5:15 PM - 6:30 PM
[HQR04-P10] Identification of middle to late Pleistocene marine sediments inferred from electric conductivity analysis of the Bando observation well core in Tokushima Plain, western Japan
Keywords:Tokushima Plain, subsurface geology, Kitajima Formation, Median Tectonic Line Fault Zone
Tokushima Plain is a coastal plain located on the western coast of the Kii Channel. Along the northern margin of the plain, the Median Tectonic Line Fault Zone is distributed, and displaces fans and Pleistocene terraces. The fault zone is a reverse fault with large right-lateral displacement and uplift of the northern side around the plain (Okada, 2020). This suggests and the plain can be regarded as a tectonic basin by the MTL fault zone. Since 1960s, many studies about the subsurface geology of the plain have been conducted (Kawamura & Nishiyama, 2019). The Quaternary subsurface geology can be roughly divided into two formations, the Tokushima and Kitajima Formations, which are related to Alluvium and the middle to late Pleistocene, respectively. However, detail sedimentary periods and environments have not been clarified.
We attempted to identify the marine sediments in the Kitajima Formation based on electrical conductivity [EC] analysis of the Bando observation well core, 502 m long core drilled in the plain in 1995 by GSJ. Matsumoto & Arai (2021) reported stratigraphic description and pollen fossil and paleomagnetic analyses of the core. These indicate that several glacial-interglacial cycles are preserved, and the Brunhes-Matsuyama [B-M] boundary may be located between 339 and 440 m depth. In this study, EC analysis was carried out on subsamples collected from lower than 90 m depth. Sample treatment and interpretation were based on Yokoyama & Sato (1987) and Niwa et al. (2009).
As the results, we found five marine sediments; M1, 93.4–102.45 m depth; M2, 228.08–237.00 m; M3, 258.83–263.98 m; M4, 2 88.06–288.60 m depth; M5, 305.50 m. Comparison with the pollen fossil stratigraphy of the Osaka Group (Hongo, 2009), the M1 layer probably deposited during the MIS5e because of the high abundance of Lagerstoemia (Matsumoto & Arai, 2021). High abundance of Cyclobalanopsis in the M3 layer (Matsumoto & Arai, 2021) suggests that this layer deposited during the MIS11. Assuming these interpretations are correct, M2 layer, located between the M1 and M3 layers, should be corresponding to the MIS 7 or 9 marine sediment. In addition, the M4 and M5 layers might deposit during the MIS 13, 15, or 17 because these are above the B-M boundary.
These insights suggest that glacial-interglacial cycles sediments after the middle Pleistocene had been recorded in the subsurface geology of the Tokushima Plain. The average subsidence rate is estimated to be ~0.7 to ~0.8 mm/yr. For clarifying detail stratigraphy of the subsurface geology in the plain, we will conduct volcanic ash, pollen fossils and paleomagnetic analyses of the core materials in future.
References
Hongo, M. (2009) Middle Pleistocene pollen biostratigraphy in the Osaka sedimentary basin, southwest Japan, with special reference to paleoenvironmental change. The Journal of the Geological Society of Japan, 115, 64–79.
Kawamura, N. & Nishiyama, K. (2019) Correlation study of the late Pleistocene and Holocene deposits underlying the major coastal plains of Shikoku, Southwest Japan. The Journal of the Geological Society of Japan, 125, 87–105.
Matsumoto, N. & Arai, T. (2021) Report on construction of facilities and equipment for groundwater monitoring for earthquake prediction in 1995, No.2*. Open-File Report of the Geological Survey of Japan, AIST, no.713.
Niwa, Y. et al. (2009) Evidence for late Holocene coseismic subsidence identified in drilling cores along the Yoro fault system, western margin of the Nobi plain, central Japan. Daiyonki-Kenkyu, 48, 339–349.
Okada, A. (2020) Median Tectonic Line Fault Zone. 368p. Kokon-Shoin, Tokyo.
Yokoyama, T. & Sato, M. (1987) Analysis of paleo-environments based on electric conductivity of stirred clayey sediments into water -On two drilled cores of Osaka Group in Senriyama Hills and Biwako Group at bottom of Lake Biwa, Japan-. The Journal of the Geological Society of Japan, 93, 667–679.
We attempted to identify the marine sediments in the Kitajima Formation based on electrical conductivity [EC] analysis of the Bando observation well core, 502 m long core drilled in the plain in 1995 by GSJ. Matsumoto & Arai (2021) reported stratigraphic description and pollen fossil and paleomagnetic analyses of the core. These indicate that several glacial-interglacial cycles are preserved, and the Brunhes-Matsuyama [B-M] boundary may be located between 339 and 440 m depth. In this study, EC analysis was carried out on subsamples collected from lower than 90 m depth. Sample treatment and interpretation were based on Yokoyama & Sato (1987) and Niwa et al. (2009).
As the results, we found five marine sediments; M1, 93.4–102.45 m depth; M2, 228.08–237.00 m; M3, 258.83–263.98 m; M4, 2 88.06–288.60 m depth; M5, 305.50 m. Comparison with the pollen fossil stratigraphy of the Osaka Group (Hongo, 2009), the M1 layer probably deposited during the MIS5e because of the high abundance of Lagerstoemia (Matsumoto & Arai, 2021). High abundance of Cyclobalanopsis in the M3 layer (Matsumoto & Arai, 2021) suggests that this layer deposited during the MIS11. Assuming these interpretations are correct, M2 layer, located between the M1 and M3 layers, should be corresponding to the MIS 7 or 9 marine sediment. In addition, the M4 and M5 layers might deposit during the MIS 13, 15, or 17 because these are above the B-M boundary.
These insights suggest that glacial-interglacial cycles sediments after the middle Pleistocene had been recorded in the subsurface geology of the Tokushima Plain. The average subsidence rate is estimated to be ~0.7 to ~0.8 mm/yr. For clarifying detail stratigraphy of the subsurface geology in the plain, we will conduct volcanic ash, pollen fossils and paleomagnetic analyses of the core materials in future.
References
Hongo, M. (2009) Middle Pleistocene pollen biostratigraphy in the Osaka sedimentary basin, southwest Japan, with special reference to paleoenvironmental change. The Journal of the Geological Society of Japan, 115, 64–79.
Kawamura, N. & Nishiyama, K. (2019) Correlation study of the late Pleistocene and Holocene deposits underlying the major coastal plains of Shikoku, Southwest Japan. The Journal of the Geological Society of Japan, 125, 87–105.
Matsumoto, N. & Arai, T. (2021) Report on construction of facilities and equipment for groundwater monitoring for earthquake prediction in 1995, No.2*. Open-File Report of the Geological Survey of Japan, AIST, no.713.
Niwa, Y. et al. (2009) Evidence for late Holocene coseismic subsidence identified in drilling cores along the Yoro fault system, western margin of the Nobi plain, central Japan. Daiyonki-Kenkyu, 48, 339–349.
Okada, A. (2020) Median Tectonic Line Fault Zone. 368p. Kokon-Shoin, Tokyo.
Yokoyama, T. & Sato, M. (1987) Analysis of paleo-environments based on electric conductivity of stirred clayey sediments into water -On two drilled cores of Osaka Group in Senriyama Hills and Biwako Group at bottom of Lake Biwa, Japan-. The Journal of the Geological Society of Japan, 93, 667–679.