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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, and measured the electrical conductivity of the pore water. Based on these 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. Because the sea level at the time was not sufficiently high for sea water to penetrate this area, it is speculated that freshwater lakes existed in the Okayama Plain and that the clay was deposited therein. Furthermore, based on its different hardness and the presence of volcanic ash from the Aira Caldera eruption, the clay layer located below the Holocene clay layer was confirmed to be of Pleistocene origin, suggesting that fresh water lakes and marshes were distributed across the Okayama Plain at the time of the last glacial stage. The above reconstructed history of the coastal water environment indicates that fresh water lakes were widely distributed across the Okayama Plain, located in the far reaches of the Seto Inland Sea, from the last glacial stage to just prior to the Jomon Transgression. These results showed that pore water in the sediment of the Okayama Plain retained a low salt content, suggesting a relatively low potential risk of groundwater salinization.

Formation process of the Holocene clay layer in a coastal alluvial plain of an enclosed sea: case study of the Okayama Plain