Paleotectonic and paleogeographic significance of the Calaveras Complex, western Sierra Nevada, California

Abstract

The Calaveras Complex of the western Sierra Nevada, as defined here, consists of a 375 km long, 35 km wide belt of metasedimentary and metavolcanic rocks, bounded on the west by the Melones fault zone and Kings-Kaweah suture, and on the east by the Sierra Nevada batholith. The Calaveras Complex forms a continuous northwest-trending belt between the Placerville area and the Merced River area. South of the Merced River the belt extends in numerous roof pendants at least as far south as the Tule River. A sequence of four lithologic units is recognized, each of which is thousands of meters thick. Precise original stratigraphic thicknesses cannot be measured because of intense soft-sediment and post-consolidation deformation. The lowest unit consists of mafic pillow lava, breccia, tuff, and argilllte, and may represent layer 2 of oceanic crust. This basal unit is overlain by a predominantly chaotic unit of argillite with variable amounts of chert and siltstone often occurring as clasts in a diamictite. Olistoliths of shallow water limestone are locally an important component of this argillite unit. The overlying chert unit contains abundant large olistoliths of rhythmically bedded chert and locally important limestone olistoliths in a matrix of streaky argillite and diamictite. The highest unit included within the Calaveras Complex contains abundant, well-bedded quartzite with abundant interbedded olistostromes containing quartzite clasts and limes tone olistoliths. Fossils from limestone olistoliths reported here indicate a maximum Permo-Carboniferous age for the upper part of the argillite unit, and a maximum late Permian age for the over lying chert unit. Published fossil data indicate the upper parts of the quartzite unit are late Triassic to early Jurassic. The argillite and chert units apparently comprise numerous olistostromes that accumulated on oceanic crust in a marginal basin that was broad enough to have been relatively free of elastic detritus derived from the basin margins. Olistostromes apparently were shed from tectonically elevated areas within the marginal basin that were denuded of their pelagic and hemi pelagic cover. The quartzite unit may represent an early Mesozoic northwestward progradation of mature continent-derived sand across the western end of the late Paleozoic marginal basin. The marginal basin is considered to have been situated between the Cordilleran miogeocline to the southeast and a volcanic arc terrane to the northwest. The late Paleozoic Havallah sequence of north-central Nevada is believed to have accumulated in the same marginal basin. The Melones fault zone and Kings-Kaweah suture represent a zone of early Mesozoic tectonic truncation a long which the Calaveras Complex is juxtaposed against upper Paleozoic ophiolitic rocks and Jurassic volcanic and epiclastic rocks. Thus, we infer that the Calaveras Complex represents the westernmost exposure of the late Paleozoic marginal basin.

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