n . s w i f t event.
T h i s t r a n s g r e s s i o n seems t o have been a v e r y
I t f l o o d e d R o t l i e g e n d b a s i n s whose s u r f a c e s may have been some 250 t o
300 rn below sea l e v e l (Smith, 1970, 1979; Z i e g l e r , 1982; G l e n n i e and B u l l e r , 1983).
A g l o b a l r i s e i n sea l e v e l seems t o have caused a p p r o x i m a t e l y simultaneous marine t r a n s g r e s s i o n s o v e r much o f t h e Russian p l a t f o r m , many o f t h e A r c t i c i s l a n d s , c e n t r a l e a s t Greenland and c e n t r a l N o r t h America, as w e l l as l a r g e p a r t s o f western Europe (Smith, 1964). T h i s e u s t a t i c r i s e was p r o b a b l y a s s o c i a t e d w i t h t h e decay o f t h e Gondwana i c e cap, and seems t o have been g r e a t enough t o breach what was p o s s i b l y no more t h a n a v a l l e y - b o t t o m sedimentary b a r r i e r somewhere between NE Greenland and t h e N o r t h Cape o f Norway. Once breached, w a t e r w i l l have poured s o u t h a l o n g t h e p r o t o A t l a n t i c r i f t (Russel, 1976; Z i e g l e r , 1978), which a l s o must have been below sea l e v e l along i t s e n t i r e length.
The ensuing f l o o d i n g o f t h e R o t l i e g e n d b a s i n s i s
i n f e r r e d t o have been f a s t enough t o have caused widespread 1 i q u e f a c t i o n w i t h i n t h e R o t l i e g e n d dune sands w i t h a s s o c i a t e d s o f t - s e d i m e n t d e f o r m a t i o n . D e f o r m a t i o n seems t o have been c o n f i n e d l a r g e l y t o dunes o f t r a n s v e r s e t y p e , which have a h i g h percentage o f under-compacted avalanche-slope sands. The s e i f dunes o f t h e Durham area, on t h e
532
o t h e r hand, w i t h t h e i r lower-angle s t y l e of c r o s s bedding and i n h e r e n t l y t i g h t e r g r a i n p a c k i n g , seem l a r g e l y t o have r e s i s t e d l i q u e f a c t i o n ( G l e n n i e and B u l l e r , 1983). IMPACT OF GONDWANA GLACIATION ON ROTLIEGEND WINDS Because of an a s s o c i a t e d e u s t a t i c r i s e i n s e a l e v e l , t h e demise of t h e Gondwana g l a c i a t i o n s seems t o have r e s u l t e d i n the end of c o n t i n e n t a l d e s e r t c o n d i t i o n s over many low-lying a r e a s of N . W . Europe. I t i s s u g g e s t e d h e r e , t h a t t h e p e r i o d s o f more i n t e n s e a r i d i t y i n t h e R o t l i e g e n d d e s e r t a r e a s were caused by s t r o n g e r g l o b a l wind s y s t e m s , which i n t u r n , were d r i v e n by p o l a r a r e a s of g l a c i a l l y - i n d u c e d high barom e t r i c p r e s s u r e o v e r Gondwana. T h i s g e n e r a l p r e m i s e , which i s i l l u s t r a t e d concept u a l l y i n Fig. 5 , has been advocated by many workers ( e . g . Lamb, 1961; G l e n n i e , 1970; Manabe and Hahn, 1977) and i s s u p p o r t e d by t h e e v i d e n c e from P l e i s t o c e n e d e s e r t s in PRESENT N
7
I
PLUVIALS
,I
DOLDRUMS
7;---
5
5
GLACIALS
INTERGLACIALS
VERY STRONG CONSTANT WINDS BUILT MAJOR DUNE SYSTEMS.
WEAK WIND SYSTEMS STRONG CONVECTION INFLUENCE NEAR COASTS & MOUNTAINS DESERT “PLUVIALS”
F i g . 5 - Conceptual D i f f e r e n c e s i n Width, Location of t h e E a r t h ’ s A i r - p r e s s u r e Belts, and t h e I n t e n s i t y of t h e A s s o c i a t e d Wind Systems i n R e l a t i o n t o t h e S i z e of t h e P o l a r I c e Caps.
533 Australia (Galloway, 1965; Williams, 1973; Bowler, 1976), S o u t h Africa (Lancaster, 1981), t h e Persian Gulf (Sarnthein, 1972, 1978) and offshore North-West Africa (Sarnthein e t a l . , 1981). Several p e r t i n e n t analogies can be made concerning wind a c t i v i t y in the Permian palaeodesert of Northwest Europe and t h e Pleistocene-Recent d e s e r t s of the world. I n a r i d d e s e r t s , t h e r e seem t o be two simple basic types of dune; the transverse dune ( o r barchan where t h e r e i s a shortage of sediment) which i s formed by winds of moderate strength a n d has i t s a x i s transverse t o the dominant sand-transporting wind; and the l i n e a r o r s e i f dune, which forms i n winds t h a t a r e too strong f o r transverse s t a b i l i t y and has i t s a x i s p a r a l l e l t o the dominant sand-transporting wind (Glennie, 1970). The w r i t e r believes t h a t t h e bulk of t h e e x i s t i n g major dunes of North a n d S o u t h Africa, Arabia, India a n d Australia originated with such b a s i c a l l y simple forms during the l a s t (Weichsel/Wisconsin) g l a c i a t i o n . Many of these g i a n t dunes have heights t h a t exceed 100 m and a width of 1 km or more. Since then, however, wind i n t e n s i t i e s have decreased, and t h e maximum s i z e of s e i f dunes b u i l t today seem t o have a n average height of l i t t l e more t h a n 5 o r 10 m. Perhaps more important in terms of t h e building a n d preservation of Pleistocene dune forms, i s t h a t a t the time of t h e i r formation, winds possibly blew with sand-transporting v e l o c i t i e s f o r much of the g l a c i a l winter; according t o Wilson (1971, p . 190; using data from Dubief, 1952) modern sand-transporting winds in a n area of Algeria blow f o r a cumulative t o t a l of only some 52 hours per year (mean deviation 39 hours). Bagnold (1941, p . 69) s t a t e s t h a t sand flow i s proportional t o the cube of the excess velocity above t h a t a t which sand begins t o move. This f a c t lays g r e a t s t r e s s on t h e transporting power of strong winds; coupled with the probable much g r e a t e r annual duration of strong winds during high l a t i t u d e g l a c i a t i o n s , t h e annual amount of sediment moved must then have been immeasurably g r e a t e r than today. The g i a n t Pleistocene dunes include a high proportion of l i n e a r forms. I n the present i n t e r g l a c i a l conditions t h e winds a r e generally weaker and the surfaces of t h e Pleistocene dunes a r e now commonly covered by much smaller p a r a s i t i c dune forms whose axes a r e transverse t o t h e prevailing winds ( s e e e.g. Cooke and Warren, 1973, P1 4.1, 4 . 2 ) . I n many places, t h e d i r e c t i o n of t o d a y ' s winds i s n o t in equilibrium with the former g i a n t s across which they blow, and t h e l a t t e r s u f f e r a g r e a t e r o r
l e s s e r degree of morphological change. As a r e s u l t , t h e axes of younger dunes may be seen t o c u t across those of the o l d e r dune systems ( s e e Breed e t a l . , 1979, Fig. 202, f o r example). Where the old and new wind d i r e c t i o n s a r e s i m i l a r , erosion of the o l d e r system of g i a n t s i s l e s s obvious, b u t the small s i z e of the r e s u l t i n g new dunes i s plain t o see ( e . g . Glennie, 1970, Fig. 7 4 ) . I n t h e Rotliegend, t h e bulk of the dunes seem t o be of the transverse type, with some suggestion of minor transverse i n s t a b i l i t y . Seif dunes a r e c l e a r l y present only a t the western basin margin in County Durham (Fig. l ) , where wind strengths presum-
(SOL 1 Sdla a3NIaw03
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Id OLL6-SLE6
NOllIWVH
N
N
14 SOt6-OSE6 Sdla R
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1-92/6t
Id OZ16-0CL6 Sdla 81
N
OSS3/113HS
Id EEZL -€ELL
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Sdla LZ
IdZEOL-SLL9
Id OL LL -2EOL
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N
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Id €ELL-OLLL Sdla t 1
N
Id SLL9 -SEE9 Sdla tl.
N
N
535 a b l y were g r e a t e r than i n t h e b a s i n c e n t r e . S e i f s may a l s o be p r e s e n t l o c a l l y i n t h e Northern Permian Basin ( G l e n n i e , 1 9 8 3 ) . The d i s t r i b u t i o n of t h e i r bedding a t t i t u d e s i n comparison w i t h t h a t of an i n f e r r e d t r a n s v e r s e dune i s i l l u s t r a t e d i n Fig. 3. The s t u d y of P l e i s t o c e n e - R e c e n t dunes shows t h a t t h e r e have been changes of both wind s t r e n g t h and wind d i r e c t i o n w i t h time. S i m i l a r v a r i a t i o n s with time can be seen from a s t u d y of t h e bedding a t t i t u d e s i n R o t l i e g e n d dune sands a s d e r i v e d from d i p m e t e r l o g s ( F i g . 6 ) . The d e r i v a t i o n of t h e palaeowind d i r e c t i o n s i s s u b j e c t - i v e r a t h e r t h a n s t r i c t l y s t a t i s t i c a l , so t h a t some of t h e i n f e r r e d d i r e c t i o n s could be q u e s t i o n e d , a l t h o u g h arguments i n t h e i r s u p p o r t have been given a t some l e n g t h by Glennie ( 1 9 8 3 ) . There can be l i t t l e doubt a b o d t the g e n e r a l wind d i r e c t i o n a t riost well l o c a l i t i e s , however. In both t h e i l l u s t r a t e d w e l l s , the dunes a r e probably o f t h e t r a n s v e r s e t y p e . E s p e c i a l l y i n well 49/26-2, t h e i n d i v i d u a l d e p t h s e q u e n c e s , e x c e p t perhaps f o r xhe d e e p e s t , seem t o conform c l o s e l y t o t h e l i m i t e d s p r e a d of bedding a t t i t u d e s t y p i c a l of t r a n s v e r s e dunes. Because of s l i g h t changes i n wind d i r e c t i o n w i t h t i m e , t h c added s p r e a d i n d i p a t t i t u d e s seen i n t h e combined d a t a f o r t h e t o t a l Rotliegend dune sequence g i v e s a f a l s e impression of d e p o s i t i o n on a barchanoid dune, w i t h i t s element of t r a n s v e r s e i n s t a b i l i t y . Simple t r a n s v e r s e dunes seem t o be l e s s l i k e l y i n t h e c a s e of well 9/28-2, e s p e c i a l l y a t t h e d e e p e r l e v e l s , which have a more barchanoid d i s t r i b u t i o n o f bedding a t t i t u d e s . Here t o o , however, i t may be a c a s e of sampling a t t o o c o a r s e an i n t e r v a l . What i s more remarkable, p e r h a p s , i s t h e r e l a t i v e l y small range of palaeowind d i r e c t i o n s i n f e r r e d f o r a time span of d e p o s i t i o n t h a t runs i n t o m i l l i o n s of y e a r s . These o b s e r v a t i o n s may r e f l e c t a s i t u a t i o n i n v o l v i n g a s t r o n g e r g l a c i a l c o n t r o l of d e s e r t winds e a r l i e r i n t h e Permian, and a weakening wind system a s t h e Gondwana i c e c3p d e c l i n e d . S i m i l a r l y , the winds of t h e l a s t P l e i s t o c e n e g l a c i a t i o n probably had much l e s s v a r i a b i l i t y i n d i r e c t i o n t h a n t h o s e of t o d a y . C e r t a i n l y , t h e d e p o s i t i o n a l r e s u l t s of weaker Permian i n t e r g l a c i a l winds a r e l i k e l y t o have been l a r g e l y removed by t h e s t r o n g e r winds r e l a t e d t o each succeeding g l a c i a t i o n o v e r Gondwana. A weakening wind system i s l i k e l y t o be the r e a s o n why, w i t h advancing t i m e , t h e a r e a of t h e R o t l i e g e n d d e s e r t l a k e i n c r e a s e d a t t h e expense of t h e dune s a n d s . The i n t e n s i t y of e v a p o r a t i o n t h a t gave r i s e t o h a l i t e p r e c i p i t a t i o n e a r l i e r d u r i n g R o t l i e g e n d d e p o s i t i o n i s t h o u g h t t o have been more c l o s e l y r e l a t e d t o d r y highv e l o c i t y t r a d e winds t h a n t o high t e m p e r a t u r e s . I n d e e d , e v i d e n c e from t h e P l e i s t o c e n e i n d i c a t e s t h a t , a l t h o u g h the snowline i n E t h i o p i a was lower by some 600-1000 m during t h e l a s t g l a c i a t i o n (Adamson e t a l . , 1 9 8 0 ) , the l e v e l of Lake V i c t o r i a was a l s o much lower than now, and i n the Sudan, l i n e a r dunes were a c t i v e p r i o r t o l a t e G l a c i a l f l o o d i n g of t h e N i l e v a l l e y ( W i l l i a m s , 1 9 7 5 ) . S i m i l a r l y , Galloway (1965) r e p o r t s t h a t , a l t h o u g h A u s t r a l i a was c o n s i d e r a b l y c o l d e r d u r i n g t h e Late P l e i s t o c e n e than now, i t was a l s o more a r i d w i t h a s s o c i a t e d s t r o n g winds c a p a b l e of b u i l d i n g s e i f dunes.
536 The old?;,
known g l a c i d t i o n i n Gondwaria i s f o u n d i n E a r l y C a r b o n i f e r o u s s t r a t a i n
t h e Acdean b e l t o f A r g e n t i n a ; t h e y o u n g e s t ( i c e - r a f t e d ) g l a c i a l d e p o s i t s were r e c o r d e d i n Tasmania and t h e Sydney B a s i n o f A u s t r a l i a and a r e o f m i d - P e r m i a n acje. Be::weri
t h e s e e x t r e m e s , g l a c i a l c o n d i t i o n s c o n t i n u e d i n t e r m i t t e n t l y f o r some 80
m i . l l i c n y e a r s , w i t h a t l e a s t 12 advances and r e t r e a t s b e i n g r e c o g n i s e d i n t h e Piirana B a s i n o f Peru ( M a r t i n , 1981). The Gondwana g l a c i a t i o n seems t o have been a t i t s maxiciutri e x t e n t a t a b o u t t h e Carboniferous-Permian t i m e boundary (Stephanian-Early Autunian).
In Western Europe,
l o c a l i n t e r m o n t a n e c o a l s o f t h e s e ages t e s t i f y t o t h e e x i s t e n c e o f r a i n f a l l w i t h i n t h e V a r i s c a n H i g h l a n d s , b u t o n t h e i r n o r t h e r n f o r e l a n d t h e r e were i n c r e a s i n g s i g n s o f aridity.
Dune sands a s s o c i a t e d w i t h e a r l i e s t Permian v o l c a n i c s o c c u r b o t h i n S.W.
E n g l a n d ( n e a r E x e t e r ) and S.W.
Scotland (Ayrshire).
A t about t h i s time i n North
A m e r i c a , t,he e r a o f P e n n s y l v a n i a n c o a l d e p o s i t i o n was d r a w i n g t o a c l o s e i n t h e A p p a l a c h i a n s , b u t f a r t h e r w e s t t h e Weber ( F r y b e r g e r , 1979) and de C h e l l y (Weber, 1 9 7 9 ) dune sands were b e i n g d e p o s i t e d .
In b o t h f o r m a t i o n s , t h e dunes seem t o be o f
t h e t r a n s f o r m t y p e , Weber ( 1 9 7 9 ) m a k i n g a c o m p a r i s o n between t h e b e d d i n g s t y l e s o f t h e de C h e l l y and t h e R o t l i e g e n d dunes o f t h e N o r t h Sea gas f i e l d s .
Thus, a l t h o u g h
w i r l d s t r e n g t h s w e r e n o t e x t r e m e , dune a c t i v i t y c o i n c i d e d w i t h t h e t i m e o f a m a j o r h i g h l a t i t u d e i c e cap. Had Permian i c e caps d e v e l o p e d c o e v a l l y i n b o t h hemispheres, as i n t h e P l e i s t o c e n e , R o t l i e g e n d w i n d s t r e n g t h s may w e l l have been g r e a t e r . The m a j o r h , i a t u s between t h e C a r b o n i f e r o u s and t h e Upper R o t l i e g e n d o f t h e S o u t h e r n P e r m i a n B a s i n c o n t a i n s an anomaly. A g r e a t e r volume o f r o c k seems t o have been e r o d e d a t t h i s t i m e t h a n can be a c c o u n t e d f o r i n t h e R o t l i e g e n d sequence. I n v e r s i o n a l o n g t h e S o l e P i t a x i s o f t h e S o u t h e r n N o r t h Sea ( G l e n n i e and Boegner, 1951) r e s u l t e d l o c a l l y i n e r o s i o n o f t h e t o t a l W e s t p h a l i a n sequence, w h i c h p r o b a b l y h a d a d e p o s i t i o n a l t h i c k n e s s a p p r o a c h i n g 1000 m ( F i g . 7 ) , b e f o r e d e p o s i t i o n o f t h e s u c c e e d i n g R o t l i e g e n d sequence. A l s o , p r i o r t o Z e c h s t e i n d e p o s i t i o n , e a r l y Permian e r o s i o n o v e r t h e u p l i f t e d M i d N o r t h Sea H i g h p e n e t r a t e d t h r o u g h t h e f o r m e r Carboni f e r o u s c o v e r deep i n t o t h e D e v o n i a n .
T h i s anomaly c a n be e x p l a i n e d i f l a r g e - s c a l e
r e m o v a l o f t h e f i n e r f r a c t i o n b y d e f l a t i o n i s a c c e p t e d as p l a u s i b l e , t h e p r o d u c t s o f d e f l a t i o n b e i n g d e p o s i t e d e l s e w h e r e i n t h e oceans o r o n l a n d as l o e s s .
On a s i m i l a r
l a r g e s c a l e , d e f l a t i o n o f much o f t h e T e r t i a r y p r o d u c t s o f e r o s i o n f r o m t h e w e s t s i d e o f t h e Oman M o u n t a i n s i n A r a b i a h a s been p r o p o s e d b y G l e n n i e e t a l . ( 1 9 7 4 ) . I n the context o f deflation, the time o f o r i g i n o f the Rotliegend desert lake i s n o t known. I t s p o s i t i o n o v e r l y i n g sequences o f W e s t p h a l i a n C t o D ( F i g . 7 ) and even S t e p h a n i a n s t r a t a (Van W i j h e e t a l . ,
198,O) s u g g e s t s t h a t i n t h e s e a r e a s , d e p o s i t i o n
may have been l o c a l l y c o n t i n u o u s a c r o s s t h e C a r b o n i f e r o u s - P e r m i a n t i m e b o u n d a r y . I f c o r r e c t , t h e n t h e l a k e i t s e l f , o r m o i s t u r e f r o m n e a r - s u r f a c e g r o u n d w a t e r , w i l l have p r e v e n t e d f u r t h e r e r o s i o n b y d e f l a t i o n ; t o t h e c o n t r a r y , t h e s e w e t a r e a s may w e l l have t r a p p e d w i n d - b l o w n c l a y and s i l t p a r t i c l e s t o f o r m some o f t h e e a r l i e s t s e d i ments o f t h e d e s e r t l a k e .
537 KM
-350
*
SOUTHERN
c
PERMIAN BASIN
c
.
DESERTLAKE
NORTH SEA
PLATFORM
CD B
A N
DI D
WESTPHALIAN
C-D
B
ROTLIEGEND
0
SANDSTONES (WADI & DUNE1
A NAMURIAN DINANTIAN DEVONIAN
DESERT LAKE CLAYSTONE & HALITE
lo00 M
E0
Fig. 7 - Sketch i l l u s t r a t i n g t h e R e l a t i o n s h i p between t h e R o t l i e g e n d Sedimentary Basin and the eroded Pre-Permian S t r a t a . Because of t h e e q u a t o r i a l l o c a t i o n of t h e i r d e p o s i t i o n a l a r e a , the Westphalian Coal Measures of N.W. Europe were probably l a r g e l y u n a f f e c t e d by t h e Gondwana g l a c i a t i o n s e x c e p t i n terms of a r e l a t i v e l y cool c l i m a t e and o c c a s i o n a l g l a c i o - e u s t a t i c r i s e s i n s e a l e v e l t o g i v e t h e d i s t i n c t i v e marine marker h o r i z o n s found i n t h e Coal Measures. A s soon a s t h i s a r e a began t o d r i f t i n t o t h e b e l t of Trade Winds, however, i n c r e a s e d a r i d i t y coupled w i t h s t r o n g winds r e s u l t e d i n s e v e r e d e f l a t i o n , which was more marked i n t h e n o r t h than t o t h e s o u t h . CONCLUSIONS
The R o t l i e g e n d s e d i m e n t a r y sequence was d e p o s i t e d i n s e v e r a l c o n t i n e n t a l b a s i n s of a n o r t h e r n hemisphere t r o p i c a l d e s e r t , the l a r g e s t of which were the Southern and Northern Permian b a s i n s . D e s e r t d e p o s i t i o n succeeded an e a r l y Permian p e r i o d t h a t involved c o n s i d e r a b l e e r o s i o n of pre-Permian s t r a t a , which was p o s s i b l y caused i n p a r t by s t r o n g d e f l a t i o n . T h i s n o r t h e r n hemisphere wind a c t i v i t y seems t o have c o i n c i d e d w i t h the e a r l y waning s t a g e s of a major p o l a r i c e cap o v e r Gondwana, which, l i k e t h e P l e i s t o c e n e g l a c i a t i o n s , may have a f f e c t e d t h e p a t t e r n s of t e m p e r a t u r e and b a r o m e t r i c p r e s s u r e on a g l o b a l s c a l e . The change from a humid C a r b o n i f e r o u s c l i m a t e t o t h e a r i d c o n d i t i o n s of t h e R o t l i e g e n d d e s e r t was probably l a r g e l y t h e r e s u l t of t h e northward d r i f t of N . W . '
Europe from t h e v i ' c i n i t y of t h e L a t e P a l a e o z o i c e q u a t o r i n t o the zone of p r e v a i l i n g n o r t h e a s t Trade Winds; thus the R o t l i e g e n d d e s e r t would a l s o be w i t h i n the r a i n shadow of t h e Variscan-Ural system of mountains. The a x i a l p a r t of the Southern Permian Basin was occupied by a major d e s e r t l a k e whose w a t e r s were f e d e s p e c i a l l y from t h e e a s t e r n Variscan Mountains. The l a k e c o n t a i n e d bedded h a l i t e i n t h e middle p a r t o f i t s sedimentary sequence. H a l i t e p r e c i p i t a t i o n seems t o have c o i n c i d e d w i t h the maximum e x t e n s i o n o f t h e b e l t of dune sands t o i t s s o u t h , t h e r e b y emphasing the r e l a t i o n s h i p between a r i d i t y and wind a c t i v i t y in t h i s d e s e r t area.
338
A r e l a t i v e l y s t r o n g g l a c i a l l y - i n d u c e d w i n d system i s t h o u g h t t o have r e s u l t e d i n a f a i r l y c o n s t a n t a n t i c y c l o n i c p a t t e r n o f mid-Permian winds o v e r t h e p r e s e n t N o r t h Sea area; e s s e n t i a l l y f r o m t h e e a s t o v e r t h e Southern Permian B a s i n and f r o m t h e west o v e r t h e N o r t h e r n Permian Basin. A t any one l o c a l i t y , t h e d i r e c t i o n o f t h e m i d Permian palaeowinds s h i f t e d s l i g h t l y w i t h time, b u t t h e o v e r a l l d i r e c t i o n seems t o have been remarkably c o n s t a n t . These m i n o r changes a r e t h o u g h t t o r e f l e c t d i f f e r e n c e s i n t h e g l o b a l wind p a t t e r n , which p r o b a b l y became l e s s u n i f o r m arid l e s s i n t e n s e as t h e Gondwana i c e cap d w i n d l e d , and r e s u l t e d i n much l o w e r a e o l i a n t r a n s p o r t r a t e s i n t h e Rotliegend desert.
A g l a c i o - e u s t a t i c r i s e i n sea l e v e l i s t h o u g h t t o have r e s u l t e d i n t h e Zechstein marine transgression.
A t t h a t time, t h e surface o f t h e Rotliegend desert l a k e i s
e s t i m a t e d t o have been some 250 m below sea l e v e l and, indeed, t h e e n t i r e r o u t e along which t h e Z e c h s t e i n marine w a t e r must have f l o w e d f r o m t h e v i c i n i t y o f t h e p r e s e n t A r c t i c Ocean must a l s o have been below sea l e v e l .
The R o t l i e g e n d b a s i n s were flooded
so r a p i d l y t h a t t h e r e was no e x t e n s i v e marine r e w o r k i n g o f t h e exposed unconsolidated R o t l i e g e n d dune sands, and much o f t h e i r f o r m e r s u b - a e r i a l r e l i e f i s now preserved beneath t h e d r a p i n g b a s a l K u p f e r s c h i e f e r .
I n t e r n a l l y , however, e s p e c i a l l y w i t h
t r a n s v e r s e dune forms t h a t were exposed above t h e w a t e r t a b l e , t h e f o r m e r avalanche bedding was s u b j e c t e d t o widespread homogenisation and t o l a r g e - s c a l e s o f t - s e d i m e n t d e f o r m a t i o n as t h e dunes were i n u n d a t e d beneath t h e r i s i n g Z e c h s t e i n Sea. ACKNOWLEDGMENTS The w r i t e r i s i n d e b t e d t o The S h e l l I n t e r n a t i o n a l e Petroleum M i j . f o r perm ssion t o p u b l i s h t h i s paper. A l s o i t i s a p l e a s u r e t o acknowledge t h a t t h e t e x t has b e n e f i t e d f r o m c o n s t r u c t i v e c r i t i c i s m by A.T. D.B.
Smith, U.K.
B u l l e r , U n i v e r s i t y o f Trondheim
and
I n s t i t u t e o f G e o l o g i c a l Sciences, and by my c o l l e a g u e s J.P.H.
K a a s s c h i e t e r , S I P M , The Hague, and J. Zimdars, BEB, Hannover, Germany.
REFERENCES Adamson, D.A., Gasse, F., S t r e e t , F.A. and W i l l i a m s , M.A.J., 1980 - L a t e Q u a t e r n a r y H i s t o r y o f t h e N i l e . N a t u r e 28: 50-55. Adrichem Boogaert, H.A., 1976 - O u t l i n e o f t h e R o t l i e g e n d (Lower Permian) i n The N e t h e r l a n d s . I n : F a l k e , H. (Ed.) quod v i d e 23-37. Bagnold, R.A., 1941 - P h y s i c s o f blown sand and d e s e r t dunes. Methuen, London, 265 p. B e l l , J . , Holden, J., P e t t i g r e w , T.H. and Stodman, K.W., 1979 - The M a r l S l a t e and Basal Permian B r e c c i a a t M i d d r i d g e , Co. Durham. Proc. Yorks. g e o l . soc. 42 ( 3 ) : 43-460. Born, A., 1921 - Uber j u n g p a l a o z o i s c h e k o n t i n e n t a l e G e o s y n k l i n a l e n M i t t e l e u r o p a s . Ab. Senkenburg. N a t u r f o r s c h . G e s e l l s c h . 37: ( 4 ) 507-583. Bowler, J.M., 1976 - A r i d i t y i n A u s t r a l i a : Age, o r i g i n s and e x p r e s s i o n i n a e o l i a n landforms and sediments. E a r t h - S c i e n c e Reviews, 12: 279-310. F r y b e r g e r , S.G., Andrews, B., McCauley, C., L e n n a r t z , F., Gebel, 0. and Breed, C.S., Horstman, K., 1979 - Regional s t u d i e s o f Sand Seas u s i n g LANDSAT (ERTS) imagery. I n : McKee, E.D. (Ed.) quod v i d e 305-397. Brennand, T.P. and Van Veen, F.R., 1975 - The Auk o i l f i e l d . I n : Woodland, A.W. (Ed.) quod v i d e 275-285.
539
Brookfield, M.E., 1978 - Revis on of the stratigraphy of Permian and supposed Permian rocks of southern Scotland. Geol. Rundschau 67 (1): 110-143. Cooke, R.V. and Warren, A., 19 3 - Geomorphology in deserts. Batsford Ltd., London. 374 p. Dubief, J . , 1952 - Le vent et e deplacement du sable au Sahara. Trav. Inst. Rech. Sahariennes 8: 123-162. Deegan, C.E. and Scull, B.J., 1975 - A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Report No. 77/25. Inst. Geol. Sci. H.M.S.O. Ellenberg, J., Falk, F., Grumbt, E., Lutzner, H. and Ludwig, A.O., 1976 Sedimentation des hoheren Unterperms der Flechtinger Scholle. Z. geol. Wiss, Berlin 5: 705-737. Falke, H., 1972 - Rotliegend, essays on European Lower Permian. International Sedimentarv Petrographical Series V. 15. Brill, Leiden. Falke, H., 1976 - Problems of the continental Permian in the Federal Republic of Germany. In: Falke, H. (Ed.) quod vide 38-52. Falke, H., (Ed.) 1976 - The continental Permian in Central, West and South Europe. NATO Advanced Study Institutes Series C, Math. Phys. Sci. 22: 352 p. Feys, R., 1976 - Le Permien et la phase saalienne dans le bassin de Brive (S.W. de la France). In: Falke, H. (Ed.) quod vide 80-90. Fryberger, S.G., 1979 - Eolian-fluviatile (continental) origin of ancient stratigraphic trap or petroleum in Weber Sandstone, Rangely Oil Field, Colorado. Mountain Geologist 16 (1): 1-36. Galloway, R.W., 1965 - Late Quaternary climates in Australia. J. Geol., 73: 603-618. Gill, W.D., 1967 - The North Sea basin. 7th World Petroleum Cong. Proc. 2: 211-219. Glennie, K.W., 1970 - Desert sedimentary environments. Developments in Sedimentology 14. Elsevier. 222 p. Glennie, K.W., 1972 - Permian Rotliesendes of north-west Europe interpreted in liqht of modern desert sedimentation stidies. Bull. Am. Assoc. Petrol. Geologists, 56: (6) 1048-1071. Glennie, K.W., 1983 - Early Permian (Rotliegendes) palaeowinds of the North Sea. Sedimentary Geology 34. Glennie, K.W. and Boegner, P.L.E., 1981 - Sole Pit inversion tectonics. In: Illing L.V. and Hobson, G.D. (Eds.) Petroleum geology of the Continental Shelf of North-West Europe. Inst. Petrol. London, 110-120. Glennie, K.W., Boeuf, M.G.A., Hughes Clarke, M.W., Moody Stuart, M., Pilaar, W.F.H. and Reinhardt, B.M., 1974 - Geology of the Oman Mountains. Trans. Roy. Ned. Geol. & Mining SOC. 423 p. Glennie, K.W., Mudd, G.C. and Nagtegaal, P.J.C., 1978 - Depositional environment and diagenesis of Permian Rotliegendes sandstones in Leman Bank and Sole Pit areas of the UK southern North Sea. J. Geol. SOC. London. 135: 25-34. Glennie, K.W. and Buller, A.T., 1983 - The Permian Weissliegend of N.W. Europe: the partial deformation of aeolian dune sands caused by the Zechstein transgression. Sedimentary Geology 35. Haubold, H. and Katzung, G., 1978 - Palaeoecology and palaeoenvironments of tetrapod footprints from the Rotliegend (Lower Permian) of Central Europe. Palaeogeog., Palaeoclimatol., Palaeoecol. 23: 307-323. Holser, W.T., 1979 - Rotliegend evaporites, Lower Permian of Northwestern Europe Geochemical confirmation of the non-marine origin. Erd6l und Kohle, 32 (4): 159-162. Holub, V.M., 1976 - Permian basins in the Bohemian Massif. In: Falke, H. (Ed.) quod vide 53-79. Hunter, R.E., 1977 - Basic types of stratification in small eolian dunes. Sedimentology 24: 361-387. Katzung, G., 1975 - Tektonik, Klima und Sedimentation in der Mitteleuropaisches Saxon Senke und in angrenzenden Gebieten. Z. Geol. Wiss. Berlin 3 (11): 1454-1472. Kocurek, G., 1981 - Significance of interdune deposits and bounding surfaces in aeolian dune sands. Sedimentology 28: 753-780. Lamb, H.H., 1961 - Fundamentals of climate. In: Nairn, A.E.M. (Ed.), 1961. Descriptive Palaeoclimatology. Interscience, New York. Lancaster, N., 1981 - Palaeoenvironmental implications o f fixed dune systems in Southern Africa. Palaeogeog., Palaeoclimatol., Palaeoecol. 33: 327-346.
540 Lovell, J.P.B., 1983 - Permian and Triassic. In: G.Y. Craig (Ed.). Geology of Scotland (2nd edit.). Scottish Academic Press. Manabe, S. and Hahn, D.G., 1977 - Simulation of the tropical climate of an ice age. J. Geophys. Res. 82 (27): 3889-3911. Martin, H., 1981 - The late Palaeozoic Gondwana glaciation. Geol. Rundschau 70 (2): 480-496. McKee, E.D., 1979a - Ancient sandstones considered to be eolian. In: McKee, E.D. (Ed.) quod vide 187-233. McKee, E.D. (Ed.) 1979b - A study of global sand seas. Geol. Surv. Prof. Paper 1052. 429 p. Nagtegaal, P.J.C., 1969 - Sedimentology, palaeoclimatology and diagenesis of posthercynian continental deposits in the south-central Pyrenees, Spain. Leidse Geol. Meded. 42: 143-238. Nemec, W. and Porebski, S.J., 1977 - Weissliegendes sandstones: a transition from fluvial-aeol ian to shallow-marine sedimentation (Lower Permian of the Fore-Sudetic Monocl ine). 1. Sedimentary structures and textural differentiation. Annales de la Soci6tP Geologique de Pologne XLVII (4): 513-544. Peacock, J.D., 1966 - Contorted beds in the Permo-Triassic aeolian sandstones of Morayshire. Bull. Gecl. Surv. G.B. 24: 157-162. Pennington, J.J., 1975 - The Geology of the Argyll Field. In: Woodland, A.W. (Ed.), quod vide 285-291. Plein, E., 1978 - Rotliegend-Ablagerungen im Norddeutschen Becken. Z. deuts. geol. Ges. 129: 71-97. Plumhoff, F., 1966 - Marines Ober-Rotlieqendes (Perm) im Zentrum des nordwestdeutschen Rot1 iegend-Beckens; neue Beweise und Fol gerungen. Erdol u. Kohle 19 ( 1 0 ) : 713-310. Pokorski, J., 1978 - Saxonian. In: Depowski, S., (Ed.) Lithofacies - Paleogeographical Atlas of the Permian of platform areas of Poland Inst. Geol. Warsaw. 30 p. Pryor, W.A., 1971a - Petrology of the Weissliegendes Sandstones in the Harz and Werra-Fulda areas, Germany. Geol. Rundsch., 60: 524-552. Pryor, W.A., 1971b - Petrology of the Permian Yellow Sands of northeastern England and their North Sea Basin equivalents. Sediment. Geol., 6: 221-254. Russell, M.J., 1976 - A possible Lower Permian age for the onset of ocean floor spreading in the northern North Atlantic. Scott. J. Geol. 12 (4): 315-323. Sarnthein, M., 1972 - Sediments and history of the post-glacial transgression in the Persian Gulf and northwest Gulf of Oman. Marine Geol. 12: 245-266. Sarnthein, M., 1978 - Sand deserts during glacial maximum and climatic optimum. Nature. 272: 43-46. Sarntheim, M., Tetzlaff, G., Koopmann, B., Wolter, K. and Pflaumann, U., 1981 Glacial and interglacial wind regimes over the eastern subtropical Atlantic and North-West Africa. Scotese, C.R., Rambach, R.K., Barton, C., Voo, R. van der, and Ziegler, A.M., 1979 Paleozoic base maps. J. Geology 87 (3): 217-277. Selley, R.C., 1969 - Torridonian alluvium and quick sands. Scott. 3. Geol. 5 (4): 328-346. Smith, D.B., 1964 - The Permian period. In: Harland, W.B., Smith, A.G. and Wilcock, B. (Eds.). The Phanerozoic Time-scale. Supplement to Q.J.G.S. 120 5: 211-220. Smith, D.B., 1970 - The Palaeogeography of the British Zechstein. In: Rau, J.L. and Dellwig, L.F. (Eds.). Third Symposium on Salt, Vol. 1. Northern Ohio Geol. SOC. 20-23. Smith, D.B., 1976 - A review of the Lower Permian in and around the British Isles. In: Falke, H. (Ed.) quod vide 14-22. Smith, D.B., 1979 - Rapid marine transgressions and regressions o f the Upper Permian Zechstein Sea. J1. geol. SOC. Lond. 136 (2): 155-156. Smith, D.B., Brunstrom, R.G.W., Manning, P.I., Simpson, S. and Shotton, F.W., 1974 A correlation of Permian rocks of the British Isles. Geol. SOC. Lond. Special Rept. 5: 45 p. Smith, D.B. and Francis, E.A., 1967 - The geology of the country between Durham and West Hartlepool. Mem. geol. Surv. U.K. H.M.S.O., 354 p. Tanner, W.F., 1970 - Triassic-Jurassic lakes in New Mexico. Mountain Geologist 7 (4): 281-289.
54 1
-
Veen, F.R. v a n 1975 G e o l o g y o f t h e Leman Gas F i e l d . I n : Woodland, A.W. (Ed.) quod v i d e 223-233. W a l k e r , A.D., 1973 - The age o f t h e C u t t i e ' s H i l l o c k Sandstone ( P e r m o - T r i a s s i c ) o f t h e E l g i n a r e a . S c o t t . J. Geol. 9 ( 3 ) : 177-183. Weber, K.J., 1979 - C o m p u t a t i o n o f i n i t i a l w e l l p r o d u c t i v i t i e s i n e o l i a n sandstone on b a s i s o f g e o l o g i c model, Leman gas f i e l d , U n i t e d Kingdom. Am. Assoc. P e t r o l . Geol. B u l l . 63 ( 3 ) : 549. 1980 - The R o t l i e g e n d i n The W i j h e , D.H. van, L u t z , M. and K a a s s c h i e t e r , J.P.H., N e t h e r l a n d s and i t s gas a c c u m u l a t i o n s . Geol M i j n b o u w 59: 3-24. W i l l i a m s , G.E., 1973 - L a t e Q u a t e r n a r y p e d i m e n t s e d i m e n t a t i o n , s o i l f o r m a t , i o n and p a l a e o c l i m a t e s i n a r i d S o u t h A u s t r a l i a . Z e i t . Geomorph. N.F. 17 (1): 102-125. 1975 - L a t e P l e i s t o c e n e t r o p i c a l a r i d i t y synchronous i n b o t h W i l l i a m s , M.A.J., h e m i s p h e r e s . N a t u r e 253: 617-618. N i l s o n , I . G . , 1 9 7 1 - D e s e r t s a n d f l o w b a s i n s and a model f o r t h e d e v e l o p m e n t o f e r g s . G e o l . J. 137: 180-197. Woodland, A.W. ( E d . ) 1975 - P e t r o l e u m and t h e C o n t i n e n t a l S h e l f o f N o r t h West Europe. V o l . 1: Geology. App. S c i . P u b l . London. 5 0 1 p. Z i e g l e r , P.A., 1978 - N o r t h - W e s t e r n Europe: t e c t o n i c s and b a s i n development. Geol. M i j n b o u w 57: 487-502. Z i e g l e r , P.A., 1982 - G e o l o g i c a l A t l a s o f W e s t e r n and C e n t r a l Europe. E l s e v i e r , 130 p.
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543
LONGITUDINAL DRAA I N THE PERMIAN YELLOW SANDS OF PmPTtl-FAST FNGLAND R I C H A M l P. STEELE, RP P e t r o l e u m nevelopment Ltd., London, EC2Y 9RU, Enqland.
1.
R r i t a n n i c House, Moor Lane,
I NTRODllCTION The Y e l l o w Sands i s a f o r m a t i o n o f l a r g e l y uncemented, cross-bedded,
aeolian
sand w i t h a 30 km l o n q N-5 o u t c r o p i n County Durham and n e i q h b o u r i n q areas i n NE Enqland. The Sands form t h e basal f o r m a t i o n o f t h e l o c a l Permian sequence, r e s t i n q unconformably on C a r b o n i f e r o u s s t r a t a , c a r b o n a t e and e v a p o r i t e sequence.
and are o v e r l a i n by a t h i c k
They are b e l i e v e d t o be l a t e E a r l v Permian i n
aqe, on t h e s t r e n q t h o f f o s s i l s c o l l e c t e d from t h e immediately o v e r l v i n q Marl S l a t e . The f o r m a t i o n i s exposed i n a number o f l a r q e q u a r r i e s , where i t i s duq f o r b u i l d i n q sand, and i n a few n a t u r a l exposures. NW Europe i s known t o have been p a r t o f a l a r q e i n t r a c o n t i n e n t a l d e s e r t d u r i n q t h e E a r l y Permian (e.q.
Glennie, 1972;
Zieqler,
1981). The Durham area
l a y near t h e NW edqe o f a l a r q e d e s e r t b a s i n which extended eastwards as f a r as Poland. E a r l y Permian a e o l i a n sands a r e known from t h e southern North Sea, t h e Nether1 ands, n o r t h e r n Germany, and from numerous l o c a l i t i e s i n western Enqland and S c o t l a n d (e.q.
G l e n n i e , 1972; Smith e t al.,
1974). The Yellow Sands f o r m
p a r t o f t h i s suite. The f o r m a t i o n has l o n q been accepted as a e o l i a n (e.q.
D a l q l i s h and F o r s t e r ,
1864; Hodqe, 1931) and has i n t h e p a s t been viewed as a s e r i e s o f s e i f dunes (Smith & F r a n c i s , 1967; Glennie, 1970, p.lO1).
A s h a l l o w marine i n t e r p r e t a t i o n
proposed by P r y o r (1971) has n o t qained currency.
The p r e s e n t paper comprises a
b r i e f d e s c r i p t i o n and i n t e r p r e t a t i o n o f t h e d i s t r i b u t i o n and sedimentary structures o f t h e formation. 2.
THE DISTRIRlJTI0N OF THE SANDS The d i s t r i b u t i o n o f t h e Y e l l o w Sands i s shown on F i q . 1. T h i s map i s compiled
f r o m o u t c r o p i n f o r m a t i o n and 208 b o r e h o l e and s h a f t records, and r e p r e s e n t s a l l t h e d a t a a v a i l a b l e a t mid-1981. From F e r r y h i l l northward t o Sunderland, t h e Sands a r e disposed i n seven w e l l d e f i n e d , p a r a l l e l r i d q e s 1.5-3.5 d i s t r i b u t e d sands 0.8-2
km wide separated by c o r r i d o r s o f t h i n , p a t c h i l y
km wide. Two p o o r l y d e f i n e d r i d q e s may e x i s t between
Sunderland and Newcastle. p a t c h y sands and b r e c c i a s .
South o f F e r r y h i l l t h e Sands a r e r e p l a c e d by t h i n , N o r t h o f t h e Tyne i n f o r m a t i o n i s sparse.
The averaqe
t h i c k n e s s o f t h e sand r i d q e s ranqes f r o m 16-33 m and t h e i r t r e n d i s a c o n s i s t e n t 055'-235".
544
LEGEND
Fiq. 1. D i s t r i b u t i o n o f t h e Y e l l o w Sands from o u t c r o p and subsurface i n f o r m a t i o n . Data p o i n t s a r e n o t dense enorrqh t o isopach b u t a l l o w d e l i n e a t i o n o f sand r i d q e s . I n s e t map shows l o c a t i o n o f area. The o u t c r o p s show t h a t t h e v a r i a t i o n i n t h i c k n e s s o f t h e Sands r e c o r d s t h e topography o f t h e upper s u r f a c e o f t h e f o r m a t i o n , n o t r e l i e f on t h e Carboniferous s u r f a c e (Smith and F r a n c i s , 1967).
3.
SEDIMENTARY STRUCTURES Three q e n e t i c a l l y s i q n i f i c a n t u n i t s may be d i s t i n q u i s h e d w i t h i n t h e a e o l i a n
p a r t o f t h e Yellow Sands on t h e b a s i s o f sedimentary s t r u c t u r e s .
These a r e shown
i n q e n e r a l i s e d sequence i n F i q . 2. The l o w e s t u n i t c o n s i s t s o f f l a t - l y i n q o r q e n t l y d i p p i n q sane showinq a m i x t u r e o f c l imbinq t r a n s l a t e n t s t r a t i f i c a t i o n (Hunter, 1977), here termed wind-
54 5 AGE
Fiq. 2. L i t h o l o q i c a l sequence and i n t e r p r e t a t i o n o f t h e Y e l l o w Sands. sequence i s q e n e r a l i s e d and does n o t r e p r e s e n t any s p e c i f i c l o c a l i t y . r i p p l e l a m i n a t i o n , and sand-sheet l a m i n a t i o n ( F r y h e r q e r
pt
a1
., 1979).
The
The l o w e r
f l a t - l a m i n a t e d u n i t i s up t o a few metres t h i c k , h e i n q b e s t seen beside t h e R i v e r Clear a t N o r t h H y l t o n , near Sunderland ( F i q . 4).
Q m i l a r m a t e r i a l i s found a t t h e
base o f o t h e r exposures where t h e C a r b o n i f e r o u s s u r f a c e may reasonably he expected t o l i e n o t f a r helow qround.
It i s n o t p r e s e n t , however, a t Tvnemouth,
where cross-bedded Yellow Sands l i e d i r e c t l y on c a r b o n i f e r o u s s t r a t a . Most o f t h e f o r m a t i o n shows l a r q e s c a l e t r o u q h cross-heddinq i n s e t s t v p i c a l l y
4-6 m t h i c k a t t h e i r maximum and 40-60 m wide (Fiq. 3a). The t h i c k e s t s e t s noted reach 11 m. extensive, t h a n 80 m).
I n sections p a r a l l e l t o palaeocurrent the sets are l a t e r a l l y l a r q e r t h a n any i n d i v i d u a l a p p r o p r i a t e l y o r i e n t e d exposure ( i .e. more Two o r d e r s o f houndinq s u r f a c e a r e d i s p l a y e d , one d e f i n i n q t h e s e t s
and one i n t e r n a l t o them. Local u n i t s o f f l a t - l a m i n a t e d sand are found w i t h i n t h e cross-bedded i n t e r v a l h u t w i t h t h i s e x c e p t i o n t h e b u l k o f t h e f o r m a t i o n i s crossbedded throuqhout.
The cross-bedded sand c o n t a i n s ,
i n o r d e r o f abundance, wind-
r i p p l e , sandflow and q r a i n f a l l l a m i n a t i o n (Hunter, 1977; Kocurek
R
D o t t , 1981).
Sandflow laminae w i t h i n cross-bedded s e t s q e n e r a l l y reach 40 mm i n t h i c k n e s s , e x c e p t i o n a l l y up t o 80 mm. The uppermost u n i t o f t h e f o r m a t i o n c o n s i s t s o f f l a t - l y i n q sand showinq windr i p p l e and r a r e sand-sheet l a m i n a t i o n .
