In the Sanriku coastal area, discrepancies have been suggested in crustal movements between long (104
years) and short (101
years) time scales. To clarify the cause of these discrepancies and reconstruct the tectonic history of this area, knowledge of incised valley fills with many radiocarbon ages provides basic and important data. Although the southern Sanriku coast has some small alluvial plains in the environments of ria coasts, the formation process of valley fills has not been discussed on the basis of a number of radiocarbon ages. In this study, a sediment core, TY1, is acquired from the lower reaches of the Tsuya Plain, southern Sanriku coast. Core sediments show a shallow marine succession influenced by Holocene sea-level change. Based on twelve radiocarbon ages, the accumulation rate is high (>5 mm/yr) at 9,000 to 7,100 cal BP, low (ca
. 1 mm/yr) at 4,080 to 2,800 cal BP, and high (3–5 mm/yr) after 2,800 cal BP. High accumulation rates in bay mouth deposits during the middle Holocene, when relative sea-level rise decelerated, indicates sedimentation from the seaward area during the period of marine transgression. This marine transgression can be explained by the deposition of terrestrial sediments in small basins upstream from the Tsuya Plain and setback in the deceleration of relative sea-level rises by the Holocene subsidence trend. In the regressive phase since the middle Holocene, a low accumulation rate in deltafront deposits and a high accumulation rate in delta plain deposits coincide with changes of accumulation rate in the coastal area where relative sea-level has risen in a millennium scale. This also implies that the Tsuya plain subsided during the Holocene.