Abstract: Regional Relations of the Mesozoic Rocks of the Gualala Block, Northern California

WENTWORTH, C. M., D. L. JONES, and E. E. BRABB, U.S. Geological Survey, Menlo Park, CA; andW. P. ELDER, California Academy of Sciences, Golden Gate Park, San Francisco, CA

New investigations of the Mesozoic rocks of the Gualala block indicate that: 1) Spilite at Black Point is a diabasic sheeted sill complex with inter-mixed pillow basalt inferred to be Coast Range ophiolite; 2) This ophiolite is overlain by Upper Cretaceous strata of Stewarts Point and Anchor Bay along a low-angle detachment fault; 3) The contact between these two units is marked by an upward change from predominantly granitic and silicic porphyry conglomerate clasts (Stewarts Point) to predominantly diabase with lesser gabbro and some pyroxenite (Anchor Bay); 4) These mafic clasts differ strongly from rocks exposed in Logan Quarry near San Juan Bautista (offset from Eagle Rest Pk?) and mafic clasts in an Eocene(?) or Miocene conglomerate at Parkfield (also derived from Eagle Rest Pk?). The Anchor Bay strata have long been known to contain abundant fragments of the tropical rudistid bivalve Coratiochama orcutti (late Campanian). The Gualala block is akin to the Great Valley sequence with a basal ophiolite, and therefore cannot be part of Salinia.

The hitherto unique Anchor Bay strata are now also recognized on Highway 92 south of San Francisco, where a small sliver of conglomerate exposed along the Pilarcitos fault consists principally of diabase and gabbro clasts and contains thick-shelled fossil fragments previously identified as rudists that have morphology similar to Coraliochama. Although earlier considered to be Franciscan, this sliver is undoubtedly Anchor Bay conglomerate.

All palinspastic reconstructions suggested by possible Neogene offsets along the San Andreas fault system place the Gualaia block in central California, where no equivalent section has been recognized. Large pre-San Andreas displacements seem indicated.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California