Land sections were made in the NE-SW direction Figure 3 and Figure 4. The Intiédougou section begins with andesitic tuffs, volcanic breccias, lapilli tuffs intersected by a brecciated rhyolite dyke. In the SW direction appear some andesitic flowed in association with volcanic breccias with fragments of andesite and basalt rocks up to 15 cm in diameter, crossed by dikes of microgranites. The Tarkwaïan sedimentary formations are surrounded by the Birimian volcanic and volcano-sedimentary units and the contacts are on a mesoscopic or macroscopic scale.

At the outcrop in the area of Nahirindon, a frank and parallel contact between these units related to N-S accidents directed and operated by thalwegs namely in the Kobidja region (Figure 3). The most remarkable features of these units are the presence of pebbles from the Birimian rocks and misclassification of the sediments (Figure 5(b) and Figure 5(c)). The pebbles have very varied petrographic natures, they have an angular shape and are sometimes oval and can reach 20 cm in diameter. The graded bedding is generally normal but in the Intiédougou sector, it is reversed.

The polygenic conglomerate is located about 1.5 km from the Intiédougou village towards the Bougouriba River. It is ash gray color composed of a little quartz-feldspar cement around pebbles of varied petrographic nature. The microscopic examination reveals a variety of pebbles, which confirms the polygenic nature: (i) andesites with spherolitic heteroganular porphyric to plagioclase texture embedded in a mesostasis rich in damourite and iron

oxide; (ii) Biotite hornfels with polygonal texture: the original rock underwent a granoblastic recrystallization and replaced by a microblaste feldspar mosaic. The matrix is rich in crystallized plagioclase sericite dotted with small strips of slightly destabilized biotite into chlorite and muscovite and opaque; (iii) Porphyroclastic spherolitic dacite with mylonite texture. The bottom is made up of fine to coarse clasts with plagioclase, biotite destabilized into opaque, chlorite and muscovite; (iv) Porphyric microdiorite sometimes of granophyric type, dacitic tuff, mylonite and quartz; (v) Crystal and/or lithic elements tuff.

Feldspathic sandstones: they form varied upper facies or the matrix of polygenic conglomerates pebbles. They contain lithic microfragments with a composition similar to macroscopic fragments. The petrographic analysis of

these rocks reveals heterogranular texture with generally pelitic and phyllitic cement. These sandstones have a constant variation of fine to coarse grains passing by medium grain terms.

The fine facies contains angular heterogranular quartz crystals, plagioclase sometimes significant amount of (35%) fully damouritized orthose with accessory biotite relicts associated with muscovite. The lithic fragments consist of porphyritic andesitic lavas and cherts, porphyritic microdiorites embedded in a sparse phyllitic cement rich in muscovite and sericite.

The middle facies presents a character that differs very slightly from the first by the size of the grains which are coarser, the presence of abundant enough opaque, lithic fragments marking the presence of diorites, of microdiorites and chert. These fragments are molded in abundant pelitic and hydrous mica cement.

The coarse facies is locally recrystallized into quartzite with hydrothermalized biotite. It has a coarse and massive grain size and well-preserved bedding, especially in the south of the area of Bonfesso. At a microscopic scale, the texture of the quartzite is heterogranular granoblastic serrated and meshed, with re-crystallized quartz crystals into sub meshed grains. The opaque and the muscovite highlight the fractures of the minerals and the rock (Figure 6).

Despite the alteration, the Tarkwaian lands spared from the ductile shearing deformation clearly present primary sedimentological structures in terms of bedding. There is a bedding which is sometimes horizontal or tabular and rhythmic or laminate sometimes monoclinal or without any form of cross-bedded stratification. The thickness of the beds is of millimeter to centimeter order within thick decimeter to meter beds. The conglomerate sandstone beds and those of feldspathic sandstone in fine to coarse facies, sometimes silts show intercaltions of sandstone pudding however well exposed in the Zanawa region. In the Tarkwaian mounds of Intiédougou, the conglomeratic levels show lateral variations of facies marked by a decrease in the size and quantity of lithic pebbles.

The sandstone conglomerates rest in gully discrepancy on the sandstone levels and this arrangement demonstrates a negative polarity and an inverse grano-grading. We observe tracks of breccias with angular fragment of green lava or ferruginized with sandstone cement corresponding to collaterals of the Birimian rocks. The Tarkwaian formations described are generally not classified and immature sandstone terms.

They are deducted from field observation of sedimentary Figure 4 that will enable an interpretation of the transportation and the depositional environment. On the field we observe the presence of poorly sorted coarse sediment, pebbles sometimes oval and oblique and intersecting stratifications and linear streaks (Figures 7(b)-(d)). The conclusions we can draw from these observations are the misclassification of sediments and the frequency of coarse and angular pebbles express a very short transportation.

