Changes in Depositional Environments during the Post-glacial Stage in Kagoshima Bay and Seas around the Northern Part of the Ryukyu Islands

Abstract

Changes in depositional environments during the post-glacial period in Kagoshima Bay and seas around the northern part of the Ryukyu Islands are deduced on the basis of the analyses of ten sediment core samples which were recovered from ten boreholes on the Kagoshima City, alluvial plain and from two boreholes in Naze Bay.
The sea-level curve for post-glacials reported from the stable continental shelf of the East China Sea suggests that the sea level rose rapidly during the period ranging from the last glacial to 7, 000yrs BP. The benthic and planktonic foraminiferal data from two sediment cores indicate that deposition in Naze Bay began about 9, 300yrs BP under very shallow inner bay conditions, scarcely influenced by open sea water; with the passage of time, the bay deepened, with concomitant increasing degrees of flow of open sea water. The change in depositional environments was caused by a small supply of sediments from rivers and tectonic subsidence of the Naze Bay area. On the other hand, the benthic and planktonic foraminiferal data from the Alluvium in Kagoshima City indicate that sea-level changes synchronized with water depths during the period ranging from 10, 000 to 7, 000yrs BP. The good preservation of shallow inner-bay environments during the period may be attributable to a large supply of sediments mainly from the pyroclastic deposits distributed in this area. During the period ranging from 7, 000 to 5, 000yrs BP, the transgression reached its maximum extent to develop an alluvial plain, the rudiments of the recent alluvial plain.