DISTRIBUTED NORMAL FAULTS IN THE NIOBRARA CHALK AND PIERRE SHALE OF THE CENTRAL GREAT PLAINS

Normal faults occur in the Niobrara Chalk and Pierre Shale Platte in the following areas: Francis Case and Sharp lakes in central South Dakota, in the Lewis and Clark Lake area of NE Nebraska, in the Harlan Reservoir area of S-central Nebraska, and in N-central Kansas. Several criteria were used to distinguish faults associated with mass wasting from those described here that reflect deeper processes. In any one area these faults have variable strikes and dip directions and throws ranging from centimeters to several tens of meters. In the Niobrara chalks the fault zones typically display well developed striae, occasional slickensides, limited fractured damage zones, and dilational jogs with greenish, coarse calcite fill. In places the faults are associated with very gentle monoclines, but elsewhere occur in horizontal strata. Due to a weathering bias the faults are likely more common than observed.

At any one locality these faults have or could be attributed to local deeper seated tectonism, differential compaction, and/or glacial rebound. If due to tectonism, taken in aggregate they indicate fairly widespread (although minor) tectonism since deposition. This would be consistent with the idea of a critically stressed continental interior with migrating sites of faulting. An alternate hypothesis is that of diagenetically driven deformation in fine-grained mudrocks. Literature suggests subsurface polygonal faults occur in the Niobrara Chalks in the Rocky Mountain region. The faults seen in outcrop here could represent a more eastern equivalent fault assemblage, but one substantially less developed due to shallower burial depths and attendant decreased diagenesis. Smectitic clays associated with polygonal faulting elsewhere, are also abundant in these units. The fault kinematics and widespread character is consistent with a diagenetic origin. Such faulting would be expected to be stratabound and not extend to depth. Timing is uncertain, but a multistage diagenetic history associated with deposition and changing pore water chemistry provides a framework of possibilities. For these strata the possibility of diagenetically driven deformation should be considered when faulting is found.