COOU Journal of Physical Sciences 4 (1), 2021

MAJOR ELEMENT ANALYSIS OF NSUGBE SANDSTONE, ANAMBRA BASIN, SOUTHEASTERN : IMPLICATION FOR PROVENANCE

1 2 Egbosi Blessing Ukamaka , Nfor, Bruno Ndicho and Chiaghanam, Osita3

1,2,3: Department of Geology, Faculty of Physical Science, Chukwuemeka Odumegwu Ojukwu University, , Nigeria

All correspondences to [email protected]*, [email protected], [email protected]

ABSTRACT The Nsugbe Sandstone is one of the several lithostratigraphic units within the Anambra Basin, South-eastern Nigeria. Geochemical evaluation was carried out on sandstone samples from different outcrops of Nsugbe Sandstone in order to determine its provenance, degree of weathering of the source area, paleoclimatic and paleotectonic settings. Result from chemical analysis using x-ray spectrometry revealed average values of the following major elemental oxides SiO2 (95.432%); Al2O3 (1.976%); Fe2O3 (1.460%); TiO2 (0.352%); CaO (0.103%); P2O3 (0.007%); K2O (0.422%); MnO (0.794%); MgO (0.012%); Na2O (0.078%). Statistical analysis of geochemical data reveals chemical index of alteration (CIA) and mineralogical index of alteration (MIA) values of 89.5% and 79% respectively implying that the source area was intensively weathered typically of semi-humid climate. The provenance discriminate function plots show that the Nsugbe Sandstone was derived from quartzose sedimentary source. Inferences drawn from the analyses carried out suggest that the Nsugbe Sandstone is quartz arenite derived from intense chemical weathering of recycled older sediment in a passive paleotectonic setting perhaps Abakaliki folded belt and Cameroon Mountains.

KEYWORDS: Nsugbe Sandstone, Provenance, Paleoclimatic Setting, Paleotectonic Setting and Geochemistry

1. Introduction All rocks are subjected to transition (Rock Cycle) among the three rock types (Igneous, metamorphic and sedimentary). Rocks exposed at the earth’s surface undergo physical, biological or chemical weathering and are broken down into sediments which may be transported, deposited and lithified to form sedimentary rock. Sedimentary rocks contain embedded clues for reconstruction of its processes. Provenance as applied to sedimentary deposits means origin from which sediments were derived. Provenance encompasses the source-to-basin relationship (with respect to distance and direction), the tectonic, the relief and the climate of the source area. Provenance study is the reconstruction of origin of sediments which involves the evaluation of compositional characteristics of sedimentary rock. Provenance study aids in basin analysis as it provides information related to the tectonic origin of a particular basin. The use of geochemical composition of clastic sedimentary rocks is a vital

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tool to understand the nature of source rocks (Roser and Korsch, 1988; Ajayi et. al., 2006), weathering and erosion dynamics (Nesbitt and Young, 1982), the tectonic setting of the depositional basins (Bhatia, 1983; Roser and Korsch, 1986) and post depositional changes (Fedo et. al., 1995, 1996). Utilizing geochemical data from sediments to understand such sedimentary processes are growing in literature. This study focuses on the application of geochemical approach in assessment of the provenance, weathering condition, paleoclimatic and paleotectonic settings of the source area.

2. Geologic Setting The Nsugbe Sandstone has been elected as a member of the Ameki Group. It is Eocene in age (Nwajide, 1980). It is underlain by Imo Formation and overlain by the Nanka Formation. It is exposed at Nsugbe and environs. The Sandstone was studied within latitude N 060 081 – N 060 181 and longitudes E 0060 451_ E 0060 551. It lies on the eastern part of Anambra state, covering parts of and Oyi L.G.A, Southeastern Nigeria. The major accessible route to the study area is the – Nsugbe expressway. The map of the area is presented in figure 1.

