The lithostratigraphy of the Cape Verde Peninsula was established from the data collected during research surveys on water or hydrocarbons, but also from outcrops observations. Thus, three formations have been identified.

a) Mesozoic Age Formations

Ÿ The Trias and Lias are characterized by the existence of salt-bearing rocks. They are associated with black and green clays, pyrite and sulfur, siderite crystals and rare quartz grains (Templeton, [7] ).

Ÿ The Jurassic is characterized by allochemical carbonate sediments in the West and detrital sediments in the East. The microfauna and microflora are very diverse (lamellibranch moluscs, gastropods, echinoderms and bryozoans, algae). The carbonates form a thick series stretching along West Africa’s margin.

Ÿ The Lower Cretaceous is characterized by the appearance of detritus in the West, despite the predominance of carbonate deposits. A series consisting of sandstones alternates with silty clays. Its thickness decreases eastwards. In the West occur carbonate facies alternating with sandstone rocks. Ostracods, pollen, many Choffatella decipiens and microplankton are observed.

Ÿ The Middle Cretaceous is characterized by detritus invading the basin. The facies that existed in the Cretaceous are still persistent. Throughout the study area, the limestones are dominant and are dated using Nummulites. The facies observed there are limestone clays, sandy limestone, clay or silica of variable thickness.

Ÿ The Upper Cretaceous is characterized by limestone facies, limestone clays or marly limestone often containing phosphate and planktonic microfauna that consists of Globorotalia cocoansis and Globigerapsis semi involuta and Catapsydrax unicavus. At the end of the Cretaceous, a thick sandy clay deposit was formed on the impermeable base. These sands contain large amounts of water used throughout the basin.

b) The Tertiary Formations

They include the following formations:

Ÿ The Lower Paleocene consists of clayey marl, calcareous marl and clays (the Madeleines Inlet). It is ten meters thick at the outcrop and over 200 meters deep. It contains a wildlife that consists of Globigerina and Globorotalia.

Ÿ The Upper Paleocene includes silicified clay called “Hospital formation” that outcrops at the Pointe Bernard, at the Pointe Diop, at the Madeleines Inlet and the Pointe de l’Abattoir (Bongrand and Elouard [8] ). Its thickness decreases from the North to the South of the head of the Cape Verde Peninsula.

Ÿ The Lower Eocene corresponds to the calcareous clay (attapulgite) of the Dakar prison. It contains a micro-fauna with Globorotalia and Cibicides.

Ÿ The Middle Eocene is basically formed with yellow to gray marly clay, with a rich microfauna consisting of Robulusincisus, Cyroidinasenegalensis and Epinideselel’atus (Bernard Beach, Bargny). Then the calcareous marl of the Bernard Beach appears.

Ÿ The Upper Eocene outcrops nowhere in the head of the peninsula. It was observed in a single survey conducted in Yoff and is entitled DK2. It is formed by a beige clay with a rich fauna consisting of uvigerina.

The Tertiary is characterized by a continental episode with intense volcanism that gave birth to many types of rocks (the tuffs of the Madeleines and the ankaratrite and basanite flows). The dating of these rocks revealed an age between 48 Ma and 5.30 Ma (Cantagrel et al. [9] ; Sarr et al. [10] ). These volcanic formations are covered by a laterite surface which marks the end of the Tertiary and the beginning of the Quaternary.

c) The Quaternary Formations

They are represented by marine, continental and volcanic facies.

The marine Quaternary is characterized by two episodes:

Ÿ The Inchirien represented by a beach-rock (Southeast Bargny) dated 32,000 years by K/Ar method;

Ÿ The Nouakchottian (6800 - 4200 years by K/Ar method) is characterized by accumulations of shells (Cape Verde Peninsula and the Tamna Lake) and a more humid climate. During that period, the sea flowed into all the depressions to form a series of shallow bays with Anadara senilis shells accumulations (Elouard et al. [11] ).

