STRATIGRAPHIC REVISION OF THE GREEN RIVER FORMATION IN FOSSIL BASIN, WYOMING: THREE DISTINCT PHASES OF FOSSIL LAKE

The Green River and Wasatch formations in Fossil Basin (SW Wyoming) represent a complex interfingering relationship between fluvial and lacustrine sedimentation in a relatively flat basin. Detailed stratigraphic and sedimentologic studies have greatly expanded our knowledge of the Green River Formation in Fossil Basin, providing impetus for stratigraphic revision of the formation in an upcoming publication. This has long been recognized with the published description of the informal lower, middle, and upper units. The lower unit will be designated as a new member. The middle and upper units will consist of revised Fossil Butte and Angelo members.

The lower unit is a previously unmapped unit and consists of over 100 m of kerogen-rich laminated micrite (“oil shale”), kerogen-poor laminated micrite, bioturbated micrite, dolomicrite, mudstone, siltstone, and sandstone. Distinctive interbeds of cross-bedded sandstone, interpreted as prograding deltas and fluvial deposits, also distinguish this unit from the overlying lacustrine sequences. The contact between the new member and the Fossil Butte Member will be designated as the top of the Sandstone Tongue of the Wasatch that is recognized at Fossil Butte as a 4 m thick sequence of thinly bedded brown mudstones. The new boundary between the Fossil Butte and Angelo members will be placed at a distinctive lithology change from calcimicrite in the Fossil Butte Member to dolomicrite with abundant evaporite mineral casts in the Angelo Member.

The proposed designations reflect three distinctive phases of Fossil Lake. Lower unit time was characterized by localized subsidence and frequent fluvial dominated events, interpreted as a balanced to over-filled lake sequence. Fossil Butte time was characterized by the maximum high stand of Fossil Lake, and marked a distinctive climate transition to wetter climate and balanced to over-filled lake conditions, supporting diverse fish faunas. Angelo time was characterized by a drier climate and a hypersaline, under-filled lake devoid of life, except for a few fresh periods when isolated fish populations returned.