The basement of West African Craton is made up of Archean and Paleoproterozoic formations mainly observed in two separated domains: the Réguibat Shield in the North and the Man Shield in the South (Figure 1). In Western Niger the basement essentially Paleoproterozic, is represented by the Birimian formations of Niger Liptako province. These latter outcropping along the eastern edge of the Man Shield, are affected by the Eburnean orogeny (2.4 to 1.8 Ga according to [42]). They are consisted ofalternating greenstone belts (Gorouol, Dargorou-Darbani, Sirba and Makalondi sectors) and granitoid massifs oriented generally NE-SW (Figure 1). These granitoid plutons expose in the following sectors: Dargol-Gothèye, Torodi and Téra-Ayorou.

Figure 1. (a) Simplified geological map of West Africa (modified from [17]). (b) Geological map of the northeastern border of the Man Shield (modified from [2]). G.G.B: Gorouol Greenstone Belt; T.A.G: Téra Ayorou Granitoid; D.D.G.B: Diagourou Darbani Greenstone Belt; D.G.G: Dargol Gotheye Granitoid; S.G.B: Sirba Greenstone Belt; T.G: Torodi Granitoid; M.G.B: Makalondi Greenstone Belt; M.G: Mossipaga Granitoid.

The greenstone belts are mainly composed of metavolcano-sediments, phylladesof sedimentary origin, metagrauwackes, metabasites and meta-ultrabasites ([49]; [24]). Whereas, the granitoid plutons are made of tonalites, trondhjemites and granodiorites (TTG), diorites, quartz diorites, monzonites and locally syenites. In the study area the Paleoproterozoic basement is characterized Birimian granitoids of Téra-Ayorou which are affected by dykes doleritic ([49]; [24]).

The Neoproterozoic sedimentary cover of West African Craton is mainly represented by the huge intracratonic Basin of Taoudenni (Figure 1). The sedimentary infilling of this basin began with the Supergroup 1 (coarse sandstones, limestones, stromatolite dolostones, clays and silts) at approximatively 1000 Ma ([17]). The Supergroup 1 is successively overlain by the Supergroup 2 and the Supergroup 3. The Supergroup 2 is characterized at its base by the triad “Tillites-carbonates-silexites”, related to glacial deposits occurred during the Late Cryogenian ([47]; [37]). While the base of Supergroup 3 corresponds to the Late Ordovician glacial deposits which marked the upper bounded of the Neoproterozoic (Miningou et al., 2010, 2017).

The Neoproterozoic cover outcrops in the peripheral basins of Gourma and Volta which correspond to the southeastern part of the Taoudenni Basin (Figure 1).

The subsiding basin of Gourma with 8000 m thickness is affected to the East by Pan African Orogeny, characterized by the thrusting of the nappes of the Gourma folded belt ([17]; [44]; [2]). This basin, in continuity with the Taoudenni basin, is mainly characterized in his southeastern border by the sedimentary deposits of Ydouban group which poorly outcrop in Béli (Burkina Faso) and Firgoun (Niger) areas (Figure 1) ([37]; [33]). According to [36], [44] and [8], this Ydouban group composed of variable rocks includes from bottom to top: conglomerates and quartzite sandstones (basal Formation of “Figrounsandstones”), coarse sandstones with conglomerates and shale intercalations ( Formation I), shales (Formation II), carbonates with intercalations of shales and sandstones (Formation III), quartzite with intercalations of shales (Formation IV) a siliceous formation (cherts and jasper) and carbonates with shales and siltsones (Formation V).