It l i e s below a s t r u c t u r e l e s s o r
b i o t u r b a t e d , s p o r a d i c a l l y f o s s i l i f e r o u s bed o f sand ( w i t h d i s a r t i c u l a t e d
54 b
SHERBURN HILL SAND PIT
3a
0
WEST FACE
(NZ 344417)
la_il West face
-
-4 Bowburn plan and locatioi
Sherburn Hill plan and l o c a t i o n
BOWBURN QUARRY (NY 327382)
7b
12m
0
0
Fiq. 3. Scale diaqrams o f p a r t s o f ( a ) Sherhurn H i 1 Sand P i t and ( h ) Rowhurn Quarry. ( a ) shows t r o u q h cross-hedded u n i t w i t h t h e face f r o m 0-50 m t r a n s v e r s e ( b ) shows the t o p a l a e o c u r r e n t and from 60-140 m s u b p a r a l l e l t o pa a e o c u r r e n t . upper f l a t - l a m i n a t e d u n i t a t t h e t o p o f t h e face. F qures ahove diaqrams i n d i c a t e t h e o r i e n t a t i o n o f t h e exposures. L i n q u l a ) , t y p i c a l 1 . y 0.3 m t h i c k , which forms t h e t o p o f t h e f o r m a t i o n , and on which t h e t r a n s q r e s s i v e Marl S l a t e r e s t s (Re11 e t al.,
1979). The upper f l a t -
l a m i n a t e d u n i t i s l a t e r a l l y e x t e n s i v e , h e i n q p r e s e n t a l o n q 70% o f t h e exposed l e n q t h o f t h e t o p o f t h e f o r m a t i o n , and ranqes up t o 3 m t h i c k .
I t i s most
a c c e s s i h l e a t Rowburn Q u a r r y ( F i q . 3h). Toqether t h e f l a t - l a m i n a t e d u n i t s make up some 10% o f t h e f o r m a t i o n , t h e r e m a i n i n q 90% h e i n q cross-bedded (e.q.
Sherhrrrn H i l l Sand P i t , F i q . 3a). A r o s e
diaqram o f measured cross-beddinq d i p azimuths shows a w e l l d e f i n e d , s.ymmetrica1, bimodal d i s t r i b u t i o n ( F i q . 5).
One mode i s d i r e c t e d S, t h e o t h e r hINW t o NW.
v e c t o r r e s u l t a n t i s d i r e c t e d t o i135O, p a r a l l e l t o t h e t r e n d o f t h e major sand r i d g e s shown i n F i q . 1.
The
547
t
N-
I
F i q . 4. P l a n a r - b e d d e d a e o l i a n sand r e s t i n q on C a r h o n i f e r o u s s a n d s t o n e s a t N o r t h H y l t o n , S u n d e r l a n d . Hammer (0.33 m l o n q ) r e s t s on plane o f unconformity. Dip i s structural.
4.
F i q . 5. C r o s s - h e d d i n q a z i m u t h p l o t f o r t h e Y e l l o w Sands. 239 measurements, c o m p i l e d f r o m a l l a v a i 1 ah1 e e x p o s u r e s .
INTERPEFTATIllN The a e o l i a n o r i q i n o f t h e Y e l l o w Sands i s u n a m h i q u o u s l y c o n f i r m e d b y t h e
d i a q n o s t i c a e o l i a n l a m i n a t i o n t h e y show t h r o u q h o u t . The d i s t r i b u t i o n o f t h e f o r m a t i o n i n r e q u l a r ,
parallel
r i d q e s i s a l s o an
aeolian feature - it i s not r e l a t e d t o t h e Zechstein transqression.
There i s no
e v i d e n c e f o r r e w o r k i n q o f any s u b s t a n t i a l q u a n t i t i e s o f sand b y t h e Z e c h s t e i n s e a ; t r a c e s o f e r o s i o n and r e d e p o s i t i o n b y w a t e r on and between t h e r i d q e s a r e c o n f i n e d t o t h e p o s s i b l e r e m o v a l o f some d u n e - s i z e d t o p o q r a p h y .
The p a r a l l e l i s m
of t h e r i d q e s w i t h t h e p a l a e o c u r r e n t r e s u l t a n t s t r o n q l y suqqests t h a t t h e y were shaped b y t h e same d e p o s i t i n q medium.
T h i s must he t h e w i n d ,
l a m i n a t i o n types w i t h i n t h e cross-heddinq.
Further,
qiven the
t h e occurrence o f t h e upper
f l a t - l a m i n a t e d zone, w i t h i t s a e o l i a n l a m i n a t i o n , shows t h a t t h e a q q r a d a t i o n o f t h e r i d q e s was l i m i t e d by an a e o l i a n p r o c e s s . Given t h i s ,
t h e r e q u l a r i t y o f t h e r i d q e s and t h e i r i n d e p e n d e n c e o f any
i r r e q u l a r i t i e s i n t h e u n d e r l y i n q Carhoniferous surface,
t h e Ye1 l o w Sands r i d q e s
m u s t be a e o l i a n b e d f o r m s . T h e i r s i z e and t h e c o n c o r d a n c e o f t h e i r o r i e n t a t i o n w i t h t h e p a l a e o c u r r e n t r e s u l t a n t ( w h i c h must be t h e n e t s a n d - d r i f t d i r e c t i o n ) i n d i c a t e s t h a t t h e r i d q e s a r e l o n q i t u d i n a l d r a a . Thus t h e r a p i d Z e c h s t e i n t r a n s q r e s s i o n h a s f o r t u i t o u s l y p r e s e r v e d hedforrns w h i c h w o u l d o t h e r w i s e have had an e x t r e m e l y l o w p r e s e r v a t i o n p o t e n t i a l m i g r a t e and c l i r n h t o p r o d u c e t h i c k ,
( o n l y t r a n s v e r s e a e o l i a n b e d f o r m s can
r e a d i l y p r e s e r v a h l e , h l a n k e t sands).
548
F i q . 6. I n t e r p r e t e d sequence of t h e Y e l l o w Sands r i d q e s . 1. I n i t i a t i o n fPom l o c a l i s e d sand p a t c h e s and m i o r a t i n q dunes. 2. A a s r a d a t i o n h y d e p o s i t i o n f r o m m i q r a t i n q dunes. 3. S t a h i l i t , y , wit.h reduced dune p o p u l a t i o n and s l o w d e p o s i t i o n from f l a t sand-sheet areas. N.R. Scale o f dunes r e l a t i v e t o t h e d r a a r i d q e s i s exaqqerated f o r c l a r i t y .
The sequence o f i n t e r n a l s t r u c t u r e s w i t h i n t h e Y e l l o w Sands r i d q e s t h e r e f o r e r e c o r d s t h e i n i t i a t i o n and q r o w t h o f l o n q i t u d i n a l d r a a . The t h r e e u n i t s i n t o w h i c h t h e f o r m a t i o n may he d i v i d e d r e c o r d t h e f o l l o w i n q sequence o f e v e n t s , i l l u s t r a t e d i n Fiq. 6 : ( 1 ) I n i t i a t i o n o f t h e r i d q e s b y t h e q r o w t h o f f l a t , d u n e l e s s sand p a t c h e s on t h e r o c k y d e s e r t f l o o r and t h e s l o w p i l i n q up o f sand b y s i n u o u s t r a n s v e r s e bedforms.
This resulted i n the lower flat-laminated unit.
(2) A q q r a d a t i o n of t h e r i d q e s h,y d e p o s i t i o n f r o m s i n u o u s t r a n s v e r s e dunes m i q r a t i n q and c l i m b i n q a l o n q t h e l e n q t h o f t h e d r a a . The b u l k o f t h e f o r m a t i o n a c c u m u l a t e d i n t h i s wa,y, d e v e l o p i n q t h e t r o u g h c r o s s - h e d d i n g .
( 3 ) C e s s a t i o n ( o r a c o n s i d e r a b l e r e d u c t i o n ) o f a q q r a d a t i o n and t h e d e v e l o p ment o f w i d e s p r e a d , g e n t l y r o l l i n q , d u n e l e s s s a n d - s h e e t s o v e r t h e r i d q e s . T h i s may i n d i c a t e t h a t t h e d r a a had reached e q u i l i b r i u m , w h o l l y sand-passinq,
w i t h t h e e r q becoming a l m o s t
r a t h e r than p a r t l y sand-trapping
as d u r i n q a q q r a d a t i o n .
The e r q was t h e n p a s s i v e l y , b u t r a p i d l . y , i n u n d a t e d by t h e Z e c h s t e i n Sea, t o a d e p t h b e l o w storm-wave base, and t h e t o p s u r f a c e o f t h e r i d q e s was c o l o n i s e d f o r a number o f ,years b y a b i o t u r b a t i n q i n f a u n a . beqan and t h e b a s i n became a n o x i c .
E v e n t u a l l y Marl S l a t e d e p o s i t i o n
54 9 6.
CONCLUSIONS T,he'Y*el l o w Sands e x p l i c i t l y and unamhiquously r e c o r d t h e e x i s t e n c e o f t h r e e
d i f f e r e n t s i z e s o f superimposed a e o l i a n hedforms i n t h e s t r a t i q r a p h i c record. R i p p l e s c o n t r i h u t e d t o t h e l a m i n a t i o n , and dunes d e p o s i t e d t h e cross-beddinq.
The
c r o s s - h e d d i o s e v i d e n t l y b u i l t up t h e l a r g e r hedforms, t h e r i d q e s , and these may be c o n v e n i e n t l y l a b e l l e d l o n q i t u d i n a l draa. Whether i t i s accepted o r denied t h a t dunes and draa a r e d i f f e r e n t ' o r d e r s ' o f a e o l i a n hedform, they were e v i d e n t l y superimposed d u r i n q t h e d e p o s i t i o n o f t h e Y e l l o w Sands. Cross-beddinq developed hv t r a n s v e r s e hedforms preserves o n l y a f r a c t i o n o f t h e o r i q i n a l h e i q h t o f t h e bedforms (Ruhin & Hunter, 1982) and thus complete t r a n s v e r s e dunes and draa a r e u n l i k e l y t o he found i n t h e s t r a t i g r a p h i c r e c o r d . The near-complete w e s e r v a t i o n deduced f o r t h e Y e l l o w Sands draa i s t h u s unusual and i s owed t o t h e r a p i d i t y o f t h e Z e c h s t e i n t r a n s q r e s s i o n (Smith, 1970, 1979). The q u a r r i e s i n t h e Y e l l o w Sands must o f f e r some o f t h e best s e c t i o n s i n t h e w o r l d t h r o u q h complete, v e r y l a r q e , a e o l i a n hedforms. 6.
ACKNOWLEDGEMENTS The work recounted i n t h i s paper forms p a r t o f t h e a u t h o r ' s Ph.D.
t h e s i s and
was undertaken a t t h e llepartment o f G e o l o q i c a l Sciences i n t h e l l n i v e r s i t y o f Durham.
It was funded by t h e N a t u r a l Environment Research Council.
Ithank the
Department o f G e o l o q i c a l Sciences a t Durham, and e s p e c i a l l y my s u p e r v i s o r
D r . A. P. Heward. The paper was w r i t t e n under t h e c a j o l i n q o f Gary Kocurek, John O'Learv and o t h e r s , and has h e n e f i t e d from d i s c u s s i o n w i t h Kocurek, Ralph Hunter and David Rubin. It i s p u b l i s h e d w i t h t h e p e r m i s s i o n and a s s i s t a n c e o f The B r i t i s h P e t r o l e u m Company p.1 .c. REFERENCES B e l l , J., Holden, J., P e t t i q r e w , T.H. and Sedman, K.W., 1979. The Marl S l a t e and Basal Permain R r e c c i a a t M i d d r i d q e , Co. Durham. Proc. Yorks. Geol. SOC., 42, 439-460. D a l q l i s h , J. and F o r s t e r , G.B., 1864. On t h e Maqnesian Limestone o f Durham. Trans. N. o f Fnqland I n s t n . Min. Enq., 13: 205-213. F r v b e r q e r , S.G., Ahlbrandt, T.S. and Andrews, S., 1979. O r i q i n , sedimentary f e a t u r e s , and s i q n i f i c a n c e o f low-anqle e o l i a n "sand-sheet" d e p o s i t s , Great Sand Dunes N a t i o n a l Monument and v i c i n i t y , Colorado. J. Sedim. Petrol., 49: 733-746. Glennie, K.W., 1970. D e s e r t Sedimentary Environments. Developments i n Sedimentoloqy, No. 14. E l s e v i e r , Amsterdam, 222 pp. Glennie, K.W., 1972. Permian R o t l i e q e n d e s o f N.W. Europe i n t e r p r e t e d i n t h e 1 ght of modern d e s e r t s e d i m e n t a t i o n s t u d i e s . R u l l . Am. Ass. P e t r o l . Geol., 56: 1048-1071. Hodqe, 1931. The Permian Y e l l o w Sands o f North-East Enqland. Proc. Univ. Durh P h i l . SOC., 8, 410-458. Hunter, R.E., 1977. B a s i c t y p e s o f s t r a t i f i c a t i o n i n small e o l i a n dunes. Sedimentoloqy, 24: 361-387.
550
J r . , 1981. D i s t i n c t i o n and uses o f s t r a t i f i c a t i o n Kocurek, G. and D o t t , R.H., types i n t h e i n t e r p r e t a t i o n o f e o l i a n sand. J. Sedim. Petrol., 51: 579-595. Pryor, W.A., 1971. P e t r o l o q y o f the Permian Yellow Sands o f N.E. Enqland and t h e i r North Sea Rasin equivalents. Sedim. Geol., 6: 221-254. 1982. Bedform c l i m b i n q i n t h e o r y and nature. Rubin, D.M. and Hunter, R.E., Sedimentoloqy, 29: 121-138. Smith, D.B., 1970. The Palaeoqeoqraphy o f t h e B r i t i s h Zechstein. In: J.L. Rau and L.F. D e l l w i q ( E d i t o r s ) , T h i r d Symposium on S a l t , 1, N. Ohio Geol. SOC. Cleveland, pp. 20-23. Smith, D.B., 1979. Rapid marine t r a n s q r e s s i o n o f t h e Upper Permian Zechstein Sea. J. Geol. SOC., 136: 155-156. 1967. Geoloqy o f t h e c o u n t r y between Durham and Smith, D.B. and Francis, E.A., 27. West H a r t l e p o o l . Mem. Geol. Surv. G.B., Manninq, P.I., Simpson, S. and Shotton, F.W., Smith, D.B., Brunstrom, R.G.W., 1974. A c o r r e l a t i o n o f Permian rocks i n t h e B r i t i s h I s l e s , Geol. SOC. London Special Report No. 5. 45 pp. Z i e q l e r , P.A., 1981. E v o l u t i o n o f sedimentary basins i n N.W. Europe. In: L.V. I l l i n q and G.D. Hobson ( E d i t o r s ) , Petroleum Geoloqy o f t h e C o n t i n e n t a l She1 o f N.W. Europe. I n s t i t u t e o f Petroleum, London.
551
PERMIAN SHORELINE EOLIAN COMPLEX I N CENTRAL ARIZONA:
TO CYCLIC SEALEVEL RONALD C.
DUNE CHANGES I N RESPONSE
CHANGES
BLAKEY and
LARRY T.
MIDDLETON,
Arizona U n i v e r s i t y , F l a g s t a f f , Arizona
Department
of
Geology,
Northern
86011
INTRODUCTION I n the
last
f i v e years
the
identification
of
ancient
e o l i a n sandstone
sequences has gone from s p e c u l a t i o n and g e n e r a l i z a t i o n t o d e f i n i t i v e i n t e r p r e tation.
Due t o t h e work o f Hunter (1977, 1980), McKee (1979a,b),
Kocurek and
D o t t (1981), Kocurek (1981a, 1981b), and o t h e r s , t h e i d e n t i f i c a t i o n o f a n c i e n t sequences as e o l i a n seems assured i n imany cases. available t o aid i n identification, the large-scale
With d e f i n i t i v e c r i t e r i a
i t i s possible t o i d e n t i f y the nature o f
c o n t r o l s t h a t produced t h e e o l i a n sandstone body.
Controls
such as c l i m a t e , base l e v e l , sand supply, and t e c t o n i c s , which a c t e d t o produce t h e geometry and i n t e r n a l s t r a t i f i c a t i o n o f e o l i a n sand bodies, can be i d e n t i f i e d and i n some cases q u a n t i f i e d .
The purpose o f t h i s s t u d y i s t o r e l a t e t h e
c h a r a c t e r i s t i c s o f an a n c i e n t e o l i a n compl ex and a s s o c i a t e d marine shore1 ine deposits t o these controls.
A l o c a l l y v e r y w e l l exposed Permian sequence was
chosen f o r s t u d y because o f excel l e n t t h r e e - d i m e n s i o n a l exposures and presence of
t h i n wide-spread
m a r i n e carbonates
useful
for
correlation
i n otherwise
unfossiliferous strata. The s e c t i o n s t u d i e d i n c l u d e s t h e upper t h i r d o f t h e Schnebly H i l l Formation and base o f t h e o v e r l y i n g Coconino Sandstone ( f i g . 1 ) .
The base o f t h e s e c t i o n
i s a r e g i o n a l l i m e s t o n e marker bed, t h e F o r t Apache Member o f t h e Schnebly H i l l Formation o f e a r l y Leonardian age (Blakey, 1980). Rim,
an
Colorado P l a t e a u ( f i g .
2).
central
Mogollon
escarpment
that
The area o f study i s i n t h e
forms t h e s o u t h e r n edge o f t h e
D e t a i l e d work was performed near Sedona, Arizona
and less d e t a i l e d work was done t o t h e southeast a l o n g t h e Mogollon Rim. sequence comprises a p p r o x i m a t e l y 100 m o f c r o s s - s t r a t i f i e d ,
The
ripple-laminated,
and wavy bedded v e r y f i n e - t o medium-grained q u a r t z a r e n i t e t o s u b f e l d s p a t h i c arenite.
The sequence d i s p l a y s a s t r o n g l y c y c l i c c h a r a c t e r w i t h r e p e t i t i o n
d i s t i n c t i v e s t r a t i f i c a t i o n and bedding types.
The v e r t i c a l
c y c l i c nature
accompanied by a m a j o r l a t e r a l f a c i e s change from n o r t h w e s t t o southeast.
Of i S
The
sequence formed i n d o m i n a n t l y e o l i a n d e p o s i t i o n a l systems i n t h e Sedona area and p r e d o m i n a n t l y s h a l l o w marine and sabkha systems t o t h e southeast where abundant sandy mudstone, 1980).
l i m e s t o n e , d o l o m i t e , and gypsum a r e p r e s e n t (Blakey,
552
KAIBAB L S
PERMIAN ... .. .. .... .. .,., . ... . ,, ,
SUPAl GROUP
lm
'EN NSY L V AN IAN
REDWALL LE MISSISSIPPIAN
F i g u r e 1. Columnar s e c t i o n of u p p e r P a l e o z o i c s t r a t a i n s t u d y a r e a showing i n t e r v a l of s t u d y ( i n b r a c k e t ) .
N
F i g u r e 2. I n d e x maps. ( a ) C e n t r a l A r i z o n a showing o u t c r o p o f i n t e r v a l o f study. ( b ) Sedona a r e a showing o u t c r o p of i n t e r v a l o f s t u d y and l o c a t i o n of B e l l Rock and West F o r k s e c t i o n s . ( c ) A r i z o n a showing l o c a t i o n o f maps a, b.
553 Tectonic S e t t i n g The area o f s t u d y l i e s on t h e e a s t f l a n k o f t h e Sedona Arch between two proininant t e c t o n i c f e a t u r e s , Basin ( f i g .
3).
t h e Grand Canyon-Emery P l a t f o r m and t h e Hol brook
The Holbrook Basin was a c t i v e d u r i n g o n l y Schnebly H i l l and
l o w e r Coconino d e p o s i t i o n as i t c o n t a i n s two t o t h r e e t i m e s t h e t h i c k n e s s o f l o w e r Leonardian rocks as s u r r o u n d i n g areas
(Blakey,
1980).
The Sedona Arch
forms t h e western edge o f t h e Schnebly H i l l - D e C h e l l y Sandstone complex.
The
Schnebly H i l l Formation t h i c k e n s across t h e southeast edge o f t h e Sedona Arch i n t o t h e Holbrook Basin and across t h e Mogollon S h e l f . i n t h e Grand Canyon,
West o f t h e Sedona Arch
t h e Schnebly H i l l Formation i s absent;
temporal c o r r e l a -
t i v e s t r a t a may be r e p r e s e n t e d by t h e l o w e r s e v e r a l meters o f Sandstone.
Across t h e a r c h ,
l e s s t h a n 100 km.
t h e Coconino
l o w e r Leonardian s t r a t a t h i c k e n t o over 100 m i n
D u r i n g t h i s t i m e i n t e r v a l subsidence r a t e was much g r e a t e r
t o t h e southeast o f t h e Sedona Arch t h a n i t was t o t h e n o r t h w e s t as evidenced by t h e t h i c k e r isopachs t o t h e southeast ( f i g . 3 ) .
A
6
\ Facies Migration
Maps showing ( a ) t e c t o n i c f e a t u r e s and ( b ) d i s t r i b u t i o n o f F i g u r e 3. environment across A r i z o n a and v i c i n i t y d u r i n g d e p o s i t i o n o f upper Schnebly H i l l Formation and l o w e r Coconino Sandstone. Deposits are t h i n , less than s e v e r a l t e n s o f meters t h i c k , west o f Sedona Arch, b u t t h i c k e n t o over 250 m i n Holbrook Basin.
554 D u r i n g Leonardian t i m e , f a c i e s p a t t e r n s on t h e Colorado P l a t e a u and v i c i n i t y were c o n t r o l l e d by t e c t o n i c elements ( f i g . f a c i e s changes on t h e e a s t
3).
The e o l i a n aspects o f sharp
s i d e o f t h e Sedona Arch and t h e i r response t o
e x t e r n a l c o n t r o l s a r e t h e s u b j e c t o f t h i s paper. Source o f Sand The Coconino e r g i s p a r t o f t h e " g r e a t sand p i l e " o f l a t e P a l e o z o i c and Mesozoic age on t h e Colorado Plateau. study ( f i g .
Paleowind d i r e c t i o n s gathered f o r t h i s
4) and by Reiche (1938) and McKee (1979b) i n d i c a t e r a t h e r c o n s i s Facies r e l a t i o n s h i p s show t h a t t h e sand was
t e n t paleowinds from t h e n o r t h .
f e d i n t o t h e area o f study from t h e n o r t h and n o r t h w e s t (Blakey,
1980).
The
m a t u r i t y o f t h e sandstone suggests t h a t much o f i t was r e c y c l e d from o l d e r
N
N
I
N
I
N
I
'E F i g u r e 4. P a l e o c u r r e n t r o s e diagrams. ( a ) P o l a r - d i p diagram o f sand-flow s t r a t a f a c i e s TWC. ( b ) P o l a r - d i p diagram o f i n t r a s e t s t r a t a f a c i e s IC. Open d o t s r e p r e s e n t E-2 surfaces. ( c ) Rose diagram o f trough-axes dips, f a c i e s TCT. ( d ) Rose diagram o f f o r e s e t d i p s o f c l i m b i n g t r a n s l a t e n t s t r a t a , f a c i e s TCT. ( e ) Foreset d i p o f p o s s i b l e marine s t r a t a , f a c i e s CC. ( f ) Summary rose diagram f o r a l l e o l i a n sand-flow f o r e s e t s and t r o u g h axes ; r i p p l e f o r e s e t s n o t in c l uded.
555 Pennsylvanian and Permian r o c k s t o t h e n o r t h i n Utah,
Wyoming,
and Montana.
Regional f a c i e s p a t t e r n s ( f i g . 3 ) suggest t h a t sand was t r a n s p o r t e d a l o n g t h e e a s t e r n margin o f t h e C o r d i l l e r a n seaway t o t h e southwest and t h e n blown i n l a n d across t h e Grand Canyon-Emery P l a t f o r m by winds from t h e n o r t h (Blakey, 1980). The e r g m i g r a t e d southward and southeastward across t h e Sedona Arch.
Here i t
encountered t h e c o a s t a l complex a s s o c i a t e d w i t h t h e Mogol l o n S h e l f and Hol brook Basin.
FACIES A N A L Y S I S Trough-Cross-Stratified ( i ) Description.
T r a n s l a t e n t F a c i e s (TCT)
The t r o u g h - c r o s s - s t r a t i f i e d
t r a n s l a t e n t f a c i e s (TCT)
comprises trough-shaped s e t s f i l l e d w i t h c l i m b i n g t r a n s l a t e n t s t r a t a ( f i g .
5).
The g r a i n s a r e m o s t l y v e r y f i n e t o f i n e g r a i n e d and m o d e r a t e l y w e l l sorted. The sandstone i s r e d d i s h orange t o moderate r e d d i s h brown. s e t s a r e 3-10
m wide and up t o 1.5
determined by a x i a l (fig.
4c).
plunge d i r e c t i o n
m thick (fig.
6).
i s bimodal-bipolar
The trough-shaped The t r o u g h axes as southwest-northeast
The l a r g e s t t r o u g h s a r e c o s e t s t h a t c o n t a i n s m a l l e r b u t s i m i l a r l y
shaped t r o u g h s t h u s c r e a t i n g a h e i r a r c h y o f s e v e r a l s i z e s o f t r o u g h s ( f i g . Surfaces t h a t bound t h e s e t s , r e g a r d l e s s o f s i z e , a r e c l e a r l y e r o s i o n a l . t r u n c a t e o t h e r trough-shaped
5). They
s u r f a c e s as w e l l as enclosed laminae and do n o t
r e p r e s e n t f o r e s e t s o f m i g r a t i n g bedforms.
The laminae t h a t f i l l and g e n e r a l l y
conform t o t h e t r o u g h shape a r e c l i m b i n g t r a n s l a t e n t s t r a t a o f Hunter (1977).
Climbing Translatent S t =Grain
Flow-Grain Fall
F i g u r e 5. Transverse and 1o n g i t u d i n a l s e c t i o n drawings o f f a c i e s TCT. Arrows show wind d i r e c t i o n as shown by f o r e s e t laminae o f c l i m b i n g t r a n s l a t e n t s t r a t a (towards southwest).
556
F i g u r e 6. C h a r a c t e r i s t i c s o f f a c i e s TCT. ( a ) Eroded t r o u g h s u r f a c e s ; w i d t h of v i e w a p p r o x i m a t e l y 2 rn. ( b ) Troughs i n t r a n s v e r s e v i e w on c l i f f w a l l ; f i g u r e f o r scale. ( c ) Oblique view o f c l i m b i n g t r a n s l a t e n t s t r a t a w i t h u n u s u a l l y w e l l preserved w i n d - r i p p l e f o r e s e t s ; wind blew from r i g h t t o l e f t ( t o w a r d s s o u t h w e s t ) ; l e n s cap 54 mm. ( d ) Longitudinal view o f climbing t r a n s l a t e n t s t r a t a showing t r u e t h i c k n e s s o f l a m i n a e . A l l p h o t o s a t West Fork.
These s t r a t a a r e exceed n g l y even and u n i f o r m t h r o u g h o u t t h e f a c i e s and d i s p l a y o c c a s i o n a l t o abundant r i p p l e f o r e s e t 1arni nae ( f i g . upwards and r a n g e f r o m 2-5 mm t h i c k . were examined i n t h e f a c i e s ,
6).
Most 1ami nae c o a r s e n
Several hundred r i p p l e f o r e s e t
laminae
and a l t h o u g h d i f f i c u l t t o measure e x a c t d i r e c t i o n
o f d i p , a l l c l e a r l y show a g e n e r a l s o u t h t o west d i p d i r e c t i o n . Not
all
of
the
strata
i n the
strata.
A
TCT
facies
are trough
climbing
translatent
laminae,
t h e forrner s u g g e s t i n g t h a t a n g l e - o f - r e p o s e
few beds d i s p l a y
shaped o r
sand-flow
and
contain
grain-fall
sedimentation took place.
Small cone-shaped sand f l o w d e p o s i t s a r e i n t e r b e d d e d w i t h c l i m b i n g t r a n s l a t e n t s t r a t a i n some s e t s ( f i g .
6).
M i n o r amounts o f s l u m p i n g and s m a l l - s c a l e s o f t -
557 sediment f a u l t i n g a l s o occur w i t h i n t h e f a c i e s . o b s e r v e d by McKee, e t a l .
S i m i l a r f e a t u r e s have been
(1971) i n modern dunes.
The t r o u g h - c r o s s - s t r a t i f i e d
sandstone comprises cosets which form t a b u l a r -
shaped beds up t o 6 m t h i c k .
The f a c i e s i s most abundant i n t h e l o w e r p a r t o f
t h e i n t e r v a l o f s t u d y , e s p e c i a l l y w i t h i n a few t e n s o f i n e t e r s o f t h e u n d e r l y i n g F o r t Apache Member ( f i g . 7 ) . (ii)
Interpretation.
TCT i s d o m i n a t e d by c l i m b i n g t r a n s l a t e n t
Facies
Sharp ( 1 9 6 6 ) , H u n t e r (1977,
strata.
1981),
and Kocurek and l l o t t ( 1 9 8 1 ) have
documented t h e w i n d r i p p l e o r i g i n f o r t h e s t r a t a .
The m a j o r c h a r a c t e r i s t i c s ,
t l i i n e v e n l y spaced l a m i n a e o f equal t h i c k n e s s , c o a r s e n i n g - u p w a r d s l a m i n a e , and rare
ripple
yeometry
foreset
and
laminae
distribution
are
of
e x t e n s i v e l y developed
cli-nbing
translatent
i n the facies.
strata
d i f f e r e n t t h a n t h a t r e p o r t e d i n modern o r a n c i e n t dunes.
in
facies
The TCT
is
There a r e t h r e e ways
i n w h i c h t r o u g h s o f t h e s i z e i n q u e s t i o n can f o r m i n e o l i a n d e p o s i t s .
1) I n
s t r o n g l y l u n a t e dunes c a l l e d a k l e - ( B r o o k f i e l d , 1977) o r o t h e r s i n u o u s - c r e s t e d t r a n s v e r s e dunes; 2 ) as m i g r a t i n g s c o u r p i t s g e n e r a l l y i n t h e l e e of l a r g e r dunes ( D a v i d R u b i n , p e r s . comm.); and 3 ) as d e p r e s s i o n s a s s o c i a t e d w i t h b l o w o u t dunes;
(Mckee,
1979a;
Ah1 b r a n d t and F r y b e r g e r ,
1980).
Migrating s l i p -
f a c e s on a k l e - dunes as d e s c r i b e d by B r o o k f i e l d w o u l d a p p a r e n t l y p r o d u c e t r o u g h cross s t r a t i f i c a t i o n .
C l e a r l y t h i s i s n o t t h e case w i t h f a c i e s TCT as s l i p -
f a c e d e p o s i t s a r e uncommon.
M i g r a t i n g scour p i t s ,
f e a t u r e s c a r v e d by v o r t e x
e d d i e s a s s o c i a t e d w i t h s i n u o u s - c r e s t e d b e d f o r m s , l e a v e a r a t h e r complex t h r e e d i i i i e n s i o n a l s w a l e and s w e l l t o p o g r a p h y r a t h e r t h a n t h e r e g u l a r e l o n g a t e p a t t e r n we o b s e r v e ( D a v i d R u b i n , p e r s . comm.). From t h e dunes,
relatively
few d e s c r i p t i o n s
smooth s c o u r e d d e p r e s s i o n s
sedimentary
structures
(McKee,
and
discussions
o f inodern b l o w - o u t
f i l l e d w i t h t r o u g h s t r a t a a r e t h e expected
1979a;
Ah1 b r a n d t
and
Fryberger,
1980).
The
s t r a t a d i p a t r e l a t i v e l y l o w a n g l e s due t o t h e i n i n o r o c c u r r e n c e o f s l i p - f a c e deposits complex
and
may
wind
Browns
Park
blow-out
than
processes.
varying
orientation
(Ahlbrandt
of
iaLisS
TC:
direction, 1980).
representing
I n t h e Miocene
t r o u g h axes formed by p r o b a b l e
a r e n o t i n phase w i t h i m m e d i a t e l y u n d e r l y i n g a v a l a n c h e - f o r m e d p e r s . comm.,
those
of
1983).
the
Schnebly
Mean t r o u g h - a x e s
D i p a n g l e s o f f a c i e s TCT a r e c o n s i d e r a b l y Hill
and
i s e'Bx,;:tl-
wituh
Coconino
formed
by
avalanche
d i p d i r e c t i o n o f f a c i e s TCT v a r i e s by a b o u t 60"
from s l i p faces o f t h e avalanche d e p o s i t s ( f i g . E . _ .
dip
and F r y b e r g e r ,
Formation o f northwest Colorado,
dunes
d i p s (Ahlbrandt, lower
show
distribution
rjlther
4).
The p a t t e r n o f t r o u g h s i n
s t r a i g h t t r o u g h axes.
The u p p e r ( u p w i n d )
p a r t o f t h e a x i s d i p s more s t e e p l y ( r a r e l y up t o 22") whereas t h e r e m a i n d e r o f -L _I I ~ ak-,\ L,'; 3=-,L.',., '$y--?-lJ1,: 1 0 - c . ~ +.h?n and t v o i c a l l y i s n e a r l y f l a t f a r t h e r down t h e t r o u g h .
T h i s i s t r u e o f b o t h t h e s o u t h w e s t - and n o r t h e a s t -
BELL ROCK
WEST FORK KEY
strata type
INTRASET H GF/SF
LAMIlATED DOLOl’ITE
facies
IC
F T . APACHE llBR
Apache Member
F i g u r e 7. D e t a i l e d columnar s e c t i o n s a t West Fork and B e l l Rock. Facies WBR shown on column where t o o t h i n t o show on drawing. F a c i e s TCT and CC a r e differentiated for clarity.
559 t r e n d i n g troughs.
T h i s suggests t h a t t h e scours were carved by bimodal winds,
p o s s i b l y onshore dnd o f f s h o r e winds,
and t h a t t h e c l i m b i n g t r a n s l a t e n t s t r a t a
t h a t f i l l t h e t r o u g h s were produced by n o r t h e a s t e r l y ( o f f s h o r e ) breezes. Intraset-Cross-Stratified ( i ) Description. by
thick
(up
to
Facies (IC)
The i n t r a s e t - c r o s s - s t r a t i f i e d
12
sedimentary s t r u c t u r e .
m)
tabular
bodies
with
f a c i e s ( I C ) i s dominated
extremely
ranges from p a l e r e d d i s h orange t o p a l e g r a y i s h orange. bound t h e i n t r a s e t s
are subparallel
t a b u l a r s e t s up t o 0.3 (figs.
4b,
8).
complex
internal
The sand i s c h i e f l y f i n e g r a i n e d and w e l l s o r t e d and
m thick;
erosional
The s u r f a c e s which
planes which d e f i n e i n c l i n e d
t h e s u r f a c e s d i p south-southwest a t 18-22'
The i n c l i n e d e r o s i o n s u r f a c e s , p r o b a b l y second-order bounding
s u r f a c e s o f B r o o k f i e l d (1977), become i n d i s c e r n i b l e near t h e base o f t h e coset and d i s a p p e a r between c l i m b i n g t r a n s l a t e n t s t r a t a ( f i g .
9b).
They a l s o become
i n d i s t i n c t between a n g l e - o f - r e p o s e laminae h i g h e r up t h e f a c e o f t h e f o r e s e t s (fig.
8).
Set
thicknesses,
as
defined
s u r f a c e s , a r e remarkably c o n s t a n t ( f i g .
by
distance
between
second-order
9).
-
0 1 2 3 M Climbing Translatent S t r a t a Ripple Laminae =Grain Flow-Grain Fall (dashed in intraset) F i g u r e 8. Transverse and l o n g i t u d i n a l s e c t i o n drawings o f f a c i e s IC. Arrows show wind d i r e c t i o n as i n d i c a t e d by f o r e s e t d i p d i r e c t i o n s (towards south). R i p p l e laminae formed by eol ian and subaqueous ri p p l es.
560
C h a r a c t e r i s t i c s o f f a c i e s IC. ( a ) Complex i n t r a s e t s a t B e l l Rock; F i g u r e 9. a p p r o x i m a t e l y 4 m o f s e c t i o n shown. ( b ) L o n g i t u d i n a l v i e w a t West F o r k ; a p p r o x i m a t e l y 2 in o f s e c t i o n shown. ( c ) O b l i q u e v i e w a t same l o c a t i o n as b ; 4 - 5 m o f s e c t i o n shown. ( d ) T r a n s v e r s e v i e w a t West Fork. Major second-order e r o s i o n s u r f a c e s d i p t o w a r d camera ( s o u t h ) a t 18-22". I n t r a s e t s d i p south-southwest. Notebook i s 17 cm t a l l .
5 The i n t r a s e t l a m i n a e between s e c o n d - o r d e r e r o s i o n s u r f a c e s d i p t o t h e s o u t h o r t o t h e west a t 16-28'
( c a l c u l a t e d inean d i p = 22.40)
a s s o r t m e n t s o f sand f l o w ,
grain f a l l ,
l a t t e r d o m i n a t e t h e base o f t h e f o r e s e t s Abundant within
reactivation the
intraset
surfaces laminae.
and
and c o m p r i s e complex
and c l i m b i n g t r a n s l a t e n t
The
where d i p s become n e a r l y h o r i z o n t a l .
ininor
These
strata.
trough-shaped
form t h i r d - o r d e r
scours bounding
are
present
surfaces
of
ripples
on
B r o o k f i e l d (1977). Minor bedding tracks
sedimentary planes
and
with
trails,
structures crests
include
oriented
pull-apart
parallel
laminae
l a m i n a e due t o c o m p r e s s i o n a l f o r c e s .
raindrop to
prints, foreset
due t o t e n s i o n a l
wind dip, forces,
unidentified and
folded
The f a c i e s f o r m s a t a b u l a r l i t h o s o m e up
561 t o 12 m t h i c k b u t t y p i c a l l y
i s much t h i n n e r due t o t r u n c a t i o n by o v e r l y i n g
strata.
be i n t e r b e d d e d o r
The
facies
I C may
gradational
with
f a c i e s TCT
p r o d u c i n g a complex s u b f a c i e s o f t r o u g h and i n t r a s e t c r o s s s t r a t i f i c a t i o n . ( i f ) Interpretation.
F a c i e s I C was d e p o s i t e d by l a r g e s o u t h w a r d - m i g r a t i n g
compound t o complex ( t e r m s o f McKee, 1979a) e o l i a n dunes. was
likely
formed
by
B r o o k f i e l d (1977). angle-of-repose
dunes
migrating
over
The u p p e r f o r e s e t s of
a
large
The i n t r a s e t p a t t e r n draa
as
described
by
t h e d r a a were l a r g e l y t h e s i t e o f
g r a i n f a l l and sand f l o w d e p o s i t i o n b u t t h e l o w e r reaches were
o c c u p i e d by dunes t h a t m i g r a t e d o b l i q u e t o t h e m a j o r s l i p f a c e o f t h e draa.
A t t h e base
Numerous examples o f dunes on d r a a s a r e p r o v i d e d by W i l s o n (1973). of
t h e dune,
w i n d r i p p l e s m i g r a t i n g a c r o s s t h e base o f t a n g e n t i a l s l i p f a c e s
p r o d u c e d c l i m b i n g t r a n s l a t e n t s t r a t a w h i c h g r a d e i n t o and wedge o u t between t h e more s t e e p l y d i p p i n g sand f l o w d e p o s i t s ( f i g .
9b,c),
a f e a t u r e common on many
modern dunes ( H u n t e r , 1977). P r o c e s s e s o p e r a t i n g on t h e s l i p f a c e of t h e l a r g e d r a a must have been v e r y complex.
As v e r y l i t t l e i s known a b o u t t h e i n t e r n a l
d i r e c t comparison i s p o s s i b l e . internal Fryberger, the
top
structure
is
1980; W i l s o n , of
the
very
intricate
1971, 1973).
thicker
(McKee,
1979a,
sequences
1977; McKee,
climbing translatent
1982;
Ahlbrandt
document 1979a).
sedimentation
ripples.
W i t h renewed a v a l a n c h i n g , Large-scale
by
s t r a t a commonly o v e r 1 i e s s e c o n d - o r d e r
s u g g e s t s t h a t between p e r i o d s o f a v a l a n c h i n g , wind
and
Where p r e s e n t h i g h e r
e r o s i o n s u r f a c e s and i s o v e r l a i n by g r a i n f a l l and sand f l o w d e p o s i t s . dune s l i p face.
no
The sand f l o w and g r a i n f a l l l a m i n a e a t
preserved
avalanching o r suspension (Hunter, on t h e i n t r a s e t s ,
s t r u c t u r e o f draas,
What l i t t l e work has been done i n d i c a t e s t h a t
reactivation
angle-of-repose or
This
r i p p l e s c l i m b e d w e l l up o n t o t h e climbing o f
strata buried the superimposed
dunes
p r o b a b l y formed t h e s e c o n d - o r d e r s u r f a c e and t h e c y c l i c sequence was repeated. The dunes w h i c h d e p o s i t e d t h e i n t r a s e t l a m i n a e moved a c r o s s ,
down,
and l o c a l l y
u p t h e f a c e o f t h e draa.
Tabu1 a r - and W e d g e - C r o s s - S t r a t i f i e d ( i ) Description.
The t a b u l a r -
F a c i e s (TWC)
and wedge-cross
s t r a t i f i e d facies
c o n t a i n s t h e l a r g e s t s e t s i n t h e S c h n e b l y H i l l and Coconino sequence.
(TWC)
The sand
i s f i n e t o medium g r a i n e d and i s v e r y w e l l s o r t e d .
Color ranges from y e l l o w i s h
o r a n g e t o p a l e g r a y i s h orange t o y e l l o w i s h g r a y .
The f a c i e s i s d o m i n a t e d by
angle-of-repose laminae ( f i g .
f o r e s e t s (mean d i p = 22.2") 10).
c o m p r i s i n g g r a i n f a l l and sand f l o w
The f o r m e r appear as e x t r e m e l y t h i n l a m i n a e between sand
f l o w l a m i n a e w h i c h r a n g e up t o s e v e r a l c e n t i m e t e r s t h i c k . and r e p t i l e t r a c k s a r e found on f o r e s e t s u r f a c e s .
Rare w i n d r i p p l e s
Slump s t r u c t u r e s o f v a r y i n g
s c a l e s , many c o m p a r a b l e t o t h o s e d e s c r i b e d by Mckee e t a l . ,
(1971) and f o u n d i n
562
-
Climbing Translatent S t r a t a Grain Flow Grain Fall
0
10 2 0 M
F i g u r e 10. T r a n s v e r s e and 1 o n g i t u d i n a l s e c t i o n d r a w i n g s o f f a c i e s TWC. Arrows show w i n d d i r e c t i o n as i n d i c a t e d b y d i p s o f s a n d - f l o w s t r a t a ( c h i e f l y towards south).
the
Coconino
of
Grand
throughout t h e facies.
Canyon
(McKee,
1979a)
are
sporadically
distributed
C l i m b i n g t r a n s l a t e n t s t r a t a f o r m a s m a l l component o f
f a c i e s TWC and a r e c h i e f l y c o n f i n e d t o t h e base o f t h e f o r e s e t s
(fig.
11).
Toes o f sand f l o w c r o s s s t r a t a commonly i n t e r f i n g e r w i t h c l i m h i n g t r a n s l a t e n t strata. The c r o s s s t r a t a a r e g r o u p e d i n t o wedge- and t a b u l a r - s h a p e d s e t s t h a t range up t o 10 m i n t h i c k n e s s ( f i g . spond t o f i r s t - o r d e r shaped s e t s .
11).
Subparallel continuous planes t h a t corre-
b o u n d i n g s u r f a c e s o f B r o o k f i e l d (1977) e n c l o s e t h e wedge-
The e r o s i o n p l a n e s t h a t
f o r m wedge-shaped
cosets d i p a t low
a n g l e s and up t o 20" down w i n d and c o r r e s p o n d t o s e c o n d - o r d e r b o u n d i n g s u r f a c e s o f B r o o k f i e l d (1977). wedge-s haped c o s e t s.
Third-order
b o u n d a r y s u r f a c e s a r e p r e s e n t w i t h i n some
Some compl ex wedge-s haped c o s e t s d i s p l a y in t r a s e t c r o s s
s t r a t i f i c a t i o n similar t o that i n facies
IC.
Trough-shaped s e t s a r e p r e s e n t
t h o u g h n o t common i n f a c i e s TWC and may be 30 m i n w i d t h . Most
foreset
l a m i n a e and many s e c o n d - o r d e r
s o u t h i n f a c i e s TWC ( f i g .
4).
The l a r g e ,
bounding surfaces
southward-dipping
n e n t among t h e t h i n n e r s e t s o f t h e o t h e r f a c i e s ( f i g .
(ii)I n t e r p r e t a t i o n . e o l i a n dunes and draas.
dip t o the
s t r a t a a r e promi-
12).
F a c i e s TWC was d e p o s i t e d by l a r g e s o u t h w a r d - m i g r a t i n g A l t h o u g h modern, a c t i v e l a r g e dunes and d r a a s c o n t a i n -
563
F i g u r e 11. C h a r a c t e r i s t i c s o f f a c i e s TWC. ( a ) Stacked s e t s i n Coconino Sandstone n e a r West F o r k ; f i g u r e n e a r b o t t o m g i v e s s c a l e . (b) Grain-fall and s a n d - f l o w s t r a t a i n l a r g e s e t a t West Fork. (c) Detail of toe of large dune. L i g h t - c o l o r e d g r a i n - f a l l and c l i m b i n g t r a n s l a t e n t s t r a t a i n t e r t o n g u e w i t h sand-flow toes (dark). Lens cap i s 54 mm. ( d ) Wind r i p p l e s on a n g l e o f - r e p o s e f o r e s t a t West F o r k .
i n g s i l i c i c l a s t i c g r a i n s have n e v e r been t r e n c h e d t o g r e a t d e p t h s ,
studies ot
s u r f i c i a l p r o c e s s e s and s t r u c t u r e s and m i g r a t i o n o f s u p e r i m p o s e d dunes p r o v i d e some c l u e s as t o t h e t y p e o f i n t e r n a l s t r a t i f i c a t i o n t h a t , n i g h t be p r o d u c e d (Wilson,
1972; H u n t e r ,
1977; McKee,
1979a; R u b i n and H u n t e r ,
1982). Ahlbrandt
and F r y b e r g e r ( 1 9 8 0 ) i l l u s t r a t e d t h e i n t e r n a l f e a t u r e s o f d i s s e c t e d b a r c h a n o i d dunes f r o m t h e Sand H i l l s o f Nebraska.
Comparison o f f e a t u r e s o f modern dunes
564
F i g u r e 12. Sequences o f f a c i e s i n u p p e r S c h n e b l y H i l l F o r m a t i o n and l o w e r Fort C o c o n i n o Sandstone. ( a ) E n t i r e i n t e r v a l o f s t u d y a t B e l l Rock. Apache L i m e s t o n e i s a t base o f p h o t o . N e a r l y 200 rn shown. ( b ) M i d d l e and u p p e r i n t e r v a l o f s t u d y n e a r Sedona a t S c h n e b l y H i l l . A p p r o x i m a t e l y 100 m shown. I n both photos l i g h t e r colored proininantly c r o s s - s t r a t i f i e d u n i t s a r e composed o f f a c i e s TWC and I C . Darker u n i t s are combinations o f facies WBK, TCT, and CC.
