Bankole and 3 co-authors reported in 2015 that the Paleoproterozoic (∼2.15 Ga) Franceville Basin, South Eastern Gabon, is a continental sedimentary basin that host unmetamorphosed sediments. This study involve detailed mineralogy, sedimentology, and petrography of the basal sedimentary units of FA and lower FB (FB1) Formations, from the basin margin to center in relation to mineral paragenesis and fluid flow.
The FA Formation conglomerate, sandstone, and mudstone consists lithofacies of mixed fluvial and fluvio-deltaic transitional origin, while the overlying FB1 Formation includes alternating organic rich black shale and sandstone of marine deposit. The medium- to coarse grained fluvio-deltaic quartz arenite in upper part of FA Formation is characterized by pervasive authigenic quartz cementation that reduced the porosity and permeability in the early stage of burial history. This provides a resistant framework for subsequent diagenetic modification and also inhibits fluid flow during burial diagenesis.
In contrast, the clay and unstable detrital grains rich fluvial arkosic to sub-arkosic sandstones that escaped early quartz cementation show considerable pressure solution at grain contacts. These arkosics were less porous and permeable when deposited but transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis and subsequent precipitation of authigenic mineral cements in the resulting secondary porosities.
From the proximal to distal basin and within sample suites, there is no considerable chemical variation in the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipitation from a near equilibrium homogenous pore-fluid. The predominance of 1Mt illite polytype in most
lithologies reflects precipitation of the clay minerals and probably other cements in an environment with high fluid/rock ratio. Dolomite, anhydrite, barite, and Fe-oxides are the main crystallized authigenic minerals aside illite and Fe-rich chlorite clay minerals.
The authors concluded that the mineralogical assemblages and textural occurrences of the rocks suggest that diagenesis and fluid flow in the FA Formation in the Franceville Basin are mainly controlled by depositional facies. The Basin represents one of the world oldest petroleum systems and ranked as a major Proterozoic accumulation of organic matter (Mossman David et al., 2005). The basin is also globally known for its unique fossils (El Albani et al., 2010, 2014) and natural nuclear fission reactors at Oklo, and widely explored for uranium and manganese ore deposits (Gauthier-Lafaye and Weber, 2003).
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