Sansikwa 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 and equivalent Terreiro Gr in Angola Basin), 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) followed by mid-Paleozoic red beds of the Inkisi Gr. (Delpomdor and Préat, 2013, in de Wit et al., eds.)

Synonym: Sansikwa Subgroup, Terreiro Gr (Angola Basin)

References: Cahen 1978; Frimmel et al. 2006; Straathof 2011

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

Sansikwa Gr (up to 1630 m-thick in DRC Lower-Congo basin) has a basal unit (ca. 4.5 m) of conglomerate; followed by mostly siliciclastic rocks: quartzites, siliceous shales, psammites, quartzites with stromatolitic and oolitic cherty beds, and shale with interbedded calcareous sandstones or alternating shales and quartzites to the top. Igneous events are associated to this succession: basaltic lavas interbedded to the bottom of the Lower Diamictite Formation and feeder dolerite sills and dykes with a tholeiitic affinity intruding both the Lower Diamictite Fm and the Sansikwa Gr (Lepersonnel, 1974; De Paepe et al., 1975). In the DRC, the succession (stratigraphic order) consists of: [Cailteux et al., 2015].

S2b (uppermost unit) = 200-250 m of dark shale with interbedded calcareous sandstone; alternating shale and quartzite.

S2a = ca. 875 m of fine-grained quartzite, purple psammite and siliceous shales in the lower part; whitish to greyish feldspathic quartzite with many interbedded cherts, frequently oolitic or stromatolitic.

S1 = ca. 500 m of purple siliceous shale and psammite, with occasional interbedded purple-gray quartzite.

S0 (basal unit) = ca. 4.5 m of basal conglomerate; clayey or arkostic-supported with interbedded siliceous shale and arkose.

In the Angola Basin, the equivalent middle and upper Tonian is the Terreira Gr of 2,400 m of siliciclastic rocks" (de Wit and de Wit, 2015; no details given).

Tonian; pre-Cryogenian (overlain by the Lower Diamictite of basal Ediacaran). The filling of the West Congo Margin basins began at about 870 Ma (Fullgraf et al., 2024), hence 45% up in Tonian.

It is interpreted as a continental rift fill because of its continental crustal basement, basal conglomerate, and the fluvial siliciclastic composition of its upper formation (Frimmel et al., 2006). It comprises a thrust-fold-belt with top-to-(north-)east transport direction from the hinterland domain in the west and grades into a foreland domain in the east with decreasing regional metamorphism from amphibolite facies (in the west) to unmetamorphosed rocks (in the east) (Frimmel et al. 2006)