| Abstract |
Genetic experiments on combined-flow ripples were carried out to provide detailed criteria of combined-flow ripples. A recirculating flume with a wave generator was used, in which the direction of uni...directional flows was reversible. Two types of combined-flow ripples, wave-dominated and current-dominated types, were recognized based on the ripple roundness. The current-dominated combined-flow ripples appeared when the unidirectional velocity exceeded approximately 19 cm/sec independent from unidirectional-flow direction. Ripple symmetry index (RSI) and ripple roundness index (RRI) were useful for distinction of combined-flow ripples from either current or wave ripples. The indicative features of combined-flow ripples were revealed as follows: (1) rounded lee and stoss sides, and also rounded troughs were remarkable within the current-dominated combined-flow ripples. The roundness became stronger as relative magnitude of oscillatory velocity to unidirectional velocity became lower; (2) the internal structures were characterized by convex-upward foreset laminae, the extremely sigmoidal shape of toes, a "joined" structure and truncated troughs, for current-dominated combined-flow ripples. Some of these were recognized in wave-dominated combined-flow ripples, although whose outer forms were very close to that of wave ripples; and (3) the character of the grain fabric of combined-flow ripples was that preferred imbrication pattern both of the wave-ripple-type and the current-ripple-type appeared in specific locality in a single foreset lamina. The unique round profiles of combined-flow ripples resulted from the characteristic movement of sand particles on migrating ripples. The vortices generated only beside lee sides of the ripples, caused by the coexistence of oscillatory and unidirectional components. The experimental results were applied to geologic records to prove those criteria available for the distinction of combined-flow ripples from wave and current ripples, and the estimation of sedimentary processes. The applications were carried out as follows: (1) distinction of wave ripples and combined-flow ripples in shoreface deposits; (2) identification of combined-flow ripples in tidal deposits; (3) examination of coarse-grained ripple shoreface deposits; (4) examination of ripples within 'hummocky sequences; and (5) paleo-current analysis for tide-dominated shelf deposits including combined-flow ripples of the Miocene Shirahama Formation in the Kii Peninsula, Japan. The most powerful criterion for the distinction was the "rounded profile" of the ripple, i.e., high RRI value. The characteristic internal structures were also useful. Wave-dominated combined-flow ripples were sometimes difficult to distinguish from wave ripples only by their outer forms. The recognition of the combined-flow ripples in 'hummocky sequences' may strongly indicate the process of HCS formation, which is still a matter of controversy.show more
|
Export Mendeley
29_01_p001