Confirmation and Characterization of a Previously Unmapped Northeast Extension of the Cheraw Fault, Southeast Colorado

Abstract:

Recent geologic field mapping, shallow geophysical surveys, and boreholes, combined with interpretation of reprocessed industry 2D seismic reflection profiles, provide new constraints for the structural characterization of the Cheraw Fault in southeastern Colorado. A previously unmapped topographic scarp extending northeast from of the mapped fault trace exhibits northeast-oriented fractures in Cretaceous Niobrara shale and an apparent ~3m down-to-the-northwest offset across a bedrock/alluvium contact observed in exposures of the scarp near of Haswell, CO. Shallow seismic surveys and a transect of 4 shallow borings across the scarp confirm the vertical displacement estimates. Two industry 2D seismic refraction profiles that cross this scarp were licensed, reprocessed, and depth-migrated. Preliminary analyses of the seismic profiles show ~ 100m or less of down-to-the-northwest displacement across a primary fault that is coincident with the topographic scarp and extends through Mesozoic and Paleozoic strata into Precambrian basement. Fault dip constraints and are pending further seismic data interpretation and analysis. These data show that 1) the Cheraw fault extends an additional 15 km from its currently mapped northern termination, for a minimum total fault length of ~60 km, 2) offset of an early (?) Quaternary pediment along the northeast extension near the town of Haswell of ~3 m, is similar in magnitude to the offset in a late Pleistocene channel reported by Crone et al. (1997) along the main trace of the fault, and 3) seismic reflection data show that the fault extends into crystalline basement rock, supporting a tectonic feature rather than dissolution collapse.