Depositional features of rhyolitic and andesitic volcaniclastic rocks of the Mineral King submarine caldera complex, Sierra Nevada, California

Abstract

Abstract Rhyolitic and lesser andesitic pyroclastic rocks and lava flows are interstratified with marine sedimentary rocks deposited in shelf to basinal environments early Jurassic time. Voluminous (> 25 km 3 ) thick tabular sheets of rhylite ash-flow tuff accumulated within caldera collapse structures. Small (1–10 km 3 ) andesitic stratocones also formed. The andesitic units show abundant features typical of subaqueous deposition, including sorting, bedding, grading of single beds or overall grading of bedded sequences, and soft-sediment deformation. The rhyolite ash-flow tuffs show much less evidence of interaction with water during transport and deposition but are nonetheless believed to have been deposited subaqueously. The andesite stratocones have a thick core facies of amygdaloidal lava flows, hyaloclastites, and massive tuff breccias and an apron facies of bedded and sorted lapilli tuffs, lahars, tuff turbidites and subaqueous fallout tuffs. The rhyolite ash-flow sheets, in contrast, consist dominantly of massive, unsorted, unbedded and pumiceous ash-flow tuffs which resemble subaerially deposited ash-flow tuffs; however, each ash-flow sheet passes gradationally upward and distally into a bedded facies which was sorted by water during deposition. The bedded facies composes only 5–15% of each ash-flow sheet. The bedding is defined by: (1) discontinuous, crudely defined layers of blocks, lapilli, and ash within single flow units and (2) fallout layers and debris flow deposits interstratified with flow units at the distal end of each ash-flow tuff sheet. The former may be segregations that formed by shear between differentially flowing laminae within a single flow unit. The latter may have formed by settling and remobilization of material which was winnowed from the distal end of each flow unit during its subaqueous transport. Turbulent mixing with the water column was important during small-volume pulsating andesitic eruptions, whereas silicic pyroclastics were deposited from highly concentrated plug or laminar flows fed by steady, voluminous eruptions.