565 w i t h t h o s e o f f a c i e s TWC c o n f i r m t h e e o l i a n o r i g i n .
Toes o f sand f l o w s t r a t a
that interfinger with climbing translatent strata are a distinctive feature o f inodern ( H u n t e r ,
1977) and a n c i e n t ( H u n t e r ,
1981) dunes.
The h i g h c o n s i s t e n c y
of d i p d i r e c t i o n i n l a r g e - s c a l e s e t s composed o f g r a i n - f l o w s t r a t a s u g g e s t t h a t
s l i p f a c e s were s t r a i g h t t r a n s v e r s e bedforms
and p a r a l l e l
4;
(fig.
b a r c h a n and t r a n s v e r s e - r i d g e (Ah1 b r a n d t
and
compares w i t h v a l u e s o f 22'
and 24'
for
dunes r e s p e c t i v e l y i n t h e Sand H i l l s o f Nebraska
1980).
Fryberger;
a characteristic o f
6 o f A h l b r a n d t and F r y b e r g e r ,
compare w i t h f i g .
The mean d i p a n g l e o f 22.2'
1980).
f r o m dune t o dune,
The
abundance
of
second-order
s u r f a c e s c o m p a r a b l e t o t h o s e d e s c r i b e d b y B r o o k f i e l d (1977)
bounding
i s attributed t o
The bedforins m i g r a t e d i n r e s p o n s e t o r e g i o n a l
dunes m i g r a t i n g a c r o s s draas. winds from t h e north.
C o m p l e x l y C r o s s - S t r a t i f i e d F a c i e s (CCY
( i )D e s c r i p t i o n .
The c o m p l e x l y c r o s s - s t r a t i f i e d
facies
(CC) i n c l u d e s a
v a r i e t y o f c r o s s - s t r a t i f i c a t i o n t y p e s o f f i n e - t o medium-grained r e d d i s h - o r a n g e sandstone.
s e t s and c o s e t s a r e up t o 1 m t h i c k and 3 o r more
Cross-stratified
meters wide ( f i g .
13).
I n d i v i d u a l s e t s r a n g e f r o m t r o u g h , t o wedge,
t o tabular
and c o m p r i s e m o s t l y m e g a r i p p l e f o r e s e t l a m i n a e t h a t d i p a t 15-20'. angle dipping lamination i s a l s o present.
Compound c r o s s s t r a t i f i c a t i o n i s
Set b o u n d a r i e s d i p i n v a r y i n g d i r e c t i o n s and t h e l a m i n a e o f a d j a c e n t
abundant.
s e t s coinmonly d i s p l a y a h e r r i n g b o n e p a t t e r n ( f i g . a r e abundant and complex. trough
A complex
axes.
current-ri pple
stratification
wedge,
o r trough-shaped sets.
to
shape
or
13d).
Reactivation surfaces
Most t r o u g h s e t s show n o r t h - s o u t h o r i e n t a t i o n o f heirarchy o f
tabular, the
Some low.
on
orientation
of
stratification
megari p p l e the
scales
stratification
within
small
compound
Many i n d i v i d u a l l a m i n a e do n o t c o n f o r m confining
set,
c o n t r a s t s w i t h f a c i e s TCT.
Some s e t s a r e s i g m o i d a l
p a r t i a l l y preserved topsets,
f o r e s e t s , and b o t t o m s e t s .
a
slump s t r u c t u r e s a r e p r e s e n t .
characteristic
that
i n c r o s s s e c t i o n showing These i n d i c a t e t h a t t h e
m e g a r i p p l e s w h i c h d e p o s i t e d t h e s e t s were 12-15 cm h i g h . and small-scale
includes
Uncommon wavy b e d d i n g
One sequence o f f a c i e s CC shows
i n t e r b e d d e d p l a n a r - t a b u l a r s e t s 0.3 m t h i c k and r i p p l e l a m i n a t e d u n i t s 6-10 cm thick (fig.
13a).
F a c i e s CC forms t a b u l a r packages up t o 4
(ii)I n t e r p r e t a t i o n . s t r a t i f i e d translatent
m thick.
F a c i e s CC s u p e r f i c i a l l y r e s e m b l e s t h e t r o u g h - c r o s s -
facies
(TCT) b u t l a c k s c l i m b i n g t r a n s l a t e n t s t r a t a and
c o n t a i n s a broader range o f s t r a t i f i c a t i o n types.
Most larninae appear t o be o f
s a n d f l o w o r i g i n d e p o s i t e d as f o r e s e t s o f m i g r a t i n g b e d f o r m s o f d i f f e r e n t s i z e s and shapes. directions. facies
D i v e r g e n t d i p d i r e c t i o n s on t h e l a m i n a e i n d i c a t e v a r y i n g c u r r e n t Because t h i s
another
depositional
facies
lacks characteristics
environment
i s suggested.
o f the other eolian Many o f t h e f o r e s e t
l a m i n a e f o r m complex b u n d l e s o f s m a l l r i p p l e c r o s s s t r a t i f i c a t i o n superimposed
566
F i g u r e 13. C h a r a c t e r i s t i c s o f f a c i e s CC. (a) P l a n a r - t a b u l a r cross s t r a t i f i c a t i o n w i t h i n t e r c a l a t e d wavy-bedded s a n d s t o n e a t B e l l Rock. F o r e s e t s d i p t o n o r t h , t h e o p p o s i t e t r e n d o f most e o l i a n s t r a t a . (b,c) D e t a i l s o f b e d d i n g and s t r a t i f i c a t i o n a t B e l l Rock. Note c o m p l e x i t y o f s e t s and abundant r e a c t i v a t i o n s u r f a c e s . ( d ) Complex polymodal foresets and p o s s i b l e swash l a m i n a t i o n a t S c h n e b l y H i l l . Scale i n centimeters, n o t e b o o k i s 17 cm h i g h .
on l a r g e r bedforms.
These f e a t u r e s p r o b a b l y formed i n subaqueous c o n d i t i o n s
and a few d e f i n i t e w a v e - r i p p l e
laminae support t h i s .
t i n u o u s s t r a t a may have been formed by wave swash. bound and l a c k s f l u v i a l associated
with
marine
Low-angle d i p p i n g conThe f a c i e s i s n o t channel
f a c i e s a s s o c i a t i o n s and t h e r e f o r e i s c o n s i d e r e d t o be conditions.
The
complex
c l o s e l y r e s e m b l e modern s h o r e 1 i n e sand b o d i e s .
stratification
P a r t i c u l a r l y c l o s e resemblance
i s n o t e d w i t h b a r r e d c o a s t l i n e s i l l u s t r a t e d by D a v i d s o n - A r n o t t (1976) and Greenwood and D a v i d s o n - A r n o t t
sequences
and Greenwood
( 1 9 7 9 ) and i n t e r t i d a l shoal deposits
567 d e s c r i b e d by Boersina and T e r w i n d t (1981).
The a p p a r e n t absence o f m a r i n e t r a c e
f o s s i l s f r o m t h i s f a c i e s m i g h t be e x p l a i n e d by r e l a t i v e l y r a p i d d e p o s i t i o n and b u r i a l o f t h e sand.
Wavy Bedded t o R i p p l e - L a m i n a t e d F a c i e s (WBR)
( i) D e s c r i p t i on.
The wavy bedded t o r i p p l e-1 ami n a t e d f a c i e s
p r i s e s t h e most h e t e r o g e n e o u s f a c i e s varieties
i n the interval
o f study.
(WBR) comThe s e v e r a l
a r e d e s c r i b e d as d i s c r e t e s u b f a c i e s b u t many g r a d a t i o n a l
forms a r e
present. Ripple-laminated
s a n d s t o n e and sandy mudstone i s t h e most w i d e l y d i s t r i b u t e d
The s u b f a c i e s c o n t a i n s s e v e r a l t y p e s o f r i p p l e-1 ami n a t e d s a n d s t o n e
s u b f a c i es.
and mudstone.
D i s t i n c t i v e wave-ripple
medium-grained
sandstone
(fig.
14).
bedding
i s most common i n f i n e -
Wave r i p p l e s may c o m p r i s e t h i n l a y e r s
l a m i n a e t h i c k o r sequences n e a r l y 1 m t h i c k .
several
to
The w a v e - r i p p l e
t u r e s d i s p l a y b u n d l e - w i s e a r r a n g e m e n t as d e s c r i b e d by de Raaf. e t a1
struc-
., ( 1 9 7 7 ) .
O t h e r r i p p l e l a m i n a e a r e l e s s d i s t i n c t as t o wave- o r c u r r e n t - r i p p l e o r i g i n and consist o f indistinct
r i p p l e laminae,
most commonly i n sandy mudstone.
The
r i p p l e s o c c u r as l e n s e s o f s l i g h t l y c o a r s e r s e d i m e n t 3-12 nnn t h i c k and 12-100 m long.
Flaser-like
l a m i n a e o f mudstone s e p a r a t e t h e r i p p l e forms.
In rare
p l a n - v i e w e x p o s u r e s , t h e r i p p l e s d i s p l a y an u n d u l a t o r y t o l i n g u o i d form. Wavy-bedded swell
s a n d s t o n e i s coinmonly
f i n e t o medium g r a i n e d and c o n s i s t s o f
and s w a l e t o p o g r a p h y w i t h a m p l i t u d e s o f up t o s e v e r a l c e n t i m e t e r s and
lengths
of
irregular
m.
0.5
Many
packages
of
variations
are present
cross-stratified
and some packages c o n t a i n
sandstone.
Laminations
a r e somewhat
i n d i s t i n c t and t y p i c a l l y y i e l d a m a s s i v e appearance. Crinkly-bedded
sandstone
i s f i n e t o medium g r a i n e d and c o n t a i n s c r i n k l e d
laminae w i t h several centimeters o f r e l i e f .
Individual
layers are extremely
c o n t i n u o u s and t h e h o m o g e n e i t y o f t h e u n i t i s c o n t i n u o u s f o r hundreds o f m e t e r s o r more. Lenticular abundant
in
bedding facies
and
flaser-bedded
WBR.
Individual
e n c l o s e d i n f i n e r sandy mudstone ( f i g .
sandstone
ripple
lenses
observed
dips
sets t h a t truncate adjacent
a r e 10".
Set
l a m i n a e a r e v e r y t h i n , 0.5-1.0
thicknesses
m.
mudstone
are
25-50
is
locally
mm l o n g and
14c).
Plane-bedded s a n d s t o n e i s an uncommon s u b f a c i e s . b r o a d wedge-shaped
and
The p l a n e beds f o r m v e r y
s e t s a t low angles.
r a n g e f r o m 7-25
Maximum
cm and i n d i v i d u a l
The s a n d s t o n e i s f i n e t o medium g r a i n e d and
very well sorted. F a c i e s WBR f o r m s an i m p o r t a n t b u t v a r i a b l e p e r c e n t a g e o f t h e u p p e r S c h n e b l y
H i l l Formation.
I n d i v i d u a l u n i t s o f t h e f a c i e s may c o n t a i n one o r more o f t h e
subfacies but a l l
were n e v e r o b s e r v e d i n t h e same u n i t .
sequence v a r i e s g r e a t l y ,
Although v e r t i c a l
a somewhat t y p i c a l v e r t i c a l s u c c e s s i o n i n c l u d e s b a s a l
568
F i g u r e 14. C h a r a c t e r i s t i c s o f f a c i e s WBR. ( a ) Wavy bedded sequence t h a t s h a r p l y t r u n c a t e s l a r g e e o l i a n dune s e t at H e l l R o c k ; pen on h e d d i q g plane i s 16 cin l o n g . ( b ) D e t a i l o f w a v e - r i p p l e l a m i n a e a t same l o c a t i o n . (c) P r o b a b l e l e n t i c u l a r b e d d i n g a t Be1 1 Rock. ( d ) T y p i c a l heterogeneous sequence w i t h v a r i o u s t y p e s o f i r r e g u l a r t o r i p p l e b e d d i n g a?d s m a l l - s c a l e c r o s s s t r a t i f i c a t i o n a t B e l l Rock.
ripple-mdrked t o flaser-bedded sandstone w i t h o r w i t h o u t stone. local
Sequences commonly ball-and-pillow
sandy inudstone,
m e d i a l wavy and r i p p l e bedded
l e n t i c u l a r b e d d i n g , and u p p e r c r i n k l y - b e d d e d c o a r s e n upwards.
structure,
and
poorly
p r e s e n t i n many sequences o f f a c i e s WBR. a l w a y s s h a r p and ,isconforrnab’e
sand-
Small - s c a l e c o n t o r t e d bedding, cross-stratified
sandstone
is
The b a s a l c o n t a c t o f a sequence i s
w i t h o r w i t h o u t basal l o a d s t r u c t u r e s .
The t o p
may be g r a d a t i o n a l o r s h a r p . Climbing t r a n s l a t e n t s t r a t a sections t o t h e northwest.
f o r m an i m p o r t a n t component o f f a c i e s WBR i n
F a r t h e r s o u t h e a s t t h e s t r u c t u r e i s r a r e o r absent.
569 ( i i ) Interpretation.
F a c i e s WBR i n c l u d e s a v a r i e t y o f i n t e r d u n e and e x t r a -
dune d e p o s i t s ( t e r m i n o l o g y o f Lupe and A h l b r a n d t , strata,
c o n t o r t e d laminae,
common i n t h e f a c i e s , 1981b).
wavy
laminae,
wave
1975).
Climbing t r a n s l a t e n t
r i p p l e s and c u r r e n t
a r e common i n modern wet i n t e r d u n e d e p o s i t s
ripples, (Kocurek,
Climbing t r a n s l a t e n t s t r a t a o f t h i s facies sharply truncate underlying
dune d e p o s i t s and g r a d e upwards and l a t e r a l l y i n t o t h e s a n d - f l o w t o e d e p o s i t s o f a d v a n c i n g dunes
(fig.
llc).
f i rst-order
bounding surfaces
Brookfield,
1977; Kocurek,
They f o r m i n t e r d u n e d e p o s i t s w h i c h o v e r l i e
o v e r w h i c h d r a a s advanced (Wi 1 son,
1971,
1972;
Some u n i t s o f f a c i e s WBR s h a r p l y t r u n c a t e
1981b).
u n d e r l y i n g f a c i e s , t y p i c a l l y t h i c k e n t o t h e s o u t h e a s t , and g r a d e l a t e r a l l y i n t o a s s o c i a t e d sabkha and s h a l l o w i n a r i n e d e p o s i t s . sequences common.
similar
Wave-generated
t o t h o s e d e s c r i b e d by de Raaf,
et al.
s t r u c t u r e s and
(1977) a r e l o c a l l y
Here f a c i e s WBR i s an e x t r a d u n e r a t h e r t h a n i n t r a d u n e d e p o s i t .
F i n e - G r a i n e d C1 a s t i c and C a r b o n a t e F a c i e s Southeastward
along
the
Mogollon
the
Rim,
eolian
and
associated
rocks
d e s c r i b e d above g r a d e l a t e r a l l y i n t o complex sequences o f f i n e - g r a i n e d c l a s t i c , carbonate, supratidal, Blakey,
and e v a p o r i t e u n i t s o f s h a l l o w m a r i n e , and
1980).
lagoonal
origin
and
r e s t r i c t e d marine,
Gerrard,
These r o c k s i n c l u d e r i p p l e - l a m i n a t e d
c r a c k s and s a l t c r y s t a l c a s t s , mite,
(Peirce
1966;
Gerrard,
sabkha, 1966;
sandy mudstone w i t h mud
f o s s i l i f e r o u s t o o o l i t i c c a l c a r e n i t e and d o l o -
l a m i n a t e d t o f e n e s t r a l d o l o m i c r i t e , and gypsum and g y p s i f e r o u s mudstone.
The f a c i e s change between m o s t l y e o l i a n s t r a t a i n n o r t h e r n Oak Creek Canyon t o nearly
totally
(Blakey,
non-eolian
strata
near
Ft.
Apache t a k e s
place over
200
km
1980).
SEQUENCES AND CYCLES V e r t i c a l Sequence F i g u r e 7 shows t h e v e r t i c a l sequence of f a c i e s and t h e i r i n t e r p r e t a t i o n i n t h e u p p e r S c h n e b l y H i l l F o r m a t i o n and l o w e r m o s t Coconino Sandstone a t West F o r k o f Oak Creek cyclic;
1.
Canyon and B e l l
Rock.
The p a t t e r n of
vertical
succession
is
t h e f o l l o w i n g s a l i e n t p o i n t s a r e noted: All
cycles
s t a r t w i t h some a s p e c t
of
f a c i e s WBR,
generally
ripple-
l a m i n a t e d sandy mudstone, w h i c h d i s c o n f o r m a b l y o v e r l i e s t h e t o p o f t h e p r e v i o u s c y c l e.
2.
C y c l e s c o a r s e n upwards.
3.
S c a l e o f c r o s s s t r a t i f i c a t i o n t e n d s t o become l a r g e r t o w a r d s t o p o f many
cycles.
4.
A t y p i c a l c y c l e a t West F o r k c o n s i s t s of a s h a r p d i s c o n f o r m i t y a t t h e
base, WBR,
TCT,
I C and TWC.
570 5.
A t y p i c a l c y c l e a t B e l l Rock c o n s i s t s o f a sharp d i s c o n f o r m i t y a t t h e
base, WBR, CC, TCT, I C and TWC. 6.
F a c i e s WBR, CC and TCT dominate t h e l o w e r i n t e r v a l and f a c i e s I C and TWC
dominate t h e upper i n t e r v a l ( f i g . 7 ) .
7.
Rocks o f t h e Schnebly H i l l Formation below t h e i n t e r v a l o f study a t B e l l
Rock a r e 150 m t h i c k and comprise d o m i n a n t l y f a c i e s WBR and m i n o r f i n e - g r a i n e d c l a s t i c and carbonate f a c i e s .
Rocks o f t h e Coconino Sandstone above t h e i n t e r -
v a l o f s t u d y a t b o t h B e l l Rock and West Fork a r e 250 m t h i c k and a r e composed o f f a c i e s TWC w i t h some I C .
The i n t e r v a l o f s t u d y r e p r e s e n t s t h e c y c l i c change
from f a c i e s WBR a t t h e bottom t o f a c i e s TWC a t t h e top. L a t e r a l Sequence F i g u r e 1 5 shows t h e somewhat g e n e r a l i z e d l a t e r a l d i s t r i b u t i o n o f f a c i e s of t h e i n t e r v a l o f s t u d y from n o r t h w e s t t o southeast o v e r a d i s t a n c e o f a p p r o x i m a t e l y 100 km. 1.
The f o l l o w i n g s a l i e n t p o i n t s a r e noted:
From southeast t o northwest,
f a c i e s WBR grades l a t e r a l l y i n t o f a c i e s CC
and TCT which i n t u r n grade i n t o f a c i e s I C and TWC. 2.
The sequence coarsens t o t h e northwest.
-
0
5 10 1
Fort Apache Member
5
E ~ S 3~Inland Dune
0Sabkha-
ESJ Coastal Dune
Extradune
E93 Marine Carbonate
F i g u r e 15. G e n e r a l i z e d c r o s s s e c t i o n o f upper Schnebly H i l l Formation and l o w e r Coconino Sandstone. I n t e r p r e t a t i o n s based on d i s t r i b u t i o n o f following facies: I n l a n d dune--TWC, I C ; Coastal dune--TCT, I C ; Sabkhainterdune-extradune--WBR, CC. Heavy h o r i z o n t a l 1 i n e s r e p r e s e n t e r o s i o n s u r f a c e carved by r a p i d marine t r a n s g r e s s i o n .
571 3.
Scale o f cross s t r a t i f i c a t i o n increases towards t h e northwest.
4.
F a c i e s WBR t h i n s and p i n c h e s o u t t o t h e n o r t h w e s t and f a c i e s TWC and I C
t h i n and p i n c h o u t t o t h e s o u t h e a s t . 5.
S u c c e s s i v e l y h i g h e r t o n g u e s o f f a c i e s I C and TWC e x t e n d f a r t h e r t o w a r d s
t h e s o u t h e a s t and s u c c e s s i v e l o w e r tongues o f f a c i e s WBR e x t e n d f a r t h e r n o r t h west. 6.
From s o u t h e a s t t o n o r t h w e s t , t h e i n t e r v a l o f s t u d y changes f a c i e s f r o m
WBR t o TWC.
T h i s i s e x a c t l y t h e same f a c i e s change t h a t t a k e s p l a c e v e r t i c a l l y
a t the Bell
Rock s e c t i o n .
The v e r t i c a l change o f f a c i e s r e f l e c t s t h e h o r i -
zontal d i s t r i b u t i o n o f facies a t a given i n t e r v a l . INTERPRETATION OF SEQUENCES Sequence o f E v e n t s Based upon t h e i n t e r p r e t a t i o n and d i s t r i b u t i o n o f f a c i e s ,
the depositional
h i s t o r y o f t h e u p p e r S c h n e b l y H i l l F o r m a t i o n and l o w e r C o c o n i n o Sandstone can be d e v e l o p e d . shows
a
F i g u r e 15 shows t h e g e n e r a l d i s t r i b u t i o n o f f a c i e s and f i g u r e 16
northwest-southeast
i n t e r v a l o f study.
cross
section
of
two
typical
cycles
base o f each c y c l e i s c l e a r l y e r o s i o n a l and t h e b a s a l f a c i e s , t h e southeast.
from
They p r o v i d e t h e b a s i s f o r t h e f o l l o w i n g d i s c u s s i o n . WBR,
the The
thickens t o
F a c i e s WBR where c l e a r l y e x t r a d u n a l as shown i n f i g u r e 16 was
d e p o s i t e d i n a m o d e r a t e l y l o w e n e r g y c o a s t a l complex w i t h i n t e r t i d a l and l e n t i c u l a r b e d d i n g ) p o s s i b l y s u b t i d a l
( c u r r e n t - and w a v e - r i p p l e
(flaser
bedding),
and s u p r a t i d a l -sabkha ( s a l t c r y s t a l c a s t s , wavy and c o n t o r t e d b e d d i n g ) e n v i r o n ments r e p r e s e n t e d . F a c i e s CC,
where p r e s e n t ,
pretation of
facies
i s a l w a y s a s s o c i a t e d w i t h f a c i e s WBR.
CC as some t y p e o f
b a r o r shoal
The i n t e r -
i n a wave-
or tidal-
d o m i n a t e d c o a s t l i n e s u g g e s t s t h a t h i g h e r e n e r g y c o a s t a l c o n d i t i o n s sometimes a f f e c t e d t h e dominantly r e s t r i c t e d shore1 ine. Facies
TCT,
interpreted
as
i s closely
blow-out
coastal
translatent
strata
f a c i e s CC.
V e r t i c a l and l a t e r a l s t r a t i g r a p h i c
associated w i t h
dunes facies
filled
with
climbing
WBR and l e s s commonly
p o s i t i o n o f f a c i e s TCT ( f i g s .
15, 16) s t r o n g l y s u p p o r t t h e c o a s t a l p o s i t i o n o f t h i s e o l i a n f a c i e s . F a c i e s I C commonly o v e r l i e s and g r a d e s i n t o f a c i e s WBR a n d / o r TCT.
As t h e
l a r g e d r a a s t h a t f o r m e d f a c i e s I C approached t h e c o a s t a l e n v i r o n m e n t s m a l l e r dunes formed on t h e d r a a s , p e r h a p s i n r e s p o n s e t o complex c o a s t a l w i n d regimes. Some o f t h e s m a l l e r dunes can be c l e a r l y seen t o have c l i m b e d up t h e f a c e o f t h e s o u t h w a r d - a d v a n c i n g draa. F a c i e s TWC,
i n t e r p r e t e d t o have been d e p o s i t e d by s o u t h w a r d - p r o g r a d i n g
l a r g e dunes and d r a a s ,
gradationally overlies facies
IC.
However,
very
higher i n
t h e s e c t i o n and t o t h e n o r t h w e s t t h e f a c i e s may d i r e c t l y o v e r l i e f a c i e s WBR. I n t h i s l a t t e r s i t u a t i o n f a c i e s WBR i s wet i n t e r d u n e r a t h e r t h a n an e x t r a d u n e
572
NW
‘West
/
Head of Oak Creek
Fork
t
Schnebly Hill
/
t
Bell Rock /
SE
0
0
Km
1 0 =Inland
Dune
=Coastal
Dune
I - 1 Sabkha -Extradune -Shallow Marine F i g u r e 16. Cross s e c t i o n and columnar s e c t i o n s showing d i s t r i b u t i o n o f f a c i e s and i n t e r p r e t a t i o n o f environments w i t h i n two i d e a l i z e d c y c l e s . Heavy h o r i z o n t a l l i n e s r e p r e s e n t s u r f a c e s o f e r o s i o n r e l a t e d t o r a p i d marine transgression. See F i g u r e 15 f o r a d d i t i o n a l e x p l a n a t i o n .
deposit.
Facies WBR i s t h e n a s s o c i a t e d w i t h t h e f i r s t - o r d e r
as d e f i n e d by B r o o k f i e l d (1977) and Kocurek (1981b).
bounding surfaces
Where TWC succeeds I C ,
t h e t r a n s i t i o n from more complex c o a s t a l dunes a f f e c t e d by s h i f t i n g land-sea and sea-land breezes t o l a r g e r b u t somewhat s i m p l e r p r o b a b l y t r a n s v e r s e i n l a n d dunes, i s documented. The sequences o f environments t h u s documented i n v e r t i c a l succession suggest t h e f o l l o w i n g events:
1. 2.
Sharp, wide-spread e r o s i o n a l s u r f a c e i s carved on t o p o f p r e v i o u s c y c l e . Low-energy w i t h o c c a s i o n a l h i g h e r energy c o a s t a l and shore1 i n e d e p o s i t s
formed on t h e surface. 3.
seaward.
As sand blew from i n l a n d , a band o f dune sand developed and prograded Complex onshore and o f f s h o r e winds formed blow-out dunes which were
f i l l e d i n with wind-ripple
deposits.
As t h e sand p i l e moved seaward, l a r g e
draas e n t e r e d t h e area and prograded o v e r t h e c o a s t a l
blow outs.
Complex
c o a s t a l winds coupled w i t h r e g i o n a l winds from t h e n o r t h formed compound and complex dunes o r draas.
573 As t h e e r g c o n t i n u e d t o s l o w l y m i g r a t e southward, l a r g e i n l a n d dunes and
4.
d r a a s m a n t l e d t h e a r e a and a complex o f d u n e - i n t e r d u n e d e p o s i t s was formed. The v e r t i c a l
5.
succession
o f dunes and c o a s t a l d e p o s i t s documents t h e
g r a d u a l change f r o m s h a l l o w m a r i n e , s a b k h a - s u p r a t i d a l , dunes.
c o a s t a l dunes, t o i n l a n d
The same p a t t e r n was p r e s e n t l a t e r a l l y o v e r a d i s t a n c e o f a p p r o x i m a t e l y
50 km a t any g i v e n t i m e . 6.
The s u c c e s s i o n
was
a b r u p t l y terminated by a p e r i o d o f
e r o s i o n and
a n o t h e r c y c l e was i n i t i a t e d . 7.
Later
e a r l i e r ones.
pulses
of
prograding
dunes
moved
farther
southeastward
than
T h i s p r o b a b l y r e f l e c t s i n c r e a s e d s u p p l y o f sand as t h e m a i n e r g
i n i g r a t e d s l o w l y southward. CONTROLS OF DEPOSITION AND CYCLES The o r i g i n of t h e e r o s i o n s u r f a c e s t h a t f o r m t h e base o f t h e c y c l i c sequence j u s t d e s c r i b e d i s c r i t i c a l t o u n d e r s t a n d i n g t h e causes and c o n t r o l s o f c y c l i c sedimentation.
Obvious f a c t o r s i n t h e c o n t r o l o f e o l i a n sedimentation i n c l u d e
1) w i n d d i r e c t i o n ,
velocity,
and c o n s i s t e n c y ;
2) sand s u p p l y ;
3) c l i m a t e and
v e g e t a t i o n ; 4 ) s i z e and shape o f t h e b a s i n o f e o l i a n d e p o s i t i o n ; 5 ) base l e v e l ;
6) w a t e r t a b l e h e i g h t ;
and 7)
r e l a t i o n s w i t h a d j a c e n t e n v i r o n m e n t s and t h e i r
g r o w t h and decay w i t h r e s p e c t t o e o l i a n d e p o s i t i o n . Changes i n Wind The m i g r a t i o n o f an e r g i s a v e r y s l o w g e o l o g i c p r o c e s s Brookfield,
1977).
(Wilson,
1972;
It w o u l d seem u n l i k e l y t h a t any seasonal o r l a r g e r c y c l i c
w i n d p a t t e r n s c o u l d p r o d u c e t h e l a r g e e x t e n t o f t h e s u r f a c e s t h a t u n d e r l i e each cycle.
R a t h e r , w i n d p a t t e r n s were p r o b a b l y r e l a t e d t o p r o x i m i t y of t h e c o a s t -
l i n e and t h u s had g r e a t c o n t r o l on dune t y p e f r o m i n l a n d t o t h e c o a s t .
Modern
c o a s t a l dune complexes l i k e t h o s e i n t h e Namib D e s e r t show g r e a t w i n d v a r i a t i o n s and changes i n dune t y p e and f o r m f r o m t h e c o a s t t o i n l a n d (Breed, al.,
et
1979). Sand S u p p l y Sand s u p p l y was d i r e c t l y r e l a t e d t o t h e p o s i t i o n o f t h e m a i n body o f t h e
Coconino e r g a t any g i v e n t i m e d u r i n g d e p o s i t i o n o f t h e i n t e r v a l o f s t u d y . This
i s d i r e c t l y shown by t h e f a r t h e r
southeastward progradation o f
inland
dunes i n y o u n g e r c y c l e s as t h e e r g complex m i g r a t e d s o u t h w a r d i n t o t h e a r e a o f study.
However,
t h e sharp basal e r o s i o n surfaces
s u g g e s t t h a t t h e r e was a
t e m p o r a r y c u t o f f o f sand s u p p l y a t t h e base o f each new c y c l e .
The e r o s i o n
s u r f a c e c u t i n t o t h e e r g f r o m s o u t h e a s t t o n o r t h w e s t and a l l o w e d s h o r e l i n e and
.
574 s h a l l o w marine c o n d i t i o n s t o move f a r i n l a n d t o t h e n o r t h w e s t o v e r p r e v i o u s e r g It t h e n t o o k t h e e r g some l o n g p e r i o d o f t i m e t o m i g r a t e back i n t o
deposits.
t h e area from t h e n o r t h w e s t , t h u s r e p l e n i s h i n g sand i n t o t h e c o a s t a l areas. C l i m a t e and V e g e t a t i o n C l i m a t e and v e g e t a t i o n were obvious f a c t o r s t h a t a l l o w e d t h e growth and m i g r a t i o n o f t h e Coconino erg.
The c l i m a t e was h o t and a r i d as shown by t h i c k
s a l t d e p o s i t s i n t h e a d j a c e n t Holbrook Basin ( P e i r c e and Gerrard, 1966). ever,
How-
t h e r e i s no evidence t h a t changing c l i m a t e o r c l i m a t i c c y c l e s i n f l u e n c e d
t h e deposits o f the i n t e r v a l o f study other than t h e s h i f t i n g c l i m a t i c b e l t s a s s o c i a t e d w i t h t h e a l t e r n a t i n g i n l a n d and c o a s t a l c o n d i t i o n s . S i z e and Shape o f t h e Basin and Base Level The Sedona Arch d i v i d e d c e n t r a l
Arizona i n t o two t e c t o n i c elements.
The
broad, p r o b a b l y t o p o g r a p h i c a l l y and t e c t o n i c a l l y f e a t u r e 1 ess Grand Canyon-Emery P l a t f o r m p r o v i d e d b o t h an avenue o f t r a n s p o r t a t i o n and s i t e f o r accumulation o f sediments o f t h e Coconino erg. across t h e p l a t f o r m .
The Coconino c o n t a i n s no known marine d e p o s i t s
The Holbrook Basin and a d j a c e n t Mogollon S h e l f were areas
o f much more r a p i d subsidence as shown by t h e 100-m-thick
zone o f i n t e r t o n g u i n g
o f t h e i n t e r v a l o f s t u d y which i s n o t p r e s e n t across t h e p l a t f o r m .
A t times
t h e Holbrook Basin and Mogollon S h e l f were low enough t o a l l o w m a r i n e d e p o s i t s t o form on t h e s o u t h e a s t margin o f t h e Sedona Arch.
The r e l a t i o n s h i p between
changing subsidence r a t e s and area o f f a c i e s change seems v e r y d i r e c t .
Thus
t h e s u b t l e change i n t e c t o n i c regime from n o r t h w e s t t o s o u t h e a s t seems t o have a strong c o n t r o l
on t h e f a c i e s .
It seems u n l i k e l y ,
subsidence
i n the
study are d i r e c t l y
cycles.
rates
however,
t h a t changing
related t o individual
Some 30 c y c l e s a r e p r e s e n t i n t h e B e l l Rock s e c t i o n ( f i g .
seems l u d i c r o u s t o c a l l cycles.
area o f
upon 30 episodes
7 ) and i t
o f c y c l i c subsidence t o form t h e
I n s t e a d , changing base l e v e l , sea l e v e l i n t h i s case, would appear t o
be a more l i k e l y cause.
The w e l l documented Permian e u s t a t i c s e a l e v e l changes
due t o g l a c i a t i o n ( C r o w e l l , 1978) would cause s e a l e v e l change t o f l u c t u a t e and these
fluctuations
would
be b e s t
recorded
i n the
s h e l v e s and b a s i n s w i t h c o n n e c t i o n t o t h e open ocean.
stratigraphic
record o f
Subsidence e a s t o f t h e
Sedona Arch as documented by t h i c k e n i n g o f s e c t i o n and i n c r e a s e d marine deposi t i o n (Blakey, level,
1980),
k e p t t h e area low enough t h a t d u r i n g h i g h stands o f sea
m a r i n e d e p o s i t s were formed.
Rapid marine t r a n s g r e s s i o n s
caused by
l a r g e - s c a l e a b l a t i o n o f c o n t i n e n t a l i c e sheets i n t h e s o u t h e r n hemisphere may have advanced r a p i d l y a c r o s s t h e Mogollon S h e l f and i n t o t h e Sedona area.
Late
Q u a t e r n a r y r a t e s o f s e a l e v e l change o f 100 m p e r 15,000 y e a r s have been documented by Curray (1961).
I f s i m i l a r r a t e s o c c u r r e d i n t h e Permian,
a rapid
t r a n s g r e s s i o n a c r o s s a reasonably s t a b l e area l i k e c e n t r a l A r i z o n a would l i k e l y
575 result
in a
"rapid
erosional
t r a n s g r e s s i o n " (Curray,
transgression"
1964).
would l i k e l y be formed.
to
"discontinuous
depositionaj
An e r o s i o n a l p l a n e mantled w i t h t h i n d e p o s i t s
Advancement of t h e e r g accompanying t h e f a l l o f sea
l e v e l would produce common o f f l a p i n t h e r e g r e s s i o n a l sequence.
Coastal f a c i e s
would o v e r l i e o f f s h o r e f a c i e s and be o v e r l a i n by i n l a n d f a c i e s .
The r a t e o f
subsidence of t h e area would equal t h e t h i c k n e s s o f sediment d e p o s i t e d between e r o s i o n s u r f a c e s d i v i d e d by t h e number o f y e a r s between t r a n s g r e s s i o n s . Leonardian was about 18 my l o n g (VanEysinga, one-quarter
1975;
Waterhouse,
1978);
The about
o f t h a t t i m e o r 5 my approximates t h e l e n g t h o f t i m e d u r i n g which
t h e l o w e r Leonardian r o c k s o f t h e i n t e r v a l o f s t u d y were d e p o s i t e d (Blakey, 1980).
Assuming a g l a c i a l c y c l i c i t y o f about 2 m a j o r c y c l e s p e r m i l l i o n y e a r s
a l l o w s t i m e f o r 10 g l a c i a t i o n s d u r i n g upper Schnebly H i l l and l o w e r Coconino time.
The B e l l Rock s e c t i o n c o n t a i n s 30 c y c l e s and t h e i n c o m p l e t e West Fork
section contains
A complete s e c t i o n i n t h e West Fork area would
13 c y c l e s .
c o n t a i n about 15 c y c l e s .
The B e l l Rock s e c t i o n i s more basinward and would
l i k e l y r e f l e c t s l i g h t f l u c t u a t i o n s o f s e a l e v e l more than t h e West Fork s e c t i o n . The 15 c y c l e s a t West Fork i s comparable t o t h e c a l c u l a t e d 10 g l a c i a l c y c l e s . The average t h i c k n e s s o f t h e West Fork c y c l e s i s 6 m. p e r 500,000 years.
y e a r s equals a subsidence
Six meters o f s e c t i o n
r a t e o f a p p r o x i m a t e l y 1 in p e r 100,000
The s e d i m e n t a t i o n r a t e was p r o b a b l y g r e a t e r as an unknown amount o f
sediment was removed by each e r o s i o n a l t r a n s g r e s s i o n .
These c a l c u l a t i o n s a r e
approximate and do n o t t a k e i n t o account f a c t o r s o f p r e s e r v a t i o n p o t e n t i a l and non-deposited c y c l e s , something about which we have no i n f o r m a t i o n . Water Table H e i g h t s The h e i g h t o f t h e water t a b l e r e p r e s e n t s t h e l o w e s t l e v e l t o which a dune f i e l d can be d e f l a t e d (Stokes,
1968).
Although B r o o k f i e l d (1977) and Kocurek
(1981b) have d i s c o u n t e d t h e Stokes model as a m a j o r c o n t r o l f o r f o r m a t i o n o f most e o l i a n bounding s u r f a c e s , 1983).
a c t i v e debate c o n t i n u e s (Loope,
1983; Kocurek,
Loope contends t h a t m i n o r f l a w s i n Stokes' drawings s h o u l d n o t j u s t i f y
d i s c a r d i n g t h e model and t h a t modern examples a r e s u p p o r t i v e o f t h e model. Kocurek agrees b u t c o u n t e r s t h a t e x t e n s i v e w a t e r - t a b l e - c o n t r o l l e d t h e a n c i e n t were formed by an " e x t r a - d u n e - f i e l d spread s u r f a c e s ,
event".
surfaces i n
He f e e l s t h a t wide-
n o t a t t r i b u t a b l e t o c l i m b i n g bedforms, " h e r a l d a m a j o r e v e n t
i n t h e a e o l i a n b a s i n " and t h a t "Stokes-type''
bounding s u r f a c e s mark " c o n t a c t s
between p e r i o d s o f a e o l i a n b a s i n e v o l u t i o n . "
H e r e i n l i e s t h e importance o f t h e
difference
between
i n t e r d u n e and
extradune
deposits.
The
base
of
upper
Schnebly H i l l c y c l e s a r e i n i t i a t e d w i t h extradune d e p o s i t i o n and a r e r e l a t e d t o m a r i n e t r a n s g r e s s i o n by drowning o f t h e source area. m a j o r event i n t h e e o l i a n basin. t i o n o f t h e dune f i e l d ,
The s u r f a c e s h e r a l d a
D u r i n g t h e ensuing r e g r e s s i o n and prograda-
t h e u n d e r l y i n g marine-sabkha d e p o s i t s and a s s o c i a t e d
576 w a t e r - t a b l e l e v e l s formed t h e l o w e s t l e v e l s t o which t h e new dune f i e l d s c o u l d be d e f l a t e d . (fig.
Some o f t h e s u r f a c e s near t h e m i d d l e o f t h e B e l l Rock s e c t i o n
7) may have formed i n t h i s manner r a t h e r t h a n as d i s t i n c t e r o s i o n a l
transgressions.
On t h e o t h e r hand, i n t e r d u n e d e p o s i t s w i t h i n t h e main Coconino
e r g a r e p r o b a b l y r e l a t e d t o bedform m i g r a t i o n and c l i m b i n g as d e s c r i b e d by I t i s these
B r o o k f i e l d (1977), Kocurek (1981b), and Rubin and Hunter (1982). differences that
i n mechanisms o f accumulation and p r e s e r v a t i o n o f dune d e p o s i t s
produced t h e s e d i m e n t o l o g i c d i f f e r e n c e s
Coconino and a l l o w d i f f e r e n t i a t i o n
of
between t h e Schnebly H i l l
t h e two d i s t i n c t
and
though g r a d a t i o n a l
geologic units. Re1a t i ons w i t h Adjacent Environments The e o l i a n d e p o s i t s strongly
influenced
of
by
s u p r a t i d a l environments. lagoons,
wave-
and
t h e upper Schnebly H i l l and l o w e r Coconino were
the
adjacent
shallow
marine,
coastal,
sabkha,
and
As e o l i a n dunes m i g r a t e d i n t o sabkha l a k e s o r c o a s t a l
ripple-laminated,
d e p o s i t s o f f a c i e s WBR were formed.
contorted,
and
wavy-bedded
sandstone
An example o f t h i s i s l o c a t e d s e v e r a l km
east o f Sedona where t h e Schnebly H i l l Road crosses t h e o u t c r o p o f t h e i n t e r v a l o f study.
A 15-20 m - t h i c k e o l i a n d e p o s i t o f f a c i e s TWC grades l a t e r a l l y i n
l e s s t h a n 200 m i n t o c r i n k l y - b e d d e d subaqueously d e p o s i t e d 10 m - t h i c k sandstone o f f a c i e s WBR ( f i g .
17).
Dune d e p o s i t s were reworked and r e d i s t r i b u t e d by
waves and t i d e s i n t o f a c i e s CC.
S i m i l a r l y c o a s t a l shoal and beach d e p o s i t s
were reworked by e o l i a n processes. A l t h o u g h adhesion r i p p l e s o r c l i m b i n g adhesion r i p p l e laminae a r e v e r y r a r e , a few poor examples a r e present.
These a r e common f e a t u r e s a s s o c i a t e d w i t h t h e
i n t e r a c t i o n o f e o l i a n and s u p r a t i d a l - s a b k h a environments.
Perhaps some o f t h e
abundant r i p p l e laminae o f f a c i e s WBR a r e a d h e s i o n - r i p p l e post-depositional
laminae o r perhaps
m o d i f i c a t i o n s have d e s t r o y e d t h e f e a t u r e s .
Upper Schnebly H i 11 , Lower Coconino Model The
interpretation
of
the
Coconino and t h e i r v e r t i c a l the depositional
history
of
facies
of
Schnebly H i l l
t h e upper
and lower
and l a t e r a l d i s t r i b u t i o n y i e l d a c l e a r model f o r these deposits
(fig.
18).
The model
depends
s t r o n g l y on o u r i n t e r p r e t a t i o n o f t h e e r o s i o n s u r f a c e s a t t h e base o f each c y c l e as b e i n g formed by r a p i d marine t r a n s g r e s s i o n .
Evidence gathered and
p r e s e n t e d i n t h i s paper i n d i c a t e s t h a t
none o f t h e o t h e r p o s s i b l e c o n t r o l s
c o u l d have caused most o f t h e widespread,
sharp s u r f a c e s i n t h e upper Schnebly
H i l l Formation.
577
F i g u r e 17. D e t a i l e d c r o s s s e c t i o n and p h o t o s showing t e r m i n a t i o n o f e o l i a n dune o r d r a a due t o m i g r a t i o n i n t o aqueous e n v i r o n m e n t , Schnebly H i l l l o c a l i t y . D r a w i n g t o s c a l e w i t h 2X v e r t i c a l e x a g g e r a t i o n .
578
NW
SE TRANSGRESSION /
-
1 inland dunes
PROGRADATION
2 (0
L
1
3 TRANSGRESSION @I
I
4
F i g u r e 18. Depositional Sedona area.
model f o r t h e upper Schnebly H i l l Formation i n t h e '
CONCLUSIONS D e t a i l e d a n a l y s i s o f t h e upper Schnebly H i l l Formation and l o w e r Coconino Sandstone near Sedona, A r i z o n a y i e l d e d t h e f o l l o w i n g f a c i e s and i n t e r p r e t a t i o n : T r o u g h - c r o s s - s t r a t i f i e d sandstone c o n t a i n i n g c h i e f l y c l i m b i n g t r a n s l a 1. t e n t s t r a t a i s i n t e r p r e t e d t o have formed as a c o a s t a l blow-out dune complex.
2.
I n t r a s e t c r o s s - s t r a t i f i e d sandstone formed i n l a r g e dunes and draas t h a t
m i g r a t e d southward i n response t o winds from t h e n o r t h .
3.
T a b u l a r - and wedge-shaped
s e t s o f h i g h - a n g l e g r a i n f a l l and sand f l o w
l a m i n a e formed i n l a r g e p r o b a b l y t r a n s v e r s e dunes and draas t h a t a l s o m i g r a t e d southward.
4.
Complexly c r o s s - s t r a t i f i e d
and wedge-shaped
sandstone w i t h m o s t l y medium-scale t r o u g h -
s e t s was d e p o s i t e d i n b a r s o r s h o a l s i n wave-
or tidal -
dominated shore1 i n e environments. 5.
Wavy bedded t o r i p p l e - l a m i n a t e d sandstone w i t h a v a r i e t y o f sma 1 - and
medium-scale
sedimentary s t r u c t u r e s formed i n a v a r i e t y o f i n t e r d u n e ,
dune, and s h a l l o w m a r i n e environments.
extra-
579 6.
The above f a c i e s grade l a t e r a l l y southeastward i n t o mudstone, carbonate
and gypsum o f marine, Vertical
and
r e s t r i c t e d inarine, and sabkha o r i g i n .
lateral
facies
distribution
indicate
that
southeastward-
p r o g r a d i n g e o l i a n d e p o s i t s were p e r i o d i c a l l y t r u n c a t e d and o v e r l a i n by marine s h o r e l i n e and a s s o c i a t e d d e p o s i t s .
The r e s u l t i n g sequence i s s t r o n g l y c y c l i c .
L a t e r e o l i a n c y c l e s prograded f a r t h e r southeastward than e a r l i e r d e p o s i t s u n t i l e v e n t u a l l y t h e e n t i r e r e g i o n was b u r i e d by t h e Coconino erg. Erosional
surfaces t h a t
marine transgressions.
base each c y c l e were carved by r a p i d e r o s i o n a l
Permian e u s t a t i c c y c l e s caused by g l a c i a l c y c l e s a r e
b e l i e v e d t o be t h e m a j o r cause.
Subsidence along t h e southeast margin o f t h e
Sedona Arch p r e s e r v e d an average o f 6 m o f sediment between c y c l i c e r o s i o n a l events. The d e p o s i t s o f t h e upper Schnebly H i l l and l o w e r Coconino r e f l e c t changing e o l i a n dune d e p o s i t s and a s s o c i a t e d environments t h a t formed i n response t o v a r i a b l e winds,
sand supply,
a d j o i n i n g environments. connections
between
base l e v e l changes,
tectonics,
and r e l a t i o n s t o
We b e l i e v e t h a t t h i s study has shown r a t h e r c l e a r
process,
product,
and cayse and may p r o v i d e c l u e s f o r
i n t e r p r e t i n g o t h e r a n c i e n t e o l i a n - m a r i n e complexes where these connections a r e more obscure. ACKNOWLEDGEMENTS T h i s paper b e n e f i t e d from reviews by Tom A h l b r a n d t , Mike B r o o k f i e l d , Loope,
Dave
and Jim Steidtmann and t h e i r suggestions f o r improvement a r e g r e a t l y
appreciated.