The Continental Quaternary is represented by shell glazes and alluvial terraces. The glazes are sometimes polygenic. The Continental Quaternary ends with aeolian deposits covering large areas with an ancient erg (Akcharien), red dunes (Ogolian) and yellow and white dunes forming a coastal strip between Dakar and Saint Louis. The quaternary volcanic activities that are observed at the outcrop are located in the head of the Cape Verde Peninsula.

The volcanism of Western Senegal is represented by small outcrops which are distributed between the Cape Verde Peninsula (Dakar) and the Thiès plateau. It covers an area of 325,500 km² (Figure 1). The outcrops are in the form of lava flows, veins, dykes, and tuffs. Tertiary volcanic rocks are distributed throughout the outcrop area while those of the Quaternary volcanism are located in the northwestern tip of the Cape Verde Peninsula. There is also evidence of volcanism in boreholes. Tertiary volcanic rocks are represented by olivine basanites, olivine alkali basalts and nephelinites. Their texture ranges from the fine microlitic porphyry-type (nephelinites basanites and olivine) to the subdoleritic type (alkaline olivine basalts to medium grain) (Dia, [1] ; Lo, [2] ). Those features reflect a petrographic evolution from nephelinites to medium-grained alkali olivine basalts. The Quaternary volcanic rocks include alkali olivine basalts and olivine basanites with fine to coarse grain. (Dia, [1] ; Lo, [2] ). Their texture is either microlitic porphyry (some alkali olivine basalts and some olivine basanites) or doleritic (coarse-grained alkaline olivine basalts). They evolved from olivine basanites to coarse-grained alkali olivine basalts (Dia, [1] ; Lo, [2] ). This evolution also resulted in a decrease in the content of mafic minerals and mesostasis (olivine and pyroxene). The Quaternary lava are therefore more differentiated.

Three categories of deep faults have been described (Dia, [1] ; Lo, [2] ; Crévola et al. [6] . Mpassi, [12] ). The first one, well represented at the horst of Diass, is oriented North-South. It corresponds to the ancient fractures hemming the blocks that were tilted during the rifting coinciding with the opening of the Atlantic Ocean. The second one, which is oriented East-West, cuts across the eastern part of the study area. This type of transform faults is related to the Mid-Atlantic Ridge (Bellion and Crévola, [13] ). The third category, which is oriented North-West-

South-East, affects the western part of the Cape Verde Peninsula. All these faults probably had a real impact and could have caused the quaternary volcanism of the head of the Cape Verde Peninsula (Dia, [1] ).

The average contents of the major elements of Western Senegal’s Quaternary lavas (Table 1) show that they are highly under-saturated in silica (SiO₂ average around 44.7%), with high levels of sodium (3.64% of Na₂O on average), medium levels of potassium (0.91% of K₂O on avarage), high levels of calcium (11.65% of CaO on average), high levels of magnesium (around 10.42% of MgO on average) and high levels of iron (11.81% of Fe₂O₃ on average). The Tertiary lavas of the Thiès Plateau (Table 2) are slightly under-saturated in silica (an average of 45.5%), with higher concentrations of alumina (an average of 13.52%), more sodium (an average of around 4.11%) and more potassium (with 1.22% on average), but with less iron (10.78% on average), less mag- nesium (8.55% of Mg on average) and lower contents of calcium (10.61% on average) than those of the Cape Verde Peninsula [SiO₂ ~ 43.95%) (Al₂O₃ ~ 12.51%) (Fe₂O₃ ~ 11.73%) (MgO ~ 11.12%) (CaO ~ 12.04%) (Na₂O

~ 3.47%), (K₂O ~ 0.79%)]. The Quaternary lavas (Table 1) are slightly more differentiated than the Tertiary lavas (Table 2) if we consider the geochemistry of the major elements. They are slightly undersaturated in silica (49.85%), with high concentrations of sodium (3.89%), calcium (9.08%), and medium concentrations of magnesium (8.35%) and iron (10.62%).

