Abstract

Normal faults occur in the Niobrara Chalk and Pierre Shale in numerous localities in South Dakota, Nebraska, and Kansas. Lakeshore outcrops provide a window into strata that otherwise outcrop poorly. Various criteria were used to distinguish mass-wasting features from faults that formed at the subsurface. Locally, these faults have variable orientations and throws that vary from centimeters to tens of meters. In the Niobrara Chalk, the fault zones typically display well-developed striae, occasional slickensides, limited to absent damage zones, and dilational jogs with coarse, microvein calcite fill. In places, the faults are associated with gentle monoclines and folds, but elsewhere they occur in horizontal strata. A weathering bias suggests these faults are more common than observed.

At any one locality, these faults have been or could be attributed to local tectonism, differential compaction, and/or glacial rebound. If due to tectonism, taken in aggregate, they indicate fairly widespread (albeit low-strain) activity, which is consistent with the idea of a critically stressed continental interior with migrating sites of faulting. An alternate and preferred hypothesis is that these are polygonal faults due to diagenetically driven deformation in fine-grained mud rocks. Subsurface polygonal fault systems have been described from the Niobrara Chalk and Pierre Shale in the Denver-Julesburg Basin area in eastern Colorado. Smectitic clays, associated with polygonal faulting elsewhere, are abundant in these units. The fault kinematics (extension in multiple directions), widespread character, and other traits are consistent with an origin by diagenetically driven deformation.

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