Emplacement and Origin of Eocene and Oligocene Submarine Pyroclastic Flows of Boninite and Rhyodacite Composition in the Bonin Fore-arc, IODP Expedition 352
Twenty felsic tuffs with thicknesses to 41 cm were observed in Oligocene sediments (143-167 mbsf). Some of these layers are massive, normally graded, and in some cases finely laminated or contain normally graded beds several centimeters thick. Others, reverse graded with pumiceous granular tops and lithic-rich bases, or reverse graded with convolute lamination, indicate PDCs. The texture of the glass shards and the shape of the vesicles reveals two major groups of tephra. The first is dominated by highly vesicular, pumiceous clasts with tubular and elongated vesicles, along with less abundant dense, blocky, and cuspate glass shards. The second is dominated by dense, blocky, and cuspate glass shards, with a moderate abundance of rounded to elongate bubbles, but only a minor amount of highly vesicular pumiceous clasts. Less vesicular pyroclasts tend to occur toward the base in the tuff sequence. The lowermost tuffs additionally include large, blocky, mostly dense glass shards, consistent with an origin in a submarine environment.
Homolithic hyaloclastite/pyroclastic flow deposits of high-silica boninite were recovered between 192–243 mbsf. They are mainly composed of black, glassy ol+opx-phyric scoria clasts (~1-2 cm diameter; 20%–30% of the deposit) in a fine grained matirix of glass and mineral fragments (ol and opx). The deposits are partly welded in places, with flow textures around coarser clasts. The rocks are interpreted as primary submarine pyroclastic flow deposits, in part resedimented by gravity flows, which resulted in rare graded horizons. The primary nature is suggested by the lack of exotic clasts, the common mineral assemblage in both the scoria clasts and the matrix, and the rock texture indicating syneruptive welding.
Compositional analysis and 3D-thinsection-petrography will identify primary versus secondary flow deposits, respective eruption and emplacement processes of the deposits and related temporal variations.