Sedimentary Facies and Depositional Ages of a Holocene Sediment Core Obtained from the Naha Alluvial Lowland, South Japan

Abstract

　Sedimentary facies and depositional ages were examined from a newly obtained core, 7.50 meters in length, drilled in Naha Alluvial Lowland, located between the Asato and Kokuba Rivers on the southwestern coast of Okinawa Island. We observed core material, radiocarbon dating, and elemental concentration analysis using an Itrax XRF core scanner of the GSJ, AIST. The recovered Holocene sediments can be divided into lower and upper layers, separated by a boundary at −0.45 m T.P. The lower layer primarily consists of muddy sediments interpreted as inner bay or tidal flat deposits, estimated to have been deposited between approximately 8,000 and 4,000 cal BP. In contrast, the upper layer displays greater vertical heterogeneity in sediment characteristics than the lower layer. It consists of muddy deposits interspersed with accumulations of coral gravel and distinct layers of fine- to medium-grained sand. The presence of mollusks and a reduced accumulation rate suggest alternating episodes of sedimentation and erosion within an estuarine tidal flat environment. A decline in sulfur content indicates that the site was no longer tidally influenced by approximately 1,777 – 1,443 cal BP at the latest.