(i )
Introducing Victorian Geology
Panorama of Bendigo, then known as Sandhurst, 1888. Numerous head frames (also known as poppet heads) can be seen. These were ereeted over deep mine shafts. At the top of each head frame is a large wheel. A wire cable travelling around the wheel was attached to a cage in which men could be raised or lowered in the shaft. The shaft was also used for hauling ore to the surface. The head frames were ereeted at intervals across the town along north-south lines as saddle reefs formed along anticlines were opened up for mining by various companies. In the foreground are dumps of mullock, i.e. waste sedimentary rock that had to be extracted to reach the gold-bearing quartz reefs. The large building on the left would have housed the battery (crushing plant) and other equipment used to treat the gold orcs. The chimneys indicate the buildings where wood was burnt to raise the steam needed to run the machinery. Behind the left hand chimney is a tailings dam, where the quartz sands were sent after the gold had been extracted. (photograph courtesy of Geological Survey of Victoria).
(iii)
Introducing Victorian Geology Edited by:
G.W. Cochrane G.W. Quick D. Spencer-Jones
Principal authors:
J.N. Rowan (Chapter
2)
J.J. Jenkin (Chapter 3) J A Webb (Chapter 4) J.G. Leonard (Chapter .
.
6)
P.G. Dahlhaus (Chapter 7)
Other authors:
W.D. Birch (Chapter I & 5), G.W. Cochrane (Chapter I & 5).
c. R. Dalgarno (Chapter I), R.C. Glcnie (Chapter 5). I. W. McHaffie (Chapter 5)
GEOLOG I CAL SOCIETY OF AUSTRALIA (Victorian Division), Mel bourne, 1995
©Geological Society of Australia Incorporated (Victorian Division) 1999. This book is copyright. Apart from any fair dealing for the purpose of private study, research or review, as permitted under the Copyright Act, no part may be reproduced in any manner whatsoever without the
written permission of the publisher.
Published by the Geological Society of Australia Incorporated (Victorian Division), P.O. Box 2355V, Melbourne, Vic 300 I.
First edition 199 I Reprinted 1995 Reprinted 1999
National Library of Australia Cataloguing-in-Publication Introducing Victorian Geology. ISBN 0 949600 33 4.
1. Geology - Victoria.
Cochrane, G W.
I. Rowan, 1. N. (J ames Nial l).
ill. Spencer-Jones, D.
TI.
IV. Quick, G. W.
V. Geological Society of Au lralia. Victorian Divi iOn
559.45 Registered in Australia for transmission by post as a book. THE PUBLISHER IS A DIVISION OF THE GEOLOGICAL SOCIETY OF AUSTRALIA 706 WYNYARD HOUSE 30 I GEORGE STREET, SYDNEY NSW 2000
See GSA home page
http://www.gsa.org.au for more publications, activities, and membership
This book is available from: The Publications Officer Victoria Division, Geological Society of Australia, P.O. Box 2355V, Melbourne, Vic, 300 I. [for direct sales enquiries, telephone (02) 9290 2194 or fax (02) 9290 2198 or e-mail
[email protected] 1 Or available from:Research Publications Pty Ltd., 27a Boronia Road, Vermont Vic 3133 phone (03) 9873 1450 or fax (03) 9873 0100 Diagrams and maps drawn by Denise Russo, Susan Drummond, with additional drawings by Katrina Sandiford and Owen Smith. Wholly set up in Australia by Research Publications Pty. Ltd. Printed by Eastside Printing Pty Ltd. Melbourne.
Acknowledgements
The following individuals assisted in the preparation of this book by providing original material or by reviewing manuscripts: N.W. Archbold, P.F. Bolger, F. Canavan, R.A.F. Cas, W.P. Cole, LG. Douglas, LA. Ferguson, P.S. Forwood, D. Klindworth, M. Learmonth, G. Markovics, R.M. Molesworth, LL. Neilson, S. Oberman, N.W. Schlei ger, A.H.M. VandenBerg, M. Williams. The following companies provided the material and illustrations, which are included in the case histories in Chapter 5: Australian Cement Ltd., Boral Resources (Vic) Ply. Ltd., Brick and Pipe Industries Ltd., C.R.A. Exploration Pty. Ltd., Darley Refractories Pty. Ltd., Macquarie Resources Ltd., Western Mining Corporation Ltd. Other assistance by: Geological Survey of Victoria, P. L. Atkinson, T.W. Dickson, P. Dowd, J.A. Ferguson, C. Laughton, M.F. Lenard, G. Krummel, R.1. Nott, G.C. Smith, J.H. Smith and S.H. Tan. Special assistance with equipment was provided by: CSIRO Division of Building, Construction and Engineering and David Mitchell Ltd. The Association of Australian Palaeontologists kindly gave their permission for the fossils illuSLrated in Figures 4-23 and 4-46 to be reproduced from the Dorothy Hill Jubilee Memoir. The Victorian Division of the Geological Society of Australia was given generous financial assistance by the following major sponsors: Geological Society of Australia Incorporated BHP Petroleum Ply. Ltd. CRA Limited The Australian Institute of Quartying Education Foundation Western Mining Corporation Holdings Limited and also by Boral Resources (Victoria) Pty. Ltd. BP Australia Limited Crushed Stone Association of Australia (Victoria) Incorporated Pasminco (Australia) Limited Ashton Mining Limited Aberfoyle Resources Limited Newmont Australia Limited Sons of Gwalia N.L. Douglas McKenna and Partners Ply. Ltd.
Affil iations of a uthors and editors
W.D. Birch (Museum of Victoria), G.W. Cochrane (David Mitchell Ltd.), C.R. Dalgarno (Geological Survey of Victoria), P.G. Dahlhaus (Ballarat University College), R.C. G1enie (petroleum geology consultant), J.J. Jenkin (formerly Soil Conservation Authority), J.G. Leonard (Geological Survey of Victoria). l.W. McHaffie (Geological Survey of Victoria), G.W. Quick (CSIRO Division of Building, Construction and Engineering), J.N. Rowan (formerly Soil Conservation Authority), D. Spencer-Jones (formerly Department of Minerals and Energy), LA. Webb (Latrobe University).
(vi)
(vii)
CONTENTS
FOREWORD
(xi)
Ch apter 1:
r.:l=n nr.:v
EPT� I Land use
1 4
................................................... . . . . . . . . . . . . ...... ....... . ..........
Composition and structure of the Earth
. . . . . . . . . . . . . . . . ...... ......... . . . . . . .........
...................... .............................................................
Magma
Soils
6 6 6 7 7 8
............................................................................................. ..........................................................................
Narure of soils The soil profile Properties of soils
.......... . . . .................................. ................... . . . . . . .
Minerals
.......... ............... . . .. ............ ..... . . . . . . . .................
....... ................ .. .............. ...... . . . . . .....................................
Formation of minerals Chemistry of minerals Important minerals
........................................................ . . . . . . .
... .............................................................
......................................... ....... ...................
Rocks
............................ ................................................... . . . .......
10 10 10 11 12 15 17 19 21 23
......................................................... . . . . . . . . . . . . . . . . .
Igneous rocks Sedimentary rocks
.......................................... ..........................
Metamorphic rocks
Fossils
...................... .................. ............... ............
................................... ......................................................
Palaeontology
Geological time
....... . . . . . ........................... . . ................................
.............................................................................
23 23
. . . . ................ . . . . . . . . . . ........ .................................
Concept of lime Relalive time in geology
................... ....................................... . . .
Determination of relalive lime
Numerical geological time
Plate tectonics
24
. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .
25 27 30
................ ...........................................
..............................................................................
Magmas and igneous activity
..................... ......................................
30 30 31 34
...................... ............................... .................
Types of magma Movemem of magmas Magmas in ViclDria
. . . . .................... . . . . . . ...... .................. . . . .......
................. ......... ........................... . . . . . .........
Deformation and metamorphism
.................. ....................................
34 36 37 38
................................................. ................... .............
Folding Faulting Jointing Regional mel amorphism
. . . . ............................................................................
.............................. ......... ................ . . . . . . . . . .................
Erosion and sedimentation Sedimenlary basins
........................................... . . . . . . . ...... .....
38 39 39 40
.. .......... . . .......... .... . . .............................. . .
.............. . . . . . . ........................... . . . . .......... ... . . . ............................................ . . . ........
Sedimenlary rocks in Vicloria Lithifaction or diagenesis
......... ...................................................
41
........ . . . . ................................................................
Geological maps
Correlalion of geological formalions Geological map nomenclature
........................ ......................
43 44
... ..................... . ...... . .......................
Chapter 2:
45 Soil formation
..............................................................................
Soil classification
. . . . . . . . . . ................................ . . . . . . . ........................ . .
Organic soils Uniform soils Gradalional soils Duplex soils
. . . . . . ......... .... ........... . . . . . . . .. . . . . . ....... . . . .................... .......... ........... .... ..................................... ........
........................ . . . . ................. ...............................
H u man impact on Soil erosion Salin.tion
oils
.................................................................
..................................... . . . . ................. ..................
................. . . ...........................................................
Acidificalion Comp.clion
............. . . . . . . . . ........... ................. ........ .............. . . . .
. . . .................... . . . . . . ........ . . . . . . . ................................
oils of the Melbourne suburbs oil geochemistry
46 47
....... . . .......................... ................... . . ..................
49 49 50 50 52 52 54 54 54 55 55
............ . . ...... ....................................
................................................................ . .........
(viii)
Chapter 3:
57 58 58 Weathering 58 Mass movement 59 Fluvial processes 60 Karst processes . . 61 Aeolian processes 61 Marine processes .................................................................... 62 Glacial processes . 62 .. . 63 Processes inside the Eanh Volcanic processes 63 Tectonic processes 63
Geomorphic processes
. . .. . .. . . . . . . . ....... . . . ....... . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Processes at the Earth's surface
. . . . . . . ......................... . . ........... . . . ....
. . . . . . . . . . . . . . . .. . . . . . . . . . . .. . ..... . . . . . . . . . . . . .. . . . . . . . . . . . ...... ......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . • . . . . . . . . . . .. • . . . . . . . . . . . . .
. . ........... . ..... . . ........ . . . . . . . . . . ....................... . . . . . . .
. . . . . . . . . . . . . . . . . .. . . . ...........
. . . . . .. . . . . . . . . . . . . . . .
. . . . .........
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .
... . . . . . . . . .. . . . . . .......... . . . . . . . . . . . .
. . . . . . . . . . . . .. . . . . . . . . . . . . . .
.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
...
. . .. . . . . . . .
. . .. . . . . .. . ..... . . . . . ............ . . ....... . . . . . . . . . . ...... ....... . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . • . • . . . . . . . . . . • . • . • . . . . . . . . . . . . . . . . . . . . .
Geomorphic divisions of Victoria
.
.. . . . . ....... . . ......... . . . . . . .
. . . . . . . . .. . .
..
. . .. ...
64
Central Victorian Uplands 66 East Victorian Uplands 66 Upland plateaus 67 Dissected uplands (Midlands)....................................................... 68 Wellington Uplands ................................................................. 69 ......... . . . . . . . . . . .. . . . . . ..... . . . . . . . . . . .. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .... ................. . . . . . . . . . . . . . . . .. . . . . . . . . . .
. . . . .. . . . . . . . . . . . . . . . . . . ...... . . . . ............ . . . . . . . . . . . . . . . . . . . . . . .
West Victorian Uplands ..............................................................70
70 74 76 South Victorian Uplands . 76 Murray Basin Plains . . 76 Riverine Plain 76 Mallee Dunefield . 78 Wimmera Plain . 79 West Victorian Volcanic Plains 80 South Victorian Coastal Plains 85 Follet Plain . . . . 85 Pon Campbell Coastal Plain 86 Bellarine Peninsula and Moorabbin plains ........................................ 86 Coastal sand barriers 88 88 South Victorian Riverine Plains . . . . . .. . . . . . . . . . . ....... . . . . . . . . . . . . . . . . . . . . .. . . . . . . . ...............
Dissected uplands The Grampians Dissected tablelands
. . ......... . . . .. . . . .. . . . . . . . . . . . . . . . . . . . . . . . ...... . . . . . . . . . . . . . . ......
... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ . . . . . . . . . . . . . . . . . . . .. . . . ......... . . .. . . . . . . . . . . . . . .
. . . . . . . . .. .
.. . . . . . . . . . . . . . ............ . . . . . .
. . . . . . ........... . . . . . . . . . . . . . . . . . . . ..... . . . . . .
. .....
. . . . . . . . . . . . . . . . . . . ... . • . • . . ..... . • . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . ............ . . . . . .. . . . . . . . . . . .
. . . . . . . .. . . . . . . . . ....... . . . . . .. . . .
. . . . . . . . . . . . . . . . . . . . . . . ....... . . . . .
. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .
. . . ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.. ......... . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . ...... . . . . . . . . ...... . . . .. . . .
. . . . . . . . . . . ......
...
.. . . . . . . . . . .
.
. . . . . . . . . . . . . . ...... ....... . . . . . . . ..... ........ . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................. . . . . . ........ . . . . . . . . . . . . . . . . . . . .. . . .. . . . . ................. . . . . . .
The Victorian coast
............ . . . . . . . . . . . . . . . . . . . . .................... . . . . . . . . . ..
..
. . . ...
88
Coastal processes ..................................................................... 89 Coastal types 91 ...... . . . . . . ..................... . . . . . . . . . . . . . . . . . .......... . . . . ....... . . .
Chapter 4:
97 Major geological divisions of Australia
. . . . . . .. . . . . .. . . . . . ...... ..... . . . . . . ..... . .. . .
Tasmo" Fold Belt
98
99
... . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... . . . . . . . . . . . . .
99 99
Pre·Cambrian history of the Australian Cralon Archaean
....
. . . . ......
........... . . ................. . . . . . .. . . . . . . . . . . . . . . . . . .
Australian Craton
. . . . . . ... . . .
. . . .
..
. .. . . . . .
.
.......
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...
99
100 . 100 101 101 101 104 106 106 I14
... . . . . . . . . . . ..... . . ............... . . . . . . ......... . . . . . ..... . . .. . . . • . . . . . . .
Proterozoic Life in the Pre-Cambrian
. . . . . . . . . . . . . ..... . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction to the Phanerozoic Palaeozoic Era
..
..... . . . . . ................. . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . .. . . . . . ............ . . . . . . . ....... . . . . . . . .
Cambrian . Mapping Cambrian greenstones from the air Thefirst Victorian miners - Aborigines Ordovician . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . .. . . . . . . . . .
.
. . . .. . . . . . . . . . . . . . . . . . . . . . . .
.... . . . . . . . . . . .
.
. . . . . ........ . . . . .
. . . . . . . . . . . . ....... . . . . . . . . . . ......
. . . . . .. . . . ......... . . . . . . . . . .... . . . . . . . .
. . . . . . . . . . .. . . . . . . . . ... . . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . .... . . . . . . . . . . . . .
. .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .....
Silurian to Middle Devonian Late Devonian to Carboniferous Xenoliths . Late Carboniferous - Permian Glaciation and the greenhouse effect. Continental drift
. . . . . . . . . . . . . . ...... ....
...
.
....... . . .
..... . . . . . . . . . . . . . .... .......
. . . . . .. . . . . . . . . . . ....... . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .......... . . . . . . . . . . . . . . ..... . . . . . .. . . . . . . . . . . . . . . . . . .
. . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . .. . . .. . . ...... . . . . . .
Mesozoic Era
. . . . . . . . . . . . . . . . . . . . . . . ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............
Triassic and Jurassic . Early Cretaceous Dinosaur Cove Late Cretaceous . ...
. . . . . . . . . .. . . . . . .. . . . . .. . . . . . . . . . .
.
. . . . . . . . .. . . . . . . . . . . . . . . . . . . .
. . ..... . . ..... . . . . . . . . . ............. . . . . . . .. . . . . . . . . . . .. . . . . . . . .
. . . . . . . . . . . . . . . . . . ... . . . . ... . . . . . . . . . . ....... . . ....... . . . . . . . . . . . . . . . . . ....
Cainozoic Era
........
Teniary . Quaternary . . ..
.
. .....
.
. . . . . ...........
.
. . . .. . . . . . . .. . . . . . . . . .. . . . . .
..
. . . . . .. . . .
. . . . . . . . .. . . . . . . . . . . . .. . . . . .. . . ..... . . . ... . . . . . . . . . . .. . . . . . . .
.
. . .. . .
.......... . . . . . . .. . . . . ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . ...
. . . ................. . . . . . . . . . . . . .. . ....... . . . . . . . . .......... . . . . . . . . . . . . . . .
129
131 135 138 141 142 142 144 149 150 151 151 161
(ix)
Chapter
169
5: .
..................................
The concept of resources
Mineral resources . . . . . . .. Economic significance of rocks and minerals .
..
....
...
..
.....
. .
. . . . . .
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..
....
. .
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. ...
.........
...
. ..
.
.
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.
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.......
170 . 171 173 174 . 174 . . 175 . 175 .. 177 .. . 180 .. .. . 181 . . . 183 186 . 187 ...
...
.
...
....
.
Extracnon of rocks and minerals
......
.....
.......................................
Control of mining and quarrying
......
....
............
Construction materials
.
..
...................................
. . .
..........................
.
. .
.......
...
...............
Location of the construction material industry Hard rock quarry productS .. .. Case history: Boral basolt quarry - Bundoora ..
......
..................
.
..
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.
............
.......................
.
................. .....
...
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...............
.
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..
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...
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. .....
Fuel minerals
....
.
..
...
....
.......
Coal . . Petroleum . ........ .
..
.......
...
.....
..
.
. .
.
....
....................
..
.
.
...
... ......
.........
.....
.
. .
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..
...
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......
..
189
.....
192
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192 197
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..
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...
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.
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..
.
.
...
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..
..
.... . . . . ...............
.....................................
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............
Metallic minerals Gold
.
...
............
...
..
..... ...................
...
.............
..
...........
........
.
.............................
Gravel . . . . . .. Sand . . . . .. .. . Clay ... . .. . . .. .. Case history: Hallam day pits . . . Limestone . . .. . . .. Case history: Limestone quarry and cemen t manufacturing plant, near Geelong . . . . .. ....
169
..................
......................................
..............................................................................
.
.....
Case history: Magdala gold mine - Stowell Joint Venrure
.
....
...... . . . . . . .
....... ................... . . ...............................................
Base metals Case history: Benambra - a mine jor tile jurure?
204 204 212 216 217 220 220 223 224 224 225 225 226 226
.............................
Heavy mineral sands . Case history: Heavy mineral sands - WIM 150 project Tin ........
......................... ..............
.
...............
...... ................
...... ............... . . . . ................................... .......... ............... .................................................. ..................................
Iron Bauxite
.
.......... . . . . . . . . ......... .........................
I ndustrial minerals and rocks Salt
.
............................. .....
.....
. . .
...............
Gemstones and specimen minerals
Chapter
.........................
............ . . .. . . ...........
Gypsum . . . . Diatomite or diatomaceous earth ........
...........................
.
............
.
..
..
....
..
.
..................
.......................
...... .............. ...................
.
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. . . . . .. . . . . .................
.
....... ...............
226
6:
229 Hydrology . .. .
..
Water cycle . .
.
....
....................................
.
...
.
........
....
. .. .
Surface water . .
...............
......................
.
.
................
Selection of sites for storage darns
Groundwater
.
.........
.. .
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.
.... ..
...
..
.
................ .......
...
..
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.
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.
..
.
.
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...
Water quality and use .
.
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.
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..
...
...
...
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.
.......
232 233
.. .. . 233 . ... .. ... .234 .... .. ... ... 234 . . 236 . . . 236
.......
...
231
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.........
...
...
...
...
..
..
...
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... ........
.......
237 . . .. 238 Groundwater versus surface water for major developmenl. .................. 238 Water in Australia . . . . . . 239 The water resources of Victoria 239 .
..
............
. .
.....
.
....
230
...
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..
............................................ .....
.
.
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.............................................
.......
.
......
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. ...
.........
...........
. 230
.............
.
. .. ..
...
.......
Porosity . . . Permeability Aquifers . Aquifer materials . . . Selection of sites for groundwater bores .
.
............
........ . . . . . ...........
.
...
....
.
.
.....
.
....
.............. ............... .......................
Treatment of domestic water supplies ..........
...
......
....
.........................
............
......
..
.......
......................
........
......................................................
. .
.........
The climate of Victoria Water balance for Victoria Surface water resources .
..
.
........... ................
Groundwater provinces of Victoria
Water use in Victoria
.
...
...
..
.....
.
...............................................
........................
Surface water supply systems Groundwater supply . .
239 240 .. , ...................... 241 242 . 250
........................................
.........................................................
.
............................
.............
.
... . . . . ...... ........... ................
............
.
........ . . . . .. . . .. . . . . .........
Urban and industrial use of water Irrigation . Stock and farm domestic use Electricity generation Mineral water . . ................ . . . .......
.
250 . 252 252 . 254 255 256 256
.............
. . ..............
........ . . . . . . . . . . ..............................
.............................
.............................
.
.
.................
.
.......
..
....... . . . . . . . . .
...............................................................
...........
...........
......................... . . .....................
Environmental problems associated with water in Victoria Pollution of water supplies The salinity problem
..
........ . . . ..................
.
..................
The future of water in Victoria
......... . . . . . .....
.
.....................
...
. . . . . . . . . . ..................... ..............
...................
.
..................................
257 257 257 261
Surface water potential . . . 261 Groundwater potential. .............................................................261 ...........................
.................
..............
(xl
Chapter 7:
E G
E �
IRONMENTAL GEOLOGY
Geology and planning Geological hazards
265
. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .
................... . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . ........................
Earthquakes Case history: Balliang Earthquake Landslides Case history: The Lake Elizabeth Landslide Case history: The Windy Point Rockslide
. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . ..
....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .... . . . . . ................ . . ...... . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........
Swelling clay soils
278 282
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
Ground subsidence
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Subsidence over old pits
265 266 266 269 271 275 277
282 283 284 284 285
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . .
...........................................
Case history: Yarraville sinking village Subsidence over old underground mines Natural subsidence
. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .
........................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .
Geology and engineering
.......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .
. . . . . . . . . . . . . .. . . . . . . ....... . . . .. . . .............. . . . . . . . . . . . . . . . . . . .
Site investigation Building foundations Case history: Westgate Bridge foundations Tunnels Case history: Melbourne Underground Rail Loop
. . . . . . . . . . . . . . . . .. . . . . . . ........... . . . . . . . . . . . . . . . . . . . . . . . ......
285 287 288 290 293 295 297 298
. . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . .
Water storage dams Case history: Dartmouth Dam
Geology and Ihe environmenl Surface water supply
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..................
. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . .
. . . . . . . . . . . . . . . . . . . .................................. . . . . . . . . . . . . . . . . . . . . . . ................. . . . . . . . . . . . . . . . . . ......... . . . . . . . . . .
Coastal development Case history: Beach restoration - Mentone, 1977 Domestic waste disposal
. . . . .. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .. . . . . ........... . . . . . . . . . . . . . . . . . . . . . . . . . . . .
INDEX:
299 299 300 302
..............................................................................................................................
305
(xi)
FOREWORD The year, 1834, saw the arrival of the first Europeans to settle permanently on the south-eastern Australian mainland. They opened up the land for farming and laid the foundations for what was to become later the State of Victoria. With reliable water supplies in the south and moderately fertile soils, especially on the western volcanic plains, the region gradually developed as a farming colony. Suddenly, less than twenty years later, alluvial gold was found at shallow depths at Warrandyte, east of Melbourne and soon afterwards, in much greater quantities, at Ballarat, Castlemaine, Bendigo and many other places in central Victoria. The era of the great Victorian gold rushes had begun. In ten years, the population of Victoria grew eight-fold, as people flooded in from many parts of the world. Many miners - perhaps most - failed to make their fortunes from gold. Nevertheless, overall the wealth created by gold mining from the 1850s onwards had a profound effect on the development of Victoria and its economy. Indeed this influence extend ed to the rest of Australia. For a period in the nineteenth century, Australia was the wealthiest per capita country in the world. For a few years, it was also the largest gold producer. In 1851, Victoria became a self-governing colony and in 1855, government by a parliament of twO houses was introduced. Using the income derived from gold, the early Governments established many institutions, some of a type that were only found in much older cities and countries around the world. It was not surprising that some of the new institutions related to geology. As the easily-won deposits of alluvial gold were worked out, it became important to find the source of this gold in the underlying bedrock. A Geological Survey of Victoria was formed and charged with the responsibility of making geological maps, first of the goldfields, and later of the whole State. It was the work of the Geological Survey, that eventually lead to the discovery of the vast brown coal deposits, which provide our modern State with most of its energy. Today the Geological Survey remains an important Government service, as modem society requires an increasingly detailed knowledge of the geological materials of the State. It is not only essential to know where the most valuable minerals, such as gold, oil, coal and so on are located . More mundane things such as crushed rock, limestone, sand and clay must be also found and protected, because they are needed in large quantities for the construction of our modern office buildings, roads, houses, power houses, bridges, dams, churches, sports stadiums and so on. The members of the first Geological Survey came from Great Britain mainly. But the early Government soon took action to train colonial Victorians in the science of geology. The University of Melbourne was established in 1853 and one of the first four schools to be set up was that of Natural Science. Its professor, Frederick McCoy, had a geological background. McCoy was also involved in spreading a knowledge of geology to a wider public, as he became the Director of the first museum in Victoria. The Museum grew to become another important centre of geological investigations. It also became a place where generations of Victorians enjoyed free displays of rocks, minerals, fossils and working models of mining machinery. During the gold mining era, a majority of Victorians lived on the goldfields or in the service centres of Melbourne and Geelong. The miners and their families resided close to their employment - the mines. This is clearly shown in the panorama of part of the Bendigo Goldfield given in the frontispiece of this book. Because the lives of so man y people were linked to gold in some way, there was widespread interest in rocks, minerals and other aspects of geology. In some goldfields towns, education in geology and mining was available at Schools of Mines, that later became centres of higher education. At that time there were also various societies for people who were interested in the natural sciences and in enjoying field excursions to sites of geological interest. Passing from the nineteenth to the twentieth century, community interest in geology declined as most wage-earners became employed in new manufacturing industries. However, there is certainly still a need for professional geologists and not only the University of Melbourne, but various other universities and colleges now provide courses in geology. But there is relatively little education in geology at school
(xii)
level and few opportunities for adults to learn about the subject and to adopt it as a hobby. Over fifty years ago, the late E. Sherbon Hills, formerly Professor of Geology at the University of Melbourne, wrote a book entitled "Physiography of Victoria", which was reprinted many times. It was primarily used by geography students, but it also became an invaluable source of information for large numbers of people interested in the geological features of Victoria. Our present book, "lntroducing Victorian Geology", is an attempt by the Victorian Division of the Geological Society of Australia to serve a similar dual purpose. It is hoped that it will revive interest in geology, both in schools and in the population at large, by providing a modern interpretation of the geological development of Victoria. It also contains chapters dealing with various aspects o f Victorian geology that are important to human existence - our soils, water supplies and economic minerals. In addition it explains how geological studies are important in construction work and in solving environmental problems. The study of geology is like the study of human history. Geology aims to reconstruct the sequence in which past natural events took place and to explain how, why and when they happened. The Earth's history began about 4600 million years ago, when the Sun and its planets began to form from a cloud o f gases and dust. Some of the oldest rocks to be formed during this history are to be found in Western Australia . By contrast, Victoria's geological history can only be traced back a little over 550 million years ago. At that time, Victoria formed the floor of a deep ocean. Today, in narrow belts across Victoria, we can see rocks that were erupted by volcanoes on that deep ocean floor. There are also rocks containing trilobites and hydroids, some of the earliest creatures to have lived in the seas above ancient Victoria. Starting some 500 million years ago, parts of Victoria became dry land. At first, this land was devoid o f life. But slowly plants became established and the first amphibious animals emerged from the sea to spend part of their lives on the nearby land. Later they were joined by reptiles, birds, mammals, a profusion of plants and insects, and finally by human beings. During its long geological history, Victoria experienced some long stable periods. Then, for tens to hundreds of millions of years, either sediments accumulated slowly on the ocean floor or the dry land was slowly worn away by the forces of rivers, winds and seas. However, there were also shorter periods of spectacular activity. There was a period when the State was covered with glaciers and icesheets, and other times when the land was shaken by violent volcanic activity. Perhaps the most remarkable geological events were those called ·orogenies'. Over periods of a few million years, forces within the Earth squeezed and crumpled thick accumulations of sediments and volcanic rocks, and lifted them above sea-level to form mountain ranges. At these times, large masses of granite solidified from a viscous melt in the roots of the mountains. The landscape we see around us in Victoria today is the result of all these past geological events. By travelling through the State, we can see the rocks that formed on ocean floors, those that were thrown out of volcanoes and those deposited by glaciers, as well as granites that once formed far below the surface. We trust Introducting Victorian Geology inspires its readers to go out into the field and see our State's geological features first hand.
Basic Concepts in Geology
Chapter 1
BASI C CONCEPTS I N GEOLOGY
Figure 1-1 Geological map of the suburbs north-east of the centre of the city of Melbourne. (From Melbourne and Suburbs geological sheet, scale 1 : 3 1 680, 1959, Geological Survey of Victoria).
B
COLLDi'ilG!"OOlllm;{;;;f"', Collingwood
Victoria
'is "0 o I
Street
c
YARRA
OJenf.rr�
I)
QUATERNARY
TERTtARY
{
Land use
[KJ m � �
[!]
SILURIAN
�- s
Kilometres
G
�
·c .
Alluvial flats. mud flats. Basalts Marine and non-marine sands, clays. ferruginous sandstones and gravels Mudstones. siltstones and sandstones Disused
pit
- Sand. Gravel. Ctay
The most enjoyable way LO study geology is to go into the country beyond the buildings and roads of the cities and LO look at the various eanh materials that occur underfoot. Crops and pastures and other vegetation cover much of the land but there are still plenty of places where rocks and soils can be inspected. In coastal areas, cli ffs are another place where there are large exposures of bare rocks. Even in large citie earth materials can be found in places such as road cuttings, the banks of streams and excavations made at construction sites. People interested in finding out what makes up Mount Buffalo or what lies below the Wimmera plains may start their investigations by buying a geological map from a Government bookshop. They will hope that the map will tell them where
2
Ch apter 1
to find different kinds of rock and that it will explain the geology of various places. But although it may appear that the different colours on the map mean different kinds of geology, the reader probably will still be bewildered by aU the unfamiliar names. On a geological map of the Melbourne district words such as Devonian, Balcombian, rhyodacite, Deep Creek Formation and so on will mean nothing to the reader. Nevertheless a stan in studying geology can be made by going to several areas represented by different colours on the map and considering what differences Ihere are between the areas - differences in natural features, in the shape of the land and in ways in which the land is used. Figure 1-1 has been copied from pan of a geological map of Melbourne and its surrounding suburbs that was published in 1959 by the Geological Survey of Victoria. The figure covers some of the north-eastern suburbs. There are four areas lhat were shown in different colours on the original map. In Figure I - I , these areas are represented by different shadings and leuers. A few of the features of each area are discussed below:
Figur. \-2 Area A on the map. at Abbol5ford.
There is a large market garden i n front of the institution i n the background. The garden is established on rich loamy soils formed on material deposited by the Yarra River ( i n the foregr ound) when it nooded over its banks in the past. The high ground on the right has a shallow topsoil over clay and it supports eucalyptus woodland. (Photograph by W. Schleiger). .
Figure \-3 Area B on the map.
at Co lli ngwood .
The nat land i the site of large fact ories and high rise buildings. Factories were fir t built in this area bec.ause the nearby Yarra Ri,er provided a reliable ource of water and a layer of hard rock at shallow depth offered strong foundation for big buildings. As in Figure 1 -2 . the land on the near side of the ri' er is hilly and more uited to parkland and lower density housing. (Photograph by . \V. Schleiger).
Figure \-4 Area D on I h e map. Dighl Falls on the Varra al Iud Ie) Park.
There are lilted la,er of 3 hard rock i n Ihe rh or ank. The original falls were compo ed of Ihe �me rock. Further expo ures of 'iii milar rocks are found in cuttings in the hills around 5IUdley Park. (Photograph by G.W. Quick).
b
Basic Concepts in Geology
3
Area A (green on the published map)
Shape the land is very flal. Use this area is used widely for recreational purposes (e.g. golf courses, spons -
-
ovals). There are not many houses. mainly deep loams. I n the past, thi son of land was used for growing vegetables and for small farms. Similar land today along the Maribyrnong River is still used for market gardens.
Sail
-
water is available at shallow depth had to be carried from the Yarra River.
Wafer
-
in weUs. In earlier days most water
Area B (pink on the published map)
Shape also flat, but a little higher than area A land. Use dense housing and many factories, especially heavy industries. Soil heavy black clayey soils, which are very sticky when wel. They are fenile -
-
-
but hard to cultivate. Brick houses sometimes develop bad cracks in the walls, because the soils shrink and swell with seasonal changes in the weather.
Wafer Mining
-
good supplies of water can be obtained from boreholes to shallow depths.
there have been some quarries in this area in a hard dark grey rock, familiarly called 'bluesfone'. The rock is crushed to give road metal. Blocks of bluestone can be seen in old gutters, bridges and buildings. In similar terrain in the western suburbs, bluestone quarries are till being worked. -
Area C (yellow on the published map)
these areas form flat tops to some of the hilly suburbs on the map. Beyond the boundary of the map, area C-type land forms most of the south-eastern suburbs, where the terrain is gently rolling.
Shape
-
rhis is also mainly residential land with only light industries. Many years ago, the land wa used extensively by market gardeners. There are many golf courses laid out on this land.
Use
-
gardening is easier on this land than anywhere else in Melbourne because the sandy surfaces are easy to cultivate.
Soil
-
rain oaks easily into the sandy soils, so water can be obtained from boreholes. I n time of drought and water restrictions, some residents use bores to obtain water for their garden .
Wafer
-
the map shows the site of past sand pits. In similar land to the south in the Dingley - Springvale area, sand-mining is still an imponant industry.
Mining
-
Area D (grey on the published map)
thi strongly contrasts with the fir t two areas: the land is a mixture of hills and valleys.
Shape
-
some of the more affluent suburbs are located on this land. Many hou es have attractive views of distant mountains to the east . It is less suitable for heavy industry.
Use
-
there i u ually a shallow topsoil over blocky clay. The soil dig and not very fenile.
Soil
-
are hard to
early residents would have found it difficult to find useful supplies of water in wells or boreholes. Rainwater would have been the only assured source of water in the higher parts.
Wafer
-
the map show there have been a few clay pir in this area. Further to the ea t on imilar land, clay excavations were more common. The material has been used for bricks and house tiles. In addition, Melbourne's only underground metal mine were on this class of land. Many year ago, gold was mined at Warrand)�e and antimony at Ringwood.
