Lake-Type Evolution of Oil Shale Deposits of the Eocene Green River Formation, Western U.S.
Alan R. Carroll
Department of Geology and Geophysics, University of Wisconsin, Madison, WI
The Green River Formation is widely acknowledged to contain the largest oil shale resources in the world, but the controls on accumulation of these important deposits have been the subject of controversy for nearly 100 years. In contrast to the archetypal Mahogany Zone, lithofacies contained within the Green River Formation actually form a wide range of associations, ranging from freshwater lake deposits and coal to evaporative salt pan deposits. Each of the three principal basins (Green River, Piceance Creek, Uinta) records a symmetrical long-term evolution from an overfilled basin (fluvial-lacustrine facies association), to balanced-fill (fluctuating profundal facies association), to underfilled (evaporative facies association), then back through balanced-fill and overfilled. A recently established high-resolution geochronologic framework based on 40Ar/39Ar dating of interbedded tuffs permits the first detailed temporal correlation between basins, and reveals that this characteristic long-term pattern occurs at different times in each basin. An overriding climatic control therefore appears to be excluded, and intra-regional drainage relationships instead appear to have governed the timing and quality of oil shale deposition. Relatively fresh-water lakes commonly drained to more saline downstream sinks, and long-term basin fill patterns record a general north to south progression of drainage through the three basins. Interestingly, the Mahogany Zone itself appears to resulted from a specific drainage capture event that occurred far to the north of Lake Uinta, possibly in central Idaho. The captured drainage initially flowed south into Lake Gosiute (Wyoming), which in turn spilled southward and caused expansion of Lake Uinta (Colorado-Utah).
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