Alluvial sediments in Bol area (Lake Chad Basin): implications for source area-weathering and tectonic settings

This paper discussed the source area-weathering and tectonic settings of alluvial sediments from the Lake Chad Basin (LCB). Four profiles of different levels characterised by variation in colours and textures have been examined. The textural variation was linked to the alternation of wet and dry periods in the LCB. Micro-textural observations by Scanning Electron Microscopy (SEM) revealed sub-rounded to angular particles with collision impact and sometimes adhered particles on their surfaces. This suggested fluvial transport and short periods of aeolian movements. The bulk mineralogy performed with X-TRA Thermo-ARL Diffractometer showed that the clayey and clayey sandy facies are dominated by kaolinite, quartz and illite whilst the sandy samples are made up of quartz, kaolinite, illite, goethite and rutile. The geochemistry of whole samples was assessed by X-Ray Fluorescence Spectrometry. Amount the major elements, SiO2, Al2O3 and Fe2O3 have the highest contents, and this is consistent with the mineralogical composition. The sediments are mature and classified as Fe-shale, Fe-sand and (sub)litharenite. Trace elements behaviour suggested a detrital origin, low sediment recycling and oxic depositional environment. The rock sources underwent variable degrees of weathering which revealed once more the impact of climate change prevailing in LCB. The discriminant plots indicated a felsic nature of the original source rocks in a context of passive margin.


Introduction
The geochemical characteristics of alluvial sediments provide many insights into sedimentological processes and factors, such as rock composition, paleoclimate, maturity, depositional environment, weathering source conditions and tectonic setting (Huyan et al., 2021;Sababa et al., 2022;Taylor & McLennan, 1985;Tian et al., 2020).The morphology of alluvial sediments and precisely their quartz grains give reliable information on their origin, the depositional environment and the paleoclimate (Armstrong-Altrin et al., 2015;Ekoa Bessa et al., 2021a;Hossain et al., 2020;Tchouatcha et al., 2022).The abundances of major and trace elements geochemistry are important for the determination of the weathering of the source materials and the provenances of these sediments (Garzanti et al., 2016;Hernández-Hinojosa et al., 2018;Hossain et al., 2017;Nesbitt & Young, 1984).Geochemical features through some logarithmic ratios (SiO 2 /Al 2 O 3 and Fe 2 O 3 /K 2 O) can help classify the sediment according to their abundance in rocks (Herron, 1988).Roser and Korsch (1988) suggest that some major elements (Ti, Al, Mg, Ca, Na, and K) can be used to calculate the discriminant functions (F1-F2) for the determination of the provenance of clastic sediments.Various indexes can be calculated using major elements of geochemistry to assess the weathering degree of the source rock and the textural maturity of alluvial sediments (Babechuk et al., 2014;Cox et al., 1995;Fedo et al., 1995;Nesbitt & Young, 1982).The less mobile trace element ratios have been also used to indicate the depositional environment (U/Th, Ni/Co, V/Cr and authigenic U; Jones & Manning, 1994;Nagarajan et al., 2007) and determine the provenance (La/Sc, La/Co, Th/Sc, Th/Co, Cr/V, Y/ Ni and Zr/Sc; Huyan et al., 2021;Ngagoum Kontchipe et al., 2021) of alluvial sediments.The log-ratio transformations of 10 major elements (TiO 2 , SiO 2 , Al 2 O 3 , Fe 2 O 3 , MnO, MgO, CaO, Na 2 O, K 2 O, P 2 O 5 , Verma & Armstrong-Altrin, 2013) and K 2 O/Na 2 O-SiO 2 plot (Roser & Korsch, 1986) are useful to discriminate the tectonic settings.
According to Toteu et al. (2004), the Lake Chad Basin is located in the Central African zone of the "Pan-African Mobile Zone".It is characterised by the deposition of sedimentary platforms as a result of lake level variations (Massuel, 2001;Mathieu, 1978;Moussa et al., 2016;Schuster et al., 2005).Lake Chad is located at the junction of the Saharan desert and the savannah.It is the main freshwater body in the heart of the African continent and shared by four riparian countries, such as Cameroon, Nigeria, Chad and Niger.It plays an important role in the organisation of the economic, social and cultural life of the basin's populations.It therefore offers important opportunities for agriculture, fishing and livestock breeding.The LCB sediments provide an opportunity to study the effect of lithological changes on the chemical composition of sediments and weathering of their source rocks.Also, the geochemistry and the microtextures of these sediments can help understand the chemical and the physical changes that occur during the deposition.On the other hand, previous studies on paleo-tectonic identification in the region attempted to unravel the geodynamic evolution of the sub-region (Ambassa Bela et al., 2023;Ekoa Bessa et al., 2021b).Findings of previous researchers have led to controversial results, which documented both collision and passive margin settings for the basement rocks.Physical, mineralogical and geochemical data of this study will contribute to clarifying this scientific debate.
The encounter of these two air mass movements triggers the rain that determines two seasons-a short rainy season (July and August) followed by a long dry season (October to June).The total annual rainfall varies from 200 to 550 mm with an annual average of 323 mm and an annual average temperature of 28 °C (Roder, 1964).The relief of the study area is a succession of dunes and depressions (Fig. 2).The water inflow to Lake Chad is around 50 billion m 3 , 79% of which comes from the Chari and Logone Rivers.These landform conditions result in the formation of raw mineral soils typical of hot deserts (Rieu, 1975).On the sandy plateaus, sub-arid brown steppe soils are found on calcareous sand and on the low plains, hydromorphic soils are found (Pias, 1962).The vegetation developed on these soils is of the Sahelian type dominated by herbaceous plants, acacias, date and doum palms as well as Sapindaceae observed around ponds and wadis.

