Geochemistry of Maastrichtian Clastic Sedimentary Rocks from Western Flank of Anambra Basin , Nigeria : Implications for Provenance , Tectonic Setting , Paleoclimate and Depositional Paleoenvironment

The geochemistry, provenance, tectonic setting, paleoclimate and depositional paleoenvironment of Maastrichtian clastic sedimentary rocks from the western flank of Cretaceous-Tertiary intracratonic Anambra Basin, Nigeria have been studied through major and trace element geochemical analysis. Ten (10) representatives outcrop samples collected from two stratigraphic sections in the study area were analyzed chemically using Inductively Coupled Plasma Atomic Emission Spectroscopy, ICP-AES (Lithium Borate Fusion). The geochemical data shows that the sediments are enriched in major oxides including SiO2, Al2O3, Fe2O3 and TiO2 but depleted in other major element oxides. Also, the sandstones have higher concentrations in SiO2, CaO and Na2O compared to the associated mudrocks whereas the mudrocks are more enriched in Al2O3, Fe2O3, MgO, K2O, TiO2 and P2O5 compared to the sandstones. The mudrock samples have higher concentration of Ba, Ni, Sr, Y, Nb, Sc in comparison to sandstones. However, sandstone samples have higher concentration in Zr (1098.50 ppm) than the mudrocks (1038 ppm). The sandstones are classified as litharenites, sublitharenites, Fe-rich sandstones, while the mudrocks are labeled as shale and Fe-rich shale based on chemical composition. Analysis of the data shows a dominantly felsic igneous to intermediate igneous and quartzose sedimentary provenances of mixed granite and granodiorite composition as indicated by various discriminant plots. The inferred provenance is corroborated by the low concentrations of Cr, Ni and Sc in the samples and other proxies. On the basis of various tectonic setting discriminant function diagrams, the Maastrichtian sediments from the study How to cite this paper: Ogbahon, O.A. and Olujinmi, O.B. (2019) Geochemistry of Maastrichtian Clastic Sedimentary Rocks from Western Flank of Anambra Basin, Nigeria: Implications for Provenance, Tectonic Setting, Paleoclimate and Depositional Paleoenvironment. International Journal of Geosciences, 10, 91-118. https://doi.org/10.4236/ijg.2019.101007 Received: November 29, 2018 Accepted: January 28, 2019 Published: January 31, 2019 Copyright © 2019 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
The study area with geographic coordinates defined by latitudes 7˚07'N and 7˚13'N longitudes 6˚21'E and 6˚31'E is situated on the western flank of Anambra Basin in southern Nigeria.The outcropping sedimentary successions investigated in the current study display the characteristics of Mamu Formation (Figure 1).The Anambra Basin is a Mesozoic-Cenozoic petroliferous and coal-bearing basin located within the southern Benue Trough which evolution is related to the opening of the Gulf of Guinea and the separation of African and South American Plates sequel to the breakup of Gwandana supercontinent during Late Jurassic to Early Cretaceous times.The NE-SW oriented basin has a roughly triangular outline and covers about 40,000 Km 2 [1] with sedimentary thickness of over 5000 m.The basinal fill consists of a wide spectrum of lithologies both in lateral and temporal relationships including limestone, marl, shale, coal, sandstone and siltstone.The occurrence of favorable stratigraphic architecture of interbedded sandstones and shales alongside sparse limestones [2] in the southern Benue Trough spurred the first interest for hydrocarbon exploration in the early 1900s.The effort was rewarded by the discovery of commercial coal deposit in the Anambra Basin in 1916.The initial discovery of petroleum was not in commercial quantity.The later discovery of commercial quantity of hydrocarbon in contiguous Niger Delta in 1956 by Shell D Arcy rekindled research interests of authors to extend the exploration efforts to other basins in Nigeria, including Anambra Basin.These efforts have led the discovery of commercial quantity hydrocarbon in the basin.Consequently, there are plethora of publications on petroleum geochemical studies [e.g.[3] [4] [5] [6] [7]] of Anambra Basin available in literature.There is however a dearth of publications on provenance studies in the basin.The few available provenance studies [e.g.[8] [9]] applied the petrographic approach which allows one to make deductions only from analysis of modal composition of the framework grains of sandstones or coarse-grained sediments while excluding the matrix component and the as- sociated mudrocks in the stratigraphic section.According to Eynatten1 et al. [10], petrographic provenance study is prone to high methodical errors (counting statistics) in the quantification of individual variables.Furthermore, the mineralogical and chemical composition of coarse grained sedimentary rocks frequently used for petrographic analysis may be significantly different from that of the parent material because of the effect of hydraulic sorting of grains controlled by grain size, grain shape and density of sedimentary particles.In consideration of the aforementioned reasons, Blatt [11] and Roser and Korsch [12] suggested that mudrocks and shale should be used more extensively in provenance study.
The sedimentary fill of a basin is controlled by a complex interplay between sediment supply, tectonic subsidence and sea level fluctuations and preserves the record of source rock composition, paleoclimate, paleogeography and tectonic history.The importance of geochemical provenance interpretations cannot be overemphasized.It provides useful information that permits better insight into the evolutionary history of a basin and it is an integral part of basin analysis.Basin analysis is an integrated programme of study that involves application of sedimentologic, stratigraphic, and tectonic principles to develop a full understanding of the rocks that fill sedimentary basins and to use this information to interpret the geologic history and evaluate the economic importance of these rocks [13].According to Jian et al. [14] provenance analysis has industrial implications such as helping to evaluate the relationship between source and sink, discriminating sedimentary systems and depositional areas and thus facilitating oil and gas exploration.Inorganic geochemical study of sediments is an important tool in the determination of provenance, environment of deposition and paleo-weathering at source area.In addition, it serves as major tool in the discrimination of tectonic settings.According to Herron and Herron [15], the introduction of modern chemical analytical techniques (X-ray fluorescence, energy dispersive X-ray spectrometry, and atomic absorption spectrometry, and so many more) has enabled a large number of high-quality chemical analyses of rocks, including sandstones and shale, to become available without corresponding detailed petrographic study.Despite its usefulness however, provenance analysis of analysis of Anambra basin fill has received only little attention.This study therefore seeks to utilize major and trace element geochemical analysis to unravel the provenance, tectonic setting and paleoclimatic conditions of sediments of Mamu Formation exposed in parts of the western flank of Anambra Basin.

