PALEOHYDROLOGY OF SPRING DEPOSITS IN THE WILKINS PEAK MEMBER OF THE EOCENE GREEN RIVER FORMATION, BRIDGER BASIN, WY

Carbonate mounds in the Eocene Green River Formation (GRF) have long been recognized, but their mode of origin and significance with respect to the lacustrine paleohydrology remain controversial. Here we present new sedimentological, geochemical, and structural evidence indicating that significant spring discharge led to the formation of a large complex of carbonate mounds in the upper Wilkins Peak Member of the GRF at Little Mesa (LM), northern Bridger Basin, Wyoming.

Carbonate mound outcrops at LM exhibit interlayered subaqueous and subaerial precipitation features that formed in response to lake level fluctuations near a spring point source. Subaqueous carbonate precipitation is illustrated by domal concentric stromatolitic mounds, 1-3 m tall. Subaerial, gravity-controlled spring precipitates occur as: 1) thin-bedded downward growing dendritic tufa that formed “ledges” at the air-water interface, and 2) sub-vertical flowstone aprons that drape tufa “ledges” and underlying domal stromatolites. Flowstones contain mini-rimstone dam structures associated with dendrites, coated grains, and coated algal sheaths, all similar to modern carbonates formed where spring waters discharge, degas CO₂, and flow downslope.

Strontium isotopes ratios (⁸⁷Sr/⁸⁶Sr) measured from LM mound carbonate range from 0.71040 to 0.71140. These values are less radiogenic than those previously reported from non-mound Wilkins Peak lake deposits, which range from 0.71195-0.71561. Subaerial dendritic and flowstone tufa have lower ⁸⁷Sr/⁸⁶Sr ratios (0.7104-0.71101) than subaqueous stromatolitic mound carbonates (0.71096-0.71140), the latter indicating precipitation from mixed lake and spring waters.

LM spring deposits and the underlying siliciclastic alluvial facies of the Wasatch Formation (WF) occur on the footwall of the Hogsback thrust, part of the Sevier orogenic thrust front and upthrown Paleozoic carbonates. Several minor thrust faults terminate at the base of the WF and likely served as groundwater migration pathways. Calcite cemented sand tufa in the WF directly below the LM spring deposits and low ⁸⁷Sr/⁸⁶Sr ratios suggest that groundwater derived from Paleozoic carbonates preferentially flowed along thrust faults, mixed with meteoric waters in the WF, and discharged near the shoreline of Wilkins Peak lakes.