NEW DETAILED GEOLOGIC MAPPING WITHIN THE SOUTHEAST PORTION OF THE UKIAH 30′ X 60′ QUADRANGLE, MENDOCINO COUNTY, CALIFORNIA

Recent geologic mapping by the California Geological Survey (CGS) within the Ukiah 30’ x 60’ Quadrangle in Mendocino County, California, yields improved detail in identifying geologic units and refined locations of faults. This work integrates three adjacent 7.5-minute quadrangles, from north to south: the Redwood Valley, Ukiah, and the Elledge Peak quadrangles. Mapping these areas consecutively enhances regional correlation and incorporation of previously published and unpublished data, resulting in a seamless and systematic geologic characterization with consistent nomenclature across the map area.

The project area contains a north-south trending alluvial valley drained by the Russian River. The City of Ukiah, Coyote Valley Dam, and Lake Mendocino are within the central quadrangle. Bedrock in the area is chiefly comprised of Mesozoic Franciscan Complex units and an overlying Paleocene sandstone. Younger units include a Pliocene to Quaternary continental unit and numerous Quaternary deposits and landslides. A prominent outcome of this new mapping is a doubling of the number of identified bedrock and Quaternary units compared to previous maps of the area. Similarly, where earlier maps depict a handful of the largest landslides, over one thousand individual landslides are identified on the new maps. In addition, Franciscan Complex units with distinctive lithologies were identified and mapped in new areas.

These 1:24,000-scale detailed geologic maps are the product of three years of fieldwork, high-quality lidar and aerial imagery interpretation, and incorporation and modification of earlier maps and thesis data. Lidar imagery is crucial to differentiate bedrock and Quaternary map units, particularly in areas with limited field access. Lidar-based interpretation also improved landslide and alluvial fan floodplain mapping and recognition of the Holocene-active Maacama Fault. These maps provide derivative geologic framework data for future hazard mapping products, including liquefaction, landslides, and active fault zone maps. This work is part of an ongoing STATEMAP cooperative effort with the U.S. Geological Survey. CGS will include these maps in a future production of the Ukiah 30 x 60-minute geologic map.