The descriptions of the major elements show:

Ÿ Higher contents of SiO₂ in the Quaternary lavas compared to the Tertiary lavas. The Tertiary lavas of the Thiès Plateau are richer in silica than the Tertiary lavas of the Cape Verde Peninsula. The basalts of the Cape Verde Islands (Table 4) have lower contents of silica than those of Western Senegal;

Ÿ High contents of Al₂O₃ in the Tertiary lavas compared to the Quaternary lavas. In addition, the Tertiary lavas of Western Senegal have higher levels of alumina in the Thiès Plateau than in the Cape Verde Peninsula. In the Cape Verde Islands, the Quaternary basalts are more aluminous than the Tertiary basalts of these islands (Doucelance et al., [14] ) and those of Western Senegal. The latter are more aluminous than the Tertiary basalts of the Cape Verde Islands;

Ÿ Relatively low MgO, FeO and CaO contents in the Tertiary lavas, the latter containing lower levels of magnesium, iron and calcium in Thiès Plateau than in the Cape Verde Peninsula. The Tertiary basalts of the Cape Verde Islands contain higher concentrations of magnesium and calcium than the lavas of Western Senegal. The Quaternary lavas of these islands contain lower concentrations of magnesium, but they have iron and calcium contents that are higher than in the Quaternary volcanic rocks of Western Senegal;

Ÿ and a relatively slight variation of Na₂O and K₂O between the Tertiary and Quaternary lavas of Western Senegal and those of the Cape Verde Islands (Figure 2).

These different features reveal two groups in the Tertiary lavas of Western Senegal: the group of the Cape Verde Peninsula Tertiary basalts (40% - 52% in silicia with an average of 44.23%) and the group of the Thiès Plateau Tertiary basalts (43% - 49% in silicia with an average of 45.6%). The basalts of the Cape Verde Islands (Doucelance et al., [14] ) are more undersaturated with silica than those of Western Senegal (Figure 2).

The data in Table 4 show, on the one hand, the properties of some major elements in the Tertiary and Quaternary basalts of Western Senegal, and on the other hand, the properties of the Tertiary and quaternary basalts of the Cape Verde Islands.

The variations observed in the lavas of Western Senegal show that the Tertiary basalts have higher magnesium levels, but are more ferriferous, more calcic, less aluminous, and less alkaline than the Quaternary basalts. The latter contain more silica, less alumina, less mafic elements, less potassium and less titanium than the Quaternary basalts of the Cape Verde Islands. The Tertiary basalts of Western Senegal have higher levels of silica with more sodium, less potassium, less magnesium, less titanium than the Tertiary basalts of those islands. The Tertiary lavas of Western Senegal and those of the islands (Sao Nicolau, Santiago, Sao Vicente and Sal) have all average contents of silica below 45%. Contrastingly, the Quaternary basalts of Western Senegal have higher concentrations of silica than the Tertiary basalts of Western Senegal or the basalts of the Cape Verde Islands. In the Cape Verde Islands, Doucelance et al. [14] have shown that the average content of magnesium in the Southern islands (Fogo and Santiago) is 7.97% and the average magnesium content in the northern islands (Sao Vicente, Sao Nicolau and Sal) reaches 13.14%. This shows that the Tertiary basalts (10.42%) and Quaternary basalts (8.35%) of Western Senegal have average contents of magnesium between those of the South Islands and North Islands. All these variations reveal that the basalts of the Cape Verde Islands are similar to the basalts of Western Senegal. The lavas of Western Senegal are alkaline to potassic trend. Tertiary lavas of the Cape Verde Islands are alkaline to sodium trend while those Quaternary are alkaline to potassic trend (Figure 3).

