(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:<.id<s mdaUic sulfides m�aUic carbonates other salls
Rocks
Example sold (Au) C3SSiteritl!' (snOJ) gal"", (PbS) magnesitC' (MgCO.) barite (BaSO.)
Economic minerals Some rock-forming minerals have industrial uses. Quartz (as sand), clays and calcite (as limestone) are especially important. Because they are so common. these minerals usually do not sell at very high prices. On the other hand, there are many other useful minerals that are valuable because they are comparatively rare. These are called
economic minerals. Economic minerals are not normal constituents of rocks. However, in certain unusual geological environments they become concen trated in mixtures of minerals called ore bodies. Rocks are ore bodies, when it is profitable to extract them from the ground and to process them in order to obtain one or more minerals needed by industry. The chemical compositions of most common economic minerals are simpler than those of the rock-forming minerals. Economic minerals are classified intO the five chemical groups given in Figure 1-13. Some economic minerals are deposited from nuids emanating from magmas. Others are concentrated by processes of rock weat hering. Rocks form the solid outer part of the Earth's crust. For convenience most geologists use the term 'rock' to include aU materials of the crust apart from water, ice and thin layers of soil. FoUowing this definition, rocks include not only hard, massive materials, but also unconsolidated deposits such as sand dunes, river gravels, thick layers of clay and 0 on. These soft or loose materials are usually young in the geological sense. However, at some time in the future, they will aU consolidate to form rocks in the popular meaning of the word. On geological maps, soft materials such as river alluvium and swamp clays are treated as geological formations in the same way as hard rocks such as granites are. 'Rock', however, has a different meaning for engineers and engineering geologists (see Chapter 7). They limit the term 'rock' to consolidated hard material, which will support heavy loads and can only be extracted from the ground by blasting with e.xplosives. They call all soft material 'soil'. On close examination, perhaps helped by a magnifying glass or a microscope, it is apparent that rocks are made up of a large number of mineral grains. Most rocks are hard because: •
•
the grains are interlocking; or because the grains are bound together by some cementing substance, such as iron oxide or silica.
In a few rocks, all the mineral grains belong to one species, e.g. limestone is largely calcite. Most rocks though contain two or more types of minerals. There are several hundred kinds of rocks. but only a few are common on all the continents. The main components of rocks are the common rock-forming minerals discussed previously - quartz, feldspars, micas, amphiboles, etc. In Victoria, there are less than ten common rocks. In some country regions, especially the Wimmera and MalIce, it is the unconsolidated materials - deep deposits of sand and clay - that predominate. Elsewhere the most widespread rocks are sandstone, shale, granite and basalt ('bluestone'). Most people will be familiar with these rock names and many would recognise one or more of them in the ground. There are also many other less common rock types in Victoria. As examples, limestone, coal and rocks with less well-known names such as siltstone, hornfels, rhyolite, dacite, gneiss, schist and conglomerate are common in some areas. Large quantities of dacile, hornfels and lime lone arc quarried in some outer caslern uburbs of Melbourne 1 0 provide crushed rock screen ings. Rocks can be classi fied in various ways to help people 10 under tand the differences bel ween Ihem. First they are divided into Ihree major groups: • • •
igneous; sedimentary; metamorphic.
This division is based on fundamental differences in the ways in which rocks are formed.
t2
C hapter
t
Figure 1-14 Granite from Wilsons Promontory.
This s i a plutonic rock, which solidified deep in the Earth's cruSt about 360 mill ion years ago. It is made up of visible crystals of clear quartz, white feldspar and the dark brown mica mineral, biotite.
Figure I-IS Basalt from a lava flow exposed in the excavation made for the Victorian Arts Centre on the south side of the Yarra Rh'er, near Princes Bridge. The site is near the end of a flow o f lava, that erupted from a volcano at Hayes Hill, 30 kilometres nonh of Melbourne, about 800 ()()() years ago. The basalt is a dark grey rock. The lava solidi fied quickly and individual crystals are hard to see with the naked eye The black patches are holes caused by steam escaping from the lava.
Figure 1-16 Magmatic differentiation. As magma moves slowly upwards through the Earth ' s crust. the mineral with the highest melt ing point stans to crystallise first. As these crystals settle, the remaining magma has a di fferent composition to the original magma The gradual separation into layers o f different composition is called magmatic di fferentiation.
WA M
AN N
IGNEOUS ROCKS Igneous rocks form by the cooling and solidification (i.e. crystallisation) of magma. As discussed in a previous section, there are two major types - granitic magma and basaltic magma It might be supposed, therefore, that there would be only two igneous rock types - granite and basalt (Figures 1-14, 1-15). While these two are very common, particularly in Victoria, there are also many other igneous rocks, although most are rare. There are three main reasons for this variety of igneous rocks:
1 . Granitic magma forms from the melting of previously solid parts of the Earth's crust. The composition of the crust varies from place to place, so magmas of different composition can form when the crust melts. 2. As magma rises from the upper mantle and lower crust, it becomes cooler. Eventually minerals with higher mel ring points begin to crystallise and sertle out. This process is called magmatic differentiatian (Figure 1-16). The composition of a magma is therefore continually Changing. The type of rock finally produced depends on the composition of the magma when it solidifies.
3. The composition of a rising magma can also change as overlying crust rocks melt and mix with it. This process is called
assimilation.
Granitic rocks are rich in silicon. • •
•
They contain much aluminium, smaller quantities of potassium, sodium, calcium and \vater as well as minor magnesium and iron. The main minerals present are therefore quartz, feldspars and micas. They crystallise from viscous magmas, which erupt violently if they reach the surface.
Basaltic rocks are poor in silicon. • •
•
They also contain much aluminium, far more iron, magnesium and calcium than are in granitic rocks, but less potassium, sodium and water. The common minerals are calcium-rich feldspars (plagioclase), biotite mica and ferromagnesian minerals (i.e. olivine and members of the amphibole and pyroxene groups). They crystallise from fluid magmas, which often erupt as relatively quiet flows of lava.
13
Basic Concepls in Geology
Classification of igneous rocks A simple classification of igneous rocks is given in Figure 1-17. It includes localities where some types can be found in Victoria.
Figure 1-17 Classification of igneous rocks.
Silica '/.
A C
>
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 <
0.004
silt
clay
Features
Name
ronglom�'ale
rounded rock fragments oflen cemented together
breccia
angular rock. fragments
SDJJdsron�
mainly quartz.
,rtY""'Qcke
M
F I
SEDIMENTARY ROCK
quanz. feldspar. rock fragmentS
siftslOlU' shale· nllldSlOrle cla),sIOIlf! marl
line quanz
}
mixtures of sill and clay
abundant dilY nkllnly day and calcltc
-The lerm shale Is used for a fine-grained rock Ibal is laminaud (i.e. very thinly-bedded) andfissilt (I.e. " spillS
along bedding planes). A mudslone lends 10 break iDlo irregular fragmenls.
Note: The ranges of diameters used for the various particle sizes by Geolo gists differ from those used by Soils Scientists (see earlier).
Figure 1-25 Conglomerate from St Helena. norlheasl of Ihe Melbourne suburb of Greensborough. II consisls of rounded pebbles of white quartz and grey sandslOne in a cement of finely crystal line quartz. clay and i ro n oxides. I I occurs i n rocks called t h e Brighton Group on geological maps o f (he Melbou rne area. They were laid down in river beds over two million years ago. These deposits are widespread in the south -eastern suburbs of Melbourne. but are moslly only hill cappings in (he nonh and nonh-easlern di st rict s .
Figure 1-26 Shale from the Bendigo district. Shale is used for fine-grained sedimentary rocks. that are fissile, i .e . they split along the bedding plane. Mudslone is used for fine grained rocks that splil into irregular blocks when hil by a hammer. Bo(h contain silt and clay.
Basic Concepls in Geology
17
Organic and chemical rocks These rocks are formed b y biological activity and chemical precipitation. The most abundant organic rock is limestone, which consists largely of calcite crystals. There are many kinds of limestone. Most are made up of broken shells and fragments of skeletons of various orgartisms, especially lhose lhat lived in the sea. The fragments may have accumulated where lhey grew (as in a coral reef) or they may have been carried away by ocean currents and deposited in layers elsewhere. There are prominent beds of porous limestone, full of shell pieces, in the coastal cliffs between Torquay and Anglesea and at Pon Campbell. Hard crystalline limestone occurs at Lilydale and in the Buchan district.
Figure 1-21 Sandslone escarpmenlS in the Wonderland Range in The Grampians. The Grampians offer some of Ihe most picLuresque sceneI)' in Vicloria and Ihey are easily accessible for 10urislS. There are many rock faces formed by beds of sandslone, as seen in Ihis pholograph. The shallow sandy soils support a varielY of low nalive shrubs, which provide a colourful noral display in lhe Spring. (photograph by G.W. Quick).
Figure \-28 Limestone rrom Buchan, in East Gippsland. II is made up of Ihe remains of corals (seen in cross-seclion) packed logelher in a matrix of fine-grai ned calcile and ot her mineral grains. These oorals evidem ly lived in 'coral gardens' wilh large numbers of conical corals silling upright. This rock is dark grey in colour and was formed in a shallow sea, some 380 million years ago. (Photograph by lA. Webb).
Coal is another imponant organic sedimentary rock. II formed from rolling vegetable mal ler, such as grasses, leaves and wood fragments, which accumulated in poorly drained areas. (See Chapter 5 for further details of coal formation). To some extent chemical rocks formed in a similar way to detrital rocks. When older rocks decomposed and were worn away by running water, some chemical substances di solved in lhe water. These substances were carried away in solution to eas or lakes. In certain circumstances, e.g. after evaporation in lakes and lagoons, the soluble substances crystalli ed as layers of salt. Rock salt (sodium chloride) and gypsum (CaSO 2H,O) form beds in some inland salt lakes. Some limestones are also chemical precipitates. •.
METAMORPHIC ROCKS In the past, orne sedimentary and igneou rocks were heated and/or squeezed by forces deep in the Earth's crust. They did not reach melting lemperatures so magma was not formed. The forces did, however, cause change in the texture and mineral compositions of Ihe rocks. This process is called metamorphism and the results are metamorphic rocks. Water contained in the minerals and pore spaces of rocks played an important part in the chemical reactions that look place during metamorphism. The overall chemical composition of a mass of rock does nOl change much during metamorphism apart from the gains or losses of \Vater. Metamorphism cannOt be seen taking place on the Earth's surface. Nevertheless metamorphic changes are always happening very slowly deep in lhe upper continental crust. The changes take place in the solid state. There are two broad types of metamorphism: •
•
contact (or thermal) metamorphism; regional metamorphism.
18
Chapter 1
Contact metamorphic rocks These were formed when magma intruded cooler sedimentary rocks. As temperarures rose, chemical reactions occurred between water in the pores of the sedimentary rocks and the mineral grains. The result was usually a denser, harder rock than the original. The zone adjacent to an intrusive rock, where contact metamorphism occu rred, is called a metamorphic aureo le. This zone may vary in width from a few tens of centimetres to many kilometres. Some contact metamorphic rocks are: a dense, hard rock formed by the recrYSlallisation of quartz grains in a sandstone.
Quartvte
-
- usually a coarse-grained rock formed by the recrystallisation of calcite crystals in a limestone.
Marble
Hornfels
-
a dense, dark grey, fine-grained rock formed from mudstones and shales.
Contact metamorphic rocks can be found in Victoria almost everywhere gran.itic rocks have intruded sedimentary rocks. Aureoles of hard rocks such as hornfels often stand out as ridges, because they resist erosion beller than granite and sedimentary rocks. Examples close to Melbourne occur at Lysterfield and near Broadmeadows.
Regional metamo rphic rocks Figure 1-29 (left) Slate from near Tanangalla in north-eastern Victoria. This is a dark grey, very fine grained rock with nat cleavage planes. The rock can be easily split along these plane . The original sedimentary rock was a mudstone, that was regionany metamorphosed to slate, probably about 400 million years ago.
These rocks formed in parts of the Earth's crust, where moderate to high pressures and temperatures existed over million of years. A wide range of new minerals may be presen t. A feature called foliation is usually developed in regional metamorphic rocks. Foliation is a roughly parallel structure along which the rock tends to split into flakes or thin plates (Figure 1-29). It usually involves either: • a parallel alignment of flat minerals (e.g. micas, Chlorite) in fine-grained rocks; or • a separation of different minerals into parallel bands in coarse-grained rocks. Foliation should not be confused with bedding in sedimentary rocks. Foliation is a response to the squeezing pressures during metamorphism; it can develop at any angle to the original bedding.
Slaty cleavage
Figure 1-30 (right) I.t) cleavage cen in folded. metamorphosed sedimentary rocks exposed in a road CUlling. A sequence of mud tones and silt tones was fo lded by squeezing. The tre ses changed the rocks to slates. They also caused naky minerals. rich i n mica, to grow into a parallel vertical alignment. This alignment produced paraJlel planes and weaknesses called cleavage. The rock will split along the cleavage planes and not along the bedding planes.
Some regional metamorphic rock
are:
a very fine-grained, dark grey rock usually formed from shale. It can be split into thin slab along parallel plane - a feature called slaty cleavage (Figures 1-29, 1-30).
Slate -
Schist - a rock
that splits along wavy planes. Platy cry tal can often be seen with the naked eye. Schists are formed under more inten e metamorphism than that producing lates. They can be produced from many types of igneous and sedimentary rocks. Schists are named after t he rna t prominent mineral pre ent, e.g. //Iica schist, amphibole schist.
a coar e-grained rock made up of parallel band of light and dark minerals. The bands may be highly cantoned. Quanz and feldspar commonly form the light band , and ferromagnesian minerals and biotite the dark bands.
Gneiss
-
Both igneous and edimentary rocks can be changed by regional metamorphism . Granite gneis and green tone (formed from basaltic rocks) are examples of former igneou rocks that are found in Victoria. Some unu ual mineral , which only form at high temperatures. may also be found in metamorphic rocks, e.g. garnet , sillimanite. cordierite.
Basic Concepts in Geology
Figure \-3\ The metamorphism of mudstone or shale.
Mudstone or shale can change to slate, schist or gneiss, depending on the intensity of temperature and pressure.
MUDSTONE
19
SHALE
layers or laminations clay minerals change to
•
•
•
lollalion cleavage mica crystals change � intensity metamorphism _
-
banding feldspar crystals
10�
01
Many sedimentary rocks in Victoria have been subjected to mild regional metamorphism, because they were buried under considerable thicknesses of other rocks and compacted. Schists and slates are common in the Pyrenees Range and in hills west of Ararat and Stawell. Gneiss is widespread in the mountains between Omeo and the Hume Reservoir.
Fossils
The word /ossil is applied to any evidence in sediments and sedimentary rocks that living organisms existed in the past. This evidence can take many forms. It might be a bone or shell of an extinct animal or it may be some indirect indication of past life, such as a burrow made by a worm or an animal footprint left in the sand. The study of the history of life on Earth, as recorded by fossils, is called palaeonlology. This science investigates the evolution of various animal and plant families since life first appeared on this planet. By human observations, it is known that many animals and plant species have become extinct over the past few hundred years. Unfortunately most of these extinctions were caused by human activity. However, similar processes were taking place long before humans appeared on this planet. Over a very long period, various life forms have competed for particular environments with dominant species even tually excluding weaker forms. The fossil record shows that different species have been the dominant forms at different times during the history of the Earth. Most living things consist of fluids, soft parts (e.g. skin, tissues, flesh) and hard pans (e.g. bones, shells, seeds). Hard pans have the best chance of surviving over a long period and of becoming fossils. Clearly, shelly fossils are more likely to be preserved than jellyfish. Whether an organism eventually becomes a fossil depends on the environment in which it lived and died. Evidence of creatures that lived in the sea is more likely to be found than thaI of land-dwelling plants and animals. This is because on land animal and plant remains are destroyed fairly quickly by the action of atmospheric agencies (e.g. oxygen, water) or by bacteria. There are also many carrion-eaters (scavengers), that is, animals that eat dead flesh. For a land dweller to have been fossilised, it was usually necessary that some catastrophe lOok place. For example, a creature may have been buried by a landslide, trapped in a bog, drowned in a flood and covered with silt, or covered under material thrown out by a volcano.
Figure \-32 Stages in the formation of fossils. (a) a primitive fish is swimming in the sea in a past 1eological period.
(c) The thickness of sediments is increasing and the fish s i now completely bur ied. The deepest sediments are slowly being convened (0 rocks. The soft pans of the fish have decomposed or were removed earlier by a scavenger. As the rocks formed, the hard parts of the fish were probably replaced by minerals or they were dissolved. In any case they left an im pression (cast) on the sediment. A nautilus is swimming in the sea above. (d) casts of the nautilus hell and the fish skeleton are preserved in the rocks, which have been uplifted above sea-level. Erosion has exposed pan of the fossil fish.
.
/
(b) the fish has died and is rapidly being covered by sediments.
:.:.. :.',.::.:,
,. '.:,0; :.
.
( a ) . '.
H....,... ... -. ., . ..� ,. � .
. ( e) '
. (b) :
. ; _:._.
:S?_��:��:':-:':::":';
I':--'·= '� =':"'.
.
.
! -S-' ": . . .�: --
.'
..
'
•
• •
• •
• �
.
.
.. . ... .
I
' .
. '
.. "
.
. . '
20
Chapter 1
The main types of fossils are: 1.
Those in which both hard and soft parts are preserved. These are very rare. However, some complete remains of animals have been preserved in peat bogs and frozen ground in Arctic regions, e.g. prehistoric mammoths found in Siberia. Insects have also been found completely encased i n amber, a resi n formed on extinct pine trees.
2.
Those with only the hard parts preserved. The shells in which marine organisms with soft bodies lived and the teeth and bones forming the skeletons of more advanced creatures are common forms o f fossils. Most shells are made of the minerals calcite or aragonite (both calcium carbonate). Calcite in panicular is stable over very long periods. Many limestone deposits are rich in fossils o f this type. For example, in western Victoria, there are extensive limestone formations, rich in shells, corals and sea mosses (called bryozoa). Sharks' teeth are found in sandy clays in the ctiffs at Beaumaris on Pon Phillip Bay.
3.
Those in which the hard parts have been replaced. I n some fo sils, the hard parts o f organisms were replaced or infilled by chemical substances, such as silica, calcium carbonate, iron oxide or iron sulfide. These chemicals were derived from water circulating in the ground. The replacement took place gradually, molecule by molecule, and so even minute details of original organic tissues were preserved. The remains of vegetation found in brown coal are an example of this kind of fossil - the original wood changed to other carbon compounds. Where the brassy yel low mineral, pyrite, replaced plants or shells, it usually decomposed rapidly when the rocks were later exposed to the air by erosion. The pyrite (iron sulfide) oxidised to iron oxides and hydroxide, so the fossils now occur as rusty red outlines. There are also many occurrences in Victoria of petrified wood, i.e. wood replaced by silica (Figure 1-33). Particularly striking forms of replacement fossils have been found on occasions in opal mines in inland Australia. Colourful shells and skeletons of fish have been found where animal skeletons were replaced by opaline ilica.
figure \-33 Petrified wood, a fossil exposed in S culling on the Princes High\\llY near Maramingo Creek, eight kilometres nonh-east of Genoa in Ea t Gippsland.
Along the cUlling there are unconsolidated quartz gravels. grits and sands. that were deposited by a river during the Tertiary period. The fo sil Iree trunk protrudes from the edimenlS and appears to extend well into the bank. It formed because iliea molecule replaced the original organic compounds in the tree. The silica was originally dissolved in water percolating through the ground. The t ree now consists largely of a variety of the mineral quanz. .J. Ro engren). (Photograph by
4.
Those in which rhe sofr or hard parrs have lefr an impression on sediments which larer become rocks_ I mpre sions of very soft creatures such as jellyfish have been found i n ancient rocks in orne pans of the world, e.g. in sandstones in the Flinders Ranges, South Australia. Fo sil foOtprints of extinct animal are another feature. The animals evidently walked across clayey or sandy ground, and their prints were later covered by deposits of sediment . For example, footprints of an unknown veneb ra te have b een found in andstone on the floor of the G en oa River in East Gippsland (Figure 1 -34).
5 . Those ill which hard parrs are preserved as casrs or moulds.
Fossil shells are often found in this form. I f a marine bivalve. (i.e. an animal enelo ed in a shell of tWO interlocking valves), died on the ea-floor and was buried by edimems, the form of the shell may have been preserved in any one of th ree ways: (a) Percolating acidic solutions may have completely di olved the calcareous shell within the sedimen t, leaving an impression in a cavity known as an
exremal mOllld_
Basic Concepts in Geology
21
(b) If the shell filled with sediment and later was dissolved, then both external and internal moulds may be preserved, i.e. outer surface and inner surface impressions of the shell (Figure 1 -35c). (c) If the cavity left by the shell (i.e. the mOUld) was later filled by silica or calcium carbonate deposited from percolating water, a cast of the original fossil may have been preserved (Figure 1 -35d). Figure \-34 Fossil tracks left by an amphibious verlebrate on sandslone on the no or of the Genoa River, East Gippsland, near the border with New South Wales. These Lracks were discovered in 1972 and are possibly the oldest footprints left by a four-footed animal found anywhere in the world. The animal walked across wet sand beside a river about 360 million years ago. The direction o f travel was from right to left. (Photograph courtesy of Zoology Department, Monash University).
6. Indirect evidence of once-living creatures supplied by their bu"oHls, trails and
droppings.
Figure \-35 Formation of fossil casts and moulds. Casts and moulds are impressions left in sedimentary rocks by former living creatures. In (he diagram, a b ivalve has died and been buried on the sea-noor. I ts shells can leave casts o r moulds i n various ways: (a) After the creature died, its soft parts decayed, leaving the shell. ( b ) The shell was buried and filled by sediment on the sea-floor.
These fossils are surprisingly common in some rocks. Ground which has been disturbed by burrowing animals, such as worms, is said to be bioturbated. Modern examples can be seen along the shoreline at the coast or along tidal river nats (Figure 1-36). An example of burrows preserved in ancient rocks formed in shallow seas is provided by abundant vertical burrowings in sandstones of the Grampians Ranges, e.g. along the track to the summit of Mount William.
PALAEONTOLOGY To study palaeontology i t is necessary to have a detailed knowledge of biology perhaps specialising in either zoology (animals) or bOlany (plants). In both sciences, classifications have been developed that divide living organisms into various groups. The naming of many fossils (and living organisms) is often difficult for beginners, because changes are made to the classification tables from time to time. Only a mall proportion of the fossils discovered have been named and described. Later in Chapter 4 of this book, various Victorian fossils are illustrated and described as the history of the development of various geological formations in this State is discussed. The main groups used in classifying plants and animals - living and fossil - are given in Figure 1 -37.
(a)
(c) The sediments around the shell were compressed by overlying material and eventually they have been converted to a sedimentary rock. The shell dissolved away. BOlh internal and external moulds of the shell surfaces were left in the sedimentary rock. ( d ) Alternatively, the cavity left by the shell in the sedimentary rock was later filled by silica or calcium carbonate from percolating groundwaters, so that a cast of the shell was produced.
CaSI 01 shell
(b)
(d)
�' ""�
External mould
(0)
t
t
0
.l::Q:)
Original shell
Sediment
�
Secondary ca/cire
22
Chapter 1
Figure 1-36 Modern bioturbation.
The faim line seen below the centimetre scale is the track left by a gastropod (sea-snail) Bembicium nanum - as it moved over wet sand from the sea to the right. Such tracks can be preserved in rocks, that is after sands have hardened to sandstones. The depression in the sand around the pebble may also be preserved. (photograph by N .W. Schleiger).
�
MO ERA
t=
PROTtSTA
rUNGI
Bacteria Cyanobacteria (stromatolites) Brown algae Diatoms Green algae Red algae Protozoans (foraminifera. radiolarians) Oinonagellatcs
Pl..ANTAE Mosses, liverworts Lycopods Horse-tails Ferns Seed [ern, Pcoloxylalcs Gymnosperms Conifers
Figure 1-37 A llIble of the main fo rms of life found as fossils.
Cycads
ANIMALlA
}
Ginkgos Angiosperms (flowering plants, grasses)
�
Porifera (spo:;ges. 5lrOmaloporoids) Coelenu:rates
fJt�!�
s
rug� corals tabulate corals hexacorals
Bryozoans (sta mosses) Brachiopods
L C
articulatcs inarticulatC5
Annelids (segmented worms) Molluscs
E
t=
as
g tropods (snails) bivalves (clams. scallops. ctc.) scaphopods (tusk shells) cephalopods (ammonoids. nautiloids. squids)
Arthropods �ruStaccans - trilobites. ostracods, crab.. Insects horseshoe crab� Echinoderms
�
����:�:
(sea lilics) slar fish brillle Slars ec:hinoids (sea-urchins)
Conodonu Hcmichordmcs
L
graplolites
L
vertebrates
Chordate!'>
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
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FORM ExrtNCT FORM CONTINUES
Finally over the past few decades, a technique known as isotope dating or radiometric dating has been used to calculate the numerical ages of rocks with confidence. There are certain unstable radioactive elements (called parent isotopes)
Figure 1-41 (left) Decay of a radioacthe isotope at an exponential rate.
that gradually lose particles from their atomic nuclei and change into atoms of other elements (called daughter isotopes). By experiment it is known that the rate of decomposition or decay from any parent to its daughter i otope is constant. Each parent-daughter pair has a distinctive rate of decay, which is measured by the time taken for half the radioactive isotope to decay. This is known as the radioactive halfliJe of the pair (Figure 14 1 ).
After each time interval, half the isotope decays. It never completely ds i appears. Figure 142 (right) A linear rate of decay. Half the candle burns in a certain time and the remaining half disappears after the same time. Sand passing Ihrough an hour glass also has a uniform straight line depletion.
All AMOUNT OF ORIGINAL ISOTOPE
1 Amounl ol candle remaining
lIz
REMAINING
''''',." ... Number of Half Lives
5--
····,,·,,·······
1
/ /
2N 4h' L---�� �O�----� 1 hall lile
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halllrv&S
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Basic Concepls in Geology
27
As an example, the isotope uranium-235 disintegrates 10 form lead-207. This decay has a half-life of 713 million years. This means that if a vein containing uranium ore was intruded 713 million years ago, the ore mineral today would contain equal amounts of uranium-235 and lead-207. The decay occurs at an exponential rate. In the next 7 1 3 million years, half the remaining uranium-235 will decay and so on. The decomposition never quite reaches the point where all the uranium disappears (Figure 1-41). This contrasts with the burning of a candle, which follows a linear decay. That is to say, if half the candle burns in two hours, the remaining half burns in the next two hours (Figure 1-42).
Figure 1-43 Radioactive isotopes used in the radiomelric dating of igneous rocks and hence in calculaling nu merical geological time. The uranium-lead pairs are often studied in zircon crystals and the other two in muscovite or bi otite crystals.
Half-life of parent
Effective daling rangt:
Daughter
(millions of )'cars)
(millions of years)
ltad-206 Lead-207 Argon-40 Strontium-87
4500 710 1300 47000
Isotopes Parent Uranium-23S Uranium-235 Potassium-40 Rubidium-87
10 10 0.1 10
· · · ·
4600 4600 4600 4600
Radioactive dating is applied chiefly to unweathered igneous rocks. It is assumed that when they were first intruded into the upper crusl, they contained no daughter isotopes. Hence if the ratio of parent to daughter isotopes is m�asured loday, a calculation can be made of the time that has elapsed since the parent firsl arrived in the crust. By combining the ages of igneous events calculated by isotope dating with the information about relative geological ages previously known, a geological time scale with numerical ages before the present time has been developed (Figure 1 -40). Present evidence suggests that the age of the Earth is about 4.5 - 4.6 billion years. This is the time that has elapsed since the Earth condensed from a large rotating ga� cloud called the solar nebula. The laboratory techniques used for measuring the concentrations of radioactive isotopes in minerals are complex. It is usually only possible 10 achieve dates that are accurate within a few per cent. There are also still disagreements between geologists about the precise positions of many boundaries shown in the geological time scale. Consequently there are usually differences in detail to be found in the time scales given in di fferent books. For example, the start of the Cambrian is variously given as 600 10 530 million years ago. In Victoria, 560 million years is considered to be close to the date. Undoubtedly, as experimental techniques and geological knowledge improve in the future, more precise dates will be allocated 10 past geological events and the duration of each period and epoch.
The term tectonics comes frorn the Greek word, tektonikes, a carpenter. It embraces all the building processes in the Earth's crust that affect the struclure of the rocks and the shapes of rock masses.
Many natural physical and chemical actions control the geological processes that shape the face of the Earth. Some of the processes already discussed briefly include the rise of magma, the changes to metamorphic rocks, the decomposition and erosion of rocks and so on. Other processes will be introduced later in this chapter. However, there is one major geological process known as plate tectonics which must be considered at this point. Full details of this process have only become understood in recent times, but within its framework, nearly all other geological processes can be explained. Plate teclOnics involves the continuous, very slow, movement of slabs of the Earth's crust called plates. These plates slide over the top of the upper mantle at the rate of centimetres per year. The concept of plate tectonics does not only explain how and why there are continuous, although almost imperceptible, movements of the continents. It also explains why, associated with these slow movements, there are periodic sudden violent events such as volcanic eruptions and earthquakes. It also shows how layers of sediments, lhat were deposited on past ocean floors, were later lifted above sea-level, crumpled, crushed and built into mountain ranges. There are many aspects of plate tectonics, which can only be satisfactorily explained at the Tertiary level of geology studies. Indeed the theories are continuously being reviewed and modified as new information comes 10 light. In simple terms, however, the main points about the theory of plate tectonics are as follow:
I . The Earth's crust is made up of about fifteen plates, seven of which are very large. All are moving at di fferent speeds and in different directions (Figure 1-44). At their surface the plates may be represented by either continents (continental crusl)
C hapter 1
28
EURA SIAN PLATE
San
Japan Trench Mananas
'" r en"""'r:o -I � a
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.J
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PA CIFIC
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.
-
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-
ARABIAN P ATE
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PLATE
(()
AFRICAN PLATE
Kermadec Tonga Trench
PLATE
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PLATE
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\ PHIL IPPINE �TE ..,�
1, y. Ctf.
Andrea Faull
--
(7
-
Rise
( v
- --l,r -
SubduClion zone
Zones of deep-focus earthquakes
Mid-ocean ridge
Figure 1-44 (aboye) The world's major tectonic plates, The plates move away (diverge) from t he mid-ocean ridges, where new oceanic crust is continuaJly being formed. The plates move towards (converge on) the subduction zones. Earthquake belts occur along many of the plate boundaries. The Pacific, Nazca and Cocos plates are mainly oceanic crust. The other plates comain bOlh continental and oceanic crust. Along its northern boundary, the Indian·Australian Plate is converging on other plates and the collision is forc ing up great mountain ranges, e.g. HimaJayas, Owen Stanley Range (papua New Guinea).
Figure 1-45 Oceanic and continental crust. The average composition of the upper part of the Earth's crust i s similar 1 0 that o f the rock granodiorite. At deeper levels, the rocks are under greater pressure. They are probably more layered with an average composition like t hat of basah. The oceanic crust has up to one kiiomci re of sediment overlying basal t . It is continuously moving away from t he mid-ocean ridge.
Olfection of plate movement
or by the floors of the oceans (Figure 1-45).
(oceanic crust) or by combinations of these two
2. The oceanic crust is made up of basaltic rocks. The continental crust consists of less dense granilic and metamorphic rocks. Although sedimentary rocks form well over half the Outcrops around the world, the total thickness of this skin is very small compared with the total thickness of the crust. Hence only the granitic and basaltic pans are discussed here.
3 . There are some long zones where two oceanic plates are diverging, that is, moving away from each other. These movements do not leave a great widening chasm in the crust under the ocean however. Instead new oceanic crust is being created continuously by the eruption of lava along the so-called mid-ocean ridges (Figure 1 -45). This eruption occurs mainly below the sea. The new crust moves outwards from the ridges by a process called sea floor spreading. Figure 1 -44 shows for example where the Indian-Au tralian Plate is plilling away from the Antarctic Plate along the South-east Indian Rise beneath the Southern Ocean. 4. Where two blocks of continental crust collide, the materials of the upper crust are pushed up to form mountain ranges. For example, the collision between the north-moving Indian-Australian Plate and the Eurasian Plate led to the formation of a long mountain belt, that include the Himalayas. The building up of CONTINENTAL
CONTINENTAL PLATFORM
�_-�_
PLATFORM
MID-OCFAN
AIOOE
mountain ranges does no t continue indefinitely of course. High mountain
are constantly being worn down by moving ice (glaciers) on their upper slopes and high rain fall and torrential river on their lower slope .
5 . A plate o f oceanic crust can move towards and pass underneath a continental crust plate. Thi occur at a subduction zone. An example occurs where the Nazca Plate meets the western edge of the South American Plate. Pushing-up
Bas;c Concepts in Geology
Figure 146 The geological processes occurring along a convergent plate boundary.
The oceanic crust on the left is converging (colliding) with the continental plate on the right along a subduction zone. Sediments on the oceanic and continental plates are crumpled into a folded mountain belt along the margin of the continen t . Older sedimentary rocks o n the cOnlin ental plate, deep in the crust, are undergoing metamorphic changes where the temperatures and pressures are high. The descending oceanic crust is partly melted and reacting with the lower crust to form new granilic magmas. These magmas rise through the folded sedimentary and metamorphic rocks of the continent to form igneous intrusions.
29
Folding and mOUnlain building Metamorphism and crustal deformalion
Sedim " en �: ts�
Oceanic C I
�===���� 'Lower c.'us!}
Granitic intrusions Con,!;n.n!,!1 plate
lowor crust
effects along this boun dary have cau ed the uplift of the Andes Mountains (Figure 1 -46). Both 4. and 5. are examples of convergent plate boundarie . 6. Some plates do not diverge or converge, but slip past one anolher, e.g. San Andreas fault system of western North America. 7. Nearly all the world's active volcanoes are situated close 10 the main subduction zones or on spreading ridges. The Hawaiian volcanoes are a notable exception. They are far from the boundaries of the Pacific Plate on which they occur.
8 . Most of the world's largest and most destructive earthquakes occur on subduction zones or where plates are coUiding or sliding past each other. In recem times severe earthquake in Iran, Yugoslavia, Turkey and Armenia all occurred along the southern boundary of the Eurasian Plate. 9 . The pattern o f crustal plates, that is recognised today, has not always existed. The Antarctic and Indian-Australian plates have only been diverging since Early Cretaceous times, some 1 30 million years ago. Prior to that, Australia and Antarctica were joined together as one continent. Earlier again aU the Southern Hemisphere continents formed part of a supercontinent caUed Gondwana. During the first half of the Palaeozoic times, Australia was near a converging plate margin to the east. There were then various periods of mountain building. 1 0 . Over the complete geological history of the Earth, there were other plate systems, within which continents were being formed, joined, broken apart and moved in different directions. Like all the other continents, Australia is a jig-saw puzzle, consisting of irregular blocks of continental crUSt added at different times. Working OUt the origin of the various pieces and when they were filled together is a task in which some geologists specialise. I f the Earth's crust was not split into various moving plates, long ago it probably would have consolidated into a uni form mass of rock covered by a universal ocean. But, becau e there are various chemical and physical forces acting in the crust and mantle, panicularly along the boundaries of the moving crustal plates, the Earth has an ever-changing face. At any time, parts of it are being built up while other pam are being destroyed. The most active regions keep changing in time and place. Today Australia is geologically a fairly stable continent with a subdued topography. This is explained because it is located weU inside the Indian-Australian Plate. But not far away along the northern coast of Papua New Guinea and extending westward acros Indonesia, some of the most powerful geological forces are manifest from time to time. There, where the Pacific Plate is converging on the Indian Australian Plate, there have been recurring violent volcanic eruptions. Nearby in Papua New Guinea great mountain ranges have been thrown up in geologicaUy recent times by the tectonic forces. During the Early Palaeozoic era Victoria probably was close 10 a converging plate boundary. Then there were many superimposed episodes of mountain building, intrusions of granite and regional metamorphism as well as volcanic activity and probably earthquakes. It is difficult to unravel this complex geological history and explain it in simple plate teclOnic terms. In the next sections, some of the main geOlogical processes, which have been referred to brieny earlier, will be discussed in more detail. Most of these processes are driven by forces related 10 plate tectonics.
30
Chapter 1
M ag mas and ig neous activity
TYPES OF MAGMA Earlier in this chapter, reference was made to the existence of granitic and basaltic magmas belQw the Earth's surface and to. the parts they played in fQrming igneous rQcks. The principles Qf plate tectonics can be used to. explain why there are different types Qf magma and hQW they mQve upwards through the Earth's crust. There are two. main SQurces Qf magmas: •
•
SQme CQme straight from the Earth's upper mantle - these are primary magmas; SQme fQrm by the melting Q f other rocks - these are secondary magmas.
Magmas Qriginate at depths between 2 and 200 kilometres belQw the Earth's surface (Figure 1-47). Figure 147 The forma1ion of magmas in the continental crust.
Basaltic magma rises from the upper mantle and is erupted through volcanoes. Granitic magma is formed by the melting of rocks in the lower crust and it intrudes n i to the upper crust. Dyke swarms may include both basaltic and granitic magma types. The boundary between the lower crust and the upper mantle is called the Mohorovicic Discontinuity or Moho. It is not a sharp junction, however, and is probably much deeper beneath continents than below the ocean. The illustration shows fealUres which have occurred during the geological history of Victoria
Folded sedimentary rocks, metamorphIC locks .,
BasaltIC YOIcanoes With lava flows
Me/ling 01 lower porllOll 01 conhnenlal crust by basalltC magma trom manlle
Upwellng basalllC magma 'rom upper manlle
I . Primary magmas - these magmas Qriginate in the Earth's partly molten upper mantle. They are usually basaltic in composition and fairly mobile. They can therefQre rise rapidly, eventually reaching the surface as vQlcanic material at one of the fQllowing outlets: • at a diverging plate bQundary provided by a mid-ocean ridge, e.g. the bQundary between the African and South American plates. The island of Iceland is a rare case Qf an oceanic ridge rising abQve sea-level at the nQrth-eastern end Qf this ridge. •
2.
where a deep fracture, cutting through a cominent, meets the surface.
- these magmas fQrm when masses of rQck are heated by Qne Q f the following ways: • by Qceanic crust being fQrced deeper into the Earth during subduction. • by IQwer cQntinental crustal rocks being intruded and melted by primary magma.
Secondary magmas
When basaltic oceanic crust, cQvered by some sediments previously depQsited Qn the Qcean floor, mQves dQwn a subduction zone towards the mantle, it is heated. Some Qf the material melts to give a new magma with a higher silica CQntent than the original basaltic magma The first magma is often formed offshore from the edge of a cQntinent. It cQmmQnly has the compQsitiQn Qf an andesite and it rises to. give a chain of andesitic vQlcanQes called an is/and arc. The Japanese islands were built in this way. Where the subduction zone continues under the margin Qf a cQntinent, increasingly silica-rich magma is formed. This magma is produced partly from mQlten oceanic crust and partly from the melting Qf the rocks Qf the crust. The result is a granitic magma, the most common type fQund at depth within cQntinen ts.
MOVEMENT OF MAGMAS All magmas move upwards because they are less dense than the surrQunding rocks. They push upwards along fractures or by melting, chemically attacking and punching through Qverlying rocks. All magmas, primary and secondary, finally SOlidify to. form igneous rocks. I. Magmas reaching the surface give volcanic rocks. Basaltic magmas are the mQst fluid. They can spread Qver large areas Qf the Qcean floor or the land surface. Granitic magmas are more visCQUS and Qften cQntain large amounts Qf trapped gas. They QccasiQnally erupt to fQrm rocks of rhyolitic or dacitic compQsition. They produce the mQst explQsive types of volcanoes.
Basic Concepts in Geology
Figure ]48 A cross-section through pari of the upper portion of the continenllll cruSI of Victoria.
mllion years ago
(a) During Devo nian times: Granitic magma has intruded as a batholith inlo folded Lower Palaeozoic sedimentary rocks. The heat of the magma has caused contact metamorphic changes in the sedimentary rocks in the aureole around the intrusion. Elsewhere there is an explosive eruption of granitic magma along a ring fracture to produce ignimbrite deposits within a subsiding caldera.
zone
Rong
/(lQnlmbnl.es) lorrntno ranges
PI� ot YOIcaOlC rocks
Rng fracture
(b) Present-day: After several hundred million years of erosion, the granite is exposed al the surface. Around i� Ihe tough contact metamorphic rocks form a low ridge. The ignimbrites formerly fined a depression bUI because they have resisted erosion more than the sedimentary rocks, they now form a mountain range. These fealures are seen in and around the Dandenong Ranges, east of Melbourne.
31
2. Magmas not reaching the surface accumulate i n magma chambers. When the magma ultimately crystallises, usually to a mass of granitic rocks, it fonns a pluton or batholith.
Dykes and sills Magma may also solidify in thin sheets along fractures in the crust. The rocks are called: • sills, if they crystallise along the bedding planes in sedimentary rocks; or, • dykes, if they cut across sedimentary beds or older olidified igneous rocks. Groups of sub-parallel dykes are called 'swarms. Individual dykes and sills may only be a few tens of centimetres thick, but they can range up to many kilometres in length.
MAGMAS IN VICTORIA The intrusion and eruption of magmas has been very important in shaping Victoria.
Granitic magma intrusions Between 500 and 370 million years ago, there were several periods, each lasting up to 10 million years, when there were numerous intrusions of granitic magma into the crust below Victoria. These plutons crystallised to give various masses of granite, granod iorite and related rocks. The magmas for these rocks formed when metamorphic rocks in the lower crust were heated to melting point. These periods occurred when there were active subduction zones close to Eastern Australia Other effects were the formation of volcanic island arcs, the development of zones of regionally metamorphosed rocks and particularly the uplifting of high mountain ranges. The granitic plutons were intruded into the roots of the mountain ranges. Since Late Devonian times (360-370 million years ago), there have been no large granitic intrusions in the Victorian region. Over this period, the sedimentary and volcanic rocks overlying the granites were gradually worn away by erosion. Today there are nearly 300 separate areas of granitic rock outcrops in Victoria, which have been exposed by erosion (Figure 1-48). A feature of the final stage of the long period of magmatic intrusion during the Palaeozoic era was the intrusion of large numbers of parallel dykes in some parts of Victoria. These dykes solidified to rocks varying in composition from ultrabasic to acidic, although intermediate compositions (e.g. diorite) were the most common. The best-known swarm of these dykes extends through mountainous country around the old gold mining towns of Woods Point and Wa lhalla in central Gippsland. Another group of ultrabasic dykes is found throughout the Bendigo Goldfield. These are of Jurassic age. The rock is called monchiquite.
Magmas erupted from volcanoes The Earth's largest and most active volcanoes are generally found along zones where crustal plates are interacting. These include subduction zones, mid-ocean ridges and some deep fault-bounded valleys on continents. Although there is no volcanic activity in Victoria today, there were many volcanic episodes through Victoria's geological
32
Chapter 1
Figure 149 Three types of volcanic activity in Victoria. (a) Island arc volcanoes A chain of voieanoes called an island arc was typical 0 f Cambrian volcanic activity. An island arc rO nTIS above a subduction zone. Oceanic crUSt and sediments pass down into a trench, where they mix and melt to produce new basaltic magma. The islands are made up of voieanic and intrusive rocks fo rmed from the new magma. The volcanic chain is separated from a nearby continent by a shallow sea, known as a back-arc basin . Sediments derived from both the cominent and the volcanic islands are deposited in the basin. (b) Large continen1al volcanoes Two major forms of volcanic activity occurred during the Silurian and Devonian periods in Victoria. In cauldron subsidence, a large, roughly cylindrical block of crust collapses along a fracture system into a magma chamber. At the surface, the sunken arca is called a caldera. Explosive eruptions of magma take p l ace a long the fracture. Thick rhyolite and rhyodacite ignimbrite or ash flow deposits accumulate within the caldera. Thin layers of sediment are deposited in occasional lakes. Mixed acid and inlermediatc volcanic rocks are found in eastern Victoria. In Figure 1 -49b(ii) a volcano erupting andesite lava is near a larger ash now eruption of rhyolite and rhyodacite. Many d i fferen t voieanic rocks may be fo und in this region. These include mud Oows, which form when pyroclastic layers are saturated by heavy rain fa ll and then move rapidly down the slopes under gravity.
(c) Small continental yolcanoes A variety of volcanoes occurred in central and western Victoria during the Cainozoic era. The highest features are scoria cones, formed by the pi ling up or basaltic cinders around the ven t. Lava Oowed from some scoria cones. bUI not as much as came from I he broad lava shield nows. Maar craters are un usual explosive forms, which do not have much lava. Many volcanoes in the Western District are combinations o f these three types.
back arc baSin 10 con�marOin .���'.�I�.2'.��Y'"
thin on
(volcamc Islands
10-100 kilometres WIde) RtAN
(20 Of more kilometres across each diagram) (i) CAULDRON SUBSIDENCE OR CALDERA COLLAPSE Oil MIXED ACID · INTERMEOIATE VOLCANISM
lava shield volcano
scoria cone With small 1a1l8 flow
100m
_
(c)
�
,
-- 1-2Km ---------, ,----
..,.. maar crater ..
2Km ---- -
history. The first was some 560 million years ago and the most recent a mere 7240 years ago. There have been three main types of volcanic activity in V ictoria: •
•
•
island arc volcanoes (Figure 149a); cauldron subsidence and caldera collapse ( Figure 1-49b); small continental volcanoes (Figure 149c).
Island arc volcanoes
The oldest rocks in Victoria are Cambrian in age; they are called greellstolles. They are probably the squeezed and metamorphosed remains of basaltic and andesitic volcanic and intrusive rocks formed on or near island arcs. The volcanoes forming these rocks would have looked very much like the chains of volcanic islands i n I ndonesia and Papua New Guinea today. Typical greenstones are found i n a narrOw belt through Colbinabbin, Heathcote and Lancefield in central Victoria. Cauldron subsidence and caldera collapse
Volcanoes of this type during Devo nian times were amongst the most speclacular and catastrophic events ever to affect Victoria. Huge cylindrical or rectangular blocks of crust, many kilometres across, collapsed along fractures into magma chambers
Basic Concepts in Geology
33
below. Magma forced its way up these fracLUres and erupLed wiLh great violence. Eruption took place not from a single crater, but in a continuous line or arc of fire along the fractures. The magma exploded into huge billowing clouds of very hot gases carrying crystals and fragments of glass and frothed-up magma known as pumice. These turbulent mixtures nowed like boiling milk over the landscape at speeds up to 100 kilometres per hour. When they eventually came to rest, the solid particles seuled into layers blanketing the surface. The resulting layer was often still hot enough for the glassy particles to stick or weld together to form a hard compact rock known as an ash flow iliff or ignimbrire. The t hickest ash now layers accumulated i n the calderas or depressions created where the crustal blocks collapsed. These deposits may be up to one or two kilometres thick. The Dandenong Ranges, Mount Macedon and the Cerberean Ranges between Marysville and Eildon are composed of rhyolite and rhyodacite ash flows, erupted in this way some 360-370 million years ago. In some of these volcanic areas, the rocks also include andesites and basalts. Much of the Snowy River valley in eastern Victoria has been cut down through large thicknesses of rhyodacite ignimbrites, which accumulated in fault-bounded valleys in the Early Devonian. There were also some Devonian acid lavas, similar in composition to the ignimbrites, but containing less gas, thal flowed quietly over the land. Some occur JUSt WeSt of The Grampians.
Figure I-SO An agglomerate or ash now tuff in the Snowy River Vol""nics, 3 kilometres north of MOllnt Cobberas, East Gippsland. The rock consists of rounded to angular fragments of rhyodacite and sedimentary rocks, including shales and slates, in a fine-grained matrix. The rounded boulders were possibly stream deposits which were Lhrown out during an explosive ignimbrite eruption in Lower Devonian times. (photograph by N.J. Rosengren).
Figure 1-5 I Two stages in the formation o f Hanging Rock and Camels Hump, near Macedon. Between 6.5 and 7 million years ago, a small volcano erupted layers of volcanic ash and then a trachyte lava Oow. Trachyte also solidified in the neck of the volcano. With Lhe passing of time. the soft ash deposits and some of the now roc k were removed by erosion. A blocky jointing pallern developed as the plug cooled; Ihis has since been accenl uated by wealhering. Trachyte is an intermediate volcanic rock containing potassium-rich feldspar and minor amphibole and pyroxene.
VISCOUS trachyte magma plugs vanl and soIolies
�
�' 1a1'lowlrom crater
-�=::::= ::: � ErOSIOn removes flanks 01 \'()lcano around plug
&. ��-�---. i::t:.: �...::::I:
Remnan! of IrachY18 flow
,=>=_ _
Small continental volcanoes
The nat plain of Ihe Western District and Ihe country north and west of Melbourne are dOl led with many volcanic cones and cra ters. These erupted al intervals through the Cainozoic era. Primary basaltic magmas from Ihe upper mantle forced Iheir way up Ihrough fractures i n Ihe continental crust to erupl as isolated volcanoes. These were small volcanoes thai may have been aClive for only a few weeks or months. Basaltic lavas are typically very nuid, so much of the volcanic activity produced long flows from cones with gentle lopes.
34
Chapter 1
Figure I-52 Volcanic bombs_
(a) Typical spindle-shaped bomb.
(b) This specimen from Mount Nooral, near Terang is 1 2 �emimer.res across. It consists of an outer shell of dark grey, vesicular basalt and a core of the green mineral, olivine. These blocks were carried up from the Earth's mantle by volcanic aClion�
Defo rmation and m Figure I -53 (below) Fold terminology.
(al Wh en sedimentary rock are
compressed evenly from opposite directions, symmetrical folds are produced. The U- haped fold or trough on the left i called a sync/ine. The arch-shaped fold on the right is an anticline. The line, where there is maximum curvature, is called the fold axis. The sides o f the folds are the limbs.
(bl This bed o f rock has not only been fo lded, but it has also been lilted in a direction at right angles to the forces of compression. A plunging fold has been produced. The angle of plunge i the angle between the fold axis and a horizontal line in a common vertical plane. (c) Gentle warping may produce structures lermed domes and basins. They also may resuh where there have been compre sive rorces rrom [Wo di fferent pairs of directions. The beds everywhere dip outwards from the eresl of a dome. They everywhere dip inwards in a basin.
(8)
•
-<
There are also a few very steep-sided lava domes, e.g. Hanging Rock and Camels Hump, near Macedon. These formed when a very viscous lava cooled and blocked the vent of the volcano. They are composed of the intermediate rock, trachyte (Figure I-51). There are also many explosive volcanoes. Some form prominent cone-shaped hills built up of outward-dipping layers of the rock, scoria, e.g. Mount Fraser at Beveridge beside the Hume Highway. In places, these layers contain aerodynamicaUy shaped blobs of magma blown from the vent. These are known as volcanic bombs (Figure I-52). They usually have olivine i n lheir core. This is material brought up from deep in the crust or the upper mantle. Some unusual volcanoes erupted fme ash and large volumes of gas (mainly steam) bUl litlle lava A wide crater formed, surrounded by a rim or ring of piled-up volcanic ash (tu/ f). These features are known as moors and IU// rings. Over 400 volcanoes have been identified in the Victorian province. This forms part of a chain of Cainozoic basaltic volcanoes, which extends from Queensland to Tasmania. Its origin is uncertain. The volcanic chain waS nOl associated with mid oceanic ridges and probably magma did not escape from the upper manLle along deep fracLUre in the crust. One possibility is that the Indian-Australian Plate, which wa drifting northwards during the Cainozoic era, passed over a sequence of fixed 'hot spots' in the Eanh's upper mantle. Magma possibly weUed up at lhese hot spots. It may have leaked up through the crustal rocks and escaped through various fractures.
De/ormation and melamorphism are term applied to the great physical and chemical changes that take place as thick piles of sedimentary and volcanic rocks are forced up to form mountain ranges. They usually involve the uplift of horizontal layers of sediments and volcanic rocks from the sea-floor to high, dry land. Similar changes can occur if these rocks are forced deep into the crust and compressed. These processes were most intense over relatively short periods in the Earth's history called orogenies. They mostly lasted five to ten millions of years. Most orogenies were accompanied by widespread intrusions of granitic magmas. The most vi ible effects of past orogenies are those produced by processes known as/aiding and/aulling. Folding is mainly restricted to sedimentary and volcanic rocks, whereas faulling affects all kinds of rock. Orogen ies are movemenlS in part of the Earth's crust which are close to ubduction zones. As was the case with major magmatic int rusions, there have been no orogenic event in Victoria since Devonian times, some 360 million years ago. However, since then lhere have been frequent more gemle SQueezing and stretching movements in the crust, which caused faults, mild form of folding and uplift of large areas, e.g. in the Otway Range. Some of the e ffects of plate tectonics can be observed in parIS o f the world tOday. The mo t obvious are active volcanoes and earthquakes. Orogenic movement , however, are far Ie active events of this kind. Mountain building movements, that take place over a period of say 10 million years, only require a few centimetres movement each year. Thi i far below human perception. evertheless there are a few plate boundaries where earth movements can be detected by careful measurements. For example, a plate boundary cro ses the South Island of New Zealand. The Southern Alps there are riding up over a ubduction zone. Their rise has been measured as up to six centimetres per year.
FOL DING During orogenies, when a pile of more or less horizontal ediment and volcanic rocks is queezed by pressures on the sides, they begin to crinkle and buckle into folded shapes.
(b)
(c) surfacf'
20: Angle 01
AnllCline
Dome
Basic Concepts in Geology
35
The lightness (or wavelength) of the folds depends on: •
• •
the size and direction of the compressing forces; the types of rocks in the layers; the total thickness of the pile.
Some of the terms that are used when describing folds are illustrated in Figure 1-53. Fine-grained rocks such as shales are more easily folded than beds of massive sandstone. Figure 1-54 A relief diagnun of plunging folds in sedjmentar}' rocks. Interbedded hard rocks (I), e.g. sandslones, and SOfl rocks (2), e.g. mudstones, have been folded by fo rces from lhe east and west. The folds are also plunging lO lhe north. The sandstones form ridges, while rivers have carved out valleys in the mudstone. The ridges have a characteristic zigzag pattern due to the effects of lhe plunging folds.
Figure 1-55 An anticline al Cape Liplrap on Ihe Soulh Gippsland coast. Many prominent folds are seen in the Early Devonian sedimemary rocks on the coast east and west of Cape Liplrap. The thick beds are sandstones, which are most resistant to ero ion. Mudstones and shales weather more easily. (photograph by G. Medwell).
Throughout Victoria, all the sedimentary and volcanic rocks of Cambrian 10 Early Devonian age were affected by pressures from the east and west during one or more orogenies. As a result, these rocks (which are mainly sandstones, siltstones and shales) are usually found wilh fairly open folds with very roughly nOrlh-soulh axes. The wavelengths of I hese folds are commonly in the range from a few hundred metres to one kilometre. I n some areas, the pressure was greater from one direclion (e.g. the west) than Ihe other. This produced asymmelricai folds or lighlly overfllmed folds (Figure 1-56). Figure I-56 A synclinal fold exposed in a cutting on the road between Wallan and Romsey. The rocks are sandslones of Silurian age. The fold is said 10 be asymmetrical, because (he dip of Ihe beds on Ihe lefl hand side is grealer Ihan Ihe dip on Ihe righl hand side. In the distance, an asymmetrical anticline is visible. (Pholograph by N.W. Schleiger).
36
Chapter 1
Figure I-57 (right) A recumbent fold in Ordovician
sandstones and shales on the beach near MaJlacoota airport, East G ippslan d_ Folds of lhis type are unusual in
Victoria Instead of even forces being applied on both sides of a fold, there has been a much greater force from the right hand side. As a resull, the fold has loppled over. The plane of the axis of the fold is nearly horizontal. (Photograph by NW. Rosengren).
Figure I-58 (above) A monocline.
The seclimentary beds bend down near the surface, but at depth they have eparated along a fault.
The layered rock that were deposited in Victoria from the Late Devonian through to Mesozoic times exhibit much more open folds than those in the older rocks. These folds were produced during gemle sagging or uplifting over broad areas. Folds of this type can be seen along the cliffs of the Otway Range coast and between Cape Paterson and San Remo. Young, horizontal or gemly inclined beds sometimes display a step-like bend: this is called a monocline or monoclinal jold (Figure I-58) . At depth, the monocline may pass into a fault.
Figure t-59 Faults in Palaeozoic rocks. (a) above - A normal raull in a cutting at Stud Ie) Park.
The wavy line F-F touching the geological pick i n normal fault in Silurian �andstones. The fault is more-<>r-Iess parallel to the bedding planes in the sandstones on the left hand ide. The sandstone beds on the right-hand ide are clearly cut off by the fault. (photograph by G.W. Quick). (b) right - A quartz ,>cinle. displaced by a re\'crse fault.
The quartz vein lei intersected Ordovician sandstones 31 Mallacoota. The fault is the sharp line pa sing from the top left to the lower right comer of the photograph. The right-hand part of the quartz ,cinlet has been pushed up and over lhe left-hand part.
FAULTING ja/lll is a fract ure in rocks, where one block has mo\'ed relative to another (Figure I-59). The simp lest fauhs are sharp brea ks along single planes. However, quite often the movement takes place within a zone that is bounded by parallel or nearly A
parallel planes. The rocks within the zone are usually broken up into fragments of varying sizes - the resulting rocks are said to be a ja/lll breccia. These zones of fauhing are called shear �ones. Such struclUres can often be seen in old gold mine workings in central Victoria; the gold was often found within shear zones. Faulting often accompanied folding \\ hen rock were compressed. However, it can also occur where rock have been stretched. Faults can pass across all kinds of rocks, induding intrusives. The common types of faulLS seen in Victoria are illustrated in Figure 1-60.
Basic Concepls in Geology
37
� �� � (a)
�I
Figure I�O Types of faull movemenls. (a) Normal faull The right hand block has dropped down relative to the left hand block. The fault movement occurred because the Earth's crust was in tension, i.e. was being strelched. Most normal faults are Sleeply inclined. A scarp or cliff i s formed along geologically recenl faults.
� ::.::.
. ,
"
: ::-
1 �r
(b) Reverse or Ihrust faull The left hand block has moved over the righl hand block. The fault movement occurred because the crust was compressed, i.e. squeezed from bOlh sides. Reveme faults often have low angles of dip. Scarps are quickly worn away.
z
..
.
. ..
(b)
.
(e) Laleral slip faull
The left hand block has moved horizontall y to the left relative 10 the right hand block. The San Andreas Fault along the coast of Californi a is a major fault of this type.
(c)
The biggesl faulls are the boundaries between adjacent plates fanning the Earth's crust. The best known is the San Andreas Fault, which extends along the Californian coast of the United States of America. There, the Pacific oceanic piaIe is moving in a north-westerly direction pasl lhe North American continental plate. The destruclive earthquakes, lhal are periodically associaled with lhis fault, result from the intermittent jerky nature of the fault movements. The movements along most faults were less noticeable than those along the San Andreas Fault. In mOSl cases they would have been imperceptible over the human life span. Vet because lhey look place repealedly over lens of millions of years, their total displacement may measure hundreds or somelimes thousand of metres. Many big faults in Vicloria separale blocks of folded sediments and volcanic rocks which were formed at di fferent times in lhe Slate'S geological history. For example, lhe Heathcote Fault, which runs north-soulh between Healhcote and Rochester, separales a block of Cambrian rocks on the western side from Lower Devonian rocks to lhe east. Like folds, small faults can often be seen in cuttings and cliff faces. Old faults are not always easily recognised on the ground. They may only become apparent when geological mapping shows layers or rocks ending abruptly against different rocks. Recently active faulls, however, may be visible at the surface due to a ridge or scarp marking the boundary between the uplifted and downthrown rock masses. An example is the Rowsley Fault along the eastern edge of the Brisbane Ranges between Bacchus Marsh and country west of Geelong (Figure 7-9). Figure I�I Joinls in Silurian sandstones (I he Ihicker beds) and mudslones (Ihe Ihinner beds) on a hillside beside Ihe lrack to Partingtons Flal. Greensborough. A major joint ( fracture) CutS across the sedimentary rocks in the middle of the photograph . Minor joints (cracks) only inte rsect one or tWO beds. Bedding planes can be traced directly across the joint. I f there was any displacement of the beds, the feature would be a small fault, not a joint. (Photograph by N W. Schleiger).
JOINTING To a greater or lesser extent, all hard rocks are crisscrossed by fractures or cracks called joinls. Joints differ from faults, because there is no displacement along joints. Most joints in sedimentary rocks are the result of past fold and fault movements and of broad-scale upwarping. On the mher hand, joints in igneous rocks were mostly produced by the rocks shrinking as they cooled. In addit ion, some probably were caused by the unloading of the rock mass when a cover of overlying rocks was eroded away. Some joints were also produced by atmospheric forces, such as the heating of rocks by day and lheir cooling by night. Two major geological processes - weathering and erosion - are greatly aided by the existence of joints in rocks. Water, oxygen, and humic acids and salts from soils penetrate into rocks along joints. These substances react with the minerals in the rocks to produce newer, usually softer minerals in the process of weathering.
38
Chapter 1
REGIONAL METAMORPHISM Folding and faulting was often accompanied by regional metamorphism. Metamorphic rocks developed in the cores or root zones of mountain chains formed from folded rocks, where pressures and temperatures were greatest . At these depths, granites may have intruded as well, adding to the metamorphic effects. The nearest Victoria had to conditions like these is probably represented by schists, gneisses and granites in the north-eastern region of the State. These are probably the remains of the root systems of a very large mountain range forced up by a coUision between crustal plates in Early Palaeozoic times.
Erosion and sedi mentation
The maj or geological processes described so far, e.g. magmatism, metamorphism, orogenesis, faulting, have all occurred sporadically through the Earth's history. Some like volcanic or earthquake activity can be seen or felt during a human lifetime. Others such as fold movements may take several million years to complete. By contrast, the last two processes to be described, erosion and sedimentation, have been going on continually over vast areas almost since the Earth was formed as a solid mass. They are bound to continue into the future. Erosion, in particular, is taking place over aU land surfaces, except those covered by deep ice or flat, permanently frozen areas. Erosion and sedimentation are always linked to one another. Mineral grains and fragments of rocks that are deposited as sediments are derived from the erosion of rocks elsewhere. The main agents of erosion are: 1 . Water: • running water on land; • the action of the sea along the coast; • moving ice. 2. Wind - whenever and wherever there is little vegetation covering the land surface. Many examples of erosion taking place can be seen in Victoria. Wind erosion is seen as dust storms or moving sand along beaches and sand dunes. Rounded waterworn boulders occur along mountain streams and finer material is carried as mud, silt and sand by rivers. An impressive example of coastal erosion occurred in 1990 when a scenic feature near Port Campbell, known as London Bridge, was demolished by the force of the sea. The previous bridge-shape had been sculptured in Tertiary sedimentary rocks, probably over a period of many hundreds of years (Figure 3-60).
Figure 1�2 The main environments where detrital and organic sediments are deposited and the processes producing them.
•
wind erosion
dunes
•
river erosion
lakes and swamps river channels alluvial fans flood plains
•
ice erosion
tills
•
river erosion
deltas shallow seas
•
shallow seas
limestone reefs and shell deposits sands and silts on continental shelf
•
deep ocean
turbidites at fOOl of contintental shelf mud and clay
Sedimentation follows erosion, as eroded material is carried away by water or wind and deposited elsewhere. The main environments where sediments may be deposited are summarised in Figure 1-62 and illustrated in Figure 1-63.
Basic Concepts in Geology
39
Desert (sand dunes, salt lakes)
���;;;�
"-. � --->i '-& =
R I v er
L agoon
""
Floodplain 0 1 river
���==� Beach -..
Conllnental shell Continental Slope"'"
Figure i-{;3 (above) The various environments where sediments ClIn be deposited on land, along the coast and on the sea-noor.
SEDIMENTARY BASINS Some of the deposits referred t o i n Figure 1 -6 2 cover small areas, e.g. sand dunes, swamp silts. But other accumulations of sediments are very thick and extend over large areas called sedimentary basins. These basins are found both on land (onshore) and beneath the sea (offshore). A basin often commences as a broad shallow depression. Eventually a great thickness of sediments may be laid down as the floor of the basin subsides under the weight of added material. Examples of present-day sedimentary basins are: •
• •
the flood plains of the Murray River and Coopers Creek; the delta of the Nile River in Egypt; The Gulf of Mexico, a shallow sea, where sediments are deposited by the Mississippi River.
Deep narrow basins may be called Iroughs. The term, rifl valley or graben is used for a depression produced when a long block of land drops between faults.
SEDIMENTARY ROCKS I N VICTORIA Most of the sedimentary environments given in Figure 1 -62 are represented by rocks in Victoria. During any one period in the State's geological history, one or two types of sedimentation were dominant. From Cambrian to Early Devonian limes, most sedimentary rocks were laid down in marine environments. From the Late Devonian to the end of the Mesozoic era, however, most sediments were deposited onshore by rivers. Both marine and continental sediments are found in Cainozoic rock formations.
Marine sediments The Early Palaeozoic marine sediments include deep water, shallow water and turbidite types. 1. The deep water rocks are black shales, which were deposited very slowly in still water on the bottom of a deep ocean basin during the Ordovician period. These rocks are finely-banded and often contain fossils of small floating organisms known as graptolites. 2. Some Silurian and Early Devonian sandstones, mudstones and shales were laid down in shallow marine basins. Ripple marks, formed by the action of waves on the sea-floor, are common in these rocks (Figure 1 -64b).
3. Turbidites are interbedded fine- and coarse-grained sedimentary rocks, which were deposited by lurbidity currents. These currents are turbulent mixtures of sediment and water that slumped off the edge of the continental shelf and flowed down the slope on to the deep ocean floor. Thrbidity currents often gouge out channelways or submarine canyons. As they become slower on reaching deep water, a layer of heavier, coarser sand or silt particles is deposited first. Gradually the finer particles are laid down until quiet conditions return. Later turbidity currents bring further alternations of coarser and finer sediments. A sequence of coarse to fine sediment is called a graded bed (Figure 1 -65). Turbidity currents often carry shells and other remains of marine animals typical of shallow water into much deeper water.
40
Chapter 1
Figure 1-64 (a) Ripple marks formed on tbe seabed in shallow water as th. tide came i n near St Kild. Pier. The ripple marks were left on the sand after the tide receded. The ripples split and branch due to changes i n the directions and speeds of the waves. During the last ebb tide, water nowing back scoured the earlier ripple marks in the lower right hand of the photograph.
(b) Sandstone from the Cathedral Range, Buxton. These ripple marks formed on a sandy seabed, aboUl 375 million years ago. The sandstone consists mainly of sand grains cemented by silica (quartz) and iron oxides. The presence of ripple marks shows the sand was deposited in shallow water, similar to the conditions prevailing in Figure 1-64a, (Photographs by N.W. Schleiger).
Continental Sediments
Figure 1-65 Graded bedding produced by lurbidity currents. Large amounts of sediments are carried down the continental slope al intervals by lurbidity currents. Each current slow down as it passes over Ihe noor of the deep ocean. Coarse sand grains sCllle first. These are followed by progressively finer sediments through silts to muds. In this figure the turbidity sequences deposited by two successive turbidity currents are shown.
The largest areas of conLinemal sediments deposited by rivers are found in the Otway and Strzelecki ranges. These accumulated in a rift-like valley during the Early Cretaceous time . The valley noor was an extensive nood plain with occasional coal swamps. Chan nelways known as braided rivers spread across the plain. Braided rivers have a series o f channels that cominually fork and rejoin. Between the channels are large sandbars. These sandbanks cominually move downstream, pushed by the river currenl. As they do so, layers of sand periodically avalanche down the lee Or downcurrem faces of the banks. These avalanching sandbanks are preserved as cross bedded sandstones, where the small-scale layering within the individual beds represems the avalanching faces o f Ihe sandbanks. In places gravels were deposited in the river channels and muds were carried beyond the river banks by nood waters. The result is a sequence of cross-bedded sandstones interbedded with occasional conglomerale and mudstone layers. I n north-easlern Victoria, east and soulh-east of Mansfield, there are Late Devonian sedimenlary rocks which were deposiled in a somewhat di fferent environment. Sands and muds were laid down in rivers and lakes in a long narrow valley, surrounded by hills bare o f vegetation. Fans of sediments spread out across Ihe valley and from small side valleys and braided streams wandered over lhe valley noor. Sand d u n es built up by winds are anolher type of continental sedimentary deposit. These also commonly exhibit cross-bedding. Pleislocene dune limeslones occur in various places along the south Viclorian coasl ( Figure 3-59).
LITHIFICATION OR DIAGENESIS Most sediments begin as loose masses of mineral grains and rock fragments. Gradually with the passage of Lime lhey slart 10 consolidate. First soft rocks, and eventually hard, massive rocks are produced. Sands become sandslones, muds become mud lones and so on. These changes are called Iilhificalion ( rock formation) or
diagenesis.
Basic Concepts in Geology
41
As sediments are deposited in a basin, the lower layers come under increasing pressure due to the weight of overlying material. They compact as the pore space between the individual grains decreases. If the basin is slowly subsiding, the compressed layers are forced deeper and are also heated. Chemical reactions begin between water in the pore spaces and the mineral grains. These produce new minerals such as clays, zeolites, feldspars and chlorite. These new minerals fill the pore spaces and cement the remaining old grains together, hardening and toughening the rock. Quam, calcite and iron oxides are also cementing materials. Figure 1-66 Formation of a sedimentary rock from unconsolidated particles. (a) There is an acctunulation of loose grains of sand and broken pieces of various shells in shallow water near a coast. The sand grains were derived by the erosion of sandstone cli ffs, while wave action gradually smashed the shells of small creatures, whjch once lived in the sea. (b) After the deposit was lifted permanently above sea-level, calcitun carbonate crystallised from groundwater percolating through the porous sediment. Gradually crystals of calcite filled most of the spaces, which previously occurred between the various particles. The mass became harder and solidified to form a sedimentary rock - in this case a siliceous (sandy) limestone.
(a)
Grains of quartz sand
Fragments of shells
Po re
s pa ce
(b)
Pore space filled by calcite cement
Geological maps
The work of a geologist in the field involves the identification of the various rocks in an area, the tracing of boundaries between di fferent rock types and the measuring of dips and strikes in sedimentary rocks. This infonnation is usually recorded first on aerial photographs, because the precise positions of many geological features can be seen on these photographs (Figure 1.{j7). Later the data are transferred to topographical maps at specific scales. Geologists may also collect rock samples to enable further invest igations to be carried out at a laboratory, e.g. identification of minerals and fossils under a microscope, chemical analyses of ore rocks, etc. Geologists pass on the information they acquire by means of geological maps, geological cross-sections and geological reports. Since it was established in 1852, the Geological Survey of Victoria has been the main public authority responsible for producing geological maps and reports of the State. These may be purchased at the Victorian Government Bookshop in Little Bourke Street, Melbourne, and at some Govern ment offices elsewhere in the State. A standard series of geological maps at a scale of 1 :250 000 has been prepared for the whole of Victoria. Each map heet covers a rectangular area endosed by 1 .5' longitude by I' latitude (nearly 15 000 square kilometres). Below each map, there is a geological cross- ection. This is a vertical slice across the map sheet, which shows the relationships between the main rock formations beneath the surface.
42
Chapter 1
Figure 1-67 The use of aerial photographs in geological mapping. (a) The photograph is taken from the air at a height o f over 7000 metres above the ground It covers very rugged, forested country around CromweU Nob, a high plateau, about 100 kilometres north·west of Bairnsdale. Geological mapping on t he ground is a slow and arduous task n i this largely uninhabited country with few roads for access. When tWO successive aerial photographs taken on the flight run are examined under equipment known as a stereoscope, a three-dimensional view of the country is obtained. This enables many geological boundaries and structures to be identified on a photograph. (Acknowledgement: Survey and Mapping, Victoria Ministry of Finance). (b) An interpretation of the geology is given on the map. The 'striped' areas arc sandstone and conglomerate beds overlying more massive volcanic rocks, all of Devonian age. These have been compressed into broad anticlinal and synclinal folds. In the deeper valleys in the north, these rocks unconformably overlie more strongly folded Ordovician sedimentary rocks. The bedding in the older rocks is not visible on the photograph. There is, however, a distinctive branching drainage pattern over these rocks. The semi-cleared (speckled) patches on the photograph are areas where logging of alpine ash or woolly butt (Eucalyptus delegatensisj has taken place recently. Alpine ash, which typically occurs in localised patches at high elevations, is at present the main source of hardwood board timber in the State. Regeneration and reseeding will produce a forest again in the future. "
The main purpose of a geological map is to tell the reader what rocks are pre em in an area and what their geological ages are. This information is presen ted by showing areas marked with different colours, shadings and symbols ( Figu r e 1 -68). The
Basic Concepts
in Geology
43
Figure 1-68 A medium-scale (1:25 000) geological map of the Bald Hill area, three kilometres north�west of Bacchus Marsh. The area shows many interesting geological features, including a rare occurrence in V ictoria of sedimentary rocks of Triassic age in the Council Trench quarry. The key to the geological for mations shown on the map from youngest to oldest is given below: Qrn
Recent river al luvium
Qrt
Recent low level river terraces
Qpt
Pleistocene high level river gravels
Qvn
Pliocene basalt
Tmg
Miocene sand, gravel
Tew
Eocene-Miocene clay, sand, gravel
1\ro
Palaeocene-Eocene basalt
TR
Triassic sandstone
P
Permian tillite, sandstone, mudstone
(Geology from 1:50 000 Bacchus Marsh geological map sheet, 1985, Geological Survey of Victoria).
'"
positions of major geological structures, such a faults, are also indicated. A geological reference beside the map explains where the various rock formation fit in the geological time scale.
CORRELAT ION OF GEOLOGICAL FORMATIONS An important aim of geological mapping is to show which rocks occurring in separated areas are of equivalent type and/or age This is done by a process called
correlarion.
Correlation of igneous rocks Outcrops of intrusive rocks may be correlated, (that is, shown to be pans of one particular batholith), if each outcrop has the same mineral composition and texture. This evidence may be supported by checking that some outcrops have similar chemical compositions. It might also be possible to measure the absolute ages of separate outcrops by radiometric dating - but this is an expensive technique.
Correlation of sedime ntary rocks The correlation of sedimentary rocks can be ba ed on either rock type and/or fossil contenl. I. Rock correlation (al If twO boreholes, drilled through horizontal strata at localities several kilometres apart, both intersect a similar sequence of roc k units, it is likely each rock unit can be correlated. (b) Similar rocks may be correlated simply because they were all formed in an unusual way. For example, all the outcrops of rocks of glacial origin in one district mighl be correlated because ice ages are rare events in the geological record.
2. Fossil correlation
Sedimentary formations at di fferent localities can be correlated if they contain the same populations of animal and plant remains (Figure 1-69).
44
C hap ter 1
Figure 1-69 Correlation of sedimentary beds using fossils. Geological mapping has identified a sequence of nat-lying beds numbered 7 (the oldest) through to I (the youngest) in a coastal cli ff, and another sequence, G to A, in the sides of an inland river valley. This example might be typical o f Tertiary strata i n the Pon Campbell - Timboon area of south-western Victoria. The first assumption would probably have been to correlate bed I with bed A, 2 with B, 3 with C, etc., because of the similar thickness and rock type in each pair of beds. However, after fossils were collected from several beds, it was clear that beds 2 and B contained different sets of fossils, whereas 2 and E contained identical fossils. Therefore 2 can be correlated with E, though the thicknesses of the outcrops and their elevations in the two areas are different. Similarly 4 correlates with G.
Lt.,lugkol (ollerllun,
.. 0 ud, �trds, lant , � 0 t t ,fe " n 1 ..:1; , r- n ak COt an to "I . Th e Ire J b tl lor c Jur reat mlcre r at I\e ru n Iml� h � pia, Ir r. u� urn rhe�c ell "'l lel ln J of Clen I "v L ik
a r
ce t and
n.
AREA 1
AREA 2
-Coa stal Cliff Outcrop-
-River Valley Outcrop-
ROCKS
� � m�{J BED 2
BED
4
Echinoid E
SB Echinoid E
Coral A
Coral 8
Crab A
CRA
CA
Snail B
CB
limestone
Mar l s Sandstone
FOSSILS BED E
BED B
CRA
BA
Echinoid E
SB Echinoid E
BB E
Coral A
CA
Coral B
SA
Bivalve A Bivalve B
Echinoid
BED G
Crab A
Snail A
Snail B
CB
[n special invest igations, some unusual scientific techniques have been used to correlate rocks over a very large area. For example, rocks from several parts of the world are correlated with an age at the Cretaceous - Tertiary boundary because they contain uncommonly high amounts of the rare metal, iridium. This metal is believed to have been derived from an extra-terrest rial body, which collided with Earth at that time. This even t may have led to the mass extinction o f life including the dinosaurs.
GEOLOGICAL MAP NOMENCLATURE A feature of most modern geological maps is that each geological unit is given a
formal name, e.g. Shepparton Formation, Coldstream Rhyolite, Mount Buffalo Granite. All rock outcrops identified by a particular name can be correlated. The first part of a name is a locality where the rocks occur - preferably a well-known town or land feature. The second part is the main rock type present. Terms such as/ormation and beds are used for sedimentary rocks, where two or more rock types are well represented in the pile. For example, on the Tallangatta 1 :250 000 map sheet, the Tambo Beds are made up of shale, sandstone, conglomerate and minor interbedded rhyolite. The derivation of some names is not immediately clear. For example, Melbourne is in part underlain by the Dargile Formation. But Dargile is not a suburb of Melbourne, i t is the name of a parish in central Victoria near Heathcote. The formation was first named after detailed mapping in that area in the 1 930s. Subsequent mapping showed that the rocks of the same type and age could be traced southwards to Melbourne.
Soils
C hapter
S O I LS
45
2
Figure 2-1 A deep, fertile soil on alluvium of the Lerde rderg River flals, Bacchus
Ma"h. This area is intensively cultivaled to produce vegetables and fruit for the Melbourne markets. A line of sprinklers is irrigating crops in the background. By contrast, the hills lopes on the left have been overgrazed and are prone to soil erosion. (pholograph by N.W. Schleiger).
Figure 2-2 A fossil soil in Ihe bank of Jacksons Creek, near Bulla. It may be thoughl that rocks are old and soils are young. However, examples can be found of soils formed in pasl geological ages. In this photograph, Ihe geol ogical pick is lying on an old clayey soil developed on a weat hered lava flow. The soil has been covered by a later basal I flow, which is not very weathered. II s i called a fossil soil, because it was rormed in a paSI geological period. (PhOlograph by NW. Schleiger).
Soils Form a very small pan of the Eanh's crust compared with the space occupied by rocks and sediments. Nevertheless soils vary more in their characteristics than rocks do. Consequently they are more difficult to classify. The composition of a soil depends not only on its parenl rocks and minerals, but also on the interplay of various geomorphological, climatic and biological faclOrs. It is also important 10 recognise that soils are continually changing with the passage of time. This means that a soil profile may gain or lose material over the years. It can also be continuaUy modified by various chemical and biological processes. Rocks, by contrast, tend to remain unchanged over long periods of geological time, apart From the weathering and erosion that takes place close to the surface of the land. The best soils For agriculture and fore try are deep, well-structured loams with adequate humus and nutrient contents. Such fertile soils only occur on geologically
46
Chapter 2
very large areas in some overseas countries. Examples are the alluvium forming the river flats and deltas of some of the world's largest rivers, such as the Nile and Ganges, and the glacial deposits on the prairies of North America. In China, there are important soils on loess, which is dust originating in glacial deposits. Hmvever, in Victoria, first-rate soils are restricted to small areas of river flats and to volcanic ash deposits. The largest river flats occur in Gippsland beside the Thomson, Macalister and Avon rivers near Maffra, the Mitchell River near Undenow and the Snowy River near Orbosl. Young volcanic ash deposits mixed with colluvium occur as aprons around volcanic cones in the Western District. ·A large area near Koroit is a particularly productive agricultural area. There are larger areas of soils, which have favourable physical properties but are less fertile. These i nclude the follmving soils: • •
• •
red, friable, clayey soils developed from colluvium on hilly areas of basaltic rocks, e.g. near Ballarat, Trentham, Warragul and Monbu lk; red, brown and yellow friable soils developed from colluvium on mountain slopes ,vit h moi t southerly and easterly aspects in the East Victorian Uplands and in the Otway and Strzelecki ranges; red and brown loamy soils developed from windblown (aeolian) deposits on the Mallee plains; well-st ructured grey clays on the W immera Plain.
