Phosphoria Formation

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Phosphoria Formation
Stratigraphic range: Early Permian
An outcrop of the Phosphoria Formation near the Big Hole River in western Montana.
Type Geological formation
Underlies Dinwoody Formation
Overlies Casper Formation, Park City Formation, Tensleep Sandstone
Area 350,000 sq. km (140,000 sq. mi)[1]
Thickness Up to 420 metres (1,380 ft)[1]
Primary Limestone, dolostone, shale
Other Chert, phosphorite, sandstone
Region Idaho, Montana, Utah, Wyoming[2]
Country  USA
Type section
Named for Phosphoria Gulch, Idaho
Named by Richards and Mansfield, 1912[3]
Uinta-Piceance Basin stratigraphic column showing the relationship of the Phosphoria Formation


The Phosphoria Formation of the western United States is a geological formation of Early Permian age.[4] It represents some 15 million years of sedimentation, reaches a thickness of 420 metres (1,380 ft) and covers an area of 350,000 square kilometres (140,000 sq mi).[1]

The formation includes phosphorite beds that are an important source of phosphorus. They have been mined in southeastern Idaho, northern Utah, western Wyoming, and southwestern Montana. Low concentrations of uranium are present in the phosphorite beds,[5] and a vanadium-enriched zone is present in western Wyoming and southeastern Idaho.[6] Many of the Phosphoria shales are rich in organic matter and are petroleum source rocks,[7] and petroleum has been produced from some of the dolostones in the formation.[8]

Stratigraphy and Lithology[edit]

Peloidal phosphorite from the Phosphoria Formation, Simplot Mine, Idaho. 4.6 cm wide.

The Phosphoria Formation was deposited under marine conditions in a foreland basin located between the Paleozoic continental margin and the North American cratonic shelf.[7]

The formation is commonly subdivided, from the top downward, as follows:

  • The Tosi chert member (chert with limestone at base).
  • The Retort phosphatic member (phosphorite, dolostone and siltstone).
  • The Rex chert member (gray limestone at base, black chert, and black cherty shale).
  • The Meade Peak phosphatic member (interbedded brown to black shale, gray dense limestone, dark brown oolitic phosphorite, and minor white calcareous sandstone).
  • The Lower Chert member (dark cherty shale).

The Phosphoria is underlain by the Pennsylvanian-Permian Casper Formation[1] or, depending on the location, by the Park City Formation or the Tensleep Sandstone,[9] and it is overlain by the Triassic Dinwoody Formation.[1] The upper boundary is placed at the top of the uppermost phosphorite bed and below the tan calcareous siltstone of Dinwoody formation. The lower boundary is marked by a thin phosphorite that contains abundant fish scales and bones.[2][9]

Thickness and Distribution[edit]

The formation reaches its greatest thickness in the Sublett Range in the Paleozoic cordilleran structural basin of southern Idaho. Within the basin the formation consists of a basal phosphorite overlain by a thick sequence of chert and cherty sandstone. To the east in western Wyoming the Phosphoria gradually decreases in thickness and is intertongued with carbonate rocks of the Park City Formation and redbed sandstones of the Chugwater Formation of Permian to Triassic age.[1] The Phosphoria units extend into the Wind River Mountains and pinch out in the Green Mountains to the east.[1]


Fossils recovered from the Phosphoria Formation include brachiopods, sponge spicules, crinoid stems, conodont elements, and fish scales and bones. The Early Permian age of the formation is based primarily on conodont biostratigraphy.[2][3][4][10]


  1. ^ a b c d e f g Blatt, Harvey and Robert J. Tracy, Petrology, Freeman, 1996, 2nd ed. pp. 345-349 ISBN 0-7167-2438-3
  2. ^ a b c U.S. Geological Survey. "Geologic Unit: Phosphoria". Retrieved 2014-12-23. 
  3. ^ a b Richards, R.W., and Mansfield, G.R., 1912, The Bannock overthrust; a major fault in southeastern Idaho and northeastern Utah: Journal of Geology, v. 20, no. 8, p. 681-709.
  4. ^ a b Behnken, F.H., Wardlaw, B.R. and Stout, L.N., 1986, Conodont biostratigraphy of the Permian Meade Peak Phosphatic Shale Member, Phosphoria Formation, southeastern Idaho. University of Wyoming Contributions to Geology, v. 24, no. 2, p. 169-190.
  5. ^ McKelvey, V.E., and L.D. Carswell, 1967. Uranium in the Phosphoria Formation. In: Anatomy of the Western Phosphate Field: A Guide to the Geologic Occurrence, Exploration Methods, Mining Engineering and Recovery Technology; AAPG Fifteenth Annual Field Conference, p. 119-123.
  6. ^ McKelvey, V.E., Strobell Jr, J.D. and Slaughter, A.L., 1987. The vanadiferous zone of the Phosphoria Formation in western Wyoming and southeastern Idaho. In: The Thrust Belt Revisited; AAPG 38th Annual Field Conference Guidebook, p. 305-308.
  7. ^ a b Maughan, E.K. 1984. Geological setting and some geochemistry of petroleum source rocks in the Permian Phosphoria Formation. Rocky Mountain Association of Geologists.
  8. ^ Campbell, C.V. 1962. Depositional environments of Phosphoria Formation (Permian) in southeastern Bighorn basin, Wyoming. AAPG Bulletin 46(4): 478-503.
  9. ^ a b McKelvey, V.E., Cheney, T.M., Cressman, E.R., Sheldon, R.P., Swanson, R.W. and Williams, J. Steele, 1959. The Phosphoria, Park City, and Shedhorn formations in the western phosphate field. In: Geology of Permian rocks in the western phosphate field, U.S. Geological Survey Professional Paper 313-A, p. A1-A47.
  10. ^ Wardlaw, B.R. and J.W. Collinson, 1986. Paleontology and deposition of the Phosphoria Formation. Rocky Mountain Geology 24(2): 107-142.