INFLUENCE OF PRE-EXISTING JOINTS ON STYLE OF DEFORMATION RELATED TO LACCOLITH DEVELOPMENT, TENT CANYON DONE, WYOMING

Laccolith development in sedimentary rocks includes brittle deformation of the overlying units typically by warping of beds and faulting. Physical laboratory models of dome formation have suggested radial fracture orientations developing in unfractured overlying materials for a circular dome or fractures developing along a preferred trend parallel to the long axis of an elliptical dome. Pre-existing joints will likely influence the nature of deformation and accommodate a significant amount of deformational response to a doming event. Tent Canyon Dome, a shallow elliptical laccolith in the Black Hills of Wyoming, exhibits deformation concentrated along pre-existing joints. Jointing in more competent lithologies can mostly be attributed to deformation prior to doming, indicating that few new fractures were developed during laccolith emplacement. Tilting and faulting were the dominant responses to the doming event, both of which utilized existing joints. Thinning of a weaker shale unit between more competent limestones and sandstones accompanied stretching of overlying units.

Parts of the Tent Canyon Dome sedimentary roof rocks broke into fault-bounded blocks, each showing a unique orientation of tilting. Bedding orientations exhibit restricted variability within individual blocks and distinct differences in the average orientations compared to adjacent blocks. Presence of these areas of relatively uniform dip suggests each block of overlying sedimentary rock acted as a somewhat coherent plate, rising and tilting as a rigid mass rather than having gradual changes in dip amount by warping. Pre-existing joints allowed the overlying sedimentary units to deform mostly by tilting of these blocks rather than internal deformation by warping. Less tilting appears associated with bounding faults along the northeastern periphery of Tent Canyon Dome, where deformation was accomplished mostly by fault displacement.