BIRTH AND DEATH OF A LATE JURASSIC VOLCANIC ARC TERRANE, WESTERN SIERRA NEVADA FOOTHILLS, CALIFORNIA

In the westernmost foothills of the Sierra Nevada, a Late Jurassic volcanic arc terrane covers an area over 300 km long, and up to 50 km wide. Based on our work and the pioneering studies of Clark (1964, 1975), we recognize four stratigraphic units that record the construction, erosion, and burial of the volcanic arc. (1) The oldest unit is a volcanic edifice-building sequence of lava flows, pillowed lava flows, flow breccias, and homolithic pyroclastic rocks of basaltic to rhyolitic composition, all of which have been intruded by hypabyssal rocks of similar composition. (2) The end of edifice construction is marked by a discontinuous unit of felsic volcanic centers, up to 5 km in diameter. These centers contain rhyolite flows, autobreccias, and felsic volcaniclastic rocks as well as felsic porphyries that intrude the underlying volcanic edifice-building sequence. (3) Overlying the edifice-building units is a volcanic edifice-erosional sequence of volcanic heterolithic conglomerates, sandstones, and finer grained volcaniclastic rocks that exhibit sedimentary structures indicative of turbidity flow deposition. Rare felsic dikes and sills occur in the lower part of the volcanic edifice-erosional sequence. (4) The youngest stratigraphic unit is a largely argillaceous epiclastic unit, with a sharp lower contact having little or no intermixing of volcaniclastic and epiclastic detritus. The epiclastic unit and the upper volcanic unit contain Oxfordian-Kimmeridgian fossils (159 ± 4 - 151 ± 3 Ma), but the upper volcanic unit is intruded by c. 162 Ma dike swarms and may be mostly Lower Oxfordian. The maximum age of the lowermost volcanic unit is unconstrained. The three volcanic units belong to the Gopher Ridge and Copper Hill formations of Clark (1964), and the epiclastic unit is equivalent to his Salt Spring Slate. The Copper Hill formation is simply a repeated section of the Gopher Ridge formation. A possible tectonostratigraphic model for this volcanic arc terrane involves the construction of a volcanic arc edifice on disrupted volcanic, sedimentary, and ophiolitic basement rocks along a continental margin, followed by its erosion, which closely followed the end of volcanic edifice construction. Deposition of volcanic detritus into an intra-arc basin was followed by deposition of quartz-rich epiclastic detritus.