Tephrochronogical Study in South Kanto

Part I Stratigraphy of tephra and its relation to the history of volcano

Abstract

It is well known that the pyroclastic-fall (tephra) layers are the best key beds for regional geologic chronology. South Kanto is a good field for Quaternary tephrochronology in Japan, since there exists a vast amount of tephra mainly supplied from the two volcanoes, Mt. Hakone and Mt. Fuji to the east, covering upon and interbedding wihtin Quaternary sediments.
There exists a considerable number of literature on the tephrochronology around Tokyo and Yokohama about 100 kilometers away from the source volcanoes. However, so many tephra beds there consisting of weathered and yet thinner fine ashes with similar brown loamy appearance make it extremely difficult to discriminate some definite time-markers of tephra.
As one goes nearer to the source volcanoes, coarser and thicker tephras are expected. In this connection, the best type field is placed in the Ooiso hills, adjacent to the Hakone volcano, where a great number of time-markers is easily distinguished even to the naked eyes and several Quaternary sediments are developed.
In this study, each sheet of time-markers which fell after the marine Shimosueyoshi or Kissawa formation was constructed, is precisely discriminated at the type field, Kissawa, eastern part of the Ooiso hills. And then each is traced and correlated eastward along the main axis of distribution to the environs of Tokyo which have been a standard Quaternary succession in Japan (Fig. 1, 2, 4, Table 1, 2, 3).
As the first part of this study, the stratigraphy of tephra and absolute ages of several time-markers are discussed in this paper together with the history of the source volcanoes. Former interpretations as to the development of the volcanoes and the marine terraces in this area would be improved considerably.
1) The tephra layers of South Kanto were formerly divided into four groups; Tama, Shimosueyoshi (Kissawa), Musashino, Tachikawa tephras. Time gaps during the accumulation of each group were estimated by soil profiles and unconformable relations to about 10, 000 years or more in length. But few data concerning large breaks of tephra falls are found at every boundary in the Ooiso hills and its neighbourhood. A buried soil profile and unconformity are inferred not to indicate a great stratigraphic hiatus but possibly to show a slow accumulation and to be apparent features revealed in the air-laid tephra deposits upon slopes and at the scarps of terraces. In addition, some absolute ages measured by Carbon-14 and Fission track datings indicate that the tephra was supplied rather successively through the late Quaternary (Fig. 10). The longest break of tephra accumulation might be less than several thousand years.
2) The Younger tephra groups consisting of the Musashino and the Tachikawa tephra are roughly estimated to have successively accumulated from about 60, 000 years to about 10, 000 years B. P. This estimation is based upon the Fission track age of the Tokyo pumice (TP) and the C-14 ages of the several horizons of the Tachikawa loam (Fig. 10). And the underlying Shimosueyoshi or Kissawa tephra layers are estimated to have successively fallen from about 130, 000 or 120, 000 years B. P. to about 60, 000 years B. P. based upon the Fission track datings of obsidian and zircon included in the several pumice beds (Fig. 10). This result seems to coincide the stratigraphic facts that the total volume of the Shimosueyoshi tephra is about the same as that of the Younger one (Fig. 2) and that some short stratigraphic hiatuses are found at several horizons such as the boundaries of the three subgroups (K₁; the lower Kissawa tephras, K_m; the middle Kissawa tephras, K_u; the upper Kissawa tephras, Fig. 6A, B). The above age of the lower limit of the Shimosueyoshi tephra layers must suggest that the Shimosueyoshi transgression was dated back to the past a little older than the former concept.