| The sedimentary rocks of the Pinda Formation were deposited in the Lower Congo basin, in environments ranging from tidal flat to shallow marine shelf. Detrital clay grains were derived from the erosion of metamorphic basement rocks of the Congo Craton, and in smectite-rich soil transported into the Lower Congo basin. Authigenic clay minerals formed either by alteration of detrital clay grains, or by direct precipitation from pore solutions (neoformation). Increasing temperature and pressure associated with burial diagenesis transformed detrital kaolinite and smectite into mixed layer illite/smectite, illite, and/or chlorite. Changes in pore water chemistry caused by dissolution of mafic minerals and/or carbonate rocks and subsequent flushing by a variety of waters through clastic units during diagenesis was also important in controlling clay mineral formation and chemistry. Successive transgressive/regressive events altered pore fluid conditions and led to the development of halite, calcite, dolomite, albite and clay mineral cements. |
