NEOGENE STRATA OF SOUTHERN SANTA ROSA MOUNTAINS, CALIFORNIA, AND THEIR SIGNIFICANCE FOR TECTONIC EVOLUTION OF WESTERN SALTON TROUGH

Neogene strata on the eastern piedmont of the Santa Rosa Mountains (SRM) between Travertine Palms Wash (TPW) and Sheep Canyon contain key evidence of Salton Trough evolution. The section comprises three conformable units totaling about 1,300 m thick, and it is dropped down against the main basement ridge of the SRM by an east-dipping normal fault (Sharp, 1979). The basal unit, exposed in a NW-dipping homocline along TPW, consists of up to 300 m of pale-gray to dark-red continental sandstone, cobble-boulder conglomerate, and rock-avalanche breccia. These deposits lap southeastward onto an irregular surface of pre-Cenozoic crystalline rocks. Dibblee (1954) lumped this basal unit with the Pliocene Canebrake Conglomerate, but it correlates better with the late Miocene Split Mountain Formation. The basal unit is overlain by about 200 m of olive-gray marine mudstone and sandstone of the Pliocene Imperial Formation. The uppermost unit is an upward-coarsening succession of continental sandstone, cobble-boulder conglomerate, and rock-avalanche breccia about 800 m thick. We accept the assignment of this unit to the Canebrake Conglomerate (Dibblee, 1954). Imbricated clasts show that northeast-flowing streams deposited the lower and upper units. Both contain clasts derived from nearby SRM basement (mylonitized granitic and metasedimentary rocks) and from rocks farther west in the Peninsular Ranges (relatively undeformed tonalite), but the locally derived materials dominate the lower unit, whereas the upper unit consists mostly (95% or more) of “exotic” tonalite. We conclude: (1) The southern SRM region lay at the west edge of the ancestral Salton Trough during the late Miocene and Pliocene; (2) a local basement high protruded from the basin floor southeast of TPW; (3) the lower and upper units were deposited as northeast-sloping coarse-grained alluvial fans; (4) deposition of the lower unit possibly records the onset of basin extension in the late Miocene; (5) extension culminated during the Pliocene with deposition of the upper unit (Canebrake Conglomerate); normal faulting translated the Neogene strata a limited distance eastward relative to SRM basement; (7) northwestward tilting of the basal unit probably reflects Pleistocene contraction that uplifted the SRM and terminated extension-related sedimentation.