長野県北佐久郡望月町観音寺における珪藻土とその形成について

概要

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Two diatomite beds are developed at the sampling station as illustrated in the columnar section. The lower bed belongs to the Uryuzaka formation of the early pleistocene Komoro group, and the upper bed was formed during the deposition of the middle pleistocene Yaehara formation. Diatom assemblages of eight samples from the two diatomite beds were examined and clarified to be composed of 72 species and 27 genera. A. Physico-chemical spectra analysis of the ecological condition indicates that these diatoms were deposited in a slightly alkaline fresh-water lake. 1. The dominant species of diatom assemblages from the lower bed are Melosira italica, Melosira granulata, Stephanodiscus niagarae. The most of them are planktonic forms. The scarcity of benthic diatoms suggests that the diatomite was deposited at depth greater than 15 meters (PRESCOTT, 1968). 2. The dominant species from the upper bed are Fragilaria construens with its form variation, and some genera of Cymbella, Epithemia, Gomphonema are included. Most of them are benthic or epiphytic forms. These diatoms were deposited at shallow part of a lake as near shore. B. On the basis of dominant and subdominant species, the fresh-water diatomites from Japan were classified into three groups (OkuNo 1952). Two diatomite beds of the lower and upper parts come under two different groups of three.
地学団体研究会の論文
1981-05-25