Meade Peak member

One concern surrounding Gulbrandsen s work was that the samples he analyzed came from different regions within the Phosphoria Formation, which leaves the possibility that different weathering patterns have affected the eastern and western portions of the Phosphoria Formation to produce the results Gulbrandsen reported. Current work on the Meade Peak Member of the Phosphoria Formation (Knudsen and Gunter 2002) supports Gulbrandsen s (1970) theory. 

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). 

In Idaho, U.S.A., at the Henry Mine (Monsanto Corporation), the producing beds of the Meade Peak Member of the Phosphoria Formation are very soft and dip at an es of 50°-60 . Bulldozers with vertical scraper blades peel off layers of ore then earthmovers scrape up the ore and move it to storage piles according to grade. The high wall is benched approximately every 50 ft vwth 10- to 15-ft benches. The ore is transported by truck to the plant on a spedal road where it is used for elemental phosphorus manufacture. 

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. 

Figure 4. Isopach map of Meade Peak (left) and Retort (right) Members of the Phosphoria Formation contour interval is 10 m. Principal overthrust faults of the Sevier thrust belt indicated by barbed line isopachs and faults are dashed where uncertain.

The Retort Member is thickest in southwestern Montana The Retort closely resembles the Meade Peak except that its thickness is about one-half that of the Meade Peak and its shorelines and apparent depocenter are displaced northward The Retort, like the Meade Peak, is believed to have been deposited along a belt that extended southward into eastern Idaho from the apparent depocenter in southwestern Montana However, the Upper Permian sequence has been eroded in most of eastern Idaho prior to deposition of Triassic strata (22) and has left the original extent of the Retort unknown ... 

Figure 10. Plot comparing organic carbon and bitumen (left), and phosphorus to organic carbon, bitumen, and hydrocarbons (right) in samples from Meade Peak and Retort Members of the Phosphoria Formation. Values in percent for organic carbon and phosphorus are from Maughan (11) bitumen and...

Figure 9. Idealized stratigraphic section of the Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation at the Enoch Valley Mine (Fig. 4). The major ore and waste units within the Meade Peak, which are recognized throughout the southeastern Idaho phosphate-mining district, are shown. This stratigraphic section (modified from Carroll et al. 1998) is most comparable to the least-weathered or less-weathered sections collected for our current work the more-weathered section generally does not contain the carbonate strata.

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. 

Grauch RI (2002) Trace-element mineral residence and paragenetic framework of the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, southeastern Idaho. In Life cycle of the Phosphoria Formation from deposition to the post-mining enviromnent. Hein JR (ed) Handbook of Exploration Geochemistry series (in press)... 

Herring JR, Wilson SA, Stillings LA, Knudsen AC, Grmter ME, Tysdal RG, Grauch RI, Desborough GA, ZieUrtski RA (2000a) Chemical composition of weathered and less weathered strata of the Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation—B measured sectiorts C and D, Dry Valley, Caribon Cormty, Idaho. U S Geol Surv Open-File Report 99-147-B... 

Herring JR, Granch RI, Seims DF, Tysdal RG, Jolmson EA, Zielinski RA, Desborongh GA, Knndsen AC, Grmter ME (2001b) Chemical composition of strata of the Meade Peak phosphatic shale member of the Permian Phosphoria Formation channel-composited and individual rock samples of measured... 

Meade Peak Phosphatic Shale Member, Phosphoria Formation Carbonaceous marine shales, western U.S. 

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