Age relationships and depositional environments of Paleozoic strata, northern Sierra Nevada, California

Abstract

Upper Paleozoic volcanic rocks in the northern Sierra Nevada rest unconformably on the lower Paleozoic Shoo Fly Complex. Late Ordovician conodonts recovered from an exotic limestone block in Shoo Fly melange have North American affinities; the Complex may have traveled little from its original location. Magmatism may have begun as early as Early Devonian and persisted into Early Mississippian time. The volcanic rocks constitute a longitudinal section through a relatively immature volcanic arc dominated by tholeiitic magmas. Two lithologically distinct members of the overlying Peale Formation are given formation status. The predominantly volcanogenic lower member is renamed the “Keddie Ridge Formation”; the name “Peale Formation” is retained for the upper chert member. Two new Late Mississippian–Early Pennsylvanian radiolarian localities in the Peale chert at the north end of the Paleozoic outcrop belt correlate well with a previously described assemblage at the south end and thus rule out time-transgressive deposition. Early Permian fusilinids in a chert-pebble conglomerate only 10 m upsection from an Early Pennsylvanian radiolarian locality tightly constrain a regional depositional hiatus. The Peale chert displays common soft-sediment deformation, slump structures, intraformational breccias, and abrupt termination of thick sequences along strike. Locally, the chert is intruded by tholeiitic basalt. These features suggest deposition in small, tectonically active basins, possibly in an extensional, back-arc regime. The overlying Permian sequence of arc tholeiites is truncated by Mesozoic strata in most areas but appears to grade into Triassic shale and limestone in the extreme northwest.