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Quaternary faulting in the Williams Fork Valley graben, north-central Colorado, and com... Page 4 of 5 Quaternary deformation. East-striking and northeast-striking linear valleys along minor tributaries occur within the Williams Fork Valley graben and are probably controlled by minor Quaternary faults. These lineaz drainages are eroded into middle and late Quaternary deposits and the underlying Miocene Troublesome Formation. In an area between the confluence of Lost Creek and the Williams Fork, the linear drainages are aligned with saddles and faceted ridges formed in Proterozoic rocks. These landforms, which are developed in hazd, resistant bedrock, may result from differential erosion along faults and hence are fault-line scarps. They support an interpretation that the shallow linear drainages resulted from post-middle Quaternary tectonism within the central part of the Williams Fork Valley graben. Fault on east side of graben: Several prior investigations in the valley described an unnamed fault that forms the eastern margin of the Williams Fork Valley graben (Tweto and Reed, 1973; Tweto and others, 1976; Kirkham and Rogers, 1981; Unruh and others, 1993; Widmann and others, 2002). As mapped, the fault on the eastern side of the graben cuts the Miocene Troublesome Formation but not Quaternary deposits. Photogeologic studies and limited field work conducted during the current project support the conclusions of the prior investigations. Therefore no additional work was conducted on this structure. Quaternary deformation in the Williams Fork Valley graben appears to be limited to the northern and central sections of the Williams Fork Mountains fault and to faults within the interior part of the late Cenozoic graben. NON-TECHNICAL SUMMARY Preliminary results suggest lazge earthquakes have occurred repeatedly in the Williams Fork Valley, north-central Colorado during the recent geologic past. Field investigations conducted along the Williams Fork Mountains fault (west side of valley) discovered numerous faults scarps related to the prehistoric earthquakes. Analysis of profiles measured across the scarps and data from pits excavated into faulted sediments will help to constrain the timing of the last fault movement and the rate of fault movement. Tilting and faulting of geologically young gravel deposits that cap mesas and ridges in the valley's center provide additional evidence of recent earthquake activity. REPORTS PUBLISHED None yet. AVAILABILITY OF SEISMIC, GEODETIC OR PROCESSED DATA No seismic or geodetic data was collected. Processing of field data will occur during the next few months. REFERENCES CITED Ellis, E.G., 1976, Surficial and environmental geology of part of the upper Williams Fork River basin, Colorado: M.S. thesis, Boulder, University of Colorado, 66 p. Kellogg, K.S., 2001, Tectonic controls on a large landslide complex: Williams Fork Mountains near Dillon, Colorado: Geomorphology, v. 41, p. 355-368. Kirkham, R.M., and Rogers, W.P., 1981, Earthquake potential in Colorado-A preliminary evaluation: Colorado Geological Survey Bulletin 43, 171 p. http://erp-web.er.usgs.gov/reports/annsum/vo144/niw/02hggr0102.htm1 5/16/2005