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

This study presents new detrital zircon and ⁴⁰Ar/³⁹Ar geochronology and stratigraphic interpretations from the Soledad Rojo formation, a moderately east-dipping section of previously undated red beds located in the Lower Colorado River Extensional Corridor of SE California. Stratigraphic relationships suggest that the Soledad Rojo formation represents an alluvial fan/braided fluvial system deposited in a syndepositional half-graben basin that developed during regional mid-Tertiary extension. This basin is bounded on the east by an inferred N-trending normal fault that uplifts mid-late Oligocene volcanic rocks of the Palo Verde Mountains in the footwall. To the west, the Soledad Rojo formation is deposited conformably above the Tuff of Black Hills, which extends further outside of the study area. New ⁴⁰Ar/³⁹Ar geochronology of the Tuff of Black Hills provides a maximum depositional age of 23.68 ± 0.25 Ma for the Soledad Rojo formation, which is roughly consistent with our detrital zircon geochronology that suggests a maximum depositional age of ca. 24-25 Ma (youngest peaks of relative age probability plots).

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.