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Econ. Environ. Geol. 2023; 56(6): 799-816

Published online December 29, 2023

https://doi.org/10.9719/EEG.2023.56.6.799

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Mineralogy and Geochemistry of Shale Deposits in the Lower Anambra Basin, Nigeria: Implication for Provenance, Tectonic Setting and Depositional Environment

Olugbenga Okunlola1, Agonsi Udodirim Lydia1, Aliyu Ohiani Umaru2,*, Raymond Webrah Kazapoe3, Olusegun G. Olisa4

1Department of Geology, University of Ibadan, Ibadan, Nigeria
2Department of Geology, University of Maiduguri, Maiduguri, Borno State
3Department of Geological Engineering, University for Development Studies, Nyankpala, Ghana
4Department of Earth Sciences, Olabisi Onabanjo University, Ago Iwoye, Ogun state, Nigeria

Correspondence to : *ualeey@gmail.com

Received: September 10, 2023; Revised: November 24, 2023; Accepted: November 27, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Mineralogical and geochemical studies of shales within the Lower Anambra Basin was conducted to unravel the depositional environment, provenance, maturity, paleo-weathering conditions, and tectonic settings. Mineralogical studies conducted using X-ray diffraction analysis revealed that the samples were composed of kaolinite, montmorillonite, chlorite, and illite. KaolinIite is the dominant mineral, constituting approximately 41.5% of the bulk composition, whereas the non-clay minerals are quartz, ilmenite, and sillimanite. Geochemical analysis showed a predominance of SiO2, Al2O3, and Fe2O3 contents of the shale samples with mean values of 52.29%, 14.09%, and 6.15% for Imo Shale (IS); 52.31%, 16.70%, and 7.39% for Mamu Shale (MS); 43.21%, 21.33%, and 10.36% for Enugu Shale (ES); 53.35%, 15.64%, and 7.17% for Nkporo Shale (NS); and 51.24%, 17.25%, and 7.78% for Agwu Shale (AS). However, the shales were depleted in Na2O, MgO, K2O, MnO, TiO2, CaO, and P2O5. The trace element ratios of Ni/Co and Cu/Zn of the shale suggest an oxic depositional environment. The average SiO2 vs. Al2O3 ratio of the shales indicated textural maturity. Compared to the PAAS standard, the shales plot below the PAAS value of 0.85, suggesting a high degree of maturity and intensive chemical weathering, further confirmed on a CIA vs. PIA plot. On log (K2O/Na2O) against SiO2 and tectonic setting discriminant function diagrams, the shales plot mostly in the field of passive continental margin tectonic setting. The discriminant function diagrams as well as Al2O3/TiO2 ratio of the shales showed that they were derived from a mixed source (mafic and intermediate igneous rocks).

Keywords Anambra Basin, provenance, tectonic setting, mineralogical, chemical, shale

  • Mineralogical studies reveal KaolinIite as the dominant mineral, constituting approximately 41.5% of the bulk composition.

  • The shales are deposited in an oxic depositional environment and are texturally mature.

  • Tectonically, the shales characterized in the passive continental margin tectonic setting and are derived from a mixed source (mafic and intermediate igneous rocks)

