Continental Red Beds (Developments in Sedimentology)

DEVELOPMENTS IN SEDIMENTOLOGY 29 CONTINENTAL RED BEDS FURTHER TITLES IN THIS SERIES VOLUMES 1. 2.3, 5, 8 and 9 are o...

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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

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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 i t e r e d bed a s s o c i a t i o n ,

It i s

g e n e r a l l y known a s t h e " R o t l i e g e n d e s " ( t h e " r e d beds" which u n d e r l i e t h e Upper Permian Z e c h s t e i n s e q u e n c e ) and r e p r e s e n t s a c o n t i n e n t a l c l a s t i c sequence d e p o s i t e d under semi-desert conditions.

R o t l i e g e n d e s f a c i e s have been r e c o g n i z e d i n England,

S c o t l a n d ( S h e r l o c k , 1926, 1928, 1 9 4 7 ) , s o u t h e r n Norway ( H o l t e d a h l , 1934; Dons and G y W , 19671, t h e Rhine Graben i n F r a n c e and w e s t e r n Germany ( L a p p a r e n t , 1906; H a r r a s s o w i t z , 1 9 2 6 ) , T h u r i n g i a and Harz i n c e n t r a l and e a s t e r n Germany (Gagel, 1926; Born, 1926) and i n Poland and o t h e r p a r t s of F r a n c e , Germany, t h e P y r e n e e s , and R u s s i a ( S h e r l o c k , 1947).

North Sea e x p l o r a t i o n h a s shown t h a t t h e s e s c a t t e r e d

109

r25 I

B

215' sea level

35"

Well Bank

340°160~280"realevel

100"

Well Bank

4/ \ \ \

0

0

'2 km

1

235" sea level

55'

Smith Knoll

0

0

1

2km

Fig.2.24. A . P r o f i l e s o f s a n d waves i n t h e s o u t h e r n s i g h t o f t h e North Sea. V e r t i c a l s c a l e p o s i t i o n e d w i t h r e s p e c t t o s e a l e v e l = Om. Lengths o f s e c t i o n s a b o u t 3800m ( A ) , 2800m (B), 900m (C) and 1220m (D). C a l c u l a t e d d i p s o f " s t e e p f a c e s " a v e r a g e s 5 O ( a f t e r McCave, 1971). B. S p a r k e r p r o f i l e s o f t i d a l c u r r e n t r i d g e s i n t h e e a s t c o a s t o f England. Although t h e y look l i k e a s y m m e t r i c a l c r o s s bedded f e a t u r e s , compare t h e h o r i z o n t a l and v e r t i c a l s c a l e s t o s e e 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 . C a l c u l a t e d s t e e p s l o p e a v e r a g e 5-60 ( a f t e r H o u b o l t , 1968).

110 o u t c r o p s form p a r t of a t l e a s t two f a u l t bounded s e d i m e n t a r y b a s i n s which l a y a l o n g t h e n o r t h e r n margin o f t h e V a r i s c a n mountain c h a i n . WNW-ESE a r e s e p a r a t e d by t h e Mid-North Sea

-

These b a s i n s

which t r e n d

RingkBbing-Fyn High.

The p a l e e o -

geography and f a c i e s d i s t r i b u t i o n o f t h e R o t l i e g e n d e s , b a s e d on Z i e g l e r (1975, 1978)

i s shown i n Fig.2.25.

Fig.2.25.

R o t l i e g e n d e s palaeogeography of NW Europe ( , a f t e r P . A . Z i e g l e r , 1978).

The Lower R o t l i e g e n d e s were d e p o s i t e d on a d e s e r t s u r f a c e c a r v e d o u t of a blockf a u l t e d s t r u c t u r a l c o n f i g u r a t i o n s i m i l a r t o t h a t d e s c r i b e d by Kent (1974, 1975) f o r e a s t e r n England and t h e North Sea.

A t t h i s t i m e (Late Carboniferous

-

E a r l y Permian)

t h e r e was widespread v o l c a n i c a c t i v i t y , p a r t i c u l a r l y i n t h e Oslo Graben and t h e a r e a t o t h e e a s t o f t h e N e t h e r l a n d s , and a l s o i n t h e s m a l l e r i s o l a t e d b a s i n s s u c h a s s o u t h west England (Laming, 1965, 1966) and t h o s e i n t h e s o u t h of S c o t l a n d (Mykura, 1965; Wagner, 1966; B r o o k f i e l d , 1 9 7 8 ) .

I n the e a s t e r n Netherlands t h i s Stephanian t o Early

Permian v o l c a n i c i t y r e s u l t e d i n t h e a c c u m u l a t i o n of o v e r 1,000m of q u a r t z p o r p h y r i e s , and s p i l i t i c l a v a s and t u f f s and t h e v o l c a n i c s a r e i n t e r b e d d e d w i t h conglomerates and c o n g l o m e r a t i c s a n d s t o n e s which r e s u l t e d from penecontemporaneous e r o s i o n .

In

some a r e a s c o n t i n e n t a l d e p o s i t i o n was c o n t i n u o u s through l a t e Westphalian-StephanianE a r l y Permian b u t o t h e r a r e a s were s u b j e c t e d t o e r o s i o n and deep w e a t h e r i n g f a r i n t o t h e Permian.

I n some of t h e s e a r e a s , such a s t h e Midlands of B r i t a i n and t h e Midland

V a l l e y of S c o t l a n d t h e deep d e s e r t w e a t h e r i n g was r e s p o n s i b l e f o r t h e secondary r e d d e n i n g of g r e y W e s t p h a l i a n Coal Measures ( B a i l e y , 1926; Mykura, 1960a). I t was

111 on t h i s d e s e r t s u r f a c e t h e n , c o n s i s t i n g m o s t l y o f W e s t p h a l i a n and S t e p h a n i a n s e d i m e n t s , i n p l a c e s w i t h contemporary v o l c a n i c s , t h a t t h e R o t l i e g e n d e s were d e p o s i t e d (Fig.2.26).

ES WLCANICS

Fluvial (Wadi) Conglomomtor, hndrtonor L Shalor

Fig.2.26. Conceptual c r o s s s e c t i o n s through t h e R o t l i e g e n d e s b a s i n i n s o u t h e r n North Sea and e a s t e r n N e t h e r l a n d s j u s t p r i o r t o t h e Z e c h s t e i n t r a n s g r e s s i o n . Cross s e c t i o n s a r e s o u t h of t h e Mid-North Sea High and Fyn G r i n s t e d High r e s p e c t i v e l y ( a f t e r G l e n n i e , 1972). The R o t l i e g e n d e s sediments a r e d i v e r s e and show many c h a r a c t e r i s t i c s analogous t o t h o s e of Recent d e s e r t s e d i m e n t s .

The f o l l o w i n g a c c o u n t of R o t l i e g e n d e s f a c i e s i s

based on S h o t t o n ( 1 9 5 6 ) , Laming ( 1 9 6 6 ) , G l e n n i e (1970, 19721, P i p e r ( 1 9 7 0 ) , Waugh ( 1 9 7 0 a , b ) , G l e n n i e e t a l . (1978), Marie ( 1 9 7 5 ) , and B r o o k f i e l d (1977, 1978). Marginal f a n g l o m e r a t e s and wadi d e p o s i t s Coarse g r a i n e d f l u v i a l c l a s t i c s i n c l u d i n g wadi-fan b r e c c i a s and f a n g l o m e r a t e s o c c u r t h r o u g h o u t , and a l o n g t h e s o u t h e r n margin of t h e V a r i s c a n mountains, and a l s o a s f l a n k d e p o s i t s around o t h e r h i g h l a n d a r e a s i n t h e Midlands o f England, t h e Lake D i s t r i c t , S o u t h Devon and t h e S c o t t i s h Basins ( F i g . 2 . 2 5 ) . d e s c r i b e d wadi d e p o s i t s from t h e s o u t h e r n North Sea.

G l e n n i e (1972) has

These c o n s i s t of a l t e r n a t i o n s

of subrounded-rounded q u a r t z i t e c o n g l o m e r a t e , brown-red s a n d s t o n e s , and dark r e d clay.

The r e d c l a y s have been c r a c k e d and c u r l e d by d e s i c c a t i o n and a l s o i n t r u d e d

112 by s a n d s t o n e dykes i n t h e same manner a s Recent wadi s e d i m e n t s . The d e s e r t f l u v i a t i l e s e d i m e n t s a r e s e e n t o b e s t e f f e c t i n some o f t h e o n s h o r e o u t c r o p s s u c h a s t h o s e i n SW England d e s c r i b e d by Laming ( 1 9 6 6 ) .

Here p o o r l y s o r t e d

w a t e r - l a i d b r e c c i a s were t r a n s p o r t e d t o t h e e a s t and n o r t h - e a s t a s i n d i c a t e d by c l a s t i m b r i c a t i o n and t h e i n c r e a s e i n roundness of l i m e s t o n e c l a s t s w h i l e i n t e r b e d d e d a e o l i a n sands show a palaeowind d i r e c t i o n t o t h e n o r t h - w e s t ( F i g . 2 . 2 7 ) .

The w a t e r -

l a i d d e p o s i t s show many o f t h e f e a t u r e s t y p i c a l of ephemeral d e s e r t a l l u v i u m They were p r o b a b l y d e p o s i t e d a s a l l u v i a l f a n s on which s h e e t f l o o d i n g

Fig.2.27. The Permian o u t c r o p of s o u t h Devon showing t h e t r a n s p o r t d i r e c t i o n s o f wadi d e p o s i t s and t h e palaeowind d i r e c t i o n ( a f t e r Laming, 1966):.

113 was of common o c c u r r e n c e .

The bedding s t y l e of t h e b r e c c i a s i s m a i n l y f l a t w i t h

trough c r o s s beds i n some down-fan a r e a s .

Other s i g n i f i c a n t sedimentary s t r u c t u r e s

i n c l u d e d e s i c c a t i o n c r a c k s , r a i n p r i n t s , and s a n d s t o n e dykes.

The w a t e r - l a i d

d e p o s i t s a r e f i r m l y cemented by c a l c i t e which was p r e c i p i t a t e d s h o r t l y a f t e r deposition a s i n other desert deposits.

In p l a c e s t h e w a t e r - l a i d a l l u v i u m i s 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 ,

These a r e

g e n e r a l l y w e l l - s o r t e d , l a m i n a t e d s a n d s which a r e r e l a t i v e l y p o o r l y cemented. The s u c c e e d i n g wadi s e d i m e n t s have s h a r p e r o s i o n a l b a s e s i n d i c a t i n g t h e e r o s i o n of wind blown sands which p r e v i o u s l y b l o c k e d wadi c h a n n e l s . A e o l i a n sands Wadi s e d i m e n t s may b e o v e r l a i n o r i n t e r b e d d e d w i t h a e o l i a n dune sands ( G l e n n i e , 1970, 1972).

I n onshore a r e a s t h e r e a r e a number of Permian s a n d s t o n e s p a r t s of

which can b e s a f e l y r e g a r d e d a s d e s e r t dune d e p o s i t s .

These i n c l u d e t h e B r i d g n o r t h

Sandstone ( S h o t t o n , 1937, 1954) and i t s e q u i v a l e n t s i n t h e Vale of Clwyd, t h e C o l l y h u r s t Sandstone ( C o l t e r and B a r r , 1975) some o f t h e s a n d s t o n e s i n t h e s o u t h Devonshire b a s i n (Laming, 1 9 6 6 ) , and t h e s a n d s t o n e s on t h e Permian b a s i n s of t h e South of S c o t l a n d i n c l u d i n g t h e Mauchline, T h o r n h i l l , Lochmaben and Dumfries and t h e C o r r i e Sandstone of Arran ( P i p e r , 1970; B r o o k f i e l d , 1977, 1978).

The Yellow Sands

o f NE England, f o r a long t i m e r e g a r d e d a s a e o l i a n (Hodge, 1 9 3 2 ) , were r e i n t e r p r e t e d a s s h a l l o w m a r i n e dunes by P r y o r (1971b) b u t a r e a l s o l i k e l y t o b e of a e o l i a n o r i g i n . These s a n d s t o n e s f u l f i l l t h e c r i t e r i a e s t a b l i s h e d by G l e n n i e (1970) f o r t h e r e c o g n i t i o n of a e o l i a n d e p o s i t i o n . The a e o l i a n dune s a n d s comprise i n d i v i d u a l u n i t s o f r e d s a n d s t o n e s e p a r a t e d by bounding s u r f a c e s ,

Each u n i t u s u a l l y s t a r t s a t t h e b a s e w i t h s u b h o r i z o n t a l , f i n e l y

l a m i n a t e d s a n d s t o n e s and p a s s e s upwards i n t o more s t e e p l y d i p p i n g f o r e s e t laminae ( s l i p f a c e s ) which may a p p r o a c h 30'

i n d i p , b u t n e v e r 34'

T h i s d i f f e r e n c e can b e a c c o u n t e d f o r by compaction.

a s s e e n i n modern dunes;

These major bounding s u r f a c e s

a r e i n t e r p r e t e d a s erosion surfaces associated with the migration of draas.

I n the

North Sea a e o l i a n dunes i n t h e R o t l i e g e n d e s a r e r e a d i l y r e c o g n i z a b l e by t h e i r welll a m i n a t e d c r o s s - b e d d i n g and p e t r o l o g i c a l c h a r a c t e r i s t i c s .

A s i n t h e onshore o u t c r o p s

t h e g r a i n s i z e of t h e s a n d s i s m o s t l y fine-medium and o c c a s i o n a l l y c o a r s e g r a i n e d n e a r t h e b a s e of i n d i v i d u a l u n i t s .

The f i n e r g r a i n s a r e g e n e r a l l y s u b a n g u l a r and t h e

c o a r s e r ones subround o r round w i t h a f r o s t e d a p p e a r a n c e .

Clay m a t r i x , a p a r t from a u t h i g e n i c c l a y , i s a b s e n t and mica g e n e r a l l y

uncommon. scarce.

Bimodal sands a r e n o t

Some of t h e 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 R o t l i e g e n d e s a e o l i a n s a n d s a r e shown

i n Fig.2.15. Another f e a t u r e worthy of n o t e a r e t h e f u l g u r i t e s which o c c u r i n t h e C o r r i e Sandstone of t h e I s l e o f Arran.

These a r e r o d s o f f u s e d s a n d w i t h a s t a r - s h a p e d

c r o s s s e c t i o n which a r e i n t e t p r e t e d a s p a l a e o - l i g h t e n i n g

f l a s h e s (Harland and Hacker,

114 1966; P i p e r , 1970). They p r o v i d e good e v i d e n c e o f t h e former e x i s t e n c e o f d e s e r t s torms

.

Continuous d i p m e t e r measurements o v e r a wide a r e a of t h e North Sea shows t h a t dune s l i p f a c e s d i p c o n s i s t e n t l y westwards ( G l e n n i e , 1972).

It i s not possible to

t e l l from c o r e s a l o n e , however, what dune t y p e s p r e v a i l e d i n t h e s e dune f i e l d s . T h i s a l s o a p p l i e s t o many of t h e o n s h o r e a r e a s , w h e r e two-dimensional o u t c r o p p r e c l u d e s t h e r e c o g n i t i o n o f dune t y p e s .

However, S h o t t o n (19561, b e l i e v e d t h a t t h e

v a s t m a j o r i t y of Permian dunes were of t h e b a r c h a n t y p e on t h e b a s i s of unimodal d i s t r i b u t i o n s of s l i p f a c e laminae and Waugh (1970b) c o n s i d e r e d t h e P e n r i t h Sandstone t o have a l s o been d e p o s i t e d a s b a r c h a n dunes.

G l e n n i e (1972) on t h e o t h e r

hand was of t h e o p i n i o n t h a t s e i f d u n e s , on t h e b a s i s o f bedding a t t i t u d e s i n b o t h o u t c r o p s and w e l l c o r e s , were p r o b a b l y more numerous t h a n b a r c h a n s on t h e h i g h e r ground n e a r e r t h e V a r i s c a n Mountains, whereas b a r c h a n s o r t r a n s v e r s e dunes were more abundant i n t h e i n t e r i o r . Permian p a l a e o m a g n e t i c d a t a ( B r i d e n e t a l . , 1970; Van d e r Voo and French, 1974) e n a b l e a r e a l i s t i c 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 t o be made d e s p i t e t h e f a c t t h a t when a n a l y s e d i n d e t a i l t h e r e a r e some d i s c r e p a n c i e s i n t h e r e s u l t s (Briden e t a l . , 1970; Turner and Vaughan, 1977).

The r e l i a b l e d a t a ( s e e Van d e r Voo and F r e n c h , 1974,

p.109) p l a c e s B r i t a i n a b o u t 10°N of t h e Permian p a l a e o e q u a t o r ( F i g . 2 . 2 8 ) and t h u s provides important corroborative evidence f o r the postulated o r i g i n s of t h e Rotliegendes.

Moreover, i f t h e palaeowind d i r e c t i o n s a r e superimposed on t h e p a l a e o -

l a t i t u d e r e c o n s t r u c t i o n t h e n i t can be s e e n t h a t t h e palaeowinds were p r o b a b l y t h e

NE t r a d e winds and t h e R o t l i e g e n d e s d e s e r t can b e c o n s i d e r e d a s a N o r t h e r n hemisphere " t r a d e wind d e s e r t " ( G l e n n i e , 1972). D e s e r t l a k e and sabkha d e o o s i t s I n t h e SE p a r t o f t h e s o u t h e r n R o t l i e g e n d e s b a s i n t h e a e o l i a n s a n d s a r e succeeded by t h e Ten Boer Member a s u c c e s s i o n of r e d mudstones w i t h minor amounts o f s i l t s t o n e and s a n d s t o n e w i t h n o d u l a r a n h y d r i t e .

The s a n d s t o n e s commonly show a f f i n i t i e s w i t h

b o t h aqueous and a e o l i a n t r a n s p o r t ; mudcracks and c u r l e d mudflakes a r e common i n d i cating subaerial desiccation.

Adhesion r i p p l e s a r e commonly a s s o c i a t e d w i t h n o d u l a r

a n h y d r i t e and t h e i n t e r p r e t a t i o n i s t h a t t h e s e d e p o s i t s r e p r e s e n t t h o s e of a n i n l a n d sabkha which was s u b j e c t e d t o a l t e r n a t e f l o o d i n g and d e s i c c a t i o n . The i n l a n d sabkha sequence g r a d e s l a t e r a l l y i n t o a c l a y - r i c h sequence c h a r a c t e r i z e d by t h e p r e s e n c e of h a l i t e .

A d e t a i l e d p a l a e o g e o g r a p h i c a l map showing t h e l a t e r a l

f a c i e s arrangement i s g i v e n i n F i g . 2 . 2 9 .

T h i s sequence i s well-known a s t h e

H a s e l g e b i r g e f a c i e s of t h e R o t l i e g e n d e s ( R i c h t e r - B e r n b u r g , 1955; Kent and Walmsley, 1970) and is t h e dominant component i n t h e c e n t r a l p a r t of t h e s o u t h e r n b a s i n where t h e R o t l i e g e n d e s may b e up t o 1500m t h i c k a s opposed t o 250m a t t h e b a s i n margin.

115

.

Fig.2.28. Permian p a l a e o l a t i t u d e s f o r NW Europe ( b a s e d on Van d e r Voo and French, 1974) w i t h superimposed palaeowind d i r e c t i o n s . The r e c o n s t r u c t i o n p e r m i t s i n t e r p r e t a t i o n t h a t R o t l i e g e n d e s dune s a n d s were d e p o s i t e d i n a n o r t h e r n hemisphere " t r a d e wind d e s e r t " . H a l i t e comprises a b o u t 30% o f t h e H a s e l g e b i r g e f a c i e s and a n h y d r i t e i s s c a r c e , o n l y o c c u r r i n g a s cement o r a n h y d r i t i c c l a y ,

By a n a l o g y w i t h Recent h o t d e s e r t s t h e

H a s e l g e b i r g e f a c i e s i s i n t e r p r e t e d a s t h e d e p o s i t s of a s a l i n e d e s e r t l a k e a l t h o u g h l a k e s of t h i s magnitude a r e n o t known a t t h e p r e s e n t b u t o c c u r r e d a t o t h e r times i n t h e p a s t ( G l e n n i e , 1972).

In o r d e r t o accumulate t h e t h i c k sequences o f Ten Boer sabkha f a c i e s and H a s e l g e b i r g e l a k e f a c i e s r e l a t i v e s u b s i d e n c e must have o c c u r r e d and k e p t pace w i t h sedimentation.

Water was p r o b a b l y s u p p l i e d t o t h e c e n t r e of t h e R o t l i e g e n d e s b a s i n

from t h e V a r i s c a n mountains i n t h e s o u t h v i a t h e m a r g i n a l a l l u v i a l f a n s and wadis. T h i s R o t l i e g e n d e s d e s e r t b a s i n was t e r m i n a t e d by t h e Z e c h s t e i n marine t r a n s g r e s s i o n which i s r e p r e s e n t e d by t h e K u p f e r s c h i e f e r and t h e l a t e r a l l y e q u i v a l e n t Marl S l a t e of NE England.

T h i s t r a n s g r e s s i o n may have been t h e c a t a s t r o p h i c f l o o d i n g

of a sub-sea l e v e l b a s i n ( T u r n e r e t a l . , 1978; Smith, 1979) b u t t h e r e a r e i n d i c a t i m s t h a t i t took p l a c e i n a number of s t a g e s .

The f i r s t e v i d e n c e of marine c o n d i t i o n s

i s found i n t h e B l n d e r s c h i e f e r (Plumhoff, 1966) which c o n s i s t s o f a t h i n band w i t h a r i c h m a r i n e fauna a b o u t 2.5m below t h e l p w e s t Z e c h s t e i n l i m e s t o n e .

Also, Glennie

(1972) r e p o r t s s a n d s t o n e s which a r e c o n s i d e r e d t o have been slumped and homogenized

116 by flowing w a t e r .

I n a number o f p l a c e s , two s u c h h o r i z o n s o c c u r , s e p a r a t e d by a

temporary development of dune sand c l e a r l y i n d i c a t i n g t h e e p i s o d i c n a t u r e of t h e transgression.

=Mudstones

and Halite

=Wadi

Sediments

B A d h e s i o n Ripples

Fig.2.29. Schematic R o t l i e g e n d e s f a c i e s d i s t r i b u t i o n o f f t h e e a s t c o a s t o f England. South of t h e lake-margin sabkha dune s a n d s form t h e dominant sediment; t h e y a r e i n t e r bedded w i t h a v a r i a b l e p r o p o r t i o n o f wadi s e d i m e n t s a l o n g a n a x i s t r e n d i n g N from t h e p r e s e n t N o r f o l k c o a s t , and w i t h sabkha s e d i m e n t s , i n c l u d i n g a d h e s i o n r i p p l e s , a l o n g a n E-W a x i s through t h e n o r t h e r n S o l e P i t a r e a . Superimposed on t h i s map a r e c o n t o u r s of t h e e s t i m a t e d d i f f e r e n c e between t h e p r e s e n t and maximum d e p t h s of b u r i a l of t h e o v e r l y i n g B u n t e r S h a l e ( s o l i d l i n e s ) and t h e amount o f p o r o s i t y d e s t r u c t i o n a t t r i b u t e d t o compaction (dashed l i n e s ) . The Dowsing f a u l t zone was p r o b a b l y a c t i v e from R o t l i e g e n d e s t i m e ( a f t e r G l e n n i e e t a l . , 1978). D i a n e n e s i s and r e d d e n i n n of t h e R o t l i e n e n d e s F o r t h e most p a r t t h e R o t l i e g e n d e s a r e u n i f o r m l y r e d i n c o l o u r .

They p a s s upwards

117 i n t o marine s a n d s t o n e s ( W e i s s l i e g e n d e s ) which were never reddened (Nemec and Porebski, 1977a,b).

The r e d c o l o u r a t i o n o f t h e R o t l i e g e n d e s s a n d s t o n e s has been

debated f o r many y e a r s .

Dunham (1953) was w e l l aware t h a t t h e c o l o u r was due t o

g r a i n c o a t i n g s and m a i n t a i n e d t h a t i t developed p r i o r t o cementation.

He r e f e r r e d

t o t h e pigmentary m a t e r i a l a s t u r g i t e and b e l i e v e d t h a t i t o r i g i n a t e d i n d e s e r t m a r g i n a l a r e a s by l a t e r i t i c w e a t h e r i n g under a s u b t r o p i c a l , humid c l i m a t e .

Dunham

s u g g e s t e d t h a t t h e t u r g i t e was t r a n s p o r t e d t o s h a l l o w lagoons where i t c o a t e d sand g r a i n s d u r i n g s e d i m e n t a t i o n ; a mechanism of reddening i n e f f e c t i d e n t i c a l t o t h e Primary r e d bed model of Krynine (1949).

Shotton (1956), w h i l s t accepting t h a t

s u c h a mechanism might a c c o u n t f o r t h e r e d c o l o u r o f some o f t h e f l u v i a l r e d beds i n t h e New Red S a n d s t o n e , c l e a r l y thought i t a n u n s u i t a b l e e x p l a n a t i o n f o r t h e a e o l i a n s a n d s t o n e s s u c h a s t h e P e n r i t h Sandstone and t h e B r i d g n o r t h Sandstone because o f t h e absence o f w a t e r i n t h e a e o l i a n environment. S h o t t o n (1956, p.454) on t h e o t h e r hand q u i t e c l e a r l y s u g g e s t e d t h a t i n t r a s t r a t a l s o l u t i o n o f i r o n - b e a r i n g m i n e r a l s was r e s p o n s i b l e f o r t h e r e d c o l o u r a t i o n of t h e a e o l i a n sandstones..."The

o n l y method I c a n s u g g e s t i s t h e c a p i l l a r y movement of

dew w a t e r and o c c a s i o n a l r a i n f a l l i n t h e top l a y e r s o f s a n d , c r e e p i n g a s f i l m s over t h e s u r f a c e s and c o a t i n g them w i t h t u r g i t e from i r o n - b e a r i n g c o n s t i t u e n t s w i t h i n t h e sand i t s e l f . ,

,'I

Subsequent r e s e a r c h , n o t a b l y by Waugh ( 1 9 7 0 a , b ) , G l e n n i e e t a l . (1978), K e s s l e r (1978) and Hancock (1978) h a s s u p p o r t e d t h e c o n t e n t i o n s of S h o t t o n a l t h o u g h t h e i r c o n c l u s i o n s a r e based mainly on t h e o v e r a l l d i a g e n e t i c p i c t u r e and p a r t i c u l a r l y t h e p r e s e n c e o f a u t h i g e n i c phases.

Diagenetic f e a t u r e s i n Rotliegendes sandstones

include:

1) g r a i n d i s s o l u t i o n and t h e replacement o f framework s i l i c a t e s by c l a y 2 ) mechanical i n f i l t r a t i o n of c l a y

3 ) f o r m a t i o n o f 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 4 ) f o r m a t i o n o f c a l c i t e and a n h y d r i t e cements Although t h e e f f e c t s o f b u r i a l d i a g e n e s i s a r e w e l l s e e n i n t h e R o t l i e g e n d e s t h e e a r l y d i a g e n e t i c f e a t u r e s a r e o f t e n extremely w e l l preserved.

There a r e many

e x c e l l e n t i l l u s t r a t i o n s o f t h e s e f e a t u r e s (e.g. Waugh, 1970a,b, 1978; G l e n n i e , e t a l . , 1978; Hancock, 1978; Kessler, 1978). Mechanical i n f i l t r a t i o n o f c l a v The r e d c o l o u r e d g r a i n c o a t i n g s i n some of t h e R o t l i e g e n d e s a e o l i a n s a n d s t o n e s can o f t e n be demonstrated t o have a g e o p e t a l f a b r i c ( F i g . 2 . 3 0 ) .

This f a b r i c u s u a l l y takes

t h e form o f a c o a t i n g on t h e p o s t - d e p o s i t i o n a l upper s u r f a c e of t h e d e t r i t a l g r a i n s and may t h i c k e n a t g r a i n c o n t a c t s o r even show meniscus b r i d g e s due t o s u r f a c e tension e f f e c t s .

When examined i n d e t a i l , u s i n g SEM, g r a i n c o a t i n g s o f t h i s type

a r e s e e n t o c o n s i s t o f l a y e r s o f c l a y f l a k e s o r i e n t a t e d p a r a l l e l t o t h e s u r f a c e of

118

Fig.2.30. Photomicrographs of r e d c l a y - i r o n o x i d e g r a i n c o a t i n g s i n R o t l i e g e n d e s a e o l i a n s a n d s t o n e s . A . Well-rounded s a n d g r a i n s showing c l a y - o x i d e g r a i n c o a t i n g s . Note t h e i r even t h i c k n e s s and a b s e n c e a t g r a i n c o n t a c t s ( C o r r i e S a n d s t o n e , A r r a n , S c o t l a n d ) . B. Clay-oxide c o a t i n g s of uneven t h i c k n e s s s u g g e s t i n g a geopedal o r i g i n ( S o l e P i t a r e a , North S e a ) . C . Clay-oxide i n f i l l i n g p i t s on a q u a r t z g r a i n s u g g e s t i n g o r i g i n a s a c l a y c u t a n ( P e n r i t h S a n d s t o n e , Cumbria). I). Deformed g r a i n of c l a y o x i d e , p r o b a b l y a c o m p l e t e l y pseudomorphed f e r r o m a g n e s i a n s i l i c a t e ( S o l e P i t a r e a , North S e a ) .

119

Fig.2.31. Photomicrographs of a u t h i g e n i c q u a r t z and f e l d s p a r i n R o t l i e g e n d e s a e o l i a n sandstones. A . Monocrystalline q u a r t z grains with o p t i c a l l y continuous overgrowths i n d i f f e r e n t e x t i n c t i o n p o s i t i o n s ( P e n r i t h S a n d s t o n e ) . B. Polyc r y s t a l l i n e q u a r t z g r a i n showing o p t i c a l c o n t i n u i t y between e a c h s u b c r y s t a l and i t s overgrowth ( P e n r i t h S a n d s t o n e ) . C. P l a g i o c l a s e w i t h o p t i c a l l y c o n t i n u o u s overgrowth ( S o l e P i t a r e a , N o r t h S e a ) .

120 t h e d e t r i t a l g r a i n ( s e e Kessler, 1978, P l a t e 2 ) . t h a t of a u t h i g e n i c c l a y .

T h i s f a b r i c i s q u i t e d i s t i n c t from

More commonly no g e o p e t a l f a b r i c i s r e c o g n i z a b l e , t h e r e d

c l a y o x i d e c o a t i n g i s o f uniform t h i c k n e s s a r o u n d t h e d e t r i t a l g r a i n s b u t a b s e n t a t g r a i n c o n t a c t s ( F i g . 2.30). Crone ( 1 9 7 5 ) , Walker (1976) and Walker e t a l . (1978) have d e m o n s t r a t e d t h a t g e o p e t a l c l a y f a b r i c s of t h i s t y p e form from t h e mechanical i n f i l t r a t i o n of c l a y by i n f l u e n t s e e p a g e of s u r f a c e r a i n o r f l o o d w a t e r s ,

I n t h e Cenozoic d e p o s i t s o f t h e

a r i d p a r t s o f N o r t h America t h i s i n f i l t r a t e d c l a y i s n o t r e d when d e p o s i t e d b u t reddens w i t h t i m e .

T h i s happens b e c a u s e o f t h e f o r m a t i o n of h a e m a t i t e by t h e

r e a c t i o n of oxygenated groundwater w i t h i r o n a d s o r b e d on c l a y s u r f a c e s and i n c l a y mineral l a t t i c e s .

There seems t o b e no r e a s o n why t h i s p r o c e s s i s n o t d i r e c t l y

a c c o u n t a b l e f o r t h e g e o p e t a l f a b r i c s observed i n t h e R o t l i e g e n d e s a e o l i a n r e d beds; i t is a l s o p r o b a b l e t h a t t h e non-geopetal

a s i m i l a r manner.

c l a y - o x i d e g r a i n c o a t i n g s were formed i n

There a r e no i n d i c a t i o n s t h a t t h e c l a y - o x i d e c o a t i n g s were formed

i n a more p l u v i a l c l i m a t e and l a t e r r e d e p o s i t e d i n a e o l i a n dunes ( c f . F o l k , 1976). Indeed many of t h e c o a r s e , well-rounded a e o l i a n g r a i n have s i n g l e c l a y o x i d e c o a t i n g s which c o u l d o n l y have formed p o s t - d e p o s i t i o n a l l 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 A u t h i g e n i c o v e r g r o w t h s , p a r t i c u l a r l y of q u a r t z o r f e l d s p a r , a r e a common f e a t u r e of t h e Permian R o t l i e g e n d e s .

Q u a r t z overgrowths a r e v o l u m e t r i c a l l y more i m p o r t a n t

t h a n f e l d s p a r and have been d e s c r i b e d i n d e t a i l from t h e P e n r i t h Sandstone by Waugh (1970a,b).

The P e n r i t h Sandstone i s s i l i c i f i e d by q u a r t z overgrowths i n t h e

n o r t h e r n p a r t o f i t s o u t c r o p i n t h e Vale o f Eden b u t remains u n s i l i c i f i e d i n t h e s o u t h e r n p a r t where i t i s i n t e r b e d d e d w i t h c a l c i t e cemented f a n g l o m e r a t e s (brockram). (Waugh, 1970b, F i g . 4 ) . The overgrowths which t a k e t h e form of b i p y r a m i d a l q u a r t z c r y s t a l s o c c u r on a v a r i e t y o f q u a r t z t y p e s , i n c l u d i n g u n s t r a i n e d , s t r a i n e d , and polycrystalline grains.

The q u a r t z type i s i n s t r u m e n t a l i n c o n t r o l l i n g t h e t y p e of

overgrowth which i s produced.

Thus, s i n g l e c r y s t a l s t r a i n e d , and u n s t r a i n e d and

p o l y c r y s t a l l i n e overgrowths were r e c o g n i z e d by Waugh (1970a,b) and t h e s e a r e shown i n Fig.2.31. silcrete.

The s i l i c i f i e d P e n r i t h Sandstone can r e a d i l y b e i n t e r p r e t e d a s a d e s e r t

Other s i l i c i f i e d deposits e.g.

t h e Lower Permian p e t r i f i e d f o r e s t s o f

Germany have been i n t e r p r e t e d a s s i l c r e t e s (Schwarzbach, 1963, p . 6 5 ) .

Silcretes are

a common f e a t u r e o f modern d e s e r t s and have been d e s c r i b e d by Woolnough (19301, Sandford (1935) and A l l e y (1977).

The s i l i c e o u s cement i n s i l c r e t e s is u s u a l l y

r e p o r t e d a s o p a l o r chalcedony b u t Williamson (1957) h a s r e c o r d e d rounded a e o l i a n grains with o p t i c a l l y continuous quartz r i m s . Waugh (1970b) a t t r i b u t e d t h e s o u r c e o f t h e s e c o n d a r y s i l i c a i n t h e P e n r i t h Sandstone t o s i l i c e o u s d u s t produced by wind a b r a s i o n .

This i s consistent with the

g r e a t i m p o r t a n c e of a i r b o r n e d u s t i n t h e f o r m a t i o n of d e s e r t s o i l s (Yaalon, 1973).

121 Waugh went on t o s u g g e s t t h a t t h i s d u s t w i t h a h i g h l y s o l u b l e s u r f a c e l a y e r was d i s s o l v e d by a l k a l i n e groundwater and d e s e r t dews which i n t u r n were drawn t o t h e s u r f a c e of dunes by e v a p o r a t i o n .

On e v a p o r a t i o n o p t i c a l l y c o n t i n u o u s q u a r t z

overgrowths were p r e c i p i t a t e d a r o u n d i n d i v i d u a l g r a i n s .

The f o r m a t i o n o f q u a r t z

overgrowths i n t h i s way a c c o u n t s f o r t h e f a c t t h a t t h e c o r e s of P e n r i t h Sandstone barchans a r e f r e q u e n t l y l e s s s i l i c i f i e d and a l s o ( b e c a u s e o f d i f f e r e n c e s i n groundw a t e r c o m p o s i t i o n ) f o r t h e o c c u r r e n c e of u n s i l i c i f i e d P e n r i t h Sandstone a s s o c i a t e d with t h e marginal fanglomerates.

Here where c a r b o n a t e r o c k s were common i n t h e s o u r c e

a r e a t h e groundwaters were l i k e l y t o have been more a c i d . The d i s t r i b u t i o n o f a u t h i g e n i c q u a r t z i n t h e a e o l i a n s a n d s t o n e s of t h e s o u t h e r n N o r t h Sea i s a l s o i r r e g u l a r .

G l e n n i e e t a l . (1978) n o t e s t h a t a u t h i g e n i c q u a r t z i s

more abundant i n dune t o p s e t s than b o t t o m s e t s b u t a t t r i b u t e d i t t o b u r i a l d i a g e n e s i s . Although t h e amount of a u t h i g e n i c q u a r t z was t o o g r e a t t o be a c c o u n t e d f o r by p r e s s u r e s o l u t i o n a l o n e ( G l e n n i e e t a l . , 1978, p . 3 2 ) s u g g e s t e d t h a t ' f e l d s p a r l e a c h i n g and a l t e r a t i o n o f u n d e r l y i n g m o n t m o r i l l o n i t e and i l l i t e / m o n t m o r i l l o n i t e , r a t h e r t h a n a b r a s i o n d u s t , p r o v i d e d the s o u r c e o f e x t r a s i l i c a .

T h i s i s l i k e l y t o be the c a a e

f o r t h e l a t e d i a g e n e t i c q u a r t z which forms d u r i n g b u r i a l d i a g e n e s i s b u t i t c a n n o t a c c o u n t f o r t h e l a r g e volumes o f a u t h i g e n i c q u a r t z which undoubtedly form d u r i n g e a r l y d i a g e n e s i s , a s i n t h e c a s e of t h e P e n r i t h Sandstone. Authigenic f e l d s p a r (Fig.2.31) i s a minor, b u t c h a r a c t e r i s t i c c o n s t i t u e n t of Lower Permian s a n d s t o n e s .

Most commonly t h e a u t h i g e n i c f e l d s p a r o c c u r s a s over-

growths on d e t r i t a l g r a i n s b u t a l s o a s a p o r e - f i l l i n g cement.

K - f e l d s p a r overgrowths

a r e commonly s t o i c h i o m e t r i c K A l S i 0 w i t h -2V of 20-25' s u g g e s t i n g p o t a s s i u m i n t e r 3 8 m e d i a t e s a n i d i n e (Waugh, 1978). A u t h i g e n i c p l a g i o c l a s e s a l s o o c c u r b u t a r e much more scarce than the K-feldspar.

Hancock (1978) d e s c r i b e d a u t h i g e n i c overgrowths of

a l b i t e a n d a l b i t e cement c r y s t a l s from t h e R o t l i e g e n d e s o f West Germany.

The

r e q u i s i t e i o n s f o r t h e development o f a u t h i g e n i c f e l d s p a r a r e a l m o s t c e r t a i n l y p r o v i d e d by i n t r a s t r a t a l d i s s o l u t i o n of d e t r i t a l s i l i c a t e s ( i n c l u d i n g f e l d s p a r s ) and v o l c a n i c r o c k fragments.

The d i a g e n e t i c h i s t o r y o f f e l d s p a r i n t h e Lower

Permian s a n d s t o n e s i s p r o b a b l y , l i k e t h a t of q u a r t z , l o n g and complex, and a u t h i g e n i c f e l d s p a r was p r o b a b l y produced d u r i n g b u r i a l d i a g e n e s i s a s w e l l a s d u r i n g e a r l i e r diagenesis. The r e c o g n i t i o n o f d i s s o l u t i o n and a u t h i g e n e s i s i n f e l d s p a r g r a i n s i s o f c r u c i a l importance t o t h e r e d bed q u e s t i o n .

I t d e m o n s t r a t e s t h a t i n t r a s t r a t a l s o l u t i o n was

a n i m p o r t a n t p r o c e s s , and t h a t 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 must a l s o have suffered its effects. Permian s a n d s t o n e s .

There a r e v e r y few r e l i c t f e r r o m a g n e s i a n g r a i n s i n tk Lower Kessler (1978, P l a t e 2 , F i g . 6 ) i l l u s t r a t e s a p a r t i a l l y d i s s o l v e d

hornblende g r a i n b u t t h i s i s r a r e .

Following Walker (1967a) one m i g h t assume t h a t

t h e p a u c i t y of u n s t a b l e f e r r o m a g n e s i a n s i l i c a t e s i s due t o t h e i r s u b s e q u e n t d i s solution.

I n s u p p o r t of t h i s i s t h e f a i r l y common o c c u r r e n c e o f g r a i n s o f c l a y w i t h

a t h i c k e r o u t e r s h e l l of haematite (Fig.2.30)

which c o u l d b e c o m p l e t e l y r e p l a c e d by

122 ferromagnesian m i n e r a l s .

However, i n c o n t r a s t t o t h e f i r s t - c y c l e a r k o s e s d e s c r i b e d

by Walker they a r e u n l i k e l y t o have been v e r y abundant i n t h e Lower Permian which was d e r i v e d mainly from W e s t p h a l i a n / S t e p h a n i a n s e d i m e n t a r y r o c k s .

I n some a r e a s

e . g . Northern Germany v o l c a n i c d e t r i t u s i s i m p o r t a n t and t h e i n t r a s t r a t a l s o l u t i o n of such g r a i n s was p r o b a b l y a n i m p o r t a n t s o u r c e o f i r o n and o t h e r e l e m e n t s . The r e l a t i v e importance o f m e c h a n i c a l l y i n f i l t r a t e d c l a y and i n t r a s t r a t a l d i s s o l u t i o n of ferromagnesian g r a i n s o r v o l c a n i c c l a s t s i n t h e r e d d e n i n g of t h e Lower Permian s a n d s t o n e s i s n o t c l e a r .

Both p r o c e s s e s c e r t a i n l y o p e r a t e d and i t i s

l i k e l y t h a t i n combination t h e y c o u l d a c c o u n t f o r t h e o b s e r v e d c o l o u r a t i o n .

In

e i t h e r c a s e r e d n e s s was produced a t a n e a r l y s t a g e of d i a g e n e s i s . The most common a u t h i g e n i c c l a y m i n e r a l s i n t h e Lower Permian s a n d s t o n e s a r e i l l i t e and c h l o r i t e and i n some a r e a s t h e r e i s minor k a o l i n i t e .

The i l l i t e o c c u r s

i n a v a r i e t y o f forms which r e a d i l y e n a b l e i t s d i s t i n c t i o n from m e c h a n i c a l l y i n f i l trated illite.

Whereas t h e l a t t e r h a s a c l a s t i c t e x t u r e p a r a l l e l t o g r a i n s u r f a c e s

( s e e Kessler, 1978, P l a t e 2 , F i g . 3 ) t h e a u t h i g e n i c i l l i t e o c c u r s a s randomly o r r a d i a l l y o r i e n t a t e d f l a k e s which may s e p a r a t e outwards i n t o f i b r e s ( s e e G l e n n i e , e t a l . , 1978, P l a t e 2 , Fig.B).

Combined i l l i t e and c h l o r i t e i s u b i q u i t o u s i n a n c i e n t

r e d beds and i t i s r e a s o n a b l e t o assume ( c f . Walker, 1976; G l e n n i e e t a l . , 1978) t h a t much of i t was o r i g i n a l l y 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 produced by silicate alteration.

The t r a n s f o r m a t i o n o f i l l i t e - m o n t m o r i l l o n i t e t o i l l i t e and

t h e i n c r e a s e i n i l l i t e c r y s t a l l i n i t y w i t h i n c r e a s e i n t e m p e r a t u r e and d e p t h of b u r i a l g i v e s some i n d i c a t i o n o f t h e d e g r e e o f b u r i a l d i a g e n e s i s (Ktlbler, 1968).

The e f f e c t

o f a u t h i g e n i c c l a y m i n e r a l s on s a n d s t o n e p e r m e a b i l i t y i s d i s c u s s e d by S t a l d e r (1973)

-

A u t h i g e n i c i l l i t e i n Lower Permian s a n d s t o n e s i s f r e q u e n t l y i r o n - r i c h and a l t h o u g h t h e r e have been no d e t a i l e d s ' t u d i e s o f i t s t h i n s e c t i o n p e t r o g r a p h y i t i s l i k e l y t o have reddened d u r i n g d i a g e n e s i s by r e a c t i o n w i t h oxygenated g r o u n d w a t e r , C h l o r i t e , o c c u r r i n g a s t h i n hexagonal p l a t e s , formed a t t h e same t i m e a s i l l i t e . There i s no d i r e c t e v i d e n c e y e t r e p o r t e d which i n d i c a t e s t h a t t h e a l t e r a t i o n o f a u t h i g e n i c c h l o r i t e c o n t r i b u t e s d i r e c t l y t o r e d bed c o l o u r a t i o n .

Few o t h e r c l a y

m i n e r a l s sre r e p o r t e d from t h e R o t l i e g e n d e s , k a o l i n i t e b e i n g noteworthy.

This often

o c c u r s a s books o f pseudohexagonal c r y s t a l s which choke p o r e s p a c e s and i t i s normally i n t e r p r e t e d a s a n i n d i c a t i o n of a n a c i d phase of d i a g e n e s i s ( p e r h a p s r e s u l t i n g from m e t e o r i c w a t e r ) r e s u l t i n g from u p l i f t a f t e r deep b u r i a l ( M i l l o t , 1970; Hancock, 1978).

A u t h i g e n i c i l l i t e and k a o l i n i t e a r e i l l u s t r a t e d i n F i g . 2.32.

Carbonate and s u l p h a t e cements Two cementing a g e n t s a r e normally s e e n i n t h e R o t l i e g e n d e s s a n d s t o n e s : and a n h y d r i t e

(Fig.2.33).

calcite

C a l c i t e t y p i c a l l y o c c u r s a s a v e r y e a r l y cement i n wadi

s a n d s t o n e s and f a n g l o m e r a t e s .

This i s c o n s i s t e n t with the f e a t u r e s observed i n

modern d e p o s i t s formed under s i m i l a r c i r c u m s t a n c e s ( B l i s s e n b a c h , 1954; G l e n n i e , 1970;

123

Fig.2.32. SEM photographs of a u t h i g e n i c i l l i t e and k a o l i n i t e i n R o t l i e g e n d e s s a n d s t o n e s from t h e North Sea a r e a . A . I l l i t e , showing t y p i c a l boxwork t e x t u r e . B. A u t h i g e n i c q u a r t z c r y s t a l s forming p o r e s p a c e s plugged w i t h pseudohexagonal f l a k e s of k a o l i n i t e and some f i b r o u s i l l i t e .

124

Fig.2.33. C a l c i t e and a n h y d r i t e cement i n Rot'liegendes s a n d s t o n e s . A . C a l c i t e cement i n a wadi b r e c c i a . Growth zones a r e d e f i n e d by i r o n o x i d e i n c l u s i o n s (South Devon). B. A n h y d r i t e cement i n a dune s a n d s t o n e ( S o l e P i t a r e a , N o r t h S e a ) .

125 S t a l d e r , 1975). I n a e o l i a n s a n d s t o n e s c e m e n t a t i o n i s n e v e r p e r v a s i v e and many o f t h e dune sandstones a r e only loosely consolidated.

There a r e i m p o r t a n t d i f f e r e n c e s between t h e

Lower Permian s a n d s t o n e s o f B r i t a i n and t h e R o t l i e g e n d e s of t h e s o u t h e r n North Sea. The former show v e r y l i t t l e i n t h e way of c a r b o n a t e o r a n h y d r i t e cements and i t must b e presumed t h a t t h e cements were removed a t some s t a g e .

I n the Rotliegendes

gypsum i s s e e n a s a n e a r l y cement i n some dunes b u t i t i s u s u a l l y c o n v e r t e d t o a n h y d r i t e on b u r i a l ( G l e n n i e e t a l . ,

1978).

The a n h y d r i t e shows a p o i k i l i t i c o r

v o i d f i l l i n g h a b i t ( F i g . 2 . 3 3 ) and i s o f t e n a s s o c i a t e d w i t h d o l o m i t e which o f t e n shows similar textures.

Dolomite a l s o o c c u r s a s b l o c k y a n h e d r a l cement f i l l i n g p o r e s p a c e s

o r a s a g g r e g a t e s of f i n e e u h e d r a l c r y s t a l s .

As w i t h a n h y d r i t e t h e r e a r e i n d i c a t i o n s

t h a t d o l o m i t e may have been a n e a r l y p h a s e t o c r y s t a l l i z e i n t h e dune s a n d s t o n e s , b u t many a u t h o r s (e.g. G l e n n i e e t a l . , 1978) r e g a r d i t t o b e a p r o d u c t of b u r i a l diagen-

esis.

The f o r m a t i o n of d o l o m i t e by r e p l a c e m e n t i n t h i s manner i f of i n t e r e s t because

of t h e well-known a s s o c i a t i o n o f r e p l a c e m e n t d o l o m i t e s w i t h i r o n hydroxide o r h a e m a t i t e cements and zones ( K a t z , 1971; S t a l d e r , 1975).

Under c e r t a i n c i r c u m s t a n c e s

t h e l e a c h i n g o f i r o n from c a r b o n a t e s d u r i n g d o l o m i t i z a t i o n c o u l d p r o v i d e a n i m p o r t a n t s o u r c e of pigmentary h a e m a t i t e , CONCLUSIONS AND FURTHER READING

Desert r e d beds i n c l u d e two d i s t i n c t i v e t y p e s of r e d b e d s , d e s e r t a l l u v i u m and a e o l i a n sands.

The former p r o v i d e c l a s s i c examples o f r e d bed f o r m a t i o n by long-

c o n t i n u e d i n t r a s t r a t a l a l t e r a t i o n i n b o t h Cenozoic a l l u v i u m and a number o f a n c i e n t r e d beds.

The r e d d e n i n g h i s t o r y of a e o l i a n s a n d s is more c o m p l i c a t e d .

Red a e o l i a n

s a n d may 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 e d i m e n t s and t h e n b e s u b j e c t e d t o i n t r a s t r a t a l a l t e r a t i o n and f u r t h e r . r e d d e n i n g ,

The g e o l o g i c a l s e t t i n g s of some

a n c i e n t r e d b e d s , s u c h a s t h e R o t l i e g e n d e s , a r e c o n s i s t e n t w i t h t h i s view and more work i s needed t o e l u c i d a t e t h e r e l a t i v e i m p o r t a n c e of p r e - d e p o s i t i o n a l a n d p o s t d e p o s i t i o n a l reddening. Comprehensive d e s c r i p t i o n s of modern d e s e r t s e d i m e n t s a r e g i v e n by t h e f o l l o w i n g . They a l s o i n c l u d e some d i s c u s s i o n of t h e problems o f r e d c o l o u r a t i o n i n d e s e r t s o i l s and s a n d s . Cooke, R.U.

and Warren, A . , 1973. Geomorphology i n Deserts. B.T.

Batsford Ltd.,

London, 374 pp. G l e n n i e , K.W.,

1970. Desert Sedimentary Environments. E l s e v i e r , Amsterdam, 222 pp.

126

CHAPTER 3 DELTA PLAIN RED BEDS

INTRODUCTION D e l t a s a r e b o d i e s of c l a s t i c s e d i m e n t formed i n s u b a e r i a l and s h a l l o w w a t e r environments by t h e i n f l o w of r i v e r s and i n which a g r a d a t i o n i n t o o f f s h o r e , g e n e r a l l y f i n e r g r a i n e d f a c i e s can be t r a c e d ,

I n many d e l t a s t h e d e p o s i t i o n a l

i n f l u e n c e of r i v e r s may b e masked by waves and t i d e s and t h e d e p o s i t s of t h e s e may b e v e r y d i f f i c u l t t o r e c o g n i z 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 .

Other d e l t a s a r e

dominated by t h e d e p o s i t i o n a l i n f l u e n c e of t h e s u p p l y i n g r i v e r s and a r e c h a r a c t e r i z e d by a s u b a e r i a l a l l u v i a l p l a i n known a s t h e d e l t a p l a i n .

Most o f t h e a n c i e n t

d e l t a i c s u c c e s s i o n s which have been d e s c r i b e d a r e o f t h i s r i v e r - d o m i n a t e d t y p e and c o n t a i n s e d i m e n t a r y sequences which can be a t t r i b u t e d t o d e p o s i t i o n i n t h e d e l t a p l a i n environment. The n a t u r e of t h e d e l t a p l a i n environment i s dependent upon s e v e r a l i n t e r - r e l a t e d v a r i a b l e s of which c l i m a t e , w a t e r d i s c h a r g e ( r a t e and v a r i a b i l i t y ) , and sediment l o a d ( q u a n t i t y and g r a i n s i z e ) a r e p a r t i c u l a r l y i m p o r t a n t .

I n A r c t i c and a r i d

c l i m a t e s where p r e c i p i t a t i o n i s e r r a t i c and v e g e t a t i o n i s s p a r s e , b r a i d e d c h a n n e l p a t t e r n s w i t h c o a r s e - g r a i n e d bed l o a d s t e n d t o develop on t h e d e l t a p l a i n and sandy o r pebbly sequences a r e d e p o s i t e d .

I n humid, t r o p i c a l c l i m a t e s p r e c i p i t a t i o n i s

h i g h and v e g e t a t i o n abundant f a v o u r i n g t h e f o r m a t i o n of meandering c h a n n e l s w i t h a h i g h suspended sediment l o a d and mud-dominated sequences predominate. Red beds may form i n any of t h e s e c l i m a t i c s e t t i n g s b u t a r e p a r t i c u l a r l y charact e r i s t i c and best-known from d e l t a p l a i n d e p o s i t s of m o i s t t r o p i c a l c l i m a t e s ( w i t h or without seasonal a r i d i t y ) .

T y p i c a l l y t h e s e r e d beds a r e developed i n m o t t l e d

o r polychrome mudstones d e p o s i t e d o v e r wide a r e a s of t h e d e l t a p l a i n . A s i n d e s e r t and a l l u v i a l r e d beds t h e r e a r e no good modern a n a l o g u e s of d e l t a p l a i n r e d b e d s , 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 a r e o f d i a g e n e t i c o r i g i n . The p r o c e s s e s l e a d i n g t o r e d d e n i n g o f muddy d e l t a p l a i n s e d i m e n t s a r e c l o s e l y dependent on ground w a t e r c o n d i t i o n s a n d , i n many c a s e s , a r e a s s o c i a t e d w i t h pedogenesis.

These p r o c e s s e s c o n t r a s t markedly w i t h r e d d e n i n g p r o c e s s e s i n f i r s t - c y c l e

arkosic alluvium.

They i n c l u d e t h e r e l e a s e o f i r o n 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 o f f i n e l y d i s s e m i n a t e d p y r i t e a s w e l l a s t h e i n t r a s t r a t a l a 1 t e r a t i o n of i r o n - s i l i c a t e s . I n t h i s c h a p t e r t h e s e d i m e n t s o f modern d e l t a s and t h e i r a n c i e n t a n a l o g u e s a r e described i n d e t a i l .

T h i s i s f o l l o w e d by a d e s c r i p t i o n of t h e o c c u r r e n c e of r e d

127 beds i n a n c i e n t d e l t a p l a i n s e d i m e n t s and a d i s c u s s i o n o f t h e i r o r i g i n .

Special

r e f e r e n c e i s made t o t h o s e r e d beds which formed i n m o i s t t r o p i c a l c l i m a t e s and a r e i n t e r - d i g i t a t e d with coal-bearing s t r a t a i n t h e Carboniferous

System o f n o r t h e r n ,

Europe. DELTA MODELS Moore and A s q u i t h (1971) d e f i n e a d e l t a a s :

"The s u b a e r i a l and submerged c o n t i -

guous sediment mass d e p o s i t e d i n a body of w a t e r (ocean o r l a k e ) p r i m a r i l y by t h e a c t i o n of a r i v e r " .

D e l t a s formed a t ocean margins a r e c o n s i d e r a b l y more i m p o r t a n t

t h a n t h o s e formed i n l a k e s and a r e t h e s o l e s u b j e c t o f t h i s c h a p t e r .

Present-day

d e l t a s a r e r e l a t i v e l y young b e c a u s e t h e y have o n l y formed s i n c e t h e p o s t - P l e i s t o c e n e r i s e i n s e a l e v e l ; f o r t h e most p a r t , however, t h e y a r e v e r y l a r g e and t h e i r s u b a e r i a l e x t e n t may b e i n e x c e s s o f 100,000km

2

(Smith, 1966).

The d i s t r i b u t i o n , o r i e n t a t i o n and i n t e r n a l geometry of d e l t a i c d e p o s i t s i s c o n t r o l l e d by 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 c l i m a t e , w a t e r d i s c h a r g e , sediment l o a d , river-mouth p r o c e s s e s , waves, t i d e s , c u r r e n t s , w i n d s , s h e l f s l o p e and t h e t e c t o n i c s and geometry o f t h e r e c e i v i n g b a s i n (Wright e t a l . , 1973).

The r e s u l t i s t h a t d e l t a s

show c o n s i d e r a b l e v a r i a t i o n and a n u n d e r s t a n d i n g o f t h e c o n t r o l l i n g p r o c e s s e s i s n e c e s s a r y b e f o r e any m e a n i n g f u l c l a s s i f i c a t i o n can b e made. C l i m a t e , w a t e r d i s c h a r g e , and sediment l o a d a r e c l o s e l y i n t e r - r e l a t e d .

I n moist

t r o p i c a l r e g i o n s p r e c i p i t a t i o n i s g e n e r a l l y h i g h r e l a t i v e t o e v a p o t r a n s p i r a t i o n and r u n o f f t e n d s t o b e h i g h and s t e a d y .

The i m p o r t a n c e of chemical w e a t h e r i n g i n such

regions leads t o high dissolved-load sediment concentrations. r i v e l y s t a b l e meandering c h a n n e l p a t t e r n s predominate.

Consequently, r e l a -

I n Arctic o r a r i d conditions

where p r e c i p i t a t i o n i s spasmodic and v e g e t a t i o n s p a r s e , b r a i d e d c h a n n e l p a t t e r n s w i t h l a r g e bed l o a d s t e n d t o predominate (Coleman a n d W r i g h t , 1975; M i a l l , 1976). When a sediment-laden r i v e r e n t e r s a body o f s t a n d i n g w a t e r , depending on t h e d e n s i t y d i f f e r e n c e s between t h e r i v e r and t h e l a k e o r s e a i n t o which i t f l o w s , one of t h r e e t y p e s of flow d i s p e r s a l may o c c u r : A ) I n f l o w more dense and forms a p l a n a r j e t a l o n g t h e bottom.

commonly r e s u l t s .

A turbidity current

The d e p o s i t s which form from t h e s e bottom c u r r e n t s a r e u s u a l l y

c l a s s i f i e d a s submarine fans.

B) I n f l o w e q u a l l y dense o f t e n o c c u r s when rivers e n t e r l a k e s .

Under t h e s e c o n d i t i o n s

t h e sediment i s d i s p e r s e d r a d i a l l y and t h e competency i s r a p i d l y l o s t . a r c u a t e zone o f a c t i v e d e p o s i t i o n t h u s r e s u l t s

-

A narrow

the c l a s s i c a l Gilbertian delta.

C) I n f l o w less dense i s t h e c a s e which u s u a l l y p r e v a i l s i n marine d e l t a s because f r e s h w a t e r i s less dense t h a n s e a w a t e r ,

The t y p e of sediment d i s p e r s a l which t h e n

t a k e s p l a c e depends on t h e s t r e n g t h o f waves, t i d e s and l o n g s h o r e c u r r e n t s . The energy c o n d i t i o n s t h a t e x i s t i n t h e s e a a t t h e r i v e r mouth e x e r c i s e t h e main c o n t r o l on t h e d e p o s i t i o n a l environment and t h e morphology of t h e r e s u l t i n g d e l t a .

128 The most u s e f u l c l a s s i f i c a t i o n of d e l t a t y p e s i s one based on t h e r e l a t i v e s t r e n g t h s of f l u v i a l and marine p r o c e s s e s ( c f . F i s h e r e t a l . , Galloway, 1975).

1969;. Coleman and W r i g h t , 1975;

F i s h e r e t a l . (1969) d i s t i n g u i s h e d between h i g h - c o n s t r u c t i v e

d e l t a s dominated by f l u v i a l p r o c e s s e s , and h i g h - d e s t r u c t i v e d e l t a s dominated by b a s i n a l processes (Fig.3.1).

H i g h - c o n s t r u c t i v e d e l t a s i n c l u d e t h e l o b a t e and b i r d -

f o o t types w h i l s t t h e h i g h - d e s t r u c t i v e d e l t a s i n c l u d e t h e wave-dominated and t i d e dominated t y p e s .

Although t h i s c l a s s i f i c a t i o n i s u s e f u l b e c a u s e i t stresses t h e

importance o f v e r t i c a l f a c i e s sequence and t h e a r e a l d i s t r i b u t i o n o f f a c i e s i t has a number of d i s a d v a n t a g e s and h a s n o t been w i d e l y a d o p t e d .

I n the f i r s t place the

scheme p l a c e s t o o much emphasis on t h e two end-members o f what i s r e a l l y a c o n t i n uous spectrum o f d e l t a - t y p e s .

Secondly, t h e u s e of t h e term h i g h - d e s t r u c t i v e i s

m i s l e a d i n g because a l l d e l t a s , by d e f i n i t i o n a r e c o n s t r u c t i v e w h i l s t a c t i v e and t h e

term i s a l s o confused w i t h t h e s o - c a l l e d " d e s t r u c t i o n a l phase" of d e l t a s ( S c r u t o n , 1960) which t a k e s p l a c e a f t e r d e l t a abandonment. A more w i d e l y used c l a s s i f i c a t i o n i s t h a t proposed by Galloway (1975) which employs a t e r n a r y diagram t o d e p i c t t h e f i e l d s o f f l u v i a l - , wave-, and t i d e dominated d e l t a s (Fig.3.2). Fluvial-dominated

Fig.3.1. (1969).

High-constructive

d e l t a s occur when waves, t i d a l c u r r e n t s , and

and h i g h - d e s t r u c t i v e d e l t a s a s d e f i n e d by F i s h e r e t a l .

129 longshore c u r r e n t s a r e weak.

Rapid seaward p r o d u c t i o n t a k e s p l a c e and a v a r i e t y of

c h a r a c t e r i s t i c f l u v i a l l y - d o m i n a t e d d e p o s i t i o n a l environments d e v e l o p s .

A t t h e mouth

o f each d i s t r i b u t o r y c h a n n e l subaqueous l e v e e s may form a s t h e j e t of w a t e r e n t e r s the sea (Fig.3.3).

The main s e d i m e n t l o a d i s t h e n d e p o s i t e d i n a d i s t r i b u t o r y mouth

b a r which becomes f i n e r g r a i n e d seawards.

With c o n t i n u e d p r o g r a d a t i o n t h e r i v e r

g r a d i e n t becomes s h a l l o w e r and t h e flow becomes less competent.

During p e r i o d s of

h i g h d i s c h a r g e t h e s u b a e r i a l l e v e e s may b e b r e a c h e d upstream forming a c r e v a s s e through which t h e f l o w may b e d i v e r t e d towards t h e s e a .

This i s often a shorter

r o u t e t h a n that p r o v i d e d by t h e former d i s t r i b u t a r y c h a n n e l and i t may r e s u l t i n t h e f o r m a t i o n of c r e v a s s e - s p l a y d e p o s i t s i n i n t e r d i s t r i b u t a r y bays.

RIVER

bY

DOMINATED

DOMINATED

Rhono

WAVE ENERGY

TIDE ENERGY

Fig.3.2. A c l a s s i f i c a t i o n o f d e l t a t y p e s b a s e d on v a r i a t i o n s i n t r a n s p o r t a t i o n p a t t e r n s on t h e d e l t a ( a f t e r Calloway, 1975). River-dominated d e l t a s t e n d t o b e of two main s u b t y p e s ,

The f i r s t i s t h e b i r d -

f o o t d e l t a which h a s r e l a t i v e l y few d i s t r i b u t a r i e s , s h o e s t r i n g s a n d s , and d i s c r e t e mouth b a r d e p o s i t s .

T h i s t y p e of d e l t a t e n d s t o form when t h e r i v e r d i s c h a r g e i s

s t e a d y and has a h i g h s u s p e n s i o n l o a d ( s e e Fig.3.1D).

The second t y p e i s normally

l o b a t e i n o u t l i n e and t h e r e a r e a g r e a t e r number of d i s t r i b u t a r i e s e a c h of which t e n d s t o be more ephemeral.

I n t h i s t y p e t h e s e d i m e n t s tend t o b e c o a r s e r g r a i n e d

w i t h a g r e a t e r p r o p o r t i o n of bed l o a d and t h e mouth b a r d e p o s i t s merge l a t e r a l l y

130 i n t o s h e e t sands ( s e e Fig.3.1C)

Fig.3.3. Subenvironments a t a d i s t r i b u t a r y mouth i n a r i v e r - d o m i n a t e d d e l t a , South P a s s , M i s s i s s i p p i d e l t a ( a f t e r Coleman and G a g l i a n o , 1965).

I n wave-dominated d e l t a s mouth b a r d e p o s i t s a r e c o n t i n u a l l y reworked i n t o a s e r i e s of superimposed c o a s t a l b a r r i e r sands (Fig.3.4A).

These w i l l t e n d t o dominate t h e

v e r t i c a l s u c c e s s i o n of f a c i e s w i t h s a n d b o d i e s a l i g n e d g e n e r a l l y p a r a l l e l t o t h e c o a s t i n c o n t r a s t t o t h e sand b o d i e s o f r i v e r - d o m i n a t e d d e l t a s which w i l l be g e n e r a l l y perpendicular t o the coast.

In tide-dominated d e l t a s t h e t i d a l r a n g e i s so h i g h t h a t t i d a l c u r r e n t s i n d i s t r i b u t a r y c h a n n e l become t h e p r i n c i p a l s o u r c e of sediment d i s p e r s a l e n e r g y .

Around,

and seaward o f t h e d i s t r i b u t a r y mouth t h e sediment may b e reworked i n t o a series of p a r a l l e l , l i n e a r o r d i g i t a t e r i d g e s which a r e a l i g n e d p a r a l l e l t o t h e t i d a l c u r r e n t s .

131 DELTAIC FACIES ASSOCIATIONS C l a s s i c a l l y t h r e e s t r u c t u r a l elements can be recognized i n d e l t a s : - t o p s e t , fores e t and b o t t o m s e t .

The t o p s e t d e p o s i t s o f a d e l t a a r e t h e most i m p o r t a n t and

comprise a v a r i e t y o f s e d i m e n t s i n c l u d i n g marsh d e p o s i t s and d e l t a - f r o n t s i l t s and sands.

These a r e c u t by r i v e r and d i s t r i b u t a r y c h a n n e l s w i t h n a t u r a l l e v e e s and

crevasse splays.

Muddy s e d i m e n t s w i t h s h e l l l a y e r s a r e o f t e n d e p o s i t e d i n

i n t e r d i s t r i b u t a r y bays ( S c r u t o n , 1960).

T o p s e t d e p o s i t s can b e d i v i d e d i n t o

t h o s e d e p o s i t e d s u b a e r i a l l y ( d e l t a p l a i n f a c i e s ) and t h o s e d e p o s i t e d subaqueously (delta front faciesl. h

A

B

-

WAVE DOMINATED

c

b

TIM -DOMINATED

Fig.3.4. A . A wave-dominated d e l t a , t h e modern Rh8ne d e l t a d e l t a , t h e modern Gulf of Papua ( a f t e r F i s h e r , 1969).

B. A tide-dominated

The f o r e s e t d e p o s i t s a r e t h o s e o f t h e p r o d e l t a o r d e l t a f r o n t s l o p e .

These a r e

p r e d o m i n a n t l y s i l t y c l a y s , sometimes w i t h s a n d , which a r e formed o f f t h e main d e l t a distributaries.

F r e q u e n t l y i n c o r p o r a t e d i n t o t h e p r o d e l t a a r e g u l l i e s 2-8m deep

132 and up t o a s much a s lkm from e a c h o t h e r i n t h e M i s s i s s i p p i b u t r a t h e r s m a l l e r on t h e R h h e d e l t a (Van S t r a a t e n , 1959). Bottomset d e p o s i t s a r e o f f - s h o r e c l a y s which a r e under t h e i n f l u e n c e of a c t i v e deltas.

These b o t t o m s e t d e p o s i t s a r e up t o 1 0 m t h i c k i n t h e M i s s i s s i p p i d e l t a and

a r e u n d e r l a i n by m a r g i n a l d e p o s i t s of P l e i s t o c e n e a g e . I n f a c t t h i s t h r e e - f o l d d i v i s i o n i s somewhat a r b i t r a r y and o n l y r e a l l y a p p l i e s t o the classical "Gilbertian delta".

The two most a c t i v e d e p o s i t i o n a l zones of a d e l t a

a r e b o t h w i t h i n t h e t o p s e t a r e a and i t i s , t h e r e f o r e , more p r a c t i c a b l e t o make a two-fold d i v i s i o n i n t o t h e d e l t a f r o n t which i n c l u d e s t h e s h o r e l i n e and seawardd i p p i n g p r o f i l e e x t e n d i n g o f f s h o r e ; and t h e d e l t a p l a i n which i s t h e low-lying a r e a developed behind t h e d e l t a f r o n t . The d e l t a p l a i n D e l t a p l a i n s a r e low-lying a r e a s which a r e c u t by a c t i v e and abandoned d i s t r i b a t a r y channels.

These c h a n n e l s t r a n s f e r t h e t o t a l d i s c h a r g e o f t h e a l l u v i a l s y s t e m

s u p p l y i n g t h e d e l t a t o t h e d e l t a f r o n t environment.

The i n t e r - c h a n n e l a r e a s c o n t a i n

a v a r i e t y of b a y s , f l o o d p l a i n s , l a k e s , t i d a l f l a t s , m a r s h e s , swamps and s a l i n a s . I n m o i s t t r o p i c a l and s u b t r o p i c a l c l i m a t e s d e l t a p l a i n s a r e t h i c k l y v e g e t a t e d and mangrove swamps, f r e s h w a t e r l a k e s , swamps and marshes may b e common ( e . g . t h e N i g e r d e l t a ( A l l e n , 1 9 7 0 ~ )and t h e M i s s i s s i p p i d e l t a s (Coleman, 1966).

I n a r i d regions,

however, v e g e t a t i o n i s s p a r s e and d e l t a p l a i n s may b e c h a r a c t e r i z e d by c a l c r e t e s e . g . t h e Ebro d e l t a (Maldonado, 1975) o r s a l i n a s w i t h gypsum and h a l i t e e.g. N i l e d e l t a ( E i n s e l e and Werner, 1968).

the

Sometimes d e l t a s i n a r i d c l i m a t e s a r e

covered by e x t e n s i v e dune f i e l d s e s p e c i a l l y i n d e l t a s w i t h b a r r i e r - b e a c h s h o r e l i n e s where sand i s eroded from t h e beach r i d g e s e . g . SZo F r a n c i s c o d e l t a .

D e l t a s formed i n

tundra r e g i o n s a r e c h a r a c t e r i z e d by p i n g o s , p a t t e r n e d ground and o t h e r c r y o g e n i c features.

Tundra v e g e t a t i o n may accomulate i n thaw ponds e.g.

t h e Mackenzie and

C o l v i l l e d e l t a s (Mackay, 1963; Naidu and Mowatt, 1975). Most d e l t a p l a i n s a r e a f f e c t e d by e i t h e r f l u v i a l o r t i d a l p r o c e s s e s and i n some c a s e s t h e r e i s r a t h e r a d e l i c a t e b a l a n c e between t h e two.

Wave-influenced d e l t a

p l a i n s a r e r a r e b e c a u s e t h e s e d e v e l o p b a r r i e r - b e a c h s h o r e l i n e s which p r o t e c t t h e d e l t a p l a i n from wave a c t i v i t y .

D e l t a p l a i n s which a r e dominated by f l u v i a l

p r o c e s s e s may p a s s downstream i n t o a t i d e dominated d e l t a p l a i n ( e . g . delta).

t h e Niger

A l t e r n a t i v e l y t h e y may b e e n c l o s e d by beach r i d g e s a t t h e seaward end (e.g.

t h e Rhone d e l t a ) o r p a s s d i r e c t l y i n t o t h e d e l t a f r o n t ( e . g .

the Mississippi deltas).

River-dominated d e l t a p l a i n s Fluvial d i s t r i b u t a r y channels The dominant f e a t u r e s a r e t h e 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 which a r e c l o s e l y

133 s i m i l a r t o channels i n conventional a l l u v i a l s e t t i n g s . I n low l a t i t u d e s where suspended-load c h a n n e l s a r e common d i s t r i b u t a r y channel p a t t e r n s a r e c l o s e l y dependent on g r a d i e n t .

I n upstream r e a c h e s t h e channel p a t t e r n

tends t o be meandering b u t a s t h e g r a d i e n t d e c r e a s e s downstream t h e channel p a t t e r n o f t e n changes from meandering t o r e l a t i v e l y s t r a i g h t ( F i g . 3 . 5 ) .

I n high l a t i t u d e s

WIDTH /DEPTH

100

40

20

10

SINUOSITY

Fig.3.5. 60186

G e n e r a l i z e d diagram o f downstream changes i n channel c h a r a c t e r i s t i c s of suspended l o a d f l w i a l systems ( a f t e r Morton and Donaldson, 1978).

where ths p r o p o r t i o n o f bed l o a d i s g r e a t e r and t h e d i s c h a r g e more s p o r a d i c b r a i d e d channels t e n d t o develop.

One of t h e major d i f f e r e n c e s between 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 a l l u v i a l channels is i n t h e abundance o f channel s w i t c h i n g o r a v u l s i o n . F l w i a l d i s t r i b u t a r y channels are f r e q u e n t l y abandoned because s h o r t e r and s t e e p e r r o u t e s are produced a s t h e d e l t a p r o g r a d e s seawards.

The abandonment o f d i s t r i b u t a r y

c h a n n e l s e n a b l e s b a s i n a l p r o c e s s e s t o t a k e o v e r i n t h e 16wer r e a c h e s o f t h e former channels and i n t h e Rh6ne d e l t a beach sands a r e d e p o s i t e d i n abandoned channel mouths ( K r u i t , 1955; Van S t r a a t e n , 1960). The f a c i e s sequences o f d i s t r i b u t a r y c h a n n e l s a r e g e n e r a l l y s i m i l a r t o those of a l l u v i a l channels and t h e s e a r e d e s c r i b e d i n d e t a i l i n Chapter 4.

Fining-upwards

sequences b e g i n n i n g w i t h a channel l a g followed by t r o u g h c r o s s bedded sands and c r o s s l a m i n a t e d s i l t s and c u l m i n a t i n g w i t h s o i l h o r i z o n s a r e t y p i c a l (Oomkens, 1967, 1974).

The fining-upwards may r e s u l t from l a t e r a l m i g r a t i o n of t h e channel o r from

channel abandonment i n which c a s e i t might b e f i l l e d by suspended l o a d sediment from overbank f l o o d i n g o f a d j a c e n t c h a n n e l s ,

One of t h e major consequences of t h e r a p i d

134 s w i t c h i n g o f d i s t r i b u t a r y c h a n n e l s i s t h a t t h e y t e n d t o be s h o r t - l i v e d compared w i t h t h e i r upstream e q u i v a l e n t s . low w i d t h t o d e p t h r a t i o .

This tends t o . r e s u l t i n sand bodies with a r e l a t i v e l y I n t h e d e l t a s o f t h e Rhine and Rh6ne t h i s r a t i o d e c r e a s e s

from a b o u t 1000 f o r a l l u v i a l c h a n n e l s t o a b o u t 50 f o r d i s t r i b u t a r y c h a n n e l s n e a r t h e s h o r e l i n e (Oomkens, 1974). I n t e r d i s t r i b u t a r y environments The i n t e r d i s t r i b u t a r y a r e a s i n r i v e r - d o m i n a t e d d e l t a p l a i n s c o n t a i n a wide v a r i e t y of environments and s e d i m e n t a r y f a c i e s . and f r e s h w a t e r l a k e s . throughcrevasses.

These i n c l u d e i n t e r d i s t r i b u t a r y b a y s , swamps

Sediment i s s u p p l i e d t o t h e s e a r e a s by overbank f l o o d i n g o r

Overbank f l o o d s r e s u l t i n t h e s h e e t - f l o w of sediment-laden w a t e r s

o v e r l a r g e a r e a s w i t h t h e d e p o s i t i o n of f i n e g r a i n e d s u s p e n s i o n l o a d d e p o s i t s . These may o r i g i n a l l y b e f i n e l y - l a m i n a t e d b u t i n t e r n a l s t r u c t u r e s a r e soon d e s t r o y e d by r o o t p e n e t r a t i o n and burrowing.

Overbank f l o o d i n g a l s o r e s u l t s i n t h e build-up

of l e v e e s b e c a u s e t h e c o a r s e r sediment i s r a p i d l y d e p o s i t e d n e a r t h e c h a n n e l banks during flood.

Repeated f l o o d i n g r e s u l t s i n t h e a l t e r n a t i o n o f t h i n e r o s i v e l y - b a s e d

sands and muds.

The sands may b e p a r a l l e l o r c r o s s l a m i n a t e d and f i n e away from t h e

c h a n n e l margin.

The muds show e v i d e n c e o f emergence i n t h e form of r o o t l e t h o r i z o n s .

C r e v a s s i n g i s a n o t h e r i m p o r t a n t f l o o d - g e n e r a t e d p r o c e s s i n which f l o o d w a t e r s e s c a p e from t h e c h a n n e l t h r o u g h d i s t i n c t c r e v a s s e c h a n n e l s i n t h e l e v e e s .

T h i s may

r e s u l t i n t h e f o r m a t i o n of c r e v a s s e - s p l a y s which a r e a r e a l l y r e s t r i c t e d wedges of sediment which form a p r o n s a l o n g t h e lower f l a n k s o f t h e l e v e e s .

Crevasse-splay

sand b o d i e s c o m o n l y show s h a r p , e r o s i o n a l b a s e s and may b e l e n t i c u l a r i n shape. u s u a l l y show c r o s s - b e d d i n g which i n d i c a t e s u n i d i r e c t i o n a l flow c o n d i t i o n s .

They

The over-

l y i n g muddy s e d i m e n t s may show e v i d e n c e of emergence. I n t h e M i s s i s s i p p i d e l t a s h a l l o w c r e v a s s e c h a n n e l s w i t h a s s o c i a t e d minor mouth b a r s a r e a n i m p o r t a n t f e a t u r e of i n t e r d i s t r i b u t a r y bays (Coleman e t a l . , 1964; Coleman and G a g l i a n o , 1964) ( F i g . 3.6).

As t h e s e c r e v a s s e / c h a n n e l mouth b a r c o u p l e t s p r o g r a d e

i n t o t h e i n t e r d i s t r i b u t a r y bay a v e r t i c a l sequence i s produced i n which b i o t u r b a t e d bay muds p a s s upwards i n t o c r o s s - l a m i n a t e d s i l t s and s a n d s .

When t h e c r e v a s s e chan-

n e l s a r e c l o s e l y s p a c e d a l a t e r a l l y e x t e n s i v e f r o n t e x t e n d s i n t o t h e bay p r o d u c i n g t h e s u b d e l t a s d e s c r i b e d by G a g l i a n o e t a l . (1971) and G a g l i a n o and v a n Beek (1975) (Fig. 3.6). I n t h e open bays of t h e M i s s i s s i p p i d e l t a t h e upper p a r t s o f c r e v a s s e sands may b e reworked i n t o s a n d s p i t s ( F i s k e t a l . , 1954).

These produce coarsening-upwards u n i t s

of w e l l - s o r t e d s a n d s w i t h f l a t l a m i n a t i o n s and w a v e - r i p p l e s . Large i n t e r d i s t r i b u t a r y a r e a s o f f l u v i a l dominated d e l t a p l a i n s a r e made up of o r g a n i c c l a y s and p e a t d e p o s i t s ( F i s k , 1960).

These s e d i m e n t s g e n e r a l l y l a c k any

w e l l d e f i n e d bedding b e c a u s e t h e y a r e e x t e n s i v e l y burrowed by p l a n t r o o t s and by

s e d i m e n t d w e l l i n g organisms.

T y p i c a l d e p o s i t s a r e t h u s homogeneous m i x t u r e s o f c l a y

135

4B

'

A Dry Cypress Bayou complex B Grand Liard complex C West Bay complex

A. A minor mouth b a r - c r e v a s s e channel c o u p l e t i n a n i n t e r d i s t r i b u t a r y bay Fig.3.6. of the modern M i s s i s s i p p i d e l t a ( a f t e r Coleman e t a l . , 1964) B. S u b d e l t a s of t h e modern M i s s i s s i p p i d e l t a ( a f t e r Coleman and G a g l i a n o , 1964). and s i l t w i t h p l a n t remains.

I n a r i d climates t h e p l a n t remains may b e much reduced

and mud-cracks and a l g a l mats may be p r e s e n t .

Detailed s t u d i e s of d e l t a p l a i n

swamp d e p o s i t s has been made by Coleman e t a l . ( 1 9 6 4 ) , Coleman (1966), Kolb and Van Lopik (1966) and Ho and Coleman (1969).

The d e l t a i c p l a i n merges i n l a n d

w i t h t h e a l l u v i a l p l a i n o f t h e lower M i s s i s s i p p i , t h e main s t r e a m s h e r e a r e t h e M i s s i s s i p p i and t h e A t c h a f a l a y a which p a r t n o r t h o f Baton Rouge.

The Atchafalaya

flows i n t o A t c h a f a l a y a Bay n e a r t h e w e s t e r n margin of t h e d e l t a i c p l a i n . A d e t a i l e d s t u d y of t h e swamp d e p o s i t s i n t h e A t c h a f a l a y a Basin of t h e M i s s i s s i p p i D e l t a has been made by Coleman (1966) and Ho and Coleman (1969). recognized f o u r t y p e s of i n t e r d i s t r i b u t a r y swamp environments:

Coleman (1966) poorly drained

swamp, well d r a i n e d swamp, f r e s h w a t e r l a c u s t r i n e , l a c u s t r i n e d e l t a f i l l and channel fill.

136

I

I

,

I

Em l= ..#.I

I

I 1

I

F i g . 3 . 7 . Physical and b i o l o g i c a l p r o p e r t i e s of a sediment core (No.199) through swamp d e p o s i t s of the Atchafalaya Basin, Louisiana ( a f t e r Coleman, 1966).

137 Swamps The swamps a r e low, f l a t a r e a s which a r e p e r i o d i c a l l y c o v e r e d o r s a t u r a t e d w i t h w a t e r and s u p p o r t a c o v e r o f woody v e g e t a t i o n :

swamps c o v e r e x t e n s i v e a r e a s , over

90% of t h e t o t a l a r e a of t h e A t c h a f a l a y a Basin.

The a r e a s n e a r t o r i v e r channels

a r e g e n e r a l l y w e l l d r a i n e d b u t i n i s o l a t e d a r e a s away from t h e r i v e r c h a n n e l d r a i n a g e i s poor.

T h i s d i f f e r e n c e i n d r a i n a g e c h a r a c t e r i s t i c s means t h a t i n w e l l d r a i n e d

swamps sediments a r e d e p o s i t e d under a l t e r n a t i n g o x i d i z i n g and r e d u c i n g c o n d i t i o n s , whereas t h e s e d i m e n t s of p o o r l y d r a i n e d swamps a r e d e p o s i t e d w h o l l y under r e d u c i n g conditions.

The r e s u l t a n t m i n e r a l o g i c a l c h a r a c t e r i s t i c s e n a b l e t h e d i s t i n c t i o n of

t h e two environments i n sediment c o r e s (Coleman, 1966). P o o r l y d r a i n e d swamp d e p o s i t s a r e made up o f b l a c k , o r g a n i c - r i c h muds w i t h o c c a s i o n a l t h i n l y - l a m i n a t e d s i l t s i n t r o d u c e d by f l o o d w a t e r s .

The sediments a r e

e x t e n s i v e l y b i o t u r b a t e d and b e c a u s e of t h e r e d u c i n g c o n d i t i o n s which p r e v a i l p l a n t m a t e r i a l i s w e l l p r e s e r v e d w i t h t h i n woody p e a t beds randomly i n t e r c a l a t e d throughout t h e sequence.

There i s abundant p y r i t e ( 0 . 5 - 3 . 5 % ) which o c c u r s a s small c u b e s ,

i s o l a t e d g l o b u l a r masses and a s a replacement of s m a l l r o o t s .

V i v i a n i t e (Fe P 0 3 2 8

8.H 0) a l s o o c c u r s commonly (up t o 2%) a s s m a l l n o d u l e s i n c l o s e a s s o c i a t i o n w i t h 2

pyrite. Well d r a i n e d swamp d e p o s i t s a r e s i g n i f i c a n t l y d i f f e r e n t from t h o s e o f t h e p o o r l y d r a i n e d swamps:

t h e c o n t e n t of o r g a n i c m a t t e r i s lower and t h e d e p o s i t s c o n s i s t of

clays with isolated s i l t lenses.

The most s i g n i f i c a n t d i f f e r e n c e , however, l i e s i n

t h e abundance and t y p e of a u t h i g e n i c m i n e r a l s .

There i s much less p y r i t e and v i v i a -

n i t e i n t h e well d r a i n e d swamp d e p o s i t s and a g r e a t e r abundance of c a l c i t e and i r o n o x i d e nodules.

The c a l c i t e n o d u l e s a r e e x t r e m e l y abundant and o c c u r b o t h a s l a r g e

(2cm) p o o r l y cemented n o d u l e s , and s m a l l ( l e s s t h a n 5mm) w e l l cemented nodules. C o n c e n t r a t i o n s of c a l c i t e a l s o o c c u r a l o n g bedding p l a n e s and o t h e r laminae. , o x i d e nodules a r e a l s o abundant i n t h e w e l l d r a i n e d swamp d e p o s i t s . u s u a l l y 0.5-6m

Iron

These a r e

i n s i z e and p r o b a b l y c o n s i s t o f well cemented i r o n hydroxide.

They

a r e s c a t t e r e d t h r o u g h o u t t h e sequence w i t h t h e l a r g e r n o d u l e s c o n c e n t r a t e d a t p a r t i c u l a r horizons.

I r o n o x i d e s a l s o o c c u r a s r i m s around p l a n t r o o t l e t s .

The f o r m a t i o n o f nodules d u r i n g t h e b u r i a l o f swamp d e p o s i t s h a s been d i s c u s s e d by Ho and Coleman (1969).

I n a c o r e o f a p p r o x i m a t e l y 35m t h e y n o t e d t h a t nodules

o c c u r r e d throughout b u t t h o s e n e a r t h e bottom of t h e c o r e were l a r g e r and b e t t e r c o n s o l i d a t e d t h a n t h o s e n e a r t h e t o p . They a l s o n o t e d a g r e a t e r number of nodules when o r g a n i c m a t t e r o r s h e l l s were p r e s e n t . P y r i t e was s i m i l a r l y c o n c e n t r a t e d i n o r g a n i c - r i c h a r e a s , a l t h o u g h r o o t l e t s and wood p a r t i c l e s had s u f f e r e d less from p y r i t e r e p l a c e m e n t i n t h e upper p a r t of t h e c o r e t h a n i n t h e d e e p e r p a r t s o f t h e core. A l s o , w e l l - c r y s t a l l i z e d p y r i t e cubes o c c u r r e d i n t h e lower p a r t s o f t h e c o r e .

138 X-ray r a d i o g r a p h s d i s p l a y v e r y w e l l t h e d i f f e r e n c e between w e l l - c r y s t a l l i z e d n o d u l e s and i n c i p i e n t n o d u l e s .

The former have w e l l - d e f i n e d b o u n d a r i e s whereas t h e

l a t t e r have d i f f u s e b o u n d a r i e s which i n d i c a t e s t h e d i f f u s i o n of t h e a u t h i g e n i c m i n e r a l s i n t o t h e s u r r o u n d i n g sediment.

D i f f e r e n t i a l t h e r m a l and X-ray d i f f r a c t i o n

s t u d i e s by Ho and Coleman (1969) showed t h a t t h e n o d u l e s were composed of s i d e r i t e perhaps w i t h some i r o n o x i d e where t h e i n t e r s t i t i a l cementing m a t e r i a l s c o n s i s t mainly of c a l c i t e and i r o n o x i d e s o f unknown s p e c i f i c t y p e . Ho and Coleman (1969) a t t r i b u t e d t h e f o r m a t i o n of t h e n o d u l e s and t h e cementing m a t e r i a l s t o i n c r e a s e d s o l u b i l i z a t i o n o f m e t a l c a t i o n s under r e d u c i n g c o n d i t i o n s a n d / o r low pH.

The main s u p p l y of m e t a l s was from d e s o r p t i o n from d e t r i t a l c l a y s

a l t h o u g h some m e t a l s may a l s o have been i n c r e a s e d from o r g a n i c m a t e r i a l s .

The

p r e s e n c e of n i t r o g e n i n t h e nodules s u g g e s t e d t o Ho and Coleman (1969) t h a t n o d u l e growth was r e l a t e d t o a d i f f u s i o n mechanism which i n v o l v e d n i t r o g e n e o u s compounds. The u t i l i z a t i o n of o r g a n i c compounds by micro-organisms a s a n e n e r g y s o u r c e f o r o x i d a t i o n - r e d u c t i o n r e a c t i o n s i s v e r y w e l l known and nodule growth p r o b a b l y began when mi'crobiological p r o d u c t i o n of d i s s o l v e d c a r b o n a t e exceeded t h e d i f f u s i o n r a t e and l o c a l s u p e r s a t u r a t i o n r e s u l t e d . 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 F r e s h w a t e r l a c u s t r i n e d e p o s i t s form i n v e r y s h a l l o w (3-4m) b u t e x t e n s i v e l a k e s which have o n l y v e r y weak wave o r c u r r e n t a c t i v i t y . dark grey o r black clays with s i l t lenses.

The d e p o s i t s a r e p r i m a r i l y

Coleman (1966) showed by means o f X-ray

r a d i o g r a p h s t h a t t h e c l a y s were f i n e l y l a m i n a t e d and s u g g e s t e d t h a t t h i s was due t o t h e a l t e r n a t i o n of f l o c c u l a t e d and n o n - f l o c c u l a t e d c l a y l a y e r s which might have r e s u l t e d from r a p i d changes i n pH, c a t i o n , and sediment c o n c e n t r a t i o n .

The f l o c c -

u l a t e d l a y e r s show s y n e r e s i s c r a c k s ; b i o t u r b a t i o n i s a cornon f e a t u r e and l a m e l l i branchs i n growth p o s i t i o n a l s o o c c u r .

I n c o n t r a s t t o t h e w e l l - d r a i n e d swamp

environment p y r i t e and v i v i a n i t e a r e abundant and i r o n o x i d e n o d u l e s a r e a b s e n t . L a c u s t r i n e d e l t a f i l l sequences a r e produced when a s t r e a m e n t e r s t h e f r e s h w a t e r lake.

T h i s b r i n g s i n c o a r s e r s e d i m e n t s and a coarsening-upwards g r a d a t i o n a l sequence

of l a c u s t r i n e c l a y , s i l t y c l a y and s a n d s t o n e i s produced.

L e n t i c u l a r bedding and

r i p p l e - b e d d i n g a r e common i n t h e c o a r s e r b e d s , a l o n g w i t h l o a d s t r u c t u r e s and s o f t sediment deformation.

Fauna and b i o t u r b a t i o n i s n o t a s common a s i n some o f t h e

other facies. Tide-dominated d e l t a p l a i n s D e l t a p l a i n s became dominated by t i d a l c u r r e n t s i n a r e a s of moderate t o h i g h t i d a l range.

During t h e f l o o d s t a g e t i d a l w a t e r flows up t h e d i s t r i b u t a r y c h a n n e l s

and b r e a k s o v e r t h e c h a n n e l banks t o f l o o d t h e i n t e r d i s t r i b u t a r y a r e a .

The d e l t a

p l a i n may t h u s b e t e m p o r a r i l y i n u n d a t e d w i t h marine w a t e r s which a r e d r a i n e d o f f

139 during t h e ebbing t i d e , Tidal d i s t r i b u t a r y channels These d i f f e r from 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 i n having a funnel-shaped form w i t h a h i g h width-depth r a t i o .

I n t h e N i g e r d e l t a ( A l l e n , 1965b, 1 9 7 0 ~ )t h e r e a r e

more t h a n 20 t i d a l i n l e t s 9-15m i n d e p t h and c u t t i n g t h e b e a c h - b a r r i e r s h o r e l i n e . The c h a n n e l flows a r e dominated by dune bed forms b u t a t t h e c o n f l u e n c e o f d i s t r i b u t a r y c h a n n e l s a r e l a r g e a r e a s of m u d f l a t s and s a n d b a r s known a s " i n n e r d e l t a s " (Fig.3.8).

I n some o t h e r tide-dominated d e l t a s s u c h a s t h e Mekong and t h e Ganges-

Brahmaputra t h e dominant bedforms of t h e t i d a l c h a n n e l s a r e l i n e a r s a n d r i d g e s aligned p a r a l l e l with the channel trend (Fisher e t a l . , 1975) ( F i g . 3 . 9 ) .

1969; Coleman and W r i g h t ,

These r i d g e s may by up t o s e v e r a l k i l o m e t r e s l o n g , a few hundred

metres wide and between 10-20m h i g h , The i n t e r n a l s t r u c t u r e of t h e s e t i d a l c h a n n e l s u s u a l l y b e g i n s w i t h an i n t r a f o r m a t i o n a l l a g c o n t a i n i n g a f r a g m e n t a l marine fauna.

These p a s s upwards i n t o s m a l l

s c a l e t r o u g h c r o s s beds and t h e n i n t o f i n e r g r a i n e d c r o s s l a m i n a t e d beds sometimes w i t h f l a s e r o r l e n t i c u l a r bedding ( T e r w i n d t , 1971).

The g r a i n s i z e g r a d u a l l y

d e c r e a s e s upwards and t h e amount of b i o t u r b a t i o n a l s o i n c r e a s e s .

The channel f a c i e -

may p a s s upwards i n t o swamp d e p o s i t s o r c o a s t a l b a r r i e r sands (Oomkens, 1974) ( F i g . 3.10). Interdistributary areas The a r e a s between t i d a l d i s t r i b u t a r y c h a n n e l s i n c l u d e i n t e r t i d a l f l a t s and lagoons which a r e c r o s s e d by minor t i d a l c r e e k s .

form mangrove swamps a s i n t h e Niger d e l t a .

The i n t e r t i d a l f l a t s may be v e g e t a t e d and Here t h e swamps a r e d i s s e c t e d by numerous

small meandering c r e e k s s e r v e d by a d e n d r i t i c d r a i n a g e p a t t e r n ( A l l e n , 1965b).

In

more a r i d r e g i o n s such a s t h e Colorado R i v e r d e l t a (Thompson, 1968; Meckel, 1975) t h e i n t e r d i s t r i b u t a r y a r e a s t e n d t o b e d e s i c c a t e d m u d f l a t s and s a n d f l a t s on which l o c a l s a l t pans develop. The d e l t a f r o n t The d e l t a f r o n t i s t h a t a r e a i n which c h a n n e l i z e d f l u v i a l c u r r e n t s e n t e r t h e b a s i n w a t e r s and undergo flow e x p a n s i o n , d e c e l e r a t i o n and d i s p e r s i o n .

The sediment

c a r r i e d by t h e r i v e r w a t e r s come9 under t h e i n f l u e n c e o f a v a r i e t y of b a s i n a l proc e s s e s i n c l u d i n g waves, t i d e s , l o n g s h o r e d r i f t and o t h e r c u r r e n t s (Wright and Coleman, 1974).

The r e s u l t i s t h a t t h e d e l t a f r o n t i s t h e most a c t i v e d e p o s i t i o n a l

zone of t h e d e l t a (Coleman and G a g l i a n o , 1965).

The r e l a t i v e importance of f l u v i a l ,

wave and t i d a l p r o c e s s e s v a r i e s c o n s i d e r a b l y i n t h e d e l t a f r o n t w i t h t h e r e s u l t t h a t a number of d i f f e r e n t d e l t a t y p e s c a n b e r e c o g n i z e d ( F i g . 3 . 1 ) .

140

1

2

0 , 1 km 3

Fig.3.8. The Forcados t i d a l d i s t r i b u t a r y c h a n n e l o f t h e Niger d e l t a showing t h e p o s i t i o n i n g of i n n e r d e l t a s a t t h e c o n f l u e n c e of two c h a n n e l s ( a f t e r NEDECO, 1961).

Accretionary bar

Fig.3.9. The Mekong d e l t a showing l i n e a r s a n d r i d g e s i n t h e lower r e a c h e s o f t i d a l d i s t r i b u t a r y c h a n n e l s ( a f t e r F i s h e r e t a l . , 1969).

141 The M i s s i s s i p p i d e l t a f r o n t i s dominated by f l u v i a l o u t f l o w c h a r a c t e r i s t i c s . The d e p o s i t i o n of s e d i m e n t a t d i s t r i b u t a r y mouths forms a series of i s o l a t e d l u n a t e mouth b a r s which p r o t r u d e i n t o t h e Gulf o f Mexico (Fig.3.11).

These comprise a

bar-back a r e a which i n c l u d e s minor c h a n n e l s , subaqueous l e v e e s and 'mid-channel b a r s ' , a b a r - c r e s t l o c a t e d o f f s h o r e from t h e d i s t r i b u t a r y mouth, and a b a r - f r o n t (sometimes c a l l e d t h e ' d i s t a l b a r ' ) which s l o p e s o f f s h o r e t o t h e p r o d e l t a (Fig.3.11).

A Metres

Mangroveswamp

B

Tidalchannel

Metres

hannel base

marine

16

So11horizon

Fig.3.10. T i d a l c h a n n e l sequences from t h e N i g e r d e l t a A. A n i n s h o r e sequence t e r m i n a t i n g i n mangrove swamp d e p o s i t s B. A s h o r e l i n e sequence t e r m i n a t i n g i n c o a s t a l b a r r i e r s a n d s ( a f t e r Oomkens, 1974).

142 Mouth b a r s advance r a t h e r l i k e d u n e s , w i t h t h e bar-back and b a r - c r e s t b e i n g e r o d e d w h i l s t t h e bar-front progrades (Coleman e t a l . ,

1974).

During t h e extreme

river f l o o d of 1973 t h e b a r - c r e s t aggraded r a p i d l y w i t h up t o 3m of s e d i m e n t b e i n g

deposited.

As t h e f l o o d s u b s i d e d t h i s s e d i m e n t was reworked by r i v e r c u r r e n t s and

r e d e p o s i t e d on t h e b a r f r o n t .

Mouth b a r p r o g r a d a t i o n was t h e r e f o r e most r a p i d

immediately a f t e r peak f l o o d (Coleman e t a l . , 1974).

0

- 30 - 60 - 90

D i s t r i b u t a r y c h a n n e l s and d i s t r i b u t a r y mouth b a r s i n t h e modern Fig.3.11. M i s s i s s i p p i d e l t a ( a f t e r F i s k e t a l . , 1954). The sequences produced by mouth b a r p r o g r a d a t i o n a r e l a r g e s c a l e c o a r s e n i n g upwards u n i t s .

A t t h e b a s e p r o d e l t a muds p a s s upwards i n t o i n t e r b e d d e d muds, s i l t

and sand r e f l e c t i n g t h e i n t e r a c t i o n between sediment-laden w a t e r s from t h e d i s t r i b u t a r y o u t l e t s and wave p r o c e s s e s .

These p a s s upwards i n t o s a n d s w i t h c u r r e n t -

produced s t r u c t u r e s ( F i s k e t a l . , 1954; F i s k , 1961; Coleman and Wright, 1975) (Fig.3.12). I n t h e M i s s i s s i p p i d e l t a t h e p r o g r a d a t i o n o f i n d i v i d u a l d i s t r i b u t a r y mouth b a r s

is v i r t u a l l y u n c o n t e s t e d by b a s i n a l p r o c e s s e s .

This has r e s u l t e d i n t h e formation

of r a d i a t i n g ' b a r f i n g e r s a n d s ' which g i v e t h e M i s s i s s i p p i d e l t a i t s c h a r a c t e r i s t i c b i r d f o o t shape ( F i s k , 1961).

The b a r f i n g e r s a n d s a r e biconvex e l o n g a t e s a n d b o d i e s

up t o 30km l o n g , 5-8km wide, and w i t h a r e l a t i v e l y uniform t h i c k n e s s of 70m ( F i g . 3,13).

143

c - Thin soils and Peat beds

li c - .

-_,-

sequences represenling rhe repeated infilltng of tnrerdtsrriburarv

tlnrerbedded muds,

silts and sands

tSlump

sheet

tHomo

enous muds. laminayed or bioturbated

Composite i d e a l i z e d sequence produced by mouth b a r p r o g r a d a t i o n i n t h e Fig.3.12. M i s s i s s i p p i d e l t a ( a f t e r Coleman and W r i g h t , 1 9 7 5 ) . I n most o t h e r p r e s e n t - d a y d e l t a s f l u v i a l p r o c e s s e s a r e n o t so dominant a t t h e d e l t a f r o n t and i n t e r a c t i n g wave and t i d a l p r o c e s s e s i n p a r t i c u l a r a r e a dominant f e a t u r e of t h e Rhone d e l t a .

The d e l t a f r o n t c o n s i s t s of l a t e r a l l y e x t e n s i v e beach

r i d g e s i n f r o n t of which i s a r e l a t i v e l y s t e e p o f f s h o r e s l o p e o f up t o 2' 1955; van S t r a a t e n , 1960) (Fig.3.54).

(Kruit,

P r o g r a d a t i o n of t h e d e l t a f r o n t t a k e s p l a c e

by t h e a c c r e t i o n of beach r i d g e s and p r o g r a d a t i o n of t h e mouth b a r producing c o a r s e n i n g upwards sequences ( L a g a a i j and K o p s t e i n , 1964; Oomkens, 1967) ( F i g . 3.15). The sequences b e g i n w i t h b i o t u r b a t e d o f f s h o r e c l a y s and p a s s upwards i n t o f i n e l y l a m i n a t e d s i l t s and t h e n i n t o r i p p l e l a m i n a t e d s i l t s and sands and t h e n f i n a l l y i n t o w e l l - s o r t e d , h o r i z o n t a l l y bedded s a n d s of b e a c h - b a r r i e r a s p e c t .

The s c a r c i t y of

d i s t r i b u t a r y c h a n n e l sands and t h e wave-dominated n a t u r e o f t h e s h e e t sands e n a b l e t h e d i s t i n c t i o n from f l u v i a l dominated d e l t a f r o n t s . The N i g e r d e l t a i s a n example i n which b o t h wave and t i d a l p r o c e s s e s a r e i m p o r t a n t a t the delta front.

The e f f e c t s o f t i d a l p r o c e s s e s a r e c o n f i n e d t o d i s t r i b u t a r y

mouth b a r s w h i l s t waves o p e r a t e o v e r a l a r g e a r e a o f t h e d e l t a f r o n t .

The s h o r e l i n e

144

Cross section Natural Levee

Fig.3.13.

Sparse to abundant faun Marsh

I

I

Bar f i n g e r sands o f the M i s s i s s i p p i d e l t a ( a f t e r F i s k , 1961).

I

F i g . 3.14.

'..

Distributary channel

,

I

The morphology of the Rh8ne d e l t a ( a f t e r van Andel and Curray, 1960).

145 t h u s c o n s i s t s predominantly o f wave g e n e r a t e d beaches o r c h e n i e r s s e p a r a t e d by t i d e dominated d i s t r i b u t a r y c h a n n e l s .

D e l t a s o f t h i s t y p e do n o t form obvious p r o t r u s i o n s

i n t o t h e s e a and have f a c i e s b e l t s p a r a l l e l t o t h e c o a s t ( A l l e n , 1965b, 1970c; Oomkens, 1974) (Fig.3.16). A

6

41-

0 50100% Sand. mainly horizontally 0 .......... bedded. that shows an upward increase in grain size

0 Upward increase in clay content

..... . . . . . .*. . . .......

-

cross bedded 10

Base of

50100%

j -

= Q

$11

Distinct upward increase 20 in number and thickness of intercalated silt and sand beds between clays

I

+i .. .. .. .. ................

silt and sand

6 + 9

i

Clay with thin silt and sand beds

m

= Horizontally bedded

- Crossbadded

6 sumpsdbeds U Burrows 6 Shells, shell debris

0 Plants, plant debris

Fig.3.15. D e l t a f r o n t c o a r s e n i n g upward sequences from t h e Rhone d e l t a A . P a s s i n g g r a d u a l l y upwards i n t o a c o a s t a l b a r r i e r s a n d B. A t r u n c a t e d sequence a t 13m o v e r l a i n by a n e r o s i v e based d i s t r i b u t a r y c h a n n e l sequence ( a f t e r Oomkens, 1967). The d i s t r i b u t a r y c h a n n e l s of t h e Niger d e l t a a r e t i d a l c h a n n e l s and have a shallow, sandy mouth b a r a t t h e e n t r a n c e which p r o b a b l y forms a s a r e s u l t of t h e expansion of tidal currents. platform’.

The mouth b a r s and beach f a c e s l o p e g e n t l y seaward t o a ‘ d e l t a f r o n t

T h i s i s a t e r r a c e between 5-10m deep and up t o 20km wide.

Beyond i t t h e

d e l t a f r o n t s l o p e s down i n t o t h e p r o d e l t a which i s a low-energy environment o n l y m i l d l y a f f e c t e d by waves, t i d a l c u r r e n t s a n d t h e Guinea C u r r e n t . F a c i e s sequences from t h e N i g e r d e l t a f r o n t have been d e t e r m i n e d from numerous c o r e d b o r e h o l e s and have been d e s c r i b e d by Weber (1971) and Oomkens (1974).

146

Fig.3.16.

D e p o s i t i o n a l environments of t h e modern Niger d e l t a ( a f t e r A l l e n , 1 9 7 0 ~ ) .

The sequences ( F i g . 3 . 1 7 ) a r e between 10-30m t h i c k and b e g i n w i t h b i o t u r b a t e d muds which p a s s upwards i n t o i n t e r b e d d e d muds, s i l t s and s a n d s ,

These p a s s i n t o e i t h e r

w e l l - s o r t e d , p a r a l l e l - l a m i n a t e d beach f a c e s a n d s o r t i d a l c h a n n e l d e p o s i t s . DELTA GROWTH AND ABANDONMENT D e l t a s o f t e n have a complex h i s t o r y i n which two p h a s e s of development can b e recognized ( S c r u t o n , 1960).

The f i r s t o f t h e s e is t h e c o n s t r u c t i o n a l phase i n which

t h e d e l t a p r o g r a d e s and b u i l d s - u p a v e r t i c a l sequence of p r o d e l t a d e p o s i t s o v e r l a i n by d e l t a f r o n t s i l t s and s a n d s and f i n a l l y d e l t a marsh d e p o s i t s and p e a t beds.

The

second i s t h e d e s t r u c t i o n a l phase which t a k e s p l a c e a f t e r t h e abandonment of d e l t a growth because o f t h e a v u l s i o n of a l l u v i a l o r d i s t r i b u t a r y c h a n n e l s .

The topmost

beds of t h e d e l t a a r e t h e n a t t a c k e d by waves and c u r r e n t s and may b e c o m p l e t e l y reworked, Compaction may r e s u l t i n l o c a l marine t r a n s g r e s s i o n s .

147

Fig.3.17. D e l t a f r o n t sequences o f t h e N i g e r d e l t a . A. T e r m i n a t i n g i n a c o a s t a l b a r r i e r s a n d B. Terminated by a t i d a l c h a n n e l i n l e t ( a f t e r Oomkens, 1974). The c o n c e p t of two-phase development of d e l t a s i s somewhat i d e a l i z e d and o n l y r e a l l y applicable t o river-dominated d e l t a s l i k e the Mississippi.

Here seven

p a r t i a l l y o v e r l a p p i n g d e l t a l o b e s c a n b e r e c o g n i z e d (Kolb and van Lopik, 1 9 6 6 ) (Fig.3.18).

These l o b e s have been s u c c e s s i v e l y abandoned d u r i n g t h e l a s t 6,000

y e a r s o r so and have undergone v a r i o u s s t a g e s of d e s t r u c t i o n .

The e a r l y s t a g e s

i n c l u d e smoothing of t h e s h o r e l i n e w i t h t h e f o r m a t i o n of b a r r i e r i s l a n d s w h i l s t more abvanced s t a g e s a r e r e p r e s e n t e d by t h e d e p o s i t i o n o f f o s s i l i f e r o u s marine c l a y s on top of t h e former d e l t a p l a i n . ANCIENT DELTAIC SUCCESSIONS I n g e n e r a l a n c i e n t d e l t a i c s u c c e s s i o n s t e n d t o show t h e f o l l o w i n g c h a r a c t e r i s t i c s :

1) t h e y a r e f r e q u e n t l y t h i c k ( s e v e r a l hundreds t o thousands of m e t r e s ) 2 ) they c o n t a i n a c o n s i d e r a b l e volume of sand o r s i l t

3 ) c o a l beds a r e commonly p r e s e n t

4 ) t h e f a u n a l c o n t e n t of i n t e r b e d d e d u n i t s may i n d i c a t e m a r i n e , b r a c k i s h and f r e s h w a t e r d e p o s i t i o n a l environments.

148

Plaqueminer

S t . Bernard

Lafourche

Teche

Maringouin

Km

The Recent M i s s i s s i p p i d e l t a system showing t h e d i s t r i b u t i o n o f d e l t a Fig.3.18. complexes and l o b e s ( a f t e r F r a z i e r , 1967).

5 ) s e d i m e n t a r y s t r u c t u r e s i n d i c a t e s h a l l o w w a t e r d e p o s i t i o n by t r a c t i o n c u r r e n t s 6 ) they o f t e n g r a d e i n an o f f s h o r e d i r e c t i o n i n t o f i n e r g r a i n e d c l a s t i c d e p o s i t s . S p e c i f i c r e c o g n i t i o n of a n c i e n t r i v e r - , wave-, o r tide-dominated d e l t a s i s p o s s i b l e by a n a l o g y w i t h Recent d e l t a s and depends h e a v i l y on v e r t i c a l and l a t e r a l f a c i e s d i s t r i b u t i o n (Fig.3.19).

I n f a c t the v a s t majority of ancient d e l t a i c

sequences which have been d e s c r i b e d a r e o f t h e r i v e r - d o m i n a t e d t y p e ; t h i s may be p a r t l y due t o t h e f a c t t h a t a n c i e n t wave- and t i d e dominated d e l t a s a r e l i k e l y t o be d i f f i c u l t t o d i s t i n g u i s h from o t h e r s h a l l o w marine s e q u e n c e s . Ancient r i v e r - d o m i n a t e d d e l t a sequences a r e r e p r e s e n t e d by r e l a t i v e l y l a r g e - s c a l e coarsening-upwards c y c l e s which r e c o r d t h e p a s s a g e from o f f s h o r e muds i n t o s h o r e l i n e sands a s t h e d e l t a f r o n t p r o g r a d e s seawards.

When complete t h e sequences a r e

t r u n c a t e d by a 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 sequence w i t h a s s o c i a t e d d e l t a p l a i n marsh d e p o s i t s ( p e a t o r c o a l ) . Ancient d e l t a i c s u c c e s s i o n s g e n e r a l l y commence w i t h a t h i c k , u n i f o r m i n t e r v a l of mudstones o r s i l t s t o n e s d e p o s i t e d a t t h e b a s e of t h e d e l t a f r o n t and f u r t h e r o f f s h o r e .

149

.

.

interdistributary shell bank distributary channel fill

marsh 1 delta distributarychannel dolta marsh stacked beach ridges

proximal distributary mouth bar

shoreface

d.pdrits distal distributary mouth bar deposits

-

pro delta shale

- WAVE DOMINATED DELTA

'

pro-deltashale

delta marsh tidal channel

d.por*

Ecoal with roots

tidal flat tidal channel

lowangle crossbedding herringbone crossbedding trough, festoon crossbedding

offrhore tidal sand ridges

TIDE DOMINATED DELTA

ripple marks shells, shell fragments bioturbation,trace fossils limestone cyclic units,showing direction of coarsening

F i g . 3 . 1 9 . Idealized v e r t i c a l sequences of r i v e r - , t i d e - , and wave-dominated ancient d e l t a s . The thickness and r e l a t i v e abundance of the f a c i e s may very considerably but t h e i r stratigraphic sequence i s more or l e s s constant ( a f t e r Miall, 1976).

150 V a r i a t i o n s i n t h e s u p p l y of suspended s e d i m e n t a r e r e f l e c t e d i n s l i g h t g r a i n s i z e v a r i a t i o n s which on o c c a s s i o n s g i v e t h e f a c i e s a s t r i p e d o r banded a p p e a r a n c e ( F i g . 3.20).

P l a n t d e b r i s i s f r e q u e n t l y abundant b u t t h e r e i s o n l y a s p a r s e m a r i n e

fauna presumably b e c a u s e of t h e h i g h s e d i m e n t a t i o n r a t e .

Thin e r o s i v e - b a s e d beds

o f f i n e g r a i n e d s a n d s t o n e sometimes o c c u r i n t h i s mudstone f a c i e s ( e . g . de Raaf

e t a l . , 1965) and p r o b a b l y r e p r e s e n t t u r b i d i t y c u r r e n t s g e n e r a t e d n e a r t h e d i s t r i b u t a r y mouth d u r i n g f l o o d p e r i o d s . Above t h e s e mudstones t h e amount o f c o a r s e s i l t s t o n e and f i n e s a n d s t o n e beds become more numerous.

These show c r o s s - l a m i n a t i o n and r i p p l e s i n d i c a t i n g depos-

i t i o n i n a g i t a t e d water above t h e wave b a s e .

I n some c a s e s t h e r e a r e c o a r s e

e r o s i v e - b a s e d s a n d s t o n e s i n which p l a n e l a m i n a t i o n s g i v e way t o c r o s s - l a m i n a t i o n . These may i n d i c a t e t h e d e p o s i t i o n of f l o o d - g e n e r a t e d s e d i m e n t s a t t h e b a s e of t h e d e l t a f r o n t under waning c u r r e n t c o n d i t i o n s .

F u r t h e r upwards t h e s a n d s t o n e beds

become t h i c k e r and more numerous and dominate t h e s u c c e s s i o n .

These s a n d s r e p r e s e n t

t h e d e l t a f r o n t and a r e t h e d e p o s i t s o f d i s t r i b u t a r y mouth b a r s .

The geometry o f

t h e s e sand b o d i e s i s most commonly t h a t of a s a n d - s h e e t b u t a n c i e n t b a r - f i n g e r s a n d s a r e d e s c r i b e d ( C o l l i n s o n and Banks, 1975). The d e l t a i c c y c l e i s completed by f a c i e s r e p r e s e n t i n g t h e d e l t a p l a i n .

These

i n c l u d e s m a l l - s c a l e (4-10m) coarsening-upwards sequences which r e p r e s e n t t h e i n f i l l i n g of i n t e r d i s t r i b u t a r y bays by c r e v a s s e - s p l a y s , c r e v a s s e channel c o u p l e t s ( E l l i o t , 1974).

l e v e e c o n s t r u c t i o n o r mouth b a r -

Such c y c l e s may b e t e r m i n a t e d by p a l a e o -

s o l s o r c o a l seams.

Large s c a l e c h a n n e l f i l l sequences w i t h b a s a l l a g d e p o s i t s and cross-bedded s a n d s t o n e s which show a n o v e r a l l f i n i n g upwards t r e n d o f t e n t e r m i n a t e r i v e r - d o m i n a t e d d e l t a i c cycles.

They r e p r e s e n t d i s t r i b u t a r y c h a n n e l sequences and a s s u c h show

u n i d i r e c t i o n a l flow c h a r a c t e r i s t i c s o f t e n w i t h abundant e v i d e n c e of s t a g e f l u c t u a t ions.

Both h i g h and l o w - s i n u o s i t y c h a n n e l s a r e p r o b a b l y r e p r e s e n t e d i n a n c i e n t

d e l t a i c sequences.

A s y e t t h e r e a r e no d e t a i l e d d e s c r i p t i o n s o f wave o r tide-dominated d e l t a s 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 Van de Graaf (1972) and Hubert e t a l . (1972) have d e s c r i b e d bimodal ( t i d a l ) c u r r e n t p a t t e r n s from C r e t a c e o u s d e l t a p l a i n d e p o s i t s i n t h e Western I n t e r i o r , USA.

I n t h e same a r e a t h e Cody Shale-Parkman Sandstone d e l t a

shows d e l t a f r o n t f a c i e s which a r e b e s t i n t e r p r e t e d a s wave-produced b e a c h - b a r r i e r s a n d s t o n e (Hubert e t a l . , 1972; A s q u i t h , 1974). A number of examples of a n c i e n t d e l t a i c d e p o s i t s a r e g i v e n i n T a b l e 3.1.

RED BEDS I N ANCIENT DELTA PLAINS Introduction Red beds i n d e l t a p l a i n environments a r e t h e least-known and most p o o r l y unders t o o d o f t h e t h r e e major a s s o c i a t i o n s o f c o n t i n e n t a l r e d b e d s ,

I n general they

151

30

Flnr grained micaceous r~pplelaminaled sandsloner and sdtstones with impoverished ripples

Erosive.based unit of trough cross bedded sandstones with interbedded SlltStOneS

25

Wavy and cross-laminated siltstones. with frequent loading

20

Planar cross qlratified sandstones wilh unidireri ional Dalaeocurrenlr

B

5

E

15

Banded or striped silty mudstones with Planolites

/....... . . ...........,.. . . .. .....;.. ..... . . ........ .-... ......... ...... . .. . . . . . ... ,

ia

,

Massive sandstones locally cross sirallfed wlth lntemal erowon surfaces and OCCasicmal soft sediment deformation

E Fissile black mudstones with abundant Plandites and thin bands and nodules (

Basal lag deposit above an erosional Surlace mth a rehef 01

6 5 m Abundant ironstone pebbles and mudstone flakes

Underlying delta front progradatlon saquenc

Fig.3.20. R e p r e s e n t a t i v e s e c t i o n s of a n c i e n t r i v e r - d o m i n a t e d d e l t a s . A . A d e l t a f r o n t sequence from t h e Coal Measures o f S o u t h Wales B. A 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 sequence l y i n g on eroded d e l t a f r o n t d e p o s i t s from t h e Namurian o f South Wales ( b o t h a f t e r K e l l i n g and George, 1971).

form t h i n , l a t e r a l l y d i s c o n t i n u o u s u n i t s i n o t h e r w i s e d r a b s t r a t a .

Their association

w i t h d e l t a i c s e d i m e n t a t i o n may n o t b e obvious b u t can be deduced from t h e f a c t t h a t t h e y a r e i n t e r b e d d e d w i t h t y p i c a l d e l t a i c s e d i m e n t s and commonly cap c o a r s e n i n g upwards ( p r o g r a d a t i o n a l ) d e l t a i c p r o f i l e s .

S i n c e t h e d e l t a p l a i n i s dominated by

TABLE 3.1. Examples of a n c i e n t d e l t a i c complexes Forma t i o n

Age

Middle Vicksburg

Lower O l i g i o c e n e

Delta type

Reference

U.S.A.

Wave-dominated

Gregory (1966)

Upper Wilcox Gp.

Eocene

U.S.A.

Wave-domina t e d

F i s h e r (1969)

Lower Wilcox

Eocene

U.S.A.

River-domina t e d

F i s h e r and McGowen (1967)

Sentinel Butte

Palaeocene

U.S.A.

River-dominated

Cherven (1978)

D i f u n t a Gp.

Upper Cretaceous - Palaeocene

Mexico

River-domina t e d

McBride e t a l . (1975)

Cody S h a l e Parkman Sands tone

Upper Cretaceous

U.S.A.

Wave-domina t e d

Hubert e t a l . (1972)

C i s c o Group

Carboniferous

U.S.A.

River-domina t e d

Galloway and Brown (1973)

-

Canyon Group

Carboniferous

U.S.A.

River-domina t e d

Erxleben (1975)

Haslingden F l a g s

Carboniferous

U.K.

River-domina t e d

C o l l i n s o n and Banks (1975)

Grindslow S h a l e s - Kinderscont G r i t

Carboniferous

U.K.

River-dominated

Walker (1966) C o l l i n s o n (1969)

Yoredales

Carboniferous

U.K.

River-domina t e d

E l l i o t (1975)

f l u v i a l d e p o s i t i o n a l environments t h e r e s u l t a n t r e d beds may c l o s e l y resemble a l l u v i a l r e d beds.

They a r e s i m i l a r i n t h a t t h e y o c c u r i n fining-upwards u n i t s and

a r e b e s t - d e v e l o p e d i n t h e f i n e member ( f l o o d p l a i n ) p a r t of t h e u n i t .

Usually there

i s c o n s i d e r a b l e e v i d e n c e of emergence i n t h e form o f mudcracks, r o o t l e t h o r i z o n s

and pedogenic f e a t u r e s .

I n C a r b o n i f e r o u s r e d beds o f t h i s t y p e a c o n n e c t i o n w i t h

d e l t a i c s e d i m e n t a t i o n i s i m p l i e d by t h e o c c u r r e n c e o f abundant s e a t e a r t h s and occasional t h i n coals within t h e red beds,

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

a l l u v i a l r e d beds i n t h a t t h e y a r e f r e q u e n t l y c o n f i n e d t o mudstones which may show a wide range of c o l o u r m o t t l i n g .

E x t e n s i v e pedogenic m o d i f i c a t i o n s ( i n c l u d i n g

c a r b o n a t e n o d u l e s , r o o t l e t s a n d s e a t e a r t h s ) and t h e i n s i t u o x i d a t i o n of p l a n t d e b r i s and p r e v i o u s l y formed p y r i t e a r e a l s o commonly observed. Although d e l t a s may a c c u m u l a t e o v e r a wide range of c l i m a t i c s e t t i n g s , a n c i e n t d e l t a p l a i n r e d beds a p p e a r t o b e p a r t i c u l a r l y c h a r a c t e r i s t i c of m o i s t t r o p i c a l climates.

T h i s i s i n d i c a t e d by t h e o c c u r r e n c e of i n t e r d i g i t a t e d c o a l s and o t h e r

e v i d e n c e of abundant v e g e t a t i o n c o v e r .

T h i s c o n t r a s t s markedly w i t h many a n c i e n t

a l l u v i a l r e d beds which a p p a r e n t l y ' formed under more a r i d c o n d i t i o n s and show many s i m i l a r i t i e s t o t h e d e s e r t a l l u v i u m d e s c r i b e d by Walker e t a l . (1978).

Therefore,

a l t h o u g h i t has been a r g u e d t h a t t h e o r i g i n of r e d beds i n d e s e r t and m o i s t t r o p i c a l c l i m a t e s may be b r o a d l y s i m i l a r (Walker, 1 9 7 4 ) , i t seems l i k e l y t h a t t h e r e may be i n t r i n s i c d i f f e r e n c e s i n t h e r e d d e n i n g of m o i s t t r o p i c a l d e l t a p l a i n s and a r i d , d e s e r t alluvium.

These a r e c o n s i d e r e d h e r e w i t h s p e c i a l r e f e r e n c e t o t h e Coal

Measures o f hW Europe. 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 i f u n t a Group o f n o r t h e a s t e r n Mexico comprises a t h i c k (4km) sequence o f s e d i m e n t s d e r i v e d from v o l c a n i c r o c k s and d e p o s i t e d i n f l u v i a l , d e l t a i c , and s h e l f environments.

The f l u v i a l systems s u p p l i e d d e l t a s t h a t prograded e a s t w a r d i n t o t h e

a n c e s t r a l Gulf o f Mexico (Fig.3.21A).

I t p r o v i d e s a n e x c e l l e n t example of t h e

f o r m a t i o n of r e d beds i n d e l t a p l a i n sediments which were d e p o s s t e d i n a sub-humid t o s e m i - a r i d c l i m a t e having s t r o n g s e a s o n a l a r i d i t y . V a r i a t i o n s i n sediment i n p u t , s u b s i d e n c e r a t e s , and sediment compaction i n t h e D i f u n t a Group r e s u l t e d i n a complex i n t e r t o n g u i n g of d e l t a i c - p a r a l i c f a c i e s ( F i g . 3.21B).

The d e l t a i c f a c i e s of t h e D i f u n t a Group a r e d e s c r i b e d by McBride e t a l .

(1975) and t h e o r i g i n o f t h e r e d beds which a r e a c h a r a c t e r i s t i c f e a t u r e of t h e d e l t a p l a i n f a c i e s , i s d i s c u s s e d by McBride (1974). The C e r r o H u e r t a , Las Ima'genes, and E n c i n a s Formations a r e mainly r e d beds w i t h minor amounts of g r e e n , p u r p l e , brown, o l i v e and y e l l o w beds t h a t a r e i n t e r p r e t e d a s d e l t a p l a i n f a c i e s (Fig.3.21C). i n c o n t r a s t t o the grey formations,

These beds a r e of uniform l i t h o l o g i c a l c h a r a c t e r These i n c l u d e t h e Ca8on de T u l e , C e r r o Grande

and Rancho Neuvo Formations which v a r y w i d e l y i n l i t h o l o g i c a l c h a r a c t e r r e f l e c t i n g

154

.Sari Antonio

#Delta

and coastal plain

k&ENES C A h

DEL TULE

CERRO HUERTA CERRO DEL PUEBLO

PAMAS

I I

I

Fig.3.21. The D i f u n t a Group of n o r t h e a s t e r n Mexico A . L o c a t i o n of P a r r a s and La Popa d e p o s i t i o n a l b a s i n s B. L i t h o s t r a t i g r a p h y . The c r o s s - h a t c h e d u n i t s a r e p r e dominantly r e d f o r m a t i o n s which t h i n o u t t o t h e e a s t . Cretaceous-Palaeocene boundary i s a b o u t t h e middle of t h e Rancho Nuevo Formation C , Environmental i n t e r p r e t a t i o n . Note t h e c l o s e a s s o c i a t i o n between r e d beds and d e l t a p l a i n / c o a s t a l p l a i n f a c i e s ( a f t e r McBride, 1974).

155 d e p o s i t i o n i n d e l t a f r o n t , d e l t a p l a t f o r m , p r o d e l t a , and s h e l f d e p o s i t s (McBride, 1974; McBride e t a l . , 1975). The d e l t a p l a i n f a c i e s The d e l t a p l a i n d e p o s i t s of t h e D i f u n t a Group a r e c h a r a c t e r i z e d by a n abundance o f r e d beds (80%) i n c y c l i c sequences of c l a y s t o n e (lo%), mudstone ( 2 0 % ) , s i l t s t o n e (20%) and v e r y f i n e s a n d s t o n e s (30%) which a l t e r n a t e w i t h c h a n n e l - f i l l s a n d s t o n e bodies.

Between 50-60% o f t h e d e l t a - p l a i n d e p o s i t s a r e a r r a n g e d i n 2-5m t h i c k

coarsening-upwards c y c l e s which b e g i n w i t h c l a y s t o n e r e s t i n g on a n e r o s i o n s u r f a c e of s a n d s t o n e and g r a d e upwards through mudstone and s i l t s t o n e t o v e r y f i n e s a n d s t o n e (Fig.3.22).

These c y c l e s a r e i n t e r p r e t e d a s f r e s h w a t e r and b r a c k i s h w a t e r l a k e and

marsh d e p o s i t s .

The g r a d u a l f i l l i n g of l a k e s and marshes by l a c u s t r i n e d e l t a s and

p r o g r a d i n g s h o r e l i n e s i s known t o produce coarsening-upwards sequences of t h i s type (Coleman, 1966; W i l l i a m s , 1968).

SILTSTONE. 80 % BURROW MOTTLED 20% CUR~ENT-RIPPLE BEDDED SILT 70%1 CALCITE 20Ki CLAY 10% COMMON CALCITE NODULES

------MUDSTONEi BEDDING SIMILAR TO CLAYSTONE CLAY 50K1SILT 30x1 CALCITE 20% 1 SPARSE CALCITE NODULESi LOCALLY ABUNCYINT OOGONIA

Fig.3.22. Schematic diagram of a t y p i c a l coarsening-upwards d e l t a p l a i n c y c l e of t h e D i f u n t a Group ( a f t e r McBride, 1974).

156 The upper p a r t of t h e s e c y c l e s f r e q u e n t l y c o n t a i n c a l c i t e n o d u l e s (mean s i z e 2cm) which become p r o g r e s s i v e l y more abundant t o comprise a b o u t 20% a t t h e t o p of some s a n d s t o n e members.

They a r e i n t e r p r e t e d a s c a l c a r e o u s p a l a e o s o l s .

Also a t t h e t o p

of t h e c y c l e s t h e r e o c c u r s i n u o u s , v e r t i c a l t u b e s which a r e i n t e r p r e t e d a s r o o t scars.

P l a n t s may have grown submerged ( m a r s h e s ) , on d r i e d l a k e b e d s , o r on over-

bank a r e a s n e a r d i s t r i b u t a r y c h a n n e l s .

The s c a r c i t y o f p l a n t d e b r i s i n t h e d e p o s i t s

c o u l d thus b e a t t r i b u t e d t o r a p i d decomposition and o x i d a t i o n p r i o r t o b u r i a l . would be favoured by w e l l - d r a i n e d swamps o r s e a s o n a l d e s i c c a t i o n .

This

The a b s e n c e o f

d e s i c c a t i o n even i n a r e a s where s u b a e r i a l e x p o s u r e i s i n d i c a t e d s u g g e s t s t h a t good d r a i n a g e i s t h e more l i k e l y c a u s e ( t h e w e l l - d r a i n e d swamp d e p o s i t s of t h e M i s s i s s i p p i d e l t a c o n t a i n v i r t u a l l y no o r g a n i c m a t t e r ) . The coarsening-upwards c y c l e s a r e p e r i o d i c a l l y i n t e r u p t e d by reddish-brown o r brown c h a n n e l - f i l l s a n d s t o n e u n i t s up t o 1Om t h i c k .

These c o n t a i n c r o s s - b e d d i n g i n

t h e lower p a r t w i t h o c c a s i o n a l i n t r a f o r m a t i o n a l l a g s and g i v e way upwards t o horizontal lamination.

These a r e i n t e r p r e t e d a s t h e d e p o s i t s of low s i n u o s i t y

d i s t r i b u t a r i e s which d r a i n e d t h e lower p a r t s of t h e d e l t a p l a i n . Colour v a r i a t i o n s and m i n e r a l o g y I n k e e p i n g w i t h o t h e r reddened d e l t a p l a i n d e p o s i t s t h e D i f u n t a Group shows a wide range of c o l o u r v a r i a t i o n s i n c l u d i n g r e d , p u r p l e , g r e e n , g r e y , o l i v e , y e l l o w and brown.

Most o f t h e d e l t a p l a i n f a c i e s i s made up of r e d beds which r a n g e from

hues of r e d (5R, 7.5R, 10R) t o r e d d i s h brown (2.5YR). of c o l o u r on g r a i n s i z e .

There i s a s t r o n g dependence

C l a y s t o n e s have s t r o n g e r s a t u r a t i o n (chroma numbers of

3 t o 5 v e r s u s 2 t o 3 f o r c o a r s e r - g r a i n e d r o c k s ) and g r e a t e r l i g h t n e s s v a l u e s .

Most

o f t h e r e d s i l t s t o n e s and c l a y s t o n e s a r e u n i f o r m l y c o l o u r e d t h r o u g h o u t a l t h o u g h a few beds have brown m o t t l e s i n burrows and r o o t s c a r s .

A l l the f i n e grained rocks

a r e c a l c a r e o u s (between 11-19’77 CaCO ) and t h e o n l y c l a y m i n e r a l s p r e s e n t a r e i l l i t e

3

and c h l o r i t e .

The r e d c o l o u r i s due t o h a e m a t i t e which i s p r e s e n t a s g r a i n c o a t i n g s

and i n c l o s e a s s o c i a t i o n w i t h c l a y m i n e r a l s .

I r o n d i s t r i b u t i o n s a r e shown i n

Fig.3.23. Green ( 5 G Y ) beds are l i t h o l o g i c a l l y s i m i l a r t o t h e r e d beds c o n s i s t i n g predomi n a n t l y of c l a y s t o n e and mudstone.

The c o l o u r i s a t t r i b u t e d t o f e r r o u s i r o n i n

c h l o r i t e and i l l i t e ( K e l l e r , 1953) i n t h e absence of h a e m a t i t e and o r g a n i c m a t t e r . The m a j o r i t y of t h e s e g r e e n beds o c c u r b e n e a t h c h a n n e l s a n d s t o n e s whose c o l o u r r a n g e s from l i g h t brown t o g r e y and t h e c o n t a c t w i t h a d j a c e n t r e d beds i s s h a r p b u t c l e a r l y t r a n s e c t s bedding p l a n e s .

T h i s f e a t u r e i s c l e a r l y of p o s t - d e p o s i t i o n a l

o r i g i n and s u g g e s t s t h a t t h e g r e e n beds have formed by t h e r e d u c t i o n o f r e d beds o r p r o t o - r e d beds by r e d u c i n g groundwaters which seeped from t h e o v e r l y i n g c h a n n e l sands.

A s i m i l a r e x p l a n a t i o n h a s been s u g g e s t e d f o r g r e e n zones u n d e r l y i n g f l u v i a l

channel s a n d s t o n e s ( F r i e n d , 1 9 6 6 ) .

The t o t a l i r o n c o n t e n t of t h e g r e e n beds i s

157

4

RED PURPLE A GREEN 0 GREY m

a& A.

OLIVE 1 YELLOW 0 ME AN VALUES

+

m n

>,,mo A

-2 2

O.0

A

o +-o

1

a+- m-t 0

Cl

,

m 2

1

moo 3

I

4

I

96 TOTAL FE

Fig.3.23. P l o t s o f t o t a l Fe v s . Fe3+ and F e 2 + v s . Fe3+ i n D i f u n t a c l a y s t o n e samples of d i f f e r e n t c o l o u r ( a f t e r McBride, 1974). v i r t u a l l y i d e n t i c a l w i t h t h a t of t h e r e d beds b u t t h e Fe2+/Fe3+ r a t i o i s v e r y d i f f e r e n t ( 2 . 8 1 a s opposed t o 0.45 i n t h e r e d b e d s ) ( F i g . 3 . 2 3 ) .

This contrasts

markedly w i t h many o t h e r g r e e n beds formed by s e c o n d a r y r.eduction which u s u a l l y show comparable Fe2+ c o n t e n t s b u t much less Fe3+ t h a n a s s o c i a t e d r e d beds.

The

d e p l e t i o n of Fe3+ i n s e c o n d a r i l y reduced g r e e n beds i s a t t r i b u t e d t o r e d u c t i o n and removal of Fe2+ i n s o l u t i o n ( s e e C h a p t e r 6 ) and t h e r e l a t i v e i n c r e a s e i n Fe

2+

i n t h e D i f u n t a g r e e n beds might i n d i c a t e t h a t i r o n has been accomodated i n a u t h i g e n i c c l a y m i n e r a l s r a t h e r than b e i n g removed (McBride, 1974).

An a l t e r n a t i v e view i s

t h a t t h e g r e e n beds were n e v e r f u l l y o x i d i z e d and t h a t t h e pigmentary h a e m a t i t e of t h e r e d beds formed by t h e complete o x i d a t i o n o f f e r r o u s i r o n - b e a r i n g i l l i t e and chlorite, P u r p l e (5RP 5 / 2 -

5P 5 / 2 ) c l a y s t o n e and s i l t y c l a y s t o n e s show s i m i l a r t e x t u r e s

and composition t o r e d b e d s o f t h e same g r a i n s i z e . 0.5-1.5m

P u r p l e beds a r e t y p i c a l l y

t h i c k and may b e t r a n s i t i o n a l between r e d and g r e e n zones.

They a r e

u s u a l l y r o o t - m o t t l e d zones w i t h a s h a r p upper c o n t a c t and a n uneven lower c o n t a c t which always t r a n s e c t s bedding p l a n e s .

Purple colouration i s a c h a r a c t e r i s t i c

f e a t u r e of r e d beds which have c o n t a i n e d s i g n i f i c a n t q u a n t i t i e s of o r g a n i c m a t t e r , s u c h a s r o o t p e n e t r a t e d h o r i z o n s , b u t i t s p r e c i s e o r i g i n i s n o t w e l l understood. The s i m i l a r i t y of t h e r e d and p u r p l e beds i n c o m p o s i t i o n and l i t h o l o g y s u g g e s t s t h a t d i f f e r e n c e s i n i r o n o r manganese c o n t e n t ( S t r a k h o v , 1958) o r t h e mixing of g r e e n and r e d m i n e r a l s (Ortlam, 1967) a r e n o t a p p l i c a b l e .

The d i s p e r s i o n of

168 pigmentary h a e m a t i t e i n mudrocks i s a n i m p o r t a n t f a c t o r i n c o l o u r a t i o n and s o i s t h e p a r t i c u l a r g r a i n s i z e of h a e m a t i t e p r e s e n t .

The b r i g h t e s t r e d c o l o u r a t i o n

r e s u l t s from f i n e l y d i s p e r s e d h a e m a t i t e of e x t r e m e l y f i n e g r a i n s i z e ( 1 - 2 ~ ) whereas p u r p l e c o l o u r a t i o n a p p e a r s t o r e s u l t from c o a r s e r g r a i n e d (3-5rm) h a e m a t i t e which i s less e v e n l y d i s p e r s e d .

These c o n d i t i o n s c o u l d r e s u l t i n r o o t d i s t u r b e d

mudrocks where t h e decomposition and o x i d a t i o n o f o r g a n i c m a t t e r i n l o c a l i z e d zones c r e a t e s v a r i a b l e Eh-pH c o n d i t i o n s w i t h p r e f e r e n t i a l growth o f h a e m a t i t e i n t h e more oxygenated a r e a s . The importance of o r g a n i c m a t t e r i s a l s o s e e n i n o l i v e , y e l l o w , and g r e y beds where i t s p r e s e n c e a c t u a l l y c o n t r i b u t e s t o rock c o l o u r a t i o n .

O l i v e beds (5GY 5 / 1 )

a r e t h e r e s u l t o f t h e c o l o u r mixing o f g r e e n c l a y m i n e r a l s and b l a c k o r g a n i c m a t t e r . The f o r m a t i o n of h a e m a t i t e i n t h e s e b e d s , a n d i n yellow b e d s , may have been i n h i b i t e d i n t h e p r e s e n c e of o r g a n i c m a t t e r and much of t h e Fe3+ c o n t e n t ( F i g . 3 . 2 3 ) may b e i n t h e form of i r o n hydroxide ( g o e t h i t e ) .

Grey beds (N4-N6) a r e n o t abundant i n t h e

d e l t a p l a i n f a c i e s b e i n g more c h a r a c t e r i s t i c of t h e marine d e p o s i t s i n t h e D i f u n t a Group.

The c o l o u r o f t h e s e g r e y beds i s p a r t l y due t o o r g a n i c m a t t e r i n t h e form

of f i n e l y comminuted p l a n t d e b r i s and a l s o t o f i n e g r a i n e d p y r i t e .

The l a t t e r o c c u r s

a s i n f i l l i n g s of f o r a m i n i f e r a n chambers and a l s o a s s c a t t e r e d framboids.

It is

l i k e l y t h a t t h e g r e y beds r e p r e s e n t p o o r l y - d r a i n e d ground i n which r e d u c i n g c o n d i t i o n s p r e v a i l e d below t h e s e d i m e n t s u r f a c e ( c f . Coleman, 1966). Brown c o l o u r e d beds a r e r e s t r i c t e d t o s i l t s t o n e and v e r y f i n e s a n d s t o n e and a p p e a r t o have formed by t h e w e a t h e r i n g of p r e v i o u s l y g r e y beds.

The c h i e f c o l o u r i n g a g e n t

a p p e a r s t o b e g o e t h i t e which h a s formed by t h e o x i d a t i o n o f p y r i t e and t h e h y d r a t i o n of h a e m a t i t e .

Such a p r o c e s s may be a n i m p o r t a n t i n t e r m e d i a t e s t a g e i n t h e f o r m a t i o n

of r e d beds i n c o a s t a l d e p o s i t i o n a l environments ( C z y s c i n s k i e t a l . , 1978). 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 I n view o f t h e f a c t t h a t no modern a l l u v i u m i s a s r e d a s a n c i e n t r e d beds (Van Houten, 1968; Walker, 1974) i t must b e presumed t h a t t h e sediment which accumu l a t e d on t h e d e l t a p l a i n was d r a b .

S i n c e t h i s sediment p r o b a b l y c o n t a i n e d

s i g n i f i c a n t amounts of i r o n h y d r o x i d e s and o t h e r i r o n - b e a r i n g m i n e r a l s i n a s s o c i a t i o n w i t h t h e abundant c l a y f r a c t i o n (Walker and Honea, 1969; Van Houten, 1972) r e d c o l o u r a t i o n most l i k e l y developed i n oxygenated c o n d i t i o n s above t h e w a t e r t a b l e . A number of f e a t u r e s s u g g e s t t h a t p e d o g e n e s i s and r e d d e n i n g were c l o s e l y r e l a t e d

w i t h i n t h i s zone.

These i n c l u d e r o o t s c a r s which a r e abundant i n p u r p l e a n d r e d

b e d s , t h e absence of b e d d i n g , and t h e abundance of c a l c i t e nodules of i n f e r r e d pedogenic o r i g i n .

The s c a r c i t y o f d i a g n o s t i c s o i l m i c r o f a b r i c s s u g g e s t s t h a t t h e

D i f u n t a s o i l s were n o t mature.

These f a c t o r s , a s w e l l a s t h e abundance of a s s o c i a t e d

r e d and g r e e n i n t r a c l a s t s i n r e d and brown c h a n n e l s a n d s t o n e s , s u g g e s t s t h a t t h e i n i t i a l reddening may have been r e l a t i v e l y r a p i d , p e r h a p s t a k i n g o n l y s e v e r a l

thousand y e a r s a s opposed t o t h e hundreds o f thousands o f y e a r s needed t o redden sandy d e s e r t a l l u v i u m (Walker, 1967a).

The r e a s o n s f o r t h i s r a p i d p o s t - d e p o s i t i o n a l

r e d d e n i n g l i e s i n t h e r e l a t i v e importance o f t h e i n s i t u a g e i n g o f f e r r i c hdyroxides and i r o n - b e a r i n g c l a y m i n e r a l s .

The c l a y - r i c h n a t u r e of t h e o r i g i n a l non-red

a l l u v i u m means t h a t i t l i k e l y c o n t a i n e d s i g n i f i c a n t amounts of f e r r i c hydroxides which c o u l d ' a g e ' i n t o h a e m a t i t e g i v e n s u i t a b l e c o n d i t i o n s ( B e r n e r , 1969a; Walker and Honea, 1969; Van Houten, 1972).

O t h e r d i a g e n e t i c p r o c e s s e s may a l s o have

c o n t r i b u t e d t o r e d c o l o u r a t i o n ; t h e s e i n c l u d e 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 b e a r i n g s i l i c a t e s such a s b i o t i t e and t h e m a r t i t i z a t i o n of d e t r i t a l m a g n e t i t e g r a i n s (McBride, 1974, p.764).

Another p o s s i b l e r e d d e n i n g mechanism i n v o l v e s t h e formation

of h a e m a t i t e by t h e o x i d a t i o n of f i n e l y d i s s e m i n a t e d p y r i t e ( C h a l l i s , 1975; C z y s c i n s k i e t a l . , 1978).

P y r i t e o x i d a t i o n h a s been e x t e n s i v e l y s t u d i e d ( S i n g e r

and M o r r i s , 1969; Smith and Schumate, 1970) and i s p a r t i c u l a r l y s i g n i f i c a n t i n t h e development of a c i d s u l p h a t e s o i l s (Van Breeman, 1973).

Pyrite w i l l rapidly

o x i d i z e when i n c o n t a c t w i t h oxygenated groundwater, t h e r a t e o f o x i d a t i o n b e i n g l a r g e l y d e t e r m i n e d by t h e o x i d a t i o n of f e r r o u s i r o n .

The mechanism i s probably

a two-stage one i n which f e r r o u s i r o n i s r e l e a s e d by p a r t i a l o x i d a t i o n :

+ 02 + Fez+ + SO:- and t h e n 2 completely oxidized: Fez++ Fe3+ and s u b s e q u e n t l y p r e c i p i t a t e d a s f e r r i c oxyhy-

FeS

droxide.

During p r o g r e s s i v e b u r i a l t h i s f e r r i c oxyhdyroxide would ' a g e ' i n t o haem-

a t i t e p r o v i d i n g t h a t o x i d i z i n g c o n d i t i o n s c o n t i n u e d t o p r e v a i l ( B e r n e r , 1969a). C a r b o n i f e r o u s r e d beds i n Europe and North America Introduction During t h e Upper C a r b o n i f e r o u s i n Europe and North America l a r g e amounts of c l a s t i c sediment accumulated i n r i v e r - d o m i n a t e d d e l t a s .

These d e l t a s s t r a d d l e d

e q u a t o r i a l l a t i t u d e s and were o f t e n covered i n l u s h v e g e t a t i o n which accumulated t o f o m p e a t and s u b s e q u e n t l y c o a l seams (Coal Measures).

Within these coal-bearing

d e p o s i t s a r e abundant i n t e r d i g i t a t e d r e d beds which were formed under m o i s t t r o p i c a l climatic conditions.

T h i s a s s o c i a t i o n o f r e d beds w i t h c o a l - b e a r i n g s t r a t a i s q u i t e

d i s t i n c t i v e from t h e red b e d - e v a p o r i t e a s s o c i a t i o n which forms under a r i d c l i m a t i c c o n d i t i o n s (Walker, 1974). The Coal Measures i n t h e UK and USA p r o v i d e e x c e l l e n t examples of r e d beds which formed w i t h i n r i v e r - d o m i n a t e d d e l t a i c s u c c e s i o n s a l t h o u g h t h e y have n o t been intensively studied.

I n NW Europe t h e Coal Measures were d e p o s i t e d i n a major p a r a l i c

b a s i n which i s now r e p r e s e n t e d by a number o f i n d i v i d u a l c o a l f i e l d s which a r e r e l a t e d t o important depositional c e n t r e s (Fig.3.24).

I n t h e Midland V a l l e y o f S c o t l a n d

1060m of Coal Measures a r e p r e s e r v e d and i s o p a c h y t e s t u d i e s i n d i c a t e a d e p o s i t i o n a l t r o u g h w i t h l o c a l a t t e n u a t i o n a t t h e north-west and s o u t h - e a s t margins.

I n the

Pennine P r o v i n c e 3050m a r e p r e s e r v e d i n t h e c e n t r e of t h e b a s i n b u t a g a i n marked

160 a t t e n u a t i o n o c c u r s i n m a r g i n a l a r e a s and i n t h e s o u t h e r n m o s t p a r t t h e Coal Measures r e s t on Pre-Carboniferous r o c k s .

S i m i l a r l y i n South Wales 244Om accumulated i n t h e

c e n t r e of t h e b a s i n n e a r Swansea and t h e r e i s r a p i d t h i n n i n g towards t h e e a s t .

Fig.3.24. Palaeogeography of NW Europe d u r i n g t h e Upper C a r b o n i f e r o u s . The t h i c k n e s s v a l u e s a r e f a r t h e t o t a l Upper C a r b o n i f e r o u s ( a f t e r P.A. Z i e g l e r , 1978). The Coal Measures accumulated mainly i n river-dominated d e l t a s and d e l t a p l a i n s and comprise b o t h coarsening-upwards and fining-upwards s e q u e n c e s .

I n fact the

complexity of l o c a l s u c c e s s i o n i s s u c h t h a t many e a r l y s t u d i e s c o n c e n t r a t e d on s t a t i s t i c a l methods i n t h e s e a r c h f o r a a e n e r a l i z e d s e d i m e n t a t i o n u n i t (Duff a n d Walton, 1962; Duff e t a l . , 1967; Doveton, 1971).

However many l o c a l s u c c e s s i o n s

have now been d e s c r i b e d i n d e t a i l and comparisons w i t h modern d e l t a s a r e now p o s s i b l e (Reading, 1978). 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 S t u d i e s of t h e o r i g i n of r e d beds i n t h e Coal Measures of t h e UK have been confused by t h e p r e s e n c e of s e c o n d a r i l y reddened ( w e a t h e r e d ) s t r a t a b e n e a t h t h e sub-Permian unconformity.

There i s good e v i d e n c e t h a t deep p e n e t r a t i o n o f o x i d i z i n g c o n d i t i o n s

b e n e a t h che a r i d Permian l a n d s u r f a c e was r e s p o n s i b l e f o r t h e r e d d e n i n g of what b o u l d o t h e r w i s e be ' n o r m a l ' g r e y Coal Measures i n a number of a r e a s ( B a i l e y , 1926; T r o t t e r , 1951; Mykura , 1960s). Goodchild (1896, p.71) o r i g i n a l l y s u g g e s t e d t h a t t h e r e d c o l o u r a t i o n of Coal

161 Measures i n S c o t l a n d was due t o d e p o s i t i o n of f e r r i c o x i d e from p e r c o l a t i n g s o l u t i o n s of f e r r o u s b i c a r b o n a t e which h e b e l i e v e d were d e r i v e d from Permo-Triassic l a k e s . The r e d d e n i n g o f t h e C a r b o n i f e r o u s beds of A r r a n was d e s c r i b e d by B a i l e y (1926) who c o n s i d e r e d t h a t t h e r e d c o l o u r i n t h e s e r o c k s had formed i n s i t u by t h e o x i d a t i o n of f e r r o u s i r o n - b e a r i n g m i n e r a l s .

The p r e s e n c e of p a r t i a l l y and c o m p l e t e l y o x i d i z e d

i r o n s t o n e n o d u l e s and reddened p l a n t d e b r i s s u p p o r t s t h i s view.

Simpson and Richey

(1936) d e s c r i b e d t h e r e d d e n i n g which a f f e c t s b o t h t h e Barren Red and P r o d u c t i v e Coal Measures i n t h e Sanquhar and T h o r n h i l l d i s t r i c t s o f S c o t l a n d .

They c o n s i d e r e d

t h e r e d c o l o u r of t h e B a r r e n Red Measures t o b e a n o r i g i n a l f e a t u r e .

Near Sanquhar,

however, Permian l a v a s r e s t d i r e c t l y on P r o d u c t i v e Coal Measures which a r e c o l o u r e d r e d d i s h - p u r p l e and l a c k a c o a l h o r i z o n ( C r e e p i e Coal) which i s p r e s e n t e l s e w h e r e a t t h e same h o r i z o n .

I n t h i s c a s e Simpson and Richey (1936, p.57-58) c o n s i d e r e d t h e

r e d d e n i n g t o be t h e r e s u l t o f d e e p l y p e n e t r a t i n g o x i d a t i o n of i r o n - b e a r i n g sediments d u r i n g t h e Permian t i m e s . The r e d d e n i n g of C a r b o n i f e r o u s s t r a t a i n NW England h a s been d e s c r i b e d by T r o t t e r (1939, 1953, 1954).

T r o t t e r demonstrated t h a t b o t h t h e C a r b o n i f e r o u s Limestone and

t h e Coal Measures a r e reddened where t h e y a r e unconformably o v e r l a i n by PermoT r i a s s i c r o c k s , t h e maximum d e p t h of r e d d e n i n g b e i n g a b o u t 600m.

The reddening

a p p a r e n t l y r e s u l t e d from t h e o x i d a t i o n of s i d e r i t e (Clayband i r o n s t o n e s and n o d u l e s , and p y r i t e ) t o h a e m a t i t e b e f o r e t h e Permo-Triassic r o c k s were d e p o s i t e d . T r o t t e r (1954) r e c o g n i z e d a zone o f complete o x i d a t i o n immediately below t h e prePermo-Triassic l a n d s u r f a c e , u n d e r l a i n e i t h e r by a zone of p a r t i a l o x i d a t i o n o r by a l t e r n a t i n g zones o f p a r t i a l o x i d a t i o n and non-oxidation.

