Abstract
Upper Triassic bivalves were discovered from the Sambosan Subterrane of a Late Jurassic to earliest Cretaceous accretionary complex along the uppermost reaches of Motoidani Gully, Itsuki-mura, Kumamoto Prefecture. The examined succession entirely comprises an accreted oceanic-rock assemblage of the basaltic volcaniclastic rocks (ca. 60 m thick) conformably succeeded by bedded black limestone (ca. 10 m thick), which is, in turn, over-lain by partly dolomitic, massive limestone (ca. 50 m thick). Two Late Triassic bivalvian assemblages were recognized in the examined succession. One is the Carnian St. Cassian-type Assemblage from the upper part of the volcaniclastic rocks and the lowermost part of the bedded black limestone. The other is the Norian Megalodontid Assemblage from the lower to middle part of the massive limestone, with an emphasis upon the successive relation of the basaltic volcaniclastic rocks to the overlying limestone. Occurrence of Gruenewaldia decussata and G. woehrmanni of the St. Cassian-type Assemblage, and the Megalodontid Assemblage implies a Tethyan affinity of the Sambosan oceanic rocks.