By contrast, there are large areas of poor soils in Victoria. These include sands in western Victoria (e.g. in the Big Desert and Little Desert), hard-setting soils in the north of the State and shallow, stony soils in the drier hills and mountains of the Cemral Victorian U p lands.
Soil format ion
OIL
UTRIENTS
Good growth in crops, vegetables and other plants depends on Lhe avai labiliLY of nULrients in soils. The following eighteen chemical elements are known to be essential for plant growth:
H
There are many types of soil in Victoria. This is caused by the great variations in the soil-forming factors - climate, parent maLerial, landform, organi ms (flora and fauna) and age - across the StaLe.
ClimaTe varies from arid
in the far north-wesL of the State to humid in the mountains. It reaches alpine conditions on the highest peaks where snow lies for many months. With increasing rai nfall and decreasing temperatures in the mountains, there are changes such as increased humus, improved st ruCLUre and increased acidity (i.e. lower pH values). Under these conditions, a1ts are dissolved out of the oils and carried away in streams. Older soils in the drier areas are red because the dryness produces a red variety of iron oxide; this coats the sand grains in the soils.
has a major effect on texture and nutrient contents. For example, granites give rise to coarse sandy soils and ba alts to clays of high fertility. Aeolian materials in the north contain considerable lime (calcium carbonate) and oluble salts. These tend to remain in the oil becau e the rainfall is too low for them to be dissolved.
ParenT material
Landform di fferences infl uence soils
greal ly, both within and between districts. For example in hilly terrain, upper lopes lend to have shallow, stony soils because of natural erosion. Red colours also occur because of the good drainage. On the other hand, soils of the lower slopes are deeper and browner because run-off water promotes denser vegetation and thus more humus. In the Mallee, in one particular paddock there can be sand on the dunes and loams on the intervening flats. The two soil types should be trealed differently during farming.
II
(C
Carbon, oxygen and hydrogen come from the air and water. The other elements mainly come from the soil. A plant may grow poorly if the soil is deficient in one or more of the above elements or if an element is prescnt but not readily available to the plant.
plants are broken down by animals and organisms such as fungi and bacteria to produce humus. Humic acids hasten the liberation of nUl rients from rocks. Rools produce pores through which water can penetrate freely. Deep roots bring nutrients to the surface. Plants also protect soils from erosion. Animals improve the structure and nutrient status of soils by adding their excreta. Even small creatures such as worms are important because they burrow through soils, thus making the soil porous. All these effects are greater in humid, mountainous regions lhan in drier areas.
Flora and fauna:
Soils
As an example, a healthy 2 kilogram cabbage should contain the following elements (approximate):
47
The age of a soil depends mainly on its position in a stream catchment. The youngest
soils are on river flats, where alluvium is still being deposited at intervals. These soils are fertile because there has been insufficient time for nutrients to be leached OUt from the sediments. On older alluvial terraces, soils with loam topsoils and clay subsoils are common. They are particularly widespread on the plains of northern Victoria. where much of the State's irrigation is carried ouL They also occupy large areas in the south-west of the State and in Gippsland. The oldest soils occur on gentle drainage divides that have been protected from erosion. Some are more than a mimon years old. They have very dense clays, which are strongly mOlLled with red, grey and off-white colours. These are scattered throughout the State, the largest single area being on the Dundas Tableland, west of The Grampians.
o I 0_
·Very much smaller amounts of sodium and chlorine are actually needed, but plants usually take up much more salt (sodium chloride) than they need simply because it is in the soil. (From CSlRO Division of Soils)
Figure 2-3. ( a bove right) Gully erosion near Darraweit Guim. The area on Palaeozoic sedimentary rocks is notably gullied. Here a gully has developed in sodic duplex soils on valley alluvium.
Figure 2-3b The Origin of soi l parent m aterials in a landscape. Various phases of weathering and erosion have produced materials in which soils have formed. The deepest soils occur on alluvial plains. colluvial slopes and plateaus. Soil development is weak on younger alluvium, so the promes are uniform, soft, porous and fertile. There is Slronger soil development on older alluvium producing duplex soils, that are hard-set ting and poorly-drained. In some areas, e.g. the broad Riverine Plain of northern VictOria. older alluvium is blanketed by dust blown eastwards from the Mallee Present-day sheet and gully erosion are removing some earlier-formed soils.
01
classificatio
Plateau wiIh resIdual sod materials
!
Raw malenal deposIted , from guilles _
' ./
.t , , ,
,
....... Old allUVium or ....... aeolian matenal In dner areas
Bedrock
Soils are classified on the basis of differences in their profiles ( ee Chapter I). The di fferences in any area are detected largely by inspecting samples from road cuttings or holes drilled with a hand auger to depths of usually not much more t an one metre. The drill holes need not be located on a systematic grid pallern. Rather, the ites are selected wherever there appear to be differences in the slope of the ground, the t)'PC of vegetation or other natural factor . Aerial photographs are used to pinpoint where each sample ite i located. The boundaries between different soil type can sometimes be seen on the photographs.
48
Chapter
2
In Victoria, four broad classes of soil profiles have been recognised, based on a system introduced by the CSIRO Division of Soils (Figure 2-4): •
•
•
•
organic soils uniform soils gradational soils duplex soils.
In the following discussion, the four broad classes are further subdivided into groups that are widespread in particular regions of the State, as shown in Figure 2-5. It is important to realise that these groups still represent broad levels of classification. Each group contains many soil types that di ffer in appearance and performance.
Figure 24 The four ki nds of soils. Soil profiles like these can be seen in road cUllings and building excavations. They usually vary from less than one metre to two metres in depth and are often dark at the lOp as a result of the accumulation of organic malter. Soil profiles in each group vary i n detail, but still conform t o the basic type.
fI�..' �����
Mainly planl remains
Sand, loam or clay particle size varies
with some sand or clay
UNIFORM
ORGANIC
:::;::::\::::::::::;::::::::::::: Sand or loam topsoil -:-_-:...-_ .: � :.: _:.: .:.' Sharp break
Soil particles become gradually finer with depth
GRADATIONAL
Figure 2-5
Common soils in Victoria. Class
Organic
Clay subsoil
DUPLEX
Parent rock - mat be fresh or weathered bedrock, sedimentary deposits or remnants of older soils
Group
Commonly used names
Occurrences
Peats
Peals
Coastal salt marshes Freshwater swamps beside rivcr<;
soils
High Plains
Unirorm
soi ls
Sands
Clays
Gradat ional .. oils
Duplex \oils
Poosals when pale ,md acid ic Sandy Malice soils ror inland
.
Coastal plains Malice dunefield
sands containing abundant lime.
Soils or heavy texture
Wimmer:t Pla;n. volcanic plains Murray Valley nood p lain�
Looms (deep)
Alluvial soils
Gippsland nood plain ..
Shallow stony loams
Skeletal soils
Celllral Victorian Uplands
Shallow stony eaTlh�
Stony soils or t he hills
Central Victorinn Uplands
Friable earlhs
Krasno7.('mS when red Mountain !loils whell brown
I-lillo; on b'l'iUh ncar Melbourne Ea�t Victorian Uplands
CaJcareOll!. eanhs
M:ll1cc soils
Malice dundicld
Red calcareous
Red brown earlh ..
Rivcrine 1>lain or northern Vietori••. Wimmera PI.lin. WcrribCt.' volcanic Illain
duplex ..oil� (alkaline) Sadie duplex �oil� ( neu lral )
Solotlic ","oil\
Acidic duplex 'iOih (acidic)
Pod.!>Olic \Oil.. l..<jteritic po<.l\Olic ..oil..
Gentler 1>lopcs of We.�t Victorian U lll:all
Lower ,lope., of East Victorian UplanJ:. Many 1>cJ ; I [crc d areali on gentle cOilchnWlI1 dividc, Large arc:t.� on I)und:" Tableland :lIld coa...lal plain)o
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 <7
Climate semi-arid sub-humid humid
The pH !rend reneets the extent to which rainwater has removed soluble substances from the soils.
There are marked contrasts in the textures and colours of the A (topsoil) and B (subsoil) horizon of duplex soils. The transition i often quite sharp and, at most, occurs over an interval of 10 centimetre . The A horizons are sandy or loamy depending largely on the parem material. There i often a subdivision into an upper, brown, A I horizon containing humus and a lower, pale-coloured, A2 horizon. Clay and iron oxide are leached from the A and concentrated in the B horizon. Calcium carbonate may be present in the lower pan of the B horizon. The clay horizons are coloured in variations of yellow, red, brow n and grey. Sometime ironstone nodules occur near the boundary of the A and B horizons. Duplex soils are by rar the commonest soils in Victoria. They occur everywhere except on the higher mountains. The profiles vary in detail from one area to another depending on their history of formation. This variation has an important bearing on their agricultural value and stability. They are subdivided imo three broad categories according to the pH o f the deeper ubsoils, as in Figure 2-7.
Soils 1.
51
A Ikaline duplex soi/s. The most widespread soils in this category are red calcareous duplex soils, also known as red brown earths. Calcium carbonate is visible in the subsoils as white flecks and nodules. These soils have developed mainly from alluvium and aeolian material on the Riverine Plain of northern Victoria, the Wimmera Plain and rhe Werribee basaltic plain. Crops and past ures are grown on these soils under dryland and irrigated agriculture. Agricultural productivity is limited by the poor structure of both rhe A and B horizons. This leads to hard-setting surfaces, waterlogging, wind erosion and salting. Plains with these soils have been almost completely cleared of the original vegetation.
2. Neutral duplex soi/s.
These soils are known as sodic duplex soils or solodic soils. The term 'sodic' indicates rhe presence of excess sodium, which gives rhe soils undesirable characteristics. Subsoils swell on wetting, blocking off soil pores and thus limiting fu nher entry of water and causing waterlogging. Rainwater runs off the surface of hilly land causing erosion of the topsoils. The sodic nature of the subsoils causes them to disperse, that is to form a suspension in water. Thus the subsoils also are readily eroded by moving water. The water becomes murky and causes silting up of streams and water storage . Because of the poor drainage, subsoils are usually yellow and often mottled with grey. Upon drying, the A horizons set hard. After heavy rains they become saturated for long periods, lose strength and it is impossible to drive vehicles across them. The main occurrences of neutral duplex soils are in the Central Victorian Uplands, (particularly the western part where the rocks are mainly Palaeozoic sandstones and mudstones), the West Victorian Volcanic Plains and the South Victorian Riverine Plains in both rhe eastern and western parts of the State. These soils are mainly used for grazing. However, the poor physical nature of the soils limits agricultural prOductivity and promotes ero ion, particularly in the West Victorian Uplands. Salting and waterlogging are also problems. 3. Acidic duplex soils.
Acidic duplex soils are often known as podsolic soils. Podsol is a Russian term meaning "ash-like below": it refers to the pale subsoil horizons. The B horizon clays are fairly porous, so that profiles are not very prone to waterlogging and erosion. Nearby streams tend to run clear. The A horizons have moderately good structure and do not set very hard when dry. The main occurrences are on lower slopes in the humid East Victorian Uplands, which remain largely forested. They are quite productive when cleared for pastures. This is especially the case when the levels of nutrients, particularly phosphorus and nitrogen. are raised and maintained. Acidic duplex soils with deep mottled clay subsoils occupy gentle catchment divides, the South Victorian Coastal Plains and the Dundas Tableland in western Victoria. The mouling is striking, with large patches of red, grey and whitish clays. Pieces of ironstone frequently occur in the upper horizons and scauered on the surface. These profiles are commonly called lateritic podso/s. The deep clay store soluble salts. When the salts are dissolved by groundwater, they can cause oil salting at downhill sites and increased salinity in streams. Figure 2-l! A common d s i tribution of soils in southern Victorian landscapes. The sand soils near the coast have low fenililY and low waler-holding capacity and I hus are not suited to agriculture or forestry. Koo-wee-rup Swamp has been drained and the pealY soils are highly prized for vegelable growing. The older alluvium funher inland has yellow duplex soils which are seasonally waterlogged and used mainly for grazing. The mountains remain foresled; here Ihe soils have favourable physical cilaracterisl ics except that Ihey arc shallow on the sleep north- and west-facing slopes.
-l>PI!.- -
-
COliSTAL SWAMP e g DUNEFIELD KOO-WEF RUP
I. ANDFORM
PIIR E N T MATERIAL
-
SAN[)
PE AT
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.
.�
� •
..
"
, .'
I
I� ,
I
, ,
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\
I \n
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
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%
-
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.
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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 !
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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
Cia)' plain w,ln �-
2 . KIlometres
In a few places, lakes were formed where streams were blocked by up-tilted blocks (Figure 3-8 and 3-32). Tectonic phenomena are also important because they define the broad framework in which other geomorphic processes operate. Block faulting and broad land subsidence have lowered large pans of the Victorian and New South Wales coastlines in the past. One effect of these movements: together with a rise of sea-level after the Pleistocene Ice Age, was to drown several old river valleys. In doing so they provided the deep water channels that enable ocean-going ships to travel up Port Phillip Bay, Western Port and Sydney Harbour.
Geom orph i c d i visions of Victo ria
The nine natural geomorphic processes, that have been described, have combined in various ways to produce the great variety of landscapes that can be seen by travelling around Victoria. The State is subdivided into six major geomorphic regions and various subdivisions (Figure 3-9). In the various geomorphic subdivisions, there are differences in land shapes, average elevations, soils, underlying geology and local climate. The di fferent landforms and soils have developed because each region has experienced di fferent combinations of geomorphic processes. These factors in turn have led to di fferent land uses in each subdivision. Because of t he great diversity of landforms, soils and land use in Victoria, it is not possible to describe every type that is found in the State. I n the limited space available in this book, discussion is restricted to a few features that are typical of each geomorphic region. Some dominant geomorphic processes in each subdivision are also described.
Some landforms can occur in more than one geomorphic subdivision. For example, granitic rocks are widespread in Victoria. Therefore landforms associated with these rocks occur in more than one geomorphic subdivision. For convenience, however, granitic landforms are only described in the section dealing with the West Victorian Uplands.
CENTRAL VICTORIAN UPLANDS
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EAsr VICTORIA UPLANDS
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WEST VICTORIAN UPLANDS
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Upland plateaus and high plains Dissected Uplands Wellington Upl,nds
MURRAY BASIN PLAINS
Dissootod Uptands (Midtands) The Grampl,ns Dundas and Merino T ablelands
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SOUTH VICTORIA UPLANDS
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Clay plaIns Southern Wlmmera Litlill Desert
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SOUTH VICTORIAN RIVERINE PLAINS
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Low CIIICllrtlOus dun(IS High silic."us dunes (Sir; Desert. SUfIS., Country)
WIMMERA PLAIN
WEST VICTORIAN VOLCANIC PLAINS
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Murray River Floodplain Shepp,rron Form,tlon
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66
Chapter 3
CENTRAL VICTORIAN UPLANDS Figure 3-10 The highest mountains in Victoria. Mountain Bogong Feathertop Nelse North Fainter South Loch Hotham Niggerhead McKay Cobberas No. Cope Spion Kopje BuUer
Elevation (metres)
1986 1922 1883 1877 1874 1861 1 &43 1&43 1838 1837 1836 1804
Extending east-west through central Victoria is a belt of relatively high country, which becomes narrower and lower towards the west. This belt is commonly called the Central Highlands, although geomorphologisls prefer the term, Uplands. Its cresl forms a sinuous divide between the rivers that flow northward to the Murray River and those that take more direct southward routes to the ocean. This divide is called the Greal Dividing Range on many maps. However it is not a range in the strict sense but a complex of plateaus, ridges and corridors. There is certainly no mountainous range, for example, at the Kilmore Gap, north of Melbourne, where the divide is only 335 metres above sea-level. This feature is important, however, because traffic on the Melbourne-Sydney railway and the Hume Highway passes through Ihis low corridor. In north-eastern Victoria, the divide stans to swing to the north. It continues close 10 the coast throughout eastern Australia 10 the top of Cape York Peninsula. The south-easlern section of Ihese uplands is generally quile rugged . The central zone from MOUn! Buller 10 beyond Mount Kosciusko in New South Wales is sometimes called the Australian Alps. This is because il includes all the mountains of Australia thaI are over 1650 metres high. Bedrock throughout the uplands is made up of various Palaeozoic sedimentary, igneous and metamorphic rocks. There are many areas of bare rock at high altitudes. Most rivers flow along deep rocky valleys. Only a few, such as the Goulburn, are accompanied by alluvial flats that are more than a few hundred metres wide. The Central Victorian Uplands are high because they have been lifted relative to the rest of Victoria by slow, periodical, teclonic movements. These involved upwarping and block faulting movements that started in the Early Cretaceous and have continued to the present day. It is conven ient to describe the East Viclorian Uplands and West Victorian Uplands separately and in each case, there are several sub-units.
Figures 3-\ 1 Mount Loch in the Viclo';.n Alps. This is one of lhe highest mountains in Victoria. It is capped by a remnant of a Tertiary basalt. now. The slopes are folded Ordovician sedimenlary rocks. The basalt displays well-
.
,
•.
...
EAST VICTORIAN UPLANDS This unit consists of a great variety of landforms, which provide some of the most outstanding scenery in Victoria The differences in the landforms result from many factors, including the past and present climate, past earth movements, structures in the rocks and variations in the erosion and weathering of rocks.
Geomorphology
Figure 3-12 The dJ1linage pallern over the Mount Buffalo gJ1lnite plateau. Streams tend to follow a rectangular pattern over the plateau because they have developed along major joints in the granite.
67
Much of the country consists of deep steep-sided valleys separated by high narrow ridges. It is mostly covered by native forests. There is also some nat to undulating country, which has been partly cleared for grazing. Small towns are mostly confined to the broader valleys, e.g. Seymour, Corryong, or to the fringes of the region, e.g. Wodonga. A few settlements are associated with recreation and tourism, e.g. Mount Buller. Others arose as centres for gold mining in the nineteenth century, e.g. Beechworth, Harrietville. There are only a few all-weather roads crossing the East Victorian Uplands. These connect towns in Gippsland in the south with towns along the Hume Highway and the Murray R i v er valley in north-eastern Victoria. Most of the roads follow the valleys of the larger streams, (e.g. the Omeo H ighway along the Tambo River). However, a few rise into the hills and pass along ridges or over plateaus (e.g. the Gelantipy road north of Buchan).
There are three subdivisions in the East Victorian Uplands:
Upland Plateaus
f
-
Il '
Figure 3-13 An aerial view of the Mount Bow Baw plateau. A rectangular drainage pattern has formed over the plateau after streams developed along joints in the Baw Baw Granodiorite. One strong set of joint directions is seen running from the top left to the lower right of the photograph. The elevation of the plateau varies from 1000 metres to 1563 metres above sea-level. Organic soils in peat swamps are a feature of the area (photograph by N.J. Rosengren).
In some parts of the Uplands, the skyline is noticeably nat over relatively small areas. There are plateaus and groups o f hilIs with summits at about the same level. These level areas represent the remnants of ancient land surfaces that once extended over wider areas. They have been lowered a little by erosion since their original formation. The highest and oldest, nat to undulating plateaus are called high plains. Their surface was formed at least 200 million years ago. Around Mount Baw Baw and Mount Buffalo, the plateaus are formed on granitic rocks, whereas parts of the Bogong High Plains are on basalt. Granitic rocks usually display patterns of joints and fractures, which follow certain preferred directions. The rocks weather most easily, where air and water penetrate along the joints. Erosion commences in the weathered rocks and this leads to the development of mountain streams. This control of drainage by joint systems is illustrated in Figure 3-12 and Figure 3-13.
68
Chapter 3
The soils on the high plains are young and immature, that is their profiles are not divided into distinct horizons. They generally contain much decomposed plant material or peat, giving typical organic soils. These organic soils are very important as they store a large amount of water, which is released slowly throughout the year. This ensures some water reaches catchment dams elsewhere in the Uplands at all times of the year. There are also some stony gradational soils, mainly near the plateau margins. Some of the high plateaus have been used for summer cattle grazing. However, in recent years, there h as been increasing Government action to restrict their usage to recreational pursuits only, such as bushwalking and skiing. This has been done to protect water catchments and distinctive high plains flora, and to reduce soil erosion. A lower level basalt plateau north and south of Gelantipy in East Gippsland, however, is cleared and used for grazing throughout the year.
Dissected uplands This is the largest region in the State; it extends from JUSt east of Melbourne in a north-easterly direction to the border with New South Wales. Between about 300 million years and 150 million years ago, Victoria underwent a prolonged period of erosion. This produced a generally low-lying, fairly flat land surface. After this period, the land, which now forms the Central Victorian Uplands, began to rise. The former sluggish streams, crossing broad Valleys on the old plain, became more active and started to cut valleys into the underlying rocks. The uplift did not occur at the same consistent rate over the whole period. Rather most of the movement was restricted to a few, intermittent, faster stages. As a result, in some places a younger valley cut down into the bottom of an older, wider, valley. This produced a feature called valley in-valley form. It is very common in the uplands, e.g. in the MacaJister River valley north of Glenmaggie.
Figure
3-14
VaHey-in-valley form along
Wembee Gorge, near Bacchus Marsh.
The effects of two periods of erosion are seen. The first produced the upper, broader valley. Following uplift of the land west of the Rowsley Fault, the upper pan of the Werribee River was rejuvenated. This produced the present-day, deeper, narrower, steep-sided valley. (After E.S. Hills, Physiography of Victoria, 1940).
In the steep country the soils are mostly young due to the continual movement of weathered material down the slopes. There are also some low granite plateaus, where the dominant soils are distinctly red, e.g. Strathbogie Ranges. There are alluvial flats along the main valleys in the dissected uplands, usually with alluvial fans along their sides (Figure 3-\5). The soils developed on the fans are thin, gradational and often stony as a result of the continual downsloP7 movem c:nl. On the valley floors, the soils have developed on accumulations of allUVIal matenals deposited during several different periods. They occur on the flood plain or on pairs of terraces, which are at the same level on opposite sides of the valley. The higher the terraces, the older is the soil.
Figure 3-15 AUuvial fans along a major valley.
river
Fast-flowing streams descending from the mountains carry large Quantities of eroded rocks. These streams lose their momentum where they meet the lowland open valleys and so they drop the coarse gravels in overlapping fans along the edge of the mountains.
Geomorphology
69
In north central Victoria, the northern margin of the Uplands meets the Riverine Plain. There, the country is lower and less steep and the climate is drier than in the north-eastern mountains. There are stony gradational soils on the steep ridges, but duplex soils dominate elsewhere. The oldest soils are duplex typeS cOnlaining ironstone, which were formed two million years ago in the Pliocene. In places, they are partly covered by a younger duplex soil, which has the older soil as its parenl material. Duplex soils are also widespread in the undulating to hilly country. They have red clays on the rises and yellow clays in the valleys and on colluvial slopes. Some features of different landfnrms, which occur in the dissected uplands and the upland plateaus of north-eastern Victoria, are shown in Figure 3-16.
(""High Plains')
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ran
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STONY GRADATIONAL SOil
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RED DUPlEX SOIL
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.
o
0
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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5
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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.
H
H
f ..J :::> ..: u.
..J ..J W o
..: o
,
H
I. ,
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
'5 !
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
o
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
:.:a;,.Jlt....
OCEAN
TERTIARYQUATERNARY
PAINCF:S
{ {D
D D •
ItIG
D •
� � J-
Swamp allUVium 8asall (valley flows). scoria 8asall Sand. alluvium. limestone Volcanic cone o ,
5 ,
KILQMETRES
10 ,
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.
•
• •
.
'
*. 0 ..
D��'"
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<.,.... (OCI - Now I
.4. N
89
Because of the commerciaf and recreational importance of the coast, it is inevitable that human acti vities wiU modify its form in many places. For instance, shipping channels may have to be deepened and wharves and other harbour installations be built to accommodate passenger and cargo ships. However, no changes should be carried out until an investigation is made of the geomorphic forces that act in an area. It is essential to know how they will affect and be affected by the new instaUations. Unfortu nately it is very easy for human activities to cause great damage to the coast. They may lead to a total loss of sand from beaches or harbours being filled with silt. For people to derive maximum long-term enjoyment from coastal recreational areas, these zones should be left in their natural state as far as possible. Vegetation should not be removed and construction should be kept to a minimum (see also Chapter 7).
COASTAL PROCESSES
The forces acting from the sea are provided by waves, tides and currents. Each type is discussed in turn.
Waves
Waves are generated by wind blowing over the surface of the sea. There are twO kinds influencing Victoria's coast: • •
swell waves: these are produced by storms in the Southern Ocean and affect the whole Victorian coastline; local waves: these are generated by local winds and they only appear along part of the coast.
The swell waves approach the Victorian coast from two directions. West of Wilsons Promontory, they come mainly from the south-west and east of the Promontory from the south-east. Although the swell waves appear to move across the open ocean, the individual particles of water do not actually travel towards the land. Instead they move with a circular or orbital motion. The diameters of the orbits are greatest at the surface of the ocean and they decrease with depth. At a certain depth, there is no disturbance of the water - this is known as wave base. Approaching the land, the wave motion becomes elliptical at the wave base. There, frictional drag retards the movement of the water mass and produces breakers (Figure 3-54). If the coast is indented, waves will meet shallow water off a headland before they reach a beach at the head of Figure 3-54 The movement of water in swell waves. A s swell waves move over the sea towards the coast, the waler itself does not move towards the shore until breakers occur. The water particles in the deep water move in a circular or orbital pat h down to the wave base. Below lhe wave base there is no movement. As the wave approaches the shore, the particle movemen t changes to an eUiptical path, especiaUy near the wave base. This produces frictional drag, eventually causing the wave crest to break. The water particles then move on to the shore and spread over the beach. The wavelength between crests decreases towards the shore and the height of the waves increases.
Deep w81er
o
Shallow water
Waler parlk:le motion
a bay. This leads to the water being retarded near the headlands first. As a result, a curving of the original straight wave crests develops. Wave refraction, as this is called, is a major factor in coastal development. It concentrates wave attack on headlands, while their force is reduced in adjacent bays (Figure 3-55). Erosion of the headlands wears away material, which is deposited nearby as spits or in adjacent bays as bars or beaches. Coastal cliffs are usually undercut by the action of waves, thus removing support for the cliff faces and eventually causing them to collapse (Figure 3-56).
Tides Tides are the alternate rise and faU of the surface of the sea caused by the gravitational pull of the Moon and, to some extent, the Sun. There are two high tides and two low tides every interval of about 24 hours and 50 minutes. In Victoria the tidal range, that is the vertical distance between high and low water, is usually I to 1 .5 metres.
90
Chapter 3
Figure 3-55 Wa\o'e refraction along a coastline. On the right hand side the waves are parallel and aU the particles have equal energy. As the waves approach the land, they first encounter shallow water off the headlands (A, N). There, the particles meet the sea-floor causing sections of the waves 10 slow down. The reduction in the velocities of the waves near the headlands causes them to bend and concentrate their attacks on A and A'. The panicles in the waves heading for B and B' meet the sea-noor later and their energy is dispersed along the beach.
Tidal ranges of up to 3 metres may occur, however, when the Moon and the Sun are aligned with the Earth and pulling in the same direction. This can happen in an area of confined sea, such as Western POrt. These are called king tides. Exceptionally low tides, which occur when the Moon and the Sun are pulling in opposite directions, are called neap tides. The movement of tidal water into a bay or inlet is a flood tide: the movement out is an ebb tide. These movements carry sediment on to tidal banks and also produce scour channels (Figure 3-65).
Figure 3-56 Entrance to Loch Ard Gorge, near Port Campbell. Small nicks can be seen at the base of the cliffs at the entrance and also along the left hand side of the narrow bay. These have formed because swell waves continuously wear away the soft sedimentary rocks and undercut the cliffs. (Photograph by NW. Schleiger).
Currents Currents are long movements of bodies of water as distinct from the orbital movements of waves. They are of three kinds: •
• •
lidol
currents, resulting from the movements of tidal water; ocean currents, which are generated by wind or differences in water density caused
by variations in temperature or salinity; longshore currents, which are produced by winds blowing along a coastline. These currents can move sediment from one locality to another. The movement is called longshore drift (Figure 3-53).
Land-related processes Wind not only generates waves and currents. It also moves sand from beaches to dunes and pushes dunes and sand sheets inland.
Geomorphology
Figure 3-S7 Mouth of the Pow lett River, near Kilcund., south-west Gippsland.
The Powleu River and its tributaries drain country as far inland as Korumburra and Leongatha. It flows slowly in a south-westerly direction across an alluvial plain and enters Bass Strait near Kilcunda. The river on the right is diverted eastwards near its mouth by a high sand barrier. Where the river meets the ocean it has dropped most of its sediments to form a river-mouth bar. (Photograph by G Wallis).
91
Onshore fluviatile processes also affect coasts in various ways. Streams bring sediment to the coast, where it is redistributed by waves, currents and wind to form beaches, offshore bars, tidal flats and dunes. Many slow-flowing Victorian streams drop most of their sediment when they reach the sea. The sediment forms river mouth bars, e.g. Powlett River at Kilcunda (Figure 3-57). Water seeping from the land lubricates cliff materials. Clays, in particular, may then slump and be carried away by waves and currents.
COASTAL TYPES There are three main kinds of coast in Victoria. These are dominated respectively by: cliffs (Figure 3-58 (A, B»; • sand barriers (beaches and dunes) (Figure 3-58 (0»; • marshes, tidal banks and mangrove swamps (Figure 3-58 (C». •
Coasts formed by cliffs Victoria has some picturesque coastal cliffs. They vary in shape, height and length. The controlling factors of cliff development are the types of rocks present and the geological structures, the direction and strength of waves, and the previous geomorphic history of the area. The main geological formations in the Victorian cliffs are:
aeolianite •
at many localities, including parts of the south-west coast and the BeBarine and Point Nepean peninsulas (Figure 3-59);
basalt •
•
as fresh, dark grey rock. e.g. west of Ponland and along the southern part of Mornington Peninsula; as deeply weathered lavas and tu ffs, e.g. around Phillip Island;
Tertiary sedimentary rocks •
•
as flat-lying limestones and marls, e.g. Warrnambool to Princetown. Anglesea to Torquay; as ferruginous sandstones, e.g. at intervals between Brighton and Momington;
Lower Cretaceous sandstones and mudstones •
along the Otway coast and from San Remo to Kilcunda;
granite •
around Wilsons Promontory and in far eastern Gippsland;
Palaeozoic sedimentary rocks •
as folded sandstones and hales at Cape Liptrap and near Mallacoota and as limestone on Waratah Bay. south o f Walkerville.
The indented Pon Campbell coast is an outSlanding example of the illfluence of previous geomorphic history. The coastal plain limestone is riddled with caves and sinkholes and intersected by numerou joints. Wave erosion has picked these features out, producing many rock stacks, coastal indentalions and sea caves. The rock stacks are pillars of solid limestone. They were left after seas invaded caves in
Chapter 3
92
J)ammanl Coastal Form.iIi
,
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••••••• Barrier ond other smut." bca('h(·.�
---
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,
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'-. ..
.
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Figure 3-58 Dominant coastal forms along the Victorian coast. There are cliffs along much of the western Victorian coast, whereas sandy beaches, often backed by sand dunes, are the commonest coastal fo rm in Gippsland. Salt marshes mainly occur around Western Port and Corner Inlet.
figure 3-59 AeoUanite or dune limestone cJjff face, behind the beach at Barwon Heads. Large-scale cross· bedding is visible. This formed as the coastal dunes migrated downwind during the Pleistocene. (Photograph by N . J . Rosengren).
/�
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the cliffs and wore away most of the walls. Eventually the roofs of the caves collapsed and the rock debris was removed by waves. The rock stacks are lefJ as small islands that the sea and winds will evenrually dest roy. Along this sectiort of the coast, there are different geological formations at the top of the cli ffs at di fferent places. Figure 3-61 shows some of the variations that can be seen in the cti ff profiles. The Otway coast is cut in Lower Cretaceous sandstones and mudstones. The forms of both its broad-scale and local fearures are controlled by geological structures. such as the dips of bedding, faults and jointing. The overall straightness of the coasl
._
Geomorphology
93
Figure 3-60 London Bridge, west of Port Campbell. (a) London Bridge in 1976 consisted of two natural arches cut by the ocean through horizonla! layers of Tertiary Iimeslone and marls. The formation is intersected by many near-venical joints and fractures. Over a long period, the force of Ihe ocean swell waves anacked these weaknesses and slowly eroded away Ihe adjacent rocks. As erosion proceeded and the origin a! cliffs were undercut, blocks of limestone would have fallen into the sea. Gradually caves became arches Ihrough Ihe headland. (phOlograph by N.J. Rosengren).
(b) Early in 1990, Ihe narrow arch, closer to Ihe mainland, collapsed crealing a new sea passage and an island. AI some future lime, Ihe ouler arch will probably also collapse and leave I wo new rock Slacks. (Pholograph by N�V. Schleiger).
from Lome to Cape Olway is due 10 a major raul! offshore. However, many folds and fauhs cuning across Ihe coasl have controlled the panern of coastal erosion. Bays have developed either belween IWO fairly close faults or within the axial zones of broad synclines. Narrow sandy be.aches have formed at the heads of the bays. The cliffs are very steep and often 100 metres or more high. Shore platforllls extend seawards from the base of the cli ffs (Figure 3-63). They are exposed between the high and low tide levels. The platforms are worn level because ahernate welling and drying of the sand tones cause them to frel. The fragment are removed by the waves. The outer ramparls of the platforms are constantly wet and thus they do not weather to the same extent as the platforms (Figure 3-58 (8».
Sand barrier coasts These coasts are characterised by longshore accumulations of and, which constitute the sand barrier (Figure 3-58 (D» . Behind the barrier there may be a lagoon, which is open to the sea t hrough entrances or ·passes. The lagoon is therefore subject to tidal innuence , but protected from the main force of the oce.an waves. Sometimes a lagoon has been closed off from the sea to form a lake or swamp. An e.xample
94
Chapter 3
Figure 3-{i) Variations in cUff profiles along the Port Campbell Umestone coast. (a) Near Lake Gillear the cliff face is rough owing to the differential weathering of hard and soft laye" in the cross· bedded Pleistocene dune limestone (aeolianite). Karst features, such as caves and sinkholes are common. Waves have cut a narrow rocky shore platform near sea·level and a notch in the foot of the cUff. (b) Buttress Point - aeolianite forms the upper pan of the cUff and it overlies Miocene marine limestone. The aeoUanite is weathered; it breaks up readily producing numerous loose blocks and smaller fragments, which tumble down the cliffs to form talus cones against the foot of the cliff. (c) Flaxmans Hill - the cliff in Miocene limestone is about 90 metres high. At headlands the cliff is being attacked by high seas. Some boulders accumulate in the sea. (d) Peterborough - the vertical Miocene limestone cliffs are topped by red·brown Pliocene sandy clays. The tops of the cliffs are bevelled. where the soft clays are eroding and being washed over the edges. In places Recent sand dunes cap the cliffs. (e) Pan Campbell - the cliffs at the headlands on either side o f the inlet plunge into deep water. Beaches are absent and the cliffs retreat by undercutting and collapse. The rubble is spread over the sea·floor. Sands at the top have a gently·sloping surface. (I) Clifton Beach - the cliff zone is up to 500 metres wide and consists of chaotically distributed blocks of aeolian ite from the top of the cli ff. These blocks have broken away and slid over the underlying calcareo us clays, some eventually arriving on the narrow sandy beach.
(d)
(b)
(e)
(1)
(e)
EZJ �
RECENT PLEISTOCENE PLIOCENE
Dune limestone (aeolianite) Clays
�
Calcareous clays
�
EARLY TERTIARY
Dune sand
� �
�
MIOCENE
Limestone
is Lake Reeve, which is a long narrow strerch of water to the east of Sale. Most of the coast between Corner Inlet and Cape Howe on the New South Wales border is a sand barrier. It includes the Ninery Mile Beach. There are also barrier coasts near Nelson in the south-western corner of the Stare and near Inverloch. In detail. the barrier consists of an ocean beach. behind which is a line of sand dunes. Behind the dunes are gemly sloping sand flats. called lVashover fans. These extend to the edge of the lagoon. The profile of the barrier beach varies with the weather co ndi ti ons . In winter. when the waves are very strong. the profile is broadly concave. During periods of relative calm. the beach is built up and a berm or bench
JjLak� . .. GIII�
WARRNA.M8QOL
sheet. Geological Survey of Victoria.
. ....:. ..:: :. ....
Tal u s cone
f'"r
Figure 3�2 Locality map showing where the cliff profiles in Figure 3�1 can be seen. From Colae 1 :250 000 geological
.
!
u·1 !
.....
3
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__
RECENT
REC£NT-PLEISTOCENE
PLEISTOCENE
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d ------______
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8..alt
e
PUOCEN[ MIOCENE
-
b---d
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3_
s.. elltl OIo/oIe a
Geomorphology
95
Figure 3�3 Shore platform at Crayfish Bay, near Cape Otway. Waves have concentrated their attack on the base of the headland. They have gradually scoured away the rocks at sea-level and so progressively undermined and eroded the overlying rocks. Cliffs are receding inland leaving a platform, which is slowly becoming wider. The plat form is fu lly exposed at low tide and covered by water at high tide. Prominent joints and bedding planes in the Lower Cretaceous sedimentary rocks are visible. A beach has formed at the head of the nearby bay where the wave action is less vigorous. (Photograph by N.J. Rosengren)
Figure 3�4 Kate Kearney Entrance, Corner Inle� South Gippsland. The entrance is cut through the long sand barrier, which separates Corner Inlet from Bass Strait. Inside the barrier, there are extensive mud islands and mangroves. These are separated by a system of shallow channels, which merge into a deeper main channel at the enlrance. Sediment has been deposited inside and outside the entrance to fo rm a ran shaped tidal delta. The outside delta forms on the ebb tide, when the sediment carried by the outgoing tide loses its energy in the open ocean. The inside delta fo rms on the flood tide. It spreads out and is not aifected by ocean wave action and swell.
is formed a little above high tide level. The seaward face o f the berm is steeply sloping. The sea often cuts into this face at high tide leaving a low sand cliff or nip. In the zone between the high and low tide marks there is often a series of beach cusps. These are low, tooth-like ri dges pointing to the sea. They are separated by shallow troughs formed by the action of swash, (the movement of water up the beach), and backwash, (the return movement of water to the sea). Behind barrier beaches, there may be a single dune, called ajoredune, or a series of dunes parallel to the coast. In front of the main dune system there is usually a low dune only partly ftxed by vegetation; this is an the incipientjoredune. If vegetation on the dunes is disturbed by fire, storm or deliberate clearing, a blowout of sand is likely to occur. A trough is sometimes excavated or a new dune, shaped like the letter 'U', is formed. The arms of the dune face upwind and the axis is parallel to the coast or at an angle to it. There are several excellent examples at Lakes Entrance. There are two main sections in the south-east Gippsland barriers - these are the Ninety Mile Beach and Corner Inlet sections. The Ninety Mile Beach extends from St. Margaret Island LO Lakes Entrance. Before construction of the artificial passage at Lakes Entrance, there was an intermittent natural entrance jusl east of the township. The sand barrier is almost continuous, the only other breaks being at Seaspray and Jack Smith Lake. In the Corner Inlet section the barrier consists o f a string o f islands, which are separated by entrances from the sea. Each entrance has an inner trunk tidal channel, which links u p with a strean1 coming from the land. In the lagoons behind the islands, the tidal channels branch to form a complex network of smaller channels separated by tidal banks. The main channel is deepened appreciably at the entrance by concentrated tidal scou r, then shallows again as it continues seaward as the outer channel. Submarine sandbanks are deposited on each side of it. The inner and outer tidal depositional complexes together form a tidal delta (Figure 3- 64).
Marsh coasts
ReClnt
I
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. :� ..' : � :::;:'
k llom... , " eo.,tal ,wamp. includlna lallm,r,,, and manorO¥"
� B.,Ch••, dun••. b.. eh L-...3 rldge• • nd I.nd M.t. )I I - !:::;j'./( Tickol chlnnell -. -
Coastal marshes form in areas t hat are protected from the direct impact of open ocean waves by headland , islands or sand barriers (Figure 3- 58 (C» . The most extensive marshes are in Western Port and Corner Lnlet. There arc also many smaller occurrences around Pan Phillip Bay, e pecially on Ihe more sheltered western side. A marsh has an outer fringe of mangroves. These are large shrubby plants which grow in mud between the high and low tide marks. The mud i deficient in oxygen . Therefore the mangroves put up numerous aerial roots so they can breathe. These are important sediment traps. The mangrove fringe is therefore very imponanl in two ways - it encourages the seaward growth of the marsh and also protect the shore from wave erosion.
96
C hapter 3
Figure 3�5 An aerial photograph of tidal banks and scour channels in Western Port, offshore from Tooradin. The dark areas along the coastal fringe are mangrove forests. Compare this photograph with the central pan of the map in Figure Hi6). (Photograph by Department of Property and Services).
Figure 3�6 Marsh coas., northern end of Western POri. The outer fringe of this coast near the towns of Warneet and Tooradin consi ts of mangroves and salt marshes. This coastline is advancing, because sediments caught in the mangroves build up and more mangroves move into the shallow water ofT the new shoreline. Conversely where mangroves are removed, as they often were in the past, the sea advances on the land. Behind this zone are either freshwater swamps or sand flats. Offshore there are extensive mud flats, which are exposed at low tide. Creeks from the land become tidal channels across the swamps and mud flats.
o
kilometres
Behind the mangroves is the marsh proper, which is flooded only occasionally by the sea. This zone usually has an outer belt of shrubby salt-tolerant plants called halophytes and an inner zone of smaller halophytes dominated by the annual, Salicomia. Large quantities of plant material are produced in a marsh. The sediments are consequently peaty clays and sands, or even peal. Landwards, a marsh gives way to grassland or tea-tree shrubland and eucalypti Banksia forest. Numerous channels cross the salt marshes. The wider and deeper ones connect streams from the land with major tidal channels offshore. They are often used as passages by small fishing boats. Both the major channels and their minor tributaries are usually flooded twice a day at high tide (Figure 3-65 and Figure 3-66). Marsh coasts are fragile. They are easily disturbed by engineering works such as drains, jetties, marinas, dredging and artificial infilling, and also by the grazing of stock. These practices often resul! in local erosion, which may become severe. They may also cause changes in coastal processes elsewhere along the coast leading to undesirable erosion or deposition there.
3
Recent
Pleistocene
Tert i�ry
1 ." I W�HJ?{:l I 8 _
Freshwater swamps and alluvium
Coastal swamps including saltmarsh and mangroves
Older sand dunes, beach ndges and sand-flats.
Sedimentary rocks - sands. clays. gravels. limestones
marls.
Geological H istory of Victoria
Chapter
97
4
GEO LOG ICAL H I STORY OF VICTORIA
Figure 4- 1 Some of Ihe oldest rocks i n Vicloria - scallered outcrops of Cambrian "olcanics in the Mount Camel Range.
The view is looking soulh along a low range of hills Ihat extends northward from Heathcote to Colbinabbin and beyond, in central Victoria. Mount Camel is on Ihe skyline. The rocks are lavas, which were erupted from volcanoes on an ancient sea-noor around 550 million years ago. At the surfaoe the rocks arc usually weat hered 10 a brown colour, but at depl h t hey arc dark green hence they are commonly called 'greenstones' . (Photograph by J.F. Bilney).
This chapter traces the history of the geological events that produced the rocks and landscapes found in Victoria today. This history has been interpreted from many kinds of geological tudies. Fossils indicale the ages of many edimentary rocks, ana sedimentary structures are used to identify the conditions under which these rocks formed (e.g. marine, glacial, elc.). Radiometric daling establishes the ages of various igneous rocks. Geological field relationships show the order in which geological events occurred in many areas, e.g. a granite is younger than adjacent sedimentary rocks if it contact metamorphosed them. Just as there are usually gaps in the recorded history of any past civilisation, so too the geological history presented here is incomplete. There are two main reasons for this: I . Past erosion has removed all traces of many rocks that were once present over
parts of Victoria. 2. Some rocks were buried beneath younger formations and their existence has not
yet been revealed . In the future, deep boreholes may reveal these previously unrecorded format ions. For the reader to un derstand all the events that occurred during Victoria's geological history, he or he must be able to picture the greal changes in the
98
Chapter 4
environment that took place over long periods. Three things in particular must be recognised if the reader is to appreciate the full significance of the events that will be described: I . Australia is not a static landmass that has always occupied the same place on the Earth's surface throughout geological time. All the continents have intermittently drifted over the surface of the Earth. At different times Australia was closer either to the Equator or to the polar regions than it is today.
2. Parts of the Earth's surface are continuously rising or subsiding. There were limes when the landmass now known as Victoria was higher above sea-level than it is today, and other times when it was partly or entirely below the surface of the ocean. Those rises and falls depended on whether forces in the Earth at a particular time were either squeezing or stretching the crust. In particular, there were occasional periods of intense deformation known as orogenies, that uplifted and crumpled parts of the crust to form mountain chains. Orogenies caused great changes i n the geography of many regions over relatively short periods of a few million years.
3 . There were times in the past when Victoria was either hotter or colder, drier or wetter than it is today. This happened not only because Victoria has occupied different positions on the Earth's surface, but also because there were considerable changes in the climate over the whole globe from time to time. In Chapter I , the concepts of plate tectonics and drifting continents were introduced. According to these ideas, ever since the Earth cooled sufficiently to form areas of continental crust, parts of this crust have been moving around. Sometimes the pieces split apart a., d sometimes they met and joined. It is difficult for geologists to reconstruct all the movements during the very long period of Pre-Cambrian time. However, from the start of the Palaeozoic era, it is easier to develop plausible reconstructions of where the continents were at any particular period of geological time. Throughout the Palaeozoic, Australia lay on one edge of the great supercontinent, Gondwana. The eastern side of Australia formed part of the coastline of Gondwana. Through the Late Palaeozoic, Gondwana progressively collided with other continents, eventually to form the very large landmass known as Pangea. In the mid-Mesozoic, Pangea began to break into smaller continents. This process of separation continued into the Cainozoic and resulted in the pattern of continents that exists today.
MAJOR GEOLOGICAL DIVISIONS OF AUSTRALIA
The Australian continent can be broadly divided into two parts, which have different geology and different landscapes: •
•
the the
A ustralian Craton or Shield Tasman Fold Belt
The boundary between these two areas is shown in Figure 4-2 .
I
\
N.T.
AUSTRALIAN
Over the Australian Pre-Cambrian Craton, Pre-Cambrian rocks occur either as extensive outcrops or beneath a cover of soils or younger sedimentary or volcanic rocks. The cralOn is composed largely of younger Pre-Cam brian (proterozoic) rocks. It also includes several blocks of older Pre-Cambrian (Archaean) rocks, the largest being the Pilbara and Yilgarn blocks. The Tasman Fold Belt consists of strongly folded Palaeozoic sedimentary and volcanic rocks and many granitic intrusive rocks.
I
I
Figure 4-2 Major geological divisions of Australia.
W.A.
PRE-CAMBRIAN '
\
,-- - - - _
I
o
I
Kilometres
1 500 I
S.A.
L
_,
CRATON
Geological History of Victoria
99
Australian Crato
A craton or shield is any extensive, stable region of a continent and it is made up mostly of complex crystalline (i.e. igneous and metamorphic) rocks. The metamomhic rocks are usually strongly deformed. The crystalline rocks may be partly covered by a relatively thin cover of younger, shallow water sedimentary and/or volcanic rocks, that are flat-lying or gently folded. Cratons are usually areas of low relief, because they have been subjected to erosion over periods of many hundreds of millions of years. The Australian Craton consists of granites and metamorphic rocks, together with sedimentary and volcanic rocks, all of Pre-Cambrian age. These are overlain in many areas by younger geological formations. The latter are often flat-lying and range in age from Cambrian to Cainozoic. For example, over a large pan of the craton in the Nonhern Territory and western Queensland there are horizontal layers of Cambrian limestones at or near the surface. In the south of the continent, the craton in the Nullarbor region is covered by nearly horizontal Tertiary sediments. This shows that since the Pre-Cambrian, portions of the Australian Craton have periodically fallen below sea-level. However, most parts of the Australian Craton were not subjected to any major folding or faulting from the Cambrian onwards.
Tasman Fold Bel
A fold belt is a long, relatively narrow zone of folded sedimentary and volcanic rocks, developed along the margin of a continental craton. The folded rocks are intersected by numerous igneous intrusions and may be broken in many places by large thrust faults. Fold belts are formed along crustal plate margins. Early in their history, they usually form high mountain chains. Later, like cratons, they are worn away by erosion. The edge of a craton may grow in a seaward direction through geological time as fold belts are welded on to it. The Tasman Fold Belt extends down the whole eastern side of the Australian continent. It consists of folded Palaeozoic sedimentary and volcanic rocks and a large number of granitic intrusives. No mine shaft or drilli ng machine has penetrated through the full thickness of Palaeozoic rocks in Eastern Australia. The Tasman Fold Belt was affected by several orogenies in the Palaeozoic and Early Mesozoic. The mountain ranges formed by the orogenies were eroded away long ago. The Great Dividing Range, which extends almost the length of the Tasman Fold Belt, was uplifted by earth movements within the last 100 million years. Subsequent erosion has reduced the height of the Great Dividing Range well below that of mountain belts formed by recent orogenies, such as the Himalayas and the Andes. Victoria lies entirely within the Tasman Fold Belt and there are no Pre-Cambrian rock outcrops in this State. The nearest are in South Aust ralia (e.g. Mount Lofty Ranges near Adelaide), ew South Wales (e.g. Broken Hill), King Island and north we tern Tasmania.
PR E-CAM BRIAN HISTORY OF TH AUSTRALIAN CRATON
The Pre-Cambrian accounts for over 86010 of the total recorded geological time. It started with the origin of the Earth, about 4600 million years ago, and lasted until the start of the Cambrian period, about 560 million years ago. Brief descriptions of some aspects of the Pre-Cambrian history of Australia are given below. The Pre-Cambrian era is composed of two major subdivisions:
Proterotoic: the period from about 2500 million years before present to the beginning of the Cambrian period, about 560 million years ago. the period from the initial consolidation of the Earth's crust (about 4600 million years ago) to the beginning of the Proterozoic. In Australia there is also a subdivision of the Proterozoic into Early and Late times, the boundary being at about 1350 million years before present.
A rchaean:
Archaean
The Archaean part o f the Australian Craton is restricted to the western half of the continent Many Archaean rocks are metamorphosed sedimentary and volcanic rocks. Some volcanic rocks are called komaliites. They are unusual, because they were erupted at high temperatures around 1700'C and are composed largely of olivine.
100
Chap1er 4
Archaean rocks of Ihe Yilgarn Block in We lern Australia contain importanl mineral deposits, e.g. gold al Kalgoorlie and many olher centres, and nickel at Kambalda and l£inster.
Proterozoic
Some of Australia's mosl valuable mineral deposits occur in Early Prolerozoic rocks: • iron ore deposils in the Pilbara region, Western Aust ralia; • uranium deposils al Ranger and labiluka in the nonh of Ihe Nonhern lerrilory; silver-lead-zinc ores at Broken • H ill (New Soulh Wales) and Mounl Isa (Queensland); • Ihe large copper-gold-uranium deposil al Olympic Dam near Roxby Downs, Soulh Aust ralia.
Life i n the D
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Figure 4-3 Oell) Dome-shaped stromatoUtes in the Jay Creek Limestone of Cambrian age. This oUlcrop is beside Ellery Creek i n Ihe MacDonnell Ranges, wesl of Alice Springs, Nonhern Terrilory. The size of Ihe slromaloliles can be gauged from Ihe camera lens cap, which is seven centimetres across. These stromatolites are very similar to species which occur in nearby Pre-Cam brian limestones.
Figure 44 (right) Modern algal stromatolites, Shark Bay. \\'estern Australia. The stromatoliles are exposed to the air al low tide and covered by the sea a l high lide. (photograph by B. Logan).
After this Archaean crust fonned, it was buried deeply in the Eanh's crust. There, the volcanic and sedimentary rocks were melted and recrystallised under conditions of high temperatures and pressures. This metamorphism produced granites, schists and gneisses that form much of the Archaean craton. Later these rocks were uplifted, and after prolonged weathering and erosion they are now exposed at the surface.
During the Early Proteroto;c, thick deposits of sediments and volcanic rocks accumulated along the edge of the Archaean craton in long marine basins called mobile belts. These rocks were deformed, intruded by granites and metamorphosed. Eventually they were welded to the earlier Australian Craton. The strongly metamorphosed Archaean and Early Proterowic rocks include complex belts of gneisses and schists. In many cases, these rocks bear little resemblance to the original sandstones, mudstones and igneous rocks, from which they were formed.
During the Late Proterotoic, a shallow sea extended across the central part of Australia from nonh to south. In South Australia, thick deposits of sandstone and mudstone, as well as dolomite and limestone, accumulated on the slowly-sinking floor of this sea There was also a period of glaciation, when tillites were deposited by advancing and retreating glaciers. These rocks were later folded and faulted, but not buried deeply enough 10 become strongly metamorphosed. They are now found through South Aust ralia and the Northern Territory, e.g. in the Mount Lofty Ranges near Adelaide, further to the north in the Flinders Ranges, and in the Roper River region, west of the Gulf of Carpentaria.
One of the Late Proterozoic sandstone formations in the Flinders Ranges contains well-preserved impressions of some of the oldest known invertebrate life forms on Earth. They include jellyfish and worms that lived in shallow seas. In some limestone beds, there are column-like structures, mostly under a metre high. These are called stromotolites (Figure 4-3). They were built by colonies of minute organisms called cyanobacteria or blue-green olgae. Cyanobacteria have also been recorded from Archaean rocks in Western Australia The oldest stromatolites are from a locality called Nonh Pole in the P ilbara district. They are about 3500 million years old. Perhaps the most remarkable thing about cyanobacteria is that there are living stromatolites at Shark Bay on the Western Australian coast that look very similar to the oldest fossil representatives (Figure 4-4). This shows that cyanobacteria have probably been living i n Australian waters for at least 3500 million years. At the close of the Pre-Cambrian, animals with shells, (e.g. brachiopods), appeared in the shallow seas. Prior to this Pre-Cambrian animals lacked hard parts. This change was important, because shells allowed various animal groups to adopt new ways of life and to spread into previously unoccupied areas of the sea-floor. Furthermore, shells are more easily preserved in rocks than the soft parts of animals. As a result, Palaeozoic sediments are much more fossiliferous than Pre-Cambrian sediments.
Geological Hislory of Victoria
I NTRODUCTION TO THE PHAN EROZOIC
1 01
The end of the Pre-Cambrian saw the development of the Tasman Fold Bell. The southern section of this belt, covering Victoria, most of Tasmania, part of south eastern South Australia and much of central and southern New South Wales, is called the Lachlan Fold Bell (Figure 4-2). From around 560 million years ago to 380 million years ago, large areas of the Lachlan Fold Belt were covered by the sea Thick sequences of sediments and volcanics built up on the sea-floor. During this long period, there were several orogenies, when the rocks in some regions were subjected to intense squeezing forces. These orogenies caused extensive folding and faulting of the sedimentary and volcanic rocks. and uplifted some areas to form mountain ranges. D ifferent regions were affected by orogenies at different times, but the overall result of these events was gradually to build extra land on the eastern side of the Australian continent. Most of the area covered by the Lachlan Fold Belt became dry land during Late Devonian and Early Carboniferous times. In Victoria, there was much explosive volcanic activity during this period. Following the volcanism, rivers deposited considerable thicknesses of sediment in broad valleys. During the Late Carboniferous and Early Permian, south-eastern Australia was affected by glaciation. The glaciers advanced from regions to the south, which are now i n Antarctica, and covered large areas of Victoria. Apart from this glaciation, there was little geological activity in Victoria from about 350 million years ago until the Cretaceous, around 100-120 million years ago. During the Early Cretaceous, there were earth movements o f a different kind to those that occurred during the intermittent Palaeozoic orogenies. Instead of being compressed, the crust was stretched and eventually split by tensional forces. These movements separated the Antarctic and Australian continents, which were joined previously as parts of Gondwana. Associated with this separation there were many volcanic eruptions, and thick sequences of sediments were deposited in basins formed by the split. At much the same time, there was uplift of the south-eastern Australian Highlands, including the Central Victorian Uplands. Throughout the Cainozoic, Australia drifted northwards away from Antarctica . During this period, Victoria remained relatively quiet. There was, however, considerable subsidence and continuing sedimentation in some areas, particularly in offshore basins to the south. Periodic eruptions of basaltic volcanoes occurred over the last 40 million years. These culminated in extensive volcanic activity over the last four to five million years, mainly to the west of Melbourne. Thus Victoria has had a long and varied geological history, spanning some 560 million years, even though this represents only the last pan of Australia's total gtOlogical record. Victoria's history will now be examined in detail by looking at eight consecutive time intervals, starting with the oldest. It should be kept in mind throughout this chapter, that there is often uncertainty among geologists as to how certain geological events should be interpreted. Different geologists may have different opinions on many topics. Future discoveries of new fossils or new outcrops, or revised radiometric dating of igneous rocks. may change some of the conclusion presented here.
PALAEOZOIC ERA
Cambrian
The Palaeozoic (from the Greek words for 'ancient life') lasted about 280-320 million years. It is subdivided into six periods, the first being the Cambrian. The word Cambrian is derived from Cambria, the Roman name for Wales, where sedimentary rocks of this age are common. Estimates about the age of the start of the Cambrian vary from 530 to 590 million years ago. However, the end of the Cambrian is generally agreed to have been 500-5 10 million years ago. During the Cambrian, the land was barren but life teemed in the sea. Animals with shells, such as trilobiles, brachiopods, bivalves and gastropods, appeared during this period. In the United States of America many strange, soft-bodied, marine animals have been found as carbonaceous impressions in shales. One of them, Hallucinogenia, had seven pairs of strut-like legs. These fossils indicate that animals with twenty-five di fferent body plans existed in the Cambrian. However, only four designs survived beyond the end of the period. Most living animals are descended from these four body plans. Red and green algae were common in the Cambrian seas, along with cyanobacteria. The green algae were probably the ancestors of all Ihe green land plants present today.
102
Chapter 4
DISTRIBUTION AND MAIN ROCK SUBDIVISIONS The oldest rocks in Viooria are Cambrian in age. They con ist of various sedimen tary and igneous rocks, which have been metamorphosed to varying degrees. Cambrian rocks are found at many localities in the State, although the areas of outcrops are frequently small (Figure 4-5). Their greatest extent is in the West Victorian Uplands and along river valleys in the Dundas Tableland.
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palaeogeography. Palaeogeography is a Lerm meaning 'ancient geography'; it refers LO Lhe relaLive posiLions of land and sea at the time. AparL from the 5t Arnaud beds i n western Victoria, most Cambrian rocks are restricted to small areas of outcrop. However, Cambrian sedimentary and volcanic rocks probably lie deep beneaL h younger rocks almost everywhere in VicLOria.
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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.
shows the northern end o f the Moun t Cam el Range where the
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Geological History of Victoria
1 05
Figure 4-7 (right) Strongly folded Cambrian shales near the Knowsley East Fault.
This exposure is about 10 metres high: it is in the Rochester Shire quarry at the northern end of Mount Carnel Range. The rocks were probably deformed during the Middle Devonian Thbberabberan Orogeny. (photograph by NW. Schleiger).
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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.
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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
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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.
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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
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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.
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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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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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,
.
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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
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y
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114
Chapter 4
II is called the Benambran Orogeny because its effects can be seen in rocks exposed around Benambra in north-eastern Victoria. The Benambran Orogeny caused major changes in geography. Areas in both western and eastern Victoria were uplifted to form dry land. As a result, the large deep water basin that existed throughout the Ordovician across central and eastern Victoria was reduced in size to two large inlets extending from the ocean to the. south-east. Besides the changes in geography, the Benambran Orogeny also had deep-seated effects. In eastern Victoria, east of the Mount Wellington Greenstone Bell, this orogeny caused the Ordovician sediments to be folded and faulted_ Most of the folds trend west-north-west to east-south-east. In addition, the h.eat and pressure associated with the deformation caused low-grade regional metamorphism of the more deeply buried areas of sedimentary rocks. There were no granite intrusions at the time, however. The original clay minerals of shales were changed into chlorite, muscovite and biotite, so these rocks became mica schists. The quartz of the sandstones was recrystallised slightly to form quartzites. These schists and quartzites outcrop ex1ensively in north-east Victoria as a broad belt from Ensay, north-east of Baimsdale, through the Bogong High Plains to Wodonga Further north, they continue into New South Wales as far as Wagga Wagga. The entire zone is called the Wagga Omeo Melamorphic Complex. Later, during the Bowning Orogeny at the end of the Silurian, the schists were intruded by granites. Higher grade metamorphism then changed the schists to gneisses. In central Victoria the Benambran Orogeny apparently had little effect. Silurian turbidites were deposited directly on the Ordovician sequence with no apparent break in sedimentation or change i n water depth. However, an area to the west, along the western margin of the Ordovician basin, was uplifted to form dry land.
S i l u rian to M iddle Devonian
The Silurian is named from another ancient Celtic tribe, the Silures. who occupied part of Wale . The term Devonian was introduced after rocks of that age were first described in the county of Devon in south-west England. The Silurian extended from between 420 and 440 million years ago until the start of the Devonian between 400 and 410 million years ago. The Devonian ended about 360 million years ago. During the Silurian, new species of most groups of animals, except the trilobites, appeared to replace tho e that became extinct at the close of the Ordovician. Brachiopods, bivalves and gastropods became very abundant, and corals and stromatoporoid (unusual-looking calcareous sponge ) built large reefs in shallow water. Swimming animal , mostly predators, were common. They included squid like ammonoids with coiled hells, as well as many species of fish, which lived in both frc h and salt water. Some fish were covered by bony armour and had well-developed jaw with teeth. One large species \Va up to 10 metres long. I n the mid-Devonian, harks and ray- finned fish appeared. The latter were the ancestors of most modern fish. In the Late Silurian, upright plant began to appear on land, although they were probably confined to wet, marshy areas. These plants had stems that contained specialised tissue to transport water and nutrients upwards from the root . The earlie t of the e plants were e sentially simple stems. but later they developed long, narrow lea\'es. ear the end of the Devonian, there was another mass extinction of animal pecies. Many pecies of brachiopods, ammonoids. corals, stromatoporoid and fish disappeared. Land plant were little affected, however.
DISTRIBUTION Silurian and Early Devonian sedimentary and volcanic rocks outcrop extensively in three separate region (Figure 4-17): I.
m>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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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
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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 ... , nn �il er Ide 0
Within the Grampians Group there is also a thin-bedded unit with some mudstones known as the Silverband Forma/ion. The mudstones are dark red due to the presence of the iron oxide mineral, hematite. Such rocks are called red-beds. Many sandstones in The Grampians are also reddish because they contain hematite. Red-beds usually fonn under arid conditions, so that the climate at this time in western Victoria may have been dry. Some sandstones wilhin the Grampians Group were deposited by extensive braided river systems. These sandstones show well-developed cross-bedding, that formed as the sandbanks in the river channels migrated downstream (Figure 4-27). From the cross-bedding it is possible to work out the direction in which the sandbanks were moving, and therefore which way the braided rivers were flowing. In cross-bedded sandstones high on the Wonderland Range and Mount Rosea, the current directions are mostly from the west. This suggest that the sand was derived from the erosion of the rocks in the Delamerian Highlands.
Figure 4-27 (right) C ross-bedding within sandstones of the Grampians Group, on the summit of the Wonderland Range.
This cross-bedding was formed by sand dunes migrating downstream within the channels of a braided stream.
Figure 4-28 (a bove) Skolithos in sandslones from the
Grampians Group. Skolithos are narrow venicaJ burrows (hat were probably occupied by marine worms. They have been found in rocks from Cambrian to J u rassic in age. U n fortunately Ihe worms I hemselves have never been found preserved as fossils. Skolithos bur rows are most common in sandstones thai were deposited along and just offshore of beaches. II is unlikely that many animaJs could have su rvived in this environ ment, which was characteriscd by pounding waves and shifting sands. Organisms that could adapt. like the Skolithos worms, had little compelition from other animals, and so t hey were often prescnt in large numbers. Sandstones with vcry abundant Skolithos burrows are somct imcs called ·pipe-rock'.
Other beds within the Granl pians G roup were deposited in shallow seas. This is shown by the presence of abundant, narrow, vertical bu rrows, which were probably worm tubes. These burrows, called Skolirhos, are often closely-spaced and stand out on weathered surfaces (Figure 4-28). They give the rocks a distinct ive spOiled or lined appearance. Elsewhere in the world, Skolithos burrows are found associated wilh shallow water gastropod and brachiopod fossils. This proves the Skolithos an l ma i s usually lived in shallow-water sands, o ffshore from a beach . The Si lverband Fortllation provides additional evidence that part of the Grampians Group formed in shallow seas. Many sandstone beds in this formation show symmetrical wave ripples, similar to those that form on tidal flats. The interbedded mudstones frequently have patlerns of mud cracks. indicating that they were deposited in very shallow water that often d ried up completely. In addition. the mudstones occasionally contain shells of lingulid brachiopods (Figure 4-29). These animals usually lived along coast lines, close to the mouths o f rivers.
124
Chapler 4
Taken logether, these fealUres indicate thaI the Grampians Group was deposited along the western edge of a shallow sea Fast-mOving braided rivers, flowing eastwards from the Delamerian Highlands, carried large quantities of sand into the shallow sea. The sand was then moved around by tides and waves and burrowed by worms. There must have been considerable subsidence in the Grampians Basin to allow a thickness of 6000 metres of sediment to accumulate there. The sedimentary structures just described prove that the Grampians Group was emirely deposited by rivers or in shallow water. Despite the considerable subsidencr no deep water basin developed in the area. Enough sediment was brought conlinually into the basin to fill the space created by the subsidence.
' -
mm
Figure 4-29 Lingula borungensis, an Early
Devonian brachiopod from Ihe Grampian.> Group. Lingula is an inarticulate
brachiopod. It is general ly less Ihan five centimel res in lenglh and has IWO Ihin, nallened shells Ihal are almosl equal in size, These shells are composed of chilin impregnated with calcium phosphale or Calcile. The oldesl species of Lingula are Cambrian in age. These arc very similar 10 Ihe species alive loday, Modern-day Lingula lives buried in Ihe mud and sand of lidal flats al river moulh . It i anchored in Ihe sediment by a fleshy slalk. Mo I fos il pecies probably inhabiled similar environments.
Figure 4-30 A specimen of Rockl ands Rhyolile from Ihe Rocklands Dam area. This is an Early Devonian acid lava wilh small cryslals of quarlz and feld par in a \ery fine-grained groundmass. This pecimen shows \\ell-d.-eloped flow banding. The folding was nOI I he resuh of laler compression, but formed when the lava was still liquid. Becau e the rhyolite lava was viscous, if formed folds as il flowed o\'er and around obslacles, (Pholograph by
W,D,
Birch),
Rocklands Rhyolite
A fler the Grampians Group rocks were deposited, there were extensive rhyolitic volcanic eruptions, mainly to the west of The Grampians. The eruptions were of two kinds: explo ive, forming ash deposits, and quiet, forming lavas. They resulted in an extensive, bUl now poorly outcropping, sheet of interbedded rhyolite lavas and ash layers, known as the Rocklands Rhyolite. Until recently, it was thought that the volcanism occurred before deposition of the Grampians Group. However, recent research has shown the Grampians Group is the older formation, Rh)Qlile lavas often display a feature known asj/OlV banding (Figure 4-30). This consists of thin parallel layers of contrasling colour, which are frequently twisted and contorted. Rhyolite lava is viscous and therefore flows lowly. As the lava gradually solidifies, crystals form within the molten rock. Flow banding is produced when the crystals are smeared Oul into thin bands by the slowly-moving lava These lavas occur within the Rocklands Rhyolite around Rocklands Reservoir, where the flow banding is well-displayed, Fossi ls in the Grampians G roup
Fossils are uncommon in the Grampians Group, although brach iopods, fish remains and small crustaceans called osrfOcods (Figure 4-3 1) have been found in the Silverband Formation, There are also some large fossils re embling tree lrunks in sandstones near Mount William (Figure 4-32). These were probably made by algae, as trees did not exist in the Early Devonian; they only evolved in the Late Devonian. Olher evidence of life has been found on slabs of sandstone used to pave a courtyard at a homeslead near The Grampians. The urfaces of these slabs reveal a variety of tracks and trails. The sizes and shapes of lhe markings indicate that mo I were probably made by arthropods, including trilobites. However, one set of poorly-preserved tracks looks like small footprints made by a four-legged vertebrate. This animal is Ihought to have been an amphibian. It was probably about one metre long, and may have resembled a mall crocodile with a short tail. I f these tracks have been correclly identi fied, Ihey represent Ihe oldesl amphibian foolprints known anywhere in the world. However, no other fossil evidence of Ihe animal that made Ihese fool prints has been found.
Geological History of Victoria
Figure 4-3 1(a, b) (below) Paraparchitts devonicus, an Early
Del>'onian ostracod from the Gmmpians Group.
aQ;>
o
1 mm
. ---''
Ostracods are tiny crustaceans related to crabs and prawns. They have two hinged shells made of calcite or chitin and are usually less than 5 millimetres long. The outsides of the shells may be smooth (as in this example) or ornamented with ridges, lumps O r spines. Ostracods live in both salt and fresh waler. They are commonest in shallow seas, where they may Occur in great numbers. OSlracoos appeared in the Cambrian and have gradually become more diverse since then. There are more species present today than at any lime in the past.
Fi "ure 4-32 (right) Fossil "logs" from near Mount William in The Grampians. These fossils look like tree trunk and are aboUl the same size and shape. However, they lack the internal structure of trees, such as growth rings. Instead they lVere probably formed by large algae. This accounts for the scaly-looking surfaces of the "logs", Many are parallel 10 each other, suggesting they lVere pushed around by the stfong river currents that deposited the sandstone.
Figure 4-33 (right) Li fe in a hallow ilurian sea. In the warm, shallow waters of pans of the COlVombat Rift, there were abundanl corals (I), algae (2) and brachiopods (3). Starfish (4) and trilobites (5) Were active on the sea-floor. Crinoids (6) grew in densely packed clumps. As they died, the crinoids built up banks composed largely of stem material. Swimming overhead were jellyfish (7) and nautiloids (8) (shown here \\ ith a slraight shell), along lVilh armoured fish (9) and conodont animals (10). The laller looked like swimming worms. The shallow seas in the Early Devo nian would have appeared Quite imilar 10 the illu tration here, excepl thaI Ihe species of animals present would have been d i fferent. (After original drawing by R. I. Molesworth).
125
1 26
Chapler 4
FAUNA AND FLORA Victoria lay close to the Equator at the beginning of the Silurian. It gradually moved southward during the Silurian and by the Early Devonian was at 40'S latitude, close to its present position. Temperatures throughout the world then may have been warmer than today. as even at the poles there appears to have been little ice. Victoria probably experienced a tropical to warm temperate climate throughout the Silurian and Early Devonian, as corals were abundant here during the Early Devonian. At similar latitudes today. the water temperature is too cold for corals to survive.
t ern
figure 4·34 CyaJhophyl/um, an Early Devonian rugose coral from limestones in the south-eastern corner of the Melbourne Trough. Two groups of corals were present in the Palaeozoic, rugosans and IObulates. Rugose corals lived as colonies or as single individuals, called solitary corols. which were generally shaped l i ke a cow's horn. Solitary rugose corals like Cyathophyl/ulll lived with the tip o f the horn embedded in soft sedimem on the sea-floor. A cross-section of a rugose coral shows many thin radiating plates called septa inside the outer wall. The septa are composed of calcite; they helped to strengthen the coral skeleton.
figure 4·36 A spiriferid ar tic ulat e brachiopod from the Early Devonian Walhalla Group, near Eildon within the Melbourne Trough Most brachiopods lived on sha1low sea-floors in quiet waters. In general they could nOl move around. Like lhe species illustrated here, they lay on muddy ,cdiment but were nOl firmly 3tL3Ched to it. However, species Ihal lived in very shaPow water affected by waves and currents somel imes ccmcnred themselves LO the sea-Ooor. (Photograph by l.A. Talent). .
figure 4·35 Favosites, a common fabulate coral in (he Early Devonian Buchan Group. All tabulate corals were colonial. They typically formed large, rounded coral heads (sh own here). up to one metre across. The coral heads were composed of large numbers of tubes closely packed together. Eaeh tube, called a comlfile, housed an indi vidual coral animal. In cross-section, the coralliles were approximately hexagonal in shape; they gave the coral the appearance o f a honeycomb. I ndeed. Favosites is often called a honeycomb coral. Tabulate corals lacked the obvious internal radiating plates (hat weTe characteristic of rugose corals. However, the corallites were subdivided by more or less horizontal plates called tabulae. These are clearly visible when the coral is cut down the length of the corallites (see longitudinal section). The tabulae give the tabulate corals their name.
Cross· se c tio n
[
/9
em
Side-on view
10 \
enlargement Corallite
9
1 mm
r
Longlt.udinal
section
Geological History of Victoria
127
Life flourished in the shallow seas (Figure 4-33). Banks of solitary and colonial rugose and tabulate corals grew on the sea-floor (Figures 4-34, 4-35). The muddy and sandy bottoms were inhabited by abundant brachiopods of many different types (Figures 4-22, 4-29, 4-36), together with some bivalves, gastropods and echinoderms. The echinoderms included groups with living representatives such as crinoids and starfish, as well as strange extinct forms, such as cystoids (Figure 4-23). Trilobites crawled or swam over the sea-floor. Floating overhead there were various graptolites. Conodont animals (Figure 4-38), nautiloids and armoured fish hunted their prey in open water. In deeper waters, there were a few animals on the sea-floor in areas where currents kept the bottom waters oxygenated. These were small colonial and solitary corals: brachiopods and some echinoderms. Soft-bodied animals, probably worms, burrowed in the soft muds. In the Silurian and Devonian, graptolites were less abundant and diverse than they were in the Ordovician. Many types of graptolites had died out in the major extinction at the end o f tne Ordovician. Those that survived were dominated by the monograptids, which have skeletons consisting of a single row of cups (Figure 4-39).
Figure 4-37 (a bo ,'e) Barag wanathia /ongijolia, a Late
Silurian and Early Devonian plant from the Wilson Creek Shale in the Melbourne Trough. This was the largest and moSt advanced land plant at the time. Barag wol1 orhia was discovered in Victoria and named after W. Baragwanath, a former Director of the Geological Survey of Victoria . The oldest occurrence of Barag wanathia in the world is from the Melbourne Trough, where it is fo und associated with Late
Silurian graptolites at a locality near Yea. Barag wanalhia remains have been fo und elsewhere, but they are all o f Early Devonian age. Barag wanathia probably grew i n shallow coastal swamps. The stems were not st rong enough to stand upright, so they lay along the ground. This specimen carne from the 19 M il e quarry on the Warburton-Mat lock road. (Photograph by J.F. Bil ney).
128
Chapter 4
This was the last group or free-swi mming or floating graptolites. However, bOllom dwelling graptolites survived into the Early Carbonirerous. The youngest graptolites
2 ,
mm.
found in ViC10ria are Early Devonian monograptids from sediments in the Melbourne Trough. In COntrast to the abundant life under the sea at this time, only a few species were able to live on land. At first there were only plants, but probably by the Early Devonian, the first terrest rial vertebrates appeared. Tidal flats were covered by algal mats, and there were primitive plants growing on exposed coastal sandbanks and in nearby marshes. one of these plants was more than two metres high. MoS! were simply leafless stems, except Baragwanathia, which had a branching stem covered i n short, thin leaves (Figure 4-37). The four-legged amphibian, recorded from The Grampians, also l ived near tne swampy flats. The hills further inland were probably bare, apart from occasional patches of algae or lichen.
Figure 4-38 Ozarkodina, an Early Devonian conodont found in l imestones i n the Melbourne Trough and Ihe Buchan Basin. Conodonts look like tiny teeth; they rormed the mouthparts or an animal that resembled a swimming worm, 5-10 centimetres long. Conodonts arc composed or calc ium phosphate, which is not soluble in dilute hydrochloric acid. Thererore conodonts can be extracted rrom limestone by dissolving the rock in hydrochloric acid. Calcite, which rorms most or the limestone, is rairly quickly dissolved, leaving a residue containing the conodonts and insoluble mineral grains, such as quam. The earliest conodonts, or Late Pre-Cambrian age, were cone shaped. They evolved rapidly into more complicated shapes like the one illustrated here. Conodonts became extinct in the Late Triassic.
BOWNING OROGENY At the close of the Silurian, eastern Victoria was afrected by a major deformation known as the Bowning Orogeny, which faulted, folded and uplifted the sediments and volcanics of the Cowombat Rifl. In central Victoria, the south-eastern corner of the Melbourne Trough was evidently uplifted at about the ame time, because pebbles of greenstone are found in Early Devonian sediments deposited in this area. However, elsewhere the Bowning Orogeny had lillie effect within the Melbourne Trough . Early Devonian sediments directly overlie the Silurian strata with no obvious break. In western ViC1oria, movement on the boundary faults of the Grampians Basin probably started during the Bowning Orogeny. The Early Devonian Grampians Group sediments accumulated in the subsiding trough. In north-eastern Victoria, metamorphism and granite intrusions were associated with the Bowning Orogeny. The Ordovician sediments in the area, which had been metamorphosed into low-grade schists during the earlier Benambran Orogeny, were subjected to even higher temperatures and pressures. The schists were changed into gneisses, and new minerals were formed, including cordierite, andalusite and sillimanite. At about the same time, (400 - 420 million years ago), many large granite masses were int ruded in north-eastern Victoria, (e.g. around Tallangalta, Koetong and Corryong). There were also some smaller intrusions in East Gippsland. The laller granites form the southern part of the large complex, Kosciusko and Berridale batholiths, which outcrop widely in south-eastern New South Wales.
TABBERABBERAN OROGENY Another major period or deformation, the
Tabberabberan Orogeny,
affected most
of Victoria around 385-395 million years ago. The Tabberabberan Orogeny was the final slage in the formation of what is now the south-eastern part of the Australian continent. It was probably the most dramatic geological event ever to occur in Victoria and it c�used major changes in the pauerns of sedimentation. Uplift associated with the ore !env caused the sea to retreat to the east, 0 that marine sedimentation ceased throughout Victoria. A major mountain range, the Tabberabberan Highlands, wa formed in eastern Victoria. By the end of the Tabberabberan Orogeny, the eastern coasl line or Australia was close to its present position. By contrast, at the beginning or the Cambrian it had been over 1000 ki lometres to the wesl in South Australia. Thus over 1000 kilometres of continental crust was added to the south-eastern margin of Australia during the period from about 560 to 390 million years ago. The Tabberabberan Orogeny caused folding and faulting of sedimentary rocks of Cambrian to Early Devonian age throughout the State. There may have been several kilometres of movement on some of these faults, e.g. t hose along the ea tern and
o I
!) !
10 ,
mm
Figure 4-39 Monograptuj' - OJ Silurian graptolilC with a single row of cups.
western sides o r t he Melbourne Trough. In the eastern part of the Melbourne Trough the sediments were folded very tightly. In the west Ihe deformation was less intense, however, and the folds are more rounded and widely- paced. Outside the Melbourne Trough, the Ordovician sediments to both the east and west are tightly folded, orten with angular fold hinges. The folds and raults generally trend approximately north south. However, there is an exception in the northern part of the Melbourne Trough, around Rushworth. There, the rold axes trend more-or-Ie.�s caSI-west. There was extensive granitic int rusive act ivity associated with the Tabberabberan Orogeny. Over 200 separate granite plutons were intruded in separate areas in western and eastern Victoria, but none in the Melbourne Trough (Figure 4-18).
Geological History of Victoria
1 29
Another event probably related to the Tabberabberan Orogeny was the intrusion of large numbers of parallel dykes. They range in composition from quartz-rich to olivine-rich. The dykes are concentrated in belts and collectively each group is called a swarm . The best known swarm extends across the eastern side of the Melbourne Trough between Walhalla and Bonnie Doon, on Lake Eildon. This is known as the Woods Point Dyke Swarm. At some localities, e.g. Woods Point and Ga ffneys Creek, fractures within the dykes are filled with quartz, which carries gold.
