靜岡県島田・掛川市付近の第三系とその浮遊性有孔虫化石群

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  • シズオカケン シマダ カケガワシ フキン ノ ダイ3ケイ ト ソノ フユウセイユウコウチュウ カセキグン
  • Tertiary Stratigraphy of the Kakegawa District, Central Japan and its Planktonic Foraminifera

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Since the stratigraphy of the Kakegawa District, Shizuoka Prefecture was first undertaken by Nakajima (1886) in his geological map "Shizuoka", many authors have contributed to the geology and paleontology of the region. Among them, Makiyama in particular has contributed to the geology, sedimentology and paleontology of the Kakegawa District. In this region, Makiyama distinguished two horizons of Lepidocyclina, one in the early and the other in the middle Miocene, the latter representing the presumed youngest horizon within the Tertiary deposits of Japan. He also recognized that the diastrophism during the Miocene in this region exceeds that found in other regions of Japan. To determine the precise horizons of the Lepidocyclina and their respective geological ages, the writer made a micropaleontological and stratigraphical study of the Kakegawa District. The present study also aims to find whether the diastrophic movements in the Kakegawa District can be correlated with similar movements in other parts of Japan. The statigraphical subdivisions of the Teritary deposits in the Kakegawa District and their biostratigraphical characteristics are shown in Plate 3, and briefly outlined below. The Mikura and Setogawa groups, mainly of an alternation of hard, medium to fine grained sandstone and siltstone reveal a complex isoclinal structure, but as a whole with NE-SW trend. Being intensely disturbed by folding and faulting the detail stratigraphical sequence of those two groups is difficult, and since fossils are very rare and not well preserved when found, determination of the geological age of those groups remains uncertain. The Ooigawa group, which comprises the Seto (lower) and Horai (upper) formations was previously considered to be early Miocene in age. The Seto, mainly of massive hard black siltstone is in fault contact with the older Setogawa group and unconformably overlain with the Horai formation. The Horai comprises conglomerate, sandstone and an alternation of sandstone and siltstone. It lies unconformably upon both the Setogawa group and the Seto formation. The planktonic Foraminiferas from the middle part of the Horai formation (Table 2) may be assigned to a part of the Globigerina ampliapertura Zone (Bolli, 1957) and be referred to early Oligocene or Lattorfian in age. Consequently, the Ooigawa group ranges from Eocene to early Oligocene in age. Accordingly, the Miocene age of the Ooigawa previously settled upon lithological similarity with other regions in Japan must be rejected. Early Oligocene deposits determined by certain paleontological evidence is only the Horai formation in Central Japan. Between the Ooigawa and Mikasa groups, three planktonic foraminiferal zones are missing and structurally, the former is more complex than the latter. Although direct relationship of the two groups cannot be observed in the field, the Mikasa is unconformably superjacent to the Ooigawa. The Mikasa group comprises five formations consiting of sandstone, siltstone, mudstone and green tuff. The stratigraphic hiatus previously reported between the Matsuba and Tozawa formations (a part of the writer's Shinzaike green tuff) is proved to be a mere assumption and is rejected (Figs. 3, 4, Chart 7). Further, four planktonic foraminiferal zones are recognized within the Mikasa group. Lepidocyclina makiyamai, which was taken to represent the middle upper Miocene (Vindobonian) by Morishima (1949) was collected from the Shinzaike formation (G. insueta/G. triloba Subzone). The lower half of the Mikasa group (Amakata to Matsuba and a part of the Shinzaike formation) is correlated with the Akahira group (Ujiie and Iijima, 1959) or Hikokubo group (Kanno, 1958) in the Chichibu Basin, Kanto Mountainland on the basis of planktonic Foraminifera. The upper half of the Mikasa is equivalent with the Oganomachi group in the Chichibu Basin and the lower part of the Sakuma group in Boso Peninsula. From the evidence of planktonic Foraminifera and the correlation above, the horizone of Lepidocyclina makiyamai is early Miocene and not middle upper Miocene as maintained by Morishima. Volcanic activity of basic rocks and intrusion of Tertiary serpentine are observed in the Mikasa group. The Kakegawa group lies unconformably upon the Mikasa goup with NW-SE general structural trend, and on the basis of its foraminiferal fauna described elsewhere by the writer (Saito, 1960), the Mio-Pliocene boundary falls within the Uchida formation, which is the middle division of the Kakegawa group. Active tectonic movement occurred between the Ooigawa and Mikasa groups, and between the Mikasa and Kakegawa groups in the present region. Particular study was made on the planktonic Foraminifera of the Ooigawa and Mikasa groups, the procedure and results are beiefly outlined in the following lines. Owing to the complex structure of the Ooigawa group sampling could not be done systematically, therefore only rocks judged to contain Foraminifera were collected and analysed. However, in the Mikasa group, rock samples of 400g. each were collected from each locality and from the following stratigraphic interval along three sampling routes (Chart 6, pls. 4-6). Towata formation ; intervals of an average of 50 meters Matsuba formation ; intervals of an average of 50 and in part of 25 meters Saigo formation ; intervals of an average of 25 and in part of 10 meters. When exposures were not found or when the rocks were too hard for treatment, sampling was not done. Rock samples were collected from quadrangles of 1 meters by 0.05 meters parallel to the bedding plane in the manner shown in Fig. 7. In treatment of the samples, first the samples from each locality were weighed to certain grams in dry condition in proportion to their foraminiferal contents. The rocks from the Ooigawa and Mikasa groups are well indurated and therefore they were treated with Glauber's salt hard rock maceration method. Three methods of rock maceration were employed according to the rock type, and the procedure is described in the text. After treatment with Glauber's salt, the sample was washed through a 200 mesh screen and then dried. The tests of the Foraminiferas contained in the original sample were picked up from the residue on the screen and then counted. The Foraminifera from each locality was divided into planktonic, benthonic calcareous and benthonic arenaceous (with siliceous test) forms. All planktonic forms were identified (Table 2, Chart 3). Considering from the stratigraphic distribution of the Foraminifera, the recognized five planktonic foraminiferal zones in the Kakegawa Tertiary deposits were found to have the same sequence as originally established in Trinidad and Venezuella by Bolli (1957) and Blow (1959). The biostratigraphic and stratigrpahic distribution of the identified planktonic Foraminifera are shown in Chart 4, and the diagnostic faunal elements of each zone are briefly noted, Globigerina ampliapertura zone occurs within the Horai formation of the Ooigawa group. In the Mikasa group, four planktonic foraminiferal zones are recognized, namely, Catapsydrax dissimilis-, Catapsydrax stainforthi-, Globigerinatella insueta/Globigerinoides triloba-, Globigerinatella insueta/Globigerinoides bispherica-Subzones. Three zones between the G. ampliapertura and C. dissimilis zone are missing in this region. On the basis of partial distribution of planktonic Foraminifera in the Towata formation (Chart 6), it is discussed ecologically that a gulf-1ike structure of the Mikasa sedimentary basin was already in being at the time of Towata deposition. Systematic descriptions of these Foraminifera will be published elsewhere at another occassion.

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