Facies architecture related to sea-level changes of the Upper Cretaceous (Santonian) Taneichi Formation, northeast Japan: evidence of actual sea-level fluctuation during the Late Cretaceous

Abstract

Well-exposed transgressive and regressive sequence of the Santonian Taneichi Formation occurs along the Pacific Coast of northeastern Honshu, Japan. Although the studied sequence is of only 60 m thickness, the outcrops show various sedimentary facies accompanied with two disconformities, suggesting a small-scale but substantial sea-level rise and fall. Orientations of some sedimentary structures, such as ripples and bipolar cross-stratification, indicate that the paleostrandline of the Taneichi Formation made a high angle with the present coastline. Compared with the Santonian Kunitan Formation that is exposed about 50 km south of the studied section and that consists of thick deposits due to an incised valley sedimentation, the thin sediments of the Taneichi Formation are thought to have been supplied from the broad coastal plain along the eastern margin of the Eurasian continent. (Communicated by Tatsuro MATSUMOTO, M.J.A.)