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The Kuroze Formation in the upper part of the Miocene Hokusetsu Subgroup contains several conglomerate beds with channel structures. Based on thin section observations, the conglomerate clasts are mainly volcanic rock and chert, with minor tuff, tuff breccia, granitoid, quartzite, siliceous mudstone, mudstone, sandstone, and mylonite. Radiolarian fossils in the siliceous mudstone clasts indicate a Middle Jurassic age. In the lower part of the succession, sediments were supplied from the Ryoke granitic and metamorphic basement rocks, whereas in the upper part of the succession, they were derived from more extensive Jurassic and older strata.

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