Volcaniclastic sediments, process interaction and depositional setting of the Mio-Pliocene Miura Group, SE Japan

The Mid-Miocene to Early Pliocene (14-3 Ma) Miura Group exposed on the Miura and Boso peninsulas around Tokyo Bay in southern Honshu comprises at least a 2 km thick succession of volcaniclastic sediments. These accumulated in a forearc basin or basins on the eastern side of a proto Izu-Bonin arc prior to and during accretion of the Miura block onto the Honshu arc and its subsequent dextral transcurrent eduction. The basin fill shows a complex admixture of facies that result from multiple processes, which operated on a relatively shallow-slope basin. The main facies groups include dark-coloured, mostly coarse-grained scoriaceous beds; pale-coloured, muddy-silty pumiceous bioturbated sediment; yellowish and whitish-coloured tuffaceous horizons; and chaotic slump, debris flow and injection units. Many of the facies and beds in the older Misaki Formation can be readily interpreted as the result of: (a) direct pyroclastic fall through the air and water; (b) downslope resedimentation via turbidity currents and related processes, in some cases derived from subaqueous base-surge pyroclastic flow or vertical density currents; (c) hemipelagic settling, commonly under the influence of thermohaline bottom currents. In other cases, composite beds are believed to result from the interaction of these processes in more complex events. The younger Hasse Formation is dominated by more shallow-water, marine-currents and storm events and in parts, by fluvial or proximal fan-delta processes. The distribution of these different facies illustrates the evolution and shallowing of the Miura-Boso forearc basins during accretion and emplacement.