Mining
-
So what do these di fferences in land appearance and land use tell people about geology? They show there are fundamental differences between the nature of the four divisions on the geological map. These differences can be t raced back to differences in the rocks and loose eanh material that underlie Melbourne. Area A, for example, is made up of alluvium deposited by nearby river when they overflowed their banks in times of flood. The dark grey rock, best seen in the quarries of area B. solidified from lava that once flowed towards the sea from volcanoes just north o f Melbourne. These volcanoes are now extinct . In the Organ Pipes National Park near Sydenham. a magnificent exposure of this volcanic rock can be seen. Hard rocks are not so easy to find in area C. In the sand pit there are clearly
4
Chapter '
deep layers of loose sand. However, there is also partly consolidated yellow and brown sandy clay in railway cuttings close to Melbourne, especially those along Ihe Dandenong and Sandringham lines. If area C is followed t o the coast, soft sandy rocks are found in the cliffs at Sandringham and Black Rock, while hard brown sandstones form low rocky ledges at the beach at Brighton. In the hilly area D suburbs, the underlying rocks can be seen in many road cuttings, e.g. at Studley Park. They are called sandstones and mudstones and typicaUy they occur in layers sloping to the east or west . The brick clays are simply decomposed mudstones. Geological maps are available for all parts of Victoria. By visiting several areas of various colours, it will soon become clear that each colour has its own distinctive appearance on the ground and is used in a particular way. In the country, potatoes may be grown mainly on land of one colour only, fruit orchards may be on another colour while other colours are left covered by forests. The different land uses all relate in some way to differences in the underlying rocks and soils, that is to the geo logy.
Composition an structure of the Earth
Our planet Earth has roughly the shape of a sphere with an average diameter of 1 2 740 kilometres. [n their everyday lives, people are concerned only with the outernlost skin of this sphere and the atmosphere above it. There are three important zones: • •
•
the gaseous part or atmosphere, which is a mixture of oxygen, nitrogen, water vapour and other minor gases; the liquid part, consisting of the waters of the oceans, lakes and streams; the solid part, consisting of rocks, ice and loose materials, such as soils and sediments. These form the dry land and the floors of the oceans and lakes.
Apart from the heat and light provided by the Sun's energy, Ihese three zones supply everything that is necessary for human existence - oxygen, water, soils, food and the raw materials for manufactured goods. Water and oxygen circulate through Ihe three zones and to some extent their molecules help 10 build up the substances in the soils and rocks. However, most of the substances from which soils and rocks are made came originally from inside the Eanh. [t is therefore appropriate to continue the study of geology by considering briefly the composition and structure of the Eanh as a whole. People have only seen directly the materials that are presenl at or very close to the Earth's surface. They have reached depths o f a little over four kilometres in the deepest mines and less than that in the deepest descents into the oceans by diving vessels. The deepest samples of rock have come from boreholes, which penetrated up to twelve kilometres below the surface. There is, however, one natural source of much deeper Eanh materials. Many lavas and gases, that emanate from volcanoes, are thought to have come from depths as great as 200 kilometres. Figure I-S A small specimen of a meteorite found al Cranboume, south-east of Melbourne. The first meteorite found in Victoria was in 1854, at Cranbourne. It was composed of coarsely-crystalline metallic iron. I t caused considerable excitement in
Europe 31 the lime. because it was the largest meteorite ever discovered. Since then, eleven similar fragment have been found along a flight path from Beacons field 10 Poareedale. Several arc preserved in the Museum of Victoria.
Other possible information on rocks deep within the Earth may come from the study of lIIeleoriles (Figure 1-5). These are extra-terrestrial rocks that come from Space, travel through the atmosphere and finally collide with lhe Earth's surface. Meteorite are believed to be fragments of small planet-like bodies t hat formed at the same time as Earth and Ihe other planets of the Solar System. However, it is clear that people have never seen and are never likely to see the great bulk of the mat erial making up the Earth. Nevertheless, scientists are able to make in ferences about the interior of the Earth from stuciies of some of its physical properties - especially its den ity, its magnetic field and the behaviour of seismic waves generated by earthquakes.
Basic Concepls in Geology
5
It is generally believed that the Earth is made up of three major concentric zones (Figure 1-6). The zones are called the crust, the mantle and the core. The materials in each zone have differem compositions and differem properties. There appear to be fairly sharp breaks between these main zones. Each zone is further subdivided into !WO. The heaviest materials are located in the innermost zone and the lightest at the surface. Brief deLails of each zone are given below.
Inner core
-
composed of solid iron and nickel at temperatures up to 50000C . Some meteorites have a similar composition.
Outer core
-
composed mainly of molten iron and some nickel at temperatures above 2000' C.
Inner mantle
-
is solid and composed mainly of silicates of iron and magnesium. This contains by far the largest part of the Earth's material.
Outer rrIilntle
-
has a similar composition to the inner mantle but it is plastic, i.e. capable of slow movement. The layer is about 600 kilometres thick. It is an important layer because it is responsible for three major geological processes which are disucssed later - eanhquakes, some volcanic activity and the bending and breaking of rocks in the overlying crust.
Crust
-
consists of two parts. The continental crust underlies the continents. It consists of the large landmasses above the level of the sea and the submerged margins of the continents called continental shelves. The outer edges of the continental shelves, at a depth of about 120 metres, slope down steeply to the floors of the deep oceans (Figure 1-63). The oceanic crust lies below the floors of the deep oceans. It consists of rocks that are heavier (denser) than those of the continemal crust. The cominental crust is commonly about 35 kilometres thick, but its thickness increases to 60 kilometres beneath some mountain ranges. The oceanic crust is only 5 to 10 kilometres thick on average.
Figure Hi The int.ernal struclure of the Earth. The structure and composition of lhe Earth's interior have been deduced from studies of irs density, irs magnetic field and the way it transmits eismic (earthquake) waves. In addition, volcanic eruptions supply samples of materials that occur 10 depths of up 10 200 kilometres.
O�
SOUTH P
EJ
I:';':?�
Crusl OUler Mantle (plastIC) Inner Manlle {SOlid}
D D
Outer Core ( liQUid) Inner Core (solid)
6
Chapter 1
The next sections of this chapter deal with the nature and origin o f the materials that make up the Earth's continental crust - soils, minerals and rocks. Before moving on to these topics, however, a special earth-material called magma must be considered. This is the parent material of rocks and minerals.
MAGMA Magma is a hot fluid mixture of chemical substances. It includes both solid and molten substances as well as water and gases in solution. Magma forms from the melting of materials in the upper mantle and the lower crust. It is lighter and more mobile than solid rock, so it tends to rise through the Earth's crust. The temperature of magma varies between 500'C and 1400'C. As it nears the surface, magma gradually cools. Some of it solidifies at depth, while the remainder forces its way up through the crust and eventually is ejected from volcanoes as lava and hot gases. Magma largely falls into two classes: I . Basaltic magma - this is dark, very hot (900' 1 400'C) and relatively fluid. It moves from the upper mantle to the surface. There it appears through volcanoes on the land and sea-floor and through long cracks, especially across the deepest parts of some oceans. The oceanic crust is derived from basaltic magma. 2. Granitic magma this is lighter in colour, more viscous and cooler (below gOO'C) than basaltic magmas. It cools to form rocks that are only found on continents and off-shore islands. Granitic magma forms when solid parts of the crust are partly or completely melted by close contact with basaltic magma from the mantle. Further discussion on magmas is given later in this chapter. -
-
Soils
Most of the Earth's crust is made up of hard rock. Some of these rocks can be seen at the surface, where they are called OlitcrOpS. Large areas of outcrops are mostly found in very arid, very high or very rugged regions or those that have been scoured by sheets of moving ice. Elsewhere, even on the floors of the oceans, hard rocks are covered by a variety of soft, unconsolidated materials. These can mostly be called sediments. However there is one special layer of loose material, which is vital to all living things that are found on dry land. This layer is known as soil.
NATURE OF SOILS Soils are rarely more than one or two metres deep. People depend on them, however, to obtain most of their food. This is supplied either directly through crops or indirectly through pastures that support animals, the source of meat and dairy products. The timber and natural fibre (wool, colton, flax, etc.) industries are also totally dependant on soil. It is the environment in which most plants and many other living organisms (including bacteria, insects and burrowing animals) live. Soils are dynamic resources because they have continual gains and losses of materials to and from the air, streams, oceans, flora and fauna. They contain plant debris and roots, small fauna of many kinds, air, water, organic matter (humus) and mineral matter. Soils are produced from two kinds of parent materials residual and -
transported. I . Residual materials are underlying solid rocks, which are broken down very slowly to form soils. In areas of temperate climate. such as in Victoria, probably less than one millimetre o f soil is produced every ten years. To develop deep soils in this way, natural erosion must clearly be almost non-existent. Deep residual soils are therefo re mainly found in fairly flat upland areas. For example, there are orne high plateaus in Victoria, where there is little erosion by streams or hillwash. 2 . Transported materials are those that are carried into an area by either water (called allu vium) or wind (called aeolian materia/) or that slip downhill under gravity (called collu vium). These materials are derived from the erosion of soils that originated elsewhere. Deep soils on transported materials can develop relatively quickly over a period of a few hundred years. In many districts, young soils on river alluvium are the most fertile. They contain freshly-decomposed rock particles. There has been insu flicient time for rain to dissolve out the chemical substances that provide plants with nourishment (i.e. plant nutrients) . Examples in Victoria are the river flats near Orbost and Lindenow in Gippsland, which are prized for dairy farming and vegetable growing. There is a special kind of transported soil material that is common in western Victoria. This wa� thrown out of volcanoes as recently as 1 0 000 years ago. The young soils on volcanic material are especially fertile. They are used intensively to produce onions, potatoes and other crops and pastures.
Basic Concepts in Geology
7
Soils are composed maioly of mineral molter. This is made up of sand grains, tiny clay particles and sill particles of intermediate size. They may also contain gravel composed of pieces of local rock or of chemical deposits such as calcium carbonate, magnesium carbonate or iron oxide. In drier regions, there may also be sodium chloride (common salt) and calcium sulfate (gypsum) in soils. Soils also contain humlls. This is mainly decomposed plant matter, but it also contains substances derived from animals. Humus is usually concentrated in the top few centimetres, giving the topsoil a dark colour. Humus supplies much of the fertility in soils and makes them easier to cultivate. There are vast numbers of small organisms in soils. These range from earthworms and small insects to microscopic plants, such as bacteria, fungi and algae. These organisms and also larger plants play an important role in the weathering of the parent rocks and sediments that produce soils. For example, plants liberate humic acids, which then dissolve in soil water. These acids promote the breakdown of minerals into soil particles, such as sand and clay, and also liberate plant nutrients, such as calcium and potassium. The depth, nature and appearance of soils vary greatly from place to place, even within the one district. I n part, these features depend on the types of rocks from which the soils were formed. Other important factors are climate (especially rainfall and temperature), drainage conditions, the slope of the land and the age of the soils. Some soils in Victoria are one million or more years old. These materials are quite distinctive, having been exposed t o climates very different from present day ones. For example, tropical conditions several million years ago led to the production of layers of ironstone and multicoloured clays - a process called lalerisaliol1.
THE SOIL PROFILE Most masses of rocks are uniform in appearance from one part to another. By contrast, soils are divided into layers called horizons. There are differences in the colour, compo ilion and other properties of the different layers. A vertical section of t he various horizons from the surface down to underlying decomposed rock is called a soil profile (Figure 1-7). These profiles can often be seen in road and railway cultings, trenches and other excavations. The main horizons are referred t o by leLLers of the alphabet as shown in Figure 1 -7 . Figure 1-7 The A, B, and soil profile.
Each horizon beneath it.
Shallow and deep ,. rooled plants
C horizons of a
grades
r-"-'''P�..::zi:Sij
into the one
�;�kn:faanTh����ll roots and organisms e 9 worms lung. bactena
Pale loam Fe .... rools Hard when dry seasonallv ·....aterloggea
B Ume and sailS accumul.ue In dner regions - Mallee Wtmmera N Plams
c
Wea thered parenl malenal Crock or unconsolidated sedlmenU
Most of the humus occurs in the A horizon, so it is the darkest layer. Percolating rainwater usually carries finely-divided clay and iron oxides out of this horizon down to the underlying B horizon. The B horizon thus becomes rich in clay and may be brightly coloured by iron oxides. Given thousands of years, this process may leave a very pale A, horizon between the uppermost humus-rich A, horizon and the underlying B horizon. This type of A, horizon �nd the lOp of the B horizon may also comain iron oxide nodules (called buckshol gravel) caused by alternate wetting and drying. In some reg ions there are chemically-precipitated layers of calcium carbonate or silica in the B horizon. I f these layers are hard, they are called hardpans.
PROPERTIES OF SOILS The properties mOst often used 10 identify soils are colour, tcxture. struct ure and consistence. Colour explain ome things about the behaviour of soils. Dark layers, for example, denote high humus content . Red colours indicate good drainage. On the other hand, alternate patches of brown and grey are an indication of waterlogging. This pre\ ents plalll roOtS and organisms, such as worms, obtaining air.
8
Chapter 1
Texture refers t o the proportions o f the particles, sand, silt, clay and gravel, that are present in a soil. For example, sands, containing more than 90"10 of sand-sized particles by weight, are called '/ighf soils. The heaviest soils, the clays, contain more than 50% clay. These particles are defined by the following ranges of grain sizes:
Particle
f
gravel sand silt clay
.,, .,
Diameter (mill imetres)
> 2 .02 - 2 .002 - .02 < .002
Texture affecls how easy it is to cultivate a soil, how easily water drains through and the amount of moisture available to plants. Soil texture therefore has an important influence on plant growth. The ideal texture is considered to be a loam, which is a mixture of sand, silt and clay particles in roughly equal quantities. Grains of various sizes are usually present in soils. Consequently it is possible to have such textures as silty loam, sandy clay, etc. Structure is the term used to describe how horizons break up into aggregates. Terms such as platy, columnar, blocky and granular are used. Soils with no observable aggregates (e.g. most sands) are termed 'single grain' when loose or 'massive' when coherent. Structure has a big influence on the ease of digging, water penetration and water retention. Granular material is crumbly and so is the easiest to cultivate; it also drains well. Consistence refers to the resistance of a soil to breaking. Terms such as 'soFt', 'hard' and 'friable' are used. Soft and friable soils are easy to cultivate and they do not hamper the spread of roots. Soils are only hard when dry. Some soils, particularly in northern Victoria, are so hard in summer that they cannot be dug with a spade. This greatly limits their productivity under crops and pastures. Major differences in soil profiles provide the basis for classifying soils into different groups. Smaller differences lead to a subdivision into soil series and types. These form the basis for mapping and classifying soils in a district. Victorian State Government departments, the CSI RO and some Universities have pubUshed soil maps of many parts of Victoria. Descriptions of the main soil groups in Victoria and problems associated with soils are given in Chapter 2.
M i nerals
I t has been shown that people could not survive without air, water and soils. However, to reach Out beyond a mere subsistence way of life, people have to use a large variety of other substances. These other resources come from the rocks of the Earth's crust. Some rocks are used in the form in which they are found in the ground. For example, buildings can be constructed from blocks of hard rock. For many uses, however, it is not rocks as a whole, but the individual grains within them, that provide people with useful substances. These grains are known as minerals. Nearly all rocks are mixtu res of minerals. A mineral is a 'naturally occurring chemical compound with a definite chemical composition and an orderly internal arrangement of its aloms'. This means: I . Each mineral has a fixed chemical composition and so it can be represented by
a chemical formula. A few minerals are chemical elements, for example: • diamond - carbon (C) • gold (Au). Some are oxides or salts with simple formu lae, for exam ple: • calcite - calcium carbonate (CaCO,); • barite - barium sulfate (BaSO.); • rutile - titanium oxide (TiO,). Most minerals, however, including some common species, have complex formulae containing four or more elements.
Basic Concepts in Geology
9
2. Each m ineral has a distinctive external crystalline shape, which reflects the orderly
internal arrangements of the atoms. Crystals are solid grains with distinctive geometric shapes, consisting of flat faces meet ing at sharp edges. Some crystal shapes can be seen with the naked eye, but most are only visible under a microscope (Figures 1 -8 to 1 - 1 0).
Figure 1-8 A cluster of needle-like crystals of the mineral aragonite (CaCO ,).
Each long crystal has six faces. The longest crystal is 4.5 centimetres. The small crystals in the background are analcime, a mineral of the zeolite family. These specimens were found at Kennon Head on Phillip Island. (photograph by J. Leach).
Figure 1-9 A perfectly-shaped crystal of the zeolite mineral, analcime, photographed using a scanning electron microscope.
This crystal is 0.5 millimetres across. Each face is nat and has four sides: one pair of sides is longer than the other. This specimen was found in a quarry at Bundoora, nonh of Melbourne.
Figure 1-10 Roughly rectangular- haped crystals of plagioclase feldspar seen under a microscope at 25 lim es magnification.
Both large and small feldspar crystals are present. (photograph by Gw. Quick).
'0
Chapter '
FORMATION OF MINERALS Most common minerals are produced from the crystallisation of magma on or below the Earth's surface. Within the Earth's crust, they are usually chemicaUy stable and remain unchanged over long periods. Near the surface, however, many minerals slowly react with water and oxygen to form new species. This process is called weachering. New minerals may also be formed when older minerals are transferred t o zones o f higher temperature and/or pressure by movements in the Earth's crust. Water plays a very important role in the formation of minerals. Without water the Earth would only have about 100 minerals. This is the number found on the Moon, where there has never been any water.
CHEMISTRY OF MINERALS Minerals can only be made up of one or more of the 90 or so naturaUy-occurring elements of the Periodic Table. However, most of these elements are rare in the Earth's crust. Most minerals are formed by combinations of two or more of only thirteen elements. Surprisingly many useful elements are not in this group, e.g. copper (Cu), sulfur (S), and zinc (Zn). Over 99070 of the Earth's crust is made up of oxygen (0), silicon (Si), aluminium (AI), iron (Fe), calcium (Ca), sodium (Na), potassium (K) and magnesium (Mg). Another 0.5% consists of titanium (Ti), hydrogen (H), phosphorus (P) and manganese (Mn). Carbon (C) is also locally abundant in rocks derived from living organisms, such as coal and limestone (mainly calcium carbonate).
figure \-\ \ The silicon-oxygen tetrahedron of atoms thai forms the basic building block of most rock forming m inerals.
The small silicon atom fits into the space between four oxygen atoms. the lanef are arranged in (he shape of a pyramid.
I MPORTANT M I N ERALS There are nearly 3500 recorded minerals on Eart h and new ones are found every year. Occasionally new minerals have been found in Victoria. Maldonice, a gold bismuth compound (Au,Bi) was named after a gold mining town in central Victoria, where the mineral was first recognised in the nineteenth century. A recently discovered mineral in Victoria, ulrichice, a copper calcium uranium phosphate, was named after an early geologist in Victoria, George Ulrich, who discovered maldonite. Most minerals are only of interest to collectors or specialist mineralogists. Less than two hundred minerals are either common or important for most people in their daily lives. These fall into three groups: •
•
•
rock-forming minerals accessory minerals economic mineral .
Rock-forming minerals As the name implies, rocks are mainly made up of one or more of these minerals. The basic framework of most rock-forming minerals is made up of atoms of oxygen and silicon. Each fundamental building block consists of four oxygen atoms equaUy spaced around a central silicon atom (Figure I-I I). These units, called cetrahedrons, are linked and stacked and combined with other atoms in many different ways to give minerals classified chemically as silicates. Aluminium atoms are of similar size to silicon atoms. In some rock-forming mineral groups one or more of the silicon atoms are replaced by aluminium atoms, giving aluminosilicates. figure
1-12
Major rock-forming silicate groups.
I . quartz
2. feldspar
3. 4.
5.
6. 7.
(a) orthoc lase (b) p lagioclase m icas pyroxenes amphiboles oliv ine c lays
There are seven major groups of rock-forming silicate minerals; they are listed in Figure 1-12. The first group consists of only one mineral, quartz. In aU the other groups there i a range of minerals. Members of each group have similar internal arrangements of atoms and similar crystal shapes, but there are differences in the atoms pre ent. Chemically they are mostly silicates or aluminosilicates of five common metallic ions - potassium, sodium, calcium, iron and magne ium. These ions link t he silicon-oxygen tetrahedra. Calcium, pOtassium and sodium ions can substitute for each other, and iron for magnesium. Quartz, feldspar and white mica are the clear or pale-coloured minerals in rocks formed fro m magma. Brown mica, pyroxenes and amphiboles are the dark minerals. Olivine i green. When rocks containing these minerals decompo e and disintegrate on the Earth's surface, the feldspars, pyroxenes and amphiboles are converted to clays. Quartz, however, i hard and chemically stable. Together with the clays, it passes inro soils. Many Victorian beaches also are made up of quartz grains. There are a few other mineral which are common in some rocks. The most important is calcite (calcium carbonate). Calcite is the main component of a widely occurring rock, limestone. Some beach sands and sand dunes are largely formed from broken pieces of sea- heUs, which in rurn are made up of calcite crystals. Calcite is also present in many soils in lower rain fall regions.
Basic Concepts in Geology
11
Accessory minerals A small number of minerals are present in minor amounts in many kinds of rocks. Many of these accessory mi nerals are very stable. The commonest are magnetite (Fe,O.), ilmenite (FeTiO,) and apatite (a complex calcium compound). :.orne accessory mmerals provIde much of the colour in rocks and clays. Yellow and brown colours are usually caused by limonite, a mixlure of iron oxide compounds containing water. Red colours are mostly due to hematite (Fe,O,). Manganese oxide (pyrolusite) produces black lines and deposits on rocks.
Figure 1-13 Important economic mineral groups.
Economk
group
mineral
naahc clements metallic o:
66'1_
I
0 I
N T
Common minerals present quartz, feldspar. mica
PLUTONIC Onhocla5� Plagioclase predominant predominant
GRANITE
GRANODIORlTE
e,g. .
'Jount Buffalo
• Wilsons
Promon tory
55-6607,
reldspar. amphibole
e.g. • Mount Alexander • Mount Saw Saw
s),miu
e.g. Bcnambra
E R M E
MINOR INTRUSIONS (dykes)
Qplil�, pegmatiu
Onhoclase predominant
VOLCANIC Plagioclase predominant
RHYOLITE
RH )'ODA CITE
DACITE
quart:. porphyry
e.g. • Warburton • l\'larysvillc
diorite
UQch)u
andesit�
e.g. • Woods Point • Walhalla
e.g.
e.g. • �'Iount SIa\cly • No.... a Nowa
• Hanging Rock •
Caslcnon
e.g. • Mount Dandcnong • Mount Mactdon
0
I
A T E B A S
45·55".
I
feldspar. amphibole. pyroxene
gabbro
do/trill
e.g. North of Murrungowcr. East Gippsland
e.g. • •
8ASA L T
e.g. Western DislriCl
Heathcote Dookie
C U L
< 45'10
pyroxene. olivine
monchiqllitt
e.g. • Bendigo mines
T
R A B A S I C
peridOtite
e.g. Aberfeldy
Rock names In capital letlers are common In Victoria Rock names in small lellers are uncommon in Victoria
Figure 1-18 The chief minerals of igneous rocks. The diagram shows the main minerals thai are likely to be found in the fo ur main classes of igneous rocks. (After H.H. Read and J. Walson, Beginning Geology, Macmillan - Allen and Unwin.
1 966).
(J)
:;t, a:
ULTRABASIC
BASIC
INTERMEDIATE
ACID
�
:E
'$ o
:;: en 80 !J! �
"
o -;
:E
� 60 a: w u. u. o w
"
�z
» r
(')
�'U
o z m z
w (J ffi 20
-; (J)
0-
�
a: 00..
I m :D ;:: Z m :D
•
Two features of igneous rocks are used to establish where each one fits il1lo the classification. I . Their chemical and mineral composition Silicon is Ihe commonest chemical element present in igneous rocks. In Figure 1 - 17, the four horizontal divisions - acid, intermediate, basic and ultrabasic - relate to Ihe amounl of silicon present as the oxide, SiO,. One or more types o f feldspars are present in mosl igneous rocks. The classification dislingui hes between rocks richer in potassium (i.e. onhoclase feldspar predominates) and those richer in sodium and calcium (i.e plagioclase feldspar predominates).
14
Chapter 1
Figure 1-19 (below) A basic dyke (right of the tree) intruding folded Lower Palaeozoic mudstones in a cutting on the Woodstock - Wan dong Road in the Merriang RiUs, north of Melbourne. East-west dykes of this sort are common in the Melbourne region. They range in age from Eocene to Miocene. The strike of the mudstones is nearly north-south, parallel to the road. At this locality the beds dip at a low angle away from the road: they are ciose to the axis of the Merriang Synciine. This fold has been traced for many kilometres. (photograph by .w. Sch leger). i
2. Where the rock formed The three wide vertical columns in Figure 1 - 1 7 indicate whether the rock solidified on or below the Earth's surface. Large bodies of rock that crystallised from magma deep below the surface are said to be of plutoniC or intrusive origin. There are also thin sheet-like bodies of magma that solidified along cracks through older rocks near the surface. These minor intrusions are called dykes. Magma that reached the surface produced volcanic or extrusive rocks. Volcanic rocks are further subdivided into lavas (which flowed from a volcano) and explosive (or pyroclastic) types which were thrown out as fragments from a volcano. Only lavas are included in Figure 1 - 1 7 . Volcanic rocks can be seen forming around many volcanoes i n the world today. By contrast, the intrusive rocks found at the surface were all formed in the past. They are only visible because other rocks, which once covered them, have been worn away over a long period. Pyroclastic rocks do not fit easily into a rock c1assification. They can be described by their chemical and mineral composition, in which case the same names as for lavas are used. For example, rocks in the Dandenong Ranges are called rhyodacites and dacites, even though they olidified from material blown out of volcanoes, not from lava flows. Alternatively, names are used for pyroclastic rocks which reflect the size of the frag ments in the deposit and the type of volcanic explosion involved. Under this system, some common rocks found in Victoria are: • •
tuff - made up of fragments generally less than two centimetres in diameter and often layered like a sediment. (Also called ash beds before consolidating to a rock); agglomerate
-
made up of fragments generally greater than two centimetres in
diameter;
•
scoria
•
ignimbrite
-
masses of frothy basalt, which almost solidified in the atmosphere;
a rock formed from the cooling of a cloud of very hot gases and volcanic fragments, that moved at high speed over the land. Much of the material is non-crystalline volcanic glass, which welded the rock together. The size, shape and orientation o f the mineral crystals in an igneous rock are determined mainly by where it solidified. These features provide the Texture of the rock. Intrusive rocks contain large crystals up to five millimetres or more acros because they formed from a slowly cooling magma beneath the Earth's surface. They have a medium- to coarse-grained texture. -
Figure 1-20 (right) Thin layer.; of basaltic tuff along the south rim of Tower HiO, a volcanic complex between \\\I rrnambool and Port Fairy. The layers consist of fine volcanic material that was Lhrown out of a crater and depo ited over the surrounding country. Occasional white layers consist of grains of calcium carbonate. (Photograph by . W. Schleiger).
In contrast, volcanic rocks formed from magma that solidified rapidly at the Earth's surface, so their crystals are much smaller than those in granitic rocks. They have a fine-grained texture. Some volcanic rocks cooled very rapidly, for example where the magma erupted under water. These rocks may consist mainly of glass, that is amorphous material without any crystalline structure. Some minor intrusive rocks caUed porphyries contain large crystals (e.g. quartz) in a matrix of fine-grained crystals . The large crystals are called phenocrySTS (Figure 1 -2 1 ) . Igneous rocks are very widespread in Victoria and form many distinctive landscape features. These include Mount Buffalo and Wilsons Promontory, made up of granitic rocks. An acid volcanic rock, rhyodacite, forms the prominent landmarks of Mount Macedon, Mount Donna Buang and the Dandenong Ranges.
Basic Concepts in Geology
15
Figure 1-21 (right) A rhyolite ignimbrite from near Eildon. There are large crystals of pink feldspar and grey quartz in a streaky, fragmental groundmass. It is an acid pyroclastic rock, which formed from material ejected by a large, explosive volcanic eruption during Upper Devonian t imes.
Figure 1-22 (below) Weathering of a dyke rock at O'Shannassy Reservoir near Warburton. This form of weathering in which layers of weathered rock are peeling off is called exfoliation or simply, onion or spheroidal weathering. The dark rock is a near-vertical porphyry dyke, aboUl one metre thick, which int ruded acid volcanic rocks. It contains larger crystals (phenocrysts) of fe ldspar and hornblende. Weathering has taken place where waler and air percolated along the margins of the dyke and aCross occasional horizontal fract ures. The deeply wealhered outside material has produced a thin residual soil, in which grass has taken root. ( Photograph by P.O. Dahlhaus).
B y contrast, very fluid basalt lava flows erupted from many volcanoes in south-western Victoria spread out over hundreds of square kilometres to form the Oat plains of the Western District. The sites of the volcanoes remain as small hills rising at intervals over the plains.
SEDIMENTARY ROCKS It has been shown that igneous rocks are produced from magma, which originated deep below the Eanh's surface. By contrast, sedimentary rocks are made up of recycled minerals, which come from material already at the surface. Sedimentary rocks to a large extent result from the two main processes, which shape the Earth's landscape: •
•
or wearing away; this takes place mainly on land and along coastlines. erosion or building up; this happens mainly under \vater, although some deposition
•
derrital or clastic rocks. (From the words, 'detritus'
-
-
occurs on land. The most distinct ive fealUre of sedimentary rocks is that they are made up of layers of minerals and (sometimes) rock fragments. Each layer is called a bed or stralllm. The planes separating beds are called bedding planes. Sedimentary rocks are classified into twO broad classes:
•
'klasLOs' (Greek) = break); organic and chemical rocks.
=
material worn a\vay and
Detrital rocks These are made up of mineral grains, which were left after older rocks decomposed and disin tegrated under the inOuence of various climatic factors. The mineral grains were carried away by Streams (or sometimes by wind or glaciers) and eventually dropped in the sea, in lakes, along river beds or elsewhere on land. Initially the material dropped was soft and unconsolidated, as in river silts, sand dunes and S\vamp clays - these are called sediments. Over millions of years, a layer upon layer of sediment was depo ited in an area, the lowermost layers became compacted by the weight of overlying material. The mineral grain were also cemented together by chemical deposits. Finally hard rocks were formed. The minerals in detrital rocks are chemically stable, as they have survived the break-up of rocks, transport and compaction. The main components are quanz (especially in sands) and clays (in muds). Feldspar , micas and fragments of the parent rocks may also be present. Detrital rocks and sediments are like oils - they can be classified in terms of the sizes of the particles that are pre ent (Figure 1-24). Because most detrital mineral grains have been rolled over and over and rubbed against other grains during transport, they are usually partly or wholly rounded. Thi contrasts with the sharp well-shaped crystal outlines in many igneous rocks. Detrital rock are widespread in Victoria. Coarse sand deposits occur on many beaches along the south coast. Sands are also heaped up in dunes along parts of the coast and over the plains of nonh-western Victoria. Offshore on the Ooors o f the bay and o n the deeper ocean floor, there are thick depo its o f muds and sands, which may become hard andstones and hale in the future. Sandstones, siltstones and shale are common rocks in many hilly and mountainous areas, and they also form steep cEff along ome part of the coast, e.g. east and west of Cape Ot\vay, and Cape Pater on to San Remo.
16
Chapter 1
Figure 1-23 Dip and strike of sedimentary beds. Strike is the bearing or compass direction of a horizontal line on a bedding plane of a sedimentary rock. The term is also used for fault planes and other planar features in rocks, e.g. joints. Dip is the angle between a horiwmal plane and a bedding plane or other type of plane in a rock. Trlle dip is measured at right angles to the direction of strike. Apparent dip is the angle measured in any other direction. Apparent dips are often seen in sedimemary rocks exposed in road cuttin gs, that are not perpendkular 10 the strike.
Some of the more attractive landscapes in Victoria are fo rmed by thick deposits of detrital rocks. Hard sandstones in particular often provide prominent escarpments. Scenic examples are found in The Grampians in western Victoria and the Moroka - Wonnangatta region in central Gippsland. Figure 1-24 Classification of detrital sedimentary rocks SEDIMENT Diamet�r of panicles and pieces (mm)
C
0
> 256
A
R S E
M E
4
-
256
2-4 0.062 - 2
Name
boulder
pebbles, cobbles gravel sand
0 I U
N E
0.004 - 0.062
jawleM fish fish w ith jawli and ,.(mour bony fish lungfi!hark� :lIId ray!'> amphibian!! (rrog!'>, !lulanHlnden) rep t i les (li ard !>, dino ur!l ) bird!l mammal'i
z
�
Basic Concepts i n Geology
Geological time
23
Geologists do not only want to know how and where minerals. rocks and fossils were formed - they are also interested in when they are produced. The matter of time can be considered in two d ifferent ways: •
•
relative time numerical time
CONCEPT OF TIME If someone says that John is older than Jane, we know that John has been living longer than Jane or that John was born before Jane was. But from this statement, John and Jane could be children, middle-aged or elderly people. Even if John and Jane are standing in front of us, we cannot teU their ages by looking at them. In the same way, it may be clear that lava from a volcano has flowed over an area of sandstones - so the volcanic rock is younger than the sedimentary rocks. But the eruption may have occurred at any time - it may have been last year or five hundred million years ago - the relative positions of the rocks could be the same in both cases. However, if the first statement is changed to John is 2 1 and Jane is 1 7 years old, we have some numerical in formation. John was born 2 1 years before now and Jane 17 years ago. Their years of birth can be calculated by counting back 21 or 17 years. Similarly rocks have ages. For example, the volcanic rock mentioned above may have solidified one million years ago; the sandstones may be much older, ay 200 million years.
RELATIVE TIME IN GEOLOGY Geology first began to be recognised as a science in Europe in the late eighteenth century. Over the next hundred years, geologists gradually established the sequence in which geological events had taken place since the Earth was formed. This led to the development of the subdivisions of time given on the left hand side of the geological time scale shown in Figure 1-40. On this scale, geological time is divided into various eras, periods and epochs. The oldest rocks on Earth are placed in the Archaean division. Going up the scale, the divisions become progressively younger, fmishing with the Recent or youngest rocks. The names Cambrian. Ordovician. Silurian and so on were given by various European geologists. The origins of these words are explained in Chapter 4. They were introduced to represent the periods during which certain prominent sequences of sedimentary rocks in E u rope were deposited. Each sequence contains distinctive assemblages of fossils. The end of one period was often indicated by the extinction of a particular fossil species or the appearance of some new form. One of the best known extinctions was the disappearance of the dinosaurs. which marked the end of the Cretaceous Period. Gradually it became recognised that the European divisions could be applied on a world-wide basis. It was reali ed for example that the Devonian Period was one of widespread limestone reef formation in many parts of the world. The Carboniferous Period was a time when thick coal depo its were formed in the continents of the Northern Hemisphere. Several things should be noted: I . When the time scale was introduced in the nineteenth centu ry. the absolute ages
of the various time divisions were not known.
2. The divisions are not of equal length. 3. Although the end of each division was sharply defined in the country in which it was first named. the sharp divisions are not necessarily present in aU parts of the world. In addition. there may have been a sudden change in the geological environment in Australia during a particular period when uniform conditions prevailed in Europe. For example, in Victoria. sediments were continuously deposited on the floor of the sea during the Silurian and early part of the Devonian Period. The later part of the Devonian and the Carboniferous periods. however. are characterised by sediments laid down by rivers over large areas of land. There was thus a more important change in geological conditions in the middle of the Devonian than there was at either the beginning or the end of that period.
