PALEOSEISMIC INVESTIGATION OF THE NORTHEAST SECTION OF THE CHERAW FAULT

The northeast section of the Cheraw fault is characterized by a distinct northwest facing topographic scarp extending from the northern terminus of the previously recognized 46 km long fault section, approximately 4 km southwest of Haswell, Colorado, to a point approximately 15 kilometers northeast. This scarp exhibits a continuous morphologic expression, similar in width and height, as the scarp associated with the recognized fault section to the southwest. Until recently, a lack of study and questions regarding its tectonic origin had led seismic hazard models to favor the original 46 km mapped length of the Cheraw fault. Our recent investigations utilizing geomorphic mapping, trenching, shallow boreholes, shallow 2D seismic refraction surveys, and interpretation of multiple 2D exploration seismic reflection profiles have confirmed the scarp to be the surface expression of a fault-related structure that extends to basement with a revised minimum Quaternary rupture length of ~60 km.

At a site near Haswell, trenches exposed early Cretaceous bedrock fold and fault structures associated with the Cheraw fault and defined offset of the basal portion of the early (?) Quaternary Nussbaum Alluvium. Two broad conclusions, focused on the northeast extension of the Cheraw fault, were obtained from this research: 1) The topographic scarp which characterizes the Cheraw fault, including the northeast section, is associated with a bedrock fault and fold structure in the Niobrara Fm., 2) Overlying early (?) Quaternary Nussbaum Alluvium at the Haswell Site is significantly deformed, with total vertical offset of the basal contact at least 5 to 6 m, and possibly up to 9 m. The present trenches do not provide evidence for the number or specific timing of events related to this deformation. Samples were collected for OSL analyses to estimate the age of Nussbaum Alluvium in the trenches, but as of late early July 2016 are pending final analysis.