Abstract
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.
