| Abstrakt: |
The Coorong Coastal Plain in the south‐east of South Australia is a series of uplifted and stranded Pleistocene, beach‐dune, aeolianite complexes (Bridgewater Formation) that range in age from Middle Pleistocene to Holocene. Aeolianite ridges are separated by elongated corridors, the sites of interdune sedimentation. The youngest corridors, sites of Holocene lacustrine deposition, are also locations of syndepositional dolomite formation. These lakes, particularly south of the Coorong Lagoon, are world‐famous and extensively studied. The older corridors, by contrast, are poorly understood. This study investigates the numerous corridor sediments and limestones between aeolianites across the coastal plain. There are two formations: (i) the Padthaway Formation, a middle Pleistocene, Marine Isotope Stage 15 to Marine Isotope Stage 2 deposit with three facies; and (ii) the Glanville Formation, a younger late Pleistocene Marine Isotope Stage 5e, unit with one facies containing warm‐water biota that is not present in the area today. These carbonates are interpreted to record deposition in palaeoenvironments ranging from lacustrine to estuarine–lagoonal. There is very little dolomite in the deposits. Aragonite and Mg‐calcite are present and most abundant in corridors younger than 350 ka. There are only seven samples of Padthaway Formation carbonates that contain any dolomite, with only two containing more than 50%. Limestone and dolomite stable isotopes are similar with negative δ13C values (−3.9 to −8.6‰) and variable δ18O numbers (+3.0 to −2.4‰). Such values are quite unlike those for younger synsedimentary dolomites. Mineralogy and geochemistry suggest original low‐magnesium calcite‐rich sediment that has been modified to varying degrees by meteoric diagenesis. Lack of synsedimentary dolomite is ascribed mainly to more humid climatic conditions in the past. The study emphasizes that lacustrine synsedimentary dolomite forms under semi‐arid conditions when lakes, fed by Mg‐rich and Ca‐rich resurging groundwaters and under bacterial mediation, are evaporated to dryness. [ABSTRACT FROM AUTHOR] |
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