Groundwater Mixing In An Alkaline Paleolake: Eocene Green River Formation, Wyoming

Tufa in the Little Mesa area of the northern Bridger Basin has been interpreted to record carbonate deposition via subaqueous and subaerial springs emanating near the shoreline of Eocene Lake Gosiute. Sedimentary facies record an overall transgression, culminating with mound structures that reach up to 9 m in height and 40 m in diameter. Mounds exhibit a strong positive, linear covariance between delta C-13 and delta O-18, defining a slope of similar to 1. Similar trends occur in many other paleolake deposits, where they are interpreted to reflect changes in evaporation, atmospheric CO2 exchange, and organic matter burial. However, delta C-13 and delta O-18 in this study also covary strongly with Sr-87/Sr-86, a new finding that is inconsistent with previously proposed mechanisms. We conclude that Little Mesa isotopic trends reflect mixing of groundwater with low Sr-87/Sr-86, delta O-18 and delta C-13 and lake water with opposite characteristics. Low Sr-87/Sr-86 in groundwater likely resulted from interaction with marine carbonate strata within the Sevier fold and thrust belt to the west, whereas drainage from Precambrian-cored uplifts that bounded Lake Gosiute to the north, east, and south was responsible for higher lake water ratios.Little Mesa carbonate facies are all less radiogenic than any time-equivalent facies near the center of the basin, implying horizontal and vertical gradients in Lake Gosiute Sr-87/Sr-86. Previous studies have shown that the lowest Sr-87/Sr-86 in basin center deposits correspond to lake highstands. Results of this study support the hypothesis that climatic modulation of surface runoff and spring emanations from the Sevier belt were principally responsible for precessional-scale expansions and contractions of Lake Gosiute. More broadly, groundwater discharge may represent an important but underappreciated contributor to covariance between Sr-87/Sr-86 ratios, delta C-13 and delta O-18 in closed paleolake systems.