The Neoproterozoic sedimentary deposits of Firgoun outcropping sporadically on both sides of the Niger River are located to the North of Niger Liptako province (eastern border of the West African Craton) approximatively 200 km to Northwest of Niamey (Figure 1, Figure 2). These rocks related to the basal Neoproterozoic deposits of Ydouban group (Gourma Basin) rest unconformably on the Northeastern border of the Paleoproterozoic basement of Man Shield (granitoids Birimian of Téra-Ayorou) in the Liptako (Figure 1, Figure 2). They are considered equivalent to the lower deposits of the Taoudenni Basin (Supergroup 1 and Supergroup 2) in the North and Volta Basin (Bombouaka and Oti supergroups) in the South ([43]; [53]; [5]; [33]).

According to these authors ([44]; [36]) the study area lithostratigraphic succession presents two units or formations: 1) the basal “FirgounSandstone” Formation, overlain by 2) the “Béli-Garous” Formation (more detailed in [44]).

The “FirgounSandstone” Formation, which is essentially detrital, is made of conglomeratic to microconglomeratic sandstones, quartzite sandstones and silt-clay sandstones ([5]; [33]). According to the recent study of [33] based on U-Pb zircon detrital measurements, the sediments of “FirgounSandstone” Formation yield about 1800 Ma age.

The “Béli-Garous” Formation, also known as “Faciès de bordure” ([44]), is composed of variable rocks (Figure 2): quartzite sandstones, silt-clay sandstones, conglomerates, alternating clayey shales and carbonates, mudstones, cherts or silexites. In this formation, the shale horizons have provided a Pan-African age of 600 Ma, obtained by the Rb-Sr method ([44]). These latter in alternating with carbonates deposits overlie the silicoclastic sediments (quartzite sandstones, silt-clay sandstones, conglomerates).

Figure 2.Location of the study area on a simplified geological map of Liptako (extract from [36]).

4.1.1. Lithostratigraphic Sections Description

On the basis of the sedimentological analysis four lithostratigraphic sections (C1, C2, C3a and C3b) have been realized in the Firgoun region.

Lithostratigraphic section C1, with approximately 15 m thickness (Figure 3), was taken from outcrops along the main Ayorou-Gao road (N14˚49'55''-E0˚53'7.6'').

With an extension of around 3 km, this section, slightly oriented SE-NW, appears to be the most representative. During our field observations, 7 lithofacies were identified (Figure 3). These are from bottom to top:

coarse quartzite sandstones, conglomeratic to microconglomeratic (lithofacies Fr1), resting unconformably on the more or less metamorphosed granites of the Birimian basement;quartzite sandstones with intercalations of silt-clay sandstone beds (lithofacies Fr2);more or less fine silt-clay sandstones (lithofacies Fr3);dark quartzite sandstones, with fine to medium grains (lithofacies Fr4);fine to medium sandstones with silt-clay interlayers (lithofacies Fr5);manganese-rich quartzite sandstones (lithofacies Fr6), associated with intermediate conglomeratic levels;and slates (lithofacies Fr7) with dolomitic marble interlayers.

Lithostratigraphic section C2(Figure 3), about 3 m thickness, was made at outcrops along the Niger River border (N14˚49'4''-E0˚51'59'' (North Bramé)). This SW-NE trending section is over 2 km long. Two types of lithofacies have been identified (Figure 3): at the bottom, quartzite sandstones identical to those described in section C1 (lithofacies Fr6) and, at the top, greenish-grey shales topped by sporadic deposits of carbonates (lithofacies Fr7).

Lithostratigraphic section C3a (Figure 3) realized south of Donkolo (N14˚50'48''-E0˚52'55.8''), is surveyed over a distance of more than 3 km, in a SE-NW direction. Four types of lithofacies are distinguished (Figure 3):

more or less fine quartzite sandstones, dark gray in color (upper term of lithofacies Fr6);shales overlain by milky-white marbles (lithofacies Fr7);shales (pelitic shales, slates, clay shales) gradually evolving into silexites;(lithofacies Fr8);and shales of varying color (lithofacies Fr9).

