FACT OR FICTION; THE HUNTERS HILL THRUST FAULT, EAST ELK RANGE, GUNNISON COUNTY, COLORADO

Versions of a NE-dipping thrust fault, the Hunters Hill (HH) thrust, are shown on regional maps published since the feature was first presented (Prather, 1964). A thrust more extensive than the original is shown in the Geologic Map of Colorado (GMC; 1979) and the map of Gunnison County (Streufert, 1999). On these maps, the HH thrust consists of three segments. Segment 1 extends east-west across Middle and East Brush Creeks to the northwestern flank of Hunters Hill and juxtaposes Pennsylvanian-Permian Maroon Fm on the southern side and Pennsylvanian Gothic Fm on the northern side. A straight trace across deep valleys and high ridges, depicted correctly on older maps (Emmons et al, 1894; GMC, 1935; Langenheim, 1952) shows that segment 1 is a high angle fault. As shown on the 1979 GMC, segments 3 and 2 connect high on the eastern flank of the ridge between Hunters Hill and the saddle north of Hill 12,022. The segment 3 trace extends southeast from Hunters Hill and eventually merges with the main structural zone of the Sawatch Uplift in upper Spring Creek Valley. A north-closing V pattern across Cement Creek valley implies a shallow N-NE dip. This feature, however, is a Laramide-age kink fold, first described by Peale (1873) and depicted (Zoerner, 1974) as a sharp, synclinal fold with gently-dipping Maroon Fm on the SW limb and vertical to overturned Maroon and Gothic Fms on the NE limb. Neither segment 1 nor 3 is a thrust fault. Segment 2, the HH thrust as originally proposed, follows along the western flank of Hunters Hill, around the southern nose of Hunters Hill, and extends a short distance to the northeast on the eastern flank of the mountain. The fault juxtaposes Belden shale on the Hunters Hill side against Morrison (Jurassic) and Dakota beds (Cretaceous) on the southwestern side. These strata are very gently dipping, and locally, Pennsylvanian Belden shales appear to overlie Dakota beds. Near the northwest end of segment 2, a fault surface dipping 80° N is exposed on the uphill side of a Dakota sandstone outcrop. Lineations show that movement was mainly dip slip. Thus segment 2 is a high-angle fault. Belden shales, displaced by mass movements and lateral flowage due to loading by strata higher on Hunters Hill, have buried the true trace of the high-angle fault. I submit that the Hunters Hill thrust, as cited in the regional literature for many years, is fiction, not fact.