The Tertiary lavas of Western Senegal have very high average levels of compatible elements (Ni = 223.45 ppm, Cr = 410 ppm, Co = 47.48 ppm, V = 244.63 ppm, Sc = 20.28 ppm) and high field strength elements (HFSE) (Nb = 83.84 ppm, Zr = 180.69 ppm, Th = 5.60 ppm, Ta = 5.73 ppm). In the Cape Verde Islands, the Tertiary basalts have average contents of compatible elements (Ni = 266.5 ppm, Co = 59.63 ppm; Cr, V, Sc not determined) and high contents of HFSE (Nb = 64.95 ppm, Zr = 250.63 ppm, Th = 5.54 ppm and Ta = 4.67 ppm). The average levels of the elements mentioned above referring to the Tertiary basalts of Western Senegal and Cape Verde Islands are higher than those of the oceanic island basalts (OIB), of the enriched (E-MORB) or normal (N-MORB) oceanic plateaus basalts (Sun and McDonough, [16] ) [Table 5]. Those high concentrations of compatible and HFS elements give the Tertiary lavas of Western Senegal and of the Cape Verde islands a primary character. The Quaternary basalts of Western Senegal have also very high average contents of compatible elements (Ni = 194.2 ppm, Cr = 344 ppm, Co = 41.3 ppm, Sc = 165.2 ppm and V = 16.66 ppm) and HFSE (Nb = 52.87 ppm, Zr = 135.7 ppm, Th = 4.22 ppm and Ta = 2.99 ppm). In the Cape Verde Islands, the average contents of the compatible elements observed in the Quaternary basalts (Ni = 60.83 ppm, Co = 45.33 ppm) and of the HFSE (Nb = 81.66 ppm, Zr = 336.5 ppm, Th = 3.5 ppm, Ta = 6.45 ppm) are also very high. The Quaternary lavas of Western Senegal and of the Cape Verde Islands display contents that are higher than those of the N-MORB, E-MORB and OIB (Sun and McDonough, [16] ) [Table 5]. These lavas reflect the primary feature of the Quaternary lavas of Western Senegal and of the Cape Verde Islands basalts. The Tertiary and Quaternary basalts of Western Senegal and those of the Cape Verde Islands have very high contents of incompatible and

HFSE. The Tertiary basalts of Western Senegal and Cape Verde Islands all have contents of these elements that are higher than the contents of the elements of the Quaternary basalts mentioned above. Small variations in the content of these elements were noted between the Tertiary lavas of Western Senegal and those of the Cape Verde Islands. In contrast, the Quaternary basalts of Western Senegal reveal average contents of these elements that are higher than the Quaternary ones in the Cape Verde Islands. This reflects the slightly differentiated nature of the Tertiary lavas in Western Senegal and in the Cape Verde Islands, compared to the type of Quaternary lavas mentioned above. These levels, which are higher than those of the OIB, E-MORB and N-MORB may reflect the contamination of these lavas by the crust that is rich in HFSE.

The C1 chondrite-normalisation patterns (Taylor and McLennan, [17] ) of the basalts in Western Senegal and in the Cape Verde Islands (Doucelance et al., [14] ) all reveal similar fractionations (Figure 4).

The basalt spectra of Western Senegal [Tertiary (e) and Quaternary (f)] and of the Cape Verde Islands [Tertiary (g) and Quaternary (h)] show a high enrichment in light rare earth elements (LREE) [60 to 150 × chondrites] and a slight enrichment in heavy rare earth elements (HREE) [5 - 20 × chondrites]. This feature reflects a high fractionation of the LREE compared with the HREE in Western Senegal’s basalts [La_N/Yb_N) Average = 17.86 (Tertiary lavas) and 17.95 (Quaternary lavas); (La_N/Sm_N) Average = 3.68 (Tertiary lavas) and 3.70 (Quaternary lavas)]. The lavas of Western Senegal and those of the Cape Verde Islands [La_N/Yb_N) Average = 18.10 (Tertiary lavas) and 17.38 (Quaternary lavas); (La_N/Sm_N) Average = 3.28 (Tertiary lavas) and 3.13 (Quaternary lavas)] show slight variations in LREE/HREE ratio. Consequently, the basalts of Western Senegal show more similarities with the basalts of the oceanic islands. Positive anomalies to a lesser degree in Eu have been observed. They characterize the fractionation of the plagioclases. Two groups of Western Senegal’s Tertiary lavas have been identified: the first group (1) which is richer in LREE, consists of the Tertiary samples PB01, PB04, PB05, PB11A, PB11B, PB16A, PB16B, PB27A and PB27B. The second group (2), which is less rich in LREE, consists of the remaining Tertiary samples. These groups are different from each other (Figure 5) in the co-variance diagrams La/Yb (i), La/Sm (j).