Geology and Stratigraphy of Anambra Bsasin
The study area is situated in the western part of Anambra Basin (Figure 1 & Figure 2).The basin is one of the structural features of the Southern Benue Trough.Topographically seen as roughly triangular shaped NE-SW trending siliciclastic wedge, Anambra Basin covers an area of about 40,000 km 2 with sediment thickness increasing southwards to maximum of roughly 6000 m [1].The International Journal of Geosciences  Anticlinorium became the topographic provenance that sourced the bulk of the sediments in the Anambra Basin [18] [19]: Prior to the Santonian tectonic event, the proto Anambra Basin was a platform thinly draped by sediments [20].Major sedimentation in the Anambra Basin began in the Campanian with a short marine transgression followed by regression (Figure 3).The basal lithostratigraphic unit of the Anambra Basin sedimentary sequence is the Nkporo Group, consisting of Nkporo shales, Enugu Shale and Owelli Sandstone as its lateral equivalent [21].
The Nkporo Shale is composed of dark grey, very fissile but soft shale and sandstone with occasional thin interbeds of sandy shales, sandstone and marl with Sulphur coatings [2].The formation is highly fossiliferous, consisting of both marine and terrestrially derived fauna, characterized significantly by dominant organo-facies responsible for hydrocarbon source system [22].The Enugu Shale is restricted to the central and northern parts of the Anambra Basin and consists of carbonaceous grey black shale and coals with interbeds of very fines and sandstone/siltstone deposited in lower flood plain and swampy environment.The Owelli Sandstone consists of massive, hard ferruginous cross-bedded sandstone.The sandstone is fine to coarse grained, poorly sorted, and may contain pebbles which are poorly sorted and well-rounded while fine grains are subrounded [2].The sandstone contains interbeds of white to brown laminated siltstones and carbonaceous claystones and it is mineralogically mature but texturally immature [23].
Subsequent marine regression in the Maastrichtian resulted in the deposition of Mamu, Ajali and Nsukka Formations.The Maastrichtian Mamu Formation (formerly called Lower Coal Measures) consists of alternation of fine to medium grained, well sorted sandstone, dirty grey shales and sandy shale, carbonaceous shale,, siltstones, including five bituminous coal seams which range from tens of centimeters to about 3.5 m in thickness in Enugu area [17].The siltstone facies contains abundant burrows of Teichichnus and Thalossinoids and other ichnogenera, mostly horizontal burrow types [17].The coal and shale facies contain abundant fossil flora especially those of continental and freshwater origin as well as fauna, especially foraminifera such as Textularia hockleyensis, Haplophrogmoids hausa [17].Also, frequent occurrence of channel cuts in the fine-graind sandstone facies was reported by Nwajide [17].Coal beds together with carbonaceous shales are present towards the basal part of the formation.The occurrence of coal suggests that the formation was deposited in paludal, deltaic and possibly shallow marine settings [24].The sedimentary succession of Mamu looks like a typical cyclotherm and is succeeded by Ajali Formation.The Ajali Formation (formerly False Bedded Sandstone) lies conformably on Mamu Formation and consists mainly of thick, friable, moderately sorted, whitish medium International Journal of Geosciences to coarse grained, burrowed sandstone, typically white with large scale occasionally clay-draped cross-bedding [25].Odundun [26], while making reference to the work of Adegoke [27]

