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Quaternary faulting in the Williams Fork Valley graben, north-central Colorado, and com... Page 3 of 5 determining landownership and obtaining consent to work on or cross private land. Most landowners were very cooperative; the Climax Molybdenum Company was especially helpful. The foot traverse started at the north end of the fault. Prominent scarps were present in all valleys, but some scarps were suspected to have non-tectonic origins such as fluvial erosion, boulder deposits associated with distal ends of debris flows, and landsliding. To distinguish tectonic scarps from non- tectonic ones, we first located the fault in bedrock on the interfluves, then traced the fault-line scarp in bedrock to the valley floor. Scarps that coincided with the fault-line scarp were assumed to be tectonic, unless other compelling evidence supported anon-tectonic origin for the scarp. If fault-line scarps could not be traced to a valley floor, then no additional work was conducted on the scarps in that valley. One section of the mountain front between about North Battle Creek and Skylazk Creek lacks well- developed faceted spurs, and fault-line scarps aze raze here. Apparently the Proterozoic bedrock in the footwall of the fault in this area is especially weak. Large landslides have broken loose from the footwall and slid across the fault, coming to rest on the hanging wall and effectively concealing or altering the tectonic geomorphology of the range front. After discovering this impedance, further work along this section of the fault was postponed. The landslides on the east side of the Williams Fork Mountains are probably similar in origin to large landslides described by Kellogg (2001) on the west side of the range. He attributed the landslides on the west side of the range to slope failures in strongly fractured Proterozoic rocks found in the overhanging block of the Lazamide-age Williams Fork Mountain thrust. About 20 topographic profiles were measured across scarps of probable tectonic origin. At some sites scarps cut deposits of more than one age. Scarp heights were lower in younger deposits and higher in older deposits, indicating recurrent fault movement during the late Quatemary. Scarp angles were commonly as steep as 20-25°. The profiles will be plotted and evaluated during the next few months. Because natural and man-made exposures of the faulted Quaternary deposits were rare, six soil pits were excavated at four sites to estimate the age of the faulted deposits. The four sites, from south to north, aze on the South Fork of Mule Creek, Middle Fork of Mule Creek, Bull Run Creek, and Johnson Gulch. All pits were dug in the upthrown or west sides of the scarps; this assured that the soil exposed in the pit was formed in a faulted deposit. The downthrown sides of the scarps in these narrow and steep valleys often had complicating effects, such as young sediment deposition or fluvial erosion. Preliminary interpretations ofcurrently available data suggest most observed scarps are in deposits of late Quaternary age; some scarps may cut Holocene deposits. Additional testing of the pedogenic soils is underway, including determination of the grain-size distribution of each described soil horizons. The clay concentrations will be compared with data from well-studied soils that formed in comparable parent material under similar climatic conditions (Shroba and Birkeland, 1983; Nelson and Shroba, 1998). This comparison will facilitate efforts to constrain the ages of the faulted deposits and estimate slip rates. Faults within the graben: Several northeast-striking and north-striking, down to the west, normal, Quaternary faults aze inferred to underlie the larger tributary valleys to the Williams Fork of the Colorado River, including Lost, Mule, and Skylazk Creeks. Middle Quaternary outwash gravels that were deposited by the Williams Fork cap several mesas and hilltops within the graben. The gravels are offset by inferred faults that underlie and control the position of the stream valleys. Many of the blocks between the faults are tilted eastward, forming a series of small half grabens that run obliquely across the larger Williams Fork Valley graben. The gravel-capped blocks aze also downdropped relative to one another, with the blocks lowering in elevation from southeast to northwest as they approach the Williams Mountains fault. No scarps were observed in Holocene and late Pleistocene alluvium along the faults, but the tilting and differential offset of the gravel-capped mesas and hilltops indicates post-middle http://erp-web.er.usgs.gov/reports/annsum/vo144/niw/02hggr0102.html 5/16/2005