Geological setting
The Lake Chad Basin (LCB) is surrounded by mountainous areas corresponding to outcrops of Precambrian rocks (Louis, 1970).Primary sandstone plateaus or Cenozoic piedmont formations are encountered, followed by deltaic deposits in vast plains (Pias, 1970;Schuster et al., 2005).The LCB was formed during the Jurassic-Cretaceous distension phase (150-120 Ma) when a large trough opened up (Black, 1992;Guiraud et al., 1992) by deep subsidence of the Precambrian basement (Louis, 1970).These sedimentary deposits are the result of the interaction between three main factors (Massuel, 2001;Moussa et al., 2016;Schuster et al., 2005): (i) lacustrine transgressions in wet periods (clay deposits); (ii) lacustrine regressions in arid periods (sand deposits); (iii) and subsidence of the basin responsible for the thickness of the layers (tectonic).
During the Quaternary, the Lake Chad Basin (LCB) was the site of aeolian, fluvial, deltaic and lacustrine sedimentary deposits (Schuster et al., 2009).These highly heterogeneous sediments include clays, sands, sandstones and diatomites (Cheverry, 1974).Pias (1970), on the base of the Cenozoic and Quaternary sedimentary formations of the LCB, distinguished three series: (i) the Bahr-el-Ghazal series, a clayey deposit of the early Plio-Pleistocene; (ii) the Soulias series consisting of aeolian sands modelled as ergs and containing lenses of lacustrine clays dating from 40,000 years; (iii) and finally, the Labdé series which fills the depressions between dunes of the northeastern shore of the present Lake Chad.The study area is located in the Labdé series.It is mostly characterised by lacustrine deposits of variable thicknesses comprising a diatom sequence and a diatom-poor sequence whose deposition began 10,000 years (Pias, 1970).

Sediment sampling
Four pits (LT1, LT2, LT3 and LT4) have been hand-dug with depths varying from 100 to 180 cm in alluvial plains of the LCB at Bol area (Table 1).The pits were realised from 30 to 40 km from the actual littoral of the current lake (Fig. 2).For each dug pit, the surface area was 100 cm × 100 cm.The presence of water flow or collapsing sand limited the extension of the pits.Sampling of alluvial sediments has been done from top to bottom according to the colour change and textural variation.The profiles were described in terms of colour, structure and hand test texture (Table 1).The colour is identified based on the Munsell soil colour chart (Table 1).The collected samples were air-dried and handled with good care to avoid any contamination.Fourteen samples from profiles LT1 (LT11, LT12, LT13 and LT14); LT2 (LT22, LT23, LT24 and LT25); LT3 (LT32, LT33 and LT34) and LT4 (LT42, LT43 and LT44) (Table 1) have been analysed.

Analytical procedures
The grain size distribution analysis was carried out in all the selected samples using the Robinson Köln pipette method in the laboratories of Geosciences of Superficial Formation and Applications (GSFA, University of Yaoundé 1, Cameroon).Briefly, it consists of destroying organic matter with H 2 O 2 ; dispersing the clay coating the particles with Na 6 (PO 3 ) 6 by mechanical agitation; then taking samples during sedimentation at a precise depth and time to isolate fine particles (clays, fine and coarse silts).Fine and coarse sands were then separated by wet sieving.The physicochemical parameters of the selected samples were measured by the method previously explained by Sababa et al. (2021) in the GSFA, University of Yaoundé 1 (Cameroon) laboratories.Ten grammes of powder samples were put in a 100 ml beaker and distilled water was added to it to determine the physicochemical parameters.The mixtures were homogenised for about 15 to 30 min.Then, an HACH-HQ11d brand electric pH metre was used to measure the values of pH (0 to 14) and Eh.Four of the selected samples were examined by scanning electron microscopy (SEM) equipped with a Tescan Mira/LMU in the Institute of Earth Sciences (ISTE) laboratories, University of Lausanne, Switzerland.The grains were positioned in a pattern with a double adhesive strip on a SEM specimen holder.This helps to pre-map the stub to easily locate the grains by SEM.The specimen holder sputter-coated with gold offers good resolution (Krinsley & Doornkamp, 1973).The coating limits beam penetration, produces sharper images and most importantly, it minimises the electrical charge on the surface.The samples have been sputtered for 300 s equipped with a current of 40 mA producing a gold coating of about 250 nm.
The bulk mineralogy was performed with an X-TRA Thermo-ARL Diffractometer using a semi-quantitative method in the laboratories of ISTE, University of Lausanne, Switzerland.The procedures have been previously described in detail by Adatte et al. (1996).The mineralogical composition of the whole alluvial samples was determined by X-ray diffraction (XRD) patterns of random powdered samples pressed into a powder holder.External standards with error margins between 5 and 10% were used for the phyllosilicate and 5% for grain minerals.The quantitative evaluation of bulk sediment mineralogy was done using the Match software after the manual identification of minerals.
All the selected samples' major and trace element contents were assessed by X-ray Fluorescence Spectrometry (XRFS) using a PANanalytical PW2400 spectrometer in the laboratories of ISTE, University of Lausanne, Switzerland.The quality of the data was verified by the inclusion of standard reference materials (NIM-G, NIM-N, SY-2, JCH-1 and BHVO).Major elements were evaluated on fused lithium tetra-borate glass discs.Samples were first heated to 1050 °C in an oven to drive out the volatiles, oxidise iron, and determine the loss on ignition (LOI) values.Then, 1.2000 ± 0.0002 g of ignited sample were mixed with 6.0000 ± 0.0002 g of lithium tetra-borate (Li 2 B 4 O 7 ) and put in a Bead machine PerlX'3 at 1250 °C to have the fused tablet.The concentrations are given in weight percentages (wt.%) for major elements.The contents of trace elements were determined on pressed discs after mixing Mowiol 2% with 15% of powdered samples.The pressed discs were then put in an oven at 110 °C for at least 6 h before analysed by XRFS.The trace element contents are given in mg.kg −1 .
The Excel programme was used to determine the correlations.The obtained Pearson's correlation matrix shows the behaviour of chemical elements.The correlations are strong when the coefficient is greater than 0.50 in absolute value.The positive values show that the elements are associated in the same processes whilst the negative coefficients show different processes of enrichment.

