Storm-dominated mixed carbonate / siliciclastic shelf sequence displaying cycles of hummocky cross-stratification, late Proterozoic Wonoka Formation, South Australia

Abstract

The regressive late Proterozoic Wonoka Formation of the Adelaide Geosyncline in South Australia contains a thick, mixed carbonate/siliciclastic interval of middle- to inner-shelf origin. The shelf was storm-dominated and displays abundant hummocky cross-stratification (HCS) of varying scales, and other wave-formed sedimentary structures. Some parts of this sequence are notably cyclic. Two kinds of cycles are described here. The first (average cycle 8 m thick) consists of the alternation of silty and argillaccous limestone with fine-grained hummocky cross-stratified calcareous sandstone. Climatic cyclicity (e.g. Milankovitch cycles) involving alternate wet and dry periods, or minor sealevel fluctuations along a low gradient shoreline, are considered the most likely cause of the observed sediment cyclicity. Carbonate deposition predominated during dry periods, or high sealevel, followed by influxes of sand from deltaic sources during wet periods, or times of low sealevel. The prevalence of HCS in the sandstones is most likely due to much slower rates of lithification in comparison to the limestones, which give evidence of rapid lithification on the seafloor. The second type of cycle (average cycle 5–10 m thick) is characterised by the cyclic appearence of red, hummocky cross-stratified glauconitic and phosphatic limestones, within a sequence dominated by grey-green stylonodular limestone tempestites. The most likely origin of the red limestone involves contributions of volcanic ash fall material. The resulting sediment did not undergo the rapid seafloor lithification of the associated grey-green limestones and thus was involved in considerable storm wave reworking before deposition. The micritic shelf carbonates of the Wonoka Formation are considered to represent direct inorganic precipitation into the middle-shelf environment, rather than reworking of detrital material from shallow carbonate banks.