Discovery of outcrop-scale fine-grained sediment waves in the lower Halang Formation, an upper Miocene submarine-fan succession in West Java

We report the first discovery of outcrop-scale, fine-grained sediment waves in the left-side muddy overbank deposits relative to the down-current direction of the adjacent channel deposits in the lower Halang Formation turbidite system in a late Miocene back-arc basin, West Java. The present outcrop examples are characterized by an average wavelength of 10.7 m and an average wave height of 0.13 m, and the aspect ratio is similar to that of modern large-scale, fine-grained sediment waves. The dimension and asymmetry of waveforms decrease in the down-current direction in association with the increase in wave steepness. In particular, aggradation and up-current migrating of waveforms are commonly observed in the proximal part of overbank deposits. Furthermore, the stoss-side deposits locally contain thicker and coarser sandstone and coarse siltstone beds (beds up to 5 cm thick) than those of the lee-side deposits. The basal part of individual waveforms does not contain any sandy bedform and/or irregularity as a precursor for developing asymmetrical waveforms from fine-grained turbidity currents. These spatial variations in geometry and component deposits of the waveforms are interpreted to have been formed under antidune or cyclic-step and standing-wave conditions from the proximal to distal overbank environments, respectively. The present outcrop-scale examples can fill the gap in dimension and formative processes between laboratory-scale bedforms and modern large-scale, fine-grained sediment waves for elucidating scale-independent processes, which are responsible for the development of fine-grained waveforms in a deep-water environment.