Stratigraphy and depositional age of the Izumi Group, Niihama area, central Shikoku, Japan

Abstract

The Izumi Group, which occurs from western Shikoku to the Izumi Mountains, Kii Peninsula, comprises sediments deposited in a strike-slip basin during the Late Cretaceous. Some differences exist in the lithostratigraphy and geological structure of the group where it occurs in western and eastern Shikoku; consequently, outcrops in central Shikoku may record evidence of the transition between the two areas. We conducted stratigraphical and geochronological surveys as part of compiling the “Niihama” geological map, and defined the following new formations: the Kussaki, Isoura, and Niihama formations. Fission-track dating of felsic tuff beds in the Niihama Formation yielded an age of 79.1±2.2 Ma. The radiolarian assemblages within mudstone of the Niihama Formation are DK assemblages, corresponding to the early-middle Campanian. The combined age data indicate a middle Campanian age for deposition of the Niihama Formation. The present results suggest that the Izumi Group was simultaneously deposited in western and central Shikoku. The lithology and geological structure in the study area are comparable with those in western and eastern Shikoku, indicating that the analyzed rocks represent strata deposited during a transitional stage of basin development and basin fill.