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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 67 (1983)

Issue: 3. (March)

First Page: 535

Last Page: 535

Title: Regional Stratigraphic and Depositional Study of Rock Units in Upper Garden Gulch and Parachute Creek Members of Green River Formation, Piceance Creek Basin, Colorado: ABSTRACT

Author(s): Janet K. Pitman

Article Type: Meeting abstract

Abstract:

The Eocene Green River Formation in the Piceance Creek basin, Colorado, is well known for its thick sequence of rich oil shale and associated saline minerals. In the subsurface, rocks of economic interest extend from the upper part of the Garden Gulch Member to the top of the Parachute Creek Member. These rocks are lacustrine deposits that represent open-and near-shore sedimentation in ancient Lake Uinta. The depocenter of the lake formed in the northern part of the basin where rates of subsidence exceeded rates of sedimentation. The Garden Gulch and Parachute Creek Members can be divided into a series of laterally gradational rock units that can be traced from the eastern part of the Piceance Creek basin westward into the south-central part of the Uinta basin, Utah. Alon the margins of the Piceance Creek basin, the lower and middle units consist of fissile argillaceous shale of low organic content, silty claystone, and siltstone. Toward the depositional center of the basin, these rocks grade into a thick sequence of carbonate-rich, kerogenous shale and lean marlstone that is interbedded with a saline facies composed of nahcolite, dawsonite, and halite. The gradational boundary between these facies represents the contact between the Garden Gulch and Parachute Creek Members. In the subsurface, the contact can be recognized by a change from a low to high resistivity log response which reflects the transition from clay-rich to carbonate-rich rock. The regional thickness distribution pattern of individual units suggest that, during the middle Eocene, Lake Ui ta gradually expanded from a shallow, relatively fresh, semirestricted body of water to a saline, alkaline lake that occupied a closed basin. A pronounced thickening of the lowermost units along the southern margin of the basin is attributed to streams that prograded northward into the basin from the southern and southwestern margin of the lake. The middle and upper units, however, thin toward the basin edge, suggesting the lake gradually expanded to the south. During the late Eocene, open-lacustrine sedimentation shifted from the Piceance Creek basin westward into the Uinta basin, Utah, due to a large influx of siliciclastic sediment (Uinta Formation). Structural analysis of individual units indicates that present day intrabasinal tectonic features were not in existence during Parachute Creek time.

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