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 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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関東山地北西縁部には,古期岩類を不整合におおう新第三系が広く分布する.それらは,下位から内山・駒込・本宿・志賀溶結凝灰岩・水落観音溶岩の各層に区分される.内山・駒込の各層は、徴化石・放射年代によって下部中新続であるとされており,フォッサマグナ中央部においては最も下位に位置する海成層である.内山層について,堆積学的な検討を加えた結果,それは基本的に9つの堆積相に区分され,それらの累重様式によって下半部と上半部の2つの堆積組相に分けられることが明らかとなった.また,各組相内部は上方細粒化と上方粗粒化のサクセッションの組み合わせからなり,それは海洋環境が浅→深→浅と変化していったことを示している.このことから,内山層の堆積期には海が浅→深→浅と変化する現象が2度繰り返されたことが明らかである.

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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  • 地球科学

    地球科学 52(2), 136-152, 1998

    地学団体研究会

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