Weathering conditions, sorting, provenance, and tectonism are interrelated phenomena that contribute to the geochemistry of clastic sedimentary rocks (Johnsson, 1993). The composition of shales is influenced by the tectonic setting of the basin of deposition as well as the chemical composition of the source rock area (Bhatia and Crook, 1986). Sedimentary geochemical data has been demonstrated to provide critical information for provenance studies through the analysis of major, trace, and rare earth elements (REEs) as well as their ratios in sedimentary rocks, which can simulate climatic, geographical, and tectonic conditions of the basins (Bhatia, 1983; Taylor and McLennan, 1985; Bhatia and Crook, 1986; McLennan, 1989; Feng and Kerrich, 1990; McLennan and Taylor, 1991; Cullers, 1994; Jahn and Condie, 1995; Girty et al., 1996; Etemad-Saeed et al., 2011; Verma and Armstrong-Altrin, 2013; Obaje et al., 2020; Waziri et al., 2020; Musa et al., 2022). Therefore, detailed studies of the geochemistry and mineralogy of fine-grained clastic sedimentary rocks is regarded as a valuable tool for determining provenance, tectonic setting, paleoweathering, and redox condition of a given source location (Ruffell et al., 2002; Ahlberg et al., 2003; Deconinck et al., 2005; Dera et al., 2009; Shettima et al., 2020a; Shettima et al., 2020b). Anambra Basin located in the southern part of Nigeria lies between the southern portion of the Benue Trough and the Niger Delta (Fig. 1). This basin has long been renowned as a coal and, later, petroleum exploration frontier (Ejeh et al, 2015). Its eastern and northwestern flanks are characterized by basement complex rocks of southwestern Niger/Bida basin and Abakaliki anticlinorium. The southern section has a post-Santonian sedimentary fill of up to >7,000 m (Whiteman, 1982). Although the tectonic setting of the Anambra Basin and the origin of the sediments have been investigated for decades, the source region, redox nature, and tectonic setting are still unclear and require more study (Reyment, 1965; Murat, 1972; Nwachukwu, 1972). As a result, the current study attempts to constrain the provenance signature, weathering, maturity, tectonic setting, as well as environment of deposition of shales within the Lower Anambra Basin which have not been thoroughly studied.

Fig. 1. Location of the Anambra Basin within the Lower Benue Trough (Modified after Murat, 1972).

The Anambra Basin is one of the intracratonic Cretaceous sedimentary basins in Nigeria and constitutes the southern portion of the Benue trough whose origin is related to the separation of Africa from South America and the opening of the South Atlantic Ocean (Obaje et al. 2004) (Fig. 1). According to Akaegbobi (2005), the sedimentation history in the lower Benue Trough is related to the evolution of the Anambra Basin depression and Abakaliki domain. The basin extends for about 402.3 km in length and almost 9 kilometers in thickness, running from Onitsha on the Niger River to Kwande on the Benue River in a NE-SW orientation (Whiteman, 1982). The Cretaceous Anambra depositional site represents a megafacie region that received sediment load over two depositional cycles spanning the Aptian to Maastrichtian periods. (Obaje et al., 1999; Obaje et al., 2004; Murat, 1972; Reyment, 1965). The sediment is indicative of fluvial-deltaic and shallow marine sedimentation on the continental scale (Akaegbobi and Schmitt, 1998). Much is unknown about the Cretaceous and pre-Santonian subsurface deposits in this basin because of the substantial post- Santonian sedimentary fill that covers much of the basin (Whiteman, 1982). On the other hand, Cretaceous outcrops, from adjoining southern Benue Trough suggest what underlies the post-Santonian deposits in the Anambra Basin. The sequence of sedimentation consists of the transgressive Albian Asu River Group which is overlain by the Cenomanian- Early Santonian strata of the Cross River Group, this includes shale, limestone, and sandstone of the Nkalagu Formation, which consist of the Eze-Aku and Awgu shale, as well as their interlingering local facies counterparts (Amasiri, Makurdi, Agala, and Agbani sandstones) (Petters and Ekweozor, 1982). In the Anambra Basin, the Agwu Shale, Enugu / Nkporo shales, Mamu Shale, Ajali Sandstone, Nsukka Formation and Imo Shale constitute the post- Santonian sedimentary fill (Whiteman, 1982; Reijers, 1996) (Table 1). The investigated area is within the Lower Anambra Basin and comprises of Agwu Shale, Enugu / Nkporo shales, Mamu Shale and Imo Shale (Fig. 2).

Fig. 2. Geological map of the Lower Anambra Basin and adjacent areas showing sample locations (Modified after Ojo et al., 2009).


