5:15 PM - 6:45 PM
[BCG06-P12] Depositional environment and age of the Torinosu-type limestone in Seiyo City, Ehime Prefecture, SW Japan
The upper Jurassic to lower Cretaceous Torinosu-type limestone is widely distributed from Kyushu in the south to Hokkaido in the north (Tamura, 1960), and generally exposed as small rock bodies in geological successions dominated by sandstone and mudstone. This means that during the late Jurassic to early Cretaceous, the supply of carbonate minerals locally exceeded the supply of terrigenous siliciclastics over a relatively large area along the eastern margin of Eurasia. A previously assumed that the formation of Torinosu-type limestone was related to a global sea level rise or eutrophication (Kakizaki et al., 2012); however, the causes that led to the increased supply of carbonate minerals are not fully understood, because the Torinosu-type limestone often lacks boundary outcrops with underlying siliciclastic rocks. In addition, information on whether such events occurred simultaneously or sporadically over a longer span is also important in order to determine the cause of the increased supply of carbonate minerals, and therefore the depositional age of the limestone must be determined with high resolution.
In this study, we conducted a detailed geological survey in Shirokawa town, Seiyo City, Ehime Prefecture, and newly discovered a continuous outcrop of Torinosu-type limestone and underlying siliciclastic rocks, which we termed Nakatsugawa section. The purpose of this study is to determine the factors that initiated and terminated the formation of the Torinosu-type limestone, by clarifying the components of the limestone and siliciclastic rocks and applying Sr isotope stratigraphy to the limestone.
Around Nakatsugawa section, the succession is composed of sandstone-mudstone alternation, sandstone with tabular and trough cross bedding, bedded limestone, and massive limestone, in ascending order. According to thin sections observation and XRD analysis, the sandstone underlying the limestone also contains about 50% calcite mainly as bioclasts and cements. The limestone contains calcite mainly as reefal metazoans, microbialite, and ooid, but quartz and feldspar are also contained about 5% in the lower part of the limestone (about 70 m from the bottom). These results imply that the increase in carbonate mineral supply that led to the formation of limestone in this section was caused by a fall of the relative sea level. Sr isotope stratigraphy based on the method of Shiraishi et al. (2005) was applied to brachiopod shells found in the central part of the limestone body, which suggested a depositional age of 147.5 (±0.5) Ma. The results will be discussed in more detail in the presentation.
References
Kakizaki, Y., Ishikawa, T., Nagaishi, K., Tanimizu, M., Hasegawa, T., Kano, A. (2012) Strontium isotopic ages of the Torinosu-type limestones (latest Jurassic to earliest Cretaceous, Japan): Implication for biocalcification event in northwestern Palaeo-Pacific. Journal of Asian Earth Sciences, 46, 140–149.
Shiraishi, F., Hayasaka, Y., Takahashi, Y., Tanimizu, M., Ishikawa, T., Matsuoka, J., Murayama, M., Kano, A. (2005) Strontium isotopic age of the Torinosu Limestone in Niyodo Village, Kochi Prefecture, SW Japan. Journal of Geological Society of Japan, 111(10), 610–623.
Tamura, M. (1960) A stratigraphic study of the Torinosu Group and its relatives. Memoir of Faculty of Education, Kumamoto University, 8, 1–40.
In this study, we conducted a detailed geological survey in Shirokawa town, Seiyo City, Ehime Prefecture, and newly discovered a continuous outcrop of Torinosu-type limestone and underlying siliciclastic rocks, which we termed Nakatsugawa section. The purpose of this study is to determine the factors that initiated and terminated the formation of the Torinosu-type limestone, by clarifying the components of the limestone and siliciclastic rocks and applying Sr isotope stratigraphy to the limestone.
Around Nakatsugawa section, the succession is composed of sandstone-mudstone alternation, sandstone with tabular and trough cross bedding, bedded limestone, and massive limestone, in ascending order. According to thin sections observation and XRD analysis, the sandstone underlying the limestone also contains about 50% calcite mainly as bioclasts and cements. The limestone contains calcite mainly as reefal metazoans, microbialite, and ooid, but quartz and feldspar are also contained about 5% in the lower part of the limestone (about 70 m from the bottom). These results imply that the increase in carbonate mineral supply that led to the formation of limestone in this section was caused by a fall of the relative sea level. Sr isotope stratigraphy based on the method of Shiraishi et al. (2005) was applied to brachiopod shells found in the central part of the limestone body, which suggested a depositional age of 147.5 (±0.5) Ma. The results will be discussed in more detail in the presentation.
References
Kakizaki, Y., Ishikawa, T., Nagaishi, K., Tanimizu, M., Hasegawa, T., Kano, A. (2012) Strontium isotopic ages of the Torinosu-type limestones (latest Jurassic to earliest Cretaceous, Japan): Implication for biocalcification event in northwestern Palaeo-Pacific. Journal of Asian Earth Sciences, 46, 140–149.
Shiraishi, F., Hayasaka, Y., Takahashi, Y., Tanimizu, M., Ishikawa, T., Matsuoka, J., Murayama, M., Kano, A. (2005) Strontium isotopic age of the Torinosu Limestone in Niyodo Village, Kochi Prefecture, SW Japan. Journal of Geological Society of Japan, 111(10), 610–623.
Tamura, M. (1960) A stratigraphic study of the Torinosu Group and its relatives. Memoir of Faculty of Education, Kumamoto University, 8, 1–40.