2018 Volume 124 Issue 5 Pages 313-329
Geology, geological age, and sedimentary environment of the Nishinoomote Formation in the Kumage Group distributed in Tanegashima Island, Kagoshima, Japan were examined on the basis of detailed mapping of lithology, biostratigraphic data of radiolarian fossils and calcareous nannofossils, and ichnological analysis. Since the Nishinoomote Formation consists mainly of sandy sediments, mudstone containing large amounts of trace fossils in the middle stratigraphic interval of the Nishinoomote is newly subdivided as the Okubo Mudstone Member. The Nishinoomote Formation correlated with the radiolarian fossil zone RP20c-RP21a (31.1-28.5 Ma) and the calcareous nannofossil zone NP23-NP24 (30.00-26.84 Ma). Therefore, geological age of the Nishinoomote Formation can be considered to be late early Oligocene (30.0-28.5 Ma).
Investigation of the distribution pattern of lithology and the geological age revealed that (1) contemporaneous stratigraphic intervals in the Nishinoomote Formation repeatedly appeared due to folds and thrusts, (2) total thickness of the Nishinoomote seems to be less than 1000 m, which is thinner than those estimated by previous authors, (3) the Nishinoomote can be correlated with not the Hyuga but the Nichinan Groups distributed in Kyusyu, and (4) depositional environment for the Nishinoomote might be a sea-floor deeper than 2000 m in water depth. Further detailed sedimentological approach must be required to clarify the origin of the Nishinoomote Formation.
