Regional guide to Victorian geology (Publication - School of Geology, University of Melbourne ; no. 1) [2 ed.] 095990610X, 9780959906103
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RegionalGuideto Vi&orianGeology
EditedbyJ, McAndrew and M.A,H.Marsden
Regional Guide to Victorian Geology Edited by
John MCAndrew Division of Mineralogy, CS I R 0 and
Marcus A.H.Marsden School of Geology, University of Melbourne
. School of Geology, University of Melbourne 1973 Second Edition
First published
1968
Second edition 1973 Wholly set up in Australia by the editors.
Cover design by Stuart Crluth, Education Technology Section, University of Melbourne.
Printed, collated and bound
in
Australia by
Hollyoak Manufacturing Company, Melbourne, for the School of Geology, University of Melbourne.
Apart from any fair dealing for the purpose of private study, research criticism, or review, as perrnitted under the Copyright Act, no part may be reproduced in any manner whatsoever without the written perrnission of theeditors.
This book is copyright.
W
,_
© John McAndrew and Marcus A, H. Marsden 1968, 1973.
School of Geology, University of Melbourne, Publication No. I.
Registered in Australia for transrnission by post as a book. ISBN 0 9599061
O
X
Available from the School of Geolo8Y» University of Melbourne Parkville, Victoria 3052, Australia.
ii
PREFACE This volume was originally prepared as the handbook for the nineteen geology excursions forming part of the 39th Congress of the Australian and New Zealand Association for the Advancement of Science, held at Melbourne in January, 1967. It provided an overall review of the geology of Victoria, and a new account of the geology of important areas other than the Grampians for which an excellent description had been published as Memoir Z5 of the Geological Survey of Victoria. The first edition of the "Regional Guide to Victorian Geology" was a revised version of the handbook. Continuing demand has led us to rearrange and revise the Regional Guide, as well as to enlarge it by the addition of a new chapter on the geology of the Melbourne district. In addition to the acknowledgements recorded in the preface to the original Excursions Handbook we are pleased to acknowledge the continued support of the School of Geology, University of Melbourne, and the Commonwealth Scientific and Industrial Research Organisation. The School of Metallurgy of the University of Melbourne and the Geological Survey of Victoria cooperated in the preparation of the Second Edition. We also acknowledge with thanks the valuable assistance received from Mesdames L. Holding, R. Howe and J.E. Richardson, Misses T. Sapountiz and N. Stewart, and Messrs D. Campbell and G. Ouick. The illustrations from early publications of the Geological Survey of Victoria are reproduced with kind permission of the Director. The photograph on p. viii is of portion of the relief model of Australia prepared in the School of Geology, University of Melbourne, under the direction of Professor E. S. Hills.
John McAndrew M.
A
.
H.
Marsden
University of Melbourne April, 1973.
iii
PREFACE TO ORIGINAL HANDBCOK This Handbook has been prepared as a guide to the Section C excursions held during the 39th Congress of ANZAAS at Melbourne in January, 1967. It provides a new account of the geology of the excursion areas and thus of most of the geologically important areas in Victoria The Handbook has been made possible through donations towards the cost of publication by a number of companies and we acknowledge with thanks the generosity of
Mount Morgan Ltd.
Aberfoyle Tin N. L. Anaconda Aust. Inc.
Anglo American Corporation (Australia) Ltd. Australian Selection (Pty.) Ltd. Broken Hill South Ltd. Consolidated Gold Fields (Australia) Pty. Ltd. Conzinc Riotinto of Australia Ltd. Esso Exploration Australia Inc. Electrolytic Zinc Company of Australasia Ltd. International Nickel (Australasia) Pty. Ltd. Kennecott Exploration (Australia) Pty. Ltd. The Mount Lyell Mining and Railway Co. Ltd.
North Broken Hill Ltd. Peko-Wallsend Investments Ltd. Sydney Smelting Co. Pty. Ltd. The Broken Hill Proprietary Co. Ltd. The Electrolytic Refining and Smelting Company of Australia Ltd. United States Metals Refining Company. United Uranium N. L. Utah Development Co. We stern Mining Corporation Ltd..
also acknowledge the support afforded individual authors by their employing organof Mineral Resources, Commonwealth Scientific and Industrial Research Organization, Geological Survey of Victoria, State Electricity Commission, The Broken Hill Proprietary Co. Ltd. , and the University of Melbourne, in the preparation of their contributions. We
isations, the Bureau
The coloured geological map of Victoria has been donated by the Department of Mines of Victoria. Through kind permission of the Australasian Institute of Mining and Metallurgy, the 'introductory paper has been reproduced, with modifications, from "Geology of Australian Ore Deposits", Second Edition, 1965. The Royal Society of Victoria has kindly permitted reproduction of figures from its Proceedings.
This volume has been prepared with the unstinted and able assistance of a number of persons which is gratefully acknowledged, in particular Miss H. Hawkins, who typed nearly all the copy, Mesdames E.A. Marsden, M.E.M. McAndrew, and F.M. Tattam, Misses P. Carolan and E. Spry, Messrs. F. Canavan, A.C. Frostick and D.J. Taylor, and Dr. J.J. Jenkin.
J. McAndrew M. A. H. Marsden Editors
O.
P. Singleton Chairman, Section
8
iv
January,
1967
C
To many, "conservation" applies to animals and plants or the environment around them, but how many readers regard the geological features of the environment as worthy of conservation? A moments reflection will show that geological features are more vulnerable, as once despoiled they are not regenerated as are living organisms. In some overseas countries, notably Britain and the United States many geological features such as fossil and mineral localities have been ruined by overzealous collectors, and public access to many such sites is now restricted. Localities have been overwhelmed by hammer-bearing groups of students and club members, and literally hacked to pieces and carried away by persons with no thought for the future. With a few outstanding exceptions this situation has not yet arisen in Victoria. However an increasing number of important geological sites have in recent years been seriously degraded. Because of the upsurge of interest in geology more people are visiting and examining the classical geological areas of Victoria for which this book serves as a guide. If you are a teacher or excursion leader please ensure that your students do not hammer or dig outcrops irresponsibly or collect specimens which will be later thrown away. Remember that since many fossils are scarce and often scientifically valuable they should be shown to a professional palaeontologist at the Geological Survey, the National Museum or one of the Universities. Likewise, many minerals, particularly well-crystallized specimens, are rare, and should be shown to a professional petrologist or mineralogist. In this way you may contribute something to the science of geology, as many have in the past. I appeal to all who use this book whether teacher, student, professional or amateur to ensure that Victorian geological sites and areas remain unspoiled for the enjoyment and education of those who will come after us.
Thomas A. Darragh, Convenor, Geological Conservation Sub-committee, Victorian Division, Geological Society of Australia.
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THE
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Chapter numbers shown-
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“
REGIONAL GUIDE TO VICTORIAN GEOLOGY
CONTENTS
Preface __ ._ ._ ._ ._ Photograph of relief model Geological map of Victoria Outline of the geology and physiography of Victoria _
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2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15.
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O. P, Singleton Geology of the Melbourne district A. H. M. VandenBerg, with contributions by M. A. H. Marsden and J. McA_ndrew Quaternary sediments of the Maribyrnong River, Keilor E- D- Gill Devonian rocks of Lilydale Editorial C0I1'€1’ib‘~1ti0I1 The Dandenong Ranges Igneous Complex O. P. Singleton _
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Geology of the Mornington Peninsula V.A. Gostin Geology of the Geelong district D- SPSIICSI'-J0neS Geology of the Bacchus Marsh district O. P. Singleton Stratigraphy and structure of the Palaeozoic of west-central Victoria J. A. Talent and D, E. _Thomas The Castlemaine-Chewton Goldfield F. C. Beavis and J. McA.ndrew Geology and petrology of the Macedon district O. P. Singleton Geomorphology of the Western District volcanic plains. lakesand coastline C. D. Ollier and E. B. Joyce Mesozoic and Tertiary stratigraphy of the Otway region O. P. Singleton Geology of South Gippsland O. P. Singleton The brown coals of the Latrobe Valley C. S. Gloe _
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Late Cainozoic geology and geomorphology of south-east Gippsland 17. Geology of East Gippsland ._ ._ ._ _
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Palaeozoic evolution of east-central Victoria 19. Palaeozoic metamorphism and igneous activity 18.
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The Riverine Plain in northern Victoria
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J. J. Jenkin J.A.. Talent M.A.. H. Marsden north-east Victoria _
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iii viii
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14 31 33
38 46 53 59
65 83
90 100 114 129 139 145
158 175
M. D. Leggo and F. C. Beavis
202
J. M. Bowler and P. G. Macumber
213
CHAPTER
l
1
OUTLINE OF THE GEOLOGY AND PHYSIOGRAPHY OF VICTORIA1 v 1__
bY
O.
P. Singleton
INTRODUCTION.
Victoria, of 87, 884 sq. miles, lies astride the Palaeozoic mobile belt of eastern Australia towards its southern end. Nowhere is the Precambrian basement exposed. Its Palaeozoic geological history was one of tectonic instability induced by primary forces directed from an easterly direction, which initiated deep depositional troughs. Progressive stabilization from the late Ordovician onwards deformed the sedimentary filling along generally meridional trends and was finally completed during the Carboniferous. A different, milder tectonic regime, producing an east-west orientation of the younger rocks, was established during the Mesozoic and has persisted to the present day as witnessed by Quaternary movements and the continued existence of Bass Strait and the highlands of Victoria. Rocks of known Cambrian age outcrop as the cores of a number of narrow, predominantly meridional belts whose boundaries are normally high-angle reverse faults. These and other "axes" played a fundamental tectonic role through the Palaeozoic as structures which were persistently but intermittently anticlinal in nature (Fig. 1). The internal structure of the axes is very complex in marked contrast to the simple though intense structures developed in the surrounding rocks. Granitic rocks of at least six definite ages occur. Certain groups of intrusives, for example, the muscovite granites of north-we stern Victoria, can be recognized, but the age of only a few can be reliably estimated. Many other granitic bodies, particularly in the east and west of the State, are of unknown age. Individual dyke suites such as the Wood's Point swarm have been accurately dated but there are many others whose age are quite unknown, some being associated with mineralization. CAMBRIAN.
