DEVELOPMENTS IN SEDIMENTOLOGY 29
CONTINENTAL RED BEDS
FURTHER TITLES IN THIS SERIES VOLUMES 1. 2.3, 5, 8 and 9 are o...
77 downloads
1963 Views
26MB Size
Report
This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form
DEVELOPMENTS IN SEDIMENTOLOGY 29
CONTINENTAL RED BEDS
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 7 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 T O 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 CHEMISTRY O F 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 17 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 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 , E d f t o r INFLUENCE O F ABYSSAL CIRCULATION ON SEDIMENTARY ACCUMULATIONS IN SPACE AND TIME 24 R.E. G R I M and N. G U V E N BENTONITES 25A 0. L A R S E N and G.V. C H I L I N G A R I A N , E d f t o m DIAGENESIS IN SEDIMENTS AND SEDIMENTARY ROCKS 26 T . SUDO and S. SHIMODA, Edftors CLAYS AND CLAY MINERALS O F JAPAN 27 M.M. M O R T L A N D and V.C. F A R M E R INTERNATIONAL CLAY CONFERENCE 1978 28 A. N I S S E N B A U M , Editor HYPERSALINE BRINES AND EVAPORITIC ENVIRONMENTS
DEVELOPMENTS IN SEDIMENTOLOGY 29
CONTINENTAL RED BEDS P. TURNER Department of Geological Sciences, The University of Aston in Birmingham, Gosta Green, Birmingham, England
ELSEVIER SCIENTIFIC PUBLISHING COMPANY 1980 Amsterdam - Oxford - New York
ELSEVIER SCIENTIFIC PUBLISHING COMPANY 336 Jan van Galenstraat P.O. Box 211, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER/NORTH-HOLLAND INC. 52,Vanderbilt Avenue New York, N.Y. 10017
Library el Congress Cataloging 10 Pnbllcation Data
Turner, P 1948 Continental red beds. (Ikvebpuents i n sedimentology j 29) Bibliography: p. Includes index. 1. Red beds. I. T i t l e . 11. Series.
&E471*l'87
552l.5 ISBn 0-4W+-blg08-X
80-1903
ISBN 0-444-41908-x (Vol. 29) ISBN 0-444-41238-7 (Series)
Elsevier Scientific Publishing Company, 1980 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 Scientific Publishing Company, P.O.Box 330, 1000 AH Amsterdam, The Netherlands 0
Printed in The Netherlands
V
PREFACE AND ACKNOWLEDGEMENTS C o n t i n e n t a l r e d beds a r e a d i s t i n c t i v e s e d i m e n t a r y f a c i e s which have e x c i t e d t h e i n t e r e s t o f g e o l o g i s t s f o r many y e a r s .
T h e i r f e a t u r e s i n c l u d e a v a r i e t y of
a l l u v i a l , d e l t a i c and d e s e r t d e p o s i t s , a h i g h l y d i s t i n c t i v e s u i t e o f d i a g e n e t i c f e a t u r e s and d i s t i n c t i v e m a g n e t i c p r o p e r t i e s .
Red beds a l s o h o s t a d i s t i n c t i v e
t y p e of m i n e r a l i z a t i o n c o n t a i n i n g c o p p e r , uranium a n d vanadium minera Is.
Perhaps
t h e most d i s t i n c t i v e , and c o n t r o v e r s i a l , f e a t u r e of r e d beds is t h e i r c o l o u r .
The
o r i g i n o f t h i s c o l o u r (and t h e o r i g i n o f r e d b e d s ) has been h o t l y d e b a t e d f o r o v e r 100 y e a r s .
E a r l y w o r k e r s , i m p r e s s e d by t h e r e d c o l o u r of some modern d e s e r t
s e d i m e n t s , t h o u g h t t h a t a n c i e n t r e d beds were d e p o s i t e d under d e s e r t , c o n d i t i o n s . L a t e r , t h e American s e d i m e n t o l o g i s t P.D. Krynine championed t h e i d e a t h a t r e d beds were formed by t h e e r o s i o n and r e d e p o s i t i o n of r e d l a t e r i t i c s o i l s i n m o i s t tropical climates.
The a s s o c i a t i o n o f t h e s e two h y p o t h e s e s r e s u l t e d i n t h e wide-
s p r e a d b e l i e f t h a t r e d beds a r e o f some p a l a e o c l i m a t i c s i g n i f i c a n c e .
Later studies
by T.R. Walker showed t h a t d i a g e n e t i c p r o c e s s e s a r e most i m p o r t a n t i n t h e f o r m a t i o n of r e d b e d s , t h a t r e d c o l o u r can form i n s i t u , and t h a t c o n t i n e n t a l r e d beds a r e n o t n e c e s s a r i l y of p a l a e o c l i m a t i c s i g n i f i c a n c e . Much of t h e e a r l y palaeomagnetic work on r e d b e d s was done p r i o r t o o u r c u r r e n t u n d e r s t a n d i n g o f r e d bed d i a g e n e s i s and i t was g e n e r a l l y assumed t h a t r e d beds were magnetized d u r i n g d e p o s i t i o n and e a r l y d i a g e n e s i s by t h e a l i g n m e n t o f d e t r i t a l magn e t i c o x i d e s ( a view c o n s i s t e n t w i t h K r y n i n e ' s h y p o t h e s i s which i s s t i l l w i d e l y h e l d to-day).
Many l a t e r palaeomagnetic s t u d i e s r e v e a l e d complex m a g n e t i z a t i o n s more
c o n s i s t e n t w i t h a d i a g e n e t i c o r i g i n f o r r e d beds.
There i s , however, much v a r i a t i o n
i n t h e d i a g e n e t i c and p a l a e o m a g n e t i c p r o p e r t i e s o f r e d beds.
This v a r i a t i o n , along
w i t h t h e b e l i e f that a review combining t h e d e p o s i t i o n a l , d i a g e n e t i c and palaeomagn e t i c h i s t o r y o f r e d beds would b e o f some v a l u e , i s t h e i n s p i r a t i o n f o r t h i s book. The book would never have been w r i t t e n w i t h o u t t h e h e l p o f numerous f r i e n d s and c o l l e a g u e s who have c o n t r i b u t e d t o d i s c u s s i o n s w i t h me o v e r t h e p a s t decade a l t h o u g h I must stress t h a t I am s o l e l y r e s p o n s i b l e f o r a l l t h e shortcomings which t h e book c o n t a i n s .
I would p a r t i c u l a r l y l i k e t o thank t h e f o l l o w i n g who
p r o v i d e d m e w i t h m a t e r i a l a s s i s t a n c e a t v a r i o u s s t a g e s of t h e work:
E.L.
Boardman, D.W.
C o l l i n s o n , R.A. I x e r , R.G.
J a c k s o n 11, R . J .
P.J.C. N a g e t g a a l , S h e l l U . K . E x p l o r a t i o n and P r o d u c t i o n , R . J . D.J.
R. A r c h e r ,
Moiola,
S t e e l , D.H.
Tarling,
Vaughan, T.R. Walker, B. Waugh and P.A. Z i e g l e r .
The p r o d u c t i o n of t h e m a n u s c r i p t would n e v e r have been p o s s i b l e w i t h o u t t h e a s s i s t a n c e of t h e s t a f f o f G e o l o g i c a l S c i e n c e s a t t h e U n i v e r s i t y o f Aston p a r t i c u l a r l y Cathy Kennedy who typed t h e m a n u s c r i p t and B e v e r l e y P a r k e r who d r a f t e d many o f t h e diagrams.
My w i f e Angela r e a d t h e m a n u s c r i p t and p r e p a r e d
VI the s u b j e c t index. Many a u t h o r s and p u b l i s h e r s have g r a n t e d me p e r m i s s i o n t o use i l l u s t r a t i o n s from t h e i r o r i g i n a l p u b l i c a t i o n s a s f o l l o w s : American A s s o c i a t i o n o f Petroleum G e o l o g i s t s ( F i g s . 2 . 1 1 ,
2.26, 3.13, 3.14, 3 . 2 9 ,
4.35, 4 . 3 7 ) American J o u r n a l of S c i e n c e ( F i g s . 1 . 2 5 , 6.12) American M i n e r a l o g i s t ( F i g . 5 . 1 7 ) Annals de Geophysique ( F i g s . 7 . 1 2 , 7.13) Annals de Physique ( F i g . 7 . 9 ) B.T.
B a t s f o r d Ltd. ( F i g s . 2 . 2 ,
2.5)
B u l l e t i n of t h e G e o l o g i c a l S o c i e t y o f America ( F i g s . 2 . 1 6 ,
2.22, 4 . 2 , 4.23, 4 . 2 9 ,
4 . 3 0 , 4 . 3 1 , 4.32, 4.38, 4.39, 4 . 4 0 , 5 . 1 4 , 5.18, 6 . 6 , 6 . 8 , 6 . 9 , 8.22, 8.23, 8 . 2 4 , 8 . 2 7 , 8.28, 8.29, 8 . 3 0 ) B u l l e t i n of t h e G e o l o g i c a l Survey of G r e a t B r i t a i n ( F i g s . 3 . 3 0 ,
3.25, 3.26, 6.28)
Canadian J o u r n a l o f E a r t h S c i e n c e s ( F i g s . 4 . 5 0 , 8 . 4 , 8 . 5 , 8 . 7 , 8 . 8 , 8.9) E a r t h and P l a n e t a r y S c i e n c e Letters ( F i g s . l . 9 ,
7.19)
Geochimica Cosmochimica Acta ( F i g . 6 . 1 1 ) Geografiska Annaler (Fig.4.11) Geologie en Mijnbouw ( F i g s . 2 . 2 4 , Geological Journal (Figs.1.22,
3.15)
4.19, 4.34, 4 . 4 4 , 4 . 5 8 , 4.59)
Geologisches Rundschau ( F i g s . l . 1 6 ,
1 . 1 7 , 1.31)
Geophysical J o u r n a l of t h e Royal A s t r o n o m i c a l S o c i e t y ( F i g s . 6 . 4 , 7.29, 7.30, 7 . 3 1 , 7.32, 7.33, 7 . 3 4 , 7.38, 7.39,
7.26,
7.27, 7.28,
7.40, 7.42, 7.43, 7.44, 8.10,
8.12, 8 . 1 3 , 8 . 1 4 , 8.15, 8.16) Geosciences Canada ( F i g . 3.19) Ground Water (Fig.6.10) John Wiley and Sons I n c . ( F i g . 2 . 1 7 ) J o u r n a l of t h e G e o l o g i c a l S o c i e t y o f London ( F i g s . l . 1 0 ,
1 . 2 1 , 2.30, 4.52, 4.53)
J o u r n a l of Geomagnetism and G e o e l e c t r i c i t y (Fig.7.5) J o u r n a l of G e o p h y s i c a l R e s e a r c h ( F i g s . 8 . 1 8 , J o u r n a l o f Sedimentary P e t r o l o g y ( F i g s . 1 . 3 6 ,
8 . 1 9 , 8 . 2 5 , 8.26, 8.31) 2 . 7 , 2.20, 2.21, 2.27, 3.11, 3.21,
3.22, 3.23, 4 . 5 , 4 . 6 , 4 . 2 3 , 4.49) Limnology and Oceanography ( F i g . 6 . 2 1 ) Marine Geology ( F i g . 3.6) Mountain G e o l o g i s t (Fig.2.23) Nature (Fig.8.6) Norsk Geologisk T i d d s k r i f t ( F i g . l . 2 0 ) P h i l o s o p h i c a l T r a n s a c t i o n s of t h e Royal S o c i e t y o f London ( F i g . 8 . 1 ) P h y s i c s of t h e E a r t h and P l a n e t a r y I n t e r i o r s ( F i g . 7 . 2 3 , P h y s i c a l Review ( F i g . 7 . 1 0 ,
7.11)
7.24,
7.25)
VII
Proceedings of the Royal Society of London (Fig.7.17) Proceedings of the Yorkshire Geological Society (Fig.4.45) Reports of Progress in Physics (Fig.6.2) Reviews of Modern Physics (Fig.7.7) Science (Figs.7.20, 1.28, 7.14, 7.15) Scottish Journal of Geology (Figs.4.41, 4.36) Sedimentary Geology (Figs.2.3, 4.9, 4.26, 4.47, 4.48, 4.56, 4.61, 4.62, 4.63) Sedimentology (Figs.2.4, 2.6, 2.9, 2.10, 2.12, 2.13, 3.10, 3.17, 4.7, 4.12, 4.13, 4.14, 4.21, 4.22, 4.46, 7.18) Society of Economic Paleontologists and Mineralogists (Figs.l.4, 1.7, 1.11, 1.12, 1.13, 1.14, 1.16, 1.18, 1.19, 1.32, 1.33, 1.34, 3.3) Springer-Verlag (Fig.4.25, 5.1) Tectonophysics (Figs.1.23, 1.24) Transactions of the Gulf Coast Association of Geological Societies (Figs.3.18, 3.1, 3.4, 3.6, 3.7) W.H. Freeman and Co. (Fig.4.28)
This Page Intentionally Left Blank
IX
CONTENTS
CHAPTER 1 THE GEOLOGICAL FRAMEWORK OF CONTINENTAL RED BEDS
i
INTRODUCTION
1
GEOSYNCLINES Pre-orogenic phase Pre-f l y s c h Flysch Mo l a s s e
2 4
4 6 6
PLATE TECTONICS AND SEDIMENTATION D i v e r g e n t p l a t e margins I n t r a c o n t i n e n t a l r i f t s and a u l a c o g e n s Intercontinenta 1 r i f t i n g Convergent p l a t e margins Forearc areas The magmatic a r c Back-arc a r e a s Transform f a u l t s Continental c o l l i s i o n
6 7 7 9 13
THE TECTONIC SETTING OF ANCIENT CONTINENTAL RED BEDS Introduction P r o t e r o z o i c r e d b e d s of w e s t e r n Canada The Caledonian orogen E a r l y Mesozoic r e d beds formed i n graben a s s o c i a t e d w i t h t h e opening of t h e A t l a n t i c Ocean Cenozoic molasse The A l p i n e molasse The molasse of t h e Canadian c o r d i l l e r a
26 26 26 29 35
13 14 15 16 21
39 40 44
THE CLIMATIC SETTING OF ANCIENT CONTINENTAL RED BEDS The s i g n i f i c a n c e of f i r s t - c y c l e a r k o s i c r e d beds The s i g n i f i c a n c e of s i z e - c o m p o s i t i o n t r e n d s
50
THE CLASSIFICATION OF ANCIENT CONTINENTAL RED BEDS I n t r o d u c t i o n and p r e v i o u s c l a s s i f i c a t i o n s Reddening and d e p o s i t i o n a l environment Palaeomagnetic c l a s s i f i c a t i o n of c o n t i n e n t a l r e d b e d s A p r a c t i c a l c l a s s i f i c a t i o n of c o n t i n e n t a l r e d beds
60 60 63 64 66
CHAPTER 2
69
DESERT RED BEDS
51 55
INTRODUCTION
69
MODERN DESERT SEDIMENTS
69
DESERT FLWIAL SEDIMENTS
71
AEOLIAN SAND DEPOSITS Ripples Dunes and d r a a s Barchans Seifs T r a n s v e r s e forms P a r a b o l i c dunes Rhourds The i n t e r n a l s t r u c t u r e of modern and a n c i e n t a e o l i a n d e p o s i t s
72 74 77 78 80 a2 a4 84 85
X The t e x t u r a l c h a r a c t e r i s t i c s of a e o l i a n sands Grain s i z e Roundness and f r o s t i n g
88 89 91
DESERT LAKES AND INLAND SABKHAS
95
THE REDDENING OF DESERT SEDIMENTS Introduction The d e t r i t a l model - Simpson D e s e r t , A u s t r a l i a The d i a g e n e t i c model Sonoran D e s e r t Early diagenetic reddening Late diagenetic reddening
96 96 100 102 104 104
ANCIENT AEOLIAN RED BEDS Introduction D i s t i n g u i s h i n g a e o l i a n from s h a l l o w m a r i n e s a n d s t o n e s The Lower Permian of NW Europe Marginal f a n g l o m e r a t e s and wadi d e p o s i t s Aeolian sands D e s e r t l a k e and sabkha d e p o s i t s D i a g e n e s i s and r e d d e n i n g of t h e R o t l i e g e n d e s Mechanical i n f i l t r a t i o n of c l a y A u t h i g e n i c q u a r t z , f e l d s p a r and c l a y m i n e r a l s Carbonate and s u l p h a t e cements
105 105 107 108 111 113 114 116 117 120 122
CONCLUSIONS AND FURTHER READING
125
CHAPTER 3
126
-
DELTA PLAIN RED BEDS
INTRODUCTION
126
DELTA MODELS
127
DELTAIC FACIES ASSOCIATIONS The d e l t a p l a i n River-dominated d e l t a p l a i n s F l u v i a l d i s t r i b u t a r y channels I n t e r d i s t r i b u t a r y environments Swamps L a c u s t r i n e and l a c u s t r i n e d e l t a - f i l l d e p o s i t s Tide-dominated d e l t a p l a i n s Tidal d i s t r i b u t a r y channels Interdistributary areas The d e l t a f r o n t
131 132 132 132 134 137 138 138 139 139 139 146
DELTA GROWTH AND ABANDONMENT ANCIENT DELTAIC SUCCESSIONS
147
RED BEDS I N ANCIENT DELTA PLAINS Introduction The D i f u n t a Group of n o r t h e a s t e r n Mexico The d e l t a p l a i n f a c i e s Colour v a r i a t i o n s and m i n e r a l o g y The o r i g i n of t h e d e l t a p l a i n r e d beds C a r b o n i f e r o u s r e d beds i n Europe and North America Introduction Secondary p o s t - d i a g e n e t i c r e d d e n i n g b e n e a t h t h e sub-Permian unconformity Red beds o f t h e Upper Coal Measures of t h e UK A. South Wales B . The Blackband and E t r u r i a Marl Groups of North S t a f f o r d s h i r e Upper c a r b o n i f e r o u s r e d beds a t J o g g i n s , Nova S c o t i a
150 150 15 3 155 156 15 8 15 9 15 9 160 163 164 168 175
CONCLUSIUWS AND FURTHER READING
178
XI CHAPTER 4
ALLUVIAL RED BEDS
179
INTRODUCTION
179
RIVER CHANNELS
180
ALLUVIAL FANS Alluvial fan deposits Debris f l o w d e p o s i t s Sheet flood deposits Stream c h a n n e l d e p o s i t s Sieve deposits
182 183 184 185 185 185
PEBBLY BRAIDED RIVERS Longi t u d i n a 1 b a r s Bars i n c u r v e d c h a n n e l r e a c h e s Sedimentary o r g a n i z a t i o n of p e b b l y a l l u v i u m on a l a r g e s c a l e
185 186 188 189
SANDY LOW-SINUOSITY RIVERS
190
EPHEMERAL STREAM CHANNELS
194
MEANDERING RIVERS Introduction Point bars
198 198 200
RIVER BANKS AND FLOODPLAINS Bank d e p o s i t s Floodplains Interfluvial areas
204 204 20 7 208
THE TRANSPORT OF IRON I N RECENT ALLUVIUM
213
ANCIENT ALLUVIAL RED BEDS The Old Red Sandstone reddened p e b b l y a l l u v i u m Internal facies The r e d d e n i n g of p e b b l y a l l u v i u m reddened sandy and muddy a l l u v i u m External facies A. Coarse members B. F i n e members The r e d d e n i n g o f sandy and muddy a l l u v i u m A . F i n e member r e d d e n i n g B. Coarse member r e d d e n i n g The Lower T r i a s s i c r e d beds o f NW Europe Upper C r e t a c e o u s r e d beds o f t h e s o u t h e r n Bohemian Basins
2 20 221 222 2 30 233 233 237 244 246 250 25 3 25 9
CONCLUSIONS AND FURTHER READING
264
CHAPTER 5
265
-
THE DIAGENESIS OF CONTINENTAL RED BEDS
INTRODUCTION
265
SANDSTONE DIAGENESIS The c o m p o s i t i o n o f n a t u r a l w a t e r s and e a r l y d i a g e n e t i c r e a c t i o n s Cementa t i o n M i n e r a l t r a n s f o r m a t i o n and replacement T e x t u r a l changes d u r i n g d i a g e n e s i s Red bed d i a g e n e s i s
265 267 269 272 273 2 74
THE MECHANICAL INFILTRATION OF DETRITAL CLAY Cenozoic a l l u v i u m o f s o u t h w e s t e r n USA and n o r t h w e s t e r n Mexico Mechanically i n f i l t r a t e d c l a y i n a n c i e n t r e d beds
275 275 2 80
DISSOLUTION OF FRAMEWORK SILICATES Feldspars Micas
2 80 282 284
XI1 Ferromagnesian m i n e r a l s
291
CLAY REPLACEMENT
293
AUTHIGENIC MLNERALS Potassium f e l d s p a r Zeolites Clay m i n e r a l s Illite-montmorillonite Illite Kaolinite-dicki te Chlorite Calcite Haematite and p r e c u r s o r o x i d e s Quartz
299 299 302 308 308 30 9 30 9 313 314 316 319
STAGES I N THE DIAGENESIS OF CONTINENTAL RED BEDS
319
CHAPTER 6
323
THE MINERALOGY AND GEOCHEMISTRY OF IRON OXIDES I N RED BEDS
IRON OXIDE MINERALS The t i t a n o m a g n e t i t e s - titanomaghemi t e s The h a e m a t i t e - i l m e n i t e s e r i e s F e r r i c oxyhydroxides
323 32 3 326 3 29
OPAQUE OXIDES I N RED BEDS Titanomagnetite Haematite-ilmenite
3 30 3 30 333
PRECIPITATION AND STABILITY OF FERRIC OXYHYDROXIDES The d e h y d r a t i o n o f f e r r i c oxyhydroxides
338 340
ENVIRONMENTAL CONTROLS ON THE PRECIPITATION AND DIAGENETIC HISTORY OF FERRIC OXYHYDROXIDES Marine c o n d i t i o n s An a n c i e n t example: The C a t s k i l l c l a s t i c wedge Lacustrine conditions A n a n c i e n t example: The . O r c a d i a n Basin
344 345 348 35 1 35 9 365
POST-DEPOSITIONAL INFLUENCE OF GROUNDWATER ON THE MINERALOGY AND GEOCHEMISTRY OF RED BEDS Reddening by groundwater Secondary r e d u c t i o n zones
365 36 7
I R O N CONTENT OF ANCIENT RED BEDS COKCLUSIONS AND FURTHER READING
376 379
CHAPTER 7
38 0
THE MAGNETIZATION OF CONTINENTAL RED BEDS
INTRODUCTION Palaeomagnetism and rock magnetism The p h y s i c a l b a s i s of magnetism Hysteresis A n i s o t r o p y , magnetic domains and t h e t i m e dependence of m a g n e t i z a t i o n
380 380 382 304 3 84
MINERAL MAGNETISM The t i t a n o m a g n e t i t e s and ti tanomaghemi t e s Haema t i t e Grain s i z e e f f e c t s The r e l a t i v e s t a b i l i t y of s p i n c a n t e d and d e f e c t moments
388 389 39 0 394 398
MAGNETIZATION PROCESSES I N SEDIMENTS D e t r i t a l remanent m a g n e t i z a t i o n The o c c u r r e n c e of DRM i n n a t u r e
40 3 40 3 408
XI11 Chemical remanent m a g n e t i z a t i o n CRM of t h e g o e t h i t e - h a e m a t i t e t r a n s i t i o n
411 413
THE MAGNETIC PROPERTIES OF CONTINENTAL RED BEDS Induced m a g n e t i z a t i o n ( J i - H ) a n d i s o t h e m a l remanence (IRM) c u r v e s Ji-T a n a l y s i s The NRM of r e d b e d s Chemical d e m a g n e t i z a t i o n Comparative t h e r m a l d e m a g n e t i z a t i o n o f p i g m e n t , s p e c u l a r i t e a n d r o c k
417 418 424 42 7 429 434
CHAPTER 8
THE PALAEOMAGNETISM OF CONTINENTAL RED BEDS
440
INTRODUCTION
440
PROTEROZOIC BASINS OF WESTERN CANADA The M a r t i n F o r m a t i o n The C h r i s t i e Bay Group
44 1 443 446
THE LATE PRECAMBRIAN OF NORTH WEST SCOTLAND
45 3
THE OLD RED SANDSTONE The Anglo-Welsh B a s i n The O r c a d i a n B a s i n
460 460 46 3
TRIASSIC RED BEDS OF THE WESTERN USA The Moenkopi F o r m a t i o n
46 8 469
LATE CENOZOIC RED BEDS OF BAJA CALIFORNIA
4 75
THE RELATIONSHIP BETWEEN DIAGENESIS AND PALAEOMAGNETISM I N CONTINENTAL RED BEDS Models of remanence a c q u i s i t i o n The p a l a e o m a g n e t i c e v o l u t i o n o f r e d b e d s Type A r e d b e d s Type B r e d b e d s Type C r e d beds
481 481 484 485 487 488
REFERENCES
493
SUBJECT INDEX
5 37
This Page Intentionally Left Blank
1
CHAPTER 1 THE GEOLOGICAL FRAMEWORK OF CONTINENTAL RED BEDS INTRODUCTION C o n t i n e n t a l r e d beds comprise a wide range o f s e d i m e n t a r y f a c i e s r e p r e s e n t i n g t h e whole spectrum of non-marine d e p o s i t i o n a l environments from a l l u v i a l f a n s , r i v e r f l o o d p l a i n s , d e s e r t s , l a k e s and d e l t a s .
They a r e n o t e x c l u s i v e l y r e d a n d o f t e n
c o n t a i n v a r i a b l e p r o p o r t i o n s of i n t e r d i g i t a t e d d r a b s t r a t a which shows a v a r i e t y of other colours.
Most commonly such c o l o u r v a r i a t i o n a r e t h e r e s u l t o f minor v a r i a t -
i o n s i n d e p o s i t i o n a l environments. Red beds a r e c o l o u r e d by f i n e l y d i s s e m i n a t e d f e r r i c o x i d e s , u s u a l l y i n t h e form of h a e m a t i t e (&-Fe 0 ) , and g e n e r a l l y have a c o l o u r between 5R and 10R a c c o r d i n g 2 3 t o t h e Rock Color C h a r t (Goddard, 1951). The o c c u r r e n c e of f e r r i c o x i d e s i n r e d beds i n d i c a t e s t h a t t h e y formed under o x i d i z i n g c o n d i t i o n s b u t i s o f no palaeoc l i m a t i c s i g n i f i c a n c e b e c a u s e r e d beds a r e known t o form i n b o t h a r i d and m o i s t tropical climates.
The a c t u a l mechanisms of r e d bed f o r m a t i o n a r e s t i l l o n l y p o o r l y
understood and i t i s now c l e a r t h a t s e v e r a l d i f f e r e n t p r o c e s s e s a r e i n v o l v e d i n t h e g e n e s i s of a p a r t i c u l a r r e d bed f o r m a t i o n .
G e n e r a l i z a t i o n s r e g a r d i n g t h e o r i g i n of
r e d beds a r e n o t t h e r e f o r e p o s s i b l e . One of t h e most i m p o r t a n t v a r i a b l e s c o n t r o l l i n g t h e f o r m a t i o n of r e d beds i s t h e n a t u r e of t h e b a s i n i n which t h e sediments a r e o r i g i n a l l y d e p o s i t e d .
The n a t u r e of
t h e sedimentary b a s i n i s l a r g e l y determined by t h e t e c t o n i c s e t t i n g which c o n t r o l s t h e r a t e of u p l i f t and e r o s i o n i n t h e s o u r c e a r e a , t h e r a t e of sediment s u p p l y t o t h e b a s i n , and a l s o t h e r a t e o f s u b s i d e n c e a n d d e p o s i t i o n w i t h i n t h e b a s i n i t s e l f . T r a d i t i o n a l l y , c o n t i n e n t a l r e d beds have been r e g a r d e d a s c l a s t i c wedges of sediment formed a l o n g t h e f l a n k s of a c t i v e l y r i s i n g mountain r e g i o n s .
The f o r m a t i o n o f t h e s e
c l a s t i c wedges c l e a r l y i m p l i e s a s t r o n g t e c t o n i c i n f l u e n c e i n t h e form of m a r g i n a l f a u l t i n g and u p l i f t of t h e s o u r c e r e g i o n s .
C o n t i n e n t a l s e d i m e n t s of t h i s t y p e a r e
a d i s t i n c t i v e f e a t u r e of t h e l a t e r s t a g e s o f mountain b u i l d i n g and r e p r e s e n t a d i s t i n c t i v e s e d i m e n t a r y f a c i e s known a s molasse.
They a l s o o c c u r i n a number of
o t h e r t e c t o n i c s e t t i n g s such a s i n t r a c o n t i n e n t a l r i f t systems and t r a n s f o r m ( s t r i k e s l i p ) s e d i m e n t a r y b a s i n s , b o t h o f which may c o n t a i n v a s t t h i c k n e s s e s o f c o n t i n e n t a l sediments w i t h a s s o c i a t e d r e d beds.
There a r e c l o s e p a r a l l e l s between t h e t e c t o n i c
s e t t i n g of Recent c o n t i n e n t a l sediments and a n c i e n t r e d beds. The f i r s t p a r t of t h i s c h a p t e r d e s c r i b e s t h e t e c t o n i c framework o f Recent s e d i m e n t a r y b a s i n s i n r e l a t i o n t o t h e g e o s y n c l i n a l and p l a t e t e c t o n i c c o n c e p t s . T h i s i s followed by a d e s c r i p t i o n of a number of a n c i e n t r e d bed b a s i n s which i l l u s t r a t e t h e c h a r a c t e r i s t i c s e t t i n g s i n which a n c i e n t c o n t i n e n t a l r e d beds a r e
found.
The l a s t two p a r t s a r e concerned w i t h t h e p a l a e o c l i m a t i c s i g n i f i c a n c e and
c l a s s i f i c a t i o n of a n c i e n t c o n t i n e n t a l r e d beds. GEOSYNCLINES Our p r e s e n t u n d e r s t a n d i n g of s e d i m e n t a r y b a s i n s owes much t o t h e development of the geosynclinal concept.
T h i s embodies t h e common a s s o c i a t i o n of t h i c k s e d i m e n t a r y
b a s i n , f o l d i n g , i n t r u s i v e and e x t r u s i v e igneous a c t i v i t y , and mountain b u i l d i n g . Most f e a t u r e s o f g e o s y n c l i n e s can b e i n t e r p r e t e d i n terms of p l a t e t e c t o n i c s , b u t a number c a n n o t , and f o r t h i s r e a s o n t h e r e t e n t i o n o f t h e c o n c e p t , w i t h i t s emphasis on t h e d e s c r i p t i o n of t e c t o n i c s e t t i n g s of s e d i m e n t a r y sequences a t c o n t i n e n t a l margins has proved most u s e f u l (HsU, 1972). The term g e o s y n c l i n e , s t r i c t l y s p e a k i n g , means a l a r g e down-warped s u r f a c e , b u t h a s always been c o n s t r u e d a s a t h e o r e t i c a l c o n c e p t o r i g i n of mountains.
-
a working h y p o t h e s i s f o r t h e
The o r i g i n a l i d e a of H a l l (1859) was t h a t s e d i m e n t a r y l o a d
l e d t o s u b s i d e n c e , which i n t u r n r e s u l t e d i n f o l d i n g , i g n e o u s a c t i v i t y a n d mountain building.
C r i t i c s p o i n t e d o u t t h a t g e o s y n c l i n e s d i d n o t s u b s i d e under s e d i m e n t a r y
l o a d b u t s y n c l i n a l o r g e o s y n c l i n a l d e p r e s s i o n s made t h e a c c u m u l a t i o n s of t h i c k s e d i m e n t a r y sequences p o s s i b l e (Dana, 1873).
Thus, i f s e d i m e n t a r y s u p p l y was slow
o r modest and c o u l d n o t keep pace w i t h s u b s i d e n c e , " g e o s y n c l i n a l " s e d i m e n t s would be b a t h y a l and p e l a g i c ( S u e s s , 1875; Haug, 1900). I t was r e a l i z e d a t a n e a r l y s t a g e t h a t orogeny c o u l d r e s u l t from l a t e r a l comp-
r e s s i o n caused by t h e movement o f ocean f l o o r towards a c o n t i n e n k a l mass and t h a t g e o s y n c l i n e s developed e i t h e r b e s i d e , o r o n , a c o n t i n e n t a l margin.
T h i s view was
s t r o n g l y i n f l u e n c e d by t h e p r e s e n t p o s i t i o n o f t h e Appalachians and t h e w e s t e r n C o r d i l l e r r a , w h e r e a s t h e e a r l y view o f Europeans was t h a t g e o s y n c l i n e s developed between and on t h e a d j a c e n t margins o f two c o n t i n e n t s .
I n t h i s context sedimentation
was c o n s i d e r e d t o t a k e p l a c e p r o g r e s s i v e l y c l o s e r t o one o f t h e c o n t i n e n t s , termed a ' f o r e l a n d ' , d u r i n g g e o s y n c l i n a l development. G e o s y n c l i n a l c l a s s i f i c a t i o n was begun by S c h u c h e r t (1923) and e x t e n d e d by S t i l l e (1936, 1940) who d i s t i n g u i s h e d o r t h o g e o s y n c l i n e s , c h a r a c t e r i z e d by a l p i n e - t y p e deformation and orogeny, and p a r a g e o s y n c l i n e s which a r e c h a r a c t e r i z e d by b l o c k f a u l t i n g and do n o t form mountain c h a i n s .
O r t h o g e o s y n c l i n e s were d i v i d e d i n t o e u g e o s y n c l i n e s
w i t h a v a r i e d igneous s u i t e i n c l u d i n g p r e - o r o g e n i c o p h i o l i t e s , s y n - o r o g e n i c a n d e s i t e s and p o s t - o r o g e n i c g r a n i t e s , and miogeosynclines which l a c k i g n e o u s r o c k s . More d e t a i l e d c l a s s i f i c a t i o n s were developed by Kay (1947, 1951) who sub-divided t h e o r t h o g e o s y n c l i n e and p a r a g e o s y n c l i n e o f S t i l l e (1936).
He retained t h e division
i n t o miogeosyncline and e n g e o s y n c l i n e and a p p l i e d i t t o t h e Appalachian mountains (Fig.l.1).
The p a r a g e o s y n c l i n e s were, however, d i v i d e d i n t o t h r e e t y p e s : exogeo-
s y n c l i n e s , s i t u a t e d on a c o n t i n e n t a l margin and s u p p l i e d by d e t r i t u s from t h e deformed o r t h o g e o s y n c l i n e ; a u t o g e o s y n c l i n e s , c o n s i s t i n g m o s t l y of i n t r a c o n t i n e n t a l
3 c a r b o n a t e b a s i n s i n d e p e n d e n t of t h e o r t h o g e o s y n c l i n e and z e u g o g e o s y n c l i n e s which Eastern New York
20 000
Vermont
New Hampshire
Maine
50
50
0 Distance (miles)
RESTORED SECTION - CAMBRIAN THROUGH MIDDLE ORDOVICIAN
100 fathom line Continental shelf
0
-
1
5000 10 000
/
15 000
20 000 Shelf __*
2 5 000
S1ope+Continental
rise
SEDIMENT THICKNESS OF ATLANTIC CONTINENTAL MARGIN
F i g . 1.1. The Appalachian g e o s y n c l i n e ( a f t e r Kay,1951) compared w i t h t h e p r e s e n t day A t l a n t i c margin ( a f t e r Drake e t a l . , 1959).
a r e i n t r a c r a t o n i c b a s i n s f i l l e d by d e t r i t u s from i n t r a c r a t o n i c mountain c h a i n s . Kay a l s o r e c o g n i z e d a number of s e d i m e n t a r y b a s i n s which h e c o n s i d e r e d t o b e c h a r a c t e r i s t i c of t h e l a t e r s t a g e s of g e o s y n c l i n a l development and termed them " l a t e cycle geosynclinesii.
These i n c l u d e d :
e p i e n g e o s y n c l i n e s d e r i v e d from
mountains which were formed from e u g e o s y n c l i n e s ; t a p h r o g e o s y n c l i n e s which a r e i n t r a c r a t o n i c b a s i n s c h a r a c t e r i s e d by fault-bounded m a r g i n s , and p a r a l i a g e o s y n c l i n e s s i t u a t e d a l o n g c o n t i n e n t a l margins and merging i n t o t h e c o a s t a l p l a i n .
This category
was c r e a t e d t o accommodate t h e Gulf of Mexico. The most w i d e l y r e c o g n i z e d o f t h e s e L a t e Cycle G e o s y n c l i n e s i s t h e Taphrogeos y n c l i n e which i s s i g n i f i c a n t b e c a u s e t h e s e r e p r e s e n t a m a j o r l o c u s o f c o n t i n e n t a l e l a s t i c d e p o s i t i o n and r e d bed f o r m a t i o n and have o f t e n been wrongly compared i n t e c t o n i c s e t t i n g w i t h p o s t - o r o g e n i c molasse.
Taphrogeosynclines a r e elongated
s e d i m e n t a r y b a s i n s which a r e i n t r a c r a t o n i c and c h a r a c t e r i z e d by t i l t e d (= t a p h r o ) f a u l t blocks.
The t y p e example i s t h e T r i a s s i c f a u l t t r o u g h o f t h e e a s t e r n U.S.A.
4
An i m p o r t a n t landmark i n t h e development of t h e g e o s y n c l i n a l c o n c e p t was t h e r e c o g n i t i o n o f t h e importance o f t u r b i d i t y c u r r e n t s (Kuenen, 1957a, 195713, 1958, 1967).
These p r o v i d e d a mechanism whereby l a r g e sand and conglomerate b o d i e s c o u l d
b e emplaced i n deep w a t e r and p r o v i d e d a n e x p l a n a t i o n of how a n i n i t i a l l y deep s e d i m e n t a r y b a s i n c o u l d be f i l l e d .
Drake e t a l . (1959) showed how t u r b i d i t y c u r r e n t s
r e l a t e d t o a n A t l a n t i c - t y p e c o n t i n e n t a l margin
-
a c o n t i n e n t a l s h e l f prism passing
oceanwards i n t o a c o n t i n e n t a l r i s e p r i s m w i t h a t h i c k s u c c e s s i o n of t u r b i d i t e s . D i e t z (1963) and D i e t z and Holden (1966) e x t e n d e d t h e A t l a n t i c margin a n a l o g y a n d s u g g e s t e d t h a t t h e l a t e Mesozoic and Cenozoic m i o g e o s y n c l i n a l s u c c e s s i o n of t h e e a s t e r n USA t h i c k e n e d seawards towards t h e c o n t i n e n t a l e d g e , forming a m i o g e o c l i n e . The A t l a n t i c m i o g e o c l i n e was t h e n compared w i t h t h e Lower P a l a e o z o i c Appalachian geosyncline (Fig.l.2). The g e o s y n c l i n a l c o n c e p t r e a c h e d a s o p h i s t i c a t e d l e v e l immediately p r l o r t o t h e r e v e l a t i o n s o f p l a t e t e c t o n i c s and many o f t h e views which p r e v a i l e d a t t h e time a r e summarized by Aubouin (1965).
He d e f i n e d a n e l e m e n t a r y g e o s y n c l i n e model w i t h
miogeosyncline and e u g e o s y n c l i n e which were f u r t h e r d i v i d e d i n t o a r i d g e and furrow. The Western A l p s , H e l l e n i d e s , Apennines and C a r p a t h i a n s e a c h formed examples and h e s t r e s s e d t h e importance o f d i s t i n c t s t a g e s o f development w i t h t h e m i g r a t i o n of s e d i m e n t a t i o n and d e f o r m a t i o n towards t h e f o r e l a n d d u r i n g orogeny. I t has been known f o r some t i m e now t h a t g e o s y n c l i n a l s e d i m e n t a t i o n i s b r o a d l y c y c l i c and f o u r d i s t i n c t i v e f a c i e s can b e r e c o g n i z e d : flysch
-
pre-orogenic
-
pre-flysch
-
molasse (HsU, 1973) a l t h o u g h t h e r e has been c o n s i d e r e b l e v a r i a t i o n i n t h e
usage o f t h e v a r i o u s terms i n v o l v e d . T h i s sequence of f a c i e s i s a p p l i c a b l e t o many o r o g e n i c b e l t s of d i f f e r e n t a g e s and i n d i f f e r e n t p a r t s o f t h e world. These exposed o r o g e n i c b e l t s o f t e n show t h e p a s s a g e from a basement ( p r e - o r o g e n i c ) p h a s e , t h r o u g h a s t a r v e d ( p r e - f l y s c h ) phase formed a s t h e b a s i n deepened, t o a deep-marine c l a s t i c ( f l y s c h ) p h a s e and f i n a l l y a c o n t i n e n t a l c l a s t i c ( m o l a s s e ) phase formed d u r i n g and a f t e r mountain u p l i f t .
Pre-orogenic phase The f a c i e s of t h e p r e - o r o g e n i c phase a r e v e r y v a r i a b l e and i n c l u d e i g n e o u s a n d / o r metamorphic c r y s t a l l i n e basement, s h a l l o w marine c l a s t i c s ( q u a r t z a r e n i t e s ) and c a r b o n a t e s and o t h e r f a c i e s c o n s i s t e n t w i t h a m i o g e o s y n c l i n a l s e t t i n g . I t i n d i c a t e s t h a t g e o s y n c l i n a l development u s u a l l y b e g i n s w i t h e s t a b l i s h m e n t of s h a l l o w marine s e d i m e n t a t i o n on c o n t i n e n t a l c r u s t . Pre-flvsch T h i s i s a v a r i a b l e f a c i e s which p r e c e d e s t h e f l y s c h s t r a t i g r a p h i c a l l y .
It
c o n s i s t s mainly of f i n e g r a i n e d sediments i n c l u d i n g b l a c k s h a l e s , s i l t s t o n e s , c h e r t s and d a r k l i m e s t o n e s sometimes r e f e r r e d t o a s t h e ' b a t h y a l l u l l ' ( G o l d r i n g , 1962).
5 Late Pracambrsan M Ordov8cian
L Y
I
.r
in 0 0 0 0
n
z
W
W
c
n
la W 7
m
.C
v)
.c W
L ln L m
Y
W
c Y 'c
W E
p:
m
.r
U
Fig.l.2.
D e p o s i t i o n and c o l l a p s e o f c o n t i n e n t a l r i s e ( a f t e r D i e t z and Holden, 1966).
I n f a c t two d i s t i n c t i v e s u b f a c i e s c a n be recognized i n t h e p r e - f l y s c h and t h e s e a r e termed t h e Becken ( b a s i n ) and Schwellen ( s w e l l ) s u b f a c i e s i n t h e Hercynian geosyncline. The b a s i n f a c i e s c o n s i s t s o f r e l a t i v e l y t h i c k sequences o f f i n e g r a i n e d sediments d e p o s i t e d as h e m i p e l a g i t e s o r l o w d e n s i t y t u r b i d i t e s whereas t h e swell f a c i e s c o n s i s t s o f condensed l i m e s t o n e s r i c h i n b i o c l a s t i c m a t e r i a l and d e p o s i t e d on a submarine rise. An i m p o r t a n t f e a t u r e i n some p r e - f l y s c h sequences i s t h e o c c u r r e n c e of o p h i o l i t e complexes.
These show t h e upward sequence:
ultramafics
p i l l o w l a v a s and a r e o f t e n a s s o c i a t e d w i t h r a d i o l a r i t e s .
gabbros
+ dykes +
The b a s i c rocks a r e
f r e q u e n t l y s e r p e n t i n i z e d and B a i l e y and McCallien (1960) gave t h e name "Steinmann T r i n i t y " t o t h e t h r e e f o l d a s s o c i a t i o n of r a d i o l a r i t e , s e r p e n t i n i t e , and g r e e n s t o n e s r e c o g n i z e d by Steinmann (1905, 1927).
6
Flysch F l y s c h i s a w i d e l y used term (HsU, 1970) which i s commonly c o n s i d e r e d t o b e a synonym f o r t u r b i d i t e .
Some a u t h o r s p r e f e r t o r e s t r i c t t h e term f l y s c h t o syn-
o r o g e n i c mass-flow d e p o s i t s found i n o r t h o g e o s y n c l i n e s ( e . g . d e R a a f , 1968; S t a n l e y , 1970) and u s e t h e term f l y s c h - l i k e o r f l y s c h o i d f o r s i m i l a r d e p o s i t s found i n l a t e g e o s y n c l i n a l s t a g e s when t e c t o n i c a c t i v i t y i s n o t known. F l y s c h c o n s i s t s m o s t l y of a l t e r n a t i n g s a n d s t o n e s and s h a l e s which a r e o f t e n r e p e a t e d through g r e a t s t r a t i g r a p h i c t h i c k n e s s e s .
A number of f a c i e s a s s o c i a t i o n s
a r e now r e c o g n i z e d ( M u t t i and Ricci L u c c h i , 1972) i n b o t h Recent and a n c i e n t t u r b i d i t e basins including:
b a s i n p l a i n , o u t e r f a n and i n n e r f a n f a c i e s
associations.
Molasse i s d e f i n e d by Van Houten (1969) a s , " C l a s t i c wedges l a i d down i n s h a l l o w marine and nonmarine environments a d j a c e n t t o r i s i n g mountain c h a i n s " .
