<国立科博専報>静岡県西部, 三笠層群の地質学的研究

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Detailed geological map (Fig. 1) of the Mikasa Group is shown strictly according to the lithological classification. Differing from the previous ones (e.g. MAKIYAMA, 1941; SAITO, 1960), the results well indicate the stratigraphic relationship between the component units and also are more practical for applied geology. The group is composed of the following formations in ascending order : Towata Mudstone, Matsuba Siliceous Shale, Kurami Green Tuff, and Saigo Mudstone. Two adjacent formations almost always show conformable or partly, especially around the boundary, interfingering relation and, therefore, the group comprises a single sedimentary cycle. This idea was proposed by UJIIE (1958 and others) and SAITO (1960) opposedly to MAKIYAMA's opinion. In the western part, however, the Saigo Mudstone covers the Matsuba Siliceous Shale with wavy base, above which cobbles derived from siliceous nodules are contained in muddy matrix. These nodules are characteristic in the uppermost part of the Matsuba Formation. Two benthonic formainiferal sequences are obtained from the typical sections across the northwestern wing of a main syncline. The results are summarized by generic composition change (Fig. 3) and show that the so-called arenaceous forms charcteristically predominate in the Towata, Kurami and particulary Matsuba Formations. On the other hand, the Saigo Formation contains abundant calcareous forms. Generally, however, both the types of fauna suggest similarly outer neritic to bathyal environment; there is no significant change of water depth. In Section 1,a sudden change of faunal composition can be recognized between Samples 13 and 14,which were taken respectively just below and above the mentioned unconformity. Compared with the sequence of Section 2,it seems that an interval equivalent to the lower half of the Kurami Formation was lost there, probably because of some subaqueous erosion and/or under no depositional condition judging from the lithology and the benthonic Foraminifera. SAITO (1960) denied the presence of this unconformity on the basis of his planktonic foraminiferal stratigraphy ranging continuously from N.5 to N.8 in term of BLOW's (1969) zonation scheme through the whole Mikasa Group. Since N.7 is represented only by the Kurami Formation (=Shinzakie Formation of SAITO), it becomes that the Saigo (N.8) contacts directly with the Matsuba (N.6). Re-examination of SAITO's results, therefore, suggests also the present unconformity, although it is limited in the western half of the basin.
国立科学博物館の論文
1980-12-01

<国立科博専報>静岡県西部, 三笠層群の地質学的研究

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