3.Stratigraphic setting Nsugbe Sandstone is one of the lithostratigraphic unit in the Anambra Basin. The sedimentological and stratigraphic settings of the sediments within the Anambra Basin are defined by three main evolutionary stages; Pre-Santonian, Post-Santonian and Tertiary ages. The first stage also called the Pre-Santonian history is characterized by a strong subsidence in the Abakiliki domain of the Benue Trough while the Anambra domain remained a platform where mud was deposited in a shallow marine environment. The second stage which occurred after the Santonian age and is characterised by the uplift of the Abakiliki sub-basin and the subsidence of the Anambra platform. This led to the deposition of the Nkporo Group and Mamu

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Formation followed by fluvio-deltaic system during the Masstrichtian which led to the deposition of the Ajalli Sandstone and Nsukka Formation. The third stage occurred during the Lower Tertiary. It was characterized by the deposition of Imo Shale and Ameki Group. A chart showing detailed stratigraphic succession in the Anambra Basin and Outcropping Niger Delta is presented in figure 2.

Fig. 2: Stratigraphic Succession in the Anambra Basin and Outcropping Niger Delta (Nwajide, 2013).

4. Methodology A systematic mapping of the area was carried out, using a field map/basemap on the scale of 1:50,000. Sandstone samples were collected systematically from different outcrops and used for geochemical analysis. XRF analytical method was used to determining the elemental composition of materials. This method measures the fluorescent x-ray emitted from sample when excited by primary x-ray source in order to determine the chemistry of the sample. Each element present in a sample has a unique characteristic fluorescent x-rays. In this study, each sample was crushed using an electric crusher and then pulverized for 60 seconds using Herzog Gyro-mill (Simatic C7-621). Pellets were prepared from the pulverized sample, first by grinding 20g of each sample with 0.4g of stearic acid for 60 seconds. After each grinding, the Gyro-mill was cleansed to avoid contamination and ensure accurate result. 1g of stearic acid was weighed into an aluminium cup to act as binding agent and the cup was subsequently filled with the sample to the level point. The cup was then taken to Herzong pelletizing equipment when it was passed at a pressure of 200KN for 60 seconds. The 2mm pellets were added into a

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sample holder of the x-ray equipment (Phillips PW-1800) for analysis. This technique is a useful tool for evaluating the sandstone provenance and tectonic setting reconstruction. The primary geochemical analysis results were further recalculated to decipher the degree of weathering in provenance region using Nesbitt and Young (1982) scheme for chemical index of alteration CIA=100* Al2O3/( Al2O3/ CaO + Na2O + K2O), Harnois, (1988), chemical index for weathering CIW=100* Al2O3/( Al2O3/ CaO + Na2O). CIA and CIW are interpreted in a similar way. CIA/ CIW equal or less than 50% indicates unweathered uppercrust while CIA/CIW values between 76-100% indicates intense weathering. Voicu et al.,(1997) scheme for mineralogical index of alteration MIA was used as a weathering parameter. MIA= 2(CIA- 50). MIA values 0-20% = onset of weathering, values 20-40% = weak weathering, values 40- 60%= moderate weathering while values 60-100= intense weathering. Roser and Korsh, (1986, 1988), Murphy, (2000) and Suttner et. al., diagrams were used to infer the provenance, paleoclimatic and paleotectonic setting of the area.

5 Results The results obtained from the geochemical analysis of the sandstone samples from the Nsugbe area are shown in Table 1. The table consists of the major oxides concentration measured directly from the laboratory, the recalculated ratio of oxides and log of ratios. The result revealed 10 elemental compositions with their concentration in weight percent. The major oxide concentrations and its averages are as follows: SiO2 (94.600-96.100), average: 95.432%, Al2O3 ( 1.840- 1.980), average: 1.976%, Fe2O3 (0.760- 1.982), average: 1.460%, TiO2 (0.209- 0.392), average: 0.352%, CaO (0.001-0.039), average: 0.103%, P2O3 (0.001-0.012), average: 0.007%, K2O (0.320-0.478) average:0.422%, MnO (0.187-1.707) average: 0.794%, MgO (0.001-0.024) average: 0.012%, Na2O (0.012-0.15) average: 0.078%. The pictoral representation of the results were presented in figures 3-6. Figures 3 showed the provenance signatures of the studied samples after Roser and Korsch, (1988). Figures 4 was used for chemical classification of Nsugbe Sandstone after Herron (1988) and Pettijohn, 1972) respectively. Figure 5 is the paleotectonic setting discriminant diagram after Murphy, (2000). Figure 8 is the paleotectonic setting discriminant diagram after Roser and Korsch, (1986) and Figure 6 is the paleoclimatic diagram showing the degree of maturity of the samples.

