[H-QR04] Quaternary, Diachronic dynamics of human-environment interactions

[HQR04-06] Paleoenvironmental change in Paleo-Tokyo Bay during MIS 5e inferred from ostracode assemblages of the Pleistocene Kioroshi Formation, Shimosa Group, northern Chiba Prefecture, central Japan.

*Mai Harashima¹, Tsutomu Nakazawa², Hirokazu Ozawa³, Minoru Kaneko, Hiroyuki Ishikawa, Masahiro Nomura⁴, Sachiko Agematsu¹ (1.University of Tsukuba, 2.Geological Survey of Japan, AIST, 3.Nihon University, 4.Surugadai University)

Keywords:Ostracoda, Kioroshi Formation, MIS 5e, Paleo-Tokyo Bay

The Pleistocene Kioroshi Formation, Shimosa Group which is distributed beneath the Shimosa Upland in Chiba Prefecture, central Japan is composed mainly of shallow marine deposits accumulated in Paleo-Tokyo Bay during MIS 5e. The depositional sequence of the Kioroshi Formation comprises a lower incised valley system and an upper barrier island system. Ostracode assemblages have been reported from the Kioroshi Formation by Yajima (1978, 1982) and Okazaki et al. (2018), but the succession of ostracode assemblages throughout the Kioroshi Formation including both incised valley and barrier island systems has not been examined so far. We report ostracode assemblages from the lower and upper parts of the Kioroshi Formation in sediment cores and interpret paleoenvironmental change in Paleo-Tokyo Bay during MIS 5e.

Fossil ostracodes were collected from two sediment cores: GS-NT-1 (Narita City, Chiba; oceanward) and GS-NY-1 (Nagareyama City, Chiba; landward) recovered from the sites within a buried incised valley. We identified 81 ostracode species from GS-NT-1 and 77 species from GS-NY-1. Based on the ostracode assemblages, the examined intervals in cores GS-NT-1 and GS-NY-1 are divided into four zones, respectively; zones NT-I–IV and zones NY-I–IV in ascending order.

Zone NT-I is characterized by Spinileberis quadriaculeata–Bicornucythere bisanensis assemblage accompanied with Cytheromorpha acupunctata, indicating an enclosed inner-bay environment near a middle bay. Zone NT-II characterized by S. quadriaculeata–B. bisanensis–C. acupunctata–Pontocythere miurensis assemblage shows an inner bay environment with coastal currents. Zone NT-III is characterized by Neomonoceratina delicata assemblage accompanied with S. quadriaculeata and C. acupunctata, seems to represent an inner bay environment under the influence of a warm current. Zone NT-IV is characterized by Aurila spp.–P. miurensis assemblage accompanied with Cythere omotenipponica, indicating an open bay environment near seaweed beds influenced by open-ocean water. According to Nakazawa et al. (2017), zones NT-I–NT-II, zone NT-III and zone NT-IV correspond to the sand bar facies which is incised valley system, the sand bar facies–the central basin facies, and the central basin facies–the flood tidal facies which is barrier island system, respectively.

Zone NY-I is characterized by S. quadriaculeata–C. acupunctata assemblage, indicating an enclosed inner bay environment slightly near a middle bay. Zone NY-II is characterized by C. acupunctata assemblage accompanied with N. delicata and S. quadriaculeata, seems to represent an inner bay environment under the influence of a warm current. Zone NY-III is characterized by Loxoconcha uranouchiensis assemblage accompanied with S. quadriaculeata and C. acupunctata, indicating an inner bay environment near seaweed beds. Zone NY-IV characterized by Aurila spp.–Loxoconcha optima assemblage accompanied with C. acupunctata and Xestoleberis hanaii shows an open bay environment near seaweed beds influenced by open water. The boundary between zones NY-I–NY-III and zone NY-IV correspond with lithologic divisions respectively the lower incised valley system and the upper barrier system.


References:

Nakazawa, T., Sakata, K., Hongo, M. and Nakazato, H. (2017) Transition from incised valley to barrier island systems during MIS 5e in the northern Chiba area, Kanto Plain, central Japan. Quaternary International, 456, 85–101.

Okazaki, H., Ishii, A., Kaneko, M., Tamura, T., Chigaku Club of Gunma Prefectural Ota Girls’ High School, Kato, H., Isaji, S. and Tanaka, G. (2018) An Excavation Report of a Baleen Whale, from the Pleistocene Kioroshi Formation, Shimosa Group - Sedimentary Facies, Microfossils, Ground Penetrating Radar Analysis. J. Nat. Hist. Mus. Inst. Chiba, 14(1), 19–28 (in Japanese with English abstract).

Yajima (1978) Quaternary Ostracoda from Kisarazu near Tokyo. Trans. Proc. Palaeontol. Soc. Japan, N. S., 112, 371–409.

Yajima (1982) Late Pleistocene Ostracoda from the Atsumi Peninsula, central Japan. Trans. Proc. Palaeontol. Soc. Japan, N. S., 146, 49–76.