Trotter attributed this

d e e p - s e a t e d o x i d a t i o n t o p e r i o d s of p r o l o n g e d d r o u g h t , d u r i n g t h i c h t h e s u b s t r a t u m i n t h e vadose zone became a e r a t e d , a l t e r n a t i n g w i t h s h o r t p e r i o d s o f r a i n f a l l i n a n a r e a of h i g h r e l i e f .

The maximum d e p t h o f r e d d e n i n g i s t h u s determined by t h e l e v e l

below which t h e pre-Permo-Triassic

w a t e r t a b l e n e v e r sank.

The a l t e r n a t i o n of

p a r t i a l l y o x i d i z e d and u n o x i d i z e d zones might be e x p l a i n e d i f t h e lower l e v e l of t h e w a t e r t a b l e were o v e r l a i n by s e v e r a l perched water t a b l e s . I n NE England t h e C a r b o n i f e r o u s s t r a t a a r e a l s o reddened b e n e a t h t h e Permian unconformity (Anderson and Dunham, 1953).

The d e p t h o f r e d d e n i n g h e r e , however,

does n o t exceed a b o u t 15m and T r o t t e r (1953) s u g g e s t e d t h a t t h i s was due t o t h e f a c t t h a t t h e pre-Permian s u r f a c e i n t h i s a r e a was a low c o a s t a l p l a i n b o r d e r i n g t h e t r a n s g r e s s i v e Z e c h s t e i n s e a and t h e upper l i m i t of permanent s a t u r a t i o n of t h e s t r a t a would b e s i m i l a r t o t h e l e v e l i n t h e a d j a c e n t s e a . The most d e t a i l e d d e s c r i p t i o n o f t h e f e a t u r e s a s s o c i a t e d w i t h s e c o n d a r y reddening

is t h a t o f Mykura (1960a) f o r t h e A y r s h i r e c o a l f i e l d . Here reddened s t r a t a o c c u r at a b r o a d l y uniform d e p t h b e n e a t h t h e Permian unconformity though t h e s t r a t i g r a p h i c a l h o r i z o n v a r i e s from t h e Upper Coal Measures down t o

t h e Upper Limestone Group.

The lower s u r f a c e o f r e d d e n i n g shows c o n s i d e r a b l e i r r e g u l a r i t y i n d i c a t i n g t h a t t h e p e r c o l a t i o n o f w a t e r and a i r was o f t e n g r e a t e r a l o n g f a u l t p l a n e s and p o r o u s , sandy

162 beds ( F i g . 3 . 2 5 ) .

The r e d d e n i n g t o o k p l a c e p r i m a r i l y by i n s i t u o x i d a t i o n b u t t h e r e

i s a l s o e v i d e n c e t h a t h a e m a t i t e p r e c i p i t a t e d from groundwater s o l u t i o n s .

This i s i n

t h e form of t h i n f i l m s of h a e m a t i t e which a r e common i n c o a l s r e p l a c e d by l i m e s t o n e and a l s o h a e m a t i t e cements which o c c u r i n b r e c c i a t e d p o r c e l l a n o u s mudstones.

COAL LIMESTONE SEATEARTH SANDSTONE REDDENED STRATA ARGILLACEOUS STRATA

Diagram i l l u s t r a t i n g t h e p o s s i b l e r e l a t i o n s h i p between f a u l t i n g and t h e Fig.3.25. lower l i m i t o f r e d d e n i n g i n t h e Coal Measures of A y r s h i r e ( a f t e r Mykura, 1960a). Reddening i n t h e A y r s h i r e Coal Measures i s u s u a l l y a s s o c i a t e d w i t h t h e r e p l a c e ment of c o a l by l i m e s t o n e .

The r e p l a c e m e n t p r o c e s s r e s u l t s i n a marked r e d u c t i o n

of t h e seam w i t h t h e o c c a s i o n a l development of 'cone i n c o n e ' s t r u c t u r e . u s u a l l y r e p l a c e d by c a l c i t e f i r s t which may i n t u r n b e d o l o m i t i z e d .

Coal i s

I n some o t h e r

a r e a s such a s t h e S o u t h D e r b y s h i r e c o a l f i e l d s ( P o t t e r y Clay Group) o x i d a t i o n of c o a l seams may r e s u l t i n c a v i t i e s c a u s i n g c o l l a p s e of t h e o v e r l y i n g s t r a t a ( M i t c h e l l , 1944).

L a t e r s t a g e s of t h e replacement i n v o l v e t h e f o r m a t i o n of h a e m a t i t e v e i n l e t s

and f i b r o u s gypsum and c a r b o n a t e s . Mykura (1960a) c o n s i d e r e d t h a t t h e g r e a t d e p t h of o x i d a t i o n below t h e Permian unconformity ( a b o u t 50Om) c a n b e p a r t l y e x p l a i n e d by t h e l o n g p e r i o d o f t i m e between t h e end of Coal Measure d e p o s i t i o n and t h e d e p o s i t i o n o f t h e e a r l i e s t Permian r o c k s w h i l s t t h e a r e a was a l a n d s u r f a c e .

The main r e d d e n i n g and t h e a l t e r a t i o n o f t h e

h i g h e r c o a l s a p p e a r s t o have t a k e n p l a c e b e f o r e d e p o s i t i o n of t h e Permian r o c k s and

163 w h i l s t t h e groundwater was r e l a t i v e l y f r e e o f c a l c i u m and magnesium.

After the

d e p o s i t i o n o f t h e Permian l a v a s w e a t h e r i n g and decomposition may have s u p p l i e d much c a l c i u m and magnesium t o t h e groundwater s o l u t i o n s which were r e s p o n s i b l e f o r t h e a l t e r a t i o n of c o a l s t o c a r b o n a t e s . Not a l l t h e r e d beds i n t h e A y r s h i r e c o a l f i e l d a r e of s e c o n d a r y o r i g i n and a number a p p e a r t o b e a s s o c i a t e d w i t h contemporary e r o s i o n s u r f a c e s .

Mykura (1960a,

p.91) d e s c r i b e s m o t t l e d r e d c l a y s t o n e s o v e r l a i n e r o s i v e l y by c o a r s e p e b b l y sands t o n e s c o n t a i n i n g fragments of reddened and non-reddened C a r b o n i f e r o u s s t r a t a . S i m i l a r l y t h e r e d beds i n t h e Barren Red Measures w i t h t h e i r a s s o c i a t e d ' S p i r o r b i s ' l i m e s t o n e s were p r o b a b l y reddened d u r i n g d i a g e n e s i s and a r e u n l i k e l y t o b e due t o sub-Permian w e a t h e r i n g .

The Coal Measures may t h e r e f o r e c o n t a i n r e d beds produced

d u r i n g two q u i t e d i s t i n c t p h a s e s of r e d d e n i n g a n d i t may b e e x t r e m e l y d i f f i c u l t t o d i s t i n g u i s h t h e two.

Secondary r e d beds produced by t h e w e a t h e r i n g of"norma1'

g r e y s e d i m e n t s a p p e a r t o b e c h a r a c t e r i z e d by t h e f o l l o w i n g f e a t u r e s :

1) a l t e r a t i o n of clayband i r o n s t o n e ( s i d e r i t e s h e e t s and n o d u l e s ) t o h a e m a t i t e and f e r r i c h y d r o x i d e s 2 ) o c c u r r e n c e o f reddened p l a n t d e b r i s and reddened non-marine l a m e l l i b r a n c h s 3 ) r e p l a c e m e n t of c o a l seams by c a r b o n a t e s , gypsum and h a e m a t i t e

4 ) a s p a t i a l r e l a t i o n s h i p o f t h e reddened s t r a t a w i t h a n unconformity o r major fault-planes. Red beds of t h e Umer Coal Measures of t h e UK

Red beds a r 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 t h e Upper Coal Measures i n t h e UK and a r e t y p i c a l l y i n t e r d i g i t a t e d w i t h non-red s t r a t a ,

They p o s s e s s a number o f d i s t -

i n c t i v e s e d i m e n t o l o g i c a l f e a t u r e s which i n d i c a t e t h a t they must have formed a t a r e l a t i v e l y e a r l y post-depositional

s t a g e ( d u r i n g d i a g e n e s i s ) and n o t by t h e sub-

s e q u e n t w e a t h e r i n g of p r e v i o u s l y l i t h i f i e d d r a b s e d i m e n t s .

Some g e n e r a l i z e d

sequences of Upper Coal Measures showing t h e s t r a t i g r a p h i c a l d i s t r i b u t i o n of r e d beds a r e i n d i c a t e d i n Fig.3.26. The widespread d i s t r i b u t i o n o f r e d beds i n t h e Upper Coal Measures and t h e i r a s s o c i a t i o n w i t h ' S p i r o r b i s ' l i m e s t o n e s i s o f t e n t a k e n t o i n d i c a t e a change t o more a r i d conditions.

T h i s may b e so, b u t s e d i m e n t o l o g i c a l s t u d i e s i n d i c a t e t h a t whereas

d e l t a f r o n t sequences a r e abundant i n t h e Lower and Middle Coal Measures t h e Upper Coal Measures a r e dominated by f l u v i a l sequences of t h e d e l t a p l a i n and i t s a l l u v i a l f r i n g e ( e . g . K e l l i n g , 1968, 1969).

The marine i n c u r s i o n marked by t h e topmost

marine band ( A n t h r a c o c e r a s c a m b r i e n s e ) is o f r e l a t i v e l y l i m i t e d a r e a l e x t e n t ( F i g . 3.27) and by t h i s t i m e r e d bed f a c i e s a s s o c i a t e d w i t h m a r g i n a l f l u v i a l sediments were a l r e a d y well-developed.

The changes which o c c u r r e d d u r i n g t h e Upper Coal

Measures f o l l o w e d a p e r i o d of f o l d i n g and f a u l t i n g i n t h e Coalbrookdale c o a l f i e l d . P r i o r t o t h i s t h e Pennine and South-west P r o v i n c e s had m a i n t a i n e d s e p a r a t e b a s i n s

164

Fig.3.26. Upper Coal Measures c o r r e l a t i o n from Somerset t o L a n c a s h i r e showing t h e s t r a t i g r a p h i c a l d i s t r i b u t i o n of r e d beds ( a f t e r P o o l e , 1977). of d e p o s i t i o n b u t t h e t e c t o n i c a c t i v i t y r e s u l t e d i n w i d e s p r e a d u p l i f t and e r o s i o n which was f o l l o w e d by d e p o s i t i o n o v e r t h e former Wales-Brabant Massif ( F i g s . 3.26, 3.27). A.

South Wales

Red beds o c c u r a t a number o f h o r i z o n s i n t h e Upper Coal Measures o f t h e S o u t h Wales c o a l f i e l d ( F i g . 3 . 2 7 ) and have been d e s c r i b e d from t h e L l y n f i and Rhondda Beds (Downing and S q u i r r e l , 1965; K e l l i n g , 1968, 1969) and a l s o t h e Swansea Beds ( A r c h e r , 1965). The Rhondda Beds were d e p o s i t e d i n a n E-W t r e n d i n g b a s i n which was e n c l o s e d on t h r e e s i d e s b u t open t o t h e w e s t .

The d e p o s i t s c o n s i s t o f sand-dominated a l l u v i u m

a r r a n g e d i n fining-upwards c y c l e s a n d , less commonly, coarsening-upwards c y c l e s

165

FACIES OF ANTHRACOCERAS CAMBRIENSE M.B. AT ACME OF INCURSION

Faunal f a c i e s a t t h e acme of t h e Top ( A n t h r a c o c e r a s cambriense) Marine Fig.3.27. Band i n B r i t a i n showing t h e p r o b a b l e l i m i t s o f t h e i n c u r s i o n and t h e s p r e a d of r e d bed f a c i e s o v e r t h e Wales-Brabant M a s s i f . The c o a l f i e l d s a r e shown i n Black (modified from C a l v e r , 1969). Fig.3.29).

The p r i n c i p a l s t r e a m s were a p p a r e n t l y meandering and r e s u l t e d i n t h e

dominance of l a t e r a l a c c r e t i o n u n i t s i n t h e c e n t r a l p a r t o f t h e b a s i n were subs i d e n c e was moderate t o r a p i d .

I n t h e same a r e a coarsening-upwards sequences which

166

SCALE FEET MERES 0 7 0 A

SOUTH WEST south walr ( W o n W o o d l o d , E m & S h p h 1957) OROVESEND SEAMS

1OOo- -Jod NSEA KJUR

North Staffordrhin(b0don EarpWl)

R e p r e s e n t a t i v e s e c t i o n s o f t h e upper p a r t o f W e s t p h a l i a n C and Fig.3.28. Westphalian D showing t h e c o r r e l a t i o n between t h e South Wales, Pennines and S c o t t i s h p r o v i n c e s ( a f t e r C a l v e r , 1969). b e g i n w i t h d a r k mudstones c o n t a i n i n g f r e s h w a t e r b i v a l v e s and o s t r a c o d e s , resemble v e r y c l o s e l y t h e l a c u s t r i n e d e l t a - f i l l sequences d e s c r i b e d by Coleman (1966). The r e d beds i n t h e Rhondda Beds a r e r e s t r i c t e d t o t h e e a s t e r n p a r t o f t h e b a s i n where t h e sequence i s a t t e n u a t e d b e c a u s e of i t s p r o x i m i t y t o t h e p o s i t i v e m a r g i n a l a r e a ( S q u i r r e l and Downing, 1964).

Here t h e s a n d s t o n e s a r e c o a r s e -

g r a i n e d o r t h o q u a r t z i t e s and j u d g i n g from t h e l o w - v a r i a b i l i t y of c u r r e n t v e c t o r s

167 and t h e n a t u r e of t h e fining-upwards sequences t h e y were d e p o s i t e d i n r e l a t i v e l y s m a l l , p r o b a b l y b r a i d e d c h a n n e l s o f s l i g h t s i n u o s i t y ( K e l l i n g , 1968).

Fig.3.29. Schematic model o f p o s t u l a t e d environments of d e p o s i t i o n i n t h e lower Rhondda Beds of South Wales. The n o r t h - s o u t h dimension i s somewhat f o r e s h o r t e r e d . Diagonal s h a d i n g aver t h e e a s t e r n p a r t o f t h e a r e a i n d i c a t e s t h e approximate d i s t r i b u t i o n o f r e d beds ( a f t e r K e l l i n g , 1968). The c o a r s e c l a s t i c s a r e mainly p a l e g r e y t o w h i t e i n c o l o u r whereas t h e i n t e r v e n i n g mudstones may b e g r e y , g r e e n o r r e d , a f e a t u r e i n common w i t h many o t h e r Coal Measure r e d b e d s .

The r e d mudstones may b e u n i f o r m l y r e d b u t more u s u a l l y

a r e m o t t l e d w i t h p a t c h e s , s t r e a k s and s p o t s of g r e e n .

The c o l o u r b o u n d a r i e s may

b e p a r a l l e l t o bedding p l a n e s b u t o c c a s i o n a l l y t r a n s e c t them.

A very 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 m o t t l e d r e d h o r i z o n s i s t h e i r a s s o c i a t i o n w i t h s e a t e a r t h s and t h e presence of s e a t e a r t h - l i k e textures.

These t e x t u r e s , u s u a l l y r e f e r r e d t o a s

" b a s t a r d s e a t e a r t h s " , c o n t a i n abundant r o o t s c a r s b u t r a r e l y show t h e p r e s e n c e of actual rootlets.

They may c o n t a i n s m a l l s i d e r i t e n o d u l e s which have a c r u d e

168 spherulitic or radial texture. The r e d beds a r e t h u s c l o s e l y a s s o c i a t e d w i t h p a l a e o s o l s , a c h a r a c t e r i s t i c f e a t u r e of r e d beds formed i n d e l t a p l a i n environments ,

The m i n e r a l o g y of t h e d i f f e r e n t

c o l o u r e d mudstones i s v e r y s i m i l a r , b e i n g composed mainly of i l l i t e and k a o l i n i t e w i t h minor amounts o f c h l o r i t e .

There a r e some i m p o r t a n t chemical d i f f e r e n c e s ,

however, w i t h t h e r e d mudstones b e i n g markedly e n r i c h e d i n f e r r i c i r o n (7% Fe 0 2 3 a s opposed t o 2.5% Fe203 i n t h e g r e e n mudstones). T h i s c l o s e a s s o c i a t i o n of r e d beds w i t h p a l a e o s o l s i s even more e v i d e n t h i g h e r up i n t h e sequence i n t h e Swansea Beds and Grovesend Beds ( A r c h e r , 1965) ( F i g . 3 . 2 8 ) . Here t h e r e d beds o c c u r a t t h r e e main s t r a t i g r a p h i c a l h o r i z o n s e a c h forming p a r t of a s i m i l a r c y c l i c u n i t (Fig.3.30).

The r e d beds a r e r e s t r i c t e d t o mudstone u n i t s and

a r e t y p i c a l l y v a r i e g a t e d w i t h reddish-brown and r e d d i s h - p u r p l e m o t t l i n g b e i n g prominent.

In p l a c e s b r i c k r e d s p o t s o c c u r and c a n b e s e e n t o s u r r o u n d ' p y r i t e

crystals.

The r e d beds show abundant r o o t s c a r s and a r e c l o s e l y a s s o c i a t e d w i t h

bastard seatearths.

These t y p i c a l l y have a soapy a p p e a r a n c e w i t h numerous l i s t r i c

s u r f a c e s and o n l y r a r e l y show p r e s e r v e d r o o t l e t s .

Spherulitic s i d e r i t e is again

common i n t h e s e s e a t e a r t h s and a s s o c i a t e d s i d e r i t e i r o n s t o n e s have a brown o r s t r a w c o l o u r e d a p p e a r a n c e whereas t h e y a r e normally g r e y i n t h e s u r r o u n d i n g g r e y s e d i m e n t s . Like o t h e r C o a l Measure examples t h e s e r e d beds c a n n o t have been d e p o s i t e d a s r e d d e t r i t u s b e c a u s e t h i s does n o t e x p l a i n t h e i r m o t t l e d a p p e a r a n c e , t h e r e d h a l o e s around p y r i t e c r y s t a l s , o r t h e o c c u r r e n c e of o c c a s i o n a l reddened p l a n t fragments. S i m i l a r l y , t h e i n t e r c a l a t i o n o f t h e r e d beds w i t h normal g r e y s t r a t a i n d i c a t e s t h a t sub-Permian w e a t h e r i n g i s a l s o a n u n l i k e l y e x p l a n a t i o n .

Archer (1965) p r e f e r r e d

a s i m i l a r mechanism t o t h a t s u g g e s t e d by McBride (1974) f o r t h e D i f u n t a Group and Downing and S q u i r r e l (1965) f o r t h e L l y n f i and Rhondda Beds.

T h i s mechanism i n v o l v e s

t h e lowering of t h e w a t e r t a b l e and t h e development of w e l l - d r a i n e d , conditions.

oxygenated

T h i s would e n a b l e t h e d e s t r u c t i o n of p l a n t m a t e r i a l t h u s p r e v e n t i n g

t h e p r e s e r v a t i o n o f c o a l seams.

Reddening c o u l d t a k e p l a c e by t h e o x i d a t i o n of

p y r i t e and a l s o by t h e p r e c i p i t a t i o n o f g o e t h i t e and h a e m a t i t e formed from i r o n r e l e a s e d from c l a y m i n e r a l s .

B.

The Blackband and E t r u r i a Marl Groups o f North S t a f f o r d s h i r e

In t h e North S t a f f o r d s h i r e c o a l f i e l d (Fig.3.27) t h e Upper Coal Measurea a r e a b o u t 1500m t h i c k and i n c l u d e two m a j o r r e d bed f o r m a t i o n s , t h e E t r u r i a Marl Group and t h e K e e l e Group,

These a r e s e p a r a t e d by o v e r 120m of g r e y mudstones and sand-

s t o n e s w i t h t h i n c o a l s (Newcastle Group) and i t i s c l e a r t h a t t h e r e d beds a r e a n i n t r i n s i c f a c i e s and n o t due t o s e c o n d a r y w e a t h e r i n g b e n e a t h t h e P e m o - T r i a s s i c unconformity. Red beds f i r s t a p p e a r i n t h e Upper Coal Measures o f N o r t h S t a f f o r d s h i r e in t h e Blackband Group.

T h i s i s a sequence o f p r e d o m i n a n t l y g r e y non-marine s t r a t a

169

Fig.3.30. Comparative s e c t i o n s of t h e L l y n f i and Rhondda Beds i n t h e n o r t h e a s t p a r t o f t h e South Wales c o a l f i e l d showing t h e d i s t r i b u t i o n o f r e d beds ( a f t e r Downing and S q u i r r e l , 1965). c o n s i s t i n g of mudstones, s i l t s t o n e s and t h i n s a n d s t o n e s sometimes a r r a n g e d i n coarsening-upwards u n i t s .

Near t h e middle p a r t o f t h e Group i s a sequence of t h i c k

c o a l s ( F i g . 3 . 3 1 ) and above t h i s a r e a number of t h i n n e r c o a l s which a r e a s s o c i a t e d with s i d e r i t i c ironstones.

These i r o n s t o n e s , known a s ' b l a c k b a n d i r o n s t o n e s '

g e n e r a l l y o c c u r immediately above t h e c o a l seams (Fig.3.31) p e l o i d a l s i d e r i t e and carbonaceous m a t t e r .

and c o n s i s t of laminated

S i d e r i t i z e d freshwater bivalves a r e

170 p r e s e n t a t some h o r i z o n s and t h e i r o n s t o n e may p a s s l a t e r a l l y i n t o l i m e s t o n e s w i t h mussels of t h e Anthraconauta p h i l l i p s i i g r o u p , o s t r a c o d e s and f i s h .

The blackband

i r o n s t o n e s c l e a r l y r e p r e s e n t widespread l a c u s t r i n e c o n d i t i o n s and t h e s u c c e e d i n g coarsening-upward u n i t s most l i k e l y i n d i c a t e p r o g r a d a t i o n a l f i l l i n g of t h e l a k e s , p o s s i b l y by l a c u s t r i n e d e l t a s .

N f W C b S I ' i GIOUP

UPPER COAL

I

L ~ '

1

/

MEASURES ILbCIWNO GROUP

MIDDLE

f

COAL MEASURES

*

7 I 1 BbNIURY M1)INl UND

LOWER

COAL MEASURES

Fig.3.31. G e n e r a l i z e d sequence o f t h e Coal Measures o f t h e North S t a f f o r d s h i r e c o a l f i e l d w i t h a d e t a i l e d s u c c e s s i o n o f t h e Blackband Group.

!

171 The v e r t i c a l and l a t e r a l d i s t r i b u t i o n of r e d beds i n t h e sequence between t h e Bassey Mine and Blackband c o a l s i s shown i n Fig.3.32.

The r e d beds a r e b e s t

developed i n t h e s o u t h e r n a n d s o u t h - w e s t e r n p a r t o f t h e c o a l b a s i n where t h e succ e s s i o n i s t h i n n e s t and become p r o g r e s s i v e l y t h i n n e r towards t h e c e n t r e of t h e b a s i n so t h a t i n d i v i d u a l r e d bed u n i t s a r e wedge-shaped w i t h a h o r i z o n t a l upper s u r f a c e and a n o b l i q u e lower s u r f a c e ( F i g . 3 . 3 2 ) . These f e a t u r e s s u g g e s t t h a t groundwater d r a i n a g e c o n d i t i o n s a t t h e t i m e o f d e p o s i t i o n e x e r c i s e d a n i m p o r t a n t c o n t r o l on t h e d i s t r i b u t i o n of r e d beds.

Marginal

a r e a s may have been of h i g h e r r e l i e f and t h u s b e t t e r d r a i n e d t h a n a r e a s i n t h e c e n t r e of t h e b a s i n .

Better d r a i n a g e i n t h e m a r g i n a l a r e a s would e n a b l e t h e o x i d a t i o n of

o r g a n i c m a t t e r and t h e development of o x i d i z i n g c o n d i t i o n s w i t h i n t h e d e p o s i t e d sediments so t h a t r e d beds c o u l d e v e n t u a l l y develop.

In t h e c e n t r e of t h e b a s i s ,

on t h e o t h e r hand, poor d r a i n a g e c o u l d r e s u l t i n t h e p r e s e r v a t i o n o f o r g a n i c m a t t e r and t h e development of r e d u c i n g c o n d i t i o n s s o t h a t r e d beds were u n a b l e t o develop. The geometry of t h e r e d beds can b e e x p l a i n e d by v e r t i c a l f l u c t u a t i o n s i n t h e l e v e l of t h e w a t e r t a b l e which may have been a s s o c i a t e d w i t h t e c t o n i c a c t i v i t y .

Relative

u p l i f t o f t h e m a r g i n a l a r e a s r e s u l t e d i n t h e w i d e s p r e a d development o f r e d bed c o n d i t i o n s ( F i g . 3 . 3 3 ) w h i l s t r e l a t i v e s u b s i d e n c e r e s u l t e d i n t h e r e c e s s i o n of r e d bed c o n d i t i o n s and w i d e s p r e a d development o f p e a t swamps and s u b s e q u e n t l y l a c u s t r i n e conditions. The Blackband Group p a s s e s upwards i n t o t h e E t r u r i a Marl Group a major r e d bed f o r m a t i o n a b o u t 30Om t h i c k .

The E t r u r i a Marl i s s t r o n g l y d i a c h r o n o u s , i t s b a s e i s

h a r d t o d e f i n e and t h e r e i s c o n s i d e r a b l e t h i c k n e s s v a r i a t i o n , p a r t l y due t o t h e 'Symon Unconformity' which o c c u r s between t h e E t r u r i a Marl and Halesowen Beds i n t h e South S t a f f o r d s h i r e C o a l f i e l d ( F i g . 3 . 2 6 ) . The E t r u r i a Marl shows many of t h e c h a r a c t e r i s t i c f e a t u r e s o f r e d beds formed I n errmciatfon with coal-bearing s t r a t a .

Much of t h e sequence i s dominated by r e d

mudatone and sandy mudetone which commonly show p a l a e o s o l f e a t u r e s .

These i n c l u d e

s e a t e a r t h s w i t h r o o t s c a r s i n r e d and p u r p l e mudstones, d i s t i n c t c o l o u r m o t t l i n g o f r e d , p u r p l e , y e l l o w , and g r e e n ( c f . McBride, 1974) and a n a b s e n c e o f bedding. t h e r e are occasional t h i n c o a l s with t h e u s u a l grey-coloured s e a t e a r t h s .

Also

Towards

t h e s o u t h , and i n t h e South S t a f f o r d s h i r e c o a l f i e l d , t h e E t r u r i a Marl c o n t a i n s s a n d s t o n e u n i t s ( e s p l e y s ) which are river c h a n n e l d e p o s i t s . based and u s u a l l y p o o r l y s o r t e d and t e x t u r a l l y immature.

These a r e e r o s i v e l y -

I n t h e North S t a f f o r d s h i r e

a r e a t h e s e channel s a n d s t o n e s c o n t a i n l a t e r a l a c c r e t i o n u n i t s and were e v i d e n t l y d e p o s i t e d by meandering s t r e a m s .

F u r t h e r s o u t h ( F i g . 3 . 2 7 ) t h e c h a n n e l sands a r e

t h i c k e r and more a b u n d a n t , and l o c a l l y d e b r i s - f l o w u n i t s o c c u r i n d i c a t i n g t h e p r e s e n c e of m a r g i n a l a l l u v i a l f a n s .

172

KEY L.B.M. 1st. Upper Bassey Mine Limestone L.B.M. Lst. Lower Borrey Mine Limestone Limestone L--L

-

J

b

5

L a

€3’

Coal Red Beds Blackband Ironstone Non-marine Lamellibranchs Ortracoda Spirorbis Ettheria Fish

W

-

0

1

2

3

4

Fig.3.32. Diagrammatic c r o s s - s e c t i o n i l l u s t r a t i n g t h e inter-wedging of r e d beds i n t o t h e North S t a f f o r d s h i r e Blackband Group s u c c e s s i o n .

173 The o r i g i n o f t h e r e d beds i n t h e E t r u r i a Marl i s c l o s e l y a s s o c i a t e d w i t h s o i l forming p r o c e s s e s and a p p e a r s t o have been s i m i l a r t o t h e D i f u n t a Group d e s c r i b e d by McBride (1974).

There i s no e v i d e n c e t o i n d i c a t e t h a t upland s o i l s on t h e Wales-

Brabant Massif were e r o d e d and s u p p l i e d r e d d e t r i t u s t o t h e E t r u r i a Marl a s s u g g e s t e d by Hoare (1960) and Earp ( 1 9 6 1 ) .

The p r i n c i p a l o b j e c t i o n t o t h i s h y p o t h e s i s i s t h a t

s o i l s a r e w e l l developed w i t h i n t h e E t r u r i a Marl and p e d o g e n e s i s and reddening might be expected t o be c l o s e l y r e l a t e d .

L14CWRlNE CONDITIONS RED BEDS RESTRICTED

I

I

SUBSIDENCE

J.

\

@

I I

LACUSTRINE REGRESSION ZONE OF RED BED FORMATION MIGRATES TOWARDS BASIN CENTRE ORMING CONDlTl

@

LACUSTRINE TRANSORESSION

CONTINUED FORMATION OF RED BEDS DURING DIAGENESIS Fig.3.33. Schematic r e p r e s e n t a t i o n of t h e c o n d i t i o n s which r e s u l t e a i n m e inLeLd i g i t a t i o n of r e d and g r e y beds i n t h e Blackband Group of t h e North S t a f f o r d s h i r e coa 1f i e l d .

174 The c h e m i s t r y and m i n e r a l o g y of t h e E t r u r i a Marl s u g g e s t s t h a t p a l a e o s o l s may b e of b o t h s e a t e a r t h and f e r r a l l i t i c (Dury, 1969) t y p e s ( F i g . 3 . 3 4 ) .

Red mudstones

of t h e E t r u r i a Marl a r e f r e q u e n t l y r i c h i n k a o l i n i t e which i s c l o s e l y a s s o c i a t e d with i r o n oxides.

K a o l i n i t e s a r e notable f o r o f t e n containing adsorbed i r o n

(Weaver and P o l l a r d , 1973) and i t i s q u i t e p o s s i b l e t h a t p i g m e n t a t i o n o f t h e E t r u r i a Marl was c l o s e l y a s s o c i a t e d w i t h t h e f o r m a t i o n of i r o n o x i d e s from d i s o r d e r e d k a o l i n i t e s which underwent c r y s t a l l i n i t y changes d u r i n g d i a g e n e s i s ( c f . S l a n s k a , 1976).

The mudstones o f t h e E t r u r i a Marl may t h u s have been reddened a t

a relatively early post-depositional stage.

T h i s i s s u p p o r t e d by p a l a e o m a g n e t i c

e v i d e n c e which i n d i c a t e s t h a t t h e mudstones were magnetized d u r i n g Upper Carboniferous times (Fig.3.35). colour.

The s a n d s t o n e s o f t h e E t r u r i a Marl may b e r e d o r d r a b i n

The d r a b s a n d s t o n e s c o n t a i n a u t h i g e n i c p y r i t e and a p p a r e n t l y were l i t h i f i e d

under r e d u c i n g c o n d i t i o n s i n o r g a n i c - r i c h groundwater.

The r e d s a n d s t o n e s , however,

show e v i d e n c e of prolonged d i a g e n e s i s under o x i d i z i n g c o n d i t i o n s .

They a r e t e x t u r a -

l l y immature a l t h o u g h much c l a y m a t r i x may have been produced by t h e i n s i t u breakdown o f v o l c a n i c rock fragments ( c f . Walker e t a l . ,

1978) and t h e r e i s abundant

a u t h i g e n i c q u a r t z , c a l c i t e , and h a e m a t i t e i n k e e p i n g w i t h many o t h e r a n c i e n t sandy r e d beds.

Palaeomagnetic r e s u l t s from s a n d s t o n e s o f t h e E t r u r i a Marl show s t e e p e r

i n c l i n a t i o n s c o n s i s t e n t w i t h l a t e r d i a g e n e s i s and more p r o l o n g e d reddening.

Si%

0

SANDSTONES MUDSTONES

Al 0 Fe203 Fig.3.34. $ernary diagram showing t h e r e l a t i v e p r o p o r t i o n s of SiOg, A1203, and Fe 0 i n s a n d s t o n e s and mudstones o f t h e E t r u r i a Marl. 2 3

175

0

SANDSTONE

A

MUDSTONE LOCAL FIELD DIRECTION

*

O

*

+

OO O

Fig.3.35. Palaeomagnetic d i r e c t i o n s ( a f t e r t h e r m a l c l e a n i n g ) from s a n d s t o n e s and mudstones of t h e E t r u r i a Marl. The mudstones i n d i c a t e e q u a t o r i a l p a l a e o l a t i t u d e s t y p i c a l of t h e Upper C a r b o n i f e r o u s o f t h e UK which s u g g e s t s r e l a t i v e l y e a r l y postd e p o s i t i o n a l reddening. The s t e e p e r i n c l i n a t i o n s of t h e s a n d s t o n e s s u g g e s t much l a t e r , p o s s i b l y Permian, r e d d e n i n g and a l o n g e r complex d i a g e n e s i s . The p r o j e c t i o n is s t e r e o g r a p h i c , open symbols = upward v e c t o r s , c l o s e d symbols = downward v e c t o r s , circumference = horizontal. 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 The C a r b o n i f e r o u s sequence a t J o g g i n s , Nova S c o t i a has been d e s c r i b e d by Duff and Walton (1973).

T h i s i s a d e l t a p l a i n sequence which prograded n o r t h - e a s t w a r d s

and was d e r i v e d from igneous and metamorphic r o c k s l i k e t h o s e s e e n t o t h e s o u t h i n t h e Cobequid Arch (Way, 1969).

The s e d i m e n t s s t u d i e d by Duff and Walton (1973)

form t h e b a s e of t h e Middle f i n e f a c i e s of Hacquebard and Donaldson (1964) and a r e considered t o be Westphalian B t o Westphalian C i n age.

A wide 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 c l u d i n g g r e y mudstone and s h a l e , c h a n n e l and s h e e t sandstone, r e d

176 and red-green m o t t l e d mudstone, non-marine l i m e s t o n e , s e a t e a r t h and c o a l . Walton (1973) r e c o g n i z e d t h r e e d i v i s i o n s :

Duff and

Lower, Middle and Upper beds and n o t e d

d i f f e r e n t f a c i e s arrangements between them ( F i g . 3 . 3 6 ) .

1

4

...... ..:... 2 .,L.

-. I . .

,

Uct1

Fig.3.36. 1973).

Sedimentary sequences a t J o g g i n s , Nova S c o t i a ( a f t e r Duff and Walton,

The s a n d s t o n e s i n t h e J o g g i n s s e c t i o n t a k e t h e form o f l e n t i c u l a r c h a n n e l sands t o n e s (mean t h i c k n e s s 4m) and s h e e t s a n d s t o n e s (mean t h i c k n e s s 2m).

The l a t t e r

a r e most commonly m u l t i s t o r e y u n i t s i n which l e a v e s o f s a n d s t o n e some 0.50m t h i c k a r e s e p a r a t e d by t h i n mudstone p a r t i n g s a l t h o u g h t h e s e c a n b e s e e n t o t h i c k e n towards t h e c h a n n e l s a n d s t o n e s .

An i m p o r t a n t f e a t u r e of t h e s a n d s t o n e s i s t h a t

t h e y a r e p e n e t r a t e d by e r e c t t r e e stumps which c o n t a i n entombed amphibians and r e p t i l e s ( C a r r o l l , 1967). O t h e r p a l a e o n t o l o g i c a l e v i d e n c e r e g a r d i n g t h e c o n d i t i o n s of d e p o s i t i o n comes from t h e o c c u r r e n c e o f t h i n b i v a l v e and o s t r a c o d e l i m e s t o n e s which a r e o f t e n i n t e r bedded w i t h c o a l s .

The most abundant o s t r a c o d e s a r e C a r b o n i t a a l t i l i s ( J o n e s and

K i r b y ) and C. c f . s a l t e r i a n a (Jones and K i r b y ) and t h e b i v a l v e s i n c l u d e ( C u r v i r i m u l a s p . a n d c f . N a i a d i t e s e l o n g a t a (Dawson) and N . P o l l a r d and M.A.

longus (Dawson) a s r e p o r t e d by J.E.

C a l v e r i n Duff and Walton (1973).

There have been many d i s c u s s i o n s

r e g a r d i n g t h e p a l a e o e c o l o g i c a l s i g n i f i c a n c e of o s t r a c o d e s and b i v a l v e s i n c o a l b e a r i n g sequences p a r t i c u l a r l y w i t h r e s p e c t t o s a l i n i t y v a r i a t i o n s and no c l e a r

177 p i c t u r e seem t o have emerged ( C a l v e r , 1968; P o l l a r d , 1969; E a g e r , 1970).

Duff and

Walton (1973) concluded t h a t t h e o c c u r r e n c e o f Curvirimula and N a i a d i t e s i n a s s o c i a t i o n w i t h C a r b o n i t a was a n i n d i c a t i o n o f b r a c k i s h t o f r e s h w a t e r c o n d i t i o n s w i t h t h e p o s s i b i l i t y t h a t t h e b i v a l v e - r i c h l i m e s t o n e s were more near-marine and t h e o s t r a c o d e - r i c h l i m e s t o n e s were more n e a r - f r e s h w a t e r . Duff and Walton (1973) compared t h e Upper and Lower beds o f t h e J o g g i n s s e c t i o n using standard transion matrix techniques. diagrams a r e shown i n Fig.3.37. t h e Lower and Upper beds.

Representative facies relationship

These i l l u s t r a t e some i m p o r t a n t d i f f e r e n c e s between

The r e d beds which show t h e f a m i l i a r a s s o c i a t i o n w i t h

s e a t e a r t h s o c c u r o n l y i n t h e Lower b e d s ; t h e s e a r e r i c h e r i n l i m e s t o n e and f o s s i l i f e r o u s mudstones.

They a r e c o m p l e t e l y a b s e n t from t h e Upper beds where

l i m e s t o n e s a r e less abundant and t h e s a n d s t o n e s much more abundant ( s a n d s t o n e / s h a l e r a t i o ' = 0.63 a s opposed t o 0.23 i n t h e Lower b e d s .

Duff and Walton (1973) a s s e s s e d

t h e o v e r a l l environment o f d e p o s i t i o n a s t h a t o f a d e l t a p l a i n w i t h back-swamps and s h a l l o w l a k e s and major d i s t r i b u t a r y c h a n n e l s .

D e p o s i t i o n a l s i t e s were p r o b a b l y

v a r i e d and i n c l u d e d t h e c h a n n e l and i t s m a r g i n s , c r e v a s s e s p l a y s , l a c u s t r i n e d e l t a s and t h e s u r r o u n d i n g f l o o d b a s i n . The d i f f e r e n c e s between t h e Upper and Lower beds p r o v i d e a n i m p o r t a n t c l u e t o t h e o r i g i n of t h e r e d beds.

The upper beds c o n t a i n i n g t h e t h i c k e s t c o a l s , abundant

s a n d s t o n e s and r e l a t i v e l a c k of non-marine f a u n a s s u g g e s t s a s i t e low on t h e d e l t a p l a i n i n which c o n t i n u o u s swamp c o n d i t i o n s a l l o w e d t h e a c c u m u l a t i o n of t h i c k sequenc e s of p e a t ,

The Lower beds on t h e o t h e r hand were c o n s i d e r e d by Duff and Walton

(1973) t o have accumulated a t a h i g h e r p o s i t i o n on t h e d e l t a p l a i n which e n a b l e d non-marine f a u n a s t o f l o i r i s h and t h e c r e a t i o n of b e t t e r d r a i n a g e c o n d i t i o n s i n which o x i d i z i n g c o n d i t i o n s and r e d bed f o r m a t i o n may have p r e v a i l e d on t h e i n t e r v e n i n g mud f l a t s which would have been above t h e w a t e r t a b l e . T h i s e x p l a n a t i o n i s c o n s i s t e n t w i t h t h a t p u t forward f o r o t h e r d e l t a p l a i n r e d beds.

S p e c i f i c r e d d e n i n g mechanisms were p r o b a b l y complex b u t t h e d e h y d r a t i o n of

d e t r i t a l c l a y - o x i d e complexes and o x i d a t i o n o f e a r l i e r formed p y r i t e a r e c o n s i d e r e d t o have been i m p o r t a n t .

4t

Shalo

t\ -COAL

I ' /

Limoston.

Fig. 3.37. F a c i e s r e l a t i o n s h i p diagrams f o r t h e C a r b o n i f e r o u s sediments a t J o g g i n s , Nova S c o t i a ( a f t e r Duff and Walton, 1973).

178 CONCLUSIONS AND FURTHER M A D I N G Although t h e r e a r e no modern examples o f d e l t a p l a i n r e d beds i t seems l i k e l y t h a t they formed i n a v a r i e t y o f c l i m a t i c s e t t i n g s .

They a r e most c h a r a c t e r i s t i c

o f a n c i e n t river-dominated d e l t a s which formed i n m o i s t t r o p i c a l c l i m a t e s and t h u s c o n t r a s t markedly w i t h d e s e r t r e d beds.

The mechanism of r e d d e n i n g a l s o c o n t r a s t s

w i t h d e s e r t r e d beds and p e d o g e n e s i i r a t h e r t h a n i n t r a s t r a t a l a l t e r a t i o n may be t h e dominant p r o c e s s . The f o l l o w i n g a r e recommended f o r f u r t h e r r e a d i n g : B r o u s s a r d , M.L.

( E d i t o r ) , 1975. D e l t a s

-

Models f o r E x p l o r a t i o n , Houston Geol.

SOC., Houston, 555 pp. Mohr, E . C . J .

and Van Baren, F.A.,

New York, 490 pp.

1954. T r o p i c a l S o i l s . I n t e r s c i e n c e Publ.,

179

CHAPTER 4 ALLWIAL RED BEDS INTRODUCTION A l l u v i a l s e d i m e n t s c o n s i s t of a g r e a t v a r i e t y o f p e b b l y , sandy and muddy d e p o s i t s . They accumulate i n a v a r i e t y of f l u v i a l d e p o s i t i o n a l environments under a l l c l i m a t i c Present-day a l l u v i a l sediments a r e not red, nor i s a l l a n c i e n t alluvium.

conditions.

A n c i e n t a l l u v i a l s e d i m e n t s a r e , however, common 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 comprise t h e most i m p o r t a n t group of a n c i e n t c o n t i n e n t a l r e d beds.

They a r e t h e most

e x t e n s i v e l y s t u d i e d r e d beds b u t a r e s t i l l o n l y p o o r l y u n d e r s t o o d and we owe much of o u r p r e s e n t knowledge t o T.R. Walker and c o l l e a g u e s who have made a n i n t e n s i v e s t u d y of Cenozoic d e s e r t a l l u v i u m i n 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.

Walker

(1967a) showed t h a t t h i s d e s e r t a l l u v i u m was reddened p o s t - d e p o s i t i o n a l l y 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 d e t r i t a l i r o n s i l i c a t e m i n e r a l s .

I t s h o u l d be s t r e s s e d

t h a t t h i s a l l u v i u m i s of f i r s t - c y c l e o r i g i n and i s r i c h i n d e t r i t a l ferromagnesian silicates,

The same c o n d i t i o n s do n o t a p p l y t o a l l a n c i e n t a l l u v i a l r e d beds.

When a p p l i e d t o a n c i e n t a l l u v i a l r e d bed s e q u e n c e s t h e d i a g e n e t i c model of Walker c a n n o t , i n i t s e l f , a d e q u a t e l y a c c o u n t f o r some o f t h e observed v a r i a t i o n s .

For

example, d e p o s i t i o n a l environment i s c l e a r l y a n i m p o r t a n t c o n t r o l l i n g f a c t o r because d i f f e r e n t environments show d i f f e r e n t c o l o u r d i s t r i b u t i o n p a t t e r n s .

A l l u v i a l fans

and c o a r s e g r a i n e d a l l u v i a l sequences f r e q u e n t l y show u n i f o r m l y r e d c o l o u r a t i o n which

i s 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 l i k e t h a t d e s c r i b e d by Walker.

Fine grained

a l l u v i a l s e q u e n c e s , l i k e t h o s e d e p o s i t e d by meandering r i v e r s , a r e g e n e r a l l y q u i t e different.

Here t h e a l l u v i u m i s d i f f e r e n t i a t e d i n t o c o a r s e ( c h a n n e l ) and f i n e ( f l o o d -

p l a i n ) members and t h e r e a r e i m p o r t a n t c o l o u r d i f f e r e n c e s between t h e two, t h e former o f t e n b e i n g d r a b and t h e l a t t e r r e d .

The s u g g e s t i o n h e r e i s t h a t d e p o s i t i o n a l and

e a r l y p o s t - d e p o s i t i o n a l p r o c e s s e s have been e f f e c t i v e i n producing t h e red- non-red differentiation.

The i m p l i c a t i o n i s t h a t t h e a l l u v i u m o r i g i n a l l y c o n t a i n e d sub-

s t a n t i a l amounts of f i n e g r a i n e d i r o n o x i d e o r i r o n h y d r o x i d e i n a s s o c i a t i o n w i t h t h e c l a y f r a c t i o n of t h e a l l u v i u m .

T h i s was c o n c e n t r a t e d i n t h e f l o o d p l a i n e n v i r -

onment and became s t a b i l i z e d under d r y , oxygenated c o n d i t i o n s , e v e n t u a l l y producing a red colour.

I n t h e s a t u r a t e d channel e n v i r o n m e n t , t h e s e i r o n o x i d e s and hydroxides

become u n s t a b l e , e s p e c i a l l y under r e d u c i n g c o n d i t i o n s , and were e v e n t u a l l y removed i n solution,

Van Houten (1972) h a s shown t h e importance o f d e t r i t a l f e r r i c hyd-

r o x i d e s i n Recent t r o p i c a l a l l u v i u m .

The s o u r c e of t h e s e hydroxides i s n o t p r e c i s e l y

known and t h e i m p o r t a n t q u e s t i o n i s whether o r n o t they have been d e r i v e d from upland r e d s o i l s . I n t h i s c h a p t e r t h e d e p o s i t i o n of p r e s e n t - d a y a l l u v i u m i s reviewed and t h e t r a n s p o r t of i r o n o x i d e s and h y d r o x i d e s i n t h e s e sediments i s d e s c r i b e d .

The

180 various post-depositional processes including the 'ageing' of f e r r i c hydroxides and t h e i n t r a s t r a t a l s o l u t i o n of i r o n s i l i c a t e s a r e a l s o d e s c r i b e d and t h e i r importance i n a n c i e n t r e d bed sequences a s s e s s e d .

Gross g e n e r a l i z a t i o n s about

t h e o r i g i n of a l l u v i a l r e d b e d s ' a r e n o t p o s s i b l e .

I n d i v i d u a l environments i n a

p a r t i c u l a r r e d bed sequence may have been reddened by d i f f e r e n t p r o c e s s e s and d i f f e r e n c e s i n 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 p o s t - d e p o s i t i o n a l h i s t o r y may e x e r t a s t r o n g c o n t r o l l i n g i n f l u e n c e on d i f f e r e n t r e d bed s e q u e n c e s .

RIVER CHANNELS When s e e n i n p l a n view r i v e r c h a n n e l s may t a k e on a v a r i e t y of forms which depend upon t h e flow s t a g e o f t h e r i v e r ,

The r i v e r c h a n n e l p a t t e r n i s c o n t r o l l e d

by a number of v a r i a b l e s i n c l u d i n g t h e sediment l o a d and i t s c h a r a c t e r i s t i c s , and a l s o t h e amount and n a t u r e of t h e d i s c h a r g e .

A f t e r t h e work o f Leopold and Wolman

(1957) t h r e e b a s i c c h a n n e l p a t t e r n s can be r e c o g n i z e d :

s t r a i g h t , meandering and

b r a i d e d which a r e d i s t i n g u i s h e d on t h e b a s i s of s i n u o s i t y ( r a t i o of Thalweg l e n g t h t o V a l l e y l e n g t h ) and t h e p r e s e n c e of a l l u v i a l i s l a n d s ,

S t r a i g h t c h a n n e l s have a

s i n u o s i t y of a b o u t 1.0 and meandering c h a n n e l s a s i n u o s i t y o f g r e a t e r than 1 . 5 . A b r a i d e d channel i s one which flows i n two o r more anastomosing c h a n n e l s around a l l u v i a l islands (Fig.4.1).

N a t u r a l channel p a t t e r n s i n t e r g r a d e w i t h e a c h o t h e r

and b r a i d e d and meandering c h a n n e l s r e p r e s e n t t h e end p o i n t s o f a c o n t i n u o u s spectrum. Even t h e same channel may show changing p a t t e r n s a l o n g i t s l e n g t h ; t h e same channel a p p e a r i n g t o meander a t b a n k f u l s t a g e s and b r a i d e d a t low f l o w s t a g e s ( R u s s e l l , 1954).

Because of t h i s continuum of c h a n n e l p a r t e r n s S c h u m (1963) proposed a more

d e t a i l e d c l a s s i f i c a t i o n proposing f i v e c l a s s e s :

straight, transitional, regular,

i r r e g u l a r and t o r t u o u s c h a n n e l p a t t e r n s . The f a c t o r s c o n t r o l l i n g c h a n n e l p a t t e r n a r e complex a l t h o u g h s l o p e and d i s c h a r g e a r e two of t h e most i m p o r t a n t .

Leopold and Wolman (1957) showed t h a t i n t h e r i v e r s

they s t u d i e d b r a i d e d and meandering c h a n n e l s c o u l d be d i s t i n g u i s h e d by t h e r e l a t i o n ship: -0.44

S = 0.06Qb

(4.1)

where S = s l o p e and Qb= b a n k f u l d i s c h a r g e .

Thus f o r a g i v e n d i s c h a r g e meanders

would be e x p e c t e d t o form on t h e s m a l l e r s l o p e s , o r a l t e r n a t i v e l y a t t h e same s l o p e a b r a i d e d c h a n n e l c o u l d be e x p e c t e d t o have a h i g h e r d i s c h a r g e t h a n a meandering channel.

T h i s i s w e l l i l l u s t r a t e d on t h e R i v e r Rhine where S c h a f e r (1973) h a s shown

t h a t t h e upper r e a c h e s a r e b r a i d e d ( s l o p e = 0.97%) and t h e lower r e a c h e s meandering (slope = 0.025%).

The r e l a t i o n s h i p , however, does n o t h o l d f o r a l l r i v e r s and o t h e r

f a c t o r s have a n i m p o r t a n t r o l e . discharge.

Amongst t h e s e a r e bank s t a b i l i t y and s e d i m e n t

Some r i v e r s have a meandering c h a n n e l p a t t e r n i n a r e a s where t h e banks

a r e s t a b i l i z e d by v e g e t a t i o n o r f o r e s t a t i o n b u t t a k e on a b r a i d e d p a t t e r n i n a r e a s where t h e banks a r e more r e a d i l y eroded.

Increased sediment load r e s u l t s i n a

181

bank L i n e d

A

B

C

b

b'

75 0 7.5 50 75 100

Fig.4.1.

The c l a s s i f i c a t i o n o f r i v e r c h a n n e l s a c c o r d i n g t o Leopold and Wolman (1957)

tendency towards b r a i d i n g and some l a r g e r i v e r s w i t h s m a l l s l o p e s have changed from a meandering t o a b r a i d e d p a t t e r n because of an i n c r e a s e i n sediment l o a d . example of t h i s i s t h e Brahmaputra R i v e r (Coleman, 1969).

A good

I t follows t h a t the

p r o p o r t i o n of bed l o a d t o a v a i l a b l e d i s c h a r g e may b e a n i m p o r t a n t cause of b r a i d i n g i n r i v e r c h a n n e l s (Morisawa, 1 9 6 8 ) . The c r o s s - s e c t i o n a l form of a r i v e r c h a n n e l depends upon a number of f a c t o r s i n c l u d i n g d i s c h a r g e and t h e n a t u r e and amount of sediment l o a d . d i s t i n g u i s h e d between bed-load,

Schumm (1968)

mixed-load and suspended-load s t r e a m s .

Fig.4.2

i l l u s t r a t e s t h e c h a n n e l morphology of t h e s e d i f f e r e n t t y p e s o f s t r e a m and t h e n a t u r e of t h e i r d e p o s i t s .

Meandering c h a n n e l s t e n d t o develop i n suspended l o a d s t r e a m s

because t h e y produce h i g h l y c o h e s i v e banks which a r e r e s i s t a n t t o e r o s i o n .

Low-

s i n u o s i t y c h a n n e l s a r e more c h a r a c t e r i s t i c of bed l o a d s t r e a m s because t h e s e have bank m a t e r i a l s which a r e g e n e r a l l y l e s s r e s i s t a n t t o e r o s i o n .

Present-day a l l u v i a l

sediments c a n be c o n v e n i e n t l y d i v i d e d i n t o f o u r major groups on t h e b a s i s of channel p a t t e r n and t y p e o f a l l u v i u m :

a l l u v i a l f a n s , pebbly b r a i d e d r i v e r s , sandy b r a i d e d

182 r i v e r s , and meandering s t r e a m s .

Each of t h e s e has a n c i e n t a n a l o g u e s i n t h e

s t r a t i g r a p h i c a l r e c o r d and forms major r e d bed s e q u e n c e s .

F i g . 4 . 2 . Scheme i l l u s t r a t i n g t y p e s of s i n g l e - c h a n n e l and m u l t i p l e - c h a n n e l s y s t e m s , a s s o c i a t i o n s , and t h e i r c h a r a c t e r i s t i c s . A . Channel morphology B. A l l u v i a l d e p o s i t s ( m o d i f i e d a f t e r Schumm, 1968).

ALLUVIAL FANS A l l u v i a l f a n s a r e cone-shaped d e p o s i t s which g e n e r a l l y develop i n a r e a s o f h i g h r e l i e f which a r e undergoing r a p i d e r o s i o n .

They u s u a l l y o c c u r a l o n g t h e f a u l t -

bounded margins of s e d i m e n t a r y b a s i n s and may c o a l e s c e l a t e r a l l y t o form a piedmont slope o r bajada.

The p r o c e s s e s forming a l l u v i a l f a n s have been most e x t e n s i v e l y

s t u d i e d i n a r i d and s e m i - a r i d r e g i o n s where r a i n f a l l i s i n f r e q u e n t and v e g e t a t i o n sparse.

Humid-climate, o r v e g e t a t e d a l l u v i a l f a n s , have r e c e i v e d much l e s s a t t e n t i o n

and a r e more p o o r l y known.

R e l e v a n t d e s c r i p t i o n s i n c l u d e t h o s e of Hoppe and Ekman

(1964) and Winder ( 1 9 6 5 ) . A l l u v i a l f a n s p a s s l a t e r a l l y i n t o a v a r i e t y of s e d i m e n t a r y environments i n c l u d i n g l a k e s , a l l u v i a l p l a i n s and marine s h o r e l i n e s .

Because of t h e i r l o c a t i o n on a c t i v e

f a u l t s c a r p s a l l u v i a l f a n s b u i l d t h i c k sequences of s y n t e c t o n i c sediments i n t e r b e d d e d w i t h t h o s e of t h e a s s o c i a t e d s e d i m e n t a r y environments.

A number of f a c t o r s , i n c l u d i n g t h e s i z e of t h e catchment a r e a , s o u r c e a r e a l i t h o l o g y and c l i m a t e c o n t r o l t h e s i z e and shape of a l l u v i a l f a n s .

I n semi-arid

r e g i o n s t h e f a n a r e a (A ) and t h e a r e a of t h e d r a i n a g e b a s i n a r e r e l a t e d by: F AF = cA: (4.2)

183 where c and n a r e e m p i r i c a l l y d e r i v e d exponents ( B u l l , 1964; Hooke, 1965; Beaumont, 1972); n g e n e r a l l y b e i n g between 0 . 8 and 1.0 and c v a r y i n g between 0 . 1 5 and 2.10 depending on s o u r c e a r e a l i t h o l o g y , t e c t o n i c a c t i v i t y and c l i m a t e . Most f a n s have a concave-up p r o f i l e when s e e n i n r a d i a l s e c t i o n and t h i s i s n o t u s u a l l y a s i m p l e curve b u t a s e r i e s o f e v e n l y s l o p i n g segments which may be due t o i n t e r m i t t e n t s o u r c e a r e a u p l i f t ( e . g . B u l l , 1964)

LOWER FAN

Y ZONE OF COALESCENCE

X MIDFAN

X

CHANNEL PROFILE

F i g . 4 . 3 . P l a n view and r a d i a l p r o f i l e o f a n a l l u v i a l f a n ( b a s e d on B u l l , 1964 and Hooke, 1 9 6 7 ) . Alluvial fan deposits Channels r a d i a t e from t h e f a n apex and d i s s e c t t h e f a n s u r f a c e .

Usually there

i s no b r e a k i n shape between t h e canyon f l o o r and t h e main channel s u g g e s t i n g t h a t d e p o s i t i o n i s i n i t i a t e d by l a t e r a l e x p a n s i o n a t t h e f a n apex r a t h e r t h a n by a decrease i n slope.

The main c h a n n e l i s commonly i n c i s e d on t h e upper p a r t of t h e

f a n and emerges a t a lower l e v e l , c a l l e d t h e ' i n t e r s e c t i o n p o i n t ' by Hooke (1967) (Fig.4.3). The d e p o s i t s of a l l u v i a l f a n s have been c l a s s i f i e d by B u l l (1972) i n t o f o u r main t y p e s :

d e b r i s flow d e p o s i t s , s h e e t f l o o d d e p o s i t s , s t r e a m channel d e p o s i t s

and s i e v e d e p o s i t s .

184 D e b r i s flow d e p o s i t s D e b r i s flows a r e d e n s e , v i s c o u s masses of s e d i m e n t i n which t h e m a t r i x can s u p p o r t and t r a n s p o r t l a r g e c l a s t s up t o b o u l d e r s i z e .

The d e b r i s flows may s o l i d i f y n e a r

t h e margins t h u s forming d e b r i s flow l e v e e s , o r i n a c e n t r a l p l u g where s h e a r i s i n s u f f i c i e n t t o overcome t h e s t r e n g t h of t h e deforming mass (Johnson, 1970; C a r t e r , 1975; Middleton and Hampton, 1976).

When t h e c e n t r a l s o l i d p l u g expands t o t h e

f u l l t h i c k n e s s of t h e flow i t w i l l c e a s e t o move. D e b r i s flow d e p o s i t i o n r e q u i r e s s o u r c e a r e a l i t h o l o g i e s which produce s u b s t a n t i a l amounts of f i n e d e t r i t u s and s t e e p s l o p e s which promote r a p i d e r o s i o n and s u r f a c e run-off.

They most commonly o c c u r t h e r e f o r e , n e a r t h e p r o x i m a l p a r t o f t h e f a n and

may b e i n i t i a t e d d u r i n g i n c r e a s e d run-off

(e.g.

Sharpe and Nobles, 1953).

The

d e p o s i t s a r e g e n e r a l l y p a r a l l e l s i d e d , u n s t r a t i f i e d beds which a r e e l o n g a t e and p a r a l l e l t o the fan-surface (Fig.4.4).

100

N \

\

\

\ \

\

\ \ \

I

I

I \

I

I I 1

/ I

I

I Probable debris flow lobm

m w i sflow ~nrrr

aPossible s h e dqmsits

Present channel DOpotitiOMl lobes probably sheet ftooc

The upper p a r t of t h e T r o l l h e i m F a n , Death V a l l e y showing t h e d i s t r i b u t i o n Fig.4.4. of r e c e n t d e b r i s flow l o b e s and l e v e e s . The unshaded a r e a s a r e mainly fragments of o l d e r f a n s u r f a c e s , There i s a r e l i e f of a b o u t lOOm o v e r t h e a r e a shown ( a f t e r Hooke, 1967).

185 C h a r a c t e r i s t i c a l l y t h e y a r e . p o o r l y - s o r t e d , m a t r i x s u p p o r t e d and show a r e l a t i v e l y r a p i d d e c r e a s e i n maximum c l a s t s i z e down t h e f a n s u r f a c e . Sheet flood deposits S h e e t f l o o d s o c c u r most commonly below t h e i n t e r s e c t i o n p o i n t and a r e r e l a t i v e l y low v i s c o s i t y f l o o d s which expand a t t h e downstream ends o f c h a n n e l s .

Locally they

develop i n t o s h a l l o w s h e e t flows w i t h u p p e r flow regime c o n d i t i o n s and p a s s downf a n i n t o p a t t e r n s of b r a i d e d c h a n n e l s and b a r s which d i s s e c t t h e upper s u r f a c e of t h e sediment.

The d e p o s i t s a r e f a i r l y w e l l - s o r t e d sand and g r a v e l w i t h i n t e r n a l

l e n t i c u l a r i t y , s c o u r i n g , minor c r o s s - b e d d i n g and c r o s s - l a m i n a t i o n . Stream c h a n n e l d e p o s i t s Stream c h a n n e l d e p o s i t s o c c u r most a b u n d a n t l y i n t h e upper p a r t of t h e f a n above t h e i n t e r s e c t i o n p o i n t ( B u l l , 1972).

Below t h i s p o i n t f l o o d s may b e l a r g e l y uncon-

f i n e d and develop i n t o s h e e t f l o o d s .

The d e p o s i t s of s t r e a m c h a n n e l s a r e m o s t l y

l e n t i c u l a r , p o o r l y - s o r t e d s a n d s and g r a v e l s w i t h i m b r i c a t i o n developed i n t h e c o a r s e r g r a v e l s and c r o s s - b e d d i n g i n t h e sandy b e d s .

They a r e t h u s e a s i l y d i s t i n g u i s h e d

from d e b r i s flow d e p o s i t s . Sieve deposits S i e v e d e p o s i t s o c c u r j u s t below t h e i n t e r s e c t i o n p o i n t where h i g h l y permeable o l d e r d e p o s i t s c a u s e t h e flow t o d i m i n i s h r a p i d l y a s i n f l u e n t seepage o c c u r s . r e s u l t i s a clast-supported gravel lobe (Fig.4.4) poorly imbricated gravel.

The

c o n s i s t i n g of w e l l - s o r t e d and

S i e v e d e p o s i t s may have l o b e s w i t h c l e a r l y d e f i n e d down-

s t r e a m margins (Wasson, 1974).

During l a t e r b u r i a l t h e i r w e l l - s o r t e d

t e x t u r e may

be d e s t r o y e d by t h e m e c h a n i c a l i n f i l t r a t i o n o f f i n e r sediment (Walker, 1976). PEBBLY BRAIDED RIVERS Pebbly b r a i d e d r i v e r s a r e well-developed i n g l a c i a l outwash a r e a s , o r c l i m a t e a l l u v i a l f a n s , and a l s o i n t h e wadis of s e m i - a r i d r e g i o n s ,

humid-

The g r o s s

morphology of t h e s e 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 systems i s u s u a l l y a s h a l l o w f a n o r v a l l e y t r a i n i n which v a l l e y w a l l s c o n f i n e t h e flow.

Much o f o u r knowledge of

t h e s e r i v e r s comes from t h e g l a c i a l outwash p l a i n s o f I c e l a n d (HjulstrMm, 1952; K r i g s t r b m , 1962; B l u c k , 1974) and n o r t h w e s t e r n Canada and Alaska (Williams and R u s t , 1969; Boothroyd, 1972; R u s t , 1972; Smith, 1974). The b r a i d e d p a t t e r n i n r i v e r s o f t h i s t y p e a r e c a u s e d by t h e development of channel bars of various s i z e s .

The p r e c i s e c a u s e of b r a i d i n g i s n o t f u l l y known, b u t

f l u c t u a t i o n s i n d i s c h a r g e ( F a h n e s t o c k , 1963; Church, 1972) which a r e a f e a t u r e of p e r i g l a c i a l a r e a s would a p p e a r t o be a n i m p o r t a n t f a c t o r .

I t i s c o n v e n i e n t t o group

186 the channel bars of pebbly braided streams i n t o t h r e e types:

longitudinal bars,

b a r s i n curved c h a n n e l r e a c h e s and t r a n s v e r s e b a r s a l t h o u g h t h e l a t t e r a r e more common i n d i s t a l r e a c h e s and w i l l b e c o n s i d e r e d i n sandy low s i n u o s i t y r i v e r s . Lonai t u d i n a l b a r s L o n g i t u d i n a l b a r s a r e c h a r a c t e r i s t i c a l l y diamond o r lozenge-shaped i n p l a n view and a r e t h e dominant bedform o f p e b b l y b r a i d e d spreams ( F i g . 4 . 5 ) .

The upper s u r f a c e s

of t h e s e b a r s may b e c o v e r e d w i t h t r a n s v e r s e r i b s (McDonald and B a n e r j e e , 1971) of c o a r s e c l a s t s w i t h well-developed i m b r i c a t i o n and t h e l o n g a x e s o f c l a s t s o r i e n t a t e d t r a n s v e r s e t o t h e flow d i r e c t i o n .

These t r a n s v e r s e r i b s t e n d t o show a r e g u l a r

s p a c i n g which i s a f u n c t i o n of g r a i n s i z e and may r e s u l t from upper flow regime a ntidune transport. The i n t e r n a l s t r u c t u r e o f l o n g i t u d i n a l b a r s c o n s i s t s of g r a n u l e t o c o b b l e g r a d e g r a v e l s which a r e m o s t l y m a s s i v e o r w i t h a c r u d e h o r i z o n t a l s t r a t i f i c a t i o n .

The

g r a v e l s may be m a t r i x - f i l l e d o r openwork (Smith, 1974) and a r e f r e q u e n t l y a r r a n g e d i n fining-upwards u n i t s .

Large s c a l e cross-bedded g r a v e l s a r e n o t f r e q u e n t l y

r e p o r t e d b u t may develop a s a r e s u l t of a v a l a n c h i n g .

These a v a l a n c h e f o r e s e t s may

show r e a c t i v a t i o n s u r f a c e s ( C o l l i n s o n , 1970) which r e s u l t from f a l l i n g s t a g e m o d i f i c a t i o n of t h e b a r f r o n t .

The g r a i n s i z e of l o n g i t u d i n a l b a r s g e n e r a l l y

d e c r e a s e s downstream and t h e downstream d e p o s i t i o n a l margins may b e s l i p f a c e s o r r i f f l e s ; o t h e r margins c o n s i s t of l a t e r a l and downstream e r o s i o n t e r r a c e s . The p r o c e s s e s of l o n g i t u d i n a l b a r i n i t i a t i o n and m i g r a t i o n a r e n o t w e l l understood.

They p r o b a b l y form by t h e s e g r e g a t i o n of c o a r s e r c l a s t s a s t h i n rhombohedra1

g r a v e l s h e e t s and grow by t h e development o f l o n g i t u d i n a l s l i p f a c e s and v e r t i c a l a c c r e t i o n t o t h e b a r top.

R u s t (1972) observed t h a t b e d d i n g d i p p e d a t less than

L1mIII

a 1-

1M M

_,.,I 0 Y

“tlC” W I Y W

s-

-l.il

-

M

wI0-u

Fig.4.5. Composite model o f a p e b b l y b r a i d e d r i v e r d e p o s i t b a s e d on t h e Donjek R i v e r i n Alaska ( a f t e r Williams and R u s t , 1969).

187 3 O i n b o t h upstream and downstream d i r e c t i o n and s u g g e s t e d t h a t t h e b a r s u r f a c e i s

the preferred depositional site.

The consequence of t h i s i s t h a t l o n g i t u d i n a l b a r s

s h o u l d m i g r a t e upstream a l t h o u g h t h i s h a s n e v e r been c o n c l u s i v e l y demonstrated because o f t h e obvious p r a c t i c a l d i f f i c u l t i e s , S t u d i e s o f t h e Donjek R i v e r i n Alaska by Williams a n d R u s t (1969) and R u s t (1972) show t h a t s a n d s a r e d e p o s i t e d on t h e t o p and f l a n k s of g r a v e l b a r s and a l s o i n adjacent channels.

These show c r o s s - s t r a t i f i c a t i o n of v a r i o u s t y p e s which i s

produced a s s a n d i s s e g r e g a t e d from g r a v e l d u r i n g lower flow s t a g e s .

Falling

w a t e r l e v e l s r e s u l t i n flow s e p a r a t i o n around t h e g r a v e l b a r and t h e e x i s t i n g a c c r e t i o n topography w i t h t h e r e s u l t t h a t c r o s s s t r a t i f i c a t i o n t r a n s p o r t v e c t o r s a r e v e r y v a r i a b l e and show g r e a t e r d i s p e r s i o n t h a n t r a n s p o r t v e c t o r s s u c h a s i m b r i c a t i o n and l i n e a t i o n which a r e produced d u r i n g h i g h flow s t a g e s (Bluck, 1974). The o v e r a l l e f f e c t i s t h a t g r a v e l b a r d e p o s i t s g e n e r a l l y show fining-upwards sequences ( F i g . 4 . 6 ) which a r e i n d i c a t i v e of waning flow s t a g e .

r

P a r a l l e l laminated

c l a y s and s i l t y c l a y s may r e s u l t from s u s p e n s i o n d e p o s i t i o n i n i s o l a t e d p o o l s which o f t e n form i n abandoned c h a n n e l s .

VERY FINE SANDS, SILTS AND CLAYS WITH HORIZONTAL LAMINATIONS

1.511 FINE-MEDIUM SANDS WITH SMALL SCALE CROSS-BEDDING

MASSIVE AND CRUDELY STRATIFIED GRAVELS

Fining-upwards sequence o f a c h a n n e l - f i l l i n a p e b b l y b r a i d e d r i v e r Fig.4.6. ( b a s e d on W i l l i a m s and R u s t , 1969).

188 Bars i n curved channel r e a c h e s Bars which a r e a t t a c h e d t o e i t h e r bank of a c u r v e d r e a c h u s u a l l y develop a s e x t e n s i o n s of t h e f l a n k s o f l a r g e r l o n g i t u d i n a l b a r s .

They a r e c h a r a c t e r i z e d by

margins which a r e s t r o n g l y o b l i q u e t o t h e main c h a n n e l t r e n d ( F i g . 4 . 7 ) and a r e o f t e n referred t o a s diagonal bars.

The upstream end o f t h e b a r may b e a t t a c h e d t o t h e

bank and t h i s c a u s e s t h e flow t o b e c o n c e n t r a t e d i n a c h a n n e l on t h e o u t s i d e which

i s r e f e r r e d t o a s a r i f f l e reach.

The r i f f l e r e a c h ends a t a s t e e p e r r i f f l e f a c e

which e x t e n d s a c r o s s t h e c h a n n e l a t a n a c u t e a n g l e . The i n t e r n a l s t r u c t u r e o f d i a g o n a l b a r s i s v e r y s i m i l a r t o t h a t o f l o n g i t u d i n a l b a r s c o n s i s t i n g of p a r a l l e l s i d e d s h e e t s of g r a v e l which may be i m b r i c a t e d o r Newly emerging bar

Small deltas 2 1

I

Fig.4.7. Morphology and s t r u c t u r e of a t y p i c a l d i a g o n a l b a r ( l ) , ( 2 ) and ( 3 ) a r e i n d i v i d u a l u n i t s of t h e b a r where (1) i s t h e newly emerging b a r which w i l l e v e n t u a l l y a t t a c h i t s e l f t o ( 2 ) ( a f t e r B l u c k , 1974). structureless.

These g r a v e l s t e n d t o p a s s i n t o cross-bedded s a n d s downstream and

may p a s s upwards i n t o r i p p l e bedded s a n d s d e p o s i t e d under waning f l o o d c o n d i t i o n s . F i n e r sediments may a l s o accumulate i n s l o u g h c h a n n e l s which a r e abandoned a t low water stages.

P r o j e c t i o n s of t h e r i f f l e f a c e i n t o t h e s l o u g h c h a n n e l s i n t h e form

of s m a l l sand l o b e s and d e l t a s a l s o r e f l e c t waning flow due t o c h a n n e l m i g r a t i o n o r f a l l i n g discharge. Diagonal b a r s accumulate by t h e l a t e r a l m i g r a t i o n o f t h e r i f f l e r e a c h w i t h t h e a c c r e t i o n of t h e newly formed b a r o n t o a n o l d e r b a r ,

This r e s u l t s i n t h e formation

o f a g r a v e l p l a t f o r m w i t h a n obvious a c c r e t i o n topography ( F i g . 4 . 7 ) .

189 Sedimentary o r g a n i z a t i o n o f 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 Pebbly b r a i d e d r i v e r s of g l a c i a l outwash a r e a s f o m l a r g e f a n s which show s y s t e m a t i c downstream c h a n g e s ,

The s l o p e of t h e f a n s u r f a c e and maximum c l a s t

s i z e both d e c r e a s e downstream a n d t h e r e a r e a s s o c i a t e d changes i n c h a n n e l and b a r morphology (Boothroyd, 1 9 7 2 ) ( F i g . 4 . 8 ) .

Diagonal b a r s may become more i m p o r t a n t

downstream and be a s s o c i a t e d w i t h a n i n c r e a s e d p r o p o r t i o n o f sand and hence c r o s s bedding. I n all, t h e d e p o s i t s o f p e b b l y b r a i d e d s t r e a m s w i l l t e n d t o show r a p i d l a t e r a l and v e r t i c a l f a c i e s changes. and g r a v e l s .

The main u n i t s w i l l c o n s i s t o f e r o s i v e l y based sands

The g r a v e l s w i l l b e m a s s i v e , o r w i t h c r u d e h o r i z o n t a l s t r a t i f i c a t i o n These w i l l p a s s

and may show well-developed i m b r i c a t i o n and fining-upwards.

l a t e r a l l y o r v e r t i c a l l y i n t o l e n t i c u l a r s a n d s which may show t a b u l a r c r o s s - b e d d i n g O c c a s i o n a l s i l t s and f i n e r g r a i n e d sediments w i l l

with reactivation surfaces.

i n d i c a t e t h e o c c u r r e n c e of s u s p e n s i o n d e p o s i t i o n i n p o o l s and abandoned c h a n n e l s .

\

Coarse gravel-> l5cm

a

0 Modium gravol-5-15cm

Fine gravel-< 5cm Mane-boddod sand Cross-boddod (largo-

P , iscale)sand

Rippled sand

Fin.-growl 5

South

4

3

II Medium-gradmidfan I

midlan 2

1

Distance

BP

1

km

2

3

4

5

6

Nor

Fig.4.8. Changes i n b a r t y p e s l o p e and s t r u c t u r e a l o n g a b r a i d e d outwash f a n ( a f t e r Boothroyd, 1 9 7 2 ) .

190 SANDY LOW-SINUOSITY RIVERS Sandy l o w - s i n u o s i t y r i v e r s a r e c h a r a c t e r i z e d by t r a n s v e r s e b a r s ( O r e , 1963; Smith, 1971b) and r e p r e s e n t t h e i n t e r m e d i a t e s t a g e between p e b b l y b r a i d e d r i v e r s and meandering r i v e r s .

They may show a b r a i d e d p a t t e r n b e c a u s e o f t h e development

of mid-channel b a r s o r show a non-braided c h a n n e l i n which t h e major thalweg g e n t l y meanders between a l t e r n a t i n g b a n k - a t t a c h e d b a r s .

The c h a n n e l p a t t e r n commonly

changes w i t h flow s t a g e becoming i n c r e a s i n g l y b r a i d e d a t low d i s c h a r g e a s t h e v a r i o u s bedforms became exposed and e r o d e d .

The c a u s e s o f b r a i d i n g i n t h e s e sandy low-

s i n u o s i t y r i v e r s a r e a g a i n n o t p r e c i s e l y known b u t may i n c l u d e a r e l a t i v e l y h i g h bed-load and r e l a t i v e l y low suspended-load.

Rapid l a t e r a l m i g r a t i o n i s t h e r e f o r e a

t y p i c a l f e a t u r e ( e . g . Gole and C h i t a l e , 1966). The bedforms of sandy l o w - s i n u o s i t y r i v e r s show c o n s i d e r a b l e v a r i a t i o n s and t h e r e i s no u n i v e r s a l l y a p p l i c a b l e c l a s s i f i c a t i o n scheme.

Detailed descriptions i n

i n d i v i d u a l r i v e r s i n c l u d e Coleman (1969) (Brahmaputra), Smith (1970) (South P l a t t e ) , C o l l i n s o n (1970) (Tana) and Cant and Walker (1978) (South Saskatchewan). The most d e t a i l e d d e s c r i p t i o n i s t h a t of Coleman (1969) f o r t h e Brahmaputra R i v e r . T h i s i s a b r a i d e d r i v e r w i t h a bed-load predominantly o f f i n e s a n d and w i t h a h i g h sediment d i s c h a r g e .

The p l a n view o f t h e c h a n n e l b a r s i s g e n e r a l l y diamond shaped

w i t h t h e long a x i s p a r a l l e l t o t h e a v e r a g e flow d i r e c t i o n i n t h e c h a n n e l of f o r m a t i o n (Fig.4.9).

They c o n s i s t o f a n amalgamation o f v a r i o u s bedforms and t e n d t o form

when t h e oncoming sediment l o a d i s g r e a t e r t h a n t h e c a r r y i n g c a p a c i t y of t h e r i v e r . Channel beds thus show r a p i d a g g r a d a t i o n w i t h t h e c h a n n e l becoming w i d e r and s h a l l o w e r c a u s i n g t h e main c u r r e n t t o change c o u r s e i n t h e s e a r c h f o r b e t t e r g r a d i e n t s and l e s s resistence. migration.

The r e s u l t i s a b r a i d e d c h a n n e l which i s c o n s t a n t l y undergoing l a t e r a l Sand b a r n u c l e a t i o n o c c u r s d u r i n g f l o o d s i n a r e a s of s l a c k w a t e r which

A s a n d b a r on t h e Brahmaputra R i v e r showing t h e change i n p o s i t i o n b e f o r e Fig.4.9. and a f t e r f l o o d of 1952. The s o l i d l i n e shows t h e l o c a t i o n of t h e s a n d b a r d u r i n g low s t a g e o f 1952 whereas t h e d o t t e d l i n e shows p o s i t i o n o f t h e same b a r a f t e r p a s s a g e of t h e f l o o d , 1952 ( a f t e r Coleman, 1969).

191 form, f o r example, i n t h e a r e a between two a c t i v e s c o u r c h a n n e l s .

M i g r a t i o n of

c h a n n e l b a r s o c c u r s d u r i n g f l o o d i n g a n d i s m a i n l y due t o t h e movement of bedforms which c o v e r t h e downstream s l o p e s o f t h e b a r s .

These bedforms i n c l u d e r i p p l e s

( h e i g h t up t o 0.3m), m e g a r i p p l e s (up t o 1.5m), dunes (up t o 7.6m) and sandwaves (up t o 15.2m). The s e d i m e n t a t i o n u n i t s produced i n t h i s way b e g i n w i t h l a r g e s c a l e cross-bedding ( F i g . 4 . 1 0 ) w i t h i n d i v i d u a l s e t s up t o l m t h i c k and t h e whole u n i t up t o 7m t h i c k . These u n i t s a r e produced by t h e m i g r a t i o n of g i a n t r i p p l e s d u r i n g f l o o d .

There may

b e s c o u r f i l l u n i t s w i t h i r r e g u l a r lower bounding s u r f a c e s a s s o c i a t e d w i t h t h i s u n i t a s w e l l a s s m a l l r i p p l e bedding and muddy s e d i m e n t d e p o s i t e d a l o n g f o r e s e t laminae and bedding p l a n e s . c r o s s bedded u n i t .

Abundant o r g a n i c m a t e r i a l may a l s o be t r a p p e d i n t h i s

8, d

Hi&ly disturbed soil zone Clay and silt with horizontal hmination and convolutr bedding

Ripplr bdding and horizontal lamination

A E’

3

Mainly l a r p scale cross-bedding

Fig.4.10. I d e a l v e r t i c a l sequence o f t h e s e d i m e n t a r y s t r u c t u r e s of a Brahmaputra R i v e r c h a n n e l b a r based on d a t a from Coleman (1969). O v e r l y i n g t h e l a r g e s c a l e c r o s s bedding i s a zone of s i l t y s e d i m e n t s mainly comp r i s i n g l e n t i c u l a r b o d i e s w i t h c l i m b i n g r i p p l e c r o s s - l a m i n a t i o n and s m a l l - s c a l e r i p p l e bedding. a l s o occur.

I n t e r c a l a t e d l e n t i c u l a r b o d i e s w i t h l a r g e - s c a l e cross-bedding may

These u n i t s a r e d e p o s i t e d a t lower f l o w s t a g e when t h e l a r g e r bedforms

have s t o p p e d moving.

Above t h i s u n i t i s a zone o f s i l t y c l a y s and s i l t showing

p a r a l l e l laminations with occasional t h i n u n i t s of climbing r i p p l e cross-lamination r e s u l t i n g mainly from s u s p e n s i o n d e p o s i t i o n .

Post-depositional deformation s t r u c t u r e s

i n c l u d i n g l o a d s t r u c t u r e s and c o n v o l u t e bedding a r e abundant i n t h i s u n i t and i n t e r p r e t e d by Coleman (1969) a s b e i n g due t o i n c r e a s e d s h e a r s t r e s s caused by a n

192 i n c r e a s e i n c u r r e n t v e l o c i t y and a sudden r i s e i n t u r b u l e n c e .

Above i s a h i g h l y

d i s t u r b e d s o i l zone w i t h burrows and r o o t s . The r e s p o n s e o f c h a n n e l b a r s t o changes i n flow s t a g e i n b r a i d e d s t r e a m s i s w e l l i l l u s t r a t e d by C o l l i n s o n ' s ( 1 9 7 0 ) work on t h e Tana R i v e r i n Northern Norway.

The

r i v e r shows a v a r i e t y o f bedforms r a n g i n g from v e g e t a t e d i s l a n d s , s i d e b a r s , l i n g u o i d b a r s , dunes, and r i p p l e s .

The l i n g u o i d b a r s a r e t h e c h a r a c t e r i s t i c c h a n n e l

b a r and may be up t o 300m l o n g and 1 5 h wide.

S l i p f a c e s on t h e b a r s a r e up t o 2m

h i g h and t h e s t o s s s i d e may b e c o v e r e d by s m a l l e r bedforms. now c o n s i d e r e d t o b e sandwaves (Harms e t a l . ,

These l i n g u o i d b a r s a r e

1 9 7 5 ) which form when t h e d e p t h and

v e l o c i t y of flow a r e above t h o s e a p p r o p r i a t e f o r r i p p l e s b u t below t h o s e f o r dunes. M i g r a t i o n of t h e l i n g u o i d b a r s t a k e s p l a c e d u r i n g f l o o d and produces t a b u l a r c r o s s bedded u n i t s .

However, d u r i n g f a l l i n g flow s t a g e c u r r e n t s become c o n f i n e d t o t h e

t o p o g r a p h i c a l l y lower a r e a s and t h e f r o n t o f t h e b a r may become m o d i f i e d by e r o s i o n of f o r e s e t l a m i n a t i o n s and t h e f o r m a t i o n of c r o s s - l a m i n a t i o n by r i p p l e s m i g r a t i n g across the bar face.

Subsequent r i s e i n flow s t a g e r e s u l t s i n t h e renewed m i g r a t i o n

of t h e b a r and r e a c t i v a t i o n of t h e c r o 5 . s - s t r a t i f i c a t i o n ( F i g . 4 . 1 1 ) .

Hlgh water stage

---__-

lntermedlate water stage

Low water stage

B

A FIo* perpendicular

Rising water stage

A

The development of r e a c t i v a t i o n s u r f a c e s by changing w a t e r s t a g e o v e r a Fig.4.11. l i n g u o i d b a r ( a f t e r C o l l i n s o n , 1970).

193 The S o u t h Saskatchewan R i v e r i n Canada i s a sandy b r a i d e d s t r e a m c h a r a c t e r i z e d by l a r g e s a n d f l a t s (5Om t o 2km i n l e n g t h and 3Om t o 450m w i d e ) , s l i p face-bounded b a r s and d u n e - f i l l e d c h a n n e l s (Cant and Walker, 1978).

A t any one l o c a l i t y t h e r e

a r e commonly one o r two m a j o r c h a n n e l s a n d s e v e r a l minor c h a n n e l s .

The beds o f t h e

major c h a n n e l s a r e f i l l e d w i t h s i n u o u s - c r e s t e d dunes up t o 1.5m h i g h and i n p l a c e s sandwaves ( H a m s e t a l . , 19751 up t o 10m l o n g and 0.3m h i g h a r e common. b a r s l i k e t h o s e d e s c r i b e d by C o l l i n s o n (1970) a r e r a r e ,

Linguoid

The major d e p o s i t s of t h e

channels a r e trough cross-beds. Slipface-bounded b a r s a r e common i n t h e S o u t h Saskatchewan R i v e r and u s u a l l y o c c u r i n a r e a s of f l o w e x p a n s i o n , f o r example a t c h a n n e l j u n c t i o n s , o r where c h a n n e l s widen.

The l a r g e r b a r s a r e up t o 2.5m h i g h and may e x t e n d s e v e r a l hundred m e t r e s d i a g o n a l l y a c r o s s t h e m a j o r c h a n n e l s . These ' c r o s s - c h a n n e l b a r s ' a s t h e y a r e c a l l e d by Cant a n d Walker (1978) produce p l a n a r - t a b u l a r s e t s of c r o s s - b e d d i n g s i m i l a r t o t h o s e d e s c r i b e d by C o l l i n s o n (1970) and Smith (1970, 1972). The s a n d f l a t s a r e a r e a s up t o 2000m long a n d 450m wide which a r e a complex of s m a l l e r f e a t u r e s s i m i l a r t o t h e exposed a r e a s of s a n d on t h e Brahmaputra (Coleman, 1969) and t h e Tana ( C o l l i n s o n , 1970).

The sand f l a t s a r e covered d u r i n g f l o o d and

r e c e n t l y emerged s a n d f l a t s r e v e a l a v a r i e t y of b a r s , sandwaves and r i p p l e s .

The

n u c l e a t i o n and s u b s e q u e n t growth of a s a n d f l a t i s a p p a r e n t l y i n i t i a t e d by t h e emergence o f a c r o s s - c h a n n e l b a r .

A s flow c o n t i n u e s s e d i m e n t i s swept around t h i s

n u c l e u s and expanding flow around t h e downstream end r e s u l t s i n i n w a r d l y d i r e c t e d sediment t r a n s p o r t (Fig.4.12). The r e s u l t i s t h e development o f horns w i t h i n w a r d l y - d i r e c t e d s l i p f a c e s . Asymmetrical s a n d f l a t s form i n t h e same way e x c e p t t h a t t h e y o r i g i n a t e on d i a g o n a l c r o s s - c h a n n e l b a r s and a r e a t t a c h e d t o a s t a b l e bank ( F i g . 4 . 1 2 ) .

Subsequent growth

of t h e s m a l l s a n d f l a t i n t o a l a r g e , complex s a n d f l a t i n v o l v e s a g g r a d a t i o n by d r i v i n g s e v e r a l l a t e r g e n e r a t i o n s o f b a r s o n t o t h e s a n d f l a t from upstream or from a d j a c e n t channels.

I t a l s o i n v o l v e s ellosion o f t h e f l a t s and t h e l i n k i n g of s e p a r a t e sand-

f l a t s by d e p o s i t i o n i n i n t e r v e n i n g c h a n n e l s . The s t r a t i f i c a t i o n sequence o f t h e South Saskatchewan R i v e r i s summarized i n F i g s . 4 . 1 3 and 4.14.

These diagrams can be used t o p l a c e i n t o c o n t e x t t h r e e

i m p o r t a n t t y p e s o f s t r a t i f i c a t i o n sequence which a r e o f g e n e r a l a p p l i c a b i l i t y t o sandy-low-sinuosity streams (Fig.4.14). maximum s a n d f l a t development,

The f i r s t sequence (Fig.4.14A)

shows

Trough c r o s s - b e d s ( c h a n n e l dunes) a r e o v e r l a i n

by a l m t h i c k p l a n a r t a b u l a r s e t ( c r o s s - c h a n n e l b a r ) .

T h i s i s o v e r l a i n by s m a l l

p l a n a r t a b u l a r s e t s w i t h minor c h a n n e l s and r e a c t i v a t i o n s u r f a c e s and a l s o r i p p l e cross-lamination (sand f l a t sequence). The second sequence (Fig.4.14B)

i s an intermediate type with deposition both

i n c h a n n e l s and on sand f l a t s and t h e t h i r d sequence (Fig.4.14C)

i s a channel

dominated sequence w i t h c r o s s - c h a n n e l b a r s which d i d n o t develop i n t o s a n d f l a t s .

194 SYMMETRICAL CROSSCHANNEL BAR

CROSS-CHANNEL BAR NlTH EMERGENT NUCLEUS

SMALL SAND FLAT

1

EXAMPLES OF COMPLEX SAND FLATS

Fig.4.12. Development of s a n d f l a t s on t h e South Saskatchewan R i v e r . S t a g e 1, submerged c r o s s - c h a n n e l b a r w i t h s l i p f a c e . S t a g e 2 emergent n u c l e u s f o l l o w i n g a lowering o f r i v e r s t a g e . I f t h e o r i g i n a l c r o s s - c h a n n e l b a r i s d i a g o n a l t h e n u c l e u s w i l l b e a s y m m e t r i c a l ( 2 A ) o r a d j a c e n t t o a s t a b l e bank (2B). S t a g e 3 , expanding flow downstream c a u s e s t h e development o f h o r n s . S t a g e 4 d e p i c t s t h e f o r m a t i o n of complex sand f l a t s ( a f t e r Cant and Walker, 1978). These sequences a c t u a l l y resemble t h e l a t e r a l a c c r e t i o n p o i n t b a r sequences i n a meandering r i v e r which may c o n s i s t of t r o u g h c r o s s - b e d d i n g a n d p l a n a r t a b u l a r s e t s d e p o s i t e d by s c r o l l - b a r s ( J a c k s o n , 1976).

T h i s emphasizes t h e

d i f f i c u l t y o f d i s t i n g u i s h i n g p r e s e n t - d a y l o w - s i n u o s i t y and meandering r i v e r s on t h e b a s i s of s t r a t i g r a p h i c sequence.

The p r e s e n c e o f a s a n d f l a t s e q u e n c e w i t h

a p l a n a r t a b u l a r c r o s s - c h a n n e l b a r d e p o s i t a t t h e b a s e p r o v i d e s an i m p o r t a n t c l u e t o t h e former p r e s e n c e o f b r a i d e d c h a n n e l s . EPHEMERAL STREAM CHANNELS

Ephemeral s t r e a m s a r e p a r t i c u l a r l y c h a r a c t e r i s t i c o f a r i d and s e m i a r i d r e g i o n s where t h e s u r f a c e run-off is spasmodic because o f t h e c l i m a t i c c o n d i t i o n s .

The

196

Block diagram summarizing t h e main m o r p h o l o g i c a l e l e m e n t s and t h e i r Fig.4.13. d e p o s i t s i n t h e South Saskatchewan R i v e r . The i n s e t shows a p l a n view. S t i p p l e d a r e a s a r e emergent. S i n g l e s h a f t e d arrows show d i r e c t i o n o f bedform movement and double s h a f t e d a r r o w s i n d i c a t e flow d i r e c t i o n . A ( s a n d f l a t ) , B (mixed sand f l a t c h a n n e l ) and C ( c h a n n e l ) r e f e r t o t h e sequences i n Fig.4.14 ( a f t e r Cant and Walker, 1978). d e p o s i t s of ephemeral s t r e a m s show many of t h e f a m i l i a r s e d i m e n t a r y s t r u c t u r e s of permanent s t r e a m flow ( P i c a r d and High, 1973) and may b e d i f f i c u l t t o d i s t i n g u i s h from t h e s e d e p o s i t s .

F r e q u e n t l y , however, t h e y show i n t e r b e d d e d a e o l i a n sands

where s t r e a m c h a n n e l s have been choked by wind-blown sand.

Also wind e r o s i o n may

l e a d t o t h e f o r m a t i o n of a pebble-strewn d e f l a t i o n l a g on t h e former s t r e a m bed ( G l e n n i e , 1970) and c u r l e d mudflakes formed d u r i n g p e r i o d s of d e s i c c a t i o n may be i n c o r p o r a t e d i n t o i n t r a f o r m a t i o n a l conglomerates ( G l e n n i e , 1970; Karcz, 1972). When d i s c h a r g e i s a b n o r m a l l y h i g h s h e e t f l o o d i n g may o c c u r and c o v e r t h e whole v a l l e y f l o o r a s a t B i j o u Creek, Colorado (McKee e t a l . , 1967).

In this particular

c a s e a s h e e t d e p o s i t between l m and Lm t h i c k was d e p o s i t e d , t h e dominant s t r u c t u r e (90-95%) b e i n g upper flow regime p a r a l l e l l a m i n a t i o n s .

I n many f l o o d p l a i n s e c t i o n s

196

SAND FLAT

MIXED INFLUENCE

CHANNEL V.A.

V. A.

FINAL CHAN. FILL, MUD IN SLOUGH-

--- - - - -

SAND FLAT ACCRETION a AGORADATI0N

CONTINUED CHANNEL AGGRADATION IN PLACES WITH C-C BAR

-

CROSS CHANNEL

BAR

-

IN CHANNEL DEPOSITION

IN CHANNEC DEPOSITION

Fig.4.14. Summary s t r a t i g r a p h i c sequences c h a r a c t e r i z i n g a r e a s dominated by sand f l a t development, a r e a s o f mixed sand f l a t and channel i n f l u e n c e , and a r e a s o f channel a n a r a d a t i o n . The arrows i n d i c a t e t h e expected p a l a e o f l o w v a r i a b i l i t y ( a f t e r Cant and Walker, 1978). p a r a l l e l l a m i n a t i o n s form a l l o f t h e s h e e t f l o o d d e p o s i t b u t i n o t h e r s cross-bedding occurs i n t h e upper p a r t of t h e p r o f i l e , i n d i c a t i n g waning flow c o n d i t i o n s .

The

l a s t s t a g e s of t h e f l o o d a t B i j o u Creek were c o n f i n e d t o t h e main channel which c u t i n t o t h e f l o o d p l a i n sands and d e p o s i t e d t r o u g h c r o s s - b e d s w i t h some p a r a l l e l l a m i n a t i o n and p l a n a r cross-bedding

(Fig.4.15).

An i m p o r t a n t f e a t u r e of ephemeral s t r e a m d e p o s i t s a r e t h e p o s t - d e p o s i t i o n a l m o d i f i c a t i o n s i n t e x t u r e which occur. well-washed sands.

The i n i t i a l d e p o s i t s a r e commonly c l e a n ,

A t d e p t h , however, t h e amount of i n t e r s t i t i a l c l a y i n c r e a s e s

due t o t h e e f f e c t s of mechanical i n f i l t r a t i o n by i n f l u e n t seepage (Walker, 1976). The o v e r a l l e f f e c t i s t o p o s t - d e p o s i t i o n a l l y d e c r e a s e t h e s o r t i n g and t e x t u r a l m a t u r i t y of t h e d e p o s i t s .

197

CONVOLUTE

BEWING

MUD tAYER CLIMBING RIPPLE LAMINATION

CONVOLUTE

BEDDING

HORIZONTAL BEDDING (LAMINATED SAND )

-

LARGE SCALE CROSS- BEDDIEIIG (DELTA FORESETS)

HORIZONTAL BEDDING MUD LAYER

CLIMBING RIPPLE LAMINATION

TE U

0

1

HORl20 NTA L BE DOING (LAMINATED SAND 1

OLDER SEDtMENTS

OLDER

SEDIMENTS

F i g . 4 . 2 5 . F l o o d p l a i n sequences formed during a s i n g l e f l o o d of Bijou Creek (Colorado) i n June 1965 (based on McKee e t a l . , 1 9 6 7 ) .

198 MEANDERING RIVERS Introduction Meandering t e n d s t o b e f a v o u r e d by r e l a t i v e l y low s l o p e s and a h i g h suspendedload/bed-load r a t i o .

Using t h e c r i t e r i o n o f Leopold and Wolman (1957) meandering

channels a r e t h o s e w i t h a s i n u o s i t y o f more t h a n 1 . 5 .

I n f a c t a number o f o t h e r

measures of s i n u o s i t y have been used; f o r example B r i c e (1964) used t h e r a t i o : c h a n n e l l e n g t h / l e n g t h of meander b e l t a x i s ( F i g . 4 . 1 6 ) and Schumm (1963) used t h e s t r e a m l e n g t h / v a l l e y l e n g t h and t h e r e i s no g e n e r a l agreement on a n o b j e c t i v e

ratio:

d e f i n i t i o n of meandering.

There a r e some fundamental r e l a t i o n s h i p s between t h e

g e o m e t r i c f e a t u r e s of s t r e a m meanders.

The meander wave l e n g t h a n d a r e a o f t h e

d r a i n a g e b a s i n a r e r e l a t e d by: aAb

A=

(4.3)

i s meander wave l e n g t h , A i s b a s i n a r e a and a and b a r e e m p i r i c a l c o n s t a n t s .

where

S i n c e t h e d i s c h a r g e o f a r i v e r i s r e l a t e d t o i t s d r a i n a g e a r e a , d i s c h a r g e and wavelength a r e also related.

x = cwm

Also (4.4)

A,

= dWn

(4.5)

X

= F R ~

(4.6)

where

3

i s wavelength, W i s c h a n n e l w i d t h , Am i s wave a m p l i t u d e , R is t h e r a d i u s of

c u r v a t u r e , and c , d , f , m , n , and t a r e c o n s t a n t s .

The exponents i n t h e s e e q u a t i o n s

approximate t o u n i t y and t h e r e l a t i o n s h i p s a r e a l m o s t l i n e a r .

-. MEI!iNDER BELT AXIS

Fig.4.16.

Geometric f e a t u r e s o f a s t r e a m meander a c c o r d i n g t o B r i c e (1964).

199 Many e x p l a n a t i o n s have been o f f e r e d a s t h e b a s i c c a u s e of meandering and a number o f f a c t o r s i n c l u d i n g t h e n a t u r e o f t h e b e d - l o a d , g r a d i e n t and d i s c h a r g e may b e i m p o r t a n t .

The dominating f a c t o r i s , however, t n e h e l i c a l flow p a t t e r n

s e e n i n meanders (Fig.4.17).

Maximim flow v e l o c i t i e s a r e found on t h e concave

bank n e a r t n e downstream end and c r o s s from one bank t o t h e o t h e r .

T h i s winding

p a t t e r n produces a c e n t r i f u g a l f o r c e p r o d u c i n g a s u p e r e l e v a t i o n o f t h e w a t e r l e v e l a t t h e o u t s i d e o f t h e bend.

The i n c r e a s e d p r e s s u r e c a u s e d by t h e e x c e s s w e i g h t of

w a t e r p i l e d up i n t h i s way i n t e n s i f i e s t h e h e l i c a l flow a t t h e bend g i v i n g a s t r o n g downward movement on t h e o u t s i d e which c a u s e s e r o s i o n .

The v e l c o i t y components

i n c l u d e a s t r o n g downward component and a much weaker l a t e r a l component.

This

l a t e r a l component o f v e l o c i t y shows a movement a l o n g t h e bottom towards t h e i n n e r p a r t of t h e bend where d e p o s i t i o n o c c u r s ,

Flow mechanisms i n meander b e l t s a r e

d i s c u s s e d by Bagnold ( 1 9 6 0 ) , Leopold e t a l . (1964) and J a c k s o n (1975).

5 Fig.4.17.

The h e l i c o i d a l flow p a t t e r n i n meander bends.

200 An i m p o r t a n t f e a t u r e of meander b e l t s i s t h e l a r g e number o f abandoned c h a n n e l s which a r e seen.

These c h a n n e l c u t - o f f s form i n two main ways ( F i s k , 1947) (Fig.4.18).

The f i r s t a r e c h u t e c u t - o f f s which form d u r i n g f l o o d s t a g e s a s t h e r i v e r s t r a i g h t e n s i t s c o u r s e by e r o d i n g c h u t e s o r s w a l e s .

As a c t i v i t y i n t h e former c h a n n e l i s reduced

i t becomes plugged by bed-load s e d i m e n t s and t h e n f i l l e d w i t h s u s p e n s i o n d e p o s i t s

(Fig.4.18A).

Fig.4.18.

The mechanisms of c h a n n e l s h i f t i n g i n a meander b e l t ( a f t e r F i s k , 1947).

Neck c u t - o f f s a r e formed when concave-bank e r o s i o n b r i n g s two bends c l o s e t o g e t h e r w i t h only a narrow neck s e p a r a t i n g them.

I f t h i s neck is b r e a c h e d t h e r i v e r w i l l

r a p i d l y abandon t h e meander and t a k e t h e s h o r t e s t r o u t e downslope.

The r e s u l t i s

an oxbow l a k e (Fig.4.18B) which becomes f i l l e d w i t h s u s p e n s i o n - l o a d sediment. Point bars P o i n t b a r s a r e t h o s e a r e a s on t h e i n s i d e o f a meander bend which r e s u l t from t h e l a t e r a l a c c r e t i o n o f sediment d u r i n g c h a n n e l m i g r a t i o n .

The s t r u c t u r e o f p o i n t b a r

d e p o s i t s has been d e s c r i b e d by many a u t h o r s i n c l u d i n g F i s k (1944, 19471, F o l k and Ward ( 1 9 5 7 ) , F r a z i e r and Osanik ( 1 9 6 1 ) , Bernard and Major (19631, Harms e t a l . (19631, A l l e n (1964a, 1 9 6 5 a , e , 1 9 7 0 a , b ) , McGowen and Garner ( 1 9 7 0 ) , Bluck (1971) and J a c k s o n (1975, 1976).

The c l a s s i c a l c o n c e p t of p o i n t bar d e p o s i t i o n assumes b a n k f u l

d i s c h a r g e and f u l l y developed h e l i c o i d a l flow around t h e bend.

Decrease i n v e l o c i t y

and d e p t h and t h e upflow component of flow o v e r t h e p o i n t b a r s u r f a c e c a u s e bed s h e a r stress t o f a l l and t h e s u r f a c e a c t s a s a n e l u t r i a t o r g i v i n g a n upslope

201 r e d u c t i o n i n g r a i n s i z e (Fig.4.19).

Floodplain Channel-fill

I

@Cross-bedding

HFIat-bedding

Fig.4.19. I d e a l i z e d diagram o f p o i n t b a r d e p o s i t i o n i n a meandering s t r e a m ( a f t e r A l l e n , 1970a). The shape and s i z e o f p o i n t b a r s i s dependent upon t h e s i z e o f t h e r i v e r channel. I n small streams they a r e simple depositional f e a t u r e s b u t i n l a r g e r i v e r s they a r e more complex and may c o n t a i n a number o f s e p a r a t e u n i t s .

The s u r f a c e o f p o i n t b a r s

may show well-developed a c c r e t i o n topography ( F i s k , 1947; Sundborg, 1956) c o n s i s t i n g of s c r o l l - s h a p e d r i d g e s ( s c r o l l b a r s ) a l t e r n a t i n g w i t h d e p r e s s i o n s ( s w a l e s ) . may be f i l l e d w i t h muddy s e d i m e n t and marshes may develop i n them. represents the depositional event of a s i n g l e flood.

Swales

Each s c r o l l b a r

The t h i c k n e s s of p o i n t b a r

d e p o s i t s may c o r r e s p o n d t o t h e maximum d e p t h o f t h e r i v e r c h a n n e l .

On t h e M i s s i s s i p p i

p b i n t b a r s up t o 25m t h i c k a r e known ( F i s k , 1944, 1947); on t h e Brazos R i v e r , 15-2Om (Bernard and Major, 1963) and 10-15m on t h e Niger R i v e r ( A l l e n , 196513).

For s m a l l e r

r i v e r s t h e t h i c k n e s s may b e o n l y 1-3m. The s t r u c t u r e and g r a i n s i z e o f p o i n t b a r d e p o s i t s f r e q u e n t l y shows a d i s t i n c t i v e v e r t i c a l arrangement.

The c o a r s e s t m a t e r i a l may b e c o n g l o m e r a t i c sediment which i s

found a t t h e b a s e o f t h e p o i n t b a r and r e p r e s e n t s a c h a n n e l l a g d e p o s i t (Fig.4.19). I f a s u f f i c i e n t l y wide range o f g r a i n s i z e m a t e r i a l i s a v a i l a b l e t h e p o i n t b a r w i l l show a g r a d u a l upward d e c r e a s e i n g r a i n s i z e .

This i n d i c a t i o n of e f f e c t i v e decrease

i n s t r e a m power i s a l s o r e f l e c t e d i n t h e sequence o f s e d i m e n t a r y s t r u c t u r e s ;

flat-

bedding p a s s e s upwards i n t o c r o s s - b e d d i n g which i n t u r n g i v e s way t o c r o s s - l a m i n a t i o n i n the f i n e r grained sands and silts.

T h i s sequence i n d i c a t e s p r o g r e s s i v e l y

202

d e c r e a s i n g flow regime and t h e t o p of t h e p o i n t b a r sequence may b e capped by a v e n e e r of muddy sediment d e p o s i t e d from s u s p e n s i o n a s a s w a l e - f i l l .

This o v e r a l l

fining-upwards i n t h e p o i n t b a r sequence i s a f u n c t i o n o f t h e l a t e r a l m i g r a t i o n o f

As l a t e r a l d e p o s i t i o n b u i l d s up on t h e i n n e r b a n k , b a l a n c i n g

t h e r i v e r channel.

e r o s i o n on t h e o u t e r bank, t h e r e i s f o r any p o i n t on t h e s u r f a c e o f t h e p o i n t b a r a d e c r e a s e i n bed s h e a r s t r e s s and s t r e a m power a s t h e thalweg m i g r a t e s l a t e r a l l y away and so f i n e r g r a i n e d d e p o s i t s r e s u l t i n t h e v e r t i c a l sequence ( A l l e n , 1970a). D e t a i l e d d e s c r i p t i o n s o f p o i n t b a r sequences s u g g e s t t h a t t h i s i d e a l d e p o s i t i o n a l model i s o n l y approximated i n n a t u r a l systems. Maryland was s t u d i e d by Leopold e t a l .

The Watts Branch p o i n t b a r i n

(1964) o v e r a p e r i o d of s i x y e a r s d u r i n g

which t h e r i v e r c h a n n e l m i g r a t e d a d i s t a n c e o f more t h a n a c h a n n e l w i d t h and t h e n e t volume o f e r o s i o n and d e p o s i t i o n were e q u a l (Fig.4.20).

The p o i n t b a r shows

a n o v e r a l l fining-upwards b u t g r a v e l l y h o r i z o n s a r e d i s t r i b u t e d a t v a r i o u s l e v e l s above t h e c h a n n e l b a s e and i n d i v i d u a l s e d i m e n t a t i o n u n i t s have a d i s c o n t i n u o u s , l e n t i c u l a r nature.

Y

f

-

33,5 -

w 31.1

1

,

,

0 0 5 1 2 DISTANCE IN METRE

5

L

SURFACE PROFILES OF MEANDER

SUCCESSIVE

Y

3

7

6

CHANNEL AND

8 POINT

9

I

10

11

BAR, 1953- 59

I

I 6,5

5,5 6 OlSrANCE IN METRE

LARGE SCALE CROSS-SECTION OF

SEDIMENTS IN

7

RELATION

GRAVEL BED OLIVE-GRAY

COARSE SAND BROWN STAINED WITH PEBBLES AND LENSES OF SILT BROWN SANOY SILT

TO PROFILES

-

1959 1958

----

1956

...-, .,-

19 5 5 19 53

FINE SAND WITH SOME SILT

Fig.4.20.

8,s

8

EXPLANAILON CLAYEY SILT WITH ORGANIC MATTER

ORANGE BROWN MATTLED SANDY SILT WITH SOME CLAY

0 0

7,s

The Watts Branch p o i n t b a r ( a f t e r Leopold e t a l , , 1964).

203 McGowen and G a r n e r (1970) i n v e s t i g a t e d c o a r s e g r a i n e d p o i n t b a r s o f t h e b i t e R i v e r , L o u i s i a n a and t h e Colorado R i v e r , Texas. (1.4 t o 1 . 7 ) bed-load

These r i v e r s a r e l o w - s i n u o s i t y

s t r e a m s w i t h a r a t h e r h i g h g r a d i e n t which t r a n s p o r t mainly

c o a r s e s a n d , and pebble-cobble s i z e d g r a v e l s .

The r i v e r s meander b e c a u s e of t h e

h i g h s t a b i l i t y o f t h e banks which a r e composed of v e g e t a t e d muddy sediment.

These

c o a r s e g r a i n e d p o i n t b a r s can b e d i v i d e d i n t o a lower a n d a n upper p o i n t b a r (Fig.4.21).

The upper p o i n t b a r i s c h a r a c t e r i z e d by c h u t e s and c h u t e b a r s e d i m e n t s .

CONVEX BANK

CONCAVE BANK

Parallel laminated to thin sand beds

-

rn

6

FLOOD STAGE

4

Parallel laminae

LOW WATER - - - - _ STAG

2 0 UPPER POINT BAR

(Chute and chute bar sediments)

0 10 I rn

Fig.4.21. 1970).

C r o s s - p r o f i l e o f a c o a r s e g r a i n e d p o i n t b a r ( a f t e r McGowen and G a r n e r ,

Chutes a r e c h a n n e l s w i t h r e l a t i v e l y s t e e p s i d e s , f l a t b o t t o m s , and s i n u o u s t r e n d s . They a r e d e e p e s t where t h e y meet t h e main c h a n n e l and d e c r e a s e i n d e p t h s t e a d i l y downcurrent where s c o u r s g i v e way t o d e p o s i t i o n i n t h e form of c h u t e b a r s . can b e 2-5m d e e p , 5-7m wide and e x t e n d f o r hundreds of m e t r e s .

These

The sequence of

sediments i n t h e lower p o i n t b a r i s s i m i l a r t o t h a t s e e n i n t h e c r o s s - o v e r ( t h e s t r a i g h t c h a n n e l r e a c h between two meander l o o p s ) ( F i g . 4 . 2 2 ) . The s t r u c t u r e of c o a r s e g r a i n e d p o i n t b a r s do n o t show t h e t y p i c a l fining-upwards

o f t h e i r f i n e r g r a i n e d e q u i v a l e n t s and t h e i r o r i g i n i s c l e a r l y r a t h e r more complicated than t h e s i m p l e l a t e r a l a c c r e t i o n model.

However, t h e t o p of t h e sequence may b e

t e r m i n a t e d by a n o r g a n i c r i c h c l a y (Fig.4.22) Unit 1

-

w i t h t h e f u l l sequence a s f o l l o w s :

trough cross-bedding (scour pool d e p o s i t ) , Unit 2

-

small s c a l e f o r e s e t

and t r o u g h f i l l c r o s s - b e d d i n g (lower p o i n t b a r / c r o s s - o v e r d e p o s i t ) , U n i t 3 s c a l e f o r e s e t cross-bedding (chute bar d e p o s i t s ) , a n d Unit 4

-

-

large

p a r a l l e l laminae,

s m a l l s c a l e f o r e s e t c r o s s - b e d d i n g and s c o u r and f i l l d e p o s i t s ( f l o o d p l a i n d e p o s i t s ) . V a r i a t i o n s from t h e i d e a l i z e d p o i n t b a r model i n n a t u r a l systems a r e p a r t l y e x p l a i n e d by J a c k s o n (1975, 1976) who has shown t h a t t h e h e l i c o i d a l flow p a t t e r n i n meander bends can b e d i v i d e d i n t o t h r e e zones: i n f l u e n c e o f t h e upstream bend p r e v a i l s bend dominates and

1) a t r a n s i t i o n zone i n which t h e

2 ) a f u l l y developed zone i n which t h e l o c a l

3 ) a n i n t e r m e d i a t e zone i n which one changes i n t o t h e o t h e r .

204

These downstream changes i n flow p a t t e r n a r e r e f l e c t e d i n t h e bedforms produced on t h e s u r f a c e of t h e p o i n t b a r and hence i n t h e s t r a t i g r a p h i c sequence of s t r a t i f i c a t i o n types (Fig.4.23).

Floodplain Deposit

t

Unit 3 Chub Bar Dsporit

Unit 2 Lower Point Bar Deposit

-

Unit 1 Scwr Pool Deposit

Fig.4.22. V e r t i c a l sequence i n a c o a r s e g r a i n e d p o i n t b a r d e p o s i t ( a f t e r McGowen and G a r n e r , 1970). The f u l l y developed zone i s c o n s i s t e n t w i t h t h e i d e a l model of p o i n t b a r d e p o s i t i o n b u t a l s o shows p l a n a r t a b u l a r s e t s produced by s c r o l l b a r s .

I n the

t r a n s i t i o n and i n t e r m e d i a t e zones t h e r e i s no g r a d u a l fining-upwards and g r a i n s i z e v a r i a t i o n s t e n d t o b e r a t h e r a b r u p t , i n p l a c e s even coarsening-upwards, RIVER BANKS AND FLOODPLAINS

Bank d e u o s i t s The most i m p o r t a n t r i v e r bank d e p o s i t s a r e n a t u r a l l e v e e s and c r e v a s s e s p l y

.

They may b e developed on e i t h e r b r a i d e d o r meandering s t r e a m s b u t a r e b e s t developed on l a r g e meandering s t r e a m s w i t h a well-developed

floodplain l i k e the Mississippi

R i v e r (Fig.4.24).

N a t u r a l l e v e e s a r e formed when t h e f l o o d w a t e r s o f a s t r e a m o v e r t o p s i t s bank. V e l o c i t y i s reduced and d e p o s i t i o n o f t h e suspended s e d i m e n t r e s u l t s .

The c o a r s e s t

m a t e r i a l i s d e p o s i t e d n e a r e s t t h e c h a n n e l and g r a i n s i z e d e c r e a s e s away from t h e channel.

The s t r u c t u r e of n a t u r a l l e v e e d e p o s i t s h a s been d e s c r i b e d by F i s k (1944,

1 9 6 1 ) , Lattman (19601, A l l e n ( 1 9 6 5 a ) , Coleman (19691, and S i n g h (1972).

I n many

ways levee s e d i m e n t s resemble t h o s e found i n t h e upper p a r t of p o i n t b a r s .

They may

b e f i n e r g r a i n e d t h a n t h e a s s o c i a t e d c h a n n e l s e d i m e n t s a n d c o n s i s t o f s m a l l scale

205 TRANSITIONAL DEPOSITIONAL FACIES .-.-.-‘niw

3

2

+

FLOW .+.+.+‘LOW FLOW XRRENT IISPERSION IOSS’ 50’ 100‘ TRAT. U

DEPOSITIONAL FACIES

-*

-.WIGH” FLOW + ++‘LOW’ FLOW DUNE HEIGHT (CMI 0 25 50 75 I

T

I

I

0

0TRANSITIONAL INTERMEDIATE

HELM-MAIER

FULLY DEVELOPED

INTERMEDIATE DEPOSITIONAL FACIES

g

I

-. -.

-. _.

-

CURRENT

_-_

‘HIGH’ FLOW FLOW

+ + +‘LOW’

CROSS- W N E HElGHT EM1 5s IW. ITRAT. 0 25 XI I5

ISPERSION

II I.I I n@ I I

MI

Y SIZE

wni)

MEAN VEL. (CM*SECI ‘HIGH‘ FLOW -‘LOW’ FLOW

-

FULLY DEVELOPED DEPOSITIONAL FACIES t_l

I:

.: .:..:I

y ,

,

,

- 3 0

3

E

MEAN S I Z E (F

F i g . 4 . 2 3 . The v a r i a t i o n of d e p o s i t i o n a l f a c i e s i n the Lower Wabash River, I l l i n o i s . The i n s e t shows the d i s t r i b u t i o n of f a c i e s w i t h i n c r e a s i n g channel curvature. V e r t i c a l f a c i e s sequences a r e shown from upstream t o downstream on a p o i n t bar Of s u i t a b l e curvature ( a f t e r Jackson, 1975, 1 9 7 6 ) .

206

r i p p l e bedded and c r o s s - l a m i n a t e d s a n d s w i t h i n t e r b e d d e d muddy l a y e r s w i t h p a r a l l e l laminations,

I d e a l l y t h e s e a r e a r r a n g e d i n r e p e t i t i o n s of t h i n fining-upwards

sequences which mark t h e r e p e a t e d submergence and emergence d u r i n g s u c c e s s i v e f l o o d s (Fig.4.25).

The sandy and muddy l a y e r s a r e of v a r i a b l e t h i c k n e s s .

The mud h o r i z o n s

o f t e n show e v i d e n c e of s u b a e r i a l e x p o s u r e i n t h e form of d e s i c c a t i o n c r a c k s , o r raindrop impressions.

Organic m a t t e r i s abundant i n t h e s e h o r i z o n s b e c a u s e l e v e e s

may s u p p o r t much p l a n t growth.

They may a l s o b e e x t e n s i v e l y m o t t l e d due t o t h e

development of s o i l h o r i z o n s . 9ML M 2 O N LAMINATE0 MUD LAYER

SMALL RIPPLE CROSS BEDMND

MUD LAVER IFINELY LAYNATED I SMALL RIPPLE CROSS M O M N O MUD

LAYER

SMALL RIPPLE

CROSS B E D D M

LAMINAILD Y N D BEDDlHo I

IWRIZDNTAL

SMALL R I P R E

CROSS B E D D I M

Fig.4.24. N a t u r a l l e v e e s a n d r e l a t e d f e a t u r e s o f a meander o f t h e M i s s i s s i p p i R i v e r ( a f t e r F i s k , 1947).

Fig.4.25. Repeated c y c l e s of fining-upward sequences i n t h e n a t u r a l l e v e e d e p o s i t s of t h e Gomti R i v e r , I n d i a ( a f t e r Reineck and S i n g h , 1973).

207

The n a t u r a l levees of t h e Brahmaputra a r e r a t h e r broad and form a n i r r e g u l a r extension i n t o t h e flood b a s i n (Fig.4.26).

Most o f t h e f l o o d w a t e r e s c a p e s from

t h e r i v e r through d i s t i n c t c h a n n e l s r a t h e r t h a n a s s h e e t - l i k e o v e r f l o w s .

The bed

l o a d sediment o f t h e f l o o d w a t e r i s t h e n d e p o s i t e d a s a tongue o r f a n of s a n d c a l l e d a crevasse-splay.

The o v e r l a p p i n g o f c r e v a s s e s p l a y s produces t h e l e v e e s which

c o n s i s t of a complex o f i ' n t e r f i n g e r i n g and o v e r l a p p i n g sandy d e p o s i t s capped by muddy s e d i m e n t ,

A. A s i n g l e c r e v a s s e s p l a y on t h e Brahmaputra B. N a t u r a l l e v e e s formed Fig.4.26. by o v e r l a p p i n g c r e v a s s e - s p l a y s on t h e Brahmaputra ( a f t e r Coleman, 1969).

The s a n d l a y e r s may show l a r g e s c a l e c r o s s - b e d d i n g , h o r i z o n t a l - b e d d i n g and c r o s s l a m i n a t i o n (Fig.4.27).

Away from t h e c h a n n e l t h e sandy l a y e r s which o u t and muddy

sediments become more i m p o r t a n t , g r a d i n g l a t e r a l l y i n t o t h e f l o o d p l a i n sediments. Penecontemporaneous d e f o r m a t i o n s t r u c t u r e s may b e common. Floodplains R i v e r f l o o d p l a i n s a r e r a r e l y i n u n d a t e d and t h e most f r e q u e n t f l o o d s a r e g e n e r a l l y one t o two y e a r s a p a r t (Wolman and Leopold, 1957).

The v e l o c i t i e s o f overbank flows

a r e u s u a l l y r a t h e r h i g h and t h e o v e r a l l s e d i m e n t a t i o n r a t e i s low.

The c o a r s e r sandy

sediment i s u s u a l l y d e p o s i t e d n e a r t o t h e c h a n n e l margins w h i l s t f i n e r m a t e r i a l i s c a r r i e d f u r t h e r on t o t h e f l o o d p l a i n a r e a .

These f i n e r s e d i m e n t s a r e d e p o s i t e d from

.suspension and even d u r i n g m a j o r f l o o d s o n l y accumulate t o a t h i c k n e s s of one o r two centimetres.

The i n t e r n a l s t r u c t u r e o f f l o o d p l a i n d e p o s i t s i n c l u d e s h o r i z o n t a l

l a m i n a t i o n and c l i m b i n g r i p p l e c r o s s - l a m i n a t i o n (McKee e t a l . ,

1967) and i n some

s m a l l r i v e r s i t may b e d i f f i c u l t t o d i f f e r e n t i a t e t h e f l o o d p l a i n and c h a n n e l d e p o s i t s . G e n e r a l l y , however, t h e f i n e g r a i n s i z e and preponderence of h o r i z o n t a l l a m i n a t i o n s make f l o o d p l a i n and c h a n n e l d e p o s i t s r e a d i l y d i s t i n g u i s h a b l e .

The d r y i n g o u t of

f l o o d p l a i n s e d i m e n t s produces d e s i c c a t i o n c r a c k s which i n d i c a t e s u b a e r i a l exposure.

208

I n some a r e a s a e o l i a n a c t i v i t y i s i m p o r t a n t and wind-blown s i l t may accumulate i n l a r g e q u a n t i t i e s on r i v e r f l o o d p l a i n s (Lambrick, 1967).

I

LAMINATED SILTY CLAYS WITH BURROWS

HORIZONTAL BEDDING 2m

LARGE SCALE CROSS BEDDING

SAND AND SILT NO INTERNAL STRUCTURE EXCEPT

Fig.4.27. Sequence of sedimentary s t r u c t u r e s i n t h e n a t u r a l l e v e e d e p o s i t s o f t h e Brahmaputra (based on Coleman, 1969). The l o w e s t - l y i n g p a r t o f a f l o o d p l a i n i s g e n e r a l l y r e f e r r e d t o a s t h e f l o o d b a s i n . This i s a f l a t , f e a t u r e l e s s a r e a which i n humid-climate a r e a s i s p o o r l y d r a i n e d and thickly vegetated.

Semi-permanent

l a k e s o r backswamps may develop.

The abundant

v e g e t a t i o n c a u s e s r o o t d i s t u r b a n c e of sediment and s o i l p r o f i l e s may develop.

Under

f a v o u r a b l e c i r c u m s t a n c e s p e a t may accumulate. Interfluvial areas S o i l f o r m a t i o n i n i n t e r f l w i a l a r e a s i s a n i m p o r t a n t p r o c e s s which can be recognized i n a n c i e n t a l l u v i a l sequences.

A t t h e p r e s e n t t i m e one of t h e moat

i m p o r t a n t c o n t r o l s o f s o i l f o r m a t i o n i s c l i m a t e and t h e s i g n i f i c a n c e of a n c i e n t s o i l s is t h e i r p o t e n t i a l value a s palaeoclimatic indicators. S o i l s may develop a s r e c e n t l y d e p o s i t e d sediments o r p o o r l y c o n s o l i d a t e d bedrock and can be regarded a s a n amalgam of t h e l i t h o s p h e r e , t h e h y d r o s p h e r e , t h e b i o s p h e r e and t h e atmosphere.

They g e n e r a l l y c o n s i s t of d i s t i n c t l a y e r s , u s u a l l y known as

h o r i z o n s which t o g e t h e r form a s o i l p r o f i l e .

The n a t u r e o f t h e v a r i o u s h o r i z o n s

and t h e i r p o s i t i o n i n t h e s o i l p r o f i l e depends upon t h e n a t u r e of t h e s u b s t r a t e and a l s o on c l i m a t i c f a c t o r s .

The changes o f s o i l - t y p e which accompany changes i n

J

209 c l i m a t e and v e g e t a t i o n a r e i l l u s t r a t e d i n F i g . 4 . 2 8 .

Some s o i l s may t a k e hundreds

of thousands o r even m i l l i o n s of y e a r s t o develop and may have w i t n e s s e d a p p r e c i a b l e c l i m a t i c change.

C o n s i d e r a b l e c a r e i s t h e r e f o r e needed i n a s s e s s i n g t h e importance

of c l i m a t e i n b o t h p r e s e n t - d a y and a n c i e n t s o i l s .

wet. cold

Dry, hot Desert soils Desert shrubs

Brown SOIIS

Chestnut soils

Charnorem soils

P d d mils

SOllS

Broadleaf forest

Conifer forest

Tundra

SOllS

Tall

Short grass

Grey brown podzolic sals

Prairie

Tundra

Limit of soil water I

I

I

I I

I

I I

I I

Alkaline soils, closed system 8

Carbonates removed

Neutral I I

soils

I I

Acid soils open system

Fig.4.28. Changes i n s o i l p r o f i l e s which accompany changes i n c l i m a t e and v e g e t a t i o n between t h e Canadian Tundra and t h e d e s e r t s of s o u t h w e s t e r n USA. A t t h e 1 0 0 t h m e r i d i a n t h e a n n u a l p r e c i p i t a t i o n a v e r a g e s a b o u t 50cm ( a f t e r Hunt, 1 9 7 2 ) . The m a j o r i t y o f p r e s e n t - d a y s o i l s have a n o r g a n i c r i c h upper l a y e r which g r a d e s downwards i n t o more m i n e r a l dominated l a y e r s .

The p r e s e r v a t i o n p o t e n t i a l of t h i s

o r g a n i c l a y e r i s g e n e r a l l y low e x c e p t i n swamps and p e a t bogs where t h e w a t e r t a b l e

i s g e n e r a l l y h i g h and t h e d r a i n a g e v e r y p o o r .

Organic m a t t e r d o e s , however, have a

s t r o n g i n f l u e n c e on t h e r e s t o f t h e s o i l p r o f i l e by changing t h e composition o f downward p e r c o l a t i n g r a i n w a t e r .

The a c i d i t y of r a i n w a t e r may b e i n c r e a s e d by thee

a d d i t i o n of v a r i o u s o r g a n i c a c i d s and a s i t moves down through t h e s o i l p r o f i l e i t removes a l k a l i and a l k a l i e a r t h i o n s i n s o l u t i o n and a i d s t h e h y d r o l y s i s of s i l i c a t e g r a i n s and t h e f o r m a t i o n of c l a y m i n e r a l s .

Downward p e r c o l a t i o n may b e improved by

t h e r o o t p e n e t r a t i o n and o t h e r a c t i v i t i e s o f growing p l a n t s .

A familiar ancient

example o f s o i l f o r m a t i o n i n t h i s way i s p r o v i d e d by t h e s e a t e a r t h s below c o a l seams s e e n i n t h e Upper P a l a e o z o i c Coal Measures. which a r e o f t e n e n r i c h e d i n k a o l i n i t e .

These a r e r o o t p e n e t r a t e d s o i l s

Comparison w i t h p r e s e n t - d a y p e a t swamps

s u g g e s t s t h a t t h e k a o l i n i t e formed i n s i t u due t o l e a c h i n g by humic a c i d s which p e r c o l a t e d downwards from t h e o v e r l y i n g p e a t ( S t a u b a n d Cohen, 1978).

210 A t t h e p r e s e n t - d a y i n c e r t a i n t r o p i c a l r e g i o n s t h e r e a r e s o i l s forming which have These a r e t h e d u r i c r u s t s (Langford-

been a major i n f l u e n c e i n t h e r e d bed problem.

Smith and Dury, 1965; Goudie, 1973) which i n c l u d e c a l c r e t e , f e r r i c r e t e , and silcrete.

These may form v e r y t h i c k p r o f i l e s , i n some c a s e s up t o lOOm t h i c k , and

a r e c h a r a c t e r i z e d by a t h i n n e r i n d u r a t e d top-zone o r hardpan.

A t the present t i m e

d u r i c r u s t s show a c l o s e r e l a t i o n s h i p w i t h c l i m a t e and form i n a r i d o r s e m i - a r i d l r e g i o n s of s t a b l e geomorphic l e v e l s a l t h o u g h many o t h e r f a c t o r s i n c l u d i n g bedrock lithology a r e a l s o important. I n many p a r t s o f t h e world t h e 50Omm a n n u a l i s o h y e t seems t o be a s i g n i f i c a n t d i v i d i n g l i n e between c a l c r e t e and o t h e r t y p e s of d u r i c r u s t .

The f o r m a t i o n of

c a l c r e t e i s g e n e r a l l y r e g a r d e d t o t a k e p l a c e by a p e r descensum mechanism.(Goudie,

1973, p.136) i n which c a r b o n a t e i s l e a c h e d from t h e upper s o i l h o r i z o n s by downward p e r c o l a t i n g r a i n w a t e r and i s p r e c i p i t a t e d a t d e p t h a s c a l c i t e o r high-magnesian c a l c i t e during dry periods.

I t i s obvious t h a t t h e a c c u m u l a t i o n s o f any s u b s t a n t i a l

t h i c k n e s s o f c a l c r e t e r e q u i r e s t h e a d d i t i o n of c a r b o n a t e t o t h e s o i l s u r f a c e .

Much

of t h i s c a r b o n a t e i s b e l i e v e d t o b e d e r i v e d from a e o l i a n d u s t s which a r e i m p o r t a n t i n s e m i a r i d r e g i o n s b o r d e r i n g d e s e r t a r e a s (Goudie, 1973, T a b l e 4.1; Yaalon, 1973). S i l c r e t e s a r e c h a r a c t e r i s t i c of a r i d r e g i o n s b u t have n o t been so e x t e n s i v e l y s t u d i e d a s c a l c r e t e s , n o r a r e t h e y so abundant i n t h e s t r a t i g r a p h i c a l r e c o r d .

The

t h i c k e r s i l c r e t e p r o f i l e s i n modern d e s e r t s a r e g e n e r a l l y r e g a r d e d a s p a l a e o s o l s ( e . g . Mabbutt, 1967).

S i l c r e t e s show b r o a d l y s i m i l a r f e a t u r e s t o c a l c r e t e s and may

a l s o o r i g i n a t e by a p e r descensum mechanism.

The s i l i c a i s i n t h e form o f q u a r t z ,

c h a l c e d o n y , o r o p a l and may o c c a s i o n a l l y show o p t i c a l c o n t i n u i t y w i t h h o s t q u a r t z grains.

The s o u r c e o f t h e s i l i c a and i t s p r e c i s e r o l e i n s i l c r e t e f o r m a t i o n i s ,

however, v e r y p o o r l y known.

Bedrock seems t o b e a n i m p o r t a n t f e a t o r because i n t h e

w e s t e r n USA v o l c a n i c r o c k s seem t o r a p i d l y develop s o i l s w i t h well-developed s i l i c a cemented h o r i z o n s , whereas a l l u v i a l s o i l s d e r i v e d from g r a n i t i c r o c k s form s i l i c a cemented h o r i z o n s , o n l y v e r y s l o w l y ( F l a c h e t a l . , 1969). of s i l i c a may b e p r o v i d e d by d e s e r t g r a s s e s .

Another i m p o r t a n t s o u r c e

These a r e known t o have e x c e p t i o n a l l y

h i g h s i l i c a c o n t e n t s and s i l i c a i s cormnonly t h e most i m p o r t a n t element i n c i r c u l a t i o n i n t h e d e s e r t ecosystem (Rodin and B a z i l e v i c h , 1965).

A r i d s o i l s i n New S o u t h Wales

c o n t a i n well-rounded s i l t - s i z e d p a r t i c l e s v e r y s i m i l a r t o t h o s e found i n t h e l e a v e s of l o c a l g r a s s e s ( H a l l s w o r t h and Waring, 1964).

Such p a r t i c l e s c o u l d c o n c e i v a b l y

be t r a n s p o r t e d by wind t o b e s u b s e q u e n t l y m o b i l i z e d , presumably under a l k a l i n e c o n d i t i o n s , and d e p o s i t e d d e e p e r i n t h e s o i l p r o f i l e . F e r r i c r e t e o r l a t e r i t e shows many m o r p h o l o g i c a l f e a t u r e s s i m i l a r t o t h o s e o f c a l c r e t e b u t a p r e c i s e d e f i n i t i o n of l a t e r i t e i s n o t e a s y t o make ( s e e McFarlane,

1976 f o r a r e v i e w ) .

A r a t h e r l o o s e usage l i k e t h a t of S i v a r a j a s i n g h a m e t a l . (1962,

p.5) i s p r e f e r r e d and t h u s i n c l u d e s : " h i g h l y weathered m a t e r i a l 2 ) poor i n humus

1) r i c h i n s e c o n d a r y forms of i r o n , aluminium o r b o t h

3) d e p l e t e d of b a s e s and combined s i l i c a

4) w i t h o r w i t h o u t

211 n o n - d i a g n o s t i c s u b s t a n c e s s u c h a s q u a r t z , l i m i t e d amounts o f w e a t h e r a b l e primary m i n e r a l s o r s i l i c a t e c l a y s ; and 5 ) e i t h e r h a r d o r s u b j e c t t o h a r d e n i n g upon exposure t o a l t e r n a t e w e t t i n g and d r y i n g " . L a t e r i t e s do show a number of developmental s t a g e s which have been d e s c r i b e d by P u l l a n (1967) and McFarlane (1976).

The weathered c r y s t a l l i n e r o c k may be s l i g h t l y

f e r r u g i n i z e d and s l i g h t l y hardened and a t t h i s s t a g e i s known a s v e r m i c u l a r i r o n s t o n e . Continued development l e a d s t o i n c r e a s e d f e r r u g i n i z a t i o n and v e s i c u l e s , 5mm i n d i a m e t e r and f i l l e d w i t h p i n k e a r t h y m a t e r i a l , become a b u n d a n t , l a t e r i t i c ironstone.

This i s a v e s i c u l a r

Continued f e r r u g i n i z a t i o n i n c r e a s e s t h e h a r d n e s s and i f t h e

m a t e r i a l i n t h e mermicules and v e s i c u l e s i s washed o u t a c e l l u l a r i r o n s t o n e i s produced.

N o d u l a r , o o l i t i c , and p i s o l i t i c s t r u c t u r e s a r e common i n l a t e r i t e s and,

a s i n c a l c r e t e s , a r e u s u a l l y cemented t o g e t h e r .

McFarlane (1969) d i s t i n g u i s h e s

between s p a c e d p i s o l i t i c l a t e r i t e and packed p i s o l i t i c l a t e r i t e .

The former

comprises d a r k r e d p i s o l i t e s and o o l i t h s s e p a r a t e d by a m a t r i x of k a o l i n i t e and quartz.

T h i s t y p e does n o t harden on exposure and i s r e a d i l y e r o d e d on exposure

t o produce a remanie of l o o s e p i s o l i t h s .

Packed p i s o l i t i c l a t e r i t e on t h e o t h e r

hand has p i s o l i t h s r e s t i n g one a g a i n s t t h e o t h e r , and t h e y a r e u s u a l l y cemented t o form a tough, hardened mass. L a t e r i t e p r o f i l e s a r e more complex t h a n t h o s e o f c a l c r e t e and i n i n s i t u l a t e r i t e s may c o n t a i n up t o s i x main e l e m e n t s . u n d e r l a i n by t h e i r o n s t o n e c r u s t ,

A t t h e s u r f a c e t h e r e i s a s o i l zone which i s

Below t h e c r u s t ( c u i r a s s e o r c a r a p a c e a c c o r d i n g

t o t h e d e g r e e o f i n d u r a t i o n ) i s a m o t t l e d zone, a p a l l i d zone, a s i l i c i f i e d zone, and t h e z e r s a t z zone.

T h i s p r o f i l e o f t e n shows marked c o l o u r v a r i a t i o n s w i t h t h e

i n d u r a t e d zone b e i n g d a r k i s h r e d o r brown (5yR 3 / 4 ) , t h e p a l l i d zone b e i n g r a t h e r p a l e (5Y 7 / 2 ) w i t h low i r o n c o n t e n t , and t h e m o t t l e d zone b e i n g somewhat mixed (10R 5 / 8 ) ( F a n i r a n , 1970).

The ' z e r s a t z ' zone c o n s i s t s of s o f t , h i g h l y a l t e r e d m a t e r i a l which

r e t a i n s t h e s t c u c t u r e of t h e r o c k from which i t formed. The r e d c o l o u r of l a t e r i t e s i s p r i n c i p a l l y due t o h a e m a t i t e ( a - F e 0 ) and 2 3 g o e t h i t e (o( -FeOOH) a l t h o u g h l e p i d o c r o c i t e ('y-FeOOH) ( d ' C o s t a e t a l . , 1966) and maghaematite ( 7 - F e 0 ) ( B o n i f a s , 1959) have a l s o been i d e n t i f i e d . O t h e r i m p o r t a n t 2 3 c o n s t i t u e n t s a r e k a o l i n i t e and h a l l o y s i t e and a l s o g i b b s i t e A1(0HI3 , boehmite y-AlO(OH),

and d i a s p o r e

%-AlO(OH)

.

Amorphous forms of aluminium o x i d e such

a s c l i a c h i t e (Maignien, 1966) a l s o o c c u r . The g e n e s i s o f l a t e r i t e i s s t i l l o n l y p o o r l y u n d e r s t o o d and t h e r e a r e s t i l l a v a r i e t y o f t h e o r i e s i n c i r c u l a t i o n ( s e e McFarlane, 1976, p .9 1 - 1 0 8 ) .

Intense

t r o p i c a l w e a t h e r i n g o r l o n g e x t e n d e d w e a t h e r i n g l e a d s t o t h e r e l a t i v e accumulation of r e s i d u a l o x i d e s o f aluminium, i r o n , manganese, and t i t a n i u m .

The k a o l i n i t e o r

h a l l o y s i t e which may b e produced by t h e w e a t h e r i n g of b a s i c r o c k s i s e v e n t u a l l y s u b j e c t e d t o s i l i c a loss and c o n v e r t e d t o g i b b s i t e .

The s i l i c a may b e l e a c h e d o u t

of t h e system i n r i v e r w a t e r , d e p o s i t e d a t d e p t h i n t h e p r o f i l e , o r t r a n s p o r t e d l a t e r a l l y f o r v a r y i n g d i s t a n c e s t o form s i l c r e t e ( S t e p h e n s , 1971).

212 Two main mechanisms have been p u t forward f o r t h e development of i n s i t u l a t e r i t e s . The f i r s t of t h e s e i s t h a t l a t e r i t e i s a residuum of t h e r e l a t i v e l y immobile endp r o d u c t s of w e a t h e r i n g . constituent oxides.

The mechanism i m p l i e s two p h a s e s of l i m i t e d m o b i l i t y of t h e

F i r s t , a s h o r t - l i v e d phase when t h e m a t e r i a l s t o b e c o n c e n t r a t e d

a r e r e l e a s e d from t h e i r p a r e n t m a t e r i a l by w e a t h e r i n g p r o c e s s e s and re-grouped i n t o r e l a t i v e l y immobile p r e c i p i t a t e s ; s u b s e q u e n t l y , a s e c o n d , m o b i l e p h a s e , d u r i n g t h e r e s o l u t i o n o r a l t e r a t i o n of t h e residuum, p r o b a b l y by groundwater. mechanism e n v i s a g e s l a t e r i t e a s a p r e c i p i t a t e .

The second main

I n t h e s i m p l e s t c a s e o x i d e s of i r o n

and aluminium a r e t h o u g h t t o be d e r i v e d from t h e p a l l i d zone and t r a n s p o r t e d upwards t o be p r e c i p i t a t e d i n a zone of e n r i c h m e n t .

C a p i l l a r i t y and s e a s o n a l f l u c t u a t i o n s

of t h e w a t e r t a b l e have b o t h been s u g g e s t e d a s t h e c a u s e of l a t e r i t e e n r i c h m e n t . There a r e a number o f problems w i t h b o t h t h e s e models of l a t e r i t e f o r m a t i o n (Goudie, 1973, p.141-144;

McFarlane, 1971, 1976, p.93-94) and most workers now

f a v o u r a compromise i n which l a t e r i t e i s c o n s i d e r e d a s a r e s i d u a l p r e c i p i t a t e ( T r e n d a l l , 1962; de Swardt, 1964; McFarlane, 1976).

Although t h e r e a r e v a r i a t i o n s

on t h e theme t h e model developed f o r Ugandan l a t e r i t e s by McFarlane (1971) s e r v e s t o i l l u s t r a t e t h e mechanism.

I n i t i a l l y i r o n is segregated i n t o p i s o l i t h s within

t h e narrow range o f w a t e r t a b l e o s c i l l a t i o n d u r i n g a l a t e s t a g e of l a n d s u r f a c e The zone of p r e c i p i t a t i o n lowers a s t h e l a n d s u r f a c e i s reduced and t h e

reduction.

p r e c i p i t a t e s accumulate a s a s h e e t a t t h e b a s e o f t h e s o i l .

Eventually the land

s u r f a c e r e d u c t i o n c e a s e s and t h e accumulated s h e e t of p i s o l i t h s becomes a l t e r e d and h y d r a t e d t o form a massive v a r i e t y o f l a t e r i t e .

On t h e i n i t i a t i o n of t h e s u c c e e d i n g

c y c l e of e r o s i o n t h e w a t e r t a b l e once a g a i n b e g i n s t o lower and p i s o l i t h f o r m a t i o n resumes.

With c o n t i n u e d l o w e r i n g t h e zone o f p i s o l i t h f o r m a t i o n lowers l e a v i n g above

i t a spread of p i s o l i t h s .

E v e n t u a l l y t h e w a t e r t a b l e i s u l t i m a t e l y lowered beyond

t h e d e p t h a t which p i s o l i t h s can form and l e a c h i n g t a k e s p l a c e t h r o u g h t h e c a r a p a c e d e p l e t i n g t h e s a p r o l i t e and forming a p a l l i d zone.

F i n a l l y deforestation leads t o

i n d u r a t i o n o f t h e l a t e r i t e and i t s loss of p e r m e a b i l i t y . L a t e r i t e a p p e a r s t o form t h e r e f o r e a s a r e s i d u a l p r e c i p i t a t e d u r i n g l a n d s u r f a c e reduction.

An i m p o r t a n t f e a t u r e of t h i s model does n o t l i m i t t h e f o r m a t i o n of

l a t e r i t e t o t h e s t a b l e p r o f i l e of a p l a n a t i o n s u r f a c e , a b e l i e f which i s s t i l l widely held. L a t e r i t e s a r e i m p o r t a n t i n r e d bed s t u d i e s b e c a u s e e a r l i e r workers b e l i e v e d t h a t they s u p p l i e d much d e t r i t a l h a e m a t i t e and g o e t h i t e t o f l u v i a l s e d i m e n t s .

This

o p i n i o n i s no l o n g e r w i d e l y h e l d mainly b e c a u s e Recent f l u v i a l s e d i m e n t s i n t h e t r o p i c s a r e n o t r e d and a l s o b e c a u s e t h e geomorphic environment of l a t e r i t e g e n e s i s does n o t f a v o u r l a r g e s c a l e f l u v i a l e r o s i o n .

There a r e , u n f o r t u n a t e l y , no d i r e c t

s t u d i e s of t h e f l u v i a l e r o s i o n o f l a t e r i t e on which t o b a s e t h i s a s s u m p t i o n .

The

p s e u d o k a r s t i c f e a t u r e s w i d e l y r e p o r t e d i n l a t e r i t e s ( s e e Goudie, 1973, p.46-48) i n d i c a t e i m p o r t a n t s u b t e r r a n e a n e r o s i o n b u t c o u l d s u p p l y o n l y r e l a t i v e l y small amounts of i r o n o x i d e t o a f l u v i a l system.

213 THE TRANSPORT OF IRON I N RECENT ALLUVIUM

Recent a l l u v i u m i s t y p i c a l l y non-red and c o n t a i n s a l a r g e p r o p o r t i o n o f c r y s t a l l i n e m a t e r i a l eroded from bedrock.

Even i n warm, s e a s o n a l l y humid c l i m a t e s

where reddish-brown s o i l s a r e common i n t h e s o u r c e a r e a , t h e a l l u v i u m i s g e n e r a l l y brown o r brownish-yellow i n c o l o u r .

Haematite m a n t l e s do n o t g e n e r a l l y develop

w i t h i n t h e zone o f i n f l u e n c e of r i v e r c h a n n e l s because s u f f i c i e n t time i s n o t available.

Consequently t h e y a r e unimportant as a s o u r c e of r e d d e t r i t u s in. Recent

a 11w i u m . T h i s c o n t r a s t s w i t h t h e view o f Krynine (1950) who r e p o r t e d examples o f Recent r e d sediments on t h e c o a s t a l piedmont of Tabasco i n s o u t h e r n Mexico and s u g g e s t e d t h a t t h e c o l o u r a t i o n was produced by " r e d mud d e r i v e d from t h e r e d s o i l s washed E a r l i e r , Krynine (1949) had argued

down t h e s l o p e s from t h e mountain i n t e r f l u v e s l ' .

t h a t t h i s was t h e p r i n c i p a l mechanism o f r e d bed f o r m a t i o n i n a n c i e n t f l u v i a l d e p o s i t s and c a l l e d such d e p o s i t s primary r e d b e d s ,

T h i s view was w i d e l y h e l d p r i o r t o t h e

p u b l i c a t i o n s o f Walker (1967a,b) who r e v i s i t e d t h e Tabasco a r e a and showed t h a t Krymine's view i s n o t s u p p o r t e d by t h e c o l o u r of t h e r i v e r a l l u v i u m o r of t h e Recent sediments. Samples were c o l l e c t e d by Walker (1967b) from a l o n g t h e e n t i r e l e n g t h o f t h e Mezcalapa R i v e r from t h e Mexico-Guatemala

b o r d e r t o t h e Gulf o f Mexico (Fig.4.29).

The c o l o u r o f t h e sediments ranges from v e r y d a r k g r e y i s h brown (10 R 3 / 2 ) t o g r e y i s h brown (10 R 5 / 2 ) , t h e c o m o n e s t c o l o u r b e i n g d a r k g r e y i s h , b r o w n (10 R 4 / 2 ) .

Walker

d i d n o t see any Recent r e d alluvium. The s c a r c i t y o f r e d a l l u v i u m i n t h i s t r o p i c a l a r e a i s due t o t h e f a c t t h a t r e d s o i l s are n o t abundant i n t h e s o u r c e a r e a .

Most of t h e soils a r e yellow o r brown

i n c o l o u r and r e d pigment o n l y o c c u r s i n h i g h l y a l t e r e d s o i l s which have formed on s u r f a c e s t h a t a r e o f P l e i s t o c e n e age o r o l d e r ( P s u t y , 1965).

Even i n t h e s e c a s e s

o n l y t h e topmost metre o r so o f t h e s o i l i s r e d and t h e m a j o r i t y o f t h e p r o f i l e i s y e l l o w o r brown i n c o l o u r .

Rapid e r o s i o n e n s u r e s t h a t t h e r e d d e t r i t u s i s q u i c k l y

d i l u t e d and masked by t h e more abundant non-red d e t r i t u s . The r e d d e s t modern a l l u v i u m o c c u r s i n some t r o p i c a l r i v e r s such a s t h o s e i n t h e Orinoco Basin (Walker, 1974).

G e n e r a l l y t h e a l l u v i u m i s y e l l o w i s h o r brownish i n

c o l o u r (2.5Y t o 1OYR) b u t d u r i n g p e r i o d s o f heavy run-off a f t e r s t r o n g storms a r e d d i s h c o l o u r e d ( 5 R 6/61 suspended l o a d i s o f t e n s e e n b u t t h i s d r i e s t o a l i g h t brown ( 7 . 5 R 6/41.

Red pigment s u p p l i e d by t h e e r o s i o n o f r e d s o i l s i s q u i c k l y

masked by non-red d e t r i t u s and some may b e l o s t by r e d u c t i o n of i r o n o x i d e d u r i n g t r a n s p o r t a t i o n (Hinze and Meischner, 1968). Examination o f Recent a l l u v i u m , t h e r e f o r e , s u g g e s t s t h a t i t i s n o t c o l o u r e d r e d by d e t r i t u s eroded from r e d s o i l s .

There i s no e v i d e n c e t h a t Recent t r o p i c a l

a l l u v i u m , or any o t h e r f o r t h a t matter, has a predominantly r e d suspended l o a d , S i m i l a r l y t h e r e i s no e v i d e n c e t h a t t h e muds and s i l t s on t h e a c t i v e f l o o d p l a i n s

214

Fig.4.29.

L o c a l i t y map of s o u t h e r n Mexico ( a f t e r Walker, 1967b).

of these r i v e r s a r e red i n colour.

I t must be c o n c l u d e d , t h e r e f o r e , t h a t t h e

mechanism of primary r e d bed f o r m a t i o n a s d e s c r i b e d by Krynine ( 1 9 4 9 ) , i s n o t s u b s t a n t i a t e d by t h e i n v e s t i g a t i o n of Recent a l l u v i u m . The f e r r u g i n o u s s o i l s of t r o p i c a l r e g i o n s do s u p p l y c l a y f r a c t i o n s pigmented w i t h yellow and brown amorphous i r o n hydroxides t o Recent a l l u v i u m (Van Houten, 1964, 1972). hydroxides.

Red pigment may be p r e s e n t and be masked by t h e more abundant brown I n t r o p i c a l a l l u v i u m eroded from igneous and metamorphic t e r r a i n s

t h e r e a r e a l s o abundant f r e s h s i l i c a t e g r a i n s i n c l u d i n g f e r r o m a g n e s i a n m i n e r a l s . I n t h i s r e s p e c t t h e a l l u v i u m o f a l l c l i m a t i c regimes i s s i m i l a r .

This i s i l l u s t r a t e d

by t h e work of Gibbs (1977) who made a d e t a i l e d s t u d y of t h e t r a n s p o r t a t i o n of i r o n and o t h e r t r a n s i t i o n e l e m e n t s ( C r , Mn, Co, N i , and Cu) i n two l a r g e r i v e r s , t h e Amazon and t h e Yukon.

These r i v e r s d r a i n b a s i n s which have a wide v a r i e t y o f rock

t y p e s and environments, and can b e c o n s i d e r e d r e p r e s e n t a t i v e of t h e i r r e s p e c t i v e

215 c l i m a t i c z o n e s , one t r o p i c a l , t h e o t h e r t u n d r a ,

Gibbs ( 1 9 7 7 ) d i s t i n g u i s h e d between:

d i s s o l v e d m a t e r i a l s , a d s o r b e d m a t e r i a l , m e t a l l i c c o a t i n g s (mainly f e r r i c h y d r o x i d e ) , o r g a n i c complexes and c r y s t a l l i n e m a t e r i a l s . I r o n i s t r a n s p o r t e d m o s t l y a s c r y s t a l l i n e p a r t i c l e s and a s f e r r i c hydroxide c o a t i n g s on t h e s e p a r t i c l e s .

A b s o l u t e v a l u e s of i r o n i n v a r i o u s t r a n s p o r t phases

a r e g i v e n i n T a b l e 4.1 and a r e l a t i v e comparison of t h e Amazon and Yukon R i v e r s i s shown i n Fig.4.30.

The Amazon c a r r i e s 47.2% of i t s t o t a l i r o n a s g r a i n c o a t i n g s and

45.5% i n t h e form o f c r y s t a l l i n e p a r t i c l e s .

I n t h e Yukon R i v e r 40.6% o f t h e t o t a l

i r o n i s c a r r i e d i n t h e form of g r a i n c o a t i n g s and 48.2% i n t h e form of c r y s t a l l i n e TABLE 4.1.

C o n c e n t r a t i o n of i r o n i n t r a n s p o r t p h a s e s i n r i v e r w a t e r and suspended sediment f o r t h e Amazon and Yukon R i v e r . R i v e r w a t e r v a l u e s i n micrograms p e r l i t r e ; s o l i d - p h a s e v a l u e s i n ppm ( a f t e r G i b b s , 1977). Fe

S o l u t i o n and o r g a n i c complexed

Amazon River Solid

34.0

Yukon River Solid

50.0

Sorbed Material

Metallic Coatings

Organic Solids

Crystalline Particles

TOTAL

-

1.3

2 ,390

1.4

26,000

333 3 ,720

2,310 25 ,800

5,065.3 55,521.4

10.0

3 ,600 25,720

976 6,970

4,270 30,500

8,906.0 63,197.5

7.5

Fe

AMAZON

YUKON

P r o p o r t i o n s of t o t a l and a v a i l a b l e i r o n t r a n s p o r t e d i n v a r i o u s p l a s e s Fig.4.30. f o r Amazon and Yukon R i v e r s ( a f t e r G i b b s , 1977).

