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Quaternary faulting in the Williams Fork Valley graben, north-central Colorado, and com... Page 2 of 5 Field investigations were conducted along the northern and central sections of the Williams Fork Mountains fault, a reported Holocene fault that bounds the western side of the graben (Unruh and others, 1993). Geologic sketch maps were prepared for several sites along the fault where scarps are present in late Quaternary alluvial deposits. Topographic profiles were measured across about 20 scarps along the Williams Fork Mountains fault. The profiles will be plotted and geometrically evaluated during the coming months. Because natural exposures aze raze where Quaternary scarps occur, six pits were dug by hand into faulted deposits on the upthrown side of the faults. James McCalpin described the pedogenic soil horizons exposed in the pits. Grain-size distributions will be determined for each soil horizon observed in the pits. The clay content of the soil horizons should provide data useful to estimate the age of the faulted deposits. The soils data will be compared to data from other pedogenic soil studies in the region that involved similar parent materials and climate (e.g. Shroba and Birkeland, 1983; Nelson and Shroba, 1998). Limited field investigations were conducted at several locations along Quaternary faults and suspected faults within the graben. A geomorphic analysis of deformed Quatemary surfaces within the graben is currently underway. Only very limited field work was performed along the fault on the eastern side of the graben. This work was undertaken to confirm observations made during the photogeologic phase of the study that suggested little or no Quaternary movement has occurred on the fault bounding the east side of the graben. The original proposal was not fully funded. Based on the recommendation of the proposal review panel, the comparative analysis with young faults near Spinney Mountain was removed from the project's scope of work. RESULTS Williams Fork Mountains Fault: Strong evidence of late Quaternary tectonism is present along the Williams Fork Mountains fault, an east-dipping normal fault that forms the western side of the Williams Fork Valley graben. During the initial photogeologic reconnaissance phase of our investigation, features suggestive of recent tectonism were observed along the northern and central sections of the fault. These features included faceted spurs on the mountain front, topographic, vegetational, and tonal lineaments, truncated ridgelines, uphill-facing escarpments, and abundant landslides, some of which were reported previously by Unruh and others (1993). These features are present as far south as Ute Park, but photogeologic evidence of young fault activity was absent or rare along the fault south of Ute Park. Due to the heavy forest cover and to the very narrow width of valleys where they cross the fault, it was difficult to discern whether topographic scarps were present in the young valley-fill deposits along the fault trace using modem aerial photography. Upon learning that lazge forest fires swept through the region in the late 1800s, a search was made to acquire older aerial photography. Black and white, 1:20,000-scale photography flown in 1938 was obtained in the hope that the forest cover would be less dense at that time and that scarps would be more appazent. Unfortunately, the resolution of these older photographs was not high, and thick stands of dominantly lodgepole pine were already well established in most, but not all, areas along the fault. Fault scarps in valley-fill deposits along the mountain front were noted on the 1938 aerial photography in a few locations along the fault, but in most valleys the ground surface was not apparent or not clearly visible in the photographs. Because of the limited usefulness of aerial photography, a decision was made to conduct foot traverses along northern and middle sections of the Williams Mountains fault to seazch for scarps in young deposits. This was atime-consuming effort, due to the rough terrain and limited public access in the area. Although much of the fault lies within land managed by the U.S. Forest Service, numerous private landowners controlled all reasonable access routes to the fault. Considerable project time was expended http://erp-web.er.usgs.gov/reports/annsum/vo144/niw/02hggr0102.htm1 5/16/2005