Franck Delpomdor

Chapter 13 Neoproterozoic sequences of the West Congo and Lindi/Ubangi Supergroups in the Congo Craton, Central Africa

Abstract The focus of this chapter is the West Congo Supergroup in the West Congo Belt (WCB), which extends along the western margin of the Congo Craton from Gabon in the north to northern Angola in the south, and the Lindi/Ubangi Supergroup of the Lindian and Fouroumbala – Bakouma Basins exposed on the northern margin of the craton. In both regions, up to two distinct diamictite horizons have been recognized, the younger of which is often associated with carbonate rocks. Geochronological constraints are generally rather poor, many of the deposits lack modern sedimentological analysis, and the glacial versus non-glacial genesis of the diamictites is a matter of debate in the literature. However, recent studies suggest a periglacial influence of diamictite deposition, particularly for the sequences in the WCB. The stratigraphy of the various basins is described, available geochemical and geochronological information collated, and recent work regarding the periglacial nature of the diamictites discussed. Finally, an updated chronostratigraphic correlation between the basins is presented. However, much more work is required, particularly in the Neoproterozoic basins on the northern margin of the Congo Craton, and more accurate geochronological constraints are required before the Neoproterozoic palaeogeography and depositional environments of the western and northern Congo Craton can be fully understood.

Sedimentology and Sequence Stratigraphy of the Late Precambrian Carbonates of the Mbuji-Mayi Supergroup in the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin (Democratic Republic of the Congo)

The late Mesoproterozoic–middle Neoproterozoic carbonate succession (1155 Ma–800 Ma) of the Mbuji-Mayi Supergroup (Democratic Republic of Congo) represents a classic late Precambrian carbonate sequence whose architecture is poorly known. Here we present new data and synthesis of microfacies analysis, sequence stratigraphy, Fischer plots coupled with C and O isotopes, to evaluate the paleoecology and sea level variations of the carbonate series of the Mbuji-Mayi Supergroup, and to establish hierarchical approach stratigraphic framework from which to resolve the evolution of the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. Our microfacies and sequence stratigraphy analyses show that the carbonate succession consists of strata accumulated on a ramp, during cyclic sedimentation across the inner ramp. Here plurimetric ‘thin’ peritidal cycles (±4 m-thick on average) record a relative maximum sea level of ca. 4 m, with fluctuations in the range around 1–4 m. This shallow-water depth and the abundance of cyanobacteria suggest that water column was oxygenated. By contrast the subtidal cyclic facies at the outer/middle ramp, preserve ‘thick’ subtidal sequences characterized by an average thickness of ±17 m. Accurate relative sea level fluctuations are difficult to assess in this ‘deeper’ environment since the facies could have been deposited in a wide range of shallow water that did not completely fill the accomodation space or available space. A probable magnitude for sea-level fluctuations here is around 10–20 m. These data are the first to place a quantitative constraint on the late Mesoproterozoic to middle Neoproterozoic carbonate deposits that have lively covered much of the Congo Shield at the end of the Precambrian, and is therefore an important type section for Central Africa.

Stromatolites and Cyanobacterial Mats in Peritidal Evaporative Environments in the Neoproterozoic of Bas-Congo (Democratic Republic of Congo) and South Gabon

The “Schisto-Calcaire Subgroup” is a muddy predominantly subtidal shelf succession that crops in the West Congolian Belt. The approximately 300-m-thick CI (Bas-Congo) and approximately 20-m-thick SCIII (Gabon) series were deposited in a very shallow marine evaporitic environment. The evidence for this interpretation includes sedimentology of dolomite and sulfate minerals and oxygen isotopes. Cyanobacteria (probable Oscillatorians) formed mats on the inland marshes fringing ponds of channeled belts. In Gabon, they are associated with abundant domal stromatolites deposited in shallow to lower intertidal settings. While diagenetic overprints (dolomicrosparitization, sulfate growth, silica replacement) may be significant, several microbial laminar mudstones retained their original fabric. SEM analysis revealed well-preserved three-dimensional (3D) cyanobacterial communities associated with the stromatolites. During progressive lithification in the upper part of shallowing-upward evaporitic sequences, the stromatolites constituted a favorable substrate which has been invaded and colonized by probable fungal hyphae. These produced characteristic features that have been reproduced in vitro in experiments.

Overview of the Neoproterozoic Sedimentary Series Exposed Along Margins of the Congo Basin

The Congo Shield is a central African large landmass with several Archean nuclei welded during the Eburnean orogeny approx. 2.1–1.8 Ga and subsequently stabilized as a coherent unit throughout late Paleoproterozoic to Meso-Neoproterozoic times. The early Neoproterozoic was marked by rifting along the margins of the Congo Shield related to the break-up of Rodinia at about 1.0 Ga, and opening of the Adamastor Ocean, followed by passive margin-type sedimentation and, finally, foreland basin deposition during the amalgamation of Gondwanaland initiated at approximately 600 Ma. We summarize the present knowledge of the lithostratigraphy of the Neoproterozoic basins exposed along margins of the Congo Basin in an attempt to establish chronostratigraphic correlations between these isolated basins in the Democratic Republic of Congo and neighboring countries.