伊豆半島の地質学的諸問題

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As the Tertiary strata developed at the Izu Peninsula, Shizuoka Prefecture, Central Japan, mainly consist of the pyroclastic rocks and are covered widely by the younger volcanic ejecta, many arguments concerning the stratigraphic sequence, geological structure and correlation have hitherto been presented. To clarify these geological problems the writers made detailed stratigraphic studies on the Tertiary strata developed in the central part of Izu Peninsula. The purpose of the present article is to describe and discuss on these problems. To avoid the confusions of the stratigraphie sequences the revised formation name and its definition are newly proposed. The Tertiary strata distributed in the central part of Izu Peninsula were classified as described below in ascending order : Ookawabata Propyrite : - Dark green propyrite, homogeneous, hard lithic volcanic breccia and lithic tuff breccia. This corresponds to the lower part of Sawamura's (1955) Yugashima Propyrite. Kaden Formation : -Comprised of stratified green tuff at the type locality. This formation consists mainly of an alternation of dark green andesitic tuff breccia and tuffaceous sandstone in the lower part, hornblende bearing dacite and its tuff in the middle part and an alternation of green patch bearing pale green or white tuff and gray to black siltstone in the upper part. Molluscs, brachiopods or larger foraminifers are contained in the calcareous sandstone or impure limestone in the lower part. Nephrolepidina japonica (Yabe) and Miogypsina kotoi Hanzawa were collected from the impure limestone developed at Nashimoto and stylolites are frequently observed at that locality (Mitsui, 1967). From the siltstone of the upper part numerous planktonic foraminifers such as Globoquadrina dehiscens (Chapman, Parr and Collins), Sphaeroidinellopsis sp., Globigerinoides sp., etc. were collected. This formation conformably overlies the Ookawabata Propyrite and partly inter-fingers with it. Shimoshiroiwa Formation : - This formation corresponds to Sawamura's Shuzenji Tuff (1955) but as its main part consists of an alternation of fossils bearing tuffaceous sandstone and pumice bearing sandy tuff, the pumice bearing massive dacitic white tuff is separated from the main part as the Shuzenji Tuff Member. From the calcareous sandstone and tuffaceous sandstone numerous larger foraminifers such as Nephrolepidina japonica and Amphistegina radiata (Fichtel and Moll) were collected in association with the planktonic foraminifers such as Globigerina nepenthes Todd, G. woodi Jenkins, Globigerinoides bollii Blow, G. obliqua Bolli, Globoquadrina altispira (Cushman and Jarvis), G. dehiscens (Chapman, Parr and Collins), Globorotalia lenguaensis Bolli, G. menardii (d'Orbigny), Sphaeroidinellopsis subdehiscens (Blow), etc. Based upon these planktonic foraminiferal assemblages it is proved that the present foraminiferal assemblage is evidently that of the Globorotalia mayeri/ Globigerina nepenthes zone. However, in the majority of cases in Japan the formations in which Nephrolepidina japonica is found, are correlated with the Globigerinatella insuetal Globigerinoides bisphericus zone, which is much lower than that of G. mayeri/ G. nepenthes. Stratigraphically the Shimoshiroiwa Formation overlies the Kaden Formation with conformity. Therefore, it is believed that the stratigraphic situation of the Shimoshiroiwa Formation is the same as the Nephrolepidina japonica bearing formations from the other localities in Japan. The above described Ookawabata Propyrite, Kaden Formation and Shimoshiroiwa Formation represent the lowest part of the Izu Peninsula strata and are called the Yugashima Group. Nekko Volcanics and Aoichi Volcanics : -The former corresponds to Sawamura's (1955) Nekko Green Andesite and consists of dacitic to andesitic volcanic ejecta distributed at the southern part of Shuzenji-machi and southern part of Amagi-Yugashima-machi. The latter corresponds to Sumi's (1958) Suzaki Andesite and consists of andesitic to dacitic volcanic ejecta distributed at the southern part of Kawazu-machi and southwestern part of Shimoda-machi. Magome Pyroclastics Member : -Comprised of rhyolitic tuff breccia or dacitic tuff breccia with an irregular alternation of tuffaceous sandstone and sandy conglomerate. This member interfingers with the middle to lower parts of the Aoichi Volcanics. Horikiri