Some d r a f t i n g was done by Debby Meier.
Raymond o f t h e B i l b y Research Center,
L o u e l l a H o l t e r and Rick
N o r t h e r n Arizona U n i v e r s i t y p r o v i d e d
. t y p i n g and p h o t o g r a p h i c a s s i s t a n c e r e s p e c t i v e l y .
F i e l d t r a v e l was f i n a n c e d by
a NRRI f a c u l t y research grant a t Northern Arizona University.
REFERENCES A h l b r a n d t , T.S., and Fryberger, S.G., 1980, E o l i a n d e p o s i t s i n t h e Nebraska Sand H i l l s . U.S. Geol. Survey Prof. Paper 1120-A, 24 pp. 1980, Pennsylvanian and E a r l y Permian paleogeography, southern Blakey, R. C., Colorado P l a t e a u and v i c i n i t y . In: Fouch, T. 0. and Magathan, E. R. ( e d i t o r s ) , P a l e o z o i c Paleogeography of West-central U n i t e d States, Rocky Mtn. Sect. SOC. Econ. Paleont. and M i n e r a l o g i s t s , pp. 239-257. and T e r w i n d t , J. H. H., 1981, Neap-spring t i d e sequences o f Boersma, J. R., i n t e r t i d a l shoal d e p o s i t s i n a rnesotidal e s t u a r y . Sedimentology, 28:151170. Breed, C. S . , e t al., 1979, Regional s t u d i e s of sand seas u s i n g Landsat (ERTS) imagery. In: McKee, E. D. ( E d i t o r ) , A s t u d y o f g l o b a l sand seas. U.S. G e o l o g i c a l Survey Prof. Paper 1052, pp. 305-398. B r o o k f i e l d , M. E., 1977, The o r i g i n o f bounding s u r f a c e s i n a n c i e n t e o l i a n sediments. Sediinentol ogy, 24:303-332.
580 C r o w e l l , J.C., 1978, Gondwanan g l a c i a t i o n , c y c l o t h e m s , c o n t i n e n t a l p o s i t i o n i n g , and c l i m a t e change. Am. J o u r . Sci., 278:1345-1372. C u r r a y , J. R., 1961, L a t e Q u a t e r n a r y sea l e v e l : a discussion. Geol. SOC. Amer. B u l l . , 72:1701-1712. C u r r a y , J. R., 1964, T r a n s g r e s s i o n s and r e g r e s s i o n s . In: M i l l e r , R. I_. ( E d i t o r ) , Papers i n m a r i n e g e o l o g y , M a c m i l l a n , New York, pp. 175-203. D a v i d s o n - A r n o t t , R. G. D., and Greenwood, B., 1976, F a c i e s r e l a t i o n s h i p s on a b a r r e d c o a s t , Kouchibouguac Bay, New B r u n s w i c k , Canada. I n : D a v i s , R. A, and E t h i n g t o n , R. L. ( E d i t o r s ) , Beach and n e a r s h o r e m a r i n e s e d i m e n t a t i o n . SOC. Econ. P a l e o n t . and M i n e r a l o g i s t s Spec. Pub. 24, pp. 149-168. G e r r a r d , T. A., 1966, E n v i r o n m e n t a l s t u d i e s o f t h e F o r t Apache Member, Supai Formation, East-central Arizona. B u l l . Amer. Assoc. P e t r o l . G e o l o g i s t s ,
50:2434-2463. R. G. D., 1979, S e d i m e n t a t i o n and Greenwood, B., and D a v i d s o n - A r n o t t , e q u i l i b r i u m i n wave-formed b a r s : a r e v i e w and case s t u d y . Can. J o u r . E a r t h Sciences, 16:312-332. H u n t e r , R. E., 1977, B a s i c t y p e s o f s t r a t i f i c a t i o n i n s m a l l e o l i a n dunes. S e d i i n e n t o l ogy, 24:361-387. H u n t e r , R. E., 1981, S t r a t i f i c a t i o n s t y l e s i n e o l i a n s a n d s t o n e s : Some In: P e n n s y l v a n i a n t o J u r a s s i c examples f r o m t h e Western I n t e r i o r 1J.S.A. E t h r i d g e , F. G. and F l o r e s , R. M. ( E d i t o r s ) , Recent and a n c i e n t noninarine SOC. Econ. P a l e o n t . d e p o s i t i o n a l environments: Models f o r e x p l o r a t i o n . and M i n e r a l o g i s t s Spec. Pub. 31, pp. 315-329. Kocurek, G., 1981a, E r g r e c o n s t r u c t i o n : t h e E n t r a d a Sandstone ( J u r a s s i c ) o f n o r t h e r n U t a h and Colorado. Palaeog. P a l a e o c l i m . P a l a e o e c o l o g y , 36:125-
153. Kocurek, G., 1981b, S i g n i f i c a n c e o f i n t e r d u n e d e p o s i t s and b o u n d i n g s u r f a c e s i n a e o l i a n dune sands. S e d i m e n t o l o g y , 28:753-780. K o c u r e k , G., 1983, R e p l y : O r i g i n o f f i r s t - o r d e r b o u n d i n g s u r f a c e s i n a e o l i a n sandstones. Sedimentology, i n press. K o c u r e k , G., and D o t t , R. H., Jr., 1981, D i s t i n c t i o n and uses o f s t r a t i f i c a t i o n types i n t h e i n t e r p r e t a t i o n o f eolian deposits. Jour. Sediment. P e t r o l .
51~579-595. Loope, D.B., 1983, O r i g i n o f e x t e n s i v e p l a n e s i n a e o l i a n s a n d s t o n e s : a d e f e n s e o f Stokes' hypothesis. Sedimentology, i n press. Lupe, R., and A h l b r a n d t , T. S., 1975, Sandstone geometry, p o r o s i t y , and permeability distribution, and fluid migration in eolian system r e s e r v o i r s . U.S. Geol. Survey O p e n - f i l e Rept. 75-357, 23pp. In: McKee, E. D. McKee, E. D., 1979a, S e d i m e n t a r y s t r u c t u r e s i n dunes. G e o l o g i c a l Survey P r o f . Paper ( E d i t o r ) , A s t u d y i n g l o b a l sand seas. U.S. 1052, pp. 83-134. I n : McKee, McKee, E. D., 1979b, A n c i e n t s a n d s t o n e s c o n s i d e r e d t o be e o l i a n . G e o l o g i c a l Survey P r o f . E. D. ( E d i t o r ) , A s t u d y i n g l o b a l sand s e a s : U.S. P a p e r 1052, pp. 187-238. McKee, E. D., 1982, s e d i m e n t a r y s t r u c t u r e s i n dunes o f t h e Narnib D e s e r t , South West A f r i c a . Geol. SOC. America Spec. Paper 188, 64pp. McKee, E. D., Douglass, J. R., and R i t t e n h o u s e , S., 1971, D e f o r m a t i o n o f l e e s i d e l a m i n a e i n e o l i a n dunes. B u l l . Geol. SOC. America, 82:359-378. P e i r c e , H. W., and G e r r a r d , T. A., 1966, E v a p o r i t e d e p o s i t s o f t h e Permian H o l b r o o k B a s i n , A r i z o n a . N o r t h e r n O h i o Geol. SOC. Second Symp. on S a l t , v. 1, pp. 1-10. d e Raaf, J. F. M., Boersma, J. R., and van G e l d e r , A., 1977, Wave-generated s t r u c t u r e s and sequences from a shallow marine succession, Lower C a r b o n i f e r o u s , County Cork, I r e l a n d . S e d i m e n t o l o g y , 24:451-483. R e i c h e , P., 1938, An a n a l y s i s o f c r o s s - l a i n i n a t i o n - - T h e C o c o n i n o Sandstone. J o u r . Geology, 46:905-932. R u b i n , D. M., and H u n t e r , R. E., 1982, Bed f o r m c l i m b i n g i n t h e o r y and nature. Sedi m e n t o l ogy , 29 :12 1- 138. Sharp, R. P, 1963, Wind r i p p l e s . Jour. Geology, 71:617-636. Van E y s i n g a , F.S.B., 1975, G e o l o g i c a l t i m e t a b l e . E l s e v i e r , Amsterdam.
581 Waterhouse, J . B . , 1978, C h r o n o s t r a t i g r a p h y f o r t h e W o r l d Perinian. In: C o n t r i b u t i o n s t o t h e G e o l o g i c Time Scale. Amer. Assoc. P e t r o l . G e o l o g i s t s S t u d i e s i n Geology, No. 6, pp. 299-322. W i l s o n , I . G., 1971, D e s e r t s a n d f l o w b a s i n s and a model f o r t h e development o f ergs. Geog. J o u r n a l , 137:180-199. Wilson, I. G., 1972, Aeolian bedforms--their development and o r i g i n s . S e d i m e n t o l o g y , 19: 173-210. W i l s o n , I. G., 1973, Ergs. Sed. Geology, 10:77-106.
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583
AEOLIAN SANDS TERMINATING AN EVOLUTION OF FLUVIAL DEPOSITIONAL ENVIRONMENT I N MIDDLE BUNTSANDSTEIN (LOWER TRIASSIC) OF THE EIFEL, FEDERAL REPUBLIC GERMANY DETLEF MADER: R B t t g e r s t r a s s e 20, D-3000 Hannover 91, Fed. Repub. Germany
INTRODUCTION A e o l i a n dune sands have formed on t h e e a r t h s i n c e t h e e a r l y Precambrian (Ross and Donaldson, 1982) i n a wide v a r i e t y o f environments. I n t h e Phanerozoic, a e o l i a n sands o c c u r most a b u n d a n t l y i n t h e C a r b o n i f e r o u s t o J u r a s s i c (McKee and B i q a r e l l a , 1979), where r e d bed dune sequences a r e c o n c e n t r a t e d i n t h e Permian and T r i a s s i c and o c c u r on most c o n t i n e n t s d u r i n g a t i m e when t e r r e s t r i a l areas were p r o b a b l y g r e a t e r i n e x t e n t t h a n a t any o t h e r t i m e i n g e o l o g i c a l h i s t o r y (Waugh, 1973; Tucker and Benton; 1983). T r i a s s i c a e o l i a n sands a r e found i n Europe, IJorth America, South America and South A f r i c a ( t a b l e 1). T r i a s s i c a e o l i a n sands i n c e n t r a l Europe o c c u r i n t h e m a i n l y f l u v i a l Lower T r i a s s i c B u n t s a n d s t e i n Formation, which was d e p o s i t e d i n a l a r q e b a s i n e x t e n d i n g from B r i t a i n t h r o u q h t h e N o r t h Sea t o t h e N e t h e r l a n d s , where i t widens and s t r e t ches f r o m France and Germany i n t h e west t o Poland and B y e l o r u s s i a i n t h e e a s t and f r o m S w i t z e r l a n d i n t h e s o u t h t o Scandinavia i n t h e n o r t h ( f i g . l ) , I n t h e c e n t r a l p a r t o f t h i s Mid-European Basin, a e o l i a n sands o c c u r ; i n t h e E i f e l area, i n t h e Saar and P f a l z areas, a t t h e e a s t e r n m a r g i n o f t h e Rhenish M a s s i f , i n t h e Hessian d e p r e s s i o n , i n Helgoland, i n J u t l a n d , and i n t h e H o l y Cross Mountains o f Poland. On t h e western margin, dune sands a r e p r e s e n t i n Cheshire and Devon, as w e l l as i n S c o t l a n d ( t a b l e 1). Most s t u d i e s o f a e o l i a n sands o f whatever s t r a t i q r a p h i c a l and palaeogeographi c a l p o s i t i o n p u b l i s h e d i n t h e l a s t decade emphasized r e c o q n i t i o n o f w i n d - l a i d d e p o s i t s (e.g.
Hunter, 1977, 1981; Kocurek and D o t t , 1981; A h l b r a n d t and F r y b e r -
ger, 1980, 1981, 1982) o r p r e s e n t e d i n t e r p r e t a t i o n s o f l o c a l , r e g i o n a l and even m u l t i - r e g i o n a l f a c i e s a s s o c i a t i o n s r e s u l t i n g i n r e c o n s t r u c t i o n o f a n c i e n t sand sea d e p o s i t s (e.g.
Adams and P a t t o n , 1979; F r y b e r g e r , 1979; Kocurek and D o t t ,
1981; Kocurek, 1981a; Blakey and Pliddleton, 1983). R e c o n s t r u c t i o n o f t h e deposi t i o n a l h i s t o r y which l e d t o sand sea f o r m a t i o n have, however, o n l y been b r o a d l y sketched o u t and n o t o u t l i n e d i n d e t a i l ( c f . Clemmensen, 1980; Clemmensen e t al., 1980; Kocurek and D o t t , 1982). T h i s paper i s t h e f i r s t t o summarize t h e e v o l u t i o n o f a f l u v i a l sedimentary environment which t o o k p l a c e d u r i n g t h e B u n t s a n d s t e i n p e r i o d i n t h e E i f e l a t t h e
584 western m a r g i n o f t h e Mid-European T r i a s s i c Basin, and t o p o i n t o u t t h e r e l a t i o n s h i p between changes i n f l u v i a l s t y l e and o r i g i n o f ai: a e o l i a n dune b e l t . The r e g i o n a l d i v e r s i f i c a t i o n o f d e p o s i t i o n a l environments i n t h e t e r m i n a l phase
of e v o l u t i o n o f t h e f l u v i a l environment i s d e s c r i b e d , and t h e r e l a t i o n s h i p s o f t h e a e o l i a n and f l u v i a l f a c i e s used t o d e f i n e a model o f sand dune f o r m a t i o n . TABLE 1.
OCCURRENCE OF TRIASSIC AEOLIAN DUNE SANDS Federal Republic of Germany Eifel Pfolz and Saor Eastern margin of Rhenish Massif Hessian Depression Helaoland
MADER 11980.1981.1982.1983) DACHROTH 11980) TIETZE (pers commun 1980) WYCISK (1983) CLEMMENSEN (1979)
Poland Holy Cross Mountains
GRADZINSKI. GAGOL 8 SLACZKA (19791
Scandinavia Jutland/Denmark Oslo Graben/Norway (?)
PEDERSEN 8 ANDERSEN (1980) RAMBERG 8 SPJELDNAES (1978)
Great Britain Cheshire/England Oevon/England Irish Sea and Cumbrio/England Moray Firth/Scotland North Minch/Scotland
THOMPSON (1969). HUOUE (1983) LAMING 11954.1966) BURLEY (pers. commun.1981). ALI (1982) TROMMESTAD (1982). GLENNIE 8 BULLER (1983) JOHNSEN (1981)
North Sea Central North Sea Basin Shetland Basin (?I
JAKOBSSON et 01. (1980). FISHER (1982) RlDD 119811, FISHER (1982)
Greenland East Greenland
CLEMMENSEN 119801
North America Nova Scotia Colorado and Utah Arizona
HUBERT 8 MERTZ (1980.1983) POOLE 11962). WAUGH 119731 MIDDLETON. BLAKEY 8 GREGG-SARGENT 11983)
South America Brazil
MABESOONE (1977). FARIA (19821
South Africa Swaziland Zululand
MINTER 8 TURNER (1981) DIJK. HOBDAY 8 TANKARD (1978)
Soviet Union Russian Platform I?) Siberian Platform ( ? I
LAPKIN et 01 11973) TUCKER 8 BENTON (1983)
China Shonxi
I?)
YEH (pers. commun. 1982)
585 GEOLOGICAL SETTING The Mid-European B a s i n i s surrounded by p r e - T r i a s s i c basement m a s s i f s ; t h e d e p o s i t i o n a l a r e a i s bounded by t h e A r d e n n i a n - G a l l i a n M a s s i f i n t h e west, t h e Fennoscandian S h i e l d i n t h e n o r t h , t h e B a l t i c - R u s s i a n S h i e l d i n t h e e a s t , and t h e Bohemian-Vindelician-Precarpathian M a s s i f i n t h e s o u t h ( f i q . 1).
F i g . 1. Palaeogeographical map o f t h e Lower T r i a s s i c i n Europe ( c f . Mader, 1 9 7 3 ~ ) 1 = basement. 2 = d e p o s i t i o n a l areas. 3 = p a l a e o c u r r e n t d i r e c t i o n s . 4 = palaeowind d i r e c t i o n s . T e c t o n i c s o m i t t e d . W i t h i n t h e b a s i n , t h e Rhenish M a s s i f a t t h e western end, and t h e RynkdbinqFyn High a t t h e iTorthern m a r g i n a r e m a j o r s w e l l s o r areas o f n o n - d e p o s i t i o n d u r i n g B u n t s a n d s t e i n t i m e . P a l a e o c u r r e n t s were n o r t h w a r d l y d i r e c t e d i n t h e southern p a r t o f t h e b a s i n , and s o u t h w a r d l y d i r e c t e d i n t h e n o r t h e r n p a r t . Palaeowinds r e f l e c t m a i n l y s o u t h e a s t e r l y t o s o u t h w e s t e r l y t r a d e winds i n t h e summer when t h e I n t e r t r o p i c a l Convergence Zone was s h i f t e d t o t h e n o r t h . Thicknesses o f Buntsands t e i n d e p o s i t s i n c r e a s e towards t h e b a s i n c e n t r e and a r e m o d i f i e d by t h e r e g i o n a l s w e l l s and t r o u g h s . The E i f e l a r e a i s s i t u a t e d a t t h e w e s t e r n m a r g i n o f t h e Mid-European Basin, i n a n o r t h - s o u t h zone between t h e A r d e n n i a n - G a l l i a n M a s s i f and t h e Rhenish M a s s i f . T r i a s s i c d e p o s i t s o c c u r i n d i f f e r e n t f a c i e s a s s o c i a t i o n s i n n o r t h e r n E i f e l (Mech-
58 6 e r n i c h a r e a ) , western E i f e l ( S t a d t k y l l area, O b e r b e t t i n g e n area, n o r t h e r n T r i e r
N 0 AECHERNICH
R T
\REA
H E R N
ITADTKVLL
ZREA
W
E >BERBETTINGEN
4REA
S
T
E VORTHERN
IRlER
R N
4REA
SOUTHERN
TRIER
AREA
S 0 U T
H E R N
60
70
80
90
100
110
120
130
F i g . 2. G e o l o g i c a l s k e t c h of t h e E i f e l (redrawn a f t e r DahlgrUn,l939; S c h r i e l , 1939; Knapp, 1980; B i n t z and M u l l e r , 1974), and l o c a t i o n o f t h e a r e a w i t h i n t h e n o r t h e a s t e r n p a r t o f c e n t r a l Europe (redrawn a f t e r Wal t h e r and Zitzmann, 1981). 1 = Devonian basement. 2 = B u n t s a n d s t e i n . 3 = Pluschelkalk and Keuper ( M i d d l e t o Uoper T r i a s s i c ) . 4 = T e r t i a r y and Q u a t e r n a r y ( o n l y i n N i e d e r r h e i n i s c h e Bucht). 5 = western boundary o f M i d d l e B u n t s a n d s t e i n d i s t r i b u t i o n . Numbers a t r i g h t and t o p - German g r i d c o o r d i n a t e s . Numbers a t l e f t and base Luxemburg g r i d c o o r d i n a t e s . I n s e t a t upper l e f t : b l a c k = o u t c r o p p i n g Buntsandstein.
587
The Buntsandstein sequence i s divided i n t o Middle and Upper Buntsandstein which in turn a r e s p l i t i n t o various members ( t a b l e 2 and f i g . 3 ) . The Middle Euntsands t e i n i s only f u l l y developed in southern E i f e l ; in western E i f e l , Karlstal-Schichten immediately o v e r l i e the eroded Variscan basement; a n d in northern E i f e l , subdivision of t h e Middle Buntsandstein i s s t i l l uncertain due t o the unusually coarse f a c i e s .
TABLE 2 .
STRATIGRAPHICAL DISTRIBUTION OF DEP0SIT I0NAL FACIE S IN THE BUNTSANDSTEIN [LOWER TRIASSIC)
OF THE EIFEL (GERMANY)
Aeolian Fluvial
I
I
Deflation lag deposits lnterdune sheet sands Dune sands Topstratum deposits Channel bar sediments Channel laa deposits
Mud-f low Alluvial-fan
The Buntsandstein in the Eifel c o n s i s t s of sediments of various o r i q i n s s c a t t ered throughout the sequence ( t a b l e 2 ) . Aeolian and l a c u s t r i n e sediments occur in t h e upper p a r t of t h e Middle Buntsandstein (Karlstal-Schichten) in regionally varying amounts and a r e associated with v a r i a b l e f l u v i a l f a c i e s .
588
STRATIGRAPHY OF THE BUNTSANDSTEIN IN THE EIFEL SAAR AREA
SOUTHERN WESTERN NORTHERN EIFEL EIFEL EIFEL
Fiq. 3. Stratigraphy of the Buntsandstein (Lower T r i a s s i c ) in the Eifel ( c f . Mader, 1981a). Dominant lithology: 1 = carbonate; 2 = s i l t and c l a y ; 3 = sandstone 4 = conglomerate; 5 = very coarse conglomerate (with numerous boulders); 6 = carbonate breccia (Brdkelbank); 7 = v i o l e t horizon ( V i o l e t t e r Horizont); 8 = carbonate concretions and c r u s t s ; 9 = pre-Buntsandstein. DISTIIKTION OF AEOL1AI.I AND FLUVIAL DEPOSITS Examination of s t r a t i f i c a t i o n types in t h e l a s t few years has r e s u l t e d in the d i f f e r e n t i a t i o n of aeolian and \rater-laid s t r a t a found in various ancient dune sequences (Hunter, 1977, 1981; Kocurek and Dott, 1981; Kocurek, 1981a,b; Ahlbrandt and Fryberger, 1981, 1982). However, t h e tliddle Buntsandstein sequence in t h e Eifel demonstrates t h a t recognition of aeolian sands i s n o t possible with a few c r i t e r i a taken alone, b u t may be d i f f e r e n t i a t k d by a combination of characte r i s t i c s , including s t r a t i f i c a t i o n , composition, i n t e r c a l a t i o n , t r a n s p o r t d i r e c t ions, petrography and t e x t u r e , and deformation. Table 3 summarizes the c r i t e r i a used f o r t h e Eifel lliddle Buntsandstein ( s e e a l s o Mader, 1982a).
AEOLIAN FACIES Aeolian sediments occur mainly in the upper p a r t of t h e Middle Buntsandstein (Karl s t a l -Schichten) and a r e c l a s s i f i e d , according t o Ah1 brandt and Fryberqer (1981), Kocurek (1981) and Hunter (1981), i n t o dune sands and interdune sediments (sandplain of Pye, 1982)(table 4 ) . Dune sands a r e divided i n t o t h i c k dune sequences and i n t o t h i n l a y e r s which occur both a t t h e t o p of f l u v i a l cyclothems and a t t h e base of l a c u s t r i n e cyclothems. Interdunes a r e broadly divided i n t o depositiona l and deflationary interdunes; t h e former beinq c l a s s i f i e d i n t o dry, damp, and wet interdunes. Dry interdune deposits a r e aeolian sheet sands: damp interdune
58 9
TABLE 3.
Criteria for distinction o f aeolian and fluvial deposits.
CRITERIA F O R AEOLIAN DEPOSITION 1. Strat i f ication 1 Presence of typical micro-lamination Occurrence of millet seed sand Rarity of trough-type pi-cross-bedding Occurrence of wedge-shaped cross-stratification Large- t o medium-scale cross-bedding 6 Partially high angles of foreset inclination 7 Occurrence of foreset and backset beds 8 Planar or slightly concave-upwards erosional surfaces 9 Absence of scour-and-fill structures 10 Occasionally remnants of dune morphology preserved
2 3 L 5
2. Composition 1 Absence of cyclic composition 2 Occurrence of stacked dune sand sequences 3 Occurrence of isolated cross-bedded dune sands in horizontal-laminated successions L Occurrence of thin aeolian sands capping fluvial cyclothems
3. Intercalations 1 Occurrence of deflation gravel lags, partially with ventifacts
2 Absence of dispersed pebbles 3 Absence of intraformational reworking horizons
1 Intercalation of thin silty-clayey lacustrine sediments 5 Intercalation of aquatically relaid dune sands, massive tp crudely stratified 6 Bioturbation and trace fossils restricted to lacustrine and fluvial intercalations 7 Occurrence of adhesion-rippled layers
8 Absence of wave- and current-ripple trains 9 Occurrence of curled mud flakes (occasionally) 10 Absence of erosional marks on bedding planes
4. Transport directions 1 Narrow distribution of local palaeowind directions 2 Concordant palaeowinds throughout the sequence
5. Petrography and texture 1 Absence of mica
2 Rarity of authigenic tourmaline and rutile 3 Weak lithification by slender quartz overgrowths
1 Abundance of nest burrows of Recent solitary bees 5 High textural and mineralogical maturity 6 Frosted grain surfaces
6. Deformation 1 Occurrence of small-scale bedding deformations 2 Slump deformations and contorted bedding 3 Slump or tensional faults
7. Miscellaneous 1 2 3 L 5
Orange to medium red colour Absence of lateral coarsening in the sedimentation area Position in evolution of fluvial sedimentary environment Large lateral extent of dune sand sequence Occurrence of aeolian deposits in various other marginal parts of the basin
590
CRITERIA FOR FLUVIAL DEPOSITION 1 Stratification 1 Absence of micro-lamination
2 Absence of millet seed sand 3 Abundance of trough-type pi-cross-bedding I Absence of wedge-shaped cross-stratification 5 Large- to small-scale cross-bedding 6 Lower angles of foreset inclination 7 Random occurrence of divergent dips 8 Occurrence of irregular erosional surfaces 9 Abundance of scour-and-fill structures 10 Bedform morphology usually destroyed by erosion
2. Composition 1 Predominance of cyclic composition
2 Occurrence of cyclic sequences and multistorey bar deposits 3 Unidirectional transition from cross-stratification to horizontal-lamination and vice versa I Occurrence of thin fluvial sediments as intercalations in aeolian sequences
3. Intercalations 1 2 3 I 5 6 7 8 9 10
Occurrence of channel lag deposits Abundance of dispersed pebbles in varying size and number Abundance of intraformational reworking horizons Thick silty-clayey sediments at the top of the cyclothems Massive to crudely stratified sandstones restricted to the base of the cyclothems Partially bioturbation in sediments and trace fossils on bedding planes Absence of adhesion-rippled layers Occurrence of numerous wave- and current-ripple trains Occurrence of curled mud flakes (very rare) Occurrence of flute casts a t the base of bar sediment series
4. Transport directions 1 Wider distribution of local palaeocurrent directions 2 Partially divergent palaeoflows in different cyclothems
5. Petrography and texture 1 Partially presence of mica
2 Occasionally abundance of authigenic tourmaline and rutile 3 Stronger lithification by broad quartz envelopes L Rarity of nest burrows of Recent solitary bees 5 High to medium textural and mineralogical maturity 6 Frosted and non-frosted grain surfaces
6 Deformation 1 Absence of small-scale bedding deformations
2 Sand dikes and injection structures 3 Recumbent cross-bedding (rare)
7 Miscellaneous 1 Medium to dark red colour
2 Distinct lateral coarsening in the sedimentation area
sediments are adhesion-rippled layers: wet interdune deposits are aquatically redeposited aeolian sands, silty and clayey pond sediments and playa deposits. Deflationary interdunes contain gravel lags. The different aeolian facies are briefly outlined below and illustrated on plates 1 to 3: more detailed descriptions and interpretations are in rlader (1982a).
591
IN THE EIFEL MIDDLE BUNTSANDSTEIN
I DUNE
I
SANDS
t
II DRY INTERDUNE
DEPOSITIONAL
DEPOSITS DAMP INTERDUNE DEPOSITS
INTERDUNE INTERDUNE
WET INTERDUNE
DEPOSITS
SEDIMENTS
DEPOSITS
I
DEFLATIONARY INTERDUNE SEDIMENTS
TABLE 4. T h i c k c r o s s - s t r a t i f i e d a e o l i a n sand sequences o r i g i n a t e d as barchanoid-type dunes (-
McKee, 1979) which accumulated i n unimodal s o u t h e a s t e r l y t o south-
w e s t e r l y t r a d e winds i n l o w n o r t h e r n p a l a e o l a t i t u d e s i n summer, when t h e i n t e r t r o p i c a l convergence zone was s h i f t e d t o t h e n o r t h . The a e o l i a n sands were m a i n l y d e p o s i t e d by g r a i n f a l l , i n r e g i o n a l l y v a r i a b l e and p r e d o m i n a n t l y m i n o r amounts a l s o b y g r a i n f l o n and s u b c r i t i c a l c l i m b i n g o f wind r i p p l e s , on l e e s l o p e s o f t r a n s v e r s e dunes w i t h r e l a t i v e l y s t r a i g h t s l i p f a c e s . According t o t h e a v a i l a b i l i t y o f l a r g e amounts o f sand, t h e a e o l i a n d e p o s i t s have n o t formed as i s o l a t e d
dunes, b u t b u i l t up l a r g e sand seas which were i n t e r s e c t e d by b r a i d e d r i v e r channel systems accompanied by narrow o r wide f l o o d p l a i n s depending on channel s i n u o s i t y and spacing. H o r i z o n t a l - l a m i n a t e d a e o l i a n sands o r i g i n a t e d as sand sheets i n d r y i n t e r d u n e areas, r a n g i n g i n s i z e f r o m l o n g , narrow c o r r i d o r s between barchanoid r i d g e s t o v e r y wide, f l a t s u r f a c e s a c c o r d i n g t o v a r y i n g spacing o f t h e dunes. S u b c r i t i c a l c l i m b i n g t r a n s l a t e n t s t r a t i f i c a t i o n p o i n t s t o a t l e a s t p a r t i a l o r i g i n o f sheet sands by wind r i p p l e m i g r a t i o n , w h i l e o t h e r h o r i z o n t a l - l a m i n a t e d d e p o s i t s were formed as p l a n e beds. T h i n a e o l i a n sands capping f l u v i a l cyclothems o r i n t e r b e d d e d w i t h t h i c k f l u v i a l d e p o s i t s o r i g i n a t e d by d e f l a t i o n o f emerged a l l u v i a l b a r sands d u r i n g e x t r a o r d i n a r i l y l o w w a t e r stages i n t h e channels. The a e o l i a n sand was blown i n t o d r y
592
Plate 1. 1 : L a r g e - s c a l e c r o s s - s t r a t i f i e d a e o l i a n dune s a n d s . The s e t s a r e s e p a r a t e d by a trouqh-shaped 2 r o s i o n a l s u r f a c e which o r i q i n a t e d by i n t e r d u n e blowout. Lenqth o f hailiiiier - 29 crn. Rocks a t t h e s o u t h e a s t e r n f l a n k of a h i l l north of t h e s p o r t s q r o u n d s o u t h e a s t o f Birqel ( S h e e t S t a d t k y l l , r 45 020, 11 76 1 8 0 ) . 2 : Larqe t o iiledium s c a l e cross-bedded a e o l i a n dune s a n d s . The s e t s w i t h i n t h e c o s e t s a r e s e p a r a t e d by h o r i z o n t a l second-order i i i t e r s e t boundaries. Some low-anqle t h i r d - o r d e r i n t r a s e t s u r f a c e s o c c u r w i t h i n t h e lower p a r t of the s e t s . The d i p a n q l e o f t e n i n c r e a s e s froiii t h e base t o t h e t o p o f t h e s e t s . Lengttl of haiiiiiier - 29 cin. Rocks a t t h e s o u t h e r n f l n a k o f t h e f l i n g e l s t e i n n e a r Biersdorf ( S h e e t b i a x w i l e r , r 31 5 6 0 , h 42 4 6 0 ) . 3 : L a r g e - s c a l e c r o s s - s t r a t i f i e d a e o l i a n dune s a n d s . The s t e e p l y d i p ping laiiiinae n e a r t h e t o p o f t h e s e t f l a t t e n c o n s i d e r a b l y i n a n q l e towards t h e b a s e . The upper f i r s t - o r d e r diastein i s o v e r l a i n by h o r i z o n t a l - l a m i n a t e d d r y i n t e r d u n e s h e e t s a n d s . Length o f liavmer - 29 ciii. P i t a t t h e n o r t h w e s t e r n f l a n k o f t h e Burgberg s o u t h e a s t of Birqel ( S h e e t S t a d t k y l l , r 45 150, h 75 3 8 0 ) . 4 : D e f l a t i o n a r y i n t e r d u n e q r a v e l l a g a t t h e t o p o f pebbly f l u v i a l channel b,ar d e p o s i t s , o v e r l a i n by a cross-bedded a e o l i a n dune sand. The pebbles were c o n c e n t r a t e d by winnowing o f t h e sand-sized p o n u l a t i o n and foriiied an i n t e r dune f l o o r pavei1,erlt inipedinq f u r t h e r d e f l a t i o n . Lenqth o f hairiirier - 29 cm. Road c u t between T r i e r and Bitburq a t t h e s o u t h e r n f l a n k of t h e Gldsqes-Berq, n o r t h west o f T r i e r - P a l l j e n ( S h e e t T r i e r , r 44 650, h 14 9 8 0 ) .
5 93
P l a t e 2. 1 : An i s o l a t e d s i n g l e - s e t , l a r g e - s c a l e c r o s s - s t r a t i f i e d a e o l i a n dune sand i s i n t e r c a l a t e d i n t o a sequence o f h o r i z o n t a l - l a m i n a t e d , a l t e r n a t i n g d r y t o w e t i n t e r d u n e s a n d s . The u p p e r a n d l o w e r f i r s t - o r d e r d i a s t e n i s a r e smooth and f l a t i n d i c a t i n q a n e a r l y f e a t u r e l e s s dune f l o o r . W i d t h o f e x p o s u r e a b o u t 9 m e t r e s . Rocks a t t h e e a s t e r n s i d e o f t h e Enz r i v e r v a l l e y , s o u t h o f S i n s p e l t ( S h e e t I l e t t e n d o r f , r 23 500, h 36 6 0 0 ) . 2 : T h i n l a c u s t r i n e c l a y l a y e r s ( 2 - 4 cni. t h i c k ) a r e i n t e r c a l a t e d i n t o h o r i z o n t a l - l a m i n a t e d , a1 t e r n a t i n g d r y a e o l i a n s h e e t sands and w e t i n t e r d u n e p l a y a s a n d s . The c l a y s e t t l e d o u t f r o m s u s p e n s i o n i n s h a l l o w i n t e r d u n e p l a y a l a k e s w h i c h o r i g i n a t e d f r o i i i heavy ephemeral r a i n f a l l o r f l u v i a l i n c u r s i o n s . H e i q h t o f e x p o s u r e a b o u t 1 m e t r e . Rocks a t t h e s o u t h e r n f l a n k o f t h e C I H l l e r s b e r g n o r t h w e s t o f L i s s i n g e n ( S h e e t G e r o l s t e i n , r 44 100, h 65 4 7 0 ) . 3 : M i c r o - l a m i n a t e d , c r o s s - b e d d e d a e o l i a n sands a r e o v e r l a i n b y m a s s i v e t o c r u d e l y s t r a t i f i e d w a t e r r e l a i d dune sands. R e d e p o s i t i o n o f a e o l i a n sands i n subaqueous e n v i r o n i i i e n t t o o k p l a c e f o l l o w i n r l h e i v y ephemeral r a i n f a l l o r f l u v i a l i n c u r s i o n s . L e n g t h o f h a i m e r - 29 ci’i. Rocks a t t h e s p o r t s g r o u n d o f B i r g e l ( S h e e t S t a d t k y l l , r 4 5 200, h 7 6 090). 4 : T h i n n i i c r o - l a i i i i : v . t e d s i n g l e - s e t c r o s s - s t r a t i f i e d a e o l i a n dune sand i s i n t e r c a l a t e d i n t o a t h i c k sequence o f h o r i z o n t a l - l a m i n a t e d , p r e d o i i i i n a n t l y wet i n t e r d u n e sands. A s m a l l b a r c h a n o i d - t y p e dune m i g r a t e d a c r o s s t h e i n t e r d u n e f l o o r when t h e p l a y a was o c c a s i o n a l l y c o m p l e t e l y d r i e d up. L e n g t h o f hammer - 29 ciii. Quarry a t the eastern side o f the Kailbach r i v u l e t valley, northwest o f iiiederk a i l ( S h e e t L a n d s c h e i d , r 52 500, h 38 9 5 0 ) .
594
Plate 3. 1 : Larqe- t o medium-scale c r o s s - s t r a t i f i e d a e o l i a n dune s a n d s . Some m c a l e s e t s a r e i n t e r c a l a t e d in t h e upper p a r t . The smooth, even secondo r d e r d i a s t e m s i n d i c a t e a n e a r l y f e a t u r e l e s s i n t e r d u n e f l o o r . Length of hammer head - 19 cm. Q u a r r y i n t h e Schmidtheim f o r e s t s o u t h e a s t o f t h e road from Dahleiii t o Blanken:ieir,i ( S h e e t Blankenheim, r 41 600, h 86 3 4 0 ) . 2 : Large- t o medium-scale cross-bedded a e o l i a n s a n d s . The wedge shaped s e t s r e s u l t from windward o r downwind d i p o f some second-order lower bounding s u r f a c e s . Height of exposure a b o u t 1 m e t r e . Same l o c a l i t y a s 1. 3 : F l u v i a l conglomerates o f t h e basal Upper B u n t s a n d s t e i n a r e trunca t i r g , with an i r r e g u l a r e r o s i o n a l s u r f a c e , a e o l i a n s a n d s . The p r e s e r v a t i o n of s c o u r pools with s t e e p f l a n k s i n t h e dune sediments r e q u i r e s s t a b i l i z a t i o n of t h e l o o s e sand by hurnidhesion. Length o f exposure a b o u t 1 . 5 m e t r e s . Q u a r r y a t t h e n o r t h e r n f l a n k o f t h e Erzberg s o u t h of B i e r s d o r f ( S h e e t Waxweiler, r 31 880, h , 42 4 0 0 ) . 4 : A s t e e p l y d i p p i n g c r o s s - s t r a t i f i e d dune sand r f i t h s e v e r a l q e n t l y i n c l i n e d t h i r d - o r d e r i n t r a s e t s u r f a c e s i s i n t e r c a l a t e d with low-angle cross-bedded and h o r i z o n t a l - l a m i n a t e d a e o l i a n s h e e t s a n d s . Length o f haninier - 29 ern. Rocks e a s t of t h e llechernich t o Satzvey r o a d , e a s t of Katzvey ( S h e e t Euskirchen, r 48 740, h 07 7 2 0 ) . 5 : F1 u v i a l channel bar sediments c o n t a i n i n g numerous i n t r a f o r r n a t i o n s l c l a y f l a k e s , c u t with i r r e g u l a r e r o s i o n a l r e l i e f i n t o a e o l i a n s a n d s . The dune f i e l d was l o c a l l y scoured by a f l u v i a l i n c u r s i o n d u r i n q t h e m i g r a t i o n of an a l l u v i a l c h a n n e l . Length o f hammer - 29 cm. Same l o c a l i t y a s p l a t e 1 , no. 2.
595 p a r t s of t h e s u r r o u n d i n g f l o o d p l a i n and was e i t h e r p i l e d up t o f o r m barchanoid d u n e l e t s m i g r a t i n g a c r o s s t h e a r e a o f overbank d e p o s i t i o n , o r formed sheet sand d e p o s i t s by s u b c r i t i c a l c l i m b i n g o f m i g r a t i n g wind r i p p l e t r a i n s . T h i n a e o l i a n sands i n i t i a t i n g l a c u s t r i n e cyclothems were l a i d down on d r y i n t e r d u n e s u r f a c e s , t h e sand m a i n l y d e r i v i n g f r o m d e f l a t i o n o f f l u v i a l i n c u r s i o n s o r r e p r e s e n t i n g r e s i d u a l sand n o t h a v i n g been i n c o r p o r a t e d i n t o l a r g e r dunes o f t h e s u r r o u n d i n g sand sea. I n c r e a s i n g dampness o f t h e p l a y a f l o o r by s l o w l y r i s i n g w a t e r t a b l e , o r a t m o s p h e r i c a l l y d e r i v e d m o i s t u r e , l e d t o t h e o r i g i n o f adhesionr i p p l e d l a y e r s by t r a p p i n g o f sand blown a c r o s s t h e damp s u r f a c e . Continuous w a t e r t a b l e r i s e , ephemeral r a i n f a l l , o r f l u v i a l i n c u r s i o n s , f i n a l l y r e s u l t e d i n f l o o d i n g o f t h e i n t e r d u n e pl.aya, and t h e wet s u r f a c e c o n d i t i o n s t e r m i n a t e d a e o l i a n deposition. D e f l a t i o n a r y i n t e r d u n e g r a v e l l a g s o r i g i n a t e d by a e o l i a n winnowing o f t h e sands i z e d p o p u l a t i o n o f p e b b l y f l u v i a l sediments, c o n c e n t r a t i n g t h e g r a v e l i n patches o r veneers on t h e i n t e r d u n e f l o o r . D e f l a t i o n o f t h e f l u v i a l sediments c o n t i n u e d u n t i l a c e r t a i n t h i c k n e s s o f g r a v e l was reached which impeded f u r t h e r winnowing. DISTRIBUTION
OF AEOLIAN SANDS
The a e o l i a n sediments a r e n o t e q u a l l y d i s t r i b u t e d e i t h e r t h r o u g h o u t t h e E i f e l area o r w i t h i n t h e M i d d l e B u n t s a n d s t e i n succession ( t a b l e 4 ) . I n p a r t i c u l a r , t h e most prominent a e o l i a n sands ( t h i c k dune sequences, t h i n d u n e l e t l a y e r s and i n t e r d u n e sheet sands) show d i f f e r e n t d i s t r i b u t i o n s i n Southern, Western and N o r t h e r n E i f e l ( f i g s . 4, 5 ) . Southern E i f e l I n t h e Southern E i f e l (Mader, 1981a), a e o l i a n sands a r e absent i n t h e e n t i r e l y a l l u v i a l T r i f e l s - S c h i c h t e n succession, which c o n s i s t s a l m o s t e x c l u s i v e l y o f
stacked c o n g l o m e r a t i c channel b a r d e p o s i t s . A e o l i a n sands o c c u r o c c a s i o n a l l y i n t h e Rehberg-Schichten sequence as t h i n l a y e r s cappinq f l u v i a l cyclothems. Widespread dune sands a r e p r e s e n t o n l y i n t h e K a r l s t a l - S c h i c h t e n ,
m a i n l y as sequences
o f 5 t o 20 m e t r e t h i c k n e s s which a r e separated by m u l t i s t o r c y p e b b l y f l u v i a l bar d e p o s i t complexes ( t h e s o - c a l l e d Felszonen o f Mader, 1981a), which sometimes scour up t o s e v e r a l m e t r e s down i n t o t h e a e o l i a n sands ( s e c t i o n s 3 and 4 o f f i g . 5 ) . The dune sequences between t h e s t a c k e d main complexes o f t e n c o n t a i n i n t e r c a l a t i o n s o f f l u v i a l cyclothems b u i l t up o f p e b b l y and/or sandy b a r d e p o s i t s and sandy, s i l t y , p a r t i a l l y b i o t u r b a t e d sediments. T h i n a e o l i a n sands o c c a s i o n a l l y cap f l u v i a l cyclothems i n t h i c k e r a l l u v i a l successions i n t e r b e d d e d w i t h t h e dune sequences, and o c c u r a t t h e base o f l a c u s t r i n e cyclothems i n t h i c k e r i n t e r dune p l a y a sediment assemblages ( s e c t i o n 5 i n f i g . 5 ) . S i m i l a r i n t e r c a l a t i o n s of
f l u v i a l conglomerates o r sandstone complexes i n t o a e o l i a n sequences a r e d e s c r i b e d by Clemmensen and Abrahamsen (1982) and H u b e r t and Metz (1982).
596
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Fig. 4. Occurrence of aeolian dune sands in the Middle Buntsandstein (Lower T r i a s s i c ) in the Eifel . 1 - thick dune sequences only divided by diastems: 2 - thick dune sequences with i n t e r c a l a t i o n s of interdune d e p o s i t s overlying erosional boundaries. 3 - thick dune sequences with i n t e r c a l a t i o n s of pebbly o r sandy f l u v i a l sediments. 4 t o 6 - t h i n l a y e r s a t t h e t o p of f l u v i a l cyclothems ( 4 - Karlstal-Schichten, Southern E i f e l . 5 - Karlstal-Schichten, Western E i f e l . 6 - Karlstal-Schichten, Northern E i f e l ) . 7 - t h i n l a y e r s a t t h e base of l a c u s t r ine cyclothems. Legend: 1 - bouldery t o cobbly f l u v i a l channel sediments; 2 - cobbly t o pebbly a l l u v i a l substratum d e p o s i t s ; 3 - pebbly t o granular f l u v i a l channel sediments; 4 - medium- t o coarse-qrained sandy a l l u v i a l substratum deposits; 5 - f i n e - t o medium-grained sandy topstratum sediments; 6 - sandy and s i l t y t o clayey overbank deposits; 7 - s i l t y t o clayey topstratum deposits; 8 - aeolian dune sands.
Fig. 5. Distribution of aeolian dune sands in the upper Middle Buntsandstein (Karlstal-Schichten) in the E i f e l . Sections 1,3,4,5 - Southern Eifel (southern T r i e r a r e a ) : s e c t i o n s 2.6.7 - Western Eifel (northern T r i e r a r e a ) : sections 8 , 9 - Western Eifel (Oberbettingen a r e a ) : section 10 - Western Eifel (Stadtkyll a r e a ) : section 11 - Northern Eifel (southeastern p a r t of Mechernich a r e a ) : sections 1 2 , 13 - Northern Eifel (western p a r t of Mechernich a r e a ) . Coordinates of t h e German Gauss-KrUger g r i d : small numbers - c o e f f i c i e n t s , l a r g e numbers - kilometres. Legend: 1 - bouldery t o cobbly f l u v i a l channel sediments; 2 - cobbly t o pebbly a l l u v i a l substratum deposits; 3 - pebbly f l u v i a l channel sediments; 4 - sandy a l l u v i a l substratum deposits; 5 - dune sands; 6 - aeolian sediments with thin l a c u s t r i n e i n t e r c a l a t i o n s ; 7 - water r e l a i d dune sands; 8 - l a c u s t r i n e deposits arranged in thick playa sequences; 9 - f l u v i a l topstratum deposits; 10 - Devonian basement; 11 - depositional area of t h e Eifel Middle Buntsandstein; 1 2 palaeowind d i r e c t i o n s ; 13 - palaeocurrent d i r e c t i o n s ; 14 - s c a l e of sections.
597
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5 98 Western E i f e l I n t h e Western E i f e l (Mader, 1980a), t h e M i d d l e B u n t s a n d s t e i n s e r i e s i s m o s t l y composed o f a e o l i a n sediment i n t h e N o r t h e r n T r i e r a r e a ( s e c t i o n s 2,6,7
in fig.
