2004 Volume 110 Issue 7 Pages 403-416
The Shimanto Supergroup in the Inomisaki- Nakamura area, western Shikoku, is favorable for the examination of geologic development 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. We have studied the petrographical features and bulk chemistry of sandstones in the area, in order to clarify the geologic developments of the Shimanto accretionary complex during the late Cretaceous and early Paleogene. These data, together with chronological and geologic data of the Sanbagawa Metamorphic Belt, led to the following conclusions.
1. The Shimanto Supergroup in the study area can be classified into the following three intervals on the basis of petrographic and geochemical features of sandstones: Interval 1 (Santonian-Early Paleocene), Interval 2 (Late Paleocene) and Interval 3 (Early and Middle Eocene).
2. The sandstones from Interval 1 were mainly derived from felsic volcanic rocks in the Inner Belt of Southwest Japan.
3. The sandstones from Interval 2 are characterized by lithic fragments of pelitic schist, siliceous schist, and weakly- to non-metamorphosed chert. An emergence and erosion of the Sanbagawa Metamorphic Belt took place in this interval along with the Mikabu Greenstones and Northern Chichibu Belt.
4. Unroofing and resulting emergence of many granitic rocks in the provenance of the Inner Belt have occurred in the depositional age of the Inomisaki Formation of Interval 3.
5. Voluminous and rapid underplating induced by a large amount of supply of clastic sediments to the trench area during the Santonian-Campanian in Shikoku might have resulted in an exhumation of the Sanbagawa Belt.
