DIGITAL GEOLOGIC MAPS AND DATABASES IN THE EAGLE MTS. 30 X 60-MINUTE QUADRANGLE, RIVERSIDE COUNTY, CALIFORNIA

Geologic mapping in the Eagle Mts. 30 x 60-minute quadrangle is being conducted and compiled under the auspices of the National Cooperative Geologic Mapping Program (NCGMP) and Southern California Areal Mapping Project (SCAMP). New mapping includes 7.5-minute quadrangles (Conejo Well, Pinto Mountain, Porcupine Wash, and San Bernardino Wash) as well as the Eagle Mts. 30 x 60-minute quadrangle. The geologic mapping is incorporated into digital geologic databases that facilitate land-use, environmental, and resource assessments of agencies active within the quadrangle, including the National Park Service, Bureau of Land Management, California Geological Survey, and San Bernardino County Museum. Joshua Tree National Park is one of the largest U.S. parks lacking a modern, detailed geologic map and mapping in the Eagle Mts. quadrangle has been undertaken in part to produce such a map.

The Eagle Mts. quadrangle straddles the boundary between two major desert ecosystems, the Mojave Desert and the Colorado Desert, and contains several local ecosystems at sites where water is relatively more abundant. Geologic mapping in the quadrangle will be used in conjunction with biological data to assess ongoing biodiversity and environmental vulnerability and recoverability studies in these fragile desert ecosystems in the southern California urban fringe area, in particular in Joshua Tree National Park. The geologic mapping also provides a foundation for the Park’s efforts to inventory its mineral and paleontological resources and as a basis for building a geologic framework for hydrologic studies in and around the Park.

The Eagle Mts. quadrangle encompasses most of the eastern Transverse Ranges physiographic province south of the Pinto Mountain fault. The province is characterized by a family of left-lateral faults that form a distinctive and important kinematic component of the greater San Andreas Fault system. Documenting the distribution of Quaternary deposits and erosion surfaces vis-à-vis basin-development along these faults ultimately will help to better constrain timing of movement. The seismotectonic significance of the left-lateral faults is not well established, yet two of these faults, the Blue Cut and Porcupine Wash–Substation faults, bracket a proposed urban landfill near the east boundary of the Park.