PRELIMINARY RESULTS FROM NEW INVESTIGATIONS OF THE CHERAW FAULT, SOUTHEASTERN COLORADO

Additional mapping, evaluation of seismic data, and new topographic profiles based on the National Elevation Data (NED) 10-meter Digital Elevation Model (DEM) across the mapped trace of the Cheraw fault and an unmapped extension to the NE show added trace complexity and consistent morphologic expression along the entire length. Most vertical slip appears to be accommodated along the main fault trace, coincident with the most prominent northwest-facing topographic scarp, but smaller, subparallel scarps, often associated with strike changes in the main scarp are common. The maximum apparent vertical offset across the scarps along much of the fault appears to be about 5-6 m. Southwest of Haswell, CO along the unmapped extension, northeast oriented fractures in Niobrara Shale suggest apparent down-to-the-northwest deformation consistent with the topographic scarp. Preliminary processing of three shallow 2D seismic lines collected at the Crone et al. (1997) trench location and three additional lines collected in areas underlain by Rocky Flats Alluvium show velocity contrasts which appear to define the fault at depth and may constrain vertical offset of the base of Pleistocene-age Rocky Flats Alluvium. Well data and several industry collected 2D time-domain seismic profiles show a down-to-the-northwest style monoclinal warp with consistent vertical throw across deformation zones of varying widths that are coincident with the mapped trace of the fault and the unmapped extension to the NE. Preliminary analysis indicates that 1) the Quaternary trace of the Cheraw fault likely extends at least 15 km further to the NE than shown by Crone et al. (1997), 2) shallow and deep seismic data indicate that apparent vertical offset along the fault is locally distributed in a broader zone that is more complex than the present single-trace map depiction, and 3) surface profiles consistently suggest 4-6 m of vertical offset along the length of the mapped fault and NE extension, of which at least 3.2 m is late Quaternary based on trench data from Crone et al. (1997). The consistency of structure and offset along the Cheraw fault and NE extension may suggest additional late Quaternary offset, and may provide a basis for testing the full history of faulting on the Cheraw fault since early Quaternary.