5 ) . F l u v i a l d e p o s i t s a r e o c c a s i o n a l l y i n t e r c a l a t e d as p e b b l e - f r e e b a r d e p o s i t s o f up t o s e v e r a l m e t r e s i n t h i c k n e s s , sometimes a l s o as w e l l - d e v e l o p e d c y c l o t h ems c o n s i s t i n g o f p e b b l y t o sandy b a r d e p o s i t s and sandy a n d / o r s i l t y t o c l a y e y topstratum deposits. M u l t i s t o r e y pebbly bar deposits occur o n l y r a r e l y along t h e western m a r g i n . I n much o f t h e O b e r b e t t i n g e n a r e a ( s e c t i o n s 8, 9 i n f i g . 5), t h e M i d d l e Bunts a n d s t e i n sequence i s m a i n l y composed o f sandy f l u v i a l b a r d e p o s i t s ; w i t h occasi o n a l cyclothems as w e l l . A e o l i a n sands a r e p r e s e n t as t h i c k complexes i n t e r b e d d e d w i t h f l u v i a l sandstone sequences, and r a r e l y a l s o as t h i n l a y e r s capping a l l u v i a l cyclothems. I n t h e S t a d t k y l l a r e a ( s e c t i o n 10 i n f i g . 5 ) , t h e M i d d l e B u n t s a n d s t e i n a g a i n c o n s i s t s p r e d o m i n a n t l y o f t h i c k dune sands; o n l y some r a r e f l u v i a l sands a r e i n t e r b e d d e d w i t h t h e dune sequence. Northern E i f e l I n t h e N o r t h e r n E i f e l (Mader, 1983a), t h e M i d d l e B u n t s a n d s t e i n succession i s c h a r a c t e r i z e d by u n u s u a l l y c o a r s e f l u v i a l b o u l d e r t o p e b b l e conglomerates o f channel b a r o r i g i n which i n p l a c e s o c c u r as m u l t i s t o r e y complexes up t o s e v e r a l t e n s o f m e t r e s i n t h i c k n e s s . A e o l i a n sands a r e r e s t r i c t e d t o t h e s o u t h e a s t e r n p a r t o f t h e Mechernich a r e a ( s e c t i o n 11 i n f i g . 5 ) . The dune sands a r e p r e s e n t as a s i n g l e t h i c k sequence o f 5 t o 20 m e t r e s t h i c k n e s s which sometimes c o n t a i n s m i n o r f l u v i a l i n t e r c a l a t i o n s , and as t h i n l a y e r s o c c a s i o n a l l y capping f d u v i a l cyclothems. I n t h e w e s t e r n p a r t o f t h e N o r t h e r n E i f e l , t h e M i d d l e B u n t s a n d s t e i n c o n s i s t s e x c l u s i v e l y o f f l u v i a l d e p o s i t s ( s e c t i o n s 12, 1 3 i n f i g . 5 ) . T r a n s i t i o n t o Upper B u n t s a n d s t e i n The K a r l s t a l - S c h i c h t e n sequence c o n s i s t s o f an i n t e r t o n g u i n g o f a e o l i a n and f l u v i a l d e p o s i t s which a r e capped t h r o u g h o u t t h e E i f e l by a t h i n zone o f dominant l y v i o l e t c o l o u r : t h e V i o l e t t e Grenzzone ( c f . Mader, 1981b) which t e r m i n a t e s t h e
M i d d l e B u n t s a n d s t e i n and marks i t s boundary w i t h t h e Upper B u n t s a n d s t e i n . W i t h i n t h e V i o l e t t e Grenzzone, which i n p l a c e s i s up t o 15 m e t r e s t h i c k , t h e f i r s t p a l a e o s o l s appear, and t h e a e o l i a n sands r a p i d l y f a d e o u t . The V i o l e t t e Grenzzone i s o v e r l a i n by p e b b l y and c o b b l y f l u v i a l sediments o f t h e basal Upper Buntsands t e i n which no l o n g e r c o n t a i n s any a e o l i a n sands. EVOLUTION OF FLUVIAL DEPOSITIONAL ENVIRONMENT AND O R I G I N OF SAND SEA Both r e g i o n a l and s t r a t i g r a p h i c a l d i s t r i b u t i o n o f a e o l i a n sands, as w e l l as t h e f a c i e s o f t h e a l l u v i a l sediments, r e f l e c t t h e e v o l u t i o n o f f l u v i a l d e p o s i t i o n a l environments i n t h e M i d d l e B u n t s a n d s t e i n o f t h e Southern E i f e l and a
599 c l i m a x o f r e g i o n a l r e g i o n a l d i v e r s i f i c a t i o n o f sedimentary m i l i e u i n t h e t e r m i n a l staqe o f t h i s e v o l u t i o n (Ilader, 1982b) ( f i g s . 6, 7 ) . F l u v i a l d e p o s i t i o n a l h i s t o r y a l s o i n c o r p o r a t e s d i s t r i b u t i o n o f a e o l i a n sedime n t s which do n o t o c c u r randomly i n t h e sequence, b u t a r e p r e s e n t a t a d i s t i n c t stage i n t h e e v o l u t i o n o f t h e f l u v i a l sedimentary environment. S i m i l a r r e s t r i c t ed p o s i t i o n s o f a e o l i a n sands w i t h i n w a t e r l a i d sequences a r e d e s c r i b e d by Hubert and Mertz (1980, 1982), Hyde (1980), H i n t e r and Turner (1981) and S z i g e t t i and Fox ( 1 9 8 1 ) .
F i g . 6. Regional d i v e r s i f i c a t i o n o f d e p o s i t i o n a l m i l i e u i n ' t h e upper M i d d l e B u n t s a n d s t e i n ( K a r l s t a l - S c h i c h t e n , Lower T r i a s s i c ) o f t h e E i f e l . R i g h t - Palaeog e o g r a p h i c a l map a t a c e r t a i n t i m e o f upper M i d d l e B u n t s a n d s t e i n d e p o s i t i o n , p a r t i a l l y c o n t r o l l e d by basement s w e l l s ( f r o m s o u t h t o n o r t h , t h e Deimlingen s w e l l , t h e Kalenborn s w e l l , t h e K a l l m u t h s w e l l ) . L e f t - d e p o s i t i o n a l models f o r t h e upper M i d d l e B u n t s a n d s t e i n i n Southern ( A ) , Western (B) and N o r t h e r n ( C ) Eifel. Legend: 1 - Devonian basement; 2 - B a s i s b i l d u n g e n ( e a r l y B u n t s a n d s t e i n sediments); 3 - b o u l d e r y t o p e b b l y f l u v i a l channel d e p o s i t s ; 4 - p e b b l y t o sandy a l l u v i a l s u b s t r a t u m d e p o s i t s ; 5 - sandy f l u v i a l channel d e p o s i t s ; 6 - s i l t y t o c l a y e y and sandy a l l u v i a l t o p s t r a t u m d e p o s i t s ( m a i n l y f l o o d p l a i n sediments); 7 - sandy f l u v i a l c r e v a s s e s p l a y sediments; 8 - l a c u s t r i n e d e p o s i t s ; 9 - a e o l i a n dune sands.
600 A e o l i a n sands appear i n t h e f i n a l phase o f t h e d e p o s i t i o n a l h i s t o r y , which i s c h a r a c t e r i z e d by i n c r e a s i n q channel spacinq and s i n u o s i t y , d e c r e a s i n g b r a i d i n q o f t h e r i v e r systems, and weakeninq s u p p l y o f coarse d e t r i t u s f r o m t h e source areas. Evolution led,
irl
M i d d l e Buntsandstein, from l o c a l a l l u v i a l f a n s v i a
c o b b l y and p e b b l y , h i g h l v b r a i d e d r i v e r s t o sandy, m o d e r a t e l y b r a i d e d streams and f i n a l l y t o an i n t e r t o n q u i n q o f a e o l i a n dunes and b r a i d e d r i v e r s matching the
M I
SOUTHERN E I F E L Pebbly t o sandy moderately braided rivers A b u n d a n c e of dune sands
WESTERN E I F E L Sandy moderately braided rivers Predominance of dune s a n d s
NORTHERN E I F E L Bouldery to pebbly highly braided rivers R a r i t y of dune s a n d s
D D L E
UPPe' part
0 U
N
middle
T
part
A N
S
lower part
T E
I
N
, REHBERG-SCHICHTEN Pebbly to sandy highly t o moderately braided r i v e r s
*
S
D
K A R L S TAL - S C HI C HT EN lntertonguing of aeolian dunes and braided streams
base
TRIFELS-SCHICHTEN Cobbly t o pebbly highly b raided streams
BASISBILDUNGEN Alluvial f a n s and mud-flows in local basement depressions
F i g . 7. E v o l u t i o n o f f l u v i a l d e p o s i t i o n a l environment i n t h e M i d d l e Buntsands t e i n (Lower T r i a s s i c ) o f t h e E i f e l .
601
climax of regional d i v e r s i f i c a t i o n of depositional milieu ( f i g . 7 ) . After a main recurrence in f l uvial s t y l e a t t h e Pliddle/Upper Buntsandstein boundary, dominantl y tr ig g e r e d by t e c t o n i c u p l i f t s in t h e source area and a chanqe from a r i d t o semi-arid c l i m a t e , in t h e Upper Buntsandstein a f u r t h e r evolution from pebbly, highly braided r i v e r s t o sandy, moderately braided streams t o o k place. The deposi t i o n a l h i s t o r y durin!, t h e Upper Buntsandstein, however, did not lead t o the formation of dune f i e l d s ; b u t increasing s i n u o s i ty of streams f i n a l l y r e s u l t e d in t h e development of meandering streams, which passed into d e l t a de posits which in turn graded i n t o t h e shallow marine environment a t the end of the Buntsandstein which inaugurated t h e Hiddle T r i a s s i c Huschel kal k tra nsqre ssion. Evolution of t h e f l u v i a l depositional environment was mainly governed by t e c t onic u p l i f t s of t h e source a r e a s and subsequent supply of d e t r i t u s t o the basin, and by qradual decrease of palaeoslope by deposition following t h e endogenic events which gave r i s e t o chanqes in f l u v i a l s t y l e . These continuous a l t e r a t i o n s of th e geometry of t h e watercourses l e d , i n Middle Buntsandstein times, t o the formation of t h e sand s ea. I n ad d i t i o n t o t h e geomorphological processes, the v e r t i c a l t r e n d s i n sedimewLation r e f l e c t an i n cr e a se in a r i d i t y during the Middle Buntsandstein, which caused i n p a r t s of t h e area a decreasing p e r e n n i a l i t y of th e r i v e r systems, and p r o g r es s i v el y ephemeral f l u v i a l sedimentation was accompanied by dune f i e l d formation during t h e climax of a r i d i t y . Evolution of t h e Middle Buntsandstein f l u v i a l environment i s outline d below. Tri f e l s-Sc hi c hten I n t h e T r i f e l s- S c h i ch t en , t h e southern source a re a s supplied l a r q e amounts o f coarse d e t r i t u s which accumulated in low-sinuosity, mu1 ti-c ha nne l, highlv braided r i v e r systems. Considerable erosion during rapid s h i f t i n g of narrowly spaced channels and slow subsidence o f t en led t o widespread reworkinq of overbank f i n e s previously l a i d down in narrow f l o o d p l ai n s ; thus p i l i n g u p thic k sequences of stacked cobbly and pebbly bar d ep o s i t s . Aeolian sands never accumulated, o r were subsequently completely eroded o f f . Rehberg-Schic hten In t h e Rehberg-Schichten, diminishing supply o f coarse d e t r i t u s from the southern source a r e a s , increasing channel spacing a nd s i n u o s i t y , and decreasing braiding of t h e r i v e r systems gave r i s e t o t h e development of e xte nsive floodp la in s . Weaker erosion during channel s h i f t i n g , and more continuous and f a s t e r subsidence favoured abundant preservation o f n ea rly complete cyclothems. Occasi o n a l l y , emergence of p a r t s of t h e f l o o d p l ai n , and e x t r a o r d i n a r i l y low water l e v e l s in t h e channels, r e s u l t e d in local d e f l a t i o n and accumulation of t h i n a e o lia n sands a t t h e top of t h e cyclothems. Sand,deflated from a l l u v i a l bars during low-stage of t h e r i v e r channels,was blown onto dry p a r t s of the adjoining overbank area and e i t h e r p i l ed u p i n t o barchanoid dune le ts o r spread over the
602 s u r f a c e as a e o l i a n sheet sands. C o n s i s t e n t s h i f t i n q o f f l u v i a l channels and f l o o d i n g o f overbank areas i n h i b i t e d development o f l a r g e r a e o l i a n sand accumulations. Karl s t a l -Schichten C o n t i n u i n g o n l a p o n t o t h e Devonian basement l e d t o d e p o s i t i o n o f t h e K a r l s t a l S c h i c h t e n t h r o u g h o u t t h e E i f e l . A r e a l e x t e n s i o n was accompanied by an e x t r a o r d i n a r i l y w e l l - d e v e l o p e d r e q i o n a l d i v e r s i f i c a t i o n o f sedimentary environments, which i s d e l i n e a t e d by d i f f e r e n t f a c i e s models f o r t h e Southern, Western and Northern E i f e l ( f i g . 6). Southern E i f e l . Here, s u p p l y o f c o a r s e d e t r i t u s f r o m t h e s o u t h e r n source areas f u r t h e r decreased. Channel spcaing and s i n u o s i t y i n c r e a s e d a g a i n , b r a i d i n g o f t h e r i v e r s d i m i n i s h e d c o n t i n u o u s l y , and widening o f t h e f l o o d p l a i n s t o g e t h e r w i t h weakening o f overbank f l o o d i n g , accompanied by i n c r e a s i n g a r i d i t y , l e d t o t h e development o f an e x t e n s i v e dune b e l t which was i n t e r s e c t e d by b r a i d e d r i v e r s . As f l u v i a l s e d i m e n t a t i o n o n l y t o o k p l a c e on overbank a r e a s d u r i n g f l o o d i n g , l a r g e amounts o f a l l u v i a l d e p o s i t s were exposed t o t h e a c t i v i t y o f t h e wind f o r l o n g p e r i o d s . On t h e o t h e r hand, abundant p r e s e r v a t i o n o f f l u v i a l cyclothems i n d i c a t e s m a i n l y s h o r t p e r i o d s o f a e o l i a n winnowing c o n t r o l l e d by t h e freqtrency o f l o w r i v e r stages and overbank emergence; i n keepinq w i t h t h e r a r i t y o f v e n t i f a c t s and g r a v e l d e f l a t i o n l a q s . P e r s i s t e n t d e f l a t i o n o f a l l u v i a l sediments through time
r e s u l t e d i n s u f f i c i e n t winnowing o f l o o s e sand t o f o r m dunes.
A s u i t a b l e r a t e o f sand s u p p l y matching n e t d e p o s i t i o n d u r i n g m i g r a t i o n , and a f a v o u r a b l e r a t e o f subsidence, l e d t o a c c u m u l a t i o n and p r e s e r v a t i o n o f t h e dune field.
Barchanoid dunes accumulated i n unimodal s o u t h e a s t e r l y t o s o u t h w e s t e r l y
t r a d e winds b l o w i n g i n summer, when t h e i n t e r t r o p i c a l converqence zone was s h i f t e d t o t h e n o r t h , and h o r i z o n t a l - l a m i n a t e d a e o l i a n sheet sands were d e p o s i t ed i n i n t e r d u n e areas. F l u v i a l i n c u r s i o n s o r heavy ephemeral r a i n f a l l o f t e n l e d t o a q u a t i c r e s e d i m e n t a t i o n o f a e o l i a n sands and t o t h e f o r m a t i o n o f s h a l l o w l a k e s i n i n t e r d u n e d e p r e s s i o n s . A l l u v i a l r e d e p o s i t i o n o f a e o l i a n sands was a1 so i n i t i a t e d by a v a l a n c h i n g o f dune sands i n t o stream channels ( c f . Andrews, 1981). I n t e r d u n e s e d i m e n t a t i o n t o o k p l a c e on a l t e r n a t i n q d r y , damp and wet surfaces w i t h r i s i n g and f a l l i n g water t a b l e , as w e l l w i t h m o i s t u r e b r o u g h t i n by epheme r a l atmospheric p r e c i p i t a t i o n and subsequent e v a p o r a t i o n . F1 u v i a l s e d i m e n t a t i o n i n m o d e r a t e l y b r a i d e d r i v e r s c o n t r o l l e d sand sea e x t e n s i o n by moderate s h i f t i n g o f channels. Subsidence g e n e r a l l y k e p t pace w i t h d e p o s i t i o n , t h u s f a v o u r i n g o f t e n complete p r e s e r v a t i o n o f cyclothems. E p i s o d i c slower sbbsidence, h i g h e r c u r r e n t i n t e n s i t y , i n c r e a s e d d i s c h a r g e and a c c e l e r a t e d s u p p l y o f c o a r s e d e t r i t u s from source areas r e s u l t e d i n t h e i n t e r c a l a t i o n o f s e v e r a l complexes o f stacked pebbly c o n g l o m e r a t i c b a r d e p o s i t s i n t o t h e a e o l i a n and f l u v i a l succession.
603 Nestern Eifel
. Here,
t h e Devonian basement was almost completely covered
during Karlstal-Schichten d ep o s i t i o n , except f o r some local swells and inse lbe rgs. The a e o l i a n sand sea reached i t s g r e a t e s t extension in t h i s p a r t i c u l a r a re a . Dune sands covered t h e marked topography of the pre-Triassic surfa c e a n d were a l s o trapped in t i n y hollows formed by p r e- T r i as s i c k a r s t i f i c a t i o n of basement carbona t e s . As dune sands accumulate b e t t e r on smooth pla ins t h a n on i r r e g u l a r surfa c e s ( c f . Tsoar e t a l . , 1979), t h e landscape during e a r l y Middle Buntsandstein times, with i t s numerous minor and major e l e v a t i o n s , probably disturbed smooth wind flow and c r e a t e d zones of erosion a n d d ep o s i t i o n . On the othe r h a n d , marked r e l i e f of some l a r g e r basement swells obviouslv acted a s b a r r i e r s where the aeolian sands have been trapped and s h el t er ed from a l l u v i a l r ede position. Sandy rinderately t o s l i g h t l y braided r i v e r s were ch ar act er i zed by widely spaced channels of low t o moderate s i n u o s i t y which s h i f t e d slowly, and the se a r e i n t e r c a l a t e d with dune sands. A t some l o c a l i t i e s , increased r a t e of a l l u v i a l channel s h i f t a n d overbank flooding r e s t r i c t e d extension of t h e dune f i e l d in p a r t s of the sequence. I n o t h e r p a r t s of t h e r eg i o n , water-laid d ep o s i t s a r e present only a s t h i n l a c u s t r i n e sediments which s e t t l e d o u t i n q u i e t temporary interdune ponds. Northern E i f e l . Here, a n increase in palaeoslope r e s u l t e d in a sudden change in f l u v i a l s t y l e . The western margin of t h e depositional area supplied l a r g e amounts of e x t r a o r d i n a r i l y coarse d e t r i t u s i n ad dition t o material derived from t h e south. ilixing of proximally derived cobbles and pebbles with sands of d i s t a l o r i g i n led t o t h e formation of bimodal sediments, c onsisting predominantly of coarse matrix-rich conglomerates. Strong erosioh during s h i f t i n g of narrowly spaced channels of t h e low-sinuosity, hiqhly braided r i v e r systems ofte n r e s u l t ed in c o n si d e r a b l e reworking o f fine-grained topstratum de posits, thus pi1 ing
u p t h ic k sequences of mu l t i s t o r ey bouldery t o pebbly bar d e p o s i t s . Aeolian sands were deposited only in a small r e s t r i c t e d dune f i e l d near.a swell, which s h e l t e r ed th e a e o l i a n sands from f l u v i a l reworking: these sands accumulated a s t r a i n s o f l e e dunes ( c f . Tsoar, 1982), on t h e northern flank of the sw e ll, and r a r e l y a s climbing dunes on t h e windward s l o p es . I n the upper p a r t of t h e succession, s u f f i c i e n t discharge and continuous s h i f t i n g of watercourses a dja c e nt t o the sand sea r e s u l t e d in breaching of t h e dune b e l t by f l u v i a l channels, and gradual extension of a l l u v i a l deposition terminated aeolian sedimentation. The f i n a l phase of t h e evolution of t h e Middle Buntsandstein f l u v i a l sedimenta r y environment ( f i g . 7 ) i s thus highlighted by a climax of regional d i v e r s i f i c a t i o n of depositional milieu ( f i g . 6 ) , r e f l e c t i n g the influences of basement morphology on f l u v i a l s t y l e which i n turn c o n t r o l s sand sea extension in the E i f e l . The dune b e l t i s l a t e r a l l y l i mi t ed by topographic b a r r i e r s ( c f . Andrews, 1981) c o n s i st i n g of t h e eas t er n and western margins of the Eifel North-South zone ( c f . f i g . 6 ) which r ep r es en t s a f i r s t - o r d e r depositional basin (Smith, 1 9 7 2 ) .
604 D u r i n g i t s l a r g e s t e x t e n s i o n , t h e sand sea reached f r o m Southern E i f e l v i a Western E i f e l t o t h e s o u t h e a s t e r n p a r t o f N o r t h e r n E i f e l , a d i s t a n c e o f a b o u t 100 k i l o m e t r e s and covered an area o f n e a r l y 2,500 square k i l o m e t r e s , w i t h a
p o s s i b l e c o n t i n u a t i o n southwards t h r o u q h t h e Saar a r e a t o t h e P f a l z r e g i o n e x t e n d i n g t h e a r e a t o o v e r 5,000 square k i l o m e t r e s . T r a n s i t i o n t o Upper Buntsandstein.
A t t h e t o p o f t h e M i d d l e Buntsandstein, t h e V i o l e t t e Grenzzone marks a h i a t u s i n t h e e v o l u t i o n o f t h e f l u v i a l d e p o s i t i o n a l environment which i s d e c i s i v e f o r t h e d i s t r i b u t i o n o f a e o l i a n sands. F i r s t appearance o f p a l a e o s o l s w i t h c a l c a r e o u s nodules and c a l c r e t e s i n i t i a t e s t h e s h i f t of t h e sedimentary m i l i e u o f t h e Upper B u n t s a n d s t e i n and i n d i c a t e s a change f r o m a r i d t o s e m i - a r i d c l i m a t e . A e o l i a n d e p o s i t i o n came t o an end; dune s e d i m e n t a t i o n faded o u t r a t h e r r a p i d l y , and t h e i n t e r t o n g u i n g o f a e o l i a n and f l u v i a l d e p o s i t i o n was r e p l a c e d e n t i r e l y by a l l u v i a l sedimentation. The disappearance o f dune sands t e s t i f i e s t o d e c r e a s i n g a r i d i t y ( c f . S u r l y k e t al.,
1981; Laming, 1982) which i s m a i n l y c h a r a c t e r i z e d by a r i s e i n annual
p r e c i p i t a t i o n encouraqing q r o w t h o f v e g e t a t i o n on emerged f l o o d p l a i n s and channel bars. The overbank a r e a s o n l y r a r e l y d r i e d o u t and t h e a l l u v i a l channels s h i f t e d more r a p i d l y . The s t a b i l i z a t i o n o f emerged b a r s and overbank l e v e e s by pedogenesis and p l a n t growth, and t h e r e d u c t i o n i n e f f e c t i v e wind s t r e n g t h made d e f l a t i o n o f sand and m i g r a t i o n o f dunes n e a r l y i m p o s s i b l e . T e c t o n i c u p l i f t i n t h e source areas r e s u l t e d i n a s u p p l y o f l a r g e amounts of c o a r s e d e t r i t u s w h i c h was stacked i n t o mu1 t i - s t o r e y c o n g l o m e r a t i c sequences a t t h e b e q i n n i n q o f Upper B u n t s a n d s t e i n f l u v i a l d e p o s i t i o n . A l l t h e s e e v e n t s caused a r a p i d r e c u r r e n t change i n f l u v i a l s t y l e which i s h g i h l i g h t e d by t h e disappearance o f a e o l i a n sands ( c f . lzlaugh, 1973;
Ross and Donaldson, 1982). GENERAL DEPOSITIONAL ENVIRONMENT OF THE AEOLIAN SAND SEA The E i f e l M i d d l e B u n t s a n d s t e i n i s c l a s s i f i e d a s f a c i e s a s s o c i a t i o n C (e.g. Tucker and Benton, 1983), c o m p r i s i n g f l u v i a l r e d beds w i t h a e o l i a n sands and p l a y a - l a c u s t r i n e sediments. The sand sea o r i g i n a t e d as an i n l a n d e r g i n l o w n o r t h e r n p a l a e o l a t i t u d e s i n t h e zone o f n o r t h w a r d b l o w i n g t r a d e winds when t h e i n t e r t r o p i c a l convergence zone was s h i f t e d t o t h e n o r t h d u r i n g t h e summer ( c f . Mader, 1980b). Winds o f m o s t l y moderate s t r e n g t h , n o t exceeding t h e v e l o c i t i e s necessary f o r t h e a c c u m u l a t i o n o f g i a n t - s i z e d t r a n s v e r s e and l o n g i t u d i n a l dunes ( a s o r i q i n a t e d i n t h e Permian R o t l i e g e n d e s d e s e r t , c f . Glennie, 1983) p i l e d up medium- t o l a r g e - s c a l e barchanoid dunes, w i t h h e i g h t s g e n e r a l l y up t o some t e n s o f m e t r e s . T e m p o r a r i l y . s t r o n g e r winds c o i n c i d i n g w i t h l a r g e r amounts o f a v a i l a b l e sand p e r m i t t e d t h e o c c a s i o n a l formation o f very l a r g e ridges.
605 Focussing o f palaeowind d i r e c t i o n s by t h e palaeomorphology r e s u l t e d i n a n a r r o w spread o f c r o s s - b e d d i n g azimuths. The t r a d e winds b l o w i n g northwards were o r i e n t a t e d d o w n v a l l e y i n t h e E i f e l North-South zone which formed a l o n g i t u d i n a l d e p r e s s i o n surrounded by l o w - r e 1 i e f margins on t h e p r e - T r i a s s i c c o n t i n e n t a l s u r f ace. ( S i m i l a r l o n g i t u d i n a l l y d o w n v a l l e y - b l o w i n g palaeowinds a r e r e p o r t e d by Hubert and M e r t z , 1982, and Kocurek and D o t t , 1982: i n c o n t r a s t , f i l l i n g o f a synsedimentary graben by t r a n s v e r s e winds i s documented by Drong e t a l . , 1982, and winds b l o w i n g from b a s i n t o u p l a n d a r e r e p o r t e d by Laming, 1982). The s t r o n g d i r e c t i o n a l c o n t r o l o f t h e palaeomorphology on t h e winds gave r i s e t o o n l y m i n o r l o c a l d e v i a t i o n s from t h e g e n e r a l p a t t e r n ( c f . Mader, 1980b), caused by small s w e l l s and i n s e l bergs. I n c o n t r a s t t o t h e R o t l i e g e n d e s d e s e r t o f t h e Southern Permian Basin, which l a y w i t h i n t h e r a i n shadow o f t h e V a r i s c a n U r a l mountains ( c f . Glennie, 1983), t h e E i f e l B u n t s a n d s t e i n sand sea was n o t s h e l t e r e d f r o m p r e c i . p i t a t i o n by a mounta i n range. The m a s s i f s which o r i g i n a t e d d u r i n g t h e V a r i s c a n orogeny had been c o n s i d e r a b l y eroded i n p r e - T r i a s s i c t i m e s , and t h e E i f e l B u n t s a n d s t e i n d e p o s i t i o n a l a r e a was surrounded by mountains o f o n l y l o w r e l i e f ( o n l y some 100 m e t r e s ) and t h e s e had no rnajQr e f f e c t on atmospheric c i r c u l a t i o n . A l s o , t h e m a r g i n a l s i t u a t i o n o f t h e E i f e l i n comparison t o t h e e x t e n s i o n o f t h e Mid-European Basin c o u l d have f a v o u r e d a c e r t a i n c o n c e n t r a t i o n o f p r e c i p i t a t i o n . The sand sea t h u s r e c e i v e d o f t e n heavy ephemeral r a i n f a l l , which r e s u l t e d i n more abundant i n t e r m i t t e n t a q u a t i c r e d e p o s i t i o n and more permanent f l u v i a l d i s c h arge, i n comparison w i t h t h e e s s e n t i a l l y d r y R o t l i e g e n d e s e r g (Glennie, 1983a, 1983b). The p e r i o d s o f i n t e n s e p r e c i p i t a t i o n were, however, o n l y s h o r t i n t e r r u p t i o n s o f t h e g e n e r a l l y p e r s i s t e n t a r i d i t y i n t h e upper p a r t o f t h e E i f e l M i d d l e B u n t s a n d s t e i n . P l a n t g r o w t h occured o n l y l o c a l l y and was r e s t r i c t e d t o t h e margins o f emerging f l o o d p l a i n s . The endemic f l o r a s c o n s i s t o f o n l y a l y c o p o d adapted t o d r y ground, and a f e r n ‘ p o s s i b l y r e s t r i c t e d t o m o i s t s u b s t r a t e s . A r i d i t y p e r s i s t e d a l m o s t t o t h e t o p o f t h e M i d d l e B u n t s a n d s t e i n and p e r m i t t e d c o n t i n u o u s g r o w t h and m i g r a t i o n o f dunes. I n c o n t r a s t , g r a d u a l l y d e c r e a s i n g a r i d i t y d u r i n g t h e c o u r s e o f t h e R o t l i e g e n d e s l e d t o a decrease i n a e o l i a n dune
abundance by expansion o f t h e d e s e r t l a k e and sabkha f a c i e s (Drong e t a l . ,
1982;
Glennie, 1983a).
A t t h e end o f t h e M i d d l e B u n t s a n d s t e i n , t h e r e v e r s i o n o f t h e processes t h a t l e d t o t h e o r i q i n o f t h e sand sea caused i t s d e s t r u c t i o n . Decreasing a r i d i t y by change f r o m a r i d t o s e m i - a r i d c l i m a t e r e s u l t e d i n r a p i d l y d e c l i n i n g wind v e l o c i t y matching t h e severe d e t e r i o r a t i o n o f f l u v i a l d e p o s i t i o n a l environments s u i t a b l e f o r a e o l i a n dune a c c u m u l a t i o n . The r e - e s t a b l i s h m e n t o f h i g h l y b r a i d e d a l l u v i a l environments w i t h r a p i d channel s h i f t i n g l e d t o e r o s i o n o f t h e sand sea, p a r t i a l r e d e p o s i t i o n o f reworked dune sand by streams, and f i n a l l y b u r i a l o f t h e e r g under a c o n t i n u o u s sheet o f f l u v i a l sediments ( c f . Waugh, 1973; Jakobsson e t a l . ,
606
1980). Once e x t i n c t , aeo l i an deposition was never resumed in the Eifel Buntsandstein. DISCUSS I O l i The Eifel sand sea which developed in t h e terminal sta qe of Hiddle Buntsands t e i n d e p o s i t i o n i s an outstanding example of intertonguinq a e olia n and f l u v i a l sedimentation. I t s formation a t t h e end of evolution of a f l u v i a l depositional environment hiqhl i g h t s t h e co n d i t i o n s necessary f o r accumulation of considerable amounts of a e o l i a n sand in a braided a l l u v i a l milie u; namely, moderate s h i f t i n g of f l u v i a l channels, ex i s t en ce of moderate t o wide floodpla ins, repeated low s t a g e s of a l l u v i a l water courses and emergence of p a r t s of the f l o o d p l a i n s . Aeolian sand accumulation i s f u r t h e r influenced by t e c t o n i c u p l i f t s in the source a r e a s , subsidence of the depositional basin and c l i m a t i c c onditions o r changes. The accumulation of dunes a n d t h e formation of t h e a e olia n sand sea was on a l a r g e - s c a l e c o n t r o l l e d mainly by topography ( e . q . Fryberger and Ah1 brandt, 1979). The r e s t r i c t i o n of t h e dune f i e l d t o the Eifel North-South depression zone, which o r ig in a t e d in p r e - T r i as s i c times by bimodal plunging of Variscan fold axes ( c f . Mader, 1981b), i n d i cat es t h a t t h i s dominantly negative topographic f e a t u r e was r e s p o n si b l e f o r trapping l a r g e amounts of wind-blown sand. On a smaller s c a l e , some p o s i t i v e topographic elements, such a s l a r g e swells on t h e pre -Tria ssic continental s u r f a c e , favoured accumulation of small l e e dune f i e l d s and she lte re d th e dunes from f l u v i a l reworking. The extension of t h e sand sea within the topogra p h i c a l l y c o n t r o l l e d sedimentary basin was r e s t r i c t e d by aqueous mechanisms. The interconnection of t h e a l l u v i a l network an d t h e migration of channels a n d floodp la in s through time and space l i mi t ed t h e expansion of t h e e r g . On the othe r hand, emerged f l u v i a l channels a n d overbank a r e a s served a s sources of sand, with the g r a in s i z e of t h e a eo l i an sands depending on t h e te xtura l parameters of the f l u v i a l sediments. During Buntsandstein d ep o s i t i o n , only s l i g h t subsidence (always being matched by sedimentation) took place in t h e Eifel North-South depression, r e s u l t i n g in progressive onlap of t h e Buntsandstein d ep o s i t s
onto t h e intra ba sina l swells a n d
onto t h e marqinal highlands. I n c o n t r a s t , rapid synsedimentary subsidence by downfaulting l e d t o r e s t r i c t i o n of some Rotliegendes aeol i a n i t e s t o grabens (Drong e t a1 ., 1 9 8 2 ) , and concentrated a l l u v i a l a n d a e olia n sediments t o elongate faul t-bounded r i f t basins (Clemmensen, 1980; Clemmensen e t a l . , 1980). During t h e course of evolution of t h e f l u v i a l depositional environment, continuously i n c r e a s i n g channel spacing, decreasing braiding of r i v e r systems, and widening of f l o o d p l a i n s progressively ameliorated t h e c onditions f o r accumulation a n d p r e s e r v a t i o n of a e o l i an sands. In a l l u v i a l networks with narrowly spaced channels, r a p id channel s h i f t i n g and narrow f l o o d p l ai n s which were a c t i v e in e a r l i e r sta ge s
607
o f t h e d e p o s i t i o n a l h i s t o r y , t h e f o r m a t i o n o f a e o l i a n sands was e i t h e r completel y i m p o s s i b l e o r l a r g e l y suppressed by a l m o s t c o n t i n u o u s f l o o d i n g o f t h e a l l u v i a l
p l a i n . Gradual a m e l i o r a t i o n o f a e o l i a n sand a c c u m u l a t i o n p o z s i b i l i t i e s due t o i n c r e a s i n g emerqence o f p a r t s o f b o t h water courses and f l o o d p l a i n s a l l o w e d l o c a l dune and sheet sand f o r m a t i o n , t h e n t h e growth o f l o c a l a e o l i a n bedforms t o l a r g e r dune and i n t e r d u n e complexes, and f i n a l l y t h e f o r m a t i o n o f an e x t e n s i v e sand sea d u r i n g t h e t e r m i n a l staqe o f a l l u v i a l e v o l u t i o n . The q e o i m r p h o l o g i c a l h i s t o r y was accompanied by c l i m a t i c changes a s s o c i a t e d w i t h p r o g r e s s i v e a r i d i t y d u r i n g M i d d l e B u n t s a n d s t e i n d e p o s i t i o n . The c l i m a t i c e v o l u t i o n was c h a r a c t e r i z e d by d e c r e a s i n g r a i n f a l l b o t h i n t h e source a r e a s o f t h e streams and i n t h e sedimentary
b a s i n , and by i n c r e a s i n g wind v e l o c i t i e s .
D u r i n g e a r l y M i d d l e B u n t s a n d s t e i n t i m e s , t h e source r e g i o n s o f t h e streams l a y an a r e a w i t h h i g h and f r e q u e n t r a i n f a l l . W i t h i n c r e a s i n g a r i d i t y , t h e p r e c i p i t a t i o n p r o g r e s s i v e l y d e c l i n e d , r e s u l t i n q i n d e c l i n i n g d i s c h a r g e o f t h e streams, which a s s i s t e d t h e geomorphological e v o l u t i o n and a l l o w e d t h e a e o l i a n d e p o s i t s t o e n l a r g e f r o m l o c a l f l o o d p l a i n dunes t o e x t e n s i v e sand seas. Thus t h e combinati o n o f t o p o g r a p h i c a l and c l i m a t i c mechanisms caused t h e development o f t h e E i f e l dune be1 t ( c f . F r y b e r g e r and Ah1 b r a n d t , 1979; Ah1 b r a n d t and F r y b e r g e r , 1982).
A s i m i l a r c o m b i n a t i o n o f d r i e r c l i m a t e and r e t r e a t o f r i v e r systems l e a d i n g t o dune sand f o r m a t i o n i s d e s c r i b e d by Clemmensen e t a1
.
(1980).
T e c t o n i c u p l i f t s i n t h e source a r e a s were p a r t l y r e s p o n s i b l e f o r t h e d e s t r u c t i o n o f t h e sand sea by r e - e s t a b l i s h i n g h i g h l y b r a i d e d r i v e r systems a t t h e beginn i n g o f t h e Upper B u n t s a n d s t e i n and p r e v e n t i n g f u r t h e r dune a c c u m u l a t i o n . T h i s recurrence i n f l u v i a l s t y l e t o c o n d i t i o n s e x i s t i n g i n the lower p a r t o f t h e M i d d l e B u n t s a n d s t e i n was accompanied by r e v e r s a l f r o m a r i d t o s e m i - a r i d c l i m a t e r e s u l t i n g i n widespread p l a n t g r o w t h and palaeosol f o r m a t i o n . The i n f l u e n c e o f subsidence, t o g e t h e r w i t h o t h e r f a c t o r s , on a c c u m u l a t i o n and p r e s e r v a t i o n o f a e o l i a n sands i s b e s t e x e m p l i f i e d by t h e c o m p o s i t i o n o f t h e K a r l s t a l - S c h i c h t e n sequence i n Southern E i f e l . D u r i n g p e r i o d s o f moderate t o r a p i d subsidence, a e o l i a n sands accumulated t o c o n s i d e r a b l e t h i c k n e s s e s and were prese r v e d t o g e t h e r w i t h i n t e r t o n g u i n g f l u v i a l cyclothems. Episodes o f s l o w e r s u b s i d ence, however, combined w i t h h i g h e r c u r r e n t v e l o c i t i e s , i n c r e a s e d d i s c h a r g e : and m i n o r t e c t o n i c movements i n t h e source a r e a s caused s l i g h t l y a c c e l e r a t e d s u p p l y o f c o a r s e d e t r i t u s , r e s u l t i n g i n d e p o s i t i o n o f e x t e n s i v e sheets o f m u l t i s t o r e y p e b b l y b a r complexes, l a c k i n g any i n t e r c a l a t i o n s o f overbank f i n e s o r a e o l i a n sands. The c o e x i s t e n c e o f a e o l i a n dunes and b r a i d e d r i v e r systems i n t h e upper p a r t o f t h e E i f e l M i d d l e B u n t s a n d s t e i n i s e s p e c i a l l y high1 i g h t e d by mutual r e d e p o s i t i o n o f wind and w a t e r - l a i d sands ( c f . Sombroek and Zonneveld, 1982; Ross and Donaldson, 1982; T a l b o t , 1980; H u b e r t and Mertz, 1982; M i d d l e t o n e t a l . ,
1982; and
c o m p i l a t i o n by blader, 1981a, 1 9 8 2 ~ ) .D u r i n g l o w stages i n t h e channel, t h e
6 08
emerged a l l u v i a l b a r sediments were s u b j e c t t o wind a c t i o n . Sand was blown o n t o t h e f l o o d p l a i n , a c c u m u l a t i n g as sand dunes a l o n g t h e streams i n d r y overbank areas and c o v e r i n g l a r g e p a r t s o f t h e s u r f a c e as a t h i n veneer o f s h e e t sand. On t h e o t h e r hand, s h i f t i n g o f a l l u v i a l channels r e s u l t e d i n a g r a d u a l approach t o , and f i n a l l y u n d e r c u t t i n g , o f t h e m a r g i n a l p a r t s o f t h e dune f i e l d s ; and t h e a e o l i a n sand was r e d e p o s i t e d i n t h e streams by b e i n g p i l e d up t o f o r m channel bars. Furthermore, d u r i n g l o w w a t e r stage, o r even d u r i n g p a r t i a l t o complete d r y i n g o u t o f some reaches, a e o l i a n dunes f r o m t h e s u r r o u n d i n g sand sea m i g r a t e d i n t o t h e channels and b l o c k e d t h e watercourses. Renewed stream f l o w m o s t l y r e d e p o s i t e d t h e l o o s e a e o l i a n sand i n channel bars: i n r a r e cases o f complete p l u g g i n g o f t h e watercourse, dune m i g r a t i o n i n t o t h e channels became a f a c t o r i n controlling the braiding
-
by i n i t i a t i n g breakthrouqhs and t h e f o r m a t i o n o f new
watercourses. T h i s r e s e d i m e n t a t i o n o b l i t e r a t e d d i f f e r e n c e s i n s i z e , s o r t i n g , roundness and s u r f a c e f e a t u r e s o f g r a i n s , and i s an i m p o r t a n t c o n s i d e r a t i o n i n t h e o r i g i n and i n t e r p r e t a t i o n o f f i n e l y i n t e r b e d d e d a e o l i a n and f l u v i a l sedimenta r y sequences such as t h e E i f e l M i d d l e B u n t s a n d s t e i n . CONCLUSIONS
1. The E i f e l M i d d l e B u n t s a n d s t e i n e r g was s i t u a t e d i n l o w n o r t h e r n l a t i t u d e s i n t h e zone o f n o r t h w a r d b l o w i n g t r a d e winds, when t h e i n t e r t r o p i c a l converqence zone was s h i f t e d t o t h e n o r t h d u r i n g t h e summer. 2. The sand sea developed i n t h e t e r m i n a l stage o f M i d d l e B u n t s a n d s t e i n d e p o s i t i o n a l h i s t o r y which was c h a r a c t e r i z e d by t h e e v o l u t i o n o f a f l u v i a l environment from l o c a l a l l u v i a l f a n s v i a c o b b l y and pebbly, h i g h l y b r a i d e d r i v e r s , t o sandy m o d e r a t e l y b r a i d e d streams, and f i n a l l y t o an i n t e r t o n g u i n g o f a e o l i a n dunes and braided r i v e r s .
3. Decreasing s u p p l y o f c o a r s e d e t r i t u s f r o m t h e source areas, i n c r e a s i n g channel s i n u o s i t y and spacing, d i m i n i s h i n g b r a i d i n g o f t h e r i v e r systems, widening o f t h e f l o o d p l a i n s and weakening o f overbank f l o o d i n g , accompanied by i n c r e a s i n q wind v e l o c i t y d u r i n g p r o g r e s s i v e a r i d i t y , r e s u l t e d i n t h e development o f an e x t e n s i v e dune b e l t i n t e r s e c t e d by b r a i d e d r i v e r s .
4. The palaeomorphology c o n t r o l l e d t h e d i r e c t i o n s o f t h e palaeowinds b l o w i n g d o w n v a l l e y i n t h e E i f e l North-South d e p r e s s i o n zone and d e t e r m i n e d t h e d i s t r i b u t i o n o f a e o l i a n sands. 5. The changing r e l i e f o f t h e m a r g i n s o f t h e sedimentary area, t h e d i f f e r e n c e s i n p a l a e o s l o p e a l o n g t h e North-South d e p r e s s i o n zone, and t h e o c c u r r e n c e o f i n t r a b a s i n a l s w e l l s gave r i s e t o v a r i a t i o n s i n f l u v i a l f a c i e s which, t o q e t h e r w i t h t h e d i s t r i b u t i o n o f a e o l i a n sands, r e s u l t e d i n an e x t r a o r d i n a r i l y w e l l developed r e g i o n a l d i v e r s i f i c a t i o n o f d e p o s i t i o n a l environment i n t h e upper p a r t o f t h e Middle Buntsandstein.
609 6. A t t h e end o f t h e M i d d l e Buntsandstein, change f r o m a r i d t o s e m i - a r i d c l i m a t e
r e s u l t e d i n i n c r e a s i n g annual p r e c i p i t a t i o n and d e c l i n i n g wind v e l o c i t i e s . T e c t o n i c u p l i f t o f t h e source a r e a s l e d t o t h e s u p p l y o f l a r g e amounts o f coarse d e t r i t u s w h i c h caused a r e c u r r e n t change i n f l u v i a l s t y l e . A l l these events gave r i s e t o t e r m i n a t i o n o f a e o l i a n d e p o s i t i o n and b u r i a l o f t h e sand sea by a c o n t i n u o u s sheet o f f l u v i a l sediment. ACKNOWLEDGEMENTS T h i s s t u d y summarizes Ph.D. and l a t e r s t u d i e s between 1977 and 1982. F o r h e l p and d i s c u s s i o n I thank my s u p e r v i s o r s , G. Fuchs and W. Dachroth, and a l s o M. Kirchmayer. J.R.L.
