Abstract: Thrust Belt of Wyoming, Idaho, and Northern Utah--Structural Geometry and Related Stratigraphic Problems

Frank Royse, Jr., M. A. Warner, D. L. Reese

Geologic and geophysical data gathered in the Wyoming-Idaho-northern Utah thrust belt can be interpreted by invoking structural concepts and geometric restraints found applicable in similar provinces, such as the Canadian Rockies and the Appalachians. The concepts include: (1) deformation is "brittle", and plastic flow or cleavage folding does not occur to a significant degree; (2) folds form concurrently with thrusting and have essentially concentric geometry; (3) thrust faults cut the stratigraphic section in the direction of tectonic transport; (4) faults tend to be parallel with bedding in incompetent rocks and oblique in competent rocks, a condition which imparts a steplike form to fault surfaces (the angle between fault and bedding on major steps is 20 to 35°); and (5) major thrusts are younger in the direction of tectonic transport of the hanging wall (east).

The structural model devised is vital to proper stratigraphic mapping because of the disruption brought about by thrust faulting. Mapping and dating of synorogenic conglomeratic "foredeep" deposits indicate that thrusting was episodic during a 90-m.y. period from latest Jurassic to early Eocene. Four major thrust-fault systems are recognized. From west to east they are: (1) the Bannock, which includes the Paris and Willard thrusts, (2) the Meade-Crawford, (3) the Absaroka, and (4) the Darby, which includes the Prospect, Darby, and Hogsback thrusts. Total shortening is about 65 mi (104.5 km) of an original 130 mi (209 km) or 50%. Younger normal (extensional) faulting occurred from Eocene time to the present. The sole of many normal faults is on thrust planes or within Jurassic salt bed .

Favorable conditions for oil prospecting are present in some areas where structural trap formation, peak oil generation, and migration occurred concurrently, and where traps are not eroded or flushed. The recent oil and gas discovery in the Nugget Sandstone at Ryckman Creek field in southwestern Wyoming is an example of exploration success in such an area.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado