EOCENE STRATIGRAPHY AND PALEONTOLOGY, OROCOPIA MOUNTAINS, SOUTHEASTERN CALIFORNIA

Abstract

Marine Eocene strata underlie about 26 square miles in the northeastern Orocopia Mountains, Riverside County, California. The newly discovered section, which is about 4800 feet thick, lies in a structural trough within basement rocks and is overlain unconformably by about 5000 feet of undated nonmarine clastic and volcanic rocks. The Eocene beds consist of interbedded siltstone, sandstone, conglomerate, and breccia with some sandy limestone and are here defined as constituting the Maniobra formation. On the east at the base of the section large granitic boulders up to 30 feet in diameter lie along the unconformity with basement granite. These give way upward to thick lenses of coarse granitic conglomerate and breccia with interbeds of buff siltstone and arkosic sandstone. The upper part of the section on the east consists of massive buff siltstone with sandstone and boulder beds. On the west the section consists largely of interbedded siltstone and sandstone with conspicuous isolated boulders of granite. Mollusks and Foraminifera, including Orbitoids, occur at many localities throughout the section. Some characteristic forms are: Turritella andersoni cf. lawsoni Dickerson, Turritella uvasana cf. applini Hanna, Clavilithes sp., Marginulina mexicana (Cushman) var., Pseudophragmina (Proporocyclina) clarki (Cushman). This fauna indicates a lower and middle Eocene age, and the strata possibly correlate with similar rocks of the Coast Ranges.