4. During any particular period. it is po sible that no sediments were deposited in many regions. despite rocks of that age having been found in some part of Europe. For example. during the Tria sic Period, the only sedimentary rocks to be preserved in Victoria were those now found over a mall area near Bacchus Marsh.
24
Chapter t
DETERMI NATION OF RELATIVE GEOLOGICAL TIME The geological time scale was built up gradually by studies of: I . Field relationships between rock units. 2. The fossils in rocks. Both sedimentary and igneous events are recorded in the time scale. I . Field relationships are between rocks of different groups or types in contact
with one another. Several simple principles may be used to determine which o f two groups of rocks i s t h e older. (a) Principle of superposition. I n a sequence of more or less nat layers o f sedimentary and/or volcanic rocks, it can be re.asonably assumed, that the oldest rocks are at the bottom and the youngest at the top. (b) Principle of cross-cutting relations (Figure 1-38). Igneous i ntrusions and geological fractures (e.g. faults) are younger than the rocks they intersect. An intrusive igneous rock is also younger than adjacent sedimentary rocks if there is a metamorphic aureole present. Conversely sediments lying over an intrusion, but not metamorphosed by it, are younger than the intrusion. These ide.as can be extended in some places to telling the relative ages of two sedimentary groups, which are close but not actually in contact with e.ach other. One group may be intersected by numerous dykes, whereas the other is not. The likelihood is that the sequence o f events was: formation of rocks not inter ected by dykes. 3 . youngest 2. t intrusion of dykes. I . oldest formation of rocks intersected by dykes.
Figure \-38 Principle of cross-cutting relations.
This principle states that a rock is younger than any other rock it cuts. In this illustration the rock unils are numbered in order of decreasing age; namely: I . older fo lded sedimentary rocks 2. large granitic intrusion 3. older dyke 4. you nger sedimentary rocks 5. younger dyke
:::::==-�::f.
2
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
(c) Principle of inclusions. One rock is younger than another, if the first rock contains fragments o f the second. For example, conglomerates deposited by fast-flowing rivers contain boulders of other rocks. The conglomerate is younger than any o f the formations from which the boulders were derived. For example, in Figure 1-25, the quartz pebbles are of Devonian age, but the conglomerate is Tertiary. Likewise, some volcanic rocks contain lumps of rocks, which they tore out of older sedimentary rock formations as they rose to the surface. This shows the volcanoes erupted after the sedimentary rocks were formed. 2. Principle of faunal succession.. Some nineteenth century geologists, who recognised the principle of superposition, also studied the fos ils contained in sedimentary rock sequences. In some areas they noted that the upper (younger) rocks contained di fferent fossils to those in the lower (older) beds - even though the rocks may have been very similar in appearance (e.g. both grey shales). Gradually it was established that rocks of di fferent relative ages usually contained di fferent assemblages of fossils. These differences were not simply explained by the rocks being deposited in d i fferent environments, e.g. one sequence in marine conditions, another in freshwater lakes. Gradually a picture was built up of so-called faunal succession - or changes in the forms of life on Earth over successive periods. Recognition of these changes enabled geologists to compare the relative ages of sedimentary rocks, even when they were not close to each other. Not all fossils are useful in determining the relative ages of rocks. Some existed
Basic Concepts in Geology
Figure 1-39 An angular unconformity between Silurian and Tertiary rocks exposed on the southern side of a railway cutting near Royal Park station. Evidence of two geological events, that occurred about 400 million years apart, is seen in this cutting, only four kilometres nonh of the centre of Melbourne. Most of the culling exposes thin beds of sandstones and mudstones of Silurian age. They were laid down in horizontal layers on the ocean floor, just over 400 million years ago. Later forces within the Earth's cruSt lifted the beds above sea-level and squeezed them, so they are now tilted to the east. Near the top of the cUlling, there are sandstones and some gravels, which were deposited in shallow sea water about live million years ago. Fossils found in similar rocks not far away indicate the ages of both these rock formations. (photograph by G.w. Quick).
25
over many periods without changes in their appearance. There are some cyano bacteria (stromatolites) that are found in rocks of all ages from the Pre-Cambrian onwards. The presence of fossil stromatolites in a rock therefore gives no clue to the age of the rock. The most valuable fossils are those that existed for a relatively shon time, but were nevertheless widespread around the world. Many species of small floating marine organisms called graptolites are very useful for this reason. Some are described in Chapter 4. Graptolites found in rocks of Victoria are identical with species found in Ordovician rocks of Great Britain; hence the Victorian rocks are also Ordovician.
Unconformities An unconformity is a surface between two rock masses, that represents a substantial break in the geological record. It indicates a period of time after earlier sedimentary rocks were deposited, when erosion removed some of the rocks and finally deposition of sediments started agai n. There are several kinds of unconformities. The most obvious type is illustrated in Figure 1-39; it is called an angular unconformity. The sedimentary rocks on the lower part of the railway cutting are older than those at the top. The older beds dip at a steeper angle than the younger ones. The unconformity marks a period when the older rocks were tilted and eroded. It is also possible to have a nonconformity, which is an unconformity where younger sedimentary rocks were deposited on an eroded surface of older igneous rocks.
NU MERICAL GEOLOGICAL TIME Towards t h e end o f the nineteenth century, after the geological t i m e scale had been developed, geologists began to look for ways by which the numerical ages of various geological divisions could be calculated. Various techniques were used, including studies of the rates at which sediments were laid down and the rate at which the salt content of the oceans increased - assuming the first oceans contained fresh water only. These methods all proved to be unsatisfactory for one reason or another. Nevertheless they pointed to a common conclusion - that the planet Eanh was of great age, probably hundreds of millions of years old.
26
Chapter 1
interval is too shan to be shown on the time scale. The Pleistocene and Recent together constitute the Quaternary Period.
of Victoria and are considered to be suitable for use in Victoria. As more radiometric dating of igneous roc ks and other geological research is carried out, there will be an increasing trend towards a universally accepted dating of the various periods and epochs. On the right are shown the ranges in lime during which various forms of life existed. Note: the last years of geological history are usually called the Recent Epoch. This
Figure 140 Geological time scale. Geologists throughout the world use the subdivisions given in this scale, although in the United States of America, the Carboniferous is divided into a younger major unit, the Pennsylvanian and an older major unit, the Mississippian. However, there is much disagreement about the ages at which each period commences and finishes. The ages given here were sup lied by the Geological Survey
10 000
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Soils
49
ORGANIC SOILS These soils are dominaled by black to dark brown decaying planl maller in the upper 30 centimetres or more. They also contain sand and clay in varying proportions. Peat is a typical organic soil. It consists mainly of plant matter that is salurated with water over long periods. Organic soils form in poorly-drained areas where dead plant material accumulates. They can occur at any level from the highest plateaus to the lowest coastal marshes. Environments, where organic soils can be found, include the following: \. Salt marshes, e.g. near Queenscliff and around the margins of Western POri, Corner
Inlet and Andersons Inlet on the South Gippsland coast. 2. Swamps formed where streams were blocked by either lava flows (e.g. Lake Condah near Macarthur in weslern Victoria) or sand drifts (e.g. swamps behind coastal dunes).
3. Deltas and sections of river valleys, where drainage has been impeded by either faulting or hard rock bars, e.g. near Heywood, Carrum, Koo- wee-rup and Tarwin Lower, in parls of the Latrobe River valley and on the Snowy River flats.
4. Valley ball oms and lower slopes where drainage waters accumulate on the High
Plains i n the East Victorian Uplands. Figure 2� A disturbed alpine bog, north of Mount Cope, in the East Victori.n U plands. In the background is a sphagnum moss bog with a shallow organic soil in the High Plains COUIHry. Excessive trampling of this area by caltle led to compaction and drainage of paris of the bog. The pealy soil was eroded, leaving a gravelly surface. (Photograph by N.J. Rosengren).
Some of the less acid organic soils have important land uses, particularly where there is a high clay content. For example, Koo-wee-rup Swamp, to the south-east of Melbourne at the head of West ern Port, was originally a waterlogged, swampy area. I t was progressively d rained between 1876 and 1920. Since then it has become an important area for market gardening, supplying Melbourne with much of its vegetables. However, with the spread of houses and factories into this area in recent years, some high quality horticultural land has been lost. Peat bogs on the High Plains are important in regulating stream flows. They absorb a great deal of rainwater and release it slowly. This decreases flooding and erosion after heavy rains and prolongs Slream flows in long dry periods. They are very acidic and have low fertility.
UNIFORM SOILS Uniform soils have no dist inct texture boundaries and only minor texture differences through their profiles. They are mostly clays and sands. There are also small areas of uniform loams, but uniform silt soils are extremely rare in Victoria. \.
Uniform clays occur over parts of the flood plains of the Murray River, its lribularies and many rivers in southern Victoria. They are also found on the Wimmera Plain and on the low-lying parts of the volcanic plains of western Victoria. Clays on the Wimmera Plain are particularly important for growing wheat, barley and oats. Both the flood plain and volcanic plain clays are subject to waterlogging. However, they can be used for dairying and grazing where they have been drained. Extensive red gum forests grow on these soils on the Murray River Ilood plain near Barmah and Cohuna. The forests are commercially important because the wood is durable and water-resistant. It is used for railway sleepers, fence posts in the country and house stumps in urban areas.
50
Chap ter 2
2. Unifonn sands are typical of sand dunes. There are two kinds: • siliceous dUlles made up of quartz grains. These are common in the Little Desert, Big Desert and Sunset Country in the north-west of the State and along parts 0 f the coast; • calcareous dUlles that are mosLly made up of calcium carbonate (lime) derived from shell fragments washed or blown from the sea-floor. They form some of the youngest coastal dunes. Calcareous dunes also occur inland in the MaUee but there the lime appears to have been deposited from groundwaters. Uniform sands are typically pale, except the lOp few centimetres, which are darkened by organ ic matter. These soils are vulnerable 10 wind erosion and have low productivity when farmed. Natural vegetation on them should nOL be cleared. Coarse uniform sands also occur on colluvium in granitic country, e.g. on the slopes of MOUn! Alexander near Harcourt and the You Yangs near Geelong.
3. Unifonn loams can be either deep or shallow. Deep loams are restricted to small areas on deposits such as young river alluvium. Poor, shal low, loamy soils, less than 15 cemimetres thick, occur between rock outcrops on the steepest hillcrests, particularly in the Central Victorian Uplands. These soils have a low capacity for storing water. They are called shallow stOIlY loams or skeletal soils.
GRADATIONAL SOILS The e are soils that become progressively more clayey with depth, but each horizon grades into the next without an obvious change. They are commonly known as 'earrils and the main occurrences are as follow: I.
Shallow stony earths occupy the steeper hillslopes in the drier parts of the Central Victorian Uplands. They are typically about half a metre deep and used mainly for sheep grazing. Even though the subsoils are clayey, they are quite porous and thus water can easily pass through them. This is important fo r groundwarer recharge, that is the addition of water to natural underground storages (see Chapter 6). However, if the country is cleared of forests, the levels of water tables may rise and possibly cause waterlogging. This is because forests absorb large quantities of water. Rising water tables also bring soluble salts into the upper soil layers. The saltS may eventually kill pastures and native vegetation.
2. Friable earths have favourable physical characteristics and reasonable chemical qualities. The most fertile are red clayey soils found on colluvium in many hilly areas of basaltic rocks, especially near Ballarat, Trentham, Warragul, Thorpdale and Monbulk. They are called krasllozems, a Russian word for red soils. They are excellent agricultural soils, that are used intensivelY for dairying and producing potatoes, vegetables, berries and OLher fruits. There are widespread red, brown and yellow friable earths developed from colluvium on humid mountain slopes in the East Victorian Uplands and the Otway and Strzelecki ranges. This country is used for timber production. Nature conservat ion is important in forested areas where magnificent eucalypts, such as mountain ash and alpine ash, grow to height of up to 100 metre . Cleared area are u ed mainly for dairying. 3. Calcareous earths have developed on dust deposits on the Mallee plains. They are used mainly for wheat growing and sheep grazing but yields are limited by the dry climate. \vind erosion and soil salting. Several irrigated areas are renowned for their grapes, oranges and grapefruit, but again there are problems with salinity.
DUPLEX SOILS
Figure 2-7 Soil pH "nd climate. Category
pH
alkaline neutral acidic
>7 7 PI!.- -
-
COliSTAL SWAMP e g DUNEFIELD KOO-WEF RUP
I. ANDFORM
PIIR E N T MATERIAL
-
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ALLUVIAL PLAIN ALLUVIUM
MOUNTAINS
COLLUVIUM FROM BEDROCK e g SandSlone Mudstone Granite
BROIID SOli
CL ASS
UNIFORM
ORGANIC
DUPLEX
GRADATIONAL
52
Chapter 2
H u m a n i m pact o n soi l s
Soils gradually change with the passing of geological time, because there are continuous changes in climatic factors and in the weathering and erosion of parent rocks. If soils and weathered rocks were not eroded naturally, most sediments, that later formed sedimen tary rocks, would not have been produced. When the first settlers, the Aborigines, carne to Victoria, soil erosion increased and soil fertility decreased because the people regularly set fire to the bush. These and other adverse changes accelerated after European settlement began. Great pressure has been placed on the land where it has been cleared for settlements, cultivation and grazing. Smaller areas have been affected by road construction, mining, timber harvesting and other activities. Agriculture has had the greatest impact. The native Victorian vegetation was adapted to the natural low fertility of most of the soils. But harvesting of new crops and grazing soon exhausted the small reserves of plant nutrients. Productivity was restored, however, by the addition of new chemical substances as fertilisers. The most important addition has been phosphorus as superphosphate. Other elements applied in fertilisers include sulfur, polaSSium and trace elements such as molybdenum, copper and zinc. A further improvement has been the introduction from overseas of grasses, clovers and other legumes, which have raised the nitrogen content and restored humus levels in soils. On well-managed farms, soil fertility can now be higher than it was before the land was used for agriculture. However, fertility has declined in intensively cropped areas. In addition to the problem of declining oil fertility, there are four other processes of soil degradation. These processes are: • soil erosion: the permanent loss of soil because it is washed or blown away; • salination: the addition of harmful salts, especially sodium chloride, to a soil; • acidification: the decrease in pH of a soil; • compaction: a process that packs soil particles tighter and impedes drainage, aeration and the spread of roots.
SOIL EROSION Soil erosion mainly occurs after: • vegetation is removed to prepare land for crops; grasses are eaten down to the roots by livestock and pest animal , particularly • rabbits. These processe leave the urface of a soil wholly or partly bare. In this State, soil can easily be removed by running \vater or wind. Figure 2-9 heel and rill erosion b) running water on cropland in northern Vicloria. Cultivation of the land had left it bare in preparation for sowing a crop laler in the year. Soil are \'er� vulnerable to running water under these condition . An area of cultivated "'il has been completely removed and many rill have also formed after water has rushed downhill from the lOp right to lower left of the photograph.
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Water erosion occurs in everal way : I . Sheet erosion - where rain water remOVe the urfa e of a hillside by the impact of raindrops, sheet now and flow along mall channels a few centimetres deep
(rills) (Figure 2-9). 2. Gully erosion - where a tream ClltS a channel into a oil, often more than a metre deep.
3. Timnelling
- where waters find passages underground and excavate caverns.
Soils
53
4. Stream bank erosion where creeks and rivers undercut their banks and the overlying material even tually collapses. Gullies and tunnels can join to destroy large areas of land. -
Wind erosion mainly causes loose soil to particles can be carried off in dust storms, kilometres. The coarser particles remain, often is most severe in northern Victoria, especially
blow away (Figure 2-\0). The finer sometimes travelling hundreds of forming sand dunes. Wind erosion the Mallee region.
Figure 2-10 Wi nd e.rosion in the Mallee.
Coarse soil particles have accumulated along a north-soulh fence afler westerly winds severely eroded paddocks. The finer particles were blown away in dusl storms. •
The net result of these various fo rms of soil erosion over 150 years is that huge amounts of soil, particularly the humus-rich layers, have been blown away or washed into streams. The soil carried away in streams has either silted up rivers and dams or been carried into the sea. A fter the Second World War, most State governments formed soil conservation
authorities. These have helped considerably to develop improved land use methods that protect soils. One technique is to reduce ploughing to a minimum, so that the land is rarely left bare. Stubbles rrom p revious crop and pastures are retained instead of being ploughed under. Many erosion gullies have been filled in and stabilised with vegetation. Revegetation has been encouraged in sheet-eroded areas. Water run-off do\\ n hillsides has been further reduced by the introduction of practices such as COIlIOUf ploughing and the con truction of COllIour banks. These all ensure that rainwater drain slowly along channel around a hillside instead of ru hing do\\ nhill cau ing maximum erosion (Figure 2- 1 1 ). Overgrazing has also been reduced by be!!er control of livestock numbers and pest animals. notably rabbits.
Figure 2- 1 1 o i l erosion pre\ention \\ orks on cropland in northern Vicloria. Soil erosion by running waler afler rain is prevented on this propeny because the movement of water down the slopes is slowed by various works. COntour banks have been construcled around Ihe hillside; these fall geml)' lowards Ihe long grassed slrip near Ihe middle of Ihe photograph. The gem Ie slopes and Ihe grass cover limil erosion. The dam also helps to slow the movement of water. At Ihe foot of the hill Ihe waler is fed safely 10 a local creek. By comrasl the hills in Ihe background are affecled b)' sheel and gully erosion due to e.xcessive clearing of nali\'e vegetation.
54
Chapter 2
SALINATION n
urn
, U 1l1\
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t I
I
11
This problem was slower to develop than erosion. This is because i t has involved the slow rise over a long period of underground water containing harmful salts. When saline water approaches within a few metres of the surface, it slows down the growth of deep-rooted crops, pastures and trees. If the groundwater rises even higher to within one to two metres of the surface, the water evaporates and salts crystallise out. These kill the vegetation, leaving bare salt-encrusted ground. The largest areas affected are the drier north-western and northern parts of the State, but there are many occurrences elsewhere (Figure 2-1 2) . The salts naturally present in the drier landscapes have been redistributed by moving groundwaters. More water moves around now under crops and pastures because there is less transpiration by deep-rooted native vegetation. Another cause of salination has been the excessive addition of water to the ground by flood irrigation. (See Chapter 6 for further discussion on salinaLion).
Figure 2-\2 Soil salinalion and erosio n on the Western District volcanic plain. A severe salinity problem developed in the soils and all vegetation died. Once the soil became bare, sheet, rill and gully erosion occurred. All three fo rms of erosion are clearly seen to be widespread. The hills in the background are volcanic cones.
Plants vary greatly in their sensitivity to soil pH. For example, camellias come from acid soils in the H imalayas; they grow poorly i f soil p H exceeds 7 . B y contraS!, luceme evolved on alkaline soils in the Medi terranean region and cannot thrive on acid soils. A few vegetables are very toleran t LO acid soils (down to a pH of 5) these are polatoes, rhubarb, shallots and walennelons. Most plants prefer soils with a pH in the range 6.0 (slightly acid) to 8.0 (slightly alkaline). MoSt trees and crops tend to prefer slightly more acid soils than do flowers and vegetables. A few examples o f recommended p H ranges i n soils for common plants are given below:
ACIDIFICATION The acidity of a soil is measu red by the concentration of hydrogen ions, expressed as pH. Most plants prefer soils to be about neutral. There are some modern farming practices which benefit soils because they provide nutrients, but due to various chemical reactions, they also increase acidity. This has the counter effect of lowering productivity. Examples are the use of clovers and nitrogenous fertilisers. Agricultural lime (finely ground limestone) can be used to neutralise the exce s acid.
COMPACTION In the drier regions of Victoria, particularly in the north, some soils are naturally compact and therefore their productivity is low. H owever, the problem of compaction has been increased on t hese and other soils by both overcropping and overgrazing and an associated decline in humus content. Productivity declines because roots cannot penetrate the hard layers, and heavy rainwaters tend to run o ff rather than enter the soils. Thus less moisture is available for plant growth.
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The quality o f soils can be restored by cultivating them as lillie as possible a technique known as minimum lillage. A red uction in livestock numbers also helps, because this encourages vigorous pasture growth which in turn improves soil structure. The addition of gypsum also leads to beller soil structure. A problem that is d i fficult to overcome is direct compaction by the hooves o f animals. This is most serious when soils are wet. It is a very serious problem in southern Victoria where the rain fa ll is high and the main land use is dairying. Government and privately-sponsored research is being carried out to solve these problems, so thal the productivity of soils can be maintained or even improved in the future.
Soils
55
Soils of the Melbourne subur
Soil type is important not only in farming country but also in urban areas. Soils affect human living in many ways. Two aspects are the stability of buildings and the management of gardens, as shown by the following examples around Melbourne Most of the hiUy areas in the north-eastern and eastern suburbs have sodie duplex soils on Silurian sedimentary rocks. The B horizons shrink and swell on drying and wetting. Shrinking is particularly great during unusually dry summers. The result is cracked walls of dwellings, particularly those with rigid brick walls. Nowadays to overcome this problem, houses are built on concrete slabs (see Chapter 7 for Further details). The A horizons o f the duplex soils are poorly structured for gardening, but they can readily be improved with compost and gypsum. Waterlogging is a problem. The higher parts of the eastern suburbs are capped by Tertiary sediments. These have acidic duplex soils, which provide stable foundations for house . They are also easier to cultivate. However, waterlogging can be a problem, often causing the sudden death of citrus trees. Clays on the basaltic plains of the western suburbs are particularly prone to soil movement on wetting and drying. This causes much damage to buildings. These clays are too tough for easy cultivation. They can be improved with large amounts of compost, gypsum or sometimes sand. Water often disappears down large cracks in the su mmer time. Sands are widespread in some baysid e suburbs and they are scattered on the Tertiary sediments of the eastern suburbs. They are not prone to move seasonally and they provide the most stable foundations for buildings. They are nOt ideal for gardening, however, requiring frequent watering because of their low water-holding capacity. They also have low fertility and therefore need heavy dressings of fertilisers and compost. Another problem is that nutrients are easily washed out of these soils. The best soils for gardening are deep loams on flood plains beside creeks and rivers, e.g. Maribyrnong River flats. Few houses are built on these oils because of the flooding hazard.
Soil geochemistr
So far soils have been discussed largely in terms of their agricultural value However, some soils are of interest to investigators in an entirely di fferent industry - that of mining and mineral exploration. Until fairly recent times, deposits of minerals containing useful metals, (such as lead and copper), could only be found by searching the su rface of the land. However, many deposits may not reach the surface or they may be covered by vegetation or soil. To find these hidden resources, geologists use techniques that depend on the physical and chemical properties of the minerals. In particular, geochemical surveys are employed to find exceptionally high concentrations of metal ions in soils and sediments. Such concentrations may be due to the presence somewhere nearby of a valuable mineral deposit. During the weathering of a mineral deposit, metal ions may be carried away in solid mineral grains or in soluble salts. I n a geochemical survey, many samples of soils or stream sediments are collected over a large area and analysed for certain metals. Geologists look for the presence of chemical anomalies, that is, concentrations of metals that are higher than the tiny amounts that normally occur in soils and sediments. In soil investigations, it is important that geologists should recognise the types of soils present. Metal anomalies are most useful where they are found in soils derived from underlying rocks. If anomalies are found in oils formed on transported parent materials, it may be difficult to detemline their source.
Figure 2·13 Loss of soil caused by a landslide near Leongalh:l. Land slumps o r this kind are prone t o occur in t h e S I r7rlccki and Ol way ranges where the original rorestS have been removed.
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Geomorphology
57
Chapter 3
GEOMORPHOLOGY .. ..
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Figure 3-1 The Twelve Apostles, Port CampbeU National Park. These rock stacks form a well known scenic attraction on the south-western Victorian coast. They are clearly composed of lhe same horizontal layers of sedimentary rocks as the nearby 60 melre high ctiffs. They have formed by lhe gradual undercutting and erosion of the coastline by the continuous fo rce o f the incoming waves. The slacks are temporary landforms existing ones will be even tually demolished by lhe ocean and new ones v.dU form where the cliffs now sland. (Photograph by J. F. Bilnoy).
Geomorphology is the branch of science that deals with the various landfonns making up the Earth's surface. The word means 'a knowledge oj (he shape oj (he Earth'. Geomorphology investigates the processes by which natural fea tures such as mountains, plains, river valleys and coastlines were formed. Another word for Ihe science is physiography: th is term is usually found in older literature on the subject. Geology mostly deals with even IS that occurred in Ihe dislant past. The human race had no influence on how or where Ihese events look place. By contrast, geomorphology is concerned with even ts thai happened in lhe recenl geological past or are happening today. These are often processes where people can in fluence the result. For example, they can either cause or prevent Ihe erosion of soil from the land, Ihey can control the flow of water along rivers by building dams and so on. There are Ihree main reasons why geomorphology is tudied: I . To satis/JI people 's curiosity aboUl how interesting and oJten unusual natural Jeatures were Jormed. Many of these places become touri I attractio ns, e.g. rock tacks near Port Campbell, strange rock shapes on Mount Buffalo, speclacular walerfalls on many Victorian rivers.
58
Chapter 3
2 . For scientific reasons. An understanding of how natural processes operate on and in the Earth's crust today can provide a key to how past geological eventS occurred. For example, a t low tide on a Melbourne beach, one can see that t h e sea-floor consists o f layers of sand grains. The surface of the sand may be marked by a series of ripples produced by the action of waves at higher tide levels. Elsewhere, in a road cutting or cliff face, one may find hard sandstone beds composed of similar mineral grains and with ripple marks on the bedding planes. The conclusion from these observations is that the sandstone was once a mass of sand in shallow sea water. 3 . To help in planning many engineering and rural projects. The sites chosen for towns, reservoirs, bridges and harbours and the routes selected for roads and raiJways are aU influenced by landforms and the processes that formed them. So too are the areas used for agriculture, the types of agriculture practised and the places developed as touriSt and recreation centres. Some landforms are renowned for their grandeur and beauty; they are acclaimed by tourists who travel from aU parts of the world to visit them. Such features include the Himalayan mountains in Asia, the European Alps, the Grand Canyon in the United States of America and the Great Barrier Reef and Ayers Rock in Australia. Victoria does not possess any outstanding scenic attractions on the grand scale o f these places. Nevertheless within a small area there is a great diversity of scenic landforms. The extensive, thickly-forested, sparsely-populated mountains, which extend from the eastern outskirts of Melbourne to the north-eastern border with New South Wales, differ greatly from the plains of western and north-western Victoria. Striking contrasts of landforms also occur along the Victorian coast. The long landscape of sand dunes and surf, that marks the Ninety Mile Beach of south-eastern Gippsland, is very di fferent from the rugged cli ffs of the Otway region in the south west. Similarly the sandy beaches and low cliffs of Port Phillip Bay comrast with the mudflats around much of Western Port, a short distance away. It is the variety and beauty of its landforms and the vegetation associated with them, that provides Victoria with so many areas of tourist imerest. This chapter discusses how some of the Victorian landforms developed and how they influenced the pattern of human seulement.
Geomorphic processes
In Chapter I it was shown that our planet, Earth, is a dynamic environmem. The various materials making up the continents and ocean floors are constantly being changed. Rocks are continuously being worn away by water, wind and ice to form sediments. These in turn are transported elsewhere and ultimately become new rocks. In many parts of the world, volcanoes also add new rocks to the surface. This chapter describes today's landforms, which have been developed after long periods by many chemical and physical processes operating in the Eart h's crust. The processes that determine the shape of the landscape are called geomorphic processes (Figure 3-2). They can be broadly grouped, according to the origin of the dominant force involved, into two main kinds: these include forces acting within the I . Processes at the Earth 's surface atmosphere (e.g. wind, sun, etc.), the dynamic effects of water in all its forms (rain, rivers, seas, glaciers, etc.) and various kinds of biological activit ies. 2. Processes inside the Earth these are forces that periodically cause the land to rise Or fall, and to be buckled or fractured in various ways. They also produce igneous rocks. These forces may act alone or in various combinations to produce a large variety o f land forms. Most of these forms have been given special names. Only a few of these names are used and defined in this chapter. The reader is referred to a dictionary of geographic or geological terms to learn the meaning of specific words that may be encountered in reading more advanced literature dealing with geomorphology. The main geomorphic processes are discussed bclow. Most are treated only briefly. because they have already been described in Chapter I in connection with the topics of weathering and the formation of rocks. -
-
PROCESSES AT THE EARTH'S SURFACE Weathering The breakdown o f rocks and sedimems by chemical reactions or physical forces is called weathering. It is an essential process in the rock-forming cycle. Weathering produces the raw geological materials that are carried away by the natural agencies of wind. water, ice and gravity to form sediments elsewhere. Weathering and later erosion can give rocks strange picturesque shapes. The rounded granite boulders called
Geomorphology
59
tors are just one example. They make interesting features at places such as Mount Bu ffalo, Wilsons Promontory and the Cobaw Range. The names given to some land features also reflect the shapes that weathering and erosion have produced, for
example Rams Horn on the far south-east coast, Mount Camel near Heathcote, Asses Ears in The Grampians.
Figure 3-2 The main geomorphic processes. The dominant agents are in italics.
WEATHERING
MASS MOVEMENT
MASS MOVEMENT
����� �RACmE /....:; :/:;::
%
-
Water & carbon dioxide
Gra vity
FLUVIAL
KARST
AEOLIAN
MARINE
MAR I N E
GLACIAL
Wave & current action
Wave ac/lon
Moving Ice
VOLCANIC
VOLCANIC
TECTONIC
Flowing water
Gentle eruplion
SCORI�
� ExplOSive eruplion
Crustal stress
Soils are also the direct or indirect product of weathering. Soils are accumulations of m ineral grains that were released after fresh rocks weathered and disintegrated. Chemically stable minerals in rocks are not changed by soil-forming processes, e.g. quanz ends up as sand grains. Other minerals are altered however, e.g. feldspars become soft clay minerals.
Mass movement Mass movement is a term used to describe the movement of eanh materials down slopes under the influence of gravity. The materials may be either fresh or weathered rocks, unconsolidated ediments or soils. They move in bulk fonn and not as separate panicles. Mass movements are most likely to occur in hilly area . The commonest type of mass movement is a landslide. Mass movements can be set off by eanhquakes or high rainfall. Rain penetrates and softens eanh materials and provides lubrication for slides. They may also commence where human activities have excavated quarries or road cUllings, leaving the ground unsupponed. Mass movements may occur rapidly or slowly. They may take many different forms and vary considerably in the size of the areas they cover. The causes and effects of landslides are described in Chapter 7. Landslides can cause enormous damage. In any area where they may occur, it is important to plan
60
Chapter 3
the siting of roads, dams, power stations, buildings and other structures carefully. Special strengthening of these structures may also be necessary.
Fluvial processes Processes related to streams are said to be fluvial. Any body of running water is called a stream. It may flow over the ground as a sheet of water, or in a small channel called a rill or gully, or in a creek or river. A sequence of events can be observed in the fluvial system. The process starts where rocks are eroded by water. Next eroded materials are transported by the stream to a valley, lake or ocean. There they are deposited as new layered sediments. Streams perform many useful functions. They provide water supplies, navigation routes and recreational areas. In some places they deposit rich alluvial soils or alluvial mineral deposits, such as gold or tin ores. Large rivers provide the energy for hydro electric schemes. These are generally considered to be the least polluting way of generating electricity.
Figure 3-3 Granite tors, Wilsons Promontory.
Piles of boulders such as these are typical of many areas where granitic rocks occur. Most granites are intersected by sets of joints running in several preferred directions. Air and water penetrate down these cracks and slowly weather the rock to a sandy clay. The clay is washed away, leaving blocks of solid rock. These often have curved outlines as weathering concentrates along original sharp edges and corners. Thin �urved sheets of weathered granite often peel off the rounded masses as seen on the right hand side of the photograph. (Photograph by G. Walli ).
figure 34 The Potholes, 8 kilometres north of Buchan. This area provides an excellent example of karst topography. The deared land is underlain by limestone and marls. Rainwater, percolating through the soils and into the underlying rocks has dissolved the limestone in many places to produce rounded depressions, called sinkholes or dolines. Many dolines open into c,ave entrances. Over 90 caves have been recorded in this district, with narrow high openings developed along joints predominating. Many of the rocks at the surface display rillenkarren, small ripples developed by solution of the limestone. (Photograph by N . J . Rosengren).
,---p.
...
Geomorphology
61
Karst processes
Limestones are widespread in geological formations of all ages. They have one characteristic property - they are more easily dissolved by water than any other common rock. As a result, limestone country often develops a distinctive landscape called karst scenery. The name, karst, comes from a limestone region in Yugoslavia. There, despite a rainfall of 5000 millimetres per year, the land is a bare, rough limestone terrain with no surface streams. When rainwater containing dissolved carbon dioxide (carbonic acid) percolates from the surface down joints and fractures in limestone, some of the rock is dissolved. The o rig in al cracks beco me en larged to form openings of various shapes and sizes; these are often given special names. The most characteristic feature of a karst landscape is a conical depression called a sinkhole or doline. Potholes are small surface holes. Underground caves or vertical openings, called shafts, may also be formed. If the water table (see Chapter 6) is high, a sinkhole and any subterranean passages leading to it may be partly filled with water. In karst areas, streams often disappear underground following a cave system only to emerge again at the surface further downstream. Karst regions are often important areas of underground water supplies. Care must be taken to ensure that the water is not contaminated. This can easily happen if sinkholes are used for waste disposal, e.g. household rubbish, animal carcasses, chemical residues.
Figure 3-5 The Pyramids, a limestone hill 6 kilometres north of Buchan. This is a prominent landform overlooking the Murrindal River. The river has cut a deep valley along the boundary between Devonian limestone to the west and Devonian acid volcanic rocks to the east. Along this section o f the stream, Ihe now is underground through cavities i n Ihe limestone. The Pyramids display some typical karsl fealUres with Ihree tall pinnacles and a deep chasm on lOp of the hill and caves not far above the bed of the river. Fossil bones of small marsupials and rodents have been found in the caves. Rainwater can freely enter the formal ion along shallow-dipping bedding planes and major vertical joints. and so slowly dissolve Ihe limeslone. (photograph by N.J. Rosengren).
L imestone caves are usually decorated by redeposited calcium carbonate as
stalactites, stalagmites, j/owstone and other shapes. These features add to the tourist and scientific interest in limestone caves. There are usually creatures living in limestone caves, that have adapted to living in dark ness. Fo ilised remains of creatures that died in caves or feU into sinkholes in earlier geological times are also found sometimes. The best examples of karst features in Victoria occur in Devonian limestones in the Buchan district, East Gippsland. They are also common in Tertiary limestones that form cliffs along parts of the south-western coast. There are unusual limestone sinkholes along the coast near Torquay. There, sinkholes have formed in Tertiary limestone beds near the edge of the coastal cli ffs. The action of Storm waves has cut away the base of the cli ffs and in places has exposed the bOlloms of the sinkholes. It is therefore possible to stand on the beach and look up through a sinkhole and see the sky above.
