Depositional age of the Tertiary Kuma Group, Shikoku, and its significance

Cenozoic clastic sediments in the Kuma area, Shikoku, southwest Japan, previously designated as the Kuma Group, are here redefined as the Hiwada-toge Formation and the overlying Kuma Group sensu stricto, considering a significant time-gap between them. The Hiwada-toge Formation is Early Eocene in age on the basis of dinoflagellate cysts, while the lower part of the Kuma Group s.s. (Sagayama Formation) was dated as late Early Miocene by fission-track dating. Organic microfossil assemblages show that the Hiwada-toge Formation contains marine strata and the lower part of the Kuma Group s.s. (Sagayama Formation) is of non-marine origin.These results provide two geochronological constraints critical to the regional tectonic history.The age of the Hiwada-toge Formation indicates that the Sanbagawa metamorphic rocks came under subaerial erosion by Early Eocene in its provenance area. The relationship between the Median Tectonic Line (M.T.L.) and the northern extension of the Miocene Kuma Group s.s. and its contiguous strata shows that a compressional activity of the M.T.L. occurred during a relatively short period in late Early to Middle Miocene.