Inkisi Gr

Cenozoic

Mesozoic

Paleozoic

Neoproterozoic

Paleoproterozoic

Inkisi Gr

Period:

Ordovician, Silurian, Devonian, Carboniferous

Age Interval:

poorly dates; post-Cambrian to pre-Karoo Permian

Province: West Congo Margin Basins

Type Locality and Naming: The term “Inkisi Formation” was derived from one of the Congo River’s tributaries in the northern part of the DRC, where Cahen (1954) first described it. Occurs in the Democratic Republic of Congo. Cosson (1955), Dadet (1969), Tack et al. (2001), and Delpomdor et al. (2019) later recognized the formation in the Republic of Congo, while Stanton et al. (1963) and Schermerhon (1981) recognized it in Angola. "In Angola, the Inkisi Subgroup is overlain by Karoo deposits and, therefore, must be older than Permian, but still belong to the Paleozoic, being a foreland-basin deposit unrelated to and younger than the Pan-African orogeny. For this reason, the Inkisi Gr (in contrast with previous papers) has been separated from the West Congolian Group and considered as an individual lithostratigraphic unit." [Tack et al., 2001].

Synonym: Inkisi Redbeds, Inkisi Sandstones, Inkisi Subgroup, Inkisi Fm

References: Cahen, 1954; Tack et al., 2001; Kadima et al., 2011; Sounga et al. 2012; de Wit et al., 2015; Delpomdor et al., 2019; Stanton et al., 1963; Schermerhon, 1981

[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]

Lithology and Thickness: Red beds. It is part of the post-Pan African foreland molasse sequences and is composed of red arkosic sandstones intercalated with conglomerate lenses and fully conglomeratic at the base (Kadima et al., 2011). It is a southward prograding fluvial-delta (de Wit et al., 2015) that displays two major sedimentary cycles from conglomerate or arkose grading upward to fine-grained quartzites and shales (the uppermost quartzitic to clayey formations are not well known in the western Congo region. [Cailteux et al., 2015]. The formation is about 920m thick (Tack et al., 2001), or reaches 600 – 700m in thickness (Dadet, 1969). The Inkisi Formation exhibits two facies: a predominantly pink facies, and a green facies (Sounga et al. 2012). Quartz is more abundant in the pink facies and feldspars are more abundant in the green facies. The principal constituents of the sandstones are quartz (68-78%), feldspar, essentially microcline (18-30%), lithoclasts commonly of quartzite (2-12%), and muscovite mica flakes (1-5%) (Sounga et al., 2012).

Lithology-pattern: Coarse-grained sandstone

Relationships and Distribution:

Lower contact: Lies unconformably on Ediacaran to lowest Cambrian siliciclastics of the Mpoika Gr (DRC Lower Congo Basin, Angola Basin) or Schisto-Gréseux Gr (RC Comba Basin). The unconformity is the local version of the Pan-African orogeny.

Upper contact: In Angola, it is directly overlain (conformably?) by the Karoo deposits of Permian age. These Karoo deposits are derived from the main Karoo Basin of Southern Africa (Oesterlen and Tessensohn, 1976; Oesterlen, 1979, 1980; Catuneanu et al., 2005).

Regional extent: The Inkisi Gr extends from the southern part of the DRC, across Congo-Brazzaville and into the north of the Republic of Angola (Sounga et al., 2012).

Province:
West Congo Margin Basins

Type Locality and Naming

The term “Inkisi Formation” was derived from one of the Congo River’s tributaries in the northern part of the DRC, where Cahen (1954) first described it. Occurs in the Democratic Republic of Congo. Cosson (1955), Dadet (1969), Tack et al. (2001), and Delpomdor et al. (2019) later recognized the formation in the Republic of Congo, while Stanton et al. (1963) and Schermerhon (1981) recognized it in Angola. "In Angola, the Inkisi Subgroup is overlain by Karoo deposits and, therefore, must be older than Permian, but still belong to the Paleozoic, being a foreland-basin deposit unrelated to and younger than the Pan-African orogeny. For this reason, the Inkisi Gr (in contrast with previous papers) has been separated from the West Congolian Group and considered as an individual lithostratigraphic unit." [Tack et al., 2001].

