Structural relationships and chronologies of two chert to clastic rock successions, Western Sierra Nevada, California

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Texas Tech University

In the western Sierra Nevada, California, an east to west rock succession of layered chert, pelitic-siliceous rock, and fine-grained clastic rock characterizes part of the Paleozoic Calaveras Formation. Along the Merced River, west of Yosemite, and at Little Bald Mountain, west of Lake Tahoe, mesoscopic structures in these lowgrade metamorphic rocks are chiefly folds and fracture cleavage in the cherts, folds and poorly defined cleavage in the pelitic-siliceous rocks, and slaty or crenulation cleavage in the fine-grained clastic rocks; lineations mainly crenulations) occur in all lithologies.

In the Merced River area, layering ( S Q ) mainly strikes northerly and dips steeply east; a metamorphic alignment of micas (S,) parallels S^; cleavages (S2) strike west-northwest and dip steeply south; and folds (F^) and lineations ( K ) are parallel, mainly plunge steeply to the southeast, and are dispersed in S^- S^ parallels F^ axial planes. The subvertical disposition f S^ may be a product of tilting or isoclinal folding (D,). Later, F^ folds, S^, S^, and 1^ developed during D^ under conditions of greenschist facies metamorphism.

In the Little Bald Mountain area, layering ( S Q) strikes mainly northeast and dips steeply south; a metamorphic alignment of micas (S^) parallels S^; F^ folds plunge shallowly with diverse trends; cleavages (Sp) strike north-northwest and dip subvertically; F^ folds plunge steeply to the south; and lineations ( K ) plunge steeply and are dispersed in S Q . S2 transects F^ folds and may be axial plane cleavage of Fp folds. 1, is arallel to ?2 ^^^^ ^^^ ^2 " ^0 "intersections. F, folds nd S^ developed in association with tilting of isoclinals folding (D,) which was accompanied by low-grade metamorphism. Later, Sp, F^ folds, and 1, formed under conditions of greenschist facies metamorphism during Dp.

The two areas are similar with respect to absence of significant penecontemporaneous deformation, rock succession, mesoscopic structures, and proposed deformational chronologies. The first (D^ and D.) and second (D- and Dp) deformational events of the two areas tentatively are correlated with the Sonoma and Nevadan orogenies, respectively.

Geology, Geology -- Sierra Nevada (Calif. and Nev.), Stratigraphic, Geology, Structural