KINEMATIC STRUCTURAL ANALYSIS OF THE CLOVERLY AND FORT UNION FORMATIONS OF THE EASTERN BIGHORN BASIN, WYOMING

The Bighorn Basin has experienced little deformation since the Laramide orogeny in Late Cretaceous-Middle Eocene time. The Early Cretaceous Cloverly Formation, especially the Greybull Sandstone Member, provides additional data and insight to Laramide-type crustal deformation, complementing other work done in the Bighorn Basin and at the Sheep Mountain Anticline in the Amsden, Madison, and Tensleep formations.

Geometries and orientations of fractures, slicken lines, and crystal growth patterns support a NE-SW principal horizontal stress direction bearing about 067°. Furthermore, the faults located in the Greybull Sandstone Member and an ash layer in the Cloverly Formation are not found in Tertiary Fort Union Formation sandstone, which itself contains a consistent fracture set bearing NW-SE. Few fracture sets disagree with the proposed 067° principal stress direction, supporting a single-stage event with a single principal stress direction during the Laramide Orogeny.