INTRUSIVE DACITIC TO RHYOLITIC HYDROCLASTIC BRECCIA COMPLEX EMPLACED AT SHALLOW LEVELS BENEATH THE SEAFLOOR IN A DEVONIAN SUBMARINE ISLAND-ARC SEQUENCE IN THE NORTHERN SIERRA NEVADA, CALIFORNIA

The Devonian Sierra Buttes Formation (SBF) occurs at the base of a thick succession of submarine Paleozoic island arc strata in the northern Sierra Nevada. Bulk eastward rotation of the succession has provided cross-sectional views of a variety of arc deposits and associated hypabyssal intrusions. Much of the SBF, which is up to 1.75 km thick, consists of volcaniclastic turbidites, ash-fall tuff, and subaqueous debris-flow deposits that accumulated in deep-marine environments and are intercalated with hemipelagic radiolarian chert. The area of concern herein is centered on the prominent glaciated Sierra Buttes peaks, from which the formation takes its name. Coeval andesitic to rhyolitic hypabyssal intrusions form a complex assemblage making up much of the SBF in this area. The assemblage extends for ~ 6 km along strike and contains large amounts of intrusive hydroclastic breccias within which pillows are chaotically dispersed. These breccias developed when ascending batches of magma were unable to penetrate thick sequences of unlithified sediment and instead intruded into and were quenched against them at shallow levels beneath seafloor.

Here we report results of detailed mapping of glaciated outcrops that occupy an area of ~ 1.5 km² within the intrusive assemblage and consist mostly of dacite and rhyolite, which show intrusive peperitic contacts to the east against a thick section of overlying submarine strata. The assemblage contains large amounts of massive fragmental material with clasts typically < 3 cm in length and ranging down to fine ash. Much of this material consists of angular, originally glassy hyaloclastite shards that formed by nonexplosive quench fragmentation of magma intruding into wet sediment. Elongate tubes several meters in length with elliptical cross-sections, as well as highly irregular, amoeboid bodies with multiple tongues occur within the breccia and are interpreted to represent parts of a branching, interconnected feeder system that supplied magma to the growing fragmental mass beneath the sea floor. In other areas, the fragmental material consists of ellipsoidal to droplet-like lapilli having quenched rims, which we interpret as spatter produced by contained explosions some distance below the seafloor; there is no evidence that this material vented onto the seafloor.