Structural Relationships of Paleozoic and Mesozoic Rocks, Northeastern Placer County, California



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Texas Tech University


In northeastern Placer County, California, along the North Fork of the American River, folded and metamorphosed Paleozoic rocks are in contact with a homoclinal succession of Mesozoic rocks. The Paleozoic rocks, part of the Blue Canyon Formation of the Calavaras Group, consist predominately of quartz metawacke, metasiltstone, slate, and metachert. The Mesozoic rocks crop out to the northeast of the Blue Canyon Formation. They are mainly of the Early to Middle Jurassic Sailor Canyon Formation, which is composed of feldspathic graywacke, siltstone, shale, and volcanogenic rocks. Between the Blue Canyon and Sailor Canyon Formations are conglomerate, limestone, and siltstone of Triassic(?) age.

In the Blue Canyon Formation layering (S^.) and foliation (S„) mainly strike northwest and dip steeply southwest; however, orientations of S_ are also distributed about a beta-axis (3), which plunges moderately to the northwest and is parallel to the plane of S„. Mesoscopic fold axes and lineations are sub parallel to 3« A. metamorphic alignment of micas (S-) parallels S„ and is transected by S_ in pelitic rocks.

In the Sailor Canyon Formation bedding (S, ) consistently strikes northwest and dips moderately to the northeast. Metamorphic foliation is absent except for a weak alignment of micas near the contact with the Triassic(?) units. At most other localities metamorphism is weak and static, and sedimentary textures are well-preserved.

The Triassic(?) units contain a foliation which parallels S^ of the Blue Canyon Formation, but bedding in these units appears conformable only with S^^ of the Sailor Canyon Formation and not with S . Apparently, V the contact between the Triassic(?) units and the Blue Canyon Formation is an angular unconformity.

Possibly, rocks of the Blue Canyon Formation were folded or tilted during an early period of deformation (D^?) which preceded deposition of the Triassic(?) units and during which S formed. A later deformational event (D_) involved penetrative shear and incomplete transposition of S and S into the plane of S in the Blue Canyon Formation and northeastward tilting, possibly in association with major fold development, in the Mesozoic units. During D , shearing associated with the development of S_ attenuated upward and eastward, dying out in the Sailor Canyon Formation.

D (?) may be an expression of the Permian-Triassic Sonoma Orogeny, D apparently corresponds to the classical Nevadan Orogeny, which affected rocks of the Sierra Nevada during Late Jurassic time. VI