Synonym: Inkisi Redbeds, Inkisi Sandstones, Inkisi Subgroup, Inkisi Fm

References: Cahen, 1954; Tack et al., 2001; Kadima et al., 2011; Sounga et al. 2012; de Wit et al., 2015; Delpomdor et al., 2019; Stanton et al., 1963; Schermerhon, 1981

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

Lithology and Thickness

Red beds. It is part of the post-Pan African foreland molasse sequences and is composed of red arkosic sandstones intercalated with conglomerate lenses and fully conglomeratic at the base (Kadima et al., 2011). It is a southward prograding fluvial-delta (de Wit et al., 2015) that displays two major sedimentary cycles from conglomerate or arkose grading upward to fine-grained quartzites and shales (the uppermost quartzitic to clayey formations are not well known in the western Congo region. [Cailteux et al., 2015]. The formation is about 920m thick (Tack et al., 2001), or reaches 600 – 700m in thickness (Dadet, 1969). The Inkisi Formation exhibits two facies: a predominantly pink facies, and a green facies (Sounga et al. 2012). Quartz is more abundant in the pink facies and feldspars are more abundant in the green facies. The principal constituents of the sandstones are quartz (68-78%), feldspar, essentially microcline (18-30%), lithoclasts commonly of quartzite (2-12%), and muscovite mica flakes (1-5%) (Sounga et al., 2012).

Lithology Pattern:

Coarse-grained sandstone

Relationships and Distribution

Lower contact

Lies unconformably on Ediacaran to lowest Cambrian siliciclastics of the Mpoika Gr (DRC Lower Congo Basin, Angola Basin) or Schisto-Gréseux Gr (RC Comba Basin). The unconformity is the local version of the Pan-African orogeny.

Upper contact

In Angola, it is directly overlain (conformably?) by the Karoo deposits of Permian age. These Karoo deposits are derived from the main Karoo Basin of Southern Africa (Oesterlen and Tessensohn, 1976; Oesterlen, 1979, 1980; Catuneanu et al., 2005).

Regional extent

The Inkisi Gr extends from the southern part of the DRC, across Congo-Brazzaville and into the north of the Republic of Angola (Sounga et al., 2012). Province: DRC Lower-Congo basin, Angola Basin, RC Sangha-Comba basins.

GeoJSON

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Fossils

Age

The Inkisi Gr is post-Pan-African orogeny, and its deposition area is unrelated to the West Congo belt trend and evolution. Poorly dated; post-Cambrian to pre-Karoo Permian. [Entire Ordovician-Carboniferous span shown here for temporary graphics.]

Age Span:

Beginning stage:

Tremadocian

Fraction up in beginning stage:

0.0

Beginning date (Ma):

486.85

Ending stage:

Gzhelian

Fraction up in the ending stage:

1.0

Ending date (Ma):

298.89

Depositional setting

Foreland continental clastic, lacustrine to fluvio-deltaic (Kadima et al., 2011). It is a southward prograding fluvial-delta (de Wit et al., 2015).

Depositional pattern:

Additional Information

Compiler:

Enam O. Obiosio, Solomon Joshua Avong and Henry Nasir Suleiman (2024) - Stratigraphic Lexicon compiled from the following books:

Geology and Resource Potential of the Congo Basin by Maarten J. de Wit, François Guillocheau and Michiel C.J. de Wit, Published Springer-Verlag Berlin Heidelberg 2015; DOI 10.1007/978-3-642-29482-2

Cailteux, J.L.H., Delpomdor, F.R.A. Ngoie Ndobani, J-P., 2015. The Neoproterozoic West-Congo “Schisto-Calcaire” sedimentary succession from the Bas-Congo region (Democratic Republic of the Congo) in the frame of regional tentative correlations. Geologica Belgica, 18(2-4): 126-146.

Structure and geological history of the Congo Basin: an integrated interpretation of gravity, magnetic and Reflection seismic data by E. Kadima, D. Delvaux, S. N. Sebagenzi, L. Tack and S. M. Kabeyaz: Basin Research (2011) 23, 499–527, doi: 10.1111/j.1365-2117.2011.00500.x