Lower Cambrian volcanics During early Cambrian time a thick sequence of basic lavas, in part at least submarine, was extruded accompanied by pyroclastics, lenticular cherts, and minor intrusives (Thomas and Singleton, 1956). Originally a suite of olivine-poor basalts and augite andesites, these greenstones have undergone low-grade dynamic metamorphism and metasomatism, and are now spilitic (Tattam, personal communication). Feldspars have been universally albitized, pyroxenes frequently replaced by chlorite and,or actinolite, and secondary minerals such as epidote, clinozoisite, and stilpnomelane developed. Locally bodies of serpentine rock have been formed. Associated minor feldspathic intrusions include diorites and albite porphyrites, with small bodies of albitized micro-granite at Heathcote. At Heathcote small intrusions of pyroxenite have been converted to talc rock containing
small veins of magnesite. Middle and Upper Cambrian sediments. The greenstones are followed by Middle to Upper Cambrian sediments characteristically without detrital quartz and granitic' accessory minerals. At Heathcote the laminated neritic Knowsley East Shales include beds of sandstone and at least one band of conglomerate, all derived from the greenstones. Besides a "dendroid" fauna, there are two Middle Cambrian trilobite assemblages which, being only 100 ft. apart, indicate some condensation in sedimentation. At Lancefield contemporaneous dark pyritic shales also contain "dendroids". In both areas the younger Cambrian is represented by some 2000 ft. of the unfossiliferous Goldie Shales which at Lancefield pass conformably up into the Ordovician. At the Dolodrook River, inte rbedded with tuffaceous rocks, is the condensed Dolodrook Limestone with trilobite-brachiopod assemblages spanning several zones of the late Middle and early Upper Cambrian. North-we st of this, at the Howqua River and Tatong on the same Mt. Wellington Axis, cherts and dark shales separate the greenstones and Ordovician sediments. 1Modified from “GEOLOGY OF AUSTRALIAN ORE DEPOSITS", Second Edition, 1965, pp. 440-449 (Eighth Commonwealth Mining and Metallurgical Congress, Melbourne).
2
Elsewhere faulting brings the greenstones into juxtaposition with post-Cambrian rocks. The lack of Cambrian outcrops outside the axes is unfortunate because, from internal evidence alone, the existence of geosynclinal conditions in central Victoria during the Cambrian cannot be demonstrated conclusively. In western Victoria there is contrasting evidence of an earlier influx of normal terrigenous detritus from a crystalline terrain, shown at Mt. Stavely, south-east of the Grampians, by interbedding of low-rank greywacke and shales with tuffaceous rocks of the presumed Cambrian greenstone suite. GRDOVICIAN. By the Ordovician, g_eosynclinal conditions were firmly established throughout the State. In central Victoria a flood of quartzose detritus entered at the beginning of the Ordovician, building up a very thick monotonous alternation of low- rank greywackes, shales and slates deposited under
anaerobic bathyal conditions. Individual lenticular greywacke beds, overall fine to medium grained and characteristically graded, are interbedded with dark shales frequently containing sedimentary pyrite., True black graptolite shales are normally absent. Hills and Thomas (1954) have shown the greywackes to be turbidity current deposits, with the dark shales accumulating during intervening quiescent periods. The greywackes contain some chert grains, soda feldspar, muscovite and accessories such as zircon and tourmaline. Volcanicity was completely absent. Although benthonic forms are virtually absent, the very rich and almost complete sequence of Ordovician graptolite faunas in central Victoria makes possible one of the most detailed zonal subdivisions in the world (Table 1). TABLE
Early Ordovician (TremadocianArenigian)
l.
Faunal stages and correlation of the Ordovician (Harris and Thomas, 1938). Yapeenian
Castlemainian Chewtonian Bendigonian
Middle
Ordovician
(LlanvirnianLlandeilian)
Darriwilian
Late Ordovician (Caradocian-
Bolindian Eastonian Gisbornian
Ashgillian) Lancefieldian Although .extremely uniform and monotonous, the sequence varies in both thickness and lithology. Hills and Thomas (1954) estimated 12, OOO ft. of Lower to Middle Ordovician at Bendigo and 12, O00 ft. of Lower to Upper Ordovician at Lancefield, but conspicuously less on the Mornington Peninsula. Individual zones may also vary greatly in thickness. In slowly deposited sequences, chert bands become conspicuous. The relative proportions of lithologies vary in different parts of the sequence. The Lancefieldian contains thick greywacke sequences and only rare intercalations of shale; the Darriwilian contains a high proportion of shale. Such variations, and the consequent behaviour of the rocks during deformation," have had a considerable influence on the structural control of quartz reefs, and at times on the localization of gold. Regional developments of the Ordovician. Between the Heathcote Axis and a line from Ballarat to Wedderburn, the sequence is predominantly Lancefieldian to Darriwilian. It is known in considerable detail (Thomas, 1939), the major structures having been mapped using graptolite stages and zones. The tight, closely spaced folds are arranged in dome-like and basin-like anticlinoria and synclinoria. Individual folds are frequently offset by one or more reverse faults which are bedded on one limb, but cross-cutting on the other, for example in the Bendigo and Chewton goldfields. These major structures are cut by a number of large meridional high-angle reverse faults with westerly hade, for example the Whitelaw Fault which forms the eastern limit of the Bendigo goldfield. South of Lancefield, the only Upper Ordovician on the western side of the Heathcote Axis is largely grits and sandstones with a fragmentary neritic fauna (Riddell Grits), indicative of marine regression from western Victoria towards the end of the Ordovician. Towards the Ballarat-Wedderburn line, Lancefieldian outcrops over a wide belt, and at Ballarat 10, OOO ft. of beds occur beneath known Upper Lancefieldian. Further westwards the bedrock sediunknown. ments are similar lithologically to those in central Victoria, but fossils are entirely that part at least are are indications in age, there to be Early Ordovician Although usually presumed probably Cambrian. The prominence of slaty cleavage and fissuring suggests greater compression than in central Victoria. On the Cflenelg River near the South Australian border, the sediments are
3
somewhat calcareous, including a thin limestone, and are intruded by basic dykes which have been subjected to the same metamorphism as the sediments (Wells, 1956). Between the Heathcote and Mt. Wellington Axes, Ordovician protrudes through the blanket of Silurian-Devonian only in the Mornington Peninsula anticlinorium, and in association with intervening major axes (Ch. 18). On the latter, the sequences are predominantly frequently in narrow fault slices and fragmentary. The Upper Ordovician is phosphate rock in dark pyritic shale. Of special significance are the occurrences of granular and of on the Howqua River, and in Darriwilian Hill (Mansfield), Lancefieldian at Phosphate Lancefieldian limestone with a trilobite-brachiopod fauna, faulted against Cambrian greenstones at Waratah Bay. These suggest to the writer that- the Ordovician on the axes was deposited on intermittently rising “anticlinal“ structures with subsequent of preservation by comparatively minor faulting. This best explains the evidence slow deposition and presence of neritic limestone. Eastwards from the Mt. Wellington Axis the bedrock consists of tightly folded greywackes and Slatés, usually very strongly crushed. Graptolite localities scattered throughout the region date them as late Darriwilian to Eastonian, although the age limits may be wider, Epi-Ordovician deformation and igneous activity_. At the end of the Ordovician the differentiation of eastern and central Victoria into eutectonic and miotectonic zones respectively first became obvious. Whereas in central Victoria the Ordovician and Silurian are essentially conformable, the whole of eastern Victoria was subjected to intense deformation. This deformation - the so-called Benambran “0rogeny" affected south-eastern New South Wales as well, and has been dated at Canberra within the limits of late Eastonian and late Llandoverian. Immediately following this deformation, a wide belt stretching from Albury in the north through Omeo to Ensay in the south was metamorphosed to a variety of schists, gneisses, and granulites, with the concurrent intrusion of a suite of granites and granodiorites, many of which show gneissic banding (Crohn, 1950). The metamorphic belt is gradational eastwards into unaltered Ordovician, whereas the we stern boundary is frequently a massive shear zone bringing gneis ses into juxtaposition with sediments (Beavis, 1962). East of the main metamorphic belt are similar granitic intrusions surrounded by wide aureoles of schist, WhiC11 are believed to be contemporaneous (Edwards and Easton, 1937). The date of deformation of the sediments we stwards from Ballarat is unknown but is quite likely to be older than the Devonian age currently ascribed to it. A metamorphic belt similar to that in eastern Victoria occurs in the far west where high grade schists contain granitic intrusions, in part gneissic (Wells, 1956). lntrusions adjacent to this belt, such as the A.rarat granodiorite and the Pyrenees granite, have similar strongly schistose aureoles. ,
SILURIAN AND LOWER DEVONIA1\L
Silurian-Devonian of central Victoria. In contrast to the Ordovician there is a profound difference between the Silurian successions in central and in eastern Victoria. In central Victoria the Silurian and Lower Devonian are an entity, conformable upon the Ordovician but deposited in a much more restricted area between the Heathcote Axis and the Mt. Wellington Axis (Fig. 1). The Heathcote Axis marks the absolute western limit of Silurian-Devonian and indications are that faulting along it delineated the margin of the rapidly subsiding trough. The rapidity of deposition is shown by Lower Silurian thicknesses of 8, 700 ft. at Lancefield and ll, 500 ft. near Melbourne (Hills and Thomas, 1954), and 24, 000 ft. of Lower Silurian to Lower Devonian at Heathcote. Low-rank greywackes and mudstones still predominate, but subtle changes in lithology are evident. Current ripplemarks and cross bedding are conspicuous while graded bedding is not nearly so common, and the argillaceous sediments are frequently greenish. ln contrast to the Ordovician, it is possible to distinguish lithological units, particularly in the Lower Devonian when regional differentiation also became marked. Vo lcanicity was entirely absent. A diachronous sheet of clean sandstones with conglomerate bands spread into the trough from the west,beginning at Heathcote in the late Upper Silurian. The conglomerate pebbles are normally of stable sediments but igneous pebbles occur near the trough margin in the Heathcote Lower Devonian and also in Lower Silurian conglomerate near Melbourne. Mudstones and silt-
4
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Stavely Axis Heathcote Axis
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2
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LOWER PALAEOZOIC LOIIEP, PALAECZCIC METAIIORPIILCS ORDOVICIAN ORDOVICIAN METAMORPHICS
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SILURIAN
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Fig. Z. Synopsis of the post-P111tcozoic structure of Victoria, sho wing the distribution of Mesozoic and Tertiary sediments and Newer Volcanic vents.
