MULTI-STRATIGRAPHIC INVESTIGATION AND PROVENANCE INTERPRETATIONS OF THE MID-TERTIARY SOLEDAD ROJO FORMATION OF THE SOUTHERN BASIN AND RANGE PROVINCE, WESTERN PALO VERDE MOUNTAINS, SOUTHEASTERN CALIFORNIA

Completed stratigraphic analysis, paleocurrent measurements, and geochronology from the Soledad Rojo formation, a red conglomerate and sandstone unit in the western Palo Verde Mountains, SE California, illustrate the development of a basin that formed during mid-Tertiary crustal extension in the southern Basin and Range province. The Soledad Rojo formation is subdivided into three distinct subunits of variable lithofacies and clast compositions that display changes in sediment provenance and depositional environments during the history of the basin: 1) a lower unit of cross bedded lithic arkose, granule-pebble conglomeratic sandstone, and matrix/clast-supported, subangular-subrounded, volcanic-metaplutonic cobble conglomerate, 2) a middle unit of clast-supported, imbricated, rounded-subrounded, metaplutonic cobble-boulder conglomerate interbedded with lithic arkose, and 3) an upper unit of conglomeratic lithic arkose and interbedded matrix/clast-supported, subangular-subrounded, metaplutonic-volcanic pebble-cobble conglomerate. Deposition of the Soledad Rojo formation is inferred to have occurred in a synextensional basin with a moderate energy alluvial/fluvial system derived from nearby sediment sources. Conglomerates were likely derived from local sources due to the large grain sizes, subangular to subrounded textures, and clast compositions similar to surrounding basement exposures; all which suggest a relatively short distance of travel following erosion. Paleocurrent analysis, thin section point-counting, and clast count data further suggests that the Soledad Rojo formation was derived locally, with clast imbrications suggesting primarily NW-directed paleoflow. Though the timing and deposition of the three subunits in the Soledad Rojo formation is difficult to constrain, new ⁴⁰Ar/³⁹Ar sanidine geochronology of volcanic rocks that underlie and intrude the Solodad Rojo formation indicates a late Oligocene to early Miocene depositional age (between 23.68 ± 0.25 Ma and 17.77 ± 0.76 Ma).