Major Elements
The result of geochemical analysis performed on selected rock samples from the study area is presented in Table  ) has proven to be a useful discriminant tool for the classification of clastic sedimentary rocks on the basis of their major element composition.
The shale samples were classified mainly as Fe-shale and shale, while the sandstone samples fell in sub-litharenite, litharenite and Fe-sandstone fields (Figure 4).According to this scheme, sandstone sample from the study area are predominantly litharenites and sublitharenites, which suggests that they are chemically immature.This deduction is however negated by the high SiO 2 and CIA values.

Trace Elements
An overview of some trace element concentration and elemental ratios are presented in

Chemical Composition
The chemical composition of siliciclastic sedimentary rocks is controlled by numerous factors such as weathering, sediment transport and depositional environment.The most essential of these factors include 1) composition of parents rocks, 2) grain size sorting effect during sediment transport and deposition [34], 3) chemical weathering during the entire sedimentary process and outcrop exposure [36], and 4) burial diagenesis and metamorphism [35].According to Jian et al. [14], it is necessary to evaluate the effects of the last three factors of chemi-International Journal of Geosciences cal composition of sediments before drawing conclusion on provenance, paleoclimate and tectonic setting of the depositional basin.
Hydrodynamic sorting of detrital particles on the bases of density, grain size and particle shape during sediment transport and deposition results in mineralogical heterogeneity and consequently chemical differentiation.The effect of grain size arising from hydrodynamic sorting can be easily observed on the geochemical data presented in Table 1 and Table 2 grain size differences is the result of hydrodynamic sorting during sediment transport and deposition leading to coarser grained sediments being generally richer in SiO 2 and K 2 O (wt%) contents as a result of their higher quartz and feldspar contents compared to finer grained ones.Compositional differences in the studied samples are manifest in the 4 various classes depicted by the geochemical classification scheme of Herron [33].The plot identified the sandstones as Fe-sandstone, sublitharenites and litharenites while the mudrocks are designated as Fe-shale, shale and sublitharenites (Figure 4).This separation of samples into different classes is the result of variation in grain size with medium-or coarse-grained sediments designated mainly as litharenite, sublitharenites and  values of samples for the present study are generally low both for the sandstones (0.00 -0.05) and shales (0.00 -0.01), suggesting the presence of insignificant amount of alkali feldspar in the original sediments.