A wide
v a r i e t y o f f a c i e s a r e r e p r e s e n t e d i n c l u d i n g a l l u v i a l f a n s , f l u v i a l , d e l t a i c , and s h a l l o w marine d e p o s i t s encompassing a wide range of l i t h o l o g i e s r a n g i n g from c o n g l o m e r a t e s , s a n d s t o n e s , mudstones, and t h i n l i g n i t e s and c o a l s . I n t h e g e o s y n c l i n a l c y c l e m o l a s s e g e n e r a l l y f o l l o w s , b u t may b e p a r t l y contemporaneous w i t h , f l y s c h s e d i m e n t a t i o n .
I t i s u s e f u l t o d i s t i n g u i s h between a l a t e -
o r o g e n i c f a c i e s now deformed and a p o s t - o r o g e n i c f a c i e s which i s undeformed. Molasse i s t h e r e f o r e one of t h e main d i a g n o s t i c f e a t u r e s of orogeny and may b e a s e n s i t i v e m o n i t o r o f d e f o r m a t i o n and u p l i f t i n mountain b e l t s . PLATE TECTONICS AND SEDIMENTATION
Most g e o l o g i s t s now a c c e p t t h e main t e n e t s of p l a t e t e c t o n i c s and i t s c o r o l l a r y c o n c e p t s o f c o n t i n e n t a l d r i f t , s e a f l o o r s p r e a d i n g , t r a n s f o r m f a u l t s and s u b d u c t i o n zones a l t h o u g h some (Beloussov, 1970; M e y e r h o f f , l 9 7 0 ) have c h a l l e n g e d t h e s e c o n c e p t s . The term p l a t e t e c t o n i c s (McKenzie, 1972; Dewey, 1972) a r o s e b e c a u s e t h e d i s c r e t e s p h e r i c a l c a p s of e s s e n t i a l l y r i g i d l i t h o s p h e r e which a r e i n r e l a t i v e motion w i t h r e s p e c t t o one a n o t h e r , and w i t h t h e less r i g i d a s t h e n o s p h e r e b e n e a t h , a r e u s u a l l y r e f e r r e d t o a s p l a t e s (McKenzie and P a r k e r , 1967). P l a t e t e c t o n i c t h e o r y l a y s emphasis on t h e h o r i z o n t a l movement of t h e l i t h o s p h e r e and t h e p l a t e t e c t o n i c s e t t i n g ( D i c k i n s o n , 1974a) of s e d i m e n t a r y b a s i n s can b e d e s c r i b e d i n terms of t h e p r o x i m i t y of t h e b a s i n t o a p l a t e margin.
Thus i n t r a p l a t e
s e t t i n g s o f c o n t i n e n t a l o r o c e a n i c c h a r a c t e r c o n t r a s t w i t h p l a t e margin s e t t i n g s . The l a t t e r i n c l u d e t h r e e d i s t i n c t t y p e s of p l a t e j u n c t i o n :
1) d i v e r g e n t where two
l i t h o s p h e r i c p l a t e s a r e moving a p a r t a s ocean f l o o r s p r e a d i n g t a k e s p l a c e ;
2)
c o n v e r g e n t where l i t h o s p h e r e i s consumed a s one p l a t e descends b e n e a t h a n o t h e r ; and
7 3) s t r i k e - s l i p o r t r a n s f o r m where p l a t e s move l a t e r a l l y p a s t one a n o t h e r . The a c c u m u l a t i o n of a t h i c k s e d i m e n t a r y b a s i n r e q u i r e s e i t h e r a n i n i t i a l depressi o n o r p r o g r e s s i v e s u b s i d e n c e , b o t h o f which imply l a r g e v e r t i c a l movements.
i s not i n c o n s i s t e n t with t h e p l a t e t e c t o n i c theory.
This
V e r t i c a l movements t a k e p l a c e
i n r e s p o n s e t o changes i n t h e t h i c k n e s s o f t h e c r u s t , i q t h e t h e r m a l c o n d i t i o n of t h e l i t h o s p h e r e , and i n t h e i s o s t a t i c b a l a n c e between l i t h o s p h e r e and a s t h e n o s p h e r e .
I t i s t h e s e v e r t i c a l movements which l a r g e l y c o n t r o l t h e e v o l u t i o n of s e d i m e n t a r y basins. D i v e r g e n t p l a t e margins D i v e r g e n t p l a t e j u n c t i o n s may o r may n o t b e a s s o c i a t e d w i t h t h e emplacement of o c e a n i c c r u s t and r e l a t e d s e a f l o o r s p r e a d i n g ,
U s u a l l y d i v e r g e n t p l a t e margins a r e
c h a r a c t e r i z e d by a domal u p l i f t w i t h a c e n t r a l r i d g e and median v a l l e y a l t h o u g h r a p i d l y s p r e a d i n g r i d g e s s u c h a s t h e E a s t P a c i f i c Rise l a c k t h e median r i f t v a l l e y . U p l i f t i s due t o thermal e x p a n s i o n , h i g h h e a t f l o w and magmatic a c t i v i t y and i s o f t e n a s s o c i a t e d w i t h t h e emplacement o f o c e a n i c c r u s t and s e a f l o o r s p r e a d i n g . Continuous s p r e a d i n g o v e r a l o n g t i m e r e s u l t s i n t h e w i d e s p r e a d s e p a r a t i o n of t h e c o n t i n e n t a l margins a s i n t h e p r e s e n t - d a y A t l a n t i c and I n d i a n Oceans.
I n other
c a s e s s p r e a d i n g may b e i n t e r r u p t e d and o n l y a c e r t a i n amount o f o c e a n i c c r u s t emplaced a s i n t h e Red Sea.
I f no o c e a n i c c r u s t i s emplaced a n i n t r a c o n t i n e n t a l
r i f t forms a n d t h i s may b e t h e s i t e of a major s e d i m e n t a r y a c c u m u l a t i o n . I n t r a c o n t i n e n t a l r i f t s and a u l a c o g e n s I n t r a c o n t i n e n t a l r i f t s a r e major g l o b a l s t r u c t u r e s w i t h a long h i s t o r y of d e f o r m a t i o n , m a g m a t i s m , and s e d i m e n t a t i o n ,
They a r e c h a r a c t e r i z e d by domal u p l i f t s ,
usually about lkm h i g h , lOOkm wide and 200-3OOkm l o n g , n e g a t i v e g r a v i t y anomalies and by a l k a l i n e v u l c a n i c i t y .
The t y p e example i s t h e E a s t A f r i c a n r i f t system which i s
o v e r 3,OOOkm long and h a s a h i s t o r y e x t e n d i n g back i n t o t h e Precambrian w i t h o u t any a p p a r e n t major l a t e r a l movement ( F i g . l . 3 ) .
The E a s t A f r i c a n r i f t i s c h a r a c t e r i z e d by
a g r a d u a l t o p o g r a p h i c r i s e towards t h e c e n t r a l r i f t which i s between 40 and 50km wide and i s a b o u t 2km below t h e s u r r o u n d i n g p l a t e a u .
W i t h i n t h e r i f t a r e a number o f
Mesozoic and Cenozoic a l k a l i n e v o l c a n i c c e n t r e s and non-volcanic s u c h a s Ruwenzori.
horst structures
T o p o g r a p h i c a l l y t h e r i f t i s complex and i s made more s o by t h e
e n e c h e l o n p a t t e r n o f t h e boundary f a u l t s .
A number of s e d i m e n t a r y b a s i n s t h e r e f o r e
e x i s t and t h e major ones a r e t h e s i t e s of l a r g e p r e s e n t - d a y l a k e s and c o n t a i n t h i c k sequences of l a c u s t r i n e s e d i m e n t s .
Because o f i t s domal s t r u c t u r e t h e maior r i v e r s
flow away from t h e E a s t A f r i c a n r i f t b u t t h e r e i s a l s o a n i n t e r n a l d r a i n a g e .
The
s e d i m e n t a r y b a s i n s a r e t h u s f i l l e d by a l l u v i a l f a n s , d e r i v e d from l o c a l t o p o g r a p h i c h i g h s , and l a c u s t r i n e s e d i m e n t s .
8
F i g . 1.3. The East A f r i c a n R i f t System ( a f t e r King,1970). I n s e t shows a r e a s of l a t e Mesozoic and Cenozoic domal u p l i f t and t r i p l e j u n c t i o n s ( a f t e r Burke and Whiteman, 1973). The B a i k a l r i f t i s a n o t h e r major r i f t system s t r i k i n g l y s i m i l a r t o t h e E a s t African r i f t .
I t s t r e t c h e s f o r a d i s t a n c e of 3,500km from n o r t h w e s t e r n Mongolia,
through t h e mountains o f s o u t h e a s t e r n S i b e r i a i n t o Yakutia and probably a s f a r a s
9
t h e s e a o f Okhotsk (King, 1976).
The r i f t s t r u c t u r e s c o n s i s t o f l i n e a r systems of
fault-bounded i n t e r m o n t a n e d e p r e s s i o n s a l o n g t h e c r e s t o f a r c h e d u p l i f t s .
There
a r e twelve l a r g e d e p r e s s i o n s , 100-7OOkm l o n g and 15-18km i n w i d t h , and numerous smaller depressions.
The b a s i n s a r e f i l l e d by up t o 5,500m o f O l i g o c e n e - P l e i s t o c e n e
c o n t i n e n t a l sediments w i t h i n t e r c a l a t e d v o l c a n i c s . nizable:
Two t e c t o n i c u n i t s a r e recog-
a lower group c o n s i s t i n g of l a c u s t r i n e , swamp and a l l u v i a l d e p o s i t s of
O l i g o c e n e , Miocene and p o s s i b l y lower P l i o c e n e a g e and a n upper group o f l a t e Pleistocene-Holocene age r e s t r i c t e d t o i n t e r m o n t a n e t r o u g h s and d e r i v e d from a h i g h e r mountain r e l i e f e v i d e n t l y under c o l d c l i m a t i c c o n d i t i o n s . O t h e r good examples o f r i f t systems i n c l u d e t h e Oslo g r a b e n w i t h i t s Permian a l k a l i c v o l c a n i c s and t h e t r o u g h s of t h e North Sea (Whiteman e t a l . , Rhine Graben i s one of t h e b e s t known.
1975).
The
I t i s o f T e r t i a r y a g e and i s l o c a t e d over
a dome forming t h e s o u t h e r n arm of a t r i p l e j u n c t i o n c e n t r e d i n F r a n k f u r t (Burke and Dewey, 1973).
I t h a s t h e u s u a l a l k a l i n e v o l c a n i c s and i s a s s o c i a t e d w i t h h i g h
h e a t flow, e a r t h q u a k e s and r e c e n t f a u l t a c t i v i t y ( M u e l l e r , 1970). A d i s t i n c t i v e type of r i f t system i s t h e abandoned r i f t o r a u l a c o g e n . were d i s c o v e r e d by S h a t s k i (1961) and developed i n t o a d o m e - t r i p l e a u l a c o g e n - r i f t c o n c e p t by Burke and Dewey (1973).
These
junction-
Aulacogens a r e deep l i n e a r
t r o u g h s which form t h e t h i r d arms of t r i p l e j u n c t i o n s and e x t e n d from a c o n t i n e n t a l margin o r g e o s y n c l i n e i n t o a f o r e l a n d p l a t f o r m o r c o n t i n e n t .
The developmental
sequence of a n a u l a c o g e n i n v o l v e s e a r l y doming and t h e f o r m a t i o n o f a plume-induced t r i p l e j u n c t i o n r i f t network. of t h e system.
S p r e a d i n g may t a k e p l a c e on any o f t h e t h r e e arms
I f i t t a k e s p l a c e on a l l t h r e e t h e n t h r e e p l a t e s a r e formed, b u t i f
o n l y on two, t h e n t h e f o r m a t i o n of two p l a t e s l e a v e s a n abandoned r i f t o r a u l a c o g e n . One of t h e b e s t known a u l a c o g e n s i s t h e Benue Trough.
T h i s formed from a
Cretaceous t r i p l e j u n c t i o n j o i n i n g S o u t h America and A f r i c a ( F i g . l . 4 ) .
Two arms
s e p a r a t e d t o form t h e S o u t h A t l a n t i c w h i l s t t h e t h i r d was abandoned and remains a s t h e Benue Trough (Burke e t a l . , 1971; Olade, 1975). s u c c e s s i o n ( o v e r 1Okm) of T e r t i a r y s e d i m e n t s .
I t has been f i l l e d by a t h i c k
These p a s s from submarine f a n d e p o s i t s
a t t h e b a s e up through d e l t a i c s e d i m e n t s i n t o c o n t i n e n t a l a l l u v i a l s a n d s t o n e s ( F i g . 1.5). Intercontinental r i f t i n g The emplacement of o c e a n i c c r u s t i n a x i a l r i f t zones i s a s s o c i a t e d w i t h s e a f l o o r s p r e a d i n g a n d s e p a r a t i o n of e a c h h a l f o f t h e r i f t .
These develop i n t o p a s s i v e
' A t l a n t i c ' t y p e c o n t i n e n t a l m a r g i n s , comprising a s h e l f , s l o p e and r i s e , and sometimes termed t r a i l i n g edges. I n t h e e a r l y s t a g e s of i n t e r c o n t i n e n t a l r i f t i n g s e d i m e n t a t i o n i s c l o s e l y a s s o c i a t e d w i t h t h e e r u p t i o n of v a s t amounts of b a s a l t i c l a v a s .
I n t h e Red S e a , which
has a complex s p r e a d i n g h i s t o r y (Richardson and H a r r i s o n , 1 9 7 6 ) , t h e r e i s a n a x i a l
10
o-(3q--+-+ of Guinea
0'
The o r i g i n o f t h e Benue Trough a s a n a u l a c o g e n abandoned d u r i n g c o n t i n e n t a l Fig.l.4. r i f t s e p a r a t i o n o f A f r i c a and S o u t h America d u r i n g t h e C r e t a c e o u s ( a f t e r Hoffman e t a l . , 1974). t r o u g h 4-3Okm wide and o v e r 1,000m deep,
I n the deeper p a r t s of t h e trough a r e h o t
b r i n e p o o l s which form where t r a n s f o m f a u l t s c u t t h e median v a l l e y ( B i g n e l l , 1975). These p o o l s d e p o s i t m e t a l l i f e r o u s s e d i m e n t s r i c h i n o x i d e s of z i n c , c o p p e r , l e a d i r o n and manganese which i n t h e A t l a n t i s I1 deep can b e d i v i d e d i n t o seven l a t e r a l l y
11
-z
I ro
I
I
\
Continental sandstones
Cretaceous rise deposits
--
I
2 '
I
km
~
~ _ _ _ _ _ _ _ ~ ~
~
~
Fig.l.5. Schematic s t r u c t u r a l c r o s s s e c t i o n showing t h e s e d i m e n t a r y f i l l of t h e Benue Trough ( a f t e r Burke, 1972). c o r r e l a t i v e f a c i e s ( B i s c h o f f , 1969). major mineralogy:
The f a c i e s nomenclature i n d i c a t e s t h e i r
d e t r i t a l , i r o n - m o n t m o r i l l o n i t e , goethite-amorphous,
m a n g a n o s i d e r i t e , a n h y d r i t e , and manganite.
sulphide,
An a d d i t i o n a l m a g n e t i t e f a c i e s was
s u b s e q u e n t l y a'dded ( H a c k e t t and B i s c h o f f , 1973).
The o r i g i n s of t h e b r i n e s and
m c t a l l i f e r o u s sediments i n t h e median v a l l e y o f t h e Red Sea has been d i s c u s s e d by
Schoell (1975).
P o s s i b i l i t i e s i n c l u d e i n t e n s e e v a p o r a t i o n of a n i s o l a t e d Red Sea
t o produce b r i n e c o n c e n t r a t i o n s , c o n n a t e b r i n e s e x p e l l e d by geothermal h e a t i n g , a s
well as e more d i r e c t v o l c a n i c o r i g i n . by s o l u t i o n of Miocene e v a p o r i t e s .
Another p o s s i b i l i t y i s t h a t t h e y o r i g i n a t e d
There i s a t h i c k (2-5km) e v a p o r i t e - c l a s t i c
sequence i n t h e Red Sea which was formed f o l l o w i n g a marine i n c u r s i o n a f t e r d e p r e s s i o n o f t h e Red Sea d u r i n g t h e l a t e Eocene and e a r l y O l i g o c e n e (Hutchinson and E n g e l s , 1970).
F u r t h e r s e a f l o o r s p r e a d i n g a n d d e p o s i t i o n of deep s e a oozes
took p l a c e d u r i n g t h e P l i o c e n e and Q u a t e r n a r y .
A s s e a f l o o r s p r e a d i n g p r o g r e s s e s two l a t e r a l l y e q u i v a l e n t s u c c e s s i o n s b u i l d up on t h e now w i d e l y s e p a r a t e d c o n t i n e n t a l m a r g i n s .
I d e a l l y t h e s e c o n t i n e n t a l margins
c o n s i s t of t h i c k sequences of c o n t i n e n t a l s h e l f s e d i m e n t s and a r e u n d e r l a i n by c o n t i n e n t a l s e d i m e n t s , i n c l u d i n g r e d b e d s , which formed i n f a u l t bounded t r o u g h s d u r i n g t h e e a r l y s t a g e s of r i f t i n g .
Oceanwards t h e r e i s a t r a n s i t i o n from c o n t i n e n t a l
r i s e , through a b y s s a l p l a i n t o t h e mid-oceanic r i d g e .
I n v e r t i c a l sequence o c e a n i c
c r u s t p a s s e s upwards through p e l a g i c s e d i m e n t s i n t o t u r b i d i t e s ( F i g . l . 6 ) .
12
CONTINENTAL SHELF
CONTINENTAL RISE
OCEANIC RIDGE
ABYSSAL PLAIN
Pillowed tholelite
C r o s s - s e c t i o n of t h e w e s t e r n A t l a n t i c showing r e l a t i o n s h i p s of c o n t i n e n t a l Fig. 1 . 6 . c r u s t , o c e a n i c c r u s t and s e d i m e n t s ( a f t e r Dewey and B i r d , 1970). Oceanic r i d g e s themselves a r e s t r u c t u r a l l y complex and may be b l o c k f a u l t e d ( e . g . M i d - A t l a n t i c r i d g e ) w i t h normal f a u l t i n g p a r a l l e l t o t h e r i d g e t r e n d and t r a n s f o r m f a u l t i n g perpendicular t o i t .
T h i s produces a complex s y s t e m of r i d g e s and b a s i n s
which a r e t r a n s v e r s e and p a r a l l e l t o t h e r i d g e t r e n d .
Some o f t h e s e a r e up t o
5,000111 deep (Van Andel and Komar, 1969; Van Andel e t a l . ,
1971) and c o n t a i n t h i c k
sequences (500-1,200m) of l a r g e l y u n d i s t u r b e d f i n e g r a i n e d t u r b i d i t e s w i t h p i l l o w l a v a s , i n t r u s i o n s and s e d i m e n t a r y b r e c c i a s a l o n g t h e boundary f a u l t
1975).
s c a r p s (Arcyana,
Away from t h e ocean r i d g e s t h e newly formed l i t h o s p h e r e c o o l s and s i n k s t o
deeper o c e a n i c d e p t h s and t h e e a r l i e r formed r i d g e s e d i m e n t s a r e o v e r l a i n by p e l a g i c sediments.
The p r e c i s e n a t u r e o f t h e s e depends upon a numberof f a c t o r s s u c h a s
o c e a n i c c i r c u l a t i o n , t h e c a r b o n a t e compensation d e p t h and t h e p r o x i m i t y of c l a s t i c sources such a s i s l a n d a r c s .
The p e l a g i c s e d i m e n t s of ocean r i d g e s and a b y s s a l
p l a i n s o f t e n c o n t a i n l a r g e amounts o f hydrogenous manganese n o d u l e s which c o n s i s t o f i r o n and manganese oxyhydroxides w i t h s i g n i f i c a n t c o n c e n t r a t i o n s o f Co, N i and Cu.
These form on t h e f l a n k s of ocean r i d g e s , seamounts and a b y s s a l p l a i n s , m o s t l y
a t d e p t h s g r e a t e r t h a n 4,000m.
13 convergent D l a t e margins Convergent p l a t e margins a r e c h a r a c t e r i z e d by t h e s u b d u c t i o n of o c e a n i c l i t h o s p h e r e b e n e a t h a v o l c a n i c a l l y a c t i v e o v e r r i d i n g p l a t e and t h e development o f a r c t r e n c h systems (Dickinson, 1974a).
Arc-trench systems c a n b e b r o a d l y equated w i t h
t h e e u g e o s y n c l i n e s o f e a r l i e r workers (Dickinson, 1974b) and have t h e f o l l o w i n g f i v e morphotectonic e l e m e n t s :
1) t h e t r e n c h , a b a t h y m e t r i c deep f l o o r e d by ocean c r u s t ;
2 ) t h e s u b d u c t i o n zone b e n e a t h t h e i n n e r w a l l of t h e t r e n c h and t h e t r e n c h s l o p e
break;
3 ) t h e a r c - t r e n c h gap, a b e l t w i t h i n which a f o r e a r c b a s i n may o c c u r between
t h e t r e n c h s l o p e b r e a k and t h e magmatic a r c ; i n t r a - a r c b a s i n s may o c c u r ;
4 ) t h e magmatic a r c , w i t h i n which
and 5 ) t h e back a r c a r e a , w i t h i n which may l i e e i t h e r
a n i n t e r a r c b a s i n f l o o r e d by o c e a n i c c r u s t and s e p a r a t e d from t h e r e a r o f t h e a r c by normal f a u l t s , on a r e t r o a r c b a s i n f l o o r e d by c o n t i n e n t a l basement and s e p a r a t e d from t h e r e a r of t h e a r c by a t h r u s t f a u l t system
(Fig.l.7).
Forearc a r e a s The f o r e a r c a r e a comprises t r e n c h , s u b d u c t i o n zone and a r c - t r e n c h gap.
The
ocean f l o o r seaward of t h e submarine t r e n c h i s made up o f p e l a g i c sediments and t u f f s o v e r l y i n g b a s a l t i c ocean c r u s t .
Within t h e submarine t r e n c h t h e r e i s o n l y a r e l a t i v e -
l y t h i n sedimentary sequence ( n o r m a l l y a b o u t 1,OOOm). Thickness v a r i a t i o n s r e s u l t
from f l u c t u a t i o n s i n t h e s u p p l y o f c l a s t i c d e t r i t u s .
When t h i s i s low t h i n sequences
(50Om) o f p e l a g i c and hemipelagic sediment a r e d e p o s i t e d and g r e a t e r s u p p l y r e s u l t s i n t h e d e p o s i t i o n of t h i c k t u r b i d i t e sequences ( 2 , 5 0 0 1 ~ ) . Seismic p r o f i l e s show t h a t t h e s e a r e l a r g e l y undeformed b u t t h i s i s t o be e x p e c t e d i f t h e r a t e o f sediment s u p p l y i s much h i g h e r t h a n t h e r e l a t i v e r a t e of p l a t e consumption and s u b d u c t i o n zones a r e masked by submarine f a n s ( S i l v e r , 1969).
I t seems u n l i k e l y t h e r e f o r e t h a t
submarine t r e n c h e s a r e t h e s i t e of t h i c k g e o s y n c l i n a l accumulations (Von Huene, 1974). The subduction zone i t s e l f i s d i r e c t l y u n d e r l a i n by deformed and u p l i f t e d oceanics t r a t a most probably c o n s i s t i n g of melanges, crumpled beds and o p h i o l i t e s , and s l i c e d by t h r u s t s .
The s u b d u c t i o n complex i s t h u s i n f e r r e d t o grow by t h e s u c c e s s i v e
increment o f o c e a n i c m a t e r i a l s which a r e jammed a g a i n s t t h e seaward f l a n k a t t h e t r e n c h a x i s o r s c r a p e d from t h e upper s u r f a c e o f t h e descending p l a t e .
The g r e a t
t h i c k n e s s e s of s u b d u c t i o n complexes would seem t h e r e f o r e t o be of t e c t o n i c r a t h e r than s t r a t i g r a p h i c o r i g i n , The a r c - t r e n c h gap ( a c c r e t i o n a r y p r i s m o f K a r i g and Sharman, 1 9 7 5 ) c o m p r i s e s a n o u t e r a r c r i d g e formed from s t a c k s o f t h r u s t s h e e t s which i n d i v i d u a l l y young towards t h e magmatic a r c b u t whose o v e r a l l a g e d e c r e a s e s towards t h e t r e n c h ( K a r i g and Sharman, 1975; Moore, 1975).
Within t h i s zone i s a f o r e a r c b a s i n which may b e
50-lOOkm wide and o v e r 5 , 0 0 0 km long.
These f o r e a r c b a s i n s c o n t a i n t h i c k sequences
of predominantly c l a s t i c sediments which may l a t e r a l l y i n t e r f i n g e r w i t h v o l c a n i c
14
TRENCH,
RA
ARC TSB FAB,+
+
TS B
C
IAB
* ARC
A IB
FTB /
I
C
500 Km I d e a l i z e d diagram i l l u s t r a t i n g t e c t o n i c s e t t i n g s o f sedimentary b a s i n s Fig.l.7. a s s o c i a t e d w i t h a r c - t r e n c h systems. V e r t i c a l e x a g g e r a t i o n i s X10; t r u e d i p a n g l e s o f p l a t e d e s c e n t a r e 600 i n A and 3 0 ' i n B ( a f t e r Dickinson, 1974a). rocks o f t h e magmatic a r c .
Sediments a r e d e r i v e d from t h e o u t e r a r c , t h e magmatic
a r c and by l o n g i t u d i n a l t r a n s p o r t from t h e a d j a c e n t c o n t i n e n t .
A v a r i e t y of f a c i e s
a r e r e p r e s e n t e d i n f o r e a r c b a s i n s and a l t h o u g h t u r b i d i t e s and s h e l f d e p o s i t s a r e abundant c o n t i n e n t a l sediments a r e a l s o r e p r e s e n t e d . f o r e a r c b a s i n s i s t h e 'interdeep'
One of t h e b e s t developed
of t h e Sunda a r c (Van B e m e l e n , 1949) ( F i g . l . 8 ) .
The b a s i n l i e s t o t h e west o f t h e Burma-Sumatra-Java
magmatic a r c and e a s t
Of
a b e l t of f l y s c h and u l t r a b a s i c r o c k s which form t h e o u t e r a r c of m e Arakan and t h e Andaman-Nicobar-Mentawai l i n e o f i s l a n d s .
I n t h e n o r t h t h e f o r e a r c b a s i n forms
t h e w e s t e r n t r o u g h o f Burma where o v e r 8,OOOm of l a t e Cretaceous t o P l i o c e n e m a r i n e , d e l t a i c and f l u v i a l sediments have accumulated.
The a r e a i s now occupied by t h e
r i v e r c o u r s e s of t h e Chindwin and lower Irrawaddy. The mamatic a r c Magmatic a r c s can be d i v i d e d i n t o i n t r a - o c e a n i c a r c s s i t u a t e d between a r e a s of o c e a n i c c r u s t and c o n t i n e n t a l margin a r c s s i t u a t e d on t h e margin o f a c o n t i n e n t
15 n e a r t h e c o n t i n e n t - o c e a n c r u s t a l boundary ( D i c k i n s o n , 1974b) ( F i g . l . 7 ) .
They
i n c l u d e i s l a n d a r c s backed by s h a l l o w e p i c o n t i n e n t a l s e a s a s w e l l a s t h o s e s t a n d i n g a l o n g t h e edges o f c o n t i n e n t a l landmasses.
20'
lrrawsddy Oslra
10'
10' INDIAN OCEAN
Fig.l.8.
The Sunda and Banda a r c s ( a f t e r Reading, 1978).
Magmatic a r c s form t o p o g r a p h i c r i d g e s i n t e r r u p t e d by fault-bounded b a s i n s .
These
i n t r a - a r c b a s i n s a r e m o s t l y e x t e n s i o n a l and may b e r e l a t e d t o v o l c a n o - t e c t o n i c s u b s i d e n c e , a r c h i n g t h a t accompanies u p l i f t o f p a r a c o n t i n e n t a l c r u s t , o r t o t h e development of a n i n c i p i e n t i n t e r a r c b a s i n .
The s e d i m e n t a r y f i l l i s t y p i c a l l y
v o l c a n i c l a s t i c and i n c l u d e s e p i c l a s t i c mass flow d e p o s i t s , c o a r s e p y r o c l a s t i c s , t u f f s and v o l c a n o g e n i c t u r b i d i t e s w i t h t h e c o a r s e r s e d i m e n t s b e i n g d e p o s i t e d around t h e volcanic centres.
C o n t i n e n t a l margin a r c s show a wide v a r i e t y of s e d i m e n t a r y f a c i e s
i n c l u d i n g c o n t i n e n t a l s e d i m e n t s and r e d beds ( D i c k i n s o n , 1974a). Back-arc a r e a s Back-arc a r e a e a r e t e c t o n i c a l l y complex a r e a s c o n s i s t i n g of r i d g e s and b a s i n s
16 b e h i n d t h e magmatic a r c .
Behind i n t r a - o c e a n i c a r c s t h e r e i s f r e q u e n t l y a remnant-
a r c which i s s e p a r a t e d from t h e magmatic a r c by a n i n t e r a r c b a s i n ( F i g . l . 7 ) . Behind t h e remnant a r c may b e a second i n t e r a r c b a s i n , u s u a l l y r e f e r r e d t o a s a m a r g i n a l b a s i n , which s e p a r a t e s t h e remnant a r c from t h e mainland.
I n continental
margin a r c s t h e r e i s no remnant a r c and t h e back-arc m a r g i n a l b a s i n e x t e n d s from t h e c o n t i n e n t a l margins t o t h e magmatic a r c .
These back-arc e n s i a l i c b a s i n s were
c a l l e d r e t r o a r c b a s i n s by D i c k i n s o n (1974a) ( F i g . l . 7 ) . Back-arc a r e a s a r e o f t e n c h a r a c t e r i z e d by h i g h h e a t flow and t h e narrow b e l t o f l i t h o s p h e r e b e h i n d t h e a r c - t r e n c h gap can b e c o n s i d e r e d a s a s e p a r a t e narrow p l a t e . P l a t e motion can t h u s r e s u l t i n s p l i t t i n g and t h e emplacement o f new o c e a n i c c r u s t c a u s i n g t h e f o r m a t i o n of a n i n t e r a r c b a s i n by back-arc s p r e a d i n g ( K a r i g , 1970, 1971). The sediments o f i n t e r a r c b a s i n s i n c l u d e d i s t i n c t i v e t u r b i d i t e a p r o n s
Of
v o l c a n i c l a s t i c d e t r i t u s d e r i v e d from t h e m i g r a t i n g f r o n t a l a r c and r e s t i n g d i r e c t l y on o c e a n i c c r u s t .
I n some i n t e r a r c b a s i n s t h e s i d e away from t h e a r c - t r e n c h system
may b e bounded by a submerged remnant a r c , no e f f e c t i v e c l a s t i c s o u r c e i s p r e s e n t , and p e l a g i c s e d i m e n t a t i o n dominates.
There i s t h u s no t e r r i g e n o u s i n p u t t o i n t e r a r c
b a s i n s and K a r i g and Moore (1975) r e c o g n i z e f o u r dominant t y p e s o f s e d i m e n t ( F i g . l . 9 ) including:
1) v o l c a n i c l a s t i c d e t r i t u s d e r i v e d from t h e magmatic a r c , v o l u m e t r i c a l l y
t h e most i m p o r t a n t and d e p o s i t e d p r i n c i p a l l y a s t u r b i d i t e s d e r i v e d from t h e v o l c a n i c c h a i n
2 ) montmorillonite clays
3 ) b i o g e n i c c a r b o n a t e ooze and
4 ) wind blown,
continentally derived dust. R e t r o a r c b a s i n s a r e d i s t i n c t from a c t i v e i n t e r a r c b a s i n s b e c a u s e t h e y leave a complex s e d i m e n t a r y r e c o r d w i t h a wide v a r i e t y o f f a c i e s .
Marine d e l t a i c and f l u v i a l
environments a r e r e p r e s e n t e d i n sequences up t o 5km t h i c k and d e p o s i t e d i n t e r r e s t r i a l lowlands and e p i c o n t i n e n t a l s e a s between c o n t i n e n t a l a r c s and landmasses.
These
s e d i m e n t s a r e m o s t l y d e r i v e d from t h e magmatic a r c a n d t r a n s p o r t e d i n a t r a n s v e r s e d i r e c t i o n towards t h e c r a t o n a l t h o u g h t h e r e a r e minor c o n t r i b u t i o n s from t h e c r a t o n itself. The C r e t a c e o u s r e t r o a r c b a s i n s o f t h e i n t e r i o r and Rocky Mountain r e g i o n of North America c o n t a i n a v a r i e t y o f marine and c o n t i n e n t a l s e d i m e n t s d e r i v e d from u p l i f t e d s t r a t a i n t h e f o l d - t h r u s t b e l t which was formed by p a r t i a l s u b d u c t i o n b e h i n d t h e a r c (Weimer, 1970).
The main h i g h l a n d s o u r c e s were f o l d e d and f a u l t e d p r e -
Mesozoic s t r a t a l y i n g j u s t west of t h e r e t r o a r c b a s i n b u t e a s t of t h e b a t h o l i t h b e l t which marks t h e main magmatic a r c (Hamilton, 1969).
The sub-Andean zone of t h e
Andes l i e s e a s t of t h e magmatic a r c and c o n s i s t s o f deep i n t e r m o n t a n e t r o u g h s .
Here
a v a r i e t y o f c o a r s e c o n t i n e n t a l c l a s t i c s e d i m e n t s a r e d e p o s i t e d i n a l l u v i a l f a n and f l u v i a l environments. Transform f a u l t s Transform o r s t r i k e - s l i p f a u l t s bound p l a t e s between which movement i s p r i m a r i l y
17 l a t e r a l and l i t h o s p h e r e i s conserved b e i n g n e i t h e r c r e a t e d o r d e s t r o y e d .
Primary
t r a n s f o r m f a u l t s ( G i l l i l a n d and Meyer, 1976) o c c u r a l o n g fundamental b r e a k s in t h e l i t h o s p h e r e and may have had a l o n g h i s t o r y o f t e c t o n i c a c t i v i t y l i k e t h e San Andreas f a u l t system which is o v e r 1,500km l o n g , 500km wide, and h a s been a c t i v e s i n c e t h e Oligocene.
Secondary t r a n s f o r m f a u l t s a r e much s m a l l e r s t r u c t u r e s which
r e s u l t d i r e c t l y from s e a f l o o r s p r e a d i n g . Volcaniclastic debris
Montmorillonite-rich brown clays
Volcaniclastic debris
- 4
I
41-
56-
1
Brown clay or siliceous
6 7. 8
Fig.l.9.
S e d i m e n t a t i o n model f o r back-arc b a s i n s ( a f t e r K a r i g and Moore, 1975).
The San Andreas F a u l t system today l i e s a t t h e s p l i n t e r e d boundary of t h e P a c i f i c and North American p l a t e s , a t e c t o n i c arrangement t h a t a r o s e i n t h e O l i g o c e n e (Fig.l.10).
Transform movement between t h e North American and P a c i f i c p l a t e s has
v a r i e d i n t h e p a s t b u t is c u r r e n t l y a b o u t 5.0cm/year (Atwater and Molnar, 1973). Southern C a l i f o r n i a and n o r t h w e s t Mexico i s t h u s b e i n g deformed a s p a r t of a broad
18
C
B
A
D
.YU
NA QU
0 QU
QU
80 F
E
:: .YU
I I
I
.YU
,
0 QU
*QU
Schematic diagrams of p l a t e arrangements a l o n g t h e w e s t e r n c o a s t of North Fig.l.10. America. P = P a c i f i c , K = K u l a , F = F a r a l l o n , N = North American p l a t e . SE = S e a t t l e , SA = Sacramento, YU = Yurna, G U = G u a d a l a j a r a . Dashed l i n e e n c l o s e s r e g i o n of p l a t e boundary t e c t o n i c complexity. Age i n Ma g i v e n a t lower r i g h t o f e a c h s k e t c h ( a f t e r C r o w e l l , 1979). t r a n s f o r m zone.
W i t h i n t h i s zone t h e o r i g i n of s e v e r a l modern s e d i m e n t a r y b a s i n s
i n c l u d i n g t h e S a l t o n Trough and t h e Gulf of C a l i f o r n i a c a n b e r e l a t e d t o t h e i r p o s i t i o n a t o r n e a r t h e p l a t e boundary ( F i g . 1 . 1 1 ) . Crowell (1974a) h a s shown how t h e s e modern, and o l d e r , s e d i m e n t a r y b a s i n s originate i n a strike-slip tectonic setting. a r e recognized:
Two main t y p e s of s e d i m e n t a r y b a s i n
p u l l - a p a r t b a s i n s and f a u l t - w e d g e . b a s i n s .
The former o c c u r where a l o n g and s t r a i g h t s t r i k e - s l i p f a u l t h a s a s h a r p d o u b l e bend (anastomosing f a u l t ) t h a t s i d e s t e p s t h e main f a u l t t r e n d .
Such double bends
can b e r e s t r a i n i n g , r e s u l t i n g i n o v e r l a p and e l e v a t i o n a t t h e bends ( F i g . 1 . 1 2 ) , r e l e a s i n g i n which a p u l l - a p a r t and s u b s i d e n c e r e s u l t s . a n i d e a l i z e d p u l l - a p a r t b a s i n a r e shown i n F i g . l . 1 3 .
01:
The c h a r a c t e r i s t i c s of
Examples i n c l u d e t h e p r e s e n t
day Gulf o f C a l i f o r n i a and S a l t o n Trough which a r e widening and l e n g t h e n i n g a s c o n t i n e n t a l t e r r a n e t o t h e west moves o b l i q u e l y away from t h e mainland p a r t o f North America.
The S a l t o n Trough c o n t a i n s o v e r 6,000m of
f l u v i a l , l a c u s t r i n e and
26
24
. i
22
30
18
. .:
:.
...:
: .
10
0
14
100 200
19 12
300
km.
40
38
\\
\
36
34
32
30 /
Major fault
6Edg, of basin floor 28
......
Continental margin
Fig.l.11. Major f a u l t s and p r e s e n t s e d i m e n t a t i o n s i t e s of C a l i f o r n i a and Baja C a l i f o r n i a ( a f t e r C r o w e l l , 1974b). marine s e d i m e n t s .
The b a s i n margins c o n t a i n c o a r s e f a n g l o m e r a t e s and a r e marked
by u n c o n f o r m i t i e s w i t h a b r u p t changes i n f a c i e s and t h i c k n e s s .
Much of t h e sediment
i n t h e S a l t o n Trough h a s been s u p p l i e d by t h e a n c e s t r a l and p r e s e n t day Colorado
20 river,
R ESTRA l NIN G BEND
RELEASING BEND
B
SHARP RESTRAINING BEND
SHARP PULL-APART
Fig.l.12. R i g h t s l i p on f a u l t w i t h marked double bends (A) r e s u l t s i n p u l l - a p a r t a t r e l e a s i n g bends (B) a n d d e f o r m a t i o n ( f o l d and t h r u s t f a u l t s ) a t r e s t r a i n i n g bends (C) ( a f t e r C r o w e l l , 1974a). Fault-wedge b a s i n s develop i n a system of anastamosing s u b - p a r a l l e l f a u l t s , a l l w i t h t h e same d i r e c t i o n o f o f f s e t .
With l a t e r a l movement t h e s e s p l i t and r e j o i n
l e a d i n g t o t h e i r convergence, compression and u p l i f t , o r t o d i v e r g e n c e , e x t e n s i o n and s u b s i d e n c e ( F i g . l . 1 4 ) . formed i n t h i s way.
The Ridge Basin ( F i g . l . 1 5 ) i s a m a j o r s e d i m e n t a r y b a s i n
I t o r i g i n a t e d a s a narrow d e p r e s s i o n bounded on t h e
southwest
by t h e s c a r p of t h e San G a b r i e l F a u l t zone and on t h e n o r t h e a s t by h i g h l a n d s r a i s e d by f o l d i n g and by movements on t h e C l e a r w a t e r , L i e b r e , and o t h e r f a u l t s ( F i g . l . 1 5 A ) . The sediments o f t h e Ridge Basin a r e o v e r 12,000m t h i c k and i n c l u d e a v a r i e d assemblage of a l l u v i a l f a n , l a c u s t r i n e and m a r g i n a l marine d e p o s i t s d e p o s i t e d d u r i n g t h e i n t e r v a l from l a t e Miocene t o e a r l y P l e i s t o c e n e .
The P l i o c e n e d e p o s i t s c o n s i s t
mainly of c o a r s e f l u v i a l conglomerates and s a n d s t o n e s d e p o s i t e d a l o n g t h e b a s i n margins and mudstone, s h a l e and s a n d s t o n e d e p o s i t e d under l a c u s t r i n e c o n d i t i o n s i n the c e n t r a l p a r t of the basin.
Rocks which f l o o r and s u r r o u n d t h e Ridge B a s i n
c o n s i s t of g r a n i t e s , g n e i s s e s , and o t h e r metamorphics a s w e l l as T e r t i a r y c l a s t i c s .
21
I
Stretched and attenuated
W
Complex unconformities and overlaps in subsurface Complex intersection Oldest intact basin f i l l Remnants of marginal -.. rocks within volcanics
1
Talus breccias 2nd rapld ’ facies changes basinward Small thrust plates -’ , , Slide blocks
Braided right -slip zone
Fig.l.13.
I
\
]Oblique
result of convergence between boundary righ! slip faults
S k e t c h map o f a n i d e a l i z e d p u l l - a p a r t b a s i n ( a f t e r Crowell, 1974a).
The V i o l i n B r e c c i a , a c o a r s e sedimentary b r e c c i a a l o n g t h e San G a b r i e l F a u l t , c o n t a i n s c l a s t a i n t h e lower p a r t which were d e r i v e d from a l i m i t e d s o u r c e a r e a . T h i s s o u r c e a r e a i s now o f f s e t by 28km a l o n g t h e San G a b r i e l F a u l t (Crowell, 1974b). Upper Miocene marine beds on t h e southwest s i d e of t h e
San G a b r i e l F a u l t have a l s o
been d i s p l a c e d from t h e i r s o u r c e a r e a by t h e same amount (Crowell, 1952).
These
f e a t u r e s a r e c o n s i s t e n t w i t h t h e o r i g i n o f t h e Ridge Basin a s a fault-wedge “hollow” r e s u l t i n g from s t r i k e - s l i p ( F i g . 1.15A). Continental c o l l i s i o n ~
~~
When c o n t i n e n t a l masses a r e b r o u g h t t o g e t h e r by s u b d u c t i o n o r s t r i k e - s l i p movement c o l l i s i o n o c c u r s r e s u l t i n g i n f o l d i n g , t h r u s t i n g and o r o g e n e s i s .
A
consequence o f c o n t i n e n t a l c o l l i s i o n s of t h i s t y p e i s t h e consumption o f n e a r l y a l l t h e o c e a n i c l i t h o s p h e r e and t h e s u t u r i n g o f t h e two a r e a s of c o n t i n e n t a l c r u s t . The c l o s u r e o f o c e a n i c a r e a s i n t h i s way i s r a r e l y c o m p l e t e l y r e g u l a r o r symmetrical and t h e r e may be r e l i c s o f ocean b a s i n p r e s e r v e d between c o l l i s i o n p o i n t s .
r e l i c t b a s i n s a r e r e f e r r e d t o a s remnant-ocean b a s i n s (Graham e t a l . , 1975).
These
As
c o l l i s i o n proceeds remnant-ocean b a s i n s a r e p a r t l y f i l l e d by sediment d e r i v e d from t h e newly formed mountains, and new b a s i n s a r e c r e a t e d .
These i n c l u d e p e r i p h e r a l
b a s i n s (Dickinson, 1974b) which form on t h e s u b d u c t i n g p l a t e o r upon t h e o v e r r i d i n g
22
p l a t e a s modified r e t r o a r c basins.
II Divergence
S k e t c h map t o show u p l i f t a t convergence and s u b s i d e n c e a t d i v e r g e n c e of Fig.l.14. s t r i k e - s l i p f a u l t s ( a f t e r Crowell, 1974a). I n a n c i e n t o r o g e n i c b e l t s i t i s common p r a c t i c e t o d i s t i n g u i s h between f o r e l a n d b a s i n s which a r e s i t u a t e d on t h e s u b d u c t i n g p l a t e from h i n t e r l a n d b a s i n s which a r e formed on t h e o v e r r i d i n g p l a t e .
F o r e l a n d and h i n t e r l a n d b a s i n s a r e a l s o c o m o n l y
d i s t i n g u i s h e d from s u c c e s s o r b a s i n s (King, 1 9 6 6 ) which a r e , i n f a c t , t h e epieugeos y n c l i n e s of Kay (1951).
S u c c e s s o r b a s i n s , b e c a u s e t h e y l i e on f o l d e d e u g e o s y n c l i n a l
r o c k s , p r o v i d e a n i m p o r t a n t r e c o r d of t h e l a t e r s t a g e s o f o r o g e n e s i s ( E i s b a c h e r , 1974). The s e d i m e n t a r y b a s i n s which r e s u l t from t h e e r o s i o n of u p l i f t e d mountain r a n g e s a r e often c o l l e c t i v e l y referred t o a s late-orogenic basins.
