C-ISOTOPES AND SEAFLOOR PRECIPITATES FROM THE POCATELLO FORMATION, SOUTHEAST IDAHO: IMPLICATIONS FOR NEOPROTEROZOIC EARTH HISTORY

The uppermost carbonate unit of the Scout Mountain Member of the Neoproterozoic Pocatello Formation (younger than 667±5 Ma; Fanning and Link, 2003) contains seafloor precipitated aragonite (pseudomorphosed) fans and records negative d¹³C values. Negative d¹³C values and macroscopic seafloor-precipitated crystal fans are relatively common in the unusual carbonates capping certain Neoproterozoic glacial strata. The carbonate fabric and isotopic compositions of the Scout Mountain Member carbonate are thus consistent with such “cap carbonates.” The unit does not, however, rest directly upon a recognized glaciogenic deposit. It is separated from an underlying diamictite-cap carbonate succession in the “Sturtian” Pocatello Formation by a regionally extensive sequence boundary and positioned well below incised valleys interpreted to represent glacioeustatic drawdown in association with the “Marinoan” glacial interval; thus, the unit in question falls in between generally accepted glacial intervals (Sturtian - ca. 750-700 Ma and Marinoan - ca. 600 Ma).

It is possible that the carbonate unit is a genuine post-glacial cap carbonate, thus requiring recognition of additional glaciation in the Neoproterozoic succession of southeast Idaho and reevaluation of correlations along the Cordillera. Furthermore, these carbon isotope data, when combined with newly reported age constraints, require additional seawater d¹³C variability between ca. 700-600 Ma than previously recognized. Most d¹³C compilations, based on little radiometric age control, display highly positive d¹³C values ca. 667; these new data confirm the presence of some negative d¹³C values between 700 and 600 Ma.