GEOCHEMICAL EVALUATION OF CAMPANIAN-MAASTRITCHIAN CLAY-SHALE SEDIMENTS OF PATTI FORMATION, SOUTHERN BIDA AND MAMU FORMATION, NORTHERN ANAMBRA BASINS

Two basins (Southern Bida and Northern Anambra Basins) were investigated to deduce weathering, paleooxygenation, provenance, depositional environment and tectonic setting, as well as to establish a relationship between the two basins. The obtained high values of calculated weathering indices such as Chemical index of alteration (CIA > 90), Chemical Index of Weathering (CIW > 90), Plagioclase Index of Alteration (PIA > 90) and the Al2O3-(CaO + Na2O)-K2O ternary relationship for the clay – shale sediments from both basins indicate intense weathering in the source area. Important geochemical ratios such as V/Cr, Cu/Zn, Ni/Co, (Cu+Mo)/Zn, revealed predominantly oxic conditions for the clay – shale sediments from both basins, although, a more reducing or an anoxic condition cannot be ruled out for the clay – shale sediments from the Southern Bida basin due to high ratios of U/Th (1.93-5.67) and Cu/Zn (0.19-5.00). In addition, the Sr/Ba ratios (0.16–3.50) for the clay-shales from the Southern Bida basin indicated an alternated marine and continental paleo-depositional settings and only continental setting (Sr/Ba ratios = 0.22 – 0.50) for the Northern Anambra basin. The Th/Sc, La/Sc, Th/Co and the LREE/HREE ratios showed a derivation of the shale and clay deposits from similar felsic-rich source rock while the log of (K2O/Na2O) vs SiO2, revealed a Passive Margin tectonic setting for the two Basins. There is insignificant differences between the geochemical classifications, weathering, source rock/provenance and tectonic settings of clay-shale sediments of both sedimentary basins, however, there exist slight disparity in their salinity conditions and redox settings.


INTRODUCTION
Geochemical signatures are crucial for determination of prehistoric and depositional events in rocks. Previous researchers have addressed questions such as depositional environment, provenance, weathering conditions and tectonic settings of the clay-shale deposits in Southern Bida and Northern Anambra Basins of Nigeria (Okunlola and Idowu, 2012;Odoma et al., 2015;Bolarinwa et al., 2019). However, the relationship between the two basins on the basis of environment of deposition and variation in chemical constituents remain unresolved. Bhatia, 1983;Roser and Korsch, 1986, used distinct ratios like Th/La and Th/Sc to discriminate tectonic setting. TiO 2 with La, Y, Sc, Cr, Th, Zr, Hf and Nb trace elements in combination are powerful tool for provenance and tectonic setting determination due to their relatively low mobility and they are not significantly redistributed in the course of sedimentation, lithogenesis, and metamorphism (McLennan et al., 1983;Fatima and Khan, 2012;Zaid, 2012). A tool that has been used to infer felsic and basic sources in clays and shales from different tectonic environments is the relative distribution of immobile elements such as La, Th Sc, Cr and Co differing in concentration (Armstrong-Altrin, 2009;Bakkiaraj et al., 2010). La and Th are felsic rock-enriched while Sc, Cr and Co are enriched in basic rocks (Wronkiewicz and Condie, 1990). Factors that control chemical composition of sedimentary rocks includes: depositional processes, source rocks, weathering, sorting, tectonic setting and paleoclimate (Bhatia, 1983;Wronkiewicz and Condie, 1990;McLennan and Taylor, 1991;McLennan et al., 1993;Armstrong-Altrin, 2009;Bakkiaraj et al., 2010). Mid-Niger Basin also known as the Bida Basin or the Nupe Basin is a NW-SE trending intracratonic sedimentary basin extending from Kontagora in Niger State of Nigeria to areas slightly beyond Lokoja in the south (Adeleye, 1974;Figs.1 and 2). On the hand, Mid-Santonian deformation in the Benue Trough displaced the major depositional axis westward which led to the formation of the Anambra Basin. Post-deformational sedimentation in the Lower Benue Trough, led to the formation of the Anambra Basin (Figs. 1 and 2)  (Obaje, 2009). The Bida Basin is assumed to be a northwesterly extension of the Anambra Basin (Akande et al., 2005). Although, the sedimentary successions of these two basins are lateral equivalents, their geochemical variations, chemical weathering in the source area, provenance, geological and depositional histories are worth studying. They will help to further understand the geology of the two basins, and to compare the two basins based on the clay-shale geochemistry (Overare et al., 2020).

