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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 64 (1980)

Issue: 5. (May)

First Page: 779

Last Page: 780

Title: Sedimentation, Biostratigraphy, and Source-Rock Potential of Deseret Starved Basin (Mississippian), Western United States: ABSTRACT

Author(s): Charles A. Sandberg, Raymond C. Gutschick

Article Type: Meeting abstract

Abstract:

Dark, organic-rich starved-basin sediments of the basal, phosphatic member of the Deseret Limestone and equivalents were deposited west of a westward-prograding carbonate platform in Osagean to early Meramecian time. These sediments comprise mainly pelletal, peloidal, oolitic, and conglomeratic phosphorite; phosphatic shale enclosing large calcareous concretions; bedded spiculitic and radiolarian chert; cherty micritic limestone; siltstone; and mudstone. The starved basin extends for more than 700 km from southeastern Nevada to southeastern Idaho. Rate of sedimentation of starved-basin sediments is calculated from the conodont zonation to be about 10 m/m.y. Slope sediments that intertongue westward with basinal sediments and eastward with carbonate-platform sediments cons st mainly of thin-bedded clinoform micrite interbedded with some debris-flow encrinite. These sediments were deposited on a gentle foreslope of 5° or less at a rate of 16 to 18.5 m/m.y. Time-equivalent carbonate-platform sediments were deposited at a rate of about 113 to 130 m/m.y.

The biota of the basinal sediments is mainly planktonic radiolarians, nektonic goniatites and conodonts, benthonic agglutinate foraminifera and sponges, and infaunal traces of burrowing organisms. The sparse shelly fauna consists mainly of small solitary corals and a few brachiopods. The bathymetry of the foreslope and shelf, considered together with the character and biota of the basin sediments, suggests that the floor of the central basin lay in the dysaerobic zone at a depth of about 300 m.

Organic-carbon and hydrocarbon content of outcropping phosphatic shales that have been deeply weathered, leached, and biodegraded are difficult to evaluate. Analyses generally produce values that are much lower than values that can be expected in the subsurface, where the same rocks have generated or are generating petroleum. Nevertheless, the following organic-carbon yields have been obtained from carefully selected outcrop

End_Page 779------------------------------

samples: 1.50 to 7.95% (4.29 median) for phosphatic shales, 0.67 to 5.11% (2.64 median) for phosphorites, and 0.40 to 3.17% (1.25 median) for micrites. Hydrocarbon analyses range from 50 to 300 ppm in areas where conodont CAI values range from 1.5 to 4.

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