Reappraisal of the carbonate microfacies, depositional environments and biostratigraphy of the lower Cambrian Second Plain Creek Member, Mernmerna Formation, Flinders Ranges, South Australia

Lower Cambrian successions in the Arrowie Basin are well preserved across the Flinders Ranges of South Australia, and many stratigraphic units have been extensively studied. An exception is the Second Plain Creek Member—originally assigned to the Wilkawillina Limestone—which has received little scientific attention since initial field-based investigations in the 1980s, largely owing to its limited outcrop. However, in the Bunkers Graben at Wilkawillina Gorge, there is excellent exposure of the Second Plain Creek Member, which directly overlies the ‘Red Crust’—a distinctive microbialite marker horizon typically associated with the regional Flinders Unconformity. Hence, resolving the lithostratigraphic, biostratigraphic and paleoenvironmental contexts of the unit is critical for a complete understanding of basin evolution, including the development of the ‘Red Crust’ and the lateral extent of the Flinders Unconformity. This study combines fossil and petrographic data, as well as carbonate microfacies analysis to determine the relative age of the Second Plain Creek Member and develop a depositional model. Small shelly fossils recovered from the unit include a diverse range of tommotiids, brachiopods, molluscs and ecdysozoans, among other taxa, that are indicative of both the Micrina etheridgei and Dailyatia odyssei zones (Cambrian Series 2, Stage 3). Microfacies associations include archaeocyath-rich grainstone, bioclastic packstone, glauconitic wackestone, limestone conglomerate and argillaceous lime mudstone, providing evidence that this unit developed in a deepening carbonate ramp environment with episodes of drowning. The Second Plain Creek Member is reassigned herein to the Mernmerna Formation owing to lithological similarities and stratigraphic continuation of fossil species into overlying members of this formation, representing a distinct change in depositional regime following the Flinders Unconformity and development of the ‘Red Crust’.