This field trip examines spectacular exposures of sedimentary serpentinites that occur in contrasting tectonic regimes of the Neogene transform-dominated San Joaquin Basin and the late Mesozoic forearc of the Great Valley Sequence, Sacramento Valley (Fig. 1). Emphasis will be on depositional mechanisms which range from intrusive/extrusive protrusions to detrital accumulations, in subaerial to deep marine environments. The interplay between tectonic events and the deposition of sedimentary serpentinite will be stressed. For it is this interrelationship--the tectonic mobilization of serpentinized ultramafic masses from deep structural levels, their forceful protrusion to the surface, and the generation of active extrusive serpentinite flows into the sedimentary environment--that underscores the importance of these deposits in the stratigraphic record. Voluminous, monomineralic accumulations of serpentinous strata, such as the Big Blue Formation and the foliate breccias of the Wilbur Springs area, should be viewed not merely as compositional curiousities but rather as unique sedimentologic responses to tectonic events.
The first day we will examine homoclinal exposures of the Big Blue Formation along the west side of the central San Joaquin Valley (Fig. 1). Six stops are planned in the Big Blue serpentinous strata. In the southern portion of the area between Anticline Ridge and Domengine Ranch, recent work by Bate describes interfingering of distal alluvial fan and tidal flat environment. Dickinson and Casey's (1976) descriptions and discussion of the Big Blue Formation in the area near Cantua Creek are recapitulated. North of Martinez Creek, they recognize a main body of subaerial protrusive serpentinite than can be traced along strike into fringing alluvial aprons that grade, with increasing distance from the source protrusion, into shallow marine facies.