AN INVESTIGATION OF A PREVIOUSLY UNMAPPED FAULT IN THE CACHE CREEK NATURAL AREA, LAKE COUNTY, CALIFORNIA

Geologic mapping has been initiated in the Cache Creek Natural Area (CCNA) located about 13 km east of Clear Lake, California. This region lies near the eastern extent of the dextral shear zone resulting from the northwestward growth of the San Andreas fault system in the wake of the passing of the Mendocino Triple Junction. The study area is bounded to the northwest by the Bartlett Springs fault system and to the southeast by the Green Valley fault system.

Our immediate task is to construct a detailed geologic base map for the CCNA, specifically targeting the fluvial terraces, mass movement features, and any active tectonic features. During our initial field investigations, we identified a previously unmapped fault, informally designated the Holokome fault in deference to a nearby prehistoric site. This fault offsets the late Plio-Pleistocene Cache Creek Formation, which in this locality consists of a loosely consolidated, matrix rich, clast supported pebble/cobble conglomerate.

To identify relative fault motion, we explored the possibility of clast rotation near the fault. Long axes orientations of clasts were measured both near the fault and a short distance away from the fault to see if any possible frictional drag rotation from their original bedding positions occurred. Preliminary data indicate that indeed there is rotation of clasts consistent with normal slip. Therefore, the N20E45W trending Holokome fault is believed to be a normal fault, parallel to the Cross Springs fault which bounds the Cache Creek Formation to the west, and is consistent with previously published stress orientations for this region (Hearn et al., 1988)