216 particles.

Only t h e o r g a n i c s o l i d p h a s e t r a n s p o r t s a s i g n i f i c a n t minor amount o f

t h e t o t a l i r o n , 6.6% i n t h e Amazon, and 11%f o r t h e Yukon R i v e r .

The i r o n p h a s e s

t r a n s p o r t e d i n s o l u t i o n and s o r b e d on s o l i d s e a c h c o n t r i b u t e l e s s t h a n 1%of t h e t o t a l i r o n t r a n s p o r t e d by t h e two r i v e r s .

The v a s t m a j o r i t y o f t h e a v a i l a b l e i r o n

i n b o t h r i v e r s is t h u s i n t h e form of t h e g r a i n c o a t i n g s ( F i g . 4 . 3 0 ) . The p o s s i b l e s i z e d i s t r i b u t i o n of i r o n t r a n s p o r t p h a s e s f o r t h e Amazon and Yukon a r e shown i n Fig.4.31.

These p h a s e s v a r y w i t h p a r t i c l e s i z e b u t t h e r e i s a remark-

a b l e s i m i l a r i t y between t h e two r i v e r s .

Fe AMAZON

I

5

h

YUKON

4-

iw 3 -

5

/-2?+&

0 0.1 PARTICLE DIAMETER (pn)

1.0 10 100 7.50 PARTICLE DIAMETER ( r m )

Fig.4.31. Concentration of i r o n i n various transporting phases f o r various p a r t i c l e s i z e s i n Amazon and Yukon R i v e r s ( a f t e r G i b b s , 1977). S u r f a c e r e l a t e d phenomena

-

g r a i n c o a t i n g s , o r g a n i c m a t e r i a l and s o r b e d m a t e r i a l

show an 80-9977 d e c r e a s e i n i r o n from f i n e t o c o a r s e r g r a i n e d m a t e r i a l ,

Crystalline

p a r t i c l e s show a d e c r e a s e o f 57% and 45% i r o n w i t h p a r t i c l e s i z e f o r t h e Amazon and Yukon R i v e r s r e s p e c t i v e l y , p r o b a b l y b e c a u s e o f i n h e r e n t m i n e r a l o g i c a l v a r i a t i o n s (Gibbs , 1967b). The importance of m e t a l l i c g r a i n c o a t i n g s a s a t r a n s p o r t o f phase of i r o n and t h e o t h e r t r a n s i t i o n m e t a l s l e d Gibbs (1977) t o a more d e t a i l e d i n v e s t i g a t i o n o f i t s n a t u r e and o r i g i n .

The t h i c k n e s s o f t h e m e t a l l i c g r a i n c o a t i n g s was c a l c u l a t e d

from t h e i r o n c o n c e n t r a t i o n and p a r t i c l e d i a m e t e r i n e a c h s i z e f r a c t i o n w i t h d e n s i t y and shape assumed.

M i c r o s c o p i c e x a m i n a t i o n s s u g g e s t e d t h e b e s t assumptions o f shape

were p l a t e s f o r m a t e r i a l less t h a n 3-5ym and cubes f o r m a t e r i a l g r e a t e r t h a n 3.5ym. From t h i s t h e s u r f a c e a r e a p e r g r a i n o f m a t e r i a l i s c a l c u l a t e d a n d , assuming a d e n s i t y

217 of 2.8 f o r Fe(OH)3, t h e t h i c k n e s s of a uniform c o a t i n g on t h e p a r t i c l e s f o r each s i z e f r a c t i o n can b e deduced ( T a b l e 4.2 ). TABLE 4.2.

C h a r a c t e r i s t i c s of t r a n s p o r t e d s e d i m e n t s i n Amazon and Yukon R i v e r s ( a f t e r Gibbs,

1977). Particle Shape

Particle diameter

Surface area ( m2/g)

(ym)

Plate

Cube

Fe c o n c e n t r a t i o n i n coating

(70)

Fe(OH3) c o a t i n g thickness

P)

Amazon

Yukon

Amazon

Yukon

0.1

5.2885 x 10l3

4.15

5.3

0.00028

0.00036

0.35

1.1916 x

4.89

3.8

0.0015

0.0011

1.25

3.337

x 10l2

4.28

3.0

0.0046

0.0032

3.5

1.1916 x 1OI2

1.75

2.3

0.0052

0.0069

12.5

1.8459 x 10"

0.37

0.5

0.0071

0.0097

47

4.909

x 10"

0.29

0.2

0.021

0.014

100

2.300

x 10"

0.21

0.33

0.033

0.051

250

9.2306 x 10

0.16

0.56

0.061

0.217

The o r i g i n of t h e s e i r o n hydroxide g r a i n c o a t i n g s i s of p a r t i c u l a r i n t e r e s t because c l a y - i r o n o x i d e c o a t i n g s a r e s u c h a c h a r a c t e r i s t i c f e a t u r e o f a n c i e n t r e d beds.

Gibbs (1977, p.838) c o n s i d e r e d two hypotheses.

The f i r s t was t h a t t h e

c o a t i n g s were p r e c i p i t a t e d i n e q u i l i b r i u m w i t h t h e r i v e r w a t e r w h i l s t t h e p a r t i c l e s were f r e e l y suspended.

T h i s i d e a i s n o t s u p p o r t e d by t h e f a c t t h a t t h e g r a i n

c o a t i n g s a r e n o t of uniform t h i c k n e s s a s one might e x p e c t them t o be.

Also physical

a b r a s i o n o r chemical d i s s o l u t i o n i s n o t a l i k e l y mechanism t o e x p l a i n t h e observed r e l a t i o n s h i p o f t h i c k e r g r a i n c o a t i n g s on c o a r s e r p a r t i c l e s .

The second h y p o t h e s i s ,

and t h e ope c o n s i d e r e d t o b e t h e most l i k e l y e x p l a n a t i o n by Gibbs (1967a), i s t h a t t h e g r a i n c o a t i n g s were formed i n a w e a t h e r i n g ( s o i l ) environment.

He a r g u e d t h a t

c o a r s e r g r a i n s developed a t h i c k e r hydroxide c o a t i n g because t h e y had a h i g h e r p e r m e a b i l i t y which a l l o w e d a g r e a t e r s u p p l y o f p r e c i p i t a t i n g i o n s , whereas t h e f i n e r p a r t i c l e s i n t h e s o i l c r e a t e d a lower p e r m e a b i l i t y and reduced t h e s u p p l y of prec i p i t a t i n g ions. Van Houten (1972) s t u d i e d t h e i r o n and c l a y c o n t e n t o f t r o p i c a l Savanna a l l u v i u m a l o n g t h e f l a n k s o f t h e S i e r r a Nevada de S a n t a M a r t a , n o r t h e r n Colombia.

Here upland

s o i l s a r e r e d d i s h t o r e d d i s h brown i n c o l o u r b u t t h e suspended l o a d of t h e l a r g e r r i v e r s d r a i n i n g t h e a r e a i s y e l l o w i s h - g r e y t o p a l e brown (cf. Walker, 1974).

Also

t h e f r e s h a l l u v i u m and v e r y immature a l l u v i a l s o i l s a r e y e l l o w i s h - g r e y t o shades of brown (10 R t o 5 Y ) a l t h o u g h some o f t h e o l d e r a l l u v i u m may b e r e d t o r e d d i s h brown

(10R t o 5YR).

The a l l u v i u m c o n t a i n s r e l a t i v e l y abundant g r a i n s o f f r e s h i r o n - b e a r i n g

218 m i n e r a l s i n c l u d i n g m a g n e t i t e , amphibole, b i o t i t e , c h l o r i t e , h a e m a t i t e , i l m e n i t e and pyroxene.

T h i s i s c o n s i s t e n t w i t h t h e r e c o g n i t i o n of u n a l t e r e d m i n e r a l s u i t e s

i n o t h e r t r o p i c a l r i v e r s l i k e t h e Amazon ( G i b b s , 1967a) and t h e E u p h r a t e s ( P h i l i p , 1968) and i s a n i n d i c a t i o n t h a t a c t i v e l y e r o d e d t r o p i c a l uplands y i e l d f r e s h m i n e r a l g r a i n s d e r i v e d from b e d r o c k ( V a n Andel, 1959; Van Houten, 1968) a s w e l l a s g r a i n s d e r i v e d from s o i l h o r i z o n s (Gibbs, 1977).

Unaltered mineral s u i t e s a r e a l s o t y p i c a l

of r i v e r s i n t e m p e r a t e r e g i o n s (Packham e t a l . , 1961). show some s i g n s of m i n e r a l a l t e r a t i o n .

The Colombian a l l u v i u m does

I n t h e B horizon of red s o i l s black oxides

a r e less abundant and much m a g n e t i t e has a p p a r e n t l y been c o n v e r t e d t o pigment; amphibole and b i o t i t e a r e a l s o reduced i n abundance b u t t h e r e a r e few obvious s i g n s of p a r t i a l l y a l t e r e d amphiboles. Clay m i n e r a l s i n t h e Colombian a l l u v i u m i n c l u d e m o n t m o r i l l o n i t e (M), i l l i t e ( I ) , mixed l a y e r I M , k a o l i n i t e and c h l o r i t e .

The mixed l a y e r c l a y s and t h e k a o l i n i t e

were a l m o s t c e r t a i n l y d e r i v e d from t h e upland s o i l s and f u r t h e r e v i d e n c e o f t h i s i s p r o v i d e d by t h e p r e s e n c e of 10-26% amorphous t o v e r y d i s o r d e r e d a l u m i n o s i l i c a t e . Such m a t e r i a l i s a c h a r a c t e r i s t i c component o f humid c l i m a t e s o i l s and i s f r e q u e n t l y found i n a s s o c i a t i o n w i t h f e r r i c hydroxide ( M i t c h e l l e t a l . , 1964; F o l l e t t e t a l . , 1965). I r o n c o n t e n t i s c o n c e n t r a t e d i n t h e c l a y f r a c t i o n s o f t h e Colombian a l l u v i u m (Fig.4.32).

I n t h e s a n d - s i l t f r a c t i o n t h e amount of t o t a l i r o n c o r r e l a t e s c l o s e l y

w i t h t h e amount 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 s u c h a s m a g n e t i t e and b i o t i t e .

The

o l d e r and r e d d e r t e r r a c e d e p o s i t s show a d e c r e a s e i n t o t a l i r o n and a l s o e v i d e n c e of t h e removal o f i r o n from amphibole and b i o t i t e g r a i n s ; most of t h e i r o n i n m a g n e t i t e i s r e t a i n e d i n s p e c u l a r h a e m a t i t e p a r t i c l e s produced by p o s t - d e p o s i t i o n a l alteration.

I n t h e c l a y f r a c t i o n s t h e amount of i r o n v a r i e s :

12.8-15.6% o c c u r s

i n r e d s o i l s , 5.5-8.0% i n a l l u v i u m on b a n k s , 4.8-9.0% i n f l o o d p l a i n s e d i m e n t s and 3.0-5.877

i n older terrace deposits.

An a v e r a g e of 75% of t h e t o t a l i r o n i s c o n t a i n e d

i n t h e c l a y f r a c t i o n s o f t h e a l l u v i u m b u t r a t h e r s u r p r i s i n g l y t h e o l d e r and r e d d e r t e r r a c e d e p o s i t s c o n t a i n l e s s i r o n i n b o t h t h e c l a y and t h e s a n d - s i l t f r a c t i o n s . T h i s i m p l i e s t h a t t h e i n c r e a s e i n r e d n e s s h a s n o t been accompanied by a n i n t r a s t r a t a l t r a n s f e r of i r o n from t h e s a n d - s i l t f r a c t i o n t o t h e c l a y f r a c t i o n .

The Colombian

a l l u v i u m c o n t a i n s a n a v e r a g e of 0.75% f r e e ( e x t r a c t a b l e ) i r o n which c o n s i s t s p r i n c i p a l l y of brown l i m o n i t i c m a t e r i a l d e r i v e d from upland s o i l s .

The main

c o n s t i t u e n t s of l i m o n i t e a r e p r o b a b l y amorphous o r p o o r l y c r y s t a l l i n e Fe(0H) g o e t h i t e (FeOOH).

3

and

The m i n e r a l s a r e known t o b e u n s t a b l e r e l a t i v e t o h a e m a t i t e i n

t h e g e o l o g i c a l environment ( B e r n e r , 1969a) and under f a v o u r a b l e c i r c u m s t a n c e s ( o x i d i z i n g c o n d i t i o n s ) w i l l i n v e r t t o r e d pigmentary h a e m a t i t e by d e h y d r a t i o n .

The

p o s t - d e p o s i t i o n a l a g e i n g of f e r r i c hydroxide i n t h i s manner has long been recognized a s a l i k e l y mechanism f o r t h e f o r m a t i o n of a n c i e n t r e d b e d s .

Post-depositional

r e d d e n i n g i s l i k e l y t o o c c u r p r o v i d i n g t h a t t h e r e i s s u f f i c i e n t f e r r i c hydroxide i n t h e sediment which i s f i n e g r a i n e d and d i s p e r s e d t h r o u g h o u t t h e m a t r i x .

Also,

219 s u i t a b l e geochemical c o n d i t i o n s a r e r e q u i r e d , i n c l u d i n g low o r g a n i c c o n t e n t and Eh-pH r a n g e s w i t h i n t h e s t a b i l i t y f i e l d o f h a e m a t i t e .

The f o r m a t i o n o f h a e m a t i t e

i s a l s o f a v o u r e d by i n c r e a s e d a d s o r b e d c a l c i u m and magnesium i n t h e c l a y f r a c t i o n ( T a y l o r and G r a l e y , 1967) and a d s o r p t i o n o f o x a l a t e i o n s by amorphous f e r r i c hydroxide (Schwertmann, 1970).

ON BANK FLOOD PLAIN LOW TERRACE

Fig.4.32. Weight % t o t a l i r o n i n a l l u v i u m from n o r t h e r n Colombia. 1. I n c l a y f r a c t i o n . 2 . I n s a n d - s i l t f r a c t i o n ( a f t e r Van Houten, 1972). P r e c i s e documentation o f f e r r i c hydroxide ' a g e i n g ' i n Recent a l l u v i u m has n o t been made.

D i f f i c u l t y a r i s e s b e c a u s e o f t h e time-dependent n a t u r e of t h e p r o c e s s .

Many o l d e r t e r r a c e d e p o s i t s a r e r e d d e r b u t i t is n o t c l e a r whether t h i s i s due t o h a e m a t i t e formed by i n t r a s t r a t a l a l t e r a t i o n of 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 o r by t h e d i r e c t a g e i n g of d e t r i t a l f e r r i c h y d r o x i d e s .

The former p r o c e s s has been

documented i n Recent t r o p i c a l a l l u v i u m by Walker (1974) and t h e amount of f e r r i c

hydroxide i n t r o p i c a l a l l u v i u m i s c e r t a i n l y s u f f i c i e n t t o produce r e d pigment p o s t - d e p o s i t i o n a l l y (Walker and Honea, 1969).

The r e l a t i v e importance o f t h e two

p r o c e s s e s i s l i k e l y t o depend on t h e g r a i n s i z e and m i n e r a l o g y o f t h e a l l u v i u m .

In

f i n e g r a i n e d a l l u v i a l f l o o d p l a i n s , w i t h abundant c l a y and s i l t , d e t r i t a l f e r r i c hydroxides and i r o n - r i c h c l a y s r e p r e s e n t a s i g n i f i c a n t p r o p o r t i o n o f t h e t o t a l i r o n b e a r i n g mineralogy.

Given f a v o u r a b l e c o n d i t i o n s much o f t h i s m a t e r i a l w i l l e v e n t u a l l y

form pigmentary h a e m a t i t e .

I n 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 of t h e t y p e d e s c r i b e d

by Walker e t a l . (1978) 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 m a t r i x a t t h e time of d e p o s i t i o n and t h e i r o n - b e a r i n g m i n e r a l o g y i s dominated by f r e s h g r a i n s o f f e r r o m a g n e s i a n minerals.

I n t h i s c a s e r e d d e n i n g p r o g r e s s e s w i t h sediment b u r i a l a s t h e i r o n - b e a r i n g

g r a i n s a r e i n t r a s t r a t a l l y a l t e r e d r e l e a s i n g i o n s i n t o t h e i n t e r s t i t i a l groundwaters which l a t e r p r e c i p i t a t e a s v a r i o u s a u t h i g e n i c p h a s e s , i n c l u d i n g h a e m a t i t e . ANCIENT ALLUVIAL RED BEDS Ancient a l l u v i a l s e d i m e n t s a r e b e s t r e c o g n i z e d by t h e i r s e d i m e n t a r y o r g a n i z a t i o n , absence of marine f o s s i l s , u n i d i r e c t i o n a l p a l a e o c u r r e n t p a t t e r n and f r e q u e n t evidence of emergence s u c h a s p a l a e o s o l s and mudcracks.

The m a j o r i t y o f a n c i e n t a l l u v i a l

sediments a r e p a r t l y r e d i n c o l o u r and t h e s e comprise t h e most i m p o r t a n t group of a n c i e n t r e d beds.

Not any one of t h e above c r i t e r i a , i s i n i t s e l f , d i a g n o s t i c of an

a l l u v i a l environment b u t i n combination t h e y can b e c o n f i d e n t l y used i n t h e i r identi f i c a tion. There a r e l i m i t a t i o n s i n s t u d y i n g a n c i e n t a l l u v i a l s e d i m e n t s by d i r e c t comparison w i t h Recent s e d i m e n t s .

S i n c e P r o t e r o z o i c times (when t h e e a r l i e s t a l l u v i a l sediments

a r e found) t h e r e have been c o n s i d e r a b l e e v o l u t i o n a r y changes i n t h e atmosphere, t h e c l i m a t i c c y c l e , and a l s o i n t h e abundance and t y p e o f p l a n t l i f e . may i n f l u e n c e a l l u v i a l p r o c e s s e s .

A l l these factors

I n pre-Devonian times, b a f o r e t h e a p p e a r a n c e of

p l a n t l i f e , e r o s i o n and s u r f a c e r u n - o f f were p r o b a b l y h i g h a n d Esoods l a r g e compared w i t h t h e p r e s e n t - d a y (Schumm, 1968).

S i m i l a r l y , t h e appearance of grasses i n t h e

L a t e Cenozoic may have p l a y e d a s i g n i f i c a n t p a r t i n s t a b i l i z i n g a l l u v i a l p r o c a g s e s . R u s s e l l (1956) r e c o g n i z e d f o u r p e r i o d s i n which e v o l u t i o n a r y changes i n t h e p l a n t kingdom c o u l d have i n f l u e n c e d a l l u v i a l p r o c e s s e s :

1. p r e v e g e t a t i o n time (pre-Devonian) 2. p r i m i t i v e v e g e t a t i o n (Devonian-Cretaceous)

3 . modern f l o w e r i n g p l a n t s (Cretaceous-Miocene)

4 . t h e appearance of g r a s s e s (Miocene-present). Despite these influences a n c i e n t a l l u v i a l sediments a r e r e a d i l y recognizable and show some remarkable s i m i l a r i t i e s t o Recent a l l u v i u m .

Many of t h e c h a r a c t e r -

i s t i c Recent a l l u v i a l environments c a n b e i d e n t i f i e d i n a n c i e n t r e d beds and t h e r e a r e a number of e x c e l l e n t summaries e.g. Reading (1978).

Here a s m a l l number of

examples a r e c o n s i d e r e d i n d e t a i l s o t h a t t h e f a c t o r s c o n t r o l l i n g a l l u v i a l

221 s e d i m e n t a t i o n and s u b s e q u e n t reddening can be c o n s i d e r e d i n d e t a i l . The Old Red Sandstone Old Red Sandstone i s t h e name g i v e n t o t h e c o n t i n e n t a l f a c i e s of t h e Devonian i n v a r i o u s p l a c e s around t h e p r e s e n t - d a y North A t l a n t i c margins. molasse f a c i e s of t h e Caledonian orogeny.

I t represents the

These p l a c e s which i n c l u d e England, Wales,

S c o t l a n d , Norway, S p i t z b e r g e n , Greenland and p a r t s o f Canada and t h e USA o r i g i n a l l y formed p a r t o f a l a r g e c o n t i n e n t a l l a n d mass which c a n be r e c o n s t r u c t e d on t h e b a s i s of palaeomagnetic e v i d e n c e ( B u l l a r d e t al., 1965; Smith e t a l . , 1973) (Fig.4.33).

30's

60'

Fig.4.33. 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 f o r t h e Lower Devonian showing t h e p r o b a b l e e x t e n t of t h e Old Red Fandstone c o n t i n e n t (based on Smith e t a l . , 1973 and Woodrow e t a l . , 1973). The Old Red Sandstone c o n t i n e n t l a y between p a l a e o l a t i t u d e s o f a b o u t 0-30's

and

t h e p a l a e o c l i m a t e , j u d g i n g from comparisons w i t h t h e p r e s e n t - d a y c l i m a t i c p a t t e r n s u s i n g l i t h o l o g y and f a u n a l and f l o r a l c h a r a c t e r i s t i c s (Woodrow e t a l . ,

1973),

ranged from h o t and s e m i a r i d i n t h e s o u t h e r n p a r t o f t h e c o n t i n e n t t o a more m o i s t t r o p i c a l c l i m a t e i n t h e n o r t h e r n p a r t o f t h e landmass.

Within t h e Old Red c o n t i n e n t

222

two t y p e s of s e d i m e n t a r y a s s o c i a t i o n can b e r e c o g n i z e d ( A l l e n e t a l . , 1967).

One

i s a n e x t e r n a l f a c i e s i n which c o a s t a l a l l u v i a l s e d i m e n t s p a s s l a t e r a l l y and i n t e r f i n g e r w i t h marine s t r a t a .

The o t h e r i s a n i n t e r n a l f a c i e s which i s c o n f i n e d t o

i n t e r m o n t a n e b a s i n s , has no a s s o c i a t e d marine s t r a t a and c o n s i s t s m o s t l y of a l l u v i a l f a n and l a c u s t r i n e d e p o s i t s .

The e x t e r n a l f a c i e s i s most i m p o r t a n t i n volume and

a r e a l e x t e n t s t r e t c h i n g from t h e n o r t h e r n p a r t o f c o n t i n e n t a l Europe through s o u t h e r n B r i t a i n t o Canada and t h e USA. These two f a c i e s o f t h e Old Red Sandstone p r o v i d e e x c e l l e n t examples o f two c o n t r a s t i n g t y p e s of a n c i e n t a l l u v i a l r e d beds.

The i n t e r n a l f a c i e s compares c l o s e l y

w i t h Recent p e b b l y a l l u v i u m d e p o s i t e d i n a l l u v i a l f a n s and b r a i d e d r i v e r s a r i d climates.

i n semi-

The d e p o s i t s a r e more o r less u n i f o r m l y r e d which c o n t r a s t s w i t h

t h o s e of t h e e x t e r n a l f a c i e s .

These more c l o s e l y resemble t h e d e p o s i t s o f meandering

r i v e r s d e p o s i t e d i n c o a s t a l a l l u v i a l p l a i n s and a r e t y p i c a l l y v a r i e g a t e d showing a l t e r n a t i o n s of r e d and d r a b sediment. Internal facies

-

reddened p e b b l y a l l u v i u m

The i n t e r n a l f a c i e s o f t h e Old Red Sandstone was d e p o s i t e d i n i n t e r m o n t a n e b a s i n s which a r e d e l i n e a t e d by major boundary f a u l t s .

These f a u l t s were a c t i v e a t t h e t i m e

of d e p o s i t i o n ( F r i e n d , 1967, 1969) and c o n t r o l l e d t h e a c c u m u l a t i o n of g r e a t t h i c k n e s s e s o f sediment ( B r y h n i , 1964a; Bluck, 1969; S t e e l e t a l . , 1977).

The r e s u l t

i s t h a t a l l u v i a l f a n d e p o s i t e ( f a n g l o m e r a t e s ) accumulated i n t h e m a r g i n a l a r e a s and

t h e s e i n t e r f i n g e r w i t h l a t e r a l l y e q u i v a l e n t a l l u v i a l p l a i n s e d i m e n t s which a r e u s u a l l y d i s p e r s e d a l o n g t h e a x i s of t h e b a s i n .

I n some i n t e r n a l b a s i n s t h i c k

l a c u s t r i n e sequences a r e d e v e l o p e d , t h e n o t a b l e example b e i n g t h e O r c a d i a n Basin of C a i t h n e s s (Donovan and F o s t e r , 1972; Donovan e t a l . , 1974) i n which o v e r 5km

of d r a b l a c u s t r i n e c l a s t i c s and c a r b o n a t e s were d e p o s i t e d d u r i n g t h e Middle Old Red Sandstone ( E i f e l i a n - G i v e t i a n ) .

The Midland V a l l e y of S c o t l a n d ( F i g . 4 . 3 4 ) r e p r e s e n t s

a l a r g e i n t e r n a l b a s i n bounded by two NE-SW t r e n d i n g f a u l t complexes, t h e Highland Boundary F a u l t s y s t e m t o t h e N o r t h and t h e S o u t h e r n Uplands F a u l t t o t h e South. The Old Red sequence rests on a b a s a l unconformity and began t o accumulate i n Gedinnian time w i t h up t o 6km of s e d i m e n t a c c u m u l a t i n g i n p l a c e s .

There i s a major

b r e a k i n t h e sequence and t h e Upper Old Red, which i s g e n e r a l l y f i n e r g r a i n e d and more e x t e n s i v e a r e a l l y , rests unconformably on t h e Lower Old Red Sandstone.

The

i m p l i c a t i o n i s t h a t t h e r e was a c o n s i d e r a b l e r e d u c t i o n i n s o u r c e a r e a r e l i e f d u r i n g t h e Middle Old Red Sandstone. Along t h e n o r t h e r n margin of t h e Midland V a l l e y b a s i n t h e Lower Old Red Sandstone c o n t a i n s massive conglomerate u n i t s which comprise up t o 70% of t h e s u c c e s s i o n and t h i n southwards ( A l l a n , 1940).

The conglomerates a s d e s c r i b e d by H i c k l i n g (1908,

19121, Campbell (1913) F r i e n d e t a l . ( 1 9 6 3 ) , Bluck (1967, 1969) show l a r g e v a r i a t i o n i n c l a s t s i z e ; i n t h e n o r t h e a s t e r n p a r t of t h e a r e a c l a s t s o v e r 2m i n s i z e o c c u r b u t

223 s o r t i n g i n t h e c o a r s e g r a i n e d p a r t s i s g e n e r a l l y good ( F r i e n d e t a l . , 1963). Roundness v a r i e s l a t e r a l l y and v e r t i c a l l y ( A l l a n , 1940) b u t u s u a l l y q u a r t z i t e c l a s t s a r e well-rounded even when a s s o c i a t e d D a l r a d i a n metamorphic fragments a r e angular.

A l l a n (1940) s u g g e s t e d t h a t t h i s might be due t o r e c y c l i n g o f p r e v i o u s l y

formed q u a r t z i t e b e a r i n g conglomerate.

The c l a s t s o f q u a r t z i t e and D a l r a d i a n

p h y l l i t e , s c h i s t and g n e i s s a r e c o n s i s t e n t w i t h a provenance i n t h e Highland a r e a s t o t h e North; however contemporaneous a n d e s i t e s and b a s a l t s w i t h i n t h e Midland V a l l e y b a s i n a r e a l s o found a s c l a s t s w i t h i n t h e c o n g l o m e r a t e s .

Upwards, D a l r a d i a n type

c l a s t s become more abundant s u g g e s t i n g p r o g r e s s i v e u n r o o f i n g o f t h e n o r t h e r n h i g h l a n d areas.

I n t h e n o r t h e a s t e r n p a r t o f t h e b a s i n Campbell (1929) found t h a t t h e lower

g r a d e metamorphic zones were p r o g r e s s i v e l y s t r i p p e d o f f u n t i l t h e high-grade zones p r e s e n t l y v i s i b l e were exposed.

Fig.4.34. Scotland

Palaeogeography o f t h e Lower O l d Red Sandstone i n t h e Midland V a l l e y o f A . S t r u c t u r a l d e t a i l s B. Suggested palaeogeography ( a f t e r Bluck, 1978).

224

On t h e s o u t h e r n f l a n k o f t h e b a s i n t h e conglomerates a r e n e i t h e r a s t h i c k n o r a s c o a r s e a s t h e i r n o r t h e r n c o u n t e r p a r t s b u t t h e y do o c c u r i n t e r b e d d e d w i t h v o l c a n i c rocks (Mykura, 1960b).

C l a s t s i n c l u d e O r d o v i c i a n and S i l u r i a n c l a s t i c s i d e n t i c a l

w i t h t h o s e of t h e S o u t h e r n Uplands a r e a t o t h e s o u t h ( M i t c h e l l and Mykura, 1962). The evidence i n d i c a t e s t h a t t h e conglomerates i n b o t h t h e n o r t h e r n and s o u t h e r n a r e a s a r e marginal a l l u v i a l f a n s d e r i v e d from t h e n o r t h and s o u t h r e s p e c t i v e l y (Fig.4.34).

The t h i c k n e s s o f t h e conglomerates i n d i c a t e t h a t downwarping went on

contemporaneously w h i l s t changes i n c l a s t composition s u g g e s t t h a t u p l i f t might have been i m p o r t a n t i n s o u r c e a r e a s . The Upper Old Red Sandstone was d e p o s i t e d i n a more e x t e n s i v e , b u t p r o b a b l y less deep Midland V a l l e y b a s i n .

Conglomerate u n i t s a r e fewer and s a n d s t o n e s and pedogenic

c a r b o n a t e s (Burgess, 1960; W a t e r s t o n , 1965) a r e more abundant.

Even so, b o u l d e r

s i z e d conglomerates o c c u r i n t h e v i c i n i t y o f t h e Highland Boundary F a u l t ( F r i e n d e t a l . , 1963).

D e t a i l e d s t u d y of t h e conglomerates by Bluck (1967, 1969) i n t h e

w e s t e r n p a r t of t h e b a s i n shows t h a t t h e y o c c u r i n sequences which b e g i n w i t h a l l u v i a l f a n d e p o s i t s and end w i t h s a n d s t o n e s and conglomerates o f f l o o d p l a i n type. Bluck (1969) recognized t h r e e such s e q u e n c e s , e a c h b e i n g a r e a l l y i s o l a t e d and i n i t i a t e d by t h e r i s i n g of a n a d j a c e n t fault-bounded b l o c k ; t h e s e e a c h have a d i s t i n c t i v e composition and a r e r e f e r r e d t o a s t h e P o r t e n c r o s s , S k e l m o r l i e , and I n v e r k i p d i s p e r s a l shadows ( F i g . 4 . 3 5 ) .

Within t h e s e a r e a s Bluck (1967) was a b l e

Fig.4.35. D i s p e r s a l shadows recognized on t h e e a s t e r n margin of t h e F i r t h o f Clyde ( a f t e r Bluck, 1969).

225 t o d i s t i n g u i s h f o u r t y p e s o f conglomerate on t h e b a s i s of s t r u c t u r e and t e x t u r e : a l l u v i a l f a n mudflows, a l l u v i a l f a n s t r e a m f l o w , b r a i d e d s t r e a m and f l o o d p l a i n ( r i v e r channel) deposits (Fig.4.36).

V e r t i c a l p r o f i l e s t h r o u g h t h e conglomerates

g e n e r a l l y show a n upward r e d u c t i o n i n g r a i n s i z e t o g e t h e r w i t h a change from f a n mudflow t o f a n s t r e a m t o b r a i d e d s t r e a m and f l o o d p l a i n d.eposition ( F i g . 4 . 3 7 ) .

-s I10 Y, -1s

5 10 15. 20 maximum particlo r m (em)

1

-

1

1

1Q 1 5 maximum particlo sir?(cm)

I

S t r u c t u r a l and t e x t u r a l p r o p e r t i e s of Upper Old Red Sandstone conglomFig.4.36. e r a t e s A . D e b r i s f l o w . B. Stream f l o w . C . B r a i d e d s t r e a m . D . F l o o d p l a i n (based on Bluck, 1967).

Alluvial fan

Braided stream

Alluwal fan

Braided stream

Maximum particle size

i

Stream channel

M.P.S. (cm)

5 10 15 20 25

D m Marl

C

Streamflood

W Mudflow

E m

A

C

M.P.S. (cm)

5 10 15 2 0 25

227 These Upper Old Red Sandstone conglomerates f o m e d a t t h e f o o t of fault-bounded mountains.

An i n i t i a l h i g h r a t e of e r o s i o n l e d t o 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

a s s h e e t f l o o d d e p o s i t s accumulated. s o u r c e r e g i o n s were c u t back

A s m a r g i n a l f a u l t a c t i v i t y grew l e s s t h e

and a l l u v i a l f a n s e d i m e n t a t i o n was r e p l a c e d by b r a i d e d

s t r e a m and f l o o d p l a i n s e d i m e n t a t i o n forming fining-upwards sequences (Read and Johnson, 1967; Chisholm and Dean, 1974).

S i m i l a r fining-upwards sequences have been

d e s c r i b e d by N i l s e n ( 1 9 6 9 ) , Deegan (19731, S t e e l ( 1 9 7 4 ) , and S t e e l and Wilson (1975). The fundamental r e l a t i o n s h i p between t e c t o n i c s and s e d i m e n t a t i o n i n i n t e r n a l b a s i n s i s w e l l s e e n i n t h e Old Red b a s i n s o f w e s t e r n Norway ( B r y h n i , 1964a,b; S t e e l , 1976; S t e e l e t a l . , 1977).

The Hornelen Basin (Fig.4.38)

c o n t a i n s a t h i c k n e s s of 25km o f sediment.

is r e l a t i v e l y s m a l l b u t

Marginal f a n g l o m e r a t e s occupy t h e n o r t h e r n

and s o u t h e r n a r e a s a d j a c e n t t o t h e bounding f a u l t s b u t most of t h e b a s i n i s f i l l e d by s a n d s t o n e s which accumulated from westward f l o w i n g r i v e r s on a broad a l l u v i a l plain.

Marginal i n t e r f i n g e r i n g of a l l u v i a l p l a i n and f a n g l o m e r a t e sediments s u g g e s t

t h a t a t c e r t a i n times t h e e n t i r e b a s i n was f l o o d e d by t h e a l l u v i a l p l a i n system. Both t h e m a r g i n a l f a n g l o m e r a t e s and t h e a l l u v i a l p l a i n sediments show a v e r y marked c y c l i c i t y (Kolderup, 1927; S t e e l e t a l . , 1 9 7 7 ) . The a l l u v i a l f a n o r i g i n o f t h e m a r g i n a l d e p o s i t s i s i n d i c a t e d by t h e wedge-shaped geometry, down-wedge d e c r e a s e i n maximum c l a s t s i z e , t h e dominance of l o c a l l y d e r i v e d c l a s t s , and t h e f r e q u e n t e v i d e n c e of s u b a e r i a l exposure (mudcracks and r a i n p r i n t s ) . The remarkable f e a t u r e of t h e s e f a n d e p o s i t s i s t h e u b i q u i t y of sequences which can b e i d e n t i f i e d a t a v a r i e t y of l e v e l s .

coarsening-upwards

A t t h e most g e n e r a l l e v e l

a r e sequences c o n s i s t i n g of s t a c k e d a l l u v i a l f a n s predominantly of t h e same t y p e

i . e . predominantly d e b r i s f l o w , s t r e a m flow o r mixed t y p e ( s e e F i g . 4 . 3 9 ) .

Individual

f a n b o d i e s a l o n g t h e s o u t h e r n margin o f Hornelen Basin a r e 50-80m t h i c k (proximal r e a c h ) w i t h a r a d i u s of a b o u t 5km.

Along t h e n o r t h e r n margin t h e f a n s a r e t h i c k e r

(100-2OOm) b u t a r e of s m a l l e r r a d i u s ( a b o u t lkm). a r e i m p o r t a n t i n b o t h t h e s e t y p e s o f f a n body.

Coarsening-upwards sequences

Also i n d i v i d u a l f a n sequences

commonly c o n s i s t of a number of t a b u l a r s u b u n i t s 10-25m t h i c k , each of which shows a c o a r s e n i n g and t h i c k e n i n g upwards t r e n d .

The f o l l o w i n g f e a t u r e s a r e c h a r a c t e r i s t i c :

1) A g e n e r a l coarsening-upwards i n d i c a t e d by a d e c r e a s i n g s a n d s t o n e p e r c e n t a g e 2 ) Coarsening-upwards of conglomerates a s i n d i c a t e d by mean maximum c l a s t s i z e

3 ) A g e n e r a l thickening-upward of conglomerate beds a s i m p l i e d from a p o s t i i v e bed t h i c k n e s s

- maximum p a r t i c l e

s i z e relationship

4 ) A s h a r p , a p p a r e n t l y p l a n a r upper boundary t o t h e f a n body o v e r l a i n by t h e f i n e s t a l l u v i a l p l a i n sediments 5 ) Lower boundary i s g r a d a t i o n a l b e c a u s e t h e r e i s g r a d u a l coarsening-upward from a l l u v i a l p l a i n s t r u c t u r e t o the f i n e s t ( d i s t a l ) fanglomerates.

T h i s has r e s u l t e d

from a l a t e r a l i n t e r f i n g e r i n g o f f a n and a l l u v i a l p l a i n s e d i m e n t s a s t h e f a n radius got l a r g e r through t i m e .

228

._

Grain Sism

F i g . 4 . 3 8 . S i m p l i f i e d map of the Hornelen Basin ( a f t e r K i l d a l , 1970). The s u c c e s s i o n i s 25km t h i c k and both the a x i a l and marginal d e p o s i t s a r e dominated b y coarsening-upwards sequences ( a f t e r S t e e l e t a l . , 1977).

229

0.5 km

I 7

7

i

20 40cm

Fig.4.39. Internal d e t a i l s of a composite debris flow dominated a l l u v i a l fan wedge which is interfingering with floodbasin sediments along the northern margin of the Hornelen basin ( a f t e r S t e e l e t a l . , 1977).

230 These r e l a t i o n s h i p s a r e b e s t a c c o u n t e d f o r by r e l a t i v e s u b s i d e n c e of t h e b a s i n f l o o r c a u s i n g i n c r e a s e d r e l i e f and d r a i n a g e a r e a i n t h e new upland r e g i o n ( B u l l , 1964, 1968; Hooke, 1968).

S t e e l e t a l . (1977, p.1130) s u g g e s t t h a t t h e main

t e c t o n i c e v e n t s c o n t r o l l i n g t h i s s u b s i d e n c e a r e marked by t h e s h a r p p l a n a r t o p of any f a n body.

The coarsening-upwards sequences t h u s r e p r e s e n t t h e i n c r e a s e d

t r a n s p o r t of d e b r i s and t h e g r a d u a l o u t - b u i l d i n g of t h e n e x t f a n i n a n e f f o r t t o e s t a b l i s h a n e q u i l i b r i u m p r o f i l e a c r o s s t h e b a s i n margin.

These a u t h o r s c o r r e l a t e d

t h e 10-25m s u b c y c l e s w i t h d i s c r e t e e p i s o d e s o f f a u l t movement s u g g e s t i n g t h a t t h i s r e f l e c t e d t h e maximurn a m p l i t u d e o f i n d i v i d u a l p h a s e s o f s u b s i d e n c e .

The

lOOm u n i t s

o v e r l a p e a c h o t h e r i n a n e a s t w a r d d i r e c t i o n i n t h e Hornelen Basin ( B r y h n i , 1964a). S t e e l e t a l . (1977) c o n s i d e r e d t h a t t h e amount of o v e r l a p of s u c c e s s i v e cyclotherms i s c o m o n l y less t h a n 0.25km and t h e r e f o r e s u g g e s t e d t h a t t h i s c o u l d be a c c o u n t e d f o r by a p e r i o d i c , r a p i d e a s t w a r d s h i f t i n t h e main l o c u s o f s u b s i d e n c e ; t h i s would a l s o a c c o u n t f o r t h e p l a n a r t o p s and asymmetry o f t h e cyclothems.

T h i s model a l s o

c o n v e n i e n t l y e x p l a i n s how s u c h a 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 (25km) c o u l d accumulate w i t h o u t t h e r e b e i n g a d i f f e r e n c e i n t h e d e g r e e of metamorphism between t h e b a s e and t h e t o p ( B r y h n i , 1964b).

The e a s t w a r d o v e r l a p of cyclothems s u g g e s t t h a t t h e base-

ment may n o t have been d e e p e r t h a n 6km below t h e s u r f a c e of t h e b a s i n .

Since t h i s

i s much less t h a n t h e l a t e r a l e x p a n s i o n o f t h e b a s i n i t seems l i k e l y t h a t sedimenta t i o n was c o n t r o l l e d by a wrench f a u l t s y s t e m ( C r o w e l l , 1974a).

S t e e l e t a l . (1977)

proposed a model i n which t h e b a s i n formed on t h e s o u t h e r n f l a n k of a r i g h t - s l i p f a u l t system (Fig.4.40). 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 The Old Red Sandstone p e b b l y a l l u v i u m i n t h e Midland V a l l e y of S c o t l a n d shows many

of t h e f e a t u r e s which t y p i c a l l y r e s u l t from 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 magnesian m i n e r a l s .

Many o f t h e s a n d s t o n e s and c o n g l o m e r a t i c s a n d s t o n e s c o n s i s t

o f framework g r a i n s and c l a y m a t r i x w i t h o u t t h e i n t e r m e d i a t e s i l t s i z e s .

The c l a y

m a t r i x may b e i n t e r s t i t i a l o r c o a t i n g t h e framework g r a i n s and c l o s e l y resembles t h e m e c h a n i c a l l y i n f i l t r a t e d c l a y d e s c r i b e d by Walker (1976).

The p o s t - d e p o s i t i o n a l

o x i d a t i o n of i n f i l t r a t e d c l a y c o u l d have been a s i g n i f i c a n t pigmenting p r o c e s s . The heavy m i n e r a l s u i t e s o f r e d s a n d s t o n e s i n t h e Midland V a l l e y b a s i n i n c l u d e h a e m a t i t e , i l m e n i t e , g a r n e t , a p a t i t e , e p i d o t e , r u t i l e , t o u r m a l i n e , z i r c o n and s c a r c e hornblende and pyroxene.

The s c a r c i t y of h o r n b l e n d e (maximum 3.3% of heavy

m i n e r a l s ) and pyroxene (maximum 2.6% b u t c o m p l e t e l y a b s e n t o v e r most o f t h e a r e a ) must b e a c c o u n t e d f o r b e c a u s e t h e r e i s a n abundant s o u r c e a r e a s u p p l y i n Dalradian metasediments and b a s i c v o l c a n i c s ( B r a i t h w a i t e and Jawad A l i , 1978).

Many of t h e

hornblende g r a i n s show cockscomb a l t e r a t i o n f e a t u r e s l i k e t h o s e s e e n i n Cenozoic d e s e r t a l l u v i u m which must b e due t o i n t r a s t r a t a l p r o c e s s e s .

It can be expected

t h a t l a r g e q u a n t i t i e s of a u t h i g e n i c pigmentary h a e m a t i t e and i n t e r s t i t i a l c l a y

231

...w'-.. Fig.4.40. A. S t r u c t u r a l e l e m e n t s i n Hornelen Basin. B. P a l a e o c u r r e n t s y s t e m ( s o l i d arrows a r e m a r g i n a l f a n g l o m e r a t i c d i s p e r s a l ) . C. A s p e c u l a t i v e model o f b a s i n development a l o n g t h e s o u t h e r n f l a n k o f a r i g h t - s l i p wrench f a u l t ( a f t e r S t e e l e t a l . , 1977). m a t r i x would b e produced by t h i s p r o c e s s .

The o t h e r heavy m i n e r a l groups a r e

c o n s i s t e n t w i t h t h e s e d i m e n t o l o g i c a l e v i d e n c e o f a provenance i n t h e metamorphosed D a l r a d i a n t o t h e n o r t h (Fig.4.41).

However, t h e a b s e n c e of k y a n i t e and s i l l i m a n i t e

s u g g e s t s t h a t t h e more d e e p l y r o o t e d metamorphic zones f u r t h e r n o r t h were n o t unroofed a t t h i s t i m e . A number of o t h e r f e a t u r e s f u r t h e r s u g g e s t s t h a t t h e s e r e d beds a r e of d i a g e n e t i c origin.

These i n c l u d e t h e a l t e r a t i o n and c l a y replacement o f f e l d s p a r s and t h e

p r e s e n c e of d i s s o l u t i o n p i t s and v o i d s i n some m e t a s t a b l e heavy m i n e r a l s such a s garnet.

Haematite i n t h e s e r e d beds 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 and w i t h i n

i n t e r s t i t i a l matrix.

I t c a n also b e s e e n r e p l a c i n g b i o t i t e and c h l o r i t e a l o n g

c l e a v a g e p l a n e s and complete pseudomorphs o f h a e m a t i t e a f t e r b i o t i t e o c c a s i o n a l l y occur.

The complete a b s e n c e o f m a g n e t i t e s u g g e s t s t h a t i t has s u f f e r e d complete

232 conversion to haematite by martitization and polished sections reveal abundant martitization textures consistent with this view.

I

Arbuthnott Group

I

T

1

Garvock Grouo A

AMPHIBOLE

P

APATITE

E G H

EPIDOTE GARNET HAEMATITE

I

R T

2

ILMENllE RUTILE TOURMALINE ZIRCON

Fig.4.41. Centres of distribution of heavy mineral groups in the Arbuthknott, Garvock, and Strathmore Groups of the Old Red Sandstone in the Midland Valley of Scotland (after Braithwaite and Jawad Ali, 1978).

233 Some o f t h e s e a l t e r a t i o n s a r e i l l u s t r a t e d i n Fig.4.42.

They r e p r e s e n t a more

advanced s t a g e o f e x a c t l y t h e same p r o c e s s e s a s t h o s e d e s c r i b e d i n Cenozoic d e s e r t a l l u v i u m by Walker (1967a; 1976) and Walker e t a l . (1978).

The same mechanism of

reddening a p p e a r s t o be a p p l i c a b l e t o a l l t h e pebbly a l l u v i u m and f a n d e p o s i t s of t h e Old Red Sandstone i n c l u d i n g che Orcadian b a s i n ( T a r l i n g e t a l . , and A r c h e r , 1977) and p r o b a b l y t h e Norwegian b a s i n s t o o .

1976; Turner

Variations i n the r a t e

of reddening and t h e e x t e n t of t h e d i a g e n e t i c p r o c e s s e s i n v o l v e d a r e t o b e expected These w i l l r e s u l t from v a r i a t i o n s i n t e c t o n i c a c t i v i t y , s o u r c e

i n different areas.

a r e a composition and t h e p a t t e r n and composition o f t h e groundwater flow.

Significant

d i f f e r e n c e s between t h e s e a n c i e n t r e d beds and Cenozoic a l l u v i u m i n c l u d e t h e more advanced o r complete a l t e r a t i o n s e e n i n t h e former, and a l s o i t s d a r k e r c o l o u r .

The

l a t t e r i s t y p i c a l of a n c i e n t r e d beds and a p p a r e n t l y r e s u l t s from long-continued growth o f h a e m a t i t e c r y s t a l s which a r e g e n e r a l l y c o a r s e r i n a n c i e n t r e d beds. External facies

-

reddened sandy and muddy a l l u v i u m

The e x t e r n a l f a c i e s o f t h e Old Red Sandstone was d e p o s i t e d on c o a s t a l a l l u v i a l p l a i n s and g e n e r a l l y l i e s conformably on marine c l a s t i c s o r c a r b o n a t e s .

I n the

B r i t i s h a r e a t h e most i m p o r t a n t e x t e r n a l b a s i n i s t h e Anglo-Welsh b a s i n which s t r e t c h e d a c r o s s s o u t h e r n B r i t a i n and a l s o extended i n t o s o u t h e r n I r e l a n d (Fig.4.43). The t r a n s i t i o n from marine t o 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 i s b a s i n has been d e s c r i b e d

by Allen (1974a).

A l l u v i a l s e d i m e n t a t i o n was i n i t i a t e d i n r e s p o n s e t o a widespread

r e g r e s e i o n which r e s u l t e d from t h e 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

Late S i l u r i a n and Lower Devonian. The a l l u v i a l d e p o s i t s of t h e Anglo-Welsh b a s i n a r e b r o a d l y s i m i l a r t o t h o s e i n

a number of other Old Red Sandstone b a s i n s i n c l u d i n g t h e Appalachians ( A l l e n and F r i e n d , 19681, S p i t z b e r g e n ( F r i e n d , 1965; Moody-Stuart, of Norway (Turner, 1974a,b).

1966) and t h e Oslo Graben

These d e p o s i t s f r e q u e n t l y show two major f a c i e s

a s s o c i a t i o n s , a 'coarse member' c o n s i s t i n g o f s a n d s t o n e s and i n t r a f o r m a t i o n a l conglomerate and a ' f i n e number' o f s i l t s t o n e s and mudstones ( A l l e n , 1965e).

These

f a c i e s a r e a r r a n g e d i n fining-upwards c y c l e s i n which t h e c o a r s e members r e p r e s e n t channel d e p o s i t s and t h e f i n e members f l o o d p l a i n and r i v e r bank d e p o s i t s . A.

Coarse members

The c o a r s e members of Old Red Sandstone fining-upwards c y c l e s c o n t a i n a number of d i s t i n c t i v e f a c i e s which have been s t u d i e d i n d e t a i l by A l l e n (1964a, 1965c, 1965e, 1970a, 1970b, 1974b).

Examples a r e shown i n Fig.4.44.

The b a s e of t h e c o a r s e members r e s t s on a scoured e r o s i o n s u r f a c e and u s u a l l y c o n s i s t s o f a t h i n conglomerate.

I n t r a f o r m a t i o n a l c l a s t s a r e common (mudstones and

c a r b o n a t e s from t h e r i v e r banks and f l o o d p l a i n ) and t h e r e may a l s o be e x t r a f o r m a t i o n a l pebbles.

The conglomerate may show i m b r i c a t i o n and i s i n t e r p r e t e d a s a channel l a g

234

Fig.4.42. Thin s e c t i o n photomicrographs showing t h e d i a g e n e t i c a l t e r a t i o n of framework s i l i c a t e s i n t h e Old Red Sandstone of t h e Midland V a l l e y of S c o t l a n d . A . Volcanic r o c k fragment e x t e n s i v e l y a l t e r e d t o h a e m a t i t e . B. D e t r i t a l g a r n e t , d i s s o l v e d and p a r t l y r e p l a c e d by c a l c i t e . C . Q u a r t z i t e fragment showing i n t e r c r y s t a l reddening. D. B i o t i t e p a r t i a l l y r e p l a c e d by h a e m a t i t e .

235

7 Intermontane Basins Coastal Plain

RCADIAN BASIN

Shallow Seas Deep Seas Upland Areas Volcanic bntrs

LORNE BAS1 HEVIOT BASIN

Pahtmgeography of t h e Lower Old Red Sandstone o f t h e B r i t i s h a r e a Fig.4.43. showing t h e Anglo-Welsh b a s i n . deposit.

Some c o n g l o m e r a t e s , c o n s i s t i n g e x c l u s i v e l y of i n t r a f o r m a t i o n a l m a t e r i a l

(mud and c a l c r e t e c l a s t s ) and l a c k i n g a q u a r t z o s e s a n d m a t r i x may r e p r e s e n t t h e d e p o s i t s o f ephemeral d r a i n a g e systems which developed from time t o t i m e on s l i g h t l y e l e v a t e d i n t e r f l u v i a l a r e a s ( A l l e n and W i l l i a m s , 1 9 7 9 ) . Above t h e b a s a l conglomerate a r e m o s t l y cross-bedded s a n d s t o n e s i n t r o u g h o r p l a n a r t a b u l a r s e t s , most commonly up t o l m t h i c k .

The former a r e more abundant

and a t t r i b u t e d t o t h e m i g r a t i o n o f s i n u o u s - c r e s t e d dunes.

T a b u l a r s e t s a r e formed

from t h e m i g r a t i o n of t r a n s v e r s e b a r s , sandwaves a n d , l e s s commonly, s c r o l l b a r s and c h u t e b a r s .

R e a c t i v a t i o n s u r f a c e s and mud d r a p e s may b e p r e s e n t i n d i c a t i n g

water s t a g e f l u c t u a t i o n s . Coarse member s a n d s t o n e s may show a n unusual t y p e of low-angled c r o s s - b e d d i n g r e f e r r e d t o by A l l e n (1963) a s e p s i l o n c r o s s - b e d d i n g .

When p r e s e n t t h i s u s u a l l y

o c c u p i e s t h e whole t h i c k n e s s of t h e c o a r s e member and i s t y p i c a l l y h e t e r o g e n e o u s ,

236 t h e i n d i v i d u a l s e t s c o n t a i n i n g c r o s s - l a m i n a t e d u n i t s and mud d r a p e s s u g g e s t i n g a These e p s i l o n u n i t s a r e g e n e r a l l y l e s s

v a r i a b l e h y d r o l o g i c a l regime ( F i g . 4 . 4 5 ) .

t h a n 5m t h i c k ( A l l e n , 1965c; Moody-Stuart, 1966) and d i p between 5 and 15'.

The

d i r e c t i o n of d i p can o f t e n b e d e m o n s t r a t e d t o be a t r i g h t a n g l e s t o t h e l o c a l palaeos l o p e and t h e f a c i e s i s t h e r e f o r e i n t e r p r e t e d a s a l a t e r a l a c c r e t i o n d e p o s i t l i k e t h o s e s e e n i n t i d a l c r e e k s (Van S t r a a t e n , 1 9 5 1 ) , t h e l a t e r a l a c c r e t i o n s u r f a c e presumably r e p r e s e n t i n g t h e p o i n t b a r s u r f a c e .

The r e l a t i v e s c a r c i t y of e p s i l o n

c r o s s - b e d d i n g , however, i n d i c a t e s t h a t n o t a l l of t h e Old Red S a n d s t o n e p o i n t b a r s iced e p s i l o n cross-bedding. C

D

E

F

Small scale cross-stratification

Large scale cross-stratification

Sandstone

Conglomerate

Flat-bedding

s~~

R e p r e s e n t a t i v e and s t a n d a r d c o a r s e member s a n d s t o n e from t h e Old Red Fig.4.44. Sandstone. (A,B) Welsh B o r d e r s , (C) Tugford Clee H i l l s , (D) M i t c h e l d e a n , (E) F o r e s t of Dean, (F) Clee H i l l s , (G) S p i t s b e r g e n . Note t h e v a r i a t i o n i n s c a l e and sequence. Where a r a n g e o f t h i c k n e s s i s g i v e n , t h e sequences a r e somewhat i d e a l i z e d s t a n d a r d s ( a f t e r A l l e n , 1965e).

237

O t h e r f a c i e s which commonly occur w i t h i n c o a r s e members a r e p a r a l l e l laminated s a n d s t o n e s and c r o s s - l a m i n a t e d

sandstones.

The former f r e q u e n t l y show primary

c u r r e n t l i n e a t i o n ( A l l e n , 196413) i n d i c a t i n g upper flow regime c o n d i t i o n s i n channel a r e a s o f h i g h v e l o c i t y o r s h a l l o w depth.

The c r o s s - l a m i n a t e d s a n d s t o n e s a r e

u s u a l l y f i n e r and more micaceous and may show c l i m b i n g r i p p l e c r o s s - l a m i n a t i o n which a l l o w s e s t i m a t i o n of t h e i n s t a n t a n e o u s r a t e o f v e r t i c a l bed a c c r e t i o n ( A l l e n , 1971).

Cross-laminated s a n d s t o n e s a r e i n t e r p r e t e d a s t h e d e p o s i t s of m i g r a t i n g

small s c a l e r i u p l e s .

LITHOLOGIES

&--,A ntnformrtional WLondomerato B M a j o r Bedding W a r m

Major bedding planrr

E p s i l o n cross-bedding i n t h e Old Red Sandstone. Fig.4.45. v e r t i c a l exaggeration ( a f t e r Allen, 1 9 6 5 ~ ) . B.

There is a l a r g e

F i n e members

There i s commonly no c l e a r d i s t i n c t i o n between c o a r s e member and f i n e member i n r h e Old Red Sandstone fining-upwards

of f a c i e s .

c y c l e s and t h e f i n e members may show a complex

They a l s o show e v i d e n c e of p o s t - d e p o s i t i o n a l m o d i f i c a t i o n i n c l u d i n g

b i o t u r b a t i o n and pedogenesis. Old Red Sandstone f i n e members f r e q u e n t l y show s a n d s t o n e s i n t e r b e d d e d w i t h s i l t s t o n e s and mudstones.

These a r e g e n e r a l l y less t h a n l m t h i c k w i t h convex tops

and s h a r p b a s e s .

L a t e r a l l y t h e y a r e i m p e r s i s t e n t ( L e e d e r , 1974) and may show

cross-bedding p a s s i n g upwards i n t o c r o s s - l a m i n a t i o n .

Since these sandstones

i n d i c a t e e p i s o d i c waning flow c o n d i t i o n s and a r e e n c l o s e d by f l o o d p l a i n s i l t s and muds t h e y a r e r e a d i l y i n t e r p r e t e d a s c r e v a s s e - s p l a y d e p o s i t s ( A l l e n , 1964a; Leeder, 1974). Another c h a r a c t e r i s t i c f a c i e s o f f i n e members i s t h e r a p i d a l t e r n a t i o n o f c r o s s l a m i n a t e d f i n e s a n d s o r s i l t s w i t h homogeneous mudstone t h e ' a l t e r n a t i n g b e d s ' f a c i e s of A l l e n , 1 9 6 5 ~ ) . T y p i c a l l y t h e i n d i v i d u a l u n i t s a r e r a t h e r t h i n ( l e s s than 25cm) and t h e s i l t s t o n e s show e v i d e n c e o f p e r i o d i c s u b a e r i a l exposure.

The sand-

s t o n e s a r e s h a r p b a s e d , sometimes e r o s i o n a l and may c a r r y a n g u l a r s i l t s t o n e c l a s t s . These r a p i d a l t e r n a t i o n s i n d i c a t e p e r i o d s o f submergence and d r y i n g o u t c o n s i s t e n t w i t h d e p o s i t i o n a s l e v e e s o r from overbank f l o o d i n g . The most c h a r a c t e r i s t i c and abundant f a c i e s of t h e Old Red Sandstone f i n e members i s r e d , r a t h e r homogeneous mudstones and s i l t s t o n e s .

Homogenization may

r e s u l t from b i o t u r b a t i o n and t h e r e i s f r e q u e n t e v i d e n c e of s u b a e r i a l exposure i n t h e form of mudcracks.

The s i l t s t o n e s and mudstones c l e a r l y r e p r e s e n t 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 o f t h e r i v e r f l o o d p l a i n s which were s u b j e c t t o p e r i o d i c f l o o d i n g and d r y i n g o u t . These f l o o d p l a i n d e p o s i t s i n t h e Old Red Sandstone c o n t a i n f r e q u e n t l y e x t e n s i v e c o n c r e t i o n a r y c a r b o n a t e u n i t s which t e n d t o be c o n c e n t r a t e d towards t h e t o p o f t h e f i n e members.

The s i m i l a r i t y of t h e s e c o n c r e t i o n a r y c a r b o n a t e s (which were formerly

known a s c o r n s t o n e s ) t o P l i o c e n e o r Q u a t e r n a r y c a r b o n a t e s o f known pedogenic o r i g i n h a s been r e a l i z e d f o r some time.and more r e c e n t l y d e t a i l e d comparisons have been made by A l l e n (1973, 1974d) and Leeder (1975, 19761, 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 c a l c r e t e s i n t h e S h r o p s h i r e a r e a have been made by A l l e n (1974a) who a p p l i e d t h e terminology developed f o r s o i l s by Brewer (1964). The c a l c r e t e s g e n e r a l l y o c c u r i n beds up t o 2m t h i c k and c o n s i s t o f c a l c i t e g l a e b u l e s (Brewer, 1964, p . 2 5 9 ) , u s u a l l y less t h a n 5cm i n d i a m e t e r s c a t t e r e d i n a groundmass o r s - m a t r i x (Brewer, 1964, p.146).

The g l a e b u l e s v a r y c o n s i d e r a b l y i n

form and i n t e r n a l l y c o n t i i n numerous c a l c i t e v e i n s o r c r y s t a l l a r i a (Brewer, 1964, p.284) a l o n g w i t h d i f f u s e p a t c h e s and broken v e i n s of m a t r i x .

Many Old Red Sand-

s t o n e c a l c r e t e s a l s o show c r y s t a l t u b e s (Brewer, 1964, p.287) up t o s e v e r a l m i l l i m e t r e s i n d i a m e t e r , o f t e n b r a n c h i n g , and l i k e t h e e l o n g a t e d g l a e b u l e s l y i n g a t a h i g h a n g l e t o t h e bedding.

The c a l c r e t e s a r e most commonly developed w i t h i n

an s - m a t r i x o f mudstone o r s i l t s t o n e , b u t i n some a r e a s (e.g. Anglesey, A l l e n , 1 9 6 5 ~ )t h e y a r e f r e q u e n t l y developed i n a s a n d s t o n e groundmass.

Usually a

c h a r a c t e r i s t i c p r o f i l e i s developed i n which r a t h e r s m a l l , s c a t t e r e d g l a e b u l e s and c r y s t a l t u b e s p r o g r e s s i v e l y i n c r e a s e i n s i z e and d e n s i t y upwards, o f t e n assuming c y l i n d r i c a l t o p r i s m a t i c forms s t e e p l y i n c l i n e d t o bedding.

The t o p of t h e p r o f i l e

i s o f t e n a massive l i m e s t o n e which h a s a s h a r p , uneven t o p and may b e o v e r l a i n by

mudstone, s a n d s t o n e o r i n t r a f o r m a t i o n a l conglomerate.

Where t h e c a l c r e t e p r o f i l e

239

B

Thick r d . coarse 8 B s t m w h I.nticles and persoatmi bad8 of wry fin. r i w bedded smdston Invertebrate burron several horizons No suncracks Abundant cilcium carbonate concretions

7 coarse stnstm mm iwanebrate burrows. rippla-b.dd.d sendston kmcbr. and convdU1e lamitwtiom No evldare ROCI

6

5

4

8

f

4

3 scoured surface in l o w r part Scattered siltstona clasts

2

1

0

Details o f Old Red Sandstone f i n e members i n t h e Anglo-Welsh b a s i n Fig.4.46. A . From Lydney B. From Abergavenny ( a f t e r A l l e n , 1964a).

i s developed i n s a n d s t o n e a s l i g h t l y d i f f e r e n t a p p e a r a n c e r e s u l t s :

on t h e lower

p a r t of t h e p r o f i l e g l a e b u l e s may b e d i f f i c u l t t o d i f f e r e n t i a t e from t h e c a l c i t e cememted h o s t r o c k and h i g h e r i n t h e p r o f i l e t h e g l a e b u l e s may b e r e s t r i c t e d t o l a r g e n o d u l a r masses ( F i g . 4 . 4 7 ) . A number of v a r i a t i o n s from t h i s s t a n d a r d c a l c r e t e p r o f i l e do e x i s t and one of

t h e s e i s t h e development o f l a r g e - s c a l e p s e u d o - a n t i c l i n e s .

I n t h e s e , curved

c r y s t a l l a r i a w i t h s h a l l o w d i p d e f i n e t h e ' l i m b s ' of t h e a n t i c l i n e and t h e g l a e b u l e s which a r e n e a r l y v e r t i c a l may b e e i t h e r i n c l i n e d s l i g h t l y inwards towards t h e a x i s

240

3

z 2 v)

2

r' I

0

20

Percent acid-solubk

40

60

80

I1

Percent acid-soluble

LEGEND Mudstone

0

Sandstone

Introformotional conglornerote

Gloebules

Parallel laminations

Irregular bonding

a

0

Limestone

Crors- bedding

Fig.4.47. V e r t i c a l c a l c r e t e p r o f i l e s i n t h e Abdon Limestone Formation. A. Nordybank (581848) B. Abdon L i b e r t y (587668) showing g r o s s l i t h o l o g y and t h e v a r i a t i o n i n w t % of a c i d - s o l u b l e c o n s t i t u e n t s (predominantly c a l c i t e ) ( a f t e r A l l e n , 1974a). of t h e p s e u d o a n t i c l i n e o r t h e y f a n outwards and upwards away from t h e a n t i c l i n a l

a x i s (Fig.4.48)

( A l l e n , 1974a, p.113-115).

Pseudoanticlines a r e , l i k e the standard

c a l c r e t e p r o f i l e , e i t h e r s h a r p l y o v e r l a i n by p l a n e l a m i n a t e d mudstone o r e r o s i v e l y o v e r l a i n by i n t r a f o r m a t i o n a l conglomerate and s a n d s t o n e s of t h e s u c c e e d i n g c o a r s e member.

These p s e u d o a n t i c l i n e s p r o b a b l y r e s u l t from t h e d i s p l a c i v e c r y s t a l l i z a t i o n

of c a l c i t e .

The c a l c r e t e s of t h e Old Red Sandstone show many p e t r o g r a p h i c f e a t u r e s v e r y s i m i l a r t o t h o s e s e e n i n Recent c a l c r e t e s .

These i n c l u d e u n d i f f e r e n t i a t e d and

a g g l o m e r a t i c c r y s t i c p l a s m i c f a b r i c s s e e n i n Recent c a l c r e t e s by Blank and Tynes (1965), G i l e e t a l . (1965) and Williams and Polach (1971) and a l s o mural f a b r i c (Brewer, 1964).

There i s abundant e v i d e n c e o f d i s p l a c i v e c r y s t a l l i z a t i o n of c a l c i t e

(Reeves, 1976; W a t t s , 1977, 1978) and e x f o l i a t e d s k e l e t o n g r a i n s o c c u r i n b o t h Recent (e.g.

Swineford e t a l . , 1958) and Old Red Sandstone c a l c r e t e s .

Cross c u t t i n g

s h e e t - l i k e c r y s t a l l a r i a a r e common i n t h e upper p a r t s of modern c a l c r e t e s and a l s o i n some o f t h e Old Red Sandstone p r o f i l e s ( G i l e e t a l . , Reeves, 1970).

Pseudo-oolitic

1966; N a g t e g a a l , 1969;

and p s e u d o - p i s o l i t i c t e x t u r e s conunonly observed i n

Recent c a l c r e t e s (Swineford e t a l . , 1958; S i e s s e r , 1973; Hay and Reeder, 1978) are a l s o s e e n i n t h e Old Red Sandstone.

241

Mudstone

%Tubes

I

' ~ i a t y ' crystallaria

I

Mudstone

IB

Mudstone

Fig.4.48.

Schematic v e r t i c a l s e c t i o n s i l l u s t r a t i n g p s e u d o - a n t i c l i n a l s t r u c t u r e s i n pedogenic c a r b o n a t e u n i t s i n t h e Old Red Sandstone A. Lydney t y p e B. Freshwater E a s t type ( a f t e r A l l e n , 1974a).

The Old Red Sandstone c a r b o n a t e u n i t s compare v e r y c l o s e l y w i t h t h e c a l c a r e o u s d e p o s i t s o f P l i o c e n e and Q u a t e r n a r y age which a r e v a r i o u s l y r e f e r r e d t o a s c a l c r e t e o r c a l i c h e ( A r i s t a r a i n , 1971a).

The mudstones w i t h s c a t t e r e d g l a e b u l e s a r e v e r y l i k e

t h e g l a e b u l a r s o i l s d e s c r i b e d by Bernard and Le Blanc ( 1 9 6 5 ) , Blokhuis e t a l . (1969) and Singh and Singh (1972).

T h i s r e p r e s e n t s S t a g e I and S t a g e I1 i n t h e morpho-

g e n e t i c scheme o f calcrete development proposed by G i l e e t a l . (1966, p.352).

The

more massive, sometimes laminated c a l c r e t e s a r e s i m i l a r t o t h e pedogenic l i m e s t o n e s d e s c r i b e d by Swineford e t a l . ( 1 9 5 8 ) , G i l e (1961, 1 9 7 0 ) , G i l e and Hawley (1966), Reeves ( 1 9 7 0 ) , A r i s t a r a i n (1971a,b) and Williams (1973) and p r o b a b l y r e p r e s e n t S t a g e 111 and S t a g e I V development '(Gile e t a l . , 1966) ( T a b l e 4 . 3 ) . The c l o s e s i m i l a r i t y o f t h e Old Red Sandstone and r e c e n t c a l c r e t e e n a b l e s some p a r a l l e l s t o b e drawn i n t h e i r c o n d i t i o n s o f f o r m a t i o n s

A t t h e p r e s e n t time t h e s e

c o n d i t i o n s a r e a n a r i d t o s e m i - a r i d climate, markedly s e a s o n a l r a i n f a l l , and a r e l a t i v e l y high mean a n n u a l t e m p e r a t u r e (Goudie, 1973, p.96-111).

These P l i o c e n e

and Quaternary c a l c r e t e s a r e most commonly formed i n t h e s h a l l o w s u b - s u r f a c e o f s t a b l e geomorphic l e v e l s (Ruhe, 1969; G i l e , 1970).

They a r e o f pedogenic o r i g i n

i n which downward p e r c o l a t i n g r a i n w a t e r d i s s o l v e s calcium c a r b o n a t e which is

242

p r e c i p i t a t e d lower i n t h e s o i l p r o f i l e a s b o t h c a l c i t e and high-magnesian c a l c i t e ( ' p e r descensum'

model o f Goudie, 1973).

J u d g i n g from t h e p r o t r a c t e d n a t u r e of

r e c e n t c a l c r e t e p e d o g e n e s i s s u b s t a n t i a l l y t h i c k c a l c r e t e p r o f i l e s i n t h e Old Red Sandstone c o u l d have t a k e n o v e r 104 y e a r s t o form. TABLE 4.3.

S t a g e s of t h e morphogenetic sequences i n t h e development o f c a l c r e t e p r o f i l e s and t h e youngest l a n d s u r f a c e s on which t h e s t a g e s o c c u r ( a f t e r G i l e e t a l . , 1966). Stage

Gravelly s o i l s

D i a g n o s t i c Carbonate Morphology Non g r a v e l l y s o i l s

Youngest Geomorphic s u r f a c e on which s t a g e o f h o r i z o n occurs

Thin d i s c o n t i n u o u s pebble coatings

Few f i l a m e n t s o r f a i n t coatings

F i l l m o r e 26005000 y e a r s

I1

Continuous p e b b l e c o a t i n g s some i n t e r p e b b l e fillings

Few t o common n o d u l e s

Leasburg 5000 years l a t e s t Pleistocene

I11

Many i n t e r p e b b l e f i l l i n g s

Many n o d u l e s and internodular f i l l i n g s

Picacho Late Pleistocene

IV

Laminar h o r i z o n o v e r l y i n g plugged h o r i z o n

(increasing carbonate impregnation)

Picacho Late Pleistocene

I

( t h i c k e n e d l a m i n a r and plugged h o r i z o n s )

J o r n a d a Mid- t o Late Pleistocene Laminar h o r i z o n o v e r l y i n g plugged horizon

La Mesa Mid Pleistocene

The fining-upwards c y c l e s i n t h e Old Red Sandstone p r o b a b l y r e c o r d d e p o s i t i o n i n a v a r i e t y of a l l u v i a l c h a n n e l t y p e s and s e t t i n g s .

Where t h e c o a r s e members a r e

i s o l a t e d i n t h e f i n e member a l l u v i u m t h e y always r e s t on a b a s a l e r o s i o n s u r f a c e . This may be f l a t o r g e n t l y concave; sometimes s t e e p , n e a r v e r t i c a l c h a n n e l margins occur.

Concave-based l e n t i c u l a r c o a r s e members a r e g e n e r a l l y r e g a r d e d t o have

formed i n l o w - s i n u o s i t y c h a n n e l s by v e r t i c a l a g g r a d a t i o n .

F l a t - b a s e d c o a r s e members,

on t h e o t h e r hand, a r e more l i k e l y t o have formed i n meandering c h a n n e l s where l a t e r a l migration ensured erosion a t a constant l e v e l .

Good examples of t h e s e

c h a n n e l t y p e s a r e found i n t h e Old Red Sandstone of S p i t s b e r g e n (Moody-Stuart, 1966) (Fig.4.49). The meandering c h a n n e l i n t e r p r e t a t i o n has been used t o e x p l a i n many Old Red Sandstone sequences which have t h i c k f i n e members w i t h pedogenic c a r b o n a t e s .

Good

examples i n c l u d e t h e Old Red Sandstone o f Anglesey ( A l l e n , 1 9 6 5 c ) , t h e Red Marls o f Pembrokeshire ( A l l e n , 1974b) and t h e Wood Bay Formation o f S p i t s b e r g e n ( F r i e n d and Moody-Stuart, 1972).

These sequences a r e , i n a g e n e r a l way, comparable w i t h

243

DEPOSITS Fine channel fill

Lww Channel Floor

Flood Basin Epsilon cross-beddin! Channel directions I '

Fig.4.49. Models i l l u s t r a t i n g t h e r e l a t i o n s h i p between c h a n n e l - t y p e and c o a r s e member s t r u c t u r e i n t h e Old Red Sandstone of S p i t s b e r g e n ( a f t e r Moody-Stuart, 1966). t h e d e p o s i t s of modern meandering c h a n n e l s which wander through t h e i r own a l l u v i u m . R i v e r s o f t h i s t y p e c u t l a t e r a l l y e x t e n s i v e , f l a t e r o s i o n s u r f a c e s which may b e l i n e d w i t h i n t r a f o r m a t i o n a l conglomerate eroded from t h e f l o o d p l a i n ( F i s k , 1944). Sand d e p o s i t i o n o c c u r s p r i n c i p a l l y by l a t e r a l a c c r e t i o n on p o i n t b a r s (Sundborg, 1956, Harms e t a l . ,

1963) and t h e fining-upwards i n t h e c o a r s e member may b e d i r e c t l y

compared w i t h t h e d e c r e a s e i n g r a i n s i z e on t h e s u r f a c e of modern p o i n t b a r s which r e s u l t s from t h e r e d u c t i o n i n bed s h e a r s t r e s s away from t h e c h a n n e l .

The f i n e

members, w i t h t h e i r e v i d e n c e o f s u b a e r i a l e x p o s u r e , b e a r much s i m i l a r i t y t o t h e t h i c k f l o o d p l a i n d e p o s i t s o f a number of r i v e r s which show e v i d e n c e of c a r b o n a t e pedogen-

esis (Bernard and L e Blanc, 1965).

The t h i n s a n d s t o n e upits w i t h i n t h e f i n e member

s i l t s t o n e s i n d i c a t e i n u n d a t i o n s o f t h e f l o o d p l a i n by f l o o d w a t e r s and a r e p r o b a b l y levee o r crevasse-splay deposits. T h i s i n t e r p r e t a t i o n i s n o t , however, w i t h o u t i t s problems.

I n laterally accreted

p o i n t b a r s e p s i l o n c r o s s - b e d d i n g might b e e x p e c t e d t o b e more abundant t h a n i t apparently i s .

A l s o , modern meandering r i v e r s a r e l a t e r a l l y l i m i t e d by e r o s i o n

s u r f a c e s on t h e c h a n n e l banks o r t h e c l a y p l u g s of abandoned c h a n n e l l o o p s .

Steep

channel margins a r e , t h e r e f o r e , l i k e l y t o b e common i n meandering s t r e a m sediments b u t a r e v e r y r a r e i n t h e Old Red Sandstone (one i s d e s c r i b e d by A l l e n (1964a) from

Tugford) and i n a n c i e n t meandering s t r e a m s e d i m e n t s as a whole.

The r e a s o n s f o r

t h e s e d i s c r e p a n c i e s a r e n o t c l e a r b u t i t i s p o s s i b l e t h a t n o t a l l fining-upwards c y c l e s of t h i s type a r e meandering s t r e a m d e p o s i t s . t h i n c o a r s e members ((2m)

Some c y c l e s show r e l a t i v e l y

which a r e l a t e r a l l y e x t e n s i v e , f l a t - b a s e d and w i t h an

abundance of p a r a l l e l l a m i n a t e d s a n d s .

These c o u l d b e t h e d e p o s i t s o f a r e a l l y

e x t e n s i v e , s i n g l e e v e n t f l o o d s which might o c c u r , f o r example, i n t h e d i s t a l r e g i o n s of large ' a l l u v i a l f a n s ' . I n some o t h e r Old Red Sandstone sequences t h e c o a r s e members a r e dominant and t h e r e i s l i t t l e p r e s e r v a t i o n of t h e f i n e member i n t e r - c h a n n e l d e p o s i t s .

These

d e p o s i t s a r e g e n e r a l l y i n t e r p r e t e d a s t h o s e of l o w - s i n u o s i t y sandy r i v e r s which may have been b r a i d e d .

The p a u c i t y o f muddy f l o o d p l a i n d e p o s i t s may b e p a r t l y due

t o t h e f a c t t h a t t h e y were o r i g i n a l l y p o o r l y developed b e c a u s e o f a low suspended l o a d (Schurmn and Kahn, 1972) o r b e c a u s e of t h e tendency o f t h e s e r i v e r s t o comb t h e i r floodplain eroding the floodplain deposits.

The i n t e r p r e t a t i o n of a n c i e n t low-

s i n u o s i t y s t r e a m d e p o s i t s i s d i f f i c u l t b e c a u s e t h e r e a r e few d e t a i l e d s t u d i e s of modern d e p o s i t s .

Cant and Walker (1976) have made a f a c i e s model of t h e B a t t e r y

P o i n t Sandstone of Quebec u s i n g a combination o f f a c i e s sequence and p a l a e o c u r r e n t information (Fig.4.50).

T h i s b e g i n s w i t h a s c o u r e d b a s e and i n t r a f o r m a t i o n a l

conglomerate which p a s s e s upwards i n t o s a n d s t o n e s w i t h t r o u g h cross-bedding.

Within

t h e sequence a r e p l a n a r t a b u l a r cross-bedded u n i t s whose p a l a e o f l o w d i r e c t i o n s d i v e r g e from t h o s e of t h e t r o u g h c r o s s - b e d s by up t o 90'. c o n s i s t s o f s m a l l - s c a l e t a b u l a r s e t s and c r o s s - l a m i n a t i o n .

The t o p of t h e sequence By comparison w i t h t h e

d e p o s i t s of South Saskatchewan R i v e r (Cant and Walker, 1978) t h i s sequence i s i n t e r p r e t e d i n t h e f o l l o w i n g manner ( s e e Fig.4.14).

The t r o u g h c r o s s - b e d s a r e due

t o s i n u o u s - c r e s t e d h i g r a t i n g dunes i n t h e c h a n n e l and t h e s e d e f i n e t h e main palaeoflow d i r e c t i o n .

The p l a n a r t a b u l a r s e t s w i t h d i v e r g e n t flow d i r e c t i o n s a r e t h e

r e s u l t of c r o s s - c h a n n e l b a r s o b l i q u e t o t h e main c h a n n e l a x i s .

The upper p a r t of

t h e sequence may r e p r e s e n t t h e b a r t o p w i t h s a n d f l a t a c c r e t i o n and c h a n n e l aggradation.

A t t h e v e r y t o p o f t h e sequence may be a t h i n zone o f v e r t i c a l

accretion (floodplain) deposits.

The complexity o f c o a r s e member-dominated sequences

i n t h e Old Red Sandstone i s s u c h t h a t many more d e t a i l e d and d i r e c t comparisons w i t h p r e s e n t - d a y l o w - s i n u o s i t y sandy r i v e r s a r e needed i f t h e y a r e t o b e p r o p e r l y understood. The r e d d e n i n g of sandy and muddy a l l u v i u m One of 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 s o f r e d beds i n t h e Old Red S a n d s t o n e i s t h e d i f f e r e n c e i n c o l o u r between t h e c o a r s e and f i n e members o f fining-upwards c y c l e s (Fig.4.51).

The f i n e members a r e a l m o s t e x c l u s i v e l y r e d (5R, 10R) whereas t h e c o a r s e

members v a r y t h r o u g h s h a d e s of g r e y (N), g r e e n i s h - g r e y (5GY), o l i v e - g r e y (5Y) and brownish-grey (5yR)

( F r i e n d , 1966; Van Houten, 1973; T u r n e r , 1974a).

245

"\ e-

\

Verjical accretion

l

I Channel floor

Fig.4.50. F a c i e s model f o r t h e B a t t e r y P o i n t Sandstone (Old Red Sandstone, Quebec). The changes i n v e r t i c a l sequence and p a l a e o c u r r e n t s a r e i n t e r p r e t e d a s t h e product o f a low-sinuosity r i v e r ( a f t e r Cant and Walker, 1976). a t h i n d r a b zone may be p r e s e n t n e a r t h e top o r t h e b a s e of t h e f i n e member (Fig.4.51) a l t h o u g h t h i s i s g e n e r a l l y a t t r i b u t e d t o secondary b l e a c h i n g .

I n some sequences,

p a r t i c u l a r l y i n Late Devonian times when p l a n t s were more abundant, drab zones w i t h i n t h e f i n e member may be a s s o c i a t e d w i t h o r g a n i c d e b r i s and c a n r e a d i l y b e i n t e r p r e t e d a s marsh o r backswamp d e p o s i t s i n which r e d u c i n g c o n d i t i o n s were developed s h o r t l y a f t e r d e p o s i t i o n (e.g.

Johnson and Friedman, 1969).

The e s s e n t i a l d i f f e r e n c e between t h e r e d f i n e member d e p o s i t s , and d r a b , c o a r s e member d e p o s i t s , i s t h a t t h e former c o n t a i n a g r e a t e r p r o p o r t i o n o f pigmentary haematite.

Much o f t h i s i s c o n c e n t r a t e d w i t h i n t h e c l a y f r a c t i o n (Walker and

Honea, 1969; Van Houten, 1973) s u g g e s t i n g t h a t t h e d e p o s i t i o n a l mechanism must have had a c o n t r o l l i n g i n f l u e n c e on t h e observed red- non-red d i f f e r e n t i a t i o n .

A t the

same t i m e t h e wide v a r i a t i o n i n c o l o u r of c o a r s e members and t h e abundance of haemati t e i n f i n e members (remembering i t s p a u c i t y i n Recent a l l u v i u m ) does s u g g e s t t h a t

p o s t - d e p o s i t i o n a l p r o c e s s e s were i m p o r t a n t and that t h e r e were d i f f e r e n c e s i n t h e

246

p o s t - d e p o s i t i o n a l h i s t o r y o f c o a r s e and f i n e members.

ENVIRONMENT

tTURES MAIN FEA-. ---

COLOUR

CHANNEL BASE

I (BLEACHED) CALCR ETE

SOIL

MASSIVE SI LTSTONE

FLOODPLAIN

CROSS- LAM1NATED

BANK DEPOSITS

fl

SI LTSTONES

I

OSS-BEDDED SANDSTONES

I

OR }NTERMEDIAT

CHANNEL

\\\k

Fig.4.51. T y p i c a l c o l o u r v a r i a t i o n s s e e n i n a n Old Red Sandstone fining-upwards cyclothem. A.

F i n e member r e d d e n i n g

F r i e n d (1966) s t u d i e d t h e c o l o u r d i s t r i b u t i o n o f fining-upwards c y c l e s i n t h e Old Red Sandstone of t h e C a t s k i l l s and S p i t s b e r g e n by a n a l y s i n g changes i n c o l o u r , c l a y m i n e r a l o g y , g r a i n s i z e and i r o n c o n t e n t ,

The c l a y m i n e r a l o g y o f t h e s e r e d

beds i s v e r y s i m i l a r t o many o t h e r a n c i e n t r e d b e d s ; t h e s a n d s t o n e s a r e r e l a t i v e l y r i c h e r i n c h l o r i t e - p l u s - k a o l i n i t e w h i l s t t h e s i l t s t o n e s from t h e f i n e members a r e relatively richer i n i l l i t e .

Although t h e r e i s some v a r i a t i o n from c y c l e t o c y c l e

t h e r e i s l i t t l e doubt t h a t a mature c l a y m i n e r a l assemblage of t h i s t y p e i s t h e r e s u l t of a u t h i g e n e s i s and 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 and t h e r e i s t e x t u r a l e v i d e n c e i n f a v o u r o f t h i s ( F r i e n d , 1966, p.279). Although t h e C a t s k i l l and S p i t s b e r g e n r e d beds show a c o n c e n t r a t i o n of haematite i n t h e c o a r s e t o medium c l a y f r a c t i o n s (1-4ym) t h e r e a r e no c l e a r r e l a t i o n s h i p s between c o l o u r and c l a y m i n e r a l o g y (Fig.4.52).

T h i s is t h e c a s e w i t h many a n c i e n t

241

lllitr 1 J

0 0 SMdstOM R d NOW

1,11,111

RIfn to

Fig.4.52. Peak h e i g h t r a t i o s f o r i l l i t e , . c h l o r i t e and k a o l i n i t e f o r r e d and drab s a n d s t o n e s and s i l t s t o n e s i n t h e Old Red Sandstone o f t h e C a t s k i l l Mountains, USA ( a f t e r F r i e n d , 1966). r e d beds (Van Houten, 1973; T u r n e r , 1974a) which i s n o t s u r p r i s i n g because t h e h a e m a t i t e i s g e n e r a l l y e u h e d r a l and most l i k e l y a u t h i g e n i c i n o r i g i n .

A number of

a u t h o r s (Van Houten, 1964, 1973; F r i e n d , 1966; T u r n e r , 1974a) have a r g u e d t h a t t h e c o n c e n t r a t i o n o f pigmentary h a e m a t i t e i n f i n e members r e f l e c t s a d e p o s i t i o n a l c o n c e n t r a t i o n o f brown-coloured, environment.

f e r r i c hydroxide p r e c u r s o r i n t h e f l o o d p l a i n

F r i e n d (1966) e x p l a i n e d t h e d i f f e r e n c e i n c o l o u r between t h e c o a r s e

and f i n e members i n t h e f o l l o w i n g manner (Fig.4.53).

I n t h e f l o o d p l a i n environment

abundant i r o n hydroxides were d e p o s i t e d w i t h t h e c l a y - r i c h suspended l o a d .

Because

of p e r i o d i c d r y i n g , and l o w e r i n g o f t h e water t a b l e , o x i d i z i n g c o n d i t i o n s p r e v a i l e d and i r o n hydroxides e v e n t u a l l y aged i n t o h a e m a t i t e .

I n t h e c h a n n e l environment,

however, t h e sediment remained m o s t l y below t h e w a t e r t a b l e a f t e r d e p o s i t i o n .

In

a r e a s where s u b s t a n t i a l o r g a n i c m a t t e r was p r e s e n t b a c t e r i a l consumption of oxygen c r e a t e d a r e d u c i n g environment i n which f e r r i c hydroxides were u n s t a b l e and removed i n s o l u t i o n and t h e c h a n n e l sediments u l t i m a t e l y became d r a b . S e v e r a l f a c t o r s s u g g e s t t h a t t h i s s i m p l e mechanism s u g g e s t e d by F r i e n d i s , i n essence, correct.

These i n c l u d e t h e p a u c i t y of h a e m a t i t e i n Recent a l l u v i u m

(Walker, 19741, t h e f a c t t h a t c l a y - g r a d e f e r r i c h y d r o x i d e s a r e u n s t a b l e r e l a t i v e t o h e m a t i t e ( B e r n e r , 1 9 6 9 a ) , t h e l a r g e amounts of t o t a l i r o n found i n f i n e members, and a l s o t h e f a c t t h a t t h e geochemical c o n d i t i o n s d e s c r i b e d by F r i e n d a r e commonly m e t w i t h i n modern a l l u v i u l s e d i m e n t s .

A c o n t r i b u t i o n from t h e d i a g e n e t i c break-

down o f s i l i c a t e g r a i n s i s a l s o t o be e x p e c t e d b u t t h e r e l a t i v e importance of t h i s

248

p r o c e s s i s d i f f i c u l t t o a s s e s s b e c a u s e o f t h e l a c k of m i n e r a l o g i c a l e v i d e n c e .

ON DEPOSITION

AFTER DEPOSITION EARLY

TAN-BROWN COLOURED FERRIC OXYHYDROXIDES

1

LATE

OXIDIZING

-b

REDUCING

-b

FERROUS IRON I N DETRITAL SILICATES

HAEMATITE FERROUS CCMPOUNDS REMOVED IN SOLUTION AND PYRITE

INTRASTRATAL ALTERATION

,

OXIDIZING

WATER TABLB

REDUCING CONDITIONS DEVELOPED IN WATERLOGGED SANDS ESPECIALLY I F THEY CONTAIN ORGANIC MATERIAL

Fig.4.53. Suggested sequence and environment o f f o r m a t i o n of i r o n compounds i n Old Red Sandstone fining-upwards c y c l e s ( a f t e r F r i e n d , 1966). A q u a n t i t a t i v e e s t i m a t e o f t h e t i m e s c a l e of r e d d e n i n g i n Old Red Sandstone

f i n e members i s p o s s i b l e b e c a u s e o f t h e o c c u r r e n c e of pedogenic c a l c r e t e u n i t s . These o f t e n o c c u r , a l o n g w i t h r e d mudstones, a s i n t r a f o r m a t i o n a l conglomerates i n t h e Old Red Sandstone ( A l l e n , 1960, 1962a) ( F i g . 4 . 5 4 ) .

O f t e n r e d mudstone c l a s t s

o c c u r i n a d r a b , sandy m a t r i x , a l t h o u g h i t i s more u s u a l t o f i n d t h a t t h e m a t r i x shows some e v i d e n c e of reddening.

I n e i t h e r c a s e t h e f a c t t h a t t h e c l a s t s were

s u f f i c i e n t l y c o n s o l i d a t e d t o s u r v i v e f l u v i a l t r a n s p o r t i s a n i n d i c a t i o n t h a t they were a l m o s t c e r t a i n l y r e d a t t h e t i m e ,

The t i m e s c a l e o f c a l c r e t e development

t h u s g i v e s some e s t i m a t e o f t h e t i m e t a k e n f o r t h e development o f r e d c o l o u r . However, t h i s t i m e s c a l e i s d i f f i c u l t enough t o estimate f o r Recent c a l c r e t e s l e t a l o n e t h o s e i n t h e Old Red Sandstone.

Radiocarbon d a t i n g by Ruhe (1967) o f d u s t -

d e r i v e d c a l c r e t e s i n New Mexico s u g g e s t t h a t c a l c i f i c a t i o n i n c r e a s e s a t t h e r a t e of 1%p e r 1000 y e a r s .

Compatible r e s u l t s have been o b t a i n e d by G i l e and Grossman

(1968) and Williams and P o l a c h (1971).

These r e s u l t s , and t h e o r e t i c a l c o n s i d e r a t i o n s ,

s u g g e s t s t h a t t h i c k c a l c r e t e s have developed o v e r some hundreds o f thousands o f years.

249

A . T h i n s e c t i o n photomicrograph of a n Old Red Sandstone c a l c r e t e showing Fig.4.54. l a r g e c r y s t a l l a r i a c u t t i n g a reddened s i l t s t o n e m a t r i x (Red Marls, F r e s h w a t e r West, Wales). B. Conglomeratic c a l c r e t e showing rounded, p r e v i o u s l y r e d d e n e d , c a r b o n a t e c l a s t s i n c a l c i t e cement (Upper Old Red S a n d s t o n e , P e r t h s h i r e , S c o t l a n d ) .

2 50 Goudie (1973, p.88-89)

c a l c u l a t e d a r a t e of l m o f 80%CaC03 p e r 300,000 y e a r s .

Under t h e s e c i r c u m s t a n c e s (assuming t h a t s i m i l a r r a t e s a p p l i e d i n Old Red Sandstone t i m e s ) i t i s n o t s u r p r i s i n g t h a t t h e mudstones were r e d by t h e t i m e t h e s u c c e e d i n g cyclothem was i n i t i a t e d .

Not o n l y i s t h i s s u f f i c i e n t t i m e f o r t h e d e h y d r a t i o n and

i n v e r s i o n of d e t r i t a l f e r r i c hydroxides i n t o h a e m a t i t e i n t h e manner s u g g e s t e d by Van Houten (1972) b u t s u b s t a n t i a l i n s i t u s i l i c a t e a l t e r a t i o n c o u l d a l s o t a k e p l a c e e s p e c i a l l y under t h e c l i m a t i c c o n d i t i o n s which p r e v a i l e d (Walker, 1967a, 1974, 1976). The r e l a t i v e c o n t r i b u t i o n of e a c h of t h e s e p r o c e s s e s i n r e d bed f o r m a t i o n c a n n o t be deduced from t h e a v a i l a b l e e v i d e n c e a l t h o u g h i t seems l i k e l y t h a t b o t h were i n opera t i o n . B.

Coarse member r e d d e n i n n

The mechanism of red-drab d i f f e r e n t i a t i o n proposed by F r i e n d (1966) does n o t s a t i s f a c t o r i l y e x p l a i n t h e f a c t t h a t many c o a r s e members a r e r e d o r brownish-red i n colour.

T h i s i s e s p e c i a l l y s o i n c o a r s e member-dominated s u c c e s s i o n s i n which

f i n e members a r e t h i n o r a b s e n t .

The i m p l i c a t i o n h e r e i s t h a t r e d u c i n g c o n d i t i o n s

were never developed and t h a t t h e groundwaters d u r i n g d i a g e n e s i s were always oxygenated.

The p o s t - d e p o s i t i o n a l h i s t o r y o f c o a r s e member s a n d s t o n e s i s f u r t h e r

c o m p l i c a t e d by e v i d e n c e which s u g g e s t s t h a t t h e r e were r e d u c i n g groundwaters m i g r a t i n g through t h e s a n d s t o n e b o d i e s u n t i l t h e i r f i n a l compaction.

This takes

t h e form of g r e e n , s e c o n d a r i l y b l e a c h e d zones which o c c u r j u s t below a n d , i n some c a s e s , j u s t above t h e c o a r s e members ( F i g . 4 . 5 1 ) .

These s u g g e s t t h a t r e d u c i n g

groundwaters were e x p e l l e d from t h e porous s a n d s t o n e b o d i e s p r o b a b l y d u r i n g compac t i o n . Colour v a r i a t i o n s i n t h e c o a r s e members of sandy a l l u v i u m i n t h e Old Red Sandstone a r e l a r g e l y due t o t h e r e l a t i v e abundance and d i s t r i b u t i o n of pigmentary clay-oxides (Table 4.4).

In some c l a y - r i c h s a n d s t o n e s t h e framework c o n s t i t u e n t s

have t h i c k c l a y - o x i d e p e l l i c l e s (Fig.4.55) Brewer (1964).

which resemble t h e c l a y - c u t a n s o f

O t h e r c l a y - o x i d e c o a t i n g s show t e x t u r e s which c l o s e l y resemble t h e

m e c h a n i c a l l y i n f i l t r a t e d c l a y t e x t u r e s d e s c r i b e d by Walker (1976) from Cenozoic alluvium.

I n b o t h t h e s e c a s e s t h e s u r v i v a l of t h e h a e m a t i t e c l a y - o x i d e m a t r i x i s

a n i n d i c a t i o n t h a t o x i d i z i n g c o n d i t i o n s p r e v a i l e d d u r i n g d e p o s i t i o n and d i a g e n e s i s . S e v e r a l l i n e s of e v i d e n c e a l s o s u g g e s t t h a t pigmentary h a e m a t i t e h a s been produced by i n t r a s t r a t a l s o l u t i o n of i r o n b e a r i n g g r a i n s i n t h e r e d c o a r s e member s a n d s t o n e s . T h i s e v i d e n c e i n c l u d e s t h e composition o f heavy m i n e r a l s u i t e s , c l a y m i n e r a l o g y , and t h e n a t u r e of t h e a u t h i g e n i c m i n e r a l p h a s e s i n t h e s a n d s t o n e s . A n o t a b l e f e a t u r e of heavy m i n e r a l a n a l y s e s i n t h e Old Red Sandstone i s t h e a l m o s t t o t a l absence of u n s t a b l e f e r r o m a g n e s i a n s i l i c a t e s such a s h o r n b l e n d e and pyroxene.

A l l e n (1974a) p u b l i s h e d a d e t a i l e d s u r v e y of heavy m i n e r a l d i s t r i b u t i o n s

i n t h e O l d Red Sandstone of t h e Anglo-Welsh b a s i n and r e c o g n i z e d t h r e e heavy m i n e r a l

251

Fig.4.55. T e x t u r a l f e a t u r e s of pigmentary i r o n o x i d e s and c l a y - i r o n o x i d e s i n Old Red Sandstone sandy a l l u v i u m . A . P a r t i c u l a t e g r a i n c o a t i n g s of h a e m a t i t e formed by p o s t - d e p o s i t i o n a l p r e c i p i t a t i o n ( R i n g e r i k e Group, S i l u r i a n , Norway). B . Thick p e l l i c l e s o f c l a y - i r o n o x i d e (Lower Old Red S a n d s t o n e , Dyfed, Wales). C. I n t e r s t i t i a l m a t r i x o f c l a y - i r o n o x i d e c o n c e n t r a t e d i n laminae (Lower Old Red Sandstone, Gamrie O u t l i e r , S c o t l a n d ) .

252

associations:

metamorphic ( g a r n e t , s t a u r o l i t e and e p i d o t e ) , igneous ( a n a t a s e ,

a p a t i t e , b r o o k i t e , r u t i l e , sphene, s p i n e l and z i r c o n ) , and p e g m a t i t i c ( t o u r m a l i n e The s t r a t i g r a p h i c a l v a r i a t i o n i n t h e s e t h r e e assemblages i s shown i n Fig.4.56.

only),

TABLE 4.4

R e l a t i o n s h i p s between whole rock c o l o u r and t e x t u r a l d i s t r i b u t i o n of c l a y - o x i d e m i s t u r e s i n Devonian r e d s a n d s t o n e s . Colour

Texture

Moderate Pink 5R7f 4

-

Moderate Orange Pink 10R7f 4

Sparse clay-oxide g r a i n coatings L i t t l e o r no i n t e r s t i t i a l m a t r i x

P a l e Red 5R6f2 10R6f2

Clay-oxide g r a i n c o a t i n g s a b s e n t t o p o o r l y developed. Opaque t o t r a n s l u c e n t i n t e r s t i t i a l m a t r i x more abundant

Moderate Reddish Orange 10R6f 6

Prominent t r a n s l u c e n t c l a y - o x i d e g r a i n c o a t i n g s , I n d i v i d u a l c r y s t a l s t o o small t o be r e s o l v e d . S p a r s e i n t e r s t i t i a l m a t r i x . I f more abundant, homogeneously distributed

Moderate Red 5R5f4 5R4f6

P a r t i c u l a t e g r a i n c o a t i n g s mainly of h a e m a t i t e . Abundant opaque i n t e r s t i t i a l m a t r i x may b e inhomogeneously d i s t r i b u t e d

-

-

Individual crystals g r e a t e r than 2 m can b e r e s o l v e d

P a l e Reddish Brown 10R5f 4

-

Dark Reddish Brown 10R3f4

Abundant t h i c k g r a i n c o a t i n g s of c l a y and h a e m a t i t e and opaque i n t e r s t i t i a l matrix

P a l a e o c u r r e n t a n a l y s i s and t h e s t u d y o f e x o t i c p e b b l e s , when combined w i t h t h e heavy m i n e r a l r e s u l t s s u g g e s t a s o u r c e a r e a t o t h e n o r t h w e s t o f t h e a r e a which i n c l u d e d Moine and D a l r a d i a n igneous and metamorphic r o c k s a s w e l l a s Lower P a l a e o c o i c sediments.

The Downton Group i s n o t a b l e f o r t h e l a r g e amounts of g a r n e t which

s u g g e s t a h i g h grade metamorphic provenance l i k e t h e Moine o r D a l r a d i a n .

The

c o m p o s i t i o n a l change a t t h e end of Downton Group times, when t h e igneous assemblage becomes much more i m p o r t a n t (Fig.4.56)

a p p e a r s t o b e due t o u p l i f t and i s o l a t i o n

of t h e d i s t a n t metamorphic s o u r c e a r e a by r i v e r c a p t u r e o r d r a i n a g e r e v e r s a l . The absence of pyroxene and hornblende i n t h e Old Red Sandstone i s q u i t e marked ( s e e a l s o F l e e t , 1925, 1926; Walder, 1941) d e s p i t e t h e f a c t t h a t i t i s l i k e l y t o have been r e l a t i v e l y abundant i n a s o u r c e a r e a which i s known t o have i n c l u d e d igneous and metamorphic r o c k s .

The observed assemblage i s dominated by m i n e r a l s

w i t h a h i g h l e v e l of p e r s i s t e n c e i n a n c i e n t s a n d s t o n e s ( P e t t i j o h n , 1975, p.496). S i m i l a r r e s u l t s a r e found throughout t h e Old Red c o n t i n e n t and i n view o f t h e known s u s c e p t i b i l i t y of hornblende and pyroxene t o i n t r a s t r a t a l a l t e r a t i o n i n o x i d i z i n g groundwaters i t seems l i k e l y t h a t complete d i s s o l u t i o n h a s t a k e n p l a c e .

The i n i t i a l

a l t e r a t i o n p r o d u c t s a r e most l i k e l y t o have been 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

2 53

and i r o n o x i d e , much of t h e i r o n o x i d e e v e n t u a l l y c o n t r i b u t i n g rowards t h e pigmentary haematite.

D i r e c t evidence o f i n s i t u s i l i c a t e a l t e r a t i o n i s p r o v i d e d by p a r t i a l l y

haematized c h l o r i t e and b i o t i t e f l a k e s ( t h e s e m i n e r a l s a r e o f comparable s t a b i l i t y t o g a r n e t i n t h e d i a g e n e t i c environment).

Advanced i n s i t u a l t e r a t i o n s a r e i n d i c a t e d

by t h e t o t a l pseudomorphing of b i o t i t e f l a k e s by h a e m a t i t e ( T u r n e r , 1974a; Turner and A r c h e r , 1977) ( F i g . 4 . 5 5 ) .

LEGEND Melomorphic Pegmatitic

DITTON AND A I D O N GPS.

Igneous

osseniblaqe assemblage

assernbtage

LEDBURY FM.

I

2

3

6

8 10

20

40

60

100

Percenl

Fig.4.56. S t r a t i g r a p h i c a l v a r i a t i o n i n t h e number p e r c e n t a g e of heavy m i n e r a l groups i n t h e Old Red Sandstone of t h e Anglo-Welsh b a s i n ( a f t e r A l l e n , 1974a). The c l a y m i n e r a l s u i t e s i n t h e Old Red Sandstone a l s o i n d i c a t e e x t e n s i v e diagenesis.

The main c l a y m i n e r a l s a r e i l l i t e , c h l o r i t e and k a o l i n i t e ; montmorill-

o n i t e b e i n g o n l y r a r e l y r e p o r t e d (e.g. A l l e n , 1974a).

I l l i t e t y p i c a l l y shows a

h i g h c r y s t a l l i n i t y index and may have f o r m e d , i n p a r t a t l e a s t , by t h e t r a n s f o r m a t i o n o f 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 which formed a s a n a u t h i g e n i c phase d u r i n g e a r l y diagenesis. The Lower T r i a s s i c r e d beds of NW Europe The lowermost T r i a s s i c ( S c y t h i a n ) of NW Europe saw t h e accumulation of a pebbly and sandy a l l u v i a l sequence on a n o r t h w e s t e r l y i n c l i n e d p a l a e o s l o p e (Audley-Charles, 1970a,b).

Northwards o f t h e London-Brabant h i g h i s a m a r g i n a l conglomeratic f a c i e s

r e f e r r e d t o a s t h e Bunter Pebble Beds (Fig.k.57)

(Brennand, 1975).

Northwestwards

t h e g r a i n s i z e d e c r e a s e s a s t h e pebbly a l l u v i u m p a s s e s downstream i n t o more sandy

254 alluvium.

There a r e a l s o marked v e r t i c a l changes i n g r a i n s i z e and i n t h e Midlands

o f England t h e Bunter P e b b l e Beds p a s s upwards i n t o a sandy a l l u v i a l sequence (Upper M o t t l e d S a n d s t o n e ) .

Above t h i s i s a renewed i n f l u x o f c o a r s e r , p e b b l y

a l l u v i u m (Lower Keuper S a n d s t o n e ) .

A t a b o u t t h e same t i m e t h e d e p o s i t i o n of 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 may have been t a k i n g p l a c e i n NW S c o t l a n d b u t t h e s e d e p o s i t s a r e n o t y e t a c c u r a t e l y d a t e d ( S t e e l and W i l s o n , 1975).

n

Maximum distribution Triassic deposits

--

Distribution solid Muschelkalk carbonates Northern limit Muschelkalk carbonates

-I

Edge Rot salt

1 Areas with Keuper salts

5w

Total Triassic thickness

Fig.4.57. Palaeogeography of t h e lower most T r i a s s i c ( S c y t h i a n ) i n NW Europe ( a f t e r Z i e g l e r , 1978). The sedimentology of t h e Bunter P e b b l e Beds h a s been d e s c r i b e d and i n t e r p r e t e d by Thompson (1970).

These t y p i c a l l y form fining-upwards c y c l e s i n which beds of

s t r u c t u r e l e s s o r i m b r i c a t e d , sometimes uncemented, conglomerate p a s s up i n t o l a r g e s c a l e cross-bedded p e b b l y s a n d s t o n e s and t r o u g h cross-bedded s a n d s (Fig.4.58). mudstones may b e p r e s e n t a t t h e t o p of t h e c y c l e .

Thin

The sequence is i n t e r p r e t e d a s

t h e d e p o s i t s o f a l o w - s i n u o s i t y b r a i d e d s t r e a m s i m i l a r t o t h o s e d e p o s i t i n g pebbly a l l u v i u m a t t h e p r e s e n t day.

T h i s view i s s u p p o r t e d by t h e f a c t t h e d e p o s i t s l i e

above concave e r o s i o n s u r f a c e s and g e n e r a l l y f i n e upwards a l o n g w i t h a d e c r e a s e

INTRAFORMATIONAL u- CONGLOMERATE

EXOTIC PEBBLES GRANULES VENTIFACTS

CHANNEL

LARGE SCALE CROSS-BEDDING

mERmIoN

.SCOURED SURFACE

LAMINATED SHALE

SMALL SCALE CROSSBEDDING

MUDCRACKS

01

01

N

mBLocKy

CHANNEL FULL SEDIMENTS DEPOSITEDAS CHANNEL BAR AND LATERAL BARS. POSSIBLE DEVELOPMENT OF POINT BARS WITH LATERAL ACCRETION SURFACES

INTRAFORMATIONAL MUD CLASTS NEAR BASE. ROLLED DREIKANTER. LARGE SCALE CROSS-BEDDING WITH FORESETS POORLY DEFINED AT BASE. SET THICKNESS GENERALLY DECREASES W A R D S ,

RAPID EROSION OF FLOODPLAIN DEPOSITS

RELATIVELY SUDDEN CESSATION OF FLOOD AND BEDLOAD TRACTION

RED? SECONDARILY BUFF OR GREY COARSE TO MEDIUM GRAINED. POORLY SORTED SANDSTONE, PEBBLY SANDSTONE AND CONGLOMERATE.

EXTRAFORMATIONAL PEBBLES. DISC AND SPHEROID SHAPE CONSISTINGOF QUARTZ, QUARTZITE. PORPHYRY. FELSITE.TUFF. CHERT,SANDSTONE, AND MUDSTONE

VERTICAL ACCRETION DEPOSITS OF LOW SINUOSITY RIVER FLOOD PLAIN

INTERPRETATION

RED MICACEOUS,LAMINATED OR BLOCKY, SILTY MUDSTONES WITH MUD CRACKS. SHARP FLAT UPPER SURFACE.AT TIMES IRREGULAR

CHANNELING

MAIN FACTS

256

i n set thickness.

The s t r u c t u r e l e s s and i m b r i c a t e d conglomerates compare c l o s e l y

with the longitudinal gravel bars of present-day coarse grained braided r i v e r s w h i l s t t h e p r e s e n c e of l a r g e s c a l e l a t e r a l a c c r e t i o n u n i t s may r e s u l t from d i a g o n a l o r l a t e r a l b a r s (Bluck, 1974) o r t h e f o r m a t i o n of g r a v e l meander l o b e s l i k e t h o s e d e s c r i b e d by Gustavson (1978). A d i f f e r e n t t y p e of sequence i s s e e n i n t h e upper p a r t o f t h e Lower Keuper Sandstone ( F i g . 4 . 5 9 ) .

Here t h e bank and f l o o d p l a i n d e p o s i t s a r e t h i c k e r and

conglomerate i s l a r g e l y c o n f i n e d t o t h e b a s e o f t h e c y c l e .

The sequence shows t h e

f a m i l i a r fining-upwards and above t h e conglomerate ( c h a n n e l l a g ) a r e c r o s s - s t r a t a which c o u l d b e p o i n t b a r d e p o s i t s o r p o s s i b l y t r a n s v e r s e b a r s o r sandwaves (Harms e t a l . , 1975).

These p a s s upwards i n t o t r o u g h c r o s s - b e d s which i n d i c a t e channel-

f i l l by s i n u o u s c r e s t e d m e g a r i p p l e s .

Above, i n t e r b e d d e d r i p p l e d s a n d s and mudstones

w i t h d e s i c c a t i o n c r a c k s and r e p t i l e f o o t p r i n t s i n d i c a t e l e v e e and f l o o d p l a i n deposition.

T h i s t y p e of c y c l e s u g g e s t s a more meandering s t r e a m c h a n n e l , p a r t l y

e n t r e n c h e d i n i t s own a l l u v i u m and w i t h well-developed banks and r i v e r f l o o d p l a i n s . I n NW England t h e d i s t a l r e g i o n s of t h i s S c y t h i a n a l l u v i a l complex r e v e a l a d i f f e r e n t p a t t e r n of s e d i m e n t a t i o n .

Here a sandy a l l u v i a l sequence ( S t . Bees

Sandstone) l i e s d i r e c t l y on a n Upper Permian e v a p o r i t i c sequence ( S t . Bees S v a p o r i t e s and S h a l e s ) which i s i n t e r b e d d e d w i t h t h i n m a r g i n a l a l l u v i a l f a n d e p o s i t s from t h e a d j a c e n t Lake D i s t r i c t Massif (brockrams) ( A r t h u r t o n and Hemingway, 1972). The S t . Bees Sandstone c o n s i s t s p r e d o m i n a n t l y o f f i n e and medium g r a i n e d sands t o n e s w i t h minor s i l t s t o n e s and mudstones.

These a r e a r r a n g e d i n l a r g e t a b u l a r

u n i t s which wedge-out l a t e r a l l y between prominent e r o s i o n a l bounding s u r f a c e s (Fig.4.60).

Between t h e bounding s u r f a c e s t h e a l l u v i a l c y c l e s b e g i n w i t h i n t r a -

f o r m a t i o n a l l a g cdnglomerate and p a s s upwards i n t o p l a n a r t a b u l a r cross-bedded u n i t s , p l a n e beds o r pseudo-plane beds (Smith, 1971a).

I n t u r n , these u s u a l l y give

way t o t r o u g h c r o s s - b e d s o r c r o s s - l a m i n a t e d s i l t s t o n e s which may g r a d u a l l y f i n e upwards i n t o a t h i n d e s i c c a f e d mudstone o r s h a l e ,

T h i s sequence c l o s e l y resembles

t h a t which i s produced by l o w - s i n u o s i t y , sandy b r a i d e d r i v e r s .

The p l a n a r t a b u l a r

u n i t s of t h e S t . Bees Sandstone f r e q u e n t l y show r e a c t i v a t i o n s u r f a c e s l i k e t h o s e d e s c r i b e d from t h e l i n g u o i d b a r s o f t h e Tana R i v e r by C o l l i n s o n (1970) and t r a n s v e r s e b a r s o r sandwaves of a s i m i l a r t y p e a r e b e l i e v e d t o have been a n i m p o r t a n t f e a t u r e of t h e S t . Bees Sandstone.

The t r o u g h c r o s s - b e d s r e p r e s e n t m i g r a t i n g s i n u o u s -

c r e s t e d dunes which were t h e main c h a n n e l - f i l l i n g bedforms. The c o l o u r of t h e s e T r i a s s i c sandy and p e b b l y a l l u v i u m i s a l m o s t u n i f o r m l y r e d e x c e p t f o r a r e a s which have o b v i o u s l y been s e c o n d a r i l y b l e a c h e d .

Although t h e

o r i g i n of t h e r e d n e s s has been d e b a t e d f o r some t i m e r e c e n t s t u d i e s ( T u r n e r and I x e r , 1977; I x e r e t a l . , 1979) s u g g e s t t h a t t h e T r i a s s i c d e p o s i t s have had a s i m i l a r d i a g e n e t i c h i s t o r y t o t h e d e s e r t sandy a l l u v i u m d e s c r i b e d by Walker (1976) and Walker e t a l . (1978).

The r e d d e n i n g a p p e a r s t h e r e f o r e t o have been produced 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 ferromagnesian minerals.

F e a t u r e s which s u p p o r t t h i s

Rrt

m m

O I I-. lmi o

c o

Hss s m m

,3

.m,

m r t a o

W FACTS

!WALL WALE CROQS-BEDDING

LARGE SCALE CROIIS-BEDDINCi

WlTW M W C l A n r .

MuwTolyE

LAMINATED W A L E

INTMFOIIMATIONAL CONGLOMERATE

MUDCRACKS

E R W O N CHANNEL

SrIAYS

A U O BANK D E W T I

B~WT#G.OCCAUONAL COARSE SANOSTONES

RIVER FLOOD PLAIN.

FWE SAIIMTONES FLAT BEfDMNG AND RIPPLE

.

M M A M I U A T E D SHALE, M U W O N E AND

MAINLY VERTICAL ACCRETnm D€W6llS OF THE

INTERCRETATION

INCLUMIYG LEVEES AND m B L Y CREVASSE

-ICY

CWCOLATE R€D,OCCMIOIIALLY GREEN,UUWINATED 01

SCOUREDSWRFACE

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258

m

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m I

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F i g . 4 . 6 0 . The l a t e r a l v a r i a t i o n i n sedimentation sequences of the St. Bees Sandstone a t Fleswick Bay, Cumbria. The sequences were d e p o s i t e d by low-sinuosity braided streams with a sandy bed load.

2 59

view a r e :

1) t h e g e n e r a l d i s t r i b u t i o n o f h a e m a t i t e a s g r a i n c o a t i n g s and i n t e r s t i t i a l m a t r i x 2 ) t h e g e n e r a l a b s e n c e of f e r r o m a g n e s i a n m i n e r a l s and heavy m i n e r a l assemblages dominated by z i r c o n , t o u r m a l i n e and r u t i l e

3 ) t h e o c c u r r e n c e of h a e m a t i t e - c l a y pseudomorphs which c o u l d b e t o t a l l y r e p l a c e d ferromagnesian mineral g r a i n s 4) t h e w i d e s p r e a d o x i d a t i o n and r e p l a c e m e n t of p h y l l o s i l i c a t e s

5 ) t h e i n s i t u replacement and d i s s o l u t i o n of f e l d s p a r

6 ) t h e p r e s e n c e of a d i s t i n c t i v e a u t h i g e n i c s u i t e of m i n e r a l s i n c l u d i n g q u a r t z , f e l d s p a r , c l a y m i n e r a l s , h a e m a t i t e , a n a t a s e and r u t i l e , and c a l c i t e . I n a l l t h e s e f e a t u r e s s u p p o r t t h e view t h a t t h i s r e d T r i a s s i c a l l u v i u m r e p r e s e n t s a n advanced s t a g e of t h e d i a g e n e t i c p r o c e s s e s d e s c r i b e d by Walker e t a l . ( 1 9 7 8 ) . The a n a l o g y i s , however, n o t p e r f e c t b e c a u s e t h e T r i a s s i c sediments were d e r i v e d from a mixed s o u r c e a r e a c o n t a i n i n g i g n e o u s , s e d i m e n t a r y and metamorphic r o c k s and u n s t a b l e f e r r o m a g n e s i a n g r a i n s were n o t a s abundant a s i n t h e Cenozoic 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.

A consequence of t h i s may have been t h a t d e t r i t a l f e r r i c

h y d r o x i d e s were more abundant and c o n t r i b u t e d d i r e c t l y t o r e d d e n i n g i n t h e f i n e r grained sediments.

The e v i d e n c e does s u g g e s t t h a t i n t h e sandy a l l u v i u m o t h e r

p r o c e s s e s were more i m p o r t a n t .

These i n c l u d e t h e m e c h a n i c a l i n f i l t r a t i o n of c l a y

and i t s s u b s e q u e n t r e d d e n i n g , t h e d i s s o l u t i o n and replacement of l a b i l e s i l i c a t e g r a i n s by c l a y l e a d i n g t o a c o n c e n t r a t i o n o f d i s s o l v e d i o n s i n t h e i n t e r s t i t i a l groundwater and t h e u l t i m a t e p r e c i p i t a t i o n of a u t h i g e n i c m i n e r a l s from t h e groundwater solutions. Upper Cretaceous r e d beds of t h e s o u t h e r n Bohemian Basins

The Upper C r e t a c e o u s i n Czechoslovakia i n c l u d e s a sequence of c o n t i n e n t a l r e d and g r e y beds (Klikov Formation) which were d e p o s i t e d i n two NW-SE b a s i n s developed on w e a t h e r e d c r y s t a l l i n e basement.

f a u l t bounded

The sequence comprises a s e r i e s

of fining-upwards cyclothems on a s m a l l and l a r g e - s c a l e and i s c h a r a c t e r i z e d by r a p i d l a t e r a l and v e r t i c a l changes (Fig.4.61)

which c l o s e l y resembles t h e s t y l e of s e d i -

m e n t a t i o n r e f e r r e d t o a s " f l y s c h i n molasse".

The sequence was d e p o s i t e d i n a l l u v i a l

and l a c u s t r i n e environments s t r o n g l y i n f l u e n c e d by t e c t o n i c u n r e s t i n t h e AlpineCarpathian geosyncline. Colour d i s t r i b u t i o n i n t h e K l i k o v Formation shows a marked dependence on g r a i n s i z e and l i t h o l o g y and t h r e e main v a r i a t i o n s can b e r e c o g n i z e d a s f o l l o w s : L i g h t g r e y , g r e e n i s h l i g h t - g r e y o r y e l l o w i s h - g r e y s a n d s t o n e s ( A ) . These a r e medium t o c o a r s e c o n g l o m e r a t i c s a n d s t o n e s which a r e p o o r l y s o r t e d and o f t e n graded. Red beds (B). These a r e reddish-brown, zones w i t h v e r y v a r i a b l e g r a i n s i z e .

sometimes p u r p l e o r g r e e n i s h - g r e y m o t t l e d They i n c l u d e p o o r l y s o r t e d c o n g l o m e r a t i c sandy

mudstones (B ) and sandy c l a y s t o n e s (B2). 1

260

Grey beds (C). These a r e g r e e n i s h - g r e y o r d a r k g r e y s e d i m e n t s w i t h c a r b o n i z e d p l a n t debris.

They i n c l u d e f i n e t o medium l a m i n a t e d s a n d s t o n e s ( C , )

and c l a y s t o n e s w i t h

s p h a e r o s i d e r i t e n o d u l e s (C 1. 2 The d i s t r i b u t i o n of t h e d i f f e r e n t f a c i e s w i t h r e s p e c t t o d e p o s i t i o n a l environment i s shown i n Fig.4.62.

Although i n d i v i d u a l c y c l e s show some s i m i l a r i t y t o t h e

s t a n d a r d fining-upwards model ( A l l e n , 1965e) t h e r e a r e some i m p o r t a n t d i f f e r e n c e s p a r t i c u l a r l y i n t h e abundance of g r a d e d bedding.

T h i s may r e p r e s e n t t h e f l o o d

d e p o s i t s of a l l u v i a l f a n s which were d e p o s i t e d a s t u r b i d i t y flows i n l a k e s ( e . g , R a t t i g a n , 1966; Duff e t a l . ,

1967).

G e n e r a l i z e d p r o f i l e of t h e Upper C r e t a c e o u s K l i k o v Formation i n s o u t h e r n Fig.4.61. Czechoslovakia showing t h e main t y p e s o f cyclothem ( a f t e r Slefnska’, 19761.

261 The r e d b e d s of t h e K l i k o v Formation a r e g r e a t l y e n r i c h e d i n i r o n r e l a t i v e t o 3+ The a d d i t i o n a l i r o n i s l a r g e l y i n t h e form of Fe ;

t h e a s s o c i a t e d d r a b beds.

whereas t h e r e d and d r a b beds have comparable Fe2+ c o n t e n t s , t h e r e d sediments have Fe3+/Fe2+ r a t i o s g e n e r a l l y g r e a t e r t h a n 5 whereas t h e d r a b beds a r e between 0.5 and 2.

Most of t h e Fe3+ i s i n t h e form of f i n e g r a i n e d pigmentary h a e m a t i t e which i n

t h e r e d beds i s c l o s e l y a s s o c i a t e d w i t h abundant w e l l - c r y s t a l l i z e d k a o l i n i t e . S d n s k a (1976) a r g u e d t h a t t h e i r o n - r i c h , k a o l i n i t i c n a t u r e o f t h e r e d beds must have been produced d u r i n g d e p o s i t i o n by s e d i m e n t a r y d i f f e r e n t i a t i o n and s u g g e s t e d t h a t t h e r e d beds formed i n o x i d i z i n g f l o o d p l a i n environments w h i l s t t h e drab beds formed i n r e d u c i n g c h a n n e l and l a c u s t r i n e environments ( F r i e n d , 1966).

Furthermore,

h e i m p l i e d t h a t h a e m a t i t e and g o e t h i t e may have formed from p o o r l y c r y s t a l l i n e i r o n r i c h k a o l i n i t e s ( C a r r o l l , 1958; Weaver and P o l l a r d , 1973) which underwent i n c r e a s e i n c r y s t a l l i n i t y during diagenesis. LITHOLOGICAL NPE

ENVIRONMENT FLUVIAL

TYPE OF

+LACUSTRINE CROSS-BEDDING LAMINATED GRADCD BEODlNG

SANDSTONES GREY BEDS

+ SILTITONES

% OF WHOLE THICKNE!

l6 %

HORIZONTAL DISCRETE BEDDING

CLAYSTONES

CONGLOMERATIC MUDDY SANDSTONIS OR @D CONOLOMERATIC IDS SANDY MUDSTONES OR SANDY CLAYSlDNES

GRADED BEDDING

38 % ! UNDlSTlNCTlVE BEDDING

SANDSTONES

MASSIVE CROSS-BEDDING GRADED BEDDING

MEDIUM to COARSE GRAINED CONGLOMERATIC

46 %

Fig.4.62. F a c i e s d i s t r i b u t i o n of t h e t h r e e main l i t h o l o g i c a l t y p e s i n t h e Klikov Formation ( a f t e r Sldnska', 1976). The i m p l i c a t i o n of t h i s , of c o u r s e , i s t h a t t h e pigmentary o x i d e s must have been d e r i v e d from soils and weathered p r o f i l e s developed i n t h e s o u r c e a r e a , a l t h o u g h t h e s e need n o t n e c e s s a r i l y b e l a t o s o l s .

Sllnska' (1976) examined s e v e r a l weathered

p r o f i l e s of basement r o c k s ( g r a n i t e s and g n e i s s e s ) , which a r e p r e s e n t e d b e n e a t h t h e Klikov Formation, i n o r d e r t o g a i n i n f o r m a t i o n a b o u t t h e c l i m a t i c environment and n a t u r e o f w e a t h e r i n g a t t h e t i m e of r e d bed f o r m a t i o n .

The weathered c r u s t s v a r y

i n t h i c k n e s s between 2 and 2Om and a r e b e s t developed where t h e o v e r l y i n g r e d beds

262 a r e thinnest.

T h i s r e l a t i o n s h i p i s t o be e x p e c t e d i n a t e c t o n i c a l l y c o n t r o l l e d

environment b e c a u s e w e a t h e r e d p r o f i l e s a r e b e s t developed i n t h o s e a r e a s which have undergone t h e l e a s t s u b s i d e n c e . The w e a t h e r e d c r u s t s d e s c r i b e d by Sla'nskg (1976) a r e r e d and m o t t l e d brownishred with greenish grey s t a i n s .

I n t h e g n e i s s e s t h r e e zones can b e d i s t i n g u i s h e d ,

from t h e t o p downwards: 1) Completely w e a t h e r e d g n e i s s a b o u t 4m t h i c k , l i g h t g r e y i n c o l o u r w i t h p a t c h e s o f b r i g h t r e d , brown and p u r p l e .

I t c o n t a i n s r e s i d u a l q u a r t z , k a o l i n i t e and i r o n

oxides. 2) Brown and r e d m o t t l e d g n e i s s a b o u t 15m t h i c k .

The p r i m a r y t e x t u r e i s s t i l l

v i s i b l e b u t t h e f e l d s p a r s a r e c o m p l e t e l y a l t e r e d and b i o t i t e i s o x i d i z e d .

3 ) Dark g r e e n i s h - g r e y b i o t i t e g n e i s s w i t h k a o l i n i z e d f e l d s p a r s and c h l o r i t i z e d biotite.

T h i s p a s s e s downwards i n t o u n a l t e r e d g n e i s s .

Chemical changes f o r two weathered p r o f i l e s a r e r e p r e s e n t e d i n F i g . 4 . 6 3 u s i n g Loughman's (1969) method.

This depicts r e l a t i v e gains o r losses during the different

s t a g e s of w e a t h e r i n g . There a r e some d i f f e r e n c e s between t h e two p r o f i l e s p a r t i c u l a r l y w i t h r e g a r d t o t h e alumina c o n t e n t of t h e w e a t h e r e d rock and t h a t of r e l a t i v e l y u n a l t e r e d rock. Assuming t h a t t h i s r a t i o i s a n i n d i c a t i o n o f t h e d e g r e e of w e a t h e r i n g t h e n t h e top zone o f p r o f i l e I1 r e p r e s e n t s a more advanced s t a g e t h a n t h e same zone i n p r o f i l e I. The s i l i c a c u r v e f o r b o t h p r o f i l e s shows r e l a t i v e enrichment i n t h e i n t e r m e d i a t e zones.

T h i s i s d i f f e r e n t t o t h e t r e n d observed by G o l d i c h (1938) f o r t h e Morton

g r a n i t e g n e i s s which shows a s t e a d y d e c r e a s e w i t h i n c r e a s e i n w e a t h e r i n g i n t e n s i t y . The Fe 0 c u r v e s show a g r a d u a l and t h e n s h a r p i n c r e a s e a s o x i d a t i o n and h y d r a t i o n 2 3 of t h e i r o n p r o c e e d s , The FeO c u r v e shows t h e o p p o s i t e t r e n d . The a l k a l i m e t a l s o r e l e a c h e d o u t d u r i n g t h e e a r l y s t a g e s of w e a t h e r i n g b u t a p p e a r t o b e more o r l e s s s t a b l e a s weathering progresses. These w e a t h e r e d p r o f i l e s t h u s a p p e a r t o b e v e r y s i m i l a r t o t h e r e d p o d z o l i c p r o f i l e s developed on c r y s t a l l i n e r o c k s , l i k e t h a t on t h e Morton g r a n i t e g n e i s s , d e s c r i b e d by G o l d i c h (1938).

The end-product o f t h i s t y p e of w e a t h e r i n g i s a

s o i l h o r i z o n e n r i c h e d i n k a o l i n i t e and i r o n hydroxide.

T h i s i n d i c a t e s a warm and

humid s o u r c e a r e a climate undergoing i n t e n s i v e w e a t h e r i n g ,

T e c t o n i c u n r e s t probably

p r e v e n t e d t h e f o r m a t i o n of t r u e l a t e r i t i c m a n t l e s b e c a u s e of t h e r a p i d e r o s i o n of the s o i l s ,

Sla'nska' (1976) concluded, t h e r e f o r e , t h a t t h e a l l u v i u m of t h e Klikov

Formation was o r i g i n a l l y brown i n c o l o u r w i t h much of t h e i r o n b e i n g i n t r o d u c e d i n t h e form o f k a o l i n i t e - i r o n o x i d e complexes.

The u l t i m a t e c o l o u r and c o l o u r d i f -

f e r e n t i a t i o n was t h e r e s u l t of s u b s e q u e n t e a r l y d i a g e n e t i c changes.

I n oxidizing

zones h a e m a t i t e c r y s t a l l i z e d from i r o n h y d r o x i d e s w h i l s t i n r e d u c i n g zones a u t h i g e n i c s i d e r i t e and b l e a c h e d p a t c h e s developed.

The s t u d y p r o v i d e s a n i m p o r t a n t example

of t h e i m p o r t a n c e of f e r r i a l i t i c s o i l s i n r e d bed f o r m a t i o n .

Such s o i l s p r o v i d e

f i n e g r a i n e d a l l u v i u m which i s r i c h i n i r o n h y d r o x i d e s and c l a y m i n e r a l s a l t h o u g h

263

t h e u l t i m a t e f o r m a t i o n o f r e d beds depends upon d i a g e n e t i c p r o c e s s e s s u c h a s t h e a g e i n g of i r o n h y d r o x i d e s i n t o h a e m a t i t e and t h e r e l e a s e of i r o n 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 . O t h e r a n c i e n t t r o p i c a l w e a t h e r i n g p r o f i l e s have been d e s c r i b e d by Blank (1976) a n d S t u r t e t a l . (1979).

"'1

CONCENTRATION RATIO b (PROFILE2)

Fig.4.63. S u c c e s s i v e c h e m i c a l changes i n i t i a t e d d u r i n g a n c i e n t w e a t h e r i n g of g n e i s s p r i o r t o t h e d e p o s i t i o n of t h e Upper C r e t a c e o u s Klikov Formation. R e s u l t s a r e r e c a l c u l a t e d on a n anhydrous b a s i s . C o n c e n t r a t i o n r a t i o i s t h e r a t i o of t h e c o n t e n t o f an o x i d e a t s p e c i f i c l e v e l t o i t s c o n t e n t i n t h e p a r e n t rock a d j u s t e d t o t h e alumina c o n c e n t r a t i o n r a t i o a s a u n i t ( a f t e r Sla'nska', 1976).

264 CONCLUSIONS AND FURTHER READING

A l l u v i a l r e d beds comprise a v a r i e t y of r e d beds formed i n d i f f e r i n g d e p o s i t i o n a l environments and a wide s p e c t r u m of c l i m a t i c c o n d i t i o n s .

Palaeoclimatic

i n d i c a t o r s , i n c l u d i n g palaeomagnetism, s u g g e s t t h a t t h e m a j o r i t y of a n c i e n t a l l u v i a l r e d beds formed i n s e m i - a r i d c l i m a t e s .

Two i m p o r t a 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 f o r m a t i o n of a l l u v i a l r e d b e d s :

t h e d e p o s i t i o n and a g e i n g of

d e t r i t a l f e r r i c h y d r o x i d e s , and 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 minerals.

The former p r o c e s s i s more i m p o r t a n t i n muddy a l l u v i u m where t h e r e i s

much f e r r i c hydroxide i n a s s o c i a t i o n w i t h c l a y s and o t h e r f i n e g r a i n e d m a t e r i a l s . I n t r a s t r a t a l a l t e r a t i o n i s more i m p o r t a n t i n sandy a l l u v i u m , e s p e c i a l l y when i t

i s m i n e r a l o g i c a l l y immature.

The r e l a t i v e i m p o r t a n c e of t h e s e two p r o c e s s e s has

n o t been a s s e s s e d i n most a n c i e n t r e d beds and t h e r e a r e i n d i c a t i o n s t h a t s o u r c e a r e a composition may be a n i m p o r t a n t c o n t r o l l i n g f a c t o r . Comprehensive d e s c r i p t i o n s of a n c i e n t f l u v i a l s e d i m e n t s a r e found i n : Miall, A.D.

( E d i t o r ) , 1978. F l u v i a l Sedimentology. Can. SOC. P e t r o l . Geol.

Memoir 5. I n f o r m a t i o n on c o a r s e p a r t i c l e o x i d e s i n a t r o p i c a l r i v e r i s g i v e n by: R i e z e b o s , P.A., 1979. Compositional downstream v a r i a t i o n of opaque and t r a n s l u c e n t heavy r e s i d u e s i n some modern Rio Magdalena s a n d s (Colombia). Sediment. G e o l . , 24: 197-225.

265

CHAPTER 5 THE DIAGENESIS OF CONTINENTAL RED BEDS INTRODUCTION:

SANDSTONE DIAGENESIS

D i a g e n e s i s i s a c o l l e c t i v e term f o r t h o s e p r o c e s s e s which t a k e p l a c e a f t e r t h e d e p o s i t i o n o f a sediment.

They i n c l u d e p h y s i c a l and c h e m i c a l p r o c e s s e s

which a f f e c t t h e d e t r i t a l m i n e r a l g r a i n s and a l s o t h e changes which t a k e p l a c e i n t h e c h e m i s t r y of t h e i n t e r s t i t i a l c o n n a t e f l u i d s .

The i n t e r a c t i o n between

t h e d e t r i t a l m i n e r a l assemblage and t h e i n t e r s t i t i a l w a t e r i s c o n s t a n t l y changing; d u r i n g t h e e a r l y s t a g e s of d i a g e n e s i s when t h e overburden i s n o t g r e a t t h e i n t e r s t i t i a l w a t e r s w i l l t e n d t o d i s s o l v e t h e more l a b i l e d e t r i t a l g r a i n s t h e r e b y i n c r e a s i n g t h e p o r o s i t y of t h e sediment and a l s o t h e c o n c e n t r a t i o n of t h e d i s s o l v e d c o n s t i t u e n t s .

When t h e 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 t h e p r e c i p i t a t i o n of m i n e r a l cements w i l l t a k e p l a c e .

Various

cements, i n c l u d i n g c l a y m i n e r a l s , q u a r t z , c a r b o n a t e s , and f e l d s p a r s commonly occur i n sandstones.

These have t h e e f f e c t o f f i l l i n g p o r e s p a c e s and t h u s r e d u c i n g

t h e o v e r a l l p o r o s i t y of t h e sediment.

F u r t h e r p o r o s i t y r e d u c t i o n may r e s u l t

from mechanical changes s u c h a s compaction which i s a n i m p o r t a n t f e a t u r e of more deeply buried sandstone.

An e x c e l l e n t o u t l i n e of t h e m a j o r f e a t u r e s of s a n d s t o n e d i a g e n e s i s i s g i v e n by P e t t i j o h n e t a l . ( 1 9 7 2 ) ;

1)

d i a g e n e s i s based on: chemical evidence.

These a u t h o r s r e c o g n i z e s i x s t a g e s of s a n d s t o n e

textural

2)

mineralogical

3)

physical

and

4)

These s t a g e s a r e summarized i n Fig.5.1.

N e a r l y a l l t h e i m p o r t a n t r e a c t i o n s which t a k e p l a c e d u r i n g d i a g e n e s i s do so i n a n aqueous s e t t i n g , e i t h e r i n w a t e r s which a r e n e a r t o t h e d e p o s i t i o n a l i n t e r f a c e , o r i n groundwaters which c i r c u l a t e i n d e e p l y b u r i e d s e d i m e n t s l o n g a f t e r deposition.

I n t h i s r e s p e c t i t i s c o n v e n i e n t t o d i s t i n g u i s h between

t h o s e p r o c e s s e s which t a k e p l a c e i n r e c e n t l y d e p o s i t e d s e d i m e n t i n s u r f a c e w a t e r s , and t h o s e which o c c u r i n more d e e p l y b u r i e d sediment i n s u b s u r f a c e w a t e r s .

The

former a r e r e f e r r e d t o a s e a r l y d i a g e n e t i c p r o c e s s e s and t h e l a t t e r a s l a t e diagenetic processes.

Because e a r l y d i a g e n e t i c p r o c e s s e s t a k e p l a c e i n s u r f a c e

w a t e r s t h e d e p o s i t i o n a l environment e x e r t s a s t r o n g c o n t r o l on t h e n a t u r e o f t h e d i a g e n e t i c r e a c t i o n s i n c l u d i n g t h e r e s u l t a n t a u t h i g e n i c m i n e r a l assemblages. For t h i s r e a s o n , t h e d i a g e n e t i c f e a t u r e s of m a r i n e and c o n t i n e n t a l sediments a r e q u i t e d i s t i n c t i v e and a r e r e a d i l y d i s t i n g u i s h e 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 . C h a r a c t e r i s t i c a u t h i g e n i c p h a s e s of marine and c o n t i n e n t a l s e d i m e n t s a r e l i s t e d i n T a b l e 5.1.

266

Immediately after deposition. Exposed to air or water of depositional environment. Original detritus, high porosity

Buried a fmv metres to tens of metres. Exposed to interstitial waters. Some compaction,some early chemical precipitates possible.

Buried to moderate depths of about lOOOm Pore water may be a brine. Chemical cements may reduce porosity,clays may be altered.

Deep burial to thousands of metres, perhaps with folding. Porosity may be very low from chemical cement and pressure solution.

Incipient metamorphism. Growth of chlorine ond other metamorphic minerals with extensive pressun solution and quartzitic texture.

After uplift and erosion,within tens of metres of land surface. Invasion by metooric water, decementation and"weathering"of clays may increase porosity.

The s t a g e s of d i a g e n e s i s i n r e l a t i o n t o d e p t h o f b u r i a l and i n c r e a s e of p r e s s u r e and t e m p e r a t u r e ( a f t e r P e t t i j o h n e t a l . , 1 9 7 2 ) .

Fig.5.1.

TABLE 5 . 1 . The c h a r a c t e r i s t i c a u t h i g e n i c m i n e r a l s u i t e s of marine s a n d s t o n e s and c o n t i n e n t a l r e d beds. Marine Sandstones

C o n t i n e n t a l Red Beds

Quartz Glauconi t e Chamosite Pyrite Calcite

Quartz Potash Feldspar I l l i te-Montmori l l o n i t e Chlorite Ha ema t i t e Calcite

-

267 The composition of 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

I n s a n d s t o n e s d e p o s i t e d under m a r i n e c o n d i t i o n s t h e i n i t i a l p o r e w a t e r s a r e s e a w a t e r whereas t h o s e i n c o n t i n e n t a l environments can be approximated t o t h e mean composition o f v a r i o u s r i v e r w a t e r s ( T a b l e 5 . 2 ) . are striking.

The d i f f e r e n c e s between t h e two

I n marine w a t e r s t h e rnCa2+/mMg2+ r a t i o i s 0.2 b u t i n r i v e r w a t e r s i s

TABLE 5.2.

Comparison of t h e m o l a l i t y o f s e a w a t e r (Mason, 1966) and mean r i v e r w a t e r ( L i v i n g s t o n e , 1963). sea water

r i v e r water

Dissolved species

PPm

molality

PPm

molality

c 1-

18,980

0.535

7- a

0.00022

Na+

K+

H BO 32+3 Sr F-

10,556

0.459

6.3

0.00027

2649

0.028

11.2

0.00012

1272

0.052

4.1

0.00017

400

0.010

15.0

0.00037

380

0.01

2.3

0.00006

140

58.8

0.00096

65

2.3~10a. 1x10- 4

0.02

2.5~10-

26

4.2~10-~

0.1-0.01

5x10-'

8 1.3

9. 1x10-5 6.8~10-

0.09

1x10-6

1

0.09

5~10-~

13.1

0.00014

H4S104 NO

0.5

1.0~10-~ a. 1x10- 6

1.0

0.00002

Fe

0.01

2x10-7

0.67

0.00001

~l (OH)&

0.01

1~10-

0.24

6 2.5~10-

Totals

34,479

1.106

120.8

0.00235

-

d+Fe 3+

2.2.

The d i f f e r e n c e i s due t o t h e removal of CaCo3 from s e a w a t e r by c a l c a r e o u s -

s h e l l e d organisms.

There is a l s o a n i m p o r t a n t d i f f e r e n c e i n t h e s i l i c a c o n t e n t .

In n a t u r a l w a t e r s , v i r t u a l l y a l l t h e s i l i c a i s i n t h e form of u n d i s s o c i a t e d o r t h o s i l i c i c a c i d (H S i 0 ) b u t i s a b o u t t e n times more abundant i n r i v e r w a t e r

4

than i n sea water.

4

The d i f f e r e n c e i s a l m o s t e n t i r e l y due t o t h e removal o f

s i l i c a from t h e s e a by marine organisms s u c h a s diatoms and r a d i o l a r i a . That s t a g e of d i a g e n e s i s d u r i n g which chemical r a a c t i o n s t a k e p l a c e d u r i n g s h a l l o w b u r i a l was r e f e r r e d t o by Dapples (1962) a s t h e "redoxomorphic s t a g e " . The redoxomorphic s t a g e i s dominated i n marine s e d i m e n t s by r e a c t i o n s i n v o l v i n g o r g a n i c m a t t e r and i n marine mudstones a number o f d e p t h r e l a t e d zones can be r e c o g n i z e d ( C u r t i s , 1977, 1978) ( F i g . 5 . 2 ) .

Beneath a t h i n v e n e e r of oxygenated

s e d i m e n t , r e d u c i n g c o n d i t i o n s p r e v a i l because of oxygen consumption by b a c t e r i a l

268

MICROBIAL REDUCTION THERMAL PROCESSES OF F.3* DECOMPOSITION I----- -I------

OCEAN

I

OXIDATION

It

SIJLPHATE REDUCTION

102m.

10m. 111 FERMENTATtON

-25)

1.0 km .

~

-

IV

DECARBO XYLATION

.. ... ... .... .... .... ..... ..... ..... ...... ...... ...... ...... ....... ...... ....... ...... ....... ...... ...... .... ....

2.5 km

b \

40km.

LIQUID HYDROCARBONS GAS

VI GRAPHITE 7METAMORPHISM

..... ... ..... .... .... .

................ ........ ....... ..... ....... ...... .... .... .....

-

-20?

--/ :

:/

::::;;/ .. :.7

RATE OF INTRODUCTION OF ORGANIC MATTER DERIVED CARBON DIOXIDEI BICARBONATE

-

Fig.5.2. The p r o d u c t i o n o f c a r b o n d i o x i d e w i t h i n d i f f e r e n t d i a g e n e t i c zones. Zones I - I V a p p r o x i m a t e l y c o r r e s p o n d t o Dapples' redoxomorphic s t a g e ( a f t e r C u r t i s , 1978). activity. (e.g. F e 3 ' j

I n t h i s zone s u l p h a t e r e d u c t i o n t a k e s p l a c e and t r a n s i t i o n m e t a l s Fe2+ and Mn4++ i 2 + ) a r e a l s o reduced.

With i n c r e a s i n g d e p t h o r g a n i c

m a t t e r , d e r i v e d l a r g e l y from p h y t o p l a n k t o n , undergoes complex t r a c t i o n s i n c l u d i n g f e r m e n t a t i o n , d e c a r b o x y l a t i o n , and t h e f o r m a t i o n of hydrocarbons.

These zones a r e

d e p t h r e l a t e d and c l o s e l y l i n k e d t o t h e s e d i m e n t a t i o n r a t e (Goldhaber and Kaplan, 1975). The redoxomorphic s t a g e i s t h u s l a r g e l y r e s p o n s i b l e f o r t h e c o l o u r o f sediments (Goldhaber and Kaplan, 1975; C u r t i s , 1978) and a c c o u n t s f o r t h e d i f f e r e n c e s s e e n between marine and c o n t i n e n t a l s e d i m e n t s ,

The a b s e n c e o f a r i c h s o u r c e o f o r g a n i c

m a t t e r ( p h y t o p l a n k t o n ) i n non-marine s e d i m e n t s means t h a t d i a g e n e t i c r e a c t i o n s can t a k e p l a c e under oxygenated c o n d i t i o n s .

T h i s means t h a t t h e i r o n o x i d e s , g o e t h i t e

and h a e m a t i t e may form and import a r e d p i g m e n t a t i o n t o t h e sediment.

I n the

p r e s e n c e of s u b s t a n t i a l o r g a n i c m a t t e r b a c t e r i a l consumption o f oxygen r e s u l t s i n r e d u c i n g c o n d i t i o n s w i t h i n t h e s e d i m e n t and most o f t h e i r o n i s p r e c i p i t a t e d i n t h e

269

f e r r o u s s t a t e , mainly a s p y r i t e .

Thus, t h e c o l o u r o f t h e s e d i m e n t s e v e n t u a l l y

becomes d r a b because of t h e p r e s e r v a t i o n o f o r g a n i c m a t t e r and f i n e l y d i s s e m i n a t e d pyrite, S u b s u r f a c e w a t e r shows much more v a r i a t i o n i n comp,osition than r i v e r w a t e r o r sea w a t e r ,

T h i s i s l a r g e l y due t o i t s complex o r i g i n , b e i n g a m i x t u r e of j u v e n i l e

w a t e r s , m e t e o r i c w a t e r s , ocean w a t e r s , w a t e r s produced from d i a g e n e t i c r e a c t i o n s , and magmatic w a t e r s (White e t a l . , 1963; White, 1965). i n s h a l l o w ((300m)

Waters which a r e p r e s e n t

s a n d s t o n e beds a r e g e n e r a l l y o f m e t e o r i c o r i g i n and c o n t a i n l e s s

+

t h a n 100 ppm d i s s o l v e d s o l i d s c o n s i s t i n g mainly of Na , Ca2+, and Mg2+ c a t i o n s and 2HCO;, SO4 , and C 1 - I a n i o n s . With i n c r e a s e d b u r i a l t h e e l e c t r o l y t e c o n t e n t of s u b s u r f a c e w a t e r s i n c r e a s e s from e s s e n t i a l l y t h a t o f f r e s h w a t e r t o more t h a n t e n times h i g h e r t h a n t h e c o n c e n t r a t i o n of s e a w a t e r . I n deeper s u b s u r f a c e w a t e r s C12r e p l a c e s SO4 a s t h e dominant a n i o n and t h e p r o p o r t i o n of C1- g e n e r a l l y i n c r e a s e s

with s a l i n i t y .

The main p h a s e s p r e c i p i t a t e d from deep s u b s u r f a c e w a t e r s a r e q u a r t z

and some c l a y m i n e r a l s such a s c h l o r i t e . Cementa t i o n A v a r i e t y o f m i n e r a l s o c c u r a s cements i n s a n d s t o n e s .

They o c c u r p r i n c i p a l l y

as p o r e f i l l i n g s and p o r e l i n i n g s and i n c l u d e c l a y m i n e r a l s , q u a r t z , c a r b o n a t e s ,

f e l d s p a r , and h a e m a t i t e .

The p a r a g e n e s i s and morphology of cements v a r i e s between

i n d i v i d u a l s a n d s t o n e s ( s e e D a p p l e s , 1967) a l t h o u g h t h e sequence

2 . q u a r t z and f e l d s p a r

3. c a r b o n a t e s i s o f t e n o b s e r v e d .

1. c l a y m i n e r a l s

However, t h e r e may be

more t h a n one phase of c e m e n t a t i o n f o r any p a r t i c u l a r m i n e r a l ( e . g . q u a r t z ) , and l a t e r cements may r e p l a c e e a r l i e r formed cements.

The most abundant cements i n

s a n d s t o n e s a r e q u a r t z and c a l c i t e . Amorphous s i l i c a i n d i s t i l l e d o r s a l i n e w a t e r s has a n e q u i l i b r i u m s o l u b i l i t y of a b o u t 120 ppm a t room t e m p e r a t u r e .

The s o l u b i l i t y r a p i d l y i n c r e a s e s w i t h

t e m p e r a t u r e and d e p t h of b u r i a l and so o p a l i n e cements i n s a n d s t o n e s a r e a good i n d i c a t i o n of s h a l l o w b u r i a l .

I n d e e p l y b u r i e d s a n d s t o n e s s i l i c a always p r e c i p i t a t e s

a s q u a r t z because t h e s o l u b i l i t y o f q u a r t z i s o n l y a b o u t 5% t h a t of amorphous s i l i c a a t room t e m p e r a t u r e and i n c r e a s e s much less r a p i d l y w i t h t e m p e r a t u r e than does t h e s t a b i l i t y of amorphous s i l i c a .

The amount of t o t a l s i l i c a i n s o l u t i o n i s

markedly dependent on pH and r a p i d l y rises a s pH i n c r e a s e s above 9 . 0

-

9.5.

Most

o f t h e s i l i c a i n n a t u r a l w a t e r s i s i n t h e form of o r t h o s i l i c i c a c i d ( H SiO ) and 4 4 i s found i n g r e a t e s t q u a n t i t y i n s u b s u r f a c e w a t e r s where t h e c o n c e n t r a t i o n i s commonly above t h e s o l u b i l i t y o f q u a r t z ( a b o u t 6 ppm). The p r e c i p i t a t i o n of q u a r t z is a n i m p o r t a n t d i a g e n e t i c p r o c e s s i n s a n d s t o n e s d e p o s i t e d under marine and c o n t i n e n t a l c o n d i t i o n s . p r e c i p i t a t e d a s a n o p t i c a l l y c o n t i n u o u s overgrowth.

Most commonly q u a r t z i s The form of t h e overgrowth i s

f r e q u e n t l y e u h e d r a l b u t may be m o d i f i e d . o r even c o m p l e t e l y i n h i b i t e d , i n t h e

270 p r e s e n c e o f g r a i n c o a t i n g s ( P i t t m a n and Lumsden, 1968; Heald and L a r s s e , 1973). Q u a r t z overgrowths a r e so a b u n d a n t , e s p e c i a l l y i n o l d e r q u a r t z - r i c h s a n d s t o n e s , t h a t t h e s o u r c e of t h e a d d i t i o n a l s i l i c a i s n o t immediately o b v i o u s . t h e d i s s o l u t i o n of s i l i c e o u s organisms sponge s p i c u l e s ) may be i m p o r t a n t .

I n m a r i n e sandstones

( d i a t o m , r a d i o l a r i a n t e s t s and s i l i c e o u s

I n continental deposits, especially those

d e p o s i t e d under a e o l i a n c o n d i t i o n s , t h e p r o d u c t i o n o f s i l i c a d u s t by g r a i n a b r a s i o n may be an i m p o r t a n t s o u r c e o f s i l i c a f o r q u a r t z overgrowths (Waugh, 1970 a , b ) .

In

more d e e p l y b u r i e d s a n d s t o n e s t h e r e i s a g e n e r a l tendency f o r t h e abundance of q u a r t z overgrowths t o i n c r e a s e c a u s i n g s i g n i f i c a n t r e d u c t i o n s i n p o r o s i t y and p e r m e a b i l i t y (FUchtbauer, 1967b).

I n t h i s s i t u a t i o n t h e c o r r e l a t i o n between t h e

d e s t r u c t i o n of m e t a s t a b l e heavy m i n e r a l s and i n c r e a s e d q u a r t z c o n c e n t r a t i o n s u g g e s t s t h a t much of t h e e x t r a s i l i c a was s u p p l i e d by t h e 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 m i n e r a l s ( P h i l l i p e t a l . , 1963).

A l s o , p r e s s u r e s o l u t i o n i s l i k e l y t o be i m p o r t a n t

i n more d e e p l y b u r i e d s a n d s t o n e s (Thomson, 1959; Weyl, 1 9 5 9 ) , where i t i s a p p a r e n t l y f a c i l i t a t e d by t h e p r e s e n c e of i l l i t i c c l a y laminae. C a l c i t e i s p r o b a b l y t h e most common chemical cement i n a n c i e n t s a n d s t o n e s and may form w i t h o r w i t h o u t d e s t r u c t i o n of t h e g r a i n s u p p o r t e d framework ( D a p p l e s , 1971). N o n - d e s t r u c t i v e c a l c i t e cement c o n s i s t s m o s t l y of a n h e d r a l s u b e q u a n t c r y s t a l s 20 m o r more i n d i a m e t e r which g e n e r a l l y f i l l p o r e s p a c e s .

