Hypotheses of oil-shale genesis, Green River Formation, northeast Utah, northwest Colorado, and southwest Wyoming

In the six decades or so of oil-shale studies on the Green River Formation (Paleocene/Eocene) in northeast Utah, northwest Colorado, and southwest Wyoming, two alternative hypotheses for the paleoenvironment have dominated the literature: (1) a stratified lake in which anaerobic conditions in the hypolimnion led to preservation of organic material and fine laminae in the oil shale; (2) a playa lake where oil shale formed in a shallow, non-stratified lake fringed by broad mud flats or playas. The stratified-lake hypothesis best explains the fine laminae and varves, laminae couplets, lateral persistence of thin laminations, wide lateral extent of correlatable strata, widespread distribution of calcium, distribution of sulfur isotopes in sulfide minerals, hydrogenated nature of kerogen, vertical uniformity in mineralogy, and lateral facies changes of strata from land to deep parts of the depositional basins. Proponents of the playa-lake hypothesis believe it best explains disrupted bedding, lack of continuous lamination, looped bedding, mud cracks, flat-pebble conglomerate, flat-topped ripple marks, interference and oscillation ripples, stromatolitic limestone, ooliths, pisoliths, crystal molds of saline minerals, and a low topographic gradient. The playa-lake advocates first formulated their ideas from studies in southwest Wyoming. They then extended the work to northwest Colorado. In contrast, the stratified-lake defenders have worked about equally in the Uinta and Piceance Creek basins. This basinal bias raises an intriguing consideration. There is little oil shale in southwest Wyoming. Organic content of the so-called oil-shale beds is very low compared with beds in the two southern basins, and there are few oil shale beds. There are lithified, blue-green algal, mat-like deposits that resemble oil shale. Thus, to a large extent, the hypothesis has evolved independent of the rock it explains. The principal workers saw little, if any, oil shale in the strata they described. Attempts to extend the playa-lake hypothesis to oil shale formation in the Piceance Creek Basin have relied greatly on shallow water lacustrine strata, stromatolitic rock, oolite, pisolite that was deposited landward of oil shale. These beds contain sedimentary structures indicative of shallow water deposition or exposure, but they yield little or no evidence of conditions during oil-shale deposition. The careful stratigraphic work in the Uinta and Piceance Creek basins has led to recognition of facies changes from fluvial strata to nearshore lacustrine to offshore lacustrine in the Parachute Creek Member. Oil shale originated in the deepest water of the basins, at times in water 200 feet or more deep. Given the lateral variations in environments basinward, the oil shale could not have been deposited in a playa-lake setting without unreasonable movement of the basin floor and undocumented rapid fluctuations in water depth.