Clastic dikes that occur within the terrestrial, Oligocene White River Group strata at localities throughout the Great Plains typically display internal mud to fine sand layers that are subparallel to the walls. Shrink-swell weathering usually obscures details of the internal layer geometry of the dikes. Recent work in the Slim Buttes area documents internal layer cross-cutting relationships that indicate tens or more of recurrent opening and injection events for thicker individual dikes. Evidence of significant dike-wall modification also exists. Source beds were unobserved despite adequate outcrops. Dikes are enclosed within the Oligocene Brule Formation. Some are truncated at or near the contact with the overlying Miocene Arikaree Group strata, constraining formation timing, whereas others have upper and lower tips within the Brule Formation. Dike strikes test as random in distribution. These dike attributes are consistent with repeated fracture opening and tip propagation from diagenetically driven shrinkage that induced episodic fluid flow which mobilized host-rock sediment (crack-fill instead of crack-seal). Sediment fill is proposed to have come from dike-wall erosion in branching tip regions during propagation events. In general, clastic dikes are polygenetic, and the diagenetically driven, recurrent formation mode evident in the White River Group examples can be considered in addition to standard injection models associated with overpressurized source beds or Neptunian infill.

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