I n some i n s t a n c e s t h e volume

of c a l c i t e cement exceeds 30%, t h e o r i g i n a l p o r o s i t y of t h e s e d i m e n t , and t h e d e t r i t a l q u a r t z p a r t i c l e s a r e c o r r o d e d , f e a t u r e s which c l e a r l y i n d i c a t e t h e d e s t r u c t i v e n a t u r e of t h e c e m e n t a t i o n . I n o r d e r t o s a t u r a t e a n aqueous s o l u t i o n w i t h r e s p e c t t o c a l c i t e , i t i s n e c e s s a r y t h a t t h e i o n a c t i v i t y p r o d u c t LCa2+][ p r o d u c t f o r t h i s compound.

CO:-J

exceeds t h e thermodynamic s o l u b i l i t y

T h e r e f o r e i n c r e a s e i n t h e a c t i v i t y o f e i t h e r calcium

i o n on c a r b o n a t e i o n may r e s u l t i n t h e p r e c i p i t a t i o n of CaC03.

S i n c e t h e amount of

calcium i n marine and s u b s u r f a c e w a t e r s i s r e l a t i v e l y s t a b l e and o n l y a f f e c t e d 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 c a l c i u m - b e a r i n g s i l i c a t e s c a l c i u m c a r b o n a t e p r e c i p i t a t i o n u s u a l l y r e s u l t s from s u p e r s a t u r a t i o n c a u s e d by a n i n c r e a s e i n t h e a c t i v i t y of t h e c a r b o n a t e i o n . C a l c i t e and q u a r t z cements commonly c o e x i s t i n s a n d s t o n e s and may show evidence of replacement of one m i n e r a l by t h e o t h e r .

This r e l a t i o n s h i p i s r e a d i l y explained

by r e f e r e n c e t o t h e p H - s o l u b i l i t y c u r v e s of q u a r t z , amorphous s i l i c a , and c a l c i t e (Fig.5.3).

The s o l u b i l i t y of s i l i c a i s u n a f f e c t e d by pH v a l u e s below a b o u t 9

whereas c a l c i t e i s v e r y s o l u b l e i n a c i d i c s o l u t i o n s .

T h e r e f o r e , changes i n pH may

cause s o l u t i o n of one phase and p r e c i p i t a t i o n o f a n o t h e r , a r e a c t i o n which commonly o c c u r s because of t h e f a c t t h a t t h e p H - s o l u b i l i t y c u r v e s i n t e r s e c t a t a v a l u e commonly met w i t h i n g r o u n d w a t e r s ; .

271

ppm

m/l

-5 ~ 1 0 ~

- 2.104 -- lo4 5 .lo3 - 2.lo3 - 103

-5~10~ - 2.102 102 -5 ~ 1 0 ~

-210.1lo1

5001

\n

a 4 20 0 Silica dissolver Calcite dissolves

4

5

6

7 PH

8

9 1 0 1 1

Fig.5.3. A . T o t a l s i l i c a and r e l a t i v e amounts of s i l i c a s p e c i e s i n s o l u t i o n i n w a t e r a t 25OC a s a f u n c t i o n of pH. B. R e l a t i o n s h i p between pH and t h e s o l u b i l i t i e s o f c a l c i t e , q u a r t z , and amorphous s i l i c a ( a f t e r B l a t t e t al., 1980).

M i n e r a l t r a n s f o r m a t i o n and r e p l a c e m e n t s The changing i n t e r s t i t i a l environment which r e s u l t s d u r i n g s a n d s t o n e b u r i a l r e s u l t s i n a number of m i n e r a l o g i c a l t r a n s f o r m a t i o n s and r e p l a c e m e n t s .

Different

i n t e r s t i t i a l c o n d i t i o n s may r e s u l t i n t h e r e p l a c e m e n t of framework c o n s t i t u e n t s o r p r e v i o u s l y formed cement by a n o t h e r ( t h e Locomorphic S t a g e of D a p p l e s , 1962). Examples i n c l u d e t h e d o l o m i t i z a t i o n of f e l d s p a r o r c a l c i t e cement and t h e r e p l a c e ment of a u t h i g e n i c q u a r t z by c a l c i t e .

The b e s t known m i n e r a l t r a n s f o r m a t i o n d u r i n g

d i a g e n e s i s a r e t h e s y s t e m a t i c m o d i f i c a t i o n s which t a k e p l a c e i n c l a y m i n e r a l s during sandstone b u r i a l (Table 5 . 3 ) .

The best-known of t h e s e i s t h e mixed l a y e r

illite-montmorillonite-illite t r a n s f o r m a t i o n ( P e r r y and Hower, 1970, 1 9 7 2 ) .

This

i n v o l v e s d e h y d r a t i o n and r e s u l t s i n m o d i f i c a t i o n of t h e p o r e w a t e r composition. TABLE 5 . 3 . Some of t h e c l a y m i n e r a l r e a c t i o n s which o c c u r d u r i n g s a n d s t o n e , d i a g e n e s i s ( a f t e r P e t t i j o h n e t a l . , 1972). Clay m i n e r a l formed

Precursor

Kaolinite

feldspar

Kaolini te

pore space

Illite

kaolinite

Muscovite

kaolinite

Illite

montmorillonite

Chlorite

montmorillonit e

Montmorillonite

volcanic glass

Glauconite

illite

Components added(+) or subtracted(-)

S t a g e of diagenesis

+(Fez+ ,Mg '+) -(Si02,H20,Na+,Ca2+)

During deep b u r i a l c l a y m i n e r a l s a r e t r a n s f o r m e d i n t o micas ( t h e Phyllomorphic S t a g e of D a p p l e s , 1 9 6 2 ) .

The f o r m a t i o n of s e r i c i t e , m u s c o v i t e , b i o t i t e and c h l o r i t e

i n t h i s manner r e p r e s e n t s t h e end p o i n t o f d i a g e n e s i s a n d marks t h e o n s e t o f metamorphism.

The f o r m a t i o n of micas i s a s s o c i a t e d w i t h t e x t u r a l c h a n g e s , i n p a r t i c u l a r

t h e development of q u a r t z i t i c t e x t u r e , a n d o n l y o c c u r s i n d e e p l y b u r i e d s a n d s t o n e s ( S t a g e s 4 , 5 and 6 of P e t t i j o h n e t a l . , 1972) ( s e e F i g . 5 . 1 ) .

213

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 The o v e r a l l f a b r i c o f a s a n d o r s a n d s t o n e i s dependent upon v a r i o u s f a c t o r s including:

1) manner of d e p o s i t i o n

2) g r a i n s i z e

and 5 ) p h y s i c a l and chemical compaction.

3) s o r t i n g

4 ) c l a s t shape,

The f a b r i c o f a s a n d s t o n e can be

s u b s t a n t i q l l y changed by compaction r e s u l t i n g from overburden p r e s s u r e .

The

importance o f compaction i s n o t o n l y t h a t i t changes t h e f a b r i c o f t h e s a n d s t o n e , c a u s i n g r e d u c t i o n i n p o r o s i t y and p e r m e a b i l i t y , b u t a l s o p l a y s a n i m p o r t a n t r o l e 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 s t r i b u t i o n o f m i n e r a l cements ( e . g . Modarresi and G r i f f i t h s , 1963).

V a r i o u s schemes have been used t o s p e c i f y s a n d s t o n e f a b r i c

(Emery a n d G r i f f i t h s , 1954; Kahn, 1956a,b; A l l e n , 1962b; Mellon, 1964; G r i f f i t h s , 19671, and most r e l y h e a v i l y on t h e n a t u r e o f g r a i n c o n t a c t s ( F i g . 5 . 4 ) .

During depo-

s i t i o n and s h a l l o w b u r i a l t h e f a b r i c i s d e t e r m i n e d l a r g e l y by t h e p h y s i c a l mechanism of d e p o s i t i o n , g r a i n s h a p e , and t h e packing h e t e r o g e n e i t y ( A l l e n , 1969; Morrow, 1971). I n s a n d s t o n e s which have undergone e a r l y c e m e n t a t i o n and s u f f e r e d l i t t l e o r no compaction t h e main t y p e s of g r a i n c o n t a c t s a r e : convex.

t a n g e n t i a l , l o n g , and concavo-

With i n c r e a s i n g d e g r e e o f compaction t h e number of long g r a i n c o n t a c t s

t e n d s t o i n c r e a s e and i n d e e p l y b u r i e d sands (>2000m) l o n g and s u t u r e d c o n t a c t s a r e abundant ( e . g . T a y l o r , 1950; P h i p p s , 1969).

S u t u r e d c o n t a c t s , caused by

Y

Fig.5.4. The d e f i n i t i o n o f g r a i n c o n t a c t s i n s a n d s t o n e s which a r e used f o r f a b r i c a n a l y s i s ( a d a p t e d from G r i f f i t h s , 1967 and P e t t i j o h n e t a l . , 1972).

274 s o l u t i o n and r e c r y s t a l l i z a t i o n , a r e a c h a r a c t e r i s t i c f e a t u r e of d e e p l y b u r i e d s a n d s t o n e s and a good i n d i c a t i o n of compaction and i n c r e a s e i n geothermal g r a d i e n t . E x t e n s i v e p r e s s u r e s o l u t i o n under t h e s e c o n d i t i o n s i s i n d i c a t e d by abundant s t y l o l i t i c boundaries.

Evidence of p r e s s u r e compaction i s demonstrated by s a n d s t o n e s i n which

t h e volume of t h e p o r e s p a c e p l u s t h e volume of chemical cement i s l e s s t h a n t h e o r i g i n a l p o r e volume o f t h e s a n d .

The o c c u r r e n c e o f h e a l e d f r a c t u r e s and b e n t

m i n e r a l g r a i n s a r e f u r t h e r good i n d i c a t i o n s o f t h e e f f e c t s o f compaction. One of t h e major p h y s i c a l e f f e c t s o f s a n d s t o n e compaction i s t h e r e d u c t i o n i n p o r o s i t y and p e r m e a b i l i t y .

The i n c r e a s e i n q u a r t z c e m e n t a t i o n and p r e s s u r e s o l u t i o n

i n d e e p l y b u r i e d s a n d s t o n e s i s c l o s e l y dependent on p h y s i c a l compaction and t h e s e p r o c e s s e s , a c t i n g i n c o n c e r t , can c o n s i d e r a b l y reduce p o r o s i t y and p e r m e a b i l i t y . C o n t r i b u t i n g f a c t o r s i n c l u d e t h e d e f o r m a t i o n o f d u c t i l e framework g r a i n s and a l s o enhanced c l a y m i n e r a l a u t h i g e n e s i s i n deep s u b s u r f a c e b r i n e s .

The e f f e c t of

compaction and p r e s s u r e s o l u t i o n on p o r o s i t y r e d u c t i o n h a s been d e a l t w i t h theor e t i c a l l y by R i t t e n h o u s e ( 1 9 7 1 a ) .

He found t h a t t h e r e l a t i v e amounts of p o r o s i t y

l o s s due t o s o l u t i o n and c e m e n t a t i o n v a r y g r e a t l y depending upon g r a i n s h a p e , a n g u l a r i t y , packing d i r e c t i o n from which t h e p r e s s u r e was a p p l i e d , and t h e amount of s o l u t i o n which had o c c u r r e d .

A good i l l u s t r a t i o n of t h e s e r e l a t i o n s h i p s i s

g i v e n by P h i l l i p e t a l . (1963). R i t t e n h o u s e (1971b) a l s o made a t h e o r e t i c a l s t u d y of p o r o s i t y r e d u c t i o n caused by t h e compactional d e f o r m a t i o n o f d u c t i l e f r a g m e n t s .

Sandstones with a higher

p r o p o r t i o n of s u c h d u c t i l e g r a i n s might b e e x p e c t e d t o l o s e t h e i r p o r o s i t y on b u r i a l more r a p i d l y t h a n q u a r t z o s e s a n d s t o n e s .

Atwater and Miller (1965) i n a

d e t a i l e d s t u d y of'Miocene and younger sands i n s o u t h e r n L o u i s i a n a showed t h a t i n c r e a s i n g d e f o r m a t i o n of less r e s i s t a n t fragments was t h e main r e a s o n f o r t h e l i n e a r decrease i n p o r o s i t y with depth.

Nagtegaal (1978) h a s a l s o shown t h a t

l i t h i c s a n d s t o n e s a r e a l s o more s u s c e p t i b l e t o a l l f o u r p o r o s i t y and p e r m e a b i l i t y reducing processes:

mechanical compaction, p l a s t i c d e f o r m a t i o n , p r e s s u r e s o l u t i o n ,

and m i n e r a l o g i c a l a l t e r a t i o n of t h e framework c o n s t i t u e n t s . Red bed d i a n e n e s i s C o n t i n e n t a l r e d beds show many of t h e d i a g e n e t i c f e a t u r e s e x h i b i t e d by s a n d s t o n e s and e v i d e n t l y have a c o m p l i c a t e d p o s t - d e p o s i t i o n a l h i s t o r y .

Much o f o u r knowledge

of r e d bed d i a g e n e s i s stems from t h e work o f Walker (1967a, 1976) and Walker e t a l . (1978) who have s t u d i e d t h e d i a g e n e s i s i n f i r s t - c y c l e a r k o s i c d e s e r t a l l u v i u m i n 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 New Mexico.

I n t h i s a r e a a number of d i a g e n e t i c

p r o c e s s e s have been d e s c r i b e d , i n c l u d i n g t h e mechanical i n f i l t r a t i o n of d e t r i t a l c l a y , t h e d i s s o l u t i o n of framework s i l i c a t e s by groundwater and t h e i r replacement by c l a y , and t h e a u t h i g e n e s i s of a s u i t e of m i n e r a l s i n c l u d i n g h a e m a t i t e .

The

u n d e r l y i n g importance of W a l k e r ' s work l i e s i n t h e f a c t t h a t i t d e m o n s t r a t e s two

275 important concepts:

1. I n t r a s t r a t a l s o l u t i o n c o n t r o l s t h e c h e m i s t r y of t h e

i n t e r s t i t i a l groundwater and hence t h e n a t u r e of t h e p r e c i p i t a t e d a u t h i g e n i c phases. 2 . The p r o g r e s s i v e d e s t r u c t i o n of l a b i l e p h a s e s means t h a t g r o s s changes i n m i n e r a l composition and t e x t u r e may t a k e p l a c e d u r i n g t h e d i a g e n e s i s of r e d b e d s . However, i t s h o u l d be s t r e s s e d t h a t n o t a l l a n c i e n t r e d beds a r e f i r s t c y c l e a r k o s e s l i k e t h o s e s t u d i e d by Walker, and many can be shown t o have been d e r i v e d from s o u r c e a r e a s less r i c h i n f e l d s p a r s and fefromagnesian m i n e r a l s .

Consequently,

t h e d i a g e n e t i c changes i n t h e s e r e d beds can be e x p e c t e d t o b e t h a t much l e s s extensive than those seen i n f i r s t cycle arkoses.

Similarly, the processes

d e s c r i b e d by Walker, because o f t h e i r time-dependant n a t u r e , c a n n o t be seen i n o p e r a t i o n i n a n c i e n t r e d beds and t h e i r p r e v i o u s e x i s t e n c e can o n l y be i n f e r r e d . N e v e r t h e l e s s , t h e s t u d y o f many a n c i e n t r e d beds p r o v i d e s compelling e v i d e n c e t h a t d i a g e n e t i c p r o c e s s e s , l i k e t h o s d e s c r i b e d by Walker, have been o p e r a t i v e and caused e x t e n s i v e p o s t - d e p o s i t i o n a l m o d i f i c a t i o n s .

This evidence includes the

p r e s e n c e of d i s s o l u t i o n f e a t u r e s , a p a u c i t y of u n s t a b l e s i l i c a t e g r a i n s and a v a r i e d a u t h i g e n i c s u i t e of m i n e r a l s i n c l u d i n g q u a r t z , f e l d s p a r , c a l c i t e and h a e m a t i t e . THE MECHANICAL INFILTRATION OF DETRITAL CLAY

Cenozoic a l l u v i u m of 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 S t u d i e s of 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 by Walker (1976) and Walker e t a l .

(1978) show t h a t when o r i g i n a l l y d e p o s i t e d i t i s e s s e n t i a l l y f r e e of i n t e r s t i t i a l clay matrix.

T h i s i s because s t r e a m flow i s a h i g h l y e f f i c i e n t p r o c e s s f o r t h e

s e p a r a t i o n o f c l a y from c o a r s e r g r a i n e d d e t r i t u s ; because c l a y has a much lower s e t t l i n g v e l o c i t y i t s t a y s i n s u s p e n s i o n i n t h e s u r f a c e w a t e r and i s c a r r i e d away a s the heavier coarser p a r t i c l e s a r e l e f t behind.

Modern a l l u v i u m from a wide

v a r i e t y of d e s e r t e n v i r o n m e n t s , and even t h a t d e p o s i t e d by ephemeral s t r e a m s , has been shown by Walker e t a l . (1978, p. 19) t o be e s s e n t i a l l y f r e e of i n t e r s t i t i a l c l a y a t t h e time of d e p o s i t i o n .

However, e x a m i n a t i o n o f samples c o l l e c t e d from

p i t s dug i n t o Holocene a l l u v i u m r e v e a l s t h a t w i t h i n a few f e e t below t h e s u r f a c e abundant i n t e r s t i t i a l c l a y o c c u r s w i t h i n t h e c o a r s e g r a i n e d d e t r i t u s .

This clay

i s commonly c o n c e n t r a t e d on t h e upper s u r f a c e s of b u r i e d p e b b l e s and shows a geopedal type f a b r i c (Crone, 1975).

The c l a y r e p r e s e n t s t h e e a r l y s t a g e s of

mechanical i n f i l t r a t i o n which o c c u r s a f t e r a p e r i o d o f s u r f a c e r u n - o f f .

Clay of

a s i m i l a r o r i g i n i s more abundant i n a l l u v i u m of P l e i s t o c e n e and L a t e T e r t i a r y a g e and i n t h i s s i t u a t i o n t h e c l a y forms w e l l - d e v e l o p e d c o a t i n g s on framework grains (Fig.5.5,

5.6).

M e c h a n i c a l l y i n f i l t r a t e d c l a y i s r e a d i l y r e c o g n i z e d by i t s h e t e r o g e n e o u s p e t r o graphic c h a r a c t e r i s t i c s .

I t c o n s i s t s o f v a r i o u s s i z e s of c l a y p l a t e l e t s which a r e

arranged p a r a l l e l t o the g r a i n surfaces.

I n t h e e a r l y s t a g e s of development t h e

c l a y may form s i n u o u s r i d g e s and b r i d g e s which form a t t h e m e n i s c i of p e l l i c u l a r

a.

CONCENTRATION IN VADOSE ZONE ,

-

0

.

o 0

O

CONCENTRATION NEAR THE WATER TABLE ,GROUND

,GROUND SURFACE SURFACE ZONE OF REWORKING

O

D

.

0

.

SURFACE

0

. .

0

- - - 0- - - - 0- - o

b.

o

0

WATER TABLE

. 0

0

.

.

IMPERMEABLE BARRIER

IMPERMEABLE 0ARRIER

c

CONCENTRATION ABOVE IMPERMEABLE BARRIERS

CONCENTRATION IN PROXIMITY TO SOURCE OF INFLUENT SEEPAGE

d d-I

MARGINAL CONGLOMERATES

...

. 7 .. c

d-2

SHIFTING DRAINAGEWAYS ACTIVE

d-3 INCISED DRAINAGEWAYS

ABANDONED

BEDROCK IMPERMEABLE BARRIER

IMPERMEABLE BARRIER

/

w a t e r between a d j a c e n t g r a i n s .

The c o m p o s i t i o n o f t h e c l a y is a l s o v a r i a b l e and

o f t e n resembles the clays i n the sediment source a r e a . Mechanical i n f i l t r a t i o n of c l a y i n t o permeable s e d i m e n t s i s p r o b a b l y v e r y i m p o r t a n t i n a r i d r e g i o n s where t h e d r a i n a g e i s i n f l u e n t b e c a u s e o f t h e low w a t e r table.

Thus whenever i n f l u e n t seepage accompanies a l l u v i a t i o n t h e mechanical

i n f i l t r a t i o n of c l a y w i l l o c c u r .

T y p i c a l l y , c l a y of t h i s t y p e i s i r r e g u l a r l y

d i s t r i b u t e d w i t h i n t h e s e d i m e n t because o f t h e v a r i a b l e n a t u r e o f i n f l u e n t seepage; t h e c l a y a c c u m u l a t i n g i n t h o s e p l a c e s where s u s p e n s i o n s e t t l i n g i s p o s s i b l e .

The

amount of c l a y which a c c u m u l a t e s depends upon a number o f f a c t o r s , i n c l u d i n g t h e amount of c l a y suspended i n t h e w a t e r a n d t h e f r e q u e n c y of e p i s o d e s o f i n f l u e n t seepage.

I n t h e Cenozoic d e p o s i t s of t h e a r i d r e g i o n s i n North America Walker (1976)

r e c o g n i z e d f o u r mechanisms which c o u l d a c c o u n t f o r t h e o b s e r v e d d i s t r i b u t i o n of mechanically i n f i l t r a t e d clay (Fig.5.5).

1) C o n c e n t r a t i o n s i n t h e vadose zone C o n c e n t r a t i o n s i n t h e vadose zone o c c u r when p e l l i c u l a r f i l m s o f w a t e r a r e n o t f u l l y developed on t h e framework g r a i n s .

Under t h e s e c o n d i t i o n s t h e i n f l u e n t

s u r f a c e w a t e r r e p l e n i s h e s t h e s e f i l m s and does n o t r e a c h t h e w a t e r t a b l e .

The

suspended c l a y i s t h u s r e t a i n e d by t h e p e l l i c u l a r w a t e r and a c c u m u l a t e s on t h e c u r f a c e s of t h e framework g r a i n s a s c o a t i n g s composed of a g g r e g a t e s of c l a y p l a t e l e t s orientated p a r a l l e l t o the surface of the grain.

I n p l a c e s c l a y - b r i d g e s may develop

between a d j a c e n t g r a i n s b e c a u s e o f t h e m e n i s c i between a d j a c e n t f i l m s of p e l l i c u l a r

water ( F i g . 5 . 6 ) . I n t h i s mechanism t h e c l a y accumulates from t h e s u r f a c e downwards and t h e t h i c k n e s s o f t h e zone o f c l a y c o n c e n t r a t i o n depends on how deep t h e i n f l u e n t seepage can p e n e t r a t e .

The s e d i m e n t s n e a r t h e s u r f a c e may be f r e e of i n f i l t r a t e d c l a y

b e c a u s e o f reworking i n c h a n n e l s and p e r i o d i c s h e e t flow.

I f t h i s occurs the

a c c u m u l a t i o n of i n f i l t r a t e d c l a y b e g i n s immediately below t h e zone o f a c t i v e reworking. 2) Concentrations near t h e water t a b l e

I n some c a s e s i n f i l t r a t e d c l a y forms nearyy h o r i z o n t a l l a y e r s which c u t primary d e p o s i t i o n a l s t r u c t u r e s s u g g e s t i n g t h a t t h e y formed n e a r former p o s i t i o n s of t h e w a t e r t a b l e (Fig.5.5B).

Walker (1976) s u g g e s t e d t h a t t h e c o n c e n t r a t i o n s could be

e x p l a i n e d by t h e d i f f e r e n t v e l o c i t i e s of w a t e r m i g r a t i o n , above and below t h e water t a b l e .

Above t h e w a t e r t a b l e i n f l u e n t s e e p a g e w a t e r , having r e p l e n i s h e d

p e l l i c u l a r f i l m s , moves r a p i d l y downward under t h e i n f l u e n c e o f g r a v i t y and c a r r i e s w i t h i t i t s s u s p e n s i o n l o a d of c l a y .

W i t h i n t h e s a t u r a t e d zone below t h e w a t e r

t a b l e , t h e m i g r a t i o n v e l o c i t y i s much lower because h e r e t h e groundwater moves under t h e i n f l u e n c e o f a v e r y low w a t e r t a b l e g r a d i e n t .

When t h e downward

218 m i g r a t i n g w a t e r r e a c h e s t h i s zone i t s v e l o c i t y i s much reduced and t h e suspended c l a y b e g i n s t o s e t t l e o u t , t e n d i n g t o become c o n c e n t r a t e d i n t h e upper p a r t o f t h e s a t u r a t e d zone where t h e i n i t i a l d e c r e a s e i n v e l o c i t y f i r s t o c c u r s .

Fluctuations

i n t h e p o s i t i o n of t h e w a t e r t a b l e w i l l t h u s i n c r e a s e t h e v e r t i c a l t h i c k n e s s of t h e zone o f c o n c e n t r a t i o n .

3 ) C o n c e n t r a t i o n s above impermeable b a r r i e r s C o n c e n t r a t i o n s o f m e c h a n i c a l l y i n f i l t r a t e d c l a y may o c c u r whenever t h e downward m i g r a t i o n o f i n f l u e n t s e e p a g e i s a r r e s t e d by a n impermeable b a r r i e r .

These b a r r i e r s

may be beds of f i n e - g r a i n e d s e d i m e n t w i t h i n c o a r s e a l l u v i u m , b u r i e d s o i l s o r even bedrock.

During p e r i o d s of i n t e r m i t t e n t s u r f a c e r u n - o f f p e r c h e d w a t e r t a b l e s may

form above such b a r r i e r s c r e a t i n g c o n d i t i o n s which a l l o w c l a y t o s e t t l e from s u s p e n s i o n i n t h e manner d e s c r i b e d i n 2 ) .

During d r y p e r i o d s when t h e i n f l u e n t

seepage t e m p o r a r i l y c e a s e s t h e p e r c h e d w a t e r t a b l e s may d r a i n away and under t h e s e c i r c u m s t a n c e s t h e remaining c l a y may form c o a t i n g s and b r i d g e s i n t h e manner d e s c r i b e d i n 1 ) . A l t e r n a t i o n of d r o u g h t and i n f l u e n t s e e p a g e c a u s e s t h e accumulation of l a r g e amounts of i n f i l t r a t e d c l a y i n t h e i n t e r m i t t e n t l y s a t u r a t e d zone; i t may u l t i m a t e l y f i l l i n t e r s t i t i a l v o i d s i n t h e a l l u v i u m above impermeable b a r r i e r s .

4 ) C o n c e n t r a t i o n s i n p r o x i m i t y t o s o u r c e s of i n f l u e n t s e e p a g e The c l o s e a s s o c i a t i o n between i n f l u e n t s e e p a g e and i n f i l t r a t i o n of c l a y c l e a r l y s u g g e s t s t h a t t h e amount of i n f i l t r a t e d c l a y s h o u l d g e n e r a l l y i n c r e a s e i n p r o x i m i t y t o p r i n c i p a l recharge a r e a s .

Thus m a r g i n a l f a n g l o m e r a t e s a r e l i k e l y t o c o n t a i n

more i n f i l t r a t e d c l a y t h a n d i s t a l s a n d s which a r e a long way from t h e r e c h a r g e a r e a and o n l y p e r i o d i c a l l y r e a c h e d by major s t o r m s .

S i m i l a r l y more c l a y i s i n -

f i l t r a t e d i n t o t h e a l l u v i u m b e n e a t h ephemeral c h a n n e l s than i t i s i n t h e i n t e r fluvial divides.

A s c h a n n e l s s h i f t p o s i t i o n s o does t h e zone of c l a y c o n c e n t r a t i o n

b u t where c h a n n e l s a r e i n c i s e d t h e y r e c h a r g e t h e same a r e a f o r a much l o n g e r p e r i o d and l a r g e r amounts of i n f i l t r a t e d c l a y accumulate ( F i g . 5 . 5 ) . The i n t r o d u c t i o n of c l a y i n t o p o r o u s , permeable s e d i m e n t s o c c u r s wherever i n f l u e n t seepage accompanies a l l u v i a t i o n .

Large amounts of c l a y c a n be added t o

c o a r s e g r a i n e d a l l u v i u m i n t h i s way and Walker e t a l . (1978, p . 2 1 ) r e p o r t t h a t t h e o l d e r P l e i s t o c e n e and T e r t i a r y d e p o s i t s 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 commonly c o n t a i n 10-20% of m a t r i x c l a y which o r i g i n a t e d i n t h i s manner.

T h i s p r o c e s s t h e r e f o r e makes i m p o r t a n t changes i n t h e t e x t u r e , m i n e r a l o g y ,

and chemical composition by a d d i n g m a t r i x t o sediment which was e s s e n t i a l l y m a t r i x f r e e when d e p o s i t e d . This p o s t - d e p o s i t i o n a l

i n f i l t r a t i o n of c l a y t h u s c a u s e s a d i a g e n e t i c d e c r e a s e

i n t h e t e x t u r a l m a t u r i t y ( F o l k , 1951) o f t h e sediment and may l e a d t o t h e f a l s e c o n c l u s i o n t h a t d e p o s i t i o n was by a mudflow o r some o t h e r p r o c e s s c h a r a c t e r i z e d by

219

M e c h a n i c a l l y i n f i l t r a t e d c l a y i n Cenozoic a l l u v i u m and a n c i e n t r e d beds. Fig.5.6. A . SEM photomicrograph of d e t r i t a l g r a i n c o a t e d w i t h c l a y s k i n (CS) which has been m e c h a n i c a l l y i n f i l t r a t e d . Upper G i l a Group ( P l i o c e n e - P l e i s t o c e n e ) , New Mexico. B . Enlargement of a r e a o u t l i n e d i n A showing broken c l a y s k i n . C . Enlargement of a r e a o u t l i n e d i n B showing c l a y p l a t e l e t s o r i e n t e d p a r a l l e l t o t h e g r a i n s u r f a c e and one s m a l l c r y s t a l o f a u t h i g e n i c f e l d s p a r , D . Thin s e c t i o n photomicrograph of m e c h a n i c a l l y i n f i l t r a t e d c l a y - o x i d e i n 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 l e P i t B a s i n , North Sea. A-C a r e produced by c o u r t e s y of T.R. Walker.

280 weak s o r t i n g a c t i o n .

S i m i l a r l y t h e b u l k chemical c o m p o s i t i o n of t h e s e d i m e n t w i l l

be changed by t h e a d d i t i o n of c l a y m i n e r a l s which a r e n o r m a l l y r i c h e r i n aluminium and lower i n a l k a l i s and a l k a l i n e e a r t h s t h a n t h o s e o f t h e o r i g i n a l sediment. M e c h a n i c a l l v i n f i l t r a t e d c l a v i n a n c i e n t r e d beds G r a i n c o a t i n g s o f c l a y and i r o n o x i d e a r 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 a n c i e n t

red beds,

The r e l a t i v e proportions of i r o n oxide (haematite) and clay varies

considerably.

I n some c a s e s t h e g r a i n c o a t i n g s c o n s i s t s o f o n l y p a r t i c u l a t e

c r y s t a l s of h a e m a t i t e which have a p p a r e n t l y been p r e c i p i t a t e d i n s i t u and i n o t h e r s t h e r e may be t h i c k c l a y - o x i d e p e l l i c l e s which resemble t h e c l a y c u t a n s of Brewer ( 1 9 6 4 ) .

Such g r a i n s c o u l d c o n c e i v a b l y have been d e r i v e d from upland

s o i l s , o r more l i k e l y , from s o i l s which formed i n a d j a c e n t f l o o d p l a i n a r e a s . More u s u a l l y t h e g r a i n c o a t i n g s c o n s i s t of a c l a y - o x i d e p e l l i c l e of v a r i a b l e t h i c k n e s s and geopedal t e x t u r e s i n c l u d i n g meniscus b r i d g e s a r e cornon., There can be l i t t l e doubt t h a t t h e s e f e a t u r e s were formed by t h e m e c h a n i c a l i n f i l t r a t i o n of c l a y i n t h e manner s u g g e s t e d by Walker ( 1 9 7 6 ) .

An i m p o r t a n t f e a t u r e of m e c h a n i c a l l y - i n f i l t r a t e d c l a y i s t h a t i t i s n o t r e d when d e p o s i t e d b u t o n l y reddens w i t h time a f t e r b e i n g i n c o n t a c t w i t h oxygenated groundwater.

I n Cenozoic a l l u v i u m t h e r e d d e n i n g of i n f i l t r a t e d c l a y i s s i g n i f i c a n t

because i t t h u s r e p r e s e n t s t h e f i r s t s t a g e of p i g m e n t a t i o n i n r e d bed f o r m a t i o n . I t seems l i k e l y t h a t i n a n c i e n t sandy a l l u v i u m t h e i n i t i a l s t a g e s of r e d d e n i n g occur

l a r g e l y a s a r e s u l t of t h e o x i d a t i o n o f m e c h a n i c a l l y - i n f i l t r a t e d and d e t r i t a l c l a y . DISSOLUTION OF FRAMEWORK SILICATES I n g e n e r a l t h e a l t e r a t i o n of s i l i c a t e s f o l l o w s G o l d i c h ' s m i n e r a l s t a b i l i t y s e r i e s which i s t h e r e v e r s e of Bowen's r e a c t i o n s e r i e s .

T h i s sequence o f a l t e r a t i o n

r e s u l t s because of t h e v a r i a t i o n i n bond s t r e n g t h s between oxygen and c a t i o n s i n the s i l i c a t e minerals (Keller, 1957).

M i n e r a l s which a r e low i n Bowen's r e a c t i o n

s e r i e s s u c h a s a u g i t e , h o r n b l e n d e , and p l a g i o c l a s e t e n d t o be a l t e r e d f i r s t w h i l s t q u a r t z , m u s c o v i t e and o r t h o c l a s e a r e t h e most s t a b l e .

The most s t a b l e bond i n

s i l i c a t e s i s t h e S i - 0 bond and d i f f e r e n c e s i n t h e s t r e n g t h o f t h i s bond between t h e d i f f e r e n t s i l i c a t e groups a r e v e r y s m a l l compared t o t h e d i f f e r e n c e s between oxygen and o t h e r c a t i o n s .

T h e r e f o r e , s i l i c a t e s w i t h h i g h S i / O r a t i o s a r e more

s t a b l e r e g a r d l e s s of which m e t a l l i c i o n s b a l a n c e t h e s t r u c t u r e e l e c t r i c a l l y . Within e a c h of t h e m a j o r s i l i c a t e groups t h e s p e c i e s of m e t a l l i c i o n p r e s e n t d e t e r m i n e s t h e p o s i t i o n of t h e m i n e r a l i n t h e w e a t h e r i n g s t a b i l i t y s e r i e s (Table 5.4).

The o r d e r of i n c r e a s i n g s t r e n g t h of t h e o x y g e n - m e t a l l i c c a t i o n bonds i s e x a c t l y t h e r e v e r s e o f t h e o r d e r of r e s i s t a n c e t o w e a t h e r i n g of t h e m i n e r a l s i n which t h e c a t i o n s a r e i m p o r t a n t .

F o r example, o r t h o c l a s e i s one of t h e more

281 r e s i s t a n t p h a s e s i n Bowen's r e a c t i o n s e r i e s y e t potassium-oxygen bonds a r e v e r y weak.

F o r s t e r i t e , on t h e o t h e r hand, w e a t h e r s r a p i d l y y e t t h e magnesium-oxygen

bond i s r e l a t i v e l y s t r o n g .

The r e a s o n f o r t h i s l i e s i n t h e s t r e n g t h of t h e s i l i c o n -

oxygen bond which p r e v e n t s t h e o r t h o c l a s e s t r u c t u r e from d i s i n t e g r a t i o n d e s p i t e t h e r a p i d removal of p o t a s s i u m i o n s .

F o r s t e r i t e h a s few s u c h bonds and w i t h o u t t h e

s t a b i l i z i n g magnesium atoms t h e s t r u c t u r e i s v e r y u n s t a b l e and r a p i d l y c o l l a p s e s . TABLE 5.4.

Bond s t r e n g t h s between oxygen and common c a t i o n s , c a l c u l a t e d from a r e f e r e n c e s t a t e o f gaseous i o n s (from t e l l e r . 1957). I on

Energy of f o r m a t i o n (Kcal/mole)

K+

299

+ Na

322

H+(in OH-)

5 15

Ca

2+

8 39 9 12

Mg2+ Fe

2+

9 19

A 1 3+

1793

*14+

1878

4+ Ti

2882

si4+

(in (in (in (in (in (in

tectosilicates) phyllosilicates) double-chain i n o s i l i c a t e s ) single-chain i n o s i l i c a t e s ) sorosilicates) nesosilicates)

3110 3123 3127 3131 3137 3147

The d i s s o l u t i o n of s i l i c a t e g r a i n s t a k e s p l a c e h y d r o l y s i s .

Sodium, c a l c i u m ,

and magnesium a r e s e p a r a t e d from t h e s i l i c a t e framework r a t h e r e a s i l y and a r e soon c a r r i e d away i n aqueous s o l u t i o n , whereas i r o n , aluminium, and t i t a n i u m t e n d t o remain behind.

T h i s i s c o n s i s t e n t w i t h t h e t h e o r e t i c a l p r e d i c t i o n ( T a b l e 5 . 4 ) and

i s u s u a l l y observed i n n a t u r e ( G o l d i c h , 1938; Wahlstrom, 1948).

The a c t u a l n a t u r e

of t h e h y d r o l y s i s r e a c t i o n i s r a t h e r c o m p l i c a t e d and depends upon t h e composition of t h e m i n e r a l i n v o l v e d . 5KA1Si308

F o r p o t a s h f e l d s p a r i t may be w r i t t e n :

+ 4H+ + 4HC03- +

16H20

+

(Feldspar)

KA15Si7020(OH)4

+ 8H4Si04 + 4K+ + 4HC03-

( I l l ite)

and under extreme c o n d i t i o n s k a o l i n i t e may form from i l l i t e : 2KA15Si7020(OH)4 + 2H+ 2HC0313H20 -p 5A12Si205(0H)4 + 4H4Si04

+

(Illite)

+

(Kaolinite)

(5.1)

+ 2K+ + 2HC03(5.2)

The h y d r o l y s i s of framework s i l i c a t e s i s f u n d a m e n t a l l y i m p o r t a n t i n r e d bed diagenesis.

Not o n l y does i t m a t e r i a l l y change t h e framework m i n e r a l o g y b u t

h y d r o l y s i s a l s o p r o v i d e s t h e major s o u r c e of i o n s from which a u t h i g e n i c m i n e r a l p h a s e s may form. Feldspars Both t h e major groups o f t h e f e l d s p a r m i n e r a l s , t h e a l k a l i f e l d s p a r s and t h e p l a g i o c l a s e s , commonly o c c u r a s d e t r i t a l g r a i n s i n r e d beds and may show e v i d e n c e of e x t e n s i v e i n s i t u d i s s o l u t i o n .

I n g e n e r a l t h e a l k a l i f e l d s p a r s t e n d t o be more

s t a b l e than t h e p l a g i o c l a s e s and t h e r e a r e a l s o v a r i a t i o n s w i t h i n e a c h group; m i c r o c l i n e i s more s t a b l e t h a n o t h e r a l k a l i f e l d s p a r s and s o d i c p l a g i o c l a s e i s more s t a b l e than c a l c i u m r i c h f e l d s p a r s .

Individual grains of feldspar frequently

r e v e a l marked h e t e r o g e n e i t i e s s u c h a s c o m p o s i t i o n a l z o n i n g , p e r t h i t e s of v a r i o u s k i n d s , p o i k i l i t i c t e x t u r e s , and replacement f i l m s and c o a t i n g s .

The i d e a l framework

s t r u c t u r e i s a l s o c o m p l i c a t e d by l o c a l and d i s t a n t o r d e r i n g i n t h e d i s t r i b u t i o n of s i l i c o n and aluminium i o n s .

The i n h e r e n t c o m p l e x i t y of t h e f e l d s p a r s must t h e r e f o r e

have a s i g n i f i c a n t e f f e c t on any d i s s o l u t i o n p r o c e s s e s . There has been a c o n s i d e r a b l e amount of e x p e r i m e n t a l work on t h e i n i t i a l h y d r o l y s i s of f e l d s p a r s (Correns and von E n g e l h a r d t , 1939; W o l l a s t , 1967; Helgeson,

1971).

Correns and von E n g e l h a r d t (1939) found t h a t p o t a s s i u m , s i l i c a , and

aluminium were r e l e a s e d i n t r u e s o l u t i o n d u r i n g t h e breakdown o f a d u l a r i a a l t h o u g h t h e r a t i o s i m p l i e d t h a t a r e s i d u a l c o a t i n g of

hydrous aluminium s i l ; c a t e ,

with a

Si/A1 g r e a t e r than t h a t of k a o l i n i t e , formed around t h e decomposing g r a i n s .

These

a u t h o r s i n f e r r e d t h a t t h e c o a t i n g m a i n t a i n e d a c o n s t a n t t h i c k n e s s and i n f l u e n c e d t h e r a t e of t h e a l t e r a t i o n r e a c t i o n , a view which i s c o n s i s t e n t w i t h l a t e r experiments on o r t h o c l a s e by W o l l a s t (1967).

Helgeson (1971) p o s t u l a t e d t h e

s e q u e n t i a l a p p e a r a n c e of i n t e r m e d i a t e r e a c t i o n p r o d u c t s such a s g i b b s i t e , k a o l i n i t e , and mica s s zoned s u r f a c e l a y e r s around t h e decomposing f e l d s p a r g r a i n s .

Such a

r e s i d u a l l a y e r h a s n o t been d i r e c t l y observed a s y e t and 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 r e v e a l t h a t t h e i n i t i a l s t a g e s o f h y d r o l y s i s a r e c h a r a c t e r i z e d by well-developed c o r r o s i o n f i g u r e s on c l e a v a g e s u r f a c e s , a f e a t u r e which i m p l i e s continuous d i s s o l u t i o n .

T h i s i s t h e c a s e i n e x p e r i m e n t a l ' c o n d i t i o n s (Tchoubar,

1965), i n s o i l s Wilson (1975), and d u r i n g t h e e a r l y s t a g e s of s a n d s t o n e d i a g e n e s i s (Walker e t a l . , 1978).

I t is a l s o apparent t h a t various feldspar species a r e

a f f e c t e d by c o n t i n u o u s d i s s o l u t i o n . I n t h e f i r s t c y c l e d e s e r t a l l u v i u m s t u d i e d by Walker e t a l . (1978) f e l d s p a r g r a i n s which have been e t c h e d o r hollowed o u t by d i s s o l u t i o n a r e a common f e a t u r e

(Fig.5.7).

Only p l a g i o c l a s e f e l d s p a r s a r e s e r i o u s l y a f f e c t e d by t h e d i s s o l u t i o n

w h i l s t potassium f e l d s p a r has n o t y e t been d i s c e r n i b l y a f f e c t e d .

The d i s s o l u t i o n

of p l a g i o c l a s e t e n d s t o b e i r r e g u l a r w i t h t h e g r a i n s showing i r r e g u l a r l y e t c h e d ,

283

Fig.5.7. P a r t i a l l y d i s s o l v e d f e l d s p a r s i n Cenozoic a l l u v i u m . A , B t h i n s e c t i o n s ; C-D SEM photomicrographs. A . P e r i p h e r a l l y d i s s o l v e d p l a g i o c l a s e g r a i n . D i s s o l u t i o n v o i d s (DV) s u r r o u n d t h e r e l i c t g r a i n a n d c l a y s k i n (CS) marks t h e o r i g i n a l g r a i n boundary. Thin s e c t i o n ? P l i o c e n e f a n g l o m e r a t e , CaZon R o j o , Baja C a l i f o r n i a . B. I n t e r n a l l y d i s s o l v e d p l a g i o c l a s e ( P I . Most of t h e o r i g i n a l g r a i n now occupied by d i s s o l u t i o n v o i d s . Same d e t a i l s a s A . C . P l a g i o c l a s e p a r t i a l l y r e p l a c e d by c l a y (RC) and s u b s e q u e n t l y d i s s o l v e d , forming d i s s o l u t i o n v o i d s (DV). Same d e t a i l s a s A and B. D . P a r t i a l l y d i s s o l v e d p l a g i o c l a s e g r a i n ( P ) showing well-developed d i s s o l u t i o n v o i d s (DV). Tesuque Formation (Miocene-Pliocene), n e a r Dixon, New Mexico. Produced by c o u r t e s y of T.R. Walker.

or fluted, surface textures.

The n a t u r e of t h e r e l i c t g r a i n s i n d i c a t e t h a t d i s -

s o l u t i o n may proceed inward from t h e p e r i p h e r y of t h e g r a i n , o r i t may s e l e c t i v e l y remove t h e i n t e r i o r o f t h e g r a i n s and produce a hollow " s h e l l " .

G r a i n s may be

completely removed by t h i s p r o c e s s l e a v i n g no c l u e of t h e i r o r i g i n a l p r e s e n c e o t h e r than t h e d i s s o l u t i o n v o i d .

Commonly, however, d i s s o l v e d g r a i n s show c o a t i n g s of

i n f i l t r a t e d o r a u t h i g e n i c c l a y which a r e more r e s i s t a n t t o d i s s o l u t i o n and t h e s e may be l e f t behind t o " g h o s t " t h e o r i g i n a l g r a i n a f t e r i t has c o m p l e t e l y d i s a p p e a r e d D i s s o l u t i o n v o i d s o f t h i s t y p e may become f i l l e d w i t h cement and t h u s p r e s e r v e d , b u t o f t e n t h e y c o l l a p s e and t h e c l a y c o a t i n g becomes mixed w i t h t h e i n t e r s t i t i a l matrix.

When t h i s happens t h e r e i s no r e c o r d of t h e p r e v i o u s e x i s t e n c e o f t h e

dissolution void. The d i s s o l u t i o n of f e l d s p a r i n a n c i e n t s a n d s t o n e s i s w i d e l y r e p o r t e d ( M i l l e r ,

1955; Heald and L a r e s e , 1973; F l e s c h and Wilson, 1974; Waugh, 1 9 7 8 ) .

Potassium

f e l d s p a r s show e v i d e n c e o f more e x t e n s i v e d i s s o l u t i o n t h a n t h e Cenozoic a l l u v i u m d e s c r i b e d by Walker e t a l . (1978) and even though p l a g i o c l a s e may s u f f e r p r e f e r e n t i a l 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 ( e . g . P i t t m a n , 1 9 6 9 ) , j u d g i n g from t h e r e l a t i v e abundance of potassium f e l d s p a r and p l a g i o c l a s e i n a n c i e n t c o n t i n e n t a l r e d beds i t would a p p e a r t h a t t h e r e has been w h o l e s a l e d i s s o l u t i o n o f t h e more unstable plagioclase feldspars (Table 5 . 5 ) .

The a v e r a g e s e d i m e n t c o n t a i n s a b o u t

5% f e l d s p a r ( g l a t t e t a l . , 1980) b u t t h i s i s c o n s i d e r a b l y less t h a n r e d beds which f r e q u e n t l y c o n t a i n more than 8-10% f e l d s p a r , t h e m a j o r i t y o f which i s p o t a s h feldspar. The p r e f e r e n t i a l d j s s o l u t i o n o f p l a g i o c l a s e i s t h u s c o n s i d e r e d t o b e a n important diagenetic process.

A s w e l l a s changing t h e b u l k m i n e r a l o g i c a l and chemical com-

p o s i t i o n of t h e s e d i m e n t i t a l s o r e d u c e s t h e a p p a r e n t m i n e r a l o g i c a l m a t u r i t y and by i n c r e a s i n g t h e amount of i n t e r s t i t i a l c l a y m a t r i x c a u s e s a n a p p a r e n t d e c r e a s e i n textural maturity. Micas The micas do n o t show obvious d i s s o l u t i o n f e a t u r e s d u r i n g i n t r a s t r a t a l a l t e r a t i o n but i n keeping with t h e complexity of t h e i r behaviour i n s o i l s , probably y i e l d d i v e r s e a l t e r a t i o n p r o d u c t s , which depend upon t h e n a t u r e o f t h e p a r e n t m i n e r a l and a l s o t h e c o n d i t i o n s i n t h e i n t e r s t i t i a l environment.

The i n f l u e n c e of s t r u c t u r e

i s w e l l - i l l u s t r a t e d by t h e f a c t t h a t t r i o c t a h e d r a l micas ( e . g . b i o t i t e ) a r e more s u s c e p t i b l e to weathering than t h e i r dioctahedral counterparts ( e . g . muscovite) (Deer e t a l . , 1966, p . 193-200; Weaver and P o l l a r d , 1 9 7 3 ) .

The t r i o c t a h e d r a l micas

a r e r e a d i l y c o n v e r t e d t o v e r m i c u l a t e d u r i n g w e a t h e r i n g , a p r o c e s s which i s a p p a r e n t l y connected w i t h t h e o r i e n t a t i o n o f t h e hydroxyl d i p o l e i n t h e o c t a h e d r a l s h e e t ( B a s s e t t , 1960).

I n t r i o c t a h e d r a l micas t h e hydroxyl d i p o l e i s normal t o the

s i l i c a t e s h e e t s o t h a t t h e p r o t o n i c c h a r g e i s a d j a c e n t t o t h e p o t a s s i u m i o n , which

285

i s t h e r e f o r e , i n an unstable p o s i t i o n .

I n t h e d i o a c t a h e d r a l micas t h e hydroxyl

group i s i n c l i n e d and moves t h e p r o t o n f u r t h e r from t h e potassium i o n which i s then i n a more s t a b l e p o s i t i o n between t h e s i l i c a t e s h e e t s . TABLE 5 . 5 .

Comparison of t h e a v e r a g e f e l d s p a r c o n t e n t of v a r i o u s p l u t o n i c i g n e o u s r o c k s w i t h t h a t o f a n c i e n t r e d beds. The r e s u l t s i n d i c a t e t h e r e l a t i v e enrichment of p o t a s h f e l d s p a r i n a n c i e n t r e d b e d s , Data a r e from 1. S e l l e y (1966) 2. Simonen and Kuovo (1955) 3. T u r n e r and McD. Whitaker (1976) 4. Krynine (1950) 5. Huckenholtz (1963).

Granite Syeni t e Granodiorit e Quartz d i o r i t e Diorite Gabbro

P o t a s h F e l d s p a r (%)

P l a g i o c l a s e (7")

40 12

26 12 46 56 64 65

15 6 3 0

Precambrian 1 T o r r i d o n i a n Sands tone J o t n i a n Sandstone'

27

3 1

Palaeozoic R i n g e r i k e Sands t o n e 3

8.3

1

Mesozoic 4 Newark Group

24

6

Cenozoic 5 Oligocene Arkose

18.5

21

0.4

There a r e many f i e l d and e x p e r i m e n t a l s t u d i e s which show t h a t t h e breakdown of b i o t i t e i s a complex p r o c e s s .

There i s ample e v i d e n c e t o i n d i c a t e t h a t b i o t i t e

responds i n d i f f e r e n t ways t o v a r y i n g c o n d i t i o n s d u r i n g d i a g e n e s i s ( T u r n e r and A r c h e r , 1977).

T h e o r e t i c a l c o n s i d e r a t i o n s s u g g e s t t h a t b i o t i t e c o u l d weather t o

v e r m i c u l i t e under a c i d c o n d i t i o n s and t o v e r m i c u l i t e p l u s m o n t m o r i l l o n i t e under n e u t r a l and a l k a l i n e c o n d i t i o n s .

T h i s i s c o n s i s t e n t w i t h t h e f a c t t h a t t h e most

abundant mica c l a y i n c a l c a r e o u s o r a l k a l i n e s o i l s i s m o n t m o r i l l o n i t e (McNeal and S a n s o t e r r a , 1964).

The t r a n s f o r m a t i o n of b i o t i t e t o v e r m i c u l i t e a l t h o u g h

known t o o c c u r w i d e l y i n s o i l s i s o n l y r a r e l y r e p o r t e d t o have o c c u r r e d i n r e d beds ( e . g . S c h l u g e r and Roberson, 1 9 7 5 ) .

The b i o t i t e - j v e r m i c u l i t e

transformation

i s known t o be complex, a n d e a r l y work (Walker, 1949) s u g g e s t e d t h a t t h e l o s s of p o t a s s i u m was accompanied by o x i d a t i o n , s u b s t i t u t i o n of hydroxyl f o r oxygen, and t h e loss of o c t a h e d r a l i r o n and magnesium.

The l o s s of l a y e r c h a r g e a s s o c i a t e d

w i t h v e r m i c u l i t i z a t i o n can be g e n e r a l l y a c c o u n t e d f o r by t h e o x i d a t i o n of o c t a h e d r a l i r o n ( N o r r i s h , 1972).

The a p p a r e n t s c a r c i t y o f v e r m i c u l i t e i n r e d beds

may be due t o t h e f a c t t h a t b i o t i t e o x i d a t i o n may n o t have been accompanied by vermiculitization.

I t i s p o s s i b l e f o r v i r t u a l l y complete o x i d a t i o n of b i o t i t e w i t h

l i t t l e o r no v e r m i c u l i t i z a t i o n even though a s u b s t a n t i a l p r o p o r t i o n of t o t a l potassium

2 86

Fig.5.8. Thin s e c t i o n photomicrographs (PPL) showing t h e f o r m a t i o n o f h a e m a t i t e a f t e r b i o t i t e A. Randomly s c a t t e r e d pseudohexagonal f l a k e s of h a e m a t i t e B. Thin t r a n s l u c e n t s h e e t s of h a e m a t i t e C . Thin opaque s h e e t s of h a e m a t i t e D. T h i c k e r , d i s c o n t i n u o u s , s h e e t s of opaque h a e m a t i t e E . P r e f e r e n t i a l development of h a e m a t i t e i n zones k i n k e d due t o compaction F. Thick c o n t i n u o u s s h e e t s o f opaque h a e m a t i t e . T h i s i s t h e s t a g e of r e p l a c e m e n t p r i o r t o complete pseudomorphing of b i o t i t e f l a k e s .

may have been l o s t (Walker, 1949; Seddoh and P e d r o , 1974).

B i o t i t e o x i d a t i o n can

thus p r o c e e d w i t h o r w i t h o u t accompanying v e r m i c u l i t i z a t i o n . must be compensated by o t h e r changes i n t h e b i o t i t e .

The v a l e n c y changes

Three p o s s i b l e mechanisms

have been p o s t u l a t e d and a r e f o r m u l a t e d below f o r a n n i t e t h e f e r r o u s end-member of t h e p h l o g o p i t e - b i o t i t e series (Farmer e t a l . ,

1971):

loss of i n t e r l a y e r c a t i o n s : ( S i 3 A 1 0 1 0 ) F e ~ ( O H ) 2K

+

f0,

+ hH20

l o s s o f hydroxyl p r o t o n s : (Si3A1010)Fe32+ (OH)2 K + %02

+

(Si3A1010)Fe22+ Fe 3+ (OH)2

(Si3A1010)Fe 2+ Fe 3+0 2

K

+ KOH

+ H20

(5.3)

(5.4)

2

l o s s of o c t a h e d r a l i r o n : (Si3A1010)Fe32+ (OH)* K

+

+ %H20 +

%02

( S i 3 A 1 0 1 0 ) F e ~ ( O H ) 2K

+ FeO

OH

(5.5)

The l o s s of i n t e r l a y e r c a t i o n s c a n o n l y be of minor importance a s t h e amount of i r o n o x i d i z e d d u r i n g v e r m i c u l i t i z a t i o n i s v e r y much g r e a t e r t h a n t h e l a y e r c h a r g e . The second mechanism which i n v o l v e s t h e c o n v e r s i o n of hydroxyl t o o x i d e i o n s

i s w e l l e s t a b l i s h e d f o r t h e r m a l o x i d a t i o n of b i o t i t e s and i t may a l s o be i m p o r t a n t i n t h e o x i d a t i v e w e a t h e r i n g o f b i o t i t e (Farmer e t a l . , 1971) a l t h o u g h a g e n e r a l a p p r a i s a l i s d i f f i c u l t b e c a u s e of t h e problem of d i s t i n g u i s h i n g e x c e s s w a t e r from c o n s t i t u t i o n a l hydroxyl (Newman and Brown, 1966; R i m s a i t e , 1967, 1970). The l o s s of o c t a h e d r a l i r o n h a s been p o s t u l a t e d f o r some time i n o r d e r t o e x p l a i n t h e lower i r o n c o n t e n t of w e a t h e r e d o x i d i z e d b i o t i t e s i n s o i l s when compared w i t h f r e s h b i o t i t e from t h e p a r e n t r o c k (Walker, 1949; Wilson, 1 9 7 0 ) .

However, o x i d i z e d

b i o t i t e s do n o t n e c e s s a r i l y have lower i r o n c o n t e n t s ( R i m s a i t e , 1967) and t h e p r e s e n c e of i n t e r l a y e r i r o n o x i d e s h a s been a b l y demonstrated by Farmer e t a l . (1971). These a u t h o r s s u g g e s t e d t h a t e l e c t r o n t r a n s f e r from t h e o c t a h e d r a l f e r r o u s i o n s i s i n i t i a l l y mediated through a hydroxyl o r o x i d e i o n .

This i n i t i a l s t e p probably

i n v o l v e s t h e t r a n s f e r o f a hydrogen atom from a hydroxyl group t o t h e o x i d a n t . L a t e r when remaining f e r r o u s i o n s a r e c o o r d i n a t e d o n l y t o o x i d e i o n s e l e c t r o n s may t r a n s f e r d i r e c t l y from o x i d e i o n s t o t h e o x i d a n t , o r i n d i r e c t l y through b r i d g i n g water molecules. (FeFO;-)5+;

I n t h i s way t h e g r o u p i n g :

(Fe2+Fe3+O:-)4+

i s converted t o

t h e r e s u l t a n t l o c a l c o n c e n t r a t i o n of p o s i t i v e c h a r g e s i s u n s t a b l e

and a (Fe3+02-)+ g r o u p i n g i s e j e c t e d through hexagonal h o l e s i n t h e s i l i c a t e s h e e t t o an i n t e r l a y e r space.

The i n i t i a l i n t e r l a y e r o x i d e i s p r o b a b l y amorphous i r o n

hydroxide o r a c r y s t a l l i n e phase o f p - F e O O H . There i s l i t t l e doubt t h a t t h e o x i d a t i v e decomposition of b i o t i t e i s a charact e r i s t i c f e a t u r e of e a r l y d i a g e n e s i s i n c o n t i n e n t a l a l l u v i u m .

The f a t e of t h e

i n t e r l a y e r o x i d e s does however v a r y presumably a c c o r d i n g t o t h e i n t e r s t i t i a l conditions.

Walker (1967a, 1976) and Walker e t a l . (1978) have d e s c r i b e d i r o n -

r i c h h a l o s around decomposing b i o t i t e g r a i n s and have concluded t h a t t h e s e r e p r e s e n t t h e d i s t r i b u t i o n o f i r o n by oxygenated p o r e w a t e r u l t i m a t e l y forming pigmentary

288

Fig.5.9. SEM photomicrographs showing t h e p r o g r e s s i v e r e p l a c e m e n t of b i o t i t e by s h e e t s of h a e m a t i t e c r y s t a l l i t e s p a r a l l e l t o (001). A l l from t h e Old Red Sandstone of t h e Gamrie O u t l i e r , S c o t l a n d , A. Oxidized b i o t i t e showing i n c i p i e n t a l t e r a t i o n on c l e a v a g e s u r f a c e . B . D e t a i l of c l e a v a g e s u r f a c e w i t h i n c i p i e n t a l t e r a t i o n . Note a b s e n c e of c l e a r l y d e f i n e d c r y s t a l forms. C. Cleavage s u r f a c e showing abundant pseudohexagonal c r y s t a l l i t e s of h a e m a t i t e , D . Advanced s t a g e of a l t e r a t i o n showing haematite c r y s t a l l i t e s coalesced i n t o a s i n g l e sheet.

289

T.S.

S.E.M. I

t I

t

I

I

I

r

I

t

f

.

L

2

a

U

I

I

Fig.5.10. Schematic diagram showing t h e model f o r t h e f o r m a t i o n o f h a e m a t i t e pseudomorphs a f t e r b i o t i t e i n Devonian r e d beds a s s e e n i n t h i n s e c t i o n (TS) and s c a n n i n g e l e c t r o n microscopy (SEM). Four s t a g e s a r e r e c o g n i z e d : 1 * S c a t t e r e d pseudohexagonal f l a k e s o f t r a n s l u c e n t h a e m a t i t e i n i n t e r l a y e r p o s i t i o n s . 2 = Thin d i s c o n t i n u o u s s h e e t s p a r a l l e l t o t h e b i o t i t e c l e a v a g e . 3 = Continuous s h e e t s of opaque h a e m a t i t e . 4 = Complete pseudomorph formed by s h e e t s of h a e m a t i t e packed p a r a l l e l t o t h e b i o t i t e cleavage..

I

U

haematite. (1974).

S i m i l a r h a l o e s have been d e s c r i b e d i n C r e t a c e o u s r e d beds by McBride

B i o t i t e s i n b o t h Cenozoic a l l u v i u m and many a n c i e n t r e d beds show

i n t e r l a y e r s h e e t s of h a e m a t i t e .

These have been s t u d i e d i n d e t a i l i n t h e Old Red

Sandstone of S c o t l a n d by T u r n e r a n d Archer (1977) who d e s c r i b e d a complete sequence of b i o t i t e a l t e r a t i o n r a n g i n g from a p p a r e n t l y f r e s h g r a i n s t o t h o s e c o m p l e t e l y r e p l a c e d by h a e m a t i t e (Fig.5.8).

The i n i t i a l s t a g e s of o x i d a t i o n a r e marked by t h e

development of t h i n t r a n s l u c e n t f l a k e s of h a e m a t i t e a l o n g c l e a v a g e p l a n e s when s e e n i n thin section.

I n k e e p i n g w i t h t h e model o f Farmer e t a l . (1971) i t i s l i k e l y

t h a t t h e s e were i n i t i a l l y

FeOOH

and have s u b s e q u e n t l y undergone i n v e r s i o n .

With p r o g r e s s i v e o x i d a t i o n t h e s e f l a k e s b e g i n t o c o a l e s c e l a t e r a l l y and e v e n t u a l l y t h i n d i s c o n t i n u o u s s h e e t s of h a e m a t i t e become v e r y a b u n d a n t .

These a r e t h i c k e r i n

more e x t e n s i v e l y a l t e r e d b i o t i t e g r a i n s s u g g e s t i n g t h a t i n d i v i d u a l s h e e t s o f h a e m a t i t e a r e s t a c k e d one upon top of t h e o t h e r .

Scanning e l e c t r o n microscope

s t u d i e s confirm t h a t t h e s e i n d i v i d u a l s h e e t s of h a e m a t i t e a r e formed from c o a l e s c e d , s m a l l pseudohexagonal f l a k e s of h a e m a t i t e ( F i g . 5 . 9 ) .

P r e c i s e i n s i t u compositional

a n a l y s i s of t h e f l a k e s i s n o t p o s s i b l e u s i n g e n e r g y - d i s p e r s i v e X-ray a n a l y s i s b u t Turner and Archer (1977) were a b l e t o show t h a t i r o n was t h e major e l e m e n t i n t h e f l a k e s and judged t h a t i r o n o x i d e was t h e most l i k e l y composition.

I n a number of

a n c i e n t r e d beds t h i s o x i d a t i o n of b i o t i t e i s s e e n t o have gone f u l l c y c l e w i t h t h e f o r m a t i o n o f complete h a e m a t i t e pseudomorphs a f t e r b i o t i t e ( T u r n e r , 1974a; Turner and A r c h e r , 1 9 7 7 ) .

Evidence from SEM and t h i n s e c t i o n s t u d i e s i n d i c a t e s

t h a t t h e pseudomorphs a r e formed by t h e s t a c k i n g of i n t e r l a y e r s h e e t s of h a e m a t i t e (Fig.5.10).

Complete pseudomorphs of t h i s t y p e a r e common i n t h e R i n g e r i k e Group

of Norway ( T u r n e r , 1974a). The r e a s o n s why b i o t i t e o x i d a t i o n y i e l d s i n t e r l a y e r i r o n o x i d e s i n some c a s e s and n o t i n o t h e r s a r e f a r from c l e a r .

I r o n - d e p l e t e d b i o t i t e s i n s o i l s a r e analogous

t o t h e l e a c h e d b i o t i t e s w i t h i r o n o x i d e h a l o e s r e p o r t e d by Walker ( 1 9 6 7 a ) , Walker

e t a l . (1978) and McBride (1974).

These p r o b a b l y form under c o n d i t i o n s i n which

i r o n s a n be r e d i s t r i b u t e d outwards from t h e decomposing g r a i n s .

I s m a i l (1969) has

shown t h a t t h e p r o d u c t s of e x p e r i m e n t a l w e a t h e r i n g of b i o t i t e depend upon t h e pH conditions.

I n a n e u t r a l environment o x i d a t i o n r e s u l t s i n a d e c r e a s e i n s u r f a c e

c h a r g e y i e l d i n g a s m e c t i t e - l i k e p r o d u c t , whereas i n a c i d c o n d i t i o n s o x i d a t i o n i s balanced by t h e r e l e a s e of o c t a h e d r a l i r o n which m a i n t a i n s t h e s u r f a c e c h a r g e a t a h i g h l e v e l t o form v e r m i c u l i t e .

I t t h u s seems l i k e l y t h a t v a r i a t i o n i n pH i n

t h e i n t e r s t i t i a l environment i s a n i m p o r t a n t f a c t o r i n d e t e r m i n i n g whether i r o n

i s released i n t o the pore waters during diagenesis o r r e t a i n e d within the b i o t i t e s t r u c t u r e t o form i n t e r l a y e r s h e e t s of h a e m a t i t e o r pseudomorphs of h a e m a t i t e a f t e r biotite.

291 Ferromaenesian m i n e r a l s Ferromagnesian m i n e r a l s a r e g e n e r a l l y u n s t a b l e i n t h e aqueous o x i d i z i n g environment and show w i d e s p r e a d e v i d e n c e of i n s i t u d i s s o l u t i o n i n b o t h Recent w e a t h e r i n g p r o f i l e s (Wahlstrom, 1948) and Cenozoic a l l u v i u m (Walker e t a l . , 1978). O l i v i n e a p p e a r s t o be l e s s s t a b l e t h a n pyroxene which i s g e n e r a l l y c o n s i d e r e d t o be l e s s s t a b l e t h a n amphibole ( G o l d i c h , 1938; Wilson, 1975).

Because o l i v i n e i s

t h e most u n s t a b l e f e r r o m a g n e s i a n s i l i c a t e i t i s n o t a commonly o c c u r r i n g heavy mineral.

During d i a g e n e s i s i t can be e x p e c t e d t o be r a p i d l y a l t e r e d , a f e a t u r e

which p a r t l y a c c o u n t s f o r i t s s c a r c i t y , The most abundant f e r r o m a g n e s i a n s i l i c a t e s i n s e d i m e n t s a r e pyroxenes and amphiboles.

SEM and p e t r o g r a p h i c e x a m i n a t i o n s show t h a t pyroxenes c h a r a c t e r i s t i c a l l y

dgvelop "cockscomb" o r "hacksaw" t e r m i n a t i o n s which a r e c o n s i d e r e d t o r e f l e c t p r e f e r e n t i a l d i s s o l u t i o n a l o n g p l a n e s t r a n s v e r s e t o t h e c - a x i s by Walker e t a l . (1978) ( F i g . 5 . 1 1 ) .

The d i s s o l u t i o n o f pyroxene has a l s o been d e s c r i b e d by Edelman

and Douglas (1931) b u t t h e s e a u t h o r s c o n s i d e r e d t h e "hacksaw" t e r m i n a t i o n s t o r e s u l t from a combination of d i s s o l u t i o n a l o n g c l e a v a g e p a r a l l e l t o t h e c - a x i s and c r y s t a l l o g r a p h i c p l a n e s a t 45-60'

t o the a-axis (Fig.5.11).

This explanation i s

more c o m p a t i b l e w i t h t h e s t r u c t u r e of pyroxene t h a n i s t h e e x p l a n a t i o n of Walker e t a l . (1978). The d i s s o l u t i o n b e h a v i o u r o f hornblende i s q u i t e d i f f e r e n t from pyroxene. Hornblende g r a i n s develop d e l i c a t e n e e d l e - l i k e t e r m i n a t i o n s which a r e p a r a l l e l t o the c-axis.

This r e s u l t s

amphibole double c h a i n .

b e c a u s e of p r e f e r e n t i a l d i s s o l u t i o n p a r a l l e l t o t h e

The l i n k s between a d j a c e n t c h a i n s c o n s i s t of a " f r e e

oxygen-cation bonds" and t h e s e r e p r e s e n t t h e p l a n e o f weakness a l o n g which d i s s o l u t i o n can proceed most r a p i d l y . The d i s s o l u t i o n of hornblende and pyroxene p r o b a b l y c o n t i n u e s f o r a l e a s t t e n s of m i l l i o n s o f y e a r s a f t e r d e p o s i t i o n .

I n t h e Cenozoic a l l u v i u m s t u d i e d by Walker

(1976) and Walker e t a l . (1978) r e l i c t s of t h e s e m i n e r a l s a r e common i n r e d beds which, on t h e b a s i s o f a s s o c i a t e d d a t e d b a s a l t s , were d e p o s i t e d between 9 and 26 My ago and a l s o o c c u r i n d e p o s i t s of Miocene a g e .

I n a n c i e n t r e d beds however d e t r i t a l

hornblende and pyroxene g r a i n s ( a l o n g w i t h o t h e r u n s t a b l e m i n e r a l s ) a r e e x t r e m e l y scarce.

T h i s has l o n g been a t t r i b u t e d t o i n t r a s t r a t a l s o l u t i o n ( P e t t i j o h n , 1941)

a c o n c l u s i o n which i s s u p p o r t e d by s t u d i e s of t h e m i n e r a l assemblages w i t h i n c a l c a r e o u s c o n c r e t i o n s ( B r a m l e t t e , 1941) and i n s h a l e s and a s s o c i a t e d s a n d s t o n e s ( B Z a t t and S u t h e r l a n d , 1969). There a r e a number of i m p o r t a n t consequences o f t h e d i s s o l u t i o n of d e t r i t a l s i l i c a t e grains.

I n t h e f i r s t p l a c e i t s u p p l i e s a n abundant s o u r c e of i o n s

i n c l u d i n g K , Na, Ca, A l , S i , Mg, F e , Mn t o t h e i n t e r s t i t i a l groundwater. of t h e s e a r e p r e c i p i t a t e d l a t e r a s a u t h i g e n i c p h a s e s . o f t h e sediment p r o v i d i n g t h a t

Many

I t a l s o increases the porosity

1) d i s s o l u t i o n v o i d s a r e n o t s u b s e q u e n t l y d e s t r o y e d

292

Fig.5.11. P a r t i a l l y d i s s o l v e d f e r r o m a g n e s i a n s i l i c a t e s i n Cenozoic a l l u v i u m . A . P e r i p h e r a l l y d i s s o l v e d h o r n b l e n d e w i t h w e l l - d e v e l o p e d d i s s o l u t i o n v o i d s (DV) and a c l a y s k i n (CS) marking t h e o r i g i n a l g r a i n boundary. A l s o n o t e t h e needlel i k e t e r m i n a t i o n s on t h e r e l i c t g r a i n . Thin s e c t i o n P l i o c e n e ? f a n g l o m e r a t e , Cazon Rojo, Baja C a l i f o r n i a . B. P a r t i a l l y d i s s o l v e d h o r n b l e n d e showing w e l l developed d i s s o l u t i o n n e e d l e s (DN) and d i s s o l u t i o n v o i d s (DV). SEM photomicrograph. Other d e t a i l s a s i n A. C. D e t a i l of hornblende d i s s o l u t i o n needles. Other d e t a i l s a s i n B. D . P a r t i a l l y d i s s o l v e d a u g i t e g r a i n showing well-developed cockscomb t e r m i n a t i o n s on r e l i c t g r a i n . SEM photomicrograph, Hayner Ranch Formation (Miocene) , New Mexico. Produced by c o u r t e s y of T.R. Walker.

293 by compactive c o l l a p s e o r

2 ) t h a t t h e p r e c i p i t a t i o n of s t a b l e a u t h i g e n i c phases

has n o t c o m p l e t e l y compensated f o r t h e newly c r e a t e d p o r o s i t y .

The m i n e r a l o g i c a l

m a t u r i t y o f t h e s e d i m e n t i s i n c r e a s e d because of t h e d i s s o l u t i o n of f e l d s p a r which incresses the quartz/feldspar r a t i o ,

The t e x t u r e i s a l s o changed because of t h e

r e d u c t i o n i n g r a i n s i z e o f t h e a f f e c t e d m i n e r a l s a n d t h i s may l e a d t o a n i n c r e a s e i n s i l t s i z e d p a r t i c l e s thereby decreasing the t e x t u r a l maturity.

The b u l k chemical

composition of t h e s e d i m e n t may a l s o be changed i f t h e r e l e a s e d i o n s a r e n o t prep r e c i p i t a t e d a s a u t h i g e n i c p h a s e s b u t c a r r i e d away by m i g r a t i n g groundwater. CLAY REPLACEMENT

The d i a g e n e t i c d i s s o l u t i o n of s i l i c a t e g r a i n s 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 i n s i t u r e p l a c e m e n t by c l a y . a l l u v i u m (Walker e t a l . ,

T h i s i s a f e a t u r e commonly s e e n i n b o t h Cenozoic

1978) and a l s o i n s o i l s (Wilson, 1975).

The form o f t h e

r e p l a c e m e n t v a r i e s a n d may t a k e p l a c e i r r e g u l a r l y , p e r i p h e r a l l y o r p r e f e r e n t i a l l y a l o n g c r y s t a l l o g r a p h i c p l a n e s , i n c i p i e n t f r a c t u r e s , g r a i n c o n t a c t s i n rock fragments and o t h e r zones of weakness.

Replacement c l a y s a r e t h e r e f o r e a u t h i g e n i c i n o r i g i n

(Wilson and P i t t m a n , 1977) and t h e i r m i n e r a l o g i c a l composition depends upon t h e n a t u r e of t h e h o s t m i n e r a l s . F e l d s p a r s i n s a p r o l i t e s and s o i l s y i e l d a v a r i e t y of d a u g h t e r c l a y m i n e r a l s depending upon t h e e x a c t composition of t h e h o s t phase and t h e i n t e r s t i t i a l c o n d i t i o n s .

In s o i l s i t i s g e n e r a l l y a s s e r t e d t h a t f e l d s p a r a l t e r s d i r e c t l y t o i l l i t e b u t t h i s i s o f t e n d i f f i c u l t t o prove because o f t h e p r e s e n c e of a s s o c i a t e d s e r i c i t i c mica. The f o r m a t i o n of mica from f e l d s p a r s a p p e a r s t o be favoured i n c o n f i n e d e n v i r o n ments l i k e t h o s e e x i s t i n g i n s a p r o l i t e s ( K a t o , 1965).

T h i s i s because of t h e

r e t e n t i o n o f potassium i o n s which a r e e s s e n t i a l f o r t h e mica s t r u c t u r e .

In more

open systems s o l u b l e p o t a s s i u m i o n s a r e removed and t h e f o r m a t i o n of o t h e r c l a y m i n e r a l s such a s v e r m i c u l i t e o r m o n t m o r i l l o n i t e may t a k e p l a c e .

These m i n e r a l s

may be formed d i r e c t l y from f e l d s p a r o r from a s s o c i a t e d mica (Meilhac and Tardy, 1970).

The k a o l i n i z a t i o n of f e l d s p a r i s a complex p r o c e s s which i n v o l v e s t h e

f o r m a t i o n of k a o l i n i t e from a s d l u t i o n phase i n t e r m e d i a t e between p a r e n t f e l d s p a r and d a u g h t e r k a o l i n i t e .

A t l e a s t two morphologies a r e i n v o l v e d :

a p l a t y , book-

l i k e k a o l i n i t e forms under " w e a t h e r i n g c r u s t " c o n d i t i o n s i n which t h e geochemical c o n d i t i o n s p e r s i s t e d e s s e n t i a l l y a t e q u i l i b r i u m o v e r a v e r y long t i m e .

Elongate

k a o l i n s , o f t e n r e f e r r e d t o a s h a l l o y s i t e a p p a r e n t l y form under r e l a t i v e l y h i g h e r l a n d c o n d i t i o n s , o r where s h o r t e r t i m e k i n e t i c p r o c e s s e s were i n v o l v e d ( K e l l e r , 1978).

The f a c t t h a t k a o l i n i z a t i o n i s p r e c e d e d by a s o l u t i o n i n r e a c t i o n w i t h t h e

s o l i d p h a s e s i s i n d i c a t e d by t h e random o r i e n t a t i o n of k a o l i n i t e w i t h r e s p e c t t o t h e p a r e n t f e l d s p a r and a l s o by t h e c o r r o s i o n f e a t u r e s r e p o r t e d by Parharn (19691,

Keller (1976) and Berner and Holdren (1977). The a l t e r a t i o n p r o d u c t s o f t h e f e r r o m a g n e s i a n m i n e r a l s a r e n o t a s e x t e n s i v e l y

294

Fig.5.12. Thin s e c t i o n photomicrographs o f s i l i c a t e m i n e r a l s r e p l a c e d by c l a y i n Cenozoic a l l u v i u m . A. P l a g i o c l a s e g r a i n (P) r e p l a c e d i r r e g u l a r l y by c l a y (RC). P l i o c e n e ? f a n g l o m e r a t e , Cat%on Rojo, Baja C a l i f o r n i a . B. Hornblende g r a i n ( H ) showing p e r i p h e r a l r e p l a c e m e n t by c l a y (RC) which h a s been squeezed between o t h e r framework C . P l a g i o c l a s e g r a i n ( P ) showing p r e f e r e n t i a l c l a y g r a i n s . O t h e r d e t a i l s a s i n A. replacement a l o n g c l e a v a g e p l a n e s . Note t h e development of s i l t - s i z e d r e l i c t s of p l a g i o c l a s e . G i l a Group ( P l i o c e n e - P l e i s t o c e n e ) , Tucson, A r i z o n a . D . G r a n i t i c rock fragment now composed m a i n l y of c l a y (RC) which h a s r e p l a c e d a l l o r i g i n a l m i n e r a l s e x c e p t q u a r t z (Q). O r i g i n a l o u t l i n e of t h e r o c k fragment i s marked by a n o r i e n t e d c l a y s k i n . Tesuque Formation ( M i o c e n e - P l i o c e n e ) , New Mexico. Photomicrographs produced by c o u r t e s y of T.R. Walker.

studied a s the feldspars.

I n g e n e r a l , t h e y a l t e r t o magnesium-rich t r i o c t a h e d r a l

expandable m i n e r a l s and c h l o r i t e , o f t e n i n a mixed l a y e r a r r a n g e m e n t .

Basham

(1974) h a s s t u d i e d t h e a l t e r a t i o n of c l i n o - and ortho-pyroxenes i n some g a b b r o i c s a p r o l i t e s i n NE S c o t l a n d and was a b l e t o d e m o n s t r a t e t h e g r a d u a l replacement of pyroxene by v e r m u c u l i t e pseudomorphs.

The v e r m i c u l i t e shows a h i g h d e g r e e of

preferred orientation indicating, as i n dissolution, the close structural control of the host mineral. Although amphiboles a r e g e n e r a l l y c o n s i d e r e d t o be more r e s i s t a n t t o w e a t h e r i n g t h a n p y r o x e n e s , they n e v e r t h e l e s s y i e l d s i m i l a r a l t e r a t i o n p r o d u c t s .

Hornblende

t y p i c a l l y a l t e r s t o c h l o r i t e and s u b s e q u e n t l y t o mixed l a y e r c h l o r i t e - v e r m i c u l i t e i n s o i l s developed on u l t r a b a s i c m a t e r i a l ( S t e p h e n , 1952; K a t o , 1965).

Wilson and

Farmer (1970) demonstrated t h e a l t e r a t i o n of i r o n - r i c h l a m e l l a e i n hornblende t o mixed l a y e r c h l o r i t e - s a p o n i t e .

The d a u g h t e r c l a y showed no o r i e n t a t i o n r e l a t i o n s h i p

w i t h t h e p a r e n t hornblende o t h e r t h a n a tendency towards p a r a l l e l i s m w i t h t h e cleavage planes. I n t h e Cenozoic a l l u v i u m s t u d i e d by Walker e t a l . (1978) m i g r a t i n g ground w a t e r s e n s u r e a more open type o f system t h a n t h a t found i n most s o i l s and s a p r o l i t e s . The r e s u l t i s a s i m p l e c l a y mineralogy c h a r a c t e r i s t i c a l l y c o n s i s t i n g of randomly i n t e r s t r a t i f i e d 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 (Reynolds and Hower, 1970).

The

c o n t a c t between t h e d a u g h t e r c l a y and r e l i c t p a r e n t m i n e r a l a r e normally s h a r p and t h e r e i s no t r a n s i t i o n a l zone of p a r t i a l r e p l a c e m e n t a t t h e margins of t h e r e l i c s . There a p p e a r s t o be no c l o s e s t r u c t u r a l r e l a t i o n s h i p between t h e o r i e n t a t i o n of t h e d a u g h t e r c l a y s and p a r e n t r e l i c and t h i s , t o g e t h e r w i t h t h e o t h e r f e a t u r e s , s u g g e s t s t h a t t h e replacement c l a y i s formed from a s o l u t i o n i n r e a c t i o n w i t h t h e p a r e n t a 1l u v i um ( F i g .5.12 )

.

The f o r m a t i o n o f replacement c l a y i n t h i s way has s e v e r a l i m p o r t a n t e f f e c t s . The amount of i n t e r s t i t i a l m a t r i x i s g r e a t l y i n c r e a s e d and t h i s d e c r e a s e s t h e t e x t u r a l m a t u r i t y of t h e s e d i m e n t s d i a g e n e t i c a l l y .

Even i n Cenozoic a l l u v i u m c l a y

of replacement o r i g i n may o f t e n comprise more t h a n 15% by volume of t h e rock and produce a g r e y wacke-type t e x t u r e i n which p o o r l y s o r t e d a n g u l a r a n d s u b a n g u l a r grains r e s t i n a clay matrix.

T h i s thus p r o v i d e s c o n v i n c i n g s u p p o r t f o r t h e

d i a g e n e t i c o r i g i n o f t h e greywacke t e x t u r e s (Cummins, 1962; Whetten and Hawkins, 1970).

Completely pseudomorphous r e p l a c e m e n t can o c c u r i n t h i s manner b u t t h e s e

a r e seldom p r e s e r v e d because t h e s o f t c l a y y i e l d s t o compactive p r e s s u r e and i s r e a d i l y squeezed between g r a i n t o become i n d i s t i n g u i s h a b l e from i n t e r s t i t i a l m a t r i x . T h i s t y p e of m a t r i x i s r e f e r r e d t o a s pseudomatrix by Dickinson

(1970a).

Any

evidence t h a t t h e d a u g h t e r c l a y formed by t h e replacement of framework s i l i c a t e s becomes o b s c u r e d w i t h time a s t h e d e g r e e of a l t e r a t i o n and d e p t h of b u r i a l i n c r e a s e s . The i d e n t i f i c a t i o n of replacement c l a y i n a n c i e n t r e d beds i s coneequently v e r y d i f f i c u l t t o prove.

296 Another i m p o r t a n t a f f e c t of t h e c l a y r e p l a c e m e n t p r o c e s s i s t h e r e d u c t i o n i n a v e r a g e g r a i n s i z e of t h e framework c o n s t i t u e n t s because o f t h e tendency f o r framework g r a i n s t o become broken down by incomplete r e p l a c e m e n t . p a r t i c u l a r s u f f e r s from t h i s p r o c e s s .

Plagioclase i n

G r a n i t i c r o c k fragments may be e x t e n s i v e l y

d i s a g g r e g a t e d because o f t h e s e l e c t i v e r e p l a c e m e n t of t h e u n s t a b l e m i n e r a l components ( F i g . 5 . 1 2 ) .

Even e x p e c t e d l y s t a b l e rock fragments s u c h a s q u a r t z i t e

fragments may b e d i s a g g r e g a t e d b e c a u s e of t h e r e p l a c e m e n t of c l a y a l o n g g r a i n boundaries.

The replacement p r o c e s s s i g n i f i c a n t l y changes t h e m i n e r a l composition

o f t h e sediments because o f t h e s e l e c t i v e removal u n s t a b l e s i l i c a t e s l i k e p l a g i o c l a s e and t h e ferromagnesian m i n e r a l s .

This r e s u l t s i n an increase i n the proportion of

t h e s t a b l e m i n e r a l s such a s q u a r t z and t h e r e b y i n c r e a s e s t h e m i n e r a l o g i c a l m a t u r i t y d i a g e n e t i c a l l y i n t h e same way a s d i s s o l u t i o n . One of t h e most i m p o r t a n t a s p e c t s of t h e c l a y r e p l a c e m e n t p r o c e s s i s t h e a s s o c i a t e d r e l e a s e of c e r t a i n elements i n t o t h e groundwater c i r c u l a t i o n v i a t h e d a u g h t e r c l a y . There have been numerous s t u d i e s o f t h e geochemical e f f e c t s o f w e a t h e r i n g ( e . g . G o l d i c h , 1938; Brock, 1943; K e l l e r , 1957; H a r r i s and Adams, 1966; Anderson and Wiklander, 1975).

I n s o i l s and s a p r o l i t e s i t i s commonly o b s e r v e d t h a t i n t h e

m o b i l i z a t i o n and r e d i s t r i b u t i o n of c a t i o n s d u r i n g w e a t h e r i n g t h e a l k a l i e s , p a r t i c u l a r l y sodium, and t h e a l k a l i n e e a r t h m e t a l s a r e more mobile t h a n many of t h e o t h e r c a t i o n s commonly found i n s i l i c a t e l a t t i c e s .

A s i m i l a r p a t t e r n emerges from t h e

geochemical s t u d y of t h e breakdown o f s i l i c a t e m i n e r a l s i n Cenozoic a l l u v i u m by Walker (1967a) and Walker e t a l . (1978).

These a u t h o r s have p a i d p a r t i c u l a r a t t e n t i o n

t o t h e geochemical a l t e r a t i o n of hornblende s i n c e t h i s i s a n i m p o r t a n t s o u r c e of i r o n f o r m i g r a t i n g groundwaters.

A combination of e l e c t r o n p r o b e m i c r d a n a l y s i s

SEM, X-ray d i f f r a c t i o n and c o n v e n t i o n a l p e t r o g r a p h y have been used t o d e m o n s t r a t e t h e s e l e c t i v e removal of c a l c i u m , magnesium and i r o n from t h e p a r e n t h o r n b l e n d e w i t h a c o r r e s p o n d i n g i n c r e a s e i n aluminium and s i l i c o n ( F i g . 5 . 1 3 ) . The d a u g h t e r c l a y (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 i r o n - r i c h and quant i t a t i v e a n a l y s e s of hornblende g r a i n s i n P l i o c e n e r e d beds by Walker e t a l . (1967) i n d i c a t e t h a t t h e i r o n c o n t e n t of t h e c l a y r a n g e s from 2.5% t o 16.5% a d j a c e n t t o decomposing g r a i n s .

F i g . 5 . 1 4 shows a s c h e m a t i c diagram o f a p a r t i a l l y r e p l a c e d

hornblende g r a i n ( G r a i n 1) w i t h t h e p o s i t i o n s of microprobe p o i n t a n a l y s e s ( T r a v e r s e

1).

The d i s t r i b u t i o n of major o x i d e s w i t h i n c r e a s i n g d i s t a n c e from t h e p a r e n t

hornblende a r e a l s o shown i n t h i s diagram.

These r e v e a l t h a t t h e d a u g h t e r c l a y

c o n t a i n s 10.8% Fe 0 n e a r t h e p a r e n t hornblende b u t t h i s d e c r e a s e s s t e a d i l y away 2 3 from t h e p a r e n t hornblende t o 3.5% a t a d i s t a n c e of 2 2 0 m. Thin s e c t i o n s of t h e s e P l i o c e n e r e d beds r e v e a l a l l s t a g e s of s t a i n i n g o f t h e a u t h i g e n i c i l l i t e - m o n t m o r i l l o n i t e by i r o n o x i d e s .

I n i t i a l l y t h e s t a i n i s c o n c e n t r a t e d aoong f r a c t u r e s i n t h e

c l a y and on t h e p e r i p h e r y o f t h e a l t e r e d g r a i n s

i s d i f f u s e d throughout t h e c l a y .

,

i n more advanced s t a g e s t h e s t a i n

The outward d e p l e t i o n of i r o n h r s been c l e a r l y

demonstrated i n a number of c a s e s and i t seems r e a s o n a b l e t o cnnclude t h a t t h i s i o n

297

I CLAY I

HORNBLENDE

I CLAY

Fig.5.13. F i v e element microprobe a n a l y s i s a c r o s s a hornblende g r a i n p e r i p h e r a l l y r e p l a c e d by c l a y i n Cenozoic a l l u v i u m , G i l a Group, Tuscon Arizona ( a f t e r Walker e t a l . , 1978).

i s a p o t e n t i a l s o u r c e o f pigmentary h a e m a t i t e a t l h o u g h i t i s f a i r t o s a y t h a t t h e e x a c t manner i n which p a r t i c u l a t e h a e m a t i t e g r a i n c o a t i n g s and e u h e d r a l c r y s t a l s of h a e m a t i t e form from i r o n - s t a i n e d c l a y i s n o t y e t e s t a b l i s h e d .

Much of t h e

i r o n o x i d e may be t r a n s p o r t e d a s amorphous Fe(OH)3 i n f i n e l y p a r t i c u l a t e form (Langmuir and Whittemore, 1971) and s u b s e q u e n t l y be p r e c i p i t a t e d a s g o e t h i t e (d(-FeOOH) o r h a e m a t i t e (W-Fe 0 ) depending upon t h e p r e c i s e geochemical c o n d i t i o n s .

2 3

The d a u g h t e r c l a y i s much d e p l e t e d i n CaO and MgO when compared w i t h t h e p a r e n t hornblende ( F i g s . 5 . 1 3 , 5 . 1 4 ) b u t t h r o u g h o u t t h e d a u g h t e r c l a y t h e y remain c o n s t a n t ; i n t h e s e o x i d e s t h e l o s s i s more r a p i d and t a k e s p l a c e a t t h e i n t e r f a c e between t h e hornblende and t h e c l a y .

A s y e t , comparative q u a n t i t a t i v e d a t a a r e n o t a v a i l a b l e f o r t h e o t h e r m i n e r a l s i n Cenozoic a l l u v i u m such a s p l a g i o c l a s e and p o t a s h f e l d s p a r .

However, t h e r e l e a s e

of e l e m e n t s from t h e s e m i n e r a l s i s w e l l documented i n t h e s u r f i c i a l w e a t h e r i n g of igneous r o c k s ( H a r r i s and Adams, 1966) and e x p e r i m e n t a l l y (Andersson and Wiklander, 1975) and t h e r e can be l i t t l e doubt t h a t t h e s e d a t a a r e a p p l i c a b l e i n a g e n e r a l way t o t h e p h r e a t i c and v a d o s e e a r l y d i a g e n e s i s i n Cenozoic a l l u v i u m .

A s a whole, t h e

breakdown and r e p l a c e m e n t of t h e s e m i n e r a l s r e l e a s e s a v a r i e t y of e l e m e n t s i n c l u d i n g sodium, p o t a s s i u m , c a l c i u m , magnesium and i r o n i n t o t h e i n t e r s t i t i a l groundwater. Depending upon t h e chemical c o n d i t i o n s i n t h e groundwater, t h e r e l e a s e d i o n s m y e i t h e r be c a r r i e d away i n s o l u t i o n o r p r e c i p i t a t e d a s a u t h i g e n i c m i n e r a l s .

298

I

/

S

20

I

I

1

I

40

60

80

100

I

I

I

I

I

1

120 140 160 180 200 220

Distance from hornblende in p

m

F i g . 5 . 1 4 . D i s t r i b u t i o n of major o x i d e s i n a hornblende g r a i n p a r t i a l l y r e p l a c e d by a u t h i g e n i c c l a y . D e t e r m i n a t i o n s by e l e c t r o n probe m i c r o a n a l y s i s . The a s t e r i s k i n d i c a t e s w e i g h t p e r c e n t i n t h e p a r e n t h o r n b l e n d e . T r a v e r s e No.1 and G r a i n No.1 of Walker e t a l . ( 1 9 6 7 ) .

S i g n i f i c a n t removal o f i o n s i n groundwater w i l l o b v i o u s l y a l t e r t h e b u l k c h e m i s t r y of t h e s e d i m e n t from i t s o r i g i n a l composition. I n a n c i e n t r e d beds c l a y r e p l a c e m e n t can a l s o be demonstrated t o have been o f considerable importance.

P o t a s h f e l d s p a r s i n p a r t i c u l a r o f t e n show e v i d e n c e of

more e x t e n s i v e c l a y r e p l a c e m e n t t h a n t h a t s e e n i n Cenozoic a l l u v i u m and i t seems r e a s o n a b l e t o assume t h a t t h i s c o u l d be due t o more prolonged exposure t o i n t e r s t i t i a l fluids.

There i s some d i f f i c u l t y , however, i n making r e a l i s t i c e s t i m a t e s

of t h e q u a n t i t a t i v e importance o f c l a y r e p l a c e m e n t d u r i n g d i a g e n e s i s .

This a r i s e s

b e c a u s e o f t h e d i f f i c u l t y of d i s t i n g u i s h i n g c l a y replacement which h a s t a k e n p l a c e d u r i n g 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 t h a t which h a s o c c u r r e d p o s t - d e p o s i t i o n a l l y . V a r i a t i o n s i n t h e normal sequence o f f e l d s p a r s t a b i l i t y may p r o v i d e a n i m p o r t a n t c l u e t o s o u r c e a r e a w e a t h e r i n g ( e . g . Todd, 1968) b u t u s u a l l y t h e replacement p r o c e s s f o l l o w s t h e same sequence d u r i n g s o u r c e a r e a w e a t h e r i n g and d i a g e n e s i s .

Despite t h i s

d i f f i c u l t y i t seems l i k e l y t h a t t h e abundance o f h e a v i l y a l t e r e d f e l d s p a r s i n a n c i e n t r e d beds i s a t l e a s t p a r t l y due t o i n t r a s t r a t a l c l a y replacement.

The

dominant c l a y m i n e r a l s i n a n c i e n t r e d beds a r e i l l i t e and c h l o r i t e , a f e a t u r e which s u g g e s t s t h a t t h e mixed l a y e r illite-montmorrilonite-illite t r a n s f o r m a t i o n i s a f e a t u r e o f l a t e r r e d bed d i a g e n e s i s . AUTHIGENIC MINERALS

The d i s s o l u t i o n and r e p l a c e m e n t of d e t r i t a l s i l i c a t e g r a i n s r e s u l t s i n t h e r e l e a s e of chemical c o n s t i t u e n t s i n t o t h e i n t e r s t i t i a l groundwater s o l u t i o n s .

The

f a t e o f t h e s e r e l e a s e d i o n s depends upon t h e i n t e r s t i t i a l physico-chemical c o n d i t i o n s . The i o n s may be s o l u b l e and be c a r r i e d away i n t h e m i g r a t i n g groundwater o r , under f a v o u r a b l e c i r c u m s t a n c e s , t h e y may be p r e c i p i t a t e d a s a n a u t h i g e n i c m i n e r a l phase. I n t h e Cenozoic r e d beds s t u d i e d by Walker e t a l . (1978) a v a r i e t y o f a u t h i g e n i c p h a s e s i n c l u d i n g q u a r t z , c l a y m i n e r a l s , z e o l i t e s , f e l d s p a r s , h a e m a t i t e , and c a l c i t e have been p r e c i p i t a t e d i n t h i s manner.

This authigenic s u i t e is a l s o a character-

i s t i c o f a n c i e n t r e d beds and i s b e l i e v e d t o have formed i n a s i m i l a r manner. Potassium f e l d s u a r A u t h i g e n i c p o t a s h f e l d s p a r i s one of t h e most commonly o c c u r r i n g d i a g e n e t i c phases i n c o n t i n e n t a l r e d beds.

I n t h e Cenozoic a l l u v i u m of SW United and NW Mexico

t h e a u t h i g e n i c p o t a s h f e l d s p a r o c c u r s i n p a r t a s overgrowths o r d e t r i t a l g r a i n s and a l s o a s s m a l l c r y s t a l s w i t h i n t h e i n t e r s t i t i a l m a t r i x i n a s s o c i a t i o n w i t h mechanically i n f i l t r a t e d c l a y and a u t h i g e n i c c l a y and q u a r t z .

I n p l a c e s , a g g r e g a t e s of f e l d s p a r

c r y s t a l s may f i l l p o r e s p a c e s o r d i s s o l u t i o n v o i d s . A u t h i g e n i c p o t a s h f e l d s p a r i s a l s o a common c o n s t i t u e n t i n t h e a l t e r e d t u f f s and t u f f a c e o u s s a n d s t o n e s of f l u v i a l and l a c u s t r i n e o r i g i n i n t h e w e s t e r n United S t a t e s (Hay, 1966; Sheppard and Gude, 1968, 1969, 1973; Moiola, 1970).

Here i t o c c u r s

300

Fig.5.15. SEM photomicrographs of a u t h i g e n i c f e l d s p a r i n Cenozoic a l l u v i u m A . A u t h i g e n i c p o t a s s i u m f e l d s p a r on d e t r i t a l (A-C) and T r i a s s i c r e d beds (D-F). g r a i n , P l i o c e n e f a n g l o m e r a t e , CaEon Rojo, Baja C a l i f o r n i a . B. Red i n t e r s t i t i a l m a t r i x w i t h a u t h i g e n i c f e l d s p a r , same l o c a l i t y . C . Enlargement of o u t l i n e d a r e a i n B. D . O r i e n t e d c r y s t a l s of a u t h i g e n i c f e l d s p a r on t h e s u r f a c e of a d e t r i t a l f e l d s p a r i n a n e a r l y s t a g e o f overgrowth f o r m a t i o n . E . A more advanced s t a g e c o m p l e t e l y c o v e r i n g t h e g r a i n s u r f a c e . F. A complete overgrowth of p u r e potassium f e l d s p a r w i t h l a r g e , smooth c r y s t a l f a c e s . A l l from Lower Keuper S a n d s t o n e ( T r i a s s i c ) , West Midlands, U.K. A-C produced by c o u r t e s y o f T.R. Walker and D-F by c o u r t e s y of A.D. A l i .

301 predominantly a s overgrowths on g r a i n s of o r t h o c l a s e and o c c a s i o n a l l y p l a g i o c l a s e b u t a l s o a s v e r y f i n e g r a i n e d , low b i r e f r i n g e n t a g g r e g a t e s (Hay and Moiola, 1963). The f e l d s p a r i s o f t e n i n a s s o c i a t i o n w i t h a u t h i g e n i c z e o l i t e s ( a n a l c i m e ) which i n p l a c e s form a s t r a t i g r a p h i c a l l y zoned sequence ( s e e F i g . 5 . 1 6 ) .

The f o r m a t i o n of

a u t h i g e n i c K - f e l d s p a r i n v o l c a n i c l a s t i c d e p o s i t s i s s t i m u l a t e d by r e l a t i v e l y high a l k a l i - i o n t o hydrogen-ion a c t i v i t y r a t i o s and by r e l a t i v e l y h i g h s i l i c a a c t i v i t i e s (Hemley, 1959, 1962; Hay and M o i o l a , 1963, 1964; Mioola and Hay, 1 9 6 4 ) . The z o n a t i o n s e e n i n t h e Esmeralda Formation

(Fig.5.16):

analcime

+

feldspar,

c l i n o p t i l o l i t e , and p h i l l i p s i t e i s a p p a r e n t l y n o t t h e r e s u l t o f i n c r e a s e d hydros t a t i c p r e s s u r e o r t e m p e r a t u r e w i t h d e p t h of b u r i a l b u t i s t h o u g h t by Moiola (1970) t o r e f l e c t d i f f e r e n c e s i n composition of t h e p r e c i p i t a t i n g groundwater. A l l the a v a i l a b l e evidence suggests t h a t the authigenic feldspar i s pure potash f e l d s p a r ( s e e f o r example, Hay and M o i o l a , 1963, F i g . 3 p . 324).

This i s consistent

w i t h t h e r e s u l t s o f a n a l y s e s of a u t h i g e n i c K - f e l d s p a r i n a n c i e n t r e d beds which a r e commonly p u r e p o t a s s i u m f e l d s p a r (Waugh, 1978). A u t h i g e n i c K - f e l d s p a r i s known from many a n c i e n t r e d bed sequences i n c l u d i n g t h e T o r r i d o n i a n Sandstone (Waugh, 1 9 7 8 ) , t h e Old Red Sandstone and p a r t i c u l a r l y t h e Permo-Triassic S a n d s t o n e s o f n o r t h e r n Europe (Reynolds, 1929; W i l l i a m s , 1973; G l e n n i e e t a l . , 1978; K e s s l e r , 1978; Hancock, 1 9 7 8 ) .

I t s h o u l d be n o t e d t h a t

a u t h i g e n i c f e l d s p a r a l s o o c c u r s i n a number o f marine f a c i e s , i n p a r t i c u l a r limes t o n e s ( K a s t n e r , 1971; Buyce and Friedman, 1975). Waugh (1978) h a s made a d e t a i l e d s t u d y o f a u t h i g e n i c K - f e l d s p a r i n B r i t i s h Permo-Triassic Sandstones.

I n t h i n s e c t i o n s t h e s e a r e developed a s overgrowths

around d e t r i t a l c o r e s o f o r t h o c l a s e o r m i c r o c l i n e , o r v e r y r a r e l y , p l a g i o c l a s e . Discrete authigenic c r y s t a l s with an a d u l a r i a n h a b i t a l s o occur within the diss o l u t i o n v o i d s of d e t r i t a l K - f e l d s p a r g r a i n s .

These a u t h i g e n i c c r y s t a l s a r e

randomly o r i e n t e d and i n d i c a t e t h a t d i s s o l u t i o n p r e d a t e s t h e i r p r e c i p i t a t i o n and als'o t h a t a d e t r i t a l s e e d c r y s t a l i s n o t n e c e s s a r y f o r t h e f o r m a t i o n of a u t h i g e n i c K - f e l d s p a r ( c f . K a s t n e r , 1971).

The overgrowths c o n s i s t of p r i s m f a c e s , u s u a l l y

developed p a r a l l e l t o l o n g a x e s of g r a i n s , t r u n c a t e d by t h e b a s a l p i n a c o i d .

The

overgrowths o f t e n d i s p l a y "hacksaw" p r o j e c t i o n s w i t h a d i s t i n c t rhombohedral o u t l i n e , p a r t i c u l a r l y a t t h e d i s t a l ends o f e l o n g a t e g r a i n s .

Those evergrowths

p o s s e s s i n g a h i g h d e g r e e o f c r y s t a l l i n e p e r f e c t i o n show a n o v e r a l l s h a p e which i s markedly rhombohedral.

SEM s t u d i e s w i t h e n e r g y - d i s p e r s i v e a n a l y s i s of X-rays

(EDAX) and e l e c t r o n microprobe a n a l y s i s by Waugh (1978) i n d i c a t e t h a t t h e overgrowths a r e v i r t u a l l y p u r e p o t a s s i u m f e l d s p a r t h e a v e r a g e o f 28 overgrowths b e i n g K 0 = 2 16.94%, A1203 = 18.45% and S i 0 2 = 64.41% and t h e composition of any i n d i v i d u a l overgrowth d i d n o t v a r y by more t h a n

_+

0.5% from t h e above a v e r a g e s .

The a n a l y s e s

i n d i c a t e t h a t t h e overgrowths a r e p u r e , s t o i c h i o m e t r i c K A l S i 0 w i t h a n o p t i c a x i a l 3 8 a n g l e (-2V) r a n g i n g from 20-25'. Using t h e nomenclature o f Smith (1974) t h e f e l d s p a r overgrowths can be c l a s s i f i e d a s p o t a s s i a n i n t e r m e d i a t e s a n i d i n e s . Shenhav (1971)

302 h a s a l s o d e s c r i b e d a u t h i g e n i c . o v e r g r o w t h s o f s a n i d i n e from Lower C r e t a c e o u s s a n d stones of I s r a e l . Waugh ( 1 9 7 8 ) r e c o g n i z e d a s e r i e s o f d i s t i n c t g r o w t h s t a g e s i s t h e d e v e l o p m e n t of t h e potash f e l d s p a r overgrowths.

The i n i t i a l g r o w t h s t a g e r e c o g n i z e d i n t h e

d e v e l o p m e n t o f s m a l l , f l a t t e n e d r h o m b o h e d r a l c r y s t a l s v a r y i n g from 3-30/rm i n l e n g t h (Fig.5.15).

The c r y s t a l s a r e c o m b i n a t i o n s o f t h e b a s a l p i n a c o i d ( 0 0 1 ) a n d p r i s m

(110), w i t h ( 0 1 0 ) b e i n g v e r y n a r r o w o r u s u a l l y a b s e n t . so-called

T h i s morphology i s t h e

' a d u l a r i a ' h a b i t ( B a s k i n , 1956) b u t t h e u s e of t h i s term i s u s u a l l y u s e d

a s a m o r p h o l o g i c a l term a n d n o t a s a d i s t i n c t v a r i e t y of K - f e l d s p a r .

A t this stage

o f d e v e l o p m e n t t h e s e p s e u d o - r h o m b o h e d r a 1 c r y s t a l s show a p r o n o u n c e d p r e f e r r e d o r i e n t a t i o n b e c a u s e o f t h e c r y s t a l l o g r a p h i c c o n t r o l e x e r t e d by t h e h o s t f e l d s p a r .

In t h e e a r l y s t a g e s t h e s e a d u l a r i a - l i k e rhombs a r e s p a r s e l y s c a t t e r e d o v e r t h e d e t r i t a l g r a i n s u r f a c e b u t a s growth proceeds t h e i r s u r f a c e d e n s i t y i n c r e a s e s .

The

Permian a e o l i a n sands such a s t h e Bridgnorth and Mauchline sandstones d i s p l a y t h i s g r o w t h s t a g e v e r y w e l l ; i n t h e f o r m e r c a s e t h e a u t h i g e n i c f e l d s p a r h a s grown d i r e c t l y on t h e d e t r i t a l clay-oxide p e l l i c l e and i n the l a t t e r c a s e t h e a u t h i g e n i c f e l d s p a r h a s grown o v e r a u t h i g e n i c i l l i t e c l e a r l y i n d i c a t i n g t h a t f e l d s p a r p r e c i p i t a t i o n postdates the formation of a u t h i g e n i c c l a y .

The s u b s e q u e n t g r o w t h

p h a s e s i n t h e development o f complete overgrowths i n v o l v e a n i n c r e a s e i n t h e n u m e r i c a l d e n s i t y o f t h e f e l d s p a r rhombs a n d t h e i r m e r g i n g t o g e t h e r t o p r o d u c e l a r g e r smooth c r y s t a l f a c e s .

A t t h i s s t a g e t h e r h o m b o h e d r a l m i c r o s t r u c t u r e may

s t i l l b e d i s c e r n i b l e a n d l a r g e a r e a s o f t h e d e t r i t a l g r a i n s u r f a c e may b e s t i l l f r e e from a u t h i g e n i c f e l d s p a r .

Growth o c c u r s o v e r l a r g e r a r e a s by t h e e x t e n s i o n

o f t h e i n d i v i d u a l c r y s t a l f a c e s w h i c h may r e t a i n a d e g r e e o f i r r e g u l a r i t y b e c a u s e o f t h e i n c o m p l e t e o v e r l a p a n d s t a c k i n g of t h e r h o m b o d e d r a l u n i t s .

A t this stage

t h e o v e r a l l rhombohedral morphology o f t h e complete overgrowt h i s s e e n ; t h e a u t h i g e n i c c r y s t a l s r a n g e i n s i z e from 0 . 2 5 t o 1 . O O m m i n l e n g t h .

The c r y s t a l s

f r e q u e n t l y show hacksaw t e r m i n a t i o n s ( F i g . 5 . 1 5 ) . Zeolites A u t h i g e n i c z e o l i t e s a r e well-known i n C e n o z o i c c o n t i n e n t a l d e p o s i t s w h e r e t h e y occur a s euhedral c r y s t a l s i n pore spaces of f l u v i a l d e p o s i t s and i n p a r t i c u l a r a s a replacement of v i t r i c fragments i n l a c u s t r i n e r h y o l i t i c t u f f s .

The z e o l i t e s a r e

h y d r a t e d a l u m i n o s i l i c a t e s of t h e a l k a l i and a l k a l i n e e a r t h m e t a l s w i t h a n i n f i n i t e l y e x t e n d e d t h r e e - d i m e n s i o n a l a n i o n network and t h u s have t h e a t o m i c r a t i o 0 : (Al+Si) =

2.

They i n c l u d e a w i d e v a r i e t y o f m i n e r a l s p e c i e s ( s e e D e e r , Howie a n d Zussman,

1 9 6 6 , p . 393) b u t t h e o n e s o c c u r r i n g more commonly i n c o n t i n e n t a l d e p o s i t s a r e l i s t e d i n Table 5 . 6 .

Hay ( 1 9 6 3 , 1 9 6 4 ) a n d Hemley ( 1 9 5 9 , 1 9 6 2 ) h a v e shown t h a t

z e o l i t e f o r m a t i o n i s f a v o u r e d by r e l a t i v e l y h i g h a l k a l i i o n / h y d r o g e n i o n a c t i v i t y r a t i o s a n d by r e l a t i v e l y h i g h s i l i c a a c t i v i t i e s .

303 TABLE 5 . 6

I

The i d e a l formulae o f z e o l i t e s which commonly o c c u r i n c o n t i n e n t a l d e p o s i t s . Name

Formula

Analcime

Na

A1Si206

Chabazite

Ca

A12Si4012

Clinoptilolite

(Ca ,K2Na2)

Erionite

(Na2,K2,Ca,Mg)4.5

Mordenite

(Na2,K2,Ca)

A12Si10024

Phillipsite

(&a ,Na ,K)3

A12Si5016

H2° 6H20

A12Si20024

5H20

AlgSi27072 7H20 6H20

I n t h e f l u v i a l d e p o s i t s s t u d i e d by Walker e t a l . (1978) a u t h i g e n i c z e o l i t e s i n t h e form of c l i n o p t i l o l i t e o c c u r wherever v o l c a n i c l a s t i c d e t r i t u s i s a b u n d a n t . I t may l o c a l l y be t h e main cementing a g e n t , b u t more commonly o c c u r s a s s m a l l ,

c l e a r e u h e d r a l c r y s t a l s w i t h low b i r e f r i n g e n c e and h i g h r e l i e f which l i n e i n t e r s t i t i a l voids (Fig.5.17).

The f o r m a t i o n o f z e o l i t e s d u r i n g d i a g e n e s i s i s p a r t i c u l a r l y

f a v o u r e d i n t h e p r e s e n c e of v i t r i c d e t r i t u s , a l t h o u g h t h i s i s n o t t h e o r e t i c a l l y an a b s o l u t e n e c e s s i t y (Hay, 1966, p . 8 3 ) . The c l o s e a s s o c i a t i o n of z e o l i t e a u t h i g e n e s i s and v i t r i c d e t r i t u s i s w e l l - s e e n i n numerous examples i n t h e Cenozoic f l u v i a t i l e and l a c u s t r i n e d e p o s i t s of t h e w e s t e r n U n i t e d S t a t e s (Hay, 1963, 1966, Moiola and Hay, 1964; Sheppard, 1971, Sheppafd and Gude, 1968, 1969, 1973; Moiola, 1970; Walton, 1975).

The z e o l i t e s a r e

most abundant i n a l t e r e d v i t r i c t u f f s and t u f f a c e o u s s a n d s t o n e s and a r e g e n e r a l l y a g r e e d t o have formed d u r i n g d i a g e n e s i s by t h e r e a c t i o n of v o l c a n i c g l a s s w i t h i n t e r s t i t i a l w a t e r , which o r i g i n a t e d a s m e t e o r i c w a t e r (Hay, 1963) o r t h e connate w a t e r of a s a l i n e l a k e ( B r a d l e y , 1929; Hay, 1 9 6 4 ) .

The z e o l i t e m i n e r a l s a r e u s u a l l y

a s s o c i a t e d w i t h o t h e r a u t h i g e n i c m i n e r a l s s u c h a s m o n t m o r i l l o n i t e and K - f e l d s p a r and may show a l a t e r a l l y , o r v e r t i c a l l y zoned s u c c e s s i o n .

The Esmeralda Formation,

f o r example, a L a t e Cenozoic (Miocene-Pliocene) 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 shows a v e r t i c a l z o n a t i o n from t h e bottom upwards of analcime c l i n o p t i l o l i t e and p h i l l i p s i t e ( F i g . 5 . 1 6 )

+ K-feldspar,

(Moiola, 1970).

A n a l c i n e o n l y o c c u r s n e a r t h e b a s e of t h e Esmeralda F o r m a t i o n , i n a s s o c i a t i o n with authigenic K-feldspar.

I t o c c u r s a s c l e a r , s u b h e d r a l - e u h e d r a l c r y s t a l s which

show no e v i d e n c e t h a t i t formed d i r e c t l y from v i t r i c m a t e r i a l o r from t h e r e placement of a n a l k a l i - r i c h z e o l i t e p r e c u r s o r s u c h a s c l i n o p t i l o l i t e ( c f . Hay, 1966; Goodwin and Surdam, 1967; Sheppard and Gude, 1 9 6 9 ) .

I t i s more p r o b a b l e

t h a t t h e analcime p r e c i p i t a t e d d i r e c t l y e i t h e r from t r u e s o l u t i o n o r a n i n t e r m e d i a t e a l u m i n o s i l i c a t e g e l (Marimer and Surdam, 1 9 7 0 ) .

304

m

Gray sandstones siltstones and conglomarmr with whim vkric tuffs with basalt flow and tuff-bnocia

PHILLIPSITE

8000

7m

Pale orange to lightgrey sandstones siltstoms 8nd shales with light gray vitric tuffs.

6OOC I-

w

W LL

z

5000

9z

Greyishoranga and greyish-yellow siltsWnes with white-grey sandstones and yellowish-gray shales intartonging with grayish-orange pebble conglomerates limestone brlccia and coarse sandstones

CLINOPTILOLITE

4000 I I-

3MM

Light grey to grayish-orenge volcanic pebble conglomerates undstones and siltstones and grey sandstones with thin tuffs.

2000

1000

0

Whitr to pda orange t u f f u c ~ ~and l s siliaous sandstones with greyish-orange shdr and yellowish-brown SandstOniL Pde ydlowish-orange to greyish-yellowfaldtprthic and lithk sandstones and siltstones Light brown to grwnish-gray pebble c o n g l ~ r a t wand brrcciu with thin interbods of sandstons siltstone and limrstoner

ANALCIME+ K-FELDSPAR

Fig.5.16. S t r a t i g r a p h y and z e o l i t e z o n a t i o n o f t h e Esmeralda Formation (MiocenePliocene) ( a f t e r Moiola, 1970). The main p a r t o f t h e Esmeralda Formation i s c h a r a c t e r i z e d by c l i n o p t i l o l i t e which o c c u r s c h i e f l y a s a pseudomorphic r e p l a c e m e n t o f g l a s s b u t a l s o a s a cement i n sandstones.

The pseudomorphic r e p l a c e m e n t s u g g e s t s t h a t d i s s o l u t i o n o f g l a s s

s h a r d s preceded t h e c r y s t a l l i z a t i o n o f c l i n o p t i l o l i t e .

The r e a s o n s b e i n g t h a t t h e

l a r g e r pseudomorphs a r e g e n e r a l l y hollow and c h a r a c t e r i s t i c a l l y c o n s i s t of a v e r y t h i n m a r g i n a l f i l m of m o n t m o r i l l o n i t e o r t h i n l a y e r of o p a l , succeeded i n w a r d l y by a thin layer of c l i n o p t i l o l i t e c r y s t a l s oriented perpendicular

td

the inner wall

of t h e s h a r d . P h i l l i p s i t e i s r e s t r i c t e d t o b a s a l t i c t u f f - b r e c c i a s n e a r t h e t o p o f t h e Esmeralda Formation where i t o c c u r s p r i m a r i l y a s a r e p l a c e e m n t of s m a l l pumice f r a g m e n t s . Moiola (1970) showed t h a t t h e v e r t i c a l z e o l i t e z o n a t i o n i n t h e Esmeralda Formation was t h e r e s u l t of t h e r e a c t i o n of m i g r a t i n g groundwater w i t h v o l c a n i c d e t r i t u s ,

305 t h e d i f f e r e n t zones r e p r e s e n t i n g d i f f e r e n c e s i n t h e c o m p o s i t i o n o f t h e groundwater i n c o n t a c t w i t h t h e s e d i m e n t s a t t h e time o f a l t e r a t i o n .

The e a r l i e s t formed m i n e r a l

was p r o b a b l y m o n t m o r i l l o n i t e which formed by m a r g i n a l h y d r o l y s i s o f g l a s s and which

i s f a v o u r e d by a r e l a t i v e l y low Na

+K

t o H a c t i v i t y r a t i o (Hemley, 1 9 6 2 ) .

The

f o r m a t i o n o f m o n t m o r i l l o n i t e i n t h i s way would i n c r e a s e t h e pH and t h e c o n c e n t r a t i o n of a l k a l i i o n s .

The groundwater s o l u t i o n became s a t u r a t e d w i t h r e s p e c t t o c l i n o p -

t i l o l i t e which t h e n p r e c i p i t a t e d .

Conditions favouring i t s formation a r e a

r e l a t i v e l y h i g h S i t o A 1 a c t i v i t y r a t i o and a r e l a t i v e l y h i g h Na

+K

to H ratio.

C o - p r e c i p i t a t i o n of o p a l i n c o n j u n c t i o n w i t h c l i n o p t i l o l i t e i n d i c a t e s t h a t t h e s o l u t i o n s were s u p e r s a t u r a t e d w i t h r e s p e c t t o q u a r t z , t r i d y m i t e and c r i s t o b a l i t e . Zonation of z e o l i t e s a r e a l s o s e e n i n t h e Barstow Formation i n t h e Mud H i l l s a r e a of C a l i f o r n i a (Sheppard and Gude, 1969) b u t h e r e t h e z e o l i t i c assemblages show a l a t e r a l zonation.

The z e o l i t e s o c c u r i n a l t e r e d v i t r i c t u f f s of Miocene a g e and

a r e predominantly a n a l c i m e and c l i n o p t i l o l i t e b u t a l s o i n c l u d e c h a b a z i t e , e r i o n i t e , m o r d e n i t e and p h i l l i p s i t e . c r y s t a l s 0.01

-

Analcime most commonly o c c u r s a s s u b h e d r a l t o e u h e d r a l

0.05mm i n s i z e and may be a s s o c i a t e d w i t h t h e o t h e r z e o l i t e s and

potash feldspars but i s never seen i n a s s o c i a t i o n with r e l i c t g l a s s . o c c u r s a s p r i s m a t i c o r p l a t y c r y s t a l s , most commonly 0.01

-

Clinoptilolite

0.02mm long and i s

a s s o c i a t e d w i t h a u t h i g e n i c c l a y s , p o t a s h f e l d s p a r , o p a l and q u a r t z .

Larger c r y s t a l s

o f c l i n o p t i l o l i t e c o n s i s t o f a uniform c o r e and a p r o g r e s s i v e l y zoned r i m , p r o b a b l y r e p r e s e n t i n g a l e s s s i l i c e o u s o u t e r zone.

C l i n o p t i l o l i t e i s often associated with

a c i c u l a r o r p r i s m a t i c c r y s t a l s o f c h a b a z i t e , e r i o n i t e and m o r d e n i t e ; p h i l l i p s i t e occurs a s small s p h e r u l i t e s 0.1

-

0.3mm i n s i z e .

Sheppard and Gude (1969) e n v i s a g e d t h e f o l l o w i n g sequence o f e v e n t s t o e x p l a i n t h e observed l a t e r a l z o n a t i o n of z e o l i t i c assemblages.

I n i t i a l l y , s o l u t i o n of

s i l i c i c g l a s s by m o d e r a t e l y a l k a l i n e and s a l i n e pore-water r e s u l t e d i n t h e d i r e c t f o r m a t i o n of z e o l i t e s ( e x c e p t a n a l c i m e ) from g l a s s by a s o l u t i o n p r e c i p i t a t i o n mechanism ( D e f f e y e s , 1959).

V a r i a t i o n s i n t h e observed z e o l i t e p a r a g e n e s i s a r e

a t t r i b u t e d t o v a r i a t i o n s i n t h e a c t i v i t y of S i 0 2 , t h e a c t i v i t y o f H 2 0 , and t h e proportions of cations i n the i n t e r s t i t i a l water during diagenesis i n a similar manner t o t h a t d e s c r i b e d by Moiola (1970) f o r t h e Esmeralda Formation.

In the

Barstow Formation t h e e v i d e n c e o f a u t h i g e n i c m i n e r a l . a s s e m b l a g e s s t r o n g l y s u g g e s t s t h e f o r m a t i o n o f analcime from a l k a l i c , s i l i c i c z e o l i t e p r e c u r s o r s , s u c h a s c l i n o p t i l o l i t e (Hay, 1966; Walton, 1975, 1977).

The f o r m a t i o n o f a n a l c i m e r a t h e r t h a n

a l k a l i c , s i l i c i c z e o l i t e s i s favoured by a h i g h Na+:H+ r a t i o (Hess, 1966) r e l a t i v e l y low a c t i v i t y of S i 0 2 (Coombs e t a l . , 1959; Senderov, 1963; Campbell and F y f e , 1965) and p e r h a p s , r e l a t i v e l y low a c t i v i t y of H 2 0 .

The c o m p o s i t i o n a l v a r i a t i o n o f

analcime h e r e a p p a r e n t l y r e f l e c t s i n p a r t t h e c o m p o s i t i o n o f t h e z e o l i t e precursor

-

r e l a t i v e l y s i l i c e o u s analcime forming from a z e o l i t e s u c h a s c l i n o p -

t i l o l i t e whereas r e l a t i v e l y aluminous a n a l c i m e may have formed from a more aluminous precursor such a s p h i l l i p s i t e .

S i m i l a r l y , a u t h i g e n i c p o t a s h f e l d s p a r i s nowhere s e e n

306

Fig.5.17. A u t h i g e n i c z e o l i t e . A. Thin s e c t i o n photomicrograph showing a u t h i g e n i c c l i n o p t i l o l i t e (Z) p a r t l y f i l l i n g i n t e r s t i t i a l v o i d s from t h e Tesuque Formation (Miocene-Pliocene), New Mexico. B. SEM photomicrograph o f t h e same sample showing a u t h i g e n i c c l i n o p t i l o l i t e ( 2 ) c r y s t a l s and a s s o c i a t e d c l a y . C . A u t h i g e n i c c l i n o p t i l o l i t e pseudomorphing g l a s s s h a r d from t h e Esmeralda Formation (MioceneP l i o c e n e ) , Nevada. D. E u h e d r a l a u t h i g e n i c a n a l c i m e from t h e Esmeralda Formation. A-B produced by c o u r t e s y of T.R. Walker and C-D by c o u r t e s y of R . J . Moiola.

i n c o n t a c t w i t h v i t r i c m a t e r i a l and a p p a r e n t l y d i d n o t form d i r e c t l y from v i t r i c m a t e r i a l b u t o n l y through a n i n t e r m e d i a t e phase such a s a n a l c i m e o r c l i n o p t i l o l i t e The f a c t o r s a f f e c t i n g t h e f o r m a t i o n of p o t a s h f e l d s p a r from z e o l i t e s a r e d i s c u s s e d by I i j i m a and Hay (1968) and e x p e r i m e n t a l s y n t h e s i s of p o t a s h f e l d s p a r from c l i n o p t i l o l i t e has been made by Nemecz and V a r j u (1962) and by Sheppard and Gude ( 1 9 6 9 ) . The l a t e r a l v a r i a t i o n i n some a u t h i g e n i c m i n e r a l o g y of t h e Barstow Formation t u f f s of:

nonanalcimic z e o l i t e s

-

analcime

-

K - f e l d s p a r i s p r o b a b l y due t h e r e f o r e t o a

v a r i a t i o n i n s a l i n i t y of t h e p o r e w a t e r s t r a p p e d w i t h i n t h e t u f f s d u r i n g d i a g e n e s i s . The o b s e r v e d z o n a t i o n of a u t h i g e n i c z e o l i t e s can i n many i n s t a n c e s be b e s t a c c o u n t e d f o r by a n open system l e a c h i n g model of d i a g e n e s i s (Hay, 1963; Walton, 1975).

According t o t h i s model, w a t e r f l o w i n g through v o l c a n i c s e d i m e n t d i s s o l v e s

g l a s s a n d becomes e n r i c h e d i n d i s s o l v e d s o l i d s t o t h e c o n c e n t r a t i o n a t which m o n t m o r i l l o n i t e and then z e o l i t e s form.

The c o m p o s i t i o n of t h e groundwater s o l u t i o n i

a t any p a r t i c u l a r p o i n t i n t h e d i a g e n e t i c system i s t h u s a f u n c t i o n of t h e l e n g t h of time t h e w a t e r has been i n c o n t a c t w i t h v o l c a n i c g l a s s , t h e i n i t i a l composition of t h e groundwater and a l s o t h e composition of t h e g l a s s .

As deposition continues

t h e d i s t a n c e which groundwater must m i g r a t e from t h e r e c h a r g e a r e a t o r e a c h t h e point gets greater.

The groundwater i s t h e r e f o r e i n c o n t a c t w i t h s e d i m e n t l o n g e r

b e f o r e i t r e a c h e s t h e f i x e d p o i n t and i s c o n s e q u e n t l y changed from i t s o r i g i n a l composition.

A s t h e composition o f t h e groundwater changed d u r i n g d i a g e n e s i s ,

s e v e r a l m i n e r a l s may form s u c c e s s i v e l y a t e a c h p o i n t , e i t h e r a s r e p l a c e m e n t s o r precipitated i n voids.

The c o m p o s i t i o n a l changes which o c c u r a s w a t e r p a s s e s through

t h e d i a g e n e t i c system a r e r e p r e s e n t e d i n F i g . 5 . 1 8 .

The f i r s t formed m i n e r a l s a r e

m o n t m o r i l l o n i t d and o p a l which form from d i s s o l v e d s i l i c a , aluminium and o t h e r cations a s the glass dissolves.

I n i t i a l h y d r o l y s i s of s i l i c a and alumina d i s s o l v e d

from t h e g l a s s r a i s e s t h e pH. I n t h e a r e a s of m o n t m o r i l l o n i t e f o r m a t i o n most of t h e s i l i c a and aluminium i s consumed, a l t h o u g h e x c e s s s i l i c a forms o p a l , and t h e s o l u t i o n becomes r e l a t i v e l y e n r i c h e d i n a l k a l i s (1-2, F i g . 5 . 1 8 ) .

When t h e changing composition of t h e groundwater

r e a c h e s p o i n t 2 t h e r a t i o of a l k a l i i o n t o hydrogen i o n i s such t h a t z e o l i t e formation i s p r e f e r r e d t o montmorillonite formation.

S i l i c a and aluminium a r e t h e n

consumed i n p r o p o r t i o n s comparable t o t h o s e i n t h e o r i g i n a l g l a s s and o p a l c e a s e s t o form.

The s u b s e q u e n t f o r m a t i o n o f a n a l c i m e p o s e s a problem i n t h a t l a r g e amounts

of aluminium need t o be added d u r i n g t h e c o n v e r s i o n .

V a r i o u s s u g g e s t i o n s have been

p u t forward i n c l u d i n g t h e f o r m a t i o n of a n a l c i m e by t h e r e a c t i o n of hydrous aluminium s i l i c a t e s w i t h a l k a l i n e p o r e w a t e r (Ross, 1928; K e l l e r , 1952) and t h e r e p l a c e m e n t of m o n t m o r i l l o n i t e by q u a r t z .

The l a t t e r e x p l a n a t i o n seems more l i k e l y ; t h i s would

lower t h e s i l i c a a c t i v i t y and r a i s e t h e a l u n i n i u m a c t i v i t y t h u s c a u s i n g c o n v e r s i o n of c l i n o p t i l o l i t e t o a n a l c i m e a t composition 4 ( F i g . 5 . 1 8 ) .

308

/'

1

P"

-

I

Fig.5.18. Changes i n w a t e r composition a s i t moves through t h e d i a g e n e t i c system. During f o r m a t i o n o f m o n t m o r i l l o n i t e g l a s s d i s s o l v e s and s o l u t i o n becomes more a l k a l i n e w i t h concomitant i n c r e a s e i n Al(0H); ( 1 - 2 ) . A t same t i m e , s i l i c a a c t i v i t y i s s e t by e q u i l i b r i u m w i t h amorphous s i l i c a phase t h a t c o n t a i n s some aluminium, b u t conc e n t r a t i o n of o t h e r d i s s o l v e d i o n s i n c r e a s e s . S o l u t i o n r e a c h e s s a t u r a t i o n w i t h c l i n o p t i l o l i t e a t 2. Rapid c o n v e r s i o n of g l a s s t o c l i n o p t i l o l i t e remains t h e same. A t 3 replacement of m o n t m o r i l l o n i t e by q u a r t z lowers s i l i c a a c t i v i t y and r a i s e s aluminium a c t i v i t y , c a u s i n g c o n v e r s i o n o f c l i n o p t i l o l i t e t o a n a l c i m e when composition 4 i s reached ( a f t e r Walton, 1975). The p r e c e d i n g d i s c u s s i o n c l e a r l y i n d i c a t e s t h a t t h e b o u n d a r i e s between a u t h i g e n i c z e o l i t e assemblages a r e d e t e r m i n e d more by groundwater hydrology and c h e m i s t r y t h a n by t e m p e r a t u r e , p r e s s u r e o r d e p t h of b u r i a l .

Even s o , t h e e v i d e n c e from d e s e r t

r e g i o n s o f t h e Western S t a t e s does i n d i c a t e t h a t a minimum t h i c k n e s s of s e d i m e n t , p o s s i b i l i t y 200m o r more, must accumulate b e f o r e groundwater i s i n c o n t a c t long enough f o r a u t h i g e n i c z e o l i t e s t o develop (Walton, 1975). Clay M i n e r a l s Illite-montmorillonite The i n t r a s t r a t a l d i s s o l u t i o n o f framework g r a i n s d u r i n g e a r l y d i a g e n e s i s produces a l u m i n a , s i l i c a , and sodium much o f which i s r e p r e c i p i t a t e d a s a u t h i g e n i c c l a y . Throughout t h e Cenozoic a l l u v i u m of SW U n i t e d S t a t e s a n d NW Mexico s t u d i e d by Walker e t a l . (1978) b o t h r e p r e c i p i t a t e d and r e p l a c e m e n t a u t h i g e n i c c l a y s a r e randomly i n t e r s t r a t i f i e d mixed-layer i l l i t e - m o n t m o r i l l o n i t e c o n t a i n i n g 80-95% expandable l a y e r s ( i . e . n e a r l y p u r e m o n t m o r i l l o n i t e ) .

X-ray d i f f r a c t i o n t r a c e s

t h u s e n a b l e t h e r e a d y d i s t i n c t i o n of t h i s a u g h i g e n i c c l a y from t h e more m i n e r a l -

309 o g i c a l l y heterogeneous mechanically i n f i l t r a t e d c l a y .

However, i n t h i n s e c t i o n

a u t h i g e n i c c l a y o f t h i s type may be d i f f i c u l t t o r e c o g n i z e ; i t o c c a s i o n a l l y shows r a d i a t i n g c r y s t a l s which l i n e i n t e r s t i t i a l v o i d s b u t more u s u a l l y t h e p r e c i p i t a t e d c l a y i s a n u n r e c o g n i z a b l e p a r t of t h e i n t e r s t i t i a l m a t r i x .

However, SEM r e v e a l s a

v e r y c h a r a c t e r i s t i c boxwork o r c e l l u l a r t e x t u r e which confirms i t s a u t h i g e n i c T h i s t e x t u r e i s f i g u r e d by B o r s t a n d K e l l e r (1969) and by

nature (Fig.5.19).

Wilson and P i t t m a n ( 1 9 7 7 ) .

I t v a r i e s somewhat depending upon t h e d e h y d r a t i o n s t a t e

of t h e a u t h i g e n i c s m e c t i t e , a h i g h l y c r e n u l a t e d a p p e a r a n c e r e s u l t i n g when t h e m i n e r a l

i s dehydrated.

Although t h i s t e x t u r e s u p e r f i c i a l l y resembles t h e honeycomb t e x t u r e

s e e n i n a u t h i g e n i c c h l o r i t e and i l l i t e i t s h o u l d n o t be confused w i t h t h e s e because i t d i f f e r s i n a n i m p o r t a n t r e s p e c t i . e . i n d i v i d u a l c r y s t a l s o f s m e c t i t e c a n n o t be

r e s o l v e d u s i n g t h e SEM, i n c o n t r a s t t o t h e c h l o r i t e o r i l l i t e honeycomb which i s formed of i n d i v i d u a l c l a y p l a t e l e t s i n t e r l o c k i n g and normal t o t h e d e t r i t a l g r a i n surface.

I n a s i m i l a r v e i n Wilson a n d P i t t m a n (1977) p o i n t o u t t h a t i t can be

m i s l e a d i n g t o u s e o n l y SEM e v i d e n c e t o d i s t i n g u i s h m o n t m o r i l l o n i t e and mixed l a y e r illite-montmorillonite.

I n g e n e r a l t h e SEM a p p e a r a n c e of 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 depends upon t h e r e l a t i v e p r o p o r t i o n s of t h e p a r t i c i p a t i n g c l a y minerals:

i f i l l i t e - r i c h t h e s t r u c t u r e may be d i f f i c u l t t o d i s t i n g u i s h from pure

i l l i t e and t a k e s t h e form of s h e e t s w i t h s h o r t l a t h - l i k e d i g i t a t e edges.

Illite

A u t h i g e n i c i l l i t e i s u b i q u i t o u s i n a n c i e n t r e d beds and i s l e s s common i n T e r t i a r y and Q u a t e r n a r y a l l u v i u m .

I n t h i n section i t is readily distinguished

from o t h e r c l a y s by i t s h i g h e r b i r e f r i n g e n c e ; i t commonly forms p o r e l i n i n g s (Fig.5.19).

I n SEM s t u d i e s i l l i t e g e n e r a l l y o c c u r s a s i r r e g u l a r f l a k e s w i t h l a t h -

like projections.

The a u t h i g e n i c growths, a t t a c h e d t o t h e s u r f a c e o f t h e sand

g r a i n s , develop a s s h e e t s t h a t c u r l away from t h e p o i n t of a t t a c h m e n t .

The edges

of t h e s e s h e e t s have p r o j e c t i o n s which may b e r e l a t i v e l y s h o r t o r t h e y may develop i n t o c u r l e d , f i b r o u s p r o j e c t i o n s up t o 10ym long and 0 . 5 ~ " i n w i d t h , t h e s o - c a l l e d fibrous o r hairy i l l i t e . Kaolinite-Dickite K a o l i n i t e and d i c k i t e a r e common a u t h i g e n i c c l a y s i n s a n d s t o n e s and t h e r e a r e a number of r e p o r t e d o c c u r r e n c e s i n r e d beds.

K a o l i n i t e o c c u r s i n t h e 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 t h e North Sea a r e a ( G l e n n i e e t a l . , 1978; Hancock, 1978;

Kessler, 1978).and a l s o t h e Old Red Sandstone ( F r i e n d , 1966).

i n t h e O l d Red Sandstone o f S c o t l a n d .

Dickite a l s o occurs

K a o l i n i t e and d i c k i t e a r e t h e most

e a s i l y r e c o g n i z e d c l a y m i n e r a l s i n t h i n s e c t i o n ( F o t h e r g i l l , 1955, p. 305; Karpova, 1969, p . 7 ) b e c a u s e t h e y a r e f r e q u e n t l y c o a r s e r than o t h e r c l a y m i n e r a l s p e c i e s . K a o l i n i t e i s n o t d i s t i n g u i s h a b l e from d i c k i t e u s i n g o n l y SEM b u t they a r e r e a d i l y

310

A u t h i g e n i c 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 n Cenozoic Fig.5.19. a l l u v i u m and a n c i e n t r e d beds. A . Large c r y s t a l s of a u t h i g e n i c c l a y forming a r a d i a l p o r e l i n i n g . Thin s e c t i o n , Cuchara Formation ( E o c e n e ) , Colorado. B. D e t a i l o f a u t h i g e n i c c l a y forming i n t e r s t i t i a l m a t r i x , SEM photomicrograph, G i l a Group, Tucson, Arizona. C . A u t h i g e n i c mixed l a y e r c l a y (A-C) developed on m e c h a n i c a l l y i n f i l t r a t e d c l a y (MIC). SEM photomicrograph, Hayner Ranch Formation (Miocene), New Mexico. D . A u t h i g e n i c i l l i t e showing t y p i c a l boxwork t e x t u r e . SEM photom i c r o g r a p h , R o t l i e g e n d e s ( P e r m i a n ) , North Sea. A-C produced by c o u r t e s y o f T . R . Walker and D by c o u r t e s y of B. Waugh.

TABLE 5.7.

C h a r a c t e r i s t i c s of a u t h i g e n i c c l a y s ( a f t e r Wilson and P i t t m a n , 1977). Morphology

Form o f Aggregate

Relationship with D e t r i t a l Grains

Thickness of Coating o r Long Dimension o f Aggregate

(ym)

pseudohexagonal

stacked p l a t e s (book)

pore f i l l i n g

2-2500 (generally 2-20)

pseudohexagonal

vermicule

pore f i l l i n g

10-2500 (generally 20-200)

pseudohexagonal

sheet

pore f i l l i n g

0.1-1

pseudohexagonal

p l a t e s (2dimensional cardhouse

pore l i n i n g

2- 10

c u r l e d equidimensional w i t h rounded edges

honeycomb

pore l i n i n g

2- 10

equidimensional w i t h a n g u l a r o r l o b a t e edges

r o s e t t e o r fan

pore. l i n i n g and f i l l i n g

4- 150

fan-shaped f i b r o u s bundles

ca bba gehea d

pore-lining and f i l l i n g

8-40

Illite

i r r e g u l a r with elongate spines

sheet

pore l i n i n g

0.1-10

Montmorillonite

n o t recognizable

wrinkled s h e e t o r honeycomb

pore l i n i n g

2-12

Mixed l a y e r montiuorilloni t e /illi t e

subequant w i t h s t u b b y spines

imbricate sheet t o r a a a e d honevcomb

pore l i n i n g

2- 1 2

K a o l i n i t e and Dickite

Chlorite

(generally 4-20)

312

Fig.5.20. A u t h i g e n i c k a o l i n i t e and c h l o r i t e i n a n c i e n t r e d beds. A. Pseudohexagonal c r y s t a l s of k a o l i n i t e forming a p o r e f i l l i n g . Thin s e c t i o n , Lower Old Red S a n d s t o n e , Gamrie O u t l i e r , S c o t l a n d . B. S t a c k e d c r y s t a l s of k a o l i n i t e o f v a r i a b l e g r a i n s i z e forming a p o r e - f i l l i n g . Note t h a t many of t h e c r y s t a l s a r e twinned a s i n d i c a t e d by n o t c h e s . SEM photomicrograph, R o t l i e g e n d e s ( P e r m i a n ) , North Sea. C . A u t h i g e n i c c h l o r i t e forming a p o r e - l i n i n g , Thin s e c t i o n , R o t l i e g e n d e s ( P e r m i a n ) , North Sea. D . A u t h i g e n i c c h l o r i t e p o r e l i n i n g s . Thin s e c t i o n , T o r r i d o n i a n Sandstone ( L a t e P r e c a m b r i a n ) , NW S c o t l a n d . B i s produced by c o u r t e s y of B . Waugh.

313 s e p a r a t e d u s i n g X-ray d i f f r a c t i o n .

Authigenic k a o l i n i t e - d i c k i t e t y p i c a l l y occurs

i n s a n d s t o n e s a s pseudohexagonal p l a t e s , m o s t l y commonly p l u g g i n g p o r e s p a c e s completely, but a l s o a s pore l i n i n g s (Fig.5.20). The most common h a b i t o f a u t h i g e n i c k a o l i n i t e - d i c k i t e i n s a n d s t o n e s i s t h e c - a x i s s t a c k i n g of pseudohexagonal p l a t e s t o form "books".

I n d i v i d u a l p l a t e s may range from

3 - 2 0 ~i n d i a m e t e r ; i n d i v i d u a l p l a t e s and books of k a o l i n i t e may show grooves o r n o t c h e s which may r e p r e s e n t twinning ( M a n s f i e l d and B a i l e y , 1972).

Kaolinite also

d i s p l a y s a v e r m i c u l a r h a b i t which c o n s i s t s o f a sequence of s t a c k e d pseudohexagonal p l a t e s t h a t may e x t e n d t h e f u l l l e n g t h o f p o r e s p a c e s .

More r a r e l y k a o l i n i t e may

o c c u r a s o v e r l a p p i n g pseudohexagonal p l a t e s c o a t i n g s a n d s i z e d d e t r i t a l g r a i n s o r o t h e r a u t h i g e n i c overgrowths. 2-lO/m i n d i a m e t e r .

These p l a t e s a r e g e n e r a l l y l e s s t h a n

pm

t h i c k and

P o r e f i l l i n g and p o r e l i n i n g k a o l i n i t e i n r e d beds i s g e n e r a l l y

c o n s i d e r e d t o be a r e l a t i v e l y l a t e d i a g e n e t i c f e a t u r e .

C h l o r i t e , a l o n g w i t h i l l i t e i s t h e most commonly o c c u r r i n g c l a y m i n e r a l i n a n c i e n t red beds.

I t o c c u r s a s p o r e l i n i n g s which may a p p e a r p a l e g r e e n o r c o l o u r -

l e s s i n t h i n s e c t i o n and a l s o w i t h i n i n t e r s t i t i a l m a t r i x .

The p o r e l i n i n g s a r e

e x t r e m e l y v a r i a b l e i n s a n d s t o n e s g e n e r a l l y and may t a k e t h e form of p l a t e s a t t a c h e d t o d e t r i t a l sand g r a i n s ( P i t t m a n and Lumsden, 1968; Hayes, 1 9 7 0 ) , r o s e t t e s , honeycombs, o r cabbagehead-like growths (Wilson and P i t t m a n , 1977). of t h e s e growth h a b i t s v a r i e s a s f o l l o w s : cabbagehead.

p l a t e s and r o s e t t e s

The i r o n c o n t e n t honeycomb

C h l o r i t e most commonly develops a s i n d i v i d u a l idiomorphic p l a t e s

a t t a c h e d on edge t o t h e s u r f a c e of t h e s a n d g r a i n s u s u a l l y r a n g i n g from 2 - 1 0 ~i n s i z e . Fig.5.20. Honeycomb s t r u c t u r e d e v e l o p s when t h e p l a t e s a r e a r r a n g e d i n a c e l l u l a r p a t t e r n due t o t h e i n t e r s e c t i o n and c u r v i n g of i n d i v i d u a l c r y s t a l s g i v i n g a honeycomb l i k e a p p e a r a n c e .

A u t h i g e n i c c h l o r i t e a l s o forms r o s e t t e s and fan-shaped

c l u s t e r s a s p o r e l i n i n g s and p o r e f i l l i n g s , u s u a l l y 5 - 2 0 ~i n d i a m e t e r , b u t o c c a s i o n a l l y up t o 1 5 0 ~ . I n d i v i d u a l c r y s t a l s t y p i c a l l y range from 4-18p i n diameter.

The r a r e s t growth h a b i t of a u t h i g e n i c c h l o r i t e i s t h e cabbagehead.

At

low m a g n i f i c a t i o n s t h e s e a p p e a r a s s m a l l e q u i d i m e n s i o n a l g r a i n s a t t a c h e d t o sand sized grains.

A t higher magnifications they appear a s concentric ribbon o r

s h e e t - l i k e l a y e r s of c h l o r i t e w i t h complex o r i e n t a t i o n s . r a n g e from a p p r o x i m a t e l y 8 t o 4

I n d i v i d u a l cabbageheads

0 and ~ most commonly form p o r e l i n i n g s .

In a n c i e n t r e d beds c h l o r i t e i s a c h a r a c t e r i s t i c a u t h i g e n i c p h a s e .

It i s

u s u a l l y a n i r o n - r i c h v a r i e t y a s i n d i c a t e d by t h e r e l a t i v e l y s t r o n g (002) and ( 0 0 4 ) r e f l e c t i o n s ( B r i n d l e y , 1961) and o c c u r s a s d i s c r e t e p o r e l i n i n g s and w i t h i n i n t e r s t i t i a l matrix.

314 Calcite A u t h i g e n i c c a l c i t e i s a common cementing a g e n t i n a n c i e n t r e d s and a l s o i n t h e i r L a t e T e r t i a r y and Q u a t e r n a r y p r e c u r s o r s . The e a r l i e s t formed c a l c i t e cements i n r e d bed p r e c u r s o r s o c c u r a s a r e s u l t of b o t h p h r e a t i c and vadose c e m e n t a t i o n i n humid-semi-arid

a l l u v i a l f a n and ephemeral

wadi s t r e a m d e p o s i t s ( B l i s s e n b a c h , 1954; S t a l d e r , 1 9 7 5 ) .

Here zones of c o n s o l i d -

a t i o n a r e closely associated with successive depositional episodes c l e a r l y indicating t h a t cementation occurred very r a p i d l y a f t e r deposition.

More r a r e l y t h e d e g r e e

o f c a l c i t e c e m e n t a t i o n may c o r r e l a t e w i t h s e d i m e n t a r y s t r u c t u r e s

-

a clear indication

t h a t primary p e r m e a b i l i t y and c a p i l l a r y f o r c e s d e t e r m i n e d by g r a i n s i z e were c o n t r o l l i n g f a c t o r s i n l o c a l i z i n g t h e cement f a b r i c .

The f a c t o r s g o v e r n i n g t h e

p r e c i p i t a t i o n of c a l c i t e from f r e s h w a t e r a r e d i s c u s s e d by MUller e t a l . (19721, B l a t t e t a l . (1972) and Folk and Land (1975).

In a l l u v i a l f a n and wadi c h a n n e l d e p o s i t s c a l c i t e cement t e x t u r e s a t t r i b u t a b l e t o b o t h p h r e a t i c and vadose c e m e n t a t i o n o c c u r ; i n p l a c e s t h e r e may be rhythmic a l t e r n a t i o n s of t h e two.

The most t y p i c a l vadose t e x t u r e i s d r i p s t o n e o r s t a l a c t i t e s

composed of l a m i n a t e d m i c r i t i c s e d i m e n t .

Blocky c a l c i t e cements r e s u l t from p h r e a t i c

cementa t i o n . S t a l d e r (1975) has made a d e t a i l e d s t u d y of t h e c a l c i t e c e m e n t a t i o n o f P l i o c e n e Q u a t e r n a r y f l u v i a t i l e c l a s t i c s i n t h e Oman Mountains. several species:

The c a r b o n a t e s h e r e i n c l u d e

c a l c i t e , high-Mg c a l c i t e and d o l o m i t e which a r e c l o s e l y r e l a t e d

t o t h e a r e a of s u r f a c e r u n - o f f . For example, i n a r e a s w i t h c a r b o n a t e o r mixed

c a r b o n a t e - o p h i o l i t e s o u r c e r o c k s t h e cement i s c a l c i t e .

I n areas with peridolite

s o u r c e r o c k s c a l c i t e , h i g h Mg c a l c i t e and d o l o m i t e o c c u r a s cements.

The o r i g i n

o f Ca2+ was t h e r e f o r e p r i n c i p a l l y from c a r b o n a t e s and from p l a g i o c l a s e s and

pyroxenes p r e s e n t i n l a r g e q u a n t i t i e s i n b a s a l t i c and g a b b r o i c s o u r c e a r e a s .

The

h i g h Mg2+ needed f o r t h e f o r m a t i o n of h i g h Mg c a l c i t e (up t o 30% MgCO,) and dolomite was p r o b a b l y d e r i v e d mainly from o l i v i n e s and pyroxenes i n p e r i d o t i t i c source rocks.

F l u c t u a t i o n s i n t h e s u p p l y of Ca2+ and Mg2+ i o n s r e s u l t e d i n t h e

chemical z o n a t i o n o f cement f a b r i c s which e n a b l e d S t a l d e r (1975) t o conclude t h a t c e m e n t a t i o n r e s u l t e d from s u p e r s a t u r a t i o n by e v a p o r a t i o n . I n a n c i e n t r e d beds c a l c i t e i s g e n e r a l l y c o a r s e l y c r y s t a l l i n e , f i l l i n g b o t h i n t e r g r a n u l a r v o i d s and d i s s o l u t i o n v o i d s .

Completely d i s s o l v e d d e t r i t a l g r a i n s

r e v e a l e d o n l y by t h e i r c l a y o x i d e c o a t i n g s ( g h o s t s ) may be f i l l e d by c a l c i t e cement, a p r o c e s s which p r e v e n t s them from compaction and o b l i t e r a t i o n . common i n a n c i e n t r e d beds ( F i g . 5 . 2 1 ) .

Such g h o s t s a r e

C a l c i t e c e m e n t a t i o n may a l s o be i m p o r t a n t

i n s e a l i n g o f f permeable zones and t h u s p r e v e n t i n g p a r t i a l l y a l t e r e d framework g r a i n s from more e x t e n s i v e d i s s o l u t i o n . Changes i n groundwater c h e m i s t r y may d i s s o l v e e x i s t i n g a u t h i g e n i c c a l c i t e and t h e r e may be complex t e x t u r a l r e l a t i o n s h i p s w i t h c a l c i t e , t h e framework g r a i n s and

315

Fig.5.21. A u t h i g e n i c c a l c i t e i n Cenozoic a l l u v i u m and a n c i e n t r e d beds. A . P a r t i a l l y d i s s o l v e d hornblende g r a i n (H) w i t h o r i g i n a l o u t l i n e marked by p r e s e r v e d c l a y s k i n . D i s s o l u t i o n v o i d i s f i l l e d w i t h c a l c i t e . Thin s e c t i o n . P l i o c e n e ? f a n g l o m e r a t e , Cai'ion Rojo, Baja C a l i f o r n i a . B. A u t h i g e n i c c a l c i t e (Ca) w i t h i n c l u s i o n s o f a u t h i g e n i c p o t a s s i u m f e l d s p a r ( A F ) . SEM photomicrograph, o t h e r d e t a i l s a s i n A. C. Probable completely dissolved s i l i c a t e g r a i n with t h e o r i g i n a l o u t l i n e marked by a p r e s e r v e d c l a y s k i n . D i s s o l u t i o n v o i d i s f i l l e d w i t h c a l c i t e which i s i n o p t i c a l c o n t i n u i t y w i t h t h e i n t e r s t i t i a l c a l c i t e . Thin D. Authigenic c a l c i t e causing s e c t i o n , Upper M o t t l e d Sandstone ( T r i a s s i c ) , U.K. p h y s i c a l d i s p l a c e m e n t of f e l d s p a r fragments by d i s p l a c i v e c r y s t a l l i z a t i o n . Thin s e c t i o n , R i n g e r i k e Group ( S i l u r i a n ) , Norway. A-B a r e produced by c o u r t e s y o f T R Wa l k e r

..

.

316 o t h e r authigenic phases.

I n p a r t i c u l a r , c a l c i t e may c o r r o d e o r p h y s i c a l l y d i s p l a c e

framework g r a i n s such a s t h e more e a s i l y d i s s o l v e d c a l c i c p l a g i o c l a s e s and o f t e n shows c o r r o s i v e r e l a t i o n s h i p s w i t h o t h e r a u t h i g e n i c p h a s e s s u c h a s q u a r t z .

