Sedimentology, geochemistry and depositional environments of the 1175-570 Ma carbonate series, Sankuru-Mbuji-Mayi-Lomami-Lovoy and Bas-Congo basins, Democratic Republic of Congo: new insights into late Mesoproterozoic and Neoproterozoic glacially- and/or tectonically-influenced sedimentary systems in equatorial Africa

was an enigmatic period characterized by the development of the first stable long-lived ~1.1-

0.9 Ga Rodinia and 550-500 Ma Gondwana supercontinents, global-scale orogenic belts,

extreme climatic changes (cf. Snowball Earth Hypothesis), the development of microbial

organisms facilitating the oxidizing atmosphere and explosion of eukaryotic forms toward the

first animals in the terminal Proterozoic. This thesis presents a multidisciplinary study of two

Neoproterozoic basins, i.e. Bas-Congo and Sankuru-Mbuji-Mayi-Lomami-Lovoy, in and around the Congo Craton including sedimentology, geochemistry, diagenesis, chemostratigraphy and radiometric dating of carbonate deposits themselves.

The Mbuji-Mayi Supergroup sequence deposited in a SE-NW trending 1500 m-thick siliciclastic-carbonate intracratonic failed-rift basin, extends from the northern Katanga Province towards the centre of the Congo River Basin. The 1000 m-thick carbonate succession is related to the evolution of a marine ramp submitted to evaporation, with ‘deep’ shaly basinal and low-energy carbonate outer-ramp environments, marine biohermal midramp (MF6) and ‘very shallow’ restricted tide-dominated lagoonal inner-ramp (MF7-MF9) settings overlain by lacustrine (MF10) and sabkha (MF11) environments, periodically

submitted to a river water source with a possible freshwater-influence. The sequence stratigraphy shows that the sedimentation is cyclic in the inner ramp with plurimetric ‘thin’ peritidal cycles (± 4 m on average) recording a relative sea level of a maximum of 4 m, with fluctuations in the range of 1-4 m. The outer/mid ramp subtidal facies are also cyclic with ‘thick’ subtidal cycles characterized by an average thickness of ± 17 m, with a probable sealevel

fluctuations around 10 to 20 m. The geochemistry approach, including isotopic and major/trace and REE+Y data, allows to infer the nature of the dolomitization processes operating in each carbonate subgroup, i.e dolomitization may be attributed to evaporative reflux of groundwater or to mixing zones of freshwater lenses. The latest alteration processes occured during the uplift of the SMLL Basin. New ages, including LA-ICP-MS U-Pb laser ablation data on detrital zircon grains retrieved in the lower arenaceous-pelitic sequence (BI group), combined with carbon and strontium isotopic analyses, yielded a new depositional time frame of the Mbuji-Mayi Supergroup between 1176 and 800 Ma reinforcing the formerly suggested correlation with the Roan Group in the Katanga Province.

In the Democratic Republic of Congo, the Sturtian-Marinoan interglacial period was previously related to pre-glacial carbonate-dominated shallow marine sedimentation of the Haut-Shiloango Subgroup with stromatolitic reefs at the transition between greenhouse (warm) and icehouse (cold) climate periods, commonly marked by worldwide glacigenic diamictites and cap carbonates. This thesis highlights that these deposists record as a deepening-upward evolution from storm-influenced facies in mid- and outer-ramps to deepwater environments, with emplacement of mass flow deposits in toe-of-slope settings controlled by synsedimentary faults. In absence of diagnostic glacial features, the marinoan Upper Diamictite Formation is interpreted as a continuous sediment gravity flow deposition along carbonate platform-margin slopes, which occurred along tectonically active continental margins locally influenced by altitude glaciers, developed after a rift–drift transition. The maximum depth of the deepening-upward facies is observed in the C2a member. The

shallowing-upward facies exibit a return of distally calcareous tempestites and semi-restricted to restricted peritidal carbonates associated with shallow lagoonal subtidal and intertidal zones submitted to detrital fluxes in the upper C2b to C3b members.

The geochemistry highlights (i) the existence of a δ13C-depth gradient of shallow-water and deep-water carbonates; (ii) the carbonate systems were deposited in oxic to suboxic conditions; and (iii) all samples have uniform flat non-marine shale-normalized REE+Y distributions reflecting

continental detrital inputs in nearshore environments, or that the nearshore sediments were

reworked from ’shallow’ inner to mid-ramp settings in deep-water slope and outer-ramp

environments, during the rift-drift transition in the basin. The pre-, syn- and post-glacial

carbonate systems could record a distally short-lived regional synrift freshwater-influenced

submarine fan derived from nearshore sediments, including gravity flow structures, which are

attributed to regional tectonic processes due to a sudden deepening of the basin caused by

differential tilting and uplifting of blocks, related to the 750-670 Ma oceanic spreading of the

central-southern Macaúbas Basin.

Combining sedimentology, isotopes and trace elemental geochemistry, the thesis highlights

that the δ13C variations in the Neoproterozoic carbonates are complex to interpret, and can be

related to: (i) the existence of a δ13C-depth gradient; (ii) the exchange between isotopically

light carbon in meteoric waters and carbonate during lithification and early diagenesis; and

(iii) isotopic perturbations due to regional metamorphism. Considering the possible englaciation of the Earth (Snowball Earth hypothesis), the Mbuji-Mayi Supergroup and West

Congolian Group seem reflected the intimate relationship between glaciations and tectonic

activity during the break-up of the Rodinia supercontinent, followed by the rift–drift

transition, and finally the pre-orogenic period on the passive continental margin.