Allen,
L. Clemmensen, R. S t e e l , M. B r o o k f i e l d , and T.S.
Ahl-
b r a n d t r e a d v a r i o u s v e r s i o n s o f t h e m a n u s c r i p t . The Deutsche Forschungsgemeins c h a f t (DFG, Bonn) and t h e O r g a n i z a t i o n Committee (McMaster U n i v e r s i t y ) p r o v i d e d t r a v e l g r a n t s t o p r e s e n t t h e s t u d y a t t h e 1 1 t h I n t e r n a t i o n a l Sedimentology Congress. M. E. B r o o k f i e l d t y p e d t h e f i n a l m a n u s c r i p t . REFERENCES
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610 DahlgrUn, F., 1939. Geol. U b e r s i c h t s k a r t e von Deutschland, 1: 200,000. B l a t t 137 Cochem. R e i c h s s t e l l e f u r Bodenforschung, B e r l i n . D i j k , D.E. v., Hobday, D.K. and Tankard, A.J., 1978. Permo-Triassic l a c u s t r i n e d e p o s i t s i n t h e E a s t e r n Karoo Basin, N a t a l , South A f r i c a . I n t e r n a t . Assoc. S e d i m e n t o l o g i s t s , Spec. Publ. 2: 225-239. Drong, H.J., P l e i n , E., Sannemann, D., Schuepbach, M.A. and Zimdars, Z . , 1982. Der Schneverdingen-Sandstein des R o t l i e g e n d e n - e i n e 8 o l i s c h e S e d i m e n t f U l l u n g a l t e r G r a b e n s t r u k t u r e n . Z. deutsch. g e o l . Ges., 133: 699-725. Eschner, T.B. and Kocurek, G.A., 1982. Marine d e s t r u c t i o n o f an a e o l i a n sand sea. J u r a s s i c C u r t i s Formation, N o r t h e a s t e r n Utah. 1 1 t h I n t e r n a t . Congr. Sedimento l o g y , H a m i l t o n , O n t a r i o , Canada. A b s t r a c t s , p. 163. F a r i a , L.E. do C., 1982. Zur S t r a t i g r a p h i e und S e d i m e n t o l o g i e d e r Permo-Trias i m Flaranhao-Becken, N . E . - B r a z i l i e n . Ph.D. Thesis, Univ. WUrzburg. 156pp. F i s c h e r , M.J., 1982. T r i a s s i c . I n : I n t r o d u c t i o n t o t h e p e t r o l e u m g e o l o g y o f t h e N o r t h Sea. J o i n t A s s o c i a t i o n f o r Petroleum E x p l o r a t i o n Courses, Course Notes, 7: F1 - F19. F r y b e r g e r , S.G., 1979. Eol i a n - f l u v i a t i l e ( c o n t i n e n t a l ) o r i g i n o f a n c i e n t s t r a t i g r a p h i c t r a p f o r p e t r o l e u m i n Ideber Sandstone, Rangely o i l f i e l d , Colorado. Mountain G e o l o g i s t , 16: 1-36. F r y b e r g e r , S.G. and A h l b r a n d t , T.S., 1979. Mechanisms f o r t h e f o r m a t i o n o f e o l i a n sand seas. Z e i t . Geomorph., 23: 440-460. G l e n n i e , K.W., 1983a.Lower Permian R o t l i e g e n d e s d e s e r t s e d i m e n t a t i o n i n t h e N o r t h Sea a r e a . T h i s volume. Glennie, K.W., 1983b. E a r l y Permian ( R o t l i e q e n d e s ) palaeowinds o f t h e N o r t h Sea a r e a . Sediment. Geol., 34: i n press. G l e n n i e , K . b J . and B u l l e r , A.T., 1983. The Permian Weissliegend o f N.#. Europe: t h e p a r t i a l d e f o r m a t i o n o f a e o l i a n dune sands caused by t h e Z e c h s t e i n t r a n s g r e s s i o n . Sediment. Geol., 35: i n p r e s s . G r a d z i n s k i , R., Gagol, J. and Slaczka, A., 1979. The T u m l i n Sandstone ( H o l y Cross Plts., C e n t r a l P o l a n d ) : Lower T r i a s s i c d e p o s i t i o n o f a e o l i a n dunes and i n t e r dune areas. A c t a Geol. P o l o n i c a , 29: 151-175. Hubert, J.F. and M e r t z , K.A., 1980. E o l i a n dune f i e l d o f L a t e T r i a s s i c age, Fundy B a s i n , Nova S c o t i a . Geology, 8: 516-519. 1982. E o l i a n sandstones i n Upper T r i a s s i c - L o w e r Hubert, J.F. and Flertz, K.A., J u r a s s i c redheds of t h e Fundy Basin, Nova S c o t i a . 1 1 t h I n t e r n a t . Congr. Sedime n t o l o g y , H a m i l t o n , O n t a r i o , Canada. A b s t r a c t s , p. 69. 1977, B a s i c t y p e s o f s t r a t i f i c a t i o n i n small a e o l i a n dunes. Hunter, R.E., Sedimentology, 24: 361-387. Hunter, R.E., 1981. S t r a t i f i c a t i o n s t y l e s i n a e o l i a n sandstones: some Pennsylvani a n t o J u r a s s i c examples f r o m t h e Western I n t e r i o r U.S.A.. SOC. Econ. P a l e o n t . M i n e r a l . , Spec. Publ., 31: 315-329. Huque, M.A., 1983. Ph.D. T h e s i s . U n i v . Keele. i n p r e p a r a t i o n . Hyde, R.S., 1980. Sedimentary f a c i e s i n t h e Archean Timiskaming Group and t h e i r t e c t o n i c i m p l i c a t i o n s , A b i t i b i Greenstone B e l t , N o r t h e a s t e r n O n t a r i o , Canada. Precambrian Res., 12: 161-195. Jakobsson, K.H., Hamar, G.P., Ormaasen, D.E. and Skarpnes, O., 1980. T r i a s s i c f a c i e s i n t h e N o r t h Sea n o r t h o f t h e C e n t r a l h i g h s . I n : Norsk P e t r o l e u m f o r e n i n g ( E d i t o r ) , The s e d i m e n t a t i o n o f t h e N o r t h Sea r e s e r v o i r r o c k s , X V I I I : 1-10. Johnsen, J.R., 1981. F a c i e s a n a l y s i s o f Permo-Triassic sediments on t h e e a s t e r n m a r g i n o f t h e N o r t h Minch Basin, N o r t h West S c o t l a n d . Cand. Real. Thesis, U n i v . Bergen. 384pp. Knapp, G., 1980. Geol, K a r t e d e r nelrdl i c h e n E i f e l 1:100,000. Geol. Landesamt Nordrhein-Westfalen, Krefeld. Kocurek, G., 1981a. S i g n i f i c a n c e o f i n t e r d u n e d e p o s i t s and bounding s u r f a c e s i n a e o l i a n dune sands. Sedimentology, 28: 753-780. Kocurek, G., 1981b. Erg r e c o n s t r u c t i o n : t h e Entrada Sandstone ( J u r a s s i c ) of n o r t h e r n U t a h and Colorado. Palaeogeogr., Palaeocl i m a t o l , Palaeoecol , 36: 125-153. Kocurek, G. and D o t t , R.H., 1981. D i s t i n c t i o n and uses o f s t r a t i f i c a t i o n t y p e s
.
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611 i n t h e i n t e r p r e t a t i o n o f e o l i a n d e p o s i t s . J. Sedim. P e t r o l . , 51: 579-595. Kocurek, G. and D o t t , R.H., 1982. J u r a s s i c paleogeography and p a l e o c l i m a t e o f t h e C e n t r a l and Southern Rocky Mountains Region. I n press. Laming, D.J.C., 1954. Sedimentary processes i n t h e f o r m a t i o n o f t h e New Red Sandstone o f s o u t h Devon. Ph.D. T h e s i s . Univ. London. 260pp. 1966. I m b r i c a t i o n s , p a l e o c u r r e n t s and o t h e r sedimentary f e a t u r e s Laming, D.J.C., i n t h e Lower New Red Sandstone, Devonshire, England. J. Sediment. P e t r o l . , 36: 940-959. Laming, D.J.C., 1982. The New Red Sandstone. I n : E.M. Durrance and O.J.C. Laming ( E d i t o r s ) , The Geology o f Devon. E x e t e r Univ. Pub1 ., pp. 148-178. Entsova, F.I., Zamerenov, A.K., L a p k i n , I.J., Blom, G . I . , G r i g o r y e v , N.V., K a l a n t a r , I . Z . , K i s n e r i u s , Y.L., K u c h t i n o v , D.A., L u t k e v i c h , E.M., Movshovich, E.V., Sokolova, E . I . , S t e r l i n , B.P., S u v e i z d i s , P . I . and Tverdochlebov, V.P., 1973. The P e r m i a n - T r i a s s i c boundary on t h e Russian P l a t f o r m . I n : A.Logan and L.V. H i l l s ( E d i t o r s ) , The Permian and T r i a s s i c systems and t h e i r mutual boundary. Canad. SOC. P e t r o l . G e o l o g i s t s Memoir, 2: 150-157. Mabesoone, J.M., 1977. Paleozoic-Mesozoic d e p o s i t s o f t h e Piaui-Maranhao synec l i s e ( B r a z i l ) : q e o l o q i c a l h i s t o r y o f a sedimentary basin. sediment. Geol., 19: 7-38. Mader, D., 1979. S t r a t i g r a p h i e und F a z i e s a n a l y s e i m B u n t s a n d s t e i n d e r W e s t e i f e l . Ph.D. Thesis. Univ. H e i d e l b e r g . 293pp. Mader, D., 1980a. A e o l i s c h e und f l u v i a t i l e Sedimentation i m M i t t l e r e n Buntsands t e i n d e r W e s t e i f e l . N. Jb. Geol. P a l d o n t . Abh., 160: 1-41. Mader, D., 1980b. P a l d o w i n d r i c h t u n g e n und Paldostrbmungsrichtungen i m M i t t l e r e n B u n t s a n d s t e i n d e r W e s t e i f e l . Geol. Rdsch., 69: 922-942. Mader, D., 1981a. A e o l i s c h e und f l u v i a t i l e S e d i m e n t a t i o n i m M i t t l e r e n Buntsands t e i n d e r S U d e i f e l . N. Jb. Geol. P a l d o n t . Abh., 161: 354-407. Mader, 1981b. Genesis o f t h e B u n t s a n d s t e i n (Lower T r i a s s i c ) i n t h e Western E i f e l . Sediment. Geol., 29: 1-30. Mader, D., 1982a. A e o l i a n sands i n c o n t i n e n t a l r e d beds o f t h e M i d d l e Buntsands t e i n (Lower T r i a s s i c ) a t t h e western m a r g i n o f t h e German Basin. Sediment. Geol., 31: 191-230. Mader, D., 1982b. D e p o s i t i o n a l h i s t o r y and e v o l u t i o n o f t h e B u n t s a n d s t e i n (Lower T r i a s s i c ) a t t h e western m a r g i n o f t h e German Basin. I.A.S. 3 r d European Mtg., Copenhagen, 1982. A b s t r a c t s , pp. 38-41. Mader, D., 1983a. A e o l i s c h e und f l u v i a t i l e S e d i m e n t a t i o n i m M i t t l e r e n Buntsands t e i n d e r N o r d e i f e l . N. Jb. Geol. Paldont. Abh., 165: 254-302. Mader, D., 198313. E v o l u t i o n of f l u v i a l s e d i m e n t a t i o n i n t h e B u n t s a n d s t e i n (Lower T r i a s s i c ) o f t h e E i f e l (Germany). I n p r e s s . Mader, D., 1983c. Paldogeographie und Genese des B u n t s a n d s t e i n s i m M i t t e l e u r o p d i s c h e n Triasbecken. I n P r e p a r a t i o n . McKee, E.D., 1979. I n t r o d u c t i o n t o a s t u d y o f g l o b a l sand seas. I n : E.D. McKee ( E d i t o r ) , A s t u d y o f q l o b a l sand seas. U.S. Geol. Survey P r o f . Paper, 1052: 3-18. McKee, E.D. and B i g a r e l l a , J.J., 1979. A n c i e n t sandstones c o n s i d e r e d t o be e o l i a n . I n : E.D. McKee ( E d i t o r ) , A s t u d y of g l o b a l sand seas. U.S. Geol. Survey P r o f . Paper, 1052: 187-238. M i d d l e t o n , L.T., Blakey, R.C. and Gregg-Sargent, C., 1982. I n t e r a c t i o n o f e o l i a n meandering systems i n t h e T r i a s s i c ( ? ) o f t h e Colorado P l a t e a u . 1 1 t h I n t e r n a t . Congr. Sedimentology, Hamilton, O n t a r i o , Canada. A b s t r a c t s p. 69. M i n t e r , W.E.L. and Turner, B.R., 1981. Diamond-bearing upper Karoo f l u v i a l sediments i n N o r t h e a s t Swaziland. I n : Modern and a n c i e n t f l u v i a l systems: sedimentology and processes. U n i v . Keele, A b s t r a c t s . Pedersen, G.K. and Andersen, P.R., 1980. D e p o s i t i o n a l environments, d i a g e n e t i c h i s t o r y and source a r e a s o f some B u n t e r Sandstones i n n o r t h e r n J u t l a n d . Danm. Geol. Unders. Arbog., 1979: 69-93. Poole, F.G., 1962. Wind d i r e c t i o n s i n L a t e P a l e o z o i c t o M i d d l e Mesozoic t i m e on t h e Colorado P l a t e a u . U.S. Geol. Survey P r o f . Paper, 450-D: 147-151. Pye, K. , 1982. N e g a t i v e l y skewed a e o l i a n sands f r o m a humid t r o p i c a l c o a s t a l dune f i e l d , N o r t h e r n A u s t r a l i a . Sediment. Geol., 31: 249-266.
612 Ramberg, I . B . and Spjeldnaes, N., 1978. The t e c t o n i c h i s t o r y o f t h e Oslo r e g i o n . I n : I.B. Ramberq and E.R. Neumann ( E d i t o r s ) , T e c t o n i c s and qeophysics o f c o n t i n e n t a l r i f t s . D. R e i d e l Pub. Co., D o r d r e c h t . pp. 167-194. Ridd, H.F., 1981. Petroleum q e o l o q y west o f t h e S h e t l a n d s . I n : L.V. I l l i n g and G.D. Hobson ( E d i t o r s ) , Petroleum geoloqy o f t h e c o n t i n e n t a l s h e l f o f N.W. Europe. I n s t i t u t e o f Petroleum, London. pp. 414-425. Ross, G.M. and Donaldson, J.A., 1982. The B i q b e a r Erq: a P r o t e r o z o i c a e o l i a n sand sea i n t h e Hornby Bay Group, Northwest T e r r i t o r i e s , Canada. l l t h I n t e r n a t . Congr. Sedimentology, H a m i l t o n , O n t a r i o , Canada. A b s t r a c t s , p. 68. (see a1 so Ross, G.M., t h i s volume). S c h r i e l , W., 1939. Geol. U b e r s i c h t s k a r t e von Deutschland 1:200,000. B l a t t 122 + 123 Aachen-Kdln. R e i c h s s t e l l e f u r Bodenforschunq, B e r l i n . Smith, H.T.U., 1972. PLayas and r e l a t e d phenomena i n t h e Saharan Region. I n : C.C. Reeves ( E d i t o r ) , Playa Lake symposium proceedings, I n t e r n a t . Center f o r A r i d and S e m i - s r i d Land S t u d i e s , Texas Tech. Univ. P u b l . , 4 : 63-87. Sombroek, W.G. and Zonneveld, I . S . , 1971. A n c i e n t dune f i e l d s and f l u v i a t i l e d e p o s i t s i n t h e Rima-Sokoto b a s i n (N.W. N i g e r i a ) . N e t h e r l a n d s S o i l Survey I n s t . , Waqeningen. S o i l Surv. Paper, 5. 109pp. 1981. P o s t - P a l e o z o i c e v o l u t i o n o f S u r l y k , F., Clemmensen, L.B. and Larsen, H.C., t h e East Greenland c o n t i n e n t a l m a r g i n . Canad. SOC. P e t r o l . Geol. Memoir, 7: 611-645. S z i q e t i , G.J. and Fox, J.E., 1981. Unkrapa Sandstone ( J u r a s s i c ) , B l a c k H i l l s , South Dakota: an e o l i a n f a c i e s o f t h e M o r r i s o n Formation. SOC. Econ. P a l e o n t . F l i n e r a l . , Spec. P u b l . , 31: 331-349. T a l b o t , M.R., 1980. Environmental response t o c l i m a t i c chanqe i n t h e West A f r i c a n Sahel o v e r t h e p a s t 20,000 y e a r s . I n : M.A.J. W i l l i a m s and H. Faure ( E d i t o r s ) , The Sahara and t h e N i l e ; Q u a t e r n a r y environments and p r e h i s t o r i c o c c u p a t i o n i n n o r t h e r n A f r i c a . Balkema, Rotterdam. pp. 37-62. Thompson, D.B., 1969. Dome-shaped dunes i n t h e Frodsham Member o f t h e s o - c a l l e d "Keuper" Sandstone Formation ( S c y t h i a n - ? A n i s i a n : T r i a s s i c ) a t Frodsham, Cheshire, England. Sediment. Geol., 3: 263-289. Trommestad, K., 1982. Ph.D. T h e s i s . U n i v . Bergen. Tsoar, H., 1983. S i m u l a t i o n o f echo and c l i m b i n g dunes i n t h e wind t u n n e l . T h i s volume. Tsoar, H., Greeley, R . and P e t e r f r e u n d , A.R., 1979. Mars:the N o r t h P o l a r Sand Sea and r e l a t e d wind p a t t e r n s . J o u r . Geophys. Research, 84: 8167-8180. Tucker, M.E. and Benton, M.J., 1983. T r i a s s i c environments, c l i m a t e s and r e p t i l e e v o l u t i o n . Palaeogeogr., P a l a e o c l i m a t o l . , Palaeoecol., i n p r e s s . Tu ner, B.R., 1982. C l i m a t i c and t e c t o n i c c o n t r o l s on c o n t i n e n t a l d e p o s i t i o n a l f a c i e s i n t h e E a s t e r n Karoo Basin, South A f r i c a . l l t h I n t e r n a t . Congr. Sedime n t o l o g y , H a m i l t o n , O n t a r i o , Canada. A b s t r a c t s , p. 149. Wa k e r , R.G.,and M i d d l e t o n , G.V., 1981. F a c i e s models 4 . E o l i a n Sands. I n : R.G. Walker ( E d i t o r ) , F a c i e s Models. Geoscience Canada R e p r i n t S e r i e s , 1: 33-41. Wa t h e r , H.W. and Zitzmann, A., 1981. Geologische K a r t e d e r Bundesrepublik Deutschland 1:1,000,000.Bundesanst. f. Geowiss. u. R o h r s t . , Hannover. Waugh, B., 1973. The d i s t r i b u t i o n and f o r m a t i o n o f P e r m i a n - T r i a s s i c r e d beds. I n : A. Logan and L.V. H i l l s ( E d i t o r s ) , The Permian and T r i a s s i c Systems and t h e i r mutual boundary. Canad. SOC. P e t r o l . Geol. Memoir, 2: 678-693. Wycisk, P., 1983. Ph.D. T h e s i s . U n i v . B e r l i n . i n p r e p a r a t i o n . 1982. G e o l o g i c a l A t l a s o f Western and C e n t r a l Europe. S h e l l Z i e g l e r , P.A., I n t e r n a t . P e t r o l . Maatsch. B.V., Den Haag. 130 pp.
613
PROCESSES
AND
ON
CONTROLS
THE
FORMATIONS, NOKTHEKN A R I Z O N A :
INTERTONGUING
MIUDLETON and RONALD C .
LARRY T.
OF THE K A Y E N T A AND NAVAJO
EOLIAN-FLUVIAL I N T E R A C T I O N S Department o f G e o l o g y ,
BLAKEY:
Arizona U n i v e r s i t y , F l a g s t a f f , Arizona
Northern
86011
I liiTKODUC T I O N The
interaction
between
eolian
and
fluvial
depositional
r e s u l t i n g f a c i e s sequences a r e i n a d e q u a t e l y s t u d i e d .
systems
and
Must s t u d i e s of modern
e o l i a n systems have f o c u s e d on wadi and wet i n t e r d u n e a r e a s ( G l e n n i e , 1 9 7 0 ) . C o m p a r a t i v e l y l i t t l e work has been d i r e c t e d a t t h e p o s s i b l e i n t e r p l a y between h i g h e r s i n u o s i t y f l u v i a l s y s t e m s s u c h as t h o s e a r o u n d t h e m a r g i n s o f i n l a n d ergs,
f o r example,
i n t h e Namib D e s e r t o f w e s t e r n A f r i c a (McKee,
1979), o r
t h o s e t h a t c o u l d be p r e s e n t d u r i n g t h e n a s c e n t s t a g e s o f e r g developmment. Such s t u d i e s a r e n e c e s s a r y i n o r d e r t o :
a) u n d e r s t a n d t h e c o n t r o l l i n g i n t r a -
and e x t r a b a s i n a l c o n d i t i o n s t h a t a r e r e s p o n s i b l e f o r t h e b u i l d u p o f a sand sea,
b ) document t h e p h y s i c a l p r o c e s s e s o f s e d i m e n t a t i o n a s s o c i a t e d w i t h t h e
i n t e r a c t i o n o f t h e s e c o n t r a s t i n g d e p o s i t i o n a l s y s t e m s , and c ) d e v e l o p f a c i e s models
that
would
enable
stratigraphic record, interaction,
detailed
r e c o n s t r u c t i o n of
Kayenta Formation ( J u r a s s i c )
in
(fig.
inferred
depositional
processes
lithologies,
stratigraphic
from
analysis
of
l a t e r a l and v e r t i c a l f a c i e s sequences,
relationships.
These
in
i n northern
turn w i l l
sedimentary and r e g i o n a l
be used t o d i s c u s s
p o t e n t i a l r e g i o n a l c o n t r o l s on i n i t i a l d e v e l o p m e n t o f t h e N a v a j o e r g . approach w i l l
type o f (Glennie,
the
1). Emphasis w i l l be on e x a m i n a t i o n
A r i z o n a and s o u t h e r n U t a h , U.S.A. of
structures,
systems
t h e i n t e r t o n g u i n g o f t h e e o l i a n N a v a j o S a n d s t o n e ( J u r a s s i c ) and
the underlying dominantly f l u v i a l the
such
T h i s s t u d y w i l l c o n c e n t r a t e on s u c h an e o l i a n - f l u v i a l
1970;
Jerzykiewicz,
McKee,
entail 1982)
1974; Horne,
comparisons as w e l l
1975;
as
w i t h modern ancient
Clemmensen,
1978;
the This
transitional facies
examples
(Gradzinski
Fryberger,
and
1979; Mader,
1982 1. REGIONAL STRATIGRAPHIC SETTING G l e n Canyon Group (i)
Introduction.
out throughout Formation
The T r i a s s i c - J u r a s s i c G l e n Canyon Group ( f i g .
n o r t h e r n A r i z o n a and s o u t h e r n U t a h .
(Triassic)
and
is
overlain
by
either
2) c r o p s
It overlies the Chinle
the
Carmel
E n t r a d a S a n d s t o n e , b o t h J u r a s s i c i n age ( H a r s h b a r g e r e t a l . ,
Formation o r
1 9 5 7 ) . The g r o u p
614
COLORADO
- - - _ _
NEVADA
\
I I
0
Flagstaff
\ CAL I FORNI
ARIZONA
d
-
>
0
Phoenix
7 \
I
\ \ \
F i g u r e 1.
O
100 200km U
I
\
MEXICO
NEW M E X I C O
I I I
I
-
-
-1-
- _ J
7 \
I n d e x map showing o u t c r o p area where i n t e r t o n g u i n g r e l a t i o n s h i p s between t h e Kayenta F o r m a t i o n and Navajo Sandstone were examined.
TRIASSIC - JURASSIC
STRATA. NE ARIZONA
ENTRADA - CARMEL
JURASSIC
- _ _ _ - - - - _ _
KAYENTA FM
_ - _ _ - - - -
MOENAVE FM
TRIASSIC CHINLE
F i g u r e 2.
FM
S t r a t i g r a p h i c r e l a t i o n s h i p s o f T r i a s s i c and J u r a s s i c s t r a t a i n n o r t h e r n A r i z o n a . Dashed l i n e s between f o r m a t i o n s o f t h e Glen Canyon Group i n d i c a t e g r a d a t i o n a l c o n t a c t s .
615 consists
of,
in
ascending
order,
Wingate
Sandstone,
Moenave
Formation,
L a y e r l t a Forbnation, and N a v a j o Sandstone. N u w r o u s i n t e r t o n g u i n g r e a t i o n s h i p s o c c u r between a l l f o r m a t i o n s
o f t h e G l e n Canyon Group i n d i c a t i n g c o n t i n u o u s
d e p o s i t i o n . The g r o u p a t t a i n s maximum exposed t h i c k n e s s ( a p p r o x i m a e l y 700 m) near t h e Utah-Arizona border Mountains of
t o a p p r o x i m a t e l y 60 m i n t h e Z u n i
and t h i n s
n o r t h w e s t e r n New M e x i c o
(Cooley e t al.,
1969).
The W i n g a t e
S a n d s t o n e wedges o u t t o t h e n o r t h w e s t and t h e Moenave, K a y e n t a and N:vajo toward t h e southeast (Cooley e t al.,
1969).
The e n t i r e sequence c o m p r i s e s a s e r i e s of e o l i a n and f l u v i a l s a n d s t o n e , f l o o d p l a i n mudstone, p a r t of
and m i n o r l a c u s t r i n e c a r b o n a t e and wudstone.
t h e Wingate Sandstone,
t h e L u k a c h u k a i Member,
member o f t h e Moenave F o r m a t i o n , t h e N a v a j o S a n d s t o n e were
t h e basal
and most o f
t h e D i n o s a u r Canyon Sandstone,
deposited within eolian
The u p p e r
parts of
systems.
These e o l i a n
d e p o s i t s a r e c h a r a c t e r i z e d b y l a r g e - s c a l e cross-bedded sandstone (Cooley e t al.,
1969). T e x t u r a l l y t h e sandstones a r e f i n e - t o medium-grained, moderately
t o w e l l s o r t e d and c o n s i s t o f w e l l - r o u n d e d g r a i n s . are extremely quartz-rich. formed
in
channel
a
buildups.
The f l u v i a l d e p o s i t s o f t h e G l e n Canyon Group
variety of
systems,
settings,
vertically
Sandstone
Most o f t h e s e s a n d s t o n e s
body
including
accreted
both braided
floodplains
geometries
within
the
l e n t i c u l a r whereas m o s t e o l i a n u n i t s a r e t a b u l a r .
and
and m e a n d e r i n g
small
fluvial
lacustrine
deposits
c o n t a i n m o r e s i l t - and c l a y - s i z e m a t r i x t h a n a s s o c i a t e d e o l i a n d e p o s i t s , g r a i n s a r e s u b a n g u l a r t o s u b r o u n d e d and p o o r t o m o d e r a t e l y s o r t e d . and l i t h i c f r a g m e n t s
are
Non-eolian sandstone u n i t s
a r e more a b u n d a n t i n t h e f l u v i a l - l a c u s t r i n e
and
Feldspar deposits,
w h i l e t h i n c a r b o n a t e and c h e r t h o r i z o n s o c c u r i n b o t h t h e N a v a j o and K a y e n t a f o r m a t i o n s . These u n i t s d e v e l o p e d i n s m a l l ephemeral l a k e s on t h e f l o o d p l a i n s and w i t h i n i n t e r d u n a l a r e a s . Some, however, r e p r e s e n t c a l c r e t e h o r i z o n s w h i c h indicate
that
parts
of
the
floodplains
periodically
were
subjected
to
p e d o g e n i c p r o c e s s e s ( L e e d e r , 1975; Bown and K r a u s , 1 9 8 1 ) . ( i i ) Kayenta Formation. of
coarse-
to
The K a y e n t a c o m p r i s e s a h e t e r o g e n e o u s assemblage
fine-grained
clastics
that
s o u t h w e s t e r n U t a h , and n o r t h e r n A r i z o n a .
crop
and n o r t h e a s t e r n A r i z o n a , H a r s h b a r g e r e t a l .
2 generalized facies.
The
l o c a l i t y a t Kayenta,
Arizona
lenticular,
lithic
sandy f a c i e s (fig.
and f e l d s p a t h i c
out
in
western
Colorado,
Based on e x p o s u r e s i n n o r t h c e n t r a l (1957) d i v i d e d t h e Kayenta i n t o
i s b e s t developed near t h e type
1) where i t c o n s i s t s o f r e d d i s h brown, sandstone
interbedded with
subordinate
amounts o f g r a y t o r e d mudstone. The s a n d s t o n e u n i t s t y p i c a l l y have c h a n n e l e d bases
and
contain
stratification.
small-
Southwest
to
and w e s t
medium-scale of
Kayenta
trough the
l a t e r a l l y i n t o t h e s i l t y f a c i e s o f Harshbarger e t a l .
and
sandy
horizontal
facies
grades
(1957), which c o n s i s t s
616
cf r e d d i s h brown s i l t s t o n e and c l a y s t o n e w i t h m i n o r s a n d s t o n e ; l i m e s t o n e beds a r e l o c a l l y a b u n d a n t .
thin
gray
General d i s t r i b u t i o n o f these f a c i e s i n
n o r t h e a s t e r n A r i z o n a i s shown i n f i g u r e 3 a l t h o u g h t h e s a n d y and s i l t y f a c i e s are often complexly interbedded. parallel
the trend
Transport directions
of K a y e n t a s e d i m e n t s
of d e c r e a s i n g g r a i n s i z e , t h a t i s , s t r e a m f l o w t o t h e
southwest. ( i i i ) N a v a j o Sandstone. U n l i k e t h e K a y e n t a , t h e N a v a j o Sandstone i s f a i r l y uniform
in
texture
and
l i t h o l o g y throughout
A r i z o n a and s o u t h e r n U t a h . we1 1 s o r t e d ,
i t s distribution
The N a v a j o i s composed l a r g e l y o f f i n e - g r a i n e d ,
and s u b r o u n d e d q u a r t z - r i c h sand.
Nedge-planar
and
i n northern
tabular-planar
Large-scale
cross-bedding,
features include
contorted
bedding,
h o r i z o n t a l l y bedded s a n d s t o n e and s i l t s t o n e , and c h e r t y l i m e s t o n e .
Regional
s t r a t i g r a p h i c and p a l e o c u r r e n t a n a l y s e s s u g g e s t t r a n s p o r t t o t h e s o u t h ( f i g .
4). A l t h o u g h t h e e o l i a n o r i g i n o f t h e N a v a j o has been q u e s t i o n e d (Freeman and Visher,
1975),
stratification
facies
relationships
with
adjacent
units,
c o n s i s t i n g o f l a r g e complex dunes and b r o a d i n t e r d u n a l a r e a s . developed
small-scale
t y p e s , and o t h e r d a t a c l e a r l y d e l i n e a t e a d e p o s i t i o n a l s y s t e m
within
the
interdunal
depressions
and
L o c a l l y , ponds
freshwater
carbonate
accumulated.
-ORADO -..-.. MEXICO Kayenta
N 0
S ILTY FAC I
ES F A C I ES
Flagstaff
30
60krn
I
F i g u r e 3.
G e n e r a l i z e d d i s t r i b u t i o n o f " s i l t y " and " s a n d y " f a c i e s o f t h e Kayenta F o r m a t i o n i n n o r t h e a s t e r n A r i z o n a ( m d i f i e d f r o m Harshb a r g e r e t a l . , 1957). These f a c i e s a r e c o m p l e x l y i n t e r b e d d e d making p r e c i s e placement o f t h e boundary impossible.
617
F i g u r e 4.
Isopachs o f t h e Navajo Sandstone i n n o r t h e a s t e r n Arizona. S t r a t a t h i n r a p i d l y t o t h e south-southeast i n t h e d i r e c t i o n o f sediment t r a n s Dor t
.
( i v ) Intertonguing Interval.
Two t o n g u e s o f t h e N a v a j o and t w o t o n g u e s of
t h e Kayerita a r e recognized i n southwestern Utah.
Near Cedar City, U t a h ( f i g .
1) t h e S h u r t z S a n d s t o n e t o n g u e o f t h e N a v a j o l i e s between 125 and 220 m b e l o w t h e base o f t h e inain b o d y o f t h e N a v a j o ( A v e r i t t e t a l . ,
1955).
Although n o t
t r a c e a b l e i n t o t h e inain b o d y o f t h e N a v a j o , t h i s t o n g u e i s l i t h o l o g i c a l l y i d e n t i c a l t o t h e N a v a j o and c o n t a i n s s i m i l a r sequences o f l a r g e - s c a l e c r o s s b e d d i n g and a s s o c i a t e d s m a l l e r - s c a l e f e a t u r e s .
It t h i n s f r o m a p p r o x i m a t e l y
105 m n e a r Cedar C i t y t o 1 9 m w i t h i n a d i s t a n c e o f 13 km t o w a r d t h e s o u t h ( A v e r i t t e t al.,
1955).
The Lamb P o i n t t o n g u e o f t h e N a v a j o ( W i l s o n ,
o c c u r s between 0 and 65 m b e l o w t h e base o f t h e N a v a j o .
1958)
T h i s tongue thickens
n o r t h e a s t o f Kanab ( f i g . 1 ) t o o v e r 160 m and e v e n t u a l l y merges w i t h t h e m a i n body o f t h e Navajo ( A v e r i t t e t al.,
1955).
S o u t h w e s t o f Kanab t h i s t o n g u e
t h i n s r a p i d l y and g r a d e s i n t o t h e s i l t y f a c i e s o f t h e K a y e n t a . The Cedar C i t y t o n g u e o f t h e K a y e n t a o v e r l i e s t h e S h u r t z t o n g u e of Navajo ( A v e r i t t e t al.,
the
1 9 5 5 ) . A l t h o u g h t h i c k n e s s i s v a r i a b l e , t h e Cedar C i t y
t o n g u e g e n e r a l l y t h i c k e n s t o t h e s o u t h and t h i n s t o t h e n o r t h i n c o n j u n c t i o n w i t h changes i n t h i c k n e s s o f t h e S h u r t z t o n g u e o f t h e N a v a j o .
The Tenney
Canyon t o n g u e o f t h e K a y e n t a i s b e s t d e v e l o p e d n e a r Kanab where i t i s up t o 35 m t h i c k ( W i l s o n , 1 9 5 8 ) . T h i s t o n g u e o v e r l i e s t h e Lamb P o i n t t o n g u e ( A v e r i t t e t al.,
1955).
B o t h tongues
o f t h e Kayenta a r e r e l a t i v e l y f i n e
g r a i n e d and s i m i l a r t o a s s o c i a t e d f a c i e s w i t h i n t h e m a i n b o d y o f t h e K a y e n t a .
Numerous
smaller
scale
intertonguing
relationships
occur
between
the
N a v a j o and K a y e n t a t h r o u g h o u t t h e s t u d y a r e a , p a r t i c u l a r l y i n n o r t h e a s t e r n Arizona.
A l t h o u g h n o t f o r m a l l y d e s i g n a t e d as t o n g u e s , t h e s e sequences a f f o r d
t h e o p p o r t u n i t y t o examine i n d e t a i l t h e c h a n g i n g n a t u r e o f t h e d e p o s i t i o n a l p r o c e s s e s as soc ia t ion w i t h t h i s f 1 uv ia1 - e o l ian t r a n s i t i o n . SEDIMENTOLOGY Facies Descriptions S i x f a c i e s are recognized w i t h i n t h e i n t e r t o n g u i n g i n t e r v a l o f t h e Navajo and
Kayenta
formations.
Facies
were
erected
based
combinations
on
l i t h o l o g y , s e d i m e n t a r y s t r u c t u r e s and u n i t g e o m e t r i e s and i n c l u d e : large-scale
cross-bedded Facies C
sandstone,
-
sandstone,
Facies
small-scale
cross-bedded
r i p p l e - l a m i n a t e d s a n d s t o n e and s i l t s t o n e ,
p l a n a r - l a m i n a t e d s a n d s t o n e and s i l t s t o n e , and mudstone,
-
B
Facies F
-
cryptalgal
Facies E
-
of
Facies A Facies C -
interbedded sandstone
l a m i n a t e d and s t r u c t u r e l e s s c a r b o n a t e .
Many of t h e s e f a c i e s c a n be d i v i d e d i n t o s u b f a c i e s based on t h e p r e s e n c e of s m a l l - s c a l e s t r a t i f i c a t i o n f e a t u r e s discussed below. Facies A comprises s i n g l e s e t s o f large-scale s e t s r a n g i n g f r o m 0.3
t o 15
b u t p l a n a r wedge v a r i e t i e s
m t h i c k ( f i g . 5). a l s o occur.
m i d d l e and u p p e r p a r t s o f t h e s e t s .
cross-bedded sandstone i n
Most s e t s a r e p l a n a r t a b u l a r
F o r e s e t d i p s a v e r a g e 20-27'
i n the
The b a s a l p a r t o f most f o r e s e t s t e n d s t o
be a s y m p t o t i c t o t h e l o w e r p l a n a r b o u n d i n g s u r f a c e , w h i l e t h e u p p e r b o u n d a r y of each s e t i s e i t h e r p l a n a r ( f i g . 5 ) o r d e e p l y s c o u r e d . u n i t s a r e composed o f f i n e - t o m e d i u m - g r a i n e d sand,
Texturally these
most o f which i s w e l l
s o r t e d and subrounded. M o s t s a n d s t o n e s a r e q u a r t z t o s u b f e l d s p a t h i c a r e n i t e s . Internally, s t r a t if ic a t ion.
these large sets consist
of
t h r e e types
These in c 1ude s a n d f 1 ow c r o s s - s t r a t a ,
climbing translatent
s t r a t a (terminology o f
Hunter,
of
smaller
scale
g r a i n f a1 1 1 ami n ae , and 1977a,
b).
Sandflow
s t r a t a a r e c h a r a c t e r i z e d b y beds up t o 1 2 cm o f s e e m i n g l y s t r u c t u r e l e s s sand ( f i g . 6 a ) . These u n i t s a r e b r o a d l y l e n t i c u l a r i n c r o s s s e c t i o n s p e r p e n d i c u l a r t o foreset dip direction (fig. t o foreset abundant
dip direction
type
of
6 a ) and a r e wedge-shaped i n s e c t i o n s p a r a l l e l
(fig.
small-scale
6b).
Sandflow c r o s s - s t r a t a
stratification
within
c o n c e n t r a t e d i n t h e m i d d l e and u p p e r p a r t s o f f o r e s e t s .
this
a r e t h e most
facies
and
are
Interbedded w i t h t h e
sandflow c r o s s - s t r a t a a r e t h i n g r a i n f a l l laminae t h a t form p l a n a r ( f i g .
6a)
o r s l i g h t l y undulatory deposits t h a t p a r a l l e l subadjacent s t r a t a .
Near t h e
b a s e o f many f o r e s e t s
climbing
are t h i n
(less
t h a n 2 cm t h i c k )
sets o f
t r a n s l a t e n t s t r a t a ( f i g . 6 b ) . These s t r a t a t y p i c a l l y wedge o u t up t h e f o r e s e t w i t h i n sandflow cross s t r a t a ( f i g . cross-laminae,
6b),
occasionally contain ripple-foreset
and o f t e n a r e i n v e r s e l y graded.
619
F i g u r e 5.
F i g u r e 6.
L a r g e - s c a l e c r o s s - b e d d e d s a n d s t o n e ( F a c i e s A) n e a r Tuba C i t y , A r i z o n a . S e t i s up t o 12 ti1 t h i c k , bounded by f i r s t - o r d e r t r u n c a t i o n s u r f a c e s and p l a n a r l a m i n a t e d s a n d s t o n e o f F a c i e s D.
( a ) S a n d f l o w c r o s s - b e d d i n g w i t h i n F a c i e s A (homogeneous s e t above hammer) o v e r l a i n by t h i n g r a i n f a l l l a m i n a e . View p e r p e n d i c u l a r t o ( b ) Uedge-shaped foreset dip direction. Hammer head i s 18 cm l o n g . s a n d f l o w c r o s s - s t r a t a p i n c h i n g o u t down f o r e s e t ( t o r i g h t ) . Sandf l o w c r o s s - s t r a t a a r e i n t e r c a l a t e d w i t h upward-tapering sets o f climbing translatent strata. View p a r a l l e l t o f o r e s e t d i p d i r e c t i o n . ( B o t h p h o t o g r a p h s f r o m o u t c r o p s n e a r Tuba C i t y , A r i z o n a )
620
Large-scale syndepositional deformational structures occur throughout this facies (figs. 7a, b). These consist of large asymmetric folds, many of which are recumbent, and numerous small thrust-like displacements of individual layers. Small breccia zones are present in the middle and lower portions of some foresets and contain clasts derived from underlying foreset beds (fig. 7c). Orientation of these clasts ranges from subparallel to random. Facies A also contains sets without sandflow, grainfall, and/or climbing translatent strata. These sets tend to be less than 50 cm thick and are both trough and planar-tabular cross-stratified (fig. 8). Most are moderately sorted and tend to be interbedded with mudstones (fig. 8). These sandstones are mineralogical ly less mature containing considerably more feldspar and lithic fragments. Small-scale cross-bedded sets (Facies B) are less than 30 cm thick and are almost entirely of the trough variety. These occur i n cosets of up to 8 sets and are commonly associated with mudstone facies. Grain size and sorting are variable ranging from poor to moderately sorted fine-grained sand to moderately well-sorted medium-grained sand. The sandstones are mostly lithic and/or feldspathic arenites. Ripple-laminated fine to very fine grained sandstone and siltstone (Facies C ) occur sporadically throughout the section. Both current and oscillation ripples are present with the former being more abundant. Rarely, well-defined adhesion ripples are present. This facies is interbedded with cross stratification of all scales. Facies D comprising planar-laminated fine-grained sandstone and siltstone occurs repeatedly throughout the intertonguing interval. Laminae range from horizontal to gently dipping (fig. 9) and occur as thin beds between largeand small-scale cross-stratified sandstone (fig. 9). I n a number of instances this facies is up to 6 m thick, although it normally comprises beds less than 2 m in thickness (fig. 9). Occasionally, thin laminae of very coarse grained sandstones and granules occur within this facies. The interbedded sandstone-mudstone facies (Facies E ) i s common in the basal parts of the intertonguing interval (fig. 10). Near the top of the interval where this facies is dominantly mudstone sand-filled V-shaped desiccation cracks up to 20 cm in length are present (figs. lla, b). The sandstone is either rippled or horizontally laminated. Mudstone lower in the section contains zones of discrete and coalesced calcite nodules. The mudstones consist largely of discrete illite, randomly interstratified mixed-layer illite-smectite with between 40 to 70% expandable smectite layers, and minor kaolinite. Mixed-layer clays increase in abundance upwards as discrete illite decreases.
621
F i g u r e 7.
( a ) Oblique c r o s s s e c t i o n o f l a r g e - s c a l e cross-bedded s e t w i t h i n F a c i e s A e x h i b i t i n g complex a s y m m e t r i c f o l d s a l o n g t h e :middle p a r t O F f o r e s e t ( v i e w p a r a l l e l t o f o r e s e t d i p d i r e c t i o n ) . Person i s 1.7 m t a l l . ( b ) O b l i q u e p l a n v i e w o f same s e c t i o n as i n f i g u r e 7a s h o w i n g f o l d e d f o r e s e t s . Small w i n d r i p p l e s above t h e harmer ( s e e (c) Brecciated clasts o f foreset a r r o w ) . Hammer i s 30 c:n l o n g . l a m i n a t i o n s t h a t were r i p p e d up and t r a n s p o r t e d down t h e f o r e s e t d u r i n g a v a l a n c h i n g o f wet sand. Lens c a p i s 50 mi. A l l p h o t o s n e a r Tuba C i t y , A r i z o n a .
622
F i q u r e 8.
C o s e t s o f t r o u g h c r o s s - s t r a t i f i e d s a n d s t o n e and o v e r l y i n g mudstone n e a r base o f i n t e r t o n g u i n g i n t e r v a l a t Tuba C i t y , A r i z o n a . Har:isier a t base o f o u t c r o p i s 30 cm l o n g .
F i g u r e 9.
P l a n a r - l a m i n a t e d s a n d s t o n e ( F a c i e s D ) between l a r g e - s c a l e c r o s s s t r a t i f i e d s e t s o f F a c i e s A n e a r Tuba City, A r i z o n a . T h i s sequence i s o v e r l a i n by a t h i n c a r b o n a t e h o r i z o n . Person i s 1 . 7 m t a l l .
623
F i g u r e 10.
F i g u r e 11.
I n t e r b e d d e d s a n d s t o n e and iiiudstone o f F a c i e s E n e a r Leupp, Arizona. Sectioti i s near t h e basal p a r t o f t h e i n t e r t o n g u i n g i n t e r v a l where Facics E c o n t a i n s inore s a n d - s i z e i l l a t e r i a l . C l i f f f a c e i s 10 iii h i g h .
( a ) C r o s s s e c t i o n o f V-shaped, s a n d - f i l l e d d e s i c c a t i o n c r a c k i n mudstone o f F a c i e s E t h a t o c c u r s b e n e a t h l a r g e - s c a l e c r o s s bedded s e t . ( b ) Underside o f sand bed i n f i g u r e I l a showing p o l y g o n a l m u d c r a c k s . Hammer h a n d l e i s 20 cm. B o t h p h o t o s froin t o p o f i n t e r t o n g u i n g i n t e r v a l n e a r Tuba C i t y , A r i z o n a .
624 C r y p t a l g a l - l a m i n a t e d c a r b o n a t e s ( F a c i e s F ) o c c u r as t h i n r e s i s t a n t beds i n the
upper
part
the
of
intertonguing
interval.
These
are
composed
of
h o r i z o n t a l t o wavy l a m i n a t i o n s . The l a t t e r o c c a s i o n a l l y have a m p l i t u d e s o f up to
11 cm ( f i g . 1 2 a ) and h a v e mudcracked u p p e r s u r f a c e s .
The c a r b o n a t e s a r e
12b).
d o m i n a n t l y m i c r i t i c w i t h some s i l i c i c l a s t i c and o s t r a c o d a l d e b r i s ( f i g . Facies Associations W i t h i n t h e zone o f i n t e r t o n g u i n g t w o f a c i e s a s s o c i a t i o n s a r e common.
A) w i t h
a s s o c i a t i o n c o m p r i s e s l a r g e - s c a l e c r o s s s t r a t i f i e d sandst.one ( F a c i e s
and p l a n a r - l a m i n a t e d s a n d s t o n e and s i l t s t o n e ( F a c i e s C
subordinate r i p p l e and D ) ,
and c a r b o n a t e ( F a c i e s F ) .
b y l a r g e - and s m a l l - s c a l e
The second f a c i e s a s s o c i a t i o n i s d c m i n a t e d
t r o u g h and p l a n a r c r o s s - s t r a t i f i c a t i o n
and 6 ) and i n t e r b e d d e d s a n d s t o n e and mudstone ( F a c i e s E ) . ripple-laminated
One
sandstone
and
mudstone
C)
(Facies
(Facies
A
M i n o r amounts o f
and
planar-laminated
sandstone (Facies D) a l s o occur. ( i ) F a c i e s A s s o c i a t i o n 1. T h i s a s s o c i a t i o n i s m o s t a b u n d a n t i n t h e u p p e r
p a r t s o f t h e i n t e r t o n g u i n g i n t e r v a l and i s p r e s e n t i n a l l o u t c r o p s examined. Although
very
large scale
cross-stratification
d e f i n i t i v e c r i t e r i o n f o r e o l i a n sedimentation, characteristics particular
of
this
significance
climbing translatent (1977a,
facies is
strata
the
has been
questioned
a r e c o n s i s t e n t w i t h an e o l i a n association
within
these
as
a
the smaller scale internal of
large
sandflow, sets.
origin.
grainfall,
Studies
by
Of and
Hunter
b ) and K o c u r e k and D o t t ( 1 9 8 1 ) have d e m o n s t r a t e d t h e u n i q u e n e s s o f
the combination o f these s t r a t i f i c a t i o n types t o e o l i a n s t y l e s o f sediment transport
and
deposition.
Sandflow
from
mass
a v a l a n c h i n g o f l o o s e , c o h e s i o n l e s s g r a i n s down t h e s l i p f a c e o f a dune.
This
process generates s t r u c t u r e l e s s ,
stratification
results
wedge-shaped d e p o s i t s t h a t p i n c h o u t down
t h e f o r e s e t ( f i g . 6 b ) . The g r a i n f a l l l a m i n a e r e p r e s e n t s u s p e n s i o n s e t t l i n g o f g r a i n s s a l t a t i n g o f f t h e dune c r e s t . p r e - e x i s t i n g 'topography
of
the
As such, t h e s e t h i n l a m i n a e c o n f o r m t o
slipface
surface.
amplitude wind r i p p l e s across t h e s l i p f a c e
The
migration
of
low
i s r e c o r d e d b y t h e p r e s e n c e of
i n v e r s e l y g r a d e d , c o n t i n u o u s l a m i n a e o f c l i m b i n g t r a n s l a t e n t s t r a t a w i t h few well preserved f o r e s e t
laminations
(Hunter,
strata without foreset
laminations
a r e common on modern dunes and a r e an
extremely useful
1977a).
c r i t e r i a t o distinguish eolian
Climbing translatent
f r o m subaqueous
climbing
r i p p l e s ( H u n t e r , 1977a; 1981; K o c u r e k and D o t t , 1 9 8 1 ) . Equally
important
in
the
environmental
interpretation
of
this
facies
a s s o c i a t i o n i s t h e p r e s e n c e o f a number o f l o w - t o h i g h - a n g l e s u r f a c e s t h a t truncate intervene
foresets
(fig.
