Overview of rocks bordering Ridge Basin, southern California

Abstract

Ridge Basin lies within a region of basement terranes overlain nonconformably by Cenozoic strata. The distribution and characteristics of these bordering rocks show that significant tectonic movements occurred before the origin of the basin, during its filling by Miocene and Pliocene sediments, and afterwards. These movements include uplift of sediment source areas, sinking of depositional sites, thrust faulting, folding, and noteworthy strike-slip faulting.Basement rocks bordering Ridge Basin on the west, constituting the Alamo-Frazier Mountain terrane southwest of the Canton and San Gabriel faults, consist of Precambrian gneiss, Mesozoic granite, and mylonite of the latest Cretaceous Alamo-Piru shear zones. A thick section of Tertiary strata nonconformably overlies this basement and interfinger westward into beds of the deep east Ventura Basin. These strata include Eocene marine Juncal and Matilija? Formations and nonmarine "Canton Canyon beds," Oligocene nonmarine Sespe Formation, and Oligocene-lower Miocene marine shale and sandstone of the Vaqueros and Rincon Formations. Coarse Sespe conglomerates containing boulders >8 m are interpreted as deposited along a fault scarp, probably the Canton fault, at the margin of the east Ventura Basin. Distinctive lithologies of the clasts, including anorthosite, Lowe Granodiorite, and Mendenhall Gneiss indicate derivation from basement terrane now exposed in the western San Gabriel Mountains. Southwest of the San Gabriel fault, the thick marine upper Miocene Modelo Formation overlies the Sespe Formation unconformably, and includes the Devil Canyon Conglomerate member containing clasts of gneiss and anorthosite and related rocks such as those exposed in the western San Gabriel Mountains. As discussed in the tectonics chapter in this volume, in order to match conglomerates with their inferred sources, dextral displacements of ~30 km are suggested on the Canton fault in mid-Miocene time before Ridge Basin originated, followed by displacements of 45 km on the San Gabriel fault as the basin received sediment. Later events in the Pliocene include thrusting along the Frazier Mountain thrust system and truncation of Ridge Basin on the north by the San Andreas fault. Terranes north of Ridge Basin across the San Andreas fault have very different histories from that of terranes south of the fault. The northern rocks include ancient gneiss and granite of Precambrian and Mesozoic age, and pendants of Jurassic? marble. Miocene volcanic rocks and marine and nonmarine strata nonconformably overlie this basement.Areas east of Ridge Basin and south of the San Andreas fault are underlain by Mesozoic granodioritic and gneissic rocks, including small masses of distinctive Triassic megaporphyritic monzogranite and "polka-dot" granite, as well as marble of uncertain but probable Paleozoic age. This basement is overlain nonconformably by