Lacustrine Microbialite Architectural and Chemostratigraphic Trends: 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 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 in the eastern Uinta basin, Colorado and Utah. Two facies associations occur within the carbonate units: Lacustrine Margin Carbonates, consisting of six packstone to rudstone lithofacies dominating littoral to upper sublittoral environments; and Lacustrine Microbial Carbonates, consisting of stromatolitic and thrombolitic lithofacies, dominating littoral to lower sublittoral zones. 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 and Piceance basins respectively.

Multiple scales of carbonate cyclicity, indicated by excursions of δ¹⁸O and δ¹³C stable isotopes correlate to characteristic microbialite facies. Bed set scale cycles, on the order of 1 to 5 m, are characterized by deepening upward lithofacies that correlate to positive excursions of stable isotopes. Large scale trends, on the order of 10’s to 100’s of meters, are also observed in this study, and relate microbialite lithofacies to lake stage evolution developed by Tänavsuu-Milkeviciene & Sarg, (2012).

Lake stage 1 (fresh to mesosaline) corresponds to initial sparse microbialite deposition, with low diversity and relatively light δ¹⁸O and δ¹³C isotopic values that indicate 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 heavier δ¹⁸O and δ¹³C isotopic values that characterize 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; suggesting high paleo-organic productivity and the greatest lake restriction and highest salinity and alkalinity conditions. Lake stage 4 (rising lake) contains the last observed microbialite deposits, 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.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018