Gravel Waves in an Ancient Canyon: Analogous Features and Formative Processes of Coarse-Grained Bedforms in a Submarine-Fan System, the Lower Pleistocene of the Boso Peninsula, Japan

摘要

Gravel waves have been observed from many modern submarine fans. However, the internal organization and formative processes of gravel waves are still controversial, because features analogous to gravel waves seen on outcrops have been poorly understood. Here, we analyzed cross-stratified pebble conglomerates in a lower Pleistocene paleocanyon-fill succession exposed on the Boso Peninsula of Japan, which exhibit features similar to those of modern gravel waves in terms of texture, size, and geometry. Gravel-wave deposits examined in this study are interpreted to have been formed as traction-sedimentation bedforms that migrated in downslope directions under gravelly, high-density turbidity currents, and are gradationally overlain by weakly graded sandstones from sandy high-density turbidity currents in single depositional events. Thus, erosion and reshaping of coarse-grained deposits to develop the wave forms by subsequent lower-density turbulent flows within single or later depositional events, as proposed by previous studies, are not evident from the present examples. The findings indicate that a formative process of this kind has also been responsible for the development of gravel waves in modern submarine fans, although later modification of the bedforms by subsequent lower-density turbidity currents cannot be excluded.