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 s