A t the present t i m e
t h e s o u t h e r n f l a n k o f t h e a c t i v e l y r i s i n g Himalayas c o n t a i n a number of b a s i n s which a r e up t o 500 km wide and e x t e n d f o r o v e r 1,000 km ( F i g . l . 1 6 ) . These b a s i n s a r e p r i m a r i l y p a r a l l e l t o t h e main t e c t o n i c t r e n d a s a r e t h e main d r a i n a g e c o u r s e s , a l t h o u g h t h e r e a r e a l s o t r a n s v e r s e elements ( F i g . 1.16);.
A l l u v i a l f a c i e s have been
a c c u m u l a t i n g i n t h e s e b a s i n s s i n c e Miocene times and t h e o l d e r d e p o s i t s a l o n g t h e n o r t h e r n margin of t h e a l l u v i a l a p r o n a r e f o l d e d and t h r u s t e d s o t h a t t h e d e p o s i t i o n a l a x i s of t h e b a s i n has m i g r a t e d p r o g r e s s i v e l y southwards w i t h t i m e .
The
23
Upth~d block
Extension & Subsidence
Fig.l.15.A.Block diagram i l l u s t r a t i n g o r i g i n o f t h e Ridge Basin a t a curve i n San Andreas F a u l t ( a f t e r C r o w e l l , 1974b). B. F a c i e s , r e l a t i o n s h i p s i n Ridge Basin showing v e r t i c a l t r a n s i t i o n of marine t o l a c u s t r i n e s e d i m e n t s and l a t e r a l p a s s a g e of a l l u v i a l f a n g l o m e r a t e s through m a r g i n a l l a c u s t r i n e f a c i e s i n t o o f f s h o r e l a c u s t r i n e f a c i e s ( a f t e r C r o w e l l , 1975; Link e t a l . , 1978). The d e t a i l e d r e l a t i o n s h i p s between t e c t o n i c s and a l l u v i a l s t r a t i g r a p h y i n one of these basins
( t h e Jhelum R e - e n t r a n t , P a k i s t a n ) h a s been i n v e s t i g a t e d by V i s s e r and
Johnson (1978).
T h i s formed p a r t o f a r a p i d l y s u b s i d i n g f o r e d e e p which developed
d u r i n g t h e Neogene and Q u a t e r n a r y a s a r e s u l t o f southward d i r e c t e d t e c t o n i s m and r a p i d morphogenetic u p l i f t .
I t p e r s i s t s today a s t h e Indo-Gangetic b a s i n and
c o n t a i n s a sequence of p r e - t e c t o n i c E a r l y Palaeogene c a r b o n a t e s o v e r l a i n by L a t e Palaeogene f l u v i a l - d e l t a i c molasse (Rawalpindi Group) and t h e younger f l u v i a l S i w a l i k Group,
The environment and r a t e of f l u v i a l d e p o s i t i o n i n t h e Upper p a r t
of t h e S i w a l i k Group i n t h e Jhelum R e - e n t r a n t was c o n t r o l l e d by l a r g e - s c a l e s y n t a x i a l t e c t o n i c s i n t h e n o r t h w e s t e r n Himalaya.
V i s s e r and Johnson (1978)
i d e n t i f i e d a n i s o c h r o n o u s i n t e r v a l of f l u v i a l d e p o s i t i o n between a c o r r e l a t i v e
PZ
25 p a i r of v o l c a n i c a s h h o r i z o n s and s p a n n i n g t h e Gauss/Matuyama p o l a r i t y t r a n s i t i o n
(2.4My) ( F k . 1 . 1 7 ) .
0REVERSED POLARITY NORMAL POLARITY
F R4
FR3
M P8
MP5
RT1
FR1
RT2
CH1
Fig. 1.17. L i t h o s t r a t i g r a p h i c v a r i a t i o n a t e i g h t l o c a l i t i e s i n t h e Jhelum R e - e n t r a n t , P a k i s t a n showing upper S i w a l i k r o c k s bounded by two v o l c a n i c a s h h o r i z o n s and t h e p o s i t i o n of t h e Gauss/Matuyama p o l a r i t y t r a n s i t i o n . L i t h o l o g i c symbols: L a t e r a l a c c r e t i o n s a n d s t o n e f a c i e s , s t i p p l e d showing dominant bedform; v e r t i c a l a c c r e t i o n mudstone f a c i e s w i t h d e p o s i t i o n a l f a b r i c p r e s e r v e d , no p a t t e r n ; mudstone-facies m o d i f i e d by p e d o g e n e s i s , h a t c h e d ( a f t e r V i s s e r and Johnson, 1 9 7 8 ) . These f l u v i a l d e p o s i t s c o n s i s t of markedly fining-upwards c y c l e s w i t h w e l l developed l a t e r a l a c c r e t i o n u n i t s ( p o i n t b a r s a n d s ) and v e r t i c a l a c c r e t i o n u n i t s ( f l o o d p l a i n mudstones).
The d e t a i l e d sequence o f t h e v e r t i c a l a c c r e t i o n d e p o s i t s
i n d i c a t e s t h a t t h e f l o o d p l a i n environment c o n s i s t e d o f a stream-proximal, sedimentation-dominant zone and a s t r e a m - d i s t a l , pedogenesis-dominant zone.
The depegdence o f t h e a l l u v i a l s t r a t i g r a p h y on t e c t o n i c s i s i l l u s t r a t e d b y two variables:
t h e p r o p o r t i o n of l a t e r a l a c c r e t i o n s a n d i n any s e c t i o n and a l s o t h e
t o t a l t h i c k n e s s of s e d i m e n t between t h e two d a t e d v o l c a n i c a s h h o r i z o n s .
The
p r o p o r t i o n of l a t e r a l a c c r e t i o n s a n d b o d i e s i s , of c o u r s e , dependent on a g r e a t v a r i e t y of e n v i r o n m e n t a l v a r i a b l e s b u t i n s p i t e o f t h i s t h e r e a p p e a r s t o b e a r e l a t i o n s h i p between t h e t o t a l t h i c k n e s s of l a t e r a l a c c r e t i o n d e p o s i t s and t h e p r o x i m i t y t o major f l u v i a l c o u r s e s .
The t h i c k n e s s of t h e i n t e r v a l e n c l o s e d by t h e
v o l c a n i c a s h e s embodies a d i r e c t measure of t h e s e d i m e n t a t i o n r a t e which can b e c a l c u l a t e d on t h e b a s i s of t h e palaeomagnetic d a t i n g .
The r a t e v a r i e s from a low
of 0.33m/1000 y e a r s i n t h e s o u t h e a s t (FR1 i n F i g . l . 1 7 ) t o a h i g h of 0.4m/lOOO y e a r s
i n t h e n o r t h w e s t (FR4 i n F i g . l . 1 7 ) .
This defines a regular trend of increasing
sediment r a t e towards t h e apex of t h e Jhelum R e - e n t r a n t i n d i c a t i n g t h a t t h e Ree n t r a n t s t r u c t u r e was a m a j o r f o c u s of molasse s e d i m e n t a t i o n .
The s i g n i f i c a n c e o f
R e - e n t r a n t s a p p e a r s t o b e t h a t t h e y form p r e f e r e n t i a l s i t e s f o r t h e p a s s a g e o f m a j o r l o n g i t u d i n a l i n t e r m o n t a n e r i v e r s i n t o t h e molasse b a s i n ,
The p r e c i s e t e c t o n i c
s i g n i f i c a n c e o f R e - e n t r a n t s i s , however, n o t y e t e s t a b l i s h e d .
The f l e x u r e o f t h e
n o r t h w e s t e r n Himalayan s y n t a x i s c o u l d be due t o a northward p r o j e c t i n g s p u r of t h e Indian lithospheric p l a t e c o l l i d i n g with the Eurasian p l a t e ,
Crawford ( 1 9 7 4 ) , however,
b e l i e v e s t h e s y n t a x i s t o b e a s e c o n d a r y f e a t u r e formed between converging E u r a s i a n c r u s t a l b l o c k s t o t h e e a s t and w e s t of t h e s y n t a x i s . THE TECTONIC SETTING OF ANCIENT CONTINENTAL RED BEDS
Introduction The p r e v i o u s s e c t i o n i l l u s t r a t e s t h e f a c t t h a t c o n t i n e n t a l s e d i m e n t s may accumulate i n a wide range of t e c t o n i c s e t t i n g s from p r e - o r o g e n i c r i f t s t o l a t e - o r o g e n i c b a s i n s . A t t h e p r e s e n t - d a y t h e s e c o n t i n e n t a l s e d i m e n t s do n o t c o n t a i n abundant r e d beds
a l t h o u g h i n some c a s e s t h e p r o g r e s s i v e reddening o f t h e o l d e r s e d i m e n t s can b e demonstrated. I n t h e s t r a t i g r a p h i c r e c o r d s t r u c t u r a l and s t r a t i g r a p h i c a n a l y s i s can b e used t o demonstrate t h a t a n c i e n t c o n t i n e n t a l s e d i m e n t s formed i n s i m i l a r t e c t o n i c s e t t i n g s t o t h e i r p r e s e n t - d a y c o u n t e r p a r t s a s f a r back a s t h e P r o t e r o z o i c .
Not a l l t h e s e a n c i e n t
c o n t i n e n t a l sediments a r e r e d b u t t h e r e i s a marked abundance of r e d beds compared t o t h e p r e s e n t - d a y , a f e a t u r e which i n d i c a t e s t h e importance of d i a g e n e t i c reddening. I n t h e s u c c e e d i n g s e c t i o n s t h e t e c t o n i c s e t t i n g o f a number o f d i f f e r e n t r e d bed basins i s described.
They a r e assembled i n s t r a t i g r a p h i c a l sequence and i l l u s t r a t e
t h e wide range o f t e c t o n i c s e t t i n g s i n which a n c i e n t c o n t i n e n t a l r e d beds a r e formed. P r o t e r o z o i c r e d beds of w e s t e r n Canada The e a r l i e s t major r e d bed sequences t o a p p e a r i n t h e s t r a t i g r a p h i c r e c o r d a r e
27
ATHAPUSCOW AULACOOEN
SLAVE PLATFORM
Sio"?"
-
0
SOUTH
D
-
D
-
x) liu
'
NORTH
F i g . l . 1 8 . Stratigraphic cross-section of the Athapuscow aulacogen and adjacent platform with structural cross-sections of the northeast half (above) and southwest half (below) ( a f t e r Hoffman e t a l . , 1974).
28
GRABEN STAGE
TRANSITIONAL STAGE
DOWNWARPING STAGE
POST-GEOSYNCLINAL STAGE
CHURCHILL PROVINCE-ATHAPUSCOW FANGLOMERATE
0
RED LlTHlC SANDSTONE GREYWACKE TURBIDITES
0
QUARTZITE
..... . PEBBLY SUBARKOSE
AULACOGEN
[lkm
10 km
-SLAVE PROVINCE OLISTOSTROME MUDSTONE CARBONATE
@
QUARTZ DlORlTE VOLCANICS
F i g . l . 1 9 . Schematic transverse c r o s s - s e c t i o n s showing the e v o l u t i o n o f the Athapuscow aulacogen ( a f t e r Hoffman e t a l . , 1974).
29
t h o s e a s s o c i a t e d w i t h t h e C o r o n a t i o n g e o s y n c l i n e and Athapuscow a u l a c o g e n (Hoffman, 1973; Hoffman e t a l . , 1974) o f w e s t e r n Canada. i n Fig.8.1.
A t e c t o n i c map of t h e a r e a i s shown
The sequence i n t h e Athapuscow a u l a c o g e n t h i c k e n s from 2200m i n t h e
S l a v e P l a t f o r m i n t h e n o r t h t o o v e r 7000m i n t h e main p a r t of t h e a u l a c o g e n .
Within
t h i s framework a v a r i e t y o f d e p o s i t i o n a l p h a s e s c a n b e r e c o g n i z e d i n c l u d i n g a major molasse sequence, t h e C h r i s t i e Bay Group ( F i g . l . 1 8 ) . i n t h e development o f t h e Athapuscow a u l a c o g e n :
Four s t a g e s c a n b e r e c o g n i z e d
Graben, t r a n s i t i o n a l , downwarping
and p o s t - g e o s y n c l i n a l which i s v e r y s i m i l a r t o t h e developmental sequence s e e n i n o t h e r a u l a c o g e n s (Ham, 1969).
The molasse p h a s e d e v e l o p s d u r i n g t h e downwarping
s t a g e and b e g i n s w i t h a g i g a n t i c o l i s t o s t r o m e i n which a n g u l a r b l o c k s o f stromatol i t i c d o l o m i t e and l i m e s t o n e a r e c h a o t i c a l l y d i s p e r s e d i n a r e d mudstone m a t r i x , I n d i v i d u a l o l i s t o l i t h s a r e up t o 45m t h i c k and a k i l o m e t r e i n l e n g t h and many a r e recumbently f o l d e d or emplaced u p s i d e down.
The o l i s t o s t r o m e i s o v e r l a i n by a 450m
t h i c k sequence o f r e d mudstone w i t h hopper-shaped h a l i t e c a s t s which p a s s e s up i n t o 850m o f r e d cross-bedded l i t h i c s a n d s t o n e s o f a l l u v i a l o r i g i n which were d e p o s i t e d by r i v e r s which flowed towards t h e n o r t h e a s t a l o n g t h e a x i s o f t h e aulacogen.
The
s a n d s t o n e s a r e o v e r l a i n by 230m of r e d mud-cracked s i l t s t o n e s w i t h h a l i t e and gypsum casts. The downwarp s t a g e was followed by a s t a g e o f m i l d c o m p r e s s i o n a l d e f o r m a t i o n ( F i g . l . 1 9 ) i n which t h e molasse and e a r l i e r s e d i m e n t s were f o l d e d .
A t h i c k sequence
o f f a n g l o m e r a t e s was d e p o s i t e d on t h e u p t u r n e d edges of t h e s e o l d e r r o c k s d u r i n g t h e post-geosynclinal stage.
T h i s sequence c o n s i s t s o f 4,OOOm of f l u v i a t l e r e d and b u f f
b o u l d e r conglomerates a n d p e b b l y s a n d s t o n e c l o s e l y a s s o c i a t e d w i t h a l k a l i c b a s a l t flows.
The conglomerate c l a s t s r e c o r d p r o g r e s s i v e e r o s i o n o f p l a t f o r m c o v e r from t h e
u p l i f t and u n r o o f i n g o f t h e basement.
They t e n d t o o c c u r i n homoclines d i p p i n g
p a r a l l e l t o t h e t r e n d of t h e f a u l t s from which t h e y were d e r i v e d .
T h i s f e a t u r e , and
t h e r e c o g n i t i o n of d e x t r a l t r a n s c u r r e n t movement on some of t h e boundary f a u l t s ( R e i n h a r d t , 19691, s u g g e s t s t h a t t h e f a n g l o m e r a t e s may have been d e p o s i t e d i n s t r i k e s l i p b a s i n s l i k e t h o s e d e s c r i b e d by Crowell (1974a). The Caledonian Orogen The Caledonian o r o g e n i c b e l t s t r e t c h i n g through G r e e n l a n d , NW Europe and North America c a n b e i n t e r p r e t e d i n terms o f t h e opening and c l o s u r e of a
proto-Atlantic
o r I a p e t u s ocean (Wilson, 1966; Dewey, 1969; M i t c h e l l and McKerrow, 1975; P h i l l i p s e t a l . , 1976; McKerrow e t a l . , 1977).
Sediments a s s o c i a t e d w i t h t h e e v o l u t i o n of
t h e Caledonian orogen range i n a g e from L a t e Precambrian t o Devonian.
Continental
r e d beds were formed i n t h e i n i t i a l phase of opening i n t h e L a t e Precambrian and a l s o d u r i n g and a f t e r t h e f i n a l c l o s u r e of the p r o t o - A t l a n t i c i n Devonian times. The Sparagmite s u c c e s s i o n o f s o u t h e r n Norway c o n s i s t s o f a complex sequence of greywackes, s h a l e s , c a r b o n a t e s , a r k o s e s , c o n g l o m e r a t e s , and t i l l i t e s up t o 3000m
30 thick.
Red b e d s , p a r t i c u l a r l y amongst t h e a r k o s i c s e d i m e n t s , a r e abundant.
The
sequence i s b e l i e v e d t o have been d e p o s i t e d i n a r i f t v a l l e y which formed d u r i n g t h e i n i t i a l opening of t h e p r o t o - A t l a n t i c (Bjbrlykke e t a l . ,
1976) ( F i g . l . 2 0 ) .
The v a l l e y i s e s t i m a t e d t o have been 60-70km a c r o s s and 2-4km deep.
I n the central
p a r t of t h e b a s i n s e d i m e n t a t i o n was dominated by s h a l e s and greywackes which p a s s upwards i n t o s h a l l o w marine sediments i n c l u d i n g t i l l i t e s .
The a r k o s e s and cong-
lomerates dominated t h e b a s i n margins and were d e p o s i t e d a s c o a r s e c l a s t i c wedges ( f a n d e l t a s ) which p e r i o d i c a l l y prograded i n t o t h e b a s i n .
The L a t e Pre-Cambrian
T o r r i d o n i a n Sandstone of NW S c o t l a n d a c o n t i n e n t a l r e d bed sequence formed i n a s i m i l a r t e c t o n i c s e t t i n g t o t h e s p a r a g m i t e s of Scandanavia.
I t formed t h e north-
w e s t e r l y margin of t h e p r o t o - A t l a n t i c ocean d u r i n g t h e i n i t i a l s t a g e s o f r i f t i n g and s e a - f l o o r s p r e a d i n g . The T o r r i d o n i a n sequence rests w i t h marked unconformity on t h e Lewisian basement
-
i n p l a c e s a p r e - T o r r i d o n i a n rugged topography w i t h a r e l i e f o f 300-4Oh i s e v i d e n t . The unconformity i s o v e r l a i n by a t l e a s t 2300m of t h e S t o e r Group c o n s i s t i n g of r e d s a n d s t o n e s and s h a l e s ,
T h i s i t s e l f i s unconformably o v e r l a i n by t h e T o r r i d o n Group Most
c o n s i s t i n g of a t l e a s t 7000m o f r e d pebbly s a n d s t o n e s , s i l t s t o n e s and s h a l e s .
o f t h e s e r e d beds a r e of a l l u v i a l o r i g i n and i n c l u d e b r a i d e d s t r e a m d e p o s i t s ( S e l l e y ,
1 9 7 0 ) , a l t h o u g h minor marine h o r i z o n s do o c c u r ( G r a c i e and S t e w a r t , 1967).
Williams
(1969a) h a s shown t h a t t h e p a l a e o s l o p e was g e n e r a l l y t o t h e e a s t and s o u t h e a s t w i t h t h e p a l a e o c u r r e n t s r a d i a t i n g from two p o i n t s o u r c e s west of t h e p r e s e n t o u t c r o p n e a r t h e l i n e of t h e Minch F a u l t ( F i g . l . 1 1 ) .
The p r e s e n c e o f a t l e a s t two a l l u v i a l f a n s
i s i m p l i e d and c o n t i n e n t a l d e p o s i t i o n i n t h i s L a t e Precambrian r i f t may have been
c o n c e n t r a t e d a l o n g a t e c t o n i c a l l y c o n t r o l l e d , n o r t h - w e s t e r l y r e t r e a t i n g Lewisian scarp slope.
T h i s i s c o n s i s t e n t w i t h c l a s t s t u d i e s o f t h e T o r r i d o n i a n Group which
confirm t h e i r d e r i v a t i o n from rocks which span t h e range o f Lewisian e v e n t s (3000-1000
My)but which a r e from a h i g h e r c r u s t a l l e v e l t h a n i s now exposed
( A l l e n e t a l . , 1974) ( F i g . l . 2 1 ) . The subsequent c l o s u r e of t h e p r o t o - A t l a n t i c ocean d u r i n g t h e O r d o v i c i a n and S i l u r i a n r e s u l t e d i n c o n t i n e n t a l c o l l i s i o n and t h e f o r m a t i o n of t h e Caledonian Mountains.
T h i s c a n be i n t e r p r e t e d t o have o c c u r r e d f i r s t i n t h e n o r t h e a s t and
l a t e r towards t h e south-west a s a t r i p l e p o i n t m i g r a t e d i n t h i s d i r e c t i o n ( P h i l l i p s e t a l . , 1976).
The e v o l u t i o n o f t h e S c o t t i s h Caledonides c a n a l s o be compared
c l o s e l y w i t h present-day n o r t h e a s t e r n Burma ( M i t c h e l l and McKerrow, 1975).
The
molasse f a c i e s o f t h e Caledonian orogen i s r e f e r r e d t o a s t h e Old Red Sandstone. I t i s a major c o n t i n e n t a l r e d b e d sequence r a n g i n g i n age from l a t e S i l u r i a n t o
Devonian which was d e p o s i t e d i n a number of d i f f e r e n t l a t e - o r o g e n i c s e d i m e n t a r y b a s i n s , some w i t h a marine c o n n e c t i o n ( e x t e r n a l b a s i n s ) and some w i t h no marine connections and i n t e r n a l d r a i n a g e p a t t e r n s ( i n t e r n a l b a s i n s ) .
31
Lower Cambrian
VanQsAs Formation O e I I a x and Coastal
Feldsoalhlc Sandstone
Moelv Tillite and Ekre Shale
< Oeoosilion 01 Glacial ' Sediments
Ring Formation
Biri Limestone and Shale
Shale
Brettuq Formation d... 1
\L
and PhOSDhate Deposits /15";"Carbonate on Shallow Shell
S i m p l i f i e d diagram i l l u s t r a t i n g t h e d e p o s i t i o n a l h i s t o r y and d i s t r i b u t i o n Fig.l.20. of f a c i e s i n t h e c e n t r a l s p a r a g m i t e b a s i n i n s o u t h e r n Norway ( a f t e r B j d r l y k k e e t a l . , 1976). The Midland V a l l e y of S c o t l a n d was a major Old Red Sandstone b a s i n which c o n t a i n s a b o u t 7000m of s e d i m e n t s and a n d e s i t i c v o l c a n i c s .
The b a s i n i s a fault-bounded
g r a b e n ( F r i e n d , 1967; Bluck, 1978) i n which t h e boundary f a u l t s a r e p a r a l l e l t o t h e main Caledonian t e c t o n i c elements. The Lower Old Red Sandstone i s a c o n g l o m e r a t e - l i t h i c a r e n i t e assemblage i n t e r bedded w i t h a c i d t o b a s i c v o l c a n i c s which was d e p o s i t e d i n a t l e s s t two b a s i n s : t h e S t r a t h m o r e b a s i n t o t h e n o r t h w e s t and t h e Lanark b a s i n t o t h e s o u t h e a s t .
Both
t h e s e b a s i n s were f i l l e d l o n g i t u d i n a l l y from t h e n o r t h e a s t (Bluck, 1978) ( F i g . 1.221, t h e S t r a t b o r e b a s i n b e g i n n i n g i n t h e n o r t h e a s t a n d e x t e n d i n g southwestwards so
32 t h a t t h e b a s i n - f i l l o v e r l a p s and t h i n s i n a s o u t h w e s t e r l y d i r e c t i o n .
-O
200 Km
i" f
4
TORRIDON GROUP alluvial fanpile TORRIDON GROUP
-
-
1.0 b.y. 1 . 8 b.y.
> 2 ' 5 b.y. Fig.l.21. R e c o n s t r u c t i o n of t h e p r i n c i p a l pre-Torridon Group u n i t s i n r e l a t i o n t o t h e a r e a of T o r r i d o n (Applecross Formation) d e p o s i t i o n i n NW S c o t l a n d ( a f t e r A l l e n e t a l . , 1974). The Upper Old Red Sandstone sediments were p a r t l y d e r i v e d from t h e Lower Old Red Sandstone over which t h e y unconfonnably l i e .
The b a s i n h a s a n E-W o r i e n t a t i o n and
was f i l l e d l o n g i t u d i n a l l y from t h e s o u t h w e s t , a major r e v e r s a l o f t h e d r a i n a g e p a t t e r n s e e n i n t h e Lower Old Red Sandstone.
The b a s i n began i n t h e Clyde r e g i o n
and was i n i t i a l l y extended by s u c c e s s i v e normal f a u l t s which formed i n r e s p o n s e t o s i n i s t r a l movement of t h e Highland Boundary F a u l t f 1 . 2 2 ) .
33
i
Upper Old Red Sandstone
Schematic diagram o f t h e d i p o f major l o n g i t u d i n a l b a s i n s i n t h e Midland Fig.1.22. V a l l e y o f S c o t l a n d i n Lower and Upper Old Red Sandstone times ( a f t e r Bluck, 1978). The Devonian b a s i n s o f w e s t e r n Norway a r e a l s o i n t e r n a l b a s i n s b u t show evidence o f h a v i n g formed i n a d i f f e r e n t t e c t o n i c regime t h a n t h e Midland V a l l e y of S c o t l a n d . 2 These b a s i n s a r e n o t a b l e f o r t h e i r s m a l l s i z e ((2,OOOkm ), v a s t s t r a t i g r a p h i c t h i c k n e s s e s (up t o 25km) a n d t h e c y c l i c n a t u r e of t h e i r c o a r s e - g r a i n e d a l l u v i a l i n f i l l i n g ( S t e e l , 1976; S t e e l and Aasheim, 1978) ( F i g . 1 . 2 3 ) .
The l o n g e s t of t h e s e
b a s i n s i s t h e Hornelen b a s i n which i s bounded by t h r u s t ( e a s t ) , h i g h - a n g l e f a u l t ( n o r t h and s o u t h ) and unconformable ( w e s t ) margins.
The s u r r o u n d i n g basement rock
h a s a n E-W Caledonian g r a i n and c o n s i s t s o f Precambrian g n e i s s e s , Cambro-Silurian m e t a b a s a l t s , s c h i s t s , q u a r t z i t e s , rnetagreywackes, g r a n o d i o r i t e s and gabbros. The b a s i n was f i l l e d l a r g e l y w i t h sandy a l l u v i u m d e p o s i t e d on a n a l l u v i a l p l a i n o r sandy f a n d e l t a growing t o t h e w e s t and n o r t h w e s t ( S t e e l and Aasheim, 1978). margins of t h e b a s i n a r e f l a n k e d by f a n g l o m e r a t e s ( S t e e l e t a l . ,
The
1977; Larsen and
S t e e l , 1978) which frequently show a coarsening-upwards theme, usually of the order of 100-20Om, and i r r e s p e c t i v e of the a l l u v i a l f a c i e s .
This i s interpreted by
S t e e l ( 1 9 7 6 ) a s an indication of rapid lowering of the basin f l o o r and the consequent
35 p r o g r a d a t i o n of a l l u v i a l s e d i m e n t a t i o n .
I n t h e Hornelen b a s i n a l l u v i a l t r a n s p o r t
was mainly a l o n g t h e a x i s o f t h e b a s i n ( F i g . 1 . 2 3 ) and t h e r e a r e i m p o r t a n t d i f f e r e n c e s i n f a c i e s , t h i c k n e s s e s , g r a d i e n t s and s i z e o f f r i n g i n g a l l u v i a l f a n s on o p p o s i t e s i d e s of t h e basin.
These f e a t u r e s and t h e dominance o f a s a n d s t o n e r a t h e r t h a n
conglomerate s u c c e s s i o n s u g g e s t s a d e p o c e n t r e s y s t e m a t i c a l l y m i g r a t i n g through time i n a s t r i k e - s l i p t e c t o n i c regime (Crowell, 1974a; S t e e l , 1976). The Solund b a s i n i s n o t so c l e a r l y o r g a n i z e d a s t h e Hornelen b a s i n b u t coarseningupwards sequences a r e p r e s e n t .
The s u c c e s s i o n i s dominated by conglomerates which
were d e p o s i t e d by c u r r e n t s flowing t r a n s v e r s e t o t h e b a s i n a x i s , f e a t u r e s which s u g g e s t t h a t t h e b a s i n was formed i n a d i p - s l i p t e c t o n i c regime. E a r l y Mesozoic r e d beds formed i n graben a s s o c i a t e d w i t h t h e opening of t h e A t l a n t i c
Ocean
The break-up of t h e s u p e r c o n t i n e n t Pangaea and t h e opening of t h e North A t l a n t i c Ocean d u r i n g t h e l a s t 200 My h a s been a s s o c i a t e d w i t h t h e f o r m a t i o n o f numerous graben (Fig.1.24).
I n a l l , over 100 s e p a r a t e graben c a n b e i d e n t i f i e d and t h e s e a r e
l i n k e d , forming a n e l a b o r a t e network of t r i p l e r i f t systems (Burke, 1976).
Some of
t h e s e r i f t s developed t o form new o c e a n i c c r u s t b u t many o f them f a i l e d and were f i l l e d w i t h t h i c k sequences o f c o n t i n e n t a l s e d i m e n t s , many o f which a r e now r e d beds. The formation of t h e s e t e n s i o n a l r i f t v a l l e y s i s commonly accompanied by b a s a l t i c volcanism and t h e r a p i d accumulation of s e v e r a l k i l o m e t r e s of sediment.
The
sedimentary f i l l i n g i s f r e q u e n t l y t r a n s v e r s e a l t h o u g h h o r s t s w i t h i n t h e graben s t r o n g l y i n f l u e n c e t h e e a r l y s t a g e s of sediment d i s p e r s a l .
Rifting generally
c o n t i n u e s u n t i l t h e o n s e t ' o f normal marine s e d i m e n t a t i o n b u t i n some c a s e s sediment d i s t r i b u t i o n may be i n f l u e n c e d by graben which a r e o v e r 100 My o l d . Graben o f T r i a s s i c age are a c h a r a c t e r i s t i c f e a t u r e o f t h e c e n t r a l A t l a n t i c a r e a which was t h e f i r s t p a r t o f t h e A t l a n t i c Ocean t o open.
These a r e b e s t known from
t h e North American s i d e o f t h e A t l a n t i c where t h e y form a l i n e a r system of f a u l t bounded b a s i n s a b o u t 2000km i n l e n g t h ( S a n d e r s , 1963).
These a r e g e n e r a l l y f i l l e d
w i t h s e v e r a l k i l o m e t r e s of r e d f l u v i a l and r e d / d r a b l a c u s t r i n e sediments of Carnian t o Lower J u r a s s i c age (Cornet and T r a v e r s e , 1975) i n t e r c a l a t e d w i t h t h o l e i t i c b a s a l t i c l a v a s , s i l l s , and dykes g i v i n g r a d i o m e t r i c a g e s between 210-170 My.
The
graben a r e normally a few t e n s o f k i l o m e t r e s wide and occur up t o 500km i n t o t h e c o n t i n e n t , t h e s t r i k e u s u a l l y b e i n g c o n t r o l l e d by t h e s t r u c t u r a l t r e n d of t h e underl y i n g Appplachian f o l d e d r o c k s , a l t h o u g h t h e Newark graben c r o s s e s t h e Appalachian strike.
Sedimentation w i t h i n t h e graben i s l a r g e l y c o n t r o l l e d by t h e marginal
boundary f a u l t s .
The t e c t o n i c h i s t o r y of any i n d i v i d u a l graben i s , however,
c o m p l i c a t e d and t h e r e i s evidence t h a t s t r i k e - s l i p components a r e i m p o r t a n t i n some c a s e s ( S a n d e r s , 1963).
36
NORTH SEA
ARGENT1
*.-
S k e t c h map i l l u s t r a t i n g t h e d i s t r i b u t i o n o f major g r a b e n a r o u n d t h e Fig.1.24. A t l a n t i c Ocean which formed i n a s s o c i a t i o n w i t h c o n t i n e n t a l r u p t u r e . The g r a b e n between t h e l i n e s V-W and X-Y developed between 210 and 170 My ago; t h o s e s o u t h of t h e l i n e X-Y formed between a b o u t 145 and 125 My ago; t h o s e n o r t h o f t h e l i n e T-Y formed a b o u t 80 My ago and t h o s e n o r t h o f R-S between 80-60 My ( a f t e r Burke, 1976). The H a r t f o r d b a s i n c o n t a i n s a 4km t h i c k s u c c e s s i o n which can b e c o r r e l a t e d f o r a d i s t a n c e o f o v e r 140km.
I t c o n t a i n s t h e E a s t B e r l i n Formation, a 145-45Om t h i c k
sequence of f l u v i a l and l a c u s t r i n e r o c k s o f Lower J u r a s s i c a g e (Hubert e t a l . , 1976). Along t h e e a s t e r n boundary o f t h e b a s i n was a s t e e p escarpment a l o n g which a l l u v i a l f a n s were developed.
These t r a n s p o r t e d d e t r i t u s e a s t w a r d s r a d i a l l y away from t h e
fault-bounded margin (Fig.1.25)
i n t o t h e c e n t r a l p a r t of t h e graben.
Dispersal
h e r e seems a l s o t o have been e s s e n t i a l l y e a s t w a r d s , j u d g i n g from t h e r a t h e r v a r i a b l e p a l a e o c u r r e n t s , which s u g g e s t t h a t t h e g r a b e n was f i l l e d t r a n s v e r s e l y .
The p a l a e o -
s l o p e of t h e g r a b e n f l o o r was g e n e r a l l y t o t h e s o u t h and t h e r e may have been s i g n i f i c a n t d i s p e r s a l o f sediment a l o n g t h e a x i s o f t h e g r a b e n ,
37
B
A
*OI
km
OJ ~ R O D E DEDGE; OF BASIN ,I
__*
Palasocurrents
a
Stream Channel Sendstone
0
Alluvial Fan conglomerats & tlldstom
A. P a l a e o c u r r e n t and p a l a e o s l o p e f o r s a n d s t o n e and conglomerates d e p o s i t e d Fig.1.25. i n s t r e a m c h a n n e l s and a l l u v i a l f a n s of t h e E a s t B e r l i n Formation B. P a l a e o c u r r e n t s f o r r e d s a n d s t o n e and s i l t s t o n e d e p o s i t e d i n s h a l l o w l a k e s on f l o o d p l a i n s . Palaeos l o p e s f o r f l o o d p l a i n r e d mudstone w i t h t h i n s a n d s t o n e l a y e r s showing t h e r e g i o n a l p a l a e o s l o p e o f t h e v a l l e y f l o o r ( a f t e r Hubert e t a l . , 1976).
38 The North Sea (Fig.1.26)
i s a n o t h e r example of a f a i l e d r i f t system a s s o c i a t e d
w i t h t h e opening of t h e North A t l a n t i c (Whiteman e t a l . , 1975; Burke, 1976) a l t h o u g h
C o n t i n e n t a l r e d beds accumulated i n t h e North Sea b a s i n
i n c l u d e t h e Permian
R o t l i e g e n d e s , a d e s e r t f l u v i a l / a e o l i a n s u c c e s s i o n d e r i v e d l a r g e l y from t h e V a r i s c a n mountains i n t h e s o u t h ( G l e n n i e , 1972) and t h e T r i a s s i c , a n e v a p o r i t e / r e d bed sequence w i t h marine i n t e r c a l a t i o n s (Brennand, 1975).
Around t h e s o u t h e r n margin
of t h e North Sea i s a t h i n a p r o n of c o a r s e c l a s t i c s w i t h conglomerates, d e r i v e d from t h e h i g h l a n d s t o t h e s o u t h .
The major p a r t o f t h e s u c c e s s i o n s c o n s i s t s o f
s h e e t s a n d s t o n e s and s h a l e s w i t h e v a p o r i t e s i n t h e upper p a r t and d i s p l a y s p a r t i c u l a r l y w e l l t h e i n f l u e n c e o f b l o c k f a u l t i n g on t h i c k n e s s v a r i a t i o n s (Fig.1.27).
I n the
m a r g i n a l a r e a s t h e T r i a s s i c i s t h i n and rests unconformably on o l d e r r o c k s whereas
39 i n t h e C e n t r a l Graben t h e s u c c e s s i o n i s much t h i c k e r .
SECTION 1 NORTHERN VIKING GRABEN CormorCnt
Dunlin
kult
L Cwnhn)
SECTION 2 CENTRAL NORTH SEA
T r i a s s i c s e c t i o n s i n t h e n o r t h e r n North Sea showing t h e i n f l u e n c e of Fig.1.27. b l o c k - f a u l t i n g on t h e r a t e of s u b s i d e n c e ( a f t e r Brennand, 1975). AS i n o t h e r f a i l e d r i f t s o f t h e North A t l a n t i c margins t h e r e i s evidence o f s t r i k e -
s l i p t e c t o n i c s i n t h e North Sea.
For example, t h e gas f i e l d s which o c c u r i n
R o t l i e g e n d e s s a n d s t o n e s of t h e s o u t h e r n North Sea a p p e a r t o have been a l i g n e d by Hercynian s t r i k e - s l i p movements ( B l a i r ; 1975). Cenozoic molasse C o n t i n e n t a l sediments formed a s c l a s t i c wedges a l o n g c r a t o n i c margins ( f o r e d e e p s
or e x o g e o s y n c l i n e s ) a r e known as molasse and a fundamental p a r t of t h e l a t e r s t a g e s
40
of orogeny.
Molasse i s w e l l - r e p r e s e n t e d i n t h e Cenozoic f o l d mountains b e l t s of t h e
world (Fig.1.28)
o f t e n forming sequences s e v e r a l k i l o m e t r e s t h i c k and c o n t a i n i n g
mtrrl
nalryas
. .
. .... . ...
. . .. L. .:’ parrlic
J
marina
I
I
m+m
Fig.1.28. A. S t r a t i g r a p h i c s e c t i o n s o f v a r i o u s molasse sequences ( s h a d e d ) and d u r a t i o n s i n My. Dots = t h i c k c o n g l o m e r a t i c u n i t s ; V = v o l c a n i c u n i t s . LM (Lower m a r i n e ) , LFW (Lower f r e s h w a t e r ) , UM (Upper m a r i n e ) , and UFW (Upper f r e s h w a t e r m o l a s s e ) . B. T h i c k n e s s of t h e same sequences ( s h a d e d ) and s u b s i d e n c e rates i n metres p e r m i l l i o n y e a r s ( a f t e r Van Houten, 1969). major r e d bed u n i t s . deposition:
There i s some e v i d e n c e t h a t t h e m a j o r p e r i o d s of molasse
Palaeocene/Eocene and Miocene/Pliocene c o r r e s p o n d t o t h e main p e r i o d s
of l i t h o s p h e r i c p l a t e motion i n t h e i r r e s p e c t i v e a r e a s (Van Houten, 1969). The A l p i n e molasse The p r i n c i p a l t e c t o n i c e l e m e n t s of a l p i n e Western Eruope a r e shown i n Fig.1.29. The Molasse n o r t h of t h e C e n t r a l and E a s t e r n Alps accumulated i n a c r a t o n i c b a s i n a b o u t 700km long and 50-14Okm wide which l a y between l a r g e Hercynian b l o c k s .
Other
i m p o r t a n t m o l a s s e b a s i n s a r e t h e A q u i t a i n e B a s i n l y i n g between t h e Massif C e n t r a l and t h e P y r e n e e s , and t h e Ebro B a s i n i m e d i a t e l y s o u t h of t h e P y r e n e e s ( F i g . 1 . 2 9 ) . The Molasse Basin succeeded e a r l y and m i d d l e O l i g o c e n e F l y s c h which p a s s e d t h r o u g h
41 a s l o p e f a c i e s ( t o n m e r g e l ) i n t o a p a r a l i c sequence (Lower Marine Molasse) a s t h e a x i s of t h e A l p i n e Foredeep was d i s p l a c e d towards t h e c r a t o n (Hagn, 1960). The s u c c e e d i n g Molasse i s c h a r a c t e r i s t i c a l l y c o a r s e g r a i n e d , mainly non-marine, and r a n g e s i n a g e from l a t e Oligocene up late-Miocene - e a r l y P l i o c e n e and i s up t o 6000111 t h i c k .
The d e p o s i t s a r e mainly wedge-shaped; i n e a s t e r n S w i t z e r l a n d , f o r
example, t h e proximal Molasse i s a b o u t 4500111 b u t 60km t o t h e n o r t h t h i n s t o o n l y 1800m (FUchtbauer, 1967a).
Along t h e n o r t h e r n margin t h e Molasse i n c l u d e s d e b r i s
from c r a t o n i c s o u r c e s and i n t e r f i n g e r s w i t h marine d e p o s i t s o f t h e Rhine Graben and the Jura d i s t r i c t .
These n e a r sea level d e p o s i t s r e f l e c t a c t i v e u p l i f t of t h e
orogen, h i g h a n g l e f a u l t i n g o f t h e basement b e n e a t h t h e f o r e d e e p and t h e northward movement o f nappes t h a t began s e v e r a l m i l l i o n s o f y e a r s a f t e r t h e main orogeny from e a r l y Eocene t o e a r l y Oligocene ( C l a r k and J P g e r , 1969; Milne, 1969; HsU and Schlanger,l971). The n o r t h e r n A l p i n e Molasse comprises two major and f i v e minor coarsening-upwards megacycles s e v e r a l t o many hundreds o f metres t h i c k (FUchtbauer, 1967a) which r e f l e c t phases o f A l p i n e deformation ( F i g . 1.30). The Lower F r e s h w a t e r and Upper F r e s h w a t e r Molasse ( F i g . l . 3 0 ) a r e s e p a r a t e d by t h e mid-Miocene p a r a l i c t o s h a l l o w marine Upper Marine Molasse which r e f l e c t s reduced d e t r i t a l i n p u t and e x t e n s i v e marine f l o o d i n g o f t h e foredeep.
The r e g i o n a l palaeo-
d r a i n a g e p a t t e r n o f t h e Molasse i s t y p i c a l o f t h a t s e e n i n Cenozoic o r o g e n i c b e l t s l y i n g p a r a l l e l t o t h e a x i s o f t h e f o r e d e e p and t h e mountains.
The Lower Freshwater
Molasse g r a d e s e a s t w a r d s i n t o p a r a l i c and marine f a c i e s whereas t h e Upper Marine Molasse and s u c c e e d i n g Upper F r e s h w a t e r Molasse were t r a n s p o r t e d westwards ( F i g . l . 3 1 ) . The Lower Marine Molasse, which comprises t h e middle Oligocene tonmergel and p a r a l i c s a n d s t o n e w i t h i t s minor i n f l u x of conglomerate ( T r b p y , 1960; FUchtbauer, 1964), f i l l e d t h e f o r e d e e p e a r l y i n mid-Oligocene t i m e ; s e v e r a l hundred metres were d e p o s i t e d a t t h e narrower w e s t e r n end and o v e r a lOOOm i n t h e wider e a s t e r n end where s u b s i d e n c e was more a c t i v e (FUchtbauer, 1964).
The s u c c e e d i n g p a r a l i c s a n d s t o n e s
show s i m i l a r t h i c k n e s s v a r i a t i o n s r a n g i n g from a few t e n s o f metres i n t h e west t o s e v e r a l hundred metres t h i c k i n t h e e a s t .
The s a n d s t o n e s a r e q u a r t z o s e and c a l c -
l i t h i c a r e n i t e s d e r i v e d from t h e F l y s c h , P r e a l p s , and Northern Calcareous Alps. A s s o c i a t e d minor conglomerates a l s o i n d i c a t e c a l c a r e o u s s o u r c e a r e a s and r e f l e c t i n c i p i e n t u p l i f t and nappe emplacement of t h e l a t e A l p i n e o r o g e n i c phase. During l a t e Oligocene and e a r l y Miocene time t h e C e n t r a l Alps were a c t i v e l y u p l i f t e d and r e s u l t e d i n t h e accumulation o f Molasse i n t h e c e n t r a l and w e s t e r n p a r t s of t h e f o r e d e e p .
This began w i t h i n t e r b e d d e d non-marine s a n d s t o n e and mudstone and
w a s followed by t h e development of a f r i n g e o f e i g h t o r n i n e l a r g e a l l u v i a l f a n s
over lOOOm t h i c k ( F i g . l . 3 1 ) s u c c e s s i v e fining-upwards
(Fiichtbauer, 1964; G a s s e r , 1966, 1968).
These comprise
c y c l e s 10-20m t h i c k which i n c l u d e c o a r s e f a n g l o m e r a t e s ,
u s u a l l y s e v e r a l metres t h i c k .
These f a n g l o m e r a t e s grade l a t e r a l l y and d i s t a l l y i n t o
v a r i e g a t e d f l u v i a l fining-upwards c y c l e s , u s u a l l y no more t h a n a few metres t h i c k ,
42
-
Hercynian blocks Cenozoic mountain belts
0
Fig.1.29.
100
200
300 km.
The main t e c t o n i c e l e m e n t s of p a r t of w e s t e r n Europe.
and i n c l u d i n g t h i n beds o f l i g n i t e and f r e s h w a t e r l i m e s t o n e s ( B e r s i e r , 1945).
The
a l l u v i a l f a n s s u p p l i e d sediment t o t h e a x i a l r e g i o n of t h e b a s i n which wss t h e n t r a n s p o r t e d e a s t w a r d s t o t h e marine a r e a which p e r s i s t e d i n s o u t h e a s t e r n Germany and a d j a c e n t A u s t r i a (Lensch, 1961; S t e p h a n , 1965). U p l i f t and d e t r i t a l i n f l u x had waned by Mid-Miocene t i m e and t h e n o r t h e r n A l p i n e f o r e d e e p was i n u n d a t e d by marine c o n d i t i o n s and o n l y a few minor f l a n k i n g a l l u v i a l f a n s remained.
