WHERE HAVE ALL THE SMALL SHELLY FAUNAS GONE? A MULTI-PROXY APPROACH TO UNDERSTANDING THE ABSENCE OF FOSSILS FROM THE TYPICALLY FOSSILIFEROUS MERNMERNA FORMATION OF SOUTHERN AUSTRALIA

The Mernmerna Formation of the Arrowie Basin, South Australia is a succession of interbedded calcareous shales and limestones representing a transgressive slope deposit with considerable lateral and stratigraphic variability. This unit comprises a considerable portion of the Hawker Group and is well-known for its abundance and diversity of early Cambrian (Series 2, Stages 3-4) small shelly faunas (SSFs) used in regional biostratigraphic correlation. Due to the considerable thickness and recovery of SSFs from this unit, several stratigraphic sections intersecting the Mernmerna Formation have been targeted for bio-, litho- and chemo-stratigraphic investigation. However, successions of the upper Mernmerna Formation in the Chace Range, Southern Flinders Ranges, have yielded little to no fossil material in either acid residues or thin sections. Due to the absence of fossil material at the Chace Range locality, relatively little geochemical work has been conducted in this upper portion of the formation. Understanding the paleoenvironmental and paleoceanographic conditions that led to the apparent absence of fossil material may provide insight to potential barriers to the preservation or occurrence of small shelly faunas in this part of the basin.

We employed a multi-proxy approach to better understand the paleoenvironmental and paleoceanographic factors that led to the lack of fossil material in the Mernmerna Formation at the Chace Range locality. Limestone blocks were collected for geochemical analysis along a 505-m thick measured stratigraphic section (CR2-taph) from the contact with the underlying Wirrapowie Limestone. We conducted stable isotopic analyses (δ¹⁸O_carb, δ¹³C_carb, δ³⁴S_CAS) and rare earth element + yttrium (REY) distributions from bulk carbonates. These geochemical proxies can be used to infer freshwater input, enhanced pyrite burial, erosion/deposition of organic-rich material, and seawater oxygenation. Additionally, we conducted a facies analysis to determine systematic changes in depositional environment. Initial results of the isotopic analyses indicate low δ¹³C_carb and high δ³⁴S_CAS values in the first 150 meters of the section, which may indicate decreased burial of organic material and increased sulfate burial, while Ce/Ce* results suggest normal oxygenation conditions.