The base of the Chibanian Stage is defined by the base of the Ontake-Byakubi-E (Byk-E) regional tephra bed, in the Chiba Section, Japan.
The Chiba section is located on the Boso Peninsula 26km south of the city of Ichihara in the Prefecture of Chiba, Japan. Latitude: 35°17'39.6'' to 35°17'36.9'' north, longitude: 140°08'47.6'' to 140°08'47.2'' east. The Chiba section is exposed along the eastern gorge wall of the Yoro River.
The Chiba section was deposited on the continental slope in an open-ocean setting. The sediments are bioturbated, gray, hemipelagic, silty, and parallel-bedded, reflecting deposition of fine-grained suspended material under quiescent bottom-water conditions. Sedimentation rates across the GSSP are high at ~890 mm per thousand years. The Byk-E tephra bed is a white pumiceous fine ash deposit ~3.5 cm thick.
The base of the Calabrian Stage is recognised locally and regionally by the base of the Byk-E tephra bed. The primary guide to the boundary globally is the Matuyama–Brunhes paleomagnetic reversal which has a directional midpoint 1.1 m (astronomical age of 772.9 ka) above the GSSP.
The GSSP occurs within MIS 19 and specifically at the top of MIS 19c. This places it immediately below stratigraphic indicators for the inception of glaciation at the onset of MIS 19b.
The GSSP has an astronomical age of 774.1 ± 5.0 ka. The Byk-E tephra bed, and hence the GSSP, has a U-Pb zircon age of 772.7 ± 7.2 ka for the eruption/deposition age which is consistent with its astronomically estimated age.
The base of the Chiba Stage defines that of the Middle Pleistocene Subseries and both were ratified by the Executive Committee of the IUGS on 17 January 2020.
Head, M.J. 2021. Review of the Early–Middle Pleistocene boundary and Marine Isotope Stage 19. Progress in Earth and Planetary Science, 8:50, 1–38. https://doi.org/10.1186/s40645-021-00439-2
Head, M.J., Pillans, B., and Farquhar, S., 2008. The Early–Middle Pleistocene Transition: characterization and proposed guide for the defining boundary. Episodes 31(2): 255–259. https://doi.org/10.18814/epiiugs/2008/v31i2/014
Head, M.J., Pillans, B., Zalasiewicz, J.A., and the ICS Subcommission on Quaternary Stratigraphy, 2021. Formal ratification of subseries/subepochs for the Pleistocene Series/Epoch of the Quaternary System/Period. Episodes 44 (3): 241–247. https://doi.org/10.18814/epiiugs/2020/020066
Suganuma, Y., Okada, M., Horie, K., Kaiden, H., Takehara, M., Senda, R., Kimura, J.-I., Kawamura, K., Haneda, Y., Kazaoka, O., Head, M.J., 2015. Age of Matuyama-Brunhes boundary constrained by U-Pb zircon dating of a widespread tephra. Geology, 43: 491–494; https://doi.org/10.1130/G36625.1
Suganuma, Y., Okada, M., Head, M.J., Kameo, K., Haneda, Y., Hayashi, H., Irizuki, T., Itaki, T., Izumi, K., Kubota, Y., Nakazato, H., Nishida, N., Okuda, M., Satoguchi, Y., Simon, Q., Takeshita, Y. 2021. Formal ratification of the Global Boundary Stratotype Section and Point (GSSP) for the Chibanian Stage and Middle Pleistocene Subseries of the Quaternary System: the Chiba Section, Japan. Episodes 44 (3): 317–347. https://doi.org/10.18814/epiiugs/2020/020080