DISENTANGLING REDOX ENVIRONMENTS AND SMALL SHELLY FAUNAL OCCURRENCES FROM THE NORMALLY FOSSILIFEROUS MERNMERNA FORMATION OF SOUTH AUSTRALIA

Early Cambrian small shelly faunas (SSFs) represent some of the earliest evidence of biomineralization in the fossil record, including organophosphatic and calcareous forms. Cambrian SSF biostratigraphic studies rely on the recovery of fossil material, though the potential biases that impact their stratigraphic and spatial distribution remain poorly understood. Determining whether paleoenvironmental and geochemical processes were conducive to the secondary mineralization of SSFs compared to influencing their primary distribution is a significant challenge. Regardless, redox proxies can provide insight into interpreting SSFs in biostratigraphic and paleoecologic studies.

The Mernmerna Formation (Cambrian Series 2, Stage 3) of the Hawker Group is an interbedded calcareous limestone and shale unit representative of transgressive deepwater slope environments and well-renowned for abundant SSF material across the Arrowie Basin, South Australia. Within the central Bunkers Ranges, the Mernmerna Formation has a well-constrained biostratigraphic framework, encompassing the Pararaia bunyerooensis Zone and Dailyatia odyssei Zones for trilobites and SSFs, respectively. However, exposure of units within the Chace Range in the southern part of the basin has yielded little to no SSF material. Prior lithofacies analyses have demonstrated little difference between the geographical regions; hence, the paucity of preserved faunas in the Chace Range may indicate that other paleoenvironmental factors are limiting the distribution or preservation of SSFs. Herein, we utilize carbonate associated sulfate (CAS) data from the Mernmerna Formation of the Chace Range and the central Bunkers Ranges to directly compare CAS and SSF abundance trends across the temporally equivalent deposits. Notably, this study presents the first δ³⁴S_CAS results from Cambrian successions of South Australia, providing pivotal insights into redox patterns across the Arrowie Basin. Moreover, these data will help bridge existing gaps in the global δ³⁴S_CAS dataset during Stage 3 of the Cambrian, allowing for further understanding of the relationship between animal diversification and changes in ocean chemistry.