Lithostratigraphic section C3b, about 4 m thick (Figure 3), was surveyed south-east of Donkolo (N14˚50'57.5''-E0˚53'34.9''). This section was surveyed over a distance of more than 4 km, in a SE-NW direction. Three types of lithofacies were identified (Figure 3):

fine quartzite sandstones (upper term of lithofacies Fr6);shales evolving vertically to milky-white marbles (lithofacies Fr7);and banded cherts or silexites (lithofacies Fr8).

Figure 3. Lithostratigraphic sections C1, C2, C3a and C3b of the studied area.

4.1.2. Lithostratigraphic Correlation

Correlation between different lithostratigraphic sections (C1, C2, C3a and C3b) of Firgoun area (Figure 3) has enabled to establish a synthetic lithostratigraphic column (Figure 4), including from bottom to top, 9 lithofacies, named Fr1 to Fr9. The lower siliciclastic unit (lithofacies Fr1 to Fr3) is assigned to the “Firgoun Sandstone” basal Formation. While the upper deposits (Fr4 to Fr9) have been linked to the “Béli-Garous” Formation.

Lower unit“FirgounSandstone”Formation

The “FirgounSandstone”Formation essentially detrital begins with conglomeratic to microconglomeratic basal sandstones (lithofaciès Fr1, about 1 m thickness) which rest uncomformably on the Paleoproterozoic basement of Liptako (Figure 4). The presence in this lithofacies of poorly sorted “matrix-supported” conglomerates, consisting of subrounded, blunty quartz granules and pebbles (size varying from 2 to 4 cm) indicates a fluvial influence of sedimentation. This lithofacies evolves towards the top to relatively finer deposits presenting in places ripple marks and oblique bedding (Figure 4).

The lithofacies Fr2, with a total thickness of 3.5 m, is represented by alternating quartzite sandstone beds and silt-clay sandstone levels, attributed to a turbiditic sequence (Figure 4). The finely bedded silt-clay sandstone levels become increasingly coarse and thick towards the top. Quartzite sandstone horizons become increasingly finer towards the top. It is characterized by the presence of oblique bedding, sometimes associated with asymmetrical and symmetrical ripples with erosive surfaces. Oblique beddings occur both in quartzite sandstone beds and in silt-clay sandstone interlayers (Figure 4).

The lithofacies Fr3, made up of relatively fine deposits, outcrops over a maximum thickness of around 1.5 m. These clayey-silty sandstone beds mark the end of the “Firgoun Sandstone” Formation. The main sedimentary structures observed in this lithofacies are hummocky cross stratifications (HCS), oblique bedding associated with asymmetrical and symmetrical ripples (Figure 4).

Upper unit“Béli-Garous”Formation

Unlike the previous deposits, which are essentially detrital, those of the “Béli-Garous” formation are made up of a diversity of rocks: conglomerates, quartzite sandstones, silt-clay sandstones, shales, silexites and more or less metamorphosed carbonates (limestones and dolomites) (Figure 4).

The “Béli-Garous” Formation begins with shallow sedimentation (lithofacies Fr4), about 4 m thick. The lithofacies Fr4 is made of blackish-brown quartzitic sandstones (Figure 4). Asymmetric to symmetric ripples sometimes associated with oblique bedding were observed at the top of the beds.

The lithofacies Fr5 corresponds to fine to medium sandstone beds with brownish silt-clay intercalations which overlined the precedent lithofacies. Asymmetric to symmetric ripples sometimes associated with oblique bedding were observed at the top of the beds (Figure 4).

Lithofacies Fr6 outcrops over a thickness of approximately 5 m. it is represented by quartzitic sandstone beds, more or less manganese-bearing (Figure 4 and Figure 5). This type of lithofacies is characterized in the intermediate levels by the presence of faceted pebble conglomerates (Figure 5) attributed to occurrences of diamictites. These are poorly sorted deposits (unclassified), composed of more or less floating pebbles in a sandstone matrix. Furthermore, it is also worth mentioning the notable presence in this lithofacies of sedimentary features such as oscillation ripples, interference ripples, herringbone bedding, and hummocky cross stratifications (HCS) (Figure 5).

