Loukoula Fm

Lower 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: Loukoula Group, Série de la Loukoula

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]

Paragneiss, schist-quartzite, amphibolite. Originally clayey sandstone as generalized lithology?

Ca. 75% to 92% up in Rhyacian (2110 to 2070 Ma) on Fig. 21 of Fullgraf et al. (2024). Geochronology of detrital zircons from migmatitic paragneiss of the Loukoula Group yielded ages between 3192 and 2069 Ma with peaks at P2 [2420–2535 Ma] and P3 [2100–2150 Ma], demonstrating erosion of Paleoproterozoic and Archaean source areas. The classification as P2 and P3 (P = plutonism) refers to the history of Paleoproterozoic magmatism in Gabon. P2 zircons appear rounder than P3 zircons suggesting their origin from distal sources. The youngest 207Pb/206Pb age of 2069 ± 17 Ma is 13% discordant and therefore only provides the minimum age of this grain. The youngest concordant 207Pb/206Pb age of 2111 ± 14 Ma provides the maximum age of sediment deposition and high-grade metamorphic event that led to the anatexis of the metapelites. Isoclinally folded gneissic sill granite yielded a concordia age of 2045 ± 7 Ma which is interpreted as time of magmatic emplacement; it constrains the maximum age of the main gneiss-forming event, isoclinal folding and low-angle shearing under medium-to high-grade conditions at this locality. The ages of several xenocrysts from this and the other sample (DOL2724-1) vary between 2200 and 2620 Ma documenting Paleoproterozoic to Neoarchean source or host rocks (Fullgraf et al., 2024).

Autochthonous