宮崎平野の更新統・通山浜層の貝類群集の分布と変遷

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A middle Pleistocene shallow marine sediment, the Toriyamahama Formation of southwest Kyushu, was investigated from the viewpoint of sedimentary features and molluscan associations. This formation, less than 36m thick, consists of unconsolidated gravel, sand, and silt, and fills both the topographic depressions in the Miyazaki Group and its surrounding submerged river channels. Upward fining structures in the sediments are recognized throughout the succession as well as within each single bed set. In addition, it is presumed that the lower part of the formation corresponds to estuary, the middle part to beach, and the upper part to tidal mud flat or bay-bottom. Therefore, the Toriyamahama Formation, as a whole, can be regraded as the sedimentary product of a transgression during the middle Pleistocene.The formation contains 74 identified species of molluscan fossils belonging to 61 genera which are mostly extant. Among them, four ecological associations are differentiated on the basis of autochthonous occurrence: Corbicula, Crassostrea-Cerithidea, Protothaca, and Turritella-Dosinia associations. The Corbicula association is composed of Corbicula japonica monospecifically, and is found in sandy mud beds which are rich in plant remains. This association corresponds to a molluscan community of the fluvial environment, which is referable to an initial stage of transgression. The Crassostrea-Cerithidea association in the lower part of the mud beds of the formation is characterized by standing Crassostrea gigas colonies in a muddy habitat, accompanied by the muddy surface dweller Cerithidea djadjariensis and sessile benthic Anomia chinensis, Barbatia virescens, and Trapezium lilatum. It indicates that the nature of molluscan association shifted to that of an uppermost infralittoral or intertidal community. The Protothaca association of the basal part of the overlying mud bed is represented Protothaca schencki, Barnea manilensis, and Saxidomus purpuratus standing directly on a small semi-hard substrate layer of the basement. The Turritella-Dosinia association is found in a massive mud bed dominated by Turritella (Kurosioia) kurosio and Dosinis angulosa, but also containg Paphia undulata, Theora fragilis, and Ringicula doliaris. This association is a shallow bottom community which represents an inbayment habitat at a later stage of transgression.For comparison, 49 species belonging to 38 genera of ostracods in muddy sediments are examined from the view of paleoecology. The Crassostrea-Cerithidea association is accompanied by two ostracod assemblages: one predominated by Sinocytheridea sp. and Spinileberis furuyaensis and the other by Spinileberis quadriaculeata. The former two species are usually found in intertidal mud flats, and the last is an inhabitant of the uppermost infralittoral or intertidal zone in the present embayment environment. In the Turritella-Dosinia association, ostracod assemblage is predominated by Neomonoceratina microleticurata of the upper infralittoral zone. Conclusively, the result of an ecological analysis for molluscan associations in the Toriyamahama Formation is not inconsistent with the result of analysis for ostracod assemblages.

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中尾賢一
徳島県立博物館

宮崎平野の更新統・通山浜層の貝類群集の分布と変遷

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