Origin of Lacustrine Rocks of Wilkins Peak Member, Wyoming

Abstract

Wilkins Peak Member of the Green River Formation, with its interbeds of trona, oil shale, and dolomitic mudstone, contrasts markedly with the predominantly oil-shale and calcitic mudstone sequences of the remainder of the succession. Much discussion has centered on the origin of these lacustrine rocks. A perennial meromictic (stratified) lake model has been replaced in recent years by a playa-lake model. The subsurface stratigraphy, however, was not well known when the models were developed. Paleogeographic reconstruction indicates that the region was subjected to variations in hydrographic conditions mainly in response to climatic fluctuations. During humid subtropical periods, widespread during the deposition of the Tipton Shale Member and Laney Member, the lakes were p edominantly open meromictic systems with rivers entering the basin from the north. These environmental conditions were modified during deposition of the Wilkins Peak Member as the climate became more arid, and along the southern margin of the basin, shallow, saline-alkaline playa lakes formed in response to these changes. The widespread occurrence in the subsurface of fluvial clastic sequences of the Wasatch Formation associated with the Wilkins Peak Member would indicate that the playa lakes were frequently flooded and freshened by southern drainage systems. Therefore, the lakes were continuously undergoing modifications under the influence of climatic and possibly tectonic processes. A composite model is supported that combines aspects of both the meromictic and playa-lake models. Beca se of these complexities, it is unlikely that one modern lake can serve as an analog for ancient Lake Gosiute.