Stratigraphic Setting of the Anambra Basin (Modified after Nwajide and Reijers,1996)


AgeSouthern Benue/Anambra BasinCycles of sedimentation
TertiaryEoceneAmeki/Nanka formationThird (3rd) cycle of sedimentation
PaleoceneImo shale
Upper CretaceousMaastrichtianNsukka formation
Ajali formation
Mamu Shale
CampanianEnugu/Nkporo formation
Santonian-ConiacianAgwu formationSecond (2nd) cycle of sedimentation
TuronianEze-Aku Group, (Keana, Markudi, Agala and Amaseri formations)
CenomanianOdukpani formation
Lower CretaceousAlbianAsu River GroupFirst (1st) cycle of sedimentation
Aptian
PrecambrianBasement complex

A total of sixteen (16) representative samples obtained from Nkporo Shale from Aguabor near Onitsha Road, Enugu Shale from Amaechi, Agwu Shale from Ituku Ozara, Imo Shale from Nibo and Umuawulu as well as shale from Mamu Formation all of which lie within the Lower Anambra Basin. The samples were collected using a chisel and geological hammer and were packed into polythene bags to prevent loss of moisture. The samples were air dried and pulverized before analysis and tests were carried out. Mineralogical analysis of the samples was carried out at the Center for Energy Research and Development (CERD), Obafemi Awolowo University, Ile-Ife, using Xray Diffractometer. The analysis was conducted with the aid of Radicon MD 10 Diffractometer and the interpretation of the diffractograms was done by comparing the peaks to standard minerals established by the International Center for Diffraction Data (ICDD), 2008 and 2009. Geochemical analysis was carried out at Acme Analytical Laboratories (Vancouver), Canada for major, trace and rare earth element using Inductive Coupled Plasma Atomic Emission Spectrometer (Perkin-Elmer Elan 6000 or 9000) following Lithium metaborate/tetraborate fusion and dilute nitric digestion to determine the major oxides (SiO2, Al2O3, Fe2O3, MgO, CaO, Cr2O5, Na2O, K2O, TiO2, P2O5 and MnO) while trace elements (Nb, Ba, Cr, Ni, Sr, Zr, Ni, Y and Sc) and rare earth elements were analyzed by ICP Mass Spectrometry. Loss on ignition (LOI) was determined for by measuring the weight loss after heating.

Mineralogy

Mineralogical analysis of shale samples from Agwu Shale reveal minerals such as sillimanite, vermiculite, dickite and kaolin. Kaolin was the dominant minerals present (Fig. 3). Enugu Shale revealed minerals such as dickite, kaolin minerals, illite, kaolinite, chlorite and chamosite. The result showed that kaolin is the dominant mineral (Fig. 4 and 5). Mamu Shale reveal minerals such as chlorite, kaolin minerals, sillimanite, quartz, vermiculite and kaolinite with kaolinite been the dominant mineral present (Fig. 6) and Imo Shale samples reveal minerals such as montmorillonite, chamosite, dickite, kaoline minerals, vermiculite, and kaolinite. Montmorillonite and kaolinite were the dominant minerals present (Fig. 7). On the average, the XRD results showed that the kaolinite group of minerals (comprising of kaolinite and dickite) are dominant in the analyzed samples and constitute about 41.5% of the entire mineralogical composition. The chlorite group (comprising amesite and chamosite) make up 17.8%. Montmorillonite make up a composition of 15.8% and illite has a composition of 6.8%. the non-clay minerals including quartz, ilmenite and sillimanite make up 3.7%, 4.0% and 10.2% respectively (Table 2).

Fig. 3. X-Ray diffractogram for Agwu shale.

Fig. 4. X-Ray diffractogram for Enugu shale.

Fig. 5. X-Ray diffractogram for Nkporo shale.

Fig. 6. X-Ray diffractogram for shale at Mamu Formation.

Fig. 7. X-Ray diffractogram for Imo shale.


Average of mineralogical composition of the shale samples


MINERALAMOUNT (%)
Kaolinite group41.5
Chlorite17.8
Montmorillonite15.8
Sillimanite10.2
Illite6.8
Ilmenite4.0
Quartz3.7


