Seafloor precipitates and C-isotope stratigraphy from the Neoproterozoic Scout Mountain Member of the Pocatello Formation, southeast Idaho: implications for Neoproterozoic earth system behavior

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Abstract Macroscopic seafloor-precipitated crystal fans are rare in post-Paleoproterozoic strata, but they are more common in the unusual carbonates that cap certain Neoproterozoic glacial strata. The uppermost carbonate and marble unit of the Scout Mountain Member of the Neoproterozoic Pocatello Formation (younger than 667±5 Ma) contains seafloor fans, records negative δ 13 C values, and lies within a transgressive sequence, all features consistent with such post-glacial carbonate units. It does not, however, rest directly upon a known glaciogenic deposit or sequence boundary. Instead, it occurs within a cyclic upward-fining succession that overlies an erosional contact on the pink dolostone post-glacial carbonate atop the late Sturtian Scout Mountain Member diamictites. It is possible that the carbonate unit in question is a cap carbonate and thus requires recognition of an additional glaciation recorded in the Pocatello Formation. Alternatively, it may imply that means independent of glaciation, such as overturn of an anoxic ocean, foster the deposition of cap-like carbonates independent of post-glacial processes. Thus, Neoproterozoic carbonate units with unusual lithofacies and negative carbon isotope values need not indicate the aftermath of low-latitude glaciation. Additionally, the radiometrically calibrated δ 13 C record of the Pocatello Formation demonstrates that the interval from 700 to 580 Ma has more variation than previous compilations would suggest.