Journal of science of the Hiroshima University. Series C, Earth and planetary sciences Volume 11 Issue 2
published_at 2003-08-25

Sedimentary environments of the Jurassic-Cretaceous Tetori Group of Nagato River area, southern Toyama Prefecture

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Sedimentology and stratigraphy were studied for the Jurassic-Cretaceous Tetori Group in Nagato River area (southern Toyama Prefecture) in order to reconstruct the sedimentary environments and processes of an alluvial depositional system. The Tetori Group in the study area unconformably overlies the Hida metamorphic rocks, and is divided into the Nagatogawa and Atotsugawa Formations in the ascending order, which correspond to the Uppermost Jurassic to Lower Cretaceous Itoshiro and Akaiwa Subgroups, respectively. The Nagatogawa Formation is subdivided into the lower Ioridanitoge Conglomerate Member (about 250 meters thick) and the upper Inotani Sandstone-mudstone Member (about 150 meters thick). The Atotsugawa Formation is subdivided into the lower Minamimatadani Conglomerate Member (about 70 meters thick) and the upper Wasabu Sandstone-mudstone Member (about 700 meters thick). Fossil assemblage, consisting of plants, freshwater mollusks, tortoise shells, and ganoid scales, indicates that the four members were deposited in non-marine environments. The sedimentary facies analysis defined five sedimentary facies (Facies A~E) from the Tetori Group in the study area. The conglomerate-dominant Facies A and Facies C were formed in alluvial fans, and in a proximal braided river, respectively. Facies B consists of the alternating beds of mudstone, sandstone and conglomerate, which were formed in the braidplain near a distal alluvial fan. Facies D is dominated by sandstone intercalating thin layers of conglomerate formed in a distal braided river environment. Facies E consists of alternating beds of sandstone and mudstone deposited in a meandering river environment. Results of the facies analysis, together with paleocurrent and composition analysis of the coarse clastics, conducted four stages of the depositional history of the Tetori Group in the Nagatogawa River area. Stage 1 is characterized by the development of alluvial fans, proximal braided river and partially distal braided river, and this alluvial system deposited the Ioridanitoge Conglomerate Member. The current of the braided river generally directed from west to east, and delivered gravel of granitic and felsitic volcanic rocks originated from the Hida Terrane. Stage 2 was the period of the Inotani Sandstone-mudstone Member, which was deposited mainly in a meandering river flowing from north to south. In Stage 3, the Minamimatadani Conglomerate Member was deposited in the distal braided river with two current directions. One was from east to west, and carried the gravels of the siliceous sedimentary rocks. Another was from north to south, and delivered the rocks of the Hida Terrane. Stage 4 is characterized by the development of a meandering river, generally flowing from north to south. This meandering river system lasted for a long time and deposited thick alternating beds of the Wasabu Sandstone-mudstone Member.