Understanding the kinematic development of basement-involved compressive structures that form at low temperature is dependent, in part, on gaining a better factual understanding of their deformational behavior. Results presented here show that the response of crystalline basement to deformation is incongruous among different structures in Colorado and Wyoming. At Big Thompson anticline and Rattlesnake Mountain anticline, Precambrian basement was not rotated in the anticlinal hinge during Tertiary folding. At both Banner Mountain and at a minor fold on Casper Mountain the basement has been rotated near the anticlinal hinge by as much as 26°. In the steep limb of all four of these monoclinal structures the basement is in fault contact with the stratified cover. At two sites, the Five Springs Creek area of the Bighorn Mountains and Casper Mountain, evidence for greater rotation of basement is clear. Precambrian dikes at Five Springs exhibit rotation of as much as 85° across the Laramide anticlinal hinge. Although the anticline at Casper Mountain shows minor folding, in the footwall a depositional contact on basement is rotated 50° from regional dip. These structures (Big Thompson anticline, Casper Mountain, and Five Springs) show minor fractures in the basement that apparently do not control the kinematics of basement deformation, but do indicate the stress field orientation. Typically σ 1 is nearly horizontal and normal to the fold axis, σ 2 is parallel to the fold axis, and σ 3 is very steep. This is the stress field normally associated with thrust faulting.

Abstract The Baxter Basin gas fields are located along the axis of the Rock Springs uplift in Sweetwater County, southwest Wyoming. Two important commercial gas fields, namely, South Baxter Basin and North Baxter Basin, have been developed to date on the Rock Springs uplift. At South Baxter Basin a total open flow of approximately 308 million cubic feet per day of inflammable sulphur-bearing gas has been developed by 11 wells drilled to the Dakota sandstone of Upper Cretaceous age. In the same field, a total open flow of approximately 48 million cubic feet per day of “sweet “gas has been developed by 8 wells completed in the Frontier formation, also of Upper Cretaceous age, and lying 400 feet above the Dakota formation. At North Baxter Basin three horizons produce “sweet” gas, namely, the Frontier formation and Dakota formation of Upper Cretaceous age and the Sundance formation of Jurassic age. Five wells have developed a total open flow of approximately 51 million cubic feet per day from these three producing horizons. Both at South Baxter Basin and North Baxter Basin, normal faulting in addition to anticlinal folding has influenced gas accumulation. Stratigraphic irregularities and unconformities as shown by varying formational thicknesses are noted. An interesting and important problem, not yet satisfactorily explained, is the presence of sulphur-bearing gas in the Dakota formation at South Baxter Basin and “sweet” gas in the Dakota formation at North Baxter Basin. Another problem of interest is the commercial gas production in the Sundance formation at North Baxter Basin and its absence in the same formation at South Baxter Basin, although the latter is approximately 1,200 feet higher structurally than the former.