Coorong Dolomites (South Australia) and Models of Ancient Coorong-Style Dolomites: ABSTRACT

Abstract

The Coorong Dolomites and associated magnesian and calcian carbonate muds form sporadic deposits over a 90 × 200 km (56 × 125 mi) area of coastal dune strand plain in southeastern South Australia. They are deposited wherever the Pleistocene dune surface intersects the water table. A reconstruction based on the hydrological setting of the Coorong is a facies mosaic model. It includes the ideas that: (a) dolomite and magnesian carbonate deposition was widespread and associated with resurging continental groundwater; and (b) although widespread, the actual volume of primary dolomite is insignificant compared to the total volume of lacustrine and other carbonate sediment. The lacustrine sediment in the interdunal depressions builds up to the highest level attained by the water surface, equivalent to the outcropping water table. The upper portion of these carbonate muds often contains one, sometimes two, indurated crusts. Such crusts contain subtle, small-scale structures called extrusion tepees. These tepees form as layers of mud are injected from below into megapolygonal cracks in the crust. Successive layers of mud are cemented to the sides of the crack as the lake desiccates each summer. Ongoing episodes of injection and cementation expand the crust volume until it overthrusts into large-scale tepee structures. An ancient dolomite is a shallowing upward mudstone sequence which shows increasing evidence of subaerial exposure in the upper levels of the cycle. The lower portion of the cycle is a relatively thick, organic rich, laminated, calcitic mudstone (2-3 m, 7-10 ft, thick). This passes up into a lighter, highly bioturbated and pelleted dolomudstone (1-2 m, 3-7 ft, thick). The upper zone is characterized by crusts containing extrusion tepees, intraclast breccias, and siliceous cements. The succession is capped by a well-developed palaeosol with portions of the uppermost zones sometimes showing evidence of dedolomitization. One should be wary of the idea that ancient Coorong-type dolomites are never associated with evaporites. In the modern Coorong lakes, gypsum and halite grow as an ephemeral phase in the near surface and surface muds during late summer. The same crystals are dissolved out by winter rains, and their molds destroyed by bioturbation and the thixotropic nature of fresh dolomite mud. In Precambrian analogs, the lack of bioturbation could lead to the retention of some evaporite pseudomorphs. Also the modern hydrology of the Coorong is highly compartmentalized. A laminated gypsarenite sequence at least 2 m (7 ft) thick is End_Page 537------------------------------ forming in a coastal salina (Halite Lake) less than 2 km (1 mi) from the area where the thickest Coorong dolomites are forming. Obviously evaporite filled ponds can occur in an ancient Coorong-type facies mosaic, and the distribution of evaporites versus dolomites could be a good palaeoslope indicator. End_of_Article - Last_Page 538------------