Schisto-Calcaire Gr

Synonym: Xisto-Calcario Gr (Angola Basin; Portuguese-language version of the French-language name); Schisto-Calcaire Subgroup; Schist-Limestone Gr. Lukala Subgroup (alternate DRC usage). In DRC Lower Congo basin, it has four ascending units of Kwilu (with Bulu Fm and Luanza Fm), Lukunga Fm, Bangu Fm and Ngandu Fm)

References: Delhaye and Sluys 1923; Bertrand-Sarfati 1972; Frimmel et al. 2006; Poidevin 2007; Straathof 2011; Delpomdor and Pre´at 2013. For RC Sangha-Comba basins and Niari-Nyanga basins – Ackouala Mfere et al. 2020. Alvarez 1992; Bertrand-Sarfati and Milandou 1989; Babet 1935; Dadet 1969; Scolari 1965; 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]

Ngandu Fm (topmost unit; “Civ”; ca. 550 Ma); is locally identified in the Luozi area close to the Nionga massif at the border area between the DRC and RC. This unit has been firstly described in the Niari Basin from the RC (Cosson, 1955; Nicolini, 1959). In the DRC, the Ngandu (CIV) unit (90 m-thick) contains, from base to top (Delhaye & Sluys, 1929; Mayor, 1951; Cahen & Lepersonnel, 1976): (1) light-greyish to -greenish clayey limestones, sometimes dolomitic, with lenticular intraformational breccia and dark cherty levels, (2) reddish to pinkish limestone beds and red calcareous claystone interbeds, (3) gray-greenish claystones with cherts, and (4) reddish calcareous claystones with very thin greenish clayey limestones and pinkish limestones, evolving to reddish sandy claystones and fine-grained feldspathic quartzites.

Bangu Fm (Ciii or C5) = up to ± 265 m-thick, is characterized by a variably dolomitic and/or organic-rich carbonate succession, with oolite, chert, shale and calcpelite beds. It is marked by: (Lower) the 10-40 m-thick Kisantu Oolite unit that occurs in the lower part of the formation and consists of lenticular beds of gray to dark-grey silicified oolites or pseudo-oolites interbedded with shales and calcpelites; and (Upper) oolitic beds, lenticular calcareous conglomerates or breccias at the base. The presence of organic matter suggests a regressive episode with deposition in more or less closed lagoonal-type basins, and the lithologies indicate a sedimentation close to the coast, partly in quiet lagoonal or wavy conditions.

Lukunga Fm (Cii or C4) = ± 300 m-thick, is marked by a sudden more clastic deposition at the base including an alternation of shales, calcpelites or sandstones with frequent ripple-marks. Sedimentation evolved to clayey limestones or dolomites, stromatolitic or oolitic limestones, and ended with a new clastic material input marked by the deposition of shales, calcpelites, calcareous sandstones containing abundant chert as beds or nodules.

Kwilu Fm unit (Ci; +/-650 m total): subdivided into three formations (C1, C2 and C3) that consist of: (Uppermost) ± 240 m-thick cyanobacterial and oolitic limestones at the top (C3 or Luanza Formation; ca. 575 Ma). (Middle) ±400 m-thick generally clayey and sandy limestones, calcpelites or sandstones, clayey or sandy pelites (C2 or Bulu Formation), (Basal) a 0 to 15 m-thick pinkish or gray cap carbonates on the Cryogenian glacial diamictite, formed by alternating dolomite and purplish thin shale beds at the top (C1 or Dolomies Roses Formation)." [Cailteux et al., 2015]

In Republic of Congo (RC) Sangha-Comba basins, the Schisto-Calcaire Gr consists of three carbonate-dominaed subgroups (SCI-SCIII, up to 1300 m thick) recording relative sea-level changes. "The most typical sedimentologic feature of the SCIc Formation is the deposition of giant stromatolitic bioherms (stacked up to 20 m) topped by ooid shoals (up to 75 m thick) deposited during high systems tract prograding and forced regressive systems tract phases that ended with a lowstand systems tract phase with evaporitic and karstic conditions at the top of the SCIc Formation." (Ackouala Mfere et al. 2020).

"In Gabon and RC, the Schisto-Calcaire Subgroup consists of four predominantly carbonate formations, called SCI to SCIV, respectively. The SCI dolomudstones Fm starts with a 6 - 12 m-thick basal member composed of cap-carbonate dolomites. … Succeeding to the Kyandamu Fm diamictite in the Central Africa Copperbelt, interpretation of Marinoan age, the Gombela Sub Gr (lower Kundelungu Gr) starts with the basal 5 to 15 m-thick Lusele Fm (Calcaire Rose Fm) cap-carbonate, followed by up to 1600 m-thick alternating carbonatic shales and siltstones of the Kanianga Fm, and by up to 150 m-thick carbonate beds of the Lubudi Fm (Lubudi limestone) at the top (Batumike et al., 2007). It displays a sedimentary succession strongly comparable to the C1 (Kwilu Fm), C2 (Bulu Fm) and C3 (Luanza Fm) formations of the Schisto-Calcaire Gr, respectively. In particular, the Lubudi Fm is marked by massive, irregularly bedded, oolitic high purity limestone beds alternating with sandy carbonate beds, showing shallow-water proximal conditions of deposition, similarly as for the C3b in Bas-Congo. … The overlying Mongwe Fm and Kiubo Fm (of Ngule Sub Gr, upper Kundelungu Gr) up to > 600 m-thick are pelite-sandstone more or less dolomitic complexes, with frequent intraformational conglomerates and upward coarsening. The sequence contains predominant pelites to the bottom (Mongwe Fm), and predominant sandstones to the top (Kiubo Fm). These units are characterized by shallow-water and evaporitic conditions, similarly as for the C4 (Lukunga Fm) and C5 (Bangu Fm) formations in Bas-Congo (Cahen, 1978)." [Cailteux et al., 2015]

In the Niari Basin – Mayombe Chain, the “group has been subdivided into three subgroups reflecting major transgression-regression cycles.” (Fullgraf et al., 2024), called Sc1-Sc3.