Small ripples in dunes regime and interpretation about dunes-missing in turbidite

Abstract

It was found from the present study that in the process of decreasing flow velocity from upper plane bed regime to the dunes regime, the bed configuration transition was not the reversal one as to time of the contrary ie increasing-velocity process. In spite of being in the dunes regime, small ripples (less than 5 cm in length and 4.8 mm in height) turned up first from the plane bed, right after the velocity was reduced from upper-plane-bed regime. These small ripples are called here 'upper ripples'. 'Upper ripples' are different inshape from the low relief bed waves (Bridge and Best, 1988). 'Upper ripples' grow large into dunes gradually with the flow velocity constant at dunes regime, so are of a transitional phenomenon. Because this growth is not quick, fully developed dunes would be difficult to remain and, only (small) ripples, 'upper ripples' and/or ordinal ripples would be preserved, if the current damps fast from the upper regime to zero. This interpretation does not contradict to and can explain the fact that dune-scaled cross bedding is rarely found between B-division and C-division in turbidites.