Syllabus Review 2 (1) (2010): 1-8

N SYLLABUS REVIEW E S Science Series

RESEARCH ARTICLE - GEOSCIENCES

Geochemistry and duricrusting of the weathering products on granite at Djoulgouf in the Lake Chad basin

Likiby B. 1, Kamgang Kabeyene Beyala V. 2, Ngapgue F.3, , Ndomé Effoudou C. 2, Parisot J.C. 4, Ekodeck G. E. 5

1Ministry of Scientific Research and Innovation, P.O. Box 1457, Yaounde, Cameroon, Email: [email protected] ; 2Higher Teacher’s Training College, University of Yaounde I, P.O. Box. 8127, Yaounde, Cameroon; 3IUT of Bandjoun, University of Dschang Cameroon; 4CEREGE, UMR 6635, University Paul Cézanne/Aix-Marseille III, P.O.Box 80, Europôle de l’Arbois, France; 5Faculty of Sciences, University of Yaounde I, P.O. Box 812, Yaounde, Cameroon

Received: 02 February 2010 / Revised: 4 May 2010 / Accepted: 7 May 2010  Ecole Normale Supérieure, Université de Yaoundé I, Cameroun

Abstract A profile located at 10 037’31” N-14 028’53’’E, around Maroua, in the Cameroon portion of the Lake Chad was studied in order to compile a geological and geochemical database on the Djoulgouf region and also to understand the duricrusting mechanisms on granite in the sudano-sahelian zone. Morphological and mineralogical studies using thin sections and X-ray diffraction, as well as geochemical analyses enabled the identification of granites overlain by three levels. These levels, from the bottom to the top, include an alteritic level, a dismantled duricrust and an organomineral level. The mineralogy of the granite revealed the occurrence of quartz, microcline, biotite, albite and rare anorthites. Microcline, goethite and traces of kaolinite and hematite were observed on the loose materials. The present investigation also revealed that the duricrusts were formed from a recombination of granitic alterites and iron derived from the nearby volcanic alterites. Moreover, there is an obvious contribution of the watertable fluctuation resulting from the various transgressive phases of the Lake Chad basin. Thus, the recombination of material sunder fluctuating water levels, together with hydrolytic weathering might have been responsible for the edification of what is observable today as the Djoulgouf duricrust. However, their present-day dismantling might be caused by thermoclastic processes rather than by geochemical processes of hematite-goethite replacements.

Key words : Geochemistry, duricrusting, weathering, Lake Chad basin, Djoulgouf.

Résume Afin d’élaborer une banque de données géologiques et géochimiques sur Djoulgouf, jadis sommaires voire inexistantes, et de comprendre les phénomènes de cuirassement sur roche granitique, dans un environnement soudano-sahélien, un profil sur le site de coordonnées 10°37’31’’N – 14°28’53’’E, situé près de Maroua et dans la portion Camerounaise du bassin du Lac Tchad a été étudié. Les études morphologique, minéralogique sur lames minces et par diffraction des RX, et géochimique ont permis de distinguer des granites sur lesquels reposent, de bas en haut, des niveaux isaltéritique, cuirassé démantelé et organominéral. La minéralogie des granites révèle le quartz, le microcline, la biotite, l’albite et de rares anorthites. Les microcline, goethite et des traces de kaolinite et d’hématite sont observés sur l’ensemble des niveaux ameublis. Ces études révèlent également que ces cuirasses se sont formées à partir d’une recombinaison des altérites granitiques et du fer venu des altérites volcaniques environnantes. A cela se sont ajoutées, les fluctuations du niveau de la nappe résultant des différentes phases transgressives du bassin du Lac Tchad. Ainsi, la recombinaison des matières associées aux phases d’immersion / émersion et l’altération hydrolitique seraient à l’origine des profils cuirassés actuels de Djoulgouf. Cependant, leur démantèlement actuel serait aussi bien du fait de la thermoclastie que de la fonte géochimique liée à la goethisation des hématites.

Mots clés : Géochimie-Cuirassement-Altération-Bassin du Lac Tchad-Djoulgouf.

