[SEM22-07] Magnetostratigraphy of the Nakakoshiki Formation (Eocene), Koshikijima islands, Kagoshima Prefecture
Keywords:Eocene, magnetostratigraphy, Koshikijima islands, Kagoshima Prefecture, Nakakoshiki Formation
The Nakakoshiki Formation occupies the basal part of the Kamikoshikijima Group and consists of red and grey mudstone, sandstone, and conglomerate. Six stratigraphic sections exposed along the coastline southern part of Kamikoshiki island were extensively sampled. 83 samples were thermally demagnetized up to a maximum of 700 °C, and the component structure of the NRM was plotted on vector end-point demagnetization diagrams. In addition, in order to identify the magnetic carriers of the rocks, rock magnetic experiments were performed.
Thermal demagnetization of the red mudstone samples revealed three distinct remanent magnetization components, referred to as components A, B, and C, respectively. The last demagnetized component (Component C) with both polarities is interpreted to be the primary magnetization, which is carried by specular hematite with unblocking temperatures between 640°C and 700°C. Based on the polarities of Component C, we established the magnetostratigraphy. Considering the mammal age and radiometric dates reported from the Nakakoshiki Formation, the magnetic polarity zonation is consistent with a correlation to the world magnetic polarity time scale at about 50–52 m.y.