Mpioka Gr

The West Congo Belt (WCB) is exposed, on 1,300 km, along the western margin of the CS. The sedimentary West Congo Supergroup (or West Congo Group in Base-Congo) comprises two main sedimentary successions: pre-Pan-African-orogeny passive-margin platform sequences (ca. 4000 m in Bas-Congo of the Sansikwa Gr, Haut-Shiloango Gr, Schisto-Calcaire Gr; with Cryogenian glacial deposits of the Lower Diamictite Fm and Upper Diamictite Fm) and a late- to post-Pan-African molasse sequences (Mpioka Gr; or equivalent Schisto-Gréseux Gr, which is French for "clayey sandstone") followed by mid-Paleozoic red beds of the Inkisi Gr. (Delpomdor and Préat, 2013, in de Wit et al., eds.)

Synonym: M'Pioka Fm, Mpioka Subgroup, Mpioka Fm. Northern Plateaux des Cataractes (formerly Bangu) facies and Eastern Inkisi River facies. Schisto-Gréseux Gr in Gabon and in Sangha-Comba basins of Republic of Congo.

References: Cailteux et al. (2015), de Wit et al. (eds.; 2015), Fullgraf et al., 2024

[Fig 1. Simplified geological map of the Congo Basin, compiled from various published map. The stratigraphic units have been assembled into major sequences (Late Neoproterozoic, Paleozoic and Cenozoic)]

[Fig 2. Tectonic setting of the Neoproterozoic basins of present-day Central Africa, compiled from the 1: 2M geological map of the Zaire (Lepersonne, 1974a) and the 1:4M map Geology and Major Ore Deposits of Africa (Milesi et al., 2006)]

[Fig 3. Lithostratigraphic synthesis for the Neoproterozoic to Early Paleozoic period. Compiled after various authors. Dotted lines between Banalia, Alolo and Galamboge formations: stratigraphic transition by recurrences]

[Fig 4. Lithostratigraphic columns for the Congo Basin established using data from the 4 wells in the central part of the basin and outcrops on its NE margin (Lindi-Ubangi and Kisangani-Kindu region), compared with the West-Congo and Katanga stratigraphy]

[Fig 5. Simplified geological map of Sub-Saharan Africa showing the Neoproterozoic basins on and around the Congo Shield (modified after De Waele et al. 2008). (LC) Lower Congo Basin, (Co) Comba Basin, (Sa) Sangha Basin, (Ny) Nyanga-Niari Basin, (An) Angola Basin, (SO) Semb-Ouesso Basin, (Ba) Ubangui Basin, (LiB) Likki-Bembe´ Basin, (Bk) Bakouma Basin, (Fou) Fouroumbala Basin, (Li) Lindi Basin, (It) Itombwe Basin, (Ma) Malagarazi-Bukoban Basin, (Mb) Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin, (Lu) Luamba Group, (Kat) Katanga Basin, (RB) Ruwenzorian Belt, KaB) Karagwe-Ankole Belt, (RuB) Ruzisian Belt, (KiB) Kibaran Belt, (UB) Ubendian Belt, IB Irumide Belt]

[Fig. 6. Western margin of the Congo Shield. (A) Sketched geological map of the West Congo Belt (modified after Frimmel et al. 2006)]

[Fig. 7a. Synthetic stratigraphic log combined with C and Sr isotopic curves of the West Congolian Group in the DRC (modified after Tait et al. 2011). Note strontium isotope ratios of the Haut-Shiloango Subgroup ranging between 0.7068–0.7072 (Frimmel et al. 2006; Poidevin 2007) and of the C3 to C5 formations of the Schisto-Calcaire Subgroup ranging between 0.7074 to 0.7084 suggesting deposition, respectively, between 800 Ma and 650 Ma and between 635 and 575 Ma (Frimmel et al. 2006; Poidevin 2007; Delpomdor and Pre´at 2013) and the negative shift of -13 ‰, consistent, in absolute values, with the negative swing of the Trezona anomaly (Halverson et al. 2007)] [Fig. 7b. Synthetic stratigraphic log of West Congolian Group in Angola. cc cap carbonates; SG Sturtian Glaciation, MG Marinoan Glaciation.] [Fig. 7c. Synthetic stratigraphic log of West Congolian Group in the RC (modified after Alvarez 1995)]

[Fig. 8. Stratigraphic correlation between the Neoproterozoic sedimentary basins in Central Africa. Correlations based on Sr isotopic data, radiometric age constraints and revised lithological relationships]

The Mpioka Group is the youngest lithostratigraphic unit of the West-Congo Supergroup (Tack et al., 2001; Frimmel et al., 2006) and records the molasse stage of the Pan-African orogeny (Scolari, 1965; Scolari and Van Daalhoff, 1965; Frimmel et al., 2006). Consists of approximately 1,000 m-thick red-bed siliciclastic rocks. "It displays a northern ~1000 m-thick “Plateaux des Cataractes” (former Bangu) facies (north of the Mbanza-Ngungu anticline), and a ≥ 370 m-thick “Eastern Inkisi River” facies (east of the Inkisi river). Both facies are marked by a basal conglomerate (Bangu Fm and Niari Conglomerate Fm), that occurs locally in the Eastern Inkisi River facies. The succession includes: (1) a Lower sequence of siltstones and shales with alternating carbonate sandstone, feldspathic quartzite and conglomerate beds (Vampa Fm, Cataracte facies) or carbonate sandstones and shales, psammites, feldspathic quartzites with conglomerate beds (Gidinga Fm and Luvemba Fm, Eastern Inkisi River facies), and (2) an Upper sequence of feldspathic quartzites and siltstones with occasional shale and conglomerate beds (Kubuzi Fm and Liansama Fm vs. Mpioka Fm sensu stricto)." [Cailteux et al., 2015]

Uncertain; late Ediacaran-early Cambrian. “Analysis of metamorphic and detrital white mica constrains the depositional age of the unit between 575 ± 8.7 Ma and 499 ± 19 Ma after the main orogenic event at 580 Ma” (Fullgraf et al., 2024); then an erosion.

Molasse-type sequences deposited into the late- to post-Pan-African orogen foreland that formed along the West Congo belt. "The siliciclastic red-beds of the Schisto-Gréseux Gr in Gabon and the Mpioka Gr in RC that overlay unconformably the Schisto-Calcaire Subgroup, start with basal conglomerates, followed by rocks ranging from claystones to conglomeratic sandstones; they are correlated to the similar Mpioka Subgroup sequence in the DRC (Préat et al., 2011; Tait et al., 2011)." [Cailteux et al., 2015]