<国立科博専報>小笠原・母島の地質 : とくにその層位学的研究

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Haha-jima (Hillsborough Island) located at about 1,000 km south of Tokyo has long been famous geologically ever since YOSHIWARA reported the occurrence of Nummulites in so early days as in the 1900's. Although several short papers on the Eocene fossils were published by such authors as YABE, HANZAWA and IWASAKI, stratigraphy of the whole island has remained obscure. This paper gives an outline of the geology of Haha-jima and, from the viewpoints of larger Foraminifera and planktonic Foraminifera, the island's stratigraphy is outlined for the first time. Haha-jima, elongated in the NNW-SSE direction, is made up mainly of volcanic breccia and lava of two-pyroxene andesite, partly basaltic, accompanied by a number of dykes of similar rocks. These volcanic layers are gently dipping or warping; the stratigraphic continuity is interrupted by a number of faults, the major ones of which seem to run obliquely to the outline of the island. Pillow lava is scarcely developed, differing from Chichi-jima at about 50 km north of Haha-jima, in the same Bonin Islands. This difference may be due to that the volcanics of Chichi-jima are more basaltic, being composed of "boninite" which is a sort of basalt bearing rich bronzite phenocrysts, and that lavas of Haha-jima may have flowed over not muddy but sandy seabed at shallower depth. Marine fossil-bearing sediments which are intercalated in these volcanics are distributed in the southern half of the island and can be divided into three formations, i.e., Yusan Formation, Okimura Formation and Sekimon Limestone in ascending order stratigraphically, from south to north geographically and also lithologically. The Yusan Formation is composed of volcanic breccia, tuff breccia and medium- to very coarse-grained sandstone and in many places overlies authobrecciated lava. The formation abounds in huge tests of Nummulites boninensis HANZAWA. The Okimura Formation, probably conformably overlying the Yusan, is predominantly coarse-grained sandstone which is sometimes composed essentially of foraminiferal shells as seen in "Rosu-ishi", utilized as a building stone before World War II. Conformably resting on the Okimura, the Sekimon Limestone is developed in the north, particularly east of Mt. Sekimon where a local karst topography is recognized. Major part of the limestone is oolite-bearing calcarenite and contains abundant Pellatispira rutteni. 21 species (including five new species) of larger Foraminifera belonging to 10 genera are distinguished and their occurrences in the three formations are shown in the form of a occurrence range chart (Fig. 3). Referring to known occurrence in the world of some characteristic species and of their allies, the Sekimon and Okimura Formations are correlated with the Biarritzian or uppermost Lutetian, or with the oldest Tertiary b if the Yusan Formation is included. Rich planktonic foraminiferal assemblages in the Okimura Formation make it possible to correlate the formation with the Orbulinoides beckmanni Zone, the upper second of five zones in the Lutetian. As a result, both kinds of Foraminifera indicate nearly the same age, namely, late Middle Eocene. Such a combined biostratigraphy must contribute greatly to the adjustment between two sorts of foraminiferal stratigraphy, when their paleontological details are published in the near future as a separate paper written in English. The age-assignment obtained thus seems to be one step older than a K-Ar age-assignment as 40.1 m.y.BP (Late Eocene, Pribonian) for a two-pyroxene andesite from the northernmost portion of Haha-jima (KANEOKA et al., 1970). This discrepancy may be due to the rock samples for the radiometric measurement, whether they were taken from a stratigraphically higher position (higher toward the north of Haha-jima in general trend) or from dyke rock of later intrusion. In any case, it must be mentioned that the late Middle Eocene is the oldest age for the basement of the Izu-Mariana Arc and even of the Phil
国立科学博物館の論文
1977-12-20

<国立科博専報>小笠原・母島の地質 : とくにその層位学的研究

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