第四紀研究
Online ISSN : 1881-8129
Print ISSN : 0418-2642
ISSN-L : 0418-2642
10 巻 , 1 号
選択された号の論文の2件中1~2を表示しています
  • 町田 洋
    1971 年 10 巻 1 号 p. 1-20
    発行日: 1971/04/30
    公開日: 2009/08/21
    ジャーナル フリー
    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, Km; the middle Kissawa tephras, Ku; 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.
  • 田村 俊和, 三浦 修
    1971 年 10 巻 1 号 p. 21-30
    発行日: 1971/04/30
    公開日: 2009/08/21
    ジャーナル フリー
    Several types of slope deposits were observed in undulating lands near the northern margin of Kitakami Massif. Stratigraphic relations of the deposits were investigated tephrochronologically. The radiocarbon ages of several tephra layers have already been known. The processes by which the slope deposits were formed were studied from their facies, their relations to landforms and lithology, and paleoenvironmental data including pollen analysis. As the result, the following four periods were discerned as 'periods of slope instability' when mass-movements on slope surfaces were more active than at the other periods.
    The period I: It is earlier than the late Pleistocene and might be warmer. Piedmont gentle slopes composed of removed materials of granodiorite were built at the foot of the mountains of the same rock.
    The period II: It is the cooler late Pleistocene when gentle slopes composed of screes were formed, smoothly continuing to or covering terrace surfaces.
    The period III: It is the latest Pleistocene and is subdivided into the subperiod III-1 about 30, 000 years B. P. and the subperiod III-2 about 15, 000 or 20, 000 years B. P.. Palynological data suggest colder climate of those days which is concordant to common knowledge. The same landforms as those of the period II were formed during the subperiod III-1, and derasional valleys were configurated through the subperiod III-1 and III-2. Those shallow concave valleys were terraced and flat-floored valleys were excavated in the Holocene. The morphological evidence suggests the intensification of fluvial action at about the time of the Pleistocene-Holocene boundary. Rocks were deeply weathered on the summits of the residual mountains and tali were formed at the foot of steep slopes during the period III.
    The period IV is in the Holocene and was the least unstable among the four periods of slope instability.
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