SMALL SHELLY FAUNAS AND REPEATED PHOSPHATIC HARDGROUNDS FROM THE LOWER CAMBRIAN, WIRRAPOWIE LIMESTONE, IKARA-FLINDERS RANGES, SOUTH AUSTRALIA

Hardground surfaces represent periods of synsedimentary lithification of the seafloor prior to the deposition of overlying sediments. Preservation of hardground surfaces via phosphatization is common during the early and middle Cambrian due to the upwelling of organic-rich bottom waters onto carbonate shelves and subsequent bacterial decay of organic matter which led to global-scale phosphate precipitation. This window of extensive phosphatization not only led to the formation of phosphatic hardgrounds but increased the preservation potential of small shelly faunas (SSFs) through secondary phosphatization. Within the Arrowie Basin of South Australia, SSFs are well documented whereas phosphatic horizons, while not uncommon, have typically been overlooked.

Herein, we document a series of repeated phosphatic horizons from a laterally extensive ~3 m-thick interval of the lower Cambrian (Series 2, Stage 3) Wirrapowie Limestone, Ikara-Flinders Ranges, South Australia. Microfacies analysis reveals a suite of distinct lithologies characteristic of periods of prolonged condensation and reworking, including glauconite-rich bioclastic wackestones, peloidal packstones, oncoidal packstones, and glauconitic grainstones. To investigate potential changes in paleoenvironment, inorganic carbon and oxygen isotopes were collected from each sample horizon, including high-resolution data collected across phosphatic omission surfaces using microsampling techniques. In conjunction with stratigraphic and isotopic data, fossil data were collected using both acid maceration and computed tomographic microscopy techniques to investigate differences in the preservation potential of constituent faunas across phosphatic horizons. The implications for these repeated episodes of condensation are twofold. Firstly, diagenetic phases of phosphogenesis in this interval enhance the preservation potential of select SSFs but also impact their means of extraction by traditional acid maceration techniques, thereby introducing both preservational and sampling biases. Secondly, determining the timing of these omission surfaces within the established SSF biostratigraphic framework may help to resolve the sequence-stratigraphic context of this succession within the Arrowie Basin.