Td. Frank et al., ISOTOPIC EVIDENCE FOR THE PALEOENVIRONMENTAL EVOLUTION OF THE MESOPROTEROZOIC HELENA FORMATION, BELT SUPERGROUP, MONTANA, USA, Geochimica et cosmochimica acta, 61(23), 1997, pp. 5023-5041
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.