Rhyolite Member : -Comprised of rhyolite lava and tuff breccia. This member inter-fingers with the middle to lower parts of the Aoichi Volcanics, Magome Pyroclastics Member and Ookamo Tuff Member. Ookamo Tuff Member : -Comprised of perlitic, rhyolitic to dacitic pumice tuff or tuff breccia. The rock facies of this member resemble those of the Shimoshiroiwa Formation and Shuzenji Tuff Member but its geological horizon is uncertain. Umenoki Tuff : -Comprised of andesitic to dacitic tuff breccia or pumice tuff with partly andesite and dacite lavas. The main part of this tuff corresponds to Sawamura's (1955) Ohmi White Tuff. Some molluscs and brachiopods which correspond to those of the Shirahama Formation in the vicinity of Shirahama, Shimoda-machi, were collected from the conglomerate cemented by calcareous sandstone. Among the molluscs Chlamys shirahamaensis (Nomura and Niino), Aequipecten vesiculo-sue (Dunker), Mizuhopecten planicostulatus (Nomura and Niino), etc. are the characteristic species. Shirahama Formation : -Comprised of an alternation of tuffaceous sandstone, pale yellowish green tuff, tuff breccia, tuffaceous to calcareous coarse-grained sandstone and tuffaceous siltstone. Conspicuous cross-bedding is well developed. From the calcareous sandstone or tuff breccia numerous molluscs such as Amussiopecten praesignis (Yokoyama), Chlamys (Mimachlamys) satoi (Yokoyama), Chlamys shirahamaensis (Nomura and Niino), Aequipecten vesiculosus (Dunker), Comptopallium izuensis (Nomura and Niino), etc. were collected in association with bryozoans, brachiopods, crustaceans, echinoids, etc. Also numerous planktonic foraminifers such as Globorotalia crassaformis (Galloway and Wissler), G. inflata (d'Orbigny), G. menardii (Parker, Jones and Brady), Globigerina bulloides d'Orbigny, Globoquadrina altispira (Cushman and Jarvis), etc. were collected in association with some benthonic foraminifers. As the present formation consists of acidic tuff and tuffaceous sandstone, its stratigraphical position had frequently been confused. Therefore, with regard to the geological age and distribution of the Shirahama Formation many arguments have hitherto been presented. However, as the molluscan shells from the vicinity of the type locality correspond evidently to those from the Dainichi, Tonohama and Takanabe formations, it can be considered that the geological age of the Shirahama Formation is Early Pliocene. Also from the planktonic foraminiferal assemblages it is apparent that the geological age of the formation is Early Pliocene. But as these fossil data are obtained from the middle part of the Shirahama, it may be considered that the geological age of the Shirahama Formation may extend from the Late Miocene to Early Pliocene. Yokoyama Siltstone : -Comprised of an alternation of bluish gray siltstone and fine-grained sandstone. This siltstone nearly corresponds to Sawamura's (1955) Jyo Formation and is restricted in its distribution to a narrow area. Molluscan shells and foraminifers are contained in the siltstone but the characteristic species for indicating the geological age are unknown. Oono Conglomerate : -This conglomerate corresponds to Sawamura's (1955) Ohno Formation and unconformably overlies the Yokoyama Siltstone. The present one comprises pebble to cobble andesite and tends to become larger in size upwards. Neo Volcanics : -Comprised of volcanic ejecta from the Late Pliocene to Recent. The volcanics are restricted in their distribution to the northern region of the Izu Peninsula. From the accounts above mentioned it is noticed that the general stratigraphical characteristics of the Yugashima Group represented by the green tuff are much related with those of the backbone area of Northeastern Honshu and also it can be considered that the Yugashima Group may have been deposited under nearly the same environmental conditions as those of the backbone area of Northeastern Honshu in the Early Miocene Time. The trend of the main geological structure of the present area is usually N 30°E. Five parallel anticlines are observed and the lowest Ookawabata Propyrite occupies the core of the anticline axis. Also the main faults extend N 30°E and N 50-70°W. The N 30°E fault may be related with the Tanna Fault and the N 50°-70°W fault may be related with the linear arrangement of the Daruma, Tanaba and Amagi Volcanoes.
東北大学の論文
1969-08-25

伊豆半島の地質学的諸問題

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