Stratigraphy and sedimentary environment of the Tetori Group in its central distribution based on nonmarine molluscan assemblages

Nonmarine molluscan fossils are considerably abundant at many horizons of the Upper Jurassic to Lower Cretaceous Tetori Group. The typical section of the group in its central outcrop area (northern part of the Gifu Prefecture, central Japan) is composed mainly of nonmarine strata intercalated with marine beds. We, therefore, can recognize changes in sedimentary environments and molluscan assemblages of the Tetori Group through time, as influenced by transgression and regression phases.The Tetori Group is lithologically divided into the Ushimaru, Mitarai, Otaniyama, Okurodani, Amagodani, Okura and Bessandani formations, in ascending stratigraphic order. The oldest succession of the group, consisting of the Ushimaru to Mitarai formations, shows estuarine, deltaic and shallow marine environments. The shallow marine Mitarai Formation bears ammonites of Callovian age and reflects the climax of transgression. Subsequently, the two younger successions of the Otaniyama to Okurodani formations and the Amagodani to Bessandani through Okura formations, reveal distributary bay and braided and meandering river environments, respectively.Nonmarine molluscan assemblages occur in the Ushimaru Formation and in the succession of the Otaniyama and Okurodani formations, interbedded with shallow marine Mitarai Formation. Five assemblages of nonmarine molluscs are recognized : [1] Ostreidae gen. et sp. indet. - Myrene (Mesocorbicula) tetoriensis, [2] Myrene (Mesocorbicula) tetoriensis, [3] Tetoria (T.) yokoyamai, [4] Sphaerium sp. and [5] Plicatounio (P.) naktongensis-Trigonioides (Wakinoa) tetoriensis. These assemblages show various grades of brackish-to freshwater environments, and the species composition of each assemblage varies with salinity.Based on the occurrence of these five molluscan assemblages, the Ushimaru Formation reflects more repeated environmental change than the succession of the Otaniyama to Okurodani formations. This indicates that there is repetition of numerous minor cycles of transgression and regression in transgression-phase, but smooth environmental changes in the regression-phase. We can recognize more precise environmental changes by using non-marine molluscan assemblages.