Late Devo n i an to Carbon iferous
The Carboniferous was so named because in western Europe rocks of this age contain large deposits of carbon as black coal. Most coal in the Northern Hemisphere was formed in the Late Carboniferous. Australia's largest black coal deposits in Queensland and New South Wales, however, were not deposited until the next period, the Permian. The Carboniferous extended from 360 million years ago until approximately 285-290 million years ago. Apart from the corals and stromatoporoids, life i n the seas recovered quickly from the mass extinction of the Late Devonian. In the Carboni ferous, ammonoids, sharks and ray-finned fish swam in open water, while brachiopods, crinoid , bryozoan s and foraminifera inhabited the sea-floor. On land, plants continued to diversify. By the Late Devonian, some plants had developed bark tissue around the stem. This enabled them to grow as tall trees up to 30 metres high and form the first forests. The extra height allowed their spores to be dispersed further by the wind. One plant group, the lycopods, became the dominant trees in the Late Devonian and Carboniferous forest . Compressed lycopod remains form the bulk of the extensive Late Carboniferous coal seams in Europe. Other plants included ferns, seed ferns and the earliest gymnosperms. Modern representatives of the gymnosperms include conifers (pine trees) and cycads. Many insects lived among the plants. The oldest insects were wingless forms of Early Devonian age. By the Late Carboniferous, flying in ects, including dragonflies, were also present, along with spiders and m illipedes. [n the Devonian, vertebrate animals began to spend some time on land. They were amphibians, that i s creatures that lived partly under water and partly on land. By the Late Devonjan and Carboniferous various four-legged amphibians were present. Many lived entirely on land as adults, but all had to return to water to breed. The largest amphibian superficially resembled a crocodile and was about 6 metres long. The earliest reptiles appeared in the Late Carboniferous. They were small animals that laid eggs. The waterproof shell around an egg enables the embryo inside to survive away from water. As a result, reptiles were able to breed on land. There, they quickly became the dominant vertebrate group.
DISTRIBUTION
Rocks of thi age fall into three groups (Figure 440): l. There are widespread Late Devonian intrusive and related volcanic rocks in central Victoria. Most of the volcanics are very resistant to erosion and so form hills and ranges. Many are well-k nown scenic localities, e.g. Mount Donna Buang and Lake Mountain, or prominent landmarks, e.g. Mount Macedon. 2. Further east, a broad belt of volcanics and sedimentary rocks stretches south east from Benalla to the hills north-west of Bairnsdale. This belt forms some of Victoria'S most remote and peclacular upland scenery and includes Mount Timbertop, Mount Cobbler and Mount Howitt. 3. There are also several small areas of edimentary rocks near Cann River in the ea tern corner of the State.
REGIONAL SETTING This part of Victoria's geological history was completely different to that of earlier periods. Following the Tabberabberan Orogeny, the whole of Victoria became dry
130
Chapter 4
Figure 440 (below) Distribution of Upper Devonian and Early Carboniferous rocks in Victoria. The position of the coastline on the map represents the maximum west ward extension of the sea during this period. For most of the time, the shoreline was probably further to the east.
land for the first time. The eastern coast line of the Australian continent probably ran approximately nonh-south. It was located somewhere to the east of the present New South Wales coastline for much of the time. However, the sea occasionally extended westwards, reaching into Victoria at least once. The Tabberabberan Highlands, a high mountain range, stretched from Orbost to Wodonga and extended into New South Wales. A large sedimentary basin, 40-50 kilometres wide and called the Mount Howill Province, developed along the south-western edge of the Tabberabberan Highlands. On the eastern side of the Tabberabberan Highlands, a broad, flat plain stretched from the ranges to the coast. It was crossed by large, easterly flowing rivers. The Late De\Unian was characterised by very extensive explosive volcanic activity. Although similar eruptions had occurred in the Early Devonian in eastern and western Victoria, the activity in the Late Devonian was more widespread.
UPPER DEVONIAN - LOWER CARBONIFEROUS
·
\
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·
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AREAS OF OUTCROPS
Direction In which the rivers lIowed
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VolcaniC rocks
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VOLCANIC ROCKS The earliest events in central and eastern Victoria during the Late Devo nian were a series of volcanic eruptions, which deposited thick, extensive sheets of ignimbrite . The ignimbrites were erupted suddenly from large bodies of molten magma that had risen very close to t h e Ea rt h's surface. During an ignimbritic eruption, large amOunt of material are rapidly removed from the magma chamber. Thi can cause the crust overlying the chamber to subside, so forming a depre ion called a caldera or cauldron. A caldera is often circular or oval and usually 20-40 kilomelre or more across. Becau e of the subsidence wilhin a caldera, there i a large area where the erupted ignimbrilic material can build up, sometime lO thicknes es of over one kilometre. During the Lale Devonian, ignimbrite calderas developed in two regions of Victoria. There was a broad area, known as the Cell/ral ViCTorian Province, and an elongate area between Benalla and Bairnsdale, called the Mount Howill Province.
Geological History of Victoria
131
Central Victorian Province In the Late Devonian at least six calderas formed across central Victoria. The best preserved are those forming the Dandenong Ranges, the Cerberean and Acheron calderas between Warburton and Eildon, and the northern part of the StraLhbogie Ranges, east of Euroa. The westernmost calderas, (Arthurs Seat on Mornington Peninsula and Mount Macedon), have only small outcrops of ignimbrite. Most of their volcanic material has been removed by erosion. By contrast, the ignimbrites of the Cerberean and Acheron calderas are about 1 200 metres thick. They are inside semicircular faults which mark the edge of the calderas . The volume of volcanics in the Cerberean Caldera alone is about 400 cubic kilometres, enough to cover the Melbourne suburban area to a depth of 150 metres. A large part of these volcanics has been eroded away since they were erupted, so the original volume must have been enormous.
Mount Howitt Province This province is about 130 kilometres long. It formed after subsidence along a major, northwest-southeast trending weakness in the Earth's crust. A very large caldera, about 45 kilomet res wide, developed at the northern end of the province. The volcanics from this caldera extend southwards from Benalla to Mount Timbertop and also outcrop to the west in the Blue Range, south of Mansfield. There are also large deposits of ignimbrites to the south-east, in the high country of the catchments of the Barkly, Macalister, Avon and Mitchell rivers, between Mount Timbertop and Bairnsdale. Some basalt lavas are interbedded with the ignimbrites. These volcanics disappear under younger sediments at lhe southern edge of the ranges, north-west o f Bairnsdale.
INTRUSIVE ROCKS Upper Devonian gran itic i ntrusions only occur in a broad area of central Victoria, except one intrusion east of Mallacoota (Figure 4-18). They are about 370 million years old, and, unlike older granites in Victoria, generally trend east-west. They form the largest, continuous Out crops of granitic rocks in the State, e.g. in the Strathbogie Ranges (sout h-west of Ben alia), and the Cobaw Range (north of Mount Macedon). All the calderas in tbe Central Victorian Province were intruded by granitic magmas, usually very soon after the volcanic eruptions ceased. For example, the dacitic ignimbrite at Art h urs Seat on Morninglon Pen insula was intruded by the Dromana Granite. Mostly, these granites represent the molten material left in the underlying magma chamber after the volcanics were erupted. The fact that the granites intruded the volcanics ind icates that large bodies of molten magma can rise close to the Earth's su rface. In Victoria, the maximum thickness of the volcanics is 1 500 metres. As the top of the volcanics formed the Earth's surface, this means that the granites intruding the volcanics probably rose to less than 1500 metres below the surface. To the west of the calderas, lhere are several Upper Devonian intrusions with no associated volcanICS, e.g. the Harcourt GranodIOrite south 01 l:Iendigo, and the You Yangs Granite nort h-east of Geelong. By contrast, the volcanics of lhe Mount Howitt Province appear 10 have no associated int rusives. However, lhe e volcanics may overlie granites, that have not yet been exposed by erosion. The Late Devonian granites represent the last lime when large volumes of the crust underneath Victoria melted to form intrusions. After the Early Carboniferous, there was little igneous activity in Victoria until basaltic lavas erupted in the Tertiary.
Xenoliths
The Cobaw Granodiorite illustrates a common feat ure of granitic rocks: the presence o f inclusions called xenoliths. These are rounded bodies, usually a few centimetres across, which are often darker and finer-grained than the granile. Many xenoliths are fragments of rocks which once surrounded the granodiorite. They were torn off and incorporated into the magma as it rose. Granites may also contain xenolilhs of older crystallised magma of different composition to the granite. This type was mixed into lhe molten granite deep within lhe Earth's crust. Exactly how this occurred is still being studied.
, 32
Chapter 4
Figure 441 rk grey
large da enolit h in the Cobaw Granodiorite.
A
x
Xenoliths in the Cobaw Granodiorite are very plentiful. The first are recrystallized pieces of Lower Palaeozoic sedimentary rocks incorporated from the magma walls. They are usually tabular in shape, fine-grained and dark in colour. They consist of interlocking crystals of quartz, plagioclase, biotite, muscovite and sometimes cordieritc. The second variety is more abundant, nearly spherical and of igneous origin. They were incorporated into the magma in a different part of the chamber. They are pale to medium grey in colour, medium-grained and consist of pyroxene, plagioclase, biotite and sometimes quartz and hornblende. Those in the photograph are of this type. (Photograph by G.W. Quick).
-,
.r
, ·1
-
I
Figure 442 Granite on the coast at Wilsons Promontory containing numerous rounded dark xenoliths.
Rocks, that contain a large number of xenoliths, have a 'plum-pudding' appearance. (photograph by G. Wallis).
SEDIMENTARY ROCKS Central Victorian Province Thin sequences of sed iments are present in most of the calderas of the Central Victorian Province. Conglomerate , sandstones and mudstones, composed largely of volcanic fragments, occur underlying and interbedded with the ignimbrites. The sediments were deposited by rivers or in lakes that formed within the rapidly subsiding calderas.
Mount Howitt Province The Mount Howitt Province is split into four di fferent areas of outcrop. The southern three areas were originally continuous and formed a long, relatively narrow basin with a large river system nowing down the middle towards the south-east. However, the northern area, extending from Benalla to Mount Timbertop, was a very large, collapsed caldera, separated from the southern basin by a ridge. The rivers draining this northern basin nowed to the north-west. Within both the Mount Howitt and Central Victorian provinces, thin layers of sedimentary rocks occur below and interbedded with the ignimbrites. However, in the Mount Howitt Provi nce, 3000 metres of Late Devonian and Early Carboniferous sediments were deposited over the volcanics by rivers. The thickness of sedimentary rocks exceeds that of t he underlying volcanic rocks. At the base of these overlying sediments are beds of conglomerate, which in places are several hundred metres thick. Conglomerate outcrops can be seen at Parad ise Falls south of Whitfield, and in the Blue Range, south of Mansfield. These conglomerates were deposited on alluvial fans (see Chapter 3). Alluvial fans formed mainly along the south-western edge of the Tabberabberan H ighlands, where streams nowed westward into the large subsiding basins of the Mount Howitt Province. The boundary between the highlands and the basins was a fault line, running south-east from Whitfield.
Geological History of Victoria
133
The conglomerates wi thin the northern basin are composed of material eroded from the Tabberabberan Highlands. Pebbles of ignimbrite are rare. This indicates that the ignimbrites were confined almost entirely within the caldera and did not extend across the Tabberabberan Highlands in this area. Figure 443 Mount Ligar (A) and Long Hill (B). about 80 kilometres north of Traralgon, between the upper MacaUsler and Wellington rivers (looking south). The rocks are sandstones, siltstones and mudstones of the Snowy Plains Formation, a thick sequence of Late Devonian sedimentary rocks overlying ignimbrites in the Mount Howitt Province. The beds dip very gently to the east. (Photograph by N.J. Rosengren).
The alluvial fan conglomerates are overlain by up to 2000 metres of finer-grained sediments deposited by braided and meandering rivers flowing Ihrough the northern and southern basins within the Mount Howitt Province. Outcrop of the e sandstone and mudstones can be seen east of Mansfield at Battery Hill, along the beds of Delatite River and Broken River, and through the high country at the head waters of the Macalister and A on rivers (Figure 4-43). The change from coarse-grained alluvial fan conglomerates to finer-grained sands and muds reflects a gradual decrea e i n the energy of the streams depositing the sediments. This occurred because the nearby ranges had been worn down to lower altitudes. At Ihe same lime the sub iding basins had filled with sediments. A a result, there was no longer such a sharp boundary bel ween the highlands and the plains. Rivers flowing from the hills became slower-moving and deposited sands and muds rather Ihan gravels. The sandstones are mostly the channel deposil of the rivers. They show abundant cross-bedding formed by the downstream movement of sandbanks on Ihe river bed. The mudstones were deposiled on flood plains beside the river channels. These sediments display mud cracks. which formed when the mud dried out after a flood. Occasionally there are ancient soil horizons within the mudslones, containing remnants of rootlets fro m plants growing on the soil. Most mUlbtones and many sandstones and conglomerates are dark red. They resemble the rocks of the Early Devonian Grampian Group. Red-beds (described previously) are more common in the Mount Howitt Province than in The Grampians. Their presence indicates t he climate in Victoria during the Lale Devonian and Early Carboniferous was probably hot and dry, especially away from Ihe coaslline. Within the flood plain mud lones, I here are often Ihin sheels of sand. These were deposited suddenly during floods, as a river spilled over its banks and spread across the flood plain. The abundance of such deposits ind icales Ihal flooding was a regular event during Late Devonian 1 0 Early Carboniferous limes. In dry, arid climates today, rain often falls as short-lived int ense slorms Ihat cause rapid flooding.
134
Chapter 4
The effect of these storms would have been particularly severe in the Devonian and Carboni ferous. At that time, few plants grew on the hills, so there was nothing to stop rainfall running straight imo the rivers and causing bad soil erosion. Flooding would have been very rapid and severe.
East Gippsland R ivers flowing eastwards from the Tabberabberan Highlands towards the sea deposited sediments o n a broad , flat coastal plain. These rocks, which are u p to 1000 metres thick, are found as four separate areas, along the Bemm, Combienbar, Cann and Genoa rivers. Sandstones of the Genoa River Beds form spectacular cliffs at Mount Nungatta to the east of the Cann Valley Highway, a few kilometres north of the border with New South Wales. The sediments accumulated in a similar pallern to those in the Mount Howitt Province to the west, except that they do not overlie volcanics. Layers of conglomerate, up to 10 metres thick, were deposited first. These were overlain by river channel sandstones and flood plain mudstones deposited by large braided river systems. On at least one occasion, the sea-level rose sufficiently for the coastline to come inland as far as the Genoa River, near the Victoria-New South Wales border. One sandstone bed in this area contains Lingula, a small brachiopod that lived in shallow estuaries, where rivers flowed into the sea.
Figure 444 Life on land in the Early Carboniferous. A large river is flowing across the foreground. I n it there are several fish, including a lungfish ( I ) and an armoured fish (2). Growing on the swampy flood plain beyond the river are numerous lycopod trees (3). Underneat h these are mosses, ferns (4) and horsetails (5). Horse-tai ls grew as tall, upright stems, with the leaves coming off the stem in circles. Only a few species of horse-tails exist today: they grow along rivers and in swamps. In the centre of the illustration is a small crocodile like amphibian (6), one of the first animals to l ive on land. In contrast to the plains along the rivers, the drier hills in the background are mostly devoid of vegetation. (After original drawing by R.M. Molesworth).
FAU NA AND FLORA Poorly-preserved plant fragments are found scallered throughout the Late Devonian and Early Carboniferous sandstones and mudstones. Primitive ferns and lycopods are the most common remains. These plants grew on flood plains along rivers (Figure 4-44). Lycopods today are small plants, usually only a few centimetres high, but in the Devonian and Carboniferous they grew as tall trees (Figure 4-45). At least 20 di fferent species of fish lived in the rivers. In a few places, their remains are well-preserved in fine-grained sedimentary rocks (Figure 4-46). Many fish were armoured with thick, interlocking bony plates, which were often decorated w i t h a pattern of raised ridges. Other fish had small scales and were more like those living today. There were also several species of lungfish. By then, there were amphibians on land, as shown by a line of footprints across a sandstone outcrop fOWld along the Genoa River (Figure 1-34). A part from the Early Devonian footprints in The Grampians (described earlier), the Genoa River tracks represent one of the earliest records of vertebrates on land in Australia. Although no bones have been discovered, a study of the footprints suggests that the amphibian was probably less than one metre long and had short legs and tail. Fossils found elsewhere in the world show that it may have resembled a small, thick-set crocodile with a blunt, rounded nose, unlike the long, pointed snout of a modern crocodile.
Geological History 01 Victoria
Figure 4-46
Figure 445 (below)
Culmactmthus stewartii! a Late
Lepidodendron, an Early
Carboniferous lycopod tree from the Mount Howitt Province, eastern Victoria. This diagram shows the bark of the tree. The outside of the trunk was covered with long narrow leaves. As the tree grew, t he leaves on the lower pan of the trunk feU off. The diamond-shaped cars on the bark, (shown in this diagram), mark the points where the leaves were originally joined to the trunk. During the Devonian, plants evolved the ability to grow upright as trees. Early Devonian lycopods such as Barag wanathia {Figure 4-39) could only grow as flat-lying stems. By contrast, Early Carboniferous lycopods such as Lepidodendron were up to 50 metres high with trunks as much as one metre in diameter.
1 35
3 .
Devonian fish from the Mount HowiJI Province. This fish swam in rivers and lakes. II did not have an armoured skin. Instead, it was protected from predator.; by very thick, sharp spines. which supported I he rins. These spines are the parts of the fish most likely to be found as fossiJs. However, at a remarkable locality near Mount Howitt, large numbers of complete fish fossils have been collected. At least ten di fferent species are present, including the one illustrated here. (Original drawing by J.A. Long).
KANIMBLAN OROGENY Towards the end of the Early Carboniferous, about 350 million years ago, there was a deformation known as the Kanimblan Orogeny. This was a wide pread but not very intense event that uplifted central and eastern Victoria To the west of the Tabberabberan Highlands, the Kanimblan Orogeny caused some faulting and folding of the sedimentary and volcanic rocks. This folding split the southern basin of the Mount Howitt Province into t hree almost separate areas of outcrop. Generally, the limbs of the folds are not steeply-dipping except along major faults (Figure 1-67). In East Gippsland the deformation caused four small blocks of Ihe Late Devonian and Early Carboniferous sediments to be down faulted. These are now the only remnants in Victoria of what was originally an extensive cover of sediments to the east of the Tabberabberan Highlands. The Kanimblan Orogeny was the last major Palaeozoic deformation to affecl Victoria. It represented the final stage in the history of the Lachlan Fold Belt. The sedimentary, volcanic and intrusive rocks of the fold belt then formed the eastern margin of the Australian continent. This remained quite stable from about 350 to 120 million years ago, when a quite different tectonic event took place. This will be described later in the Lower Cretaceous section.
Late Carboniferou - Perm ian
The term Permian was derived from Perm, a Russian province on the western side of the Urals. The Permian lasted from between 285 and 290 million years ago until about 250 million years ago. The great continent of Gondwana straddled the South Pole during the Late Carboniferous and Early Permian, and experienced an ice age then. Gondwana consisted of the present-day lands of Africa, South America, Australia, Antarctica and India. Other landmasses along the equator had a tropical climate, similar to that earlier in the Palaeozoic. Life in the shallow tropical seas was similar 10 that in the Carboniferous. Brachiopods and bryozoans were particularly common inhabitants of the sea-floor. On land, reptiles diversified and began to replace amphibians, probably because reptiles could run faster and had better jaws and teeth. One group of reptiles may have been warm-blooded and covered in hair. These animals were the ancestors of mammals. Because of the differences in climate between the poles and the equator, separate floras developed in the two areas. In Gondwana the dominant plant was a deciduous gymnosperm called Glossopteris, whereas the tropical flora consisted largely of evergreen coni fers. Towards the end of the Permian, the climate around the world became increasingly arid, causing gradual changes in the floras. The Permian ended with whal may have been the greatest mass extinction in the Earth's history. This mainly affected groups living in the sea, bUI also included some on land. Many reptiles, ammonoids, brachiopods, bryozoans and crinoids died out, as did the last trilobites and some groups of corals and foraminifera. Land floras were less severely modified.
136
Chapter 4
DISTRIBUTION Outcrops of Late Carboniferous-Permian sedimentary rocks are concentrated in four areas (Figure 4-47). The most accessible exposures are in the Bacchus Marsh area, especially in and around Werribee Gorge and along Lerderderg River, below the gorge Outcrops are also present on the Dundas Tableland, around Derrinal, and between Wodonga and Glenrowan. Rocks of this age have been found also in drill holes and mines in other parts of the State beneath Tertiary sediments or basalt.
LPJ'E CARBONIFEROUS - PERMIAN
I,
AREAS OF OUTCROPS
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PALAEOGEOGRAPHY
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Occurrences of Late Carboniferous - Permian rocks under younger rocks
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Figure 447 Distribution of Lale Carboniferous and Permian rocks in Victoria. On most published maps, these rocks are shown as Permian in age. However, fossil spores and pollen of Late Carboniferous age have been discovered recently within the basal part of the sedimentary sequence. When the glaciers were at their maximum extent in the Early Permian, they reached almost to the Murray River. leebergs floated in the sea to the north. As the glaciers retreated towards the end of the Early Permian, a shallow sea formed in the central part o f southern Victo r ia .
•
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25 50 75 100 ! ! ! Kilometres
,
Originally Late Carboniferous-Permian sedimentary rocks may have covered much of Victoria, but they have been largely eroded away. Where these sediments have been preserved. they are either in down faulted blocks (eg. at Bacchus Marsh) or in deep valleys eroded into the Permian landscape (e.g. Derrinal).
REGIONAL SETTING The Lale Carboniferous and Pennian was a period of relative stability for V ictoria. There was no major faulting or folding. Most of Victoria was dry land, probably consisting of low ranges of hills and occasional mountains. In eastern V ictoria the remnants of the Tabberabberan Highlands possibly formed a higher mountainous region. There was a large, shallow sea to the north. The southern edge of this sea extended up to 50 kilometres south ofthe Murray River into V ictoria. At one stage, another shallow sea covered the. southern pari of the State, reaching as far north as Bacchus Marsh. During the Late Carboniferous and Early Permian, parts of Gondwana, including Victoria, lay close to the South Pole and were largely covered by glaciers. The glaciers began to advance across Victoria from the south during the Late Carboniferous. They reached their maximum extent in the Early Permian, when either an extensive icesheet or a series of smaller interconnected icecaps covered most of the State Some higher mountains protruded through the ice, and icebergs floated in the shallow sea to t he north.. A t that time, V ictoria probably looked much like the coastal areas of present-day Antarctica, with occasional mountain ranges rising out of a largely ice-covered landscape (Figure 4-48). There were several minor advances and retreats of the g.laciers until they finally retreated and disappeared towards the end of the Early Permian.
Geological History of Victoria
1 37
Figure 448 The Victorian landscape during the Permian. The icesheet is in the distance, with glacier tongues flowing out between the hills and mountains along the edge of the sheet. It is mid-summer and the melting ice is feeding braided stream systems flowing away from the ends of the glaciers. The land is covered with rubble of all sizes, from boulder to clay. This material was dropped by the glaciers as they melted. Growing beside the streams are small stands of Glossopteris trees. These were almost the only plants that could withstand the cold conditions close to the glaciers.
Figure 449 'The Str.mger', a large glacial erratic near Lake Eppalock. The erratic is a 100 tonne block of granite deposited on Ordovician sedimentary rocks. It has grooving and striations on its upper surface formed by ice in glaciers passing over it. The term 'erratic' indicate the granite is foreign to the district. 'The Stranger' can be seen from a sign-posted vantage point on the Mclvor Highway between Heathcote and Knowsley. (Photograph courtesy or Geological Survey of Victoria).
The glaciers changed the Victorian landscape great ly. The effects can still be
seen today i n a few places. For instance, glacial pavements are present aJ several localil ies, e.g. around Lake Eppalock (Figure 3-6) and along the Lcrderderg River near Bacchus Marsh. The surfaces of Ihe pavements are covered by grooves or
striations thai
formed parallel 10 Ihe direction in which the ice was moving. In Victoria
the striations are oriented IOwards the north and north-east. This shows the glaciers mUSI have t ravelled from t he somh and soul h-west . When Ihe glaciers fillally melted, I hey left behind the material carried by the ice. This included large boulder ca l led erratics ( Figure �-l9). Ollle erratics are now scallered over parts of Victoria, particularly around Derrina!. Most of the erratics are rocks from nearby outcrops, such as sandstones. mudstones and granites. Others, however, are high-grade metamorphic rocks. These rocks have not been found
anywhere as outcrops in V ictoria. btil I hey do occur loday in Antarctica. This suggests Ihat SOllle Permian erratics in VieJOria may have been carried hundreds of ki lometre by glaciers moving slowly nort hward from mountains in AI1larctica.
138
Chapler 4
Glaciation and the greenhouse effect
Glaciers are large rivers of ice, that creep slowly downhill. They form i n areas where snow is present all the year round. There are no glaciers in Australia now, because the snow that falls in the mountains each winter melts during the summer. However, glaciers are present in high mountainous parts of the world, such as the South Island of New Zealand and in areas close to the North and South poles. Although glaciers are relatively widespread today, for most of the Earth's history they were absent, probably even at the poles. The present Antarctic icesheet only began to form 40-50 mimon years ago. It did not reach its present extent until 1 3 -1 5 million years ago. Australia has been affected by only four periods of glaciation during its long geological history. Two were in the Pre-Cambrian and are therefore not recorded in Victoria. The most recent glaciation in Australia occurred in the Pleistocene, but these glaciers did not extend into Victoria. The Late Carboniferous to Early Permian ice age was therefore the only one to have a major affect on Victoria. What causes glaciers to advance and retreat is not completely understood. During each major period of glaciation, minor advances and retreats of the glaciers occur in a pattern related to variations in the tilt of the Earth's axis and the shape of the Earth's orbit around the Sun. For example, if the orbit becomes more circular, winters are less cold and the glaciers melt and retreat. On the other hand, if the shape of the orbit is more elliptical, winters are colder and more intense As a result, the glaciers advance. These cyclic changes are quite rapid and happen over periods varying from tens of thousands to hundreds of thousands of years. They are known as Milankovic cycles, after a Yugoslav astronomer, Milutin Milankovic, who helped to explain what causes them. However, such rapid, small-scale changes cannot explain the major periods of glaciation in the Earth's history, when glaciers covered a large part of the land surface for millions of years. It seems likely that variations in the amount of carbon dioxide in the atmosphere may have been responsible. This is the so-called 'greenhouse effect'. At present the level o f atmospheric carbon dioxide is increasing because carbon dioxide is produced by the burning of carbonaceous fuels such as wood, coal and petrol. The effect of this increase in carbon dioxide is to reduce the amount of heat rising through the Earth's atmosphere to be lost in space. As a result, the Earth is very slowly heating up. This will eventually cause the glaciers to melt and retreat. I t is thought that there were also changes in atmospheric carbon dioxide levels in the past before humans appeared. These could have been due to volcanic or biolOgical processes, or a combination of both. Small amounts of carbon dioxide are emitted by volcanoes. Thus times of widespread volcanic activity may have caused high levels o f carbon dioxide in the atmosphere. The result would have been higher temperatures at the Earth's surface and an absence of glaciers. Alternatively, changes in carbon dioxide levels may have resulted from biological activity. All animals and plants remove carbon dioxide from the atmosphere to make their body tissues and often their skeletons. if larger amounts of atmospheric carbon dioxide were used up in this way at certain times, the Earth's surface would have cooled because more heat escaped into space. As a result, advances of the glaciers could have occurred. However, the relative importance of the volcanic and biological processes, and exactly how much effect they had on carbon dioxide in the atmosphere, are still being investigated.
ROCK TYPES The Late Carboni ferous and Permian rocks in Victoria were all deposited underneath or close to glaciers. Two main rock types are present: •
pebbly sandstones (tillites); interbedded sandstones and mudstones.
I.
Pebbly sandstones: These consist of pebbles and boulders, up to one metre in diameter, set in a matrix of mudstone or fine- to medium-grained sandstone (Figure 4-50). The pebbles in general do not touch each other. Most are rounded, except where they have fiat, striated faces caused by the grinding action within a glacier. The pebbles appear to have been originally beach or river gravels. They must have been picked up by glaciers as they moved across the landscape. The mud and sand between the pebbles were produced when rock fragments within the glacier ground against each other and the underlying bedrock. The fme-grained material produced in this way is called 'rock flour.
•
Geological History of Victoria
139
The pebbly sandstones were deposited when the glaciers melted and dropped all the fragments and rock flour carried within the ice. Such deposits are called Iil/s. They later consolidate to form the rock tillite. When the ice melts all this material is deposited together. Till is therefore composed of fine and coarse fragments mixed i n a random manner. As glaciers retreat, they leave behind large mounds of till called moraines. These are often quite unstable, because they are saturated with water and not held together by vegetat ion. Large portions of these mounds may suddenly move downslope as mudflows or landslides. Many Permian tiUites in Victoria seem to have slumped in this manner. A thick sequence of tillites is well-exposed in the Bacchus Marsh district, particularly on the slopes of Bald Hill and along Werribee Gorge and Korkuperrimul Creek. Some of these beds display irregular folds. These folds were not formed by tectonic movements, but by a glacier, which advanced over recemly deposited sands and clays while they were still soft. The irregular folds developed where the slow-moving ice dragged the underlying soft sediment along with it. Such folding is present in several different beds within the sediments at Bacchus Marsh. Each bed represents an advance of the glaciers. This indicates that the Victorian glaciers advanced and retreated several times during the Early Permian.
Figure 4-50 Tillite from Werribee Gorge, near Bacchus Marsh. This rock is made up of rock debris left behind by a Permian glacier. II contains boulders of various sizes and rock types in a fine-grained 'rock flour' matrix. (Photograph by L.L. Q uick).
2. Sandstones and mudstones: These rocks for me d in several different sedimentary environments. o Thin sandstone and mudstone bed are occasionally present amongst the pebbly sandstones. These sediments were deposited by rivers flowing northwards from the glaciers and fed by melting ice. The sands and mud were the liner-grained material washed out of the moraines as the rivers flowed over t helll. o
Some sediments were deposited in shallow lakes, that formed behind mounds of till left by retreating glaciers. Thinly-bedded mudstone accumulated on the lake floors. Some mudstones have alternating Iight- and dark-coloured layers. These are called varves. Varves can be seen in outcrops near Coleraine, and along the Lerderderg River near Bacchus Marsh. The light-coloured layers were deposited in ummer and are composed of silt washed into the lake. The dark coloured layers formed in winter, when the lakes were frozen over. The finer grained clay gradually settled out beneath the ice, together with t he remains of any small organisms t hat lived in the lake waters during summer. The carbon from the organic remains helped to give the winter layers their dark colour. Each pair of light and dark layers was deposited over one year. By counting the layers i n a sequence of varved shales, i t is possible to work out how many years it took for the deposit to accumulate.
o
Interbedded sandstones and mudstones that outcrop on the upper slopes of Bald Hill, near Bacchus Marsh, were deposited in a shallow sea in front of the glaciers. At the base of the sequence is a thin conglol1lerate. which contains a few shallow marine fossils. Thi� bed was most l i kely " beach gravel. The overlying sandstone s and mudstones were p robn bly dcp
140
Ch apler 4 with each sandstone - mudstone pair representing deposition over one year. As the ice mehed at the beginning of each summer, the rivers flowing from the ends of the glaciers would have carried large amounts of sand and mud into the shallow sea to the north. Sediment-laden currents moved across the sea-floor from the river mouths and deposited a layer of sand. The movement of the current over the ea-floor caused ripples to form in the sand. These ripples can be clearly seen in many sandstone beds. The finer-grained muds remained suspended in the sea water over summer. During the following winter, the rivers and perhaps also the sea would have frozen over, cutting off the supply of sediment. A layer of mud then settled on the sea-floor over the sand deposited during the summer.
FAUNA AND FLORA The Permian sediments of Victoria do not contain many fossils, because much of the landscape was covered by ice. It was too cold for many plants and animals to survive. Spores (Figure 4-5 1 ) have been extracted from the Permian sediments found in some drill holes, and poorly preserved leaves of Glossopteris have been collected from river-deposited sandstones at Bald Hill. These leaves are tongue-shaped and 10-20 centimetres long. The leaf veins form a distinctive pattern in which they repeatedly fork and rejoin (Figure 4-52). Glossopteris trees grew along rivers and in swamps. The leaves were probably shed each winter, so the trees were deciduous. Although Glossopteris was relatively rare in Victoria, it grew as dense forests covering extensive swamps in central ew South Wales and Queensland. These areas were not covered by ice a the temperatures were probably warmer than in Victoria. The trees there were up to 40 metres high and the trunks up to 1 .5 metres across. They grew close together and had specialised roots to cope with the waterlogged conditions. The leaves and trunks accumulated as thick peat deposits, that later compacted to form black coal seams. These seams are now the major black coal deposits in Australia with many mines in the Sydney and Bowen basins. The few marine fossils found in the Permian sediments in Victoria were almost all collected in the Bacchus Marsh area. They are mostly brachiopods, and only one type is pre ent. However, in the warmer waters to the north in ew South Wales and Queensland, a large variety of animals lived in the hallow eas including brachiopods, bivalves, bryozoans and trilobites.
figu," 4-5 1 Spores extracted from marine Permian sedimenlary rocks oblained from . drill hole in northern Victoria. These pores were probably produced by ferns or horse-Iails, which grew underneat h Glossopteris trees on the Permian landscape (see Figure 4-48). Because spores are smali and lighl, they can be easily carried over long distances by wind. Therefore. spores can be blown offshore and buried in sedimenlS building up on Lhe sea-floor.
figure
o 50 �m 1..____...1
50 �m o 1_____01· 10
4-5 2
Glossopteris. a Permian plant from
Bacchus M."h. The distinctive patlern of veins is shown only for Ihe lower lef! hand portion of Ihe leaf. Some specie of Glosscpreris did nOI have Ihe prominent mid-vein shown in (hi illustralion. These species were formerly known as Gongomopleris. Glosscpreris leave have been found all over Ihe former supercontinent Gondwana, from present-day India 10 Anlarclica.
o
1 0 mm
�
Geological History of Victoria
Continental drift
1 41
The concept of drifting continents appeared in the middle of the nineteenth century, when the first reliable world maps suggested to scientists how certain continents could be fitted together like pieces of a jig-saw puzzle. The mo t detailed evidence for the theory was published in 1915 by Alfred Wegener, a German meteorologist. Some of the most convincing evidence supporting Wegener's ideas came from the Southern Hemisphere. For example, Glossopteris has been found in Permian sediments in the southern parts of Africa and South America, as well as in Madagascar, Antarctica, Australia and India. Plants such as Glossopteris cannot spread across the large expanses of open ocean that now separate lhese areas. Their seeds were several millimetres in diameter, too large to be blown over long distances by strong winds. Wegener's conclusion was that the regions where Glossopteris had been found were joined to each other during the Permian. This led to the suggestion lhal a supercontinent, now called Gondwana, existed at thaI t ime. Additional evidence for the existence of Gondwana comes from lhe distribution of Permo-Carboniferous glacial sediments (Figure 4-53). The directions of glacial ice flows in the southern continents and India can also be explained if the landmasses are fitted together as proposed by Wegener.
Figure 4-53 The maximum extent of the Late Carboniferous-Permian glaciers across Gondwana. The southern continents are restored to the relative positions they occupied when they were part of Gondwana. The arrows indicate the directions of t h e Late Palaeozoic glaciers, as revealed by evidence found in the glacial rocks of this age.
Direction Of movement of glac iers
I I
Maxi m u m extent of Permo C a r boniferou s glaciation
c:::!�':J
DEFORMATION During the Late Carboniferous and Permian, Victoria was stable with little folding Or faulting. However, faulting during the Tertiary tilted the Permian beds, usually
at low angles. The Late Carboniferous-Permian sediments in Victoria rest unconformably on lhe older Palaeozoic rocks of the Lachlan Fold Belt. This unconformity can be seen in Werribee Gorge, where gently dipping tillites overlie strongly-folded Ordovician sandstones and shale .
1 42
Chapler 4
MESOZOIC ERA Tri assic and Jurassic
Figure 4-54 Distribution of Mesozoic rocks in Victoria.
The map also shows Ihe approximate position of the boundaries of the rift valley that extended across southern ViclOria during Early Cretaceous times.
The Mesozoic era (from the Greek words for 'middle life') lasted about 185 million years. It is divided into the Triassic, Jurassic and Cretaceous periods. The name Triassic is derived from the Latin word 'tres', meaning three, because the Triassic rocks in Germany are subdivided into three distinctive stratigraphic units. The Jurassic was named after the Jura Mountains between France and Switzerland, where rocks of this age are well-exposed. The Triassic extended from around 250 million years ago to the beginning of the Jurassic between 200 and 205 million years ago. The Jurassic ended approximately 130-145 million years ago. Representatives of many animal groups died out al the end of the Permian. However, the bivalves and gastropods were not severely affected and [hey were abundant on shallow sea floors in [he Early Triassic. By the Late Triassic, the seas were richly populated by fish, sharks, ammonoids, brachiopods, sea-urchins and hexacorals. The hexaconals, which appeared in the Triassic, are the same coral group that forms reefs today. Large carnivorous reptiles also i nhabited the oceans during the Triassic and Jurassic, some being similar in size and shape to seals and dolphins. Land plants were not great ly affected by the Late Permian extinction. Nevertheless the Mesozoic floras were quile different from the earlier Palaeozoic ones. Ferns and seed ferns were very abundant. The foresl trees were dominated by the cycads, conifers and ginkgos. All three of these groups are present today, although they have become less common. Land animals recovered slowly after the extinctions. In the La[e Triassic, two new groups appeared - dinosaurs and mammals. Many varieties of dinosaurs developed, ranging from ones as small as a chicken [0 others over 26 metres long. The largest carnivorous dinosaurs stood 6-7 metres high. Flying dinosaurs (pterosaurs) were also present. In conlrasl lo the dinosaurs, lhe mammals remained small and uncommon throughout the Mesozoic. The largest was aboul the size of a house cal. Crocodiles, frogs and turtles also became established during lhe Triassic and Jurassic. The earliest bird, Archaeopteryx, was found in Late Jurassic sedImentary rocks. I I teeth and long tail show that i[ evolved from the dinosaurs.
M ESOZOIC r-'Mildura
I I
a II Earlyi Cretaceous rocks EJ [J Tnasslc intruSive D boundanes Cretaceous baSin D ApprOXimate D sed ment ry rocks
,
AREAS OF OUTCROPS
\
,
I i
Tnasslc sedimentary
rocks
JurassIc volcaniC rocks
,
I I I I I I
01
Early
Direction In whIch the rivers flowed
Wodonga
8endigo o Benambra
Yandoit TO
D
Bacchus Marsh
T oO
o !
25 50 75 100 ,
,
,
t Kilomelres
Dinosaur Cove
,
Geological History of Victori a
143
DISTRIBUTION Very few Triassic and Jurassic rocks are exposed in Vicwria. There are minor occurrences of Triassic sedimentary rocks at Bald Hill, near Bacchus Marsh, and near Yandoit, north of Daylesford. A group of Triassic intermediate intrusive and extrusive igneous rocks forms some prominent mountains near Benambra, in eastern Victoria. In western ViC1oria, Middle Jurassic volcan ic rocks outcrop in creek beds and at plateau level on the Dundas Tableland, north-easl of Caslerton. Lale Jurassic dykes were found in many gold mines through central Victoria. Sedimentary rocks of Late Jurassie age underlie Early Cretaceous sedimentary rocks in south-western VielOria. These rocks do not outcrop, bUI have been found in drill holes.
REGIONAL SETTING After the glaciers retreated at the end of the Early Permian, Victoria was dry land until the end of the Jurassic. Low hills probably covered most of the State, with perhaps some higher mountains. These ranges extended southwards into Antarclica, which was still joined to Australia as part of Gondwana. ViC10ria was very stable from the mid-Permian to the Jurassic. There was no subsidence to give areas where sediments could build up, so very little sediment accumulation occurred. However, tectonic activity and sediment accumulation started again in the Late Jurassic and continued into the Early Cretaceous (see later).
TRIASSIC SEDIMENTARY ROCKS Thin deposits of river gravels and sands were deposited in small areas near Bacchus Marsh and Yandoit. They contain poorly preserved fragments of plant fossils of probable Late Triassic age- By contrast, much thicker depo its of Triassic sediments were laid down in eastern New South Wales and Queensland by large river sy terns flowing eastward towards the sea.
TRIASSIC IGNEOUS ROCKS The only Triassic igneous activity in Victoria occurred near Benambra. Several small intrusions of syenite now form prominent hills, such as Mount Leinster and The Brothers. Unusual quartz-feldspar rocks at The Sislers are of the same age (Figure 4-55). There are also smaller areas of trachyte lava flows in the di trict. Syenite and trachyte contain abundant large orthoclase crystals, but little quartz or plagioclase feldspar. Igneous rocks with thi composition are uncommon in Victoria.
Figure 4-55 The islers, Ihree hills composed of Triassic inlrush'e rocks. The hills are IS kilomelres north easl of Omeo in Ihe Bowen Range. The rocks are pale-coloured quartz feldspar porphyry: Ihey consist of large quarlz and feldspar crYSlais in a fine-grained matrix of Ihe same minerals. The Si leT rise 10 over 1200 metres above the lower hill fo rmed by Ordovician sedimentary rocks. The porphyry is part of the youngesl intrusive complex in Victoria. (Photograph by N.J. Rosengren).
JURASSIC IGNEOUS ROCKS In the Middle Juras ic, trachyte lavas erupted on the Dunda Tableland in western ViclOria. These are dark dense rocks when fresh, bUl they weather to a soft brown material. They outcrop in creek valley north-east of Casterton, e.g. Wennicott Creek. During the Late Jurassic, many basic and ultrabasic dykes, moslly less than two metres wide, were intruded into Ordovician sediments in central Victoria. These rocks are composed largely of olivine, pyroxene and feldspar. They are very dark when lhey are unweathered. However, they weat her to a brown clay at the surface.
, 44
Chapter 4
Early Cretaceous
Cretaceous is derived from the Latin word 'creta', meaning chalk. Extensive deposits of chalk (a very fine-grained, white limestone) accumulated across large areas of the Northern Hemisphere during this period. For exam ple, the White Cliffs of Dover, along the south coast of England, are composed of Cretaceous chalk. This period lasted from 1 30-145 million years ago until around 65 miUion years ago. The animal and plant life of the Cretaceous, both in the seas and on land, was a mixture of modern and ancient forms. Many fish were strikingly modern in appearance but they lived alongside a variety of ammonoids and huge carnivorous reptiles up to 1 5 metres long. On the sea-floor, bivalves, gastropods and bryozoans dominated the faunas. On land, dinosaurs continued to predominate. There were enormous herbivorous and carnivorous species, along with pterosaurs (flying reptiles). Mammals were still small and few ill number. Nevertheless the first marsupial and placenta Is, the groups to which modern mammals belong, appeared then, as did snakes. The land plants un derwent a major change in the Cretaceous. Jurassic floras had been dominated by gymnosperms, but they were largely replaced by flowering plants (angiosperms) in the Cretaceous. The sudden abundance of flower producing plants caused a parallel increase in the number of insect species, which fed on the nectar. There was another severe episode of extinction at the close of the Cretaceous. The dinosaurs died out, as well as ammonoids, many marine reptiles, several groups of bivalves and many other marine animals. It has been postulated that this extinction was the result of a comet or meteorite striking the Earth somewhere in the Caribbean Sea. The h uge volumes of dust and water thrown into the air by the impact would have covered the sky around the world. As a result, sunlight would have been blocked out almost completely and the Earth's surface would have become quite cold. The animals that became extinct would have been those lhat could not cope with the sudden cold.
DISTRIBUTION Early Cretaceous sandstones and mudstones outcrop most exten ively in the Otway and Strzelecki ranges, and along the Glenelg and Wan non river valleys in western Victoria. The best outcrops are in cliffs along the Otway coast and from San Remo to Inverloch in south-west Gippsland (Figure 4-54). The sandstones and mudstones weather easily to give deep soils. The landscape developed over these rocks consists of rounded hills with few outcrops.
Figure 4-56 Early Cretaceous sandstones exposed at Johanna Beach, on (he Otway coast. On the left of the pholOgraph, the sandslOnes show well-developed honeycomb weal hering. When sea spray dries on the surface of the rocks, salt crystals form. As these crystals grow, they wedge apart the grains of the sandstone. This forms small pockets in the sandstone surface, that become deeper and wider with l i me. Evcmually honeycomb weat hering is produced. The sandstone on the right hand side of the photograph docs not show honeycomb weathering. This outcrop is morc strongly cemented than the adjacent sandstone, by minerals such as calcite. Spherical "can non-balls", which often weather out of Early Cretaceous sandstones in t he coastal cli ffs, arc also composed of well-cemented sandstone. These bodies are called concretions. They fo rmed after the sandstone was deposited; they arc not large bouldcr!l that were deposited within the sandstones.
Geological History of Victoria
145
REGIONAL SETTING
Figure 4-57 The Victorian landscape in the Early Cretaceous. There are mountain ranges on both sides of the central rift valley. The streams nowing out of these ranges have built alluvial fans along the foothills. Volcanoes are active in the distance. Eruptions of these volcanoes supplied the ash and cinders deposited by the large braided river system flowing through the rift valley. Growing on the hills are tall pine trees (I) and shorter pentoxylalean trees (2), with a small plant-eating dinosaur (3) running underneath them. It is mid-summer, so the snow on the mountains has all melted. After the nooding caused by the snow melt, the water levels in the braided river system have dropped. There are occasional tree ferns (4) on the sandbanks between the river channels. Clumps of Phyl/opleroides ferns (5) are growing on the banks of the channels.
After the end o f the Permian, the supercontinent of Gondwana gradually broke up into the continents that exist today. The first to break away were Africa and South America in the Late Jurassic. They were followed in the Early Cretaceous by India, which drifted northward to collide eventually with Asia In the latest Jurassic and Early Cretaceous, Antarctica and Aust ralia began to split apart (Figure 4'{)9). This was the most important tectonic event to affect southern Australia in Mesozoic and Cainozoic times. The first stage of this split formed a series of parallel east-west faults running along the southern margi n of Victoria. The area between these faults subsided, forming a basin over 600 kilometres long but only 100 kilometres wide in places. On either side of the basin there were mountain ranges. Volcanic eruptions began as the basin started to form and they continued for over 20 million years. As Australia and Antarctica moved apart, the central part of the basin subsided gradually. It was slowly filled by over 3000 metres of Early Cretaceous sediments. The up-faulted block of the Mornington Peninsula now divides the sediments deposited in this basin into two separate areas - the Gippsland Basin in eastern Victoria and the Otway Basin in western Victoria. However, it should be noted that during the Early Cretaceous the Otway and Gippsland Basins were joined as one continuous basin (Figure 4-54).
ROCK TYPES Sedi mentary Rocks The most abundant Early Cretaceous rocks are sandstones deposited by fast-flowing rivers. The channel systems of these rivers were very wide, sometimes over one kilometre across. They consisted of many individual shallow channels. These channels were usually about 1 0-20 metres wide and 1-2 metres deep. They continually forked and rejoined to form braided river systems. Between the channels were sandbars. As the sandbars moved downstream, they deposited cross-bedded layers of sand. Cross-bedding is visible in many Early Cretaceous sandstones exposed in sea cliffs and road cuttings. The directions of movement of the sandbars indicate the Early Cretaceous rivers were flowing mostly towards the west and south-west in the western part of the Otway Range. This suggests that the rivers flowed more or less down the centre of the long, narrow, east-west basin (Figure 4-57).
146
Chapter 4
There were extensive flood plains on both sides of the wide channel systems. During floods, these areas were covered with water and thin beds of mud were deposited. Occasionally, the river channels changed their courses during major floods, abandoning one channel and scouring another elsewhere on the flood plain. The pieces of mud eroded from the plain as the river cut a new channel usually accumulated at the bottom of the channels. Here they formed thin beds of mud chip conglomerate. After a river changed course, the abandoned channel often became a series of small lakes or billabongs. Fine-grained muds accumulated on these lake floors. The mudstones that formed are often very fossiliferous. The most common fossils are leaves, stems and trunks from the plants that grew on the flood plain and the nearby mountain ranges. Occasionally the remains of fish and aquatic insects are also found. There are localised deposits of coarse conglomerate along the north-eastern edge of the Early Cretaceous basin. These built up as alluvial fans where streams flowed from the adjacent highlands on to the flat plains of the basin. Exposures of these conglomerates can be seen along the Tyers River, north-east of Yalloum.
Volcanic rocks and the origin of the sedimentary rocks Volcanic eruptions supplied most of the enormous volume of sediment lbat was deposited during the Early Cretaceous in Victoria. The sandstones consist largely of small fragments of basalt or andesite, as well as plagioclase feldspar crystals derived from the erosion of volcanic rocks. Most of this volcanic material was probably cinders and ash from explosive eruptions. It was washed off the sides of the volcanoes into nearby rivers. Radiometric dating of small volcanic grains from sandstones has shown there were two main periods of volcanism, one about 123 million years ago and the other 106 million years ago. There were also smaller eruptions between these two periods. It is uncertain where these volcanoes were located, because there are no outcrops of Early Cretaceous lavas in Victoria. Basalts of this age have been found in a drill hole south of Bairnsdale, where they underlie sandstones at the base of the Early Cretaceous sequence. However, the lavas there are too thin and restricted in distribution to represen t a major volcanic area. MoS! of the volcanoes probably lay further to the east, off the present-day Australian coast. As a result of plate tectonic movements, the part o f the Earth's cruS! that was in this region during the Early Cretaceous has since shifted. It now forms an underwater plateau around Lord HO\ve Island. Drilling of the sea-floor on this plateau has indeed found Early Cretaceous volcanics. It therefore seems likely that erosion of volcanic rocks, originally located to the east of Victoria, produced the Early Cretaceous sandstones. The volcanic nature of the sandstones explains why these rocks weather so easily. When the andesitic and basaltic fragments are exposed to the rain and the atmosphere, they quickly alter to clay. As a result, Early Cretaceous sandstone is not very durable when used as a building tone. St Paul's Cathedral, Melbourne, was constructed of this material, and many original blocks in the building crumbled and had to be replaced.
COAL A large variety of plants grew on the flood plains beside the Early Cretaceous rivers and on the surrounding hills (Figures 4-58 to 4-61). Leave and stems from these plants were buried in the sediments deposited during frequent floods. Small pieces of carbonised wood, often lying parallel to the bedding or cross-bedding in sandstones, can be found in many Early Cretaceous rock outcrops. There were also swamps in places. The buried accumulations of plant material in these S\vam ps eventually compacted to form thin seams of black coal. These are most common in the Wonthaggi-Kilcunda area in south-west Gippsland. Because the Early Cretaceous rivers often changed their courses, the swamps were buried by channel sands before large thicknesses of plant material could build up. The coal seams are therefore all thin (see also Chapter 5).
FLORA The drier areas on the hills and flood plains were covered by forests, consisting of many different conifers (pine trees) (Figure 4-58). Some of them were related to species that still occur in rai n forest s in northern Australia, e.g. Hoop Pine, Bunya Pine. Ferns and mosses grew underneath the conifers. The wetter, s\vampy pans of the flood plains were also covered by ferns and shrubby pine trees. The plants of the region gradually changed over the 20 million years of the Early Cretaceous. Conifers gradually became more abundant and there were changes in the species. Many fer n species died out and were replaced by others. The leaves of one fern, Phyllopteroides, changed shape with time. In the earliest Cretaceous, the
Geological History of Victoria
i� ,.,
Figure
147
leaves had smooth edges (Figure 4-60), but by the mid-Cretaceous the edges were notched to look like a tiny saw-blade. The Cretaceous floras contain rare angiosperms (flowering plants) (Figure 4-62), which appeared in southern Australia at this time. A very small, delicate flower from the Koonwarra locality (see later) has been described as the oldest angiosperm flower in the world. Today angiosperms dominate the vegetation throughout the world, and conifers, particularly those types present in the Cretaceous of Victoria, are relat ively uncommon.
'"
4-58
Geinitzia (a) and Brachyphyl/um (b) shoots of two conifers found in Early Cretaceous sedimentary rocks of southern Victoria.
About 20 different species of conifers grew on the hills and flood plains of the region at that time.
Figure 4-59 (below)
7beniopteris daintreei, Early Cretaceous. o 10 IIn1 -------
This long, strap-shaped leaf belonged to an extinct group of plants called the Pentoxylales. These trees had shon, branched trunks; the leaves and cones grew from the ends of the branches. This species was named after Richard Daimree, who became a member of the Geological Survey of Victoria soon after it was formed in the 1 850s. In 1 869, Dainlree was appointed Government Geologist of North Queensland.
o
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o 10 IM'I ----
(b)
(a)
Figure 4�O (a, b) Phyl/opteroides, a rern that grew as dense clumps along slream banks in the Early Cretaceous. Three different species of Phyllopteroides are presem i n the Early Cretaceous rocks of Victoria. The leaves of the olde t species, (illustrated here), had smooth margins. whereas the younger species had leaves wit h notched or loothed edges.
5 hew !
Centimetres
Lwl U
CentllTlCtres , ,
,
,
Figure 4�t Ginkgo australis. Early
Cretaceous. These fan-shaped leaves were on large trees, which grew both on flood plains and surrounding hills. Ginkgo was abundant in the Early Cretaceous throughout the world and there were a number of species. Today there is just one species. which grows naturally in China. However, it is widely planted as an ornamental tree (maidenhair tree) and many grow in gardens in Victoria.
Figure
�-62
Cla,'atipo/lemtes, the pollen of an angiosperm (flowering plant) from
the Early Cretaceous rocks of southern Victoria. The wall structure of angiosperm pollen is distinctive and easily recognised. The oldest angiosperms yet found in Australia have come from Koonwarra in south-west Gippsland. However, angiosperms were rare in Victoria during the Early Cretaceous. Elsewhere in the world at this time they were much commoner. This suggests I hal angiosperms spread into Australia from other lands.
148
Chapter 4
FAUNA Many different types of insects lived during the Early Cretaceous, including dragonnies, bugs, beetles, fleas, nies and wasps (Figure 4-63a,b». Most are related to insects now in Australia. There were also at least five di fferent types of dinosaur, including two carnivorous (meat-eating) species (Figure 4-64). The other dinosaurs were smaller plant-eaters; one was only the size of a large chicken. In addition there were plesiosaurs, large reptiles that lived in the rivers with amphibians, turtles and various fish, including lungfish. There were also plerosaurs (flying reptiles) and at least one species of bird. In 1959, the number of Early Cretaceous fossil species found in Victoria increased greatly with the discovery of a richly fossiliferous bed of mudstone in a road cutting near Koonwarra, south of Leongat ha. The mudstone was deposited in a small lake in an abandoned river channel. It contains abundant plant fossils and many fish and insect remains. Koonwarra is now a world famous fossil locality because of the number of new species found and the excellent pre ervation of many specimens, particularly the insects. Perhaps the most exciting discoveries are a few small bird feathers, indicating that at least one species of bird lived in the area Feathers are very rare as fossils.
Figure 4-63 A beetle (a) and a wasp (b) from the Early Cretaceous sedimentary rocks al Koonwarrn. These insects are closely related to species living today. They would have lived in the vegetation on the flood plains beside rivers and lakes. Over 70 pecies of insects have been collected at Koonwarra, including one pecies of flea. The fleas presumably lived on reptiles, or perhaps birds, that inhabited the area at the time.
(b )
(a)
O,,-....:_...;2 mm
9 "1:0
.....
_ _
...;2 mm
_ _
Figure 4-64 AlloSOlUUS, an Early Creta ceous meat-eating dinosaur. Only one bone of an Allosaurus skeleton has been found in Victoria, and this was from a mall individual. Allosaurus weighed as much as two IOnnes and grew to (en metres in length, Half its length consisted of a well developed tail, which probably helped to counterbalance the body of the dinosaur when it was running on its hind leg . A llosaurus had powerful jaw with large pointed teeth. Therefore it could take large bites out of its prey, which included other dinosaurs.
2 o 1 3 Metres ....__..........t
Geological History of Victoria
149
Dinosaur Cove
_ _ ".... .. _ .... _ . _ .;a" I._ .. _ .....
Figure 4-65 Dinosaur Cove. During the Early Cretaceous, Dinosaur Cove was in the southern polar region adjoining Antarctica. (After New Scientist, April 1989).
Of all the groups of extinct animals, it is the dinosaurs that have gripped the imagination of most people. Dinosaurs have been the subject of numerous books and museum displays, and they are frequently figured in science fiction stories and films. Dinosaurs evolved from reptiles in the Late Triassic. They finally died out at the end of the Cretaceous, about 65 million years ago. Their name means 'terrible lizards', and the largest dinosaurs would indeed have been terrifying to see. The carnivorous ljrannosaurus was over six metres high when it stood upright. It could probably have run rapidly in pursuit of its prey. Bronlosourus, a piant-eruing dinosaur, stretched 24 metres from head to tail when fully grown and weighed up to 50 tonnes. However, many dinosaurs were only between 20 centimetres and 2 metres long. In the 1 980s, research scientists from Monash University and the Museum of Victoria found dinosaur bones in a cUff face on the rugged coastline along the Otway Range (Figure 4-65, 4-66). This site is now known as Dino aur Cove because at least three different species of small dinosaurs have been found here. The dinosaur bones occur in a thin conglomerate bed that was deposited in a river channel. They are mostly small and broken, as a result of being transported by a turbulent, flooding river. However, one skull has been found and some leg bones that were still joined together.
Figure 4-66 Dinosaur Cove.
Fossilised bones 0 f several dinosaurs have been fo und in the cliff face, which i s exposed to the storms of Bass Strait. The Early Cretaceous sedimentary rocks are nearly horizontal at this locality. The rock platform provides access at low tide. A small rock stack has been left after erosion of the ctiff. (photograph by .J. Rosengren).
To gain access to the fo sil-bearing conglomerate behind the outcrop in the cliff face, it was necessary to use explosives to open up hon tunnel . Since 1984, many volunteers have helped the cientists in thi operation, with the as istance of mining consultants and construction companie . Only isolated teeth and bone were found during the first three seasons of excavation . However, towards the end of the 1987 dig, about 35"10 of the skeleton of a new species of a small plant-eating dinosaur called Hypsilophodon was discovered (Figure 4-67). One shin-bone was particularly intere ting, becau e there was evidence that the bone had been fractured and then knitted again during the lifetime of the small dinosaur.
Figure 4-67
Hypsilophodofl, • small E.rly Cretaceous dinosaur. This animal was about the size of a large chicken. It ate leaves and stems of plants. Fossil remains of Hypsilophodofl have been fo und at Dino aur Cove.
o
50 ems 25 _ '... .... 1 .... . _ _1
150
Chapter 4
CLIMATE Geophysical evidence i ndicates that during the Cretaceous Victoria lay within the Antarctic Circle, where Antarclica is now. Today there are no plants bigger than mosses growing in most of Antarctica. However, in the Early Cretaceous, conifer forests were present in the south polar region. This suggests that the climate was warmer than it is now, not only at the south pole, but probably over the whole world. One of the most accurate ways to determine past temperatures uses the oxygen isotopic composition of calcite. Measurements made on calcite from Early Cretaceous sandstones in Victoria indicate that temperatures were below freezing in winter. The highland areas were snow-covered and the rivers froze. At this time in South Australia, rivers flowed northwards from the Flinders Ranges into a shallow sea which covered much of inland Austral ia. As the winter ice in these rivers melted and broke up each spring, small icebergs floated down the rivers and out to sea. Many icebergs carried river gravels that were frozen into the ice. These gravels were dropped on the sea floor when the icebergs melted. As a result, many shallow marine shales, that were deposited across northern South Australia in the Early Cretaceous, contain patches of river pebbles and boulders. Summer temperat ures in southern Australia during the Early Cretaceous must have been well above freezing, because forests of large trees were present then. Present day trees can only grow if the average temperature during the whole year is over IO'e. As winter temperatures dropped below O'C in Victoria in the Cretaceous, summer temperatures must have been greater than lO'e. However, the summers were probably still fairly cool. The nature of the Early Cretaceous sediments also suggests evidence about the prevailing climate. They were mostly deposited by flooding rivers. This indicates that there were regular floods, which may have been the result of seasonal summer melting of the winter snows on the highlands. Overal l it seems likely that the Victorian climate in the Early Cretaceous was colder than it is now, but not as severe as it is in Antarctica today. There would have been snow and ice on the highlands in winter, but Victoria was not permanently snow-covered. At present, Earth's polar regions have very short days and very long nights in winter. This would have been the same in the Cretaceous, even though the polar regions were warmer at the time. The presence of polar forests in Victoria during the Cretaceous shows the trees were able to cope with long periods of almost total darkness over winter. I t is the cold rather than the darkness that StOps plants from growing at the poles today. Dinosaurs that lived in Victoria during the Cretaceous would also have had to cope with dark, cold winter months. Some dinosaurs may have hibernated, like modern-day bears in the Northern Hemisphere. Alternatively they may have migrated northwards to warmer climates, as reindeer migrate today. However, it is also possible that some dino aur species spent the whole year in the area. The Hypsilophodoll skull found at Dinosaur Cove had very large eyes. This may have been a special adaption fo r living through the months of darkness. Probably the greatest problem facing any overwintering dinosaurs would have been lack of food. However, some plants may have kept their leaves during the winter darkness.
Late Cretaceous
In the middle of the Cretaceous period there was a further stage in the break-up of Australia and Antarctica. The main split between the two continents developed to the west and south of Tasmania. As Australia moved nort hwards a)vay from Antarctica, the stresses and strains along the southern margin of Australia caused parts of the Early Cretaceous basin in Victoria to be folded. faulted and uolifled to form ranges. The present Otway and Strzelecki ranges are remnants of the Late Cretaceous ranges. Most folds there are broad, open structures. The beds generally dip at low angles (Figure 4-68). Other portions of the Early Cretaceous basin subsided and the sea nooded in. Late Cretaceous shallow marine sediments were deposited in the areas of subsidence. The latter are now largely beneath Bass Strait. Late Cretaceous sedimentary rocks have only been found in offshore oil explorat ion drill holes. Fossils from some drill hole samples show that bivalves and gastropods (snails) crawled over or burrowed into the sea-noor at the time, while fish and ammonoids swam in the waters above. Other samoles contain abundant plant fossils, showing those sediments were deposited by rivers. The plants growing beside the rivers were mostly conifers, although angiosperms were becoming increasingly abundant. Climatic conditions were cool and wet, as in the Early Cretaceous.
Geolog ical History of Victoria
151
Figure 4�8 The Gable, Otway coast, western Victoria. The darker coloured rocks in the lower part of lhe cliff are Early Cretaceous sandstones. They were deposited by braided rivers. Overlying these are Tertiary sands and gravels. These sediments were laid down in shallow seas and on deltas. The bedding in the Tertiary sed.lments IS not quite parallel to the bedding in the Early Cretaceous sandstones. This indicates that there is a slight angular unconformity between the two, i.e. the Cretaceous sedimentary rocks were tilted slightly by earth movements before the Tertiary sediments were deposited. The Cretaceous sandstones are well-cemented and form almost vertical cliffs. By contrast, the slope, that has developed on the 1ertiary sedi ments. is not as steep. This is because the Tertiary sands and gravels are much less resistant to weathering. (photograph by N.J. Rosengren).
I n t h e mid-Cretaceous another major split developed within the southern part of the Australian Plate, this time along the south -eastern coast of Australia. A large area of continental crust broke away along a north-south rift to form the Lord Howe Rise. This is now a mostly submerged plateau, some 1 500 k ilometres off the Australian coast. New Zealand lies on the southern margin of the Lord Howe Rise and Lord Howe Island is located near its centre. The Tasman Sea between Australia and New Zealand was formed by this episode of rifting (Figure 4-69). The South-Eastern Highlands of Australia were probably uplifted during the rifting between Australia and the Lord Howe Rise. The uplift was probably rapid and it formed an asymmetrical mountain range. The eastern slopes of the Highlands, which face the sea, are generally steep, whereas the inland western slopes are gentle.
CAINOZOIC ERA Tertiary
The name Cainozoic comes from the Greek words for 'new life'. The Cainozoic is generally subdivided into the Tertiary and Quaternary periods. The Tertiary period is further split into five epochs - the Palaeocene, Eocene, Oligocene, Miocene and Pliocene, from oldest to youngest. The Quaternary period is divided into the Pleistocene and Recen t epochs. Alternatively, some geologists divide the Cainozoic into the Palaeogene (comprising the Palaeocene, Eocene and Oligocene) and the Neogene (consisting of the M iocene, Pliocene, Pleistocene and Recent). The word Tertiary comes from the Latin 'teniarius', meaning third. I t was the third subdivision in an early stratigraphic classification. It extended from 65 million years ago to 1 .6 million years ago. In the sea, many groups o f creatures that survived the extinction at the end of the Cretaceous quickly increased their numbers. The sea-floor was populated by bivalves, gastropods, sea-urchins, foraminifera and bryozoans. Hexacorals built extensive reefs. Fish and sharks swam in open water. Whales first appeared in the Eocene. The great development of flowering plants continued on land. Grasses evolved during the Tertiary. Because Australia became an island continent in the Late Cretaceous after splitting from Antarctica, the Australian flora developed i n isolation. As a result, many plants are restricted t o Australia and nearby islands, e.g. gum trees (eucalypts) and wattles (acacias). The mammals, having inherited the world from the dinosaurs, underwent a remarkably rapid diversification. At the start of the Tertiary, the biggest mammal was the size of a dog. By the end of the Eocene, many modern mammal groups were present, e.g horses. elephants and bats. There were also many large and unusual mammals, including an enormous four-legged marsupial standing up to seven metres high. Marsupials had evolved before the break-up of Gondwana. In the isolation of Australia they remained the dominant group of land animals. Elsewhere in the world, placental mammals superseded the marsupials during the Tertiary.
152
Chapter 4
96 M I L L IO N YEARS AGO END OF EARLY CRETACE OUS
a.
AUSTRALIA
7
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RIFTING BEGAN IN LATE JURASSIC
NORTHWARD ORIFT OF AUSTRALIA
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ruFTlNG BEGAN IN EARLY CRE TACEOUS
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E N D OF LATE E O C E N E
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,
6 M I L L I O N YEARS A G O E N D OF LATE M I O C E N E
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DRIFT CONTINUES TO RECENT
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HOWE RISE
36 M I L L I O N YEARS AGO
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66 M I L LION YEARS AGO E N D OF LATE CRETACEOUS
b.
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BASIN
COAl.
, , I \ "
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BASS BASIN
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M V
SEA
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Fi�ure 4-69 Separation of Australia and Antarctica during the disintegration of Gondwana. Four stages are shown in the drift apan of the Australian and Antarctic continents during Late Mesoloic and Cainozoic limes. During the Early Cretaceous, twO large ri ft valleys developed by faulting along the south-eastern margins of 'Australia'. Seaways. (narrow marme straits), developed as the cont inents broke apart permanently. Antarctica and Australia have continued to drift further apart throughout the Cainozoic era. Tertiary coalfields formed in low-lying swamps close to the new coastlin� while the oil and gas deposits slowly developed in the ncarby offshore marine sediments. In each diagram, the location of oil andlor gas containing rocks of thal age are sh()INn.
SOU THERN
PETROlEUM
•
eal1tnl: r ..o.;8
OCEAN
+
t t t
ANTARCTICA LIES TO THE SOUTH
TASMAN SEA
D 0
w
ote: M - Melbourne, S - Sydney, H - Hoban, A - Adelaide. (From original drawing by R.C. Glenie).
DISTRIBUTION Tertiary rocks cover about one half of Victoria, although they mostly OCcur beneath a veneer of younger (Quaternary) sands and clays (Figure 4-70). The regions where the Tertiary rocks were deposited are now mainly low-lying plains. However, there are also many small areas of outcrop in the Central Victorian Uplands, especially in the Midlands. The thickest Tertiary deposits are beyond the southern margin of the State, beneath the waters of Bass Strait and the Tasman Sea (Figure 5-28). However, good exposures of horizontal to shallow-dipping Tertiary sediments can be seen along the coast between Anglesea and Torquay, and further west along the Port Campbell coastli ne. There are also outcrops in cli ffs along the lower parts of such rivers as the Glenelg north of Nelson, the Barwon near Geelong and the Mitchell north-west of Bairnsdale. Thick Tertiary deposits also occur in nort h-west ern Victoria, but they are generally hidden beneath the Mallee and Wimmera plains. Some exposures of these rocks occur in cli ffs along the Mu rray River, although the best outcrops are downstream in South Australia.
Geological History of Victoria
153
Tertiary rock formations are of great economic importance to Victoria. They contain all the State's brown coal and most of the oil and gas resources. A major part of the gold won in the past came from Tertiary sands and gravels. Recently, major heavy mineral sand deposits have been discovered within Tertiary sediments in western Victoria.
·
\
·
PALAEOGEOGRAPHY
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TERTIARY
Shallow sea Approximate pOSition 01 coastline
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o 25 50 75 100
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WIlsons Figure 4-70 (above) Distribution of Tertiary rocks in Victoria. The level of the sea rose and fell limes during [he Tertiary. The coastline shown in the illustration represents the maximum extent of the shallow seas, i.e. the highest ea-Ievel across Victoria. The western pan of the Gi ppsland Basin (lhe Latrobe Valley) was never covered by the sea. The southern margins of the Gippsland and basins lie to the south of Victoria undernealh Bass Strait.
many
REGIONAL SETTING Throughout the Cainozoic, Australia continued to drift nort hward away from An tarctica (Figure 4-71 ). At first this movement was rather slow - less than one centimelre per year. However, from about 40 million years ago, it became much faster at 5-6 centimetres per year. As a result, the Southern Ocean grew from a narrow east-west seaway to a width of over 2500 kilometres. The split between the Lord Howe Rise and Aust ralia, which began in the mid-Cretaceous, stopped about 57 million years ago. Since then the Tasman Sea, which formed in the rift area, has remained more or less umlorm III size. As a result, ew Zealand and Australia have retained the san)e relat i ve positions for the last 57 million years.
Otway
Figure 4-71 Stages in thr steady northward drift of Australia away from Antarctica during the Cainozoic Era. In order to simplify Ihe diagram, only IwO posilions of New Zealand are shown, 31 1 0 million years ago and Ihe presen!. Tasmania was attached by dry land to Ihe mainland on several occasions during Ihe Cainozoic. (After Ludbrook, Department of and Energy. Soulh Auslralia. 1980).
N.M. Mines
FigUN'1f indicaU! agf!1f in million �'Or8 before pN'lfl!tlt
154
Chapter 4
In the Cretaceous, during the early stages of the rifting between Australia and Antarctica, braided rivers crossed the floor of the subsiding rift. Subsequently, the two continents moved further apart i n the Late Cretaceous. Some areas of the enlarging rift were uplifted while others continued to subside. As a result, three interconnected basins formed along the southern edge of the Australian continental plate - the Otway, Gippsland and Bass basins (Figure 5-28). Great quantities of sediments accumulated in these basins during the Tertiary. The Otway and Gippsland basins extend roughly east-west through south-western and south-eastern Victoria respectively. Parts of these basins now lie onshore, but large areas are located beneath the waters of Bass Strait . The Bass Basin is entirely offshore to the south and west of Wilsons Promontory. Rocks of the Bass Basin have only been investigated by a few oil exploration boreholes. The three basins are separated from each other by ridges of older rocks known as highs. The Gippsland and Bass basins are divided by the Bassiafl High, formed largely of Palaeozoic granite. This ridge extends from Wilsons Promontory through various small granite islands to Flinders Island and the north-eastern corner of Tasmania. The Bass Basin is separated from the Otway Basin by an underwater high running south-westward from Mornington Peninsula to ](jng Island in Bass Strait. All three basins had a similar history. Parts of them were flooded by the ocean in the Late Cretaceous. However, large areas remained dry land, where sediments were deposited in swamps and on river deltas throughout the Late Cretaceous and Early Tertiary. By the Miocene, all three basins were almost covered by shallow seas. At that time the coastline along southern Victoria lay weU inland of its present position in many places. Since the Miocene, the sea-level has moved up and down several times. As a result, alternating beds of continental and marine sediments have been deposited along the coastal areas of Victoria as the shoreline periodically advanced and retreated. Tertiary sediments also accumulated in the Murray Basin. This developed in the Early Tertiary, later than the other basins. It was a broad, gentle sag that occupied a large area of north-western Victoria and extended into western New South Wales and eastern South Australia.
ROCK TYPES Tertiary sedimentary rocks in Victoria were laid down in several di fferent environments. For simplicity they are discussed in the following groups: I . Continental sediments • river deposits • swamp deposits 2. Deltaic sediments, laid down where rivers flowed into the sea 3. Marine sediments • shallow water limestone • shallow and deep water sands and clays In addition, volcanic rocks of both basaltic and intermediate compositions were erupted over parts o f Victoria during the Tertiary.
River deposits After the uplift of the Sout h-Eastern Highlands, extensive river systems formed over the pans of Victoria not covered by shallow seas. Throughout the Tertiary, river sediments were deposited on broad alluvial plains that stretched north and south from the uplands. These plains merged into deltas in some places along the coast. Gravel and coarse sand accumulated in the river channels. During floods, finer-grained sands, silts and clays were deposited on the low-ly ing plains beside the rivers. The rivers were fast-flowing close to the newly-elevated mountains. There, they eroded deep channels in the landscape and these filled with coarse gravels. Similar coarse-grained sediments can be seen today in the beds of many rivers in eastern Victoria, e.g. Snowy and Avon rivers (Figure 4-72). Tertiary river gravels are present throughout much of Victoria (Figure 4-7 3). They occur in the uplands within old river channels that have been buried by later sediments or basalt lava flows. They are also preserved as river terraces along present-day river valleys. The gravels have been important in many districts because they contained panicles of gold. The gold was originally present in quartz veins intruding the Palaeozoic bedrock (see Chapter 5). It was released during the long period of weathering that affected the Victorian landscape for most of the Mesozoic (at least 1 50 million years). The gold was then washed into the fast-flowing rivers during the Tertiary. Present-day rivers in the Midlands are generally muddy and slow-flowing (e.g. Loddon River), in contrast to the much faster- flowing Tertiary streams which deposited the gOld-bearing gravels.
Geological History of Victoria
155
Figure 4-72 Avon River beside the Princes Highway at Stratford, East Gippsland. The river bed is over one hundred melres wide and covered with a thick layer of coarse gravels. Many of the pebbles are Early Carboniferous and Late Devonian sedimentary and volcanic rocks, which have been carried over distances of more than 30 kilometres from the ranges to the north. (photograph by G.W. Quick).
As the uplands were worn down by erosion after the Cretaceous uplift, they decreased in elevation and so the energy of the rivers also decreased. The slower flowing rivers could transport only silt and mud, so finer-grained sediments were deposited over the earlier gravels. In Gippsland there are extensive sheets of partly-cemented gravel, called the Haunted Hills Cravel. They cover the plains south of the uplands. The gravel are exposed in many road cuttings throughout southern Gippsland. They formed as a series of overlapping alluvial fans, where fast-flowing streams ran from the uplands on to the plains. It is likely that these alluvial fans formed after a minor uplift of the highlands in the area about five million years ago.
Figure 4-13 Tertiary gravels and sands (upper dark rocks) overlying steeply dipping Ordovician sandstones and shales (lower light rocks) with a strong anguJar unconformity. The cutling is east of Bruthen, near an old railway crossing on the Buchan Road, East Gippsland. The gravels are cross-bedded and partly cemented with iron oxides. They were deposited by a fast-flowing river flowing south from the Central Victorian Uplands. There are numerous reef quartz pebbles in the deposit. Several minor faults crossing the Ordovician rocks are also visible. (Photograph by N.J. Rosengren).
Swamp deposits There were swamps in all [he Tertiary basins, especially in the Gippsland Basin, where thick beds of brown coal accumulated. In the Latrobe Valley, which stretches eaSllvards from Moe, these sediments are known as the Latrobe Valley Croup. They contain vast brown coal rieposits (see Chapter 5). The Latrobe Valley swamps developed in tne broad valley separatmg the cast Victorian Uplands to the north from the Strzelecki Ranges to the south. In the Tertiary, rivers flowed eastwards through this valley imo Bass Strait, just as the Latrobe River does today. During periods when the sea-level was rising, the advancing waves pushed in front of them ever-increasing amoums of sand, acting like a giam bulldozer. As a result, high coastal sand dunes were formed. These dammed the rivers within the valley and led to the development of extensive swamps (Figure 5-24). The well-preserved plant fossils found in the swamp sediments indicate the kinds of vegetation that grew in the swamps and on the surrounding mountains. Large pieces of wood are present in many coals, and the interbedded clays contain abundant leaves, stems and fruit (Figure 4-74, 4-75). Pollen is present in all the sediments (Figure 4-76). This shows that rainforest covered the mountain ranges and the higher
156
Chapter 4
parts of the plains. I n contrast, the vegetation i n and around Ihe swamps was probably low and scrubby. Small conifers grew i n lhe shallow lakes. In the drier parts of the swamps, there were rushes and ferns, as well as trees such as Casuarina (she-oak) and Banksia. Rainforest species were not present within the swamps, despite their presence on the surrounding mountains. This was probably partly due to the influence of bush fires caused by lightning strikes. Charcoal particles in the coal indicate that the drier parts of the swamps were periodically burnt by bush fires. This prevented the growth of rai nforest species, which are not adapted to the effects of fire.
q
f L:
em
Figure 4-74 A leaf of an angiosperm from the Tertiary coal-bearing sediments of southern Victoria. Angiosperms have leaves with a characteristic pattern of veins, that distingui hes them from other plan! group . In angiosperms, between the major veins there is a network of finer veins. Therefore, it is usually easy to deduce that a fossil leaf came from an angiosperm. However, the leaves from any one pecies of nowering plan! are often very similar to the leaves from many other species. A a result, it is usually difficult to ident ify the type of angiosperm pre ent in Tertiary sedimenlary
The floors of the swarnps slowly subsided throughout much of the Tertiary. After the plants growing there died, their remains were buried beneath new growth and slowly formed peat . Gradually considerable thicknesses of decaying vegetation built up. The weight of the overlying sediments compressed the peat to form brown coal. In the Latrobe Valley, there are several very thick layers (seams) of brown coal. The e are separated by thin beds of sandstone and mudstone deposited by rivers. There is very little sand or mud within the coal seams. The thick vegetation around the edges of the swamps filtered out any sediment being brought in by rivers. Eventually, the subsidence of the swamps stopped, so no further coal was formed. For a while the top of the coal was exposed at the surface. When lighlning strikes caused bush fires, the coal outcrops sometimes caughl alight and holes were burnt in them. These holes lhen filled with rainwater. They became lraps for unwary animals which occasionally fell into them and drowned. During coal mining operations, kangaroo bodies, some with well-preserved skin and fur, have been found in the sediments filling these holes. Coal seams are also present in pans of the Murray and Olway basins (e.g. at Bacchus Marsh, Altona, Lal Lal and Anglesea). These seams are thinner and less common than those in the Gippsland Basin.
rocks if only fallen lea\'es are preserved.
Figure 4-75 (right ) Angiosperm leaf from Tertiary Oood plain sedimenls in (he Anglesea brown coal mine.
It is uncertain which group of modern-day plants is mo t closely related to thi pecies. The shape of the leaf is similar to the leaves from some species o f the family Proteaceae. which include stich plants as Banksia, Grt!vi/lea and Telopea (\Varatah). (Photograph by J.G. Dougla ).
Deltaic sediments Along the coastline in the southern part of the Gippsland Basin. river built up large deltas where lhey flowed into the ea. The fine-grained carbonaceou sands, ilts and clay deposited on the deltas merged shoreward into river and wamp sediments, which were forming at the same time. The deltaic sediments commonly contain small pyrite crystals and al 0 large amount of plant debri and pollen, that were wa hed and blown i n from the nearby forest on land. Most of the deltaic deposits now lie beneath Bas Strait. where they contain the extensive offshore Bass Strait oil and gas field ( ee Chapter 5). There are also deltaic sediments in the Otway Basin. They occur along the sOUlh we t Otway coa t near Princetown (Figure 4-68).
