The Angelo Member of the Eocene Green River Formation represents the culminating phase of lacustrine sedimentation in Fossil Basin. During Angelo Member time Fossil Basin existed as a dynamic, hypersaline-alkaline lake in a closed, low topographic gradient basin bounded on the east by the Wyoming thrust belt. The lithofacies and paleoenvironments of the Angelo Member are consistent with an under-filled model of lacustrine sedimentation and vary dramatically from those of earlier phases including the familiar Fossil Butte Member.

Lithofacies of the Angelo Member grade laterally through siliciclastics, bioturbated micrite, kerogen-poor laminated micrite, kerogen-rich laminated micrite and kerogen-rich laminated dolomicrites, to kerogen-rich chemoturbated dolomicrite. Time-synchronous units exhibit lateral changes from bioturbated and laminated calcimicrite in marginal facies to dolomicrite with evaporite mineral casts at the center of the basin. In one unit, dolomicrite grades into dolomicrite with Magadi-type chert and calcite pseuedomorphs after saline minerals. Rapid transgressions represented by kerogen-rich dolomicrites with sharp bottom contacts were followed by sequences deposited during more arid phases. Mud-cracks, flat-pebble conglomerates, strandlines with stromatolite-coated logs, evaporite minerals and Magadi-type chert all were deposited during periods of desiccation.

Wasatch Formation fluvial and carbonate mud-flat paleoenvironments interfinger with, and grade into, Angelo Member maginal lacustrine environments. Marginal lacustrine paleoenvironments are characterized by siliciclastics, bioturbated facies, stromatolitic material, and fossils; whereas open lacustrine paleoenvironments lack fossils and are dominated by dolomite and evaporites. Depositional systems during upper unit time fluctuated between siliciclastic, evaporite and carbonate and evaporite domination. The lithofacies, and paleoenvironmens of the Angelo Member suggest that the latter stages of Fossil Lake exhibited dramatic fluctuations in deposition, precipitation, and areal extent. These data are consistent with an under-filled lake model and were most probably produced in response to more arid climate conditions.