A barred-basin marine evaporite in the Upper Proterozoic of the Amadeus Basin, central Australia



The Ringwood evaporite is part of the 900 m.y. old Bitter Springs Formation, a warm-water shallow-marine sequence of stromatolitic dolomite and limestone, microfossiliferous chert, red beds, quartzite, and evaporites. The evaporite at Ringwood comprises two parts: (i) a lower 127 m characterized by brecciated pyritic bituminous dolomite, together with smaller amounts of dolomite-gypsum breccia, friable chloritic dololutite, coarsely crystalline anhydrite, and satin-spar gypsum; and (ii) an upper 133 m which is similar except that bituminous dolomite forms only one bed, and the characteristic rock-type is dolomite-gypsum breccia. The evaporite is overlain by limestone breccia and massive stromatolitic limestone, interpreted as an algal reef. Gypsum is secondary after anhydrite, and the ratio of gypsum to anhydrite increases upwards. The evaporite shows none of the features of a sabkha or desiccated deep ocean basin deposit, and instead is interpreted as the filling of a barred basin which was cut off from the ocean by growth of an algal barrier reef. As circulation became restricted, bituminous dolomite deposited in the lagoon behind the reef, together with pyrite from the destruction by anaerobic bacteria of algal debris derived from the reef. With continued evaporation, brine concentration increased and gypsum precipitated. Occasional dust storms contributed wind-blown clay to the deposit. The barrier reef transgressed diachronously across the evaporite lagoon, and was eventually drowned when normal marine conditions became established. Burial of the evaporite to about 7000 m beneath the succeeding sediments of the Amadeus Basin converted gypsum to anhydrite, and formed chlorite by reaction of clay with dolomite. Late Palaeozoic tectonism folded and brecciated the rocks, and was followed by erosion which eventually exposed the evaporite to ingress of meteoric water. Hydration of anhydrite to gypsum ensued, the reaction becoming less complete with increasing depth from the ground surface.