GEOLOGY AND STRATIGRAPHY
The origin of the Bida Basin is connected with possibly the Santonian orogenic movements of southeastern Nigeria and the Lower Benue Trough of Nigeria (Ojo and Ajakaiye, 1989). It is a NW-SE basin, which extends from Kontagora, Niger State, Nigeria to slightly beyond Lokoja in the south (Figs. 1, 2 and 3). This basin experienced a northeast and southwest separation by the basement complex but connected with Anambra and Sokoto basins containing post orogenic molasse facies and few thin unfolded marine sediments (Adeleye, 1974). The Bida basin can possibly be regarded as the northwestern extension of the Anambra Basin, with its deposition during the major third transgressive cycle of southern Nigeria at the Late Cretaceous times (Agyingi, 1991). Stratigraphically, the Bida basin is divided into two sectors: the Northern Bida and the Southern Bida basin (Agyingi, 1991). The Northern Bida basin is made up of Bida sandstone, Sakpe ironstone, Enagi siltstone and Batati ironstone (Fig. 3) while the Southern Bida basin consists of Lokoja sandstone, Patti and Agbaja ironstone Formation (Agyingi, 1991). On the other hand, the Anambra Basin is located in the southeastern part of Nigeria. It is bordered to the north by Bida Basin (Fig. 3), a NE-SW trending, folded, aborted rift basin that runs obliquely across Nigeria (Obaje, 2009;Fig. 3). Hence its origin was linked to the tectonic processes that accompanied the separation of the African and South American plates in the Early Cretaceous (Murat, 1972;Burke et al., 1971).

METHODOLOGY
Twelve samples (six clay and six shale) were collected from an exposed clay-shale section along Ahoko and Abaji along Lokoja/Abuja highway within the Southern Bida Basin and another twelve samples (six clay and six shale) at Ojodu along Ayingba/Itobe highway within the Mamu Formation in the Northern Anambra Basin. Representative clay and shale samples were collected from different vertical section of the exposures (Figs. 2 and 3) avoiding weathered horizons. Samples were pulverized to 0.07 mm size for XRD and XRF analyses at Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) Hannover, Germany. XRD pattern of the representative claystone-shale samples were determined using a PANalytical X'pert PRO MPD diffractometer equipped with a variable divergence slit (20 mm irradiated length), primary and secondary soller, scientific X' Celerator detector (Active length 0.59), and a sample changer (Sample diameter 28 mm). The samples were investigated from 2 to 85 2 theta with a step size of 0.0167 2theta and a measuring time of 10 s. per step. For the specimen preparation the top loading technique was used For the XRF analysis, powdered samples were analyzed using a PANalytical Axios. Samples were prepared by mixing a flux material and melting into glass beads. The beads are analyzed by wavelength dispersion X-ray fluorescence spectrometry (WD-XRF) for the following oxides and elements determination; SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, MgO, TiO, P 2 O 5 , K 2 O, MnO and LOI, Trace elements such as As, Ba, Co, Cr, Cu, Ga, Hf, Nb, Ni, Pb, Sr, Th, U, Zr and rare earth elements such as Heavy rare earth elements (HREE) e.g. Y and Sc and Light rare earth elements (LREE) e.g. Ce, La, Nd and Sm. To determine loss on ignition (LOI) 1000 mg of sample material was heated to 1030˚C for 10 min. after mixing the residue with 5.0 g lithium metaborate and 25 mg lithium bromide, it is fused at 1200˚C for 20 min. the calibrations are validated by analysis of Reference materials. Monitor samples and 130 certified reference materials (CRM) are used for the correction procedures.

