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Abstract

Many types of penecontemporaneous dolomites have been explained in the literature by involving the well-known sabkha model which is typically associated with an arid climate. The various carbonates now precipitating in the ephemeral lakes of the South Australian Coorong Lagoon, however, are the products of a more humid climatic and hydrologie regime. The distribution of carbonates in the Coorong region is largely controlled by the hydrology of the depositional environment. Characteristic suites of sedimentary structures are formed in specific parts of the Coorong system, including desiccation features and stromatolites. These suites can also be identified with a remarkable degree of confidence in the 1600 m.y. old Yalco Formation of the McArthur Group, Northern Territory, Australia. The following conclusions can be drawn from the comparison. (1) All penecontemporaneous dolomites are not necessarily formed in an arid sabkha-like environment; the Yalco Formation dolomites formed in a more humid environment analogous to that of the Coorong, in which distinct climatic and seasonal controls prevail. (2) The lack of evaporite minerals or evaporitic casts in an ancient dolomite sequence does not mean that concentrated brines were never present. Evaporite minerals may be precipitated during dry summer months, but are flushed out during winter by a reflux mechanism as occurs in the Coorong dolomite region. (3) By comparison with the Coorong system, ancient water table movements can be inferred from vertical sequences of sedimentary structures in the Yalco Formation, in conjunction with evidence from the underlying and overlying formations. The stratigraphy of the Lynott Formation, Yalco Formation, and Stretton Sandstone can be explained as a diachronous regressive sequence using the hydrological model developed in this paper.

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