Exoperimental study of estimation for paleohydraulic conditions based on sedimentary structures in strata.

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Generic experiments on combined-flow ripples were carried out to provide detailed criteria of ripples. A recirclating flume with a wave generator was used, in which the direction of unidirectional flows was reversible. Two types of ripples, wave-dominated and current-dominated combined-flow ripples, were recognized based on the ripple roundness. The current-dominated ripples appeared when the unidirectional velocity exceeded approximately 19 cm/sec. 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 ripples. The roundness became stronger as relative magnitude of oscillatory velocity to undirectional velocity became lower. A larger unidirectional velocity often resulted in a higher asymmetry ; (2)the internal structures were characterized by convex-upward foreset laminae, the extremely sigmoidal shape of toes and truncated troughs, for current-dominated ripples. Some of these were recognized in wave-dominated ripples, 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- and current-ripples appeared from part to part in a single forest lamina. The unique round profiles resulted from the characteristic movement of sand particles on migrating ripples. The most important phenomenon was vortices generated only beside lee sides of the ripples, caused by the coexistence of the 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 sedimetary processes and depositional environments.