CENOZOIC SEDIMENTATION HISTORY OF THE CONGO BASIN REVISITED
Résumé
The Congo Basin in Central Africa is one of the major river basins of the world. Its history over
the last 75 Myr should record global and major regional events, including the Paleocene-Eocene
Thermal Maximum at ~55 Ma and the Miocene aperture of the Western branch of the East
African Rift System along its eastern border at ~25 Ma. Available data for associated off-shore
deposits show that the Congo River delta experienced a starvation period during the Midto
Late Cretaceous and Paleogene, with endorheic lacustrine to desert environments in the
upstream basin, followed by a period marked by high rates of drainage and sediment supply
in the Neogene.
Here we complement this sedimentation history with a multi-proxy analysis of an on-shore
Paleogene section along the west coast of Central Africa (Landana, Cabinda Enclave, Angola)
and with new age determination results for paleosurface-related manganese oxide deposits
from the southern margin of the Congo Basin in the DR Congo.
Newly obtained 39Ar-40Ar ages for cryptomelane from Katanga (Kasekelesa) and Kasaï (Mt
Mwatshimwa) reveal an old (Cretaceous?) surface that developed before ~76 Ma and they suggest
the existence of at least two younger Eocene denudation episodes, during the Lutetian
(~45 Ma) and the Priabonian (~35 Ma). They furthermore confirm the identification of a series
of Mio-Pliocene denudation surfaces that were previously recognized by the same methods for
the Kisenge area, Katanga (De Putter et al., 2015). It is likely that the newly obtained Cretaceous
age records the subcontinental ‘African Surface’ that had previously not been identified for
Central Africa.
The Landana section presents a condensed (~45 m) Paleogene-Neogene sequence. The Paleogene
deposits (~25 m), dated by the rich fossil assemblage they contain (Solé et al., in prep.),
record a shallow marine carbonate environment with minor to moderate fine-grained terrigenous
detrital input. Sediment provenance, as documented by εNd, does not change significantly
in the course of the Paleogene, whereas weathering, as documented by εHf and δ30Si,
increases slightly from the Danian to the Lutetian (Bayon et al., 2016). This time interval corresponds
roughly to the assumed period of deposition of the Kalahari Group of the western
part of the Congo Basin, where it occurs as a ~150 m thick sand accumulation. The top of the
Lutetian beds of the Landana section is a major discontinuity, marked by strong silicification,
which would correspond to the Lutetian denudation surface in Katanga.
Sedimentation recorded by the Landana section changes drastically after this hiatus with Lutetian onset, becoming dominated by coarse-grained siliciclastic sediments. A major change
in sediment source is confirmed by εNd, whereas isotopic proxies of weathering (εHf, δ30Si)
document a major decrease in weathering intensity. The sharp increase in sediment discharge
is attributed to uplift along the southern and eastern margins of the Congo Basin, preceding
the opening of the East African Rift in the Oligocene.
A combination of sedimentological, geochemical and geochronological proxies provides new
constraints for a Paleogene time interval that is only poorly documented for the Congo Basin.
During this period, sediments, including those of the Kalahari Group, were deposited on
a denudation surface tentatively identified as the Cretaceous African Surface. The uplift that
occurred before opening of the East African Rift resulted in increased terrigenous off-shore
sedimentation after the Lutetian.
De Putter et al., 2015. Ore Geol. Rev. 71, 350-362
Bayon et al., 2016. Goldschmidt Conf. 2016, abstract book, 181
This abstract is a contribution to the Brain.be PalEurAfrica project: http://www.paleurafrica.be