Research into the origin and geological history of the Lake Hamana

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In order to elucidate the depositional process of the Holocene sediments distributed in and around Hamana Lake, we have started a multidisciplinary research project. As a part of this project, four boring core samples were collected from four sites in the lake. Cores of 85H-1(-12m in depth), 85H-2(-5m in depth), 85H-3(-5m in depth), and 86H-4(-3m in depth) are 16.45m, 41.46m, 50.45m and 23.96m long, respectively. The coredd sediments are all Holocene age, and the bottoms of all four holes reached the boundary between Holocene and Pleistocene.The lithofacies of the boring core samples were described in detail. The geological age of the samples were determined by tephrochronological and 14C methods.Oki volcanic ash (ca.9,300 yr BP) was found with thickness of 30mm in 44.58m(H-3). Ah volcanic ash (ca.6,300 yr BP) was also found in 8.98m(H-2), 14.25m(H-1) and 15.75m(H-4) with 10 to 15mm in thickness.A pair of tephras, Os scoria (ca.2,800 yr BP) and Kg pumice (ca.3,000 yr BP) was observed in 6.52 - 6.57m(H-1) and 4.86 - 4.95m(H-4).By using the AMS (Accelerator Mass Spectrometry) technique, many radiocarbon dates were obtained from very small shell fragments and wood chip samples in the core sediments. The 14C data for the bottom of the H-1 core is 7140 140 yr BP at a depth of 28.15m, and for the H-2 core is about 10,000 yr BP at a depth of 46.4m.These data suggest that during the early Holocene there was not just one but two discrete marine transgressions, first before about 8,500 to 8,000 yr BP and the second after 8,000 to 7,000 yr BP, and the marine regression after the highest sealevel occurred rapidly from 4,000 to 3,500 yr BP.