Bu5 - Coimadai - Lake Merrimu - Dolomite

This information has been developed from this publication:

Sites of Geological and Geomorphological Significance in the Western Region of Melbourne (1986) by Neville Rosengren

Geological heritage sites, including sites of geomorphological interest and volcanic heritage sites, are under regular revision by the Geological Society of Australia, especially in the assessment of significance and values. Reference should be made to the most recent reports. See the Earth Science Heritage (external link) section of the Geological Society of Australia website for details of geological heritage reports, and a bibliography.

Location:	Bullengarook - 787337 to 791321. Shire of Bacchus Marsh. Eastern shore of Lake Merrimu storage.
Access:	From Coimadai to Diggers Rest road.
Ownership:	Crown land administered by Rural Water Commission
Site Description:	Outcropping above the eastern shoreline of Lake Merrimu are Permian glacial rocks overlain by sediments of the Werribee Formation. The uppermost unit of the Werribee Formation is the Coimadai Dolomite Member, a pale-coloured dolomite (magnesian limestone) that occurs in hard (recrystallised) as well as soft bands. The dolomite is thin-bedded and includes interbedded tuff, clay sand and gravel, and has a gentle dip towards the west. The dolomite has been the subject of a number of geological papers offering slightly different interpretation as to the age, origin and stratigraphic status of the dolomite. The most recent evaluation is that the dolomite has member status as part of the Werribee Formation and is a chemical precipitate deposited in a lake. Although there are tuff bands associated with the deposit, it is older than, and not associated with , the lava flows from Mount Bullengarook.
	Fossil bones (wombats, microbodies and diprotodont) have been reported from the dolomite deposit. Several quarries have operated in the dolomite and soft beds are exposed in abandoned pits. The harder dolomite outcrops as thin and discontinuous bands around the eastern shore of Lake Merrimu.
Significance:	Regional. The site is the only worked dolomite deposit in the study area and one of the few large areas of freshwater magnesian limestone in Victoria.
Management:	Class 2. Much of the site will be flooded when the new full supply level of 175 m is reached in Lake Merrimu. Areas of dolomite remaining above this level include thin hard bands parallel to the lake shoreline. A requirement of any recreation or other development scheme around this shoreline should be to determine the specific location of the dolomite outcrops and design a management scheme that will retain the outcrops.
References:	Coulson A (1924) Keble RA (1925) Gill ED (1964) Roberts PS (1984, 1985)

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