11:45 AM - 12:00 PM
[MZZ52-05] Formative processes of the Tosa-suzuri inkstone in Mihara Village, Kochi, Japan: implications of burial and thermal diagenetic history of the Southern Shimanto Belt
Keywords:inkstone, Mihara Village, Accretionary Prism, Raman spectroscopy
The inkstone has important cultural values in East Asian countries, and is necessary for traditional calligraphy and brush painting to make liquid ink by grinding solid ink with water. In Japan, the inkstone deposits widely occur along the outer zone in Southwest Japan1. The materials for the inkstone are taken from mudstone in the alternating sequence of sandstone and mudstone of the Cenozoic accretionary prism, but to date, the formation of the inkstone in terms of geological processes is not clearly understood due to the lack of scientific analysis. In this study, to verify the formative processes of the inkstone, we examined the burial and thermal diagenetic processes of the Cenozoic accretionary prism in the Mihara Village and adjacent areas of Kochi, SW Japan, which is known as the producing district of the Tosa-suzuri inkstone.
The Eocene-Oligocene Southern Shimanto Belt is widely exposed in the studied area, with the strata generally striking northeast-southwest2. Also, the Southern Shimanto Belt is intruded by the middle Miocene granitic rocks, which crop out at the toe of the Otsuki Peninsula, SW Kochi. In this study, we collected sandstone samples from the Southern Shimanto Belt because of the occurrence of relatively larger carbonaceous fragments in sandstones than those in mudstones, which take advantage of the thermal diagenetic analysis. Obtained samples were cut to make slabs, and the cut surface was analyzed by using a laser micro-Raman spectrometer and estimated the experienced maximum temperature of the samples during diagenesis3. The obtained Raman spectra of carbonaceous fragments show that the samples experienced maximum temperatures from 244 to 406℃ in the studied area. Also, a systematic decreasing trend of the temperature is observable with the distance of the possible heat source of the intruded middle Miocene granite within the Paleogene accretionary prism.
Based on the age of sedimentation and thermal diagenesis of the studied succession, we consider that mainly two step-wise diagenesis is essential for the formation of the inkstone. First, burial consolidation took place since the deposition of the strata at the latest in the Oligocene, and this process was prolonged at least ca. 10 m.y. Then, thermal diagenesis is due to the intrusion of magma in the middle Miocene. These two processes harden enough the strata into the production of the inkstone. Furthermore, a similar geological setting, i.e., distribution of the Cenozoic accretionary prism with the intrusion of the middle Miocene granite, is well-known in the inkstone-producing district along the outer zone in SW Japan such as Nachiguro-suzuri inkstone in the Kii Peninsula and Kokei-suzuri inkstone in the Kyushu Island4,5. The combination of burial consolidation and thermal diagenesis is essential for the formation of the traditional inkstone along the outer zone in SW Japan.
References: 1Shirono, 1886, J. Inkstone, 2, 234–282; 2Geol. Soc. Japan, ed., 2016, Regional Geology of Japan 7, Shikoku Region. 708 p; 3Kouketsu et al., 2014, Island Arc, 23, 33–50; 4Geol. Soc. Japan, ed., 2009, Regional Geology of Japan 5, Kinki Region. 453 p. 5Geol. Soc. Japan, ed., 2010, Regional Geology of Japan 8, Kyushu and Okinawa Region. 619 p.
The Eocene-Oligocene Southern Shimanto Belt is widely exposed in the studied area, with the strata generally striking northeast-southwest2. Also, the Southern Shimanto Belt is intruded by the middle Miocene granitic rocks, which crop out at the toe of the Otsuki Peninsula, SW Kochi. In this study, we collected sandstone samples from the Southern Shimanto Belt because of the occurrence of relatively larger carbonaceous fragments in sandstones than those in mudstones, which take advantage of the thermal diagenetic analysis. Obtained samples were cut to make slabs, and the cut surface was analyzed by using a laser micro-Raman spectrometer and estimated the experienced maximum temperature of the samples during diagenesis3. The obtained Raman spectra of carbonaceous fragments show that the samples experienced maximum temperatures from 244 to 406℃ in the studied area. Also, a systematic decreasing trend of the temperature is observable with the distance of the possible heat source of the intruded middle Miocene granite within the Paleogene accretionary prism.
Based on the age of sedimentation and thermal diagenesis of the studied succession, we consider that mainly two step-wise diagenesis is essential for the formation of the inkstone. First, burial consolidation took place since the deposition of the strata at the latest in the Oligocene, and this process was prolonged at least ca. 10 m.y. Then, thermal diagenesis is due to the intrusion of magma in the middle Miocene. These two processes harden enough the strata into the production of the inkstone. Furthermore, a similar geological setting, i.e., distribution of the Cenozoic accretionary prism with the intrusion of the middle Miocene granite, is well-known in the inkstone-producing district along the outer zone in SW Japan such as Nachiguro-suzuri inkstone in the Kii Peninsula and Kokei-suzuri inkstone in the Kyushu Island4,5. The combination of burial consolidation and thermal diagenesis is essential for the formation of the traditional inkstone along the outer zone in SW Japan.
References: 1Shirono, 1886, J. Inkstone, 2, 234–282; 2Geol. Soc. Japan, ed., 2016, Regional Geology of Japan 7, Shikoku Region. 708 p; 3Kouketsu et al., 2014, Island Arc, 23, 33–50; 4Geol. Soc. Japan, ed., 2009, Regional Geology of Japan 5, Kinki Region. 453 p. 5Geol. Soc. Japan, ed., 2010, Regional Geology of Japan 8, Kyushu and Okinawa Region. 619 p.