PREVIOUSLY UNREPORTED OUTCROPS OF NEOGENE IMPERIAL FORMATION IN SOUTHERN SANTA ROSA MOUNTAINS, CALIFORNIA, AND IMPLICATIONS FOR TECTONIC UPLIFT

New geologic mapping in the SE Santa Rosa Mountains (SRM) documents about 200 m of Neogene marine Imperial Formation (IF) exposed sporadically along 10 km of the eastern range front between Travertine Palms Wash (TPW) and Barton Canyon (BC). Previous maps showed only one small outcrop of IF, near TPW about 2 km southwest of State Hwy 86. Those deposits and other basal strata along strike to the southwest consist of yellow- to olive-gray marine sandstone and conglomerate that overlie and interfinger with nonmarine sandstone, conglomerate, and rock-avalanche breccia derived from the west. They contain a diverse suite of bivalves, gastropods, echinoids, barnacles, and corals. The main body of marine strata consists of gypsiferous olive-gray mudstone and very fine sandstone containing shark and ray teeth, Neogene benthic forams, and recycled Cretaceous coccoliths. Intercalated beds of coquina and coarse sandstone contain disarticulated oysters and clams, barnacles, sand dollars, and sparse gastropods. A variegated sequence of gray, brown, pink, and orange paralic sandstone and mudstone with sparse clams and oysters caps the marine section. The overall succession resembles Pliocene IF elsewhere in the western Salton Trough, which previous workers have ascribed to influx of coarse detritus from the Peninsular Ranges, followed by influx of deltaic mud and sand spread westward from the ancestral Colorado River. Near BC, the marine beds are conformably overlain by about 800 m of continental sandstone, conglomerate, and avalanche breccia derived from the west. A major east-dipping normal fault separates the IF and overlying continental sediments from the pre-Cenozoic basement core of the SRM. The IF outcrops lie at elevations of 60-625 m above sea level. Allowing for late Neogene eustatic sea-level fluctuations, the highest outcrops apparently have been uplifted at least 500-700 m. Other workers have proposed that the adjacent normal fault is a detachment structure related to extensional opening of the Salton Trough. However, the elevated position of IF beds is not readily accounted for by extensional deformation alone. We suggest that the entire SRM complex (basement core, normal fault, and Neogene section) may have been uplifted to its current position by contractional deformation subsequent to extensional faulting.