There are two groups: the Ségou group and the Madina Kouta group (Figure 2). The Segou Group rests unconformably on the Birimian bedrock and extends along a line from Fongolembi to Pelel Kindessa. It forms most of the Meso-Neoproterozoic sedimentary cover in Senegalese territory. It is subdivided by [7] into two formations: a basic conglomerate discordant on the basement, surmounted by sandstones, oolitic limestones (F. de Pélel) and decimetric to plurimetric

Figure 2. Synthetic lithostratigraphic section of the Ségou Madina Kouta Supergroup (modified, after [7]).

sandstone bars alternating with thin clay-silty interbancs (F. de Dindéfélo). [6] described the basic conglomerate in detail and made a third formation, the Kafori formation. Madina Kouta’s Group is transgressive on the Segou Group and sometimes discordant on the base. It is only observed on the Fongolembi plateau and on the border with Guinea south of Ségou. It is made up of three Formations: (i) the Fongolembi Formation consisting of fine clay-silty sandstone topped by an alternating silt-sandstone; (ii) the Kanta Formation, consisting of fine to medium silty sandstones with intersecting oblique stratifications; (iii) the Dira Formation consisting of siltstones, calcareous sandstones and coarse yellow sandstones with silty intercalations followed by fine sandstones and purple siltstones.

It was divided by [4] into two formations: the Hassana Diallo Formation, which contains the members of Pelel and Diagoma, all glacial origin, and the Nandoumari Formation, which includes the members of Tanagué, Bowal and Fougon, all probable marine origin. This lithostratigraphic division was taken up by [6] who distinguished two groups (i) the Walidiala Group, which begins with an alternation of sandstone and pelitic (F. de Walidiala 1) topped by massive tillites and laminated tillites with dropstones, capped by calcareous silt-sandstone argillites in centimetric wafers (F. de Walidiala 2) and quartz arenites with convoluted beddings (F. de Walidiala 3); (ii) the Mali Group subdivided into three formations: the Mali Limestone Dolomite Formation 1, the Mali Chert Formation 2 and the Mali Purple Pelite Formation 3. The recent work of [21] and [22] has led to the proposal of a new division in the Walidiala Group with two formations instead of three: the Walidiala Formation 1, alternating massive tillite and laminated silty clay with dropstones, and the Walidiala 2 Formation consisting of quartz arenites in channels. This work has also proposed two (2) formations in the Mali Group: The Formation of dolomitic limestones and calcareous dolomites with celestine and barite (M1) sometimes evaporitic and the purple pelitic-cherts Formation (M2). The continuity of these formations in the Landiene and Bandafassi sectors has been proven by the work of [22].

It shows from bottom to top three types of facies: silty clay and purple greywackes, sometimes cornified with centimetric to metric levels of fine white to pinkish sandstone (Figure 3(A)); a clear alternation of clay and sandstone with a predominance of sandstones towards the summit where they amalgamate, showing levels of interbedded intraformational conglomerates. The sandstone facies consist of

Figure 3. Silty clay and purple greywackes with intercalations of fine sandstone levels of DF1 (A-B), fine clayey sandstone with clayey interbeds of DF2 (C); Quartz sandstone with clay-sandstone interbeds, from DF3 (D); monogenic tectonic breccias of DF3 (E); ia = clayey intercalations; ags = Silty clays, grf = Fine sandstones; bt = tectonic breccias.

10 to 30 cm beds of white or pink quartz sandstone or friable brown to pink clayey sandstones (Figure 3(A) & Figure 3(B)). It is common to find in these fine facies centimetric pastes of very coarse to microconglomerate sandstones. Intraformational conglomerate levels are 3 to 5 m thick. These are breccias with granular support and contain intraclasts made up of large quartz grains, centimetric to decimetric sandstone fragments, caught in a dark sandstone-microconglomerate matrix rich in volcanic material and a ferruginous cement.

It is made up of decimetric to metric beds of friable medium clayey sandstone, mauve, purple, or pinkish white, interspersed with centimetric and decimetric beds of pelites and argillites. The clay interbeds are strongly eroded and leave spaces between the sandstone banks constituting zones of weakness (Figure 3(C)), creating favorable conditions for differential erosion characteristic of the sandstones of the Dindefelo Formation.

It outcrops spectacularly at the top of the hills, all along the Senegalese Guinean border, from Tépéré to Pelel where it disappears completely. On the ground, the whole of Member 3 is structured in tiers made up of plurimetric sandstone units separated by centimetric clay-sandstone flats (Figure 3(D)). There are also monogenic breccias pockets (Figure 3(E)). From the base to the top, the sandstone banks increase in thickness and granulometry, to the detriment of the soft clay-sandstone beds. These banks are easily recognizable by their very characteristic conjugate fractures giving rise to a ruiniform landscape (Figures 4(A)-(D)). The

Figure 4. Overview of the ruiniform sandstones of DF3: Multi-metric ruiniform sandstone of the Dents de Dandé (A-B); General view of the strongly fractured ruiniform sandstones, of the Dindéfélo waterfall (C-D).

monogenic breccias associated with the sandstones are very localized and very resistant; their lithoclasts, very angular and small, are made up of fragments of quartzite sandstone observed north-east of Ségou and east of Dindéfélo where they are quite often located along small faults.

