Megafossil-bearing mudstone blocks in the Oriai Formation of the Upper Jurassic Imaidani Group in the Shirokawa area, Ehime Prefecture, Southwest Japan

愛媛県城川地域に分布する今井谷層群下相層において，泥岩中に異地性岩塊として混在する含大型化石泥質岩を発見した．含大型化石泥質岩岩塊は，生物擾乱を受けた塊状の砂質シルト岩からなり，腕足類，二枚貝，ウニ，アンモナイトおよび材化石などの大型化石を含む．岩塊は，周囲の泥岩と密着しており，漸移関係は示さない．エッチング処理を施した泥質岩を観察した結果，含大型化石泥質岩岩塊は基質の泥岩よりも粗粒な炭質物を含み，特異的であることが明らかになった．含大型化石泥質岩の初生的な堆積場は，陸棚以浅の浅海環境が想定される．含大型化石泥質岩は，浅海域での斜面崩壊に伴いブロック化し，下相層の形成場であるより深い斜面域の堆積盆へと移動し，泥岩中に混在したものと考えられる．岩塊に含まれるアンモナイトと今井谷層群の年代から，混在化の時期はジュラ紀末期（Tithonian前期）と推定される． / The mode of occurrence and lithofacies were examined in detail for some megafossil-bearing blocks found in mudstones of the Oriai Formation, in the lower part of the Imaidani Group in the Shirokawa area, western Shikoku, which is an equivalent of the Torinosu Group. These sandy siltstone blocks have irregular contacts with mudstone. These blocks are composed mostly of intensively bioturbated, massive or poorly-stratified sandy siltstone, and yield megafossils, such as brachiopod, bivalve, echinoid, ammonite, plant fossils and so on. Ammonite Hybonoticeratid species newly found from a few blocks indicate middle Kimmeridgian to early Tithonian age. Microscopic observation of HF-etched surface of argillites shows that the grain-size of plant debris contained in the sandy siltstone blocks are larger than those in the surrounding mudstone. The mode of occurrence of the sandy siltstone blocks and differences in litho-and biofacies between the blocks and the surrounding mudstone indicate that the sandy siltstones are not interbeded with mudstone but are included as allochthonous blocks. These blocks were originally accumulated in shallower marine, probably environment and were brought into a deeper environment of shelf slope. The mixing of the allochthonous blocks with surrounding mudstones took place in early Tithonian