Shallow marine limestones Shallow marine limestones are present in all the Victorian Tertiary basins. They are moderately re istant to weathering and form steep cliffs along the south-western Victorian coast, e.g. near Port Campbell and Torquay. Limestones are formed predominantly from the broken remains of marine animals uch a hell fish and corals. Corals live mainly in warm waters. The Great Barrier Reef is a typical limestone deposit formed in tropical walers. However, lime tones can also be deposited in cooler waters. a in Victoria. The cool water
Geological History of Victoria
Figure 4-76 No/ho/agus pollen gmins, found in
Tertiary river and swamp sediments of southern Victoria.
o 10 �m ....,
o 10 �m '-'
Several species 01 No/llo/agus trees are still present in the rainforests of eastern Australia. One species, known as myrtle beech, grows in small areas 0 1 the Otway Range and Central Victorian Uplands. However, in the Tertiary, Notllo/agus was abundant and widespread in the rainforests which covered mOst of the Victorian highlands. Notho/agLis trees produce millions 0 1 pollen grains each year. As a result, Notho/agLiS pollen is often the most abundant component in pollen samples Irom Tertiary sediments in Victoria.
157
limestones are composed mainly of fragments of shells, bryozoans and calcareous algae. Bryozoans grow in colonies on the sea bottom and often have branching skeletons (Figure 4-77). Modern bryozoans are very abundant on the floor of pans of Bass Strait. Their remains have built up large underwater sandbanks. The Tertiary limestones in Victoria formed close to the shoreline in a similar way to the present-day bryozoan banks. The wave action along the shore broke up the shells and bryozoans into small sand-sized pieces, which were swept short distances offshore to form large sandbanks in shallow water. The Batesford Limes/one is a typical bryozoan limestone; it is quarried for cement manufacture near Geelong (see Chapter 5 ) . I:Iryozoan limestones also occur extensively i n the Lower South-East region o f South Australia and i n the south-western corner of Victoria (Figure 4-78). A karst landscape has developed on these limestones. Many caves and sinkholes are present, including tourist caves at Naracoorte (South Australia) and along the Glenelg River.
Figure 4-77 A bryozoan from the Middle Tertiary Jan Jue Marl in a coastal cliff at Bird Rock, west of Torquay. Bryozoans are colonial animals. They first evolved in the Ordovician and several thousand species are alive today. I ndividual bryozoan animals are very small. They generally live in calcareous tubes within the overall bryozoan skeleton. The openings to these tubes are the small dots on the bryozoan skeleton in the photograph. Most bryozoans lived on shallow sea floors. Figure 4-78 (right) Tertiary Bryozoan limestone in a cliff face along the Glenelg River, near the border between Victoria and South Australia. (Photograph courtesy 01 Department of Conservation and Environment).
Marine sands and muds Large thicknesses of sands and muds were deposited in the shallow seas that covered all four of the Tertiary basins in south-eastern Australia. The character of the sands varied according to the environment in which they were laid down and the source of the sediment. For example, a unit variously called the Brigh/on Group and the Baxter Sandstone outcrops in the cli ffs on the eastern side of Pon Phillip Bay, e.g. Black Rock and Mornington. This formation was deposited in the easternmost pan of the Otway Basin. It is a very coarse sandstone, consisting of well-rounded quartz grains cemented together by iron oxide. The sandstone contains a few marine snails and bivalves. Probably, most of it was deposited on beaches. The continuous to-and-fro action of the waves rounded the quanz grains to their present shape.
Chapter 4
158
Figure 4-79 Polycope sanctacather;nae, a
Tertiary ostracod from the Middle Miocene Fyansford Formation at Fossil Beach, Mornington. (a) outside of the shell, which is ornamented with interconnected ridges; (b) inside of the shell. In the centre there is a group of small round scars, which mark the points where muscles were allached to the shell . The shells of this species are about 0.4 millimetres long. (Photograph by M. Warne).
In the Murray Basin there is an unusual but important sand layer called the
Parillo Sand. This sand is very fine-grained and consists mainly of quartz. However, it also contains heavy minerals (ilmenite, rutile, zircon, monazite), which are an important economic resource (see Chapter 5). They occur within the sand as thin dark layers, which show that the overall sand body is extensively cross-bedded. The type of cross-bedding present is called hummocky cross stratification. It indicates that these beds were deposited in fairly shallow water, which was periodically affected by major storms (Figure 4-24). In contrast to the sands, finer-grained Tertiary muds were deposited further offshore in quieter waters, at depths down to 200 metres. For example, at Geelong, the muddy Fyansford Formation was deposited over the shallow water Batesford Limestone as the sea-level rose and the water deepened. The mudstones contain abundant fossils, including marine snails, ostracods (Figure 4-79) and bivalves (Figure 4-80).
(a)
( b)
Figure 4-80 Glycymeris, a bivalve from Tertiary marine sedimentary rocks in southern Victoria. The first bivalves are found in Ordovician rocks, but t hey did nOt become abundant until the Tertiary. The twO shells of a bivalve are hinged by a series of teeth and sockets, and are held together by strong muscles. Most bivalves are marine and many burrow into the soft sediments on shallow sea floors. Glycymeris was a burrowing type.
InSide 01 shell
OutSide of shell
9
fem
Basaltic rocks Volcanic activity occurred intermittently throughout the Cainozoic almost to the present day. Basaltic eruptions began in the Late Cretaceous, about 90 million years ago, and there was a major period of activity between 20 and 40 million years ago. A fter this, few eruptions occurred until 6-7 million years ago, when a new phase of volcanism commen<:ed with some trach)llic lavas (see later). These were followed by widespread basaltic activity, which peaked at about 2 million years ago and continued into the Quaternary. For convenience, Tertiary basalts in Victoria older than 7 million years are grouped together as the Older Volcanics. Basalt lavas younger than this age are called the Newer Volcanics (see next section). Outcrops of Older Volcanics are scattered across southern Victoria, either within or adjacent to the Tertiary basins. The lava nows are generally interbedded with river and swamp sediments . The greatest thickness of basalts occurs along the southern pan of Mornington Peninsula. There, one drill hole intersected over 400 metres o f lavas. These basalts are well-exposed in the cliff around Flinders and Cape Schank. Other outcrops of Older Volcanics can be seen along the Phillip Island coast and in part of the Strzelecki Ranges (e.g. Thorpdale and Leongatha areas). Basalts of the Older Volcanics are also found in the East Victorian U plands, where they were mostly erupted over Palaeozoic rocks. Many of these lavas nowed down river valleys that exi ted at the time. They now occur, often along ridge , as long lines of outcrops, marking the former course of rivers. Small lava plains ometimes formed around the eruption points. These are preserved as plateaus in the high country in the east of the State, e.g. Dargo High Plains. Few of the volcanoes from which the Older Volcanics were erupted have been recognised. This is because the volcanic ash deposits usually associated with volcanoes have generally been eroded away. Many Older Volcanic lavas are deeply weathered because they have been exposed at the surface over a long time. They often outcrop as grey, brown or red clayey material, and deep soils have developed on some of these weathered basalts. However, some valley nows are still fresh and can be used to produce crushed stone.
Geological History of Victoria
1 59
I ntermediate volcanic rocks Trachytic lavas were erupted about 6 million years ago i n the Mount Macedon area. They now form the well-known scenic localities of Brocks Monument and Hanging Rock. The trachyte lava was quite viscous. Therefore it did not flow far and formed a steep-sided dome where it was erupted. As the lava cooled it shrank slightly. I n places the cracks that formed i n the rock were arranged i n a pattern o f irregular columns. Erosion of the trachytic dome at Hanging Rock has made these columns clearly visible (see Chapter I).
SEA·LEVEL CHANGES Within each Tertiary sedimentary basin, the relationships between onshore (river and swamp) sediments and offshore (shallow and slightly deeper marine) sediments were governed to a large extent by a series of world-wide changes in sea-level. The effect of these sea-level oscillations can be clearly seen in the Otway Basin sediments exposed along the cliffs between Torquay and Aireys Inlet. The shallow water Poin t Addis Limestone was deposited as sandbanks just offshore from a beach. However, one horizon within this limestone is more resi tant to weathering, because it is well cemented. This layer represents a time when the beach and offshore sediments were exposed to the atmosphere by a faU in sea-level. The period of exposure allowed the sands 10 be cemented by calcite. After this faU in ea-Ievel, the sea rose again and more limeslOne formed on tOp of the well-cemented layer. As the sea continued to rise, muds were deposited on lOp of the limestone in deeper, quieter water. Afterwards, the sea-level dropped again and more shallow water limestOnes were deposited on top of the deeper water muds.
FLORA AND FAUNA
Figu re 4-8 1 Corystus. a n Oligocene echinoid from the Point Addis Limestone
near TorQuay.
Echinoids, (also called sea-urchins), have skeleton that are hollow and shaped either like a hean (as illustrated here), a globe or a disc. They consist of plates of calcite, closely fitted together. Many of the plates are perforated by tin)' holes, through which extend lube feel. Echinoids use the tube feet to move around slowly and to help with feeding. The outside of an echinoid skeleton is covered with moveable spines. Like the plates of (he skeleton, [he spines also consist of single calcite crystals. Some echinoids have long, sharp spines that act as a defence against predators. A rter an echinoid has died, the spines quickly fall off. The specimen illustrated has lost its spines. All cchinoids live in the sea. The globular forms prefcr rocky coasts. The hearl-shaped echinoids burrow into sandbanks, where they e.xtract fine organic mal ler.
During the Tertiary, the mountains and parts of the plains in Vicloria were covered with rainforests. Some trees i n these forests are today found only in tropical rainforests in north Queensland. A remarkable variety of animals inhabited the forests. Many were closely related to species present in Australia tOday, (e.g. kangaroos, possums and goannas), but they were generally much larger than the living representatives. For example, some Tertiary kangaroos were over four metres tall. I n addition, there were many strange animals that are now extinct. Diprotodol1, a large marsupial about the size of a small cow, grazed on trees and shrubs. The fierce Thy/aeo/eo, which resembled a leopard, preyed on the smaller animals in the forests. Along the coastline, exten ive bryozoan banks built up in the shallow waters offshore. Because of the un stable surface of these Shifting sandbanks, only specialised animals could live on them. Sea-urchins, some up to 1 5 centimetres across, burrowed into the sand (Figure 4-81). Today Iheir skeletons can still be found preserved in their burrows in the bryozoan limestone. Scallops (Figure 4-82), small sponges and oysters lived on the su rface of the sandbanks along with ostracods and disc-shaped foraminifera (Figure 4-83). Sometimes the calcareous shells of the foraminifera accumulated i n sufficient numbers 10 form limestone beds, e.g. the uppermost part or the Bates/ord Limes/one. There were sharks and fish swimming above the sandbanks. I hetr remams, particularly the teeth or Ihe sharks, are sometimes found in the limeslO ne. Offshore, in deeper water, bivalves (Figure 4-80), gastropods (Figure 4-84), rusk shells (Figure 4-85) and corals (Figure 4-86) were abundant on the muddy sea-floors. The corals were small and lived as separate individuals in moderately deep, muddy waler. They were di fferent 10 the large colonies of corals on the present-day Great Barrier Reef, which thrive i n warm, clear shallow water.
:_ : ���:{<: : r.;}: , ·:,:·,:·,: : <::�:·.;:.;J:t� {�e::./:i> . i · .
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'60
Chapter 4
Figure 4-82 Notochillmys,
Figure 4-83 •
bivalve from Tertiary marine sedimentary rocks in southern Victoria.
Cibicides thiara,
foraminiferid from marine Miocene rocks.
''''''-''''�'lY,1.\
o
This fossil belongs lO a group of bivalves called pectens or scallops. One living species of pecten has a large muscle holding the shells i sold i n together. This muscle s fish shops as scallop. Pectens are also well-known through one oil company, which has a scallop shell as its logo. Scallops usually lie on sandy sea floors. When they are escaping from predators or fishing boats, they can swim for shon distances by clapping their shells together.
Figure 4-8S Dentalium, a lusk shell from marine Tertiary mudstones of southern Victoria.
a
Columbarium gastropod (snail)
from Miocene mudstones in sout hern Victoria.
End view
Side view
0.1
Figure 4-84
0.2
0.3
mm
Foraminifera are tiny, single-celled organism& They are usually less than one millimetre across, but a few reach several centimetres. Most have a chambered shell made of calcite. Some foraminifera float in the upper levels of the open ocean , whereas others (such as Cibicides) live o n the sea-floor. Foramin ifera first evo lved i n the Cambrian. They were diverse and widespread in the Teniary, and very abundant in some areas. Some Tertiary li mestones are aImost completely made up of foraminiferan shells. Foraminifera, particularly the floating plank IOnic forms, evolved quickly. Some species lived for periods as short as two million years. As a result, floating forami n i fera are very useful fo r dating ll:rtiary sedimentary roc ks, in the same way that graptolites are used for Ordovician roc ks. Because fo raminifera are so tiny, even small rock samples from drill holes can contain thousands of specimens. This makes fo raminifera very important in the oil industry. where they are used to date samples of sedimentary rocks obtained from drill holes.
o 2cm !__.6=d! � Gastropods generally have a calcareous coiled shell, which may be ornamented with ribs and spines. Gastropods first appeared in the Cambrian. However, it was not until the Tertiary that they multiplied rapidly and became highly diversified. Today there are more gastropods living on the sea floor than any other group of shelled animals, such as bivalves and brachiopods. Most gastropods live in shal low seas. Many prefer (0 crawl over quiet, muddy boltoms, where they eat fine organic maner contained in sediment. The 'Tertiary mudstones of Victoria freq uently contain abundant, well-preserved gastropods.
Figure 4.,'!6 (left) Placofrochus. a solitary cor.tI from
Tertiary mudslones of soulhern Vicloria.
Side view
�=="l,Omm . ,J, 0' _ 1. Tusk shells are related to gastropods and bivalves. They have a sim ple, tusk-shaped shell and live partially buried in soft sediments on the sea-noor. Only the narrow tip of the shell protrudes inlo lhe sea water. Most tusk shells live in the quiet. deep waters on the offshore pari of the continental shelf. They are often found a1isociated with ga'il ropads, burrowing bivalves and small solitary corals in Vi('10rian Te niary mudstones.
Top View
· ... 2_;,; 1,Omm i.... ?
Modern-day corals called hexacorals appeared in the Triassic, after the rugose and tabulate corals of the Palaeozoic had died out. Hexacorals have well-developed radial plates inside the outer wall of the skeleton. These plates, called sepIa, are in multiples of six. The sepIa in rugose corals were in mul tiples of four. Many hexacorals, including those living on coral reefs, have tiny algae living in their soft tissue. These algae are important for the corals' growth. Some corals live in deeper, muddier walers lhat are 100 dark fo r the algae to survive. Hexacorals without algae are mostly small and live as individuals. Placolrochus is an example.
Geological Hislory of Vicloria
161
CLIMATE V ictoria experienced a warm, wet climate during much o f the 1l:rtiary period. This explains the presence of temperate rainforest over most of the State then. Around 20 million years ago, ocean temperatu res were probably 5-IO'C higher than they are now, even though Australia was some 1000 kilometres closer to the South Pole. Nevert heless, the water was not warm enough for coral reefs to develop. As Australia gradually moved northwards away from Antarctica, it experienced a great change in climate about 5-10 million years ago. This was related to changes in the atmospheric circulation above Australia and a cooling of the Southern Ocean. This cooling was caused by a large continental icesheet that developed in Antarctica. The rainfall over much of Australia decreased greatly and the climate became more and more arid. The rain forests of the Early Tertiary, which existed even in central Australia, gradually died out. The large animals, that inhabited the forests, became extinct. They were replaced by the inland desertS and smaller animals of today. One effect of the climate change was to alter the predominant wind direction over Victoria. In the Early Tertiary the winds over much o f southern Australia blew mainly fro,/, the east. However, in the Late Terliary they changed to westerlies.
DEFORMATION 1l:rtiary sediments and volcanic rocks are mostly horizontal or gently-dipping. Steeply dipping beds are rare and only found associated with faults or monoclines.
Quaternary
The name Quaternary is derived from the Latin word 'quatuor', meaning four. It was the fourth period in an early stratigraphic classification. The Quaternary extends from 1.6 million years ago to the present and is divided into the Pleistocene and Recent. The boundary between these subdivisions is placed at 10 000 years ago. There were several ice ages during the Quaternary. They caused considerable rises and falls in sea-level. During cold periods, many plants in Europe died out and the migration patterns of animals were affected. Some animals, such as woolly mammoths, developed long, hairy coats to cope with the cold. Life in the sea, however, remained much the same as it had been in the Late Tertiary. Ape-like ancestors of modern humans appeared in Africa about 7 million years ago (Late Miocene). Primitive humans did not evolve until the beginning of the Pleistocene, about 1 .6 million years ago. The oldest fossils of modern h umans (Homo sapiens) discovered so far are about 100 000 years old. However, modern humans may have appeared in Africa earlier, perhaps 200 000 years ago. Humans spread from Africa to Europe and Asia during the later part of the Pleistocene. They probably reached Australia around 50 000 years ago.
DISTRIBUTION Even over the relatively short time of the Quaternary period, there have been major changes in the geology of Victoria. These are particularly noticeable along the coastal areas. Nevertheless, the basic geological processes i n operation today are those that have been active over the last two million years. Quaternary sediments are widespread, particularly as sheets of windblown sand and silt in north-western Victoria and along the coast. There are also extensive areas 01 channel and lIood plain sediments along many rivers, particularly those Ilowing north to the Murray. Most sediments are so young they have not yet been cemented to form rock. The widespread volcanic activity, which commenced in the Tertiary, continued throughout the Quaternary, particularly over south-western Victoria and the Melbourne and Ballarat-Maryborough districts (Figure 4-87).
REGIONAL SETTING Throughout the Quaternary, Australia has been al much the same latitude a II 1S today. There has been no large-scale tectonic activity and only minor fault movements and tilting. The main influences on the geological h istory were glacial episodes and intermittent volcanic activity.
GLACIATION AND ITS CLI MATIC EFFECTS To understand how and why Quaternary sediments were deposited in Victoria, it is necessary to consider the worldwide climatic conditions at the time. The Quaternary was a period of major glaciat ion in the Northern Hemi phere. Glaciers advanced and retreated a number of Li mes. At their maximum extent, they formed a continuous icecap over the northern parts of North America, Europe and Asia, reaching as far south as the United States of America, France and Germany. In Australia, by contras� there were few glaciers, because Au tralia was too close to the Equator. Australian glaciers were con fined to small areas in the Snowy Mountains in New South Wales and the highland areas of Tasmania. There were none i n Victoria. As the icecap in the Northern Hemisphere grew larger, the water level in all
162
Chapler 4
QUATERNARY
AREAS OF OUTCROPS
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Sedimentary rocks
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Newer Volcanics - basalIS erupled within the last 6-7 mHlion years
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,
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y 2r Sf' 7f 1qo Kilometres
Figure 4-87 Distribution of Quaternary sediments and
e\o\'er Volcanics in
Victoria.
The basalt hown all belong to the ewer Volcanic� They were erupted o'''r the last 6-7 million years, that is during the Late Tertiary and Quaternary times.
the oceans dropped. This happened because water was taken from the seas by evaporation and added to" the glaciers as snow. However, when lhe icecap melled, the sea-levels rose because waler returned to the ocean . The most recent expansion of the icecap reached its maximum extent about 17 000 - 20 000 years ago. At this time, the sea-level along the Victorian coast wa aboUl 120 melres below present-day sea-levels. Between Victoria and Tasmania there was a low-lying dry plain, so humans and animals could mO\'e freely between the two regions. Pon Phillip Bay and Western Port were dry land also. After their maximum expansion 17 000 - 20 000 years ago, lhe Nonhern Hemisphere glaciers retreated and large partS of the icecap melted. As a result, lhe sea rose very rapidly between 14 000 and 6000 years ago, until it was probably one to two metres above present sea-level. After that, it dropped lowly to about its present level. However, in the last 100 years it has begun to rise slowly again. As the icecap melted and lhe seas rose, there was a dramatic change along the coast. The coastline moved rapidly inland. The advancing ea pushed the coastal dunes in front of it. These dune systems now act as barriers acro s the mouths of the major rivers such as the Murray and the Snowy. They al 0 form the barrier sand dunes on the seaward side of the Gippsland Lake . As the sea-level rose, it flooded the nearshore parts of the valleys covered by rivers uch as the Yarra and Murray. These valleys still exist on lhe sea-floor and can be seen on bathymetric charts showing the shape of the ea-floor. The advances and retreats of the glaciers were al 0 reflected in changes in the vegetation. During the colde t period, about 1 7 000 - 20 000 years ago, the climate through much of V ictoria was cold, dry and windy. There were fewer tree , and extensive grasslands covered the low-lying areas. Becau e of the lack of plants and low rainfall, the topsoil was easily blown away by the wind . This was a time of great dust storm , when drought killed much of the vegetation in western Victoria. The windblown ilt and clay (called loess) blanketed the country ide as a thin sheet of sediment. I n the we tern suburbs of Melbourne and towards Werribee, a layer of loess, less than one metre thick, covers the Quaternary basalt flows. Much of the loess was later wa hed into rivers. This sediment supply was so great that some rivers were choked by it. Instead of being carried out to ea, the transported loess built up in the river c hannels, forming thick bed in places. Climatic changes aI 0 affected the lakes in Victoria. During the weller periods, there were many shallow lakes through western and nonhern V ictoria. Then, as now, the predominalll winds blew from the west. They built up beach sand deposits on
Geological History of Victoria
1 63
the eastern sides of lakes. As the climate became more arid, the lakes dried out. The winds blew the silt and clay from the lake floors, piling this material into low crescent-shaped lunettes around the eastern sides of the lakes (see Chapter 3).
TYPES OF SEDIMENTS River and lake sediments The thickest and most extensive Quaternary river sediments were deposited on the Riverine Plain along the Murray River, to the east of Kerang. These deposits of sand, gravel and clay are cal led the Shepparlon Formalion. They often contain soil profiles that were buried by a sudden influx of sediment, probably during flood periods. The channels of the rivers that deposited the Shepparton Formation are barely visible on the ground, because they have been fill ed by later sediments. However, on aerial photographs they often stand out clearly. The vegetation growing on the channel sediments is different from that on the adjacent flood plains. During the wet ter periods of the Quaternary, low-lying areas of north-western Victoria were filled with shallow, stagnant lakes. Some of these lakes still exist but evaporation has increased their salt contents. Rivers flowing south from the uplands have less extensive Quaternary sediments associated with them. These rivers carried loess washed from the surrounding hills, particularly in the central parts of Vicroria. For example, along the Maribyrnong River, near Melbourne, there are extensive deposits of a light brown sediment called the DOl/lla Galla Sill. This represents loess washed into the river 1 8 000 to 13 000 years ago. The Doulta Galla Silt is imponant because Aboriginal bones have been found within it. The oldest of these, known as the Keilor Skull, was from an Aborigine who lived about 1 5 000 years ago. Many stone tools with harp flaked edges have also been found in the Doutta Galla Silt. South of the uplands. lakes formed behind either coastal and barriers or basalt lava flows which blocked many rivers. Sands, silts and occasionally limestones were deposited in these shallow lakes.
Inland windblown sands In nonhern and north-western Vicr oria, thin sand sheets cover large areas. These are divided into two units: I. Crescent-shaped and straight, white to yellow sand dunes are common in the Big Desen, Little Desert and the Sunset country. They are up to 20 metres high and are called the Lewan Sand. 2. Between the desertS of Lowan Sand and the Murray River to the north-east, there are red-brown sands, silts and clays of the Woorinen Formalion. These occur as both elongated dunes and thin sheets of sediments. Lunettes occur along the eastern sides of many inland lakes. Aboriginal artifacts and skeletons have been found in some of these lunettes. There is an important site at Kow Swarnp, between Kerang and Echuca. When the lakes were full, there would have been abundant animal and plant life in the area, providing food for Aborigines.
Coastal deposits Windblown dune deposits are common along most of the Victorian coast. They are made up of sand blown off the nearby beaches. These dune sands show large internal cross-beds, that formed as the dunes migrated downwind. West of Wilsons Promontory, the sand on many beaches is composed of small shell fragments. When blown into dunes this material is readily cemented by calcite to form dune limeslone or aeo/ianile (Figure 3-59). Dune limestone forms prominent outcrops along pans of the central and western Victorian coas� e.g. Sorrento ocean beach, Barwon Heads, Warrnambool and Portland. I t also occurs as a series of low ridges parallel to the coast across south-western Victoria and the south-eastern region of South Australia. Each ridge line represents an old coastline. The ridges have been preserved because this region has been lowly uplifted. The old li nes of dunes were raised above sea level, where they could not be dest royed by waves and tides. Sub-horizontal red layers can often be seen in dune limestones exposed in road cuttings or cliffs. Sometimes these layers contain small, white, venical tubes, usually a few centimetres across. The red horizons are old soils. They represent times of abundant rainfall, when t he dunes were stabilised and vegetation grew over them. When the climate became drier and the vegetation died, winds blew across the dunes and buried the soils. The white vertical tubes consist of calcite. They formed around tree roots that grew in the soil at the time. Dune limestones near Portland contain numerous mall caves, particularly at Bans Ridge . Some contain bones of extinct marsupials and birds. About 6000 years ago, ea-level was one to twO metres above it present height, as previously mentioned. Evidence for the higher sea-level can be found at several places on the Victorian coast. There are hell beds at about this height in paddocks
164
Chapter 4
on Bellarine Peninsula, east of Geelong. Radiocarbon daring of these shells has given an age around 6000 years. In addition, at the mouth of the Yarra River, barnacles have been found encrusting rocks about two metres above present sea-level. Nearby shell beds at about the same height overlie older river sediments, which contain tree trunks dated at about 8000 years old. . n he fma [Xhl Hlon elu \1el oume, n 1871 . one f the ex/ltbl's "as a bas.:Jt tre. Man'. 1(0. y ou an cf year Cal Ii h('l a'a had O\\cJ ar und .1 t runK. ard cooled .lSt en ough te 10 m a l:' o Id )Un the ' ree trur.k l-e�ore e �ood urned Then. when he \\c d t d burn d\\a). hole had been , \\her< he tT'Jn� �Jd been. A _ter flov 01 basalt I"" th., "ole and r Ime
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 <'l t he dee un ne s r Is WI h a m:, ('Iokm' m t lun r e ked 11\ 1anl.l On 10 km� ) 5CC \\ ,at I "as, I fo Ihe rri•• \\� o'u eJ by 1 1. toot of I hark enocd cd n no " I of h l un n , a harK \\ h" r ct:OOC ,,"Cal age ad keen Ydrr. n 10 t haT 11ca ,t\ latcr }\II n c 'he r t bal k f lhe fI"cr M lh)Tn n P, k r"oale he ke eton ot a d)/� h n. \\he, ' e \\O rkm cn dlSCOVc cU t he r b . c thought t lev ad found I " mar. 'hen \\her. the) t.:ame UI 01 the !likuJ 'hl..")1 !Say, at 11 lad a long beaK and I hut; '1t I musl .., l 10o,;sll t'lirtl. bu t or 'sC('IV 'mg the beak wa.' lined with llanV l Ih thC\ were qUite puule as what the arllmal cc uld be I I was Delphlnlls 1e1phmll.s. t he \,.omm 1 ..rolph,c Of 11; pan 01 Ihe v.orld . : (from a broa(kaSl on A B.C radiO hy r.D. Gill, 'lalla �h.: eum of V ctor-a). •
Most tectonic movements during the last two million years were small-scale. Nevertheless, some significant faulting occurred. For example, the western side of the Cadell Fault, which runs north-south through Echuca, was uplifted about 30 000 years ago. This movement temporarily dammed the Murray River as a large lake (Figure 3-32). There was also broad uplift of orne regions, particularly south-western Victoria. The uplift is indicated by the presence of many raised beaches along the coast. Further evidence of uplift can be seen in the sea cliffs near Nelson, close to the South Australian border. The cliffs display sub-horizontal notches, each notch representing a former sea-level. Some of these notches have been uplifted as much as six metres above present sea-level.
FAU NA AND FLORA The giant marsupials, that lived in Australia during the Late Tertiary, survived through most of the Quaternary but finally became extinct about 10 000 years ago. The increasing dryness of the climate probably caused these animals to die out. They were apparently adapted to weller climates and lusher vegetation. However, their smaller relatives, such as present-day kangaroos and wallabies, were able to survive the changes. The dryness of the climate also caused great changes in Ihe vegetation. Rainforests, which once covered vast areas of Australia, gradually died back. They are now con fined to the wettest parts of the continent, including areas in eastern Gippsland.
ABORIGINES Aborigines have been living in Victoria for at least 30 000 years. Artefacts, including ground and flaked stone tools, are widespread over much of Victoria. There are also many Aboriginal middens. These are piles of shells at feasting sites, that are usually beside beaches or rivers. At a few places there are remains of stone traps built to catch fish and eels. The Aborigines coped successfully with the climalic changes that occurred during the Quaternary, including the great dust torms and the rapid rises in sea-level . The latter would have caused coastal tribes to move inland. During falls in sea-level, the Aborigines were able to walk across Bass Sirait to Tasmania. Recelll excavations have shown that Aborigines were present in Tasmania over 30 000 years ago. Aborigines also saw the most recent volcanic eruption at Mount Schank, about 4000 years ago. Legends from the tribes in Ihe area vividly describe Ihe eruplion.
Soils
in Victoria
Soils of Victoria Plale 1
Plate
2
Plate I Unirorm soil in the Mallee.
There s i a uniform sand texlUre throughout the profile. The sand is easy to cullivate but it is low in nutrienl.s, has a low water-holding capacity and a high wind erosion hazard.
Plate 2
Gradational soil in the Milllee. The texture is loam at the surface and it gradually becomes clay with depth. without obvious change. The white patches in the subsoil are lime ael"Umulalions. These soils have good physical and chemical Qualities ahhough sail concentrations are sometimes excessive.
Plate Plale
3
3
Plate 4 Plate 4 Podsol on South Vicloriun
Duplex soil on a gen lle: hillslope nCllr Bendigo.
Coastal Pluin, near Otway Range.
The horizons and their approximate depths are: A,: ().8 centimetres. Brown loam with moderate blocky structure.
This has a sand texture throughout and therefore
A�: 8·)0 centimetres. Pale brown loam
with stones presem, weak Structure. seasonally hard-selling, a sharp break
is a uniform soil. The horizons and their
approximate depthS arc: AI:
to the horizon below.
B:
C:
it
0·20 centimetres. Black sand. soft, weak
30-80 centimetres. Red brown clay
structure.
with moderate blocky structure. 80 cemimelres onwards. Weathered
AI:
20·90 centimetres. Off·
while sand, single grain struclllrc, soft. There is an unusually uneven boundary to the horizon belo..... , with 'pipes' of
Ordovician shales.
Al material ('.'
B:
90 ccntimetres on.....ards.
Yellow sand, single grain Structure, soft. The irregular sur face of this horil.on is coated by dark organic mailer. Snong acidity and Strong profile dc"elopmcm sugge<;t that this is an old soil.
'·'utc 5 An erosiun I(ull)' on a
Iuw('r I('nllc
hillslupc ncur I-Ic-athl'oll'.
Tunnelling has mntribtllcd to the developmel11 of this wide gully. The soil i.. .1 '\odic duplex type with a vcry pale, k30liniti� deeper U hori/on. Thcre is a prominent layer of iron oxide nodules ncar the boundary bctwl'e!l the A and B hori.l.Ons. The pate material deposited ncar the fen�e i.. m'ller;,,1 eroded from the gully.
Platc S
165
166
Geological Features along the Victorian Coast
Geolog ical Features along the Victorian Coast
Plate 6
Plate
6
Plate 7 A coastal cliff S()uth-west of Anglesea.
Eagle Rock. a rock stack near Split POint. Aireys Ink!1.
The cliffs along the surf coast from Torquay to Aireys Intet show excellent exposures of various Tertiary formations within the Otway Basin. At Eagle Rock, there are two units of Late Oligocene age separated by an unconformity. The dark rock at the base is olivine basalt. liS uneven eroded surface is overlain by buff-coloured Poin! Addis Limestone. a sandy bryozoan limestone containing echinoids. (Photograph courtesy of Geological Survey of Victoria).
Pink clays and siltstones below the white bed belong to the Anglesea Member of Eocene-Oligocene age. The white rock and overlying pink beds are part of the Angahook Member. which contains tuffs and
sediments derived from volcanic rocks, as well as sands. clays and gravels. The beds dip towards the coast at a low angle. There is a tendency for landslips to occur down-dip along wet clay surfaces. e.g. JUSt above the white bed. This feature is apparent at nearby Soapy Rocks. (photograph counesy of Geological Survey of Victoria).
Plate 9
landslips nfar the roast !>etwfen Anglesea and Point Addis. Attack by storm waves continuously undercuts the cliffs formed by beds of the Anglesea Member. Groundwater penetrates down fractures and along bedding planes and blocks of ground slip over clay beds towards the cliffs and topple to the beach. (photograph courtesy of Geological Survey of Victoria).
Plale A basaltic dyke of Early Tertiary age is cutting across Devonian granitf on the coast near Point Hicks in East Gippsland. The dyke has been displaced a shon distance by a cross-fault, There are several directions of strong jointing in the granite, (Photograph courtesy of Geological Survey of Victoria).
Plate 10
Platf to Steeply-dipping. thin-bedded Ordo\'ician cherts and shuJes form an unusual coaslline near Mallacoota in East Gippsland.
(PholOgraph by J .A. Webb).
Plale 1 1
Plille I I A view looking t o thf east across Lakes Entrance, On the left. tretS covet the stranded cliffs of a former coastline near Jemmys Point. al Kalimna. The cliffs consist of Pliocene calcareous sandstones with tWO prominent shell beds. Other features are the Nonh Arm on the lefl. Cunninghame Arm on the right. Bullock Island in the foreground and the Ninety Mile Beach, Recent uplifts of the coast led to the formation of the inner and outer sand barriers.
Various Geological Features around Victo ria
167
Various Geological Feat u res Around Victoria
Plate
Plate
12
Plate
12
An unconformil)' between Pleistocene gravels and clays and Permian tillile in cliffs along the Werribee Rh'er near Bacchus Marsh, There wa� a large lime break and change in the environment belween the deposition of the lower
pale-coloured jXlOdy .
bedded rocks by OJ glacier and Ihe overl yi ng. well-bedded brown strata
of nuv;al origin, ( Photog rap h by J . F . Bi lney)
.
Plate
IJ
13
Columns of rhyodacite on the noor of t he 5no\\ y River along ilS gorge north of Buchan.
Columnar structures are most commonl>' secn in Cainozoic basalts, as in the Organ Pi pes Park near Melbourne. Howcvcr. t hey are also found in some Devonian acid volcanic rocks of the Snowy River Volcanics, (Photograph cOurlesy of Geological Survey of Victoria).
Plate 1 4 (left)
1
Plale 14
Paradise Falls on a tributary of King River, south of Whitfield.
H igh cliffs of gently-folded Late Devonian 10 Early Carboniferous sedimentary and a<:id volcanic rocks form magnificent scenic escarpments through Gippsland, bUI most of t he coumry is lacking in access roads. Paradise Falls in nOrlh-eastern Victoria is one local ity that can be reached easily by tourists. Thick beds of conglomerates represent alluvial fans deposited in Carboniferous times, (Photograph by J .A. Webb). Plate 15 (right) Complex folding and minor hulling in metamorphosed sedimentary rocks al Copes Hills. wesl of Ararat.
These rocks belong to the St Arnaud Beds of Cambrian age.
(Photograph courtesy of Geological Survey of VictOria).
Plate
16
PIBte 17
16 Lepfoleps. i an Earl)' Cretaceous fish from (he
Plate
Koonwarra rossil locality.
Plate 1 7 Mount Bogong. the highest mountain in V ictoria.
The high COllOtr�' is composed of schists and gneisses of t he Wagga-Omeo Metamorphic Complex.
Plate 18 Tilled De,'o nian sandstones in The Grampians.
This is a typical cuest a landform with a long. gently·inclined dip slope and a steeper escarpment on the right . Rocks in The Grampians typically have a reddish colour due to the presence o f iron oxides. This renects the fact that they were mostly deposited by rivers under arid conditions.
Plale 1 8
1 68
Some Aspects of Economic Geology in Victoria
Some Aspects of E conomic Geology i n Victoria
Plate 20
Plate 19
Plate 19 A trench excavated in Parilla Sand at Ihe \VIM 150 heavy mineral p rospect nur Horsham .
The darker bands are concentrations of valuable heavy minerals. which are a potential source of titanium. zirconium and rare�arth minerals. The noar of the pit is in clay, which underlies the sands throughout the area. This area may become Victoria's moSt important mining field after the Latrobe Valley coal fields. if problems relating to the very fine sizes of the sand grains can be overcome. (Photograph by J.A. Webb).
Platt 20 A wei-mining opel1lltion al Scarsdale, south·west of Ballaral.
Boral Resources (Vic) Pty. Ltd. extract sand and gravel from sediments deposited in an old river valley in Tertiary limes. Some alluvial gold is also recovered. The sediments are broken down by directing a slrong jet of water on to them from a 'monitor', (centre rear of photograph). The material is washed into a sump and then pumped to a plant. where various size fractions are separated. This form of mining is called 'sluicing'. (Photograph courtesy of Geological Survey of Victoria).
Plate 2 1
Platt 22
Plate 2 1 Cores of rocks obtained from a n UploNliion drill hole o n Stawell Goldfield.
Nowadays before a decision is made to mine or quarry any kind of mineral deposit. it is usually necessary to prove the extent and grade of the deposit by a series of drill holes. In the photograph, solid cores of rock have been obtained by using a cutting bit embedded with small fragments of diamonds. The white cores arc quart'l, where the gold is most likely to occur. The dark cores are metamorphosed sedimentary rocks. intersected by a few vein lets of Quartz. (Photograph courtesy of Western Mining Corporation Holdings Ltd.). Plale 22 Chakop)'rile, a common ropper mineral, in quartz from an old mine dump on 8elhang" goldfield. east of Wodonga.
Plale 23
Bethanga is unusual among Viclorian goldfields insofar as it is one of the few where the gold is accompanied by large quantities of sulfide minerals. Between 1877 and 1 9 1 6 , small Quantities of coppt.r were: produced intermittemly from the gold·sulfide Ores at smelters on this field. Chalcopyrite is a brassy yellow colour when fresh, but in the photograph the mineral has tarnished 10 various reddish colours due to exposure to the air. (Photograph courtesy of Geological Survey of Victoria). Plate 23 Blocks or hematite ore at a small disused open-cul mint about eight kilometres north of o,,'a So"a In Easl Gippsland.
About six million tonnes of magnetite·hematite orc "",ere discovered by drilling an area indicated by a magnetic anomaly. Most of it occu rs benealh 25 to 80 metres of De,'onian volcanic rocks. Only small quantities of thi ore have been extracted for minor mdU5trial uses. The reserve i� too small and covered by too much overburden to 'Aarrant developing it for sales to steelmaking companies.
Platt 24(a)
PiaIe 24 Ornamenlal ruck, rrum Gipp..lllnd.
Plate 2�(b)
(a) A bright red granite from Gabo hiand wa, u..ed for ornamental ..toncwork in wme early building.� in Melbourne. (b) Th(.�c pieces or poli..hcd, Silurian rccry,talli ..cd limc'tollc were obtaill(."tI from the I.imC\tone Creek-Clare Creek area in lIunh ·ca\ tern Victoria. Thi"l arca probably contouR' the mo..t attractive variety of colour(.'(j limestones in the State. Early attcmpt'" to dcvclop �I ' ma rblc' indu\try in thi, area fa iled bccau'e of the high Iran\port cmt.. involved in carrying the rock, ,liang poor trad, down the T;unbo Rivcr valley to the railhead ott Ilruthcn. In reccll! t imc�. a propo�al to develop Ihc\c dC(lO\ i t!-. lap-.cd when the urea W;J\ p lac(."tI in a muional rark. I PhOlograrh (.'Ounc�y Mu�cum 01 Vktoria).
Economic Geology
1 69
Chapter 5
ECONOMIC G EOLOGY Figure 5·1 The 'Southern Cross' oil well drill i ng rig carrying out exploration in Bass Strai t for ESSO·BHP.
A
The Southern Cross is a semi· submersible drill·rig, which i s towed between sites b y a large work boat. Between 1 98 1 and 1 990, the rig drilled 54 wells in the Gippsland Basin. While drilling it
is anchored in a precise position and
stabilised by five automated self· correcting rotors controlled by computerised equipment. It is designed to drill below up to 450 meters of water on the continental shelf. It can operate with waves up to seven metres high and can penetrate up to 6 kilometres below the sea·floor. A typical offshore oil wel l costs between $5 m i l l ion and $ 1 0 m i l l ion dollars to drill but sometimes much more. Some features of the drilling rig are: A. drill mast B . winch for lifting the drill pipes C. stack of drill pipes D. hetideck (for helicopters) E. crane (photograph courtesy of BHP Petroleum Ltd).
The concept of resources
A very large n umber of chemical substances occur naturally on our planet, Eart h. They can be divided into: • •
organic substances (plants, animals) - formed by biological processes; inorganic substances (rocks, m inerals) - produced by geological processes.
Many of these substances are useful to mankind and they are then called
resources.
Thousands of years ago, the needs of primitive humans were very limited. Then, the only things regarded as resources were air, water, a few rocks and t rees used for weapons and tools, mineral pigments for paints and a few plants and animals, which provided food and clothing. As people advanced from the SlOne Age to Ihe Bronze and Iron ages, various metals came to be used and pottery was made from clay. Since then, as people increased their skills and standards of living, their demands for new types and greater quantit ies of resources have grown continuously. In today's industrial societies, biological resources supply most of man kind's food and medicines, many textiles, t imber, leather, hides and skins. All other needs are provided by geological resources. These supply the raw materials for nearly all construction work, modern forms of transport, domestic 100is and appliances, industrial machines, coml11unication systems and even arts. spons and leisure items.
Chapter 5
170
Most of the fuel and energy consumed by industrial societies is also derived from geological materials. Resources formed by geological processes are commonly called mineral resources. The branch of geology, which is concerned with the study of the resources, is called economic geology. In modern industry, the term 'mineraf is not limited to its strict scientific meaning of 'a narurally occurring, pure chemical substance': it also embraces various mixtures of minerals that form useful rocks and sediments. In this broad sense, the mineral industry is di fferentiated from other primary i ndustries, such as agriculture, forestry and fisheries.
Figure 5-2 Ten commonest elements in the Earth's crust. (After B. Mason, Principles of Geochemistry, John Wiley and Sons, 1966) .
M I NERAL RESOURCES Occurrence of mineral resources There are over 90 naturally-occurring chemical elements and over 3000 known minerals. It might be thought therefore that there would never by any scarcity of minerals to satisfy all mankind's needs. Unfortunately there are several reasons why this statement is not necessarily true: I . The various elements, minerals and rocks are not distributed evenly through the Earth's crust. Crushed rock, used for road-building, might seem a commonplace commodity. But throughout most of the Victorian Mallee and Wimmera regions there are no hard rocks. There, thin layers of hard calcium carbonate within some oils have to be used for road metal.
(
la n t
of energy - petroleum, coal and uranium - are also comparatively scarce.
m
l. Il;TI
2. Many valuable elements are rare in the Eanh's crust, e.g. copper makes up < 0.0070/., zinc < 0 .0 1 3 % and silver <0.0000 1 % of the crust. The main sources
r, H
3. Some useful elements are common in the Earth's crust, but they cannot be extracted ea i1y from most of the minerals in which Lhey occur. For example, aluminium is a valuable metal, which forms 8 . 1 % of crustal rocks. It occ urs widely in clays, feldspars and micas. However, it is only economic to extract aluminium from one relatively uncommon mineral, bauxite (a mixture of hydrated aluminium oxides). It is an unfortunate fact that mo I minerals, that people want, are either rare or they are 0 par ely caltered through rock , that it is not practicable to extract them. Most useful concentration of mineral resources were produced by rare geological events. These events occurred only because there were special combinations of favourable chemical and physical condition prevailing at a particular time in the Eanh's history. Minerals only become geological resources where they are sufficiently concentrated to justify the cost of removing them from the ground. A mineral resource must be old in the marketplace for a sum that exceed the total cost of extracting it from the ground, passing it through a treatment plant and delivering it to the market. Except in time of national emergency, there must also be scope for the mineral producer to achieve a reaso nable margin of profit, otherwi e investment funds will not be directed to the development of mineral resources.
Classification of mineral resources For convenience of di cussion, economic minerals are usually divided into several categories, which are only loosely defined. These groups are discu sed under the headings of constrllction materials, fuel or energy minerals, metallic minerals and industrial minerals and rocks. Thi chapter describes mineral resource that are available in Victoria. Constfllction materials are of twO kind : •
•
rock that are sold after simply being either cru hed or cut into block : edimem used in the building indu try, e pecially and, gra\'el and clay.
Fuel or energy minerals are used to produce electrical, mechanical and thermal (heat) energy for home , offices, indu tries and tran pon ervice . The main energy minerals used in Vi toria are fo il fuels of biological origin - coal, natural gas and petroleum liquids. Water, when used to generate hydro-electricity, is also an energy resource. Metallic minerals are t hose from which pure metals are produced, usually after various stages of proces ing. For example, hematite (Fe,O,) is a metallic mineral, because it i mined and treated to yield some form of metallic iron (Fe), such as steel or ca t iron. Industrial minerals all d rocks comprise all other useful earth materials. They in lude rocks, which are u ed as they occur in the ground, (e.g. magne ite, dOlomite), and minerals which are con\'ened to a wide range of chemicals u ed in factories,
Economic Geotogy
171
on farms and in homes. Mineral salts such as halite or common salt (NaCl) and calcite (CaCO,) are not regarded as metallic minerals, but as industrial minerals. This is because they usually are not convened to the metals sodium or calcium, but to various other chemical substances comaining these metals.
ECONOMIC SIGNIFICANCE OF ROCKS AND MINERALS Australia The discovery, development and supply of rock and mineral resources are essential for the survival, prosperity and comfon of mankind. The constant quest for bener living standards by people throughout the world and the continual growth in the total world population (Figure 5-3) leads to an ever-increasing demand for the Eanh's raw materials. The production of minerals not only satisfies human needs but it also creates wealth for the countries which export minerals to other nations. In 1 988/89. the profits generated by the minerals industry in Ausnralia accounted for 7 % of the Gross Domestic Product or GOP. (The GOP is the total value of all goods and services produced by a country's economy during the period). A major item in the GOP is the difference between the value of expons and the value of impons. Figure 54 shows that mineral products for the 1988/89 period accounted for 46.5% of the total value of Australian expons. Since the 1 980s, Australia's balance of payment ha been in debt. that is the cost of impons has greatly exceeded the income generated by expons. Nevenheless this financial problem would have been far worse, but for the large income coming fTom expons of minerals and metallic products manufactured from minerals. M ineral commodities that earn over one bill ion dollars annually for Australia are black coal. petroleum prOducts, gold, aluminium (and alumina) and iron ore. Other mineral expons, that are valued at over 100 million dollars annually, are copper. lead. zinc. steel. uranium, mineral sands (rutile and zircon). manganese, nickel. salt and diamonds. Apart from the export earnings derived from minerals, the industry also contributes more to the national economy in payments of taxes and charge than most other industries. This is so, despite the fact that some tax concessions have been made to the gold mining industry. Most mining industries, besides paying normal company taxes, also pay royalties to State Governments and sometimes to private landowners. Royalties are payment made to the owners of the minerals on every tonne of mineral produced. Most minerals belong to the Crown (a repre ented by State Governments), although in Victoria construction materials on private land belong to the landowner. Many mining operations al 0 make large contribution to Government revenue because they are major users of railways and ports. They also provide the total infrastructure (commercial, dome tic and public buildings, roads, etc.) for many inland mining town at a cost which is usually 45-50"10 of the total project cost. Figure 5-3 Projected world population growth. (From Lynch A . J . , 1989, Aus! . I . M . M . Annual Con ference) .
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r' I"-
,
Victoria Victoria is generally not considered to be a mllllllg state. Manufacturing, grazing, agriculture and commerce are regarded as the main activitie . Certainly Victoria has never yielded a wide variety of minerals. Neverthele , throughout the history of the State, a few minerals have played vital roles in the local economy. The key commoditie have been gold, brown coal, construction materials and petroleum. Soon mineral ands may become another major Victorian indu try. Gold
The first indications of gold were found in Victoria in 1 839, only a fe\\ years after the arrival of the first British senlers. However, it was not until mid- I SS I that mining started at Clunes, north of Ballarat and in Andersons reek, Warrand)'le, near Melbourne. Soon a fterwards, large amounts of gold were found in shallow soils and st ream sediments at Ballarat . Bendigo, Mount Alexander (Castlemaine) and other localities. The news of these discoveries soon spread around the world and huge numbers of people nocked to Victoria hoping to make quick fonune . This wa, the period of the great Victorian gold rushe , vhen thousands of men hastened to each ne\\ area where gold was reponed. The gold discoveries had a profound effect on the economy of the Australian colonies and that of Victoria in particular. alcs of gold brought a great Ilow o f money into the colony. Early shallow gold mining I' as so successful, that its profit largely financed a later. more cost ly pha,e of deep underground mining. Relatively
172
Chapler 5
linle overseas capital was needed to finance Victorian gold mining and the profits were mostly ploughed back into the Australian economy. Mining also stimulated investment in many new industries, which supplied goods, equipment and engineering plants to the mines. The financial system grew rapidly as many new banks were opened. Melbourne became the financial centre of the continent, a position it still holds. The new wealtb was used to finance the construction of many large buildings, not only in Melbourne but al o in the larger gold mining towns. Education developed quickly and the Universily of Melbourne was opened only a few year after gold mining commenced. The population of Victoria grew from 77 000 in early 1 85 1 10 540 000 ten years later. When gold mining eventually declined around the end of the nineteenth century, the large population mostly remained and became available for other work, especially in the growing manufacturing industries.
Figure 54 Value of Australian exporls
Total
1988-1989.
n% • Agrlcuhl,l1.1 ProdUCII o PrllNry Mineral Products IJ MetalS & SlfT1pie Melal ManufacllXInQ � OIhet Nco·Aural ProdUCIs o Machinery & Equipment CJ Other Manufacturers
29.26% u
9 91%
• o iii ED o • 8
Figure 5-5 Value of minerals produclion in Victoria (excluding construelion malerials and pelroleum). (From Annual Reporl. 1988-19 9, Depanmenl of Indu Iry, Technology and Resources).
17"0 • o 0 La 0 • B
Brov.n Coal 1l9 0� Minerals production in Victoria 1988/89 S Value
Cool I
Go<>
A'I.!i'1"Aum
..... Z,nc 0<"'"
Copper irOI' &
5tHI
Con truclion materials Deposit of rocks and minerals uilable for construction work are found throughout Victoria, except in Ihe nonh-western pari of lhe Stale. During the earliest gold rushes, Ihe miners lived under canvas, while local limber was used 10 build larger accommodalion. BUI 0011 more substantial permanent buildings were constructed from various building SlOnes Ihal were available near Melbourne and Ihe main gold mining centre . Many subslantial nineleenth century building , made of basalt, granile, sandstone and limestone, can still be een. Laler when Ihe use of building stone became less fa hionable, there were ample supplies of malerials needed for modern melhods of construction, e.g. limestone, sand and crushed rock for concrete, and clays for bricks and tile . Basalt and some acid igneou rock (e.g. granite, dacite) provided crushed stone for railway ballast and later, road metal, as t ranspon sy terns were developed throughout the State.
Economic Geology
173
In this regard, Melbourne was more fortunate than Sydney, where sandstone is the only common rock type suitable for construction close to the city.
Brown coal Timber was the first fuel to be used for heating and cooking and to generate steam
f
n Ja h
power for industry in Victoria. By 1 860, black coal from New South Wales was being burnt to produce gas for heating and lighting and later to generate electricity. However, the discovery of brown coal in the Yallourn area and the decision soon after World War I to use this resource as the fuel to produce electricity made Victoria independent of interstate supplies of steaming coal. With local brown coal being the main source of energy, the cost of electricity has always been relatively cheap in Victoria. Huge reserves of brown coal guarantee that supplies of electricity will remain stable in to the foreseeable future. This has made Victoria an attractive state in which to set up new industries, which use a lot of energy. Major examples of recently developed power-intensive industries are the aluminium smelters at Portland and Point Henry. The discovery that carbon dioxide effluent from coal-burning power plants contributes to the so-called greenhouse effect may place some uncertainty over the future of the brown coal i ndustry in Victoria. It is pos ible that alternative cleaner forms of energy generation will eventually be adopted.
Petroleum
Commercial quantities of petroleum were found in the Gippsland Basin off the south eastern Victorian coast in the 196Os. Since then, the development of the Victorian Oil industry has yielded great benefits for the Australian economy as a whole. The discovery of Gippsland oil could nOt have come at a better time for Australia. It happened when the main overseas oil-producing countries (OPEC) had decided to greatly increase the price of crude oil to help their economies. Australia was spared the financial problems endured by many oil-deficient countries, which were faced suddenly with large i ncreases in the costs of their importS. Despite the heavy Federal Government taxes and charges on oil production in Australia, consumers still enjoy some of the lowest prices for petrol in the world. The Victorian economy also benefits from royalties paid to the State Government on oil and natural gas produced in the Gippsland Basin. In 1 987/88 this region produced 84"70 of Australia's crude oil and 39"70 of its natural gas. The royalties on this production paid t o the State Treasury amounted to $ 1 47.2 million.
mn l l Moreo' m per n e w rd qu I' bal c bee d r
The future
e I ac pu " h
all r th W ok I 21 2'"1 A
Extraction of rocks and m i nerals
During the 1 990s, the production of oil from the Gippsland Basin is expected to decline substantially, but tbe role of supporting the Australian economy wiil probably be taken over by fields 0 ff the north-west coast of the continent. The Victorian economy will suffer by the decline in royahie,. One new mineral industry, however, promises to become of major importance as a source of import earnings. Extensive deposits of heavy mineral sands, containing titanium, zirconium and rare earth metal compounds have been discovered during the past decade over north-western Victoria. This resource is discussed in detail later in the chapter.
Various terms are used to describe industrial operations, where rocks and minerals are extracted from the ground. A mine is an excavation to obtain metallic minerals, coal and some industrial minerals. A quarry is an excavation that is open to the air and used for extracting construction materials and most industrial minerals. Strictly speaking, the term quarry is limited to a series of benches cut into a hillside. Where there is a large open working below surface, it should be called a pit. Any mineral deposit can be obtained from a pit. Mines are of several kinds: I.
Underground mine, where access to workings is obtained thro ugh a vertical shaft or a declined tunnel.
2 . Open
CUI, which is an open hole, worked from a series of benches reached by
ramps cut into the sides of the cut. 3 . Wei mine, where the mineral deposit is under water. Various kinds of machinery are used to dredge, pump or lift mineral
from wet mines.
174
Chapler 5
Control of m i ning and quarrying
I n most states o f Australia, all minerals are owned b y the State and most forms of mining and quarrying are controlled under a Mines Act. Petroleum and coal may be covered by separate Acts. In Victoria, however, a distinction is also made between mining and extractive industries. The Victorian Mineral Development (formerly M ines) Act only covers metallic and industrial minerals and coal. Construction materials are controlled by an Extractive Industries Act. These Acts are administered by a State Government department, (presently the Department of Manufacturing and Industry Development). An individual or a company wishing to develop a mineral deposit in Victoria must first obtain a mining title or an extractive industry title. These titles give the holders permission to conduct an operation for a certain period, say 10 or IS years. Titles are usually renewed i f the whole deposit has not been removed during the first period. There are also other forms of titles, which allow the holders to carry out exploration for minerals before a decision is made to commence mining. [n general, before a lease to mine or licence to extract is granted, the applicant must present a proposal covering some or all of the following aspects of the operation: I. Evidence that a workable deposit o f mineral(s) is present, based on geological investigations and results of testing. 2. A working plan describing how the deposit will be extracted.
3. A rehabilitation plan, setting out how the site will be restored to another use after mining ceases.
4. A landscaping plan to reduce any adverse visual impact from the operation. S. An environmental i m pact statement may also be required to show that the oper ation wil not be detrimental to the surrounding environment. Various Acts of Parliament and related regulations control how mining and quarrying are carried out, e.g. regulations on safety matters deal with the handling and storage of explosives, the use of heavy equipment, etc. Local government authorities use their planning powers to determine where mining and quarrying can or cannot lake place. The Department of Manufacturing and Industry Development sets special conditions under which these activities operate, after consullation with local government planners and other State authorities, e.g. Environment Protection Authority. The conditions relate partly to environmental issues, e.g. measures to suppress noise and dust, and limitations on working hours. To reduce public awareness of operations, especially in urban areas, owners are usually required to plant tree screens or erect earth bunds around the boundaries of their properties. Restrictions may also be placed on the height and colour of buildings. Mining and quarrying are only temporary uses of land. Most resources are eventually worked out and the land becomes available for other uses. Many disused pits, especially in the Melbourne metropolitan area, have been used for rubbish disposal before being redeveloped for parks or playing fields. Figure 5-6 Crushing and screening plant at an Oaklands Junction quarry operated by Readymix Group (Vic) Ply. Ltd.
A new crushing and screening planl was opened in 1990 al this quarry, where granite and hornfels are eXlracted. Conveyor belts carry the various sizes of rock fragments on 10 different stockpiles. Two aspects of modern environmental controls have been incorporated in the design of the plant. Noise and duSl hazards are reduced by having crushing machinery completely enclosed inside insulated build ings. The
visual impact is reduced by painting the buildings pleasant green and khaki colours. (PhOlograph by G. W. Quick).
Construction materials
Most materials used in modern con't ruction work except wood are obtained from rocks or unconsolidated sedimentary deposit�. Apan from steel, they arc all relalively low cost prod ucts. Const ruclion rocks and sedimenl� arc handled in bulk al pils and quarries. lillIe processing is involved aparl from crushing and screening 10
Economic Geology
175
provide products within certain size ranges . Some quarry products are used directly, e.g. for road foundations, railway ballast. Others are mixed and often heated, so that chemical reactions take place to give building materials, e.g. concrete, bricks. Some construction materials are called aggregales. Aggregale is a term used for hard, chemically-stable rock fragments or grains. Aggregate is either used alone or mixed with a binding material (e.g. cement, bitumen, clay) to form a hard product (e.g. concrete, mortar, road pavement). Aggregates are divided into two groups: • coarse aggregate, i.e. crushed rock and gravel; • fine aggregate, i.e. construction sand. Quarries where crush ed rock is produced are often called hard rock operations to differen tiate them from the SO/I rock industries at sand and clay pits.
LOCATION OF THE CONSTRUCTION MATERIAL INDUSTRY R
Large quantities of construction materials are needed by all modern societies. In Victoria, about 9 lOnnes of construction materials are used annually for every head of population. I t is essential that they remain relatively cheap commodities otherwise the costs of such things as housing and road construction will rise considerably. Low selling prices are possible because the costs of operating large quarries are also low. However, bulky products such as construction materials are expensive to transpon. Therefore to k eep elling prices reasonably low, it is desirable to have quarries near markets. Fortunately this is usually possible in Victoria, where resources are large and widespread . Ideally in their planning schemes. municipalities should reserve for extractive use adequate land whert deposits of construction materials have been identified by geological surveys. This was not done in the south-eastern suburbs of Melbourne however. There, potential sand resources were overrun by residential developments, which made more intensive use of the land. To an increasing extent, Melbourne's large and requirements must be carried now from deposits in west Gippsland at higher costs to consumers.
HARD ROCK QUARRY PRODUCTS The main kinds of products sold by hard rock quarries are:
I . Crushed rock for road construction. The material used in the lower parts of a road pavement has a wide range of particle sizes. Road surface material, however, has a narrow range of sizes (Figure 5-7). 2 . Crushed rock for con crete made with Portland cement. This material ha narrow, well-defined range of particle sizes.
a
3 . Ballast or coarse crushed rock for railway construction.
4. Filter and bedding materials used in dam walls and pipe laying. 5. Large blocks and slabs of up to 20 tonnes in weight u ed to prevent erosion at dams and seawalls.
figure 5-7
Cross-section through
Terms used in road con truction.
A sealed road resembles a sedimentary deposit - it is made up of a series of layers, each consisting o f different geological materials. Special names are given 10 each layer. The prime purpose of the surface course is to resist the wear and tear caused by traffic and climatic effects (e.g. rain). I t i s made up o f a strong-wearing coarse aggregate and asphalt mix. The base COurse carries most of the load provided by traffic. I t consists o f coarse aggregate thal, for reasons of economy, may be of lower quality than the material used at the surface. The sub-base i a layer of coarse, granular material (e.g. gravel, sand) or non plastic soil. which protects (he base course and separates it from the subgrade. The subgrade is natural undisturbed rock or soil basement or selected fill.
8
sealed road
Portion traversed by vehicular Iralhe
ERGE
ROADWAY OR CARRIAGEWAY
- PAVEMENT. - '-c , SHOULDER SHOULDER
ht:=='---- FORMATION -ROAD
Trimmed or prepared surface
.....
--i--
Og � o_o .c> �.
'-
��
,:, " '::'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.<;u m
DSlur')' M�'i����3:iCIMn A,r & Cases f�:·.�· .:1 Clinker DCcmenl
192
Chapter
5
starts to decompose at the cooler end. At the hotter end, CaO, AI,O" SiO, and iron oxides react to form a fused mixture of cement compounds known as clinker. Carbon dioxide released from the limestone, combustion gases and steam pass back up the kiln and are removed as exhaust gases. After pouring from the kiln, the clinker is cooled and stored. Some gypsum (Ca,SO •. 2H,0) is then added and the mixture is crushed, fine ground and passed to a series of storage silos. The silos hold up to 30 000 tonnes of product. Different varieties of cement, such as fast-acting and slow-setting types, can be produced by varying the proportions of raw materials used. Because a slurry is fed to the kiln, this operation is known as a weI process. At some other cement plants, a dry process is used. Quality control
It is essential that each cement product should always be of uniform, specified composition. To achieve this, at intervals samples are sent for chemical analysis from various points in the line of production. Sampling starts at the drill holes prepared for blasting and continues on the overland conveyor belt, in the slurrying section, kilns, cement mill and at the despatch section. Environment protection measures
The greatest nuisance in a cement plant is dust, which is present in the ernuent gases from the kilns, in the grinding mills and wherever the process materials are moved from one section of Lhe plant to another. Various techniques are used to keep the du t emission below levels set by the Environmental Protection Authority. The noor of the quarry is 80 metres below the level of the Moorabool River and every day about 25 ()()() tonnes of groundwater seepage have to be removed to maintain reasonably dry working conditions. The salt content of the quarry water is continuously monitored. Provided the a1t content does not exceed a maximum level set by the Environmental Protection Authority, the water can be pumped back into the Moorabool River, downstream from the quarry. Water with excessive salt must be pumped further to Corio Bay. Run-off water from the cement plant is collected in settling ponds. After olid particle ettie, clear water is pumped into the river. This article is based on in formal ion provided by Australian Cement Limited.
Fuel minerals
Coal and petroleum, which are discussed in this section, are called fossil fuels. This means they were formed by the decomposition of living matter. They are used to produce energy for lighting, heating and cooling and to drive a vast range of machines. Energy can also be obtained by harnessing natural forces (e.g. Sun's heat, wi"d power, running water) and from other natural resources (wood, plant oils, uranium ore). Thousands of years ago, when people needed energy, they had few alternatives. Wood was used for heating and wind for propelling ships. By the Middle Ages coal was also widely used, at least in Europe. In a modern society, however, there are many way of supplying energy. Householders for example can heat their homes by using natural gas, wood, electricity, oil, solar energy or bottled gas. Public authorities may use fossil fuels, nuclear energy or the force of moving water to generate electricity on a large-scale. Although there is sometimes much debate, at community or domeslic level, before a decision is reached, eventually a particular form of energy is selected because it is the most efficient available. In Victoria, brown coal is used to produce most of the State's electric power, because it is the cheapest fuel available and reserve are unlikely to run out in the foreseeable future. Petroleum is another important commodity, because so many machines are designed to operate on petroleum prouucts.
COAL Nature of coal Coal is a sedimentary rock formed from the partial decomposition of vegetation. If a piece of coal is examined under the microscope, it is seen to consist largely of pieces of organic material; these are called macerals. The plant remains include wood fragments, leaf cuticles, spores and resins. They consist of various organic chemical compounds containing mainly carbon, oxygen and hydrogen atoms. Nitrogen and sulfur arc minor constituents. Water and small amounts of sedimentary materials, such as quarLl grains and day, are also present ill coal. These mineral grains form the ash content of a coal.
Economic Geology
193
Varieties of coal There are various kinds of coal. They differ in appearance, composItIon and efficiency as a fuel. The most important property of a coal is its net specific energy. This is a measure of its energy or heating value. The net specific energy increases as the amount of decomposition of the original plant material becomes greater. This is called an increase in the rank of the coal. Four of the commonest types of coal are shown in order of increasing rank in Figure 5-22. There is a progressive change in appearance from peat, which is a dark brown, wet, spongy material to anthracite, which is hard, black and shiny. Figure 5-22
Composition of coals and wood.
Nel speci fie energy (Megajoules/kilogram)
Carbon' 'I.
Oxygen' 'I.
Hydrogen' 'I.
Water ('I. by weighl)
45·50
40-45
6-7
variable
Peal
50·60
3540
5-6
75-80
5
Brown coal
60-75
20-30
5-6
50-70
5· 1 5
Black coal
75-90
1 0-15
4-5
5·10
24-33
Anthracite
90-95
2-3
2-3
2·5
25-38
Wood Coals (in increasing rank)
*These percentages are calculated on
dry.
ash-rree samples.
Uses of coal The energy in coal is converled to other forms of energy. Its most important use is in the generation of electricity at thermal power stations. The heat of burning coal is used to convert water to steam. The steam operates turbines, which in turn drive generators to produce electric power. Coal used for this purpose is called steaming coal. Coal is also burnt to provide heat in many other industries. Another major use of some hard, high rank coals is in coke manufacture. Coke has a very high carbon content. It is used as a reducing agent in a blast furnace to produce metallic iron from iron ore. Anthracite and black coal are the mOst efficient coals for producing energy. Victorian brown coal has a much higher water content than black coal and so it has a lower net specific energy. II would be uneconomic to transport the water, which has no value, over long distances. Victorian brown coal also gives off much heat when exposed to the atmosphere. It therefore tends to catch fire spontaneously if transported far. Hence brown coal is usually u cd near the mines. Alternatively the water is partly removed by converling the coal to briquelles, which are safe to handle. Most electricity used in Victoria comes from power stations, where brown coal is burnt to provide heat energy. A power line grid distributes electricity throughout the State from power stations on the Latrobe Valley coalfields in Gippsland. Brown coal can also be used to make coal gas for domestic and industrial consumption. During the 1950s and 1 960s. gas was produced in the Latrobe Valley and transported in pipes [0 Melbourne. This was replaced by cheaper natural gas found offshore in Bass S trait. Petroleum and petrochemicals can also be produced from brown coal by liquefaction methods. The cost of these processes at present preclude them from competing with petroleum and petrochemicals produced from crude oil.
Formation of coal Dead plants in the open air slowly decompose 10 water and carbon dioxide and finally disappear. However, if the decomposition takes place under stagnant water, where there is very lillie oxygen. partial decay into peat and coal occurs.
194
Chapter 5
Coal formation occurred during periods of high rainfall and humid climate, when dense forests grew in and around large low-lying swamplands. Decaying plants accumulated in the swamps and gradually became buried by younger material. A combination of pressure by compaction and minor rises of temperatures changed the vegetation to coal. Where large thicknesses of coal are found, it is likely that the floor of the swamp was slowly sinking over a long period. In that way, layer upon layer of rotting vegetation built up under the water and slowly turned to peat. Because water flowed slowly through the swamps, little sand and clay were washed in and so almost pure organic malter accumulated. The climate and other geological conditions in the peat swamps eventually changed. Later lavas flowed over or younger sediments were deposited in the swamps. They buried the peat and coal deeper in the Earth's crust and subjected them to higher temperatures and pressures. Eventually higher ranks of coal were formed. The process of coal formation has been going on in swamps around the world for about 400 million years. There are no older coals because that is when plants first spread rapidly on land. By contrast, petroleum generation from marine life started some 800 million years ago in the Late Pre-Cambrian. The name, Carboniferous, was given to the period when extensive coal deposits were formed in many regions in the Northern Hemisphere. In Australia and other Southern Hemisphere continents, the main period of black coal formation was the Permian. Black coal also occurs in some Mesozoic rocks and brown coal in Lower - Middle Tertiary sediments in Australia. The difference between the ages of the black coals of the Northern and Southern Hemispheres is probably a result of the drift of the supercontinents that existed in Palaeowic times. The present-day northern continents experienced tropical conditions during the Carboniferous, when Gondwana (including Australia) was in the southern polar region. During the Permian, the southern part of Australia was still joined to Antarctica in the deep southern latitudes. However, there were evidently wet, high humidity conditions in the north-east, which were suitable for the rapid growth of the diverse vegetation needed for coal formation. The Carboniferous floras of the northern continents were dominated by tall trees (club-mosses, horse-tails, tree-ferns) that grew to 30 and 40 metres. Their fossilised remains show no growth rings, indicating tropical environments. The Permian flora of Gondwana were smaller plants, mainly seed-ferns like Glossopteris (Figure 4-5 1 ). In coal deposits, the fossilised remains of these plants have growth rings, indicating seasonal growth in temperate regions.
Black coal deposits i n Victoria Thin seams of black coal are interbedded with gently-dipping sandstones and mudstones of the Strzelecki Group of Early Cretaceous age in south-west Gippsland. The coal is a steaming variety and its ash content is rather high. Between 1 880 and 1968, about 23 million tonnes of black coal were mined at ten localities. The greatest production was at Wonthaggi, where the Victorian Government operated mines to supply coal for the steam locomotive engines formerly used on the State's railway system. These mines closed in 1968, when diesel power replaced steam on the railways. Mining of Wonthaggi coal was never profitable, because of the high costs involved in mining seams which were less than three metres thick and intersected by numerous small faults. Black coal was also mined near Korumburra, Ouurim, Kilcunda and Coalville for use in local industries and domestic heating. Testing by hundreds of bores failed to find any unworked deposits that could be mined profitably.
Brown coal deposits in Victoria The largest resources of brown coal in Australia have been found in Tertiary formations within the Gippsland, Otway and Murray sedimentary basins (Figures 5-23(a) and 5-28) . The heating value of this coal is only about a quarter of that of most Australian black coals. Nevertheless it is more economical to use brown coal for most power generation in Victoria than to bring black coal from New South Wales. This is because brown coal is extracted at low cost from open cuts by huge earth-moving machinery and transport costs to the nearby power stations are minimal. Gippsland Basin
At intervals over a period of nearly 40 million years during the Tertiary period, geological and climatic conditions were ideal for the formation of great thicknesses of coal in the western part of the Gippsland Basin (see Chapter 4). The Gippsland Basin was a slowly-sinking, fault-bounded trough in which sediments were deposited on land in the west and beneath the sea in the east. Thick seams of brown coal, interbedded with clay, sand and basalt flows, form a 700 metre thick
Economic Geology
I'��
Figure 5-23 (a) Brown coal deposits in
\
,
Victoria.
I I
(b) Brown coal deposits in the Latrobe Valley. Gippsland.
,
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.,
Kerang 0
'I I
MURRAY
£
,
BASI�- �-�,� o
I
Shepparton
,
o
!
Brown coa l
)
-
"-
-
100 ,
TASMAN SEA
Kilometres
(a)
o 10 '-----'
Kilometres
Less than 9Cm overburden Minor deposits or deeply buried Poorly defined deposits Marine sediment margin
(b)
Coalfield (existing/potential)
LATROBE VALLEY
Figure 5-24 Formation of coal in a Gippsland swamp.
o 1
10 ,
Kilometres
D
f=-� o
20 !
\ I
Clayey depoSilS Coarse and line sandy depOSits
1
..........
Peal. coal accumulation
F
I
--
lake deposns Sea Faults
195
196
Chapter 5 sequence of rocks known as the Latrobe Valley Coal Measures. These sediments overlie Lower Cretaceous rocks unconformably. The trough extends from Yallourn in the west to country east Sale, and from the south-eastern flanks of the Strzelecki Ranges to the coast (Figure 5-23(b» . After the Miocene epoch, the Latrobe Valley Coal Measures were lilted and gently folded into broad structures by earth movements. Subsequently. fast-flowing rivers eroded the uplands to the north and covered the coal measures with sands and gravels known as the Hat/nJed Hill Gravel. Finally, ero ion during Quaternary times bought some of the thick coal seams c lose to the present land surface. Latrobe Val ley coal is a complex material. It consists of a groundmass of very fine plant debris in which partly decomposed plant remains are embedded. There are substantial variations in its composition in the different seams and at different localities. The net speci fic energy, reflecting the car bon content, increases with the increasing depths at which the coal occurs. Moisture usually decreases with increasing depth. After detailed drilling over many years, the areas with the thinnest cover of soils and sediments over the coal have been defined and selected for large open-cut mines. Today major mines operate in the nonh-western part of the basin at Yallouro, Morwell and Loy Yang, mainly producing fuel for nearby power stalions. Other
Figure 5-25 Loy Yang open cut and power station, Latrobe Valley.
In the foreground a bucket-wheel excavator is removing overburden of sand and gravel from above the coal. The buckets drop the overburden on to a continuous rubber belt, which transfers the material to the far end of the machine. There it drops on to another moving belt, which carries the overburden out of the mine. In the middle of the photograph, another bucket-wheel excavator is digging coal, which is carried by a moving belt to the power station area. Each excavator digs up to 60 000 tonnes per day. In the background, the Strzelecki Range, formed largely by Lower Cretaceous sedimentary rocks, is on the upthrown side of the Yarragon Fault. (photograph counesy of State Electricity Commission).
Economic Geology
197
potentially mineable deposits have been found further lO the east at Rosedale and Stradbroke, and at Gelliondale and Alberton near Corner Inlet. It has been estimated that between 1 50 000 and 200 000 million tonnes of coal exist beneath the Latrobe Valley. With the current technology available, over 40 000 million tonnes can be economically extracted by open-cUl mining. Coal seams over 100 metres thick, with overburden thicknesses averaging 20 metres, provide a vast and easily accessible resource. Annual production amounts lO about 40 million lOnnes. Otway Basin
Figure 5-26 Maddingley No_ 2 brown coal open cut, 3 kilometres south-east of Bacchus Morsh The open cut is worked on twO levels. A feature of the faces is the strong rectangular pattern of fractures, which breaks the coal into rough cubic shapes. Many of the cubes have an outward bending (convex) face. There are numerous major venical joints cutting lhe coal . Some of these are filled with deposits of clay, calcite or magnesite. The coal is overlain by up to 10 metres of river alluvium. Maddingley coal contains about 6OOf. moisture and 5% ash. It is of Early Miocene age and comains abundant plant remains. The coal is excavated by electric and diesel shovels with buckets of 2.5 cubic metres capacilY. The coal is crushed locally to blocks of 8 centimetres across. (photograph by N.W. Schleiger). _
There are two small sub-basins of Tertiary sediments in the eastern part of the Otway Basin between the Otway Range and Mornington Peninsula. These are known as the Torquay and Port Phillip sub-basins. Brown coal has been found in the Torquay Sub-basin on the soUlh-eastern side of the Otway Range at Deans Marsh, Wensleydale, Benwerrin and Anglesea. It is associated with sands, clays and gravels of the Eastern View Formation and is Palaeocene lO Eocene in age. The largest deposit is north of Anglesea where it is mined by open cut. The ruel is used at a nearby power station, which provides electricity to the Alcoa aluminium refinery at Point Henry, near Geelong. Anglesea brown coal is the highest-grade brown coal deposit in Victoria. The moisture content averages 46070 , which is less than that of any of the seams in the Latrobe Valley. Within the Port Phillip Sub-basin, brown coal of Oligocene to Miocene age occurs at several localities from A1lOna lO Bacchus Marsh. Some production was obtained from an underground mine at Altona from 1 9 1 0 to 1 9 1 9, but most of the deposits are too deep to warrant exploitation. However, there is mining at Mllddingley near Bacchus Marsh. There, seams of 25 to 43 metres thickness have been extracted beneath 10 lO 25 metres of sediments in open cuts. The coal is mainly used as a fuel to raise steam in industrial plants. There are about 400 million tonnes of coal near the present open cut. Over half of this is covered by basalt, which would be uneconomic lO remove.
Murray Basin
During recent years, exploratory drilling has located extensive coal seams beneath the sediments of the Murray Basin near Kerang and Shepparton. The coals and sediments are Palaeocene lO Miocene in age within the Olney Formation. The coal seams are of variable quality and at depths of over 300 metres. They are therefore unlikely to be mined in the foreseeable future.
PETROLEUM The word 'petroleum' means 'rock air . It is often used as a synonym for 'oil', the substance from which petrol is obtained. However, strictly speaking, petroleum is a collective term covering compounds in the three states of mauer, i.e. natural gas (gas), crude oil (liquid) and asphalt or bitumen (viscous liquid to solid). These compounds have a common source. Over the past two decades, Victoria's most important mineral industry in economic terms has been the petroleum industry. To understand the geology of the Victorian deposits, it is neces ary lO can ider some general aspects of the petroleum industry. The formation and movement of petroleum compounds within the Earth is a complex and at times controversial subject.
Composition Petroleum is a complex mixture of hydrocarbons, that
.
tS
.
'
orgamc compounds whIch
198
Chapl.r 5
contain mainly carbon and hydrogen, but occasionally other elements. Details of the main petroleum compounds are given below: natural gas
petroleum
paraffins
open chain molecules with the general formula C.H2n" (e.g. methane, CH.)
naphthenes
carbon ring molecules with the general formula C.H2n (e.g. cyclopemane, C,H ,.)
crude oil
·of minor imponance in Victoria Natural gas is either: • dry gas - mainly methane, which is a paraffin, or • \\leI gas - other paraffi ns, especially ethane, propane, butane. Most crude oils, including those of Victoria, are mixtures of paraffins and naphthenes. The paraffins are the most valuable components but are relatively scarce. The term condensate is used for any liquid hydrocarbon produced with wet gas at the head of a petroleum well.
Properties of crude oil There are three imponant physical properties of crude oil: Density: The specific gravity of most oils is between 0.7 and 0.7 Ughl oils are rich in paraffins and are suitable for the production of transport • fuels (petrol, diesel). • Heavy oils are rich in asphalt and are used for lubricants and bitumen. Note: the Gippsland offshore is now ( 1 992 ) producing less than four-fifths of Australia's demand. Heavy oils mu t be imponed. Viscosity: This property determines how easily oil wiU flow. It is caused by the friction
between the oil molecules and depends on both the density and composition of the oil. Viscous, high-wax oils are difficult to pump through pipelines. This type occur.; in the offshore Gippsland fields. Colour: Paraffin oils are usually pale in colour, whereas asphalt-based crude oils
(i.e those dominated by naphthenes) are dark.
Uses All industrial societ ies are heavily dependant on petroleum and its products.
90"7. oil and natural gas
-
energy:
as engine fuels (petrol, kerosene, diesel) and to generate heat and electric power; they account for about two thirds of the energy used in the world.
10"7.
-
petrochemical industry:
converted to lubricants, plastic , waxes, symhetic rubber, medicines, paints, sealants, chemical detergents, inks, cosmetics, bitumen road seals, etc.
