Biostratigraphic and chemostratigraphic constraints of the Mbuji-Mayi Supergroup (Meso-Neoproterozoic age), Democratic Republic of Congo.

The Mbuji-Mayi Supergroup is a sedimentary sequence unaffected by regional metamorphism [1]. It was deposited between 1174 ± 22 Ma and ca. 800 Ma in the intracratonic failed-rift SMLL “Sankuru-Mbuji-Mayi- Lomami- Lovoy” basin [2] which extends from SE to NW between Katanga and Kasai Provinces. And overlies the Mesoproterozoic Kibaran Belt Supergroup (in the eastern part of SMLL basin) while in the Western part, where we focused our work, it rests unconformably upon Archean Dibaya Granitic Complex [3]. The amygdaloidal basaltic pillow lavas (948 ± 20 Ma) overlie the Mbuji-Mayi Supergroup, at the confluence of Mbuji-Mayi and Sankuru rivers [4]. Lithostratigraphically, this Supergroup consists in two distinct successions: a lower siliciclastic sequence (~500m) of BI Group and an upper carbonatic sequence (~1000m) with stromatolitic build-ups and black shales of BII Group [2]. Our own and previous sedimentological observations [5] indicate facies ranging from subtidal, low-energy stromatolitic environments to overlying intertidal to supratidal evaporitic settings of lagoon and sabkha.
Here we present data on microfossil diversity and carbon isotope chemostratigraphy from the Kanshi, Lubi and Kafuku drillholes. The well-preserved and diverse assemblage of acritarchs and filamentous forms includes prokaryotes and eukaryotes, and is similar to other coeval assemblages described worldwide outside of Africa. The presence of the acanthomorph acritarch Trachyhystrichosphaera aimika is significant as it is indicative of the late Meso- to early Neoproterozoic age elsewhere, and is reported for the first time in Central Africa. So far, 56 species belonging to 31 genera were identified, dramatically increasing the previously reported diversity [6, 7]. Chemostratigraphy based on δ13Ccarb values for 290 samples, records, for the BI Group, predominantly negative values down to -8 to -9 ‰ VPDB with few samples having more positive, up to +3 ‰, values. Although the siliciclastics-rich sediments in the lower part of the BI Group likely record early diagenetic signal, carbonates in the upper part of the BI Group show similar patterns in both the Lubi and Kafuku drill cores with the sharp fall from +1 to +3 ‰ values to -8 to -7 ‰ and recovery back to +1 ‰ values over 40 to 70 m of section. The BII Group shows a less dramatic rise from -1 ‰ to +4 to +5 ‰ over more than 150 m of section. These large-scale variations differ from the steady-state carbon cycle of the late Mesoproterozoic [8] and are typical of the early Neoproterozoic record [9]. The project is supported by the EU FP7 ERC Stg ELITE.