PALEOPROTEROZOIC HIGH δ 13C DOLOMITES FROM THE LASTOURSVILLE AND FRANCEVILLE BASINS (SE GABON): STRATIGRAPHIC AND SYNSEDIMENTARY SUBSIDENCE IMPLICATIONS

With the discovery in the 1970s of two-billion year old nuclear reactors in Oklo, Gabon, the Paleoproterozoic of the Franceville basin (Gabon), neglected for many years, attracted the attention of the international community. This led to the intensive study of the basin for about ten years. Between 2006 and the present time we participated in a large mapping project sponsored by the European Union (SYSMIN funds). Our new data for the isotopic composition of carbon in shallow marine sedimentary carbonates in southeastern Gabon (Franceville and Lastoursville sub-basins) indicate that the value of δ¹³C in these sediments underwent a large positive excursion around 2.1 Ga. These carbonate rocks belonging to the FC Formation in the Francevillian Group were deposited along the margin of the Archean Chaillu Craton in open-marine shelf and evaporitive settings and experienced, locally, lower greenschist facies metamorphism. Four microfacies (MF1-MF4) are recognized from subtidal cyanobacterial dolobindstones to inter- and supratidal collapse breccias (MF4). The dolomites exhibit elevated ¹³C values, most lying above 0.50 ‰, the higher value is 9.74‰ and the average value is near 4 ‰ vs. VPDB, consistent with deposition during the large worldwide positive excursion between 2.22 and 2.06 Ga. These positive δ¹³C values are independent of the microfacies types. Such high ¹³C values are also recorded in reworked carbonates (dolomites) embedded in coarse-grained sandstones and conglomerates interstratified in the black shales of the underlying FB Formation suggesting that the dolomites were at least stratigraphically equivalent to the black shale deposits. Interaction between early structural development and sediment fill lead to the formation of several sub-basins with distinct differences in term of amount and timing of subsidence. The ¹³C-enriched dolomites are probably linked to high burial rates of organic carbon starting with the deposition of the FB sediments in this extensional geotectonic context. As a consequence the cherty and dolomitic FC Formation cannot longer be considered as a marker interval. The end of the positive excursion is located near the FB-FC transition and could be used in future studies as a potential chemostratigraphic tool in the Paleoproterozoic.