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Many types of penecontemporaneous dolomites have been explained in the literature by involving the well-known sabkha model. The various carbonates now precipitating in the ephemeral lakes of the South Australian Coorong Lagoon are the products of a more humid climatic and hydrologic regime. The distribution of carbonate rocks in the Coorong region is largely controlled by the hydrology of the depositional environment. Both primary and early diagenetic mineralogy can be related to regional hydrology as it has varied throughout the Quaternary. Characteristic sedimentary structures (including stromatolites) are formed in specific parts of the Coorong system, and these can be confidently identified in an ancient analogue, the 1,600-m.y.-old Yalco Formation of the McArthur Gro p of Australia. The resemblance between the ancient and modern environments, in terms of both sedimentary structures and mineralogy, is striking. The following conclusions can be drawn from the comparison:
1. All penecontemporaneous dolomites are not necessarily formed in an arid sabkha environment; a significant number may be formed in a more humid environment analogous to that of the Coorong, in which distinct climatic and seasonal factors prevail.
2. The lack of evaporite minerals or evaporitic casts in an ancient dolomite sequence does not mean that concentrated brines were never present. In the modern Coorong system, minor evaporite minerals are precipitated in the dolomite lakes during dry summer months, but are flushed out during winter by a reflux mechanism.
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