Joint Meeting: University of Wyoming Department of Geology and Geophysics Wyoming Geological Association Geological Survey of Wyoming: April 2-4, 1982 Laramie, Wyoming: Subsurface Practices in Geology and Geophysics Abstracts of Papers - Compiled by James R. Steidtmann

Field and subsurface studies of the Rocky Mountain foreland indicate that the boundaries of many uplifted basement blocks are characterized by: steeply dipping basement/cover contacts, reverse faults placing basement over cover, and attenuated cover-rock slivers beneath these reverse faults. The observations can be incorporated into a mechanically coherent picture of “splinter blocks”. Splinter blocks are defined here as slivers of basement which are: fault-bound, tapered downward, often spoon-shaped in plan view, and often severely rotated. The response of the cover rocks to the motions of the splinter blocks is much like that which occurs in the case of simple block boundaries (drape folds), except that the splinter blocks′ narrow width (and hence tight synclinal folds) can cause extensive reverse faulting of the cover—faulting which does not project to basement level. Other features like “pop-ups” (folds caused by room problems) are also sometimes found. The general aspects of splinter-block tectonics are illustrated by examples taken from the north and east margins of the Beartooth Mountains, the northwestern Wind River Mountains, the Wind River Basin, and the Front Range of Colorado.

While the concepts described here are not new, they have not been routinely applied to exploration in the Rocky Mountain basins. Splinter-block tectonics has important implications to both prospect generation and evaluation. The examples will be used to illustrate these implications.