Palaeoecological effects of back-arc spreading on Miocene benthic communities, SW Japan Arc

• The lower to middle Miocene Misaki Group, South-western Japan, characterised by low ichnodiversity and little bioturbation, is studied.
• The group was deposited in a forearc basin under the strong influence of extremely active tectonics, in terms of spreading of two backarcs in the vicinities, and the forced subduction of the young, hot oceanic plate.
• Rapid and frequent sedimentation across the entire basin, induced by the tectonics, was the most likely controlling factor for the colonisation of depauperate fauna, and considerably diluted their traces.

During the early to middle Miocene, rapid development of backarc basins, now known as the Sea of Japan, occurred in the NE margin of the Asian Continent. Then the entire SW Japan Arc (the arc) was rotated clockwise very rapidly. Such an event was associated with the opening of another backarc basin located to the south of the arc. Immediately after this opening, the very young, hot oceanic plate began to subduct underneath the arc. This subduction caused rapid uplifting and enhanced the denudation of the arc, in the post-early Miocene.The lower to middle Miocene Misaki Group is a 3,000 m-thick siliciclastic forearc basin fill, consisting of wave-dominated shallow-marine and overlying braid-plain deposits. Its defining features are the low ichnodiversity and paucity of ichnofabrics. For instance, up to 96% of the total thickness of the measured offshore deposits are non- or very slightly-bioturbated, despite the other “normal” offshore deposits are generally highly bioturbated. Although the less bioturbated sediments were previously considered as anoxic deposits, anoxic water was unlikely to develop within the highly open wave-dominated shallow-marine settings.The highly active tectonics associated with the spreading of the two backarc basins and the forced subduction most likely caused sediment mass-production in the hinterland, as evidenced by the sedimentological features of the coeval Kuma Group, and over-sedimentation (highly frequent and rapid sedimentation) that was taken place across the entire Misaki Basin. This over-sedimentation, supported by a series of evidence, such as the pervasive soft-sediment deformation across the group, estimated rate of net aggradation (more than 136 to 230 cm/kyr), etc., most likely induced the colonisation of depauperated fauna, and diluted their ichnofabrics, across the entire basin. The extremely active tectono-sedimentation played an important role in the development of the basin-wide anomalous benthic ecosystems and ichnofabrics during the early to middle Miocene.

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