The origins of thick, monotonous sequences of fine-grained, massive, volcaniclastic deposits in the rear arc of the Izu-Bonin arc (IODP Exp. 350) are addressed. Units I, III, and V (65% of U1437) are composed overwhelmingly of a single lithofacies (“tuffaceous mudstone”). The great thickness, pervasive bioturbation, variable but significant alteration, and fine-grained nature of the tuffaceous mudstone lithofacies make interpretations of its composition(s), origin(s), and depositional mechanism(s) challenging. Whole-rock geochemistry indicates that the lithofacies is overwhelmingly composed of Izu-derived tephra, and is dacitic in average composition. Within Unit V, some tuffaceous mudstone intervals are gradational with demonstrably eruption-fed, rhyolitic turbidites. Each interval exhibits reverse coarse-tail grading of dark green fiamme against a normally graded (fine-sand to silt) matrix of angular ash shards, crystals, and foraminfers. Each has a sharp, sometimes scoured, basal contact with the underlying tuffaceous mudstone horizon and grades upwards into more massive, greenish, and weakly bioturbated tuffaceous mudstone that is visually indistinguishable from the tuffaceous mudstone lithofacies. Core scanning shows the same compositional gradients (carbonate and heavy mineral proxies) in both the volcaniclastic turbidites and the intercalated tuffaceous mudstone intervals. Trace element analysis of the ash shards indicates that these turbidites originated in the Izu arc front. Uniform crystal and glass compositions within each interval support transport from a single source and event.
We infer that they were sourced from shallow submarine explosive eruptions at the Miocene arc front based on grain size and provenance. We infer that drilling closer to the axis of the channel would recover thicker and coarser volcaniclastic turbidites, and that the U1437 intervals are overbank deposits. Presumably similarly fine-grained turbidites were transported out into the deeper Shikoku Basin to the west. The supply of arc front density currents into the rear arc supports interpretations that the intra-arc rift was not then open, and that there was a largely unobstructed channel westward from the arc front as the rear arc seamount chains developed.