水底堆積テフラからみた鮮新・更新世火山活動の変遷 : 新潟堆積盆小国町地域の例

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Transition of volcanism from Pliocene to early Pleistocene was revealed by means of subaqueous tephra layers intercalated in the marine, brakish and fluvial formations in the Oguni region, Niigata sedimentary basin. These formations, namely the Hachikokusan, Suganuma and Hachioji Formations and the Uonuma Group in ascending order, pile up conformably and attain 2000m in total thickness. Rhyolitic to andesitic tephras of more than 200 layers are intercalated in these formations. To visualize the transition of volcanism, the authors divided these formations into 10 Tephrozones (TNO-I〜TNO-X) in respect to the distinctive features of tephras, that is (1) frequency of appearence of tephra layers, (2) petrography and rock types and (3) their mode of occurrence and lithofacies. In latest Miocene to earliest Pliocene (TNO-I), only two rhyolitic thin layers are intercalated among nonvolcanogenic sediments of 280m thick. In early Pliocene, andesitic tephras with lava flows are dominant in the lower part (TNO-II), but only rhyolitic tephras are interbedded in the upper part (TNO-III). Middle Pliocene can be characterized by the scanty of tephra layers, which are not found in the lower part (TNO-IV) and a few rhyolitic thin layers can be found in the upper part (TNO-V). In late Pliocene (2.9-2.1 m. y.), with the deposition of the Uonuma Group, pumiceous deposits of more than 10m thick are occasionally intercalated (TNO-VI). Rhyolitic volcanism gets to be intensive from latest Pliocene to earliest Pleistocene (2.1-1.7 m. y.) (TNO-VII), in which rhyolitic tephras occupy more than 50% of total thickness in some parts. In early Pleistocene, rhyolitic and hornblende dacitic ones are occasionally interbedded in the lower part (1.7-1.3 m. y.) (TNO-VIII), but tephras are rarely intercalated in the ascending formations (1.3-0.9m. y.) (TNO-IX). In TNO-X (0.9-0.7m. y.), airfall tephras by eruptions at remote regions can only be intercalated. It is characteristic, however, that pink-colored dacitic ones are not uncommon in TNO-X, which is indicative of the transition of volcanism in the source areas. Transition of volcanism as revealed in the Oguni region is almost commonly noticed through the Niigata sedimentary basin, while in the southern uplifting area adjacent to the basin, andesitic volcanism was successively in active since late Pliocene.
地学団体研究会の論文
1986-05-25

水底堆積テフラからみた鮮新・更新世火山活動の変遷 : 新潟堆積盆小国町地域の例

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