Are coarse-grained sediment waves formed as downstream-migrating antidunes? Insight from an early Pleistocene submarine canyon on the Boso Peninsula, Japan

Gently undulating waveforms were identified in conglomerates and pebbly sandstones of the lower Pleistocene Higashihigasa Formation, which represents an infill of a submarine canyon on the Boso Peninsula, Japan. On the basis of dimension, geometry, and texture, these waveforms are interpreted as bedforms that are analogous to coarse-grained sediment waves in modern deep-water environments. The studied waveforms exhibit symmetrical or dune-like asymmetrical forms, in association with minor antidune-like asymmetrical forms with the lee sides being longer than the stoss sides. The waveforms are also characterized internally by bedding gently inclined in the downstream direction to the northeast. Laterally, washout-dune and humpback-dune deposits are locally developed in association with planar bedding. The coarse-grained sediment-wave deposits are interpreted to have developed as downstream-migrating antidunes in the upper-flow-regime condition of turbidity currents. On the basis of the empirical relationship between the wavelengths and mean flow depths of downstream-migrating gravel antidunes, the waveforms are interpreted to have been formed by waves at the interface between the denser bottom-flow and the less-dense upper-flow within a turbidity current. It is possible that some of the coarse-grained sediment waves in modern deep-water environments may have been formed as downstream-migrating antidunes, regardless of their plan-view geometry.