Aeolian processes Landforms produced by wind are said to be aeolian in ong,". rhey are seen mostly in deserts and close to coastlines. They include dunes, sand and dust sheets, sand blowollls and dej/ation hollows. The laller form where wind has blown away loose surface material. Sand dunes are the best known of the e feature .
62
Chapter 3
Dunes may be: •
stable, i.e. they remain the same shape and in the same position over a long period;
•
active, i.e. they move slowly across the land.
or,
Dunes are stabilised if: • they are held together by the roots of vegetation or covered by vegetation; or, • the sand grains are cemented together by mineral substances, e.g. iron oxides, silica or calcite. Dunes are active where grains are loose, the ground is bare and continuously-blowing winds slowly shift the sands. Distinctive scenic landforms can also be produced where strong winds carrying hard sand grains gradually wear away rocks. The process is called sand blasting. Where all the fine material is blown away, an area, bare of vegetation and covered only by wind-polished pebbles, is left. This is called a gibber plain or gibber desert. In Australia, gibber pia ins are only found in arid, inland regions. Aeolian processes are important in relation to environmental protection issues, both along the coast and in inland lower rainfall regions. Because they mainly consist of unconsolidated mineral grains, aeolian deposits are very sensitive to any disturbance, particularly to the removal of vegetation. They may erode severely, especially wbere they are cleared for cultivation or subjected to excessive or unwise recreational use, e.g. where dune buggies or trail bikes are driven over them. There are serious erosion problems on the margins of the Sunset Country, the Big Desert and Little Desert of north-western Victoria due to excessive land clearing for farms. In times of drought and strong winds, dust storms can originate in these areas.
Marine processes Wave, current and tidal actions in the ocean are marine processes. They combine to shape the coastline and the offshore sea-floor. They also interact with forces from the land to modify existing landforms. The resulting coastline may be dominantly constructional with sandy beaches, tidal flats and salt marshes being built up. Alternatively, it may be desrructional, with cliffs and shore platforms be.ing formed as rocks are eroded away. Commonly both effects are seen along a particular stretch of coastline. For exam ple, rocky headlands alternate with sandy beaches along the shores of Port Phillip Bay. The coast is an important recreation zone, but human activities can cause changing panerns of erosion and deposition. (A more detailed discussion on processes acting along the coast is given later in this chapter). The sea-floor is not usually thought of as a landform and most people are unaware of its shape. Nevertheless knowledge of the form of the sea-floor is essential in ocean navigation, for investigating fishery resources and in the search for certain minerals that occur in sea-floor sediments.
Glacial processes Moving ice in the form of glaciers and ice caps, together with streams of water derived from melting ice, produce characteristic ero ional and depositional landforms. ice caps are large sheets of slow-moving ice which obscure the underlying rocks. They are only found nowadays in the polar regions, but during past ice ages they extended far out from the poles. There are no glaciers in Australia today. However, many landforms and deposits can be observed, which were produced by glaciers during Pleistocene (Tasmania and the Snowy Mountains), Permian (south-eastern Australia) and Pre-Cambrian times (South Australia). A glacier typically excavates a deep valley with a U-shaped cross-section. This contrasts with the usual V-shaped valley cut by an active stream in mountainous country. In Tasmania, U-shaped valleys in the western part of the island are evidence of Pleistocene glaciation. In Victoria, there are only small-scale glacial landforms caused by Permian glaciation. The thick mass of ice in a glacier, and the grinding action of the rock debris that it carries, can scour out large quantities of rock from a valley. Gravel and boulder debris can scratch and cut grooves (called striations) in the underlying bedrock. The resultant scoured surface is known as a glacial pavemelll (Figure 3-6). The striations on the scoured bedrock are parallel to the direction of ice movemen t. However, if the rock debris is pulverised rock flour, silt or sand, the underlying rock surface is polished rather than scratched. There are examples of glacial pavements along Werribee Gorge, near Bacchus Marsh, and beside Lake Eppalock. Where the ice melts at the end of a glacier, the boulders previously carried may be dumped in an area of entirely different rock types. Such boulders are called matics. An example is a 100 tonne granite block known as 'The Stranger', which was left on a hillside of Ordovician rocks at Derrinal, near Heathcote, in central Victoria (Figure 449).
Geomorphology
63
Figure 3-6 D unn's Rock, near Knowsley, a glacial pavement formed on Lower Ordovician sandstone bedrock. An area north and south of Lake Eppalock (east of Bendigo) contains the most outstanding Permian glacial features in Victoria. Dunn's Rock was found in 1892 and later named after its discoverer, E.J. Dunn. a former Victorian Government Geologist. There are numerous sub-parallel striations and other small grooves, that were scoured out by rock debris in a slowly-moving flow o f ice. Glacial pavements are best preserved under soil cover - they tend to deteriorate rapidly i f exposed to atmospheric weathering.
PROCESSES INSIDE THE EARTH Volcanic processes Although there are no active volcanoes in Victoria now, western Victoria has one of the World's largest young volcanic plains. The shape of the present-day country is nearly the same as it was when the volcanic period ceased several thousands of years ago. The monotony of the flat low-lying plains is relieved by numerous old volcanic hills. Lavas flowed and volcanic ashes were ejected from these vents during Pliocene to Recent times. Some of the volcanic hills are well-known scenic localities. Several, including Tower Hill, near Koroit, are protected as State parks. Hanging Rock, east of Woodend, formed by an exceptionally viscous lava, is also a tourist attraction. The volcanic plains are important economically. They are often covered by fertile soils, which are suitable for certain types of intensive cropping, especially potatoes. They also provide excellent pastures for grazing and dairy stock.
Tectonic processes Stresses occurring from time to time in the Earth's crust result in the lifting, breaking and bending of rocks. The movements are expressed at the surface as various kinds of mountains, basins and escarpments. These features have special names such as fault scarp, rift valley (or graben), tilt block and so on. Many examples are found in Victoria For instance, the road from Cranbourne to Phillip Island along the eastern side of Western Port is mostly flat, except where it rises over several tilt blocks (Figure 3-7). The Rowsley Fault scarp is another example: it is crossed by the Western Highway just west from Bacchus Marsh (Figure 7-9).
64
Chapter
3
Figure 3-7 Control of stream patterns by geological structures in west Gippsland.
i-
N
On the eastern side of Western Port, faults in a northeast southwest direction have produced tilted blocks of land. These form low lines of hills. The slopes on the north-western sides of the faults are fairly steep but those on the south-eastern sides are much gentier. The shape of the land controls the course of the Bass River. The stream nows along the foot of the Bass Fault escarpment. o ,
5 !
KllOMEl'AES
10 ,
Figure 3-8 Lake Omeo, near Benambra, north-easlern Victoria. The lake formed when the Morass Creek Fault caused the Benambra ridge to block a small tributary stream of Morass Creek. The ridge rose faster than [he stream could cut down into its valley. The lake is usually dry because the rate of evaporation exceeds the rate of water supply from its small catchment.
I
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RED GRADATIONAL SOIL
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RED·BROWN GRADATIONAl SOil
Figure 3-16 Important landforms and soils in the East Victorian Uplands. This diagram shows the differences in the typical landforms, soil types, bedrock geology, land use and environmental problems that are found in various partS of north
eastern Victoria. Typical localities for each landform are shown across the top of the diagram.
Many of Victoria's larger water storages are in Ihe dissected uplands, e.g. Hume, Dartmouth, Upper Yarra dams. Human activities are usuaUy excluded from their catchments to restrict erosion and pollution. Timber CUlling is an important activity in some areas with eucalypts supplying hardwood for the housing industry. The mountain treams are also popular for recreational fishing, e.g. the Goulburn River.
Wellington Uplands This region covers a belt of rugged country. which extends from Mansfield in a south easterly direction to ranges north of Maffra, in Gippsland. Much of the area is forested, difficult of access and uninhabited. It is made up of several basin-shaped areas of massive, hard, Upper Devonian and Carboniferous sandstones, conglomerates and acid volcanic rocks (see Chapter 4). The geological Slructure of the sandstones influences Ihe landforms developed. The rocks are mostly flat-lying or gently dipping. They often form plateaus or isolated flat-topped mountains (mesas), e.g. Mounl Ballery, Mansfield. The thick, resistant rock formations often present steep escarpments on the outside of the basin. In a few places, there are razorback ridges where the ranges have been dragged up along boundary faults. In the mountainous country, there are shallow loams on the ridges and scarps. Alpine humus soils are common on the high plateaus. The region s i part of lhe catchments for Eildon and Glenmaggie reservoirs. Outside these areas logging and saw-milling are carried out. A large area is reserved as parks, e.g. Wonnangalla Moroka National Park.
70
Chapter 3
The lower country around Mansfield in the north of the area is formed on softer red mudstones and shales of Carboniferous age. Grazing is the most important industry. Before the land was cleared, the vegetation was open woodland and forest, growing on red duplex soils.
WEST VICTORIAN UPLANDS
The West Victorian Uplands were formed by similar tectonic processes to those causing the uplift of lhe East Victorian Uplands. The West Victorian Uplands, however, are generally much lower and less rugged than the country in eastern Victoria. The highest mountains are just over l lOO metres, e.g. Mount William (l l67 metres) in The Grampjans. There are three subdivisions of the West Victorian Uplands.
Dissected uplands This region broadly covers the country, which is usually called the Midlands or the Central Goldfields. It extends from Ballarat and Gisborne in the south to Bendigo and St Arnaud in the north. Much of it consists of Lower Palaeozoic granodiorite and folded sandstones and shales. The dominant features are low north-south ranges (e.g. Pyrenees Ranges) and intervening broad, relatively low-lying corridors of valleys, plains and undulating country. The urnt is separated from the East Victorian Uplands by a major fault zone, which passes through Heathcote and country east of Lancefield. Figure 3-17 Moolort corridor in the Midlands. The Moolon corridor is a north· south belt of generally flat country formed by lava flows and alluvial plains beside the Loddon River, Thllaroop Creek and other streams. It separates hilly areas around Maryborough in the west and Maldon in the east.
D Alluvium � I22Zl
Ordovician rocks
v
5 I Kilometres
Landfonns on basalt flows: The corridors are occupied mrunly by basaltic lava flows and river alluvium (Figure 3-17). It is clear along some corridors lhat the divide between the north- and south-flowing streams in Victoria is not a range. It may not be a visible feature at all, where it crosses flat, open paddocks, e.g. through country just north of Ballarat. Tertiary lava flows, which once flowed down older river valleys, also occur in the hilly country. In the country around Daylesford and Trentham there are many examples of these long valley flows. They are often visible above the level of nearby present-day gullies. Some lava flows have been gradually eroded leaving a series of flat-topped residual hills, e.g. north of Daylesford. The Guildford Plateau, south west of Castlemrune, is a large isolated remnant of a basalt flow. The basalt overlies
Geomorphology
71
aUuvial gravels, that indicate a former higher level course of the Loddon River. In many places, a stream has cut its course along the boundary between basalt and older Palaeowic bedrock. Such streams are caUed laterals. They have formed where water flowed off the gently-curved, upper surface of a basalt flow to the edges. Where there are streams on both sides of a lava flow, they are caUed twin laterals (Figure 3-18). Examples are Goodmans Creek and Pyrites Creek on either side of the Mount BuUengarook basalt flow north of Bacchus Marsh (Figures 3-19, 3-20). Figure 3-18 (right) Development of late",1 and twin lateral streams.
3.
I. A stream is nowing through a valley in folded sedimentary rocks. 2. A volcano erupts and lava pours into the valley thus blocking the stream. A lake is formed. 3. The stream cuts a new valley along the edge of the lava now and forms a lateral stream. 4. Alternatively. streams cut valleys along both sides of the lava now and produce twin lateral streams.
Figure 3-19 (below) Mount Bullengarook lava now, north-east of Bacchus Marsh.
A now of Newer Basalt poured southward down an old river valley for 20 kilometres from Mount Bullengamok (673 metres above sea-level). The eruption occurred in hilly country formed by folded
Ordovician sedimentary mcks.
4.
� Streams, which formed after the lava solidified. preferentially cut down in the softer sedimentary rocks on each side of the now. These streams are called twin laterals. Thus there is now a ridge of basalt where there was once a valley. (Photograph by N . ) . Rosengren).
... , 'oj Roads
Figure 3-20 (above) Twin laternl slreams. north-easl of Bacchus Marsh. Goodmans Creek and Pyrites Creek are twin lateral streams that cut valleys alongside the Mount Bullengamok lava now.
72
Chapter 3
Some streams flow across wide volcanic areas. Where they tumble over the eroded edge of a mass of basalt, there are sometimes spectacular waterfalls. At the base of the fall there is often a plunge pool excavated by the force of falling water and debris. Behind the faIl is a notch worn away by the backspray. Examples can be seen at Trentham Falls, La! Lal Falls, Turpins FaIls near Barfold, Sailors FaIls at Daylesford. Figure 3-21 Trentham Falls near Trentham in central Victoria. The 15 metres high falls drop over a basalt flow, which formed several million years ago. It flowed down an old valley containing Thrtiary alJuvial sedimems, thal in turn overlie Ordovician sedimentary bedrock. The falls were originally formed about two kilometres downstream, where a new stream (now the Coliban River) flowed over the edge of the recently solidified basalt. The back-splash from the faUing water slowly eroded the sediments exposed beneath the basalt in the cli ff-face. The overlying basalt collapsed along vertical and horizontal joints. As a result of this progressive erosion of the cliff, the falls have been slowly retreating upstream. There is a cOntrast between the broad, open valley upstream and the steep-sided valley down tream. (Photograph by P.O. Dahlhaus) .
.. N
Land/onns on granitic and metamorphic rocks: Granitic rocks are common in the
HARCOURT
dissected uplands and there can be great contrasts in the landforms associated with them. They form the highest hills in some areas including the Langi Ghiran - Mount Cole group (east of Ararat), Mount Korong (near Wedderburn) and Mount Alexander (near Harcourt) (Figure 3-23). By contrast, some granitic rocks are deeply weathered and have been excavated by streams 10 form shallow basins. The Murphy's Creek area west of Tarnagulla is a good example (Figure 3-24).
5 r
10
Kilometres
Figure 3-22 Annular stream pattern near Metcalfe in central Victoria. The Coliban River has cut its course in Harcourt Granodiorite., near its boundary with contact metamorphosed Ordovician sedimentary rocks.
,
Around the granitic intrusions, the Ordovician sandstones and shales have been converted by the high temperatures of contact metamorphism 10 quartzites and hornfelses. The very hard metamorphic rocks often form conspicuous ridges or high peaks. This is particularly so through the country between Maryborough and Wedderb urn. In this belt, the prominent peak of Mount Moliagul is formed by hornfels, although the slopes are mainly granOdiorite (Figure 3-24). Mount Ararat near Ararat and Mount Tarrengower at Maldon (Figure 3-23) are of similar origin. Metamorphic aureole ridges usually have fairly steep slopes with poor, stony, gradational soils. Sheet erosion is likely to occur where they are cleared. The boundary between contact metamorphic and granitic rocks is often a zone that is easily eroded by running water. It is frequently followed by streams . Where there are curving courses around intrusions, these slreams are said to follow an annular drainage panern (Figure 3-22).
73
Geomorphology
Figure 3-23
...
Geological plan and cross-section of the country between Maldon and Harcourt in the Midlands.
...
In the east, Mount Alexander stands out east of the Calder Highway as a high landmark composed of granodiorite. By contrast on the western side, the granitic rocks occur in low hiUs, well below the level of the summit of Mount Thrrengower. This mountain i formed by hornfels, i.e. folded Ordovician sedimentary rocks, that were hardened by the contact metamorphic effects of the nearby granitic intrusion.
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Figure 3-24 Gt..'Ological plan and cross-section of the Murphys Creek area in Ihe Midlands. Murphys Creek is a cleared farming area belween the small settlements of Moliagul and Tarnagulla 10 the west of Bendigo. There are few outcrops of Ihe underlying Murphys Creek Granite (0 be seen, because it is generally deeply weathered. The contact metamorphosed Ordovician rocks in the hills near Tarnagulla and particularly tho e at Mount Moliagul to the west fo rm higher ground than the granitic country.
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74
Chapter 3
Granitic terrain also show other weathering features, which are not restricted to the dissected uplands. These include: • exfoliation domes, where sheets of rock are separated from the main mass of granite along curved joints parallel 10 the surface. • tors, which are rounded forms scattered over the ground (Figure 3-25). Til/en, which are gutters formed by a combination of weathering and erosion. There • are often deep crevices also. • caves: where this term is used in granite country, it refers not to underground openings, as in limestones, but to sheltered undercut platforms beneath roc k overhangs, e.g. Melville Caves, west of Inglewood.
Figure 3-25 Gnmodiorile lors in the Cobaw Range, north of Laneefield .
Prolonged wealhering and erosion of the granodiorile have len count less rounded outcrops induding some balancing rocks. Similar fealures on a larger scale are seen on the Mount Bu ffalo plaleau. (Pholograph by GW. Quick).
The Grampians The shapes of landforms in The Grampians have been largely determined by geological structures. These spectacular ranges consist of prominent ridges of resistant Devonian sandstone . The intervening valley have been cut in either soft shales or deeply weathered granite. Where the beds dip at angles up to 45', thc resullant landform has a steep escarpment and a gentler backslope. This feature is called a Cllesta. The Mount William, Serra and Wonderland ranges are the main examples. Where the beds have been affected by boundary faulls, Lhey dip very steeply or even vertically. The result is a more-or-Iess symmetrical ridge called a hogback, e.g. The Terrace, near Halls Gap.
Figure 3-26
Mount Abrupt al the southern end of The Grampians, near Dunkcld.
This form of hill is called a cueSla. It is formed by beds of sandstone, whieh dip lO the west at aboul 30°. The gentle len hand hil l,lopc is paraliel lO the bedding while there is a steep cscarpmcni on th e
eastern (right hand side).
(Photograph by G. Wa llis).
Geomorphology
75
A large synclinal basin surrounded by the Mount Victory Range is the main catchment basin of the McKenzie River. The river was dammed in the late nineteenth century upstream from McKenzie Falls to form Lake Wartook. Figure 3-27 Wartook Reservoir, The Grampians. The McKenzie River flows southward along the axis of a broad synclinal fold in the sandstone of the Grampians Group. The fold plunges gently to the south. The small reservoir was constructed in 1887, the first major water storage in The Grampians. Wartook Reservoir is one of several storages in and around The Grampians, which distribute water through the Wimmera-Mallee Domestic and Stock Water Supply System to a large semi-arid but agriculturally important region through a series of earthen pipes and channels. (photograph by N.J. Rosengren).
The regional strike of hard and soft beds controls the overall form of ranges and valleys in The Grampians. H owever, jointing in the sedimentary rocks has had a strong influence in shap;ng the tributary stream patterns and minor landforms (Figure 3-28). The combination of these factors has produced a (reI/is dmil/age parrern (Figure 3-29).
Figure 3-28 Oeft)
A deep crevasse on the Wonderland to Pinnacle walking track in The Grampians. This striking feature in the landscape developed from deep weathering and erosion along a vertical joint crossing gently. dipping sandstone beds. (Photograph by GW. Quick).
Figure 3-29 (right) Trellis drainage pattern in The Grampians. In this drainage pattern, streams flow for long distances in one direction, where they are parallel to the strike o f bedding planes in the sandstones. Where they turn suddenly at right angles they are probably following a major joint direction in the rocks. The resulting pattern resembles a garden trellis.
J 1
J
Chapter 3
76
Dissected tablelands
Figure 3-30 A typical soit profile in the dissected tablelands. A duplex soil B ferricrete C mottled clay zone with ironstone nodules D pal lid clay zone E parent rock. The duplex soils may be up to twO metres th ick, but they are easily eroded. Severe erosion leaves the hard ferricrete layer at the surface . The ironstone is sometimes cal led laterite, and the soil profile a lateritic profile.
The hard ironstone capping is not easily eroded. Much of the original flat land surface is therefore preserved. From the Pleistocene onwards, streams have cut deep, narrow valleys across the tablelands to expose a variety of parent rocks. The ironstone often forms low steep cli ffs at the tops of the valleys. Open woodland vegetation grows naturally on the ironstone soils on the tablelands. It has been partly cleared for pastures and some crops. Soils on the valley sides are quite di fferent to those on the tableland. They are mostly dark, well-structured clays called black earths. These soils suppon rich pastures used for sheep and cattle grazing. The steep valley slopes are subject to landslides during periods of prolonged rainfall. Gully erosion is common in the alluvium of the valley floors, where originally thick scrub held the soils in place.
SOUTH VICTORIAN UPLANDS
A
This unit COvers much of the country between Geelong and the south-west Victorian coast, and between the south-eastern side of Pon Phillip Bay and Wil ons Promontory. The South Victorian Uplands owe thei r elevation and shape to block fault movements during Tertiary to Recent times. For example, most of Mornington Peninsula is an upthrown fault block (Figure 3-3 1). Similarly the Strzelecki and Hoddle ranges between the Latrobe Valley and the coast are bounded by faults and monoclines, which broadly trend north-east to south-west.
°11
c
The Dundas and Merino tablelands to the west and north-west of Hamilton are the extensive remnants of an ancient land surface. During the Pliocene, a very thick soil developed on this surface because of deep intense weathering. The weathering affected aU rock types from granite to Tertiary marine sands. A typical profile shows fou r different zones over bedrock (Figure 3-30). Iron oxide from the decomposing rock has been concentrated near the top of the profile, mainly as a hard ironstone layer calledJerricrete. Iron oxide nodules also occur in the underlying clayey mottled zone.
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o
o
o
o
o
Figure 3-31 Momington Peninsula, a horst or up-faulted block. Movements along the Selwyn and Tyabb faults during the Cainozoic upli fted most of Momington Peninsula relative to the Pon Phillip Sunkland to the west and the Western Pon Sunk land to the east. The Nepean Peninsula and the Hastings coastal zone are low lying because they are on the down faulted su nklands.
nlXll.lme ej
lMJOI' ••1111 b.. C.rrow POWlII to ., __• ,• • _01'1 do¥o-nUvown ....,. PORT PHILL IP BAY
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Wilsons Promontory is called a granite residual rollge. It was formerly at the nonhern end of a much higher granite range that extended to north-eastern Tasmania. However, after east-west down faulting, the level of the land between Victoria and Tasmania was reduced and Bass Strait was formed. This left a chain of granite islands between Wilsons Promontory and Tasmania. Soils in the South Victorian Uplands vary greatly, depending mainly on the nature of the u'lderlying rock and the local geomorphic history. The Otway and South Gippsland ranges are made up of Cretaceous sand tones and mudstones. The soils on the ridges are mainly fenile gradational soils. On the lower slopes there are yellow or red duplex soils. These areas are used for forestry, grazing and as water catchment areas. For example, on the lower northern slopes of the Otway Range, the West Barwon Dam on the Barwon River supplies much of Geelong's water needs. Deep, well-structured, red clay soils called krosnozems occur on basalt in central Gippsland and near Flinders on the Mornington Pen in ula. Around Warragul, Thorpdale and Leongatha, krasnozems are used for dairying and market gardening, especially potato growing. Soils containing ferricrete layers are common on the Momington Peninsula and around the margins of the Ot\vay Range. They al 0 occur at a few places in South Gippsland.
MURRAY BASIN PLAINS This region covers the whole of northern and north-western Victoria north of the Central Victorian Uplands. There are three divisions of the Murray Basin Plains, each of which was formed by a different set of processes. The divisions are the R iverine Plain, the Mallee Dunefield and the Wimmera Plain.
Riverine Plain The Riverine Plain is dominantly of fluvial origin, that is it \vas built from alluvium deposited by rivers (Figure 3-32). There are two main levels in the plain: I. An extensive, older, higher level flood plain formed on an accumulation of Pleistocene alluvial sediments known as the Shepparton Formation. 2. Younger, generally narrow, lower level Ilood plains along the main rivers, especially the Murray River. These occur where the rivers have cut down into the older flood plain. The higher level of the Riverine Plain is also crossed by various low winding ridges. These mark the meandering courses of older streams. The laller are known as prior or ancestral streams. They are generally unrelated to presen! Streams. The meanders of ancestral streams form much larger curves than those of the exisling streams. This is because the size of a river meander is related to the amount of waler flow ing along the river, which in turn is related to the prevailing climate. The large meanders show that there were greater river Ilows during very wet periods in the
Geomorphology
Figure 3-32 Features of the Murray Basin riverine plain.
77
past. As meanders developed, the outer parts of the bends were eroded. At the same time, sediments were deposited on the inner sides forming a succession of crescent· shaped banks called point bars (Figure 3-33). Sand dunes close to the rivers are another feature of this terrain. During dry periods, winds blow the sand from the beds of the streams to t h e dunes.
This region is flat and featureless. The extensive high·level alluvial plain is crossed by narrow low-level plains formed by present·day, slow· oils called red·brown earths characterise the Shepparton Formation. They have flowing rivers and low winding duplex profiles and contain lime in the clay horizon. These soils are extensively ridges. The latter are natural levee irrigated for dairying, fruit growing and market gardening, e.g. in the Goulburn Valley. banks, which formed along the They are also used for dry farming. In recent times, salting and waterlogging have eastern banks of ancient river . become serious problems in the irrigation areas (see Chapter 6). To try to combat Faults in Recent times produced these threats, extensive drainage schemes have been constructed to remove the saline escarpments up to 5 metres high. waters. Some of the fauh movements were The soils in the ancestral valleys and on the present flood plains are grey with relatively rapid. Upli ft of a block high sodium contents. Their main u e is for grazing. of country north·west of Echuca forced the Murray and Goulburn rivers to change their cour es by swinging suddenly to the south across the _ _ _ _ _ _ _ _ _ _ _ _ _ ... _ _ _ ...._ -:_ downthrown
S
Echuca Depression.
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f ..J :::> ..: u.
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,
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H
J.Flor:HFSTF R
(
c::B:J Higher aI/uvial plain deposited by Pleistocene streams r:;h;I LoweraI/uvial plain deposfted by modern streams lSI Abandoned Pleistocene stream courses with nalural levee banks r>rul Former lake o 5 '0 E�3Swamps ! I I Kilometres � Lunettes
__ Recent fault escarpment
N
H
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20 I
Geology Irom Bendigo ' :250,000 sheet
(Geological Survey of Victoria) and J.M. Bowler and LB. Harford
78
Chapter 3
Figure 3-33 Poinl bar deposits in a meandering
stream. Most rivers tend to meander because water flows are turbulent. The faster now on the outside of
any bend (a, b) causes the stream to undermine its banks. Sediments are deposited by the stream, where the flow rate is slowest on the inside bends (c, d). The deposits occur on the point of the meander and are called point bars. The stream valley slopes downstream, so erosion is greatest on the downstream end of each meander bend (e, I). Meanders gradually migrate
Mallee Dunefield There are two subdivisions in this region. They are dominated respectively by low calcareous sand dunes and high siliceous sand dunes (Figure 3-34). These landforms are formed by wind action. The word, 'calcareous', means the dunes contain abundant calcium carbonate. Siliceous dunes are made up of quartz grains. Low calcareous dunes
The low calcareous du nes are elongated in a west-east direction. This is about the same direction as the dominant westerly wind, which moves the sand. Such dunes are said to be longitudinal. The dunes were probably formed when the climate was drier than it is today. The calcareous dunes often contain several layers of calcium carbonate. This shows the dunes were built up in stages, with alternating periods of stability and wind activity. Older soils developed during the stable periods are called palaeosols. For many thousands of years, water has been discharging from the ground into low areas between the dunes. This water has dissolved saiLS from the underlying sandy materials. After it reaches the surface, much of the water evaporates, especially during the hot summer periods. This leaves salt lakes (salinas) and gypsum jlats. On the eastern side of each salt lake, there is usually a low crescent-shaped ridge called a lunelle (Figure 3-36). It consists either of clay, silt and fine sand or powdery gypsum (called COpt). This material has been both blown from the lake floor by prevailing westerly winds and carried by wind-generated waves. Like the longitudinal dunes, lunettes have been built up in stages and they often contain palaeosols.
ideways and
downstream.
fi gure 3-34
b
Landforms of the Mallee Dunefield and northern Wimmera.
East-West dunes
\\\
The main features of the Mallee Dunefield are east-west longitudinal calcareous dunes and
) )
Coastal ridges Arcuate dunes
intervening low-lying sandy nats, dOlled with small shallow salt
a,b Cut bank c,d Point bar
e,f Direction of point bar migration
lake . Curved siliceous du nes predominate in the 'desert' counrry. There are various swam p and lake deposits in the Wimmera. There are also stranded coastal ridges formed during the Pliocene in both the Wimmera and the
,
.., ,
,
Mallee.
BIG DESERT ,
)
)
,
)
.,
,
)
LdAe , Hmdrndfstl
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50 KIlometres
Figure 3-35 \Vyperfeld Nat ion a l Park in the southern port of the Malice Dunefield. High siliceous dunes of the type seen in the background extend across red sandv flats. The soils
r
are not very fe tile but they support a varied native nora. Native pines (Callilris) are in (he mid-di lance and the low vegetation includes the yellow flowering plants Senecio laullls (Variable Groundsel) and Glischrocaryoll behr;; (golden pennants), and some clumps of spini fex. This country is very susceptible to wind erosion i f cleared for farming. (photograph by I. Dunn).
100
Geomorphology
79
Figure 3-36 Block diagram showing the stages in the growth of a clay luneUe The luneue forms on the downwind side of the lake. The 'crest' gradually migrates away from the lake as successive layers are built up. The 'beds' have a low angle of rest. There is a low eli ff on the upwind side, where groundwater leaks OUl and causes the toe of the slope to recede. (After J.M. Bowler, Proc. Roy. Soc.
SECTION
Vicl., 95, 1983).
PLAN
The low west-east calcareous dunes have been almo t entirely cleared for growing crops and grazing. The soils are dominantly reddish sands overlying compact loam. In the drier northern part of the Mallee, cropping is a marginal occupation. High siliceous dunes High siliceous dune either extend at right angles across the general west to ea t direction of the prevailing winds or t hey have the shape of a parabola. They are a feature of the Big Desert to the north of Nhil\' The soils on t h e high siliceous dunes are infertile sands and sandy podsols. If t hey are cleared, t hey become very susceptible t o wind erosion. Consequently l i t t le clearing of t imber ha taken place, although t here is limited grazing in ome areas. They do, however, carry a large variety of native vegetation. Exten ive areas have been set aside as parks or as other reserves, e.g. Pink Lakes State Park, Wyperfeld ational Park (Figure 3-35), Big Desert Wilderness Area, Red Bluff Wildlife Reserve.
Wimmera Plain This division extends to the north and outh of the Western H ighway over the country between Horsham and Ihe border with South Australia. The clay plains of the northern and eastern Wimmera are a mixture of aeolian, lake and swamp deposits. They arc nat to undulati ng w i t h some low west-east dunes. To t he south o f N h i l l , t he L i t t le Desert i a dunefield consist ing o f fine- to medium-grained quartz sand. Some of t he dunes have the shape of a parabola, but t here are also many irregular forms. In the southern Wim mera, which extends southwards from t he Goroke area towards Edenhopc, there arc nort h-west to sout h-ea t dune ridges and nats of swamp, lake and lagoon origin. There are many small lakes on the nat . Each has a lunette at its eastern edge. Another feature of the Wimmcra Plain, and t o some extent of the Malice regio:1, is a series of parallel straight to curving ridges (Figure 3-37). The,. extend into the Lower South-East region o f South Australia. These ridges were formed along the shorelines of ancient coasts during Pliocene t imes. During much of the Tertiary period, a large gulf extended from t he open sea across sout h-eastern South Australia, north western Victoria and western New South Wales. The sea retreated in stages and each ridge indicates a temporary shoreline. Examples of slumping and bedding feat u res associated wit h shoreline deposit ion can be seen in road cutt ings along t he Western H ighway at Kiata and Lawloit, near The \V illlllll'ra Plain i�
hill.
co\cn:d bv t!rcv. brown and red
h.: arco ll s day soils.
(a
They arc highly prod uctive and suppo; , ; l ilfiving wheal rind graLing. indu i ry. On t he other hand, pale acid sands of t he Little Desert arc not fertile, so t hev arc on I" lI�cd for farming to a small extent. However. because the soils rarry a grcrl l \'ariC'I
;'
80
Chapter 3
Figure 3-37 Old coastal ridges in western Victoria.
75
,
100
I
Many low ridges form prominent landforms across parts of wes tern Victoria. These are deposits of sands left along the coastline at various times, when the sea-level was relatively higher than it i s today. The sea reached its maximum extent inland during the Miocene The ridges fo rmed as the sea retreated in stages during the Pliocene· and Pleistocene times. Between Portland and Cape Otway. scattered occurrences of Pleistocene marine sedimems and Recent raised shell beds are further evidence of higher sea-levels in the past. There are also some low escarpments formed by Quaternary fault movements.
WEST VICTORIAN UPLANDS
SOUTHERN OCEAN CAPE OnrAY
MaXimumManneTransQres�tQns
-...._.. MIOCene
- . - Pliocene
Faults, monoclines and strucrurallineamenrs
CoastalForms
':::::: Pliocene coastal ridges .;::: Pleistocene coastal ridges
CoastalPfP05�S
\b. Quaternary coastal zone sediments *
J;.
Pleistocene marine sedIments · isolated occurrences Emerged Recent shell beds
of native plan IS, a large area south of hill has been set aside a Ihe LillIe Desert Nalional Park. In the southern Wimmera, the sand ridge are dominated by pale acidic sands wilh a podsol profile. By contrast, Ihe intervening Oals have yellow sodie duplex soils. The lerm, sodie, indicates a high proportion of odium ion . The e disperse the clay subsoils when they are weI, leading 10 poor drainage. Grazing is the main agricultural aClivilY with ome cropping.
WEST VICTORIAN VOLCANIC PLAINS The volcanic plains streIch we Iward from Melbourne almost to the South Australian border in a belt averaging about 100 kilometres wide. Arm of Ihis plain also exlend up valleys to Ihe north of both Ballarat and Melbourne, where lavas Oowed from volcanoes near Ihe presenl drainage divide. The volcanic plains are Oal to undulating and dOlled wilh many hill formed by eXlinct volcanoes. N umerous, relalively thin, basalt Oows form the bulk of the plain. Volcanic ash deposits are also a sociated with many volcanic hills. The volcanic
Geomorphology
81
material was derived from eruptions which mostly occurred 2 to 4.5 million years ago. Sporadic volcanic activity also continued through the Pleistocene into Recent times. 1L has been calculated that the youngest volcano at Mount Napier, south of Hamilton, occurred only about 7240 years ago.
Figure 3-38 Mount Cotteril, 10 kilometres south of Melton. The gently-sloping shield volcano of Mount Coueril is about 8 kilometres in diameter and formed by radial flows of fluid basaltic lava. (Photograph by G. W. Quick).
-.
_ 11 ...
