Abstract
The Shimanto Supergroup in the Inomisaki-Nakamura area, western Shikoku, is favorable for the examination of geohistory around the boundary between the Cretaceous and the Paleogene, because the Maastrichtian-Paleocene strata, which are not exposed in most of the Shimanto Belt, occur as good outcrops in this area. Geological survey of the area and radiolarian fossils extracted from black or gray shale, acidic tuff, red shale, green shale, siliceous shale and rubbles of shale led the authors to the following conclusions. 1) The studied area can be divided into the following twelve tectonostratigraphic units on the basis of lithofacies and depositional ages inferred from radiolarian assemblages : Nonokawa Fm. (early Campanian), Oyu Melange (late Campanian), Nakamura Fm. (late Campanian-early Maastrichtian), Warabioka Fm. (Santonian-early Campanian), Ukibuchi Fm. (late Campanian-) Maastrichtian), Ishimiji Fm. (Early Paleocene), Ida Fm. (Late Paleocene), Kamochi Fm. (Early-Middle Eocene), Inomisaki Fm. (Middle Eocene), Tanokuchi Fm. (Middle-Late Eocene), Hiromi Complex (Middle Eocene), and Arimisaki Fm. (Late Oligocene-Early Miocene). 2) Depositional ages of the tectonostratigraphic units become younger southward except for the Warabioka Formation which is presumed to be slope basin deposits. It is possible that the Warabioka Formation is made up of megablocks of slope failure origin. 3) The chronological relationships among black (gray) shale, red shale, green shale and siliceous shale are suggestive of the subduction of a very young oceanic plate during the late Campanian to Early-Middle Eocene. 4) Pebbly shales with large rubbles of sandstone and shale of slope-failure origin begin to increase after the Late Campanian. The ages of shale rubbles in pebbly shale generally become younger corresponding to the depositional age of the host strata.
