Sedimentological consequences of arc-arc collision: An example from Mio-Pliocene Miura Group, Japan

This paper presents brief sedimentological inventory of the Misaki and the overlying Hasse Formation (constituting the Miura Group) based on observations made at Jogashima Island, Miura Peninsula and infers the nature of the stratigraphic transition from tectonic viewpoint. The lower part of the Misaki Formation is made of very coarse to coarse-grained scoriaceous, normally graded sandstones, fine-grained sandstones/siltstones, and mudstones and indicates a deep sea submarine canyon depositional setting. There is no evidence of storm reworking (shallow marine processes) in the lower parts of the Misaki Formation at least in the Jogashima Island as suggested by Stow et al. (1998). However, the upper part of the Misaki succession formed in a shallow marine depositional setting. The overlying Hasse Formation characterized by brownish coloured, very coarse to coarse-grained large-scale trough cross stratified, very poorly sorted sandstones, very poorly sorted polymictic and matrix dominated mass flow conglomerates with siltstone/fine-grained sandstone clasts derived from the underlying Misaki Formation and occasional reverse grading. In significant contrast to the underlying Misaki Formation the Hasse Formation lacks very fine-grained sandstone/siltstone/mudstone facies. Unlike the tholeiitic basalt composition of the lapilli and pebbles of the Misaki sandstone beds, the scoriaceous lapilli and pumiceous beds of the Hasse Formation are of calc-alkaline composition. The Hasse sedimentation probably took place in much shallower paleogeography compared to the underlying Misaki Formation, most probably in an alluvial fan-braided fluvial setting. The Misaki-Hasse stratigraphic contact is very sharp and erosional, and represents a subaerial unconformity (i.e., depositional sequence boundary). Our sedimentological data from the Jogashima Island clearly indicate progressive shallowing of the depositional setting during the terminal phase of the Misaki sedimentation and much abrupt shallowing across the Misaki-Hasse transition possibly as a consequence of ongoing collision between the Izu-Bonin and Honshu arcs.