VIMP Report 25 - The geology and prospectivity of the Orbost 1:100 000 map area, Eastern Victoria
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Product description:Download The downloadable version of this report is supplied in PDF format. Abstract This report describes the geology of the area covered by the Orbost 1:100 000 mapsheet in eastern Victoria. It gives a comprehensive account of the geological history, Palaeozoic and Cainozoic rock units, structural, metamorphic and economic geology and of the geophysical information, together with an extensive list of references. The region lies in the eastern part of the Lachlan Fold Belt and covers the southeastern extremity of the Tabberabbera and southern portions of the Buchan and Mallacoota structural zones. Exposure is generally much poorer than in the mountainous terrain further north, and in the south the Palaeozoic rocks disappear under Tertiary subaerial and marine sediments. The oldest rocks are Ordovician and are part of a sheet of turbidites and black shales that cover much of the eastern Lachlan Fold Belt. The older Adaminaby Group (Lower Ordovician) forms the bedrock over most of the area and consists of thick to thin bedded quartz-mica turbidites and siltstones with occasional intervals of thin bedded grey chert. The overlying Bendoc Group (Upper Ordovician) is only preserved as fault bound slices in a number of fault zones. It consists of three formations; Sunlight Creek Formation, Warbisco Shale and Akuna Mudstone, all dominated by black to dark grey shale, chert and siltstone. The Lower Silurian Yalmy Group (probably unit 3) occurs in a fault slice at Nowa Nowa. Upper Silurian? limestone occurs in a single drillhole. The complex Lower Devonian Snowy River Volcanics, preserved in two segments of the Buchan Rift, include subaerial fanglomerates, pyroclastics and minor fluvial sediments and submarine volcanic mass flow deposits, shales, breccias and intrusives. They are subdivided into 21 formations, most of which are grouped into five subgroups (Timbarra, Mount Dawson, Ninnie, Tara Range and Little River). They are overlain by the Lower Devonian Buchan Group, a shallow marine carbonate and mudstone sequence. The Lower Palaeozoic rocks are overlain by an extensive sheet of Tertiary gravels which cover much of the southern part of the mapsheet. Granites form a significant part of the area. Most are I-types, of probably Middle Silurian age, with one, the Colquhoun Granite, probably Lower Devonian. These are discussed and described in detail. The area has been affected by at least three deformation episodes: the Benambran Deformation, at the beginning of the Silurian: production of bedding parallel fabric in Lower Ordovician Pinnak Sandstone; the Quidongan Deformation, in the Middle Silurian, with tight folding of the Ordovician rocks; and the Tabberabberan Deformation in the Middle Devonian, in which the Lower Devonian rocks were folded and faulted. Newly introduced units include the Ninnie and Tara Range subgroups, Fluke Knob, Tomato Creek, Kanni, Stonehenge and Rankin Road ignimbrites, Boggy Creek and Hospital Creek sandstones, Nowa Nowa Conglomerate, and Castor Oil Lava formations, Amberley Park Member, and Eleven Bob, Kaerwut, Crohn, Mollys Plains and Case granites. The geophysics chapter of this report contains an interpretation of the aeromagnetics and radiometrics. The economic geology of the region has been compiled and is discussed in a number of sections covering mineral exploration, mining history, mineralization style and economic potential and prospectivity. Bibliographic reference VandenBerg, A.H.M., Hendrickx, M.A., Willman, C.E., Magart, A.P.M., Oranskaia, A, Rooney, S., and White, A.J.R., 1996. The geology and prospectivity of the Orbost 1:100,000 map area, Eastern Victoria. Victorian Initiative for Minerals and Petroleum Report 25. Department of Natural Resources & Environment. Related products:
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