Grain fabric of hummocky and swaley cross-stratification

Abstract

Bed-form and grain fabric of two types of hummocky and swaley cross-stratification from the Pleistocene formations in the Kanto Plain, cental Japan, display clearly distinctive flow conditions of storm (typhoon)-generated combined flow. The grain alignment in the vertical profile of a domal structure of hummocky cross-set, together with variable strike directions of a lamina, suggest that the cross-stratification was produced under the waning stage of oscillatory-dominant combined flow. Elsewhere, the preferred grain orientation along the maximum dip direction of a large swaley structure shows a small angle of upcurrent imbrication both on the stoss and lee sides of the laminae. The stratification within the swale is asymmetric, but the strike directions of a lamina are persistently oriented in a preferred direction. The fabric and the structure of the swale indicate that the cross-stratification was formed as a very low-angle mega ripple under unidirectional-dominant combined flow. The grain fabric of both cross-sets clearly shows that in spite of a superficial resemblance to antidunes, neither of the two structures was formed in flow conditions for antidunes.