The r e s u l t i n g Upper Marine Molasse i s less t h a n lOOOm t h i c k b u t
e q u i v a l e n t d e p o s i t s a t t h e e a s t e r n end of t h e f o r e d e e p where s u b s i d e n c e was s t i l l g r e a t e r ( s c h l i e r m e r g e l ) a r e more t h a n 2OOm t h i c k .
I n t h e west t h e Upper Marine
Molasse c o n s i s t s of s h a l l o w m a r i n e t i d a l s a n d b o d i e s ( B r i e l , 1962; Van d e r Linden, 1963). The C e n t r a l and E a s t e r n Alps were u p l i f t e d i n l a t e Miocene-Early P l i o c e n e times and t h e H e l v e t i c nappes were t r a n s p o r t e d northwards o v e r i d i n g t h e f o r e d e e p by a
43 EBRO
-Pontian hrmatian Tortonian Helvetian Burdigalian E Aquitanian
-
-ChattianScampian - Ruplian N
Lnnoirsian-Lartorfian Ludian
Priabonian
Et
Lutetian
Y pratian-Cuitian
Fig.l.30. S t r a t i g r a p h i c d i s t r i b u t i o n o f t h i c k Cenozoic conglomerates i n t h e A l p i n e , A q i i i t a i n e and Ebro Basins. The r a n g e s o f conglomerates and o r o g e n i c p h a s e s depend on t h e a b s o l u t e time span a s s i g n e d t o Cenozoic epochs and on t h e assumption t h a t t h e A q u i t a n i a n and P o n t i a n S t a g e s a r e Miocene i n a g e 1. Main and l a t e A l p i n e orogeny (Milne, 1969). Arrow p o i n t e d l i n e s a r e S t i l l e ' s A l p i n e p h a s e s o f deformation ( R i c h t e r , 1927; B r e y e r , 1960) 2. P o s t - F l y s c h , coarsening-upward megacycles (Ftlchtbauer, 1967a) 3 . N o r t h Pyrenean o r o g e n i c p h a s e s ( S c h o e f f l e r , 1971) 4. South Pyrenean o r o g e n i c p h a s e s ( R i b a , 1955; S o l e r and P u i d e f a b r e g a s , 1970; Henry e t a l . , 1971) 5. Conglomeaates from c r a t o n i c b l o c k s ( F e r r e r e t a l . , 1968; G r o s s , 1968) 6. European Cenozoic S t a g e s ( a f t e r Van Houten, 1974). few t e n s of k i l o m e t r e s .
O l d e r proximal molasse d e p o s i t s were f o l d e d and f a u l t e d
b e n e a t h t h e nappes a s t h e a x i s of t h e f o r e d e e p was s h i f t e d t e n s o f k i l o m e t r e s t o t h e north.
The exposed Molasse was e v e n t u a l l y reduced t o l e s s t h a n h a l f i t s o r i g i n a l
width.
The Upper F r e s h w a t e r Molasse was d e p o s i t e d a l o n g t h e l e n g t h of t h e d i s p l a c e d
f o r e d e e p a s l a r g e a l l u v i a l f a n s d u r i n g T o r t o n i a n and P o n t i a n times,
Some of t h e s e
f a n s , such a s t h o s e a t Napf and H b r n l i a r e o v e r 1500m t h i c k ( M a t t e r , 1964, 1970). With i n c r e a s e d u p l i f t of t h e E a s t e r n Alps c l a s t s d e r i v e d from t h e Northern Calcareous Alps dominate t h e e a s t e r n f a n g l o m e r a t e s ( J a n o s c h e k , 1963) and t h e e a s t w a r d d i s p e r s a l p a t t e r n s e e n i n t h e Lower Molasse was now c o m p l e t e l y r e v e r s e d so t h a t e a s t e r n d e t r i t u s was now s p r e a d a c r o s s t h e J u r a D i s t r i c t ( F i g . 1 . 2 9 ) .
The a c c u m u l a t i o n of t h e Upper
F r e s h w a t e r Molasse p r o b a b l y c o n t i n u e d i n t o P l i o c e n e time f i l l i n g t h e f o r e d e e p t o o v e r lOOOm above s e a l e v e l ,
D i s t a l molasse was f o l d e d i n t h e J u r a s and much of
t h e l a t e r r e c o r d was d e s t r o y e d by deep e r o s i o n .
44
Fig.l.31. D i s t r i b u t i o n o f a l l u v i a l f a n s and d i s p e r s a l p a t t e r n s i n t h e n o r t h e r n A l p i n e f o r e d e e p d u r i n g a c c u m u l a t i o n o f Lower Freshwatel; ( C h a t t i a n - A q u i t a n i a n ) and Upper F r e s h w a t e r ( T o r t o n i a n P o n t i a n ) Molasse ( a f t e r FUchtbauer, 1967a). The molasse of t h e Canadian c o r d i l l e r a Molasse d e p o s i t i o n i n t h e Canadian c o r d i l l e r a shows some p a r a l l e l s w i t h t h e A l p i n e molasse.
I t c o n s i s t s of two o r o g e n i c b e l t s :
The P a c i f i c Orogen on t h e West
and t h e Columbian Orogen on t h e E a s t (Wheeler and G a b r i e l s e , 1972).
These two
b e l t s a r e s e p a r a t e d by t h e I n t e r m o n t a n e B e l t which i s c h a r a c t e r i z e d by v o l c a n i c and g r a n i t i c t e r r a i n u n c o n f o m a b l y o v e r l a i n by non-volcanic s u c c e s s o r b a s i n s of Middle J u r a s s i c t o e a r l y T e r t i a r y age.
The v a s t m a j o r i t y of t h e C o r d i l l e r a n m o l a s s e i s
r e l a t e d t o t h e Columbian Orogen and was l a i d down i n f o r e l a n d b a s i n s , i n t r a m o n t a n e v a l l e y s and e u g e o s y n c l i n a l s u c c e s s o r b a s i n s ( E i s b a c h e r e t a l . ,
1974).
The palaeo-
d r a i n a g e p a t t e r n and d e p o s i t i o n o f t h e m o l a s s e was c l o s e l y c o n t r o l l e d by a c t i v e l i n e a r t e c t o n i c elements.
The d i s t r i b u t i o n of m o l a s s e f a c i e s i n t h e Columbian
Orogen i s shown i n F i g . 1 . 3 2 .
A c e n t r a l zone of u p l i f t is f l a n k e d on t h e west by
F o r e l a n d Basins and on t h e e a s t by S u c c e s s o r B a s i n s ; r e l a t i v e t e c t o n i c t r a n s p o r t w i t h i n t h e s e two a r e a s were t o t h e e a s t a l o n g t h e e a s t e r n edge and t o t h e w e s t a l o n g t h e w e s t e r n edge; t h e c e n t r a l zone o f u p l i f t i s a l s o t h e r e f o r e a zone of s t r u c t u r a l divergence.
E a s t of t h e a x i s of t h i s zone t h e r o c k s of t h e Columbian Orogen c o n s i s t
mainly of deformed m i o g e o c l i n a l c a r b o n a t e - q u a r t z i t e s u c c e s s i o n s , whereas west o f t h e a x i s , high-grade metamorphic complexes a r e dominant.
Other major f e a t u r e s of t h e
Columbian Orogen a r e l o n g i t u d i n a l f a u l t z o n e s , well-known s i n c e McConnells' (1896) d e s c r i p t i o n , and now r e g a r d e d a s l o n g - l i v e d , r e p e a t e d l y r e a c t i v a t e d d e e p - c r u s t a l f r a c t u r e s (Monger e t a l . , ( F i g . 1.32).
1972) and a l s o s t r u c t u r a l r e - e n t r a n t s and s a l i e n t s
45
,
j; ...:;.::. :..... , .: :.. , ’.:.: .... .(. ./
. .
::.:, . . ..,
i. ,
Pml Reentrant
... ’.’ .;.;.. :. ... . ....
Molassa Flcies High grade maamorphicn
rm!Y km.
F i g . 1 . 3 2 . Main t e c t o n i c elements i n the Canadian C o r d i l l e r a and d i s t r i b u t i o n o f molasse f a c i e s i n s u c c e s s o r b a s i n s , intramontane v a l l e y s and f o r e l a n d b a s i n of the Columbian Omgen ( a f t e r Eisbacher e t a l . , 1974).
46
The s o u t h e r n F o r e l a n d b a s i n of t h e Columbian Orogen c o n t a i n s two major molasse sequences e a c h r e p r e s e n t i n g a p r o g r a d a t i o n o f c o a r s e g r a i n e d , non marine c l a s t i c s o v e r marine s h a l e s and s a n d s t o n e s (Fig.1.33).
The lower sequence i n c l u d e s t h e m a r i n e
I f T
'W l y River- Psrkapoo' Assemblago
'Alborta' Assemblage
'Kootenay- Bkirmorr' Aswrnblam
1
Fig.1.33. The f o u r l i t h o l o g i c a l assemblages o f t h e s o u t h e r n F o r e l a n d B a s i n of t h e Columbian Orogen ( a f t e r E i s b a c h e r e t a l . , 1974). Upper F e r n i e Assemblage and t h e non-marine Kootenay-Blairmore Assemblage.
The upper
sequence c o n t a i n s t h e m a r i n e A l b e r t a Assemblage and t h e non-marine B e l l y RiverPaskapoo Assemblage.
Only one molasse sequence i s p r e s e n t i n t h e n o r t h e r n f o r e l a n d
b a s i n and c o n s i s t s o f a n Upper C r e t a c e o u s t o Lower T e r t i a r y c o n t i n e n t a l c l a s t i c wedge o v e r l y i n g Lower C r e t a c e o u s m a r i n e c l a s t i c s . The Lower Molasse (Kootenay-Blairmore Assemblage) i n t h e Canadian C o r d i l l e r a conformably s u c c e e d s s l o p e d e p o s i t s i n t h e Upper p a r t o f t h e F e r n i e Formation; t h e Lower d e p o s i t s (Kootenay Formation) r e p r e s e n t d e l t a i c coal-swamp c o n d i t i o n s ( J a n s a ,
1972) and a r e followed b y a l l u v i a l s e d i m e n t s o f t h e Blairmore Group which c o v e r e d e x t e n s i v e a r e a s of t h e s o u t h e r n f o r e l a n d b a s i n and l i e w i t h e r o s i o n a l d i s c o n f o r m i t y on t h e o v e r l y i n g s e d i m e n t s , a l t h o u g h t h e s e a r e n o t f o l d e d ( N o r r i s , 1964).
I n the
v i c i n i t y of t h e Crowsnest r e - e n t r a n t t h e Lower Molasse i s 150Om t h i c k and h e r e r e d beds a r e developed i n t h e a l l u v i a l p l a i n sediments
(Mellon, 1967; H o l t e r and Mellon,
1972); t h e molasse t h i n s e a s t w a r d s and northwards g r a d i n g i n t o marine s h a l e s i n t h e north (Fig.1.32).
47
L
KOOTENAY
- BLAIRMORE MOLASSE
I BELLY RIVER-PASKAWO MOLASSE
F i g . 1 . 3 4 . I n f e r r e d palaeodrainage p a t t e r n s during the d e p o s i t i o n of KootenayBlairmore molasse (A) and B e l l y River-Paskapoo molasse (B) ( a f t e r Eisbacher e t a l . , 1974).
48 D e p o s i t i o n o f t h e Lower Molasse a l o n g t h e mountain f r o n t was accompanied by t h e n o r t h w e s t e r l y p r o g r a d a t i o n of f l u v i o - d e l t a i c s a n d b o d i e s from t h e e a s t e r n margin o f t h e b a s i n and t h i s i n d i c a t e s g e n e r a l emergence of t h e f o r e l a n d b a s i n .
These e a s t e r n
c l a s t i c s a r e t h e Athabasca O i l Sands (McMurray Formation) and were d e r i v e d from t h e c r y s t a l l i n e Precambrian s h i e l d ( C a r r i g y , 1966, 1967). s t r u c t i o n (Fig.1.34)
P a l a e o g e o g r a p h i c a l recon-
based on H o l t e r and Mellon (1972) and S t o t t (1972) shows t h a t
sediment t r a n s p o r t was t o t h e n o r t h a l o n g t h e a x i s o f t h e F o r e l a n d b a s i n w i t h l a t e r a l i n p u t of d e t r i t u s from l a r g e a l l u v i a l f a n s which developed i n t h e v i c i n i t y of s t r u c t u r a l r e - e n t r a n t s s u c h a s t h e Peace and Crowsnest. The d e p o s i t i o n o f t h e Lower Molasse c u l m i n a t e d i n a w i d e s p r e a d m a r i n e t r a n s g r e s s i o n d u r i n g A l b i a n time.
T h i s p r o g r e s s e d southwards from t h e n o r t h and r e s u l t e d i n t h e
d e p o s i t i o n o f a b o u t lOOOm o f f i n e g r a i n e d s h a l l o w marine c l a s t i c s which i n c l u d e t h e c o a s t a l sands o f t h e well-known Cardium Formation ( M i c h a e l i s and Dixon, 1969; S i n h a , 1970). The t r a n s i t i o n from t h i s m a r i n e A l b e r t a Assemblage t o t h e Upper Molasse ( B e l l y River-Paskapoo Assemblage) took p l a c e g r a d u a l l y d u r i n g t h e Campanian; a t t h e same t i m e t h e r e was s p o r a d i c m o l a s s e d e p o s i t i o n w i t h i n some o f t h e i n t r a m o n t a n e valleys.
The Upper Molasse i s a b o u t 3000m t h i c k and i s t h i c k e s t n e a r t h e Rocky
Mountain F r o n t n e a r t h e Crowsnest r e - e n t r a n t ; e a s t w a r d s i t t h i n s and i n t e r f i n g e r s w i t h marine d e p o s i t s .
Conglomerates a r e r a r e i n t h e Upper Molasse and o c c u r m a i n l y
i n t h e Eocene and O l i g o c e n e where t h e y c a n b e r e l a t e d t o t h e f i n a l morphogenic u p l i f t of t h e Rocky Mountain Belt.
The s u c c e s s i o n i s dominated by f l u v i a l and d e l t a i c
s a n d s t o n e s and s h a l e s , and i n t h e w e s t e r n margin o f t h e f o r e l a n d b a s i n a r e i n t e r bedded Upper C r e t a c e o u s v o l c a n i c s .
D e t a i l e d p e t r o l o g i c a l i n f o r m a t i o n i s g i v e n by
Lerbekmo (19631, Campbell and Lerbekmo (19631, Nelson (1968) and C a r r i g y (1971). The p a l a e o d r a i n a g e p a t t e r n o f t h e Upper Molasse i n t h e s o u t h e r n F o r e l a n d B a s i n was predominantly a x i a l and from t h e n o r t h ( F i g . 1 . 3 4 ) .
I n the northern Foreland
B a s i n , n e a r t h e P e e l r e - e n t r a n t a r e Upper C r e t a c e o u s t o T e r t i a r y e l a s t i c wedges 200Om t h i c k and e q u i v a l e n t t o t h e B e l l y River-Paskapoo Assemblage (Mountjoy, 1967; Young, 1971, Green, 1972).
These were d e r i v e d from deformed P r o t e r o z o i c and Palaeo-
z o i c s u c c e s s i o n s t o t h e west and p r o b a b l y t r a n s p o r t e d i n a n e a s t e r l y f l o w i n g d r a i n a g e network (Young, 1971). The f i n a l phase of molasse d e p o s i t i o n i n t h e f o r e l a n d b a s i n s o f t h e Columbian Orogen o c c u r r e d d u r i n g Eocene-Oligocene t i m e when f a u l t - c o n t r o l l e d i n t r a m o n t a n e b a s i n s r e s u l t e d i n t h e development o f c o a r s e g r a i n e d a l l u v i a l f a n s ( P r i c e , 1965; J o n e s , 1969).
These c o n g l o m e r a t e s r e s t w i t h unconformity on t h e B e l l y River-Paskapoo
c l a s t i c s ( C a r r i g y , 1971) and r e p r e s e n t t h e t r a n s i t i o n from l o n g i t u d i n a l t o s u p e r imposed t r a n s v e r s e d r a i n a g e , r e f l e c t i n g t h e f i n a l morphogenic u p l i f t of t h e Columbian Orogen ( B a l l y e t a l . , 1966). The most remarkable f e a t u r e o f t h e Upper Molasse i s t h a t t h e s o u t h e r n F o r e l a n d Basin r e c e i v e d much of i t s s e d i m e n t from t h e n o r t h e r n p a r t o f t h e Columbian Orogen. T h i s was p r o b a b l y due t o f a u l t i n g and u p l i f t i n t h e n o r t h e r n Rockies which t e r m i n a t e d
49
d e p o s i t i o n and i n i t i a t e d e r o s i o n i n t h a t a r e a .
This d i r e c t reversal of the a x i a l
d r a i n a g e d i r e c t i o n is a n a l o g o u s t o t h a t s e e n i n t h e A l p s , c o i n c i d e s w i t h e x t e n s i v e i n t r a m o n t a n e f a u l t i n g and a p p e a r s t o b e a c h a r a c t e r i s t i c f e a t u r e o f Cenozoic molasse The s i m i l a r i t y o f m o l a s s e b a s i n development i n two q u i t e
deposition (Fig.1.35).
d i s s i m i l a r o r o g e n i c b e l t s i s remarkable and may r e f l e c t two m a j o r p h a s e s of c r u s t a l s h o r t e n i n g a l o n g o p p o s i t e o r o g e n i c f l a n k s , e a c h p r o d u c i n g a major molasse c y c l e . The same phenomenon i s a l s o s e e n i n o l d e r m o l a s s e b a s i n s s u c h a s t h e Old Red Sandstone o f t h e Midland V a l l e y o f S c o t l a n d (Bluck, 1978).
S I
h
COLUMBIAN OROGEN
The two molasse megacycles of t h e s o u t h e r n Columbian Orogen and t h e Fig.1.35. n o r t h e r n Alps. The o n s e t of i n t r a m o n t a n e d e p o s i t i o n i s i n d i c a t e d by 'XI. The arrows i n d i c a t e t h e d i r e c t i o n o f sediment t r a n s p o r t a l o n g t h e b a s i n a x e s ( b a s e d on Schmidt-Thome, 1963 and S t o t t , 1 9 7 2 ) .
50
THE CLIMATIC SETTING OF ANCIENT CONTINENTAL RED BEDS
The c l i m a t i c s i g n i f i c a n c e o f a n c i e n t c o n t i n e n t a l r e d beds i s a c o n t r o v e r s i a l subject.
The o l d e s t c o n t i n e n t a l r e d beds i n t h e s t r a t i g r a p h i c r e c o r d a r e p r o b a b l y
t h o s e i n t h e Dharwar g r e e n s t o n e b e l t s of I n d i a which a r e a b o u t 2500 My o l d ( S r i n i v a s a n and S r e e n i v a s , 1972).
Red beds do n o t , however, become abundant i n
t h e s t r a t i g r a p h i c r e c o r d u n t i l a b o u t 1800-2000 My ago which has been t a k e n a s a n i n d i c a t i o n t h a t f r e e O2 began t o accumulate i n t h e atmosphere a b o u t t h i s time (Cloud, 1968). Red beds a r e c e r t a i n l y a n i n d i c a t i o n o f a n oxygenated atmosphere b u t t h e q u e s t i o n remains whether t h e y a r e a more s p e c i f i c p a l a e o c l i m a t i c i n d i c a t o r .
Traditionally,
r e d beds have been a s s o c i a t e d w i t h h o t , d r y c l i m a t e s and a r i d o r s e m i - a r i d c o n d i t i o n s . E a r l y a u t h o r s were much i m p r e s s e d w i t h t h e o c c u r r e n c e of r e d d e s e r t s e d i m e n t s (Dawson, 1848; Crosby, 1885; G o o d c h i l d , 1896) and r e d beds were s u b s e q u e n t l y associated with a h o t , dry climate.
I t was o n l y u n t i l r e l a t i v e l y r e c e n t l y t h a t
t h e o c c u r r e n c e o f r e d beds i n b o t h d r y and m o i s t t r o p i c a l c l i m a t e s has been s t r e s s e d . D i r e c t e v i d e n c e of t h e n a t u r e o f t h e c l i m a t e w i t h i n t h e d e p o s i t i o n a l b a s i n may b e p r o v i d e d by f a u n a l o r f l o r a l e v i d e n c e .
There may a l s o b e l i t h o l o g i c a l e v i d e n c e of
c l i m a t e and Walker (1974) has d i s t i n g u i s h e d between: a ) a d e s e r t - e v a p o r i t e r e d bed a s s o c i a t i o n i n which r e d beds a r e a s s o c i a t e d w i t h a e o l i a n s a n d s , d e s e r t f l u v i a l s e d i m e n t s and e v a p o r i t e s formed i n p l a y a l a k e s and i n l a n d sabkhas a n d , b ) a m o i s t c l i m a t e - r e d bed a s s o c i a t i o n i n which r e d beds a r e i n t e r b e d d e d and i n t e r f i n g e r with coal-bearing s t r a t a Although t h e s e two a s s o c i a t i o n s a r e c l i m a t i c a l l y q u i t e d i s t i n c t t h e y b o t h i n d i c a t e d e p o s i t i o n w i t h i n 30'
of t h e e q u a t o r and t h e q u e s t i o n a r i s e s whether r e d beds a r e
g e n e r a l l y i n d i c a t i v e of low p a l a e o l a t i t u d e s .
T h i s i s i m p o r t a n t f o r many r e d bed
f o r m a t i o n s which show no d i r e c t e v i d e n c e of t h e n a t u r e of t h e p r e v a i l i n g c l i m a t e i n the depositional basin. Many a u t h o r s have a t t e m p t e d t o make p a l a e o c l i m a t e a s s e s s m e n t on t h e b a s i s o f m i n e r a l o g i c a l composition and t e x t u r e .
T h i s m i n e r a l o g i c a l method i s aimed a t t h e
i d e n t i f i c a t i o n of s o u r c e a r e a composition and t h e n a t u r e o f t h e s o u r c e a r e a weathering.
The r e s u l t s c a n n o t , t h e r e f o r e , b e compared d i r e c t l y w i t h o t h e r p a l a e o c l i m a t e
i n d i c a t o r s because t h e r e could be important differences i n t h e climates of t h e source a r e a and t h e d e p o s i t i o n a l b a s i n .
There i s a l s o t h e a d d i t i o n a l d i f f i c u l t y of p o s t -
d e p o s i t i o n a l changes i n mineralogy and t e x t u r e which c o u l d modify any d i a g n o s t i c f e a t u r e s i n h e r i t e d from t h e s o u r c e a r e a . N e v e r t h e l e s s , t h e r e i s e v e r y i n d i c a t i o n t h a t a n c i e n t c o n t i n e n t a l r e d beds have b o t h a d i s t i n c t i v e d e p o s i t i o n a l and d i a g e n e t i c mineralogical s i g n a t u r e , although the q u a n t i t a t i v e s i g n i f i c a n c e of each i s n o t y e t fully established.
51 The s i g n i f i c a n c e of f i r s t - c y c l e a r k o s i c r e d b e d s There a r e many a n c i e n t examples o f r e d beds which w e r e d e r i v e d d i r e c t l y from c r y s t a l l i n e r o c k s o f a p p r o x i m a t e l y g r a n i t i c c o m p o s i t i o n and which a r e c o n s e q u e n t l y rich i n feldspar.
These s a n d s t o n e s a r e g e n e r a l l y r e f e r r e d t o a s ' a r k o s i c ' o r
' a r k o s e s ' a l t h o u g h t h e r e i s some c o n f u s i o n a s t o t h e e x a c t meaning of t h e term ( O r i e l , 1949).
I n h i s o r i g i n a l d e f i n i t i o n B r o n g n i a r t (1826) u s e d t h e term a r k o s e
t o d e n o t e a c o a r s e c l a s t i c r o c k w i t h abundant f e l d s p a r b u t d i d n o t s p e c i f y t h e e x a c t amount.
Most workers f o l l o w t h e usage o f P e t t i j o h n (1949) and r e s t r i c t t h e
term a r k o s e t o s a n d s t o n e s w i t h more t h a n 25% f e l d s p a r i n t h e framework c o n s t i t u e n t s a l t h o u g h h e l a t e r r e d e f i n e d t h e term a s a s a n d s t o n e w i t h 25% o r more f e l d s p a r r o c k fragments i n which t h e f e l d s p a r formed a t l e a s t h a l f .
+
R e c e m t usage of t h e
term ' a r k o s i c ' s u g g e s t s t h a t i t i s b e s t r e s t r i c t e d t o c o n t i n e n t a l d e p o s i t s s o a s t o
a v o i d c o n f u s i o n w i t h f e l d s p a t h i c greywackes ( s e e P e t t i j o h n , 1975, p.214) which form i n q u i t e a d i f f e r e n t manner.
The term ' f e l d s p a t h i c ' o r s u b a r k o s i c i f o f t e n used
t o d e n o t e thobe s a n d s t o n e s i n which t h e f e l d s p a r c o n t e n t i s between 10-25%. Although a r k o s e s may form by a v a r i e t y o f c l o s e l y r e l a t e d p r o c e s s e s
(Pettijohn,
1975 p.217) t h e y a r e g e n e r a l l y r e g a r d e d , b e c a u s e o f e a s y h y d r o l y s i s , t o r e q u i r e high r e l i e f , r a p i d e r o s i o n , t r a n s p o r t a t i o n , and b u r i a l b e f o r e d e c o m p o s i t i o n c a n t a k e p l a c e i n lowland s o i l s .
They t y p i c a l l y develop a s a l l u v i a l f a n and f l u v i a l d e p o s i t s
a t t h e margins o f t e c t o n i c a l l y a c t i v e fault-bounded g r a b e n s ,
Examples i n c l u d e t h e
L a t e Precambrian T o r r i d o n i a n Sandstone of NW S c o t l a n d ( S e l l e y , 1 9 6 6 ) , t h e Old Red Sandstone i n p a r t s o f S c o t l a n d (Mackie, 1899) t h e Pennsylvanian Lyons and F o u n t a i n Formations o f Colorado ( H u b e r t , 1 9 6 0 ) , t h e T r i a s s i c Newark S e r i e s of C o n n e c t i c u t and o t h e r e a s t e r n s t a t e s (Krynine, 1950) and p a r t s of t h e T e r t i a r y molasse of s o u t h e r n Germany and S w i t z e r l a n d ( G a s s e r , 1968).
The amount o f f e l d s p a r i n a r k o s i c r e d beds
t h u s a p p e a r s t o b e c o n t r o l l e d by t h r e e m a j o r f a c t o r s : a ) t h e proportion of f e l d s p a t h i c c r y s t a l l i n e rock i n t h e source area b ) t h e c l i m a t e w i t h i n t h e s o u r c e a r e a and d e p o s i t i o n a l b a s i n c ) t h e amount o f p o s t - d e p o s i t i o n a l d e s t r u c t i o n o f f e l d s p a r s by i n t r a s t r a t a l s o l u t i o n V a r i a t i o n i n one o r more of t h e s e f a c t o r s a c c o u n t s f o r t h e observed v a r i a t i o n i n t h e f e l d s p a r c o n t e n t o f a r k o s e s which i s c o n s i d e r a b l e ( s e e P e t t i j o h n , 1975 p.215). T r a d i t i o n a l l y a r k o s e s a r e t h o u g h t t o form when e i t h e r c l i m a t i c c o n d i t i o n s were too d r y o r t o o c o l d t o a l l o w much w e a t h e r i n g w i t h t h e r e s u l t t h a t t h e f e l d s p a r remained l a r g e l y undecomposed; o r t h e s o u r c e a r e a was u p l i f t e d and e r o d e d so r a p i d l y t h a t t h e r e was n o t s u f f i c i e n t t i m e f o r f e l d s p a r w e a t h e r i n g .
These a r e r e f e r r e d by Folk
(1968a) a s t h e C l i m a t i c Arkose and t h e T e c t o n i c Arkose r e s p e c t i v e l y .
I n practice,
c l i m a t i c a r k o s e s form on s t a b l e s h i e l d a r e a s and a r e m o s t l y s h a l l o w m a r i n e , beach o r dune d e p o s i t s and a r e r a r e i n n a t u r e . T e c t o n i c a r k o s e s form p r i m a r i l y i n b l o c k - f a u l t e d mountainous a r e a s a s a l l u v i a l f a n s o r piedmonts.
S i n c e t h e s e t e c t o n i c a l l y a c t i v e a r e a s a r e c h a r a c t e r i z e d by r a p i d
e r o s i o n and d e p o s i t i o n , and i f c r y s t a l l i n e r o c k s a r e exposed i n t h e s o u r c e a r e a , f e l d s p a r w i l l b e a b u n d a n t l y p r e s e r v e d and may comprise up t o 50% o f t h e framework constituents.
R e l a t i v e u p l i f t i n t h e s o u r c e a r e a s may b e o f t h e o r d e r o f 5000-8OOOm
and r e s u l t i n t h e exposure o f basement g r a n i t e s and g n e i s s e s .
Rates of u p l i f t a r e
d i s c u s s e d by Crowell ( 1 9 7 4 a , b ) and S t e e l e t a l . (1976). The combination o f rugged r e l i e f and r a p i d e r o s i o n means t h a t p r e d o m i n a n t l y f r e s h d e t r i t u s , i n c l u d i n g f e l d s p a r s , i s formed.
I n humid c l i m a t e s t h e r e may b e a t h i c k
weathered m a n t l e and s o i l i n t h e i n t e r f l u v i a l a r e a s and Krynine (1949) has shown how t h i s can e x p l a i n t h e m i x t u r e o f weathered and unweathered f e l d s p a r s o f t h e same species.
I n a r i d c l i m a t e s r e d s o i l s a r e l a r g e l y a b s e n t i n t h e s o u r c e a r e a s (Walker,
1967b) because t h e r e i s i n s u f f i c i e n t r a i n f a l l f o r t h e development of l a t o s o l s .
In
such a r e a s t h e d e t r i t u s i s n o t r e d , b u t g r e y , y e l l o w o r t a n i n c o l o u r , and o n l y reddens through s u b s e q u e n t b u r i a l and d i a g e n e s i s . I n a n c i e n t a r k o s i c sequences t h e r e i s o f t e n l i t t l e o r no e v i d e n c e of t h e c l i m a t e i n t h e s o u r c e and d e p o s i t i o n a l a r e a .
Of c o u r s e , t h e b e s t e v i d e n c e l i e s i n t h e
p r e s e n c e of fauna o r f l o r a o r s e d i m e n t a r y p a l a e o c l i m a t i c i n d i c a t o r s such a s a s s o c i a t e d e v a p o r i t e s , b u t f o r a v a r i e t y o f r e a s o n s t h e s e may b e a b s e n t . Sedimentary p e t r o l o g i s t s have l o n g been i n t e r e s t e d i n u s i n g t h e mineralogy o f a r k o s e s t o i n t e r p r e t p a l a e o c l i m a t i c c o n d i t i o n s a t t h e t i m e of d e p o s i t i o n .
Important
c l u e s may b e p r o v i d e d by t h e d e g r e e and n a t u r e of f e l d s p a r a l t e r a t i o n (Krynine, 1947; F o l k , 1 9 6 8 a ) p r o v i d e d t h a t t h e e f f e c t s due t o c l i m a t e and d e p o s i t i o n a l p r o c e s s e s can be i s o l a t e d from p r e w e a t h e r i n g e f f e c t s ( e . g . due t o d e u t e r i c o r hydrothermal a l t e r a t i o n ) and d i a g e n e t i c e f f e c t s ( e . g .
t h e i n t r a s t r a t a l d i s s o l u t i o n and r e p l a c e m e n t o f
f e l d s p a r by c l a y j . Krynine showed t h a t i m p o r t a n t c l u e s may b e p r o v i d e d by g r a i n r o u n d n e s s , which g i v e s some i n d i c a t i o n o f t h e r a t e of e r o s i o n and s o u r c e a r e a r e l i e f , t h e a v e r a g e d e g r e e of f e l d s p a r a l t e r a t i o n and a l s o t h e homogeneity o f f e l d s p a r a l t e r a t i o n , t h e homogeneity b e i n g t h e d e g r e e of w e a t h e r i n g between i n d i v i d u a l f e l d s p a r s p e c i e s . F o l k (1968a) h a s shown how f e l d s p a r a l t e r a t i o n m i g h t be i n t e r p r e t e d i n terms o f s o u r c e a r e a topography and c l i m a t e . I n humid c l i m a t e s w i t h rugged topography abundant a n g u l a r f e l d s p a r i s produced and g e n e r a l l y c o n s i s t s o f a m i x t u r e of f r e s h and weathered g r a i n s b e c a u s e o f t h e e r o s i o n of weathered m a n t l e and f r e s h bedrock.
I n a r i d climates fresh feldspar i s
produced, and well-rounded f r e s h f e l d s p a r s may b e a n i n d i c a t i o n of a n a r i d c l i m a t e and low topography (peneplane) c o n d i t i o n s . There have been a number o f s p e c i f i c a p p l i c a t i o n s of f e l d s p a r w e a t h e r i n g and a r k o s e mineralogy o f p a l a e o c l i m a t e i n t e r p r e t a t i o n .
Todd (1968) h a s shown t h a t under
f a v o u r a b l e c i r c u m s t a n c e s m i n e r a l o g i c a l e v i d e n c e can b e used t o p r o v i d e e v i d e n c e o f ancient climates.
H e i n v e s t i g a t e d t h e k a o l i n i t i c w e a t h e r i n g o f t h e Domengine
s a n d s t o n e , one o f t h e Eocene a r k o s i c sequences i n C a l i f o r n i a .
The method i n v o l v e d
i d e n t i f i c a t i o n of f e l d s p a r g r a i n s i n t h i n s e c t i o n u s i n g t h e u n i v e r s a l s t a g e
53
G r a i n s were i d e n t i f i e d a s o r t h o c l a s e ,
measurement o f a x i a l a n g l e s and o p t i c p l a n e s .
o r t h o c l a , s e m i c r o p e r t h i t e , p l a g i o c l a s e , p e r t h i t e and m i c r o c l i n e and i n a l l a t o t a l of 1050 g r a i n s were examined, o f which 19% were d e t e r m i n e d u s i n g t h e U-stage.
Todd
measured t h e g r a i n s i z e , g r a i n r o u n d n e s s , and t h e t y p e and d e g r e e of w e a t h e r i n g f o r each grain.
The d e g r e e of w e a t h e r i n g was d e t e r m i n e d by measuring t h e p e r c e n t a g e
a r e a of t h e g r a i n which showed v i s i b l e a l t e r a t i o n .
A w e a t h e r i n g i n d e x y = logz A
where A i s t h e p e r c e n t a g e v i s i b l e a r e a showing a l t e r a t i o n was c a l c u l a t e d and used 0-1 f r e s h , 1-2 s l i g h t l y w e a t h e r e d , 2-3 m o d e r a t e l y
with the following verbal scale: w e a t h e r e d , 3-4 s t r o n g l y weathered.
The r e s u l t s a r e s m a r i z e d i n T a b l e 1.1.
TABLE 1.1. T e x t u r a l , c o m p o s i t i o n a l and w e a t h e r i n g c h a r a c t e r i s t i c s of f e l d s p a r g r a i n s of t h e Domengine Sandstone (Eocene) Sacramento V a l l e y , C a l i f o r n i a ( a f t e r Todd, 1 9 6 8 ) . No. Grains
Average Mean Size 0
Average Mean Roundness
Composition Mean
Composition S.D.
Mean
S.D.
Y Y
459
1.88
2.67(SA)
Ab+An 33%
2.24
0.66
Orthoclase M i c r o p e r t h i t e 410
1.75
2.94(SA)
Ab+An 39%
2.44
0.74
Plagioclase
2.11
3.13(SR)
Orthoclase
100
An
37%
An 7.7%
1.50
0.78 ~
The r e s u l t s c l e a r l y show t h a t t h e a v e r a g e roundness of t h e t h r e e f e l d s p a r s p e c i e s
i s s i m i l a r b u t t h e p o t a s h f e l d s p a r i s somewhat c o a r s e r t h a n t h e p l a g i o c l a s e .
Rather
s u r p r i s i n g i y t h e o r t h o c l a s e m i c r o p e r t h i t e i s , on a v e r a g e , t h e most weathered o f t h e t h r e e f e l d s p a r s p e c i e s , c l o s e l y f o l l o w e d by o r t h o c l a s e , w i t h p l a g i o c l a s e t h e f r e s h e s t of a l l .
The w e a t h e r i n g of t h e f e l d s p a r s i s p r e d o m i n a n t l y k a o l i n i t i c ; 10% of t h e
g r a i n s shown s e r i c i t i c a l t e r a t i o n , and some t h e "bubbly" a l t e r a t i o n o f Folk (1955). T h i s sequence o f o r t h o c l a s e more weathered t h a n p l a g i o c l a s e ( a n d e s i n e ) i s o f c o u r s e c o n t r a r y t o t h e g e n e r a l l y a c c e p t e d sequence of m i n e r a l s t a b i l i t y . A p l o t of a v e r a g e s i z e of e a c h s p e c i e s o f f e l d s p a r from a number of t h i n s e c t i o n s , when p l o t t e d a g a i n s t a v e r a g e w e a t h e r i n g , shows a number of i n t e r e s t i n g t r e n d s . Although o r t h o c l a s e m i c r o p e r t h i t e i s o v e r a l l , t h e most w e a t h e r e d s p e c i e s , g r a i n s s m a l l e r t h a n 2.0
0
a r e fresher than the equivalent sized orthoclase.
c o n s i s t e n t w i t h t h e i r more a n g u l a r a p p e a r a n c e ,
This i s
The p l a g i o c l a s e w e a t h e r i n g t r e n d
p a r a l l e l s t h o s e o f t h e two a l k a l i f e l d s p a r s b u t i s c l e a r l y f r e s h e r t h r o u g h o u t , except f o r the very coarsest orthoclase grains.
The e s t i m a t e d t r e n d f o r each s p e c i e s
shows a tendency t o show i n c r e a s e d w e a t h e r i n g w i t h i n c r e a s e i n g r a i n s i z e up t o a p o i n t a t which same d e g r e e o f w e a t h e r i n g i s m a i n t a i n e d .
Again t h i s i s c o n t r a r y t o
t h e g e n e r a l l y a c c e p t e d b e l i e f which h o l d s t h a t t h e f i n e r g r a i n s b e c a u s e of t h e i r g r e a t e r s u r f a c e area/volume r a t i o s h o u l d normally show a h i g h e r d e g r e e of weathering.
54
3.0 3-5 2.5
1
0 ORTHOCLASE 0 ORTHOCLASE MICROPERTHITE
A PLAGIOCLASE
-
-
M%o 1.5
-
1.0
-
0.5
-
. .....
I
-1.0
I
I
I
1
0.0
1.0
2.0
3.0
1
-uo
Md R e l a t i o n s h i p between a v e r a g e d e g r e e o f w e a t h e r i n g (My)and a v e r a g e g r a i n Fig.1.36, s i z e (Md) o f f e l d s p a r s p e c i e s i n t h e Domengine Formation (Eocene) Sacramento V a l l e y , C a l i f o r n i a ( a f t e r Todd, 1968). Todd (1968) showed t h a t t h e s e d a t a c o u l d b e r e a d i l y e x p l a i n e d p r o v i d e d t h a t due c o n s i d e r a t i o n was p a i d t o t h e c r y s t a l l o g r a p h i c s t r u c t u r e of t h e i n d i v i d u a l f e l d s p a r s p e c i e s and a l s o t h e a c t i v i t y of d i s s o l v e d i o n s i n t h e s o i l w a t e r , i n c l u d i n g d i s t i n c t i o n between w e a t h e r i n g a n d l e a c h i n g r a t e .
The e x p e r i m e n t a l d a t a and t h e o r e t i c a l
c a l c u l a t i o n s of Hess (1966) show t h a t d u r i n g t h e w e a t h e r i n g of K-feldspar a t l e a s t f o u r d i f f e r e n t sequences o f m i n e r a l p h a s e s c o u l d a c h i e v e s t a b i l i t y depending upon
+ + concentration.
the log K /H
With d e c r e a s i n g s i l i c a t h e s e a r e :
K-feldspar-
montmorillonite+kaolinitej
K-feldspar+
kaolinite
gibbsite
+gibbsite
+ K-mica + g i b b s i t e + + decreases with decreasing
K-feldspar
and i f l o g K /H K-feldspar+
silica
K-mica j k a o l i n i t e 3 g i b b s i t e
For example, a h i g h w e a t h e r i n g r a t e t o g e t h e r w i t h a h i g h l e a c h i n g r a t e may r e s u l t i n r a p i d loss o f K+ from o r t h o c l a s e w i t h l i t t l e o r no development of secondary mica because o f t h e h i g h H+ c o n c e n t r a t i o n . of K
+ would
I f t h e l e a c h i n g r a t e were r e t a r d e d b u i l d up
r e s u l t i n t h e development of secondary mica on t h e f e l d s p a r s u r f a c e ,
A l b i t e , on t h e o t h e r hand, would be more s t a b l e t h a n K - f e l d s p a r i n a n environment
+ +
i n which t h e l o g Na +/ H + c o n c e n t r a t i o n was r e l a t i v e l y h i g h and t h e l o g K /H c o n c e n t r a t i o n was r e l a t i v e l y low.
The more h i g h l y w e a t h e r e d s t a t e of o r t h o c l a s e
55
m i c r o p e r t h i t e r e l a t i v e t o o r t h o c l a s e c a n b e e x p l a i n e d i n terms o f c r y s t a l l o g r a p h i c structure,
P e r t h i t e l a m e l l a e b o u n d a r i e s c a n b e c o n s i d e r e d a s a r r a y s of d i s l o c a t i o n s
(Weertman and Weertman, 1964) which a r e more s u s c e p t i b l e t o w e a t h e r i n g t h a n a r e a s with perfect crystal lattice.
As e x p e c t e d such a r e a s show e v i d e n c e of more e x t e n s i v e
w e a t h e r i n g (Todd, 1968 p.840). These m i n e r a l o g i c a l d a t a can b e t a k e n t o i n d i c a t e t h a t t h e w e a t h e r i n g of t h e Domengine s o u r c e r o c k s i n v o l v e d s l u g g i s h l e a c h i n g o f a l k a l i - r i c h p a r e n t r o c k s under low-relief subtropical conditions.
Todd (1968) a r g u e d t h a t t h e h i g h sodium-low
p o t a s s i u m a c t i v i t y i n s o i l w a t e r c o u l d b e b r o u g h t a b o u t by abundant non-seasonal r a i n f a l l and s u f f i c i e n t v e g e t a t i o n t o u t i l i z e much o f t h e p o t a s s i u m f o r p l a n t growth. S i n c e P l a n t s do n o t u s e sodium b u t r e q u i r e p o t a s s i u m i n abundance, t h e p r e s e n c e of low run-off and l u s h v e g e t a t i o n c a n a d e q u a t e l y e x p l a i n t h e h i g h sodium r e l a t i v e t o p o t a s s i u m a c t i v i t y i n t h e s o i l and t h e observed w e a t h e r i n g c h a r a c t e r i s t i c s of t h e feldspar species.
T o d d ' s h y p o t h e s i s i s s u p p o r t e d by i n d e p e n d e n t p a l a e o b o t a n i c a l
e v i d e n c e ( A x e l r o d , 1966) which shows t h a t t h e s o u r c e a r e a ( S o u t h e r n S i e r r a Nevada) a t t h i s time w i t h 700m peaks was covered on i t s w e s t e r n s l o p e s by b r o a d - l e a f e d e v e r g r e e n s which p r e f e r r e d a warm-temperate c l i m a t e w i t h f r o s t l e s s w i n t e r s w i t h a n a n n u a l t e m p e r a t u r e r a n g e of 15-22OC.
The c l i m a t e p r o b a b l y d i d n o t show marked
s e a s o n a l i t y and s o i l m o i s t u r e l e v e l s were p r o b a b l y r e a s o n a b l y c o n s t a n t w i t h a n a n n u a l p r e c i p i t a t i o n between 60 and 70 i n c h e s p e r y e a r . Todd (1968) a l s o made a n e x a m i n a t i o n of t h e Upper Miocene Lower Modelo s a n d s t o n e i n C a l i f o r n i a which was d e r i v e d from a v a r i e t y o f p l u t o n i c r o c k groups ( S u l l w o l d , 1960).
The o r d e r of f e l d s p a r s t a b i l i t y i s o r t h o c l a s e - m o r e - s t a b l e - t h a n
plagioclase
(An (50%) w i t h a mean w e a t h e r i n g i n d e x ( M y ) of 1 . 3 4 f o r o r t h o c l a s e and 1.56 f o r plagioclase.
Observed w e a t h e r i n g p r o d u c t s i n c l u d e p o t a s s i u m mica ( i l l i t e ) and l e s s e r
amounts of k a o l i n i t e .
These f e a t u r e s c o u l d s u g g e s t a more t e m p e r a t e c l i m a t e , a c t i v e
l e a c h i n g (needed t o produce t h e k a o l i n i t e ) b u t w i t h r e s t r i c t e d removal of potassium i o n s ( t o produce t h e i l l i t e ) .
Comparison o f t h e h a b i t a t s of t h e modern e q u i v a l e n t s
o f t h e S i e r r a Madrean f l o r a which dominated b o t a n i c a l a s s o c i a t i o n i n s o u t h e r n C a l i f o r n i a a t t h i s time ( l a t e Miocene) ( A x e l r o d , 1957) s u g g e s t s p r e c i p i t a t i o n of 75-87cm p e r y e a r and a mean a n n u a l t e m p e r a t u r e of 2OoC.