The

f r e q u e n t c o e x i s t e n c e of t h e s e two m i n e r a l s i n r e d bed s a n d s t o n e s i s n o t s u r p r i s i n g because t h e s o l u b i l i t y c u r v e s of q u a r t z and c a l c i t e meet a t a pH o f a b o u t 7.5 which i s commonly found i n n a t u r a l groundwaters. Haematite and p r e c u r s o r o x i d e s F i n e l y d i s p e r s e d a u t h i g e n i c h a e m a t i t e i s t h e p r i n c i p a l c o l o u r i n g a g e n t of cnntinental red beds.

I t s a u t h i g e n i c o r i g i n i s confirmed by t h e e u h e d r a l n a t u r e

of t h e h a e m a t i t e c r y s t a l s and t h e f a c t t h a t t h e y o c c u r a s p o r e l i n i n g s and w i t h i n d i s s o l u t i o n v o i d s , p a r t i c u l a r l y of f e r r o m a g n e s i a n m i n e r a l s . o c c u r a s t h i n , pseudohexagonal p l a t e s a b o u t 1~ i n l e n g t h . on g r a i n s u r f a c e s o r a s s m a l l c l u s t e r s of c r y s t a l s Fig.5.22.

The h a e m a t i t e c r y s t a l s They may o c c u r d i s c r e t e l y Much o f t h e i r o n which

forms a u t h i g e n i c h a e m a t i t e i s r e l e a s e d v i a 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 o f ferromagn e s i a n m i n e r a l s s u c h a s h o r n b l e n d e and p r o b a b l y pyroxene.

The f a t e of t h e i r o n , once

r e l e a s e d from s i l i c a t e l a t t i c e s , depends upon t h e chemical c o n d i t i o n s i n t h e i n t e r s t i t i a l ground w a t e r .

I n Eh-pH c o n d i t i o n s which c o r r e s p o n d t o t h e s t a b i l i t y f i e l d

o f f e r r o u s i r o n then t h e i r o n may be t r a n s p o r t e d i n s o l u t i o n i n t h e i n t e r s t i t i a l groundwater.

P r e c i p i t a t i o n w i l l o c c u r when Eh and pH changes and e n t e r s t h e

s t a b i l i t y f i e l d of h a e m a t i t e .

The i n i t i a l p r e c i p i t a t e i s l i k e l y t o b e amorphous

i r o n hydroxide o r g o e t h i t e (Langmuir, 1 9 7 1 ) , which s u b s e q u n n t l y a g e s i n t o h a e m a t i t e ( B e r n e r , 1969a).

I n s u p p o r t of t h i s is t h e f a c t t h a t i n Cenozoic a l l u v i u m w i t h t h e

youngest d e t e c t a b l e r e d d e n i n g ( L a t e P l e i s t o c e n e ) h a e m a t i t e c a n n o t be d e t e c t e d by SEM o r X-ray d i f f r a c t i o n a n a l y s i s (Walker, 1 9 7 6 ) .

A u t h i g e n i c e u h e d r a l h a e m a t i t e i n abundant i n a n c i e n t r e d beds and has been i d e n t i f i e d by a number of a u t h o r s i n c l u d i n g , F r i e n d ( 1 9 6 6 ) , T u r n e r (1974a1, Wilson ( 1 9 7 1 ) , Walker (1976) and Walker and L a r s o n (1976)

( s e e F i g s . 5 . 2 2 and 5 . 2 3 ) .

I x e r e t a l . (1979) have c o n v i n c i n g l y d e m o n s t r a t e d t h e a u t h i g e n i c n a t u r e of h a e m a t i t e i n t h e T r i a s s i c S t . Bees Sandstone of n o r t h e r n England.

Here e u h e d r a l h a e m a t i t e

c r y s t a l s o c c u r n o t o n l y a s f i n e g r a i n e d pigment b u t a l s o a s overgrowths which o c c u r a s rhombohedral and p r i s m a t i c p r o j e c t i o n s o r a s s y n t a x i a l r i m cements ( F i g . 5 . 2 3 ) . Complete overgrowths of h a e m a t i t e develop i n a s i m i l a r manner t o t h a t d e s c r i b e d by Waugh (1970b, 1978) f o r s i l i c a t e g r a i n s .

I n t h e e a r l y s t a g e s t h e authigenic haematite

c o n s i s t s of f l a t rhombohedral c r y s t a l s r a n g i n g from 2-20 m i n l e n g t h , t h e c r y s t a l s being arranged perpendicular t o the d e t r i t a l g r a i n surface.

The o r i e n t a t i o n o f t h e

a u t h i g e n i c c r y s t a l s i s c o n t r o l l e d by t h e s t r u c t u r e of t h e h o s t g r a i n such t h a t i n monocrystalline d e t r i t a l g r a i n s , a l l the authigenic c r y s t a l s a r e arranged i n p a r a l l e l fashion.

In p o l y c r y s t a l l i n e d e t r i t a l g r a i n s t h e d i f f e r e n t o r i e n t a t i o n s

of t h e d i f f e r e n t s u b c r y s t a l s i s r e f l e c t e d i n t h e o r i e n t a t i o n of t h e a u t h i g e n i c

317

Fig.5.22. A u t h i g e n i c pigmentary h a e m a t i t e i n Cenozoic a l l u v i u m and a n c i e n t c o n t i n e n t a l r e d beds. A. SEM photomicrograph of h a e m a t i t e c r y s t a l s l i n i n g a d i s s o l u t i o n v o i d . Hayner Ranch Formation (Miocene), New Mexico. B. E n l a r g e d view of o u t l i n e d a r e a i n A. C . E n l a r g e d view o f o u t l i n e d a r e a i n B showing c l u s t e r s of e u h e d r a l h a e m a t i t e c r y s t a l s . D . SEM photomicrograph o f a u t h i g e n i c h a e m a t i t e i n t h e Moenkopi Formation ( T r i a s s i c ) , Arizona. E . C h a r a c t e r i s t i c t e x t u r a l a p p e a r a n c e of a u t h i g e n i c h a e m a t i t e pigment c o a t i n g d e t r i t a l q u a r t z g r a i n . Thin s e c t i o n photomicrograph, R i n g e r i k e Group ( S i l u r i a n ) , Norway. F. D e t a i l of e u h e d r a l a u t h i g e n i c h a e m a t i t e ( R i n g e r i k e Group). A-D a r e produced by c o u r t e s y of T.R. Walker.

318

Fig.5.23. A u t h i g e n i c i r o n a n d t i t a n i u m o x i d e s i n t h e S t . Bees Sandstone ( T r i a s s i c ) , Cumbria, U.K. A . D e t r i t a l g r a i n w i t h a u t h i g e n i c overgrowth c o n s i s t i n g of s y n t a x i a l rhombohedra1 c r y s t a l s o f h a e m a t i t e . R e f l e c t e d l i g h t . B. D e t r i t a l g r a i n w i t h s y n t a x i a l overgrowth c o n s i s t i n g o f a s i n g l e rhombohedra1 c r y s t a l . R e f l e c t e d l i g h t . C. D e t r i t a l g r a i n w i t h p a r a l l e l rhombohedra1 c r y s t a l s o f a u t h i g e n i c h a e m a t i t e and p e r f e c t l y e u h e d r a l a n a t a s e ( t o p r i g h t ) . SEM. D. P o l y c r y s t a l l i n e d e t r i t a l h a e m a t i t e w i t h a u t h i g e n i c rhombohedra1 c r y s t a l s o f h a e m a t i t e showing t h r e e d i f f e r e n t , c r y s t a l l o g r a p h i c a l l y c o n t r o l l e d , o r i e n t a t i o n s . E. D e t a i l of a u t h i g e n i c rhombohedra1 h a e m a t i t e . F. D e t a i l o f a u t h i g e n i c a n a t a s e c r y s t a l showing t h e b a s a l p i n a c o i d a n d pyramidal f a c e s .

319 h a e m a t i t e c r y s t a l s (Fig.5.23).

Complete overgrowths o f h a e m a t i t e p r o b a b l y form

by t h e growth and s t a c k i n g t o g e t h e r o f i n d i v i d u a l rhombohedral c r y s t a l s i n a s i m i l a r manner t o t h a t s e e n i n t h e f e l d s p a r s .

I x e r e t a l . (1979) showed t h e t t h e

h a e m a t i t e overgrowths show some i m p o r t a n t c o m p o s i t i o n a l v a r i a t i o n .

I n particular

t i t a n i f e r o u s overgrowths w i t h a b o u t 5% T i 0 2 a r e a common f e a t u r e o f some T r i a s s i c red beds. Titanium o x i d e s a r e a l s o a n i m p o r t a n t a u t h i g e n i c phase i n r e d beds.

These

commonly o c c u r i n a n c i e n t r e d beds a s s m a l l d i s c r e t e c r y s t a l s of a n a t a s e o r r u t i l e up t o 4

0 i n~ s i z e ( F i g . 5 . 2 3 ) and l i n i n g p o r e s p a c e s .

a n a t a s e and r u t i l e a l s o o c c u r ,

S y n t a x i a l overgrowths of

A u t h i g e n i c t i t a n i u m o x i d e p r o b a b l y forms i n t h e

same manner a s h a e m a t i t e a l t h o u g h a r e a d y s u p p l y of t i t a n i u m i o n s from framework s i l i c a t e s i s n o t immediately o b v i o u s .

I x e r e t a l . (1979) s u g g e s t e d t h a t t i t a n i u m

i o n s might be r e l e a s e d i n t o t h e i n t e r s t i t i a l groundwaters d u r i n g t h e m a r t i t i z a t i o n of d e t r it a l titanomagnetite. Quartz Q u a r t z i s a n abundant a u t h i g e n i c phase i n r e d beds a n d i s commonly a s o o c i a t e d with authigenic feldspar.

The p r e c i p i t a t i o n of a u t h i g e n i c q u a r t z a c c o u n t s f o r

most of t h e f r e e s i l i c a which i s r e l e a s e d by s i l i c a t e d i s s o l u t i o n , t h e remainder b e i n g used i n c l a y m i n e r a l s and f e l d s p a r . I n t h e Cenozoic a l l u v i u m s t u d i e d by Walker e t a l . (1978) a u t h i g e n i c overgrowths on d e t r i t a l q u a r t z g r a i n s a r e s c a r c e and most of t h e a u t h i g e n i c q u a r t z o c c u r s i n two d i s t i n c t c r y s t a l forms.

These a r e :

1) t a b u l a r c r y s t a l s t h a t a r e f l a t t e n e d

p a r a l l e l t o t h e b a s a l pinacoi'd and which have u n e q u a l l y developed p r i s m f a c e s t h a t g i v e s them a pseudo-orthorhombic form and 2 ) p r i s m a t i c c r y s t a l s which a r e t e r m i n a t e d w i t h rhombohedral f a c e s .

Walker e t a l . (1978) concluded t h a t t h e two c r y s t a l forms

r e f l e c t e d d i f f e r e n t concentrations of s i l i c a i n the groundwaters, o r o t h e r d i f f e r e n c e s i n t h e i n t e r s t i t i a l geochemical c o n d i t i o n s . D i s c r e t e a u t h i g e n i c c r y s t a l s a l s o o c c u r i n a n c i e n t r e d beds b u t s y n t a x i a l overgrowths a r e more abundant ( F i g . 5 . 2 4 ) .

I n some a e o l i a n s a n d s t o n e s t h e s e a r e s o

abundant a s t o resemble s i l c r e t e d u r i c r u s t s of modern d e s e r t s .

The f o r m a t i o n of

q u a r t z overgrowths of t h i s t y p e has been d e s c r i b e d by Waugh ( 1 9 7 0 a , b ) . STAGES I N THE DIAGENESIS OF CONTINENTAL RED BEDS The s t u d y o f d i a g e n e s i s i n f i r s t c y c l e d e s e r t a l l u v i u m made by Walker and h i s c o l l e g u e s p r o v i d e s a s t r i k i n g . e x a m p l e o f t h e way i n which m i n e r a l systems may respond t o changes i n t h e i r p h y s i c a l and chemical environment.

Many of t h e m i n e r a l s

a f f e c t e d by t h e s e d i a g e n e t i c p r o c e s s e s were formed under c o n d i t i o n s much d i f f e r e n t from t h o s e i n which they a r e now found,

Thus, many o f t h e m i n e r a l s a r e n o t i n

e q u i l i b r i u m w i t h t h e i r environment a n d a r e undergoing a l t e r a t i o n s .

These a l t e r a t i o n s

320

Fig.5.23. SEM photomicrographs o f a u t h i g e n i c q u a r t z i n Cenozoic a l l u v i u m and a n c i e n t r e d beds. A . T a b u l a r c r y s t a l s of a u t h i g e n i c q u a r t z which a r e f l a t t e n e d p a r a l l e l t o b a s a l p i n a c o i d and have u n e q u a l l y developed p r i s m f a c e s . P l i o c e n e f a n g l o m e r a t e , Ca8on R o j o , Baja C a l i f o r n i a . B. P r i s m a t i c c r y s t a l s of q u a r t z t e r m i n a t e d w i t h rhombohedra1 f a c e s . E l R i t o Formation ( E o c e n e ? ) , New Mexico. C . A u t h i g e n i c q u a r t z ( Q ) e n c r u s t i n g a u t h i g e n i c p o t a s s i u m f e l d s p a r (AF). Same d e t a i l s a s i n A. D . E u h e d r a l overgrowth of q u a r t z . S t . Bees Sandstone ( T r i a s s i c ) , U.K. A-C a r e produced by c o u r t e s y of T.R. Walker.

321

Fig.5.25. Schematic r e p r e s e n t a t i o n of t h e t h r e e s t a g e s o f d i a g e n e t i c a l t e r a t i o n 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 ( a f t e r Walker e t a l . , 1 9 7 8 ) . c a n be e x p e c t e d t o c o n t i n u e u n t i l t h e m i n e r a l assemblages i s i n e q u i l i b r i u m w i t h i t s physico-chemical environment.

The e x i s t e n c e o f p a r t i a l l y a l t e r e d m i n e r a l

g r a i n s i n d e p o s i t s which a r e tend of m i l l i o n s of y e a r s o l d i n d i c a t e s t h e e q u i l i b r i a t i o n p r o c e s s t a k e s p l a c e v e r y slowly; i n d e e d t h e r e may be r e p e a t e d e p i s o d e s o f e q u i l i b r i a t i o n a s t h e p r e c i p i t a t i o n o f a u t h i g e n i c p h a s e s changes t h e c o n d i t i o n s i n t h e i n t e r s t i t i a l w a t e r and i n i t i a t e s a new phase of a l t e r a t i o n .

I t seems l i k e l y

t h a t i n t r a s t r a t a l a l t e r a t i o n s a r e most r a p i d d u r i n g t h e e a r l y s t a g e s a f t e r d e p o s i t i o n because t h i s i s t h e p e r i o d when t h e most u n s t a b l e d e t r i t a l g r a i n s a r e most abundant. The i n t e r s t i t i a l environment may a l s o be a f f e c t e d by e x t e r n a l c a u s e s s u c h a s c l i m a t e and t e c t o n i s m .

C l i m a t e may a f f e c t t h e amount and t e m p e r a t u r e o f c i r c u l a t i n g

ground w a t e r and a l s o t h e amount of o r g a n i c m a t t e r i n t h e d e p o s i t e d s e d i m e n t , and t h e s e w i l l i n t u r n a f f e c t t h e r a t e of i n t r a s t r a t a l a l t e r a t i o n , the concentration of d i s s o l v e d i o n s and t h e Eh-pH c o n d i t i o n s .

Tectonism may a f f e c t t h e r a t e of

groundwater c i r c u l a t i o n and a l s o t h e i n t e r s t i t i a l t e m p e r a t u r e and p r e s s u r e ; u p l i f t o f t h e s o u r c e r e g i o n s t e n d s t o i n c r e a s e t h e c i r c u l a t i o n r a t e and d e c r e a s e t h e t e m p e r a t u r e and p r e s s u r e ; b u r i a l on t h e o t h e r hand t e n d s t o d e c r e a s e t h e c i r c u l a t i o n r a t e and i n c r e a s e t h e t e m p e r a t u r e and p r e s s u r e .

Hence c l i m a t i c o r t e c t o n i c changes

322 a r e l i k e l y t o produce a n immediate r e s p o n s e o f t h e m i n e r a l assemblage t o t h e changes i n t h e i n t e r s t i t i a l environment. Walker e t a l . (1978) have p r e s e n t e d a s c h e m a t i c r e p r e s e n t a t i o n of t h e d i a g e n e s i s changes which have been observed o r i n f e r r e d i n t h e i r s t u d i e s o f Cenozoic s a n d s t o n e s and conglomerates i n t h e s o u t h w e s t USA and n o r t h w e s t e r n Mexico (Fig.5.25). stages a r e recognized:

Stage 1

-

mechanical i n f i l t r a t i o n of c l a y , S t a g e I1

Three

-

s i g n i f i c a n t a l t e r a t i o n of p y r o x e n e s , amphiboles and c a l c i c p l a g i o c l a s e , S t a g e 111 complete l o s s of p y r o x e n e s , amphiboles and most c a l c i c p l a g i o c l a s e .

-

K - f e l d s p a r and

q u a r t z may show e v i d e n c e of a l t e r a t i o n , newly formed c l a y m i n e r a l s . Many a n c i e n t c o n t i n e n t a l r e d beds show e v i d e n c e of S t a g e I11 a l t e r a t i o n s and can be i n f e r r e d t o have r e s u l t e d from a c o n t i n u a t i o n o f t h o s e p r o c e s s e s c o n s i d e r e d t o be o p e r a t i v e 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 .

T h i s e v i d e n c e may be summarized a s

follows :

1) a b s e n c e of f e r r o m a g n e s i a n s i l i c a t e s such a s h o r n b l e n d e and pyroxene i n r e d beds d e r i v e d from s o u r c e r o c k s i n which t h e y were p l e n t i f u l

2 ) s a n d - s i z e d c o n c e n t r a t i o n s of c l a y squeezed between r e s i s t a n t framework g r a i n s . I f t h e c l a y i s h a e m a t i t e - r i c h i t may r e p r e s e n t a c l a y pseudomorph a f t e r a f e r r o -

magnesian s i l i c a t e

3) absence of s i l t grade d e t r i t u s . framework g r a i n s w i t h a c l a y m a t r i x .

Many a n c i e n t r e d beds c o n s i s t of s a n d - s i z e d Such t e x t u r e s a r e n o t produced d e p o s i t i . o n a l l y

and a r e b e s t e x p l a i n e d i f t h e c l a y i s t h e p r o d u c t of i n f i l t r a t i o n a n d / o r a u t h i g e n e s i s

4 ) d i s s o l u t i o n and c l a y r e p l a c e m e n t of f e l d s p a r s i n d i c a t e s t h a t i n t r a s t r a t a l s o l u t i o n must have p r o g r e s s e d t o t h e p o i n t when r e l a t i v e l y s t a b l e g r a i n s a r e b e i n g a t t a c k e d ; t h u s a n i n d i c a t i o n t h a t l e s s s t a b l e g r a i n s might have been c o m p l e t e l y d i s s o l v e d .

5 ) e u h e d r a l a u t h i g e n i c h a e m a t i t e i s common i n a n c i e n t r e d beds.

It i s frequently

c o a r s e enough t o be o b s e r v e d i n t h i n s e c t i o n and w i t h She SEM i s s e e n t o form p l a t y hexagonal c r y s t a l s and r o s e t t e s of hexagonal c r y s t a l s

6 ) a c h a r a c t e r i s t i c a u t h i g e n i c s u i t e o f q u a r t z , K - f e l d s p a r , c a l c i t e and c l a y m i n e r a l s i s p a r t i c u l a r l y common i n a n c i e n t r e d beds and t e s t i f i e s t h e former e x i s t e n c e o f i n t e r s t i t i a l w a t e r r i c h i n d i s s o l v e d i o n s which were r e l e a s e d by i n t r a s t r a t a l dissolution.

323

CHAPTER 6 THE MINERALOGY AND GEOCHEMISTRY OF IRON OXIDES I N RED BEDS I R O N OXIDE. MINERALS

Abundant i r o n - t i t a n i u m o x i d e m i n e r a l s a r e one o f 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 s of c o n t i n e n t a l r e d beds.

They a r e n o t o n l y r e s p o n s i b l e f o r t h e

c o l o u r a t i o n o f t h e s e r o c k s b u t a l s o r e p r e s e n t t h e end p r o d u c t s of t h e i r d i a g e n e s i s and a r e r e s p o n s i b l e f o r t h e rock magnetism and hence p a l a e o m a g n e t i c p r o p e r t i e s of red beds,

I r o n - t i t a n i u m o x i d e s which o c c u r i n r e d beds show a wide range of

t e x t u r a l and c o m p o s i t i o n a l v a r i a t i o n b u t can c o n v e n i e n t l y b e c o n s i d e r e d i n two g r o u p s : f i n e p a r t i c l e pigmentary o x i d e s , p r i n c i p a l l y c o n s i s t i n g o f h a e m a t i t e b u t a l s o c o n t a i n i n g o t h e r f e r r i c oxyhydroxides, and c o a r s e p a r t i c l e opaque o x i d e s which contain a v a r i e t y of iron- titanium oxide minerals.

The m i n e r a l o g - i c a l p r o p e r t i e s

o f t h o s e i r o n - t i t a n i u m o x i d e s which commonly o c c u r i n r e d beds a r e l i s t e d i n T a b l e 6.1. The i r o n - t i t a n i u m o x i d e s can b e r e p r e s e n t e d by t h e FeO-Ti02-Fe 0 t e r n a r y 2 3 The t h r e e end p o i n t s o f t h e diagram a r e r e p r e s e n t e d by diagram ( F i g . 6 . 1 ) . h a e m a t i t e and maghemite (Fe 0 ) , r u t i l e ( T i 0 2 ) and w U s t i t e (FeO), W i t h i n t h e 2 3 t e r n a r y diagram t h r e e i m p o r t a n t s o l i d s o l u t i o n s e r i e s a r e r e p r e s e n t e d : 1) t h e t i t a n o m a g n e t i t e s v a r y i n g between m a g n e t i t e (Fe 0 ) and ulvklspinel (Fe T i 0 ) which 3 4 2 4 2 ) t h e h a e m a t i t e - i l m e n i t e s e r i e s v a r y i n g between

have t h e s p i n e l s t r u c t u r e

h a e m a t i t e (Fe 0 ) and i l m e n i t e (FeTiO ) which have t h e rhombohedra1 s t r u c t u r e and 3 2 3 3) t h e p s e u d o b r o o k i t e s e r i e s (Fe T i 0 ) w i t h orthorhombic s t r u c t u r e . 2 5 The titanomannetites-titanomazhemites W i t h i n t h e FeO-Ti0 -Fe 0 t e r n a r y diagram t h e most i m p o r t a n t group o f m i n e r a l s 2 2 3 a r e t h o s e w i t h t h e s p i n e l s t r u c t u r e which l i e i n t h e Fe304-Fe2Ti0 Fe2Ti0 -Fe 0 45 2 3 quadrilateral. T h i s s y s t e m c o n t a i n s t h e t i t a n o m a g n e t i t e s and t h e i r o x i d i z e d e q u i v a l e n t s t h e titanomaghemites ( F i g . 6 . 2 ) .

The l e f t - h a n d s i d e of t h e q u a d r i l a t e r a l

r e p r e s e n t s t h e t i t a n o m a g n e t i t e s and t h e b a s e r e p r e s e n t s t h e s o l i d s o l u t i o n between m a g n e t i t e and maghemite (l/-Fe 0 ). The c o o r d i n a t e s ( x , z ) a r e used t o s p e c i f y 2 3 p o i n t s w i t h i n t h e q u a d r i l a t e r a l where p o i n t s on t h e l e f t - h a n d s i d e c o r r e s p o n d t o z = 0

( O < x < l ) and t h o s e on t h e b a s e t o x = 0 ( O L z < l ) .

The p a r a m e t e r z i s

t h e r e f o r e a n o x i d a t i o n p a r a m e t e r and when z = 1 a l l t h e i r o n i s p r e s e n t a s Fe

3+

T h i s i s t h e c a s e w i t h maghemite which may be r e p r e s e n t e d by t h e formula: Fe3+ a13 0-: ( x = 0 , z = I ) , where r e p r e s e n t s a v a c a n t l a t t i c e s i t e : Maghemite may

a

.

D113

t h e r e f o r e b e d e s c r i b e d a s a c a t i o n d e f i c i e n t s p i n e l and i s one of a whole s e r i e s of i r o n - t i t a n i u m c a t i o n d e f i c i e n t s p i n e l s which a r e r e f e r r e d t o a s t h e

324

Ruth

TAN- BROWN

*BLUE

Hagnotito

Haomatito Maghaomit.

Fig.6.1. The FeO-Ti02-Fe203 t e r n a r y system, showing t h e p r i n c i p a l s o l i d s o l u t i o n series. The dashed l i n e s are some l i n e s of c o n s t a n t Fe:Ti r a t i o a l o n g which o x i d a t i o n may proceed. I s o t r o p i c , a n i s o t r o p i c , and t h e c o l o u r v a r i a t i o n s are t h e o p t i c a l p r o p e r t i e s as viewed i n r e f l e c t e d l i g h t oil-immersion. titanomaghemites.

Those s p i n e l s w i t h a c a t i o n f a n i o n r a t i o o f 314, i.e. when

z = 0 a r e referred to a s the titanomagnetites.

I n n a t u r e s u c h compounds a r e

probably r a t h e r s c a r c e and i t i s common p r a c t i c e t o use t h e term t i t a n o m a g n e t i t e f o r b o t h t h e t r u e t i t a n o m a g n e t i t e s , and a l s o f o r t h e i r o x i d i z e d e q u i v a l e n t s , t h e titanmaghemites.

To overcome t h e c o n f u s i o n O ' R e i l l y (1976) s u g g e s t s r e s t r i c t i n g

t h e term t i t a n o m a g n e t i t e t o t h o s e s p i n e l s w i t h a n o x i d a t i o n parameter 240.1. I r o n - t i t a n i u m s p i n e l s w i t h z h 0 . 1 s h o u l d b e r e f e r r e d t o a s titanomaghemite.

A

g e n e r a l formula f o r a l l t h e i r o n - t i t a n i u m s p i n e l s may be w r i t t e n : 3+ 2+ 02Fe 2-2x + z ( I + x ) R Fe ( I + x ) ( l - z ) R 3(1-R) 4 where R = 8 1 8 + z ( l + x ) . The c r y s t a l s t r u c t u r e of t h e s e s p i n e l s c o n s i s t s of a c l o s e packed a n i o n l a t t i c e w i t h c a t i o n s having f o u r f o l d c o o r d i n a t i o n ( t e t r a h e d r a l s i t e s ) and s i x f o l d coordination (octahedral s i t e s ) .

Each a n i o n has t h r e e o c t a h e d r a l s i t e c a t i o n s

and one t e t r a h e d r a l s i t e c a t i o n a s n e a r e s t neighbours. The c a t i o n d i s t r i b u t i o n f o r m a g n e t i t e i s u s u a l l y w r i t t e n Fe 3+LFe3+, Fez+]

O4

where c a t i o n s o u t s i d e t h e b r a c k e t a r e i n t e t r a h e d r a l s i t e s and t h o s e i n s i d e a r e i n octahedral s i t e s .

I n a c t u a l f a c t t h e c a t i o n d i s t r i b u t i o n i s more c o r r e c t l y

325

113 Fo304 magn6titi

haomatito maghomito

The Fe 0 -Fe T i 0 -Fe T i 0 -Fe 0 q u a d r i l a t e r a l r e p r e s e n t i n g t h e Fig.6.2. c o m p o s i t i o n a l zone f o r s i n g l e phase compounds w i t h s p i n e l s t r u c t u r e below a b o u t 400OC. The h a t c h e d r e g i o n r e p r e s e n t s t h e same zone a t a b o u t 1 2 O O O C ( a f t e r O ' R e i l l y , 1976).

I

w r i t t e n Fe3+

04 b e c a u s e above t h e Verwey t r a n s i t i o n (420°K)

e l e c t r o n hopping between c a t i o n s on o c t a h e d r a l s i t e s r e s u l t s i n i n t e r m e d i a t e oxidation state.

Below t h e Verwey t r a n s i t i o n t h e s t r u c t u r e becomes orthorhombic

and t h e r e a r e f o u r o c t a h e d r a l s p e c i e s s i n c e two o c t a h e d r a l s i t e s now o c c u r .

The

c a t i o n d i s t r i b u t i o n i n i n t e r m e d i a t e members of t h e t i t a n o m a g n e t i t e s e r i e s i s o n l y p o o r l y known and v a r i o u s models have been proposed (Akimoto, 1954; Ngel, 1955; B a n e r j e e a n d O ' R e i l l y , 1966).

B a n e r j e e e t a l . (1967) concluded t h a t t e t r a h e d r a l

s i t e s c o n t a i n b o t h Fe2+ and Fe3+ i o n s t h r o u g h o u t most of t h e series.

A useful

r e v i e w of t h e s u b j e c t i s g i v e n by L i n d s l e y (1976). I n t h e m a g n e t i t e - u l v b s p i n e l series t h e r e i s complete s o l i d s o l u t i o n a t t e m p e r a t u r e s above 6OO0C ( F i g . 6 . 4 ) .

During c o o l i n g t h e s o l i d s o l u t i o n i s much

more r e s t r i c t e d and t h e r e i s a tendency f o r e x s o l u t i o n o f t h e two p h a s e s t o occur.

True e x s o l u t i o n t e x t u r e s a r e , however, r e l a t i v e l y s c a r c e i n m a g n e t i t e -

ulvijspinel s o l i d s o l u t i o n s .

The commonly o b s e r v e d i n t e r g r o w t h s of i l m e n i t e i n

t i t a n o m a g n e t i t e a r e now c o n s i d e r e d t o form d i r e c t l y as a r e s u l t of o x i d a t i o n a t low t o moderate p r e s s u r e s and t e m p e r a t u r e s above 60OoC.

These i l m e n i t e i n t e r g r o w t h s

i n t i t a n o m a g n e t i t e may b e d i v i d e d i n t o t h e f o l l o w i n g t e x t u r a l forms a c c o r d i n g t o t h e Buddington and L i n d s l e y (1964) c l a s s i f i c a t i o n :

326 a)

trellis types

b)

composite ( g r a n u l a r ) t y p e s

c)

sandwich t y p e s

The t r e l l i s t y p e s a r e t h e most common and a r e c l e a r l y o x i d a t i o n p r o d u c t s o f magnetite-ulv8spinel.

The composite and sandwich t y p e s a r e l e s s common and

p r o b a b l y c o n t a i n i l m e n i t e s which formed a s p r i m a r y p r e c i p i t a t e s .

A well-illustrated

d e s c r i p t i o n of t h e v a r i o u s t e x t u r e s i s g i v e n by Haggerty ( 1 9 7 6 a ) .

Fig.6.3. P a r t o f t h e s p i n e l u n i t c e l l . Large open c i r c l e s r e p r e s e n t a n i o n s , s m a l l open c i r c l e s , o c t a h e d r a l s i t e c a t i o n s , and s m a l l h a t c h e d c i r c l e s , t e t r a h e d r a l s i t e c a t i o n s . The broken l i n e shows t h e t r i g o n a l a x i s f o r t h e c e n t r a l o c t a h e d r a l site. A d i f f e r e n t o x i d a t i o n mechanism which o p e r a t e s d u r i n g d e u t e r i c c o o l i n g r e s u l t s i n

f o r m a t i o n of c a t i o n d e f i c i e n t s p i n e l s ( t i t a n o m a g h e m i t e s ) . a t t e m p e r a t u r e s i n t h e r a n g e 400-600°C

This takes p l a c e i n i t i a l l y

and i s a common f e a t u r e of submarine b a s a l t s .

This t y p e o f o x i d a t i o n ( m a g h e m i t i z a t i o n ) a l s o c o n t i n u e s d u r i n g submarine w e a t h e r i n g . This i s s u g g e s t e d by p i l l o w l a v a s which show r a d i a l v a r i a t i o n s i n t h e d e g r e e of m a g h e m i t i z a t i o n , t h e s u r f a c e s b e i n g most h i g h l y o x i d i z e d ( R y a l l and Ade-Hall,

1975).

The h a e m a t i t e - i l m e n i t e s e r i e s Haematite (er-Fe203) and i l m e n i t e (FeTiOg) a r e rhombohedra1 o x i d e s which form a s o l i d s o l u t i o n s e r i e s t h a t i s complete a t h i g h t e m p e r a t u r e s ( c e r t a i n l y above 96OoC and p r o b a b l y above 80OoC) b u t i n t e r r u p t e d by a m i s c i b i l i t y gap a t lower t e m p e r a t u r e s ( L i n d s l e y , 1973).

Haematite has t h e corundum s t r u c t u r e which c o n s i s t s o f a

321

t

-v 1200

Fig.6.4. 1974).

Phase diagram f o r t h e b i n a r y t i t a n o m a g n e t i t e system ( a f t e r Hauptman,

hexagonal-close

packed a n i o n l a t t i c e w i t h c a t i o n s f i t t e d i n t o t h e i n t e r s t i c e s ,

t h e rhombohedra1 u n i t c e l l c o n t a i n i n g f o u r c a t i o n s and s i x a n i o n s .

All c a t i o n

s i t e s a r e e q u i v a l e n t w i t h d i s t o r t e d o c t a h e d r a l symmetry which a r i s e s b e c a u s e of e l e c t r o s t a t i c r e p u l s i o n between n e a r e s t - n e i g h b o u r c a t i o n s i n a d j a c e n t b a s a l planes (Fig.6.5). The c r y s t a l s t r u c t u r e of i l m e n i t e i s v e r y s i m i l a r t o t h a t o f h a e m a t i t e b u t t h e morphology of n a t u r a l i l m e n i t e c r y s t a l s has shown t h a t t h e s p a c e group must have a lower symmetry and R? ( a subgroup of R ~ c )was chosen f o r i l m e n i t e o n - t h i s basis,

Compared t o t h e h a e m a t i t e s t r u c t u r e , one h a l f of t h e i r o n i o n s a r e

r e p l a c e d by t i t a n i u m i n s u c h a way t h a t e v e r y Fe-03-Fe u n i t , and t h e sequence of c a t i o n s a l o n g any @ O O g

u n i t becomes a Fe-03-Ti

axis is

.. ., . Fe-Ti-LI) -Ti-Fe-n-Fe

where t h e hyphens r e p r e s e n t i n t e r v e n i n g p l a n e s o f oxygen l a y e r s .

As i n h a e m a t i t e ,

e v e r y t h i r d p o t e n t i a l o c t a h e d r a l s i t e i s empty; t h e n e a r e s t c a t i o n s t o a n empty

s i t e along arranged i n

0001 0001

b e i n g two i r o n o r two t i t a n i u m .

The Fe-0 - T i u n i t s a r e so 3 t h a t oxygen l a y e r s a l t e r n a t e w i t h m e t a l l a y e r s , which themselves

a l t e r n a t e between l a y e r s of i r o n and l a y e r s of t i t a n i u m . Haematite and i l m e n i t e form a complete s o l i d s o l u t i o n s e r i e s a t h i g h t e m p e r a t u r e s . 2+ The c o m p o s i t i o n may be e x p r e s s e d Fe3+ Fe T i 03, where x i s t h e mole f r a c t i o n of 2-2x x i l m e n i t e . I n t e r m e d i a t e s o l i d s o l u t i o n s c o n t a i n Fe2+ and Fe3+ i n amounts which a r e a p p r o x i m a t e l y p r o p o r t i o n a l t o x and (2-2x) r e s p e c t i v e l y , so i t i s p r o b a b l e t h a t 4+ titanium i s present only a s T i Solvus r e l a t i o n s h i p s o f t h e h a e m a t i t e - i l m e n i t e

.

s e r i e s were e s t a b l i s h e d by Carmichael (1961) on n a t u r a l m a t e r i a l , and a r e v i s e d , e x p e r i m e n t a l s o l v u s was p r e s e n t e d by L i n d s l e y (1973).

The c o n s o l u t e t e m p e r a t u r e

328

Fig.6.5. The corundum s t r u c t u r e . The c a t i o n s ( s m a l l h a t c h e d c i r c l e s ) l i e i n t h e i n t e r s t i c e s between t h e close-packed a n i o n l a t t i c e . Broken l i n e s i n d i c a t e t h e t r i g o n a l a x i s . The d i s t o r t i o n o f oxygen t r i a n g l e s and d i s p l a c e m e n t o f c a t i o n s from t h e median p l a n e a r e a l s o i n d i c a t e d . ( c r i t i c a l p o i n t f o r t h e s o l v u s ) may b e lower t h a n 800°C and t h e m i s c i b i l i t y gap, a l t h o u g h n o t y e t d e t e r m i n e d p r e c i s e l y , o c c u p i e s a narrower f i e l d t h a n t h a t s u g g e s t e d by Carmichael (1961) ( F i g . 6 . 6 ) . E x s o l u t i o n of h a e m a t i t e from i l m e n i t e and v i c e v e r s a i s r e s t r i c t e d t o deeps e a t e d i n t r u s i o n s and i s p a r t i c u l a r l y c h a r a c t e r i s t i c of a n o r t h o s i t e s a n d o t h e r b a s i c s u i t e s , but a l s o occurs i n g r a n i t i c s u i t e s .

The p l a n e o f e x s o l u t i o n i s

p a r a l l e l t o t h e (0001) rhombohedra1 d i r e c t i o n , t h e e x s o l u t i o n t a k i n g p l a c e i n a c c o r d w i t h decomposition a s a consequence o f slow c o o l i n g and s o l v i - i n t e r s e c t i o n . S i n g l e - p h a s e compounds o f i n t e r m e d i a t e composition a r e r a t h e r r a r e i n n a t u r e b u t o c c u r i n some quench-cooled e x t r u s i v e i g n e o u s r o c k s (Haggerty, 1976b). The terminology of Buddington e t a l . (1963) i s c o n v e n i e n t t o u s e f o r i n t e r mediate members o f t h e h e a m a t i t e - i l m e n i t e series: F e r r i a n i l m e n i t e i s a one-phase g r a i n o f i l m e n i t e c o n t a i n i n g v a r y i n g amounts of Fe 0 i n s o l i d s o l u t i o n . 2 3 T i t a n h a e r n a t i t e i s a one-phase g r a i n of h a e m a t i t e c o n t a i n i n g v a r y i n g q u a n t i t i e s of FeTiOg i n s o l i d s o l u t i o n . Haemo-ilmenite i s a two-phase g r a i n c o n s i s t i n g o f t i t a n h a e m a t i t e l a m e l l a e e n c l o s e d i n a ferrianilmenite host. Ilmeno-haematite i s a two-phase g r a i n c o n s i s t i n g o f f e r r i a n i l m e n i t e l a m e l l a e enclosed i n a titanhaematite host. T i t a n h a e m a t i t e s a r e o f p a r t i c u l a r i n t e r e s t b e c a u s e o f t h e i r remarkable m a g n e t i c p r o p e r t i e s which i n c l u d e s e l f - r e v e r s a l (Wescott-Lewis and P a r r y , 1971 a , b ) .

329 Hoffman (1975) proposed a mechanism by which s e l f - r e v e r s a l m i g h t o c c u r .

I

I

8001

700

600 00

60

40

Fe Ti O3 Ilmenite

Mole Percent Fe Ti

O3

1

20

Fe203 Hciematite

Fig.6.6. The j o i n Fe203-FeTiO showing t h e h a e m a t i t e - i l m e n i t e m i s c i b i l i t y gap of Carmichael (1961) (hoop-shaped c u r v e ) and L i n d s l e y ’ s (1973) e s t i m a t e of t h e r e g i o n ( s t i p p l e d a r e a ) w i t h i n which t h e m i s c i b i l i t y gap must l i e . P means t h a t p s e u d o b r o o k i t e s o l i d s o l u t i o n s , R t h a t r u t i l e , was p r e s e n t i n t h e r i m p r o d u c t s ( a f t e r L i n d s l e y , 1973). F e r r i c Oxvhvdroxides The n a t u r a l l y o c c u r r i n g forms o f f e r r i c oxyhydroxide a r e l i s t e d i n T a b l e 6.1. T h e i r o c c u r r e n c e , e x c e p t f o r a k a g a n g i t e , have been d e s c r i b e d by P a l a c h e e t a l . (1944). AkaganLite i s a r a r e m i n e r a l , o c c u r r e n c e s h a v i n g been d e s c r i b e d by Van T a s s e l (1959) and Chandy (1962).

I t i s noteworthy b e c a u s e i t o c c u r s i n m e t e o r i t e s and l u n a r rocks

a s a n o x i d a t i o n p r o d u c t o f l a w r e n c i t e (FeC1 ) and n a t i v e n i c k e l - i r o n ( T a y l o r e t a l . , 2 1974). Amorphous f e r r i c oxyhydroxide i s u s u a l l y g i v e n t h e formula Fe(OH)3 f o r convenience.

P o o r l y c r y s t a l l i n e , c o l l o i d a l m a t e r i a l , w i t h a composition n e a r

2.5 FegO3 4.5 H 2 0 , and a p p a r e n t l y w i t h a d i s t o r t e d h a e m a t i t e - t y p e s t r u c t u r e , i s

g i v e n t h e name f e r r i h y d r i t e (Towe and B r a d l e y , 1967).

I t may b e a s s o c i a t e d w i t h

b a c t e r i a l o x i d a t i o n o f f e r r o u s i r o n s o l u t i o n s (Chukrov, 1973).

Amorphous f e r r i c

oxyhydroxide and g o e t h i t e a r e t h e main c o n s t i t u e n t s of l i m o n i t e which i s t h e name

330 TABLE 6.1. N a t u r a l l y o c c u r r i n g forms of f e r r i c oxyhydroxide F e r r i c oxyhydroxide

I d e a l Formula

Amorphous

i n d e f i n i t e Fe (OH)

Goethite

FeOOH

Akaganeite

FeOOH

Lepidocrocite

FeOOH

Haematite

Fe203 Fe 0 2 3

Maghemi t e

g i v e n t o t h e yellow t o brown c o l o u r e d f e r r i c oxyhydroxide complexes which form a s p r e c i p i t a t e s i n n a t u r a l w a t e r s and a s a n o x i d a t i o n p r o d u c t of i r o n - b e a r i n g minerals.

On d e h y d r a t i o n , o r a g e i n g , g o e t h i t e u s u a l l y i n v e r t s t o h a e m a t i t e .

Haematite i s t h e most s t a b l e f e r r i c o x i d e and i s t h e p r i n c i p a l c o l o u r i n g a g e n t o f r e d beds.

Maghemite o c c u r s mainly a s a n o x i d a t i o n p r o d u c t o f m a g n e t i t e , and

more r a r e l y by t h e d e h y d r a t i o n o f l e p i d o c r o c i t e .

T h i s l a t t e r f e r r i c oxyhydroxide

o c c u r s a s a p r e c i p i t a t e i n n a t u r a l w a t e r s , and i s found r a r e l y i n c o n t i n e n t a l r e d beds.

OPAQUE OXIDES IN RED BEDS

U n f o r t u n a t e l y 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 opaque o x i d e s i n e i t h e r Recent s e d i m e n t s o r a n c i e n t r e d beds and we owe much of o u r p r e s e n t knowledge t o t h e c o n t r i b u t i o n by Van Houten ( 1 9 6 8 ) .

O t h e r o b s e r v a t i o n s have

been made by Chamalaun ( 1 9 6 3 ) , P i c a r d ( 1 9 6 5 ) , and Turner and I x e r (1977). A c c u r a t e c h a r a c t e r i z a t i o n and u n d e r s t a n d i n g o f t h e opaque o x i d e s i n r e d beds i s e s s e n t i a l f o r a p r o p e r u n d e r s t a n d i n g of t h e i r 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

history.

Opaque o x i d e s can p r o v i d e u s e f u l i n f o r m a t i o n a b o u t t h e provenance of

c o n t i n e n t a l a l l u v i u m and t h e y a r e a l s o r e s p o n s i b l e f o r t h e m a g n e t i c p r o p e r t i e s of most c o n t i n e n t a l r e d b e d s .

Consequently t h e o c c u r r e n c e o f t h e v a r i o u s o x i d e s

i n igneous r o c k s , and t h e i r d e r i v e d e q u i v a l e n t s i n c o n t i n e n t a l r e d b e d s , a r e described systematically.

In g e n e r a l , t h e c o m p o s i t i o n s a n d modal abundances o f

p r i m a r y opaque o x i d e s i n igneous r o c k s depend upon t h e i n i t i a l b u l k c h e m i s t r y o f t h e h o s t r o c k , on t h e d e p t h of emplacement, and on t h e oxygen f u g a c i t y o f t h e c r y s t a l l i z i n g magma.

S i n c e FeO, T i 0 2 and Cr203 d e c r e a s e w i t h S i 0 2 , b a s i c r o c k s

t e n d t o c o n t a i n l a r g e r c o n c e n t r a t i o n s of o x i d e s t h a n t h e i r i n t e r m e d i a t e s u i t e s

o r a c i d end members.

The m i n e r a l o g i c a l p r o p e r t i e s of t h e commonly o c c u r r i n g

i r o n - t i t a n i u m o x i d e s a r e l i s t e d i n T a b l e 6.2. Titanomagnetite T i t a n o m a g n e t i t e o c c u r s commonly i n i n t e r m e d i a t e and b a s i c i g n e o u s r o c k s where

331 TABLE 6.2. Mineralogical p r o p e r t i e s of iron-titanium oxides i n polished s e c t i o n . Name, chemical formula and symmetry

Optical properties

Reflectance i n % (wavelength = 589nm)

Microindentat i o n hardness

Magnetite -304 Cubic

Grey, c o m o n l y w i t h brownish t i n t . Much d a r k e r t h a n haematite. Isotropic

20.3 ( A i r ) 8.00 ( O i l )

592 ( l o a d of 100)

Ulvbspinel Fe2TiOq Cubic

Brown-reddish brown I s o t r o p i c . Usually found a s f i n e e x s o l ution bodies i n ti tanomagneti t e

16.35 ( A i r )

No d a t a

Maghaemi t e y-,Fe203 Cubic

B l u i s h - g r e y . Darker than haematite. Isotropic

22.8 ( A i r ) 9.00 ( O i l )

412 ( l o a d of 5 0 )

Ha ema t i te oc-Fe203 Hexagona 1

Grey-white w i t h a b l u i s h t i n t . Anisot r o p i c : greyish-b lue g r e y i s h yellow. Deep red internal reflections

29.15-25.1 ( A i r ) 14.4-11.0 ( O i l )

1038 (mean of 3 r a n g e s 100, 200, 300 )

Ilmenite FeTi03 Trigonal

Brown sometimes w i t h a pink o r v i o l e t t i n t . Anisotropic: l i g h t greenish grey, brownish g r e y

20.2-17.4 ( A i r ) 7.6-5.8 ( O i l

519-533 ( l o a d o f 100) 659-703

Goe t h i t e

Dull grey t o b r i g h t grey. Anisotropic: g r e y - b l u e , greyy e l l o w , brown, greenish- grey. Brownish-yellow internal reflections

16.1-15.0 5.05-4.0

(Air) (Oil)

Greyish-whi te. Anisotropic: shades o f g r e y . Reddish internal reflections

17.4-11.1 5.55-2.0

(Air) (Oil)

u - FeOOH

Orthorhombic

Lepidocrocite Y-FeOOH Orthorhombic

402 (mean of 3 r a n g e s 100, 200, 300)

i t o f t e n comprises up t o f i v e p e r c e n t by volume.

Increased s u b s t i t u t i o n of

t i t a n i u m i s c h a r a c t e r i s t i c of b a s i c igneous r o c k s and h e r e t i t a n o m a g n e t i t e s 3+ 2+ (Fe2-2x Fel+x T i 4 + 02-)w i t h x a p p r o x i m a t e l y i n t h e range 0.54xCO.8 a r e t y p i c a l . x 4 Titanium poor m a g n e t i t e s a r e u s u a l i n a c i d igneous r o c k s . O t h e r c a t i o n s a r e u s u a l l y p r e s e n t i n t i t a n o m a g n e t i t e s , i n c l u d i n g manganese, n i c k e l , z i n c , and chromium ( C r e e r and I b b e t s o n , 1970).

O r i e n t e d i n c l u s i o n s and o x i d a t i o n l a m e l l a e

p a r a l l e l t o (111) a r e common and i n c l u d e h a e m a t i t e , i l m e n i t e and r u t i l e .

The

o c c u r r e n c e o f t i t a n o m a g n e t i t e s i n igneous rocks i s d e s c r i b e d i n g r e a t d e t a i l by Haggerty (1976b).

Other u s e f u l references i n c l u d e

V i n c e n t and P h i l l i p s ( 1 9 5 4 ) ,

Katsura and Kushiro ( 1 9 6 1 ) , Buddington and L i n d s l e y ( 1 9 6 4 ) , and B u t l e r (1973). T i t a n o m a g n e t i t e i s r e a d i l y o x i d i z e d i n t h e s e d i m e n t a r y environment and u n a l t e r e d g r a i n s a r e s c a r c e i n Recent sediments and s o i l s . i n alluvium.

F r e s h g r a i n s most commonly o c c u r

Van Andel (1959, p. 155), Walker (1967a? p. 365) and Van Houten

(1968, Table 1 and 2 ) a r e amongst t h e few a u t h o r s who r e p o r t t h e o c c u r r e n c e o f r e l a t i v e l y u n a l t e r e d t i t a n o m a g n e t i t e g r a i n s ; some of t h e s e are e u h e d r a l and are less a l t e r e d t h a n t i t a n o m a g n e t i t e g r a i n s i n s o i l s and s a p r o p e l s , a f e a t u r e

i n d i c a t i n g d i r e c t c o n t r i b u t i o n from l i t t l e weathered bedrock.

Unfortunately,

d e s p i t e v a s t numbers o f heavy m i n e r a l a n a l y s e s o f Recent a l l u v i u m , t h e r e i s v i r t u a l l y no i n f o r m a t i o n on t h e d e t a i l e d mineralogy o f i r o n o x i d e s ; most a u t h o r s have been c o n t e n t t o group them a l l a s "opaques" d e s p i t e t h e f a c t t h a t t h e y o f t e n form t h e m a j o r i t y o f t h e heavy m i n e r a l f r a c t i o n .

Most of t h e i n f o r m a t i o n r e g a r d i n g

t i t a n o m a g n e t i t e comes from d e t r i t a l c o n c e n t r a t e s i n beach sands.

Wright (1964) h a s

d e s c r i b e d t h e o x i d a t i o n of t i t a n o m a g n e t i t e d e r i v e d from v o l c a n i c rocks i n New Zealand beach sands.

These show p a r t i a l t o complete a l t e r a t i o n t o homogeneous

t i t a n h a e m a t i t e o r t o t i t a n h a e m a t i t e and pseudobrookite.

Some o f t h e h a e m a t i t i z a t i o n

was c e r t a i n l y i n i t i a t e d i n t h e p a r e n t r o c k , b u t p o s t - d e p o s i t i o n a l a l t e r a t i o n a l s o takes p l a c e (Wright, 1964, p. 431).

Rapid o x i d a t i o n o f m a g n e t i t e and t i t a n o -

m a g n e t i t e t a k e s p l a c e i n t h e weathered zone o f igneous r o c k s and, i n r e s i d u a l p l a c e r s o f o x i d e m i n e r a l s o v e r 50 p e r c e n t h a e m a t i t i z a t i o n i s n o t uncomon (e.g. T s u s u e and I s h i h a r a ,

1975).

I n l a t e r i t i c s o i l s t i t a n o m a g n e t i t e s may b e weathered

t o s t r u c t u r e l e s s p e l l e t s o f brown and r e d o x i d e ( B r i g g s , 1960) and a l l t h e g r a i n s u s u a l l y show some e v i d e n c e o f h a e m a t i t i z a t i o n .

T h i s u s u a l l y t a k e s t h e form o f

r e d f i n e g r a i n e d rims o f h a e m a t i t e , o r t h i n h a e m a t i t e l a m e l l a e n e a r t h e g r a i n margin (Van Houten, 1968, p. 401). (Katsura e t a l . ,

S o i l s developed on b a s i c v o l c a n i c r o c k s

1962; C a r r o l l and Hathaway, 1963; Matsusaka e t a l . , 1965) c o n t a i n

much t i t a n o m a g n e t i t e which shows p a r t i a l t o complete o x i d a t i o n t o titanomaghernite.

In a n c i e n t r e d beds u n a l t e r e d g r a i n s o f m a g n e t i t e a r e v i r t u a l l y unknown.

Those

m a g n e t i t e s which do o c c u r form r e l i c t c o r e s w i t h i n h a e m a t i t e s and show e x t e n s i v e signs of oxidation.

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 r a p i d h a e m a t i t i z a t i o n

( m a r t i t i z a t i o n ) o f m a g n e t i t e t a k e s p l a c e i n t h e d i a g e n e t i c environment.

This i s

333 s e e n i n t h e p r o g r e s s i v e i n c r e a s e o f m a r t i t e r e l a t i v e t o m a g n e t i t e i n more a n c i e n t r e d beds. Haema t i t e - I l m e n i t e The o c c u r r e n c e o f members o f t h e h a e m a t i t e - i l m e n i t e series i n i g n e o u s r o c k s i s d e s c r i b e d by Haggerty (1976b), and i n metamorphic r o c k s by Rumble (1976).

Members

o f t h e h a e m a t i t e - i l m e n i t e series form a minor c o n s t i t u e n t o f most i g n e o u s r o c k s ; a c i d s u i t e s , g r a n i t i c p e g m a t i t e s and c a r b o n a t i t e s t e n d t o b e r i c h e r i n p y r o p h a n i t e (MnTiO ) w h i l s t h a e m a t i t e - r i c h s o l i d s o l u t i o n s a r e r e s t r i c t e d t o a c i d i n t r u s i v e 3 s u i t e s and a n o r t h o s i t e s , where h a e m a t i t e s o l i d s o l u t i o n s a r e e x s o l v e d , and t h e s e coexist with ilmenite-rich solutions.

I n metamorphic r o c k s h a e m a t i t e - i l m e n i t e

s o l i d s o l u t i o n s a r e found i n a l l metamorphic zones i n a wide v a r i e t y of rock t y p e s . E x s o l u t i o n l a m e l l a e of i l m e n i t e i n h a e m a t i t e , o r h a e m a t i t e i n i l m e n i t e i s u s u a l l y observed.

A s m a l l amount o f p y r o p h a n i t e i s u s u a l l y p r e s e n t and i n magnesian-rich

r o c k s a n a p p r e c i a b l e c o n t e n t o f g e i k i e l i t e (MgTiO

i s n o t e d ( S p r i n g e r , 1974). 3 I n t h e s e d i m e n t a r y environment h a e m a t i t e is t h e most i m p o r t a n t i r o n oxide.

Coarse p a r t i c l e s o c c u r i n i n t i m a t e a d m i x t u r e w i t h o t h e r f i n e g r a i n e d o x i d e s i n many k i n d s o f soil, i n c l u d i n g l a t e r i t e s .

The v a s t m a j o r i t y of h a e m a t i t e i n

w e a t h e r e d p r o f i l e s a p p e a r s t o b e formed by t h e o x i d a t i o n of t i t a n o m a g n e t i t e . Under w e a t h e r i n g c o n d i t i o n s members o f t h e h a e m a t i t e - i l m e n i t e s e r i e s a r e more s t a b l e t h a n m a g n e t i t e (Mohr and Van Baren, 1954; Hartman, 1959; Katsura e t a l . , 1962).

A f t e r p r o l o n g e d o x i d a t i o n i l m e n i t e i t s e l f may b e o x i d i z e d ; i l m e n i t e

e x s o l u t i o n l a m e l l a e i n h a e m a t i t e may b e c o n v e r t e d t o h a e m a t i t e p l u s r u t i l e and i l m e n i t e i t s e l f may b e a l t e r e d t o form ' l e u c o x e n e ' .

This a p p e a r s milky w h i t e i n

r e f l e c t e d l i g h t and normally c o n s i s t s o f f i n e l y d i v i d e d r u t i l e , o r more r a r e l y brookite.

B a i l e y e t a l . (1956) r e c o g n i z e d t h r e e s t a g e s o f i l m e n i t e o x i d a t i o n i n

beach s a n d s : p a t c h y i l m e n i t e

amorphous i r o n - t i t a n i u m o x i d e

r u t i l e , a n a t a s e , and sphene ( l e u c o x e n e ) .

finely crystalline

I n a n c i e n t r e d beds i l m e n i t e i s s c a r c e

and u n o x i d i z e d g r a i n s a r e l a r g e l y c o n f i n e d t o t h e d r a b zones i n r e d bed sequences ( M i l l e r and Folk, 1955; Van Houten, 1968). I n r e c e n t and a n c i e n t sediments t h e r e i s l i t t l e e v i d e n c e t h a t p r i m a r y c o a r s e p a r t i c l e haematites a r e volumetrically important.

I n f a c t haematites of t h i s type

may b e d i f f i c u l t t o r e c o g n i z e u n l e s s t h e y show obvious rhombohedra1 e x s o l u t i o n textures.

The h a e m a t i t e s w i t h " o l e a n d e r - l e a f " shaped e x s o l u t i o n l a m e l l a e of

i l m e n i t e d e s c r i b e d by P h i l l i p s ( i n Van Houten, 1964, p.659) from t h e T o r r i d o n i a n Sandstone are p r o b a b l y primary g r a i n s d e r i v e d from i g n e o u s o r metamorphic r o c k s . , I n a n c i e n t r e d beds t h e opaque o x i d e g r a i n s are u s u a l l y r e f e r r e d t o a s specularite.

They show a wide range o f t e x t u r e s , b u t t h e v a s t m a j o r i t y show

e v i d e n c e o f having formed a f t e r t i t a n o m a g n e t i t e ( s e e F i g . 6 . 7 ) .

Such g r a i n s

a r e r e f e r r e d t o a s m a r t i t e 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 from m a g n e t i t e i s

334

Fig.6.7. R e f l e c t e d l i g h t photomicrographs of t y p i c a l c o a r s e - p a r t i c l e i r o n o x i d e s i n c o n t i n e n t a l r e d beds. ' A . O c t a h e d r a l pseudomorph o f h a e m a t i t e a f t e r m a g n e t i t e . B . Haematite s h o i i n g m a r t i t i z a t i o n t e x t u r e ( b o t h from S t . Bees S a n d s t o n e , T r i a s s i c , U.K.). C . M a r t i t e ( R i n g e r i k e Group, S i l u r i a n , Norway). D . Haematite w i t h e x s o l u t i o n d i s c s o f i l m e n i t e and s y n t a x i a l a u t h i g e n i c h a e m a t i t e ( S t . Bees S a n d s t o n e ) E . Rhombohedra1 d e t r i t a l h a e m a t i t e . F . Haematite w i t h rhombohedra1 e x s o l u t i o n l a m e l l a e of i l m e n i t e ( b o t h from R i n g e r i k e Group). A l l g r a i n s a r e 1 0 0 - 2 0 0 ~i n diameter.

335 r e f e r r e d t o a s m a r t i t i z a t i o n (Ramdohr, 1969, p. 9 0 6 ) .

In p a r t i a l l y r e p l a c e d

m a g n e t i t e s , s u c h a s o c c u r i n Recent s e d i m e n t s and Cenozoic r e d b e d s , t h i n p l a t e s o f h a e m a t i t e a r e formed a l o n g t h e o c t a h e d r a l (111) p l a n e s of t h e m a g n e t i t e , resulting i n a triangular alteration pattern.

More r a r e l y t h e m a r t i t i z a t i o n may

develop p a r a l l e l t o growth zones w i t h i n t h e m a g n e t i t e c r y s t a l .

In more a n c i e n t

r e d beds i t i s more u s u a l t o f i n d t h a t o r i g i n a l m a g n e t i t e g r a i n s have been c o m p l e t e l y r e p l a c e d by h a e m a t i t e which s t i l l r e v e a l s t h e t r i a n g u l a r m a r t i t i z a t i o n texture i n polycrystalline grains.

F r e q u e n t l y such g r a i n s show e x s o l u t i o n zones Trellis

of i l m e n i t e which may b e a l t e r e d t o h a e m a t i t e w i t h r u t i l e o r a n a t a s e .

t e x t u r e i n p o l y c r y s t a l l i n e h a e m a t i t e g r a i n s a l s o i n d i c a t e s f o r m a t i o n of h a e m a t i t e a f t e r magnetite (Fig.6.7). Although m a r t i t i z a t i o n may o c c u r o v e r a wide t e m p e r a t u r e range i t i s c l e a r t h a t i t must b e mainly o f low-temperature o r i g i n i n a n c i e n t r e d beds.

However

t h e a c t u a l mechanism of m a r t i t i z a t i o n i n a n c i e n t r e d beds has n o t been s t u d i e d i n any d e t a i l and i t i s n o t c l e a r w h e t h e r h a e m a t i t e fX-Fe203)

forms d i r e c t l y

from m a g n e t i t e (Fe 0 ) o r whether a n i n t e r m e d i a t e p h a s e of maghemite (Y-Fe2O3) 3 4 o r a compound i n t e r m e d i a t e i n c o m p o s i t i o n between Fe304 and Fe 0 i s i n v o l v e d .

2 3

Colombo e t a l . (1964) m a i n t a i n e d t h a t below 6OO0C m a g n e t i t e n e v e r o x i d i z e d t o k - F e 2 0 3 u n l e s s c r - F e 2 0 3 n u c l e i were p r e s e n t b u t G a l l a g h e r e t a l . (1968) have c l e a r l y d e m o n s t r a t e d t h e d i r e c t o x i d a t i o n o f Fe3Opw-Fe2O3 in p a r t i c l e s l a r g e r t h a n 300011 and t e m p e r a t u r e s a s low a s 18OoC.

Under l a b o r a t o r y c o n d i t i o n s t h e

t r a n s i t i o n from Fe30C t o Y-Fe203 h a s been r e s t r i c t e d t o f i n e l y d i v i d e d , submicron s i z e d m a t e r i a l (Readman and O ' R e i l l y , 1975).

1972; Johnson and M e r r i l l , 1973; O'Donovan,

P a r t i c l e s below a b o u t 30002 i n d i a m e t e r always y i e l d )'-Fe203

whilst

p a r t i c l e s above t h i s s i z e a r e o x i d i z e d d i r e c t l y t o o+Fe203 v i a compounds i n t e r m e d i a t e i n composition between Fe304 and Fe203 ( G a l l a g h e r e t a l . , 1968). I m p u r i t i e s , e s p e c i a l l y aluminium and a l s o a d s o r b e d w a t e r , may be i m p o r t a n t i n s t a b i l i z i n g t h e s p i n e l s t r u c t u r e d u r i n g t h e Fe304 Y-Fe203 t r a n s i t i o n i n s m a l l p a r t i c l e s ( s e e B a t e , 1975, f o r a r e v i e w ) . The e x p e r i m e n t a l 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 view t h a t m a r t i t i z a t i o n most commonly p r o c e e d s w i t h o u t t h e f o r m a t i o n of a n i n t e r m e d i a t e Y-Fe 0 phase. 2 3 Van Houten (1968) has made t h e o n l y d e t a i l e d s u r v e y of opaque o x i d e s i n Recent a l l u v i u m and a n c i e n t r e d beds ( F i g . 6 . 8 ) .

The s e d i m e n t s s t u d i e d ranged from

P r e c a m b r i a n - P l e i s t o c e n e i n a g e and i n c l u d e d r e d beds and a s s o c i a t e d drab zones. F o r d e t a i l e d r e s u l t s of i n d i v i d u a l f o r m a t i o n s t h e r e a d e r i s r e f e r r e d t o Van Houten (1968, T a b l e s 4 , 5 and 6 ) .

When s t u d i e d a s a whole t h e r e s u l t s

r e v e a l two main t r e n d s ( F i g . 6 . 9 ) which have l a r g e l y been s u b s t a n t i a t e d by l a t e r workers and which may be summarized a s f o l l o w s :

1) Red beds c o n t a i n abundant h a e m a t i t i z e d b l a c k o x i d e g r a i n s and show a p r o g r e s s i v e i n c r e a s e i n h a e m a t i t e / m a g n e t i t e and h a e m a t i t e / i l m e n i t e r a t i o s i n s u c c e s s i v e l y o l d e r r e d beds.

T h i s d e p l e t i o n t r e n d c o u l d p a r t l y be due t o t h e f a c t t h a t many of t h e

336

Sequ*nr.r M

Sands

I

Fig.6.8. R e l a t i v e abundance o f h a e m a t i t e (H), m a g n e t i t e (M) and i l m e n i t e (I). A . Upland s o i l s B. Recent s e d i m e n t s , c o a s t a l Sonora, Mexico C . Cenozoic r e d beds D. Mesozoic r e d beds E . P a l a e o z o i c r e d beds F. Precambrian r e d beds G . Ancient d r a b sequences H. Black sands i n beach p l a c e r s ( a f t e r Van Houten, 1968).

337 more r e c e n t samples came from v o l c a n i c - r i c h f o r m a t i o n s (Van Houten, 1968, p. 403) b u t a number o f o t h e r f a c t o r s s u g g e s t t h a t t h e t r e n d is real.

These i n c l u d e t h e

f a c t t h a t m a g n e t i t e i s a l s o p r e s e n t i n Recent sediments and some Mesozoic r e d beds, w h i l s t i t i s v i r t u a l l y a b s e n t i n Precambrian and P a l a e o z o i c r e d beds.

The u b i q u i t y

o f m a r t i t e s i n a n c i e n t r e d beds a l s o s u g g e s t s t h a t m a g n e t i t e has been c o n v e r t e d t o h a e m a t i t e i n s i t u t h u s c o n t r i b u t i n g t o t h e observed d e p l e t i o n t r e n d . 2 ) Drab mudstone and s a n d s t o n e s i n r e d bed sequences g e n e r a l l y c o n t a i n v e r y few b l a c k o x i d e s and many o f t h e s e a r e i l m e n i t e o r leucoxene.

When compared w i t h t h e i r

a s s o c i a t e d r e d beds t h e s e d r a b zones n o t o n l y show fewer t o t a l g r a i n s b u t a g r e a t e r p r o p o r t i o n o f i l m e n i t e and T i 0 2 , f e a t u r e s which s u g g e s t t h a t p o s t - d e p o s i t i o n a l d i s s o l u t i o n o f b o t h pigmentary and c o a r s e - p a r t i c l e o x i d e s h a s t a k e n p l a c e ( M i l l e r and F o l k , 1955; Van Houten, 1968, p. 404), i n d i c a t i n g t h e g r e a t e r r e s i s t e n c e of i l m e n i t e and t i t a n i u m o x i d e s t o t h e r e d u c i n g s o l u t i o n s which remove i r o n o x i d e s from r e d beds.

MAGNETITE

HEMATITE

lLMENlTE

SILICATES

-*b
IN ALLUVIUM TRANSIT

YOUNQER

OLMR

Fig.6.9. O r i g i n and f a t e o f opaque o x i d e s and pigment i n r e d bed sequences. C o n f i g u r a t i o n s s u g g e s t changes i n abundance o f e a c h component r e l a t i v e to amount i n s o u r c e r o c k s ( a f t e r Van Houten, 1968).

338 These d a t a r e v e a l a n i m p o r t a n t p r o c e s s which i s i n s t r u m e n t a l i n p r o d u c i n g t h e c h a r a c t e r i s t i c o x i d e m i n e r a l o g y o f r e d beds.

This process i s t h e i n s i t u

c o n v e r s i o n of m a g n e t i t e t o h a e m a t i t e ; i t o p e r a t e d under o x i d i z i n g i n t r a s t r a t a l c o n d i t i o n s and c o n t i n u e d f o r a s long a s such c o n d i t i o n s c o u l d b e m a i n t a i n e d , perhaps even f o r m i l l i o n s of y e a r s . PRECIPITATION AND STABILITY OF FERRIC OXYHYDROXIDES The p r e c i p i t a t i o n o f f e r r i c oxyhydroxides i s t h e s i n g l e most i m p o r t a n t p r o c e s s responsible f o r t h e colouration of continental red beds.

T h i s may t a k e p l a c e a t

v a r i o u s s t a g e s i n t h e d e p o s i t i o n and d i a g e n e s i s o f r e d beds b u t two of t h e more i m p o r t a n t ones a r e t h e p r e c i p i t a t i o n o f f e r r i c oxyhydroxides from i n t e r s t i t i a l w a t e r s d u r i n g d i a g e n e s i s and l a t e r p r e c i p i t a t i o n from c i r c u l a t i n g ground w a t e r s . The f i r s t - f o r m e d p h a s e s a r e u s u a l l y m e t a s t a b l e compounds, such a s amorphous Fe(OH)3 o r g o e t h i t e (FeOOH); t h e s e do n o t n o r m a l l y p e r s i s t i n t h e g e o l o g i c a l environment b u t become d e h y d r a t e d t o form t h e s t a b l e i r o n o x i d e , h a e m a t i t e . The commonest oxyhydroxides found a s p r e c i p i t a t e s i n n a t u r a l w a t e r s a r e g o e t h i t e , amorphous Fe(0H)

and more r a r e l y , l e p i d o c r o c i t e . These oxyhydroxide p r e c i p i t a t e s 3 a r e u s u a l l y m i x t u r e s whose a p p a r e n t thermodynamic s t a b i l i t y o r s o l u b i l i t y depends l a r g e l y on t h e s o l u b i l i t y o f t h e l e a s t s t a b l e p h a s e p r e s e n t ( u s u a l l y t h e amorphous material).

N a t u r a l oxyhydroxide p r e c i p i t a t e s c o n t a i n s u b s t a n t i a l amounts o f

c o l l o i d a l m a t e r i a l and p a r t i c l e s i z e s r a n g i n g from m o l e c u l a r Fe(OHl3 t o microns i z e d c r y s t a l s o r c r y s t a l a g g r e g a t e s i s p o s s i b l e (Langmuir, 1969).

Particle size

i s p r o b a b l y t h e most i m p o r t a n t c o n t r o l on t h e thermodynamic s t a b i l i t y of t h e

oxyhydroxides; o t h e r s b e i n g m i n e r a l o g y , c r y s t a l l i n i t y and t h e p r e s e n c e of impurities. S o l u t i o n r e a c t i o n s f o r a l l t h e oxyhydroxides can b e r e p r e s e n t e d b y * Fe(OH)3 = Fe3+

+ 30H-

(6.1)

OtFeOOH + H20 = Fe3+ + 30H112 6X-Fe203)

(6 * 2)

+ 3/2H20 = Fe3+ + 30H-

(6 * 3 )

The n e g a t i v e l o g a r i t h m o f t h e a c t i v i t y p r o d u c t f o r e a c h of t h e s e r e a c t i o n s , assuming p u r e w a t e r and s o l i d s , mav b e w r i t t e n :

1'

PK = - 1 0 g [ ~ e 3 + 1 [ ~ ~ -

when t h e a c t i v i t y of w a t e r i s n e a r u n i t y .

(6.4) T h i s pK i s , i n e f f e c t , a s o l u b i l i t y

p r o d u c t and v a l u e s r a n g e from a b o u t 37.1 f o r f r e s h , r a p i d l y p r e c i p i t a t e d amorphous m a t e r i a l , t o p r o b a b l y 44 o r more f o r c r y s t a l l i n e g o e t h i t e and h a e m a t i t e (Langmuir, 1969, 1971).

Langmuir and Whittemore (1971) have shown t h a t c o - e x i s t i n g oxyhyd-

r o x i d e s can r e v e r s e t h e i r r e l a t i v e s t a b i l i t i e s b e c a u s e of p a r t i c l e s i z e e f f e c t s . For example, f o r e q u a l s i z e d c r y s t a l s of g o e t h i t e and h a e m a t i t e , t h e former i s more s t a b l e above 7601 b u t becomes less s t a b l e below t h i s p a r t i c l e s i z e . The b e h a v i o u r o f p r e c i p i t a t e d f e r r i c oxyhydroxides i n s o l u t i o n can most

339 c o n v e n i e n t l y b e d e s c r i b e d i n terms of Eh and pH (Hem and Cropper, 1959; G a r r e l s , 1960).

F i g . 6 . 1 0 shows a n Eh-pH diagram w i t h t h e s t a b i l i t y f i e l d s o f s o l i d s and

predominant i o n i c s p e c i e s i n t h e s y s t e m F e - H ~ o - 0 ~ -C02.

Ion-ion b o u n d a r i e s a r e drawn

a t e q u a l i o n a c t i v i t i e s and i o n - s o l i d b o u n d a r i e s a r e drawn f o r 10-3M

dissolved

i r o n s p e c i e s which i s a t y p i c a l c o n c e n t r a t i o n found i n i r o n - r i c h groundwaters (Langmuir and Whittemore, 1971; Whittemore and Langmuir, 1975).

The f e r r i c - f e r r o u s -

f e r r i c oxyhydroxide b o u n d a r i e s are drawn f o r pK v a l u e s of 37.1 and 44.2.

The

d i f f e r e n c e means t h a t f o r a g i v e n Eh and pH a w a t e r can c o n t a i n a b o u t seven o r d e r s of magnitude more d i s s o l v e d i r o n i n e q u i l i b r i u m w i t h f r e s h amorphous m a t e r i a l t h a n i t can i n e q u i l i b r i u m w i t h w e l l c r y s t a l l i z e d g o e t h i t e o r h a e m a t i t e .

I n general

t h e Eh and pH of i r o n - r i c h n a t u r a l w a t e r s p l a c e s them on t h e f e r r i c o r f e r r o u s species

-

f e r r i c oxyhydroxide boundary.

The groundwater i n n a t u r a l a q u i f e r s , most

commonly has a pH between 5 and 8 , and Eh between 0 . 3 and -0.1 v o l t s .

Under t h e s e

c o n d i t i o n s i r o n i s p r e s e n t a s Fe2+ i o n s and s o l i d f e r r i c oxyhydroxide p h a s e s . Acid w a t e r s s u c h a s t h o s e from mine d i s c h a r g e s u s u a l l y have a pH between 2 and 5 and Eh from w i t h i n t h e Fe3+ f i e l d down t o 0 v o l t s . Eh v a l u e s measured i n t h e f i e l d a r e o f t e n between t h o s e p r e d i c t e d from t h e s o l u b i l i t y of Fe(OH)3 and t h e s o l u b i l i t y of h a e m a t i t e (Back and Barnes, 1965; Barnes and Back, 1964).

These a u t h o r s i n t e r p r e t e d t h i s as a n i n d i c a t i o n t h a t

f e r r i c oxyhydroxides s p e c i e s e x i s t e d i n v a r y i n g d e g r e e s of d e h y d r a t i o n and t h a t low Eh c o n d i t i o n s l e a d t o s e l e c t i v e l e a c h i n g o f more s o l u b l e f e r r i c oxyhydroxide p h a s e s l e a v i n g h a e m a t i t e a s t h e l a s t f e r r i c phase t o d i s s o l v e (Hem and Cropper, 1959; G a r r e l s and C h r i s t , 1965; Back and B a r n e s , 1965). The d e h y d r a t i o n s t a t e o f f e r r i c oxyhydroxides has a r o u s e d c o n s i d e r a b l e i n t e r e s t i n t h e c o n t e x t of r e d bed f o r m a t i o n .

I n Recent temperate-humid c l i m a t e s o i l s

g o e t h i t e r a t h e r than h a e m a t i t e i s formed (Schwertmann, 1965).

Under c e r t a i n

c o n d i t i o n s yaghemite may a l s o form ( T a y l o r and Schwertmann, 1974; M u l l i n s , 1977); i t may b e t h a t t h e f o r m a t i o n of h a e m a t i t e is i n h i b i t e d by t h e p r e s e n c e of f i n e

g r a i n e d o r g a n i c matter a s s u g g e s t e d by Schwertmann (1965). I n Recent sediments t h e r e i s abundant yellow o r brown f e r r i c hydroxides. a n c i e n t r e d beds t h e s e are s c a r c e and are r e p l a c e d by abundant h a e m a t i t e .

In This

h a s l e d a number of a u t h o r s t o a r g u e t h a t d e t r i t a l f e r r i c hydroxides a g e t o h a e m a t i t e d u r i n g d i a g e n e s i s by some d e h y d r a t i o n r e a c t i o n (Van Houten, 1964; 1972; Schmalz, 1968; T u r n e r , 1974a).

The p r e c i s e m i n e r a l o g y of i r o n h y d r o x i d e s i n

Recent f i n e - g r a i n e d a l l u v i u m i s v e r y p o o r l y known.

Some a u t h o r s have assumed

t h a t much of i t i s amorphous ( B l a t t e t a l . , 1 9 7 2 ) ; o t h e r s assume t h a t i t c o n s i s t s p r i n c i p a l l y of g o e t h i t e ( B e r n e r , 1971).

Undoubtedly much of t h e f e r r i c hydroxide

i n Recent s e d i m e n t s i s c l o s e l y a s s o c i a t e d w i t h c l a y m i n e r a l s ( C a r r o l l , 1958; S o i l e u and McCracken, 1967; Deshpande e t a l . ,

1968; Walker and Honea, 1969) and

t h e e v i d e n c e does s u g g e s t t h a t p o o r l y c r y s t a l l i n e p h a s e s a r e more abundant than

340

Fig.6.10. Eh-pH r e l a t i o n s h i p s between s o l i d s and predominant i o n i c s p e c i e s i n t h e The p o s i t i o n of t h e F e z + - f e r r i c oxyhydroxide boundary i s system Fe-H20-02-C02. shown f o r pK's o f 37.1 and 4 4 . 2 . T o t a l d i s s o l v e d c a r b o n a t e s p e c i e s e q u a l 10-3M. S o l i d s p e c i e s i n p a r e n t h e s e s are dashed l i n e f i e l d s o n l y ( a f t e r Whittemore and Langmuir, 1975). t r u l y amorphous m a t e r i a l ( F o l l e t t , 1965).

Amorphous m a t e r i a l may be a d s o r b e d on

t h e s u r f a c e s o r edges of c l a y m i n e r a l s o r i n o r g a n i c complexes.

E l e c t r o n micro-

scope s t u d i e s (Greenland e t a l . , 1968) a l s o r e v e a l t h a t c l a y p l a t e l e t s m y be covered by minute p a r t i c l e s o f f r e e f e r r i c o x i d e s which may r e p r e s e n t t h e p o o r l y c r y s t a l l i n e phases many o f which a p p e a r t o b e h y d r a t e d f e r r i c o x i d e r e l a t e d t o h a e m a t i t e (Towe and B r a d l e y , 1967).

The most abundant c r y s t a l l i n e phase i n t h e

ferric hydroxide o f f i n e g r a i n e d a l l u v i u m i s p r o b a b l y g o e t h i t e .

The d e h y d r a t i o n o f f e r r i c oxyhydroxides Yellow o r brown i r o n h y d r o x i d e s i n t h e c l a y f r a c t i o n s of a l l u v i u m are b e l i e v e d

341 t o be a b l e t o c o n v e r t i n t o r e d pigmentary h a e m a t i t e by d e h y d r a t i o n d u r i n g d i a g e n e s i s and t h u s produce r e d beds ( F r i e n d , 1966; Van Houten, 1968).

Since t h i s i s a

d e h y d r a t i o n r e a c t i o n many a u t h o r s have m a i n t a i n e d t h a t a n a r i d o r a t l e a s t s e a s o n a l l y dry climate i s necessary f o r i t t o proceed, thus f o s t e r i n g t h e b e l i e f t h a t ancient r e d beds a r e a p a l a e o c l i m a t i c i n d i c a t o r .

F o r example, Schmalz (1968) a r g u e d t h a t

h a e m a t i t e might form from amorphous i r o n hydroxide o r g o e t h i t e by one of t h e following reactions: 2Fe(OH)3 --+Fe203 + 3H20

(6.5)

2Fe (OH) -+2HFe02 + 2H20 +Fez03 2HFe02-+Fe203

+ 3H20

(6.6)

+ H20

(6.7)

Using a v a i l a b l e d a t a Schmalz (1968) q u o t e d a Gibbs f r e e e n e r g y (aGo) of between 4 . 2 and +2.2 k c a l / m o l e f o r r e a c t i o n ( 6 . 7 ) and concluded t h a t h a e m a t i t e could n o t

form s t a b l y i n t h e p r e s e n c e o f w a t e r . E a r l y phase e q u i l i b r i u m s t u d i e s of t h e d e h y d r a t i o n o f g o e t h i t e ( P o s n j a k and Merwin, 1919; Smith and Kidd, 1949; Schmalz, 1959) s u g g e s t e d t h a t t h e d e h y d r a t i o n t e m p e r a t u r e was a b o u t 130OC.

The l o w e s t t e m p e r a t u r e a t which t h e r e a c t i o n has been

o b s e r v e d e x p e r i m e n t a l l y was r e c o r d e d by Tunnel1 and Posnjak (1931) who c o n v e r t e d n a t u r a l g o e t h i t e t o h a e m a t i t e i n a few weeks a t 100°C i n 0. I N H C l .

However, i n one

of t h e s e c a s e s was t h e e q u i l i b r i u m d e l i n e a t e d by r e v e r s i n g t h e r e a c t i o n : h a e m a t i t e could n o t be hydrated.

Moreover t h e method u s e d : t h e a g e i n g o f f r e s h l y p r e c i p i t a t e d

f e r r i c h y d r a t e g e l s a t v a r i o u s t e m p e r a t u r e s and w a t e r p r e s s u r e s does n o t t a k e due a c c o u n t of f a c t o r s s u c h as pH, which could i n f l u e n c e t h e k i n e t i c s o f t h e r e a c t i o n , n o r t h e a b i l i t y of f o r e i g n i o n s t o c a t a l y s e t h e c r y s t a l l i z a t i o n o f h a e m a t i t e o r g o e t h i t e ( T a y l o r and G r a l e y , 1967; Schwertmann, 1970).

I n f a c t Smith and Kidd

(1949) showed t h a t t h e d e h y d r a t i o n t e m p e r a t u r e was markedly dependent upon pH. Berner (1969) Kas shown t h a t t h i s e q u i l i b r i u m d e h y d r a t i o n t e m p e r a t u r e o f 13OoC i s much t o o h i g h and t h a t f i n e g r a i n e d g o e t h i t e i s u n s t a b l e r e l a t i v e t o h a e m a t i t e

under v i r t u a l l y a l l g e o l o g i c a l c o n d i t i o n s .

H i s method was t o d e t e r m i n e AGO by

measuring t h e r e l a t i v e s o l u b i l i t i e s of g o e t h i t e and h a e m a t i t e i n H C 1 s o l u t i o n s . The r e s u l t s c l e a r l y i n d i c a t e d t h a t f i n e g r a i n e d g o e t h i t e i s more s o l u b l e t h a n h a e m a t i t e a t 85OC ( F i g . 6 . 1 1 ) .

Calculations using d i f f e r e n t i a l s o l u b i l i t y enabled

B e r n e r (1969) t o show t h a t t h e AGO f o r r e a c t i o n ( 6 . 7 ) a t 25OC i s -0.66 k c a l / m o l e and a t O°C i s -0.40 k c a l / m o l e .

He f u r t h e r c a l c u l a t e d t h a t t h e maximum t e m p e r a t u r e

a t which f i n e g r a i n e d g o e t h i t e i s s t a b l e r e l a t i v e t o h a e m a t i t e p l u s p o r e w a t e r i s

a b o u t 4OoC.

Assuming t h a t t h e s e d a t a a r e c o r r e c t , i t i s c l e a r t h a t f i n e g r a i n e d

g o e t h i t e i s u n s t a b l e r e l a t i v e t o h a e m a t i t e under v i r t u a l l y a l l g e o l o g i c a l c o n d i t i o n s . The d e h y d r a t i o n o f f i n e g r a i n e d g o e t h i t e i s t h e r e f o r e l i k e l y t o t a k e p l a c e f a i r l y r a p i d l y , e s p e c i a l l y d u r i n g s e d i m e n t b u r i a l and c o u l d a l s o t a k e p l a c e i n t h e p r e s e n c e of water.

I t i s n o t p o s s i b l e t h e r e f o r e t o draw any c o n c l u s i o n s r e g a r d i n g climate

o r p a l a e o c l i m a t e from t h e d e h y d r a t i o n r e a c t i o n .

342

A Goethita 0 Haematite

8.0

3.0 2.0 1.0

0 0

2

4

6

8

10

12

14

16

18

20

Time Days

F i g . 6 . 1 1 . D i s s o l u t i o n and p r e c i p i t a t i o n c u r v e s f o r g o e t h i t e and h a e m a t i t e i n 0 . 1 2 M H C l a t 85 2 l 0 C . Data a r e p l o t t e d o n l y f o r r u n s where d i s s o l u t i o n was n o t e s s e n t i a l l y complete ( a f t e r B e r n e r , 1969a). The e f f e c t of p a r t i c l e s i z e e f f e c t on t h e d e h y d r a t i o n r e a c t i o n has been s t u d i e d by Langmuir (1971) and Langmuir and Whittemore (1971).

Langmuir's (1971) thermo-

chemical r e s u l t s a r e i n g e n e r a l agreement w i t h t h e s o l u b i l i t y r e s u l t s o f B e r n e r (1969).

F i g . 6 . 1 2 shows t h e p a r t i c l e s i z e e f f e c t on t h e Gibbs f r e e e n e r g y of t h e

goethite dehydration r e a c t i o n a t 25OC.

Langmuir (1971) showed t h a t c o a r s e - g r a i n e d

g o e t h i t e ( > p m ) i s s t a b l e r e l a t i v e t o c o a r s e g r a i n e d h a e m a t i t e i n w a t e r up t o 8OoC a t the E a r t h ' s surface, but t h a t f i n e grained goethite i s unstable r e l a t i v e t o coarse grained haematite.

Langmuir (1971) a l s o showed t h a t f o r k i n e t i c r e a s o n s ,

h a e m a t i t e , once formed, w i l l n o t r e h y d r a t e t o form g o e t h i t e .

This i s c o n s i s t e n t

w i t h t h e f a c t t h a t g o e t h i t e pseudomorphs a f t e r h a e m a t i t e a r e n o t known t o o c c u r (Palache e t a l . , 1944).

The measurement of p a r t i c l e s i z e s i n t h e r e g i o n 100-10008

i s e x t r e m e l y d i f f i c u l t and c a r e must b e t a k e n i n e x t r a p o l a t i n g t h e d a t a t o geological conditions.

Langmuir and Whittemore (1971) found i n t h e l a b o r a t o r y

t h a t f r e s h l y p r e c i p i t a t e d f e r r i c oxyhydroxides c o n s i s t e d of amorphous m a t e r i a l , l e p i d o c r o c i t e , and g o e t h i t e .

Based on X-ray d i f f r a c t i o n and e l e c t r o n microscopy

g o e t h i t e occurred a s rod o r needle-like c r y s t a l s elongated p a r a l l e l to the c-axis w i t h t y p i c a l dimensions b e i n g 50-1002 i n d i a m e t e r and 500-20002 i n l e n g t h ( s e e Lamgmuir and Whittemore, 1971, F i g . 5 ) .

Lepidocrocite occurred with i t s t y p i c a l

l a t h - s h a p e d h a b i t r a n g i n g from 60-2002 t h i c k a l o n g t h e b - a x i s , a v e r a g i n g 4002 p a r a l l e l t o t h e c - a x i s and 23001 long i n t h e c - d i r e c t i o n (Langmuir and Whittemore,

1 9 7 1 , Fig.5;

s e e a l s o t h e e x c e l l e n t e l e c t r o n micrographs of Schwertmann, 1 9 7 0 ) .

Langmuir and Whittemore (1971) a l s o made a s t u d y of n a t u r a l l y o c c u r r i n g f e r r i c

343

40001-T---7 3000

1000

-

- 1000 -

m

0.01

hematite > lp goethite < IN

0.1

1

Particle Size of Cubes (microns)

Fig.6.12. P a r t i c l e s i z e e f f e c t on t h e Gibbs f r e e e n e r g y of t h e g o e t h i t e d e h y d r a t i o n r e a c t i o n ( a f t e r Langmuir, 1971). oxyhydroxides and i r o n c o n t e n t i n 24 w e l l w a t e r s from t h e c o a s t a l p l a i n of New J e r s e y and Maryland,

The suspended oxyhydroxides h e r e a r e p r o b a b l y amorphous

m a t e r i a l and g o e t h i t e w i t h a n a v e r a g e g r a i n s i z e , based on f i l t r a t i o n s t u d i e s , of 10,000-20,000~. A number of i m p l i c a t i o n s emerge from t h e work of Berner (1969a) and Langmuir

(1971).

Although h a e m a t i t e may c r y s t a l l i z e d i r e c t l y from w a t e r d i s s o l v i n g g o e t h i t e

a t low t e m p e r a t u r e s ( c 1 0 O o C ) ,

kinetic considerations indicate that the reaction

must b e e x t r e m e l y s l o w , and t h i s p r o b a b l y e x p l a i n s why i t has n o t been observed i n the laboratory.

The t r a n s f o r m a t i o n of l e p i d o c r o c i t e t o g o e t h i t e , on t h e o t h e r hand,

h a s been shown by Schwertmann and T a y l o r (1972) t o t a k e p l a c e by t h e d i s s o l u t i o n of l e p i d o c r o c i t e , and t h e s u b s e q u e n t n u c l e a t i o n and growth o f g o e t h i t e c r y s t a l s .

The

f o r m a t i o n o f h a e m a t i t e by d e h y d r a t i o n of g o e t h i t e i s l i k e l y t o t a k e p l a c e r e a d i l y i n t h e s e d i m e n t a r y environment, e s p e c i a l l y d u r i n g d i a g e n e s i s , and w i l l be enhanced by i n c r e a s e d t e m p e r a t u r e o r lower w a t e r vapour p r e s s u r e , o r a combination of t h e s e . There have been no c o n v i n c i n g d e m o n s t r a t i o n s of t h e i n s i t u a g e i n g of g o e t h i t e t o haematite, nor indeed i s there convincing d i r e c t evidence t h a t the haematite i n a n c i e n t r e d beds has formed from g o e t h i t e d e h y d r a t i o n .

The r e l a t i v e importance of

g o e t h i t e and amorphous f e r r i c hydroxide a s h a e m a t i t e p r e c u r s o r s i s u n c l e a r . Amorphous f e r r i c hydroxide i s u s u a l l y l e s s s t a b l e than f i n e g r a i n e d g o e t h i t e and thermodynamic and k i n e t i c c o n s i d e r a t i o n s i n d i c a t e t h a t h a e m a t i t e f o r m a t i o n from s u c h m a t e r i a l i s more f a v o u r a b l e t h a n h a e m a t i t e f o r m a t i o n from g o e t h i t e .

I n t h o s e r e d beds which c o n t a i n e d s u b s t a n t i a l amounts o f d e t r i t a l c l a y - o x i d e , f o r example, t r o p i c a l savanna a l l u v i u m , t h e d e h y d r a t i o n o f g o e t h i t e i s l i k e l y t o have been an i m p o r t a n t mechanism of h a e m a t i t e - f o r m a t i o n .

D i r e c t formation of

h a e m a t i t e from amorphous f e r r i c hydroxide i s l i k e l y t o have been i m p o r t a n t i n r e d bed a q u i f e r s . ENVIRONMENTAL CONTROLS ON THE PRECIPITATION AND DIAGENETIC HISTORY OF FERRIC OXYHYDROXIDE S

I r o n i s t r a n s p o r t e d i n t h e s e d i m e n t a r y environment i n a v a r i e t y o f forms. amounts may o c c u r i n t r u e s o l u t i o n e s p e c i a l l y i n more a c i d w a t e r s .

Small

The m a j o r i t y ,

however, i s t r a n s p o r t e d a s c r y s t a l l i n e i r o n hydroxide g r a i n c o a t i n g s and w i t h i n s i l i c a t e mineral l a t t i c e s .

O t h e r i m p o r t a n t mechanisms i n c l u d e t h e t r a n s p o r t of

i r o n which i s adsorbed on t h e s u r f a c e and edges of c l a y m i n e r a l s , and a l s o t h a t The p r o p o r t i o n of i r o n t r a n s p o r t e d by

which i s a s s o c i a t e d w i t h o r g a n i c complexes.

t h e s e d i f f e r e n t mechanisms v a r i e s a c c o r d i n g t o t h e d e p o s i t i o n a l environment.

In

m i n e r a l o g i c a l l y immature s a n d s t o n e s d e r i v e d from c r y s t a l l i n e basement, much of t h e i r o n i s bound i n t h e l a t t i c e s of ferromagnesian s i l i c a t e s .

In f i n e g r a i n e d a l l u v i u m ,

a g r e a t e r p r o p o r t i o n o f t h e i r o n i s t r a n s p o r t e d a s hydroxide g r a i n c o a t i n g s and i n association with clay minerals. The u l t i m a t e f a t e o f t h e s e v a r i o u s forms of i r o n depends upon a number of f a c t o r s i n c l u d i n g t h e geochemical environment 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 , and a l s o s u b s e q u e n t l y d u r i n g sediment b u r i a l .

The p o t e n t i a l f o r p r o d u c i n g r e d sediment

i s l a r g e l y d e t e r m i n e d d u r i n g e a r l y d i a g e n e s i s and s u b s e q u e n t b u r i a l .

Iron is

sedimented p r i m a r i l y a s i r o n h y d r o x i d e s and c r y s t a l l i n e p a r t i c l e s ; t h e f a t e of t h e s e i r o n h y d r o x i d e s , which c o n s t i t u t e t h e r e a d i l y a v a i l a b l e i r o n , i s d e t e r m i n e d by t h e Eh-pH c o n d i t i o n s below t h e sediment s u r f a c e .

I n continental alluvium oxidizing

c o n d i t i o n s commonly p r e v a i l , above and below t h e sediment s u r f a c e ,

Under t h e s e

c o n d i t i o n s m e t a s t a b l e i r o n h y d r o x i d e s may ' a g e ' i n t o h a e m a t i t e , s i l i c a t e o x i d a t i o n may c o n t i n u e d u r i n g s e d i m e n t b u r i a l , and pigmentary o x i d e s may b e p r e c i p i t a t e d i n p o r e s p a c e s and i n t e r s t i t i a l m a t r i x t h u s p r o d u c i n g r e d sediment.

I n many marine

and l a c u s t r i n e environments t h e s p e c i a l r o l e which i r o n p l a y s w i t h i n t h e s u l p h u r c y c l e means t h a t q u i t e a d i f f e r e n t d i a g e n e t i c h i s t o r y r e s u l t s .

Available i r o n ,

whether i t b e i n c o l l o i d a l form, a s amorphous Fe(OH)3, o r even a s c r y s t a l l i n e hydroxides o r o x i d e s , may s u f f e r r e d u c t i o n , d i s s o l u t i o n o r s u l p h i d a t i o n and e v e n t u a l l y form i r o n s u l p h i d e ( p y r i t e ) .

A m a j o r consequence o f t h i s i s t h a t t h e s e

sediments a r e g e n e r a l l y d r a b i n c o l o u r .

The i n f l u e n c e which d e p o s i t i o n a l e n v i r o n -

ment e x e r t s on t h e f o r m a t i o n o f f e r r i c oxyhydroxides, and u l t i m a t e l y on s e d i m e n t c o l o u r , i s w e l l - s e e n i n t h e c o l o u r d i f f e r e n t i a t j o n which e x i s t s between i n t e r b e d d e d marine and non-marine s e d i m e n t s and a l s o between i n t e r b e d d e d f l u v i a l and l a c u s t r i n e sediments.

346

Marine c o n d i t i o n s The f a t e o f a v a i l a b l e i r o n i n t h e marine environment i s i n t i m a t e l y bound up with the sulphur cycle.

Kaplan e t a l . (1963) showed t h a t b a c t e r i a l s u l p h a t e

r e d u c t i o n i s t h e most i m p o r t a n t p r o c e s s i n t h e s u l p h u r c y c l e w i t h t h e main genera i n v o l v e d b e i n g D e s u l p h o v i b r i o and Desulphotomaculum.

One o f t h e most i m p o r t a n t

s p e c i e s i s D e s u l p h o v i b r i o d e s u l p h u r i c a n s which can t o l e r a t e s a l i n i t y v a r i a t i o n s between 0 and 12% ( L i t t l e w o o d and P o s t g a t e , 1957) and i s p r i m a r i l y r e s p o n s i b l e f o r t h e r e d u c t i o n o f s u l p h a t e i n marine w a t e r s ( T r u d i n g e r , 1976). B a c t e r i a l s u l p h a t e r e d u c t i o n o c c u r s o n l y i n t h e absence o f oxygen.

These

c o n d i t i o n s a r e commonly met w i t h j u s t below t h e s u r f a c e o f marine sediments because o f t h e r a p i d consumption o f oxygen d u r i n g a e r o b i c r e s p i r a t i o n r e l a t i v e t o t h e r a t e of r e p l e n i s h m e n t . The i n i t i a l p r o d u c t s o f s u l p h a t e r e d u c t i o n , H2S and HS- may combine w i t h i r o n , which i s reduced t o Fe2+ and t h u s made more mobile under t h e s e c o n d i t i o n s , t o form a n " i n t e r m e d i a t e q qm e t a s t a b l e i r o n s u l p h i d e such as mackinawite ( t e t r a g o n a l FeS) a n d / o r g r e i g i t e ( c u b i c Fe3S4) ( F i g . 6 . 1 3 . ) . -

,

Subsequent t r a n s f o r m a t i o n of t h e s e i n t e r m e d i a t e s t o t h e more s t a b l e p y r i t e ( c u b i c FeS ) o c c u r s by t h e a d d i t i o n o f e l e m e n t a l s u l p h u r (Goldhaber and Kaplan, 2 1974, 1975). The r a t e o f s u l p h a t e r e d u c t i o n under l a b o r a t o r y c o n d i t i o n s i s c o n t r o l l e d by t h e abundance o f e l e c t r o n donor ( o r g a n i c m a t t e r ) a v a i l a b l e t o t h e b a c t e r i a (Goldhaber and Kaplan, 1975).

I n s e d i m e n t s , t h e p o s i t i v e c o r r e l a t i o n between % s u l p h u r and

% carbon i n a v a r i e t y o f s e d i m e n t a r y environments (Sweeney, 1972) a l s o s u g g e s t s a

c o n t r o l on p y r i t e p r o d u c t i o n by o r g a n i c carbon.

i s d e r i v e d mainly from marine p l a n k t o n .

Organic matter i n marine sediments

During i t s t r a n s p o r t and b u r i a l t h e

o r g a n i c m a t t e r undergoes t r a n s f o r m a t i o n s i n t o a series of f r a c t i o n s : degraded c e l l u l a r m a t e r i a l d w a t e r s o l u b l e complexes c o n t a i n i n g amino a c i d s , f a t t y a c i d s , and s u g a r s d u l v i c acids-whumic acidsAerogen-(Nissenbaum and Kaplan, 1972). T h i s pathway r e p r e s e n t s i n c r e a s i n g l y p o l y m e r i z a t i o n o f t h e o r g a n i c m a t e r i a l .

It

t h u s f o l l o w s t h a t t h i s more complex o r g a n i c m a t e r i a l i s less s u s c e p t i b l e t o b a c t e r i a l a t t a c k because o f t h e l a c k o f s u i t a b l e s o l u b i l i z i n g and enzyme systems (Alexander,, 1971).

The t o t a l sediment accumulation r a t e may t h e r e f o r e be c l o s e l y The main r e a s o n s f o r t h i s being:

r e l a t e d t o t h e rate of b a c t e r i a l metabolism.

1. t o t a l o r g a n i c carbon p r e s e r v e d i n sediments t e n d s t o be p o s i t i v e l y c o r r e l a t e d w i t h high s e d i m e n t a t i o n r a t e ( B e r n e r , 1972)

2. less complexing and rearrangement

3. f o r t h e most p a r t r a p i d l y

of organic m a t e r i a l occurs during r a p i d b u r i a l

accumulating marine sediments tend t o o c c u r a t s h a l l o w d e p t h s which l e a d s t o a lower p r e s s u r e , s h o r t e r t i m e of exposure t o oxygenated water d u r i n g t r a n s p o r t , and a g e n e r a l l y h i g h e r bottom w a t e r temperature.

This c o r r e l a t i o n between b a c t e r i a l

r a t e and s e d i m e n t a t i o n r a t e i s shown i n Fig.6.14.

346 A-

I

I

4

J.

Fe S2

/

(Fmmboidr)

The pathway o f s e d i m e n t a r y p y r i t e f o r m a t i o n ( a f t e r Goldhaber and Fig.6.13. Kaplan, 1974). The above d i s c u s s i o n i n d i c a t e s t h a t r e a c t i o n s o c c u r r i n g d u r i n g s u l p h u r d i a g e n e s i s c o n s i s t o f a number of t r a n s f o r m a t i o n s i n v o l v i n g s u l p h u r and i r o n i n b o t h t h e s o l i d and aqueous p h a s e s .

Both d i s s o l v e d s u l p h i d e and a c i d v o l a t i l e s u l p h i d e (FeS) a r e

i n v o l v e d i n t h e pathway o f p y r i t e f o r m a t i o n which may b e s i m p l i f i e d : 2-

SO4 +H2S

"Fe"

SO

FeS +FeS2

This sequence emphasises t h e f a c t t h a t d i s s o l v e d s u l p h i d e and a c i d v o l a t i l e s u l p h i d e a r e t r a n s i e n t i n t e r m e d i a t e s which a r e c o n t i n u o u s l y b e i n g formed and removed a t any given t i m e .

P o r e w a t e r d a t a show a s y s t e m a t i c d e p l e t i o n o f d i s s o l v e d s u l p h a t e w i t h

d e p t h ( t i m e ) and t h i s i s a s s o c i a t e d w i t h a n e x p e c t e d i n i t i a l i n c r e a s e i n d i s s o l v e d sulphide (Fig.6.15).

Below t h e d e p t h of z e r o s u l p h a t e , s u l p h i d e p r o d u c t i o n from

t h i s s o u r c e must c e a s e , and d i s s o l v e d s u l p h i d e d e c r e a s e s r e f l e c t i n g p r o g r e s s i v e consumption by r e a c t i o n w i t h s o l i d components, and a l s o d i f f u s i o n of d i s s o l v e d sulphide.

S u l p h i d e consumption i s a l s o o c c u r r i n g a t d e p t h s s h a l l o w e r t h a n t h a t a t

which s u l p h a t e d i s a p p e a r s , b u t i n t h e upper 150cm s u l p h i d e p r o d u c t i o n i s g r e a t e r t h a n t h e s u l p h i d e removal r a t e , hence t h e i n c r e a s e i n t h i s zone.

Since sulphide

p r o d u c t i o n ( s u l p h a t e r e d u c t i o n ) c o r r e l a t e s w i t h s e d i m e n t a t i o n r a t e , one m i g h t e x p e c t t h e maximum c o n c e n t r a t i o n of d i s s o l v e d s u l p h i d e t o show a s i m i l a r r e l a t i o n s h i p . 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 i s i s t h e c a s e , t h e h i g h e s t maximum d i s s o l v e d s u l p h i d e c o n c e n t r a t i o n s b e i n g found i n t h e most r a p i d l y d e p o s i t e d s h a l l o w marine sediments.

The v a r i a t i o n i n maximum s u l p h i d e c o n c e n t r a t i o n is n o t s o l e l y r e l a t e d

347

0

-51 0

I

I

I

I

200

400

600

800

1000

0

0

0-

1200 1400 1600 1800 2000 2200 2400

Sedimentation rate cm. per 1000 yrs.

Fig.6.14. R e l a t i o n s h i p between s u l p h a t e r e d u c t i o n r a t e v e r s u s s e d i m e n t a t i o n r a t e . S e d i m e n t a t i o n r a t e s : Somes Sound, Marine 0 and Long I s l a n d Sound @ ( B e r n e r , 1 9 7 2 ) , (Koide e t a l . , 1972; Emery, 19601, Carmen Basin A and S a n t a Barbara B a s i n P e s c a d e r o Basin (Van Andel, 1 9 6 4 ) , C a r i a c o Trench 0 ( a f t e r Goldhaber and Kaplan, 1975). t o v a r i a t i o n i n production r a t e but a l s o t o the nature of the dissolved sulphide removal mechanism.

T h i s mechanism m a i n l y i n v o l v e s r e a c t i o n w i t h i r o n o x i d e o r

i r o n h y d r o x i d e (see Goldhaber and Kaplan, 1974 f o r a r e v i e w ) .

R i c k a r d (1974) h a s

s t u d i e d t h e s u l p h i d a t i o n of g o e t h i t e under l a b o r a t o r y c o n d i t i o n s and found t h a t t h e r a t e o f t h i s r e a c t i o n i s dependent upon s u l p h i d e c o n c e n t r a t i o n , pH, and t h e surface area of goethite. The abundance of a c i d v o l a t i l e s u l p h i d e s i s s i m i l a r l y d e t e r m i n e d by t h e r e l a t i v e r a t e s of p r o d u c t i o n and consumption.

S i m i l a r arguments t o t h o s e which a p p l y t o

d i s s o l v e d s u l p h i d e a r e a l s o r e l e v a n t add s i m i l a r r e l a t i o n s h i p s e x i s t (Goldhaber and Kaplan, 1975) F i g . 6 . 1 6 ) .

The s i t u a t i o n i s a c t u a l l y a l i t t l e more c o m p l i c a t e d

b e c a u s e t h e c o n v e r s i o n t o p y r i t e i n v o l v e s a d d i t i o n of e l e m e n t a l s u l p h u r (Fig.6.13). T h e r e f o r e t h e low abundance o f a c i d v o l a t i l e s u l p h u r r e l a t i v e t o p y r i t e s u l p h u r a t low r a t e s o f d e p o s i t i o n may b e a f f e c t e d by a n abundant s u p p l y of z e r o v a l e n t s u l p h u r r e s u l t i n g from l o n g e r e x p o s u r e t o o x i d i z i n g c o n d i t i o n s n e a r t h e sediment-

348

Sulfate mM

Sulfide mM +O

300

t

300L

Fig.6.15. V a r i a t i o n i n d i s s o l v e d s u l p h a t e and s u l p h i d e i n p o r e w a t e r s of a c o r e from Carmen B a s i n , Gulf o f C a l i f o r n i a ( a f t e r Goldhaber and Kaplan, 1 9 7 5 ) . w a t e r i n t e r f a c e ( B e r n e r , 1964).

Under c e r t a i n c o n d i t i o n s t h e f o r m a t i o n o f p y r i t e

does n o t i n v o l v e an i n t e r m e d i a t e s u l p h i d e .

These c o n d i t i o n s a r i s e when t h e

s o l u b i l i t y p r o d u c t o f t h e l e a s t s o l u b l e a c i d v o l a t i l e s u l p h i d e i s n o t exceeded ( p y r i t e i n e q u i l i b r i u m w i t h e x c e s s e l e m e n t a l s u l p h u r i s many o r d e r s o f magnitude l e s s soluble than g r e i g i t e o r mackinawite).

Such u n d e r s a t u r a t i o n c o u l d c o r r e l a t e

w i t h a low s e d i m e n t a t i o n r a t e ; s i m i l a r l y low s e d i m e n t a t i o n r a t e s c o u l d r e f l e c t a reduced p r o p o r t i o n of p y r i t e s u l p h u r h a v i n g p a s s e d t h r o u g h t h e a c i d v o l a t i l e s t a g e . 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

The Upper Devonian C a t s k i l l Formation i n C e n t r a l P e n n s y l v a n i a p r o v i d e s a f i n e example of c o l o u r change i n r e s p o n s e t o a l t e r n a t i n g marine/non m a r i n e c o n d i t i o n s . I n t h i s a r e a t h e C a t s k i l l Formation c o n s i s t s o f two m o t i f s motif c o n t a i n i n g marine- non-marine i n t e r c a l a t i o n s and

2.

1. t h e I r i s h V a l l e y t h e a l l u v i a l motif

c o n s i s t i n g of fining-upwards cyclothems and d e s c r i b e d by A l l e n and F r i e n d (1968). The I r i s h V a l l e y m o t i f i s n o t a b l e b e c a u s e i t d i f f e r s from many modern c o a s t a l d e p o s i t s and shows no e v i d e n c e of the b a r r i e r i s l a n d - t i d a l

f l a t model proposed b y

A l l e n and F r i e n d (1968), n o r any e v i d e n c e of a v i g o r o u s l y p r o g r a d i n g sandy d e l t a ; t h e t i t l e " C a t s k i l l Delta" i s t h e r e f o r e something of a misnomer. The C a t s k i l l complex r e p r e s e n t s a r e g r e s s i v e sequence b u i l t i n t o t h e Upper Devonian

349

I

1

1

1

200 300 400 Sdirnontation rat. ern. per 1000 p a r s 100

.

1

500

Fig.6.16. R e l a t i o n s h i p between maximum p e r c e n t a g e o f t o t a l reduced s u l p h u r i n t h e Data o f Sweeney ( u n p u b l i s h e d ) from a c i d v o l a t i l e form a g a i n s t s e d i m e n t a t i o n r a t e Marine Del Ray Harbor, Los Angeles 0 Data o f Kaplan e t a l . (1963). Sedimentation Goldhaber a n d Kaplan (1975) ( a f t e r Goldhaber and Kaplan, r a t e from Emery (1960) 1975).

A

b a s i n a s a r e s u l t of t h e Acadian orogeny.

In t h e Susquehanna V a l l e y a r e a o f

c e n t r a l P e n n s y l v a n i a a n i n i t i a l marine t r a n s g r e s s i o n submerged o l d e r , s h a l l o w marine o r s h o r e l i n e d e p o s i t s ( T u l l y Limestone and i t s e l a s t i c c o r r e l a t i v e s , Johnson and Friedman, 1966).

The s u c c e e d i n g r e g r e s s i v e sequence can b e d i v i d e d

i n t o f o u r main t y p e s : 1. a t t h e b a s e , g r e y , m a r i n e s i l t y mudstones ( l o w e r p a r t of Trimmers Rock F o r m a t i o n , a b o u t 80m) Formation, a b o u t 45Om)

2 . t u r b i d i t e s ( b u l k o f Trimmers Rock

3 . a l t e r n a t i n g a g i t a t e d - w a t e r marine and non-marine beds

( I r i s h V a l l e y Member of t h e C a t s k i l l Formation, a b o u t 60Om); and 4. a t t h e top a l l u v i a l p l a i n s a n d s t o n e s and mudstones (Buddys Run Member o f t h e C a t s k i l l F o r m a t i o n , a b o u t 1300m).

The s t r a t i g r a p h i c t e r m i n o l o g y i s a f t e r Hoskins e t a l .

(1963) and Dyson (1963). A t y p i c a l u n i t i n t h e I r i s h V a l l e y m o t i f (Fig.6.17)

has an abrupt base overlain

by b i o t u r b a t e d s a n d s t o n e , m a r i n e o l i v e g r e e n s h a l e s and s i l t s t o n e s , r e d s i l t s t o n e s w i t h minor s a n d s t o n e s , and a cap o f r e d mudstones w i t h r o o t t r a c e s d e s i c c a t i o n cracks, and calcareous nodules.

Walker and Harms (1971, p. 387) e n v i s a g e t h e I r i s h

V a l l e y m o t i f forming i n f i v e s t a g e s . c o a s t a l p l a i n o f low r e l i e f . reworking o f winnowed sand.

The p r e s e n c e o f b r a c h i o p o d s a n d c r i n o i d a l d e b r i s

i m p l i e s a normal m a r i n e environment. i n a gradually shoaling sea.

F i r s t , r a p i d marine transgression over a

Second, a s t a g e o f v e r y slow d e p o s i t i o n and b i o l o g i c a l T h i r d , m a r i n e d e p o s i t i o n o f mud took p l a c e

I n i t i a l d e p o s i t s were o l i v e g r e e n f i s s i l e s h a l e s , b u t

a s t h e s e a s h o a l e d , t h i n w e l l - s o r t e d s a n d s t o n e s were d e p o s i t e d .

F o u r t h , t h e r e was

emergence o f mud f l a t s , w i t h many r o o t s p e n e t r a t i n g t h e s e d i m e n t , some s h a l l o w , ephemeral l a k e s and no m a r i n e f a u n a ,

Symmetrical r i p p l e s formed i n ephemeral l a k e s

350

FINING UPWARD ALLUVIAL MOTIF MAY BE PRESENT AT THIS POSITION I N SOME OF THE MAIN MOTIFS.

'RANSGRESSION

I

11-1

bASTAL PLAIN

I

4 "4

6. MASSIVE, BLOCKY RED MUDSTONE WITH ROOT TRACES, FILLED (?DESSICATIONICRACKS, AND TAN CALCAREOUS NODULES. SMDSTONE RARE UNLESS IN FINING-UPWARD MOTIF. 5. DRAB RED SILTSTONES WITH MANY ROOT TRACES AND SOME FILLED (?DESSICATION)CRACKS. BEDDING IS COMMONLY APPARENT! THERE ARE SANDSTONE BEDS 2-IOCM. THICK WITH SMALL SYMMETRICAL RIPPLES. MARINE fOSSILS ABSENT,VERTICAL AND HORIZONTAL BURROWS AND TRAILS O N MANY BEDS 4. GREEN SILTSTONES AND MUDSTONES WITH FINE-GRAINED SANDSTONE BEDS 10-10 CM.THICK, LOW ANGLE CROSSBEDDED,HORIZONTALLY LAMINATED OR RIPPLE CROSS- LAMINATED, SYMMETRICAL RIPPLES WAVELENGTH 10-15 CM. PRESENT, BRACHIOPODS RARE, BURROWS AND TRAILS COMMON. THIS UNIT IS THIN OR ABSENT IN MANY MOTIFS

-

3. OLIVE GREEN FISSILE SHALE, SCATTERED BRACHIOPODS

4

AND CRINOIDS. THIS UNIT MAY REST DIRECTLY ON BASAL SURFACE OF MOTIF. /OIOTURBATE SANDSTONE WITH SCATTERED OUARTZ GRANULES, 2, IACHIOPODS. CRINOIDS, PHOSPHATIC NOOULES. M3NE FRAOMENTS. \BASAL SURFACE.PLANAR, COMMONLYSHUP. IN PLACES BURROWED AND IRREWLAR.

@*

Wfl

METERS

Fig.6.17.

u bG

I d e a l i z e d I r i s h V a l l e y m o t i f ( a f t e r Walker and Harms, 1971).

and d e s i c c a t i o n c r a c k s i n d i c a t e a l t e r n a t e f l o o d i n g and d r y i n g of t h e c o a s t a l f r i n g e . The c o l o u r of t h e s e f i r s t non-marine d e p o s i t s i s r e d , c o n t r a s t i n g w i t h t h e o l i v e g r e e n and d r a b marine s h a l e s and s a n d s t o n e s b e n e a t h .

F i f t h , h i g h e r on t h e c o a s t a l

p l a i n , t h e r e a r e r e d s i l t s t o n e s and mudstones w i t h d e s i c c a t i o n c r a c k s , r o o t s , and t a n c a l c a r e o u s n o d u l e s , s u g g e s t i n g i n c i p i e n t s o i l development.

A few t h i n f i n i n g -

upwards c y c l e s o c c u r and t h e s e d i m e n t s a r e p r e d o m i n a n t l y r e d . The a b s e n c e of s a n d i n t h e s e s h o r e l i n e s e d i m e n t s s u g g e s t a s i m i l a r i t y w i t h t h e p r o g r a d a t i o n a l C h e n i e r P l a i n of Southwestern L o u i s i a n a (Byrne e t a l . ,

1959; Gould

and McFarlan, 1959) and a l s o t h e n o r t h e r n Colorado R i v e r D e l t a (Thompson, 1968). The p r o g r a d a t i o n o f t h i s C a t s k i l l muddy s h o r e l i n e p r o b a b l y took p l a c e by l o n g s h o r e s u p p l y of sediment from more a c t i v e d e l t a i c a r e a s t o t h e n o r s h ( G l a e s s e r , 1970). There a r e numerous ( a b o u t 25) r e p e t i t i o n s o f t h e I r i s h V a l l e y m o t i f i n t h e Susquehanna V a l l e y area a l t h o u g h i t i s n o t c l e a r whether t h e s e w e r e c o n t r o l l e d sedimentologically o r tectonically.

I n t h e former c a s e l o c a l t r a n s g r e s s i o n and

351 r e g r e s s i o n and hence r e p e t i t i o n of t h e m o t i f c o u l d b e e f f e c t e d by t h e r e l a t i v e p r o x i m i t y of major r i v e r c h a n n e l s .

When t h e s e were i n a r e l a t i v e l y d i s t a n t a r e a

o f t h e c o a s t a l p l a i n r e g i o n a l s u b s i d e n c e and compaction would r e s u l t i n l o c a l transgression.

R e g r e s s i o n would o c c u r where t h e r a t e o f sediment s u p p l y by longshore

d r i f t exceeded t h e r a t e of s u b s i d e n c e .

Such a mechanism i s well-documented i n t h e

L o u i s i a n a C h e n i e r P l a i n and M i s s i s s i p p i d e l t a (Gould and McFarlan, 1959). I n t e r m i t t e n t e p i s o d e s of s u b s i d e n c e , p e r h a p s r e l a t e d t o t e c t o n i c a c t i v i t y i n t h e Acadian Mountain Chain may a l s o have been i m p o r t a n t . I n any c a s e , t h e o v e r a l l r e s u l t was t o produce a s e r i e s of i n t e r f i n g e r i n g marine and non-marine s e d i m e n t s i n t h e I r i s h V a l l e y f a c i e s , which a r e r e f l e c t e d i n t h e red- non-red d i f f e r e n t i a t i o n ( F i g . 6 . 1 8 ) .

I t seems l i k e l y t h a t t h e i m p o r t a n t f a c t o r s

c o n t r o l l i n g s e d i m e n t c o l o u r a t i o n under t h e s e c o n d i t i o n s were t h o s e which were l i n k e d t o t h e d e p o s i t i o n a l environment, n o t a b l y redox p o t e n t i a l and s u l p h i d e a c t i v i t y .

In

t h e marine s e d i m e n t s g r e e n o r g r e y c o l o u r s p r e v a i l e d s i n c e pigmentary i r o n o x i d e s and h y d r o x i d e s c o u l d n o t form b e c a u s e of s u l p h i d a t i o n and r e d u c t i o n b e n e a t h t h e sediment s u r f a c e ,

The non-marine sediment e v e n t u a l l y became r e d b e c a u s e of t h e l a c k

of o r g a n i c matter and o x i d i z i n g c o n d i t i o n s which e n a b l e d i r o n o x i d e s and h y d r o x i d e s t o e v e n t u a l l y form s t a b l e pigmentary o x i d e s .

These r e s u l t s t e n d t o s u p p o r t t h e view

t h a t e n v i r o n m e n t a l l y c o n t r o l l e d c o l o u r d i f f e r e n t i a t i o n of t h i s t y p e i n d i c a t e s t h a t s u b s t a n t i a l amounts o f r e a d i l y a v a i l a b l e f i n e g r a i n e d i r o n o x i d e s and hydroxides must have been p r e s e n t a t t h e time o f d e p o s i t i o n .

The r o l e o f t h e s e d e t r i t a l i r o n

o x i d e s and h y d r o x i d e s i n p r o d u c i n g r e d s e d i m e n t i n r e d beds o f t h i s t y p e i s t h e r e f o r e c o n s i d e r e d t o have been more i m p o r t a n t t h a n t h e s u b s e q u e n t d i a g e n e t i c breakdown of i r o n s i l i c a t e s .

T h i s view i s s u p p o r t e d by t h e f a c t t h a t t h e heavy m i n e r a l s u i t e s

o f a number o f r e d beds a r e much t h e same a s t h e i r marine e q u i v a l e n t s . Lacustrine conditions S i n c e i r o n i s a n e s s e n t i a l m i c r o n u t r i e n t , i t s c y c l i n g i n t h e l a c u s t r i n e environment i s i n t i m a t e l y i n v o l v e d w i t h o r g a n i c a c t i v i t y .

Moreover s i n c e t h e o x i d a t i o n of

Fe2+ t o Fe3+ i s e x e r g o n i c and c a p a b l e o f s u p p l y i n g e n e r g y i t i s u s e d by some microorganisms a s a s o u r c e o f energy.

Only s m a l l amounts o f f e r r o u s i r o n a r e p r e s e n t

i n s o l u t i o n i n oxygenated l a k e w a t e r s b i t i n t h e r m a l l y s t r a t i f i e d l a k e s (Fig.6.19) r e d u c i n g c o n d i t i o n s i n t h e hypolimnion may c o r r e s p o n d t o h i g h e r c o n c e n t r a t i o n s o f ferrous iron i n solution.

Much o f t h e i r o n i n normal l a k e w a t e r i s p r e s e n t a s

f e r r i c h y d r o x i d e i n f l o c c u l a n t form and i s removable by f i l t r a t i o n w i t h membranes of a p o r o , s i t y o f 0.5um (Hutchinson, 1957; Hem and Cropper, 1959; Hem and Skougstad, 1960).

C o l l o i d a l Fe(0H)

3

w i t h a p a r t i c l e s i z e 0.001 t o 0.5ym a l s o o c c u r s .

These

p a r t i c l e s commonly have a p o s i t i v e c h a r g e a l t h o u g h a n e g a t i v e l y charged sol can o c c u r a t h i g h pH.

The p o s i t i v e l y charged i o n s i n s o l u t i o n and n e g a t i v e l y charged

c l a y p a r t i c l e s and o r g a n i c c o l l o i d s , c a n n e u t r a l i z e t h e c h a r g e s on t h e hydroxide

352

ACADIAN MOUNTAINS

A

GREEN MUDS-MARINf

I RED SILTS k MUDS-NON MARINf

Fig.6.18. A model of Upper Devonian s e d i m e n t a t i o n i n s o u t h - c e n t r a l P e n n s y l v a n i a . Red- non-red d i f f e r e n t i a t i o n o c c u r s i n t h e I r i s h V a l l e y f a c i e s and r e f l e c t s nonmarine c o n d i t i o n s as t h e C a t s k i l l s h o r e l i n e p r o g r a d e d northwestwards ( a f marine Walker and Harms, 1971).

-

colloid particles.

These uncharged a g g r e g a t e s may form a r a p i d l y s e t t l i n g

p r e c i p i t a t e , and m e t a l s s u c h a s copper i o n s may b e a d s o r b e d and c o p r e c i p i t a t e d with t h e f e r r i c hydroxide p r e c i p i t a t e . The s o l u b i l i t y and a v a i l a b i l i t y o f i r o n i n l a k e w a t e r s i s g r e a t l y enhanced by t h e f o r m a t i o n o f i r o n complexes w i t h c e r t a i n o r g a n i c m o l e c u l e s .

Many o r g a n i c

bases form s t r o n g s o l u b l e i r o n complexes w i t h f e r r o u s ( G j e s s i n g , 1964) and f e r r i c ions.

I r o n enrichment i s commonly found i n s u r f a c e w a t e r s w i t h a h i g h c o n t e n t

of dissolved organic matter.

Such h i g h c o n c e n t r a t i o n s o r complexed s o l u b l e i r o n

a r e a s s o c i a t e d w i t h h i g h l e v e l s of humic a c i d s ( S h a p i r o , 1 9 5 7 ) , t a n n i c a c i d s (Hem, 1960) and o t h e r l i g n i n d e r i v a t i v e s .

The i n t e n s e yellow-brown c o l o u r o f

bog-waters i s p a r t l y a s s o c i a t e d w i t h t h e s e complexes.

S h a p i r o (1964, 1966, 1969)

a53

EPlLlMNlON

25t I % k

Ib

1;

o;

215

3'0

Tempomtun ('C ) Fig.6.19.

A t y p i c a l thermally s t r a t i f i e d lake.

h a s s t u d i e d t h e complexing o f i r o n w i t h humic d e r i v a t i v e s , e s p e c i a l l y w i t h yellow o r g a n i c a c i d s of low m o l e c u l a r w e i g h t , and c o n c l u d e s t h a t t h e p r i m a r y mechanism i s p e p t i z a t i o n i n which i r o n i s d i s p e r s e d i n a s o l u b i l i z e d form o f Fe(0H)

as a 3 r e s u l t of a d s o r p t i o n o f t h e o r g a n i c a c i d s o n t o t h e s u r f a c e s o f t h e p a r t i c l e s .

Some i r o n i s c h e l a t e d w i t h o r g a n i c a c i d s by weak chemical bonding b u t t h i s i s ~ o t t h e p r i m a r y complexing mechanism. The b e h a v i o u r o f i r o n i n l a k e w a t e r s 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 a t o f manganese and i t i s p e r t i n e n t t o d i s c u s s t h e b e h a v i o u r o f manganese h e r e . T h e o r e t i c a l s t u d i e s by Hem (1963, 1964) and Stunrm and Morgan (1970) a r e c o n s i s t e n t w i t h t h e b e h a v i o u r o f manganese i n l a k e s .

Although manganese o c c u r s i n s e v e r a l

v a l e n c e s t a t e s Mn3+ i s thermodynamically u n s t a b l e i n aqueous s o l u t i o n s under normal c o n d i t i o n s and Mn4'

compounds a r e i n s o l u b l e a t most e n v i r o n m e n t a l pH v a l u e s .

Like i r o n , d i v a l e n t Mn2+ o c c u r s a t low redox p o t e n t i a l s and pH (Fig.6.20).

Some

form of o x i d i z e d manganese w i l l be i n e q u i l i b r i u m w i t h Mn2+ under o x i d i z i n g c o n d i t i o n s o f h i g h pH and Eh, and some form o f Mn2+ may b e i n e q u i l i b r i u m w i t h manganese c a r b o n a t e under r e d u c i n g c o n d i t i o n s o f low pH and Eh. a r e s l o w e r f o r manganese t h a n f o r i r o n .

R a t e s of o x i d a t i o n

Above a pH of 8.5 a n i n t e r m e d i a t e o x i d e

complex forms i n which Mn2+ i s a d s o r b e d o n t o manganese o x i d e s .

The Mn2+ of t h e s e

o x i d e complexes can r e a c t r e l a t i v e l y r a p i d l y w i t h o t h e r a n i o n s and p r e c i p i t a t e a s MnC03, MnS, and Mn(OH)2.

Manganese i s a d s o r b e d on i r o n o x i d e s and c o p r e c i p i t a t e s

w i t h f e r r i c hydroxides when t h e pH i s g r e a t e r than 6 t o 7 .

Manganese forms

s o l u b l e complexes w i t h b i c a r b o n a t e and s u l p h a t e and i n c r e a s e d b i c a r b o n a t e a c t i v i t y d e c r e a s e s manganese s o l u b i l i t y , b u t a t h i g h c o n c e n t r a t i o n s i t has been found t o

3 54

;j$/ 3 10 12 14 P

Fig.6.20. D i s t r i b u t i o n o f manganese s p e c i e s in r e l a t i o n t o pH and redox p o t e n t i a l (modified from Stumm and Morgan, 1970). reduce the oxidation r a t e .

Manganese i n t h e form of Mn2+ forms s t a b l e complexes

w i t h o r g a n i c m a t t e r , i n a s i m i l a r manner t o i r o n , b u t t h e o p e r a t i o n o f t h e s e in a q u a t i c systems a r e n o t a s w e l l u n d e r s t o o d a s t h o s e w i t h i r o n .

Manganese i s

r e l a t i v e l y abundant i n a l k a l i n e s o i l s a s h y d r a t e d o x i d e s and o r g a n i c complexing p l a y s an i m p o r t a n t r o l e i n r e t e n t i o n of manganese i n a d i s s o l v e d form s u i t a b l e for transport.

Drainage from f o r e s t l i t t e r o f t e n has a h i g h manganese c o n t e n t ,

a g a i n p o i n t i n g t o t h e i m p o r t a n c e of o r g a n i c complexes a s a t r a n s p o r t i n g medium of e a r l y o x i d i z e d m e t a l i o n s . The range of t o t a l i r o n found i n most n e u t r a l o r a l k a l i n e l a k e s v a r i e s from a b o u t 5 0 t o 200 g / l i t r e and c o n s i s t s l a r g e l y of Fe(OH)3, o r g a n i c a l l y complexed i r o n , and a d s o r b e d i r o n i n p a r t i c u l a t e form.

Under o x i d i z e d c o n d i t i o n s , a s i n

t h e e p i l i m n i a of l a k e s , v e r y l a r g e amounts o f i r o n a r e found o n l y i n v e r y a c i d i c w a t e r ( p H 4 3 ) s u c h a s i n l a k e s of v o l c a n i c o r i g i n .

The range of manganese

c o n c e n t r a t i o n s ( a b o u t 10-850 g / l ) i s a l s o v a r i a b l e b u t t h e a v e r a g e q u a n t i t y 35 g / l i s somewhat l e s s t h a n t h a t o f i r o n .

The Fe:Mn r a t i o o f l a k e w a t e r i s

g e n e r a l l y much l e s s t h a n t h e a v e r a g e c r u s t a l r a t i o ( 5 0 : l ) i n d i c a t i n g t h e r e l a t i v e enrichment of manganese w i t h r e s p e c t t o i r o n , and i n agreement w i t h t h e r e a c t i o n e q u i l i b r i a ( H u t c h i n s o n , 1957; L i v i n g s t o n e , 1963). The v e r t i c a l d i s t r i b u t i o n o f i r o n and manganese is r e f l e c t e d i n t h e d i s t r i b u t i o n of redox p o t e n t i a l s .

I o n i c i r o n o f oxygenated w a t e r s o f o l i g o t r o p h i c l a k e s ,

e p i l i m n i a of more p r o d u c t i v e l a k e s , and of c i r c u l a t i n g w a t e r s i s v e r y low.

Ferrous

ions d i f f u s e r e a d i l y from s e d i m e n t s when t h e y a r e reduced t o redox p o t e n t i a l s below

355

a b o u t 200m

and m i g r a t i o n o f manganese may o c c u r a t somewhat g r e a t e r redox

potentials,

T h i s i s w e l l s e e n i n e n t r o p h i c l a k e s (Mortimer, 1971) ( F i g . 6 . 2 1 )

where t h e r e l e a s e of manganese p r e c e d e s t h a t o f i r o n . STRATIFICATION

OVERTURN

8 4

2 1

05 2 1

6 4

2

a 4

8

-04

4

-0 2

0.4

.70

0 2

.8.6

-+

30

.80

20 10

.80

S e a s o n a l d i s t r i b u t i o n i n composition mg/l and w a t e r p r o p e r t i e s of w a t e r Fig.6.21. w i t h i n 3Ocm o f t h e s e d i m e n t s a t a d e p t h o f 14m i n E s t h w a i t e W a t e r , England ( a f t e r Mortimer, 1971). I n v e r y p r o d u c t i v e l a k e s , t h e redox p o t e n t i a l of t h e h y p o l i m n e t i c w a t e r s may f a l l , a s t h e decomposition o f t h e hypolimnion f a l l s t h r o u g h o u t t h e p e r i o d of stratification.

Under t h e s e c o n d i t i o n s s u l p h a t e i s reduced t o hydrogen s u l p h i d e

and H S may b e f u r t h e r produced by b a c t e r i a l decomposition of s u l p h u r c o n t a i n i n g 2 o r g a n i c compounds. S i n c e t h e r e i s s i g n i f i c a n t r e l e a s e of f e r r o u s i r o n a t redox p o t e n t i a l s g r e a t e r t h a n t h o s e needed f o r s u l p h a t e r e d u c t i o n t h e r e i s u s u a l l y much Fez+ i n t h e hypolimnion a t t h e time o f s u l p h i d e f o r m a t i o n .

The f o r m a t i o n o f i r o n

s u l p h i d e s and o t h e r m e t a l s u l p h i d e s can r e s u l t i n a s i g n i f i c a n t r e d u c t i o n o f i r o n towards t h e end of summer. The c y c l i n g and d i s t r i b u t i o n o f i r o n and manganese i s c o n t r o l l e d mainly by t h e

3 56 redox p o t e n t i a l .

B a c t e r i a l and p h o t o s y n t h e t i c metabolism g r e a t l y i n f l u e n c e t h e s e

c o n d i t i o n s and t h e r e b y i n d i r e c t l y r e g u l a t e t h e c y c l i n g o f t h e m e t a l s .

The d i r e c t

u t i l i z a t i o n of i r o n and manganese by b a c t e r i a i n e n e r g e t i c t r a n s f o r m a t i o n s i s o n l y of r e l a t i v e l y minor importance.

The c h e m o s y n t h e t i c u t i l i z a t i o n of e n e r g y from

i n o r g a n i c o x i d a t i o n s i n C02 f i x a t i o n i s r e l a t i v e l y i n e f f i c i e n t .

For example t h e

o x i d a t i o n of Fez+ t o Fe3+ by i r o n b a c t e r i a g i v e s a n e n e r g y f i e l d o f o n l y The c y c l i n g o f i r o n and manganese i s i n f l u e n c e d by two p r o c e s s e s

11 k c a l / m o l e .

(Kuznetsov, 1970).

F i r s t , r e d u c t i o n o f t h e o x i d i z e d combined m e t a l o c c u r s under

a p p r o p r i a t e redox c o n d i t i o n s a s f e r r o u s b i c a r b o n a t e or i s p r e i p i t a t e d a s a s u l p h i d e , example r e a c t i o n s b e i n g : Fe203 FeS

+

+

3H2S +2FeS

+

3H20

+S

(6.8)

+

H2S

(6.9)

2H2C03+Fe(HC03)2

Second, s h e a t h e d and s t a l k e d b a c t e r i a , a l g a e , p r o t o z o a n f l a g e l a t e s , and s p e c i f i c t r u e b a c t e r i a p r e c i p i t a t e f e r r i c and manganic o x i d e s on t h e i r c e l l s . i r o n b a c t e r i a o c c u r i n i r o n - r i c h w a t e r s o f n e u t r a l o r a l k a l i n e pH.

The t r u e Characteristic

r e a c t i o n s of chemoautotrophic b a c t e r i a which d e p o s i t h y d r o x i d e s and o x i d e s a r e : 4Fe(HC03)2 4MnC03

+

+ O2 +

02+

+ 4H2C03 + 4C02

6H20+4Fe(OH)3

2Mn203

+

(+ 58 k c a l )

4C02 (+ 76 k c a l )

(6.10) (6.11)

Some s p e c i e s of L e p t o t h r i x can o x i d i z e b o t h f e r r o u s and manganous s a l t s , whereas G a l l i o n e l l a (Spirophyllum) i s r e s t r i c t e d t o i r o n .

S i n c e a t n e u t r a l pH, and i n t h e

p r e s e n c e o f oxygen Fez+ i s s p o n t a n e o u s l y o x i d i z e d ; t h e i r o n o x i d i z i n g b a c t e r i a a r e r e s t r i c t e d t o a r e a s o f s t e e p redox g r a d i e n t , and a r e e f f e c t i v e l y competing w i t h oxygen f o r t h e reduced i r o n .

They a r e t h e r e f o r e , g e n e r a l l y r e s t r i c t e d t o t h e

i n t e r f a c e a r e a s o f i r o n - r i c h r o c k s e e p s , swamps, and bogs where t h e redox p o t e n t i a l

i s s u f f i c i e n t l y low f o r reduced i r o n t o o c c u r , and t o upper h y p o l i m n e t i c a r e a s . O t h e r groups o f b a c t e r i a i n v o l v e d i n t h e p r e c i p i t a t i o n o f i r o n and manganese a r e h e t e r o t r o p h i c forms s u c h a s C l a d o t h r i x and some L e p t o t h r i x which a r e f i l a m e n t o u s and d e p o s i t i r o n and manganese on t h e c e l l i n s h e a t h d u r i n g t h e metabolism o f o r g a n i c compounds.

The c o l o n i a l , c o c c o i d c e l l s o r s h o r t r o d s o f S i d e r o c a p s a a r e a

common form o c c u r r i n g a t t h e oxic-anoxic i n t e r f a c e zone of hypolimnion-metalimnion, e s p e c i a l l y i n i r o n m e r o m i c t i c l a k e s (Dubinina e t el., 1 9 7 3 ) .

Siderocapsa i s a l s o

w i d e l y d i s t r i b u t e d i n t h e oxygenated zones o f s t r e a m s and l a k e s (Hardman and T h i s genus i s n o t a b l e b e c a u s e i t i s c a p a b l e o f m i n e r a l i z i n g

Henrici, 1939).

humates and i n c r e a s e s i n numbers c o i n c i d e n t w i t h i n c r e a s e s i n i r o n humates d u r i n g p e r i o d s of h i g h r a i n f a l l .

Another w i d e l y d i s t r i b u t e d i r o n - o x i d i z i n g b a c t e r i u m i s

T h i o b a c i l l u s t h i o o x i d a n s which o x i d i z e s i r o n s u l p h i d e (FeS) t o f e r r i c s u l p h a t e

(ZoBell, 1973) : FeS2

+ 3%02 + H20+FeS04

2FeS04 +

10, +

H2S04

+ H2S04

Fe2(S04)3

+

(6.12) H20

(6.13)

357 A l s o , F e r r o b a c i l l u s f e r r o o x i d a n s , which i s found i n v e r y a c i d w a t e r s (pH<3), and which o x i d i z e s f e r r o u s c a r b o n a t e t o f e r r i c hydroxide: 4FeC03 + O2 + 6H20

4Fe(OH)3 + 4C02

(6.14)

A few h e t e r o t r o p h i c s p e c i e s of i r o n o x i d i z i n g b a c t e r i a of t h e g e n e r a S p h a e r o t i l i s , L e p t o t h r i x , C l o n o t h r i x , and S i d e r o b a c t e r a l s o d e p o s i t o x i d i z e d manganese i n t h e i r sheaths.

Some s p e c i e s o f t h e genus Metallogenium o b t a i n p a r t o f t h e i r e n e r g y

r e q u i r e m e n t s from t h e o x i d a t i o n of manganese compounds; o t h e r s a r e h e t e r o t r o p h i c and a l o n g w i t h o t h e r b a c t e r i a c o n t r i b u t e t o t h e f o r m a t i o n of manganese and i r o n o x i d e s i n l a k e s e d i m e n t s (Oborn, 1964; P e r f i l ' e v and Gabe, 1969; Kuznetsov, 1970). Metallogenium i s one of t h e dominant microorganisms i n v o l v e d i n t h e d e p o s i t i o n of manganese n o d u l e s i n l a k e s (Sokalova, 1961; S o r o k i n , 1970). A g e n e r a l summary of t h e i n o r g a n i c and b a c t e r i a l r e l a t i o n s h i p s i n o l i g o t r o p h i c ( o x i c hypolimnion) and meso- o r e u t r o p h i c l a k e s ( r e d u c e d o r a n o x i c hypolimnion) i s g i v e n i n Fig.6.22.

In o l i g o t r o p h i c l a k e s t h e bottom s e d i m e n t s a r e poor i n o r g a n i c

m a t t e r and t h e sediment s u r f a c e may b e c o v e r e d i n i r o n and manganese o x i d e s which a r e d e p o s i t e d a l o n g w i t h c l a y s , s i l t s and c a r b o n a t e s (Kuznetsov, 1975; MUller e t a l . , 1972).

The f o r m a t i o n of t h e i r o n manganese d e p o s i t s i s n o t s t r a i g h t f o r w a r d .

After

i n i t i a l p r e c i p i t a t i o n on t h e bottom t h e o x i d e s a r e reduced and t h e i r m o b i l i t y i n c r e a s e d through t h e a c t i o n o f Bacterium c i r c u l a n s , B . polymyxa,Desulfovibrio and other bacteria.

D i f f u s i o n t o t h e a e r o b i c upper sediment l a y e r s may t h e n t a k e p l a c e

where f a v o u r a b l e c o n d i t i o n s f o r b i o g e n i c o x i d a t i o n e x i s t .

The i o n s a r e d e p o s i t e d

i n t o iron-manganese c o n c r e t i o n o r l a m i n a t i o n s (Sokolova-Dubinina and D e r j u g i n a 1967, 1 9 6 8 ) . I n e u t r o p h i c l a k e s t h e decomposition of o r g a n i c matter produces s u l p h a t e and phos-

p h a t e from p r o t e i n s , b a c t e r i a of t h e genus P r o t e u s b e i n g i m p o r t a n t p r o t e i n decompo s i n g forms ( B u t l i n , 1953).

In t h e r e d u c i n g c o n d i t i o n s o f t h e bottom d e p o s i t s t h e

a c t i v i t y of s u l p h a t e r e d u c i n g b a c t e r i a such as D e s u l f o v i b r i o and Desulfotomaculum produces abundant hydrogen s u l p h i d e .

Iron sulphides a r e thus p r e c i p i t a t e d i n the

bottom s e d i m e n t s and a r e u l t i m a t e l y s t a b i l i z e d i n t h e form of FeSZ ( p y r i t e o r m a r c a s i t e ) , p r o b a b l y i n a s i m i l a r manner t o t h a t s e e n i n t h e m a r i n e environment. A t t h e same t i m e , p r e c i p i t a t i o n of FeP04 and i r o n and manganese c a r b o n a t e s may

a l s o take place. The d i f f e r e n t m i n e r a l o g i c a l t r a n s f o r m a t i o n s which o c c u r i n t h e bottom sediments of l a k e s and which r e s u l t from d i f f e r e n c e s i n o r g a n i c p r o d u c t i v i t y a r e most i m p o r t a n t i n c o n t r o l l i n g t h e c o l o u r of l a k e s e d i m e n t s .

In e u t r o p h i c l a k e s , most

o f t h e a v a i l a b l e i r o n i s u l t i m a t e l y f i x e d a s i r o n s u l p h i d e s , c a r b o n a t e s and phosphates and t h e c o l o u r o f t h e s e d i m e n t s i s d r a b .

Minor i n t r i n s i c c o l o u r

v a r i a t i o n s due t o l a c u s t r i n e v a r v e s and o t h e r m i n e r a l o g i c a l l a m i n a t i o n s , such as c a r b o n a t e o r o r g a n i c - r i c h l a m i n a e , may b e abundant.

In oligotrophic lakes with

low o r g a n i c p r o d u c t i v i t y i r o n o x i d e s can form d u r i n g e a r l y d i a g e n e s i s ; i f

3 58

Surface run off

Y-lc Outflow Fe, M n

~

i

ZONE

Precipitation and Sedimentation I

without transport to

OLIGOTROPHIC LAKES (Oxic hypolimnion)

MESO-OR EUTROPHIC LAKES (Hypolimnetic O2 reduced or Anoxic)

The i r o n and manganese c y c l e s i n l a k e s of low and h i g h p r o d u c t i v i t y emphasizing m i c r o b i a l i n t e r a c t i o n s ( m o d i f i e d from Kuznetsov, 1970).

Fig.6.22.

359 s u b s e q u e n t c o n d i t i o n s a r e s u i t a b l e t h e n f o r m a t i o n of s t a b l e p i g m e n t a r y o x i d e s w i l l occur during l a t e r diagenesis.

The i m p l i c a t i o n h e r e i s t h a t a n c i e n t l a c u s t r i n e

s e d i m e n t s may o r may n o t b e r e d and t h e c o l o u r v a r i a t i o n s may p r o v i d e a good i n d i c a t i o n o f t h e former l e v e l s o f l a k e p r o d u c t i v i t y ; r e d s e d i m e n t s may i n d i c a t e low l e v e l s of p r o d u c t i v i t y w h i l s t d r a b s e d i m e n t s s u g g e s t r e l a t i v e l y h i g h l e v e l s of l a k e p r o d u c t i v i t y . An a n c i e n t example:

The O r c a d i a n Basin ~

The O r c a d i a n B a s i n was a m a j o r i n t r a m o n t a n e b a s i n d u r i n g t h e d e p o s i t i o n o f t h e Old Red Sandstone and a t h i c k sequence of i n t e r b e d d e d f l u v i a l and l a c u s t r i n e sediments accumulated d u r i n g t h e Middle Old Red S a n d s t o n e s ( E i f e l i a n - G i v e t i a n ) . Remnants of t h e b a s i n are p r e s e r v e d i n n o r t h e a s t e r n S c o t l a n d , t h e Orkneys and the Shetland I s l a n d s (Fig.6.23). chief key ORS outcrop

[wick

@ Achanrrrrr Lit. end correlatives

0 oilfield middle ORS palmocurrants

/ / / wells

4 '

,

Fwlt

I

masibla irnit of )RS

F e l l s Boundary

.*

*

G n a t Glen

C

Bwhm field

0 ria

0

F i g . 6.23.

Palaeogeography of t h e O r c a d i a n Basin.

100 km

360 These m i d d l e Devonian s e d i m e n t s i n t h e O r c a d i a n B a s i n a r e r e f e r r e d t o a s t h e Caithness Flagstones.

The s t r a t i g r a p h y i n n o r t h e a s t e r n C a i t h n e s s (Donovan e t a l . ,

1974) i s s u m a r i z e d i n T a b l e 6.3.

TABLE 6.3. L i t h o s t r a t i g r a p h y of t h e C a i t h n e s s F l a g s t o n e s ( a f t e r Donovan e t a l . ,

John 0 ' G r o a t s Sands t o n e (627m)

1974).

Red and b u f f s a n d s t o n e s w i t h minor amounts of d r a b F l a g s t o n e and laminite - - - - ? - - - - - - - - - - - - - - - - - - - - - - - - -

- - - - ? - - - - -

MeY 553m

Upper C a i t h n e s s Flagstone ( 150 7m+)

Grey and g r e e n F l a g s t o n e s and sands t o n e s . Red s h a l e s and s a n d s t o n e s i n h i g h e r beds

Ham-Scarfskerry 750m

Grey and g r e e n F l a g s t o n e s and laminites. Grey/black laminites with dolostones - - - - ? - - - - - - - - - - - - - - - - - - - - - - - - La t h e r o n 174mi-

Grey s a n d s t o n e s , g r e e n mudstones and grey/black laminites

Robbery Head Lamini tes and t h i n l y bedded Flagstones 15 7m+ - - - - ? - - - - - - - - - - - - - - - - - - - - - - - - -

Lower C a i t h n e s s Flagstone (2336m+)

Lybs t e r 8 70m+

Grey s a n d s t o n e s and F l a g s t o n e s w i t h g r e y / b l a c k l a m i n i t e s . Carbonate l a m i n i t e and d o l o s t o n e

H i l l h e a d Red Bed 159m Clyth 1150m+ Sarclet (434m+)

Red F l a g s t o n e s and s a n d s t o n e s Grey F l a g s t o n e s and s a n d s t o n e s G r e y / b l a c k F l a g s t o n e s and l a m i n i t e s Red f l u v i a t i l e muds t o n e s , s a n d s t o n e s and conglomerates

The lowermost d e p o s i t s a r e r e d f l u v i a t i l e mudstones, s a n d s t o n e s and conglomerates of t h e S a r c l e t Group.

These a r e o v e r l a i n by a p r e d o m i n a n t l y d r a b l a c u s t r i n e

sequence obout 4km t h i c k and e x h i b i t i n g complex f a c i e s v a r i a t i o n s .

The l a c u s t r i n e

sequence c u l m i n a t e s i n a n o t h e r group of p r e d o m i n a n t l y f l u v i a t i l e r e d b e d s , t h e John o 1 G r o a t s Sandstone Group.

The l a c u s t r i n e f l a g s t o n e s u c c e s s i o n i n g e n e r a l

e x h i b i t s p e r s i s t e n t rhythms, i n v o l v i n g l i g h t g r e y , red-brown and g r e y - g r e e n cross-bedded s a n d s t o n e , f l a g g y s a n d s t o n e , and g r e e n i s h - g r e y s i l t s t o n e w i t h greyb l a c k s i l t y l a m i n i t e and i n f r e q u e n t d a r k g r e y l i m e s t o n e (Crampton and C a r r u t h e r s , 1914, p. 90-97).

The fauna and f l o r a of t h e C a i t h n e s s F l a g s t o n e s a r e c h a r a c t e r i s t i c

of a n e n c l o s e d P a l a e o z o i c b a s i n b e i n g r e s t r i c t e d t o f i s h , a l g a e , v e s c u l a r p l a n t

361 f r a g m e n t s , s p o r e s , e u r y p t e r i d s ( P t e r y g o t u s ) and t h e small c o n c h o s t r a c h a n Asmussia murchisoniana.

The s e d i m e n t a r y s t r u c t u r e s which i n c l u d e l a c u s t r i n e - t y p e v a r v e s

(Rayner, 1963) and s y n e r e s i s c r a c k s , (Donovan and F o s t e r , 1 9 7 2 ) a r e f u r t h e r i n d i c a t i o n s o f a l a c u s t r i n e environment. The rhythmic p a t t e r n of s e d i m e n t a t i o n s e e n throughout t h e C a i t h n e s s F l a g s t o n e s o c c u r s a s a s e q u e n t i a l arrangement of a number o f p a r t i c u l a r l i t h o l o g i c a l a s s o c i a t i o n s which have been d e s c r i b e d by Donovan e t e l . ( 1 9 7 4 ) .

I n t h e Lower

C a i t h n e s s F l a g s t o n e Group t h e f o l l o w i n g a s s o c i a t i o n s o c c u r : A. L a m i n i t e s i n which c a l c i t e o r d o l o m i t e and c l a s t i c laminae e a c h 0.1

thick, alternate.

-

1.OOm

Organic d e t r i t u s may b e abundant i n t h e c l a s t i c laminae.

Colour N 4 - N 5 . B. C l a s t i c l a m i n i t e , c o n s i s t i n g o f a l t e r n a t i o n s o f medium-coarse s i l t s t o n e and

f i n e s i l t s t o n e o r mudstone 1.OOmm t h i c k . C. A l t e r n a t i o n s of c o a r s e s i l t s t o n e (N5-N7)

averaging 7 m thick.

Colour N 3 - N 4 . and f i n e s i l t s t o n e ( N 3 - N 4 )

i n pairs

Abundant subaqueous s h r i n k a g e c r a c k s .

D. A l t e r n a t i o n s of c o a r s e s i l t s t o n e ( N 5 - N 7 )

and f i n e s i l t s t o n e i n p a i r s 5 - 3 O m .

Subaqueous and s u b a e r i a l d e s i c c a t i o n c r a c k s common a l o n g w i t h orange-weathering d o l o s t o n e s 5-4Orrrm t h i c k . E . A l t e r n a t i o n s o f c o a r s e s i l t s t o n e s and f i n e s a n d s t o n e s , 5-5Omm t h i c k , w i t h f i n e

s i l t s t o n e s and mudstones o f s i m i l a r t h i c k n e s s . g r e y ) N5-N7

Colour v a r i a b l e , N 3 - N 4 ( d a r k

( l i g h t g r e y ) , g r e y - g r e e n ( s h a d e s o f 5GY) and o c c a s i o n a l l y r e d (GR).

S u b a e r i a l d e s i c c a t i o n c r a c k s a r e common a s a r e subaqueous s h r i n k a g e c r a c k s i n t h e dark grey sediments. F. Grey ( N 5 - N 7 )

f i n e t o medium-grained s a n d s t o n e , upwards o f 5Omm t h i c k , i n t e r -

bedded w i t h t h i n n e r s i l t s t o n e s and mudstones u s u a l l y g r e y , d a r k g r e y o r grey-green i n colour. lamination.

Sandstones l a t e r a l l y p e r s i s t e n t w i t h a b r u p t b a s e s and showing c r o s s F r e q u e n t s u b a e r i a l d e s i c c a t i o n c r a c k s i n mudstones.

G . Red beds ( s h a d e s o f 5 R ) r a n g i n g from f i n e l y l a m i n a t e d mudstones t o l e n s e d medium

grained sandstones.

R i p p l e marks and s u b a e r i a l d e s i c c a t i o n c r a c k s .

Most o f t h e l i t h o l o g i e s p r e s e n t i n t h e Lower C a i t h n e s s F l a g s t o n e Group, w i t h t h e e x c e p t i o n o f B , D and G , c h a r a c t e r i z e t h e Upper C a i t h n e s s F l a g s t o n e Group, t o g e t h e r w i t h some a d d i t i o n a l l i t h o l o g i e s which a r e : H. C a r b o n a t e / c l a s t i c l a m i n i t e s s i m i l a r t o A b u t w i t h amorphous ' c o l l o p h a n i t e ' i n

c l a s t i c laminae and conspicuous p h o s p h a t e - c h e r t n o d u l e s .

Colour N 4 - N 5 .

