Geology and Geochemistry of Sediments from Lewumeji and Idogun Wells, Eastern Dahomey Basin Southwestern Nigeria

Selected composited samples from Lewumeji (0-111m) and Idogun (054m) Abeokuta Group, Eastern Dahomey Basin, were subjected to detailed lithologic and geochemical studies. The studies aimed at determining the lithological sequence, paleoenvironments of deposition, tectonic settings, provenance, weathering history and the classification of sediments. The core samples were subjected to lithological description and geochemical analysis were done following standard procedure through the use of Inductively Coupled Plasma Mass Spectrometry (ICPMS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) through lithium metaborate fusion method.The lithologies from both wells consist of reddish to brown colour, rounded to sub rounded sandstone, brownish clay, and dark grey shale denoting possibly fluvial, brackish, lagoonal and marine environments. The sandstone revealed dominance of quartz which were classified as quartz arenites to sublitharenite and subarkoses, this suggests a recycled orogen source for the sandstones. Shale in the study area shows patches of CaCO3 indicating the presence of biogenic materials. And the reddish brown colouration of clay suggests ferruginization. The geochemistry revealed low values for K2O/Al2O3 ratio coupled with high average values of Plagioclase Index of Alteration (81.64% and 73.80%), Chemical Index of Alteration (79.22% and 71.52%) and Chemical Index of Weathering (82.41 % and 75.03%) for Idogun and Lewumeji wells respectively. This values indicate intense weathering condition. The plots of Log (Fe2O3/K2O) against Log (SiO2/Al2O3) revealed sediments dominated by Fe sand and Fe shale. Also, Al2O3-(K2O+CaO+MgO)-(Fe2O3+MgO), (AKF) ternary plots reveals that the sediments are derived from continental environment. The Log (K2O/Na2O) against SiO2 and the discriminant function plots indicate sediments deposited within the passive margin. Also, the Trace elements ternary plots of Th-Sc-Zr/10 and Th-Co-Zr/10 reveal deposition within Continental Island Arc and Passive Margin. The discriminant function plots characterized the sediments as Mafic Igneous rock and quartoze sedimentary provenances with minor input from Intermediate and felsic igneous provenances. The chondrite normalized REE plots show enrichment of Light REE over Heavy REE in the study area with negative Europium and Cerium anomalies greater than 1. This indicated an oxidising and a shallow marine environments. The REE pattern is similar to those for the Upper Continental Crust sediments (UCC). Conclusively, the study shows that the sediments in the study area have multiple provenances subjected to high weathering conditions and were deposited within an oxidizing and continental to shallow marine settings.

abundance and large variations of SiO2 ranging from 90.72% -36.39% with an average value of 50.18% for the lewumeji well and 99.14% -42.48% with an average of 70.81% for Idogun well. This average value islower in Lewumeji and higher in Idogun well than the value for the upper continental crust (UCC), Al2O3 is moderately high ranging from 3.7% -19.8% (average: 14.09%) and 1.66% -19.72% (average: 12.7%) for Lewumeji and Idogun well respectively, the concentration of Aluminosilicate is a good measure of detrital flux (Nagarajan, et. al., 2007) and the mean high concentrations of Al2O3 may indicate a high Kaolinite/illite ratio within the study area (Besly and Clearl, 1997). Concentration of Fe2O3 in Idogun well and lewumeji varies from 8.6% -1.04% (average: 5.21%) and 2.41% -9.55% (average: 5.9%) This appreciable value of Fe2O3 can be from the source rock, indicating that the source rock may contain an appreciable biotite and hornblende, it can also indicate oxidation condition (Bassey and Eminue 2014). CaO ranges between 0.07% -11.79% (average: 4.3%) in Idogun and 0.12% -15.11% (average: 5.6%). Also the MgO concentration is relatively low in Lewumeji it ranges between 0.05% -5.19% (average: 3.39%) and Idogun ranges between 0.03% -0.7% (average: 0.45). Which implies that the sediments contain minute amount of ferromagnesian minerals such as mica, hornblende and haematite. The low value of MgO and CaO contents indicates associated carbonate or dolomitization. However the concentration of TiO2, Na2O, K2O, P2O5, MnO and Cr2O5 are relatively low. The low K2O content indicated low amount of illite or K-feldspar present in the sediment (Akpokodje et al., 1991) and the low phosphate (P2O5) content could be attributed to lower amount of accessory phases such as apatile and monazite. The summary of major element of Lewumeji and Idogun wells is given in Table 1 and 2 respectively.
