GSV Report 89 - Colac 1:50 000 map area geological report
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Product description:Download The downloadable version of this report is supplied in PDF format and is a large file (19.2MB). Abstract Mapping of the Colac area has resulted in a 1:50 000 geological sheet accompanied by these explanatory notes. The notes describe the physiography, stratigraphy, structure, economic geology and geological hazards. The geology of the Cretaceous and Tertiary formations in particular is intimately related to the separation of Australia and Antarctica. The dynamics of this separation caused different areas to be either upfaulted or downfaulted at various times up until the end of the Tertiary. These movements have controlled the nature of the sedimentation. The oldest unit outcropping in the area is the Early Cretaceous Otway Group consisting predominantly of volcanogenic, fluviatile sandstone. Overlying this with mild unconformity is the Eastern View Formation which ranges in age from Early Palaeocene to Late Eocene. It is predominantly sand and in the main fluviatile in origin. Following a period of weathering and erosion, sedimentation continued with fine grained marine sediments prevailing. This period lasted from the Late Eocene to the Middle Miocene and is represented by the Demons Bluff Formation, Clifton Formation and Gellibrand Marl. A minor volcanic event occurred in the Late Oligocene with submarine tuff and pillow basalt of the Yaugher Volcanics interbedded in the upper part of the Demons Bluff Formation. The volcanic vent which was the source of these rocks is also preserved. Although there is evidence for faulting prior to and throughout the Tertiary, the main faulting event took place at the end of the Tertiary when a series of northeast-southwest horsts and grabens were formed. These are thought to have resulted from right lateral strike slip movement along the Colac Monocline. The Tertiary beds are folded into broad anticlines and synclines on the horsts and grabens respectively and as a rule are not disrupted by faults but are monoclinally folded. In the Quarternary, basalt and pyroclastics of the Newer Volcanics were erupted during two main phases. The Younger phase is best preserved and includes the "stony rise" basalt and a maar/scoria cone complex at Red Rock. The outflowing of the Newer Volcanics disrupted the drainage pattern on the northern part of the map area leading to the formation of many lakes with lunettes on their eastern shorelines. Two phases of lunettes were mapped, clay and sand. Extensive lake floor sediments are associated with the sand lunettes while clay lunettes formed as a result of deflation of the adjacent dry lake beds. Deposits of sand, limestone, rock aggregate, clay for brick making and bentonite have been described. Brown coal is present in the Eastern View Formation and although no major deposits have been found, there is a possibility of small scale deposits occurring at shallow depth in areas near the limits of the formations outcrop. Groundwater is a significant resource in the area, particularly in the Barwon Downs Graben. The main aquifer is the Eastern View Formation which is presently being tested as a supplementary water supply for Geelong. The yields and water quality of various other aquifers are also discussed. Several of the geological formations on the mapsheet are prone to landslides. The types of failures are described and the potential for future landslides is discussed. Bibliographic reference Tickell, S.J., Cummings, S., Leonard, I.G. & Withers, J.A., 1991. Colac 1:50 000 map geological report. Geological Survey of Victoria Report 89. Related products:
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