Abstract
Small-scale bed waves under unidirectional currents can be classified into two types; Type I well-developed bed waves formed under a flow with a certain velocity, and sand grains move by avalanche on the lee slope; Type II: immature bed waves with short wavelength and low height formed at the early stage after the flow velocity decreases from the upper-plane-bed-regime, and grains on the bed move as a sheet flow. Experiments demonstrated that Type II bed waves could be preserved in the presence of ambient sedimentation and resulted in small-scale cross-stratifications similar to Type I formed under a relatively slow flow. Grain fabric analyses showed that the dominant direction of long axes of grains in the deposit of Type II bed waves dipped upstream against the lee slope, unlike Type I deposits. It is possible to identify the type of small-scale cross-stratifications by its grain fabric. We found an ancient example of ripples categorized into Type II in a turbidite sequence of the Plio-Pleistocene Kakegawa Group, in Japan.
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