The Isokusa Formation and its Late Upper Jurassic and Early Lower Cretaceous Ammonite Fauna
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概要
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The Isokusa Formation, the uppermost part of the Shishiori Group can be divided into three units at Niranowaki on the east coast of Oshima Island, southern Kitakami Massif, northeast Japan : 1 ) Sandy mudstone of the lowest part (IK-1) yielded Berriasella sp., an unidentified gigantic perisphinctid and fossils of pelecypod, echinoid and coral ; fragmental fossil plants also occur, 2) Clayey and silty shale in the middle (IK-2) yielded abundant Substeueroceras sp. and a few trigonians, and 3) Black fine grained sandstone in the upper (IK-3) yielded one Berriasella sp. from the lowermost part and one Thurmanniceras isokusense from the upper. The black fine grained sandstone and sandy mudstone at the sea coast of Isokusa on the west coast of Oshima Island, corresponds to IK-3 at Niranowaki in lithofacies. Ptychophylloceras sp., Berriasella spp. and Protacanthodiscus akiyamai occurred from the lower part of the Isokusa sequence and Thurmanniceras isokusense abundantly from the upper part. Fine grained sandstone and black mudstone at Nagasaki on the east coast of Oshima Island, yielded Olcostephanus sp., Spiticeras cf. binodiqer, Berriasella sp. (ex. gr. B. berthei), Protacanthediscus aff. malbosi and Kilianella sp. Among them Olcostephanus sp. is abundant in the middle part and the others are of uncertain horizons, but probably from the lower part of the sequence where the sediments are somewhat coarser and rich in pelecypod fossils. The boundary between the Tithonian (Jurassic) and Berriasian (Cretaceous) is tentatively drawn between Ik-2 and Ik-3 in the Niranowaki sequence. The horizon of Olcostephanus is Valanginian in age according to universal acceptance of ammonite genera.
- 東北大学の論文
- 1973-02-28
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