2007 年 113 巻 6 号 p. 266-269
Red mudstone in the accretionary complex in Yakushima Island is dated as middle Middle to early Late Eocene by co-occurrences of Dictyoprora mongolfieri, Theocyrtis cf. perpumila, Calocycloma cf. ampulla.
As most of the accretionary complex in Yakushima Island underwent contact metamorphism by Middle Miocene Yakushima Granite, no fossils were obtained from the accretionary complex. We extracted radiolarian fossils from the non-metamorphosed red mudstone and a hard mudstone nodule included in mudstone in the eastern part of the Yakushima Island. The radiolarian fossils from the red mudstone are poorly preserved but sufficient for age determination.
Compared with the Hyuga Group of the Shimanto accretionary complex in southeastern Kyushu, which the original stratigraphy are known well, the red mudstone of Yakushima Island is just younger than the early Middle Eocene red mudstone with basalt.
Although the turbidite of the Hyuga Group is middle Middle Eocene in southeastern Kyushu, the age of the whole Hyuga Group's terrigenous sediments is Eocene to Early Oligocene. This implies that there is some age difference between the red mudstones. As the lithology and structure of the accretionary complex in Yakushima Island is very similar to that in southeastern Kyushu, the accretionary complex in Yakushima Island is compared to the Hyuga Group.