EVOLUTION OF FOSSIL LAKE, WYOMING DURING DEPOSITION OF THE EOCENE GREEN RIVER FORMATION

Fossil Lake developed in a structurally dynamic, piggyback basin in the Idaho-Wyoming thrust belt. Lake sediments form the Road Hollow, Fossil Butte and Angelo members of the Eocene Green River Formation.

The Road Hollow Member was deposited as the pre-existing fluvial system migrated into the basin from the south to form Fossil Lake. This member is characterized by interbedded laminated micrite, claystone, bioturbated limestone and sandstone. Carbonate sedimentation was interrupted by pulses of deltaic siliciclastic input into the southern part of the basin. Lacustrine sedimentation was initiated by subsidence in the southern part of Fossil Basin where lacustrine deposition was focused. Topographic relief was high with a relatively high depositional gradient. Precipitation/Evaporation (P/E) rate was fairly high resulting in an overfilled lake basin. Late in Road Hollow time, a dramatic influx of deltaic siliciclastics dampened the high topographic gradient.

A basin-wide expansion of Fossil Lake initiated deposition of the Fossil Butte Member. Fossil Lake expanded to its maximum extent, drowning alluvial fans in the western and northern parts of Fossil Basin. The lacustrine depocenter migrated into the northern part of Fossil Basin. This member is characterized by organic-rich, laminated micrite that grades laterally into bioturbated micrite nearshore. High P/E rates gradually decreased as Fossil Lake developed into a balanced-fill lake basin. Geochemical and sedimentological data suggest that an isolated embayment existed in the northeastern part of the basin, bounded on the west by a peninsula composed of folded and uplifted Mesozoic rocks.

The lake became more saline-alkaline at the onset of Angelo Member deposition. This member is characterized by evaporites, laminated to massive dolomicrite and volcaniclastic mudstone. Widespread deposition of evaporites suggests that Fossil Lake shallowed and maintained a large areal extent with a low topographic gradient in an underfilled lake basin.

Lake size and depth was controlled by the interplay of sedimentation, P/E and subsidence rates. Depositional rates greatly increased at the onset of Bullpen Member (Wasatch Formation) deposition, filling in Fossil Lake.