Revisiting depositional models for the Ediacara Member of the Rawnsley Quartzite in South Australia

The Ediacara Member of the Rawnsley Quartzite hosts one of the best preserved and most diverse assemblages of the Ediacara Biota. In it, soft-bodied organisms are preserved across various depositional environments, with a proposed connection between sedimentary facies and fossil assemblages. Recent studies have questioned previously-established facies models, undermining links between paleoenvironment, paleoecology, and taphonomy. Here, we revisit these models using field observations from across the central Flinders Ranges, supplemented by two new cores drilled through the Ediacara Member at the Nilpena fossil site. The two drill sites are 2 km apart and span strata from the top of the underlying Chace Member through the overlying fossiliferous facies of the Ediacara Member. These two cores are easily correlated to surface outcrops and provide the most complete record of the deposition of the Ediacara Member thus far. The core drilled at “One Tree Hill” (OTH-1) reaches a depth of 65 m and records characteristic “petee laminations” below the erosional contact with the Ediacara Member, which is marked by a breccia horizon. The basal breccia of the Ediacara Member gradationally passes into thinly laminated planar to slightly wavy siltstone that then transitions into alternating thin beds of siltstone and thick beds of massive sandstone often affected by soft-sediment deformation. These beds grade into wavy-laminated siltstone interbedded with thin beds of arenite. Forming the top of the core are thick beds of massive arenite. The second drill core (MR-1) spanning 75.8 m records analogous facies with changing thickness and siltstone/sandstone ratio but lacks a breccia horizon at the base of the Ediacara Member. Both cores highlight repeated cycles of alternating deposition of sandstone and siltstone often obscured in the surface exposure. We investigate an array of sedimentary structures observed in the cores and surface exposures in thin sections, exploring the role of microbial matgrounds and silica cementation in sediment binding and transport. Both are critical for any depositional model developed for the Ediacara Member across the Nilpena site and central Flinders Ranges, its accumulation rate, sediment sources and potential triggers for repeated channelized flows observed throughout the unit. A unified depositional model built across this basin will be critical to further untangle the complex interplay between time, changing taxonomic diversity, water depth, and paleoenvironment at the dawn of animal life.