The West African basins (Taoudeni, Tindouf, Madina–Kouta, Gourma, Ougarta, etc.) and belts (Mauritanides, Rokelides, Trans-Saharan belt, Anti-Atlas belt, etc.) were always studied separately. Eastern Senegal and western Guinea were studied during the early twentieth century (De Chetelat, 1933; Renaud and Delaire, 1955), but a coherent scheme including the whole western margin was delivered after 1960. Sougy (1962) yielded the first global interpretation and distinguish two belts: a northern belt (Mauritanides) running from the “Spanish Sahara” to the northern Guinea Bissau and involving the northern part of the Palaeozoic Bové basin (including the Devonian deposits) and a southern belt (Rokelide) unconformably covered by this Palaeozoic Bové basin. So, the first belt was linked to a “Hercynian” orogen and the second one to a “Caledonian” orogen. Bassot (1966, 1969) confirmed this interpretation in southeastern Senegal and northern Guinea.

Then, the Soviet-Guinean cartographic program (1968–1975) performed several 1/200,000 geological maps in western Guinea, including the “Bassaride” belt area (Petrovsky et al., 1973). However, Villeneuve (1980, 1984) evidenced a regional unconformity: the KLU (Koubia–Lessere Unconformity) between the underlying metamorphic Bassaride formations and the overlying sedimentary covers beginning by a mixed molassic and “glaciogenic” level. Dallmeyer and Villeneuve (1987) dated the metamorphism at circa 650 Ma and consequently linked this metamorphic formations to the “Panafrican-I orogen”. To the south, the Rokelide belt (Allen, 1969; Culver and Williams, 1979) experienced a metamorphism event dated at circa 550 Ma linked to the Panafrican-II orogen (Villeneuve and Dallmeyer, 1987).

From 1998 to 2007, a “German–Guinean” cartographic team (PAGEM 2) worked on the Guinean Bassaride area (Brinckmann and Meinhold, 2007). But the last investigations in the Bassaride area took place with the cartographic “PASMI” project (2009–2010) in eastern Senegal (Fullgraf et al., 2013). These two cartographic projects provide numerous radiometric data that turn down the Villeneuve and Dallmeyer (1987) interpretation and lead us to rework this “Koubia–Lessere” unconformity in order to re-evaluate its extension, age, and geological significance.

The KLU is evidenced in three different areas: the Bassaride ranges, the Kemberra inlier and the Tominé inlier. All are located in Fig. 1a.

3.1 In the Bassaride area

The five main cross-sections located in Fig. 1b and shown in Fig. 2 allow us to present the “Koubia–Lessere” unconformity in the eastern side of the Bassaride belt with, from the north to the south: the “Nepin Peuhl” cross-section (1 in Fig. 2) showing deformed shales with isolated clasts to the base and a flat “Bouma” sequence to the top, the Termesse–Lessere cross-section (2 in Fig. 2), where the “Koubia–Lessere” unconformity was first evidenced, the “Lampoukenn–Kouragnaki” cross-section (3 in Fig. 2) interpreted from the geological map and showing an unconformable contact between the underlying “Termesse group” and the overlying “Mali group”, the “Hamdallaye–Bantala” cross-section (4 in Fig. 2), which is based on a free interpretation of the Brinckmann and Meinhold (2007) geological map and, finally, the Kounsitel–Hore Pete cross-section (5 in Fig. 2), which is a re-interpretation of the eastern part of a Brinckmann and Meinhold (2007) cross-section and displays a structural discrepancy between the folded underlying “Termesse group” and the flat overlying “Mali Group”.

3.2 In the Kemberra inlier

The “Kemberra Inlier” is located 30 km to the south-west of Gaoual (Fig. 3a). It was investigated by Villeneuve (1979). In the southern part (Fig. 3b), this inlier shows several formations below the flat Pita group sandstones. There is a granitic basement similar to the Niokolo–Koba one (of the Bassaride areas). This granitic basement is associated with a metasedimentary succession (Tagirata formation) including breccias, sandstones and volcanic metatuffs exhibiting a sericitic schistosity and stratigraphic layers with dips between 60 to 80° to the east (Fig. 3c). On the western side, the granitic basement is capped by a set of mylonitic and blastomylonitic metatuffs. Along the Netivel stream, a palaeovalley dug in the Tagirata sandstones is infilled with breccias linked to the “Illiassa polymict breccias” outcropping at the base of the Mali group. On the eastern side, the Mali group beginning by the “Illiassa polymict breccias” and overridden by argilites and siltites shows a 30 to 40° dip to the east below the horizontal Pita group. The KLU is located on top of the “Tagirata sandstones” and at the base of the “Illiassa polymict breccias”.

