Landforms and Geology of Granite Terrains

333 Pages·2005·29.976 MB·English

by  Charles Rowland Twidale, Juan Ramón Vidal Romani

LANDFORMS AND GEOLOGY OF GRANITE TERRAINS
Copyright 2005 by Taylor & Francis Group
LANDFORMS AND
GEOLOGY OF GRANITE
TERRAINS
C.R. Twidale
University of Adelaide, Australia
J.R. Vidal Romaní
University of Corun~a, Spain

A.A. BALKEMA PUBLISHERS LEIDEN/ LONDON/ NEWYORK/ PHILADELPHIA/ SINGAPORE
Copyright 2005 by Taylor & Francis Group
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Cover Illustrations
Front: Acuminate and ensiform residual, Mt. Manypeaks area, near Albany, Australia.
Photograph by C.R. Twidale.
Back: Domed granite inselberg from the Bolson of the Sen~or de la Pen~a Valley, Anillaco,
República Argentina. Photograph by J.R. Vidal Romaní.
Copyright © 2005 Taylor & Francis Group plc, London, UK
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Published by: A.A. Balkema Publishers Leiden, The Netherlands, a member of
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ISBN 04 1536 435 3
Printed in Great Britain
Copyright 2005 by Taylor & Francis Group
Table of Contents
Preface and acknowledgements IX
1 Characteristics and foundations 1
1.1 Typical landforms and landscapes 1
1.2 Previous work 5
1.3 Occurrences of granite 9
1.4 Granite – definition and composition 10
1.5 Physical characteristics 13
1.6 Orthogonal fracture systems 16
1.7 Fractures and drainage patterns 18
2 Sheet fractures and structures 27
2.1 Terminology 27
2.2 Description and characteristics 28
2.3 Theories of origin 32
2.3.1 Exogenetic explanations 33
2.3.2 Endogenetic explanations 41
2.4 Summary 47
3 Weathering 49
3.1 Definition and significance 49
3.2 Physical disintegration 49
3.3 Chemical alteration 54
3.4 The course of weathering in granite 54
3.5 Controls of weathering 58
4 Plains – the expected granite form 63
4.1 Weathering and surfaces of low relief 63
4.2 Plains of epigene (subaerial) origin 63

4.2.1 Rolling or undulating plains 64
4.2.2 Pediments 66
4.2.3 Relationship between pediment and peneplain 72
4.3 Etch plains in granite 72
4.4 Very flat plains 74
4.5 Multicyclic and stepped assemblages 75
4.6 Exhumed plains 78
4.7 Summary 79
5 Boulders as examples of two-stage forms 81
5.1 The two-stage or etching mechanism 81
5.2 Boulders – morphology and occurrences 83
5.3 Subsurface exploitation of orthogonal fracture systems 83
5.4 Tectonic and structural forms 95
5.5 Types of peripheral or marginal weathering 97
Copyright 2005 by Taylor & Francis Group
VI Table of Contents
5.6 Causes of peripheral weathering 98
5.7 Evacuation of grus 102
5.8 Boulders of epigene origin 103
5.9 Summary 105
6 Inselbergs and bornhardts 109
6.1 Definitions and terminology 109
6.2 Bornhardt characteristics 115
6.3 Theories of origin 118
6.3.1 Environment 118
6.3.2 The scarp retreat hypothesis 120
6.3.3 Tectonics and structure: faulting and lithology 121
6.3.4 Variations in fracture density 127
6.3.5 Differential subsurface weathering and the
two-stage concept 127
6.4 Evidence and argument concerning origins of 131
bornhardts
6.4.1 Contrasts in weathering between hill and plain 131
6.4.2 Incipient domes 131
6.4.3 Subsurface initiation of minor forms 131
6.4.4 Flared slopes and stepped inselbergs 132
6.4.5 Regional and local patterns in plan 139
6.4.6 Coexistence of forms associated with
compression/shearing 140
6.4.7 Topographic settings 140
6.4.8 Occurrence in multicyclic landscapes 140
6.4.9 Fracture-defined margins 142
6.4.10 Age of inselbergs and bornhardts 142
6.5 Exhumed bornhardts and inselbergs 146
6.6 Antiquity and inselberg landscapes 149
6.7 Summary 149
7 Other granitic residuals and uplands 153
7.1 Isolated residuals 153
7.1.1 Nubbins 153
7.1.2 Castle koppies 155
7.1.3 Large conical forms or medas 159
7.1.4 Towers and acuminate forms 162
7.2 Massifs 163
7.3 Regions of all slopes topography 169
7.4 Discussion 171
8 Minor forms developed on steep slopes 173
8.1 Flared slopes 173
8.1.1 Description and characteristics 173
8.1.2 Origin 177
8.1.3 Changes after exposure 183
8.2 Fretted basal slopes and other variants 184
8.3 Scarp-foot weathering and erosion, and the piedmont angle 188
8.4 Rock platforms 190
8.4.1 Description 190
8.4.2 Origin 190
Copyright 2005 by Taylor & Francis Group
Table of Contents VII
8.5 Scarp-foot depressions 190
8.5.1 Description 190
8.5.2 Origin 192
8.6 Flutings or grooves 193
8.6.1 Description 193
8.6.2 Origin 196
8.6.3 Surface or subsurface initiation? 200
8.6.4 Inversion 203
9 Minor forms developed on gentle slopes 207
9.1 Rock basins 207
9.1.1 Description 207
9.1.2 Nomenclature 211
9.1.3 Origin 211
9.1.4 Differentiation of major types 216
9.1.5 Evacuation of debris 219
9.1.6 Rate of development 219
9.2 Plinths and associated blocks and boulders 220
9.2.1 Description 220
9.2.2 Origin 221
9.3 Pedestal rocks 222
9.3.1 Terminology 222
9.3.2 Origin 222
9.4 Gutters or runnels 224
9.4.1 Terminology 224
9.4.2 Description 224
9.4.3 Origin 226
9.5 Rock levees 228
9.6 Rock doughnuts 230
9.6.1 Description 230
9.6.2 Origin 231
9.6.3 Evidence and argument 232
9.7 Fonts 232
10 Caves and tafoni 235
10.1 General statement 235
10.2 Caves associated with corestones and grus 235
10.3 Caves associated with fractures 236
10.4 Tafoni 238
10.4.1 Description 238
10.4.2 Process 245
10.4.3 Stages of development 249
10.4.4 Case-hardening and other veneers 250
10.5 Speleothems 251
11 Split and cracked blocks and slabs 259
11.1 Split rocks 259
11.1.1 Description 259
11.1.2 Origin 260
11.2 Parted and dislodged blocks 264
11.3 Dislocated slabs 266
11.3.1 A-tents 266
Copyright 2005 by Taylor & Francis Group
VIII Table of Contents
11.3.2 Overlapping slabs 271
11.3.3 Displaced slabs 271
11.3.4 Chaos 275
11.3.5 Wedges 276
11.3.6 Origin of the forms 278
11.3.7 Relationship of A-tents and pressure ridges 282
11.4 Polygonal cracking 282
11.4.1 Description 282
11.4.2 Previous interpretations 285
11.4.3 Evidence 287
11.4.4 Explanations 287
11.5 Tesselated pavements 291
12 Zonality,azonality and the coastal context 293
12.1 Introduction 293
12.2 Lithological zonality and azonality 293
12.3 Climatic zonality and azonality 309
12.4 The coastal context 313
13 Retrospect and prospect 327
About the authors 352

