ABSTRACT: Post-Eocene West-Stepping Thrust Faulting in the Utah-Wyoming Thrustbelt

Arthur E. Berman

1311 MMBOE were discovered between 1975 and 1982 in the Utah-Wyoming Thrustbelt on the Absaroka thrust, but no commercial discoveries have been made on other thrust sheets. Structural field relationships suggest that assumptions about timing of thrusting and petroleum generation and migration may have been incorrect. This in turn led to exploration models that proved invalid outside of the productive Absaroka fairway. Significant oil and gas accumulations may be present on thrust sheets west of the Absaroka.

Thrust faulting probably began in western Wyoming and eastern Utah with movement on the Absaroka thrust during the Late Cretaceous. It seems likely that the Darby thrust became active shortly thereafter. Intraplate deformation of the Absaroka package occurred during or after the Eocene resulting in belts of folds and faults along the Commissary, Tunp, and Crawford trends. This Absaroka intraplate deformation resulted from compression produced by movement on the Ogden thrust to the west. The Ogden thrust package is exposed along the Wasatch fault between Ogden and Salt Lake City, Utah, and extends as far east as the Devils Slide area, west of Croyden, Utah. This thrust package has not previously been recognized. The Precambrian crystalline rocks of the Wasarch Mountains are carried on he Ogden thrust along with Paleozoic and Mesozoic sedimentary rocks that outcrop east of the range. The Willard thrust moved after the Ogden and initiated still farther to the west. The Willard includes Proterozoic and lower Paleozoic rocks not recognized on any of the more easterly thrust sheets.

The unique occurrence of oil and gas within the Absaroka package is related to many factors including its initial geometry and the remigration of fluids associated with the post-Eocene modification of this geometry. Application of these observations to exploration models might lead to reevaluation of the potential for future resource discoveries in the Wyoming-Utah Thrustbelt.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990