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The AAPG/Datapages Combined Publications Database
CSPG Special Publications
Abstract
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Phosphoria Formation (Permian) and Its Resource Significance in the Western Interior, U.S.A.
Abstract
The Phosphoria Formation forms part of the Permian in the Western Interior of the United States and includes phosphorite beds associated with two organic carbon-rich mudstone units. The Phosphoria was deposited during marine transgressive-regressive cycles in the Sublett basin, an embayment centered in northern Utah and southern Idaho and extending into adjacent areas of the broad western shelf of the North American craton. Westward movement of the North American plate during assembly of the Pangean continent developed a foreland basin along its western margin in Permian time that was partially restricted by tectonic highlands and by a probable island arc from the open ocean farther west.
Geographic conditions were favorable for upwelling currents within the basin and for the entrapment and reduced influx of terrigenous sediments into the basin from land areas to the east. The configuration of the depositional system suggests slow subsidence during Phosphoria deposition and a favorable environment for abundant biotic production. Sapropelic mud accumulated throughout the basin, but the highest concentration of Corg suggests areas of upwelling adjacent to the Idaho-Wyoming border during deposition of the Meade Peak Member of the Phosphoria Formation, and in southwestern Montana during deposition of the Retort Member. The supply of organic nutrients brought into the zone of photosynthesis by upwelling was enhanced by probable volcanic sources to the west as well as by leaching of minerals from the rocks in highly alkaline lagoonal areas along the eastern fringes of the basin. Solutes from intensely weathered rocks of adjacent land areas may have supplied additional nutrients.
Economic phosphorite resources are the result of winnowing of the organic mud that left concentrated apatite lags in areas of elevated sea floor. Economic petroleum resources resulted from burial of the organic-rich mud to depths of more than 2.5 km, leading to hydrocarbon generation. Economic concentration of vanadium in these organic-rich beds, and possibly of other trace metals such as silver and chromium are additional resources in these Permian rocks.
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