BRULE FORMATION CLASTIC DIKES OF THE GREAT PLAINS, YET ANOTHER TYPE?

Clastic dikes occur locally within the Brule Formation of the Great Plains, and are well know from Badlands National Park. Arguably they are poorly understood. Their sediment fill source, timing, mechanics and reason for localization are all uncertain. 3 traits stand out: internal layering, a lack of identified source beds, and a fairly uniform strike distribution. Details of the internal layering are typically obscured by expansive weathering associated with their smectite content. Recent work at Slim Buttes, SD, demonstrates that in a typical ≈10 cm thick dike there are tens of layers that often cut each other, indicating recurrent emplacement of cm to mm thick layers. Dikes that cut each other increases the number of emplacement events. At Slim Buttes, smaller dikes are stratabound within the Brule, while larger dikes are truncated at the contact with the overlying Arikaree Group. No dikes reached into the underlying Chadron Fm. A lack of distinct source beds despite badlands exposures is taken to indicate that they do not exist. Instead of source beds, we propose that the sediment is sourced from the fracture walls of the main dike and from contributing smaller fractures and dikelets in tip regions. Abundant non-matching and irregular wall geometries are attributed to wall erosion. If not propped open by fill continued compaction could close some dike portions. Strike dikes at Slim Buttes cannot be distinguished from a random/uniform distribution. Those at Badlands National Park have weakly developed preferred orientations with a large inferable uniform distribution component. Both are consistent with uniform strain in a horizontal plane. Observations place dike formation early during diagenesis. Our working model is that diagenetically driven and opening mode fractures incrementally propagate and grow within differentially lithified Brule sediments. Their interaction with groundwater pressure gradients induces liquefaction and flow, and sediment transport from dike tip regions, propping open the dike interiors. In addition to Neptunian clastic dikes with infilling from above, and injection/liquefaction events associated with input of additional energy (e.g seismic, storm waves, rapid loading), the Brule Formation clastic dikes may exemplify an additional and diagenetically driven mechanism.