Facies analyses, chronostratigraphy and paleoenvironemental reconstructions of jurassic to cetaceous sequence of the Congo basin
Résumé
The Congo Basin is characterized by an extensive and relatively thick (ca. 1 km) succession of Jurassic-Cretaceous sedimentary sequences that preserves a unique record of the tectonic and climatic evolution of central Africa during the main period of break-up of Gondwana and the emergence of the Indian and South Atlantic Oceans. New facies analysis and detailed correlations of these ‘Congo’ sequences are described from field observations in the southwestern Congo Basin and by re-logging cores and well logs from four deep boreholes drilled in the center of the basin in the 1950s and 1970s. The lowermost Upper Jurassic sequence (the Stanleyville Group) records a short marine incursion of the proto-Indian Ocean into the northern Congo Basin, and is in turn overlain to the south by widespread aeolian dune deposits (the Lower Kwango Group), which correlate well with other Upper Jurassic to Lower Cretaceous aeolian sequences in Namibia and eastern Brazil, attesting to a giant ‘Sahara-like’ paleo-desert across central West Gondwana, just before the separation of Africa from South America. U-Pb detrital zircons geochronology from this aeolian sequence in the Congo Basin dates mid-Silurian (ca. 430 Ma), Permian-Triassic (ca. 240 and 290 Ma) and Jurassic (ca. 190 Ma) magmatic zircons, here proposed to have been sourced from abundant volcanic activity along the proto-Andes, in southernmost Gondwana. Two successive middle Cretaceous lacustrine sequences in the center of the Congo Basin (the Loia and Bokungu Groups), first analcime-rich and episodically anoxic, and then more carbonated, are interpreted to record an episode of basin stagnation following the eruption of the Paraná-Etendeka Large Igneous Province and a subsequent hot/humid climate maximum during the opening of the South Atlantic Ocean. Late Cretaceous sedimentation in the Congo Basin terminated with fluvial sediments (the Upper Kwango Group) suggesting marginal uplifts during the Kalahari epeirogeny. The top of these sequences is truncated by a regional Cenozoic peneplanation surface.