Major Element Geochemistry

The geochemical results of sixteen (16) shale samples from the Lower Anambra Basin comprising of Agwu Shale (2), Nkporo Shale (2), Enugu Shale (2), Mamu Shale (1) and Imo Shales (9) (Table 3) show that they are on average relatively rich in SiO2, Al2O3 and Fe2O3 and have a mean value of 52.29%, 14.09% and 6.15% for Imo Shale (IS), 52.31%, 16.70% and 7.39% for Mamu Shale (MS), 43.21%, 21.33% and 10.36% for Enugu Shale (ES), 53.35%, 15.64% and 7.17% for Nkporo Shale (NS) and 51.24%, 17.25% and 7.78% for Agwu Shale (AS). The SiO2 content is higher in Imo Shale (IS) with SiO2 value ranging from (22.85-75.87wt%) the sample L2S1 records the highest SiO2 value of 75.87wt% indicating a high siliceous content. This is followed by Nkporo Shale (NS) (51.92-54.78wt%), Mamu Shale (MS) (52.31wt%), Agwu Shale (AS) (51.06-51.41wt%), and Enugu Shale (ES) (41.90-44.51wt%). Enugu Shale records the highest Al2O3 content of (21.24-21.41wt%) while IS (9.48-18.87wt%) and AS (16.87-17.63wt%). According to Cullers and Podkovyrov, (2000) such high Al2O3 content may be due to the dilution effect of quartz. The Al2O3 content is lowest in NS (14.82-16.46 wt%), followed by IS (14.08%) and MS (16.70wt%). ES records the highest Fe2O3 content of (9.23-11.48wt%) followed by sample L2S16, IS (1.66-9.27wt%), AS (7.64-7.92wt%), MS has 7.39% while NS has the lowest Fe2O3 content of (7.07-7.26wt%). High content of Fe2O3 suggests the presence of iron-bearing minerals as well as deposition under a reducing condition (Akpokodje et al.,1991). The shales are however low in Na2O, MgO, K2O, MnO, TiO2, CaO and P2O5. ES samples such as L5S4, L5S1 and IM samples L2S6 and L2S14 recorded the lowest K2O content. This low K2O value may be due to low amount of illite, montmorillonite or feldspar present in small proportions (Akpokodje et al.,1991; Adamu et al.,2022). Imo Shale records the lowest P2O5 content on sample L2S1 and L2S7 this may be due to the lower amount of accessory phases present such as apatite and monazite (Okunlola and Idowu, 2012). The low content of MgO and CaO as recorded on NS and IS sample L2S1 suggest they had no associated carbonates or dolomitisation (Okunlola and Idowu, 2012). The average MnO, K2O, Na2O and SiO2 contents are very close for NS and MS (Table 6). The NS shales are higher in SiO2 than MS but lower in Al2O3, MgO and TiO2. The 19.58% average Loss on ignition (LOI) for the Anambra shales is high, which reveals the potential of the shales for carbonaceous compounds. On the Fe2O3/K2O vs. SiO2/Al2O3 chemical classification diagram (Fig. 8; Herron, 1988), Anambra shales plot in the Fe shale field and Fe sands confirming the field description of the samples. Three samples of Imo Shale plot in the field of Fe sands and can be attributed to the high SiO2 content (samples L2S16, L2S3 and L2S1) in comparison with the other shale samples (Table 3).

Fig. 8. Geochemical classification of the shales using log (SiO2/Al2O3) vs log (Fe2O3/K2O) diagram (after Herron, 1988).


