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Description
Mesozoic and Cenozoic rocks near Atascadero, California, straddle the junction between the Nacimiento and Salinia structural provinces and comprise a Late Mesozoic and Cenozoic sedimentary section exceeding 15,000 feet in thickness. The Mesozoic rocks are, from oldest to youngest, the Franciscan Assemblage (Late Jurassic-Early Cretaceous), the Toro Formation, a 3,000-plus foot section of Late Jurassic-Early Cretaceous sandstone and shale, and the Atascadero Formation, 1,400 feet of lithic sandstone and conglomerate of late Cretaceous age. The Toro and Atascadero Formations (the Great Valley Sequence) are separated by a Middle Cretaceous unconformity. The Franciscan Assemblage, a melange of unknown thickness, is everywhere in fault contact with the Great Valley Sequence and is unconformable beneath the Tertiary strata. Large masses of diabase have intruded into the Toro and Franciscan, and the Late Mesozoic rocks contain serpentine lenses, tectonically emplaced along faults. The Tertiary section unconformably overlies the Late Mesozoic sedimentary rocks west of the Salinas River or Cretaceous granitic rocks east of the River and consists of about 5,000 feet of fossiliferous marine sandstone and shale. The formations are, from oldest to youngest, the Vaqueros Sandstone (Oligocene (?)-Lower Miocene), the Monterey Shale (Middle Miocene), the Santa Margarita Formation (Upper Miocene) and the Paso Robles Formation (Pliocene-Pleistocene). Lithology provides a basis for subdividing the Vaqueros Sandstone into two members, a characteristic transgressive marine sandstone and a coarse fanglomerate member which occurs only east of the Salinas River. The Monterey Shale, a 4,000-foot section of phosphatic and siliceous sediments, is composed of a basal sandy shale, a middle diatomite and an upper procelaneous shale. The Santa Margarita Formation is largely a friable and unconsolidated regressive marine sandstone which interfingers with a coarser facies and is unconformable with the Monterey Shale. A lacustrine gravel, the Paso Robles Formation and the Quaternary alluvial deposits blanket most of the region east of the Salinas River and were not further subdivided. Structural and stratigraphic discontinuities reflect alternate periods of mountain building and formation of deep marine basins arising from east-west compression on the continental margin. Tectonism has been relatively continuous from the Jurassic to the Pliocene; the oldest rocks are the most severely deformed, and the youngest deposits are relatively undisturbed. Deformation during the Middle to Late Miocene bowed the region into a broad syncline, and provided for the emplacement of hundreds of clastic dikes and sills in the basal unit of the Monterey Shale. Most of the complex structures within the Monterey Shale are related to late Miocene deformation; however, the regional structures in the area were in existence prior to Miocene time. Relative quiescence during the Ouaternary has allowed for development of geomorphic features which are strongly controlled by the geology.
