グリーンタフ地向斜発生期における陥没と火山活動と沈降運動との関係 : 富山県布施川上流地域の第三系の地質構造
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概要
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At the base of the Nirehara Formation, ranging in age from the latest Oligocene to the earliest Miocene, there is a member of ill-sorted breccias, which abuts unconformably against the Hida Metamorphic Rocks of the basement complex. A zone of concealed sheared rocks, about 100 m wide and apparently produced by faulting, is found in the basement area near the unconformity. The zone, running from north to south, is nearly parallel to the boundary line, i.e. the line of the unconformity, and is overlain unconformably, in some areas, by the Nirehara Formation. These facts suggest that there was, in the initial stage of the Nirehara sedimentation, a fault scarp along the margin of the sedimentary basin, and that the above-mentioned breccia member was being laid down in forms of talus breccias or fanglomerates (or rather piedmont alluvial fans). The breccia member at the base of the Nirehara Formation is overlain conformably by conglomerates with round boulders, which, in turn, are overlain conformably by sandstone member. The Nirehara Formation is succeeded conformably by the Iwaine Formation, which is made up of pyroclastic rocks of the so-called "green tuff formation". In the southern part, where no breccias are found, these members and formations overlap successively the subjacent ones and finally lie on the gently inclined surface of the Hida Metamorphics. These facts suggest that the fault scarp had been worn down when the round-stone conglomerates were being deposited, and that either basin subsidence of sea-level rise succeeded the conglomerate deposition. The sequence of sediments from the base of the Nirehara Formation to the Iwaine Formation reveales a tectonic sequence of, 1) collapse, 2) subsidence, and 3) volcanism. A different scheme of tectonic sequence is proposed by FUJITA et al. (1970), in which, 1) collapse resulting from rising volcanic magma, 2) volcanism, and 3) subsidence occur successively. The relations of the subsidence in this area to the preceding collapse as well as to the following volcanism are not well understood as yet, and are merely noticed in this paper.
- 地学団体研究会の論文
- 1978-07-25
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