Lower and Middle Tertiary stratigraphic units of the San Emigdio and western Tehachapi Mountains, California

Abstract

Lower and middle Tertiary stratigraphic units exposed in the San Emigdio and western Tehachapi Mountains, Calif., consist of the Tejon Formation (Eocene), San Emigdio Formation (late Eocene and Oligocene), Pleito Formation (Oligocene), Tecuya Formation (late Eocene (?) to early Miocene), Temblor Formation (early Miocene), and the Monterey Shale and an unnamed conglomerate unit (both middle and late Miocene). These units, well exposed in the central part of the report area, constitute one of the few fossiliferous, continuously deposited marine sedimentary sequences in California of early and middle Tertiary age. This sequence is moderately thick, and the marine strata interfinger eastward into nonmarine beds that contain fossil mammals and include radiometrically dated volcanic rocks. The marine San Emigdio and Pleito Formations and the nonmarine Tecuya Formation are redefined in this paper according to lithology, thickness, stratigraphic relations, depositional environments, ages, type sections, and type localities. Most of the marine Eocene, Oligocene, and early Miocene strata were deposited on a shallow marine shelf that deepened westward and northward. The nonmarine fluvial strata of the Tecuya Formation to the east were probably deposited as alluvial fans fringing the shallow marine shelf. Climatic and tectonic changes affecting the supply of sediment resulted in a migrating and oscillating shoreline. Tectonic uplift in the southwestern part of the San Emigdio Mountains during Oligocene and early Miocene time produced a local angular unconformity. INTRODUCTION The lower and middle Tertiary stratigraphic units exposed in the San Emigdio and western Tehachapi Mountains form a sequence of sedimentary and volcanic rocks that is valuable for related stratigraphic and paleontologic studies in California. The section is moderately thick, complete, and well exposed in most of the area and