Figure 4. Synthetic lithostragraphic column of the Firgoun area. 1, Hummocky cross-stratifications (HSC); 2, herring bones; 3, current ripples; 4, wave ripples; 5, planar cross-bedding; 6, planar cross tangential at the bottom (modified from [33]; [5]).

The lithofacies Fr7, locally overlying the preceding deposits, is composed of slates, with intercalations of carbonate horizons. Particular sedimentary structures, notably “cryoturbation figures”, are frequently associated with slates (Figure 5).

Carbonate deposits are poorly exposed (approx. 0.5 m). These carbonate deposits sandwiched between the diamictites of lithofacies Fr6 and the silexites of lithofacies Fr8, represent the upper part of the “Béli-Garous” Formation.

Figure 5. Deposits of glacial lithofacies Fr6 and slates of lithofacies Fr7 presenting cryoturbation features. Herringbones (Fr6-a), current ripples (Fr6-b) structures observed in the lithofacies Fr6. Occurrence of diamictite marked by facetted pebbles (fp) in the lithofacies Fr6.

Two types of carbonate have been founded in the Firgoun region (Figure 6): 1) dolomitic limestones (variegated carbonates), unmetamorphosed, and 2) massive marbles, milky white to purplish pink in color. The variegated carbonates (dolomitic limestones) are soft rocks of varied color (whitish, yellow, purple, brownish) presenting centimetric cavities (Figure 6(a)).

The discontinuous marble layer is around 0.5 m thick. It is affected by a dense network of fractures (Figure 6(b)). Laminar structures identical to diagenetic stylolithes (Figure 6(c)) have been observed in pink, slightly metamorphosed dolomitic marbles. They have blackish brown fillings, attributed to barite concretions (Figure 6(d)). According to [11], diagenetic stylolithes commonly develop in limestone horizons, along stratification planes or at the boundary between limestones and clays (in the case of nodular limestones).

Microscopic observation of dolomitic marbles indicates the presence of rhombohedral crystals of dolomite in a calcitic matrix (Figure 7(a), Figure 7(b)). Although dolomite can also exist in the form of cement. In this case it could be a secondary replacement mineral.

Figure 6.Carbonate deposits: (a) Variegated carbonates with a cavernous appearance (dissolution figures), (b) White marbles showing a dense network of microfractures with secondary siliceous or calcitic filling. The pen point indicates a North orientation, (c) and (d) Purplish pink marbles showing diagenetic stylolithes (sty) and barite in concretions (Ba).

Microscopic observation in dolomitic limestones (variegated carbonates) shows microsparite cement with a reddish pigment. This latter is due to the presence of oxides (Figure 7(c)). Small fragments of dolomite recrystallize within the microsparitic cement.

Indice of copper minerals have been observed in the dolomitic marbles in places (Figure 7(d)). Microscopic observation of these marbles indicates minerals such as chalcopyrite and azurite (Figure 7(e), Figure 7(f)).

Lithofacies Fr8 corresponds to massive or banded, greenish-black silexites (Figure 5). These are silexites with a conchoidal fracture. The gradual transition from the shales of the preceding lithofacies Fr7 to the silexites of lithofacies Fr8 is remarkable in some places.

Figure 7.Photomicrograph in the carbonate deposits of lithofacies Fr7. (a) and (b): Thin section in of dolomite marbles observed in reflected light; (c): thin section in variegated carbonates observed in reflected light. Cal: calcite, Dol: dolomite, Si: silica; msp: microsparite, cla: clay, Ox: oxides. (d): indice of copper minerals in dolomitic marbles, (e), (f): polished sections showing the presence of azurite (az) and chalcopyrite (cp).

