Abstract

We have recognized several age groups of radiolarians in cherts in a belt of rock in the Sierra Foothills between the Melones-Gillis Hill fault and the Bear Mountains-Wolf Creek-Big Bend faults. They are Late Permian, Late Triassic, Early Jurassic, Middle Jurassic, and late Middle Jurassic in age. The biostratigraphic control provided by recognition of these groups, combined with study of lithologic assemblages related to them, has enabled us to divide this belt in the area between the Cosumnes River and the Middle Fork of the American River into five terranes; Mother Lode, French Creek, Mount Ararat, Cool Quarry, and American River. The slate and volcanic breccia of the Mother Lode Terrane (Mariposa and Logtown Ridge Formations) are depositionally linked to unmetamorphosed Late Permian basalt, chert, and gabbro which we propose represents basement of the stratigraphie sequence. In addition, the late Middle Jurassic Colfax Formation is depositionally linked to the early Middle Jurassic olistostrome (Clipper Gap Formation) in the American River Terrane. Recognition of separate terranes and the stratigraphy within them indicate that terrane amalgamation (probably during accretion to North America) is constrained to be younger than Callovian, and that the Nevadan orogeny was the result of accretion of multiple terranes to the continental margin during the period of 163-143 Ma.

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