Major elements distribution of shale samples from Anambra Basin


Parameters (% Oxide)Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo ShaleMean value
L4S1L4S2L1S1L1S4L5S1L5S4L3S1L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17
SiO251.4151.0654.7851.9241.944.5152.3175.8750.1945.760.2653.1347.9122.8564.750.0751.15
Al2O316.8717.6314.8216.4621.4121.2416.79.4817.1117.8414.8417.1718.874.649.5417.315.75
Fe2O37.647.927.077.2611.489.237.393.927.488.337.474.625.51.669.277.116.83
MgO0.570.590.330.530.510.470.560.482.623.081.132.893.251.5922.471.44
CaO0.260.230.070.060.390.440.060.040.10.410.170.130.3234.50.190.232.35
Na2O0.280.270.370.360.120.140.360.040.020.030.010.030.040.010.060.030.14
K2O1.321.31.331.320.790.851.320.440.440.760.390.840.810.160.90.780.86
TiO21.431.451.441.141.651.721.161.241.051.171.211.091.260.770.731.11.23
P2O50.070.080.080.090.170.180.110.050.120.070.080.060.070.280.190.110.11
MnO0.060.060.030.030.030.030.030.020.020.210.020.030.030.220.060.120.06
Fe2O3/K205.786.095.315.514.5310.855.598.91710.9619.155.56.7910.3710.39.11-
SiO2/Al2O34.052.893.693.152.952.093.1386.937.564.065.092.534.926.782.894.35
K2O/Na2O4.714.813.693.676.586.073.67112225.33392820.25161526-
Al2O3/TiO211.7912.1610.2914.4412.9712.3514.397.6516.2915.2412.2615.7514.976.0313.0615.72-
CIA90.0690.7389.3390.4394.2793.6990.5694.896.8393.6996.394.4994.1681.889.2494.3292.16
ICV0.60.5880.620.580.6220.5250.5820.5210.6240.7180.6190.4970.5278.2191.3080.62-
PIA96.6497.0296.8497.397.5897.2397.3499.1299.2897.4898.7699.0298.0481.4997.1898.4596.79
CIW96.8997.2497.1197.5197.6797.3497.5499.1699.397.5998.899.0798.1291.8597.4498.5197.57
LOI19.919.219.420.621.32119.88.220.622.214.219.821.73312.120.4-

CIA: Chemical Index of Alteration = Al2O3Al2O3+CaO*+Na2O*100 after (Nesbitt and Young, 1982), CaO*=CaO103*P2O5 after (McLennan et al., 1993), PIA: Plagioclase index of alteration = 100*Al2O3K2OAl2O3+CaO*+Na2OK2O (after Fedo et al., 1995), CIW: Chemical index of weathering = Al2O3Al2O3+CaO*+Na2O*100 (After Harnois, 1988), ICV: Index of Chemical Variability = Fe2O3+K2O+Na2O+CaO+MgO+MnOAl2O3




Average chemical composition of Anambra shales compared to shale from other sedimentary basins in Nigeria


OxidePresent studyBida Shale (Okunlola & Idowu,2012)Asu River Group (Amajor, 1987)Ezeaku Shale (Amajor, 1987)Auchi Shale (Fagbamigbe, 2013)Ifon Shale (Ajayi et al.,1989)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
SiO251.2453.3543.2152.3152.2961.2669.9444.9151.6863.3
Al2O317.2515.6421.3316.7014.0816.881015.7118.7618.47
Fe2O37.787.1710.367.396.153.754.046.244.671.26
MgO0.580.430.490.562.170.160.872.584.390.82
CaO0.250.070.420.064.010.053.3815.421.90.09
Na2O0.280.370.130.360.030.060.40.420.930.42
K2O1.311.330.821.320.611.391.152.361.162.36
TiO21.441.291.691.161.071.740.520.651.951.02
P2O50.080.090.180.110.110.080.170.460.250.46
MnO0.060.030.030.030.080.020.040.060.060.01
LOI19.552021.1519.819.114.29.2111.114.0511.6
Total99.8299.7799.8199.899.799.5999.6999.9199.8799.81


When compared to average value of shale from different sedimentary basins within Nigeria, it is observed that Agwu Shale (AS), Nkporo Shales (NS), Mamu Shales (MS), and Imo Shale (IS) had a higher SlO2, Al2O3, Fe2O3 and TiO2 content than Ezeaku shale. In contrast, Ezeaku shale had a higher MgO, CaO, Na2O and K2O content than shales from the Anambra Basin. AS showed similarity to Auchi shale in terms of SiO2, Al2O3 a nd M nO. Auchi s hale i s lower in Fe2O3, MgO, K2O but higher in CaO, Na2O, TiO2 and P2O5 content. Agwu Shale (AS) is similar to Bida Shale in P2O5 content while Bida Shale has a higher SiO2, K2O, TiO2 and lower in terms of Al2O3, Fe2O3, MgO, CaO, Na2O and MnO. When compared to shales from sedimentary basins from other parts of the world (Pettijohn, 1957; Turekan and Wedephol 1961; PAAS (Taylor and McLennan, 1985; Gromet et al., 1984), Anambra shales had a lower average of SiO2, MgO, CaO except for Imo Shale (IS) which had a higher Al2O3, Fe2O3 and TiO2 content (Table 3).