The Fr9 lithofacies, corresponding to marine deposits of the top of “Béli-Garous” Formation, is made up of shales, clays and silexites of varying color. Theses rocks show locally lenses of quartz sandstone (Figure 5).

These geochemical analyses are focus on the uppermost “Béli-Garous” Formation, which include variable rocks such as cataclased quartzitic sandstones, filled with oxides (FI11), shales (FI3), silexites (FI7), variegated carbonates (FI7) and dolomitic marbles (FI12).

Major elements

The results of the geochemical analysis of major elements in the sediments sampled as part of this study show high levels of SiO₂ (55.20%), Al₂O₃ (5.50%), Fe₂O₃ (13.94%), CaO (6.15%), MgO (3.69%) and relatively low levels of K₂O (1.59%), Na₂O (0.50%) and MnO (0.82%) (Table 1).

Table 1.Geochemical analyses in major elements.

The high SiO₂ content (55.20%) reflects quartz enrichment, while the high Al₂O₃ content (5.50%) is due to the clay mineral content of some of the samples taken at Firgoun, particularly the shales. The Al₂O₃/SiO₂ ratio values are less than 1, confirming the quartz enrichment according to [16] classification.

These high SiO₂ and Al₂O₃ values suggest a felsic source for the sediments. The high iron content, 13.94% in average, can be linked to hematite enrichment in the samples studied. The high CaO and MgO values, ranging respectively from 6.15%, and from 3.69%, attest to the carbonate nature of some of the samples studied, particularly the limestone and dolomitic marble samples.

The diagram (Al₂O₃ + K₂O + Na₂O₃ versusSiO₂), commonly used by [50] to discriminate the climatic conditions of the depositional paleoenvironment, was adopted for the study of the Firgoun sediments.

The different samples, plotted in the Al₂O₃ + K₂O + Na₂O₃ versus SiO₂diagram, define a field corresponding to either humid or arid conditions (Figure 8).

Trace elements

Trace elements analysis shows very high values for Ba (8789 ppm) and Sr (1703 ppm) in the dolomitic marbles (FI12), while for all the samples studied, concentrations are moderate for Rb (1 to 150 ppm) and low for Cs (0.1 to 6.3 ppm).

Figure 8.Characterization of the climate of the depositional environment of the Firgoun sediments, deduced from the samples projected in the diagram of [50].

The trace element profile of the Firgoun samples, normalised to the UCC (Upper Crust Continental, [52]), show positive Ba peaks, which are more pronounced in the dolomitic marbles (Figure 9), suggesting that they are enriched in Ba.

Figure 9.Spectrum of trace elements normalized to UCC values ([52]).

The negative peaks in Rb, K and Sr recorded in the trace element spectra of the majority of samples reflect their depletion in the samples investigated. Whereas, the exceptional recording of positive peaks in K and Sr, respectively at the level of the spectra of schists and dolomitic marbles, suggests their enrichment.

Ba, Rb, K and Cs belong to the LILE (Large Ion Lithophile Elements) group. Due to their highly mobile nature, enrichment in LILE could be the consequence of alteration phenomena ([41]). The variations in LILE elements in the samples taken at Firgoun could be due to weathering. The high Sr content (1703 ppm) recorded in the dolomitic marbles (FI12) and the low Fe/Sr, Mn/Sr and Ca/Sr ratios (<0.01) indicate, according to [22], very limited post-sedimentary alteration.

Geochemical data also indicate low contents of Hf (0.4 to 4.3 ppm), Nb (1.3 to 6.4 ppm), but a high concentration of Zr (205 ppm). These elements belonging to the HFSE group (High Field Strength Elements) have an immobile character.

C and O Isotope Analysis of Carbonates

C and O isotope analyses of the carbonate deposits (sample FIR3) were used to clarify the stratigraphic position of the Firgoun carbonates during the Neoproterozoic glaciations. Given the apparent absence of biostratigraphic markers and the lack of radiometric dating for Neoproterozoic carbonate deposits, the use of C and O isotopes represents an important tool for making stratigraphic correlations for this period.

