GSV Report 118 - Omeo 1:100 000 map area geological report

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Abstract This report describes the geology of the area covered by the Omeo 1:100 000 mapsheet (8423) 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 and includes the southern end of the Omeo structural zone as well as parts of the Tabberabbera and Buchan structural zones. Although part of the region is very rugged, access 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 as small fault bounded slices in three fault zones. It is generally not differentiated into formations. The Lower Silurian Yalmy Group is also present only in fault slices. During the Benambran Deformation at about 430 Ma, the Ordovician and Lower Silurian rocks were strongly deformed, with deformation accompanied by intrusion of S-type granites and production of the high-T, low-P schists, gneisses and migmatites of the Omeo Metamorphic Complex.

Late Silurian rocks are preserved in the large Limestone Creek Graben and the smaller Scrubby Creek Syncline in which the Mount Tambo Group was deposited. Probable Late Silurian rocks occur also in a newly discovered fault slice along the Barmouth Fault. The Limestone Creek Graben contains a lower sequence of silicic volcanics and intrusives (Thorkidaan Volcanics), overlain by marine and nonmarine clastics with volcanics and intrusives, the Enano Group. The Mount Tambo Group is a very thick sequence of mainly subaerial conglomerate, sandstone and mudstone with a prominent felsic volcanic component. The Barmouth Group along the Barmouth Fault consists of a volcaniclastic unit (Koomberar Formation) overlain by marine sandstone and mudstone (Nobby Road Sandstone).

The Silurian rocks were folded and faulted during the Bindian Deformation at the end of the Silurian, followed 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 at Mount Elizabeth, the Mount Elizabeth Caldera Complex was formed. Tiny remnants of Tongio Munjie Volcanics at Swifts Creek suggest volcanism was much more widespread. At Bindi, deposition of the largely ignimbritic Avonmore Subgroup of the Snowy River Volcanics was followed by pronounced subsidence during which the marine Buchan Group was deposited. Both the Snowy River Volcanics and Buchan groups were folded and cleaved during the Middle Devonian Tabberabberan Deformation.

The Triassic Mount Leinster Igneous Province lies mostly within Benambra 1:100 000 map area but several of the granites extend into Omeo 1:100 000 map area and there are also trachyte plugs and dykes.

The late Mesozoic and Tertiary history is mainly one of block faulting, deep incision and stream diversion, but in the Eocene there was eruption of extensive Older Volcanics basalts, now preserved only on the Nunniong Plateau. Extensive colluvial fans are developed along the large Livingstone Creek fault scarp, and extensive alluvial flats and terraces occur along the Tambo River, with more narrow flats along many of the smaller streams.

Newly named and/or defined units include the Sapper Nose Rhyolite Member of the Silurian Thorkidaan Volcanics, the Silurian Barmouth Group (Koomberar Formation and Nobby Road Sandstone), the Berrawan Conglomerate, Moonip Sandstone Member, Blackfellows Flat Conglomerate, Vitric Ignimbrite Member, Shanahan Sandstone and Old Mill Ignimbrite Member of the Upper Silurian Mount Tambo Group, the Lower Devonian Avonmore Subgroup of the Snowy River Volcanics (Attunga Paringa Formation, Carriage Range Ignimbrite, Tin Pot Ignimbrite, Quindalup Ignimbrite and Roadsend Formation); the Lower Devonian(?) Tongio Munjie Volcanics and the Neogene Oriental Claims Gravel. Newly named granites and suites include the Cobungra, Kimberly Park, Mountain Maid and Mount Burt granites; the Connleys Track, Moscow Villa, Cocks Break and Bentleys Plain granodiorites; the Jim and Jack Tonalite, Saturday Morning Tonalite, the Commins Track Leucogranite, the Rileys Creek Suite (Swifts Creek Igneous Complex: Rileys Creek Granodiorite, Brookville Granite, O’Dell Tonalite, Bayliss Spur Tonalite, Tongio Quartz Diorite; Mount Baldhead Igneous Complex: Mount Baldhead Granodiorite and Marthavale Tonalite; and the unassigned Old Sheep Station Granodiorite and Mungobabba Tonalite), the Polar Star Suite (Polar Star Tonalite, Dry Hill Granodiorite, Livingstone Creek Tonalite and Hallets Road Tonalite), the Ensay ‘Suite (Doctors Flat Tonalite, Bald Hills Creek Tonalite, Reedy Flat Tonalite, Connors Creek Tonalite, Emu Vale Tonalite, Holstons Tonalite and previously named Tambo Crossing Tonalite), and the buried Hells Hole Creek, Camp Oven Gap and Alpine Road plutons. The Sisters Granite and Day Hill Syenite are newly named Triassic intrusions.

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 Willman, C.E., Morand, V.J., Hendrickx, M.A., VandenBerg, A.H.M., Haydon, S.J., Carney, C., 1999. Omeo 1:100 000 map area geological report. Geological Survey of Victoria Report 118.