第四紀研究
Online ISSN : 1881-8129
Print ISSN : 0418-2642
ISSN-L : 0418-2642
14 巻 , 2 号
選択された号の論文の4件中1~4を表示しています
  • 大森 博雄
    1975 年 14 巻 2 号 p. 63-76
    発行日: 1975/06/30
    公開日: 2009/08/21
    ジャーナル フリー
    Erosional low-relief surfaces on the mountains of the Oshima Peninsula are divided into two levels. The higher surface is called the Matsumae surface and the lower one is called the Kikonai surface in the Matsumae Peninsula of the southern part of the Oshima Peninsula. Coastal terraces spread below and around these erosional low-relief surfaces. The Esashi district is one of the areas where coastal terraces are typically developed in the Oshima Peninsula. The author discussed the physiographic development and the crustal movement of the Esashi district.
    The coastal terraces in the Esashi district are classified into five surfaces as follows:
    Esashi surface (Pleistocene); 340-140m a. s. l., 1500m wide.
    Oyama surface (Pleistocene); 170-80m a. s. l., 2000m wide.
    Ohma surface (Pleistocene); 80-40m a. s. l., 500m wide.
    Jinya surface (Pleistocene); 45-20m a. s. l., 250m wide.
    Kitamura surface (Holocene); 12-8m a. s. l., 100m wide.
    The Esashi surface is presumed to have been formed by the transgression over the Kikonai surface which was the lower erosional low-relief surface.
    The Oyama surface is the widest marine erosional surface in the Esashi district. This terrace was formed by the transgression over the Esashi surface which was gently sloping toward the sea.
    The Ohma surface was formed by the transgression with larger than 20m rise of the sea level. The coast at that time, however, was a ria coast.
    The Jinya surface was formed about 30, 000 years B.P. At that time, the rise of the sea level was larger than 7m. It is suggested that the minor transgression took place at the Würm interstade about 30, 000 years B.P.
    The Kitamura surface was built by the Postglacial transgression with larger than 50m rise of the sea level. A part of the Kitamura surface is presumed to be a marine terrace formed by a minor transgression about 3, 000 years B.P.
    From the height of the old shorelines of these terraces, the crustal movement of the tilting toward the north in the area of Palaeozoic sedimentary rocks, the warping in the area of Miocene sedimentary rocks in the Esashi Hill of the northern part of the Amano River and the tilting toward the south in the Kaminokuni Hill of the southern part of the Amano River are recognized. The crustal movement is presumed to become relatively active after the period between the formation of the Oyama terrace and that of the Ohma terrace.
  • 町田 洋, 松島 義章, 今永 勇
    1975 年 14 巻 2 号 p. 77-89
    発行日: 1975/06/30
    公開日: 2009/08/21
    ジャーナル フリー
    The tephrochronological study of the eastern foot of Mt. Fuji (Fig. 1) would provide abundant data for analyzing not only eruptive history of Fuji volcano but also other late-Quaternary events, because there lies a large quantity of tephra derived mainly from the volcano. Several marker pumice layers are found sandwiched within great numbers of scoria sheets and are correlated with those in south Kanto, where stratigraphic position and radiometric age of them were well established (Table 1, 2 and Fig. 3). The revised stratigraphy and chronology in this area should play significant role in discussing several Quaternary problems around there.
    1) Mt. Fuji volcano became active ca. 80, 000 y. B.P., when the most important marker pumice, the Ontake Pm-I, showered. Since then, the eruption of “Older Fuji stage” had occurred more or less continuously until ca. 10, 000 y. B. P. without any significant periods of quiescence. The Older Fuji tephras are estimated at approximately 250km3 in volume, which are distributed extensively in south Kanto (Fig. 6). These explosive activities ended at about 10, 000 y. B. P. and there followed a long quiescent period, succeeded by “Younger Fuji stage”.
    2) The fluvial formation formerly called “Suruga Gravels” is a deposit of the ancient Sakawa river ca. 80, 000 y. B. P. The ancient river took its rise in Tanzawa mountains and might have flowed west- or southwestward into Suruga bay, instead of Sagami bay, the present course flows into. The change in river course appears to have taken place during the period from 80, 000 to 60, 000 y. B. P., associated with the growth of the volcanic edifice of Mt. Fuji (Fig. 5).
    3) The Kannawa fault, one of the most important tectonic lines in central Japan, runs east to west along the northern margin of the area. It branches westwards into two faults; Kn and Ks. Along the Kn fault a thrust is found dipping northward, whereas along most of the course the fault plane is nearly vertical (Fig. 9). The movements of the both faults are younger than the Suruga Gravels, which is displaced vertically more than 50 metres during the last 80, 000 years.
  • 秋山 純
    1975 年 14 巻 2 号 p. 91-106
    発行日: 1975/06/30
    公開日: 2009/08/21
    ジャーナル フリー
    A submarine late Quaternary succession off the west coast of the Okinawa Island disclosed by anchor hole drilling and test boring is described (Figs. 1, 2). The late Quaternary sediments constituting the sea bottom have an average thickness of ca. 12m, being represented mostly by highly calcareous sands more or less rich in gravel-sized fragment of reef-building corals. Simplified biofacies analysis reveals that these sediments can be subdivided into several major facies (Figs. 4, 7) which in turn correspond in a rough approximation to the stratigraphic subdivision (Fig. 10, units A-C). The upper two units, A and B, are both assigned to Holocene on the basis of their stratigraphic position, high carbonate content and low L/H ratio (ratio of low-Mg calcite to high-Mg calcite) (Fig. 8; Tabs. 2, 3). The 14C dates prove that this part of the succession has resulted from the Flandrian (Jomon) transgression since early Holocene (Tab. 4, samples 1, 3-7). The lowest unit C can be tentatively assigned to latest Pleistocene, probably Bölling to Alleröd interstades, as inferrid from the 14C date (Tab. 4, sample 8), the position of near-reef facies more distant from the present shoreline, the presence of finer, back-reef sediments, the low carbonate content and high L/H ratio (Fig. 8). No distinct unconformity has been found between the units B and C and this stratigraphic relation remains open to future studies. From a further discussion on the age of the buried topographic surfaces covered by the late Quaternary sediments, early Würmian age is suggested for the lowest buried surface (-50 to -55m below sea level).
  • 田村 俊和, 片山 〓五郎
    1975 年 14 巻 2 号 p. 107-114
    発行日: 1975/06/30
    公開日: 2009/08/21
    ジャーナル フリー
    The previously circulated views on the stratigraphy of terrace deposits along the southeastern coast of the Kii peninsula are amended and the periods of slope instability in the same area are discussed in this paper.
    Sequence of coastal terraces with stratigraphy of terrace deposits is summarized in Fig. 6. Sealevel change including crustal movement, which may be continuous uplift with tilting to the north, as shown in Fig. 7 is derived from the sequence. Three major transgressions in the middle and late Pleistocene are noticeable. The Shingu Gravel indicates the development of alluvial fan in the age just prior to the maximum of the regression which precedes the transgression exhibited with the Kumanoura formation.
    Old debris-flow deposits are laid on a part of dissected piedmont gentle slopes and colluvio-fluvial deposits fill small valleys dissecting the terraces. Stratigraphic relation of such slope deposits to terrace deposits suggests the periods when a lot of slope deposits are produced, namely the periods of slope instability. Increase of fluvial deposits as the Shingu Gravel may also reflect such a condition. The periods of slope instability in the area seem to coincide with the ages of low sealevel.
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