PALEOENVIRONMENTAL AND TAPHONOMIC IMPLICATIONS FOR LOWER CAMBRIAN PHOSPHATIC HARDGROUNDS FROM SOUTH AUSTRALIA (Invited Presentation)

The resolution in our understanding of stem-group metazoans is owed in part to the exceptional taphonomic circumstances present during the Cambrian Period. With respect to millimetric small shelly faunas (SSFs), this window into the fossil record manifests as a consequence of extensive phosphatization that spanned approximately 35 million years. Increased delivery of organic-rich waters to shallow carbonate shelves and liberation of phosphates via bacterial decay within the sediment created optimum microenvironments for the precipitation of phosphates on a global-scale. Consequently, this enhanced the preservation of originally calcareous SSFs as well as lightly mineralized to organic tissues via secondary phosphatization, capturing a rare array of microfossil constituents in exceptional detail.

Herein, we report a laterally extensive ~3-meter interval of repeated phosphatic hardgrounds from the lower Cambrian Wirrapowie Limestone, Ikara-Flinders Ranges, South Australia. Phosphatic hardgrounds are not uncommon for the Arrowie Bain, but their stratigraphic and taphonomic significance has been largely overlooked. Using a combination of micro-CT, microfacies analysis, and fossil residues, we investigate the microstratigraphy and geometry of phosphatic omission surfaces in conjunction with taphonomic characteristics of the SSFs contained therein. From this work, it is possible to (1) elucidate various depositional phases associated with changes in hydrodynamic regimes that favored phosphogenesis; (2) tie these intervals to regional-scale paleobathymetric trends; and (3) reveal specific conditions involved in SSF preservation and potential biases imposed on biostratigraphic and paleoecological studies of such fossil assemblages.