A Preliminary Discussion of Fault Styles in the Southwest Uinta Basin, Utah

By

Craig D. Morgan and Kevin P. McClure

Geological Survey, Salt Lake City, UT

S. Robert Bereskin

Tesseract Corporation, Salt Lake City, UT

Mary McPherson

McPherson Geologic Consulting, Grand Junction, CO

Aspects of structural geology were observed during the investigation of the regional stratigraphic trends in the Tertiary Green River Formation in the SW Uinta Basin. The area is dominated by 3o-5o NE regional dip into the basin, interrupted by only a few E-W and NW-SE trending faults. Most faults exposed at the surface in the SW Uinta Basin can be grouped into two types: shallow, hingeline, Duchesne-graben type, and oblique-slip faults.

The Duchesne fault zone (DFZ) is an E-W trough with a master fault that dips steeply northward and has ~200 m of net displacement. This feature coincides with the southern hinge of the basin structural trough. The fault terminates just above the Mahogany oil shale of the Green River at ~1400 m, or flattens into a low-angle detachment zone at ~1000 m. At Sand Wash, southeast of the DFZ, another graben is exposed in several cliff faces, terminating just above the Mahogany. The graben is ~150 m wide and has only a few meters of throw. We believe this graben represents the downward termination of a DFZ type system.

Faults along the southern flank of the Peters Point anticline are exposed in Cedar Ridge Canyon near the junction with Desolation Canyon. These parallel faults have a throw of 1-40+ m, and a wide deformation zone indicating possible significant lateralmovement. These oblique-slip faults are associated with a deeper, basement-involved movement. We interpret the faults to be part of a flower structure related to the Garmesa wrench-fault zone that forms the northern margin of the Uncompaghre uplift.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming