The Mio-Pliocene is widely distributed in the Higashikubiki district of Niigata Prefecture, which is located in the northernmost area of the Fossa Magna. The Mio-Pliocene, attaining to more than 3,000 meters in total thickness, is lithologically divided into seven formations, that is, the Matsunoyama, Taruda, Sugawa, Tamugikawa, Shobu, Higashikawa, and Naradate formations. Each of them is mainly composed of the lithofacies as follows respectively, that is, the Matsunoyama Formation is of alternating beds of dacitic tuff breccia and tuff with dark grey mudstone, the Taruda Formation of alternating beds of sandstone and mudstone rich in mudstone, the Sugawa and Shobu formations of massive mudstone, the Tamugikawa Formation of alternating beds of sandstone and mudstone, and thick-bedded sandstone, the Higashikawa Formation of sandy siltstone, and the Naradate Formation of alternating beds of conglomerate, sandstone, and siltstone with volcanic rocks in the top. They have generally the conformable relationship each other. The tephra marker beds show the flysch-type alternation of the Tamugikawa Formation changes laterally into mudstone of the upper part of the Sugawa Formation and the Shobu Formation. From F. T. ages of volcanic tuff layers and foraminifera data, each formation is correlative with the Neogene strato-type of the Niigata sedimentary basin as follows, that is, the Matsunoyama and Taruda formations with the Upper Miocene Teradomari Formation, the Sugawa Formation with the Upper Miocene Shiiya Formation, the Tamugigawa Formation with the Lower Pliocene Hamatsuda Formation, the Shobu and Higashikawa formations with the lower part of the Plio-Pleistocene Nishiyama Formation, and Naradate Formation with the Lower-most and Lower Formation of the Plio-Pleistocene Uonuma Group. It is considered that the sedimentary environment changed as follows, that is, from the lower bathyal basin plain in the Late Miocene, through the upper bathyal trough and slope in the Early Pliocene and the littoral shelf in the Middle Pliocene, to the neritic and partial terrestrial environment in the Late Pliocene. The change in the movement of sedimentary basin took place in earliest Pliocene age.
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