Thus p r e c i p i t a t i o n and
t e m p e r a t u r e were b o t h somewhat less i n t h i s s o u r c e a r e a t h a n t h a t of t h e Domengine Sandstone.
A s a whole t h e e v i d e n c e i s c o n s i s t e n t w i t h t h e change i n c l i m a t e i p
C a l i f o r n i a from a composition macroclimate i n E a r l y Eocene t o a n i n c r e a s i n g l y c o n t i n e n t a l one t h r o u g h o u t t h e Miocene a s r e l i e f i n c r e a s e d and c l i m a t i c b a r r i e r s became more i m p o r t a n t , The s i g n i f i c a n c e o f s i z e - c o m p o s i t i o n t r e n d s Another method of u s i n g m i n e r a l o g i c a l d a t a t o i n t e r p r e t p a l a e o c l i m a t e c o n d i t i o n s has been d e s c r i b e d by Young e t a l . (1975) and depends upon comparison w i t h s i z e -
56 composition p l o t s of framework g r a i n s i n f i r s t - c y c l e s a n d s t o n e s .
The main
d i f f e r e n c e s between s e m i - a r i d and humid c l i m a t e s a r e shown i n Fig.1.37)
r 60
t I
\
Fragments
Monocrystolline Ouortz
Feldspar
Polycr ysta I line Ouartz
t
Distinction
Total Pyroxene Amphibole. Micos, Heavier, Ooaqws
r
-
0
M
F
kh
L C
C
M
F
C
H
F
C
M
F
--
Grain Size
High-rank metomorphic Low-rank metomorphic Plutonic
Arid
Humid
V a r i a t i o n i n composition o f f i r s t c y c l e Holocene f l u v i a l s a n d from semiFig.1.37. a r i d and humid climates. Weathering i n s e m i - a r i d c l i m a t e s y i e l d s g r e a t e r amounts of r o c k f r a g m e n t s , f e l d s p a r and a c c e s s o r y m i n e r a l s . Weathering i n humid c l i m a t e s produces more p o l y c r y s t a l l i n e and m o n o c r y s t a l l i n e q u a r t z . The r e l a t i o n s h i p s h o l d r e g a r d l e s s of c r y s t a l l i n e s o u r c e rock-type ( a f t e r Mack and S u t t n e r , 1977). between s e m i - a r i d and humid p a l a e o c l i m a t e s may b e p o s s i b l e u s i n g t h i s method.
Mack
and S u t t n e r (1977) a p p l i e d t h e t e c h n i q u e i n a s t u d y o f t h e P e n n s y l v a n i a n F o u n t a i n Formation i n t h e Colorado F r o n t Range.
The F o u n t a i n Formation was d e p o s i t e d a s
a l l u v i a l f a n s which f l a n k e d t h e e a s t e r n s i d e o f t h e A n c e s t r a l R o c k i e s , a n a r e a o f c r y s t a l l i n e Precambrian Rocks s i t u a t e d n e a r t h e l o c a t i o n o f t h e p r e s e n t Colorado F r o n t Range ( T i e j e , 1923; K n i g h t , 1929; H u b e r t , 1960; Howard, 1966) ( F i g . 1.38). A number o f a u t h o r s i n c l u d i n g Wahlstrom (1948) and Hubert (1960) b e l i e v e d t h a t
t h e F o u n t a i n Formation was d e p o s i t e d under humid c l i m a t e c o n d i t i o n s whereas Raup (1966) and Mallory (1972) a r g u e d t h a t i t was d e p o s i t e d under a r i d t o s e m i - a r i d conditions. Mack and S u t t n e r (1977) have a p p l i e d t h i s t e c h n i q u e i n o r d e r t o compare t h e provenance and palaeogeography of t h e F o u n t a i n s a n d s t o n e s w i t h Holocene s a n d s i n t h e same a r e a .
Having e s t a b l i s h e d t h a t t h e Holocene s a n d s were d e r i v e d from t h e same
EXPLANATION
I I
p?)Alluvial fan
I
I Fig.1.38. Middle Pennsylvanian palaeogeography i n t h e Colorado a r e a b a s e d on Krumbein and S l o s s (1963) and D o t t and B a t t e n (1971). s o u r c e r o c k s ( s e e a l s o H u b e r t , 1960, and Howard, 1966) Mack and S u t t n e r (1977) then compensated f o r d i a g e n e t i c m o d i f i c a t i o n such a s f e l d s p a r and hornblende breakdown (Walker, 1967a, 1973) by s u b t r a c t i n g t h e modal amounts and n o r m a l i z i n g t h e r e s t of t h e framework g r a i n s t o 100%. Thus t h e n o r m a l i z e d p o p u l a t i o n s of q u a r t z g r a i n s and rock fragments i n t h e F o u n t a i n Formation and Holocene s a n d s c o u l d r e f l e c t t h e d i f f e r e n c e s ( i f a n y ) i n t h e e f f e c t s of w e a t h e r i n g a n d c l i m a t e i n t h e s o u r c e a r e a . I n a l l t h e s i z e f r a c t i o n s r o c k fragments a r e more abundant i n t h e Holocene sands than i n the Fountain sandstone (Fig.1.39).
Following t h e i n t e r p r e t a t i o n
s u g g e s t e d by Young e t a l . (1975) t h i s r e l a t i o n s h i p was t a k e n by Mack and S u t t n e r (1977) a s a n i n d i c a t i o n t h a t t h e F o u n t a i n Formation c l i m a t e was more humid t h a n t h e Holocene.
Rigorous chemical w e a t h e r i n g i n humid c l i m a t e s more r a p i d l y d e s t r o y s
l a b i l e m i n e r a l s i n r o c k fragments t h e r e b y b r i n g i n g a b o u t t h e i r r a p i d d e s t r u c t i o n . Humid c l i m a t e s a r e c h a r a c t e r i z e d by lower amounts of rock fragments i n a l l sand size classes. P o l y c r y s t a l l i n e q u a r t z fragments show e x a c t l y t h e o p p o s i t e t r e n d t o t h e rock fragments.
I n a l l t h e s i z e f r a c t i o n s p o l y c r y s t a l l i n e q u a r t z i s more abundant i n
t h e F o u n t a i n Sandstone t h a n i t i s i n t h e Holocene sand.
I n p a r t i c u l a r the coarse
s a n d f r a c t i o n shows a much h i g h e r p r o p o r t i o n o f p o l y c r y s t a l l i n e q u a r t z t h a n t h e Holocene sand which produces a much h i g h e r s l o p e i n t h e s i z e - c o m p o s i t i o n t r e n d , These p o l y c r y s t a l l i n e q u a r t z r e s u l t s a u p p o r t t h e humid-climate h y p o t h e s i s and t h e
58 s l o p e d i f f e r e n c e of t h e s i z e - c o m p o s i t i o n p l o t can be e x p l a i n e d by t h e p r e f e r e n t i a l mechanical d e s t r u c t i o n d u r i n g t r a n s p o r t a t i o n of l a b i l e m i n e r a l s on t h e edges o f p o l y m i n e r a l i c rock fragments i n t h e c o a r s e s a n d f r a c t i o n .
T h i s p r o c e s s would produce
a h i g h e r r a t i o of frequency p e r c e n t p o l y c r y s t a l l i n e q u a r t z i n t h e c o a r s e f r a c t i o n t o frequency p e r c e n t i n t h e f i n e f r a c t i o n i n a humid c l i m a t e .
I
PLUTONIC SOURCE
60 *O
1
Rock Fragments
10
0 5
C
M
F
100
Monocrys1.
EXPLANATION Colorado Springs, Colo. Fountain Fm. A Holocene A Boulder, Colo. Fountain Fm. 0 Holocene
-L C
40
2o O
M
C
M
F
F
.
METAMORPHIC SOURCE 100
Golden, Colo. Fountain Fm. 0 Holocene Loveland, Colo. Fountain Fm. 0 Holocene
40
20
0
C
M
F
C
M
F
C
M
F
Grain Size Fig.1.39. V a r i a t i o n i n c o m p o s i t i o n of Holocene f l u v i a l s a n d and s a n d s t o n e s from t h e F o u n t a i n Formation. Each p o i n t i s based on s e p a r a t e 200 p o i n t modal a n a l y s e s of t h e i n d i v i d u a l s i z e f r a c t i o n s ( a f t e r Mack and S u t t n e r , 1977). There i s l i t t l e marked d i f f e r e n c e i n t h e mean frequency p e r c e n t of m o n o c r y s t a l l i n e q u a r t z between t h e F o u n t a i n and Holocene sands ( F i g . 1 . 3 9 ) .
However, a l l s i z e f r a c t i o n s
of t h e F o u n t a i n s a n d s t o n e c o n t a i n more m o n o c r y s t a l l i n e q u a r t z than t h e Holocene s a n d
from t h e same a r e a .
This f e a t u r e i s t a k e n by Mack and S u t t n e r (1977) a s f u r t h e r
s u p p o r t f o r t h e i r argument t h a t t h e F o u n t a i n c l i m a t e was more humid t h a n t h e Holocene because Young e t a l . (1975) have shown t h a t more i n t e n s e chemical w e a t h e r i n g i n humid
c l i m a t e s r e l e a s e s more m o n o c r y s t a l l i n e q u a r t z t h a n i n a r i d o r s e m i - a r i d r e g i o n s . Mack and S u t t n e r (1977) i n t e r p r e t e d t h i s d a t a t o i n d i c a t e t h a t t h e c l i m a t e on t h e e a s t f l a n k of t h e F r o n t Range d u r i n g d e p o s i t i o n o f t h e F o u n t a i n Formation was more humid t h a n i t i s a t p r e s e n t .
T h i s d i f f e r s from t h e c o n c l u s i o n s o f Raup (1966) who
s u g g e s t e d on t h e b a s i s of c l a y m i n e r a l o g y t h a t t h e c l i m a t e was a r i d t o s e m i - a r i d a t t h e time.
Raup's e v i d e n c e was m a i n l y t h e abundance of i l l i t e and mixed l a y e r
i l l i t e - m o n t m o r i l l o n i t e and t h e o n l y minor amounts of k a o l i n i t e i n t h e F o u n t a i n Formation which s u g g e s t e d t o him i n c o m p l e t e l e a c h i n g i n a n a r i d o r s e m i - a r i d c l i m a t e ( c f . Todd, 1968).
The u s e of c l a y m i n e r a l assemblages i s however s e r i o u s l y i m p a i r e d
by d i a g e n e t i c m o d i f i c a t i o n and Walker (1973) h a s a r g u e d t h a t much o f t h e c l a v i n t h e F o u n t a i n Formation, i n c l u d i n g t h e i l l i t e and mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e ,
i s of d i a g e n e t i c o r i g i n . A l s o Van Houten (1972) h a s shown t h a t c l a y m i n e r a l assemblages l i k e t h o s e d e s c r i b e d by Raup (1966) may a l s o b e found i n Recent a l l u v i u m d e r i v e d from c r y s t a l l i n e s o u r c e r o c k s and d e p o s i t e d i n a t r o p i c a l savanna c l i m a t e w i t h 30-40 i n c h e s of r a i d y e a r .
Raup a l s o n o t e d t h a t t h e c l a y m i n e r a l o g y of t h e
F o u n t a i n Formation was s i m i l a r t o t h a t o f t h e Minturn Formation, a F o u n t a i n e q u i v a l e n t on t h e w e s t f l a n k of t h e F r o n t Range which c o n t a i n s e v a p o r i t e s and s u g g e s t e d t h a t t h i s was f u r t h e r e v i d e n c e o f a n a r i d c l i m a t e d u r i n g t h e d e p o s i t i o n o f t h e F o u n t a i n Formation. T h i s paradox i s e a s i l y r e s o l v e d when t h e d e t a i l e d palaeogeography i s c o n s i d e r e d . C o n t i n e n t a l r e c o n s t r u c t i o n s f o r t h e L a t e C a r b o n i f e r o u s - E a r l y Permian ( D o t t and B a t t e n , 1971; Van d e r Voo and F r e n c h , 1974) show t h a t t h e A n c e s t r a l Rockies were within 1 0'
of l a t i t u d e of t h e contemporary e q u a t o r .
I n t h i s s i t u a t i o n , with the
A n c e s t r a l Rockies s i t u a t e d j u s t s o u t h o f t h e e q u a t o r , t h e p r e v a i l i n g e q u a t o r i a l e a s t e r l y winds on a s c e n d i n g t h e A n c e s t r a l Rockies would c a u s e l a r g e amounts of prec i p i t a t i o n on t h e e a s t e r n f l a n k , whereas p r e c i p i t a t i o n on t h e w e s t e r n f l a n k would b e low because of t h e r a i n shadow e f f e c t , t h e r e b y p r o d u c i n g c o n d i t i o n s c o n s i s t e n t w i t h e v a p o r i t e f o r m a t i o n i n t h e Minturn Formation.
I n d e p e n d e n t e v i d e n c e of t h e o r o g r a p h i c /
r a i n shadow i n t e r p r e t a t i o n i s d i s c u s s e d by Vaughn (1964, p.581-582).
I n north-
c e n t r a l Texas t h e f l o r a o f t h e Wolfcampian Lower W i c h i t a Group p r o v i d e s e v i d e n c e o f a humid c l i m a t e whereas Wolfcampian d e p o s i t s i n t h e Four Corners a r e a ( H a l g a i t o S h a l e ) r e f l e c t s d e p o s i t i o n under more a r i d c o n d i t i o n s (Baars', 1962). There a r e many e x c e l l e n t Recent examples of t h e o r o g r a p h i c / r a i n shadow e f f e c t i n t r o p i c a l l a t i t u d e s (Haunuitz and A u s t i n , 1944; Kendrew, 1 9 5 3 ) . Although t h e m i n e r a l o g i c a l d a t a i n t h e s t u d i e s of Todd (1968) and Mack and S u t t n e r (1977) e n a b l e p a l a e o c l i m a t i c i n t e r p r e t a t i o n s which a r e s u p p o r t e d by f a u n a l and f l o r a l e v i d e n c e l i m i t a t i o n s a r e imposed b e c a u s e of t h e d i a g e n e t i c d e s t r u c t i o n o f p o l y c r y s t a l l i n e q u a r t z and r o c k f r a g m e n t s (Walker, 1974; Walker e t a l . , 1978).
The s e l e c t i v e
d e s t r u c t i o n of l a b i l e m i n e r a l s d u r i n g d i a g e n e s i s and t h e r e p l a c e m e n t of i n t e r i o r g r a i n c o n t a c t s by a u t h i g e n i c c l a y may r e s u l t i n t h e d i s a g g r e g a t i o n o f rock fragments and p o l y c r y s t a l l i n e q u a r t z g r a i n s i n t o t h e i r d a u g h t e r m o n o c r y s t a l l i n e q u a r t z u n i t s .
60
Walker (1978) has p r e s e n t e d p e t r o g r a p h i c e v i d e n c e which i n d i c a t e s t h a t s u c h d i a g e n e t i c m o d i f i c a t i o n h a s been i m p o r t a n t i n t h e F o u n t a i n Formation b u t h e h a s n o t p r e s e n t e d any q u a n t i t a t i v e i n f o r m a t i o n t o s u p p o r t h i s c o n c l u s i o n t h a t t h e t e c h n i q u e d e s c r i b e d by Mack and S u t t n e r (1977) c a n n o t b e a p p l i e d t o t h e F o u n t a i n Formation. Such q u a n t i t a t i v e i n f o r m a t i o n on t h e d e g r e e of d e s t r u c t i o n d u r i n g d i a g e n e s i s may b e d i f f i c u l t o r even i m p o s s i b l e t o c o l l a t e b u t d e t a i l e d s t u d i e s a r e c l e a r l y needed i n t h i s f i e l d . There a r e s e v e r a l p o s s i b i l i t i e s . I n t h e f i r s t p l a c e some i n t e r n a l measure i s p o s s i b l e by comparing t h e p r o p o r t i o n s of framework g r a i n s . For example, i n t h e F o u n t a i n Formation t h e r e i s 1 b 2 h times more p o l y c r y s t a l l i n e q u a r t z t h a n i n Holocene Sands from t h e same s o u r c e a r e a
-
a f e a t u r e which s u g g e s t s t h a t d i a g e n e t i c
d i s a g g r e g a t i o n o f p o l y c r y s t a l l i n e g r a i n s c o u l d h a r d l y have been q u a n t i t a t i v e l y important.
Another method might b e t o compare t h e framework m i n e r a l o g y o f zones
which were r a p i d l y cemented and e s c a p e d e x t e n s i v e i n t r a s t r a t a l s o l u t i o n ( s e e Walker
e t a l . , 1978) w i t h uncemented zones which c o u l d have undergone more e x t e n s i v e i n t r a s t r a t a l solution.
The d i f f e r e n c e s would g i v e some i n d i c a t i o n o f t h e e x t e n t
t o which rock fragments and p o l y c r y s t a l l i n e q u a r t z g r a i n s had become d i s a g g r e g a t e d during diagenesis. THE CLASSIFICATION OF ANCIENT CONTINENTAL RED BEDS I n t r o d u c t i o n and p r e v i o u s c l a s s i f i c a t i o n s The p r e v i o u s d i s c u s s i o n s i n t h i s c h a p t e r i n d i c a t e t h a t c o n t i n e n t a l r e d beds o c c u r i n a wide r a n g e o f d i s t i n c t i v e t e c t o n i c s e t t i n g s .
These i n c l u d e f a i l e d c o n t i n e n t a l
r i f t s ( a u l a c o g e n s ) , s t r i k e - s l i p b a s i n s on t r a n s f o r m f a u l t s and a v a r i e t y of l a t e orogenic sedimentary basins.
W i t h i n t h e s e s e t t i n g s a wide r a n g e of c o n t i n e n t a l
sediments a r e deposited i n d e l t a i c , a l l u v i a l and a e o l i a n environments.
Red beds a r e
r e p r e s e n t e d i n a l l of t h e s e d e p o s i t i o n a l environments and t h e a v a i l a b l e e v i d e n c e s u g g e s t s t h a t t h e y may form i r r e s p e c t i v e o f t h e p r e v a i l i n g c l i m a t i c c o n d i t i o n s . D e s p i t e t h e s e c l e a r l y d e f i n e d d i f f e r e n c e s i n c o n t i n e n t a l r e d beds p r e v i o u s c l a s s i f i c a t i o n s have been based a l m o s t e x c l u s i v e l y on c o l o u r v a r i a t i o n s o r t h e g e n e t i c o r i g i n of pigmentary h a e m a t i t e .
T h i s i s u n s a t i s f a c t o r y b e c a u s e i t may
c o n f u s e r e d beds which a r e o f d i v e r s e t e c t o n i c o r s e d i m e n t a r y a s s o c i a t i o n s i m p l y because t h e y have s i m i l a r c o l o u r c h a r a c t e r i s t i c s .
A l s o , t h e u s e of t h e o r i g i n of
pigment a s a c l a s s i f i c a t i o n c r i t e r i o n i s u n s a t i s f a c t o r y b e c a u s e i t c a n h a r d l y l e a d t o mutually exclusive categories.
I t i s now g e n e r a l l y r e c o g n i z e d t h a t t h e v a s t
m a j o r i t y of pigmentary h a e m a t i t e i s o f d i a g e n e t i c o r i g i n ( t h e e x c e p t i o n s i n c l u d e some a e o l i a n s a n d s which c o u l d b e d e r i v e d from l a t e r i t i z e d a l l u v i a l s o u r c e r o c k s ) . T h i s view d o e s , of c o u r s e , c o n t r a s t markedly w i t h t h o s e of e a r l i e r workers. Krynine (1949) proposed a comprehensive c l a s s i f i c a t i o n of r e d beds which u t i l i z e d h i s own i d e a s on t h e o r i g i n o f i r o n o x i d e pigment i n r e d b e d s .
Krynine e n v i s a g e d
t h r e e p o s s i b l e means by which pigmentary o x i d e s c o u l d c o n t r i b u t e r e d c o l o u r a t i o n :
61 d e t r i t a l p a r t i c l e s , p r e c i p i t a t i o n from s o l u t i o n and t h e f o r m a t i o n of a u t h i g e n i c o x i d e s by i n s i t u o x i d a t i o n .
From t h e s e he developed a f o u r - f o l d c l a s s i f i c a t i o n
( F i g . 1.41) which he a d m i t t e d was "pragmatic" and " n o t e n t i r e l y l o g i c a l " (Krynine
RED SOILS
c. 0x1DATlON
I
Primary DETRITAL Continental
I-
.\ RED BEDS
\ Marine or bog / CHEMICAL PRECIPITATES
Fig.l.40. K r y n i n e ' s (1949) c l a s s i f i c a t i o n of r e d beds b a s e d on g e n e s i s and provenance o f f e r r i c o x i d e pigment. 1949, p . 6 0 ) .
These f o u r d i v i s i o n s may b e summarized a s f o l l o w s . I n t h i s c a t e g o r y r e d pigment was b e l i e v e d t o b e produced by
1) P r i m a r y r e d b e d s .
w e a t h e r i n g i n t h e s o u r c e a r e a and i n c o r p o r a t e d d i r e c t l y i n t o t h e d e p o s i t e d sediment e i t h e r w i t h i n t h e r e g o l i t h (Primary R e s i d u a l Red Beds) o r by l o c a l reworking, ( P r i m a r y Reworked Red Beds) o r a f t e r e r o s i o n and t r a n s p o r t (Primary D e t r i t a l Red Beds) 2) Post-depositional
a n d p o s t - d i a g e n e t i c r e d beds.
Produced by t h e o x i d a t i o n of
non-red d e t r i t u s e i t h e r immediately a f t e r d e p o s i t i o n ( P o s t - D e p o s t t i o n a l Red Beds) o r a f t e r b u r i a l , emergence and deep s u b - s u r f a c e o x i d a t i o n ( P o s t - D i a g e n e t i c Red Beds)
3) Secondary r e d beds.
Produced by t h e e r o s i o n a n d r e d e p o s i t i o n of p r e - e x i s t i n g r e d
beds. 4 ) Chemical r e d beds.
Produced by chemical p r e c i p i t a t i o n w i t h i n t h e d e p o s i t i o n a l
basin under 'marine' o r !freshwater' conditions, o r ' a u t h i g e n i c a l l y and i n t r a s t r a t a l l y
62
by i n f i l t r a t i o n w i t h i n t h e s e d i m e n t ' . U n f o r t u n a t e l y , a s i s t h e c a s e w i t h many c l a s s i f i c a t i o n schemes, K r y n i n e ' s c l a s s i f i c a t i o n of r e d beds h a s s u f f e r e d much a b u s e .
Many workers o n l y d i s t i n g u i s h
between ' p r i m a r y ' and ' s e c o n d a r y ' r e d beds and u s e t h e terms i n a d i f f e r e n t s e n s e from t h a t o r i g i n a l l y i n t e n d e d by Krynine.
Primary r e d beds i s t h e term o f t e n a p p l i e d
t o f a c i e s - c o n t r o l l e d r e d beds whether t h e y b e of d e t r i t a l o r d i a g e n e t i c o r i g i n ( i n t h e s t r i c t s e n s e t h e former c o u l d b e denoted p r i m a r y i f t h e y were d e r i v e d from weathered s o u r c e r o c k s w h i l s t t h e l a t t e r a r e p o s t - d e p o s i t i o n a l r e d beds i n K r y n i n e ' s S i m i l a r l y t h e term s e c o n d a r y r e d beds i s o f t e n used t o d e n o t e r e d
classification).
beds formed by i n s i t u w e a t h e r i n g of s e d i m e n t s a f t e r b u r i a l and emergence.
Such
r e d beds were termed p o s t - d i a g e n e t i c r e d beds by Krynine a n d t h e term s h o u l d b e r e s t r i c t e d t o i t s o r i g i n a l meaning t o d e n o t e t h o s e r e d beds formed by t h e e r o s i o n and r e d e p o s i t i o n of o t h e r r e d beds (e.g. Duchesne R i v e r Formation, Anderson and P i c a r d , 1974). The problem w i t h K r y n i n e ' s g e n e t i c c l a s s i f i c a t i o n i s t h a t i n t h e V a s t m a j o r i t y of To overcome t h i s C l a r k (1962) proposed a f i e l d
cases i t i s d i f f i c u l t t o apply.
c l a s s i f i c a t i o n i n which t h e c o l o u r was r e l a t e d t o s e d i m e n t a r y s t r u c t u r e s , t e x t u r e s and f o s s i l c o n t e n t .
He d i s t i n g u i s h e d t h e f o l l o w i n g t y p e s o f r e d b e d s :
1) Shale-pebble r e d .
B r i c k - r e d t o deep r e d i n c o l o u r t y p i c a l l y w i t h p e b b l e s of r e d
material i n a less-red matrix.
Red c o l o u r becomes d e e p e r towards t h e s o u r c e a r e a
and away from t h e s o u r c e a r e a t h e c o l o u r i s c o n c e n t r a t e d i n s h a l e beds. 2 ) Red c l a y conglomerates.
B r i c k r e d t o deep r e d i n c o l o u r and a s s o c i a t e d w i t h a
c r y s t a l l i n e igneous s o u r c e a r e a .
3) V a r i e g a t e d r e d . and s t r e a k s .
Shales generally purple t o red i n colour with greenish lenses
Sandstones w h i t e , l i g h t - g r e y o r g r e e n i s h when f r e s h , f r e q u e n t l y weath-
e r i n g t o d a r k brown.
C o n c r e t i o n a r y h o r i z o n s and v e r t e b r a t e f o s s i l s a r e o f t e n
abundant.
4 ) Cinnamon r e d .
S h a l e s deep r e d w i t h g r e e n i s h m o t t l i n g a n d abundant r i p p l e marks,
mud c r a c k s and v e r t e b r a t e t r a i l s .
S a n d s t o n e s dark-red t o l i g h t g r e y , u s u a l l y f i n e -
medium g r a i n e d and l a t e r a l l y p e r s i s t e n t .
5 ) Brick red. gypsum.
S i l t s t o n e s b r i c k r e d t o o r a n g e t o ochreous y e l l o w , sometimes w i t h
Sandstones l i g h t r e d t o l i g h t brown and o f t e n l e n s o i d .
6) P a s t e l red.
Laminated l a v e n d e r , y e l l o w , p u r e w h i t e ; l i t t l e r e d c o l o u r .
Mainly
f i n e grained, well-sorted quartzose sandstone. 7) Spattered red.
Red c o l o u r i n c o n c e n t r a t i o n s and i n complex r a m i f y i n g s t a i n s i n
a v a r i e t y of g r e e n i s h c o l o u r e d l i t h o l o g i e s . C l a r k (1962) i n t e r p r e t e d t h e s h a l e - p e b b l e , r e d c l a y conglomerate and v a r i e g a t e d r e d beds a s t y p e s of a l l u v i a l r e d beds formed under d i f f e r e n t climatic c o n d i t i o n s . Cinnamon r e d s and b r i c k r e d s were i n t e r p r e t e d a s e s t u a r i n e , d e l t a i c o r s h a l l o w marine r e d beds formed under d e s e r t c o n d i t i o n s and p a s t e l r e d s were c o n s i d e r e d t o b e o f lacustrine origin.
S p a t t e r e d r e d beds were b e l i e v e d by C l a r k t o b e t h e t y p i c a l
r e s u l t o f p o s t - d i a g e n e t i c w e a t h e r i n g and t h u s o f no e n v i r o n m e n t a l s i g n i f i c a n c e . Although t h i s c l a s s i f i c a t i o n scheme i s p r e f e r a b l e t o t h e p u r e l y g e n e t i c scheme o f Krynine i t h a s t h e major f a i l i n g of a t t a c h i n g t o o much s i g n i f i c a n c e t o t h e c o l o u r o f r e d beds.
We now know t h a t pigmentary o x i d e s may form i n r e d beds i n
a v a r i e t y o f ways and so r e d bed c l a s s i f i c a t i o n s b a s e d on c o l o u r o r t h e o r i g i n of colour a r e l i k e l y t o be misleading. Reddening and d e p o s i t i o n a l environment I n Recent s e d i m e n t s i r o n o x i d e i s most c o m o n l y p r e s e n t a s a tan-brown c o l o u r e d compound p r o b a b l y c o n s i s t i n g of amorphous Fe(0H)
and g o e t h i t e . F i n e g r a i n e d 3 h a e m a t i t e i s a p p a r e n t l y n o t p r e s e n t i n s u f f i c i e n t abundance t o i m p a r t a r e d c o l o u r t o t h e sediment (Walker, 1967b). The importance o f d e t r i t a l i r o n h y d r o x i d e s i n p r o d u c i n g r e d beds h a s been demonstrated i n a series o f p a p e r s by Van Houten (1961, 1964, 1 9 6 8 ) , p a r t i c u l a r l y i n t r o p i c a l savanna a l l u v i u m , which c o n t a i n s s u f f i c i e n t t a n o r brown c o l o u r e d i r o n hydroxide i n t h e s i l t and c l a y f r a c t i o n t o t u r n t h e s e d i m e n t r e d a f t e r i t s c o n v e r s i o n i n t o haematite. (1969a).
The f e a s i b i l i t y of s u c h a n ' a g e i n g ' p r o c e s s i s d e s c r i b e d by Berner
The e n r i c h m e n t o f i r o n h y d r o x i d e s i n t h e c l a y and s i l t f r a c t i o n of
f l u v i a t i l e r e d beds goes some way t o e x p l a i n i n g t h e f a c t t h a t r e d s i l t s t o h e s and mudstones g e n e r a l l y c o n t a i n much more i r o n t h a n a s s o c i a t e d s a n d s t o n e s .
The p o s t -
d e p o s i t i o n a l a g e i n g of i r o n h y d r o x i d e s i s l i k e l y t o b e i m p o r t a n t i n a n c i e n t f i n e g r a i n e d a l l u v i u m p a r t i c u l a r l y i n c o a s t a l a l l u v i a l p l a i n s w i t h well-developed f l o o d plain deposits.
I t i s a l s o o f c o n s i d e r a b l e i m p o r t a n c e i n t h o s e r e d beds which a r e
predominantly mudstones and o c c u r i n t e r d i g i t a t e d w i t h non-red c o a l and c o a l - b e a r i n g s t r a t a i n d e l t a i c a s s o c i a t e d sequences,
Ageing of d e t r i t a l hydroxides i s u n l i k e l y
t o b e a n i m p o r t a n t p r o c e s s i n a e o l i a n s a n d s t o n e s b e c a u s e of t h e winnowing of t h e finer fractions. The o t h e r major s o u r c e o f pigment i n r e d beds i s the i n t r a s t r a t a l breakdown of d e t r i t a l i r o n bearing minerals.
T h i s produces a number of a u t h i g e n i c m i n e r a l s ,
i n c l u d i n g pigmentary h a e m a t i t e , a n d has been most a b l y demonstrated by Walker (1967a, 1976) and Walker e t a l . (1978) i n a series of f i r s t c y c l e a r k o s e s d e p o s i t e d under d e s e r t c o n d i t i o n s d u r i n g t h e Cenozoic of t h e s o u t h w e s t e r n USA and n o r t h w e s t e r n Mexico.
F r e s h l y d e p o s i t e d a l l u v i u m c o n t a i n s no d e t r i t a l c l a y a l t h o u g h t h i s may be
mechanically i n f i l t r a t e d during e a r l y diagenesis.
I n t h e s e sediments reddening
p r o g r e s s e s w i t h time as d e t r i t a l f e r r o m a g n e s i a n s i l i c a t e s a r e a l t e r e d by i n t e r s t i t i a l groundwater.
Most o f t h e pigmentary h a e m a t i t e i s t h u s a u t h i g e n i c and many
a n c i e n t f i r s t - c y c l e a r k o s i c r e d beds were p r o b a b l y produced i n a s i m i l a r manner (e.g.
S c h l u g e r a n d Roberson, 1975; Walker and L a r s o n , 1976; Hubert and Reed, 1978).
64
The r e l a t i v e importance of a u t h i g e n i c h a e m a t i t e depends upon a number of f a c t o r s i n c l u d i n g t h e o v e r a l l t e c t o n i c s e t t i n g , c l i m a t e , s o u r c e a r e a , c o m p o s i t i o n and d e p o s i t i o n a l environment.
I t i s l i k e l y t h a t t h e r e a r e i m p o r t a n t amounts of a u t h -
i g e n i c h a e m a t i t e i n most r e d beds b u t i t i s most s p e c t a c u l a r l y developed i n f i r s t c y c l e d e s e r t a l l u v i u m d e r i v e d from c r y s t a l l i n e s o u r c e r o c k s .
Such a l l u v i u m may
o r i g i n a l l y be composed of t o t a l l y f r e s h , u n o x i d i z e d d e t r i t u s because o f r a p i d e o o s i o n and d e p o s i t i o n .
During s u b s e q u e n t b u r i a l h a e m a t i t e would b e produced by
i n t r a s t r a t a l a l t e r a t i o n and t h e pigment c o u l d be c o m p l e t e l y of a u t h i g e n i c o r i g i n . The d i a g e n e t i c model of r e d d e n i n g a l s o a p p e a r s , i n g e n e r a l , t o b e a p p l i c a b l e t o aeolian sands.
Most p r e s e n t - d a y a e o l i a n sands a r e n o t r e d i n c o l o u r b u t t h e
m a j o r i t y of a n c i e n t ones a r e , a f e a t u r e s u g g e s t i n g t h a t d i a g e n e t i c p r o c e s s e s p l a y a n i m p o r t a n t p a r t i n reddening.
There a r e , however, i m p o r t a n t e x c e p t i o n s i n t h a t
some p r e s e n t - d a y a e o l i a n s a n d s a r e d i s t i n c t l y r e d . F o l k (1976) h a s been a b l e t o d e m o n s t r a t e t h a t t h e r e d a e o l i a n s a n d s t o n e s i n t h e
Simpson Desert o f A u s t r a l i a a r e l a r g e l y d e r i v e d from T e r t i a r y f l u v i a l s e d i m e n t s which were p r e v i o u s l y reddened d u r i n g a p e r i o d of r e g i o n a l l a t e r i t i z a t i o n (Schmidt and Embleton, 1976).
Krynine (1949) had p r e v i o u s l y r e c o g n i z e d t h a t many o f t h e
Recent " r e d s a n d s o f t h e d e s e r t " were reworked r e d f l u v i a l s a n d s e.g. S a h a r a , where t h e y a r e d e r i v e d from r e d O l i g o c e n e f l u v i a l s a n d s .
i n the Northern
There i s e v i d e n c e
t h a t some a n c i e n t a l l u v i a l r e d beds may a l s o have been d e r i v e d from p r e v i o u s l y formed r e d beds ( e . g . Anderson and P i c a r d , 1974) b u t t h e s e u s u a l l y show e v i d e n c e of a d d i t i o n a l a u t h i g e n i c pigment. The pigment i n ' a n y p a r t i c u l a r r e d bed sequence may t h u s b e of d i v e r s e o r i g i n . Colour v a r i a t i o n s , o r t h e o r i g i n o f pigment, a r e t h e r e f o r e c o n s i d e r e d t o b e u n s a t i s f a c t o r y methods of c l a s s i f y i n g c o n t i n e n t a l r e d beds. Palaeomagnetic c l s s s i f i c a t i o n o f c o n t i n e n t a l r e d beds C o n t i n e n t a l r e d beds have been used i n p a l a e o m a g n e t i c s t u d i e s f o r many y e a r s and t h e i r magnetic p r o p e r t i e s have been i n v e s t i g a t e d i n d e t a i l ( C o l l i n s o n , 1965a, 1967, 1968b, 1974).
They g e n e r a l l y p o s s e s s a r e l a t i v e l y s t r o n g ( l o m 4 t o
n a t u r a l remanent m a g n e t i z a t i o n (W) which i s c a r r i e d by h a e m a t i t e .
G cm3 g-')
There h a s been
c o n s i d e r a b l e d e b a t e r e g a r d i n g t h e o r i g i n o f h a e m a t i t e i n r e d beds a s well a s t h e o r i g i n ot t h e m a g n e t i z a t i o n and i t s t i m i n g r e l a t i v e t o t h e d e p o s i t i o n a l a g e o f t h e sediments. Krynine (1949) b e l i e v e d t h a t r e d beds c o u l d form by d i r e c t d e r i v a t i o n o f h a e m a t i t e from r e d t r o p i c a l s o i l s ( l a t o s o l s ) .
I n t h i s c a s e ( i f i t were t r u e ) t h e m a g n e t i z a t i o n
o f t h e r e d beds would b e a d e t r i t a l remanent m a g n e t i z a t i o n (DRM) produced by t h e a l i g n m e n t of magnetic p a r t i c l e s d u r i n g s e d i m e n t a t i o n a n d i t would a c c u r a t e l y r e c o r d t h e ambient geomagnetic f i e l d a t t h e time o f d e p o s i t i o n .
Van Houten (1964) b e l i e v e d
t h a t t h e h a e m a t i t e c o u l d form d i a g e n e t i c a l l y by t h e i n s i t u d e h y d r a t i o n of y e l l o w
65 o r brown i r o n hydroxides d e r i v e d from d e e p l y weathered s o i l s b u t n o t n e c e s s a r i l y latosols.
T h i s c o n v e r s i o n i s thought t o t a k e p l a c e r a p i d l y i n t h e g e o l o g i c a l
s u b - s u r f a c e and i f i t d i d o c c u r would produce a chemical remanent m a g n e t i z a t i o n (CRM) by t h e growth o f h a e m a t i t e through i t s c r i t i c a l g r a i n s i z e .
I t would
thus r e c o r d a s h o r t - t e r m i n t e g r a t i o n of t h e ambient geomagnetic f i e l d , a l t h o u g h i t would have s u f f i c i e n t p r e c i s i o n t o be g e n e r a l l y a c c e p t a b l e f o r r o u t i n e palaeo-
magnetic purposes.
More r e c e n t l y , Walker (1976) has a r g u e d t h a t r e d beds i n both
t h e a r i d and warm m o i s t c l i m a t i c a s s o c i a t i o n s form due t o t h e a u t h i g e n e s i s of h e a m a t i t e which h a s formed d i a g e n e t i c a l l y owing t o t h e i n s i t u a l t e r a t i o n o f d e t r i t a l s i l i c a t e minerals.
S i n c e t h i s p r o c e s s c o n t i n u e s w i t h b u r i a l , any m a g n e t i z a t i o n of
t h e r e d bed would b e a CRM.
Moreover, i t would be a long-term i n t e g r a t i o n of t h e
ambient geomagnetic f i e l d , p o s s i b l y c o n t a i n i n g multicomponent m a g n e t i z a t i o n s , and a s s u c h would have l i t t l e o r no palaeomagnetic v a l u e .
Walker has p r e s e n t e d convincing
evidence i n s u p p o r t o f h i s h y p o t h e s i s b o t h f o r t h e T e r t i a r y a r k o s i c sediments of Baja C s l i f o r n i a and a l s o t h e a l l u v i a l d e p o s i t s o f Holocene and P l e i s t o c e n e age i n P u e r t o Rico and t h e Orinoco Basin, The g e o l o g i c a l evidence i n f a v o u r of t h e d i a g e n e t i c o r i g i n o f h a e m a t i t e i n r e d beds i s convincing; a v a r i e t y o f d i a g e n e t i c p r o c e s s e s i n c l u d i n g pigment p r o d u c t i o n , t h e pseudomorphing o f s i l i c a t e s and t h e o x i d a t i o n o f m a g n e t i t e t a k e p l a c e a t v a r i o u s
times throughout t h e d i a g e n e t i c h i s t o r y o f c o n t i n e n t a l r e d beds.
The paleeomagnetic
e v i d e n c e i n d i c a t i n g t h a t h a e m a t i t e c a r r i e s t h e NRM of c o n t i n e n t a l r e d beds i s e q u a l l y compelling and i t must be concluded t h a t t h e m a g n e t i z a t i o n of r e d beds i s a c q u i r e d during diagenesis.
However, t h e magnetic c h a r a c t e r i s t i c s o f r e d beds a r e extremely
complex and g e n e r a l i z a t i o n s r e g a r d i n g t h e t i m i n g o f m a g n e t i z a t i o n i n a i l r e a Deas s h o u l d n o t be made a s t h e y can o n l y be m i s l e a d i n g .
Three b r o a d t y p e s o f r e d bed
m a g n e t i z a t i o n have been r e c o g n i z e d by Turner (1979) which r e f l e c t p r o g r e s s i v e degrees of d i a g e n e t i c a l t e r a t i o n .
These i n c l u d e :
Type A m a g n e t i z a t i o n s which c o n t a i n
e s s e n t i a l l y a s i n g l e component of m a g n e t i z a t i o n and may show d i s c r e t e zones of normal and r e v e r s e d p o l a r i t y .
E a r l y Mesozoic sandy a l l u v i u m a f f o r d s p a r t i c u l a r l y good
examples o f t h i s t y p e o f m a g n e t i z a t i o n ( H e l s l e y and S t e i n e r , 1974; Turner and I x e r , 1977).
Type B m a g n e t i z a t i o n s a r e t y p i c a l l y composite o r multicomponent and were
a c q u i r e d o v e r a l o n g e r t i m e i n t e r v a l , spanning a t l e a s t one geomagnetic f i e l d reversal.
They t h e r e f o r e r e p r e s e n t a more advanced s t a g e of d i a g e n e s i s t h a n t h a t
r e p r e s e n t e d by Type A m a g n e t i z a t i o n s .
Examples a r e most commonly found i n t h e
Upper P a l a e o z o i c ( e . g . Roy and P a r k , 1974; Turner and A r c h e r , 1975).
Type C
m a g n e t i z a t i o n s a r e t h o s e which can be demonstrated t o be complete replacements o f t h e o r i g i n a l m a g n e t i z a t i o n and t h e r e f o r e b e a r no r e l a t i o n s h i p t o t h e d e p o s i t i o n a l a g e of t h e r o c k s .
They a r e most commonly found i n v e r y a n c i e n t r e d beds which have
suffered extensive diagenetic alterations. Precambrian ( e . g . Lomax and B r i d e n , 1977).
A number of examples a r e known from t h e
These t h r e e t y p e s o f m a g n e t i z a t i o n r e p r e s e n t s t a g e s i n t h e c o n t i n u o u s d i a g e n e t i c m o d i f i c a t i o n of c o n t i n e n t a l r e d b e d s .
The p a l a e o m a g n e t i c c h a r a c t e r i s t i c s o f r e d
beds p r o v i d e s a s e n s i t i v e means of d e t e c t i n g d i a g e n e t i c changes and t h u s e n a b l e s t h e d i s t i n c t i o n between r e d beds which have s u f f e r e d d i f f e r e n t d e g r e e s o f d i a g e n e s i s . Many f a c t o r s i n c l u d i n g a g e , m i n e r a l o g i c a l c o m p o s i t i o n and t e x t u r a l p r o p e r t i e s a f f e c t t h e r a t e and e x t e n t of d i a g e n e s i s .
These p r o p e r t i e s a r e , however, d i f f i c u l t t o
monitor and t h e p a l a e o m a g n e t i c t e c h n i q u e a l l o w s a r e l a t i v e l y s i m p l e means of c l a s s i f y i n g them i n t o meaningful g r o u p s , A p r a c t i c a l c l a s s i f i c a t i o n of c o n t i n e n t a l r e d beds
The more r e a l i s t i c approach t o t h e c l a s s i f i c a t i o n of r e d beds must t a k e i n t o a c c o u n t a l l s i g n i f i c a n t c h a r a c t e r i s t i c s i n c l u d i n g d e p o s i t i o n a l , t e x t u r a l and m i n e r a l o g i c a l p r o p e r t i e s , and d i a g e n e t i c f e a t u r e s .
The i n i t i a l d e p o s i t i o n a l
environment i s i m p o r t a n t i n c o n t r o l l i n g t h e s u b s e q u e n t d i a g e n e t i c h i s t o r y .
The
main d e p o s i t i o n a l environment i n which c o n t i n e n t a l r e d beds a c c u m u l a t e , t o g e t h e r w i t h t h e i r main m i n e r a l o g i c a l , t e x t u r a l and d i a g e n e t i c p r o p e r t i e s a r e s u f f i c i e n t l y d i s t i n c t t o e n a b l e them t o b e c l a s s i f i e d i n t h e manner shown i n 7 a b l e 1.2. major a s s o c i a t i o n s of c o n t i n e n t a l r e d beds a r e r e c o g n i z e d :
Three
a l l u v i a l , d e s e r t and
d e l t a p l a i n r e d beds which a r e r e a d i l y d i s t i n g u i s h e d on t h e b a s i s o f t h e i r characteristic facies. A l l u v i a l r e d beds comprise a wide range of d e p o s i t i o n a l f a c i e s r a n g i n g from marginal a l l u v i a l fans and coarse grained braided r i v e r s t o a l l u v i a l p l a i n s with l o w - s i n u o s i t y o r meandering r i v e r s .
A l l u v i a l f a n s and c o a r s e g r a i n e d b r a i d e d r i v e r s
t y p i c a l l y develop i n t e c t o n i c a l l y a c t i v e s e d i m e n t a r y b a s i n s and a r e a p a r t i c u l a r f e a t u r e of s t r i k e - s l i p b a s i n s developed on t r a n s f o r m f a u l t s .
