THE ORIGINS OF CLASTIC DIKES WITHIN BADLANDS NATIONAL PARK, SD

Although clastic dikes have been observed and studied for over a century, they are still relatively poorly understood features. Clastic dikes are thought to form in two basic ways: they may form either by infilling from above, or from injection of clastic sediments below. This study tests the hypothesis that abundant clastic dikes exposed in a roughly 2 square kilometer region of the Sage Creek Basin area of Badlands National Park, South Dakota, were injected in response to seismic activity on normal faults located several kilometers away from the observed dikes. Our test used both kinematic/geometric analyses derived from fieldwork, as well as mechanical modeling. In this study, for kinematic/geometric analysis, we constructed a high resolution GIS map incorporating data such as dike position, dike orientation of, grain-size of injected material, dike apertures, internal structures in the dikes, and cross cutting relationships between interacting dikes. We used the map and GIS database to reconstruct the sequence of dike formation, and attempted to determine if any patterns involving orientation, age or any other physical characteristics are present. These characteristics and patterns must be incorporated into an integrated mechanical model of the dike formation process. In conjunction with the GIS map, we conducted a series of mechanical models in Poly 3D to evaluate the plausibility of known local normal faults triggering the formation of some or all of the dikes in the study area. In these models, fault trace length was varied and known fault orientations were used to analyze the effect of fault position and length on the local stress field in the area where the dikes are observed. The models show that these particular faults cannot account for all of the dike orientations. It would be reasonable to assume that there were other mechanisms at work here, perhaps seismic activity on a larger more remote fault or faults.