Evolution of the Congo rift basin, West Africa: an inorganic geochemical record in lacustrine shales

The inorganic geochemistry and mineralogy of synrift lacustrine shales from the Early Cretaceous Congo rift basin provide insight into source terrane, palaeoclimate and evolution of rift topography. The basin formed in a Late Proterozoic metamorphic belt as South America and Africa began to separate. Within the synrift section, a late rift sequence (Marnes Noires and Argilles Vertes Formations) can be distinguished from an underlying active rift sequence (Vandji, Sialivakou and Djeno Formations), based on diminished faulting and more uniform subsidence.

Provenance did not vary significantly until deposition of the youngest part of the rift sequence. Al2O3/TiO2 and K2O/Na2O ratios, generally constant, rise sharply in upper Marnes Noires and Argilles Vertes, demonstrating decreased contribution from Proterozoic volcanic sources. Parameters including (quartz+feldspar)/total clay and SiO2/Al2O3 suggest that depositional systems reorganized during lower Djeno deposition, possibly due to rejuvenation of faulting. Grain size decreases in the late rift section; however, the parameter SiO2/Al2O3 increases. This is attributed to chemical or biogenic deposition of silica.

The proportion of chemical sedimentation increases upward in the synrift section, peaking in the Marnes Noires Formation, where the concentrations of organic carbon and carbonate and the proportion of SiO2 to siliciclastic‐associated elements reach a maximum. This is interpreted as resulting from input of high dissolved chemical load in the late rift stage and is attributed to increased chemical weathering in the basin as rift topography diminishes. The slower flow of ground and surface water to the rift lakes enhanced weathering and dissolution of minerals in rocks and sediments surrounding the rift lake. The carbon isotopic composition of carbonate decreases from +6 at the base of the rift section, associated with carbon from Late Proterozoic carbonates, to 0, indicating increasing contribution of light carbon; the source of light carbon is interpreted as vegetation, which increases as rift topography degrades.