Q
5
stones with rich shelly faunas characterize the Lower Devonian at Kinglake and Lilydale (type Ye ringian), with a lenticular coral-stromatoporoid limestone intercalated at Lilydale. In the eastern portion of the trough the early Lower Devonian is typified by pelagic faunas, the Monograptus-Baragwanathia association, now regarded as early Lower Devonian (Jaeger, 1962), and the specialised “Tanjilian" fauna with the lamellibranch Panenka, orthoconic nautiloids, and “pteropods". The widespread occurrence of land plants in these and in the succeeding Walhalla Series implies the existence of a contiguous land area, almost certainly to the eastward. The very thick Walhalla Series consists of shales and slates with rhythmic sandstone and spasmodic thick beds of relatively clean fossilife rous grit, echoes of the Basal Grits. The "Basal Grits" of the Walhalla Series are a persistent horizon of fossilife rous shallow-water conglomerates and sandstones, and lenticular limestones which accumulated on local shoal areas. Cambrian greenstone pebbles in some of the conglomerates indicate that one of the Cambrian axes to the east was exposed to erosion. In the Silurian-Devonian, neritic shelly faunas become progressively more conspicuous. Conversely graptolites,ubiquitous in the Ordovician, occur only spasmodically, usually in narrow bands. All in all, deposition was predominantly neritic with local intermittent areas of deeper water as, for example, while the Monograptus-Baragwanathia dark shales accumulated. The youngest assemblages known indicate that sedimentation finally ceased late in the Emsian or perhaps early in the Eifelian. In the The structure of the Silurian-Devonian shows a contrast between west and east. western sector the folds are simple persistent structures, with several simple domes and basins. Cleavage and fissuring are subordinate. Deformation was more intense in the eastern sector with the production of anticlinoria and synclinoria. The Mt. Easton and Mt. Useful Axes flanking the Walhalla Synclinorium are complex faulted structures bringing up Ordovician in their cores (Ch. 18). Cleavage and fissuring become progressively more intense eastwards particularly in the eastern limb of the Walhalla Synclinorium which has been severely crushed against the trough margin. Major structure pattern in eastern Victoria. Eastern Victoria and contiguous parts of New South Wales entered a rather different tectonic regime because of the stabilization at the end of the Ordovician with development of a more competent substrate. East of the metamorphic belt, large faults divide the region into a mosaic of large phacotic blocks. One such fault has been traced from Bindi down the Indi River and into New South Wales as far as the latitude of Canberra. This pattern of augen-like blocks is considered to be the result of "flattening" by compression from the east, with considerable shearing in the bounding fault zones. We stwards into the less disturbed metamorphic belt the pattern changes into two conjugate fault sets, one NNW-SSE parallel to the regional trend, the other NE-SW. These faults were certainly active at the end of the Silurian and probably earlier as determinants of Silurian sedimentation. Variations in both thickness and lithology suggest that various blocks behaved differently, either sinking rapidly or slowly, or remaining positive. Silurian of eastern Victoria. The Silurian in this eutectonic region contrasts strongly with that in central Victoria. At Wombat Creek, north of Omeo, a Silurian sequence occupying a narrow graben begins with the thick sub-aerial Mitta Mitta River rhyolites, including ignimbrites with xenoliths of Ordovician sandstone and slate, and of granite. This is overlain by a massive conglomerate formation containing rhyolite pebbles in addition to sandstone, slate, and granite. The upper fossiliferous beds are mainly shales, with interbedded conglomerates, sandstones, and lime stones. At Limestone Creek near the New South Wales border the Silurian is represented by very thick low rank greywackes and mudstones, with subordinate lime stones and clean quartz sandstones (Talent, 1959). At Quedong near Bombala, N.S.W. a simple gentle basin structure contains a relatively thin sequence of quartz sandstones, followed successively by limestones and calcareous shales. An outlier of the basal sandstone forms the monadnock of Mt. Delegate near Bendoc. The shelly faunas in all these sequences indicate a Middle or Late Silurian age but the beginning and end of sedimentation cannot be dated (accurately. Epi-Silurian deformation and igneous activitl. At or about the end of the Silurian another period of deformation affected eastern Victoria - the so-called Bowning "Orogeny" - but because of the more competent bedrock, it was expressed largely as major faulting. The thick Limestone Creek sequence was strongly folded whereas ,
6
the thin Quedong sequence was protected. Likewise the Wombat Creek sediments, while steeply inclined, escaped strong folding because of the underlying competent rhyolites and conglom-
erates.
The Limestone Creek Silurian has been intruded by the southern end of the Kosciusko mas sif and weakly metamorphosed to low-grade schists and marble (Talent, 1959). The relationships between the major fault pattern and intrusions, such as the Murrumbidgee and Kosciusko Batholiths in New South Wales, suggest that foundering of fault blocks has been responsible for their emplacement. Metamorphism has been insignificant in comparison to the size of the intrusions. LOWER AND ,MIDDLE DEVONIAN.
East of the Mt. Wellington Axis, Silurian rocks were apparently absent over a wide belt, for at Tabberabbera transgressive Lower, to early Middle, Devonian rests with violent unconformity upon Upper Ordovician (Talent, 1963). This neritic sequence, some 8,000 ft. thick, links the geosynclinal Lower Devonian of central Victoria, already discussed, with the marine Devonian of eastern Victoria. Basal conglomerate and coarse quartz sandstones pass up through thick silty sandstones and fhales, with an Emsian shelly fauna, into dark shales and minor lime stone bands of Eifelian age. The whole is folded into a strongly plunging synclinorium with a remarkable northerly extension as a long simple syncline. Further east the epi-Silurian deformation and intrusion was followed by another deposition cycle similar to that of the Silurian, but showing the effects of still greater bedrock stability. At Limestone Creek both the Silurianiand succeeding granodiorite are overlain with strong unconformity by the sub-aerial Snowy River Volcanics (Talent, 1959). These extend southwards to Buchan and Nowa Nowa in a general synclinal structure and consist of albitized rhodacites with sub-ordinate pyroclastics and latite flows (Ringwood, 1955), and local intercalations of non-marine conglomerates and sandstones (Fletcher, 1963). The volcanics, normally 2000 to 2500 ft. thick, reach a maximum of 10, 000 ft. about 30 miles north of Buchan where they represent flood extrusions from fault fissures. South of Buchan thin intercalations of fossiliferous sandstone presage the marine transgression which followed. At Buchan the neritic marine sequence, up to Z, 800 ft. thick (Teichert and Talent, 1958), began with thin clastic sediments including arkose, rapidly succeeded by the uniform Buchan Caves Limestone, the lower part of which is dolomitized. The upper bedsgrade laterally southwards from bedded lime stones, dark calcilutites, and calcareous mudstones into slightly deeper water mudstones and nodular muddy lime stones. In places richly fossiliferous massively bedded lime stones have been interpreted as bioherms, but, in fact, true reefs are not present. The Shelly faunas, principally of corals, stromatoporoids, and brachiopods, range in age from Emsian in the Buchan Caves Limestone to Upper Eifelian. Similar Devonian sequences are found at Bindi, Limestone Creek, and on the Erinundra River. Because of the thick competent lavas the major structures are broad and open, with strike faulting and disharmonic folding of the sediments forming a synclinorium at Buchan. At Waratah Bay, Lower Devonian Siegenian lime stones with a gritty basal phase re st unconformably upon greenstones of the Cambrian axis and are followed disconformably, by dark Emsian lime stones (Talent, personal communication). This sequence is separated by a major boundary fault from homologues of the Walhalla Series - the Liptrap Formation - which may extend into the early Middle Devonian._ This contemporaneity with the limestones emphasises the contrast between shallow water deposition on the Cambrian axis and the accumulation of clastic sediments in the flanking deeper water trough.