Provenance
The composition of sediments is typical of certain assemblages of igneous, metamorphic and sedimentary provenances that have unique style of sedimentary history [34].Some major and trace elements are considered to be very useful for constraining the composition of provenance and tectonic setting as a result of time in saline water [29].Since these elements are thought to fractionate only a little during continental erosion, they are distributed proportionately into clastic sediments, thereby reflecting the chemical signature of the parent materials [29] [41].Two commonly used provenance discrimination diagrams are those of [12] and [42].Roser and Korsh [12] introduced two major elements discriminant function diagrams to constrain provenances and compositions of source rocks.
The first diagram is best suited for sediments containing little or no biogenic fraction while the second one is adapted for sediments with significant biogenic content.The diagrams differentiated four types of source rocks: 1) Mafic igneous (basaltic and subordinate andesitic detritus), 2) intermediate igneous (dominantly andesitic detritus) 3) Felsic (acidic plutonic and volcanic detritus), and 4) quartzose sedimentary (recycled detritus).Samples for this study contain no significant biogenic content based on geochemical signature of very low CaO concentration and judging from megascopic examination of samples.Plots on the relevant provenance diagram of [12] indicates that majority of the samples fall well within the quartzose sedimentary provenance field with only one sample falling in the felsic igneous field, suggesting tectonic uplift and sedimentary recycling (Figure 5).The spread of the samples may result from compositional variations in the protoliths.This result suggests a significant contribution from recycled detritus or intensive sediment reworking.Recycled sedimentary provenance interpretation is in conformity with the consensus claim by some authors in the basin, [e.g.[4] [18]] that recycled sediment from the Abakaliki anticline located E of the study area was the major source of terrigenous input to the Santonian Anambra Basin.The provenance indicated by [12] discriminant diagram is however different from the one depicted by [42].Hayashi et al. [42] employed bivariate plot of TiO 2 against Al 2 O 3 to discriminate among four provenance types: basalt, basalt-granite, granite-basalt and granite sources.The plot shows that half of the samples (five) fell in the granite field, two in the granite-basalt field, two in the basalt-granite field and one in an undefined field (Figure 6).
They also employed bivariate plot of TiO 2 vs. Zr to discriminate three provenance types: mafic igneous, intermediate igneous and felsic igneous rock provenances.Provenance interpretations from TiO 2 and Zr ratios is premised on the fact that these elements Ti and Zr) are associated with some of the most stable heavy minerals (rutile and Zircon) in siliciclastic rocks and are therefore thought to preserve the geochemical signatures of the parent rocks [37] [43].This diagram (Figure 7) indicates that most of the analyzed samples fall in the felsic igneous field with only one sample falling in the intermediate igneous rock field, indicating a provenance consisting dominantly of felsic igneous rocks with minor contribution from intermediate igneous rocks.A comparison of the sample distributions in the two [42] provenance schemes indicates that there are no significant differences in outcomes if it is assumed that their basalt-granite and granite-basalt fields are equivalent to intermediate igneous field except for one sample with an undefined provenance.It is however difficult to decide the provenance of the sample that fell in the undefined field.Hayashi et al. [42] failed to recognize quartzose sedimentary (recycled detritus) provenance.

Tectonic Setting
Deductions on the tectonic setting of the depositional basin can be made by evaluating the geochemical signatures of the sediments.to discriminant among samples from three main tectonic settings namely ocean island arc margin (ARC, active continental margin (ACM), and passive margin (PM).The samples plot both in passive margin (60%) and active continental margin (40%) regions (Figure 8).This result is consistent with the low to high and two low silica types.The discriminant functions diagrams of [45] have been used to constrain the tectonic setting of the sediments of the study area (Figure 9).All silica-rich samples plotted in the continental rift region.One of the two low silica samples plotted in the continental rift field while the other one plotted in collision region of [45] diagram.We prefer the result of the tectonic setting depicted in [45] diagrams because their diagrams have been proven to have higher success rate compared to that of [44].Their diagrams were able to successfully discriminate among sediments collected from different known tectonic settings at success rate of 84.5% to 93.6% whereas the former diagram recorded a maximum success rate of 62% which is considered unsatisfactory [45].Therefore the analyzed samples from the study area were deposited mainly in continental rift/ passive margin setting.

Source Area Weathering and Paleoclimate
The effect of chemical weathering on the composition of siliciclastic sedimentary rocks can be evaluated by measuring the degree of chemical weathering.Various weathering indices are available in literature but the most commonly used ones are the chemical index of alteration, CIA; [36] and chemical index of weathering, CIW; [46].The CIA monitors the gradual conversion of feldspars to clay minerals.The applicability of this index is premised on the fact that feldspars are the most abundant minerals of the upper continental crust and that the major process involved in chemical weathering is the alteration of feldspars to form clay minerals [30]  Nesbitt and Young [36] applied CIA index to the study of fine-grained sediments only and not sandstones because the composition of mudrocks is thought to be more similar to that of the parent rocks compared to coarse-grained clastic sediments [30].According to [36] [47] and [35] pyroxene) that also contain these elements are present in the parent rock, the application of the CIA becomes unreliable [50].
As noted by [46], the unpredictable character of K + during weathering can be a source of error in CIA since [36]

Figure 1 .
Figure 1.Geologic map of the Southern Benue Trough and Anambra Basin showing the location of the study area (modified after Nwajide and Reijers, [1]).

