Evolution of the Congo rift basin, West Africa_ an anic geochemical record in lacustrine shales.pdf

Basin Research (2000) 12, 425–445
Evolution of the Congo rift basin, West Africa: an
anic geochemical record in lacustrine shales
N. B. Harris
Department of Geosciences, The Pennsylvania State
University, 503 Deike Building, University Park, PA 16802,
USA (e-mail: ******@)
ABSTRACT
The anic 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 anized 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 anic 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 w