The Silica-Alumina ratio (SiO2/Al2O3) of the studied shale sample is shown in table 3. According to Le Maitre, (1976) The SiO2/Al2O3 ratio is about 3 in basic rocks (basalts and Gabbros) and it is around 5 in the acidic end member (granites and rhyolites) Ratio more than 5 or 6 in sedimentary rocks provided evidence of sedimentary maturation (Roser et al., 1996). The SiO2/Al2O3 ratio for Lewumeji and Idogun shale samples varies between 2.95 -3.23 (average: 3.09) and 2.24 -3.03 (average: 2.24) respectively which is within the range of a basic rock. Sandstone in both wells has a significantly high value of 24.26 for Lewumeji and 11.16 -54.71 (average: 33.93) for Idogun wells, indicating a high mature sediment. Which is comparable to the value of 30 reported in the modern sediment (Valloni and Maynard, 1981;McLennan et al., 1990) that interpreted as highly mature sediments. The Potassium -Alumina ratio (K2O/Al2O3) is an index of mudrock composition which can be used as an indicator of original composition of ancient sediments (Popoola et. al., 2014). The K2O/Al2O3 has different ratios for clay minerals and feldspar which ranges between 0.0 to 0.3 and 0.3 to 0.9 respectively, The very low value of K2O/Al2O3 of the study areas varies from 0.02 to 0.10 for lewumeji and 0.04 -0.07 for Idogun sediments generally indicating the preponderance of clay minerals over the K-bearing minerals such as K-feldspar and micas. The concentration of TiO2 increased with Al2O3, indicating that TiO2 is associated with Phyllosilicate especially with Illite (Dabard, 1990). The concentration of Fe2O3 and TiO2 (Pearse et al., 1999) indicate the presence of irontitanium minirals (haematite, anatase and rutiles) .The ratios of TiO2/ Al2O3 in Lewumeji and Idogun well (0.05 -0.10, 0.04 -0.16) and Na2O/K2O (0.06 -0.10, 0.04 -0.10) respectively suggests that the sediments are highly chemically mature (Jenner et al., 1981). Also, the relative high K2O/Na2O ratios (8.00 -15.00) are attributed to the relatively common presence of K-bearing minerals such as K-feldspar, Illite and some micas (Pettijohn et al., 1963, Kalsbeek et al., 2008Nath et al., 2000;Zhang, 2004;Osae et al., 2006). The Al2O3/ TiO2 ratio are essentially used to infer the source rock composition of most clastic rocks this is because Al2O3/ TiO2 ratio increases from 3 to 8 for mafic Igneous rock (SiO2 -45.52%), 8 to 21 for intermediate rocks (SiO2 = 53%-66%) and 21 to 70 for felsic rocks (SiO2= 66% -76%) (Hayashi et al., 1997). Therefore, the Al2O3/ TiO2 ratio of the sediment ranges from 9.84 -21.31 (mean = 18.16) for Lewumeji and 6.14 -20.93 (13.53) suggesting that they were possibly derived from a mafic to intermediate Igneous source.
The loss on ignition (LOI) for Lewumeji and Idogun sandstone ranges between 2.4 -4.1%, shales from the study wells ranges between 16.90 -25.5%, and clay is 9.7%. The LOI for shale is high, indicating that the shale has a higher potential for carbonaceous compounds.

Classification of sediments
The standard plot of Herron (1988) using Log (Fe2O3/K2O) against Log (SiO2/Al2O3) was employed to classify the sediments of the study area. From the plot (Fig.5), the shale and the clay samples from Idogun and lewumeji well dominantly falls within the Iron shale (Fe-shale) field and the sandstone from the study well falls within the Iron sand (Fe -sand) compartment which is an indication of ferruginization. The sediments that fall within the Iron -sand (Fe -sand) field further suggest an incursion of fluvial current and this is consistent with the presence of the brownish sandstone and clay observed from the lithological description.
Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol. 10, No.5, 2020   It can be observed that the clay and shale samples from both well falls mainly in the litharenite regions, the Idogun sandstone falls within the subarkose and quartzarenite while the lewumeji sandstone falls in the sublitharenite region (Fig.6).The sample that plotted within the quartzarenite region is an indication of the maturity and quartz enrichment in the sample.  (Osae et al., 2006). The intensity of chemical weathering or alteration of sediments source can be calculated using the following formular: The CIA, PIA and CIW Values (table 3) for Lewumeji samples range from ( 42.73% -94.68%,42.25% -96.56%, 44.08% -96.64%) with averages of (71.52%, 73.80% and 75.03%) respectively while Idogun samples range from (56.16% -98.40%,56.54% -99.54%, 57.68%-99.54% ) with an averages of (79.22% , 81.64% and 82.41%). These values are variable and it may be as a result of multiple provenances for sediments which have variable proportions of source area weathering and related processes or may be due to low concentrations of the alkalis and alkaline earth elements. From the two well, majority of the samples show CIA, PIA and CIW values greater than 70% suggesting moderate to high weathering either at the source or during transport before deposition (McLennan, 1993;Fedo et al., 1995), this show that the sediments are geochemically and texturally mature. The mobility of elements during the progress of chemical weathering can be plotted using the molar proportion of Al2O3, Na2O, and CaO* (CaO in silicate fraction) in a A-CN-K ternary diagram (Fig.7). The plots shows clusters of data pointing towards the Al2O3 apex and oriented smoothly parallel to the A-CN join suggesting the removal of Ca, Na, and K in the parent material and abundance mineral phase with composition close to Plagioclase feldspar, Smectite and Kaolinite (Ogbahon et al., 2019).

Provenance Indices
The main aim of sedimentary provenance studies are to reconstruct and to interpret the history of sediment from the initial erosion of parent rocks to the final burial of their detritus. The provenance discriminant function plot used for this study defined four (4) main provenance zones; mafic Igneous provenance (basaltic and subordinate andesitic detritus), Intermediate,Igneous provenance (dominantly andesitic detritus), Felsic Igneous provenance (acidic, plutonic and volcanic detritus) and quartzose sedimentary provenance (recycled detritus).
Roser and Korsch, (1986) introduced two major element discriminant function diagram to constrain provenance and compositions of source rock. The first is the use of oxides of Ti, Al, Fe, Mg, Ca, Na and K to effectively differentiate the sediments into four zones and the discriminant functions based upon the ratios of individual TiO2, MgO, Fe2O3, and K2O to Al2O3. The ratio plot is not as effective as the one base on raw oxides. In the ratio plot biogenic CaO and SiO2 in provenance determination could be eliminated. Using the formula for The plots using raw oxides (Fig 9) show that the study sediments from the two wells (Idogun and Lewumeji well) were sourced mainly from Mafic igneous rock and Quartzose sedimentary provenance implying a mixed source. The lewumeji shale, Idogun shale and clay plotted within the the mafic igneous provenance while the Idogun sandstone,lewumeji sandstone and few of lewumeji shale plotted within the quartzose sedimentary zone while the ratio plots (fig 10) shows the sediments plotting in the four zones (quartzose sedimentary zone, mafic, felsic and intermediate zone). In this case the mafic input also must have come from first cycle basaltic and minor andesitic detritus and the recycled sources represent quartzose sediments of mature continental provenance and the derivation of the sediments could be from a highly weathered granite-gneiss terrain and or from a pre-existing sedimentary terrain.(Roser and Korsch 1988),

Trace Element Geochemistry
The trace element concentrations in sediments result from the influence of provenance, weathering, diagenesis etc. Certain trace elements because of their relatively low mobility have been used to distinguish depositional environment, source rock composition and tectonic setting. The immobile trace elements (Zr, Y, Hf, Nb) are useful for provenance and tectonic setting determinations (Bhatia and Crook, 1986

Provenance and Tectonic setting
The concentration of Zr is used to characterize the nature and source rock composition (Hayashi et al., 1997;Paikaray et al., 2008). The TiO2/Zr ratios can be used to differentiate between the three (3) different source rock types, i.e., felsic, intermediate and mafic. The TiO2 versus Zr plot (Fig. 12) shows Idogun samples plotting in the Mafic, intermediate and a sample of sandstone in felsic zone while the Lewumeji samples plotted in the Mafic and Intermediate zone. The abundance of Th, Y, Zr and Nb in source rocks will increase as their chemistry becomes more developed (Madukwe et al., 2016). Th/Sc ratio is a sensitive index of the bulk composition of the source, Th/Sc ratio for post-Archean rocks is usually 1, and greater than 1 for granitic rocks; for Archean and basic rocks the ratio is less than 1 (Taylor and McLennan, 1985). The Th/Sc ratio of lewumeji and Idogun sediments averages 0.96 and 1.31 respectively, this suggest that lewumeji sediments may have come from Mafic source while the Idogun sediments is likely to be from the granitic rocks.
