Fossils from the channel-fill deposits of the Upper Cretaceous Saku Formation in the Teshionakagawa area, Hokkaido, Japan, and their significance.

Abstract

Channel-fill deposits in the Saku Formation of the Teshionakagawa area, northern Hokkaido, are correlated to the Upper Turonian. Eubostrychoceras cf. japonicum, Inoceramus cf. pedalionoides and I. pictus minus occurred from gravels filling the channel structure. The former two species co-occurred in a single gravel. We suggest the following three interpretations based on these results and the ages suggested for E. japonicum (Middle to Late Turonian) and I. pictus minus (Late Cenomanian) by previous studies:(1) The gravels containing I. pictus minus demonstrate that the channel structure scored as deep as the Cenomanian in the upper reaches, or the large-scale regression in this area denuded the underlying Cenomanian in the land and/or the shallow shelf area;(2) the first occurrence of I. pedalionoides may date back to the early part of the Late Turonian-Middle Turonian ; and (3) the calcareous concretion (gravel) containing E. cf. japonicum should have been consolidated within the time between the first occurrence of E. japonicum (Middle Turonian) and the sedimentation of the channel-fill deposits (Late Turonian).