Bikossi Fm

Period:

Paleoproterozoic

Age Interval:

Orosirian

Province:
West Congo Margin Basins

Type Locality and Naming

Uppermost formation in the Loeme Super Gr. The Nyanga-Niari Basin in the northern part of the West Congo Belt on the western margin of the Congo Shield comprises two distinct geologic domains from west to east, i.e. the metasedimentary hinterland domain and the unmetamorphosed internal domain, i.e. the Nyanga syncline. The Mayumba basin is exposed on the Mabouine´ and Mayumba complexes. All rocks of Palaeoproterozoic (Eburnean orogeny) age are now combined into the Loeme Super Gr (Loémé Supergroup); the unit is further subdivided into the autochthonous Loukoula Fm (Group) intruded by the calc-alkaline Bilinga Suite Gr, and the allochthonous Bikossi Fm (Group).

Synonym: Bikossi Gr

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

[Fig 1. Lithostratigraphic synthesis for the Neoproterozoic to Early Paleozoic period. Compiled after various authors (see text).Dotted lines between Banalia, Alolo and Galamboge formations: stratigraphic transition by recurrences.]

[Fig 2. 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 3. 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. 4. 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

Quartzite, conglomerates with dyke/sill swarm = molasse (Fullgraf et al., 2024; de Wit et al., 2015).

Lithology Pattern:

Sandstone

Relationships and Distribution

Lower contact

Rests (?) on Bilinga Suite Gr of granitic intrusions and/or the Loukoula Fm of clayey sandstone (now paragneiss, etc.)

Upper contact

Termination of Eburnean orogeny was around 1970 ±40 Ma. Major unconformity of nearly 1 billion years that included the Rodinia breakup. Unconformably overlain by the volcanics of the Nemba Complex Fm of the basal Sounda Gr.

Regional extent

Niari Basin (Gabon) and Mayombe Chain (RC and Gabon)

GeoJSON

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Fossils

Age

Ca. 8% to 28% up in Orosirian (2030 to 1980 Ma) on Fig. 21 of Fullgraf et al. (2024). Termination of Eburnean orogeny was around 1970 ±40 Ma. “Deposition of the Bikossi Group post-dates Eburnean plutonism and pre-dates Tonian magmatism. U-Pb SHRIMP and LA-ICP-MS analysis of detrital zircons from basal quartzite constrains maximum deposition ages of 2043 ± 13 Ma and 1971 ± 40 Ma, respectively being indicative for the erosion of Eburnean middle crust and the interpretation as a molasse (Fullgraf et al., 2024).

Age Span:

Beginning stage:

Orosirian

Fraction up in beginning stage:

0.08

Beginning date (Ma):

2,030.00

Ending stage:

Orosirian

Fraction up in the ending stage:

0.28

Ending date (Ma):

1,980.00

Depositional setting

Molasse (coarse-grained erosional products into foreland basin)

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

Fullgraf, T., Callec, Y., et al. (2024) Structural architecture and metamorphism of the Mayombe Chain and Niari Basin (West Congo Belt) in Congo Brazzaville. Jour. Afr. Earth Sci., 220: article 105369.