Abstract
Sandy and/or gravelly reworked deposits are intercalated in the Holocene bay-floor muds around the southern Kanto region of central Japan. These deposits consist of poorty sorted muddy sand and sandy gravel beds in increasingly fine upward sequences, with abundant transported shells and rip-up clasts, unconformably covering bioturbated bay-floor muds yielding in situ and indigenous molluscs.
These deposits are observed in six drilling cores at three alluvial plains and can be combined into seven isochronal layers, denoted T1 to T7 in ascending order, based on 79 radiocarbon ages of shell and wood samples dated by AMS.
Molluscan and ostracode assemblages in normal sediments above and below layers T3 to T7 indicate that the depositional environment was a muddy floor which occupied the central part of the bay at a depth of 10-20m. However, layers T3 to T7 include molluscs which inhabited the rocky coast or shore platforms, mixed with muddy bay-floor assemblages. Molluscan shells in these layers have reversed radiocarbon ages with respect to the underlying muds.
Layers T1 and T2 show sedimentary structures and fossil contents similar to the preceding anomalous deposits. In particular, they include ostracode assemblages indicating the invasion of open marine water into the brackish embayment.
The sedimentary facies, sedimentary structures, fossil assemblages, and radiocarbon ages of these layers indicate that submarine and coastal erosion and transportation of reworked sediments into the bay floor were accomplished by strong water currents.
Moreover, layers T3 to T7 show ages for that match the rapid emergence of five Holocene marine terraces in this region caused by great earthquakes centered around the Sagami Trough (trench) south of Kanto. The age agreements strongly suggest that the anomalous reworked layers are earthquake-induced tsunami deposits. The tsunamis occurred at intervals of 400 to 2, 000 years during the past 10, 000 years.
