Lake-stage control on lacustrine microbialite morphology spanning theEarly Eocene Climate Optimum, Uinta Basin USA

Morphological attributes of lacustrine microbialites are highly dependent on local factors—water chemistry, faunal competition, and depositional setting—while broader secular patterns vary widely and are driven by extra-basinal controls like climate and sediment supply. This study undertakes a field-based chemostratigraphic approach in characterizing the marginal lacustrine carbonates in the Green River Formation of the eastern Uinta Basin. The present study evaluates how microbialite development, carbonate cyclicity, and isotopic variability record lake-level dynamics and climate forcing during and immediately following the Early Eocene Climate Optimum.

Three facies associations are recognized spanning upper littoral to lower sublittoral environments: (1) delta-proximal, non-microbial carbonates dominated by quartz-rich bioclastic, peloidal, and intraclastic packstones to grain-/rudstones with local sandy oil shale; (2) non-microbial carbonates that interfinger with microbialites, including ostracodal, oolitic, and peloidal packstones–grainstones and intraclastic textures; and (3) diverse microbial carbonates comprising stromatolitic and thrombolitic lithofacies. These associations capture shifts in clastic influx, water energy, and accommodation along the lake margin.

High-frequency (∼1–5 m) cycles show systematic deepening-upward successions paired with positive δ18O and δ13C shifts, consistent with short-term increases in salinity/restriction. Superimposed, larger-scale (tens to hundreds of meters) trends record: (Stage 1) sparse, low-diversity microbialites with isotopically light values under fresher conditions; (Stage 2) enhanced microbialite abundance and meter-scale biostromal/biohermal buildups with isotopic enrichment indicating rising salinity/alkalinity; (Stage 3) peak microbialite diversity coincident with the heaviest isotope values, reflecting maximum restriction and chemical concentration; and (Stage 4) waning microbialite development and isotopic reversal toward lighter values during lake deepening and freshening.

Microbialite macro-structure, facies stacking, and isotopic excursions track shifts in water chemistry, accommodation, and clastic delivery along the lake margin, linking EECO-era climate variability to carbonate productivity and fabric. The congruence of high-frequency cycles with isotopic enrichment suggests climatically paced hydrologic restriction at bed-set scales, while long-term trends express basin-scale lake evolution. These relationships provide a predictive framework for mapping microbialite-prone intervals and for interpreting salinity, alkalinity, and productivity fluctuations in ancient lake systems.