3 Table 1: Major Oxide Composition of Nsugbe Sandstone. 4 Major 5 N1 6 N2 7 N3 8 N4 9 N5 10 Average Oxides 11 SiO2 12 95.460 13 95.300 14 96.100 15 95.700 16 94.600 17 95.432 18 Al2O3 19 1.900 20 1.890 21 1.840 22 1.670 23 1.980 24 1.976 25 Fe2O3 26 0.867 27 0.892 28 0.760 29 1.200 30 1.982 31 1.460 32 TiO2 33 0.273 34 0.230 35 0.209 36 0.392 37 0.248 38 1.352 39 CaO 40 0.039 41 0.036 42 0.001 43 0.020 44 0.240 45 0.103 46 P2O5 47 0.001 48 0.001 49 - 50 0.001 51 0.012 52 0.003 53 K2O 54 0.380 55 0.340 56 0.320 57 0.478 58 0.390 59 0.422 60 MnO 61 1.030 62 1.707 63 0.558 64 0.189 65 0.487 66 0.794 67 MgO 68 0.024 69 0.001 70 0.001 71 0.003 72 0.001 73 0.150 74 Na2O 75 0.150 76 0.018 77 0.012 78 0.15 79 0.061 80 0.078 81 CIA 82 89.64 83 88.88 84 89.23 85 89.36 86 90.47 87 89.51 88 MIA 89 79.28 90 77.76 91 78.46 92 78.72 93 80.94 94 79.033 95 Function 96 -0.85684 97 -7.83319 98 -9.137107 99 -7.740045 100 -8.598812 101 -8.273 1 raw

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102 Functio 103 -6.01087 104 -6.76798 105 -6.48133 106 -6.82689 107 -6.2180 108 -6.4612 n 2 raw 109 Functio 110 3.64545 111 10.54307 112 4.824329 113 7.439856 114 8.66813 115 8.27123 n 1 ratio 116 Functio 117 10.54306 118 6.890867 119 7.876555 120 6.623456 121 8.2435632 122 7.55132 n 2 ratio 4 123 CIA= Chemical index of alteration ie 0-50(weak), 50-75(moderate), 76-100(intense); MIA= Mineralogical index of alteration ie 0-20(incipient), 20-40% (weak), 40- 60%(moderate), 60-100% (intense).

6.0 Discussion. The chemical composition of clastic sedimentary rocks reflects the provenance, weathering condition of the source rock, paleoclimatic and paleotectonic setting of the area.

6.1 Weathering of the Source Area. As presented in Table 1, the average CIA and MIA values are 89.5% and 79.03% respectively which indicates intense weathering of the source area. This is also an indication that the sediments of Nsugbe Sandstone has undergone intense weathering and a long period of transportation from source. Low values of Na2O, CaO and MnO also suggests that the sediments have undergone intense weathering and recycling (Joo et. al., 2005; Jin et. al., 2006). Low K2O in Nsugbe Sandstone may be described as a result of intense chemical weathering which enhanced leaching of soils and removal of alkali in solution. Also a very low K2O/Al2O3 ratio average of 0.03% indicates intense chemical weathering of the source rocks (Potter et. al., 1980). However, intense chemical weathering of first cycle detritus can produce sandstone as mature as those of the Nsugbe Sandstone.

6.2 Provenance indication. Major elemental composition was used to infer the provenance of Nsugbe Sandstone. Roser and Korsch (1988), used major element discriminant functions to discriminate four provenances namely mafic, Intermediate, felsic and quartzose recycled provenances. Plotting the present data into this diagram, all the samples fall within the quartzose recycled field (figure 3). The diagrams show that Nsugbe Sandstone was derived from quartzose sedimentary provenance. Position of the plots within the recycled sediment field is similar to that observed for several recycled suites elsewhere (Roser et. al., 1996). This was interpreted as the effect of recycling with progressive loss of feldspar and relative increase in quartz. The plot position of the studied samples also reflect more intense weathering and maturation of Nsugbe Sandstone.

