Abstract
Hummocky cross-stratification (HCS) is one of the most common and widely recognized structures in ancient storm-dominated sequences. It has been thought to be formed in shallow marine environments during high-energy storm events. There is little knowledge of sedimentary processes of it because modern shelf observation during storms is limited and experimental approaches had never succeeded in forming HCS. U-shaped wave duct was commonly used in forgoing experimental studies. This is well-suited to generating oscillatory flow but not to supplying sediments because it consists of a closed duct.
New circular flume used in this study was designed for observation of sedimentary processes with sediment supply under long-period oscillatory flow as high-energy wave in storm event. This flume can generate flow by paddle movement and can keep hydraulic condition constant even in bed aggradation.
Three runs with different rates of sediment supply were conducted. Quartz sands with the mean diameter of 0.3 mm are used. Period of oscillatory flow is 15 sec and maximum near-bed velocity is 80 cm/sec. It was observed that the development of hummocky bedform was suppressed under aggrading conditions with the rate of 1-3 mm/min. This rate is not exceptional in modern shelf. In the run with temporal change of supply rate from 0 to 1 mm/min, HCS-like structure was formed. Upward flattening laminae were observed in each bed, which is common in ancient rock record of HCS. This result suggests that upward flattening laminae in HCS might be formed with increasing of sediment supply. Although further confirmation that the effect of sediment supply is not an artifact of experimental design is needed, this new criteria for sediment supply rate contributes to better reconstruction of ancient marine sedimentary processes.
