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[BCG06-P05] Formation factors of the Upper Jurassic–Lower Cretaceous Torinosu-type Limestone
The upper Jurassic to lower Cretaceous Torinosu-type limestone is widely distributed from Kyushu in the south to Hokkaido in the north (e.g. 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. However, the causes that led to the increased relative supply of carbonate minerals are not fully understood, because the Torinosu-type limestone often lacks boundary outcrops with overlying and underlying siliciclastic rocks, and the limestone bodies that have been precisely dated is still limited. 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, and Kikunotani section was also established in the second largest limestone body after the limestone body exposed at the Nakatsugawa section. The objective of this study is to elucidate the factors that initiated and terminated the formation of the Torinosu-type limestone, by clarifying the components of the limestone and siliciclastic rocks, evaluating the environmental changes and the diagenetic effects during limestone formation from carbon and oxygen isotope ratios, and applying Sr isotope stratigraphy to these limestone bodies to date them with high resolution in these sections.
Around Nakatsugawa section, the succession is composed of sandstone-mudstone alternation with hummocky cross bedding, sandstone with trough cross bedding, bedded limestone, and massive limestone in ascending order. These results suggest that limestone formation initiated as a result of a decrease in water depth. According to XRD analysis and point counting, lithofacies of Nakatsugawa section in ascending order is composed of grainstone consisting mainly of terrigenous siliciclastics and peloids, wacke/packstone consisting mainly of limemud and terrigenous siliciclastics, flamestone consisting mainly of reef-builders, microbial crusts, microencrusters, wackestone consisting mainly of matrix including terrigenous siliciclastics, grains probably radiolarians. Carbon isotope ratios exhibit increasing trend in the lower part of limestone body. This implies that bioproductivity have increased at the initiation of limestone formation. While Kikunotani section consists only of massive limestone and all horizons contained more than 90% calcite and less than 7.2% terrigenous siliciclastics based on XRD analysis. The lower terrigenous siliclastics content than in Nakatsugawa section may be caused by the lack of exposure of the transition part with overlying and underlying siliciclastic rocks of limestone body.
These results imply that limestone formation initiated by decreasing current velocities and the growth of calcareous skeletal organisms in a shallow water environment terminated by increasing water depth. Sr isotope stratigraphy based on the method of Shiraishi et al. (2005) was applied to brachiopod shells found in the middle part of the limestone body, which suggested a depositional age of 147.5 (±0.5) Ma. This means that it is coeval with depositional duration of other limestone bodies estimated by previous studies using Sr isotope stratigraphy.
References
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
Around Nakatsugawa section, the succession is composed of sandstone-mudstone alternation with hummocky cross bedding, sandstone with trough cross bedding, bedded limestone, and massive limestone in ascending order. These results suggest that limestone formation initiated as a result of a decrease in water depth. According to XRD analysis and point counting, lithofacies of Nakatsugawa section in ascending order is composed of grainstone consisting mainly of terrigenous siliciclastics and peloids, wacke/packstone consisting mainly of limemud and terrigenous siliciclastics, flamestone consisting mainly of reef-builders, microbial crusts, microencrusters, wackestone consisting mainly of matrix including terrigenous siliciclastics, grains probably radiolarians. Carbon isotope ratios exhibit increasing trend in the lower part of limestone body. This implies that bioproductivity have increased at the initiation of limestone formation. While Kikunotani section consists only of massive limestone and all horizons contained more than 90% calcite and less than 7.2% terrigenous siliciclastics based on XRD analysis. The lower terrigenous siliclastics content than in Nakatsugawa section may be caused by the lack of exposure of the transition part with overlying and underlying siliciclastic rocks of limestone body.
These results imply that limestone formation initiated by decreasing current velocities and the growth of calcareous skeletal organisms in a shallow water environment terminated by increasing water depth. Sr isotope stratigraphy based on the method of Shiraishi et al. (2005) was applied to brachiopod shells found in the middle part of the limestone body, which suggested a depositional age of 147.5 (±0.5) Ma. This means that it is coeval with depositional duration of other limestone bodies estimated by previous studies using Sr isotope stratigraphy.
References
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