Late Middle Devonian deformation. Eastern and central Victoria we re subjected to the (last major deformation - the socalled Tabberabberan "Orogeny" which can be dated within the limits of late Middle and early Upper Devonian. The resultant structures are much more spectacular in central Victoria where they developed in unconsolidated sediments, whereas in eastern Victoria movement was largely absorbed by the major fault lines. Epi-Middle Devonian igneous activity. An episode of igneous intrusion followed the late Middle Devonian deformation but antedated all the Upper Devonian rocks. The Woods Point dyke swarm (Hills, 1952) is concentrated in the ,
7
western limb of the Walhalla Synclinorium, dykes decreasing in abundance westwards and being absent from the more strongly crushed eastern limb. The dykes generally follow regional strike but are cross-cutting in section. Diorites and hornblende lamprophyres predominate although the composition ranges from quartz porphyry to perknite and peridotite. Many dykes have suffered later compression with the development of conjugate shear-plane fractures which have acted as loci for the introduction of auriferous quartz reefs - the ladder veins of the Morning Star and other mines. Similar diorite dykes at Tabberabbera are accurately dated, cutting Eifelian but truncated by the Upper Devonian. A petrologically related hornblende granodiorite at Mt". Buller, east of Mansfield, antedates Upper Devonian lavas (see Ch, 18) and the similarly situated Mt. Taylor granite porphyry stock near Bairnsdale is probably contemporaneous. UPPER DEVONIAN AND LOWER CARBONIFEROUS. With the whole of Victoria largely stabilized, thick non-marine sedimentation during the Late Devonian and Early Carboniferous was restricted to two graben structures, one in the Grampian Ranges of western Victoria, the other on the eastern flank of the erstwhile SilurianDevonian trough of central Victoria. Typical fluviatile red beds are characteristic, with plant fossils and occasional fish faunas. Upper Devonian cauldron volcanics and intrusives. Acid Volcanics are wide spread in the Upper Devonian. ln central Victoria volcanicity was concentrated in a number of rounded and polygonal cauldron subsidences, several skirted by ring dykes, which contain piles of lavas up to 5, 000 ft. thick (Hills, 1959). Initially, relatively thin acid lavas and ignimbrites, with subordinate ande site and basalt, were extruded from central vents. Pyroclastics occur spasmodically and one of the rare lenses of clastic sediment contains Late Devonian fishes at Taggerty. At the northern end of the eastern Upper Palaeozoic belt (north of Mansfield) conglomerates are interbedded with rhyodacites and have been locally folded prior to the main cauldron lavas (Brown, 1961). During the second phase, the final cauldron collapse, flood extrusions of nevadite, rhyodacite, and hypersthene dacite, individually up to 2000+ ft. thick, filled the cauldrons in one or several stages. Finally, granite and granodiorite intrusions were emplaced by major stoping adjacent to and into the Volcanics. North of Mansfield, granite intruding unfolded Upper Devonian cauldron lavas is in turn overlain unconformably by Lower Carboniferous sediments (Brown, 1961). A number of other high level discordant granitic batholiths in central Victoria, sometimes showing evidence of the same emplacement mechanism, can be reliably correlated with these epi-Upper Devonian intrusives. All have unstressed hornfels metamorphic aureoles. Hills (1959) has recognized a cauldron subsidence in north-easte rn Victoria where the thick Jemba nevadite is surrounded by a quartz porphyry ring dyke. This, together with the nearby Pine Mountain and Mt. Mittamatite red granites, which transect a consanguineous swarm of NESW trending dykes ranging from quartz porphyry to diorite and labradorite porphyrite (Edwards and Easton, 1937), is likely to represent slightly older igneous activity (Ch. 19), Upper Palaeozoic sedimentary sequences. The Upper Palaeozoic sequence in we ste rn Victoria began with extensive rhyolites and minor trachytes. The overlying Grampians Sandstones were deposited mainly in a rapidly subsiding graben, with thinner sections on the flanking aprons. Within the graben their base is covered ; even so 20,000 ft. of beds are exposed. A thin basal phase of conglomerates, with pebbles of rhyolite and granite as well as sediment, is followed by a remarkably uniform succession of pale cross-bedded quartz sandstone, siltstone, and shale (Spencer-Jones, 1961). Near the top of one of the se - the Silverband Formation - is a thin marine inte rcalation with a small undiagnostic fauna (Talent and Spencer-Jones, 1963). The Grampians Sandstones certainly include Upper Devonian and are likely to extend into the Lower Carboniferous. Within the graben they have been down-faulted and tilted to the we st. Where the boundary faults converge towards the north strong drag on both margins has produced a closed syncline. Within the graben, they have been intruded by sills of quartz porphyrite and stocks of granite and granodiorite. In the eastern Upper Palaeozoic belt, more complex in history and structure, the depositional basin approximates to a graben trending NNW-SSE with the bounding structures evident in places. The belt is subdivided by conjugate anticlinal structures trending NNE-SSW.
8
In the South Blue Range at Mansfield the Upper Devonian sediments, interbedded sandstones and mudstones with fish, are underlain by basal muddy conglomerates lacking igneous pebbles and capped by a thin rhyodacite ignimbrite. These beds have been folded into a tight synclinorium prior to the deposition of the Carboniferous (see Ch.l8`).: The Carboniferous and of the Mansfield Basin begins with sandy basal conglomerates, containing igneous pebbles
intercalated arkoses where overlying lavas, followed by a return to red-bed sedimentation. Lower Carboniferous fish occur some distance above the base. In the remainder of the belt, Upper Devonian and Lower Carboniferous are concordant and have not been differentiated yet. The basal phase, of proven Upper Devonian age, consists of widespread volcanics with local interbedded sediments, largely conglomerates. Individual flows are thin and predominantly rhyolitic, with subordinate basalts. Further conglomerates are followed by many thousands of feet of red beds. Locally lava flows persist for some distance and, in contrast to the Grampians sequence, pale sandstones are subordinate. Upper Palaeozoic deformation. Internally these Upper Palaeozoic rocks have been only gently deformed into broad open folds but in proximity to some of the Lower Palaeozoic contacts, particularly on the western margin of the belt, strong deformation has occurred. These boundaries are usually fault folds In places clean which have even caused local overturning in the Carbonife rous (see Ch. 18). been thrust greenstones have Barkly River Cambrian fractures form the boundary, and on the 1954). and Thomas, Palaeozoic (Harris over the Upper This compression along the we stern margin of the belt, the shearing of the Woods Point dykes, and reversed faulting of some of the epi-Upper Devonian quartz reefs, for example in the Wattle Gully Mine, Chewton, mark the final pulses of tectonic activity in the Victorian Palaeozoic. After the Lower Carboniferous, tectonic instability connected with the eastern Australian mobile belt finally ceased. PERMIAN AND
TRIASS_IC_=
period of tectonic quiescence intervened during the Permian and early Mesozoic. The Permian is represented by glacial and fluvio-glacial sediments usually preserved from erosion by down-faulting. They are normally flat-lying and, in the absence of any overburden, often thin, so poorly lithified as to resemble Pleistocene tills. The sequences are comparatively graben, an east-west feet occupy except at Bacchus Marsh where several thousand the first indication of a new tectonic regime. A thin marine band with Notoconularia has recently containing been discovered towards the top of this sequence (Thomas, l9f69' ), which is capped by beds Triassic plants. syenite, At Benambra, near Omeo, are a number of stocks of granite porphyry and quartz base 1, OOO ft. As the tuffs for up to trachytic lavas and some of which intrude their associated were intruded time, plutonic rocks surface of the to the erosion the lavas corresponds of Australian National well above the surrounding bedrock. A recent K-Ar dating by the(McDougall, personal University gave a late Triassic age of 202 and 207 million years the State. acidic intrusive in youngest known communication), making it the A
JURASSIC AND CRETACEOUS.
Jurassic and, or Lower Cretaceous. of The establishment of a new tectonic regime during the Mesozoic led to the initiation Gippsland basin is The South portion of the State across the southern basins spread depositional which is continDistrict basin, from the Western separated by the positive Mornington Peninsula basin is uous between its outcrops in the Otway Ranges and Casterton area. The South Gippsland from the structure, distinct is thus a separate rimmed on the south by Palaeozoic rocks and present Bass Strait. The exclusively With over 9,000 ft. of deposition, subsidence was extremely rapid. approximating feldspathic sandstones cross-bedded fluviatile sediments consist of greenish grey and Baker, 1943). shales (Edwards mudstones and alternating with grey to arkoses,monotonously the Grains of ande site of unknown source occur in the arkoses. Exotic conglomerates occur at The Wonthaggi-Korumburra throughout. are common conglomerates base while autochthonous fossils area of South Gippsland contains thin bituminous drift coals, formerly mined. Plant -
9
are common, with rare fish remains. On the basis of plant remains these rocks were assigned to the Jurassic, but recent palynological studies have dated them as between the base of the .Cretaceous and the Aptian to Albian (Dettmann, 1963). These sediments now outcrop in late Cainozoic structures discussed below. There is little discordance between them and the flanking'T€1`tia1`Y1`°C‘kS» but in the Balook Dome of South Gippsland overstepping by Eocene basalts indicates quite considerable pre-Tertiary upwarping (Edwards, 1942). Upper Cretaceoug ln a separate episode, up to 3, OOO ft. of neritic Upper Cretaceous was deposited during a marine transgression on to the coastal belt of western Victoria between Port Campbell and Nelson on the South Australian border (Baker, 1961; Taylor, 1964). Around Port Campbell a simple marine cycle of shallow water sandstones and siltstones grades outwards into dark mudstones, in part glauconitic and pyritic, containing Turonian and Santonian foraminife ra and an ammonite. These are completely overlapped without any obvious break by marine Paleocene. ln the Nelson bore quartz sandstones predominate. ,
CAINOZOIC
.