Indeed, periods of glaciation are generally associated with a strong reduction in organic activity, which drives the fractionation of carbon isotopes ([41]). As a result, C and O isotope compositions can be used to estimate the extent of physico-chemical alterations (diagenesis) that the sediment undergone after its formation.

The results of isotopic analysis on carbonates (sample FIR3 of dolomitic limestone taken from the upper levels of the “Beli-Garous” formation) provided δ¹³C (−7.1‰) and δ¹⁸O (−15.6‰) values. According to [21], the δ¹⁸O of meteoric waters is essentially controlled by evaporation/condensation processes (Rayleigh distillation), which depend on geographical latitude and altitude (or depth) and have values ranging from −40‰ to +5.7‰ (mantle: 5.7‰ ± 0.3‰). Therefore, the δ¹⁸O (−15.6‰) value obtained suggests a probable influence of meteoritic waters on the carbonates. On the other hand, the negative δ¹³C values of the Firgoun carbonates (δ¹³C = −7.1‰) indicate a drastic decrease in organic activity ([28]) and testify to a significant period of glaciation in the Firgoun area during the Neoproterozoic.

This formation is characterized by marine sedimentation with brief fluvial episodes ([5]). The model of depositional environment could be illustrated in Figure 10.

In fact, the lithofacies Fr1 is characterized by poorly sorted matrix-supported conglomeratic to microconglomeratic horizons at the bottom. The presence of blunt and subrounded pebbles in the lithofacies Fr1 of Firgoun area points to a fluvial influence ([5]).

Figure 10. Model of the depositional environment proposed for the Firgoun sediments.

Whereas marine sedimentation is manifested by the presence of a flyschoid-like succession attributed to the Bouma turbiditic sequence in the intermediate deposits (lithofacies Fr2) and the presence of tidal dynamics sedimentary structures characteristic (HCS, oblique bedding associated with symmetrical and asymmetrical ripples) observed in the upper deposits (lithofacies Fr3) of the “Firgoun Sandstone” Formation. Symmetrical or oscillation ripples generally result from bidirectional wave movements ([45]; [5]).

The “Béli-Garous” formation is characterized by marine sedimentation under glacial influence ([5]) (Figure 10).

The presence of sedimentary structures characteristic of tidal dynamics, notably asymmetrical ripples with rounded or flattened crests, linguoid undulating ripples, sometimes associated with hardgrounds at the tops of benches, herringbone structure (lithofacies Fr4), bundles of intersecting oblique bedding (lithofacies Fr5) and hummocky cross stratifications (lithofacies Fr6) indicate a shallow marine sedimentation. However, the intermediate levels of lithofacies Fr6 are marked by the presence of matrix-supported faceted pebble conglomerates, attributed to glaciogenic deposits (diamictites) ([45]; [10]; [13]). The glacial origin of the diamictites is attested by the presence of faceted pebbles ([35]). The lithofacies Fr6, characterized by occurrences of diamictites associated with marine-type deposits, would have been set up in a mixed glacio-marine environment. In such an environment, the arrival of large boulders seems to be accidental. Only ice rafts, which drop sedimentary material as they melt, can give rise to sedimentation of this type ([33]).

Marine sedimentation continues in the upper lithofacies, notably in lithofacies Fr7 (slates with intercalated carbonates), lithofacies Fr8 (silexites) and lithofacies Fr9 (shales). The carbonate deposits in the Firgoun region, notably the variegated carbonates and the purplish-pink dolomitic marbles of lithofacies Fr7, often show centimetric cavities interpreted as dissolution figures comparable to the “bird-eyes limestone” of [40]. These types of structures generally appear in intertidal environments 5 ([11]; [40]). According to [11], these structures result from trapped air bubbles. Indeed, the abundance of solution cavities and fractures associated to carbonates capping glacial sediments have been described in the Taoudenni basin ([4]).