RESULTS AND DISCUSSION
The major oxide concentrations, values of plagioclase index of alteration (PIA), chemical index of alteration (CIA) and chemical index of weathering (CIW) as well as calculated geochemical ratios for the studied Cretaceous sediments from Southern Bida and Northern Anambra Basins are presented in Tables 1 and 2. The obtained results were compared to average shales worldwide (Pettijohn, 1957), NASC (Gromet et al., 1984;Turekan and Wedephol, 1961) and shales from other parts of Nigeria (Tables 3 and 4). The investigated sediments are characterized by high contents of SiO 2 , moderate Al 2 O 3 , small variation in Fe 2 O 3 (Tables 1 and 2) but low in TiO 2 , CaO, Na 2 O and K 2 O. The low K 2 O values indicated a lack of expandable clays in both sediments, such as montmorillonite (Akpokodje et al., 1991) while the low content of TiO 2 and CaO and MgO for all the samples is ascribed to strong weathering (Roy et al., 2008).    The interrelationships between major oxides, some trace and rare elements are given in Tables 5 and 6. The abundance of silica and alumina are attributed to the clayey-silty nature of the samples as well as presence of biogenic SiO 2 as indicated by the SiO 2 -Al 2 O 3 association (Fig. 4A). The observed strong negative correlation between SiO 2 and Al 2 O 3 ; r = -0.96 (Table 5) and r = -0.58 (Table 6) for the sediments from Southern Bida Basin and Northern Anambra Basin respectively, suggest a terrigenous origin for the studied sediments (Moosavirada et al., 2011). This is also supported by the Fe 2 O 3 /TiO 2 vs.  Table 6), this could be attributed to lack of Feoxides in the Northern Anambra Basin. This is true as ironstone of Agbaja Formation is associated with the Southern Bida basin as documented by Adeleye and Dessauvagie (1972).   (Barbera et al., 2006), (B). Bostrom (1973) diagram; analysed sediments are compared to argillite (T) and hydrothermal (H) end members whose mixing is modelled by the H-T curve. PAAS (Taylor McLennan, 1985) and UCC (Rudnick and Gao, 2003) data are reported for comparison, and (C). K2O-Fe2O3--Al2O3 compositional space (Date after Condie, 1993) showing the major element distribution in the clay-shale sediments for both Southern Bida and Northern Anambra basins respectively.
The detrital features and continental crust precursor of the analysed clay-shales are further supported by the high values (0.49-0.97; Tables 1 and 2) of the parameter D*=Al 2 O 3 /(Al 2 O 3 +MnO+Fe 2 O 3 ; Machhour et al., 1994), which connects Al 2 O 3 , strongly assembled in the continental crust (D*=0.79; Taylor and McLennan, 1985). The ternary plot of K 2 O-Fe 2 O 3 -Al 2 O 3 (Fig. 4C) reveals that all of the investigated clay and shales plots close to Al 2 O 3 region, suggesting enrichment in Al 2 O 3 , which also indicates that clay minerals in these clayshale sediments largely controlled the abundance of elements (Wronkiewicz and Condie, 1987). MgO content ranges between 0.01 to 0.14% (Table 1) for clay-shale sediments from Southern Bida Basin and 0.02 to 0.11% (  (Tables 1 and  2). The high variation in value of TiO 2 in some of the investigated samples is an indication of the detrital nature of the sediments in the presence of titanium minerals such as rutile and ilmenite, transported and deposited mechanically (Liu et al., 2009). The positive correlation between Al 2 O 3 and TiO 2 (r = 0.49) for the Southern Bida Basin and Northern Anambra Basin (r = 0.87) suggest terrigenous origin (Saccà et al., 2011). There is an observed general depletion in oxides of Na, K and Ca in the investigated samples (Tables 1 and 2) compared to the Upper Continental Crust values, (3.9 %, 3.4 % and 4.2 %). This could be due to their hydration energy resulting in high mobility during weathering process (Cullers, 1995 (Liu et al., 2013). This is supported with the enrichment of the clayey constituents as Cr, Co, Ni and V elements are readily adsorbed onto clay minerals during weathering process. There was an observed strong positive correlation, (r = 0.69; 0.85; 0.62 and 0.62 ) of Al 2 O 3 concentration with V, Cr, Ni and Co for sediments of the Southern Bida Basin respectively (Table 5). This strongly suggests their presence as adsorbed ions in clay minerals and strongly controlled by the nature of the source rocks (EL-Wekeil and Abou El-Anwar, 2013). On the other hand, within the Northern Anambra Basin, there was a strong positive correlation of Al 2 O 3 with V and Cr (r = 0.72 and 0.91) but insignificantly correlated with Ni and Co (r = r = 0.13 and -0.24) ( Table 6). This must be as a result of renewed tectonic activity within the Northern Anambra Basin during the mid Santonia when the Lower Benue trough was displaced from its major depositional axis westward, thereby disturbing the sedimentation dynamics, possibly resulting in sediment mixing and