3.3 In the Tominé inlier

The “Tominé inlier” surrounded by the Youkounkoun sandstones, crops out in the centre of the Youkounkoun basin (Fig. 4). It was successively investigated by Torchine (1976), Villeneuve (1984) and Brinckmann and Meinhold (2007). It consists (Fig. 4a) of a basement made of granitoid rocks intruded by rhyolitic and dacitic dykes belonging to the “Panampou group”. This “Panampou Group” is linked to the Niokolo–Koba volcanic arc. This volcanic arc is intruded by a microgabbro patch outcropping north of Tabadian. This basement is unconformably overlain by the “Lumbalumbito” conglomerate (partly of glacial origin) and by the “Batapa” argilites that are linked to the Mali group (Fig. 4b). The KLU separates the basement from the Lumbalumbito conglomerate.

The first radiometric data came in the 1960s, first in Senegal (Bassot et al., 1963) and, later on, in Sierra Leone (Allen, 1969; Allen et al., 1967), and in Mauritania (Lille, 1967).

Rb/Sr radiometric dating on red argilites evidenced an age at circa 1022 ± 20 Ma on argilitic rocks sampled at the base of the Madina–Kouta group and a Panafrican age (645 ± 39 Ma) on biotites extracted from a grano-dioritic sample belonging to the Niokolo–Koba magmatic complex (Bassot et al., 1963). They also evidenced a Late Precambrian age (560 ± 50 Ma on argilitic fraction) from pelitic rock of the Youkounkoun group and several Hercynian metamorphic events (358 ± 33 and 206 ± 6 Ma) in northeastern Senegal (Bakel group). Later on, Bassot and Caen-Vachette (1983) indicated a 683 ± 18 Ma age (isochron on whole rock) for the “Niokolo–Koba” protolith.

Then, Dallmeyer and Villeneuve (1987) provided thirteen ³⁹Ar/⁴⁰Ar radiometric ages from southern Senegal, among which three ⁴⁰Ar/³⁹Ar radiometric ages on muscovites extracted from the Guinguan group quartzites.

Later on, Brinckmann and Meinhold (2007) provided new radiometric data from northern Guinea: 15 U/Pb on zircons (6 on the Bassaride ranges), six Rb/Sr (five on the Bassaride ranges) and one Ar³⁹/Ar⁴⁰, also on the Bassaride ranges. Moreover, 10 new U/Pb age data on zircons (three in the Niokolo–Koba area and seven in the Bassaris range) were obtained by the “PASMI” cartographic project in Senegal (Fullgraf et al., 2010).

4.1 In the Bassaride area

4.2 In the Kemberra inlier

4.3 In the Tominé inlier

New radiometric data from Senegal and Guinea suggest a volcano-sedimentary basin in the Bassaride area that operated between 700 Ma and 494 Ma, involving the lower Bassaride groups (Termesse, Guinguan, Panampou, Nandoumba). These groups experienced the same Panafrican-II orogenic event and, consequently, they belong to the Panafrican-II (Cadomian) orogen, which includes the Panafrican-I metamorphic event (Villeneuve and Dallmeyer, 1987). On another hand, the “KLU” separating these lower Bassaride groups from the overlaying “Mali” group is younger than 494 ± 5.2 Ma (age of the Nandoumba basaltic flows) and 493.3 ± 8 Ma (age of the Tabadian microgabbro). Consequently, the “Mali” group is younger than the Panafrican-II orogen and has likely been deposited after 493 Ma. This hypothesis is illustrated in Fig. 5, where the previous (Villeneuve, 1980) interpreted cross-section (Fig. 5b) located in Fig. 5a is now completed (Fig. 5c) noticeably between the Bassaride ranges and the Madina–Kouta basin areas. In this cross-section, the “Mali” group is capping the Madina–Kouta basin, the Bassaride belt, and the Niokolo–Koba volcanic arc. However, the “Mali” group is deformed only in the Bassaris range and in the Niokolo–Koba volcanic arc. These deformations are related to a Late Cambrian or Early Ordovician tectonic event. The overlying and flat “Youkounkoun group” could represent the “molassic” stage associated with this tectonic event. Thus, the KLU should be differentiated from the Walidiala erosional unconformity (WU) that separates the Madina–Kouta formations correlated with the Super Group 1 (SG1) of the Taoudeni basin from the “Triade” ascribed to the worldwide “Marinhoan” glacial event (635 Ma).

Our new hypothesis consisting in rejuvenation (from 635 to 490 Ma) of the KLU previously correlated with the Walidiala Unconformity. So, there are two diachronous unconformities, each one overlain by conglomeratic levels bearing evidences of a “glaciogenic” environment. That allows us to propose new correlations inducing several geological modifications on the western margin of the West African Craton.

Until now, the geological correlations across the West African craton were supported by a fundamental “stratigraphic” postulate coming from the Mauritanian Atar lithostratigraphic succession used as a referential scale for the whole West African area (Trompette, 1973). This lithostratigraphic chart took into account only one Precambrian/Cambrian glacial event (the Jbeliat tillite). Thus, all “tillitic” or “mixtitic” sediments deposited in West Africa in this time bracket were then linked to this event, itself correlated with the worldwide “Marinhoan” glacial event (650–635 Ma). Despite some local attempt to shake this rule, no consistent arguments prevent this postulate. The last synthetic paper devoted to the West African glacial events (Deynoux et al., 2006) validated the “triade” as a Marinhoan glacial deposit. Since this time, every geologist working in West Africa has to be in accordance with this stratigraphic scale.

