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[HCG28-P05] Microcodization in a Kasimovian section of the Omi Limestone (Niigata Prefecture, central Japan), the Upper Carboniferous
Keywords:Microcodium, Carboniferous, Kasimovian, Omi Limestone, subaerial exposure, sea-level change
INTRODUCTION
Microcodium is structure consisting of carbonate minerals, which are biologically induced via decomposition of organic matter (plant roots, fungi, corroded organic matter) by microorganisms in Ca-rich soil (Kabanov et al., 2008). Therefore, occurrence of microcodium from marine limestone is a powerful indicator of subaerial exposure.
Here, we report the occurrence of microcodium from the Gongen section of the Omi Limestone, a Caeboniferous–Permian seamount-type limestone in Niigata Prefecture, central Japan. In Japan, microcodium had been known as "blackish brown sparry calcite" (Machiyama, 1994). Hasegawa and Goto (1990) reported blackish brown sparry calcite is laterally and continuously occurred from the lower part of the Triticites zone (Kasimovian, the Upper Carboniferous) in the Omi Limestone. Takeuchi and Nakazawa (2010) also reported the occurrence of microcodium from the Upper Carboniferous in the Gongen section. However, they did not show the mode of occurrence and horizon.
AGE OF GONGEN SECTION
Schwagerinids (Fusuliniod foraminifer) are occurred from the middle (11 m horizon) and the uppermost part (28 m horizon) of the Gongen section, which exposes about 30 m–thick shallow marine limestone. They are identified as Montiparus sp., based on the following characters: 1) Small (about 2 to 3 mm diameter) and oval test with massive chomata and the relative primitive forms in the genus Triticites (s. l.) in the axial section, 2) Coiling tight in the early stage and gradually coiled with growth, and the dark colored secondary deposits near the septa and at the bottom of the chambers in the sagittal section. Hence, the middle to the uppermost horizons of the Gongen section can be correlated to the middle Kasimovian, the Upper Carboniferous (Davydov, 1990).
OCCURRENCE & CHARACTERISTICS OF MICROCODIUM
Microcodium is occurred in the upper horizon (~ 27 m horizon) of the Gongen section. The host rock of the microcodium is the Bioclastic Pack–wackestone, which is rich in fragments of crinoids and peloids. On the surface of the outcrop, the microcodium agglutination shows amorphous to gathering clouds–shape, and the gathering clouds–shaped agglutination partly extends to the direction of the bedding plane. In the microscopic views, microcodium shows a “corn-cob texture” with a diameter of ~2 mm, and is frequently occurred with solution voids filled with blocky cement.
ISOTOPIC RATIO OF MICROCODIUM
The bulk carbon isotope ratios (the δ13C values) of Bioclastic Pack–wackestone, the host rock of microcodium, range from +0.8 to +3.5 ‰ (Ave. +1.94 ‰), and the bulk oxygen isotope ratios (the δ18O values) range from -6.9 to -5.5 ‰ (Ave. -6.05). Meanwhile, the δ13C values of microcodium range from +0.9 to +2.5 ‰ (Ave. +1.68 ‰), and the δ18O values range from -6.6 to -2.6 ‰ (Ave. -4.40 ‰). On average, the microcodium has a lower δ13C value (-0.26 ‰ lower) and a higher δ18O value (+1.65 ‰ higher). The δ13C values of microcodium are lower than those of host rock, probably because more 12C were fixed in microcodium via microbial corrosion and decomposition process (Kabanov, 2008).
MICROCODIUM FROM OTHER SEAMOUNT-TYPE LIMESTONE IN JAPAN
From the above, it is interpreted that the Gongen section was subaerially exposed just after the middle Kasimovian, and then pedogenesis and microcodization were progressed on the surface of the limestone. Microcodium is also occurred from the Moscovian to the Kasimovian sections of the Akiyoshidai Limestone (Hasegawa, 1988; Machiyama, 1994; Sano et al., 2004). In particular, Sano et al. (2004) suggested that the Akiyoshidai Limestone was subaerially exposed during the late Kasimovian, based on the lack of the upper Kasimovian fusuline biozone. The microcodium has also been reported from seamount-type limestone in the Ashio Belt, which is thought to be the Kasimovian (Uchiyama et al., 2010). As the future works, it will be necessary to discuss about synchronism of the subaerial exposure and the microcodization of seamount-type limestone, including sea-level changes and the paleogeographic position of seamounts, comprehensively.
REFERENCES
Davydov, 1990, Paleontol. Jour. 2, 13-25; Hasegawa, 1988, Reports of KAKENHI Grants-in-Aid for Scientific Research (C) by JSPS, 14pp. 3 figs., 3pls.; Hasegawa & Goto, 1990, Guidebook for geol. excurs. of 97th Annual Meet. of Geol. Soc. Japan, Toyama, pp. 227 – 260; Kabanov et al., 2008, Sediment. Geol. 205, 79–99; Takeuchi & Nakazawa, 2010, In: Nagamori et al. (eds.), Explan. booklet 1:50,000 Geol. Map ”Kotaki”, pp. 15–28; Sano et al., 2004, Palaeogeogr., Palaeoclimatol., Palaeoecol. 213, 187–206; Uchino et al., 2010, Jour. Geol. Soc. Japan 116, 118-123.
