A geochemical investigation into the genetic history of the lower Cambrian Flinders Unconformity and red crust
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The Flinders Unconformity (FU) represents a significant regional disconformity in the Early Cambrian carbonate successions of the Arrowie Basin of South Australia. However, the genetic history associated with the FU and conspicuous ferruginous microstromatolitic "red crust" that caps the disconformity surface has yet to be unraveled. Herein, we investigate the environmental conditions of both the FU and red crust's formation using a suite of geochemical proxies combined with targeted sampling of carbonate cements in tandem with microstratigraphic sections across the unconformity surface. Lithological samples were analyzed for stable isotopes [delta]^13C and [delta]^18O and rare earth elements and yttrium (REY). Results show both stratigraphic and lateral differences indicative of environmental shifts across the boundary. On average across all localities, [delta]^13C trends negatively across the FU with a slight shift towards 0[percent] above the red crust horizon. This is, however, dependent on lateral variations in lithology. Little to no change in [delta]18O is observed, regardless of the lithology. The normalized REY data reveal relatively flat curves, excluding notable positive Ce anomalies, which increase across the red crust horizon. With limited evidence for meteoric diagenesis from the stable isotope data and no conclusive evidence for normal seawater conditions, evidence of subaerial exposure in connection to the FU remains inconclusive. However, together, the Ce anomaly and low Y/Ho ratios across the FU suggest anoxic conditions just prior to and during the red crust formation.
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M.S.