長野盆地西縁部の第四系 : 長野盆地の形成史に関する研究

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In the western marginal area of the Nagano Basin, there are widely developed marine and lacustrine deposits of early to late Pleistocene age. They are classified into the Uppermost Sarumaru, the Iizuna, the Toyono, and the Minamigo Formations in ascending order. Among them, the Toyono Formation can be subdivided by lithofacies into following members, from the lower, the Takaoka sand and silt, the Kannonyama silt and the Hiraide sand and gravel. The name of the Minamigo Formation is newly proposed as the stratigraphic unit in this article. The chief purpose of the present article is to give detailed stratigraphic descriptions in the studied area. Concerning the geological history of the Nagano basin, it is noteworthy that the conglomerate of the Uppermost Sarumaru Formation lies upon the Susobana Formation of middle Miocene age with significant unconformity in the western marginal part. It may also be mentioned that the sediments referred to the Uppermost Sarumaru are well developed in the bottom of the Nagano Basin. From this evidence it is clear that at the beginning of the Pleistocene age an intense crustal movement accompanied with the subsidence occurred in and around the present basin and the marine transgression took place in the depression area. The marine condition changed to the lacustrine during the Toyono Age, when the Toyono Formation deposited in the Palaeo-Toyono Lake. The east area out of the eastern margin of the Nagano Basin have been belonged to the uplifting block, where the Quartz-Diorite massif distribute. On the other hand, the western area of this Basin subsided and the major fracture zone developed along the western margin, which was researched by the explosion seismic studies of the underground structure in the Matsushiro swarm earthquake area. The first activity of the above mentioned fracture line occured during the Latest Sarumaru Age, and this movement repeated after this Age, which formed the Tago, the Kaesa and the Nagaoka Faults. These faults are characterized by the thrust up from west to east, and the strike direction of faults are almost paralled to the general trend of the fractured zone. The activity of the fracture line are assumed to have continued through the Pleistocene to the Present in different grades. The Nagano Basin have been controlled structurally by the movement of the fractured zone and the origin of Basin dates at least back to the middle Miocene.
1977-02-21

長野盆地西縁部の第四系 : 長野盆地の形成史に関する研究

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