Stratigraphy of mid-Paleozoic island-arc rocks in part of the northern Sierra Nevada, Sierra and Nevada Counties, California
New stratigraphic data for the mid-Paleozoic island-arc succession in the northern Sierra Nevada are presented from a well-exposed area in Sierra and Nevada Counties. The Grizzly Formation of Diller is shown to be a laterally discontinuous but regionally extensive epiclastic unit at the base of the arc succession. Newly discovered macrofossils and conodonts date the formation as Late Devonian (Frasnian–mid-Famennian), and it records a widespread erosional episode that affected the underlying Shoo Fly Complex prior to the onset of arc volcanism. The Grizzly Formation is conformably overlain by the Sierra Buttes Formation, a sequence of silicic to andesitic submarine ash-fall tuffs; tuffaceous turbidites; and thick, nonwelded, subaqueous volcaniclastic mass-flow deposits intercalated with radiolarian cherts. Coeval hypabyssal intrusions show evidence of rapid chilling against wet, unlithified sediment, and some consist of complex masses of intrusive hyaloelastite breccia and pillows. Newly discovered conodonts, together with previously described fossils, show that the Sierra Buttes Formation was deposited largely or entirely in the mid-Famennian. Andesitic tuffs and tuff-breccias of the Upper Devonian-Lower Mississippian(?) Taylor Formation abruptly overlie the Sierra Buttes Formation, with no evidence for an intervening Elwell Formation. Like the Sierra Buttes Formation, the Taylor Formation contains abundant coarse-grained subaqueous volcaniclastic mass-flow deposits interbedded with finer-grained rocks. The two formations together constitute a thick sequence of volcaniclastic debris that accumulated rapidly below storm-wave base in an active island-arc setting.