Figure 2 .
Figure 2. Road and drainage map of the study area showing the location of outcrop exposures as red triangles (Modified from Federal Survey map, Nigeria).
of mudrock and sandstone were collected from all sedimentation units during field work.Samples were studied megascopically to document the lithological and textural attributes of the rocks.Selection of rock samples for geochemical analysis was based on stratigraphic and lithologic considerations in such a manner as to capture the entire stratigraphic sections and rock types.A total number of 10 samples comprising six sandstones and four mudrocks collected from the two (2) outcrops were analyzed for their major and trace element composition using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) (Lithium Borate Fusion).Rock samples were firstly ground to powder with the aid of agate mortar and pestle at the Geochemistry Laboratory of The Federal University of Technology, Akure, Nigeria following which samples were packaged and sent to Canada.Precisely 0.25 g aliquots of each sample were mixed with a flux of lithium metaborate and lithium tetra borate and fused in an induction furnace at 1000˚C.The molten melt was immediately poured into a solution of dilute HNO 3 .The digested samples were analyzed for their major oxides and the trace elements using ICP-AES facilities at the Mineral Laboratory of Acme Laboratories Limited, Vancouver British Columbia, Canada.The oxides of 10 major elements namely, Si, Al, Mg, Ca, Fe, K, Na, P, Mn, and Ti were analyzed.Trace elements measured were Nb, Ba, Cr, Ni, Sr, Zr, Ni, Y and Sc.Major element data is re-International Journal of Geosciences ported in normalized percentages and trace element data is reported in parts per million (ppm).Loss on ignition (LOI) was determined by igniting a sample split and then measuring the weight loss.This was done after roasting the sample at 1000˚C for 2 hours.Quality control was guaranteed by totals ranging between 99.95 and 99.88 wt% for major and trace elements.Sample preparation was handled with caution to avoid sample contamination.Further details of the analytical procedures, precision, accuracy and standards used for instrumental calibration can be obtained from Acme Laboratories Limited, Vancouver, Canada.

O
. A. Ogbahon, O. B. Olujinmi DOI: 10.4236/ijg.2019.101007105 International Journal of Geosciences their immobile nature during weathering and transport and their short residence

Figure 5 .
Figure 5. Discriminant function diagram for provenance signatures of sandstone-shale samples from the study area using major elements, fields after [12].
, CIA values of 50 to 60 indicate low weathering; CIA values of 60 -80 indicates intermediate weathering; CIA values of 80 -100 reflects extreme weathering.The CIA of unaltered feldspar is 50.According to[35], CIA of 100 indicates complete conversion of feldspar to aluminous clay minerals such as kaolinite[47].Chemical weathering is relevant only under humid condition resulting in substantial leaching of mobile cations (Na + , K + and Ca 2+ ) and a concomitant enrichment of Si and Al in the residuum.Climate and rate of tectonic uplift are the main factors controlling the degree of chemical weathering.According to[48] and[49], increased intensity of chemical weathering may suggest decrease in tectonic activity and/or change of climate towards warm and humid climatic conditions which are more favorable for chemical weathering in the source region.In arid or cool condition, sediment is produced primarily by abrasion and attrition which involve mere physical disintegration of the parent materials with insignificant chemical alterations.Also, the degree of chemical weathering was evaluated using chemical index of weathering (CIW).The use of the CIA index should be treated with caution since it O.A.Ogbahon, O. B. Olujinmi DOI: 10.4236/ijg.2019.101007111 International Journal of Geosciences was founded on restrictive assumptions which may, in some instances present a challenging obstacle to its practical applicability [50].The usability of CIA is based on the assumptions that the initial geochemistry of the unweathered parent rock is invariant and that Al 2 O 3 , CaO, K 2 O and Na 2 O resides exclusively in feldspars.Therefore, if significant quantity of other minerals (e.g.amphibole, samples respectively.The CIW values are higher for shales than sandstones and mimic the pattern of CIA.The high CIW values (close to 100) for the analyzed samples reflect intense chemical of the sauce area.The CIW index increases with the degree of depletion of Na and Ca in sediments, relative to Al[46].According to[46], the difference between the CIW index values for the parent rock and the sediment indicates the amount of chemical weathering experienced by the weathered material.Although CIA and CIW indices can be used to determine the degree of source area weathering of sediment, they however cannot be used to deduce the trends of paleo-weathering conditions, especially in sediments with variable provenances because they are mere numerical values.Therefore the CIA was incorpo-rated into A (Al 2 O 3 ) − CN (CaO* + Na 2 O) − K (K 2 O) compositional diagramwhich can effectively evaluate chemical weathering, diagenesis, metamorphism and source composition of clastic sediments[35].The concentrations of A-CN-K in moles of samples from the study area were plotted on a ternary A-CN-K diagram to determine the trend of chemical weathering, composition of parent materials and the likely effect of metasomatism (Figure10).The plot indicates a tight cluster of data-points very close to the Al 2 O 3 apex and oriented roughly parallel to the A-CN join suggesting substantial loss of Ca, Na and K in the parent material and abundance of mineral phases with compositions close to kaolinite, smectite and illite in the samples.The observed trend that roughly paralleled the A-CN join suggests that potassium metasomatism played insignificant role in the sedimentary process.According to Nesbitt and Young[47], the interpretation of the paleo-weathering trends in sediments can be used to deduce the climatic regimes influencing the composition of sediments.The occurrence of kaolinite in terrigenous sediments indicates tropical weathering conditions(Nesbitt and Young, 1982) Following the method proposed by[51] the composition of the parent materials was established by drawing a line of best fit through the data points and projecting it backwards to the Plagioclase-potassium

