Abstract

Detailed mapping in the foothills of the central Sierra Nevada has revealed that the Lower Jurassic Peñon Blanco Formation and the newly named Jurassic-Triassic Jasper Point Formation occupy the core of a previously unrecognized anticline of Late Jurassic age that plunges gently southeastward, exposing the Upper Jurassic Mariposa Formation on its limbs. The Jasper Point Formation consists of >900 m of massive, pillowed, and brecciated basalt overlain by as much as 100 m of varicolored radiolarian chert; these units resemble layers 2 and 1 of the oceanic crust, respectively. The Peñon Blanco Formation is composed of as much as 5 km of coarse, augite- and plagioclase-bearing volcaniclastic rocks and lesser flows that appear to have originated at a common vent. Fine tuff at the base of the formation is interbedded with, and overlies, the Jasper Point chert gradationally.

A restored section of the Peñon Blanco Formation suggests that the volcanic rocks define a single, composite volcano that probably originated as part of an island arc. Voluminous volcanidastic detritus that composes most of the volcano may have been produced at great depth, possibly by hyaloclasis, autoclasis, or mass wasting. Much of the detritus was deposited as turbidites of tuff and tuff breccia that define an extensive volcanic apron that thickens and coarsens upward over hundreds of metres, suggesting progradation. Submarine clastic processes thus dominated the construction of the volcano. Fragmentation of most of the erupted material probably occurred at or near the vent, and subsequent distribution of the detritus around the vent by epiclastic processes formed a distinctive Stratigraphic sequence. These hypotheses may be tested by dredging young, deeply submerged island-arc volcanoes.

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