銚子半島高神低地の完新世における珪藻群集の推移と古海水準

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概要

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• The author observed fossil diatom assemblages of 138 samples of Holocene deposits in Takagami Lowland, central part of the Choshi Peninsula. The Holocene deposits in Takagami Lowland can be divided into 5 zones of diatom assmeblages (Fig. 8). The character of each zone can be described as follows.Zone I (-15m--10m)Fresh-water species are dominant and comprise more than 60%. But fresh-brackish species and marine species can be observed together. A ratio of marine species increases toward upper horizon. Dominant species are Achannthes lanceolata, Cocconeis placentula, Navicularadiosa, all of which live in fresh-water pond, lake and river.Zone II (-10m--2.5m)Marine species are dominant and comprise more than 90%. Dominant species are Thalassiosira excentrica, Cyclotella striata, Actinoptychus undulatus, all of which are planktonic and live in inner bay and ocean.Zone III (-2.5m-3.5m)Marine species are dominant and comprise more than 90%, too. But Cocconeis scutellum, which lives in polyhaline lagoon, is dominant.TR Zone (3.5m-4.7m)TR Zone can be observed in Loc. 1 and Loc. 6. This zone is a transitive zone from Zone III to Zone IV. Rhopalodia gibberula, which lives in oligohaline lagoon, is dominant.Zone IV (3.5m-10m)Fresh-water species are dominant and comprise more than 90%. Marine species cannot be observed. Dominant speceis are Achnanthes minutissima, Fragilaria construens, Fragilaria brevistriata, Melosira granulata, all of which live in fresh-water pond and lake.The paleo-environmental succession in Takagami lowland is shown in Fig. 9. About 8, 000-10, 000y.B.P., marine transgression started and a drowned valley began to be formed (I). Due to rapid transgression, the drowned valley was spread widely till 6, 000y.B.P. (II). About 5, 500-6, 000y.B.P., the beach ridge was formed in the eastern part of this drowned valley, so that the depositional environment of this area was changed from inner bay to polyhaline lagoon (III). About 5, 000y.B.P., in the middle and eastern part of Takagami lowland fresh-water marsh was formed, and a second transgression did not happen (IV-1). In the northwestern part, marine sand with shell fragments and diatom valves of marine species were deposited frequently on the freshwater marsh by storm surges till about 3, 000y.B.P. (IV-2).

著者

• 鹿島 薫

  東京大学理学部

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