Introduction contrasting seasons: 3 to 4 months of rainy season The Maroua region, located in the Extreme North of and 8 to 9 months of dry season (Suchel, 1978). The Cameroon, is an important zone of the Lake Chad mean annual rainfall and temperature for the period conventional basin that comprises Niger, Chad ranging between 1991 and 2003 are 730 mm and Nigeria and Cameroon (Batello et al., 2006) (Fig. 1). 26°C, respectively. The major superficial formation The Djoulgouf site is located within this region of the region are the vertisols (or karrals), raw between latitude 10°15’ and 10°45’N, and longitude mineral soils and ferruginous soils. The latter often 14°00’ and 14°30’E. The thorny savannah landscape show a duricrust layer which is currently of the Diamare plain (Seignobos, 1982; Letouzey, undergoing dismantling at Djoulgouf and Mindif. 1985) occupies the central and the eastern portions The main geological formations are the Precambrian limited to the west by the Mandara Mountains. The basement and the sedimentary cover (Dumort and climate is the sudano-sahelian type with two Peronné, 1966; Tillement, 1970; Pias, 1970).

Geochemistry and duricrusting of weathering products 2

are found mainly in the Mandara Mountains and in 10° 15° 20° 25° NIGER the Djoulgouf inselbergs made up of granitic facies CHAD Gourgour (Fig. 2). Kanem l 15°N za Noko ha Gouré N’Guigmi G Zinder e Mao r l L ah The Djoulgouf duricrust is formed on iron poor ak B Diffa e Moussoro Nguru Ch atha ad Ati B Gashua granite which is rather intriguing considering the

Massakori N Kano bé Lake Alifa o Kornou m

Y a H A u Maidugurir Fitri a o e g s Ab-Touyour

d u d N’Djamena D fact that iron is the basis of the formation of an iron e o e j t i d Y a a a m m ïd la o e Melfi a U K B C S e L h L ar S o i duricrust. To explain the formation of the duricrust, g o Nubir n e 10°N Laï kt Abuja ou a soil profile located at 10°37”31’ and 14°28”53’ A Garoua Niellim hr NIGERIA Pala Kélo Ba Lake of was studied. From the results obtained through Lagdo

Enugu Ngaoundéré morphological, mineralogical and geochemical analyses, a genetic and evolutive model is proposed CAMEROON CENTRAFRICAN REPUBLIC for these materials. Atlantic Douala Ocean 0 100 200 300 400 500 600 700 800 900 5°N Yaounde Malabo Kilometers

LEGEND Material and methods

HYDROGRAPHIC BASIN River The macroscopic study of the Djoulgouf superficial CONVENTIONAL BASIN formations involves the observation and the Capital description of the different soil horizons. It takes Locality into consideration the size, colour, structure and Fig. 1. Location map of the Lake Chad region. texture of the various constituents. The description (Modified from Batello et al . 2006 ). is followed by the description and classification of the material as well as the interpretation of the main

results obtained. The description of thin sections is 2 completed by x-ray diffraction analyses using a 10°45 Philips MPD 3710 diffractometer with a cobalt α γ cathode (K = 1.79 Å) and a hind monochromator.

γm The Global chemical composition of the material is expressed in percentage of the most stable oxides. 2 The relative quantitative determination of the

M2 minerals was done according to Beer-Lambert’s method for which the net surface area of a mineral’s Gazoua 10°30 peak is proportional to its abundance in the sample.

So S Chemical analyses were done by fluorescence X method. All these analyses were carried out at the “Centre Européen de Recherche et d’Enseignement P θ ζ des Géosciences de l’Environnement (CEREGE)”

14°00 14°30 of the Aix-Marseille University in Aix-en-Provence LEGEND (France) and in the SEAMIC laboratory in Laterite Anatexic rock Tanzania. Alluvia Anatexic granite Following the results obtained from the above

Lacustrine clays Heterogeneous calco-alcalin granite analyses a geochemical study was carried out to