Here, we are changing this rule by proposing another kind of correlation. We admit two regional unconformities underlined by two different conglomeratic levels with “tillitic” or “conglomeratic” evidences. The older one (Walidiala tillite) located at the base of the lower Bassaris Groups (Termesse or Faleme groups) is correlated with the “Marinhoan” glacial event (Shields et al., 2007) and the younger one (Toulel Sigon conglomerate) located at the base of the “Mali” group is not yet correlated with a worldwide event. Sometimes this conglomerate presents glacial characters and sometimes there is a non-typical glaciogenic conglomerate and sometimes there is no clastic sediment at all. However, several indices of glacial occurrences in the Ediacaran to Cambrian times were evidenced in several West African places like in the Anti-Atlas (Vernhet et al., 2012), in the NE Taoudeni basin (Bertrand-Sarfati et al., 1995), in the Tanezrouft (Caby and Fabre, 1981), and in the Tamale group of the Volta basin (Affaton, 1987). The Toulel Sigon conglomerate may be coeval with one of them.

Fig. 6 illustrates the new correlations between the Bassaris area and the Adrar of Mauritania. In this figure:

• • the “Termesse” group is correlated with the “Teniagouri” group and the “Walidiala” tillite is correlated with the “Jbeliats” tillite of northern Mauritania;
• • the “Mali” group is correlated with the “Atar cliff” group and the “Youkounkoun” group to the lower part of the “Oujeft” plateau group;
• • the Pita group (base of the Bové basin) is correlated with the Oued Touergua group.

Thus, these stratigraphic changes have fundamental consequences on the geological map (Fig. 7). For example, the southern “Rokel River” formations of Sierra Leone should be correlated with the “Mali” group, but not with the “Faleme” (or Termesse) group and the “tillitic” glacial levels setting at the base of this “Rokel River”group (Tabe formation) should be correlated with the “Toulel Sigon” conglomerate, but not with the “Walidiala” tillite. Consequently, the Rokelide basement of southern Guinea and Sierra Leone is older than the Rokel river trough. This is different from previous interpretations of the Rokelide belt. Similarly, the KLU is correlated with an angular unconformity evidenced by Dia (1984) in the central Mauritanide belt. This unconformity separates the underlying “Tichilit al Beida” group (equivalent of the Faleme or Termesse groups) from the overlying “Mejeria” group (equivalent of the Mali group). The Jbeliat unconformity is located at the base of the “Tichilit al Beida” group and the KLU at the base of the Mejeria group. Finally, our correlations support four main cycles linked to orogens or tectonic events: Eburnean cycle (2000 Ma), Panafrican-II (or Cadomian) cycle (700 to 550–530 Ma), Mali (or Kolente) cycle (between 490 and 450 Ma), and Hercynian cycle (450 to 330–270 Ma). All are pointed out in Fig. 6.

New age data provided by recent cartographic projects change our knowledge of the West African domain. The main consequences are:

• • the “Koubia–Lessere” unconformity (KLU) is not equivalent to the “Walidiala Unconformity” (WU), as previously assumed;
• • the Walidiala unconformity (WU) separating the basement (including the Madina–Kouta sedimentary group) from the Bassaris groups (Termesse, Walidiala, Faleme and Panampou) is involved in the Bassaride tectonic orogen;
• • the KLU that is separating the Bassaris groups from the overlying “Mali” and “Rokel River” groups is, at least, 100 Ma younger than previously accepted;
• • volcanic intrusions occurred at many times, from the Mesoproterozoic to the Devonian.

Accordingly:

• • the “Walidiala triade” of the Taoudeni basin (Marinhoan glacial event) is not a correlative of the “Toulel Sigon” conglomeratic formations (Villeneuve, 1984) and the last one should be linked to an unknown Late Cambrian or Early Ordovician glacial event;
• • the Bassaride belt is linked to the Panafrican-II (Cadomian) orogen;
• • the Rokelide belt should be reinterpreted by taking in consideration that Allen (1969) and Latiff et al. (1997) have demonstrated that the “Rokel River” group is unconformably covering the Marampa group, which is now linked to the Bassaride groups (Lytwyn et al., 2006);
• • the tectonic event related to a sinistral strike-slip movement affecting the “Mali” and “Rokel River” groups is not linked to the main Rokelide metamorphic event (550–530 Ma) but to a younger one that occurred before the deposition of the “Youkounkoun” group (post- 490 Ma and ante-Silurian).

Finally, new consistent radiometric data in West Africa recorded since 2007 have greatly enhanced our knowledge of the western part of the Taoudeni basin.