APPLYING GEOCHEMICAL PROXIES IN TEASING APART THE GENETIC HISTORY OF THE FLINDERS UNCONFORMITY AND RED CRUST, FLINDERS RANGES, SOUTH AUSTRALIA

The Early Cambrian Flinders Unconformity (FU) of the Arrowie Basin, South Australia is a significant regional boundary, though its origins have not been definitively determined. Previous workers have described it as a subaerial erosional surface while others have attributed it to a syndepositional marine hardground associated with platform drowning. Perhaps the most characteristic feature of the FU is the overlying “red crust” composed of a ~10cm-thick deposit of ferruginous microstromatolitic laminae containing small shelly fossils (SSF) and calcareous sponges. The microstromatolites range from columnar to domal to Frutexites fabrics with Fe and Mn oxides.

To further unravel the genetic formation of the red crust at Wirrealpa Mine, we conducted several geochemical analyses, measuring Rare Earth Element and Yttrium (REY) concentrations, and δ¹³C and δ¹⁸O isotopes from limestone samples directly above, below and at the unconformity surface. REYs have been used as proxies for inferring patterns in seawater geochemistry, in particular redox conditions, while δ¹³C isotopes provide insights into the organic content and burial rates in carbonate sediments. Samples were categorized petrographically by cement and fabric (laminar calcrete, Mn-rich, micrite, microbial, spar), then assessed for similarities. Results of the δ¹⁸O analyses reveal no significant difference between fabrics, but δ¹³C values in microbial fabrics were significantly lighter than in spar or micrite. Spider and nMDS plots of the REY data suggested two groupings, one of microbial and laminar calcrete textures and one of micrite and microbial fabrics containing SSF. The microbial and calcrete group tended to have higher concentrations of REY than the micrite and SSF microbial group. Except for Mn-rich fabrics, which exhibited a slight negative anomaly, all otherwise exhibited positive Ce anomalies of >3.6. This has been interpreted by Reitner (2018) as indicative of a dys- to anoxic environment. The adsorption of Ce³⁺ onto Mn⁴⁺ could influence the anomalies, but this requires more investigation. We propose that the formation of the FU and the deposition of the red crust are separate events, the former by subaerial exposure and the latter by inundation and a shallow-water environment, possibly combined with an influx of manganous waters.