Textural and mineralogical features
The summary of macroscopic organisation of the different samples of each profile is presented in Table 1.Different levels are characterised by the variation in colour, texture (clay, sandy and sandy clay) and thickness (Table 1; Fig. 3).Fine roots are encountered in the superficial part of the profiles.Profile LT1 is particularly characterised by a thick sandy layer with an abundance of fine roots at the top of the profile.The predominance of sands in alluvial sediments may result from intense dynamic processes during the transportation and the deposition (Mohtar et al., 2017).Similar results were obtained in the northern and central part of the LCB (Moussa et al., 2016;Sebag et al., 2013).This could also result from the nature of the surrounding soils which may be sandy in the context of local sedimentation.Three different facies are observed: clayey layer (06 samples), clayey sandy layer (05 samples) and sandy layers (03 samples).These sedimentary deposits, characterised by layers of different compositions, could be linked to the alternation of wet and dry periods in the LCB (Massuel, 2001).Indeed, lacustrine transgressions in wet periods favour clay fraction deposits whilst lacustrine regressions in arid periods are responsible for sand deposits (Moussa et al., 2016;Schuster et al., 2005).In the classification diagram of Picard (1971), the samples fall into several fields of textural classes, such as claystone, sandstone, sandy claystone, silty claystone, clayey mudstone and clayey sandstone (Fig. 4a).They also show low permeability and variable porosity (Fig. 4b) as a result of variable proportion of clay, silt and sand (Table 1) indicating deposition by settling.The omnipresence of sandy fractions is related to the nature of the substratum, mostly composed of sand dunes and aeolian allochthonous input (Sebag et al., 2013).Sand is thus almost exclusively of wind origin, either directly trapped in the inter-dune depression like aeolian depositions or indirectly, due to reworking from the dune slope (i.e.runoff).
The mineralogical composition of samples from each profile is shown in Fig. 5.The clayey and clayey sandy facies were dominated by kaolinite, quartz and illite with few amounts of goethite and rutile (Table 2).The amounts of each mineral varied from one bank to another: quartz values vary from 6 to 63%; kaolinite represents 7-62%; goethite amounts are from 1 to 39%; illite represents 6-31% of the total amount of minerals and illite represents < 10% of the mineral assemblage (Table 2).Kaolinite contents are higher than 50% in certain clayey and clayey sandy facies.Illite represents more than 20% in most of clayey and clayey sandy facies.The mineral composition of alluvial sediments reflects the regional soils (Sababa et al., 2022;Sebag et al., 2013).The whole mineralogy reflects the relative amounts of textural fractions in sediments dominated by a sandy fraction with high proportions of clay and silt.The presence of quartz could indicate major inputs of sandy fraction in the lake originated both from fluvial and aeolian transport (Ekoa Bessa et al., 2021b).

