09:15 〜 09:30
[BCG07-02] ガボン前期原生代フランスヴィル層群の炭酸塩炭素同位体層序
★招待講演
キーワード：前期原生代、炭素同位体比、ガボン
The large fluctuations of δ13C_carb in the Paleoproterozoic are important in terms of the speculation on the atmospheric oxygen level and the global stratigraphic correlations. The interval characterized by the generally high values of δ13C_carb up to +10 ‰ is called Lomagundi-Jatuli Event (LJE; ca. 2.2-2.0 Ga) and interpreted as reflecting high rates of oxygen production (e.g. Karhu & Holland, 1996; Martin et al., 2011). The drop of δ13C_carb and δ13C_org at the end of LJE are reported from Shunga in Fennoscandia, and the negative shift of δ13C_org is correlated with that of Franceville in Gabon (Kump et al., 2011); however, the δ13C_carb data in Gabon is still missing because of the scarcity of the carbonate rocks. The Paleoproterozoic Francevillian Group in Gabon consists of FA (fluvial sandstones), FB (black shales/carbonates), FC (cherts), FD (black shales), and FE (sandstones) in ascending order. The lithostratigraphic correlation between the Franceville, Lastoursville, and Okondja basins suggests that the FA-FB-FC units are in different depositional settings and the FD-FE units are in common facies, which indicates the final opening of the intracratonic basins. In order to explore the global correlation ofδ13C_carb, the analyses in the FD-FE units are required. In this study, we conducted two drillings in the Lastoursville and Okondja basins to obtain the fresh core-samples of FD. The lithological observations clarified that numbers of carbonate layers are intercalated in black shales in the FD unit in the both areas. We analyzed δ13C_carb of these carbonate layers, and filled in the stratigraphic blanks in the FD unit. In this presentation, we will discuss the propriety of the δ13C_carb stratigraphic correlations.