かん国うつ陵島のテフラ

Abstract

A trachytic tephra fall deposit, the Ulreung-Oki ash, was first discovered at the Torihama archaeological site, in the northern Kinki district, where it forms whitish ash layer in strata bearing artifacts of the earliest Jomon ceramic culture. Occurrence of alkali feldspar and specific amphibole (probably kaersutite) clearly shows that this tephra is alkalic in composition. Correlatives of this tephra have been recognized at many localities in central Honshu and in piston cores from adjacent to Ulreung island to Oki bank. The grain-size distribution clearly suggests the source to be Ulreung volcano.This paper mainly deals with late Quaternary eruptive history of Ulreung volcano based on tephra studies in Ulreung island.Ulreung or Dagelet island is a large Quaternary stratovolcano situated in the west Japan Sea, 130km off the eastern coast of the Korean peninsula. Though the volcano has been considerably dissected, it has a remarkable caldera with a longer diameter of 3.5km and a central cone. The caldera has had a series of silicic tephra eruptions, forming a sequence of airfall and airflow deposits in late Quaternary. At least seven coarse-grained airfall units are discriminated and named from upper to lower as U-1, U-2, …and U-7.Approximate dating of these tephras is possible because the two time-marker tephras of Kyushu origin are found within the soils between the pumice falls: the Aira-Tn ash (AT, 21, 000-22, 000 YBP) between U-4 and U-5, and the Kikai-Akahoya ash (K-Ah, 6, 300 YBP) between U-1 and U-2.These Ulreung tephras are trachytic in composition, containing considerable amounts of alkali feldspar, kaersutite, biotite and clinopyroxene as phenocrysts. Chemical composition of pumiceous glass for these tephras is completely different from that of such subalkalic tephras as AT and K-Ah, etc. However, compositional variation from layer to layer is too small to be distinguished each other.In the late Quaternary age before ca. 22, 000 YBP, many plinian type eruptions took place producing such pumice falls as U-5, 6 and 7, which are not well preserved in the island due to erosion but in abyssal sediments in the Japan Sea. The three pumice falls, U-2, -3 and -4, recognized between the AT and K-Ah ashes were respectively formed by plinian, phreato-plinian and plinian eruptions. The last two are associated with pyroclastic flow deposits. This multicyclic explosive volcanism should result in the formation of caldera.Of the younger three tephras, U-2 seems to represent the largest recent eruption of the caldera. Distribution and stratigraphic position of this tephra indicates that U-2 can be correlated with the U-Oki ash. Three radiocarbon dates of ca. 9, 300 YBP were obtained at the Torihama site and south port of Osaka for this ash. U-Oki or U-2 is used as an excellent time-marker in analyzing paleo-sea level and paleo -oceanographic environments in the Japan Sea.The youngest activity of Ulreung volcano took place after 6, 300 YBP, forming the U-1 scoria falls, Al-bong cinder cone and lava flows.