INVESTIGATIONS OF POSSIBLE QUATERNARY FAULTING ON COLORADO’S HIGH PLAINS – WHAT IS THE ORIGIN OF THE ANTON SCARP?

Colorado's High Plains, consisting of a broad expanse of Miocene and younger sediments that dip gently eastward into Kansas and Nebraska, are crossed by numerous lineations and scarps, many of which have a NW-SE orientation. The largest of these scarps, informally named the Anton scarp, is at least 135 km long and up to 20-25 m high. During 2004, the Colorado Geological Survey and its research partners conducted different types of GIS and field investigations in order to assess the origin, age, and evolution of this feature, including the possibility that it was formed, at least in part, by faulting.

At the main study site, 11 km NE of Anton, the scarp is 25 m high, about 200 m wide, and has a maximum slope angle of seven degrees. Fieldwork included scarp profile measurement, GPR and refraction seismic surveys, trenching and trench wall logging, sampling for C14 and optically stimulated luminescence dating, and borehole drilling and core logging. A 180 m long, 4.5-6.0 m deep, four-level trench was dug down the fall line of the scarp beginning at the crest. The trench exposes 22 meters of stratigraphic section, almost all of which dips gently to the west and thus is truncated by the scarp. Near the bottom of the trench, however, the older strata dip gently eastward. No direct evidence of faulting was found in the trench. Continuous cores of up to 12 m deep were drilled along the trench and to the east, within a closed depression, to extend the depth and lateral extent of lithofacies correlations. There is some evidence of stratigraphic interruption in the subsurface beneath the depression; this is a likely area for future trenching.

Other investigations included GIS terrain modeling and soil mapping, a 45-km traverse along a 2.6-m deep, natural gas pipeline trench to describe lithofacies relationships and look for seismogenic sedimentary features, and reconnaissance mapping of exposures of “algal limestone” in the Ogallala Formation. Where the pipeline trench crossed the Anton scarp, the base of the scarp is underlain by a steeply dipping, organic-rich, extremely heavily burrowed zone that may represent a biologically modified fault zone. Numerous sand-filled cracks and wedge-like features occur along this trench; they are interpreted to be Pleistocene ice wedges and not of seismogenic origin.