I . Laminated c a l c a r e o u s f i n e s i l t s t o n e s and mudstones o f d a r k medium g r e y c o l o u r

(N3-N5)

w i t h 50m t h i c k d o l o m i c r i t e s .

S c a t t e r e d f i s h s c a l e s and c o p r o l i t e s

common.

J. Fine-medium c a l c a r e o u s s i l t s t o n e s .

Colour N 3 - N 5 .

K. F i s s i l e f i n e s i l t s t o n e s and mudstones w i t h s u b a e r i a l d e s i c c a t i o n c r a c k s .

Colour N3-N5 o r g r e e n i s h - g r e y ( s h a d e s of 5 G Y ) .

362 L. Massive g r e y and b u f f fine-medium g r a i n e d s a n d s t o n e s 0.10-2.0m

thick,

a l t e r n a t i n g w i t h p a l e g r e e n and ( l e s s commonly) r e d mudstones.

Sandstones

have e r o s i v e b a s e s , c r o s s - b e d d i n g , p a r t i n g l i n e a t i o n and penecontemporaneous deformation s t r u c t u r e s .

Mudstones show s u b a e r i a l d e s i c c a t i o n c r a c k s and y i e l d

p o o r l y p r e s e r v e d p l a n t fragments. The l a c u s t r i n e o r i g i n of t h e C a i t h n e s s F l a g s t o n e has l o n g been r e a l i z e d ( G e i k i e , 1879; Crampton and C a r r u t h e r s , 1914) and i s s u p p o r t e d by more r e c e n t a n a l y s i s by Donovan (1975).

Carbonate cemented b r e c c i a s which o c c u r a t a number

o f l o c a l i t i e s a r e i n t e r p r e t e d a s m a r g i n a l lake-beach d e p o s i t s which o c c u r r e d d u r i n g p e r i o d s of l a k e l e v e l s t a b i l i t y d u r i n g a g e n e r a l l a c u s t r i n e c y c l e of t r a n s g r e s s i o n / regression.

The d a r k g r e y l a m i n i t e s a r e i n t r r p r e t e d a s " n o n - g l a c i a l v a r v e s "

r e s u l t i n g from a s e q u e n t i a l a n n u a l l a c u s t r i n e rhythm (Rayner, 1963; F a n n i n , 1969; Donovan, 1975).

T h i s p r o b a b l y i n v o l v e d s e a s o n a l a l g a l blooms which l e a d t o

i n c r e a s e d p h o t o s y n t h e s i s , a r i s e i n pH and c a r b o n a t e p r e c i p i t a t i o n .

Subsequent

a l g a l d e a t h r e s u l t e d i n a c c u m u l a t i o n of o r g a n i c m a t t e r on t h e l a k e f l o o r .

There

was a l s o c o n t i n u o u s o r p e r i o d i c accumulation o f c l a s t i c d e t r i t u s . During p e r i o d s o f l a m i n i t e d e p o s i t i o n t h e O r c a d i a n Lake was p r o b a b l y s t r a t i f i e d , w i t h a c o o l , dense a n a e r o b i c hypolimnion and a warm p h o t i c a l g a e - s u p p o r t i n g epilimnion.

Such a s t r a t i f i c a t i o n i s s u g g e s t e d by t h e p r e s e r v a t i o n o f u n d i s t u r b e d

f o s s i l f i s h and by t h e d e l i c a t e l a m i n a t i o n .

N o n g l a c i a l v a r v e s of t h e t y p e under

d i s c u s s i o n a r 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 t h e q u i e t hypolimnia of l a k e s whose w a t e r s a r e permanently s t r a t i f i e d , (Anderson and K i r k l a n d , 1960; B r a d l e y , 1973).

In t h i s s i t u a t i o n t h e t h e r m o c l i n e would have e f f e c t i v e l y a c t e d a s a n o r g a n i c m a t t e r f e n c e (Eh=O) below w h i c h . t h e p r o c e s s e s o b s e r v e d i n Recent l a k e s would have o p e r a t e d . These would i n c l u d e t h e b a c t e r i a l decomposition of p r o t e i n s which wuuld produce s u l p h a t e s and p h o s p h a t e s , b a c t e r i a l r e d u c t i o n o f s u l p h a t e , and t h e f o r m a t i o n of i r o n s u l p h i d e s w i t h i n t h e o r g a n i c r i c h sediment.

The abundance o f o r g a n i c c a r b o n

(somestmes more t h a n 2 o r 3%) and a u t h i g e n i c m a r c a s i t e and p y r i t e g r a i n s i n t h e carbonate laminites (Fig.6.24),

a l o n g w i t h t h e less common o c c u r r e n c e o f a u t h i g e n i c

p h o s p h a t e s , t e s t i f i e s t o e a r l y d i a g e n e s i s under r e d u c i n g c o n d i t i o n s and t h e o p e r a t i o n o f s i m i l a r p r o c e s s e s i n t h e C a i t h n e s s F l a g s t o n e s t o t h o s e s e e n i n the hypolimnion o f Recent l a k e s . The complex f a c i e s r e l a t i o n s h i p s s e e n i n t h e C a i t h n e s s F l a g s t o n e s can b e a s s e s s e d i n terms of f i v e l i t h o l o g i c a l a s s o c i a t i o n s :

1) a n uppermost f l u v i a l s a n d s t o n e sequence w i t h a n e r o s i v e o r e x t r e m e l y a b r u p t base

2 ) thinly-bedded l a t e r a l l y p e r s i s t e n t s i l t s t o n e s d e p o s i t e d by l a c u s t r i n e t u r b i d i t y flows

3 ) c a r b o n a t e l a m i n i t e s o f deeper w a t e r l a c u s t r i n e o r i g i n

363 4 ) f l a g s t o n e s and t h e i r m a r g i n a l e q u i v a l e n t s d e p o s i t e d i n s h a l l o w w a t e r of f l u c t u a t i n g depth

5 ) b r e c c i a s and s a n d s t o n e s o f f l u v i a l o r i g i n

A. Thin s e c t i o n photomicrograph of a t y p i c a l c a r b o n a t e l a m i n i t e , t h e Fig.6.24. Achannaras Limestone. B. R e f l e c t e d l i g h t mhotomicrograph showing abundant a u t h i g e n i c p y r i t e and m a r c a s i t e . The f a c i e s changes a r e r e a d i l y i n t e r p r e t e d i n terms o f a l a c u s t r i n e t r a n s g r e s s i o n / regression (Fig.6.25). Following a n i n i t i a l low l a k e l e v e l d u r i n g which f l u v i a l s e d i m e n t a t i o n was dominant and s u b a e r i a l s c r e e s formed a t t h e l a k e m a r g i n , t h e l a k e began t o f i l l and t r a n s g r e s s .

When a s u f f i c i e n t d e p t h was r e a c h e d thermal s t r a t i f i c a t i o n

d e v e l o p e d , o r g a n i c p r o d u c t i o n was h i g h , and most o f t h e a v a i l a b l e i r o n t r a n s p o r t e d a s c o l l o i d a l Fe(OH)3, a d s o r b e d on c l a y m i n e r a l s , and a s o r g a n i c complexes was reduced i n t h e hypolimnion and f i x e d a s i r o n s u l p h i d e s .

A t t h i s t i m e carbonate

p r e c i p i t a t i o n was h i g h and c l a s t i c i n p u t g r e a t l y r e t a r d e d ; g r a d e d laminae s u g g e s t i n g t h a t m i c r o d e n s i t y flows i n t r o d u c e d much of t h e sediment.

764

aerobic early diagenesis

Fig.6.25.

-

increasing influence of microdensity flows producing elastic laminae

S e d i m e n t a t i o n model f o r t h e C a i t h n e s s F l a g s t o n e s ( a f t e r Donovan, 1 9 7 5 ) .

During t h e s u b s e q u e n t l a c u s t r i n e r e g r e s s i o n r i v e r s t r a n s p o r t e d more s e d i m e n t i n t o t h e l a k e and c a r b o n a t e p r e c i p i t a t i o n c e a s e d .

T u r b i d i t y f l o w s were g r e a t e r

i n s i z e and g r a d e d s i l t s t o n e s and mudstones were d e p o s i t e d w h i l s t towards t h e b a s i n c e n t r e s o r t i n g of s e d i m e n t s by l a c u s t r i n e s h o r e p r o c e s s e s would have been e f f e c t i v e . A t t h i s time a r e l a t i v e l y b r o a d a l l u v i a l margin may have been p r e s e n t ; e a r l y

d i a g e n e s i s under o x i d i z i n g c o n d i t i o n s and t h e a b s e n c e o f abundant o r g a n i c m a t t e r would a i d t h e development o f pigmentary o x i d e s .

These c o u l d a l s o form d u r i n g more

p r o l o n g e d d i a g e n e s i s p r o v i d i n g t h a t s u i t a b l e i r o n - b e a r i n g d e t r i t u s was p r e s e n t and o x i d i z i n g c o n d i t i o n s were m a i n t a i n e d .

Red b e d s c o u l d r e a d i l y form under t h e s e

conditions. The i n t e r b e d d i n g o f r e d f l u v i a l and l a k e margin s e d i m e n t s w i t h d r a b l a c u s t r i n e

-

s e d i m e n t s can t h e r e f o r e b e r e a d i l y e x p l a i n e d i n terms o f f l u c t u a t i n g l a k e l e v e l . D e t a i l e d e n v i r o n m e n t a l a n a l y s i s shows t h a t c o l o u r v a r i a t i o n i n t h e C a i t h n e s s F l a g s t o n e s a r e c o n t r o l l e d p r i m a r i l y by t h e c o n d i t i o n s d u r i n g e a r l y d i a g e n e s i s . These c o n d i t i o n s have a v i t a l i n f l u e n c e on l a t e r d i a g e n e s i s by removing a v a i l a b l e

365 i o n s from c i r c u l a t i o n and changing t h e c h e m i s t r y of i n t e r s t i t i a l w a t e r s .

Since

e a r l y d i a g e n e t i c c o n d i t i o n s a r e themselves c o n t r o l l e d by t h e d e p o s i t i o n a l environment i t f o l l o w s t h a t t h e whole spectrum o f p o s t - d e p o s i t i o n a l changes was l i n k e d by a c o n t i n u o u s c h a i n t o t h e e n v i r o n m e n t a l c o n d i t i o n s which p r e v a i l e d during deposition.

I t i s not surprising t o f i n d therefore t h a t red/drab i n t e r -

c a l a t i o n s i n t h e C a i t h n e s s F l a g s t o n e s c o r r e s p o n d s o c l o s e l y t o changes i n d e p o s i t i o n a l environment.

S i m i l a r changes and e x p l a n a t i o n s have been used t o

a c c o u n t f o r o t h e r complex sequences i n v o l v i n g l a c u s t r i n e s e d i m e n t s .

These

i n c l u d e t h e T r i a s s i c Lockatong Formation of t h e Newark Group (Van Houten, 1964, 1965) and t h e Eocene Green R i v e r Formation o f Wyoming ( P i c a r d and High, 1972). POST-DEPOSITIONAL INFLUENCE OF GROUNDWATER ON THE MINERALOGY AND GEOCHEMISTRY OF RED BEDS Reddening by groundwater A number o f a u t h o r s have s u g g e s t e d t h a t t h e m i n e r a l o g y and c h e m i s t r y o f some

r e d bed sequences i s r e l a t e d t o t h e i r p r e s e n t o r p a l a e o groundwater c h e m i s t r y and f l o w s y s t e m ( B e r n e r , 1969a; Horowitz, 1971; S c h l u g e r and Roberson, 1975).

The

most e f f e c t i v e d e m o n s t r a t i o n o f t h e s e r e l a t i o n s h i p s has been made by S c h l u g e r and Roberson (1975) u s i n g much of t h e e a r l i e r i n f o r m a t i o n s u p p l i e d by Mack ( 1 9 6 2 ) , and Barnes and Back (1964) and Back and Barnes (1965).

T h e i r s t u d y c o n c e n t r a t e d on t h e

P a t a p s c o Formation ( C r e t a c e o u s ) of t h e Potomac Group which c o n s i s t s of u n c o n s o l i d a t e d g r a v e l , s a n d , s i l t and c l a y of f l u v i a t i l e and s h a l l o w marine o r i g i n and c r o p s o u t between Washington D . C . ,

B a l t i m o r e , and Annapolis i n e a s t e r n U.S.A.

The p r e s e n t

groundwater s y s t e m i n t h e P a t a p s c o Formation i s t h o u g h t t o b e s i m i l a r t o t h e one which e x i s t e d a t t h e t i m e o f d e p o s i t i o n .

Based on t h e c o l l e c t i o n of 87 o u t c r o p

s a m p l e s , S c h l u g e r and Roberson (1975) made d e t a i l e d a n a l y s i s o f v a r i a t i o n s i n c o l o u r , c l a y m i n e r a l o g y , i r o n o x i d e m i n e r a l o g y and i r o n a n a l y s e s and r e l a t e d t h o s e t o t h e groundwater f l o w s y s t e m ( F i g . 6 . 2 6 ) .

The c o l o u r of t h e P a t a p s c o s e d i m e n t s was

a s s e s s e d by p l o t t i n g isochromes b a s e d on c o l o u r and v a l u e .

Yellowish-red sediments

(YR isochrome) a r e d i s t r i b u t e d mainly a l o n g t h e margins o f t h e b a s i n w h i l s t t h e

l i g h t r e d s e d i m e n t s (LR isochrome) a r e n e a r e r t o t h e c e n t r e of t h e b a s i n .

The

d a r k e s t r e d s e d i m e n t s (DR isochrome) a r e l o c a t e d i n t h e v e r y c e n t r e o f t h e b a s i n . A s i m i l a r d i s t r i b u t i o n i s observed i n t h e c l a y m i n e r a l o g y . 7?./108

Variation i n the

peak h e i g h t r a t i o o f i l l i t e and k a o l i n i t e ( t h e two most abundant c l a y

m i n e r a l s ) , r e v e a l s a t r e n d o f r e l a t i v e i l l i t e c o n c e n t r a t i o n downdip ( i . e . t h e c e n t r e of t h e b a s i n ) f o r b o t h r e d and d r a b s e d i m e n t s ( F i g . 6 . 2 6 ) .

towards

Other

n o t a b l e f e a t u r e s of t h e c l a y m i n e r a l o g y i n c l u d e t h e d i s t r i b u t i o n of mixed l a y e r c l a y s which a r e found i n 11.6% o f t h e r e d s e d i m e n t s and a r e a b s e n t i n t h e d r a b sediments.

366

BALTIMOIII. MD.

ANNAPOLIS. MD.

ANNAPOLIS. MD.

WASHINGTON D.C.

ANNAPOLIS. MD.

Sml.

WASHINGTON D.C.

0

Skl.

Sml 0

5km.

Fig.6.26. Relationships between mineralogy, geochemistry and groundwater flow pattern in the Patapsco Formation. A . Isochrome map showing colour distribution. YR = yellow-red; LR = light red; DR = dark red. B. 72/102 peak height ratio. Solid lines reddish clays; dashed lines drab clays. C. Weight percent Fez03 in whole rock samples. D. Weight percent FeO in whole rock samples. E. Potentiometric surface in feet (after Schluger and Roberson, 1975).

367

A l l s t u d i e d samples c o n t a i n e d h a e m a t i t e a n d / o r g o e t h i t e .

I n a few s a m p l e s ,

l e p i d o c r o c i t e was i d e n t i f i e d , and based on t h e amounts o f e x t r a c t a b l e f r e e i r o n p r e s e n t S c h l u g e r and Roberson (1975) judged t h a t a s i g n i f i c a n t p r o p o r t i o n of amorphous o r p o o r l y o r d e r e d f e r r i c oxyhydroxides must b e p r e s e n t .

No s i m p l e

p a t t e r n i n i r o n o x i d e d i s t r i b u t i o n emerged b u t X-ray e v i d e n c e s u g g e s t s t h a t h a e m a t i t e i s r e l a t i v e l y e n r i c h e d i n t h e a r e a o f t h e DR isochrome.

Iron analyses

r e v e a l e d a c l o s e r e l a t i o n s h i p w i t h t h e o t h e r m i n e r a l o g i c a l v a r i a b l e s , b o t h Fe3+ ( e x p r e s s e d a s Fe 0 ) and Fez+ ( e x p r e s s e d a s FeO) d e c r e a s i n g towards t h e c e n t r e of 2 3 the basin (Fig.6.26). The P a t a p s c o Formation l i e s w i t h i n t h e humid t e m p e r a t e c l i m a t i c b e l t o f t h e United S t a t e s w i t h a mean a n n u a l p r e c i p i t a t i o n o f 111.7cm.

There i s n e g l i g i b l e

w a t e r e n t e r i n g t h e f o r m a t i o n by s t r e a m flow b e c a u s e t h e a q u i f e r i s f u l l and continuously discharging i n t o streams.

The p o t e n t i o m e t r i c s u r f a c e i s h i g h e s t i n

t h e w e s t e r n o u t c r o p a r e a and d e c r e a s e s towards t h e s o u t h e a s t (Fig.6.26) 1962).

(Mack,

Back and Barnes (1965) showed a g e n e r a l d e c r e a s e i n Eh from a h i g h of

700mV i n r e c h a r g e a r e a s t o 145mV i n a r e a s o f d i s c h a r g e t o t h e s o u t h - e a s t .

These

a u t h o r s a l s o showed a n e g a t i v e c o r r e l a t i o n between Eh and groundwater i r o n c o n t e n t ; Fe2+ r a n g i n g from 0 ppm t o 20 ppm, l o w e s t Eh and h i g h e s t i r o n c o n t e n t o c c u r r i n g i n water a s s o c i a t e d w i t h drab sediments i n the south-east of the area. The ground w a t e r f l o w p a t t e r n and g e o c h e m i s t r y t h u s show a remarkable p a r a l l e l i s m w i t h t h e c o l o u r , c l a y m i n e r a l o g y , and i r o n o x i d e c o n t e n t of t h e P a t a p s c o Formation.

S c h l u g e r and Roberson (1975) e n v i s a g e d t h e f o r m a t i o n of

v e r m i c u l i t e and mixed-layer c l a y m i n e r a l s by t h e d i a g e n e t i c a l t e r a t i o n of b i o t i t e and f e r r o m a g n e s i a n s i l i c a t e s i n t h e n o r t h - e a s t e r n p a r t o f t h e a r e a .

Associated

w i t h t h i s was t h e d i s s o l u t i o n o f i r o n which m i g r a t e d s o u t h e a s t w a r d s w i t h t h e ground w a t e r flow u n t i l d i s s o l v e d i r o n e n t e r e d t h e s t a b i l i t y f i e l d o f f e r r i c oxyhydroxides which were p r e c i p i t a t e d presumably as amorphous Fe(OH)3 i n i t i a l l y , and which l a t e r aged i n t o l e p i d o c r o c i t e , g o e t h i t e and h a e m a t i t e .

The s t u d y t h u s p r o v i d e s a

c o n v i n c i n g d e m o n s t r a t i o n of t h e i n t e r a c t i o n between r e d bed f o r m a t i o n s and groundw a t e r g e o c h e m i s t r y and f l o w p a t t e r n . Secondary r e d u c t i o n zones A c h a r a c t e r i s t i c f e a t u r e of many c o n t i n e n t a l r e d bed sequences i s t h e

o c c u r r e n c e of g r e y , g r e e n o r w h i t e m o t t l e d zones w i t h i n a predominantly r e d succession.

Such zones f r e q u e n t l y t r a n s e c t d e p o s i t i o n a l b o u n d a r i e s and t h e y a r e

t h e r e f o r e g e n e r a l l y c o n s i d e r e d t o b e p o s t - d e p o s i t i o n a l o r secondary r e d u c t i o n zones.

White a r e a s of s a n d s t o n e a r e known t o r e s u l t from t h e l e a c h i n g of i r o n

(Hartmann, 1963) w h i l s t g r e y o r g r e e n c o l o u r c a n b e a t t r i b u t e d t o s u l p h i d e s o r c l a y m i n e r a l s (mainly c h l o r i t e ) which form under r e d u c i n g c o n d i t i o n s . b a s i c t y p e s can be r e c o g n i z e d :

Three

368 1. I s o l a t e d s p h e r i c a l o r i r r e g u l a r l y shaped zones u s u a l l y r e f e r r e d t o a s r e d u c t i o n spots o r reduction mottling.

T r a d i t i o n a l l y t h e s e a r e t h o u g h t t o r e s u l t from

e a r l y 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 of f e r r i c oxyhydroxide by r e s i d u a l o r g a n i c m a t e r i a l ( S h e r l o c k , 1947; T r o t t e r , 1953) 2. H o r i z o n t a l , more o r less c o n t i n u o u s w h i t e o r g r e e n h o r i z o n s , which show some r e l a t i o n s h i p t o l i t h o l o g i c a l b o u n d a r i e s and o c c u r r i n g i n s a n d s t o n e s of f i n e to coarse grain size.

These zones a r e r e l a t e d t o t h e m i g r a t i o n o f r e d u c i n g

ground w a t e r 3. Large z o n e s , c o m p r i s i n g whole s t r a t i g r a p h i c a l u n i t s and p o s s i b l y up t o a few hundred m e t r e s i n t h i c k n e s s .

The l o n g - c o n t i n u e d a c t i v i t y o f r e d u c i n g ground

w a t e r s i s t h e l i k e l y c a u s e o f s u c h l a r g e s c a l e r e d u c t i o n zones I n a d d i t i o n , v e r t i c a l p i p e s of s e c o n d a r i l y reduced s e d i m e n t a r e commonly found r e l a t e d t o p r e s e n t - d a y l a n d s u r f a c e s and t h e s e a r e formed by r e a c t i o n w i t h r e d u c i n g s o i l w a t e r moving around r o o t s o r down f i s s u r e s (Downing and S q u i r r e l , 1963). When t h e i r o n c o n t e n t o f r e d beds i s compared w i t h t h a t of a s s o c i a t e d , s e c o n d a r i l y reduced d r a b z o n e s , t h e l a t t e r a r e always c o n s i d e r a b l y d e p l e t e d i n i r o n . Reduction s p o t s a r e u s u a l l y i r r e g u l a r l y - s h a p e d t o s p h e r i c a l b o d i e s of w h i t e o r g r e e n i s h d r a b s e d i m e n t up t o a few c e n t i m e t r e s i n d i a m e t e r and o f t e n w i t h a d a r k speck o r " f i s h - e y e " of o r g a n i c m a t e r i a l i n t h e c e n t r e ( F i g . 6 . 2 7 ) .

They f r e q u e n t l y

c o n t a i n more calcium c a r b o n a t e and l e s s c l a y m a t r i x t h a n t h e i r h o s t r e d sediment. The t o t a l i r o n c o n t e n t of r e d u c t i o n s p o t s i s l e s s t h a n t h a t i n t h e s u r r o u n d i n g r e d sediment ( K e l l e r , 1929; P i c a r d , 1965).

F r e q u e n t l y t h e f e r r o u s i r o n l e v e l s are

s i m i l a r , o r s l i g h t l y g r e a t e r i n t h e r e d u c t i o n s p o t , and t h e main d i f f e r e n c e l i e s i n t h e g r e a t e r f e r r i c i r o n c o n t e n t o f t h e h o s t r e d sediment ( T a b l e 6 . 4 ) .

This i s

normally a t t r i b u t e d t o i n s i t u r e d u c t i o n o f Fe3+ t o Fe2+ w i t h a s s o c i a t e d d i s s o l u t i o n of pigmentary o x i d e s .

One of t h e most remarkable f e a t u r e s o f r e d u c t i o n s p o t s and

zones i s t h e o c c u r r e n c e o f uranium, vanadium, and copper c o n c e n t r a t i o n s w i t h i n t h e i r organic-rich nuclei.

These a r e well-known i n t h e Permo-Triasic r e d beds o f

b o t h t h e United S t a t e s ( F i s c h e r and S t e w a r t , 1 9 6 1 ) , p a r t i c u l a r l y i n Utah and t h e Colorado P l a t e a u , and a l s o i n G r e a t B r i t a i n ( H a r r i s o n , 1975) and Canada (Van de P o l l and S u t h e r l a n d , 1976).

Various mechanisms have been s u g g e s t e d f o r t h e

formation of these metal-enriched reduction s p o t s , including t h e a c t i v i t y of H S 2 and s u l p h a t e - r e d u c i n g b a c t e r i a , and a l s o t h e e n r i c h m e n t o f s u l p h a t e i o n s i n groundwaters.

The common a s s o c i a t i o n o f o r g a n i c m a t e r i a l s , m e t a l e n r i c h m e n t ,

and Fe3+ r e d u c t i o n , s u g g e s t s t h a t o r g a n i c m a t t e r p l a y s a n i m p o r t a n t r o l e i n t h e formation of reduction spots.

There i s no e v i d e n c e t h a t r a d i o a c t i v e e l e m e n t s

have a n y t h i n g t o do w i t h Fe3+ r e d u c t i o n ; on t h e c o n t r a r y H e i n r i c h (1958) found t h a t r a d i a t i o n o x i d i z e d f e r r o u s i r o n . One of t h e most f u l l y documented o c c u r r e n c e s o f r e d u c t i o n s p o t s i s i n t h e Permo-Triassic r e d bed sequence o f South Devon, U . K .

Here, t h e r e d u c t i o n s p o t s

369

Fig.6.27. Reduction zones i n r e d beds. A. Large r e d u c t i o n s p o t i n r e d s i l t s t o n e s from t h e Permo-Trias of South Devon w i t h a m i n e r a l i z e d c o r e ( ' f i s h e y e ' ) c o n t a i n i n g o r g a n i c matter, copper s u l p h i d e s and uranium and vanadium m i n e r a l s . The lens cap i s 5.3cm i n d i a m e t e r . B . Reduction zone a t t h e t o p of a fining-upwards cyclothem from t h e Old Red Sandstone of South Wales. Dark r e d mudstones ( f l o o d p l a i n ) a r e o v e r l a i n by grey-green s a n d s t o n e s ( c h a n n e l ) t h e j u n c t i o n b e i n g marked by a t h i n bleached zone a t t h e top of t h e mudstones. The s c a l e i s 0.5m. C . Thin (5-10cm) d i s c o n t i n u o u s r e d u c t i o n zones i n r e d sandy a l l u v i u m o f t h e S t . Bees Sandstone o f Cumbria.

370 a r e found i n a sequence of r e d mudstones and t h i n s a n d s t o n e s ( L i t t l e h a m Mudstones and Exmouth Sandstones and Mudstones) o f p r o b a b l e Upper Permian a g e .

Details a r e

p r o v i d e d by P e r u t z (19391, H a r r i s o n ( 1 9 7 5 ) , and Durrance e t a l . , 1978.

These

r e d u c t i o n s p o t s c o n t a i n c o p p e r , vanadium, and uranium i n c o n c r e t i o n a r y c o n c e n t r a t i o n s and may have formed i n a manner v e r y s i m i l a r t o most o t h e r r e d u c t i o n s p o t s .

TABLE 6.4.

The i r o n c o n t e n t o f s e c o n d a r y r e d u c t i o n zones and h o s t r e d s e d i m e n t s . 1. Chugwater Formation ( P i c a r d , 1965) 2. L i t t l e h a m Mudstone Formation (Durrance e t a l . , 1978) 3,4. S t . Bees Sandstone 5. J u n i a t a and Bald E a g l e Formations (Thompson, 1 9 7 0 ) . ~~

Reduction S p o t s

1 2

Drab Sediment

Fe203 F eO Fe203 FeO

Red Sediment

0.97

1.67

0.93

0.93

1.45

4.77

0.96

0.86

0.75

2.26

0.88

0.61

Reduction Zones

3 4

Fe203 F eO Fe203 FeO

0.73

2.05

0.92

0.42

0.50

1.71

1.44

1.43

Large s c a l e r e d u c t i o n zones

5

Fe203 F eO

The m e t a l e n r i c h e d c o n c r e t i o n s have b e e n c l a s s i f i e d by P e r u t z (1939, p.150-151) as: I. i r r e g u l a r

11. s p h e r o i d a l - a s t e r o i d a l , and 111. s p h e r o i d a l - c o n c e n t r i c .

The

s i z e dimensions of t h e c o n c r e t i o n s and t h e s u r r o u n d i n g r e d u c t i o n h a l o e s a r e summarized i n Fig.6.28.

There a p p e a r s t o b e no r e l a t i o n s h i p between s i z e o r

morphology of t h e c o n c r e t i o n s and t h e h o s t r o c k l i t h o l o g y b e c a u s e a l l t h e v a r i a n t s a r e s e e n b o t h i n impermeable m a r l s and porous s a n d s t o n e s .

The c o l o u r of t h e Host

m a r l s r a n g e s from 5YR 6 / 4 ( l i g h t - b r o w n ) t o 5 R 4 / 2 ( g r e y i s h - r e d ) and c o n t r a s t s w i t h the reduction haloes 5Y 6 / 1 ( l i g h t o l i v e grey) t o 5GY 6 / 1 ( g r e e n i s h g r e y ) .

The

r e l a t i o n s h i p s o f t h e b l e a c h e d zones t o bedding p l a n e s c l e a r l y i n d i c a t e t h e i r p o s t d e p o s i t i o n a l f o r m a t i o n s ; a l s o t h e tendency f o r c o n c r e t i o n s t o have l o n g a x e s l y i n g p a r a l l e l t o bedding p l a n e s s u g g e s t s t h a t they might have formed p r i o r t o f i n a l compaction. The t e x t u r e and c o m p o s i t i o n of t h e c o n c r e t i o n s i s h i g h l y complex a l t h o u g h t h e y g e n e r a l l y a r e b l a c k , d a r k g r e y , brown o r o l i v e when f r e s h l y f r a c t u r e d and show a n

371 annular structure.

M i n e r a l s i d e n t i f i e d by H a r r i s o n (1975) i n c l u d e : n a t i v e c o p p e r ,

m a u c h e r i t e (Ni A s ) , n a t i v e s i l v e r , n i c c o l i t e (NiAs), r a m m e l s b e r g i t e (Ni,Co,Fe)As 3 2 2’ c h a l c o p y r i t e (Cu,FeS ) , c h a l c o c i t e (Cu2S), c o v e l l i n e (CuS) , b o r n i t e (Cu5FeS4) ,

2

v a n a d i a n m i c a , c o f f i n i t e (USiO ) and v a r i o u s secondary m i n e r a l s .

4

Radioactive

m i n e r a l s i n t h e c o n c r e t i o n s have been known f o r some time ( C a r t e r , 1931).

Perutz

(1939) t h o u g h t uranium complexed w i t h vanadium perhaps a s c a l c i o c a r n o t i t e (CaO, U03, V203, nH20) o r r a u v i t e the radioactivity.

Ca(U02) V12 OZ3 20H 0 ) was mainly r e s p o n s i b l e f o r 2 H a r r i s o n (1975) showed by a l p h a - t r a c k measurements t h a t t h e

main e n e r g i e s a r e due t o decay p r o d u c t s o f t h e uranium-238 s e r i e s : 238U, 218Po, *I8Rn, 214Bi,

226Ra,

214P0, 210Po and i d e n t i f i e d t h e u r a n y l s i l i c a t e c o f f i n i t e .

T h i s p r o b a b l y a c c o u n t s f o r some of t h e r a d i o a c t i v i t y of t h e c o n c r e t i o n s , o t h e r p o s s i b l e c o n t r i b u t i o n s coming from o t h e r m i n e r a l s s u c h a s v a n a d i a n mica. compounds i n c l u d i n g CH

Organic

and p o s s i b l y s e c o n d a r y amide, k e t o n e , a c i d o r e s t e r

2 c a r b o n y l were i d e n t i f i e d i n t h e c o n c r e t i o n s u s i n g i n f r a - r e d s p e c t r o s c o p y , a l t h o u g h

o n l y i n v e r y s m a l l amounts.

71

I 1 2 3 4 5 R,(cm)

Fig.6.28. R a d i i of c o n c r e t i o n s (Rn) v e r s u s r a d i i of t h e i r r e s p e c t i v e h a l o e s (Rh) i n r e d u c t i o n s p o t s from t h e Permian of South Devon ( a f t e r H a r r i s o n , 1975).

372 A d e p o s i t i o n a l c o n t r o l i s e v i d e n t b e c a u s e of t h e c l o s e a s s o c i a t i o n o f t h e

r e d u c t i o n s p o t s w i t h porous s a n d s t o n e h o r i z o n s and s a n d s t o n e dykes.

Percolating

f l u i d s , p r o b a b l y i n a r e d u c i n g c o n d i t i o n were r e s t r i c t e d i n low p e r m e a b i l i t y s i l t s t o n e s where s h a r p r e d / d r a b c o n t a c t s a r e s e e n ; i n more porous u n i t s t h e i r r e d u c i n g e f f e c t s a r e more w i d e l y s e e n .

These r e d u c i n g groundwaters were p r o b a b l y

i n operation s h o r t l y a f t e r deposition.

S t r o n g l y r e d u c i n g f o c i were p r o b a b l y

produced i n a r e a s o f r e l a t i v e l y h i g h o r g a n i c m a t t e r c o n c e n t r a t i o n and h e r e Fe3+ was reduced t o t h e more s o l u b l e Fez+ and removed i n s o l u t i o n , p o s s i b l y i n p a r t i n o r g a n i c complexes.

A t t h e same t i m e o r g a n i c c a t a l y s t s may have f a c i l i t a t e d t h e p r e c i p i t a t i o n

of m e t a l s u l p h a t e s , a r s e n a t e s and c a r b o n a t e s a s s u l p h i d e s , a r s e n i d e s a n d n a t i v e metals i n concretions.

A l s o t h e r e d u c t i o n of V5+

t o t h e less s o l u b l e V4+ o r V3+

(Hartmann, 1963) would f a c i l i t a t e i t s p r e c i p i t a t i o n w i t h i n t h e c o n c r e t i o n a r y zone. C o n c e n t r i c z o n a t i o n i n c o n c r e t i o n s s u g g e s t s t h a t some d i f f u s i o n mechanism, outward from t h e n u c l e u s , was r e s p o n s i b l e f o r t h e r e d u c t i o n s p o t s and t h a t t h e r e was a n outward i n c r e a s e i n Eh and pH.

A t same p o i n t t h i s r e a c t i o n became n e u t r a l i z e d by

t h e h o s t sediment t h u s marking t h e r e d / d r a b r e d u c t i o n s p o t i n t e r f a c e . S t u d i e s o f t h e r e d u c t i o n s p o t s u s i n g Mbssbauer s p e c t r o s c o p y (Durrance e t a l . ,

1 9 7 8 ) and chemical a n a l y s e s shows t h a t t h e h o s t r e d mudstones c o n t a i n a n a v e r a g e t o t a l i r o n c o n t e n t o f 5.5%, w i t h a n Fe2+/Fe3+ r a t i o o f 0.16 w h i l s t t h e r e d u c t i o n s p o t s have a n a v e r a g e t o t a l i r o n c o n t e n t o f 2.7% w i t h a n Fe

2+

/Fe3+ r a t i o o f 1.2.

Haematite i s p r e s e n t a s g r a i n c o a t i n g s and a l s o w i t h i n s i l i c a t e ( c l a y ) l a t t i c e s i n t h e r e d mudstone b u t i s a b s e n t from t h e r e d u c t i o n s p o t s .

Here i r o n i s p r e s e n t o n l y

w i t h i n c l a y m i n e r a l l a t t i c e s ; t h e d i f f e r e n c e , however, i s n o t simply due t o t h e f a c t t h a t t h e r e d mudstone c o n t a i n s h a e m a t i t e b u t i s a l s o b e c a u s e a b o u t 50% o f t h e i r o n i n c l a y m i n e r a l l a t t i c e s i n t h e r e d u c t i o n s p o t s i s o x i d i z e d t o Fe3+ i n t h e r e d mudstone.

T h i s change i n o x i d a t i o n s t a t e does n o t c o r r e s p o n d w i t h t h e r e d / d r a b

boundary b u t b e g i n s w i t h i n t h e r e d u c t i o n s p o t .

Quite c l e a r l y , the implication i s

t h a t t h e r e d u c t i o n s p o t s a r e a r e a s i n which r e d d e n i n g has n e v e r t a k e n p l a c e .

i s c o n s i s t e n t w i t h t h e p r e s e n c e of o r g a n i c m a t t e r i n t h e r e d u c t i o n s p o t s .

This

This

would c r e a t e r e d u c i n g c o n d i t i o n s d u r i n g b u r i a l and f a c i l i t a t e t h e r e d u c t i o n and removal of r e a d i l y a v a i l a b l e i r o n i n s o l u t i o n . However t h e l a r g e d i f f e r e n c e i n t o t a l i r o n c o n t e n t between t h e r e d u c t i o n s p o t s and h o s t s e d i m e n t s s u g g e s t s t h a t s u b s t a n t i a l amounts o f i r o n have been removed from t h e reduced a r e a .

S i n c e i t seems l i k e l y t h a t t h e r e d u c t i o n s p o t s have n e v e r been

p o s t - d e p o s i t i o n a l l y reddened i t f o l l o w s t h a t a s u b s t a n t i a l p r o p o r t i o n o f t h e i r o n o x i d e i n t h e s e mudstones must have been of d e t r i t a l o r i g i n , p r o b a b l y i n t h e form of g o e t h i t e and amorphous Fe(0H)

3' Haematite f o r m a t i o n may have been i n h i b i t e d i n t h e p r e s e n c e of o r g a n i c m a t t e r ,

a s s u g g e s t e d by Schwertmann (1965) and t h e subsequent r e d u c t i o n o f Fe3+ would b e made t h a t much e a s i e r s i n c e g o e t h i t e i s t h e more s o l u b l e m i n e r a l .

The e v i d e n c e

i n d i c a t e s , t h e r e f o r e , t h a t t h e s e r e d u c t i o n s p o t s were formed a t a r e l a t i v e l y e a r l y s t a g e of d i a g e n e s i s and must a t l e a s t have been i n i t i a t e d p r i o r t o reddening.

A

s i m i l a r o r i g i n f o r o t h e r r e d u c t i o n s p o t s seems l i k e l y , e s p e c i a l l y where t h e y a r e d i r e c t l y associated with nuclei of organic matter. L a r g e r s c a l e r e d u c t i o n zones p r o b a b l y have a s i m i l a r o r i g i n .

They f r e q u e n t l y

show a c l o s e a s s o c i a t i o n w i t h permeable/impermeable b o u n d a r i e s e . g . b l e a c h e d zones a l o n g t h e upper o r lower margins o f permeable s a n d s t o n e s which a r e bounded by impermeable s i l t s t o n e s o r c l a y s t o n e s ( e . g . P i c a r d , 1965; F r i e n d , 1966).

Laterally

d i s c o n t i n u o u s r e d u c t i o n zones o f t h i s t y p e have been s t u d i e d i n t h e S t . Bees Sandstone, a Lower T r i a s s i c f l u v i a t i l e r e d bed sequence which c r o p s o u t i n Cumbria,

U.K.

Here t h e zones are up t o Im wide and u s u a l l y show s h a r p c o n t a c t s w i t h t h e

s u r r o u n d i n g r e d sediment.

Sometimes t h e c o n t a c t s are r a t h e r d i f f u s e due t o secondary

r e m o b i l i z a t i o n of pigmentary o x i d e s .

F i g . 6 . 2 4 shows t h e t y p i c a l o c c u r r e n c e of t h e

d r a b zones a t t h e b o u n d a r i e s of t a b u l a r u n i t s of cross-bedded s a n d s t o n e where t h i n impermeable r e d s i l t s t o n e s and s h a l e s o c c u r . a n a l y s e s o f red-drab p a i r s of s a n d s t o n e .

Table 6 . 4 shows t h e r e s u l t s of i r o n

The d r a b s a n d s t o n e s c o n s i s t e n t l y show

d e p l e t i o n of Fe3+ c o n t e n t and i n t h e m a j o r i t y o f c a s e s a n e n r i c h m e n t o f Fe

2+

.

Heavy m i n e r a l a n a l y s e s i n d i c a t e t h a t t h e d r a b zones a r e d e p l e t e d n o t o n l y i n pigmentary h a e m a t i t e , b u t a l s o i n b l a c k , c o a r s e p a r t i c l e s of h a e m a t i t e and i n s i t u r e d u c t i o n and d i s s o l u t i o n h a s c l e a r l y been a n i m p o r t a n t p r o c e s s .

There a r e no

i n d i c a t i o n s o f o r g a n i c m a t t e r c o n c e n t r a t i o n i n t h e d r a b zones n o r any e v i d e n c e of sulphide o r other mineralization.

The e n r i c h m e n t o f Fe2+ may have been brought

a b o u t b e c a u s e reduced i r o n was n o t removed i n s o l u t i o n b u t became t r a p p e d i n l o c a l p o r e w a t e r s t o become a d s o r b e d on e x i s t i n g c l a y m i n e r a l s o r i n c o r p o r a t e d i n t o authigenic c l a y minerals.

The morphology o f t h e r e d u c t i o n zones i s c o n s i s t e n t w i t h

t h e i d e a t h a t s t a g n a n t , r e d u c i n g ground w a t e r s were c o n c e n t r a t e d a t permeable/ impermeable b o u n d a r i e s . Some reduced zones i n t h e Permo-Trias o f England a r e s p a t i a l l y r e l a t e d t o s t r a t a b o u n d copper m i n e r a l i z a t i o n and i t i s c l e a r t h a t reduced ground w a t e r s w i t h m i n e r a l i z i n g c a p a b i l i t y were o p e r a t i v e i n whole s t r a t i g r a p h i c a l u n i t s and n o t j u s t a t concretionary concentrations.

Perhaps t h e best-known examples o f t h i s i n B r i t a i n

a r e t h e r e d u c t i o n and m i n e r a l i z a t i o n which o c c u r s w i t h i n t h e B u n t e r and b a s a l Keuper s a n d s t o n e s .

I n t h e b a s a l Keuper s a n d s t o n e s a t A l d e r l e y Edge (Dewey and

Eastwood, 1925; Warrington, 1965) f a u l t e d s a n d s t o n e s and c o n g l o m e r a t i c s a n d s t o n e s which are m i n e r a l i z e d a r e markedly g r e y , y e l l o w o r w h i t e i n c o l o u r when compared 'with t h e n o n - m i n e r a l i z e d normal reddish-brown s a n d s t o n e s .

There i s a d i s t i n c t

r e l a t i o n s h i p between t h e b l e a c h e d and m i n e r a l i z e d s a n d s t o n e s and t h e p o r o s i t y / permeability r e l a t i o n s h i p s of t h e sediments.

The m i n e r a l i z e d s a n d s t o n e s c o n t a i n

t h e g r e a t e s t c o n c e n t r a t i o n s o f m a l a c h i t e , a z u r i t e , b a r y t e , g a l e n a , pyromorphite and a n g l e s i t e a t permeable/impermeable i n t e r f a c e s .

The p r e s e n c e o f s u l p h i d e s

374 a l o n g j o i n t s and p a r a l l e l t o bedding p l a n e s i s a c l e a r i n d i c a t i o n of t h e a c t i v i t y of r e d u c i n g ground w a t e r i n permeable h o r i z o n s .

A mechanism o f m i n e r a l i z a t i o n

s i m i l a r t o t h a t a t Budleigh S a l t e r t o n i s e n v i s a g e d a l t h o u g h t h e o c c u r r e n c e of o r g a n i c m a t t e r has n o t been documented a t A l d e r l e y Edge. The a s s o c i a t i o n of m i n e r a l i z e d a n d s e c o n d a r i l y reduced r e d beds i s a w i d e s p r e a d phenomenon and c e r t a i n l y n o t c o i n c i d e n t a l .

In p a r t i c u l a r t h e o c c u r r e n c e o f copper

i s a c h a r a c t e r i s t i c f e a t u r e o f r e d beds ( E r n s t , 1970) and a summary o f t h e

d i s t r i b u t i o n of red-bed c o p p e r d e p o s i t s i s g i v e n by Lindgren (1933, p. 403-409) and B a s t i n (1933).

Examples o c c u r i n Oklahoma, New Mexico, Utah ( B a s t i n , 1933)

Pennsylvania ( B u t l e r , 1938; McCauley, 1961), Nova S c o t i a ( P a p e n f u s , 1931) i n B o l i v i a a t Corocoro (Ljunggren and Meyer, 1964) i n R u s s i a (Bakun e t a l . 1964; Narkelyan e t al.,

1969, 1971; F e o k t i s o v and Kochin, 1972).

Rose (1976) h a s d i s c u s s e d t h e

o r i g i n of r e d bed copper d e p o s i t s and concluded t h a t t h e y a r e r e l a t e d t o e v a p o r i t e s which p r o v i d e d a s o u r c e of b r i n e s .

Copper i s r e l a t i v e l y i n s o l u b l e i n normal f r e s h

-

w a t e r s b u t i t may form s o l u b l e cuprous c h l o r i d e complexes (CuC12- and CuC12-) i n c h l o r i d e b r i n e s of i n t e r m e d i a t e o x i d a t i o n s t a t e a t low t e m p e r a t u r e s .

3

Because of

t h e s o l u b i l i t y , c h l o r i d e r i c h b r i n e s c o u l d flow f r e e l y through permeable r e d beds and l e a c h i n g of t h e h o s t rock c o u l d p r o v i d e a n a d e q u a t e s o u r c e of copper; Rose and Suhr (1971) and Rose (1976) have shown t h a t t h e d e t r i t a l i r o n o x i d e component i n Recent s t r e a m s e d i m e n t s may c o n t a i n 100-1000 ppm copper.

P r e c i p i t a t i o n of copper

under t h e s e c o n d i t i o n s would o c c u r when t h e ground w a t e r e n c o u n t e r e d a r e d u c i n g zone e i t h e r by s u l p h a t e r e d u c t i o n o r r e p l a c e m e n t o f p y r i t e .

The a s s o c i a t i o n w i t h

e v a p o r i t e s and ground w a t e r of s u i t a b l e c o m p o s i t i o n t h u s make r e d b e d s i d e a l h o s t s f o r m i n e r a l i z a t i o n of t h i s (Moench and S c h l e e , 1967; Moghul, 1974; Morton, 1974).

In a number of i n s t a n c e s s e c o n d a r y r e d u c t i o n o f r e d s e d i m e n t s has o c c u r r e d o v e r a s u f f i c i e n t l y wide a r e a and w i t h i n s u c h w e l l - d e f i n e d l i t h o l o g i c a l b o u n d a r i e s t h a t t h e r e d u c t i o n zone has been a t t r i b u t e d some s t r a t i g r a p h i c a l s i g n i f i c a n c e . Thompson (1970) made a d e t a i l e d s t u d y o f s u c h a c o l o u r boundary between t h e Upper O r d o v i c i a n J u n i a t a a n d Bald E a g l e Formations i n c e n t r a l P e n n s y l v a n i a U . S . A . t h e J u n i a t a Formation i s p r i m a r i l y r e d i n c o l o u r (5R 3/2 t o 10R 6/2) and o v e r l i e s t h e d r a b , grey-green

(5Y 5/2 t o 10 Y/2) Bald E a g l e Formation.

The d i s t i n c t i o n

between t h e s e two f o r m a t i o n s i s b a s e d s o l e l y on c o l o u r d i f f e r e n c e d e s p i t e t h e f a c t t h a t t h e c o l o u r boundary t r a n s g r e s s e s m a j o r l i t h o f a c i e s b o u n d a r i e s and v a r i e s more than 700 f e e t i n i t s v e r t i c a l p o s i t i o n (Fig.6.29).

Thompson has p r e s e n t e d a

v a r i e t y o f e v i d e n c e which i n d i c a t e s t h a t t h e d r a b c o l o u r of t h e Bald E a g l e Formation

i s a secondary f e a t u r e produced by aqueous r e d u c t i o n and d i s s o l u t i o n o f p r e v i o u s l y a l l - r e d sediment by c i r c u l a t i n g ground w a t e r s .

P e r t i n e n t l i n e s of e v i d e n c e i n c l u d e :

1. T e x t u r a l r e l a t i o n s h i p s s u g g e s t t h a t t h e d r a b c o l o u r a t i o n i s l a t e r t h a n t h e r e d colouration.

The main r e d pigments a r e p a r t i c u l a t e B r a i n c o a t i n g s o f h a e m a t i t e

which o c c u r c o n s i s t e n t l y a t g r a i n - t o - g r a i n

contacts suggesting the occurrence of

375 a predepositional haematite precursor.

The d r a b pigment c o n s i s t s o f t h e c l a y

m i n e r a l s i l l i t e and c h l o r i t e which a l s o o c c u r a s g r a i n c o a t i n g s and m a t r i x . Some c h l o r i t e o c c u r s between f i r s t and second s t a g e cements c l e a r l y i n d i c a t i n g a diagenetic origin. The opaque o x i d e s of r e d and d r a b s a n d s t o n e s a r e s i m i l a r i n c o m p o s i t i o n c o n s i s t i n g m a i n l y of h a e m a t i t e and h a e m a t i t e - i l m e n i t e i n t e r g r o w t h s b u t t h e t o t a l c o n t e n t of opaque o x i d e s i s much less i n t h e d r a b beds.

The d r a b beds

a l s o c o n t a i n s m a l l amounts of d i a g e n e t i c p y r i t e . I r o n a n a l y s e s show t h a t t h e d r a b beds a r e d e p l e t e d i n t o t a l i r o n and f e r r i c iron.

Average c o m p o s i t i o n s a r e i n d i c a t e d i n T a b l e 6.4.

The composition o f c h l o r i t e i n r e d and d r a b beds i s d i f f e r e n t .

Using t h e

methods of Brown (1955) and Schoen (1962) Thompson compared t h e o c t a h e d r a l c a t i o n composition o f c h l o r i t e s i n r e d and d r a b s a n d s t o n e s i n t h e p r o x i m i t y o f t h e c o l o u r boundary.

I n d r a b - r e d p a i r s s p a n n i n g t h e c o l o u r boundary c h l o r i t e s

from t h e d r a b s a n d s t o n e s a r e cormnonly r i c h e r i n o c t a h e d r a l i r o n than t h e r e d sandstone c h l o r i t e s .

Also t h e d r a b s a n d s t o n e s have a lower i l l i t e / c h l o r i t e

r a t i o than a s s o c i a t e d r e d sands t o n e s .

SPRING RUN

Fig.6.29. L i t h o f a c i e s c o r r e l a t i o n of Upper O r d o v i c i a n u n i t s a c r o s s c e n t r a l P e n n s y l v a n i a u s i n g t h e boundary between t h e R e e d s v i l l e - B a l d E a g l e Formations (Lithof a c i e s B-C) c o n t a c t . The c o l o u r boundary between t h e J u n i a t a and Bald E a g l e Formations i s shown a s a dashed l i n e ( a f t e r Thompson, 1970).

376 Using t h i s e v i d e n c e Thompson was a b l e t o conclude t h a t t h e Bald E a g l e - J u n i a t a sequence was i n i t i a l l y a l l r e d , due t o t h e i n i t i a l d e p o s i t i o n o f h a e m a t i t e o r a h a e m a t i t e p r e c u r s o r and t h e s u b s e q u e n t f o r m a t i o n of d i a g e n e t i c h a e m a t i t e .

The

Bald Eagle Formation was s u b s e q u e n t l y c o n v e r t e d t o a d r a b c o l o u r by t h e a c t i o n of groundwaters w i t h low Eh and low pH.

Under t h e s e c o n d i t i o n s h a e m a t i t e pigment and

c o a r s e opaque o x i d e s were reduced and removed i n s o l u t i o n .

Some of t h e a v a i l a b l e

f e r r o u s i r o n was i n c o r p o r a t e d i n t h e d i a g e n e t i c c h l o r i t e and p y r i t e p h a s e s . The danger o f a s c r i b i n g s t r a t i g r a p h i c a . s i g n i f i c a n c e t o c o l o u r b o u n d a r i e s i s clearly evident i n these results.

The Bald E a g l e Formation h a s n e i t h e r chrono

o r li t h o s t r a t i g r a p h i c a l s i g n i f i c a n c e . IRON CONTENT OF ANCIENT RED BEDS

T o t a l i r o n a n a l y s e s have been used by numerous a u t h o r s t o a i d t h e i n t e r p r e t a t i o n of a n c i e n t r e d bed f o r m a t i o n s .

Useful summaries a r e p r o v i d e d by Van Houten (1961,

1 9 7 3 ) , James ( 1 9 6 6 ) , and Walker and Honea (1969).

The t o t a l i r o n c o n t e n t of r e d

s a n d s t o n e s and mudstones shows c o n s i d e r a b l e v a r i a t i o n and q u o t e d a v e r a g e s ( T a b l e 6.5) range from 1.E (Walker, 1967) t o 3.5% (Van Houten, 1961).

TABLE 6.5.

Weight p e r c e n t i r o n i n modern a l l u v i u m and a n c i e n t r e d beds ( a d o p t e d from Van Houten, 1973). 1 T a b l e 3 i n Walker (1967a)., 2 T a b l e s 4 , 5 i n Van Houten (1961). Hot D e s e r t

T r o p i c a l Savanna

A n c i e n t Red Beds

mean

range

mean

range

2.6

0.7-4.7

5.25

3.1-7.6

1.7 3.5 0.67

Bulk Sample Total Iron Free ( e x t r a c t a b l e )

0.38

0.1-2.3

0.75

0.3-1.5

Clay F r a c t i o n Total Iron Free (extractable)

4.5 0.8

2.5-7.5 0.1-4.1

6.6 1.5

3.0-9.0 0.7-3.2

mean

range 1 0.2-5.1 2 0.7-7.0 1 0.1-2.1

I n d i v i d u a l samples may show even more extreme v a r i a t i o n and r e p o r t e d v a l u e s o f 10% o r even 12% t o t a l i r o n are n o t u n u s u a l .

The a v e r a g e t o t a l i r o n c o n t e n t of a l l

s a n d s t o n e s i s a b o u t 2.65%, P e t t i j o h n (1963) g i v i n g a f i g u r e of 2.35% and Middleton (1960) a n a v e r a g e of 2.9%.

The e v i d e n c e i n d i c a t e s l i t t l e d i f f e r e n c e i n t h e t o t a l

i r o n c o n t e n t of r e d beds and s a n d s t o n e s a s a whole.

The c l e a r i m p l i c a t i o n h e r e i s

t h a t t h e r e i s no e v i d e n c e of any o v e r a l l p r o c e s s o f i r o n - e n r i c h m e n t o p e r a t i n g i n red sediments.

This might b e e x p e c t e d i f r e d b e d s were d e r i v e d from l a t e r i t i c

s o u r c e a r e a s , a s s u g g e s t e d by Krynine (1949). One of t h e most remarkable f e a t u r e s of i r o n a n a l y s e s i n r e d b e d s , i s t h e a l m o s t u n i v e r s a l dependence o f i r o n c o n t e n t on g r a i n s i z e w i t h i n c r e a s i n g Fe2+ and Fe3+ i,n t h e f i n e r g r a i n s i z e s , (Van Houten, 1961, 1973; P i c a r d , 1965; Walker and Honea, 1969;

377

A . ..

6 VERY FINE - 5 R A I N E O

S.STMIE

A WELL-SORTED SILTSTONE 0 SILTY CLAYSTONE 4.00

3.00

2.00

I

4.0

1.20 1.60 PER CENT F.0

8.0

2.00

2.40

1.00

I

I

I

I

2.00 3.00 4.00 1.00 PER CENT TOTAL I R O N (F.1

B X

Total Iron FeO

h

Fe2*

3-

8

2-

1 1

A

I I

I

318

T u r n e r , 1974aj.

This r e l a t i o n s h i p i s i n d i c a t e d i n Fig.6.30 which shows t h e

i r o n c o n t e n t s of v a r i o u s r e d beds as a f u n c t i o n of g r a i n s i z e . This r e l a t i o n s h i p h o l d s n o t o n l y f o r t h e t o t a l i r o n c o n t e n t b u t a l s o f o r t h e amount o f f r e e ( e x t r a c t a b l e ) i r o n i n r e d s e d i m e n t s .

Walker and Honea (1969) i n

t h e i r s t u d y of a n c i e n t r e d beds and s o i l s from t h e Sonoran D e s e r t removed t h e r e d pigment ( e x t r a c t a b l e i r o n ) w i t h r e d u c i n g a g e n t s .

The a g e n t s u s e d were sodium

d i t h i o n a t e and sodium c i t r a t e (Mehra and J a c k s o n , 1960) o r where t h i s d i d n o t remove a l l t h e pigment, c o n c e n t r a t e d o x a l i c a c i d was used a l t h o u g h t h i s d i d n o t cause any n o t i c e a b l e a l t e r a t i o n 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 such a s m a g n e t i t e , ilmenite o r iron silicates.

Walker and Honea (1969) made some i m p o r t a n t

o b s e r v a t i o n s u s i n g t h i s method.

They showed t h a t t h e amount of i r o n p r e s e n t i n

pigmentary m a t e r i a l i n a n c i e n t r e d b e d s v a r i e d from 0.1 t o 2.1% w i t h an a v e r a g e of 0.7%.

I n p a r t i c u l a r t h e y p o i n t e d o u t t h a t some b r i g h t r e d samples (10R 4 / 6

and 7.5R 4 / 6 ) c o n t a i n e d o n l y 0.1% e x t r a c t a b l e i r o n t h u s c l e a r l y r e v e a l i n g t h e e x c e e d i n g l y s m a l l amount o f pigmentary m a t e r i a l needed t o i m p a r t b r i g h t c o l o u r t o a r e d bed.

Walker and Honea (1969) a l s o demonstrated t h a t Recent s e d i m e n t s i n t h e

Sonoran d e s e r t a l s o showed t h e c h a r a c t e r i s t i c enrichment of i r o n i n t h e f i n e r g r a i n e d sediments.

They concluded t h a t t h i s f e a t u r e c o u l d n o t b e t a k e n t o i n d i c a t e a

provenance i n d e e p l y w e a t h e r e d s o u r c e a r e a s b e c a u s e t h e s e were n o t p r e s e n t i n t h e s o u r c e a r e a of t h e Sonoran a l l u v i u m . F e r r i c / f e r r o u s i r o n r a t i o s have been used a s a n i n d e x of t h e d e g r e e o f o x i d a t i o n of r e d beds by a l a r g e number of a u t h o r s ( e . g . P i c a r d , 1965; F r i e n d , 1966; T u r n e r , 1974a; SlLnskL, 1976).

I n g e n e r a l r e d b e d s show e x c e s s f e r r i c i r o n and Fe203/Fe0

r a t i o s g r e a t e r t h a n two are common. e x c e s s f e r r o u s i r o n (e.g.

In a number of c a s e s , however, t h e y do show

T u r n e r , 1974a, Table IV) which may b e r e l a t e d t o t h e

p r o p o r t i o n o f u n o x i d i z e d f e r r o m a g n e s i a n s i l i c a t e s , and a l s o f e r r o u s i r o n i n t h e l a t t i c e s of c l a y m i n e r a l s s u c h a s i l l i t e and c h l o r i t e .

V a r i a t i o n s i n provenance

and d i a g e n e t i c environment must have a n i m p o r t a n t e f f e c t on t h e Fe 0 /FeO r a t i o s 2 3 of r e d beds. F e r r o u s i r o n i n r e d beds however does show t h e same g e n e r a l tendency a s f e r r i c i r o n t o be enriched i n t h e f i n e r g r a i n f r a c t i o n s (Fig.6.30).

The r e a s o n f o r t h i s

g r a i n s i z e dependence i n f e r r o u s and f e r r i c i o n i s c l e a r l y t h e i r c l o s e a s s o c i a t i o n w i t h t h e c l a y f r a c t i o n s of t h e s e d i m e n t s .

The e x t r a c t a b l e i r o n i s p r o b a b l y

c o n t a i n e d w i t h i n d i s c r e t e , e x t r e m e l y f i n e - p a r t i c l e pigmentary h a e m a t i t e ; some o f i t may a l s o b e f i n e l y c r y s t a l l i n e g o e t h i t e a l t h o u g h i n a n c i e n t r e d beds X-ray

d i f f r a c t i o n s u g g e s t s t h a t h a e m a t i t e i s p r o b a b l y f a r more abundant ( F o l k , 1960; Van Houten, 1964, p.652).

Some o f t h i s f r e e i r o n might a l s o b e adsorbed on c l a y m i n e r a l s

as amorphous m a t e r i a l which would n o t r e a d i l y b e d e t e c t e d by X-ray d i f f r a c t i o n . t h e c o n c e n t r a t i o n of i r o n i n f i n e g r a i n e d s e d i m e n t s c a n b e r e a d i l y e x p l a i n e d b y

Thus

normal p r o c e s s e s o f s o r t i n g and s e d i m e n t a t i o n i n a low-energy regime.

A similar

phenomenon i s o b s e r v e d i n , f o r example, d r a b marine s e d i m e n t s d e p o s i t e d under reducing conditions

-

here the i r o n i s f i n a l l y located a s authigenic pyrite.

Obviously no i m p l i c a t i o n s r e g a r d i n g t h e o r i g i n s o f r e d beds can be drawn from these results

-

t h e y g i v e no i n d i c a t i o n s of p r e - o r p o s t - d e p o s i t i o n a l r e d d e n i n g .

CONCLUSIONS AND FURTHER READING C o n t i n e n t a l r e d beds c o n t a i n i r o n o x i d e m i n e r a l s i n two i m p o r t a n t t e x t u r a l forms, f i n e p a r t i c l e pigment and c o a r s e - p a r t i c l e , opaque g r a i n s .

Pigment, a l t h o u g h

of d i v e r s e o r i g i n , i s a s u b t l e i n d i c a t o r o f geochemical c o n d i t i o n s 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 and i t s p r e s e r v a t i o n o r removal i s i n t i m a t e l y r e l a t e d t o Eh-pH c o n d i t i o n s and t h e s u l p h u r c y c l e i n b o t h marine and non-marine environments. Opaque o x i d e s i n c o n t i n e n t a l s e d i m e n t s a r e dominated by detrita1,titanomagnetite b u t i n a n c i e n t r e d beds h a e m a t i t e i s t h e dominant o x i d e , a r e s u l t of e x t e n s i v e i n s i t u , low-temperature o x i d a t i o n .

This f e a t u r e i s c o n s i s t e n t with t h e diagenetic

e v i d e n c e and h a s i m p l i c a t i o n s f o r t h e i n t e r p r e t a t i o n of palaeomagnetic d a t a from c o n t i n e n t a l red beds. Comprehensive c o v e r of t h e c h e m i s t r y of f o r m a t i o n and mineralogy of i r o n o x i d e s a r e g i v e n by t h e f o l l o w i n g : Berner, R.A.,

1 9 7 1 . P r i n c i p l e s of Chemical Sedimentology. McGraw-Hill, New York,

240 pp.

Rumble, D. I11 ( E d i t o r ) , 1976. Oxide M i n e r a l s . Min. SOC. America, S h o r t Course Notes

.

380

CHAPTER 7 THE MAGNETIZATION OF CONTINENTAL RED BEDS

INTRODUCTION Palaeomaenetism and Rock Magnetism I n r e c e n t y e a r s palaeomagnetism h a s become i n c r e a s i n g l y u s e d i n t h e s t u d y of sedimentary rocks.

T h i s i s p o s s i b l e b e c a u s e v i r t u a l l y a l l s e d i m e n t s and s e d i m e n t a r y

r o c k s c a r r y s m a l l amounts of m a g n e t i c i r o n o x i d e s which g i v e t h e r o c k s a permanent o r f o s s i l magnetism.

Whilst the strength (usually referred t o a s i n t e n s i t y ) of

t h i s f o s s i l magnetism i s v e r y weak when compared, f o r example, w i t h i g n e o u s r o c k s , t h e development of i n c r e a s i n g l y s o p h i s t i c a t e d measuring d e v i c e s means t h a t t h e f o s s i l magnetism of v i r t u a l l y a l l s e d i m e n t s can be a c c u r a t e l y measured.

The s t u d y

of f o s s i l magnetism i s r e f e r r e d t o a s palaeomagnetism. Although most rock-forming m i n e r a l s a r e non-magnetic t h o s e c o n t a i n i n g i r o n ( o r i n some c a s e s , o t h e r t r a n s i t i o n m e t a l s ) e x h i b i t some m a g n e t i c p r o p e r t i e s when placed i n an applied magnetic f i e l d .

The s t u d y of t h e s e b u l k m a g n e t i c p r o p e r t i e s

together with s p e c i f i c i n v e s t i g a t i o n s of t h e t o t a l magnetic mineralogy a r e r e f e r r e d t o a s rock magnetism.

Rock m a g n e t i c i n v e s t i g a t i o n s a r e e s s e n t i a l t o any s u c c e s s f u l

palaeomagnetic study. The f o s s i l magnetism o f a r o c k i s termed t h e n a t u r a l remanent m a g n e t i z a t i o n (NRM).

Rocks may a c q u i r e NRM by a v a r i e t y o f r a t h e r complex p r o c e s s e s .

The

m a g n e t i z a t i o n a c q u i r e d by t h e c o o l i n g of a m a g n e t i c m i n e r a l through i t s C u r i e p o i n t i s r e f e r r e d t o thermoremanent m a g n e t i z a t i o n (TRM).

The f o r m a t i o n o f magnetic

o x i d e s by low-temperature chemical r e a c t i o n s i s chemical remanent m a g n e t i z a t i o n (CRM).

The a l i g n m e n t o f m a g n e t i c p a r t i c l e s d u r i n g d e p o s i t i o n o r by r o t a t i o n i n

u n c o n s o l i d a t e d sediment produces a d e t r i t a l remanent m a g n e t i z a t i o n (DRM).

One of

t h e b a s i c assumptions o f palaeomagnetism i s t h a t r o c k s a r e magnetized by one o r o t h e r o f t h e s e p r o c e s s e s a t t h e time o f t h e i r f o r m a t i o n and t h u s i n c o r p o r a t e t h e d i r e c t i o n of t h e ambient geomagnetic f i e l d .

T h i s NRM d i r e c t i o n p o s s e s s e s t h r e e

fundamental p r o p e r t i e s : a d e c l i n a t i o n ( D ) measured e a s t w a r d s from t r u e N o r t h , a n i n c l i n a t i o n ( I ) reckoned p o s i t i v e downwards and n e g a t i v e upwards and a n i n t e n s i t y (Int.).

The d i r e c t i o n s a r e r e p r e s e n t e d by v e c t o r s a t t h e c e n t r e of a s p h e r e and

p l o t t e d g r a p h i c a l l y a s p o i n t s where t h e v e c t o r c u t s t h e s p h e r e ,

E i t h e r an equal

a r e a (Schmidt) p r o j e c t i o n o r e q u a l a n g l e (Wulf) p r o j e c t i o n can b e used.

It is

u s u a l t o d i s t i n g u i s h between upward ( n e g a t i v e l y ) d i p p i n g v e c t o r s and downward ( p o s i t i v e l y ) d i p p i n g v e c t o r s by u s i n g open and c l o s e d symbols r e s p e c t i v e l y (Fig.7.1).

A l t e r n a t i v e l y v e c t o r o r Z i j d e r v e l d diagrams c a n b e used t o p l o t t h e

381 r e s u l t s (HalLs, 1 9 7 9 ) .

Fig.7.1. P l o t t i n g o f p a l a e o m a g n e t i c d i r e c t i o n 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 , The c i r c u m f e r e n c e = h o r i z o n t a l ; open symbols = upward ( n e g a t i v e ) d i p s ; c l o s e d symbols = downward ( p o s i t i v e ) d i p s . The o t h e r fundamental assumption of palaeomagnetism i s t h a t t h e E a r t h ' s magnetic f i e l d , when a v e r a g e d o v e r p e r i o d s of s e v e r a l thousand y e a r s o r s o , conforms t o t h a t

of an a x i a l geocentric dipole i . e . a geocentric dipole d i r e c t e d along t h e r o t a t i o n a l a x i s of t h e E a r t h ( F i g . 7 . 2 . ) .

An i m p o r t a n t consequence of t h i s model i s t h a t :

tan I = 2 tan Lat

(7.1)

where I i s t h e i n c l i n a t i o n of t h e f i e l d and L a t . t h e g e o g r a p h i c l a t i t u d e a t t h a t p o i n t and t h i s means t h a t t h e p a l a e o l a t i t u d e o f a g i v e n s i t e can e a s i l y be c a l c u l a t e d from t h e mean i n c l i n a t i o n o f i t s NRM.

Fig.7.2.

The m a g n e t i c f i e l d of an a x i a l g e o c e n t r i c d i p o l e .

382 I n f a c t , the E a r t h ' s magnetic f i e l d i s n o t t h a t of a simple a x i a l geocentric d i p o l e ; a b o u t 80% of t h e f i e l d a t t h e E a r t h ' s s u r f a c e can b e r e p r e s e n t e d by a d i p o l e f i t t e d a t l l k O w i t h r e s p e c t t o t h e r o t a t i o n a x i s , and t h e remaining 20% i s then r e p r e s e n t e d by n o n d i p o l e terms i n a s p h e r i c a l harmonic expansion.

The

geomagnetic f i e l d a c t u a l l y undergoes s i g n i f i c a n t long-term v a r i a t i o n s ( g r e a t e r than a y e a r ) , g e n e r a l l y r e f e r r e d t o a s geomagnetic s e c u l a r v a r i a t i o n , and t h e time-averaged f i e l d can b e e x p e c t e d t o b e s i g n i f i c a n t l y d i f f e r e n t from t h e i n s t a n t aneous f i e l d .

Analyses o f Recent geomagnetic d a t a

(Cox, 1962, 1970) show

t h a t a l o n g l i n e s o f l a t i t u d e p r o g r e s s i v e l y n e a r e r t h e E q u a t o r , geomagnetic f i e l d d i r e c t i o n s become p r o g r e s s i v e l y more d i s p e r s e d .

Thus t h e magnitude of s e c u l a r

v a r i a t i o n should be g r e a t e r nearer t o t h e Equator.

S e c u l a r v a r i a t i o n h a s been

c o n v i n c i n g l y d e m o n s t r a t e d i n Cenozoic s e d i m e n t s , i n c l u d i n g l a k e s e d i m e n t s ( C r e e r e t a l . , 1972) and deep-sea s e d i m e n t s (Opdyke e t a l . , and Cox, 1965).

1 9 7 2 ) , and l a v a flows ( D o e l l

The p e r i o d i c i t y o f t h e s e v a r i a t i o n s v a r i e s somewhat; changes i n

d e c l i n a t i o n commonly v a r y between 2,000 and 3,000 y e a r s (Thompson, 1975; C r e e r e t a l . , 1976) and s i m i l a r changes a r e s e e n i n i n c l i n a t i o n b u t t h e s e may show a l a r g e r p e r i o d i c i t y (Opdyke e t a l . , 1972). Palaeomagnetic measurement o f a sample r e p r e s e n t i n g a s h o r t p e r i o d of t i m e ( w i t h i n one s e c u l a r v a r i a t i o n c y c l e ) i s used t o c a l c u l a t e a v i r t u a l geomagnetic pole (VGP),

t h e e q u i v a l e n t of t h e p r e s e n t geomagnetic p o l e s .

I n order to estimate

t h e geographic p o l e i t i s n e c e s s a r y t o use t h e average palaeomagnetic f i e l d 5 ( l o 4 10 y e a r s i s c o n s i d e r e d s u f f i c i e n t ) o r t h e a v e r a g e s of t h e c o r r e s p o n d i n g

-

VGP's. D e t a i l e d a n a l y s e s o f t h e time-averaged p a l a e o m a g n e t i c f i e l d show, i n f a c t , t h a t t h e geomagnetic f i e l d i s n o t t h a t o f a s i m p l e a x i a l g e o c e n t r i c d i p o l e .

McElhinny

and Merrill (1975) showed t h a t n e a r l y h a l f o f t h e p a l a e o m a g n e t i c p o l e s f o r t h e p e r i o d up t o 5 My ago were a s s i g n e d 95% c i r c l e s o f c o n f i d e n c e t h a t d i d n o t i n c l u d e the present geographic pole.

However, r e g i o n a l l y a v e r a g e d p a l a e o m a g n e t i c p o l e s

were always s t a t i s t i c a l l y i n d i s t i n g u i s h a b l e from t h e g e o g r a p h i c p o l e .

These

o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e view t h a t t h e r e a r e l o n g p e r i o d s o f s e v e r a l m i l l i o n y e a r s a s s o c i a t e d w i t h t h e geomagnetic s e c u l a r v a r i a t i o n .

The a x i a l

g e o c e n t r i c d i p o l e h y p o t h e s i s i s o n l y s t r i c t l y v a l i d p r o v i d i n g t h e time t a k e n f o r averaging i s a s long a t these long periods, a d e q u a t e p e r i o d o f time.

I n most c a s e s 5 My i s p r o b a b l y a n

A d e t a i l e d review o f t h e time-averaged p a l a e o m a g n e t i c

f i e l d , and i t s o r i g i n s , i s g i v e n by M e r r i l l and McElhinny (1977). The u h v s i c a l b a s i s o f magnetism The magnetic d i p o l e moment of a n atom c o n s i s t s of a c o n t r i b u t i o n from t h e o r b i t a l magnetic moment due t o e l e c t r o n o r b i t around i t s n u c l e u s and a l s o t h e s p i n m a g n e t i c moment due t o e l e c t r o n s p i n a b o u t i t s own a x i s .

I n most m a t e r i a l s

383 t h e f o r m a t i o n of a molecule o c c u r s when atoms w i t h opposing e l e c t r o n s p i n s a r e b r o u g h t t o g e t h e r ; t h e s e a r e compensated and z e r o magnetic moment r e s u l t s .

Such

m a t e r i a l s a r e c h a r a c t e r i z e d by a s m a l l n e g a t i v e s u s c e p t i b i l i t y and t h e e f f e c t i s c a l l e d diamagnetism.

Many common rock-forming m i n e r a l s s u c h a s c a l c i t e and

quartz a r e diamagnetic. I n many o t h e r s u b s t a n c e s , p a r t i c u l a r l y t h e t r a n s i t i o n m e t a l s which have u n p a i r e d e l e c t r o n s , t h e atoms p o s s e s s r e s u l t a n t m a g n e t i c moments which become a l i g n e d i n t h e p r e s e n c e of a n a p p l i e d f i e l d .

An induced m a g n e t i z a t i o n r e s u l t s i n

t h e d i r e c t i o n of t h e a p p l i e d f i e l d g i v i n g a p o s i t i v e s u s c e p t i b i l i t y .

This e f f e c t

i s paramagnetism and many i r o n - b e a r i n g m i n e r a l s s u c h a s i r o n - s i l i c a t e s and i r o n carbonates a r e paramagnetic. Some s u b s t a n c e s , i n p a r t i c u l a r i r o n and c o b a l t , have a permanent magnetic d i p o l e e v e n i n t h e a b s e n c e o f a n a p p l i e d f i e l d and t h u s e x h i b i t spontaneous m a g n e t i z a t i o n . T h i s a r i s e s b e c a u s e t h e exchange f o r c e s between t h e u n p a i r e d e l e c t r o n s i n t h e s e s u b s t a n c e s a r e s u c h t h a t t h e i r s p i n s a r e a l i g n e d and r e i n f o r c e e a c h o t h e r even i n t h e a b s e n c e of a n a p p l i e d f i e l d .

T h i s e f f e c t i s r e f e r r e d t o a s ferromagnetism.

T r u l y f e r r o m a g n e t i c m a t e r i a l s a r e , however, q u i t e r a r e i n n a t u r e and t h e term i s o f t e n used i n palaeomagnetism t o d e n o t e any s u b s t a n c e w i t h a spontaneous magnetization.

The a l i g n m e n t p r o c e s s i n f e r r o m a g n e t i c m a t e r i a l s i s l o s t due t o

thermal a g i t a t i o n a t a c r i t i c a l temperature f o r each substance c a l l e d the Curie temperature o r Curie point. Some o t h e r s u b s t a n c e s a r e c h a r a c t e r i z e d by a s u b - d i v i s i o n i n t o two magnetic s u b l a t t i c e s which a r e a r r a n g e d i n a n a n t i p a r a l l e l manner and t h u s c a n c e l e a c h o t h e r o u t a n d no r e s u l t a n t moment o c c u r s .

This e f f e c t i s c a l l e d antiferromagnetism.

A n t i f e r r o m a g n e t i c m a t e r i a l s do n o t have a C u r i e p o i n t b u t t h e o r d e r i n g of t h e atomic moment is d e s t r o y e d a t t h e Ngel t e m p e r a t u r e o r Ndel p o i n t above which t h e y behave l i k e an ordinary paramagnetic. magnetic.

Many o f t h e f e r r i c oxyhydroxides a r e a n t i f e r r o -

I f t h e magnetic s u b l a t t i c e s a r e n o t b a l a n c e d t h e n a n e t spontaneous

m a g n e t i z a t i o n r e s u l t s known a s f e r r i m a g n e t i s m .

F e r r i m a g n e t i c m i n e r a l s such a s

m a g n e t i t e have a C u r i e p o i n t and a l l t h e same p r o p e r t i e s a s f e r r o m a g n e t i c m a t e r i a l s . The m a g n e t i z a t i o n o f a m a t e r i a l i s g e n e r a l l y made up o f two components, t h e remanent m a g n e t i z a t i o n (remanence) which i s t h a t remaining a f t e r t h e removal of a n a p p l i e d f i e l d , and t h e induced m a g n e t i z a t i o n which i s t h a t induced i n a n a p p l i e d f i e l d , b u t which d i s a p p e a r s a f t e r t h e removal o f t h e f i e l d .

The i n t e n s i t y of t h e s e

m a g n e t i z a t i o n s i s g i v e n by t h e m a g n e t i c moment p e r u n i t volume.

-5 o f remanence range from a b o u t 10 to Gauss cm3 f o r i g n e o u s r o c k s .

lo-*

Typical i n t e n s i t i e s

Gauss cm3 f o r s e d i m e n t s , and

to

of t h e v e c t o r sum o f t h e remanence J

and t h e m a g n e t i z a t i o n i n d u c e d i n t h e E a r t h ' s

m a g n e t i c f i e l d J . where: J = Ji

+ Jn

lom5

The t o t a l m a g n e t i z a t i o n o f a rock i s a c t u a l l y made up

(7.2)

384 The induced m a g n e t i z a t i o n J

i

i s proportional t o the applied f i e l d H, t h a t i s

Ji = I H

where

xis a

(7.3) constant of p r o p o r t i o n a l i t y c a l l e d t h e magnetic s u s c e p t i b i l i t y .

The

Koenigsberger r a t i o (Q) i s d e f i n e d a s t h e r a t i o o f t h e remanent t o i n d u c e d magnetization:

Q,

H

(7.4)

Qt = Jt/ H

(7.5)

= Jn/

or where Qn i s t h e r a t i o of t h e i n t e n s i t y of NRM ( J n ) t o t h a t i n d u c e d i n t h e E a r t h ' s f i e l d a t t h e sampling s i t e and Q t i s t h e r a t i o of thermoremanence (J ) a c q u i r e d i n t

t h e f i e l d H t o t h e m a g n e t i z a t i o n i n d u c e d by t h e same f i e l d a t room t e m p e r a t u r e . Hysteresis The m a g n e t i c p r o p e r t i e s of f e r r o m a g n e t i c m a t e r i a l s can b e c o n v e n i e n t l y d e s c r i b e d by r e f e r e n c e t o t h e i n i t i a l m a g n e t i z a t i o n c u r v e and h y s t e r e s i s loops ( F i g . 7 . 3 ) .

When

a specimen of f e r r o m a g n e t i c m a t e r i a l i n a demagnetized s t a t e , i s p l a c e d i n a n a p p l i e d f i e l d H , i t f i r s t shows a l i n e a r i n c r e a s e i n m a g n e t i z a t i o n J a s H i s i n c r e a s e d ( a ) . I f H i s reduced t o z e r o a t t h i s p o i n t J f a l l s t o z e r o and t h e i n i t i a l s u s c e p t i b i l i t y of t h e specimen i s g i v e n by t h e s l o p e a t a .

I f H i s i n c r e a s e d t o b and t h e n reduced

t o z e r o J does n o t f a l l t o z e r o b u t p o s s e s s e s a n i s o t h e r m a l remanent m a g n e t i z a t i o n (IRM) g i v e n by J .

Continued i n c r e a s e o f H beyond d would produce no f u r t h e r

i n c r e a s e i n J and a s a t u r a t i o n m a g n e t i z a t i o n i s produced.

I f h i s reduced t o z e r o

(which would b e a l o n g c ) a s a t u r a t i o n I N , J r s would r e s u l t .

I f H i s then applied

i n t h e o p p o s i t e d i r e c t i o n t h e IF3 i s l o s t and J i s reduced t o z e r o by a f i e l d H which i s c a l l e d t h e c o e r c i v e f o r c e o r c o e r c i v i t y of t h e m a t e r i a l .

This i s a

measure o f t h e m a g n e t i c h a r d n e s s o f t h e m a t e r i a l , h i g h e r c o e r c i v i t y b e i n g a measure of g r e a t e r magnetic hardness.

Repeated c y c l i n g of t h e f i e l d c a u s e s a h y s t e r e s i s

l o o p , t h e l a r g e s t one o c c u r r i n g i f s a t u r a t i o n i s r e a c h e d w i t h s m a l l e r ones ( a s shown i n F i g . 7 . 4 i f s a t u r a t i o n i s not r e a c h e d ) .

A n i s o t r o p y , magnetic domains and t h e time dependence o f m a g n e t i z a t i o n , G e n e r a l l y s p e a k i n g , t h e work done t o m a g n e t i z e a m a g n e t i c m a t e r i a l t o s a t u r a t i o n

i s dependent on t h e d i r e c t i o n i n which t h e f i e l d i s a p p l i e d , t h a t i s t h e m a t e r i a l

i s magnetically anisotropic.

A n i s o t r o p y a r i s e s from two main s o u r c e s which a r e due

t o shape (shape a n i s o t r o p y ) and c r y s t a l s t r u c t u r e ( m a g n e t o c r y s t a l l i n e a n i s o t r o p y ) . A n i s o t r o p y may a l s o b e i n d u c e d by m e c h a n i c a l s t r e s s , o r by t h e a p p l i c a t i o n of a n a p p l i e d f i e l d w h i l s t c o o l i n g through a c r i t i c a l t e m p e r a t u r e . The work done t o m a g n e t i z e a specimen may b e w r i t t e n :

E

.

anis

= Eo

+ Klfl

(0)

+ K2f2

(4)+

.....

(7.5)

386 where E

i s a c o n s t a n t , a d e f i n e s t h e d i r e c t i o n o f i n t e r e s t , and K1, K2, e t c a r e

i s a minimum i n t h e ' e a s y ' d i r e c t i o n o r p l a n e the anisotropy constants. Eanis and i s a maximum i n t h e ' h a r d ' d i r e c t i o n o r p l a n e ( O ' R e i l l y , 1976). In some c a s e s

K

2

and h i g h e r terms a r e s u f f i c i e n t l y s m a l l t o b e n e g l e c t e d and t h e magnitude of K

1

i n d i c a t e s t h e d e g r e e o f a n i s o t r o p y w i t h t h e s i g n o f K1 i d e n t i f y i n g t h e e a s y a x i s , I n m a g n e t i t e t h e e a s y d i r e c t i o n i s a l o n g t h e c1111 a x i s and t h e r e i s a l s o a p p r e c i a b l e shape a n i s o t r o p y i n t h a t n o n - s p h e r i c a l p a r t i c l e s a r e more e a s i l y magnetized a l o n g t h e i r long axes.

I n h a e m a t i t e t h e e a s y d i r e c t i o n i s normal t o t h e c - a x i s and t h e r e

i s no a p p a r e n t s h a p e a n i s o t r o p y .

Fig.7.3. I n i t i a l m a g n e t i z a t i o n c u r v e and h y s t e r e s i s l o o p s ( J - H l o o p s ) f o r a f e r r o m a g n e t i c m a t e r i a 1. Magnetic domains form i n m a g n e t i c m a t e r i a l s i n o r d e r t o a c h i e v e a lower energy state.

When a magnetic body g i v e s r i s e t o a n e x t e r n a l f i e l d i . e . c a r r i e s remanence

i t has a c e r t a i n m a g n e t o s t a t i c e n e r g y o r e n e r g y o f s e l f - d e m a g n e t i z a t i o n .

The

demagnetizing f i e l d i s p o r p o r t i o n a l t o a f a c t o r r e l a t e d t o t h e shape o f t h e body c a l l e d t h e demagnetizing f a c t o r .

F o r a u n i f o r m l y magnetized e l l i p s o i d of i n t e n s i t y

J t h e m a g n e t o s t a t i c e n e r g y p e r u n i t volume i s g i v e n by: Em = 112 NJ 2 where N i s t h e demagnetizing f a c t o r i n t h e d i r e c t i o n o f m a g n e t i z a t i o n .

(7.6)

N has v a l u e s

386 r a n g i n g from 0 t o 4 3 .

I f a f e r r o m a g n e t i c body i s s a t u r a t e d t h e i n t e r n a l f i e l d

c r e a t e s a l a r g e m a g n e t o s t a t i c e n e r g y , b u t i f t h e body was s u b d i v i d e d i n t o two r e g i o n s w i t h opposing m a g n e t i z a t i o n t h e i n t e r n a l f i e l d and m a g n e t o s t a t i c e n e r g y

w i l l be decreased (Fig.7.4).

These d i f f e r e n t r e g i o n s a r e termed m a g n e t i c domains

and t h e b o u n d a r i e s between them, domain w a l l s ; s u f f i c i e n t w i l l form t o a c h i e v e t h e minimum e n e r g y s t a t e of t h e system.

S i n c e t h e s e domain w a l l s have a f i n i t e t h i c k -

n e s s , u s u a l l y between 100-10008, i t f o l l o w s t h a t f o r v e r y s m a l l p a r t i c l e s no domain w a l l s can o c c u r . particles.

These p a r t i c l e s a r e r e f e r r e d t o a s single-domain o r monodomain

P a r t i c l e s w i t h o n l y a few domains may show some o f t h e p r o p e r t i e s o f

s i n g l e domain g r a i n s and a r e s a i d t o l i e i n t h e pseudo-single-domain ( S t a c e y and B a n e r j e e , 1974, p. 60-62). r e f e r r e d t o a s multidomain g r a i n s .

(PSD) range

L a r g e r p a r t i c l e s w i t h more domain w a l l s a r e

I n g e n e r a l t h e r e i s r e a s o n a b l y good agreement

between e s t i m a t e s of t h e c r i t i c a l g r a i n s s i z e o f s i n g l e domain b e h a v i o u r b a s e d on t h e o r y and e x p e r i m e n t a l o b s e r v a t i o n .

For magnetite t h i s g r a i n s i z e i s about

and f o r h a e m a t i t e i t i s much l a r g e r p r o b a b l y l y i n g i n t h e range 10-100~. M u l t i domain h a e m a t i t e p a r t i c l e s a r e t h u s l i k e l y t o b e r e l a t i v e l y s c a r c e i n r e d beds whose m a g n e t i z a t i o n i s dominated by single-domain p a r t i c l e s .

trjl+l-i B

A

t *l t vl t

t

C

1

- - + +

t l f l

- + - +

Fig.7.4. S u b d i v i s i o n o f a f e r r o m a g n e t i c g r a i n i n t o domains. Domain m a g n e t i z a t i o n s a r e i n d i c a t e d by t h e s t r a i g h t s o l i d a r r o w s . The dashed arrows i n d i c a t e t h e demagnetizing field. There a r e some i m p o r t a n t d i f f e r e n c e s i n t h e p r o p e r t i e s of multidomain and s i n g l e domain g r a i n s .

Multidomain g r a i n s have r e l a t i v e l y low c o e r c i v i t i e s b e c a u s e t h i s

i s governed by t h e r o t a t i o n of domain w a l l s .

F o r multidomain m a g n e t i t e t h e

c o e r c i v i t y c a n n o t exceed a b o u t 900 Oe (Evans and McElhinny, 1969) and some m u l t i domain s i n g l e c r y s t a l s of h a e m a t i t e may s a t u r a t e i n f i e l d s o f o n l y a few t e n s of o e r s t e d s ( E a t o n a n d M o r r i s h , 1969).

S i n g l e domain p a r t i c l e s have much h i g h e r

387 c o e r c i v i t i e s , f o r m a g n e t i t e c o e r c i v i t i e s i n e x c e s s of 1000 Oe a r e u s u a l l y a t t r i b u t e d t o t h e s h a p e a n i s o t r o p y of s i n g l e domains (Evans e t a l . , 1968); t h e c o e r c i v i t y of some f i n e p a r t i c l e h a e m a t i t e powders may b e a s h i g h a s 30 KOe (Roquet, 1954). Although b o t h s i n g l e domain and multidomain p a r t i c l e s a r e known t o o c c u r w i d e l y i n n a t u r e (Evans and McElhinny, 1969; Evans and Wayman, 1970; McElhinny and Opdyke, 1973; T u r n e r , 1975) i t i s t h e s i n g l e domain p a r t i c l e s , b e c a u s e of t h e i r h i g h e r c o e r c i v i t i e s , which a r e e s p e c i a l l y i m p o r t a n t i n palaeomagnetism.

S i n g l e domain

p a r t i c l e s o f m a g n e t i t e a r e r e s p o n s i b l e f o r t h e p r i m a r y , s t a b l e remanence o f many igneous r o c k s .

The m a g n e t i z a t i o n of c o n t i n e n t a l r e d beds i s formed a l m o s t

e x c l u s i v e l y from s i n g l e domain h a e m a t i t e . The i m p o r t a n c e of c o e r c i v i t y i n palaeomagnetism i s 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 time dependence o f m a g n e t i z a t i o n .

An assemblage of magnetized p a r t i c l e s i n z e r o

f i e l d does n o t r e p r e s e n t a n e q u i l i b r i u m s t a t e and i n s u c h a s y s t e m t h e approach t o e q u i l i b r i u m i s a t i m e dependent m a g n e t i z a t i o n .

V a r i o u s e n e r g y b a r r i e r s r e s i s t such

changes i n c l u d i n g t h o s e which oppose r o t a t i o n i n s i n g l e domain p a r t i c l e s o r domain w a l l t r a n s l a t i o n s i n multidomain p a r t i c l e s and a l s o b a r r i e r s t o i o n i c d i f f u s i o n and changes i n m i c r o s t r u c t u r e .

When t h e r a t e o f growth o r decay o f m a g n e t i z a t i o n can b e

observed i n t h e l a b o r a t o r y i t i s r e f e r r e d t o a s m a g n e t i c v i s c o s i t y ( N e e l , 1949). I n s i n g l e domain g r a i n s t h e i n t e r n a l m a g n e t i z a t i o n e n e r g y depends upon t h e o r i e n t a t i o n of t h e moment w i t h r e s p e c t t o c e r t a i n a x e s .

For m a g n e t i c a l l y u n i a x i a l

g r a i n s , t h e e n e r g y E i s g i v e n by: 2 E = Kvsin 8 where v i s volume and

4 the

(7.7)

a n g l e between t h e m a g n e t i c moment and t h e a x i s .

maximum e n e r g y b a r r i e r i s r e p r e s e n t e d by

2

90'

There The

a r e two p o s s i b l e moment o r i e n t a t i o n s f o r minimum energy,-Q =Oo and$ =180°. and has a v a l u e E r g i v e n by:

E r = vK

(7.8)

The t h e r m a l f l u c t u a t i o n s i o n s of e n e r g y E t a r e g i v e n by: E t = kT

(7.9)

where k i s Boltzmann's c o n s t a n t and T i s t h e a b s o l u t e t e m p e r a t u r e . Thus Er, i n a n assemblage o f s i n g l e domain g r a i n s w i t h u n i a x i a l a n i s o t r o p y and s a t u r a t e d a t

The magnetic moment can b e r o t a t e d from one p o s i t i o n t o t h e o t h e r i f E t t i m e t = O t h e remanence w i l l decay a c c o r d i n g t o :

Mr = Mo exp (-t/?') where M

r e l a x a t i o n t i m e is r e l a t e d t o t h e o t h e r two e n e r g i e s E

-

Y

=

(7.9)

is t h e moment remaining a f t e r time t a n d r i s t h e r e l a x a t i o n time.

exp

'r -

The

and E t by: (7.10)

Et

(7.11)

Y

=

exp

vK

where C i s a f r e q u e n c y f a c t o r of a b o u t l o l o s e c - l .

The a n i s o t r o p y c o n s t a n t K i s r e l a t e d t o t h e c o e r c i v i t y H Hc =

by t h e s i m p l e r e l a t i o n s h i p : C

2K

(7.12)

JS

and s o Y m a y b e w r i t t e n :

(7.13) showing t h a t t h e r e l a x a t i o n time r i s d i r e c t l y r e l a t e d t o c o e r c i v i t y .

T h i s i s one

of t h e fundamental c o n c e p t s of palaeomagnetism and forms t h e b a s i s o f m a g n e t i c c l e a n i n g a s a t o o l t o i s o l a t e p r i m a r y remanence i n r o c k s .

Progressive demagnetization

randomizes t h o s e domains w i t h low c o e r c i v e f o r c e , which f o r any g i v e n g r a i n volume a r e a l s o those with the s h o r t e s t relaxation t i m e . P a r t i c l e s f o r which t h e combined v a l u e s o f K, v and T r e s u l t i n T < l O O s i . e . when r e l a x a t i o n t i m e i s very s h o r t a r e s a i d t o be superparamagnetic because a f t e r t h e removal of a n a p p l i e d f i e l d , t h e remanence decays v e r y q u i c k l y .

The r e l a x a t i o n t i m e

becomes v e r y s h o r t when T i s v e r y l a r g e o r v i s s m a l l and s u c h g r a i n s a r e u s u a l l y therefore very f i n e grained.

F o r any g r a i n of volume v , t h e r e i s t h e r e f o r e a

c r i t i c a l b l o c k i n g t e m p e r a t u r e Tb a t w h i c h r b e c o m e s s m a l l b u t which i s below t h e Curie temperature.

S i m i l a r l y a t any g i v e n t e m p e r a t u r e T t h e r e i s a c r i t i c a l b l o c k i n g

volume db a t w h i c h r a g a i n becomes s m a l l .

A p r a c t i c a l consequence o f t h i s t h e o r y i s s e e n i n t h e m a g n e t i z a t i o n changes which r e s u l t from t h e b u r i a l and u p l i f t o f r o c k s .

During p r o g r e s s i v e b u r i a l , remanent

m a g n e t i z a t i o n i s g r a d u a l l y l o s t a s t e m p e r a t u r e i n c r e a s e s and g r a i n s become unblocked. With s u b s e q u e n t u p l i f t a new remanence w i l l r e s u l t a s t h e t e m p e r a t u r e f a l l s below the blocking temperatures of these grains.

The t h e o r y and p r a c t i c a l consequences o f

m a g n e t i z a t i o n changes caused by b u r i a l and u p l i f t a r e d i s c u s s e d by P u l l a i a h e t a l . (1975). MINERAL MAGNETISM

I r o n i s t h e most abundant t r a n s i t i o n e l e m e n t i n t h e E a r t h ' s c r u s t and o c c u r s i n a wide v a r i e t y o f m i n e r a l s i n c l u d i n g s i l i c a t e s , c a r b o n a t e s and o x i d e s .

The i r o n

o x i d e s which a r e p r i n c i p a l l y f e r r i m a g n e t s o r n o n - c o l l i n e a r a n t i f e r r o m a g n e t s w i t h C u r i e p o i n t s above a b o u t 20°C a r e r e s p o n s i b l e f o r v i r t u a l l y a l l t h e magnetic remanence observed i n r o c k s .

M i n e r a l s w i t h C u r i e p o i n t s less t h a n a b o u t 2OoC do

n o t c o n t r i b u t e t o t h e remanence of r o c k s , n o r do t h e c a r b o n a t e s o r s i l i c a t e s , which f o r t h e most p a r t a r e p a r a m a g n e t i c o r d i a m a g n e t i c and t h u s n o t c a p a b l e of c a r r y i n g permanent m a g n e t i c moments.

Some of t h e s u l p h i d e m i n e r a l s , n o t a b l y m o n o c l i n i c

p y r r h o t i t e (Fe S ) and g r e i g i t e (Fe S ) a r e f e r r i m a g n e t i c and may c o n t r i b u t e 3 4 7 8 remanence on a minor s c a l e . The most i m p o r t a n t remanence c a r r y i n g m i n e r a l s a r e t h e t i t a n o m a g n e t i t e s , and t h e i r oxidized equivalents t h e titanomaghemites.

These f e r r i m a g n e t s a c c o u n t f o r

t h e remanence o f i g n e o u s r o c k s and a r e a l s o i m p o r t a n t m a g n e t i z a t i o n c a r r i e r s i n s e d i m e n t s , i n c l u d i n g some r e d b e d s .

Haematite, a non-collinear antiferrornagnet,

389 i s r e s p o n s i b l e f o r a t l e a s t p a r t o f t h e remanence i n some h i g h l y o x i d i z e d igneous r o c k s and c a r r i e s t h e b u l k o f t h e remanence i n many s e d i m e n t s , p a r t i c u l a r l y r e d beds.

I n t e r m e d i a t e members o f t h e h a e m a t i t e - i l m e n i t e s o l i d s o l u t i o n s e r i e s a r e

r a r e i n n a t u r e a l t h o u g h t h e y do o c c u r i n some r a p i d l y c o o l e d v o l c a n i c and i n t r u s i v e igneous r o c k s where t h e y a r e i m p o r t a n t remanence c a r r i e r s .

Another f e r r i c oxyhy-

d r o x i d e , g o e t h i t e , a l s o p o s s e s s e s a weak remanence and c o n t r i b u t e s t o t h e m a g n e t i z a t i o n of some r e d b e d s , The t i t a n o m a e n e t i t e s a n d t i t a n o m a e h e m i t e s The t i t a n o m a g n e t i t e s a r e f e r r i m a g n e t i c w i t h a s a t u r a t i o n m a g n e t i z a t i o n of 3 1 92 Gauss cm gThe f e r r i m a g n e t i s m r e s u l t s b e c a u s e t h e r e a r e two o c t a h e d r a l (B)

.

s i t e c a t i o n s f o r e a c h t e t r a h e d r a l (A) s i t e c a t i o n which g i v e s two unequal subl a t t i c e s and t h u s a n e t , permanent m a g n e t i z a t i o n .

The C u r i e t e m p e r a t u r e of p u r e

m a g n e t i t e i s 58OoC b u t t h i s d e c r e a s e s w i t h i n c r e a s i n g t i t a n i u m s u b s t i t u t i o n (Fig.7.5).

T i t a n o m a g n e t i t e s commonly o c c u r r i n g i n b a s a l t i c igneous r o c k s have

composition p a r a m e t e r i n t h e range 0 . 5 r x c 0 . 8 which c o r r e s p o n d s t o C u r i e p o i n t s between a b o u t 200°C and room t e m p e r a t u r e .

U l v b s p i n e l i s p a r a m a g n e t i c a t room

t e m p e r a t u r e and a n t i f e r r o m a g n e t i c a t low t e m p e r a t u r e s w i t h a N e h l p o i n t o f (-153OC).

Complete s o l i d s o l u t i o n i n t h e m a g n e t i t e - u l v b s p i n e l s e r i e s o n l y e x i s t s

above 60OoC.

A t lower t e m p e r a t u r e s t h e r e i s a tendency f o r t h e e x s o l u t i o n of

t i t a n i u m r i c h p h a s e s t o t a k e p l a c e l e a v i n g a more i r o n - r i c h h o s t phase. I n p r a c t i c e many of t h e n a t u r a l l y o c c u r r i n g t i t a n o m a g n e t i t e s show a c e r t a i n d e g r e e of o x i d a t i o n and a r e more c o r r e c t l y r e f e r r e d t o a s titanomaghemites.

These

titanomaghemites a r e i m p o r t a n t i n o c e a n i c b a s a l t s where o x i d a t i o n t a k e s p l a c e i n t h e t e m p e r a t u r e r e g i o n 600-10OO0C d u r i n g c o o l i n g and a l s o a s a r e s u l t of submarine w e a t h e r i n g ( M a r s h a l l and Cox, 1972; R y a l l a n d Ade-Hall, 1975). O x i d a t i o n o f t i t a n o m a g n e t i t e s t o titanomaghemite i s accompanied by a r i s e i n C u r i e t e m p e r a t u r e (Readman and O ' R e i l l y , 1972).

The most c h a r a c t e r i s t i c f e a t u r e

of t h e titanomaghemites i s t h a t thermomagnetic c u r v e s show i r r e v e r s i b l e behaviour due t o i n v e r s i o n , maghemite i n v e r t i n g t o h a e m a t i t e , and titanomaghemites t o a s t a b l e i n t e r g r o w t h o f h a e m a t i t e and i r o n - t i t a n i u m o x i d e s a t t e m p e r a t u r e s i n t h e r e g i o n o f 350-400°C.

This i r r e v e r s i b l e bahaviour has enabled t h e i d e n t i f i c a t i o n

o f titanomaghemites i n a number o f r e d bed f o r m a t i o n s

(e.g. Waage and S t o r e t v e d t ,

1973). The c r y s t a l a n i s o t r o p y o f m a g n e t i t e a n d t h e t i t a n o m a g n e t i t e s shows a s t r o n g dependence on t e m p e r a t u r e and composition.

The f i r s t c u b i c a n i s o t r o p y c o n s t a n t

5 (K1) has a v a l u e o f a b o u t 10 e r g cmm3 a t 80°K (-193OC) and axis. (-1.5

[log

i s the easy

I n p u r e m a g n e t i t e , K changes s i g n a t 130°K (-143OC) and becomes n e g a t i v e 1 5 x 10 e r g cmm3) and Clll] i s t h e e a s y a x i s . T h i s low t e m p e r a t u r e t r a n s i t i o n

p r o v i d e s a u s e f u l means o f i d e n t i f y i n g m a g n e t i t e i n r o c k s even when o n l y v e r y

390 minute q u a n t i t i e s a r e present

( M a u r i t s c h and T u r n e r , 1975).

Fig.7.5. Contours o f C u r i e t e m p e r a t u r e and c e l l edge p a r a m e t e r f o r titanomagn e t i t e s a n d t h e i r o x i d i z e d e q u i v a l e n t s , t h e titanomaghemites ( a f t e r Readman and O ' R e i l l y , 1972). Haema t i t e The m a g n e t i c p r o p e r t i e s o f h a e m a t i t e have been s t u d i e d by many d i f f e r e n t workers f o r o v e r 50 y e a r s b u t t h e r e i s s t i l l no comprehensive u n d e r s t a n d i n g of t h e m i n e r a l . I n p a r t i c u l a r , t h e r e i s a g r e a t deal of v a r i a t i o n i n t h e reported magnetic

p r o p e r t i e s of s y n t h e t i c and n a t u r a l m a t e r i a l s , which s u g g e s t s t h e r e may b e more s t r u c t u r a l a n d c o m p o s i t i o n a l v a r i a t i o n i n n a t u r a l h a e m a t i t e s t h a n h a s g e n e r a l l y been realized.

U s e f u l reviews a r e t o b e found i n F u l l e r (1970) and J a c o b s e t a l . (1971).

A t room t e m p e r a t u r e , h a e m a t i t e i s n o t p u r e l y a n t i f e r r o m a g n e t i c and a weak

ferromagnetism r e s u l t s from a n o n - c o l l i n e a r , o r c a n t e d , a n t i f e r r o m a g n e t i c s p i n system.

There i s a l s o a v a r i a b l e , e x t r i n s i c , c o n t r i b u t i o n t o t h e m a g n e t i z a t i o n ,

u s u a l l y a t t r i b u t e d t o s t r u c t u r a l d e f e c t s , and r e f e r r e d t o a s i s o t r o p i c o r 3 ferromagnetism. The s u s c e p t i b i l i t y of h a e m a t i t e i s a b o u t 20 x G cm 3 and t h e s a t u r a t i o n m a g n e t i z a t i o n a b o u t 0 . 4 G cm g-' ( c f . m a g n e t i t e which 3 92 G cm ) . The C u r i e t e m p e r a t u r e of h a e m a t i t e i s u s u a l l y q u o t e d a s 680°C

defect -1 - 1 Oe g is

but

r e p o r t e d v a l u e s may be somewhat h i g h e r , p e r h a p s up t o 71OoC ( C h e v a l l i e r , 1951).

391 Most a u t h o r s a r e of t h e o p i n i o n t h a t t h e C u r i e p o i n t and

Ndel p o i n t c o i n c i d e

somewhere between 675OC and 69OoC. I n t e r m e d i a t e members of t h e h a e m a t i t e - i l m e n i t e s e r i e s show complex magnetic b e h a v i o u r and f o r d e t a i l e d d i s c u s s i o n s t h e r e a d e r i s r e f e r r e d t o t h e work o f Ishikawa and Akimoto ( 1 9 5 8 ) , S h i r a n e e t a l . (19591, and Hoffman (1975).

As the proportion

of i l m e n i t e i n c r e a s e s , t h e c e l l dimensions i n c r e a s e and t h e C u r i e t e m p e r a t u r e decreases (Fig.7.6).

I l m e n i t e i s p a r a m a g n e t i c a t l i q u i d n i t r o g e n t e m p e r a t u r e and

a n t i f e r r o m a g n e t i c a t v e r y low t e m p e r a t u r e s w i t h a rhombohedra1 u n i t c e l l p a r a m e t e r of 5.523g.

I n t h e range 0 t o 5 0 p e r c e n t t h e s o l i d s o l u t i o n s a r e c a n t e d a n t i -

f e r r o m a g n e t i c s , w h i l s t between a b o u t 45 and 95 p e r c e n t i l m e n i t e t h e y become f e r r i m a g n e t i c , r e v e r t i n g t o a n t i f e r r o m a g n e t i c n e a r 100 p e r c e n t i l m e n i t e .

Synthetic

specimens w i t h compositions c o n t a i n i n g 45 t o 60 p e r c e n t i l m e n i t e a r e of p a r t i c u l a r i n t e r e s t b e c a u s e t h e y show s e l f - r e v e r s i n g p r o p e r t i e s (Wescott-Lewis and P a r r y , 1971a; Hoffman, 1975).

Members of t h e h a e m a t i t e - i l m e n i t e s e r i e s w i t h such compos-

i t i o n s a r e , however, u n l i k e l y t o b e found i n c o n t i n e n t a l r e d beds. The i n t r i n s i c ferromagnetism o f h a e m a t i t e shows a t r a n s i t i o n a t a b o u t 10°C (263'K)

which i s u s u a l l y r e f e r r e d t o a s t h e Morin t r a n s i t i o n (Morin, 1950) a l t h o u g h

i t was a c t u a l l y d i s c o v e r e d by e a r l i e r workers (Honda and Song, 1914; Charlesworth

and Long, 1939).

Above t h e Morin t r a n s i t i o n (T ) t h e s u s c e p t i b i l i t y measured i n

m

-6 G cm3 0e-I g-'.

t h e b a s a l p l a n e and a l o n g t h e c - a x i s i s a b o u t 20 x 10

Below Tm,

f a l l s t o v e r y low v a l u e s p a r a l l e l t o t h e c - a x i s , b u t remains a b o u t t h e same i n the b a s a l plane (Fig.7.7)

(Ne'el and P a u t h e n e t , 1952).

- 5.50

1

0 Fo20

0.2

,

1

1

1

1

1

0.4 0.6 0.0 'Moh fraction of FeTiOg

.

1.0 FeTiO3

Fig.7.6. V a r i a t i o n o f C u r i e t e m p e r a t u r e and u n i t c e l l p a r a m e t e r ( a r h ) w i t h composition of t h e h a e m a t i t e - i l m e n i t e series.

392

1

I

0

1

I

200

I

I

0

I

200

I

I

400 Tomporntun

I

I

I

400

600

Temperature

I

1

800

O K

I

I

I

600

I

I

800

I

OK

-

Fig.7.7. V a r i a t i o n o f s a t u r a t i o n m a g n e t i z a t i o n ( a ) and s u s c e p t i b i l i t v (-b ). o f a h a e m a t i t e s i n g l e c r y s t a l w i t h i n t h e b a s a l p l a n e (1) a n d a l o n g - t h e t r i g o n a l a x i s ( 2 ) ( a f t e r N C e l , 1953). T h i s i s c o n s i s t e n t w i t h o b s e r v a t i o n s from n e u t r o n d i f f r a c t i o n ( S h u l l e t a l . , and Mbssbauer s p e c t r o s c o p y d a t a ( B l u m e t a l . , 1965; Kllndig e t a l . ,

1951)

1966) which

i n d i c a t e t h a t s p i n s a r e p a r a l l e l t o t h e c a x i s below T b u t normal t o i t above Tm. m Dzyaloshinsky (1958) showed t h a t above Tm c a n t i n g o f t h e s p i n s u b l a t t i c e s w i t h i n t h e b a s a l p l a n e would a c c o u n t f o r t h e o b s e r v e d i n t r i n s i c ferromagnetism and t h a t t h e l o s s of m a g n e t i c moments through T

m

i s due t o t h e d i s a p p e a r a n c e of t h e s p i n

c a n t i n g , s o t h a t below t h i s t e m p e r a t u r e h a e m a t i t e i s p u r e l y a n t i f e r r o m a g n e t i c ( L i n , 1962) ( F i g . 7 . 8 ) . Remanence a s s o c i a t e d w i t h t h e weak ferromagnetism a l s o d e c r e a s e s r a p i d l y below Tm ( F i g . 7 . 7 )

b u t does n o t d i s a p p e a r c o m p l e t e l y ( F l a n d e r s and Remeika, 1965).

This

was i n t e r p r e t e d by N6el (1953) t o i n d i c a t e t h a t t h e magnetism o f h a e m a t i t e c o n t a i n e d

393 two components, one a n i s o t r o p i c and l i n k e d t o t h e a n t i f e r r o r n a g n e t i s m ( t h e i n t r i n s i c s p i n c a n t e d moment) and t h e o t h e r i s o t r o p i c and i n d e p e n d e n t o f t h e Morin Transition.' The t e m p e r a t u r e o f t h e Morin T r a n s i t i o n i s r a i s e d by i n c r e a s i n g p r e s s u r e b u t t h e r e i s some d i s c r e p a n c y i n t h e r e s u l t s o b t a i n e d

and Ono, 1966).

( c f . Whorlton e t a l . , 1967 and Kawai

The t r a n s i t i o n i s a l s o a f f e c t e d by s l i g h t c o m p o s i t i o n a l v a r i a t i o n s

.

r e s u l t i n g from s u b s t i t u t i o n by o t h e r c a t i o n s which u s u a l l y lower T Only 0.3 mole m % Ti4' s u p p r e s s e s t h e t r a n s i t i o n c o m p l e t e l y (Haigh, 1957; Kaye. 1960; T a s a k i and 3+ I i d a , 1961; Besser e t a l . , 1967). Rhodium (Rh ) a p p e a r s t o b e unique i n t h a t i t r a i s e s t h e t e m p e r a t u r e of t h e Morin T r a n s i t i o n (Kr& e t a l . , 1965).

The s e n s i t i v i t y

of t h e t r a n s i t i o n t o t i t a n i u m doping i s e x p l a i n e d by B e s s e r e t a l . (1967) i n terms o f l a t t i c e d i s t o r t i o n produced by t h e l a r g e s i z e of Ti4+ c a t i o n s .

0 Fe 0 0

Fig.7.8. The m a g n e t i c s t r u c t u r e of h a e m a t i t e (a-Fefl3) above ( A ) and below (B) t h e Morin t r a n s i t i o n . S l i g h t c a n t i n g of t h e s p i n s u b l a t t i c e s produces t h e i n t r i n s i c ferromagnetism of h a e m a t i t e . Below t h e Morin t r a n s i t i o n t h e s p i n s a r e a l i g n e d p a r a l l e l t o t h e t r i g o n a l a x i s and p u r e a n t i f e r r o r n a g n e t i s m r e s u l t s ,

394 The d e f e c t moment i s l e s s e a s y t o e x p l a i n t h a n t h e i n t r i n s i c s p i n c a n t e d moment. I t v a r i e s i n magnitude from sample t o sample and i n n a t u r a l m a t e r i a l s t e n d s t o b e l a r g e r i n n a t u r a l c r y s t a l s and s m a l l e r i n f i n e g r a i n s ( G a l l o n , 1968a; Dunlop, 1 9 7 1 ) . I n f a c t t h e u s e of t h e term i s o t r o p i c f o r t h e d e f e c t moment i s a misnomer because G a l l o n (1968a) h a s found t h a t t h e low-temperature moment o f s y n t h e t i c c r y s t a l s and of n a t u r a l c r y s t a l s a f t e r a n n e a l i n g l i e s i n t h e b a s a l p l a n e .

The u s e o f t h e term

s t r u c t u r e s e n s i t i v e o r d e f e c t moment (Smith and F u l l e r , 1967) i s t h u s p r e f e r r e d . The f a c t t h a t t h e d e f e c t moment i s a b s e n t i n p u r e s t o i c h i o m e t r i c s y n t h e t i c c r y s t a l s below Tm, and t h a t i t can b e reduced on a n n e a l i n g i n d i c a t e s t h a t s t r u c t u r a l d e f e c t s o r , p o s s i b l y , f e r r o u s i r o n i m p u r i t i e s p r o v i d e a good e x p l a n a t i o n of i t s o r i g i n . I n t h e o r y , t h e Morin T r a n s i t i o n n o t o n l y p r o v i d e s a r e a d y means o f i d e n t i f y i n g c o n t r i b u t i o n s of h a e m a t i t e t o r e d bed NRM b u t a l s o p r o v i d e s a method of a s s e s s i n g t h e r e l a t i v e importance of i n t r i n s i c and d e f e c t moments.

I n p r a c t i c e though, r e d

beds r a r e l y e x h i b i t t h e Morin T r a n s i t i o n a l t h o u g h i t has been o b s e r v e d i n l a c u s t r i n e s e d i m e n t s ( C r e e r e t a l . , 1972; Thompson, 1977). t h e d e f e c t moment i s more i m p o r t a n t i n r e d beds.

The u s u a l e x p l a n a t i o n i s t h a t I t may b e t h a t t h e t r a n s i t i o n i s

s u p p r e s s e d because of s m a l l c o n t e n t s o f T i 4 + o r o t h e r c a t i o n s i n n a t u r a l h a e m a t i t e s . Grain s i z e e f f e c t s The domain s t r u c t u r e of h a e m a t i t e s i n g l e c r y s t a l s has been i n v e s t i g a t e d by a number o f workers u s i n g a v a r i e t y of methods (Williams e t a l . , 1958; Blackman e t a l . , 1959; G u s t a r d , 1967; G a l l o n , 1968b, and Eaton and M o r r i s h , 1969).

I n synthetic

c r y s t a l s domain w a l l s a r e r e g u l a r and d e f i n e domain p a t t e r n s which t a k e t h e form o f p a r a l l e l s l a b s o r c y l i n d e r s u s u a l l y s e e n on f a c e s p e r p e n d i c u l a r t o t h e b a s a l p l a n e (Eaton and M o r r i s h , 1969).

I n n a t u r a l c r y s t a l s the walls tend t o be i l l - d e f i n e d

and more c o n v o l u t e d ( G u s t a r d , 1967).

Domain w a l l s p a c i n g s o f 100-500r, a r e found

and t h i s s u g g e s t s t h a t t h e c r i t i c a l single-domain s i z e , d o , must b e much l a r g e r t h a n f o r normal f e r r o m a g n e t i c s , p r o b a b l y l y i n g i n t h e 1 0 - 1 0 0 ~range (Dunlop, 1 9 7 1 ) . The Morin T r a n s i t i o n a p p e a r s t o have l i t t l e e f f e c t on room t e m p e r a t u r e domain w a l l p a t t e r n s which p e r s i s t below T

w i t h l i t t l e o r no o b s e r v e d w a l l movement. m S y n t h e t i c c r y s t a l s a r e , however, r e a d i l y s a t u r a t e d i n f i e l d s of o n l y 30 Oe o r s o .

N a t u r a l c r y s t a l s , a s e x p e c t e d , a r e somewhat h a r d e r and s a t u r a t e a t s l i g h t l y h i g h e r fields.

Q u i t e c l e a r l y , e x p e r i m e n t s on l a r g e s i n g l e c r y s t a l s a r e n o t r e l e v a n t when

c o n s i d e r i n g t h e o r i g i n of t h e h i g h c o e r c i v i t i e s found i n f i n e g r a i n e d assemblages of h a e m a t i t e which c o n s i s t e n t i r e l y o f single-domain p a r t i c l e s .

O b s e r v a t i o n s on t h e

c o e r c i v i t y o f r e d beds and t h e domain s t r u c t u r e o f h a e m a t i t e a r e q u i t e c o n s i s t e n t : most r e d beds used i n p a l a e o m a g n e t i c s t u d i e s p r o b a b l y c o n s i s t e n t i r e l y o f s i n g l e domain assemblages. T h i s s t i l l does n o t e x p l a i n t h e o r i g i n of t h e c o e r c i v i t i e s o f many KOe found i n f i n e p a r t i c l e h a e m a t i t e (Roquet, 1954; Haigh, 1957).

These may b e a s h i g h a s

395 20-30KOe and p r o v i d e a r e a d y means o f d i s t i n g u i s h i n g m a g n e t i t e from h a e m a t i t e .

Also i n t h e single-domain s i z e range t h e c o e r c i v i t y o f h a e m a t i t e i s g r a i n s i z e dependent ( C h e v a l l i e r and Mathieu, 1943).

These a u t h o r s showed t h a t t h e weak

Fig.7.9. The e f f e c t of g r a i n s i z e on t h e m a g n e t i z a t i o n o f h a e m a t i t e ( a f t e r C h e v a l l i e r and Mathieu, 1 9 4 3 ) . ferromagnetism of h a e m a t i t e d e c r e a s e d w i t h d e c r e a s i n g g r a i n s i z e ( F i g . 7 . 9 ) .

The

c o e r c i v e f o r c e was found t o be a b o u t 300 Oe f o r g r a i n s of 3 0 0 ~ ( p r e p a r e d by c r u s h i n g a n a t u r a l c r y s t a l ) and i n c r e a s e d t o 1500 Oe a t 1 t o 400 Oe f o r l p g r a i n s .

5 b u~t t h e n d e c r e a s e d

T h i s peak i n c o e r c i v e f o r c e i s c o n s i s t e n t w i t h e s t i m a t e s

o f c r i t i c a l single-domain s i z e of lO-lOO/vm made by Dunlop (1971) b u t p r o v i d e s no e x p l a n a t i o n of t h e h i g h c o e r c i v i t i e s o b s e r v e d i n s y n t h e t i c h a e m a t i t e powders. Shape a n i s o t r o p y can b e d i s c o u n t e d b e c a u s e t h e s a t u r a t i o n m a g n e t i z a t i o n i s s o s m a l l . A m a g n e t o e l a s t i c e x p l a n a t i o n has been s u g g e s t e d by s e v e r a l workers ( e . g . Merri11,1968)

and P o r a t h (1968b) h a s a r g u e d t h a t i n t e r n a l s t r e s s becomes more i m p o r t a n t i n f i n e g r a i n s and i s c a p a b l e of e x p l a i n i n g t h e o b s e r v e d c o e r c i v i t y .

However t o e x p l a i n

c o e r c i v i t i e s o f a b o u t 1OKOe a n i n t e r n a l s t r e s s of a b o u t 1 kb i s r e q u i r e d and t h i s In single

c o u l d o n l y b e e x p e c t e d in t h e i m e d i a t e v i c i n i t y o f d i s l o c a t i o n s .

c r y s t a l s t h e b a s a l p l a n e a n i s o t r o p y i s dominated by u n i a x i a l terms of m a g n e t o e l a s t i c

396 o r i g i n ( B a n e r j e e , 1963) w i t h superimposed, and g e n e r a l l y much s m a l l e r , t r i a x i a l terms u s u a l l y a t t r i b u t e d t o t r i g o n a l m a g n e t o e l a s t i c c o u p l i n g ( I i d a and T a s a k i , 1964;

B e s s e r e t a l . , 1967).

Dunlop (1971) c o n s i d e r s m a g n e t o c r y s t a l l i n e a n i s o t r o p y t o b e

a more n a t u r a l e x p l a n a t i o n of t h e t r i a x i a l t e r m s , d e s p i t e c o n s i d e r a b l e v a r i a t i o n i n r e p o r t e d v a l u e s o f K , t h e a n i s o t r o p y c o n s t a n t , and s u p p o r t e d h i s c o n t e n t i o n w i t h t h e r e s u l t s of a n n e a l i n g e x p e r i m e n t s on s y n t h e t i c h a e m a t i t e powders.

One o f t h e

problems h e r e i s t h a t t h e m a g n e t i c p r o p e r t i e s o f s y n t h e t i c h a e m a t i t e c r y s t a l l i t e s a r e s e n s i t i v e t o t h e method o f p r e p a r a t i o n (Takada e t a l . , 1965); t h e r e a r e a l s o d i f f e r e n c e s i n h a e m a t i t e produced by t h e d e h y d r a t i o n o f g o e t h i t e and t h a t formed by o t h e r methods (Dunlop, 1971). The weak ferromagnetism of h a e m a t i t e d e c r e a s e s f o r c r y s t a l l i t e s s m a l l e r t h a n 10002 and d i s a p p e a r s a t a b o u t 2008 ( K h d i g e t a l . ,

1971).

1966; Hedley, 1968; B a n e r j e e ,

I n f a c t t h e r e i s some v a r i a t i o n i n r e p o r t e d v a l u e s o f t h i s c r i t i c a l s i z e

f o r s u p e r p a r a m a g n e t i c b e h a v i o u r ( d ) , and t h e c o n v e n t i o n a l e s t i m a t e o f 0.lym (Strangway e t a l . , 1967) may b e t o o l a r g e .

The superparamagnetic/ferromagnetic

t r a n s i t i o n h a s been s t u d i e d by Ktlndig e t a l . (1966) u s i n g Mbssbauer s p e c t r o s c o p y f o r supported@-Fe 0 p a r t i c l e s ( F i g . 7 . 1 0 ) . Samples w i t h p a r t i c l e s l e s s t h a n 2 3 100.8 d i a m e t e r gave a s p e c t r u m w i t h o n l y a s i n g l e q u a d r u p o l e s p l i t d o u b l e t peak and I

-10

-5

0

I

I

+5

+10

V E L O C I T Y IN mmlSEC.

Fig.7.10. Mbssbauer s p e c t r a of o(-.Fe 0 a s a f u n c t i o n of p a r t i c l e s i z e 2 3 ( a f t e r KUndig e t a l . , 1966).

397 a r e completely superparamagnetic.

A s t h e p a r t i c l e s i z e i n c r e a s e s t h e s i x peak

(Zemann s p l i t ) s e t o f l i n e s a s s o c i a t e d w i t h magnetic o r d e r i n g becomes p r o g r e s s i v e l y P a r t i c l e s o f 1801 _f 102 gave a

more p r e p o n d e r a n t o v e r t h e q u a d r u p o l e d o u b l e t .

spectrum c o r r e s p o n d i n g t o t h a t e x p e c t e d from a n e a r l y p u r e ferromagnet.

Kllndig

e t a l . (1966) a l s o d e m o n s t r a t e d t h a t t h e Morin t r a n s i t i o n was i n h i b i t e d i n v e r y f i n e

p a r t i c l e s ( F i g . 7 . 1 1 ) and b e l i e v e d t h a t t h i s was because g r a i n s u r f a c e s a c t e d a s d e f e c t s and were c a p a b l e o f p i n n i n g t h e e l e c t r o n s p i n s .

S c h r o e e r and N i n i n g e r (1967)

Artman e t a l . (1965) and Besser e t a l . (1967)

h a v e , however, a r g u e d a g a i n s t t h i s .

have shown t h a t t h e Morin t r a n s i t i o n , and i t s dependence on i m p u r i t i e s i s a r e s u l t of t h e c-axis a n i s o t r o p y being t h e

d i f f e r e n c e o f two l a r g e q u a n t i t i e s , t h e d i p o l a r

and s i n g l e - i o n a n i s o t r o p i e s (Kmd and Ksi

respectively).

Above Tm t h e n e g a t i v e K md

i s dominant, whereas below T t h e p o s i t i v e K i s dominant. Hedley (1970) has m si c o n s i d e r e d t h e g r a i n s i z e dependence of K and Ksi and used i t t o e x p l a i n t h e md d e p r e s s i o n and d i s a p p e a r a n c e of t h e Morin t r a n s i t i o n a s g r a i n s i z e d e c r e a s e s .

I

-10

I

-5

I

0

I

+5

I

+10

Velocity in mm/sec

Fig.7.11. Mbssbauer s p e c t r a o f sample D a t v a r i o u s t e m p e r a t u r e s showing t h e a b s e n c e of t h e Morin t r a n s i t i o n ( a f t e r KUndig e t a l . , 1966). There a p p e a r s t o be some fundamental d i f f e r e n c e i n t h e s u p e r p a r a m a g n e t i c s i z e , and b e h a v i o u r , o f h a e m a t i t e s produced by g o e t h i t e d e h y d r a t i o n and t h o s e produced by o t h e r methods.

For example, t h e r e s u l t s of C h e v a l l i e r (1951) s u g g e s t d

near

0.1 m , w h i l s t Strangway e t a l . (1967) s u g g e s t e d a v a l u e of 0 . 5 ~ f o r h a e m a t i t e s of

Y

t h i s type.

However, p a r t of t h e problem i s c e r t a i n l y due t o i n a c c u r a t e g r a i n s i z e

determination.

The assumption t h a t d a u g h t e r h a e m a t i t e i s t h e same s i z e a s t h e

p a r e n t g o e t h i t e has been shown by Dubey and Dunlop (1971) t o be i n v a l i d . Cohen e t a l . (1962) and Strangway e t a l . (1968) have d e s c r i b e d a s t a b l e t h e m o remanence i n u l t r a - f i n e g r a i n e d K - F e 0 which t h e y have a t t r i b u t e d t o s u p e r a n t i 2 3

398 ferromagnetism o r s p i n imbalance (N&,

1962).

N e h l (1962) p o i n t e d o u t t h a t a

remanence c o u l d r e s u l t i n u l t r a f i n e p a r t i c l e s i f t h e number o f s p i n s i n t h e two s u b l a t t i c e s were n o t b a l a n c e d ,

Such a n imbalance moment would b e e x p e c t e d t o l i e

p a r a l l e l t o t h e s u b l a t t i c e m a g n e t i z a t i o n whereas t h e s p i n c a n t e d moment i s perpendicular.

P o r a t h (1968a) has used t h i s t o i n d i c a t e t h a t imbalance remanence

r a t h e r t h a n s p i n c a n t e d remanence d e v e l o p s ino(-Fe203 d u r i n g i n v e r s i o n from

Also, B a n e r j e e (1970) concluded t h a t t h e s t a b l e remanence of O<-FeOOH

3/-Fe203.

( g o e t h i t e ) d i s c o v e r e d by Strangway e t a l . (1968) was p a r a l l e l t o t h e s u b l a t t i c e s m a g n e t i z a t i o n a n d was t h u s a n imbalance remanence.

One might e x p e c t t h a t i f s t a b l e

imbalance ferromagnetism can e x i s t i n t h e p r e s e n c e o f a s u p e r p a r a m a g n e t i c s p i n c a n t e d moment, t h e n i t t o o m i g h t become s u p e r p a r a m a g n e t i c a t e x t r e m e l y f i n e g r a i n sizes.

The superparamagnetism r e p o r t e d by Creer ( 1 9 6 1 ) i n a specimen o f Keuper

Marl c o u l d be o f t h i s t y p e . 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 a n d d e f e c t moments 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 i s o f c r u c i a l importance i n r e d bed p a l a e o m a g n e t i c s t u d i e s b u t u n f o r t u n a t e l y has n o t f r e q u e n t l y been i n v e s t i g a t e d ,

Haigh (1957) was t h e f i r s t t o make i n v e s t i g a t i o n s o f t h i s k i n d

and conducted two e x p e r i m e n t s on a h a e m a t i t e powder o f u n s p e c i f i e d g r a i n s i z e : 1. A r e v e r s e f i e l d was a p p l i e d t o a s a t u r a t e d sample and t h e r e s u l t a n t remanence was m a i n t a i n e d on c o o l i n g below Tm.

By v a r y i n g t h e r e v e r s e f i e l d and r e p e a t i n g

t h e c o o l i n g s e p a r a t e DC d e m a g n e t i z a t i o n c u r v e s c o u l d be deduced f o r t h e d e f e c t and s p i n c a n t e d moments 2 . The DC d e m a g n e t i z a t i o n ' c u r v e of t h e d e f e c t moment above was measured d i r e c t l y a t t e m p e r a t u r e s below T ,

m

Haigh (1957) found t h e remanent c o e r c i v e f o r c e H

o f t h e s p i n c a n t e d moment was cr 4800 Oe w h i l e t h e d e f e c t moment was c o n s i d e r a b l y s o f t e r w i t h Hcr of a b o u t 500 Oe. Dunlop (1971) has i n v e s t i g a t e d t h e 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 i n a v a r i e t y o f s y n t h e t i c h a e m a t i t e powders u s i n g a n n e a l i n g e x p e r i m e n t s : "Ferrite" reds "Copperas" r e d s "Goethite" reds Powder A

-

produced from Fe 0 +c(-FeZ03 3 4 produced from FeS04. 7H20

- unknown

produced from N-FeOOH origin

The samples were a n n e a l e d i n a i r a t p r o g r e s s i v e l y h i g h e r t e m p e r a t u r e s between 650-950°C and i s o t h e r m a l remanent m a g n e t i z a t i o n (IRM) c u r v e s measured a f t e r e a c h annealing.

Although t h e r e a r e some d i f f e r e n c e s i n t h e m a g n e t i c p r o p e r t i e s o f t h e

d i f f e r e n t powders s t u d i e d by Dunlop t h e i r r e s p o n s e t o t h e a n n e a l i n g p r o c e d u r e i s b r o a d l y s i m i l a r e x c e p t f o r t h e g o e t h i t e r e d s whose p r o p e r t i e s have o f t e n been r e c o g n i z e d a s anomalous ( c f . C h e v a l l i e r , 1951). c u r v e s of Fig.7.12

The m a j o r f e a t u r e i n t h e IRM

i s t h e progressive i n c r e a s e i n magnetic hardness with annealing

399

POWDER A

0

5

10

15

H ( k0.)

COPPERAS RED Or (omuig 1

GOETHITE RED

//--

Fig. 7.12. Isothermal remanence curves of various synthetic fine particle haematites initially (curve 1) and after annealing at successively higher temperatures. In Powder A the temperatures are 2 (65OOC) 3 (75OOC) 4 and 5 (85OOC) 6 (92OOC) and 7 (95OOC). For all the others the temperatures are 2 (70OOC) 3 (80OOC) and 5 (950°C)(after Dunlop, 1971).

400 i.e.

a f t e r e a c h t e m p e r a t u r e i n c r e m e n t a h i g h e r f i e l d is r e q u i r e d t o produce a

g i v e n l e v e l o f remanence.

The i n c r e a s e d c o e r c i v i t y v a r i e s from sample t o sample,

f o r t h e f e r r i t e r e d s t h e i n c r e a s e i s o n l y moderate b u t t h e c o p p e r a s r e d beds show a v e r y marked i n c r e a s e i n c o e r c i v i t y w i t h a n n e a l i n g .

The g o e t h i t e r e d s show a n

o p p o s i t e t r e n d which can b e r e a d i l y i n t e r p r e t e d a s growth of s u p e r p a r a m a g n e t i c m a t e r i a l , by s i n t e r i n g , through a g r a i n s i z e d p r o p e r t i e s appear.

a t which s t a b l e f e r r o m a g n e t i c

S i n c e t h e g r a i n s i z e of t h e g o e t h i t e r e d s was known t o b e

s i m i l a r t o t h a t o f t h e o t h e r h a e m a t i t e s t h i s may r e p r e s e n t e v i d e n c e t h a t d h a e m a t i t e s produced by t h i s method i s r a t h e r l a r g e r t h a n t h e d

for

i n haematites

produced by o t h e r methods. Dunlop (1971, p. 18-19) d i s c u s s e d s e v e r a l p o s s i b i l i t i e s which would a c c o u n t f o r t h e observed i n c r e a s e i n H saturation magnetization

cr

and t h e r a t i o of s a t u r a t i o n remanence (Crs) t o

(PSI.

Fig.7.13. F e r r o m a g n e t i c h y s t e r e s i s c u r v e s b e f o r e a n n e a l i n g ( s o l i d ) and a f t e r a n n e a l i n g a t 95OoC (open c u r v e s ) ( a f t e r Dunlop, 1971). Dunlop a r g u e d t h a t i f t h e m a j o r e f f e c t of a n n e a l i n g i s t o r e l a x i n t e r n a l stress then t h e changes i n Hcr and Prs c a n b e i n t e r p r e t e d a s a r e s p o n s e t o a change i n t h e m a g n e t o e l a s t i c c o n t r i b u t i o n t o b a s a l p l a n e a n i s o t r o p y o r a change i n t h e d e f e c t ( s t r u c t u r e s e n s i t i v e moment).

S i n c e o n l y t h e d e f e c t moment would c a u s e

changes i n t h e s a t u r a t i o n m a g n e t i z a t i o n

CWs)

Dunlop measured h y s t e r e s i s c u r v e s

of powders b e f o r e and a f t e r a n n e a l i n g ( F i g . 7 . 1 3 ) . remanence, a n i n c r e a s e i n c o e r c i v e f o r c e H

These show l i t t l e change i n

<

and a d e c r e a s e i n i n i t i a l s u s c e p t i b i l i t y .

Most i m p o r t a n t l y though, t h e h y s t e r e s i s c&es

reveal a s i g n i f i c a n t decrease i n

401 on a n n e a l i n g d e m o n s t r a t i n g t h a t t h e d e f e c t moment must be changing. Dunlop's a n n e a l i n g e x p e r i m e n t s on f i n e p a r t i c l e h a e m a t i t e s c a n b e i n t e r p r e t e d i n terms o f a g r a d u a l a n n e a l i n g o u t o f a r e l a t i v e l y " s o f t " d e f e c t moment whose o v e r a l l c o e r c i v i t y i s lower t h a n t h a t of a " h a r d e r " i n t r i n s i c s p i n c a n t e d moment, The d i f f e r e n c e i n c o e r c i v i t y spectrum of t h e d e f e c t and s p i n c a n t e d moments i s n o t g r e a t (Fig.7.13) w i t h Haigh (1957).

and t h e terms hard and s o f t a r e o n l y r e l a t i v e i n agreement Dunlop's (1971) d a t a c l e a r l y show, however, t h a t t h e a n i s o t r o p i c

s p i n c a n t e d moment i s h a r d e r t h a n t h e d e f e c t moment i n f i n e p a r t i c l e h a e m a t i t e . I n s i n g l e c r y s t a l s however, t h e r e v e r s e a p p e a r s t o be t h e c a s e (Smith and F u l l e r , 1967).

The specimen s t u d i e d

(Specimen 2 3 o f Kobayashi and Smith, 1965) was a

s i n g l e c r y s t a l o f h a e m a t i t e w i t h a b o u t 8% t i t a n i u m .

Smith and F u l l e r f i r s t

s u b j e c t e d t h e specimen t o continuous thermal d e m a g n e t i z a t i o n o f a s a t u r a t i o n IRM. T h i s i s shown a s Curve 1 on Fig.7.14.

Here t h e s a t u r a t i o n IRM i s l o s t a t t h e

N6el p o i n t of 65OoC (about 4OoC h i g h e r t h a n t h e C u r i e t e m p e r a t u r e f o r t h i s specimen).

Temperature ('C 1

Fig.7.14. Continuous thermal d e m a g n e t i z a t i o n of s a t u r a t i o n IRM of a h a e m a t i t e s i n g l e c r y s t a l w i t h a b o u t 8% t i t a n i u m ( a f t e r Smith and F u l l e r , 1967). Curve 2 i s a r e p e t i t i o n of t h e f i r s t experiment; t h i s shows p r o g r e s s i v e loss of t h e h i g h - t e m p e r a t u r e moment and enhancement o f t h e s p i n - c a n t e d moment.

Smith

and F u l l e r (1967) t h e n went on t o show t h a t t h i s high-temperature moment has a h i g h e r remanent c o e r c i v i t y t h a n t h e s p i n c a n t e d moment. The specimen was p l a c e d 4 i n a f i e l d o f 1.1 x 10 Oe and t h e n i n a r e v e r s e f i e l d s u f f i c i e n t l y s t r o n g t o s w i t c h t h e d i r e c t i o n o f t h e b u l k m a g n e t i z a t i o n and t h e composite m a g n e t i z a t i o n t h u s produced was t h e r m a l l y demagnetized. s e e n below 612OC (Fig.7.15)

The s p i n - c a n t e d moment which can be

i s i n the sense of t he sm al l er negat i ve f i e l d but t he

4

h i g h t e m p e r a t u r e moment r e t a i n s t h e d i r e c t i o n o f t h e o r i g i n a l f i e l d o f 1.1 x 10 O e and would n o t have been s w i t c h e d by t h e n e g a t i v e f i e l d .

402 Temperature f'C II

I

-

+.01k

5

I

P i

Demonstration of h i g h t e m p e r a t u r e moment i n a s i n g l e c r y s t a l of Fig.7.15. h a e m a t i t e ( w i t h 8% t i t a n i u m ) between t h e C u r i e p o i n t (615OC) and t h e Nee1 p o i n t (65O0C) and i t s h i g h c o e r c i v i t y ( a f t e r Smith and F u l l e r , 1967). These e x p e r i m e n t s were a l s o performed on samples w i t h less t h a n 0.05% t i t a n i u m and a l l t h e r e s u l t s i n d i c a t e t h a t t h e h i g h - c o e r c i v i t y remanence a t room t e m p e r a t u r e

i s a s s o c i a t e d w i t h t h e h i g h - t e m p e r a t u r e moment and n o t w i t h t h e s p i n c a n t e d moment. Smith and F u l l e r (1967) f u r t h e r concluded t h a t t h e s t a b l e remanence i n h a e m a t i t e

i s due t o s m a l l f e r r i m a g n e t i c r e g i o n s caused by l a t t i c e i m p e r f e c t i o n s ; t h i s e x p l a n a t i o n i s a t t r a c t i v e b e c a u s e i t e x p l a i n s t h e memory e f f e c t which a r i s e s b e c a u s e of a t r a n s i t i o n zone of s p i n s t h a t c o u p l e t h e f e r r i m a g n e t i c r e g i o n s t o the antiferromagnet. An a l t e r n a t i v e view h a s been p u t forward by Bucur (1978) who s t u d i e d t h e d e f e c t moment i n two s y n t h e t i c h a e m a t i t e powders.

The p r o p e r t i e s o f t h e d e f e c t moment

were s t u d i e d i n weak and moderate f i e l d s w i t h r e s p e c t t o t h e s a t u r a t i o n f i e l d of h a e m a t i t e , and a l s o a s a f u n c t i o n of a n n e a l i n g .

Two d e f e c t components were

separated: a ) a component of low c o e r c i v i t y due t o l a t t i c e d e f e c t s and which d i s a p p e a r s a t t h e Morin t r a n s i t i o n b ) a component w i t h h i g h e r c o e r c i v i t y which p e r s i s t s below t h e Morin t r a n s i t i o n and which i s u n a f f e c t e d by a n n e a l i n g .

T h i s component i s c o n s i d e r e d t o b e due

t o non-magnetic i m p u r i t i e s . The s i g n i f i c a n c e o f t h e s e e x p e r i m e n t a l s t u d i e s l i e s i n t h e i n s i g h t which t h e y can g i v e i n t o t h e o r i g i n of t h e NRM of r e d beds.

I f t h e d e f e c t moment i s t h e more

s t a b l e i n r e d beds ( t h i s might b e e x p e c t e d i f c o a r s e p a r t i c l e h a e m a t i t e was more i m p o r t a n t than pigment i n a p a r t i c u l a r specimen) then i t s p a l a e o m a g n e t i c r e l i a b i l i t y must be viewed w i t h t h e f a c t t h a t s u c h a moment might b 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 g e o l o g i c a l stress o r h e a t i n g .

A l t e r n a t i v e l y , i f t h e s p i n c a n t e d moment i s t h e

more s t a b l e ( t h i s would b e e x p e c t e d i f t h e pigment was t h e dominant m a g n e t i c c a r r i e r ) , t h e n p a l a e o m a g n e t i c r e s u l t s u n a f f e c t e d by s t r e s s o r h e a t might be e x p e c t e d .

403 U n f o r t u n a t e l y t h e r e have been few s t u d i e s o f t h e r e l a t i v e i m p o r t a n c e of s p i n c a n t e d and d e f e c t moments i n r e d b e d s , so a g e n e r a l a s s e s s m e n t of t h e i r palaeomagnetic s i g n i f i c a n c e i s n o t p o s s i b l e a t t h i s s t a g e . 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 S t u d i e s on t h e remanent m a g n e t i z a t i o n of u n c o n s o l i d a t e d sediments have shown t h a t a v a r i e t y of p r o c e s s e s a r e c a p a b l e o f p r o d u c i n g a n a l i g n m e n t of magnetic p a r t i c l e s and t h u s i m p a r t i n g a d e t r i t a l remanent m a g n e t i z a t i o n o r DRM t o t h e sediment.

O r i g i n a l l y t h e terms d e t r i t a l remanent m a g n e t i z a t i o n and d e p o s i t i o n a l

remanent m a g n e t i z a t i o n were c o n s i d e r e d synonymous (Nagata, 1953) b u t I r v i n g (1957) r e c o g n i z e d t h a t d e t r i t a l m a g n e t i z a t i o n i n v o l v e d p r o c e s s e s which o p e r a t e d i n u n c o n s o l i d a t e d sediment a f t e r d e p o s i t i o n and i n t r o d u c e d t h e term p o s t - d e p o s i t i o n a l d e t r i t a l remanent m a g n e t i z a t i o n .

Now t h e d i s t i n c t i o n i s made between:

depositional

DRM which r e f e r s t o t h e m a g n e t i z a t i o n a c q u i r e d a s t h e m a g n e t i c . p a r t i c l e s s e t t l e

from s u s p e n s i o n , i n t e r a c t w i t h t h e s e d i m e n t s u r f a c e a t t h e s e d i m e n t / w a t e r i n t e r f a c e and t h e n come t o r e s t on t h e s u b s t r a t e and p o s t - d e p o s i t i o n a l DRM which r e f e r s t o t h e m a g n e t i z a t i o n which is a c q u i r e d by a s e d i m e n t a f t e r magnetic p a r t i c l e s come t o r e s t on t h e s u b s t r a t e i . e . below t h e s e d i m e n t / w a t e r i n t e r f a c e , DRM h a s been s t u d i e d e x p e r i m e n t a l l y and t h e o r e t i c a l l y i n much more d e t a i l than

chemical remanent m a g n e t i z a t i o n (CRM) b e c a u s e o f t h e r e l a t i v e s i m p l i c i t y of d e s i g n i n g laboratory experiments.

N e v e r t h e l e s s t h e r e s u l t s o f DRM e x p e r i m e n t s have n e v e r

proved e a s y t o i n t e r p r e t i n terms o f n a t u r a l s e d i m e n t a r y environments.

One of t h e

key q u e s t i o n s i n t h e s t u d y o f DRM i s t h e a c c u r a c y w i t h which sediments r e c o r d t h e d i r e c t i o n o f t h e ambient magnetic f i e l d .

Nagata e t a l . (1943) a t a n e a r l y s t a g e

showed t h a t c r u s h e d b a s a l t fragments f a i t h f u l l y r e c o r d e d t h e d e c l i n a t i o n of a n a p p l i e d f i e l d when d e p o s i t e d i n w a t e r .

Many e a r l y e x p e r i m e n t s r e d e p o s i t e d n a t u r a l

s e d i m e n t a r y m a t e r i a l s , and r e v e a l e d t h a t a l t h o u g h d e c l i n a t i o n c o u l d be f a i t h f u l l y reproduced t h e i n c l i n a t i o n was o f t e n much l e s s t h a n t h e a p p l i e d f i e l d .

Johnson

e t a l . (1948) d i s c o v e r e d t h i s f o r a r e d e p o s i t e d v a r v e d c l a y and Clegg e t a l . (1954) d i s c o v e r e d a n '8

i n c l i n a t i o n e r r o r i n a redeposited T r i a s s i c red sandstone.

This

e f f e c t was c a l l e d " i n c l i n a t i o n e r r o r " by King (1955) who observed t h a t t h e i n c l i n a t i o n i n t h e sediment Io, c o u l d b e r e l a t e d t o t h e i n c l i n a t i o n of t h e a p p l i e d field I

f

by t h e r e l a t i o n s h i p :

tan I = F tan I f where f i s a c o n s t a n t ( F i g . 7 . 1 6 ) .

(7.14) F o r t h e t y p e s of sediment he s t u d i e d King

d e t e r m i n e d f Z 0 . 4 and e x p l a i n e d t h e i n c l i n a t i o n e r r o r by assuming t h a t t h e sediment c o n s i s t s of a m i x t u r e of s p h e r i c a l and d i s c - s h a p e d p a r t i c l e s whose magnetic moment l a y i n t h e p l a n e of t h e d i s c ( t h i s would o n l y b e n e c e s s a r i l y so i n t h e c a s e of

404 m a g n e t i t e which p o s s e s s e s a p p r e c i a b l e shape a n i s o t r o p y ) .

The s p h e r i c a l p a r t i c l e s

a r e a l i g n e d w i t h t h e E a r t h ’ s magnetic f i e l d on t h e s u b s t r a t e s u r f a c e b u t t h e disc-shaped p a r t i c l e s r o t a t e s o t h a t t h e y l i e i n t h e h o r i z o n t a l p l a n e .

I f both

s e t s of p a r t i c l e s have e q u a l m a g n e t i c moments t h e n (7.14) r e s u l t s and f can b e i d e n t i f i e d a s the f r a c t i o n of spherical p a r t i c l e s .

1

0

U

E0

c

tan

If

R e l a t i o n s h i p between i n c l i n a t i o n o f r e d e p o s i t e d g l a c i a l v a r v e s I, and Fig.7.16. i n c l i n a t i o n of a p p l i e d f i e l d I f ( a f t e r King, 1955). An a l t e r n a t i v e e x p l a n a t i o n was g i v e n by

G r i f f i t h s e t a l . (1960) who showed

experimentally t h a t the i n c l i n a t i o n e r r o r i s independent of p a r t i c l e s i z e (Fig.7.17).

tf

t

I

0

10

+ I

20

3

Mean Particle Diameter (microns) Fig.7.17. Independence o f i n c l i n a t i o n and p a r t i c l e s i z e . Each s i z e f r a c t i o n o b t a i n e d from a s i n g l e v a r v e l a y e r (from G r i f f i t h s e t a l . , 1 9 6 0 ) .

405 G r i f f i t h s e t a l . (1960) s u g g e s t e d t h a t t h e i n c l i n a t i o n error a r i s e s when a n a l i g n e d p a r t i c l e t o u c h e s t h e bottom and t h e n r o l l s i n t o t h e n e a r e s t d e p r e s s i o n . To c a l c u l a t e t h i s e f f e c t r e q u i r e s t h e a v e r a g i n g o v e r p a r t i c l e s which r o l l i n a l l d i r e c t i o n s and i n f a c t t h e p a r t i c l e s whose a x e s o f r o t a t i o n a r e p a r a l l e l t o t h e h o r i z o n t a l component o f t h e a p p l i e d f i e l d have t h e g r e a t e s t i n f l u e n c e on t h e inclination error. tan I where

= ( 2 cos

The r e s u l t

0) / (1

+

cos

(Griffiths e t al.,

0)

1960) i s :

t a n If

(7.15)

0 i s t h e a v e r a g e a n g l e t h r o u g h which t h e p a r t i c l e r o l l s .

Although t h i s " r o l l i n g s p h e r e s 1 ' model a p p e a r s v e r y d i f f e r e n t from t h e "shape a n i s o t r o p y " model, King and Rees (1966) have s u g g e s t e d t h a t t h e y can b e c o n s i d e r e d a s v a r i a n t s o f t h e same p r o c e s s whereby g r a v i t y overcomes t h e magnetic o r i e n t a t i o n a t the i n s t a n t of deposition. The d i r e c t i o n of m a g n e t i z a t i o n o f l a b o r a t o r y d e p o s i t e d s e d i m e n t s i s i n f l u e n c e d by t h e a n g l e of s l o p e of t h e b e d d i n g p l a n e .

T h i s was f i r s t d i s c o v e r e d by King

(1955) who c a l l e d i t "bedding e r r o r " and d e f i n e d i t a s t h e d i f f e r e n c e between t h e i n c l i n a t i o n o b s e r v e d on a d i p p i n g bed and t h a t o b s e r v e d on a h o r i z o n t a l bed. Hamilton and King (1964) showed t h a t b e d d i n g e r r o r i s a p p r o x i m a t e l y e q u a l t o t h e s l o p e o f t h e bedding p l a n e ; p a r t i c l e s r o l l i n g downslope r o l l through a n a n g l e

(0

c< i s t h e s l o p e of t h e bedding p l a n e w h i l s t p a r t i c l e s r o l l i n g u p s l o p e r o l l through a n a n g l e o f o n l y (0 -4. The b i a s c r e a t e d by t h e d i f f e r e n c e i n t h e s e +6()

where

two a n g l e s l e a d s t o t h e bedding e r r o r (King, 1955; G r i f f i t h s e t a l . ,

1960).

I n a d d i t i o n t o i n c l i n a t i o n e r r o r and bedding e r r o r King (1955) a l s o r e p o r t e d t h a t d e p o s i t i o n i n f l o w i n g w a t e r a f f e c t e d t h e d i r e c t i o n of m a g n e t i z a t i o n of t h e sediment.

G r a n a r (1958) a t t r i b u t e d t h i s t o p a r t i c l e r o t a t i o n a b o u t a n h o r i z o n t a l

a x i s , p e r p e n d i c u l a r t o t h e f l o w d i r e c t i o n and c a u s e d by s h e a r s t r e s s i n t h e laminar flow layer.

I f t h e d i r e c t i o n o f flow i s i n t h e v e r t i c a l p l a n e o f t h e magnetic

f i e l d , o n l y a n e r r o r i n i n c l i n a t i o n w i l l be produced; however, i f t h e flow i s o b l i q u e t o t h e f i e l d , e r r o r s i n b o t h i n c l i n a t i o n and d e c l i n a t i o n w i l l r e s u l t .

Griffiths

e t a l . (1960) and Rees (1961) made q u a n t i t a t i v e measurements o f s u c h " c u r r e n t r o t a t i o n e f f e c t s " and i n one c a s e , Rees was a b l e t o c r e a t e a n e r r o r i n d e c l i n a t i o n

0' of 3

by a c u r r e n t which made a n a n g l e o f 145O w i t h t h e a p p l i e d f i e l d .

The t h r e e e f f e c t s d e s c r i b e d above a r e a l l a s p e c t s o f d e p o s i t i o n a l DRM.

The

p o s s i b i l i t y of p o s t - d e p o s i t i o n a l DRM was f i r s t r a i s e d by I r v i n g (1957) from a n i n v e s t i g a t i o n o f slumped beds of T o r r i d o n i a n Sandstone.

The f a c t t h a t t h e s e

d i s t u r b e d beds have a uniform d i r e c t i o n o f m a g n e t i z a t i o n l e d I r v i n g (1957) t o p r o p o s e t h a t f i n e d e t r i t a l magnetic p a r t i c l e s were f r e e t o r o t a t e i n w a t e r f i l l e d v o i d s a f t e r d i s t u r b a n c e and t h u s i m p a r t a p o s t - d e p o s i t i o n a l DRM.

A practical

l a b o r a t o r y d e m o n s t r a t i o n o f p o s t - d e p o s i t i o n a l DRM was made by I r v i n g and Major (1964) who produced a s t a b l e m a g n e t i z a t i o n i n a w e t s y n t h e t i c s e d i m e n t c o n s i s t i n g o f q u a r t z and m a g n e t i t e .

