Provenance, Deposition, and Deformation of the San Benito Gravels, California
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Deposition
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Medium sand samples from well cores taken in the Santa Clara Valley, California, were studied to determine their composition and, if possible, their provenance. Sand samples were taken from various depths from five wells distributed over the western and central parts of the valley. The oldest of these samples is known to date to about 800 ka.
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The Punchbowl block is a fault-bounded crustal sliver in the eastern San Gabriel Mountains of southern California with important implications for conflicting reconstructions of the San Andreas fault system. Detailed mapping, determination of conglomerate-clast and sandstone compositions, and dating of detrital and igneous zircon of Oligocene–Miocene strata define two distinct subbasins and document initiation of extension and volcanism ca. 25–24 Ma, followed by local exhumation of the Pelona Schist, and transition from alluvial-fan to braided-fluvial deposition. Strata of the Punchbowl block correlate with those of other regions in southern California, confirming 40–50 km of dextral slip on the Punchbowl fault, and supporting reconstructions with 60–70 km of dextral slip on the San Gabriel/Canton fault and ∼240 km of dextral slip on the southern San Andreas fault. Provenance and probable correlations of Punchbowl-block strata argue against 80–110 km of dextral slip on the San Francisquito–Fenner–Clemens Well fault and limit the time interval during which such slip could have occurred. Synthesis of these findings with previous work produces paleogeographic reconstructions of the Punchbowl block and its probable correlatives through time.
Conglomerate
Alluvial fan
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San Joaquin
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San Joaquin
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New single-grain detrital zircon U-Pb age data from sandstone lenses in the Upper Jurassic Mariposa Formation of the Sierra Nevada foothills metamorphic belt indicate that: (1) the earliest phase of clastic sedimentation mainly involved material derived from the Bragdon and Baird Formations of the Eastern Klamaths and the Paleozoic miogeocline of Nevada ± sources farther to the east, with modest input from the Sierra Nevada arc; (2) the arc became the dominant sediment source for the upper turbidite interval in the Mariposa Formation; and (3) the youngest zircon ages constrain the onset of clastic deposition at 152 ± 1 Ma. Zircon age data also suggest that the local drainage divide migrated westward, resulting in a higher proportion of detritus derived from the Sierra Nevada arc over time. New geologic mapping in the central Sierra Nevada foothills shows that the Mariposa Formation thickens eastward, and that the number of coarse-grained sandstone bodies increases up section. These observations indicate that a topographically low Sierran volcanic arc gradually began to rise, providing increasing amounts of clastic debris to the Mariposa depositional basin.
Detritus
Foothills
Volcanic arc
Pebble
Siliciclastic
Lithic fragment
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Island arc
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Sedimentation
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