Holocene geomorphic development and sea-level change in the Tamana plain, southeastern coast of Ariake bay, Western Japan

The paleogeography and sea-level change during the Holocene in the Tamana Plain along the Kikuchi River are investigated on the basis of aerial photographs, borehole data, sulfur and diatom analyses, and ¹⁴C dating. The Tamana coastal plain along the southeastern coast of Ariake Bay is a typical alluvial plain formed in the Holocene. The plain is geomorphologically divided into three parts : flood plain, delta, and reclaimed land. The flood plain includes natural levees, active and abandoned channels, and flood basins. The delta has two sandy ridges parallel to the coast which are old bars and sand dunes. The distribution of archeological sites on the sandy ridges indicates that the inner ridge had emerged earlier than ca. 2, 000 yr. B. P. and the outer one earlier than ca. 500 yr. B. P. The reclaimed land along the coast was constructed after the 17th century. Recent formations beneath the plain are divided into four members : basal gravel member (BG), lower sandy member (LS), middle clayey member (MC), and upper sandy member (US) in ascending order. The BG is fluvial gravel beds which were deposited in the basal valley bottom during the stage of the lowest sea-level, ca. -100 m, in the last glacial age. The LS is sandy sediments formed in a delta or a flood plain during the early transgressive stage in the last glacial period. The MC is clayey sediments with many marine molluscs. The MC formed in the delta and the bay during the main stage of the Holocene transgression. The ¹⁴C dating of moluscan fossils in the MC and the sulfur and diatom analysis of the upper part of the MC shows that the sea-level was about -20 m in 9 ka, rose to 2 m of the maximum level of transgression in 5 ka, and lowered to O. 2 m at 3. 5 ka. The US is fluvial or deltaic sandy sediments after 5 ka in the slow regression. The sea-level change indicates the uplifting of the Tamana Plain during the past 6,000 years. The uplifting corresponds to the hydro-isostatic adjustment model around the Japan Islands. Thus asthenospheric material flows from the ocean side of the East China Sea to the inland side of the Tamana Plain by meltwater loading due to the last deglaciation of large polar ice caps. The uplift is consistent with this model.