Zr/Sc ratio is highly sensitive to accumulation of zircon and can be used to identify heavy mineral concentrations (Taylor and McLennan, 1985). The average Zr/Sc ratio of the Idogun sediments is 70.1 and 26.9 for the Lewumeji sediments. These values are lower than the UCC and PAAS values suggesting that the sediments are moderately enriched in zircon. All elements involved in the ratios are also resistant to weathering processes (Taylor and McLennan, 1985)  The U/Th and Ni/Co ratio are sensitive to the paleoredox conditions of ancient sediments (Nagarajan et al., 2007, Rimmer, 2004. Sedimentary rocks derived from oxic conditions are characterized by U/Th ratio below 1.25 Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol. 10, No.5, 2020 whereas values above 1.25 suggest suboxic and anoxic condition. (Nath et al., 2000). The U/Th ratio for Idogun and lewumeji sediments ranges between 0.15 -1.04 and 0.20 -0.36 respectively indicating that the sediments were deposited in an oxic environment. Dypvik, (1984) and Dill, (1986) have used Ni /Co ratio as a redox indicator. However, it is noted that Ni/Co ratio can easily be altered by diagenetic reactions. Ni/Co ratio below 5 indicate oxic environments, whereas ratios above 5 indicate suboxic and anoxic environments (Jones and manning, 1994). Thus Ni/Co ratio ranges between 0.75 -2.94 and 1.43 -1.91 for the Idogun and Lewumeji sediments respectively suggesting an oxic environment.
According to Bhatia and Crook (1986) Th-Sc-Zr/10 and Th, Co and Zr/10 concentration can be used to discriminate sediments derived from oceanic Island arc, continental Island arc and passive and active continental margins. From the ternary plots of Th-Co-Zr/10 (Fig 13), the Lewumeji shale, Idogun shale and sandstone plotted in the continental Island arc while the Idogun clay and shale plotted in the passive margin. From the Th -Sc -Zr/10 plot (fig 14) all the shale samples from both well and Idogun sandstone plotted in the continental Island arc and the Idogun clay plotted in the passive margin region.
Both plot indicate that the Idogun clay is derived from passive margin while the shale from both well and a sandstone from is derived from continental island arc.

Rare Earth Element Geochemistry (REEs)
These are the least soluble trace element which are relatively immobile during low-grade metamorphism, weathering and the residence times of REEs in seawater are short (50-600 years) (McLennan et al., 1993,). They are effective indicators of sediment source when compared to Upper continental crust (UCC), Mantle material and Oceanic crust. Rare earth element are highly resistant to fractionation during weathering and diagenesis, their low solubility and short residence time in ocean makes them to be preserved in terrigenous sediment and validate their use as provenance indicator (Sethi et al., 1998). Because of near quantitative transfer of REE from a source region to the depositional site, terrigenous sediments reflect the average composition of source rocks.
The Light rare earth element (LREE) has atomic no 57 to 61, while the heavy rare earth elements (HREE) has atomic no 62 to 71. Their abundance in rocks are usually normalized to a common reference standard and are then expressed as the logarithm to the base 10 of the value, which are mostly made up of the values for chondritic meteorites. The chondrite normalization remove the abundance of variation (Zig-Zag pattern) between even and odd atomic elements and also allows any fractionation of the REE group relative to chondrite meteorites to be identified. Distribution of the sediments normalized to chondrite values (in ppm) are plotted against the atomic numbers of rare earth element ( fig. 15a and b) which produce a Europium anomaly. The chondrite normalizing factor from Sun and Mcdonough, (1989).
The Lewumeji sediments and Idogun sediments show large variation in REE contents (49.33 -235.34ppm) and (37.29 -269 ppm) respectively. In the study wells there is high enrichment of Light rare earth element (La, Ce and Nd) to heavy rare earth element (Er, Dy and Gd). The light rare earth element like Tb, Ho, Tm and Lu are the least enriched. (Table 5). Chondrite normalised (Gd/Yb)N ratio of sandstone, clay and shale in the study well is low (1.10 -2.20) for lewumeji sediments and (1.34 -2.32) for Idogun sediments which suggest that the sediments are derived from HREE depleted source rocks and the ratios of LREE/HREE, (Ce/Yb)N, (La/Yb)N, values are higher in Lewumeji sandstone ( 14.4 ,14.7 and 15.9 ) and Idogun sandstone (11.3, 11.5 and 15.9) respectively than in Lewumeji shale (10.1, 11.1, and 14.1) and Idogun shale (9.4 , 10.6 and 12.62) respectively indicating that the REE in sandstone is more fractionated than in shale, however, the higher REE content in the sandstone maybe due to dominance of fine sand, silt and clay fractions ( Cullers et al .,1974, Taylor andMcLennan, 1985) . Eu/Eu* is a parameter used to evaluate the abundances of plagioclase in an igneous or its sedimentary derivative (McLennan et al., 1990). Europium (Eu) value greater than 1.0 indicate a positive anomaly and Eu value less than 0.9 indicate negative anomaly (Adeigbe and Jimoh, 2013). As shown in (table 6), the mean value for Eu anomaly of the Idogun and Lewumeji sediments is 0.66 and 0.71 respectively, which is quite typical to that of UCC (0.65) and PAAS (0.66), indicating a negative Eu anomaly. The enrichment of LREE and moderately negative Eu anomaly reflect their relative abundance in the upper continental crust (Goldschmidt, 1954).
