AN ALTERNATE INTERPRETATION OF THE PROPOSED THRUST FAULT ON HUNTERS HILL, GUNNISON COUNTY, COLORADO

Some versions of the Hunters Hill (HH) thrust are shown to have included 2 non-thrust segments, a vertical E-W fault that extends westward from the saddle north of Hunters Hill, and a sharp, lower monoclinal bend that extends to the SE from the SE flank of Hunters Hill. The dipping monoclinal beds do not extend under Hunters Hill. The segment along the western and southern flanks of Hunters Hill, the thrust as originally defined, corresponds to a subhorizontal contact between Pennsylvanian rocks above and Mesozoic strata below. This contact is suggested to mark the edge of dark shales, judged to be Belden on geochemical and geological grounds, displaced downslope by mass wastage and subsurface flowage. About 600m SW of Hill 12,705, a near-vertical fault places Belden against Dakota beds. To the SE and E, this fault trace is assumed to be buried by displaced Belden shales. A vertical E-W fault that crosses Cement Creek about 1.1 km upstream from the monoclinal bend separates vertical to overturned strata to the south from gently dipping strata west of Cement Creek. This fault was probably continuous with a steeply dipping, E-W fault west of the central Italian Mountain pluton.

In upper Cement Creek basin, calcite from Belden strata and veinlets show Sr, C, and O isotopic values consistent with Sr derived mainly from the carbonate host rocks, a C component derived by oxidation of organic matter, and interaction with a fluid that had a meteoric water component. Veinlet calcites locally show sheared-out textures and closely-spaced, bent twin lamellae and are quite different isotopically and texturally from calcite in metasomatized Belden adjacent to the northern granite. This veined zone roughly coincides with a decrease in thickness and dip of the Belden Fm, but does not match any locations proposed for the HH thrust. Proponents of the proposed thrust should be obliged to show the trace of the proposed thrust from the SE flank of Hunters Hill toward Cement Creek and northward, and to explain how a gently-dipping sheet of Pennsylvanian strata comprising the proposed upper plate was fortuitously emplaced directly over similar strata of the same age in the proposed lower plate.