Figure 3-39 Mount Elephant, near Derrinallum, western Victoria. This steep-sided scoria COne is one of the highest extinct volcanoes in Victoria. The crater is 90 metres deep and the summit about 240 metres above the surrou nding plains. There are two breaches in the volcanic cone, where small flows of basalt lava emerged. The fragments thrown out by the volcanic activity range in size from fine ash to coarse bombs and blocks. (Photograph by N.J. Rosengren).
..
,.
--•
( Figure 340 View looking south from a lookout at Red Rock, 1 2 kilometres north-west of Co lac. The Red Rock volcanic complex consists of various maars and scoria cones. There are several lakes, where the craters are deep enough to expose t he water table. The maars are surrounded by low tuff rings. Red Rock lookout is at the lOp of a scoria cone. Lake Corangamile is in the distance. I n this area, there were lava flows first, then maar explosions and finally scoria eruptions.
•
82
Chapter 3
Volcanoes are either quiet or explosive. About half of them were lava volcanoes, which are characterised by gently sloping sides, e.g. Mount Coneril, south of Melton (Figure 3-38). These volcanoes probably erupted quietly, with streams of molten lava flowing down their sides and across the plains. Scoria cones are the other common type of volcano. These are composed of scoria, made up of irregular lumps of basalt lava, full of gas bubbles. Scoria volcanoes are up to 90 metres high and have steep slopes, e.g. Mount Elephant north-west of Colac (Figure 3-39). The scoria cones erupted as "fire mountains". During these eruptions, blocks of red hot lava were continually spraying OUt of the mouth of the volcano to land on its slopes. These lumps of frothy lava then cooled and solidified to form scoria. At many scoria cones, there was a final period of quiet volcanic activity, when lava broke through one side of the cone. This produced a breached cone. There are about tWO hundred breached cones in Victoria. The third type of volcano in Victoria is called a maar. There are about 40 maars, mostly between Colac and Port Fairy (Figure 340). These volcanoes have large circular craters, up to 2 kilometres across and often filled with lakes, e.g. Tower Hill, north east of Port Fairy. The raised rim of the crater is composed of layers of volcanic ash and thin deposits of this ash can extend for several kilometres a,vay from the crater. These volcanoes \vere formed by very explosive eruptions, approaching small nuclear explosions in force. As molten magma intruded the sedimentary rocks underlying the crater, it suddenly encountered ,vater within the rocks, perhaps filling caves developed in Tertiary limestone. The water \vas superheated to steam and exploded with devastating force, blowing fragments of magma and pieces of limestone into the air. These fel l to the ground as the layers of ash that surround the maar crater. The prevailing winds during the eruption caused most of the ash to be deposited on one side of the crater. It is notable that most Victorian maars have thicker ash deposits on their eastern sides, reflecting the dominant westerly wind direction. Examples of various kinds of volcanic cones are given in Figure 3-41. Figure 341 Examples of volcano types on the West Viclorian Volcanic Plains.
lYpes
Examptes
Locality
Scoria cone
Mount Elephant Mount Napier
near Derrinallum south of Hamilton
Breached cone
Mount Franklin Mount Eccles Mount oorat Mount Shadwell
near near near near
Lava
Mount Bainbridge Mount Blackwood Mount Cotteril
near Hamilton nonh-west of Bacchus Marsh south of Melton
Maar
Lake Purrumbete Moun t Leura Tower Hill Lake Keilambete Lake Terang
near Camperdowp near Camperdown near Warrnarnbool at Terang at Terang
Daylesford Macarthur Terang Monlake
Surface features of the original lava flows have sometimes been preserved, especially on the younger ones. The surface is either rough and blocky or it may be fairly smooth. Smooth surfaces have small winding or contoned ridges, which look like rope. The latter type is called ropy lava, (e.g Harman Valley flow from Mount Napier). After the surface olidified, molten lava sometimes kept moving inside a flow and pushed up hillocks of consolidated lava called IUlI/uli (Figure 3-42). An example occurs near Wallacedale, south-west of Hamilton. If the lava beneath the solid crust drained away, a lava tunnel was left (Figures 3-43 and 3-44). Commonly the crusts of the tunnels collapsed leaving a trough and ridge terrain locally known as stony rises. Lava tunnels and stony rises occur at Skipton. Mount Hamilton. Byaduk, Mount Eccles and Stonyford. Where the lava flows were thick, they usuaUy cooled slowly and developed a regular, close pallern of joints. When viewed from the side, these now appear as columns. If they are exposed in the floor of a valley. a pavement of hexagonal blocks is seen. Good examples occur in the Organ Pipes National Park near Sydenham (Figure 3-45) and at a locality three kilometres east of Romsey beside the WaUan road. Lakes and swamps orten formed inside the depressions produced at maars. There are also many others in shallow, generally irregular depressions on and close to the lava flows. Some formed where existing creeks were blocked by lava flows. For example the Condah and Whittlebury swamps, south of Hamilton, were formed where basalt
Geomorphology
83
flowed west from Mount Eccles along Harman Creek valley and blocked an ancestor of Darlot Creek and its tri butaries (Figure 3-46). Extensive swampy flats also occur behind lava flows at Wallan and south of Whittlesea. Lake Corangamite, the largest lake in Victoria, is a remnant of a much larger older lake, which was partly filled by lava flows (Figure 3-47). Figure 342 La"" tumulus at Wallacedale, south-west of Hamilton. Thmuli are mounds up to 20 metres across and 10 metres high on a lava flow. They formed after the basah surface had solidified. A concentration of gas pressure developed in the underlying cooling lava, which buckled and cracked the overlying lava crusl. The inner lava is frothy. This tumulus formed on a basalt flow from Mount Napier. They are very rare features. (photograph by L . B . Harris).
Figure 343 Lays cave near Byaduk, western Victoria A lava flow, extruded from MOUn! Napier to the east, nowed westwards down an old 10 metre deep, steep-sided river valley and just overflowed the top of the banks. The uppermost skin of the lava solidified as it was cooled by the atmosphere. The underlying lava was insulated by the valley walls enclosing il. This lava therefore remained molten and continued to flow down the valley. After the eruption had ceased and all the lava in the valley had nowed away, a cave was left beneath the surface skin. The skin eventually weathered and collapsed to expose the cave. (photograph by N.W. Schleiger.)
Figure 344 Lake Surprise, Mount Eccles National Park. A volcanic crater developed along a fissure from coalescing vents. A lava tunnel extended away from one end of the fissure. The craler is made up of ahernating layers of scoria and blocky basalt lava. A lake now fills the crater. At the southern (left) end of the fissure, the hill is a spaller cone formed from scoria and volcanic ash. (Photograph by N .W. Schleiger.)
84
Chapter 3
Figure 345 The Organ Pipes,
ydenham.
The vertical columns of basalt are part of a lava flow which was erupted from a nearby volcano. The lava filled an old vaUey in folded Palaeozoic rocks to a depth of 70 metres and then spread over the adjacent area; both vertical and horizontal cracks developed due to the rock shrinking as it cooled. Cracks extending down from the surface and up from the base joined to give the high columns. Jacksons Creek cut a new valley into the lava plain and exposed The Organ Pipes.
Figure 346 Swamps fonned along the edge of the Mount Eccles lava flow in western Victoria. Condah, Whinlebury, Homerton and other mall swamps were formed after creeks were blocked by the lava from Mount Eccles. Darlot Creek is a lateral stream along the edge o f the basalt flow in places.
..
WOOLSTHORPE /SWAMP
QUATERNARY
o
Tyrendarra flow
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OCEAN
TERTIARYQUATERNARY
PAINCF:S
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Swamp allUVium 8asall (valley flows). scoria 8asall Sand. alluvium. limestone Volcanic cone o ,
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KILQMETRES
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Geomorphology
Figure 347 Lake Corungamite on the West Victorian Volcanic Plains. The presenl Lake Corangamite is the remnant of a much larger older lake, thai formed when an ancestor of the Barwon River was blocked by a flow of basahic lava in Pliocene times. Laler small flows of basah extended into the lake reducing the area of waler considerably. Some of Ihe ridges in the lake are stony rises or narrow longues of solidified lavas. The lake has three limes the content o f dissolved sahs as Ihe sea and is less than two metres deep. Crescent-shaped lerraces and ridges along its eastern shore show that past c1imalic changes controlled Ihe rise and fall of the water level. Figure 3-48 Main soils on the "'est Victorian Volcanic Plains.
5 !
10 15 I ! __' H
85
ZO ,
Campetdown
__l""" 01 '" �''' �.f' C"'�. v>gL .""
The soils on the volcanic plain are variable depending on the ages of the volcanic flows, Iheir elevalions, their history of erosion, the past and present climale and the nature of any sediments deposited after the lavas solidified (Figure 3- 48). Land ""
E.�p�
i croppng. grazing
Hamilton nonhwards
Pleistocene
cropping. grazing
Melton to Geelong
yellow brown sodic duplex
Pleistocene
grazing
Camperdown 10 Skipton
)'cllow acidic: gradational
20 (XX)
years
foresu)', grazing. poI31oes
Warrnambool Koroil
grey .sadie c13.)'5
10 000
grazing
",KJe:sprnd
red and b ro ... ..n stony gradational
)�
10 000 )'eafS
grazing
wKJe:sprnd
landrorm
Soil
Higher plain
mt duplex with iroru;to�
ImermedialC� plain
� plain 5100)'
rises
calcareous
sadie duplc.x
Ag. Pliocene to about 2 million )'tar5
SOUTH V I CTORIAN COASTAL PLAINS A coas/QI plain is flat-lying land near the coast, that was once benealh the sea. The plain emerged above lhe present sea-level, because there was either an uplift of Ihe land or a fall In sea-level or both in recent geological limes. There are two large coastal plains in south-western Victoria and two smaller o�� beside POrt Phillip Bay. Large sand barriers are also included in this geomorphic diVISion. They occur along much of lhe South Gippsland coast and lhe coast to the west and east of Portland. Each of the plains and the and barriers are discussed in turn.
Follet Plain This is in the south-west corner of the Stale beyond Hamillon. It continues to the west across lhe Lower South-Easl region of Soulh AUSlralia. It consists of a series of long, low, narrow ridges, which are parallel to the present coast and separated by sandy and swampy flats. The ridges were originally dunes formed by cross-bedded, wind-blown, calcareous sand. They are made up largely of small fragments of shells. The dunes consolidaled to form the rock, aeolianite, after lhe original grains were cemented logether by calcium carbonale. The lower Glenelg River and its lr ibutaries have eroded deep valleys into the plain exposing underlying Tertiary and Pleistocene sediments. There is a variely of landforms on lhe plain. Consequently the dislribulion of soil types is also complex. The dune ridges moslly carry pale acidic sandy podsols. Their pH is less than 7. In some places, however, there are lime-rich soils called terra rossas or red earths. The dune soils support limited grazing. Soils on the flats are moslly humic acidic sands or mottled duplex IYpe . They are poorly drained. Agricultural development i therefore limited becau e the soils are often walerlogged. However, this landform- oil complex supports many flowering plants. A large area has been reserved as the Lower Glenelg Nalional Park.
86
Chapter 3
Port Campbell Coastal Plain In mid:reniary times, this dissected plain extended from the coast and the Otway Range as far inland as the West Victorian Uplands. However, a large pan of it was later covered by the lava flows and tuffs of the West Victorian Volcanic Plains_ The coastal plain is terminated on the seaward side by spectacular sea cliffs (Figure 3- 49). The flat-lying limestones and marls that form the base of the plain, were originally deposited on the floor of the sea. After uplift, they were largely covered by clays and sands laid down by rivers. Some of the sand has been subsequently reworked by the action of winds to form dunes and sand sheets. Some of the limestone areas show typical features of karst terrain, even where they are covered by river clays. There are many sinkholes and interconnected caves (Figure 3-50).
Figure 349 (below) Features of the Port Campbell coastal plain and coastline in the Peterborough district. The plain is clearly underlain by widespread limestone. The large numbers of sinkholes provide a karst landscape. Groundwater has gradually dissolved blocks of limestone as it percolated down joints in the rock. In many places, overlying river clays collapsed into lhe sinkholes after lhe limestone was removed. Many small sinkholes expose the water table. Along the coast, storm waves continually attack the limestone cliffs, gradually eroding them away. Remnants of former cliffs are left as picturesque offshore rock stacks in the ocean. (Figure 3-1). There are also extensive Pleistocene and Recent dunes along this section of the Victorian coast. Curdies Inlet is a shallow body of water, formed because Curdies River is almost blocked off from the ocean by a sand barrier.
� •
The plain has been dissected by streams rising in the western Otway Range. The trends of their valleys have been in fluenced by four factors: I . Pliocene coastal ridges, which were left as the sea retreated across the plain.
2. Tectonic movements that produced broad domes and depressions over the plain. 3. The diversion of streams by lava flows. 4. The building of sand barriers along the coast. The soils on the coastal plain frequently contain large ironstone concretions called buckshot gravel. The gravel is mostly loose but sometimes is cemented into a massive layer. The topsoils are sandy and poor in plant nutrients. In the Gellibrand River catchment and sporadically across the remainder of the plain, there are sandy duplex soils overlying clay or a hardpan. The hardpan consists of clay cemented by iron oxides. The main agricultural activities are sheep and cattle grazing with some dairying. The Heytesbury land settlement area was developed for dairying in the 1950s in a formerly heavily-forested area to the nonh of Pan Campbell. However, the soils are not very fertile and large quantities of artificial fertilisers have had to be applied. I n addition, soil erosio n and a build-up of salt have developed. Unfortunately these problems were not foreseen at the planning stage.
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CJPrrUf'll 0Hd1H D���",, � l_ PfN,� cftI lOp !lIArs
Id.II «t . D r=.c:onKI lc1q, ,s..".. ,..,
.,IiCU wrtI� �IOWY>'G .... RQdc •
.....-. C "fdQ),UI
1000 1500 ! ! Melres
2000 !
Bellarine Peninsula and Moorabbin plains These plains beside Port Phillip Bay are made up of sandy dune ridges and sheets, with intervening clay swamps. On the Moorabbin Plain, a series of low, parallel sandy ridges can be traced across the south-eastern suburbs of Melbourne. The ridges mark the positions of successive shorelines; they \vere stages in the retreat of the sea in Late Pliocene times. Bellarine Peninsula has a central core of Teniary basalt overlying older rocks. The surrounding areas consist of Teniary sediments and Quaternary dunes, sand sheets and swamps.
Geomorphology
Figure 3-50 Limestone cave at Loch Ani Gorge, near Port Campbell. Wave action has eroded the foot of the limestone cli ffs and formed caves. Groundwater percolating down through the rocks dissolves some of the limestone. Where the groundwater appears at the roof of the cave, evaporat ion leaves a deposit of calcium carbonate. This gradually forms a stalactite. Some drops of water fall to the noor. Again evaporation occurs and gradually a stalagmite is built up. In places a stalactite and stalagmite grew so large that they mel 10 fonn a column. (photograph by N.W. Schleiger).
Figure 3-5) Ewing Marsh and coastal saod barrier, east of Lakes Entrance. A dune-capped sand barrier (A) extends for over 50 kilometres from Red Bluff, near Lake lYers, to Point Ricardo, east of Marlo. Behind the barrier is Ewing Marsh (B), a swampy lagoon and 200-300 metres further inland is a low, former coastal lerrace (C), eroded in Tertiary sedimentary rock . The sand barrier is 80-100 metres wide and has a narrow, teeply-sloping beach in front of it. The dune is mostly stabilised by vegetaLion, but in the foreground there is a btowout� which carries sand into the swamps. In the mid-distance is Hospital Creek (D), a stream that drains into the marsh because it lacks sufficient erosive energy 10 breach the sand barrier. I t is one of several creeks along this seclion o f the coast, that fai ls to reach the ocean. (Photograph by N.J. Rosengren).
87
SS
C h ap ter 3
Coastal sand barriers Long accumulations of sand are common along the Victorian coast. They were built by the action of waves across bays and river mouths and have been modified by tides and winds. In East Gippsland, where they are best developed, there is a succession of barriers ranging from Late Pleistocene to Recent in age. The sandy barriers are favoured sites for holiday developments at such localities as Loch Sport, Woodside Beach and Marlo. However, at some of these resorts, considerable problems with sand blowouts have arisen where natural vegetation has been removed.
SOUTH VICTORIAN RIVERINE PLAINS The riverine plains of south-eastern Victoria have been built up by alluvium deposited by rivers flowing southward from the East Victorian Uplands across Gippsland to Bass Strait. They commonly form extensive swampy flats, especially at the northern end of Western Port, e.g. Koo-wee-rup Plain. There are three levels of the riverine plains - the present flood plain and two higher levels of terraces. The terraces are the remnants of earlier flood plains, which were cut into by the rivers when the land was uplifted. These older riverine plains are extensive in south-east Gippsland. The lower of the two, known as the intermediate terraces, carry red duplex soils. The sandy courses of earlier streams form minor rises in an otherwise extremely flat landscape. There are also slight depressions occupied by grey or pale yellow swampy soils. The colour results from iron in the soils being in the reduced state due to intermillent waterlogging. Nevertheless large areas of pastures are irrigared and have become a major source of dairy products, e.g. Warragul-Drouin area. The higher terraces represent a former extensive flood plain with alluvial fans at its inner margin. The terraces are crossed by roughly parallel sandy ridges that are separated by swampy depressions. Most of the towns in the Latrobe Valley are on these higher areas which are relatively well-drained. There are also extensive pine plantations, especially on the sandy soils south of the Latrobe River and north o f Lake Wellington. Elsewhere sheep and callie graz.ing i s dominant.
The Victorian coast
• "i gure 3·52 The coastal zon� where forces from the land meet forces from th e sea.
There is an alternative to studying the geomorphology of Victoria in terms of the various geomorphic divisions. One can concentrate on zones where one geomorphic process predominates. For instance, all the landforms along a particular river valley could be studied. In this book, one zone - the coastal belt of Victoria - is selected for a detailed investigation. This is where the most important processes are of marine origin. The coastal zone is the strip at the edge of the land where forces from the sea meet and i nteract with those from the land. The zone is up to several kilometres wide (Figure 3-52) .
f"
Forces from the sea
..
Coastal zone
---
-
F orces tf orn Ins lane
--===-
Coastal plain
000" ",0'" ... "
Continental slope
A large proportion of the population of Victoria lives close to the coast. As in most tropical and temperate countries, the coastal zone is probably the most popular recreational area for Victorians. The coast is also important because it provides ports and harbours. Australia is dependent on world trade for its prosperity. It is therefore essential that deep-sea ships should have access to sheltered harbours at intervals along the coast. It must be remembered, however, that coastlines are nOl static unchanging environments. At any one time, coastlines may be either building out\vards or being worn away. These changes are caused by various natural forces, especially those o f the sea. The sea exerts enormous forces on the coast. To a large extent these forces cannOl be controlled by human activities. Some of the natural changes along a coast are seasonal and have no permanent effects. For example, winter storms may carry sand from one side of a bay to another; but in the summer, winds and waves from a different d irection will return the sand to its original location (Figure 3-53). On the other hand over a longer period, imperceptible rises in sea-level may lead to enormous quantities of sand being carried away and this material may nOl return in the short term.
Geomorphology
Figure 3-53 Longshore sand drift at Black Rock beach o n the north ...stern side or Port Phillip Bay. From April to November, northerly to north-westerly winds cause waves to carry sand towards the southern end of the beach. Predominant south to south· westerly winds from November to March drive the waves in a north easterly direction and the sand returns to the northern end of the beach. (After E.C.F. Bird, I 976). Black Roc/.. P()lfIt
Nov -Mar WAVES
EOiI' 01 Be. t h "Ulufft" ( 1.4 . , -AD< - -- So
Tatong
�
\ \'0 0 l.cola
\.
Bai rnsdaleo
o ,
Portla�d
Figure 4-5 Distribution of Cambrian rock outcrops in Victoria and the probable Cambrian
�
Greenstone and Sedimentary rocks
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AREAS OF OUTCROPS
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�o Rochester
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25 50 75 100 !
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,
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r
The Cambrian rocks or Victoria fall into rour main groups: I.
Greenstone belts: narrow, discontinuous belts or dark green, altered igneous rocks
of Early to Middle Cambrian age. 2 . Fossiliferous sediments: fossiliferous sed imentary rocks of Middle to Late Cambrian age, thaL overlie the greenstones in Central Victoria.
3. Glenelg River Beds: un fossiliferous, metamorphosed sedimentary rocks, that outcrop north or CasLertOn in western Victoria. 4. St Arnaud Beds: unrossiliferous, slightly metamorpho ed sedimentary rocks, that occupy a large area between the eastern margin of The Grampians and a line approximately joining Charlton, Avoca and Ballarat. This line is sometimes called the Wedderburn Line. Fossils in the sedimentary rocks associated with the greenstone belts prove the c rocks are Cambrian in age. The unro siliferou Glenelg River Beds and SI Arnaud Beds are thought to be Cambrian because or indirect evidence discussed later.
REGIONAL SETTING During the Cambrian, Victoria was covered by an ocean that extended far to the east and was deep in most places. The nearest coastline was to the north-west, extending along the eastern side or Ihe present Mount Lorty Ranges and continuing in a north-easterly direction past Broken Hill. Mountain ranges were present inland in both South Aust ralia and New South Wales. Erosion of the rocks in Lhese ranges probably provided the material to form the St Arnaud Beds and other Cambrian sedimentary rocks on the noor or the ocean. There were also large volcanoes scaLlered across the sea-noor. Some or Lhese built Lheir cones above the surface of the sea, where they were eroded by wave action. The eroded volcanic material contributed
Geological History of Victoria
103
to the sediments accumulating on the surrounding sea-floor. Between the volcanoes, there were also extensive eruptions of basalt lavas from long cracks in the bed of the sea. Cambrian limestone beds i n the Dolodrook River valley, north-east of Licola in Gippsland, indicate that the sea was shallow in at least one area.
ROCK FORMATIONS Greenstones Greenstones occur as discontinuous outcrops along narrow belts, only a few kilometres wide. Most of these belts trend north-south to northwest - southeast across the Central Victorian Uplands (Figure 4-5). In addition, narrow subsurface greenstone belts have been traced through country west and north-west of Horsham by geophy ical surveys and a few borehole intersections. The boundaries of the belts are generally major faults. There are also small, isolated outcrops of greenstones in southern Victoria along the western coast of Waratah Bay, (south of Walkerville South), and in the Barrabool Hills, west of Geelong. Cambrian igneous activity in Victoria started near the beginning of the period and probably finished in the Middle Cambrian. Both volcanic and intrusive rocks are present and they range in composition from intermediate to ultrabasic. The main minerals originally present i n these rocks were plagioclase feldspar, pyroxenes and often olivine. Quartz is absent. The igneous rocks were later modified by low-grade regional metamorphism ,caused by i ncreased pressure and temperature. These changes resulted both from the weight of large thicknesses of younger, overlying sediments and from deformat ion during later earth movements. The metamorphism produced new minerals, e.g. chlorite, actinolite, talc and serpentine. The metamorphosed igneous rocks are called greenstones, because the new minerals are generally dark green in colour. The volcanic rocks are mostly basalt lavas. There are also andesites near Glenthompson and in the Heathcote and Mount Wellington greenstone belts. The lavas are often interbedded with thin layers of sedimentary rocks, including tuffs, mudstones, shales, cherts and sandstones. The sandstones consist of eroded volcanic material. Some of the lavas show columnar jointing. Others contain structures called pillows. These are rounded, bulbous masses of lava, up to 30 centimetres across, formed when molten basalt solidifies as it flows into sea water. Basic intrusive rocks are common in the Heathcote and Mount Wellington belts. They include dolerite sills and larger bodies of gabbro. Some ultrabasic rocks are also present. They were originally composed of olivine and pyroxene, which altered to serpentine. It is difficult to di fferentiate between the various igneous rock types, because of their general dark green colour. The greenstones are mostly deeply weathered in central and western Victoria, and commonly covered by a dark red-brown clayey soil. The largest outcrops of unweathered greenstones are along the Mount Wellington Belt in the beds of large rivers such as the Howqua. Fresh rock can be inspected more easily along the beach on the south-western side of Waratah Bay.
Fossiliferous sedimentary rocks Narrow belts o f sedimentary rocks containing Cambrian fossils overlie greenstones along the Heathcote and Mount Wellington greenstone belts. They are deep water marine sediments, mostly black shales, chertS and turbidites. Lenses of red jasper are as ociated in places with both the sedimentary rocks and the underlying volcanic rocks. There are also some fossiliferous sandstone beds, e.g. in the Knows/ey East Formation, a few kilometre north of Heathcote. The youngest Cambrian unit is the Goldie Cheri, which outcrops in the Lancefield - Romsey area. It consists o f chert and siliceous mudstone. Along the Heathcote Greenstone Belt, in the Mount Camel Range, Cambrian shales are highly contorted due to their proximity to the nearby Knowsley East Fault (Figure 4-7). The Dolodrook Limestone is i nterbedded with Cambrian sedimentary rocks overlying the Mount Wellington Greenstone Belt, north-east of Licola in Gippsland. This is the oldest limestone in Victoria. [n contrast to most other Cambrian sediments in Victoria, this unit appears to have formed in shallow water.
Glenelg River Beds These are distinctive, unfos iliferous edimemary rocks found only in the far western part of the State, north of Casterton. They are exposed along the sides of the Glenelg River, the Wando River and a few of their tributaries. I n the south-west towards Castenon, the rocks are weakly-metamorphosed sandstones and mudstones, originally
104
C hapter 4
deposited as turbidites. There are also occasional volcanic ash and dolomite layers. Further to the nonh -east the regional metamorphic effects are more intense. There, the rocks include biotite schist, garnet biotite schist and schists containing Olher metamorphic minerals, e.g. staurolite, andalusite, hornblende and diopside. Amphibolite (metamorphosed basalt) is also present. The Glenelg River Beds are intruded in many places by granitic rocks. Radiometric dating has shown that the granites are about 500 million years old (earliest Ordovician) - hence the Glenelg River Beds are older and therefore Cambrian in age. The Glenelg River rocks resemble metamorphosed turbidites which outcrop extensively along the eastern side of the Mount Lofty Ranges in South Australia. The latter rocks, called the Kanmantoo Group, are Middle Cambrian in age.
The Cambrian greenstones have an interesting characteristic. They usually contain the strongly magnetic mineral, magnetite. As a result, greenstones are usually more magnetic than the surrounding rocks. This properly can be measured by an instrument called a magnetometer. To cover large areas quickly, magnetometers are frequently mounted in aircraft, which fly low over the ground, measuring the magnetism of the rocks below. The results are plotted as magnetic imensity maps. On these maps, the greenstone belts show up as zones of higher than normal magnetic intensity, called magnetic anomalies. Magnetic anomalies can be recorded even where the greenstones are covered by younger rocks. For example, the northern extension of the Heathcote Greenstone is obscured by Cainozoic sediments, but can be clearly traced on the magnetic intensity map of this area (Figure 4-6).
Mapping Cambrian greenstones from the air
Figure 4� Maps of an area
between Colbinabbin and Rochester in north-centraJ Victoria.
Formation, near Heathcote. Hydroids are related to corals. They have a soft. branching skeleton made of chitin. which is a horny material often resembling fingernails in appearance. Hydroids are usually only a few centimetres high. They are colonial. that is a group of similar organisms live together on the same skeleton. Each branch on the hydroid skeleton houses an individual organism called a polyp. Nearly twenty species of hydroids have been collected from the Cambrian rocks of the Heathcote Greenstone Belt.
Figure 4-9 J(ootenia, a Cambrian Irilobile.
O __ "m
St Arnaud Beds The St Arnaud Beds are similar to the less metamorphosed parts of the Glenelg River Beds. They were originally sandstones and mudstones, which were deposited in moderately deep water by turbidity currents. Later these rock were converted to schists, slates and phyllites by weak regional metamorphism. The sandstones are composed largely of quartz, probably derived from the erosion of the moumains to the west and north-west in South Australia and r;:.v South Wales. The best exposures of the e rocks are in the Pyrenees Ranges to the west and north·west of Avoca and i n the hills west of Stawell and Ararat. On older published geological maps. the areas occupied by the St Arnaud Bed are shown as Ordovician in age (e.g. Ballarat 1 :250 000 geological sheet. 1973 edition). However. this is unlikely. as the rocks are almost completely unfossiliferous. To the east of the St Arnaud beds. between Ballarat and Bendigo. similar rocks comain abundant fossils known as graptolites (see next ection). I n Victoria the oldest graptolites are Ordovician in age. lt is therefore assumed that the absence of graptolite in the west means the unfossiliferous St Arnaud Beds were deposited during Cambrian times. before graptolites evolved.
FAUNA AND FLORA
In the deep water environments covering mo I of Victoria during the Cambrian. only a rew organisms lived either on the sea bOllom or in the open waters. Animals called hydroids were locally abundant on the sea·floor. e.g. in the Knowsley Eost Formation (Figure 4-8). Siliceous sponges also grew on the sea bOllom in places.
Thi fossil occurs in the Kllowsley East Formation wi t h fo ur other pecies of trilobites and a variety of brachiopods. Trilobites are aJl extinct group that lived from the Cambrian 1.0 the Permian. They are related to insects and crabs, as (hey had jointed legs and a tough outer coat. called an exoskeleton. In trilobites, the exoskeleton was composed mainly of calcite. Most trilobites were only 5-8 centimetres long. but some reached 70 centimetres or more. They mostly had large heads with well· developed compound eyes. so they
could probably see very well. The trilobite body was divided into several segments. and a pair of legs was auachcd to each segment. The legs were delicate and are rarely preserved as fossils. The tail consisted of several segments fused into a single plate. Tr ilobites lived only in the sea. Most crawled over the sea-noor in shallow Waler. Early Palaeozoic sandstones deposited in this environment often contai n tracks and burrows made by trilobites. However, some trilobites were able t.o live in deeper water, because they could toierate high pressures and lack of light.
t 06
Chapter 4
I n shallower waters, brachiopods were common i n areas o f little wave o r current activity. Algae were also present. Trilobites crawled over the sea-floor (Figure 4-9). By cOntrast, on the dry land to the west of Victoria there were neither plants nor animals, just bare rocks and deposits of weathered material.
DELAMERIAN OROGENY In latest Cambrian and earliest Ordovician times, earth movements called the Delamerian Orogeny affected south-eastern South Australia, westernmost Victoria and central-western Tasmania. Deposition of sediments ceased in these areas. The Cambrian rocks there were faulted, folded and uplifted to form mountain ranges called the Delamerian Highlands (Figure 4-11). Rocks deep in the roots of these ranges were subjected to moderately high temperatures and pressures. As a result, the original clays, sands and dolomite recrystallised to produce the metamorphic rocks of the Glenelg River Beds. The name Delamerian comes from the township of Delamere, south of Adelaide, where folded Cambrian rocks are well-exposed. Associated with the Delamerian Orogeny there were widespread granite intrusions, both in the south-eastern part of South Australia and in western Victoria around the upper Glenelg River. These rocks are dated at about 500 million years old, making them the oldest granitic plutons i n Victoria. The Delamerian Orogeny had little effect in central and eastern Victoria. These regions remained under deep water during the Cambrian and Ordovician. Along the Heathcote Greenstone Belt, Ordovician rocks were apparently deposited over Cambrian rocks with little or no interruption in the processes of sedimentation. However, the earliest Ordovician rocks are coarser and more quartz-rich than the underlying Cambrian sediments. This reflects an influx of sediments eroded from the newly uplifted Delamerian Highlands to the west.
The first Victorian miners At several places along the Heathcote Greenstone Belt, the greenstones are fine grained, very tough rocks consisting of densely interlocking actinolite needles. - Aborigines
Actinolite is a calcium magnesium iron silicate belonging to the amphibole mineral family. Victorian Aborigines quarried this greenstone to make axe heads. The toughness of the rock ensured that the axes did not chip easily and they took a good edge when they were ground. The material was quarried at several localities in Victoria and particularly at Mount William near Lancefield in the Heathcote Greenstone Belt. The quarries were worked until the 1 840s.
O rdovician
The Ordovician is named after the Ordovices, an ancient Celtic tribe of central Wales. Ordovician rocks are prominent in Wales, where they were studied by the early British geologists. The Ordovician extended from 500-5 10 million years ago until 420-440 million years ago. The Ordovician was one of the last periods to be given a formal name. Earlier it was called the Lower Silurian. The latter age is used on most nineteenth century maps of the Victorian goldfields for rocks now considered as Ordovician. In the Ordovician seas there was a great diversity of animal groups, including many forms that continued to flourish in later Palaeozoic periods. Trilobites were less common than in the Cambrian, but brachiopods became more abundant. They were joined on the shallow sea floors by rugose and tabulate corals, crinoids and bryozoans. The first fish had evolved in the Cambrian, but in the Ordovician some species developed heavy, bony armour. Small, primitive plants probably appeared in wetter areas on land. At the end o f the Ordovician, more than half the species of brachiopods and bryozoans died out. This event may have been linked to a period of glaciation which caused oceans around much of the world to become cooler. At that time, North Africa lay at the North Pole and was covered by an ice-cap. Periods of extensive glaciation, even at the poles, are very uncommon in the Eanb's history. The Ordovician ice age was the first to occur after the Pre-Cambrian.
Geological History of Victoria
D Hill
D D D
{WAANl
1 07
ORDOVICIAN
Deep Sea ShaUow sea
. . . . ..
AREAS OF OUTCROPS
Mountains Approximate position o' coastline
Sed,mentary ,ocks
� MetamorphiC rocks � : IntrUSive rocks
� ; :�
Horshamo
I W " .�
....
\S . . :
o 25 50 75 tOO ,
Figure 4·10 (above) Distribution of Ordovician rock outcrops in Victoria and the probable Ordovician palaeogeography. Ordovician rocks underlie younger sedimentary rocks through central· eastern Victoria. The metamorphic rocks i n eastern Victoria were rormed rrom Ordovician sedimentary rocks during the Benambran (Late Ordovician) and Bowning (Late Silurian) orogenies.
1
!
1
DISTRIBUTION Ordovician sedimentary and metamorphosed sedimentary rocks are widespread over t he State (Figure 4 · 1 0). However, Ordovician granitic rocks intruded during the Delamerian Orogeny are confined to the south·western part of Victoria. The western limit of Ordovician sedimentary rocks in Victoria is the line between Charlton and Baliarat, which also forms the eastern boundary of the Cambrian St Arnaud Beds. There are extensive Ordovician outcrops in cemral Victoria between this line and the Heathcote Greenstone Belt. By contrast, to the east of the greenstone belt there are few outcrops of Ordovician rocks for a distance of 100·1 40 kilometres. This is becau e the Ordovician rocks there are largely covered by younger Silurian and Devonian sedimentary and, in places, volcanic rormations. Where Ordovician
A A
Broken 0 HIli
A A �A A� �A AlZ ,.
Figure 4·1 1 Distribution of land and sea across soutb�tern Australia i n the Ordovician. The eastern coastline of the Australian Craton in (he Ordovician was far to the west of its present position. Most of Victoria was covered by deep ocean. To the south. shallow marine shelves extended over much or Tasmania. Orrshore 10 the east, there was a line of volcanic islands.
