湖成層の珪藻化石群集を用いた水城古環境解析 : その1 大分県玖珠盆地宝泉寺層・野上層(下部〜中部更新統)

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The Hosenji and Nogami Formations (Lower and Middle Pleistocene, respectively) are lacustrine deposits in the southern part of the Kusu Basin in Oita Prefecture. The paleoenvironments of these formations were determined by pollen and plant macrofossils (Iwauchi and Hase, 1987). Fossil diatom assemblages of the Hosenji, Nogami and Asono Formations suggest a lacustrine environment of a lake making up their predominantly shared environment in the early and middle Pleistocene. Aquatic conditions were made evident through fossil diatom assemblage analysis in this study. The fossil diatom assemblages suggest a lake environment, where both the Hosenji and Nogami Formations left oligotrophic deposits, suggesting a relatively sustained stability due to a lack of appreciable flow. The rate of Cyclotella comta was 85-95% at the basal part of the formations. In the middle part of the formations, however, diatoms were eutrophic, and their rate increased. We think that the eutrophication of these lakes rapidly occurred during the deposition of the Hosenji and Nogami Formations. The Nogami and Asono Formations are of such similarity of diatom assemblage that it is clear they were borh deposited in the Nogami-Asono Lake (Iwauchi and Hase, 1987). Picea pollen rate increased representing cool-temperate conditions at both horizons of the Asono and Kuroiga areas, although this same condition did not occur at the corresponding horizon of the Nogami locality, where the type locality of the Nogami Formation exists. Although markers of cooking climate were evident around the shore area of the lake such as foud at the Asono and Kuroiga lacalities, they were much less evident in the Nogami locailty toward the center of the lake. The fossil diatom assemblages of the Hosenji Formation at the Shishimuta locality along the Kusu River were similar to the assemblages from the Nogami Formation at the Nogami locality. It is, therefore a reasonable assumption that these formations were deposited in the same sedimentary basin.
熊本大学の論文
2004-03-31

湖成層の珪藻化石群集を用いた水城古環境解析 : その1 大分県玖珠盆地宝泉寺層・野上層(下部〜中部更新統)

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