CHRONOSTRATIGRAPHIC CORRELATION OF LACUSTRINE DEPOSITS USING (SR)-S-87/(SR)-S-86 RATIOS, EOCENE GREEN RIVER FORMATION, WYOMING, USA

摘要

The reconstruction of detailed, basin-scale depositional histories from sedimentary rocks fundamentally depends on the availability of reliable time markers. Unlike marine strata, lacustrine strata typically lack rapidly evolving, cosmopolitan fauna or flora that might serve this purpose. Depending on their geologic context, lacustrine strata may also lack tephras that could provide isochronous markers or radioisotopic age. Variations in Sr-87/Sr-86 ratios could potentially provide an alternative means of chronostratigraphic correlation for carbonate-rich lake deposits, based on the hypothesis that Sr isotopes are well mixed in a lake and do not experience significant fractionation. To test this hypothesis we measured Sr-87/Sr-86 ratios in 114 samples from two drill cores of the upper Wilkins Peak Member from the Green River Formation that are located similar to 23 km apart. These cores can be independently correlated using distinctive tephras and organic-carbon rich mudstone horizons. Measured Sr-87/Sr-86 ratios range from 0.71154 to 0.71504, and vary inversely with lake-water depth, as interpreted by sedimentary lithofacies characteristics. Lower ratios of Sr-87/Sr-86 are found in lithofacies deposited during lake highstands, which are marked by laminated dark-gray mudstone and elevated organic-carbon enrichment (as measured by Fischer Assay analysis). Higher Sr-87/Sr-86 ratios occur in lithofacies deposited during lake lowstands, which are marked by organic-lean gray-green mudstone(. 87)Sr/Sr-86 in approximately time-equivalent samples from the two cores show a strong positive correlation (r = 0.68), despite the likely presence of small temporal mismatches between approximately correlative samples. We conclude that lake-water was consistently well mixed with respect to Sr across distances of at least 23 km. These results suggest that Sr-87/Sr-86 can serve as a powerful tool to aid high-resolution chronostratigraphic correlation of lake deposits.