Control of fluid-pressure on early deformation structures in the Paleoproterozoic extensional Franceville Basin (SE Gabon).
Résumé
The Franceville Basin (2100–2000 Ma) has been widely studied for its uranium deposits most of which are located close to the major faults at the transition from the FA to FB (FB1, FB2a and FB2b) Formations. These formations deposited during the Paleoproterozoic and represents volcano-sedimentary series. Well-preserved outcrops may be found in quarries (i.e. Oklo, Mikouloungou and Socoba) and along roads and railways’ track (i.e. M’vengue). Our study identifies footwall anticlines and hangingwall forced synsedimentary synclines (i.e. progressive unconformities) associated with a NW–SE trending longitudinal normal faults. It also contributes to the identification of main (MPB) and secondary permeability barriers (SPB) promoting fluid-pressure at the FA–FB1 and FB2a–FB2b stratigraphic transitions. Multiscale and multidisciplinary approaches of outcrop studies allowed to describe synlithification and postlithification fluid-pressure related structures. Synlithification faults in Oklo quarry and sand injectites at M’vengue underlined the former structuration, while quartz veins and stylolites observed in Socoba quarry are associated with a postlithification stage. The observation from outcrop to centimeter scales of intraformational breccia reworked during postlithification extensional episodes (i.e. quartz veins and stylolites) close to synsedimentary faults at Oklo and Mikouloungou suggest a near-surface postlithification deformation model (i.e. chemical compaction). We propose a possible reconstruction model of fluid-pressure structuration at the basin scale. Solid bitumen and associated uranium ores are restricted to fluid-pressure structures related to hydraulic fracturing induced by oil migration. They are spatially restricted by permeability barriers within footwall forced anticlines.