Major and Trace Element Evaluation of the Campano-Maastrichtian Sediments of Anambra Basin Exposed around Enugu , Nigeria

An integrated geochemical and sedimentological study of the CampanoMaastrichtian sediment of Ajali and Owelli Formation exposed within the southern portion of the Anambra basin was undertaken to determine the sandstone provenance, tectonic setting, and paleo-weathering conditions. All sandstone samples are enriched in quartz (Q) but poor in feldspar (F) andlithic-fragments (L). The major-element concentrations of these sandstones reveal the relative homogeneity of their source. Geochemically, the sandstones are classified as quartzarenite, sub-litharenite, and subarkose. Tectonic setting discrimination diagrams suggest a passive continental margin. The sediments are enriched in SiO2 and depleted in Na2O, CaO and TiO2.Chemical index of weathering (CIW) of the sample suggests intense recycling and high degree of weathering in a humid climatic condition. Results of the elemental ratios critical of provenance compared to those of felsic and basic derived sediment of UCC and PAAS values suggests a felsic source rock. Cu/Zn, U/Th, Ni/Co and (Cu+Mo)/Zn paleo-oxygenation ratios used as redox parameters depicts an oxic environment. Finally, presence of minerals, like Anatase, Rutile and Microcline depicts an igneous and metamorphic source terrain. DOI: https://dx.doi.org/10.4314/jasem.v23i1.17 Copyright: Copyright © 2019 Haruna and Ojo. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 17October 2018; Revised: 05 January 2019; Accepted 17 January 2019

The Anambra Basin is one of the Cretaceous sedimentary Basin of Nigeria, bounded on the Southwestern flank by the Niger Delta hinge line, northwest by the Benue flank and southeast by the Abakaliki fold belt.The Basin is roughly triangular in shape and covers an area of about 40,000 square kilometres with sediment thickness increasing southwards to a maximum thickness of 12,000m in its central part.It was developed during the late Campanian to early Maastrichtian allocyclic incursion of the sea and this has been of great significance to several workers interested in the post-Santonian deposition.The basin is one of the subbasins of the Benue rift structure which form a part of the West African Rift System (WARS).The combination of petrographic and geochemical data of sedimentary rocks can reveal the nature, source regions, tectonic setting and paleoclimatic conditions of sedimentary basins (McLennan et. al., 1993;Armstrong-Altrinet. al., 2004).Paleoenvironments of deposition of the sediments in the basin were deduced on the basis of textural attributes of strata inferred by Nwajide (1990) and Nwajide and Reijers, (1996).Fluvio-deltaic to fluviatile environment has been suggested for Ajali Formation by various workers, which include Hoque and Ezepue (1977).Paleocurrent direction and paleogeography of the formation based on inferred exposed crossbedded sandstones indicated that the sediment transport was generally in NE-SW direction.Hoque and Ezepue (1977) have classified the Ajali Sandstone as a quartz arenite on the basis of four parameters earlier mentioned.The present scheme is based on log plots of Fe2O3/K2O and SiO2/Al2O3 of Herron (1988).This study focuses on the Campano-Maastrichtian (Ajali and Owelli) sediments of Anambra basin exposed around Enugu in order to infer the provenance and paleotectonics using certain analyses such as X-ray diffraction, X-ray florescence, and petrographic analysis.The study area falls between latitudes 6°00 I N-6°30 I N and longitudes 7°12 I E-6°42 I E (Fig. 1).

Geologic Setting and Stratigraphy of the Study Area:
The Benue -Abakaliki Trough, originated in the early Cretaceous as a failed rift associated with the opening of the south Atlantic (Wright et al, 1985).In early Albian -Coniacian periods, two stable areas could be distinguished on either side of the Benue Abakaliki Trough, namely, the Anambra and the Ikpe platforms in the west and east of the southern trough respectively.The origin, tectonic evolution and stratigraphic settings in the Benue Trough and the Anambra Basin have been reviewed and widely discussed by several authors (Murat, 1972;Kogbe, 1976;Petters, 1977;Hoque and Nwajide, 1984;Benkhelil, 1987;Reijers and Nwajide,1998).The sedimentary infilling of the Southern Benue Trough can be grouped into three (3) major unconformity bounded depositional successions viz;(a) The first phase (Albian -Cenomanian), (b) The second phase (Turonian -Coniacian), (c) The third phase (Campanian -Pliocene) (Table .1).Methodology: The method adopted in this study involved both field studies and laboratory analyses.The mapping exercise involved identifying the various rock types and establishing stratigraphic succession of the rocks on the basis of their field relationships.Field observations including texture, colour, grains orientation, mineralogical composition, cross-beddings, thickness andlateral extent of beds, and logging of exposed vertical sections were done .Ten (10) samples from Ajali and Owelli formations were selected for Bulk geochemical analysis using standard preparatory techniques.The analysis sampled for major elements (oxides), trace and rare earth elements (RRE) using X-ray Fluorescence (XRF).The x-ray diffraction was analyzed using a PanalyticalX'Pert Pro powder diffractometer with an X'Celerator detector and variable divergence and fixed receiving slits with Fe filtered Co-Kα radiation (λ=1.789Å).The phases were identified using X'Pert High score plus software.Petrographic analysis was carried out in order to access the maturity of the sandstone at the Geology laboratory of the University of Ibadan, Ibadan, Oyo State.

