徳島県南部の四万十累帯白亜系
スポンサーリンク
概要
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The Cretaceous eugeosynclinal sediments developed in the southern part of Tokushima Prefecture arc assigned to the lower half of the Shimanto Supergroup. They are divided into the Akamatsu, Hinotani, Taniyama, Mugi and Hiwasa Formations in ascending order. In the Akamatsu Formation muddy rocks arc dominant, accompanied with chert and greenstones. The Hinotani Formation conformably overlies the Akamatsu Formation and is rich in sandstone. These two formations form a coarsening- and thickening-upward sequence as a whole. On the basis of bivalve and radiolarian fossils, the two formations are assigned to the Lower Cretaceous. The Taniyama Formation is composed of muddy rocks accompanied with greenstones, acidic tuff and sandstone. It is assigned to the Upper Cretaceous, judging from the radiolarian fossils. The Mugi Formation is composed of muddy rocks accompanied with greenstones. The Hiwasa Formation, which overlies conformably the Mugi Formation, is very rich in sandstone and conglomerate. These two formations also form a coarsening- and thickening-upward sequence. On the basis of Gaudryceras (Vertebrites) sp. and radiolarian fossils, the Mugi and Hiwasa Formations are assigned to the Upper Cretaceous (Santonian-Maestrichtian). The properties of conglomcrates in the Hinotani and Hiwasa Formations were studied in detail at 17 localities. Modal compositions of sandstone were analyzed on 190 samples which were collected from the whole area of study. The properties of these coarse sediments show distinct stratigraphical change. The characteristics of the coarse clastics of the Hinotani Formation indicate that the hinterland was composed mainly of sedimentary rocks, acidic and intermediate volcanic rocks and granitic rocks, whereas those of the Hiwasa Formation indicate that the hinterland was widely covered by acidic volcanic rocks forming a large scale acidic volcanic-plutonic complex. This igneous activity is inferred to be contemporary with the sedimentation of the Hiwasa Formation and to correspond to the late Cretaceous igneous activity in the Inner Zone of Southwest Japan.
- 日本地質学会の論文
- 1981-05-15