Ancient sand Calco-alcalin granite understand the behaviour of elements along the profile, that is, to determine their distribution and Tufed schists and quarzite Biotite granite their evolution in the different levels. This approach Predominant volcanic rock Syenite was done through the elaboration of geochemical Quartzite leptinitic gneiss Basalt balances to assess the gain and loss of matter by the So Soapstone, amphibolite and serpentinite River concentration rate method, triangular diagrams and Road Embrechite gneiss T Talc briefly outlining of outstanding features. Scale: 1/750 000 The interpretation and discussion of the main results have provided an insight into the genesis and the Fig. 2. Geological map of the Maroua region. evolution of the studied supergene material. (Modified from Dumort and Péronne, 1966). Results Globally, the geological formations of the Maroua Morphostructural organisation of the profile region can be classified into the Precambrian The Djoulgouf profile is made up of three levels of basement, the oligo-quaternary cover and the rocky superficial materials that overlie a granitic rock. The massives (Ségalen, 1967; Eno Belinga, 2001).The following materials are observed on this granite sedimentary cover is rich in aeolian deposits. The from the base to the top: an isalterite, a duricrust basic rocks include vesicular andesites and currently undergoing dismantling and a surface rhyodacites found around Maroua town. Acid rocks organic layer (Fig. 3).

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duricrust level. It also constitutes the transition point Organomineral material between the latter and the other horizons of the soil profile. The brown duricrust is constituted by a set Dismantled Duricrusted fine duricrust level gravelly matrix of very hard nodular blocks whose diameters go (DL1) Duricrust beyond 50 cm. The diameter of the nodules varies between 0.5 and 1 mm. The iron nodules, quartz and remains of feldspar phenocrystals are identified. Clay s sandy alterate The loose organo-mineral level is discontinuous, Alteritic level (DL2) 50 cm thin (20 cm), dark brown in colour and shows some roots (Fig. 3). The pedological features and minerals Parent rock, 50 cm granite (DL3) that are visible to the naked eye include quartz, iron nodules and pisoliths.

Fig. 3. Sketch of the Djoulgouf dismantling duricrusted profile. Mineralogical characterisation of the material The microscopic observation of thin sections of the parent rock (granite) revealed the presence of quartz, alkaline feldspars (orthoclase), sodic plagioclase (albite), biotite and opaque minerals. X-ray diffraction analyses (Fig. 5) of the granite confirmed the presence of quartz via its strong reflections at 3.34 Å, 4.26 Å and 1.81 Å. In addition to quartz, there are feldspars such as microline with strong reflections at 3.24 Å, albite with reflections at 6.38 Å, 3.77 Å and 3.19 Å as well as traces of anorthite that peak at 3.191 Å 3.035 Å. From integral check of the intensity of the peaks and based on the Beer-Lambert’s principle, it can be

Fig. 4. Djoulgouf duricrust concluded that quartz is the most abundant mineral, followed by albite, microcline, anorthite and biotite. The Djoulgouf granitic substratum therefore carries It is worth noting that the Djoulgouf isalterite has an isalterite marked by the complete conservation of mineral composition nearly underlying granite. the structure of this granite on which has been However, in this horizon, biotite and albite tend to developed a more than 1 m thick iron duricrust disappear, while the relicts of micas, quite oxidised, presently undergoing dismantling. The 40 cm thick are ferruginised. sandy clayey material acts as a matrix on which are The analysis of the sandy clayey matrix with iron embedded gravely iron nodules and residual blocks nodules of the duricrust level (Fig. 6) revealed resulting from the breakdown of the brown duricrust intense reflections of quartz materialised by peaks at (Fig. 4). This horizon is overlain by a loose 3.34 Å, 4.26 Å and 1.817 Å. The lower reflections discontinuous organo-mineral level which is correspond to microcline with peaks of 3.24 Å and actually the nodular matrix crossed by few plant 3.83 Å, goethite with peaks at 4.16 Å, 2.69 Å, 2.45 roots. and 1.71 Å, kaolinite with reflections at 7.16 Å, 3.57 The base of the Djoulgouf soil profile is a Å and 2.33 Å and lastly hematite marked by leucocratic granite with a granular to porphyraceous reflections at 2.69 Å and 2.51 Å. The global analysis granular texture. Biotite grains, often present in one of the intensities of the peaks portrays a significant of the granite facies are sparse and small in size (0.5 amount of quartz in the loose level followed by to 1 mm). microcline relicts and kaolinite. The alteritic materials are constituted by quartz The diffractograms obtained from the duricrust grains which are well visible within a grey clayey materials are similar to those of the overlying loose material made up of feldspar pseudomorphs in the fine gravely sandy clayey layer. However, it should course of weathering: the original petrographic be noted that kaolinite is remarkably low or texture is relatively well preserved. Towards the inexistent. On the other hand, the spectral analysis upper part, the biotite minerals are presently has revealed a predominance of quartz, followed by undergoing weathering at this level and creating rust microcline, hematite and goethite. patches on the surface of the rock and thus contributing to weaken it. This structure is gradually Geochemistry breaking down into a loose material as one move Bulk chemical analyses towards the contact zone with overlying duricrust, The chemical analysis of the supergene materials and the fabric of the parent rock disappears. has as main objective to understand and evaluate the The sandy clayey matrix in which are embedded the evolution and the distribution of major chemical nodules and the duricrust blocks represent the iron

