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The basal Great Valley sequence in Napa and southern Lake Counties, California, is a mappable chaotic unit composed largely of ophiolitic debris. Serpentinite flows and breccias, mafic breccias and associated finer-grained clastic rocks, and blocks of extrusive greenstone, mafic breccia, chert, bedded and unbedded clastic sedimentary rocks, phyllites, actinolitic greenschists, and hornblende amphibolites are mixed with Great Valley sequence mudstone and serpentinous mudstone. The chaotic unit extends along strike for at least 50 km. Cross-sections indicate that it extends for at least 20 km across strike and is up to 1 km thick. It is involved in complex folds caused by imbricate thrust faulting. The unit lies directly above the serpentinite that represents the Coast Range Ophiolite within the study area and below the well-bedded Great Valley sequence of Upper Jurassic and Cretaceous age. Its lower contact is enigmatic but is probably depositional; the upper contact is sheared and gradational. Locally the unit represents the entire Tithonian Stage.

Ophiolitic detritus in the lower Great Valley sequence is also found elsewhere in the Northern California Coast Ranges—near the Geysers, in Rice Valley, near Wilbur Springs, along the Bartlett Springs Road near Walker Ridge, near Cooks Springs, and at Crowfoot Point west of Paskenta. Other accumulations of ophiolitic debris are inter-layered in the Great Valley sequence at various stratigraphic levels in and near the study area. This detritus takes four forms, which may be mixed together: (1) sedimentary serpentinite debris flows; (2) mafic breccias; (3) basaltic sandstones; and (4) polymict, polymorphous chaotic units with blocks-in-matrix texture, like the rocks in Napa County described here.

Widespread detrital textures and the common occurrence of spaced, rather than penetrative, shear foliation in its matrix demonstrate that the chaotic unit in Napa County is not a tectonic melange, and I interpret it to be an amalgam of olistostromes. These ophiolitic olistostromes are a facies distinct from the overlying turbidites. Thus, the basal Great Valley sequence in this area is composed of two different rock types: very proximal ophiolitic debris flows, and substantially more distal subsea-fan rocks derived from a volcanic arc.

Ophiolites may form at mid-ocean ridges, in back-arc or forearc basins, or in island arcs. Ophiolitic detritus may be eroded and deposited on ophiolitic basement in any environment in which the oceanic crust is deformed. The geology of surrounding terranes and the petrologic features of the ophiolitic basement below the Great Valley sequence suggest that the basement was formed in a back-arc basin. The stratigraphy of the chaotic rocks that overlie the basement in Napa County suggests that they were deposited on deeply eroded basement in a technically active forearc basin. Large volumes of rock stuffed under the hanging-wall slab after the onset of subduction may have uplifted the forearc basin and subjected its basement to erosion. A wave of uplift may have passed across the basin, so that debris shed from eroding oceanic basement was deposited directly on freshly exposed harzburgite tectonite. Some blocks may have been carried completely across the forearc basin and into the trench, and incorporated into the Franciscan melange wedge, which is also rich in ophiolitic blocks. The change from back-arc to forearc basin was probably caused by collisional tectonics and the establishment of a new subduction zone off the western coast of California during the Late Jurassic Nevadan orogeny.

Stratigraphic relationships in and above the Coast Range Ophiolite are unusual through much of the Northern Coast Ranges. Nearly complete ophiolites are the exception rather than the rule, and in many areas only serpentinite is present. In some areas, ophiolitic debris different from that described here overlies the serpentinite. In other areas, arc-derived submarine fan rocks of the Great Valley sequence directly overlie serpentinized harzburgite tectonite. The relationships described here suggest that many of these contacts are not tectonic, and that the Coast Range Ophiolite does not owe its fragmentary nature to tectonic dismemberment. Rather, it is likely that the ophiolitic basement below the Great Valley sequence was deeply eroded during Mesozoic time. Many of the contacts throughout the Coast Ranges along which sedimentary rocks overlie serpentinite—which must represent deep layers of the oceanic crust or the upper mantle—are in the main nonconformities.

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