They a l s o o c c u r a s a
m a r g i n a l f a c i e s w i t h i n f a i l e d c o n t i n e n t a l r i f t s and i n a v a r i e t y of l a t e - o r o g e n i c basins.
C l i m a t i c a l l y , a l l u v i a l r e d beds have no s p e c i a l s i g n i f i c a n c e a l t h o u g h
palaeomagnetic e v i d e n c e does s u g g e s t t h a t t h e y may have been d e p o s i t e d w i t h i n 20'-40'
of t h e p a l a e o - e q u a t o r , a f e a t u r e which s u g g e s t s t h a t t h e y do n o t
r e a d i l y form a t h i g h p a l a e o l a t i t u d e s i n c o l d c l i m a t e s .
There a r e no p r e s e n t -
day a l l u v i a l r e d beds and t h e a n c i e n t o c c u r r e n c e s a r e c o n s i d e r e d t o b e a l m o s t e x c l u s i v e l y of d i a g e n e t i c o r i g i n .
I n many examples t h e i n t r a s t r a t a l a l t e r a t i o n o f
d e t r i t a l i r o n - s i l i c a t e s h a s been c o n v i n c i n g l y demonstrated and t h e r e can be l i t t l e doubt of t h e a u t h i g e n i c n a t u r e of t h e pigmentary i r o n o x i d e s .
I n t r a s t r a t a l solution
i s , however, most i m p o r t a n t i n c o a r s e g r a i n e d a l l u v i u m i n which t h e r e i s l i t t l e
i n t e r s t i t i a l c l a y and s i l t y m a t r i x .
Such a l l u v i u m i s e f f e c t i v e l y a p o r o u s , open
system through which o x i d i z i n g groundwater can p a s s a s i t a l t e r s t h e framework mineralogy.
I n f i n e r g r a i n e d a l l u v i u m such a s t h e f l o o d p l a i n s o f meandering r i v e r s
t h e emphasis is s l i g h t l y d i f f e r e n t .
Here t h e s e d i m e n t s c o n t a i n abundant c l a y and
s i l t which c o n t a i n s d e t r i t a l f e r r i c h y d r o x i d e s .
Dehydration of such m a t e r i a l c o u l d
TABLE 1.2. The c l a s s i f i c a t i o n o f c o n t i n e n t a l r e d beds i n t o t h r e e major a s s o c i a t i o n s based on d e p o s i t i o n a l environment, colour c h a r a c t e r i s t i c s and d i a g e n e t i c f e a t u r e s . Depositional C h a r a c t e r i s t i c s
Redness C h a r a c t e r i s t i c s
Diagenetic C h a r a c t e r i s t i c s
A l l u v i a l Fan-Braided R i v e r s
Pebbly a l l w i u m Mostly m u l t i s t o r e y channel deposits interbedded debrisflow and stream Flood deposits
Uniformly red. F i n e r g r a i n e d horizons more i n t e n s e l y red
I n t r a s t r a t a l s o l u t i o n of s i l i c a t e grains. Increase i n mineralogical maturity. Decrease i n t e x t u r a l m a t u r i t y Authigenic q u a r t z , f e l d s p a r , c a l c i t e , clays
A l l u v i a l P l a i n s w i t h high o r low-sinuosity streams
Sandy and muddy alluvium i n FU c y c l e s . Sand u n i t s a r e channel and bank d e p o s i t s . Mudstones a r e f l o o d p l a i n deposits. Pedogenic modifi c a t i o n of f l o o d p l a i n d e p o s i t s common
Variegated. Sandy u n i t s r e d o r drab. Mudstone u n i t s r e d o r variegated
A s above. Also i n v e r s i o n of f e r r i c hydroxides - haematite i n muddy f l o o d p l a i n d e p o s i t s . C a l c r e t e a common f e a t u r e of floodplain deposits
DESERT RED BEDS
Cross s t r a t i f i e d w e l l - s o r t e d sands w i t h s t e e p f o r e s e t . i n c l i n a t i o n s . Interbedded w i t h i n l a n d sabkha and d e s e r t l a k e (gypsum and a n h y d r i t e ) and poorly-sorted wadi deposits .
Uniformly r e d
I n t r a s t r a t a l s o l u t i o n of f e l d s p a r s and ferromagnesian m i n e r a l s . Authigenic q u a r t z and f e l d s p a r . Gypsuma n h y d r i t e cements
CU and FU c y c l e s . S i l t s t o n e s frequently associated with s e a t e a r t h s and o t h e r pedogenic m o d i f i c a t i o n s . Mainly floodp l a i n , well-drained swamp and lacustrine delta deposits
Variegated. Red beds u s u a l l y confined t o A mottled mudstones. appearance i s common
Early diagenetic pyrite. Formation of k a o l i n i t e by humid a c i d leaching. S i d e r i t e . Pedogenic nodules
ALLUVIAL RED BEDS
Alluvial, aeolian sabkhas, d e s e r t l a k e s
DELTA PLAIN RED BEDS D e l t a p l a i n s of river-dominated deltas
produce s i g n i f i c a n t q u a n t i t i e s o f h a e m a t i t e and t h u s c o n t r i b u t e d i r e c t l y t o t h e r e d d e n i n g of t h e sediment. Desert r e d beds i n c l u d e a wide range of f a c i e s d e p o s i t e d i n a h o t , a r i d c l i m a t e .
They i n c l u d e t h e d e p o s i t s o f l a k e s , i n l a n d s a b k h a s , a e o l i a n dunes, and a v a r i e t y of d e s e r t a l l u v i a l sediments. on s t a b l e c r a t o n i c a r e a s . occur.
Desert b a s i n s may b e t e c t o n i c a l l y a c t i v e o r developed
I t i s t h e l a t t e r on which t h e l a r g e s a n d s e a s o f t h e world
Desert a l l u v i a l s e d i m e n t s a r e e s s e n t i a l l y t h e same a s t h e i r c o a r s e g r a i n e d
counterparts i n the general a l l u v i a l category.
They d i f f e r i n b e i n g l a r g e l y ephemeral,
c o n s i s t i n g o f p o o r l y s o r t e d d e t r i t u s which may b e i n t e r b e d d e d w i t h a e o l i a n s a n d s . The r e d d e n i n g o f c o a r s e g r a i n e d d e s e r t a l l u v i u m t a k e s p l a c e by t h e i n t r a s t r a t a l a l t e r a t i o n of i r o n s i l i c a t e s . A e o l i a n s a n d s a r e w i d e l y r e g a r d e d a s one of t h e most d i s t i n c t i v e f a c i e s of c o n t i n e n t a l r e d beds a l t h o u g h t h e y a r e s t i l l p o o r l y u n d e r s t o o d .
They may be more
d i f f i c u l t t o recognize i n t h e s t r a t i g r a p h i c record than i s generally appreciated and i t i s n o t p o s s i b l e t o make g e n e r a l i z a t i o n s a b o u t t h e o r i g i n of t h e i r r e d colouration.
P r e s e n t - d a y a e o l i a n s a n d s e x i s t i n v a r i o u s c o l o u r s t a t e s from r e d
t o d r a b and t h e r e i s e v i d e n c e t h a t b o t h d e t r i t a l a n d d i a g e n e t i c r e d d e n i n g may be important processes. The l a s t r e d bed f a c i e s a s s o c i a t i o n which i s r e c o g n i z e d a r e d e l t a p l a i n r e d beds. These a r e d e p o s i t e d i n d e l t a i c complexes a t c o n t i n e n t a l margins and i n a v a r i e t y o f tectonic settings.
A s t h e d e p o s i t s of f l u v i a l d i s t r i b u t a r y c h a n n e l s and f l o o d -
p l a i n s t h e y b e a r many c h a r a c t e r i s t i c s i n common w i t h some a l l u v i a l r e d beds.
They
a r e d i s t i n g u i s h e d by t h e i r a s s o c i a t i o n w i t h d e l t a i c s e d i m e n t s and a number of distinctive features.
The l a t t e r i n c l u d e a n abundance o f c o l o u r m o t t l e d mudstones,
r o o t l e t h o r i z o n s , s e a t e a r t h s and o c c a s i o n a l c o a l h o r i z o n s .
These f e a t u r e s
i n d i c a t e a m o i s t t r o p i c a l climate a n d many of t h e d e s c r i b e d a n c i e n t d e l t a p l a i n r e d beds were d e p o s i t e d under t h e s e c o n d i t i o n s .
I t i s , however, l i k e l y t h a t d e l t a
p l a i n r e d beds a l s o formed under o t h e r c l i m a t i c c o n d i t i o n s and have been d e s c r i b e d from a r i d c l i m a t e s and t r o p i c a l c l i m a t e s whth s e a s o n a l a r i d i t y .
They may b e
i n d i c a t i v e o f r e l a t i v e l y low p a l a e o l a t i t u d e s . The r e d d e n i n g p r o c e s s e s i n d e l t a p l a i n r e d beds c o n t r a s t w i t h t h o s e o f t h e a l l u v i a l and d e s e r t a s s o c i a t i o n s .
Pigmentary o x i d e s may form by i n t r a s t r a t a l s o l u t i o n
o f d e t r i t a l i r o n s i l i c a t e s b u t t h e predominance o f mudstone u n i t s means t h a t a number o f o t h e r p r o c e s s e s may b e more i m p o r t a n t .
These i n c l u d e t h e d e h y d r a t i o n o f g o e t h i t e ,
t h e r e l e a s e of o x i d e s d u r i n g c l a y m i n e r a l t r a n s f o r m a t i o n s and t h e o x i d a t i o n of f i n e l y disseminated p y r i t e .
Pedogenic p r o c e s s e s p l a y a n i m p o r t a n t r o l e i n t h e p o s t -
depositional reddening of d e l t a p l a i n sediments.
69
CHAPTER 2 DESERT RED BEDS INTRODUCTION
Desert r e d beds comprise a wide v a r i e t y of s e d i m e n t a r y f a c i e s d e p o s i t e d under a r i d and s e m i - a r i d c o n d i t i o n s i n a range of d e p o s i t i o n a l environments.
They i n c l u d e
a l l u v i a l f a n s , d e s e r t f l u v i a l (wadi) d e p o s i t s , a e o l i a n s a n d s and t h e d e p o s i t s of d e s e r t l a k e s and i n l a n d sabkhas w i t h t h e i r a s s o c i a t e d e v a p o r i t e s .
Red c o l o u r a t i o n
i s a u b i q u i t o u s f e a t u r e o f a n c i e n t d e s e r t s e d i m e n t s and many e a r l y workers b e l i e v e d t h a t a l l r e d beds were formed under d e s e r t c o n d i t i o n s .
T h i s b e l i e f was based l a r g e l y
on t h e m i s c o n c e p t i o n t h a t Recent d e s e r t s e d i m e n t s a r e r e d i n c o l o u r .
I n fact the
majority of d e s e r t sediments a r e not red a t t h e present t i m e (although there a r e a few n o t a b l e e x c e p t i o n s ) and r e c e n t work i n d i c a t e s t h a t many of t h e w o r l d ' s d e s e r t s a r e s l o w l y reddening w i t h time a s a r e s u l t of d i a g e n e t i c a l t e r a t i o n . Those a e o l i a n s a n d s which a r e a c t u a l l y r e d i n c o l o u r , a good example b e i n g t h e Simpson Desert of A u s t r a l i a , c a n n o t b e a c c o u n t e d f o r by d i a g e n e s i s a l o n e and a p p e a r t o have been d e r i v e d from f l u v i a l s e d i m e n t s which were p r e v i o u s l y reddened d u r i n g a Pleistocene pluvial climate.
Recent d e s e r t s e d i m e n t s t h e r e f o r e a p p e a r t o show two
d i s t i n c t r e d d e n i n g p r o c e s s e s and t h i s must b e a n i m p o r t a n t f a c t o r when c o n s i d e r i n g t h e o r i g i n o f a n c i e n t d e s e r t r e d beds. I n t h i s c h a p t e r t h e s e d i m e n t a r y environments and f a c i e s of modern d e s e r t sediments a r e reviewed w i t h s p e c i a l r e f e r e n c e t o a e o l i a n s a n d d e p o s i t s
The r e d d e n i n g of
d e s e r t s e d i m e n t s i s d e s c r i b e d u s i n g t h e d e s e r t a l l u v i u m of t h e s o u t h w e s t e r n USA and t h e a e o l i a n sands of t h e Simpson Desert a s examples.
A s e c t i o n i s devoted t o a n c i e n t
a e o l i a n r e d beds and i n c l u d e s d i s c u s s i o n s o f t h e problems of t h e r e c o g n i t i o n of a e o l i a n sands i n t h e s t r a t i g r a p h i c r e c o r d and t h e o r i g i n of t h e i r r e d c o l o u r a t i o n . An a n c i e n t example, t h e Lower Permian o f n o r t h w e s t Europe, i s d e s c r i b e d i n d e t a i l .
MODERN DESERT SEDIMENTS Modern a r i d d e s e r t s a r e b a r r e n t r a c t s of l a n d o v e r which r a i n f a l l i s t o o l i m i t e d t o support vegetation.
The upper l i m i t of r a i n f a l l i s a b o u t 25cm a y e a r and t h e r a t e
o f e v a p o r a t i o n f a r exceeds t h e r a t e of p r e c i p i t a t i o n .
D e s e r t s c o v e r a b o u t 20% of
t h e w o r l d ' s l a n d s u r f a c e a n d a r e c o n c e n t r a t e d between t h e l a t i t u d e s of about 10'
30'
and
n o r t h and s o u t h o f t h e e q u a t o r i n t h e r e g i o n s o f p r e v a i l i n g t r a d e winds ( F i g . 2 . 1 ) .
D e s e r t s o c c u r between t h e s e l a t i t u d e s b e c a u s e t h e r e l a t i v e humidity of t h e descending h i g h - p r e s s u r e a i r of t h e h o r s e l a t i t u d e s (30'
n o r t h and s o u t h o f t h e e q u a t o r )
d e c r e a s e s a s t h e a i r i s compressed a d i a b a t i c a l l y and g i v e s r i s e t o c l o u d l e s s s k i e s . P a r t o f t h e a i r flows towards t h e e q u a t o r and i s d e f l e c t e d westwards by t h e C o r i o l i s
OL I m p o r t a n t f e a t u r e s of d e s e r t s a r e t h e d i u r n a l extremes of t e m p e r a t u r e and humidity which promote w e a t h e r i n g p r o c e s s e s ( O l l i e r , 1 9 6 9 ) .
During t h e c o o l e r hours
before
dawn t h e r e l a t i v e humidity may be a s h i g h a s 100% and heavy dews f r e q u e n t l y c o v e r rock o u t c r o p s .
These dews promote chemical c o r r o s i o n .
M o i s t u r e i s a l s o drawn t o
t h e s u r f a c e by c a p i l l a r i t y and e v a p o r a t i o n r e s u l t s i n t h e growth o f gypsum and h a l i t e c r y s t a l s which c a u s e e x p a n s i o n and mechanical w e a t h e r i n g of t h e h o s t r o c k . Rapid d i u r n a l changes i n t e m p e r a t u r e c a u s e d i f f e r e n t i a l e x p a n s i o n and c o n t r a c t i o n s t r e s s e s between t h e s u r f a c e and t h e u n d e r l y i n g r o c k , r e s u l t i n g i n s p a l l i n g of t h e rock s u r f a c e and t h e s p l i t t i n g of b o u l d e r s .
Igneous rock s u r f a c e s a r e t y p i c a l l y
rounded by e x f o l i a t i o n . The s i l t and s a n d - s i z e d p r o d u c t s of w e a t h e r i n g a r e s e t i n t o motion by t h e wind i n deserts,
T r a n s p o r t t a k e s p l a c e b y s u s p e n s i o n . s a l t a t i o n and s u r f a c e c r e e p
71 (Bagnold, 1941).
The c o a r s e r , more r e s i s t a n t w e a t h e r i n g p r o d u c t s a r e l e f t on t h e
d e s e r t s u r f a c e a s a l a g d e p o s i t t h a t may b e a b r a d e d by s a n d - l a d e n winds i n t o v e n t i facts.
Lag g r a v e l s on t h e d e s e r t s u r f a c e may e f f e c t i v e l y p r e v e n t most of t h e
u n d e r l y i n g s e d i m e n t s from b e i n g removed by t h e wind. D e s e r t sediments may accumulate i n b o t h a c t i v e t e c t o n i c s e t t i n g s where f a u l t controlled basins a r e rapidly subsiding (e.g. cratonic areas (e.g. Sahara).
s o u t h w e s t e r n USA) o r on s t a b l e
Many of t h e d e s e r t b a s i n s have i n l a n d d r a i n a g e
p a t t e r n s which may b e l a r g e l y i n h e r i t e d from P l e i s t o c e n e p l u v i a l e p i s o d e s when t h e c l i m a t e was more humid and s u r f a c e run-off
l e s s ephemeral.
number of d e p o s i t i o n a l zones can be r e c o g n i z e d .
A t t h e present-day a
Around t h e margins of d e s e r t b a s i n s
f l u v i a l p r o c e s s e s f r e q u e n t l y dominate and i n t e c t o n i c a l l y a c t i v e a r e a s a l l u v i a l f a n s may be abundant.
I n t h e c e n t r a l r e g i o n s a e o l i a n p r o c e s s e s dominate and i n t h e s t a b l e
i n t r a c r a t o n i c r e g i o n s l a r g e s a n d s e a s o r ' e r g s ' (Wilson, 1973) may,form.
Desert
l a k e s and i n l a n d sabkhas may a l s o b e found i n t h e c e n t r a l r e g i o n s because t h e w a t e r t a b l e h e r e may b e n e a r t o t h e s u r f a c e . A e o l i a n s a n d s a l s o form i n c o a s t a l r e g i o n s forming e l o n g a t e d dune f i e l d s p a r a l l e l to the coast. and sabkhas.
Here t h e y a r e a s s o c i a t e d w i t h o t h e r c o a s t a l sediments s u c h a s beaches Although t h e s e c o a s t a l dunes g e n e r a l l y have a low p r e s e r v a t i o n p o t e n t i a l
because o f t h e e f f e c t s of m a r i n e t r a n s g r e s s i o n t h e y might be more abundant i n t h e s t r a t i g r a p h i c record than i s generally r e a l i z e d .
The problem of d i s t i n g u i s h i n g
a n c i e n t a e o l i a n sands which p a s s l a t e r a l l y i n t o s h a l l o w marine s a n d b o d i e s a r e considerable.
DESERT FLWIAL SEDIMENTS Heavy r a i n s t o r m s a r e i n f r e q u e n t i n t h e d e s e r t and b e c a u s e of t h e l a c k of v e g e t a t i o n t o b i n d t h e sediment t h e y have a d r a m a t i c e f f e c t when t h e y do occur. Large amounts o f sediment may b e t r a n s p o r t e d , v i a a l l u v i a l f a n s and w a d i s , from t h e h i g h l a n d t o t h e lowland a r e a s .
I f t h e r a i n s t o r m i s p a r t i c u l a r l y heavy t h e
flowing w a t e r may e x t e n d t h e l e n g t h of t h e wadi and e v e n t u a l l y r e a c h t h e s e a ; o f t e n t h e p a s s a g e of w a t e r i s b l o c k e d by a e o l i a n s a n d and temporary l a k e s form.
During
m i l d e r r a i n s t o r m s t h e w a t e r s o a k s i n t o t h e wadi s e d i m e n t s and t h e downward p e r c o l a t i o n may r e s u l t i n t h e d e p o s i t i o n of c l a y i n what were o r i g i n a l l y m a t r i x - f r e e s e d i m e n t s . Under t h e s e c o n d i t i o n s t h e wadi may l o c a l l y become o v e r l o a d e d w i t h sediment.
Over-
l o a d i n g may a l s o o c c u r a t t h e n i c k p o i n t s between h i l l and p l a i n ; h e r e t h e r a p i d r e d u c t i o n i n w a t e r v e l o c i t y a s t h e flow widens l a t e r a l l y f r e q u e n t l y c a u s e s c h a n n e l b r a i d i n g and t h e f o r m a t i o n of a l l u v i a l f a n s .
D e b r i s flows a r e a common o c c u r r e n c e
on a l l u v i a l f a n s when t h e s e d i m e n t / w a t e r r a t i o i s h i g h .
A s t h e w a t e r s u p p l y i n t h e wadi d i m i n i s h e s much of i t s o a k s i n t o t h e sediment f i l l i n g t h e channel.
T h i s w a t e r i s commonly s a t u r a t e d w i t h r e s p e c t t o calcium
c a r b o n a t e and e v a p o r a t i o n r e s u l t s i n r a p i d c e m e n t a t i o n w i t h i n t h e v a d o s e zone.
The
72 s u r f a c e sands and s i l t s d r y o u t v e r y r a p i d l y and a r e u s u a l l y n o t cemented.
The
l a s t a r e a s t o d r y up a r e s t a g n a n t p o o l s i n which mud accumulates from s u s p e n s i o n . These become d e s i c c a t e d and mud c r a c k s form; t h i n mud l a y e r s r e s u l t i n g i n t h e f o r m a t i o n of f r a g i l e c u r l e d mud f l a k e s , which a r e e a s i l y blown away by t h e wind, w h i l s t t h i c k e r and h e a v i e r mud f l a k e s may b e p r e s e r v e d i n s i t u by a c o v e r o f wind blown sand. Because o f t h e ephemeral n a t u r e of d e s e r t f l u v i a l p r o c e s s e s d r y wadi c h a n n e l s a r e f r e q u e n t l y reworked o r f i l l e d by a e o l i a n sand and i n v e r t i c a l p r o f i l e t h e r e may be r a p i d a l t e r n a t i o n of w a t e r - d e p o s i t e d and wind-deposited s e d i m e n t s .
Although
a e o l i a n sands a r e g e n e r a l l y much b e t t e r s o r t e d t h a n f l u v i a t i l e s a n d s t n e reworking o f a e o l i a n s a n d s d u r i n g f l o o d s and t h e d e r i v a t i o n o f a e o l i a n sand from d r y wadi c h a n n e l s can l e a d t o some i n h e r i t a n c e o f t e x t u r a l c h a r a c t e r i s t i c s .
This often
makes t h e i r d i s t i n c t i o n d i f f i c u l t and G l e n n i e (1970) h a s l i s t e d a number o f c r i t e r i a which e n a b l e t h e r e c o g n i t i o n of w a t e r - l a i d d e p o s i t s i n t h e d e s e r t environment:
1) The bedding may be t h a t o f normal stream-flow s e d i m e n t s 2 ) Unsorted mudflow conglomerates may o c c u r 3) Channels and a c c r e t i o n f o r e s e t s a r e common
4 ) Clay laminae o r c l a y d r a p e s a r e common 5 ) Many sands a r e n o t w e l l - s o r t e d , b e i n g sometimes a r g i l l a c e o u s o r p e b b l y 6 ) They a r e commonly cemented by c a l c i t e
7 ) Sand and s i l t l a y e r s a r e commonly m i s s i n g from g r a d e d conglomerates 8) M a j o r i t y of q u a r t z g r a i n s t h a t a r e n o t cemented by c a l c i t e e x h i b i t f r o s t e d ( p i t t e d ) s u r f a c e s under t h e microscope The f o l l o w i n g f e a t u r e s may a l s o e n a b l e t h e r e c o g n i t i o n of s u b a e r i a l e x p o s u r e of w a t e r - l a i d sediment i n d e s e r t environments 1 ) P r e s e n c e o f c u r l e d ( g e n e r a l l y concave-up) c l a y f l a k e s 2 ) Comon p r e s e n c e o f c l a y p e b b l e s
3 ) P r e s e n c e of mud c r a c k s w i t h a sandy i n f i l l 4 ) P r e s e n c e of s a n d s t o n e dykes 5 ) A e o l i a n sand i n t e r b e d d e d w i t h w a t e r - l a i d d e p o s i t s .
The c o n t a c t s may show
e v i d e n c e of f l u v i a l e r o s i o n o r d e f l a t i o n . For e x c e l l e n t i l l u s t r a t i o n of t h e s e and many o t h e r f e a t u r e s o f d e s e r t f l u v i a t i l e sediments t h e r e a d e r i s r e f e r r e d t o t n e books o f G l e n n i e (1970) and P i c a r d and High (1973). AEOLIAN SAND DEPOSITS
A e o l i a n s a n d d e p o s i t s a r e t h e most c h a r a c t e r i s t i c f e a t u r e of d e s e r t s . o c c u r s n o t i n i s o l a t e d dunes b u t i n l a r g e a r e a s known a s s a n d s e a s ( t h e
The s a n d of t h e
n o r t h w e s t e r n S a h a r a ) which c o v e r between o n e - q u a r t e r and o n e - t h i r d of t h e w o r l d ' s deserts.
Wilson (1970) e s t i m a t e d t h a t 99.8% of a l l ' a c t i v e ' a e o l i a n s a n d was found
73 i n e r g s whose a r e a was g r e a t e r t h a n 125km2 and f u r t h e r l a r g e amounts o f a e o l i a n sand a r e t o b e found i n ' f i x e d ' e r g s on many d e s e r t margins ( F i g . 2 . 2 ) .
The e r g s
LARGE DUNE AREAS
Fig.2.2. 1973).
World map of p r i n c i p a l f i x e d and a c t i v e e r g s ( a f t e r Cooke and Warren,
d e r i v e t h e i r sand by t h e winnowing o f p e r i p h e r a l and i n t e r n a l d e s e r t f l u v i a t i l e sediments and by t h e a t t r i t i o n o f s o u r c e r o c k s .
The low d e n s i t y and v i s c o s i t y o f
wind make i t a n e f f i c i e n t sediment s o r t e r ; o n l y s a n d - s i z e d p a r t i c l e s c a n b e moved a s bed-load.
C o a r s e r g r a i n s a r e g e n e r a l l y l e f t behind a s a d e f l a t i o n l a g and f i n e r
sediment goes i n t o s u s p e n s i o n and may b e t r a n s p o r t e d beyond t h e b a s i n of d e p o s i t i o n thus contributing t o t h e general d e f l a t i o n of d e s e r t basins, The n a t u r e of t h e bedforms i n e r g s a r e c o n t r o l l e d by b o t h g r a i n s i z e and t h e t h i c k n e s s o f t h e s a n d c o v e r (Wilson, 1972, 1973) ( F i g . 2 . 3 ) .
I n t h e t h i c k accum-
u l a t i o n s , a s i n t h e Sahara and A r a b i a , t h r e e s c a l e s o f bedform:
r i p p l e s , dunes,
and d r a a s a r e developed b u t i n d e s e r t s where t h e c o v e r i s t h i n , a s i n t h e A u s t r a l i a n d e s e r t , t h e l a r g e r d r a a s a r e n o t developed and dunes form t h e l a r g e s t o b s e r v e d features.
These d i f f e r e n t bedforms a r e o f t e n i n t i m a t e l y r e l a t e d and show h i g h l y
complex p a t t e r n s .
T h i s c o m p l e x i t y r e s u l t s from t h e e f f e c t s of t o p o g r a p h i c s l o p e ,
v a r i a b l e wind d i r e c t i o n s , and t h e f a c t t h a t l a r g e r dunes and d r a a s r e q u i r e a l o n g time t o b e remoulded i n t o a new s h a p e a f t e r changes i n wind d i r e c t i o n and wind speed.
Another problem a r i s e s b e c a u s e of t h e r e l a t i v e l y h i g h t h r e s h o l d v e l o c i t i e s
needed f o r s a n d e n t r a i n m e n t .
T h i s means t h a t g r a i n movement i s a n i n t e r m i t t e n t
74 p r o c e s s and t h e s t r o n g winds which move s a n d r a p i d l y may have a d i f f e r e n t d i r e c t i o n from t h e weaker winds which o p e r a t e through a g r e a t e r p r o p o r t i o n of t h e time (Sharp, 1966).
The mechanics of s a n d t r a n s p o r t by wind a r e s t i l l p o o r l y under-
s t o o d and we s t i l l r e l y h e a v i l y on B a g n o l d ' s (1941) c l a s s i c work.
I n the aeolian
t r a n s p o r t of s a n d g r a i n b a l l i s t i c s and i n t e r g r a n u l a r c o l l i s i a n s a r e t h e dominant p r o c e s s e s whereas i n aqueous t r a n s p o r t f l u i d t u r b u l e n c e i s more i m p o r t a n t . The h i e r a r c h y of bedforms i n a e o l i a n s a n d g e n e r a l l y m a n i f e s t s i t s e l f a s a series o f superimposed forms w i t h d i s t i n c t s i z e c l a s s e s and w i t h v e r y few forms of i n t e r mediate s i z e .
T h i s i s s i m i l a r t o t h e h i e r a r c h y observed i n aqueous bedforms
( A l l e n , 1968) and t h e e x i s t e n c e of a t h r e e - f o l d h i e r a r c n y of r i p p l e s , dunes and sand waves i n s h a l l o w marine environments (Houbolt, 1968) i s m o s t l y c l o s e l y comThe a e o l i a n bedform h i e r a r c h y i s d i s c u s s e d
p a r a b l e t o t h a t s e e n i n a e o l i a n bedforms. I
i n d e t a i l by Wilson (1972) whose c l a s s i f i c a t i o n i s a d o p t e d h e r e ( T a b l e 2 . 1 ) .
This
h i e r a r c h i c a l arrangement a p p l i e s t o t r a n s v e r s e , l o n g i t u d i n a l and o b l i q u e forms and a p a r t from t h e f o u r t h o r d e r i t a l s o a p p l i e s t o e r o s i o n a l a s w e l l a s d e p o s i t i o n a l bedforms (Cooke and Warren, 1973).
Although t h e h i e r a r c h i c a l groups proposed by
Wilson o v e r l a p i n t h e i r s i z e r a n g e s , when t h e wavelength i s p l o t t e d a g a i n s t g r a i n s i z e a s i n F i g . 2 . 4 t h e y can b e s e e n t o form t h r e e d i s t i n c t groups.
I n e a c h of t h e s e
c l a s s e s t h e g r a i n s i z e o f t h e s a n d i n c r e a s e s w i t h t h e wavelength o f t h e bedform. S i n c e a g r e a t e r windspeed i s needed t o move c o a r s e r s a n d i t f o l l o w s t h a t bedform wavelength w i l l i n c r e a s e w i t h windspeed ( s e e a l s o , S h a r p , 1963). Ripples Although a e o l i a n r i p p l e s o c c u r w i t h s i m i l a r v a r i e t y t o t h e i r subaqueous c o u n t e r p a r t s t h e y a r e most commonly a s i m p l e arrangement o f p a r a l l e l r i d g e s a n d t r o u g h s t r a n s v e r s e t o t h e wind d i r e c t i o n .
i s r a r e l y v e r y pronounced.
They may b e s t r a i g h t o r s i n u o u s b u t t h e s i n u o s i t y
Commonly, t h e y a r e asymmetric w i t h a s t e e p e r l e e s l o p e
o r v e r y s m a l l s l i p f a c e and t h e h e i g h t / w a v e l e n g t h r a t i o i s u s u a l l y between 0.02 and 0.08 which i s r a t h e r s m a l l e r t n a n t h a t o f o t n e r a e o l i a n bedforms (Ellwood e t a l . , 1975).
Ridge c r e s t s a r e a l m o s t i n v a r i a b l y c o a r s e r g r a i n e d t h a n t h e t r o u g h s .
E a r l y workers ( e . g . B o u r c a r t , 1928) b e l i e v e d t h a t r i p p l e s , l i k e o t h e r a e o l i a n bedforms, were t h e r e s u l t of aerodynamic i n s t a b i l i t y and t h e r e i s some e v i d e n c e (Bagnold, 1941; W i l s o n , 1972; Cooke and Warren, 1973) t o i n d i c a t e t h a t a s m a l l p r o p o r t i o n of r i p p l e s may b e of t h i s type. o r fluid-drag interface.
These a r e t h e s o - c a l l e d 'aerodynamic'
r i p p l e s which r e s u l t from aerodynamic i n s t a b i l i t y n e a r t h e s a n d / a i r
They t e n d t o have a s t r o n g l o n g i t u d i n a l component i n t h e s h a p e and a l s o
t o be s e v e r a l times l a r g e r t h a n t h e t r a n s v e r s e impact r i p p l e s .
Most a e o l l a n r i p p l e s
a r e r e c o g n i z e d t o have formed by t h e r e g u l a r l y s p a c e d impact of s a n d g r a i n s a s d e s c r i b e d by Bagnoid (1941, p.146-153).
I n wind t u n n e l e x p e r i m e n t s Bagnold found
t h a t t h e r e was e q u i v a l e n c e between t h e l e n g t h o f t h e c h a r a c t e r i s t i c f l i g h t p a t h o f
75
Oriental NORTH AFRICA
Fig.2.3. The Erg O r i e n t a l , A l g e r i a showing i t s p o s i t i o n i n r e l a t i o n t o r e g i o n a l p a t t e r n s of s a n d t r a n s p o r t and t h e r e l a t i o n s h i p between sand c o v e r and bedforms w i t h i n i t , A . Flow d i r e c t i o n s medium s a n d , B. Flow d i r e c t i o n s f i n e s a n d (arrows i n d i c a t e d i r e c t i o n s t a k e n from aedforms) , C . Prominent d r a a t r e n d s ( d i f f e r e n t t r e n d s a r e r e l a t e d t o g r a i n s i z e d i f f e r e n c e s ) , D . P r o p o r t i o n of sand c o v e r , E . Mean s p r e a d - o u t sand t h i c k n e s s (m), and G . Mean d r a a wavelength (km) ( a f t e r Wilson, 1 9 7 3 ) .
76 s a l t a t i n g s a n d g r a i n s and r i p p l e wavelengths.
I n the f i e l d t h e s u r f a c e of r i p p l e s
i s o f t e n covered by c o a r s e r g r a i n s t r a v e l l i n g a s s u r f a c e c r e e p .
When t h e s e g r a i n s
r e a c h r i p p l e c r e s t s t h e y c a n n o t move on b e c a u s e t h e y e x p e r i e n c e fewer bombardments by s a l t a t i n g g r a i n s .
Sharp (1963) has c l e a r l y demonstrated t h e c o n c e n t r a t i o n of
c o a r s e r g r a i n s i n a e o l i a n r i p p l e c r e s t s by s e c t i o n i n g impregnated r i p p l e s . e n v i s a g e d a sequence of r i p p l e f o r m a t i o n a s f o l l o w s :
He
a s t h e wind moves o v e r a f l a t
s a n d bed, s a l t a t i n g s a n d moves t h e s u r f a c e c r e e p ; t h i s e n c o u n t e r s chance i r r e g u l a r i t i e s and p i l e s up; t h e p i l e s of s a n d grow u n t i l a n e q u i l i b r i u m h e i g h t i s r e a c h e d ; t h i s c h a r a c t e r i s t i c h e i g h t and t h e a n g l e of i n c i d e n c e of t h e g r a i n s c o n t r o l t h e wavelength. TABLE 2.1. H i e r a r c h i c a l c l a s s i f i c a t i o n o f a e o l i a n bedform elements ( a f t e r W i l s o n , 1972). Order
Wavelength
Height
Orientation
Possible Origin
Name
1st
300-55OOm
20-45Om
long. o r transverse
aerodynamic instability
draas
2nd
3-600m
0.1- lOOm
long. o r transverse
aerodynamic instability
dunes
3rd
15-250cm
0.2-0.5cm
long. o r transverse
aerodynamic instability
aerodynamic ripples
4th
0.5-2,000cm 1.0- 3,000cm
0.05-100cm 0.05-100cm
transverse long.
impact secondary Taylor-G'ir t l e r vortices
impact r i p p l e s secondary ripple sinuosity
G r a n u l e - r i p p l e s ( S h a r p , 1963) have been d i s c u s s e d by a number o f a u t h o r s (Bagnold, 1941; G l e n n i e , 1970).
They have much g r e a t e r wavelengths t h a n o r d i n a r y
s a n d r i p p l e s ; Bagnold (1941) observed some i n t h e Libyan Desert w i t h a wavelength of up t o 20m and a n a m p l i t u d e of o v e r 60cms.
Granule-ripples usually contain
l a r g e p r o p o r t i o n s o f g r a i n s o v e r 2mm; which may make up t o 20% of t h e r i p p l e a s a whole and up t o 80% of t h e r i p p l e c r e s t .
They a r e f r e q u e n t l y symmetrical i n c r o s s -
s e c t i o n and t h e c o n c e n t r a t i o n s o f c o a r s e g r a i n s i n t h e r i p p l e c r e s t i s c h a r a c t e r istic.
Bagnold (1941) thought t h a t g r a n u l e - r i p p l e s formed when t h e r e had been
long-continued d e f l a t i o n s o t h a t much of t h e f i n e s a n d was removed from t h e s u r f a c e layers.
I t was t h o u g h t t h a t a s u p p l y of f i n e s a l t a t i n g g r a i n s from upwind
m a i n t a i n e d t h e s u r f a c e c r e e p and s i n c e t h e c o a r s e g r a i n s c o u l d n o t move beyond t h e r i p p l e c r e s t t h e upward growth o f t h e g r a n u l e - r i p p l e would s l o w l y c o n t i n u e ; a c c o r d i n g l y he t h o u g h t t h e y were v e r y o l d .
The p r o b a b l e c o n t r o l l i n g f a c t o r of t h e s i z e of
granule-ripples is t h e i r grain size.
The f l i g h t p a t t e r n s of s a l t a t i n g g r a i n s i n
c o a r s e m a t e r i a l a r e g r e a t e r because of t h e i r b e t t e r bounce o f f t h e c o a r s e g r a i n s .
77
S c a t t e r diagram o f g r a i n s i z e (P20 = c o a r s e twenty p e r c e n t i l e ) a g a i n s t Fig.2.4. ripples, B dunes, wavelength f o r a e o l i a n bedforms showing t h e h i e r a r c h y : A C d r a a s ( a f t e r Wilson, 1972).
-
-
-
Because of t h i s t h e wavelength i s g r e a t e r and c o a r s e g r a i n s accumulate a t granuler i p p l e c r e s t s i n t h e same way a s i n normal wind r i p p l e s (Ellwood e t a l . , 1975). Another k i n d of r i p p l e which c o m o n l y o c c u r s i n t h e d e s e r t environment i s t h e a d h e s i o n r i p p l e ( G l e n n i e , 1970) a l t h o u g h t h e s e were f i r s t d e s c r i b e d from c o a s t a l f l a t s i n n o r t h e r n Germany (Reineck, 1955).
Adhesion r i p p l e s form most commonly
on sabkhas and i n damp i n t e r - d u n e a r e a s where sand g r a i n s a d h e r e t o t h e s u r f a c e because of t h e m o i s t u r e c o n t e n t .
G r a i n s t h e n c o n t i n u e t o b e t r a p p e d on t h e upwind
s i d e s of t h e r i p p l e s a n d . c m o n l y grow up t o 5cm h i g h a l t h o u g h t h e y may be o c c a s i o n a l l y much l a r g e r ( G l e n n i e , 1970). ripples is r a t h e r irregular.
The i n t e r n a l s t r u c t u r e o f a d h e s i o n
I t t e n d s t o b e l e n t i c u l a r , which i n s a b k h a s , produces
a n i r r e g u l a r wavy l a m i n a t i o n . Dunes and d r a a s Dunes and d r a a s show v e r y s i m i l a r morphological t y p e s and t h e e s s e n t i a l d i f f e r e n c e between them i s one of s i z e (Fig.2.4). described together,
For t h i s r e a s o n t h e y c a n b e c o n v e n i e n t l y
Dunes have wavelengths measured i n 10's o r 100's of metres and
h e i g h t s i n metres, w h i l e d r a a s have l e n g t h s o f t h e o r d e r of k i l o m e t r e s and h e i g h t s i n 10's of m e t r e s .
The r a t e of movement of b o t h is r e l a t e d t o t h e i r s i z e w i t h t h e
s m a l l e r forms b e i n g a b l e t o move more q u i c k l y (Long and S h a r p , 1964).
Draas move 6 years).
q u i t e s l o w l y ( a b o u t lcm/year) and p r o b a b l y t a k e a long t i m e t o develop (104-10
They p r o b a b l y o r i g i n a t e by t h e n u c l e a t i o n of a number o f dunes and a r e u s u a l l y covered on b o t h windward and leeward f a c e s by dunes.
Those dunes on t h e l e e s i d e
t e n d t o have l a r g e s l i p f a c e s and c l e a r l y c o n t r i b u t e t o t h e m i g r a t i o n of t h e draa. Dunes o r i g i n a t e i n a e o l i a n sand because o f chance i r r e g u l a r i t i e s (Bagnold, 1941).
78 Such i r r e g u l a r i t i e s may b e a g e n t l e d i p i n t h e d e s e r t s u r f a c e o r some u p s t a n d i n g foreign object,
Once a p a t c h of sand i s n u c l e a t e d i n t h i s way i t b e g i n s t o modify
t h e s u r r o u n d i n g a i r flow p a t t e r n . o f 10-15'
I n t h i s way a b a c k s l o p e , u s u a l l y w i t h a n a n g l e
i s developed and w i t h c o n t i n u e d growth d e p o s i t i o n on t n e l e e s l o p e g e t s
p r o g r e s s i v e l y n e a r e r t h e s m i t u n t i l t h e a n g l e of r e p o s e f o r s a n d i s exceeded and s l i p p i n g r e s u l t s (McKee e t a l . ,
1971).
Depending upon a v a r i e t y of f a c t o r s i n c l u d i n g wind c o n d i t i o n s , s a n d t y p e , and s u p p l y of s a n d , dunes may a c q u i r e a v a r i e t y of s h a p e s and s i z e s .
Many a u t h o r s have
a t t e m p t e d t o c l a s s i f y sand dune t y p e s (Melton, 1940; Bagnold, 1941; Holm, 1960; McKee, 1966; Cooper, 1967).
A commonly used c l a s s i f i c a t i o n i s t h a t of McKee (1966)
b u t t h i s has t h e d i s a d v a n t a g e of n o t d i s t i n g u i s h i n g d r a a - s i z e d f e a t u r e s :
1. Barchan 2. T r a n s v e r s e 3. P a r a b o l i c 4. S e i f 5. S t a r 6. Dome-shaped 7. Reversing A more comprehensive c l a s s i f i c a t i o n which can be a p p l i e d t o dune and d r a a - s i z e d f e a t u r e s i s g i v e n by Cooke and Warren (1973) ( s e e F i g . 2 . 5 ) . Bar c hans Barchans a y e c r e s c e n t shaped s a n d mounds formed i n a n u n d i r e c t i o n a l wind by a v a l a n c h i n g of s a n d on t h e s l i p f a c e .
The e x t r e m i t i e s ( h o r n s ) of t h e dune e x t e n d
downwind b e c a u s e t h e y m i g r a t e more r a p i d l y t h a n t h e main body.
S e v e r a l measurements
have been made of t h e r a t e o f b a r c h a n m i g r a t i o n which i s r e l a t e d t o h e i g n t ( F i n k e l 1959; N o r r i s , 1966; H a s t e n r a t h , 1967) and a l s o t o t h e shape o f t h e dune; narrower dunes a r e l i k e l y t o advance more r a p i d l y t h a n b r o a d e r o n e s ,
(Long a n d S h a r p , 1964)
a l t h o u g h b a r c h a n s do p r e s e r v e t h e i r p l a n shape d u r i n g m i g r a t i o n (McKee, 1966). A c t u a l r a t e s o f advance which have been q u o t e d v a r y from 17-47m/annum ( H a s t e n r a t h , 1967) and 15m/annum ( B e a d n e l l , 1 9 1 0 ) . Barchan dunes u s u a l l y o c c u r i n f a i r l y complex p a t t e r n s .and may form b o t h dune and d r a a - s i z e d f e a t u r e s .
I s o l a t e d b a r c h a n s t e n d t o develop i n a r e a s where t h e s u p p l y
of sand i s d i m i n i s h e d such t h a t t h e u n d e r l y i n g pavement c a n n o t be c o m p l e t e l y covered. The s a n d s u p p l y c o n t r o l s t h e s p a c i n g of t h e d u n e s , t h e less t h e amount of s a n d t h e wider t h e dune s p a c i n g , b u t t h e a n g l e of d i p o f s l i p f a c e s i s c o n s i s t e n t l y n e a r 34' n e a r t h e c e n t r a l p a r t of t h e dune and somewhat l e s s i n t h e h o r n s .
These s l i p f a c e s
r e s u l t i n p l a n a r - t a b u l a r u n i t s of c r o s s - b e d d i n g ( F i g . 2 . 6 ) and d i p w i t h i n a n a r c of l e s s t h a n 150' and u s u a l l y a b o u t 90'.
The unimodal d i s t r i b u t i o n of f o r e s e t d i r e c -
t i o n s when p l o t t e d on a s t e r e o g r a p h i c p r o j e c t i o n e n a b l e s barchans t o be d i s t i n g u i s h e d
79
Fig.2.5. The main t y p e s o f a e o l i a n dune and d r a a . The terms u s e d i n B-H may be a p p l i e d t o dunes a s w e l l a s d r a a s . E f f e c t i v e wind d i r e c t i o n i s i n d i c a t e d where a p p r o p r i a t e . I n C and I no one d i r e c t i o n dominates ( a f t e r Cooke and Warren, 1 9 7 3 ) .
80 from o t h e r t y p e s of dunes i n a n c i e n t d e p o s i t s , p a r t i c u l a r l y s e i f dunes ( s e e Fig.2.8).