The entire Tertiary succession is represented in Victoria but in no single section (maximum thickness 4, 500 ft.) is it complete. The principal depositional areas, now lowlands, are the Gippsland Basin with a synclinal extension we stwards into the Latrobe Valley, the Westernport and Port Phillip sunklands separated by the Mornington Peninsula, the Western District plains, and the Mallee and Wimmera regions in the north-we st of the State which form the south-easte rn portion of the Murray Basin. Complications in the stratigraphy are largely the result of the interplay between three depositional environments. The coal measures environment is represented by stable clastic sediments from conglomerates to clays, with the accumulation of extremely thick seams of brown coal in favourable situations. Coal measures deposition was widespread during the Paleogene and persisted locally into the Early Miocene. The marine deltaic environment, typified by the Angle sea Sand and Dilwyn Clay, is represented by fine-grained carbonaceous pyritic clastics deposited rapidly under anaerobic bottom conditions, due more to an excess of organic matter than to poor circulation. The carbonaceous matter and abundant pollen was supplied by plant debris washed and blown in from the adjacent land. A benthonic fauna is absent, the foraminifer Ha_plo_phra_gm_o_ides being the only common fossil. However occasional thin intercalations of better aerated sediment do carry small shelly faunas. Sediments of this environment range in age from Paleocene to Early Oligocene, being contemporaneous with the coal measures forming on the adjacent land. As such, they occur either overlying or seawards from the coal measures sequences. The normal marine environment is further divisible into shallow water neritic, characterized by polyzoal lime stones with echinoid-brachiopod-pectinid faunas, and slightly deeper water neritic with glauconitic marls and clays containing rich molluscan faunas. Marginal grits, sometimes glauconitic and invariably fe rruginized in outcrop, are subordinate. Thin phosphatic nodule beds occur at the base of transgressive units and periods of condensed deposition are marked by nodule beds or greensands. From the Paleocene to Early Oligocene because of the wide spread marine deltaic environment and the influx of clastic detritus, normal marine deposits were localized and lime stones uncommon. They became wide spread following marine transgression which was intensified in the Late Oligocene (Janjukian) and reached a :maximum in the Miocene. Deposition of Upper Oligocene to early Upper Miocene limestones, marls and clays with the entry of clastic detritus at a minimurn, indicates low relief in the highlands over this time. During the Late Miocene the sea began to fill up and a greater influx of detritus led to the deposition both then and in the Lower Pliocene of regressive shallow water sands and clays, and an absence of limestone. The sea was then expelled from most areas except the far south-west, where a separate local transgression in the Late_Pliocene (Werrikooian) and Early Pleistocene is represented by shallow water shell beds and oyster limestones. Rejuvenation of the highlands which began in the Late Miocene reached a climax in the Late Pliocene and Pleistocene, shown by widespread conglomerates and sands, such as the "Torrent Gravels“ in Gippsland. Similar, perhaps older, deposits constitute the overburden to the Latrobe Valley coal measures and have been displaced by local structures. Quaternary sediments include widespread dune and sheet ,
10
sands in the Mallee and south-western Victoria, alluvial deposits in the Wimmera, Northern Plains and Gippsland Plains, and Pleistocene coastal dune limestone. Gypsum and rock salt deposits occur in playas in the Mallee. Palaeogene_ coal measures contain some of the thickest brown coal seams in the world. In the Latrobe Valley several major seams, separated and subdivided by impersistent clay splits reach a maximurn total thickness of 1000 ft. (Gloe, 1960). Thick seams also occur around the South Gippsland Hills, ufnder the We rribee Plains between Melbourne and Bacchus Marsh, and on the eastern and northern flanks of the Otway Ranges (Thomas and Baragwanath, ,
1949-51). On the Gippsland Shelf petroleum and natural gas occur in Cretaceous and Tertiary horizons (James and Evans, 1971; Griffith and Hodgson, 1971). lt is of historical interest that the first oil produced and marketed in Australia came from a Lower Oligocene greensand at Lakes Entrance.) Cainozoic volcanic activity. The Cainozoic was a period of basic volcanicity grouped about two maxima in the Eocene and Plio-Pleistocene, but linked by spasmodic flows in the Oligocene and Miocene. The Eocene Older Volcanics are concentrated to the east of Melbourne, both in South Gippsland and the highlands, whereas the Newer Volcanics lie almost entirely to the west of Melbourne, forming the Werribee and Western District lava plains, and valley flows in the western highlands. The Older Volcanics, locally more than 1100 ft. thick, are olivine basalts averaging 45 per cent SiO2 with unsaturated differentiates such as nephelinite, tinguaite, and phonolite. On the other hand the Newer Volcanics, up to 500 ft. thick, average 50 per cent SiO2, with saturated alkaline differentiates including alkaline basalt, trachyte and soda trachyte. While the Newer Volcanics began in the Pliocene, they invariably overlie the marine Tertiary, including Upper Pliocene marine, and are largely Pleistocene, continuing into the Holocene. The physiographic features of the youngest volcanoes and flows are practically unaltered by weathering. Cainozoic tectonic activity; Mild tectonic activity occurred intermittently through the Tertiary with downwarping in the sedimentary basins and active faulting in the Palaeozoic rocks, f01‘ example 011 Se1WY1'1'S Fault The most obvious movements, however, took place during and after the Pliocene. A series of en echelon domal uplifts elongated in an ENE°°WSW direction and bounded by monoclines and faults is located on the site of the former Lower Cretaceous troughs. Initiated prior to Upper Cretaceous sedimentation, these were rejuvenated and accentuated during the late Cainozoic. They continue out on to the Gippsland Shelf where structures form reservoirs for major natural gas and petroleum accumulations At the same time the highlands of Victoria were elevated along an E-W axis, and N-S faulting, some of it very recent, occurred along pre-existing fault lines or on new structures such as the Rowsley Fault. ,
,
.
PH YSIOGRAPHY .
The east-to-we st backbone of the Victorian Highlands separates the Murray Basin Plains north from a series of lowlands grouped together by Gregory as the "Great Valley" of Victoria, in turn flanked on the south by the youthful uplifts of the Southern Uplands (Hills, 1940, 1955). on the
Victorian Highlands.
While continuous topographically with the NNE trending highlands of New South Wales, the Victorian Highlands owe their origin to the tectonic regime which controlled Mesozoic and Tertiary sedimentation rather than to the primary westerly directed forces operating along the east coast of the continent. They approximate to a broad E-W anticlinal arch with a gentle westerly plunge, modified in detail by pre-existing Palaeozoic trends. Remnants of at least three erosion surfaces are recognizable, the youngest of which, carrying Older Volcanic residuals, is at the latest early Tertiary in development, implying that the two higher surfaces are pre-Tertiary and perhaps of great antiquity. The highlands are conveniently divided into eastern and we ste rn sections by the Kilmore Gap north of Melbourne. The eastern highlands are similar to contiguous New South Wales, and preserve extensive tracts of the higher erosion surfaces, particularly the middle one, forming the
Fig.
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Fig. 3 (upper). Melbourne area.
Diagrammatic cross-section illustrating the stratigraphic relationships of the principal Tertiary formations in the (Not to scale, vertical dimension greatly exaggerated. adapted from Kenle~ 1967).
Fig. 4 (lower).
Cross-section of the Yarra Delta between Stony Creek, Spotswood and Princes Bridge (after Neilson, 1967).
N W
24 Viaduct Sands at Geelong (Ch. 7), possibly also with the littoral Marina Cove Sand at Mornington (Ch. 6). 3.
Late Tertiary Terrestrial Phase.
(a) Brighton Group - Red Bluff Sands. The Red Bluff Sands outcrop in a wide area between Essendon, Camberwell, Dandenong and the coastline, and extend under the New Volcanics southwest of Melbourne. Gill (1950, 1958) suggested a disconformity between it and the underlying Black Rock Sandstone, possibly with an intervening period of laterisation. The formation is 24 m thick at the type locality near Sandringham, but is somewhat thicker in the Ormond-Oakleigh area. It consists of poorly consolidated sands, silts, clays and gravels, often with cross-bedding and rapid changes of grain size. Individual bands have been ferruginised by weathering. Gill (1957) postulated a partly fluviatile, partly nearshore lagoonal or paludal origin for the formation. However, a predominantly fluviatile environment is likely. The sparse fauna and flora includes a fresh-water spong spicules and bivalves, wood fragments, and pollen. An acritarch (Hystrichosphaeridium tubuliferum; Gill, 1957) from Red Bluff is indicative of a marine environment, although the associated remaining flora is terrestrial. The Red Bluff Sands are probably of Middle to Late Pliocene age (Singleton, 1941). In the Geelong area, its equivalent is represented by the upper part of the Moorabool Viaduct Sands, and on the Mornington Peninsula it is represented by the Baxter Sandstone. Undifferentiated thin, discontinuous deposits of fluviatile silty sand and sandy gravel, often partly silicified, underlie the Newer Volcanics northwest of Melbourne. These become more continuous southwards and merge laterally into the Red Bluff Sands. Near Bulla, they contain Eucalyptus spp., Acacia spp., and ferns. The deposits are regarded as Pliocene, although definite indication of their age has yet to be found. (b) Newer Volcanics. Erosion of the Red Bluff Sands and underlying rocks produced only slight relief southwest and east of Melbourne. Subsequent volcanicity formed the Werribee Plains a continuous sheet of Newer Volcanics up to 60+ m thick. In contrast, the volcanics were restricted to valley flows in the undulating topography fringing the Werribee Plains. The Newer Volcanics have been extruded from a large number of basalt and scoria cones, and comprise numerous flows, with interbedded pyroclastics, sand, and soil horizons. Most of the flows in the district are olivine basalts of various types, but limburgites are also known (Edwards, 1938; Condon, 1951; Hanks, 1955). A range of more siliceous alkaline variants occurs in the Gisborne-Macedon district (Ch. 11). Condon (1951) suggested two phases of activity in the Werribee Plains, early extensive sheet flows of olivine and pyroxene basalt, possibly from dyke feeders, and later small tongue flows of olivine and iddingsite basalt from central vents. As many of the flow boundaries, however, are not visible on aerial photographs, and others are traceable only for short distances, this interpretation is uncertain, particularly as the sequence north of Melbourne does not show any clear division into earlier sheet flows and later central eruptions (Hanks, 1955). In this area the youngest flow (possibly the youngest in the entire district) extends probably from Mt. Fraser to the Yarra Delta, a distance of 50 km. The Pliocene-Pleistocene age of the basalts has been confirmed by radiometric dating (Page, 1968). The oldest dated basalt is from the Essendon area, with an age of 4.5 m. y.. Basalts from Newport and Albion gave dates of 2. 5 m. y., and a basalt from Northcote, probably originating from one of the vents near Craigieburn, gave an age of 2.2 m. y.. The Mt. Fraser-Yarra Delta flow has been dated at Alphington at 0.81 m. y .. Most of the numerous volcanic cones in the district are of the flat-topped type (see page 97), for example Mt. Kororoit. A variety of scoria cones occurs, for example Mt. Fraser, with multiple deep craters and relatively steep slopes, and Mt. Mary with a low breached cone. The Newer Basalts show a variety of well-preserved structures. The basalts are typically vesicular, for example, on shore platforms south of Williamstown, where vesicles are arranged along confused flow lines. Columnar jointing is widespread, unusual examples of which along Jacksons Creek include the well-known Organ Pipes. Pillow lavas occur at the base of flows at Exford (Condon, 1951) and Footscray. A fine example of a radial squeeze-up rising some 5 m occurs on Mt. Kororoit. On the Mt. Fraser flow east of Merri Creek, in the BeveridgeDonnybrook area, crescentic tumuli are well developed,and unusual, broad flat-topped stony rises lie above the general level of the basalt. .