Morphological characterisation
A total of 125 quartz sand grains and aggregates were selected for detailed information regarding the micro-textures.The particles range from sub-rounded to angular and some appear to be polished (Fig. 6).They show fluvial and aeolian transport in a context of short transport distance in general.Most of the gains are fresh, un-weathered, and a few show solution etchings, presumably as a result of pre-wetting prior to local emplacement in the deposited area.Aggregates producing neo-formed clay minerals (Badapalli et al., 2022) are found around the sand grains (Fig. 6a and b).The grains in the aggregates occur as sub-rounded grains with only a slight solution attack.Some grains show the greatest range of angularity/roundness values on the Shephard and Young (1961) scale with grains ranging from angular to sub-angular and shallow rounded edges showing little or no evidence of attrition (Fig. 6c and d).The occasionally well-rounded with high sphericity grains indicate abrasion during aeolian transport (Fig. 6c and d).There are numerous sub-rounded grains with large amounts of secondary silica precipitation on their surfaces (Fig. 6e and f).This may be due to the ambiguity of the incrustations (Smith & Whalley, 1981).In some cases, the dust may be cemented to the host particles by silica precipitation (e.g.Fig. 6e and f).Close examination of rounded sand grains reveals that the rounding is due to crystal form and outgrowth.This suggests low attrition of the grains as they appear to be the product of in situ weathering of the underlying basement complex.The well-rounded grains (Fig. 6g) are evidence of aeolian transport due to the Harmattan winds that blow over the region during the dry season.Roundness is attributable to the deposition of silica on formerly sharp grain edges (Fig. 6h).The same aspect of grain has been recorded in sediments from Bodélé depression around the LCB (Moreno et al., 2006).The grain shapes have been described in sediments from Lake Ngaoundaba in the Adamawa plateau in the neighbored Nord of Cameroon (Ekoa Bessa et al., 2021b).The origin of these grains thus suggests a 'grus' that has undergone little transport.The sub-rounded and rounded grains show the well-marked edge rounding and upturned plates of the aeolian rounding.The modification by aeolian action probably represents only short periods of movement.Roundness and upturned surface patches indicate sustained and energetic wind transport.Some particles are very likely to be of local origin and may derive from the surrounding weathered rocks.Others, on the other hand, would have undergone varying transport with some having travelled further.They may have come from the recycled sands which occasionally contain wellrounded grains.Relatively energetic but short-lived transport of material is currently observed in the LCB, particularly at the beginning of the rainy season, when downdrafts associated with intense thunderstorms drive material from the bare ground surface in localised dust storms (Moreno et al., 2006;Sebag et al., 2013).The frequency and intensity of this movement decrease as the rainy season progresses and vegetation and soil moisture increase.Currently, wind erosion prior to storms is favoured by land clearing, but under the moisture conditions envisaged for the formation of the    basins.Several factors may have increased soil erodibility.These include an overall decrease in vegetation and effective rainfall for soil moisture recharge and a possible decrease in storm frequency during the shorter rainy season, which would have allowed surface soils to dry out between storms (Ekoa Bessa et al., 2021b;Sebag et al., 2001).

Geochemical characterisation
Major element contents in alluvial sediments from Lake Chad Basin are listed in Five groups of trace elements were distinguished on the basis of their mean contents: (1) elements with concentrations higher than 100 mg.kg −1 are Ba (average 229.48 mg.kg −1 ) and Zr (average 150.57mg.kg −1 ); (2) elements with concentrations between 50 and 100 mg.kg −1 are Cr, V, Sr, Ce and Rb; (3) elements whose the contents vary between 20 and 50 mg.kg−1 are Zn, La, Ni, Nd, Cu and Ga; (4) elements with concentrations between 5 and 20 mg.kg −1 are Co, Sc, Th, Nb, Y, Pb, Br, Sn and Sm; (5) and elements of low concentrations below 5 mg.kg −1 are As, Ag, Cs, U, Hf, Mo, W, Ta, Ge and Yb (Table 4).The distribution of trace and rare-earth elements reveals a felsic composition of the alluvial sediments.In fact, trace elements, such as Sc, Cr and Co, are associated with mafic composition whilst Ba, Zr, La, Ce, and Th are linked to felsic materials (Armstrong-Altrin et al., 2004;Bolarinwa et al., 2019;Mbanga Nyobe et al., 2018;Zaid & Al Gahtani, 2015).Trace elements from these sediments strongly suggest a detrital origin due to the high presence of clay minerals in modern Lake Chad sediments transported sometime by the Chari Logone River and other time deposited by wind from Saharan desert (Moreno et al., 2006;Sebag et al., 2013).
The correlations between elements are presented in Table 5 as Pearson's correlation matrix.As confirmed by high SiO 2 content, the predominance of silica minerals led to negative correlation between SiO 2 and most of the elements.The negative correlations are especially strong with Al 2 O 3 , Fe 2 O 3 , LOI and Sr.Conversely, Al 2 O 3 is strongly bound to Fe 2 O 3 and LOI due probably to their affinity for clay minerals (Bhuiyan et al., 2011;Sababa et al., 2022).Loss on ignition (LOI) assesses the quantity of volatile matter (water, carbonate, organic matter) which are lost during heating to high temperature (Sababa et al., 2021).It also increases with the degree of weathering as a result of clay minerals formation.Thorium, Rb, Nb, Sc, V, Cr, Co and Ni display strong positive correlation, whilst W is negatively correlated with Sc, V, Co, Ni and positively with Cr.There are also positive correlations between Rb and Sr, U, between Ta and LOI, between Hf and Mo, between Zr and Hf, Mo.Barium is positively correlated with W, Sc, V, Cr and negatively with U and Co. Molybdenum shows positive correlations with W, Ta and negative correlation with Sc and V. Positive correlations could be linked to the hosting of elements by the same minerals or compound and negative correlations show different nature of hosting mineral or different processes of enrichment (Mbale Ngama et al., 2019).

Sediment classification and maturity
The classification diagram of alluvial sediments suggested by Herron (1988) uses the logarithmic ratios of major elements (Log SiO 2 /Al 2 O 3 vs.Log Fe 2 O 3 /K 2 O).Most of samples fall in the fields of Fe-shale, Fe-sand and (sub)litharenite compositions of this plot (Fig. 7).The Na 2 O/K 2 O and SiO 2 /Al 2 O 3 ratios show significant values and indicate mature sediments (Roser et al., 1996) as confirmed by the Index of Compositional Variability (ICV < 1; Cullers & Podkovyrov, 2000;Fig. 8).Such character is associated with strong hydrodynamic settings or long-distance transport and high weathered source (Huyan et al., 2021).Micro-textural observation of sand grains indicated energetic and short transport distance.

