Basement-Controlled Deformation in Wyoming Province of Rocky Mountains Foreland

Abstract

The Laramide deformation of the Rocky Mountains foreland in Montana, Wyoming, Colorado, and New Mexico is characterized by a distinctive structural style. Because the Laramide structure of Wyoming east of about 110° W. Long. well typifies that structural style, the term Wyoming Province is here used for the conterminous area characterized by it. The Precambrian basement of the Wyoming Province was closely involved in the Laramide deformation of the region. During the Laramide deformation the basement was covered generally by 8,000-15,000 feet of sedimentary rocks. The dominant structural style of the Wyoming Province is basement block faulting of diverse trends and great structural relief. Individual blocks are bounded by upthrust faults which commonly die out upward into monoclinal flexures in the overlying sedimentary rocks. Tilting of basement fault blocks produces a characteristic asymmetry of individual uplifts and basins. The deformation of the sedimentary rocks is mostly a direct consequence of differential movement of discrete fault blocks in the underlying basement. Originally horizontal sedimentary beds have been ch nged in attitude by (1) bodily tilting in unity with the underlying basement block, (2) drag along faults, (3) substitution of flexing for faulting along the strike of, or updip along, basement faults which extend into the overlying sedimentary rocks, and (4) buckling in local zones of horizontal compression related to upthrust faulting. Field studies in the Wyoming Province do not substantiate the widely held concept that there is a cause-and-effect relationship between either inherent fabric anisotropies or lithologic inhomogeneities in the basement and the style and locus of Laramide deformation. However, present inability to evaluate definitely such factors in controlling the style and locus of Laramide deformation of both basement and overlying sedimentary rocks does not necessarily invalidate that concept. Experimental rock deformation and other lines of evidence ultimately may provide a basis for relating inherent basement features to subsequent deformation.