GSV Report 114 - Benambra 1:100 000 map area geological report

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Abstract This report describes the geology, geophysical character and mineral potential of the area covered by the Benambra 1:100 000 map area in eastern Victoria. It is an explanation of the accompanying 1:50 000 geological maps and provides a comprehensive account of the geological history, Palaeozoic, Mesozoic and Cainozoic rock units, structural, metamorphic and economic geology and of the geophysical information, together with an extensive list of references. Brief descriptions of the best geological sites are given in Appendix 1.

The region lies in the eastern part of the Lachlan Fold Belt at the margin between the Omeo and Buchan structural zones. Most of the region is very rugged and has poor access, although exposure is generally good.

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 in major synclines and as fault bounded slices in a number of fault zones. It consists of three formations; Sunlight Creek Formation and Warbisco Shale, dominated by black to dark grey shale, chert and siltstone, and New Country Sandstone, with a high proportion of dark sandstone. During the Benambran Deformation at about 420 Ma, the Ordovician rocks were strongly deformed, with deformation accompanied by intrusion of S-type granites and production of the high-T, low-P schists and migmatites of the Omeo Metamorphic Complex.

Late Silurian rocks are preserved in two large grabens, the Wombat Creek and Limestone Creek grabens, which together make up the Cowombat Rift. Both contain a lower sequence of silicic volcanics and intrusives (Mitta Mitta Rhyolite and Thorkidaan Volcanics, respectively), overlain by marine and nonmarine clastics, the Wombat Creek and Enano groups, of which the latter includes volcanics and intrusives.

The Silurian rocks were folded and faulted during the Bindian Deformation at the end of the Silurian, and again this was accompanied by igneous activity in the broad meridional Cravensville Igneous Province, which appears to have been a zone of extension. Numerous large and small I-type intrusions were emplaced, and the large Dartella and small Besford calderas were formed. The complex Dartella Caldera contains the Dartella Volcanics, which show an upward trend from highly silicic to intermediate subaerial ignimbrites, whereas the Besford Caldera contains a single rhyolitic ignimbrite. The much thicker and more complex Mount Tambo Group, of which only a small portion lies within the Benambra 1:100 000 map area, also appears to have been deposited at this time. Of the Devonian rocks, only the Mount Tambo Group has been deformed to any extent: it was folded into a tight syncline, presumably during the Middle Devonian Tabberabberan Deformation.

The Triassic Mount Leinster Igneous Province lies mostly within the Benambra 1:100 000 map area and consists of K-feldspar–rich granites and syenites, together with patchily preserved trachytes and volcaniclastic sediments (Teapot Creek Formation).

The late Mesozoic and Tertiary history is mainly one of block faulting, deep incision and stream diversion, but in the Pliocene there was eruption of a long valley flow of basalt, the Morass Creek Basalt. This itself shows evidence of post-basaltic faulting and in places is covered by a thin veneer of Japan Creek Gravel. Extensive colluvial fans are developed along several of the large fault scarps, and alluvial and swamp sediments occur in the Morass Creek and Buenba Creek valleys.

Newly named and/or defined units include the Upper Ordovician New Country Sandstone, the Lower Devonian Mount Tabor Volcanics, Larsen Creek and Besford ignimbrites and Eight Mile Loop Rhyolite, the Triassic Teapot Creek Formation and Little Mount Tambo Trachyte Member, the Silurian Wallaby, Ludrik Munjie and Wattle Grove granites and Mac Creek Granodiorite, the Devonian Bingo Munjie Suite comprising the Bingo Munjie and Charlestown quartz diorites and the Post Office Granite, the Devonian Ludrik Munjie, Hinno Munjie, Lower Tableland and Wattle Grove granites, Burrungabugge Granodiorite, Eustace Creek Diorite and Silver Flat and Mowamba porphyries, and the Triassic MacFarlane, Mole Hill, Bung Bung and Grand View syenites and Beloka Gap and Percydale granites.

The geophysical character of the various rock types is given with the lithological descriptions, and a geophysical interpretation of the aeromagnetic and radiometric data is provided.

The economic geology of the region has been compiled and is discussed in a number of sections covering mineral exploration, mining history, mineralisation style and economic potential and prospectivity.

Bibliographic reference VandenBerg, A.H.M., Hendrickx, M., Willman, C.E., Magart, A.P.M., Simons, B.A. and Ryan, S.M., 1998. The geology and prospectivity of the Benambra 1:100 000 map area, eastern Victoria. Geological Survey of Victoria Report 114.