福島県太平洋岸,塚原海岸における最終間氷期最盛期の海進海退過程とその時期降下したテフラについて Transgression and Regression of MIS 5.5and Its Related Tephras Recognized at the Tsukabara Coast on Pacific Ocean, Northeast Japan
Marine terraces are well preserved in the coastal region facing the Pacific Ocean of northeast Japan. Their geomorphic development has been studied to clarify the history of sea-level changes and crustal movements. The Tsukabara coast located in the southeast part of Haramachi City, Fukushima Prefecture, has a marine terrace named the Kobama surface. Previous studies concluded that this terrace was formed in the last interglacial period. In the basal part of the Tsukabara Formation, which is marine sediment stratigraphically just below the terrace deposit of the Kobama surface, an air fall tephra consists of fine pumice lapilli was discovered. This study clarifies that this tephra contains mainly a well-vesiculated fiber type of glass shard with distinctive low indices (n=1.495-1.499) and small mounts of phenocrysts (orthopyroxene, hornblende, biotite, quartz, plagioclase). These characteristic properties and the chemical composition of the glass shards show that this tephra is correlated to the Tagashira Tephra (TG), which is distributed in the area from southern Fukushima Pref. to north Tochigi Pref., and also to Yokomori Volcanic Ash (YkA) distributed on the easten base of Adatara Volcano.<BR>Fission track dating and stratigraphic positions of TG and its related tephra suggest that the Tsukabara Formation and the terrace deposit of Kobama surface were formed during marine isotope stage 5.5. The Tsukabara Formation is most likely to be marine bay sediment, which buried a dissecting valley formed in the low stand of sea-level of MIS 6. Judging from the stratigraphic position of TG in the Tsukabara Formation, TG deposited between the low sea-level period when the Tsukabara coast was dry land and higher sea-level period when this area was submerged in the bottom of the bay with a depth of less than 20m, was associated with the transgression from MIS 6 to MIS 5.5. It is most likely that the present altitude of sea-level at the deposition of TG is about 2 m. This estimate and the elevation of the former strandline of MIS 5.5 indicate that the sea level at the deposition of TG was 28m lower than that at the culmination of MIS 5.5. On the basis of highresolution chronostratigraphy of Martinson et al.(1987), the age of TG is estimated to be135-125ka.<BR>After the deposition of TG, the sea invaded landward, resulting in the formation of a bay, with continuous deposition of fine marine silt of the Tsukabara Formation. The regression from MIS 5.5 to MIS 5.4 took place after the culmination of MIS 5.5, associated with the emergence of the Kobama surface. Subsequently, Ad-D tephra estimated to be 125-100ka was deposited. At that time sea-level was at least 17m lower than that of the MIS 5.5 culmination. Paleosealevel of MIS 5.3 is estimated to be less than-8m from the present altitude.