Jurassic submarine arc-apron deposits and associated magma/wet-sediment interaction, northern Sierra Nevada, California

Jurassic metavolcanic rocks of the Northern Sierra terrane in northern California are part of an extensive Triassic–Jurassic arc constructed along the western margin of North America. In the English Mountain area, Nevada and Sierra Counties, a well-exposed volcaniclastic sequence 3.6 km thick records Jurassic island-arc activity in a submarine environment. In the lower part of the sequence, thinly bedded andesitic volcanic sandstone turbidites and mudstones of the Lower to Middle Jurassic Sailor Canyon Formation were deposited below storm wave base in a long-lived marine basin. This unit is overlain conformably by the Middle Jurassic Tuttle Lake Formation, which records rapid accumulation of coarse-grained, calc-alkaline volcanic debris in the proximal parts of a submarine apron around a major volcano that developed within the earlier Sailor Canyon basin. The formation consists primarily of massively bedded, matrix-supported, polymict, basaltic to andesitic volcanic breccias deposited from submarine debris flows. Intercalated pillow-hyaloclastite breccias formed during local extrusion of lavas on the seafloor. Numerous intrusions cut the sequence and are petrographically and chemically identical to clasts in the debris-flow deposits. Peperite occurs along intrusion margins and as abundant isolated pockets, indicating intrusion of magma into wet, unconsolidated sediments. Elongate intrusive pods to 13.6 m in length are associated with smaller intrusive pillows and are inferred to represent a network of feeder tubes that extended from larger, coherent intrusions and supplied magma to complex zones of magma/wet-sediment interaction at shallow levels beneath the seafloor. Syndepositional intrusive activity of this type, including formation of abundant peperite, probably is a characteristic feature of submarine volcaniclastic sequences developed in proximal island-arc settings.