Q u a r t z g r a i n s from c o a r s e s i l t t o f i n e s a n d s i z e were mixed w i t h

406 m a g n e t i t e p a r t i c l e s w i t h t h e r a t i o o f maximum d i a m e t e r s r a n g i n g from 1.0 t o 0.5. The m i x t u r e s were f l o o d e d and t h e n d r a i n e d and d r i e d i n a n a p p l i e d f i e l d ; t h e r e s u l t s c l e a r l y i n d i c a t e t h e p r o d u c t i o n o f a p o s t - d e p o s i t i o n a l DRM ( F i g . 7 . 1 8 ) . S i m i l a r e x p e r i m e n t s have been d e s c r i b e d by Tucker (1979) who showed t h a t t h e f i n e r ( h i g h c o e r c i v i t y ) g r a i n s were p r e f e r e n t i a l l y r o t a t e d .

Fiold Inclination

Fig.7.18. P o s t - d e p o s i t i o n a l DRM of a s y n t h e t i c sediment o f q u a r t z and m a g n e t i t e d r i e d i n t h e presence of an applied f i e l d a c c u r a t e l y records t h e i n c l i n a t i o n of t h e f i e l d ( a f t e r I r v i n g and M a j o r , 1964). Although no s a t i s f a c t o r y model has been developed on a q u a n t i t a t i v e b a s i s t h e q u a l i t a t i v e t h e o r y of I w i n g (1957) s u g g e s t e d t h a t magnetic g r a i n s would be f r e e t o r o t a t e within water f i l l e d pore spaces. experiments1 observations.

This i s c o n s i s t e n t with s e v e r a l

L d v l i e ( 1 9 7 4 ) , f o r example, d e p o s i t e d a c l a y from

s u s p e n s i o n i n a known f i e l d and mid-way t h r o u g h t h e e x p e r i m e n t r e v e r s e d t h e d i r e c t i o n of t h e f i e l d and d e p o s i t e d a c a l c i u m c a r b o n a t e marker bed.

When t h e

d e p o s i t e d s e d i m e n t was measured t h e e x p e c t e d p a l a e o m a g n e t i c boundary was found t o b e d i s p l a c e d downwards w e l l below t h e marker bed ( F i g . 7 . 1 9 ) . a r e c o n s i s t e n t w i t h t h e p r o d u c t i o n of

These r e s u l t s

p o s t - d e p o s i t i o n a l DRM and s i m i l a r r e s u l t s

have been o b s e r v e d i n deep s e a s e d i m e n t coves.

Dymond ( i 9 6 9 ) demonstrated t h a t

t h e Brunhes-Matuyama palaeomagnetic boundary was d i s p l a c e d 160cm downward w i t h r e s p e c t t o t h e c o r r e s p o n d i n g r a d i o m e t r i c boundary.

T h i s was a t t r i b u t e d t o slow

d e w a t e r i n g of t h e sediment b u t Opdyke (1972) h a s q u e s t i o n e d Dymond's i n t e r p r e t a t i o n of t h e r a d i o m e t r i c d a t a .

The i m p o r t a n c e o f c r i t i c a l w a t e r c o n t e n t i n t h e f o r m a t i o n

of p o s t - d e p o s i t i o n a l DRM has been d e m o n s t r a t e d by Khramov (1968) who d e p o s i t e d r e d c l a y s from s u s p e n s i o n .

For some samples he i n c r e a s e d t h e i n t e n s i t y of t h e a p p l i e d

f i e l d a f t e r s e d i m e n t a t i o n was completed; t h i s i n c r e a s e d t h e i n t e n s i t y of magneti z a t i o n f o r sediment w i t h a 70% w a t e r c o n t e n t b u t t h e r e was no e f f e c t on t h o s e

401 samples w i t h o n l y a 30% w a t e r c o n t e n t .

These r e s u l t s a r e c o n s i s t e n t w i t h t h e

d i f f i c u l t y of o b t a i n i n g r e l i a b l e p a l a e o m a g n e t i c r e s u l t s from v e r y wet sediments (Kent, 1973). 65

-a

60

1

55

50 45 40 00

900 Dadination

1800

Fig.7.19. Displacement o f a p a l a e o m a g n e t i c boundary i n a l a b o r a t o r y d e p o s i t e d sediment. The a p p l i e d f i e l d (dashed l i n e ) was r e v e r s e d a f t e r 62 days. Subsequent measurement o f t h e remanent magnetism shows t h a t sediment a l r e a d y d e p o s i t e d responded t o t h e change i n f i e l d d i r e c t i o n i n d i c a t i n g t h e f o r m a t i o n of a p o s t d e p o s i t i o n a l DRM ( a f t e r L b v l i e , 1974). The i m p l i c a t i o n of t h e s e r e s u l t s a r e q u i t e c l e a r .

I f the i n i t i a l water

c o n t e n t of a sediment i s low and below t h e c r i t i c a l v a l u e needed f o r p o s t d e p o s i t i n n a l DRM t o o c c u r t h e n t h e o r i g i n a l DRM ( i f p r e s e n t ) i s l i k e l y t o be preserved.

On t h e o t h e r hand i f t h e w a t e r c o n t e n t i s above t h e c r i t i c a l v a l u e

then p o s t - d e p o s i t i o n a l r o t a t i o n s may o c c u r and a p o s t - d e p o s i t i o n a l DRM r e s u l t . The p r e s e n c e o r a b s e n c e of a n i n c l i n a t i o n e r r o r may be u s e f u l i n d i s c r i m i n a t i n g one type of m a g n e t i z a t i o n from t h e o t h e r .

i s important i n magnetization fixation.

I t f o l l o w s t h a t sediment d e w a t e r i n g

Although d e h y d r a t i o n has been a l m o s t

e x c l u s i v e l y used t o d e w a t e r s e d i m e n t s i n l a b o r a t o r y experiments compaction i s much more i m p o r t a n t i n n a t u r e .

There have been few e x p e r i m e n t a l s t u d i e s of

compaction a l t h o u g h Yaskawa (1974) h a s e s t i m a t e d t h e f i x a t i o n d e p t h of p o s t d e p o s i t i o n a l DRM u s i n g a v i s c o u s f l u i d model i n which v i s c o s i t y i s a f f e c t e d by g r a v i t a t i o n a l compaction.

Models o f t h i s k i n d a r e o n l y u s e f u l i f t h e y pay due

r e g a r d t o t h e i n h e r e n t m a g n e t o m i n e r a l o g i c a l p r o p e r t i e s o f s e d i m e n t s and t h e g r o s s n a t u r e of t h e s e d i m e n t a r y environment.

I n p a r t i c u l a r the r o l e of d i f f e r e n t

magnetic c a r r i e r s must b e c o n s i d e r e d a s must t h e m a g n e t i c g r a i n s i z e spectrum. The r o l e o f g r a i n s i z e v a r i a t i o n s i n DRM has been d e m o n s t r a t e d by a number of s t u d i e s of l a c u s t r i n e and g l a c i a l s e d i m e n t s .

Thompson and Kelts ( 1 9 7 4 ) , f o r

example, r e p o r t e d t h a t t h e b a s a l p a r t s o f t u r b i d i t e l a y e r s i n Lake Zug, S w i t z e r l a n d have lower m a g n e t i c i n t e n s i t i e s and lower i n c l i n a t i o n s t h a n t h e l a m i n a t e d upper portions.

The l i k e l y e x p l a n a t i o n o f t h i s i s t h a t t h e c o a r s e r p a r t i c l e s i n t h e s e

b a s a l l a y e r s were a l i g n e d w i t h t h e c u r r e n t d u r i n g d e p o s i t i o n and were u n a b l e t o r o t a t e i n t o t h e ambient f i e l d a f t e r d e p o s i t i o n b e c a u s e o f t h e i r c o a r s e g r a i n s i z e .

408 The o c c u r r e n c e o f DRM i n n a t u r e D e s p i t e t h e s i m p l i c i t y of t h e t h e o r y t h e a c t u a l o c c u r r e n c e of DRM o r p o s t This i s l a r g e l y because

d e p o s i t i o n a l DRM i s a c t u a l l y v e r y d i f f i c u l t t o prove.

o f t h e complex m a g n e t i c and t e x t u r a l v a r i a t i o n s shown by s e d i m e n t s .

Even i n

Recent s e d i m e n t s i t i s p o s s i b l e t h a t s i g n i f i c a n t p o r t i o n s o f t h e NRM may be of chemical o r i g i n and c a r r i e d by a u t h i g e n i c h a e m a t i t e .

However, i t i s now known

t h a t t h e a p p a r e n t Morin t r a n s i t i o n s o b s e r v e d i n Lake Windermere s e d i m e n t s ( C r e e r e t a l . , 1972) were n o t t r u e t r a n s i t i o n s b u t due t o t h e d i s t u r b a n c e o f m a g n e t i c f a b r i c d u r i n g t h e thawing of t h e s e d i m e n t ( C r e e r e t a l . , 1979). Under s p e c i a l c i r c u m s t a n c e s DRM may b e i n f e r r e d i f t h e r e i s e v i d e n c e t h a t t h e m a g n e t i z a t i o n was s t a b i l i z e d v e r y s h o r t l y a f t e r d e p o s i t i o n .

Verosub (1975), f o r

example, h a s compared t h e m a g n e t i z a t i o n of contemporaneous deformed and undeformed v a r v e s and was a b l e t o show t h a t t h e d e f o r m a t i o n had o c c u r r e d l e s s t h a n t h r e e y e a r s a f t e r d e p o s i t i o n of t h e v a r v e .

The m a g n e t i z a t i o n of t h e v a r v e s i s t h u s l i k e l y t o

b e a DRM o r v e r y e a r l y p o s t - d e p o s i t i o n a l DRM ( F i g . 7 . 2 0 ) .

Glacial varves represent

a r a t h e r s p e c i a l i z e d s e d i m e n t a r y environment i n which d e p o s i t i o n a l DRM and p o s t d e p o s i t i o n a l DRM a r e commonly p r e s e r v e d .

In l a c u s t r i n e s e d i m e n t s DRM and CRM a r e 1972; Thompson and K e l t s ,

both important magnetization processes (Creer et a l . ,

1974; Thompson, 1977) and i n t h e marine realm DRM i s a major p r o c e s s of m a g n e t i z a t i o n i n a v a r i e t y of environments (Kent, 1973; Graham, 1974; Blow and Hamilton,

1974).

A u s e f u l review o f DRM i n Recent s e d i m e n t s i s g i v e n by Verosub (1977). Une of t h e most u s e f u l methods i s t o compare t h e t h r e e - d i m e n s i o n a l m a g n e t i c f a b r i c w i t h t h e d i r e c t i o n of remanence.

T h i s can b e done by measurement of t h e

m a g n e t i c s u s c e p t i b i l i t y a n i s o t r o p y (Hamilton a n d Rees, 1970).

The m a g n e t i c

a n i s o t r o p y of many s e d i m e n t s can b e e s t i m a t e d by a second r a n k symmetric t e n s o r K ( G r a n a r , 1958) which r e l a t e s t h e m a g n e t i z a t i o n Ji i n d u c e d i n a specimen by a

f i e l d H:

(7.16)

J i = K i j Hj

The t e n s o r d e s c r i b e s a r e p r e s e n t a t i v e o r i e n t a t i o n f o r a l l t h e m a g n e t i c g r a i n s i n a sediment and g i v e s some i n d i c a t i o n o f t h e d e g r e e of a l i g n m e n t .

The s u s c e p t i b i l i t y

can b e c o m p l e t e l y s p e c i f i e d by t h e magnitude and o r i e n t a t i o n o f t h r e e o r t h o g o n a l p r i n c i p a l a x e s Kmax, Kint, two e l e m e n t s :

and Kmin.

The magnetic f a b r i c of many s e d i m e n t s c o n t a i n s

a h o r i z o n t a l f o l i a t i o n due t o t h e o r i e n t i n g e f f e c t o f t h e g r a v i t -

a t i o n a l f i e l d and a l i n e a t i o n due t o p r e f e r r e d g r a i n o r i e n t a t i o n s which r e s u l t from depositional currents.

S i n c e m a g n e t i t e i s most e a s i l y magnetized a l o n g i t s l o n g

a x i s b e c a u s e o f s h a p e a n i s o t r o p y any a x i s o f p r e f e r r e d g r a i n o r i e n t a t i o n w i l l c o i n c i d e w i t h t h e d i r e c t i o n o f t h e maximum s u s c e p t i b i l i t y a x i s . n a t u r a l s e d i m e n t s ( R e e s , 1965) and l a b o r a t o r y d e p o s i t e d m a t e r i a l

Measurements on (Rees and Woodall,

1975) i n d i c a t e t h a t s u s c e p t i b i l i t y a n i s o t r o p y i s a good measure of t h e s e d i m e n t a r y f a b r i c o f c o a r s e s i l t s and f i n e s a n d s .

A p r i m a r y d e p o s i t i o n a l f a b r i c may t h u s be

409

. .;

,

. N

W

Fig.7.20. Evidence f o r d e p o s i t i o n a l DRM i n g l a c i a l v a r v e s from New England. Above a v e r t i c a l c r o s s s e c t i o n shows f o l d e d v a r v e s and below t h e remanence d i r e c t i o n s a r e shown 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 . ' U n f o l d i n g ' t h e f o l d c a u s e s t h e d i r e c t i o n s t o c l u s t e r around t h e d i r e c t i o n s of n e a r l y undisturbed varves. D e t e r m i n a t i o n of t h e number o f v a r v e s i n v o l v e d i n t h e f o l d p u t s a l i m i t on t h e number o f y e a r s between d e p o s i t i o n and d i s t u r b a n c e ( a f t e r Verosub, 1975). e s t a b l i s h e d w i t h t h e p r i n c i p a l a x i s o f maximum s u s c e p t i b i l i t y l y i n g p a r a l l e l t o t h e b e d d i n g p l a n e and i n d i c a t i n g t h e p a l a e o c u r r e n t d i r e c t i o n .

A number o f palaeo-

c u r r e n t s t u d i e s have been made i n t h i s way (Hamilton, 1963; Hamilton and Rees, 1970).

T h i s does n o t n e c e s s a r i l y mean, of c o u r s e , t h a t t h e remanent magnetism

i s of d e p o s i t i o n a l o r i g i n and i t i s n e c e s s a r y t o compare t h e m a g n e t i c f a b r i c w i t h t h e NRM d i r e c t i o n . g r e a t e r than say

Coarse g r a i n e d s t r o n g l y a n i s o t r o p i c s e d i m e n t s ( t o t a l a n i s o t r o p y

lo%), which

show i n c l i n a t i o n e r r o r may be c o n s i d e r e d t o show

DRM i f t h e remanence and s u s c e p t i b i l i t y d e c l i n a t i o n s a r e c l o s e l y c o i n c i d e n t .

practice t h i s situation i s rarely realized.

In

I t i s more u s u a l t o f i n d t h a t t h e

t o t a l s u s c e p t i b i l i t y a n i s o t r o p y of s e d i m e n t s i s r a t h e r small (2-5%) and i s c a r r i e d by a d i f f e r e n t g r a i n s i z e f r a c t i o n t h a n t h e remanence.

Elongated magnetite g r a i n s ,

f o r example may b e a l i g n e d on d e p o s i t i o n and be t o o c o a r s e t o b e r o t a t e d i n t o t h e ambient f i e l d p r o v i d e d t h a t t h e g r a v i t a t i o n a l hydrodynamic f o r c e s exceed t h e m a g n e t i c c o u p l e due t o t h e geomagnetic f i e l d .

Very s m a l l m a g n e t i t e p a r t i c l e s may

be o r i e n t a t e d l a r g e l y by t h e magnetic f i e l d b e c a u s e t h e y may be s i n g l e domain

410 g r a i n s w i t h l a r g e r m a g n e t i c moments, and b e c a u s e t h e y a r e f r e e t o r o t a t e a f t e r d e p o s i t i o n t o produce p o s t - d e p o s i t i o n a l DRM.

The same r o c k may t h u s c a r r y

magnetic g r a i n s which c a r r y p a l a e o m a g n e t i c i n f o r m a t i o n and r e s i s t AF ' c l e a n i n g ' p r o c e d u r e s and l a r g e r magnetic g r a i n s which c a r r y p a l a e o c u r r e n t i n f o r m a t i o n and dominate t h e magnetic s u s c e p t i b i l i t y a n i s o t r o p y .

The c r i t i c a l g r a i n s i z e f o r

p o s t - d e p o s i t i o n a l a l i g n m e n t i s l i k e l y t o v a r y w i t h w a t e r c o n t e n t and m a t r i x composition b u t may commonly b e i n t h e r e g i o n o f 20-40 m

(Stupavsky e t a l . , 1974;

T u r n e r , 1975). Comparison o f t h e s u s c e p t i b i l i t y a n i s o t r o p y and remanence d i r e c t i o n s may b e used t o h e l p i n t h e r e c o g n i t i o n o f p o s t - d e p o s i t i o n a l DRM. shown, f o r example, t h a t i n some l i m e s t o n e s ( F i g . 7 . 2 1 )

Turner (1975) h a s

t h e p r i n c i p a l a x i s of

maximum s u s c e p t i b i l i t y l i e p a r t l y w i t h i n t h e bedding p l a n e i n d i c a t i n g a p r i m a r y d e p o s i t i o n a l f a b r i c and a r e a l s o s t r u n g o u t between t h e bedding p l a n e and t h e d i r e c t i o n o f remanence.

The n e a r c o i n c i d e n c e o f t h e p r i n c i p a l a x i s o f maximum

s u s c e p t i b i l i t y a n i s o t r o p y a n d t h e remanence d i r e c t i o n i s a good i n d i c a t i o n t h a t t h e m a g n e t i z a t i o n i s a DRM and t h e s p r e a d of d i r e c t i o n s i n d i f f e r e n t specimens a n e q u a l l y good i n d i c a t i o n o f t h e o c c u r r e n c e o f p o s t - d e p o s i t i o n a l g r a i n r o t a t i o n s .

k tl I

I I I l l 1

I I I I I I

I

I

1

Fig.7.21. Comparison o f t h e d i r e c t i o n s o f remanence and m a g n e t i c s u s c e p t i b i l i t y a n i s o t r o p y a x e s i n some C a r b o n i f e r o u s l i m e s t o n e s ( a f t e r T u r n e r , 1975). The o c c u r r e n c e o f DRM i n r e d beds has been s u g g e s t e d by a number of a u t h o r s ( I r v i n g , 1957; Opdyke, 1961; Roy and P a r k , 1972, 1974; Dunlop, 1972; C o l l i n s o n ,

1974) b u t t h e i r c o n c l u s i o n s have been l a r g e l y b a s e d on t h e f a c t t h a t s p e c u l a r i t e (opaque h a e m a t i t e p a r t i c l e s ) c a r r i e s a s i g n i f i c a n t p r o p o r t i o n o f t h e NRM.

This

assumption i s c l e a r l y i n c o r r e c t b e c a u s e much of t h e s p e c u l a r i t e i n r e d beds i s demonstrably o f d i a g e n e t i c o r i g i n a f t e r m a g n e t i t e (Van Houten, 1968).

I t does

seem l i k e l y , however, t h a t DRM may have been i m p o r t a n t i n many r e d b e d s , p r i o r t o d i a g e n e s i s b e c a u s e many of t h e o r i g i n a l d e t r i t a l o x i d e s were p r o b a b l y m a g n e t i t e .

411 R e l i c t g r a i n s o f m a g n e t i t e a r e known t o o c c u r i n a number of a n c i e n t r e d beds and b e c a u s e of t h e dominance o f s h a p e a n i s o t r o p y o v e r m a g n e t o c r y s t a l l i n e a n i s o t r o p y even i n r o c k s c o n t a i n i n g a l a r g e f r a c t i o n of h a e m a t i t e a p r i m a r y d e p o s i t i o n a l f a b r i c may b e p r e s e r v e d . Although a number of w o r k e r s , i n c l u d i n g t h e a u t h o r , have made measurements of t h e s u s c e p t i b i l i t y a n i s o t r o p y of r e d b e d s , t h e r e a r e few r e c o r d s of t h e p r e s e r v a t i o n of primary depositional f a b r i c s .

One of t h e few e x c e p t i o n s i s t h e Precambrian

M a r t i n Formation o f Saskatchewan, Canada which h a s been s t u d i e d by Evans and Bingham (1973) and Aziz-ur-Rahman e t a l . (1975).

Measurement of t h e magnetic

s u s c e p t i b i l i t y a n i s o t r o p y showed a w e l l - d e f i n e d primary s t y l e f a b r i c w i t h maxima (K1) o f magnetic s u s c e p t i b i l i t y p a r a l l e l t o t h e bedding p l a n e s , and minima (K3) grouped around t h e p a l a e o v e r t i c a l a f t e r c o r r e c t i o n f o r g e o l o g i c a l d i p e t a l . , 1975).

(Aziz-ur-Rahman

A s i s comonly the case i n sedimentary rocks, the s u s c e p t i b i l i t y

e l l i p s o i d s t e n d t o b e o b l a t e r a t h e r t h a n p r o l a t e ; i n o t h e r words t h e r e l a t i v e d i f f e r e n c e between K 3 and K

2

i s g r e a t e r t h a n t h a t between K1 and K2.

f o r a number of s i t e s a r e shown i n Fig.7.22

The K

1

axes

and when t h e s e a r e compared w i t h paleoo-

c u r r e n t e v i d e n c e deduced from c r o s s - b e d d i n g and t h e M a r t i n Formation b a s i n geometry t h e y can b e s e e n t o be a r e a s o n a b l e r e f l e c t i o n o f t h e p a l a e o c u r r e n t p a t t e r n .

All

t h e i n d i c a t i o n s h e r e a r e t h a t t h e s u s c e p t i b i l i t y a n i s o t r o p y i s a shape a n i s o t r o p y due t o a s m a l l p r o p o r t i o n o f m a g n e t i t e i n r o c k s whose o x i d e mineralogy i s dominated by haema t i t e . The s t a b l e NRM o f t h e M a r t i n Formation i s c a r r i e d by h a e m a t i t e (Evans and Bingham, 1973) which i s i n d i c a t e d by t h e r e s i s t a n c e o f t h e NRM t o AF c l e a n i n g . direction:

Dec = 322',

I n c = -29'

magnetic s u s c e p t i b i l i t y a n i s o t r o p y .

The mean

shows no r e l a t i o n s h i p t o t h e d i r e c t i o n s o f These r e s u l t s g i v e some i n s i g h t i n t o t h e

m a g n e t i z a t i o n of r e d beds; t h e o r i g i n a l sediment may have been r i c h i n m a g n e t i t e p o s s e s s i n g a d e p o s i t i o n a l magnetic f a b r i c and a DRM.

However, a s d i a g e n e s i s ,

i n c l u d i n g m a g n e t i t e o x i d a t i o n , p r o g r e s s e d t h e DRM was r e p l a c e d by a CRM c a r r i e d by h a e m a t i t e b u t t h e d e p o s i t i o n a l magnetic f a b r i c ( c a r r i e d p r i n c i p a l l y by t h e c o a r s e r m a g n e t i t e g r a i n s ) was. a t l e a s t p a r t i a l l y p r e s e r v e d . Chemical remanent m a g n e t i z a t i o n The i n s i t u m a g n e t i z a t i o n of s e d i m e n t s o r r o c k s by t h e chemical growth of m a g n e t i c m i n e r a l s i s r e f e r r e d t o a s chemical remanent m a g n e t i z a t i o n (CRM). Although t h e r e has been some d e b a t e a s t o t h e r e l a t i v e importance o f DRM and CRM i n r e d beds C o l l i n s o n (1965a) i n r e v i e w i n g t h e a v a i l a b l e e v i d e n c e concluded

t h a t CRM was t h e mechanism t h r o u g h which t h e m a j o r i t y of r e d bed f o r m a t i o n s were magnetized.

T h i s view i s s t i l l w i d e l y h e l d a t t h e p r e s e n t time.

The p r o c e s s of

CRM i s most c o n v e n i e n t l y c o n s i d e r e d t h e n u c l e a t i o n and s u b s e q u e n t g r a i n growth of haematite.

During t h e e a r l y s t a g e s t h e g r a i n s a r e so small t h a t t h e r e l a x a t i o n

41 2

Fig.7.22. Sampling s i t e s of t h e M a r t i n Formation i n Saskatchewan. The t h i c k arrows show t h e p r i n c i p a l a x e s of maximum s u s c e p t i b i l i t y a n i s o t r o p y and t h e t h i n arrows p a l a e o c u r r e n t d i r e c t i o n s b a s e d on c r o s s - s t r a t i f i c a t i o n ( a f t e r Aziz-ur-Rahman e t a l . , 1975). i s v e r y s h o r t and t h e g r a i n s a r e s u p e r p a r a m a g n e t i c .

A s t h e g r a i n grows i t p a s s e s

through t h e c r i t i c a l b l o c k i n g d i a m e t e r db and t h e r e l a x a t i o n t i m e i n c r e a s e s v e r y rapidly.

A t t h i s p o i n t t h e d i r e c t i o n of t h e ambient m a g n e t i c f i e l d i s f r o z e n i n

and s u b s e q u e n t changes i n f i e l d d i r e c t i o n w i l l have no e f f e c t upon i t s d i r e c t i o n of magnetization. From Eq.7.11 a s a s i n g l e domain grows a t t e m p e r a t u r e T , i t s m a g n e t i z a t i o n i s s t a b i l i z e d a t volume V

b

=

b

when

lnCy2:30

(7.17)

kT

where

Y

= 1000s and C = 10

10

S-

.

1

P u t t i n g K = 1250 e r g cm-3 (Strangway e t a l . ,

1 9 6 8 ) and assuming t h e g r a i n s a r e s p h e r e s , t h e c r i t i c a l b l o c k i n g d i a m e t e r a t 3OoC

-0,lym.

From Eq.7.17 g r a i n s w i t h a d i a m e t e r o f 0 . 1 4 ~have a r e l a x a t i o n t i m e of

lOI8 y e a r s c l e a r l y i n d i c a t i n g t h a t CRM can b e s t a b l e on a g e o l o g i c a l time s c a l e .

In p r a c t i c e t h e a c q u i s i t i o n i s p r o b a b l y much more c o m p l i c a t e d t h a n i s s u g g e s t e d

413 by t h i s s i m p l e model.

I n p a r t i c u l a r there a r e a v a r i e t y of diagenetic processes

which produce CRM i n r e d beds and n o t a l l of them i n v o l v e t h e s i m p l e n u c l e a t i o n and g r a i n growth of h a e m a t i t e .

The most i m p o r t a n t of t h e s e p r o c e s s e s a r e :

1.

The a g e i n g o f f e r r i c oxyhydroxide

2.

The p r e c i p i t a t i o n and growth from s o l u t i o n of f e r r i c oxyhydroxides

3.

The i n t r a s t r a t a l f o r m a t i o n of f e r r i c oxyhydroxides from s i l i c a t e a l t e r a t i o n

4.

The pseudomorphism o f s i l i c a t e s by

5.

The f o r m a t i o n o f a u t h i g e n i c

6.

I n s i t u o x i d a t i o n of m a g n e t i t e

Fe203 Fe 0 overgrowths 2 3

These p r o c e s s e s f a l l n a t u r a l l y i n t o two g r o u p s : p a r t i c l e h a e m a t i t e and

2.

1.

Those producing pigmentary f i n e

Those p r o d u c i n g b l a c k s p e c u l a r i t e g r a i n s .

Because of

t h e p r a c t i c a l d i f f i c u l t i e s o n l y t h e f i r s t group of p r o c e s s e s have been s t u d i e d under experimental conditions. Hedley (1968).

The CRM o f pigmentary FeOOH a n d FeZ03 has been s t u d i e d by

Hedley (1968) used s y n t h e t i c FeOOH mixed w i t h p l a s t e r of P a r i s

moulded i n t o 1 x 2.5cm d i s c s .

These were h e a t e d i n s t e p s from 220'

t o o v e r 6OO0C

i n t h e E a r t h ' s f i e l d and a l l o w e d t o c o o l i n f i e l d f r e e s p a c e i n o r d e r t o produce CRM.

A f t e r e a c h h e a t i n g and c o o l i n g c y c l e t h e remanence and s u s c e p t i b i l i t y o f t h e

d i s c s was measured.

Phase changes i n v o l v e d i n t h e d e h y d r a t i o n and a l t e r a t i o n o f

t h e i n i t i a l FeOOH were m o n i t o r e d u s i n g D i f f e r e n t i a l Thermal A n a l y s i s (DTA), D i f f e r e n t i a l Thermogravimetric A n a l y s i s (DTG) and Thermomagnetic measurements. X-ray d i f f r a c t i o n and E l e c t r o n Microscopy was used t o i d e n t i f y s p e c i f i c phases and t h e i r g r a i n s i z e . CRM of t h e g o e t h i t e

-

haematite t r a n s i t i o n

Hedley s t u d i e d s e v e r a l samples o f g o e t h i t e and i n common w i t h o t h e r a u t h o r s n o t e d some d i f f e r e n c e s i n t h e b e h a v i o u r o f d i f f e r e n t samples.

The r e s u l t s f o r a

w e l l - c r y s t a l l i z e d g o e t h i t e , G , a r e shown i n Fig.7.23. T h i s i s a 20% mix of 3 -1 g o e t h i t e and shows remanence o f cm g , i n i t i a l m a g n e t i c s u s c e p t i b i l i t y of

2Op

3 0 F cm3 0e-I g-'

and a decomposition t e m p e r a t u r e o f 290°C.

T h i s sample followed

t h e ambient f i e l d from d e h y d r a t i o n t o 45OoC and shows a well-marked double peak on t h e DTG r e c o r d c o r r e s p o n d i n g t o Van O o s t e r h o u t ' s (1965) Type A g o e t h i t e . 3 -1 and Most g o e t h i t e s , however, showed much weaker remanence of a b o u t 8 ~ c m 4 g a l s o more s c a t t e r e d d i r e c t i o n s , d e s p i t e having

s i m i l a r i n i t i a l magnetic s u s -

c e p t i b i l i t y , decomposition t e m p e r a t u r e a n d g r a i n s i z e . example (MYLL 100) a r e shown i n Fig.7.24.

R e s u l t s from a t y p i c a l

T h i s d i f f e r e n c e i n b e h a v i o u r could be

a c c o u n t e d f o r i f G 1 c o n t a i n e d a s m a l l amount of 7-FeOOH which would i n v e r t t o maghemite on d e h y d r a t i o n and t h u s c a r r y a s t r o n g e r , more s t a b l e CRM.

Since

y - F e 0 i s n o t s e e n on t h e thermomagnetic c u r v e o f G1 (Fig.7.23) a more l i k e l y 2 3 e x p l a n a t i o n l i e s i n t h e d e f e c t n a t u r e o f s y n t h e t i c g o e t h i t e s . T h i s d e f e c t was proposed t o e x p l a i n t h e v a r i a t i o n i n t h e r m a l and m a g n e t i c p r o p e r t i e s of g o e t h i t e

414

N

021 04'*

0

-

I

A

I

L

I

400

,

I"'"

I

,

800

800

n

Fig.7.23. CRM o f t h e W-FeOOH+ b(-FegOg t r a n s i t i o n , same15 GI. I. DTG c u r v e , dm/dt i n mg/min 11. DTA, d b l d t = 2.5OImin 1II.y P G cm 0 e - l g - l ) IV. 3 -1 CRM i n Y G a u s s cm g ( a . f t e r Hedley, 1968). (Van O o s t e r h o u t , 1965).

H e d l e y ' s r e s u l t s s u g g e s t t h a t i t may i n f l u e n c e t h e

magnetic p r o p e r t i e s o f t h e r e s u l t a n t h a e m a t i t e , a r e s u l t which i s of some s i g n i f i c a n c e i n t h e palaeomagnetism o f r e d beds i n view o f t h e l a r g e v a r i a t i o n i n m a g n e t i c p r o p e r t i e s which t h e y e x h i b i t . F i n e d-FeOOH was p r e p a r e d by a d d i n g sodium hydroxide t o molar f e r r i c c h l o r i d e solution.

The p r e c i p i t a t e c o n s i s t e d of p o o r l y d e f i n e d a c i c u l a r g r a i n s 2000 x 6508

mixed w i t h 2508 p a r t i c l e s .

The CRM r e s u l t s f o r t h i s , m a t e r i a l (50% mix) a r e q u i t e

u n l i k e t h o s e produced from t h e o t h e r g o e t h i t e samples ( F i g . 7 . 2 4 ) .

The remanence is

much h i g h e r by a n o r d e r o f magnitude and t h e r e i s a c o r r e s p o n d i n g i n c r e a s e i n t h e i n i t i a l magnetic s u s c e p t i b i l i t y .

The v a r i a t i o n o f remanence w i t h t e m p e r a t u r e shows

two d i s t i n c t maxima a t 27OoC and 45OoC w i t h CRM d i r e c t i o n s s c a t t e r e d a r o u n d t h e ambient f i e l d d i r e c t i o n a t 273'

and 335OC b u t show no g r o u p i n g a t o t h e r t e m p e r a t u r e s .

The maximum i n remanence a t 27OoC was shown by X-ray e v i d e n c e t o r e s u l t from

IM 0 p o s s i b l y a b o u t 1008 i n s i z e . Hedley (1968, p. 113) used R 2 3 measurements t o show t h a t s u c h f i n e m a t e r i a l c o u l d c a r r y remanence and a t t r i b u t e d ultra-fine#-Fe

415

N

-!:

6

4 2 200

00

4

600

800

1°C)

Fig.7.24.

CRM o f t h e q-FeOOH-, t<-Fe203 t r a n s i t i o n .

i t t o s p i n imbalance ( N g e l , 1962).

D e t a i l s same a s i n Fig.7.23.

The second maxima a t 45OoC is p r o b a b l y t h e

r e s u l t o f g r a i n growth o f t h e u l t r a - f i n e h a e m a t i t e from 1002 t o a b o u t 2002 a t which IRM i n c r e a s e s r a p i d l y .

I n summary H e d l e y ' s r e s u l t s show a number of f e a t u r e s

which a r e o f i n t e r e s t i n t h e m a g n e t i z a t i o n of r e d b e d s .

I n the f i r s t place the

g r e a t v a r i e t y i n thermal b e h a v i o u r o f t h e CRM produced by d e h y d r a t i o n o f t h e f o u r f e r r i c oxyhydroxide polymorphs i s c o n s i s t e n t w i t h t h e complex t h e r m a l d e m a g n e t i z a t i o n b e h a v i o u r o b s e r v e d i n r e d beds.

1. Aciculara(-FeOOH.

T h i s v a r i a t i o n may b e summarized:

O f t h r e e samples o n l y one a c q u i r e d a CRM on d e h y d r a t i o n which

was c l o s e t o t h a t o f t h e a p p l i e d f i e l d .

T h i s CRM showed a maximum i n m e d i a t e l y a f t e r

d e h y d r a t i o n when t h e h a e m a t i t e was a b o u t 4002 i n g r a i n s i z e 2. F i n e O(-FeOOH produced u l t r a f i n e N - F e 2 0 3 on d e h y d r a t i o n which showed a v e r y high remanent i n t e n s i t y n e a r t o t h e d i r e c t i o n o f t h e a p p l i e d f i e l d and p r o b a b l y due t o s p i n imbalance.

Above 35OoC t h e remanence d e c r e a s e s and t h e d i r e c t i o n s s c a t t e r ,

possibly a s a r e s u l t of s i n t e r i n g . S t u d i e s o f t h e o t h e r FeOOH polymorphs by Hedley (1968) a l s o r e v e a l e d p r o p e r t i e s which c o u l d b e of p r a c t i c a l s i g n i f i c a n c e i n t h e m a g n e t i z a t i o n of r e d b e d s ,

For

example, p-FeOOH produced a p p r e c i a b l e remanence o n l y a f t e r h e a t i n g t o 6OO0C

416

N ..

(b)

t*\

I

m

2oo

100

0 150

W

.

"

'

-

IX

5

Fig.7.25. CRM of f i n e d-FeOOH+c(-Fe203 a s Fig.7.23.

t r a n s i t i o n , sample 82.

D e t a i l s same

s u g g e s t i n g t h a t t h e R - F e 0 produced by t h i s d e h y d r a t i o n may b e c h e m i c a l l y and 2 3 m a g n e t i c a l l y d i f f e r e n t from t h a t produced by t h e d e h y d r a t i o n o f D(-FeOOH. The remanence produced a t 6OO0C was s t a b l e and i n t h e ambient f i e l d d i r e c t i o n . y-FeOOH showed m a g n e t i c b e h a v i o u r w i t h a s t r o n g dependence on c r y s t a l h a b i t . T a b u l a r c r y s t a l s produced s u p e r p a r a m a g n e t i c Y-Fe203 w h i l s t a c i c u l a r c r y s t a l s followed t h e ambient f i e l d t o 5OO0C a t which r e v e r s a l o c c u r r e d .

I f t h i s were

t r u e s e l f - r e v e r s a l , p o s s i b l y due t o some form o f m a g n e t o s t a t i c c o u p l i n g , t h e n i t c o u l d b e of some p a l a e o m a g n e t i c s i g n i f i c a n c e e s p e c i a l l y i f a p p r e c i a b l e q u a n t i t i e s of l e p i d o c r o c i t e were o r i g i n a l l y p r e s e n t i n a p a r t i c u l a r r e d bed f o r m a t i o n . These e x p e r i m e n t a l r e s u l t s show t h a t t h e d e h y d r a t i o n o f g o e t h i t e may b e a n i m p o r t a n t means of p r o d u c i n g CRM i n h a e m a t i t e .

However, i t s i m p o r t a n c e i n n a t u r e

i s v e r y d i f f i c u l t t o a s s e s s , e s p e c i a l l y i n view o f t h e wide v a r i e t y o f g o e t h i t e s which a r e found i n r e d b e d s .

D e t r i t a l f i n e g r a i n e d g o e t h i t e i n t h e form o f

' l i m o n i t e ' c o u l d a g e d i r e c t l y t o produce h a e m a t i t e w i t h a CRM.

Goethite a l s o

forms a s a m e t a s t a b l e i n t e r m e d i a t e d u r i n g t h e o x i d a t i o n of f e r r o m a g n e s i a n s i l i c a t e s . I t may be r e s p o n s i b l e f o r a s u b s t a n t i a l p a r t of t h e remanence i n s u c h c a s e s

e s p e c i a l l y i n r e l a t i v e l y young (Cenozoic) r e d beds ( e . g .

Larson a n d Walker, 1975).

417 I n g e n e r a l , t h e r e f o r e , i t seems l i k e l y t h a t CRM produced by t h e d e h y d r a t i o n o f g o e t h i t e i s i m p o r t a n t i n r e d beds.

The r e l a t i v e importance o f t h i s p r o c e s s and

t h e d i r e c t p r e c i p i t a t i o n of h a e m a t i t e from s o l u t i o n v a r i e s c o n s i d e r a b l y from one example t o a n o t h e r .

The d i r e c t p r e c i p i t a t i o n of h a e m a t i t e from groundwater

s o l u t i o n s may r e s u l t i n t h e f o r m a t i o n o f CRM i n a manner v e r y s i m i l a r t o t h a t o u t l i n e d i n t h e t h e o r y above.

This process i s important i n coarse grained red

beds which were porous and allowed t h e p a s s a g e o f groundwater s o l u t i o n s .

Both

pigmentary and opaque h a e m a t i t e c o u l d be p r e c i p i t a t e d i n t h i s way. THE MAGNETIC PROPERTIES OF CONTINENTAL RED BEDS

The magnetic p r o p e r t i e s o f r e d beds a r e d e s c r i b e d by C r e e r (1962a) and i n a series o f p a p e r s by C o l l i n s o n (1965a, b; 1966a, b; 1967; 1968a, b ) .

These s t u d i e s were

made b e f o r e r e c e n t advances i n o u r knowledge o f c o n t i n e n t a l r e d bed d i a g e n e s i s and t h e r e have been r e l a t i v e l y few r e c e n t s t u d i e s which have c o n s i d e r e d t h e magnetic p r o p e r t i e s of r e d beds i n r e l a t i o n t o t h e i r d i a g e n e t i c f e a t u r e s . The b u l k magnetic c h a r a c t e r i s t i c s o f r e d beds have been s t u d i e d u s i n g a v a r i e t y of methods which e n a b l e t h e i d e n t i f i c a t i o n o f t h o s e i r o n o x i d e s which c o n t r i b u t e towards t h e NRM and a l s o , i n some c i r c u m s t a n c e s , t h e i r t e x t u r a l c h a r a c t e r i s t i c s . Most f r e q u e n t l y thermomagnetic a n a l y s i s , e i t h e r by thermal demagnetization of t h e NRM ( I r v i n g and Opdyke, 1964 f o r example), o r more commonly by measurement of t h e

t e m p e r a t u r e dependence of t h e b u l k s a t u r a t i o n m a g n e t i z a t i o n (Schwarz, 1969 f o r example) h a s been used.

Thermomagnetic methods a r e o f l i t t l e v a l u e i n s e p a r a t i n g

d i f f e r e n t forms of h a e m a t i t e and a r e a l s o l i m i t e d by t h e f a c t t h a t h e a t i n g whole r o c k s c a u s e s magnetic m i n e r a l changes which make i n t e r p r e t a t i o n d i f f i c u l t .

Low

t e m p e r a t u r e t r a n s i t i o n s a r e u s e f u l i n a v o i d i n g phase changes (Nagata e t a l . , 1964; M a u r i t s c h and T u r n e r , 1975) b u t a r e l i m i t e d by t h e f a c t t h a t t h e t r a n s i t i o n s may be s u p p r e s s e d i n v e r y f i n e p a r t i c l e s .

The fundamental d i f f e r e n c e i n c o e r c i v i t y

s p e c t r a between h a e m a t i t e and m a g n e t i t e means t h a t t h e i r s e p a r a t e c o n t r i b u t i o n s may be i s o l a t e d by t h e measurement o f m a g n e t i z a t i o n (Ji-H) curves :

c u r v e s o r IRM (Jir-H)

m a g n e t i t e p a r t i c l e s , even when single-domain r a r e l y have c o e r c i v i t i e s

which exceed 1 KOe whereas t h e c o e r c i v i t y o f h a e m a t i t e may be a s high as 20 o r 30 kOe (Roquet, 1954).

I n a d d i t i o n t h e c o e r c i v i t y of h a e m a t i t e i n t h e s i n g l e -

domain range i s s u f f i c i e n t l y g r a i n s i z e dependent ( C h e v a l l i e r and Mathieu, 1943) t h a t t h e s e methods s h o u l d t h e o r e t i c a l l y e n a b l e t h e s e p a r a t i o n o f c o a r s e g r a i n e d s p e c u l a r i t e and f i n e p a r t i c l e pigment i n r e d beds. Another method o f a n a l y s i n g t h e c o e r c i v i t y spectrum i s by t h e d e t e r m i n a t i o n of r o t a t i o n a l h y s t e r e s i s , Wr ( t h e work done i n one c y c l e of a r o t a t i n g f i e l d H) a s a f u n c t i o n of H. O ' R e i l l y (1970) have used Wr-H

Brooks and

c u r v e s t o d i s t i n g u i s h between m a g n e t i t e , s p e c u l a r i t e ,

and pigment i n r e d beds and by comparing t h e i r r e s u l t s w i t h type c u r v e s f o r s y n t h e t i c h a e m a t i t e s , have e s t i m a t e d p i g m e n t / s p e c u l a r i t e r a t i o s .

418 The a p p l i c a t i o n of t h e s e t e c h n i q u e s has shown t h a t d e s p i t e c o n s i d e r a b l e complexity r e d beds have a number of c h a r a c t e r i s t i c m a g n e t i c p r o p e r t i e s .

These

p r o p e r t i e s e n a b l e i d e n t i f i c a t i o n of t h o s e m i n e r a l p h a s e s which c a r r y t h e NRM o f r e d b e d s , and when c o n s i d e r e d w i t h s e d i m e n t o l o g i c a l and d i a g e n e t i c e v i d e n c e a l l o w some i n t e r p r e t a t i o n o f t h e o r i g i n of t h e m a g n e t i z a t i o n i n c o n t i n e n t a l r e d beds

.

Induced m a g n e t i z a t i o n (J1-H) and i s o t h e r m a l remanence (IRM) c u r v e s C o l l i n s o n (1968a, b ) has shown t h a t J i - H

c u r v e s of r e d beds i n t h e r a n g e

0-8 KOe have a s h a l l o w c u r v a t u r e compared w i t h t h a t of n a t u r a l and s y n t h e t i c h a e m a t i t e s , and a s t r a i g h t l i n e p o r t i o n between a b o u t 3 KOe and 8 KOe ( F i g . 7 . 2 6 ) . These f e a t u r e s s u g g e s t t h e p r e s e n c e o f s u b s t a n t i a l p a r a m a g n e t i c m a t e r i a l , p o s s i b l y i n c l u d i n g i r o n - r i c h c l a y s , p h y l l o s i l i c a t e s , and i l m e n i t e .

The s u s c e p t i b i l i t i e s

of some common p a r a m a g n e t i c c o n s t i t u e n t s a r e g i v e n i n T a b l e 7 . 1 .

*r

-

cy

P x

c I

m

3

6

9

Field (K.Oe)

Fig.7.26. T y p i c a l p a r t i a l h y s t e r e s i s c u r v e s o b t a i n e d from r e d beds. M a g n e t i z a t i o n i s corrected f o r diamagnetic contribution o i n c r e a s i n g f i e l d odecreasing f i e l d ( a f t e r C o l l i n s o n , 1968b).

419 TABLE 7 . 1 The m a g n e t i c s u s c e p t i b i l i t i e s of some p a r a m a g n e t i c c o n s t i t u e n t s which commonly o c c u r i n s e d i m e n t a r y rocks.

Mineral

S u s c e p t i b i l i t y YG 0 e - l g - l

Fe203(%)

FeO(%)

Illite

1.4

4.7

12

Montmorillonite

2.8

3.0

11

Chamosite

14.0

40.0

70

Biotite

19.2

7.9

63

Orthopyroxene

24.0

1.0

40

I lmeni t e

46.4

-

85

The h i g h - f i e l d s u s c e p t i b i l i t y o f h a e m a t i t e i s one o f i t s most c h a r a c t e r i s t i c -1 -1 p r o p e r t i e s and has a v a l u e o f a b o u t cm3 Oe g C o l l i n s o n ( 1 9 6 8 a ) has

2YG

.

used t h i s p r o p e r t y t o c a l c u l a t e t h e h a e m a t i t e c o n t e n t o f a number of r e d bed f o r m a t i o n s ( T a b l e 7 . 2 ) and has made d e t a i l e d i n v e s t i g a t i o n s of t h e r e l a t i o n s h i p s TABLE 7 . 2 The h a e m a t i t e c o n t e n t of v a r i o u s r e d beds based on magnetic a n a l y s i s . Formation

T o t a l Fe203(%)

Ha ema ti t e (% )

Taiguati (Carboniferous, Brazil)

2.01

1.46

Pimenteira (Devonian, B r a z i l )

8.70

6.67

Cutler (Permian, USA)

1.82

1.12

Chinle ( T r i a s s i c , USA)

2.22

1.06

Miller Peak ( P r e c a m b r i a n , USA)

3.54

1.31

420

between f e r r o u s / f e r r i c i r o n r a t i o s and i n d u c e d m a g n e t i z a t i o n i n a number of r e d bed f o r m a t i o n s ( C o l l i n s o n , 1968b).

F i g . 7 . 2 7 shows t h e dependence o f t h e a v e r a g e

s p e c i f i c induced m a g n e t i z a t i o n , Jsi w i t h i n a f o r m a t i o n .

The p l o t t e d v a l u e s a r e

based on from 4 - 1 3 samples f o r e a c h f o r m a t i o n and t h e b a r s i n d i c a t e t h e r a n g e o f variation.

The r e s u l t s show a c l e a r p o s i t i v e c o r r e l a t i o n when t h e a v e r a g e s p e c i f i c

R IM (IRM p e r gram o f Fe20j) a c q u i r e d by samples from e a c h f o r m a t i o n i s compared with the corresponding average f e r r o u s / f e r r i c i r o n r a t i o a negative c o r r e l a t i o n i s apparent (Fig.7.28).

This indicates t h a t the mineral controbuting t o the increased

induced m a g n e t i z a t i o n i s p r o b a b l y p a r a m a g n e t i c o r h a s o n l y a v e r y weak IRM, s i n c e i t i s e a s i l y shown t h a t m a g n e t i t e o r maghemite would a l s o i n c r e a s e t h e s p e c i f i c

IRM i f i t was t h e c a u s e of t h e i n c r e a s e d Jsi.

t

T

t

TB .

T..

1

lW

1

0

10

20

3

4

5

6

Fe O/Fe203(%)

Fig.7.27. Formation means of s p e c i f i c induced m a g n e t i z a t i o n , J s i p l o t t e d a g a i n s t mean f e r r o u s / f e r r i c o x i d e r a t i o ( a f t e r C o l l i n s o n , 1968b). IRM c u r v e s a r e u s e f u l i n d e t e r m i n i n g t h e m a g n e t i c m i n e r a l o g y of r e d beds

(Dunlop, 1972).

T h i s i s b e c a u s e m a g n e t i t e and maghemite a r e r e l a t i v e l y ' s o f t '

and even f o r e l o n g a t e d s i n g l e domain p a r t i c l e s have a maximum remanent c o e r c i v i t y

(Her)

about cr 2 KOe f o r 40 m g r a i n s and f i n e p a r t i c l e pigmentary h a e m a t i t e i s by f a r t h e h a r d e s t of a b o u t 1500 Oe.

Coarse h a e m a t i t e ( s p e c u l a r i t e ) i s ' h a r d e r ' w i t h H

material with H

i n e x c e s s of 19 KOe and p r o b a b l y much h i g h e r . Using t h i s cr i n f o r m a t i o n Dunlop (1972) h a s a n a l y s e d t h e c o e r c i v i t y s p e c t r a o f r e d beds and

a r b i t r a r i l y d i v i d e d them i n t o t h e i r ' s o f t ' ( 0 < H c r

< IKOe),

'intermediate'

421

0'

I

I

I

I

10

20

30

40

I

50

60

FeO/Fe2O3(%)

Fig.7.28. Formation means o f s p e c i f i c IRM p l o t t e d a g a i n s t mean f e r r o u s / f e r r i c o x i d e r a t i o ( a f t e r C o l l i n s o n , 1968b). (1
Fig.7.29

shows t h e IRM c u r v e s o f a number o f Upper

B u n t s a n d s t e i n (Lower T r i a s s i c ) r e d beds from t h e Vosges Massif o f e a s t e r n France

L

I

I

5

10

15

H ( KOe) Fig.7.29. T y p i c a l IRM c u r v e s o f unheated r e d beds from t h e Upper B u n t s a n d s t e i n ( a f t e r Dunlop, 1972).

422 D e t e r m i n a t i o n of t h e remanent i n t e n s i t y (18 KOe) J r and mean c o e r c i v i t y ( f i e l d r e q u i r e d t o produce 1 / 2 J ) sandstones.

Hcr

from t h e s e c u r v e s g i v e v a l u e s t y p i c a l of r e d 3 G cm and 3cr from 2.57 t o 5.20 KOe.

J r r a n g e s frzm 9.9 t o 34.7 x

These r e s u l t s s u g g e s t t h a t t h e s a n d s t o n e s c o n t a i n i m p o r t a n t amounts o f s p e c u l a r i t e and pigment and o n l y minor amounts of m a g n e t i t e and maghemite.

terms t h e m a g n e t i t e c o n t e n t need o n l y be 0 . 5

-

In quantitative

1%of t h e h a e m a t i t e c o n t e n t i n o r d e r

t o make a n e q u a l c o n t r i b u t i o n t o s a t u r a t i o n remanence, s o i n t h e s e r o c k s t h e maximum amount must b e a f r a c t i o n of one p e r c e n t of t h e t o t a l o x i d e c o n t e n t .

In

o r d e r t o a n a l y s e t h e c o e r c i v i t y s p e c t r a , a s deduced from IRM c u r v e s , i n more d e t a i l Dunlop (1972 p. 39) c a l c u l a t e d t h e i n c r e m e n t a l i s o t h e r m a l remanent moments mr 3 (G c m ) i n 1 KOe A H i n t e r v a l s and p l o t t e d t h e r e s u l t s a s a h i s t o g r a m . These i n c r e m e n t a l c o e r c i v i t y s p e c t r a ( F i g . 7 . 3 0 ) r e v e a l o n l y minor amounts o f m a g n e t i t e as expected, but a l s o considerable v a r i a t i o n s i n coercivity spectra i n the

10-18 K O e r a n g e .

T h i s i s u n l i k e l y t o b e due t o t h e i n f l u e n c e o f s p e c u l a r i t e and

must be due t o i n t r i n s i c v a r i a t i o n s i n t h e c o e r c i v i t y spectrum o f t h e pigment which a r e p r o b a b l y due t o g r a i n s i z e v a r i a t i o n s , s t r u c t u r a l d i f f e r e n c e s and impurity content.

These v a r i a t i o n s a r e q u i t e c o n s i s t e n t w i t h t h e e x p e r i m e n t a l

e v i d e n c e from s y n t h e t i c h a e m a t i t e s .

0

5

10

15

Fig.7.30. T y p i c a l c o e r c i v i t y s p e c t r a o f two s a n d s t o n e s from t h e Upper B u n t s a n d s t e i n , d e t e r m i n e d from t h e c u l v e s o f Fig.7.29 ( a f t e r Dunlop, 1972).

423 T h i s method of i n c r e m e n t a l c o e r c i v i t y spectrum a n a l y s i s i s o f c o n s i d e r a b l e v a l u e i n m o n i t o r i n g chemical and s t r u c t u r a l changes whicn t a k e p l a c e d u r i n g t h e h e a t i n g of r e d beds.

I n t h e B u n t s a n d s t e i n s t u d i e d by Dunlop (1972) a f t e r e a c h

s t a g e of h e a t t r e a t m e n t t h e m a g n e t i c mineralogy was determined from t h e c o e r c i v i t y spectrum.

T y p i c a l s p e c t r a f o r one s a n d s t o n e a r e shown i n Fig.7.31.

The most

obvious changes o c c u r i n t h e m a g n e t i t e (0-1 KOe) and pigment ( 3 - 1 8 KOe) t r a c t i o n s . Heating i n a i r t o 52OoC d e s t r o y s a b o u t t w o - t h i r d s o f t h e o r i g i n a l m a g n e t i t e c o n t e n t , presumable by o x i d a t i o n t o h a e m a t i t e , b u t h e a t i n g t o 62OoC and above r e s u l t s i n magnetite production.

The p r o d u c t i o n o f m a g n e t i t e on h e a t i n g r e d beds

h a s been f r e q u e n t l y r e p o r t e d i n t h e l i t e r a t u r e ( S t e p h e n s o n , 1967; Schwarz, 1969, 1970) and i s p r o b a b l y due t o t h e breakdown of i r o n - s i l i c a t e s (Dunlop, 1972, p. 4 6 ) . Such a p r o c e s s i s l i k e l y t o c a u s e s e r i o u s problems d u r i n g t h e r m a l d e m a g n e t i z a t i o n b e c a u s e of t h e p r o d u c t i o n o f l a b o r a t o r y m a g n e t i z a t i o n s .

Unannealed

c

0 2-

620°C

I I I r I 1 I7-m

0

I

, ,

.

i

-g

I

(Y

0, X

i

a

+ *

0

5

10

15

H (kOe) E v o l u t i o n of t h e c o e r c i v i t y spectrum of a t y p i c a l s a n d s t o n e (BD3) when Fig.7.31. h e a t e d i n a i r t o t h e g i v e n t e m p e r a t u r e and a n n e a l e d 2-3hr ( a f t e r Dunlop, 1972).

424

The c o e r c i v i t y spectrum of t h e pigment changes o n l y ,a l i t t l e between 520 and 85OoC b u t then i n c r e a s e s s h a r p l y i n t h e 94OoC a n n e a l .

This increase i n coercivity

spectrum of t h e pigment i s c o n s i s t e n t w i t h D u n l o p ' s (1971) r e s u l t s from s y n t h e t i c f i n e p a r t i c l e haematites.

I t i s i n t e r p r e t e d a s e i t h e r t h e a n n e a l i n g o u t of d e f e c t

ferromagnetism o r p o s s i b l y t h e i n c o r p o r a t i o n o f i m p u r i t i e s such a s Ti4+ i n t o t h e haematite l a t t i c e . J.-T analysis Thermal decay c u r v e s of induced m a g n e t i z a t i o n i n r e d beds can g e n e r a l l y be i n t e r p r e t e d a s a combination o f p a r a m a g n e t i c components, d e c a y i n g a c c o r d i n g t o t h e C u r i e Law, i . e .

i n v e r s e l y a s t h e a b s o l u t e t e m p e r a t u r e , and a f e r r o m a g n e t i c

c o n t r i b u t i o n from h a e m a t i t e which decays s l o w l y a t lower t e m p e r a t u r e s and t h e n drops rapidly i n t h e v i c i n i t y of the N e h l point.

F i g . 7 . 3 2 shows Ji-T

show t y p i c a l paramagnetic and f e r r o m a g n e t i c b e h a v i o u r .

c u r v e s which

The m a g n e t i z a t i o n s t i l l

p r e s e n t a t 75OoC can b e e x p l a i n e d by t h e p a r a m a g n e t i c component which w i l l f a l l t o a b o u t 30% of i t s room t e m p e r a t u r e v a l u e by 750°C and t h e f a c t t h a t h a e m a t i t e

i s paramagnetic above t h e Nkel p o i n t .

0

400

800

Temperature ('C)

Fig.7.32. T y p i c a l examples of t h e t h e r m a l decay of i n d u c e d m a g n e t i z a t i o n i n . samples showing h i g h , i n t e r m e d i a t e and low h a e m a t i t e c o n t e n t ( P 4 0 5 83%, T14 52%, D12 17% of t o t a l f e r r i c o x i d e r e s p e c t i v e l y . D 1 2 shows n e a r p u r e l y p a r a m a g n e t i c b e h a v i o u r ( a f t e r C o l l i n s o n , 1968a).

425

Some a u t h o r s ( e . g . Dunlop, 1971) c o n s i d e r t h a t t h e Ne'el p o i n t and C u r i e p o i n t O t h e r workers c o n s i d e r t h e NLel p o i n t t o

of h a e m a t i t e a r e c o i n c i d e n t a t 68OoC.

b e a l i t t l e h i g h e r , n e a r t o 69OoC ( C o l l i n s o n , 1968a) and t h e C u r i e p o i n t i s o f t e n quoted a s 675OC a l t h o u g h h i g h e r v a l u e s a r e commonly r e p o r t e d .

The work of a number

of a u t h o r s i s c o n s i s t e n t w i t h a c o i n c i d e n t NLel p o i n t and C u r i e p o i n t a t 725'C (Uyeda, 1958; Hedley, 1968; Al-Khafaji a n d Vincenz, 1971).

I n t h e i r s t u d y of t h e

Cambrian Lamotte Formation, Al-Khafa j i and Vincenz (1971) r e p o r t e d t h e predominant C u r i e t e m p e r a t u r e a t 725OC, a l t h o u g h v a l u e s o f 675OC and some i n t h e range 550-600°C were a l s o found,

Al-Khafa j i and Vincenz (1971) r e i n t e r p r e t e d t h e d a t a

of Nagata and Akimoto (1956) showing t h e r e l a t i o n s h i p between C u r i e t e m p e r a t u r e and i l m e n i t e c o n t e n t i n t h e h a e m a t i t e - i l m e n i t e s e r i e s (Fig.7.34).

The C u r i e

t e m p e r a t u r e a t 675OC would t h u s c o r r e s p o n d t o a n i l m e n i t e c o n t e n t of a b o u t 6%; and t h a t a t 55OoC t o a n i l m e n i t e c o n t e n t o f 17-20%.

Curie p o i n t s i n t h e

55O0C-60O0C range a r e commonly found i n r e d beds and i t seems t h a t t h e s e a r e most l i k e l y due t o t i t a n i u m s u b s t i t u t i o n i n h a e m a t i t e .

This i s consistent with

t h e m i n e r a l o g i c a l and geochemical e v i d e n c e which s u g g e s t s t h a t t i t a n h a e m a t i t e s may be more abundant i n t h e s e d i m e n t a r y environment 3 t h a n has g e n e r a l l y been r e a l i z e d . C u r i e p o i n t s i n t h e r a n g e 0-500°C a r e n o t

w i t h up t o a b o u t 5-10% FeTiO

commonly r e p o r t e d from r e d bed s t u d i e s .

These would b e e x p e c t e d i n t h e p r e s e n c e

of t i t a n i u m - r i c h h a e m a t i t e s o r t i t a n o m a g n e t i t e s and i t must b e deduced t h a t s u c h phases a r e correspondingly r a r e i n red beds.

l~t-0-L

--

iZ'9-

-e+

warm grained drt. aminad rdrt.

--A-+-- fino

.\

I 0

200

400

600

800

Tnnporature ('C )

Fig.7.33. C u r i e t e m p e r a t u r e a n a l y s i s of t h e Cambrian Lamotte Formation (USA). H e a t i n g i n nominal vacuum o f 10-3m, A p p l i e d f i e l d 1 KOe. The r e s u l t s a r e f u l l y d e s c r i b e d i n t h e t e x t ( a f t e r Al-Khafaji and Vincenz, 1971).

426

0

Fe203

20

40 60 TiFe03mol %

00

100 TiFe03

Fig.7.34. R e l a t i o n z h i p between C u r i e t e m p e r a t u r e and chemical composition i n t h e h a e m a t i t e - i l m e n i t e s e r i e s . S o l i d l i n e and t h e d a t a i n d i c a t e d by open and c l o s e d c i r c l e s a f t e r Nagata and Akimoto (1956). Dashed l i n e i s a l e a s t s q u a r e s f i t f o r s y n t h e t i c samples which a r e shown by t h e f u l l c i r c l e s . N a t u r a l m i n e r a l s a r e shown by open c i r c l e s ( a f t e r Al-Khafaji and Vincenz, 1971).

427 A f e a t u r e n o t e d i n a number of thermomagnetic a n a l y s e s o f r e d beds i s t h e i r r e v e r s i b i l i t y o f t h e .Ti-T c u r v e s .

100

200

A t y p i c a l example i s shown i n Fig.7.35.

300 400

500 800

700

T'c

Fig.7.35. I r r e v e r s i b l e thermomagnetic b e h a v i o u r i n a Devonian r e d s a n d s t o n e from t h e O r c a d i a n b a s i n , S c o t l a n d ( a f t e r Waage and S t o r e t v e d t , 1973). These c u r v e s u s u a l l y show a t r a n s i t i o n i n t h e r e g i o n o f 200-4OO0C repeated during cooling.

which i s n o t

A l s o , t h e r e i s u s u a l l y a s i g n i f i c a n t loss of magnet-

i z a t i o n a f t e r t h e h e a t i n g and c o o l i n g c y c l e .

These f e a t u r e s a r e r e a d i l y i n t e r p r e t e d

a s a n i n d i c a t i o n o f t h e p r e s e n c e o f maghemite o r titanomaghemite which undergoes i n v e r s i o n t o h a e m a t i t e and o t h e r i n v e r s i o n p r o d u c t s a t t h e t r a n s i t i o n t e m p e r a t u r e , which i s u s u a l l y between 300 and 40OoC.

The loss of m a g n e t i z a t i o n i s due t o t h e

l a r g e d i f f e r e n c e i n s p e c i f i c m a g n e t i z a t i o n between titanomaghaemite and h a e m a t i t e . T h i s d i f f e r e n c e does mean, of c o u r s e , t h a t t h e amounts of titanomaghemmite p r e s e n t i n r e d beds a r e n o t n e c e s s a r i l y l a r g e , p o s s i b l y i n t h e r e g i o n 0.5-1% when p r e s e n t . The NRM o f r e d beds The NRM o f r e d beds v a r i e s i n i n t e n s i t y between a b o u t 0.10 and 100 G .

The

upper l i m i t i s t h e t h e o r e t i c a l maximum which a rock specimen c o n t a i n i n g one p e r c e n t h a e m a t i t e by volume ( c h e m i c a l l y produced) c o u l d o b t a i n a t a t m o s p h e r i c p r e s s u r e s i n t h e E a r t h ' s f i e l d ( s a y 0 . 5 0 e ) ( S t a c e y , 1963).

The most c h a r a c t e r i s t i c

f e a t u r e of t h e NRM of r e d beds i s i t s e x t r e m e l y h i g h c o e r c i v i t y ; t h i s i s c o n s i s t e n t w i t h t h e view t h a t t h e m a g n e t i z a t i o n i s c a r r i e d by s i n g l e domain h a e m a t i t e .

An

i m p o r t a n t consequence of t h i s i s t h a t r o u t i n e l a b o r a t o r y methods of AF demagneti z a t i o n ( r a n g e 0-1KOe) c a n n o t b e used i n t h e p a r t i a l d e m a g n e t i z a t i o n (magnetic c l e a n i n g ) o f r e d beds and o t h e r methods s u c h a s t h e r m a l and chemical d e m a g n e t i z a t i o n must b e used.

R e s u l t s o f t h e AF t r e a t m e n t o f some t y p i c a l r e d b e d s , d e m o n s t r a t i n g

t h i s h i g h c o e r c i v i t y , a r e shown i n F i g . 7 . 3 6 .

428

Direction difference from NRM

Fig.7.36. The r e s u l t s of i n c r e a s i n g AF t r e a t m e n t on t h e NRM i n t e n s i t y o f some t y p i c a l c o n t i n e n t a l r e d beds (The T o r r i d o n i a n S a n d s t o n e ) ( a f t e r S t e w a r t and I r v i n g , 1974). Thermal d e m a g n e t i z a t i o n has been most w i d e l y u s e d i n t h e p a l a e o m a g n e t i c s t u d y of r e d beds.

U s u a l l y specimens a r e h e a t e d i n s t e p w i s e i n c r e m e n t s o f 50 o r 100°C

and m a i n t a i n e d a t t h e d e s i r e d t e m p e r a t u r e f o r a b o u t 5 minutes.

By h e a t i n g t o

p r o g r e s s i v e l y h i g h e r t e m p e r a t u r e s t h e lower c o e r c i v i t y components a r e removed u n t i l , n e a r t h e C u r i e p o i n t o f h a e m a t i t e , o n l y t h e h i g h e s t c o e r c i v i t y components o f t h e NRM remain.

(Eq.7.13)

S i n c e t h e c o e r c i v i t y i s d i r e c t l y r e l a t e d t o r e l a x a t i o n time

i t i s g e n e r a l l y b e l i e v e d t h a t s t a b l e NRM components i s o l a t e d i n t h i s

way approximate t o t h e p r i m a r y remanence.

The t h e r m a l decay c u r v e s o f t h e NRM i n

r e d beds v a r y w i d e l y depending upon t h e number o f d i s c r e t e components which may be p r e s e n t and a l s o on t h e i r b l o c k i n g t e m p e r a t u r e s p e c t r a ,

Blocking t e m p e r a t u r e s

may b e t h e r m a l l y d i s t r i b u t e d o r t h e r m a l l y d i s c r e t e ( I r v i n g and Opdyke, 1964).

In

t h e former c a s e a wide d i s t r i b u t i o n of b l o c k i n g t e m p e r a t u r e s r e s u l t s i n a s t e a d y drop i n NRM i n t e n s i t y d u r i n g t h e r m a l d e m a g n e t i z a t i o n , whereas i n t h e l a t t e r c a s e t h e r e i s l i t t l e loss o f NRM i n t e n s i t y u n t i l t e m p e r a t u r e s i n e x c e s s o f 6OO0C when a l a r g e drop o c c u r s ( F i g . 7 . 3 7 ) .

The common o c c u r r e n c e of b l o c k i n g t e m p e r a t u r e s

o v e r 6OO0C i n r e d beds i n d i c a t e s t h a t h a e m a t i t e must b e t h e p r i n c i p a l m a g n e t i z a t i o n carrier. Although t h e r m a l d e m a g n e t i z a t i o n may b e u s e f u l i n removing s e c o n d a r y m a g n e t i z a t i o n s from r e d beds d e t a i l e d s t u d i e s show t h a t i t i s n o t always p o s s i b l e t o i s o l a t e t h e s t a b l e p r i m a r y NRM.

T h i s i s h a r d l y s u r p r i s i n g i n v i e w o f t h e complex phase

changes and growths of h a e m a t t t e which t a k e p l a c e d u r i n g d i a g e n e s i s .

These o f t e n

r e s u l t i n a v e r y complex NRM which i s i m p o s s i b l e t o r e s o l v e i n t o d i s c r e t e m a g n e t i c components and which d e f i e s s t a n d a r d p a l a e o m a g n e t i c i n t e r p r e t a t i o n s .

429

1.0

-

M ~ 0.5 o -

1

I

I

100

200

1

I

300 400 Tempemture

1

I

I

500

600

700

(‘t)

Fig.7.37. The t h e r m a l decay o f t h e NRM of two specimens o f t h e St.Bees Sandstone ( T r i a s s i c , UK). SB 19 shows t h e r m a l l y d i s t r i b u t e d components. SB 10 shows t h e m a l l y d i s c r e t e components. The complexity of r e d bed m a g n e t i z a t i o n s has meant t h a t t h e r e have been v e r y few s t u d i e s of t h e d e t a i l e d s t r u c t u r e o f t h e NRM i n i n d i v i d u a l r e d bed f o r m a t i o n s , and i n p a r t i c u l a r o f i t s r e l a t i o n s h i p t o d i f f e r e n t t e x t u r a l p h a s e s of h a e m a t i t e . The few s t u d i e s u n d e r t a k e n have c o n c e n t r a t e d on t h e r e l a t i v e importance of s p e c u l a r i t e and pigment a s NRM c a r r i e r s .

Two t e c h n i q u e s have been u s e d :

a)

chemical d e m a g n e t i z a t i o n and b ) t h e c o m p a r a t i v e thermal d e m a g n e t i z a t i o n of s e p a r a t e d c o n c e n t r a t e s o f s p e c u l a r i t e , pigment and t h e p a r e n t rock. Chemical d e m a g n e t i z a t i o n The t e c h n i q u e o f chemical d e m a g n e t i z a t i o n was o r i g i n a l l y d e v i s e d by C o l l i n s o n (1965b) and minor m o d i f i c a t i o n s were made by Park (1970).

It involves the treatment

o f specimen c y l i n d e r s o r d i s c s w i t h H C 1 and assumes t h a t t h e two forms of h a e m a t i t e a r e dissolved a t different rates.

The specimens a r e removed from s o l u t i o n a t

p e r i o d i c i n t e r v a l s t o b e remeasured and t h e amount o f i r o n d i s s o l v e d c a n b e d e t e r m i n e d by a n a l y s i s of t h e a c i d s o l u t i o n .

W h i l s t some u s e f u l r e s u l t s have

been o b t a i n e d u s i n g chemical d e m a g n e t i z a t i o n i t s g e n e r a l s u i t a b i l i t y i s open t o q u e s t i o n f o r a number o f r e a s o n s .

I n t h e f i r s t p l a c e i t i s l i m i t e d because o n l y

specimens w i t h s u i t a b l e p o r o s i t y and p e r m e a b i l i t y c a n be used.

A l s o , because

of t h e s l o w p e n e t r a t i o n o f t h e a c i d , a s i t u a t i o n o f t e n a r i s e s whereby t h e pigment and s p e c u l a r i t e a r e c o m p l e t e l y d i s s o l v e d n e a r t h e margins of t h e specimen b u t remain v i r t u a l l y untouched n e a r t h e c e n t r e o f t h e specimen.

I n inhomogeneously

magnetized r e d beds s u c h a s i t u a t i o n c o u l d e a s i l y l e a d t o v e r y m i s l e a d i n g r e s u l t s However some u s e f u l r e s u l t s have been o b t a i n e d which s u g g e s t t h a t b o t h pigment and s p e c u l a r i t e a r e i m p o r t a n t NRM c a r r i e r s i n d i f f e r e n t r e d bed f o r m a t i o n s . C o l l i n s o n (1965b) i n a s t u d y o f t h e T r i a s s i c Chugwater Formation showed t h a t

430

70-80% o f t h e NRM s t i l l remained a f t e r l e a c h i n g o f t h e pigmentary h a e m a t i t e (Fig.7.38)

and t h u s concluded t h a t t h e s p e c u l a r i t e c a r r i e d t h e b u l k of t h e NRM.

I n t h e T a i g u a t i Formation ( C a r b o n i f e r o u s , B o l i v i a ) C o l l i n s o n (1966a) s u b j e c t e d a number of samples t o chemical l e a c h i n g a n d showed a b a s i c s i m i l a r i t y i n t h e shape o f c u r v e s r e p r e s e n t i n g NRM decay and loss o f i r o n c o n t e n t ( F i g . 7 . 3 9 ) .

1 .oo

1.00

NRM 0.75

0.75

0.50

MIM,

t

0

0.25

A 10

15

20

25

0 10

Time (hr)

15

20

Time (hr)

Fig.7.38. Comparison of t h e NRM decay and d e c r e a s e i n i r o n c o n t e n t d u r i n g chemical l e a c h i n g o f two samples o f t h e T r i a s s i c Chugwater Formation ( a f t e r C o l l i n s o n , 1965b).

Also 30-45% o f t h e t o t a l i r o n o x i d e c o n t e n t o f t h e r o c k s remained a f t e r t h e NRM had decayed t o z e r o and C o l l i n s o n concluded t h a t t h e pigment was r e s p o n s i b l e f o r t h e NRM.

Later experiments a l s o suggest t h a t pigment.carries a l a r g e proportion

of t h e NRM ( C o l l i n s o n , 1974, F i g . 4 ) . The chemical d e m a g n e t i z a t i o n t e c h n i q u e i s most u s e f u l when compared w i t h d i r e c t i o n a l i n f o r m a t i o n r e g a r d i n g t h e NRM.

T h i s o f t e n r e v e a l s t h a t t h e pigment

and s p e c u l a r i t e c a r r y d i f f e r e n t d i r e c t i o n s o f m a g n e t i z a t i o n .

Since these could

o n l y o c c u r a f t e r changes i n t h e geomagnetic f i e l d d i r e c t i o n , i t s u g g e s t s t h a t a t l e a s t one of t h e m a g n e t i z a t i o n s i s of d i a g e n e t i c o r i g i n . (Permian, U.S.A.)

I n t h e S u p a i Formation

t h e r o c k s c a r r y a m a g n e t i z a t i o n o f Q u a t e r n a r y a g e and t h e

o r i g i n a l Permian d i r e c t i o n c a n n o t be r e v e a l e d by t h e r m a l d e m a g n e t i z a t i o n , even up t o t h e C u r i e p o i n t o f h a e m a t i t e .

However, d u r i n g chemical l e a c h i n g o f t h e

431 -0-

NRM

IRON CONTENT

-%-

Stag.

Fig.7.39. Comparison o f NRM decay and d e c r e a s e o f i r o n c o n t e n t d u r i n g chemical l e a c h i n g o f samples from t h e C a r b o n i f e r o u s T a i g u a t i Formation o f B o l i v i a ( a f t e r C o l l i n s o n , 1966a). i r o n o x i d e , t h e d i r e c t i o n s of m a g n e t i z a t i o n moved towards t h e Permian f i e l d d i r e c t i o n o f North America ( F i g . 7 . 4 0 ) .

These r e s u l t s a r e r e a d i l y I n t e r p r e t e d

i f one assumes t h a t t h e b u l k o f t h e Q u a t e r n a r y m a g n e t i z a t i o n i s c a r r i e d by t h e pigment, which i s removed d u r i n g t h e l e a c h i n g , w h i l s t t h e o r i g i n a l Permian m a g n e t i z a t i o n i s c a r r i e d p r i n c i p a l l y by t h e s p e c u l a r i t e . Roy and P a r k (1972, 1974) have made d e t a i l e d chemical l e a c h i n g experiments on Hopewell Group ( M i s s i s s i p p i a n - P e n n s y l v a n i a n ) o f N e w Brunswick, Canada.

These r e d

beds o f t e n c o n t a i n two s t a b l e m a g n e t i z a t i o n s w i t h i n t h e same specimen c l e a r l y i n d i c a t i n g t h e a c q b i s i t i o n of a t l e a s t p a r t o f t h e remanence d u r i n g d i a g e n e s i s . F i g . 7 . 4 1 shows t h e chemical l e a c h i n g of a s i n g l e specimen.

The NRM d i r e c t i o n ,

i n i t i a l l y of s h a l l o w upward i n c l i n a t i o n and d i r e c t i o n towards t h e s o u t h , moves northwards d u r i n g t h e l e a c h i n g t o r e v e a l a n o r t h e r l y d i r e c t e d component, and then back t o t h e s o u t h e r l y p o s i t i o n r e v e a l i n g a t h i r d component, t h i s t i m e w i t h s h a l l o w downward i n c l i n a t i o n .

Although Roy and Park (1972) r e f e r r e d t o t h e s e t h r e e

m a g n e t i z a t i o n s a s CRMA, CRM

B

m a g n e t i z a t i o n i s a DRM.

and DRM t h e r e i s no e v i d e n c e t h a t t h e l a s t r e v e a l e d

I n view o f t h e e v i d e n c e c o n c e r n i n g changes which might

a f f e c t t h e s p e c u l a r i t e f r a c t i o n i t does n o t n e c e s s a r i l y f o l l o w t h a t t h e most chemically r e s i s t a n t i r o n oxides i n t h e rock, o r t h e c o a r s e s t g r a i n s , c a r r y a d e t r i t a l m a g n e t i z a t i o n o r even t h a t t h i s i s t h e p r i m a r y m a g n e t i z a t i o n .

Two of

t h e t h r e e m a g n e t i z a t i o n components ( h e r e d e s i g n a t e d A and B , F i g . 7 . 4 1 ) a r e removed a t a p o i n t which c o i n c i d e s w i t h t h e removal o f t h e r e d pigment t h r o u g h o u t t h e specimen and Roy and Park (1972) t h u s concluded t h a t t h e s e two m a g n e t i z a t i o n s were c a r r i e d by t h e pigment and must be of chemical o r i g i n .

I t c a n n o t b e assumed a s many a u t h o r s do (e.g. E l s t o n and P a r u c k e r , 1979) t h a t chemical d e m a g n e t i z a t i o n p r e f e r e n t i a l l y removes pigment and l e a v e s specu l a r i t e i n t a c t i n a l l r e d beds.

I n f a c t t h e r e i s some e v i d e n c e t h a t t h e t o t a l

432 1 .oo

0

N

0.75

0.50

0,25

0 100

200

5

300

Time (min.)

Fig.7.40. Chemical l e a c h i n g of t h e Permian S u p a i Formation, A . L O S S of NRM ( c i r c l e s ) and i r o n c o n t e n t ( c r o s s e s ) , B. D i r e c t i o n a l changes showing t h e p r o g r e s s i v e movement of t h e specimens towards t h e USA Permian f i e l d d i r e c t i o n ( a f t e r Collinson, 1965b). TN

Change Scale & Acid Strength BNtj+lON

Hour8 in Acid

Fig.7.41. Chemical l e a c h i n g of a sample from t h e Hopewell Group. The d i r e c t i o n a l B) changes r e v e a l t h r e e superimposed m a g n e t i z a t i o n s A , B and C , two o f which (A a r e a s s o c i a t e d w i t h t h e r e d pigment ( m o d i f i e d from Roy and P a r k , 1972).

+

433 amount of m a g n e t i z a t i o n l o s t may have a s i g n i f i c a n t c o n t r i b u t i o n from t h e s p e c u l a r i t e fraction.

I n t h e S t . Bees S a n d s t o n e pigment-dominated and s p e c u l a r i t e - d o m i n a n t

m a g n e t i z a t i o n s behave r a t h e r d i f f e r e n t l y d u r i n g chemical d e m a g n e t i z a t i o n .

Sandstones

w i t h no pigment show a more r a p i d i n i t i a l loss of NRM c l e a r l y i n d i c a t i n g t h e d e s t r u c t i o n of t h e s p e c u l a r i t e (Fig.7.42).

I n r e d b e d s w i t h a pigment-dominated

NRM

t h e r e i s a l e s s r a p i d i n i t i a l loss b u t a c r i t i c a l p o i n t i s soon r e a c h e d when t h e r e i s a s h a r p drop i n NRM i n t e n s i t y ( F i g . 7 . 4 2 ) .

D e t a i l e d p e t r o l o g i c a l examination o f

c o m p l e t e l y l e a c h e d specimens shows t h a t r e s i d u a l NRM may b e c a r r i e d by pigment s i t e d between d e t r i t a l g r a i n s and a u t h i g e n i c overgrowths which c a n n o t b e a t t a c k e d by a c i d . These r e s u l t s from chemical l e a c h i n g e x p e r i m e n t s thus p r o v i d e a n i m p o r t a n t c l u e t o t h e d i f f e r e n t r o l e s o f pigment and s p e c u l a r i t e a s NRM c a r r i e r s i n r e d beds.

They

i n d i c a t e t h a t b o t h are i m p o r t a n t b u t t h e r e l a t i v e p r o p o r t i o n s of NRM c a r r i e d v a r y

N

N

+

I

i

\

Fig.7.42. Chemical d e m a g n e t i z a t i o n o f a d r a b ( s p e c u l a r i t e - d o m i n a n t MM) s a n d s t o n e (SB18) and a r e d (pigment-dominant NRM) s a n d s t o n e (SB1) from t h e S t . Bees Sandstone (Triassic). The thermal d e m a g n e t i z a t i o n c u r v e s a r e shown f o r comparison.

434 c o n s i d e r a b l y n o t o n l y between f o r m a t i o n s , b u t a l s o from sample t o sample w i t h i n any i n d i v i d u a l f o r m a t i o n .

Inherent mineralogical variation including the t o t a l

p i g m e n t / s p e c u l a r i t e r a t i o and v a r i a t i o n s i n g r a i n s i z e and c o m p o s i t i o n of pigment and s p e c u l a r i t e a r e l i k e l y t o d e t e r m i n e t h e i r r e l a t i v e i m p o r t a n c e a s NRM c a r r i e r s . The l e a c h i n g e x p e r i m e n t s a r e a l s o i m p o r t a n t b e c a u s e t h e y i n d i c a t e t h a t t h e pigment

i s of post-depositional o r i g i n

-

a feature q u i t e consistent with t h e diagenetic

evidence. Comparative thermal d e m a g n e t i z a t i o n of pigment, s p e c u l a r i t e and rock T h i s t e c h n i q u e was d e v i s e d by C o l l i n s o n (1974) s p e c i f i c a l l y t o d e t e r m i n e t h e r o l e of pigment and s p e c u l a r i t e a s NRM c a r r i e r s .

I t i n v o l v e s c r u s h i n g a sample

o f t h e rock and s e p a r a t i n g t h e s p e c u l a r i t e u s i n g a h e a v y - l i q u i d a n d slow-speed The remanent magnetism i s t h e n r e c o n s t i t u t e d by a l l o w i n g t h e

centrifuge, specularite

+

heavy m i n e r a l s ( h e r e a f t e r r e f e r r e d t o a s s p e c u l a r i t e ) and pigment

+

l i g h t f r a c t i o n ( h e r e a f t e r r e f e r r e d t o a s pigment) t o s e t t l e through a l i q u i d column i n a known m a g n e t i c f i e l d ; t h e r e s u l t i n g s e d i m e n t s t h e n p o s s e s s a DRM which can be s t u d i e d and compared w i t h t h e NRM p r o p e r t i e s of t h e whole r o c k . D e t a i l s a r e g i v e n by C o l l i n s o n (1974, p. 254). A v a r i e t y o f r e d bed f o r m a t i o n s were i n v e s t i g a t e d by C o l l i n s o n (1974) w i t h v a r i o u s r e s u l t s , which may have p a r t l y depended on t h e e f f i c i e n c y o f t h e s p e c u l a r i t e separation.

The most remarkable r e s u l t s o b t a i n e d by C o l l i n s o n were t h a t t h e

s p e c u l a r i t e was more s t a b l e t h a n t h e pigment i n b o t h AF and t h e r m a l d e m a g n e t i z a t i o n . F i g . 7 . 4 3 shows t h e t h e r m a l d e m a g n e t i z a t i o n of a s e r i e s of f i n e g r a i n e d s a n d s t o n e s and s i l t s t o n e s .

C l e a r l y t h e s p e c u l a r i t e i s m a g n e t i c a l l y h a r d e r i n a l l t h e s e samples

and t h e c l o s e s i m i l a r i t y i n t h e r m a l decay of t h e s p e c u l a r i t e and t h e whole r o c k s u g g e s t s t h a t t h e s p e c u l a r i t e i s c a r r y i n g t h e NRM i n t h e s e r o c k s .

This conclusion

does make t h e assumption t h e r e i s no m a g n e t i c a l l y h a r d component of t h e pigment, Such a component m i g h t c a r r y remanence i n t h e rock b u t c o u l d be o b s c u r e d i n t h e

DRM sample b e c a u s e o f a g r e a t e r q u a n t i t y o f ' s o f t ' m a t e r i a l which i s n o t a l i g n e d i n t h e rock b u t becomes a l i g n e d i n t h e DRM p r o c e s s .

Also, the 'hard' magnetic

p a r t i c l e s which dominate t h e DRM o f t h e s p e c u l a r i t e c o u l d b e c a n c e l l e d out: i n t h e whole rock and so n o t c o n t r i b u t e t o t h e o v e r a l l NRM.

I n o r d e r t o show t h a t t h e

DRM s e p a r a t i o n s were r e p r e s e n t a t i v e C o l l i n s o n r e p e a t e d t h e d e p o s i t i o n u s i n g low f i e l d s i n t h e range 0.1-2.0

O e ( a s opposed t o t h e r o u t i n e 10 Oe) and showed t h a t

t h e r e was no s i g n i f i c a n t d i f f e r e n c e i n t h e t h e r m a l d e m a g n e t i z a t i o n o f t h e DRM produced.

Another method which c a n b e used t o d e t e c t d i f f e r e n t m a g n e t i c p r o p e r t i e s ,

e s p e c i a l l y t h e p r e s e n c e of s i g n i f i c a n t amounts o f m a t e r i a l w i t h low o r h i g h b l o c k i n g t e m p e r a t u r e s , i s t o examine t h e p a r t i a l thermoremanence (PTRM) spectrum of t h e pigment and s p e c u l a r i t e .

The PTRM i n t e n s i t y a c q u i r e d i n any t e m p e r a t u r e i n t e r v a l

can then be compared w i t h t h e DRM i n t e n s i t y l o s s i n t h e same i n t e r v a l d u r i n g

435

0 ROCK

X SPECULARITE

0 PIGMENT

0,4L 0

100

200

300

400

500

0

600

100

200

300

400

500

600

Tmmpwaturm ('C)

Fig.7.43. Thermal d e m a g n e t i z a t i o n o f t h e whole rock NRM and t h e DRM of e x t r a c t e d s p e c u l a r i t e and pigment f o r a group of r e d beds i n which s p e c u l a r i t e c a r r i e s a s i g n i f i c a n t p r o p o r t i o n of t h e NRM. Red beds r e p r e s e n t e d a r e : Chugwater Formation (R5, HC6, G 3 ) Moenkopi Formation ( 0 7 ) Supai Formation ( S 3 1 ) Bonito Canyon Q u a r t z i t e (B17) ( a f t e r C o l l i n s o n , 1974). d e m a g n e t i z a t i o n ; i f t h e r a t i o s of PTRM g a i n e d t o DRM l o s t a r e s i m i l a r i n t h e same t e m p e r a t u r e i n t e r v a l t h e n t h e r e i s e v i d e n c e t h a t t h e p r o p e r t i e s of t h e DRM a r e r e p r e s e n t a t i v e of t h e b u l k magnetic p r o p e r t i e s o f t h e two s e p a r a t e s . f o r two samples a r e shown i n F i g . 7 . 4 4 .

The r e s u l t s

These samples were h e a t e d i n t h e u s u a l

100°C i n t e r v a l s and a l l o w e d t o c o o l i n a 1 Oe f i e l d , t h e PTRM a c q u i r e d i n e a c h

i n t e r v a l b e i n g d e t e r m i n e d by s u b t r a c t i o n .

The r e s u l t s c l e a r l y show t h e absence

of any components w i t h h i g h b l o c k i n g t e m p e r a t u r e s and t h u s confirms t h a t t h e s t a b l e

NRM i n t h e s e specimens i s c a r r i e d by t h e s p e c u l a r i t e .

Using t h e same t e c h n i q u e

T u r n e r and Archer (1975) have shown t h e i m p o r t a n c e o f s p e c u l a r i t e a s an NRM c a r r i e r i n t h e Old Red Sandstone and s i m i l a r r e s u l t s a r e p r e s e n t e d by T u r n e r and I x e r (1977) f o r p a r t of t h e T r i a s s i c S t . Bees Sandstone. C o l l i n s o n (1974) d e s c r i b e d a n o t h e r group of r e d beds which showed a d i f f e r e n c e i n t h e r m a l decay between t h e NRM and DRM.

In t h e s e r o c k s i t was n o t p o s s i b l e t o

make s u c h a c l e a r s e p a r a t i o n of pigment from s p e c u l a r i t e and i t seems l i k e l y t h a t

436

DRM 0 Pigment x Specularite

PTRM

+ Pigment A Specularite

0.2'

1

100

200

300

400

500

600

Tempomtwo ('C )

Comparison of t h e r m a l d e m a g n e t i z a t i o n o f DRM and PTRM spectrum o f Fig.7.44. pigment and s p e c u l a r i t e f r a c t i o n s i n two r e d beds from t h e S u p a i Formation (S31) and Chugwater Formation ( R 5 ) ( a f t e r C o l l i n s o n , 1974). b o t h pigment and s p e c u l a r i t e c o n t r i b u t e towards t h e NRM. i n Fig.7.45

The r e s u l t s a r e shown

ZY60 and ZY64 can be e x p l a i n e d by t h e NRM o f t h e pigment b e i n g r e v e r s e d

r e l a t i v e t o t h e NRM of t h e s p e c u l a r i t e .

I n ZY60 t h e g r e a t e r s p e c u l a r i t e magnet-

i z a t i o n c o u p l e d w i t h t h e more r a p i d l o s s of t h e pigment m a g n e t i z a t i o n r e s u l t s i n t h e whole r o c k NRM i n c r e a s i n g a s t h e c a n c e l l a t i o n e f f e c t o f t h e two a n t i p o d a l l y opposed v e c t o r s i s reduced. t e m p e r a t u r e i s approached.

A s h a r p drop i n t h e NRM and DRM i s s e e n a s t h e C u r i e I n ZY64 t h e t h e r m a l d e m a g n e t i z a t i o n b e h a v i o u r i s b e s t

e x p l a i n e d by assuming t h a t t h e pigment NRM dominates i n t h e rock.

The r a p i d loss

of whole rock NRM a t h i g h e r t e m p e r a t u r e s c a n be e x p l a i n e d i f t h e NRM o f t h e r o c k

is t h e d i f f e r e n c e between t h a t o f t h e pigment and s p e c u l a r i t e i n which c a s e i t s p r o p o r t i o n a l d e c r e a s e can b e g r e a t e r t h a n t h a t o f e i t h e r of i t s components. e x p l a n a t i o n i s a l s o a p p l i c a b l e t o P397.

This

S i n c e t h e s e r e s u l t s c o n t a i n b o t h normal

and r e v e r s e d components of m a g n e t i z a t i o n one of them a t l e a s t must be a CRM .and have been a c q u i r e d a f t e r t h e time t a k e n f o r a f i e l d r e v e r s a l t o o c c u r .

The r e s u l t s t h u s

q u i t e c l e a r l y i n d i c a t e t h e d i a g e n e t i c o r i g i n o f t h e h a e m a t i t e pigment i n ZP60 and ZY64. C o l l i n s o n ' s t e c h n i q u e i s p o t e n t i a l l y v e r y v a l u a b l e i n r e l a t i n g t h e NRM of r e d beds t o t h e i r d e p o s i t i o n a l and d i a g e n e t i c h i s t o r y .

T h i s i s e s p e c i a l l y so when t h e

thermal d e m a g n e t i z a t i o n and d i r e c t i o n a l c h a r a c t e r i s t i c s o f t h e whole rock NRM a r e

437 0 PIGMENT

0 ROCK

0

100 200

300 400 500 600 700

Temperature ( C '

# SPECULARITE

L

0

1

100 200 300 400 500 600

Temperature

["c )

Fig.7.45. Thermal d e m a g n e t i z a t i o n o f t h e whole rock NRM and t h e DRM of e x t r a c t e d s p e c u l a r i t e and pigment f o r a group o f r e d beds i n which b o t h pigment and s p e c u l a r i t e a p p e a r t o c a r r y a s i g n i f i c a n t p r o p o r t i o n o f t h e NRM. Red beds r e p r e s e n t e d a r e t h e T a i g u a t i Formation (ZY60, ZY164) Brunswick Beds, T r i a s s i c , USA (T2) and P i c o s Beds, Devonian, B r a z i l (P397) ( a f t e r C o l l i n s o n , 1974). compared w i t h t h e t h e r m a l d e m a g n e t i z a t i o n t r e n d s of e x t r a c t e d pigment and s p e c u l a r i t e fractions.

I t i s t h e n p o s s i b l e t o g a i n some i n d i c a t i o n o f t h e time of a c q u i s i t i o n

of t h e NRM c a r r i e d by i n d i v i d u a l s e p a r a t e s and deduce something of t h e m a g n e t i z a t i o n h i s t o r y o f t h e whole rock.

Turner and A r c h e r (1975) a p p l i e d t h i s method i n a s t u d y

of t h e Old Red Sandstone Gamrie O u t l i e r i n S c o t l a n d .

A l l t h e r e d beds i n t h i s a r e a

have complex m a g n e t i z a t i o n s ; some c o n t a i n components which can b e i n f e r r e d t o have been a c q u i r e d i n Devonian t i m e s ; o t h e r s have been c o m p l e t e l y remagnetized p r o b a b l y d u r i n g t h e L a t e Carboniferous-Permian p e r i o d . (P9) i s shown i n F i g . 7 . 4 6 .

An example o f one of t h e s e s a n d s t o n e s

The NRM of t h i s s a n d s t o n e i s s t a b l e and has SSE

declination with steep negative inclination.

T h i s i s q u i t e d i f f e r e n t from known

N

+

0 Whole Rock

A D

Specularite > 53 y m Specularite < 53 y m

X Pigment

0.5

\. I

I

I

I

I

I

1

100 200 300 400 500 600 700 TEMPERATURE (

C)

F i g . 7 . 4 6 . Thermal d e m a g n e t i z a t i o n o f t h e NRM of a remagnetized Devonian Sandstone and t h e DRM of i t s e x t r a c t e d pigment, c l a y - o x i d e , and s p e c u l a r i t e f r a c t i o n s . S p e c u l a r i t e c a r r i e s t h e NRM and must have been remagnetized a s i n d i c a t e d by t h e d i r e c t i o n a l a n a l y s i s o f t h e whole rock NRM i n post-Devonian times, p r o b a b l y by t h e i n s i t u o x i d a t i o n of m a g n e t i t e . Devonian d i r e c t i o n s and t h e s a n d s t o n e must have s u f f e r e d post-Devonian remagn e t i z a t i o n , p r o b a b l y i n Permian t i m e s , j u d g i n g from t h e d i r e c t i o n s of a number

o f s i m i l a r specimens ( T u r n e r and A r c h e r , 1975, p. 248). of s e p a r a t e d pigment, c l a y

+

Thermal d e m a g n e t i z a t i o n

i r o n o x i d e and s p e c u l a r i t e f r a c t i o n s shows t h a t t h e

NRM must r e s i d e i n t h e s p e c u l a r i t e f r a c t i o n .

Hence t h e s p e c u l a r i t e f r a c t i o n must

have been remagnetized i n Permian times and m i n e r a l o g i c a l e v i d e n c e s u g g e s t s t h a t t h e o x i d a t i o n of m a g n e t i t e was r e s p o n s i b l e . CONCLUSIONS AND FURTHER READING The m a g n e t i c p r o p e r t i e s of c o n t i n e n t a l r e d beds i n d i c a t e t h a t h a e m a t i t e i s t h e p r i n c i p a l magnetization c a r r i e r .

Although b o t h t e x t u r a l p h a s e s of h a e m a t i t e ,

439 pigment and s p e c u l a r i t e , a c t a s remanence c a r r i e r s i n p a r t i c u l a r c a s e s , t h e s p e c u l a r i t e i s g e n e r a l l y more i m p o r t a n t .

There i s no e v i d e n c e t h a t t h i s s p e c u l a r i t e

c a r r i e s any m a g n e t i z a t i o n (DRM o r PDRM) which r e l a t e s t o d e p o s i t i o n a l o r e a r l y post-depositional

events.

I n d e e d t h e e v i d e n c e of e x t e n s i v e s p e c u l a r i t e d i a g e n e s i s

( m a r t i t i z a t i o n and a u t h i g e n i c overgrowth) s u g g e s t s t h a t t h e m a g n e t i z a t i o n i s i n t i m a t e l y r e l a t e d t o red-bed d i a g e n e s i s .

T h i s i s c o n s i s t e n t w i t h t h e abundance

of multicomponent m a g n e t i z a t i o n s i n r e d beds which s u g g e s t a long-continued p r o c e s s o f d i a g e n e s i s and a s s o c i a t e d m a g n e t i z a t i o n . The t h e o r y of r o c k magnetism and palaeomagnetism i n c l u d i n g m a g n e t i z a t i o n p r o c e s s e s i n s e d i m e n t s a r e d e s c r i b e d by: McElhinny, M.W.,

1973. Palaeomagnetism and P l a t e T e c t o n i c s . Cambridge U n i v e r s i t y

P r e s s , 358 pp. S t a c e y , F.D.

a n d B a n e r j e e , S.K., 1974. The P h y s i c a l P r i n c i p l e s of Rock Magnetism

E l s e v i e r , Amsterdam, 195 pp. Verosub, K . L . ,

1977. D e p o s i t i o n a l and p o s t - d e p o s i t i o n a l p r o c e s s e s i n t h e

m a g n e t i z a t i o n of s e d i m e n t s . Rev. Geophys. Space. P h y s . , 15.: 129-143.

440

CHAPTER 8

THE PALAEOMAGNETISM OF CONTINENTAL RED BEDS INTRODUCTION

C o n t i n e n t a l r e d beds a r e of s p e c i a l s i g n i f i c a n c e i n palaeomagnetism. were t h e f i r s t s e d i m e n t a r y r o c k s t o be s t u d i e d i n any d e t a i l ( e . g .

They

Clegg e t a l . ,

1954; Runcorn, 1955) and many p o l a r wander p a t h s and 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 r e l y h e a v i l y on p a l a e o m a g n e t i c d e t e r m i n a t i o n s from r e d beds.

Many of t h e s e r e s u l t s

were made p r i o r t o t h e r e c e n t advances i n o u r knowledge of r e d bed d i a g e n e s i s b u t t h e y have s u b s e q u e n t l y c o n t i n u e d and g e n e r a l l y w i t h l i t t l e r e g a r d t o t h e d i a g e n e t i c p r o c e s s e s which a r e r e s p o n s i b l e f o r t h e m a g n e t i z a t i o n of r e d beds.

The rock magnetic

e v i d e n c e q u i t e c l e a r l y i n d i c a t e s t h a t h a e m a t i t e i s t h e p r i n c i p a l remanence-carrying phase i n r e d beds.

S i m i l a r l y the diagenetic evidence i n d i c a t e s t h a t haematite i s

mainly an a u t h i g e n i c phase and i t must b e concluded t h a t t h e magnetism of c o n t i n e n t a l r e d beds i s a c q u i r e d d u r i n g d i a g e n e s i s . Palaeomagnetic s t u d i e s have been made on r e d beds of a l l g e o l o g i c a l a g e s , throughout t h e P r o t e r o z o i c and t h e P h a n e r o z o i c , i n c l u d i n g t h e Cenozoic d e s e r t a l l u v i u m o f t h e s o u t h w e s t e r n USA (Larson and Walker, 1 9 7 5 ) .

I n many i n d i v i d u a l

r e d bed f o r m a t i o n s which have undergone e x t e n s i v e d i a g e n e t i c c h a n g e s , t h e p a l a e o magnetic p r o p e r t i e s may p r o v i d e t h e most d e t a i l e d r e c o r d of t h e s e changes.

This

may be i n t h e form of d i f f e r e n t components of m a g n e t i z a t i o n c a r r i e d by d i f f e r e n t t e x t u r a l p h a s e s of h a e m a t i t e which were produced a t d i f f e r e n t s t a g e s o f d i a g e n e s i s . Other magnetic p r o p e r t i e s , such a s blocking temperature s p e c t r a , o r s u s c e p t i b i l i t y changes d u r i n g h e a t i n g , may g i v e i m p o r t a n t c l u e s t o t h e d e p t h o f t e m p e r a t u r e s t o which t h e r o c k s have been h e a t e d .

buria1,or the

The p a l a e o m a g n e t i c s i g n i f i c a n c e

of continental red beds, t h e r e f o r e , i s n o t t h a t they represent an accurate record of t h e ambient geomagnetic f i e l d a t t h e time of d e p o s i t i o n b u t t h a t t h e y a r e e x t r e m e l y s e n s i t i v e i n d i c a t o r s of t h e n a t u r e and e x t e n t of r e d bed d i a g e n e s i s The palaeomagnetic p r o p e r t i e s o f r e d beds a r e e x t r e m e l y v a r i a b l e b u t a number of d i s t i n c t i v e t y p e s o f m a g n e t i z a t i o n ( b a s e d on t h e number of d i s c r e t e components p r e s e n t and t h e t i m e of t h e i r a c q u i s i t i o n ) can b e i d e n t i f i e d .

These d i f f e r e n t

t y p e s of r e d bed m a g n e t i z a t i o n show no s y s t e m a t i c v a r i a t i o n through g e o l o g i c a l time.

I n f a c t , t h e p a l a e o m a g n e t i c s t u d y o f r e d beds of v a r i o u s a g e s p r o v i d e s a n

i m p o r t a n t c o n t r i b u t i o n t o t h e o r i g i n o f r e d beds.

I t shows t h a t d i a g e n e s i s must

be a r a t e p r o c e s s i n which t h e r a t e , i n c l u d i n g t h e r a t e of a c q u i s i t i o n o f magn e t i z a t i o n , i s determined by:

1. t h e o r i g i n a l m i n e r a l o g y o f t h e s e d i m e n t ; m i n e r a l o g i c a l l y immature s e d i m e n t s b e i n g s u s c e p t i b l e t o more e x t e n s i v e a l t e r a t i o n and p r o l o n g e d m a g n e t i z a t i o n

441

2 . t h e d e p o s i t i o n a l c o n d i t i o n s ; f i n e r g r a i n e d s e d i m e n t s b e i n g less s u s c e p t i b l e t o e x t e n s i v e a l t e r a t i o n and p r o l o n g e d m a g n e t i z a t i o n

3. t h e p r e v a i l i n g c l i m a t e ; warm a r i d c l i m a t e s b e i n g f a v o u r a b l e f o r t h e longc o n t i n u e d a l t e r a t i o n and m a g n e t i z a t i o n o f r e d beds. These f e a t u r e s c a n b e d e m o n s t r a t e d by r e f e r e n c e t o r e d beds of v a r i o u s g e o l o g i c a l a g e s and d e p o s i t e d i n a v a r i e t y o f d e p o s i t i o n a l environments under d i f f e r e n t climatic conditions. PROTEROZOIC BASINS OF WESTERN CANADA The n o r t h w e s t e r n p a r t o f t h e Canadian s h i e l d i s s u b d i v i d e d i n t o t h e S l a v e , C h u r c h i l l , and Bear P r o v i n c e s on t h e t e c t o n i c map o f Canada ( S t o c k w e l l , 1968). E a r l y P r o t e r o z o i c s u p r a c r u s t a l r o c k s a r e now p r e s e r v e d m a i n l y i n t h r e e s t r u c t u r a l t h e Epworth b a s i n s o u t h w e s t o f C o r o n a t i o n G u l f , t h e Goulburn b a s i n around

basins:

B a t h u r s t I n l e t , and t h e G r e a t S l a v e b a s i n around t h e e a s t arm of G r e a t S l a v e Lake (Fig.8.l).

These s u p r a c r u s t a l r o c k s formed w i t h i n t h e C o r o n a t i o n g e o s y n c l i n e a l o n g

t h e w e s t e r n margin o f t h e S l a v e P r o v i n c e and a r e now b e s t exposed i n t h e Epworth basin.

Thick s u p r a c r u s t a l r o c k s c o r r e l a t i v e w i t h t h o s e i n t h e g e o s y n c l i n e a l s o

accumulated f a r t o t h e e a s t o f t h e Bear P r o v i n c e i n two a u l a c o g e n s , now exposed

in t h e Goulburn and G r e a t S l a v e b a s i n s . These P r o t e r o z o i c r o c k s , d e s c r i b e d by Hoffman (1973), a r e remarkable i n t h e i r resemblance t o P h a n e r o z o i c g e o s y n c l i n a l sequences.

They imply t h e p r e s e n c e o f a

c o n t i n e n t a l margin and g l o b a l p l a t e i n t e r a c t i o n s d u r i n g t h e E a r l y P r o t e r o z o i c . G e o s y n c l i n a l d e p o s i t i o n began w i t h t h e d e p o s i t i o n o f o r t h o q u a r t z i t e s on a westwardf a c i n g c o n t i n e n t a l s h e l f w i t h o v e r l y i n g s t r o m a t o l i t i c dolomite.

West of t h e s h e l f

e d g e , t h e d o l o m i t e p a s s e s a b r u p t l y i n t o a much t h i n n e r mudstone sequence w i t h d o l o m i t e d e b r i s - f l o w s , and t h e o r t h o q u a r t z i t e i n t o a t h i c k l a m i n a t e d s i l t and muds t o n e sequence w i t h q u a r t z i t e t u r b i d i t e s .

Foundering o f t h e c o n t i n e n t a l s h e l f

r e s u l t e d i n t h e d r a p i n g o f a t h i n l a m i n a t e d p y r i t i c b l a c k mudstone sequence overl a i n by a westward-thickening

c l a s t i c wedge which r e s u l t e d from t h e i n t r u s i o n and

e r o s i o n of b a t h o l i t h s t o t h e w e s t .

The c l a s t i c wedge b e g i n s w i t h a t h i c k sequence

o f t u r b i d i t e s t h a t p a s s e s e a s t w a r d s i n t o c o n c r e t i o n a r y mudstone on t h e p l a t f o r m . The mudstone g r a d e s upward i n t o l a m i n a t e d s h a l y l i m e s t o n e , minor t u r b i d i t e s , o v e r l a i n by cross-bedded r e d l i t h i c s a n d s t o n e s . The p l a t f o r m i s marked by two a u l a c o g e n s

-

deeply subsiding f a u l t troughs t h a t

e x t e n d a t h i g h a n g l e s from t h e g e o s y n c l i n e f a r i n t o t h e i n t e r i o r o f t h e p l a t f o r m . These a u l a c o g e n s r e c e i v e d much t h i c k e r s e d i m e n t a r y sequences and were n e v e r s u b j e c t e d t o b a t h o l i t i c i n t r u s i o n s , r e g i o n a l metamorphism, o r low-angle overt h r u s t i n g c h a r a c t e r i s t i c of t h e g e o s y n c l i n e .

The Athapuscow a u l a c o g e n i n t h e

G r e a t S l a v e Lake a r e a i s i n t e r p r e t e d a s h a v i n g been a n i n c i p i e n t r i f t l o c a t e d over a c r u s t a l a r c h during the c o n t i n e n t a l s h e l f s t a g e of t h e geosyncline, b u t

442

...... ,.... ..,... ...... ..... ,..... .. . . , Middle Proterozoic and younger cover rocks. ...*.....*. ..... Comagmatk granitic and vdcenic rocks of the epizoni Great B u r

itholith

Motamorphorrd eerly Proterozoic wpracrustalrocks of the continental rise and clastic mdy of the Corontion 0.ocyncline intruded by the mowzonal Hapburn batholith Thick u r l y Proterozoic wpracrustel rocks of the continental shelf end ciastic wedge of the Coronation coryncline

Thkk eerly Proterozoic suprrruitd rocks deposited in anlacogens

a 3

Thin only Protarozok wprrmstd rocks of tha continentel platform

Mainly exhumed Archaem k r m n t subjected to eerly Proterozoic cetrlestic and retrogressive metamorphism

3

ExhumadAmhwn basement

Fig.8.1. T e c t o n i c s u b d i v i s i o n and s e d i m e n t a r y b a s i n s of e a r l y P r o t e r o z o i c r o c k s i n t h e n o r t h w e s t e r n Canadian S h i e l d ( a f t e r Hoffman, 1 9 7 3 ) .

443 sagged t o become a c r u s t a l downwarp d u r i n g t h e c l a s t i c wedge s t a g e . F i n a l l y , t h e aulacogen became p a r t o f a r e g i o n a l t r a n s c u r r e n t f a u l t s y s t e m , a l o n g which t h i c k f a n g l o m e r a t e s accumulated i n l o c a l t r o u g h s . The molasse o f t h e C o r o n a t i o n g e o s y n c l i n e c o n s i s t s of a l l u v i a l r e d beds d e r i v e d from t h e b a t h o l i t h i c b e l t t o t h e west.

A t t h e b a s e a r e 125m o f r e d mudstone w i t h

t h i n c r o s s - l a m i n a t e d and mud-cracked b u f f s i l t s t o n e s .

These a r e o v e r l a i n by o v e r

500111 o f r e d mudstone w i t h fining-upwards u n i t s of f r i a b l e r e d l i t h i c s a n d s t o n e s many metres t h i c k .

The s a n d s t o n e s , which commonly show t a b u l a r s e t s o f c r o s s -

bedding up t o 2m t h i c k , a r e g e n e r a l l y w e l l - s o r t e d , c a l c i t e - c e m e n t e d and c o n t a i n abundant a n g u l a r t o sub-rounded c l a s t s of s e d i m e n t a r y r o c k s , i n t e r m e d i a t e t o s i l i c i c v o l c a n i c r o c k s , and minor p l u t o n i c and metamorphic r o c k s . I n t h e Authapuscow a u l a c o g e n t h e molasse phase b e g i n s w i t h 650m of r e d mudstone w i t h h a l i t e c a s t s and e x t e n s i v e o l i s t o s t r o m e s w i t h a n g u l a r b l o c k s o f s t r o m a t o l i t i c d o l o m i t e and l i m e s t o n e .

Above a r e 850m of r e d l a m i n a t e d l i t h i c s a n d s t o n e i n

t a b u l a r cross-bedded u n i t s up t o 5m t h i c k w i t h abundant mud-cracks and r i p p l e marks. The s a n d s t o n e i s o v e r l a i n by 230m of r e d mud-cracked mudstone w i t h b u f f s i l t s t o n e beds and abundant h a l i t e and gypsum c a s t s .

The m o l a s s e phase c u l m i n a t e s w i t h up

t o 4000m of r e d and b u f f f a n g l o m e r a t e s and s a n d s t o n e s which a r e n e a r l y f l a t - l y i n g except c l o s e t o major f a u l t s .

L i t h o l o g i c a l l y t h e sandstones a r e indistinguishable

from t h o s e of t h e g e o s y n c l i n e . These P r o t e r o z o i c r e d b e d s , b e c a u s e o f t h e i r unmetamorphosed n a t u r e , o f f e r a u n i q u e o p p o r t u n i t y o f s t u d y i n g t h e palaeomagnetism of o l d e r Precambrian r e d beds. The o c c u r r e n c e of contemporaneous l a v a flows a l s o p r o v i d e s some independent c o n t r o l on t h e s i g n i f i c a n c e o f t h e p a l a e o m a g n e t i c d a t a . The M a r t i n f o r m a t i o n The M a r t i n Formation c o n s i s t s o f a t l e a s t 3960m of f l u v i a l c o n g l o m e r a t e s , a r k o s i c s a n d s t o n e s , s i l t s t o n e s , a n d e s i t i c - b a s a l t i c l a v a f l o w s , and g a b b r o i c s i l l s preserved i n a small fault-bounded b a s i n t o the south-east of the Great Slave b a s i n ( F r a s e r e t a l . , 1970).

The c l a s t i c s e d i m e n t s a r e unmetamorphosed b u t t h e

v o l c a n i c r o c k s a r e h i g h l y a l t e r e d , p r o b a b l y due t o d e u t e r i c a l t e r a t i o n (Tremblay, 1968).

The M a r t i n Formation r e s t s w i t h a n g u l a r unconformity on t h e metamorphosed

T a z i n Group and a n upper a g e l i m i t i s p r o v i d e d by a Rb/Sr whole rock i s o c h r o n from p e g m a t i t e s c u t t i n g t h e T a z i n Group which y i e l d e d a n a g e o f 1975 (Sassano e t a l . , 1972).

F

20 My

A g a b b r o i c dyke c u t t i n g t h e lower p a r t of t h e M a r t i n

Formation h a s been d a t e d (K/Ar whole r o c k ) a t 1490 flows has y i e l d e d a K / A r whole r o c k a g e of 1635

2

t

100 My and one o f t h e b a s a l t

180 My.

F r a s e r e t a l . (1970)

r e g a r d e d t h e M a r t i n Formation a s having formed between 1830 and 1650 My ago. These r o c k s a r e p r o b a b l y t h e o l d e s t unmetamorphosed c o n t i n e n t a l r e d beds anywhere i n t h e world.

444 The palaeomagnetism of t h e M a r t i n Formation i s descrbbed by Evans and Bingham (1973) and t h e magnetic s u s c e p t i b i l i t y a n i s o t r o p y by Aziz-ur-Rahman e t a l . (1975). Although t h e r e d beds were d e p o s i t e d i n f l u v i a l environments and p r o b a b l y formed by t h e d i a g e n e t i c breakdown o f i r o n - b e a r i n g m i n e r a l s , t h e r e a r e , u n f o r t u n a t e l y , no detailed diagenetic studies reported i n the literature. Evans and Bingham (1973) p r e s e n t e d p a l a e o m a g n e t i c r e s u l t s from r e d b e d s , v o l c a n i c s and dykes from a number o f l o c a l i t i e s i n t h e M a r t i n Formation (Fig.8.2).

Fig.8.2.

Sampling l o c a l i t i e s i n t h e M a r t i n Formation ( a f t e r Evans and Bingham, 1973).

S i t e mean d i r e c t i o n s f o r t w e l v e o f t h e t h i r t e e n s e d i m e n t a r y s i t e s sampled a r e shown

i n F i g . 8 . 3 a f t e r thermal c l e a n i n g a t 500-660°C.

The specimen d i r e c t i o n s o f t h e

t h i r t e e n t h s i t e were random u s i n g W a t s o n ' s (1956) c r i t e r i a . r i b u t i o n s i n Fig.8.3.

The s i t e mean d i s t -

show a tendency t o form two groups w i t h NW d e c l i n a t i o n / u p w a r d

d i p and SE declination/downward d i p .

The r e s u l t s a r e , however, r a t h e r ' s t r e a k e d ' o u t

and t h e p o s i t i o n of s i t e M P and MH s u g g e s t a g r e a t c i r c l e s c a t t e r o f d i r e c t i o n s . Evans and Bingham (1973, p. 1488) thought t h a t s i t e s MP and MQ ( t h e one w i t h random d i r e c t i o n s ) had been magnetized d u r i n g a r e v e r s a l w h i l s t t h e geomagnetic f i e l d was i n an intermediate p o l a r i t y stage.

I n s u p p o r t o f t h i s argument they p r e s e n t e d t h e

r e s u l t s o f i n d i v i d u a l p i l o t specimens which show good thermal ' e n d p o i n t s ' s p r e a d a l o n g a g r e a t c i r c l e p a s s i n g c l o s e t o t h e o v e r a l l mean d i r e c t i o n ( F i g . 8 . 3 ) and v e r y

44 5 N

MARTIN FORMATION N

Fig.8.3. Palaeomagnetic r e s u l t s of t h e M a r t i n Formation (Precambrian) of Saskatchowan p l o t t e d on e q u a l - a r e a s t e r e o g r a p h i c p r o j e c t i o n s . Above t h e behaviour of p i l o t specimens f o r s i t e s MQ and MP i s shown d u r i n g s u c c e s s i v e t e m p e r a t u r e i n c r e m e n t s . Below t h e mean d i r e c t i o n s o f sedimentaxy and igneous s i t e s i s shown a f t e r thermal c l e a n i n g . The open symbols i n d i c a t e upward d i p p i n g v e c t o r s and t h e c l o s e d symbols downward d i p p i n g v e c t o r s ( a f t e r Evans and Bingham, 1 9 7 3 ) .

446 s i m i l a r t o t h e g r e a t c i r c l e s s u g g e s t e d by t h e d i s t r i b u t i o n o f s i t e means.

This

however i s o n l y p a r t l y t r u e b e c a u s e some o f t h e p i l o t specimens from t h e MP s i t e do n o t show s t a b l e end p o i n t s , a f e a t u r e which s u g g e s t s t h e p r e s e n c e o f more than

one component of m a g n e t i z a t i o n . The i n t e r p r e t a t i o n t h a t t h e s e i n t e r m e d i a t e d i r e c t i o n s r e p r e s e n t m a g n e t i z a t i o n s a c q u i r e d d u r i n g a f i e l d r e v e r s a l i s o n l y one p o s s i b i l i t y and one t h a t seems e x t r e m e l y u n l i k e l y i n view of t h e g e o l o g i c a l e v i d e n c e .

This hypothesis requires

t h a t t h e r o c k s were magnetized w i t h i n a few thousand y e a r s of d e p o s i t i o n (assuming t h a t Precambrian and Phanerozoic f i e l d r e v e r s a l s proceeded a t t h e same r a t e ) .

The

a v a i l a b l e p e t r o l o g i c a l e v i d e n c e s u g g e s t s t h a t t h e s e r e d beds have s u f f e r e d d i a g e n e t i c m o d i f i c a t i o n c o n s i s t e n t w i t h t h a t s e e n i n many o t h e r f i r s t - c y c l e a r k o s i c r e d beds. A more p l a u s i b l e a l t e r n a t i v e i s t h a t t h e s i t e mean d i r e c t i o n s c o n t a i n superimposed

normal and r e v e r s e d components which y i e l d i n t e r m e d i a t e r e s u l t a n t v e c t o r s a t some s i t e s (MH, M P , MQ).

I n t h e s e c a s e s i t may be t h a t normal and r e v e r s e d components

a r e of comparable magnitude, whereas t h o s e s i t e s which a r e a n t i p o d a l l y opposed might be dominated by e i t h e r t h e normal o r r e v e r s e d component. c o n s i s t e n t with the geological evidence.

This hypothesis i s

I f t h e m a g n e t i z a t i o n was a c q u i r e d d u r i n g

d i a g e n e s i s t h e n i t i s more t h a n l i k e l y t h a t normal and r e v e r s e d f i e l d components would be i n t e g r a t e d ( i f a r e v e r s a l o c c u r r e d ) i n t h e r e d bed sequence b e c a u s e of t h e t i m e s c a l e of d i a g e n e s i s (Walker, 1967a, 1976).

If o n l y a n t i p a r a l l e l components a r e

i n v o l v e d i n t h i s way t h e n t h e g r e a t c i r c l e d e f i n e d by t h e s i t e mean d i r e c t i o n s may be a r e l i a b l e e s t i m a t e of t h e d i p o l a r a x i s of m a g n e t i z a t i o n which e x i s t e d d u r i n g M a r t i n Formation d i a g e n e s i s ( s i n c e a l l t h e r e s u l t a n t v e c t o r s would l i e i n t h e same plane).

R e s u l t s from contemporaneous igneous r o c k s i n t h e M a r t i n Formation s u g g e s t

t h a t t h i s c o u l d be t h e c a s e b e c a u s e , a l t h o u g h somewhat s c a t t e r e d , t h e y show t h e same a x i s of m a g n e t i z a t i o n a s t h e s e d i m e n t a r y s i t e s .

The s i g n i f i c a n c e of p o l e

p o s i t i o n s from t h e M a r t i n Formation i s d i s c u s s e d by Evans and Bingham (1973, p.

1490-1492).

The C h r i s t i e Bav Group The G r e a t S l a v e Supergroup comprises o v e r 12,000m of unmetamorphosed s e d i m e n t a r y r o c k s and v o l c a n i c s p r e s e r v e d i n t h e E a s t A r m F o l d B e l t of t h e G r e a t S l a v e Lake (Fig.8.l).

In many p l a c e s t h e s t r a t a a r e t i g h t l y f o l d e d and u n c o n f o m a b l y o v e r l a i n

by t h e Et-Ehen Group.

The C h r i s t i e Bay Group i s t h e uppermost of t h e four groups

( S o s a n , K a h o c h e l l a , P e t h e i , C h r i s t i e Bay) r e c o g n i z e d by Hoffman (1969) t o make up t h e G r e a t S l a v e Supergroup.

I t c o n s i s t s o f f o u r conformable f o r m a t i o n s , which a r e

i n d e c r e a s i n g a g e , t h e S t a r k , Tochatwi, P o r t a g e I n l e t , and P e a r s o n Formations. T o g e t h e r t h e s e make up t h e m o l a s s e f a c i e s o f t h e Athapuscow a u l a c o g e n (Hoffman, 1973). Palaeomagnetic r e s u l t s have been r e p o r t e d f o r t h e two o l d e s t f o r m a t i o n s of t h e

441

C h r i s t i e Bay Group, t h e S t a r k and t h e Tochatwi Formations by Bingham and Evans (1976) and Evans and Bingham (1976) r e s p e c t i v e l y . i n Fig.8.4.

Sampling l o c a l i t i e s a r e shown

The S t a r k Formation c o n s i s t s of a t l e a s t 60Cm o f f i n e l y bedded r e d

mudstones w i t h a b u n d a n t , d i s c o n t i n u o u s , p a l e g r e e n r e d u c t i o n s l a y e r s and s p o t s . R i p p l e marks, c r o s s - b e d d i n g , and h a l i t e c a s t s a r e common.

Hoffman (1969) c o n s i d e r e d

t h e S t a r k Formation t o have been d e p o s i t e d i n a s h a l l o w s u b l i t t o r a l marine e n v i r onment o f h i g h a r i d i t y .

K / A r d a t e s from a s s o c i a t e d i g n e o u s r o c k s s u g g e s t t h a t t h e

S t a r k Formation i s between 1600 and 1800 My o l d (Hoffman, 1968).

Sampling l o c a l i t i e s used i n t h e palaeomagnetic s t u d y of S t a r k Formation Fig.8.4. and t h e Tochatwi Formation. Compiled from Bingham and Evans (1976) and Evans and Bingham (1976). D e t a i l e d p a l a e o m a g n e t i c r e s u l t s from t h e S t a r k Formation a r e p r e s e n t e d by Bingham and Evans (1976).

A t o t a l of 160 hand samples from 55 s t r a t i g r a p h i c a l l y

d i s t i n c t s i t e s were used i n t h e s t u d y (Fig.8.4).

The p r e l i m i n a r y r e s u l t s showed

t h e p r e s e n c e of two zones o f d i f f e r e n c e magnetic p o l a r i t y w i t h a t r a n s i t i o n a b o u t 280m above t h e b a s e of t h e S t a r k Formation.

P i l o t specimens from b o t h zones

were s u b j e c t e d t o b o t h AF and t h e r m a l d e m a g n e t i z a t i o n . I n b o t h c a s e s t h e r e was l i t t l e d i r e c t i o n a l change d u r i n g t h e d e m a g n e t i z a t i o n t r e a t m e n t b u t t h e i n t e n s i t y p a t t e r n s , a r e d i f f e r e n t f o r r e v e r s e d specimens and f o r normal specimens.

The p i l o t

448 specimens from t h e r e v e r s e d s i t e s g e n e r a l l y show a d e c r e a s e i n i n t e n s i t y whereas t h e normal s i t e s show a s l i g h t i n c r e a s e i n i n t e n s i t y a t 400 Oe (Fig.8.5).

The

t h e r m a l d e m a g n e t i z a t i o n a g a i n r e s u l t e d i n l i t t l e d i r e c t i o n a l change u n t i l temp e r a t u r e s were above 66OoC and t h e same c o n t r a s t i n b e h a v i o u r between normal and r e v e r s e d s i t e s was o b s e r v e d

.

Large d e c r e a s e s i n i n t e n s i t y o n l y o c c u r a t t e m -

p e r a t u r e s o v e r 6OO0C ( F i g . 8 . 5 ) i n d i c a t i n g t h a t h a e m a t i t e must c a r r y t h e m a j o r p a r t o f t h e remanence.

1.0

1.0

J/

Jo

Om5i

\

REVERSES SITES

J/ JO

0.1

REVERSED SITES

\ NORMAL SITES

I 0.01 L. I I I I ' I

0

goo

1200

1800

20

I

Peak firld(00)

200

400

600

Temperature (c)

Fig.8.5. Normalized i n t e n s i t y changes d u r i n g AF and 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 l o t specimens from t h e S t a r k Formation ( a f t e r Bingham a n d Evans, 1976). The i n t e n s i t y changes which o c c u r d u r i n g d e m a g n e t i z a t i o n s u g g e s t t h e p r e s e n c e of a s e c o n d a r y component o f m a g n e t i z a t i o n , a p p r o x i m a t e l y p a r a l l e l t o t h e mag-

n e t i z a t i o n of t h e r e v e r s e d s i t e s b u t opposing t h e normal s i t e s .

Bingham and Evans

(1976, p. 572) a p p l i e d v e c t o r s u b t r a c t i o n of t h e components removed from a number of s i t e s between 0 and 400 Oe. form a c l o s e group (347, f 6 7 ,

With t h e s t r a t a i n t h e i r p r e s e n t p o s i t i o n t h e s e R g 5 = 6 ) n e a r t o t h e p r e s e n t f i e l d (030, +63) a t t h e

c o l l e c t i n g l o c a l i t y , a f e a t u r e s t r o n g l y suggesting t h e a c q u i s i t i o n of a secondary component i n r e l a t i v e l y r e c e n t times.

A s i m i l a r a n a l y s i s of t h e v e c t o r s removed

by h e a t i n g t o 5OO0C a l s o y i e l d e d a group (239, +80,

o(

95

= 4')which

showed l i t t l e

1

449

s c a t t e r b e f o r e any c o r r e c t i o n was made f o r g e o l o g i c a l t i l t .

The s c a t t e r o f t h i s

group i s much g r e a t e r when c o r r e c t e d f o r bedding t i l t and a g a i n a p o s t - f o l d i n g secondary component o f m a g n e t i z a t i o n a p p e a r s t o b e p r e s e n t . The r e s u l t s o f s i t e mean d i r e c t i o n s a f t e r thermal c l e a n i n g a t 5OO0C a r e shown i n Fig.8.6.

The most obvious f e a t u r e of t h e s e r e s u l t s i s t h e p o l a r i t y r e v e r s a l

which o c c u r s a b o u t mid-way through t h e s e c t i o n .

S i t e means i n t h e v i c i n i t y o f t h e

r e v e r s a l show i n t e r m e d i a t e d i r e c t i o n s and t h e r e i s one anomalous s i t e mean d i r e c t i o n i n t h e lower, normal i n t e r v a l of t h e s e c t i o n .

The f e a t u r e s a r e s i m i l a r t o f i e l d

r e v e r s a l s documented i n much younger sediments a n d , i n p a r t i c u l a r , show v i r t u a l geomagnetic p o l e s (VGP's) which f o l l o w a g r e a t c i r c l e p a t h d u r i n g t h e p o l a r i t y t r a n s i t i o n ; t h e pre- and p o s t - t r a n s i t i o n a l VGP's a r e e s s e n t i a l l y a n t i p o d a l and t h e remanent i n t e n s i t y i s reduced by a f a c t o r o f 3 w i t h i n t h e t r a n s i t i o n zone. hiination

Inclination ooD-4so oo +UP +so06

Intensity (lO%mU om3,

10

60

[.-.

Fig. 8.6. S t r a t i g r a p h i c p r o f i l e o f t h e r m a l l y c l e a n e d , s i t e mean d i r e c t i o n s f o r t h e S t a r k Formation a f t e r c o r r e c t i o n f o r g e o l o g i c a l d i p . The i n t e n s i t y i s r e p r e s e n t e d by a f i v e p o i n t moving a v e r a g e ( a f t e r Bingham and Evans, 1 9 7 5 ) .

450 These r e s u l t s have been i n t e r p r e t e d a s a f a i t h f u l r e c o r d o f a n e a r l y P r o t e r o z o i c geomagnetic f i e l d r e v e r s a l and t h e anomalous s i t e mean d i r e c t i o n h a s been i n t e r p r e t e d a s a geomagnetic e x c u r s i o n (Bingham and Evans, 1975; 1976).

I f t h i s were

t r u e then m a g n e t i z a t i o n must have been completed d u r i n g d e p o s i t i o n and e a r l y diagenesis.

The o n l y i n d e p e n d e n t e v i d e n c e f o r t h e a g e o f t h e m a g n e t i z a t i o n o f t h e

S t a r k Formation comes from t h e n e a r b y Mackenzie i n t r u s i o n s . Mackenzie p o l e i s 70'

The f a c t t h a t t h e

from t h e S t a r k Formation p o l e s u g g e s t s t h a t r e g i o n a l r e h e a t i n g

d i d n o t c a u s e r e m a g n e t i z a t i o n of t h e S t a r k Formation.

The a g e of t h e Mackenzie

i n t r u s i o n s (1200 My) p u t s a n upper l i m i t on t h e a g e o f t h e m a g n e t i z a t i o n .

There

i s no d i r e c t e v i d e n c e of t h e p r e c i s e a g e of t h e remanence of t h e S t a r k Formation.

Although t h e palaeomagnetic e v i d e n c e i s c o m p a t i b l e w i t h t h e a c q u i s i t i o n o f remanence s h o r t l y a f t e r d e p o s i t i o n , i t does n o t p r e c l u d e t h e p o s s i b i l i t y of m a g n e t i z a t i o n a t a l a t e r s t a g e of d i a g e n e s i s . Formation i.e.

Some o f t h e 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 t h e S t a r k

intermediate d i r e c t i o n s during t h e p o l a r i t y t r a n s i t i o n , t h e presence

of u n s t a b l e m a g n e t i z a t i o n s n e a r t h e p o l a r i t y t r a n s i t i o n and t h e d i s s i m i l a r b e h a v i o u r of normal and r e v e r s e l y magnetized specimens i s e q u a l l y c o n s i s t e n t w i t h a much l a t e r

m a g n e t i z a t i o n i n which a r e v e r s e d zone was superimposed on a 'normal zone'. The Tochatwi Formation (Hoffman, 1968) c o n s i s t s o f up t o 80Om o f f i n e g r a i n e d r e d - b u f f s a n d s t o n e s w i t h s u b o r d i n a t e s h a l e s and conglomerate r e s t i n g conformably The a g e o f t h e Tochatwi Formation i s n o t

on t h e u n d e r l y i n g S t a r k Formation.

p r e c i s e l y known b u t may b e a b o u t 1700 My.

Most o f t h e d e p o s i t s a r e c o n s i d e r e d t o

be o f f l u v i a l o r i g i n by Hoffman (1969) e x c e p t f o r t h o s e n e a r t h e b a s e which may r e p r e s e n t a s h a l l o w marine t i d a l o r i n t e r t i d a l environment. Sampling l o c a l i t i e s used i n t h e p a l a e o m a g n e t i c s t u d y by Evans and Bingham (1976) a r e shown i n Fig.8.4. study.

I n a l l , 29 s t r a t i g r a p h i c a l l y d i s t i n c t s i t e s were u s e d i n t h e

During t h e r m a l d e m a g n e t i z a t i o n t h e m a j o r i t y o f specimens i n t h e Tochatwi

Formation ( r e p r e s e n t e d by t h e N. changes.

s e c t i o n i n Fig.8.4)

show s y s t e m a t i c d i r e c t i o n a l

The d i r e c t i o n s move a l o n g g r e a t c i r c l e s by 20-50'

e n d - p o i n t s between 5OO0C and 60OoC.

before reaching s t a b l e

They a l s o r e v e a l t h e r m a l l y d i s t r i b u t e d b l o c k i n g

t e m p e r a t u r e s p e c t r a w i t h o n l y 1-9% of t h e NRM remaining a t 6OO0C (Fig.8.7). On t h e b a s i s o f t h e r e s u l t s from t h e t h e r m a l d e m a g n e t i z a t i o n 46 spceimens (one from e a c h sample) were h e a t e d t o 5OO0C and 60OoC.

The r e s u l t s showed h i g h e r

w i t h i n - s i t e p r e c i s i o n (Watson, 1956) a t 5OO0C and t h e d i r e c t i o n s f o r t h e e l e v e n non-random s i t e s o f t h e N-section

( e x c l u d i n g s i t e NA) a r e shown i n Fig.8.8

and a f t e r , c o r r e c t i o n f o r g e o l o g i c a l t i l t .

before,

Q u i t e c l e a r l y , t h e r e s u l t s a r e much

b e t t e r grouped b e f o r e c o r r e c t i o n f o r g e o l o g i c a l d i p (Mean d i r e c t i o n 165, +78 w i t h

k = 16) than a f t e r c o r r e c t i o n (k decreased t o 4).

The d e c r e a s e i n k i s s i g n i f i c a n t

a t t h e 99% l e v e l (McElhinny, 1964) i n d i c a t i n g a n e g a t i v e f o l d t e s t (Graham, 1949). The r e s u l t s i n d i c a t e t h a t t h i s N-section i n t h e Tochatwi Formation h a s been remagnetized a f t e r f o l d i n g .

A s i m i l a r p o s t - f o l d i n g r e m a g n e t i z a t i o n h a s been

r e c o g n i z e d i n t h e Kahochella Group, some 70km t o t h e e a s t o f t h i s s e c t i o n

451 (McMurray e t a l . , 1973). The d i r e c t i o n o f m a g n e t i z a t i o n i n t h e Kahochella Group

i s v e r y c l o s e t o t h a t s e e n i n t h e N - s e c t i o n of t h e Tochatwi Formation, i n d i c a t i n g t h a t t h e r e m a g n e t i z a t i o n was o f some r e g i o n a l s i g n i f i c a n c e .

The most l i k e l y

p o s s i b i l i t y i s t h a t r e g i o n a l r e h e a t i n g , d u r i n g a m a j o r i g n e o u s e v e n t , was t h e c a u s e of t h e r e m a g n e t i z a t i o n .

L

I

I

I

200

I

I

Temperature

I

600

400

f

( O C)

Fig.8.7. Thermal d e m a g n e t i z a t i o n c u r v e s f o r t h e Tochatwi Formation (N-section) F o u r t e e n specimens a r e r e p r e s e n t e d by t h e shaded a r e a ( a f t e r Evans and Bingham, 1976). Not a l l of t h e Tochatwi Formation h a s s u f f e r e d complete r e m a g n e t i z a t i o n .

This

i s i n d i c a t e d by a number of s i t e s N A , FA-FE, AC and AD which y i e l d d i r e c t i o n s s i m i l a r t o t h o s e of t h e S t a r k Formation,

S i t e s AC and AD have s u f f e r e d some d e g r e e

o f t h e r m a l metamorphism by i n t r u s i v e b o d i e s of a s i m i l a r a g e t o t h e Mackenzie Diabase ( I r v i n g e t a l . ,

1972).

The p r o g r e s s i v e thermal d e m a g n e t i z a t i o n o f some of

t h e s e specimens i s shown i n F i g . 8 . 9 .

I n i t i a l l y t h e y have a s h a l l o w i n c l i n a t i o n

towards t h e WSW ( t h e Mackenzie d i r e c t i o n f o r t h e a r e a ) b u t move p r o g r e s s i v e l y towards t h e o p p o s i t e q u a d r a n t on h e a t i n g t o h i g h e r t e m p e r a t u r e s .

S t a b l e end-points

w i t h a d i r e c t i o n v e r y s i m i l a r t o t h a t s e e n i n t h e S t a r k Formation s u g g e s t s t h a t t h i s i s t h e o r i g i n a l remanence of t h e Tochatwi Formation,

The mean d i r e c t i o n s of

t h e s e s i t e s a r e s c a t t e r e d b e f o r e c o r r e c t i o n f o r d i p , b u t form a c o h e r e n t group a f t e r c o r r e c t i o n , w i t h a mean d i r e c t i o n 030, -11 ( k = 14,Wg5 = 15').

The remanence

o f t h e s e s i t e s t h e r e f o r e p r o b a b l y p r e c e d e s t h e f o l d i n g , and a l s o t h e d e p o s i t i o n of t h e o v e r l y i n g E t - t h e n Group s e d i m e n t s . The a g e of t h e m a g n e t i z a t i o n of t h e S t a r k Formation and Tochatwi Formation cannot be more p r e c i s e l y d a t e d .

The a g e of t h e E t - t h e n s e d i m e n t s i s n o t p r e c i s e l y known.

They a r e younger t h a n t h e d i o r i t e s and s y e n i t e s which i n t r u d e t h e G r e a t S l a v e Supergroup, which a r e d a t e d a t 1630-1845 My, and o l d e r t h a n t h e Mackenzie d i a b a s e s

452

0 0

.

O. 0.

i-0 0

Fig.8.8. Palaeomagnetic r e s u l t s f o r t h e Tochatwi Formation ( N - s e c t i o n ) a f t e r thermal c l e a n i n g . The v e c t o r s a r e p l o t t e d b e f o r e ( a b o v e ) and a f t e r (below) c o r r e c t i o n f o r g e o l o g i c a l t i l t . The c l o s e g r o u p i n g of t h e v e c t o r s p r i o r t o c o r r e c t i o n f o r g e o l o g i c a l t i l t s u g g e s t s t h a t t h e r o c k s were remagnetized a f t e r f o l d i n g ( a f t e r Evans and Bingham, 1 9 7 6 ) .

453

which c u t them and a r e d a t e d a t 1250 My ( F a h r i g and J o n e s , 1969).

Accepting a n

age of 1700-1750 My f o r t h e S t a r k and Tochatwi Formations means t h a t t h e a g e o f t h e m a g n e t i z a t i o n c a n n o t , f o r c e r t a i n , b e d a t e d t o c l o s e r t h a n w i t h i n a b o u t 500 My of d e p o s i t i o n .

The c l o s e s i m i l a r i t y o f t h e r e s u l t s , p a r t i c u l a r l y t h o s e from t h e

S t a r k Formation, w i t h r e s u l t s from P h a n e r o z o i c r e d beds does s u g g e s t t h a t t h e m a g n e t i z a t i o n o c c u r r e d w i t h i n a few m i l l i o n y e a r s o f d e p o s i t i o n .

The r e s u l t s a r e

q u i t e c o m p a t i b l e w i t h t h e a c q u i s i t i o n of remanence d u r i n g r e d bed d i a g e n e s i s .

N

D1 NRM 200

300 400 450 500 550 575 600

0 3 +C3 NRM

NRM

500 500 575 600 630 660

400 450 500 550 575

C4 NRM

600

200 300 400 450 500 575

630 660

600 630

630 660

D i r e c t i o n a l changes d u r i n g t h e r m a l d e m a g n e t i z a t i o n )oE s i t e s AC and AD Fig.8.9. i n t h e Tochatwi Formation. These r e v e a l s t a b l e d i r e c t i o n s s i m i l a r t o t h o s e i n t h e S t a r k Formation ( a f t e r Evans and Bingham, 1976).

THE LATE PRECAMBRIAN OF NORTH WEST SCOTLAND The palaeomagnetism o f t h e Precambrian s e d i m e n t a r y r o c k s of NW S c o t l a n d has been i n v e s t i g a t e d by I r v i n g ( 1 9 5 7 ) , I r v i n g and Runcorn (1957) and S t e w a r t and I r v i n g (1974); t h e s e r o c k s were amongst t h e f i r s t t o b e s t u d i e d i n d e t a i l , t h e r e s e a r c h b e i n g i n i t i a t e d by S . K . Runcorn i n J u n e 1951.

The r o c k s i n v e s t i g a t e d i n c l u d e t h e

S t o e r Group c o n s i s t i n g o f a t l e a s t 2300m o f r e d s a n d s t o n e s and r e s t i n g unconformably on Lewisian basement.

T h i s i s unconformably o v e r l a i n by t h e T o r r i d o n Group which

c o n s i s t s of a t l e a s t 700Om o f r e d s a n d s t o n e s and s h a l e s .

It i s divided i n t o three

4 54 f o r m a t i o n s ( D i a b a i g , A p p l e c r o s s and A u l t b e a ) and i s o v e r l a i n by t h e Lower Cambrian (Peach e t a l . ,

1907) ( F i g . 8 . 1 0 ) .

G e o c h r o n o l o g i c a l s t u d i e s show t h a t t h e Lewisian basement l a s t s u f f e r e d r e g i o n a l metamorphism a t 1500-1800 My.

Mica and whole r o c k K / A r a g e s o f 1120

2

40 My have

been r e p o r t e d from p a r t s of t h e basement and a l s o from d e t r i t u s i n t h e S t o e r and T o r r i d o n Groups s o t h e s e d i m e n t s must b e younger t h a n t h i s .

Rb/Sr whole r o c k

measurements on s h a l e s and s i l t s t o n e s from t h e S t o e r and T o r r i d o n Groups y i e l d e x c e l l e n t i s o c h r o n s and a g e s of 9 9 1 2 24 My and 796

2

24 My r e s p e c t i v e l y (Moorbath

e t a l . , 1967; Moorbath, 1969).

Fig.8.10. G e n e r a l i z e d s e c t i o n through t h e Caledonian f o r e l a n d showing r e l a t i o n s h i p s of t h e S t o e r and T o r r i d o n Groups t o t h e basement ( a f t e r S t e w a r t and I r v i n g , 1 9 7 4 ) . S e d i m e n t o l o g i c a l s t u d i e s o f t h e r e d f a c i e s o f t h e S t o e r and T o r r i d o n Groups show t h a t t h e y were d e p o s i t e d on a pediment s u r f a c e which formed a t t h e f o o t o f a r e t r e a t i n g Lewisian s c a r p ( W i l l i a m s , 1969 a , b ) (Fig.8.11).

I t i s probable t h a t

s h a l l o w marine environments a r e r e p r e s e n t e d i n p l a c e s ( G r a c i e and S t e w a r t , 1967). The s e d i m e n t s c o n t a i n a v a r i e t y of f i n e - c o a r s e g r a i n e d a r k o s i c s a n d s t o n e s .

The

c o a r s e s a n d s t o n e s o c c u r i n l a t e r a l l y p e r s i s t e n t s h e e t s w i t h e r o s i v e b a s e s and showing f l a t b e d d i n g , t a b u l a r and t r o u g h cross-bedding.

A t t h e b a s e of t h e u n i t s

t h e r e may b e c o n c e n t r a t i o n s of i n t r a - and e x t r a - f o r m a t i o n a l conglomerate.

Finer

sandstones tend t o be l e n t i c u l a r i n n a t u r e occurring with interbedded red s h a l e s and c r o s s l a m i n a t e d s i l t s t o n e .

The s a n d s t o n e s o f t e n show penecontemporaneous

d e f o r m a t i o n s t r u c t u r e s ( S e l l e y , 1969). oned c h a n n e l s .

The r e d s h a l e s a r e o f t e n s e e n t o f i l l aband-

The s a n d s a r e i n t e r p r e t e d a s b r a i d e d c h a n n e l d e p o s i t s o r r e l a t i v e l y

455 h i g h s t r e a m Dower ( S e l l e v . 1965. 1970).

Suggested palaeogeography of n o r t h w e s t S c o t l a n d i n T o r r i d o n i a n ( l a t e Fig.8.11. Precambrian) times ( a f t e r W i l l i a m s , 1969a). The S t o e r and T o r r i d o n i a n r e d beds a r e t h u s f l u v i a l r e d beds ( s e e Chapter 4 ) . P e t r o l o g i c a l l y they a r e f i r s t - c y c l e arkosic sandstones being derived p r i n c i p a l l y from t h e Lewisian basement ( S e l l e y , 1966). S t e w a r t and I r v i n g (1974) p r e s e n t e d p a l a e o m a g n e t i c r e s u l t s f o r f o u r l o c a l i t i e s (Fig.8.12)

which c o v e r e d t h e S t o e r Group i n d e t a i l and t h e lower p a r t of t h e

T o r r i d o n Group.

The NRM o f t h e r e d s a n d s t o n e s i n t h e S t o e r and T o r r i d o n Groups

i s g e n e r a l l y v e r y s t a b l e w i t h b l o c k i n g t e m p e r a t u r e s commonly h i g h e r t h a n 6OO0C a l t h o u g h i n some c a s e s t h e t h e r m a l decay o f t h e NRM i s more r a p i d w i t h b l o c k i n g t e m p e r a t u r e s a s low a s 3OO0C. i n Fig.8.13. haematite.

T y p i c a l t h e r m a l d e m a g n e t i z a t i o n r e s u l t s a r e shown

The r e s u l t s q u i t e c l e a r l y i n d i c a t e t h a t t h e NRM i s c a r r i e d by Only i n t h e S t a c Fada Member, a v o l c a n i c mudflow b r e c c i a , is t h e r e

456

5 km

Fig.8.12. Sampling s i t e s and geology of l a t e Precambrian s e d i m e n t s i n Nw S c o t l a n d . ( a f t e r S t e w a r t and I r v i n g , 1974). any e v i d e n c e t o t h e c o n t r a r y ; h e r e lower b l o c k i n g t e m p e r a t u r e s (50O0-6OO0C) and much lower c o e r c i v e f o r c e s (median d e s t r u c t i v e f i e l d 400-500 Oe) s u g g e s t t h a t m a g n e t i t e o r haemoilmenite c o u l d b e a remanence c a r r i e r .

Although m a g n e t i t e has

been r e p o r t e d from r e d s a n d s t o n e s i n t h e T o r r i d o n Group on t h e b a s i s o f thermomagnetic a n a l y s i s ( P a r r y , 1957; Schwarz, 1 9 6 9 ) , more w i d e s p r e a d i n v e s t i g a t i o n s ( I r v i n g , 1957; S t e w a r t and I r v i n g , 1974) show t h a t i t i s l i k e l y t o b e v e r y s c a r c e . S t e w a r t and I r v i n g (1974, p. 56-57) showed t h a t h a e m a t i t e was t h e o n l y c o a r s e g r a i n e d i r o n o x i d e phase and by p l o t t i n g t h e r m a l s t a b i l i t y (M5oo/%)

a g a i n s t volume

p e r c e n t a g e of s p e c u l a r h a e m a t i t e showed t h a t t h e most s t a b l e specimens were t h o s e w i t h most s p e c u l a r h a e m a t i t e (Fig.8.14).

The s p e c u l a r i t e a l m o s t always shows

m a r t i t i z a t i o n t e x t u r e s when viewed i n p o l i s h e d s e c t i o n s i n d i c a t i n g t h e l i k e l i h o o d t h a t t h e h a e m a t i t e formed by t h e o x i d a t i o n of m a g n e t i t e . The d i r e c t i o n s o f m a g n e t i z a t i o n f o r e a c h sampling s i t e which a r e g i v e n i n Fig.8.14

( s e e S t e w a r t and I r v i n g , 1974, T a b l e s 1 and 2 f o r d e t a i l s ) c o n f i r m t h e

e a r l i e r r e s u l t s of I r v i n g and Runcorn (1957).

The S t o e r Group d i r e c t i o n s a r e a l l

457

DIRECTION DI F F ERE NC E FROM NRM

1.0

A<1Oo 0 < 2 0 ° @ ) 2 0 '

I/lO 0.5

Stoer Gp. Stac Fada Mbr. 0

LO,-+

200

Stoer Gp.

J

400

TEMPERATURE

I

I

I

200

400

600

600 ( O C

800

)

Fig.8.13. Thermal d e m a g n e t i z a t i o n c u r v e s o f l a t e Precambrian sediments. o f t h e NRM ( G) i s g i v e n i n b r a c k e t s ( a f t e r S t e w a r t and I r v i n g , 1974).

Intensity

o f normal p o l a r i t y - a n d d i f f e r e n t from t h e T o r r i d o n Group which a r e o f normal and r e v e r s e d p o l a r i t y ( I r v i n g and Runcorn, 1957).

A conglomerate t e s t (Graham, 1949)

on b o u l d e r s o f r e d s a n d s t o n e shows t h a t t h e m a g n e t i z a t i o n o f t h e S t o e r Group p r e d a t e s t h e T o r r i d o n Group ( I r v i n g and Runcorn, 1957; S t e w a r t and I r v i n g , 1974, p. 58).

S t e w a r t and I r v i n g (1974) were f i r m l y of t h e o p i n i o n t h a t t h e m a g n e t i z a t i o n

of t h e s e Precambrian s e d i m e n t a r y r o c k s o r i g i n a t e d a t t h e t i m e t h e y were l a i d down b u t s e v e r a l l i n e s o f e v i d e n c e s u g g e s t t h a t t h i s is n o t t h e c a s e and t h a t t h e magnetization originated during diagenesis.

I n the f i r s t place, f i r s t cycle

a r k o s i c r e d beds of f l u v i a l o r i g i n l i k e t h e s e , must have been o r i g i n a l l y r i c h i n d e t r i t a l magnetite.

The abundance o f m a r t i t i z a t i o n t e x t u r e s i n t h e T o r r i d o n i a n

s a n d s t o n e s i n d i c a t e s t h a t i n s i t u o x i d a t i o n of m a g n e t i t e must have t a k e n p l a c e on a wide s c a l e , and p r o b a b l y o v e r a c o n s i d e r a b l e l e n g t h o f t i m e (Van Houten, 1968). When t h e d i r e c t i o n s o f m a g n e t i z a t i o n f o r e a c h sampling l o c a l i t y a r e a n a l y s e d i n d i v i d u a l l y (Fig.8.15)

t h e y c a n b e s e e n t o b e ' s t r e a k e d ' i n a way which s u g g e s t s

t h a t t h e m a g n e t i z a t i o n i s composite and t h e i n d i v i d u a l components have n o t been resolved.

For example, t h e r e s u l t s f o r t h e S t o e r Group show a d i s t i n c t ' s t r e a k i n g '

towards t h e n o r t h a t S t o e r Bay, and a t A c h i l t i b u i e t h e y a r e s t r e a k e d towards t h e NW.

The T o r r i d o n Group r e s u l t s a r e s t r e a k e d towards t h e ENE.

I t t h u s seems

p r o b a b l e t h a t t h e m a g n e t i z a t i o n i s c a r r i e d by s p e c u l a r i t e g r a i n s of d i a g e n e t i c o r i g i n , and w i t h a l l components having a narrow r a n g e of b l o c k i n g t e m p e r a t u r e s c l o s e t o t h e C u r i e p o i n t o f h a e m a t i t e i t i s u n l i k e l y t h a t i n d i v i d u a l components could be e a s i l y resolved.

The f a c t t h a t t h e S t o e r Group was magnetized p r i o r t o

458

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t

lmO o STAC FADA MEMBER

rTORRlDON GROUP

- 0 0

0.5

1.0

1.5

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96

Thermal s t a b i l i t y (M500/%) v e r s u s volume p e r c e n t a g e o f s p e c u l a r i t e i n Fig.8.14. l a t e Precambrian s e d i m e n t s ( a f t e r S t e w a r t and I r v i n g , 1974). t h e d e p o s i t i o n of t h e T o r r i d o n Group i s h a r d l y s u r p r i s i n g b e c a u s e t h e geochrono l o g i c a l e v i d e n c e i n d i c a t e s t h a t t h e unconformity between them r e p r e s e n t s a time i n t e r v a l of some 200 My.

T h i s i s c e r t a i n l y s u f f i c i e n t time t o complete t h e

d i a g e n e t i c m a g n e t i z a t i o n of t h e S t o e r Group. On t h i s b a s i s i t i s d i f f i c u l t t o a s s e s s t h e p a l a e o m a g n e t i c r e l i a b i l i t y of t h e s e Precambrian s e d i m e n t a r y r o c k s .

