GEOLOGIC MAPPING OF GRAVITY SLIDE BLOCKS IN THE ADAMS HEAD 7.5-MINUTE QUADRANGLE, UTAH

The Sevier Gravity Slide (SGS) is one of three, large gravitational slides within the Marysvale gravity slide complex (MGSC). The oldest of the three mega-scale (>1000 km²) collapse structures, the SGS contains abundant field evidence of a single catastrophic emplacement at high velocity. These include the presence of clastic dikes, pseudotachylyte, breccia development, severe damage within the basal shear zone and the overlying slide block, and evidence of slide transport aided by thermal pressurization. We have been mapping within the SGS in the Adams Head 7.5’ quadrangle located in Garfield County, UT. Special focus was given to the Upper Sanford Creek valley, located in the west-central portion of the mapping area where we mapped volcanic and volcaniclastic strata at a 1:12,000 scale. At this site, a near-complete exposure of the basal zone is provided by Basin and Range normal faults that strike parallel to the slide transport direction. This mapping area was most recently included in the highly detailed “Geologic Map of the Adams Head-Johns Valley Area, Southern Sevier Plateau, Garfield County, Utah” (Rowley et al, 1987), at a scale of 1:50,000. However, the map predates recognition of the three MGSC slides by Bob Biek (Utah Geological Survey) and collaborators. As such, many fine-scale details useful in understanding the emplacement of these gravity slides are either omitted or unable to be delineated at such a large scale. Our mapping, supported by the USGS EDMAP program, delineates the upper allochthonous and lower autochthonous strata of the Sevier Gravity Slide in a region where allochthonous strata are arranged in a series of thrust duplexes, truncated by an upper slide plane, and overlain by subhorizontal allochthonous strata. Based on our preliminary map, we interpret these thrust duplexes to be the remnants of individual slide blocks that were emplaced at cessation of the slide.