Abstract: Sedimentary Serpentinite of the Miocene Big Blue Formation Near Cantua Creek, California

Tom Ann L. Casey, William R. Dickinson

Between Anticline Ridge on the south and the Ciervo Hills on the north, serpentinous strata of the Big Blue Formation lie stratigraphically between middle Miocene Temblor Formation and upper Miocene Santa Margarita Formation in homoclinal exposures of Tertiary strata that flank the western margin of the Great Valley. Mapping of three distinctive lithologic units within the Big Blue Formation near Cantua Creek defines key aspects of its unusual origin. Foliate but unstratified serpentinite breccia, thickest on the outcrop between Salt and Martinez Creeks, forms the body of a protrusive serpentinite extrusion that flowed as a sheared mass across an unconsolidated substratum of Temblor sand. The substratum partly was scraped away beneath the mass-flow and locally was plowed nto chaotically rumpled folds that piled up at the advancing margin of the protrusive mass. Bedded serpentinite-clast conglomerates and breccias, with intercalated serpentine-grain sandstones and associated serpentinous debris-flow deposits, were formed by fluvial reworking of the protrusive serpentinite. Detrital serpentinite locally underlies, but more commonly overlies and most typically flanks, the protrusive serpentinite as a facies equivalent. Interbedded serpentinite-grain sandstones and serpentinous claystones of probable marine origin exposed laterally along strike apparently formed as an extensive facies fringe of serpentinite detritus dispersed widely from a central core of protrusive and coarser detrital serpentinite. Santa Margarita beds lie conformably on the finer detrital serpentinite but unconformably on protrusive serpentinite and associated deposits. Easterly paleocurrents from clast imbrications in detrital serpentinite suggest a serpentinite source west of the present outcrop belt. Dispersal of serpentinite debris perhaps was fed by protrusive diapiric movement of Mesozoic serpentinite mobilized in the core of an ancestral Joaquin Ridge anticline. The initial growth of this anticline may have coincided with the Miocene onset of rapid motion along the late Cenozoic San Andreas fault system.

AAPG Search and Discovery Article #90976©1976 AAPG-SEPM-SEG Pacific Section Meeting, San Francisco, California