j
25
QUATERNARY Thin Quaternary sediments are scattered throughout the district and include alluvial, swamp, deltaic, fault apron and dune deposits. The Darley Gravels and Werribee flood plain deposits consist of sediments eroded from the upthrown blocks of the Rowsley and Coimadai Faults. The Darley Gravels grade from fault apron sandy silts, sands and gravels to alluvial sandy gravels along the Werribee River and Toolern Creek. The Werribee flood plain consists of poorly bedded, well sorted overbank silts with minor sand interbedded in the lower part. Three Pleistocene stream courses are still traceable on the surface of the flood plain. Thin (not more than 1 m) wind-blown silts 'and sandy silts, probably derived from these Quaternary sediments, form a veneer over the Newer Volcanics"extending eastwards into the western suburbs. Pliocene or Pleistocene rejuvenation of the Yarra Fault developed aggrading conditions in the Yarra River between Yarra Glen and Healesville with consequent formation of an extensive flood plain. The high-level alluvium along the Yarra River downstream from Templestowe, and along Gardiners Creek, was deposited following damming of the lower reaches of the Yarra by the Mt. Fraser - Yarra Delta flow. Younger low-level alluvium was deposited following the cutting of a new stream course. In the Yarra Delta, sedimentation occurred in a deep, broad drowned valley system, cut into Pliocene basalts and older rocks (Neilson and Jenkin, 1967). The maximum vertical thickness of sediment in depressions and channels is about 45 m, but usually is little more than 30 m and may be as little as 8 m. The development of the delta reflects fluctuating Quaternary sea levels, and includes one major break in deposition.
The earliest deposition was of widespread fluviatile gravels and sands (Moray Street Gravels), probably during a glacial low sea level. Marine deposition during an interglacial period of high sea level produced the overlying silty clays and clays of the Fishermens Bend Silt, following which a succeeding low sea level caused a major break in deposition, with sub-aerial exposure indicated by the over-consolidation, oxidation and fissuring of the silty clays. During this break, lava from Mt. Fraser (0.81 m. y.) flowed down the valley as far as the Yarra Delta. A return to marine-estuarine conditions is shown by the soft, dark grey silts and clays of the Coode Island Silt, lying disconformably on the Fishermens Bend Silt, and on the basalt flow {.seen inbores at Jolimont). A radiocarbon date on tree material within the Coode Island Silt of approximately 8500 years, gives an estimate of the age of this interglacial high sea level. The uppermost unit is the Port Melbourne Sand, from 20 ft. to 40 ft. thick, consisting of medium to fine, littoral to near-shore sands containing shell beds, on the surface of which sand ridges were developed. Present sea level is approximately 1 m below the topmost marine sands, indicating the extent of the retreat of sea level since Coode Island Silt time. The Carrum Swamp comprises a thin sequence of black clays and silts and minor shell beds, probably of Pleistocene age, reflecting a swampy lagoonal environment with minor marine inte rcalations. PHYSIOGRAPHIC EVOLUTION The physiographic evolution of the Melbourne district was governed by combinations of erosion, faulting, warping and volcanic activity. West of Melbourne, the topography has low relief and is determined by the late Cainozoic volcanicity and by faulting. In contrast, east of Melbourne, extensive areas of outcropping bedrock have higher elevations. Prolonged erosion has produced a step-like succession of partially preserved erosion surfaces, dissection of which has resulted in considerable relief. Topographic effects of Cainozoic faulting are also apparent. Areas east of Melbourne. Three main cycles of erosion are recognisable in central Victoria, culminating in turn in the Baw Baw Surface, the KinglakE" Surface and the Nillumbik Terrain. Late- Tertiary
26 uplift (Kosciusko Uplift) initiated further dissection. The Baw Baw Surface (considered either Cretaceous or Triassic by Hills, 1934, 1955) is the oldest recognisable feature and is only preserved in resistant rock types, for eXanlple, around Mt. Baw Baw (1500 Ill), Mt. Torbreck (1500 Ill) to Mt. Donna Buang (1200 Ill) and possibly Mt. Macedon (1000 Ill). The Kinglake Surface (the Triassic Erosion Surface of Neilson, 1970; but possibly up to Early Tertiary in age) is represented by two extensive plateaux. The higher Gregory Plateau (ranging froIll 900 to 1200 Ill) slopes gently westward froIll the Matlock district to Toolangi and GeIllbrook, possibly with Mt. DisappointIllent as an outlying reInnant, and is flanked by the slightly lower Kinglake Plateau (500 to. 600 Ill) of the Kinglake - Flowerdale Trawool district, with outlying reInnants in the Dandenong Ranges (600 Ill) and the Warranlate Hills (400 Ill) (Garratt, 1973a). The NillUIllbik Terrain is the proIllinent erosion surface shown by the generally concordant level of ridges in the area of Silurian-Lower Devonian bedrock east and northeast of Melbourne. It rises in elevation froIll about 20 III at Melbourne to about 200 III at the foot of Mt. Sugarloaf, which is part of the well-defined escarpIllent at the southern edge of the higher Kinglake Plateau. It is also present further east in the 'Woori Yallock Basin (p. lSI). The NilluIllbik Terrain has been dissected by the Yarra River and its tributaries and to a lesser extent by the Dandenong Creek. North and west of the Dandenong Ranges, extensive flats along the Yarra Valley, and the broad valley of Olinda Creek-Dandenong Creek, together forIll a belt of low elevation separated by scarps froIll the higher NilluIllbik Terrain to the west. ' The Yarra Fault has been shown to be a Illajor structure with a total easterly downthrow of 1500 III (Garratt, 1973 a ) rather than an erosional scarp (Keble, 1915; Hills, 1934). Renewed IlloveIllent in the? Pleistocene produced the present fault scarp in the Yarra Glen district with a relief of about 240 Ill. As suggested by Jutson (1911a), IlloveIllent was slow enough for the Yarra to Illaintain its course through the upthrown block by excavating the deep Yarra and Yering Gorges. Extensive developIllent of alluvial flats occurred upstreaIll. The lowlands of the Olinda Creek-Dandenong Creek,valleys and the strike-ridge scarps to the west have been attributed to either differential erosion or to faulting. Between Glen Waverley and Dandenong, the Wheelers Hill Fault has prod'l1ced a proIllinent scarp, whose IllaxiIllUIll relief of 100 III is partly due to subsequent erosion by Dandenong Creek. Pliocene Red Bluff Sands cap the Wheelers Hill Scarp and extend across the line of the Dandenong Valley with a relatively flat base, suggesting that there was no Illajor valley in the Pliocene. In the dissected area east of Melbourne, Tertiary sediIllents occur only as residual hill cappings, but to tile southeast they forIll the continuous low relief Brighton Coastal Plain (Neilson, 1967). The ill-defined drainage pattern of the coastal plain is largely deterIllined by a series of distinct parallel low ridges trending NW, the Illain streaIll being Elster Creek (Elwood Canal). The ridges are parallel to the coastline and have been variously referred to as dunes (Whincup, 1944), as folds in the Black Rock Sandstone (Kenley, 1967) or as structures akin to beach ridges within the Black Rock Sandstone (Kenley, in VandenBerg, 1971). They are absent east 'of the Melbourne Warp. Area west of Melbourne - Newer Volcanic Plains. The Newer Volcanic Plains are divisible into two physiographic entities, firstly, the Werribee Plains, entirely covered by Newer Volcanics or post-volcanic sediIllents, and., secondly, a wide outer zone of Newer Volcanics covering an undulating terrain of Palaeozoic bedrock which frequently protrudes. The plain has a gently coastward gradient of 0.5% to 0.8% and is sIlloothly undulating. In the outer zone, undulations often Illark the boundaries of flows (e. g. as on MMBW 1 :24,000 contour Illaps of the Bulla-MicklehaIll-Kalkallo area) but Illay also be found within flows. The strongly Illeandrine courses of the larger streanlS reflect the gentle initial slope of the terrain. Subsequent entrenchIllent has produced deep gorge-like valleys with
J
• Kellor Nillumbik Terrain
o
Kinglake Surface
v"'~ldspar-hornbh.. ndr. porphyrite dyke.
