Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming

The Wilkins Peak Member of the Green River Formation of Wyoming has been examined in outcrop with the object of reconstructing its depositional environment. Based on their assemblages of sedimentary structures, seven rock units are described, six of which define depositional subenvironments. These units are (1) flat-pebble conglomerate, (2) lime sandstone, (3) mudstone, (4) oil shale, (5) tronahalite, (6) siliciclastic sandstone, and (7) volcanic tuff. Their respective subenvironments are (1) rapid transgression of a shallow lake, (2) lake shore oscillating over a mud flat (slow transgression), (3) playa mud flats, (4) shallow lake with occasional desiccation, (5) seasonally dry salt lake, (6) braided stream, and (7) not specific.

These subenvironment deposits are arranged in depositional cycles. We have observed four types of cycles involving flat-pebble conglomerate (A), oil shale (B), mudstone (C), lime sandstone (D), and also trona. These cycles are I: A-B-C, II: D-B-C, III: D-C, and IV: B-trona-C. Individual cycles have been correlated over distances of up to 24 km.

The Wilkins Peak Member is thought to have been deposited in a playa-lake complex, which consisted of a shallow, central playa lake that was surrounded by vast, normally exposed mud flats fringed by alluvial fans. Evaporative concentration of bicarbonate-rich inflow waters led to saturation with respect to calcite, most of which must have been deposited as cement within alluvial fans. Evaporation continued in the capillary zone of the mud flats, precipitating calcite first, then magnesian calcite, and eventually protodolomite. The carbonates accumulated as a soft micritic mud at the fringes of the playa mud flats.

During periods of desiccation, the muds were subject to cracking, and the mud-crack polygons contributed sand- and silt-size dolomitic micrite intraclasts that were transported to the central lake by the next storm. When the central lake was large, oil shale accumulated in it, with the organic matter derived from a flocculent ooze consisting of bottom-dwelling blue-green algae and fungi. During dry periods the lake shrank, and trona and halite precipitated in the central portions.

An understanding of the Wilkins Peak sediments can be achieved only by considering as inseparable the hydrologic, sedimentary, geochemical, and biologic processes responsible for their formation.