AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Chemostratigraphy of the Mudstone-Dominated Upper Green River Formation in the Eastern Uinta Basin of Utah
(1) University of New Brunswick, Fredericton, NB, Canada.
Bulk geochemical analyses can be used to identify gradual trends, distinct intervals, and marker beds within thick (~200 m) intervals of mudstone such as the upper Green River Formation (GRF) of the Uinta Basin, Utah. The interval chosen for study extends from the Mahogany Oil Shale Zone/Ledge (MOSZ) at the base, up through a lower "lean" (generally lacking in oil shale) mudstone zone, a median "rich" mudstone, and an upper "lean" mudstone. In addition, gradual westward progradation of Uinta Formation (UF) interbedded sandstone and mudstone has meant that at some locations interfingering (UF-GRF) mudstone constitutes the uppermost sampled interval. A single metre-thick sandstone interval also occurs near the top of the median zone and nahcolite-rich strata are present near the base of the upper mudstone. Sections have been measured, and samples taken, from outcrop of the entire study interval at several locations in the eastern part of the basin. First results, from the study interval at Gate Canyon, indicate that a few of the oil-shale beds contain anomalously high phosphorus(up to 9.0 wt % as P2O5), high uranium, and distinct rare-earth signatures. Potassium (K, %) v. rubidium (Rb, ppm) plots identify a high coefficient of determination (r2) for values about the regression line in the upper zone compared to low r2 about a higher K:Rb ratio regression line for the middle and lower zone. Manganese (as MnO) content displays a general increase upsection, and particularly concentrated in the interfingering mudstone. Mn tends to precipitate from pore waters, ultimately as an oxide, under conditions intermediate between the reducing conditions that likely predominated during lacustrine deposition (GRF), and the more prevalent oxidizing conditions during later deposition of terrestrial strata (UF). Rb can substitute for K in some detrital alkali feldspars, micas, clays and zeolites, or be adsorbed onto other clays during deposition or diagenesis. High r2 suggests very few K-Rb-bearing minerals were being deposited/diagenetically formed higher in the GRF. Although P and U would initially concentrate in organic detritus, its subsurface degradation releases P that usually remains mobile in anoxic porewaters. However, it appears that the P was somehow fixed as Ca-fluorapatite (identified in XRD) within the oil shale, possibly with tetravalent U replacing Ca, under reducing conditions.