1996 Volume 105 Issue 1 Pages 67-76
Non-marine to marine deposits of the Lower Cretaceous Idaira Formation are exposed in north of the Lake Hamana, the western part of Shizuoka Prefecture. The Idaira Formation unconformably overlies the Chichibu Supergroup and is in fault contact with the supergroup. It is divided into three members, namely the lower, middle and upper members.
The lower consists of the basal massive conglomerate and cross-bedded sandstone, and the upper black shale and intervening lenticular-or flaser-bedded sandstone. The latter part abundantly contains such brackish molluscan fauna as Protocardia ibukii, Costocyrena otsukai and Cassiope neumayri, and the Ryoseki-type flora. The two parts are interpreted to be alluvial fan and estuary/ lagoon deposits respectively.
The middle member is composed of large-scale cross-bedded, conglomeratic sandstone with well-sorted sandstone and laminated shale, sometimes including the Ryoseki-type flora. Its sedimentary environment is thought to be braided river on fan delta.
The upper is represented by outer shelf, laminated to massive black shale intercalated with cross-laminated sandstone layers. It yields Barremian ammonite (Shasticrioceras nipponicum), and offshore thin-shelled bivalves (Parvamussium kimurai) and echinoids.
