--> Fluvial-Deltaic Deposition within an Evolving Lacustrine Basin, the Farson Sandstone Member of the Eocene Green River Formation, Southwestern Wyoming, USA

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Fluvial-Deltaic Deposition within an Evolving Lacustrine Basin, the Farson Sandstone Member of the Eocene Green River Formation, Southwestern Wyoming, USA

Abstract

In overfilled lacustrine basins deltas are deposited at the level of the outflow sill where rivers enter the lake. Thus, they are ideally positioned to record even subtle fluctuations in lake level caused by changes in discharge and basin subsidence. In balanced-fill basins the fluvial-deltaic environment can migrate tens of kilometers laterally almost instantaneously since lake level is not pinned at the elevation of the outflow sill. The Farson Sandstone Member, deposited in the Greater Green River Basin (GGRB) of Wyoming, is a fluvial-deltaic sandstone unit which covered nearly one third of the basin during a period of lake basin evolution between overfilled and balanced-fill conditions. In addition to recording basin-scale changes in subsidence and sediment supply, detailed stratigraphic study of the Farson Sandstone and laterally equivalent Tipton Shale Member has shown that paleotopography on the basin floor influenced the distribution of lithofacies. In particular, recognition of a paleohigh seems to explain the presence of a series of enigmatic mounded facies possibly related to spring discharge and microbial activity. Ten detailed stratigraphic measured sections of the Farson Sandstone Member and interbedded Tipton Shale Member were described in the summer of 2014 in southwestern WY and integrated with those of previous studies. This combined data set provides a high resolution two-dimensional stratigraphic framework to define lateral and vertical lithofacies distributions and significant stratal surfaces along a depositional dip transect. This data allows for a better understanding of the depositional history of the GGRB as it relates to basin subsidence, water discharge, and sediment supply. Interpretations produced by this study can act as analogues for other lacustrine basins.