Table 1 .
1 & Table 2.The bulk geochemistry of the main SiO 2 while the shale are slightly depleted in SiO 2.However, one of the shale samples (B10) has SiO 2 concentration of 70.57% which is higher than the average UCC and PAAS.The sandstones are highly depleted in Al 2 O 3 with respect to average UCC and PAAS values, whereas the shales are enriched in Al 2 O 3 .One of the shale samples (B6) has significantly Major element concentrations (wt%) of samples from Upper Maastrichtian Mamu Formation on the western flank of Anambra Basin. 2 O 3. All the analyzed samples show depletion in elemental oxides like MgO, Na 2 O, K 2 O and TiO 2 with respect to the average UCC and PAAS values.P 2 O 5 SiO 2 concentration has a strong negative correlation with Al 2 O 3 and Fe 2 O 3 (Table3), pointing to the fact that much of the SiO 2 occurs either as detrital (38 elements of 6 sandstone and 4 shale samples reflects the mineralogy of the samples.The sandstone samples have relatively higher SiO 2 concentrations (78.7% -94.23%; average 87.99%) compared to the shale samples(38.41%-83.4%;average 61.17%).One of the shale samples (B6) has significantly low SiO 2 concentration of 38.41%.Comparatively, the shale samples are more enriched in Al 2 O 3 (7.82%-30.36%;average 20.73%) compared to the sandstones (3.51% -10.69%; average 6.27%).Immobile oxides like Al 2O 3 and TiO 2 show slightly higher values compared to other oxide, and they even exceed the PAAS normalization value in the shale samples.Al 2 O 3 and K 2 O indicate the occurrence of potassium feldspars, mica and clay minerals like smectite and kaolinite.All the associated shale or mudstone are relatively enriched in Fe 2 O 3 , MgO, K 2 O, TiO 2 and P 2 O 5 (average 3.56%, 0.18%, 0.7%, 1.34% and 0.09% respectively) in comparison with the sandstones (average 1.71%, 0.03%, 0.23%, 0.74% and 0.05% respectively) indicating their higher concentrations in shale.On the contrary, the sandstone samples have higher concentrations of CaO and Na 2 O (average 0.08% and 0.11%) compared to the shale samples (average 0.03% and 0.03%).An increase in the abundance of Fe-bearing clay minerals or iron oxide minerals like magnetite, leucoxene can result in high concentration of Fe 2 O 3. Both the sandstone and shale samples are strongly depleted in MnO and P 2 O 5 , and there are no significant differences in their concentrations among samples.By comparison with the average upper continental crust (UCC) and Post Achaean Australian shale (PAAS), which represents continentally derived sediments [29] [30], the sandstones are enriched higher concentration of Al 2 O 3 (30.36%)with respect to both average UCC and PAAS values.An increase in the abundance of Fe-bearing clay mineral or iron oxide minerals like magnetite, leucoxene can result in high concentration of International Journal of Geosciences CIA: Chemical index of alteration, CIW, Chemical Index of Weathering, ICV: Index of compositional variability, UCC: average upper continental crust, PASS: Average Post Achaean Australian Shale, after [29] [30].Fe 2 O, K 2 O and CaO implies destruction of both plagioclase and K-feldspars as a consequence of chemical weathering in the provenance or during sediment transport to basin.O. A. Ogbahon, O. B. Olujinmi DOI: 10.4236/ijg.2019.101007 100 International Journal of Geosciences

Table 2 .
Trace element concentrations (wt%) of samples from Upper Maastrichtian Mamu Formation on the western flank of Anambra Basin.

