Bishop Tuff
Bishop Tuff
middle Pleistocene
Highly voluminous rhyolitic ignimbrite erupted from Long Valley caldera in the eastern Sierra north of Bishop, CA. Thermally and compositionally zoned (71-78% SiO2); variably nonwelded to densely welded; phenocryst content ranges from 1-25% and consists of sanidine, quartz, plagioclase, biotite, allanite, Fe-Ti oxides and zircon (Hildreth and Fierstein, 2016). Erupted from a cataclysmic, caldera-forming eruption at ~767 ka, with outflow sheets emplaced radially about the caldera, substantial ponding of pyroclastic material within the caldera, pyroclastic flows emplaced >20 km down the San Joaquin River drainage, and pyroclastic fall dispersed widely (e.g., Hildreth and Fierstein, 2016). Eruptive volume was ~600-650 km3, approximately half of which ponded within the subsiding caldera (Hildreth, 2004; Hildreth and Fierstein, 2017). An Ar-Ar date of 767 ± 2 ka reported by Rivera and others (2011) represents the most precise date on this unit. Tuff ponded within the caldera is now buried by younger volcanic, fluvial and lacustrine units; pyroclastic fall is also present as unmapped interbeds within alluvial, glacial, and other surficial deposits, and serves as an important stratigraphic marker in coeval sediments across a broad swath of the western U.S.