NEW GEOCHRONOLOGY AND STRATIGRAPHIC INTERPRETATIONS OF THE MID-TERTIARY SOLEDAD ROJO FORMATION IN THE LOWER COLORADO RIVER EXTENSIONAL CORRIDOR, WESTERN PALO VERDE MOUNTAINS, SE CALIFORNIA
The Soledad Rojo formation consists of three stratigraphic units: 1) a lower unit of cross bedded lithic arkose, granule-pebble conglomeratic sandstone, and matrix/clast-supported, subangular-subrounded cobble conglomerate, 2) a middle unit of clast-supported, imbricated, rounded-subrounded cobble-boulder conglomerate interbedded with lithic arkose, and 3) an upper unit of conglomeratic lithic arkose and interbedded matrix/clast-supported subangular-subrounded pebble-cobble conglomerate. Clast compositions indicate a metaplutonic-volcanic sediment sources, although volcanic clasts are scarce in the middle unit. Detrital zircon ages suggest sediments were derived from late Cretaceous (ca. 75 Ma), late Jurassic (ca. 153 Ma), and Proterozoic (ca. 1.2 Ga and 1.7 Ga) sources, potentially from late Mesozoic and Proterozoic metaplutonic basement rocks exposed in the region adjacent to the basin. Growth strata (fanning bedding dips) and paleocurrent data suggesting westward-directed paleoflow supports our syndepositional half graben interpretation, where volcanic/metaplutonic detritus was shed off the uplifting footwall and basement rocks surrounding the basin.