Main trench, Northwest wall
sw
NE
feet
20
----I
0
1
10
20
30 40
50
60
70
t Base of
I
I
80
90
dune I
I
100 110 120 130 l40feet
Sido tronch, Northeast wall
,
I
dun. I
150 160 170 180 l90 200 210 220 230 240 250 260 270 280 -Bounding
surface
-Selected cross
- stratum
foot
Fig.2.6. I n t e r n a l s t r u c t u r e of a barchan dune a s observed i n e x c a v a t i o n s i n t o t h e gypsum dunes of t h e White Sands N a t i o n a l Monument, New Mexico ( a f t e r McKee, 1966).
Seifs
S e i f s a r e l o n g i t u d i n a l forms e l o n g a t e d p a r a l l e l t o t h e p r e v a i l i n g wind d i r e c t i o n . They cover e x t r e m e l y l a r g e a r e a s of some d e s e r t s such a s t h e Rub a 1 K h a l i ( G l e n n i e , 1970), t h e w e s t e r n Sahara and Libyan d e s e r t s (Bagnold, 1933) and t h e A u s t r a l i a n
deserts (Folk, 1971a,b).
Many of t h e s e a r e d r a a - s i z e d f e a t u r e s which a r e a s s o c i a t e d
w i t h s i n u o u s , v e r y e l o n g a t e d c r e s t l i n e s which a r e known a s s e i f dunes from t h e Arab usage ( s e i f meaning sword).
The c r e s t l i n e s may b i f u r c a t e i n a n upwind d i r e c t i o n
and f r e q u e n t l y shows t h e development o f a l t e r n a t i n g s l i p f a c e s . The o r i g i n o f s e i f s has been a m a t t e r of some d i s c u s s i o n .
Bagnold (1941) b e l i e v e d
t h a t t h e y a r e produced when s t r o n g winds blow from a q u a r t e r o t h e r t h a n t h a t o f t h e g e n e r a l d r i f t of sand caused by t h e more p e r s i s t e n t g e n t l e winds.
McKee and T i b b i t t s
(1964) b e l i e v e d t h a t s e i f s were produced i n t h e v e c t o r o f two converging wind d i r e c t i o n s blowing from two q u a r t e r s a b o u t 90'
apart.
Other a u t h o r s have p r e s e n t e d
81 e v i d e n c e i n f a v o u r o f t h i s i d e a (Madigan, 1946; Wopfner a n d Twidale, 1967). However, a number o f a u t h o r s ( e . g . G l e n n i e , 19701, have p r e s e n t e d e v i d e n c e t h a t l o n g i t u d i n a l dunes a r e produced p a r a l l e l t o winds o f uniform d i r e c t i o n and most workers now f a v o u r t h i s e x p l a n a t i o n .
The s e i f form s u g g e s t s t h a t p a i r e d s p i r a l
v o r t i c e s w i t h a x e s p a r a l l e l t o t h e flow a r e i m p o r t a n t i n t h e i r f o r m a t i o n .
For
example, i n S a u d i Arabia d r a a - s i z e d l o n g i t u d i n a l dunes have smaller s e i f s a l i g n e d d i a g o n a l l y a c r o s s t h e s i d e s o f t h e m a j o r r i d g e s a t a n a n g l e which would b e e x p e c t e d i f three-dimensional vortex flow operated.
I n t n e Simpson Desert t h e minor long-
i t u d i n a l f e a t u r e s and a l s o t h e Y - j u n c t i o n s e x h i b i t a n g l e s o f 30-50'
which would
b e e x p e c t e d from secondary f l o w ( F o l k , 1971b; W i l s o n , 1972). The s t r u c t u r e of s e i f dunes has been d e a l t w i t h by McKee and T i b b i t t s (1964) (see Fig.2.7)
and by G l e n n i e (1970).
I d e a l l y t h e y c o n s i s t of cross-bedded u n i t s
d i p p i n g n e a r l y normal t o t h e e l o n g a t i o n o f t h e r i d g e t h u s p r o d u c i n g two modes a b o u t 120' a p a r t .
T h i s f e a t u r e e n a b l e s t h e d i s t i n c t i o n of b a r c h a n s a n d s e i f s i n v e r t i c a l
s e c t i o n s (Fig.2.8).
mStructunlarr sand
Near-vertical laminae between
inches
B
Encroachmont Dopotit
Accntion Doporit
Fig.2.7. A . I n t e r n a l s t r u c t u r e of a s e i f dune ( a f t e r McKee and T i b b i t t s , 1964). B. G e n e r a l i z e d s t r u c t u r e of a s e i f dune ( a f t e r Bagnold, 1941).
82
D mretjon mminae
l B-B'
Dips only reach 34O in
N
W
E
S
5
Fig.2.8. I d e a l i z e d p l a n view and s t e r e o g r a p h i c p l o t of s l i p f a c e o r i e n t a t i o n s i n barchans and s e i f s . The s i n g l e s l i p f a c e of t h e b a r c h a n produces a unimodal d i s t r i b u t i o n of f o r e s e t d i p d i r e c t i o n whereas t h e a l t e r n a t i n g s l i p f a c e s of t h e s e i f produces a bimodal d i s t r i b u t i o n o f f o r e s e t d i p d i r e c t i o n s ( a f t e r G l e n n i e , 1970). T r a n s v e r s e forms T r a n s v e r s e dunes a r e a l m o s t s t r a i g h t , e l o n g a t e d r i d g e s of sand o r i e n t e d a t r i g h t a n g l e s t o t h e p r e v a i l i n g wind d i r e c t i o n .
They a r e r e g u l a r l y spaced and have
broad i n t e r d u n e a r e a s which sometimes form i n l a n d sabkhas.
G l e n n i e (1970)
d i s c u s s e d t h e o r i g i n of t r a n s v e r s e dunes and s u g g e s t e d t h a t they o r i g i n a t e d i n i n l a n d sabkha a r e a s where t h e damp sabkha s u r f a c e i n h i b i t e d t h e development of barchan horns.
The i n t e r d u n e sabkhas e v e n t u a l l y d i s a p p e a r and a s a n d s e a w i t h
t r a n s v e r s e dunes i s produced ( s e e G l e n n i e , 1970, F i g . 7 7 ) .
Bagnold (1954) c o n s i d e r e d
t r a n s v e r s e dunes t o b e u n s t a b l e forms which broke up i n t o s e i f s o r b a r c h a n s . However, Cooper ( 1 9 5 8 ) , McKee ( 1 9 6 6 ) , and G l e n n i e (1970) a l l b e l i e v e them t o be s t a b l e forms.
For example, t h e y a r e w e l l - s e e n i n t h e e a s t e r n p a r t of t h e Rub a 1
83 K h a l i ( G l e n n i e , 1970, p . 9 5 ) where t h e dunes a r e s e p a r a t e d by i n l a n d s a b k h a s . I n a r e a s of complete sand c o v e r t r a n s v e r s e forms may develop more complex s h a p e s . These normally have a v a l a n c h e f a c e s on t h e i r l e e s i d e s b u t t h e d r a a - s i z e d forms may have g e n t l y s l o p i n g l e e s i d e s wnicn a r e covered w i t n superimposed dunes.
A common
v a r i a t i o n on t h e s i m p l e s t r a i g h t c r e s t e d form i s t h e a k l e p a t t e r n ( F i g . 2 . 5 ) made up o f a l t e r n a t i n g convex and concave downwind s e c t o r s . The i n t e r n a l s t r u c t u r e of t r a n s v e r s e dunes i s d i s c u s s e d by McKee (1966) who n o t e s t h a t cross-bedding i s of t h e planar t a b u l a r type (Fig.2.9) with s t e e p l y dipping
(30-34')
The f o r e s e t s a r e more
f o r e s e t laminae which a r e t y p i c a l l y l o n g and even.
o r l e s s p a r a l l e l t o t h e a c t i v e s l i p f a c e and a r e c u t by i n t e r n a l e r o s i o n s u r f a c e s which a r e comparable t o t h e r e a c t i v a t i o n s u r f a c e s o f f l u v i a l b a r s .
These d i p a t
low a n g l e s which range from n e a r - h o r i z o n t a l n e a r t h e dune c r e s t t o o v e r 20' lower p a r t o f t h e s e t .
i n the
These s u r f a c e s a r e p r o b a b l y t h e p r o d u c t s of r e v e r s e d o r
t r a n s v e r s e wind e p i s o d e s .
Dome-shaped dunes (McKee, 1966; Thompson, 1969) a p p e a r
t o form by g r a d u a l lowering of t h e s l i p f a c e o f t r a n s v e r s e dunes by s u c c e s s i v e erosional events. Detail of asymmetrical trough, Side trench
SE
feet
NW
:I 5
2 0 1
ft.
sw
I
I
I
I
I
0
5
10
15
20
Main trench, North wall
- Bounding surface
-Selected cross-stratum
Fig.2.9. I n t e r n a l s t r u c t u r e of a t r a n s v e r s e dune a s observed i n e x c a v a t i o n s i n t o t h e gypsum dunes of t h e White Sands N a t i o n a l Monument, New Mexico ( a f t e r McKee, 1966).
84 P a r a b o l i c dunes P a r a b o l i c dunes a r e U o r V shaped s a n d r i d g e s w i t h t h e concave s i d e towards t h e wind.
They a r e n o t so common a s some of t h e o t h e r dune t y p e s .
The m i d d l e p a r t o f
t h e dunes t h u s moves forward w i t h r e s p e c t t o t h e s i d e s and McKee (1966) b e l i e v e d t h a t t h i s was because t h e s i d e s were anchored by v e g e t a t i o n t h u s a l l o w i n g t h e c e n t r a l p a r t of t h e dune t o move forward more r a p i d l y .
Characteristically the s l i p
f a c e i s convex upwards. The i n t e r n a l s t r u c t u r e i s d e s c r i b e d by McKee (1966).
Large s c a l e c r o s s - b e d d i n g
i s a g a i n dominant w i t h e r o s i o n s u r f a c e s b e i n g s e p a r a t e d by b u n d l e s of f o r e s e t laminae.
F o r e s e t laminae i n p a r a b o l i c dunes a r e low-angled compared w i t h o t h e r
dunes and t y p i c a l l y , b e c a u s e of t h e o r i e n t a t i o n of t h e s l i p f a c e , t h e y a r e concave downwards ( F i g . 2 . 1 0 ) . Moin trench, Northwest wail
sw
0
NE
10
M
30
10
M
M)
70
80
.)
901cetio3
Main h m h . Wthwest WON NE
sw
I M
0
I0
20
Y)
LO
M
€0
?a
80
'
I
93IediW
'/
Side trench, Southwest wall SE
F i g . 2 . 1 0 . I n t e r n a l s t r u c t u r e o f a p a r a b o l i c dune a s s e e n i n e x c a v a t i o n s i n t o t h e gypsum dunes of t h e White Sands N a t i o n a l Monument, New Mexico ( a f t e r McKee, 1 9 6 6 ) . Rhourds Rhourds ( F i g . 2 . 5 )
and o t h e r complex i n t e r f e r e n c e p a t t e r n s o c c u r where t h e wind
p a t t e r n i s highly variable.
The i n t e r f e r e n c e p a t t e r n i s sometimes r e c o g n i z a b l e a s
t h e i n t e r s e c t i o n of two o r more l i n e a r t r e n d s b u t t h e s e commonly b r e a k down i n t o a
s e r i e s of i s o l a t e d peaks which when d r a a - s i z e d a r e c a l l e d rhourds.
These may b e
o v e r 200m h i g h and most commonly r e s u l t from two dominant wind d i r e c t i o n s which a r e
85
s e p a r a t e d by a n a c u t e a n g l e ,
They a r e b e s t - d e v e l o p e d i n a r e a s of complete sand
c o v e r and a d e s e r t f l o o r l a g o f c o a r s e s a n d u s u a l l y s e p a r a t e s i n d i v i d u a l rhourds. Rhourds may show a complex p a t t e r n o f s l i p f a c e s and i n many c a s e s c a r r y s u p e r imposed dunes.
O t h e r s may t a k e t n e form o f smooth dunes w i t h no well-developed
s l i p f a c e s (McKee, 1966). The i n t e r n a l s t r u c t u r e o f modern and a n c i e n t a e o l i a n d e p o s i t s I n k e e p i n g w i t h t h e mechanism of m i g r a t i o n o f s a n d dunes two t y p e s of sand beds c a n be r e c o g n i z e d .
These were r e f e r r e d t o by Bagnold (1941) a s :
1. A c c r e t i o n d e p o s i t s 2. Encroachment o r a v a l a n c h e d e p o s i t s A c c r e t i o n d e p o s i t s r e p r e s e n t t h e windward g e n t l e s l o p e of sand dunes and c o n s i s t o f r a t h e r t h i n s u b - h o r i z o n t a l laminae which d i p g e n t l y (3-10')
upwind.
They
g e n e r a l l y c o n s i s t of f i r m l y packed s a n d a r e a r e n o t commonly p r e s e r v e d i n a n c i e n t a e o l i a n sandstones. Encroachment d e p o s i t s a r e t n e f o r e s e t laminae produced by a v a l a n c h i n g of sand on t h e s l i p f a c e .
They form t h e f a m i l i a r c r o s s bedding of a e o l i a n s a n d d e p o s i t s .
Laminae t e n d t o b e r a t h e r t h i c k (2-5cm) and d i p 25 t o 34'
downwind.
They
c o n s t i t u t e t h e major p a r t o f a dune body and a r e most commonly p r e s e r v e d i n a n c i e n t aeolian sandstones.
Tne s t e e p l y d i p p i n g f o r e s e t laminae t e n d t o f l a t t e n o u t n e a r
t h e base of t h e s l i p face producing t h e c h a r a c t e r i s t i c asymptotic f o r e s e t s . The laminae o f dune s a n d s a l s o show d e f o r m a t i o n s t r u c t u r e s .
These a r e thought
t o be due t o t h e movement o f p a r t i a l l y c o h e s i v e s a n d masses on t h e s l i p f a c e a f t e r r a i n has f a l l e n (McKee e t a l . , 1971; B i g a r e l l a , 1972; McKee and B i g a r e l l a , 1972). D e t a i l e d d e s c r i p t i o n s o f t h e i n t e r n a l s t r u c t u r e o f i n d i v i d u a l dune t y p e s have been g i v e n i n t h e p r e v i o u s s e c t i o n s .
These a r e based l a r g e l y on t h e dunes o f gypsum
sand a t White Sands, New Mexico and i t i s n o t c l e a r how t y p i c a l t h e y a r e o f a e o l i a n dunes i n g e n e r a l .
A v a i l a b l e e v i d e n c e ( B i g a r e l l a e t a l . , 1969; G l e n n i e , 1970)
s u g g e s t s t h a t t h e y c l o s e l y compare w i t h q u a r t z i t i c dunes.
These d a t a can be used
t o i d e n t i f y d i f f e r e n t dune t y p e s i n a n c i e n t a e o l i a n sequences and u n d e r i d e a l c i r c u m s t a n c e s e n a b l e t h e d i s t i n c t i o n of t r a n s v e r s e from l o n g i t u d i n a l t y p e s ( S h o t t o n , 1956; G l e n n i e , 1970).
T r a n s v e r s e dunes s u c h a s b a r c h a n s produce a s i n g l e mode
o f d i r e c t i o n s when t h e f o r e s e t a t t i t u d e s a r e p l o t t e d on a s t e r e o g r a p h i c p r o j e c t i o n . L o n g i t u d i n a l d u n e s , however, produce a bimodal p a t t e r n b e c a u s e of t h e a l t e r n a t i n g s l i p f a c e s a l t h o u g h t h i s might be more c o m p l i c a t e d because o f t h e i n f l u e n c e of superimposed dunes.
S t u d y o f t h e Lower Permian dune bedding i n NW Europe by G l e n n i e
(1972) s u g g e s t s t h a t l o n g i t u d i n a l dunes may dominate upland a r e a s whereas t r a n s v e r s e t y p e s a r e more abundant i n t h e c e n t r a l p a r t o f t h e d e s e r t b a s i n ( F i g . 2 . 1 1 ) . Trough cross-bedded u n i t s a r e n o t commonly d e s c r i b e d from a e o l i a n s a n d d e p o s i t s b u t have been d e s c r i b e d i n some a n c i e n t examples ( B i g a r e l l a , 1972).
They a r e
86 c o n c e i v a b l y produced by t h e m i g r a t i o n of a n a k l e dune o r d r a a p a t t e r n i n a manner analogous t o t h e p r o d u c t i o n of aqueous t r o u g h c r o s s - b e d d i n g by t h e m i g r a t i o n of l i n g u o i d o r l u n a t e e l e m e n t s ( A l l e n , 1968; B r o o k f i e l d , 1977).
Fig.2.11. S t e r e o g r a p h i c p r o j e c t i o n s ( u p p e r hemisphere) of p o l e s t o Lower Permian dune bedding. A and B a r e from o u t c r o p s i n n o r t h e r n England. C and D a r e from w e l l s i n t h e s o u t h e r n North Sea. Deduced palaeowind d i r e c t i o n s a r e shown. Dashed l i n e s i n d i c a t e a r e a s o f t y p i c a l t r a n s v e r s e ( a ) and l o n g i t u d i n a l dune bedding a t t i t u d e s ( b ) ( a f t e r G l e n n i e , 1972). Bounding s u r f a c e s i n a e o l i a n s a n d d e p o s i t s have been d i s c u s s e d by a number o f authors.
A c l a s s i f i c a t i o n , b a s e d p r i m a r i l y on a n c i e n t examples h a s beefl proposed
by B r o o k f i e l d (1977) ( F i g . 2 . 1 2 ) .
H e d i s t i n g u i s h e s t h r e e o r d e r s o f a e o l i a n bounding
s u r f a c e s which a r e a r r a n g e d i n a h i e r a r c h y a n a l a g o u s t o t h e bedform h i e r a r c h y o f Wilson (1972). First-order
These a r e termed f i r s t , second, and t h i r d - o r d e r bounding s u r f a c e s . bounding s u r f a c e s a r e f l a t - l y i n g o r convex-up bedding p l a n e s c u t t i n g
a c r o s s c r o s s - b e d d i n g and o t h e r dune s t r u c t u r e s ,
The d i s t a n c e s between t h e s e p l a n e s
r a n g e s from 0.3-17m and t h e y t e n d t o b e r e g u l a r l y s p a c e d i n one p a r t i c u l a r formatior Opdyke and Runcorn (1960) c o n s i d e r e d them t o be due t o t h e i n t e r a c t i o n of a e o l i a n d e f l a t i o n and d e p o s i t i o n i n a d j a c e n t a r e a s .
S t o k e s (1968) thought t h a t t h e y were
= o
50
Fig.2.12. S k e t c h diagrams of q u a r r y f a c e s i n t h e Lower Permian dune s a n d s of t h e Dumfries b a s i n , S c o t l a n d showing a e o l i a n bounding s u r f a c e s : F i r s t - o r d e r surfaces t h i c k l i n e s , second-order surfaces - t h i n l i n e s , third-order surfaces dashed l i n e s ( i n s e t 9 ) . Cross laminae a r e shown by d o t t e d l i n e s ( a f t e r B r o o k f i e l d , 1977).
-
-
c o n t r o l l e d by groundwater l e v e l s i n t h e f o l l o w i n g manner:
a s t h e s u r f a c e of
s e d i m e n t a t i o n r i s e s s p a c e i s c r e a t e d f o r groundwater s a t u r a t i o n and w i t h n e a r s u r f a c e w a t e r t a b l e and s t r o n g e r t h a n u s u a l wind a c t i o n t h e d r y s a n d w i l l b e s t r i p p e d o f f and t h e zone of s a t u r a t i o n exposed.
A change i n wind s t r e n g t h would
c a u s e dunes t o m i g r a t e o v e r t h e s u r f a c e t h u s c r e a t i n g a bedding p l a n e . seems u n l i k e l y
-
This process
t h e major o b j e c t i o n b e i n g t h a t t h e w a t e r t a b l e i n s a n d s e a s i s n o t
h o r i z o n t a l b u t forms a subdued r e p l i c a o f t h e bedform geometry (McKee and Moiola, 1975).
B r o o k f i e l d ( 1 9 7 7 ) a t t r i b u t e s f i r s t - o r d e r bounding s u r f a c e s t o d r a a m i g r a t i o n .
A s dunes m i g r a t e and climb o v e r e a c h o t h e r a n d a c r o s s i n t e r d u n e a r e a s t h e b a s e s of
e a r l i e r bedforms a r e n o t c o m p l e t e l y removed and a p p e a r a s t r u n c a t e d f o r e s e t s i n t h e
88 a r e a s of d e f l a t i o n (McKee, 1966; McKee and Moiola, 1975).
Examples of f i r s t - o r d e r
bounding s u r f a c e s i n t h e Permian s a n d s t o n e s o f t h e D u m f r i e s h i r e b a s i n i n s o u t h e r n S c o t l a n d and t h e T r i a s s i c s a n d s t o n e s a t Frodsham, C h e s h i r e a r e g i v e n by B r o o k f i e l d (1977).
B r o o k f i e l d (1977) a l s o g i v e s d a t a on t h e r a t e of m i g r a t i o n of d r a a s and
d i s c u s s e s t h e e f f e c t s of c l i m b i n g bedforms and s u b s i d e n c e r a t e s on t h e b u i l d - u p of f i r s t - o r d e r bounding s u r f a c e s . Second-order bounding s u r f a c e s a r e c u t by f i r s t - o r d e r bounding s u r f a c e s a n d c o n t a i n w i t h i n them cross-bedded u n i t s ( F i g . 2 . 1 2 ) .
Although o n l y s p a r s e d a t a i s
a v a i l a b l e B r o o k f i e l d s u g g e s t s a c h a r a c t e r i s t i c s p a c i n g of between 1 and 1.5m w i t h i n c l i n a t i o n a n g l e s from 0-15'.
They a r e c o n s i d e r e d by B r o o k f i e l d (1977) t o mark
t h e p a s s a g e of dunes a c r o s s d r a a s , o r t o l o n g i t u d i n a l dunes m i g r a t i n g a c r o s s t h e lower l e e s l o p e s of d r a a s o r a l t e r n a t i v e l y t o t h e c l i m b i n g of dunes down a d r a a which does n o t have a well-developed s l i p f a c e .
This draa i n t e r p r e t a t i o n f o r
second-order bounding s u r f a c e s a c c o u n t s f o r t h e t y p i c a l sweeping f o r e s e t laminae c h a r a c t e r i s t i c of many a n c i e n t a e o l i a n s a n d s t o n e s w i t h o u t having t o i n v o k e prolonged reworking by cross-winds ( S h a r p , 1966). T h i r d - o r d e r bounding s u r f a c e s a r e c o n s i d e r e d by B r o o k f i e l d (1977) t o b e r e a c t i v ation surfaces.
These a r e d i s c o n t i n u i t i e s i n t h e l a t e r a l sequence of c r o s s - b e d s
which t a k e t h e form o f e i t h e r a p l a n a r o r convex-upwards e r o s i o n s u r f a c e .
The
c r o s s - b e d s u n d e r l y i n g t h e r e a c t i v a t i o n s u r f a c e a r e t r u n c a t e d , whereas t h o s e above a r e conformable ( A l l e n , 1 9 7 4 ~ ) . R e a c t i v a t i o n s u r f a c e s i n a e o l i a n sands may i n d i c a t e change i n wind d i r e c t i o n o r v e l o c i t y , o r a l t e r n a t i v e l y , t h e a c t i o n of v o r t i c e s g e n e r a t e d by bedforms upstream.
There i s l i t t l e e v i d e n c e t o s u p p o r t t h e f i r s t o f
t h e s e hypotheses and i t seems more l i k e l y t h a t t h e r e a c t i v a t i o n s u r f a c e s a r e t h e r e s u l t of l o c a l a i r flow changes caused by c o n f i g u r a t i o n a l changes i n dune p a t t e r n s . For example, a dune coming under t h e i n f l u e n c e of e d d i e s t o t h e l e e o f upwind dunes c o u l d have i t s c r e s t rounded o f f and t h e s l o p e o f t h e upper lee s i d e c o n s i d e r a b l y reduced, thus producing a r e a c t i v a t i o n surface. The t e x t u r a l c h a r a c t e r i s t i c s o f a e o l i a n sands The s i m i l a r i t y of a e o l i a n and aqueous c r o s s - b e d d i n g makes i t v e r y d i f f i c u l t on occasions t o recognize a n c i e n t a e o l i a n sands,
The d i f f i c u l t y may be p a r t l y overcome
by c a r e f u l s t u d y o f t h e t e x t u r a l p r o p e r t i e s o f a e o l i a n sand which may b e v e r y distinctive.
D i s c r i m i n a t i o n of a e o l i a n sand from o t h e r t y p e s of s a n d u s i n g s c a t t e r
diagrams of g r a i n s i z e p a r a m e t e r s ( e . g . Friedman, 1961) has now l a r g e l y been abandoned b e c a u s e of t h e o v e r l a p s between d i f f e r e n t e n v i r o n m e n t s , p a r t i c u l a r l y beaches (Moss, 1962, 1963).
A more r e a l i s t i c approach i s t o c o n s i d e r t h e whole
g r a i n s i z e spectrum i n c l u d i n g t h e p r o p o r t i o n s of d i f f e r e n t s u b p o p u l a t i o n s .
Folk
(1968b), f o r example, h a s shown t h a t d e s e r t f l o o r s a n d s a r e t y p i c a l l y bimodal and supermature and 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 connnon i n a n c i e n t a e o l i a n s a n d s t o n e s .
89 O t h e r t e x t u r a l p r o p e r t i e s which a r e commonly o b s e r v e d i n a e o l i a n s a n d s i n c l u d e t h e h i g h d e g r e e of roundness and t h e s u r f a c e p i t t i n g ( f r o s t i n g ) o f q u a r t z g r a i n s .
The
roundness o f a e o l i a n sand g r a i n s r e s u l t s from t h e h i g h l e v e l o f p h y s i c a l a t t r i t i o n which t h e y e x p e r i e n c e .
The f r o s t i n g c a n n o t b e a t t r i b u t e d t o sample impact marking
and i s more l i k e l y t h e r e s u l t o f s m a l l s c a l e s o l u t i o n and p r e c i p i t a t i o n of s i l i c a during t h e d i u r n a l temperature cycle.
Some of t h e s e problems a r e now c o n s i d e r e d
i n more d e t a i l . Grain s i z e Wind i s a p a r t i c u l a r l y good s o r t i n g medium and i t s e f f e c t s can b e b e s t i l l u s t r a t e d by c o n s i d e r i n g t h e t h r e e r e l a t e d s i z e - f a m i l i e s of s e d i m e n t a r y p a r t i c l e s a s s o c i a t e d with deserts.
These a r e :
1) c o a r s e s a n d and g r a v e l which a r e e a s i l y r o l l e d by t h e wind b u t a r e t o o l a r g e t o s a l t a t e ; t h e s e accumulate i n w e l l - s o r t e d groups (e.g.
ripple crests)
2 ) medium and f i n e sand which t r a v e l s l a r g e l y by s a l t a t i o n and which accumulates a s w e l l - s o r t e d s a n d dunes 3) v e r y f i n e s a n d and c o a r s e s i l t which, i f u n d i s t u r b e d and on a f l a t s u r f a c e , i s t o o s m a l l t o b e e a s i l y moved by t h e wind; b u t when d i s t u r b e d may t r a v e l g r e a t distances (e.g. dust storms). The s a l t a t i o n p o p u l a t i o n , 2) a b o v e , i s t h e p r i n c i p a l c o n s t i t u e n t o f a e o l i a n sand dunes.
Udden(1898) showed t h a t dune c r e s t s were p r e d o m i n a n t l y i n t h e 0.25-0.125mm
s i z e (2-30) r a n g e r e g a r d l e s s o f l o c a l i t y and was w e l l aware of t h e good s o r t i n g of a e o l i a n sands.
Many o t h e r a u t h o r s have commented on t h e f i n e g r a i n s i z e and
good s o r t i n g of a e o l i a n dunes b u t t h e r e a r e s u r p r i s i n g l y few s y s t e m a t i c a n a l y s e s f o r i n l a n d d e s e r t s and r a t h e r more f o r c o a s t a l dunes.
Alimen (1957a,b) and
C h a v a i l l i o n (1964: have s t u d i e d many samples from t h e S a h a r a , Moiola and Weiser (1968) s t u d i e d 30 samples from s e v e r a l d e s e r t s and i n v e s t i g a t e d t h e r e l a t i o n s h i p s between mean g r a i n s i z e , s o r t i n g , and t h e h i g h e r moment measures.
W a i t t (1969)
c o n s i d e r e d s t u d i e d g r a i n s i z e p a r a m e t e r s i n r e l a t i o n t o dune p o s i t i o n i n a West Texas dune f i e l d .
Folk (1968b, 1971b) has s t u d i e d t h e a e o l i a n s a n d s of t h e Simpson
Desert i n A u s t r a l i a i n some d e t a i l .
F u r t h e r d a t a i s p r o v i d e d by McKee and T i b b i t t s
( 1 9 6 4 ) , Sharp ( 1 9 6 6 ) , Warren (1971, 1972) and Skozek a n d S a a d a l l a h (1972).
The
g r a i n s i z e p a r a m e t e r s of a e o l i a n d u n e s , a s r e p o r t e d by t h e s e a u t h o r s , can b e c o n s i d e r e d a s a continuum between two d i s t i n c t i v e t y p e s .
The b e s t known o f t h e s e
i s t h e f i n e g r a i n e d v e r y w e l l - s o r t e d t y p e d e s c r i b e d by Udden ( 1 9 9 8 ) .
This i s
t y p i c a l l y p o s i t i v e l y skewed w i t h a s h a r p c u t - o f f towards t h e c o a r s e r g r a i n s and has a t a i l of f i n e m a t e r i a l (Fig.2.13E,F).
T h i s f i n e m a t e r i a l i n a e o l i a n dunes i s
p r o b a b l y due t o t h e s e t t l e m e n t o f d u s t a f t e r t h e c e s s a t i o n o f winds (Sokolow, 1894). Another v e r y common sand found i n dune a r e a s i s a bimodal sand w i t h a c o a r s e mode a t a b o u t 0 . 6 m (0.750) and a f i n e mode a t a b o u t 0.12mm ( 3 . 0 0 ) (Bagnold, 1941;
90
Fig.2.13. R e p r e s e n t a t i v e g r a i n s i z e d i s t r i b u t i o n s ( s o l i d l i n e s ) from a e o l i a n sands o f t h e Simpson Desert, A u s t r a l i a , The dashed l i n e s show how i n d i v i d u a l d i s t r i b u t i o n s can b e broken down i n t o c o a r s e (C), medium (M), and f i n e ( F ) G a u s s i a n s u b p o p u l a t i o n s ( a f t e r F o l k , 1971b).
91 F o l k , 1968b, 1971b; Warren, 1971, 1972).
Typically t h e g r a i n diameter r a t i o i n
t h e s e bimodal s a n d s i s between 4 : l and 8 : l and t h e y a r e b e s t developed i n f l a t i n t e r d u n e a r e a s , p a r t i c u l a r l y downwind o f s e i f s ( F o l k , 1971b; Warren, 1972) (Fig.2.13B,C).
Many z i b a r a r e a l s o bimodal (Cooke and Warren, 1973) b e c a u s e t h e
c o a r s e g r a i n s e f f e c t i v e l y s e a l o f f t h e s u r f a c e so t h a t o n l y s t r o n g winds can b u i l d dunes. The o r i g i n o f bimodal s a n d s on t h e d e s e r t f l o o r h a s been a m a t t e r o f some discussion.
F o l k (1968b) a r g u e d t h a t t h e b i m o d a l i t y r e s u l t e d from t h e s e l e c t i v e
removal of t h e s a l t a t i o n p o p u l a t i o n from i n i t i a l l y u n s o r t e d d e t r i t u s .
H e thus
c o n s i d e r e d t h e c o a r s e mode t o r e p r e s e n t t h e s u r f a c e c r e e p and t h e f i n e mode t o r e p r e s e n t c o h e s i v e m a t e r i a l w i t h a h i g h c r i t i c a l t h r e s h o l d f o r movement ( F i g . 2 . 1 4 ) . F i n e r s e d i m e n t s do i n d e e d have h i g h c r i t i c a l t h r e s h o l d f o r movement (Bagnold, 1941, p.88).
However t h i s i s p a r t l y due t o i n t e r p a r t i c l e c o h e s i o n (Smalley, 1964, 1970)
and o n l y a p p l i e s t o v e r y w e l l s o r t e d f i n e s a n d s ; when t h e f i n e sand e x i s t s i n a p o o r l y s o r t e d m i x t u r e i t w i l l b e c o n s t a n t l y bombarded by s a l t a t i n g c o a r s e r g r a i n s . An
The d i s t u r b a n c e c r e a t e d i s t h u s l i k e l y t o c a u s e e n t r a i n m e n t of t h e f i n e sands.
a l t e r n a t i v e , and more s a t i s f a c t o r y , e x p l a n a t i o n of b i n o d a l i s m has been p u t forward by Warren (1972).
T h i s h a s been c a l l e d t h e " p r o t e c t i o n i s t theory".
I n t h i s theory
t h e s a n d s i n s a l t a t i o n i n c l u d e some p a r t i c l e s t h a t a r e f i n e enough t o f i t i n t o t h e i n t e r p a r t i c l e v o i d s of t h e c r e e p l o a d , so t h a t when t h e y s t r i k e t h e bed t h e y p e n e t r a t e t h e v o i d s and a r e p r o t e c t e d from f u r t h e r bombardment.
The s l i g h t l y c o a r s e r f r a c t i o n
o f t h e s a l t a t i o n l o a d i s t o o l a r g e f o r t h e v o i d s so t h a t i t remains on o r n e a r t h e s u r f a c e of t h e bed where i t i s bombarded by o t h e r p a r t i c l e s and moved onward. s a n d i s t h u s f i l t e r e d o u t and a c c u m u l a t e s on dunes.
This
This p r o t e c t i o n i s t theory
e x p l a i n s t h e e x i s t e n c e o f bimodal s a n d s on dunes a s w e l l a s on a r e a s where t h e r e h a s been n e t d e f l a t i o n .
I t a l s o e x p l a i n s t h e o c c u r r e n c e o f c o r e s o f f i n e sand w i t h i n
r i p p l e s ( S h a r p , 1963) and bimodal s a n d s on t h e h o r n s of b a r c h a n s ( V e r l a q u e , 1958). These g r a i n s i z e c h a r a c t e r i s t i c s a r e w i d e l y o b s e r v e d i n a n c i e n t a e o l i a n s a n d s t o n e s . Some of them, from t h e Lower Permian d e s e r t of NW Europe a r e i l l u s t r a t e d i n Fig.2.15. Roundness and f r o s t i n g The most c h a r a c t e r i s t i c f e a t u r e s o f a e o l i a n s a n d g r a i n s a r e t h e h i g h d e g r e e of roundness and t h e i r f r o s t e d s u r f a c e a p p e a r a n c e .
These f e a t u r e s a l o n e have o f t e n been
used t o i d e n t i f y a e o l i a n e n v i r o n m e n t s , a p r a c t i c e which i s n o t t o b e recommended b e c a u s e a e o l i a n sand g r a i n s can be d e r i v e d and reworked i n t o s h a l l o w marine environments and a l s o b e c a u s e s i m i l a r f e a t u r e s may r e s u l t from subaqueous t r a n s p o r t (Margolis and K r i n s l e y , 1971).
However well-rounded g r a i n s w i t h well-developed
s u r f a c e f r o s t i n g do p r o v i d e a n i m p o r t a n t c l u e t o a n a e o l i a n o r i g i n ( G l e n n i e , 1972). There have been a number of s c a n n i n g e l e c t r o n microscope (SEM) s t u d i e s of t h e s u r f a c e f e a t u r e s of a e o l i a n s a n d g r a i n s ( M a r g o l i s and K r i n s l e y , 1971; K r i n s l e y and
92
UNSORTED FLUVIAL DETRITUS SOURCE MATERIAL
.
.
Md Sd.+ Fn. Sd MOVED EASIEST
DEFLATION PROCESS DUNE
BIMODAL PRODUCT Bimodal Cr. Sd.+VFn.Sd.
Unimodal Fn. -Md. Sd.
Fig.2.14. I d e a l i z e d diagram d e p i c t i n g t h e o r i g i n of bimodal a e o l i a n s a n d s by d e f l a t i o n . F i n e sand i s removed from u n s o r t e d f l u v i a l d e t r i t u s b e c a u s e t h i s i s most e a s i l y s a l t a t e d . F i n e r s e d i m e n t remains b e h i n d b e c a u s e i t i s t o o c o h e s i v e w h i l s t c o a r s e r g r a i n s c a n n o t b e moved by s a l t a t i o n ( a f t e r F o l k , 1968b). Doornkamp, 1973) and i t i s now r e c o g n i z e d t h a t b o t h g r a i n impacts and s o l u t i o n / r e p r e c i p i t a t i o n a r e important i n t h e i r formation.
K r i n s l e y e t a l . (1976) r e c o g n i z e d
f o u r t y p e s of t e x t u r a l f e a t u r e s t h a t seem t o b e c h a r a c t e r i s t i c of a e o l i a n q u a r t z g r a i n s from modern h o t d e s e r t s f F i g . 2 . 1 6 ) .
1. "Upturned p l a t e s " .
These a r e p a r a l l e l r i d g e s r a n g i n g i n l e n g t h from a b o u t 0.5Ym
t o a b o u t 1Ofm and c o v e r i n g t h e s u r f a c e s of most g r a i n s g r e a t e r t h a n 4 0 0 - 5 0 0 ~i n diameter.
These u p t u r n e d p l a t e s a r e i n t e r p r e t e d a s c l e a v a g e s c a r p s r e s u l t i n g from
" a b r a s i o n f a t i g u e " ( P a s c o e , 1961) due t o s a l t a t i o n i m p a c t s .
2 . Equidimensional o r e l o n g a t e d e p r e s s i o n s 20 t o 250 m i n l e n g t h .
These d e p r e s s i o n s
tend t o be on s m a l l e r g r a i n s and a r e i n t e r p r e t e d by K r i n s l e y e t a l . (1976) a s p o s s i b l y b e i n g due t o d i r e c t r a t h e r t h a n g l a n c i n g i m p a c t s between s a l t a t i n g o r creeping grains.
3. Smooth s u r f a c e s on s m a l l e r g r a i n s , 90-400ym i n s i z e .
These a r e due t o p r e -
c i p i t a t i o n and s o l u t i o n o f s i l i c a and a r e u n a f f e c t e d by a b r a s i o n t o any g r e a t e x t e n t because such g r a i n s a r e t r a n s p o r t e d mainly i n s u s p e n s i o n .
4 . A r c u a t e , c i r c u l a r o r p o l y g o n a l c r a c k s (Lucci and Casa, 1968) commonly found on
93
Fig.2.15. Thin s e c t i o n photomicrographs showing some c h a r a c t e r i s t i c t e x t u r e s o f a e o l i a n sands. All from t h e R o t l i e g e n d e s (Lower Permian) of t h e S o l e P i t a r e a , North Sea. A. F i n e g r a i n e d , w e l l - s o r t e d s a n d from a dune t o p . B . Medium g r a i n e d s a n d from t h e i n t e r m e d i a t e p a r t of a dune. C. Bimodal sand from a dune base. Note t h a t t h e f i n e r mode i s a n g u l a r and t h e c o a r s e r mode rounded.
94
Fig.2.16. A. A q u a r t z s a n d g r a i n from t h e Algodones dunes ( s o u t h e r n C a l i f o r n i a ) . The g r a i n i s smooth and rounded w i t h a s u g g e s t i o n o f u p t u r n e d p l a t e s on t h e lower r i g h t s i d e B. Closeup of A. Note u p t u r n e d p l a t e s (UP) s l i g h t l y rounded by s o l u t i o n C. Quartz sand g r a i n from T r i a s s i c o f Devonshire showing e l o n g a t e o r rounded d e p r e s s i o n s and upturned p l a t e s on t h e u p p e r l e f t - h a n d c o r n e r D. Closeup o f C. Note s i m i l a r i t y t o B e x c e p t t h a t t h e u p t u r n e d p l a t e s (W) a r e l a r g e r ( a f t e r K r i n s l e y e t a l . , 1976).
95 g r a i n s 90 t o 150um i n s i z e .
These f e a t u r e s might b e t h e r e s u l t of p h y s i c a l o r
chemical w e a t h e r i n g and a r e n o t p r e s e n t on l a r g e r g r a i n s b e c a u s e i t would p r o b a b l y b e removed by a b r a s i o n d u r i n g s a l t a t i o n o r c r e e p . These f e a t u r e s have been r e c o g n i z e d i n a v a r i e t y o f a e o l i a n s a n d s from t h e d e s e r t s of Libya, A r a b i a , A u s t r a l i a , New Mexico and N e w Mexico and were a l s o d e s c r i b e d from E a r l y T r i a s s i c r e d beds from South Devonshire:
c o n s i d e r e d by t h e a u t h o r s t o b e t h e
o l d e s t known example of a e o l i a n s u r f a c e t e x t u r e s unmodified by d i a g e n e s i s . DESERT LAKES AND INLAND SABKHAS The t o p o g r a p h i c a l l y l o w e s t a r e a s o f many d e s e r t s o f t e n c o n t a i n ephemeral s a l i n e lakes.
More permanent l a k e s r e s u l t b e c a u s e o f t h e i n l a n d d r a i n a g e p a t t e r n and t h e
f a c t t h a t t h e w a t e r t a b l e i s v e r y c l o s e t o t h e s u r f a c e i n t h e c e n t r a l p a r t of t h e
INFLUENT SEEPAGE
EFFLUENT SEECAOE
INFLUENT SEEPAGE
/
ALLUVIAL FANS PLAYA EVAPORITES
LACUSTRINE
A
A//
BEDROCK
Fig.2.17. Schematic diagram of a d e s e r t b a s i n showing t h e d i s t r i b u t i o n of s e d i m e n t a r y f a c i e s and t h e w a t e r c i r c u l a t i o n ( a f t e r Davis and De Wiest, 1 9 6 6 ) . d e s e r t basin (Fig.2.17).
The d i s t r i b u t i o n o f s e d i m e n t a r y f a c i e s and groundwater
c i r c u l a t i o n a r e closely interdependent.
I n t e c t o n i c a l l y a c t i v e basins a l l u v i a l fans
a r e developed around t h e margins and t h e s e have i n f l u e n t d r a i n a g e .
These a r e
f o l l o w e d , towards t h e c e n t r e of t h e b a s i n , by a b e l t o f a e o l i a n s a n d and i n t h e v e r y c e n t r e of t h e b a s i n where t h e d r a i n a g e becomes e f f l u e n t , d e s e r t l a k e s may develop. They a l s o m i g h t form where dune s a n d s b l o c k a wadi c h a n n e l f o r example ( s e e G l e n n i e , 1970, F i g . 4 9 ) .
These d e s e r t l a k e s a r e r e f e r r e d t o a s p l a y a s i n North America and
i n l a n d sabkhas i n t h e Middle E a s t . They range i n s i z e from j u s t a few s q u a r e metres 2 up t o thousands o f km , Lake E y r e , A u s t r a l i a , f o r example, had a maximum a r e a l e x t e n t
of a b o u t 8,000km2 (Twidale, 1972).
The more r e c e n t h i s t o r y o f p r e s e n t - d a y p l a y a s i s
96 h i g h l y complicated and many of them were p e r e n n i a l l a k e s i n t h e p a s t ( S t o e r t z and E r i k s o n . 1974) e s p e c i a l l y d u r i n g P l e i s t o c e n e times. When t h e d e s e r t l a k e d r i e s up, d u r i n g t h e h o t s e a s o n , t h e s i l t a n d c l a y d e p o s i t e d when t h e l a k e was f u l l becomes d i s r u p t e d .
T h i s may b e by t e r r e s t r i a l a n i m a l s o r o t h e r
a g e n c i e s which l e a v e abundant t r a c k s on t h e sediment s u r f a c e .
Wind blown s a n d g r a i n s
may s t i c k t o t h e damp sabkha s u r f a c e and a s m o i s t u r e c o n t i n u e s t o r i s e by c a p i l l a r i t y a d d i t i o n a l g r a i n s w i l l a d h e r e and a d h e s i o n r i p p l e s w i l l form.
These a r e one o f t h e
most c h a r a c t e r i s t i c sedimentary s t r u c t u r e s o f i n l a n d sabkhas.
D e s i c c a t i o n polygons
develop and may c u r l up a t t h e edges l e a v i n g c r a c k s which a r e f i l l e d from above by wind blown sand o r from below by t h e i n j e c t i o n of a s l u r r y o f wet sand (Oomkens, 1966; G l e n n i e , 1970).
E v a p o r a t i o n of t h e s a l i n e l a k e w a t e r l e a d s t o t h e c o n c e n t r a t i o n
of s a l t s and a c r u s t of h a l i t e may form on t h e former l a k e bottom; gypsum c r y s t a l s ma) grow w i t h i n t h e sediment c a u s i n g f u r t h e r d i s r u p t i o n o f bedding s t r u c t u r e s .