13a)
and
essentially
between
thick
sets
of
cross-strata
horizontal (Facies
A)
surfaces or
that
separate
625
( a ) Wavy a l g a l - l a m i n a t e d c a r b o n a t e mudstone o f F a c i e s F, Tuba City, Arizona. T h i s b e d i s 8 0 cm t h i c k a n d c o n t i n u o u s f o r a t l e a s t 100 m. The t o p s u r f a c e i s t y p i c a l l y mudcracked. ( b ) Photomicrograoh o f t h i s carbonate bed showing coarse sparf i l l i n g o f o s t r a c o d e s h e l l a n d p a r t i a l l y neomorphosed c a r b o n a t e mud.
F i g u r e 12.
horizontally
laminated
stratification
sandstones
(Facies A)
(figs.
C)
(Facies
13a, b ) .
froin
thick
sets
of
cross
The i n c l i n e d and s l i g h t l y c o n c a v e
upward s u r f a c e s s e p a r a t i n g s e t s o f c r o s s s t r a t a have been t e r m e d s e c o n d - o r d e r bounding surfaces b y B r o o k f i e l d (1977).
The s u r f a c e s p a r a l l e l t h e d i r e c t i o n
of
angle
foreset
dip
although
at
a reduced
(fig.
13a).
Brookfield
has
a t t r i b u t e d t h e i r g e n e s i s t o t h e p a s s a g e o f dunes a c r o s s t h e s u r f a c e o f a d r a a w i t h each b o u n d i n g p l a n e m a r k i n g t h e s u p e r p o s i t i o n o f an o v e r r i d i n g dune on a
previous bedform.
The n e t r e s u l t
cross-stratification nuinerous
wherein
bounding planes.
!nodern (McKee,
i s the generation o f
individual
foreset
Such s u r f a c e s
1 9 8 2 ) and t h e a n c i e n t
are
laminae
well
a complex f o r i n o f are
truncated
documented
( W a l k e r and Harms,
1972;
by
i n both the Steidtmann,
1974). First-order
bounding
surfaces
(terininology
of
Brookfield,
1977)
are
represented by t h e h o r i z o n t a l d i s c o n t i n u i t i e s t r u n c a t i n g i n d i v i d u a l sets o r separating
horizontal
and c r o s s - b e d d e d
sets
(figs.
13a,
b).
Studies
by
B r o o k f i e l d ( 1 9 7 7 ; 1 9 7 9 ) and K o c u r e k ( 1 9 8 1 a ) h a v e s u g g e s t e d a c l i m b i n g b e d f o r m iiiodel f o r
genesis o f these surfaces.
draas o r i n d i v i d u a l ,
I n t h i s model s e q u e n t i a l passage o f
l a r g e dunes o v e r downwind dune c o m p l e x e s o r i n t e r d u n a l
areas i s r e s p o n s i b l e f o r t h e h o r i z o n t a l bedding p l a n e break. erosion o f
I n t h e s e cases
t h e downwind dune i s f o l l o w e d b y s u p e r p o s i t i o n o f t h e f o l l o w i n g
626
Fiqure 13.
( a ) Horizontal f i r s t - o r d e r bounding surfa c e s ( 1 ) se pa ra tin5 t h i c k co s et s o f c r o s s - s t r a t a ; concave-upward second-order surfaces ( 2 ) d i v i d e individual s e t s . Foresets a n d secondo r d e r bounding planes d i p toward the w u t h - s o u t h e a s t . Person i s 1.7 m t a l l . ( b ) F i r s t - o r d e r bounding surfa c e ove rla in by planar-laminated sandstone. Hammer i s 30 cm.
627 i n t e r d u n a l d e p o s i t s o r draa.
S t o k e s ( 1 9 6 8 ) p o s t u l a t e d t h a t some of
these
b o u n d i n g s u r f a c e s a r e r e l a t e d t o d e f l a t i o n o f d r y sand down t o a s a t u r a t e d zone
(water
table).
The
common
association
of
horizontally
laminated
s a n d s t o n e s ( F a c i e s D) o v e r l y i n g l a r g e - s c a l e c r o s s beds i n t h e i n t e r t o n g u i n g interval
conforms
well
with
a
climbing
bedform s t y l e
of
generation
of
first-order surfaces. Near t h e b a s e o f t h e i n t e r t o n g u i n g i n t e r v a l f i r s t - o r d e r b o u n d i n g s u r f a c e s a r e corninon and s e c o n d - o r d e r
surfaces r e l a t i v e l y r a r e suggesting t h a t during
e a r l y s t a g e s o f e r g c o n s t r u c t i o n , s i m p l e u n m o d i f i e d dunes were t h e p r i n c i p a l bedforms.
Higher
in
the section the proportion o f
second-order
surfaces
i n c r e a s e s r e f l e c t i n g i n c r e a s i n g c o m p l e x i t y o f b e d f o r m s and p o s s i b l y a more v a r i e d wind regime. Some of
the
large-scale deformational structures
(figs.
7a,
b) present
w i t h i n t h e i n t e r t o n g u i n g i n t e r v a l a r e s i m i l a r t o those r e p o r t e d from t h e main b o d y of
t h e Navajo
(Doe
and
Dott,
1980;
g e n e r a t e d e x p e r i m e n t a l l y (McKee e t a l . , the
Horowitz,
1982)
and
t o those
1971). O f p a r t i c u l a r s i g n i f i c a n c e a r e
b r e c c i a t e d zones composed o f c l a s t s d e r i v e d f r o m u n d e r l y i n g f o r e s e t s
(Figure 7c).
P r e s e n c e of t h e s e c l a s t s i n d i c a t e s m o i s t c o n d i t i o n s where t h e
s a n d was somewhat c o h e s i v e p r i o r t o s l o p e f a i l u r e ( H u n t e r , and D o t t ,
1980; K o c u r e k and D o t t ,
have formed (Horowitz,
1981).
1977a, 1981; Doe
While t h e c o n t o r t e d bedding c o u l d
b e l o w t h e w a t e r t a b l e when t h e s a n d was c o m p l e t e l y s a t u r a t e d 1979;
Doe and D o t t ,
1980),
the association with the brecciated
zones s u g g e s t s t h a t d e f o r m a t i o n o c c u r r e d a t t h e a i r - w a t e r conditions
interface.
were r e l a t i v e l y common d u r i n g t h e e a r l y s t a g e s
of
Moist
Navajo erg
development. The m o d e r a t e d e g r e e o f s o r t i n g o f t h e p l a n a r - l a m i n a t e d s a n d s t o n e f a c i e s ( a s compared t o t h e w e l l - s o r t e d presence
of
thin
laminae
texture of
composed o f
c l iinbing t r a n s l a t e n t s t r a t a ,
p o i n t t o an e o l i a n o r i g i n f o r F a c i e s D. and F r y b e r g e r ,
1981;
deposits
and K o c u r e k ,
(Walker
large-scale
granule-size
grains,
c r o s s beds), r a r e sets
of
and i n t e r b e d d i n g w i t h t h e l a r g e r c r o s s - s t r a t a S i m i l a r f e a t u r e s a r e documented from
modern i n t e r d u n a l a r e a s ( F r y b e r g e r e t a1 ancient
the
1981a)
and Harms,
K o c u r e k , 1981b, B l a k e y and M i d d l e t o n ,
., 1979;
1972;
McKee, 1979, 1982; A h l b r a n d t
and a r e r e p o r t e d f r o i n numerous Steidtmann,
1974;
Hunter,
1981;
t h i s volume).
The d e g r e e o f m o i s t u r e i n t h e s e i n t e r d u n a l a r e a s c a n be e s t i m a t e d u s i n g c r i t e r i a o u t l i n e d i n K o c u r e k (1981a, T a b l e 1). gravels
and
climbing
translatent
strata
The p r e s e n c e o f d e f l a t i o n l a g
suggest
a
dry
interdune floor.
However, t h e i n t e r c a l a t i o n o f r i p p l e - l a m i n a t e d s a n d s t o n e and s i l t s t o n e w i t h r a r e adhesion s t r u c t u r e s (Facies C), desiccation cracks
i n t e r b e d d e d mudstone w i t h l a r g e V-shaped
( F a c i e s E ) and l a m i n a t e d c a r b o n a t e ( F a c i e s F )
indicate
628
t h a t the interdunal f l a t s were p e r i o d i c a l l y wet and occasionally ponded. Thin, l e n t i c u l a r carbonate buildups have been described in d e t a i l from several ancient interdune complexes (Hanley and Steidtmann, 1973; and Fryberger, 1979). The carbonates within the intertonguing i n t e r v a l o v e r l i e interdunal sandstones and/or f i r s t - o r d e r bounding planes. Petrographically they are ostracodal and peloidal wackestones a n d carbonate mudstones. The wavy algal laminations a n d f e n e s t r a t e porosity are s i m i l a r t o those repcrted by Kocurek (1981a) f o r modern deposits from Padre Island, Texas. As suggested by Hanley and Steidtmann (19731, t h e presence of these carbonate lenses i s l i k e l y the r e s u l t of decreasing s o l u b i l i t y of c a l c i t e w i t h increasing s a l i n i t y of lake waters. Such conditions probably developed w i t h gradual drying of ponded areas. Mudcracked surfaces and disrupted laminations on t o p of these carbonate horizons support t h i s hypothesis.
I n summary, f a c i e s association 1 records t h e i n t e r p l a y of eolian dune and interdune processes. Alternating wet and dry conditions are indicated with the l a t t e r being more common near t h e contact w i t h t h e main body of t h e Navajo. ( i i ) Facies Association 2. The second f a c i e s association i s characterized by an abundance of large- t o small-scale trough and planar cross s t r a t i f i c a t i o n , interbedded sandstone and mudstone, and minor amounts of r i p p l e - and planar-laminated sandstone and mudstone. Sets of large-scale t r o u g h and planar c r o s s - s t r a t i f i c a t i o n in t h i s association are l e s s than 50 cin thick and sandflow, g r a i n f a l l and cl imbing t r a n s l a t e n t interbeds a r e
absent. Cosets o f up t o 15 sets o f trough c r o s s - bedding are common throughout the lower parts of t h e intertonguing i n t e r v a l : f i g . 8 ) . These cosets form l e n t i c u l a r sandstone lithosomes and grade i n t o finer-grained 5aridstone and mudstone ( f i g . 1 4 ) . Rarely do these c o s e t s e x h i b i t a pronounced change i n c r o s s - s t r a t a l
vertical
thickness.
Paleocurrent d i r e c t i o n s
from
the southwest whereas large-scale c r o s s - s t r a t i f i e d s e t s i n f a c i e s association 1 a r e toward the
trough
axis
orientations
indicate
flow
toward
southeast. Facies and l i t h o l o g i c r e l a t i o n s h i p s suggest deposition in a f l u v i a l system characterized by r e l a t i v e l y shallow channels and f l u c t u a t i n g flow conditions. Similar sequences are described by S m i t h (1970) and M i a l l (1977) from l i n g u o i d a n d t r a n s v e r s e bars w i t h i n the braided P l a t t e River system. The mudstone sequences were deposited away from t h e channel d u r i n g periods of overbank flooding. A number of mudstones contain small c a l c i t e nodules of probable pedogenic o r i g i n . Their presence indicates t h a t although f l o o d i n g was common, enough time elapsed between floods t o allow local development of s o i l s (Leeder, 1975; Bown and Kraus, 1981).
629
F i g u r e 14.
L e n t i c u l a r trociitiand p l a n a r cross s t r a t i f i e d sandstone o f b r a i d e d c h a n n e l o r i g i n g r a d ng i n t o f l o o d p l a i n niudstone, Tuba C i t j , A r i z o n a . P e r s o n i s 1 . 6 rn t a l l .
;it i i i c a t i o n 5eqiier;ce
involve5 cosets
~i truu;)!i c r o i s - s t r a t a w i t h upward deireas i i i y s e t t h i c k n e s s a s s o c l d t e d w i t h
irisliiied
ii~rfJcc"j ( f i y .
i r e d ba;,il
sJrfaces
IS")
tiiari
15).
These
are
interbedded w i t t i
c o n t a c t s w i t h u n d e r l y i i i r j iiiudstones, tiirougiiout
ttieir
iriailstorie,
and e x h i b i t
exterit
15).
(fig.
tizve
low-angle Clasts
i i t t i ) i r j y i c a i l y i d e n t i c a l t o t h e u n d e r l y i n g iriiidstones a r e cornrrioii a t tiit b a s e . itratification
Trirl
(epsilon) s u r f a c e s tlisciJs,;rd fur.iri
iri
t ti e
are
similar
to
ttiose
reported
r e p r es e ri t in g
1ow - an g 1 e s u r f d c es
associated w i t h p o i n t bar inigi-ation
froin i n e a n d e r i n g 1 a t e r a 1 - sc c r e t i on
1963).
(Allen,
AS
by A1 l c r i ( 1 9 8 3 ) , l a t e r a l a c c r e t i o n s u r f a c e s o f l i m i t e d e x t e n t c o u l d
i31icjy
br-,iided
systeins v i a d e p o s i t i o n a l o n g b d r m a r g i n s .
i n s t a n c e s t e n d s t o be l e s s
in IUCII
ext:?rit the
styles
s y i t eiii s w i t t i
5t r e xi
thdri
10
The l a t e r a l
tiiires s e t t h i c k n e s s .
intertorigijirig i n t e r v a l t h e l a t e r a l extent o f these bodies
Within
i s between 10
arid 20 t i n i e s t h a t o f t h e s e t t h i c k n e s s arid t h e r e f o r e i s i n t e r m e d i a t e between those
r-eportetl
for
r w ~ t i d e r i i i q\ L r e a i i i s
irr
any
systeirr
streaiii
iwteroyencous
iaridy
braided
(A1 l c n , 1 9 8 3 ) . where
1 ittioloyy,
deposits
there
vertical
those
of
higher
sinuosity
is
sufficient
decrease
bar
channel
scdle
of
curvature,
the
the
and
sets,
likewise
1 . i t e r - d l Jiccri,t
depcisi t s a r e t h i c k i r i u i i i t o r i e > a r i d o c c d s i o n a l l y t h i n c a l c r e t e
tiorizons.
Tt;c~;c f i n e - q r a i n e d
point
iii
ctidririelired base iiiii
suggest
and
W t r i l e l a t e r a l - a c c r e t i o n s u r f a c e s can f o r m
f a c i e s were
development.
d e p o s i t e d on
Overlying
floodplains
these
during
630
F i g u r e 15.
i s t e r a l d c c r e t i o n sLirFace r e p r e s e n t i n g i n - c h a n n e l b a r d e p o s i t i o n w i t h c h a n n e l e d base ( a r r o w ) and o v e r l y i n g i n t e r b e d d e d s a n d s t o n e arid inrudstone o f o v e r b a n k o r i g i n s . E n t i r e l a t e r a l a c c r e t i o n package i s 6 . 5 iii O u t c r o p i s n e a r Leupp, A r i z o n a .
Parallel-lmiiiated coiniiiori
within
this
sandstone facies
and
interbedded
association.
inudstme
Horizontal
(Facies
laminated
E)
are
fine-
to
:iisdiurn-yrained s a n d s t o n e f o r m e d d u r i n g h i g h - v e l o c i t y f l o w w i t h i n and a d j a c e n t to
the channel systems.
parallel-laniinated epiierrierdl c h a n n e l .
sand
Tunbridgc deposited
(1981) has documented up t o 1.5 m o f
during
a single
flood
event
within
an
S m i t h ( 1 9 7 0 ) and M i a l l ( 1 9 7 7 ) a l s o r e p o r t e d t h e common
occurrence o f p l a n a r l a m i n a t i o n s i n sandy b r a i d e d systems. Facies
association
meandering
fluvial
2
records
systems.
Near
deposition
r e l a t i v e l y t h i c k sequences o f i n - c h a n n e l coiiiirion.
Higher
incdridcirjI 3
iii
in
t h e base o f
both the
braided
and
interval
sands o f b r a i d e d s t r e a i n o r i g i n a r e
t h e s e c t i o n l a t e r a l and v e r t i c a l a c c r e t i o n d e p o s i t s o f
stream systein occur.
Interbedded
with the v e r t i c a l l y
iniuditories a r e t h i n p l a n a r - l a i n i t i a t e d s a n d s t o n e s r e p r e , e n t i n g siiial 1 ephemeral c h a n n e l s . wi!h
sandy
intertonguing
a
accreted
deposition within
F a c i e s a s s o c i a t i o n 1 i s m o s t cominonly i n t e r b e d d e d
t h e l a t t e r two s t y l e s o f f l u v i a l s e d i m e n t a t i o n .
631 SEDIMENTOLOGIC S Y N T H E S I S The
reasons
for
the
transition
from
a
predominantly
fluvial
system
( K a y e r i t a F o r m a t i o n ) t o an e o l i a n one ( N a v a j o S a n d s t o n e ) a r e p o o r l y known. S p e c u l a t i o n s have ranged f r o m t e c t o n i c adjustments source,
r e g i o n a l c l i m a t i c changes,
i n t h e b a s i n and/or t h e
combined t e c t o n i c / c l i m a t i c
effects,
i.e.
r i s i n g o r o g r a p h i c b a r r i e r s t o t h e west promoting increased a r i d i t y i n t h e Kayenta-Navajo basin,
t o s u b t l e base l e v e l changes.
climatic
were
variations
desertification, ascertain.
from
interval,
sedimentologic
facies
however,
parameters
While i t i s l i k e l y t h a t
responsible
the overriding control
Information
intertonguing
chiefly
on
the
trend
tgward
t h e s e changes i s d i f f i c u l t
types
and
provide
associated
for
relationships
insight
with
the
into
early
to
within
the stages
the
changing of
erg
development.
As p r e v i o u s l y d i s c u s s e d ,
parts o f
facies
2 a c c u m u l a t e d on
association
t r a n s v e r s e and l i n g u o i d b a r s w i t h i n a s a n d y b r a i d e d system.
Such f l u v i a l
s y s t e m s a r e common i n s e m i - a r i d and a r i d t e r r a i n s where t h e y o f t e n o c c u r on alluvial
fans
or
around t h e
margins
of
inland deserts
(Glennie,
1970).
Likewise, t h e i r association w i t h eolian deposits i n the stratigraphic record has
been documented
i n a number
o f studies
(Gradzinski
and J e r z y k i e w i c z ,
1974; Horne, 1 9 7 5 ) . The e x i s t e n c e o f h i g h e r s i n u o s i t y s t r e a m s w i t h i n a r i d d e p o s i t i o n a l systems i s somewhat u n u s u a l . The s h i f t f r o m a b r a i d e d t o a m e a n d e r i n g c h a n n e l p a t t e r n o o u l d r e s u l t f r o m i n c r e a s e d and p e r s i s t e n t w a t e r
discharge
i n the drainage
basin. The abundance o f v e r t i c a l l y a c c r e t e d f l o o d p l a i n mudstone i n d i c a t e s t h a t o v e r b a n k f l o w s w e r e numerous. These m e a n d e r i n g s t r e a m s were l o c a t e d around t h e s o u t h e r n border o f t h e encroaching Navajo erg. T h e i r southwestward
f l o w o r i e n t a t i o n p a r a l l e l s t h e e a s t e r n b o r d e r of N a v a j o d e p o s i t s ( f i g . 4). The s h i f t from m e a n d e r i n g t o s m a l l , ephemeral b r a i d e d c h a n n e l s o c c u r s n e a r t h e t o p of t h e i n t e r t o n g u i n g i n t e r v a l and l i k e l y r e s u l t e d f r o m t h e i n c r e a s i n g i n f l u x of w i n d - b l o w n material
sand f r o m t h e s o u t h e a s t w a r d m i g r a t i n g N a v a j o e r g .
e v e n t u a l l y c l o g g e d many c h a n n e l s
processes.
disrupting
flow
This
and m e a n d e r i n g
B l o c k a g e r e s u l t e d i n d i v e r s i o n o f f l o w i n t o many s m a l l c h a n n e l s
and l o c a l pond d e v e l o p m e n t . During the
early
p e r i o d i c a l l y wet large
dune
as
fields
i n t e r d u n a l areas
stages
the erg
evidenced and
by
interdunal
margin
and
aojacent
interbedded desiccated ponds.
While
the
floodplain mudstones
abundance o f
were
within t h e wet
( f l a t s and p o n d s ) c o u l d be r e l a t e d t o r i s i n g g r o u n d w a t e r
t a b l e s t h e i n t i m a t e a s s o c i a t i o n and a p p a r e n t p r o x i m i t y o f c h a n n e l s t o t h e e o l i a n d e p o s i t s a r g u e f o r o v e r b a n k f l o o d i n g f r o m n e a r b y c h a n n e l s as a s o u r c e of
i n t e r d u n e waters.
Other c r i t e r i a t h a t i n d i c a t e moist conditions during
e a r l y N a v a j o d e p o s i t i o n a r e t h e b r e c c i a zones
on
some dune
foresets
and
632 presence o f well-preserved
v e r t i c a l and h o r i z o n t a l
trace fossils
on some
f o r e s e t s ( M i d d l e t o n and B l a k e y , i n p r e p . ) . The i n t e r b e d d i n g o f t h e f l u v i a l and e o l i a n d e p o s i t s r e s u l t e d f r o m p e r i o d i c advance and r e t r e a t o f t h e N a v a j o e r g .
Such s h i f t s a p p a r e n t l y w e r e common
d u r i n g e a r l y e r g d e v e l o p m e n t and f i n a l l y t e r m i n a t e d w i t h advancement o f t h e m a i n mass o f t h e N a v a j o sand sea. ACKNOWLEDGEMENTS Reviews
and/or
discussions w i t h Michael
Brookfield,
Loope, and James S t e i d t m a n n i m p r o v e d t h e m a n u s c r i p t . t h e manuscript
and D e b b i e M e i e r
Mary
Kraus,
David
Connie Erickson typed
and R i c k Raymond ( B i l b y R e s e a r c h C e n t e r )
a s s i s t e d w i t h t h e g r a p h i c s and p h o t o g r a p h y .
P a r t i a l s u p p o r t f o r t r a v e l was
p r o v i d e d b y an O r g a n i z e d R e s e a r c h G r a n t ( N o r t h e r n A r i z o n a U n i v e r s i t y )
to
i*l id d 1e t o n .
RE ERENCES
Ah b r a n d t , T. S., and F r y b e r g e r , S. G., 1981. S e d i m e n t a r y f e a t u r e s and significance o f interdune deposits. In: F. G. E t h r i d g e and R. M. F l o r e s ( E d i t o r s ) , R e c e n t and A n c i e n t Nonmarine D e p o s i t i o n a l E n v i r o n m e n t s . Models f o r E x p l o r a t i o n . SOC. Econ. P a l e o n . M i n e r a l . Spec. Pub. 31, 293-314. A l l e n , J . R. L., 1963. The c l a s s i f i c a t i o n o f c r o s s - s t r a t i f i e d u n i t s , w i t h n o t e s on t h e i r o r i g i n . S e d i m e n t o l o g y , 2: 93-114. J. R. L., 1983. Studies i n f l u v i a t i l e sedimentation: bars, Allen, b a r - c o m p l e x e s and s a n d s t o n e s h e e t s ( l o w - s i n u o s i t y b r a i d e d s t r e a m s ) i n t h e B r o w n s t o n e s (L. D e v o n i a n ) , Welsh B o r d e r s . Sed. Geology, 33: 237-293. A v e r i t t , P., Detterrnan, J . S., H a r s h b a r g e r , J. W., Repenning, C. A,, and W i l s o n , R . F., 1955. R e v i s i o n s i n c o r r e l a t i o n and n o m e n c l a t u r e of T r i a s s i c and J u r a s s i c f o r m a t i o n s i n s o u t h w e s t e r n U t a h and n o r t h e r n A r i z o n a . Amer. Assoc. P e t . G e o l . B u l l . , 39: 2515-2535. B l a k e y , R. C., and M i d d l e t o n , L. T., 1983. P e r m i a n s h o r e l i n e e o l i a n c m p l e x i n c e n t r a l A r i z o n a : dune changes i n r e s p o n s e t o c y c l i c sea l e v e l changes. I n : M. E. B r o o k f i e l d and T. S. A h l b r a n d t ( E d i t o r s ) , A e o l i a n Sediments and P r o c e s s e s . E l s e v i e r , Amsterdam, and K r a u s , M. J . , 1981. Lower Eocene a l l u v i a l p a l e o s o l s Bown, T. M., and t h e i r s i g n i f i c a n c e f o r ( W i l l w o o d F o r m a t i o n , n o r t h w e s t Wyoming, U.S.A.) paleoecology, paleoclimatology, and basin analysis. Palaeogeog., P a l a e o c l i m . , P a l a e o e c o l . , 34: 1-30. B r o o k f i e l d , M. E., 1977. The o r i g i n o f b o u n d i n g s u r f a c e s i n a n c i e n t a e o l i a n s a n d s t o n e s . S e d i m e n t o l o g y , 24: 303-332. Anatomy o f a Lower P e r m i a n a e o l i a n s a n d s t o n e B r o o k f i e l d , M. E., 1979. complex, s o u t h e r n S c o t l a n d . S c o t . Jour. Geol., 15: 81-96. 1978. A l t e r n a t i n g a e o l i a n , sabkha, and s h a l l o w - l a k e Clemmensen, L. B., d e p o s i t s f r o m t h e M i d d l e T r i a s s i c G i p s d a l e n F o r m a t i o n , S c o r e s b y Land, E a s t G r e e n l a n d . Palaeogeog., P a l a e o c l i m . , P a l a e o e c o l . , 24: 111-135. C o o l e y , M. E., H a r s h b a r g e r , J . W., A k e r s , J . P., and H a r d t , W. F., 1969. R e g i o n a l h y d r o g e o l o g y o f t h e N a v a j o and H o p i I n d i a n R e s e r v a t i o n s , A r i z o n a , New M e x i c o , and U t a h . U.S. Geol. S u r v . P r o f . P a p e r 521-A, 6 1 pp. Doe, T. W., and D o t t , R . H., 1980. G e n e t i c s i g n i f i c a n c e o f d e f o r m e d c r o s s b e d d i n g - - w i t h examples f r o m t h e N a v a j o and Weber S a n d s t o n e s o f U t a h . J o u r . Sed. P e t r o l o g y , 50: 793-812.
633 Freeman, W. E., and Visher, G. S., 1975. Stratigraphic analysis of the Navajo Sandstone. Jour. Sed. Petrology, 45: 651-668. Fryberger, S. G., 1979. Eolian-fluviatile (continental) origin of ancient stratigraphic trap for petroleum in Weber Sandstone, Rangely oil field, Colorado. Moun. Geologist, 16: 1-36. Fryberger, S. G., Ahlbrandt, T. S., and Andrews, S., 1979. Origin, sedimentary features, and significance of low-angle eolian "sand sheet" deposits, Great Sand Dunes National Monument and vicinity, Colorado: Jour. Sed. Petrology, 49: 733-746. Glennie, K. W . , 1970. Desert sedimentary environments. Developments in Sedirnentology, n. 14, Elsevier, Amsterdam, 222 pp. Gradzinski, R., and Jerzykiewicz, T., 1974. Dinosaur- and mammal-bearing aeolian and associated deposits of the Upper Cretaceous in the Gobi Desert (Mongolia). Sed. Geology, 12: 249-278. Hanley, J. H., and Steidtmann, J.R., 1973. Petrology o f limestone lenses in the Casper Formation, southernmost Laramie Sasin, Wyoming and Colorado. Jour. Sed. PetroloGy, 43: 428-434. Harshbarger, J. W., Repenning, C. A., and Irwin, 3. H., 1957. Stratigraphy of the uppermost Triassic and the Jurassic rocks of the Navajo Country. U.S. Geol. Surv. Prof. Paper 291, 74 pp. Horne, R. R., 1975. The association o f alluvial fan, aeolian and fluviatile facies in the Caherbla Group (Devonian), Dingle Peninsula, Ireland. Jour. Sed. Petrology, 45: 535-540. Horowitz, D. H., 1982. Geometry and origin of large-scale deformation structures in some ancient wind-blown sand deposits. Sedimentology, 29: 155 - 180. Knter, R. E., 1977a. Basic types of stratification in small eolian dunes. Sed iineri t o l o y y , 24: 361-388. Hunter, R. E., 1977b. Terminology of cross-stratified sedimentary layers and climbing-ripple structures. Jour. Sed. Petrology, 47: 697-706. Hunter, R. E., 1981. Stratification styles in aeolian sandstones: some In: Pennsylvanian to Jurassic examples from the Western Interior U.S.A.. F. G. Ethridge and R. M. Flores (Editors), Recent and Ancient Nonmarine Depositional Environments: Models for Exploration. SOC. Econ. Paleon. Mineral. Spec. Pub. 31, 315-329. Kocurek, G., 1981a. Significance of interdune deposits and bounding surfac-s in aeolian dune sands. Sedimentology, 28: 753-780. Kocurek, G., 1981b. Erg reconstruction: the Entrada Sandstone (Jurassic) of northern Utah and Colorado. Palaeogeog., Palaeoclim., Palaeoecol., 36: 125-153. Kocurek, G., and Dott, R. H., 1981. Distinctions and uses of stratification types in the interpretation of eolian sand. Jour. Sed. Petrology, 51: 579-595. Leeder, M. R., 1975. Pedogenic carbonates and flood sediment accretion rates: A quantitative model for alluvial arid-zone lithofacies. Geol. Mag., 112: 257-270. Mader, D., 1982. Aeolian sands in continental red beds of the Middle Buntsandstein (Lower Triassic) at the western margin of the German Basin. Sed. Geology, 31: 191-230. McKee, E. D., 1979. A study of global sand seas. U.S. Geol. Surv. Prof. Paper 1052, 429 pp. McKee, E. D., 1982. Sedimentary structures in dunes of the Namib Desert, Southwest Africa. Geol. SOC. America Spec. Paper 188, 64 pp. McKee, E. U., Douglass, J. R., and Rittenhouse, S., 1971. Deformation o f lee-side laminae in eolian dunes. Geol. SOC. America Bull., 82: 359-378. Miall, A. O . , 1977. A review of the braided river depositional environment. Earth-Sci. Rev., 13: 1-62. Middleton, L. T., and Blakey, R. C., in preparation. Variations in trace fossil assemblages in a fluvial-eolian transition, Kayenta and Navajo formations, northern Arizona.
634
Smith, N . O., 1970. The braided stream depositional enviornment: comparison of the Platte River with some Silurian clastic rocks, north-central Appalachians. Geol. SOC. America Bull., 81: 2993-3014. Steidtmann, J. R., 1974. Evidence for eolian origin of cross-stratification in sandstone of the Casper Formation, southernmost Laramie Basin, Wyoming. Geol. SOC. America Bull., 85: 1835-1842. Stokes, W. L., 1968. Multiple parallel-truncation bedding planes--a feature of wind-deposited sandstone formations. Jour. Sed. Petrology, 38: 510-515. Tunbridge, I . P., 1981. Sandy high-energy flood sedimentation--some criteria for recognition, with an example from the Devonian of S.W. England. Sed.Geology, 28: 79-95. Walker, T. R., and Harms, J. C.. 1972. Eolian oriqin of flaqstone beds, Lyons Sandstone (Permian), type area, Boulder County; Colorado: Moun. Geologjst, 9: 279-288. Wilson, R. F., 1958. The stratigraphy and sedimentology of the (Jurassic) Kayenta and (?Triassic) Moenave formations, Vermillion Cliffs region, Utah and Arizona. Unpub. Ph.D. dissertation, Stanford Univ., 167 pp.
635
CHANGING WIND AND HYDROLOGIC REGIMES D U R I N G DEPOSITION OF THE NkVAJO AND AZTEC SANDSTOiiES, JURASSIC ( ? ) , SOUTHWESTERN UNITED STATES JOHN E . MARZOLF:
Department of Geology, Southern I l l i n o i s University, Carbondale, I l l i n o i s 62901
I NT RO DUCT I0 Pi The Navajo and Aztec Sandstones form a wedge of predominantly crossbedded sandstone t h a t thickens westward from i t s zero edge in western Colorado, and northeastern Arizona t o over 650 ni in exposures along the eastern edge of the foreland fold and t h r u s t b e l t in southern Nevada. Further t o t h e west, t h i s sandstone wedge becomes progressively s t r u c t u r a l l y dismembered o r i s absent due t o erosion. ~
Uhere present in western Colorado, northern Arizona, and southern U t a h , the Navajo Sandstone i s unconformably overlain by t h e Temple Cap Formation, Page Sandstone, t h e marine Carfiiel Formation, o r younger s t r a t a (Fig. 1 ) . I t does not, as previously t h o u g h t , i n t e r f i n g e r with overlying marine s t r a t a (Peterson and Pipiringos, 1979). The Navajo Sandstone i s t h e uppermost formation of the Llen Canyon Group which, in southeastern Utah, a l s o includes, i n ascending o r d e r , t h e Wingate Sandstone and the Kayenta Formation. I n southwestern U t a h , t h e lowest formation of the Glen Canyon Group i s the lloenave which i n t e r f i n gers with the Wingate Sandstone i n south-central Utah and north-central Arizona (Williams, 1954) (Fig. 1 ) . I n southwestern Utah, well defined tongues of t h e Navajo Sandstone extend southwestward i n t o t h e Kayenta Formation. Where t h e contact i s s t r a t i g r a p h i c in southern Nevada, the Aztec Sandstone i s unconformably overlain by continental s t r a t a of Cretaceous and T e r t i a r y age. Elsewhere, the Aztec Sandstone i s in t h r u s t contact beneath rocks of e a r l y Paleozoic age. Although not formally named, tongues of Aztec Sandstone i n t e r f i n g e r with redbeds equivalent t o t h e Hoenave and Kayenta Formations (Wilson and Stewart, 1967) in t h e Valley of F i r e a n d t h e Wilson C l i f f s of southern Nevada. Hewett (1956) extended t h e Aztec Sandstone and underlying Chinle Formation t o t h e Mescal Range in the flojave Desert and Grose (1939) suggested t h a t 7,000 f t ( 2 , 1 2 1 rn) of interbedded quartz sanstone and volcanics in the Soda ilount a i n s represented a western f a c i e s of the Aztec Sandstone. Within the l a s t f i v e y e a r s , crossbedded quartzose sandstone and interbedded volcanics o f demo n s t r a b l e T r i a s s i c o r J u r a s s i c age have been recognized f u r t h e r and f u r t h e r
i 636
SE UTAH
S W UTAH
-1
W
CRETACEOUS FORMATIONS _s_
0 W -1
0
Figure 1. East-west cross-section of lower Mesozoic rocks of southern Utah and northern Arizona. (After Peterson and Pipiringos, 1979) west in t h e Mojave Desert of California (Dunne, 1977; Novitski-Evans, 1978; Miller, 1978; Miller and Carr, 1978; Cameron, e t a l . , 1979) and f u r t h e r south in the eastern Mojave and south-central Arizona (Bilodeau and Keith, 1979; Haxel and o t h e r s , 1980; Hamilton, 1982; M i l l e r , Keith, and John, 1982). These rocks a r e presumed t o represent f u r t h e r extensions of the Aztec and Navajo Sandstones i n t o an e a r l y Nesozoic orogenic-volcanic a r c . CHARACTERISTICS AND STRATIGRAPHIC RELATIONSHIPS OF THE NAVAJO AND AZTEC SANDSTONES Eastern Faci es The Navajo Sandstone i s a f e l d s p a t h i c quartz a r e n i t e t o quartz-rich subarkose composed of rounded t o well-rounded quartz sand and containing l a r g e t o very large-scale trough, tabular-planar, and wedge-planar c r o s s - s t r a t i f i c a t i o n . Although c r o s s - s t r a t i f i e d sandstone i s t h e most conspicuous and most common sedimentary s t r u c t u r e , other sedimentary f e a t u r e s which a r e important t o the i n t e r p r e t a t i o n of t h e depositional environment of the Navajo Sandstone a r e volumetrically s i g n i f i c a n t . These f e a t u r e s include: Limestone l e n s e s , desiccation-cracked horizontally s t r a t i f i e d sandstone and horizontal erosion surfaces, contorted s t r a t i f i c a t i o n , and s t r u c t u r e l e s s sandstone. Limestone lenses. A l t h o u g h only a small percent of t h e t o t a l volume of the Navajo Sandstone, limestone lenses having a maximum thickness o f 1 t o 2 m a r e
637
n o t uncommon i n p a r t of o r throughout v e r t i c a l se c tions of the sandstone. C h a r a c t e r i s t i c s of t h es e limestone l en s es have been summarized by Picard (1975, p . 1 1 7 ) .
I n v ar i ab l y , they were deposited in shallow depressions a few
thousand square meters o r l e s s i n area (Fig. 2 A ) .
Most a r e underlain by
shaly-bedded, muddy s i l t s t o n e ( F i g . 26) a n d some contain limestone breccias around t h e i r margins. i'lany contain a freshwater fauna and f l o r a (Harsbarger, e t a l . , 1957) and a few l en s es have been reported t o preserve dinosaur t r a c k s . Horizontal s t r a t i f i c a t i o_ _n . Several types of horiz onta lly s t r a t i f i e d sand___-__ stone which, a t l e a s t , have been modified by a n aqueous environment can be i d e n t i f i e d i n t h e Navajo Sandstone. They comionly a r e dark reddish-brown, s i l t y sandstone containing a higher than average content of t h e l e s s than 43 pr1i sediinent Fraction. These s t r a t a e i t h e r appear s t r u c t u r e l e s s or a r e wavy bedded o r d i sp l a y shaly p ar t i n g ( F i g . 2 C ) . Commonly, t h e i r surfa c e s a r e marked by d e s i c c a t i o n cracks or adhesion r i p p l e s and adhesion warts (Kocurek a n d F i e l d e r , 1982) and, a t one l o c a l i t y i n eas te rn Utah, a s i n g l e l a r g e dinos a u r t r a c k ( F i g . 2E).
A t Zion Canyon, horizontal s t r a t i f i c a t i o n near the base of t h e sandstone i s bioturbated and marked by dish s t r u c t u r e s . Horizontally s t r a t i f i e d sandstone i s commonly o v er l ai n by 1 arge-scale c r o s s - s t r a t i f i c a t i o n , b u t l a t e r a l l y may u n d er l i e l en s es of limestone which, in turn, a r e overlain by
large-scale cross-stratification.
I n ad d i t i o n , l a t e r a l l y - e x t e n s i v e , desicca-
tion-cracked erosion s u r f a c e s , with or without limestone le nse s upon them, mark areas of i n t e r r u p t ed sedimentation. Contorted s t r a t i f i c a t i o n and s t r u c -__ t u r e l e s s sandstone.
Individual zones of
contorted s t r a t i f i c a t i o n extend l a t e r a l l y along outcrop surfa c e s f o r dista nc e s of a few t e n s o f meters t o over a thousand meters.
Zones range from a few A s i n g l e zone commonly encloses b u t a s i n g l e s e t of well-defined, r e l a t i v e l y simple f o l d s co n s i sting of wide, rounded synclines separated by narrow, sharp-crested a n t i c l i n e s s i m i l a r i n a l l re spe c ts b u t s c a l e t o convolute lamination t y p i cal of t u r b i d i t e s (Fig. 3A and B ) . Elsewhere, a zone may comprise more complexly folded laminations t h a t cannot c l e a r l y be d e l i n e a t ed i n t o a s i n g l e s e t of f o l ds. The more complexly-folded contorted zones commonly contain laminations of l e s s f r i a b l e , ferruginous sandstone which, i n ad d i t i o n t o having been f o l de d, have been f a u l t e d and displaced v e r t i c a l l y ( F i g . 3C). Marzolf (1970, p . 83) described s t r u c t u r e l e s s sandstone in t h e Navajo Sands t o n e a s a sedimentary s t r u c t u r e which, i n outcrop, appears devoid of laminat i o n s ( F i g . 3D), and suggested i t s common a s s o c i a t i o n with la rge -sc a le cont o r t e d s t r a t i f i c a t i o n . Sanderson (1973) mapped zones of contorted s t r a t i f i c a t i o n and s t r u c t u r e l e s s sandstone c l e a r l y showing the r e l a t i o n s h i p o f these two sedimentary s t r u c t u r e s t o each o t h er and t o t h e la rge -sc a le c ross-
meters t o over 35 m t h i c k .
638
F i g u r e 2. A. Limestone l e n s i n s h a l l o w d e p r e s s i o n along h o r i z o n t a l e r o s i o n s u r f a c e , b a s a l Navajo S a n d s t o n e , Zion National Park, Utah. B . Thin-bedded limestone overlying shaly, h o r i z o n t a l l y s t r a t i f i e d sandstone. Diapiric s t r u c t u r e s i n s a n d s t o n e s u g g e s t f l u i d e s c a p e . C . Horizontal e r o s i o n s u r f a c e o v e r l a i n by t h i n - b e d d e d , h o r i z o n t a l l y s t r a t i f i e d s a n d s t o n e , North Wash, Utah. D. S a n d - f i l l e d d e s i c c a t i o n c r a c k s on h o r i z o n t a l l y s t r a t i f i e d s a n d s t o n e i n C. S c a l e i s 15 cm long. E. S i n g l e d i n o s a u r t r a c k , form genus A n c h i s a u r i p u s _ rninisculus _ o r t u~ b e r o s u s (Donald E a i r d , p e r s . conim., 1 9 6 4 ) , found on t h e d e s i c c a t i o n - c r a c k e d s u r f a c e i l l u s t r a t e d i n F i g s . 2C and 0. S c a l e i s 1 5 cm l o n g .
639
F i g u r e 3 . A. C o n t o r t e d s t r a t i f i c a t i o n u n d e r l a i n and o v e r l a i n by l a r g e - s c a l e c r o s s - s t r a t i f i c a t i o n in t h e basal Navajo Sandstone near Kanab, Utah. Contorted zone i s a p p r o x i m a t e l y 10 111 t h i c k . B. S i n g l e zone o f c o n t o r t e d s t r a t i f i c a t i o n showing broad, f l a t s y n c l i n e s and s h a r p - c r e s t e d and r u p t u r e d a n t i c l i n e s , s o u t h e a s t e r n San Rafael S w e l l , Utah. Zone i s approximately 2 in t h i c k . C. Ferruginous l a y e r s i n zone of c o n t o r t e d s t r a t i f i c a t i o n c u t by soft-sedii;ient f a u l t i n g , C a p i t o l Reef National P a r k , Utah. Zone o f c o n t o r t e d s t r a t i f i c a t i o n i s a p p r o x i m a t e l y 14 II~ t h i c k and i s o v e r l a i n by approximately 3 ni of s t r u c t u r e l e s s s a n d s t o n e . D. S t r u c t u r e l e s s s a n d s t o n e o v e r l y i n g recumbent c r o s s - s t r a t i f i c a t i o n , North Wash, Utah, s t r a t i f i c a t i o n which e n c l o s e s both.
Sanderson f u r t h e r demonstrated by X -
radiography t h a t s t r u c t u r e l e s s s a n d s t o n e c o n s i s t s of f a i n t wavy l a m i n a t i o n s . D i s t r i b u t i o n _______ of non-crossbedded l i t h o l o g i e s
~-
I n t h e eastern f a c i e s , nine vertical sections describe the vertical d i s t r i -
C,,tizn 3f sedi:;entary f e a t , J r e s w i t h i n tl-e BlzvaJo Sandstone ( F i g s . 5 3nd 6 ) . Location of v e r t i c a l s e c t i o n s i s i l l u s t r a t e d i n F i g u r e 4. These a r e s p e c i f i c r a t h e r than generalized s e c t i o n s .
They a r e r e p r e s e n t a t i v e o f t h e o v e r a l l
64 0 v e r t i c a l d i s t r i b u t i o n o f n o n - c r o s s b e d d e d l i t h o l o g i e s w i t h i n an a r e a o f a f e w s q u a r e ! k i l o m e t e r s a l t h o u g h s p e c i f i c 1i t h o l o g i e s e n c o u n t e r e d m i g h t v a r y o v e r l a t e r a l d i s t a n c e s o f 100 t o 1000 in.
BW
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I I
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Calif.
CR-Capitol Reef
BW-Buckhorn Was
RC-Comb Ridge HD-Hart Draw MR-Mescal Range
CH-Cowhole Mtns DB-Dewey Bridge ER-Escalante Rivf
NW-North Wash
OD-Old Dad Mtn
PR-paria River
SM-Soda Mountah
ZC-Zion Canyon
VF-Valley o f Fire WC-Wilson C l i f f s
I
F i g u r e 4. Names and l o c a t i o n s o f c r o s s b e d - o r i e n t a t i o n measurements and measured s e c t i o n s r e f e r r e d t o i n t e x t and i n F i g u r e s 5, 6, 7 , 8, 9, 11 and 12. S e c t i o n s w e r e l o c a t e d where t o p o g r a p h y was s u f f i c i e n t l y s t e e p t o p e r m i t s t r a t i g r a p h i c l o c a t i o n w i t h i n sections b u t n o t too steep t o preclude traversi n g them,
These r e q u i r e m e n t s g r e a t l y l i m i t e d t h e number o f s u i t a b l e l o c a t i o n s
and, as i n t h e c a s e o f t h e V a l l e y o f F i r e , c o u l d n o t a l w a y s b e met. Wilson C l i f f s ,
I n the
s t e e p t o p o g r a p h y made measurement d i f f i c u l t between 900 and
1 2 0 0 m e t e r s above t h e b a s e o f t h e s e c t i o n .
S i g n i f i c a n t e r r o r i n measured
thickness i s l i k e l y within t h i s interval. N o n - c r o s s b e d d e d f e a t u r e s a r e d i s t r i b u t e d t h r o u g h o u t f i v e o f t h e s i x sect i o n s measured i n t h e N a v a j o S a n d s t o n e i n s o u t h e a s t e r n Utah.
Non-crossbedded
l i t h o l o g i e s a v e r a g e 32 p e r c e n t o f t h e t o t a l t h i c k n e s s o f t h e measured s e c t i o n s
w i t h a r a n g e o f 1 5 t o 50 p e r c e n t .
H o r i z o n t a l l y s t r a t i f i e d sandstone i s most
common n e a r t h e b a s e o f t h e N a v a j o Sandstone; whereas, contorted stratification,
limestone lenses,
s t r u c t u r e l e s s s a n d s t o n e and d e s s i c c a t i o n - c r a c k e d
e r o s i o n s u r f a c e s a r e m o r e common, t h o u g h n o t r e s t r i c t e d t o , t h e m i d d l e and
641
VERTICAL DISTRIBUTION OF PRIMARY FEATURES SOUTHEASTERN UTAH
EXPLANATION TABULAR PLANAR CROSS STRATA
T R OU GH C R OSS- ST R A T A HORIZONTALLY SANDSTONE
CAPITOL REEF Page
Ss
STRATIFIED
CARBONATE LENS S T R U C T U R E L E S S SA N D ST ON E RECUMBENT CROSS-STRATA UNDULATORY CROSS-STRATA CONTORTED ST R A T A
1 1
NORTH WASH
loom
BUCKHO RN W A S H COMB RIDGE HART DRAW
Om
F i g u r e 5. V e r t i c a l d i s t r i b u t i o n o f sedimentary s t r u c t u r e s i n t h e Navajo Sandstone, s o u t h e a s t e r n Utah. Crossbed o r i e n t a t i o n i n 15 and 30 m e t r e increments i n d i c a t e d by a r r o w s . N o r t h i s towards t h e t o p o f t h e f i g u r e .
