Abstract

The Aoshima Formation, the uppermost part of a Neogene forearc basin fill, is well exposed along the Nichinan Coast of Kyushu Island, SW Japan. It consists of alternating hemipelagic mudstones and sandy sediment-gravity-flow deposits. In the present study, 36 stratigraphic logs of this alternation were collected over 700 m in the paleocurrent direction. The correlation of these logs revealed cyclic lateral changes of bed thickness and sedimentary facies along the paleocurrent direction, which is indicative of hundred-meter-scale sediment waves. Four primary structures were identified: (1) upstreamward migrations of distributions of rip-up mud clasts, (2) depositional undulations with relief of < 2 m, (3) slope gradients of depositional undulations, suggesting that the thickening of beds occurred at sites with larger values of slope gradient, and (4) differences in sedimentary facies, grain-size distributions, and bed thicknesses between the upstream and downstream flanks. These results suggest that hydraulic jumps of sediment gravity flows caused the wave-like sedimentary topographical constructions, induced by the formation of cyclic steps or antidunes. Based on the thickness distributions and sedimentary facies distributions in the studied stratigraphic interval, the depositional topography was attributed to sediment waves as an immature and short-lived antidune topography.