大すみ半島北部から宮崎平野に分布する後期更新世テフラ

Abstract

Southren Kyushu has been the region of intense volcanism at least since Pliocene time. One of the most characteristic features is the prevalence of the large-scale pyroclastic flow eruptions which originated from such gigantic calderas as Aira, Ata, Kikai and Kakuto.There exist a considerable number of literature on the stratigraphic sequence and distributions of the pyroclastic flow deposits in South Kyushu. However, relatively small number of reports are available on air-fall tephra deposits, which are useful for establishing Quaternary chronology both of source volcanoes and of marine or fluvial sediments in the coastal regions such as the Miyazaki Plain. In this study, each bed of maker-tephras which erupted during the time from ca. 100, 000 to 25, 000y.B.P., is precisely discriminated and described in the northern part of the Osumi Peninsula, Kagoshima Prefecture first. And then each tephra is traced northeastward along the main axis of distributions to the Miyazaki Plain.Of many tephras, the following four well-dated tephras are used as fundamental timemakers because of their widespread occurence; Ata pyroclastic flows, originated from Ata caldera in 95, 000-90, 000y.B.P. ; Kikai-Tozurahara ash falls, originated from Kikai caldera in 75, 000y.B.P. ; Aso-4 pyroclastic flows, originated from Aso caldera in 70, 000y.B.P.; Ito pyroclastic flows and AT ash, originated from Aira caldera in 22, 000-21, 000y.B.P. Several air-fall tephras from the Aira and Kirishima volcanic centers are identified in detail and roughly dated from their stratigraphic positions between these fundamental maker-beds.About 75, 000-70, 000y.B.P., explosive activity of Aira caldera occurred resulting in the formation of plinian pumice fall deposit, Fukuyama pumice falls, which is found from the Osumi Peninsula to the Miyazaki Plain. During ca. 60, 000-25, 000y.B.P., intermittent eruptions occurred forming five sheets of tephras, of which the Iwato eruption was greatest in producing pumice falls, pyroclastic surges and pyroclastic flows. Iwato pumice falls mantle extensive area from the Osumi Peninsula to the Miyazaki Plain. Cataclysmic eruption occurred from Aira caldera, producing Osumi pumice falls, Tsurnaya and Ito pyroclastic flows and AT ash 22, 000-21, 000y.B.P. Most of these eruptions were accompanied with phreatomagmatic ones.Eruptive history of Kirishima volcano is divided into two stages deduced from the tephra sequence. At ca. 40, 000 y.B.P., older stage of activity started with ejection of relatively felsic pumice falls, Iwaokoshi pumice fall, and graded to more mafic and frequent eruptions, Awaokoshi scoria fall. Younger stage began with the plinian eruption of Kobayashi pumice fall at ca. 15, 000y.B.P.Of many terraces in Miyazaki Plain, Sanzaibaru terrace is the most extensive one and is accompanied with transgressive marine deposits. Stratigraphic relation with tephra sequence shows that Sanzaibaru terrace was emerged before the Ata pyroclastic flow eruption, ca. 95, 000y.B.P., probably indicating the Last Interglacial Stage. Most of terraces younger than Sanzaibaru are of fluvial origin, except for Nyutabaru II and probably III terraces which are partly of marine origin, and are largely devided into two groups, older and younger. Older terraces, Nyutabaru terrace group, formed during the time from the Ata eruption to the Aso-4 eruption, were chracterized by the profiles with more gentle gradient. Younger ones which were chracterized by the profiles with steeper gradient, were formed after the Aso-4 eruption and before the Kobayashi pumice fall. The difference of their profiles reflects the sea level after the maximum stage in the Last Interglacial Age.