11-7 Deep marine andesitic/felsic partly intrusive hyaloclastite complex in a Devonian submarine island-arc sequence in the northern Sierra Nevada, California

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes

Presenting Author:

Authors:

Abstract:

The Sierra Buttes Formation (SBF) records the onset of Devonian submarine Paleozoic island arc volcanism in the Northern Sierra Terrane, located in the northern Sierra Nevada. The SBF 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 sequence, which is up to 1.75 km thick, is spectacularly exposed at the prominent glaciated Sierra Buttes peaks, from which the formation takes its name. Bulk eastward rotation of the succession and associated coeval andesitic to rhyolitic hypabyssal intrusions that make up much of the SBF in this area, has provided an almost continuous cross-sectional view of the formation at Sierra Buttes. Of interest to this study is a complex of eruptive and intrusive, deep marine felsic to andesitic rocks with a complicated emplacement history exposed on the northern slope of Sierra buttes.

The complex extends for ~ 6 km along strike, containing a minimum of six eruptive and

intrusive events, starting with generation of felsic lapillistone from lava fountaining on the seafloor. Before the lapillistone could lithify, a massive felsic lobe-hyaloclastite unit erupted, bulldozing the lapillistone deposit, which produced a gradational mixed contact between the two. A second felsic lobe-hyaloclastite unit with a different phenocryst content mixed with the first unit along their contacts. This was followed by a period of hemipelagic and volcaniclastic deposition before intrusion of a third felsic hypabyssal unit that forms a carapace of intrusive hyaloclastite and dispersed intrusive pillows that grade into peperite along the upper contact, and at rare locations along the basal contact. Along basal contacts where no peperite or sediment are preserved, the third felsic lobe-hyaloclastite unit and intrusive pillows mixed with that of the previous felsic lobe-hyaloclastite units. The intrusion of quartz porphyry into the complex obscures what would have been the base of the complex. Lastly, fluidal partly interconnected dike-like intrusions of andesite consisting of three petrographic types snake throughout the complex. The andesitic intrusions have peperitic contacts where they mixed with different hyaloclastite units. This example illustrates the complexities that can develop where extrusive deposits and synvolanic intrusions interact in submarine arc settings.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9186

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