Littoral Molluscan Assemblages during the Post-glacial Jomon Transgression in the Southern Kanto, Japan

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  • 南関東における繩文海進に伴う貝類群集の変遷
  • ミナミ カントウ ニ オケル ジョウモン カイシン ニ トモナウ カイルイ グ

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Abstract

Six different types of shallow water molluscan assemblages have been distinguished from the embayment deposits accumulated during the maximum stage of the post-glacial Jomon Transgression (ca. 6, 500-5, 000y.B.P.) in the southern Kanto, central Japan. These assemblages are also recognized in the embayment deposits of the very early stage as well as in the later stage of the transgression. In detail, however, their rise and fall are in harmony with the sedimentary history during transgression. In the early stage of the transgression, ca. 10, 000-7, 000y.B.P., the Crassostrea gigas-Anadara granosa-Batillaria zonalis assemblage and the Dosinella penicillata-Paphia undulata-Anadara broughtonii assemblage predominated in accordance with the development of muddy facies on the bottom of drowned valley. On the contrary, the sandy bottom assemblages, such as, the Meretrix lusoria-Mactra veneriformis-Umbonium moniliferum and the Meretrix lamarcki-Umbonium gigantium-Glycymeris albolineata assemblages, became dominant after the maximum stage of the transgression, from about 4, 500 to 2, 000y.B.P., during the reclamation stage of the embayment by the sandy materials provided from the adjoining rivers.<br>The intertidal assemblage, the Crassostrea assemblage, found in the innermost part of the bay is characterized by the association of several warm water species. Two steps of invasion of the warm water species are recognizable in the southern Kanto region. At the first step, ca. 9, 500-8, 700y.B.P., Anadara granosa, Anomalocardia squamosa and some other warm water molluscs appeared following the advancement of the Crassostrea assemblage into the embayment. The second step is at the beginning of the maximum stage of the Jomon Transgression, about 6, 500 to 6, 000y.B.P.. Molluscs of the tropical nature are the invaders of this step. They are represented by Ostrea pauluciae and Tellinimactra edentula. These warm water species rapidly declined in the later stage of maximum transgression, between 5, 000 and 4, 000y.B.P., and most of the species disappeared after 4, 000 years. Cause of disappearance of the warm water species has been said to be the lowering of water temperature. But it is known that the main cause of the disappearance is most probably attributable to the disappearance of their habitats, shallow muddy environment, rather than the lowering of water temperature.<br>There are many shell-mounds formed during the Jomon Era along the west coast of Tokyo Bay. Their situation and duration, relative abundance, and the change in species composition of shells can be explained by changes of paleogeography, sedimentary facies, and associated molluscan assemblages in and adjoining coastal areas of the embayments formed during the Jomon Transgression.

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