Table 3
Major element compositions (in wt.%) and some calculated indexes of the alluvial sediments from Bol area (Lake Chad Basin)      which increase the concentrations of certain elements (Armstrong-Altrin et al., 2012).Thus, the low Th/Sc ratios (Table 6) associated with the low Zr, Hf, TiO 2 and P 2 O 5 contents (Tables 3 and 4) indicate low degree of sediment recycling as a result of low chemical differentiation.The Th/U vs. Th and Th/Sc vs. Zr/Sc diagrams illustrate sediment recycling and weathering degree (McLennan et al., 1993).During sediment recycling and/or weathering, U is rapidly leached relative to Th and leads to high Th/U value.The alluvial sediments of LCB have similar composition to that reported for the upper continental crustal composition (Fig. 9a; Th/U = 3.8; Taylor & McLennan, 1985).In addition, the low degree of sediment recycling is confirmed by the micro-textural character and the fact that the samples do not follow the weathering trend (Fig. 9a).There is no correlation between Th/Sc and Zr/Sc (Fig. 9b) as a result of low sediment recycling.In fact, significant chemical differentiation leads to rapid increase of Zr/Sc values compared to Th/Sc as a result of significant sediment recycling.Positive correlation between Th/Sc and Zr/Sc indicates igneous differentiation trend and rapid Zr/Sc ratio increase expressing important sediment reworking, which is accompanied by zircon enrichment (McLennan et al., 1993).

Depositional environment
The physicochemical parameters of the alluvial sediments of the Lake Chad Basin have been determined.They are moderately acid and basic (pH = 6.58-9.15;Table 1) with variable electrode potential (Eh = − 129 to + 23 mV).Except for some samples of profile LT2 which fall in the acidic-reductant field, all samples show basic and reductant character (Fig. 10).Previous study on Eh and pH in Lake Chad (Mothersill, 1975) shows very similar results with those of this study indicating alkaline conditions.Generally, the lower pH measurements occurred in the slightly deeper-water area where the Eh values of the sediments are also lower.The current nature of the alluvial sediments may not reflect the paleo-environment during deposition due to post-depositional external sources as solutions and solids (Bourman & Ollier, 2002).
Trace element ratios are used to infer the paleo-environmental conditions (Jones & Manning, 1994;Nagarajan et al., 2007;Wignall & Myers, 1988).In the depositional environment studies, U/Th, Ni/Co and V/Cr ratios indicate the paleo-redox conditions.High values (U/Th > 1.25 and V/ Cr > 4.5) suggest an anoxic environment and low values (U/ Th < 0.75 and V/Cr < 2) are indicative of oxic depositional environment (Armstrong-Altrin et al., 2015;Jones & Manning, 1994).The alluvial sediments of the LCB have low U/ Th and V/Cr values (U/Th = 0.20-0.34;V/Cr = 0.40-132; Table 6) indicating an oxic depositional environment.In addition, oxic environment is characterised by Ni/Co ratio below 5, whereas a suboxic and anoxic environment are revealed by high values (Ni/Co > 5) (Jones & Manning, 1994).In this study, the Ni/Co ratios are all below 5, suggesting once more an oxic paleo-environmental conditions.The authigenic U (AU) values of alluvial sediments have been also calculated to analyse the oxic/reducing characteristics of the depositional environment.This parameter was estimated according to the formula of Wignall and Myers (1988): Au = (total U)-Th/3; Table 6.High authigenic values (AU > 5) represent anoxic conditions whilst low values (AU < 5) indicate oxic depositional conditions (Jones & Manning, 1994).Definitely, all the proxies indicate oxic paleo-environmental conditions as confirmed by the presence of diatom in this environment, as observed by Pias Fig. 8 Index of Compositional Variability (ICV) vs.Chemical Index of Alteration (CIA) plot (Long et al., 2012) of alluvial sediments from Bol area (Lake Chad Basin).See Fig. 6 for symbols

Table 6
Trace element ratios and authigenic uranium (AU) of the alluvial sediments from Bol area (Lake Chad Basin)    1970).In fact, diatoms are important oxygen producers and play a vital role in marine environments, as they form the basis of food webs for many species (Mann & Droop, 1996).
In the SiO 2 vs. Al 2 O 3 + K 2 O + Na 2 O diagram (Fig. 11; Suttner & Dutta, 1986), the samples of alluvial sediments of LCB fall in the fields of semi-arid and semi-humid environment.The samples follow the increasing chemical maturity trend.These results confirm that the sediments are mature.They formed under climate variation from humid to arid environment.Grain size and shape of selected grain clearly show that the sediments probably have been reworked many times with only the minerals which are most stable in the sedimentary environment remaining and minerals from surrounding soils and rocks which adhered the grains.

