2009 Volume 115 Issue 3 Pages 130-140
A total of 25 species belonging to 21 genera of marine bivalve fossils were obtained from the Sebayashi Formation of the Sanchu Cretaceous System in the Chichibu Composite Belt, Sebayashi area, Gunma Prefecture, Kanto Mountains. The bivalve fauna includes Neithea matsumotoi and Nemocardium yatsushiroense indicative of the Tethyan fauna typically found in the Albian Yatsushiro Formation of the Pre-Sotoizumi Group in west Kyushu. The same locality yielded an ammonoid fauna of Barremites otsukai, Pseudohaploceras japonicum and Crioceratites (Paracrioceras) asiaticum. These co-occuring ammonoids suggest that Neithea matsumotoi, Nemocardium yatsushiroense and Yabea akatsui range down into the Barremian. The Tethyan bivalve fauna in the Sebayashi Formation cannot be explained soley by sinistral translation along the Kurosegawa Tectonic Zone. The pattem of bivalve occurrence suggests that the Sebayashi Formation was deposited in an environment storongly affected by increasing seawater temperature.