N 1 N N N N

Fig. 3: Discriminant function diagram for provenance signatures of the studied samples: A- using raw oxides; B- Ratio of oxides (after Roser & Korsch 1988) Website:www.coou.edu.ng 109

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6.3 Chemical composition/ classification Table 2: Chemical Classification of Nsugbe Sandstone (After Blatt et al., 1972; Hebron, 1988; Pettijohn et al., 1972; Potter, 1978; Akinmosin and Osinowo, 2008; Obiefuna and Orazulike, 2011; Akiyemi et al., 2014). S/N Log of ratio of oxides Type of Sandstone 1 Log(SiO2/ Al2O3) >1.5 Arenite 2 Log((SiO2/ Al2O3) >1 and Log(K2O/ Na2O) <0 Greywacke 3 Log((SiO2/ Al2O3) >1 and Log(K2O/ Na2O) >0 Arkose and Log Fe2O3+ MgO/ Na2O+K2O 4 Log((SiO2/ Al2O3) >1 and Log(K2O/ Na2O) <0 Lithic arenite including subgrey and Log Fe2O3+ MgO/ Na2O+K2O >0 wacke and proto quartzite

According to Table 2, Nsugbe sandstone is classified as quartz arenite using the above classification scheme. Also Pettijohn (1972) Chemical classification of sandstones diagram and Chemical classification of the sandstones based on log (SiO2/Al2O3) against log (Fe2O3/K2O) diagram of Herron (1988) were used to classify the sandstone as quartz arenite as presented in figure 4.

N 1 N2 N3 N4 N5

Arenite

Fig. 4: Chemical classification of sandstones: A- Log (Na2O/K2O)/ Log (SiO2/Al2O3; After Pettijohn, 1972); B- log (SiO2/Al2O3) against log (Fe2O3/K2O) diagram of Herron (1988)

6.4. Paleotectonic Reconstruction. Several discrimination diagrams of tectonic setting using major oxides composition have been proposed for clastic sediment. In this study, the Roser and Korsch 1986 diagram of K2O/ Na2O against SiO2 and Murphy (2000) were used to reconstruct the tectonic setting. The diagram classified Nsugbe Sandstone into passive margin as in figure 5.

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N 1 A B N 2 N 3

N 4 N 5

Fig. 5:Paleotectonic setting discriminant diagrams using A- SiO2/ Al2O3 against K2O/ Na2O; after Murphy, 2000. B- Log(K2O/ Na2O) against SiO2; after Roser and Korsch, 1986.

6.5 Maturity and Paleoclimatic reconstruction. The analyzed samples are dominated by SiO2 which ranges from (94.60-96.10), average: 95.43%. Silica enrichment is a measure of sandstone maturity and also a reflection of the duration and intensity of weathering and destruction of less stable minerals during transportation (Lindsey, 1999). As the percentage of quartz increases, the mineralogical maturity does too. Maturity of sandstone can also be reflected by the SiO2 /Al2O3 index. Values of SiO2 /Al2O3 increase because of increase in quartz at the expense of less resistant components during sediment transport and recycling. High ratios indicate mineralogical mature (quartzose, rounded) sediment while low ratios represents chemically immature sediments. The average value of SiO2 /Al2O3 ratio for the Nsugbe Sandstone is 51.6 which is significantly high indicates mature sediments. The dominant quartz arenite is an additional criteria for their high maturity. Suttner and Dutta (1986) plot was used to evaluate the maturity of Nsugbe sandstone as a function of climate. The plotted samples showed semi humid climatic condition with the sandstone increasing maturity as presented in figure 6.

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N 1 N2 N3 N4 N5

Fig. 6: Paleoclimatic diagram showing the degree of maturity of the samples expressed by bivariate plot of SiO2 versus Al2O3+ K2O+ Na2O. After Suttner & Dutta, (1986)

7. Conclusion Geochemical analysis helped to ascertain the chemical composition, degree of weathering, provenance, paleoclimatic and tectonic setting under which Nsugbe Sandstone was deposited. The weathering indices MIA and CIA are 89.3% and 79% respectively implying intense weathering under semi humid climatic condition. Roser and Korsch (1986) diagram of K2O/ Na2O against SiO2 and Murphy (2000) classified Nsugbe Sandstone into passive margin. Provenance discriminant diagram shows that Nsugbe Sandstone was from quartzose sedimentary source probably from Abakiliki anticlinorium.

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