Paper No. 4
Presentation Time: 9:00 AM
GEOCHEMISTRY OF THE GREEN RIVER FORMATION, PICEANCE CREEK BASIN, COLORADO
The Green River Formation of Colorado, Utah and Wyoming constitutes one of the richest and thickest petroleum source rock deposits in the world. Although Lakes Uinta and Gosiute, where these rocks were deposited, were unusual, even extreme, in their chemistry, the rocks provide clues to conditions required to produce and preserve source rocks. We analyzed major and trace element chemical data from our work (82 samples) and U. S. Geological Survey studies (172 samples), integrating them with stratigraphic and mineralogic studies at the Colorado School of Mines and Binghamton University. A rise in Na2O and Na/Al in the basin center during Stage 2 (of six lake stages) coincides with decrease in clay mineral content and successive occurrence of dawsonite, nahcolite and halite, reflecting transition from mesosaline to hypersaline conditions. Na2O drops near the top of Stage 3, reflecting dissolution of nahcolite/halite across a stratigraphically transgressive surface. Na2O and Na/Al remain elevated due to authigenic albite. Dawsonite and quartz abundance decrease in Stage 3, reflecting increased silica activity with continued hypersaline conditions. Nahcolite and halite form in the water column, as crusts at the brine bottom, and displacively. Chemical measures of redox potential support the conclusion that the Colorado portion of Lake Uinta was stratified, with a deep zone (basin center samples) that was persistently dysoxic, and commonly anoxic, and a shallow zone (basin margin samples) that was less persistently dysoxic. Correlation of Fe to Al indicates origin from clastic constituents (clay and oxide/ hydroxide minerals), but Ca/[Ca+Mg+Fe*] (Fe* = non-pyrite Fe) for basin center samples match carbonate mineralogy, and the Mg/[Mg+Fe*] indicate ferroan dolomite [XFe averaging (by unit) 0.11-0.26] as the main Fe reservoir. Metal/Al ratios show enrichment with respect to average shale, in the order: Mo>U>Cu>V>Co. These data, along with thick organic rich zones and presence of buddingtonite support a model of chemical stratification, brackish-mesosaline-hypersaline conditions and dysoxia/anoxia from the Garden Gulch Member onward. Potentially fresh water conditions in part of the basin margin section were likely local features.