Geochemistry and duricrusting of weathering products 4

each of the soil horizons. Those elements include Al 2O3, CaO, Fe 2O3T, FeO, Fe 2O3, K 2O, MgO, MnO, Na 2O, P 2O5, SiO 2 and TiO 2. The fire loss and the free water contents have also been determined. The chemical analysis of the major elements of the Djoulgouf duricrust profile was performed on four types of materials which include the parent rock, the isalterite, the duricrust and the loose fine gravely matrix. The main results obtained show a difference in silica content between the parent rock and the isalterite of about 75 % and a variation of 50 % between the duricrust level and the loose material (Table 1). In these materials, ferric iron (Fe 2O3) contents are close to 0.75 % in the granite and 0.85 % in the isalterite, 22 % in the duricrust and 19 % in

Fig. 5. Diffractogram of parent rock. Keys: Biotite (B), the loose material. It is important to remark here that Anortite (An), Quartz (Q), Albite (A) and Microcline (M). ferrous iron (FeO) is inexistent. Aluminium (Al 2O3) seems to be stable between 14 and 12 % with a global decreasing tendency from the parent rock to the surface. Free water content and fire loss increase from the parent rock towards the superficial material with a global range between 0.3 and 3.4 % and 0.7 and 9.57 %, respectively. Potassium and calcium display a contrary behaviour to that of water, passing from 5.16 to 0.40 and 0.90 to 0.033, respectively. Magnesium (MgO), manganese (MnO) and phosphorus(P 2O5) and titanium all show contents below 0.5 % and can be considered as stable due to the error margin of the chemical analyses which stands at 0.31 %. These elements have therefore

enabled the calculation of balances based on the Fig. 6. Diffractogram of the fine gravelly sandy clays material concentration rate so as to appreciate the loss and of the durcrusted level. Keys: Kaolinite (K), Quartz (Q), gain of matter. The chemical elements of the parent Goetite (G) and Hematite (H) rock are considered as a reference.

Table 1: Bulk chemical analysis of Djoulgouf materials (100 % molare concentration)

N° Ech % free water % LOI % Al 2O3 % CaO % Fe 2O 3T % K 2O % MgO % MnO % Na 2O % P 2O5 % SiO 2 % TiO 2 3 3,30 9,57 12,22 0,03 18,18 0,40 0,24 0,13 0,03 0,05 54,43 0,41 Matrix 2 L0/Dl2 3,48 9,81 12,50 0,03 19,71 0,40 0,24 0,13 0,04 0,07 53,18 0,40 1 2,75 8,98 12,97 0,04 20,60 0,59 0,26 0,15 0,04 0,07 53,96 0,41 3 2,05 8,46 12,70 0,04 21,99 0,72 0,26 0,22 0,44 0,08 52,62 0,43 Duricrust 2 L1/DL3 1,95 8,12 12,93 0,04 24,09 0,72 0,27 0,19 0,05 0,08 51,11 0,44 1 1,82 7,99 12,82 0,04 23,81 0,81 0,26 0,20 0,07 0,08 51,69 0,41 3 0,20 0,92 12,59 0,45 0,73 4,98 0,41 0,23 4,40 0,05 74,25 0,79 Isalterite L2/DR2 2 0,19 0,86 12,13 0,46 0,73 4,90 0,20 0,25 4,33 0,02 75,85 0,09 1 0,23 0,81 13,26 0,52 0,78 5,20 0,21 0,27 4,62 0,09 72,89 0,10 3 0,31 0,65 13,70 0,90 0,86 5,16 0,25 0,13 4,87 0,81 72,25 0,11 Parent rock L3/DR9 2 0,29 0,68 13,37 0,82 0,86 5,00 0,28 0,13 4,81 0,08 73,58 0,11 1 0,30 0,72 13,82 0,80 0,77 5,14 0,30 0,13 5,01 0,09 72,82 0,10 Table 2 : Djoulgouf balance