Copyright 2005 by Taylor & Francis Group
Preface and acknowledgements
The authors owe a great debt of gratitude to many individuals and organisations withoutwhose sup-
port and assistance the research, and particularly the field investigations, on which this book is based
could not have been carried out. The Australian Research Council (and its several predecessors)
have supported investigations of various aspects of granite landform evolution. The universities
of Adelaide and of Coruña have also provided support and facilities, and through study leave and
similar schemes, supported the travel, consultation and collaboration without which this book
would not have been possible.
It is impossible to acknowledge specifically the interest and encouragement of the many individ-
uals,in various parts of the world, who have given us part of their time, as well as the benefit of their
local knowledge; but we thank them all. Special thanks are due to Dr Liz Campbell and Dr Jennie
Bourne, both of whom have, over a period of many years, been enthusiastic, lively and critical field
collaborators and companions, as well as co-authors of many of the papers that form the foundation
of this larger work. Mr Peter Moss, Dr Liz Campbell, Mrs Noreen Shepherd and Ana Martelli that
read the book in draft form and helped eliminate many errors and inconsistencies. Debbie Haggar
is responsible for drafting many of the line drawings, Jacie Davis for reproducing them.

Copyright 2005 by Taylor & Francis Group
About the authors
Charles Rowland Twidale (1930) obtained his Doctoral
degree in Geology from the University of Bristol (1957) and
is Honoris Causa at the Complutense University of Madrid
(1983). At present he is Emeritus Professor at the University
of Adelaide, South Australia, and Research Leader of the
University Institute of Geology “Isidro Parga Pondal” of the
University of Corun~a. He has worked on geomorphology
subjects in North America, Australia, Africa and Europe to
gain insight in structural geomorphology, granitic geomor-
phology, eolian deposits in deserts areas and etching
processes in continental and marine environments. In addi-
tion, he worked on the development of models of landscape
evolution in intraplate continental areas. As a specialist in
granitic geomorphology, he has published numerous papers
in international scientific journals and numerous books on
this topic and on landscape evolution. Twidale is the author
of Granite Landforms(1982, Elsevier, Amsterdam).
Juan Ramón Vidal Romaní(1946) obtained his Doctoral
degree in Geology from the Complutense-University of
Madrid (1983). He is a Professor in Geodynamics at the
University of Corun~a and Director of the University Institute
of Geology “Isidro Parga Pondal”. He has worked on granitic
geomorphology and on its relation to the particular charac-
teristics of landscapes, like glacial, coastal and continental
landscapes, either in past or present climates. He has devel-
oped new research methods for cosmogenic chronology, ero-
sive granite surfaces, granitic pseudokarst processes and for
the genesis of the granitic forms. By field work in Argentina,
Australia, Madagascar, Portugal, Spain and North African
countries, he has become a specialist in the interpretation of
the origin of granitic forms in relation to their geodynamic
environment.
Copyright 2005 by Taylor & Francis Group
1
Characteristics and foundations
1.1 TYPICAL LANDFORMS AND LANDSCAPES
Many familiar landforms are developed on granite as well as in other lithological environments. Thus,
fault scarps and fault-line scarps, valleys (Figs 1.1a, b, and c) and other features (Figs 1.1d and e) are
(a)
Figure 1.1. (a) The Meckering fault scarp, Yilgarn Province, Western Australia was a complex of dirt scarps
in granitic rocks, and formed on October 20, 1968. All the scarps are now degraded (West
Australian Newspapers).
Copyright 2005 by Taylor & Francis Group