SEDIMENTARY SERPENTINITE OF THE LOWER GREAT VALLEY GROUP, CALIFORNIA: UPDATES ON DISTRIBUTION AND CHARACTERISTICS

Sedimentary serpentinite horizons in lower Great Valley Group (GVG) forearc basin deposits were identified decades ago in the eastern part of the northern California Coast Ranges where they border the Central Valley. Recent field studies show that these deposits are more widely distributed. Newly identified localities include one in the northern Diablo Range, several exposures in the San Leandro and Hayward Hills of the eastern San Francisco Bay area, and several exposures in Sonoma County of the central-western northern Coast Ranges. These detrital serpentinite horizons can be distinguished from intact slabs of serpentinite peridotite associated with the Coast Range Ophiolite (CRO) or Franciscan subduction complex on the basis of their internal sedimentary textures, mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks, and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG. High-pressure/high-temperature and high-pressure/low temperature metamorphic blocks occur sparsely in the type detrital serpentinite localities of the northeast Coast Ranges, but are more common in some of the western localities, such as those in the Hayward Hills and Sonoma County. The detrital serpentinite horizons, including those in the type area, have locally abundant blocks of antigorite schist exhibiting a mylonitic texture and including exotic sand-sized clasts such as hornblende. Such antigorite schist and high-pressure metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite schist may be mistaken for serpentinite matrix in localities with limited exposure. The detrital serpentinite horizons in the type region reach a km or more in thickness, whereas the western exposures range from several hundred meters (Sonoma County, Hayward Hills) to about ten meters in thickness for a northern Diablo Range exposure. GVG detrital serpentinite has been suggested to represent forearc serpentinite mud volcano deposits, consistent with their field relationships. Diapiric feeders for these deposits, whose contacts should cross cut GVG and/or CRO stratigraphy/pseudostratigraphy, have proved difficult to identify.