2017 Volume 123 Issue 11 Pages 925-937
We have investigated the Mugitani Formation located in the structurally upper part of the Cretaceous accretionary complex in the Shimanto Belt, central Kii Peninsula. The Mugitani Formation is newly divided into two units: the Izuo Unit and the Sebato Unit. The Izuo Unit is composed mainly of mudstone, broken beds of sandstone and mudstone, and mixed rock with minor amounts of sandstone and chert. The Sebato Unit is composed mainly of mudstone with minor amounts of broken beds of sandstone and mudstone, basalt, and mixed rock. The bedding planes generally strike WNW-ESE and dip moderately northward. In addition, we also carried out U-Pb dating of detrital zircons separated from three sandstones to clarify the depositional age of the Mugitani Formation. The youngest zircon grain ages and/or the youngest cluster ages indicate ca. 100-110 Ma, presenting latest Early Cretaceous (Albian). Furthermore, we found no Late Cretaceous zircon grains that are known to have been produced by the intensive igneous activity in Southwest Japan. Based on the youngest zircon grain ages, the youngest cluster ages, and age compositions, the depositional age of the Mugitani Formation corresponds to that of the Yukawa Formation, ranging from Albian to Cenomanian.