This implies that the source of sediment would be very close to the depositional environment. The petrographic study shows that the pebbles of the conglomerate come from fragments of older rocks. The repeated presence of the oblique and cross-bedded stratification forming during the depositions of sediments shows either a fluvial transportation regime or torrential regime (Figure 6). In the petrographic analysis, there was high proportion of quartz and feldspar fragments on the one hand and on the other hand the abundance of lithic fragments mainly volcanic and sub-volcanic rocks that reflect the diversity of Birimian source eroded lands. All these arguments are in favor of a fluvial transportation regime whose deposition is much like those of torrential cones.

The deformation structures in the Tarkwaian system of the study area are much limited. At the outcrop some structural elements were inventoried. These elements are schistosity (S1) stratification (S0), the lineation of the pebbles according to the schistosity, pressure shadows, fractures and cracks. Rocks as a whole do not seem to be deformed due to the scarcity of structures. A well developed major schistosity S1 through the strip is observed. It is towards NNE-SSW direction with a dip that varies between 30˚ and 60˚ alternately at the NE and SW. To this cleavage is added a NNW-SSE direction crenulation cleavage at N-S and sloped at 80˚. Schistosity S1 is superposed to the stratification in some places.

Microfolds can be observed as kink bands at Intiedougou. The surface of some quartz shows some striations of N40˚ overall direction whose extension ranges from 10˚ to 20˚ sub-horizontal. The presence of slits in echelon observed in the area of Zanawa and the stretching of the quartz pebbles sometimes fractured seems to indicate a shearing zone. The analysis of these structures suggests a NW-SE horizontal compression perpendicular to schistosity (S₁) and a horizontal extension NE-SW. The main constraint axis that caused this deformation seems to be a sinistral displacement or a gradual transition from a non-deformed area with a strongly deformed one. The

major event of deformation affecting formations known as Tarkwaian is the regional sheared schistosity (S₁). They are also affected by other styles of deformation ranging from semi-ductile to brittle whose result is superposed structures.

The results of the chemical analyses of six (06) samples are shown in Table 1. The analysis of major elements and traces of Tarkwaian sediments shows that the sandstones show a normal siliceous character with contents of SiO₂ between 80% and 88%. The proportion of Fe₂O₃ varies from 2.29% to 4.29% and the proportions of MgO (≤1%) and CaO (<1%) are very low. The amounts of Na₂O (0.011 to 1.812) and low K₂O (1 à 3) could suggest intermediate mafic source.

The feldspathic sandstones of Intiédougou are richer in alumina (7% - 110%) than those of Bonfesso (average 5%). This difference is controlled by the amount of feldspar and mica. The iron oxides higher (4%) in the sandstone of Bonfesso than in the sandstone of Intiédougou (2% to 3.5%) is accounted for by the quantity of the remainders of biotite and the opaques. Potash two to three times higher (in feldspathic sandstones of Intiédougou) is probably partly controlled by potassium feldspar and biotite white mica of these rocks. Some major elements exist in infinite quantity near their detection limit. The case of CaO shows the lack of basic plagioclase. The Ba values between 380 and 590 ppm higher in the feldspathic sandstones of Intiédougou are replaced by the plagioclase; the tungsten (W) imposes itself in some samples in proportions reaching 390 ppm.

The data of the major element analysis allowed from Na₂O-MgO-Fe₂O₃ + K₂O [26] (Figure 8(a)) to show arkosic sandstones and lithic sandstones. This representation confirms the petrographic character highlighted above. In order to understand the origin of the sediments, in literature [27] proposed a discrimination diagram that considers seven (7) major elements Al₂O₃, Fe₂O₃, TiO₂, MgO, K₂O, Na₂O and CaO. They establish two functions F1 and F2 (Figure 8(b)) by using this diagram one easily understands that the represented samples probably originate from a mafic to intermediate source, which assumes the Birimian volcanic and subvolcanic formations.

The log chart (K₂O/Na₂O) versus SiO₂ Figure 8(c) [27] enables to separate the field of the oceanic insular arc, the field of the active continental margin and the passive margin. This diagram classifies the sediments of the study area in the field of the active continental margin. This diagram relates primarily to the maturity of the sediments. For mature clastic sediments are derived from the stable continental blocks and deposited in like rifts basins while immature sediments come from mixed sources and are stored on the active continental margin. The representation of the composition of the points of the samples indicates a signature of active continental margin.

The ternary Th-Sc-Zr/10 diagram [28] using and La-Th-Sc diagram Figure 9 a signature of the oceanic island arc agrees with the geodynamic Birimian context. The representation of the multi-elements spidergram of the samples of normalized N = 1 sandstone [29] shows that most of the samples of the study area are depleted in Rb, Sr

and P probably close to a light contribution the continental crust Figure 10. The sharp depression of the uranium in a sandstone sample could be explained by the process of alteration and the recycling of source rocks resulting from the clastics terrigenous sediments. Sr and Tb deficiency shows the scarcity of the plagioclase in the sample. K, Ba, and Nd enrichment are due to greater accumulation of the feldspars and micas.