Microcodium is structure consisting of carbonate minerals, which are biologically induced via decomposition of organic matter (plant roots, fungi, corroded organic matter) by microorganisms in Ca-rich soil (Kabanov et al., 2008). Therefore, occurrence of microcodium from marine limestone is a powerful indicator of subaerial exposure.
Here, we report the occurrence of microcodium from the Gongen section of the Omi Limestone, a Caeboniferous–Permian seamount-type limestone in Niigata Prefecture, central Japan. In Japan, microcodium had been known as "blackish brown sparry calcite" (Machiyama, 1994). Hasegawa and Goto (1990) reported blackish brown sparry calcite is laterally and continuously occurred from the lower part of the Triticites zone (Kasimovian, the Upper Carboniferous) in the Omi Limestone. Takeuchi and Nakazawa (2010) also reported the occurrence of microcodium from the Upper Carboniferous in the Gongen section. However, they did not show the mode of occurrence and horizon.
AGE OF GONGEN SECTION
Schwagerinids (Fusuliniod foraminifer) are occurred from the middle (11 m horizon) and the uppermost part (28 m horizon) of the Gongen section, which exposes about 30 m–thick shallow marine limestone. They are identified as Montiparus sp., based on the following characters: 1) Small (about 2 to 3 mm diameter) and oval test with massive chomata and the relative primitive forms in the genus Triticites (s. l.) in the axial section, 2) Coiling tight in the early stage and gradually coiled with growth, and the dark colored secondary deposits near the septa and at the bottom of the chambers in the sagittal section. Hence, the middle to the uppermost horizons of the Gongen section can be correlated to the middle Kasimovian, the Upper Carboniferous (Davydov, 1990).
OCCURRENCE & CHARACTERISTICS OF MICROCODIUM
Microcodium is occurred in the upper horizon (~ 27 m horizon) of the Gongen section. The host rock of the microcodium is the Bioclastic Pack–wackestone, which is rich in fragments of crinoids and peloids. On the surface of the outcrop, the microcodium agglutination shows amorphous to gathering clouds–shape, and the gathering clouds–shaped agglutination partly extends to the direction of the bedding plane. In the microscopic views, microcodium shows a “corn-cob texture” with a diameter of ~2 mm, and is frequently occurred with solution voids filled with blocky cement.
ISOTOPIC RATIO OF MICROCODIUM
The bulk carbon isotope ratios (the δ13C values) of Bioclastic Pack–wackestone, the host rock of microcodium, range from +0.8 to +3.5 ‰ (Ave. +1.94 ‰), and the bulk oxygen isotope ratios (the δ18O values) range from -6.9 to -5.5 ‰ (Ave. -6.05). Meanwhile, the δ13C values of microcodium range from +0.9 to +2.5 ‰ (Ave. +1.68 ‰), and the δ18O values range from -6.6 to -2.6 ‰ (Ave. -4.40 ‰). On average, the microcodium has a lower δ13C value (-0.26 ‰ lower) and a higher δ18O value (+1.65 ‰ higher). The δ13C values of microcodium are lower than those of host rock, probably because more 12C were fixed in microcodium via microbial corrosion and decomposition process (Kabanov, 2008).
MICROCODIUM FROM OTHER SEAMOUNT-TYPE LIMESTONE IN JAPAN
From the above, it is interpreted that the Gongen section was subaerially exposed just after the middle Kasimovian, and then pedogenesis and microcodization were progressed on the surface of the limestone. Microcodium is also occurred from the Moscovian to the Kasimovian sections of the Akiyoshidai Limestone (Hasegawa, 1988; Machiyama, 1994; Sano et al., 2004). In particular, Sano et al. (2004) suggested that the Akiyoshidai Limestone was subaerially exposed during the late Kasimovian, based on the lack of the upper Kasimovian fusuline biozone. The microcodium has also been reported from seamount-type limestone in the Ashio Belt, which is thought to be the Kasimovian (Uchiyama et al., 2010). As the future works, it will be necessary to discuss about synchronism of the subaerial exposure and the microcodization of seamount-type limestone, including sea-level changes and the paleogeographic position of seamounts, comprehensively.
REFERENCES
Davydov, 1990, Paleontol. Jour. 2, 13-25; Hasegawa, 1988, Reports of KAKENHI Grants-in-Aid for Scientific Research (C) by JSPS, 14pp. 3 figs., 3pls.; Hasegawa & Goto, 1990, Guidebook for geol. excurs. of 97th Annual Meet. of Geol. Soc. Japan, Toyama, pp. 227 – 260; Kabanov et al., 2008, Sediment. Geol. 205, 79–99; Takeuchi & Nakazawa, 2010, In: Nagamori et al. (eds.), Explan. booklet 1:50,000 Geol. Map ”Kotaki”, pp. 15–28; Sano et al., 2004, Palaeogeogr., Palaeoclimatol., Palaeoecol. 213, 187–206; Uchino et al., 2010, Jour. Geol. Soc. Japan 116, 118-123.