The slates of the lithofacies Fr7 locally display upturned beds or cryoturbation structures ([33]; [5]). These peculiar structures, attributed to freeze-thaw phenomena, represent an important indicator of periglacial environments ([6]; [40]). Both, the intebedded diamictites of lithofacies Fr6, the carbonates of lithofacies Fr7 and the silexites of lithofacies Fr8 form the terms of the “Tillites-Carbonates-Silexites” triad, a stratigraphic marker characteristic of the Taoudenni, Gourma and Volta basins. As a result, the interbedded diamictites (lithofacies Fr6) of the Firgoun region have been assimilated to the Neoproterozoic glaciation (probably the Late Cryogenian glaciation, [33]).

In the Taoudenni basin, the carbonate deposits covering the neoproterozoic glaciogenic sediments have been interpreted as one of the characteristics of the beginning of the postglacial eustatic transgression ([46]).

The result from the projection of the samples in the Al₂O₃ + K₂O + Na₂O₃ versus SiO₂ diagram ([50]) reveals mixed paleoclimatic conditions (humid and arid) for the sediments of uppermost “Beli-Garous” Formation. According to [39], intense chemical weathering is strongly associated with a warm, humid climate. The diagram [50] has been used in order to establish the paleoclimatic conditions on the basis of lithogeochemic characteristics of variable rocks from the “Beli-Garous” sequence. The interpretation of the diagram reveals:

On the one hand, a humid climate characterized by high chemical maturity, likely due to the high SiO₂ contents (over 65%) in the lower siliciclastic deposits of the Béli-Garous Formation. Furthermore, the low Al₂O₃, K₂O, and P₂O₅ contents imply a considerable terrigenous input.On the other hand, an arid climate characterized by low chemical maturity, which is reflected in high Al₂O₃, K₂O, and P₂O₅ contents. This characterizes the upper deposits of the Béli-Garous Formation, implying a low terrigenous input, particularly in the carbonates (lithofacies Fr7) deposited in a marine paleoenvironment under arid conditions.

These observations are in agreement with hypothetic boundary or transition between glacial deposits and cap carbonates ([26]; [32]; [28]). The result is supported by the identification of the carbonate deposits, in particular the marbles and dolomitic limestones (post-glacial carbonate) of lithofacies Fr7under arid climat condition, which systematically follow glacial deposits (interbedded diamictites of lithofacies Fr6) under humid climat.

The carbonate rocks of the lithofacies Fr7 overlie in places the interbedded diamictite (lithofacies Fr6) and locally onto slates exhibiting cryoturbation structures. The transition from these slates presenting cryoturbation features to carbonate deposits in the Firgoun region is very similar to the horizon of green pelites intercalated within carbonate beds attributed to the last glacial phase of the Infracambrian in the Majâbat region (Taoudenni basin, [19]). This abrupt transition between glacial sediments and carbonates is at the origin of the Neoproterozoic climatic paradox, suggesting a rapid transition from glacial (“ice-house”) to tropical (“green-house”) conditions ([26]; [32]; [28]). Therefore, these authors attributed the overlying carbonates to marine warm-water facies ([43]).

The Firgoun carbonates have been classified as cap carbonates based on field observations, particularly lithological criteria, due to their belonging to the “Neoproterozoic Triad”. They are part of a typical sequence associating tillites/diamictites (glacial formations), carbonates (limestones and dolomites), and silexites/cherts ([33]).

The carbonate deposits of the Firgoun region, similar to their equivalents deposits in the Neoproterozoic basins of Taoudenni ([47]), Volta ([1]; [43]) and Gourma ([23]; [37]), are closely associated with “triad” glacial deposits that is considered as an evidence of major stratigraghic marker for Neoproterozoic glaciations in West Africa. The triad has been founded at the base of the Supergroup 2 (Taoudenni basin) and the base of the Oti Supergroup (Volta basin). Indeed, these transgressive carbonate deposits observed in Firgoun region locally overlie shales exhibiting cryoturbation features characteristics of periglacial environment.

