Cyclic sea-level changes based on fossil ostracode faunas from the Upper Pliocene Sasaoka Formation, Akita Prefecture, northeast Japan

摘要

The paleoenvironments in the Upper Pliocene Sasaoka Formation (2.20-2.05 Ma), Gojome area, Akita Prefecture, northeast Japan, are reconstructed on the basis of the ostracode assemblages and sedimentary facies. The upper part of this formation is divided into five units representing sedimentary facies. Each unit shows coarsening-upward sequences. Q-mode principal component analysis of ostracodes suggests the following: (1) the Upper Pliocene Sasaoka Formation was deposited entirely under cold-water influences; and (2) paleoenvironments of this formation are interpreted as combinations of a coastal area in sublittoral zone, open-water environment in upper bathyal zone, a bay in upper bathyal zone, and a bay in sublittoral zone. Each of four sedimentary units records one cycle of relative sea-level change based on ostracode assemblages, whereas one sedimentary unit (II) is barren. The sea-level changes reflected in the upper two units (IV and V) might have been shorter in duration than those of the lower two units (I and III). Relatively large-amplitude oscillation is recognized in the middle part by presence of lag deposits, absence of the upper part of a cycle in unit below the lag deposit, and no correlation between ostracode assemblages and paleodepth just above the lag deposits. This large shift might have been caused by the global glacial event at 2.08 Ma. The paleodepth between upper bathyal and sublittoral zone shifts to shoreface at the remarkable boundary. The upper half is characterized by a shallowing-upward sequence based on occurrences of Sinocytheridea latiovata.