EOCENE BALLENA GRAVEL CLAST POPULATION, PALEOGEOMORPHOLOGY AND TECTONIC RECONSTRUCTION, WESTERN SALTON TROUGH, CALIFORNIA USA

Previously unreported occurrences of the Eocene Ballena Gravel east of the Elsinore Fault provide limits for determining right-lateral slip and rotational history along a portion of the western Salton Trough. Ballena Gravels were named by Fairbanks (1893) for the east-west trending linear outcrops of gravels deposited on the western slopes of Peninsular Ranges granitic basement. These gravels are present to within 11 km west of the Elsinore Fault. Mapping of Ballena Gravels east of the Elsinore fault indicates right-lateral offset of 14 km with clockwise rotation of at least 35°.

Eocene Ballena Gravel (Eb) is preserved in fault-bound paleovalley segments extending from near the Elsinore Fault Zone southeastward to near the San Jacinto Fault Zone. Gravel occurrences are characterized in terms of clast type proportions and paleogeomorphologic setting. In addition, Eb clast proportions are compared to gravels/conglomerates from undifferentiated Miocene (Mc) and upper Neogene (Nc) strata.

Eb gravel clast population consists of 37% extra-regional clasts (Poway, Owl Creek and Las Palmas types) and 63% Peninsular Ranges basement clasts (plutonic and metamorphic types). Of the extra-regional clasts, Poway clast counts compare favorably with clast populations from west of Elsinore fault (83% compared to 86%). Mc and Nc clast proportions contrast with Eb: Mc clast proportions are 6% extra-regional, 68% Peninsular Ranges basement and 26% Miocene Alverson volcanic types; Nc gravel proportions are predominantly local basement plutonic types with extra-regional clasts being very rare.

Eb valley paleogeomorphology is reconstructed from detailed geologic mapping at 5 localities. Eb and locally Mc and Nc gravels are in nonconformable contact with basement rocks. Paleogeomorphologic elements consist of inner and outer valley walls locally separated by strath terraces. Although complicated by younger structural deformation, element dimensions are estimated as follows: outer valley widths are highly variable (450-1,250 m); inner valley widths average 270 m; strath terrace relief from inner valley floor to terrace top average 60 m. Eb coarser textured gravels typically rest on strath terrace surfaces. Strath terrace development seems to correspond to base level changes recorded in the Eocene stratigraphy of coastal southern California.