Permian volcanic sequence
Permian volcanic sequence
early Late Permian to late Early Permian
Consists chiefly of Arlington Formation (volcaniclastic sandstone, slate, conglomerate, and breccia), which locally grades up into the Goodhue Formation (pillow basalt, basaltic tuff breccia, and tuff). These two formations, together with the locally overlying Reeve Formation (plagioclase-rich andesitic tuff, flows, and hypabyssal intrusive rocks, and lesser volcaniclastic sandstone, chert, and conglomerate) and Robinson Formation (volcaniclastic conglomerate, sandstone, shale, and minor limestone) make up the Permian volcanic arc sequence (Harwood, 1992). Unit occurs in various thrust-fault bounded slices, with the older parts of the unit only cropping out in the northern blocks, the Arlington Formation pinching out north of the Middle Fork Feather River, and the Goodhue Formation pinching out near the North Yuba River. Between the Middle Fork Feather River and the American River area northwest of Lake Tahoe, the unit consists of only the Reeve and Robinson Formations, and is unconformably overlain by the Sailor Canyon Formation and associated units (unit NSTS) along a Permian to Triassic unconformity. The base of the Arlington Formation has yielded late Early Permian (early Leonardian [Kungurian]) conodonts and other fossils in the northwestern block, and a sliver of the formation has yielded probable equivalent age fossils in the northeastern block along the basal contact with the underlying Carboniferous rocks (Harwood, 1992). The Goodhue Formation lacks fossils or other internal age control. In the northernmost block, the Reeve Formation has yielded Early Permian (Leonardian) radiolarians and other fossils of similar age, in the northeastern block late Early Permian (early Leonardian) McCloud affinity fusulinids along with other late Early Permian and early Late Permian fossils, south of the Middle Yuba River late Early Permian (middle to late Leonardian) fossils, in the southernmost block late Early Permian or early Late Permian (Leonardian or Roadian-Wordian) fossils (Harwood, 1992). Spurlin and others (2000) collected detrital zircons from sandstone in the block south of the Middle Yuba River that they correlated with the Arlington Formation, but fossils found nearby are too young to support that correlation. Detrital zircons have an MDA of ~357 Ma (earliest Mississippian), much too old to represent the depositional age of the unit, and probably reflective of redeposition of detritus derived from underlying units. In the northeastern block, the Robinson Formation has yielded early Late Permian (Wordian to Capitanian) conodonts (Harwood, 1992).