Sedimentary detrital deposits made up of polygenic sandstone conglomerates, sandstone or pudding micro-

conglomerate, sedimentary breccia and feldaspathic sandstones with fine, medium to coarse facies form thin strips alongside the Birimian belts, Houndé belt in particular.

These formations have been described by (Bonkoungou, [13] ; Bossière et al., [14] ; Baratoux et al., [30] ) and attributed to the “Tarkwaian” system on the basis of petrological correlation especially with conglomeratic levels. Comparing these formations to the reference Tarkwaian (Table 2) where the lithostratigraphy begins with the Kawere group conglomerate, the quartzites group of the Banket, the phyllite group of Tarkwa and ends with the Huni sandstone. At the stage of the current knowledge on the field, it is difficult to find the equivalent of all the

units in Burkina Faso. In this country and the neighboring regions, these units defined in Ghana could correspond to lateral or longitudinal variations of facies. Sedimentary rocks contain fragments of volcanic and volcano- sedimentary and volcanic rocks, hornfel, mylonite, quartz and feldspar that probably come from the erosion of the surrounding rocks. The geochemical character indicates that the sediments originate from mafic to intermediate source and the geotectonic context would be in an active continental margin. This seems to be close to the model of Baratoux et al. [30] . In their geodynamic model on the greenstone belts in the south-western Burkina Faso, Baratoux et al. [30] presented two scenarios: (1) each of the Southwest belts approximately corresponds to a volcanic arc and (2) the other to a single large arc consisted of undifferentiated granitoids within.

The structural will be established during a first tectonic sheared phase and the low-intensity second phase which tighten the whole set. The sediments of the area consist of a mixture of volcano-detrital and volcanic elements in varying proportions. The geodynamic significance of this volcanism could be attached to syntectonic parameters leading to the formation of rifting or pull apart type basins. These riftings will be the seat of the clastic and volcanic sediments. From a structural perspective (Castaing et al., [15] , Bonkoungou, [13] , Milesi et al., [20] ) agree on the existence of an angular unconformity between the Birimian and the Tarkwaian in the south-west of Burkina Faso. They use structural arguments namely the dip difference between the first subvertical and oblique for the second to show the existence of a mega bend whose flank may have preceded dipping of 45˚ to justify the assumptions.

From a geochronological point of view, scattered U/Pb ages obtained on detritical zircon in Ghana 2081 ± 25 and 1968 ± 49 Ma by (Hirdes et al. [31] [32] ) and 2194 - 2132 ± 20 Ma by (Davis et al. [33] ) are similar with the ages found in Burkna Faso by (Bonkoungou, [13] ) 2151 ± 20 Ma 1991 ± Ma suggest multivariable source of birimian rocks and contribution to Tarkwaian deposits. The differences can be found in the interpretation of geodynamic contexts of the regions.

In the Kobidja area, the Birimian volcanic-Tarkwaian clastic contact appears to be frank and parallel. An angular structural and non-stratigraphic unconformity is admissible considering the rectified character of the sheared volcanics and the S₀ stratification structures of the sandstones surrounding and in contact.

The basic and felsic volcanic and hypovolcanic formations and their associated quartz veins of the Birimian chains are the main source of the Tarkwaian sediments. The quartz and chert pebbles and the more or less rolled fragments of microdiorite, diorite and microgranite alternating from this Birimian contribution is the same for volcanoclastite with dacitic tuffs with crystals and dominant lithic elements, rhyolitic and arenite tuffs less often. The abundance of debris of spherolitic dacitic flows and spherolitic to porphyry microlithic andesites in these terrigenous sediments is correlated with the predominance of these facies in the Birimian volcanism. The majority of these lithic fragments underwent shear deformation and hydrothermal alteration rather than metamorphism. These assumptions are corroborated by the presence of volcanic quartz that is gulfed or twinned or even in the form of dagger blade. Although limited, the participation of the granite base is conceivable whose marks are found in coarse quartz debris poecilitic igneous of plagioclase polycrystalline debris or in digestion, the quartz fragment in mosaic. The contribution of old birimiandetrital sediments or volcano-sediments is still to be elucidated with physico-chemical tracers.

The calc-alkaline geochemical signatures of the Birimian volcanics and their subvolcanic vein associated Eburnean granites are like emanations of geodynamic contexts of intra-oceanic subduction with insular arc [15] . The products derived from these formations may have influenced the composition and the definition of the geodynamic setting of the island arc of the Tarkwaian sediments. This model is accepted by authors who supported it with lithostructural arguments especially for the greenstone belts of the West and Southwest of the country. The structuring of the Birimian system is induced by a first phase of ductile brittle shearing around 2100 Ma, known in the West as the Houndé-Ouahigouya rift and in the East as the Markoye-Tiébélé rift have regional extension. These accidents have worked in multiple tectonic reactivations to which are likely linked rifts, a megastructure favourable to the deposits of the Tarkwain terrigenous sediments [15] .