Other
m i n e r a l s p e c i e s may b e i m p o r t a n t i n d e s e r t l a k e s and t h e s e may show a z o n a l a r r a n g e ment w i t h t h e more s o l u b l e s a l t s b e i n g d e p o s i t e d towards t h e c e n t r e of t h e b a s i n (Hunt e t e l . , 1966; Amiel and Friedman, 1971). I n Death V a l l e y , C a l i f o r n i a t h r e e main m i n e r a l f a c i e s c a n be r e c o g n i z e d e a c h w i t h s u b - f a c i e s determined on t h e b a s i s o f s u r f a c e morphology o r t h e amount o f admixture (Hunt e t a l . , 1966).
clastic
C a r b o n a t e s , s u l p h a t e s , and c h l o r i d e s show a r o u g h l y
c o n c e n t r i c z o n a t i o n w i t h t h e c h l o r i d e f a c i e s occupying by f a r t h e l a r g e s t c e n t r a l a r e a (Fig.2.18).
H a l i t e i s v o l u m e t r i c a l l y t h e dominant m i n e r a l i n t h e b a s i n and t h e
c a r b o n a t e and s u l p h a t e f a c i e s a r e d e f i n e d on t h e b a s i s of r e l a t i v e l y minor amounts of c a l c i t e and gypsum.
The c a r b o n a t e f a c i e s e r c u r s a t t h e t r a n s i t i o n from d i s t a l
f a n t o s a l t p a n (Fig.2.19), fine i n t o the basin.
t h e c a l c i t e o c c u r r i n g w i t h i n t h e c l a s t i c s e d i m e n t s which
These c a l c i t e - b e a r i n g s i l t s a r e e n c r u s t e d on t h e s u r f a c e by
h a l i t e b e n e a t h which i s a l a y e r o f gypsum nodules.
I n t h e s u l p h a t e f a c i e s gypsum
i s p r e c i p i t a t e d a t t h e s u r f a c e a s e i t h e r a massive l a y e r o r a s a c r y s t a l mush w i t h
a n i r r e g u l a r m i c r o r e l i e f (Fig.2.18).
The c h l o r i d e s c o n t a i n a number o f s u b - f a c i e s ,
t h e most e x t e n s i v e o f which c o n s i s t s of a smooth s i l t l a y e r up t o 15cm t h i c k which
i s u n d e r l a i n by up t o 30cm o f h a l i t e and rests on c l a s t i c sediment.
The s i l t l a y e r
c o n t a i n s s m a l l nodules of s u l p h a t e s and b o r a t e s sometimes cemented by c a r b o h a t e and h a l i t e and forming a hardpan.
The h a l i t e probably formed a t t h e t o p o f t h e c a p i l l a r y
f r i n g e of t h e w a t e r t a b l e . THE REDDENING OF DESERT SEDIMENTS
Introduction The reddening o f d e s e r t dune sands i s c e n t r a l t o t h e problem o f r e d bed formation. E a r l y workers impressed by r e d dune sands i n d e s e r t s were o f t h e o p i n i o n t h a t a n c i e n t r e d beds were a l s o of d e s e r t o r i g i n (Dawson, 1848; Crosby, 1885, 1891; Goodchild, 1896; B a r r e l l , 1908).
I t was g e n e r a l l y b e l i e v e d t h a t t h e r e d n e s s o f d e s e r t s a n d s was
L6 Fig.2.18. M i n e r a l z o n a t i o n i n t h e Death V a l l e y p l a y a of C a l i f o r n i a . I n t h e s u l p h a t e zone massive gypsum has a t h i n s k i n o f a n h y d r i t e . T h i s i s u n d e r l a i n by c a l c i t i c sandy s i l t . I n t h e c a r b o n a t e zone (expanded s e c t i o n ) s u r f a c e h a l i t e o c c u r s a s a b l i s t e r - l i k e c r u s t above t h e gypsum o f t h e s u l p h a t e l a y e r . The c a l c i t e of t h e c a r b o n a t e l a y e r o c c u r s a s s m a l l e u h e d r a l c r y s t a l s i n a s i l t y and sandy m a t r i x ( a f t e r Hunt e t a l . , 1966). caused by t h e low w a t e r t a b l e and l a c k of o r g a n i c m a t t e r which r e s u l t e d i n a n o x y g e n a t i n g environment (Dawson, 1848).
I n t h i s environment t h e d e s e r t sun was
presumed t o c a u s e t h e d e h y d r a t i o n of yellow f e r r i c h y d r o x i d e s t o r e d f e r r i c o x i d e (Crosby, 1885, 1891; B a r r e l l , 1908). Although t h e r e are many r e c o r d s o f r e d s u r f a c e m a t e r i a l s i n d e s e r t s t h e y a r e by no means e x c l u s i v e l y so ( G l e n n i e , 1970).
I n t h e a r i d Sudan Buursink (1971) found
v e r y few r e d o r r e d d i s h s o i l s and i n A u s t r a l i a , L i t c h f i e l d (1962, 1963) found drab soils.
V a r i e d c o l o u r s a r e much more t y p i c a l ; Warren (1970) r e p o r t e d 5YR 516 and
even r e d d e r c o l o u r s i n t h e f i x e d a e o l i a n s a n d s of t h e s e m i - a r i d Sudan and White (1971) n o t e d c o l o u r s of 5YR 516-8 and 2.5yR 416-8 i n s i m i l a r f i x e d a e o l i a n s a n d s i n t h e Republic of Niger.
Worral (1969) summarized much o f t h e work on t h e sands of
t h e s o u t h e r n S a h a r a and p o i n t e d o u t t h e wide r a n g e i n c o l o u r f r o n r e d , r e d d i s h - y e l l o w a n d reddish-brown.
G l e n n i e (1970) r e p o r t s r e d a e o l i a n s a n d s from t h e T r u c i a l Coast.
The r e d a e o l i a n sands o f t h e A u s t r a l i a n d e s e r t s a r e well-known ( L i t c h f i e l d , 1962, 1963; F o l k , 1976, 1978).
86 M 3E)NW INlWVNVd
3
Fig.2.19. R e l a t i o n s h i p between f r i n g i n g a l l u v i a l f a n s , s a l t p a n s and t h e d i s t r i b u t i o n of p l a n t s i n Death V a l l e y ( a f t e r Hunt e t a l . , 1966). Enlargement shows a h y p o t h e t i c a l c r o s s - s e c t i o n of l a r g e s c a l e d e s i c c a t i o n f i s s u r e s found i n some p l a y a s ( a f t e r N e a l , 1965). and e l s e w h e r e i n Mexico c o l o u r s of 10R 614 a r e known (Walker, 1967a).
The b r i g h t e s t
c o l o u r s i n r e d d e s e r t s o i l s a r e o f t e n found i n t h e B h o r i z o n a f e a t u r e which can b e a t t r i b u t e d t o more advanced w e a t h e r i n g , i r o n i l l u v i a t i o n , and a l s o t o t h e f a c t t h a t t h e r e i s no c o l o u r masking by o r g a n i c m a t t e r i n t h i s h o r i z o n . Many a u t h o r s have n o t e d t h a t a e o l i a n sand dunes become r e d d e r w i t h t i m e .
A
number o f examples a r e g i v e n by N o r r i s and N o r r i s (1961) and t h e s u b j e c t is d i s c u s s e d by N o r r i s ( 1 9 6 9 ) , G a u t i e r (1935, p . 4 4 ) .
T r i c a r t and Brochu (1955) have observed
t h a t smaller, presumably more a c t i v e and younger, Saharan d u n e s , a r e y e l l o w , whereas t h e l a r g e r e v i d e n t l y f i x e d dunes t e n d t o b e r e d i n c o l o u r . confirmed G a n t i e r ' s o b s e r v a t i o n s on a q u a n t i t a t i v e b a s i s .
Alimen (1957a,b) Logan (19601, i n t h e
d e s e r t s of s o u t h w e s t A f r i c a , showed t h a t younger dunes on t h e c o a s t were p a l e y e l l o w w h i l s t t h e o l d e r , i n l a n d dunes were r e d i n c o l o u r .
Many o t h e r workers have made
s i m i l a r o b s e r v a t i o n s i n c l u d i n g T e r r y (1957) f o r w e s t e r n A u s t r a l i a , Stephens (19611, J a c k s o n (1962) and Wopfner and Twidale (1967) f o r c e n t r a l A u s t r a l i a ; P r i c e (1962) f o r s o u t h e r n Texas, Dolan (1970) i n n o r t h C a r o l i n a , and G l e n n i e (1970) f o r t h e T r u c i a l Coast.
I t i s n o t c l e a r i n a number of t h e s e c a s e s whether t h i s i s due t o
t h e l a c k o f t i m e needed f o r t h e development of pigment o r t o t h e removal o f pigment by a b r a s i o n from t h e more a c t i v e , younger dunes.
For a l o n g t i m e t h e r e d c o l o u r of d e s e r t dune s a n d s has been a t t r i b u t e d t o haematite grain coatings.
T h o u l e t (1881) made chemical a n a l y s e s of h a e m a t i t e - c o a t e d
Saharan sands and P h i l l i p s (1882) r e p o r t e d t h a t r e d s a n d s from t h e Arabian d e s e r t c o n t a i n e d 0.21% Fe203.
P h i l l i p s (1882) a l s o made t h e i m p o r t a n t o b s e r v a t i o n t h a t
t h e r e d pigment must have been a c q u i r e d a f t e r t h e rounding of t h e s a n d o r i t would have o t h e r w i s e been worn away.
However t h e r e have been few d e t a i l e d s t u d i e s of t h e
o x i d e m i n e r a l o g y of r e d d e s e r t s o i l s and dunes.
Walker (1967a) c o u l d n o t d e t e c t
c r y s t a l l i n e h a e m a t i t e i n many of t h e r e d s o i l s of t h e Sonoran d e s e r t and i t i s l i k e l y t h a t more p o o r l y c r y s t a l l i n e o x i d e s o r h y d r o x i d e s , p r o b a b l y i n i n t i m a t e a s s o c i a t i o n with clay minerals, a r e important colouring agents.
These may e x i s t a s g r a i n c o a t i n g s
o r a s i n t e r s t i t i a l i r o n stained c l a y matrix. There i s c o n s i d e r a b l e e v i d e n c e t h a t modern d e s e r t s e d i m e n t s become r e d d e r w i t h age.
F i n e g r a i n e d h a e m a t i t e g r a i n c o a t i n g s and h a e m a t i t e - s t a i n e d c l a y m a t r i x a r e
responsible for the colouration.
These f e a t u r e s a r e c o m p a t i b l e w i t h a r e d d e n i n g
p r o c e s s which i n v o l v e s t h e i n s i t u p r o g r e s s i v e h y d r o l y z a t i o n of ferromagnesian m i n e r a l s ( R e i f e n b e r g , 1947, 1950; R i m , 1951; S h o t t o n , 1956; P r i c e , 1962; Walker, 1967a; N o r r i s , 1969).
Three i m p o r t a n t f a c t o r s , h e a t and m o i s t u r e , time a r e
i m p o r t a n t i n t h e i n t r a s t r a t a l a l t e r a t i o n of f e r r o m a g n e s i a n m i n e r a l s and subsequent d e g r e e o f r e d d e n i n g which might r e s u l t .
Heat i s i m p o r t a n t i n s p e e d i n g up chemical
r e a c t i o n s ; i n d e s e r t s u r f a c e s e d i m e n t t e m p e r a t u r e s between 70-85OC a r e commonly r e c o r d e d ( W a l t h e r , 1900; Mohr and Van Baren, 1954; Cloudsley-Thompson, 1968) and some workers have commented t h a t r e d n e s s was most i n t e n s e n e a r t h e s u r f a c e because o f s o l a r h e a t i n g ( P a s s a r g e , 1904). Moisture i s c l e a r l y important i n t h e formation of hydrated weathering products and a c e r t a i n m i m i m u m amount i s needed i f i n t r a s t r a t a l s o l u t i o n i s t o proceed. There i s c o n s i d e r a b l e e v i d e n c e o f t h e a b i l i t y o f d e s e r t s a n d s t o r e t a i n m o i s t u r e f o r l o n g p e r i o d s under t h e s e c o n d i t i o n s ( C v i j a n o v i c h , 1953).
The c o n d e n s a t i o n of m o i s t u r e
t o form i n t e r n a l dew is a l s o well-known ( C o r n i s h , 1914; R i m , 1951; V e r l a q u e , 1958; Engel and S h a r p , 1958); t h e consequence i s t h a t even d u r i n g t h e d r y s e a s o n dunes may be damp j u s t a metre o r so below t h e s u r f a c e and u n s t a b l e s i l i c a t e m i n e r a l s may b e i n prolonged c o n t a c t w i t h m o i s t u r e e s p e c i a l l y i n f i x e d dunes.
A l t e r a t i o n of f e r r o -
magnesian m i n e r a l s and t h e f o r m a t i o n o f h a e m a t i t e c o u l d t a k e p l a c e under t h e s e conditions, The e v i d e n c e does s u g g e s t however t h a t t h i s a l t e r a t i o n p r o c e s s i s a r e l a t i v e l y slow one and o n l y s e e n t o good e f f e c t i n P l e i s t o c e n e and P l i o c e n e sediments which have been i n p r o l o n g e d c o n t a c t w i t h oxygenated ground w a t e r .
Walker (1967a) i n
h i s c l a s t i c s t u d y o f t h e Sonoran D e s e r t a l l u v i u m showed t h a t t h e f o r m a t i o n of c r y s t a l l i n e h a e m a t i t e and r e d c o l o u r a t i o n c o u l d t a k e up t o 106 y e a r s .
The impl i c a t i o n h e r e i s t h a t t h e r e l a t i v e s c a r c i t y o f modern r e d dune sands i s due t o t h e
f a c t t h a t s u f f i c i e n t t i m e has n o t been a v a i l a b l e f o r t h e i n t r a s t r a t a l a l t e r a t i o n t o develop. The l o n g p e r i o d o f time which i s needed f o r i n s i t u r e d d e n i n g makes i t
100 d i f f i c u l t t o a c c o u n t f o r t h o s e r e d a e o l i a n s a n d s which do o c c u r a t t h e p r e s e n t time; f o r example, some of t h e l a r g e r dunes i n t h e Saharan e r g s c a n be no more t h a n a few thousand y e a r s o l d (Wilson, 1970) and i n t h e Sudanese f i x e d dunes s t a b i l i t y i s c o n s i d e r e d t o have l a s t e d f o r o n l y some 9,000 y e a r s (Warren, 1970).
The p r e s e n t l y
a c t i v e r e d sand dunes of t h e A u s t r a l i a n d e s e r t s a r e a l s o d i f f i c u l t t o e x p l a i n by t h i s d i a g e n e t i c model. A p o s s i b l e c l u e t o t h i s problem l i e s i n t h e once w i d e l y h e l d b e l i e f t h a t r e d n e s s
i n deserts is associated with wetter ciimates i n the past.
This i s p a r t i c u l a r l y
a p p l i c a b l e t o A f r i c a (Grabham, 1926; B o u r c a r t , 1928; S a n d f o r d , 1935; Kubiena, 1955; Meckelein, 1957; Cooke, 1961) and A u s t r a l i a ( C r o c k e r , 1946; Madigan, 1946; Mabbutt, 1965).
T h i s d e t r i t a l model f o r t h e o r i g i n of r e d d e s e r t s a n d s h a s been r e c e n t l y
r e v i v e d by F o l k (1976). The d e t r i t a l model
- Simpson Desert, A u s t r a l i a
The Simpson Desert is a r e d e r g i n c e n t r a l A u s t r a l i a , a b o u t 300 miles i n d i a m e t e r , which c o n s i s t s o f NEJW t r e n d i n g s e i f dunes developed on P l e i s t o c e n e f l u v i a l s e d i m e n t s . The d e s e r t i s t h e r e s u l t of t h e d e s i c c a t i o n o f o l d e r a l l u v i a l p l a i n s which were formed a t t i m e s o f much h e a v i e r r a i n f a l l and l e s s e n e d e v a p o r a t i o n , e s p e c i a l l y d u r i n g t h e P l e i s t o c e n e (Madigan, 1946).
F o l k (1976) s t u d i e d t h e n o r t h w e s t e r n edge of t h e
e r g and sampled t h e c r e s t s and f l a n k s of a c t i v e dunes and a l s o t h e i n t e r v e n i n g deflationary f l a t s o r reg,
The dunes, composed o f w e l l - s o r t e d f i n e s a n d , a r e b r i g h t
r e d i n c o l o u r whereas t h e c o a r s e r r e g sediments a r e d u l l e r and d a r k e r .
This i s
confirmed by q u a n t i t a t i v e measurement of hue, v a l u e and s a t u r a t i o n u s i n g t h e Munsell S o i l Colour C h a r t on s i e v e d
40
grades.
The r e s u l t s a r e summarized i n Fig.2.20.
In
t h e dune s a n d s t h e f i n e and v e r y f i n e s a n d s (between 2.00 and 4.50) a r e a l m o s t a l l of hue 2.5YR ( r e d d i s h - o r a n g e ) , v a l u e 4/ ( d a r k ) and s a t u r a t i o n / 7 ( v e r y s t r o n g ) . The c o a r s e r s e d i m e n t s i n t h e dunes (up t o 1.00) a r e more o r a n g e w i t h hue 3.5YR, v a l u e
5 / (medium) and s a t u r a t i o n / 6 .
Similar r e s u l t s a r e seen i n t h e reg sediments but
t h e hue i s s l i g h t l y more o r a n g e , t h e v a l u e s l i g h t l y d a r k e r and t h e s a t u r a t i o n a l m o s t a f u l l shade weaker (Fig.2.20).
F o l k (1976) n o t e d on i n s p e c t i o n of s i n g l e s a n d g r a i n s
w i t h t h e b i n o c u l a r microscope t h a t t h e c o l o u r d i s t r i b u t i o n i s bimodal; c o a t s on t h e g r a i n s a r e e i t h e r s t r o n g " r e d d i s h orange" (10R t o 2.5YR) o r p a l e r " y e l l o w i s h orange" (7.5YR) w i t h o n l y a few g r a i n s of i n t e r m e d i a t e hue.
The d a r k e r , r e d d e r l i n e s o c c u r
i n p i t s i n t h e g r a i n s ( s e e F o l k , 1976, F i g . 1 ) and a r e c o v e r e d by a t h i n c o a t i n g of chemically-precipitated "turtle-skin"
silica.
The more y e l l o w hues a r e caused by
a s o f t " f l u f f " of l i m o n i t i c m a t e r i a l (Fig.2.21). Folk (1976) showed t h a t t h e r e d d i s h m a t e r i a l i n t h e p i t s i s a c u t a n o r c l a y i r o n - o x i d e s k i n (Brewer, 1964) and a r g u e d t h a t t h e r e d c u t a n and y e l l o w " f l u f f " a r e of d i f f e r e n t a g e s .
The r e d c u t a n s a r e c l e a r l y o l d e r and a r e t h o u g h t t o have o r i g -
i n a t e d d u r i n g a p e r i o d o f P l e i s t o c e n e ( o r e a r l i e r ) l a t e r i t i z a t i o n whereas t h e y e l l o w
101
I
I
DUNES
I
I
REG
1
5
1
6
I
7
I
0
SATURATION
Fig.2.20. Raw c o l o u r d a t a f o r samples from dune and r e g environments i n t h e Simpson Desert. G r a i n s i z e of t h e sample f r a c t i o n shown on t h e v e r t i c a l s c a l e . Each r e a d i n g of hue, v a l u e ( l i g h t n e s s ) and s a t u r a t i o n (chroma) shown by a d o t . Mean c o l o u r r e a d i n g s a s a f u n c t i o n o f g r a i n s i z e shown by curved l i n e s . Both dune and r e g show r e d d e s t , d a r k e s t , and most i n t e n s e c o l o u r s f o r t h e f r a c t i o n between a b o u t 28 and 40. Bottom diagram superimposes t h e mean t r e n d l i n e s f o r b o t h environments; g r e a t e s t d i f f e r e n c e between dune and r e g i s t h e weaker s a t u r a t i o n o f t h e r e g sediments ( a f t e r F o l k , 1976). " f l u f f " i s Holocene and s t i l l accumulating today.
The c o n t r a s t i n c o l o u r between
f i n e r and c o a r s e r sand f r a c t i o n s i s a t t r i b u t e d t o t h e f a c t t h a t t h e f i n e s a n d , because i t r e p r e s e n t s t h e s a l t a t i o n p o p u l a t i o n , i s c o n s i s t e n t l y mobile and does n o t a l l o w t h e accumulation of t h e l i m o n i t i c f l u f f .
There i s c o n s i d e r a b l e evidence
i n f a v o u r o f t h e widespread f o r m a t i o n o f l a t e r i t e d u r i n g p r e - P l e i s t o c e n e o r P l e i s t o c e n e time i n c e n t r a l A u s t r a l i a and l a t e r i t e s and i r o n p i s o l i t e s a r e r e p o r t e d by T e r r y (19341, C a r r o l l ( 1 9 3 9 ) , Whitehouse ( 1 9 4 0 ) , Crocker ( 1 9 4 6 ) , Madigan (1946), and Mabbutt (19h5). The s i g n i f i c a n c e of p r i o r l a t e r i t i z a t i o n and c l i m a t i c change i n t h e formation
of o t h e r r e d a e o l i a n s a n d s i s n o t c l e a r because s u f f i c i e n t s t u d i e s have n o t y e t been carried out.
Many d e s e r t b a s i n s a r e s i t e d on a n c i e n t a l l u v i a l p l a i n s , a f e a t u r e
102
10R- 25YR red orange
-
S k e t c h showing t h e sequence of g r a i n c o a t i n g s on reddened a e o l i a n sand Fig.2.21. from t h e Simpson Desert 1) H a e m a t i t i c c l a y c u t a n , formed i n P l e i s t o c e n e l a t e r i t i c s o i l and p r e s e r v e d o n l y i n p i t s ; 2 ) t h i n s i l i c a c o a t w i t h bumpy s u r f a c e , g i v i n g a g r e a s y l u s t r e o r i g i n a t i n g i n t h e p r e s e n t d e s e r t regime; 3 ) l i m o n i t i c s u r f a c e " f l u f f " of Holocene o r i g i n ( a f t e r F o l k , 1976). which s u g g e s t s t h a t t h e p r o c e s s c o u l d b e i m p o r t a n t e l s e w h e r e , p a r t i c u l a r l y i n p l a c e s l i k e t h e Sahara where dune s a n d s a r e known t o show downwind c o l o u r v a r i a t i o n s . The d e t r i t a l model has n o t y e t been f u l l y i n v e s t i g a t e d i n a n c i e n t a e o l i a n s a n d s t o n e s , a l t h o u g h many e a r l i e r workers c o n s i d e r e d i t i m p o r t a n t ( e . g . Dunham, 1953). The d i a g e n e t i c model
-
Sonoran D e s e r t
The Sonoran d e s e r t of n o r t h w e s t e r n Mexico and t h e s o u t h e a s t e r n USA c o n t a i n s a sequence of l a t e T e r t i a r y and Q u a t e r n a r y d e s e r t a l l u v i u m a s s o c i a t e d w i t h m a r g i n a l marine d e p o s i t s and a e o l i a n s a n d ( F i g . 2 . 2 2 ) .
The d e s e r t a l l u v i u m h a s been s t u d i e d
i n d e t a i l by Walker (1967a, 1976) and Walker e t e l . (1978).
I t c o n s i s t s of f i r s t -
c y c l e a r k o s i c d e t r i t u s d e r i v e d from f r e s h c r y s t a l l i n e basement which l a c k s any s i g n i f i c a n t s o i l development.
The a l l u v i u m i s non-red a t t h e t i m e of d e p o s i t i o n
and t h e e a r l i e s t d e t e c t a b l e r e d d e n i n g o c c u r s i n d e p o s i t s of l a t e P l e i s t o c e n e a g e . I n t h i s e a r l y s t a g e of r e d d e n i n g , which a l s o i n c l u d e s l a t e T e r t i a r y d e p o s i t s , t h e pigment c o n s i s t s o f amorphous f e r r i c o x i d e which c a n n o t b e i d e n t i f i e d by X-ray d i f f r a c t i o n a n a l y s i s (Walker, 1976, p.281). reddish-yellow
( e . g . 5yR 6 / 6 ) .
A t t h i s stage the deposits a r e typically
B r i g h t r e d c o l o u r a t i o n ( e . g . 8R 4 / 6 t o 9R 4 / 6 1 and
f i n e l y c r y s t a l l i n e h a e m a t i t e o n l y d e v e l o p i n o l d e r Miocene d e p o s , i t s .
103
A
GYNITIC ROCK$
LOW-TIDE TERRACE SAND
I
Fig.2.22. A. L o c a t i o n map B. Diagrammatic c r o s s s e c t i o n showing f a c i e s r e l a t i o n s h i p s o f R e c e n t , P l e i s t o c e n e , and P l i o c e n e s e d i m e n t s i n n o r t h e a s t e r n Baja C a l i f o r n i a , Mexico ( a f t e r Walker, 1967a). Walker (1967a, 1976) p r e s e n t e d c o n v i n c i n g e v i d e n c e t h a t t h e s e r e d beds formed i n s i t u by t h e p r o g r e s s i v e a l t e r a t i o n of d e t r i t a l ferromagnesian s i l i c a t e m i n e r a l s . S i n c e t h e r e has been no a p p r e c i a b l e c l i m a t i c change t h r o u g h o u t t h e d e p o s i t i o n a l h i s t o r y of t h e a r e a t h e s t u d y p r o v i d e s c o n v i n c i n g e v i d e n c e t h a t r e d beds can form i n a h o t , a r i d c l i m a t e and t h a t s e a s o n a l l y h i g h r a i n f a l l o r p r i o r l a t e r i t i z a t i o n a r e n o t p r e r e q u i s i t e s f o r t h e i r formation. The d i a g e n e t i c p r o c e s s e s l e a d i n g t o t h e f o r m a t i o n o f r e d beds i n t h i s a r e a a r e d e s c r i b e d by Walker e t a l . (1978).
They depend v e r y h e a v i l y on t h e t y p i c a l ground-
water c i r c u l a t i o n p a t t e r n seen i n d e s e r t b a s i n s (Fig.2.17).
Owing t o t h e h i g h
p e r m e a b i l i t y o f m a r g i n a l a l l u v i a l f a n s t h e w a t e r t a b l e g r a d i e n t s a t t h e b a s i n margin a r e low and t h e s u r f a c e d r a i n a g e p a t t e r n i s i n f l u e n t . o c c u r s w a t e r flows i n t o t h e a l l u v i u m ,
Whenever s u r f a c e r u n o f f
E f f l u e n t seepage only occurs near the c e n t r e
o f t h e b a s i n where t h e s a t u r a t e d zone l i e s n e a r e r t o t h e s u r f a c e and groundwater i s l o s t by e v a p o t r a n s p i r a t i o n . These c o n d i t i o n s e n s u r e t h a t groundwater m i g r a t e s towards the centre of t h e basin.
Two t y p e s of d i a g e n e t i c p r o c e s s e s o c c u r which l e a d t o t h e r e d d e n i n g o f t h e s e
104 d e s e r t sediments.
These a r e ' e a r l y ' d i a g e n e t i c and ' l a t e ' d i a g e n e t i c reddening.
E a r l y d i a g e n e t i c reddening The i n f l u e n t seepage i n m a r g i n a l a l l u v i u m r e s u l t s i n t h e mechanica of d e t r i t a l c l a y i n t o p r e v i o u s l y c l a y - f r e e sediment.
infiltration
This clay coats d e t r i t a l
g r a i n s and f r e q u e n t l y c o n t a i n s small amounts o f i r o n a s hydroxide c o a t i n g s and i n the c l a y mineral l a t t i c e .
A f t e r d e p o s i t i o n t h i s c l a y i s more o r less c o n t i n u o u s l y
bathed i n oxygenated groundwater and w i t h t i m e t h e c l a y b e g i n s t o redden due t o t h e formation of i r o n o x i d e .
The i r o n o x i d e may form by ' a g e i n g ' o f t h e d e t r i t a l i r o n
hydroxide o r by o x i d a t i o n of t h e c l a y m i n e r a l l a t t i c e .
I n e i t h e r case i t i s t h i s
type of e a r l y d i a g e n e t i c reddening which i s mainly r e s p o n s i b l e f o r t h e r e d d i s h yellow (5YR 6 / 6 ) pigment t h a t c h a r a c t e r i z e d much o f t h e younger ( P l i o - P l e i s t o c e n e ) d e s e r t a l l u v i u m o f t h e s o u t h e a s t e r n USA. Late d i a e e n e t i c reddeninn When u n s t a b l e framework s i l i c a t e s a r e i n prolonged c o n t a c t w i t h oxygenated groundwater t h e y a r e s u s c e p t i b l e t o a l t e r a t i o n by h y d r o l y s i s .
The a l t e r a t i o n
i n v o l v e s t h e f o r m a t i o n o f d i s s o l u t i o n v o i d s and t h e replacement of framework g r a i n s by mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e .
I r o n - b e a r i n g s i l i c a t e s such a s h o r n b l e n d e ,
pyroxene, and b i o t i t e c a n b e observed i n v a r i o u s s t a g e s of a l t e r a t i o n and e l e c t r o n microprobe a n a l y s e s show t h a t t h e replacement c l a y c o n t a i n s i r o n o x i d e which i s d i f f u s i n g outwards, away from t h e decomposing g r a i n .
This process causes the
r e l e a s e o f i o n s ( i n c l u d i n g sodium, potassium, c a l c i u m , magnesium, aluminium, s i l i c o n , and i r o n ) i n t o t h e c i r c u l a t i n g groundwater.
As t h e groundwater moves t h r o u g h t h e
sediment i t s composition i s modified and whenever t h e l o c a l c o n c e n t r a t i o n of d i s s o l v e d i o n s i s s u f f i c i e n t l y g r e a t a u t h i g e n i c m i n e r a l phases a r e p r e c i p i t a t e d i n t h e p o r e spaces.
The d e s e r t a l l u v i u m o f n o r t h w e s t e r n Mexico and t h e s o u t h w e s t e r n USA c o n t a i n s
a d i s t i n c t i v e s u i t e of e u h e d r a l a u t h i g e n i c m i n e r a l s i n c l u d i n g : mixed l a y e r i l l i t e - m o n t m o r i l l o n i t e ,
potassium f e l d s p a r ,
q u a r t z , c a l c i t e and h a e m a t i t e .
It i s the
c r y s t a l l i n e , a u t h i g e n i c h a e m a t i t e formed by t h i s l a t e d i a g e n e t i c p r o c e s s which g i v e s t h e o l d e r (Miocene) d e p o s i t s o f t h e r e g i o n t h e i r b r i g h t r e d (9R 4 / 6 1 c o l o u r a t i o n . The c o n d i t i o n s needed f o r t h e f o r m a t i o n of r e d beds by i n t r a s t r a t a l a l t e r a t i o n of framework s i l i c a t e s have been summarized by Walker (1967a) a s f o l l o w s :
1) occurrence o f i r o n - b e a r i n g d e t r i t a l g r a i n s 2) post-depositional conditions favouring i n t r a s t r a t a l a l t e r a t i o n of the i r o n bearing
grains
3 ) i n t e r s t i t i a l Eh-pH c o n d i t i o n s t h a t a r e s u i t a b l e f o r t h e f o r m a t i o n o f i r o n o x i d e s
4 ) absence of subsequent r e d u c t i o n o f t h e f e r r i c i r o n 5 ) enough t i m e f o r i n t r a s t r a t a l a l t e r a t i o n s of t h e i r o n - b e a r i n g g r a i n s and t h e f e r r i c hydroxide formed by t h e a l t e r a t i o n t o be c o n v e r t e d i n t o h a e m a t i t e , and p o s s i b l y
105 6 ) r e l a t i v e l y warm (above 65-70°F)
temperature.
Red beds can form wherever t h e s e c o n d i t i o n s a r e m e t .
The i m p o r t a n t f a c t o r i s t h a t
c l i m a t e i s n o t a n i m p o r t a n t r e q u i s i t e f o r r e d bed f o r m a t i o n .
These r e s u l t s t h u s
c o n t r a d i c t t h e c l a i m t h a t r e d s o i l s i n d e s e r t s a r e t r a n s p o r t e d from, o r r e l i c t s o f , l a t e r i t i c s o i l s which formed i n p r e v i o u s h o t , m o i s t climates. The m a j o r i t y o f a n c i e n t r e d beds a r e now c o n s i d e r e d t o b e of d i a g e n e t i c o r i g i n and formed i n a manner s i m i l a r t o t h a t d e s c r i b e d by Walker (1967a, 1 9 7 6 ) .
The model
a p p l i e s e q u a l l y w e l l t o b o t h a n c i e n t a e o l i a n s a n d s ( e . g . Kessler, 1978) and a n c i e n t d e s e r t a l l u v i u m ( e . g . G l e n n i e e t a l . , 1978). ANCIENT AEOLIAN RED BEDS Introduction The m a j o r i t y o f t h e ' c l a s s i c ' examples o f a n c i e n t a e o l i a n s a n d s t o n e s come from t h e l a t e P a l a e o z o i c and E a r l y Mesozoic of t h e w e s t e r n USA ( T a b l e 2 . 2 ) .
O t h e r examples
i n c l u d e t h e T r i a s s i c Botucatu Sandstones of B r a z i l (Almeida, 1953; B i g a r e l l a , 1972). and t h e Lower Permian ( R o t l i e g e n d e s ) s a n d s t o n e s o f B r i t a i n and t h e North Sea b a s i n (Laming, 1966; G l e n n i e , 1970, 1972; Waugh, 1970b) which o c c u r i n a s s o c i a t i o n w i t h d e s e r t a l l u v i a l and i n l a n d sabkha d e p o s i t s . The r e c o g n i t i o n o f a n c i e n t a e o l i a n s a n d s t o n e s i s by no means a n e a s y m a t t e r and t h e c r i t e r i a d e v i s e d by G l e n n i e (1970) may b e u s e f u l l y a p p l i e d :
1. Bedding h o r i z o n t a l o r l a r g e and small s c a l e c r o s s - b e d d i n g , showing c o n s t a n t o r multiple orientation 2. I n d i v i d u a l laminae w e l l - s o r t e d , e s p e c i a l l y i n f i n e r g r a i n s i z e s ; s h a r p g r a i n s i z e d i f f e r e n c e s between laminae a r e common
3 . G r a i n s i z e commonly r a n g e s from s i l t (60 m) t o c o a r s e sand.
Maximum s i z e f o r
wind t r a n s p o r t i s c . l.Ocm
4. L a r g e r s a n d g r a i n s t e n d t o b e w e l l - s o r t e d 5 . Clay d r a p e s a r e v e r y r a r e 6. Sands a r e f r e e of c l a y 7. Uncemented q u a r t z g r a i n s a r e f r o s t e d
8. Mica i s g e n e r a l l y a b s e n t Perhaps t h e most c h a r a c t e r i s t i c ( b u t n o t n e c e s s a r i l y d i a g n o s t i c ) f e a t u r e of a n c i e n t a e o l i a n s a n d s t o n e s i s t h e l a r g e s c a l e of t h e cross-bedding.
These a r e most
commonly p l a n a r t a b u l a r u n i t s a l t h o u g h t r o u g h s ( e . g . K n i g h t , 1929) a r e a l s o described.
I n d i v i d u a l s e t s may b e up t o 35m t h i c k and m u l t i p l e s e t s may occupy
g r e a t s t r a t i g r a p h i c thicknesses (Table 2.2).
One o f t h e b e s t d e s c r i p t i o n s of a n c i e n t
a e o l i a n c r o s s - b e d d i n g has been made by Walker and Harms (1972) from t h e Permian Lyons Sandstone of Colorado ( F i g . 2 . 2 3 ) .
I n v e r t i c a l cross s e c t i o n t h e cross-bedding appears
t o be p l a n a r t a b u l a r b u t on t h e exhumed l e e f a c e t h e r e a r e v e r y low a m p l i t u d e sand
106 TABLE 2.2.
C h a r a c t e r i s t i c s of t h e c l a s s i c a e o l i a n s a n d s t o n e s o f t h e w e s t e r n USA. Formation
CASPER
Age
Penn-Perm
Approx.Max. Thickness
Thick
Dip
Type
Ref e r e nce
240m
min 15m,
15-25'
troughs
S t e i d tmann
,
1974 COCONINO
Perm
3 30m
CEDAR MESA
Perm
450m
DE CHELLY
Perm
WHITE R I M
huge"
25- 30'
randomly B a a r s , 1962 oriented wedge-shaped sets
up t o 30m
up t o 25
planar tabular
B a a r s , 1962
3 30m
up t o 35m
15-35'
planar tabular
B a a r s , 1962
Perm
200m
up t o 20m
19-27'
planar tabular
Baars and S e a g e r , 1970
LYONS
Perm
40m
up t o 13m
25-28'
planar tabular
Walker and Harms, 1972
WINGATE
U.Trias
130m
up t o 15m
up t o 30'
NAVAJO
L.Jurassic
700m
up t o 30m
20-30'
U.J u r a s s i c ENTRADA ( S l i c k Rock Member)
280m
up t o 8 m
Dane, 1935 planar S a n d e r s o n , 1974 tabular Freeman and i n lower V i s h e r , 1975 p a r t (3-10m) trough i n upper p a r t (up t o 6m ) "wedging set" "sweeping aeolian c r o s s beds"
r i p p l e s t h a t s u g g e s t t h a t s a n d was blown a c r o s s t h e dune f a c e .
C r a i g and Shawe, 1975
The p l a n view shows
t h a t t h e e n t i r e dune advanced down d i p n o t by a v a l a n c h i n g down t h e s l i p f a c e b u t by t h e a d d i t i o n t o t h e l e e f a c e of wedges of s a n d blown i n t o p l a c e a c r o s s t h e dune. According t o Walker and Harms (1972) t h i s a l s o e x p l a i n s t h e low d i p a n g l e (25') t h e c r o s s beds which would b e n e a r e r t h e a n g l e of r e p o s e f o r d r y s a n d (34')
of
i f dune
m i g r a t i o n had been by a v a l a n c h i n g . I n f a c t t h e a n g l e of 34'
i s r a r e l y a t t a i n e d by a n c i e n t a e o l i a n s a n d s t o n e s a l t h o u g h
t h i s c o u l d be due p a r t l y t o p o s t - d e p o s i t i o n a l r e d u c t i o n by compaction ( G l e n n i e , 1972). The most abundant t y p e of a n c i e n t a e o l i a n c r o s s - b e d d i n g i s t h e p l a n a r t a b u l a r t y p e a p p a r e n t l y produced by t r a n s v e r s e dune t y p e s i n c l u d i n g barchans ( e . g . S h o t t o n , 1956).
107
Fig.2.23. Geometry of a e o l i a n c r o s s - b e d d i n g a s s e e n i n t h e Lyons S a n d s t o n e , Lyons Quarry (Permian, Colorado) ( a f t e r Walker and Harms, 1972). L o n g i t u d i n a l dune t y p e s a r e s u r p r i s i n g l y r a r e i n t h e s t r a t i g r a p h i c r e c o r d a l t h o u g h t h e Permian Yellow Sands o f NE England p r o v i d e a p o s s i b l e example ( G l e n n i e , 1972; Magraw, 1975). D i s t i n g u i s h i n g a e o l i a n from s h a l l o w marine s a n d s t o n e s A number of a n c i e n t s a n d s t o n e u n i t s p r e v i o u s l y r e g a r d e d a s a e o l i a n i n o r i g i n
( i n c l u d i n g some i n T a b l e 2 . 2 ) have r e c e n t l y been r e i n t e r p r e t e d a s s h a l l o w marine deposits.
Baars (19621, f o r example, c r i t i c i z e d t h e a e o l i a n i n t e r p r e t a t i o n of
t h e Cedar Mesa s a n d s t o n e s a s d i d Baars and S e a g e r (1970) f o r t h e White R i m sandstones ( s e e Table 2.2). cross-bedding.
I n b o t h c a s e s t h e i r argument was b a s e d on t h e s t y l e of
The Cedar Mesa i s c h a r a c t e r i z e d by .."moderate t o low a n g l e c r o s s -
s t r a t i f i c a t i o n , v e r y low a n g l e t o h o r i z o n t a l t h i n s a n d b e d s , long sweeping c u r v e s from m o d e r a t e l y d i p p i n g c r o s s - s t r a t a i n t o h o r i z o n t a l f o r e s e t b e d s , t h i n s i m p l e s e t s of low a n g l e c r o s s - s t r a t a "
( B a a r s , 1962, p.178).
The d e p o s i t s were r e i n t e r p r e t e d
a s l i t t o r a l o r beach sands ( B a a r s , 1962) b u t t h i s i n t e r p r e t a t i o n c a n n o t a c c o u n t f o r t h e s e t s up t o 30m t h i c k which a r e unknown i n any p r e s e n t - d a y beach environment. The c r i t i c i s m of t h e a e o l i a n i n t e r p r e t a t i o n o f t h e White R i m Sandstone was based on t h e i r r e g u l a r b a r - l i k e topography on t h e t o p of t h e White R i m .
The b a r topography
i s mantled by a v e n e e r o f wave-rippled sand which was a l m o s t c e r t a i n l y d e p o s i t e d i n
a marine environment.
A p p a r e n t l y i g n o r i n g t h e 20m t h i c k c r o s s beds i n t h e White R i m
Sandstone Baars and S e a g e r (1970) r e i n t e r p r e t e d t h e whole f o r m a t i o n a s s h a l l o w marine on t h e b a s i s of t h e b a r - l i k e topography and v e n e e r o f wave r i p p l e d sand.
A more
s a t i s f a c t o r y i n t e r p r e t a t i o n would b e t h a t t h e White R i m Sandstone i s a n a e o l i a n sand
108 which has s u f f e r e d marine reworking i n i t s upper p a r t . The s h a l l o w marine d e p o s i t s which most c l o s e l y resemble a e o l i a n s a n d s and which t h u s c r e a t e t h e most c o n f u s i o n i n i n t e r p r e t a t i o n a r e submarine s a n d waves and t i d a l current ridges.
These a r e common on many s h e l f a r e a s (Houbolt, 1968; McCave, 1971;
S w i f t , 1975) and f r e q u e n t l y show c r o s s - b e d d i n g which i t i s c l a i m e d c l o s e l y r e s e m b l e s a e o l i a n dune bedding ( S t a n l e y e t a l . , 1971, p . 1 3 ) ( F i g . 2 . 2 4 ) .
Thus, w i t h i n c r e a s i n g
knowledge of cross-bedded submarine s a n d s a number of s a n d s t o n e s c l a s s i c a l l y r e g a r d e d a s a e o l i a n have been r e i n t e r p r e t e d a s s h a l l o w marine s a n d waves o r t i d a l c u r r e n t ridges.
The most n o t a b l e amongst t h e s e i s t h e J u r a s s i c Navajo S a n d s t o n e , b u t t h e
r e i n t e r p r e t a t i o n o f Freeman a n d V i s h e r (1975) h a s been s t r o n g l y c r i t i c i z e d by many a u t h o r s ( F o l k , 1977; P i c a r d , 1977; R u z y l a , 1977; Steidmann, 1977).
The r e i n t e r p -
r e t a t i o n of t h e Permian Yellow Sands a s a t i d a l c u r r e n t r i d g e by P r y o r (1971b) i s a l s o open t o c r i t i c i s m .
P r y o r (1971a) l i k e n e d t h e W e i s s l i e g e n d e s of Germany t o t h e
t i d a l c u r r e n t r i d g e s d e s c r i b e d by Houbolt (1968) c l a i m i n g t h a t Houbolt (1968) showed .."cross-stratification i n thickness..".
i n t h e submarine sand r i d g e s o f t h e North Sea t o exceed 40m
I n f a c t H o u b o l t ' s (1968) c r o s s s e c t i o n s show c o n s i d e r a b l e v e r t i c a l
e x a g g e r a t i o n and t h e t r u e d i p on t h e " s t e e p " f a c e s i s o n l y 5 o r 6'
In
(Fig.2.24).
g e n e r a l , t h e t r u e deep o f " s t e e p " f a c e s on submarine sand waves and r i d g e s i s r a t h e r s h a l l o w (up t o 1 0 ' )
and does n o t b e a r c l o s e comparison w i t h t h e v e r y t h i c k , s t e e p l y
d i p p i n g cross-beds which t y p i f y a n c i e n t a e o l i a n s a n d s t o n e s . many a e o l i a n u n i t s has c e r t a i n l y been o u t of p l a c e .
The r e i n t e r p r e t a t i o n of
I t may b e t h a t we r a t h e r under-
e s t i m a t e t h e amount of a e o l i a n s a n d i n t h e s t r a t i g r a p h i c r e c o r d . A l l t h e a e o l i a n s a n d s which have been s u b j e c t t o s e r i o u s r e i n t e r p r e t a t i o n l a t e r a l l y i n t e r f i n g e r w i t h s h a l l o w marine d e p o s i t s and do n o t c o n t a i n abundant d e s e r t s e d i m e n t s of o t h e r f a c i e s .
A e o l i a n s a n d s t o n e s can b e c o n f i d e n t l y i d e n t i f i e d when t h e y o c c u r i n
i n t e r n a l b a s i n s a s s o c i a t e d w i t h d e s e r t f l u v i a l and i n l a n d sabkha d e p o s i t s a n d , of c o u r s e , show t h e t y p i c a l t e x t u r a l p r o p e r t i e s and c r o s s - b e d d i n g ,
An e x c e l l e n t example
i s a f f o r d e d by t h e Lower Permian of NW Europe. The Lower Permian of NW Europe The Lower Permian o f Northwestern Europe i n c l u d i n g t h e North Sea a r e a i s one of t h e best-known a n c i e n t exantples of t h e d e s e r t - e v a p o r
