The Polygonal Fault System of the Niobrara Formation

Nicholas Kernan
Colorado School of Mines

For close to one hundred years geologists have known about randomly oriented normal faults in the outcrops of Niobrara Formation in Western Kansas. These faults have dips close to forty-five degrees or less and a few hundred feet of displacement. They have been interpreted as being slump features due to salt dissolution within underlying Permian strata. Recently, 3D seismic data in the Denver Basin has helped identify a polygonal fault system within the Niobrara. Polygonal faults are layer bound extensional features that develop in fine grained deep water sequences. Faults mapped in outcrop were related to faults interpreted on seismic over the Wattenberg and Silo fields of Colorado. We interpret the faults in Kansas outcrops to be an extension of the polygonal fault system identified in the subsurface Niobrara and not related to salt dissolution. This would be the first outcrop of a polygonal fault system documented in North America. Theories behind polygonal faulting are still debated, as such, this research aims to further the overall understanding of how these fault systems initiate and develop. The paragenesis and fracture history of the Niobrara formation was investigated using petrographic and seismic interpretation, together with field and core descriptions. A primary focus was put to understanding potential causes for the nucleation and development of the polygonal fault system. Observations suggest that volumetric contraction, due to early burial dissolution, could have made the Niobrara formation susceptible to polygonal fault nucleation. Subsequent burial, compaction, and water expulsion would have then caused these faults to develop. The role of bioturbation in polygonal fault development is also considered.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013