The spectra of rare earth elements average compositions (Figure 6) normalized by C1-chondrites (Taylor and McLennan, [17] ) of the Tertiary and Quaternary lavas of Western Senegal compared to those of the Cape Verde Islands (DOUCELANCE [14] ), and those of the alkaline basalts of Cameroon (Dongmo [18] ) and Ethiopia (Treuil and Joron [19] ), and those of the OIB (Sun and McDonough [15] ) and the BHVO-2 reference of Kalauea, Hawaii (from GEOREM data base), show that the lavas of this volcanic provinces have very similar HREE levels expected the Quaternary lavas of Western Senegal. On the contrary, compositions in LREE differ rmarkedly; they are closer to those of the intraplate alkaline basalts of Cameroon and OIB (Sun and McDonough [15] ). As for the Quaternary ones of Western Senegal they have lower levels of rare earths elements than the Tertiary ones of Western Senegal, of the Cape Verde Islands, of Cameroon, and the OIB, and are closer to the BHVO-2 reference.

The behaviour of the major elements with regard to the differentiation index variation reveals a geochemical continuum that suggests a high probability of co-geneticism between the Tertiary and Quaternary lavas of Western

Senegal. MgO, CaO, TiO₂, P₂O₅ and Fe₂O₃ have negative correlations with SiO₂ (figure not included) and positive correlations with the Mg# (Figure 7). The Na₂O, K₂O and Al₂O₃ show positive correlations with SiO₂ and negative correlations with Mg# (Figure 7). The decreasing contents in MgO, CaO, TiO₂, Fe₂O₃ and P₂O₅ could be related to the separation of olivines, pyroxenes, and plagioclase from the titanomagnetite and from the ilmenite through fractional crystallization. The successive appearance of these minerals led, during the magma evolution, to the modification into basanites, alkali olivine basalts and nephelinites of the chemistry of these basalts.

The average Nb, Zr, Th, Ta contents of the Tertiary basalts of Western Senegal (Table 5) are higher than the contents of the oceanic island basalts (Sun and McDonough, [16] ). The Nb, Zr, Th, Ta contents of Western Senegal’s Quaternary basalts show levels that are very similar to those mentioned above. The variations of the Ni and Co contents of Western Senegal’s basalts reveal that they have more similarities with the less evolved magmas. In the Cape Verde Islands, the Ni contents of the Quaternary basalts (Fogo) range from 34 to 113 ppm while the Ni contents of the Tertiary basalts range from 34 to 313 ppm (Santiago), 205 - 553 ppm (Sao Vincete), 163 - 349 ppm (Sao Nicolau), 232 - 611 ppm (Sal) (Doucelance et al., [14] ).

The trace elements of the Tertiary and Quaternary basalts of Western Senegal and the Cape Verde Islands normalised to C1 chondrites (Taylor and McLennan, [17] ), are shown in Figure 8.

The spectra of the Tertiary lavas which are subparallel to the spectra of the Quaternary volcanic rocks of Western Senegal suggest that they are co-genetic in nature. The basalts of the Cape Verde Islands also have spectra that are subparallel to the spectra of Western Senegal (Tertiary and Quaternary), which also bear evidence of their possible co-genetic character. The K, Rb, and Pb negative anomalies and the Eu (weak), Nb, Sr and Ta positive anomalies observed here are typical of the basalts of the oceanic islands. Similar anomalies of the same elements have been described in the islands of Sao Nicolau and Santiago (Millet, [20] ). In addition, the C1 chondrite-normalized data collected by Doucelance et al., [14] from the different islands of Cape Verde also show K and Rb negative anomalies and Nb, La, Sr and Ta positive anomalies [Figure 8(q) and Figure 8(r)]. The K anomalie are due to the fractionation of the plagioclase.

The broadened spectra of compositions of trace elements in the alkaline basalts of Western Senegal compared to those of the Cape Verde Islands (DOUCELANCE [14] ), the intra plate alkaline basalts of the Cameroon (Dongmo [18] ), the Ethiopian rift (Joron winch and [20] ), to those of the OIB (Sun and McDonough [16] ) and the BHVO-2 reference standard (Menard [21] ), show that they are all similar and have positive Ba, Th, U, Sr, Zr anomalities, and strong negative Ta, Rb, K, Pb anomalities. However, enrichment in the more incompatible elements is highly variable from one zone to another. The BHVO-2 benchmark is less enriched in Ba, Th, U, Nb and Ta than the alkaline basalts of theafore mentioned volcanic provinces.

