Isotopic evidence for the paleoenvironmental evolution of the Mesoproterozoic Helena Formation, Belt Supergroup, Montana, USA

High-resolution isotopic and geochemical data constrain long-debated marine vs. lacustrine depositional models for the Helena Formation, a carbonate unit within the Mesoproterozoic Belt Supergroup, Montana. Paired carbon and oxygen isotope and minor element analyses of microsamples from a diverse array of petrographic components effectively distinguish between primary and secondary isotopic signatures. delta(13)C values reflecting primary depositional conditions increase progressively from an average of -0.2 parts per thousand near the base of the Helena Formation to an average of +1.4 parts per thousand near the top, over a stratigraphic thickness of 100-200 m. Because the base of this 1.6 parts per thousand shift coincides with a basin-wide facies change marked by the appearance of stromatolitic units and an increased occurrence of features indicative of evaporative conditions and subaerial exposure, the shift toward more positive delta(13)C values is interpreted to reflect an increase in biologic productivity coupled with increasingly evaporative conditions in the eastern part of the Belt basin. delta(13)C values from the lower part of the Helena Formation are similar to those from other marine carbonates of similar age and suggest that the Belt basin was marine during deposition of the Helena Formation. Taken as a whole, these relations suggest that Helena sediments were deposited in a shallow marine basin that was increasingly isolated from the open Mesoproterozoic ocean. Original delta(18)O values have been retained only in syndepositional and early diagenetic intergranular cements in oolitic grainstones that occur in the upper part of the Helena Formation. The most positive delta(18)O values derived from these cements (similar to -6.1 parts per thousand PDB) are up to 3 parts per thousand higher than previous estimates for the delta(18)O value of Mesoproterozoic marine calcite. However, because these phases are associated with facies deposited under restricted marine conditions, interpretation of delta(18)O values from these cements with regard to the composition of the open Mesoproterozoic ocean awaits study of additional coeval marine carbonate successions. Copyright (C) 1997 Elsevier Science Ltd.