Modern Cascade arc rocks, old
Modern Cascade arc rocks, old
early Pleistocene to Pliocene
Consists of a wide range of mafic to silicic, predominantly andesitic to rhyolitic, lava flows, cinder cones, and lava domes distributed across the northern map area, south of Lassen Peak.
Volcanic units are all calc-alkaline but include a broad range of compositions and phenocryst assemblages. Basalt and basaltic andesite (of subordinate volume) are commonly sparsely olivine-phyric; andesite typically contains hypersthene ± augite phenocrysts; dacite typically contains hornblende ± hypersthene ± pyroxene ± biotite phenocrysts; rhyodacite (of subordinate volume) is coarsely porphyritic with augite ± hypersthene; rhyolite is typically sparsely porphyritic with hypersthene ± hornblende ± biotite (Clynne and Muffler, 2010).
Includes rocks of Maidu volcanic center, Dittmar volcanic center, and Rockland caldera complex, among others (Clynne and Muffler, 2010). Oldest dated unit has K-Ar age of 1.78 Ma; youngest has K-Ar age of 803 ± 27 ka (Clynne and Muffler, 2010). Broadly coeval with, interfingers with, and overlies late Pliocene Tuscan Formation (map unit tus). Represents more proximal volcanic deposits, whereas the Tuscan Formation represents more distal volcaniclastic debris flow and fluvial deposits.