Occurrence Petroleum is found in sedimentary basins. These are thick accumulations of sediments, which may be faulted and gently folded, but they have not experienced any major orogeny. There are more than 700 sedimentary basins spread across the continents and continental shelves of the world. About 50 are in Australia. Only about half of the world's basins have been explored for petroleum to any extent. About 1 50 of these are commercially productive with ten being in Australia. However, most of the world's oil and gas resources occur in a few basins only. Three quarters of known oil and two-thirds of known gas reserves are found in just three regions. These are centred on the Persian Gulf, the Gulf of Mexico and the Ural Mountains (U.S.S.R.). The most productive basins are those of younger Mesozoic
Economic Geology
199
and Cainowic age. I n Australia the richest known basins are Gippsland (Late Jurassic to Early Eocene) and the Northwest Shelf (Permo-Triassic to Cretaceous). Sedimentary basins are divided into two main groups: I . Interior basins are saucer-like depressions within a craton, which were filled with
sediments brought in by an internal drainage system. The major petroliferous basins of this kind in Australia are the Amadeus Basin (Ordovician sedimentary rocks in Central Australia), the Cooper Basin !Permo-Triassic rocks in South Australia and Queensland) and the Eromanga Basin (Jurassic-Cretaceous rock s also i n South Australia and Queensland. 2. Marginal basins are elongated accumulations of sediments within down-warped troughs on the edge of a craton. The sediments were either deposited as river deltas or as layers forming on the continental shelf. In Victoria, both the oil- and gas-rich Gippsland Basin and the prospective Otway Basin are of this type. So too are the major gas fields in late Palaeozoic to Mesozoic sediments on Australia's Northwest Shelf, whlch are being developed for liquefied natural gas (LNG). Major oil fields also have been discovered there in recent years.
Origin of petroleum Various theories have been put forward to explain the origins of oil and natural gas. Like coal, crude oil and natural gas are generally considered to have developed by the decay of living matter after burial in sediments over a long period. The only evidence, however, of organic matter in oil is provided by complex molecules, which are common to both plant and animal tissues. It seems likely that minute aquatic animals (e.g. krill or micro-crustaceans), diatoms, algae and bacteria are the sources of most oil. These organisms grew in marine or freshwater habitals as vast noating masses. However, the chemical composition of some oil indicates that it was derived from land plant material that had formed coal seams or had dispersed as coaly particles in sediments. It is now thought that most Gippsland oil was generated from carbonaceous malter located in the deeper parts of the sedimentary basin.
Formation, migration and trapping of petroleum The change from noating living organisms to accumulations of oil and natural gas somewhere in a sedimentary basin rock sequence takes place in several stages: I . The organisms die and fall to the noor of the sea (or lake), where they are buried
by sediments.
COMMON UNITS AND CONVERSIONS
Although metric measures are gradually becoming universal in the petroleum industry, non-metric units are still commonly used in Australia, particularly in financial and press reportS. •
barre (bll , O . • <9 k 01 t kL) 1 < gdl", , ( m ><:nal 4� galloP' k 'J
n.
Oilfield production and reserves are measured in thousands or millions of barrels (Mbbl and MMbbl) or kilolitres (kL x 10' and kL x 10'). "
, blC co I J fl l ..I 02�1 .. bu,; met r
rr )
Gas field produclion and reserves are customarily measured in billions or trillions of cubic feel (bcf and ICt) or cubic metres (m' x 1 0' and m' x 10").
2. To produce useful concentrations of oil and gas, vast quantitie of the organisms musl accumulate and slowly decompose. The conditions must prevent their destruction by animal bOllom-dwellers or their dispersal by currents and wave . The most favourable environments are stagnant or poorly-circulating waters of river deltas, estuaries, coastal lagoons, swamps, tidal nats and lakes, where there is little oxygen and fine-grained sediments predominate. Such environments were present during Early C retaceous to Early Tertiary times in the Gippsland Basin. The most common source rocks for petroleum are carbonaceous shales and carbonaceous limestones. 3. After the organisms are deeply buried in the mass of sediments, they decompose due to increasing temperarures and pressures. Gradually, various fats, waxes and oils containing proteins are produced. These are rich in carbon and hydrogen , and they become the parent materials for the hydrocarbons in petroleum. The process is called maJUro/ion. There is a critical range of temperatures between 60°C and 1 20 °C (called the window) at which oil and gas are formed. Outside this range, the temperatures are either too low for enough decomposition to occur or they are so high that decomposition is excessive and the petroleum is dissipated. Time is also a vital factor affecting maturation. If the proces continues for too long, decomposition may go too far . This is one reason why Cainozoic and Mesozoic basins are more likely to contain oil than older (palaeozoic) rocks. 4. Sub equently, the oil or gas is expelled from the fine-grained, impermeable source rocks in either one of two ways: (a) as additions of sediments to the basin bury the source rocks deeper, the increasing pressure (compaction) squeezes the petroleum OUt of the source rocks. (b) the generation of petroleum from the original organisms involves great increases in volume. This in turn fractures the source rocks and allows the hydrocarbons to escape.
200
Chapter 5
Figure 5-21 Four types of petroleum traps in sedimentary rocks. Porous rocks (sandstones) provide the reservoirs and impervious rocks (shales) provide the seals. Most petroleum fields in the Gippsland Basin are trapped by combinations of unconformities and anticlines. Dots in the diagrams indicate sands or sandstones. Lines and da hes indicate shales or marls.
5 . After being driven from the source rocks, the petroleum migrates upwards umil it either: • reaches the land surface, where it forms a seepage of gas, oil or asphalt, or • reaches a trap or barrier in the rocks, where it forms an accumulation of oil and/or natural gas, known as an oil or gas pool. (Multiple, adjacent pools related to a common geological feature are called a field) . 6. Three essential componems are needed to form a petroleum pool: (a) reservoir: reservoir rocks must be both porous, that is capable of holding large quantities of petroleum, and permeable, that is capable of releasing the petroleum when they are intersected by a borehole. (See Chapter 6 for funher
discussion on porosity and permeability of rocks). Commercial oil and gas fields world-wide are either in sandstones (60%) or carbonate rocks (limestones and dolomites) (40"0). (b) cap or seal: impermeable rocks such as shale (clays, silts) or marls (calcareous muds) form the best seals. Petroleum cannot easily pass through a seal. (c) trap : a particular arrangement of the rocks is needed to ensure that the petroleum cannot escape from the reservoir by finding some path around the seal. Some common traps are formed by anticlines, faults, unconformities or a change up the dip of sedimentary rocks from coarse- to fme-grained rocks (Figure 5-27).
7. Apart from being trapped beneath seal rocks, petroleum pools are often held in place by the pressure of groundwater in the reservoir rocks. Within the trap, the gas separates out on top of the oil, which in turn floats on the water.
Exploration
ANTICLINAL TRAP
FAULT TRAP
The aim of oil exploration is to find combinations of geOlogical structures and rock types, which might trap petroleum - these are called plays or prospects. Exploration of a sedimentary basin commences with a compilation of the geological data available from surface mapping and the records of intersections of rocks in water supply boreholes. This helps to identify potential source and reservoir rocks. Geophysical surveys are the main means of defining geological struct ures suitable for traps. Airborne magnetic and gravity surveys are used at the 'reconnaissance stage to provide information about a basin's shape at depth, the locations of concealed volcanic rocks, the trends of the main folds and any fault structures. Seismic surveys are used in the follOW-Up exploration to define suitable structures more precisely. In these surveys, sound waves are sent into the ground or sea-floor, where they are reflected back by successive rock layers. The reflected waves are received back at the surface of the land or sea by microphones (called respectively geophones or hydrophones). Evemually the most prospective petroleum plays are tested by drilling an exploration well. Petroleum wells are drilled by a rotary method using a rock bit connected to rotating drill pipes. A special heavy mud is pumped through the pipes to flush the rock cuttings to the surface between the column of drill pipes and the hole wall. The outflowing mud fluids are sampled for rock cuttings and con tandy monitored for the presence of hydrocarbons. When oil or gas is detected, cores of the potential reservoir beds are talken for examination. Tests are then run over the prospective beds to obtain samples of the hydrocarbons and to determine their flow rates. Finally, these beds may be developed for petroleum production. Gas rises naturally to the surface but oil u ually has to be pumped.
Petroleum in Victoria There are three connected sedjmentary basins of Mesozoic-Cainozoic age extending across southern Victoria and through Bass Strait (Figure 5-28). The developmen t UNCONFORMITY TRAP of these basins accompanied the separation of Australia and Antarctica during the OR TRUNCATION TRAP Cretaceous and is described in Chapter 4. The main sequences and important rock units present are shown in Figure 5-29. The Gippsland Basin contains Australia's main petroleum-producing province. Exploration of the Otway and Bass basins has been largely unsuccessful to date. The Late Cretaceous to Eocene sediments at each end of Bass Strait contained carbon-rich organic matter favourable for the generation of petroleum. These are the Latrobe Group sediments deposited on river fl ood plains and deltas in the Gippsland Basin and the Sherbrook Group deltaic sediments in the eastern Otway Basin. Unconformities were produced in the Eocene-Early Oligocene beds after an Early Tertiary advance and a later retreat of the sea over the earlier ediments. The combination of these unconformities with anticlines produced du ring subsequent L ...:tI.���::...:..c... : ""-.:;;;::::I ::' gentle folding provided large structures suitable for trapping migrating petroleum. BEDDING BARRIER TRAP The fold structures in both basins mostly trend in a north-east to south-west direction. OR PINCH-OUT TRAP Overall the structures are complex and many imerpretations have been made. _ _ _
201
Economic Geology
Later in their histories, the basins were again invaded by the sea and the shallow water limestone and marl sequences of the Seaspray and Heylesbury Groups were deposited.
Figure 5-28
o Thm Basm Sedimems
The Cretaceous - Tertiary sedimentary basins of Victoria.
Oil
D ThK:Ic BaSin Sedments
is most likely to be found
where there are thick sequences of sediments.
NEW SOUTH WALES
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- -- - - TAS�lANIA - - - - - - -BASS BASIN
Gippsland Basin
The oil and gas resources that have been discovered in the Gippsland Basin are large by world standards. The fields are in the offshore pan of the basin, contrasting with the coalfields that are onshore. Oil was first found by accident in 1 924 in a water well, Lake Bunga o. I , at Lakes Entrance. This discovery led t o sporadic exploratory drilling, but u p till 194 1 only 3063 barrels of oil had been produced. An experimental shaft, with horizonLal radiating collector holes, was sunk between 1948 and 1 951 at Lakes Entrance townsh ip. This produced an additional 4935 barrels before the project was abandoned. The geological target of this exploration was a fine-grained sandstone bed in the Lakes En/ranee Formalion (Late Oligocene) at the base of the marine Seaspray Group. This reservoir sandstone is unfortunately lighl, that is, it has low permeability. However, a commercial flow may be obtained from this locality in the future if modern techniques to force out the oil are applied. Geological studies of the basin continued during the 1 950s, when the first airborne magnetic surveys were also carried out. These included offshore surveys in 1951 and 1 956. By 1 962- 1 963, the first seismic survey had been conducted. This revealed the basic geological structure of the Gippsland Basin, including the major anticlinal structures that later proved to be petroleum-bearing. Drill ship 'Glomar lII' commenced Barracouta No. 1 well in late 1 964 and drilling reached a final depth of 2652 metres after five months. This was Australia's first offshore well and it was a success. A major gas field was discovered in an elongated dome-shaped trap beneath a regional unconformity at the top of the Latrobe Group. By 1970, some 20 000 kilometres of seismic survey lines using improved techniques had revealed numerous targets for testing. Forty-three wells (100 000 metres of drilling) were completed for six oil, six gas and three combined oil and gas discoveries. Three major gas fields (Barracouta, Marlin and Snapper) and two major oil fields (Kingfish and Halibut) were brought into production (Figures 5-30 and 5-3 1). Exploration continued over the next ten years with 1 5 000 kilometres of seismic and thirry-eight wells. There were fou r oil, two gas and two other combined discoveries. By 1 980, two additiona.l oil fields (Cobia and Fortescue) were being developed for production . One well ( Hapuku No. 1) was driUed in deep water (3840 metres) on the continental slope.
202
Chapter
5
SHALLOW MAR/fIE SfELF MAINLY CARBONATES
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The sediments were deposited in various geological environments and were affected by several kinds of tectonic movements, including uplift, faulting and,in some areas, folding. Certain environments were favourable for the production of coal and petroleum.
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Correlation chart of the sedi mentary rock formations in the Otway and Gippsland basins during Cretaceous and Tertiary times.
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In the 1 980s new improvements to seismic survey techniques enabled a complex three-dimensional picture of the geological structures to be obtained. Sixty more wells were drilled including the deepest well in the basin, Volador No. I, which reached 461 1 metres. Many small oil and gas discoveries were made and some earlier finds, (e.g. Bream and Tuna), were brought into production. Despite advances in exploration methods, the declining rate of successes in d rilling suggests that the chances of making additional major discoveries in the Gippsland Basin are not good. To slow the final depletion of some wells, enhanced recovery can be achieved by maintaining the reservoir pressure with an injection of water or even of steam down adjacent wells. Other basins
The other Tertiary basins do not appear to have as many favourable features as the Gippsland Basin but they have not been tested to nearly the same extent. Small Continued on page 204
Economic Geology
203
t
SYSTEM
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Figure 5-3 t (below)
Figure 5-30 (above) Oil and natural gas discoveries in Bass Strail.
Geological cross-sections through oilfields in the Gippsland Basin. MOSl oil and nalural gas are in lraps at the top of Eocene sedimentary rocks beneal h an unconformity wilh Oligocene Slrala . There are many block faults in the Palaeozoic basemenl rocks and the older formal ions of lhe basin. A' EAST KISGFISH
The petroleum fields occur within thick sequences of Tertiary sedimentary rocks on the continental shelf. There are major east-west fault SYSlems along the nonhern and southern boundaries of the area. A WEST
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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
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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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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
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Economic Geology
207
Figure 5-34 A west-east geological cross-section across Wattle GuUy, Chewton Goldfield, near Castlemaine.
The main gold producer was the Wattle Gully reef, a fissure reef up to 30 metres thick, developed along a west-dipping shear zone. Other smaller bedded reefs were also mined. (After 'Geology of Australian Ore Deposits'. AUSl. I.M.M., 1953).
Figure 5-35 Saddle reef in the North Deborah mine, Bendigo.
Gold was mined in both the thick saddle reef and the other thin bedded reefs, known as the 'outer west' and 'outer east' legs. The monchiquite dyke (a basic rock) was intruded long after the folding of the Ordovician sedimentary rocks and the intrusion of the quartz reefs. (After 'Geology of Australian Ore Deposits', Ausl. I.M.M., 1953). Figure 5-36 (below) Stacked saddle reefs. Great Extended Husllers mine. Bendigo.
2fi 5,0 7,5 190
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Several low·dipping faults have displaced the anticline short distances. (After 'Victoria Gold and Minerals', Mines Dept. , Victoria, 1935).
o
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'5 I
2 . Bedded reefs: these occur along bedding planes in folded rocks. rYSIaliisation occurred particularly where adjacent sedimentary beds tended 10 separale and leave opening , e.g. along the axes of folds. Arch-shaped bedded reefs developed along anliclines are called saddle reefs (Figure 5-35). The largesl , mOSI numerous and mOSI produclive saddle reefs are found on Ihe Bendigo Goldfield. There, thirteen parallel anliclines and syncline occur wilhin an area of aboul 18 kilomel res north-SOU lh by 5 kilomelres eaSl west. Long gold-bearing saddle reefs were mined along mosl of these anticlines. There are repealed saddle reefs al various depths on many anliclines (Figure 5-36). In many places, thin-bedded reefs extend away from Ihe saddle reefs down Ihe limbs of Ihe anticlines - these are known as 'legs' (Figure 5-35). Large masses of gold-bearing quartz in saddle reefs helped 10 make Bendigo by far Ihe largesl gold-producing centre in V icloria. Smaller saddle reefs also occur in other goldfields in Ihe Midlands, such as Casllemaine, Ballaral Wesl, Taradale, ele.
208
Chapter 5
3 . Spurry reefs: these are groups of short, closely-spaced, gold-bearing quartz veins, which have filled fracture systems in older rock (Figure 5-37). The veins may be sub-parallel to each other or form an irregular network, called a stockwork. Although the veins may be separated by barren host rocks, if they are numerous and close to the surface, the whole mass of quartz-veined rock may be suitable for open-cut mining. Gold is usually scattered erratically through the veins. Figure 5-37 (left) Series of short spur reefs, Carlisle Mine, Bendigo. (After 'Geology of Aust ralian Ore Deposits', Ausl. I.M.M., 1953).
o
5
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15
� Slate D Sandstone D O anz u
Figure 5-38 (right) A spurry reef exposed in a road cutting betwLoen Frye.rstown and Campbells Creek in the Midlands. A zone comAining white, narrow, low-dipping quartz veinlets and irregular 'splashes' intersects steeply-d ipping Ordovician . W. siltstones. (photograph by Schleiger).
Figure 5-39 arrow gold·bearing vein between the two walls of • dyke.
4 . Reefs associated with Devonian dykes: in some areas, gold and quartz occur within or very close to dykes of Devonian age. The best-known examples are associated with the Woods Point Dyke Swarm in central Gippsland. Similar reefs typified small goldfields at Blakeville, (south-east of Daylesford), Clonbinane, (east of Kilmore), Foster (in South Gippsland) and Tanjil (north of Moe). Some of the variations in this group are: (a) the reef follows the wall of a dyke, e.g. the Long Tunnel mine at Walhalla, which produced mOre gold than any other mine in Victoria. (b) a parallel series of short, narrow gold-bearing veins cro es the dyke between the walls (Figure 5-39). (c) the dykes of the Woods Point Dyke Swarm extend for many kilometres in length but are mostly only one or two metres wide. In a few places, however, the dykes widen out into steeply-i nclined cylindrical bodie , called bulges. Zigzag patterns of gOld-bearing veins formed in some dyke bulges, e.g. at two of Victoria's most productive mines in the past, the A l mine at Gaffneys Creek and the Morning Star mine at Woods Point (Figure 5-40).
There are three fealUres of quartz reefs that are important in gold mmmg: I . Quartz reefs are common throughout the Lower Palaeozoic rocks of Victoria. Most, however, consist of massive white quartz that does not carry any gold.
2. Many fissure reefs consist of i rregular 'splashes' of quartz within a sheared zone of crushed sedimentary rocks. Gold may occur both within the quartz patches and along fractures in the sedimentary rocks.
3 . Gold is not scattered evenly through quartz reefs . Payable quantities are restricted to zones of irregular shape called 'ore shoots' .
Alluvial deposits
Nearly two thirds of t he gold found in Victoria came from secondary deposits. I t was this shallow alluvial gold, that att racted the several hundred thousand people
Economic Geology
Figure 540 Cross-section through the main shaft, Morning Star mine, Woods Poinl. The gold occurred in thin quartz reefs deposited along fissures in the diorite dyke bulge. A lillie gold was also found in the dyke adjacent to the reefs . The fissures probably formed as the diorite cooled after it was intruded. (After Geology of Australian Ore Deposits, Aust. I . M . M . 1953).
209
who took part in the nineteenth century gold rushes. Gold-bearing quartz reefs originally crystallised within Lower Palaeozoic folded sedimentary rocks deep below the surface. However, by the Tertiary period, prolonged erosion during Upper Palaeozoic and Mesozoic times had exposed h undreds of gold-bearing reefs at the surface. Like other rocks, when exposed to air and water, the primary reefs began to weather and erode. These processes have continued to the present day. Quartz and gold are chemically stable, so weathering of reefs mainly leads to a disintegration of the quartz into fragments and a liberation of any gold grains present . The gold may become concentrated in one of the following environments: I . Soils: heavy grains of gold may accumulate in soil close to a parent reef. Some early miners excavated shallow residual and colluvial soils near reefs, broke up the clay with water and recovered loose particles of the gold. This form of mining was called surfacing. Unfortunately it badly damaged the natural environment by leaving areas of bare stony ground (Figure 5 -4 1 ).
2. Present-day river beds: there are many gold-bearing quartz reefs in the high rainfall region of the East Victorian Uplands. Periodical, strong run-off of water down the steep-sided mountains gradually erodes particles of gold from the reefs. The gold is eventually transported into large rivers, such as the Goulburn. There it may be trapped in crevices in the rocks along the river beds. This gold can be recovered by using an edue/or dredge, equipment that acts like a vacuum cleaner on a river bed.
3 . Tertiary-Pleistocene ri,'er beds: during this period, conditions were favourable for the production of large alluvial gold deposits, because: there were long periods of high rainfall; • the mountains were higher than they are today and fast-nowing rivers ran down their sides. Across the Central Victorian Uplands, mountain torrents carried away vast quantities of rock debris, including gold particles and quartz boulders. This material was deposited downstream along the river beds as large masses of sand and boulders. Evidence of this vigorous erosion can be found in many parts of Victoria. This includes the Midlands, where only sluggi h rivers now today. The Tertiary-Plei tocene river deposits occur at several differem levels in the landscape: •
(a) Lines of rock and quartz gravels occur at hallow depths close to the headwater sections of many present-day creeks. Where these old shallow river gravels carry gold, they are called shallow leads. They can be traced over hundreds of kilometres, especially in the M idlands, by the remains of pits and small waste rock dumps, that were left behind by the diggers in the gold rushes. The shallow lead are much broader than nearby present-day streams. (b) Going downstream, the Tertiary streams cut progressively deeper valleys. The wash layers were laler covered by sands and silts deposited by slower-nowing rivers. These rivers did not have enough velocity to carry gold particles downstream. Cainozoic basalt lava nows sometimes nowed along these valleys, adding another layer to the material overlying the early gravels. As the early miners followed the shallow leads downstream, they eventually reached a point where it became toO wet at the bOllom of their pits to continue work. From there downstream, the leads were known as deep leads. The gold bearing deep leads were worked by companies, which had to pump large quantities of water from their mines to reach the gold-bearing wash below the permanent water table.
Soli
Weathered reel
Figure 541 Different forms of secondary gold deposits. Gold particles may be fo und in soils and nearby alluvial sediments along past and present-day streams. Because gold is much heavier than qU3nz. clay and other minerals, mosl secondary gold panicles occur close to bedrock or in coarse gravels deposited by fast Oowing rivers. Some gold settle inLo crevices in the bedrock.
Folded PalaeOZOIc sedlmentarv rocks Gold bearing Quartz reel
Gold In old allUVial deep lead
GOld In reSidual SOil
GOld In present
day stream
Chapter 5
210
The most extensive gold·bearing deep lead systems drained much the same country in north-western Victoria as do the Loddon and Avoca rivers today. Basalt·covered deep leads in the Ballarat-Creswick area were especially rich in gold, e.g. Madame Berry Lead. There are also deep leads in north ·eastern Victoria, e.g. along the Ovens River valley at Harrietville, below Reedy Creek at Eldorado, and the Chiltern Valley Lead in the Chiltern and Rutherglen districts.
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(c) Gold·bearing gravels may occur along the sides of hills and as cappings o n low ridges i n Lhe Midlands. [n some places, these gravels form a hard conglomerate layer because they have been cemented together by iron oxides and silica. They were originally deposited along the beds of Tertiary rivers. The land was subsequently uplifted and because the cemented high-level gravels were more resistant to Quaternary erosion than the surrounding rocks, they are now found above present·day streams. High-level gravels were mined for gold in north·western Victoria between Avoca and St A rnaud, nea, Tarnagulla and north-east of Moliagul. In north eastern Victoria, gold was obtained from thick deposits of gravels, uJa[ formed terraces along some river valleys, such as the Mitta Mitta.
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Gold production reached its highest levels in Victoria in the years from 1 85 1 to 1 87 1 . Since the early 1 900s, gold mining has been only a minor industry in this State. There were several reasons for the decline in gold mining. One major cause was the depletion of the shallow, easily-found, alluvial deposits, which had sparked off the first rushes. Underground mining later declined because until the early 1 970s the price of gold was fixed by Government authorities around the world. Particularly after World War I I , rising inflation caused the costs of labour and goods to increase continuously. Eventually the costs of gold mining exceeded the income at most gold mines, forcing them 10 close. Around twenty years ago, the price of gold was deregulated. Since then the price has been sufficiently high to make the prospect of opening new gold mines attractive once agai n . The revival in gold mining, however, has taken place mainly in Western Australia, Queensland and the Northern Territory, where the geological conditions are most favourable for the development of open·cut mines. Alluvial deposits
er I
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Figure 542 Small, disused bucket dredge used for gold·mining at Porcupine Flat, east of Maldon. The dredge was used fo r mining alluvial gold contained in Tertiary gravels deposited on Ordovician bedrock. A continuous moving chain o f buckets excavated the complete bank of the pond at 'he fran! of the dredge (left hand side). The gold was separated from the sands and gravels (called toilings). The latlef were discharged from hoses on a large boom al the rear of the dredge (right hand side), thus refilling a m i n ed-out area, Dredges were used on many goldfields in ViclOria, e .g. Yackandandah and Loddon River near Guildford. Most dredges in the world are now fo und along the western ccaSI o f Malaysia and Thailand, where they recover allu vial lin from river deltas and the shallow sea·noor). (PholOgraph by N. W. Sch leiger).
During the 1 980s, large numbers of people equipped with metal detectors walked over the old, shallow alluvial goldfields and ground close to OUtcropping reefs searching for pieces of gold missed by earl ier prospectors. A few people were amply rewarded for t heir labours and a few large nuggets were found. In recent times, one mining company carried out an extensive exploration program aimed at discovering unworked sections of deep alluvial gold deposits. The company explored buried river valleys by drilling lines of boreholes at intervals downst ream from old mines. They proved that a large unworked gold resource does exist, mainly along the Loddon Valley system . The company estimated that there are at least 700 kilom etres of deep leads in Victoria with an average gold content of 4 grams per cubic metre. The wash layers are up to 5 metres in thickness and one kilometre in width.
Economic Geology
21 1
The company had hoped to introduce a technique of metal extraction called solution mining, which has been used successfully overseas to work sedimentary
uranium deposits (Figure 5-43). In this method, the metal being sought is extracted from the ground by dissolving it in a suitable solution and then pumping. Laboratory experiments showed that the most effective solvent for gold was a solution containing cyanide ions. 4Au + 8CN- + O,(g) + 2H,0 -+ 4Au(CN), + 40HThe project was abandoned after authorities judged that it would be unwise to introduce a poisonous liquid into the groundwater, even though precautions to prevent escape of cyanide from the mining area were proposed.
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Figure 543 The concept of solution mining for gold. Small grains of gold occur in the gravels deposited in an old river valley. In solution mining, a solution containing a substance able to dissolve gold would be pumped down the injection wells. The solution would circulate through patches of gravels and then be withdrawn up the extraction wells. Gold dissolved by the solution would be re-precipitated in a treatment' plant at the surface. Water pumped from the monitor wells would be tested regu larly to determine whether there was any escape of toxic substances beyond the injection-extraction zone. (After 'Solution Mining Technology', eRA Exploration Pty. Ltd.).
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In recent years, new gold mines have opened at Stawell (see case history), Maldon, Fosterville and Nagambie after drilling programs indicated significant resources of ore. The lirsl three mines are extensions of old workings, but Nagambie is a new discovery. At Union Hill, Maldon, an open cut is being worked in zones of mall spur veins, which extend for dis tances of up to 1 5 metres on both sides of the 2 to 1 0 metre wide Eaglehawk Reef (Figure 5- 44). Between 1 857 and 1 906, the Eaglehawk Reef had been worked i n underground mines by various companies to depths o f over 200 metres. A few remnants o f the reef are being removed in the open-cut mine. Figure 544 Union Hill gold mine, Maldon. A group of short, gold·bearing. quartz spur veins cut across Ordovician sedimentary rocks, which dip 60° to the east (right of the photograph). The spurs are locally called 'spurries'. Their dark colour is caused by iron oxide, fo rmed by the oxidation of pyrite (iron sulfide) in the spurries. (P hotograph courtesy of Triad Minerals N L. ) .
212
Chapter 5
At Nagambie, a wide zone of spur reefs was discovered in broken ground between two parallel faults. The gold is fme and is being recovered by leaching dumps of crushed rock with cyanide solution in the open air. There is another open-cut mine, similar to the Union Hill operation, at Fosterville, 20 kilometres east of Bendigo. Eight miUion dollars were spent to establish a mine that has a planned production of 900 kilograms of gold per annum. Long cuttings through parts of this country reveal where past open-cut mining took place in spurs branching off a long fissure reef.
Future of gold mining i n Victoria For various social and geological reasons, it seems unlikely that gold mining will again become a major industry in Victoria. [n the nineteen th century, people migrated to the goldfields in search of wealth and employment. Families settled as close as possible to the mines where the men worked. Mines were as much a pan of the urban landscape in many parIS of Victoria, as were the houses, shops and other buildings. [f the crushing planlS were noisy or the waste dumps dusty, these inconveniences were accepted, because a thriving mining industry provided security of employment. Nowadays many communities tend to see mines and quarries in a different light. The emphasis is on hiding them from public view and hearing. Even after an ore deposit has been discovered there is often a long lead-in time to the commencement of mining due to the many approvals that have to be obtained from various authorities for work to take place. For the most part, exploration companies tend to direct their funds at regions further 'outback' where mining is more readily accepted. In States such as Western Australia and Queensland there is usually less competition fo r land between mining interests on the one hand and farming, industrial and land conservation groups on the other. Most primary gold lodes in Victoria appear to be relatively narrow vein deposits, which are not suitable for the large-scale production methods favoured by modern mining companies. There are a few fieldS, where unworked gold-bearing reefs, one metre or less in thickness are known to exist, e.g. Bethanga Goldfield near Hume Reservoir. Probably these could only be reopened by using high cost , labour-intensive underground mining methods. Such development is unlikely in the foreseeable future. Wide spurry zones, which can be worked by open-cut mining, appear to be less common in Victoria than in Western Australia.
Case h istory: Magdala gold mine Stawell Joint Venture
History
Stawell Goldfield is the most westerly of the major goldfields of Victoria. Alluvial gold was discovered in 1853 at Pleasant Creek on the western side of the present town. A typical 'rush' took place and for a short period up to 1859, there were an estimated 25 000 miners on the field. Most of the alluvial gold came from river gravels that were buried beneath up to 30 metres of Tertiary sands and clays. · The source of some of the alluvial gold was found in qu a rtz reefs at Big Hill, situated 2 kilometres east of Pleasant Creek. Eventually two main systems of gold bearing reefs were found. These supported underground mining until 1 920. The total production of gold from Stawell to that time is estimated to have been about 1 .9 million ounces (59 tonnes). This would have a presen t-day value of well over 900 million dollars. Exploration
[n 1946, a new investigation of the goldfield commenced. This aimed at finding repetitions of the lodes by drilling to the north and east of the old workings. Despite some successful intersections of gold-bearing reefs, the results did not warrant the cost of reopening the old mines. However, after rises in the price of gold in the 1 970s, drilling was intensified and more encouraging results were obtained. This led eventually to a decision to carry out exploratory underground development of the Magdala lode. In addition, two small ore zones were extracted by open-CUI mining and old tailing dumps were retreated. Unlike most modern exploration for metals, which involves considerable use o f geophysical and geochemical techniques, the search for gold at Stawell involved only a study of the records of past mining and drilling from surface and underground locations. Geo[ogy
StaweU is located in an area of Cambrian rocks consisting of basic volcanics overlain by fme-grained turbidite sediments. Strong deformation led to folding along north west to south-east axes and the development of shear zones, particularly in the sedimentary rocks. [n places metamorphism converted the sedimentary rocks to schists.
Economic Geology
213
Figure 545 Central lode on the 382 metre level of the Magdala mine, Stawell. Samples have been cut from the three diagonal lines to confirm the width of rock to be excavated and sent to the treatment plant. From left to right the lines are along: • •
•
unmtneralised footwall schist; sheared gold-bearing quartz sulfide lode between well defined fault planes; upper portion of the lode comprising irregular masses and veinlets of quartz and sulfides with some gold.
The wavy white line and arrow on the right hand side are a guide to control the width of mining. Note the sampler is carrying various items of safety equipment, which are compulsory for underground employees. These include safety helmet, ear-muffs, safety glasses and on his belt, an air- purifying device to be used in the unlikely event of ap underground fire. He also wears plastic waterproof trousers and carries a cap lamp powered by a sealed lead acid battery attached to his belt. (photograph counesy of Western Mining Corporation Limited).
Figure 546 A typical cross-section through the Magdala mine, Stawell. It shows several gold-bearing 'vertical' and 'nat' reefs (e.g. Scotchman's Vertical Reef, No. 2 Flat) and the wider Magdala Reef in a shear zone along the boundary between Cambrian basalt and schist. Rocks and reefs above the Magdala zone are said to be on the hanging lVall side, those underneath the zone are on the footlVall side.
The lode systems are largely confined to the schists. Most of the past gold carne from so-called 'vertical' quartz reefs along 60° west-dipping shear zones, and associated 'flat' reefs. The latter branch off the underside of the vertical reefs at low angles of dip. Both types of reefs are only 1-3 metres thick and gold was obtained to depths of up to 600 metres. These reefs were high-grade, averaging over one ounce (3 1 grams) of gold per tonne of quartz. In later years , the wider but lower-grade Magdala reef was mined. This occurs in a major shear zone at the contact of the basic volcanics and the overlying sedimentary rocks. The Magdala Reef averages 4 metres in thickness but in places is 1 5 metres wide (Figure 5-46). Unlike the narrower reefs the Magdala contains plentiful sulfide minerals. The gold is associated with pyrrhotite, pyrite and arsenopyrite.
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C R O S S S E CTION 288N
214
Chopter 5
Mining
Figure 547 A perspective view of the underground workings at the Magdala Gold Mine, Stawell.
It shows the large number o f openi ngs that provide access to the Magdala Central Lode. The largest is the decline, which enables trucks to travel down from the surface to the various levels, where ore is being mined. It has a gentle slope and twists and turns to remain as close as possible to the are zone. At each level, a cross-cut has been driven from the decline out to the gold -bear ing reefs. There are also drives along or close to the ore zones. Air is pumped down the ventilation shaft and it circulates through the various workings.
At the open cuts, payable ore zones were delineated by patterns of close-spaced drill holes. Shallow layers of rock were then removed by drilling, blasting and the use of an excavator. The are zones were marked by white lines on the ground and selectively mined. Access to the main underground mining area is gained via a 6 metre wide by 5 metre high. winding tunnel. which SIODes downwards at a gradient of I in 9 (Figure 547). This opening is called a decline. A decline was preferred to a vertical shaft - the type used in former days - because the large machines used in modern mining and ore transport can be readily moved between the surface and various levels being mined. The levels are 30-50 metres apart vertically. The heavy equipment used includes 40 tonne haulage trucks, front-end loaders and large mechanised drilling machines. The trucks can only enter openings 4 metres or more in width. The wider ore bodies are mined using a technique known as long-hOle sloping. (Sloping is the word used for any form of ore extraction underground). A large drilling machine bores a series of holes up to 30 metres long up the dip of the are body. The holes are filled with explosives and mass-fired, so breaking large quantit ies of roc k . Front-end loaders place the broken ore on diesel-powered trucks, which take the material up the decline to the treatment plant at the surface. The narrower ore bodies are blasted using shoner holes drilled by miners with hand-held machines. The fallen ore is scraped along the stope 1100r to the level and loaded into trucks waiting in the decline_
MUllOCK DUMP
Underground M i n e D e v elopment on t h e M ag d a l a Centra l Lode
Ore treatment Gold is extracted from the ore at the treatmcnt mill using a r�'Covery tcch nique k nown as carbon-ill-leach (the C. I. L. met hod ) . The proce�s involves t he following stages:
crushed ancl finely ground w i l h waler to fo rm a slurry. This liberates fine gold panicles from Ihe are.
I . The are is
2. The slurry is pa�sed inlo a lank where sodium cyanide Solulion is added. The gold p art ic l es dissolve in the cyanide solution.
Economic Geology
215
Figure 5-48 Underground mining at StaweU on the 382 met ... level. The miner is drilling a series of holes at the end of a drive along an orc zone. The machine is known as an air/ego The holes will be filled later with explosives and blasting will break the ore. Gold is associated with sulfide minerals, (pyrite, arsenopyrite, and pyrrhotite) in both the 'nat' dipping quartz reef and in the underlying narrow quartz stringers. The laner is said to be in the footwall of the reef. The reef is pan of the Magdala lode system . (Photograph courtesy of Western Mining Corporation Limited).
3 . Gold can be readily adsorbed on to the surface of small pellets of a porous type of carbon called activated carbon. The slurry containing the gold cyanide complex ions is therefore pumped through a series of five tanks containing activated carbon to extract the gold. The barren slurry is then pumped to a tailings dam.
4. The carbon pellets with their load of gold are creened from the tank liquids. Gold is stripped off their surfaces using chemical and electrolytic processes leaving a crude gold product. 5 . This marerial is smelted at a high temperarure, causing impurities to separate as slag and leaving molten gold. This is cooled to give bars of gold bullion, which are forwarded to a refinery to remove silver and other metal impurities.
Social and environmental impact
In rhe nineteenth century, the town of Stawell grew up around the mines. Nowadays, wherever mining is being revived in established townships, the pre sure is on the operators to make the plant as inconspicuous as possible. The environmental issues, that had to be considered when the Stawell venture started, included possible adverse affects of noise, vibration and dust associated with the mining and treatment operations. Other possible concerns related to vegetation clearing and landscape alteration, the storage of mine water and the disposal of tailings. Various measures to prOleCt the environment were introduced by the company. During dry weather, dust is suppressed by watering roads, stockpiles and bare areas. A minimum amount of vegetation was cleared when mining facilities were being constructed and revegetalion has been carried out wherever possible. Vegetation provides noise and dust barriers. Additional sound barriers have been created by erecting wooden fences and earth embankments. Mine water containing iron and other metallic salts is pumped into evaporation ponds to avoid pollution of local creeks. Topsoil was removed from areas of surface mining and later spread back over mullock (waste rock) dumps and dam walls to assist regeneration of vegetation. So-called set-back procedures were adopted. These involved planning the height and location of mullock dumps, stockpiles and buildings, so that they are screened from public view by natural topographic or vegetation features. Various environmental monitoring procedu res are carried out to ensure that standards are maintained. The levels of dust, noise and vibration due to undergro und blasting are regularly measured at several locations. Groundwater samples are collected from borehole near the tailings dam to check that no leakage of deleterious elements occurs. Routine sampling of water in mine storage dams, evaporation ponds and surrounding streams is also undenaken . The information and iUustrations in this article are based on mruerial supplied b) Western Mining Corporation LimilC.x1 for the Stawell Joint Venture partners, Great Boulder Holdings Limited and Central Norseman Gold Corporat ion.
L
Chapter
216
5 BASE METALS Copper, lead and zinc are called base melals in the mining industry. The term is used in contrast to precious metais, such as gold and silver, which command higher prices but are less reactive chemically.
Occurrence The three metals commonly occur as the sulfide minerals - chalcopyrite (CuFeS,), galena (PbS), and sphalerite (ZnS). Galena and sphalerite are often found together
in ore bodies. Chalcopyrite may either accompany these minerals or occur in deposits, where copper is the main metal of value. Silver minerals and minor gold are usually present in base metal ore bodies. Copper, lead and zinc minerals are found in many different geological environments. These include layers of sulfides within sequences of sedimentary and volcanic rocks, veinlets and blobs in limestone (mainly lead and "inc sulfide) and widespread low-grade disseminations through igneous intrusions (especially copper sulfide). Many o f the largest copper mines in the world are developed on huge ore bodies containing around I OJo copper metal in the form of copper sul fide minerals. These so-called 'porphyry' ore bodies are found in acid igneous rocks, which were int ruded close to the surface or extruded as volcanic rocks.
Figure 549 Base metal and lin ore occurrences
in Victoria.
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Base metal depOSits i n Victoria In Victoria, there are occurrences of base metal sulfides in similar geological environments to those that contain large ore bodies elsewhere. None of the Victorian deposits s i of possible economic importance at present, however, apart from two
Economic Geology
217
recent discoveries at Benambra in north-eastern Victoria. The largest number of base metal occurrences in Victoria is in the Snowy River country of East Gippsland. There, the minerals are associated with both Early Devonian volcanic activity and granitic intrusions. Small quantities of ore were mined at a few localities such as: I . Deddick - an area cent red about two kilometres south-west of McKillop Bridge
over the Snowy River. Patches of silver-bearing galena Occur in narrow quartz veins that intersect granodiorite. 2. Accommodation Creek seven kilometres east of McKillop Bridge. Chalcopyrite occurs sporadically in quartz-barite lodes developed along several narrow shear zones cutting across fo lded Ordovician quartzites and hornfelses. -
3 . Buchan District galena was produced at several small mines, the largest being at Back Creek, seven kilometrees east of Buchan. Silver-bearing galena, sphalerite and more abundant pyrite occur as veins and small lenses of ore along bedding planes and joints within dolomitic limestones near the base of the Buchan Caves Limestone formation. -
In the late 1 960s - early 1 970s, several mining companies searched for possible 'porphyry-type' copper mineralisation in eastern Victoria. Drilling of several targets
in East Gippsland revealed small tonnages of granodiorite with low concentrations of chalcopyrite disseminated through the rock, e.g. near Double Bull Creek , 1 9 kilometres nonh-west o f Orbos!. Beyond East Gippsland, most o f the small production o f base metal ores in Victoria came from two localities. I . Bethanga one of the richest goldfields in Victoria occurs between Bethanga and Talgarno, 25 kilometres east of Wodonga. The gold-bearing reefs are unusual because they intersect schists and gneisses of the Wagga-Omeo Metamorphic Complex and the gold is associated with high concentrations of sulfide minerals. The sulfide minerals are mostly pyrite, pyrrhotite and arsenopyrite, but locally there are sections of reefs with considerable chalcopyrite. Small smelters were operated intermittently between 1 866 and 1 9 1 6 to treat the chalcopyrite ores, but none was financially successful. -
2. Coopers Creek as recently as 1 97 1 , a small copper smelter was operated at the Coopers Creek or Thomson River copper mine, located five kilometres south south-west of Walhalla. The copper ore at this mine is in a different geological environment to any of the other base metal sulfide deposits discussed earlier. A small concentration 0 f chalcopyrite, associated with nickel, gold, platinum and palladium minerals, occurs within a bulge in an ultrabasic dyke belonging to the Woods Point Dyke Swarm. The combined value of the metals present in a tonne of Coopers Creek ore is considerable, but the two known ore shoots were too small for profitable mi ning. -
Case history: Benambra - a mine for the future?
In the mid - 1 96Os, much interest was taken in the possibility of finding deposits o f metallic minerals other than gold i n Victoria. B y that time, the swarms of early gold prospectors had been replaced by small exploration parties carrying out geochemical and geophysical surveys. The search was concentrated in belts of Lower Palaeowic volcanic rocks similar to those which contain ore bodies in Tasmania and New South Wales. Geochemistry is used to investigate large areas in a short time by collecting widely-spaced sanlples of either fine sediments along stream beds or soils on hillsides. Labo ratory instruments can detect the lead, zinc and copper contents in these samples in concentrations as low as a few parts per million. Many wnes of anomalous (i.e. higher than normal) metal values were found in various parts of Gippsland. One company was attracted to an area east of the small settlement of Benambra, at the head of the Tambo River. There, metal concentrations in soils as high as 4000 pans per million (0.4"70) zinc, 3000 parts per million (0.3%) copper and 1 600 parts per million (0. 16"70) lead were found. These values suggested the possibility that metal ions were leaking from a buried base metal sulfide ore body. The anomalies were over interbedded sedimentary and volcanic rocks of Silurian age. This increased interest in the area, because in southern and central New South Wales large base metal mines have been worked in similar rocks. Geochemical sampling al closer intervals together with ground and airborne geophysical surveys identified targets for drilling. After drilling many holes, twO wnes o f base metal sulfide mineralisation were out lined and called CurralVong and Wi/gao The following resources were announced:
218
Chapter 5
Tonnes
Currawong
Figure 5-50 Formation of base metal sulfide Ofe bodies by submarine volcanic act.ivity. There has been a series of volcanic eruptions from a large vent under the sea in fairly deep water. This has produced layers of acid lavas and tu ffs. The most recent activity has involved a large, viscous 'dome' of rhyolite and fluids containing a lot of water and metal ions. Where the hot material mel the sea waler, steam and other gases shattered the volcanic rocks. The solvent action of circulating water (originally from both the sea and the magma) has dissolved trace amounts of heavy metal ions from the int rusive magma and brought them to the surface. Several smaller vents formed on the flanks of the main volcano. Hot springs, called fumaroles, emanated from these vents. Heavy metal ions in the fluids precipitated as sulfides and sank to the sea-floor to for m masses of ore. Subsequently the ore bodies may be covered by younger volcanic deposits and sea floor sediments. Eventually after uplift of the rocks on the sea-floor and deformation (folding and faulti ng), the ore deposits may become pan of a folded rock sequence, as at Benam bra.
Wilga
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The minerals at Wilga are in paraUel layers of differing composition, so that the ore bodv looks like a sedimentary rock. It was probably deposited on the sea floor from fluids that emanated from a submarine volcano (Figure 5-50). The ore body is a sulfide mineral-rich layer, which occurs near the boundary between underlying acid tuffs and overlying interbedded dacite, dacitic tuffs and mudstones (Figure 5-5 1). Quartz, chlorite and white mica accompany the sulfides. There is also considerable silica in the form of chert adjacent to the ore body. Chert is typically part of a mineralising system produced by submarine volcanic activity.
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Menne se(jmenfs
Deposits of this sort are said to be polymetalliC, because they contain several valuable metallic minerals in sufficient quantities For each metal to be worth extracting. In other parts of the world, e.g. Japan, lodes of the size and grade of the Benambra ore bodjes have been profitably mined. However, the company that made the discoveries chose not [0 develop [he deposits. Subsequently they were acquired by another co mpany, Macquarie Resources Limited. This company carried ou[ additional drilling, which increased [he re ource indicated at Currawong. In 1 990 they were still assessing the feasibility o f mining [he deposits. A tunnel was excavated in the side of the steep mountain into the Wilga ore body to obtain a bulk sample of [he ore. Further closely-spaced diamond drilling was completed from underground sites (Figure 5-5 1 ) . Although the ore tonnage was not increased, the grade and therefore the [Olal metal contained were proved to be greater [han the earlier estimate. In considering whether to mine this deposit, four selS of factors present problems:
I. 2. 3. 4.
Geographic factors Environmental FaClOrs Me[allurgical factors Economic factors.
I . The area is high in the East Victorian Upland and far from established towns, railways, ports and major roads. It would be nece sary to build a new town to house mine employees and [heir Families and to construct all-weather roads to the nearest highway leading to a port. The mine products would have [0 be either shipped overseas or transported by road to an Australian smelter. The cost of [his construction work was estimated to exceed [welve million dollars in 1990.
Economic Geology
SOUTH
E3 � ---" .
NORTH
Baso'
.�'...' ,"..':'
.. .... 1.
TAM80
� l=....::....:.. p;=J � p.;;: �:;J r-'1 �
D
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l)W34.----f F F ----
219
Cheri Massive sulphIdes - maInly pynte Cha\copyflle nc:h sulphIdeS and chk:lnle
UW10
Chert strongly sheared RhyolitIC luff
Dnlt.ole trom sorlace Dnlhole trom undergrW>d o
,
Faull
Figure 50S 1 Geological cross-seclion Ihrough Ihe Wilga ba5O' melal ore body on Ihe northern slope of Macdougall Spur,
Layers of sulfide minerals (pyrite, chalcopyrite, sphalerite) are imerbedded with tuffs and cherty rocks. These arc overlain by acid lavas and fine-grained sedimcmary rocks. The ore layers have been traced by holes drilled from the surface and from (WQ sites in the tunnel. (Primed by courtesy of Macquarie Resources Limited).
50
,
METRES
100 1 19
.
2. Being in very rugged country at Ihe head of Ihe Tambo River catchment, development of the ite would be expensive, because of the need to protect the environmenl. It would be necessary to avoid any soil erosion or leakage of harmful metal or acid ions iRlo the river system from the mine. Several years ago, Macquarie Resources Slaned environmental monitoring and protection programs, which include studies of the meteorology, hydrology, flora, fauna, soils, stream biology and other factors . These investigations COSt half a million dollars each year.
3 . The metallurgical factors are related to the separation of the mineral componem imo separate saleable products, called 'collcentrales'. Chalcopyrite, pyrite, galena and sphalerite are locked together in fme-grained panicles and it will be expensive to separate them, especially the sphalerite. The ore will require crushing and fine grinding to liberate individual mineral particles. Each mineral will have to be concentrated in turn. In 1 990, the estimatcd cost to build a processing plant to extract the minerals and to build dams and other facilities was about 25 million dollars.
4. Although the costs of mine development can be calculated fairly accurately, it is difficult 10 assess the potential profits of mining the complex Benambra ore . The prices of the various melal products vary considerably from one year to another. The currency exchange rate is another crit ical factor. Mining companies benefit if the value of the Australian dollar i relatively low compared with that of the US dollar. At the end of 1 990, no decision to mine had been made. This arlicle is large!)' based on informat ion supplit..� by Mal-quarir Resources Limiled.
220
Chapter 5
HEAVY M I NERAL SANDS The term, heavy mineral, i used for various minor rock-forming minerals with densities (specific gravities) higher than those of the common rock-forming minerals. Heavy minerals concentrated in marine, beach or dune sand deposits along or near present-day or past coast lines are called heavy mineral sands. The main minerals of economic value are ilmenite (FeTiO,), rutile (TiO,), zircon (ZrSiO.), monazite (a phosphate of various rare earth minerals, which also contains thorium) and leucoxene (a weathering product of ilmenite). The minerals probably originated in granitic rocks, but they were concentrated by one or more cycles of weathering, erosion and sedimentation. In particular, after they were carried by rivers into the ocean, they were concentrated along beaches by wave action after stormy weather. Most Australian production of heavy minerals has come from deposits along the northern New South Wales - southern Queensland coast and old raised beaches near the south-western coast of Western Australia. In recent years, a very large resource of heavy minerals has been found in shallow sand layers within the sediments of the Murray Basin in Victoria.
R
Case history: Heavy mineral sands - WIM 1 50 project
Titanium minerals are used to make titanium dioxide pigments (the whitest known materials) for the paint, paper and plastic industries, titanium metal for jet planes and rockets and rutile coatings on welding rods. Zircon is used as a heat resistant material and in various ceramics. The rare earths have special chemical uses including the manufacture of the red colouring agent in television screens.
Exploration In the early 1 980s, CRA Exploration Pty Ltd commenced an investigation of the
mineral potential of the Murray Basin. Then, the only known minerals of economic value in the region were gypsum and other salts. Some brown coal in water bores and heavy mineral sands at Tyrrell Ridge. north-west of Birchip, were also known. In 1 98 5 , the company announced the discovery of a large mineral sand depo it. Up till 1 989, some $ 1 2 million had been spent on the investigation. It is estimated that about $300 million capital will be required to bring the resource into commercial production. Initial exploration was for gold, diamonds, uranium, brown coal and heavy minerals. In return for being granted exploration rights over a large region, the company was required to relinquish substantial areas at regular intervals. Consequently, there was an early emphasis on the use of rapid aerial geophysical methods of exploration, notably aeromagnetic and radiometric surveys. Records of water bores were reviewed and many vertical holes were bored, principally to search for coal. The heavy mineral deposits were discovered by geologists inspecting drill hole samples and taking geophysical measurements in the holes. Anomalous radioactivity revealed the presence of the mineral monazite, which contains the weakly-radioactive element thorium. Monazite had not been detected by the airborne radiometric surveys, because the deposits are covered by a thin layer of clay. Heavy minerals became the main exploration objective. To locate the deposits it was necessary to embark on a continuous drilling campaign. Heavy mineral resources appear to be widespread in the Murray Basin, but exploration and testing are most advanced at one deposit called WIM 150. This is located near Drung South on the Western Highway, 1 5 kilometres south-east of Horsham. Some 800 holes totalling 1 7 000 metres of drilling were needed to outline this deposit. Subsequently other large deposits to the north-east and south-west of WIM 1 50 were partly outlined by drilling (Figure 5-52).
Geology
The deposition of marine and freshwater sediments in the Murray Basin during Tertiary times is described in Chapter 4. The groundwater resources in the basin are summarised in Figure 6-19 in Chapter 6. The local geology at W[M 1 50 is shown in Figure 5-53. The Tertiary sediments are in sub-horizontal layers. Heavy minerals occur in the Parilla Sand throughout the south-eastern part of the Murray Basin, particularly in an arc-shaped band, that extends over 1 40 kilometres from north-west of Birchip to south-west of Horsham (Figure 5-52).
Economic Geology
Figure 5-52 Distribution of heavy mineral resources in the south-eastern part of the Murray Basin. It is expected that the WIM 150
deposit, located south-east of Horsham, will be the first deposit to be mined. In April 1 990, it was announced that additional heavy mineral sand deposits, called WIM 050, WIM 100, WIM 200, WIM 250 had been partly delineated. Their combined reserves exceed five times the reserve in WIM 150.
Figure 5-53
(a) Geology of the WIM 150 heavy
o LIIIIIIill
Kerang
Palaeozoic basement outcrops
•
(a)
mineral sand deposit.
o
Heavy mmeral occurrences
Partly delineated dePOSits
o
PLAN O l3tomary ..
g Sandy day
Devonian
� Gramplans
Pliocene Dp/iJrW/a Sand
This plan shows the distribution of zones with different proportions of heavy minerals within the ore body.
sandstone &
(b) A typical geological cross section through the WIM 150 heavy mineral sands deposit, near Horsham.
The vertical scale is greatly exaggerated. The Geera Clay wiU form an impervious bottom to the proposed pond in which a floating bucket dredge will be used to mine the Parilla Sand.
(b)
CROSS-SECTION
Boreholes
OuaternalY Darown - 9rey sandy clay Panlla Saoo
t' t"=
..,.,,,",, und
Q Sarrsn (ute sand
Ha
s..." mlflfHa/zontJ
Geera Clay �Cafbor!aceous day
Renmark Group
GramplaflS Group
o
I
2.5 ,
Us.,nor carbona",ovs day �Sand$lone. shale
7.5 5 1 I Kilometres
,0 ,
OWlf sand and mll'lOl' day
221
222
Chapter
5 Heavy mineral sands mined in Western Australia, Queensland and New South Wales are long linear deposits situated along or not far inland from the coasLline. These heavy minerals were clearly concentrated in dunes formed by wave and wind action along the present or former beaches. The Murray Basin beach sand deposits by contrast were spread as layers over wide areas on the floor of a shallow sea. They tend to be concentrated within 30 kilometres of the coasLline of Palaeozoic rocks. Unusual cross-bedding in the sands is taken to indicate that they were formed offshore during large storms below the normal fair-weaLher base of wave action . Details of the WlM ISO deposit The deposit is made up of numerous lenses of heavy mineral-rich sands, which are separated by nearly barren sand. Individual lenses are from 2 millimetres to 2 metres thick and they extend over distances of up to several hundred metres. They occur one above the other within a zone Lhat varies from 6 metres thickness in the souLh to 15 metres in the nonh. The uppermost Hi metres of Parilla Sand is mostly barren.
Figure 5-54 Test trench at WIM 1SO_ This was excavated to obtain bulk samples of the heavy mineral layer in the Parilla Sand.
Figure 5-55 The heavy mineral ore zone at
WIM 1SO_
)
Dark bands containing abundant heavy minerals are separated by barren, lighter-coloured sand.
/� .
, \
Figure 5-56 Average compOSition of heavy mineral fraction, WIM ISO deposit. Titanium is present in oxides (rutile, anatase) and iron·lilaniom oxides (ilmenite, leucoxene). Leucoxene is morc valuable than ilmenite, because it contains more titanium. Mon azite and xenOlime contain rare earth metals.
I n t I '" Lcul,;JX Rr '( lrd "na!)
-
Monal C Xe", m "'-io. c ,POr,11.. heavy ,IOcrlb lrol' )X de mr ma hne. PY lie. ell,; J
lOT
l
' 1 (1 I' I h �
OO C
The sediments oonsist of fine quartz and heavy mineral grains with minor mica flakes in a matrix of very fine quartz, clay or iron oxides. Up to 400/. of some bands are heavy minerals (Figure 5-55). The total reported resource at WIM 1 50 is shown below: Tonnes
one billion tonnes sand
-[
rutile and other high titanium minerals ilmenite zircon monazite xcnotime
8 000 000 1 2 500 000 5 1 00 000 500 000 1 70 000
M i nin g Topsoil and the underlying clay will be stripped down to the LOP of the Parilla Sand using scrapers and then transported to stockpiles. Much of the Parilla Sand is saturated with groundwater forming an extensive shallow aquifer on top of lhe Geera Clay, which is impervious. It will lherefore be possible to carrY out large-scale mining using a bucket dredge floating in a pond of water. A continuous chain line of buckets
Economic Geology
223
will excavate the sand down 10 the lOp of the underlying clay. After the heavy minerals are removed in a nearby floating concentrating plant, the barren sand will be returned to the area previously excavated by the dredge. The dredge will work out a block of ground by moving up and down a series of parallel lines. A dredge capable of processing 20 million tonnes of sand each year may be used. This will mean working an area of about one square kilometre each year. Treatment plant
1000 microns
I millimetre
The WlM 1 50 deposit differs from those at most other heavy mineral mines because the sands are extremely fine-grained. Typically heavy mineral grains mined elsewhere are 100-200 microns in diameter, whereas W I M 1 50 minerals average 50 microns. The fine grain size causes two problems. First, it is difficult to separate the individual grains and second, the grain size of the product is not suitable for the processes used in existing titanium pigment plants around the world. Other Australian beach sand producers extract and separate the heavy minerals by techniques that depend on their high specific gravities and certain electrical properties. Tests have shown these methods would not be suitable for the WIM 1 50 sands. It is likely that separation will be achieved by flotation. Flotation is a technique used 10 separate valuable lead and zinc sulfide minerals from complex ores at such places as Broken Hill and Mount Isa. The process depends on the fact that certain minerals will float to the surface of a slurry on bubbles of air. Chemicals are added to control the order in which the different minerals are separated. There are only a few major pigment-producing plants round the world . To supply titanium products that can be used in these processes, it will be necessary to agglomerate the grains of high-titanium minerals. This means the fine grains will size to the be formed inlO small pellets like coffee granules, which are of similar . rutile grains now used in the pigment industry. Environmental and social issues The country underlain by the heavy mineral depositS is mostly used for grazing and wheat growing. At any one time, only a small part of it will be involved in the dredging operation. After the heavy minerals are extracted, the various strata will be replaced in their original order of topsoil over clay over sand. After reseeding with grass and some tree planting the area will again be available for farming. CRA has bought the land at Drung South and will farm the area while mining is in progress. Later the land will be sold. On a regional basis, most groundwater in the Parilla Sand is 100 saline for use. However, there is some pumping for stock use along the southern margins o f the Murray Basin. Some fresh water from catchments i n The Grampians will have to be brought in for use in the treatment plants and 10 maintain a high enough water level in the dredge pond. Research is currently being undertaken to determine the environmental effects connected with water usage at the project. The project will provide a major new decentralised industry for Victoria and one which should continue through to the next century and beyond. It will bring economic growth to Horsham, where people working on the project will live. I n the long term, other major industrial projects may evolve using the minerals produced
at WIM 150. Possible downstream industries include titanium pigment and zirconia powder manufacture, rare earth metal processing and perhaps the production of titanium and zirconium metals. This article is based on information provided by eRA Exploration PlY Ltd and Wimmera industrial Minerals PlY Ltd.
TIN Occurrence Most tin metal is obtained from the mineral, cassiterite (SnO,). Like gold, cassiterite occurs both as primary deposits in hard rocks and in secondary alluvial deposi tS. A large part of the world's tin ore production comes from alluvial deposits in South-East Asia, especially Malaysia. In hard rocks, mo t cas iterite deposits crystallised during the final stages of cooling of silica-rich granitic magmas. Cassiterite is often associated with minerals such as tourmaline, topaz and muscovite. They occur in quartz, aplite and pegmatite dykes and in quartz-mica rocks called greisell .
224
Chapter
5
Tin deposits i n Victoria A l luvial The largest production of tin ore (about 10 000 tonnes) in Victoria came as a by product of gold mining along the valley of Reedy Creek between Beechworth and Eldorado. It occurred in both shallow and deep alluvial leads of Tertiary age. Minor amounts also came from deep leads in the nearby Chiltern-Rutherglen area. The gold and cassiterite were derived from the weathering of different rocks in nearby areas. The gold came from quartz reefs intersecting Ordovician sedimentary rocks, whereas the tin ore was associated with the Mount Pilot Granite. The most productive locality was Eldorado. There, in the nineteenth century, gold- and tin-bearing gravels were mined from up to five separate levels in several alluvial mines. The maximum depth worked was on bedrock at 79 metres. These levels reflected five different periods during the Tertiary, when the nearby ranges were uplifted, and rejuvenated rivers carried large quantities of sands and gravels eroded from the weathered bedrock. During a later phase of mining between 1 936 and 1 954, a bucket dredge floating in an artificial lake at Eldorado excavated alluvium down to a depth of 29 metres. It recovered gravels containing gold and cassiterite which had been left by the early miners on the upper four levels. Alluvial tin, on its own, between 1 892 and 1 940, but there have been other small cassiterite is associated with
was mined at Toora in South Gippsland on occasions the field is believed to be worked out. Intermittently operations in the uplands east of Tallangalla, where the Koetong Granite.
Lodes On several occasions, attempts have been made to work open-cut mines in large tin-bearing greisen, aplite and pegmatite dykes at Walwa in nOrlh-eastern Victoria near the Murray River. The grade of the ore has proved to be too low for profitable mining, however. Cassiterite-bearing pegmatite dykes are numerous in a belt of Ordovician schist country in north-eastern Victoria between Mount Wills, in the south, and Milta Milla and Tallandoon in the north. The concentrations of cassiterite are too patchy and small for profitable mining.
IRON Iron, in the form of steel, is the most imporlant metal used in the construction and manufacturing industries. Vast sedimentary deposits of iron ore, in the form of hematite (Fe20,), occur in Pre-Cambrian formations in the Pilbara rgion of West em Australia. Several large open-cut mines make major contributions to Austral ia's export trade.
Iron ore deposits in Victoria There are several small limonite and limonite-manganese oxide deposits to the north and south of Buchan. At one locality, McRae's Quarry, about 1 2 000 tonnes of iron ore containing 32"10 iron were excavated between 1955 and 1970. The mineral was used as a scrubber for town gas in the period before natural gas was introduced. The 10 metres thick deposit is in terbedded with limestones and tu ffs near the base of the Buchan Caves Limestone formation. The limonite merges down-
BAUXITE Bauxite is hydrated aluminium oxide formed by rock decomposition under special conditions, usually in tropical climates. Silica is largely dissolved out and minerals such as feldspar are altered to clays. Very large deposits of bauxite are mined in Northern Australia and in the Darling Ranges in the south-west of Western Australia. The material is both exported overseas and used in Australia for the production of alumina (AhO,) and refined aluminium metal. Aluminium smelting uses large amounts of electric power. Bauxite
Economic Geology
225
is brought from Western Australia to feed aluminium smelters at Portland and Point Henry, near Geelong, where cheap electric power is available.
Bauxite deposits in Victoria The reserves of bauxite found in Victoria are insignificant compared with those in Northern and Western Australia. Nevertheless over 40 small deposits have been identified in the Strzelecb Ranges of South Gippsland, especially in the Boolarra - Mirboo North area. They evidently formed by the decomposition of Older Volcanic basalt and tuff during wet, humid periods in the Tertiary. Gippsland bauxite is used mainly in the cement industry when additional alumina is needed to achieve the correct balance of oxides in the kilns. The deposits contain
50-53"70 Aha,.
Industrial miner and rocks
Industrial minerals and rocks (or non-metallies, as they are often called), form a group covering all the minerals and rocks used by modern societies except those described in the previous three sections. There is a wide range of rocks and minerals in this group, including many used in the chemical and fertiliser industries. Often, it is the physical properties of minerals that make them important. For example, mica is an insulator, industrial diamonds are very hard and used for CUlling, garnet is an abrasive material.
SALT Common salt (NaCl) occurs naturally in many sedimentary rock successions as the mineral halite or rock sail. Halite belongs to a group of rocks and minerals known as evaporites. These are deposits formed by the evaporation of water from salt-rich fluids (brines).
It
has been est mareJ that the ",orld's lll.;CanS co. le-'n 1 r 01 r (\ over 18 milh n I,;ul h,; kilc..mM or t.:omlron sal t A \olume tii O larger than the en Ife ort Amem:an conllnenl alJ(\\( sea·Ie\ ("I
Halite is also the commonest salt present in sea water and underground water. Sea water contains an average of 3.6"70 of dissolved salts. Chloride and sodium are the most abundant ions present, the others being mainly sulfate, magnesium, calcium and potassium with a lillie iron and bicarbonate. Most evaporite deposits originated by the evaporation either of sea water in confined coastal bays or of lake waters in arid regions.
In Australia, most common salt for domestic markets is produced close to major cities, either by evaporation of sea water in artificial ponds or of groundwater in natural inland lakes. Manufacturing is best carried out in areas where the rainfall is not high and there are long hours of sunlight.
Salt deposits in Victoria In Victoria, over 100 000 tonnes of salt are produced each year from the sea in Port Phillip and Corio Bay and from salt lakes in the north-west of the State. In both areas, the annual evaporation exceeds the rainfall, so salts crystallise from solution and are harvested each year.
I. Various salts are produced from sea water near Geelong and Werribee by a process known as Jractional crystallisation. This depends on the fact that different salts have different solubilities in water. Sea water is pumped into artificial ponds excavated on nat land beside the sea. The Sun's heat causes the water to evaporate. The least soluble compounds, iron oxide and calcium carbonate, are the first to precipitate. When the water is reduced to 19"70 of its original volume, calcium sulfate crystallises out. After this crystallisation is oomplete, the concentrated brine is pumped into another pond. There. after reduction of the original volume to 9.5"70. sodium chloride crystallises. Before the volume of brine falls below 4% of the original. it is pumped elsewhere for further fractional crystallisation of potassium and magnesium salts. Sodium chloride. which forms about 78% of the total salts. is harvested by mechanical scrapers. Magnesium salts are the next common. being 15.7"70 of the tOtal on average.
2. During winter, many lakes in north-western Victoria are fed by saline groundwater seeping out of the shallow Parilla Sand formation. These lakes are called salinas. Provided there are no heavy rains, the lakes dry out in summer leaving deposits of salts, which can be collected by scrapers. There is a large company salt harvesting operation at Lake Tyrrell, north of Sea Lake in the Malice. and other small producers in the Kerang area. Since 1984 there has been greater production of salt from salinas than from the sea.
226
Chapter 5
GYPSUM Gypsum is hydrated calcium sulfate (CaSO •.2H,0). It is another evaporite mineral. A major part of the gypsum used in Australia comes from dry lakes close to the west coast of South Australia. The Lake Macdonnell deposit near Penong alone contains over 500 milljon tonnes of hard, crystalline gypsum in a layer averaging almost 4 metres thick. Gypsum deposits i n Victoria Gypsum is recovered from salt deposits of late Quaternary age in the Mallee region. They are found in shallow playa lakes (see Chapter 3). Unlike the salinas, where groundwater seeps into the lake, the water table is about a metre below the floor of the playas. The groundwater is drawn IOwards the surface by capillary action. There, fractional crystallisation takes place because the water evaporates. A layer of gypsum about one metre thick forms a few centimetres below the floor of the playa. The largest crystals are near the base of the deposit - they are often two to five centimetres long. Fine-grained gypsum is also carried across the lakes by winds and relocated i n arc-shaped dunes. Fairly pure crystalline gypsum used to be mined at many localities, including Hattah, Cowangie, Nypo and Raak Plain, for use in plaster manufacture. Gypsum from Cowangie is still used as a cement additive. Most gypsum in Victoria is now used as a soil conditioner in which high purity is not essential. Many farmers extract it from their properues for their own use or for local sale. Their production amounts to about 180 000 tonnes out of a total yearly production of about 250 000 tonnes.
DIATOMITE OR DIATOMACEOUS EARTH Diatomite is an unusual kind of very fine-grained sediment consisting mainly of silica (SiO,). It is derived from the remains of microscopic organisms called diaroms. There are thousands of species of diatoms, some of which thrive in salt water and some in freshwater. Deposits of diatomite are rarely more than a few metres thick. Diatontite has useful properties of low specific gravity (0.4 - 0.6), very high porosity and absorptive capacity, and low thermal conductivity; it is also chemically inert. Its greatest use is in filtering suspended solids from fluids, e.g. in swimming pool filters. Diatomite deposits in Victoria A number of deposits have been found in Victoria in freshwater lakes associated with Newer Basalt lava flows. The lavas caused the lakes to form by damming treams. They also provided a source of silica, which enabled the diatoms to flourish. Deposits tend to be lens-shaped in cross-section and vary from one to six metres in thickness. They are usually covered by basalt but may be partly exposed by erosion. The material is soft and finely granular, its colour is either brilliant white, or cream to brown if impurities are present. In the past, small deposits were worked in the Kilmore, Woodend, Redesdale, Linton and Avoca districts. Since 1 973 the only quarry has been at LiUicur, 20 kilometres south-west of Maryborough. Lillicur diatontite is used to absorb phosphoric acids for safe handling and storage, and also as a filler in paint and plastics manufacture.
Gemstones and specimen mineral
Most economic minerals anain their value because they are converted by various industries into useful products needed by people. But there are other minerals that are traded simply because they are collectors' items. Throughout history various minerals, (most of them coloured), have been used as objects of ornamentation, curiosity or symbolism. These collectables fall into three categories: 1_ Gemstones
These are used mainly for personal adornment, because they are beautiful, durable and often transparent, Fragments are polished and usually faceted. The most valuable gems (precious stones) are rare and provide the greatest sparkle in jewellery. These include diamonds, the sapphire and ruby varieties of corundum, emerald (the green variety of beryl) and precious opal. Less lustrous, semi-precious stones, which are commoner in rocks, include garnet, topaz, aquamarine and zircon.
Economic Geology
227
2. Polished stones These are colourful pieces of opaque minerals and rocks, which can be polished, usually in a tumbler. They are used for costume jewellery and various ornaments. 3. Specimen minerals Some individual crystals or, more commonly, clusters of crystals of one or more mineral species may be put on display because of their aesthetic appeal or else collected for their scientific interest. Specimen minerals usually occur as well formed crystals. They may be cleaned and trimmed, but are otherwise presented as they are found in nature. There are several imponant areas of commercial gemstone mining in Australia, notably opal fields in inland South Australia and New South Wales, sapphire fields in Queensland and northern New South Wales and diamond mines at Argyle in north-western Australia. There are no gemstone mines in Victoria and few localities where large crystals of sp ecimen minerals can be collected. In the past however, small mines at Edi and Cheshunt near Mansfield produced good quality turquoise that was sold for jewellery in London. M INERALS IN VICTORIA In Victoria, most of the nrinerals of interest to collectors come from four geological environments: granites, quartz veins, basalts and alluvial gravels. Special mineral localities within these rocks are rare and usually found by accident. Granites Only a few of the several hundred granite masses in Victoria have provided beautiful crystals of both common and unusual minerals. These granites are usually pink or cream varieties and they often contain patches and veins of pegmatite. This is a very coarse-grained rock, composed of large crystals of quartz, feldspar, mica, possibly tourmaline and less common species such as fluorapatite, topaz and beryl. The large mineral grains developed under unusual conditions. The parent granite magma was rich in gases such as steam and compounds containing chlorine, tluorine and boron. These created open spaces in the rock. During the fmal stages of cooling, large crystals grew in the cavities. The best localities for finding well-formed crystals are granite quarries near Lake Boga and Pyramid Hill in north-western Victoria. At Lake Boga, unusual phosphate minerals containing copper, magnesium, calcium and uranium form beautiful small crystals in cavities. Similar granites occur at Cape Woolamai on Phillip Island and in the Beechwonh district. At Tallangalook in the Strathbogie Ranges, a pegmatite mass was mined during World War II for transparent quartz crystals, which were used in radio receivers and transmitters. Occasionally interesting crystals also form in rhyolite. Unusual, transparent, smoky brown quartz crystals occur in small spherical cavities in a rhyolite near MoraUa in the Black Range, south of Horsham. Quartz veins There are tens of thousands of quartz veins intersecting Palaeozoic rocks in Victoria, Many have been mined for gold, which sometimes occurred as large masses in the quartz and more rarely as beautiful crystals. As well, some magnificent groups and large individual crystals 0 f quartz were collected during gold mining, particularly at Ballarat, Bendigo, Castiemaine, Fryerstown and Matlock. Small crystals of sulfide minerals, such as stibnite, molybdenite, galena, sphalerite and chalcopyrite, were also fauna in quartz veins. Basalts Minerals belonging to the zeolite group are widespread in Victoria in both Older and Newer Volcanics with some localities being world famous. Zeolites are hydrous aluminosilicates of sodium, calc.ium and potassium. They often occur as groups of small, well-formed, colourless, white or pink crystals in cavities in basalts. These minerals crystallised at temperatures below 250'C from solutions trapped in cavities as the lavas were cooling. They are usually accompanied by crystals of calcite or aragonite. Zeolites are common in basalts forming cliffs along the southern coastlines of Phillip Island and Mornington Peninsula between Flinders and Cape Schanck. The most common varieties are analcime, chabazite, natrolite, phillipsite and thomsonite, but at least nine other rarer zeolites are known. Olivine bombs are an interesting collectors' item found on some Western District volcanic hills, e.g. Mount Noorat near Terang. These are small semi-rounded lumps of basalt lava, which contain a core of green olivine crystals.
228
Chapter 5
Alluvial gravels Prolonged weathering and erosion of basalts and granites have released small crystals of gem minerals. These are found in many stream gravels in central Victoria, particularly the deep lead gravels in the Ararat, Ballarat, Daylesford and Chiltem districts. Bright blue to near-black sapphires, orange to reddish-brown zircons and rare red rubies have come from basalts. Granites have yielded red garnets, colourless to pale blue topazes and coloured varieties of quartz such as amethyst, citrine and cairngorm. The greatest range of gemstones probably occurs in the Woolshed Valley section of Reedy Creek, near Beechwonh. There, sapphires, garnets, zircons, rubies, topazes, tourmalines, coloured varieties of quartz and rare diamonds may be found in the stream gravels. The source of the diamonds is unknown, making it one of the biggest puzzles of Victorian geology. Pebbles of colourful agates and jaspers are widespread in Victoria and come from various sources. Around Beechworth, grey to yellowish banded agates have weathered from Permian glacial conglomerates. At Moonlight Head, east of Pon Campbell, agates and other attractive pebbles are probably derived from Cretaceous conglomerates. In the Avon and Snowy Rivers in East Gippsland there are agates from Devonian rhyolites. Jasper occurs along Boggy Creek at Nowa Nowa. On the south coast of Phillip Island, agates have been weathered from cavities in basalts. All these pebbles take an attractive polish. Figure 5-57 A cluster of smoky Quartz, orthoclase and albite (feldspar) crystals collected at a granite Quarry at Lake Boga, north weSlern Victoria.
Water
229
Chapter 6
WATER Figure 6-1 Rehabilitation of a town water supply bore at Wyatt Street, Portland, 1988.
This bore was drilled in the late 1950s to supply part of the water used in the town of Portland. After several decades, the supply declined because the iron pipes down the hole had rusted. The Department of Industry Drilling Unit rehabilitaled the bore by removing the old casing and screens and replacing them with new slainless sleel equipment. A large rotary drill was used to raise and lower tools in the hole. Usually these machines are used to drill to deplhs of up to 1500 metres. With this machine, drilling mud (bentonite) was passed down the hole lhroughOUl the operation. The mud served various purposes. In particular it stopped the flow of hot artesian water while work was in progress, by filling the pores of the surrounding rocks. In the photograph, rehabilitation has been completed. HOI water and sleam were allowed to gush from the hole until the mud was cleaned out. (Photograph courtesy of Department of Industry). I
Figure 6-2 (right below) Distribution of water around the world.
The tOlai volume of water in lhe world is about 1500 million cubic kilometres. Most of this water is too salty for human consumption. Only a small amounl is readily available freshwater and over 96% of this occurs underground.
Saline water in oceans: 97.2070
Water is one of the most important resources on Earth. It is essemial to maintain all forms of life - human, animal and plant. Water is not only needed by every individual for drinking and washing, it is also required for gardening, farming and a wide variety of industrial activities. But, like any other mineral resource, water is unevenly distributed over the Earth's surface. Some areas have large, readily accessible supplies - elsewhere water is scarce or absent. Most city dwellers in Australia probably take il for gramed that they always will have a supply of fresh water available at the turn of a tap. Yet water is not a limitless resource. Australia is one of the world's driest lands, where water is a resource that must be managed, i.e. collected, handled and used efficiently. Storage component
volume (km'xlO') I 458 ()()()
oceans
Available freshwater
Tow ....'alCr store
underground """Her groundwater soil moisture surface water takes streams
1
9300 7:
0.62 0.005
9300 not available
345 '5
0.023 0.00'
500 000
0.00' 100.0
�.
nOI available not available
'5
atmospheric Water
97.2
volume (km'xllY)
2.15
32 250
icecaps and glaciers
�.
345 15
96.2
3.6 0.2
not ayailable 9660
100.0
Most human population centres around the world have developed where:
• 6
•
lee caps and glaciers: 2. t 5""0 Groundwater: 0.6250']'0 Surface Water: 0.024010 Atmospheric Wa.er: 0.001 '10
•
there are permanent supplies or water; the water is of adequate quality.
The term, adequate quality, means the water: • does not contain excessive amounts of suspended muddy material; • does not contain harmful dissolved salts or other chemicals; • does not have any objectio nable odour or taste.
230
Chapter 6
Unfortunately people cannot readily use the largest accumulations of water on our planet. The vast oceans are too salty for most uses and, so far, it is too expensive to purify sea water because of the high energy costs involved. The polar ice caps are also large storages of pure water, because they are formed from snow and hail. However, no practical method has been found to transfer this water to the populated arid parts of the world where it is needed. Apart from its uses as a mineral commodity, water also plays a major role in many natural physical and chemical processes. Some of these processes have adverse effects on mankind and wherever possible, efforts are made to control them. For example, the power of nowing water over land can lead to destructive soil erosion, nooding rivers can cause widespread damage to land and property, and the seas are constantly attacking many coastlines. Because nowing water can carry large loads of sediment, silt may eventually fill reservoirs, lakes, channels and harbours. The quality of water can be degraded because water can readily dissolve and transport harmful salts and other polluting chemicals.
Hydrology
The science of water is called hydrology. It concerns the occurrence, distribution, movement and properties of water in its three physical forms - solid, liquid and gas - on and below the Earth's surface and in the atmosphere. Just as some economic geologists specialise in the search for new depOsits of metallic ores or fuel minerals, there are others whose interest is chieny water. Such specialists are called hydrogeologists or simply hydrologists. Their main tasks are to identify where suitable water supplies occur and to ensure that water supplies will remain available in the future. Hydrologists are also concerned with environmental problems that involve water. In this chapter, discussion is concentrated on the two categories of water that are of most value to people for their domestic, farming and industrial uses. These are: water collected at the surface of the land in natural and man-made storages; • water extracted from natural re ervoirs beneath the surface of the land. This is • termed groundwater. Some of the environmental issues relating to water are also considered. These include pollution and the loss of fertility in agricultural land due to waterlogging and the build-up of harmful salts in soils.
Water cycle
The water cycle (also known as the izydrologic cycle) is a concept used to show that all water is involved in an endless cyclical movement. As illustrated in simple terms in Figure 6-3, all water that falls on the Earth's surface, whether as rain, hail or snow, eventually returns to the Earth's atmosphere as water vapour. The energy for this circulation is provided by two major forces: I. Force of gravity - this causes water to fall through the atmosphere and to move downwards over and under the Earth's surface. 2. Heat radiated by the Sun - this causes water to evaporate from the land and ocean, and water vapour to rise in the atmosphere. The water cycle has no beginning nor end, but it is convenient to describe it by starting with the oceans, which contain most of the world's water. Water evaporates from the surface of the oceans. The amount for a given area is greatest near the equator where the Sun's heat is most intense. Water vapour is nearly pure as it contains only small amounts of salts. Warm air currents lift the moist air masses through the atmosphere. Eventually at higher levels, these masses become cooler causing the water vapour to condense as rain, snow or hail. These three forms are collectively called precipitation. Precipitation may return water directly to the oceans or it may fall over land. Not all precipitation reaches the Earth's surface. Some evaporates as it falls through warmer air. Part of it is intercepted close to the surface of the land by either man made structures, e.g. buildings, or by living things, e.g. plants. Some of this water also returns to the atmosphere by evaporation. Precipitation that reaches the ground surface becomes part of the so-<:alled land phase of the water cycle. It is this phase that is of most interest to the human race. Some of the water on land does not travel far. It may be temporarily retained in pools and puddles on clayey soils, in hollows and crevices in rocks, or in the upper layers of soils. This water is largely lost by evaporation. Most of the water falling on land, however, moves downwards under the innuence of gravity. Part moves across the land, where it is termed overland flow. The remainder penetrates the ground by a process called infiltration.
