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The Permian Phosphoria Formation in the northwestern interior United States contains two phosphatic and organic-carbon-rich shale members--the Meade Peak Phosphatic Shale Member and the Retort Phosphatic Shale Member. These 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. Upwelling is indicated to have occurred in the Phosphoria sea by the presence of sapropel that was deposited adjacent to shoals near the east flank of the depositional basin.
Maximum organic-carbon concentration is as much as 30 wt. % in the organically richest beds in the shale members and the maximum average in each member is about 10 wt. %. A close association occurs in the distribution of the organic carbon, silver, chromium, molybdenum, nickel, titanium, vanadium, and zinc. Phosphorous differs slightly from the distribution of organic carbon and by contrast seems typically associated with copper, lanthanum, neodymium, strontium, yttrium, and ytterbium.
Burial of the sapropelic muds by Triassic and younger sediments and the consequent rise in ambient temperature has led to catagenesis of hydrocarbons from the kerogen in these rocks. In some areas of southwestern Montana, hydrocarbons have not been generated; however, burial has been minimal and temperatures have remained low. Consequently, these rocks remain organic-rich shales that have the potential for producing synthetic oil and gas.
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