Lyste rfield Hills.
G.C. Carlos. 1956.
Lyste rfield.
G.C. Carlos. 1956.
14. Hornblende porphyrite dyke.
42
Kalorama Rhyodacite. On Mt. Dandenong road above Montrose the Mount Evelyn and Kalorama Rhyodacites are separated by a band of tuff (Fig. 4) from which Hills (1941) recorded indeterminate plant remains.
Fig. 4. Position of tuffs on the MontroseKalorama Rd. (after Hills, 1940).
The Kalorama Rhyodacite is a uniform dark rock, 800-1 OOOft. thick, with a chilled top and relatively free of rock fragments. It appears to be essentially a single extrusion, marking the fir st stage of final cauldron collapse. Petrologically it is a quartz - biotite rhyodacite with phenocrysts of feldspar, quartz and biotite.
Similar but fragmental rocks, up to 1000 ft. thick, outcrop near The Patch, followed by up to 2000 ft. of pyroclastics which are in turn overlain by the Upper Dacite. The pyroclastics consist of agglomerates with fragments of rhyolite and toscanite interbedded with tuffs and mudstones. On the western side, north of The Basin, a thin band of tuff separates Kalorama Rhyodacite from Ferny Creek Rhyodacite. This steep-dipping band contains boulders of the underlying rhyodacite which Hills has interpreted as surface blocks buried by the tuff and subsequently rolled into it by shearing during formation of the Montrose monocline. Edwards refers to the presence in places of a zone of severely weathered rock at this. horizon,. which suggests a hiatus in volcanic activity prior to the extrusion of the Ferny Creek R;hyodacite. Ferny Creek Rhyodacite. This hypersthene rhyodacite represents the main stage of cauldron subsidence and accords in composition and position to similar extremely thick final flows in several other cauldrons. Though there is a progressive change upwards in composition it is apparently a single extrusion. It is more than 1000 ft. and could be up to 5000 ft. thick, involving the extrusion of between 7 and 35 cubic mile s of lava. The basal 200 ft. of the flow is black to blue-black in colour and is conspicuously chilled. Phenocrysts of zoned plagioclase with labradorite cores and hypersthene are set in a dense glassy base. Higher in the flow the groundrnass become s increasingly crystalline and the proportion of phenocrysts increases. With slower cooling biotite appears at the expense of hypersthene, as phenocrysts and frequently fringing hypersthene, until in the upper portion of the unit the rock is a light blue-grey hypersthene-biotite rhyodacite. Hornf~ls fragments occur mainly in the basal part. INTRUSIVE ROCKS. Lyste rfield and Silvan Granodiorite s. The Lysterfield Granodiorite is a small very high-level batholith with sharp undisturbed boundaries and was doubtless emplaced by a proce ss of major stoping involving subterranean cauldron collapse (d. Hills, 1959). Along seven miles of its northern boundary it intrudes the volcanics but while sedimentary xenoliths in varying stages of assimilation are abundant, only one of volcanic origin has been recognized. This suggests that the batholith was not roofed by volcanics but developed as a separate structure in juxtaposition to the volcanic cauldron subsidence. Small outcrops of granodiorite, scattered over about a square mile to the north-west of Silvan reservoir, may belong to a partially deroofed cupola - the Silvan Granodiorite. Quartz Porphyrite Intrusions. A quartz porphyrite intrusion, 500 yards across, occurs to the west of Silvan reservoir and metamorphoses the c-hilled·base o{the Ferny Creek Rhyodacite. A similar rock, now obscured, occurs near the Silvan darn site intruding basement sediments. A third similar
•
43 body intrudes the Kalorama Rhyodacite just west of the Evelyn Fault. Lysterfield Dyke Swarm.
,.
A small dyke swarm intrudes the north-western corner of the Lysterfield Granodiorite and adjacent metamorphic aureole, and similar dykes occur sparsely along the northern boundary of the granodiorite. More than sixty dykes have been mapped, varying from 2-20 ft. wide and up to a mile long. F~ldspar-hornblende porphyrites predominate. A few dykes are aplitic. Petrogene sis. Edwards considered that the differentiation of rock types was the result of fractional crystallization of lime-rich plagioclase and pyroxene with gravitational settling, influenced by changes in magma composition due to assimilation of argillaceous sediments. Differentiation following similar trends was renewed after each period of extrusion, the successive magmas being of slightly different bulk composition. Oxide ratio variations show parallel serial trends for the Mount Evelyn and Ferny Creek Rhyodacites, and granodiorites, less clearly for the Kalorama Rhyodacite and dyke rocks, suggesting derivation from a common magma similar to the hornblende-bearing granodiorite. For the extrusives the order in each series corresponds to the sequence of extrusion. There is a progressive relative increase in CaO and MgO from the Mount Evelyn Rhyodacites to the granodiorites, while the Kalorama and Ferny Creek Rhyodacites are enriched in iron relative to the Mount Evelyn Rhyodacite and the granodiorites. The Coldstream Rhyolites however are relatively enriched in iron and low in CaO and particularly MgO relative to iron, indicating that they are not normal differentiates. While the CaO/AI 2 0 3 ratios in Victorian hypersthene rhyodacites and related granodiorites are generally similar, the former tend to be richer in both oxides, suggesting gravitational sinking of Ca-plagioclase into the hypersthene rhyodacite magma. The hypersthene rhyodacite is enriched in iron presumably by. the accumulation of ferromagnesian minerals settling from higher magma levels during differentiation producing the earlier rhyodacites. Edwards considered that the presence of hypersthene rather than augite or hornblende was caused by the CaO being withdrawn in the cores of strongly zoned plagioclase phenoc,rysts during or before the crystallization of ferromagnesians, which are thus lime-free. Further he suggested that at the temperature then prevailing the magma was saturated in A1203. Subsequent reaction between hypersthene and the potassic residuum led to the formation of biotite, whereas in the slower cooling granodiorites release of CaO from early formed plagioclase allowed the formation of some hornblende. pyrogenetic almandine, indicative of Al 2 0 3 saturation, occurs only in those lavas with quartz phenocrysts and relatively large amounts of ferromagnesians and has formed at a late stage in cooling prior to extrusion. Edwards sugge sted that assimilation of argillaceous Palaeozoic sediments, in combination with normal differentiation processes, would explain both the ~aturation in A1 2 0 3 and the abnormal compositional features of the rhyolites. Analyses of the shales show them to be non-calcareous and to contain 1-30/0 MgO and about 70/0 iron oxide. METAMORPHOSED RHYODACITES. Silurian-Devonian sediments intruded by the Lysterfield and Silvan Granodiorites have been converted to a variety of hornfelses, including biotite and cordierite-bearing types, best seen in the Lysterfield Hills where the aureole is about half a mile wide. Schistose and "gneissic" textures in the hypersthene rhyodacite in the contact zone: 300 'to 500 yards wide,with the Lysterfield Granodiorite are the result of two distinct consecutive processes (Berger, 1961). Firstly, movement along the Selby Fault during cauldron collapse caused intense shearing in a relatively narrow zone, which with contemporaneous or later recrystallization produced strongly foliated rocks. Secondly, later emplacement of the granodiorite in juxtaposition to the down-faulted cauldron superimposed a weaker thermal metamorphism b·::>th within and outside the sheared zone. The shear zone has been traced from east of Selby westward to a point south of Upwey where it is truncated by a northward extension of the granodiorite. The degree of metamorphism decreases more or less symmetrically on either side of the central section between Selby and South Belgrave, which corresponds to the region of greatest movement on the SelbyFault. Beyond the shear zone to east and west,metamorphism was predominantly
44 thermal. Edwards has described essentially similar rocks where hypersthene rhyodacite has been metamorphosed by the Silvan Granodiorite and quartz porphyrite, and in which hypersthene is either rimmed or replaced by biotite. Within the shear zone the intensity of metamorphism depends on the amount of shearing the rocks have undergone and bears no relation to the proximity of granodiorite. Lenses of weakly altered unsheared rhyodacite similar to rocks outside the zone occur clol!!e to the granodiorite contact (Fig. 5), whereas high grade schists often occur in the northern half of
~1~~------
____________ 50' ______________________
~~
Narre Warren Road
Fig. 5.