,
Kilometres
-
'V' 'V' 'V' 'V' 'V'
'V' 'V' 'V' 'V'
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land
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VolcaniC Island 'V'
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Deep sea ApPloxlmale posilton 01 coastline
____
Presenl d:lV coaSlline
108
Chapter 4
outcrops occur, they are usually present as small up-faulted blocks, e.g. Mornington Peninsula between M oorooduc and Red Hill. Most Ordovician beds in central Victoria are deeply weathered. Consequently they tend to form low, rounded hills, e.g. between Bendigo and Ballarat. However, where these rocks were later contact metamorphosed by granitic intrusions, they became harder and now form prominent lines of hills, e.g. ranges in the Dunolly area. Where exposed in road and railway cuttings, weathered unmetamorphosed Ordovician rocks are usually pastel shades of brown, yellow and light grey. However, below the zone of weathering, the same rocks are hard and dark grey in colour. Unweathered specimens from deep below the surface can be found on dumps beside many old gold mines. In the eastern part of Victoria, beyond a line through Numurkah, Benalla and Stratford, Ordovician rocks are common except where they are covered by younger rocks or intruded by granites. This belt of Ordovician rocks extends northward into south·eastern and central New South Wales. The Ordovician rocks of eastern Victoria include areas of metamorphosed sediments (gneisses), which are very resistant to erosion. Most of the high country of eastern Victoria, (e.g. the Victorian Alps at Falls Creek and Mount Bogong), is underlain by these rocks. Rocky coastal outcrops between Marlo and Mallacoota in far eastern Victoria consist of slightly metamorphosed Ordovician sedimentary rocks.
REGIONAL SETTIN G During the Ordovicia n, most of V ictoria was part o f a deep marine basin that extended north ward into New South Wales and southward into Tasmania (Figure 4- 1 1 ). To the east, there was a line of volcanic islands. Lava nows, interbedded with sedimen tary rocks formed by erosion of the volcanoes, occur intermittently in a belt between Tumut and Gundagai in southern New South Wales and further to the north near Sofala. The edge of the volcanic deposits extends into north eastern Victoria, 20-30 kilometres east of Benambra. There, dacites and andesites occur within a narrow belt of rocks called the Blueys Creek Formation. The coastline along the western edge of the deep Ordovician basin probably ran more or less north·south through western Victoria, passing between Hamilton and Ballarat and continuing northwards towards Broken Hill. To the west were the Delamerian Highlands in southern South Australia and western Victoria. Along the southern edge of the basin there was another mountainous area in central Tasmania. Along the northern and western margins of these Tasmanian mountains there was an extensive, shallow marine shelf. This was up to 30 kilometres wide in places and covered by sandbars and coral reefs. Thick limestone and sandstone formations, representing remnants of these deposits, extend discontinuously from northern Tasmania near Devonport down the west coast to Precipitous Bluff in the south· west corner.
ROCK TYPES Nearly all the Ordovician rocks in Victoria are of deep water sedimentary origin. Most of them are either: •
•
interbedded sandstones, mudstones and minor shale of turbidite origin; or thick sequences of black shales.
There are also some areas where chertS are interbedded with the sandstones and shales, and one locality where Ordovician limestone is present.
Turbidites Sandstones and mud tones are the commonest Ordovician rock types. These sediments were depo ited by rurbidity currents along the foot of the continental slope. This is shown by sedin1entary structures found in the sandstone beds, including scours, ripples and graded bedding. •
• •
Scour are fonned '.,·here the turb ulence of the turbidity current erodes into the muddy sea bottom. The sand deposited by the cu rrent then infills the scours and preserves them. Ripple form in the sand bed as it is deposited. They move in the direction of the current. Graded beds show a progressive decrea e in the grain size of the particles from the base of the bed to the top. This is produced as the turbidity current gradually slows down. First the coarsest, heaviest particle , which need the mOSt energy to transport them, are deposited and later the finer, lighter grains (Figure 1 -65).
During the Ordovician, high mountains o f the Delamerian Highlands were eroded by fast-nowing rivers draining to the east. These rivers dropped their loads
Geological History of Victoria
109
of sediment on the narrow continental shelf. From there, the material was carried down the continental slope into deeper water by turbidity currents. This conclusion is su pported by two lines of evidence: I . From the scours and ripples in a turbidite it is possible to determine the direction in which the turbidity current was Oowing. Studies of Ordovician rocks from many parts of Victoria show that the turbidity currents travelled from the western side of the basin towards the east. This implies that most of the sediment being brought into the basin came from an area to the west . The most likely source was the Delamerian Highlands. 2. The Ordovician sandstones are composed mainly of quartz grains. The rocks in the Delamerian H ighlands were mostly quartz-rich Cambrian and Pre-Cambrian sedimentary and metamorphic rocks. Remnants of these rocks are found today in the Glenelg River Beds in sOUlh-western Victoria and further west in South ' Australia. During the Ordovician, the turbidites built up across the sea-Ooor as enormous bodies of sediment thousands of metres thick called submarine fans. There were probably many of these fans, overlapping with each other They built outwards from the western and, to some extent, the southern margins of the basin. In far eastern Victoria, the turbidity currents Oowed mainly northwards from the Tasmanian landmass i n the south. Pre-Cambrian quartzites i n the Tasmanian mountains contrib uted quartz sand to these turbidites. Turbidites accumulated throughout the Ordovician on the deep ocean floor that extended through central Victoria. However, in the eastern half of the State, turbidite deposition almost stopped after the Early Ordovician .
B lack shales In the Late Ordovician, the naLUre of sedimentation changed in eastern Victoria and black shales largely replaced turbidite sandstones. These shales outcrop mainly as small areas bounded by faults in some rugged, remote parts of Gippsland. Several of these blocks are shown on the Warburton 1 :250 000 geological sheet along a zone known as the Mount Easton Fault Belt. Others are scattered across East Gippsland, between Nowa Nowa in the south and Bonang near the New South Wales border. Occasional thin beds of black shale are also found interbedded with turbidites throughout the Ordovician sequence in central Victoria. The black shales were probably deposited on the deeper parts of the ocean floor beyond the fans built up by turbidity flows. The water at these depths was very still, because it was away from the inO uence of waves and ocean currents. As a result, fine-grained silt and clay settled slowly out of suspension. The shales contain abundant organic mauer, giving them their typical black colour. However, this organic material was not derived from creatures living on the deep sea-Ooor. There was so little oxygen in the stagnant water that no plants or animals could survive there. Instead the organic matter came from organisms which originally swam or Ooated in the upper levels of the ocean. After they died, these organisms sank to the sea-Ooor; they now occur as fossils i n the black shales. The main forms are graptolites and conodonts (see later descriptions). Their remains are well-preserved because the stagnant bottom waters contained insufficient oxygen to allow them to decay. Pyrite is also commonly present in the black shales. It was produced as a result of bacterial action. The bacteria in the mud on the sea-Ooor lived on the organic matter in the sediment and formed pyrite at the same time.
Cherts Thin-bedded cherts occur occasionally in the Ordovician rocks, most commonly in eastern Victoria (Figure 4-12). These cherts are made up of very fine-grained silica derived from radiolarians (Figure 4-13). Radiolarians are tiny organisms that Ooat in large numbers in the upper levels of the ocean. They have a spherical skeleton made of opal, a variety of silica. Radiolarians are extremely abundant where ocean waters are rich in nutrien tS and silica. As they die, their skeletons accumulate in enormous numbers on the sea-Ooor. There are up to 100 000 skeletons per gram of sediment. Because most cherts occur in eastern Victoria, the waters there must have been richer in silica. This probably reOected the presence of the line of volcanic islands to the north, remnants o f which are preserved in ew South Wales (Figure 4-1 1 ) . Silica is often released during volcanic eruptions and afterwards by weathering of the lava Oows and ash deposits. As the beds of radiolarians were buried by overlying sediments, the pressure broke up most skeletons and only a few were preserved . The pressure al 0 caused the opal to recrystallise to quartz, a different form of silica. The Ordovician chens in Victoria are now composed largely of very fine-grained quartz with few recognisable radiolarian remains.
110
Chapter 4
Shallow water l imestones and sandstones The only limestone of Ordovician age in Victoria is at Digger Island, near Walkerville South on the western side of Waratah Bay. Brown, muddy limestones, about 60 metres thick, were deposited directly on top of Cambrian greenstones. The limestones contain abundant trilobites and brachiopods, but no corals. This suggests that deposition occurred in quiet, moderately deep water below the influence or breaking waves. In Tasmania, Ordovician shallow water limestones and sandstones are common. The limestones formed as coral reefs and banks of calcium carbonate sand. The sandstones are quartz-rich and contain narrow, vertical burrows, probably made by marine worms. These sediments formed on an extensive shallow shelf, which was periodically swept by storms blowing from the open sea to the north. Apart from the limestone at Digger Island, there are no other shallow water Ordovician sediments in Victoria similar to those in Tasmania. Their absence in western Victoria is puzzling. This area must have been the western coastline of the deep-water basin covering most of Victoria. Probably shallow water sediments were deposited along this coastline, but later were removed by erosion, so that no trace of them remains today.
Figure 4-12 (right) Thin-bedded radiolarian cherI of Ordovician age. This outcrop is at Fisherman's Rocks, on the coastline west or MallacoOla in East Gipp land. The beds or chert were originally horizontal, but later were folded and tilted into an almost vertical orientation, probably during the Middle Devonian Tabberabberan Orogeny. (photograph by C.l.L. Wilson).
. ... .. .. .
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o
I
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Figure 4-13 (abovej
Cenosphaera, a Palaeo7.oic radiolarian very similar 10 the
radiolarians present in the Ordovician cherts of eastern Victoria.
Radiolarians are single-celled
ani mals, up to two millimclrcs in
diameter. Most radiolarian skeletons consist of a spherical shell with a variable number of radiating spines. Radiolarians first appeared in the Cambrian and they still exist today. Radiolarians are most diverse and abundant in tropical waters but they arc also very common in cold subpolar seas.
Geological History of Victoria
111
FAUNA AND FLORA From geophysical studies, it has been deduced that during the Ordovician, Victoria was within 20" of the Equator. Consequently the climate in Victoria then was tropical to subtropical. Life teemed in the shallow seas that covered much of Tasmania. There were trilobites on the seabed and brachiopods lay on or burrowed into the sands and muds. Algae were also probably common. In Victoria, these plants and animals are only found in the Digger Island Limestone.
Figure 4-14 Life in the Ordovician sea. Jellyfish (I) and graptolites (2) floated in the open ocean. When graptolites died, they sank to the sea-floor (3). Their skeletons were eventually buried within the layers of sediment accumulating there. I n the distance i s a coiled nautiloid (4). Nautiloids are closely related to squid. They had good eyesight and could swim rapidly in pursuit of their prey, which they caught with their tentacles. The nautiloid is in the background of the illustration; it was considerably larger than the graptolites in the foreground. In shallower waters (on the left in the diagram), algae grew on the sea-bed and brachiopods (5) were often present in great numbers. Trilobites (6), with their distinctive segmemed bodies, crawled over the sea-floor. They probably fed on the dead organisms which settled there. A s a result, few graptolites are found preserved i n shallow water sediments. (After original drawing by R.M. Molesworth).
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�
,
.
1
---.
Many different organisms swam in the Ordovician seas (Figure 4-14). NaUliioids, squid-like animals with saucer-sized, coiled or straight, cone- haped shells, hunted other animals for food. The internal chambers in their shells contained gases, largely nitrogen, which allowed the nautiloids to control their buoyancy. There were also animals resembling swimming worm , 5-10 centimetres long, with a narrow fin on each side of the tail. The mouth of these animals contained a complicated set of tiny plates and teeth, each one only a few millimetres long. These teeth, which are known as conodOnlS, were composed of calcium phosphate, similar to some bone material (Figure 4-38). They are often preserved in sediments, even though the soft parts of the conodont animal have rOlled away. Conodonts are common in black shales and cherts in Victoria, but they can only be seen under a microscope. A lew fossils of whole conodont animals have been found in rocks overseas, but none in Australia. There were also abundant tiny organ isms, called grapro/ires, floating in the upper parts of the ocean. Graptolites look like miniature saw blades (Figures 4- 1 5, 4-16). They were colonial, that is many animals lived together on each skeleton. Individual animals were housed in small cups on the skeleton. They were connected to each other by a primit ive type of backbone. It is possible that some graptolites could swim lowly by beating tiny whips around the openings of the cups. However, most were probably carried around by ocean currents. The keletons of graptolites were made of organic materia� which usually decayed quickly after they died. They could be preserved, though, in stagnan t, oxygen-poor water where black shales accumulated. G raptolites therefore are frequently found in deep-water black shales, fossilised as shiny black films of carbon. Victoria is famous for its well-preserved graptolite fauna, which is one of the richest and rna t varied in the world. Thousands of graptolite localities have been found, not only in eastern Victoria, where Ordovician black shales are abundant, but also in central Victoria, where thin black shales are interbedded with turbidites.
1 12
Chapter
4
figure 4-15 Early Ordovician graplolites from Vicloria.
(a) Rhabdinopora scitulum (2
x
enlargement)
,
(Photographs by A . H . M . V anden Berg) .
, ./.._---
(b) P,mdeograptus /rUlicosus (actual
size) (e) Pseudisograptus gracilis ( 1 . 5 x enlargement)
The first graptolites appeared in the Late Cambrian. but they became most abundant in the Ordovician. Graptolite species evolved very quickly. A few survived for periods as short as one to two million years before beco ming ext inc!. Each species followed another in a particular order. As a resul� graptolites are very useful for determining the age of the ediments in which they are found. Because black shales are widespread i n the Ordovician rocks of Victoria, graptolites are an important way to work out the age of these rocks. Furthermore, because ome Victorian graptolite species also occur overseas, the Ordovician sedimentary rocks of Victoria can be compared with overseas Ordovician sequence . In the Midlands, graptolites have been used extensively to correlate the Ordovician rocks. Without graptolites it would nOI have been possible to unravel either the patterns of folding in these areas or the sequence in which the beds were laid down. The results of this mapping are seen on the Bendigo 1 :250 000 geological sheet published by the Geological Survey of Victoria.
BENAMBRAN OROGENY
After the Delamerian Orogeny al the end of the Cambrian, the next orogeny to affect Victoria occurred during Late Ordovician - Early Silurian times, about 420440 million years ago. Then, a major period of deformation affected large areas of the State.
Geological History of Victoria
lATEORJ)QVlCIAN
Figure 4-16 Representative Ordovician gruptolites from Victoria. Many Ordovician graptolites had skeletons with several branches called stipes. Species with twO branches were common, and some had four or more stipes. However, in the Silurian most graptolites had simple skeletons consisting of one row of cups. Graptolites were most diverse and abundant in warmer waters. During the late Ordovician glaciation, when the oceans became colder, most graptolites died oul. Only five or six pecies survived. They diversified again in the Early Silurian but became less common in the late Silurian and Devonian. Finally they became extinct in the Early Carboniferous. (After original drawings by A . H .M . V�ndenBerg).
1 13
1
y
Dicranograptus nicholsoni
DispJacanthograptus spiniferus
L-...J
L--.J 5mm
5mm
Dicranograptus i i
k rk L-...J 5mm
Ortbograptus comutus
L-...J
L-.J
5mm
5mm
Climacograplus caudatus
MIPDI'S1ern ViclOria: they form The Grampians and outl)�ng hills uch as Black Range and Mount Dundas to the west and Mount Arapiles to the north-west. They also occupy lower-lying country between Hamilton and the Rocklands Reservoir, and along the Hopkin River valley, near Wickliffe
Geological H istory of Victoria
115
2 . Central Victoria: sedimentary rocks extend across a broad north-south belt bounded to the west by the Heathcote Greenstone Belt and to the east by major faults near the Mount Wellington Greenstone Belt. Isolated outcrops are also found on Mornington Peninsula and between Cape Liptrap and Foster in South Gippsland.
3 . Eastern Victoria: Silurian and Devonian sedimentary and volcanic rocks cover much of the country between the Thmbo and Snowy rivers. They extend from Nowa Nowa northwards to the headwaters of the Mu rray and Mitta Mitta rivers. These rocks also occupy smaller areas along the Mitchell and Wentworth rivers, around Dartmouth Dam, and to the north of Club Terrace. In addition, granites o f Late Silurian and Early and Middle Devonian age are widespread (Figure 4-18). They extend in a broad belt from The Grampians almost to Swan Hill. They are also common in eastern Victoria between Benalla and Mallacoota. Granite at Wilsons Promontory is also of this age.
D Deep sea D Shallow sea D Mountams
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coastline
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Figure 4-17 (above) Distribution of Silurian - Middle Devonian rock oUlcrops in Vicloria and the probable palaeogeography at (he time. The posit ion of the coastline in Victoria changed greatly during the Silurian and Early Devo nian, and by the Middle Devonian most of Victoria was dry land. The coastline shown on the diagram represents the maximum extenl of the seas during this period. The area of dry land in the soulh eastern corner or the Melbourne ll'ough was present only in the Early Devon ian. A seaway is shown connecting the Grampians Basin and the Melbourne Trough, but its existence is uncertain.
,
,
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KIlometres
•
REGIONAL SETTING
The uplift movements associaled with the Benambran Orogeny at the end of t h e Ordovician reduced the earlier deep marine basin t o two smaller marine trough the Melbourne Trough and the Cowombar Rifr (Figure 4-17). Further changes in the distribution of land and sea resulted from another orogeny (the Bowning Orogeny) at the end of the Silurian. In eastern Victoria, the Cowombat Rift was uplifted. However, renewed subsidence in the Lower Devonian formed the Buchan Basin in much tne same region. In addition, an area in western Victoria began to subside to form the Grampians Basin (Figure 4-19). The Bowning Orogeny, however, did not affect the Melbourne Trough in central Victoria. Sediments in that basin accumulated almost without imerruption from at least the beginning of the Ordovician through until the Early Devonian, a period of over 100 million years. In far western Victoria and south-eastern South Australia, it is likely Ihat the Delamerian Highlands continued as a mountain range through the Silurian and Early Devonian. They were probably a major source of Ihe sedimenl deposited in the western basins during that period.
116
C h apter
,
4
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Figure 4-18 (above) Zones of granitic rocks
I
in
4. Late Devonian granites
I
Vicloria.
- 370 are
million years old - many
associated with ignimbrites
3. Middle Devonian granites 385-400 million years old associated wilh the Tabberabberan Orogeny.
2. Late Silurian and Early Devon ian granites - 400-420
million years old - associated
with lhe Bowning Orogeny.
I.
Early Ordovician granites -
500 million years old associated with the Delamerian Orogeny. Many granites have not been dated radiometrically. so the boundaries between the zone are only approximate. There may be changes when more dales become avai lable. In particular, it is possible that the eastern boundary
of Zone 4 (Late Devonian granites) in ceOlTal Victoria should be moved 75 kilometres east of the position shown on the map. The term 'granite' is used here to cover all acid imTusive rocks - it includes granodiorites and other coarse-grained varieties.
r#
Each of the four Silurian-Lower Devonian basins had a different geological history (Figure 4-20). They are therefore di cussed eparately beginning with the most persistent one, the Melbourne Tro ugh.
Melbourne Tro ugh Boundaries of the Melbourne Trough
During the Benambran Orogeny, the regions to the east and west of the Melbourne Trough were uplifted above sea-level. However, the [rough i[self escaped uplift and remained as a deep IVater basin into the Silurian and Early Devonian. The presen t-day margins of the sedimentary rocks of the Melbourne Trough are major faults. On [he western side faults separate the Silurian-Early Devonian rocks from the Heathcote Greenstone Bell. Along the eastern side the geology is more complex. Silurian and Devonian rocks are in contact along faults with either Ordovician rocks or Cambrian greenstones of the Mount Wellington Bell. However, it is not certain where t he original shorelines of the Melbourne Trough were located. They were probably beyond the faults and greenstone belts just referred to. To the west, in the Early Devonian, shallow seas may have extended to the Grampians Basin, where the Grampians Group sediments were deposited in shallow water (see la[er). In the east there are deep-water Silurian to Devonian sedimentary rocks near the Mount Wellington Greenstone Belt, 0 the eastern edge of the Melbourne Trough must have been further to the easl. It is even possible that shallow arms of the sea could have stretched eastwards to connect the Melbourne Trough with the Cowombat Rift or Buchan Basin at various times. During the Early Devonian, an area of dry land was present in the south-eastern corner of the Melbourne Trough (Figure 4-1 9). There are several reasons for this conclusion: • Shallow-water coralline lime tones of Early Devonian age can be seen growing directly on top of Cambrian greenstone in outcrops on the south-western side of Waratah Bay. • There are various small areas of limestone further to the north, e.g. between Tyers and Walhalla, and al Toongabbie. Some of these contain large blocks of reef limestone that fell into the deep waler basin from the edge of a stromatoporoid reef growing in shallow water nearby.
Geological History of Victoria
•
117
I n the Tyers River area there are also conglomerates, which contain pebbles of greenstone. Therefore, at this time there must have been a nearby landmass, where greenstones were exposed to erosion by rivers or wave action.
The limestone and conglomerate beds become thinner and gradually disappear as they are traced towards the north and north-east. This indicates that the area of dry land in the Early Devonian was restricted to the south-eastern corner of the Melbourne Trough. The Melbourne Trough extended southwards into Tasmania. The western boundary probably ran south-easterly through central Tasmania. Silurian and Devonian sediments in north-east Tasmania were deposited in the southern part of the trough. The eastern margin of the basin may have terminated in present-day Bass Strait, where the Melbourne Trough merged with the ocean to the easl.
Figure 4-19 Distribution of land and sea across south·eastern Australia i n the Earb' Devonian. Braided rivers flowed from the mountains to the west into the shallow Grampians Basin. The Melbourne Trough, which may have been con nected to the Grampians Basin, extended southwards through central Victoria and across north-eastern Tasmania. In t he Buchan Basin to the east, shallow seas flooded acrOSS extensive deposits of ignimbrite volcanics. Some of the huge ignimbrite cal deras, from which the volcanics were erupted, were clearly visible on the landscape. Figure 4-20 Sedimentary basins in different parts of Victoria during Silurian and Devonjan times.
WESTERN VICmRLA
EARLY DEVONLAN
GRAM PlANS BASI N freshwater and shallow marine sediments, followed by terrestrial volcanics
Bowning Orogeny? SIWRLAN
CENTRAL V ICmRLA
MELBOURNE TROUGH mainly deeper water marine sedi ments
land
EASTERN VICmRLA
BUCHAN BASIN shallow water marine sediments, overlying mainly terrestrial volcanics
Bowning Orogeny COWOMBAT RIFT deep and shallow water marine sediments and volcanics.
Sediments of the Melbourne Trough
Most sediments entering the Melbourne Trough came from the wesl. Large braided river systems fiowed eastwards from the Delamerian Highlands, supplying sediments to the Grampians Basin (described later) and the Melbourne Trough. There may also have been areas of upli fted Ordovician sedimentary rocks in western Victoria, that were drained by rivers fiowing to the easl. In the Silurian, sediments within the Melbourne Trough were deposited mostly by turbidity currents. Extensive submarine fans built out into the trough from the
118
Chapter 4
west and probably to a lesser extent from the east. Turbidites, thousands of metres thick, accumulated as in terbedded sandstones and mudstones, with occasional channel conglomerates. Thick siltstones were deposited on the quiet, inactive pans of the submarine fans, as very fine-grained sediments settled on the sea-floor. Many road cunings in the eastern suburbs of Melbourne are either in Silurian turbidites or siltstones (Figure 4-2 1 ) . Throughout most of the Melbourne Trough, current directions in the turbidites are from the west. Turbidity currents flowing from the east have only been recorded close to the eastern margin of the trough. There are occasional fossiliferous beds within the turbidites. Many of these outcrop in the Kinglake-WaUan area, north of Melbourne, e.g. at Middendorp's Quarry. These beds contain a diverse and well-preserved fauna of brachiopods (Figure 4-22), trilobites and echinoderms (Figure 4-23). Echinoderms usually disintegrate quickly once they die, yet the specimens in these fossil bands are intact and well-preserved. This seems to indicate that the animals were buried alive. Many echinoderms lived in shallow water environments on the continental shelf. They were evidently carried from the shelf by turbidity currents down to the deeper ocean floor where they died. However, some echinoderms, particularly the starfish, were probably deep-water dwellers. They were smothered by the sediments deposited by the turbidity currents.
Figure 4-2 1 Folded and faulted andstones and mudstones of the Late il urian Oarg ile Formation. The cUlling is in Studley Park, Melbourne. These sedimentary rocks were deposited as turbidites in the Melbourne Trough. They were faulted and folded during the Middle Devonian Tabberabberan Orogeny. The deformat ion i n this cuning is intense. because it is close to a major fault line.
. , .. �
\ I F
F
•
F
Figure 4-22 (below)
Howellela lat;sulcata, a Late
Silurian brachiopod from the Melbourne Trough.
. ,
Brachiopods have two unequal shells hinged togelher, they enclose the soft pans of the animal. Brachiopods resemble bivalves (pelecypods) that are common on the seashores today, but the bodies
1 0 mm
of each group have very different internal struclures. Brachiopods first evolved in the Cambrian and they dominated the shallow sea noors throughout the Palaeozoic. Since then Ihey have gradually declined and today they arc greatly outnu mbered by the bivalves. Nevertheless, there arc still about 200 species of brachiopods at present. Brachiopods are divided into
1 wo groups, the inarticulates and
Side-on view of both shells joined together
articulates. The shells of articulate brachiopods are hinged logelher, small teeth on one shell fit into
sockets on the other. Inarticulate brach iopods lack these feat ures.
Howellella is an QfliL'U/Ule brachiopod, with twO thick, curved shells made of calcite. One of the hells ,vas usually larger and more convex than the other, unlike the shell of modern-day bivalves, which are generally bolh the same size and shape.
A system of muscles opens and closes the shells of a brachiopod. The muscles decay quickly after the death of the animal, and the shells are easily separated. Fossilised brachiopods fo und with the shells tightly closed wcre probably buried alive, e.g. by a turbidity current. The outsides o f brachiopod shells are often ornamented with thick ribs (as in Howellella) or spines.
Geological History of Victoria
119
Figu", 4-23 (right) Henicoc)'Slis, an Early Devonian
cystoid from the Melbourne lrough. Cystoids are an eXlinct group of echinoderms, related to starfish and sea-urchins. They originated in the Cambrian and died out in the Devonian. Most cystoids were fixed by their bases to the sea-floor. However, some species such as Henicocystis had a flexible lapering stem. This could sweep slowly from side to side, pushing the cystoid across the sea-floor. Cystoids generally lived in shallow waters and often in muddy conditions. (Original drawing by P.A. Jell).
l
5-2ocm ____-, -f 30cm-3m Figure 4-24 (above) Hummocky cross-stratification. This is a type of cross-bedding characterised by the development of low mounds or hummocks, usually less than 20 centimetres high and 30 centimetres to three metres apart. Thin layers of sand are deposited over the hummocky surface, giving broad, low-angle, curved eros -bedding. As the hummocks move, they cut into the underlying cross-bedding. Hummocky cross-stratification forms during storms under the influence of storm-driven water currents. Sandstone beds with hummocky cross-stratification are usually deposited in hallow water near the coast. Mudstones are often interbedded with these sandstOnes. The mud sellies out on the sea-floor during the calm periods between Slonns.
Most o f the Silurian sediments in the Melbourne Trough were deposited in moderately deep water. However, by the Late Silurian and Early Devonian, the water depths had decreased considerably in the west. This shallowing may have been caused simply by the build-up of sediment within the western pan of [he basin. Despite the change in the environmem, sedimema[ion was continuous from the Silurian [0 the Early Devonian. There are tWO lines o\" evidence showing that the depth of Water became shallower on the western side of [he Melbourne Trough in the Late Silurian: •
•
Some beds within the Humevale Siltstone near Whittlesea, north of Melbourne, exhibit a sedimentary structure called hummocky cross-stratification (Figure 4-24). This term is applied to a broad low-angle type of cross-bedding. Studies of ancient and modern storm-affected sedimentS uggest that this feature is developed in sedimentS deposited by storms in water less than 50 metres deep. The Humevale Siltstone contains Late Silurian and Early Devonian graptolites, trilobites and brachiopods. A patch of shallow Water limestones and marls, showing features such as ripple marks and mud cracks, was exposed in the David M itchell Limited quarry at Lilydale, east of Melbourne. An Early Devonian age is indicated by the corals, brachiopods, bivalves and slarfish that have been found in limestones in [he quarry.
In the middle of the Early Devo nian, there was widespread deposition through the Melbourne Trough of a distinctive unil called the Wilson Creek Shale. It consist mainly of black shale and outcrops extensively around the upper Yarra River. This unit accumulated under very similar conditions to the Late Ordovician black shales discussed previously. Few turbidity currentS flowed into this pan of the basin at the time, and line-grained sediments seuled Out on [he sea-floor to form black, organic rich muds. The Wilson Creek Shale conlains both well-preserved land plant and graptolite fossils. The graptolites floated or swam in the upper levels o f the open ocean ,vaters and sank to the bottom when they died. The plantS grew along the coastal plains beside the basin. They were torn up by periodic floods and floated intO the middle of the basin before they sank, to be buried in the mud on the sea-floor. By the end of [he Early Devonian, so much sediment had accumulated within [he Melbourne Trough [hat the original deep-water basin had become an area of dry land crossed by rivers. The final sediments deposited in the trough were sandstones and mudstones of the Cathedral Group, which outcrop in the Cathedral Range, south of Eildon. The sandstones are cross-bedded and were probably deposited by braided river .
Eastern Victoria In the Silurian and Early Devonian, eas[ern Victoria was panly covered by sea. There were probably large bays extending nonhwards from the ocean to the south. Marine sedimem and volcanic rocks accumulated in areas that were actively ubsiding, olien along fault lilles. There were tWO major basins, the Cowombat Rift, which contains a Silurian sequence, and the Buchan Basin with mainly Early Devonian rocks
120
Ch apter 4
(Figure 4-17). There are also several smaller areas of outcrop; namely: • • •
Late Silurian-Early Devonian volcanics at Mount B urrowa, north-west of Corryong; A belt of Early Devonian sedimentary rocks along the Mitchell and Wentworth river valleys north-west of Bairnsdale; Early Devonian sediments and volcanics at Boulder Flat, north of Club Terrace i n East Gippsland.
Most of these areas are in rugged remote country. The geology is complicated by numerous faults and geologists have only begun to unravel the history in recent years. Cowombat Rift
This basin extended across eastern Victoria and into New South Wales. The rocks that were deposited were later affected by many faults and are now found in several fault-bounded blocks. The three largest blocks are around Dartmouth Dam, between Reedy Creek and Limestone Creek to the east of Omeo, and north of the Yalmy River, a major eastern tributary of the Snowy River. Sedimentation began in the Early Silurian, when deep-water sandstones and shales were deposited as turbidites in the central and southern portions of the basin. These are now found north of the Yalmy River and along Reedy Creek. In the central portion of the Cowombat Rift around Limestone Creek, these sedimentary rocks are overlain by rhyolitic lavas and some volcanic ash deposits, which were erupted over the sea-floor. Similar volcanic rocks also occur in the northern extension o f the basin around Dartmouth Dam. They probably partly filled the basin at the time, particularly the central part, because there they are overlain by shallow water sandstones and limestones with abundant corals and brachiopods. The limestones outcrop well along the Mitta Mitta and Gibbo rivers and along Limestone Creek. After the early volcanic activity stopped, the central and northern areas of the basin continued to subside. The depth of the water increased and deeper water mudstones and turbidites were deposited over the earlier shallower water sediments. The youngest unit in the central area is the Gibsons Folly Formalion. This shows a renewal of volcanic activity, with in terbedded andesitic and dacitic lavas, tuffs and fine-grained sediments being deposited in deep water. Within this formation, near the head of the Tambo River, there are two zones of sulfide minerals called the Wilga and Currawong deposits (see Figure 5-5 1 ). They were probably formed on the sea-floor close to volcanic vents. Deposillon within. the Cowombat Rift stopped at the entl of the Silurian, when the Bowning Orogeny (described later) caused the area to be uplifted. Buchan Basin
A new north-south depression, called the Buchan Basin, developed across eastern Victoria after the earth movements of the Bowning Orogeny. The Buchan Basin partly overlapped the older Cowombat Rift. Rocks which formed in the basin are found between Nowa Nowa, near the coast, and the New South Wales border at the headwaters of the Murray River. The basin contains two major rock units: the Snowy River Volcanics, dominated by rhyolitic ignimbrites, and the overlying Buchan Group, a shallow water limestone and mudstone unit.
Snowy River Volcanics: The topography in eastern Victoria before the outpouring of the Snowy River Volcanics was rugged, with deep valleys separated by steep hills. As the Buchan Basin began to subside, these valleys filled with river gravels and sands, which were then overlain by volcanic rocks. The first eruptions produced andesitic lavas. These were followed by very extensive rhyolitic ignimbrites and small lava flows. The volcanic rocks are interbedded with river and lake deposits. These rocks include conglomerates with pebbles of volcanic rocks, along with quartzite, shale and granite. The non-volcanic pebbles came from the mountainous area around the ba in. The ignimbrites were explosively erupted from many separate volcanic centres as very fast-moving, extremely hot clouds of ash, crystals and gas. When the ignimbrites were deposited over the surrounding country, the ash and crystals were so hot that they welded together to foml dense tough rocks. These are very resistant to weathering and erosion. Ignimbrites occupy much of the rugged forested country north of Nowa Nowa. They are exposed along the deep gorge of the Snowy River ( Figure 4-25) and o n bare high peaks such as Mount Cobberas. Subsidence inside the Buchan Basin was so rapid at this time that the southern part was below sea-level. Sandstones and mudstones, composed of eroded volcanic material, were deposited by turbidity currents on the sea-floor, together with th ick-bedded sandstones slumping off the flank of a nearby volcano. These sediments are well-exposed near Mount Johnson, nort h-west of Buchan. When subsidence in this area stopped, the build-up of sediments caused the water
Geological History of Victoria
' 21
Figure 4-25 Cliffs of Snowy River Volcanics in Tulloch Ard Gorge, 26 kilometres north-east of Buchan, East Gippsland. The gorge is pari of the valley of the Snowy River, which is in the foreground. The eli ffs are composell of thick beds of ignimbritic volcanics, which are dipping west at a shallow angle. (Photograph by N.J. Rosengren).
to become shallow. The southern part of the Buchan Basin then became dry land and further ignimbrites were erupted over the area. There are also extensive deposits of volcanic ash and pumice within the Snowy River Volcanic . Around Wulgulmerang, ash and pumice fell or were washed into a large lake which filled a crater. These ash deposit are well-exposed in road cuttings between Wulgulmerang and Little River Falls, and at the LillIe River Gorge lookout. Ash and pumice are produced during explosive volcanic eruptions, when lava disin tegrates into fragments of volcanic glass and crystals. This material falls out of the eruption cloud above a volcano, often many kilometres down-wind, to be deposited as beds of silt- or sand- ized material. Buchan Group: As volcanism gradually ceased, the Buchan Basin continued to subside. The sea again flooded in from the south and deposited sediments of the Buchan Group. These are best exposed in the Buchan area. In the shallow water, limestones of the Buchan Caves LimesTolle were laid down. Some of the e limestones were probably deposited on tidal flats, where algal mats grew over black mud . Further offshore. corals and brachiopod lived on the sea-floor in slightly deeper water. Small sandban ks, composed largely of pellets of lime mud, were also built up. The pellets are mostly of faecal origin: they were produced by organism living in the shallow seas, such as snails and worms. The fossils in these shallow-water limestones are visible in the walls of the tourist caves at Buchan. Similar limestones are found further north-west near Bindi, (on the upper reaches of the Tambo River), and at everal smaller areas in East Gippsland. As sub idence continued, the water became deeper. Fine-grained mudstones of the Taravale FormaTioll were deposited in quiet waters, directly over the limestones. These mudstones can be seen in road cUllings just north of Buchan. The Taravale Formation contains fossil of animal that floated or swam in the sea. They include conodonts, large swimming nauti loids and tiny, cone-shaped. float ing tentaculitids. North of Buchan, a big limestone bank of corals and stromatoporoids built up along the shoreline during a period when the sea-level remained stable for some time. On the northern or landward side of this bank, there was a sheltered lagoon.
