Geology of the Sebastopol Quadrangle, California Classic Reprint

Excerpt from Geology of the Sebastopol Quadrangle, California

The rocks in the area, predominately sedimentary, range in age from Upper Jurassic to Recent. The oldest rocks, the franciscan-knoxville group, form the basement in the region and crop out primarily in the northwestern quarter of the quadrangle. They consist chiefly of sandstones of the graywacke type and interbedded shale, chert, and altered volcanic rocks; they have been intruded by diabase and serpentine. Meta morphic rocks of the glaucophane schist type are locally developed in the vicinity of serpentine and in a few places where no serpentine is apparent. In the southeastern part of the quadrangle, greenish clay of the Petaluma formation (middle or upper Pliocene) is in fault contact with franciscan-knoxville rocks. Both of the formations are unconformably overlain by the Merced formation (upper Pliocene) or by thin basalt flows which locally are interbedded with the Merced formation. The basalt flows are part of a thick series, the Sonoma volcanic rocks (upper Pliocene), which crop out primarily east of the Sebastopol quadrangle. The Merced formation, consisting chiefly of very fine-grained, massive, marine sandstone and some fresh water sand and gravel, forms a thin blanket over most of the central and southern part of the quadrangle. In many places it includes a thin bed of tuff-breccia. Overlying these formations are marine and river terrace deposits and valley alluvium.

The franciscan-knoxville rocks are complexly folded and faulted, but some broad northwest - trending folds can be recognized. The Petaluma formation is Sharply folded, probably by the late Pliocene orogeny. The Merced formation has been broken by a series of high-angle faults as a result of Pleistocene uplift, northeast tilting, and settling movements of the San francisco-marin block which the quadrangle occupies. The large streams draining the area flow westward across the regional structure. Recent subsidence (or rise in sea level) is evidenced by the drowned mouths of these streams.

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