Water
231
I
Figure 6-3
--"6/�_
The water cycle.
There is a continuous interchange of water between the land, the ocean and the atmosphere.
/
y:;,(
atmosphere
I
solarradlalion
,
I
evapora110n 01 ram droplets preClprtallon (snow faU) preCipitatIon
(ralnlall)
evaporation from land
ocean evaporation
lakes. and waler courses
Overland flow consists of rainwater and, in some places, water derived from melting snow or glaciers. Most of this water drains into streams and eventually reaches lakes, swamps, dams or oceans. Some water is lost from flowing streams, however, by evaporation or by percolation into the beds of the streams. If rainwater does not evaporate from the ground or run into streams, it seeps downwards into the ground. Infiltration is most likely to occur where there are no fast-flowing streams, the land is fairly flat and the surface soils are porous. Some water clings to soil particles and may be drawn into the roots of plants: this is called soil moisture. After the plants use the water, it evaporates back into the atmosphere, mainly through the leaves, by a process known as transpiration. The remaining moisture continues to move downwards through successive layers of soil, weathered rock and fresh rock. This movement is possible because there are spaces provided by fractures, j oints and the pores between the various mineral grains. At some depth, known as the water table, all the openings in rocks and soils are saturated with water. Below the water table, the movement of water changes from a vertical downward movement to a predominantly sideways motion down a very gentle gradient. It slowly moves laterally through interconnected passages until it discharges as springs or seepages into streams, lakes, swamps or oceans. Although water is constantly moving and changing its state in the water cycle, the movements are erratic, both in time and place. The relatively fast now rate of many surface streams contrasts with that of groundwater, which may take many thousands of years to reach the sea. The water cycle may be modified by human activities in various ways. Land clearing for agricultural or other purposes results in decreased interception and transpiration by natural vegetation. The accelerated run-off of rainwater may cause soil erosion. Alternatively there may be greater infiltration leading to increased storage of water underground. The passage of water from the land to lakes and seas can be interrupted by the construction of dams, stormwater drainage schemes and irrigation channels, and by the sinking of bores and wells. Most water used by people, however, evemually returns to the oceans to perpetuate the hydrologic cycle.
Surface water
People may obtain water directly from streams and lakes, or they may collect it first in dams built across stream valleys. Water flowing along a stream is a mixture of water running off the land into the stream and former groundwater, that has emerged as springs or seepages in the banks of the stream. Groundwater discharges may provide the only waler feeding a stream during dry periods.
232
Chapter 6
After precipitation falls on the ground, it may either evaporate, run off into streams or percolate down into underlying soils or rocks. The main factors, which determine whal proportion of the water fOllows each of these three routes, are climate, topography, geology and vegetation. Climate: The total amount of rain and snow falling in an area each year is important and also the frequency, intensity and duration of the falls and the time of year when they occur. Most heavy rain from thunderstorms is likely to run off into streams, whereas genlle rain over a long period usually penetrates the surface. Temperature is also important, as higher temperatures lead to greater evaporation of surface water. Temperature also determines when and how quickly snow melts. Topography: Generally there is considerable run-off from steep terrains, whereas on gentle slopes most waler infiltrates into underground storages. The altitude and orientation of water drainage basins (or watersheds) are also importanl indirectly, because of their influences on temperature and precipitation. Temperature usually falls with increasing altitude and this leads to greater falls of rain or even snow in the uplands. Evaporation, however, decreases in mountainous areas. Because the Sun is always to the north of Victoria, evaporation is greater on the northern than on the southern mountain slopes. Geology: This is mainly important because of its effect on present-day topography. The type of rocks in an area, their structures and the nature of the overlying soils also control the amount of water that passes below the surface. The more porous rocks and soils are, the easier it is for water to infiltrate them. Vegetation: Trees, plants and grass absorb a lot of water through their roots. They also retard run-off after rain so that greater quantities of water percolate into the ground. Vegetation also reduces soil erosion on steep slopes so that water flowing through forested country is usually clear.
SELECTION OF SITES FOR STORAGE DAMS In Australia, many communities are provided with permanent water supplies. These are obtained either from storage dams constructed across watercourses or, in a few places, from lakes. Holding river water in dams also helps to reduce potential flood damage that may occur after heavy rains. In some places the force of falling water released from dams can be used to generate hydro-electricity. Engineering, economic and social factors must be considered when the site for a new water storage dam is being selected. Research is carried out to eSlabli h the water requirements of the communities the dam wiU serve and to ensure that any adverse effects on the natural environment will be minimised. It is also desirable that no valuable agricultural land should be lost beneath the waler and that people downstream from the dam should not lose their customary supplies. Figure 6-4 The dam wall of Lake Eildon near the township of Eildon. This is a large earth and rock fill dam that impounds the water.; of the Goulburn, Delatite and other rivers draining the East Victorian Uplands. The first dam was built in 1927 and it was enlarged in 1955 to become the second largest reservoir in Victoria. The hill at the end of the wall is the Sugarloaf formed by south-west dipping, thick-bedded
andstones
and siltstones. The darn was constructed mainly 10 supply waler to the Goulburn valley irrigation area but it is also a popular recreational area For boating and fishing.
Water
233
Geographic factors tend to control the selection of the site for a dam. The wall should be built preferably where the valley is deep, so that adequate water is impounded by the structure, The valley should also be narrow to minimise construction costs and to ensure that the surface area exposed to evaporation is kept to a minimum. However, geological factors often determine where the fmal site will be. Detailed geological mapping is carried out to identify any zones of porous or fractured rocks that would allow water to leak away from the dam. The geological work carried out to assist dam selection is discussed further in Chapter 7.
Groundwater
As explained in the earlier discussions about the water cycle, part of the water that falls on land infiltrates below the surface. Some of this water is used by plants, but most of it penetrates deeper under the force of gravity. On its downward path the subsurface water encounters either unconsolidated earth materials, (soils and layers of loose sands, gravels and clays) or consolidated earth moterials, (hard rocks of different ages and types). There are two zones of water underground, an upper lInsatllraled zone and a lower saturated zone, called the groundwater zone. In Victoria, the unsaturated zone is rarely more than 50 metres thick and in some places it is very thin. However, elsewhere in Australia the thickness can range up to several hundred metres, particularly in some arid regions. The uppermost pan of the unsaturated zone is called the soil water zone. It is commonly less than 2 metres thick and is important because it supports the growth of plants. In the saturated zone, all interconnected openings between the minerals forming the rocks or sediments are filled completely with waler. This zone may be up to 2000 metres thick. At still greater depths rocks are usually lOO compaci to contain any significant water. The upper surface of the zone of saturalion is called Ihe water lable. Supplies of groundwater are obtained by excavating wells or drilling bores into the saturated rocks below the water table. Water can be extracted from shallow weUs and bores by pumps driven by windmills, whereas mechanically-or electrically driven pumps are connected to deeper boreholes. Before considering the behaviour of groundwater in detail, it is necessary lO understand the significance of two terms which are commonly used in groundwater studies. These are porosity and permeability.
POROSITY
Porosity is the property of rocks that explains Iheir ability 10 SlOre water. Porosity is supplied by spaces between mineral grains that form rocks and soils. The e spaces can be filled with water_ Where the spaces are interconnected, they serve as channelways along which groundwater moves.
Figure (}OS Types of rock porosity.
Examples of differenl kinds of spaces in rocks where water can be stored. A·D primary porosity . • spaces presen! when the rock is formed. E·F
secondary porosity. • spaces appeared afler the rock formed.
A Welr-sorled sedimentary deposit havin g high ptirnary porOSity.
Poorly-sorted sedimentary depoSil haVIng low primary poroSIty
E
D Basal! honeycombed by veSicles (small gas caY'lIes) WhICh,
If
Interconnected, prOVIde htgh permeability and
fall poroSlly
Secondary poroSl[Y Increased by SO!ULon. as m
�mes!oncs
Well-sorted sedmentary dePOSIt Wit"" POl'OSlty o:mlnlshed by the
minerai maner In tne mters�oos. e.g durmg consolida:on
depoSll!on of
F Secondary porosity by fraclunng and 1 0n: n9 8Sln graMe
234
Chapter 6
Porosity may be either primary or secondary (Figure 6-5). Primary porosity, (also known as intergranu/ar porosity), was produced by the original geological processes that formed a rock or soil. Unconsolidated sediments, such as sands and gravels, have good primary porosity because there is a lot of space between the panicles. They are therefore capable of holding large amounts of water. On the other hand, consolidated rocks usually have low primary porosities. Many igneous rocks, such as granites, are completely crystalline and contain few pores. The same applies to many sedimentary rocks where the spaces between mineral grains are usually filled with cementing materials, such as quartz or limonj[e. Secondary porosity, (sometimes known as fracture porosity), only occurs in consolidated rocks. The spaces were developed after the rocks were formed. They can consist of fractures, joints and fault zones in all kinds of massive rocks and also solution feaNres, such as caves, in limestones.
PERMEABILITY Earth materials are said t o be permeable if water passes through or can be pumped from them easily. The permeability of a rock is controlled by the sizes of the spaces within it and by the extent to which they are interconnected. It is important to note that rocks with the same porosity need not have the same permeability. For example a gravel and a silt may have the same porosity, because the total space between the gravel grains may be the same as that between the much finer particles of silt. Thus equal volumes of gravel and silt can store equal quantities of water. However, gravel can release more of its water and release it at a faster rate when pumped, because it is much more permeable than silt. It is difficult to pump water from silt or clay because the water is more strongly held within the tiny pore spaces by molecular and capillary forces.
AQUIFERS The groundwater zone may be imagined as a huge natural reservoir in rocks. Its storage capacity is equal to the total volume of pores or openings that are filled with water. At any one location, groundwater may be found either in one continuous body or in several distinct rock layers. Porous water-bearing rocks, which can yield useful supplies of water, are called aquifers. Less permeable rocks, that restrict the movement of groundwater into or out of adjacent aquifers, are termed aquitards or simply confining beds (Figure 6-8). In any area, where an aquifer is present, the nature of the rocks and the geological structures conuol: • • •
the amount of water that can be stOred in the aquifer; the rate of movement of groundwater through the aquifer; the yield of \ ater that can be pumped from the aquifer.
Even in areas of high rainfall, if the geological conditions are unsuitable, groundwater supplies are limited. For example, the East VictOrian Uplands is a high rainfall region, where Palaeozoic sedimentary rocks outcrop extensively. These rocks contain good quality groundwater but, because they have low permeability, bore yields are small. Conversely, with favourable geological conditions, substantial supplies of good quality groundwater may be obtained even in areas of low rainfall. For example, the town water supply for Alice Springs is obtained from the alluvium of the Todd River and the deep Mereenie SandstOne of the Amadeus Basin, both permeable formations. Aquifers perform two functions: • •
they serve as natural stOrage reservoirs; they distribute water from place to place.
They are stOrage reservoirs because they contain water that can be tapped by bores or wells. They are also distribution systems, because water enters aquifers where they outcrop, travels through underground openings and eventually discharges at springs, rivers, lakes or the oceans. The outcrops of an aquifer are called its intake or recharge areas (Figure 6-6). Aquifers can carry water from intake areas in distant high rainfall areas to arid regions, where surface nows rarely occur. Most groundwater nows very slowly through the ground - usually at a rate of only a few centimetres to several metres per day. Aquifers are commonly classified as either unconfined or confined (Figures 6-7 and 6-8).
Water
Figure (Hi An aquifer recharge area near Murrindal in East Gippsland.
The cleared country is on Lower Devonian limestone. There are no permanent streams in this area. Most water percolates into the ground through fractures in the limestone. Two large open sinkholes (dolines) can be seen. The limestone provides an aquifer with good quality w3ter, and it is covered to the south by more clayey rocks of the Taravale Formation. In the background, the forested hills are On west-dipping ignimbrites of the Snowy River Volcanics. These rocks are 'tighter'. that is groundwater cannot penetrate them as readily as it does the limestones. Surface streams are commoner in this country. (photograph by N.J. Rosengren). Figure 6-7 Occurrence of groundwater in an unconfined aquifer.
I I
Groundwater occurs in gravels and sands of Tertiary age that were deposited in a valley cut by a river through folded Palaeowic sedimentary rocks, e.g. Avoca Deep Lead at Avoca, Burnt Creek Deep Lead, south-east of Dunolly. Water enters the aquifer through the surface sands and particularly through the bed of the river.
-._ unCOnfined aqutfor __ -If e.g Tertla1y Deep L(;ad ___ rccharge
area
----�I
for unconhnoo aCl'J,lcr �
Dry bore
PrOch.,:c,ng bora
(Too shallow)
Into aQ�lfcr
1
I
����___'UNSATUR�TED lONE
Figure 6-8 (below) Occurrence of groundwater in a confined aquifer.
SATURATED ZONE
Groundwater Occurs under pressure in a gently dipping sandy formation of Tertiary age, which is largely overlain by an aquitard of clays and silts of low permeability. The Palaeowic basement rocks are also impermeable. Water tapped by bores will rise to the level of the potentiometric surface.
IL PC� oaso,Qocks "'-""./\.\ Low
lhlY
(' g folded Pal.leOZOlc shalt:;s
______________ ... ..__ . _
Conlincd Aquifer e.g. Margin 01 Tertiary Basin
Dry bore (Too shallow) Non-Ilowing bore I (Sub-artesian)
/
f
Flowing bore (artesian)
•
235
236
Chapter 6
Unconfined aq uifer An unconfined aquifer is a permeable geological formation that extends from the land surface down to an impermeable base. It is generally only partly filled with water. The water is at atmospheric pressure at the water table. This means that if a bore penetrates below the water table in an unconfined aquifer, the water will not rise up the hole. An unconfined aquifer can be recharged by infiltration of water anywhere over the area in which it occurs. The Tertiary sands, that occur in Melbourne's south-eastern suburbs, are an example of an unconfined aquifer.
Confined aq u ifers A confined aquifer is a permeable geological formation, which is largely overlain by less permeable confming beds. The latter act as a seal preventing the groundwater from escaping upwards. A confined aquifer is fully saturated except in the intake area where it outcrops. Groundwater in confined aquifers is under the pressure of the atmosphere at the intake area plus that of a head of water from the intake area. Therefore, when a confined aquifer is penetrated by a bore, water rises above the top of the aquifer. The level to which the water rise is called the potentiometric surface and it reflects the pressure in the aquifer at that locality. If the pressure is su fficient the water will rise above ground level to produce an artesian or flowing bore. In sub-artesian bores, the water rises but does not reach the surface. Confined aquifers occur in all the major Tertiary sedimentary basins in Victoria.
AQUIFER MATERIALS For simplicity, groundwater can be considered to occur either in porous rock aquifers or fractured rock aquifers. The groundwater in both of these types can be either unconfined or confined.
Porous rock aq u ifers In Victoria, the mOSI important aquifers are more-or-Iess horizontal beds of unconsolidated sands, gravels and shell fragments, which have good primary porosity. These occur mainly in Tertiary sedimentary basins, where t hey are mostly interbedded with finer-grained confining beds. There are also good aquifers over smaller areas in sand dunes and in alluvial deposits along the courses of ancient buried streams. Some ]0 - 30070 of these rocks i s occupied by spaces and they can yield large supplies of water when tapped by bores. Some Mesowic sandstones and Palaeozoic sandstones and limestones, especially those that are not greatly folded, also have sufficient pore space for them to act as moderately good aquifers. Some vesicular basalts hold large quantities of water.
Fractured rock aq uifers Igneous and metamorphic rocks, and completely cemented sedimentary rocks can form aquifers where they are strongly fractured. Ou tcrops of fractured rocks in valleys generally provide favourable groundwater conditions as the fractures in these areas tend to be wider. In Victoria, some Palaeowic sedimentary rocks in the uplands and the extensive Newer Volcanic basalt plains of western Victoria fo rm fractured rock aquifers.
SELECTION OF SITES FOR GROUNDWATER BORES If a borehole is drilled at any locality in the State, it will even tually at some depth reach the water table and below that will pass intO rocks saturated with water. However, the chances of finding useful quantities of good quality water at sufficiently shallow depths to justify the costs of boring and installation of a pump vary greatly between different districts. It is necessary to have a knowledge of the local geology to be able to predict whether a groundwater aquifer is likely to be found at any particular site. Consider a landowner, who wants to sink a bore in a d s i trict where there are already scattered successful bore . It may be po sible to antic ipat e [he yield. alinity and depth to the water table on the property by considering [he results from the other bores, provided the geology is similar. This is possible because the characteristics of aquifers only vary gradually from place to place. For example, the salt content i ncreases slowly from the intake area to the final discharge zone . On the other hand, if local information is not available Or if nearby boreholes have yielded varying results, the landowner would be advised to seek geological advice. Over the years, variou Government authorities have built up records about the State's Water re ources. This infornlation i a\'ailable to landowners at the Melbourne o ffice of the Rural W ate r Commission. Geological advice is particularly desirable, where there are di fferent type of rocks below [he surface of a property.
Water
Figure t>-9
Use of water in houses in Victoria.
"
J b th
10 k
237
No discussion on the search for groundwater would be complete without mention of water diviners. These people claim to be able to detect the presence of underground water with the aid of a divining rod. Some also claim to be able to predict the depth at which water will be found and even its salinity. Diviners work on the assumption that groundwater flows in underground streams and that they can detect the locations o f these streams. This is a fallacy, for underground water is normally distributed over a wide area in a particular rock formation. Its occurrence does not resemble a surface stream, but a vast buried lake. There is thus rarely one specific site on a property that is more favourable for the discovery of underground water than aU other possible sites. It is true that many bores drilled on the advice of diviners are successfu l . However, this is generally in areas where supplies are plentiful and a bore at any site would intersect an aquifer. There are some situations, where the best water supplies are restricted to certain narrow zones. This happens where particular geological units are more porous or fractured than the surrounding rocks. Examples are gravels at the base of deep lead sediments along old valleys, shattered rocks along fault zones and wide, fractured quartz reefs. Geological skills may be particularly helpful in tracing these features below the ground.
Water quality and use
People using water naturally hope it will cause no harmful effects. For instance they do not wish to become ill through drinking water, they do not want laundry water to soil clothes, they do not want irrigated crops to wilt nor stock to die. When water is used in industry, it must not corrode the machinery. Th� quality of water is the net result of various chemical, biological and physical factors, which affect it as it passes through the water cycle. When water evaporates into the atmosphere from the Earth's surface, it is pure, consisting solely of water molecules. By the time it returns to the ground as precipitation it will probably have collected some impurities, such as dust or dissolved salt. The latter is most likely to be present in coastal areas, where salt spray is blown into the air from the sea.
l
11 (
Figure t>-lO Quality srandards for drinking water
The World Healrh Organisation has recommended that water consumed by humans should meet the standards given in the table below. In some areas some substances may be present in higher concentrations than those recommended without any ill effects resulting, e.g. bore water containing up to 2000 pans per million total soluble salrs is drinkable. u
Continuing through the water cycle, water either passes over or under the surface of the land. I n doing so, it dissolves salts and organic substances from the soils and rocks it encounters. Water usually becomes progressively more saline as it travels away from its source. Surface water usually contains living organisms and suspended muddy material derived from the erosion of soils. It can also become discoloured by dissolving iron salts or organic matter. Harmful impurities also may be introduced by human activities giving polluted water. For every process in which water is used - be it domestic, agricultural or industrial - there is a limit to the amount of contam ination that can be tolerated. I . Dom estic d rinkin g w aler stan dards are described by upper limits for a large number of bacterial, physical and chemical constituents. Domestic drinking water should contain less than 1 500 mg/L of dissolved salts. Some metal ions are highly toxic and their maximum allowable concentrations are very low in drinking water. Nitrates, which may be derived from human or animal wastes, are also very harmful. Water should also be colourless and free of any cloudiness caused by suspended sediment. One property of water, that can be undesirable although not necessarily harmful, is hardn ess. Water is said to be hard, if it will not lather when soap is introduced. Hardness is caused by calcium and magnesium ions in solution. Hardness is temporary, if it can be removed by boiling. This is the case if calcium is present in solution as calcium bicarbonate. Boiling causes the calcium to precipitate as calcium carbonate. However, i f other anions such as sulfate are present, the hardness is perman ent: this can only be removed by water softeners. Hard water is common in limestone country, as in the Warrnambool - Mount Gambier region.
2. A gricultural standards for water are based upon the effects of the chemical \1
constituents on animals, soils and plants. There are di fferent upper limits of salinity for different crops and animals. Generally plants are more sensitive to salts than animals (Figure 6- 1 1 ).
3. Industrial stan dards vary with the particular indu try. As examples, salt contents up to only 100 mg/L and 1 50 mg/L are allowed in the rayon and paper manufacturing industries respectively, whereas up to 850 mg/L may be used i n carbonated drinks.
238
Chapter 6
Figure 6- 1 1 Salt tolel'llnces i n water used for rarm animals and crops.
Usage Irrigation tobacco citrus uees, aarden plants leBumes (e.,. sU'8wberry clo\'C'r) vines. grasses. cabbages lucerne, cotton barley w�at maize farm animals poultry pigs horses milking COW'S and ewes in lamb beef cattle other sh«p Fish gold fish rainbow trout brown trout
Upper Umit ofsalinity TOS (mglL)
so 1 000 I 500 I 500 2 500 8 000 6 000 1 100 J lOO lOO 6 500 1 000 1 1 000 IS 000 4
8 800 9 300 3 700
·Note: the sah tolerances for crops and animals are guidelines only. Actual results dtptnd on many factors. such as the proportions of individuaJ salts. the composition of pastures, etc.
TREATMENT OF DOMESTIC WATER SUPPLIES The suitability of natural water for human consumption is only poor to fair throughout most of Victoria. Consequently t17e water held in surface and underground storages must be treated in various ways before it is distributed through household supply systems. The following are t17e common treatments applied in Victoria:
In the United States of America, it has been estimated that an average person requires about I gallon (U.S.) (3.8 litres) of water each day to survive. But the tOlal usage works out at about 1800 times this amount per person, when the water used for all domestic, farm and industrial purposes is taken into consideration. Wat er use in Australia is probably similar.
I. Chlorination: Diseases can be transmined by bacteria and viruses in water; hence water supplies must be disinfected. Chlorine gas or sodium hypochlorite solution is added to kill harmful micro·organisms. The total chloride ion content must not exceed 600 mg/L however.
2. Filtration: Surface water is often cloudy due to the presence of clay, silt or organic matter in suspension. A1t17ough these substances may not be harmful, their presence is a frequent cause for consumer complaints. They may also interfere with t17e disinfecting process. Coarser suspended particles can be removed by filtering the water through sand; it is also often necessary to add a chemical such as alum (aluminium sulfate). The suspended panicles combine with the alum to form a denser mass, which then settles out - the process is known as flocculation. Sometimes calcium hydrate (Ca(OH),) or soda ash (NazCO,) is also added to promote effective settling and to correct t17e acidity of the water. Finally t17e water is filtered through layers of sand and gravel to remove the flocculent.
3 . Decolouration: Iron compounds
can cause problems, e pecially in groundwater supplies. Although they are not harmful to humans, they can discolour water and cause brown staining of clothes and porcelain ware. They can also cause an unpleasant taste sometimes. Iron is removed by aeration, filtration or the addition of lime. The laner increases the pH of water, causing iron oxides to precipitate.
4. Fluoridation: In recen t years, sodium fluoride has been added to many water supplies to reduce dental decay.
5. Some groundwaters contain sufficient hydrogen sulfide to produce an obnoxious odour. This can be removed by spraying the water into the air to release the gas.
6. In a few supply schemes using groundwater, the water is either cooled or the hardness is reduced .
Groundwater versus surface water for major development
I n the past, it was usual for public authorities to think in terms of damming rivers as new water supplies were needed. However, in many areas alternative permanent supplies are available underground in aquifers. There are advantages and disadvan tages associated with developing groundwater as opposed to surface water.
ADVANTAGES OF USING GROUNDWATER Vast water storages are usually available in aquifers and these are not affected greatly by annual variations in rainfall. Groundwater storages are highly efficient because
Wale,
239
t here are no losses due to evaporation or leakage and there is no silting up as time passes. The machinery required to tap groundwater is compact and the number of bores can be increased gradually as demand increases. Aquifers may be available immediately below the district to be supplied and so costly pipelines from distant reservoirs are not required. Overall, groundwater supply schemes involve less environmental disruption than do surface schemes.
DISADVANTAGES OF USING GROUNDWATER It is more difficult to evaluate the storage and pumping capacities of aquifers. Groundwater usually contains higher concentrations of dissolved salts and is more likely to be hard. The costs of operating groundwater schemes can be higher than those from surfaoe reservoirs, especially if water is pumped from considerable depths. There are no side benefits from underground storages, as they cannot be used for recreation activities or hydro-electricity generation.
JOINT USE OF SURFACE WATER AND GROUNDWATER From the discussions so far, it is apparent that surface water and groundwater are two related resources. To make the best use of both sources of water, it is desirable to prevent losses of good quality water to the ocean. To do this, joint surface and underground supply systems should be developed wherever possible. There are
Water in Austral ia Figure 6- 1 2 A verage rainfalls of the continents and the proportions Ihal are retained as run-off and losl by evaporation and transpiration.
Continent Africa N. America S. America Asia Europe Australia
ter resources of Vi ctoria
of course some areas where useful supplies of only one kind of water are available. Over much of Victoria, however, yields could be substantially increased by employing joint schemes. Surface water should be stored to meet most requirements in years of average to high rainfall, whereas groundwater should be retained primarily for use in years of low rainfall. Such usage depletes groundwater storage when natural recharge is lowest. The drawdown in the water level creates storage spaoe that can be replenished later, either by natural recharge in years of more plentiful rainfall or by artificial recharge. Aquifers can be recharged anificially by pumping water into their intake areas from surface dams. Water can also be injected under pressure into confined aquifers. Australia is overall the d riest inhabited continent. This explains why it is also the leasl populated continent and likely to remain that way. Not only are its water resources scanty but they are also very unevenly distributed over its area. Rainfall supplies, the main source of surface and groundwater, vary considerably from place to place, from season to season and from year to year. Although the average annual rainfall is 420 millimetres, one-third of Australia receives on average less than 250 millimetres of rain each year. The limited extent of Australia's surface water re ources compared with those of other continents is shown in Figure 6- 1 2 . Australia not only has the lowest average rainfall, but it also has the greatest rainfall losses by transpiration and (mainly) evaporation.
Area [kin')
Average annual rainf all (mm)
3 0 2 1 0 000
660
24 UiO (XX) 17 790 000 44 030 000 9 710 000 7 690 000
660 1350 610 580 420
Average annual run-o f f (1),'0 rainfall) (mm) 158 264 486 220 232 55
24 40 36 36 40 13
Evapo Iranspiralion (rom)
(0'/0 rainfall)
502 396 864
76 60 64 64 60 8i
390 348 365
Although many people believe that Melbourne has a very cold, we[ climate, this is not the case. Figure 6- 1 3 shows that of all the Australian capital cities, only Adelaide, Hobart and Canberra have lower rainfalls than Melbourne. Victoria is cenainly better endowed with water than South Australia and mosl of [he inland of the continent. However, its water resources are appreciably lower [han those of the eastern half of New South Wales, much of Queensland and Northern Australia and the western part of Tasmania. At regular intervals along the eastern coast of Australia, major rivers carry large volumes of water to the Pacific Ocean from high rainfall areas in the Eastern Highlands. Most of [his water is not used. These rivers contrast with the smaller Victorian streams which enter Bass Strait . Most of the water used in Victoria is derived from rain, snow and hail, which falls within the borders 0 f this small State. Consequently, the climatic patterns over different pans of Victoria determine how much water is available to the population. The only exceptions are [he Murray River, which receives pan of its water from tributary streams flowing from southern New South Wales, and the Snowy River, the Slate border into East Gippsland. which flows southward acro
THE CLIMATE OF V ICTORIA To understand the distribution of water in Victoria, it is useful to consider the State's
240
Chapter 6
Figure 6-13 Average annual rainfall for Australian capital cities.
(
12
Figure 6-14 Distribution of rainfall in Victoria.
/sohyels are lines connecting localities with equal average annual rai nfalls measured in millimetres. The interval between the isohyets is increased to 300 miUimetres in the higher rainfall areas of the Uplands to avoid having c1osely·spaced isohyets. There are also fewer recording stations there.
climate, particularly its precipitation. Victoria's weather is largely determined by cells or zones of atmosphere at relatively high pressure (called highs) that move from west to east across the southern part of Australia. Between these highs there are troughs of lower pressure, called lows. Separating these air masses of different characteristics, there is usually afront. The fronts are often moisture-bearing systems. The highest rainfalls occur where moist air masses coming from the west and south west are forced to rise over mountainous country. The rising air is cooled and so cannot carry as much moisture. This results in falls of rain or snow. In summer, Victoria can also experience the after-effects of tropical storm clouds driven down from northern Australia. East Gippsland is less affected by the west to east moving pressure systems. Most of its rain comes from large pressure cells that form off the south-east coast.
i i i i i i i i i i i i i .-(
-400- IsohjlBt With avtJl1Jt}B annual ramlallIn millim9''''s
Swan H,n
",_,...::A!bu:;.:.�-F'", WOOCIIIQ8
HC'S�,iL"'.
....
.....
.....
MELBOURNE
Average annual precipitation over Victoria ranges from less than 250 millimetres over parts of the Mallee plains to more than 2500 millimetres in parts of the uplands of north-eastern Victoria (Figure 6-1 4). The rainfall map shows a pattern of isohyets that is similar to that of the contours on a topographical map of the State. The main land feature influencing precipitation is the Central Victorian Uplands. Moist air masses, moving inland from the Southern Ocean in an easterly to north easterly direction, quickly rise to cross the Divide. This causes them to drop a large part of their moisture. Precipitation is therefore higher and more reliable on the southern and western slopes. Local rainfall highs also occur in other elevated areas such as the Otway Range, Wilsons Promontory, The Grampians, the Strzelecki Ranges and the far south-eastern part of Gippsland. There is a rapid fall in precipitation going north from the Divide. Because most rain falls on the western sides of mountains and high ridges, there are many areas of comparatively low rainfall ( rain shad ows) on the downwind or eastern slopes. Rainfall shadows occur in many valleys and basins in the uplands as well as in broad low-lying areas protected in the west by ranges. For example, the Werribee Plain between Melbourne and Geelong is a drier area shadowed by the Otway Range to the south-west. The northern part of the Snowy River valley in Victoria is also in a rain shadow. Victoria's annual precipitation varies quite markedly from year to year. Long periods of low precipitation occur from time to time leading to a drought every five to eight years. Droughts occur most frequently in the north-west of the State. They are least common in coastal areas. WATER BALANCE FOR VICTORIA Figure 6- 1 5 shows what happens to the precipitation that raUs in Victoria. Most of it returns directly to the atmosphere through evaporation and transpiration. Only a small proportion infi ltrates to aquifers. About 1 5 "70 runs into streams and of this about one fifth is diverted to reservoirs. The remaining four fifths eventually reaches the Southern Ocean, partly via coastal lakes. The rates or evaporatton ana transpiration in Victoria are much higher than the averages for most countries o f Europe and North America.
Water
Figure 6-15
Average annual volume of precipitation in Victoria Average annual flow in Victorian rivers Annual recharge to groundwater systems Evaporation and transpiration
Water balance for Victoria.
1 50 22 1 126
000 500 500 000
000 000 000 000
24t
ML ML ML ML
A megalitre (ML) i s one million litres or 1000 cubic metres (approximately the volume of water in an Olympic size swimming pool). It is a convenient measure in hydrology as one M L o f water covers one square kilometre to a depth of one millimetre.
SURFACE WATER RESOURCES Figure 6-16 Streamflow in the major Victorian rivers. River
Length
(km) Goulburn Glenelg Loddon Mt.Emu Creek Wimmera Hopkins Avoca Mitchell Latrobe Campaspe
Yarr,1
563 454 392 305 290 280 269 250 250 245 245 233 227 245 219
Wannon Ovens Broken Creek Milia Mitta Thomson 208 Macalister 201 11Imbo 198 Broken 192 Barwon 187 Ki(!Y.'a 184 182 Maribyrnong 162 Snowy in Victoria (total length 432) Murray (NSW) 2530 (SoUItt Rural Water Commission)
In Victoria there are 3820 named watercourses with a combined length of 56 000 kilometres. The amounts of water flowing in Victorian streams are recorded at more than 500 stream gauging stations. The measured streamflow characteristics of some major Victorian rivers are given in Figure 6- 1 6. Minimwn annual discharge
Average annual streamnoYo' (ML)
Maximwn annual streamflow (ML)
(year)
I 680 000 637 000 186 000 61 100 136 000 294 000 47 600 936 000 860 000 203 000 722 000 235 000 I 1 1 0 000 70 500 I 230 000 305 000 460 000 324 (XX) 236 000 236 000 665 000 I I I (XX) I 740 000
5 930 000 I 630 000 461 000 232 000 570 000 953 000 129 000 2 420 000 I 800 000 886 000 I 440 000 528 000 2 880 000 107 000 3 860 000 738 000 I 500 000 I 190 000 I 110 000 663 000 I 450 000 342 000 5 930 000
1974 1956 1974 1923 1956 196{) 1973 1974 1956 1974 1924 1983 1956 1978 1956 1970 1952 1974 1911 1978 1974 1916 1950
Average annual now into South Australia
(yea,)
(ML) 228 38 36 1
000 800 200 240
Nil
15 500 3 480 209 000 276 000 2 830 128 000 8 690 195 000 39 600 354 000 48 900 48 100 43 800 4 61 0 17 100 193 000 5 860 159 000
1972 1967 1967 1967 1902 1967 1982 1982 1982 1902 1982 1982 1982 1967 1979 1982 1982 1982 1943 1982 1967 1982 19�2
Streamflow t't'Corded
Recordings started
At
{Year}
McCoy's Bridge Danmoor Kerang Skipton Horsham Hopkins FaUs Quarobruook GlenaladaJe Rosedale Rochester Warrandyte HenlY Wangarall3 Rices Weir Tallandoon Wandocka Olcnmaggie Ramrod Creek Goorambat Pollocksrord Bandiana Keilor Jarrnhmond
1965 1948 1953 1921 1889 1955 1963 1937 1937 1886 1891 1967 1886 1965 1934 1969 1919 1965 1916 1906 1965 1908 1922
S 793 712 ML
The pattern of run-off, and therefore streamflow, closely follows the pattern of rainfall distribution, as might be expected. The only rivers with high flows are those rising in The Grampians, the Otway Range and the East Victorian Uplands. The large, mainly mountainous, region east and north-east of Melbourne contains 80% of the State's total surface water re!.ources. Of this, about equal quantities flow south to the ocean and north to the Murray River. There is a low percentage of run-off in the north-western sector because the terrain is mostly flat and the rainfall is under 500 millimetres per year (Figure 6- 1 7). Figure 6-17 Contrasting streamnows in three sectors of Victoria.
�
Beno.go. - _ _ _ _ _ _ __ _ _ _ _ _
t i', III'
Aft',l .15 it percentage of Ihe IOliJI of VIC/Dna
1 1 I 1 I 1 1
" " , ,'" 1, L !....J K..,.".",ts
242
Chapter 6
Stream flows vary considerably throughout the year and from year to year. They are usually greatest between July and October, especially in the western half of the State. In general, streams in eastern Victoria have th,e most reliable flows. The quality of water in Victorian streams is measured periodically, mostly at the stream gauging stations. The results show that there are large variations in quality across the State, although in general, the quality is better in the east than in the west. However, in western Victoria, low salinity streamwater is available in the Otway, Grampians and Macedon ranges. Water quality generally deteriorates downstream due to the combined effects of natural and human influences. Streams within the Central Victorian Uplands, in East Gippsland and in south western Victoria are generally clear and contain low amounts of mud. On the IJther hand, streams in the north-western part of the State and those draining farmlands and major urban centres are generally muddy. This is because much of the native vegetation has been removed in these areas and so the soil is easily eroded. GROUNDWATER PROVINCES OF VICTORIA Victoria's groundwater resources come mainly from Cainozoic aquifers in the Murray, Otway, Western Port, Port Phillip and Gippsland basins and Palaeozoic fractured rock aquifers in the uplands (Figure 6-1 8). The generalised hydrogeology of the main groundwater provinces in Victoria is given in Figures 6-1 9 to 6-24 and illustrated in Figures 6-2 5 to 6-27. Figure 6- 1 8 Groundwater provinces of Victoria with directions of groundwater flows. The letters A AI, B-8 1 , etc. refer to the geological cross-sections shown in Figures 6-25 to 6-27 inclusive.
The volume of groundwater in Victoria is estimated to be between 400 and 1 000 million megalltres (400 to 1 000 cubic kilometres), enough to cover an area the size of the State to a depth of between two and four metres. Unfortu nately the groundwater is not evenly distributed; there are considerable variations in the depths, yields and salinities of the d ifferent aquifers.
Groundwater
Province
e��mated recharge
Authorized groundwater
(MUyr)
extraction.
1 987-88 (fAL'yr)
Murray Basin
105 000
264 810
Otway Basin
425 000
86 :121
Pon Phillip Basin
45 000
1 9 601
Western Port Basin
25 000
59350
Tarwin Basin
20 000
Gippsland Basin
435 000
Uplands
4"5 000
Generalised ground.valer flow direcMn In seaimemary baSinS
77 576 30828
15 000 000
538
•
be about 25% oI lhal. alJ�lwlised
a !
25 !
50 75 100 ,
,
Kilometres
,
Geological unit fonning aquifer
'vlain occurrence
Depth to aquifer
Aquifer thickness
Rock types
Aqui;er type and fonn
Common salinity range (maiL TO$)
Range of bore yields (Lisee )
Grou ndwater
Shepparton ronnation
octensive occurrence
outcropping or 5utxropping·
formation 25 to 125 m thick. iodividual sand and gravel beds art less than 5 rn thick
sand and gravtl interspersed in a clay and silt matrix
indivi:tual sand and gJavei aquiftrs range from isolated ribbon-like bodies to sem i· continuous sheets. ShallOrN aquifers art unconfined, detper aquifers are confined
highly \ariable
generally less than 5
irrigat ion. stock and domestic usage: groundwater i5 also pumped from aquifm shallower than 25 m to lower the water table to control salinily.
40 to ISO m but
sand and silt with interbedded htavy mineral bands up to one metre thick
unconfined 5heet-like sand
I(XX) to 40 000 mostly greater than 5000
2 to S
ground.... ilter mostly 100 saline for u� o:cq>t alona the southern margins of Ihe basin where it is used for stock watering and to
in the Riverine Plain. also along the margins of the Western Uplands in the S, ArnaudDimboola·Horsham
uses
....
Parilla Sand
Cali vii Fonnwion
Ouddo Limestone
Warina Sand
.. idespread occurrence; main development is to nonh·west of line through Echuca and
generally covered by about 15-20 m of Quaternary sediments, outcrops near Kerang
Horsham
and in southern part of basin
similar distribution to that of Shepparton FonnaLion
25 to 130 m
Mallee and Wimmera regions (mostly west of nonh·south line joining Robinvale and Murloa)
SO to 190 m
basal Tertiary
ISO 10 420 m
aquifer occurs throughout m051 of the basin
aquifer
supply the 10wns or Ooroke and Pine "Iills,
2010SOm
sand and gnwel
less than 500 up to 40000
up 10 125
used around basin margins for irrigalion and scock watering, and for town supplies for Elmore. Katunga, Strothl11erton, and Chiltern.
up to 15
solution cavities
1000 to 3.500 (becomes progressi\'(1 more saline 10 the t.a5I)
some �ock v.'atering and tOrNn supplics for Murrayville. Cowangie, Lillimur, Kanh-a, Minull. Nhill.
confined shott-like aquifer
1000 to 12 000
up to SO
generally not used because of the depth of the aquifer and ils marginal to poor water quality,
unconfined in the south bul
confined to the north by the overlying Shepparton Fonnmion. Aquifer consists of separated alluvial valleys close to the Uplands. Further nor th these valleys coalesce to form a shttt-like fOrmation
·Subcropping r leans the formation is present below a �allOYo' (()\.'er of soil.
figure 6-19 Generalised hydrogeology of Ihe main Cainozoic aquifers in the Murray Basin.
generally betw«n 60 to 80 m thick
SO t o 130 m
up to 200 m thICk in the Mllduf3 area but is c:on...iderably thinner undrr the Riverine Plain
ilmeslon�
-.and itnd gm\'t\
confined sheet-like limestone aquifer, poros ity increased by development of
(icrnogK.:al 111111
Main or.:currcrll'r
))Cf'IIh 10 aqUlfcr
l>OlaJl 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<;
t--
Wc<;lcrn Pon Cirotlp (ll:axtCI, Shcr\loood and
'�llIod, li)nn:uion,,)
the Cr-.moourIIl! !1.nd LUll,: I..:t ng arca�
lonS"'arry
to 1>.1lmon·
(x:ctlr� Ihroughoul the
W�crn l'on Basin
oull'roppinl,:
outcropping to "'lb· outcropping (l'\cr Inf)l:l
thin. I1lClolly less
sand. medium 10
th:lI1 611\
�():trse
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
�lI1d, gruvcl. limotone. day,
combined a qui fer system of slu�et·like fonn. which is
300
3000
..ilt und lignitc
gcnemlly wlconfined c:<.CCpl where OIo'erlain by clayey soils
10
10 to
40
\Ioidespn:ad (),:currelll,.'e
OUICTOp� in
throu.t;houl ba.�in
CnUlbournC' area and along lI('alh 11111 Fault. C0\1:I'l''I:I by up
11\
ba..ult, ba.omhk CL1),
1ured fr.u ..
basalt aquifer L'Onfined b)' basaltic clay and o\1:r ly ing saiimcnls
250 11\ III celllrn] potrt of b....tn
hmllalion
L
main occurrem:c b in Yallock·)bnnathan· L.lng L.lng art'.l
Figure 6-21 Generalised hydrogeology of the main Cainozoic aquifers in the Western Port Basin.
SO
10 150 111
(ulldcrlit'\ Older Volcanic,)
siock and domestic supply
stock :md domcslic than supply 7 m
mninly used for irrigation parlicularly in the Dal more Com l.,ynn area; elsewhere used •
�od: WId domestic supplies; thi� aquifer system supplies more than 80�. of the jVoundwutcr used in the Western POr1 Bruin
10 to H
less
than
2000 in
weslcrn half of basin; 1000 to
2000
2
to
15
irrigation of mar ket gardens in the Cr.tllbourne Oyde nrea, and •
stod. supplies
in C25tern
In Qtca nonh·wesl
of Com Lann
half
10
(. IHlde"
uses
fOI
h:M Older Volcani�
Grdundw;utt
shetl·lik� rorm
aquifers of shoe,slring fonn illlerbeddcd in clay
b) up 10 75 III or day in the ....\'Stern
yields (L/SC'C)
SmSOI11
.... nd and grovel
\.\llh lignIte and clay bcd�
confina1 sand aquifer
and gr'3\'(1
500 to
2000
2.S 10 2S
"mcnally not Ulilised exctpt 10 provIde .... ater . for Lang Lang IOwn supply
Figure 6-22 Generalised hydrogeology of Ihe main Cainozoic aquifers in the Port Phillip Basin.
Gcolog;cnJ unil forming aquifer
Main occurrence
Wcrribce Della
:.out It of Wcrrilx.'c
'"
I)epth to lIt1uifcr
outcropping
Aquifer type and form
�ih. sand. grovel.
unconfined sa n d and grovel aquifers of shoe-siring form. interbedded with day
500 to
\1 o nd
unconl'ined sand aquifer, limited areal extent
less Ihan 1000
less Ihan
sand. �ndy calcarenite. shelly sand, mud. clay
unconlined sand aquifer. shttt-like fonn
300 10 1200
up to 25. most recorded yields are less than
gcntmlly between SO and ISO m
ba\3It, scoria. pyrocltlstics
multi-layer fractured rock aquifer 5)'51em with sheet· like basall aquifers separaled by clay ia)t:rs: uppermOSl aquifer is unconlined. IO"-'ef aquifers are conlined
up to 40 but generally less thWl 1·2
l�\ than 30
\:and, CL1)'C)' �nd. .;andy day, grovel, quanlile. sandy limestone
unconlincd aquifer in outcrop areas bUI confined when: covered by basalt; sheet-like form
greater than 3000
mostly less Ihnr 1.6
small utilisation only; some stock WUltring. walenna of foreshore reserve on the OcU"rine Peninsula, sall produclion al Uinl
up 10 30
til
clay; ,ilty \3nd in lower P.1rt of dcposiI!>; minor grJvcl and sand of levees dune dcpo�ih
soulh-ca5lcrn lluburb, of Me l bourne bct"'ttn Mordialloc and Frankston and in Oeaumari\ area
Outcropping
.. Ihan Ihlll, les
Bridgewaler l':ormallon
Nepean l\:nm\ula Wei:t of Selwyn Fault
oUlcroppmg
200
NCYo'Cr Volcanic,
Wernlx't' Plain\ �\t
or Melbourne
Moornbool Viaduci Formalion
Fyan\ford l:arm"t Ion
�
oUlcropping
Ol
In
R.1niC of bore
mnge
yields
(mgll TDS) 6CX)()
(L/s("C)
main ly used for irrigation of nHlrkcl gardens. washing dairies
.5 to 15
and stock watering
0.2
1-2
100 to kss than
6(XX)
moslly grtaler than 2500
garden Vo'3lering especially during »triods of "'aler restrictionli
domestic use, garden wmenng, irrigution in Boneo area. siock wale ring
stock watCf and minor irrigation of sail tolerant crops in rural areas. low grode industrial USI!':5 In western suburbs of Melbourne
elthcr OUlcropplllg or
ca\lern and \()Ulh·
outcropplnS
20108010
sand. gmvel. silt, elllY, shelly �Inds, calcarenite. Irm�lone
unconfined to confined sand, grovel and limestone aquifers - coarser sedimous ltnd to be lentitular although the deposits as a whole are sheet-like
100 to 6800 average aOOm 1500
up to 18, Iyplcally less than 2.6
household garden w,umng cspecially during �riod5 of .... <)Ier �rklions. Imlation and Slock watering in rurnl areas, minor industrial and (''Ommen.'ial dairy washing, golr course walering
10104() In
basuh
fractured rock aquifer, unconlincd in Cranbournt area. elsewhere confined beneath )Ounger sediments
](X) to 8000,
up to IS. typically less than !i
siock supplies and Irrigstion of market gardens and pastures in thc Lyndhurst-Cranbourne area, golf course WIllenn, al Cranbourne
$lInd, gr.wcl, cia),. lignite
unconfined 10 confined aquirer of sand and gravel, sheet-like fonn
2000 to 5000 west
III
ulHJerlyil1g N\.....'Cr .. Volcal1k ba�lt\
ca.....ern �ubu(b\ (If
Ok.lc.r Vokanio
'iOUI h--<:a.\1 or clbournc from Ja l1bournc 10 I\)n Phillip IJay
O\1I1.:ro(>, in
WOj of Melbourne rrom Bacchus Mar.h 10 A Ilona: MelhQurne ,uburb� bct .... \,"C1l MeOione and Fr'.lIlk\lon: Nepcan I\:nilhula w(:\1 of Selwyn P.wll
ouh;ro]ls in 1}U\.'dlU\ M;Ir.h arca only - up 10 4(X) III deep under Werri1xc Plains; 40 10 90 111 on Morninglol I"\:nimula; 500 to 9(X) III on Nellenn 1\:lIin\ul:1
Werritx:t' Form:tlioll
6
COllllllon salinil)'
""'C.\t of Melbourne, 9�long arc:I and Bcllarinc I\:ninsula
Brighton GrOllp Ua"ter Sand..IO�
Melbourne. and inlamJ 10 the DandenonsCmnbouf'll\: area
I
G rou ndwat er uso
Rock typo
AquIfer ,hlckncs\
��
Cr:mbourne ;tea; up t o 90 '" d\."CI) low:rrd, POtt Phillip Bay
i:real.:r thwi ISO AI Ilncchus MIt�h area: 20 10 80 III under Werrlbcc Plain"; 10 to 40 111 ..outh--<:3S1 of Melbourl1�: 2S0 111 Ne pcan l\onil 1sul a
average about
2(0)
of Melbourne and Nepean A!ninsula; ISOO 10 3000 SOUl h�ast of Melbourne
up to
SO
some industrial usage in western suburbs of Melbourne. used 10 top-up Cherry lakes al Altona, irrigation of salt-tolerant crops in Ihe ilacchus Marsh area, watering or golf courses in Ihe south-easttrn suburbs or Melbourne
Geological unit rorming aquirer
Main occurrence
Dcplh 10 aquirer
Aquirer thickness
Quau:mary alluvium and HauOled Hill Gravel
CCJ\.'trs most of the
outcropping
5 to U m
BoisdaJe Fbrmaaion
occurs throughout basin to the east of Ttaralgon
.$ to 15 m
Gippsland UmC5tone
occ urrence restriaed 10 soulh�asttm pan 0( basin bqIond 3 line joining 'mrmm-SaleBairrudale-Orbosl
150
narrow belt cttending north-south acrOSS basin in Rosedale nrea
100 to ISO
250 10480 m
occurs throughout
ncar surface (less thanSOm) in eastern part or basin but deeper (between 600 and 900 m) tOYl'IHtis the coaS!
100 to
8aJook R>rmation
Latrobe v...lley Group ('mllourn. Morwell and 1l"aralgon formations)
basin
basin
10 250 m
Rock types
Aquirer type and rorm
sand. gmvel, silt
s::md Ilnd gravel beds unconfined aquifers
50 to ISO
sand. silt. day. minor gravel and coal
sand aquirer ronfined �lh days in upper pan or formaalon or in overlying alluyium
100 to SOO m
limestone, marl
confined limestone aquirer. sheet-like rorm
1000 to
sand
confined sand aquifer
less Ihan 1000
s:\nd. gravel. silt
sand and gravel aquifm or lens-like fonn are confined by interbedded coal and day
less than 900
at least
tU1d clay
no m
and clay with major brown coal .seams
are
Common .salinity
mng' (mJl/L TDS)
Range of bort yields (Uscc)
ill er uses Ground ....
irrigation (panicularly aJong the Mitchell Rhoer) and siock watering
gencrally 1000
less than
highly '
generally
less than
5 to 20
SOO
2SOO
up to 10 but
gcnemlly leu than 2
m
unknown (up to 201) up 10 SO I
m ror irrigation in the Sale mainly rot Sale. Wurruk. Boisdale: and Briagolong
� and lown supplies
flOt utilised because it Is O\'ttlain by mort producth't aquirers containing better quality groundwater
stock .... 'alec-. possible irrigation; not greally lliUsed because of rtlath-ely small etten! groundwater from Mo!V.'C1l Fonnat.ion
used ror lhIfalgar town supply;
runher east in the Latrobe VoJ.lley Dep�ioo the aroundwater is used for irrigation: mort Ihnn 7:1 000 MUyr art pwnped rrom sands below the brown coal open cut mines 3t MorweU and Loy �ng to reduce the to upward pressun: of ground ....ater . maintain stability in the coal faces and Slop heaving of pit noor
Thorpdale \bkanics
western part of basin. main occurrence is in the small Moe Swamp Basin in the MoeDarnum-Willow Grove area
up toSOm
up to60m
basalt. basaltic clay. turf (interbedded with either 1rn.mlgon or Monvell r'Ormmion sediments)
confined fractured basaJt aquifers, valley-like ronns
less titan 1000
highly vJriable bul gener.llly less than 4
probably minor Mock \\"nlcring and irrigation III Thorpdale
Childers R>rmation
similar distribution to Thorpdnle Vokanics
15 to 20 m in northern pan of Moe Swamp Basin; up to 250 m in the )brragon:rtafalgar area
5 to 40 m
sand, gravel. silt wld clay
confined sand aquirer or shm·likc form
1css than 1000
less than 5
somt $Cock walering and irrig ation in the Moe Swamp Basin
Figure 6-23 Generalised hydrogeology of (he main Cainozoic aquifers in the Gippsland Basi n.
Figure 6-24 Generalised hydrogeology of the Uplands Province aqui fers.
Region
Geological unit fanning aquifer
SoUl hern Uplands Older Vok:anics •
Otway and Strzelecki mnges
lo.... u Cretaceous (0'.... '3)' and Strzelecki ,roups)
Main occurrence
Depth to aquifer
Str'ldecki RAnges
outcropping
Aquifer thickness
(salUnilai
Rod.. typC'S
Aquifer type and (onn
b3sa11. bas.111ic
unconfined fractured basah mostly less than aquifer 1000
thkknc:ss) 101060m
clay
only. mainly between l..congnt hit .uld Thorpdnlc and in the Warmaul area
main rock Wlil.s in the Ol� and Stn.clccki nmgC$
outcropping (C':.'ICCcp( where QI."erlain by Older Volcanics)
200-300 m?
.san d�O I\c'
sillSlOnc. mudstone, conglomcnue,
alluvial deposits
o:lensive occurrences in the upland tmcts of the Murray.
sand, gravel. silt
{Uld clay
in part covered
up to
.s
irrigation princ ip ally for potatoes and some pastures, slock wal.ering
0.1 to 1.3
nCM greally utilised: minor stock ....lltcring
unconfined (outcrop areas) to confined (buried areas)
100 10 1000
as high as 125
used mainly for Slock walering and irrigatioll JXlrlicularly in the O�ns Rh'er Y.tlley
stock and domestic use. irrigation of market gardens. non-citru.\ on:hards., pastures and fodder
sand aquifers of valley-like
Ovens and Qoulburn
by )'Ounger rocks, mainly Newer \blcanic basalt
Older Volcanics and sub-basaltic sand
Silvnn-W\lJldin area
outcropping
basalt. 10 to SO m; sub-basaltic sand up to20m
ba'iah and basahic clay Q\ulying sand. &mvel. sill and clay
unconfinoo rraclurtd basalt aquifer hydmulically connected with Wlderl)ing sand aquifer
100 to 1300
up 10 IS. gcnernlly less than S
Palaeozoic sedimentary
main rocks of the
mostly Olilcropping
200-300 m1
sandSlone. siltstone, mud!ltone. shale; g mnitt�. &r:anooiorite
unconfined fractured rock aquifer
SOO to 3000
0.1 to IS. mostly less
Milia Millo.. Kiewa.
Groundw;ucr uses
1000 to 3000 groundwater in the Sirlclecki Group lends 10 be III the higher end of the range
Ihin black coal
outcropping or
Range of bore yields (LIsee)
unconfined fractured rock aquifer
seam, East Viaorian Uplands
Common $.."llinil)' r".tnge (mg/L TDS)
fonn
Rivers
and igneous rocks
East V iaorian Uplands
West V ictorian
Nc:Yo� Volcanics
Uplands
alluvial deposits
Silurian - DcYonian
Palaeozoic sedimentary
and igneous rocks
cxlensi\'t in thl:! Woodend-Kilmott and Sn.llamt-DaylcsfordMasyborough aleas
oulcropping
most sign i fica nt nrc
Loddoll River
O\'trlaln by Newer Volcanic
deposits
bamh
The Grnmpiam
outcropping
main rocks of the
WCSI Vlaorian
Upbmds
most outcropping except where covered by
alluvial dcposilS and/or basall
generally less Ihan 1000 In the higher rainfall �as
than )
crops
mainly used for stock and
domestic supplies o:cept in the Kinglakc and MOllbulk arcus where groundwater is used cxlensi\''(I),
for irrigation
basalt. basaltic cia),
unconfin«l fractured rock aqu i fer
SOO 10 3500
moo: than 10
where hi&lll)' fractured but generally less than 2
usc, irrigalion particularly in the RallumlDa)'lcsford area; tCM'n ""'ater 9Jppl)' for L1nccncld. Woodend. 1l'entham. Gordon. Mt Egerloll. Learmol1lh. Waubra and Moca
s..1l1d. grovel. silt And cia)'
eonnnl:!d beneath Newer Volcanic basalt
SOO 10 1500
S to IS
stock. domestic and irriSlIIlon
200-300 m?
sandslone, sillSton� mudstone:. conglomerate
unconfined sand (scree:) and fraaured rock aquifer system
SOO to 2000; 10000'cr
up 10 10
Ialldy wlused otCCp l for SOnle Slock watering and as town water supplies for WiUnura, Lake Babe. MQ)'SCon. Glel1lhompson. Wlckcliffc: aquifer also provides back-up SUDply for Hamilton
2IJO..)00 m?
sandstone:. s illStone. mudslone. shale.
unconfined to confined
geoemlly less
used for Slock watering WId minor irrigBlion; where m ineralised as in the Hepburn Springs-Daylcsford area. the groundwater is bollied and sold commCTtially
10 m 10 100 rn mostly around 50 m
gra.nllc.
granodiorite
fractured rock aquifers
salinity aroundwalcr in scree on slopes of The Grampians
mostl)' 1000 to 3000: includes
mineral w.'\Iers or central Victoria
Ihnn 2
stock and domC!ilic
mainly
Water
249
The locations of the cross-sections in Figures 6-25 to 6-27 inclusive are shown
Figure 6-25 Murray Basin: cross-sections showing the geological formations thaI are aquifers and confining beds.
on Figure 6-18.
"
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200 0 on
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::;;
200
AquIfer
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200 'i:
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Confmmg beds
Duddo lImes/one
AqUifer
Olney Formallon
Confimng beds
Pre- Tortlary basemont
0
on
Mmor aqUifer (Grou"d!i...ater basement)
1
�
� .200
Name o( Borehole
.400
Figure 6-26 Otway Basin: cross-sections showing the geological formations that are aquifers and confining beds.
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250
Chapter 6
Figure 6-27 Gippsland Basin: cross-sections showing the geological formations that are aquifers and confining beds.
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Water use in Victoria
Victoria occupies only 3"70 of Australia's land area but it contains over 25% of the national population, making it the most densely populated State. There is thus a more concentrated demand for water in Victoria than anywhere else in the continent. Most Victorians live in Melbourne and districts close to Pon Phillip Bay. However, they use water mainly for domestic and industrial purposes and so account for only 20% of the State's water consumption. By contrast, 78% of the total is used for irrigated agriculture nonh of the Central Victorian Uplands.
SURFACE WATER SUPPLY SYSTEMS A dam provides a means of collecting surface water from rivers, so the water can be transferred later to areas where it can be used. I f the annual stream now of a river is reliable, sufficient water can be stored during the cooler, wetter months to cope with demand in the hotter, drier periods of the year. In most parts of Victoria, however, this method does not provide an assured water supply, even allowing for restrictions on use in drought years. The generally high annual variability of rainfall over the State has made it necessary to build reservoirs with large storage capacities relative to annual streamnows and average water usages. Many of Victoria'S large dams therefore have been designed to provide 'over-year' storages, that are sufilcient to fulfil needs over periods equal to the longest sequences of dry years ever recorded. Unfortunately large storages are costly to build and they suffer high evaporation losses. Most Victoria'S storages, however, also play an important role in controlling streamnows, so reducing potential nood damage. The first major water storage in Victoria was Yan Yean Reservoir. It was constructed in 1857 to supply the rapidly expanding Melbourne metropolitan area. Other works began soon afterwards to provide water to provincial towns and later to irrigation areas. Since then many urban water supply systems have been developed. This is because there has always been a public demand to have so much water stored that rationing can be avoided, even in times of drought. This popular desire has never been fully realised and probably never will be. The total capacity of Victoria's major stora ges exceeds 15 000 000 megalitres, which is equivalent to more than 70°/" of the mean annual discharge of all Victorian rivers (Figures 6-28),
Waler
251
Figure 6-28 Major surface water storages in Victoria and the rivers o n which Ihey are buill.
•
• Mlldura
"
12 13 14 15
•
Tullaroop Cann Curran
20 GlenmaggE
'0 "
Eppalock
•
3 ,
5
)
I
•
7
9
I I I I I
J 9
\
I I
I"
w.okoan
16 W�ham 17 BullalO
23
Thomson
2'
t.;pper Yarra
25
O'Shannass y Maroondah Sugarloaf
2.
Hovell
.7 2.
18 HUrTle
Van Yean
Greenvale
29 30 31
19 Oarlmoulh
Silvan Cardlnaa
21 Blue Rock 22 Tal
• •• Wodonga ." ,< -9 %-r,.,
�
�
},
HorsM..me
\\
Waraoga ElIdon NIUahCoolie
Albu ",
,
il
i
Me:1bQllrnes ' S,! t!g'X
Rura!Sl!Pplles T_ Rocklands BeUlield West Barwon Lal Lal Mellon Memmu PykesCrel!k
"
"
%
..
"
3 2
"ant'lO"
ftN(!1
�
POn'a!'ld
�
J
J
MI
.t
o
1
25 50 75 100 !
!
!
l
Map reference
Storage
I
Toolondo Rocldands BeUfield Wesl 8arwon
2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Lal Lal
Melton Merrimu Pykes Creek Tullaroop Cairn Curran EppaJock Waranga Eildon NilIahcoolie Mokoan William Hovell BuffaJo Hum, Danmouth Glenmaggie Blue Rock Tango Thomson Upper Yarra O'Shwmassy Maroondah Sugarloaf Van Yean Greenvale Silvan Cardinia
28
29 30 31
�stS:
+ slatIon
Construction completed (enlarged)
1953 1953 1966 1965 1972 1916 1969 1911 1959 1956 1962 1910 ( 1926) 1927 (1955) 1968 1971 1971 1965 1936 1978 1929 1983 1969 1983 1957 1928 1927 1980 1857 1971 1932 1973
Stream
Orr-SHearn Glenelg Fyans West Barwon West Moorabool Werribee Pyrites pykes Tullaroop Loddon Campaspe off-stream Goulburn Broken off-stream King Buffalo Murray Milia Milia Macalister
Tanjil
Tango Thomson Yarra O'Shannassy Walts off-stream Plenty orf-stream orf-stream off-stream
•
Full storage capacity (megalitres)
106000 348000 78 500 22000 59 500 17000 19000 24000 74000 148000 312000 4 1 I !XX) 3 390000 40000 3M000 13 500 24000 3038000 4 000000 190000 200000 37 SOO 1 1 10000 200000 4000 22000 95000 30000 27000
40 000
287000
Main uses of waler+
I. D&S 1. D&S I. D &S
T T
1 T. I T
1
I. E 1
1 1. E
1
I
1
1 I. E
1. E
D&S C. T. I T T.I T
T T T T T T T
I irrigation; T town supply; D&S domestic and stock; E electricity generation; C cooling water for power
Two very large integrated syslems dominate the State's water supply. The largest in volume is the Goulburn-Murray Irrigation District of northern Victoria. It draws on a number of major headwater storages and is operated as one in lerconnected supply system. The largest in economic terms is the Melbourne metropolitan system in south-central Victoria, which supplies water to abom 701170 of all Victorians. Smaller systems provide dDmestic or farm water in other parts of the State (Figure 6-29).
252
Chapter 6
Figure 6-29 Major public water supply systems in Victoria.
Urban Supply Dlstncts
Itflgahon DistrICts
i '"""-..r. ",-""'"
101 GolJb.rtn Camp."'&;)O ........ 2 Murray VillIOy 3 TOl'lumbafry 4iJ Nyah 40 RobInvaie
M Id..ra
I
! I i
5
I I
100tNa)' 11 GQcIOllQ Bcllalltlfl PClWlsula 12 6all.ll'lI 1 J Mclbourno 14 MOfl\IIlDIOIl Ponmsula & Oos'"Cf IS LaIIOOO Valley
M�Lsler WeI/bee BacchllS Marsh C ...,," Horsh.:lm �. 'urlo,a
DomestiC & Stock Supply
16 Wmlloora Mallco
Albury
Wodonga
i i
Be!'\O.go.
I I
, 8-.0 o
i
I
�
Por.:al'ld
Ba,rnidalee
.0
Gee10ng "
�
25 SO 75 100 , , ! ! KJIornelres
A complex network of channels and pipelines is used both to transfer water from storages to areas where it is needed and to release water down natural river systems. Many pipelines within each system are interlinked, so that water arriving at one location can be from more than one storage or from different storages at different times. Pipelines in Melbourne and other urban areas are mostly underground. On the other hand, in the country most distribution systems for public irrigation and stock and domestic supplies are open channels. There are about 21 000 kilometres of water pipelines in Victoria, over half of which supply the Melbourne metropolitan area. There are also 23 000 kilometres of open channels in rural supply systems, including 4000 kilometres required for drainage and flood protection. Most water storages provide scenic, recreational areas. Boating and fishing are permitted on some. A few such as Reedy Lake, north of Nagambie, are managed primarily to assist the breeding of water birds.
GROUNDWATER SUPPLY There are more than 69 000 bores and wells extracting groundwater in Victoria. Without them, primary industry and some other rural developments would not be possible over those parts of the State that are not linked to surface supply schemes. About 250000 megalitres of groundwater are extracted annually in Victoria. In the early days, groundwater was mainly used for watering farm animals. Nowadays, 150000 megalitres obtained from 6000 high-yielding bores are used for irrigating various fodder crops and market gardens. In addition, 40 000 megalitres are used for town supplies. An important use of groundwater is to supplement surface water supplies during droughts. For example, in the Melbourne suburbs many private water bores are used for watering gardens during periods of water restrictions. This kind of groundwater use has increased greatly since higher excess water rates were introduced in 1987.
URBAN AND INDUSTRIAL USE O F WATER Reticulated water systems upply about four million people around melbourne and in 345 country towns. Most water comes from surface storages, but 34 tOwns, including Ponland, Sale and Nhill, are dependant on groundwater aquifers (Figure 6-30). A further 28 towns, including Geelong, add groundwater to their surface water supplies. In addition, 29 towns have emergency supply bores.
Water
253
Figure 6-30 Viclorian lowns with public groundwater bores and reticulated supply systems.
it i
Swank
• •2
I I
II:
i:\ i .,
.
'""1 ." 1 ;\- . .;c .. ��
'","",---'l-'� \'
:; • ... 6
.
- .
70
I.� I .,) III
'eli\.._
"
,'" II
I
a·,
•
,�t;;.
e;U
�!lG
Groundwater province Murray Basin
Bore num�r on map
t
2 3 4
S
6 7 8 9
to II
12
IJ
Murra)"iIIe Cowangie Lillimur Kaniva Miram Nhill Goroke Apsley Pine Hills Elmore Kalunga Sirathmerton Chil l ern
21 22 2J 24 2S 26
Heathfield Heywood Ponland Pon Fairy Koroit Penhursl Dunkeld Caramul Monla"!! Curdie Vak· Peterborough Pon Campbell Sarwon Downs
27 28 29
Lismore lre:uham Scocl)'ard HIli
W(Scm l>On Basin
JO
Lallg tang
Gippsland Basin
31 J2
.. 'l1 lrnfalg
Otwa)' Basin
14
IS 16 17 18 19
20
)) 34 Up!;ltlds
)S 36 17 38 39
Sale
Bos i dah:
I bore I borc
Duddo Limestone Duddo Limestone Duddo Limestone Duddo Llmt:stone Duddo Umrslont Duddo limcstone PariUa Sand Duddo Limestone PariUa Sand Calivil Formation Cali\'il Formation Ca!i\'il Formation Calh'il Formation
Murrayville Coy,nnglc Lillimur KaOl\3 Miram hill Goro"e Apsley Harrow Elmore K31uoga Sitalhmenon Chillern
I bore
2 bor" 1 bore 6 bores 2 bores bore bore 2 bores I bore bore ) bores
I I
I
I bore 2 bores
Pon Campbell Limestone Oilwyn Formation Oilw)'n Formation Oilwyn Formation Pon Campbell Limestone (\\Cf Volcanics �cr Volcanics �er Voli.:ank.... Newer Vobnil"S Oil .... )l1 Fonnation Oil",,)n Formation Dil""yn Formation Dil""}l1 Formallon
Castcrton. SanMord Heywood Ponland Pon Fairy
4 bores
) bores ) bores
1
bore &spring
I bore I bore I bore: I bore I bore I bore:
&: spring &spring
.. borcs
'ipring 1 bores
spring
I bore I bore
5 bores
I
bore
NC'\'oCT \'O"''3ni� , (\\0'"
Voli...mil.�
t"YoCT Vok:anK.":'l
Trnfnlg.ar
2bon:s
ba..emcllI
Moullt MlK.'CdulI Trentham Gordon L..carmoolh
J bore...
dadte
I
bore
200ft'S
l10isdalc I'ormation
,,'Yocr Vok:nnk� 3I1d lxtWI1lCllI
Waubra
I
Bung Bong MO)�on \\c:sc
2 ......
44
Healh
1 bore>
ba\(I11Cnl
·8oft'flCld 10 1IK'f'C'3.'ie "";lIef
\uppl) lOr
\\arrn:ll'n�)1 Il'x ng
I
bore 2 ""'"
bore 1 bore
11I3Ur.l. I""!.l'
(ull rull "."
p an full (ull (ull filII
full lull
Barna\\anha Lan...'Clidd Woolkl1d In-nlh:un ('ionlon. MoullI L:J;,.wn ... lr.mllOl1lh Waubm \\
full pa " full pa"
p."
BOil-lillie Briagolol1);
f"�lo:a
pari full full full (ull
S.,k, WUlTul
Il('lb�
\10\'1(111. Gk."·lIhllml""lIl. \\ .d ••: hflc
\ll'nllO
"h...'1\1(' rruSXkc."li .
(ull full full
l.lplon
�IOnAcli ltonllation
2 bart...
full
Penhursl Ounkeld Cammul Monlal.e \\ arrnambool Peterborough Pon Campbell. Tlffiboon Gt'\.'iong, Angl�. Torqua� . B�on Hrads. o...'t'3ll Gl'O\e [)ry�ak. Portarlington. Quec:n...:liff. lco['lOk1 PoIOI lonsJalc:. \\il\\:hclsc:3, InJrm� Hrad LIsmore SII'\:3tham I.ang Lang
Boisd:llc l'Onmuion Boisd:de Formmi oll
(ull (ull full full (ull full full full full
"'oroil
Childl."f'tt Fonn31ion
ilriagolollg.
Poim
Supply
Toy,ns supplied
42 43
41
-..
Aquifer tapped
"....cr .. \Ulc3nk"S NC\ ..cr Volcanks Nev.'CT Vuk.'1lnk:! N��er \'obnics bast:ll1ml b:bm\cnl
40
•
Ty�
Uarnawatlha
Lan�'Cfickl
=' ��::a. 3:'· -.;j2
"� \J
Supply source Locality
"
full p.n pari
"'"
rull lull full
"'"
p
254
Chapter 6
Most water used in urban areas is for domestic purposes. There are also some water-intensive industrial activities, such as fruit canning in north-eastern Victoria, power generation and brown coal mining in the Latrobe Valley, melal refining near Geelong and textile produclion in the Geelong, Ballarat and Warrnambool areas. The Geelong district and Portland urban supply systems have groundwater components that are of special interest. At Portland, four bores extract groundwater from the confined Dilwyn Formation aquifer at depths of between 1 100 and 1 300 metres below the surface (Figure 6- 1 ). They supply a population of over 1 0 000 persons with domestic and industrial water. The groundwater has a temperature about 60°C, To make use of this heat, groundwater from the newest bore, drilled in 1 983, is passed through heat exchangers. These extract heat energy before the water enters the reticulation system. The energy is used for heating a number of large municipal buildi.ngs and the local swimming pool. The existing water supply system has a potential heat output of 1 600 kilowatts, of which only 600 kilowatts are currently used. In years of average to high rainfall, water for Geelong and the Bellarine Peninsula comes from dams in the catchments of the Barwon and Moorabool rivers. The supply system also includes four bores at Barwon Downs, some 50 kilometres west of Geelong, near the headwaters of the Barwon River. No suitable groundwater is available closer to Geelong. The aquifer tapped by this borefield has a large volume of groundwater in storage but its annual recharge is fairly small. Consequently groundwater from the aquifer is only used during times of drought. The groundwaler storage is allowed to recover in non-drought years. Measures to replenish the aquifer storage artificially are being tested.
IRRIGATION Irrigation is by far the largest user of water in Victoria. Nearly aLi of it comes from surface storages (Figure 6-3 1 ). The total annual consumption of about 3 5 50000 megalitres by irrigation would be enough to supply the Melbourne metropolitan area for about eight years. The combined full capacity of the three largest irrigation storages, Dartmouth, Eildon and Hume, is nearly six times the volume of water held in Melbourne's nine storages. Unfortunately there are substantial losses by evaporation and seepages from open channels and farm dams.
figure 6-3 1 Irrigation in Vic toria.
No. on
Figu re
In many areas there wal) a significant increase th rough the
6·29
1 980s in the amount of water used t o irrigate farmlands.
la Ib 2
Approx. arC;1
Irrigation Area
under
Volume applkd 1987188
irngmion (ha)
(ML)
Goulburn-Murray Irrigation DbtriCI:
I SO 000
Goulburn-CalllfXL.<;:pc Loddoll Murray Valley
J
Torrumbarry
Sub-IOlnl Direct pumping rrom Murray River: Nyah 43 4b Robinvalc 4c Mcrbcin·Rcd cli rr�
.
Tomt ,urra�c walcr Private groundwal<:r divcr,iOIl''''
Total - c-.tirn3led 3., 10°/0 or volume - c,lim,ltl'"
a,
91'" 000
438 000
2
555
J09
2 109 500
7 J06 1 M 98-1 120 89-1
7 500 20 100 US 000
100
aU \Ouro.:eo.
)01 SOO
328 500 S05 SOO
1 9 400
'-17
18-1
165 600
32 (XX)
190 SSI I I 913
1-10 000
) 100 I 500 J 200
Private ,urfal"C water divcr\ioil Pumping from privatc ofr·,(rearn dam,-
••
705 381 731 492
1 6 000
OirCl:t pumping from M ilia O'ven.., Kie"u
•
1 147 415 412 579
I
M\lcali\tcr Wcrribee Bacchu!> 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>
2().l 56'1
SKI 100
3 402 990
2 109
�J IXX)
ISO 000 ) m 990
1 10 (XX) 2 1J29 'XX)
34 OOU h 000
621 100
n7 -166 25 (XX)
lin 000 153 000 I; 000 'lOO
from OI1" lr<:al11 ,toro,lg<:' .
30°;0 or :lUtlluri",-'" c-.:tractlt)ll volume.
Most irrigated land is located north of the Central Victorian Uplands and is supplied by water from the Goulburn and Murray River systems. The largest irrigation district south of the Central ViclOrian U plands is the Macalister system in Gippsland. Two small irrigation districts at Bacchus Marsh and Werribee in central Victoria use water from the Werribee River and its tributaries. Areas supplied by government-cont rolled public irrigation systcms are shown on Figure 6-29. There are also some private irrigated areas scattered t hroughout the State. These divert watcr directly from nearby rivers and creek s.
Water
255
Irrigated farmland covers only 4"70 of the total area devoted to farming in Victoria, but it produces about one-quarter of the State's agricultural production. About four-fifths of all irrigation water is used on pasture , that feed meat and dairy animals. Only 10% is used for intensive cropping, but these products have a high value. The main crops produced are fresh canning and jam fruits, vegetables, tobacco, table and wine grapes, hops and citrus fruits. Higher levels of productivity are achieved on irrigated farms than woulct be pos ible if the farms relied on rainfall only. The average value of production per hectare per megalitre from Victoria's irrigation area is fairly low compared \�ilh \that from mo t other irrigation areas within Aust ralia and overseas. Thi is because most of the water is used for low value pastures grown on red-brown soils of northern Victoria. These soils cannOt be used for intensive cropping under irrigation because: • the clay subsoils retard drainage and become waterlogged after over-irrigation or wet weather; • after several years of cultivation the soils set hard, making it difficult for irrigation water to infiltrate, for roots to grow and for seedlings to emerge. Figure 6-32 Spray irrigation of citrus trees from a private channel in the Swan Hill district.
It is more efficient to irrigate by sprays than by sheet nooding. Less water is needed and there is less danger of salting developing. Large losses of water occur due to evaporation along the open supply channels, especially during the long, hot, summer months. (Photograph counesy of Rural Water Commission.
Figure 6-33 Spray irrigation of pastures at Lake Boga.
( Photograph courtesy of Rural Water Commission).
STOCK AND FARM DO MESTIC USE OUI ide the irrigalion areas, Ihere are ome public waler supply works, which caler mainly for domeslic usage and the walering of stock on rural holdings. The e are mainly in Ihe north-western part of the Stale, where the rainfall is low, there are no permanent surface waler supplies and usable groundwater supplie are limited. In this region, waler is di�tributed from dam in The Grampians and, to some extent, directly from Ihe Murray River (Figure 6-29 ). The system of channels in the Wimmera and Mallee region is one of the largest of its kind in the world, extending over some 28 500 square kilometres with a total length of more than 10 000 kilometres of mostly open channels. Large farm dam and town storages are filled from these channels once a year. usually in winter.
256
Chapter 6
In the Millewa system, which is supplied by pumping from the Murray River to Lake Cullulleraine, open channels were replaced by pipelines in the early 1 970s. This eliminated losses by seepage and evaporation and so greatly reduced the quantity of water pumped through the system.
figure 6-34 An irrigation channel lock and footbridge across a channel in the Mildul'1l district.
Irrigation water is obtained directly from the Murray River and distributed along open channels. Gates are opened in the locks to regulate the now of water down the channels. (photograph courtesy of Rural Water Commission).
ELECTRICITY GENERATION Water plays an important role in the generation of Victoria's electricity supplies in two ways. Large volumes of water from the Hazelwood Pond age, near Morwell, and Blue Rock Reservoir, on the Tanjil River, are used for cooling purposes at the large coal-fired power stations in the Latrobe Valley. At Morwell open cut, groundwater is pumped from a sand aquifer below the coal and used for power station cooling and fire prevention. In nonh-eastern Victoria, there is minor production of hydro-electricity. This is generated when the force of falling Water turns turbines. The Rocky Valley Reservoir on the Bogong High Plains is the main storage for the Kiewa hydro-electric scheme. In addition, power stations at four other dams, including the Hume Reservoir, generate electricity intermittently when water is being released for irrigation. No water is lost when hydro-electricity is generated.
MINERAL WATER
Mineral waler was defined in the Victorian Groundwater (Mineral Water) Act 1 980 as . . . "groundwater which in its natural state contains carbon dioxide and other soluble maller in sufficient concentration to cause effervescence and impan a distinctive taste". Nam ral (unbottled) mineral water also has a slight odour. When it is left in a bonle for some days, a brown deposit of iron oxide usually appears. This is the product of a reaction between dissolved iron salts and the water when it is exposed to air. There are about 1 20 known mineral springs, mostly c1u tered in the Daylesford district of cent ral Victoria. About half the springs are on public land. The policy of the State Government is to protect this resource for the people of Victoria. They have free access to springs on public land and free use of mineral water there. Commercial companies have also sunk bores on private land to obtain large supplies of nalUrai mineral water. This is treated to remove any objectionable odour or taste and to stop the precipitation of iron oxides. It is then bottled and sold as commercial mineral water. Springs and bores yielding mineral water are located in folded and fractured Ordovician sedimentary rocks, especially sandstones. Some are also found in younger sediments and basalts. Mineral springs occur where the water table intersects the land surface, usually along creeks. Boreholes for mineral water are often sunk into aquifers near natural prings. Mineral water contains from 1 000 to 1 0 000 mg/L total dissolved solids with an average of about 2500 mg/L It differs from most Victorian groundwater in that it has higher bicarbonate and lower chloride concentrations, and abundant carbon dioxide is present. Mineral springs were discovered during the gold rushes in the Midlands region. For a long while, there was controversy among geologists about their origin. It was thought the water mignt be of volcanic origin, because there are many valley flows of Newer Basalt in the ranges. However, it is now generally accepted that mineral
Water
257
water is normal groundwater recharged by rainfall and stream infiltration. The salts present are either contained in rain or they are dissolved from rocks through which the groundwater passes. Explanations differ as to the origin of the carbon dioxide. It is mOSl likely formed by the oxidation of carbonaceous matter in Ordovician rocks. The slight odour and taste present in most mineral water is caused by hydrogen sulfide, which may be formed by the oxidation of pyrite ( FeS, ) in the rocks. Some pyrite may be converted to soluble iron salts. However, rhere are still some people who think the carbon dioxide might be derived from deep-seated magmatic activity.
