茨城県大子地域における中新世植物化石群

概要

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Plant-bearing terrestrial to shallow marine strata of Early to earliest Middle Miocene age are described from the Daigo area of Ibaraki Prefecture. They are divided into the Kitatake Formation (lower), which includes the Osawaguchi Tuff Member in its uppermost part, and the Asakawa Formation. A total of 54 species, in 26 families and 45 genera, are recognized from the two formations. They are composed mainly of deciduous dicots, associated with evergreen dicots, conifers, and herbaceous plants. Based on the floral composition and on components of evergreen and warm deciduous dicot elements, the plant assemblages are subdivided into three floras, which occur in the Kitatake Formation and the lower and upper parts of the Asakawa Formation, respectively. These three floras indicate upward warming from a warm-temperate climate to a warm-temperate to subtropical one, judged from the composition of evergreen and warm deciduous elements, and the percent of entire-margined dicot species. In addition to the floral characteristics, evidence from marine mollusks in the upper part of the Asakawa Formation and from radiometric ages indicate that these three floras represent floral and paleoclimatic changes during approximately 18 to 16Ma, within the Daijima-type floras.
2008-10-01