Cenozoic volcanic products in Hokuriku district, central Japan, with special reference to the pyroclastic flow deposits

Abstract

The Cenozoic volcanic activity in Hokuriku district is divided into four stages. The age and the products of each stage are as follows: Stage I - Early Miocene - consists of lava flows and pyroclastic fall deposits of pyroxene andesite, and dacite welded tuff occurs in the lower part. Stage II - Middle Miocene - is mainly composed of deposits of pyroclastic flow and fall of rhyolite and dacite, associated with small amount of lava flow of rhyolite～dacite and basalt. Stage III - Late Miocene～Early Pliocene - lava flow and pyroclastic fall deposits of pyroxene andesite are common; pyroclastic flow deposits also exist. Stage IV - Latest Pliocene～Quaternary - several volcanic cones of pyroxene-hornblende andesite were formed (Volcano Tateyama, Hakusan, etc.) and pyroclastic flows also erupted in this stage. Present surface distribution of above volcanic products is shown in Fig. 1. Volcanism of Stage I, II and III took place in the Neogene sedimentary basin, and the distribution areas of their volcanic products became narrower and more localized from Stage I to later. Plio-Pleistocene (Stage IV) volcanism erupted on the uplifted Hida mountainland. Estimated total volumes of erupted materials in each stage are given in Table 1. The proportion of three types of volcanic products are also described. Eruption of pyroclastic flow played significant role in every stage, especially in Stage II, and ranges from pumice flow, some of which are strongly welded, to nuee ardantes (s. s.) or pelean type. Pyroclastic deposits consisting of ill-sorted, no welded essential ejecta, which is characteristic of pyroclastic flow deposits, are often inserted in the clastic sediments. The nature of these deposits, probably a type of subaqueous pyroclastic flow deposits, should be studied in future. The study of these pyroclastic strata is of prime importance to the Cenozoic stratigraphy as well as the volcanology in Japan.