Haut-Shiloango Gr

Named after the river in western Central Africa that forms the westernmost part of the border between the Democratic Republic of the Congo and the Republic of Congo (Shiloango in Congo-lese; Chiloango in Portuguese; or called Louang in Congo). 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, 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.). The Haut-Shilango Gr has a lower Bembezi Fm of siliceous shales and an upper Sekelolo Fm of quartzites to limestone.

Synonym: Haut-Shiloango Subgroup, Alto-Chiloango Gr (Same name, Chiloango is Portuguese and Shiloango is a German version), Upper Louango Gr. Bouenza Gr or the Bouenza Fm or Subgroup of Mayombe Gr (RC Sangha-Comba basins, and Gabon - Nyanga-Nairi Basin; where the shales, sandstones with minor carbonate rocks subdivided into the eastern Bouenza and western Louila subgroups)

References: Frimmel et al., 2006; Poidevin 2007; Delpomdor and Pre´at 2013

[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]

In DRC Lower-Congo basin, the Haut Siloango Gr is up to 1050 m of a "carbonatic lower sequence that reflects a typical post-Sturtian open marine carbonate platform deposition (Frimmel et al., 2006), and forms two transgression-regression cycles (Cahen, 1954; Le Personne, 1974). The Bembezi Fm (former “Mouyonzi” or “Petite Bembezi”) (500 to 700 m-thick), in the lower part, consists of bottom to top of basal sandy-calcareous matrix-supported conglomerates and quartzites, passing to bedded siliceous or dolomitic siliceous shales with calcareous sandstones and finely bedded limestones, and to clayey dolomites at the top. The upper part, the "Sekelolo" unit (200 to 350 m-thick), includes: (1) grained feldspathic quartzites passing to siliceous shales, (2) shales with clayey carbonate nodules or banded calcareous siliceous shale, (3) limestone breccias, stromatolitic-like limestones, and nodular clayey limestones at the top." (Caiteux et al., 2015). s

In the DRC Lower-Congo basin, it consists of an upward succession of the 500-700 m Bembezi Fm (S1) of Sh1 sandy-calcareous matric-supported conglomerate and quartzite; Sh2 of greenish-grey to dark-grey siliceous shales; Sh3-Sh4 of greenish to greyish siliceous frequently calcareous shale with interbedded calcareous sandstone and finely-bedded limestone; and Sh5 of reddish siliceous shale banded dolomitic shale and clayey dolomite. The Sekelolo Fm (200-350 m) is Sh6 of whitish to brownish feldspathic quartzites; Sh7 of greyish-green siliceous shale, followed by dark-grey siliceous shales with clayey limestone nodules, with limestone breccia at top; and Sh8 of dark-grey nodular limestone, stromatolitic limestone with limestone breccia at the top. [Cailteux et al. 2015]. However, the "stromatolitic" is disputed – see Fossils below.

The Bouenza Gr (or Subgroup; RC Sangha-Comba basins, and Gabon - Nyanga-Nairi Basin; where shales, sandstones with minor carbonate rocks) is "divided into five different formations. The formations Bz0 to Bz3 are mostly siliciclastic while the formation Bz4 contains mostly oolithic and stromatolitic carbonates (Charles et al., 2015)."

Delpomdor, Kant and Préat (2014. Neoproterozoic uppermost Haut-Shiloango Subgroup (West Congo Supergroup, Democratic Republic of Congo): Misinterpreted stromatolites and implications for sea-level fluctuations before the onset of the Marinoan glaciation. Jour. African Earth Sciences, 90: 49-63.) conclude it does NOT contain stromatolites or stromatolitic bioherms, in contrast to earlier interpretations.

late Cryogenian; bounded by Sturtian and Marinoan glaciations

Deepening-upward succession of a sloped/steepened storm-influenced outer-ramp. "The series consists of three shaly and carbonate lithofacies: (i) alternating limestones and claystones (lithofacies 1); (ii) nodular wackestones (lithofacies 2); and (iii) clast-supported conglomerates and breccias (lithofacies 3). Lithofacies 1 is an open marine low-energy mid/outer ramp system with hummocky cross-laminations and distal tempestites; lithofacies 2 is a distal slope facies with synsedimentary contorted structures, slided and slumped semi-consolidated limestone beds; lithofacies 3 consists of debris flows deposited in a basinal setting controlled by synsedimentary faults. None of the facies exhibits petrographic evidence of biogenicity such as stromatolitic laminar-reticulate fabrics and/or associated sediments (e.g. peloids, oncoids, ooids) or typical features such as mudcracks or clotted fabrics. The uppermost Haut-Shiloango Subgroup is made up from the stratigraphic succession of the three lithofacies and corresponds to a deepening-upward evolution from storm-influenced lithofacies in mid- and outer-ramp to deep-water environments, with emplacement of mass flow deposits in toe-of-slope settings. These processes occurred along tectonically active continental margins locally influenced by altitude glaciers, developed after a rift–drift transition." (Delpomdor, Kant and Préat, 2014. Neoproterozoic uppermost Haut-Shiloango Subgroup (West Congo Supergroup, Democratic Republic of Congo): Misinterpreted stromatolites and implications for sea-level fluctuations before the onset of the Marinoan glaciation. Jour. African Earth Sciences, 90: 49-63.)