64 2
VERTICAL DISTRIBUTION OF PRIMARY FEATURES SOUTHWESTERN UTAH AND SOUTHERN NEVADA
WILSON C L I F F S
VALLEY OF FIRE
I
/
c
J
J
Z I O N CANYON
J J 100m
J
J
J
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J
EXPLANATION T A B W PLANAR CROSSSTRATA
-El 1 (
WEDGE PLANAR CROSS-STRATA TROUGH CROSS-STRATA HORIZONTALLY STRATIFIED SANDSTONE
J
1
CARBONATE LENS STRUCTURELESS SANDSTONE
”
RECUMBENT CROSSSTRATA
Q%l 1 1 1
UNDULATORY CROSS-STRATA CONTORTED STRATA GYPSIFEROUS SILTSTONE
WAVY BEDDED SANDSTONE
Figure 6. Vertical d i s t r i b u t i o n o f sedimentary s t r u c t u r e s In t h e kavajo d ~ i ; Aztec Sandstones, southwestern Utah and southern Nevada. Crossbed o r i e n t a t i o n in 30 m increments i n d i cat ed by arrows. North i s t o top o f t h e f i g u r e .
64 3
VERTICAL SECTIONS OF AZTEC SANDSTONE AND VOLCANICS MOJAVE DESERT, CALIFORNIA SODA MOUNTAINS
COWHOLE MOUNTAINS
OLD D A D MOUNTAIN
MESCAL RANGE
t t
f
t t
EXPLANATION
"\ CROSS-BEDDED SANDSTONE
\
FLAT BEDDED WXCANlCLAgTlC SILTSTPALEOZOIC CARBONATES LIMESTONE BRECCIA ANDESITIC TO DAClTlC FLOWS DAClTlC TO RHYODACITIC FLOWS
1':
FELSlTlC DIKES AND SILLS
1-1 MAFIC HYPABYSSAL ROCKS m qCONGLOMERATE FLAT BEDDED VOLCANICLASTTIC SANDSTONE AND CROSS-BEDDED SANDSTONE
meters
4 FAULT Y
Figure 7. Vertical sectioris of Aztec Sandstone and interbedded volcanics, southeastern California. Crossbed o r i e n t a t i o n in 30 m increments indicated by arrows. North i s t o top of t h e f i g u r e .
644
upper p a r t of t h e sandstone.
W i t h t h e exception of the south-westernmost p a r t
of t h e r e g i o n , represented by Capitol Reef, non-crossbedded l i t h o l o g i e s a r e d i s t r i b u t e d t h r o u g h o u t the e n t i r e Navajo Sandstone.
A t Capitol Reef, non-
crossbedded f e a t u res a r e absent between approximately 342 m above the base of t h e sandstone and t h e unconformity, 439 m above t h e base, which se pa ra te s the Navajo Sandstone from t h e overlying Page Sandstone. The s t r i k i n g contorted s t r a t i f i c a t i o n which i s c l e a r l y v i s i b l e near t he c onta c t with t h e Carnie1 Formation i s a c t u a l l y i n t h e Page Sandstone r a t h e r than the Navajo. From southwestern Utah t o southern Nevada, non-crossbedded l i t h o l o g i e s a r e r e s t r i c t e d t o t h e basal portion of t h e Navajo and Aztec Sandstones (Fig. 6 ) . Within t h i s region, as i n southeastern Utah, horiz onta lly s t r a t i f i e d sandstone i s most common a t the base of t h e Navajo a n d Aztec s e c t i o n s .
A t Zion Canyon,
a l l non-crossbedded l i t h o l o g i e s a r e represented in the lower 100 m.
I n the
Wilson C l i f f s , t h e only non-crossbedded l i t h o l o g i e s present a r e contorted s t r a t i f i c a t i o n and s t r u c t u r e l e s s sandstone. absent.
Limestone lenses a r e notably
The Valley of F i r e s e c t i o n comprises thre e p a r t i a l se c tions located
along approximately 1 kin of t h e Valley of F i r e escarpment.
The basal Aztec
Sandstone intertongues with h o r i zo n t al l y bedded s i l t y sandstones, s i l t s t o n e s , and gypsiferous s i l t s t o n e s .
The s i l t y sandstones and s i l t s t o n e s thicken west-
ward a t t h e expense of t h e Aztec tongues i n t h e two dimensional east-west exposures. The north-south r e l a t i o n s h i p s a r e n o t exposed. Approximately the basal 30 m of t h e main body of t h e Aztec Sandstone contains wavy, horiz onta lly bedded sandstone and contorted s t r a t i f i c a t i o n . About one kilometer e a s t of t h e measured p a r t i a l s e c t i o n s , a s i n g l e calcareous marlstone le ns crops o u t in th e lower p a r t of t h e sandstone.
Because i t i s separated from the measured
s e c t i o n s by a t l e a s t one normal f a u l t of unknown displacement, i t s exact posit i o n within t h e measured s ect i o n i s unknown. From s o u t h e a st e r n Utah t o southern Nevada, the b o u n d a r y between t h a t p a r t of t h e Navajo and Aztec Sandstones containing non-crossbedded l i t h o l o g i e s and t h a t which i s e x c l u s i v el y crossbedded i s not a d i s t i n c t l i n e b u t r a t h e r a zone of t r a n s i t i o n . Crossbed o r i e n t a t i o n Crossbed o r i e n t a t i o n has been measured i n v e r t i c a l increments of 15 t o 30 m i n nine v e r t i c a l s e c t i o n s of t h e Navajo Sandstone from southeastern Utah t o s o u th e a st e r n C a l i f o r n i a.
The crossbed-dip azmith and dip angle were corrected
f o r regional d i p and s t r i k e where necessary and then were p l o t t e d on a ste re ographic Wolfe net. Eighteen hundred and f i f t y measurements p l o t t e d on 105 s t e r e o g r a p h i c p r o j ect i o n s were summarized by visual inspection.
A t Dewey Bridge, Hart Draw, North Wash, Buckhorn Wash, Zion Canyon, the
64 5 W i l s o n C l i f f s , Mescal Range and t h e Cowhole Modntains, t h e e n t i r e t h i c k n e s s o f t h e Navajo o r A z t e c Sandstone was d i v i d e d i n t o v e r t i c a l increments.
Because
o f d i f f i c u l t y o f access i n t h e W i l s o n C l i f f s , t h e t h i c k n e s s o f t h e v e r t i c a l increments between 900 and 1200 m above t h e base o f t h e sandstone may be i n e r r o r by as much as 25 p e r c e n t t o o t h i c k .
A t Z i o n Canyon and t h e V a l l e y of
F i r e , crossbed o r i e n t a t i o n measurements were made i n t h e basal Lamb P o i n t Tongue o f t h e Navajo Sandstone and unnamed basal tongues o f t h e Aztec Sandstone, r e s p e c t i v e l y ,
A t t h e V a l l e y o f F i r e , a G e r t i c a l gap o f u n c e r t a i n
t h i c k n e s s b u t n o t exceeding 100 m e x i s t s 50 m above t h e base o f t h e main body o f t h e Aztec Sandstone.
Because o f s t r u c t u r e and topography, s t r a t i g r a p h i c
p o s i t i o n c o u l d n o t be m a i n t a i n e d w i t h s u f f i c i e n t accuracy t o w a r r a n t crossbed o r i e n t a t i o n measurements i n an unknown t h i c k n e s s from t h e t o p o f t h e s e c t i o n measured t o t h e t o p o f t h e Aztec Sandstone a t t h i s l o c a l i t y .
I n addition t o
t h e l o c a l i t i e s l i s t e d above, p r e l i m i n a r y o r i e n t a t i o n measurements have been made i n v e r t i c a l i n c r e m e n t s f r o m t h e t o p and b o t t o m o f s e c t i o n s a t t h e P a r i a R i v e r and E s c a l a n t e R i v e r i n Utah. The v a r i a t i o n i n v e c t o r - r e s u l t a n t crossbed o r i e n t a t i o n f o r i n d i v i d u a l i n crements a t each o f t h e n i n e l o c a l i t i e s i s i l l u s t r a t e d i n F i g u r e s 5, 6 and 7. W o l f e - n e t s t e r e o g r a p h i c p l o t s o f grouped d a t a and t h e i r v e c t o r r e s u l t a n t s a r e shown i n F i g u r e s 8 and 9.
I n s o u t h e a s t e r n Utah, t h e v e c t o r r e s u l t a n t s of
crossbed a z m i t h f o r a l l t h e v e r t i c a l increments a t each o f f o u r l o c a l i t i e s d i p t o t h e southeast.
The range o f azmiths i s f r o m 75 t o 171 degrees.
Vector
r e s u l t a n t s f o r t h e e n t i r e t h i c k n e s s o f each o f t h e f o u r l o c a l i t i e s a r e shown i n F i g u r e 8.
The range o f t h e azmiths i s f r o m 111 t o 139 degrees.
From Z i o n Canyon i n southwestern Utah t o t h e Wilson C l i f f s i n s o u t h e r n Nevada, crossbed d i p a z m i t h r o t a t e s c l o c k w i s e , southeastward t o southwestward, f r o m basal tongues o r base t o t h e t o p o f t h e Navajo and Aztec Sandstones. The c l o c k w i s e r o t a t i o n o f grouped v e c t o r r e s u l t a n t s f r o m bottom t o t o p o f t h e s e c t i o n s ranges f r o m 58 t o 80 degrees.
A t Z i o n Canyon, t h e grouped-increment
v e c t o r r e s u l t a n t f o r t h e base o f t h e Navajo Sandstone i s 157 degrees and t h a t f o r t h e t o p i s 237 degrees, a c l o c k w i s e r o t a t i o n o f 80 degrees.
The basal
Lamb P o i n t Tongue o f t h e Navajo Sandstone y i e l d s a r e s u l t a n t o f 145 degrees.
A t t h e V a l l e y o f F i r e , t h e v e c t o r r e s u l t a n t a z m i t h f o r t h e h i g h e s t basal tongue i s 163 degrees.
V e c t o r r e s u l t a n t s o f a l l v e r t i c a l increments w i t h i n
t h e main body o f t h e Aztec Sandstone have azmiths i n t h e southwest quadrant. The r e s u l t a n t f o r t h e e n t i r e t h i c k n e s s has an a z m i t h o f 221 degrees.
The
c l o c k w i s e r o t a t i o n o f v e c t o r r e s u l t a n t a z m i t h f r o m t h e basal tongue t o t h e main body o f t h e Aztec Sandstone i s 58 degrees.
I n t h e Wilson C l i f f s , t h e
v e c t o r r e s u l t a n t a z m i t h f o r t h e base o f t h e Aztec Sandstone i s 166 degrees and f o r t h e t o p i s 237 degrees, a c l o c k w i s e r o t a t i o n o f 71 degrees.
646
(-J(+JO
N O R T H WASH
H A R T DRAW
D E W E Y BRIDGE
lO6m
Om
O m
ESCALANTE RIVER
Top-86
O m
BUCKHORN WASH
om
8Om
Om
Figure 8. Wolfe-net stereographic projections of crossbed-dip azmith and magnitude f o r e n t i r e thickness of the Navajo Sandstone a t Dewey Bridge, Hart Draw, North Wash and Buckhorn Wash and f o r t h e bottom 60 m and top 85 m a t Escalante River. North i s a t the top o f the projections. The two l o c a l i t i e s i n the Mojave Desert show apparently i n c o n s i s t e n t res u l t s . Vertical increments from the anomalously t h i n section i n t h e Mescal Range y i e l d vector r e s u l t a n t s which dip c o n s i s t e n t l y southeastward and can adequately be represented by a s i n g l e vector r e s u l t a n t azrnith of 126 degrees. I n t h e Cowhole Mountains, grouped increment vector r e s u l t a n t s f o r t h e lower and upper p a r t o f t h e Aztec Sandstone show clockwise r o t a t i o n o f 60 degrees b u t the azrnith of the lower 120 m of t h e sandstone i s 312 degrees and t h e
overlying 395 m i s 1 2 degrees.
647 WILSON CLIFFS
V A L L E Y OF FIRE
ZION CANYON
PARlA RIVER
826m
Too
00 0 :r) 0 Q n=77
n=168
Om
640m
n=OO
18Om
6Om
n=17
Basal Tongue
n=114
46m
Om
n=30
Om
Om
COWHOLE
n=l@
a (3 M 0 U N T A IN S
TopilOOm
240m
MESCAL RANGE
Lamb Polnt TonOue
1371-11
n=74
n=108
0 120m
o m
n=24
Om
Figure 9. llolfe-net stereographic projections of dip azmith and magnitude f o r grouped increnients i n t h e Havajo and Aztec Sandstones, southwestern U t a h , southern Nevada, and southeastern California. North i s a t t o p of projection.
648
Western f a c i e s I n southern Nevada, redbeds conformably underlying the Aztec Sandstone o r i g i n a l l y were assigned t o t h e Chinle Formation (Longwell, 1928; Hewett, 1931).
More r e c e n t l y Wilson and Stewart (1967) have separated a Moenave-
Kayenta equivalent from t h e Chinle Formation, as defined by Longwell, as f a r west as t h e Wilson C l i f f s .
I t i s uncertain whether any p a r t of the redbeds
underlying t h e Aztec Sandstone i n t h e eas t er n Mescal Range and assigned t o the Chinle Formation by Hewett (1956) belong e i t h e r t o t h e Chinle Formation o r Moenave-Kayenta equivalents.
Redbeds conformably underlying the Aztec Sand-
stone c o n s i s t of dark reddish-brown, co ar s e, q u a rtz s i l t s t o n e containing numerous lenses and pockets of volcanic rock fragments and a r e unlike e i t h e r t h e Chinle Formation o r Moenave-Kayenta eq u i v al e nts.
Harzolf (1980) c orre -
l a t e d these s t r a t a with s i m i l a r redbeds in t h e lower p a r t of the Aztec Sands t o n e i n t h e Cowhole Mountains.
I n t h e southeastern corner of t h e Mescal Range reddish-brown s i l t s t o n e s and mudstones with one o r two i n t er b ed s of v o l can i cs, or possibly hypabyssal rocks, s t r u c t u r a l l y u n d e r l i e t h e Aztec Sandstone a n d a r e in s t r u c t u r a l c onta c t above th e Moenkopi Formation. These s t r a t a may be t h e westernmost expmures of Moenave-Kayenta e q ui v al en t s in t h i s p a r t of t h e Mojave Desert ( i i a r z o l f , in press). Further t o t h e west a t Old Dad Mountain, and in t h e Cowhole and
h i d
Moun-
t a i n s , t h e Aztec Sandstone i s interbedded with intermediate t o s i l i c i c volcani c s (Marzolf, 1981, 1982b).
A t Old Dad Mountain and in t h e Cowhole Mountains,
t h e Aztec Sandstone r e s t s unconformably on deformed Paleozoic carbonates o r T r i a s s i c volcanics.
The basal p a r t of t h e s ect ion contains one or more hori-
zons of limestone breccia co n s i s t i n g of angular blocks of Birdspring limestone in a sand matrix.
A t t h e northern end of t h e Cowhole Mountains, the limestone
breccia i s o v e r l a i n by redbeds of q u a r t z s i l t s t o n e containing coarse sand-size volcanic c l a s t s .
Further t o t h e south t h e redbeds r e s t on crossbedded qua rtz
sandstone which, i n turn, o v e r l i e s and contains lenses of limestone bre c c ia , The basal breccia r e s t s on volcanics which have been c u t by numerous dikes. Throughout t h e Cowhole Mountains, t h e redbeds a r e ove rla in by la rge -sc a le The
crossbedded sandstone interbedded with and o v er la in by volcanic flows.
lower flows a r e a n d e s i t i c t o d a c i t i c and t h e upper and overlying flows a r e dacitic to latitic. I n t h e Soda Mountains, Grose (1959) described over 7,000 f t (2,120 m ) of interbedded, crossbedded q u ar t z sandstone and a n d e s i t i c t o d a c i t i c volcanics believed t o be c o r r e l a t i v e with t h e Aztec Sandstone.
I n t h e southeastern
corner of t h e e a st e r n Soda Mountains, t h e Aztec Sandstone c o n s i s t s of l i g h t yellowish-brown crossbedded, q u ar t z sandstone and flat-bedded, grayish-purple
64 9
q u a r t z sandstone containing p o r p h y r i t i c volcanic c l a s t s from coarse-sand t o cobble s i z e interbedded with p o r p h y r i t i c d a c i t i c flows (Marzolf, 1981).
The
sequence i s overlain,conformably by p o r p h y r i t i c qua rtz l a t i t e with q u a r t z and
A t t h e base of t h e interbedded sandstone and volcanics, a v o l c a n i c l a s t i c sedimentary breccia o v e r l i e s a mu1 t i ply fa ulte d sequence of a n d e s i t i c t o d a c i t i c flows, flow b r ecci as , a n d hypabyssal rocks o f uncertain thickness. Near t h e bottom of t h i s sequence, s l i v e r s of metamorphosed limestone and c a l c - c i l i c a t e s a r e enclosed i n t h e hypabyssal rocks. A t the base of th e sequence, volcanic and hypabyssal rocks a r e i n f a u l t and i n t r u s i v e contact with steeply-dipping Paleozoic carbonates and c a l c - s i l i c a t e s . I n t h e western Soda Mountains, t h e interbedded crossbedded, quartz sandstone and volcanics o v e r l i e shallow-water marine limestones and s i l t s t o n e s assigned by Grose (1959) t o t h e Early T r i a s s i c Moenkopi Formation. f e l d s p a r phenocrysts.
INTERPRETATION O F EASTERN AND WESTERN FACIES Origin of non-crossbedded l i t h o l o g i e s Of the f o u r types of non-crossbedded l i t h o l o g i e s , limestone lenses a r e most c l e a r l y t h e r e s u l t of aqueous deposition.
The presence of dinosaur tra c ks on
limestone bedding planes and t h e l o cat i o n of limestone lenses in shallow depressions on desiccation-cracked s u r f aces i n d i c a t e t h e bodies of water in which limestone weredeposited were shallow. The f o s s i l fauna and f l o r a contained i n some limestode l en s es , as well as boron and vanadium concentrations ( G i l l a n d , 1979), i n d i c a t e f r es h water d ep o s i t i on. Horizontal erosion s u r f aces and h o r i zo n t al l y s t r a t i f i e d sandstone a r e n o t o f themselves i n d i c a t i v e of an aqueous environment, b u t may be the r e s u l t of
normal e o l i a n deposition (Brookfield, 1977)
Erosion surfa c e s and horizon-
t a l l y s t r a t i f i e d sandstone having s u r f aces marked by de sic c a tion cracks were undoubtedly deposited i n o r modified by an aqueous environment. The anomalo u sl y high content of sediment having a g r ai n s i z e l e s s than 43 wn represents windblown d u s t trapped by a wet s u r f ace and adhesion r i p p l e s r e s u l t from sand g r a i n s adhering t o t h e wet s u r f ace (Kocurek a n d F i e l d e r , 1982).
The absence
of s m a l l - s c a l e c r o s s - s t r a t i f i c a t i o n or r i p p l e marks within horiz onta lly s t r a t i f i e d sandstone suggest t h e sand was not deposited by flowing water a n d t h e absence of o s c i l l a t o r y r i p p l e s on t h e i r s urfa c e s suggest the sand was s a t u r a t e d without a standing body of water upon them.
These observations
imply t h a t h o r i z o nt al l y s t r a t i f i e d sandstone i s a rnetadepositional f e a t u r e , i.e.,
i t r e s u l t s from modification of previously deposited sand by the upward
and perhaps l a t e r a l slow p er co l at i o n of water through t h e sand.
The abundance
of h o r i z o n t a l l y s t r a t i f i e d sandstone near t h e base of t h e Navajo and Aztec
650 Sandstones a l t e r n a t i n g w i t h l a r g e - s c a l e c r o s s - s t r a t i f i c a t i o n suggests i t s f o r m a t i o n r e p r e s e n t s a normal s t a g e i n t h e t r a n s i t i o n f r o m f l u v i a l t o e o l i a n deposition. The o r i g i n o f c o n t o r t e d s t r a t i f i c a t i o n and s t r u c t u r e l e s s sandstone i s controversial.
F l a r z o l f (1978) a s c r i b e d c o n t o r t e d s t r a t i f i c a t i o n and s t r u c t u r e -
l e s s sandstone, r e s p e c t i v e l y , t o 1 i q u e f a c t i o n and f l u i d i z a t i o n r e s u l t i n g from o v e r - p r e s s u r e d groundwater i n e o l i a n sand.
Over-pressuring i s believed t o
have r e s u l t e d f r o m f l u v i a l r e c h a r g e o f t h e e o l i a n sand.
H o r o w i t z (1982)
a t t r i b u t e d c o n t o r t e d s t r a t i f i c a t i o n i n t h e Aztec Sandstone t o c o l l a p s e o f dunes o v e r g r o u n d w a t e r - s a t u r a t e d sand l i q u e f i e d by earthquakes.
Doe and D o t t
(1930) a t t r i b u t e d c o n t o r t e d s t r a t i f i c a t i o n i n t h e Weber and Navajo Sandstones t o l i q u e f a c t i o n below t h e w a t e r t a b l e and suggested s e v e r a l mechanisms which might t r i g g e r t h e deformation.
A l t h o u g h Doe and D o t t suggest c o n t o r t e d
s t r a t i f i c a t i o n and s t r u c t u r e l e s s sandstone nay be produced i n d r y sand, l i k e F l a r z o l f and H o r o w i t z , t h e y f a v o r t h e i n t e r p r e t a t i o n o f d e f o r m a t i o n i n w a t e r s a t u r a t e d sand. These f o u r s e d i m e n t o l o g i s t s a ( r w thac c o n t o r t e d s t r a t i f i c a t i o n and s t r u c t u r e l e s s sandstone most l i k e l y o r i o i n a t e d below t h e w a t e r t a b l e ,
Where t h e y
d i s a g r e e concerns t h e n a t u r e o f t h e mechanism r e s p o n s i b l e f o r t r i g g e r i n g deformation. S i g n i f i c a n c e o f crossbed o r i e n t a t i o n The v e c t o r - r e s u l t a n t c r o s s b e d - d i p a z m i t h c a l c u l a t e d f r o m a s u f f i c i e n t l y l a r g e sample o f crossbed o r i e n t a t i o n measurements i s assumed t o i n d i c a t e paleowind d i r e c t i o n .
Thus, t h e crossbed o r i e n t a t i o n d a t a f r o m s o u t h - c e n t r a l
and southwestern Utah and s o u t h e r n Nevada i n d i c a t e wind d i r e c t i o n changed f r o m n o r t h w e s t e r l y t o n o r t h e a s t e r l y d u r i n g d e p o s i t i o n o f t h e Navajo and Aztec Sandstones i n t h i s r e g i o n . An a l t e r n a t i v e e x p l a n a t i o n i s t h a t crossbed o r i e n t a t i o n changed as a r e s u l t o f change i n dune s i z e o r morphology o r t h e s u p e r p o s i t i o n o f s m a l l e r upon l a r g e r dunes ( H u n t e r and Rubin, t h i s volume).
A comparison o f s t y l e and s c a l e
o f crossbedding between Z i o n Canyon and t h e W i l s o n C l i f f s f a v o r s t h e former hypothesis.
A t Z i o n Canyon, t h e a m p l i t u d e o f crossbed s e t s i n c r e a s e s n o t a b l y
and wedge-planar c r o s s - s t r a t i f i c a t i o n g i v e s way t o t a b u l a r - p l a n a r c r o s s s t r a t i f i c a t i o n f r o m t h e bottom t o t h e t o p o f t h e s e c t i o n .
I n t h e Wilson
C l i f f s , t h e geometry and s c a l e o f s e t s o f c r o s s - s t r a t i f i c a t i o n appear t o change l i t t l e t h r o u g h t h e s e c t i o n , y e t t h e c l o c k w i s e r o t a t i o n o f crossbed o r i e n t a t i o n from bottom t o top o f t h e s e c t i o n i s almost i d e n t i c a l t o t h a t a t Z i o n Canyon.
Change i n v e c t o r - r e s u l t a n t crossbed o r i e n t a t i o n appears t o be
independent o f s t y l e o r s c a l e o f crossbedding.
651
I n t h e Mojave Desert s e c t i o n , crossbed o r i e n t a t i o n in t h e Cowhole Mountains a l s o shows a clockwise r o t a t i o n from the base t o the t o p of the s e c t i o n , b u t the lower p a r t of t h e section y i e l d s a v e c t o r - r e s u l t a n t dip d i r e c t i o n t o the northwest; t h e upper p a r t , t o t h e northeast. Apparently, t h i s i s the r e s u l t of post-Aztec t e c t o n i c r o t a t i o n of t h e Cowhole Mountain block r a t h e r than the r e s u l t of a dramatic change in wind d i r e c t i o n (Marzolf, 1982a). The c o n s i s t e n t southeastward dip of crossbeds in the anomalously-thin section i n t h e Hescal Range i s believed t o be the r e s u l t of post-Aztec - pre-Delfont erosion of t h a t p a r t of the Aztec Sandstone which presumably was deposited by northeasterly winds (Marzolf, 1980, 1982a). Within U t a h , a comparison of crossbed o r i e n t a t i o n in t h a t p a r t of the Navajo Sandstone containing f e a t u r e s of groundwater o r i g i n with t h a t p a r t which i s exclusively crossbedded i n d i c a t e s t h e former was deposited by northwesterly winds and t h e l a t t e r by northeasterly winds. The v e r t i c a l change in crossbed o r i e n t a t i o n appears t o be gradual and continuous and so, t o a l e s s e r e x t e n t , i s t h e change from crossbedded sandstone interbedded with f e a t u r e s of groundwater o r i g i n t o exclusively crossbedded sandstone. Whether these changes were causally r e l a t e d i s , a t present, uncertain. Significance of the western f a c i e s The r e l a t i o n s h i p of Aztec Sandstone t o interbedded volcanics and underlying rocks from the Mescal Range t o the Soda Mountains i n d i c a t e Aztec sands were being blown i n t o a developing volcanic t e r r a i n . Relationships in the Cowhole Mountains i n d i c a t e a s u r f a c e of r e l i e f of a l e a s t 226 m had been developed on t h r u s t - f a u l t e d Paleozoic carbonates p r i o r t o deposition of the Aztec sandstone. Topographic lows were f i l l e d with volcanic flows upon which the windblown sand was deposited. Topographic highs protruded above t h e volcanics t o be buried d i r e c t l y by the Aztec Sandstone. The d a r k reddish-brown s i l t s t o n e appears t o be of f l u v i a l o r i g i n . The volcanic c l a s t s apparently were washed from the flanks of t h e developing volcanoes, perhaps as the r e s u l t of orographic p r e c i p i t a t i o n . Nowhere in t h e eastern Soda Mountains i s Aztec Sandstone in contact with Paleozoic o r Early T r i a s s i c sedimentary rocks. The dune sand accumulated near a volcanic c e n t e r where hypabyssal rocks had invaded Paleozoic s t r a t a . Volcanics completely buried the Paleozoic t e r r a i n p r i o r t o deposition of eolian sand.
As i n t h e Cowhole Mountains, Moenkopi s t r a t a e i t h e r had never
been deposited o r had been eroded. A t l e a s t three times during deposition of Aztec sand, eolian deposition was interrupted by volcanic eruptions. Accumul a t e d sand was reworked by local sheet flooding which introduced locallyderived volcanic c l a s t s i n t o the sand and s i l t . I n places, volcanic c l a s t s
652
were transported by t h e wind and accumulated i n crossbedded dune sand. Age of t h e Navajo and Aztec Sandstones Maximum and minimum ages of t h e Navajo and Aztec Sandstones a r e based on varied a n d meager d at a from widely separated l o c a l i t i e s .
I n southeastern
Utah, t h e evidence of t e r r e s t r i a l v e r t e b r a t e f o s s i l s in both t h e Kayenta Formation and t h e lower p a r t of t h e Navajo Sandstone i s c ontrove rsia l (Lewis, e t a l . , 1963, Galton, 1971, Olsen and Galton, 1977).
Late T r i a s s i c and e a r l y I n southwestern Utah, palynomorphs in t h e middle Whitmore Point Member of t h e Moenave Formation i n d i c a t e an Early J u r a s s i c age - probably Sinemurian t o Pliensbachian (Cornet & Peterson and Pipiringos, 1979). An upper age l i m i t of l a t e middle Bajocian f o r t h e Navajo Sandstone in southern Utah i s based on marine i n v e r t e b r a t e s in the limestone member of t h e Carmel Formation. Peterson and Pipiringos (1979) have shown t h a t t h e crossbedded sandstone with which t h e Carmel Formation i n t e r f i n g e r s a t i t s base i s separated from the Navajo Sandstone by a regional unconformity. They have named t h i s sandstone th e Page Sandstone. Westward, t h e regional unconformity a t the base of t h e Page Sandstone t r u ncat es another unconformity se pa ra ting t h e Temple Cap Formation (formerly t h e Temple Cap Member of t h e Navajo Sandstone) from the Navajo Sandstone and, i t s e l f , s ep ar at es t h e Carmel Formation from the Temple Cap Formation. Eastward, t h e same unconformity trunc a te s progressively o l d e r strata. The l i t h o l o g i c c o r r e l a t i o n of t h e Temple Cap Formation with t h e Gypsum Spring Formation (Peterson and P i p i r i n g o s , 1979) does not n e c e s s i t a t e the two formations be time equivalent. Ad d i t i o n al l y , t h e unconformities a t t h e t o p of th e Navajo Sandstone may well be time t r a n s g r e s s i v e . F i n a l l y , the presence of th e two unconformities between t h e top of t h e Navajo Sandstone a n d the Carmel Formation need not be i n t e r p r e t e d t o mean t h a t t h e Navajo Sandstone i s o l d e r than t h e late-middle Bajocian age assigned t o t h e o l d e s t f o s s i l i f e r o u s p a r t of t h e limestone member of t h e Carmel Formation. All t h a t can be s a i d with c e r t a i n t y i s t h a t t h e Navajo Sandstone i s o l d er than late-middle Bajocian in south-central Utah. I n t h e same f as h i o n , t h e probable Sinemurian t o e a r l y Pliensbachian age of t h e CJhitmore Point llember of t h e Hoenave Formation in southwestern Utah and assignment of a Late T r i a s s i c o r Early J u r a s s i c ace in southeastern Utah only i n d i c a t e t h e overlying s t r a t a i n the se two regions a r e younger than e a r l y Pliensbachian and Late T r i a s s i c t o Early J u r a s s i c r e s p e c t i v e l y . Thus, t h es e l o c a l i t i e s serve a s points in space and time between and beyond which t h e Navajo and Aztec Sandstones can be placed with some l a t i t u d e . These age r e l a t i o n s h i p s a r e i l l u s t r a t e d i n Figure 10, J u r a s s i c ages a r e tenable.
653
stage
Southweatern Utah a n d
N o r t h e a e t e r n Arlrona a n d
northweetern Arlrone
northweatern New M e x l c o
SAN RAFAEL GROUP Balocian MI-MARINE INVERTEBRATE8 TV-TERRESTRIAL VERTEBRATE8
Toarclan
Pllenabachlan
Hettangian
CHINLE Fm
Figure 10.
Age r e l a t i o n s h i p s of lower Nesozoic s t r a t a across southern Utah
a n d northern Arizona (Modified a f t e r Peterson and Pipiringos, 1979).
Gradational boundary s ep ar at es crossbedded sandstone containing noncrossbedded l i t h o l o g i e s below from ex cl u s i v el y crossbedded l i t h o l o g i e s above. I n western f a c i e s , attempts t o determine t he age of t h e Aztec Sandstone
have been based on radiometric ages of interbedded or overlying volcanics,
A
minimum K-Ar d a t e of 155 Ma was obtained by S u t t e r (Novitshy-Evans, 1978) f o r t h e Delfonte Volcanics overlying t h e Aztec Sandstone in the Mescal Range.
In
southern Arizona, t h e o l d e s t volcanics as s o ci a te d with possible Aztec Sandstone have been dated (Wright and o t h e r s , 1981) a t 190 Ma.
J u s t across
the border i n northern Sonora, Mexico, S i l v e r and Anderson dated sim ila r volcanics a t 180- 185 Ma (Anderson, pers. cornm., 1981).
These a r e Early t o
Middle J u r a s s i c ages.
I n a d d i t i o n t o t h e u n c e r t a i n i t y over t h e age of the boundaries of the Navajo and Aztec Sandstone, t h e lack of intraforrnational s t r a t i g r a p h i c control makes uncertain whether v a r i a t i o n i n wind d i r e c t i o n and depth of water t a b l e were e x c l u si v e l y temporal o r temporal and s p a t i a l ,
654
PALEOGEOGRAPHIC INTERPRETATION Several l i n e s of evidence support t h e contention t h a t the g r e a t e r p a r t of t h e Navajo and Aztec Sandstones i n southwestern Utah and southern Nevada i s younger than t h e Navajo Sandstone in southeastern Utah.
Both t h e unconformity
a t t h e top of t h e Navajo Sandstone which c u t s olde r s t r a t a from south-central Utah t o western Colorado (Peterson and P i p i r i n gos, 1979) and t h e southwestward intertonguing of t h e Navajo Sandstone with redbeds of t h e Kayenta Formation i n southwestern Utah suggest t h e Navajo Sandstone in southwestern Utah and the Aztec Sandstone i n southern Nevada a r e younger t h a n t h e Navajo Sandstone in southeastern Utah.
I f i t i s assumed t h a t change in crossbed o r i e n t a t i o n i s
temporal and n o t a t a l l s p a t i a l , then t h e Lamb Point Tongue and lowest 50 m of t h e main body of t h e Navajo Sandstone a r e coeval with the e n t i r e Navajo Sandstone e a s t of t h e Paria River i n south-central Utah.
The t o t a l thickness
of s t r a t a from t h e base of t h e Lamb Point Tongue t o 50 m above t h e base of the Navajo Sandstone ( A v e r i t t and o t h e r s , 1955) i s approximately equal t o t h e e n t i r e thickness of t h e Navajo Sandstone along the Paria River. A l t h o u g h r a t e s of sedimentation f o r t h e eo l i an a n d f l u v i a t i l e sediments were probably
n o t equal, i t i s d i f f i c u l t t o believe t h i s r e s u l t i s mere coincidence. I n addition t o t h e southeastward dip of c r o s s - s t r a t a t h r o u g h the e n t i r e Navajo s e c t i o n e a s t of t h e Paria River, sedimentary f e a t u r e s of groundwater o r i g i n a l s o a r e d i s t r i b u t e d throughout t h e e n t i r e s e c t i o n ; whereas; t o the west, th e se same f e a t u r e s a r e found lower and lower in t h e s e c t i o n .
The
d i s t r i b u t i o n of f e a t u r e s of groundwater o r i g i n was c ontrolle d by regional base l e v e l . I f t h i s i n t e r p r e t a t i o n i s c o r r e c t , i t follows t h a t t h e lowering of regional base level took place simultaneously and in a ge ologic a lly s h o r t period of time from southeastern Utah t o southern Nevada.
Thus, the boundary between
crossbedded sandstone and non-crossbedded l i t h o l o g i e s below a n d e xc lusive ly crossbedded sandstone above i s a time l i n e ( F ig. 1 0 ) . Two paleogeographic maps permitted by t h i s i n t e r p r e t a t i o n o f age r e l a t i o n s h i p s a r e i l l u s t r a t e d in Figures 11 and 1 2 . Although changes i n wind d i r e c t i o n and t h e depth of regional base level might have r e s u l t e d from changes i n global atmospheric c i r c u l a t i o n o r changes i n continental p o s i t i o n o r o r i e n t a t i o n , regional changes in paleogeography appear t o o f f e r an equally p l a u s i b l e explanation. The e a r l y sta ge re pre se nts a time i n t e r v a l within t h e l a t e Rhaetian t o Sinemurain and t h e l a t e sta ge an in te r v a l within t h e Sineniurian t o e a r l y Bajocian. During t h e e a r l y s t a g e o f deposition ( F i g . l l ) , streams which deposited sediments of t h e tloenave and Kayenta Formations flowed west, northwest, and
655
Figure 1 1 . Paleogeographic map i l l u s t r a t i n g r e i a t i o n s h i p s of e o l i a n , f l u v i a l , and eolian and volcanic f a c i e s during Rhaetian t o Sinenurian ticie. L i t t o r a l zone has n o t been preserved. Marine f a c i e s a f t e r Speed (197G). Arrows i n d i c a t e wind d i r e c t i o n s .
n o r t h t o t h e marine basin in northwestern Nevada. I n the l i t t o r a l zone, f i n e s were winnowed from tile f l u v i a l sediment and c a r r i e d offshore t o the west and northwest. These f i n e c l a s t i c s a r e probably represented by t h e Late T r i a s s i c and Early J u r a s s i c c l a s t i c s of shelf and basinal t e r r a i n s i n western Nevada (Speed, 1973). The sand was transported southeastward u p t h e paleoslopc by northwest winds. During t h i s stage of deposition, t h e shallow groundwater
656
F i g u r e 12. Paleogeographic map i l l u s t r a t i n g r e l a t i o n s h i p s o f e o l i a n e r o s i o n and d e p o s i t i o n t o r e g i o n s o f e o l i a n d e p o s i t i o n and volcanism d u r i n g Sinemurian time. R e l a t i o n s h i p of r e g i o n o f " v o l c a n i c s on1y"to marine f a c i e s i s u n c e r t a i n . i l a r i n e f a c i e s a f t e r Speed ( 1 9 7 8 ) . Arrows i n d i c a t e wind d i r e c t i o n . t a b l e was m a i n t a i n e d n e a r t h e s u r f a c e by r e c h a r g e from t h e f l u v i a l environment.
A1 though vol cani sm may have begun burying t h e eroded P a l e o z o i c t e r r a i n
a l o n g the western margin of the e o l i a n sand s e a s , t h e v o l c a n i c a r c had n o t become well enough developed t o block t h e s a n d - t r a n s p o r t i n g northwest winds nor had t h e a r c been extended s u f f i c i e n t l y f a r t o t h e s o u t h e a s t t o c u t o f f streams flowing t o t h e north.
657 D u r i n g t h e -lace s i a + oi' eo.lian d e p o s i L i o n (Fi.;.
I ? ) , which may have
extended f r o m Sinemurian t o B a j o c i a n o r p o s s i b l y as l a t e as C a l l o v i a n t i m e i n s o u t h e r n and western Nevada and s o u t h e a s t e r n C a l i f o r n i a , sand d e p o s i t e d d u r i n g t h e e a r l i e r s t a g e o f e o l i a n s e d i m e n t a t i o n was eroded f r o m n o r t h e r n and e a s t e r n Utah and t r a n s p o r t e d southwestward, up t h e p a l e o s l o p e , b u r y i n g t h e f l u v i a l environment i n s o u t h e r n Nevada and b u i l d i n ? t h e s u r f a c e o f accumulation above r e g i o n a l base l e v e l .
A t t h e same time, v o l c a n i s m a c c e l e r a t e d , e l e v a t i n g t h e
v o l c a n i c a r c and e x t e n d i n g i t f u r t h e r southeastward i n t o s o u t h e r n A r i z o n a and n o r t h w e s t e r n Sonora.
The e x t e n s i o n o f t h e v o l c a n i c a r c may have p l a y e d a r o l e
i n l o w e r i n g t h e groundwater t a b l e i n t h e e o l i a n b a s i n by c u t t i n y o f f f l u v i a l r e c h a r g e o f t h e e o l i a n sand sea.
Redbeds s t r u c t u r a l l y j u x t a p o s e d w i t h Aztec-
l i k e sandstone and v o l c a n i c s i n s o u t h e r n A r i z o n a a r e p o s s i b l y l a t e r a l l y e q u i v a l e n t ( B i l o d e a u , p e r s . con:m.,
l!KG!).
These r e l a t i o n s h i p s may r e p r e s e n t
an e a r l y s t a g e o f t r u n c a t i o n o f t h e F l i u v i a i system.
Once developed, e o l i a n
sand t r a n s p o r t e d southwestward was deposited a p a i n s t t h e v o l c a n i c a r c ,
The
sand sea need n o t have covered t h e e n t i r e area fror,i s o u t h e r n Utah t o s o u t h e r n A r i z o n a b u t , may have been r e s t r i c t e d t o a b e l t i n s o u t h e r n and southwestern A r i z o n a and s o u t h e a s t e r n C a l i f o r n i a .
co t i c L us I ON s 1 . Non-crossbedded l i t h o l o g i e s w i t h i n t h e Navajo and Aztec Sandstone a r e i n d i c a t i v e o f d e p o s i t i o n i n o r m o d i f i c a t i o n by an aqueous environment r e s u l t i n g f r o m a s h a l l o w groundwater t a b l e . 2. D i s t r i b u t i o n o f sedimentary f e a t u r e s o f groundwater o r i g i n w i t h i n t h e Navajo and Aztec Sandstone was c o n t r o l l e d by r e g i o n a l base l e v e l .
3. Crossbed o r i e n t a t i o n changes v e r t i c a l l y w i t h i n t h e Navajo and Aztec Sandstones. 4. The change i n crossbed o r i e n t a t i o n r e s u l t e d from change i n wind d i r e c t i o n d u r i n g d e p o s i t i o n o f t h e Navajo and Aztec Sandstones. 5. Anomalous crossbed o r i e n t a t i o n i n t h e Elojave D e s e r t i s t h e r e s u l t o f tectonic rotation. Because o f t h e u n c e r t a i n i t y o f age r e l a t i o n s h i p s w i t h i n t h e Navajo and Aztec Sandstones, o n l y t e n t a t i v e c o n c l u s i o n s can be reached r e g a r d i n g temporal versus s p a t i a l v a r i a t i o n s i n sedimentary f e a t u r e s and crossbed o r i e n t a t i o n . The w r i t e r acknowledges t h a t i n t e r p r e t a t i o n s o t h e r t h a n those below a r e possible
.
6. The change f r o m crossbedded sandstone i n t e r b e d d e d w i t h non-crossbedded
l i t h o l o g i e s t o e x c l u s i v e l y crossbedded sandstone r e s u l t e d from a g e o l o g i c a l l y i n s t a n t a n e o u s l o w e r i n g o f r e g i o n a l base l e v e l .
658 7. Change i n crossbed o r i e n t a t i o n , hence w i n d d i r e c t i o n , was e x c l u s i v e l y temporal. 3. F i n a l l y , a l t h o u g h change i n r e g i o n a l base l e v e l and wind d i r e c t i o n may have
r e s u l t e d f r o m change i n atmospheric c i r c u l a t i o n p a t t e r n s o r change i n c o n t i n e n t a l p o s i t i o n o r o r i e n t a t i o n r e l a t i v e t o atmospheric c i r c u l a t i o n p a t t e r n s , change i n r e g i o n a l paleogeography d u r i n g d e p o s i t i o n o f t h e Navajo and Aztec Sandstones o f f e r s a more p l a u s i b l e e x p l a n a t i o n .
Among changes which may have
e f f e c t e d a l o w e r i n g o f r e g i o n a l base l e v e l a r e :
Westward r e t r e a t o f t h e m a r i n e
s h o r e l i n e i n western Nevada; c u t o f f o f f l u v i a l r e c n a r g e o f t h e sand sea by e x t e n s i o n o f t h e v o l c a n i c a r c t o t h e s o u t h e a s t ; b u r i a l o f t h e f l u v i a l system by southwestward m i g r a t i n g dunes; and b u i l d u p o f t h e s u r f a c e o f e o l i a n - s a n d above r e g i o n a l base l e v e l . Reasons f o r t h e change i n w i n d d i r e c t i o n a r e more d i f f i c u l t t o assess b u t might a l s o include:
liestward r e t r e a t o f t h e marine s h o r e l i n e i n western
Nevada; c r e a t i o n o f an o r o g r a p h i c b a r r i e r by t h e d e v e l o p i n g v o l c a n i c a r c ; and low atmospheric p r e s s u r e o v e r new ocean c r e a t e d by c o n t i n e n t a l t r u n c a t i o n . ACKNOWLEDGEMENTS I w i s h t o acknowledge h e l p f u l d i s c u s s i o n s o f v a r i o u s aspects o f t h e Navajo and Aztec Sandstones w i t h Fred Peterson, Ronald L. Shreve, and Eugene 11. Shoemaker.
C l a r k B u r c h f i e l d i r e c t e d me t o t h e Cowhole Mountain l o c a l i t y so
c r i t i c a l t o i n t e r p r e t a t i o n o f t h e western f a c i e s .
I w i s h t o thank 14. E,
B r o o k f i e l d f o r many h e l p f u l s u g g e s t i o n s i n t h e w r i t i n g o f t h e m a n u s c r i p t . Also, I am i n d e b t e d t o t h e l a t e W i l l i a m
W. Rubey who p a t i e n t l y endured
my
e a r l y a t t e m p t s t o understand t h e s i g n i f i c a n c e o f non-crossbedded l i t h o l o g i e s . F i n a l l y , I w i s h t o acknowledge t h e a s s i s t a n c e o f B r i a n Hanke i n p r e p a r a t i o n of the figures. RE FE REll C ES A v e r i t t , P., Detterman, J.S., Harshbarger, J.W., Repenning, C.A., and W i l s o n , R.F., 1955, R e v i s i o n s i n c o r r e l a t i o n and nomenclature o f T r i a s s i c and J u r a s s i c f o r m a t i o n s i n southwestern Utah and n o r t h e r n Arizona. B u l l . Am. Assoc. P e t r o l . G e o l o q i s t s , 39: 2515-2535. Bilodeau, L.I.L., and K e i t h , S.B., 1979, I n t e r c a l a t e d v o l c a n i c s and e o l i a n " A z t e c - N a v a j o - l i k e " sandstones i n s o u t h e a s t A r i z o n a : Another c l u e t o t h e J u r a s s i c - T r i a s s i c p a l e o t e c t o n i c p u z z l e o f t h e southwestern U.S. Geol. SOC. America Abs. w i t h Proqr., 11:70. Bilodeau, W.L. and K e i t h , S.B., 1981, I n t e r c a l a t e d v o l c a n i c s and e o l i a n " A z t e c - N a v a j o - l i k e ' ' sandstones i n s o u t h e a s t A r i z o n a : Another c l u e t o t h e J u r a s s i c - T r i a s s i c p a l e o t e c t o n i c p u z z l e o f t h e southwestern U.S. I n : S.B. K e i t h , F. L o n g o r i a and W.L. B i l o d e a u ( E d i t o r s ) , Mesozoic Through E a r l y T e r t i a r y S e d i m e n t a t i o n a l and T e c t o n i c P a t t e r n s o f N o r t h e a s t Sonora and Southeast A r i z o n a , Geol. Soc. Am., C o r d i l l e r a n S e c t i o n , Annual Meeting, F i e l d Guide t o T r i p No. 12.
659
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