EARLY CRETACEOUS NON-MARINE BIVALVE FAUNAL GROUPS FROM CENTRAL AND EAST SHIKOKU, JAPAN

Cretaceous non-marine faunas in Central and Eastern Shikoku can be classified into the Masaki and Sakashu faunal groups on the basis of their taxonomic components and a new stratigraphy and tectonic subdivision of the Chichibu Superbelt. The non-marine bivalves from the Monobegawa Group in the Masaki Belt belong to the Masaki Faunal Group which are identified the Hauterivian Tatsukawa fauna, Late Barremian Yunoki fauna and Early Aptian Hibihara fauna. The non-marine bivalves from the Nankai and Takegatani groups of the Sakashu Belt belong to the Sakashu Faunal Group which are identified the Hauterivian Shobu fauna. The three faunas of the Masaki Faunal Group in Central and East Shikoku are found in stratigraphic order. The Masaki Faunal Group is characterized by the brackish- and freshwater mollusks. On the other hand, the Sakashu Faunal Group is characterized by the brackish-water mollusks. It suggests that salinity is the main differential factor between the Hauterivian faunas in the Masaki and Sakashu faunal groups.
A Hauterivian mixed assemblage of the Tatsukawa and Shobu faunas was found in Central Shikoku. The finding suggests that the two faunas were formed in different salinity environment and adjoining areas.

四国中・東部秩父累帯下部白亜系の非海生二枚貝類について種構成や群集の生息環境を考察し，再編された地帯区分との対応を検討した．これらの白亜紀非海生動物群は新たな秩父累帯の地帯区分及び種構成に墓づいて，互いに共通種を持たない正木動物群(Masaki Faunal Group)と坂州動物群(Sakashu Faunal Group)に区分される．正木帯に属する物部川層群の非海生二枚貝類は正木動物群に帰属し，立川フォーナ(Hauterivian)，柚ノ木フォーナ(Late Barremian)，日比原フォーナ(Early Aptian)で構成される．坂州帯に属する南海層群，竹ヶ谷層群の非海生二枚貝類は坂州動物群に帰属し，菖蒲フォーナ(Hauterivian)で特徴づけられる．四国中・東部における正木動物群は，汽水生一淡水生軟体動物で特徴づけられ，坂州動物群は汽水生軟体動物で特徴づけられる．このことは，坂州動物群は正木動物群より塩分濃度の高い環境で生息したことを示す．また，四国中央部では両動物群の混在が確認され，正木動物群と坂州動物群は，隣接して存在し，塩分濃度の異なる環境に生息したことが推測される．