The Paleoproterozoic (∼2.15 Ga) Franceville Basin, South Eastern Gabon, is a continental sediment... more The Paleoproterozoic (∼2.15 Ga) Franceville Basin, South Eastern Gabon, is a continental sedimentarybasin 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 basinmargin to centre in relation to mineral paragenesis and fluid flow. The FA Formation conglomerate, sand-stone, and mudstone consists lithofacies of mixed fluvial and fluvio-deltaic transitional origin, while theoverlying 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 charac-terized by pervasive authigenic quartz cementation that reduced the porosity and permeability in theearly stage of burial history. This provides a resistant framework for subsequent diagenetic modificationand also inhibits fluid flow during burial diagenesis. In contrast, the clay and unstable detrital grainsrich fluvial arkosic to sub-arkosic sandstones that escaped early quartz cementation show considerablepressure solution at grain contacts. These arkosics were less porous and permeable when depositedbut transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis andsubsequent 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 variationin the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipita-tion from a near equilibrium homogenous pore-fluid. The predominance of 1Mtillite polytype in mostlithologies reflects precipitation of the clay minerals and probably other cements in an environmentwith high fluid/rock ratio. Dolomite, anhydrite, barite, and Fe-oxides are the main crystallized authigenicminerals aside illite and Fe-rich chlorite clay minerals. The mineralogical assemblages and textural occur-rences of the rocks suggest that diagenesis and fluid flow in the FA Formation in the Franceville Basin aremainly controlled by depositional facies.
The Paleoproterozoic (∼2.15 Ga) Franceville Basin, South Eastern Gabon, is a continental sediment... more The Paleoproterozoic (∼2.15 Ga) Franceville Basin, South Eastern Gabon, is a continental sedimentarybasin 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 basinmargin to centre in relation to mineral paragenesis and fluid flow. The FA Formation conglomerate, sand-stone, and mudstone consists lithofacies of mixed fluvial and fluvio-deltaic transitional origin, while theoverlying 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 charac-terized by pervasive authigenic quartz cementation that reduced the porosity and permeability in theearly stage of burial history. This provides a resistant framework for subsequent diagenetic modificationand also inhibits fluid flow during burial diagenesis. In contrast, the clay and unstable detrital grainsrich fluvial arkosic to sub-arkosic sandstones that escaped early quartz cementation show considerablepressure solution at grain contacts. These arkosics were less porous and permeable when depositedbut transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis andsubsequent 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 variationin the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipita-tion from a near equilibrium homogenous pore-fluid. The predominance of 1Mtillite polytype in mostlithologies reflects precipitation of the clay minerals and probably other cements in an environmentwith high fluid/rock ratio. Dolomite, anhydrite, barite, and Fe-oxides are the main crystallized authigenicminerals aside illite and Fe-rich chlorite clay minerals. The mineralogical assemblages and textural occur-rences of the rocks suggest that diagenesis and fluid flow in the FA Formation in the Franceville Basin aremainly controlled by depositional facies.
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