1 0 0 0 OA 北部フォッサ・マグナ中央部の新第三系の堆積盆
The Middle Miocene to Early Pleistocene marine sediments have a wide distribution in the northern part of Nagano Prefecture. In this area basin-forming movements became conspicuous since the Aoki age. Consequently three large sedimentary basins, that is the Komiji, the Takafu and the Hikage basins, were formed. These basins have a general trend of NNE-SSW. Since the Middle Pliocene time, another new tectonic movement took place in the central part of this area. The Orihashi sedimentary basin with a NE-SW direction was formed as a result of this movement. Deposits of the early stage in this basin were a large quantity of andesitic lava flows and pyroclastics (Arakurayama pyroclastics), which covered unconformably the lower Shigarami formation in some parts. It is remarkable that the younger basin-forming movement of the Pliocene time was characterized by a violent volcanism. Similar volcanism and tectonism are found also in the Hijiri (Komiji) and the Togakushi (Hikage) districts.