Abstract
High-resolution seismic reflection profiles of the shelf off the Kuji River in northeast Japan show unconforming two sedimentary units lying on Tertiary basement rocks. The boundary surface of both units, which clearly truncates seismic reflections in the lower unit, is a ravinement surface created during the Late st Pleistocene-Early Holocene transgression.
These units are Unit Ka and Unit Kb in ascending order. Unit Ka consists of fluvial and coastal sediments deposited during the last period of low sea levels, which is estimated to be 80-85m below the present level and during the following transgression. Unit Kb, which overlies Unit Ka, is composed of shelf sediments deposited during the transgression after the formation of the ravinement surface and present high sea levels. As the distribution and thickness of Unit Kb show ridge and trough topography, 0-5m high, 1-3km apart, angles of 15-25° with the regional trend of depth contours, it is concluded that Unit Kb consists of transgressive sand ridges.
These transgressive sediments and a characteristic bounding surface indicate that the ravinement surface can be distinguished from the transgressive surface and that transgressive sediments can be divided into two units bounded by the diachronous ravinement surface.
[Correction] Jour. Sed. Soc. Japan Vol.38 No.38 (1993) pp.142b-142b