Cerium anomaly (Ce/Ce*) can be used to infer the environmental conditions at the time of deposition since higher values (>1.0) tentatively depict an oxidizing environment (Piper, 1974;Milodowski and Zalasiewicz, 1991;McDaniel et al., 1994). The Ce anomaly for the study well ranges between 0.99 -1.10 for Lewumeji and 1.00 -1.08 for Idogun indicating an oxidising environment. The Higher LREE/HREE ratios and the negative Eu anomaly are generally found in felsic rocks while Mafic rocks have a lower LREE/HREE ratios and positive Eu anomaly (Cullers, 1995). Higher LREE/HREE and negative Eu anomalies is an indication of a felsic source of the study area.   Fig. 15: Chondrite normalized rare earth elemental plot of sediments from (a)Lewumeji well (b) Idogun well

Conclusions
The Lithostratigraphy and geochemical analysis has been appropriately employed to study the sediments of Abeokuta group a part of Eastern Dahomey basin through the use of core samples from Lewumeji and Idogun well with depth ranging from 0 -111m and 0-54m respectively. Detailed Information has been derived from the work, there by establishing the depositional environment, weathering condition, tectonic setting, transportation history, provenance and the classification of sediment from the well.
The wells were examined lithologically and five units were delineated which can be further grouped into three for Idogun well two units of shale, two units of sandstone and a clay unit while the lewumeji well has a lithology of sandstone and shale. Both well are dominated by fissile to blocky, light to dark grey colour shale and the sand grain varies from medium to fine grained texture showing dominance of quartz and the clay unit covers a small interval having a reddish brown colouration. This lithology denote Marine, fluvial and Lagoonal or brackish environment respectively The bulk geochemical study of the sediments revealed that SiO2 is the dominant oxide followed by Al2O3, Fe2O3 and CaO. The sediments in the study wells show relatively high value of K2O/Na2O ratio indicating the presence of plagioclase. The relatively high Fe2O3 and TiO2 is an indication of iron-titanium minerals (haematite, anatase and rutiles) whereas very low value of K2O/Al2O3 suggest sediment recycling or increase in the source area weathering. The weathering indices (CIA, CIW, and PIA) and the A-CN-K Ternary diagram indicate a sediment that has been subjected to high degree of weathering. The sediments are inferred as highly mature sediments evidenced from their high SiO2/Al2O3 ratio The use of Herron's model classified the sediments as Fe-shale and Fe-sand. Also the AKF plots indicated that the sediments were gradually moving from the continental environment before been deposited shallow marine environment which is in agreement with the palynological deduction for paleoenvironment. This is an indication that the sediment has undergone moderate to high transportation under oxidising condition. the discriminant function plots of Roser and Korsch for the provenance signature studies reveals sediments dominants in mafic and quartzose sedimentary provenance with few inputs from intermediate igneous, as well as felsic igneous provenances. However, the bivariate plot of TiO2 versus Zr also indicates that most of the sediments from the wells are majorly from mafic source with minor from the intermediate and felsic source. The Higher LREE/HREE and negative Eu anomalies from the chondrite normalization is an indication of a felsic source of the study area. The Ce anomalies and the values of Ni/Co and U/Th show that the redox condition during the sedimentation were oxic. Discriminant function diagram After Bhatia 1983 reveal the sediments are from passive margin this is also conforming to tectonic discriminant plots of Roser and Korsch, 1986. The use of Th-Sc-Zr/10 after (Bhatia and Crook, 1986) reveals that sediments of both wells plotted in the Continental Island Arc and Passive margins it therefore implies that the sediments were deposited in plate interiors or in an intracratonic basin.