The o n l y d a t a which t h e y can b e u s e f u l l y compared

w i t h a r e t h o s e from t h e Precambrian r o c k s of t h e L a u r e n t i a n s h i e l d o f which t h e Lewisian p l a t f o r m was once a p a r t (DuToit, 1937; Wynne-Edwards and Hassan, 1970). These d a t a a r e p r i n c i p a l l y from i g n e o u s r o c k s (DuBois, 1962; B l a c k , 1963; Robertson and F a h r i g , 1971; I r v i n g and P a r k , 1972) a n d , f o r t h e p e r i o d 500-800 My a g o , y i e l d widely d i f f e r e n t pole p o s i t i o n s .

A l s o t h e h i g h d e g r e e o f a g e u n c e r t a i n t y means

a n e q u a l l y h i g h d e g r e e of p o l a r u n c e r t a i n t y e x i s t s even f o r d a t a w i t h h i g h p a l a e o magnetic p r e c i s i o n ( S p a l l , 1971; Brock, 1973). Assuming a r a t e of a p p a r e n t p o l a r wander of 0.3'

p e r m i l l i o n y e a r s (Donaldson e t a l . ,

+ 50 My i s e q u i v a l e n t t o a p o l a r u n c e r t a i n t y of 15'. Fig.8.16.

1973) a n a g e u n c e r t a i n t y of Some of t h e s e a r e shown i n

They a r e g r a d e d i n terms o f r e l i a b i l i t y a c c o r d i n g t o c r i t e r i a d e s c r i b e d

by S t e w a r t and I r v i n g (1974, p. 65).

None o f t h e s e c r i t e r i a a r e a b l e t o e s t a b l i s h

beyond any doubt t h a t t h e m a g n e t i z a t i o n was a c q u i r e d a t t h e time t h e r o c k was formed.

The conglomerate t e s t and baked c o n t a c t t e s t which a r e i n c l u d e d i n t h e

c r i t e r i a of S t e w a r t and I r v i n g (1974) (Graham, 1949) o n l y y i e l d a minimum a g e which c o u l d be s u b s t a n t i a l l y d i f f e r e n t from t h e t r u e a g e of m a g n e t i z a t i o n .

Further

problems a r i s e from p o s t - m a g n e t i z a t i o n i n t r a - c r a t o n i c r o t a t i o n of i n d i v i d u a l f a u l t blocks,

The e l a b o r a t e p o l a r wander p a t h s c o n s t r u c t e d from t h e s e Precambrian d a t a

( e . g . S t e w a r t and I r v i n g , 1974, Fig.10) must be viewed w i t h t h e s e l i m i t a t i o n s i n mind.

Even i f t h e s e p o l a r wander c u r v e s a r e a c c e p t e d , t h e S t o e r and T o r r i d o n Group

p o l e s do n o t c l o s e l y a g r e e w i t h them ( F i g . 8 . 1 6 ) .

I t must b e concluded t h a t t h e

v a l i d i t y of t h e T o r r i d o n i a n p a l a e o m a g n e t i c r e s u l t s c a n n o t b e s u b s t a n t i a t e d by

459

'i. I

\

Fig.8.15. Palaeomagnetic r e s u l t s from t h e T o r r i d o n i a n Sandstone ( L a t e Precambrian) of NW S c o t l a n d . Mean d i r e c t i o n s of non-random s i t e s a r e shown a f t e r thermal c l e a n i n g a t 500OC f o r t h e l o c a l i t i e s a t S t o e r Bay (A) and A c h i l t i b u i e (B) i n t h e S t o e r Group and v a r i o u s l o c a l i t i e s (C) i n t h e b a s a l T o r r i d o n Group. Closed symbols i n d i c a t e downward d i p p i n g v e c t o r s and open symbols upward d i p p i n g v e c t o r s . The c i r c u m f e r e n c e i s t h e bedding p l a n e and t h e g r e a t c i r c l e s a r e f i t t e d by i n s p e c t i o n ( d a t a p l o t t e d from S t e w a r t and I r v i n g , 1 9 7 4 ) .

Precambrian palaeomagnetic p o l e s f o r L a u r e n t i a from Cambrian t o t h e Fig.8.16. end of t h e Hudsonian e v e n t ( a b o u t 1830 My). The p o l e s a r e graded on a n i n e p o i n t s c a l e ; normal p o l e s s o l i d , r e v e r s e d p o l e s open symbols and mixed p o l e s i n d i c a t e d by 'bull's e y e s ' ( a f t e r S t e w a r t a n d I r v i n g , 1975)

460 comparison w i t h r e s u l t s from r o c k s of a s i m i l a r a g e e l s e w h e r e .

The l i k e l i h o o d i s

t h a t t h e m a g n e t i z a t i o n s o f t h e s e Precambrian s e d i m e n t s were a c q u i r e d o v e r a c o n s i d e r a b l e l e n g t h of t i m e (by d i a g e n e t i c p r o c e s s e s ) . THE OLD RED SANDSTONE The Anglo-Welsh B a s i n The sedimentology and d i a g e n e s i s of t h e Old Red Sandstone i n t h e Anglo-Welsh Basin h a s been d i s c u s s e d i n C h a p t e r 4.

Palaeomagnetic r e s u l t s were o r i g i n a l l y

p r e s e n t e d by C r e e r (1957) who d i s c o v e r e d t h a t t h e limbs o f s t e e p l y f o l d e d Devonian r e d beds i n Pembrokeshire, S o u t h Wales, had d i f f e r e n t d i r e c t i o n s of m a g n e t i z a t i o n and t h a t t h e s e were d i f f e r e n t from t h e d i r e c t i o n s o b s e r v e d i n n e a r b y , g e n t l y dipping s t r a t a .

Creer (1957, 1962b) e x p l a i n e d t h e s e d i f f e r e n c e s by s u g g e s t i n g t h a t

t h e m a g n e t i z a t i o n c o n s i s t e d of two components:

one a p r i m a r y component a c q u i r e d

p r i o r t o f o l d i n g , and t h e o t h e r a s e c o n d a r y component supposed t o have been s u p e r imposed a f t e r f o l d i n g i n a n e a r l y h o r i z o n t a l f i e l d

I

I

( F i g s . 8 . 1 7 and 8 . 1 8 ) .

V

Outcrop of Old,Red Smdrtons

BRECON BEACONS

NORBIER BAY

BRISTOL CHANNEL

\

Fig.8.17. L o c a l i t y map a n d p a l a e o m a g n e t i c d i r e c t i o n s o f t h e Old Red Sandstone i n t h e Anglo-Welsh B a s i n ( d a t a from Chamalaun and Creer, 1964).

461

BEDDING PLANE

BAY)

Schematic r e p r e s e n t a t i o n of t h e C r e e r two-component.hypothesis used t o Fig.8.18. e x p l a i n d i f f e r e n t d i r e c t i o n s of f o l d e d Old Red Sandstone i n Pembrokeshire ( a f t e r Chamalaun and C r e e r , 1 9 6 4 ) . From a s t a t i s t i c a l a n a l y s i s of t h e d i r e c t i o n s Creer (1962b) concluded t h a t t h i s r e m a g n e t i z a t i o n most p r o b a b l y o c c u r r e d d u r i n g t h e Permo-Triassic and thought t h a t t h e s e c o n d a r y m a g n e t i z a t i o n might be a p a r t i a l thermoremanent m a g n e t i z a t i o n i n d u c e d by b u r i a l t e m p e r a t u r e s of up t o 20OoC.

Thermal d e m a g n e t i z a t i o n s t u d i e s

by Chamalaun and C r e e r (1964) showed t h a t t h i s h y p o t h e s i s of C r e e r ' s was i n c o r r e c t b e c a u s e t h e y were u n a b l e t o i s o l a t e t h e r e s p e c t i v e components.

I n Fig.8.19

r e s u l t s f o r Manorbier and F r e s h w a t e r West a r e shown f o r comparison.

their

I n both cases

d i r e c t i o n s move a l o n g g r e a t c i r c l e s towards t h e NE and above 55OoC have upward inclinations.

These d i r e c t i o n s a r e c l o s e l y grouped a f t e r c o r r e c t i o n f o r bedding

t i l t and a l s o compare c l o s e l y w i t h d i r e c t i o n s from t h e Lower Devonian l a v a s of

t h e Midland V a l l e y of S c o t l a n d ( S t u b b s , 1958); Chamalaun and Creer (1964, p. 16141615) t h e r e f o r e a r g u e d t h a t t h e h i g h - t e m p e r a t u r e d i r e c t i o n r e p r e s e n t e d t h e Devonian f i e l d d i r e c t i o n .

S i m i l a r r e s u l t s were o b t a i n e d from t h e Brecon Beacons,

Cwmbran, and P o r t i s h e a d ( F i g . 8 . 1 9 ) which t o g e t h e r y i e l d e d a mean h i g h t e m p e r a t u r e d i r e c t i o n of 0 6 6 , -37'.

I n some r e s p e c t s t h i s r e s u l t , a l t h o u g h i n c l o s e agreement

w i t h t h a t from t h e l a v a s , i s open t o c r i t i c i s m ; i n p a r t i c u l a r i n a number of specimens t h e h i g h t e m p e r a t u r e d i r e c t i o n i s n o t b a s e d on a s t a b l e end p o i n t .

I n t h e s e r o c k s t h e main component i s c o n s i d e r e d t o be a secondary m a g n e t i z a t i o n acquired during Permo-Triassic remagnetization.

Widespread r e m a g n e t i z a t i o n o f t h e

Old Red Sandstone took p l a c e a t t h i s t i m e ( C r e e r , 1968; Waage and S t o r e v e d t , 1973; T u r n e r and A r c h e r , 1975; T a r l i n g e t a l . ,

1976).

Indeed Creer's r e m a g n e t i z a t i o n

h y p o t h e s i s ( C r e e r , 1968) i s l a r g e l y b a s e d on t h e r e s u l t s j u s t d i s c u s s e d , and t h e i r

462

BRECON BEACONS

Fig.8.19. D i r e c t i o n a l changes d u r i n g thermal d e m a g n e t i z a t i o n f o r specimens from t h e Old Red Sandstone a t t h r e e l o c a l i t i e s . Some of t h e low t e m p e r a t u r e d i r e c t i o n s from M5 a r e o m i t t e d f o r c l a r i t y . Closed symbols i n d i c a t e downward d i p p i n g v e c t o r s and open symbols upward d i p p i n g v e c t o r s . C o r r e c t e d f o r g e o l o g i c a l d i p ( a f t e r Chamalaun and Creer, 1964).

s i m i l a r i t y w i t h r e s u l t s from r o c k s of t h e same a g e i n t h e USSR and USA.

However

some of t h e s e r e s u l t s s u g g e s t t h a t t h e r e m a g n e t i z a t i o n may n o t have been a s p e r v a s i v e a s f i r s t t h o u g h t and a l s o t h a t t h e e x t e n t o f r e m a g n e t i z a t i o n may have depended i n some way on t h e i n i t i a l r e d bed d i a g e n e s i s .

C o n s i d e r i n g t h e Manorbier

Bay and F r e s h w a t e r West s i t e s ( F i g . 8 . 1 9 ) i n c a n b e s e e n t h a t t h e i n i t i a l d i r e c t i o n s of m a g n e t i z a t i o n ( t h e main component) a r e q u i t e d i f f e r e n t .

Whereas t h e d i r e c t i o n

a t F r e s h w a t e r West c o r r e s p o n d s t o t h e P e r m o - T r i a s s i c f i e l d d i r e c t i o n (so do t h o s e i n t h e Brecon Beacons) t h e Manorbier Bay d i r e c t i o n s do n o t , n o r can t h e y b e c o n s i d e r e d t o be a r e s u l t a n t o f a Devonian and a Permo-Triassic d i r e c t i o n .

Note

a l s o t h a t the high temperature d i r e c t i o n s i s o l a t e d a t t h i s l o c a l i t y a r e nearly antiparallel t o the i n i t i a l direction.

T h i s n o t t h e c a s e a t F r e s h w a t e r West. A

p o s s i b l e e x p l a n a t i o n f o r t h i s d i s c r e p a n c y i s t h a t t h e samples from Manorbier Bay might n o t have s u f f e r e d Permo-Triassic r e m a g n e t i z a t i o n a t a l l .

The i n i t i a l

d i r e c t i o n s w i t h SW d e c l i n a t i o n and downward d i p s a r e c o m p a t i b l e w i t h a r e v e r s e d Devonian d i r e c t i o n and t h e h i g h t e m p e r a t u r e d i r e c t i o n s i n t h e NE q u a d r a n t w i t h a normal Devonian d i r e c t i o n .

R e v e r s a l s a r e known t o have been f r e q u e n t i n t h e Lower

Devonian (Sallomy and P i p e r , 1973) and s i n c e t h e p e t r o g r a p h i c e v i d e n c e i n d i c a t e s t h a t d i a g e n e s i s p r o b a b l y c o n t i n u e d f o r a l o n g time a f t e r d e p o s i t i o n , t h e a c q u i s i t i o n of two components i n t h i s way seems a l i k e l y p o s s i b i l i t y .

The p r e s e r v a t i o n of a

composite m a g n e t i z a t i o n of t h i s t y p e may have depended upon t h e f o r m a t i o n of a s t a b l e m a g n e t i z a t i o n i n t h e pigment ( s e e a l s o T u r n e r and A r c h e r , 1975).

T h i s would

make t h e e x i s t i n g m a g n e t i z a t i o n more r e s i s t a n t t o r e m a g n e t i z a t i o n p r o c e s s e s whether t h e s e were a s s o c i a t e d w i t h t e m p e r a t u r e o r chemical changes. The O r c a d i a n Basin The sedimentology and d i a g e n e s i s of t h e Old Red Sandstone i n t h e Orcadian Basin a r e d i s c u s s e d i n C h a p t e r 6.

Palaeomagnetic r e s u l t s f o r t h e Old Red Sandstone o f

t h e O r c a d i a n B a s i n , NE S c o t l a n d have been r e p o r t e d by Stubbs (19581, S t o r e t v e d t and P e t e r s e n ( 1 9 7 2 ) , Waage and S t o r e t v e d t ( 1 9 7 3 ) , T u r n e r and Archer ( 1 9 7 5 ) , T a r l i n g e t a l . ( 1 9 7 6 ) , T u r n e r (1977) and S t o r e t v e d t and Carmichael (1979). One of t h e most d e t a i l e d s t u d i e s was t h a t of T u r n e r and Archer (1975) who i n v e s t i g a t e d t h e Lower Old Red Sandstone i n t h e Gamrie O u t l i e r on t h e s o u t h s i d e of t h e Moray F i r t h ( F i g . 8 . 2 0 ) .

I n a l l 2 3 s i t e s were s t u d i e d , a l l from t h e Crovie

Group, a v a r i a b l e sequence of predominantly r e d a l l u v i a l f a n and f i n e r g r a i n e d f l u v i a t i l e deposits.

The o r i g i n of t h e r e d beds has been d i s c u s s e d by Turner and

Archer (1975, p.242;

1977).

They s u g g e s t e d t h a t pigmentary o x i d e s were produced

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 s i l i c a t e g r a i n s ; h o r n b l e n d e s and pyroxene a r e e x t r e m e l y s c a r c e o r a b s e n t s u g g e s t i n g complete removal and p h y l l o s i l i c a t e s show abundant e v i d e n c e of r e p l a c e m e n t by h a e m a t i t e .

Similarly the specularite

g r a i n s a r e n e a r l y a l l m a r t i t e s and i t i s l i k e l y t h a t t h e r e h a s been e x t e n s i v e

464 i n s i t u o x i d a t i o n of m a g n e t i t e . The s t r u c t u r e of t h e NRM i n t h e G r o v i e Group s e d i m e n t s i s e x t r e m e l y complex. I n i t i a l d i r e c t i o n s a p p e a r e d random b u t Turner and A r c h e r (1975) were a b l e t o r e s o l v e t h r e e t y p e s of p a r t i a l t h e r m a l d e m a g n e t i z a t i o n b e h a v i o u r

( F i g . 8.20)

c h a r a c t e r i z e d by t h e f o l l o w i n g d i r e c t i o n s , a f t e r c o r r e c t i o n f o r g e o l o g i c a l d i p :

1) m o d e r a t e l y s t e e p downward i n c l i n a t i o n s 2 ) s o u t h e r l y d e c l i n a t i o n s and s h a l l o w i n c l i n a t i o n s

3) s o u t h w e s t e r l y d e c l i n a t i o n s w i t h m o d e r a t e l y s t e e p upward i n c l i n a t i o n s . Each of t h e s e i s now c o n s i d e r e d i n t u r n 1) Specimens w i t h t h i s t y p e o f NRM a r e found t h r o u g h o u t t h e C r o v i e Group b u t a r e

p a r t i c u l a r l y c h a r a c t e r i s t i c o f t h e Pennan Sandstone.

Progressive treatment of

p i l o t specimens y i e l d s d i r e c t i o n s which a r e c l o s e l y grouped up t o 6OO0C d e s p i t e t h e f a c t t h a t t h e NRM i n t e n s i t y may f a l l s h a r p l y o n l y t o b e f o l l o w e d by an e q u a l l y l a r g e i n c r e a s e (Fig.8.20).

Above 6OO0C t h e d i r e c t i o n moves and i n some

c a s e s assumes a n a p p r o x i m a t e l y a n t i p a r a l l e l o r i e n t a t i o n .

The i n t e r p r e t a t i o n of

t h e s e r e s u l t s o f f e r e d by T u r n e r and A r c h e r (1975) was t h a t t h e m a g n e t i z a t i o n c o n s i s t e d of two o r more a n t i p a r a l l e l components w i t h s i m i l a r b l o c k i n g t e m p e r a t u r e spectra.

T h i s e x p l a n a t i o n a c c o u n t s f o r t h e r a p i d i n t e n s i t y changes by assuming

t h a t t h e r e s u l t a n t v e c t o r i s reduced o r i n c r e a s e d i n magnitude a s one o r t h e o t h e r

o f t h e a n t i p a r a l l e l components becomes unblocked.

Application of the Collinson

t e c h n i q u e ( C o l l i n s o n , 1974) s u g g e s t e d t h a t b o t h pigment and s p e c u l a r i t e c o n t r i b u t e d t o t h e s t a b l e NRM and T u r n e r and Archer (1975) s u g g e s t e d t h a t a t l e a s t one o f t h e a n t i p a r a l l e l components might r e s i d e i n pigment which was produced d i a g e n e t i c a l l y d u r i n g a contemporary geomagnetic f i e l d r e v e r s a l .

These a u t h o r s were of t h e o p i n i o n

t h a t t h i s m a g n e t i z a t i o n was of Devonian a g e ; however t h i s c a n n o t b e v e r i f i e d by any independent e v i d e n c e ; moreover s i n g l e components w i t h s t a b l e end p o i n t s have n o t been i s o l a t e d . 2 ) . T h e second group have a NRM c h a r a c t e r i z e d by s o u t h e r l y d e c l i n a t i o n s w i t h s h a l l o w inclination.

The d i r e c t i o n s a r e s t a b l e up t o 500 o r 6OO0C b u t above t h i s t e m p e r a t u r e

t h e d i r e c t i o n s become s c a t t e r e d s u g g e s t i n g t h a t s i n g l e , s t a b l e components have n o t been i s o l a t e d .

The m a g n e t i c p r o p e r t i e s of t h i s group a r e i n t e r m e d i a t e between t h o s e

o f Group 1 and 3.

3 ) The NRM of Group 3 i s c h a r a c t e r i z e d by s t a b l e d i r e c t i o n s of m a g n e t i z a t i o n w i t h a SW d e c l i n a t i o n and moderate upward i n c l i n a t i o n .

I n some c a s e s t h e d i r e c t i o n s a r e

s t a b l e up t o 65OoC s u g g e s t i n g a s i n g l e component of m a g n e t i z a t i o n ; i n o t h e r s t h e r e i s a s l i g h t i n c r e a s e i n s c a t t e r a t 65OoC s u g g e s t i n g t h a t o t h e r minor components may be p r e s e n t .

T h i s i s a l s o s u g g e s t e d by t h e t h e r m a l decay c u r v e s which show s l i g h t

i n c r e a s e s and d e c r e a s e s i n i n t e n s i t y d e s p i t e c l o s e l y d e f i n e d b l o c k i n g t e m p e r a t u r e spectra.

465

Fig.8.20. Palaeomagnetic r e s u l t s from t h e Lower Old Red Sandstone of t h e Gamrie O u t l i e r , S c o t l a n d . A. The s t a b l e d i r e c t i o n s of 2 1 p i l o t specimen i s o l a t e d d u r i n g thermal d e m a g n e t i z a t i o n and t h e i r d i v i s i o n i n t o t h r e e g r o u p s : 1 ( s t e e p l y d i p p i n g p o s i t i v e v e c t o r s ) , 2 ( s o u t h e r l y d i r e c t e d v e c t o r s w i t h s h a l l o w d i p ) and 3 (moderately s t e e p upward d i p p i n g v e c t o r s . B , C and D show t h e d i r e c t i o n a l and normalized i n t e n s i t y changes o f groups 1, 2 and 3 r e s p e c t i v e l y . The a s t e r i s k shows t h e l o c a l d i r e c t i o n o f t h e p r e s e n t E a r t h ' s f i e l d ( a f t e r T u r n e r and A r c h e r , 1 9 7 5 ) .

466 The p r e c i s e p a l a e o m a g n e t i c s i g n i f i c a n c e of t h e s e t h r e e d i f f e r e n t t y p e s o f NRM i s i n doubt b e c a u s e of t h e d i f f i c u l t y o f i s o l a t i n g d i s c r e t e , s t a b l e components

of m a g n e t i z a t i o n .

Only t h e Group 3 d i r e c t i o n s can b e c o n s i d e r e d t o have been

s a t i s f a c t o r i l y r e s o l v e d l y i n g c l o s e t o t h e Permo-Triassic d i r e c t i o n f o r t h e a r e a . I n o r d e r t o e x p l a i n t h e r e s u l t s T u r n e r and A r c h e r (1975) a r g u e d t h a t t h e s a n d s t o n e s

had a long and complex d i a g e n e t i c h i s t o r y o f which t h e f o l l o w i n g were t h e main features:

1) P o s t - d e p o s i t i o n a l o x i d a t i o n o f s i l i c a t e s and d e t r i t a l i r o n o x i d e s p r o d u c i n g Devonian m a g n e t i z a t i o n s .

Continued d i a g e n e s i s r e s u l t e d i n t h e a c q u i s i t i o n o f b o t h

normal and r e v e r s e d components a s a r e s u l t of contemporary geomagnetic f i e l d fluctuations.

I n t h i s way a composite NRM t y p i f i e d by s c a t t e r e d d i r e c t i o n s was

produced; i t i s now r e p r e s e n t e d by t h e Group 1 d i r e c t i o n s .

2 ) Continued o x i d a t i o n d u r i n g b u r i a l r e s u l t e d i n f u r t h e r m o d i f i c a t i o n o f t h e o r i g i n a l NRM and some s a n d s t o n e s s u f f e r e d p a r t i a l r e m a g n e t i z a t i o n .

These a r e now

r e p r e s e n t e d by t h e Group 2 d i r e c t i o n . 3 ) Some s a n d s t o n e s were c o m p l e t e l y remagnetized i n P e r m o - T r i a s s i c times (Group 3 directions).

C o n t r i b u t o r y f a c t o r s were t h e e x t e n s i v e i n s i t u o x i d a t i o n o f m a g n e t i t e

t o h a e m a t i t e and o t h e r long-term d i a g e n e t i c changes i n v o l v i n g t h e p r o d u c t i o n and g r a i n growth of h a e m a t i t e .

These may have been a i d e d by a w i d e s p r e a d t h e r m a l e v e n t

i n S c o t l a n d d u r i n g Permian t i m e s which was a s s o c i a t e d w i t h t h e R o t l i e g e n d e s v o l c a n i c s and c e r t a i n i n t r u s i v e e v e n t s (Brown, 1975). Waage and S t o r e t v e d t (1973) and T a r l i n g e t a l . (1976) have made comprehensive palaeomagnetic s u r v e y s o f a number of l o c a l i t i e s w i t h i n t h e main p a r t of t h e Orcadian B a s i n ,

T h e i r r e s u l t s a r e i n g e n e r a l agreement and show t h e predominance

of a r e v e r s e d d i r e c t i o n n e a r 1 9 0 , -35' U p p e r Old Red Sandstone.

i n t h e a l l u v i a l f a c i e s of t h e Middle and

By comparison w i t h t h e p o l a r wander p a t h f o r B r i t a i n

T a r l i n g e t a l . (1976, p . 131) c o n s i d e r e d t h i s m a g n e t i z a t i o n t o be of Permian a g e and t o have been g e n e r a t e d by p e n e t r a t i v e o x i d a t i o n b e n e a t h t h e Permian d e s e r t l a n d s u r f a c e ( c f . Chamalaun and C r e e r , 1964; Cornwell, 1966; B r i d e n e t a l . , 1973). A f t e r p a r t i a l thermal d e m a g n e t i z a t i o n a t 6OO0C T a r l i n g e t a l . (1976) found no s y s t e m a t i c p a t t e r n i n specimen d i r e c t i o n s .

Waage and S t o r e t v e d t (1973) b e l i e v e d

t h a t t h e r e m a g n e t i z a t i o n i n t h e O r c a d i a n Basin was Upper Devonian i n a g e and c a u s e d by moderate t e m p e r a t u r e i n c r e a s e s due t o b u r i a l i n t h e manner d e s c r i b e d by Chamalaun (1964). Waage and S t o r e t v e d t (1973) a p p l i e d h i g h t e m p e r a t u r e t h e r m a l d e m a g n e t i z a t i o n and i n a s m a l l number of c a s e s t h e y were a b l e t o remove t h e s e c o n d a r y component and i s o l a t e a h i g h t e m p e r a t u r e component ( F i g . 8 . 2 1 ) .

The d i r e c t i o n of t h i s component

i s s i m i l a r t o c e r t a i n p a l a e o m a g n e t i c r e s u l t s from t h e Old Red Sandstone o f Norway

and t h e Orkney i s l a n d s ( S t o r e t v e d t and H a l v o r s e n , 1966; S t o r e t v e d t e t a l . , 1968; L i e e t a l . , 1969; S t o r e t v e d t and P e t e r s e n , 1972) and S t o r e t v e d t ( 1 9 7 0 a , b ) has

467

N

N

-

CA47- BI

C A 4 4 82

600

400

630

600

.

640 650

E

' 0

0 '

\ ::: \

640 650 660

620

4 E

-2

CA67-A

s

N

-

200 650'C

C A 5 6 -A1 I

;.E

I

S

S F i g . 8.21. D i r e c t i o n a l changes d u r i n g t h e t h e r m a l d e m a g n e t i z a t i o n o f Old Red Sandstone specimens from t h e O r c a d i a n b a s i n o f C a i t h n e s s , S c o t l a n d . C o r r e c t e d f o r g e o l o g i c a l d i p ( a f t e r Waage and S t o r e t v e d t , 1 9 7 3 ) .

468 a r g u e d t h a t i t r e p r e s e n t s t h e t r u e Devonian f i e l d d i r e c t i o n s r a t h e r t h a n t h a t deduced from Lower Devonian v o l c a n i c r o c k s . Palaeomagnetic d i r e c t i o n s o f S c o t t i s h and Norwegian Old Red S a n d s t o n e r e d beds and v o l c a n i c r o c k s a r e shown f o r comparison i n F i g . 8 . 2 1 .

The s t r i k i n g f e a t u r e o f

t h e s e r e s u l t s i s t h e s h a r p d i f f e r e n c e between t h e v o l c a n i c s and t h e r e d beds which

i s q u i t e c o n s i s t e n t w i t h w i d e s p r e a d Permian r e m a g n e t i z a t i o n o f t h e s e d i m e n t s .

The

i m p o r t a n t q u e s t i o n i s whether t h e t r u e Devonian d i r e c t i o n i s t h a t r e p r e s e n t e d by t h e v o l c a n i c s o f t h e Midland V a l l e y o f S c o t l a n d and t h e Lorne P l a t e a u ( b y some i n t r u s i v e r o c k s ) o r t h a t based on t h e Norwegian and C a i t h n e s s s t u d i e s o f S t o r e t v e d t and co-workers.

By comparing t h e p o l e p o s i t i o n s based on t h e s e d i r e c t i o n s from Norway

w i t h t h o s e i n C a i t h n e s s S t o r e t v e d t (1974) p o s t u l a t e d a l a r g e post-Devonian s i n i s t r a l d i s p l a c e m e n t a l o n g t h e G r e a t Glen F a u l t o f some 200-300 Km b u t t h i s h a s been o b j e c t e d t o on a number of grounds (Mykura, 1975; Donovan e t a l . ,

1976; T u r n e r e t a l . , 1976).

The r e s u l t s from t h e Devonian l a v a s of t h e Midland V a l l e y a r e c o n s i d e r e d t o b e t h e b e s t r e p r e s e n t a t i o n o f t h e Devonian f i e l d b e c a u s e o f t h e f o l l o w i n g r e a s o n s :

1) There a r e a l a r g e number o f r e s u l t s i n c l o s e agreement and which show e x a c t l y a n t i p a r a l l e l , normal and r e v e r s e d d i r e c t i o n s 2 ) The r e s u l t s a r e s u p p o r t e d by f i e l d tests a n d l a b o r a t o r y t e s t s of s t a b i l i t y

3 ) The p a l a e o l a t i t u d e deduced from t h e s e r e s u l t s p l a c e s B r i t a i n a b o u t 20-25'

south

o f t h e p a l a e o e q u a t o r and t h i s i s q u i t e c o n s i s t e n t w i t h t h e s e d i m e n t o l o g i c a l e v i d e n c e (Chapter 4 ) . The r e m a g n e t i z a t i o n o f t h e Old Red Sandstone d u r i n g t h e L a t e P a l a e o z o i c o r e a r l y Mesozoic a p p e a r s t o have been a w i d e s p r e a d e v e n t i n t h e O r c a d i a n B a s i n and a p p a r e n t l y a f f e c t e d t h e Upper Old Red Sandstone and t h e a l l u v i a l f a c i e s w i t h i n t h e predominantly l a c u s t r i n e Middle Old Red Sandstone sequence ( T a r l i n g e t a l . ,

1976).

Only p a r t s of

t h e Lower Old Red S a n d s t o n e s u c h a s t h e S a r c l e t l o c a l i t y s t u d i e d by Waage and S t o r e t v e d t (1973, p. 940) and T u r n e r (1977) and t h e Gamrie O u t l i e r a p p e a r t o have r e s i s t e d extensive remagnetization.

The l a c u s t r i n e s e d i m e n t s have been s t u d i e d by

T u r n e r (1977) and T u r n e r a t a l . (1978) who showed t h a t t h e s e t o o had been p a r t i a l l y remagnetized b u t p r o b a b l y d u r i n g Mesozoic times. a t l e a s t two d i s t i n c t e p i s o d e s o f r e m a g n e t i z a t i o n .

We t h u s have t h e p r o b a b i l i t y o f With t h e s e superimposed on

i n i t i a l l y complex d i a g e n e t i c m a g n e t i z a t i o n s i t i s n o t s u r p r i s i n g t h a t t h e r e i s no g e n e r a l a c c e p t a n c e o f a Devonian p o l e p o s i t i o n f o r B r i t a i n . TRIASSIC R E D BEDS OF THE WESTERN USA The T r i a s s i c r e d beds of t h e Colorado P l a t e a u a r e a i n t h e w e s t e r n USA have been e x t e n s i v e l y s t u d i e d and p r o v i d e t h e most d e t a i l e d p a l a e o m a g n e t i c r e c o r d of any r e d beds anywhere i n t h e world.

The r e d bed s e q u e n c e , r a n g i n g i n a g e from Upper

C a r b o n i f e r o u s t o Upper T r i a s s i c , i s a l m o s t f u l l y documented (McMahon and Strangway, 1968a,b; F a r r e l l and May, 1969; H e l s l e y , 1969; Baag and H e l s l e y , 1974a, H e l s l e y

469 and S t e i n e r , 1974; S t e i n e r and H e l s l e y , 1974a,b; Van d e r Voo and Grubbs, 1977) and t h e T r i a s s i c r e s u 1 , t s a r e n o t a b l e f o r t h e i r c o n s i s t e n c y w i t h r e s u l t s from T r i a s s i c sequences i n o t h e r p a r t s of t h e world i n c l u d i n g Europe (Burek, 1968; T u r n e r and I x e r , 1 9 7 7 ) , Greenland (Reeve e t a l . , 1974) and I n d i a ( B h a l l a and Verma, 1969). These r e s u l t s a r e c h a r a c t e r i z e d by a b u n d a n t , a p p a r e n t l y d i s c r e t e , zones o f normal and r e v e r s e d p o l a r i t y , a f e a t u r e which c o n t r a s t s markedly w i t h t h e o t h e r m a j o r Phanerozoic r e d beds l i k e t h e Old Red Sandstone. The Moenkopi Formation The r e g i o n a l s t r a t i g r a p h y o f t h e Moenkopi Formation, which c r o p s o u t i n t h e Colorado P l a t e a u a r e a , i s d e s c r i b e d by McKee (1954) and Blakey (1973).

I n the

w e s t e r n p a r t o f i t s o u t c r o p , t h e Moenkopi Formation i s c h a r a c t e r i z e d by a marine sequence i n c l u d i n g g r e y s h a l e s , c a r b o n a t e s , and e v a p o r i t e s w i t h i n t e r c a l a t e d r e d and maroon s h a l e s .

Eastwards t h e f o r m a t i o n becomes more c l a s t i c and i n e a s t e r n

Utah and w e s t e r n Colorado i t c o n s i s t s of brown and red-brown s a n d s t o n e , s i l t s t o n e and s h a l e , w i t h minor c o n g l o m e r a t e s .

The c l a s t i c f a c i e s of t h e Moenkopi has been

d i v i d e d i n t o f o u r members by Shoemaker and Newman (1969). bottom t o t o p ) :

These a r e (from

t h e T e n d e r f o o t Member, a sequence of m a s s i v e l y bedded bimodal

s a n d s t o n e , r e g i o n a l l y e q u i v a l e n t t o t h e H o s k i n n i n i Member ( S t e w a r t , 1959); t h e A l i Baba Member, a sequence of i n t e r b e d d e d s a n d s t o n e , s i l t s t o n e , and conglomerate;

t h e Sewemup Member, a sequence o f f i n e g r a i n e d s a n d s t o n e , s i l t s t o n e and s h a l e ; and t h e P a r i o t t Member, a sequence o f m u l t i c o l o u r e d s i l t s t o n e , s a n d s t o n e , and conglomerate The Moenkopi Formation i s Lower T r i a s s i c i n a g e , t h e P e r m i a n - T r i a s s i c boundary l y i n g somewhere w i t h i n t h e T e n d e r f o o t Member (Dane, 1935; Shoemaker and Newman, 1959). The palaeomagnetism o f t h e Moenkopi Formation has been s t u d i e d b u t t h e r e a r e s t i l l c o n f l i c t i n g i n t e r p r e t a t i o n s of t h e r e s u l t s (Larson e t a l . , 1976; E l s t o n and P a r u c k e r , 1979).

A f t e r t h e e a r l y work of Runcorn ( 1 9 5 6 ) , K i n t z i n g e r (1957) and C o l l i n s o n and

Runcorn (1960) H e l s l e y (1969) and H e l s l e y and S t e i n e r (1974) made a d e t a i l e d s u r v e y o f t h e s e c t i o n s s o u t h of Gateway, Colorado.

They e s t a b l i s h e d a complete magnetic

r e v e r s a l s t r a t i g r a p h y c o n s i s t i n g o f f i v e normal (N--N5)

and f i v e r e v e r s e d zones

(Rl-R5) ( F i g . 8 . 2 2 ) . The lower p a r t of t h e Moenkopi Formation c o n s i s t s of a t h i c k r e v e r s e d zone (Rl) f o l l o w e d by a series of t h i n n e r p o l a r i t y zones.

S i m i l a r l y t h e upper p a r t of t h e

Moenkopi Formation i s c h a r a c t e r i z e d by a t h i c k r e v e r s e d zone (R4) followed by t h r e e t h i n p o l a r i t y zones ( F i g . 8 . 2 2 ) .

The d i r e c t i o n s of m a g n e t i z a t i o n of t h e R3-N5

i n t e r v a l , b e f o r e and a f t e r t h e r m a l d e m a g n e t i z a t i o n , a r e shown in Fig.8.23. The i n i t i a l d i r e c t i o n s a r e r a t h e r s c a t t e r e d and t h e mean d i r e c t i o n s of t h e normal i n t e r v a l s (Dec. = 348'; I n c . = )'2

I n c . = +24')

a r e o n l y a b o u t 160'

s e c o n d a r y component.

and t h e r e v e r s e d i n t e r v a l s (Dec. = 159';

a p a r t suggesting the presence of a substantial

The most h i g h l y s c a t t e r e d NRM d i r e c t i o n s c o i n c i d e w i t h

470 s t r a t i g r a p h i c i n t e r v a l s w i t h abundant c o a r s e s a n d s t o n e u n i t s , a f e a t u r e which s u g g e s t s t h a t t h e p r e s e n c e o f o x i d i z i n g groundwater s o l u t i o n s d u r i n g d i a g e n e s i s was p a r t l y responsible f o r secondary magnetization.

N5 R5 N4

R4

N3 R3 R3 N2

I I

N1

9GNETIC ZONE Fig.8.22. L o c a t i o n map and sampling s i t e s o f t h e Moenkopi Formation i n c e n t r a l Western Colorado and t h e g e n e r a l r e v e r s a l s t r a t i g r a p h y ( a f t e r H e l s l e y and S t e i n e r ,

1974). A f t e r t h e r m a l d e m a g n e t i z a t i o n a t 55OoC c o n s i d e r a b l y reduced t h e i n i t i a l s c a t t e r t h e mean d i r e c t i o n s o f t h e normal i n t e r v a l s (Dec = 342', t h e r e v e r s e d i n t e r v a l s a r e 176' m a g n e t i z a t i o n h a s been removed.

I n c . = +17O) and

a p a r t suggesting t h a t s u b s t a n t i a l secondary The b e h a v i o u r of p i l o t specimens d u r i n g i n c r e m e n t a l

t h e r m a l d e m a g n e t i z a t i o n i s d e s c r i b e d by H e l s l e y (1969).

A characteristic feature

of t h e r e s u l t s i s t h a t t h e normal d i r e c t i o n s changed v e r y l i t t l e d u r i n g t h e dem-

471 a g n e t i z a t i o n p r o c e d u r e whereas t h e r e v e r s e d d i r e c t i o n s showed l a r g e changes i n d i r e c t i o n and i n t e n s i t y ( F i g . 8 . 2 4 ) . N

N

N

N

N

N

N

N

Fig.8.23. D i r e c t i o n s and means of p o l a r i t y zones R3-NS i n t h e Moenkopi Formation b e f o r e and a f t e r t h e r m a l d e m a g n e t i z a t i o n a t 55OoC ( a f t e r H e l s l e y and S t e i n e r , 1974).

472

+

I

1

LOWER

+ UPPER

5

\

2x166

I

3 h sbo 700 TEMPERATURE-'C

Olbo

W PRESENT

Fig.8.24. The b e h a v i o u r of s e l e c t e d specimens t o i n c r e m e n t a l thermal d e m a g n e t i z a t i o n : A . Changes i n d i r e c t i o n , B. Changes i n i n t e n s i t y . Note t h e d i f f e r e n c e i n b e h a v i o u r between t h e n o r m a l l y and r e v e r s e l y magnetized specimens ( a f t e r H e l s l e y , 1969).

In a number of t h e specimens w i t h r e v e r s e d p o l a r i t y s t a b l e end p o i n t s a r e n o t reached d u r i n g t h e t h e r m a l d e m a g n e t i z a t i o n .

These specimens a r e a l s o c h a r a c t e r i z e d

by weaker NRM i n t e n s i t i e s t h a n t h e normal i n t e r v a l s , a combination of f e a t u r e s which s u g g e s t s t h a t t h e r e v e r s e d d i r e c t i o n s c o n t a i n a superimposed normal component of magnetization.

T h i s e x p l a n a t i o n a l s o a c c o u n t s f o r zone ( A ) , which o c c u r s i n

t h e middle of R3, and c o n s i s t s of n e a r v e r t i c a l d i r e c t i o n s w i t h a p o s i t i v e (downward) d i p .

T h i s v i e w , however, i s n o t h e l d by H e l s l e y and S t e i n e r (1974) n o r by

Baag and H e l s l e y ( 1 9 7 4 a ) , who p r e s e n t e d d a t a which s u p p o s e d l y i n d i c a t e d t h a t t h e Moenkopi Formation was magnetized penecontemporaneously. D e t a i l e d s t u d y of f o u r unbroken cove segments and a series of measurements from seven c o r e s , c r o s s i n g s i x p o l a r i t y t r a n s a c i o n s i n t h e Moenkopi F o r m a t i o n , a r e r e p o r t e d by Baag and H e l s l e y ( 1 9 7 4 a ) . i n Fig.8.25.

The r e s u l t s o f t h e c o r e segments a r e shown

The v a r i a t i o n o b s e r v a b l e i n t h e s e c o r e s c o r r e s p o n d t o V i r t u a l

Geomagnetic P o l e a n g u l a r s t a n d a r d d e v i a t i o n s of 5.6',

6.6',

5.1'

and 4.3'.

At

t h e p r e s e n t time t h e a v e r a g e a n g u l a r s t a n d a r d a t a comparable l a t i t u d e ( a b o u t 10' f o r t h e Moenkopi F o r m a t i o n , I r v i n g , 1964, p. 201) i s a b o u t 8' 1974a, p. 3309).

(Baag and H e l s l e y ,

The VGP d i s t r i b u t i o n s of t h e f o u r c o r e segments have a n e l o n g a t e d

o v a l shape ( a b o u t a 2 : l e l l i p s e ) ; a time-averaged m a g n e t i z a t i o n would b e e x p e c t e d a c i r c u l a r ( o r F i s h e r i a n ) VGP d i s t r i b u t i o n .

Baag and H e l s l e y (19748)

a r g u e d t h a t t h e s e r e s u l t s i n d i c a t e d t h a t e a c h specimen was magnetized i n less t h a n one time p e r i o d o f n o n d i p o l e s e c u l a r v a r i a t i o n ( a b o u t 1800 y e a r s ) ; t h e y f u r t h e r s u g g e s t e d t h a t t h e o v a l d i s t r i b u t i o n of VGP's i n d i c a t e d t h a t d i p o l e v a r i a t i o n s 6 (Brock, 1971) w i t h a p e r i o d o f lo4 - 10 Years had l i t t l e o r no a v e r a g i n g e f f e c t o f t h e observed d a t a (Baag and H e l s l e y , 1974b).

413 602-551'C

514-574'C

0.5

0 180

90 OEC

-m o

+30

INC

IN1

135 90

511-602'C

DEC

I80 -60 -)o INC

0

3 ~ 1 1mW3 0 ~ 3.10-3 INT

457

75

50

25

_.

0

90

I80

90 OEC

INC

IN1

110 -45

DEC

133

0

INC

m-*

10-3 1n1

211

Fig.8.25. Stratigraphic plots of declination (DEC) inclination (INC) and intensity (INT) variations of four long core segments after thermal demagnetization. Intensity is in emu per specimen of about l2cm3 (after Baag and Helsley, 1974a). Serial correlation tests (Epp et al., 1971) of the magnetic variations in the four core segments indicates that cores 135 and 457 are serially correlated at the 5% level whereas 133 and 211 are not significant (Fig.8.25).

Baag and Helsley

(1974a) considered this to be further evidence that the NRM acquisition had been very rapid.

Using Fourier analyses of the four core segments and assuming that

each specimen was magnetized in 250 years they recognized periodicities of 950

-

1840 Years in declination and 1420

-

3680 Years in inclination. The periods

in declination are very similar to those calculated from the present-day westward

474

d r i f t of t h e non-dipole f i e l d a l t h o u g h i t must b e s t a t e d t h a t t h i s depends o n l y on t h e NRM a c q u i s i t i o n r a t e and t h i s has o n l y been a r b i t r a r i l y d e f i n e d a t 250 Years/

2.2cm specimen. Baag and H e l s l e y (1974a) s t u d i e d a number of t r a n s i t i o n p a t t e r n s (changes from normal t o r e v e r s e d d i r e c t i o n s o f m a g n e t i z a t i o n o r v i c e v e r s a . up t o 2.2m) r e c o r d a s i n g l e t r a n s i t i o n (Fig.8.26)

S i x l o n g c o r e s (up

and one l o n g c o r e (4.9m) c o n t a i n e d

These t r a n s i t i o n s show a number of i n t e r e s t i n g f e a t u r e s

two t r a n s i t i o n s .

which a r e open t o v a r i o u s i n t e r p r e t a t i o n s .

For example t h e t r a n s i t i o n from t h e

R , t o N1 magnetozones which were r e c o r d e d i n two c o r e s WK7 and WK8 d r i l l e d a b o u t

o n l y 5m a p a r t ( F i g . 8 . 2 6 )

cannot be a c c u r a t e l y c o r r e l a t e d i n d e t a i l .

The d e c l i n a t i o n

p l o t s , i n p a r t i c u l a r , a r e v e r y d i f f e r e n t and t h e r e a r e minor v a r i a t i o n s i n i n t e n s i t y and i n c l i n a t i o n .

Most o f t h e t r a n s i t i o n s show some e v i d e n c e of t h e p r e s e n c e of

i n t e r m e d i a t e d i r e c t i o n s of m a g n e t i z a t i o n and lower i n t e n s i t y v a l u e s d u r i n g t h e t r a n s i t i o n i n t e r v a l (good examples a r e shown i n F i g . 8 . 2 6 ) .

WK8

537'C

th -90 DEC

0 4 0 INC

10-4 lo-3 10-2 IN1

3

' ,

0

90 180 270 -90 0 +W DEC

INC

10-410-3 10-2 IN1

NlIRl

N1 I R 1

Fig.8.26. D e c l i n a t i o n (DEC), i n c l i n a t i o n (INC) and i n t e n s i t y (INT) changes a f t e r R 1 t r a n s i t i o n a s observed i n two long thermal d e m a g n e t i z a t i o n a d j a c e n t t o t h e N 1 c o r e s , WK7 and WK8 ( a f t e r Baag and H e l s l e y , 1974a).

-

415 There a r e two a l t e r n a t i v e e x p l a n a t i o n s o f t h e s e r e s u l t s .

F i r s t l y , these

v a r i a t i o n s could r e p r e s e n t g e n u i n e i n t e r m e d i a t e f i e l d d i r e c t i o n s which o c c u r r e d during t h e geomagnetic t r a n s i t i o n .

S e c o n d l y , t h e v a r i a t i o n s c o u l d be due t o a

composite zone o f m a g n e t i z a t i o n which r e s u l t e d from t h e i m p o s i t i o n of a r e v e r s e d m a g n e t i z a t i o n on a p r e - e x i s t i n g normal m a g n e t i z a t i o n .

I n t h e f i r s t c a s e t h e NRM

a c q u i s i t i o n would have t o be v e r y r a p i d and b a s i c a l l y be o f d e p o s i t i o n a l o r v e r y early post-depositional

o r i g i n . The second c a s e would be 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 of t h e NRM b u t no s p e c i f i c c o n c l u s i o n s r e g a r d i n g t h e a g e of t h e NRM c o u l d

be drawn.

Although Baag and H e l s l e y (1974a) p r e f e r r e d t h e f i r s t h y p o t h e s i s , t h e i r This lies i n the f a c t t h a t the

r e s u l t s do i n f a c t c o n t a i n x i d e n c e f o r t h e second.

l o w - i n t e n s i t y specimens from t h e t r a n s i t i o n i n t e r v a l s a r e l e s s s t a b l e t h a n t h o s e from t h e normal o r r e v e r s e d zones.

I n s t a b i l i t y d u r i n g t h e r m a l d e m a g n e t i z a t i o n of

red beds i s most f r e q u e n t l y due t o t h e m i g r a t i o n o f a r e s u l t a n t o f two o r more v e c t o r s a s one of them i s p r e f e r e n t i a l l y removed.

There i s f u r t h e r e v i d e n c e t h a t

the m a g n e t i z a t i o n of t h e Moenkopi Formation was n o t a c q u i r e d penecontemporaneously. Larson e t a l . (1976) s t u d i e d a N-R

t r a n s i t i o n i n t h e lower p a r t o f t h e Moenkopi

Formation n e a r Gray Mountains A r i z o n a .

T h i s boundary, when t r a c e d l a t e r a l l y ,

migrated a c r o s s bedding p l a n e s ( l o c a l time l i n e s ) and m u l t i p l e specimens from n e a r the t r a n s i t i o n showed v a r i o u s d i r e c t i o n s , some N , some R and some i n t e r m e d i a t e b u t a l l f a l l i n g on t h e g r e a t c i r c l e between t h e T r i a s s i c N and R d i r e c t i o n s .

Larson

e t a l . (1976) a l s o showed t h a t s t e p w i s e t h e r m a l d e m a g n e t i z a t i o n of t h e Wupatki Sandstone commonly produced p r o g r e s s i v e m i g r a t i o n from N t o R T r i a s s i c d i r e c t i o n s along a g r e a t c i r c l e p l a n e .

These a u t h o r s concluded t h a t t h e Moenkopi Formation

samples c o n t a i n two o r more a n t i p a r a l l e l components of T r i a s s i c remanence

. 'Where

one o f t h e s e components i s dominant a r e g u l a r T r i a s s i c N o r R p o l a r i t y r e s u l t s ;

where t h e two components a r e e q u a l o r n e a r l y e q u a l a n i n t e r m e d i a t e d i r e c t i o n r e s u l t s , LATE CENOZOIC RED BEDS OF BAJA CALIFORNIA The palaeomagnetism of t h e L a t e Cenozoic r e d beds i n Baja C a l i f o r n i a has been d e s c r i b e d by Larson and Walker (1975).

Their study provides an important p a r a l l e l

with t h e d i a g e n e t i c s t u d i e s of Walker (1967a, 1976).

The d e p o s i t s s t u d i e d a r e of

probable l a t e P l i o c e n e a g e and c o n s i s t of i n t e r b e d s o f s i l t y mud, s i l t and sand which i n t e r f i n g e r w i t h f a n g l o m e r a t e s and l o c a l l y w i t h g y p s i f e r o u s s a n d s t o n e .

The

sediments have many c h a r a c t e r i s t i c s i n common w i t h t h e modern i n t e r t i d a l d e p o s i t s n o r t h of San F e l i p e d e s c r i b e d by Thompson (1968) ( F i g . 8 . 2 7 ) .

Two o u t c r o p s were

s t u d i e d by Larson and Walerr; 56 o r i e n t e d hand samples were c o l l e c t e d from 183m

of s e c t i o n ( d e s i g n a t e d a s s e c t i o n SF +. 35a) and s i x samples were c o l l e c t e d from a s e c t i o n (SF

+

35b) 1.6km t o t h e West which was b e l i e v e d t o be a b o u t lOOm

s t r a t i g r a p h i c a l l y below t h e b a s e of SF

+

35a.

The s e d i m e n t s i n b o t h t h e s e o u t c r o p s

a r e l o c a l l y s t a i n e d by y e l l o w and r e d f e r r i c oxyhydroxides, p r i n c i p a l l y g o e t h i t e

9LP

and h a e m a t i t e which t e n d t o be c o n c e n t r a t e d i n s a n d and s i l t laminae w i t h i n t h e muds and i n a u r e o l e s a d j a c e n t t o t h e s e laminae.

Locally, concretions of goethite

and a h y d r a t e d manganese-barium o x i d e ( h y d r o p s i l o l e m l a n e ) o c c u r .

Walker (1967a)

has c o n v i n c i n g l y d e m o n s t r a t e d t h e d i a g e n e t i c o r i g i n o f t h e pigment i n t h e s e sediments and Larson and Walker (1975) were a b l e t o show t h a t t h e r e d and yellow pigments and t h e c o n c r e t i o n s i n c l u d i n g t h e g o e t h i t e and hydropsilomelane were a l l i m p o r t a n t c a r r i e r s o f chemical remanent m a g n e t i z a t i o n .

A variable d e t r i t a l

c o n t r i b u t i o n t o t h e remanence i s made by m a g n e t i t e , a p p r o x i m a t e l y 50% o f which shows o x i d a t i o n t o maghaemite. The NRM d i r e c t i o n s a r e shown i n F i g . 8 . 2 8 a f t e r thermal d e m a g n e t i z a t i o n a t 3OO0C and 60OoC.

i n i t i a l measurement and p a r t i a l

The i n i t i a l NRM d i r e c t i o n s a r e q u i t e

s c a t t e r e d b u t show a d i s t i n c t g r o u p i n g i n t h e NE q u a d r a n t w i t h a downward d i p . Some specimens i n t h e s o u t h e r n hemisphere have upward d i p ; many of t h e s e a r e from t h e g o e t h i t e - r i c h lower p a r t o f t h e SF of r e v e r s e d d i r e c t i o n s .

+

35a s e c t i o n b u t t h e r e i s no c l e a r grouping

Thermal d e m a g n e t i z a t i o n a t 3OO0C r e d u c e s t h e s c a t t e r

c o n s i d e r a b l y m a i n l y b e c a u s e o f t h e removal of t h e r e v e r s e d component c a r r i e d by t h e g o e t h i t e - r i c h specimens i n t h e lower p a r t o f t h e s e c t i o n .

A f t e r thermal demag-

n e t i z a t i o n a t 6OO0C t h e r e i s some i n c r e a s e i n s c a t t e r and t h e c l u s t e r i n t h e NE q u a d r a n t s b r e a k s up.

These r e s u l t s can b e e x p l a i n e d i n t h e f o l l o w i n g way.

First

t h e C u r i e t e m p e r a t u r e o f m a g n e t i t e and t h e i n v e r s i o n t e m p e r a t u r e of maghaematite

were b o t h exceeded a t t h i s t e m p e r a t u r e .

Secondly t h e thermomagnetic a n a l y s e s of

Larson and Walker show t h a t t h e s a t u r a t i o n moment of h a e m a t i t e would a l s o have been much reduced by h e a t i n g t o 6OO0C and t h i r d l y t h e b l o c k i n g t e m p e r a t u r e of most of t h e f i n e g r a i n e d a u t h i g e n i c h a e m a t i t e would a l s o have been exceeded.

I n t h i s way

t h e r e l a t i v e c o n t r i b u t i o n of random o r r e v e r s e l y magnetized components was enhanced and i n t e r m e d i a t e d i r e c t i o n s r e s u l t e d ( F i g . 8 . 2 8 ) . Larson and Walker (1975) were a b l e t o p r e s e n t c o n v i n c i n g e v i d e n c e of m u l t i p l e components o f m a g n e t i z a t i o n l i k e t h o s e observed i n many o l d e r r e d bed sequences (Fig.8.29).

I n one example, t h e c e n t r a l two l a y e r s of a sample a r e r e v e r s e d i n

d i r e c t i o n , whereas t h e bottom and t o p p a r t s a r e normal and i n t e r m e d i a t e i n d i r e c t i o n (Fig.8.29a).

I n a n o t h e r example, a reddened burrow i s r e v e r s e l y magnet-

i z e d , b u t most of t h e sample has a normal p o l a r i t y ( F i g . 8 . 2 9 b ) .

Analyses of two

p a r t s of a n o t h e r sample when measured s e p a r a t e l y show t h a t t h e two component p a r t s have m a g n e t i z a t i o n s which a r e a n t i p a r a l l e l t o e a c h o t h e r ( F i g . 8 . 2 9 ~ ) . Most spect a c u l a r though a r e t h e r e s u l t s from a sample composed of l a m i n a t i o n s of r e d f i n e g r a i n e d s a n d s t o n e and l e s s r e d s i l t s t o n e ( F i g . 8 . 3 0 ) . sample i s s t e e p l y downward towards t h e E S E , i . e .

The m a g n e t i z a t i o n o f t h e whole

intermediate i n direction.

sample was s e p a r a t e d i n t o f i v e l a y e r s and e a c h l a y e r measured s e p a r a t e l y ,

The Layers a

and c a r e r e v e r s e d w h i l s t l a y e r s b , d , and e a r e normally magnetized; a l s o one of t h e r e d zones ( c ) i s r e v e r s e d i n p o l a r i t y , whereas t h e o t h e r two r e d zones (b and e )

Fig.8.28. Palaeomagnetic r e s u l t s o f Cenozoic a l l u v i u m from Baja C a l i f o r n i a . On t h e l e f t s e c t i o n SF + 35a and on t h e r i g h t SF 35b. The r e s u l t s a r e shown i n i t i a l l y and a f t e r p a r t i a l t h e r m a l d e m a g n e t i z a t i o n a t 3OO0C and 6OO0C ( a f t e r Larson and Walker, 1 9 7 5 ) .

+

479

A wnoim 0

b

concreflon

B

Infmnsify. emu/g

wnoim b (contoins red

burra)

c

3.1 a 10-a

0

4.4

b

6 . 5 I lo-’

c

1.3 I 10-O

I

lo-#

C

wholm 0

b

Fig.8.29. Palaeomagnetic results obtained by measuring samples of Cenozoic 3 alluvium wholly and in parts. A. Sample 1 (section SF + 35a). The cube is lcm B. Sample 60 (section SF + 35b). C. Sample 23. D. Sample 7. E. Sample 11 (all from section SF + 35a) (after Larson and Walker, 1975).

.

480 a r e normally magnetized.

T h i s f e a t u r e s u g g e s t s t h a t e a c h zone a c q u i r e d i t s

remanence a t t h e time i t became reddened and t h a t t h e r e d d e n i n g of e a c h l a y e r was independent of t h e o t h e r s .

I t c l e a r l y shows t h a t t h e NFW of t h e sample i s m u l t i -

component, even a f t e r thermal d e m a g n e t i z a t i o n a t 60OOC.

N

f iI whole

WORM BURROW

+

a

worm burrow

Fig.8.30. D i r e c t i o n s of remanence i n two samples of Cenozoic a l l u v i u m a f t e r thermal d e m a g n e t i z a t i o n a t 60OoC. Equal-area p r o j e c t i o n : c l o s e d c i r c l e s downward v e c t o r s ; open c i r c l e s upward v e c t o r s ( a f t e r Larson and Walker, 1975).

Larson and Walker (1975) concluded from t h e i r s t u d i e s t h a t t h e s e s e d i m e n t s were i n t h e p r o c e s s o f a c q u i r i n g a CRM which o b s c u r e s any m a g n e t i z a t i o n t h a t t h e sediments had when t h e y were d e p o s i t e d .

Three m i n e r a l s :

g o e t h i t e , h a e m a t i t e , and hydro-

p s i l o m e l a n e c a r r y t h e CRM and whenever one of t h e s e components i s h i g h l y developed t h e remanence o f t h e bed i s s t r o n g and s t a b l e b u t i t may b e e i t h e r normal o r reversed.

When two of t h e s e a u t h i g e n i c components a r e p r e s e n t i n a s i n g l e sample

t h e r e s u l t i n g remanence may be weak and i n t e r m e d i a t e i n d i r e c t i o n a s t h e two components c a n c e l e a c h o t h e r o u t .

Small s c a l e v a r i a t i o n s i n d e c l i n a t i o n , i n c l i n a t i o n

and i n t e n s i t y can t h e r e f o r e be a t t r i b u t e d t o v a r i a t i o n s in t h e r a t e , o r d e g r e e of CRM a c q u i s i t i o n and a r e n o t due t o s e c u l a r v a r i a t i o n s o f t h e E a r t h ' s magnetic f i e l d .

The remanence c a r r y i n g a u t h i g e n i c m i n e r a l s i n t h e s e d e p o s i t s a r e t h e p r o d u c t s of i n t r a s t r a t a l a l t e r a t i o n of d e t r i t a l i r o n - b e a r i n g g r a i n s s u c h a s hornblende and

481 biotite.

I t t h u s seems l i k e l y t h a t t h e r a t e o f CRM a c q u i s i t i o n depended upon t h e

r a t e o f i n t r a s t r a t a l a l t e r a t i o n t h a t c a u s e d t h e r e l e a s e of i r o n and manganese from these minerals.

T h i s r a t e i s u n l i k e l y t o have been uniform b e c a u s e t h e a l t e r a t i o n

p r o c e s s e s depend upon a v a r i e t y o f h i g h l y v a r i a b l e f a c t o r s s u c h a s t h e f l o w - r a t e and chemical c o m p o s i t i o n of t h e groundwater, p e r m e a b i l i t y and p o r o s i t y o f t h e sediments and t h e n a t u r e of t h e d e t r i t a l m i n e r a l s . T h i s model o f CRM development u s e d by L a r s o n and Walker ( 1 9 7 5 ) goes some way t o e x p l a i n i n g t h e v a r i a b i l i t y o f p a l a e o m a g n e t i c r e s u l t s from r e d beds. c l e a r l y t h e c r u c i a l f a c t o r i s t h e amount and r a t e of a c q u i s i t i o n of CRM.

Quite In

d i f f e r e n t r e d bed sequences where t h e r e a r e fundamental d i f f e r e n c e s i n t h e d e t r i t a l mineralogy and t h e s u b s e q u e n t p a t t e r n of d i a g e n e s i s CRM w i l l b e a c q u i r e d a t a d i f f e r e n t r a t e , and i n a d i f f e r e n t manner, and t h i s w i l l b e r e f l e c t e d i n t h e o v e r a l l pa l a e o m a g n e t i c r e s u l t .

THE RELATIONSHIP BETWEEN DIAGENESIS AND PALAEOMAGNETISM I N CONTINENTAL RED BEDS The g e o l o g i c a l e v i d e n c e 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 of 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 of 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 f e r r i c hydroxide d e h y d r a t i o n , s i l i c a t e a l t e r a t i o n , and t h e o x i d a t i o n o f m a g n e t i t e can t a k e p l a c e a t v a r i o u s times d u r i n g t h e d i a g e n e s i s o f c o n t i n e n t a l r e d beds.

The p a l a e o m a g n e t i c

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 o f c o n t i n e n t a l r e d beds i s e q u a l l y c o m p e l l i n g and i t must b e concluded t h a t t h e m a g n e t i z a t i o n o f r e d beds

i s acquired during diagenesis.

The a g e of t h e m a g n e t i z a t i o n i n r e d beds (which i s

t h e s i n g l e most i m p o r t a n t f a c t o r from a p a l a e o m a g n e t i c p o i n t of view) i s t h e r e f o r e a d i r e c t r e s u l t of t h e r a t e o f a c q u i s i t i o n of remanence.

The complexity o f r e d

bed m a g n e t i z a t i o n s l a r g e l y r e s u l t s from t h e f a c t t h a t t h i s r a t e i s v a r i a b l e because i t r e p r e s e n t s t h e sum of s e v e r a l d i s c r e t e d i a g e n e t i c p r o c e s s e s which a r e a l l a c q u i r i n g remanence o t a d i f f e r e n t r a t e . Models o f remanence a c a u i s i t i o n I n t h e s i m p l e s t c a s e , i f t h e r e i s a uniform r a t e of s e d i m e n t a t i o n and t h e a c q u i s i t i o n o f remanence i s r a p i d , t h e n a n a c c u r a t e r e c o r d of any geomagnetic f i e l d v a r i a t i o n w i l l b e produced (Fig.8.31a)

and t h e r e c o r d e d a m p l i t u d e s would b e somewhat

s m a l l e r t h a n t h e a c t u a l v a r i a t i o n depending upon t h e p r e c i s e s p e e d o f m a g n e t i z a t i o n . T h i s s i t u a t i o n i s o n l y l i k e l y t o o c c u r i f t h e remanence i s a DRM and t h e r e i s no c o n v i n c i n g e v i d e n c e o f t h e o c c u r r e n c e o f DRM i n r e d beds.

I f t h e remanence was

a c q u i r e d v e r y s l o w l y t h e n minor geomagnetic f l u c t u a t i o n s would b e i n t e g r a t e d and t h e r e c o r d would show l i t t l e d i r e c t i o n a l d i s p e r s i o n .

Any major v a r i a t i o n s , such a s

geomagnetic r e v e r s a l s , would be r e c o r d e d a s i f t h e r e v e r s a l o c c u r r e d lower i n t h e s e c t i o n and e a r l i e r i n t i m e ( F i g . 8 . 3 1 b ) .

I f a r e d bed c o n t a i n e d b o t h r a p i d l y and

s l o w l y a c q u i r e d remanence componenes t h e n t h e p a l a e o m a g n e t i c r e c o r d would b e

482 considerably complicated.

Assuming t h a t t h e components were a c q u i r e d d u r i n g a

f i e l d r e v e r s a l , and c o n t r i b u t e d e q u a l l y t o t h e t o t a l remanence ( F i g . 8 . 3 1 c ) ,

then

t h e r e c o r d would show a c o m p l i c a t e d and l a r g e d i s p e r s i o n zone b e f o r e t h e geomagnetic reversal.

The d i s p e r s i o n would i n c r e a s e a s t h e a c t u a l r e v e r s a l approached owing

t o the gradually increasing

c o n t r i b u t i o n of t h e s l o w l y a c q u i r e d component added

t o t h e r a p i d l y a c q u i r e d component t h a t c a r r i e d t h e minor geomagnetic

field

fluctuations.

FIELD VARIATION DEC

RAPID ACQUISITION DEC

Q

SLOW ACQUISITION IN1

DEC

b

C

RAPID & SLOW ACQUISITION DEC

INT

d

Fig.8.31. Schematic r e p r e s e n t a t i o n o f a geomagnetic p o l a r i t y t r a n s i t i o n and i t s r e c o r d i n u n i f o r m l y d e p o s i t e d sediment. I f t h e s e d i m e n t a c q u i r e s remanence r a p i d l y t h e n t h i s w i l l be a n a c c u r a t e r e c o r d of t h e t r a n s i t i o n , More s l o w l y a c q u i r e d remanence w i l l be d e p r e s s e d s t r a t i g r a p h i c a l l y and show less d i s p e r s i o n . Remanence c h a r a c t e r i z e d by a combination o f r a p i d and slow components i s a l s o shown ( a f t e r Baag and H e l s l e y , 1 9 7 4 a ) . T h i s same r e a s o n i n g can be a p p l i e d t o t h e v a r i o u s d i a g e n e t i c p r o c e s s e s which o p e r a t e i n c o n t i n e n t a l r e d beds.

Assuming t h a t DRM i s a b s e n t , o r d e s t r o y e d p o s t -

d e p o s i t i o n a l l y , t h e most r a p i d l y a c q u i r e d remanence i n r e d beds p r o b a b l y r e s u l t s from t h e d e h y d r a t i o n of d e t r i t a l f e r r i c hydroxide i n a l l u v i u m o r m e c h a n i c a l l y i n f i l t r a t e d c l a y (Van Houten, 1972; Walker, 1976).

The breakdown of i r o n - s i l i c a t e s

a l s o produces f i n e g r a i n e d pigmentary h a e m a t i t e b u t t h e remanence produced by t h i s p r o c e s s i s p r o b a b l y a c q u i r e d a t a s l o w e r r a t e t h a n t h e d i r e c t a g e i n g of f e r r i c hydroxides.

Another i m p o r t a n t d i a g e n e t i c p r o c e s s , t h e i n s i t u o x i d a t i o n of

483 m a g n e t i t e i s d i f f i c u l t t o a s s e s s i n terms o f t h e r a t e of remanence a c q u i s i t i o n . T h i s r a t e depends upon t h e r a t e o f o x i d a t i o n which i n t u r n w i l l v a r y a c c o r d i n g t o g r a i n s i z e and t h e i n t e r s t i t i a l geochemical environment.

I f we assume t h a t

remanence i s a c q u i r e d more s l o w l y by m a g n e t i t e o x i d a t i o n t h a n t h e o t h e r d i a g e n e t i c p r o c e s s e s t h e n w e can model t h e a c q u i s i t i o n o f remanence i n r e d beds i n terms o f these three diagenetic processes.

I n F i g . 8 . 3 2 geomagnetic f i e l d f l u c t u a t i o n s

i n c l u d i n g two t r a n s i t i o n s a r e r e p r e s e n t e d on a v e r t i c a l a r b i t r a r y t i m e s c a l e .

The

r e l a t i v e r a t e o f a c q u i s i t i o n of remanence f o r t h e t h r e e d i a g e n e t i c p r o c e s s e s i s a l s o i n d i c a t e d on t h e v e r t i c a l s c a l e .

I t i s assumed t h a t remanence a c q u i s i t i o n

b e g i n s a t t h e b a s e and i s uniform f o r e a c h o f t h e t h r e e p r o c e s s e s .

The o v e r a l l

e f f e c t i s t h a t t h e more s l o w l y a c q u i r e d remanences a r e d i s p l a c e d s t r a t i g r a p h i c a l l y downwards a s i n F i g . 8 . 3 1 .

Now t h e a c t u a l p a l a e o m a g n e t i c r e c o r d p r e s e r a e d i n t h e

r e d beds w i l l depend upon t h e r e l a t i v e c o n t r i b u t i o n of e a c h o f t h e s e components.

STRATIGRAPHICAL DISTRIBUTION OF OBSERVED PALAEOMAGNETIC RECORD

INTERMEDIATE

REVERSED

NORMAL

F i g . 8.32. Schematic r e p r e s e n t a t i o n of t h e r e l a t i v e r a t e o f remanence a c q u i s i t i o n by t h r e e d i f f e r e n t d i a g e n e t i c p r o c e s s e s d u r i n g two geomagnetic p o l a r i t y t r a n s i t i o n s . Note t h a t t h e more s l o w l y a c q u i r e d remanence i s a p p a r e n t l y d i s p l a c e d s t r a t i g r a p h i c a l l y downwards. Also t h e p a l a e o m a g n e t i c r e c o r d i n t h e whole rock b e a r s l i t t l e resemblance t o t h e f i e l d v a r i a t i o n a t t h e time o f d e p o s i t i o n .

I f t h e y a l l c o n t r i b u t e e q u a l l y t o t h e t o t a l remanence t h e n t h e r e s u l t a n t d i r e c t i o n s w i l l be a n i n t e g r a t i o n of t h e s e components and would b e d i s p l a c e d s t r a t i g r a p h i c a l l y downwards.

I f t h e r e a r e any v e r t i c a l ( s t r a t i g r a p h i c ) v a r i a t i o n s i n t h e r e l a t i v e

484 c o n t r i b u t i o n s o f t h e t h r e e i t c a n b e shown t h a t t h e s e may b e combined i n s u c h a way t h a t a palaeomagnetic r e c o r d , c o m p l e t e l y u n l i k e t h e o r i g i n a l geomagnetic f l u c t u a t i o n s , c a n be produucd.

F o r example, i n t h e c a s e shown i n F i g . 8 . 3 2 , assume

t h a t t h e r e l a t i v e c o n t r i b u t i o n s of p r o c e s s e s B and C v a r y i n s u c h a way t h a t C dominates t h e lower h a l f of t h e s e c t i o n and B t h e upper h a l f o f t h e s e c t i o n .

The

r e s u l t a n t p a l a e o m a g n e t i c r e c o r d ( F i g . 8 . 3 2 d ) b e a r s l i t t l e resemblance t o t h e t r u e

It

geomagnetic f i e l d f l u c t u a t i o n s and c o n t a i n s a ' s p u r i o u s ' r e v e r s a l of p o l a r i t y . i s c l e a r t h a t s t r a t i g r a p h i c v a r i a t i o n s i n t h e r a t e of remanence a c q u i s i t i o n by

d i f f e r e n t d i a g e n e t i c p r o c e s s e s can produce c o m p l e t e l y anomalous p a l a e o m a g n e t i c results.

These v a r i a t i o n s may r e s u l t from p r i m a r y l i t h o l o g i c a l d i f f e r e n c e s , f o r

example i n d e t r i t a l m a g n e t i t e c o n c e n t r a t i o n s , o r from v a r i a t i o n s i n t h e r a t e o f o x i d a t i o n o r h y d r o l y s i s which might be more r a p i d i n porous h o r i z o n s . The palaeomagnetic e v o l u t i o n o f r e d beds The p r e c e d i n g d i s c u s s i o n i n d i c a t e s t h a t i n o r d e r t o make any c o n c l u s i o n s a b o u t t h e age of t h e m a g n e t i z a t i o n i n r e d beds some e x t e r n a l c o n t r o l i s n e c e s s a r y .

The

p r e s e n c e of a p p a r e n t geomagnetic f i e l d f l u c t u a t i o n s a l o n e i s n o t s u f f i c i e n t e v i d e n c e t o deduce t h a t m a g n e t i z a t i o n f o l l o w e d r a p i d l y a f t e r d e p o s i t i o n .

External

c o n t r o l s which c o u l d be a p p l i e d i n c l u d e t h e comparison o f t h e r e s u l t s w i t h t h o s e from igneous r o c k s o f t h e same a g e , o r w i t h a known p o l a r i t y sequence d e r i v e d from marine magnetic a n o m a l i e s .

The l a t t e r i s a & t e n t i a l l y v a l u a b l e t o o l f o r s t u d y i n g

t h e p o l a r i t y sequences o b s e r v e d i n Cenozoic r e d beds ( e . g . Barndt e t a l . , 1978). Many r e d beds show c l a s s i c a l p a l a e o m a g n e t i c r e s u l t s .

They have two groups of

d i r e c t i o n s , a n t i p o d a l l y opposed, w i t h p o s i t i v e and n e g a t i v e p o l a r i t i e s a p p a r e n t l y a c q u i r e d d u r i n g normal and r e v e r s e d geomagnetic f i e l d s .

The normal and r e v e r s e d

zones may b e l a t e r a l l y p e r s i s t e n t and i t may b e p o s s i b l e t o c o r r e l a t e them i n a way which does n o t c o n f l i c t w i t h t h e l i t h o s t r a t i g r a p h y .

The d i r e c t i o n s i n r e d beds

of t h i s t y p e a r e c o n s i s t e n t w i t h t h o s e from a s s o c i a t e d i g n e o u s r o c k s and f i e l d t e s t s of s t a b i l i t y u s u a l l y i n d i c a t e t h a t t h e m a g n e t i z a t i o n formed p r i o r t o f o l d i n g ( a l t h o u g h t h i s o f f e r s p r e c i s i o n of no b e t t e r t h a n a few m i l l i o n y e a r s ) .

Red beds

of t h i s type w i l l b e r e f e r r e d t o a s Type A r e d beds f o l l o w i n g ( T u r n e r , 1979).

They

o c c u r t h r o u g h o u t t h e s t r a t i g r a p h i c r e c o r d and a r e g e n e r a l l y b e l i e v e d t o have been magnetized d u r i n g o r s h o r t l y a f t e r d e p o s i t i o n . A l a r g e number of r e d beds show more c o m p l i c a t e d p a l a e o m a g n e t i c r e s u l t s .

They

c a n n o t g e n e r a l l y b e r e s o l v e d i n t o two groups of o p p o s i t e p o l a r i t y and u s u a l l y r e v e a l many i n t e r m e d i a t e d i r e c t i o n s which d e s c r i b e a g r e a t c i r c l e p l a n e ; more complex d i s t r i b u t i o n s of d i r e c t i o n s may be s e e n .

I n d i v i d u a l specimens can b e

shown t o have composite m a g n e t i z a t i o n s , e i t h e r by specimen s p l i t t i n g o r by chemical o r thermal demagnetization.

O f t e n , i n d i v i d u a l components may b e a n t i p a r a l l e l i n

which c a s e t h e r e s u l t a n t l i e s a l o n g t h e a x i s o f m a g n e t i z a t i o n .

Intermediate

485 d i r e c t i o n s r e s u l t from two o r more components which a r e n o t a n t i p o d a l l y opposed. Red beds w i t h m a g n e t i z a t i o n s o f t h i s type a r e r e f e r r e d t o a s Type B r e d beds. I t may be p o s s i b l e t o i s o l a t e i n d i v i d u a l components by t h e r m a l o r chemical demagn e t i z a t i o n and i n some c a s e s t h e s e may b e p a l a e o m a g n e t i c a l l y s i g n i f i c a n t .

Type B

r e d b e d s , l i k e Type A r e d beds t e n d t o be d i s t r i b u t e d throughout t h e s t r a t i g r a p h i c record. A number of r e d beds a r e c h a r a c t e r i z e d by a m a g n e t i z a t i o n which c o n s i s t s o f a s i n g l e , w e l l grouped d i r e c t i o n and which i s s t a b l e t o t h e r m a l and chemical demagnetization.

F i e l d t e s t s o f t e n i n d i c a t e t h a t t h e m a g n e t i z a t i o n formed a f t e r f o l d i n g

and t h e d i r e c t i o n i s o f t e n w i d e l y d i v e r g e n t from t h a t o f a s s o c i a t e d o r comtemporaneous i g n e o u s r o c k s .

These r e d beds a r e r e f e r r e d t o a s Type C; t h e d i r e c t i o n i s

o f t e n c l o s e t o t h e p r e s e n t E a r t h ' s f i e l d , a f e a t u r e t a k e n t o i n d i c a t e t h a t t h e NRM was a c q u i r e d i n r e l a t i v e l y Recent times. Some g e n e r a l comments r e g a r d i n g t h e r e l a t i o n s h i p s between d i a g e n e s i s and palaeomagnetic p r o p e r t i e s o f r e d beds might be p o s s i b l e i f t h e s t r u c t u r e o f t h e NRM and t h e d i a g e n e t i c f e a t u r e s i n t h e s e t h r e e d i f f e r e n t t y p e s of r e d beds c a n b e r e l a t e d . I n p a r t i c u l a r i t i s i m p o r t a n t t o i d e n t i f y t h a t t e x t u r a l phase of h a e m a t i t e which i s r e s p o n s i b l e f o r c a r r y i n g t h e major p o r t i o n o f t h e NRM and then t o deduce a t what s t a g e of d i a g e n e s i s i t was formed.

P o s s i b l e c o n t r i b u t i o n s from o t h e r p h a s e s and t h e

p o s s i b l e e f f e c t s of r e m a g n e t i z a t i o n must a l s o be c o n s i d e r e d . Tvoe A r e d beds

Moenkopi Formation o f t h e USA and S t . Bees Sandstone of t h e UK, and a l s o i n c l u d e t h e P r o t e r o z o i c r e d beds of Canada.

A l l t h e s e f o r m a t i o n s a r e c h a r a c t e r i z e d by d i s c r e t e

zones of normal and r e v e r s e d m a g n e t i z a t i o n and i n some c a s e s t h e a n a l y s i s of s m a l l s c a l e d i r e c t i o n a l changes and p o l a r i t y t r a n s i t i o n s has been used a s e v i d e n c e of penecontemporaneous m a g n e t i z a t i o n ( e . g . Baag and H e l s l e y , 1974a; H e l s l e y and S t e i n e r , 1974).

However, d e t a i l e d a n a l y s i s of t h e s e f e a t u r e s s u g g e s t s t h a t they may t r a n s -

g r e s s l o c a l t i m e b o u n d a r i e s ( e . g . Larson e t a l . ,

1976) and i t must b e concluded

t h a t t h e NRM was a c q u i r e d d u r i n g d i a g e n e s i s a l t h o u g h t h e palaeomagnetic f e a t u r e s o f r e d beds of t h i s t y p e s u g g e s t t h a t t h e NRM a c q u i s i t i o n must have been r e l a t i v e l y 6 r a p i d , s a y w i t h i n 10 -lo7 Years. The s t r u c t u r e of t h e NRM i n r e d beds of t h i s type h a s been i n v e s t i g a t e d by C o l l i n s o n (1974) and Turner and I x e r (1977).

I n both

t h e s e s t u d i e s , c o a r s e p a r t i c l e s p e c u l a r i t e and m a r t i t e g r a i n s were shown t o be t h e most i m p o r t a n t NEW c a r r i e r s .

T h i s is t h e c a s e f o r a number o f f o r m a t i o n s i n c l u d i n g

t h e Moenkopi, t h e Chugwater and t h e S t . Bees Sandstone. Although a u t h i g e n i c pigmentary h a e m a t i t e o c c u r s i n a l l t h e s e f o r m a t i o n s and has been s t u d i e d i n d e t a i l i n two of them (Walker and L a r s o n , 1976; I x e r e t a l . , 1980) i t a p p e a r s , i n g e n e r a l , t o be o n l y a minor NRM c a r r i e r p o s s i b l y a c c o u n t i n g f o r o c c a s i o n a l u n s t a b l e and anomalous

486

d i r e c t i o n s of m a g n e t i z a t i o n . I t remains t o be s e e n t h e n how t h e s p e c u l a r i t e and m a r t i t e g r a i n s c a n a c q u i r e

t h i s s t a b l e magnetization.

I t i s extremely u n l i k e l y t h a t the s p e c u l a r i t e f r a c t i o n

can c a r r y any s i g n i f i c a n t DRM.

I n t h e f i r s t p l a c e , t h e mechanical t o r q u e s d u r i n g

d e p o s i t i o n would i n t r o d u c e a s i g n i f i c a n t i n c l i n a t i o n e r r o r , and s e c o n d l y , PDRM c o u l d n o t develop b e c a u s e of t h e c o a r s e g r a i n s i z e s i n v o l v e d .

Furthermore, t h e

e v i d e n c e o f m i n e r a l phase changes ( o x i d a t i o n of m a g n e t i t e t o h a e m a t i t e ) s u g g e s t s t h a t i f any DRM o r i g i n a l l y d i d e x i s t t h e n i t would b e l a r g e l y d e s t r o y e d by l a t e r d i a g e n e t i c changes.

On b a l a n c e , t h e r e f o r e , t h e e v i d e n c e does s u g g e s t t h a t t h e NRM

was produced by t h e r a p i d i n s i t u o x i d a t i o n o t m a g n e t i t e and o t h e r d e t r i t a l i r o n bearing grains.

The r a t e of CRM a c q u i s i t i o n i n t h i s way must have been s u f f i c i e n t l y

r a p i d t o e n a b l e t h e development of d i s c r e t e zones o f normal and r e v e r s e magnetiza t i o n (although there a r e overlaps i n c e r t a i n areas).

An e s t i m a t e of t h e r e v e r s a l

f r e q u e n c y i n any p a r t i c u l a r c a s e w i l l t h u s e n a b l e some l i m i t s t o b e p l a c e d on t h e r a t e of CRM a c q u i s i t i o n .

The geomagnetic f i e l d h a s undergone f r e q u e n t r e v e r s a l s

i n t h e p a s t and t h e r e i s no obvious p o l a r i t y b i a s .

During t h e p a s t 10 My t h e a v e r a g e

r e v e r s a l f r e q u e n c y was a b o u t one e v e r y 0 . 2 3 My (Cox, 1969) a l t h o u g h t h i s has v a r i e d c o n s i d e r a b l y t h r o u g h o u t P h a n e r o z o i c time. one e v e r y 0 . 9 4 My.

From 145 t o -10.5 My t h e f r e q u e n c y was

The c a l c u l a t i o n of t h e s e f r e q u e n c i e s i s p o s s i b l e b e c a u s e o f t h e

o c c u r r e n c e of m a g n e t i c anomaly " s t r i p e s " on t h e f l o o r of t h e ocean b a s i n s .

Estimates

of t h e r e v e r s a l f r e q u e n c y d u r i n g t h e Mesozoic a r e much more d i f f i c u l t t o make because t h e s e must b e based l a r g e l y on t h e p a l a e o m a g n e t i c s t u d y o f c o n t i n e n t a l sequences. E s t i m a t e s of t h e r e v e r s a l f r e q u e n c y d u r i n g t h e Lower T r a i s s i c a r e p o s s i b l e because of t h e l a r g e amount o f p a l a e o m a g n e t i c d a t a which e x i s t s f o r t h i s p e r i o d (Burek, 1970; Johnson e t a l . , 1972; Pechersky and Khramov, 1973; C r e e r , 1975).

In a d e t a i l e d s t u d y of t h e B u n t s a n d s t e i n of SW Germany, Burek (1968) i d e n t i f i e d 9 r e v e r s e l y magnetized zones and c o m p i l a t i o n s o f t h e e x t e n s i v e d a t a from t h e Colorado P l a t e a u (Burek, 1970; McElhinny and Burek, 1971) and t h e USSR (Pechersky and Khramov, 1973) s u g g e s t t h a t t h e r e c o u l d be a t l e a s t 1 7 d i s c r e t e zones of normal o r r e v e r s e d p o l a r i t y i n t h e Lower T r i a s s i c ( i e . . 215-225 My a g o ) .

This corresponds

t o a r e v e r s a l f r e q u e n c y of one f o r e v e r y 1.18 My which, even assuming a r e l a t i v e l y slow remanence a c q u i s i t i o n r a t e , c o u l d be s u f f i c i e n t time f o r t h e development of a s t a b l e , s i n g l e component NRM.

The work of C o l l i n s o n (1974) would seem t o imply

t h a t a s t a b l e NRM was dependent upon t h e r a p i d o x i d a t i o n of d e t r i t a l m a g n e t i t e because s p e c u l a r i t e c a r r i e s t h e b u l k of t h e NRM i n r e d beds of t h i s t y p e .

Although

t h e r e a r e no d e t a i l e d d a t a on t h e r a t e of o x i d a t i o n of m a g n e t i t e i n modern a l l u v i u m , 1.18 My might be a s u f f i c i e n t time f o r t h e development of t h e s t a b l e m a g n e t i z a t i o n of t h e s p e c u l a r i t e .

The development of such a s t a b l e m a g n e t i z a t i o n may depend upon

i t comprising components which r e s u l t e d from a s i n g l e d i a g e n e t i c p r o c e s s .

Because

of t h e d i f f e r e n t r a t e s of t h e v a r i o u s d i a g e n e t i c p r o c e s s e s multicomponent magn e t i z a t i o n s a r e most l i k e l y t o form i n m i n e r a l o g i c a l l y immature,

first-cycle,

sediments w i t h abundant d e t r i t a l m a g n e t i t e and f e r r o m a g n e s i a n s i l i c a t e s ,

I t may

b e , t h e r e f o r e , t h a t Type A r e d beds a r e t h o s e which o r i g i n a l l y e x h i b i t e d some degree of m i n e r a l o g i c a l m a t u r i t y and i n which h a e m a t i t e a u t h i g e n e s i s was o f r e l a t i v e l y minor importance. TvDe B r e d beds The boundary between Type A and Type B r e d beds i s somewhat a r b i t r a r y because some Type A r e d beds may show l o c a l e v i d e n c e o f multicomponent m a g n e t i z a t i o n .

The

e s s e n t i a l d i f f e r e n c e between t h e two i s t h a t Type B r e d beds do n o t show two c l u s t e r s of a n t i p a r a l l e l d i r e c t i o n s , nor a w e l l - d e f i n e d a x i s of m a g n e t i z a t i o n . The NRM d i r e c t i o n s a r e o f t e n i n t e r m e d i a t e i n d i r e c t i o n , s c a t t e r e d , and of v a r i a b l e intensity.

A l s o t h e r e i s u n e q u i v o c a l p r o o f from l a b o r a t o r y t r e a t m e n t , of t h e

widespread development o f multicomponent m a g n e t i z a t i o n s .

There a r e numerous

examples of t h e p r e s e n c e of a n t i p a r a l l e l , normal and r e v e r s e d components i n r e d beds of t h i s t y p e (Chamalaun and C r e e r , 1964; Roy and P a r k , 1972, 1974; Larson and Walker, 1975; T u r n e r and A r c h e r , 1975) and t h e r e can be no doubt t h a t t h e NRM was acquired during diagenesis.

The e s s e n t i a l d i f f e r e n c e s between Type A and Type B

r e d beds c o u l d b e e x p l a i n e d i n a number of ways.

I f we assume s i m i l a r r a t e s of

CRM a c q u i s i t i o n then Type B r e d beds may r e p r e s e n t sequences which were formed

d u r i n g p e r i o d s c h a r a c t e r i z e d by a v e r y h i g h r e v e r s a l f r e q u e n c y

-

s o high t h a t i t

o u t p a c e d t h e r a t e of CRM a c q u i s i t i o n and normal and r e v e r s e d components became superimposed on e a c h o t h e r .

T h i s view i s n o t favoured b e c a u s e Type B r e d beds a r e

known t o o c c u r i n time i n t e r v a l s when t h e r e v e r s a l frequency was n o t e x c e s s i v e l y g r e a t ( e . g . Lower Devonian, s e e McMurray, 1970; Sallomy and P i p e r , 1973).

Also t h e

Cenozoic r e d beds s t u d i e d by Larson and Walker (1975) a r e Type B r e d beds which developed o v e r a p e r i o d of p r e d o m i n a n t l y normal p o l a r i t y . A more l i k e l y e x p l a n a t i o n i s t h a t Type B r e d beds r e p r e s e n t sequences i n which t h e o r i g i n a l s e d i m e n t s were f u n d a m e n t a l l y d i f f e r e n t i n terms of m i n e r a l o g i c a l and textural maturity.

For example, t h e f i r s t c y c l e a r k o s i c r e d beds s t u d i e d by Walker,

( 1 9 6 7 a ) ; Walker e t a l . (1978) and Larson and Walker (1975) c o n t a i n e d abundant d e t r i t a l i r o n s i l i c a t e s i n c l u d i n g h o r n b l e n d e s , pyroxenes and p h y l l o s i l i c a t e s .

Intrastratal

s o l u t i o n would t h u s produce l a r g e amounts of a u t h i g e n i c h a e m a t i t e and t h e c o n t i n u e d s u p p l y of i r o n i n t h e p o r e f l u i d s would e n s u r e t h e growth of t h i s a u t h i g e n i c h a e m a t i t e to coarser grain sizes.

I n t h i s way t h e pigmentary h a e m a t i t e would c a r r y a more

s t a b l e NRM whose d i r e c t i o n r e p r e s e n t e d a long-term i n t e g r a t i o n of contemporary geomagnetic f i e l d f l u c t u a t i o n s including r e v e r s a l s .

This i n t e r p r e t a t i o n i s supported

by s t u d i e s of t h e NRM s t r u c t u r e i n Type B r e d beds; b o t h T u r n e r and Archer (1975) and Larson and Walker (1975) showing t h a t pigment was r e s p o n s i b l e f o r components

488 a c q u i r e d d u r i n g d i a g e n e s i s a n d a f t e r s i g n i f i c a n t geomagnetic v a r i a t i o n s . The time t a k e n f o r t h e development of Type B r e d beds depends upon a number o f f a c t o r s , n o t l e a s t o f which i s t h e o r i g i n a l d e g r e e o f m i n e r a l o g i c a l m a t u r i t y .

In

immature s e d i m e n t s multicomponent m a g n e t i z a t i o n s p r o b a b l y develop r a p i d l y , s a y i n

7 y e a r s , and t h i s i s c l e a r l y t h e c a s e i n t h e Cenozoic r e d beds i n Baja

l e s s t h a n 10

C a l i f o r n i a which have t h i s t y p e o f m a g n e t i z a t i o n and a r e l e s s t h a n 5 My o l d .

In

sediments w i t h g r e a t e r m i n e r a l o g i c a l m a t u r i t y t h e r e i s less p o t e n t i a l f o r t h e development of t h i s t y p e o f m a g n e t i z a t i o n s i n c e complete s i l i c a t e a l t e r a t i o n c o u l d t a k e p l a c e i n a s h o r t e r p e r i o d o f time and a t some s t a g e p r o d u c t i o n of new i r o n o x i d e s w i l l c e a s e and t h e d i a g e n e s i s and m a g n e t i z a t i o n w i l l become a r r e s t e d .

This

concept of " a r r e s t e d " o r completed d i a g e n e s i s i s s u p p o r t e d by t h e o c c u r r e n c e of Type A and B r e d beds t h r o u g h o u t Precambrian and P h a n e r o z o i c s t r a t a . Tvue C r e d beds Many r e d beds show well-grouped d i r e c t i o n s o f m a g n e t i z a t i o n which c o n s i s t e s s e n t i a l l y of a s i n g l e component of m a g n e t i z a t i o n .

I t can r e a d i l y be proven t h a t

t h e a g e of t h e m a g n e t i z a t i o n i s c o n s i d e r a b l y l a t e r t h a n 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 ; t h e r o c k s have t h e r e f o r e been s u b j e c t e d t o complete r e m a g n e t i z a t i o n .

These

r e m a g n e t i z a t i o n p r o c e s s e s a r e n o t w e l l - u n d e r s t o o d b u t i n a number o f examples s u c h a s t h e L a t e Precambrian ( o r Cambrian) r e d beds o f t h e Longmyndian (Lomax and B r i d e n ,

1977) and t h e J u r a s s i c Navajo Sandstone o f t h e USA ( J o h n s o n , 1976) t h e r o c k s have been s t a b l y remagnetized i n t h e d i r e c t i o n of t h e p r e s e n t E a r t h ' s f i e l d .

I n t h e c a s e of

t h e Navajo Sandstone t h e r e m a g n e t i z a t i o n a p p e a r s t o b e a s s o c i a t e d w i t h t h e r e d i s t r i b u t i o n of pigmentary o x i d e s a f e a t u r e c o n s i s t e n t w i t h t h e views o f T u r n e r (1977) who s u g g e s t e d t h a t porous s a n d s t o n e s were more s u s c e p t i b l e t o r e m a g n e t i z a t i o n because of t h e i n c r e a s e d p e n e t r a t i o n by o x i d i z i n g a g e n t s . O t h e r c a s e s of r e m a g n e t i z a t i o n would a p p e a r t o b e l e s s s i m p l e .

Devonian r e d

beds i n t h e L a u r a s i a n c o n t i n e n t s s u f f e r e d w i d e s p r e a d r e m a g n e t i z a t i o n d u r i n g PermoC a r b o n i f e r o u s times (Chamalaun and C r e e r , 1964) whereas t h o s e i n t h e Gondwanan c o n t i n e n t s d i d n o t ( C r e e r , 1964).

In o r d e r t o a c c o u n t f o r t h i s d i f f e r e n c e C r e e r

(1968) f o r m u l a t e d t h e " r e m a g n e t i z a t i o n h y p o t h e s i s " i n which h e s u g g e s t e d t h a t t h e n o r t h e r n c o n t i n e n t s which l a y i n low l a t i t u d e d u r i n g t h e C a r b o n i f e r o u s and e a r l y Permian, s u f f e r e d widespread l a t e r i t i z a t i o n which e f f e c t e d t h e r e m a g n e t i z a t i o n . Many l a t e P a l a e o z o i c l i t h o l o g i e s , p a r t i c u l a r l y l i m e s t o n e s , escaped t h e i n f l u e n c e o f t h i s r e m a g n e t i z a t i o n e p i s o d e (McElhinny and Opdyke, 1973; T u r n e r , 1975; T u r n e r

e t a l . , 1979) end t h e c l o s e a s s o c i a t i o n o f t h e r e m a g n e t i z a t i o n phenomenon w i t h r e d f l u v i a t i l e s a n d s t o n e s does s u g g e s t t h a t d i a g e n e t i c p r o c e s s e s and t h e r e m a g n e t i z a t i o n a r e closely linked.

The importance o f r e g i o n a l l a t e r i t i z a t i o n i n c a u s i n g widespread

r e m a g n e t i z a t i o n has been a b l y demonstrated by Schmidt and Embleton (1976) who showed t h e L a t e P a l a e o z o i c and Mesozoic f o r m a t i o n s o f t h e P e r t h Basin i n Western A u s t r a l i a

489

have been c o m p l e t e l y remagnetized by t h e Cenozoic l a t e r i t i z a t i o n i n t h e a r e a .

This

would seem t o l e n d some s u p p o r t t o Creer's h y p o t h e s i s . The s t r u c t u r e o f t h e NRM i n remagnetized Devonian r e d beds has been i n v e s t i g a t e d by Turner and A r c h e r (1975).

They were a b l e t o show t h a t s p e c u l a r i t e and m a r t i t e

g r a i n s were i m p o r t a n t c a r r i e r s of t h e r e m a g n e t i z a t i o n v e c t o r s and a r g u e d t h a t any r e m a g n e t i z a t i o n p r o c e s s must t h e r e f o r e a c c o u n t f o r changes i n t h e e x i s t i n g o x i d e mineralogy r a t h e r t h a n t h e s i m p l e p r o d u c t i o n o f new o x i d e p h a s e s .

T h i s i s con-

s i s t e n t w i t h t h e v i e w t h a t r e g i o n a l h e a t i n g o r b u r i a l were i m p o r t a n t i n t h e remagne t i z a t i o n . S i n c e a v a r i e t y o f p r o c e s s e s can c a u s e complete r e m a g n e t i z a t i o n t h e development of Type C r e d beds may t a k e p l a c e a t v a r i a b l e r a t e s . i f e r o u s r e m a g n e t i z a t i o n took o v e r greater.

lo8

The w i d e s p r e a d Permo-Carbon-

Years and i n o t h e r c a s e s t h i s time gap was even

I n t h e Precambrian i t i s o f t e n n o t p o s s i b l e t o d a t e t h e r e l a t i v e age of

t h e m a g n e t i z a t i o n and d e p o s i t i o n more p r e c i s e l y t h a n t h i s and i n v e r y a n c i e n t r e d beds i t may b e t h a t t h e r e a r e many u n d e t e c t e d Type C r e d b e d s . Examples of t h e NRM s t r u c t u r e f o r e a c h of t h e t h r e e t y p e s of r e d bed m a g n e t i z a t i o n a r e i l l u s t r a t e d i n Fig.8.33.

The t y p e A example i s from t h e S t . Bees Sandstone

( T r i a s s i c ) of n o r t h e r n England.

I t c o n s i s t s o f a s i n g l e component of m a g n e t i z a t i o n

and t h e t h e r m a l decay of t h e whole r o c k and t h e s e p a r a t e d pigment and s p e c u l a r i t e f r a c t i o n s i n d i c a t e t h a t t h e l a t t e r c a r r i e s t h e s t a b l e NRM. A t i m e i n t e r v a l o f lo6 107 Years i s s u g g e s t e d a s t h e t i m e r e q u i r e d f o r t h e s t a b l e development o f

-

m a g n e t i z a t i o n s of t h i s t y p e ,

The Type B example, from t h e Old Red S a n d s t o n e , shows

a c l e a r multicomponent m a g n e t i z a t i o n c a r r i e d p r i n c i p a l l y by s p e c u l a r i t e .

In this

c a s e t h e thermal decay o f pigment and s p e c u l a r i t e i s q u i t e d i f f e r e n t and a g r e a t e r p r o p o r t i o n of t h e pigment m a g n e t i z a t i o n i s r e t a i n e d o v e r 500°C, s u g g e s t i n g t h a t t h e T h i s is c o n s i s t e n t w i t h

h i g h t e m p e r a t u r e component c o u l d be c a r r i e d by t h e pigment.

t h e view t h a t r e d beds o f t h i s t y p e were m i n e r a l o g i c a l l y l e s s mature than Type A and c o n t a i n a g r e a t e r p r o p o r t i o n o f a u t h i g e n i c h a e m a t i t e .

The development of Type B

m a g n e t i z a t i o n s i n which d i f f e r e n t t e x t u r a l p h a s e s a l l c a r r y a s i g n i f i c a n t p r o p o r t i o n 8 of t h e NRM p r o b a b l y r e q u i r e s more t i m e t h a n Type A and a t i m e span of lo7 10 Years

-

i s suggested. Red Sandstone.

The Type C m a g n e t i z a t i o n shown i n F i g . 8 . 3 3 i s t a k e n from t h e Old

I t c o n s i s t s e s s e n t i a l l y of a s i n g l e component o f m a g n e t i z a t i o n w i t h

a d i r e c t i o n which i s c l o s e t o t h e Permo-Triassic been c o m p l e t e l y remagnetized.

f i e l d d i r e c t i o n and has e v i d e n t l y

The s t r u c t u r e of t h e NRM i s d i f f e r e n t from t h e Type A

and Type B r e d beds shown i n t h a t t h e pigment f r a c t i o n shows much h i g h e r thermal s t a b i l i t y and p r o b a b l y c a r r i e s a g r e a t e r p r o p o r t i o n o f t h e NRM.

This is consistent

w i t h t h e view t h a t long-continued a u t h i g e n e s i s and growth of h a e m a t i t e i s a n i m p o r t a n t f a c t o r i n t h e f o r m a t i o n o f Type C r e d beds.

I t seems l i k e l y t h a t a s

d i a g e n e s i s p r o g r e s s e s t h e m a g n e t i z a t i o n 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 becomes more complex e v e n t u a l l y p a s s i n g through t h e sequence Type A (Fig. 8.34).

-

Type B

-

Type C

490

A

B N

I

I

I

I

I

I

I

I C I I I I I I I I -

0

Fig.8.33. T y p i c a l s t e r e o g r a p h i c p r o j e c t i o n s of Type A , B a n d C r e d bed magnetiza t i o n s . A . From t h e Chugwater Formation ( T r i a s s i c , USA) w i t h specimen d i r e c t i o n s shown a f t e r t h e r m a l c l e a n i n g (Van d e r Voo and Grubbs, 1977). B. From t h e Cenozoic r e d beds of Baja C a l i f o r n i a (Larson and Walker, 1975) and C . From t h e Longmyndian ( P r e c a m b r i a n , U.K.) (Lomax and B r i d e n , 1977). A and B a r e c o r r e c t e d f o r g e o l o g i c a l t i l t . C d i r e c t i o n s a r e shown i n s i t u . Circumference i s h o r i z o n t a l ; c l o s e d symbols p o s i t i v e d i p s ; open symbols n e g a t i v e d i p s .

491 These t h r e e t y p e s o f r e d bed 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 d i a g e n e t i c evolution of c o n t i n e n t a l red beds.

The t r a n s i t i o n sequence Type A

-

Type B

- Type C

i s thought t o t a k e p l a c e a s h a e m a t i t e g e n e s i s and growth p r o g r e s s e s t o a more advanced s t a t e of d i a g e n e s i s .

The p r o c e s s i s n o t n e c e s s a r i l y time-dependent b u t

proceeds a t d i f f e r e n t r a t e s i n d i f f e r e n t s e d i m e n t a r y b a s i n s because of v a r i a t i o n s i n local conditions.

These c o n d i t i o n s i n c l u d e :

d e p o s i t i o n a l environment, t h e

o r i g i n a l m i n e r a l o g y o f t h e s e d i m e n t , p o r o s i t y r e d u c t i o n by c l a y m i n e r a l d i a g e n e s i s and c e m e n t a t i o n , and s u b s e q u e n t e f f e c t s s u c h a s d e p t h o f b u r i a l and r e g i o n a l h e a t i n g . For example, Type A m a g n e t i z a t i o n s a r e found i n r e d beds a s o l d a s t h e S t a r k Formation (1750 My, Bingham and Evans, 1976) a s w e l l a s t h r o u g h o u t t h e Mesozoic. Similarly B

-

Type m a g n e t i z a t i o n s a r e found i n t h e Cenozoic r e d beds o f Baja

C a l i f o r n i a (Larson and Walker, 1975).

The advanced s t a t e of d i a g e n e s i s i n t h e

l a t t e r i s due t o t h e f a c t t h a t t h e s e s e d i m e n t s a r e f i r s t - c y c l e a r k o s e s w i t h a h i g h proportion of unstable s i l i c a t e minerals.

I n sediments which a r e t e x t u r a l l y and

MINERALOGICAL MATURITY

-b

AT THE TIME OF DEPBSITION

DEPOSITIONAL MAGNETIZATION c

Fig.8.34. red beds.

c-

Schematic r e p r e s e n t a t i o n of t h e p a l a e o m a g n e t i c e v o l u t i o n of c o n t i n e n t a l

m i n e r a l o g i c a l l y more mature t h e r a t e o f d i a g e n e s i s , and hence m o d i f i c a t i o n o f t h e m a g n e t i z a t i o n c a n b e e x p e c t e d t o t a k e p l a c e a t a much s l o w e r r a t e .

Also, the

t h r e e t y p e s of m a g n e t i z a t i o n a r e n o t n e c e s s a r i l y m u t u a l l y e x c l u s i v e ; two Types may be p r e s e n t i n t h e same F o r m a t i o n , and i n d e e d a t t h e same sampling s i t e , i n d i c a t i n g t h a t t h e s e p a r t i c u l a r r e d beds a r e i n a t r a n s i t i o n a l s t a t e ( e . g . T u r n e r and A r c h e r , 1975). A c r u c i a l q u e s t i o n i s t h e p r o p o r t i o n of Type

p a u c i t y of comparable d a t a .

-

C r e d beds which a r e u n d e t e c t e d

This i s d i f f i t u l t t o a s s e s s , l a r g e l y because of the

i n t h e palaeomagnetic record.

E x t e n s i v e l y s t u d i e d Precambrian r e d beds s u c h a s t h e

T o r r i d o n i a n Sandstone have s t r o n g , s t a b l e NRM's which a r e well-grouped b u t do n o t o f t e n c o r r e s p o n d w i t h e s t a b l i s h e d p o l a r wander p a t h s ( e . g . S t e w a r t and I r v i n g , 1974). I t i s q u i t e p o s s i b l e t h a t t h e m a g n e t i z a t i o n of t h e T o r r i d o n i a n Sandstone and some o t h e r a n c i e n t r e d beds i n t h e p a l a e o m a g n e t i c r e c o r d i s of Type

-

C and hence

completely unrelated t o the depositional age of these rocks. From t h i s work i t i s p o s s i b l e t o make some comments r e g a r d i n g t h e r e l i a b i l i t y of palaeomagnetic r e s u l t s from c o n t i n e n t a l r e d b e d s .

Type

-

A magnetizations a r e

c l e a r l y s u i t a b l e f o r p a l a e o m a g n e t i c work and c a n be e x p e c t e d t o y i e l d f a i r l y r e l i a b l e results.

Although t h e r e may b e some d i s c r e p a n c y between d e p o s i t i o n a l and magnetic

age t h e y s h o u l d g i v e p o l e p o s i t i o n s o f s u f f i c i e n t p r e c i s i o n t o b e of p a l a e o m a g n e t i c value.

However, s u c h m a g n e t i z a t i o n s a r e u n s u i t a b l e f o r s t u d i e s o f contemporaneous

p o l a r i t y t r a n s i t i o n s and s e c u l a r v a r i a t i o n s ( c f . Baag and HelsLey, 1974a). o t h e r hand Type

-

B and Type

-

On t h e

C m a g n e t i z a t i o n s must b e r e g a r d e d a s b e i n g g e n e r a l l y

unsuitable f o r palaeomagnetic conclusions. d i r e c t i o n s from r e d beds of Type

-

I t may b e p o s s i b l e t o i s o l a t e meaningful

B by d e t a i l e d t h e r m a l d e m a g n e t i z a t i o n b u t no

palaeomagnetic s i g n i f i c a n c e s h o u l d b e a t t a c h e d t o them w i t h o u t c o r r o b o r a t i v e evidence.

Type

-

C m a g n e t i z a t i o n s a r e o f no p a l a e o m a g n e t i c s i g n i f i c a n c e when

c o n s i d e r e d i n t h e c o n t e x t of 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 :

some p u b l i s h e d

palaeomagnetic d a t a may b e m i s l e a d i n g b e c a u s e of t h e p r e s e n c e o f u n d e t e c t e d Type magnetizations i n a n c i e n t c o n t i n e n t a l red beds.

-

C

493

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D i a g e n e t i c o r i g i n o f c o n t i n e n t a l r e d b e d s . I n . H. F a l k e ( E d i t o r ) , The C o n t i n e n t a l Permian i n C e n t r a l , West, and S o u t h Europe. D . R e i d e l , DordrechtH o l l a n d , pp.240-282. Walker, T.R., 1978. P a l e o c l i m a t e i n t e r p r e t a t i o n from a p e t r o g r a p h i c comparison o f Holocene Sands and t h e F o u n t a i n Formation ( P e n n s y l v a n i a n ) i n t h e Colorado F r o n t Range: A d i s c u s s i o n . J . Sediment. P e t r o l . , 48:lOll-1013. Walker, T.R. and Harms, J . C . , 1972. E o l i a n o r i g i n of f l a g s t o n e b e d s , Lyons Sandstone ( P e r m i a n ) , t y p e a r e a , Boulder County, Colorado. Mountain G e o l o g i s t , 9:279-288. Walker, T.R. and Honea, R . M . , 1969. I r o n c o n t e n t of modern d e p o s i t s i n t h e Sonoran D e s e r t : A c o n t r i b u t i o n t o t h e o r i g i n of r e d b e d s . Geol. SOC. Am. B u l l , , 80. 535-544. Walker, T . R . and L a r s o n , E . E . , 1976. Hematite a u t h i g e n e s i s i n t h e Moenkopi Formation ( T r i a s s i c A g e ) , Colorado P l a t e a u : A c o n t r i b u t i o n t o t h e o r i g i n of magnetism i n r e d beds. Geol. S O C . Am. A b s t r a c t s w i t h Programs, p.1158. Walker, T . R . , Ribbe, P . H . and Honea, R.M., 1967. Geochemistry o f hornblende a l t e r a t i o n i n P l i o c e n e r e d b e d s , Baja C a l i f o r n i a , Mexico. Geol. SOC. Am. B u l l , , 78: 1055-1060. Walker, T . R . , Waugh, B. and Crone, A . J . , 1978. Diagenesus i n f i r s t - c y c l e d e s e r t a l l u v i u m o f Cenozoic a g e , s o u t h w e s t e r n United S t a t e s and n o r t h w e s t e r n Mexico. Geol. SOC. Am. B u l l . , 89:19-32. W a l t h e r , J . , 1900. Das G e s e t z d e r WUstenbildung. D i e t r i c h Reimer, B e r l i n , 175 pp. Walton, A.W., 1975. 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53 5

Way, J . H . , 1969. Bed t h i c k n e s s a n a l y s i s o f some C a r b o n i f e r o u s f l u v i a l s e d i m e n t a r y r o c k s n e a r J o g g i n s , Nova S c o t i a . J . Sediment. P e t r o l . , 38:424-433. Weaver, L.E. a n d P o l l a r d , L . D . , 1973. The Chemistry o f Clay M i n e r a l s . Developments i n Sedimentology 1 5 , E l s e v i e r , Amsterdam, 213 pp. Weber, K . J . , 1971. S e d i m e n t o l o g i c a l a s p e c t s o f o i l f i e l d s of t h e N i g e r d e l t a . Geol. M i j n b . , 40:569-576. Weertman, J . and Weertman, J . R . , 1964. Elementary D i s l o c a t i o n Theory. MacMillan Co., New York, 213 pp. Weimer, R . J . , 1970. R a t e s o f d e l t a i c s e d i m e n t a t i o n and i n t r a b a s i n d e f o r m a t i o n , Upper C r e t a c e o u s o f Rocky Mountain r e g i o n . I n : J . P . Morgan ( E d i t o r ) , S e d i m e n t a t i o n Modern and A n c i e n t . S.E.P.M. Spec. Publ. 1 5 , pp.270-292. 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537

SUBJECT INDEX Abandoned r i f t

-

see a u l a c o g e n

Accretionary prism, 13 Acid v o l a t i l e s u l p h i d e , 346, 347 Adhesion r i p p l e s , 7 7 , 9 6 , 114 A d u l a r i a , 301 A e o l i a n bedforms, 77-88 A e o l i a n bounding s u r f a c e s , 8 6 , 8 7 , 88 A e o l i a n s a n d s , 69, 72-94

-

bedform h i e r a r c h y , 74, 76

-

g r a i n s i z e , 89

-

i n t e r n a l s t r u c t u r e , 8 5 , 86 reddening, 98

-

r i p p l e s , 74

-

r o u n d n e s s , 89

-

Simpson d e s e r t

- - grain - surface -

s i z e d i s t r i b u t i o n , 90 p i t t i n g , 89

t e x t u r a l c h a r a c t e r i s t i c s , 88-95

AF d e m a g n e t i z a t i o n , 428 A k a g a n b i t e , 329, 330 Akle dune p a t t e r n , 86 Albite, 54, 121 A l d e r l e y Edge, 373, 374 A l l u v i a l f a n s , 182-185

-

A l p i n e m o l a s s e , 44

-

d e p o s i t s , 1 8 3 , 184

A l l u v i a l r e d b e d s , 6 6 , 6 7 , 6 8 , 179-264

538 Amazon R i v e r , 215, 216, 217 Amorphous Fe (OH)3, 297, 329, 330, 338, 344 Amorphous s i l i c a , 269 A n a l c i m e , 301, 303, 305, 306, 307, 308 A n a t a s e , 318 A n c i e n t a e o l i a n d e p o s i t s , 105-125

-

i n t e r n a l s t r u c t u r e , 86, 87, 88 r e d b e d s , 105-125

A n c i e n t a l l u v i a l r e d b e d s , 220-263 A n c i e n t d e l t a i c s u c c e s s i o n s , 147-150

-

c h a r a c t e r i s t i c s , 147

-

r i v e r - d o m i n a t e d , 15 1

A n d e s , 16 Anglo-Welsh b a s i n , 460-463 A n i s o t r o p y , 384, 385 A n n i t e , 287 A n t i f e r r o m a g n e t i s m , 383 Appalachian geosyncline, 3, 4 A q u i t a i n e b a s i n , 40, 43 Arc-trench

s y s t e m s , 14

A r k o s e , 5 1 , 52

-

climatic, 51

-

tectonic, 51

A t c h a f a l a y a B a s i n , 135, 136, 137 Athapuscow a u l a c o g e n , 29, 441, 443 A u g i t e , 280 A u l a c o g e n s , 7 , 9 , 10, 29, 441, 443 A u t h i g e n i c m i n e r a l s , 266, 299-319

-

c a l c i t e , 104, 314-316

-

c h l o r i t e , 3 1 2 , 313

539

-

h a e m a t i t e , 104, 316-319

-

i l l i t e , 309-311

-

-

i l l i t e - m o n t m o r i l l o n i t e , 1 0 4 , 308-311 kaolinite-dickite,

309-312

p o t a s h f e l d s p a r , 300-302 q u a r t z , 104, 319, 320 z e o l i t e s , 302-308

Autogeosynclines, 2 A x i a l g e o c e n t r i c d i p o l e , 381, 382 A y r s h i r e c o a l f i e l d , 161-163 Back-arc b a s i n , 1 3 , 1 5 , 1 6 , 1 7 Bacterial sulphate reduction

-

f r e s h w a t e r , 355-359 m a r i n e , 345-347

Baikal r i f t , 8 , 9 Baja C a l i f o r n i a , 476 Bald E a g l e Formation, 374, 375 B a n d e r s c h i e f e r , 115 Barchans, 78, 8 0 , 82 Bar f i n g e r s a n d s , 142, 144 Barren Red Measures, 163 Barstow Formation, 307 B a t t e r y P o i n t S a n d s t o n e , 245 Bedding e r r o r , 405 Belly River

-

Paskapoo m o l a s s e , 46, 4 7 , 48

Benue Trough, 9 , 10, 11 B i j o u Creek, 195, 196, 197 Bimodal s a n d s , 9 1 , 92 B i o t i t e , 285, 287 B i r d - f o o t d e l t a , 129

540 Blackband Group, 168-173 Blackband i r o n s t o n e , 1 6 9 , 170 Blairmore Group, 46 Blocking t e m p e r a t u r e , 428 Blocking volume, 412 Bond s t r e n g t h s , 281 Botucatu S a n d s t o n e s , 105 Brahmaputra R i v e r , 1 8 1 , 1 9 0 , 191 Braided c h a n n e l s , 1 8 0 , 181 B r i d g e n o r t h S a n d s t o n e , 113 Bunter P e b b l e Beds, 2 5 4 , 2 5 5 , 256 B u n t s a n d s t e i n , 4 2 1 , 4 2 2 , 423

C a i t h n e s s F l a g s t o n e s , 3 6 0 , 3 6 1 , 3 6 2 , 363, 364 C a l c i t e , 138, 2 7 0 , 2 7 1 , 314, 315 C a l c r e t e , 2 3 8 , 2 3 9 , 2 4 0 , 2 4 1 , 2 4 2 , 2 4 8 , 249 Caledonian o r o g e n , 29 Canadian C o r d i l l e r a , 44

-

f o r e l a n d b a s i n s , 4 4 , 45 m o l a s s e , 4 4 , 4 5 , 4 6 , 4 7 , 4 8 , 49 s u c c e s s o r b a s i n s , 4 4 , 45

Canon R o j o , 283, 2 9 2 , 294, 3 1 5 , 320 Canyon Group, 152 Carbon d i o x i d e , 268 C a r b o n i f e r o u s r e d b e d s , 159-173 Casper Sands t o n e s , 106 C a t s k i l l e l a s t i c wedge, 348 Cedar Mesa S a n d s t o n e s , 1 0 6 , 107 Cementation, 269 Cenozoic a l l u v i u m , 2 9 0 , 2 9 5 , 303

-

d i a g e n e s i s , 102-105

541

-

pa l a e oma gne t i s m , 4 75- 48 1

C h a b a z i t e , 3 0 3 , 305 Chemical d e m a g n e t i z a t i o n , 429-434

-

Hopewell Group, 4 3 1 , 432

-

S t . Bees S a n d s t o n e , 433

-

Supai F o r m a t i o n , 4 3 0 , 4 3 1 , 432

-

T a i g u a t i Formation, 4 3 0 , 431

Chugwater F o r m a t i o n , 430

Chemical remanent m a g n e t i z a t i o n , 6 5 , 380, 411-417 C h l o r i t e , 2 4 7 , 2 7 2 , 295 C h r i s t i e Bay Group, 446 Chromium, 332 Chugwater Formation, 378 C i s c o Group, 1 5 2 C l a s s i f i c a t i o n o f a n c i e n t r e d b e d s , 60-80 Clay r e p l a c e m e n t , 293-299 C l i m a t i c s e t t i n g o f a n c i e n t r e d b e d s , 50-60 C l i n o p t i l o l i t e , 301, 303, 304, 305, 306, 307, 308 Coal Measures, 152 Coarse member r e d d e n i n g , 250 Coarsening-upwards s e q u e n c e s , 227

-

D i f u n t a Group, 155

-

Hornelen b a s i n , 2 2 7 , 2 2 8 , 229

-

Rhone d e l t a , 145

Coastal dunes, 7 1 Coconino S a n d s t o n e s , 106 Cody S h a l e

-

Parkman S a n d s t o n e , 152

C o e r c i v e f o r c e , 384 C o e r c i v i t y , 399, 400 C o e r c i v i t y s p e c t r a , 4 2 2 , 4 2 3 , 424

542

C o l l y h u r s t s a n d s t o n e , 113 Columbian o r o g e n , 4 4 , 4 6 , 48 Compaction, 274 C o n s t r u c t i o n a l p h a s e , 146 C o n t i n e n t a l r e d beds

-

c l a s s i f i c a t i o n , 60-68

- -

o f C l a r k , 62

- -

of K r y n i n e , 6 1

- -

p a l a e o m a g n e t i c , 64-66

-

m a g n e t i c p r o p e r t i e s , 6 4 , 417-439

Convergent p l a t e m a r g i n s , 6 , 1 3 Copper, 1 0 , 368, 370, 371, 373 C o r r i e s a n d s t o n e , 113 Corundum s t r u c t u r e , 328 Crevasse-splays,

1 2 9 , 207

C r e v a s s i n g , 134 C r y s t a l l a r i a , 238 Cuchara F o r m a t i o n , 310 Curie temperature

-

-

,

383, 389

h a e m a t i t e - i l m e n i t e , 391 r e d b e d s , 4 2 4 , 425 t i tanomagneti t e - t i t a n o m a g h e m i t e ,

C u t a n , 100 Death V a l l e y , 9 6 , 97, 98

-

a l l u v i a l f a n s , 98 m i n e r a l z o n a t i o n , 97 p l a n t z o n a t i o n , 98

D e b r i s f l o w d e p o s i t s , 184 De C h e l l y s a n d s t o n e s , 106 D e f e c t moment, 390, 394, 401

390

543 D e h y d r a t i o n of g o e t h i t e , 416 D e l t a f r o n t , 139 Delta f r o n t progradation

-

M i s s i s s i p p i d e l t a , 143

D e l t a f r o n t s l o p e , 131 D e l t a growth and abandonment, 1 4 6 , 147 D e l t a l o b e s , 1 4 7 , 148 D e l t a m o d e l s , 127-130 D e l t a p l a i n r e d b e d s , 6 6 , 6 7 , 6 8 , 126-178 D e l t a s t r u c t u r e , 131, 132 Deltaic deposits

-

c o n t r o l l i n g f a c t o r s , 127 f a c i e s , 131-146

Deltas

-

high-constructive,

-

h i g h d e s t r u c t i v e , 128

-

d e f i n i t i o n , 127 f l u v i a l - d o m i n a t e d , 1 2 8 , 129 128

t i d e - d o m i n a t e d , 128 wave-dominated, 128

D e p o s i t i o n a l DRM, 403, 409

Desert b a s i n s

-

groundwater c i r c u l a t i o n , 95 Sonoran d e s e r t , 103 t e c t o n i c s e t t i n g , 71

Desert f l u v i a l s e d i m e n t s , 7 1 , 72 D e s e r t l a k e s and i n l a n d s a b k h a s , 9 5 , 96

Desert r e d b e d s , 5 0 , 6 6 , 6 7 , 6 8 , 69-125 D e s e r t s e d i m e n t s , 69-71

-

l a t i t u d e , 69

544

-

-

r a i n f a l l , 69 r e d d e n i n g , 96-105 t e m p e r a t u r e , 70

D e t r i t a l remanent m a g n e t i z a t i o n , 6 4 , 380, 403-411, 481 Dharwar g r e e n s t o n e b e l t s , 50 D i a g e n e s i s and m a g n e t i z a t i o n , 481 D i a g e n e s i s o f c o n t i n e n t a l r e d b e d s , 265

- Cenozoic d e s e r t a l l u v i u m , 102-105, 319, - Old Red S a n d s t o n e , 230-233, 244-253 - R o t l i e g e n d e s , 116-125 -

321, 322

T r i a s s i c , 256, 259

D i a g e n e t i c r e a c t i o n s , 267 Diagonal b a r s , 188 Diamagnetism, 383 D i f u n t a Group, 1 5 2 , 153-159

-

c o l o u r v a r i a t i o n s , 156, 157, 158

D i s s o l u t i o n of framework s i l i c a t e s , 280-293

-

f e l d s p a r s , 282-284 f e r r o m a g n e s i a n m i n e r a l s , 280, 281, 291-293 m i c a s , 284-290

' D i s s o l u t i o n v o i d s , 283 D i s t a l b a r , 141 D i s t r i b u t a r y c h a n n e l p a t t e r n s , 133 D i s t r i b u t a r y mouth, 130 D i s t r i b u t a r y mouth b a r s , 142, 150 Divergent p l a t e margins, 6, 7 Dolomite, 314 Domains, 385, 386 Domengine s a n d s t o n e , 5 3 , 5 4 Dome-shaped d u n e s , 8 3

545

Donjek R i v e r , 1 8 6 , 187, 188 D r a a s , 77, 79 Dunes, 7 7 , 78, 79 D u r i c r u s t s , 210 E a r l y d i a g e n e t i c r e d d e n i n g , 104 East African r i f t , 7, 8 E a s t B e r l i n Formation, 3 6 , 37 Ebro b a s i n , 40, 43 Eh-pH c o n d i t i o n s

- i r o n , 340 - l a k e s , 354-356 - manganese, 354 E l R i t o F o r m a t i o n , 320

E n t r a d a Sands t o n e s , 106 Ephemeral s t r e a m s , 194-197 E p i e u g e o s y n c l i n e s , 3 , 22 E p s i l o n c r o s s - b e d d i n g , 237, 243 Erg O r i e n t a l , 75 E r g s , 71, 73, 100 Esmeralda F o r m a t i o n , 301, 302, 303, 304, 305, 306 Es thwai t e , 355 E t r u r i a Marl, 1 7 1 , 173, 1 7 4 , 175 Eugeosynclines, 2 E u t r o p h i c l a k e s , 357 Exogeosynclines, 2 E x s o l u t i o n i n t i t a n o m a g n e t i t e s , 325 Fault-wedge b a s i n s , 1 8 , 20, 2 1 F e l d s p a r , 282-284 F e r r i a n i l m e n i t e , 328

546 F e r r i c h y d r o x i d e s , 1 5 9 , 2 1 5 , 2 1 7 , 2 1 8 , 2 4 7 , 2 4 8 , 2 5 0 , 3 5 1 , 3 5 2 , 357 F e r r i c oxyhydroxides, 3 2 9 , 3 3 0 , 3 3 8 , 3 8 3

-

dehydration, 340-344 e n v i r o n m e n t a l c o n t r o l s , 344-365 f o r m a t i o n i n groundwater, 3 6 5 - 3 6 7 p r e c i p i t a t i o n , 338-340

F e r r i c r e t e , 210

-,

see a l s o L a t e r i t e

F e r r i h y d r i t e , 329 Ferromagnesian s i l i c a t e s , 2 9 1 - 2 9 3 , F e r roma gne t i sm , 3 8 3 F e r r o u s / f e r r i c r a t i o , 3 7 8 , 420 Ferruginous s o i l s , 214 F i n e member r e d d e n i n g , 2 4 6 Fining-upwards c y c l e s , 2 3 3

-

A l p i n e m o l a s s e , 41

-

Old Red S a n d s t o n e , 2 3 6 , 2 4 2 , 2 4 3

--

colour v a r i a t i o n , 2 4 4 , 246

F i r s t - c y c l e a r k o s e , 2 7 4 , 287 F l o o d p l a i n s , 2 0 7 , 238 F l u v i a l d i s t r i b u t a r y channels, 132 Flysch, 6 Forearc basins, 13 Foreland b a s i n s , 22 Fors t e r i t e , 281 Fountain Formation, 5 6 , 60 F r o s t i n g of q u a r t z g r a i n s , 9 1 Fulgurites, 113 Gamrie O u t l i e r , 4 6 3 , 4 6 4 , 4 6 5 G e i k i e l i t e , 333

322

547 G e o s y n c l i n e s , 2-6 G i b b s i t e , 5 4 , 282 G i l a Group, 2 9 4 , 2 9 7 , 310 G i l b e r t i a n d e l t a , 1 2 7 , 132 Glaebular s o i l s , 241 G l a e b u l e s , 2 3 8 , 239 Glauconite, 272 G o e t h i t e , 2 1 8 , 2 6 8 , 2 9 7 , 3 1 6 , 3 3 0 , 3 3 1 , 3 3 8 , 3 4 0 , 3 4 1 , 3 4 2 , 3 4 2 , 3 8 9 , 4 1 4 , 4 1 5 , 416 Gomti R i v e r , 2 0 8 Grain c o n t a c t s , 273 Granule-ripples,

76

G r e i g i t e , 345, 388 Greywacke, 295 Groundwater , 104, 366 Grovesend Beds, 1 6 8 Gulf o f C a l i f o r n i a , 1 8 , 1 9 Gulf of Papua, 1 3 1 Gypsum d u n e s , 85 Gypsum n o d u l e s , 9 6 H a e m a t i t e , 2 8 0 , 2 9 7 , 3 2 2 , 3 2 3 , 3 2 7 , 3 3 0 , 3 3 6 , 3 3 8 , 3 4 0 , 3 4 1 , 3 4 2 , 3 4 3 , 3 7 2 , 411, 4 1 5 , 4 2 5 , 4 2 6 , 477

-

authigenic, 64 diagenetic origin, 231 J u n i a t a and Bald E a g l e F o r m a t i o n s , 375

- m a g n e t i c p r o p e r t i e s , 390-403 - - g r a i n s i z e e f f e c t s , 394-398 - magnetic s t r u c t u r e , 393 - pigmentary, 60, 6 3 , 2 2 0 , 2 4 5 , - s a t u r a t i o n m a g n e t i z a t i o n , 392 - s u s c e p t i b i l i t y , 392

2 4 7 , 3 1 7 , 4 2 1 , 482

64 8

Haematite after biotite, 2 8 6 , 2 8 7 , 2 8 8 , 289 Haematite-ilmenite series, 3 2 6 - 3 2 9 , 3 9 1 Haemo-ilmenite, 328 Hartford basin, 36 Haselgebirge facies, 114 Haslingden Flags, 152 Hayner Ranch Formation, 2 9 2 , 3 1 0 , 317 Helicoidal flow, 1 9 9 Highland Boundary Fault, 32 High temperature moment, 4 0 2 Himalayas, 2 2 , 2 4 Hinterland basin, 22 Hornblende, 1 2 1 , 2 8 0 , 2 9 1 , 2 9 2 , 2 9 5 , 2 9 6 , 2 9 7 , 2 9 8 Hornelen basin, 3 3 , 3 4 , 3 5 , 2 2 7 , 2 2 8 , 2 2 9 , 2 3 0 , 2 3 1

-

alluvial deposits, 33 origin, 35

Hydropsilomelane, 477 Hypolimnion, 3 5 1 . Hysteresis, 3 8 4 , 3 8 5 , 400 Illite, 5 5 , 2 4 7 , 2 7 2 , 2 8 1 , 2 9 3 Illite-montmorillonite, 1 2 1 , 2 7 2 , 2 9 6 , 3 0 8 Ilmenite, 3 2 3 , 3 2 4 , 3 2 5 , 3 2 7 , 3 3 4 , 3 3 6 , 337 Ilmeno-haematite, 3 2 8 Inclination error, 4 0 4 , 405 Indo-Gangetic basin, 2 3 Induced magnetization

-

Ji-H curves of red beds, 418-424

J.-T curves of red beds, 424-427

Initial susceptibility, 3 8 4 Inland sabkhas, 82

549 I n t e r a r c basin, 1 3 , 16 Intercontinental r i f t i n g , 9 I n t e r d i s t r i b u t a r y bays, 129 I n t e r f l u v i a l a r e a s , 208-212 I n t r a - a r c b a s i n , 1 3 , 15 Intracontinental r i f t s

-

see a u l a c o g e n s

Intraformational conglomerate, 1 9 5 , 248 I n t r a s t r a t a l a l t e r a t i o n , 104, 1 2 1 , 2 3 0 , 2 7 0 , 2 7 5 , 2 9 1 , 3 0 8 , 3 2 1 , 4 8 0 , 4 8 1

-

f e r r o m a g n e s i a n m i n e r a l s , 6 3 , 99

-

i n d e l t a p l a i n red beds, 126

-

T r i a s s i c , 256

I r i s h V a l l e y m o t i f , 3 4 8 , 3 4 9 , 3 5 0 , 3 5 1 , 352 I r o n , 1 0 , 2 8 5 , 2 8 7 , 3 5 4 , 3 5 5 , 3 5 6 , 3 7 2 , 373

-

o r g a n i c complexing, 3 5 2 , 353

-

t r a n s p o r t i n Recent a l l u v i u m , 2 1 3

-

transport phases, 215, 216

c o n t e n t of a n c i e n t r e d b e d s , 1 5 7 , 1 5 8 , 376-379 c y c l e i n l a k e s , 358 f e r r i c , 3 7 7 , 378 f e r r o u s , 3 5 1 , 3 7 7 , 378

I r o n hydroxide, 1 3 7 , 214 I r o n o x i d e s , 2 6 8 , 323-330 I r o n oxide nodules, 138 I r o n o x i d i z i n g b a c t e r i a , 356 I s o t h e r m a l r e m a n e n t - m a g n e t i z a t i o n , 3 8 4 , 3 9 8 , 399

-

red beds, 418, 420, 421

Jhelum r e - e n t r a n t , 2 3 , 2 4 , 2 5 , 26 Ji-T a n a l y s i s , 424-427 Joggins red beds, 175-177

550 J o h n 0 ' G r o a t s S a n d s t o n e , 360 J o t n i a n S a n d s t o n e , 285 J u n i a t a Formation, 3 7 4 , 375 K a o l i n i t e , 5 4 , 5 5 , 5 9 , 1 7 4 , 2 0 9 , 2 4 7 , 2 7 2 , 2 8 1 , 2 8 2 , 293 Klikov Formation, 2 5 9 , 2 6 0 , 2 6 1 , 2 6 2 , 2 6 3 Koenigsberger r a t i o , 384 Kootenay

-

Blairmore m o l a s s e , 4 6 , 4 7 , 4 8

K u p f e r s c h i e f e r , 115 Lake E y r e , 95 Lamotte Formation, 425 L a t e d i a g e n e t i c r e d d e n i n g , 104, 105 Late-orogenic b a s i n , 22 L a t e Precambrian of n o r t h w e s t S c o t l a n d , 453-460 L a t e r a l a c c r e t i o n d e p o s i t s , 2 6 , 2 0 1 , 2 0 2 , 2 3 5 , 2 3 6 , 237 L a t e r i t e , 100, 2 1 0 , 2 1 1 , 2 1 3 , 3 3 2 , 333 L a w r e n c i t e , 329 Lead, 10 Lepidocrocite,

-

330, 331, 338, 342, 343,

P a t a p s c o F o r m a t i o n , 367

Leucoxene, 3 3 3 , 337 Levees, 2 0 4 , 2 0 5 , 2 0 6 , 207 Libyan d e s e r t , 7 6 Linguoid b a r s , 1 9 2 L i t t l e h a m Mudstones, 3 7 0 , 3 7 1 , 372 Longitudinal b a r s , 1 8 6 , 187 Lower Keuper S a n d s t o n e , 2 5 6 , 2 5 7 , 373 Lower Modelo S a n d s t o n e , 55 Lower Old Red S a n d s t o n e , 2 2 2 , 235 Lower Permian

-

see R o t l i e g e n d e s

551 Lower T r i a s s i c , 2 5 4 Lower T r i a s s i c r e d b e d s , 253-259 Lower Wabash R i v e r , 205 Low-sinuosity r i v e r s , 1 9 0 - 1 9 4 Lyons s a n d s t o n e , 1 0 5 , 1 0 6 , 1 0 7 Mackinawi t e , 345 Maghemite, 3 3 0 , 3 3 1 , 335 Magmatic a r c , 1 3 , 14 Magnesium, 2 8 1 , 285 Magnetic v i s c o s i t y , 387 Magnetism, 382-384 Magnetite,

11, 2 1 8 , 3 3 1 , 3 3 6 , 4 0 8 , 4 0 9 , 411

Magnetite-ulvbspinel,

389

M a g n e t i z a t i o n of c o n t i n e n t a l r e d b e d s , 380-439

-

Type A r e d b e d s , 6 5 , 485-487 Type B r e d b e d s , 6 5 , 4 8 7 , 4 8 8 Type C r e d b e d s , 488-489

M a g n e t i z a t i o n p r o c e s s e s i n s e d i m e n t s , 403-417 Manganese, 10, 3 3 2 , 3 5 3 , 3 5 4 , 3 5 5 , 356

-

i n l a k e s , 358

-

n o d u l e s , 12

Manganosideri t e , 11 Marginal b a s i n s , 1 6 M a r t i n Formation, 411, 4 1 2 , 4 4 3 , 444, 4 4 5 , 446 M a r t i t e , 3 3 4 , 3 3 5 , 3 3 7 , 485 M a r t i t i z a t i o n , 1 5 9 , 2 3 2 , 3 1 9 , 3 3 2 , 3 3 3 , 3 3 5 , 457 Meandering c h a n n e l s , 1 8 0 , 1 8 1 Meandering R i v e r s , 1 9 8 - 2 0 4 M e c h a n i c a l l y - i n f i l t r a t e d c l a y , 104, 2 3 0 , 275-280

-

i n a n c i e n t red beds, 280

552 Mekong d e l t a , 140 M e t a l - e n r i c h e d c o n c r e t i o n s , 3 7 0 , 371, 372 Microperthi t e , 53 Middle Old Red S a n d s t o n e , 359 Middle V a l l e y o f S c o t l a n d , 1 5 9 , 2 2 2 , 223 Mineral magnetism, 388-403 M i n e r a l o g i c a l m a t u r i t y , 284 M i n e r a l t r a n s f o r m a t i o n s and r e p l a c e m e n t s , 272 Miogeosynclines, 2 M i s s i s s i p p i d e l t a , 132

-

c r e v a s s e c h a n n e l s , 1 3 4 , 135 d i s t r i b u t a r y c h a n n e l s , 142 d i s t r i b u t a r y mouth b a r s , 142 i n t e r d i s t r i b u t a r y b a y s , 134 s u b d e l t a s , 135

Modern d e s e r t s e d i m e n t s , 69-71 Moenkopi Formation, 3 1 7

-

palaeomagnetism, 469-475

Molasse, 6

-

A l p i n e , 4 0 , 41

- Lower F r e s h w a t e r ,

4 0 , 41

- -

Lower Marine, 4 0 , 4 1 , 4 3 , 44

--

Upper F r e s h w a t e r , 4 0 , 4 1 , 4 3 , 44

- -

Upper M a r i n e , 4 0 , 42

-

Canadian C o r d i l l e r a , 4 4 , 4 5 , 4 6 , 4 7 , 4 8 , 49 Caledonian o r o g e n , 221

-,

see a l s o Old Red Sandstone

Molasse B a s i n , 4 0 , 43 M o n o c r y s t a l l i n e q u a r t z , 58 M o n t m o r i l l o n i t e , 11, 5 4 , 2 7 2 , 304, 305, 308

553 M o r d e n i t e , 303, 305 Morin t r a n s i t i o n , 3 9 3 , 394, 397 Mtlssbauer s p e c t r a of h a e m a t i t e , 396, 397 Mudf lows, 225 Multicomponent NRM, 4 7 7 , 4 7 9 , 480 M u s c o v i t e , 2 7 2 , 280 N a t u r a l l e v e e s , 2 0 4 , 2 0 5 , 2 0 6 , 208 N a t u r a l remanent m a g n e t i z a t i o n , 380

-

r e d b e d s , 427-429

Navajo S a n d s t o n e , 1 0 6 , 1 0 8 , 488 Nee1 t e m p e r a t u r e , 383 Newark Group, 285 N i g e r d e l t a , 1 3 2 , 1 4 0 , 1 4 1 , 1 4 3 , 1 4 5 , 1 4 6 , 147 N o r t h e r n Columbian a l l u v i u m , 2 1 7 , 218 N o r t h S e a , 3 8 , 39

NRM

-

s e e N a t u r a l remanent m a g n e t i z a t i o n

Old Red S a n d s t o n e , 3 0 , 3 2 , 221-253, 4 6 0 , 4 6 1 , 4 6 2 , 4 6 3 , 4 6 4 , 4 6 5 , 4 6 6 , 4 6 7 , 468

-

c l a y - o x i d e p i g m e n t , 251 c o a r s e members, 2 3 3 , 235, 2 3 6 , 237 e x t e r n a l b a s i n s , 30 e x t e r n a l f a c i e s , 233

- -

-

r e d d e n i n g , 244

f i n e members, 2 3 7 , 239

-

Gamrie o u t l i e r , 2 8 8 , 312

-

heavy m i n e r a l s , 232

-

i n t e r n a l b a s i n s , 30

-

i n t e r n a l f a c i e s , 222

fining-upwards c y c l e s , 2 3 6 , 2 4 2 , 243

heavy m i n e r a l c o n t e n t , 2 5 0 , 2 5 2 , 253

554

- - r e d d e n i n g , 230 - intrastratal alteration, - Lanark b a s i n , 3 1 , 33 -

2 5 0 , 252

palaeomagnetism, 460-468 S t r a t h m o r e b a s i n , 3 1 , 33

O l i g o t r o p h i c l a k e s , 357 O p a l , 304 Opaque o x i d e s , 330-338 O r c a d i a n b a s i n , 2 2 2 , 359, 463 Organic c a r b o n , 345 Organic m a t t e r , 1 3 7 , 1 5 6 , 2 0 9 , 3 4 5 , 3 7 2 , 373 Orogenic b e l t , 22 O r t h o c l a s e , 5 3 , 280 Orthogeosynclines , 2

Oslo g r a b e n , 9 , 110 Overbank f l o o d i n g , 134 P a c i f i c o r o g e n , 44 Palaeomagnetic e v o l u t i o n o f r e d b e d s , 484-492 Palaeomagnetism and r o c k magnetism, 380-382

-

Cenozoic r e d b e d s , 475-481 M a r t i n F o r m a t i o n , 443-446

- Moenkopi Formation, -

469-475

Old Red S a n d s t o n e , 460-468 S t a r k Formation, 447-450 Tochatwi Forma t i o n , 450-45 3 T o r r i d o n i a n Sands t o n e , 453-460

Palaeomagnetism of c o n t i n e n t a l r e d b e d s , 440-492 Pangaea, 35 P a r a b o l i c d u n e s , 84 Parageosynclines

,

2

556

Paraliageosynclines, 3 P a t a p s c o Formation, 365, 366, 367 Pebbly b r a i d e d r i v e r s , 185-190 P e d o g e n e s i s , 1 5 3 , 158 Pelagic sediments, 12 P e n r i t h sandstone, 114, 117, 118, 119, 120, 121 P e r i p h e r a l b a s i n s , 21 Permian p a l a e o l a t i t u d e s , 1 1 4 , P e r t h i t e , 55 P h i l l i p s i t e , 301, 303, 3 0 4 , 305 Phyllomorphic s t a g e , 272 Pigmentary h a e m t i t e , 2 1 3 , 220, 2 4 5 , 2 4 7 , 317, 4 2 1 , 482 P l a g i o c l a s e , 5 3 , 2 8 0 , 2 8 4 , 2 8 5 , 297 P l a t e t e c t o n i c s and s e d i m e n t a t i o n , 6-26 P l a y a s , 95 Point-bars,

200-204

P o l a r i t y t r a n s i t i o n s , 482 P o l y c r y s t a l l i n e q u a r t z , 5 8 , 59 P o r o s i t y , 2 6 5 , 274 P o s t - d e p o s i t i o n a l DRM, 4 0 3 , 4 0 5 , 4 0 6 , 4 0 7 , 4 0 8 , 410 P o t a s h f e l d s p a r , 2 8 4 , 2 8 5 , 2 9 7 , 2 9 9 , 302 Pre-Flysch, 4 P r e s s u r e s o l u t i o n , 270 P r o d e l t a , 131 " P r o t e c t i o n i s t " theory, 91 P r o t e r o z o i c b a s i n s of w e s t e r n Canada, 441-453 P r o t e r o z o i c r e d b e d s , 2 6 , 441-453

-

Athapuscow a u l a c o g e n , 2 7 , 2 8 , 2 9 , 4 4 1 , 443 C h r i s t i e Bay Group, 29

5 56 P r o t o - A t l a n t i c o c e a n , 29 Pseudo-anticlines,

239, 240, 241

P s e u d o b r o o k i t e , 332, 324 Pseudo-single domains, 386 P u l l - a p a r t b a s i n s , 18, 2 0 , 2 1 P y r i t e , 137, 346 P y r i t e o x i d a t i o n , 159 Pyroxene d i s s o l u t i o n , 291 Pyrrho t i t e , 388 Q u a r t z , 9 2 , 9 4 , 269, 270, 319, 320 Rawalpindi Group, 23 R e a c t i v a t i o n s u r f a c e s , 8 8 , 1 8 6 , 1 9 2 , 235 Red beds

-

a e o l i a n , 105-108

-

a r k o s i c , 30

- c l i m a t i c s i g n i f i c a n c e , 50 - d i a g e n e s i s , 265, 274 - E a r l y Mesozoic, 35 - a n c i e n t d e l t a p l a i n s , 150-178 - m o i s t c l i m a t e s , 50 - m a g n e t i c p r o p e r t i e s , 417-439 -

-

magnetic s u s c e p t i b i l i t y , 419 mechanisms of f o r m a t i o n , 1 t e c t o n i c framework, 1

Reddening and d e p o s i t i o n a l environment, 63 Reddening and l a t e r i t i z a t i o n , 64 Red M a r l s , 242 Red S e a , 9 , 11 Red s o i l s , 5 2 , 9 7 , 218

557 Reducing c o n d i t i o n s , 138 R e d u c t i o n s p o t s , 368-372 Reduction z o n e s , 367-376 Redoxomorphic s t a g e , 267, 268 Re-entrants,

26

R e l a x a t i o n t i m e , 387 R e m a g n e t i z a t i o n , 461, 462, 463, 481, 482, 483, 488 Remanence a c q u i s i t i o n i n r e d b e d s , 481-484 Remnant a r c , 16 Remnant-ocean b a s i n s , 2 1 R e t r o a r c b a s i n , 1 6 , 22 Rhine d e l t a , 134 Rhine g r a b e n , 9 Rhondda Beds, 1 6 4 , 1 6 6 , 167 Rhourds, 84 Ridge b a s i n , 20, 23 R i f f l e r e a c h , 188 R i n g e r i k e Group, 285, 315, 317, 378 R i v e r banks and f l o o d p l a i n s , 204-212 R i v e r c h a n n e l s , 180-182 River-dominated d e l t a s , 1 3 0 , 1 3 2 , 1 4 8 , 149 R i v e r w a t e r , 267 R o t a t i o n a l h y s t e r e s i s , 417 R o t l i e g e n d e s , 108-125, 279, 309, 310, 312

-

a e o l i a n s a n d s , 1 1 3 , 114 a n h y d r i t e cement, 124, 125 a u t h i g e n i c c l a y , 120, 1 2 2 , 123

-

a u t h i g e n i c f e l d s p a r , 1 1 9 , 120, 1 2 1

-

a u t h i g e n i c q u a r t z , 119, 120, 121

-

c a r b o n a t e cement, 1 2 2 , 124

-

c l a y - o x i d e g r a i n c o a t i n g s , 118

558

-

d e s e r t l a k e s and s a b k h a s , 114-116

-

d i a g e n e s i s and r e d d e n i n g , 116, 117

- m a r g i n a l f a n g l o m e r a t e s and - mechanical i n f i l t r a t i o n of - palaeogeography, 110 - reddening, 116, 117

w a d i s , 111-113

-

S o l e P i t a r e a , 116

-

s o u t h w e s t England, 110, 112

c l a y , 117

Roundness, 91, 93, 94

Saharan d u n e s , 98 S a l t o n t r o u g h , 18, 19 S a l t p a n s , 96 San Andreas F a u l t , 23 Sandstone d i a g e n e s i s , 265-275 Sandwaves, 108, 109 Sandy, l o w - s i n u o s i t y r i v e r s , 190-194 San G a b r i e l F a u l t , 20 S a n i d i n e , 121, 301, 302 Sao F r a n c i s c o d e l t a , 132 S a r c l e t Group, 360 Sea f l o o r s p r e a d i n g , 11 S e a t e a r t h s , 167 Sea w a t e r , 267 Secondary r e d d e n i n g , 160-163 S e c u l a r v a r i a t i o n , 382 S e i f s , 80-82 S e l f - r e v e r s a l , 329 S e n t i n e l B u t t e F o r m a t i o n , 152 S h e e t f l o o d d e p o s i t s , 185 S i e v e d e p o s i t s , 185

559 S i l c r e t e , 210 S i l i c a , 2 6 7 , 2 7 0 , 2 7 1 , 307, 308 Simpson d e s e r t ,

-

c o l o u r , 100, 101, 102 r e d a e o l i a n s a n d s , 64 s e i f s , 81

S i n g l e domains, 386

-

h a e m a t i t e , 394, 427 m a g n e t i t e , 387

S i w a l i k Group, 2 3 , 26 S i z e - c o m p o s i t i o n t r e n d s , 55-58 S o i l s , 208-212, 262 Solund b a s i n , 34 Sonoran d e s e r t , 9 7 , 9 9 , 1 0 2 , 103 S o u t h e r n Mexico, 214 South Saskatchewan R i v e r , 1 9 3 , 194 Sparagmites, 2 9 , 30, 31 S p e c i f i c induced m a g n e t i z a t i o n , 4 2 0 , 421 S p e c u l a r i t e , 4 2 1 , 4 2 2 , 4 3 0 , 4 3 1 , 4 3 3 , 4 3 5 , 4 3 8 , 458 S p i n c a n t e d moment, 390, 391, 401 S p i n e l u n i t c e l l , 326 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, 398-401 S t a g e s o f d i a g e n e s i s , 266 S t a r k Forma t i o n , 447-450 S t . Bees S a n d s t o n e , 2 5 6 , 2 5 8 , 3 1 6 , 318, 320, 334, 373, 4 2 9 , 489 ”S teinmann T r i n i t y ” , 5

S t o e r Group, 4 5 4 , S t r a i g h t c h a n n e l s , 1 8 0 , 181 S t r i k e - s l i p f a u l t s , 22 Subaqueous l e v e e s , 129

560 Subduction zone, 1 3 Sub-Permian i n c o n f o r m i t y , 160 S u b s u r f a c e w a t e r , 269 S u c c e s s o r b a s i n s , 22 Sudan, 97 S u l p h a t e r e d u c i n g b a c t e r i a , 357 S u l p h a t e r e d u c t i o n , 268 Superparamagnetism, 388, 3 9 6 , 397 S u s c e p t i b i l i t y a n i s o t r o p y , 4 0 8 , 4 0 9 , 410 Swamp d e p o s i t s , 1 3 5 , 1 3 6 , 1 3 7 Swansea Beds, 168 S y n e r e s i s c r a c k s , 361

T a i g u a t i Formation, 419 Taphrogeosynclines, 3 T e c t o n i c s e t t i n g of a n c i e n t r e d b e d s , 26-49 Ten Boer Member, 114 Tesuque Formation, 2 8 3 , 294, 306 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 , 2 7 3 , 274 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 s a n d s , 88-95 T e x t u r a l m a t u r i t y , 2 8 4 , 295 Thermal d e m a g n e t i z a t i o n , 4 0 1 , 429

- Cenozoic r e d b e d s , 478 - M a r t i n F o r m a t i o n , 445 - Moenkopi Formation, 471

- pigment and s p e c u l a r i t e , 4 3 4 , -

4 3 5 , 4 3 6 , 4 3 7 , 438

Old Red S a n d s t o n e , 4 6 2 , 4 6 5 , 467 S t a r k F o r m a t i o n , 449 Tochatwi F o r m a t i o n , 451 T o r r i d o n i a n Sands t o n e , 457

Thermomagnetic a n a l y s i s

56 1

-

see Ji-T

analysis

Thermoremanent m a g n e t i z a t i o n , 380 T i d a l d i s t r i b u t a r y c h a n n e l s , 139, 141 Tide-dominated d e l t a s , 1 3 0 , 1 3 1 , 1 3 8 , 1 3 9 , 149, 150 T i t a n h a e m a t i t e , 328, 332 Titanomaghaemate, 323-326, 388, 389, 390 T i t a n o m a g n e t i t e , 323-327, 333, 388

-

m a g n e t i c p r o p e r t i e s , 389, 390 i n r e d b e d s , 332

Tochatwi F o r m a t i o n , 451, 452, 453 T o r r i d o n i a n S a n d s t o n e , 30, 32, 285, 333, 453-459 T r a i l i n g edges, 9 Transform f a u l t s , 7 , 1 2 , 1 7 T r a n s p o r t o f i r o n i n a l l u v i u m , 213-220 Transverse dunes, 82, 83 T r i a s s i c g r a b e n , 35, 36 T r u c i a l c o a s t , 97, 98 Turbidity currents, 4 Type A r e d b e d s , 485-487 Type B r e d b e d s , 487, 488 Type C r e d b e d s , 488, 489 U l v b s p i n e l , 331 Upper C a r b o n i f e r o u s palaeogeography, 160 Upper Coal Measures, 163-166 Upper C r e t a c e o u s r e d b e d s , 259-263 Upper G i l a Group, 279 Upper M o t t l e d S a n d s t o n e , 254, 315 Upper Old Red S a n d s t o n e , 224, 225, 226 "Upturned p l a t e s " , 92 Uranium, 368, 370, 371

562 Vanadium, 368, 370, 371, 372 V e r m i c u l i t e , 285, 293 Violin Breccia, 21 V i r t u a l geomagnetic p o l e , 382 V i v i a n i t e , 137 Wave-dominated d e l t a s , 131, 149, 150 Weathering i n d e x , 153 Western A t l a n t i c , 1 2 Western North America, 18 White R i m S a n d s t o n e , 1 0 6 , 107 Wilcox Group, 152 Wingate Sands t o n e s , 106 Wood Bay Formation, 242 Wupa t k i Forma t i o n , 475 Yellow S a n d s , 108 Y o r e d a l e s , 151, 152 Yukon R i v e r , 215, 216, 217

Z e c h s t e i n t r a n s g r e s s i o n , 115 Zeugogeosynclines , 3 Z i j d e r v e l d d i a g r a m s , 380 Z i n c , 10, 332