Microbial Fossils, Phosphatization, Heavy Rare Earth Element and Uranium Enrichment: Early Diagenesis of an Upper Green River Formation Oil Shale, Uinta Basin, Utah
David Keighley, Chris McFarlane, and Tim E. Ruble
University of New Brunswick
Nearly 300 samples of shale and oil shale from outcrop and core of the Parachute Creek Member, Green River Formation (GRF), in the Uinta Basin have been analyzed using inductively-coupled-plasma mass spectrometry (ICP-MS). Some beds of oil shale are found to be relatively enriched in phosphorus, uranium, and rare-earth elements (REE). Spider plots of REE abundance have allowed for a tentative inorganic geochemical correlation of particular oil shale beds between outcrops spaced 80 km apart. One such phosphatic oil shale, from outcrop 128 m above the base of the Mahogany Oil Shale Zone (MOSZ) at Buck Canyon in the SE of the basin, also contains micro-slump-like structures. This shale has been analyzed by bitumen extract gas chromatography (GC), gas-chromatography mass-spectrometry (GC-MS), scanning electron microscopy (SEM), x-ray diffraction (XRD), and laser ablation (LA) ICP-MS. GC and GC-MS indicate both algal and bacterial derived biomarkers are typical of other GRF oil shale formed in an offshore stratified closed lake, suggesting that elemental enrichments are not mediated by an unusual biocommunity. XRD has identified abundant calcium fluorapatite (CFA) and carbonate, minimal silicate and zeolite. Microcrystalline CFA phases are imaged in SEM as filling pores, replacing grains, and fossilizing aggregates of well-preserved globular microbes. Most other organic matter is imaged as amorphous stringers, implying that, comparatively, the fossilized microbes had not degraded and possibly were alive until the phosphatization event. LA-ICP-MS indicates a persistent negative Eu anomaly suggesting an anoxic hypolimnion and substrate conditions. The working hypothesis is that phosphate-storing chemotrophic coccoid bacteria preferentially adsorbed heavy REE on their cell walls in a very shallow anoxic substrate. Slumping caused changes in pore-water conditions that killed the microbes. Phosphorus released into porewaters reached saturation, quickly precipitating (precursor) CFA that incorporated U and REE into its lattice.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013