Phosphorites Permian Phosphoria formation

The Permian Phosphoria Formation in the northwestern Interior United States contains two phosphatic and organic-ncarbon-rich shale members, the Meade Peak Phosphatic Shale Member and the Retort Phosphatic Shale Member. Ihese rocks were formed at the periphery of a foreland basin between the Paleozoic continental margin and the North American cratonic shelf. The concentration, distribution, and coincidence of phosphorite, organic carbon, and many trace elements within these shale members probably were coincident with areas of optimum trophism and biologic productivity related to areas of upwelling. In the Phosphoria sea upwelling is indicated to have occurred by sapropel that was deposited adjacent to shoals near the east flank of the depositional basin. 

The Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation is comprised generally of phosphorites, dolostones, shales, and siltstones, overlain by the Rex Chert Member of the Phosphoria Formation, and it overlies the Grandeur Dolostone Member of the Permian Park City Formation (Figs. 8 and 9). Because individual strata are difficult to trace between the measured sections, ore zones dominated by phosphorites, and waste zones dominated by dolostones and silicic-clastics, are used to compare the measured sections. Schematic box and whisker plots of the concentrations of CFA, total carbonates, and total silicates plotted across the recognized ore and waste zones (Fig. 10) summarize the dominant mineralogy and lithology of the more-, less-, and least-weathered sections. 

Maughan EK (1980) Relation of phosphorite, organic carbon and hydrocarbons in the Permian Phosphoria Formation, western USA. In Geologic comparee des gisements de phosphates et de petiole. Doc B.R.G.M 24 63-91... 

Very slow deposition rates are necessary to create economic-grade phosphorites. Slansky (1986) estimates sediment accumulation rates of economic-grade phosphorites between 2x10 and 1x10 years per meter of deposit. This slow deposition rate is seen in Permian sediments of the Rocky Mountains where the Phosphoria Formation is about 30 m thick in the most phosphatic zone, whereas the same time period is represented by over 1000 m in non-phosphatic sections of the Permian in the Rocky Mountains. 

General geology. The Phosphoria Formation lies above the Permian Park City Formation, formed largely of limestone, and is overlain by the Triassic Dinwoody Formation (Fig. 8). The Phosphoria Formation contains two primarily phosphatic shale members, the Retort and the Meade Peak. The Meade Peak Member is the larger of the two main phosphorite bodies, and is the source of samples considered in our current work discussed here. The Meade Peak Phosphatic Shale Member is comprised of phosphorites, phosphatic shales, dolostones, siltstones, and mudstones (Fig. 9). 

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