Geology of the Paleogene Kumage Group distributed around the Tanowaki–Okigahamada area in the northern part of Tanegashima Island, Kagoshima, SW Japan

Abstract

We investigated the stratigraphy, geological structure, and age of Paleogene marine strata, which were previously thought to be part of the Kadokura-zaki Formation in the Kumage Group, ditributed in the northern part of Tanegashima Island, Kagoshima, Japan, based on a detailed analysis of the lithologies and biostratigraphic data of radiolarian fossils. The strata are distributed in the coastal and inland areas. The Kadokura-zaki Formation, ditributed in the inland area, is divided into four structural units by reverse faults. Radiolarian biostratigraphic data were obtained from the upper part of the uppermost structural unit, which consists mainly of muddy sediments. The data indicate a late Middle Eocene age (radiolarian fossil zone RP16). On the other hand, the age of strata in the coastal area where the Kadokura-zaki Formation was previously thought to be distributed was found to be late Early Oligocene in age (lower part of the radiolarian fossil zone RP21a). Based on lithology and age, these strata are assigned to the Nishinoomote Formation. This suggests that the distribution, stratigraphy, and structure of the Kumage Group are more complex than previously thought. To understand the geology of the Kumage Group, it is necessary to consider the stratigraphy, structure, and age of the formation based on more detailed geological surveys in the middle–southern parts of Tanegashima Island.