Recent Formations and Holocene Paleogeography of Nagasaki Bay and the Nagasaki Lowland, Nagasaki, Western Japan

摘要

The paleogeography and sea-level chan- ge during the Holocene in Nagasaki Bay are discussed on the basis of aerial photo- graphs, borehole data, molluscan fossils, and ~14C dating. Recent formations in Nagasaki Bay and the Nagasaki Lowland are divided into four members. Urakamigawa Basal Gravel : Urg, Dejima Clay: Djc, Matsuya- ma Gravel: Mtg, and the artificial deposit: Ard. Urd is composed of fluvial gravel beds which were deposited in the basal valley during the last glacial period. The Djc is clayey to silty sediments with many marine molluscs, and was formed in the delta and the bay during the Holocene transgression. Mtg is fluvial sand and gravel beds including archeological mate- rial from the Holocene. Ard is artificial material beds deposited after A.D. 1571 when Nagasaki Port opened. The Ard member includes the high-temperature oxidized red sandy layers formed by the atomic bombing in 1945. The ~14C dating for intertidal molluscan fossils shows that the sea level was -16 m in ca. 8,000 yrs BP, and rose to 0.5 m below the maximum level of the Holocene trans- gression in ca. 4,000yrs BP. Sea level since 4,000 yrs BP has been stable.