Lower Miocene Stratigraphy and its paleo-environments of the Ame and Nukui River Areas, northwestern part of the Kanto Mountains, central Japan

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  • 関東山地北西部に分布する下部新第三系の層序と堆積環境

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Abstract

The Ame and Nukui river areas at the eastern range of Nagano Prefecture, is a northwestern part of the Kanto Mountains geologically. In this area pre-Tertiary sedimentary rocks belonging to the Chichibu Belt and Sanchu Graben, are widely distributed. The Tertiary sedimentary and volcaniclastic rocks rest unconformably upon these pre-Tertiary rocks. The Tertiary rocks which ranges in age from early Miocene to Pliocene are studied stratigraphically and sedimentologically. They are divided into five units. The lower unit consists of the Uchiyama and Komagome Formations, the upper unit comprises of the Motojyuku, Shiga welded tuff and Mizuochi-kannonn Lava Formations in an ascending order. The Uchiyama Formation, which is Early Miocene in age, is composed mainly of fine- to coarse-grained clastic sediments that may be deposited in shallow and deep-sea embayment. The Uchiyama Formation, which has many successions of sedimentary facies, is divided into the Lower and Upper Members lithologically. These two members consist of a pair of upward-fining and upward-coarsening sedimentary successions respectively. Molluscan and foraminiferal fossils occur in fine-grained sediments of the Uchiyama Formation. By the data of sedimentological analysis and molluscan faunal change, it is clear that the sedimentary environments of the Lower Member of the Uchiyama Formation has changed from fluvial passing through shelf or basin floor to shallow marine environments. The Upper Member changed from shallow marine passing through shelf to shallow marine environments. Sedimentological and radiometric chronostratigraphic data of the Uchiyama and Komagome Formations indicate that the first Miocene transgression to this region may have happened at 17Ma prior to the one of the Nishikurosawa Stage (15〜16 Ma).

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