RESULTS AND DISCUSSION
Sandstone Classification:Major and trace elements analyses of the 10 sandstone samples from Ajali and Owelli formation in the studied area are listed in Table 2. Results showsmost of the samples are high in SiO2with the Ajali sandstone having a range of91.59 to 95.2%and the Owelli sandstone ranges from55.59 to 95.4%.The Al2O3 contents is low (Ajali;and Owelli;).Ajali and Owelli formations have low contents of TiO2, MnO, CaO,Na2O, Fe2O3 and MgO respectively.The high K2O/Na2O ratios (Table 3) are attributed to the relative presence of K-bearing minerals such as Kfeldspar and some mica (Kalsbeeket al., 2008;McLennan et al., 1983;Nathet al., 2000;Zhang, 2004;Osaet al., 2006).A positive correlation between K2O and Al2O3 implies that the concentrations of the K-bearing minerals have significant influence on Al2 distribution, this suggest that the relative abundance of these elements is primarily controlled by the contents of the clay minerals (McLennan et al., 1983).Based on these ratios, some of the studied sandstones samples can be classified chemically as sub-arkose (En2), Fe-sand (Enb1, Enb2, Enb4, Awgb1, Uwak1, Uwak2, Ota1) and Awgb2 as Fe-shale (Fig. 2a and 2.b).These results are further supported by low Al2O3/SiO2 ratios which further classifies some of the samples as being quartz-arenites (Ena1) as shown in Fig. 2.a and 2.b (Pettijohnet al., 1987)  Mineralogy: The X-ray diffractograms (XRD) for the whole rock samples from Ajali and Owelli formation shows the presence of minerals such as Quartz, Kaolinite, Microcline, Hematite Zircon, Rutile, Anatase, pyrite and Muscovite.Quartz is the dominant mineral in all the samples analysed, followed by kaolinite while, Microcline, Hematite, Zircon, Rutile, Anatase, pyrite and Muscovite are present as minor minerals (Table 4).Higher percentage of Quartz in the mineralogical analysis (XRD) and presence of minerals like Microcline, Anatase and Rutile enhances the compositional maturity of these sediments.The kaolinite proportion that follows may be derived from igneous rock, metamorphic rock and sedimentary rocks during frequent rainfall and sufficient hydrolysis under tropical to subtropical climate (De Visser, 1991).
Intense weathering of the source makes kaolinite present as its end product.Kaolinite is also a common product of weathering and degradation of feldspar and the other alumina-silicates, occasionally proceeding to Aluminium oxides and hydroxides (Grim, 1968).Therefore, it can be concluded that these sediments are from mixed environments of felsic and an intermediate igneous and metamorphic origin.The low concentrations of ferromagnesian trace elements such as Cr, Ni, Sc and V in the Ajali and Owelli formation (Table 2) indicates very minimal mafic rocks were exposed in the source area.The unusual Co enrichment in sample Uwak1 with respect to average upper continental crust (Table 2) may suggest some input of mafic materials from the source terrain; however, the simultaneous depletion of Cr, Ni, and V in all the samples suggests that other factors such as post-depositional alterations might have played a role in concentrating Ni in the sandstones.Furthermore, ratios of, La/Sc, Th/Sc, and Cr/Th are significantly different in mafic and felsic source rocks and can therefore, provide information about the provenance of sedimentary rocks (Amstrong-Altrin et al., 2004).These ratios in both formation are similar to those of sediments derived from felsic source rocks (Table 5).The Th/Co, Th/Sc and La/Sc ratios for Ajali and Owelli samples from this study (  Source-Area Weathering: Alteration of igneous rocks during weathering results in depletion of alkali and alkaline earth elements and preferential enrichment of Al2O3 in sediments.Therefore, the weathering history of ancient sedimentary rocks can be evaluated in part by examining relationships among the alkali and alkaline earth elements (Nesbitt and Young, 1982).A good measure of the degree of chemical weathering can be obtained by calculating the Chemical Index of Alteration (Nesbitt and Young, 1982) and Plagioclase Index of Alteration (Fedoet al., 1995).The CIA (chemical index of alteration) and CIW (chemical index of weathering) values have been established as a general indicator of the degree of weathering in any provenance region (Nesbitt and Young, 1982).High CIA and PIA values (i.e., 75-100) indicate intensive weathering in the source area whereas low values (i.e., 60 or less) indicate low weathering in source area.In the studied samples of Ajali and Owelli formation, the CIA and PIA values of both formations are high (i.e., 78-99; 97-99 and 97-99; 99-100 respectively) as