reworking and finally forming the Anambra Basin (Obaje, 2009). The strong positive correlations between Co and Ni with Fe 2 O 3 (r = 0.77 and r = 0.77; Table 5) for the sediments from Southern Bida Basin and (r = 0.99 and r = 0.88; Table 6) for the Northern Anambra Basin sediments strongly suggest possible association of these two elements with Fe-oxides minerals. Sr and Ba show a general depletion pattern (Tables 7  and 8) compared to UCC (350 ppm and 550 ppm, Fig.  5), this is probably due to their hydration energy resulting in their preferential loss during weathering and erosion (Cullers, 1995 andLiua et al., 2013). The observed positive correlation between contents of Fe 2 O 3 and Ba for the Southern Bida Basin (r=0.799) and the Northern Anambra Basin (r = 0.074) indicates that Ba is mainly associated with Fe-oxides (EL-Wekeil and Abou El-Anwar, 2013). This is in good agreement with reports documented by Adeleye and Dessauvagie (1972) that Southern Bida Basin is a host to deposits of ironstone. The observed negative correlation between Sr and Ba with Zr (r = -0.11 and-0.35;    Ba  47  82  95  127  68  48  61  198  180  179  115  281  Sr  19  24  30  31  20  24  23  45  50  53  48  63  Ni  2  3  6  6  4  2  2  4  6  6  14  7  Co  2  2  3  3  2  2  3  4  3  4  15  4  Cu  10  9  15  20  11  6  10  13  10  11  18  13  Cr  31  40  62  74  53  34  44  48  57  52  43  62  Ga  13  17  22  23  19  14  16  16  16  16  13  18  Hf  9  20  17  13  13  14  20  14  21  21  19  25  Sc  8  9  10  10  8  8  8  8  8  8  11  11  Zn  5  6  11  11  8  6  7  10  16  14  56  15  V  22  41  77  75  50  32  46  44  76  80  48  70  Zr  455  768  644  651  623  640  685  .65%, respectively. The high PIA indicates that nearly all of the plagioclase has been transformed into clay minerals. This is consistent with the calculated CIA and CIW indices that reveal strong or prolonged weathering in the source area (Nesbitt and Young, 1982;Fedo et al., 1995; Tables 1, 2 and 9).  6A) of Nesbith and Young (1982) and Fedo et al. (1995) permit the segregation in compositional variations as related to chemical weathering and/or source rock composition , it demonstrated intense weathering history. The clay-shale sediments sourced from both Southern Bida and Northern Anambra Basins plot close to the high Al 2 O 3 contents (A-Apex), thus revealing a high level of weathering to a point where there is liberation of major amounts of alkali and alkali earth elements from the sediments (Overare et al., 2020). The plot of CIA vs. SiO 2 (Fig. 6B) also proved a similar trend with the investigated samples plotting typically in the upper part between illite and kaolinite, signifying a high degree of weathering. To identify climatic conditions which prevailed in the provenance, a bivariant plot of SiO 2 against total (Al 2 O 3 +K 2 O+Na 2 O) proposed by Suttner and Dutta, (1986) was used. The plot revealed semi-humid and arid conditions for the clay and shales in both basins ( Fig.  6C1 and 6C2). The index of compositional variation (ICV) can be used to categorize the original nature and maturity of the sediments with the prevailed climatic conditions (Cox et al., 1995).  1982;Fedo et al., 1995), (B). Plot of CIA vs. SiO2 (Nesbitt and Young 1982), SiO2 Vs (AL2O3+K2O+Na2O) for claystone-shale sediments of C1: Southern Bida and C2: Northern Anambra Basin showing trend of maturity with reference to climatic conditions (Suttner and Dutta, 1986)

ICV = (Fe 2 O 3 +K 2 O+Na 2 O+ CaO +MgO +MnO)/Al 2 O 3
Values of ICV < 1 are characteristic of minerals such as kaolinite, illite and muscovite whereas higher values points toward plagioclase, K-feldspar, amphiboles and pyroxenes Also, the more mature the sediment, the low the ICV values. Tables 1 and 2 illustrate that the documented ICV values for the investigated Southern Bida basin and Northern Anambra basin sediments ranged from 0.06 to 0.33 and 0.05 to 1.10, respectively. Hence, these sediments are considered to be highly matured and are derivative of intense chemical weathering. This may also imply distinctive clay rich mature sediments associated with either tectonically inactive or intracratonic setting, where sediment recycling is dynamic (Cox et al., 1995) or from intensely weathered crystalline basements, which is consistent with Fig. 6D established by Long et al. (2012) for the estimation of sediment maturity and weathering intensity. This also conforms well to the results obtained from the association between SiO 2 and (Al 2 O 3 +Na 2 O+ K 2 O) shown in Fig. 6C1 and 6C2. Relationship between Th/U ratio and Th concentration can be used to estimate the degree of weathering in sedimentary rocks. The observed result indicates intense weathering in the source areas or sedimentary recycling for the clay in both basins but the shale indicative of moderate to high weathering (Tables 10  and 11).

