Freshwater Molluscan Fossil Assemblages of the Middle Pleistocene Kiyokawa Formation, Shimosa Group, with Special Reference to the pH Conditions of Their Habitat

The floodplain deposits of the Kiyokawa Formation in Yoshinoda, Sodegaura City, Chiba Prefecture, yield freshwater molluscan fossils including Parafossarulus manchouricus (Bourguignat), Gyraulus cf. chinensis (Dunker), Lymnaeidae gen. et sp. indet., Unio douglasiae (Griffith & Pidgeon), Anodonta cf. woodiana (Lea) and Corbicula leana (Prime). In the silty muds, the shells of bivalves are commonly conjoined, and attain their adult size. In addition, unionid bivalves are often found in life position, and represent autochthonous or parautochthonous assemblages. These fossils indicate that there was a stagnant pond or shallow lake environment, which had continued long enough to allow development of a freshwater molluscan fauna.The shells of freshwater molluscs are also the key to knowing the pH conditions of their habitats. In the acidic conditions that often prevail in freshwater, molluscs suffer shell corrosion at their umbonal regions during life. In the case of unionid fossils from the Kiyokawa Formation, they have suffered no corrosion at their umbonal regions. This fact indicates that the paleo-pH conditions of the interstitial water of the floodplain deposits appear to have been higher than neutral. Such a chemical condition may be an important factor in preventing calcareous skeletons from undergoing pre-fossilization dissolution in sedimentary processes of the fossiliferous beds in the floodplain deposits.