THE RELATIONSHIP OF CLASTIC DIKES IN NORTHWESTERN NEBRASKA TO THOSE IN BADLANDS NATIONAL PARK

In the summer of 2011, we conducted fieldwork to compare clastic dikes found in northwestern Nebraska to those previously described in the Cedar Pass area of the Badlands National Park (Diggins et al., 2010) in an effort to further our understanding of the genesis of these enigmatic features.

Monroe Creek is the study site chosen, located just north of Harrison, Nebraska. At this location is a suite of clastic dikes, along with faults containing calcite ornamentation. In this area, the clastic dikes and faults cut the upper Chadron and lower Brule Formation strata, a lower level than seen in the Badlands National Park area. The clastic dikes are primarily mud filled in the smaller dikes and composite in the larger dikes, often with associated green alteration zones in the surrounding wall rock. Within many of the clastic dikes are brecciated chalcedony material mixed in with the clastic dike sediments. The chalcedony material inside of the continuous dike is not continuous, suggesting local derivation of the chalcedony. Clastic dikes have been observed to pinch out upwards, which indicates an injection mode for filling the dikes from the bottom or sides instead of infill from above.

Clastic dikes in this area tend to be clustered. There are three preferred orientation directions of the dikes. These were found to be near 72 degrees, 123 degrees, and 175 degrees. Common dike fill, overlapping crosscutting relationships, and dike interactions suggest that the three dikes directions are basically coeval.

Some of the faults in this study area formed parallel and locally within the dikes. Many other faults offset the clastic dikes; clearly the faults postdate the dikes. The orientation of the clastic dikes and faults in the Monroe Creek area are similar to structural orientations found at Toadstool and Rock Bass areas (northwestern Nebraska), and are recording the local stress field.