Table 3 .
(a) correlation matrix of major and trace element concentration of sandstone samples from the study area; (b) correlation matrix of major and trace element concentration of shale samples from the study area.Al 2 O 3 Fe 2 O 3 MgO CaO Na 2 O K 2 O TiO 2 P 2 O 5 MnO Cr 2 O 3 Al 2 O 3 Fe 2 O 3 MgO CaO Na 2 O K 2 O TiO 2 P 2 O 5 MnO Cr 2 O 3 The ratio of SiO 2 /Al 2 O 3 is highly variable ranging from 7.50 to 26.85 and it shows high silica to alumina content.The K 2 O/ Al 2 O 3 ratio in the sandstones is low (0.01-0.05) and it is an indication of low concentration of K-bearing minerals relative to alumina bearing ones.The concentration of Al 2 O 3 in the samples shows strong positive correlation with Fe 2 O 3 , MgO, K 2 O, TiO 2 , and P 2 O 5 (r = 0.61, 0.94, 0.96, 0.82 and 0.82 respectively) but shows very strong negative correlation with Na 2 O and CaO (r = −0.63 and −0.98).The association of Al 2 O 3 with oxides Fe 2 O 3 , K 2 O, TiO 2 , P 2 O 3 suggests their derivation from alu-

Table 2 .
The shales samples have higher concentration of Ba, Ni, Sr, Y, value of 20 ppm as that of UCC for sandstone samples, however, it is depleted when compared to the average PAAS concentration(55 ppm).on the other hand, the average concentration Ni in shales (26 ppm) is higher than the average UCC (20 ppm) but lower than the average PAAS value of 55 ppm.Average concentrations of Y, Nb in both shales (49.50, 20 ppm) and sandstone(33.17,35.25 ppm) sample are higher than the average PAAS (27, 19 ppm) and UCC (22, 12 ppm) values.meaningbothshale and sandstone samples are enriched in Y and Nb.The sandstone samples are depleted in Sc compared to average PAAS (16 ppm) and UCC (13.6 ppm) values, whereas, the shale samples are slightly enriched in Sc in comparison to average UCC (13.6 ppm) but slightly depleted compared to average PAAS value of 14.50 ppm.One of the major characteristics of the trace elements distribution is the generally strong positive correlation of Ni Sr and Sc with A 2 O 3 indicating that these elements are associated with aluminosilicates.Ba, Y and Nb show very weak positive correlation with Al 2 O 3 while Zr concentration indicates very strong negative correlation.On the other hand, Zr abundances show strong positive correlation with SiO 2 (Table3).This is because Zr is associated with zircon, an ultrastable heavy mineral that usually get concentrated with detrital quartz.In a similar vein, Sr, Ni and Sc show strong or moderate positive correlation with Fe 2 O 3 while Zr show strong negative correlation.Ba, Y and Nb display weak negative correlation with Fe 2 O 3 .
. The sandstones have more SiO 2 contents but less Al 2 O 3 Fe 2 O 3 and TiO 2 than the associated mudrocks.SiO 2 concentrations increase with increasing grain size as a result of quartz enrichment while Al 2 O 3 concentration increases with decreasing grain size because fine-grained sediments are enriched in clay minerals.According to Ben-Awuah et al. [37], [39]iO 2 , Al 2 O 3 , TiO 2 and Fe 2 O 3 .This trend can be attributed to the high mobility character exhibited by K + , Na + , and Ca 2+ ions in humid climatic setting.They tend to leach easily in contrast to the immobile Ti2+and Al 3+ ions, since the latter have high resistance to erosion.SiO 2 /Al 2 O 3 ratio increases with grain size and it is an indicator chemical maturity of the sandstones.SiO 2 is resistant to chemical weathering.According to Bhatia,[38], Al 2 O 3 is a common member of mature residual product of weathering as a result of its immobility.Cox et al.