Chemostratigraphic Subdivision and Diagenesis in the Upper Green River Formation, Southern Uinta Basin, Utah
Keighley, David; Vanden Berg, Michael D.; Yan, Guohua
The Eocene Green River Formation (GRF) of the Uinta Basin in eastern Utah comprises a several km thick lacustrine succession predominantly of variably shaley, marly mudstone. Few marker beds (thin interbedded oil shale, sandstone and tuff beds) are present that can be correlated across the Basin: one such marker is the Mahogany Oil Shale Zone that forms the basal ~ 30 m of the upper member of the GRF. An extensive sampling program has focused on the lowermost 200 - 300 m of the upper GRF at four outcrop localities and one cored borehole. These five sites form a roughly E-W transect that stretches ~80 km across the southern part of the Basin. Bulk geochemical analyses (using ICP-MS) of over 300 samples from these sites, backed up with 16 XRD and 3 SEM analyses, have provided a basis for chemostratigraphic subdivision of the upper GRF. Both specific marker beds and diagenetic intervals have been identified. For example, a few of the oil shale beds record anomalously high abundances of phosphorus, in combination with elevated uranium and rare-earth elements, that are associated with microcrystalline calcium fluorapatite in these beds. Specific phosphatic oil shale beds are correlated based on differing patterns of REE enrichment. The early diagenetic occurrence of the calcium fluorapatite suggests that these anomalies reasonably can be equated to depositional time-lines. The resulting correlation scheme indicates a significant thickening of the succession occurred during deposition of the lower part of the upper GRF from east to west. As another example, an abrupt reduction upsection in the magnesium-calcium ratio is also identified at different stratigraphic positions at different sites. The boundary is considered a later diagenetic feature, which crosses time lines and marker beds, and that is probably related to deep burial or pre-uplift (de-) dolomitization.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013