Road cutting south of Selby with granodiorite contact (after Berger, 1961).
the contact zone adjacent to normal unmetamorphosed rhyodacite. In the central, most altered. sector the contact zone is an interleaved plexus of schistose rhyodacite. schist. augen schist and a few rocks with banded gneissic texture, together with lenses and pods of unsheared rhyodacite. Original rhyodacite features are still recognisable in the schistose rocks which have deveioped a good biotite foliation. Initial textures have been destroyed in the Rchists, now' composed of quartz, feldspar and biotite. Relict plagioclase phenocrysts are still present . with the foliation sweeping around them, whereas ilmenite and hypersthene have been completely replaced by biotite. and biotite phenocrysts are corroded or have recrystallized. Quartz and orthoclase have developed as porphyroblasts and in augen. OLDER VOLCANICS. Older Volcanics were extruded on the Tertiary erosion surface and occur as residuals .a'round th'; foot of the Dandenong Range s - capping Melbourne Hill, Lilydale. in a large area on the eastern side of the range near Silvan, between Emerald and Gembrook, and to the south near Pakenham and Berwick. These lavas belong to a suite of olivine ,basalts whose petrology was described by Edwards (1939). This Older Volcanic suite differs from the late Cainozic Newer Volcanic suite in being less siliceous and less potassic, the development of unsaturated differentiates, the abundance of titanaugite and the rarity of iddingsite. ITINERARY. By Maroondah Highway to Lilydale (24 miles). Brushy Ck. scarp at Croydon N. (18m). Melbourne Hill (23 m.) - capping of Older Basalt - view of Cave Hill quarry in L. Devonian limestone lens with basalt overburden, and Dandenong Ranges with foothills to N. of Coldstream Rhyolite, overliJ.in by Mount Evelyn Rhyodacite. LOCALITY 1. -
Black's Quarry, Coldstream.
Coldstream Rhyolite ("Lower Toscanite") showing strong vertical jointing and fine flow ,banding dipping 100 SE. LOCALITY 2. -
% mile
S.E. of Lilydale.
Basal rhyolites of Mount Evelyn Rhyodacite, with fragmental ignimbritic rocks.
"
45 LOCALITY 3.
.
Mt. Dandenong Rd., l~ m. above Montrose.
Grey quartz rhyodaci~e member of Mount Evelyn Rhyodacite, with quartz, feldspar. and garnet phenocrysts, angular rock fragments. and interbedded agglomerates. dipping SE on Montrose Monocline . LOCALITY 4.
,.
-
-
~ m. beyond Loc. 3 (Fig.3).
Greenish feldspathic quartz rhyodacite member of Mount Evelyn Rhyodacite. Lavas with abundant feldspar phenocrysts and chloritised ferromagnesians. overlain by tuffs with plant remains. then by dark quartz-biotite rhyodacite at base of Kalorama Rhyodacite. LOCALITY 5.
- lJtm. beyond Loc.4.
Top of Kalorama Rhyodacite separated by thin tuffs from chilled base of Ferny Creek Rhyodacite. dipping 70 0 _ 75 0 SE. Panorama showing erosion surfaces and Yarra River valley. LOCALITY 6.
- Summit of Mt. Dandenoni (2.078 ft.)
Panorama. LOCALITY 7.
-
Mt. Dandenong Hotel.
Ferny Creek Rhyodacite - upper portion of flow containing biotite phenocrysts. Sherbrook Forest to Kallista. cross John's Hill to Selby.' LOCALITY 8.
-
Pass
~ m. S. of Selby.
Contact zone of Lysterfield Granodiorite in central sector where metamorphism most intense - fine schist. augen schist. schistose rhyodacite, and lenses of unsheared rhyodacite (Fig. 5). Lysterfield Granodiorite. LOCALITY 9.
-
Glenfern Quarry. Ferntree Gully.
Upper member of Mount Evelyn Rhyodacite. with steeply dipping feldspathic quartz rhyodacite flows with thin seams of tuff. View to Lysterfield Hills to S. composed of metamorphosed sediments. REFERENCES. Berger, A. R., 1961. Studies on Dacite-Granodiorite Contact Relationships in the Dandenong Ranges and Warburton Areas. Victoria. Unpublished Thesis. Univ. Melbourne. Edwards, A. B .• 1936. On the Occurrence of Almandine Garnets in some Devonian Igneous Rocks of Victoria. Proc.Roy.Soc.Vic .. 49(1): 40-50. Edwards. A. B. > 1937. Quartz Diorite Magma in Eastern Victoria. Ibid, 50(1) : 97-109. Edwards. A. B.; 1956. The Rhyolite-Dacite-Granodiorite Association of the Dandenong Ranges. Ibid. 68 : 111-149. Hills. E. S .• 1941. Note s on the occurrence of Fossilife rous Devonian Tuffs in the Dandenong Rang~s. Ibid. 416-422. Hills. E. S:. 1959. Cauldron Subsidences. Granitic Rocks and Crustal Fracturing in S.E. Australia. Geol. Rundschau. 47 : 543-561. Morris. M .• 1914. On the Geology and Petrology of the District between Lilydale and Mount Dandenong. ' Proe. Roy. Soc. Vic .• 26(2) : 331-336. Richards. H. C .• 1909. the Separation and Analysis of Minerals in the Dacite of Mount Dandenong. Victoria. Ibid. 21(2) : 528-539. Skeats. E. W .• 1910. Gneisses and Dacites of the Dandenong District. Quart. Journ.Geol.Soc. London. 64 : 450-469. Valliullah, M., 1964. A Study of Upper Devonian Volcanic Complexes in Central\rictoria . . Unpublished Thesis. Univ.Melbourne. VandenBerg. A. H. M. 1971. Explanatory notes on the Ringwood 1:63.360 geological map. Mines Dept. Vic .• Geoi. Surv. Rept. 1971/1
On
46 CHAPTER
6
GEOLOGY OF THE MORNINGTON PENINSULA by V.A. Gostin GENERAL. The bedrock of the Mornington Peninsula is well exposed along its axis, and consists of strongly folded Ordovician and Silurian sediments, intruded by granitic plutons of probable Upper Devonian age. The Peninsula is essentially a horst with a prominent graben to the west (the Port Phillip Sunkland) and a lesser negative area to the east (the Western Port Sunkland). The strike of the major faults is NNE-SSW, parallel to the trend of the folded Palaeozoic sediments, although several cross faults and diagonal faults are known (Keble, 1950). Several faults have shown recurrent movement and probably date back to the Palaeozoic. Many faults were active during the Tertiary and this movement has persisted to Recent times, with earthquake trerrlOrs originating in the Selwyn Fault zone. This fault, on the western edge of the Peninsula, is very im.portant having a throw of over 2,000 ft. during the Cainozoic.
t
Melbourne 12 miles
Quaternary Sediments
Sediments
]
Older Volcanics
Tertiary
Mesozoic Sediments
Granite Sediments
] L. Palaeozoic
4 miles
PORT PHILLIP BAY
N
N f
BASS
STRAIT
Schanck
Fig. 1.
Geological map of the Mornington Peninsula (after Keble, 1950).
47 SEDIMENTARY
ROCKS
Balcombe Bay
··. EZJ .-..' ..
Pleistocene
1:- ;·1
Tertiary
1+ '" ~ 1
Upper Ordovician
'
•• •
- •-
~ Lower Ordovician IGNEOUS ROCKS ~ Older Volcanics
1+++1 o
Granites Martha
I
Miles Dromana Bay N
1
+
+t + 1\
+
/'
+
1\
!\
~I\ ct
II
1\
!\ !\
'\
1\
/\
'\
1\
f\ '\
\ .\
'\
1\ f\
,".
/\
,"-,
"-
."-
"
A
/\
., Fig. 2.
A
'\
.\
Geological Map of the Arthurs Seat and Red Hill area (Keble. 1950).
Port
48 PALAEOZOIC. The Palaeozoic sediments are geosynclinal and have been folded into a broad anticlinorium with its main axis striking at 020 0 , parallel to the length of the Peninsula. The axis passes through McIlroy's Quarry, 4 miles east of Dromana, where the oldest strata of Lancefieldian (La2) age are exposed (Locality 1). East of this axis progressively younger beds are exposed including those of Bendigonian and Castlemainian age, followed by Middle and Upper Ordovician, and some Silurian sediments on the eastern side of the Peninsula (Keble, 1950). The Ordovician sediments are estimated to be some 10,000 ft. thick, and consist of silty, generally fine grained sandstones (greywackes), graptolitic shales and slates, in a monotonous thin-bedded sequence deposited mainly under deep marine anaerobic conditions. A succe ssion of beds within the Lancefieldian consists of light coloure d unfos silife rous sandy shales and medium sandstones, and is thought to have been deposited under shallow marine conditions ("Kangerong Stage" of Keble, 1950). The greywackes are quartz rich, with some mica, chert and soda feldspar. They were probably turbidity current deposits whereas the interbedded dark shales accumulated during the quiet periods and frequently contain graptolites and pyrite (Hills and Thomas, 1953). The Ordovician strata have been silicified to a certain degree, but this has not affected the graptolites. No conglomerates, limestones or volcanic-derived sediments are known. The succeeding Silurian sediments consist of lighter coloured mudstones, shales, medium to coarse grained sandstones and a conglomerate. Fossils include brachiopods, crinoids, polyzoa and some graptolite s. Shallow marine ne ritic conditions are indicated. Igneous rocks of probable Upper Devonian age inClude the Dromana Granite with associated dacites at Arthur's Seat, the Mount Martha Granodiorite and the Mount Eliza Granodiorite. Around the summit of Arthur's Seat is a small patch of rhyodacite and a hornblende dacite, which have been intruded by the Dromana Granite (Baker, 1938) (Locality 3). The granite is a medium, even-grained rock with abundant greenish orthoclase. In thin section it consists of quartz, orthoclase pe rthite, oligoclase and biotite. The oligoclase is often blocky due to inte rgrowth with orthoclase and sometimes possesses saussuritized cores. Xenoliths are scarce. Joints and small faults are common, especially to the north-west near Selwyn Fault. The Mount Martha Granodiorite is generally grey, medium grained and consists of quartz, zoned poikilitic oligoclase, orthoclase microperthite and abundant biotite. Some hornblende is associated with biotite in dark coloured clots and has been largely altered to biotite. The Mount Eliza Granodiorite is similar but contains a greater proportion of biotite (Keble, 1950).
s
... ...0
'"ro
"" ::l 0 ·... I-