Sample N° % free water % LOI % Al 2O3 % CaO % Fe 2O3T % K2O % MgO % MnO % Na 2O % P2O5 % SiO 2 % TiO 2 Matrix L0/Dl2 11,945 14,4 0,9352 0,041 23,0404 0,08 0,833 1,046 0,0079 0,848 0,723 3,606 Duricrust L1/DL3 6,672 11,9 0,9669 0,046 28,1601 0,144 0,959 1,543 0,0094 0,9808 0,695 3,983 Isalterite L2/DR2 0,6545 1,26 0,9075 0,557 0,84791 0,981 0,712 2,017 0,9002 0,1806 1,031 0,784 Parent rock L3/DR9 1 1 1 1 1 1 1 1 1 1 1 1 elements in the different layers of the studied soil Balance calculation by concentration rate method profile. In order to understand and to explain the The balance of chemical elements of the Djoulgouf different innate correlative parameters in the present profile shows (Table 2, Fig. 7 to 9): work, twelve (12) major elements are expressed, in - a loss of sodium, calcium, potassium, magnesium, the ponderal percentage of the most stable oxides in phosphorus and aluminium in all the horizons;

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- a gain in total iron, constitutional water and titanium in all the weathered materials, except the alterite. - a loss in silica in all horizons except in the isalterite where silica is seemingly stable ; - a gain in free water and manganese in the whole profile. Globally, there is loss and gain of matter with respect to the element considered. The increasing

order of evacuation of chemical elements in the Fig. 7. Balance of iron, titanium and water material is as follows: Na 2O>CaO>K 2O>MgO>P 2O5>Al 2O3>SiO 2>MnO. The increasing order of accumulation is as follows: + TiO 2>H 2O>H 2O >Fe2O 3.

Geochemical evolution of the profile by triangular diagrams Due to the normalisation of the percentage oxides in millication in which the sum of the three elements studied as total value corresponds to the 100 % content, three triangular diagrams have been realised in the Fe-(Na+K)-Mg, Si-Al-Ca+Na, Si-Al- Fig. 8. Balance of magnesium, potassium, calcium and Fe and Si-Fe+Mg) (Fig. 10). aluminum The analysis of the four diagrams shows that the parent rock is located in the alkaline and acid pole. The duricrust and fine gravely products are ferruginous. Two groups of materials are noted to be perfectly superposed; one close to the iron pole and the other near the sodopotassic pole. The first material corresponds to the ferruginous products while the second corresponds to the parent rock and the isalterite. Two groups of superimposed elements in the Si-Al-Fe system are also noted. The parent rock and the isalterite are located in the silica pole while the duricrust and its weathered products are at

Fig. 9. Balance of silica, phosphorus, sodium and manganese the centre of the Si-Fe or Si-Fe+ Mg which seem identical. Considering the three triangular diagrams, which are almost superimposed on one another, the influence of magnesium is negligible compared to that of iron on an evolutive tendency of the weathering products. On the other hand, the weathered products are closer to the alkaline pole than to the aluminous pole. Generally, the weathering of the granite under

the sudano-sahelian climate in the extreme north (a): Fe-Na+K-Mg System (b): Si-Al-Ca+Na System region of Cameroon leads to an impoverishment in aluminium and to an enrichment in iron from the bottom to the top of the profile.

