Abstract

During the late Pleistocene, a flood-tidal delta landward of barrier islands developed at the eastern mouth of Paleo-Tokyo Bay, central Japan. The sediments deposited formed sand wedges tapering into lagoonal mud, and are composed of flood-tidal delta and washover facies.

The flood-tidal delta facies is composed of three major subfacies: delta-bottom facies, delta-front facies, and deltatop facies. The delta-bottom facies comprises alternations of fine to very fine grained sand and mud. Strong bioturbation is conspicuous. Sedimentary structures such as ripple lamination and mud drapes are, however, rarely preserved. The deltabottom facies grades gradually into the delta-front facies, which displays lagoonward (landward)-inclined crossbedding with a lot of burrows. The delta-top facies consists of parallel laminated, medium to coarse grained sand, intercalated with some tabular crossbedding and trough crossbedding. A stacking of the subfacies shows a coarsening-upward sequence generated by progradation of the flood-tidal delta.

The washover facies overlies the flood-tidal delta and is distributed more lagoonward. The washover facies comprises proximal and distal subfacies. The proximal washover deposits consist of parallel-laminated, medium to fine grained sand with shell fragments at the base. Convoluted laminae are common. The distal-washover facies consists of very coarse to medium grained sand characterized by steep foreset structures with syndepositional cracks.

Enormous quantities of washover sand were deposited in a very short time interval. Some units contain features suggestive of earthquake activity during deposition. Moreover, the powerful waves responsible for the washovers may have been earthquake-generated tsunami waves.