ISOTOPIC EVIDENCE FOR THE PALEOENVIRONMENTAL EVOLUTION OF THE MESOPROTEROZOIC HELENA FORMATION, BELT SUPERGROUP, MONTANA, USA

High-resolution isotopic and geochemical data constrain long-debated m arine vs. lacustrine depositional models for the Helena Formation, a c arbonate unit within the Mesoproterozoic Belt Supergroup, Montana. Pai red carbon and oxygen isotope and minor element analyses of microsampl es from a diverse array of petrographic components effectively disting uish between primary and secondary isotopic signatures. delta(13)C val ues 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 mar ked by the appearance of stromatolitic units and an increased occurren ce of features indicative of evaporative conditions and subaerial expo sure, the shift toward more positive delta(13)C values is interpreted to reflect an increase in biologic productivity coupled with increasin gly evaporative conditions in the eastern part of the Belt basin. delt a(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. T aken as a whole, these relations suggest that Helena sediments were de posited in a shallow marine basin that was increasingly isolated from the open Mesoproterozoic ocean. Original delta(18)O values have been r etained only in syndepositional and early diagenetic intergranular cem ents in oolitic grainstones that occur in the upper part of the Helena Formation. The most positive delta(18)O values derived from these cem ents (similar to -6.1 parts per thousand PDB) are up to 3 parts per th ousand higher than previous estimates for the delta(18)O value of Meso proterozoic marine calcite. However, because these phases are associat ed with facies deposited under restricted marine conditions, interpret ation of delta(18)O values from these cements with regard to the compo sition of the open Mesoproterozoic ocean awaits study of additional co eval marine carbonate successions. Copyright (C) 1997 Elsevier Science Ltd.