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Data from the Great Valley Group (sequence) represent the most complete information regarding sandstone petrology of sediment derived from a magmatic arc. This information is useful in documenting tectonic and magmatic events within the arc and related terranes, and forms the basis for the establishment of petrostratigraphic units for mapping and correlation. Sandstone and conglomerate compositions are controlled by changes in provenance, many of which were basinwide and synchronous. Clay-mineral composition is controlled primarily by burial metamorphism. Careful attention to sample collection, sample preparation, and petrographic techniques is essential for uniform results. Seven petrographic parameters (P/F, Lv/L, M, Qp/Q, Q, F, and L--listed in decreasing importance to petrofacies discrimination) define eight petrofacies (Stony Creek, Platina, Lodoga, Grabast, Boxer, Cortina, Los Gatos and Rumsey--listed in approximate order of decreasing age).
The Upper Jurassic-Lower Cretaceous petrofacies (Stony Creek, Platina, and Lodoga) contain higher lithic contents (supracrustal sources), whereas the Upper Cretaceous petrofacies (especially the Rumsey) contain higher proportions of plutoniclastic components (quartz, feldspar, and micas). The proportion of potassium-feldspar increases from near zero in the Upper Jurassic to nearly 50% of all feldspars in the uppermost Cretaceous.
The lower part of the Great Valley Group (Upper Jurassic and Lower Cretaceous) contains significant quantities of sedimentaclastic and metamorphiclastic material eroded from accreted and deformed terranes ("tectonic highlands") formed by the arc-arc collision (Nevadan orogeny) that occurred prior to initiation of the Franciscan-Great Valley-Sierra Nevada arc-trench system. The Klamath Mountains area provided a major proportion of this detritus. Ophiolite and serpentinite detritus was deposited locally near the base of the Great Valley Group as a result of deformation along the east side of the growing Franciscan subduction complex. Volcaniclastic detritus was fed into the entire forearc basin as magmatism increased in the Sierra Nevada area during the Cretaceous. As the volcanic cover was stripped off, plutoniclastic and metamorphiclastic detritus from the underlying batholithic terranes was provided in abundance to the forearc basin. Crustal components were more "continental" in the southern Sierra Nevada and more "oceanic" in the northern Sierra Nevada, as demonstrated by the higher proportions of metamorphiclastic detritus and by the more felsic nature of volcaniclastic detritus to the south. By the middle of the Late Cretaceous, extensive batholithic terranes provided potassium-feldspar-rich arkosic detritus to the entire forearc basin. By the Paleogene, arc magmatism had migrated eastward sufficiently that deeper levels of the California part of the arc were exposed by erosion, tectonic activity decreased in the forearc basin, and the basin was filled to sea leve in most parts.
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