PRELIMINARY STRATIGRAPHIC INTERPRETATIONS OF THE SOLEDAD ROJO FORMATION IN THE LOWER COLORADO RIVER EXTENSIONAL CORRIDOR, WESTERN PALO VERDE MOUNTAINS, SOUTHEASTERN CALIFORNIA

This study presents preliminary geologic mapping, stratigraphy, and provenance data from the “Soledad Rojo formation”, an informal name given by previous workers to a moderately east-dipping section of early Neogene(?) pre-Bouse Formation red beds on the western side of the northern Palo Verde Mountains of southeastern California. This unit has been correlated to the Tolbard Fanglomerate located ~13 SW of the study area, which was likely deposited during early Miocene extensional deformation in the lower Colorado River region; however, this association is uncertain due to the lack of depositional age controls and the geographic distance between these two formations.

The Soledad Rojo formation likely represents deposition in adjacent alluvial fan and braided fluvial systems in a normal fault basin that developed during regional early Neogene extension. This basin is bounded by Oligocene-age intermediate volcanic rocks of the Palo Verde Mountains to the east, and the late Oligocene tuff of Black Hills to the west. The base of the formation is generally not exposed; however, in one locality it overlies a welded tuff that is similar in appearance to the tuff of Black Hills. Our study subdivides the formation into three stratigraphic units: 1) a lower alluvial unit, consisting primarily of trough cross bedded brick red coarse-grained lithic arkose and granule-pebble conglomeratic sandstone, interbedded with matrix- and clast-supported, subangular-subrounded, moderate-poorly sorted cobble conglomerate with meta-plutonic and volcanic clasts; 2) a middle fluvial unit of clast-supported, imbricated, rounded-subrounded, moderate-well sorted, cobble-boulder conglomerate with primarily meta-plutonic clasts, interbedded with brick red lithic arkose similar to lower unit; and 3) an upper alluvial unit of light gray-buff conglomeratic lithic arkose and interbedded matrix- and clast-supported pebble-cobble conglomerate with subangular-subrounded meta-plutonic and volcanic clasts. In the northern study area, bimodal volcanic rocks are intercalated in the section. Following (and possibly during) deposition, the Soledad Rojo formation was extended and gently tilted east by NW-trending, W-dipping normal faults. In the northern part of study area, a younger dacitic intrusion crosscuts the tilted section.