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Stratigraphy, paleontology, and geology of the central Santa Cruz Mountains, California Coast Ranges

1981; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp1168

2330-7102

Joseph C. Clark,

Geology and Paleoclimatology Research

Southwest of the San Andreas fault and northeast of the San Gregorio fault in Santa Cruz and San Mateo Counties is one of the most complete Tertiary rock sections in the California• Coast Ranges.Overlying a Salinian basement complex of granitic and older metasedimentary rocks is a succession of arkosic sandstone and mudstone units, ranging in age from Paleocene to Pliocene and having a composite thickness of as much as 7,390 m.This Tertiary section is divisible into four sedimentary rock sequences which are virtually continuous, each of which is bounded by unconformities of regional extent.Resting on the basement complex, the oldest sequence consists of erosional remnants of the Locatelli Formation of Paleocene (Ynezian) age.Basal sandstone beds of this formation contain relatively shallow-water mollusks associated with calcareous foraminifers and grade upward into siltstone of deeper marine origin that contain abundant arenaceous foraminifers.The n~xt younger sequence ranges from early Eocene (Penutian) to early Miocene (Saucesian) age and consists of the Butano Sandstone, San Lorenzo Formation, Zayante Sandstone, Vaqueros Sandstone, and Lambert Shale.Bathyal deposition was followed by local shallowing during Oligocene (Zemorrian) time.The two younger sequences are the products of two separate and successive marine cycles of sedimentation.The older cycle was a middle Miocene (Relizian and Luisian) event that produced a widely transgressive, basal sandstone unit, the Lompico Sandstone, and an overlying organic mudstone unit, the Monterey Formation.The younger cycle was initiated in late Miocene (Mohnian) time and likewise produced a transgressive, basal sandstone unit, the Santa Margarita Sandstone, and an overlying siliceous mudstone unit, the Santa Cruz Mudstone.The basal sandstone beds of each of these two sequences were deposited in a nearshore, shallow-marine environment, whereas the overlying mudstone beds were laid down in deeper water, but probably at neritic depths.A later and shallower phase of the younger cycle is recorded by the Purisima Formation of Pliocene age.Southwest of the San Gregorio fault, typical crystalline rocks of the Salinian block are not exposed, and porphyritic rhyolite that crops out near Pescadero to the north may form part of the basement.There, more than 2,600 m of Upper Cretaceous marine strata of the Pigeon Point Formation is overlain unconformably by more than 500 m of marine clastic sedimentary and volcanic rocks that range in age from Oligocene (Zemorrian) to Holocene.This Cenozoic section includes the Vaqueros(?),Monterey, and Purisima Formations, which are complexly folded and faulted in exposures north and east of Aiio Nuevo Point.The San Gregorio fault is part of an active zone, about 3 km wide, east of Ano Nuevo Point that includes at least five northwesttrending faults from the AnoN uevo fault on the west to minor faults at Greyhound Rock to the east.Marked contrasts in the Mesozoic and Cenozoic sections across the San Gregorio fault suggest past large-scale lateral displacement.The Zayante fault was an important structure during Oligocene (Zemorrian) time that displaced the crystalline basement approximately 2 km and strongly influenced the Oligocene stratigraphy of the area.South of this fault, Salinian basement is extensively exposed and occurs at relatively shallow depths, with the overlying Tertiary strata tilted and deformed into broad, open folds.North of the Zayante fault, the basement is not exposed and the strata are more strongly deformed and locally overturned.' Pierce (1972) and Barron (1976) believe that the Delmontian is partly coeval with the Mohnian Stage.