1 22
Chapter 4
where delicate coral colonies flourished. Th e southern side faced the open sea and was battered by occasional storms. The limestones deposited on the bank and in the lagoon are called the Murrindal Limestone. They are preserved around Rocky Camp Quarry, north of Buchan. Later the sea-level rose again and the bank and lagoon were covered by fine grained deeper water sediments. Finally, in the Middle Devonian, the area was uplifted and became dry land. Wentworth Group
There are also Lower Devonian sedimentary rocks in sparsely-populated country north-west of Bairnsdale. The best exposures are along a narrow synclinal belt north of Tabberabbera, to the west of the Wentworth River. This sequence of rocks, known as the Went worth Group, was deposited largely by turbidity currents in a deep-water arm of the sea, about 60 kilometres west of the Buchan Basin. However, the uppermost unit is a calcareous sandstone containing abundant brachiopods. This was probably deposited in shallow water as the basin filled up with sediments.
Grampians Basin The Grampians Basin is an elongate north-south structure, about 60 kilometres wide. Its north-eastern margin is marked by a major fault, which forms the sharp eastern boundary of The Grampians. To the north and south, the rocks of the Grampians Basin disappear under Cainozoic sediments and volcanics which conceal the true extent of the basin. The age of rocks deposited in the Grampians Basin has been difficult to determine because fossils are rare. It was formerly believed to be Late Devonian to Early Carboniferous, because sandstones in The Grampians look similar to red-bed sediments of this age in central and eastern Victoria. A Lower Carboniferous age is given to the Grampjans Group on several 1 :250 000 geological map sheets (e.g. Horsham, Hamilton), pUblished by the Geological Survey of Victoria in the 1 970s. However, these sedimentary rocks were intruded by several granitic intrusives, that have been dated by radiometric measurements as Early Devonian. The sedimentary rocks are therefore of Early Devonian age or older. A few fish scales, spines and teeth found towards the base of the sedimentary sequence also indicate an Early Devonian age.
Figure 4-26 Aerial photograph of the orea around Rose Gap at the north end of The Grampians. ote the syncline plunging to the south-east and the strong joints in the sandstones. ( P hotograph courtesy of Survey and Mapping Victoria, Department of Finance).
Geological History
of Victoria
123
Grampians Group
The Grampians Basin subsided between major faults, which trended more or less north-south. About 6000 metres of both shallow marine and fluvial sediments were deposited in this trough t o form the Grampians Group. The sedimentary rocks are mainly medium- to th ick-bedded sandstones, composed almost entirely Of quartz grains cemented together by more quartz. They are therefore very resistant to weathering and form picturesque jagged features such as the Serra Range, Wonderland Range and the Victoria Range. These sandstones also occur at Black Range, Mount Dundas and Mount Arapiles.
Braided "veTS consist of a con ,tc se ries of channels that e mir. .1311\ for k anu re]OI around large .anu. anks ll'd ,maU bnds. Th n\er are gc lCrJJ ) fasl 00\'0 Inb and carry large lJU('urt of san.... Imlividu" \. annes \\ilhir a braide r iver are csual orl} tc....s of met "Cs wide and ;,1 few met res deep, I'ul t,e \\ hole
VOLCANOES AND LAVA FLOWS Volcanic activity has had a major impact on the Victorian landscape. From Seymour in central Victoria to west of Portland, there are about 400 extinct volcanoes, representing eruptions over Ihe last six to seven million years. These volcanoes and their lavas and ash deposits belong to the Newer Volcanics. Mosl of these volcanoes erupted within the last two million years, i.e. mainly in the Quaternary. The most recent eruptions were 4000-5000 years ago at Mount Schank and Mount Gambier in south-eastern South Australia and 7000-8000 years ago at Mount Napier, south of Hamilton. It is possible that more small eruptions will happen in the future. However, it is unlikely lhat the next eruption will be soon, as there are no obvious warning signs, such as sulfurous hot springs or small, frequent earthquakes. Most Quaternary volcanic eruptions were small-scale and short-lived. Altogether they covered an area of 1 5 000 square kilometres with lava, although the total thickness of the flows in any area rarely exceeds 50 metres. The lava flows were almost all basalts and many travelled long distances. For exam ple, flows from Mount Rouse, east of Penshurst, and Mount Eccles, east of Heywood, extended 60 kilometres and 40 kilometres respectively. The Mount Eccles flow travelled 1 6 kilometres beyond the present coastline, when sea-level was lower than it is now. Many basalt flows moved down river valleys, often ftlling them. Thbutary streams to the valleys were dammed by the lava and so lakes and s\vamps were formed. Sediments began to accumulate at the bottom of the s\vamps as soon as the lava flow cooled. As a result, radiocarbon dating of the vegetation in these basal sediments gives a good indication of the age of the lava flows. In this way the lava flow at Mount Eccles has been dated as 20 000 years old (see Chapter 3).
DEFORMATION
Deep ewcrage �a\all , Manb, nDng Park a c\\ Year a revealed the prescn!;e of cxtc v ell beds. \\' ile I "J..! I.:f \\ l i ng Ihrou one �.
'-
��
,:, " '::'9; .:.
Sub-base
Subgrade (natural soil
--' or lock or selected
_ _ _ _ _ _
compelCI (IU)
176
Chapter 5
6. Shaped pieces of rock used for buildings, paving and tombstones. These are called building stones or dimension stones. Most ViclOrian hard rock quarries supply one or more producLS i n the first four calegories.
Suitability of various rocks for coarse aggregate Many factors must be considered when deciding whether a rock can be used for a certain pu rpose, e.g_ the minerals present, their grain sizes, the rock texLUre, the nature and spacing of fraclUres and the extent to which minerals are weathered. To be suitable for coarse aggregate, a rock must be physically durable, chemically stable and uniform in its properties. Strict specifications are placed on the rock producLS bought by Government authorities and other users; these depend
on the type of structure lo be built and how long it is designed to last. For example, materials used in a highway must remain table under heavy loads for up lo fifteen years with little maintenance. Specifications may cover the sizing, shape, abrasion resistance, strength, toughness and soundness of the rocks. The material used for
Figure � A granite quarry, 6 kilometres north of Baimsdale. An investigation was carried out to delermine lhe mo l erficient procedures for blasling in lhe quarry. When any rock is being excavated by blasting, it is essential to break as much material as possible, while producing fragments thal can be handled easily al lhe neXl slage of lhe operation. I n the granite in this phOlograph lhere are well developed joints (cracks) in several direclion . The quarry face is aligned parallel lO one vertical joint direclion, thu producing a clean face. Thi is a safe face because there are no loose fragments which mighl fall unexpecledly. (Pholograph courtesy of R. McKean, CSIRO Division of Geomechanics). road surfaces must also be capable of being sealed by bilU men. I f rock particles disintegrate in use, the whole structure will fail. To meet the specific.1tions, a quarry operator mu t have a suitable deposit of rock and a crushing and screening plant, which can supply rock pieces in the required size ranges. Fresh, unweathered igneous and metamorphic rocks have the greatest mechanical strength because they consist of interlocking crystal . Basalt, granite, rhyodacite and hornfels are widespread in Victoria and hence are the commonest rocks crushed for coarse aggregate. ot a l l occurrences, however, are suitable for construction material . Problems may ari e for any one of the following reasons: I . Some rocks are unsuitable for the aggregate in concrete, because they contain minerals that slowly react with small amounts of potassium or sodium in cemenLS. This causes the concrete to expand and crack.
2. The minera.is in many basalts slowly weather to form clays that swell when they arc wet and shrink when they are dry. If such basalt is used in concrete, the clays may cause the product to crack because of the shrinking and swelling effects. I f they are used in road making. the clays may cau e the rock particles to break down and move against eac h other, thus making the rock pavement unstable. Some of the harmful clays have a green colour and arc called chlorite-smectite minerals.
3. Feldspars in acid igneous rocks may weather to kaolinitic clays. This reduces the bonding st rength between the minerals in a rock and so it does not provide a sl rong aggregate.
(
)
Crushed rock in the Melbourne district Fortunately, various rocks are available to supply the large quantities of crushed
Economic Geology
Figure S-9 Hard rock quarries supplying markets in the Melbourne and metropolitan ares. Rocks from all the major geological formations, exeept the softer sedimentary rocks, are crushed to provide a range of products. (After Buckland, G . L . and Fielding, B . J . (eds.) 1986. Extractive Industries Strategy Plan for Melbourne - A Draft Repon).
1 77
rock needed in the Melbourne metropolitan area (Figure 5-9). Newer Basalt is the main source of crushed rock from quarries in the western and nonhern suburbs. This basalt is widespread, it produces good quality products where the rock is fresh and it is readily blasted and crushed. Quarries in the volcanic plains are inconspicuous and can be used later for waste disposal. However, in many areas unweathered rock is confined to a single, near-surface basalt flow of Recent age. This means that some quarries are shallow but extensive excavations. By contrast, rhyodacite. granitic rocks. homfel and Older Basalt . which occur mainly in the hills to the east and south-east of Melbourne. offer scope for large quarries with several benches. When the upper, weathered material is stripped away, fresh hard rock usually extend to a great depth. As the urban development o f Melbourne i s skewed t o the east and south-east o f the city it might be expected that these rock types would be used to a greater extent than Newer Basalt. However, this is not so, probably because the potential for conflict between quarrying and other land uses is great in attractive areas such as the Dandenong Ranges and urban growth areas such as the Berwick corridor. Nevenheless there are large quarries at Lysterfield (hornfels), Kilsyth and Coldstream (acid volcanics), Pakenham (basalt) and Dromana (granite).
[3 Ouaternary & Ter1lary SGdIments
� Ouaternary - Tel1�ry etlS31 & Scona � (NC'N(ltVobrucs)
� Tert13ryBasan (0IcSer VOicancs I CJ Devonian Acid Vobmcs I: : : : Devonian Granrte Rocks � Me50IOJC & PaJaeozOK: Sed.mentary Rocks PaloeozOlc Hornfels
PORr Pfl/l,LIP HAl'
•
,.
30
K*>Inelres
Case history: Boral basalt quarry - Bundoora
4.
A large basalt or bllieSlOlle quarry is situa t ed within a bend of the Darebin Creek off McKimmies Road in Bundoora, a nonhern Melbourne uburb. The quarry measures aboul 800 by 550 metres and is worked to a depth of 32 met res . It i typical of many basalt quarries, which have supplied the city with crushed rock products for well over one hundred years. The quarry was established by Mr D. Toohey in the late I 960s. Lalcr it was operated by Readymix. B . M . G . Resources and more recen tly Boral Resources (Victoria) Pty Ltd became the owner. Quarrying is expected to cease around 1 995-96 when the accessible stone will be exhausted.
1 78
Chapter 5
Geology
The basalt solidified from a lava flow that originated from a volcano at Hayes Hill. near Mernda, 30 kilometres north north-west of Melbourne. Between 4_5 million and 0.8 million years ago, several lava flows erupted from volcanoes in lhal area_ The lavas flowed southwards along former valleys of Darebin and Merri creeks. The Darebin valley flow is the youngest. It reached Lhe Yarra River near Fairfield and continued down the Yarra valley as far as the Melbourne city area. Within the Bundoora quarry, four di tinct near-horizontal breaks can be seen . These divide the basalt into layers and represent an interval of time between successive flows. The breaks show up either as changes in the pattern of jOinting in the basalt or as bands of blocky or very vesicular basalt. Columnar structure is not well-developed. A soft clay is exposed beneath the basalt at the bOllom of the quarry. The clay was probably on lhe floor of the old Darebin Creek valley. The quarry has recently attracted attention because atlractive, sparkling crystals of zeolite minerals have been found in vesicles in the basalt _ The mineral species include analcime, phillipsite, chabazite, thomsonite, gonnardile and nalrolite. Various forms of yellow or brown calcite also occur. Method of quarrying
An hydraulic percussion drill rig is used to drill a series of vertical holes on the bench above the quarry face _ These are filled wilh explosives, which are then detonated to blast down the face. Two front-end loaders lift the broken basalt on to 35-tonne
Figure 5-10 Geological map or the northern suburbs or Melbourne_
v
v
v
v
The map shows the extent of the Hayes Hill lava now and the location of the Bundoora basalt Quarry, where pari of (he now is quarried. (After Hanks. W. 1955. Proc. Roy. Soc. Vicl.. 67).
A.
N
o I
! ,
,
t
5 I
KILOMETRES
AllUVium, sand Basall Hayes HIli lava lIow Sand. sandy clay
TERTIARY DEVONIAN SILURIAN
� �
Granite Sandstone. SIltstone mudstone
Economic Geology
179
dump trucks, which take the material to jaw crushers. A jaw crusher is a rectangular frame with a fixed steel jaw plate at one end and a second moving jaw. The latter swings around internally and crushes large pieces of rock against the fixed plate. The spacing between the jaws can be adjusted (0 produce pieces of rock of a requ i red size range. The crushed basalt falls on to a moving belt and then passes through two screens with different mesh sizes. The stone left On the first screen returns to the crusher for further reduction in size. Quarry products
Two size ranges of crushed rock, called A (controlled) and B (uncontrolled) provide aggregate for concrete and roadmaking. Some Bundoora basalt is also used to produce bluestone pavers. These are cut from large boulders by monumental masons. Environmental concerns
The visual impact of this large quarry has been softened by trees planted around the perimeter. In the past, a few complaints were received about drilling and blasting, but improvements to the techniques used reduced noise levels below the limits set by the Environmental Protection Authority. When the quarry is worked out, it will probably become a municipal refuse tip. After the pit is filled with refuse, the remaining stockpiles of overburden and quarry Waste rock will be spread over the top of the rubbish. Finally, the area will be planted with grasses and trees to provide parkland near the creek. Figure 5- 1 1 Bundoora basalt quarry. Three layers are visible in the 1 2 metre high face. The breaks between the layers (indicated by arrows) are roughly horizontal , but some depressions in the surfaces of the middle and lower layers can be seen. Each break represents a time interval between flows. Prominent vertical and some horizontal joints in the rock are visible. These were produced as the lavas cooled and contracted. (Photograph by W . D . Birch).
1Dimension stone or building stone As Victorian towns developed during the nineteenth century, many buildings, bridges, gutters, pavements and other structures were constructed from blocks of stone. In towns to the north of Melbourne. such as Kyneton and Kilmore, and many in the Western District, there arc still numerous churches, public buildings, houses and monuments built from dark grey basalt. By contrast, granite was used extensively in Beechworth and in some towns in the Midlands, e.g. Castlemaine, giving lighter coloured buildings. Nowadays, dimension stone has been largely replaced by concrete and steel in buildings, and concrete is used instead of basalt blocks for road kerbing. However, various dimension stones can still be found around Melbourne and other Victorian cities. Even modern office blocks often have an ornamental veneer of thin slabs of either natural stone or imitation stone formed by cementing rock chips together. These are called j(lcillg or claddillg Stones. Some of the rock types used in buildings in the inner pan of Melbourne arc described below: Basalt: This came mostly from quarrie at FOOlscray. Malmsbury and Lethbridge.
Malmsbury basalt was used for paving and cladding in the City Square. FOOlscray basalt was used extens;vcJy in St Patrick's Cathedral and in the base course of St Paul's Cathedral, Flinders St reet Railway Station and many other buildings. It also forms the walls of the National Gallery on St Kilda Road. Granite and granodiorite: Most of the granitic rocks in Melbourne buildings were
extracted from quarries that closed many years ago. They include ro(k....s from Arthurs Seat , near Dromana (g reen ish- ora nge). Cape W oolamai (mediulll 10
1 80
Chapter
5
coarse-grained pink), Colquhoun, north of Lakes Entrance (brick red) and Gabo Island (rich red). However, Harcourt Granodiorite, a grey rock with dark segregations of biorite and feldspar, is still quarried at Mount Alexander. It is found in Flinders Street Railway Station and the Colonial Mutual Life Building and is also used widely i n cemeteries for tombstones and rock chips over graves.
Sandstone: These, with some limestones, are termed freestones, because they can be easily cut into blocks. Grampians Sandstone, a strong and durable rock, was used in the Law Courts and Stale Parliament House. A Lower Cretaceous sandstone from the Barrabool Hills near Geelong is found in St Paul's Cathedral and SCOLS Church, as well as in various Geelong buildings. These sandstones were deposited by fast-flowing rivers, which washed away most of the clay.
Limestone: Soft , porous bryozoan limestones of Miocene age from Batesford (near Geelong) and Warrnambool were sawn into blocks and used in local buildings. Similar rocks in the Mount Gambier district are used extensively for house construction in South Australia.
Marble: The name marble should be used only for metamorphosed limestone, in which all traces of fossils have been obliterated. However, in the building industry the term is applied to any limestone, which has an attractive appearance when it is polished. Grey Devonian limestones from the Buchan district were used on interior walls and staircases in the Shrine of Remembrance, Melbourne Town Hall, Museum of Victoria and the State Public Library. Nu merous fossil fragments provide an interesting feature of this rock. Figure 5-12 Camerons QuarQ', Soulh Buchan, 1930. Long blocks of Devonian limeslOne were skilfully extracted at the quarry for lise as columns in the Shrine of Remembrance, Melbourne. Limestone quarrying 10 produce dimension stone ceased man y years ago. (Photograph from Geological Survey of Victoria).
GRAVEL Gravel is a natural coarse aggregate, consisting of accumulations of rounded, waterworn pieces of rock deposited by large, fast-flowing rivers. There is alway a lot of sand and minor amounts of silt and clay mixed with gravel. By definition the gravel fraction consists of the pieces with diameters in the range, 4.75 to 256 millimetres. Larger boulders may al 0 occur.
Uses Gravel i sometimes used as it is found for surfacing secondary road in the country. More often, it is crushed to produce more uniform size ranges of coarse aggregate.
Gravel deposits i n Victoria Some geological environments where there are gravel quarries are: I . Early Tertiary gravels capping low hills in the Midland , e.g. Tarnagulla. Some of these deposits had been worked for gold. 2. Late Tertiary fan and sheet deposits, which are widespread over the central
Gippsland plains and along the foot of the ranges to the north. They form the Haunted Hill Gravel format ion. 3. Quaternary gravel and sand deposits up to 30 metres thick occur on both sides
181
Economic Geology
of the Murray River flood plain downstream from Wodonga. They are mostly below the permanent groundwater level. 4. Early Quaternary gravels and sands occur along old stream channels, which cross
wide flood plains. Over the flat country north of Western Port, sand and gravel are excavated along old stream channels, which were ancestors of the present day Bunyip River system. Figure 5-13 Bora 1 sand and gravel pit, Darley, 3 kilometres norlh of Bacchus Marsh. A . East face of the pit: Up to 30 metres of poorly·
Quartz is common in gravels because it is resistant LO wear and very widespread as veins cutting Lower Palaeozoic rocks in the Central Victorian Uplands. The other rocks in gravels depend on the geology of the country that was eroded. Pieces of quartz and acid igneous rocks are usually rounded, whereas sedimentary rock fragments mostly have angular outlines.
sorted medium and coarse sands with interbedded fine sands, silts and gravels and white clay lenses were deposited by a fast- flowing river during Miocene times. The sands show cross-bedding: this feature can be used to deduce that the river flowed from the nOrlh to the south. The uppermost part of the deposit is rich in clay and many vertical erosion rills are visible.
.....�:, .'
,. \
\.,1.
�
��\'l; '"
\
B. Sand screening and washing plant: The naturally-occurring mixture of clay, sand and gravel is treated at this plant to yield products in a variety of sizes, which are then used for di fferent purposes. Water is piped to the plant from Lake Merrimu. The sand is washed through a series of revolving cylindrical screens with different-sized apertures. The clay and silt are removed and the remaining materials go to various stockpiles. The major products are concrete sand, packing sand, fine and coarse gravel, and boulders for landscape gardening. (Photographs by N . W . Schleiger).
SAN D Figure 5-14 Classification of industrial sand sizes. t n
Large quantities of sand are produced for the building and road making industries because it is a hard, durable, Chemically-inert material .
Nature of sand Natural sands are sedimentary deposits formed by the action of flowing rivers, winds or waves and currents in the sea. They were derived from the weathering and erosion of older quartz-rich rocks, such as granite, rhyolite or sandstone. In geology the term sand applies to any mineral or rock particles in the size range, 0.06 - 2.00 millimetres. However, in industry the range is u ually 0.075 4.75 millimetres. Commercial sand consists mainly of quartz, often with small amounts of other minerals such as feldspar, mica, calcite, ilmenite, rutile, monazite and garnet . The quartz grains may be either: • edimentary processes, or rounded, because they have been worn down by angular (so-called sharp sand), because they have been derived directly from • weathered igneous rocks. -
There are also lime sand deposits consisting largely of shell fragments.
Uses of sand Each year over seven mil.lion tonnes of sand are produced in Victoria from more than 250 sand pits. (ost sand users require detailed information about the sizes of particles that are present in a quarry product . A sample o f the sand is therefore passed through a series of eight sieves with the apertures shown in Figure 5-14. The
182 T , th
Chapter 5 n
n
we'ght 01 material retained on each sieve is measured and converted to a percentage of the total sample. Some industries prefer sand grains to be fairly uniform in size, e.g. filter sand should not have smaller grains filling the cavitie between the larger grains. Other industries require well-graded sand with a wide range of grain sizes, e.g. the sand used in building mortar. A few of the many uses for sand are given below: I.
Coarse sand:
•
•
2.
•
3.
Fine sand: • •
4.
- this is in greatest demand: as fine aggregate in the manufacture of concrete, where it is mixed with Portland cement and coarSe aggregate; as packing material under paving and concrete labs, and as trench refill around pipes and underground tanks; in road making, sand is used alone in road bases and mixed with asphalt or concrete for seal ing roads.
Medium sand
•
•
( n
for sand blasting; as a filter medium (e.g. in septic tanks, swimming pool filters and aquariums).
in mortar, fine- to medium-grained sand is mixed with Portland cement, quicklime and water to produce a medium to bind bricks together; sheet plaster is formed mainly from gypsum mixed with hydrated lime, fine sand and sometimes animal hair.
Ultra-fine sand: •
Used in the manufacture of abrasive cleaners, CUlling compounds, toothpaste , paper impregnation, fibreglass compounds and glass.
Sand deposits in Victoria Because sand is a low-value commodity, deposits can only be worked economically where they outcrop or are close to the surface. In Victoria, most unconsolidated sands are of Cainozoic age. There are large deposits on beaches and dunes along the coast. These are unlikely to be exploited commercially because of their recreational value and environmental sensitivity. Sands and gravels are also common along many fast-flowing rivers that drain the Victorian uplands. Again it is usually undesirable to extract these on environmental grounds, although there are exceptions. Most industrial sand deposits occur in one of the following environments: 1 . River (alluvial) deposits (a) Tertiary (b) Quaternary 2 . Windblown (aeolian or dune) deposits Tertiary river sands
h ., n , ( J p )(.:� h rJ· n I11n mdl If\ an \ d r , h n t ,I an , 0 , c r du. ,ar u _r I I en! lZ r11' f"\
These are the most important deposits in the State. They are widespread in the southern and west-central regions, where they provide construction sand to the larger cities and roadmaking material for rural areas. They are found at many levels in Tertiary sedimentary sequences. Coarse sands and gravels were deposited by fast-flowing waters along the central channels of former river systems. Finer sands and silts were mostly laid down on the banks and floodplains. Because velocity and hence the load-carrying ability of these streams changed rapidly, the sediments vary greatly in grain size. After classification. they provide products suitable for many uses. In the past, most sand for the Melbourne market came from the outer south eastern suburbs, e.g. Heathenon, Springvale, Dingley, Clayton. Its main use was in concrete and concrete products. This sand is part of the Brighton Group of P liocene age; it was deposited by streams eroding the uplands to the east of Melbourne. The deposits vary considerably in grain size, both vertically and laterally. Supplies from these districts are decreasing due to a depletion of reserveS and the higher value placed Oil the land for residential development. Increasing amounts of sand for the melbourne market are now coming from the Bacchus Marsh and The Gurdies - Lang Lang - Grantville areas. The latter deposits, on the eastern side of Western Port, are of Early to Mid-Tertiary age. They are quarried, where beds have been dragged up along major north-south faults. In the future, lare. ,-Ipn"'its ill the Anglesea area may also become important.
Economic Geology
183
Quaternary river sands
Sand deposits occur along present-day stream courses, on Oood plains and terraces, and along the abandoned channels of older Pleistocene streams. These deposits are usually smaller than those of Tertiary age. Extensive sand and gravel deposits along the Murray River Oood plain downstream from Albury and Wodonga are an important exception. Several commercial deposits near Melbourne were shed directly from granite, e.g. on the Oanks of the You Yangs and at Labertouche in west Gippsland . Quaternary dunes
Extensive deposits of coastal and inland dune sands occur in southern, north-western and western Victoria. The most important are dune fields in the coastal regions. Examples are found in t he Portland, Cranbourne-Langwarrin and Lang Lang districts and on Mornington Peninsula and Wilsons Promontory. These sands were probably derived from older Tertiary sands during arid periods. A lack of vegetation at such times allowed winds to erode the older sands and relocate them in dune systems, sometimes up to 20 metres high. These are now mainly fixed by vegetation. The sorting process of wind action produced sands of fine- to medium-grain size with relatively lillie clay. Dune sands have fewer uses than alluvial ands because of their more uniform, fine grain size. Because they contain lillie clay and have low levels of iron and titanium oxides, some pure quartz sands are used in the production o f clear glass. They can also be used either alone as foundry and bedding sand or blended with coarser material as an aggregate for monar, plaster, asphalt and concrete.
CLAY Clay is a relatively low-cost, common commodity with many uses, especially in the construction industry. Victoria has abundant supplies of clays of various types, but to be of value a deposit must be near an industry that can use it.
Nature of clay The term clay is used in three different senses: •
•
•
a nalUral, earthy material, which is sticky and plastic when wet; four related groups of minerals, which have similaritie in their crystalline tructure and propenies; all soil and sediment particles, that are less than 0.002 millimetres in diameter. The clay fraction may contain organic maller and very fine grains of quanz, mica and other crystalline minerals, as well as mixtures of the four clay mineral groups.
Figure 5-15 Inlernal ionic slruclure of a kaolinite crystal. Silicon, aluminium, oxygen and hydroxyl ions are in a layered arrangement, typical of all clay minerals. There are strong forces connecting the units within each layer bUl weaker force between the layers.
(OH)
AI (OH)+ 0
Si o
Clay minerals are all hydrous aluminium silicates containing aluminium, silicon, oxygen and hydroxyl (OW) ion arranged in parallel sheets or layers (Figure 5 - 1 5). The sheets in the four group of clays have different compositions. The sheets may be strongly held together or only weakly bonded through sheets of water molecules. The groups are: consists only of the essential ion described above. The sheets are well-bonded with no intervening Water.
Kaolinite group
-
Illite group contains the same ions as kaolinite as well as potassi um and some Water between the sheels. The sheets are bonded togelher weakly. -
contains magnesium and sometimes calcium between lhe sheet with surrounding waler molecules. Iron and magnesium can enter the sheels by replacing orne of the aluminium. The sheets are poorly-bonded.
MOl1llllorilionite group
-
184
Chapter 5
similar to the montmorillonite group but forms mainly from the weathering of biotite mica.
Vermiculite group
-
Illites are the most abundant clays in nature, but kaolinites and montmorillonites are the most usefuL
Properties of clays The widespread use of clays depends largely on two of their properties: • •
after water is added, they become plastic, i.e. they can be worked into various shapes; when they are 'fired', (i.e. heated to temperatures over I050·C in a kiln), they lose all their combined water. At the same time, they shrink and form a hard product. Partial melting may occur to give a strong, glassy binding materiaL Va.rious chemical reactions take place to form crystalline minerals, which help to bond the fired particles together.
10 addition, each group of clays has some distinctive properties, which influence the uses to which any clay mineral can be put, e.g. the swelling property of montmorillonites makes them suitable to add to drilling fluid in boreholes to fill cracks in the rock and hence retain water required to circulate during drilling.
Uses of clays Clays can either be used as they are or they can be burnt in kilns to form new products. Many users of clays blend several kinds to obtain the best quality products.
10 terms of value and use, clays may be divided into two main categories: I.
Low-value clays
worth less than $5 per tonne at the pit. These are mixtures of several clay minerals, which are used in cement production and to make bricks, sewer pipes and roofing tiles (structural clays). The brickrnaking industry consumes 88"10 of all clay produced in Victoria. These clays are used directly as they are quarried.
2. High-value clays sell for prices from $40 to $200 per tonne depending on the
clay type and the extent to which they have been processed. They usually consist of a single clay mineral type, which has special properties and uses. Most high value clays are washed through screens to remove coarser particles, such as quartz grains. Kaolinite (china clay) is the most widely-used high-value clay. It is used for coating paper, as a filling material in paper, rubber, paint and plastics, and in the manufacture of whiteware, tableware, insulators, wall tiles and heat resistant ware. Important properties of kaolinite are its softness, whiteness, low absorption of moisture and chemical inertness at room temperatures. For most uses, pure kaolinite is not sufficiently plastic, so other clays must be added. Bentonite is a high-value montmorillonite type clay. It is used as a bonding agent in moulding sand at foundries, as a sealant in darns to minimise water loss by seepage and in drilling mud to exclude water.
Brick manufacture Bricks are made from weathered shale, residual or sedimentary clays (see later), or
often, a mixture of two or more clay types. Besides clay minerals (kaolinite, illite, etc.), there are always some non-clay minerals (e.g. quartz) in bricks. There are several stages in the conversion of clay to brick. One widely-used process involves the following steps: I . One or more natural clays are ground up and mixed.
2 . The ground material is mixed with water to give a plastic mass. 3. The plastic material is squeezed (eX1ruded) through a die of rectangular shape and cut by wires or knives into so-called green bricks. 4. The green bricks are stacked and left for drying. 5. The dried bricks are heated in a kiln for some days and then withdrawn after a cooling period. Clays are suitable for brickmaking if they fulfil the following conditions: •
• •
•
•
they are plastic when wet, so they can be made into any shape; they form hard products (bricks) after being fired at temperatures of 9O()OC to 1 1 5()oC depending upon the type of clay used and the required product. a desirable colour is produced after firing; there is little shrinkage during drying and firing, so the original soft, plastic mass becomes a hard product of similar size; the bricks remain stable and strong over a long period.
Economic Geology
185
The types and proportions of clay minerals present determine the plasticity of the mixture and the colour and strength after firing. Iron oxides also innuence colour. Non-clay minerals reduce plasticity, but they help to decrease the shrinkage that occurs when clays are dried and fired. Too much quartz, however, may cause the products to crack as they cool in the kilns. Calcite, dolomite, pyrite, siderite, coaly mailer and soluble salts can also cause harmful effects, such as cracking, black spots, salt encrustations, etc.
Clay deposits in Victoria Industrial clays are of two kinds: I. Residual clays, formed by the weathering of underlying rocks. Over millions of years, most rock-forming minerals, except quartz, break down to form clays.
2. Sedimentary clays, formed by the erosion of residual clays and weathered rocks and their transport and deposition elsewhere. Residual clays MoS! industrial clays are the result of intermittent weathering of older rocks to depths
of up to 30 metres over the past 50 million years. The commone t parent rock are Lower Palaeozoic marine siltstones, mudstones and shales, granitic rocks, Older Volcanics and Early Tertiary river and lake sediments. Some Lower Cretaceous sedimentary rocks also have weathered to useful clays. During the long period of weathering, feldspars and muscovite altered to kaolinite and illite, and ferromagnesian minerals to montmorillonite-type clays.
Two periods of weathering during lhe Cainozoic were particularly significant in the development of commercial clays: I. In the Early Tertiary, a very extensive, deep weathering profile developed, which was partly or wholly removed by later erosion in many places. A feature of this
profile is a white kaolinised (pallid) zone, commonly 20 to 30 metres thick. In places, where the profile is developed over Lower Palaeozoic sedimentary rocks, it provides pale-firing, residual clays suitable for brickmaking. The popularity of cream and pale-pink bricks over the past 40 years led to the development of white clays at Campbell field and Craigieburn (north of Melbourne), Warragul South and Bendigo. Some of the whitest bricks in the State are manufactured at Stawell using weathered Ordovician shale. At a few localities, a deep, high purity kaolinite of low plasticity formed over granite. At Piltong, west of Ballarat, kaolinite is separated from the quartz grains by washing and then sold as a high-value product. Similar clay has been extracted intermittently at Lal LaI, south-east of Ballarat.
2. In the Middle Tertiary after the outpouring of the Older Volcanics, there was a period of high rainfall and intensive weathering in Victoria. This produced an iron-rich upper clay zone over a deep mottled zone. Low plasticity kaolinite-illite clays of the mottled zone are used in Melbourne's brick, pipe and tile manufacturing industry. They formed on Devonian and Silurian mudstones and siltstones and mostly tire to a red colour. There are also brick plants at Traralgon and Ballarat on Palaeozoic shales. Small plants at Bendigo and Glenthompson (in the Western District) use weathered Ordovician or Cambrian shales and some Tertiaty alluvial depo its derived from them.
Figure 5-16 Bonll c1.y pi!,
P.rw.n V.lley,
soulh-wesl of Bacchus Marsh.
A large expanse of white clay of Early Tertiary age has been exposed in the pit. The clay is overlain by cross-bedded sands and clays, and higher again by basalt. The m31criai extracted from the pit contains line quartz sand with lip to 400"/0 kaolinite and 1 5% coarse mica. The clay is stockpiled on an area of basalt and transported to the company's brickworks in Melbourne as required. There it is blended with other clays. (Photograph by N. W. Schleiger).
186
Chapter 5
Sedimentary clays
Large parts of the Early Tertiary, pallid zone, residual clays were eroded, carried away by rivers and deposited on flood plains, in lakes and basins and on the sea floor. Further changes in the composition of the clays occurred during transport. Some of these clays contain a high proportion of a plastic variety of kaolinite and sometimes small amounts of organic matler. They fire to a white colour and are known as ball clays. Because of their plasticity they are often blended with residual kaolinite clays to give strength to ceramic wares before and after firing. Clays formed along Tertiary rivers are worked in pits at: •
•
Axedale (east of Bendigo): a white plastic clay is excavated from deposits laid down by an ancestor of the Campaspe River. The clay was eroded from weathered Ordovician sedimentary rocks and granite. Campbellfield: a white pia tic clay, deposited along an Early Tertiary river, occurs beneath a basalt flow.
