Lithofacies, diagenesis, and chemostratigraphy of the microbialite and marginal lacustrine carbonate units within the Green River Formation, eastern Uinta Basin, Colorado and Utah

Abstract

Marginal lacustrine carbonates of the Green River Formation are well exposed in the eastern Uinta basin, where they are interbedded with fluvial and marginal lacustrine sand and shale of the Douglas Creek Member. This study examines the stratigraphic architecture, lithofacies, and chemostratigraphy of the microbialite and other associated carbonate beds of the Green River Formation at White Face Butte and adjoining Park Canyon sites, in the eastern Uinta basin, Colorado and Utah. Field observations along a 255 m vertical section at the White Face Butte location, measured by Tänavsuu-Milkeviciene et al., (2017), have led to the identification of two main facies associations within the carbonate units: Lacustrine Margin Carbonate, and Lacustrine Microbial Carbonates. These are consistent with earlier work done by Swierenga et al., (2015) and Sarg et al., (2013) on similar beds in the surrounding area within the Uinta basin and Piceance basin respectively. This study characterizes the vertical and local lateral variability of the carbonate units within a full section of the Green River Formation in eastern Uinta basin. Multiple scales of carbonate cyclicity are observed through this study, indicated by positive and negative excursions of δ18O and δ13C stable isotopes correlated to characteristic microbial facies. Large scale trends, on the order of 10’s to 100’s of meters, are observed in the study relating microbialite lithofacies along with δ18O and δ13C stable isotopes to lake stage evolution developed by Tänavsuu-Milkeviciene & Sarg, (2012). Lake stage 1 (fresh to mesosaline) corresponds to initial sparse microbialite deposition, with low microbialite diversity and relatively light δ18O and δ13C isotopic values; indicating initial fresh water conditions and relatively low paleo organic productivity. Lake stage 2 (transitional lake) corresponds to moderate microbialite diversity, larger biostromal and biohermal build ups, and slightly heavier δ18O and δ13C isotopic values; indicating more saline conditions and higher paleo organic productivity in the lake. Lake stage 3 (highly fluctuating lake) contains the highest microbialite diversity and marks the interval of heaviest δ18O and δ13C isotopic values; indicating high paleo organic productivity and the most lake restriction and saline conditions recorded in this study. Lake stage 4 (rising lake) contains the last microbialite deposits observed in the Green River Formation in the Uinta basin, and marks the lowest microbialite diversity and a reversal in trend of δ18O and δ13C isotopic values; indicating freshening conditions and a decrease in paleo organic productivity.