shown in Table 6.Most of the samples have CIA and PIA values greater than 90 indicating very high and intense weathering conditions in the source area.Based on the Q-F-R ternary diagram of Suttner et. al. (1981), the climatic setting at the time of depositionof bothAjali and Owelli plot on the metamorphic humid field (Fig. 2), which suggest that the parent rocks were situated in a humid climatic setting.Rb/Sr ratio value of the samples from the investigated areas ranges between 0.37ppm -5.22ppm (Table 5), as compared to standards of Mclennan (1983) suggests highly weathered source area.Mclennan(1993)   Paleotectonics: Sedimentary rocks have been used to confine Provenance and Paleotectonic setting by many specialists (Dickinson et al, 1983;Bhatia 1983;McLennan et al, 1993).The main assumption behind sandstone provenance studies is that, different tectonic settings contain characteristic rock types which when eroded produce sandstones with specific compositional ranges (Dickinson, 1985).The analysis of sandstones with known provenance has been used to define these ranges from which the provenance of other samples can be deduced.With this, Dickinson and co-workers have related detrital sandstone compositions to major provenance types such as stable cratons, basement uplifts, magmatic arcs and recycled orogens (Dickinson and Suczek, 1979;Dickinson et al., 1983).In the QFL and QmFLt ternary diagrams after Dickinson et al. (1983) shows that the analyzed samples plot exclusively in the craton interior field (Fig. 6).As pointed out by Dickinson et al. (1983), sandstones plotting in this field are mature sandstones derived from relatively low-lying granitoid and gneissic sources, supplemented by recycled sands from associated platform or passive margin basins.The studied samples of Ajali and Owelli units plot within the passive margin using a discriminant function diagram (Bhatia 1983).(Fig 4).The Roser and Korsch (1986) plot of log (K2O/Na2O) VsSiO2 discrimination diagram indicated a passive-margin tectonic setting (Fig. 7).The passive-margin comprised Atlantic-type rifted continental margins developed along the edges of the continent, remnant ocean basins adjacent to collision orogens, and inactive or extinct convergent margins.Intra-cratonic and rift-bounded grabens (e.g. the Benue trough) were formed on thick continental crusts which were included in the passive-margin type tectonic setting.The studied Ajali and Owelli samples Passive-margin type sandstone are generally enriched in SiO2 and depleted in Na2O, CaO and TiO2, suggesting that they are highly recycled and mature in nature (Bhatia, 1983).The major element geochemistry of Ajali and Owelli samples discussed in terms of discrimination diagrams to characterize the tectonic setting as proposed by Bhatia (1983) and Kroonenber (1994), confirmed that the Ajali and Owelli Sandstone ware deposited in a passive continental margin.
Conclusion:Owelli Formation were derived from felsic source rocks, probably a quartzose recycled provenance from plate interior or stable continental areas while Ajali samples were derived from mafic to intermediate sources.A passive continental margin was suggested for both Formations.Cu/Zn ratiossuggest oxidizing condition or oxic environments.U/Th values shows deposition in an oxic environment.CIA and PIA values indicate very high and intense weathering conditions in the source Q-F-R ternary diagram plotted suggests humid climatic setting for the parent rocks.Quartz mineral greater than 90% suggests maturity of the sandstones with an angular to sub angular which suggests that they have not being transported far away from their source.

Fig. 1 :
Fig.1: Location Map of study Area On the south eastern-most part of Trough, the NW-SE trending structure, the Calabar flank consisting of the Ikang Trough, the Itu High as well as the Eket platform persisted without significant change right into the Tertiary (Murat, 1972).Tectonic movement resulted in the Santonian epierogenic uplift and folding of the Albian -Coniacian sediments into the

Table 2 .
Results of major oxides, trace and REE (ppm) of Ajalli and Owelli Formation

Table 3 .
Ratios of some major and trace element in the studied samples

Table 4 .
List of minerals identified by XRD on different samples of the two formations

Table 5 .
Range of Ajali and Owelli Formation elemental ratios in this study as compared to ratios of similar fractions derived from felsic rocks, mafic rocks, Upper Continental Crust and Post-Archean Australia shale

Table 6 .
PIA and CIA Values Used in Deducing Source-area

Table 7 .
Showing XRD mineralogical composition of studied sample