Due to the scarcity of radiometric, paleomagnetic and paleontological (traces of stromatholite fossils) data, the stratigraphic position of the cap carbonates is difficult to constrain. In West Africa, the glacial and postglacial succession marking the transition from tillite deposits to cap carbonates was related to the Cryogenian-Ediacaran boundary ([47]).

A Marionan or late Cryogenian age (ca.635 Ma, [17]; [47]) has been assigned to the Neoproterozoic glacial deposits at the base of the carbonates on the basis of stratigraphic correlations supported by some radiometric, isotopic and paleontological data. [34] reported two U-Pb radiometric ages (609.7 ± 5.5 Ma, U/Pb on zircon and 604 ± 6 Ma, U/Pb SHRIMP) in the volcanic tuffs of the Téniagourou formation, which overlies the glacial deposits of the Jbéliat formation in Mauritania (Taoudenni basin). In the Volta basin, a Lu-Hf age of 576 ± 13 Ma was also obtained in the phosphorites overlain by cap carbonates and glacial deposits ([7]). As a result, the Neoproterozoic glacial deposits overlain by cap carbonates in the Taoudenni and Volta basins were finally correlated with other glacial formations observed elsewhere in the world, in particular the famous Elatina-Ghaub-Nantuo glacial formation (ca. 635 Ma, [47]).

Compared with their equivalents in the Béli region (Gourma Basin SE border, [23]; [37]), which present stromatolitic structures characteristic of tidal environments (Miningou et al., 2010, 2017), the carbonate deposits in the Firgoun region do not show stromatolites traces. This makes correlations difficult between the carbonates of the Firgoun region and the Cap carbonates of the Taoudenni basin ([47]), and Béli region in Gourma (Miningou et al., 2010, 2017; [23]) considered age late-Cryogenian (ca. 635 Ma, [47]).

However, preliminary stable isotope results (δ¹³C and δ¹⁸O) reveal a probable correlation between carbonates from the Firgoun region and those obtained in Cap carbonates from the Taoudenni ([47]), Volta ([43]) basins and Béli area in the Gourma basin ([23]). The δ¹³C isotopic signature of the carbonates from the Firgoun region indicates a negative value of −7.1‰, different with those provided by the post-Marinoan Cap dolomites in the Taoudenni and Volta basins global average around −4‰ ([43]; [47]). However, exceptional isotopic value of −8.5‰ is obtained from the Volta basin carbonate sediments ([43]). In the Taoudenni basin, the rare isotopic values reported from the post-Marinoan carbonates, notably −6.8‰ from the Cliffs (Mauritania) ([48]) and Kayes (Mali) ([4]) localities and −6.4‰ in Walidiala valley (Senegal-Guinea, [47]) are near to that obtained in the Firgoun carbonates. The Cap dolomites of Jbéliat in the Mauritanian Adrar provided distinct values (δ¹³C values between −3.7‰ and +1.3‰, Taoudenni basin, [48]) to those obtained in the Firgoun carbonates.

While the δ¹⁸O value (−15.6‰) is close to the lowest value provided by the Taoudenni Cap dolomites δ¹⁸O (−13.4‰, in the Mauritanian Adrar, [48]), but slightly different from those of the Volta Basin Marinoan Cap dolomites (δ¹⁸O between −9.20‰ and −2.41‰, [43]). Isotopic values similar to those of the Firgoun carbonates have also been reported in the Kayes area cap carbonates, Taoudenni basin (−11.8‰ and −5.0‰, [4]). These observations support probably the Rb-Sr age (600 Ma, [44]) obtained in shales interbedded with carbonates from the lower part of the Ydouban Group.