The Tertiary lava Nb/Th and Nb/Ta ratios (15.7; 15.1) of Western Senegal bring them closer to the basalts of the Hawaii Archipelago (Nb/Th > 15; Nb/Ta = 15.8) [50 ± 14; Sims and De Paolo [22] and Mevel. [23] . On the contrary, those of the Quaternary lavas (12.8) bring them closer to those of Ruturu Islands, Mangaia, Tubuai (12 - 15) which are HIMU type OIBs (Graham [24] ; Mevel [23] ).

The study of the fractionation process and the analysis of the behavior of the major elements with regard to the Mg# and to some trace elements show that the major geochemical property of Western Senegal’s volcanism is controlled by the fractional crystallization of the olivine, pyroxene and plagioclase. The olivine, pyroxene and plagioclase as well as the opaque minerals constitute the major mineral phases observed in the various lavas, with variable ratios from one facies to another. The basanites contain much more olivine and pyroxene than the alkali olivine basalts, which brings evidence of a differentiation through fractional crystallization. The Tertiary lavas are more undersaturated and undifferentiated than the Quaternary lavas. The Cape Verde lavas are more undersatured in slicia than those of the Western Senegal’s lavas. This differentiation may have been caused by the existence of several lavas exiting points for the same magma, with conditions of formation and chemistry that are different from one area to another. The magmatic differentiation occurred through the fractionation of olivine and pyroxene as evidenced by the gradual decrease of the calcium and the increase in silica and alumina in the less magnesian lavas (Quaternary lavas). This is illustrated by the shift in the diagrams (Mg# versus major elements) of the Quaternary lava towards higher silica, alumina and sodium contents and towards lower iron, calcium, magnesium and titanium values for an identical Mg# (Figure 5). This can be interpreted as being caused by the effect of one or more components with weak iron, calcium and magnesium contents and high silica, alumina and sodium contents during the rifting process of the Atlantic Ocean. The REE enrichment of the Western Senegal basalts is very close to that of the basalts of the oceanic islands because of the high LREE enrichment and slight HREE enrichment. The negative anomalies (K, Rb and Pb) and positive anomalies (Nb, Ta, Sr, and Eu) observed in the trace elements patterns bring them closer to the basalts of the oceanic islands. The average levels of silica in the Tertiary basalts of Western Senegal are comparable to the basalt contents of Fogo, Sao Vicente, Sao Nicolau, Sal islands (SiO₂ < 45%) (Doucelance et al., [14] ). The MgO average contents of the Tertiary lavas are located in-between the contents of the southern islands (7.97%) [Fogo, Santiago] and those of the northern islands (13.14%) (Doucelance et al. [14] ). The Quaternary basalts of Western Senegal are more differentiated than all the basalts of the Cape Verde Islands although their MgO contents remain comparable. Like Western Senegal’s basalts, the basalts of the Cape Verde islands show more similarities with the less evolved magmas. These are related to the fractionation of the olivine and clinopyroxene. The Ni and Cr contents are included in these minerals. The Ni and Cr contents of the Tertiary and Quaternary basalts of Western Senegal, and those of Santiago, Fogo, Sao Vicente, Sao Nicolau and Sal islands, show similarities with rocks originating from primary magmas. The different features listed above show a possible genetic link between the basalts of Western Senegal and the basalts of the Cape Verde Archipelago. The latter are generated from a lower-mantle-type source (Millet, [20] ). The distribution of K₂O based on the number of samples from the lavas of Western Senegal and the Cape Verde islands is shown in Table 6.

The Tertiary and Quaternary basalts of Western Senegal range from medium to high concentrations of potassium as well as the Tertiary basalts of the Cape Verde Islands. The Quaternary basalts of the Cape Verde Islands show only high potassic contents.