The purple and greywacke silty clay in places of DF1 are made up of small mono- and polycrystalline angular quartz with ferruginous cement sandstone infiltrations (Figure 5(A)). The sandstones, fine to very fine, have a pelitic matrix and a ferruginous cement; their classification is poor and their porosity extremely high (Figure 5(B)).

Figure 5. Microscopy of the DF1 facies: Purple silty claystones with infiltrations of coarse material and ferruginous cement (A); Exceptionally fine and very porous quartz arenites (B). ar: claystone; ox: iron oxide; Po: porosity; qz: quartz.

Petrographically, the figurative elements of the DF2 clay sandstones are made up of quartz grains with alternating very fine-grained beds and fine-to-medium-grained beds (Figure 6(A)): i) the fine-grained beds are made up of angular elements of the size of very fine arenites, not joined, bound together by a very ferruginous cement; (ii) coarse-grained beds consist of fine to medium-sized, rounded to subangular, cracked and heavily worn quartz grains, bound together by a matrix of very fine quartz grains. They are sometimes joined and often interlocked, with undulating extinction and sometimes feeding. In the rock, there are also a few rare plagioclases. Glauconia and iron oxides are well represented in some slides. It is a quartzo-silty arenite, with glauconia and high porosity and iron oxide film.

DF3 sandstones are fine to medium-fine quartzite sandstones, well sorted, hard, with smooth breaks, whitish pinks. They are made up of angular to subangular grains of mono- and polycrystalline quartz with undulating extinction, intimately welded, scratched, serrated, and meshed (Figure 6(B)). The matrix is very scarce, and the cement is siliceous and ferruginous. These banks are easily recognizable by their very characteristic conjugate fractures giving rise to a ruiniform landscape (Figure 4(C) & Figure 4(D)). It is therefore a sandstone with good mineralogical and petrographic maturity.

Figure 6. Microscopy of DF2 sandstones showing alternating very Joined fine-grained beds and fine- to medium-grained beds (A) and fine- to medium-grained quartzics arenites of DF3 (B); qz: quartz; m = Matrix.

Examination of the sedimentary structures internal to the DF 1 sandstone bars shows undulating beddings, very frequent HCS, flat fine laminations, oblique and intersecting laminations, very frequent herringbones towards the top, flaser-beddings, surfaces with asymmetric current ripples and waves at the top of the banks, and a discreet normal grading. The argillites at the top of fine sandstone beds exhibit parallel laminations and are often heavily eroded.

Within the clayey sandstones of DF2 we find parallel stratifications on a large scale, oblique laminations, sometimes intersecting on a large scale, herringbones at the base, flaser beddings and undulating beddings, but also load figures at the base of the beds. On the surface of the banks, there are asymmetric current ripples, desiccation cracks but also bioturbations. All these structures could be referred to as a nipple intersecting stratification zone or HCS.

The quartzite sandstones of DF3 are rich in sedimentary structures and figures, marked by tidal bundles which are very abundant. The present forms include symmetrical, asymmetrical ripple marks (Figure 7(A) & Figure 7(B)), linguoïds, ripples, sometimes climbing ripples (climbing ripples stratification) as well as imprints of current ripples (Figure 7(C) & Figure 7(D)). There are also many bioturbations (Figure 8(A) & Figure 8(B) and microbial mats. Wrinkles as well as desiccation cracks (Figures 8(C)-(E)) and hardened surfaces are very frequent

Figure 7. Sedimentary structures and figures associated with DF2: Wave Ride (A-B); Fossilized Ripples and Ripple Prints (C-D). (C) for [23].

Figure 8. Sedimentary structures and figures associated with DF3: Bioturbations not known (A-B); Desiccation slots (C-E); Herringbones structures (F); tidal dunes and anti-dunes (G-H); tidalites and tidal gutters (I). (H) for [23].

and better expressed than in the other limbs, unlike fishbone structures (Figure 8(F)). As bedding, DF3 exhibits large-scale parallel stratifications, intersecting, sometimes arcuate oblique stratifications, flaser-beddings, HCS, sandbars, dunes, anti-dunes (Figures 8(G)-(H)) and tidalites and tidal gutters (I). The basal surfaces of the sandstones are erosive and irregular with load imprints of centimeter depth, erosion gutters of centimeter to decimeter depth. Many hard ground surfaces covered with lateritic crusts are found at different altitudes.