[H-QR05] Quaternary, Diachronic dynamics of human-environment interactions

[HQR05-14] Sedimentary environment of the Aso-4 tephra interbedded layer in the Pleistocene Karato Formation, Himeshima Island, Oita Prefecture, Japan

*Tomonori Naya¹, Kiyohide Mizuno¹, Yu Horiuchi² (1.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology , 2.Oita Himeshima Geopark Promotion Office)

Keywords:Late Pleistocene, Aso-4 tephra, Diatoms, Seto Inland Sea

Himeshima Island, located near the boundary between the Suo-Nada and Iyo-Nada Seas in the western Seto Inland Sea, consists of the Maruishibana Formation, Kawashiri gravel bed, Karato Formation and lava, dike, volcanic clastics and crater lake deposits of the Himeshima volcano group (Ito et al., 1997). Recently, it has been recognised that the Karato Formation consists of several depositional cycles, including shallow and freshwater successions (Mizuno, 2018; Naya & Mizuno, 2021). The depositional age of the Karato Formation has been considered to be Early to Middle Pleistocene (Ito et al., 1997), but Naya et al. (2024) reported that the Karato Formation is intercalated with Aso-4 tephra that was deposited in MIS 5b (Aoki, 2008), indicating that the Karato Formation contains Upper Pleistocene. Naya et al. (2024) reported that Aso-4 tephra is intercalated in a freshwater layer between two shallow marine sediments. However, later diatom analysis of a densely sampled interval revealed that the Aso-4 tephra layer in the Ukisu is intercalated with marine strata. Here we report on the depositional environment of the Karato Formation in Ukisu and the stratigraphic relationship of the tephra beds containing Aso-4 tephra.

References: Aoki (2008) Quaternary International, 178, 100–118; Ito et al. (1997) Quadrangle Series Scale 1:50,000, GSJ; Mizuno (2018) Chikyu Mon. Extra ed., 69, 48–54; Naya & Mizuno (2021) Phytotaxa, 505, 85–96; Naya et al. (2024) The 131st Annual Meeting of the Geological Society of Japan, T15-O-21.