Source weathering conditions
The weathering intensities of source rocks are strongly linked to the chemical behaviour of upper crustal materials (Huyan et al., 2021).The mobile elements (e.g.K, Na, and Ga) easily exit rocks during weathering whilst the immobile elements (e.g.Al and Fe) are bound to the residual sediments with few losses (Fedo et al., 1995;Nesbitt & Young, 1982).The chemical variations are generally quantified and expressed by several indices: e.g.Chemical Index of Alteration (CIA; Nesbitt & Young, 1982); Plagioclase Index of Alteration (PIA; Fedo et al., 1995) and Mafic Index of Alteration (MIA; Babechuk et al., 2014).
Chemical Index of Alteration determines the quantitative assessment of weathering degree.It assesses the progressive hydrolysis of feldspar minerals into clay minerals (Bhaskar et al., 2015) and expresses in molar proportions: (1)  (Suttner & Dutta, 1986) of the alluvial sediments from Bol area (Lake Chad Basin).See Fig. 6 for symbols CaO* represents only the CaO content in the silicate fraction.The correction method suggested by McLennan et al. (1993) lead to remove the CaO content include in the authigenic carbonate.The estimated CIA values are interpreted as follows: (i) 50-60% for low weathering degree; (ii) 60-80% related to moderate weathering; (iii) and > 80% indicate high or extreme degree of weathering.High weathering degree is generally associated with gibbsite and aluminous clay minerals (illite and kaolinite) fond in humid tropical and equatorial climates.The alluvial sediments collected from LCB have CIA index between 61 and 93% with an average value of 80%.These values are higher than the average upper continental crust (UCC = 50%; Bhuiyan et al., 2011) and indicative of moderate to high degree of weathering of source rocks.This weathering character of source rocks can be confirmed by the rounded shape of grains and the presence of clay minerals, such as kaolinite and illite, found in almost all the studied samples.
Plagioclase Index of Alteration also measures the degree of alteration in the source area.Fedo et al. (1995) have proposed the calculation formulae of PIA values using the molar proportions and CaO* as for CIA: The PIA values vary from 50% as very low plagioclase hydrolysis to 100% synonym of total plagioclase hydrolysis.The hydrolysis of plagioclase leads to the formation of kaolinite and gibbsite.The values for this study, ranging from 65 to 98% (average 86%), indicate moderate to high chemical alteration of the source rocks.The Post-Archean average Australian Shale (PAAS; Pourmand et al., 2012) shows lower degree of plagioclase alteration (PIA = 79%).
Mafic Index of Alteration (MIA) takes into account the chemical alteration of ferromagnesian minerals, such as garnet, pyroxene, olivine and epidote.It improves the CIA and PIA proxies by adding elements (Mg and Fe) related with mafic minerals to the assessment of chemical weathering.It has been proposed by Babechuk et al. (2014) and calculated with molar proportions and CaO*: As for CIA and PIA, higher MIA values indicate more weathering of the source rocks of sediments.Thus, 100% of MIA indicates total loss of mobile elements from feldspars as well as mafic minerals.MIA values, in this study, ranged between 45 and 72%, with an average of 61% indicating a low to moderate weathering degree of feldspars and ferromagnesian minerals. (2) CIA and PIA indicate moderate to high weathering whilst MIA indicates low to moderate weathering degree of source rocks.The controversial values of MIA may be attributed to the felsic nature of the rock sources.In fact, MIA mostly assesses the weathering of mafic minerals.Nevertheless, these indices have given useful information on the composition of the original materials.So, integrating mafic mineral in the weathering process is not appropriate in this context.Otherwise, the significant variation in the degree of weathering is related to the alternation of wet and dry periods in the LCB (Massuel, 2001).In general, the dry memory fades rapidly during the wet period in the supergene environment.The high degree of weathering is then related to the humid tropical paleo-environmental conditions in the LCB.This is confirmed by the SiO 2 vs. Al 2 O 3 + K 2 O + Na 2 O diagram (Fig. 11; Suttner & Dutta, 1986).