Paleo-Environmental Conditions and
Provenance of the Clay-Shale deposit Sr/Ba ratio have proven to be a very important tool employed by previous studies to estimate paleo-salinity (Chen et al., 2016). Sr/Ba ratios >1, 0.5-1, and < 0.5 propose seawater, brackish water, and freshwater conditions, correspondingly ( Li et al., 2018). The Sr/Ba ratios (0.16-3.50; Table 10) for the investigated clay-shales from the Southern Bida basin reveal an array of depositional paleo-environment that alternated between marine and continental settings. On the other hand, the recorded Sr/Ba ratios (0.22 -0.50; Table 11) revealed a continental setting for the Northern Anambra basin. In addition, K 2 O/Al 2 O 3 and MgO/Al 2 O 3 diagram of Roaldset (1978) was used to differentiate between marine and non-marine sediments. The Southern Bida and Northern Anambra basin's clayshales sediments plot in the non-marine to slightly near marine environment (Fig. 7A), suggesting a transitional/mixed environment of deposition, consistent with bivariant plot of V vs. Al 2 O 3 ( Fig. 7B; Mortazavi et al., 2014). This is supported by the chemical classification on the basis of (Al 2 O 3 )-(K 2 O+Na 2 O+CaO)-(Fe 2 O 3 +MgO) contents (AKF) proposed by Englung and Jorgensen (1973) were the clay -shale sediments under investigation plots in the continental zone and in an argillaceous area (Fig. 7C1-C2). Trace element geochemical ratios like Ni/Co and V/Cr have proved to be reliable tools for redox determination in depositional environments (Jones and Manning, 1994; 108 ATABO NATHANIEL ODOMA AND SUNDAY OJOCHOGWU IDAKWO Nagarajan et al., 2007;Madhavaraju et al., 2016). Jones and Manning (1994) proposed that V/Cr ratios < 2 infer oxic conditions, 2-4.25 dysoxic conditions, and > 4.25 suboxic to anoxic conditions. They also found that < 5 Ni/Co ratios assumed oxic conditions, 5-7 means dysoxic conditions while > 7 imply suboxic to anoxic conditions. The Ni/Co and V/Cr of the investigated clayshale sediments (Tables 10 and 11; Fig. 7D) suggest oxic environments. In addition to the paleo-environmental conditions, other trace elemental ratios like U/Th and Cu/Zn were also considered. According to Jones and Manning, (1994), ratios of U/Th < 0.75 infers oxic conditions, while U/Th ratios > 1.25 indicate anoxic conditions. Moreso, Hallberg (1976) proposed that low ratios of Cu/Zn in a basin reveals oxidizing depositional conditions, whereas high Cu/Zn ratios suggests reducing depositional conditions. The primarily high ratios of U/Th (1.93-5.67; Table 9) and Cu/Zn (0.19-5.00; Table 9) for the Southern Bida basin affirms a more reducing or an anoxic condition of deposition for the studied clay-shale sediments while the predominantly low ratios of U/Th (0.17 -0.25; Table 10) and Cu/Zn (0.19 -2.00; Table  10) for the Northern Anambra reveals an oxic condition of deposition for sediments studied. Authors like Taylor and Mclennan, 1985;Condie et al, 1992;Cullers, 1995;Armstrong-Altrin et al., 2004;Madhavaraju et al., 2016;Chen et al., 2016 Bas et al., 1986) was employed. The investigated clay -shale sediments for both basins plot in both intermediate and felsic fields (Fig. 8A), indicating contributions from both sources. Trace elements such as Y, Th, Nb, Zr, Hf, and Sc with REE in clastic sedimentary rocks are useful tool for their source rock interpreting. These elements have very low mobility during sedimentary processes, and are probably quantitatively