Environmental problems associated with water in Victoria
Through storms and floods, water can often have an adverse affect on the natural environment. Problems with water can also arise due to careless human actions. Two major environmental issues are discussed below.
POLLUTION OF WATER SUPPLIES As water passes over or through the ground, it may dissolve or carry contaminating substances, which can threaten life systems using the water. Pollution may be of biological or chemical origin. Biological pollution may occur where bacteria and viruses, derived from human and animal excreta, are introduced into water by inadequate waste disposal systems. This form of pollution is minimised in the extensive Melbourne and Metropolitan Board of Works supply systems by excluding people and stock animals from the catchment areas. Fortunately biological pollution is usually very low in groundwater, because micro-organisms are killed rapidly as water passes through rocks and soils. Nevertheless serious local contamination can occur where a herd of farm animals congregates on porous ground that forms the intake to a shallow aquifer. There is a greater risk of groundwater pollution occurring in limestone country. Polluted surface water can pass down quickly through various holes and fractures to an aquifer. There it can flow through caves to other parts of the aquifer without any filtration occurring. A wide range of chemicals may cause pollution of either surface or underground waters under various circumstances. For example, pesticides and fertilisers can become dangerous substances if they are dissolved by water percolating into streams or down to the water table. Dangerous heavy metal ions can enter water systems from such things as old cars dumped in gullies. Liquid effluent from certain factories must not be disposed of in streams or tormwater drains. Care must always be taken in siting and designing rubbish dumps to avoid the possibility that percolating water will dissolve contaminating substances and carry them into shallow aquifers (see also Chapter 7 ).
THE SALINITY PROBLEM In recent years a major environmental problem has gradually been recognised in Victoria and adjacent States. Dissolved salts (especially sodium chloride) in soils and water are killing natural vegetation and reducing agricultural productivity. The problem has arisen because of two reasons: I . The water level in shallow aquifers in many districLl have risen close 10 or above
Ihe land surface (Figure 6-39).
2. The concentrations of salts in both surface and underground waters have increased above the levels which can be tolerated by many plants. Most crops, pastures and natural species of plants become stunted or even die when their roots absorb saline water. As a result, there are many rural areas where trees have died off and farming has become unprofitable due 10 low yields from crops. The areas affected by the salinity problem are shown in Figure 6-35 . There are olher less common problems associaled with saline waters. For instance, during summer some dams in western Victoria become so salty that they cannot be used by stock. In nonhern and western Victoria, household equipment has been damaged by salty water supplies. Saline waters in streams and wetlands may adversely affect fish, water plants and wildlife.
258
Chap te r 6
Figure 6-35 Areas in Victoria affected by excessive salts in 5Oi1$ and groundwaters.
D D D
Irngalion regions Imgaffon saftmg
Sall-alfecled land. . In northern Irrigahon regJOns. 70%
of the Kerang and 20% ol lhe Shepparton regions ate salt prone. In dryland areas, numerous outbreaks of
Dryland saltmg Isolateel dryland saltIng occurrences
salinity occur throughout the zones shown on Ihis fIGure. Many Isol:Ued occurrences are In areas where Ihere is (X)nslCferable polenllal lor salinny 10 spread
Kerang .
•
• Shepparton
•
• •
6enoigo e
l"-
� •
•
•
•
...... ...... • •
•
MELBOURNE •
Geeleng .
.
• • •
..
•
Figure 6-36 Land near Kerang severely affected by salinity problems.
The water table has risen close to the surface in this district and the salt content of the water has increased. The salt has killed a forest of red river gum trees, a species that usuaUy thrives on river Oats. (Photograph by P.G. Dahlhau ).
••
•
Q t
25 SJ i5 lCJ !
'
Krlo:'T1e��es
"-
•
• •
8airnsdale e • •
"- -
I
"-
Water
259
Figure 6-37 Low succulent vegetation on a salt pan near Kerang.
A high concentration of salts
occurs in the soils on the floors of lakes and depressions in northern Victoria. The salt occurs where groundwater leaks to the surface and evaporates. The nature of the vegetation depends on the concentration of sallS. The succulent shrubs in the photograph have no value for feeding stock. (Photograph courtesy of Rural Water Commission).
Figure 6-38 Interlocking sodium chloride (common salt, halite) crystals developed in a soil at Lake Boga.
The crystals were left after the evaporation of saline groundwater. (Photograph courtesy of Rural Waler Commission).
The causes of the salinity problem
Natural salinity Throughout recent geological time, there have been salt marshes, salt pans. salt lakes. sall-affecled soils and saline Slreams and aquifers in some partS of Vicloria. They are especially common in the northern and weSlern regions. The highesl concentrations of salt OCcur in lhe Mallee, because lhe rain fall is low and Cainozoi� marine
ediments of lhe Murray Basin inherently contain a lot of salt. Howe\'er,
overall nalural saline features only cover a small area and have not seriously limiled the extent of land available for farming.
Induced salinity
A fter Ihe early gold r u she s of Ihe 1 850s. increasing numb.'rs llf E u n1 l'l'an �ellkrs began clearing the �ountry for farming. II appeared Ihal mOSI of Ih� Siall' outside I he Central Victorian U p lands was uitable for eilher agri�ulture
\11'
animal �razing .
This was so, because soils were arable, water supplies were adequ ate and t he land was nat to rolling. The ba ic causes of t he modern salinity problem commenced in the middle of the nineteenth cen lury. Opening up the country for farming led to the dearing o f large areas of forests. Trees, parlicularly eucalypts, take in and laler transpire much more water than do grasses and crops. As a resu l i . in cleared areas. a higher percentage of the rainfall infiltrated 10 the water table than happened where forest trees absorbed the water. The resuli was thai waler was added 10 Ihe groundwaler zones in the agricultural areas fasler Ihan il wa discharged illio streams Ihrough springs. Consequellily Ihe levels of many waler lables gradually rose. In many areas. t ill.' water table has now rcarileti 1 ill." ground slIrfact:. forming. swamps and bog.gy
arcas. These did nOI exist before European sClliel11Cnt began (Figurc
6-:19).
260
Chapter 6
Figure 6-39 The effects of land use changes on the level of the water table.
There is a greater danger that salinity problems will occur when the water table rises close to the surface. The clearing of native forests in groundwater recharge areas to provide farmland has reduced the amount of water removed as transpiration and increased the amoum of water percolating down to the water table. Tilis has caused the water table to rise, thus increasing the danger that a salinity problem will develop in the irrigation area.
HI Typ ical 1940s l...:.md Pl'ofi lc --
\.I'OU nd w:Jl('!' n·(:II:lq�('
Gl"o�lnd\\'atcl' di:,;chn r:. �t'
zune.'
zune.'
I nll1--pinillUtl
h,' clt·n,...·
\·l'�I'I.11 inn 1'(-II\II. :t-:
IIlfi l t ratlCln In \\:lh'r lahll·
Groundw:llCI' d i:..:d1:.1,.,.!l'
Will'
,',';ld,,-,. IlIcn:a,," 11 dl:.ch;I I·�1.' tf) :-ln';uu
�
b)
..ul ;' 1'\';1,.. ; ... 11111'1'1\\ ; lll- I' lal,ll' , .. du.. tlt,I I ;
O"'I:,.:,lI l11n ;tn'd�:
.uldlt ll1l);,1 \1'lh'l
Groundwall' l' n'Cha l'J,.!l' ZIIIH'
I";ilI1II1\' Ilet'II,.
.
\\;lh'l' ••• hl .
,·
, .. 1.IIIfJ :-l1tl'.tCT
...
'
q _-: . ......i. . ,l. � .. *"� \.:.\;,: \;;,' \\,'\ L,...� r.�
----
_---
-:;. \0,\\\1:
� ,...Ill
� -�
'J'ypicaI 1980s Lund
Pl'Ofilc
Much of the groundwater ciose to the surface in Victoria is saline, particularly in western Victoria. There are two reasons for this. Groundwaters usually contain more salts than surface waters, because they are in the ground for longer periods and in contact with greater quantities of soils and rocks. In addition, the heat of the Sun evaporates water from soils down to a depth of about two metres. The water that remains therefore becomes increasingly salty. Even where there was good quality groundwater originally, prolonged evaporation can lead to a build-up of salts in the soils. Many native Australian trees (e.g. eucalypts) can withstand moderately salty groundwater. However, as the water table rises to the surface, the groundwater becomes permanently waterlogged. Under these circumstances, trees gradually die. Grasses and crops are even less tolerant of salts, so they also die when the water table reaches the surface. The spread of saline waters has therefore caused a loss of farmland and a faU in the yields on some remaining farms. In addition, there are dwindling numbers of birds and other wildlife in the salt-affected areas. Irrigation has increased the spread of salinity in some areas, because more water is being added to the land than would be from rainfall alone. Saline groundwater has seeped into many streams, thus extending the problem downstream. About 2400 square kilometres in Victoria are affected by salinity problems at present, an area equal to the combined surfaces of Port Phillip Bay and Western Port. It has been estimated that this area will double by the year 2000 . The value of agricultural products lost each year increases by more than $50 million. Losses of farm revenue inevitably lead to financial losses by other sections of the community.
Control of salinity problems There are no easy or cheap ways of reducing I h e concentrations of salts that have accumulated in soils and waters over a long period. Considerable research is being carried out by Federal and State organisations (0 identify the extent and causes of the problems and to produce solutions. It appears (hat attention will have ( 0 be given both to removing existing saline waters and to reducing the quantities of water entering the shallow aquifers. Some of the control measures, which will have to be undertaken at either regional or local farm levels, are indicated below. Regional control schemes I . The water table is being lowered in some areas by pumping groundwater into
lakes, where evaporation takes place. There are over 80 evaporative ponds close to the Murray River.
Water
261
2. A grander scheme is being investigated, which would aim to transfer excess water in a system of pipelines from affected farmlands in New South Wales, Victoria and South Australia to a discharge locality in the ocean near the mouth of the Murray River. 3. Extensive revegetation is being carried out in some areas where there is an intake of water to aquifers. This is especially applicable to land outside the irrigated areas.
Farm-based controls I . Improved drainage of low-lying areas and land grading, so that excess water flows off the land instead of soaking in and raising the water table.
2. The sealing of irrigation channels to reduce leakage of water into the ground.
3. Pumping and reuse of irrigation water before it dissolves more salts or evaporates. 4. Change from flood irrigation to more economical watering techniques, e.g. trickier systems.
5 . Planting of salt-tolerant trees (e.g. Swamp Yate, Albacutya Red Gum) around farm boundaries, near water channels and dams, and on salt-a ffected land. This is more appropriate to dry land farmers. 6. Planting salt-tolerant pastures and deep-rooted species such as lucerne. 7. Fencing off severely-salted areas to prevent stock killing the remaining plants, which bind the soil and help prevent erosion.
8. Modifying cropping practices to minimise the amount of fallowed land. More water infiltrates bare land than that covered in grass or stubble.
.9. Revegetation of groundwater intake areas, especially those on south-facing slopes, where there is less evaporation.
The future of water in Victoria
Throughout the past century, the volume of water stored in Victorian reservoirs has doubled with each generat ion. The growth rate of water consumption is greater now than it has ever been in the past. Some measures to conserve water have been introduced. For example, it is now compulsory to install dual-flush toilets in new homes in the Melbourne metropolitan area. In some rural areas, waste water is being successfully reused to grow crops. However, even with water conservation, Victoria will need more water to satisfy future demands. In the long term, the extent to which the State's population can go on expanding will be limited by the availability of water.
SURFACE WATER POTENTIAL It has been estimated that only 43070 of the total flow in all Victoria's streams could be diverted for major water supplies. Just over half of this has already been developed. The remaining water is unsuitable for use either because of its poor quallty or the high costs needed to develop it. This applies particularly to the lower reaches of streams, where the land is generally flat and unsuitable for dam construction. The rivers with the most potential for future damming are those draining the uplands of central and eastern Gippsland, and the Otway and Strzelecki ranges. The surface water resources of north-eastern Victoria are already extensively developed and in north-western Victoria there are no significant rivers. In the south west, the stream waters are generally of unsuitable quality. The future cost of collecting water from streams will be much greater than it was in the past. Most untapped resources are located a long way from cities where water is likely to be needed. The development of untapped resources will require substantial capital investment for transporting water over long distances. The present distribution is largely achieved by gravitation. However, in the future, expensive tunnelling or pumping or both will be needed to transport water between di fferent river basins. I t is also probable that the major supply systems will be interconnected. This will increase management flexibility and capacity utilisation, improve reliability and reduce costs. Apart from the problems of cost, there will also be environmental issues to consider. Areas such as the East Victorian Uplands and the Otway Range (e.g. Aire River) have the greatest potential for new storages. However, they are also regarded as conservation and recreation areas. The planning of Victoria's most recent storages, Thomson and Blue Rock dams, included provisions for maintaining certain minimum flows downstream of the storages. All future developments will probably be accompanied by similar rigorous environmental flow requirements. This will reduce the yield of the storages for other purposes and further supplies will be needed at even greater costs.
GROUNDWATER POTENTIAL Groundwater now accounts for only a small part of the water used each year in Victoria. Since surface water is heavily committed, it is certain that use of
262
Chapter 6
groundwater will increase in the future, especially from basins that offer large yields of low-salinity water. Care must be taken to ensure that problems of over-development and pollution do not occur in Victoria, as they have in some overseas countries. Schemes to develop more groundwater must recognise that the resource is replenished by the infiltration of only a small portion of the State's annual rainfall. The average recharge rate is about I 500 000 megalitres/year, although the total amount of water stored underground is certainly hundreds and possibly thousands of times greater than this. The maximum volume of groundwater tbat could be used each year is the amount available when long term inputs by recharge equal the outputs by natural discharge and groundwater extraction. In some places, it may become possible to use artificial recharges to increase the rate of replenishment, thereby increasing the sustainable yield. The estimated annual extraction of groundwater at present is 250 000 megalitres, which is only about 17"10 of the estimated recharge. There is clearly considerable scope for further development of Victoria's groundwater resources. However, not all the State's groundwater could be extracted economically nor is it all of good quality. The largest undeveloped groundwater resources are in the Gippsland and Otway basins and in the nortb-east of the State. By contrast the useful groundwaters in the northern and north-western regions, the Western Port Basin, the Werribee Delta and the Ballarat and Silvan-Wandin areas are already heavily committed. In particular, excessive use in the Cora Lynn-Dalmore area of the Western Port Basin has depleted the groundwater store, creating a risk of intrusion of sea water from Western Port. In 197 1 , this basin was declared a Groundwater Conservation Area to enable extraction to be controlJed and so halt the intrusion of sea water. Environmental issues are also becoming increasingly more important in designing major ground water extraction systems. For example, at the Barwon Downs borefield, local streamflow, discharge from springs and the elevation of the land surface (to indicate any land subsidence) are all monitored to detect any adverse effects of pumping on the environment.
Figure 6-40 Torrumbarry Weir on the Murray River, east of Gunbower.
River water is retained behind the weir and distributed through irrigation channels to properties on the Riverine Plain. (photograph courtesy of Rural Water Commission).
Figure 64 1 B u ffalo Dam on the Buffalo River in north-eastern Victoria.
This small dam was built in 1 965, mainly to conserve waler for irrigated farming. Excess waler can be released down Ihe spillway in Ihe middle o f Ihe dam. Coarse pieces o f rock o n Ihe oUlside o f Ihe dam wall prolecl Ihe inner materials from erosion by waves. The highesl range in the background is composed of granile (A). The lowesl hills are Ordovician sedimentary rocks (C). Belween A and C, Ihe lower range consists of conlacl melamorphosed Ordovician rocks (B) in an aureole adjacent 10 Ihe granile. (Pholograph by courtesy of the Rural Waler Commission).
Figure 6-42 Lake Tyrrell, a salt lake near Sea Lake in nortb-western Victoria. In this area, the water table is near the surface. Highly saline ground waler discharges from Ihe banks of Ihe lake on 10 Ihe normally dry bed. Evaporation of this water leaves a thick crust of salt that can be harvested. (Photograph by P.O. Dahlhaus).
Engineering and Environmental Geology
Chapter
265
7
ENGINEERING AND ENVIRO MENTAL GEOLOGY figure 7-1 Houses destroyed by a slow landslide at an Remo between 1980 and 1985. The landslide occurred because water percolated through weathered basalt, forming a slippage plane beneath the foundation of these houses. They were situated near the top of low cliffs along the waterfront to Western Port. The slide began as a slow movement in 1980, when cracks began to develop in the house walls. The ground below the houses slipped downhill along the wet, soft basalt surface. By 1985, the houses were totally destroyed. Enginering geologists were engaged to investigate the cause of the problem. An even more important function of engineering geologists is 10 slUdy Ihe geology of siles where construction work will be carried out, so that mishaps of this Iype are avoided.
Geology and planning
The previous chapter of Ihis book dealt with the Iradilional aspeclS of geology. These included the ludy of geological processe , the formal ion of geological malerials, the interpretation of geological hislory and Ihe application of geology in Ihe search for water and mineral resources. However, over the past few decade , some new fields of geology have developed. These include Engineering and Environmental Geology, which use the science and lechniques of geology 10 assi t the planning and development of cilies, Ihe con Iruclion of engineering projects and Ihe proleclion of Ihe environment. They also involve Ihe de ign of mea ures to counter Ihe effecls of natural di asters such a floods and earthquakes. Engineering geologists help to increa e the safely of many a pecl of modern living and Irave\. For in (anee, Ihey identify any weaknes e in Ihe rocks or soil beneath building , bridges and dams, which could cau e failures in Ihese Slructures. They also look for polential landslips and rock falls on hillsides, road cunings. quarry and cliff faces and in tunnels. Many problems studied by environmental geologisls involve \laler; for c"mple, Ihe control of floods along river valley , the proleclion of beache and other pans of Ihe coaslline and Ihe prevention of pollulion in waler upplie . These geologisl, also invesligale whal happens 10 rocks and soils when Ihe nalUral land surface is ahered by foresl clearing, roadmaking, danlming of rivers, building of harbours or olher human aClivilie . Engineering and environmental geologists arc primarily concerned wilh Ihe dislribulion of geological formations and Iheir physical and chemical propenies. The geological processes Ihat are laking place al Ihe present lime arc commonly more importanl to these geologisls Ihan questions of the age and origin of Ihe geological formations presenl. The mosl important aspecls of an invesligation can simply be whelher the rocks are hard or soft and whelher Ihey arc weI or dry. During mOSI of Ihe hil.tory of Ihe human race, people did nOI consciously use geolog� as Ihe basi> 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.
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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
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Large earthquakes measuring more than 4 on the Richter Scale are rare in Victoria. Since 1975, a number of very sensitive micro-earthquake seismographs have been installed at various localities in the State. These record the locations of all Victorian earthquakes larger than magnitude 2, as well as all those larger than magnitude I within 100 kilometres of Melbourne. Within this short time, a large number of small earthquakes has been located by this network, enabling geologists to study their geographic distribution (Figure 7-6). Earrhquake activity is not very regular or constanr in time. Many more earthquakes were felt each year in Victoria between 1883 and 1909 than in any year since then. These included two in 1903 with a maximum magnitUde of just over 5. Both caused considerable damage to properry over a small area around Warrnambool. Because the Warrnambool earthquakes were shallow, their intensities reached vrr in some places. Many walls cracked, much plaster was dislodged, spires on churches were twisted, chimneys collapsed and railway lines buckled. In the cemetery, one hundred and twenty tombstones and monuments were displaced. A pedestrian suspension bridge over the Merri River at Warrnambool was also affected. On the 10th May 1897, the largest earthquake ever recorded in Victoria had its epicentre near Kingston on the south-eastern coast of South Australia. This gave intensities of up to 8 and it has been estimated that its magnitude was close to 7. Tremors were felt over the whole western half of Victoria. Even in an area that appears to be seismically quiet, energy may slowly build up over tens or even hundreds of years until eventually a large earthquake occurs. To idenrify such areas, a seismologist studies the distribution of faults that have been active in recent geological times. These faults may indicate the possible locations of future large earthquakes. An area that is experiencing many small earthquakes may be less hazardous than an inactive one, as energy is continuously released in the first area. Earthquake
Anchorage'. A1a.ska Chile San Francisco. U A �1(',icc Ciry Tangshan. China Nonhem Peru Guatemala
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Engineering and Environmental Geology
to minimise the risk of damage in the future, aU high rise buildings in Victoria should be designed and built to withstand the effects of possible ground shaking.
Figure 7'(' Victorian earthquakes recorded between 1976 and 1988 by micro earthquake seismogrnphs. Most of these earthquakes were very small. The concentration of earthquakes in central Victoria is innuenced by the greater number of recording stations in that region. (Map courtesy of G. Gibson, Preston Institute of Technology).
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269
Scale 1:6 milli on
During the early morning of 3rd December 1977, an earthquake was felt over a wide area of Victoria, including towns as far apart as Warrnambool, Echuca and Inverloch. The records left by the earthquake on seismographs showed that the earthquake epicentre was near Balliang, a small settlement south of Bacchus Marsh. The focus was at a depth of about 18 kilometres and the magnitude was 5 on the Richter scale (Figure 7-7). The only reported injury was incurred by a child who suffered a broken arm after falling from an upper bunk in a caravan. In the Balliang area, slight damage was recorded, including several broken windows and cracked brickwork. At Anakie, to the south, a 50 tonne slab of granite feU from an outcrop 15 metres above the ground and destroyed some trees. About twenty after-shocks were recorded after the main event, most with magnitudes less than 2. No fore-shocks were recorded immediately before the main shock but some minor seismic effects had been noted in the area in the preceding tWO years. The eanhquake occurred only a few kilometres east of the Rowsley Fault. This fault i a major geological structure, that forms the western boundary of the Pon
270
Chap ter 7
Phillip Sunkland (Figure 7-8). Its position is indicated by the prominent escarpment that marks the eastern edge of the Brisbane Ranges (Figure 7-9). The movement along the Rowsley Fault mostly occurred during the Tert i ary period. However, it is possible the Balliang earthquake was related to renewed movement alo ng the Rowsley Fault, although this is nOl certai n. Movement along the Selwyn Fault, on the eastern side of the Port Phillip Sunk land, was probably indicated by a stronger eanhquake, which reached 5.5 on the Richter scale at Momington on 3rd September, 1932. Figure 7-7 Isosei mal map for the BaI�ang Earthquake, 3 December, 19TI. The isoseismal �nes enclose areas of equal intensity on the Modified Mercalli scale. The data were compiled from infonnation supplied by several hundred people who noted some effects of the earthquake. (From G. Gibson, V. Wesson and R. Cuthbertson, J98J, J. geol. Soc. AUSL v28).
BASS STRAIT
Figure 7-8 The Port Phillip Sunkland.
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The country between the RowsJey and Selwyn faults sank during the Tertiary , leaVing high land to the west in the Brisbane Ranges and to the east along Momington Peninsula. Port Phillip Bay i s a drowned river valley, because before the faulting occurred, the Yarra River nowed over low lying land. which now forms the noor of the Bay.
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Figure 7-9 The Rowsley Fault esc.arp ment seen looking south from Coimadai.
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The forested Lerderderg Ranges in the background are on the upthrown side of the fault. The nearer country on the downthrown side is on the Mount Bullengarook lava now. Goodmans Creek, a lateral stream is in the middle ground. (Photograph by N.W. Schleifer).
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,
Engineering and Environmental Geology
271
LANDSLIDES Figure 7-10 Types of mass wasting or landslides. These diagrams show the ranges of speeds at which the masses of rock and soil move down hillslopes.
Over a period. any piece of loose rock or accumulation of weathered rock. sediment or soil may move down a sloping land surface. Such movement is largely caused by the force of gravity. It is often greatly accelerated when the ground becomes very wei due 10 heavy rainfall or melting snow. Waler softens and weakens the material and adds weight to the moving mass. There are many different types of land movements and they have been described by such names as slip. slump. slide. rock slide. rock avalanche. debris slide. mudslide. mudj/ow. rock/all. earthj10w and soil creep. Technically these processes are called mass wasting or more commonly. landslides. Some examples a r e illustrated in Figure 7-10.
Rock fall. extremely fast
Rock topple. slow to fast
Rock slump. extremely slow to moder ate
Figure 7-11 (bottom right) Hillside creep exposed in a cutting on the Melton-Gisbome Road. Ordovician sandstones are dipping at an angle of 75' to the east in the side of the hill. The dips at the topS of the beds appear to be less where they are bending downslope due to hillside creep. The soil on the rocks is also slipping downhill. (photograph by N. W. Schleiger).
Figure 7-12 (bottom lerl) A culling on Ihe Jamieson-Licola mountain road afler Ihe debris from . rock faU had been cleared away. The faU was triggered by rain percolating down a bedding plane and a major joint in the steeply dipping Palaeozoic sedimentary rocks. The result is known as a wedge failure. The overhanging rock at the top of the wedge is in danger of falling later.
Soil
Earth flow. very slow to fast
creep. extremely slow
Siump-earthfiow. very slow to moderate
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272
Chapter 7
Most of the movements occur very slowly as a result of natural forces and they do not cause any serious damage. The least noticeable is hillside creeo or soil creeo (Figures 7·10, 7-11). This is an imperceptible but continuous downhill movement of soil, that occurs on most slopes. Sometimes underlying layers of tilted or folded sedimentary rocks will also bend a linle in the downward direction. Any damage caused by soil creep is usually minor and easily remedied. For example, small bends may develop i n fences but they can be realigned if necessary. At the other extreme, there are occasions when huge masses of soil and rock move rapidly downhill. Speeds of several tens of metres per second are possible. Sometimes these large-scale landslides have affected populous areas causing enormous damage and loss of life. Such events are often triggered by other nalural catastrophes, such as earthquakes or exceptionally heavy rains. Two major examples of this son of disaster occurred in the province of Kansu in China earlier this century. In 1920, an earthquake there started a landslide, which turned deep, moist, silty soils into soupy mud. The mud swept downhill and quickly buried about 200 000 cave-dwellers and villagers. Seven years later a similar landslide in the same region caused the deaths of a further 100 000 people.
The causes of landslides Rocks and soils on sloping ground are usually in a mechanically stable state. There is no tendency for movement to occur, apart from the slow hillside creep referred to previously. However, under certain geological conditions an area may be susceptible to more destructive downslope movements. Landslides may result from either natural causes or human activities. Natural landslides have occurred throughout geological time. It is considered that Lake Tali Karng, a remote scenic area on the south-western slope of MOllnt Wellington in Gippsland, was formed after a large landslide blocked a deep valley in the past. Other examples of natural slides have been identified in the forested hills of the Otway Range. For simplicity, landslides can be considered in two categories: I. Movement oj weathering products. Natural weathering processes and erosion may produce loose accumulations of soil, rock scree and other products on a hillside. If this material becomes saturated with water after heavy rain , then il can behave as a nuid and now rapidly downhill. The most devastating effects have occurred in some overseas countries. There have been slides of weathered material containing slippery clay, which lead to great mudnows. Disasters have also occurred where high dumps o.f waste rock beside old mines have slipped and nowed down valleys after becommg very wet.
Figure 7·13 Lake Tali Kamg, 17 kilomelres nor1h.eaSi of Licola - a lake formed b) a landslide, (looking soulh). A landslide (e) on Ihe weslern (righl hand) side blocked IwO small Slreams, Nigolhoruk and Nighlingale creeks, 10 form Ihe lake. II is believed Ihe slide occurred al leasl 1500 years ago. Lake Tali Karng is Ihe only permanent. deep, highland lake in AUSlralia, Ihat is nOI of glacial or volcanic origin. The mounLains south of the lake are Gable End (I\) and The Sentineis (B) - they are capped by a Devonian formation called Ihe Wellington Rhyolite. (Photograph by N.J. Rosengren).
Engineering and Environmental Geology
Figure 7-14 (below left) A landslide on farmland on Ihe slopes of Ibe Barham River valley, near Apollo Bay. This is a typical curved landslide with a near-vertical slip-surface at the upper end and a mound of slumped material at the lower end. The landslide was probably triggered by heavy rains. The bedrock is Lower Cretaceous: sandstones and mudstones.
Figure 7-15 (above right) Hummocky terraceUes developed on the side of • valley below Busty Road. Apollo Bay. The terracettes have formed because there is a slow creep of soil down the steep hillside. They are used as tracks by farm animals moving around the hill. The underlying rocks arc gently dipping Lower Cretaceous sandstones and mudslOnes. This ground would be unsuitable for houses or farm buildings.
273
2. Movement 0/ blocks 0/ ground along definite natural planes. Water can percolate along various kinds of surfaces [hat occur naturally in the ground. Where these surfaces are bedding planes, joints or faults, they are flat (i.e. planar). In clays and weathered rocks, the surfaces are usually curved. Percolating water can soften rocks adjacent to these surfaces. On sloping land, this weakening of the rocks by water can cause a mass movement. If the movement occurs on a curved plane, the upper end of the landslide is often marked by an open vertical crack. AI the lower end, the landslide material moves outwards (Figure 7-14). A landslide may occur where the bedding planes in interbedded sandstones and mudstones dip in the same direction as the land slope, but at a shallower angle. Rainwater may infiltrate down through fractured or porous sandstone beds until it encounters an impervious mudstone or clay bed. There, it will probably change direction and percolate in a down-dip direction along the top of the mudstone. The latter becomes a soft surface. The overlying massive sandstone beds may slip down over the mudstone causing a landslide.
A landslide is commonly triggered by a period of exceptionally high rainfall. is most likely to occur a fter a long dry period, when large cracks had developed the soils due to shrinkage of sub oil clays. Examples of human activities which may lead to landslides are: • changing the shape of a slope by excavations such as road cuttings. These may undermine the uphill m ate rial; • overloading the slope by constructing embankments and spoil h eap s. The extra weight may upset the stability of the slope;
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•
causing hocks and vibration that affect the equilibrium of slopes. This may happen after explosives are b las ted: • changing the pallern of surface drainage or water infiltration rate by irrigation or the construction of water storages. The measures. that sometimes have to be taken to prevent landslide, can be very costly. They usually involve excavations to increase the drain ag e from an area of sloping land. A large drain may be driven into a hillside and a series of wide diameter drill holes may be bored radially into the roof. The latter are called chimney drains.
Landslides in Victoria
tlte kind that have at times countrie . However. there have been landslides in this State that involved similar volumes of moving earth materials to those in the overseas disaster areas. The major Victorian landslides fortunately did not occur in settled areas (see later case hiStory of Lake Elizabeth landslide). T here have also been many small landslides A maj or landslide, that affected the activitie of many people. occurred a rew kilometres from the summit on t he main access road to Mount Buller in J u ly. 1986. The weig ht of heavy snow on th e hillside is believed to have triggered the slide, which blocked access to the s k i resort ror IWO t o Ihrcc days.
Victoria has not experienced any disastrous landslides of caused great losses of lives in some Asian and European
.
274
Chapter 7
The Otway and Strzelecki ranges and a large area near Casterton are particularly susceptible to landsliding. These areas are mainly composed of gently-folded, interbedded feldspathic sandstones and mudstones of Early Cretaceous age. They are drained by many deep gullies. Sandstone beds, dipping at angles of 20 °-30 0, can slip down over clayey mudstone beds. Landslides particularly occur where the beds outcrop on the sides a f steep hillsides and they dip in the same directions as the hillslopes. The extent to which the clearing of vegetation for farming affects landslides is sometimes misunderstood. Land clearing is only a major problem if it allows greater percolation of rain water into rocks that are already susceptible to slides, e.g. in the Otway Range. Where rocks are massive, (e.g. granite) or steeply-dipping, (e.g. Lower Palaeozoic sediments), the risk that land-clearing will increase the landslide hazard is low. Removal of vegetation will probably lead to soil erosion however. If deep erosion gullies are formed, shallow blocks of soil may slip into the gullies during wet weather. This is not landsliding as described above however.
Effects of landslides Although catastrophic landslides are rare, smaller slips can cau e damage to land, buildings and structures such as roads, tunnels, dams and canals. Water, sewerage and gas pipes, telephone and electrical cables may also be affected. Rocks falling on roads not only damage the road surface but are also a danger to passing traffic. A landslide can block the valley of a stream creating a temporary lake. Eventually the stream may flow over the blockage and wash it away. This in turn may cause flooding downstream. Landslides may also increase the rates at which dams silt up.
Prevention of damage caused by landslides An engineering geologi.st investigates whether an area is likely to be susceptible to landslides, and if so, whether or nO! various kinds of structures can be built there safely. The geologist stu die the causes, nature and development of any previous landslides in the area so that predictions about the safety of a slope can be made. Areas of past or recent landsliding can usually be recognised by the appearance of the landscape. Landslides often give slopes a hummocky or terraced appearance (Figure 7-15). Areas where new landslides might occur can frequently be identified by the similarity of geologic materials and conditions to areas of known landslide activity. In assessing whether landslide may occur in an area, a geologist examines such features as:
• the shape of the slope, to interpret whether movement either ha occurred in the past or i s now accu rring; the rock and soils, to determine if the slope is underlain by strata likely to slide; • the ge010gical structure, to mea ure the orientation of such features as bedding planes, joint surfaces and shear planes; • the groundwater condition , to determine whether the ground is saturated close to the surf ace. Even in districts prone to landslides, most construction work can be carried out safely if the risks are reduced or eliminated by taking appropriate precautions. The most common method is to change the shape of the slope by reducing the steepnes . In VictOria, road cultings are often sloped back at a low angle to prevent landslides occurring. In ome cases. a specially designed wall, termed a relainillg 11'0//, is built at the foot of a slope to hold uphill materials in place. Other means •
of stabilising cuttings a re :
• pla n ting grass and shrubs, so thai their rOOIS bOlh hold the soil tOgether and
absorb e xc es moistu r e; construction of efficient dr ai n ag e sy t em s . so that water does nOl sarurate soils, therebv lowering their strength and their normal resistance to movement: • cove rin g uns ta ble secdons a layer of concrete or a brick wallj • removal of unstable rocks; • construction of 'catch' fences to prevent rocks rolling funher downslope; • spreading teel mesh and slio/cre/e ( sp ra yed concrete) o,·er emire slopes; • in enion of c oncr ete piles or steel rods through overlying weathered ground into the underlying stable rock mass. This pins the weathered material in to the s lop e. •
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Combinations of these techniques may also be u ed. Recommended practices for construction on slopes. where there are risks of
slides in rock or
o il. are
hown in Figure 7-16.
Engineering and Environmental Geology
Figure 7-16 The use of good and bad building practices on a steep hillside, where rock and soil slides may occur.
275
GOOD HllLSIDE PRACTICE
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/ Case history: The Lake Elizabeth landslide - a natural landslide
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POOR Hll..LSIDE PRACTICE
Lale in June 1952, one of Ihe largesl landslides known in ViclDria occurred near the town of Forrest, some 24 kilometre south-east of Co lac. The landslide involved about 6 million cubic met res of Lower Cretaceous sedimentary rock with a surface area of approximately 48 hectares. This material slid into the East Branch of the Barwon River. The rocks slipped along the bedding of the strata, which dip 12' to 20' towards the river. The toe of the slide was about 400 metres wide at river
level and t he material fo rmed a natural dam over 35 metres high (Figure 7-17) . The land lide occurred during the wettest year on record. The annual rainfall was 1683 millimetres compared with a mean of 1037 millimetres. June had a highest ever rainfall of 415 millimetres, nearly four times the average of 113 millimetres. Undoubtedly the landslide resulted from the ground being saturated over a long period. A natural re crvoir was formed behind the slumped material and named Lake Elizabeth by the local people. It filled with water quickly during the following year. On the 5th of August 1953, the water spilt over the natural wall and the flow rapidly breached the top 26 metres of the landslide dam. A wall of water rushed down the river. It was estimated to be 7 metres high about 10 kilometres downstream. An extensive one metre thick layer of silt was depo ited over the paddocks along the
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river. Fortunately, there were no houses or large sheds along the river flats and only minor damage to farm fences resulted. A smaller lake, one-fifth the volume of the original, remains. This landslide was relatively large. Ones of similar size in other countries have caused considerable loss of life. Figure 7-17 Plan of the East Barwon River landslide, which formed Lake Elizabeth. Note the steep-sided crevice at the top of the slide and the mounds of slum ped soil and rock. The road was displaced distances o f 1 00 t o 200 metres. (After A.M. Cooney).
Undislu.b�d �nd
flmll1d(ln; ofhl('d�nfl(Jntl h..,,,,., mOl't'ml'lIf IImlnt/an' r('Oml' blMk. (lfftr m(ll ('ml'/It
.,.... ..... ..... Cdgf! o( 51o�
The lake now contains aboUi one fifth of the volume of water that was present soon after [he landslide occurred. Gullies have developed within the landslide and small slips continue 10 occur from time to time. The bare patch on the edge of the slide is a small quarry. (P hotograph by .J. Rosengren).
Sllghlly d.lu'....d l.nd
-I .
___
Figure 7-18 The large natural landslide of forested land that blocks the East Barwon River to rorm lake Elizabeth.
--
......- Old road
"'"
Engineering and Environmental Geology
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In late 1968, a small landslide occurred across the Great Ocean Road at Windy Point, Case history: The Windy Point rockslide near Lome, after a small quantity of rock was removed during road reconstruction. There were several more small rockslides during the foll0:-vin ? months. On each a landslide caused by occasion, the fallen rock was pushed off the road over the chffs mto the sea. Durmg human acfIVI'ty this period, signs of a larger-scale movement were observed on the slope up to 70 _
metres above the road. During 1970 and 197 1 about 3000 tonnes of rock began sliding towards the road at rates of up to 2 centimetres per day. This was part of a larger mass of 150 000 tonnes o f Lower Cretaceous sandstone blocks, which threatened to slide over a thin clay (weathered mudstone) bed. In this area, the rocks dip at 27' towards the road.
Figure 7-19 Aerial view of the Windy Point rockslide, Great Ocean Road, 1970.
The mass of rock on Ihe hillside was slowly moving downhiU over the years, 1968- 1 97 1 . It consists of Lower Cretaceous sandstones overlying thin weathered mudstone beds, Ihal dip at an angle of 27' lowards Ihe sea. The bulldozer has pushed fallen rock over Ihe edge of the cliffs. (PholOgraph courtesy of Vic Roads).
Because of the unsafe conditions, the Great Ocean Road was closed in July
197 1 . A geological investjgation identified the cause of the slide. Engineers then designed a means to hold the sliding blocks of sandstone to the slope (Figure 7-20). Long steel cables were used to pin the larger blocks to the stationary rock below the lip plane. The cables, termed anchors, were cemented i11l0 holes drilled through the sliding blocks deep into the underlying rocks (Figure 7-2 1). The cables were then pulled tight, thus pinning the sandstone blocks to the slope and preventing funher sliding. Movement ceased oon after the first seven anchors were installed in October 1 971, notwithstanding heavy rain at the time. The road wa re-opened in December 1 9 7 1 and no funher problem have occurred.
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Figure 7-20
The upper sandstone beds are
A cross-section through the Windy Point rockslide, showing the measures taken to stop the slide.
thin and cross bedded and show minor elfects 01 slid e movement Numerous joints have opened
E: Exploratory boreholes were drilled to identify the rock layers. D:
Drainage boreholes were drilled horizontally to drain water out of the rocks.
A : Anchor boreholes were drilled deep into the rocks below the slide. Steel cables (anchors) were cemented into the boreholes and tightened to pin the overlying sliding rocks to the stable lower beds.
Major fissure
The massive sandstone beds are severely cracked between road level and SOm higher uphill
i
t_
"""': � �Cking is visbl•. a lissure
Major pint
UnderlYing stable sandstone Most 01 Ihls ground is very disturbed WIth surface subSidence
(After Williams and Muir, 1972).
Silty clay beds 12 and 20cm Ihick al road level
Sea level
Approxi mate
natural surface
� � o
Hard jointed sandstone Crushed, IraC1u red sandstone Silly clay (wea Ihered shale)
·
Anchor bo re
·
Drainage bore
·
Exploratory bore
o
I
20 !
4,0
Horizontal & Vertical Scale in Metres
Figure 7-2 1 Anchor cable used to stop the Windy Point landslide. The black tops of several cables cemented into boreholes are visible, e.g. twO immediate1y right of the man. The holes were bored through unst able slabs of sandstone slipping over clayey mudstOne beds and they bottomed in underlying stable sandstone.
Swelling clay so
Earthquakes and landslides can cause national disaslers in extreme cases. By contrast the next Iwo sections deal with problems, which usually have much less serious consequences. Lives are unlikely to be 10Sl, all hough property damage wilhin a small area may be considerable. Nearly all soils contain clay. When clay is moistened. il usually becomes plastic and rather sticky. A clay soil may include a variety of minerals. but the commonest are all hydrous aluminium silicates, i.e. compounds conlaining aluminium oxide, silicon oxide and water.
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This section deals with the problems that may occur where soils comain certain clays, which expand as they become wet and shrink as they dry out. Swelling soils are also called expansive, shrinking and swelling, unslable or reactive soils. In Adelaide, they have received the picturesque name of Bay of Biscay soils. The effect is greatest where the clays belong to a group of minerals known as montmorillonites. When these clays become wet, water molecules are taken up between the sheets of ions which make UT' the normal clay crystal structure. The water molecules effectively push the other ions apart, thus causing the montmorillonite to sweU. When the soil dries out, the clay particles return to their previous size. Because of this behavio ur, swelling soils move every time they either become wet or dry out. As a result, structures built on these soils tend to move also. Clay soils that expand and contract on wetting and drying are widespread in Victoria. There are few soils that are not subject to this problem in some degree. The greatest movements occur in dark clays formed on basalt of the West Victorian Volcanic Plains (Figures 7-22, 7-23). This region takes in most of Melbou rne's western suburbs and much of the Western District. There are also extensive areas of swelling soils over the Wimmera and Riverine plains, where they are developed on unconsolidated sedimentary deposits.
Figure 7-22 Cracks in a swelling clay soU at Mellon. Cracks up to 50 millimetres wide opened in a soil developed over basalt during the summer drought
1 982·83. The cracks closed after rains later in 1983. This is a typical swelling clay soil. Movements in these soils can cause great damage to structures built on them unless appropriate measures have been taken, e.g. use of thick concrete fo undation sl abs . of
Figure 7-23 Soil subsidence at Melton. In this area, the deep roots o f trees dried out clayey soils during a drought period. This caused the soils to shrink and eventually to collapse, producing open holes in places. The land is called 'crabhole' or 'gi/gai' country, because it becomes hummocky.
Swelling is least noticeable in sandy and most stony soils and in soils where the clay is a non- welling variety, such as kaolinite. This mineral is commonly found in soils derived from gran ite. Each year in Victoria, more than one million dollars wonh of damage is caused to buildings and other struclUre by swelling oils. Movement of the soils can cause cracks in the wall of buildings, driveways, paths and pipelines (Figure 7-24). Roads may e.x hibit a heaving effect. Cracks in the walls of buildings can lead to more serious structural damage. The destructive forces may be upward, horizontal or both. Ligh tweight structures, such as houses, are affected more often than heavier structure , such as multistorey buildings. The heavier Structures can re ist the pressures generated when the swelling soils expand.
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Figure 7-24 Se\'ere CMicks in the exterior and interior walls of a house in Brunswick. Left: Outside wall Right: Inside wall. This house was built on a montmorillonite-rich clay soil developed over Newer Basalt. Eucalyptus trees, planted close to the house, had extracted all the moisture from the soil during dry periods. This caused severe shrinking of the clay soil fo undations, which in turn led to movements of the footings of the house. Funher movement occurred after heavy rains, when the soil again expanded.
During a long drought in 1982 - 1983, cracks appeared in the walls of thousands of Melbourne houses built on swelling soils. The soils began to shrink as they dried out. The worst affected districts were those underlain by basalt, ego Footscray, Melton and Keilor. Problems also appeared in suburbs located on Silurian and Tertiary clayey sedimentary rocks, especially where large trees, such as eucalypts and wattles, had been planted close to houses. The tree roots extracted moisture from the soils that was not replaced by rainfall.
\
\
\
Problems have usually arisen where the presence or swelling soils was not recognised until a rter construction work was co mple ted . Co n sequent l y soil testing shou ld always be part of an initial siLe i n vest iga t ion . I f swell in g soils are identi l'ied,
appropriate design, construction and landscaping measures can be a dopt ed to meet the situation ( Figure 7-25). Structures should be designed to withstand the worst conditions they are likely to experience. For example, thick concrete foundations or more rein forcing steel may be used to coumer the effect of the swelling soils (Figure 7-26). Remedial measures can also be taken after construction has been completed but these often i nvolve high costs.
Engineering and Environmental Geology
Figure 7-25 Good and bad building and gardening practices on swelling clay soils.
;E
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SMALL TREES ONLY ANO NTEO AWAY FROM OUSE
EB DESIGNED CONCRETE SLAB CONCRETE CUT-OFF WAll ROOF DRAINAGE KEPT PREVENTS TREE ROOTS FROM AWAY FROM HOUSE DRYING SOIL NEAR FQUNDAnONS
rOOF DRYING
o
CLAY SOIL DRYING CAUSES SH RINKAGE
WATERING GARDEN NEXT TO HOUSE, wenlNG FOUNDATION SOILS
WETTING O F CLAY SOil CAUSES EXPANSION
.-igure 7-26 A concrele slab footing used for a new house at Melton. This type of footing, sometimes called a rafl. is fo unded al a shallow deplh in a swelling basaltit clay soil. II is Ihe mOSl suilable fooling for fo undalions in shrinking and swelling clay soils. Any movements in the soil are transmitted evenly to the structure of the house.
UNDER flOOR DRYING
t
WETTING OF SOIL CAUSES EXPANSION
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It is poor practice [0 build on swelling soils during autumn and late summer when they are usually very dry. Damage often occurs during the following spring after the winter rainfall has soaked into the ground and caused the clay minerals 10 swell. People should take care when they plan gardens around new houses_ Water loving shrubs and trees should not be planted too close to buildings. During dry seasons, trees extract large amounts of water from soils, which may cause house foundations to subside. Moreover, garden beds, which require frequent watering, should not be placed next to walls unless good drainage is provided (Figure 7-25).
Ground subsiden
In the last section, it was shown that small rises and falls may occur naturally in some clayey soils. These movements can cause cracks and fractures in constructions built on these soils. Somewhat similar and occasionally much greater damage can result where the deeper ground beneath structures sinks, usually in an irregular way. Ground subsidence may occu.r over either natural or man-made cavities. The cavities may have always been subsurface (e.g. natural caves in li mestone, old mine workings) or originally they may have been surface holes that were subsequently filled (e.g. old quarries, dolines (sinkholes) in limestones). In Victoria the problem is most common in former gold mining cities, such as Bendigo and Ballarat, where there are many underground excavations beneath the suburbs. Subsidences have also occurred in some Melbourne suburbs, where former sand and clay pits and basalt quarries were filled with waste materials and later the ground was used for parks, residences or other purposes. There have also been subsidences over abandoned underground coal mines and areas where large quantities of groundwater were pumped from bores. There have been problems where buildings were erected on old swamps and other natural low-lying areas, which had been filled with rubbish. Subsidence may occur either abruptly or gradually over many years. It may occur evenly over a wide area or as local depressions or 'sinkholes'. If the subsidence leads to cracks and depressions in the surface before any construction at the site begins, the newly-formed openings can be easily filled in. However, if subsidence is slow and intermittent, the cracks are likely 10 reappear.
SUBSIDENCE OVER OLD PITS Pits and quarries are abandoned when either:
• the SlOne or mineral resource is totally extracted, or • the resource is no longer required.
Figure 7-27 Damage caused to buildings erected over an old basalt quarry site due to a gl1ldual settling of the refuse used to fill the Quarry_ A. Quarry in operation: an hydraulic shovel loads broken basalt into a t ruck after blast n i g. B. The quarry has been abandoned and the excavation is being used as a waste disposal site. C. The refuse was covered by a thin layer of soil and the site has been developed as a new housing estate. D. Because successive layers o f waste had n o t been adequately compacted, the refuse continued to settle over the years. The house fo undations subsided, causing much damage lO buildings located over the Quarry. A nearby house on solid basalt has nOl been affected by the subsidence.
Old pits in gently-sloping to nat land often provide convenient sites for the disposal of domestic refuse or industrial waste. Some swamps and naturally low lying land can be used for the same purpose. After pits are filled, the sites can be used for recreation activities or other developments. Many houses, faclOries, playing fields and parks in Melbourne suburbs are on sites where sand, clay or basalt were previously extracted. Unfortunately subsidence has occurred at some old quarry sites, usually due 10 the slow settling of the materials used for filling (Figure 7-27). This has sometimes occurred many years after the quarry was closed to tipping and the site was redeveloped. The weight of buildings and vibration of traffic on a filled site can accelerate the subsiden ce.
A
m0SA· 9.;d;:�?·1'1':"rt:': :M� '� '': . g2ti;;
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c
D
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Some pits were worked below shallow water tables, (e.g. sand operations in south eastern Melbourne suburbs) and others partly ruled with water after they were abandoned. Where groundwater was later pumped from the area, sinking sometimes took place. This happened because the space previously occupied by water contracted. causing the ground to settle. Certain precautions should be taken to minimise the risk of subsidence occurring over filled pits, especially if construction of buildings is planned. It is not sufficient simply to roll and compact the dumped materials after the pit has been filled. Stable conditions suitable for later construction can be achieved only if the fill i ng is added in a sequence of layers. Each layer must be compacted in turn using a roller. However, high load-bearing foundations (e.g. large buildings) should never be placed on fill material, even if it has been compacted. This is because further settlement may occur slowly under high pressures. Another way of avoiding damage due to land subsidence is to construct buildings on piles, which pass right through the filling and are secured into the underlying rock. Special construction methods can also be used for roads and pipetines spanning subsidence-prone land. It is possible to make the construction flexible so that it can bend considerably without breaking.
Case history: Yarraville sinking village
Figure 7-28 Houses in the Ya rrsville sinking village. Note the subsidence of the ground near the entrance to the house on the left, the large cracks in the fence and walls of the houses and the timber supports needed to prop up 'he walls. (photograph by T. McCormac k).
Probably the most costly example of subsidence ever experienced in Victoria involved a housing estate, which came to be called the Yarraville sinking village. The estate was condemned five years after it was completed, because subsidence caused irreparable damage to many houses and installations. The area is in Yarraville and bounded by Williamstown Road, High Street and Anderson Street. It is only 9.5 kilometres from the centre of Melbourne. A basalt flow had been quarried at the site during the nineteenth century. The hole was filled between 1910 and 1959, but no records were kept of the materials dumped into the quarry. After it had been filled, the vacant land was used for many years as a transport depot and also as a site for visiting circuses. In 1970 the land was sold to a developer, who built forty, weU-appointed, twO bedroom, brick veneer villa units in 1971. These were mostly sold to elderly people. The only site investigation done at the time was one test bore to a depth of 5 0 centimetres. In February 1972, after particularly heavy rainfalls, the ground staned to subside. Several brick walls cracked so badly, they had to be re-built. After further heavy rainfalls in the following year, the underground plumbing system fractured. Water and sewage flowed into the filled ground. As a result much of the plumbing had to be relocated above ground at great expense. It was also necessary to stick tape across house windows, which had splintered with the movement and sent glass fragments into the air.
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A geological investigation showed lhat the houses were built on an old quarry, which had been filled with wrecked car bodies and industrial waste. Most of the waste was a form of calcium carbonate called 'sugar mud' or 'sugar sludge'. This material had been discarded from a nearby sugar mill. The calcium carbonate was initially dissolved by the heavy rainfall and later by the water and sewage leaking from broken pipes. Over a period of five years, pil es of rubble grew as the distorted buildings sank into the subsiding fLU. Finally, in February 1976, the entire development was con demned. Later that year, contractors began to remove the units, some of which had been almost demolished. After long legal proceedings, the residents of the estate were compensated for the loss of their houses. This distressing situation would not have occurred if a prior geological investigation of tbe site had been carried out. Examination of aerial photographs and a few deep boreholes would have indicated that the area was a filled quarry. A site of this kind was unsuitable for intensive residential development.
figure 7-29 Damage 10 a house caused by subsidence over an old gold mine. A. A typical small underground gold mine. Two reefs of gold bearing quartz intersect inclined beds of sandstone and shale. Two levels have been driven out from the main shaft to intersect the ore bodies. The roofs of the drives are supported by timber posts.
SUBSIDENCE OVE R OLD UNDERGROUND MINES The likelihood of subsidence occurring over old mines depends on how deep the workings are and on the types of rocks present. Below the water table, conditions are usually stable. There, the unweathered rocks are strong and the old workings are filled with water. I n the weathered zone above the water table, however, there may be a continuous tendency for rocks to fret away from the walls and roofs of the mine openings. This can occur particularly along fault and shear zones and where shales and other fine-grained rocks have weathered to clayey materials. These rock falls may even tually extend through to the surface. Subsidence is particularly likely to occur around old shafts after heavy rain, when water streams down from the surface (Figure 7-29).
B. The mine has been abandoned. The shaft has been sealed by a concrete slab covered by soil. A house has been built at a locality where the upper quartz reef was worked close to the surface.
Otd shaft capped I
C. The shaft seal has collapsed due to rainwater slowly eroding the soil and soft rock. The roofs and walls of many of the old workings in the soft weathered rocks have frel led away. The shallow bridge of rock left over the upper working has collapsed, causing the foundations of the house to subside. (Not to scale).
A
B
c
For safety reasons, the tops of many old gold mine hafts were capped with concrete blocks and then covered with soil or rock spoil. It is often difficult to locate the ites of these old min es. By referring to old mine plans, geological maps and historical records, planners can sometime be alerted to areas that may subside. In theory, it would be possible to backfill all the hole left by underground mining. In practice, however, such action i tOO expensive, except in the case of shallow shafts and small open CUtS.
NATURAL SUBSIDENCE This is common in limestone country, where there are many caves and clay-filled depressions. It is, however, often hard to predict when and where subsidence wiU occur.
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Geology and engineering
285
For thousands of years civilisations over many parts of the world have built a large variety of structures from timber and various geological materials. These structures led to the growth of cities and villages, where buildings were used as places of government, administration, trade, industry, commerce, domestic living, education, worship and recreation. Today such buildings are commonly lin ked by power and telephone lines, as well as water supply, sewerage and often gas pipelines. Transport systems have been developed which involve the construction of roads, railways, airports, shipping ports and docks. Engineering geologists are concerned with the effects which geological features and processes have on all the structures produced during modern building and engineering projects. These geologists provide information about possible geological hazards that may damage the structures. They also investigate the chemical and physical properties of the rocks and soils, that form the foundations at a construction site. Before designing any new construction, an engineer must consider all the information available about the geological processes, features and materials at the site. If any critical geological factors are ignored, sooner or later some damage to the construction may resuJt. The damage may range from small cracks in the \valls of a house, which can be patched up at no great cost, to the complete collapse of a large building or the wall of a dam. AU structures, from Melbourne's high rise buildings to minor country roads, are planned and designed by engineers and architects, so that the loads that they carry are transferred to the ground below. The construction of any civil engineering structure usually commen ces with the excavation of soil and rock. This is followed by the building of the structure. This, in turn, places a load on the ground below the excavation. The underlying geological materials must be able to support the load that the structure places on them. If they cannot do this, the structure will fall down or break apart during or after construction. Local geological conditions can affect the feasibility, planning, design, construction, cost and safety of a project. Consequently, before any construction work is undertaken, it is necessary to carry out a geological site investigation.
SITE INVESTIGATION An engineering geologist conduCts a site investigation to establish a three-dimensional picture of the geological conditions at a construction site. The invest igation begins during the planning stage and continues throughout the project until the construction is complete. Site investigations by engineering geologists are often called geotechnical investigations. The emphasis is on identifying the types of soils and rocks present, their relationships to each other and their properties, in particular their strength and other mechanical properties. It is also important to identify geological structures, such as faults and shear zones, and to gain information about the local groundwater. A full geotechnical investigation may involve the following stages:
1. Collection and interpretation of the existing inFormation on the site.
Maps and repons dealing with the geology, soils and groundwater conditions of many areas have been prepared over a long period by various State Government departments and authorities. Several of these orgartisations also have databases of borehole and test-pit records and flies on previous geotechnical and geophysical investigations. Geological journals, old newspapers and mining repons can also be useful sources of information. In addition, aerial photographs of the site should be studied. These will probably be available at more than one scale from the Ministry of Finance. For some areas air photography dating back to the 1 930s is available. These historical records may provide u eful information about the former nature of the site. For example, old quarries or swamp may be identified, even though they have been subsequently filled or reclaimed. New aerial photography can be commissioned from a commercial company if the existing Government photography is out of date or at an unsuitable scale.
2. Detailed geological survey of the surface of the site.
The different soils and rock s on the site should be identi fied and the boundaries between them mapped. This may not be possible in a built-up area. It then becomes important to determine the nature of any filling that was introduced in the past. Any nearby gullies, road or railway cunings and other excavations may provide useful information about the subsurface materials.
3. Geophysical surveys to interpret the subsurface geology.
Geophysics is the application of the principles of physics to the study of the Earth. The geophysical methods selected for a site inveStigat ion depend on which physical properties of the rocks or soils are of most importance. For example, a seismic survey can be used to interpret the distribution of hard and soft rocks below the surface. This technique mea ures the speed at which shock waves travel through
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the ground. The shock waves are usually generated by mechanical means or by detonating explosives. Such information helps engineers and builders to decide what machinery will be needed for carrying out later excavating works. 4. Boreholes and excavations such as test-pits, trenches and shafts. Excavations or drill holes extend the information gained from previous investigations by providing samples of the subsurface materials. It is desirable to use drilling equipment that can recover undisturbed samples of the ground intersected by the hole. Undisturbed samples can also be cut from the sides of excavations. The samples are sent to a geotechnical laboratory for further investigations (Figure 7-30). 5. Testing of soil and rock samples in the field or at an engineering laboratory. The main tests carried out on soils and rocks assess their suitability for supporting new structures. It is important to measure mechanical properties such as strength, compressibility and permeability. Tests on soil consolidation are also necessary. From these properties it is possible to calculate both how far a proposed Slructure will settle and the rate it will seule.
Figure 7-30 Site investigation drilling at Buninyong, near Ballarat. A diamond drill is being used to obtain a solid core of weathered Ordovician bedrock at this site for a new home in bushland. The samples were tested for strength to determine the safe bearing capacity for the house foolings. A diamond driU uses an annula r-shaped bit embedded with industrial diamonds, that can cut through any type of rock. Cylindrical pieces of rock (cores) pass up into a hollow sampling tube. (photograph by K. Schmidt).
At major projects, all the preceding steps would be carried ou!. On the other hand, for small projects or where the site has been built on previously, some steps may be omitted. For example, in Victoria, a site investigation for an average suburban house would normally require only steps I , 4 and 5. If an inadequate geotechnical investigation has been carried out at a construction site, then the engineers may choose an unsuilable design for the struClUre. It may Ihen be overdesigned and cost much more than il hould. Worse still, il may be underdesigned. This could lead later to the failure of some part of the structure or even collapse of the whole construction with a consequent endangering of human lives. Details of the geological investigations performed at several kinds of projects are described in the following sections.
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BUILDING FOUNDATIONS The word foundation refers to the ground which supports an engineering structure. The part of the construction, which transfers the load from the structure to the foundation, is usually called the footing. A geologist investigates the geological structures and materials within the foundation, so thai an engineer can design appropriate footings. When a structure is built, it is said to be founded on the geological material. The geological conditions at any construction site are usually one of three general types: I . Solid rock may occur at or close to the surface. Structures can then be founded upon rock with little e.xcavation being required. 2. Bedrock may be present below the surface at moderate depth. Loads can be transferred to the bedrock, using specially designed footings where necessary. 3. Bedrock may be so far below the surface that it would be uneconomical or even impossible to transfer loads to it. I n such a case all footings must be founded upon the overlying soil. Where type 2 exists, it is not always necessary to transfer loads to the underlying rock. For example, lightweight structures such as roads and footpaths can be treated as a variation of type 3. In deciding whether Ihe geological condition at a construction site will form a suitable foundation, an engineer must determine the depth to which the load will affect the ground. A building exerts a force (measured in t017nes weight) over the area of the foundation (measured in square metres). This load is called the bearing pressure (expressed in tonnes per square metre) . The bearing pressure transmitted by a structure is graduall y dissipated through the continuous mass of geological materials making up the foundation. A model showing how the pressure gradually decreases as it spreads out from the foundation is shown in Figure 7·3 1 . This model, known as the bulb of pressure, is used to calculate the load exerted by a structure on the foundation. The bulb of pressure indicates the depth to which the geological conditions below a structure must be known. The wider the foundation is, the deeper is the intluence of the bulb of pressure, provided the bearing pressure in tonnes per square metre is the same.
Figure 7·31 Bulb of pressure.
Load 5 Tonnes per square melre
This model shows how the bearing pressure exerted by a load (such as a building) is distributed through the supporting soil and rock foundations.
It is essential fo r a construction engineer to know how much sel/ling can be expected in the foundation materials after the load of a structure is placed on them. I f the foundations are in rock, sel/lement or consolidation is only likely to be a problem if the load is very high. Settlement is common, though, where the foundations are soil or unconsolidated rock. Most engineering structures can tolerate a little settlement provided that it occurs evenly over the site. Major struclures are designed to withstand settlement up to a defined limit without damage. Problems arise, however, if one end of the slructure settles more than another. This is known as differential sel/lemelll. A well·known example of such uneven settlement is the leaning tower of Pisa in Italy. Examples of how differential settlement may affect a house are illustrated in Figure 7·32.
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Figure '·32 Four ways in which damage may be caused to a house due to uneven settling of the foundations.
WEATHEAED BEDROCK (compressible)
B. The hard, unweathered bedrock does not seule but the filling does. C. The hard, unweathered bedrock does not settie but the soil does.
as
EB
A. Weathered bedrock seules unevenly because the filling is much heavier than the house.
seUles
On. sld8 01 the bUIlding 1M ground settles, caUSing cracks \0 develop
A
Clay hlhoO provides a load on natural ground
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The oyerlyino load causes Ihe nalural ground \0 sa\lle
B
Side 01 'he bUilding sallies. cauSing cracking
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D. The tiUing seules to a greater extent than the soil. (IncompreSSIble)
C
One side 01 bUilding ...,... � cauSIng cracking
sallies.
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Case history: Westgate Bridge foundations
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Until the I 950s, the most direct connection for vehicles between Port Melbourne and the western suburbs was a small ferry, which crossed the Yarra River at the western end of Williamstown Road. In December 1961, the Victorian Government requested the Country Roads Board (now known as Vic Roads) to investigate the feasibility of constructing a new major crossing of the Yarra River between Port Melbourne and Spotswood. The area selected for investigation was bounded by Craig Street, Spotswood, in the south and the junction of Stony Creek and the Yarra River to the north. Stony Creek is a small tributary of the Yarra nowing through Tottenham and Yarraville. These limits were chosen after consideration of the locations of the existing and probable future major public utilities in the area, including shipping facilities on the ri ver. Field investigations were carried out by the Country Roads Board, the Geological Survey of Victoria and the CSIRO Division of Geomechanics. The aims were to select the best location for either a bridge or a tunnel after consideri ng the geological materials present and to identify the engineering problems likely to be encountered . A systematic drilling program determined the nature and thickness of each geological formation in the area. Attention was concentrated on the sections that might be used for bridge foundations. Six drilling machines were used during the initial exploration to drill a total of 2500 metres. Samples of the different geological materials intersected were collected continuously throughout each hole. Geophysical surveys were used to infer the geology between the drill holes. Little was known about the local geology before this investigation. The incomplete records of some shallow bores suggested that various river delta deposits
Engineering and Environmental Geology
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of Quaternary age were present It was thought that thicknesses of up to 120 metres of soft alluvial sands, silts, clays and gravels were overlying interbedded sandstones and shales of the Silurian basement. The drilling program, however, showed that the Quaternary deltaic sediments were only 45 metres thick on average. Below them, sands and silts of the Late Tertiary Brighton Group and basalts of both the Newer and Older Volcanics were found above the Silurian basement rocks. Several drilling techniques were used, so that samples of both the soFt and hard materials could be collected. Diamond drilling, which recovers a core of solid rock, was introduced after basalt was found. A few boreholes were continued into the Silurian basement to investigate whether those rocks were broken by faults or joints. Such features might have influenced the stability of the foundations. The Brighton Group sediments proved very di fficult to recover as they consist of weakly-cemented fme-medium sands and light clayey silt with pockets of coarse sand and clay. In Figure 7-33 an interpretation of the drilling results i s shown in a cross-section along the eventual line of Westgate Bridge.
Figure 7-33 Westgate Bridge foundalions. A plan and a geological cross section, showing the locaLions of test bores along the route selected for the bridge. The information about the rock layers was obtained by sampling the drill holes. The bridge foundations are provided by basalt flows - mainly the Older Volcanics.
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The option of constructing a tunnel through the relatively soft Tertiary and Quaternary materials was considered but then discarded. It might have been more costly to construct a tunnel than a bridge. It was also thought that collisions might occur in the tunnel involving tankers from a nearby oil refinery. A bridge was therefore recommended, even though it had to be very long to avoid steep slopes along the approaches. It was also necessary to leave sufficient clearance for shipping to pass below the bridge. Several geological formations were considered for the bridge foundations. The shallowest material is the Moray Street Gravels within the deltaic deposits. Some bridges over the Mississippi River in the U nited States of America have been successfully founded in gravel lenses. The Moray Street Gravels were judged to be unsuitable, however, because they contain irregular zones of clay. Such material are likely to be compressible under high loads. They therefore would have been unsatisfactory for the heavy loading of the bridge foundation. The Silurian basement of fairly fresh, hard grey mudstone was another possible foundation, but it was the deepest formation. A fter considerable tesling and detailed analysis of lhe geological conditions, basalt of the Older Vo lcanics was finally selected for mo t of the Westgate Bridge
290
Chapter 7
foundations. This basalt was used because it is the shallowest hard rock. West of the Yarra, a shallower flow of basalt of the Newer Volcanics was discovered. The bridge is founded on rhis basalt, where it is present, because the rock is hard and at shallow depth. There are variations in the extent to which the basalt of rhe Older Volcanics is weathered and in the frequency of joints. Some basalt was weathered almost to clay. The presence of columnar jointing was also detected. The rock had to be tested to satisfy the construction engineers that a failure would not occur along any vertical joints. Fortunately the columns were found to have rough sides, which tended to produce an interlocking effect. The presence of some lenses of sands and clays within the basalt and of highly compressible peaty silty clay immediately below it was also of concern. There was the possibility that differential settlement of the foundations might occur. Therefore the design had to take account of any potentially weak zones in the Older Volcanics and place each footing at such a depth as to avoid them. The Newer Basalt west of the river presented no problems because it is uniformly unweathered and hard.
T UNNELS For several thousand years, men have extracted valuable minerals from the ground by excavating vertical shafts, horizontal tunnels and drives, and various other openings. In many places, there are dangers associated with underground mining. These may result from the presence of noxious or explosive gases or, more commonly, because the rocks are unstable and liable to coUapse into the mine workings. The maintenance of safe working conditions in mines forms part of the study of mining engineering. Apart from seeking minerals underground, people have used natural caverns or excavated underground chambers for many other purposes, including shelter, storage, tombs, prisons and passageways. In some hot parts of the world, water has been transferred over long distances along underground channels cut through rocks to minimise losses by evaporation. With the development of modern cities, the need for tunnels in civil engineering projects has increased. In Melbourne, tunnels have been constructed for water supply, sewerage, storm water drains, telephone services, electrical power transmission and railways. For many decades, the city'S cable tram system was drawn by long wire cables, which travelled along shallow underground tunnels. Tunnels also form an important part of the Snowy Mountains hydro-electric system and some Victorian water distribution systems. For exam ple, water is pumped westward from the Thomson Dam in central Gippsland through a tunnel in the mountains to the Upper Yarra Dam and thence into the Melbourne and Metropolitan supply system. Safety in tunnelling The general size, depth and route chosen for any tunnel obviously depend on the purpose for which it is needed. Usually various alternatives are considered during the planning stage. However, the geological conditions are often an important factor when the final selection of a route is made. This is especially so when there are various rock types and geological struct ures present and where the degree of weathering changes. The geology determines how expensive and how safe excavation along a particular route will be. The two most important geological factors affecting the safety of tunnelling are the groundwater conditions and the positions of major rock dejects (see below). The most frequent type of accident encountered during tunnelling through hard ground is one involving a sudden, unexpected faU of rock. This usually occurs where there are rock defects. These are planes along which rock may slip or move apart. They may be bedding planes, cleavage planes, fracture planes, schistosity and foliation in metamorphic rocks, fau lts, joints, shear zones or the walls of narrow dyke . A project geologist measures the number, size and orientation of rock defects as a tunnel progresses. The engineers can then determine where rock falls may occur and how the roof and walls should be supported to prevent such accidents. Sometimes rock defects may be so frequent that they affect large masses of rock, causing unstable conditions over a large area. Alternatively occasional defects may only cause local instability, which may lead to single rocks falling from the roof. I f explosives are used in the excavation of a tunnel, the presence of rock defects may cause a quantity of rock to fall out beyond the tunnel outline. This is called overbreak. It is also importam to know whether a tunnel will be located beneath the water table, as that will determine whether pumping will be required during excavation.
Engineering and Environmental Geology
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Where a tunnel is being driven below the water table through saturated porous ground, the conditions may be difficult to work in but they are usually predictable within one rock type. On the other hand, where the work is in fractured ground and water has percolated along rock defects, there may be unexpected rushes of water, possibly accompanied by sudden rock falls (Figure 7-34). Tnis can create unexpected danger for the people working underground. Large amounts of water may also be released if a tunnel penetrates caves in limestone country or buried river gravels below the water table. Figure 7-34 A sudden burst oC water into a tunnel after a drill bole penetrated a quartz-filled Cault wne containing a large supply oC underground water under pressure. A. A tunnel is being excavated
through hard, tilted sedimentary rocks. The workman is drilling a series of holes into the end of the tunnel. These will be filled with explosives, so that the rock ahead can be broken. The work is being carried out below the water table. Wide zones of faulted rocks commonly contain more water than normal sedimentary rocks.
Before
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B. The drill hole intersected a fault. A burst of rock and
water under pressure occurred, causing a bad accident.
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Another geological factor to consider is whether the rocks or soils exposed in a tunnel will be affected by exposure to air. For example, they may quickly dry out and crack or fret. Chemical reactions may also occur to give off a noxious gas. Tunnelling techniques Thnnels in hard rock, called hard ground, require different excavation techniques to those used in soils and soft rock, called soft ground. It is usually easier to tunnel through soft ground, but the conditions may be more dangerous than they are in hard ground, particularly if work is below the water table. Considerable suppon may have to be given to the roof and walls in soft ground to prevent rock falls. Extra care is needed if the ground is clean, dry, running and or nowing sand saturated with water. The air pressure may have to be increased to keep out groundwater and to stabilise the ground. Excavat ion in soft. ground is usually carried out by u ing a tunnel-boring machine filled with a rotating CUlling head. As the head revolve , cutting spokes shave small slices of the soft ground off the tunnel face and these are fed back through slots in the head. A the machine advances, supportS are placed in the tunnel behind the machine to prevent the roof from collapsing. Hard ground is usually s afe to work in and the main concern is the rat e of progre s. The harder the ground, the more difficult it is to penetrate and so the job becomes more expensive. In the past, hard ground was usually excavated using explosives. A pattern of hole was drilled into the rock face, the holes were charged with explo ives and then tired. In modern tunnelling practice, tunnel boring machines are preferred for most larger excavations. They are not only much fa ter than drilling and blasting, but als o safer lor the workers. These machines are equipped with special tungsten carbide cutters for cru hing the rock, which is then passed through slots in the CUlling head. The machines are very large and expensive and often designed specially for one particular tunnelling project (Figure 7-35).
292
Chap ter
7
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Figure 7-35 A Robbin's tunnel boring machine at the Bendigo Ordnance Factory.
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This machine was used to excavate the M.M.B.W. Western Trunk Sewer. The head o f the machine turns at about five revolutions per minute. Rock cullers at the front chip the rock. The fragmems are passed t hrough the slots on to a conveyor belt, which carries them to the rear of the machine. The head is cominually forced against the rock face. (Photograph courtesy of Melbourne and Metropolitan Board of Works)
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If rocks that form the tun nel walls are fractured and loose, they must be supported. Sometimes loose rocks are pinned to the stable rocks using steel rods called rock bolls (Figure 7-36). In other cases, timber, steel beams or reinforced concrete are used to provide support in a tunnel (Figure 7-37). Figure 7-36 (left side) Rock bolts - a temporary means of preventing rock ralls in underground openings. I here are var i ous designs at rock bolts. After the bolt is placed in the hole, one part of it is made to expand so that it grips the walls o f the hole. The small steel plate supports the rock around the hole and steel mesh may be placed behind a number of bolts to extend t he cover and support. Figure 7-37 (right side) Three ways of providing suppor1 in underground openings. Supports of this kind are particularly needed in soft sediments and weathered rocks. A. Timber sets are used in most small mines where t here is danger that pieces of rock could fall from the roof or walls. Each set consists of two thick wooden posts (or legs), tilted inwards, and a t imber cap. Notches are cut to fit the pieces together instead of using steel bolts. Additional horizontal slabs (called lagging) are placed between the rocks and the main pieces of the set.
EXPANSION
WEDGE AND BOLT
ANCHORING DEVICE
THREE PIECE TIMBER SET
THREE PIECE STEEL SET
B. There are many designs o f flexible steel arches, which may be rigid or yielding. The yielding arches may incorpo rate springs belween each piece in the set. C. A concrele lining and a steel frame not only provide strong support 10 the opening on all sides but also prevent any �ccpage o r groundwater into the opening.
CONCRETE LINING INSIDE STEEL FRAME
Engineering and Environmental Geology
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During the past gold mmmg era in Victoria, one of the skills needed by underground miners was limbering. Lengths of timber were set up in many different ways to protect the men and equipment from falling rocks. Close timbering was used mostly to prevent rock falls near the surface, where the mine was in soft, unconsolidated sediments or weathered Palaeozoic rocks. The deeper ground in unweathered sandstones and siltstones was usually safe, even if left unsupported. Shales and faulted rocks required timber supports however.
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Figure 7-38 Excavation of the portal of • tunnel in the side of the pump well on the M_M.B.W. Western Trunk Sewer project, Hoppers Crossing, south�west of Melbourne.
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The rock walls of the tunnel were sprayed with shotcrete (sprayed concrete) to provide stability. Additional support was provided by curved steel ri ng s . Rock bolts were driven into the walls of the pump well to prevent rock falls. Several rock bolts can be seen near the top of the photograph to the left of the tunnel. (Photograph by T. McCormack).
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Case history: Melbourne Underground Rail Loop
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Prior to the construction of the Melbourne Underground Railway Loop, some suburban trains terminated at either Flinders Street or Princes Bridge Stations, but others travelled through the City between the western and eastern suburbs. Trains from Sandringham, for example, passed through Flinders Street going to Essendon and Broadmeadows. The decision to build the Loop was made in the 1 960s, when it was anticipated that there would be a strong growth in the nu mbers of passengers using the suburban railway system. The number of commuters to the City from the south-eastern suburbs was aboUl twice the number from the western suburbs. The Loop was therefore introduced to allow more trains to travel between the Central B usiness District and stations on the eastern and south-easter n lines. Construction details
The loop was built between 1970 and 1982. It consists of fo ur tunnels and three underground stations with connections to the surface tracks. Each tunnel has an inside diameter of 5.95 metres. The construction was a complex engineering task, because the tu nnels are not only close together but also constantly changing their relative positions. The geology below the City is very variable. This made the development of the Melbourne underground railway relatively more expensive and more di fficult than the construction of similar facilities in Sydney and London. The Sydney tunnels and stations were excavated from massive gently-d ipping Hawkesbury Sandstone and could be left unsupported. By contrast, every tunnel and shaft on the Melbourne system had to be fully supported by steel and concrete within one hour of the surface being exposed. Each station had to be constructed by a different method because of the di fferent sequence of geological formations at each si te.
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Chapter 7
JOLIMONT •YARDS ..
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Figure 7-39 A. A simplified geological profile along the route of the Melbourne Underground Rail Loop. The pro file shows the variations in the rock types along the route and changes in the pattern of folds in the Silurian
Venita! Di.tortion 20:1
QUATERNARY
sedimentary rocks. (After A.J. Neyland and A.G. Bennett,
1974).
TERTIARY
B. Geological cross-section through Flagstaff Station. (After A.J. Neyland,
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Geology The main elements of t he geology are shown in Figure 7-39. Details of the fomlations
I is the oldest formation and Unit 5, the youngest. Modern artificial filling. 4. Quaternary -- silty clay underlain by alluvial and colluvial sandy gravels with
present are given below. Unit
5.
minor silty clay lenses. The materials are fairly stable, being above the water table. (Includes the Elizabeth Street Formation).
3.
horizontal basalt nows and tuff depOSits, generally 12 metres to 15 metres thick. These generally provide hard ground. Locally the basalt is weathered along horizontal and vertical joints giving a blocky 'onion' structure with cores of hard rock encased in soft, clayey material. The basalt contains some clay and sand seams.
2.
Tertiary -- ,*"ibee Formation -- unconsolidated sands and gravels grade upwards into consolidated silty clay. This unit is 3 metres to 7 metres thick and is an aquifer.
Tertiary -- Older Volcanics --
I . Silurian -- Dargile Formation -- the bedrock consists of gently-folded sandstones,
siltstones and mudstones. These were cia sified into five zones, each characterised by a different degree of weathering. This formation is thin ly-bedded and intersected by numerous closely-spaced joints and thin, deeply-weathered dykes.
Geotechnical investigations Exploratory drilling from the surface was carried out along Ihe proposed route of the tunnels to give the broad distribution of the various strata. Seven inspection shafts. each 0.9 metres in diameter, were then sunk at selected sites to enable the various
Engineering and Environmental Geology
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rocks to be inspected closely and tested. A pilOl tunnel was also driven to enable engineers to assess the probable behaviour of the rocks during tunnelling. Detailed geological mapping of tunnel faces and walls was carried out as work progressed underground. This assisted the engineers to forecast the conditions that lay ahead of each advance. The information collected included data on rock types, weathering, the spacing and orientation of joints and the rate of water inflows. Fluctuations in the level o f the water table were monitored and regular chemical analyses were made. The analyses were necessary because saline water can cause corrosion of steel linings and chemical reactions with concrete. The ground below the water table had to be dewatered during construction. It was therefore necessary to determine whether the ground would settle after the ,vater was pumped out.
Figure 740 A concrete arch dam.
WATER STORAGE DAMS The first official attempt to colonise Victoria was made near Sorrento by British settlers under Lt-Col. David Collins in 1803. It was abandoned after four months due to lack of water. A later auemot bv another oartv to settle at Corinella on the ,eastern side of Western POrt was also abandoned, again because of the lack of ,permanent water on the weathered basalt headland. The availability of plentiful
The pressure of water impounded by a dam of this type is transmilled through the thin concrete arch to the rocks forming the abutments. These rocks must be strong and unweathered to withstand the great concentration of pressure. water pressure v a arch action of the dam c cantilever action in the highest vertical section cf displacement of c section after the reservoir is filled with waler
fresh running water was the main reason Melb6urne subsequently became the IlrSt site for permanent settlement. The population initially obtained their water supply from the Yarra River at 30 cents per barrel of 550 Iitres. With a rapidly growing population, however, the Yarra soon became too polluted to be used for drinking purposes. Yan Yean Reservoir was therefore constructed in 1857 to secure a safe water supply for Melbourne. It was one of Victoria's earliest major engineering achievements. As described in Chapter 6, at intervals since then further dams have been constructed in various parts of the State to provide permanent water supplies for domestic, farming and industrial purposes.
