Tertiary Lake Bunyan, Northern Monaro, NSW, Part I: Geological setting and landscape history

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Abstract
The Cainozoic Lake Bunyan Basin occupies part of the Cooma‐Canberra corridor, a lowland which is bounded in the W by the Murrumbidgee Fault escarpment and in the E by N‐S trending ranges. The Murrumbidgee and Numeralla Rivers are the main streams entering the basin. Lake Bunyan covered an 8 km wide strip of this lowland from near Bredbo to 30 km SE in the vicinity of Cooma. It formed when the Murrumbidgee River became fault‐dammed 5 km S of Bredbo, during the late Oligocene or early Miocene, and it was finally breached in the late Miocene. Five sedimentary facies were identified in the Lake Bunyan sequence. A marginal facies of stratified gravels, sands and sandy clays unconformably overlies bedrock or weathered bedrock at the base of the sequence and also overlaps other facies. A lignitic facies is thin and often interbedded with clay. A volcanogenic facies is up to 50 m thick, has strong red colours, and consists of stratified clays and gravels, the latter containing volcanic debris. A 40 m thick clay facies occurs in the upper part of the sequence and is very fine grained. A diatomite facies at the top of the sequence is up to 13 m thick. The sediments of Lake Bunyan generally occur below the present 780 m contour and were deposited over a vertical interval of at least 210 m. The occurrence of Lake Bunyan, when considered with fault‐dammed Lake George and scarp‐producing movements along the Berridale Wrench Fault, indicates Cainozoic tectonics in the South East Highlands of NSW that were episodic, localized and not synchronous.

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