Lower Diamictite Fm

Northwest Angola; West Congo Belt on the western margin of the Congo Shield. Middle formation in the Mayombe Gr of the Niari Basin and Mayombe Chain of Gabon and RC. The poorly constrained West Congo Supergroup comprises from the youngest to oldest: (i) 2,400 m-thick siliciclastic Terreiro Gr; (ii) a Lower Diamictite Fm in the lower part of approximately 1,250 m-thick siliciclastic and carbonate Alto-Chiloango Gr; (iii) approximately 2,000 m-thick carbonate sequence of the Xisto-Calcario Gr starting by 10–80 m-thick Upper Diamictite Fm capped by a cap carbonate unit; and (iv) 3,250 m-thick exclusively siliciclastic Xisto-Gresoso Gr subdivided into M’Pioka Fm and Inkisi Fm.

Synonym: Lower Mixtite; Lower Tillite; Lower Tillite Subgroup; M’Beia Fm,

References: Fullgraf et al., 2024; de Wit, Guillocheau and de Wit, 2015.

[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]

Up to 400 m of poorly sorted mud-supported conglomerates with angular and rounded clasts. " Tholeiitic basalts (including pillow lavas and hyaloclastic breccias) are interlayered in the Lower Diamictite Formation, while feeder sills and dykes of the basalts intrude the underlying Sansikwa Subgroup." (Baudet et al., 2013, CAG abstract)

The Lower Tillite Subgroup, which is correlated at global scale with the Sturtian glaciation. "The Mwale (Grand Conglomerate), Varianto (Chuos) and Lower Diamictite formations in the Central Africa Copperbelt, Kaoko-Damara Belt in Namibia and West Congo Belt, respectively, are all correlated with the 750-713 Ma global Sturtian glaciation (Key et al., 2001; Hoffman et al., 1996; Frimmel et al., 2006), and represent robust geochronological markers." [Cailteux et al., 2015]