Sedimentary rocks of the Green River Formation in the Piceance Creek Basin, Colorado, were deposited on a broad playa fringe and in a shallow-water lake which underwent frequent periods of desiccation. The shallow-water nature of the depositional environment is demonstrated by widespread stromatolitic limestone, ooliths, pisoliths, and interference and oscillation ripples. Frequent periods of desiccation are indicated by mud cracks, flat-pebble conglomerate, ripples with flattened crests and mud cracks in troughs, and crystal molds of saline minerals. These assemblages of sedimentary structures are consistent with deposition in a playa-lake complex and inconsistent with a deep, permanently stratified lake.
The saline minerals interbedded with high-yield oil shale in the center of the Piceance Creek Basin are explained in terms of modern playa-lake hydrology and mineral equilibria in the system NaHCO3-Na2CO3-NaCl-H2O. The physical, chemical, and biological evidence strongly supports cyclic deposition and seasonal, as opposed to permanent, chemical stratification of the lake.