transferred to the clastic sediments during the weathering, transportation, and diagenetic processes, thus reflecting the signatures of the precursors (e.g., Bhatia and Crook, 1986;McLennan, 1989;Armstrong Altrin et al., 2004). Different authors have also employed geochemical variations between elements such as La and Th (felsic source rocks) and Sc and Cr (mafic source rocks) to distinguish between felsic and mafic origin (Ramachandran et al. 2016). The plot of Th/Co vs. La/Sc (Fig. 8B) proved that the clayshale sediments for both basins were derivative of felsic origin. This was supported with Hf vs. La/Th diagram ( Fig. 8C; Floyd and Leveridge, 1987), TiO 2 (wt. %) Vs. Ni (ppm) diagram ( Fig. 7D; Floyd et al., 1989), V-Ni-Th*10 ternary relationship ( Fig. 9A; Bracciali et al., 2007) and the Th/Sc vs. Zr/Sc relationship ( Fig. 9B; McLennan et al. 1993;Willan, 2003). The plot of Th/Sc vs. Zr/Sc indicated an igneous differentiation trend, close to PAAS composition for the clay -shale sediments under investigation, therefore, no significant signs of heavy mineral concentration due to zircon enrichment. Thus, the recycling of older mature sediments of felsic and crustal origin probably did not play a significant role (Mikes, 2006). In addition, Co/Th vs. La/Sc diagram (Fig. 9C) demonstrated a main distribution between felsic and andesitic sources, while the La-Th-Sc relationship ( Fig. 9D; Cullers, 1994a, b) revealed contributions from felsic sources, intermediate sources or possible mixing between felsic and basic source rocks.  Ternary plot for the shales after Cullers (1994a) compared with Post-Archean Average Shale (data is from Taylor and McLennan, 1985).
La/Sc, Th/Co and Th/Sc ratios are significantly different in felsic and basic rocks which can be used to infer provenance composition (Wronkiewicz and Condie, 1990;Cox et al., 1995;Cullers, 1995). The Th/Co, Th/Sc and La/Sc ratios for shale and clay samples from this study were compared to those of felsic and basic rockderived sediment (fine fraction) upper continental crust (UCC) and PAAS values (Tables 10, 11 and 12). These comparisons also indicated that such ratios came within the range of felsic source rocks.  Roser and Korsch, 1986) reveals that the clay -shale sediments for both basins plot entirely in the field of passive margin.
Passive-margin type sediments are generally enriched in SiO 2 and depleted in Na 2 O, CaO and TiO 2 , thereby revealing their highly recycled and matured nature (Bhatia, 1983). Major element analysis of the studied shale and clay samples confirm this, as all these samples were enriched in SiO 2 but depleted in Na 2 O, CaO and TiO 2 .  The compositional values for clay and shale samples' rare earth element (REE) are given in Tables 13 and 14. A slight variation in sum of REE content was observed between the claystone and shale samples;  and (267-459) for Southern Bida, (138-255) and (142-234) for the Northern Anambra Basins, respectively, these values were compared with notable world averages (Table 15). The enrichment of LREE and moderately negative Eu anomaly reflect their relative abundance in the crust, while the depletion of the HREE is due to their ability to form soluble complexes in seawater.  Ce  78  191  70  221  34  54  196  174  205  154  252  101  La  43  110  49  143  28  35  111  73  93  67  147  53  Nd  26  84  25  82  12  15  83  66  75  36  62  26  Sm  15  19  13  17  13  13  13  13  13  13  13