Fossil molluscan assemblages and sedimentary environment of the Pleistocene Furuya formation in Makinohara upland, Shizuoka Prefecture, Central Japan

The Pleistocene in Makinohara Upland, Shizuoka Prefecture, central Japan, consists of the Furuya Formation, the Kyomatsubara Formation, the Ochii Formation, and the Makinohara Formation, in ascending order. The sequence is considered to form a depositional sequence primarily controlled by sea level changes. The Furuya Formation is mainly composed of silt and clay deposits, which corresponds to transgressive systems tract filling the drowned valley, 'the Paleo Sagara Bay'. This study makes clear temporal and spatial distribution of fossil molluscan assemblages in the Furuya Formation. We identified fossil mollusks of 272 species in 184 genera, and recognized the following three assemblage types: 1) Tidal inlet type, dominated by Crassostrea gigas, Tegillarca granosa, Cerithideopsilla cingulata, C. djadjariensis, and Batillaria zonalis; 2) Subtidal to shallow embayment type, characterized by abundant occurrence of Paphia undulata, Raetella pulchella, Dosinella corrugata, Theora fragilis, and Ringicula doliaris; and 3) Outer bay type, characterized by common occurrence of oceanic species, such as Nucula paulula and Microcirce dilecta. On the basis of the distributions of the three assemblage types, local difference of paleoenvironment in 'the Paleo Sagara Bay' becomes clear. The northern part (innermost part) of the bay had been tidal inlet during the transgression, whereas the central to southern area (central to outer part) changed its bathymetry from intertidal, via shallow embayment, to outer bay. The difference is controlled by depositional setting in 'the Paleo Sagara Bay', and by the balance between sedimentation rate of clastics and speed of sea-level rise. During the transgression, seawater invaded from the south of the southern branch of 'the Paleo Sagara Bay', and the Furuya Formation deposited onlapping northwards. The occurrence of outer bay type assemblage is restricted to the uppermost or upper part of the Furuya Formation. This suggests that the embaymental condition had continued during the deposition of the Furuya Formation, but oceanic water abruptly intruded into the central part of the bay in the last depositional stage of the Furuya Formation.