Sediment provenance
The major and trace element geochemistry of alluvial sediments are widely used to infer their provenance since they reflect in a certain way the source rock compositions (Armstrong-Altrin et al., 2015;Huyan et al., 2021;Jafarzadeh et al., 2014).The F1-F2 diagram (Roser & Korsch, 1988;Fig. 12a) shows that the alluvial sediments from LCB derived from two mixed provenances (quartzose sedimentary and mafic igneous).Quartzose sedimentary provenance reflects highly weathered source materials because feldspar decreases during the weathering process and quartz increases.The sources have an acid and intermediate composition with some contribution of basic components according to the K 2 O vs. Rb diagram (Fig. 12b).The samples fall in the field of the differentiated magmatic suites (main trend K 2 O/Rb = 230; Shaw, 1968).The values of Al 2 O 3 /TiO 2 grow from mafic (Al 2 O 3 /TiO 2 = 3-8) to intermediate (Al 2 O 3 / TiO 2 = 8-21) and felsic (Al 2 O 3 /TiO 2 = 21-70) source rocks (Nagarajan et al., 2015).This study's Al 2 O 3 /TiO 2 ratio varies from 11 to 43 (average 30) (Table 3).This character indicates both intermediate and felsic provenances.This controversial interpretation can be explained by the low credibility of the major elements in the provenance study of alluvial sediments, as highlighted by Verma and Armstrong-Altrin (2016).
Several trace elements (e.g.La, Y, Ni, Cr, V, Th, Sc, U, Zr, Co, Nb, Rb, Sr) reflect better the source rock compositions due to their coherent behaviour during surface processes (Mbanga Nyobe et al., 2018;Mbog et al., 2022;Ndjigui et al., 2018;Sababa et al., 2022;Workman & Hart, 2005).The ratios of these trace elements help to discriminate between mafic and felsic source provenance.The La/Sc, La/Co, Th/Sc and Th/Co ratios are lower in mafic rocks and higher in felsic sources (La/Sc = 2.50-16.3;La/ Co = 1.80-13.8;Th/Sc = 0.84-20.5;Th/Co = 0.67-19.4)(Nagarajan et al., 2017;Ngagoum Kontchipe et al., 2021).Contrary to the behaviour of major elements, these values fall all in the ranges of felsic source rock compositions (Table 6).TiO 2 and Zr are also useful for provenance study for their bearing minerals are resistant during the surface process.According to the TiO 2 vs. Zr diagram, most of the samples plot in the field of felsic igneous rocks (Fig. 12c).A few samples plot in the boundary of intermediate and felsic igneous rocks (Fig. 12c).The Th/Co vs. La/Sc diagram  12 Bivariate plots for geochemical characterisation of rock sources of the alluvial sediments from Bol area (Lake Chad Basin): a major element provenance discriminant plot (Roser & Korsch, 1988), b: K 2 O vs. Rb (Floyd & Leveridge, 1987) with K/Rb ratio = 230 (main trend of Shaw, 1968), c TiO 2 vs. Zr (Hayashi et al., 1997), d Th/Co vs. La/Sc (Cullers, 2002), e Cr/V vs. Y/Ni (Hossain et al., 2017;McLennan et al., 1993), f Th/Sc-Zr/Sc plot (McLennan et al., 1993) with (1) average basalt, (2) low-silica andesite, (3) andesite, (4) dacite, (5) average upper continental crust (UCC), ( 6) rhyolite, phanerozoic granite and felsic volcanic, (7) I-type granite, (8) S-type granite (Condie, 1993;Taylor & McLennan, 1985;Whalen et al., 1987).See Fig. 6 for symbols characterises the composition of the source rocks and shows that the alluvial sediments derive from silicic rocks (Fig. 12d).This composition confirms the high SiO 2 contents in all the samples.All the samples, except those of profile LT2, fall in the trend of felsic rock sources in the Cr/V vs. Y/Ni diagram (Fig. 12e).In the Th/Sc vs. Zr/Sc distribution diagram (Fig. 12f), all the data are plotted close to the compositions of dacite, average upper continental crust (UCC), I-type and S-type granites.This indicates or confirms the felsic nature of the sources as shown by most of the discriminant plots (Fig. 12).
Linking this previous information to physical data and regional geology, quartz grain morphology indicates that the major inputs of sand to the lake originated from fluvial transport, whilst a few parts originated from aeolian transport.The similarity of mineralogy and extended geochemical elements data and their correlations between all the samples suggest that the same basement rock was eroded and then the products deposited (Moussa et al., 2016).At the scale of the cores, this last result, coupled with the lack of cross-beddings and the absence of coarse sand deposits, indicates that the sampling sites are far from any major river delta during the total period of sediments' deposition.These days, the detrital sediments deposited in the LCB were essentially transported by the Chari Logone system and mostly originate from the erosion of the southern part of the watershed, which is characterised by important reliefs of the Adamawa mounts, the Cameroon Volcanic Line (CVL) and mean annual rainfall of 1200 mm (Olivry et al., 1996).This part of the drainage basin is covered by ferralitic and tropical ferruginous soils in which kaolinite is the dominant clay mineral (Moussa et al., 2016).

Tectonic setting
Geochemical data (major, trace and rare-earth elements) of clastic sediments are useful for determining the tectonic settings in a sedimentary basin (Bhatia, 1983;Bhatia & Crook, 1986;Roser & Korsch, 1986;Verma & Armstrong-Altrin, 2013).Bhatia and Crook (1986) proposed many discriminatory plots to discriminate between active and passive margin tectonic settings using immobile elements (La, Sc, Th, and Zr) in ternary diagrams.Using of ternary diagrams to identify tectonic settings is limited to some statistical errors (Verma, 2012) and might lead to multiple tectonic features (Huyan et al., 2021).Two widely used plots have been proposed to discriminate better the tectonic settings (Roser & Korsch, 1986;Verma & Armstrong-Altrin, 2013).The K 2 O/ Na 2 O vs. SiO 2 plot discriminates between passive margin (PM), active continental margin (ACM) and volcanic island arc (ARC).Based on that binary diagram, all the samples plot in the passive margin field, except samples from profile LT1 which plot in the field of volcanic island arc (Fig. 13a).
This could be linked to the position of this pit in the landscape and the sediments could have been derived from fluvial transport and sometimes have the fingerprint of the active margin.The DF1 vs. DF2 plot uses the log ratio of ten major elements to suggest the tectonic settings (Verma & Armstrong-Altrin, 2013).The studied alluvial sediment samples from LCB plot mostly in the collision field (Fig. 13b).The rest fall in the boundary of rift-collision and the triple point volcanic island arc (ARC)-rift-collision.This result contradicts the interpretation of the passive margin in the K 2 O/Na 2 O vs. SiO 2 diagram.Verma and Armstrong Altrin (2016) state that the passive margin represents the sediments derived from the rift setting.The plotting of the samples in the collision field on the tectonic discrimination diagrams (Fig. 13b) is not consistent with the geology of the LCB.
The passive tectonic setting is consistent with the tectonic history of the LCB.On the other hand, the collision setting may reflect the complex history of the Pan-African basement source rocks of the studied sediments.These rocks experienced the tectonic-thermal event of Pan-African (500 Ma) (Toteu et al., 2001).The Pan-African belt is interpreted to result from a continental collision between the West African and Congo Craton in central Africa (Toteu et al., 2001(Toteu et al., , 2004)).A similar explanation has been made in recent studies and suggests that sediments can also reflect the tectonic settings of source rocks (Armstrong-Altrinet al., 2015;Ngueutchoua et al., 2019;Tchouatcha et al., 2023).In addition, the Cameroon Volcanic Line, which crosscut the LCB may have played an important role in these controversial tectonic settings.There is no consensus on the origin and volcanism along the Cameroon Volcanic Line.The most recent interpretation models propose: (1) a complex interaction between at least two successively reacting mantle rims and lithospheric fractures that induce oblique alignments of magma complexes (Begg et al., 2009;Koch et al., 2012;Ngako et al., 2006); (2) and the hypothesis of a hot spot in a sub lithospheric mantle (Déruelle et al., 1991;Reush et al., 2010).The faults penetrating the lithosphere and thinning the continental crust could have been associated with the Atlantic Ocean's opening (Tchouatcha et al., 2023;Torsvik et al., 2009).As a result, the Pan-African orogeny and the Cameroon Volcanic Line activities explain the several tectonic backgrounds associated with the sediments of LCB to the passive margin as the main sediment sources.