[39]demonstrated how the present chemical composition can be used to deduce the original detrital mineralogy of shale samples.One way of achieving this is to use the index of chemical variability, ICV = (Fe 2 O 3 + k 2 O + Na 2 O + CaO + MgO + TiO 2 ):Al 2 O 3 ) and the ratio of K 2 O:Al 2 O 3 .According to Cox et al. [39] and Culler and Podkovyrov [40] non-clay minerals have a higher ratio of the major cations to Al 2 O 3 than clay minerals and so the non-clay min-Al 2 O 3 greater than 0.5 suggest a significant quantity of alkali feldspar relative to other minerals in the original shale; those with ratios of K 2 O:Al 2 O 3 less than 0.4 suggest minimal alkali feldspar in the original shale.The K 2 O:Al 2 O 3 [40].Ogbahon, O. B. Olujinmi DOI: 10.4236/ijg.2019.101007104InternationalJournal of Geosciences significant enrichment erals have a higher ICV.They stated that ICV decreases in the order of pyroxene and amphibole (c. 10 -100), biotite (c.8), alkali feldspar (c.0.8 -1), plagioclase (c.0.6), muscovite and illite (c.0.3), montmorillionite (c.0.15 -0.3), and kaolinite (c.0.03 -0.05).Cox et al. [39] and Culler and Podkovyrov [40] further stated that immature shales,which are typical of first cycle deposits in tectonically active settings, have a high percent of non-clay silicate minerals and therefore contain ICV values of greater than one while more mature mudrocks with mostly clay minerals have lower ICV values of less than one.The average ICV of the analyzed shale samples is 0.30 (range 0.17 -0.43), suggesting the original shales contained dominantly muscovite and clay minerals and is compositionally mature and may have been recycled.The ICV values also suggest deposition of sediments in passive margin tectonic settings.In addition, K 2 O/Al 2 O 3 ratios may indicate how much alkali feldspar vs. plagioclase and clay minerals may have been present in the original shales[39].In decreasing order of values, the K 2 O:Al 2 O 3 ratios of minerals are alkali feldspars (0.4 -1), illite (0.3), other clay minerals (roughly 0)[39] [40].According to Cox et al.[39]and Culler Culler and Podkovyrov[40], shales with ratios of K 2 O: [44]erent chemical characteristics are displayed by sediments deposited in different tectonic settings.According to [[13], p. 576] and[37], the K 2 O/Na 2 O ratios and SiO 2 content of siliciclastic sedimentary rocks are particularly sensitive indicators of geotectonic setting and this formed the basis of Roser and Korsch[44]tectonic setting discriminant diagram.Roser and Korsch [44] used the plot K 2 O/Na 2 O against SiO 2 DOI: 10.4236/ijg.2019.101007108 International Journal of Geosciences [47].The index was calculated using molar proportion in the equation [CIA = Al 2 O 3 /(CaO* + Na 2 O + K 2 O], where CaO* is the concentration of CaO occurring in the silicate minerals (adjusted for CaO in carbonates and phosphates).Even though the petrographic investigation was not undertaken, the low concentration of CaO and the low LOI values (Table 1) suggest that the CaO in the samples are associated silicate with silicate phases.The CIA index values of analyzed sandstones (range 87.38 -95.51; average, 93.07) and shales (range 93.32 -98.09; average 95.97) samples from the study area (Table 1) are high and clearly indicate intense or deep chemical weathering at the source area in a humid climatic condition.It should be noted that the associated shale samples have a higher CIA values compared to the sandstones.This is probably reflecting the effect of grain sorting.Sandstones contained mainly quartz grains and little clay matrix (Al 2 O 3 ) since much of the fine clay particles were winnowed away to be redeposited further downstream with muds during sediment transport and deposition and thus can greatly affect the CIAs of sandstones.
assumed K 2 O to be a mobile component.According to [46], K + can be used in the formation of K-minerals or adsorbed onto other clay minerals through ion exchange even after entering solution.This informed the calculation of chemical index of weathering.CIW = [Al 2 O 3 /(Al 2 O 3 + Na 2 O + CaO)] × 100.The CIW values of analyzed samples (Table 1) range from 89.82 -99.53 (ave 95.89) and 99.11 -99.90 (ave 99.62) for sandstones and shale