Clays that accumulated in Tertiary lakes and swamps cover larger areas than the river clays and are often much thicker. They also were derived mainly from the weathering of Ordovician to Devonian fine-grained sedimentary rocks and Devonian granitic rocks. White clays of this type have been extracted from down faulted basins in the Bacchus Marsh district, (e.g. Parwan River valley and Darley) and at Lal La!. Sedimentary clays of Quaternary age are another important source of structural clays. Generally they contain less kaolinite than the Teniary clays. Lake deposits south of Ballarat provide plastic clay for use in brick, sewer pipe, floor tile and potlery manufacture. River valley silty clays at Shepparton, Euroa, Wodonga and Swan Hill have also been used for brickmaking.
Case history: Hallam clay pits
In the south-eastern outskins of Melbourne, two companies, Brick and Pipe Industries Pty. Ltd. and Darley Refractories Pty. Ltd., have clay pits near the Gippsland railway between General Motors and Hallam stations. Two different products are extracted - afireclay near the surface and a deeper brick and Iile clay. Fireclay is burnt to make firebricks and other refractory ware. Firebricks are used in furnaces and kilns because they can withstand high temperatures. A fireclay is distinguished from a brick clay by being relatively pure kaolinite and capable of retaining its stability at the high temperatures of over 12()()OC found in industrial furnaces. When quarrying commenced at Hallam in the early 1950s, fireclay was the only material being produced. At that time the demand for fireclay·based refractory products was rising strongly and an existing quarry at Dandenong was nearly worked out. The material was used in State Electricity Commission boilers in the Latrobe Valley, cement kilns, glassmaking furnaces, metal foundries, brick kilns, boilers, incinerators and by the Victorian Railways for its locomotives and its workshops. In recent years the demand for fireclay-based refractories has declined considerably as industrial processes have changed. However, Brick and Pipe Industries found another clay underlying the fireclay that could be blended with clays from other districts to make bricks, roofing tiles and pavers. Much greater quantities of brick and tile clay than fireclay are now produced. Geology
The clay pits are near the south-western corner of the Lysterfield GranOdiorite, an intrusion covering a wide area north-east of Dandenong. This rock is a medium grained biotite granodiorite, consisting of quartz, orthoclase and plagioclase feldspar, biotite and some hornblende. South of the Princes Highway, the granodiorite is deeply weathered. It is mostly covered by up to 5 metres of Pliocene sands, sandy clays and gravels, which were deposited as an alluvial fan. There is a transition from fresh granodiorite at depth to fireclay near the surface. In the first stage of weathering, the feldspar crystals are altered to kaolinite, but the original shapes of the feldspars are retained. Externally the rock still looks like a granodiorite. This rock passes upwards into a khaki-coloured, micaceous, sandy clay, which consists of quartz and mica crystals in a matrix of kaolinite - this is the brick and tile clay. Fireclay is at the top of the weathering profile. There, the original feldspars, mica and hornblende have all broken down to a kaolinitic clay. Quartz crystals remain scattered through the clay. Most metallic ions (sodium, potassium, iron and magnesium) contained in the original minerals have been dissolved out. The depth of fireclay rarely exceeds five metres, but the brick and tile clay occurs to a further depth of 10 15 metres. The total depth of granite weathering is thought to be about 50 metres.
Economic Geology
187
Quarrying methods
Both companies operate only in the drier months, because it is difficult to dig and drive trucks in wet clay. Clay is easily extracted using a mechanical excavator. A front-end loader transfers the material to trucks. Darley Refractories allows the clays to partly dry in an open shed before trucking it to a firebrick factory a t Darley, near Bacchus Marsh. The Brick and Pipe lndustries clays are taken to stockpiles at brickworks at Scoresby and Burwood. Low-iron clays, that burn to a cream colour, are dug out and stockpiled separately from red brown, higher-iron clays, which burn to an orange or red colour. Manu facturing
The companies use similar processes to manufacture their products. The Hallam clay is blended with other clays in different mixtures to give various products. The clays contain several percent water naturally. More water is added after they are mixed and crushed. The plastic mixture is then extruded and either cut off or pressed into the final shapes (e.g. bricks). The shapes are dried carefully at low temperatures before being fired to a high, constant temperature in a kiln. The firing temperature is 1080 - 1 130°C for bricks and tiles, and 1350°C for firebricks. The modern use of firebricks is mainly for boilers, kilns and furnaces in industries concerned with steam generation, incineration, metal melting and heat treatment. Land use
When quarrying commenced at Hallam over thirty years ago, the district was mainly used for farming. Now most of the land has been developed for factories and beyond them are housing estates_ Some of the land underlain by deep clay has been sold because its value for property development is greater than it is for the production of low-value clays. Tree screens have been planted around the quarry properties to reduce their vi ual impact.
l
This article is based on informar.ion supplied by Brick and Pipe Industries PlY Ltd and Darley Rerractories Pty Ltd.
LIMESTONE Limestone is a rock made up largely of crystals of calcite - calcium carbonate. Some magnesium is always present in the mineral dolomite, (CaCO,MgCO,). The commonest impurities are quartz and clay. Small amounts of siderite (FeCO,), sulfide minerals (e.g. pyrite FeS,) and limonite may also be present. Most lime stones formed on the floors of shallow warm seas.
Uses Limestone, like sand and clay, has a large number of uses. For most purposes, rocks with more than 90"70 calcite are required. Limestones are common rocks, but many deposits have no value because they conlain excessive silica (quartz) or magne ium (in dolomite), or they are located toO far from markets. The main uses of limestone in Vicloria fall into several categories: I. Calcium carbonate is converted to other calcium compounds.
(a) Manufacture of cemelll: The cement industry is the main consumer of limestone, because of the widespread use of cement and concrete in the construction induslrY. (See later case history for a description of cement manufacture). (b) Manufacture of qllicklime and hydrated lime: When limestone is heated in a kiln to just over I 000 °C, carbon dioxide is driven off, leaving calcium oxide (qllicklime). CaCO,(s) - CaO(,)
+
CO,(g)
I f quicklime is treated with Waler, a whilC, nearly insoluble powder is produced. This is calcium hydroxide, known also as hydraled lime.
CaO(s)
+
H,O - Ca(OH},(s)
BOlh quicklime and hydrated lime are chemically basic or alkaline subslances, i.e. Ihey react wilh acids 10 form salts. They have manv COmmon uses and in indu Iry arc both called lime. Quicklime is used w her� vigorous chemical aClion is required. Hydraled lime is easier and safer 10 ha nd lc. In the construction industry, lime is lIsed in the ma n ufal.: t l I re of mortar, plasler lime silica bricks and in sul ation malericls. Because of ils properties as a base, it is used to neulralise acidic subslances generaled by many industrial ,
, 88
Chapter
5
processes and to absorb sulfur dioxide from exhaust gases at smelters and power generation plants.
2. Calcium carbonate is a source of calcium in fertilisers, stockfeed and poultry grit. Calcium is an essential plant nutrient for plants and animals. Finely ground limestone, known as agricultural lime, is spread over many farm soils to restore fertility in areas affected by soil acidity (see Chapter 2). In Victoria, agricultural lime is required to contain a minimum of 650/. CaCO,. This enables many lower-grade limestones to be sold because they occur close to farming areas that need the product. High-grade limestones, however, are more effective.
3. Other uses depend on the physical (rather than the chemical) properties of calcium carbonate. In particular it is used as a white filler in paper, carpets, paint, rubber and other materials. Limestone is also used as crushed rock, e.g. for paths and as road base material.
Production of limestone and limestone products The extraction and processing of limestone at quarries is similar to that of crushed rock. It is crushed, ground and screened to produce the particle size ranges required by particular consumers. These vary from fist-sized lumps burnt in some lime kilns to very fine material needed for agricultural lime.
Figure
5-17
Limestone quarries operating in Victoria.
There are many other old quarries not shown on this map. where limestone was extracted in the pasl.
I
Operating Ouarries
'-- '---�Mlldu,a
I
i
I
i
Swan HIU
I
Cement
Main Usage
Ago Ohgocenfl-Mlocene
Ouickllme
D6vofllan
(Devonian at Tyers)
Agncultutallime
MIocene, Pleistocene mamly
Dolomitic agncvftutalJlme
MIocene-Pliocene
Shel/gol
Recent
i
Albury Wodonga
I
i i
Horshame
I
i i
�
Porlland
Ham ilton.
Ballarat_ MELBOURNE .lllydale Geelong Corac.
Mo,.
Balrnsdalee .Sale
� 2,5 Sf 7f lqo Kilometres
Palaeozoic li mestone deposits in Victoria
Most Palaeozoic IimeslOnes are mas ive, grey, crystalline rocks. The only ones now worked are of Early Devonian age. The largest quarry is at Lilydale, east of Melbourne; it is al 0 probably the oldest quarry in any rock type in Victoria, having been operated continuously by one family company for over 110 year . Limestone is burnt on site for the production of quicklime and hydrated lime. The rock is also crushed and ground to provide many other products (Figure 5-18). At Rocky Camp, five kilometres north of Buchan, there is a bare limestone hill, formed from a submarine bank of fos il fragments. It is a high quality deposit, containing over 97% CaCO,. This limestone is crushed to feed a lime kiln at a paper mill at Maryvale, near Traralgon. Some rock is al 0 ground fine and used for agricultural lime, stock feed and in ceramic tiles. Many years ago, other quarries south of Buchan produced a dimension stone known as Buchan marble. Some Lower Devonian limestone is also quarried near Tyers in central Gippsland and sent to a cement plant at Traralgon. Of hi torical interest are the remains of old lime kilns on the waterfront at Walkerville SOUlh on Waratah Bav which burnt Lower Devonian limestone obtained from nearby cliffs until the 1 9 0s. Large Silurian limestone formations in north-eastern Victoria have not been
2
Economic Geology
Figure 5-18
189
'!"
Lilydale limestone quarry.
A well-bedded sequence of
Devonian limestones of varying grades imerbedded with thin dolomite and marl layers is exposed on the lower faces. The beds dip eastward al 60° benealh deeply-weathered Devonian clayey sandstones, showing white on the upper faces on the left. The Palaeozoic rocks are overlain by varying thicknesses of weathered Older Basalt, the dark material on the horizon. When sold as filling, the weathered basalt is called salamander.
The quarry faces are mostly 10 metres high. A percussion drill is preparing one face for blasting by drilling lines of short holes lhal will be filled with explosives. On the floor of the quarry, a rock breaker is reducing large pieces of rock to smaller sizes. (photograph courtesy of David Mitchell Ltd.) .
used because of their distance from Melbourne. Some of these rocks in the Limestone Creek country near the head of the Murray River exhibit attractive colours when they are cut and polished. Past attempts to develop these deposits as a source of ornamental marble were not succes ful. Cainozoic limeslone deposits in Victoria
Cainozoic limestones are softer, less consolidated and more porous than Palaeozoic formations; they are often very fossiliferous. Most Cainozoic limestone fall into one of the following groups:
I . Pale brown, yellow and buff limestones are common in the Miocene rock of
the Gippsland and Otway sedimentary basins. They range from very high-grade bryozoan limestones, (e.g. Warrnambool and Mount Gambier districts) to hard, high-grade rocks interbedded with lower-grade marls. Miocene lime tones and marls at Waurn Ponds and Batesford (near Geelong), and Merrimans Creek (south of Rosedale in Gippsland), supply most of the feed to Victoria'S three cement plants. Clay in the marls supplies alumina, which is needed in cement manufacture. Miocene bryozoan limestone has also been used as a building stone. It i white and very pure in the western part of the Otway Basin. It is easily cut into building blocks, which form a strong, durable, construction material, especially for houses.
2. Pleistocene aeolianite has been exca ated at many places for agricultural lime, e.g. Yanakie near Wilsons Promontory, Portland, Warrnambool. It is also useful
for making secondary roads.
Case history: Li mestone quarry and cement manufacturing plant, near Geelong. Australian Cement Ltd.
Cement is a grey powder that sets solid when mixed with water. Selling takes place gradually over many hours and usually it takes about one month for full strength to be achieved. Cement is manufactured mainly because it is an essential component of cOllcrele, one of the most widely-used construction materials in the building industry. Concrete is made by mixing water, crushed rock (coarse aggregate), clean coarse quartz sand (fine aggregate) and sometimes some industrial waste. The cement binds together the other less costly components. The latter are inert but give bulk and strength to concrete. Chemistry of cement Cement is a complex mixture of various calcium compounds, including calcium silicate, calcium aluminosilicate and calcium ferrite. They are formed at high temperatures, when calcium oxide (CaO) reacts with silica (SiO,), alumina (AhO,) and iron oxide (Fe,O,). The CaO is produced from the decomposition of limestone when it is heated above I DOO·C. Alumina is usually obtained from clay, and sand
provides silica. Both compounds may be present together in a sedimentary rock, such as shale, or CaO, AI,O, and SiO, may occur in about the right proportions
190
Chapter
5 in some impure limestones or marls. Sometimes bauxite (hydrated aluminium oxides) is added to give more AJ,O,. Iron can be introduced as naturally-oc curring oxides or as some form of scrap iron or steel. L i mes to ne q ua rry Australian Cement Limited obtains most of its raw materials from Batesford on the western bank of the Moorabool River, nonh-west of Geelong. The only imported component is iron scale, which is a by-product from a steel rolling mill at Hastings. The bedrock on the quarry floor is Dog Rocks Granite of Devonian age. This is overlain in the quarry faces by a succession of horizontal Cainozoic formations. ear the surface, there is the overburden, that is material not used in cement manufacturing. This is chiefly a mottled sandy clay of Pliocene age, called the Moorabool Viaduct Formation. In places this is overlain by a Newer Basalt flow. The white quarry faces are formed by Bates/ord Limestone which interfingers with clays and marls of the Fyans/ord Formation; these are both of Miocene age (Figure 5-19).
Figure 5·19 Geological map of the country west of Geelong and a geological cross·section through the Batesford limestone quarry.
The limestone formed during lhe Miocene in a quiet shallow sea on the eastern side of an island formed by the Dog Rocks Granite. (Geology from Geelong 1:63 360 geological sheet, 1963. Geological Survey of Victoria).
SECTION A
-
B
Batcsford Limestone is a friable rock, containing the reJllain� of many animals that lived on the sea-floor, including bryozoa, echinoids, bivalves and foraminifera. Occa ional shark teeth and whale bones have aI 0 been found. The limestone was formed in warm shallow water around islands formed by the granite. Sandy limestone containing weathered granitic material occurs at the base of the formation on the western side of the quarry. The limestone grades upwards and laterally to the south east into the Fyansford Formation. The laller was deposited in deeper, quieter water. The fossils in the Fyansford Formation include the skeletons of both sea-floor and floating animal . Quarrying
The overburden is stripped by scrapers or diesel shovel" loaded into truck� and transferred to dumps beyond the limits of the quarry. T he overburden dump� are contoured, planted with grass and used to g rate animals. The limestones and marls arc harder than the o verbu rde n. To eXlract them, it b necessary to drill lines of ho les, which arc loaded with explosives and then detonated. A!"ter blasting, the broken rock is carried by trucks to a cr us her on the quarry floor. The crusher reduces the sto n e to pieces, which arc a little smaller than tennis balls. Rocks from the Uatesford Limestone and Fyansford Formation arc blended to provide the correct proportion, of IimeMone, sand and clay needed to produce cement. The cru,hed ro ck i, convcyed ovcr four kilometres on a scric; of convcyor belt, to the �cmcllt plant at Fyansford. Cement manufacturing plant
Cru,hed lime\lonc and marl plus minor amoun" 01 bau,ite and i ro n pow da
ar�
Economic Geology
191
Figure 5-20 Batesford Limestone quarry.
Dark grey basalt and pale sands and clay of the Moorabool Viaduct Formation form .he overburden above pale grey Fyansford Marl and white Ba.esford Limes.one. The marl and limestone are used in the manufacLUre of cement at nearby Fyan ford. All .he formation are nearly horizontal. The marl partly overlies and par.ly merges into .he limestone. The high ground in the background is on the upthrown side of .he Lovely Banks Monocline.
t�
.I
I
' -..
thoroughly mixed with water in a grinding mill 10 produce a slurry. The slurry is fed into a tilted, rotating kiln made of steel and lined with hea t-resisting bricks. Natural gas is burnt at the lower end to produce a temperature of aboUl 1 500oe. The slurry moves slowly downwards into the honest section of the kiln. The limestone figure 5-21 A now sheet showing the \'orious stages in the manufacture of cement at .he quarry and plant of Austrail an Geelong.
Overburden
Cement Limited, near
Stockpiles or Raw Materials
Raw �Iaterial Storage
,: Stack
Cooler Exhaust
�Llmestone DBou;nte f':�":..� Iron OXIde �G_\p.OI.I'HlS ,
EARLVCRETACEOts
UTE CHI:..ACEOt.:S
o
10
20
30
40
so
Kdomelres FOtITESCn: HAI.II1L.,. KINGf'ISH )IArKt:Rt;J. OBIA FIJ'H 'SIWR
B SOUTH
�
n'NA
)UOCEst:
B' NOHl'H Sen Levcl
Sal.· Crimp 5rospr'OJ GNIU/I
.000
IAtrohl- Group
I.ATE cl:t:T'\n:ol'�
OUTIIER:\'
FA L'L T ZOSE
1 5 .000
SORTflER.\'
FAULT ZO.\'E
204
Chapter
5
Figure 5-32 Fortescue oil production platform in Bass Strait. This ESSO·BHP platform was built berween 1 980 and 1 982 to produce 44 gigalitres (280 million barrels) of oil, that had been discovered by an exploration well drilled in 1 978. Although this was a large field, it contained only enough crude oil to satisfy all of Australia's needs for about 46 days. Oil production commenced in 1983 and initially about 100 000 barrels were obtained each day from 27 development wells drilled below the platform. Production is now declining and 30% of the liquid recovered is water. The oil is trapped below an unconformity in the Tertiary sedimentary rocks . Fonescue is a fixed platform on pylons driven into the sea· floor.
Some fearures of the photograph are: (A) a supply vessel which can tie up beside the platform using the hawsers (C). (B) a crane for loading and unloading the vessel and for moving heavy items over the platform. (D) a flare, where gases uch as
methane, produced with the oil, are burnt off. There is insu fficient gas in the Fortescue field to justify collecting it. Small amounts of waxes (heavy fraction) are separated and stored in holding tanks. The oil is transferred to the shore by a submarine pipeline. (photograph courtesy of BHP Petroleum Ltd).
commercial gas fields have been found in the onshore pan of the Otway Basin north of Port Campbell and near Penola in South Australia. The Bass Basin is mainly in Tasmanian waters and limited exploration drilling has encountered a few minor amounts of oil and gas. The inland Murray Basin has good source and reservoir rocks, as well as widespread seals. Nevertheless its hydrocarbon potetllial is thought to be limited, because of insufficient depth of burial, a scarcity of structural traps and the likelihood that groundwater would have flushed out most oil and gas. However, the whole Tertiary sequence of the Murray Basin may provide a seal to underlying Palaeozoic formations that might contain commercial hydrocarbons. Some oil and gas traces have been found in underlying Permian and Devonian sedimentary rocks, which occur in narrow buried tfoughS.
Metallic minerals
Gold is so soft and malleable that one troy ounce (3 1 . 1 grams) can be stretched imo a wire 80 kilometres long, or hammered into a sheet so thin, it cover over 9 square metres. It is so rare thaI only an estimated 90 000 tonnes have been taken from the Earth during all of recorded history. More steel is poured in one hour than gold has been poured since the beginning of time. (From The Gold Information Centre. ew York, U.S.A.).
Metallic minerals and fuel minerals receive mo t auention in the news media, because they comribule so much 10 Ihe export earnings of Australia and hence 10 the prosperily of the nalion. In all the Australian Slales, excepI Vieloria, Ihere are large melalliferous mines Ihal are important in the nalional minerals economy. The only metal mining in Vicloria loday involves a few small gold mines. II was different in Ihe middle to lale nineleenth cenrury, however, when gold mining was Ihe dominanl industry in Ihe Slale.
GOLD Gold is a valuable, much wanted metal. Its most important properties are its amaclive yellow colour and ils durabililY. Overali lhe metal is very rare, forming on average only 0.000005 0 07. of the Eanh's cru t . Nevertheless, it has amacted people for Ihousands of years because il has been used : •
•
as a store of weal t h , e.g. coins, bars; for jewellery, ornaments and decoration.
Until the twentieth century, most developed countries used gold (and silver) owadays, mosl gold is hoarded by for Iheir coins, (e.g. Brilish sovereigns).
Economic Geology
Some important metal ore commodities produced in other states of Auslralia are as follows:
205
governments and individuals. A remarkable aspect of gold is that, apart from accidental losses, (e.g. shipwrecks), aU the metal ever mined in the world is still in use.
Occurrence of gold Most gold occurs as the native metal. Native gold invariably has other metals alloyed (mixed) with it - usually silver and copper, but occasionally bismuth, mercury and others. As an exception, part of the gold from Kalgoorlie in Western Australia, (the largest goldfield in Australia), is combined with tellurium in minerals such as calaverite (gold silver telluride - (AuAg)Te,). I . Primary deposits
These are rocks containing gold which precipitated from hot solutions in wa ter, called hydrothermal fluids. The solutions originated from either cooling magmas, fluids circulating during metamorphism, or from sea- or ra inwater circulating and being heated in t he Earth's crust. It is thought that large v olumes of hydrothermal f luids dissolved gold that occurs in minute amounts in many rock . The solutions penetrated the upper crust along channelways provided by zones and planes of weakness in the rocks, e.g. fault zones, bedding planes, fractures. A lthough itself insoluble in water, the gold was probably transported in a soluble form as negatively-charged complex ions conta ining gold and either chlorine or hydrogen and sulfur. Precipitation occurred as temperatures and pressures dropped near the surface. Special conditions in some rocks probably helped to deposit gold from these solutions. T hese may have included contact with carbon in carbonaceous shales (a chemical condition) and confinement within an anticlina l structure (a geological structural condition). The introduction of primary gold was often accompanied by large quantities of silica (quartz), some carbonate minerals (e.g. calcite) and sulfide minerals, (especially pyrite, arsenopyrite and stibnite). 2. Secondary deposits T hese are gold-bearing sediments, which formed after older gold-bearing rocks were weathered and eroded, and the decomposed ma terial was redeposited elsewhere. Being a very heavy metal, most gold particles cannot be carried very far by running water. Hence secondary gold is often found a long river beds close to areas of primary gold occurrences. Large pieces of gold may a lso be found in soils near primary deposits. Since the I 970s, gold production has increased substantially in Western Australia, Queensland and the Northern Territory. Many new mines have been opened up on old mining fields. In earlier days most primary gold came from relatively rich concentrations in narrow veins. Operations were costly because large numbers of men were involved in mining various small zones scattered throughout each mine. Modern exploration often deals with wide zones of low-grade ma terial left between the rich gold-bearing veins previously mined. Such large zones can be worked cheaply by open-cut mining methods. Large earth-moving machinery is used to extract big tonnages of low-grade ore each day.
Gold deposits in Victoria Most of the gold mined in Vict oria has come from the fol lowing geological environments: I. Primary vein deposits, which intersect Cambrian to Lower Devonian folded sedimentary rocks or, in some areas, igneous rocks. 2. Secondary alluvial sands and gravels of Cainozoic age. Vein dep osits These are usually called quartz reef deposits in Victoria, because quartz is the main mineral present. The term covers deposits of many different sizes and shapes. A simple fourfold classification is given below: I . Fissure reefs: these a r e the commonest type. Aqueous fluids containing gold,
sulfide minerals and quartz penetrated and crystallised along fractures and zones of broken rocks, where faulting and shearing had taken place. Most reefs are sub-parallel to the bedding of folded Ordovician to Lower Devonian sedimentary rocks and are fairly steeply-dipping. However, some reefs are gently-dipping or even horizontal. Fissure reefs mostly range in width from very narrow to three metres, but a few are up to 20 metre or more in width (Figure 5-34). There are a few goldfields, where gold occurs in quartz veins intersecting fractured Early Devonian granitic rocks, e.g. Granya, north of Tallangatta.
Chapter 5
206
f-'--.r---"'L_
\ \_�-� \ \ �\ ;< \' � '\ \, � ,I � I
MINE OPERATING IN 1990'S STAWEll (MAGDALA MINE) MALDON (UNION HILL MINE) FOSTERVILLE MINE NAGAMBIE GOLD MINE GAFFNEY'S CREEK (A 1 MINE) BOUNDARV OF GOLD-BEARING COUNTRY GOLDFIELDS GOLD AND TIN FIELDS
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• Chiltern
. Wedderbu,"
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•
,
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Bendigo
• Castlemaine
Stawell
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• Beechworth
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Ararat
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('
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• Woods Point
Ballarat
'J
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Barrnsda .
. Walhalla
Figure 5-33 (above) Goldfields of Victoria.
Silver is Ihe only metal aparl from gold 10 have been produced over a long period in Victoria. In nature, some silver is always m i xed (alloyed) wilh gold; it is therefore always a by-producl of Ihe gold mining induslry. On a few Victorian goldflelds, Ihe gold comains high silver contents (Le. 2040'70 silver), e"s" SI Arnaud" Silver was also extracted from sulflde-rich ores al Belhanga, Glen Wills and Ca silis"
Antimony metal occurs mostly as Ihe mineral Slibnile (Sb,S,) " The price of antimony tends to be volalile" Long periods of relalively low prices are interspersed with brief periods, when high prices are paid for antimony ore. This makes stibnile an unat tractive ore to search for on its own aCCOUn l . Slibnite often occurs with gold in quartz reefs. Small bUI rich stibnite lodes have been worked on occasions at the Costerfield goldfield, northeast of Healhcote. It was also mined al Rmgwood In the eastern Melbourne suburbs man y years ago.
The map shows the main areas where sold mining look place in Ihe pasl as well as the local ions of flve m n i es operating in 1 990. In north-easlern Vicloria, both gold and tin were obtained together from orne alluvial deposits.
l nits u..d ror gold Pnce LS
",
tr
I •
en
Grade "1 ho:rOlaJl OCl;"untOl.:CO; in
outCIUI>PllIg
Aquifer Ihtcknc.'SS
Rock Iypo
Aquif er 1)'pC and form
Common salinicy rnnge (milL TOS)
Range of bore
unconfined sand aquifer,
less th an IIXlO
up to
up 10 10
rorl1llllg aqUlrt:r dune dc(XNt..
.. 11\1\ iill dcro'll.1lmon·
(x:ctlr� Ihroughoul the
W�crn l'on Basin
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outcropping to "'lb· outcropping (l'\cr Inf)l:l
thin. I1lClolly less
sand. medium 10
th:lI1 611\
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k.')5 .
clay, �and and gl'll\'et
20 10
175 rn
of Ihe c:r"crn 1'Klrl of the b;1\in; cO\'C'red
tlUtlrlJ
2.,
unconfinttl �d and gr3\'e1
highly vnrinblr 500 10
SOOO
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combined a qui fer system of slu�et·like fonn. which is
300
3000
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0
10 4p 6�0 1�0
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Qj3> Mar,h Coliban H or ..ham-Munou
5 6 7 8 9
1977·86
( ML/yr)
85 000 59 000 1 14 000
2 JOO Sub-Iowl
A V\."fagl" US:lgc
; 562
15 181} 20 217
12 7 12 IX
000 300 500 00)
2 <JOO -II IXX>
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SKI 100
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2 109
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ISO 000 ) m 990
1 10 (XX) 2 1J29 'XX)
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621 100
n7 -166 25 (XX)
lin 000 153 000 I; 000 'lOO
from OI1" lr for deciding \lhere Ihey sellled. e,'ertheless 010 I groups consider�d nalural faclors \I hen Ihey decided where they \lould scllle. From earliesl limes people congregated \lhere Ihere \lere fertile soil�. permanent fresh \laler upplies. mild c1imales and landforms Ihat formed nalural defences againsl ill' aders. Later.
266
Chapter 7
geomorphological features became important in selecting trade routes, e.g. passes through mountain ranges, safe harbours and navigable rivers. The value of certain rocks for building stones was recognised. People also discovered that metals could be obtained from certain minerals and used for weapons, tools and implements. The most successful civilisations were those that had plentiful natural resources and used them wisely. However, history also records many examples of people, who thought that they were living in attractive areas, but who later suffered great losses because of unexpected natural disasters, e.g. the destruction of the ancient Roman city of Pompeii by volcanic activity. These disasters were caused by geological processes, that occurred at infrequent and unpredictable intervals. Even in modern times, there is often news that towns or villages have been demolished by catastrophes such as major eanhquakes, great river floods, volcanic eruptions or large landslides. These natural events may cause greater losses of lives than some wars, but tbey are neither caused by human actions nor can they be controlled by actions of man. An engineering geologist tries to predict when and where these events may occur and to design measures to restrict the resulting damage. There are also disasters that are triggered by human activity, e.g. some landslides. The engineering geologist advises town planners and civil engineers how to minimise the risk that such problems will occur. Since Europeans first settled in Australia, most people have chosen to live close to the eastern and south-eastern coastlines and in the south-western corner of the continent. Fortunately these regions lie outside the major earthquake-volcano belt that circles the Pacific Ocean. Consequently few Australians lose their lives from natural disasters. Nevertheless, with hindsight, it is clear that some settlements were poorly located given certain aspects of their geographic environment. For example, some towns in New South Wales and Queensland were built too close to major rivers, which rise in high rainfall areas. At intervals considerable damage to property occurs because of major flooding by these rivers. In general, Victoria is fairly free from natural disasters apart from those related to climate, such as occasional floods, bush fires and droughts. There are, however, a few geological problems that may occur; some of these are described later in this chapter. When modern towns and major constructions are planned in Victoria, measure should be taken to ensure that no future damage will be caused by geological problems. Nowadays all developments throughout Victoria are subject to planning schemes administered by local Government authorities and the State Ministry of Planning and Urban Development. Each planning scheme sets out how every area of land can be used. Each new construction must be approved by the planning authorities before it can proceed. It is desirable, especially in densely-populated urban areas, that planning schemes should take into account the geomorphology, geology and soil panerns to ensure that the best use is made of the land. These features influence the siting of building developmems, water supply works, drainage and waste disposal schemes. The geology of an area also indicates where construction rocks and minerals can be found.
Geological hazar
Those respon ible for town and coumry planning chemes should be aware of any areas that may be affected by geological hazard , even if the potential dangers are not high. These hazards can be caused either by natural forces alone or by a combination of natural forces and human actions.
EARTHQUAKES During the twentieth century, many towns in Yugo lavia, Armenia, Turkey, Iran, California, Japan, ew Zealand and other countries have experienced major earthquakes. The e catastrophes all caused great destruction and loss of life. They originated at depths up to 60 kilometres below the Earth' surface. The earthquakes w ere caused by collision along boundaries between some of the great plates that form the Earth's crus\. Fortunately no pan of Australia is close to a major plate boundary. As a result. in geologically recent times. Victoria has never experienced a catastrophic earthquake. However, there are other earthquakes, which occur at shallower depths of around 15 kilometres. These earthquakes are generally associated with movements along major faults. Faults where movements still occur from time to time are called active fault . In Victoria, earthquakes along a series of nonh-east to south-west faults across the State have caused a large number of small tremor and a few stronger tremors. These earthquakes only caused minor damage. possibly because they occurred mainly in less-populated areas. The eanhquake that struck ewcastle in December, 1989, was an example of one associated with a fault.
Engineering and Environmental Geology
267
When they are designing and building large structures in a district, engineers must consider what the chances are that an earthquake will occur some time in the future. Special attention must be given to the safety of tall office buildings and institutions, such as schools, hospitals and prisons, where large numbers of people may be present. Large reservoirs and hazardous waste storage and handling facilities also must be built in such a way that they will not be damaged if earth tremors occur.
The nature of earthquakes Earthquakes are sudden motions or tremblings in the Earth produced by vibrational waves that are transmitted through the ground. Earthquakes occur when forces within the Earth slowly build up to such an extent that they exceed the strength of the rocks. When this happens, the rocks fracture and release energy, which is felt as an earthquake. Much of this energy is radiated through the Earth as waves, which can be recorded by sensitive instruments called seismographs. The brittle fracture of the rocks may be expressed as a geological fault. Any fault is likely to be reactivated during subsequent earthquakes. For a large earthquake, the fault may show several metres of movement over many square kilometres of a fault plane. The location of an earthquake is shown on a map as a single point termed the epicentre. The epicentre is the point on the Earth's surface directly above the earthquake focus (Figure 7-2). The focus is where maximum movement along the fault occurs.
Figure 7-2 The epicentre and focus of an earthquake which resulted from sudden movement along a faull plane.
earthquake epicentre
point on surface directly above the focus ground suriace
fault
Figure 7-3 Modified Mercalli intensity scale.
I \ \
I"
Measurement of earthquakes Each earthquake is measured by its intensity and magnitude. The intensity of an earthquake is a measure of the degree of shaking at a particular location on the Earth's surface. An earthquake has different intensity values at different localities. Intensity is generally greatest at the epicentre and it decreases with increasing distance away from the epicentre. lt is usually measured by estimating the visible geological effects of an earthquake and the destruction of property. The scale most commonly used is the Modified Mercalli intensity scale (Figure 7-3). It comprises grades of intensity or destructiveness from 1 (not directly felt) to Xll (total destruction). This scale has only limited use, because assessment of the degree of destruction depends on the human point of view. It also depends on the standard of building construction and how many people experience the event. Other contributing factors include the depth of the earthquake al1d the local surface geology. For example, weak alluvium is more susceptible to shaking than solid bedrock. To overcome the uncertainties of the Modified Mercalli scale, geophysicists use a scale that is independent of human estimates of damage. It is based upon the magnitude of earthquakes recorded on a seismograph instrument. The magnitude indicates the total amount of energy released by the earthquake at its source. The most commonly used magnitude scale was introduced in 1935 by a geophysicist, Charles Richter, in the United States of America. It is now known as the Richter scale. The range is from less than I for the smallest earthquakes to the largest known at about 9. The scale is logarithmic, that is, the energy released by the earthquake increase tenfold for each higher number (Figure 74).
Chapter 7
268
Figure 74 Richter scale.
Effects of earthquakes The energy waves generated during a large earthquake are termed seismic waves. When they reach the nearby surface of the Earth, they may cause considerable damage, especially if they occur in populated areas. Severe shaking of the ground in the region around the epicenrre usually occurs. In some areas. the ground may fracture and trigger landslides. Buildings are often damaged and may even collapse, dams may fail and cause flooding, fires can start from ruptured gas lines, water supply systems may be cut and health problems may occur if sewerage pipes are broken. By conrrast minor earthquakes may only be indicated by the ranling of windows.
9 (
Earthquakes in Victoria
6
4
f t
J