Conclusions
This study gives new information about depositional environments through weathered conditions and tectonic settings which were rarely documented in the sub-region of central Africa.The sediments are characterised by variation in texture due to seasonal conditions with the alternation of wet and dry periods in the Lake Chad Basin.This seasonal variation is the result of (i) fluvial transport of mature sediments from felsic sources consisting of sand grains and clays; (ii) long-range transport of dust which inevitably tend to alter the bulk aerosol chemistry by mixing and size fractionation, thus obscuring geological signatures from specific source areas; (iii) regular dust intrusions that move westwards across the Atlantic Ocean from the African coast from different sources and therefore potentially contain different mineral mixtures.These sediments have been deposited in an oxic environment during the Pan-African orogeny with the fingerprint of an active margin.Like in a few studies in the sub-region, it remains undocumented whether eolian dust carried by dry air outbreaks can, at times retain a recognisable rock source signature.

Fig. 1
Fig. 1 Geological map of: a Chad showing the position of Lake Chad, b the study area showing the sampling sites

Fig. 2
Fig.2Regional map of the study area showing the relief variation and morphology of the current Lake Chad

Fig. 3
Fig. 3 Synthetic log of the alluvial sediments (profiles LT1, LT2, LT3 and LT4) of Bol area (Lake Chad Basin) showing the sampling points

Fig. 4
Fig. 4 Clay-sand-silt diagrams for characterisation of alluvial sediments from Bol area (Lake Chad Basin): a texture, b porosity and permeability

Fig. 5
Fig. 5 X-ray diffraction patterns of samples of: a profile LT1, b profile LT2, c profile LT3 and d profile LT4 of alluvial sediments from Bol area (Lake Chad Basin) Fig. 6 Scanning electron microscope images showing the micro-textures and morphology of alluvial sediments from Bol area (Lake Chad Basin) 54

Fig. 7
Fig. 7Geochemical classification diagram of the alluvial sediments from Bol area (Lake Chad Basin) using log ratios of SiO 2 /Al 2 O 3 -Fe 2 O 3 /K 2 O(Herron, 1988).Lozenges represent samples of profile LT1, squares represent samples of profile LT2, Triangles represent samples of profile LT3 and circles represent samples of profile LT4 and Y Th represent the contents of U and Th respectively(Wignall & Myers,

Table 1
Colour, particle size distribution, texture, pH, Eh and EC of the alluvial sediments from Bol area (Lake Chad Basin)

Table 2
Mineralogical composition (in %) of the alluvial sediments from Bol area (Lake Chad Basin)

Table 3 .
(Nesbitt & Young, 1984)or elements is consistent with the grain size distribution and the mineralogical composition of samples.The alluvial sediments are characterised by high SiO 2 contents (54.15-98.16wt.%)andlow to moderate concentrations in Al 2 O 3 (0.94-18.29 wt.%) and Fe 2 O 3 (0.19-7.93 wt%).Sandy samples have the highest SiO 2 contents whilst the highest Al 2 O 3 and Fe 2 O 3 were found in the clayey samples.The high SiO 2 contents characterise the predominance of quartz minerals and the high intensity of pre-and post-depositional weathering(Nesbitt & Young, 1984).Other oxides like Na 2 O, CaO, K (Moussa et al., 2016)021b)icant contents (> 1 wt.%), especially in clayey samples.The significant content in Al 2 O 3 as the oxide with the second highest content confirms the high proportion of clay minerals like kaolinite and illite in these alluvial sediments(Ekoa Bessa et al., 2021b).The SiO 2 /Al 2 O 3 (average 14.70), Al 2 O 3 /TiO 2 (average 30.11) and Al 2 O 3 /Na 2 O (average 48.59) ratios confirm the predominance of SiO 2 and Al 2 O 3 over the other major elements (Table3).Similar results are found in other areas of LCB(Moussa et al., 2016).The variations in SiO 2 and Al 2 O 3 contents are linked to the amounts of quartz and kaolinite.In fact, natural kaolinite crystals contain low amounts of Fe 2 O 3(Mestdagh et al., 1980)and illite is a minor component.A major part of Fe 2 O 3 is therefore associated to goethite.Between 10 and 20 wt.% of the Al 2 O 3 amounts come from kaolinite.The MgO contents were low and essentially related to goethite.As illite is a minor component of samples, a part of K 2 O should be associated with the kaolinite phase.

Table 5
Pearson's correlation matrix for major and trace elements of the alluvial sediments from Bol area (Lake Chad Basin)