Discovery of megalodontoid bivalves in the Shiriya area, northern Honshu, northeast Japan, and its geological implications

Three species of megalodontoid bivalves: large and small species of megalodontids and Dicerocardium sp., were discovered from the allochthonous limestone block in the Shiriya area, Northeast Japan. Since they are probably different from other Japanese megalodontoids, which were previously described by Tamura (1983) from Southwest Japan, Japanese megalodontoids were possibly more diverse than previously thought. Although it has been considered that all the limestone blocks in this area are of Late Jurassic age, the discovery of megalodontoids suggests a Late Triassic age for these limestone blocks. Besides the presence of megalodontoids, 1) predominance of micritic lithofacies with rare fossils; 2) association of basaltic rocks with limestone; 3) presence of alternating beds of limestone and chert; 4) very small amount of insoluble residue in the limestone; 5) occurrence of limestone conglomerates around limestone bodies, also indicate that at least larger limestone blocks in this area originally deposited on the top or foot of accreted seamount, possibly located in the Panthalassa. Since these characters are also similar to those of other Late Triassic limestone bodies in Northeast Japan, correlation between the Shiriya Complex and other accretionary complex in Northeast Japan should be reconsidered based on the presence of Late Triassic limestones in these units.