Interpretation and discussion The interpretation of the diffractogram reveals that quartz is the most abundant mineral in the profile. Feldspars such as microcline and albite are represented in the entire profile. Their proportions, less than those of quartz, decrease from the base to

(d): Si-Al-Fe System (c): Si-Al-Fe+Mg System the top of the profile. Hematite and goethite appear in the duricrust and loose levels. Biotite, present in Keys the parent rock, completely disappears at the level of the duricrust. The parental minerals are quartz, Fig. 10 Triangular diagrams

Geochemistry and duricrusting of weathering products 6

microcline, biotite, albite, anorthite and accessory fluctuations in the watertable, precipitation- rutile. The orthoclase feldspars identified in the thin hardening, thanks to multidirectional illuvial sesqui- sections of the granite are mainly microcline whose oxides and the leaching of the non-hardened parts presence portrays plutonic conditions in a low under a tropical climate with contrasting seasons. temperature environment (Roubault et al ., 1982). This point of view according to which the The weathering first affected the biotite, then fluctuation in the watertable associated with a anorthite and finally albite. These minerals regress contrasted climate is necessary for the duricrusting considerably to finally disappear at the duricrust is in agreement with Nahon (1991) and Tardy level where the appearance of hematite and goethite (1993). This phenomenon can be transposed to takes the relay. In addition to these two minerals are Djoulgouf which is under the influence of the same some traces of clay minerals probably kaolinite sudano-sahelian climatic conditions as Burkina within the loose matrix of the duricrust material. In Faso. general, the differential weathering of the It seems that the formation of duricrusts is not mineralogical cortex is superimposed on Goldich’s limited to the tropical climates with contrasting series (1938). seasons. In effect, the works of Temgoua et al . The duricrust and the matrix enclose pisolites. These (2002) show that vacuolar duricrust formation at the pisolites might have been formed by the complex footslope is well documented in the South successive transformations (mineralogical and Cameroon humid forest region and appears as the structural) from a massive duricrust where the role present day expression of their sequence of of iron is primordial (Boulangé and Bocquier, evolution. According to these authors, iron which 1983). determines the formation of present day carapaces is Although boehmite has not been formally derived from the destabilisation of upslope demonstrated in the present investigation, the duricrusts dated to the Cretaceous. mineral seems to exist in trace form. According to Originally the duricrusts are haematitic (Ambrosi et Keller (1964), Boulangé and Bocquier (1983), Nahon, 1986), and the hematite nodules are held ferruginous pisolitic facies are always related to the together by haematitic bridges or cement and loose presence of boehmite and hematite. matrix .derived from the hydrated internodular The chemistry of the whole material is dominated cement (Bitom et Volkoff, 1993). Thus, the by three elements: silica that predominates in the hydration of hematite, favoured by a humid climate, parent rock and the isalterite gradually decreases in leading to goethitisation might be responsible for the hardened materials and their loose matrix. There the destabilisation of the nodular ferruginous is an impoverishment of alumina in the isalterite, duricrusts into a carapace, then finally into a loose and a gain in the duricrust and its derived product. material bearing fine gravely nodules. The The latter are also characterised by enrichment in destabilisation balance is characterised by a iron. hydration of hematite into goethite and a subsequent The low iron contents in the parent rock, its loss of iron (Bauvais et Tardy, 1991). This abundance in the duricrust layers and the existence summarises two parallel evolutionary processes of two groups of distinct points in the triangular marked by a consecutive deferruginisation (Bitom et diagrams, make it difficult to explain the origin of Volkoff, 1993; Bitom et al ., 2003). iron during the formation of the Djoulgouf duricrust. Nevertheless, the Djoulgouf duricrust are presently It might have been derived from a mechanical submitted to a dismantling process under tropical breakdown followed by chemical weathering of the climate with two contrasting (dry and humid) Djoulgouf neighbouring volcanic massifs and seasons that might have favoured the duricrusting. finally by recombination with the autochthonous The dismantling of the material generates fine alterites of granites to form the present day gravely products bearing iron nodules and duricrust duricrusts and laterites of the studied site. blocks as currently reported in the humid tropical The origin of the Djoulgouf duricrust is not well forest region (Marti, 1967; Muller et al ., 1981; known. It might date back to the Pliocene (60 to 70 Bocquier et al ., 1984). However, the actual climate million years) or to the Eocene which is the period of Djoulgouf, although contrasting, is hotter and during which the region began to be part of the drier than what used to be in the Palaeocene tropical climate with well contrasted seasons (Hervieu, 1970 (a) et 1970 (b)). (Parrish and ziegler, 1982; Kamgang, 1998) but The question is therefore why is there a duricrust which today is under the sudano-sahelian climate. dismantling at Djoulguof in climatic condition However, the processes of formation of the duricrust which should rather be favourable for their could be explained, at least in part, by the works of formation as observed in other regions of the Dya (1993). According to this author, the duricrusts intertropical zone The possible explanation takes of Burkina Faso were formed from the remains of into consideration, in addition to the factors cited the mineral accumulation horizon (B) of a above, the morphology of the landscape at the palaeosoil, and the duricrust process favoured by the period of the formation of the duricrusts. It is a

