ESTIMATION OF SEDIMENTATION RATE AND FRESH-SALINE ENVIRONMENT IN A COASTAL ALLUVIAL PLAIN, USING BORING CORES OF ALLUVIUM IN THE CENTRAL PART AREA OF SETO INLAND SEA, JAPAN

Abstract

To estimate the sedimentary environment between the last glacial stage and the Holocene epoch of an enclosed sea in the Okayama Plain, which constitutes a portion of the Seto Inland Sea coast, we collected 4 boring cores at depths of 6 to 19 m, performed radiocarbon dating of the organic matter contained in the clay, identified the volcanic glass from the Aira Caldera eruption, and measured the electrical conductivity of the pore water. Based on these dating analyses, it was evident that the Holocene clay layer that is widely distributed throughout the Okayama Plain had already accumulated prior to 10,000 years ago. In addition, the different hardness and the presence of Aira volcanic ash in the clay layer indicated the sedimentation since Pleistocene. Because the sea level at the time was lower than this clay sedimentation level, those facts suggest freshwater lakes existed in the Okayama Plain which was dammed up by the shallower granite basement in the mouth of Kojima Bay. Based on the long clay core and date for the last 30,000 years, the sedimentation rates were estimated to be 0.18 mm/ year before 8,100 years ago and 1.19 mm/year for the last 8,100 years, respectively. In the sea water rising after 7,000 years ago, the sea level exceeded the dam and the bay became a saline condition.