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AAPG Bulletin

Abstract

AAPG Bulletin, V. 86, No. 7 (July 2002),Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

An upwelling model for the Phosphoria sea: A Permian, ocean-margin sea in the northwest United States

David Z. Piper,1 Paul Karl Link2

1U. S. Geological Survey, M/S 902, Menlo Park, California, 94025; email: [email protected]
2Department of Geosciences, Idaho State University, P.O. Box 8072, South 8th Street, Pocatello, Idaho, 83209; email: [email protected]

AUTHORS

David Piper is a geologist with the U.S. Geological Survey, having joined the Survey in 1975. He received his B.S. degree from the University of Kentucky, M.S. degree from Syracuse University, and Ph.D. from Scripps Institute of Oceanography. Before joining the Survey, he was on the faculty of the Department of Oceanography, University of Washington. His research has focused on the geochemistry of seawater and marine sedimentary ore deposits of modern and ancient oceans.

Paul Karl Link is a regional sedimentary geologist at Idaho State University, where he has been since 1980. He comes from a complex clan of geologist Links, including uncles Theodore (Ted) of Calgary and Victoria, Canada, and Walter of LaPorte, Indiana, and cousins Peter of Tulsa, Andy of Houston, and George of Monroe, Louisiana, among others.

ACKNOWLEDGMENTS

The manuscript was reviewed by L. Codispoti, W. Dean, G. Filippelli, and F. Poole; however, errors and interpretations are solely our responsibility.

ABSTRACT

The Permian Phosphoria Formation, a petroleum source rock and world-class phosphate deposit, was deposited in an epicratonic successor basin on the western margin of North America. We calculate the seawater circulation in the basin during deposition of the lower ore zone in the Meade Peak Member from the accumulation rates of carbonate fluorapatite and trace elements. The model gives the exchange rate of water between the Phosphoria sea and the open ocean to the west in terms of an upwelling rate (84 m yr-1) and residence time (4.2 yr) of seawater in the basin. These hydrographic properties supported a mean rate of primary productivity of 0.87 g m-2 d-1 of carbon in the uppermost few tens of meters of the water column (the photic zone) and denitrifying redox conditions in the bottom water (below approximately 150 m depth). High rain rates, onto the sea floor, of the organic matter that hosted the phosphate and several trace elements contributed to the accumulation of phosphorite, chert, and black shales and mudstones. Evaporation in the Goose Egg basin to the east of the Phosphoria basin ensured the import of surface seawater from the Phosphoria sea. Budgets of water, salt, phosphate, and oxygen, plus the minor accumulation of the biomarker gammacerane, show that exchange of water between the two basins was limited, possibly by the shallow carbonate platform that separated the two basins.

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