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palaeohorizon (B) of accumulation whose Blot A. 2004. Caractérisation des chapeaux de fer en milieu indurations led to the formation of the present day latéritique cuirassé. Géoscience 336 : 1473-1480. Bocquier G., Muller J.P., Boulangé B. 1984. Les latérites : duricrust. The erosion resulting from the connaissances actuelles et prospectives sur les mécanismes transgressive phases of the Lake Chad basin has de leur différenciation. Sciences du sol, livre jubilaire du removed the ancient overlying levels thus exposing cinquantenaire de l’Association Française des sols, 123-138. the ancient duricrusts which outcrop today. The Boulangé B., Bocquier G. 1983 . Le rôle du fer dans la formation des pisolites alumineux au sein des cuirasses bauxitiques change in the condition of duricrust formation and latéritiques, Sciences et Géologie, Strasbourg, 29-36. the aggressiveness of the arid climate have resulted Dumort J.C., Peronne, Y. 1966. Notice explicative sur la feuille in water deficient conditions restricting the Maroua, Direction des Mines et de la Géologie, Yaoundé, 48 underlying watertable to the deepest horizons and p., 1 carte géologique de reconnaissance au 1/500 000. Dya C.S. 1993 . Connaissance des cuirasses au Burkina Faso : hence favouring the dismantling of the Djoulgouf géomorphologie et utilisation. Ed. Berichte des duricrust by thermoclasty and goethitisation. Sondersforschungsbereichs, Bd.1, Frankfurt, 117-131. Eno Bélinga S.M. 2001. Histoire géologique du Cameroun, Conclusion Classiques Camerounais, 136 p. Hervieu J. 1970a. Influences des changements de climat The Djoulgouf duricrusts have been laid down quaternaires sur le relief et les sols du Nord-Cameroun, mainly by an accumulation of rare iron from the Bulletin de l’Association Sénégalaise de l’Etude du weathering of biotite enclosed in the underlying Quaternaire Ouest africain, Daka, 25 : 97-105. granite, in addition to a concrete and significant iron Hervieu J. 1970b. Le quaternaire du nord-cameroun schéma d’évolution géomorphologique, Cahier de l’ORSTOM, Série contribution from weathered diorites and volcanic Pédologie , Volume III, 3-15 breccias situated around the site. Besides the Keller W.D. 1964. The origin of high-alumina clay minerals. presence of authiginous and allogenous iron, the review Ingerson . Clays and Clay Minerals, New York 5:129- post-Palaeocene climatic conditions have favoured 151. Letouzey R. 1985. Carte phytogéographiques du Cameroun au the haematitic duricrusting at the base of the profile. 1/500 000 avec notice explicative, IRA (Herbier National), The pre-Eocene transgressive phases of the Lake Yaoundé et INCIV, Toulouse- France, 63-94. Chad have eroded the outcropping horizons and Martin D. 1967. Géomorphologie des sols ferrallitiques dans le therefore favoured goethitisation and geochemical centre Cameroun. Cahier de l’ORSTOM, série Pédologie Volume 1 : 189-218 melting; this exposed the duricrusts to the influence Muller D., Bocquier G., Nahon D., Paquet H. 1981. Analyse des of the climate. The water deficit has led to a drastic différenciations minéralogiques et structurales d’un sol lowering of the watertable, marked by a significant ferralitique à horizons modulaires du Congo. Cahier de shifting of the of the Lake Chad limits towards the l’ORSTOM, série Pédologie , XVII, 2 : 87-109 Nahon D. 1991. 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