南関東の完新世火山灰土壌の母材 一次鉱物組成と元素組成による判定

Abstract

Volcanic ash soils occur widely on South Kanto district. The surface soil has black color and high humus content. The parent material is derived from Holocene volcanic ash. There are some buried humic horizons resulted from a succession of volcanic ash accumulation and plant residue supply leading to the accumulation of soil organic matter or humus. It has commonly been considered that Mt. Fuji supplied parent material of those soils.A huge amount of Holocene volcanic ejecta (tephra) with a large particle size accumulates at the eastern foot of Mt. Fuji, Tephrochronological studies enabled us to identify each scoria layers.However, little information of the tephrochronological study is available for soils formed in the region away from Mt. Fuji, such as on Musashino Plateau. A small amount of volcanic ash accumulation with a small particle size, compared with those in the eastern piedmont region close to Mt. Fuji promoted soil formation in this region, it is difficult to identify the characteristic tephra on Musashino Plateau on the contrary to the tephra of piedmont region which can easily be identified. Furthermore, it is assumed that more than one volcano in addition to Mt. Fuji such as Mt. Asama and Mt. Haruna supplied volcanic ash with a different rock type from that of Mt. Fuji in Musashino Plateau region. It is important to claryfy to what extent other volcanoes contributed in south Kanto district as a source volcano of the parent material of soils.In this paper, we compared morphological features, primary mineral composition, and element composition of three profiles which were locatd in Fujisawa City, Kanagawa, Fuchu City, Tokyo, Hidaka Town, Saitama respectivery. They are located almost in the same ditsance but the different direction from Mt. Fuji.At first, we directed our attention to buried humic horizon which was often identified as a horizon where it covered the archaeological site and remnants of the Nara-Heian period in Fuchu City, Tokyo. The horizon has some morphological and mineralogical characteristics; most blackish color in the solum, fine granular structure, and high olivine content. We gave the horizon tentative name as H-1BB (Holocene 1st Black Band).The results are summarized as follows:(1) On the basis of primary mineral composition, following were concluded: The dominance of heavy minerals and high content of olivine indicated that, parent material of Fujisawa pedon was supplied from Mt. Fuji. The dominance of light minerals, high content of non-colored volcanic glass and clinopyroxene, parent material of Hidaka pedon was supplied from Mt. Asama, Fuchu pedon indicated the transitional character between Fujisawa and Hidaka.(2) H-1BB is identified morphologically in all profiles based on relatively strong blackish color and structure. There was high content of transparent olivine in this horizon in every pedon. Because Fujisawa pedon indicated the highest olivine content, We assumed that H-1BB in three pedons were derived from more or less the same parent material erupted from Mt. Fuji.(3) From the viewpoint of chemical classification of volcanic rock, Si contents themselves, didn't directly reflect primary mineral composition of these soils. However, the profile which abounded in the parent material from Mt. Fuji had high Ca and Sr content. H-1BB in three profiles had high Ca and low K proportion in base element.