(After Zaruba Q, and Menel C., 1976, "Engineering Geology",
Types of dams
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The terms, dam and reservoir, are often used indiscriminately in everyday language. Strictly speaking, a dam is a wall constructed across a valley to impound a large quantity of water in a reservoir. Dams may be built either of concrete or earth and rock. The choice of which type of dam to build is dependent on the geology at the site. A thin concrete arch dam is usually cheaper and easier to construct, but it must be supported by strong, stable rocks, because the foundation bearing pressures will be extremely high (Figure 7-40). This is particularly true in the abU/mems, that is where the ends of the dams are built against the sides of the valley. The pressures are transmitted to the foundation rocks by the arch action at the abutments. Thick dams made of compacted earth materials are more uitable where the rock foundations have lower strength, because the pressure is pread over a large area. Small earth dams can be founded in part on loose river sediments. However, the cemral section, known as the core, is usually founded on underlying rock, especially if the sediments are shallow and can be easily removed. Large dams are nearly ahvays founded on strong, solid rock. Before the design of a dam is selected, it is necessary to determine the depth of loose materials and weathered rocks and the strength of the underlying unweathered rocks anhe proposed site. The cost of scraping away any soft materials may become a significant amount in the construction budget.
Geology of a dam site The rocks and soils around and below a reservoir site must form a watertight basin. Water should not leak OUt into the sides of the valley or under the dam wall. Leakage under a dam is especially serious because the pressure of the impounded water may produce an uplift pressure on the foundations through buoyancy. In severe cases, the base of the dam may tend to slide downstream because of this uplift. The geology of an area is favourable for dam construction and a reservoir if it fulfils the following requirements:
• the rocks in the foundations will withstand the forces that the structure and water •
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will exert on them; the foundation rocks will resist sliding and preferably be uniform in composition; the rocks in the foundations and throughout the reservoir will be watertight and resist erosion and weat hering. Porous sands and gravels are unsuitable because ,vater can leak through them;
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Chapter 7
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the valley slopes wiU be stable when the reservoir is full and the rocks become saturated with water. It is essential that waterlogging will not cause landslides. These would reduce the capacity of the reservoir and might even suddenly displace enough water to cause flooding downstream of the dam; rocks and soils throughout the reservoir catchment area will not be prone to erosion. They should not contain minerals likely to release metallic ions, which might adversely affect water quality; favourable geological conditions will be present at the sites of the spiUway and the outlet tunnel. This means the rocks will be stable and not liable to erode; the geological materials needed for the construction. such as sand, clay and crushed rock, are available reasonably close to the site.
The rocks. that are most likely to meet these conditions, are massive (ypes, such as granite or steeply-dipping sandstones or metamorphic rocks. Rocks that are suongly fractured, cleaved, thinly-bedded or deeply-weathered are less satisfactory. There are only a limited number of river valleys in Victoria where all the conditions are suitable for water storages. It is therefore sometimes necessary to select the best available site for a dam even though the geological conditions are not ideal. A site can be improved by removing masses of loose, weathered or fractured rocks and by sealing crevices and porous zones with concrete. Figure 741 Construction of the Thomson Reservoir, 52 kilometres north of Moe in Gippsland. This dam was the most recent to be built by the Melbourne and Metropolitan Board of Works to augment the supply of water to Melbourne and its suburbs. The dam is on the Thomson River, north of the old gold mining town of Walhalla. The reservoir is the largest in the Melbourne and Metropolitan Board of Works supply scheme. Water is pumped from the reservoir through a 37 kilometre long tunnel through the mountains to the headwaters of the Yarra River and the Upper Yarra Dam. The layers being built up to form the wall of the darn are dearly visible on the right. There is an inner, w3tertigh4 white clay zone bou nded by crushed rock filters and rock and earth fill zones. On the outside are slabs of granite called riprap. On the left, the spillway and 'saddle darn' are being constructed. A large quantity of loose alluvium and weathered bedrock were removed from the valley before construction began. Rock slides provided a problem that had to be overcome. The final foundations are on folded, unweathered Lower Devo nian sandstones and iltstones. The bedrock was 'growed' (injected) with cement to make the joints and bedding planes watertight. (Photograph courtesy of M.M .BW.).
Details of earth·fill dams The construction of a dam using natural geological malerials is a complex task. The wall must be impervious (i.e. must not leak) and sufficiently massive to withstand lhe pressure of waler when lhe reservoir is full. I n addition, there must be no possibility that il will erode or slump at any place. To fulfil these requirements, engineers design a wall made up of various layers (Figures 7-41, 7 B). In t he centre of lhe dam is an impervious core made from the best quality clay available. This core acts as the water barrier of the darn. On either side of the core are thin layers of gravel to provide porous drains. The drains divert water away from the core preventing ..
Engineering and Environmental Geology
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erosion. The bulk of the dam consists of rock and soil, which add tremendous weight and stability to the structure. Finally, the wall is capped with a shield of erosion resistant rock to protect it from wave action and rainfall. The rock slabs are called
riprap. The most critical test of a dam wall comes when the reservoir fills for the first time. Small earth tremors often occur as old structural weaknesses in the rock foundations (e.g. faults below the reservoir) are reactivated by the huge weight of water on them. In addition, small cracks or washaways along any weaknesses in the dam wall (e.g. zones of soft or bro ken mate ria l ) may develop as the water pressure increases. These must be patched with concrete grou t.
Between 1972 and 1977 Dartmouth Dam, a high earth and rock fill embankment, was built on the Mitta Mitta River, a major tributary of the Murray River in north eastern Victoria. The dam holds 4 000 000 megalitres of water when full. It provides a carry-over storage. This means the water is available in times of drought to supplement Lake Hume downstream and to increase supplies to the River Murray system. The high quality water from the Dartmouth reservoir is used both for irrigation, to boost South Australia's water supply via the Murray River and to help control salinity in the River Murray system. The project also includes a power station to generate hydro-electricity when water is released from the dam.
Case history: Dartmouth Dam
Figur. 742 Locality diagram for Dartmouth Dam.
The geological investigation required for the construction of the dam involved the following activities:
r--""�=::-r-----, Regional mapping
As the region had never been mapped in detail, little was known about the geology of the dam site, the reservoi r area or the catchment. Features mapped included the distribution of various soil and rock types and the positions of folds, faults and major joints in the rocks. Details o f the geology around the future reservoir were needed to indicate any areas where landslips or reservoir leakage might occur after the reservoir filled. The amount of erosion by streams in the catchment was measured, so that an estimate could be made of the amount of sediment which would later wash into the reservoir. The regional geology proved to be complex and affected by several major faults. The oldest rocks are Ordovician sandstones, siltstones and mudstones, which have been modified to varying extents by regional and contact metamorphism. A large pan of the reservoir is located on a down faulted block called the Wombat Creek Graben. This structure is filled with various Silurian and Lower Devonian sedimentary and volcanic rocks. There are also several small igneous intrusions of acid to intermediate composition. Earthquake investigations A study of the past records of earthquakes showed that this \vas one of the more
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active regions of Australia. There was also a possible danger that movementS might be triggered in the bedrock by water infi ltrating and lubricating fault planes and --J by the stresses caused by the huge weight of water. To study the effects of filling Dartmouth reservoir, a number of instruments, including seismographs, were installed around the reservoir site. The monitoring commenced about tWO years before filling and continued for about five years afterwards.
_ _ _ _
Detailed geological investigations A geological investigation was carried OUt for each stage of the project. Initially the work related to the establishment of a township at Dartmouth to house the construction personnel. Si te investigations were undertaken to determine where and how roads, buildings, an airstrip, a water supply dam and a sewerage treatment works would be built. Exploration was also carried out for construction materials such as sand and rock aggregate. The next step involved geological investigations for the construction of a diversion tunnel and a small dam, called a coffer dam. These excavations were needed, so that water could be directed around the site where the main dam was to be buill. Outlet works for the main dam also had to be constructed. This involved an outlet Illl1 nel with large gates, so that each release o f water from the storage could be controlled after it filled. The site selected for the main dam embankment is occupied by Dartmouth Granite, a grey rock with numerous xenoliths. Detailed mapping revealed that the foundation was a clo ely-jointed, sheared and in places foliated rock. intersectcd by
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Fi�ure 743 SecHon through the embankment of Dartmouth Dam. The embankment contains three main zones. Zone One consists o f over 2 5 00 ()()() cubic metres of thoroughly-compacted, non expansive clay, derived from deeply-weathered Bannimboola Diorite. This rock occurs to the west of the reservoir. It provides a watenight seal. Crushed Dartmouth Granite was used for the other two zones. Zone two materials are hard, durable rock fragments, that are used as filters. The filter zones protect the clay from erosion because any percolating water escapes through the sand. Zone three material forms the main rockfill of angular shaped rocks of various sizes. These blocks interlocked after compaction to form a dense and durable wall, that provides strength and also protects the clay core. The riprap protects the whole structure from wave action - it consists of large blocks of Dartmouth Granite. (After State Rivers and Water Supply Commission).
minor diorite dykes. Geologists mapped the rock defects because these might have provided paths for seepage, which would have damaged the dam foundation. Permeable and compressible materials were removed and replaced with either concrete or well-compacted fill. In some areas, a layer of concrete was sprayed over finely fissured rocks. Major clayey shear zones were also identified in the foundations. Some of them were at right angles to the dam axis. The rocks adjacent to the shears were closely jointed and highly permeable. Cement was therefore forced into the porous rocks to make them watertight - a process known as grouting. This removed the risk that water might percolate through the jointed rocks and erode the clay in lhe nearby shear zones. Dartmouth was planned to be a very large dam, so it was known that considerable water pressure would be directed at the foundations. Possible leakage under the dam was therefore the main concern. Most of the granite at the dam site was highly weathered, so it had to be removed. The dam is now founded on slightly to moderately-weathered rock, which has a much lower permeability than the original material. The completed dam embankment is 1 80 metres high, 670 metres long and contains over 1 4 000 000 cubic metres of material.
Full supply lovel -----, I 'STI{I-:A�1
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ZONE 3
Geology and the envi ronment
D Fine filler D Coarse filter I§CiO
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Danmourh GranIte
So far, this chapter has explained how planners and engineers should consider the effects of natural geological processes and the distribution of geological materials, when they design a new town or a single structure such as a house or a road. If appropriate geological investigations are carried out before construction work begins, the risks of later disasters and costly failures can be minimised. In other words the geological work is mainly carried out for economic and safety reasons.
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However, the growth of cities and the building of various structures also affects our environment. As new land is developed, there is often a progressive transformation of virgin bush to farmland to, eventually, residential land. Urbanisation causes a loss of vegetation and a modification of the local water systems. After cities and suburbs are built, rainfall, that once percolated into the ground or trickled downhill into streams, mostly flows off pavements and gutters into Stormwate, drains. Many small creeks disappear from the landscape as they are replaced by underground pipes. For example, in the city of Melbourne, Elizabeth Street occupies a former valley, where a small creek once flowed southward into the Yarra River. During rare, exceptionally heavy rainstorms, Elizabeth Street has become a rushing creek again, because the storm drains could not cope with aU the water.
Environmental geologists study the impact that human actions have on the physical environment. The aim of their studies is to achieve some acceptable balance between the necessary use of land for human settlement and the preservation of an adequate pan of the natural landscape. Their work is panicularly relevant to the development of water supplies, the treatment and disposal of domestic and industrial waste and sewage, the use and development of coastlines and the impact of primary industries on the land. Although mining and timber cutting often attract the most publicity when environmental issues are raised, it should not be overlooked that farming accounts for nearly aU the bushland that has been modified since European settlement commenced in Australia. The most widespread environmental problems of soil erosion, soil acidity, soil compaction, soil waterlogging and soil and water salinity have arisen in the farming areas. SURFACE WATER SUPPLY There is little doubt that in the future Victoria will need additional dams and reservoirs if the present practices of water use are continued. Even if farmers, industrial consumers and house dwellers all try to use less water by reducing waste and recycling whenever possible, it is certain that total water consumption will continue to grow as the population increases. As a result, controversies over the construction of more dams are likely to occur. Water supply engineers may view a particular river gorge as the perfect site for a new dam, because the maximum amount of water can be impounded at the lowest possible cost. The site may also be the closest available to the area where the water supplies are needed. On the other hand, some people may see the gorge as a wilderness area or a possible recreation site for future generations, which must be preserved in its natural state. The conflict is particularly pointed because the sites most swtable for darns are often areas of spectacular scenery. Eventually, perhaps after water rationing has been introduced and it has been determined that there are no alternatives, (such as tapping underground supplie ), it may become necessary to build the dam in the gorge. Whenever a large dam and reservoir is buill, a portion of the environment is changed forever. Part of a forested river valley will become a lake and the kinds of fish and birds inhabiting the area may change. The flows of water and sediment are altered, as are the physical and biological habitats. The change in seasonal water levels in the river may affect the environment of a wetland swamp many kilometres downsl ream. Therefore before the final site for a danl is selected, environmental geologists collect information about existing geological processes in the area. They investigate such things as the rates at which sediments are deposited or eroded and the relationship between the river and the groundwater systems. Measurements are made of the amount of sediment being transported by the river. The information collected is included in a model of the present river system. The model is used to predict what changes will occur if dams of various sizes are built al different locations. The aim is to construct a dam, which will fulfil its primary requirement of supplying water to certain communities, while ensuring it has the leasl harmful effects on the natural environment. Environmental geologists conducting this work are usually part of a larger team, which includes biologists, chemists and hydrologists.
COASTAL DEVELOPMENT Coastlines are dynamic environments, where the landscape is continuously being modified by the fonees exerted by wave action, tides and currents. These aCI as agents of both erosion and deposition (see Chapter 3). Climatic fonees, such as wind and rain, are also involved in the changes. At any time in geological history, a coastline may be either extending outwards (aggrading) or being eroded a\vay (degrading). Any interference with the natural processes by human actions will probably accelerate the changes. For example, Ihe construction of a jetty may provide a trap for moving sand (Figure 7-44). The construction of a breakwater may cause erosion by deflecting strong currents to\vards the coa t.
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Figure 744 A. The coast at Hampton, a south-eastern suburb of Melbourne, looking north. A former 30 metres wide beach has disappeared in recent years, due 10 the effects of two construction projects. In its former, natural state, the beach sand was continuously produced by erosion of sandstone cliffs at the rear of the beach. The sand drifted down the coast during the winter under the influence of waves generated by the prevailing north-westerly winds. South-westerly winds brought the sand back to Hampton during the summer months, however. The supply of sand was reduced greatly though, when a seawall was built along the foot of the low sandslOne cliffs.
In planning coastal developments, there are two differing points of view. One is that the dynamic coastal processes can be overcome by engineering. This approach has been taken in the Netherlands, where immense effons have been made to protect and reclaim low-lying areas from the sea. The alternative view is that along a coastal zone engineers should use flexible planning and land use measures, which take into account the natural processes occurring in Ihe nearby sea. In this view, all structures on an eroding coastal zone should be temporary and expendable.
""" ' " " " " -----..;,;:;;:;1
B. Sandringham harbour and breakwater, Port Phillip Bay, (looking south). In the 1950s, a long breakwater was buill out from Picnic Poin t, Sandringham, to form a boat harbour. This breakwater trapped the sand carried southward by waves from HamplOn beach during the wimer. The bay is very shallow, due 10 this build-up of sand. t he torce at the waves trom the sout h-west during the summer is now dissipated against the break water. The waves no longer have the energy to carry sand back to Hampton. (Photographs by G.w. Quick).
Case history: Beach restoration - Mentone, 1 977
Aparl from the development of a few small pons and harbours, most of the Victorian coastline outside Western Port and Port Phillip Bay has not altered significantly since the State was settled by Europeans. Nevertheless there have been a few major changes along the Bass Strait coastline. These include the silting up of the small ships harbour at Warrnambool over the years, the excavation of a permanent channel to the open sea at Lakes Entrance and the construction of berths for deep sea ships at Portland. Inside Western Port and Port Phillip Bay, however, there have been mOre extensive port developments and many modifications to the beaches and cliffs. There are some problem sections, especially in the south-eastern suburbs of Melbourne. Eroding cliff faces are receding close to some roads and home sites that were built too dose to the sea. Some beaches have also disappeared in recent times. For instance, a once popular swimming beach at the end of North Road, Brighton, has been replaced by a seawall construc ted of basalt blocks and separated from nearby houses by an area 0f clay and sparse trees and grass. Environmental geologists have been employed by the State authorities at various times to study the dynamics of the Victorian coastli ne. The geologists study the movement of sediment along the coast, the rates of erosion and deposition and the effects of previous sea-level changes. The information is used to create models of the present processes, so that decisions can be made on how best to use and protect Ihe shorelines.
At intervals around Pon Phillip Bay, there are low cliffs formed by sandy clays and ferruginous sandstones of Pliocene age. These rocks also occur over parts of the floor of the Bay. On geological maps they are variously referred to as the Brighton Group (near Melbourne), the Baxter Sandstone (Mornington Peninsula) and the Moorabool Viaduct Formation (BeUarine Peninsula). Slow erosion of the cliffs by
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the action of rainwash and wave attack has produced the yellow quartz sand deposits that typify most beaches around the Bay. In their natural environment, these beaches were fairly stable over a long period, although there were seasonal changes in their shapes and sizes. During the winter months, waves generated by winds are mainly directed on the eastern coast of Port Phillip Bay from a north-westerly direction. These waves cause a drift of sand along the coast from north to south. However, the wind direction is mainly from the south west in the summer and much of the sand is taken back to its original location. Slow erosion of the cliffs replenished any sand losses on the beach due to the action of occasional storms. Unfortunately after European settlemem spread around the Bay, many homes and roads were built close to the coast. Subsequently it was realised that these constructions would be endangered if cliff erosion proceeded too far inland. One suburb, where the problem arose, was Mentone, located 20 kilometres south-east of Melbourne. Mentone Beach forms the northern end of a 19 kilometre long crescent shaped beach. In 1936, when the beach was about 18 metres wide, it was considered that urgent action had to be taken to stop erosion of the steep cli ffs. To achieve this, a stone wall with a stone apron front was built at the back of the beach to protect the cliffs from storm waves. In addition, small jetties called groynes were constructed to prevent sand drift along the coast. Later the upper part of the cliffs was cut back to reduce the slope and revegetation was carried out. The stone walls saved the cli ffs but they also destroyed the beach. The walls prevented sand, which was washed off the cliffs, from reaching the beach. At the same time, reflection of the waves off the walls caused turbulent conditions, which accelerated the removal of sand from the beach to offshore (Figure 7-45). After several decades, only a small, narrow beach composed of medium-grained sand was left.
Figure 745 Reflection of waves off the Mentone seawall.
A
A. The seawall was built because waves at high tide were eroding the base of the cli ff, causing the cliff to recede towards the road. B. After some years, the backwash of the reflected wave from the wall had scoured most of the sand from the beach and redeposited it in the sea. (After E.C.F. Bird).
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In the early 1 970s a program to restore the beach was started. Sampling of the sediments on the floor of Pon Phillip Bay showed that offshore there are zones with sands of different grain sizes. Suitable sand for restoring the beach was found at depths of 5 metres to I I metres below sea-level and distances of 1.4 kilometres to 2 kilometres offshore from Memone. This sand was selected because the grains are coarse, with diameters in the range 1 . 2 millimetres to 1 .8 millimetres. Coar�e sand gives greater resistance to wave action than fine sand and therefore is less likely to be carried off the beach. The offshore coarse-grained sands were deposited during the last sea-level rise some 6000 years ago. They experience very lillie wave action on the sea-floor, so their removal has had lillie effect on the present-day sediment transport system. The sand was dredged from the sea-floor and stockpiled temporarily in a trench excavated below 6 metres of water about 600 metres offshore. From there, a smaller dredge transferred the material to the shore, where it was spread by bulldozers. By J uly, 1977, a new beach was formed which was 30 metres wide, 2 metres high and 1800 metres long. A beach at least 25 metres wide was needed to prevent storm waves reflecting off the stone walls and scouring away the sand. A semi-permeable groyne was constructed later at the outhern end of the beach to restrict the southward drift of sand.
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Initially the new beach sand was dirty due to clay and silt coating the sand grains. There were also some large shells and lumps of clay and sandstone. However, the appearance of the beach has improved with time, as wave action and rain have washed the clay away. The behaviour of the restored beach was closely monitored for many years. Because there is a long-term movement of the beach sand along the coastline to the south, a new beach quickly formed at the southern end of the restored section. However, there was negligible movement of the sand away from the shore.
DOMESTIC WASTE D ISPOSAL Modern societies produce vast quantities of domestic waste that must be disposed of at reasonable cost without creating health or environmental problems. This presents a problem to municipal planning engineers, because most people do not wish to have a waste disposal facility located near their home. A few city councils have installed expensive efficient incinerators, but most still rely on open rubbish pits. The commonest sites are abandoned pits and quarries in cities and, in the country, open cut mines. Old clay and basalt pits are favoured in the suburbs of Melbourne. The main concern of a geological nature with rubbish pits is that substances called leachates should not contaminate any underlying aquifers (Figure 746). A leachate is the liquid emerging from a waste deposit after water has infLItrated through it. It may be an obnoxious, mineralised liquid capable of transporting disease-carrying bacteria. It can form where percolating surface water dissolves dangerous metal ions and organic and inorganic contaminates from the filling. The strength of the leachate depends on how long the water is in contact with the rubbish.
Figure 746 Percolation of l1linwater through aD old rubbish tip is causing poUution of the underlying groundwater.
Rain water percolates through rubbish. dissolves pollutants nQ..inll!lrale�.JQ. wateIJable
____ _
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Selection of sites for waste disposal Factors which should be considered include topographic relief, amount and frequency of rainfall, depth to water table, the surface drainage pattern and the types of soils and rocks present. Favou rable features are:
• • • •
a low rain fall district, so that a minimum amount of leachate is produced; a deep water table, so that a leachate becomes diluted before it enters an aquifer; a dry clay pit, as the floor and walls tend to be impervious; Oat upland areas, where drainage is slow and away from the site. Unfavourable features are: • an area with a high water table; • swampy ground; • old limestone and fractured rock quarries, as well as sand and gravel pits, because the rocks are porous and make good aquifers. Leachates can pa s easily from a rubbish tip to the water table under these conditions. To minimise the risk of groundwater contamination near rubbish pits, it is possible to seal the floor and walls before dumping commences. The surfaces can be covered by one to three metres of clay of very low permeability compacted over a high.censity polyethylene layer covering the surface. The clay catches 70"l0 - 90% of the leachate and the remainder is held behind the polyethylene. Such pits must be kept dry by pumping out any water present and then treating it before it is released into creeks or drains. One or more boreholes should be sunk ncar the pit, so that the quality of ground water nearby can be checked regularly.
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Pollution of g roundwater by chemical spills and leaking tanks There are various ways in which underground waters can be contaminated by tiquids released by industrial accidents. Figure 747 illustrates one example where groundwater poUution has been caused by a spill of petroleum from a road tanker. Figure 747 Pollution of groundwater due to a spill of petrol. After a road accident in which a petrol tanker overturned, leaking petrol percolated down through the soil and unconsolidated sediments to the waler table. Petrol floats on water and is only slightly soluble, although some components such as benzene can dissolve in water. The upper part of the groundwater has been polluted by the pool o f petroleum. (After Feller, 1 987).
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These problems are often hard to treat because of the random way in which they may occur and the variety of substances which may be involved. Two important factors are the solubility and density of the spilled liquid. Soluble liquids pass into an aquifer easily. They are likely to be withdrawn by pumping elsewhere if they are light and float near the aquifer surface.
Septic tanks The greatest source of groundwater contamination in rural Victoria is septic tanks (Figure 748). They are the commonest form of household sewage disposal in areas where a reticulated sewerage system is not available. In a septic tank system, the household wastes are flushed into a concrete tank, where they are broken down by bacterial action. The resulting effluent is run into shallow trenches in the soil, where it is absorbed. The effluent often finds its way to the groundwater system. There, pollution may be caused by three kinds of substances: •
•
•
pathogens from toilet wastes (pathogens are substances causing diseases); phosphates from soap and detergents; nitrates from food scraps. Borewater
o
Figure 748 eptic tank groundwater contamination.
in danger of contamination "
stem
Groundwater may become contaminated by effluent from a septic tank, if the water table is high and the ground is porous. I t would be unwise to draw drinking water from a bore located a short distance downstream from the septic system.
Water table ----Plume of contaminated water The pathogens, in theory, may lead to outbreaks of typhoid or cholera, if the groundwater is used for drinking purposes. However, the risk is usually low because oxygen and soil bacteria quickly destroy the germs. Phosphates and nitrates are more likely to become a menace, because they are both nutrients for plants. Where polluted groundwater discharges into a river or stream, excessive growth of algae and other plants occurs, leading to the death of fish. Septic tanks are most likely to contribute to groundwater contamination where: • •
•
•
t h ere are many of them;
there is a combination of thin soils and permeable rocks; they are installed in permeable gravel or sand; there is a high water table.
The main precaution to take when instaUing a new septic tank is to make certain that there i no nearby well or bore, from which domestic ,vater supplies are drawn. It is important to know the direction of ground,vater flow in the area.
Figure7-49 The beach .t Middle Brighton on Port Phillip Bay. part oC a coastline aCCected greatly by human activities.
Before European scnlement, the norlh�castern section of the coast consisted of a series of beaches and low rocky headlands. Sand for the beaches was supplied by the erosion of Pliocene sandstones and sandy clays that formed cliffs. After settlement began houses and roads were built close to the coast, as seen in this photograph. Many of these structures eventually became endangered by erosion of the cliff. Various constructions, such as piers. swimming baths, seawalls, boat ramps, breakwaters and groyncs have combined to modify the natural environment over the years. In the foreground and mid-distance of the photograph, beaches have been maintained and a sandy spit has been built up because the shore is now protected from storm waves by a long curving pier and breakwater Ollt of sight to the left. However, further north, beaches have disappeared because the sand was carried away by longshore drift and not replaced by cliff erosion. (Photograph by G.W. Quick).
SUPPLEMENT TO: "Introducing Victorian Geology" Edited by G.W. Cochrane, G.W. Quick and D. Spencer-Jones Geological Society of Australia (Victoria Division). 1 99 1
GENERAL INDEX Aborigines 106, 1 63, 1 64 Aeolian processes 6 . 46. 59, 6 1 , 62 Aerial photograph 41. 42 Aeromagnetic map 104 Aggregate 175, 176, 179, 180. 1 82. 1 83, 1 89 Agricultural lirne 54, 1 89 Alluvial fan 68, 88, 145 Alluvium 6, 47, 88 Anceslral slream 76 Antimony 3, 206 Aquifer 234-236, 242-250, 254. 256, 257 confmed 234, 235, 236. 239 confIning bed 234. 235, 249. 250 intake area 234. 23 6 potentiometric surface 23 5 , 236 recharge area 50, 234, 235, 260 storage capacity 234 unconfmed 234, 235, 236 Aquitard 234, 235 Artcrsian bore 235, 236 Assimilation 1 2 Back-arc bas in 32 B all clay 1 86 Bar 89. 9 1 Base metals 216-219 Batholith 31 Bauxite 190, 224. 225 Beach restoration 300-302 Beach 10. 89, 91 , 92 Bedding plane 15 Berm 94, 95 Bioturbation 21, 22 Black shale 109, 1 1 1 Bluestone 3, 177 Braided river 40. 1 23, 137, 145 Breached cone 82 Brickrnaking 3, 1 84-1 87 Brine 225 Buckshot gravel 7, 86 Building foundations 287-290 Building stone 1 68, 176, 179, 1 80, 189 Bulb of pressure 287 Caves 74, 83, 86. 87 Cement (manufacture) 1 82, 184, 1 87, 1 89- 192, 225 Cement (Sedimentary rocks) 4 1 Chlorination 238 Classification of sand 1 82 Clay (industrial) xi, 1 83- 187 Cleavage 18, 19 Coal 17, 23. 140, 146, ISS, 1 73, 1 92197, 202, 256 anthracite 193 black 193, 194 brown xi, 20, 192-197 peat 193, 194 Coal rank 193 Coastal cliff 89, 91 -93, 94 Coastal development 299-302 Coastal dune 92 Coastal forms in Victoria 92 Coastal marsh 91. 92, 95, 96
Coastal plain 85. 86, 88 Coastal processes 89-9 1 Coastal ridge (old) 79. 80, 86 Coastal sand barrier 85. 86, 87, 88, 9 1, 92, 93-95 Coastal terrace 87 Coastal zone 88 Colluvium 6, 46. 47, 50 Concrete 1 82, 1 83, 1 87, 189 Concretion 144 Construction material 172, 173, 174179 Continental crust 5, 27, 28, 29, 30, 3 1 Continental drift 141 Continental platform 28 Continental shelf 5, 88 Continental slope 88 Contour bank 53 Contour ploughing 53 Copi 78 Copper 21 6-218 Core of the Earth 5 Correlation of igneous rocks 43 Correlation of sedimentary rocks 43, 44 Corridor 70 Craton 99 Cross-bedding 92, 1 23 Crushed rock xi, 1 1. 1 75- 177 Crust of the Earth 5, 6, 29 Crystals 9 Cuesta 74. 167 Current 90 Deflation hollow 6 1 Defonnation 34-37 Delta 49 Deposition 15 Diagenesis 40. 4 1 Diatomaceous earth 226 Diatomite 226 Dimension stone 179. 1 80. 1 88 Dip 1 6 Doline 60, 6 1 Drainage pattern 67. 7 1 .72, 75 Dune 40, 50. 53. 6 1 , 62. 78-80. 85. 86, 92 calcareous 50. 78, 79 longitudinal 78 lunette 78. 79 siliceous 50, 78, 79 Dust sheet 61 Dyke 14, 3 1 . 129, 166. 207 Earth composition and structure 4-6 Earthflow 271 Earthquake 4. 28. 29. 37. 266-270. 272. 297 Earthquake epicenlre 267, 268 Earthquake focus 267 Earthquake intensity 267. 268 Earthquake magnitude 267. 268 Economic mineral 1 1 Erosion 15. 37. 38, 47, 52. 53. 69. 1 65 . 274 Evaporite 225. 226 Exfoliation 15, 74
Exfoliation dome 74 Exoskeleton 105 Extractive lnduslI'ies Act 174 Fault breccia 3 6 Faulting 36. 37, 59. 63, 64. 76, 1 1 8, 166, 266 Ferricrete 76 Firebrick 1 87 Fireclay 1 86 Flocculation 238 Flood plain 69, 88 Flow banding 124 Flowstone 6 1 Fluoridation 23 8 Fluvial processes 59, 60 Fold belt 99 Folding 34-36. 1 67 Foliation 1 8, 19 Foredune 95 Formation 43, 44 Fossil cast and mould 20. 21 Fossil correlation 43. 44 Fossil extinctions 19, 23, 44 Fossil soil 45 Fossils 19-22. 24 Fossils (cited) Allosaurus 148 Amplexograptus differtus 1 13 Archaeolofoea 105 Atopograptus woodwardi 1 13 Banksia 156 Baragwanathia longifolia 127. 128 Brachyphyllum 147 Bronlosaurus 149 Casuarina 156 CenospJuJera 1 10 Cibicides thiara 1 60 Clavalipollenites 147 Climacograptus caudaJus 1 1 3 Columbarium 1 60 Corystus 159 Culmacanthus stewartii 135 Cyalhophyllum 126 Den1alium 1 60 Dicranograptus Jeir/ci 1 1 3 Dicranograptus nichoIsoni 1 13 Didymograptus protobifidus 1 13 Didymograptus superstes 1 13 Diprotodon 159 Displacanlhograptus spiniferus 1 13 Favosiles 126 Geinilzia 147 Ginkgo australis 147 Glossopteris 135, 137, 140. 141 Glycymeris 158 Hallucinogema 101 Halophytes 96 Henicocystis 1 19 Homo sapiens 161 Howellela laJisuicaJa 1 18 Hypsilophodon 149, 150 Isograptus victoriae 1 1 3 KOOlenia 105 Lepidodendron 135
ii
Supplement
Leptolepis 1 67 Lingula borungensis 124, 134 Monograptus 128 Nothofagus 157 NotochIamys 1 60 Oncograptus upsilon 1 13 Orthograptus calcaratus 1 1 3 Orthograptus conWlus 1 13 OzarkotiintJ 1 28 Paraparchites devonicus 1 25 PendeograptusfrUlicosus 1 12 PendeograptusfrUlicosus 1 13 Phyllopteroides 145-147 Placotrochus 1 60 Polycope sanctacatherinae 158 Pseudisograptus groeilus 1 1 2 Pseudophyllograptus angustifolius 1 13 Rhabdinopora scilulum 1 12 Salicornia 96 Skolithos 123 Taeniopteris dainlreei 147 Tetragraptus sura 1 13 Thyloeoleo 159 TyrannosQlU'us 149 Zygograplus junori 1 13 Fossil groups: algae 22. 101. I l l. 125 anunonoid 22 amphibian 22. 124, 134 angiosperm 22, 147, 156 annelid 22. 123 arthropod 22 bird 22, 148 bivalve 22, 158, 1 60 blue-green algae 100 brachiopod 22. 100, 1 1 1. 1 1 8, 1241 26 brittle star 22 bryozoan 22, 157 cephalopod 22 conifer 22. 147 conodont 22. I l l , 125 coral 22, 125, 126, 1 60 crinoid 22. 125 crustacean 22, 125 cyanobacteria 100 cycad 22 cystoid 22, 1 19 dinosaur 22, 145, 148- 1 50 echinoderm 22, 1 19 echinoid 22 fern 22, 134. 140. 145 fish 22, 125, 134. 135. 1 67 foraminifera 22, 1 60 gastropod 22, 1 60 ginkgo 22, 147 graptolite 22. 25, 109. 1 1 1 - 1 13. 127, 128 gymnosperm 22 horse-tail 22, 134. 140 hydroid xii. 22, 105 insect 22. 148 jellyfish 22, I l l. 125 lycopod 22. 129. 134. 135 manunal 22 moss 22, 134 nautiloid 22. 1 1 1 , 125 ostracod 22. 125. 128. 158 pentoxylale 22, 145, 147 petrified wood 20 radiolarian 22, 109, 1 10 scaphopod 22, 1 60 seed fem 22 spore 140 star fish 22. 125 stromatolite 22. 25. 100
stromatoporoid, 22. 1 14 trilobite xii, 22, lOS, 1 1 1 . 125 Fractional crystall isation 22S Freestone 180 Gemstone 226-228 Geochemical survey 55, 217 Geological cross-section 41 Geological map 41 -44 Geological time 23-27 nwnerical 25-27 relative 23-25 Geological time scale 26 Geomorphic divisions of Victoria 6488 Geomorphic processes 58-64 Geophysical survey 285, 288 Geotechnical investigation 285. 294, 295 Gibber plain 62 Glacial erratic 62, 137 Glacial pavement 62, 63, 137 Glacial processes 59, 62 Glacial striations 137 Glaciation 100, 101, 138; 1 6 1 - 1 63 Glacier xii, 62, 136- 140 Gold ii, xi. 3, 1 68. 171. 172. 204-215, 224 alluvial 1 68. 208-21 1 bedded reef 207 deep lead 209-21 1 fissure reef 205 Magdala mine, Stawell 212-215 nugget 210 reef deposit ii, 21 1 -215 saddle reef 207 shallow lead 209 spur reef 212 spuny reef 208 units 206 Gold ore treattnent - carbon-in-Ieach method 2 14 Gondwana 29, 98, 135. 141 , 145, 152. 194 Graben 39 Graded bedding 39, 40, 108 Granitic rocks in Victoria 3 1 , 1 1 6 Granitic terrain 74 Gravel (industrial) 1 68. 180. 1 8 1, 1 83 Greenhouse effect 138 Ground subsidence 279, 282-284 Groundwater 230, 23 1, 233-237, 238, 239, 242-250, 252-254, 256, 257, 258, 260, 261 , 262. 282. 291 , 299, 303 Groundwater provinces of Victoria: 242-250 Gippsland Basin 242, 247, 250, 253, 262 Murray Basin 242. 243, 249. 253, 259 Otway Basin 242, 244, 249. 253, 262 Port Phillip Basin 242, 246 Tarwin Basin 242 Uplands 242. 248, 253 Western Port Basin 242, 245, 253, 262 Gypsum flat 78 Hardness (water) 237 Hardpan 7, 86 Headland 90 Heavy mineral sand 168, 173, 220-ID Hillside creep 27 1 , 272 Hogback 74 Horst 76 Hummocky cross-stratification 1 19, 158
Hydro-electricity 256 Hydrologic cycle 230 Hydrology 230 Ice cap 62 Igneous intrusion 3 1 Igneous rocks 12-15 agglomerate 14, 33 andesite 13 aplite 13 ash flow tuff 33 basalt 12, 13, 1 n, 179 classification 13 composition 13 dacite 13 diorite 13 dolerite 13 extrusive 14 fonnation 14 gabbro 13 granite 12. 13. 179 granodiorite 13, 179 greenstone 18, 32, 97. 102, 103. 104, 106 ignimbrite 14, 3 1 , 32. 33. 1 20, 121, 130, 131 intrusive 14 komatiite 99 monchiquite 13, 3 1 , 207 pegmatite 13 peridotite 13 phenocryst 14. 15 plutonic 14 porphyry 14 pwnice 33 pyroclastic 14 quartz porphyry 13 rhyodacite 13 rhyolite 13, 124 scoria 14, 34, 82 syenite 13. 143 texture 14 trachyte 13, 33, 34, 143. 159 tuff 14, 34 volcanic 14 Iron 224 Irri gation 252, 254-256. 258, 260, 26 1, 263 Island arc volcano 30-32 Isohyet 240 Isotope 27 Isotope dating 26, 27 Jointing 37. 67, 82, 1 66 Karst (topography) 60, 61, 86, 87, 94 Karst processes 59-6 1 Lagoon 87, 92. 93 Land use 1-4, 69 Landslide 59, 265. 266. 268, 271-278. 296 Landslip 1 66 Lateral stream 7 1 Laterisation 7 Lateritic soil profile 76 Lava 6, 14 Lava columns 82, 84, 1 67 Lava tunnel 82. 83 Leachate 302 Lead 216-21 8 Lime 1 87, 1 88 Lime sand 1 8 1 Limestone (industrial) xi, 1 87-192 Lithifaction 40, 41 Lode 205 Loess 46, 162 Longshore drift 89. 90 Lmlette 78. 79 Magma 6, 30-34 basaltic 6, 12, 30
General Index
granitic 6, 12. 29. 30, 3 1 primary 30 secondary 30 Magma chamber 3 1 Magma movement 30, 3 1 Magmatic differentiation 1 2 Magnetic anomaly 1 04 Magnetometer 104 Mangrove 92, 95, 96 Mantle of the Earth 5, 6, 29 Marine processes 59, 62, 89-91 Mass movement 59. 60 Mass wasting 271 Meander 76, 78 Mesa 69 Metamorphic aureole 18, 24, 72. 73 Metamorphic rock 17 -19 gneiss 1 8 greenstone 18, 32, 97, 102, 103, 104, 106 hornfels 1 8. 72. 73 marble 1 8, 168, 180 quartzite 1 8 schist 18 slate 1 8 Metamorphism 17 contact 17, 18 regional 17. 1 8, 38 Meteorite 4 Mid--ocean ridge 28. 30 Mine 173 Minerals 8- 1 1 , 13, 226-228 accessory 1 1 agate 228 aluminosilicate 10 amethyst 228 amphibole 10 analcime 9, 178, 227 apatite 1 1 aquamarine 226 aragonite 9, 227 barite 8. 1 1 bentonite 184 beryl 226, 227 caimgorm 228 calcite 8, 10, 1 1 . 1 87 , 227 cassiterite 1 1 , 223, 224 chabazite 178, 227 chalcopyrite 1 68, 21 6-219, 227 citrine 228 clay 10. I I, 1 83 corundum 226 diamond 8. 226, 228 dolomite 1 87 economic 1 1 emerald 226 feldspar 9 fluorapatite 227 galena 1 1, 2 1 6-219, 227 garnet 226. 228 gemstone 226-228 gold 8. 1 1 gomardite 178 gypsum 1 7, 226 halite (rock salt) 225 hematite 1 1, 168. 224 illite 183. 1 85 ilmenite 1 1 . 220 jasper 228 kaolinite 1 83. 1 85, 186. 279 leucoxene 220 limonite 1 1, 224 magnesite 1 1 magnetite 1 1. 1 68. 224 maldonite 10 mica 10 molybdenite 227
monozite 220 montmorillonite 1 83. 1 8S. 279, 280 natrolite 1 78. 227 olivine 10, 34 opal 226 phillipsite 178, 227 pyrolusite 1 1 pyroxene 10 quartz 10. 1 1 rock-forming 10 ruby 226. 228 rutile 8, 220 sapphire 226. 228 silicate 10 sphalerite 216-219. 227 stibnite 206. 227 thomsonite 178. 227 topaz 226-228 tounnaline 227. 228 turquoise 227 ulrichite 10 vermiculite 1 84 zeolite 9 . 178. 227 zircon 220. 226. 228 Mineral resource 170, 171 Mineral water 256, 257 Mobile belt 100 Modified Mercalli intensity scale 267. 268, 270 Mohorovicic Discontinuity 30 Monoclinal fold 36 Moraine 139 Mountain building 28, 29 Mudflow 27 1, 272. 275 Mullock ii Nickel 217 Nonconformity 25 Numerical geological time 25-27 Nutrients 6 Oceanic crust 5. 6. 28. 29. 30 Olivine bomb 34, 227 Ore 170 Ore body 1 1 . 170 Ore shoot 208 Orogeny xii, 34. 98 Palaeogeography 102 Palaeontology 19. 21. 22, 26 Palaeosol 78 Palladium 2 1 7 Pangea 9 8 Permeability 234. 235 Petroleum 173, 194, 197-204 asphalt 197. 198 bitumen 197 cap 200 crude oil 197 -199 field 200 gas pool 200 maturation 199 natural gas 193. 197-199, 203 oil xi, 169. 197, 203. 204 oil pool 200 play 200 reservoir 200 trap 200 Physiography 57 Pillow lava 103 Pit 173 Plate tectonics 27-29. 30 diverging plate boundary 28, 30 convergent plate boundary 28, 29 Platinum 217 Playa lake 226 Plunge pool 72 Pluton 3 1 Point b ar 77. 78 Pore space 41
iii
Porosity 233, 234 Principle of cross-cutting relations 24 Principle of faunal succession 24 Principle of inclusions 24 Principle of superposition 24 Prior stream 76 Quarry 173 Quicklime 1 87, 1 88' Radiometric dating 26. 27 Rain shadow 240 Rampart 92, 93 Recumbant fold 36 Red-bed 123. 1 33 Relative geological time 23-25 Residual range 76 Resource 1 69 Richter scale 267, 268. 269, 270 Rift valley 39, 142. 145. 152 Rillen 74 Rillenkarren 60 Ripple marks 39, 40, 108 River terrace 69, 88 Road construction 175 Rock bolt 292 Rock correlation 43 Rock defects 290 Rock flour 62, 138 Rock outcrop 6 Rock salt 17 Rock stack 57, 91 Rock texture 14 Rock-forming minerals 10 Rockfall 271 Rocks 1 1 -19 Rocks (Engineering Geology) 1 1 Rockslide 271 , 277, 278. 298 Ropy lava 82 Salamander 1 89 Salina 78, 225 Salination 52 54 Salinity problem 52, 54, 257-26 1 Salt 225, 264 Salt marsh 49. 92, 96 Sand (indusb'ial) xi. 3, 168. 1 8 1 -1 83 Sand blowout 6 1 , 88, 95 Sand sheet 6 1 Scour charmel 90 , 96 Scours 108 Sea floor spreading 28 Sedimentary basin 39 Sedimentary bed 15 Sedimentary rocks 15-17. 41 aeolianite 85. 91, 92, 94, 163. 189 breccia 1 6 chemical I S , 17 chert 109. 218 claystone 16 conglomerate 16 detrital (clastic) 15. 16 d\Qle limestone 92 94. 1 63 greywacke 1 6 limestone 17. 20, 23, 156, 157, 1 80 marl 1 6 mudstone 16 organic 15. 17 sandstone 1 6. 1 80 shale 1 6. 109, 1 1 1 siltstone 1 6 Sedimentary strattun 15 Sedimentation 38-41 Sediment 6 continental 38. 40 estuarine 38 marine 38-40 Seismic survey 285 Seismograph 269
iv
Supplement
Separation of Australia and Antarctica 152, 153 Septic tank 303 Shear zone 36 Shield 99 Shore platform 92, 93, 95 Sill 3 1 Silver 206, 21 6 Sinkhole 60, 61, 86, 235, 282 Site inves tigation 285, 286. 297 Slaty cleavage 1 8, 19 Sluicing 1 6 8 Soil acidity 52, 54, 188 Soil classification 47-5 1 Soil colour 7 Soil compaction 52, 54 Soil consistence 8 Soil creep 27 1 . 272, 273 Soil erosion 45, 52, 53, 230, 23 1, 232, 274 Soil formation 46, 47 Soil group 8 Soil horizon 7 Soil nub'ients 46 Soil pH 50, 54 Soil profile 7, 48, 69, 76, 1 65 Soil properties 7, 8 Soil series 8 Soil structure 8 Soil subsidence 279 Soil texOJre 8 Soil types 8, 47-51 Soils: 3, 6-8 , 45, 55, 68, 69, 76, 85 alluvial 48, 5 1 alpine humus 69 black earth 76 calcareous earth 48, 50 duplex 48, 50, 5 1 . 55, 68, 69, 76, 85. 86, 88 friable earth 48. 50 gradational 48, 68 , 69, 76, 85, 1 65 humus 7 lcrasnozem 48, 50, 76 lateritic podsolic 48, 5 1 mountain 48 organic 48, 49 peat 48, 49 podSol 48, 5 1 , 79, 85 red calcareous duplex 48, 5 1 red earth (terra rossa) 85 red-brown earth 48, 51, 77 residual 6 shallow stony earth 48, 50 skeletal 48, 50 sodic duplex 47, 5 1, 55, 80 solodic 48, 5 1 stony gradational 4 8 , 6 8 , 69 transported 6 uniform 48, 49-50, 69. 165 uniform clay 48, 49 uniform loam 48, 50 uniform sand 48, 50 waterlogging 7 Solar nebula 27 Solution mining 21 1 Spit 89 Stalactite 61 , 87 Stalagmite 61. 87
Stockwork 208
Stony rise 82 Stoping 2 14 Strike 1 6 Sub-artesian bore 23 6 Subduction zone 28, 29. 30, 32 Submarine canyon 39 Submarine fan 109 Surfacing 209
Swamp 49 Swelling clay soil 278-282 Talus cone 94 Tectonic proc esses 59, 63, 64 Tectonics 27 Terracette 273 Tidal bank 90, 9S, 96 Tidal channel 9S, 96 Tidal delta 9S Tidal flat 91 Tide 89. 90 Till 1 39 Tillite 138. 139 Time scale 26 Tin 206. 2 1 6, 223. 224 Tor 59, 60, 74 Trough 39 Tuffring 34 Tumulus 82.83 TUIUlel 290-295 Turbidite 39, lOS, 109 Turbidity current 39, 108 Twin lateral streams 7 1 Unconformity 25, 1 5 1 . 155. 1 66. 167 Valley-in-valley fonn 68 Varve 139 Vein 205 Victorian Mineral Development Act 174 Volcanic bomb 34 Volcanic landform 80-85 Volcanic processes 59. 63 Volcano 4. 6, 29, 3 1 -34, 80-85 caldera collapse 32. 33, 1 30, 1 3 1 cauldron subsidence 32, 33, 130, 131 continental 32-34 island arc 30-32 lava (shield) 8 1 , 82 maar 32, 34. 8 1 , 82 scoria cone 8 1 . 82 Washover fan 94 Waste disposal 302, 303 Water cycle 230, 23 1, 237 Water diviner 237 Water pollution (groundwater) 303 Water pollution (surface) 257 Water quality and use 237, 238 Water storages (surface) 232, 233, 250252. 295-298. 299 Water supply systems in Victoria 252. 253·256 Water table 23 1 , 233, 236, 258, 260, 264. 290, 29 1. 295. 302, 303 Waterfall 72 Wave base 89 Wave refraction 89. 90 Waves 89 Weathering 10. 15. 37, 58. S9 honeycomb 144 spheroidal I S WlM 150 heavy mineral deposit 1 68, 220-223 Xenolith 1 3 1 . 1 3 2 Zinc 21 6-2 1 8
Stratigraphic and Structural Index
v
STRATIGRAPHIC AND STRUCTURAL INDEX Acheron Cauldron 1 3 1 Almurta Fault 64 Amadeus Basin 199 Angahook Member 1 66 Anglesea Member 1 66 Australian Craton 98, 99 Avoca Deep Lead 23S B alook Formation 247, 250 Bannimboola Diorite 298 Bass Basin 153, 154. 200. 201 . 204 Bassian High 153. 154 Batesford Limestone 157-159, 190. 1 9 1 Baw Baw Granodiorite 67 Baxter Sandstone 157. 245. 246, 300 B enambran Orogeny l l2. 1 14. 1 1 5 Blueys Creek Formation 108 Boisdale Formation 247. 250. 253 Bookpurnong Beds 249 . Bowning Orogeny 1 1 5, 1 1 6. 1 28 Bridgewater Formation 244, 246 Brighton Group 16, 1 57. 1 82, 246, 289. 300 Buchan Basin l l5, 1 1 7, 120-122 Buchan Caves Limestone 121 . 224 Buchan Group 121, 122, 126 Burnt Creek Lead 235 Cadell Fault 77. 1 64 Calivil Formation 243. 249, 253 Cathedral Group 1 19 Central Victorian Province 1 3 1 . 132 Cerberean Cauldron 131 Childers Formation 245, 247, 253 Chiltem Valley Lead 210 Cobaw Granodiorite 1 3 1 Cooper Basin 199 Corangamite Group 202 Cowombat Rift l IS, 1 17, 120. 128 Dargile Formation 44, 1 1 8. 294 Dartmouth Granite 298 Delamerian Highlands 106- 109. 1 1 5 Delamerian Orogeny 106. 107. 1 1 6 Digger Island Limestone 1 1 1 Dilwyn Formation 244. 249, 253 , 254 Dogs Rocks Granite 190 Dolodrook Limestone 103 Doutta Galla Silt 1 63 Dromana Granite 1 3 1 Duddo Limestone 243. 249. 253 Eaglehawk Reef 21 1 Eastern View Formation 197 Echuca Depression n Elizabeth Street Formation 294 Eromanga B asin 199 Ettrick Marl 249 Fyansford Formation 158. 190. 246 Fyansford Marl 191 Geera Clay 221 , 222 Gellibrand Marl 244. 249 Gena Clay 249 Gibsons Folly Formation 120 Gippsland Basin 153, 154, 189. 194, 196. 197. 199. 200. 201-203 Gippsland Limestone 247, 250 Glenelg River Beds 1m, 104, 109 Goldie Chert 103
Grampians Basin 1 15. 1 17. 1 22-124. 128 Grampians Group 75. 123- 125. 128 Harcourt Granodiorite 72, 1 3 1 . 180 Haunted Hill Gravel 1 55. 1 80. 196. 246. 250 Heath Hill Fault 64. 245 Heathcote Fault 37 Heathcote Greenstone Belt 102, 1 04. 106 Heytesbury Group 20 1 . 202 Humevale Siltstone 1 1 9 Ian Iue Marl 157 Iemmys Point Formation 250 Kanimblan Orogeny 1 35 Kanmantoo Group 104 Kennon Head 9 Knowsley East Fault 1 03. 105 Knowsley East Formation 103. 105 Koetong Granite 24 Lachlan Fold Belt 101. 1 35 Lake Wellington Formation 250 Lakes Entrance Formation 201. 202. 250 Latrobe Valley Coal Measures 196. 202. 250 Latrobe Valley Group 1 55. 200, 202, 247, 250 Lovely Banks Monocline 191 Lowan Sand 1 63 Lysterfield Granodiorite 186 Madame Berry Lead 210 Melbourne Trough 1 1 5-1 19, 126. 128 Mepunga Formation 244, 249 Moorabool Viaduct Formation 1 90, 191. 246, 300 Morass Creek Fault 64 Moray Street Gravels 289 Morwell Fonnation 247 Mount Easton Fault Belt 1 09 Mount Howitt Province 130, 1 3 1. 1321 35 Mount Pilot Granite 224 Mount Wellington Greenstone Belt 1 02 Muekleford Fault 73 Murphys Creek Granite 73 Murray Basin 77. 153. 154, 194, 195. 197. 201, 204. 220-223 Murrindal Limestone 122 Narrawaturk Marl 249 Newer Basalt 177. 190. 256. 280 Newer Volcanics 158. 1 62. 1 64. 236. 244. 246. 248. 249. 253. 289 Nirranda Sub-group 202 Older Basalt 177 Older Volcanics 158. 245. 246. 248. 250. 289. 294 Olney Formation 197. 249 Otway Basin 153. 154. 1 89. 1 94. 195. 197, 200, 201 . 202. 204 Otway Group 202, 248 Otway Rift 152 Parilla Sand 158. 1 68. 220-223. 225. 243. 249, 253 Pebble Point Formation 244. 249
Pember Mudstone 249 Pilbara Block 98 Point Addis Limestone 159. 166 Port Campbell Limestone 244. 249. 253 Port Phillip Su�basin 197 Port Phillip Sunkland 76, 270 Rocklands Rhyolite 124 Rowsley Fault 37, 63. 68. 269. 270 Sale Group 202 Seaspray Group 201 , 202 Selwyn Fault 76, 246. 270 Shepparton Formation 76. 77, 1 63, 243. 249 Sherbrook Group 200. 202 Sherwood Formation 244 Silverband Formation 123. 124 Snowy River Volcanics 120. 121, 167. 235 St Amaud Beds 105, 1 67 S lI'Zelecki Group 194. 202, 248 Tabberabberan Highlands 128. 130, 132. 133. 136 Tabberabberan Orogeny 1 1 6. 128, 129 Tambo Beds 44 Tambo River Fonnation 250 Taravale Formation 121 . 235 Tasman Fold Belt 98. 99 Thorpdale Volcanics 247 Torquay Sub-basin 197 Torrumbany Clay 249 Traralgon Formation 247 Tyabb Fault 76 Wagga-Omeo Metamorphic Complex 1 14, 217 Walhall a Group 126 Wangerrip Group 202 Warina Sand 243. 249 Wellington Rhyolite 272 Wentwonh Group 122 Werribee Formation 246, 294 Western Port Group 245 Western Port Sunkland 76 Wilson Creek Shale 1 19 Winnambool Formation 249 Wombat Creek Graben 297 Woods Point Dyke Swarm 129. 208, 217 Woorinen Formation 1 63 Yallock Formation 244 Yallourn Formation 247 Yarragon Fault 196 Yilgam Block 98 You Yangs Granite 1 3 1
Locality Index
vii
LO CALITY INDEX Abbotsford Aberfeldy
2 13 Accommodation Creek 216. 217 Acheron River 216 Aire River 26 1 Aireys Inlet 159. 1 66 Alberton 197. 202. 250 Albury 183 Altona 156, 197, 246 Andersons Creek 171 Andersons Inlet 49 Anglesea 17, 91, 152. 156. 166. 1 82. 197, 253 Apollo Bay 273 Apsley 253 Ararat 19. 72, 105. 167, 216. 228 Arthurs Seat 13 1, 179 Asses Ears 59 Australian Alps 66 Avoca 102. 105, 2 10, 226, 235, 248, 253 Avoca River 241 Avon River 46, 13 1, 133, 154, 155, 228 Axedale 186 Bacchus Marsh 23. 43, 45, 62, 63. 68, 71, 82. 136, 137, 139. 140, 143. 156, 167, 181. 182. 185, 186. 197. 246. 254, 269 Back Creek 21 6 Bairnsdale 129-131. 146, 153. 247 B ald Hill 43, 139, 140, 143 Ballarat � 46. 50, 70, 80. 107. 161. 168. 171. 1 85. 186. 210. 227, 228. 248. 254, 262, 282, 286 Ball arat West 207 BalHang 268, 269, 270 Barfold 72 Barham River 273 Barkly River 131 Bann ah 49 Bamawartha 253 Banabool Hills 102, 103, 142, 180 Barwon Downs 244 , 253. 254 Barwon Heads 92. 163. 253 Barwon River 76, 152. 241, 254, 275 B ass River 64 B atesford 180, 189-191 Battery Hill 133 Batts Ridges 163 Beaconsfield 4 Beaumaris 20, 246 Beechworth 67, 179, 216,
228
224. 227,
Beenak 216 Bellarine Peninsula 86. 91, 164, 246, 254 Bemm River 134 Benalla US. 129-132 Benambra 13, 64. 107, 1 14. 142, 143, 21 6. 217-219 Bendigo a" xi. 13. 16. 3 1 , 70, 73, 107. 165. 171, 1 85, 207, 208. 227. 282 Benwerrin 197 Bethanga 168, 206, 212, 216. 217
Big Desert Wilderness Area Big Desert 46. 50, 78, 79 Bindi 121
79
Birchip 220
Black Range 1 14. 123. 227 Black Rock 4. 89. 157 Blakeville 208 Blue Range 1 3 1, 132 Blue Rock Dam 25 1 , 26 1 Bogong High Plains 66, 67. 69, 256 Boisdale 247. 253 Bonang 109 Boneo 246 Bonnie Doon 129 Boolarra 225 Boulder Flat 120. 216 Bowen Range 143 Branxholme 84 Broadmeadows 18 Briagolong 247. 253 Brighton 91. 304 Brisbane Ranges 37. 270 Brocks Monument 159 Broken Creek 241 Broken River 133, 241, 25 1 Brunswick 280 Bruthen 1 55 Buchan 17, 60, 61. 121. 122. 180. 188.
217, 224
Buckleys Swamp 84 Buffalo Dam 251. 263 Buffalo River 25 1 . 263 Bulla 45 Bundoora 9, 177-179 Bung Bong 253 Buninyong 286 Bunyip River 181. 216 Burrowye 216 Buttress Point 94 Byaduk 82. 83, 84 Camels Hump 33. 34 Campaspe River 77. 1 86. 241. 25 1, 254 Campbellfield 185. 186 Camperdown 82, 249 Cann River 129. 134 Cape Howe 94 Cape Lipttap 35, 91. 1 15 Cape Otway 15. 80, 93, 268 Cape Paterson 15, 36 Cape Schanck 158, 228, 268 Cape Woolamai 179, 227 Caramut 244, 253 Cardinia Reservoir 25 1 Carisbrook 70 Carrwn 49 Cassilis 206, 216 Casterton 13, 102, 1m, 142, 143. 244.
253, 274
Castlemaine xi, 70. 171, 179, Cathedral Range 40, 1 19 Centtal Goldfields 70 Cerberean Ranges 33 Cheshmlt '1:27 Chewton 207
207, 227
Chiltem 210, 224, 228, 243. 253 Clare Creek 168 Clayton 182 Clifton Beach 94 Clonbinane 208 Club Terrace 115, 120 Clunes 171 Clyde 245 Coalville 194 Cobaw Range 59. 74, 131 Cobden 249 Cobungra 69 Codrington 249 Cohuna 49 Coimadai 270 Colac 8 1 . 82 Colbinabbin 32, 97, 102, 104 Coldstream 177 Coleraine 136, 139 Coliban 254 Coliban River 72 Collingwood 2 Colquhoun 180 Combienbar River 134 Condah Swamp 49, 82, 84 Coopers Creek 39. 217 Copes Hills 167 Cora Lyrm 245, 262 Corinella 295 Corio Bay 225 Comer Inlet 49, 92, 94, 95 Corryong 67. 1 15. 120. 128 Costerfield 206 Cowangie 226, 243. 253 Craigieburn 185 Cranboume 4. 183, 245, 246 Crayfish Bay 95 Creswick 210 Cromwell Nob 42 Cudgewa 69 Curdie Vale 249. 253 Curdies Inlet 86, 249 Curdies River 86, 244, 249 Dalmore 245. 262 Dandenong 186. 246 Dandenong Ranges 14, 3 1. 33. 131 Darebin Creek 177. 178 Darley 181, 186, 1 87 Darlot Creek 83. 84 Darnwn 247 Darraweit Guim 47 Dartmouth Dam 69. 1 15, 120, 251. 254, 297. 298 Daylesford 70, 72. 82, 228, 248. 256 Deans Marsh 197 Deddick 216. 217 Delatite River 133. 232 Derrinal 62, 136, 137 Derrinallum 81. 82 Digger Island 107, 11 0, 1 1 1 Dights Falls 2 Dirnboola 243 Dingley 3. 182 Dinosaur Cove 142, 149
viii
Supplement
Dolodrook River 103 Dookie 13, 102 Double Bull Creek 2 1 6, 217 Dromana 177. 179 Drouin 88 Dnmg South 220 Drysdale 253 Dundas Tableland 47. 48, 5 1 , 76, 102 Dunkeld 74. 244, 253 Dunn's Rock 63 Dunolly 108, 235 Dutson Downs 250 East Barwon River 276 Behuca 77, 1 63, 164. 243, 249, 269 Eddington 70 Edenhope 79. 249 Edi 227 Eildon 15. 33. 1 15. 1 19. 126. 1 3 1 . 232, 268 Eldorado 210, 21 6. 224 Elmore 243, 253 Ensay 1 14 Euroa 1 3 1 . 186 Ewing Marsh 87 Fairfield 178 Falls Creek 1 08 Flaxmans Hill 94 Flinders 76. 1 58. 227 Footscray 1 79, 280 Forrest 275 Foster 1 15. 208 Fosterville 206, 21 1. 212 Frankston 246 Fryerstown 227 Fyans Creek 25 1 Gable End 272 Gabo Island 1 68, 1 80 Gaffneys Creek 129, 206, 208 Geelong 76, 1 52, 157, 158. 1 89-192, 225, 244. 246, 253. 254. 268 Gelantipy 68 Gellibrand River 86, 244 Gelliondale 197, 202 Genoa River 20, 21, 134 Gibbo River 120 Gisbome 70 Glen Wills 206, 216 Glenelg River 85, 103, 144. 152. 157. 24 1, 249, 25 1 Glerunaggie 68 Glenmaggie Reservoir 69, 25 1 Glenrowan 136 Glenthompson 102, 103, 1 85, 248, 253 Goodmans Creek 71. 270 Goon Nure 250 Gordon 248, 253 Goroke 79, 243. 253 Goulbum River 66, 69, 77, 209, 232. 24 1 , 248. 25 1. 254 Grampians (The) 17. 33. 47. 59, 74. 75 , 1 14, 1 1 5. 122, 124, 125. 1 34. 1 67. 240-242. 248, 2S5 Grantville 64. 1 82 Granya 205 Great Dividing Range 66, 99 Green Valley Range 73 Greensborough 37 Greenvale Dam 25 1 Guildford 2 1 0 Guildford Plateau 70 Gunbower 262 Hallam 186, 187 Halls Gap 74, 75 Hamilton 76, 8 1 , 82. 83, 85, 1 14, 1 64. 244. 248 Hampton 300 Hanging Rock 13, 33. 34, 63, 1 59
Harcourt SO, 72. 73 Harman Creek 82. 83 Harrietville 67. 2 1 0 Harrow 253 Hastings 76 Hattah 226 Hayes Hill 12. 178 Heath Point 253 Heathcote 13. 32. 37. 44. 59, 62. 70. 97. 102, 1 03, 1 05. 1 65 Heatherton 182 Heathfield 253 Heathmere 249 Hepburn Springs 248 Heytesbury 86 Heywood 49. 1 64. 244. 253 Hoddle Range 76 Homerton Swamp 84 Hopkins River 1 14, 241 , 249 Hoppers Crossing 293 Horsham 79, 103. 1 68. 220. 221, 243, 254 Hospital Creek 87 Howqua 102 Howqua River 103 Hume Reservoir 19. 69, 25 1 , 254, 256, 297 Indented Head 253 Inglewood 74 Inverloch 94, 144. 269 Jack Smith Lake 95 Jacksons Creek 45, 84 Johanna Beach 144 Kadnook 136 Kaniva 243. 249, 253 Kate Kearney Entrance 95 Katunga 243, 253 Keilor 280 Kerang 1 63. 225, 243. 249. 258, 259 Kiata 79 Kiewa hydro-electric scheme 256 Kiewa River 24 1 . 248, 254 Kilcunda 9 1 , 146, 194 Killarney 249 Kilmore 179, 226. 248 Kilmore Gap 66 Kilsyth 177 King River 25 1 IGnglake 1 1 8. 248 Kingston (S. Aust.) 268 Knowsley 63 Koetong 69, 128. 2 1 6 Koo-wee-rup 49, 88 Koo-wee-rup Swamp 49. 5 1 Koonwarr a 142. 147, 148, 167 Korkuperrimul Creek 139 Koroh 46, 63, 244, 253 Konunburra 91, 194, 202 Kow Swamp 1 63 Kyabram 77 Kyneton 179 Laanecoorie Reservoir 70. 73 Labertouche 183 Lake Bellfield 2S 1 Lake Boga 227, 228, 255, 259 Lake Bolac 248, 253 Lake Bullen Merri 249 Lake Cairn Curran 70. 25 1 Lake Colongulac 249 Lake Corangarnite 8 1 . 83, 85 Lake Cullulleraine 256 Lake Eildon 69, 129. 232, 25 1 . 254 Lake Elizabeth 275, 276 Lake Eppalock 62, 63, 1 37. 25 1 Lake Gillear 94 Lake Gnoruk 249 Lake Keilambete 82
Lake Merrimu 2S 1 Lake Mountain 129 Lake Omeo 64 Lake Punumbete 82 Lake Reeve 94 Lake Surprise 83 Lake Tali Karng 272 Lake Terang 82 Lake Tyers 87. 250 Lake Tyrrell 225, 249. 264 Lake Victoria 250 Lake Wartook 75 Lake Wellington 88, 250 Lake William Hovell 25 1 Lakes Entrance 87, 95, 166, 20 1 , 202, 250. 300 Lal La1 1 5 6, 185, 186 La1 La! Falls 72 La! La! Reservoir 25 1 Lancefield 32, 70, 74, 102, 103. 1 06, 248, 253 Lang Lang 182. 183. 245, 253 Langi Ghiran 72 Langwarrin 183 Lara 246 Latrobe River 49. 88. 216, 241 Latrobe Valley 76, 88, 1 53. 155, 1 93 , 195, 197, 254 Lawloit 79 Leannonth 248, 253 Leongatha 76, 91, 148, 158, 248 Leopold 253 Lerderderg Ranges 270 Lerderderg River 45, 1 36. 137, 1 39 Lethbridge 179 Licol&, 103, 272 Lillicur 226 Lillimur 243. 252 Lilydale 17. 1 1 9, 188, 1 89 Limestone Creek 120. 1 68, 1 89 Lindenow 6, 46 Linton 226 Lismore 244. 253 Little Desert 46. 50, 78, 79 Little River Falls 121 Little River Gorge 1 2 1 Loc h Ard Gorge 87, 90 Loch Sport 88 Loddon River 70, 7 1 , 73, 154, 210, 24 1 . 248, 25 1 . 254 London Bridge 38, 93 Long Hill 1 33 Longford 202 Longwarry 245 Lome 93, 268, 277 Lower Glenelg National Park 85 Loy Yang 196, 202, 247 Lyndhurst 246 Lysterfield 1 8, 177 Macalister River 46, 68, 131, 133, 241 . 25 1 . 254 Macarthur 49. 82, 84 Macedon Ranges 242 Maddingley 197 Maffra 46, 69 Magdala gold mine. Stawell 212-2 1 5 Maldon 70, 72, 73, 206, 2 10, 21 1 Mallacoota 36, 9 1 , 1 08, 1 10, 1 15, 1 3 1 , 1 66 Mallee 46, 50, 53, 76, 78-80, 243, 255, 259 Malmsbury 179 Mansfield 40, 69, 70, 13 1 , 133. '127 Maramingo Creek 20 Maribymong River 55, 1 63. 241 Marlo 87. 88, 108 Maroondah Dam 25 1
Locality Index
Maryborough 70, 72, 1 6 1 , 248 Marysville 33 Maryvale 1 88 Matlock 127, 227 McKenzie Fall s 75 McKenzie River 75 Melbourne Underground Rail Loop 293-295 Melbourne 1. 4, 55, 80, 1 6 1 . 1 63. 246. 295 Melton 8 1 . 82. 279. 280. 281 Melton Reservoir 25 1 Melville Caves 74 Mentone 246, 300-302 Merbein 254 Merino 244. 253 Merino Tableland 76 Memda 178 Merri Creek 178 Merriang Hills 14 Merrimans Creek 189 Metcalfe 72 Merung 250 Midlands 70. 73 Mildura 243. 256 Miram 243. 253 Mirboo North 225. 268 Mitchell River 46, 1 15, 120, 1 3 1 , 152, 241. 247 Mitta Mitta 2 1 6, 224 Mitta Mitta River 1 15. 120. 210, 24 1 . 248. 25 1, 254. 297 Moe 247 Mokoan Reservoir 25 1 Moliagul 73 Monbullc 46. 50, 248 Moolort corridor 70 Moonlight Head 228 Moorabbin 86 Moorabool River 192. 254 Moralla 227 Morass Creek 64 Mordialloc 246 Momington 9 1 . 157, 158, 268 , 270 Mornington Peninsula 76, 91, 108, l I S, 1 3 1. 145, 1 5 8, 1 83. 197. 246, 270 Moroka 69 Mortlake 82, 244. 253 Morwell 1 96, 202, 247. 250 Mount Abrupt 74 Mount AJexander 13, 50, 72, 73, 1 7 1 . 1 80 Mount Arapiles 1 14, 123 Mount Ararat 72 Mount Bainbridge 82 Mount Battery 69 Mount Baw B aw 13. 67 Mount Blackwood 82 Mount Bogong 66, 107, 108, 1 67 Mount Buffalo 1 3 , 14, 57, 59, 67. 74 Mount BuUengarook 71, 270 Mount Buller 66, 67, 273 Mount Burro wa 120 Mount Camel Range 59, 97, 103, 104 Mount Cobberas 33, 66, 120 Mount Cobbler 1 29 Mount Cole 72 Mount Cope 49. 66 Mount Cotteril 8 1 , 82 Mount Dandenong 13 Mount Donna Buang 14, 129 Mount Dundas 1 14, 123 Mount Ecc les 8� 83. 84. 1 64 Mount Egerton 248, 253 Mount Elephant 8 1 . 82 Mount Emu Creek 241 Mount Fainter South 66
Mount Feathertop 66, 268 Mount Franklin 82 Mount Fraser 34 Mount Gambier (S. Aust.) 164. 180. 1 89, 237, 244 Mount Hamilton 82 Mount Hotham 66. 268 Mount Howitt 129 Mount Johnson 1 20 Mount Korong 72 Mount Kosciusko (N.S.W.) 66 Mount Leinster 143 Mount Leura 82 Mount Ligar 133 Mount Loch 66 Mount Macedon 13, 14. 33, 129. 13 1 , 1 59. 253 Mount McKay 66 Mount Moliagul 72, 73 Mount Moolort 70 Mount Napier 8 1 , 82. 83, 84. 1 64 Mount Nelse North 66 Mount Niggerhead 66 Mount Noorat 34, 82, 227 Mount Nungatta 134 Mount Rosea 1 23 Mount Rouse 1 64 Mount Schank ( S . Aust) 1 64 Mount Shadwell 82 Mount Spion Kopje 66 Mount Stavely 13 Mount Tarren gower 72. 73 Mount Timbertop 129. 13 1 . 132 Mount Victory Range 75 Mount Wellington 103. 272 Mount William (near Lancefield) 106 Mount William (The Grampians) 70, 74, 125 Mount Wills 224 Moyston 248, 253 Murphys Creek 72, 73 Murray River 39. 49, 76. 77, 152. 1 62164. 1 8 1 . 1 83, 239, 241 . 248, 25 1 , 254-256 , 260, 262. 297 Murrayville 243, 249, 252 Murrindal 235 Murrindal River 61 Murrungower 1 3 Murtoa 243, 254 Nagambie 206, 2 1 1, 212, 252 Nelson 94, 152. 153. 1 64, 249 Nepean Peninsula 76. 9 1 , 246 Nhill 79. 243, 252, 253 Nillahcootie Reservoir 251 Ninety Mile Beach 58, 94. 95. 1 66 Nowa Nowa 13. 107, 109. 1 1 5 . 120. 1 68 , 224 Nyah 254 Nypo 226 O'Shannassy Reservoir 1 5 , 25 1 O'Shannassy River 25 1 Oaklands Junction 174 Ocean Grove 253 Olney 249 Omeo 19. 1 20, 143 �bost 6, 46. 1 30, 247 �gan Pipes National Park 3. 82. 84 Otway coast 9 1 , 92. 95. 1 44. 1 5 1 Otway Range 34, 36, 40, 46. 50, 76. 86, 142, 144, 149, ISO, 1 65, 197. 240. 241 , 242. 244. 248, 261 . 272, 274 Outtrim 194 Ouyen 249 Ovens River 69. 210. 241 . 248, 254 Pakenham 1 77 Paradise Falls 132. 1 67
ix
Parwan Valley 185, 1 86 Pearcedale 4 Penhurst 1 64. 244. 253 Penola (S. Aust) 204 Peterborough 86. 94. 244. 253 Phillip Island 9. 91, 158. 227, 228 Pine Hills 243. 253 Pink Lakes Nationa! Park 79 Pittong 1 85 Pleasant Creek 2 12 Plenty River 25 1 Point Henry 173, 197, 225 Point Hicks 1 66 Point Londsdale 253 Point Ricardo 87 Port Campbell 17, 57, 86. 87. 90. 9 1 . 93. 94. 1 5 2, 153. 156. 204, 228. 244. 249, 253 Port Fairy 82, 244, 253 Port Melbourne 288 Port Phillip Bay 58, 64, 85, 86, 89, 225. 270, 301 Portarling ton 253 Portland 80, 9 1 . 1 63 , 1 64, 173. 1 83, 189, 225, 229. 244 , 252, 253, 254, 300 Powlett River 91 Princetown 9 1 . 249 Pykes Creek Reservoir 25 1 Pykes Creek 25 1 Pyramid Hill '127 Pyrenees Ranges 1 9, 70. 105 Pyrites Creek 7 1, 25 1 Queenscliff 49. 253 Raak Plain 226 Rams Hom 59 Red Bluff 79, 87 Red Cliffs 254 Red Rock 8 1 Redesdale 226 Reedy Creek 120. 210, 224, 228 Reedy Lake 252 Ringwood 3. 206 Robinvale 243, 254 Rochester 37, 77, 102, 1 04, 1 05 Rocklands Reservoir 1 14, 1 1 5, 124, 25 1 Rocky Camp 122. 188 Rocky Valley Reservoir 256 Romsey 35, 82. 103 Rosedale 197, 202, 247, 250 Royal Park 25 Rushworth 1 15, 128 Rutherglen 210. 216, 224 Sailors Falls 72 Sale 94, 247, 250, 252. 253 San Remo 15. 36, 9 1 , 144. 265 Sandford 244, 253 Sandringham 4. 300 Scarsdale 168 Sea Lake 225. 264 Seacombe 250 Seaspray 250 Serra Range 74, 123 Seymour 67, 164 Shepparton 186 Silvan 248. 262 Silvan Reservoir 251 Skipton 82. 253 Snowy River 33, 46. 49, 1 15, 120, 121, 154. 1 62, 1 67, 217, 228, 239, 240, 241 Sorrento 1 63. 295 South Buchan 1 80 South Gippsland Ranges 76 Spotswood 288 Springvale 3, 182
x
Supplement
St Arnaud 70, 206, 210, 243. 268 St Helena 1 6 St Kil da 40 S1 Margaret Island 9S Stawell 19, 102, 105, 1 68, 1 85, 206.
21 1. 212-21S
Stockyard Hill 244, 253 Stonyford 82 Stradbroke 197. 202 Stratford ISS S trathbogie Ranges 68, 131. 227 Strathmerton 243, 253 Streatham 244, 253 Strzelecki Ranges 40, 46, 50. 76. 142.
144. 150, 196.. 225. 240, 248, 248, 26 1, 274 Studley Park � 4, 36, 1 1 8 Sugarloaf Dam 25 1 Sunset Country 50 Surveyors Creek 216 Swan Hill 1 15. 186. 25S Sydenham 3, 82, 84 Tabberabbera 1 15. 122 Tallandoon 224 Tallangalook 227 Tallangatta 1 8. 128. 224 Tambo River 67. 1 1 5. 120. 121. 219. 241 Tanjil 208 Tanjil River 25 1 Taradale 207 Tarago Reservoir 251 Tarago River 25 1 Tamagulla 72. 73. 1 80, 210 Tarwin Lower 49 Terang 34, 82, 227 The Brothers 143 The Gable 15 1 The Gurdies 182 The Potholes 60 The Pyramids 61 The Sentinels 272 The Sisters 143 The Stranger (erratic) 62. 137 The Terrace 74 The Twelve Apostles 57 Thomson Reservoir 251. 261 , 296 Thomson River 46, 216. 241. 25 1 , 296 ThoqxbUe 50, 76, 158. 247, 248 Timboon 244, 249, 253 Toolondo Reservoir 251 Toongabbie 1 1 6 Toora 216, 224 Tooradin 96 Torquay 17, 61, 91, 152, 153. 156. 157. 159, 253 Tomunban:y 254 Tower Hill � 14, 63 Trafalgar 247, 253 Traralgon 1 85, 188, 247, 250 Trentham 46, 50, 70, 72, 248. 253 Trentham Falls 72 Tullaroop Creek 70, 25 1 Tullaroop Reservoir 70. 25 1 Tulloch Ard Gorge 121 Turpins Falls 72 Tyers 1 1 6. 188 Tyers River 146 Tyrendarra 84 Upper Yarra Dam 69, 25 1, 296 Victoria Range 1 23 VVtihalla 3 1, 1 15, 129, 208, 217, 296 Walkerville South 9 1 , 103, 1 10. 1 88 Wallacedale 82. 83 Wallan 35, 83, 1 18 Walwa 224 Wandin 248, 262
Wando River 103 Wannon River 144, 241 Waranga Reservoir 251 Waratab Bay 9 1, 103, 1 10. 1 1 6, 188 Warburton 1 3 , 127, 13 1 Warr agul 46. 50, 76. 88. 248 Warragul South 1 85 Warrandyte xi, 3 . 1 7 1 VVarrnambool 82. 91. 163, 1 80. 189,
237, 244, 249. 253, 254, 268. 269, 300 VVartook Reservoir 75 Watts River 25 1 VVaubra 248, 253 Waum Ponds 1 89 Wedderburn 72 Welshpool 250, 268 Wennicott Creek 143 Wensleydale 197 Wentworth River 1 1 5, 120. 122 Werribee 225, 246, 254 Werribee Gorge 62, 68. 136, 139, 141 Werribee Plain 240, 246 Werribee River 68. 1 67, 246, 25 1 , 254. 262 West Barwon Reservoir 25 1 West Barwon River 25 1 West Moorabool River 25 1 Western Port 64. 88, 92, 96 Westgate Bridge 288-290 Whitfield 132, 1 67 Whittlebury Swamp 82, 84 Whittlesea 83 Wickliffe 1 14. 248, 253 Willaura 248. 253 Willow Grove 247 Wilsons Promontory 12, 13. 14, 59. 60, 76, 89. 9 1 . 1 1 5. 132, 183, 216. 240 Wimmera 46, 49, 5 1 , 76. 78-80. 243, 255 Wimmera River 241 Winchelsea 253] Windy Point 277. 278 Wodonga 67. 1 14. 130, 136, 1 68. 1 8 I . 183. 186 Wonderland Range 74. 123 Wonnangatta 69 Wonthaggi 146, 194, 202 Woodend 63. 226. 248, 253 Woods Point 13. 3 1 , 129, 208, 209 Woodside Beach 88, 250 Woolshed Valley 228 Woolsthorpe Swamp 84 Wulgulmerang 121 Wurruk 247, 253 Wyperfield National Park 78. 79 Yackandandah 210 Yallock 245 Yalloum 1 96. 202 Yalmy River 120 Yan Yean Reservoir 250. 25 1 , 295 Yanakie 189 Yandoit 143 Y8IUlathan 245 Yarra River 3, 1 19, 164, 241. 25 1 . 270, 288. 295, 296 Yarragon 247 Yarram 247 Yarraville 283. 284 Yea 127 You Yangs 50, 1 3 1. 1 83