Descriptions of Field Trip Stops 4, 5, and 6
Tom Ann L. Casey, William R. Dickinson, 1984. "Descriptions of Field Trip Stops 4, 5, and 6", 1984 Midyear Meeting San Jose, California, Ralph E. Hunter, H. Edward Clifton, N. Timothy Hall, John L. Chin
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Bedded serpentinite-clast conglomerate and sedimentary breccia with subordinate debris flow deposits form most of the Big Blue Formation between Salt and Cantua Creeks (see Fig. 32). Rusty brown outcrops in resistant ledges are characteristic (Fig. 39). A total thickness of 50 to 100 m is estimated. The bedded serpentinite-clast rudites, in general, lap depositionally upon the flank of the thick lens of protrusive serpentinite. About 15 m of foliate serpentinite breccia persists locally beneath bedded serpentinite-clast conglomerate and breccia as far north as the Lillis Ranch (Fig. 40).
The bedded rudites consist of subangular to sub-rounded serpentinite-clast gravel associated with interstitial and interstratified serpentine-grain sand and cemented by carbonate (Fig. 41). The largest clasts are about 1 m in diameter, but most are much smaller. The texture of the sandy matrix is plainly fragmental (Fig. 42), and there is no hint of a shear fabric. Lenticular bedding and channel structures characteristic of braided-stream deposits are prominent (Figs. 43-45). Nearly all clasts are serpentinite, although rare fragments, of other rock types occur in some beds, and the chrome garnet, uvarovite, is common in the sand fraction.
The proportion of serpentinite-clast conglomerate and sedimentary breccia declines and the proportion of serpentine-grain sandstone increases with distance laterally away from the sequence of foliate serpentinite breccia. In the canyon of Cantua Creek (see Fig. 33), more than half the unit is composed of serpentine-grain sandstone that is intercalated between more prominent layers of coarser serpentinite detritus.
Crudely stratified serpentinite-clast rudites that represent debris-flow deposits locally occur strati graphically between foliate breccia and bedded conglomerate and breccia.