Paleoproterozoic high 13C dolomites from the Lastoursville and Franceville basins (SE Gabon): stratigraphic and synsedimentary subsidence implications

Abstract : New data for the isotopic composition of carbon in shallow-marine sedimentary carbonates in southeastern Gabon (Franceville and Lastoursville sub-basins) indicate that the seawater value of 13C in these sediments underwent a large positive excursion around 2.1 Ga. These carbonate rocks belong to the FC Formation of the Francevillian Group deposited along the margin of the Archean Chaillu Craton in openmarine shelf and evaporite 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 values are above 0.50‰, the highest 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 13C values are independent of the microfacies type. 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 terms of amount and timing of subsidence. The 13C-enriched dolomites are probably linked to high burial rate 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 top of the Francevillian Series (i.e. FE) and could be used in future studies as a potential chemostratigraphic tool in the Paleoproterozoic