Abstract

Geologically significant and spatially extensive deposits of primary lacustrine dolomite are found over a 90 x 200 km region of the coastal plain in the vicinity of the Coorong Lagoon, in southeastern South Australia. The microcrystalline dolomites and associated cabronates, which range in age from Pleistocene to Recent, occur beneath interdune flats between stranded barriers. The most active but by no means only region of present-day microcrystalline dolomite formation is the linear zone of shallow. ephemeral alkaline lakes which occupies the regional groundwater discharge area, immediately landward of the Coorong Lagoon and along the southeastern extension of its interdune corridor. Comparative studies between sediment cores from this region and a range of contemporary sedimentary environments show that it is possible to characterize the sedimentary sequence which is being generated by Holocene sedimentary regression. In its ideal form this sequence passes upward from a basal marine or lagoonal unit, through a protodolomite + Mg-calcite lacustrine unit, to culminate in an uppermost dolomite or dolomite + magnesite unit which may be capped by a soil. Hydrological considerations, which imply the presence of a landward-dipping continental groundwater-marine groundwater interface immediately inland from the coast, show the above sedimentary sequence to be the result of a progressive seaward movement of that interface, caused by sediment accretion and consequent shoreline regression. The position of a particular microcrystalline carbonate-forming environment at any point in time, with respect to the interface, defines its carbonate mineralogy. These observations clearly indicate that optimal conditions for Coorong-type dolomite formation are found in ephemeral lakes which are fed solely by evaporitically-modified continental groundwater and which occur inland from the interface. The relatively large amounts of Quaternary primary dolomite in the area render it somewhat of a geological anomaly in terms of present-day dolomite-forming environments. This study implies that the main reasons for this anomaly may be a favourable combination of factors which include an abundant, renewable source of magnesium for the seaward-flowing groundwaters, an active grounwater regime, and a climate which seasonally varies from humid to semiarid. The most likely source of magnesium is hypothesized to be from weathering of areally extensive Quaternary basic volcanics which occur in groundwater source areas marginal to the coastal plain. From a geological viewpoint it is suggested that many microcrystalline dolomite occurrences in the geological record may be of the Coorong-type, formed in shallow migrating ephemeral lakes in a relatively humid climate on regressive coastal plains, overlying beach deposits. Such dolomites, which contain no obvious traces of evaporite minerals, are contrasted with superficially similar sabkha-type dolomites which, at least in part, are early diagenetic replacement dolomites and which contain abundant evaporitic associates such as gypsum and anhydrite.

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