PIETRAS, JEFFREY T., UW-Madison, Dept. of Geology and Geophysics, Madison, WI

ABSTRACT: Sediment and Water Provenance and Controls on Lacustrine Sedimentation, Wilkins Peak Member, Green River Basin, WY

Ancient lacustrine basins have proven to be important sources of both oil and natural gas, as well as providing detailed records of climatic change and paleobiological evolution. The Wilkins Peak Member can provide a case example for underfilled basins, from which reservoir and source rock potential models can be constructed.

The Wilkins Peak Member of the Green River Formation records deposition in Eocene Lake Gosiute during a period of underfilling as basin subsidence outpaced sediment accumulation. This evaporative-lacustrine basin periodically expanded into a deeper lake that may have been freshened by increased runoff and drainage over a sill. These shifts have been attributed to both climatic change and tectonic basin reorganization, however detailed histories are poorly developed.

The aim of this project is to understand Wilkins Peak paleohydrology by: 1) constructing detailed cross-sections from outcrop measured sections and correlating into the subsurface, and 2) developing a provenance history of both sediment and water entering Lake Gosiute during deposition of the Wilkins Peak Member using paleocurrent measurements, sandstone petrography, and strontium fingerprinting of primary carbonates and fine-grained siliciclastic material.

Preliminary analysis of carbonate samples from the Green River Formation show that ⁸⁷Sr⁸⁶Sr ratios record closing of the basin in Wilkins Peak time (0.71490) and reconnection with isotopically light extrabasinal drainages during subsequent deposition of the Laney Member (0.71163). This large range in values suggests that discriminating isotopic values between beds within the Wilkins Peak Member will be possible.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid