有明海南東岸玉名平野の地形発達史と完新世海面変化 Holocene Geomorphic Development and Sea-Level Change in the Tamana Plain, Southeastern Coast of Ariake Bay, Western Japan
有明海南東岸の菊池川下流,玉名平野の完新世における海面変化および地形発達を,航空写真判読,ボーリング調査・貝化石の<sup>14</sup>C年代測定,硫黄・珪藻分析などの結果に基づいて考察した.平野は三角州と自然堤防帯からなり,それらを構成する沖積層は基底礫層 (BG) ・下部層 (LS: 砂・シルト)・中部層 (MC: シルト・粘土)・上部層 (US: 砂)に分けられる.この内, MCは発達の良い完新世の海進堆積物である. MCを形成した海進では,初期の約9000年前には海水準は-20m付近にあり,その後は5.5m/10<sup>3</sup>yrの急激な速度で上昇し,約4900年前には2mの最高海水準に達した.それから現在まで,海水準は緩やかに低下し,現海水準に達している.この海退に伴い上部砂層が堆積した.このように,完新世最高海水準が見かけ上現在よりも高くなる海水準変動は,海面上昇に伴うハイドロアイソスタシーによる陸側の隆起を示している.
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 <sup>14</sup>C dating.<br> 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.<br> 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. -100m, 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 <sup>14</sup>C 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 20m in 9 ka, rose to 2m of the maximum level of transgression in 5 ka, and lowered to 0.2m at 3.5 ka. The US is fluvial or deltaic sandy sediments after 5 ka in the slow regression.<br> 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.
- 地理学評論. Ser. A
地理学評論. Ser. A 70(5), 287-306, 1997-05
The Association of Japanese Geographers