Trace and Rare Earth Element Geochemistry

Trace element concentrations of shales within the Lower Anambra Basin are presented in Table 4 while average trace elements of the Anambra shales compared to some world averages are presented in Table 8. Based on the available trace elements data, Nkporo Shale records the highest concentration in Ba, Co, Cu and Pb but depleted in Ni and Zn content compared to Agwu Shale and Mamu Shale. A comparison with world averages (Vine and Tourtelot, 1970; Turekan and Wedephol, 1961 and PAAS, Taylor and McLennan, 1985) shows that the world average values are higher in terms of Ba, Cu, Ni, Pb, and Zn. All the shales recorded higher Co values compared to average global values. Also, Enugu Shale records higher Zn value than the global average. The rare earth elements geochemical data are presented in (Table 5), while the averages are presented in (Table 9) alongside average values of shale from other regions (Okunola and Idowu, (2012), McLennan and Taylor (1980), Levinson, (1974)) and PAAS (Taylor and McLennan, 1985) for comparison. Comparison with shales from other regions such as Bida Shale, it was observed that Bida Shale show a higher REE content (La-Lu) than Anambra shales while that of PAAS (Taylor and McLennan, 1985), McLennan and Taylor, (1980) and Levinson, (1974) show a higher La, Sm, Tb and Lu values than Anambra shales but depleted in Ce and Nd (Table 9).


Trace elements (ppm) distribution of shale samples from Anambra Basin


AgwuShaleNkporoShaleEnuguShaleMamu ShaleImo Shale
Trace (ppm)L4S1L4S2L1S1L1S4L5S1L5S4L3S1L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17
Ni23.127.424.32822.224.730.14.123.423.67.723.713.2245.827.5
Ba334298376353211254351204268325119199120541317329
Co22.227.435.626.423.12431.35.421.252.710.126.616.632632.5
Cu27.534.177.458.750.429.842.11.84.27.75.28.45.613.83.812.5
Pb19.116.620.819.819.71820.97.68.811.710.95.57.74.91212.1
Zn8510355661529984191011164112170409876
Ni/Co1.0410.681.060.961.020.960.761.10.450.760.890.790.671.760.85
Cu/Zn0.320.331.410.890.330.30.50.090.040.060.130.070.080.350.040.16



Rare earth element (ppm) distribution of shales from Anambra Basin


ElementsNkporoShaleImo ShaleMamu ShaleAgwuShaleEnuguShale
L1S1L1S4L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17L3S1L4S1L4S2L5S1L5S4
La19.822.410.216.118.115.022.818.815.417.822.422.218.018.520.621.6
Ce116.1129.155.791.8107.387.4159.4109.644.1109.5129.0128.0102.3104.8117.0120.6
Nd34.136.415.826.032.320.658.932.528.139.940.437.430.030.135.335.9
Sm1.962.00.861.511.901.113.302.01.562.422.452.11.661.792.122.14
Eu0.160.150.060.120.150.080.240.150.110.190.210.160.130.150.170.17
Tb0.070.050.030.050.060.040.090.060.050.090.100.060.060.060.070.08
Yb0.860.730.430.510.550.500.710.50.260.721.00.760.740.780.810.82
Lu0.020.020.010.010.010.010.020.010.0060.020.020.020.020.020.020.02
Total173.1190.983.1136.1160.4124.7245.5163.689.6170.6195.6190.7152.9156.2176.1181.3



Average trace element chemical composition of Anambra shale compared to shale from other sedimentary basins


Present studyBida Shale (Okunlola & Idowu,2012)Levinson (1974)Vine & Tourtelot (1970)Turekan & Wedephol (1961)PAAS(Taylor and McLennan, 1985)NASC (Gromet et al., 1984)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
Ni25.2526.1523.4530.11919.375070685558
Ba316364.5232.5351269.1394.23300700580650636
Co24.83123.5531.321.533.421020_23n.a
Cu30.868.0540.142.1714.4570504550n.a
Pb17.8520.318.8520.99.0222.2820n.an.a20n.a
Zn9460.5125.58475.77116.393001009585n.a
Srn.an.an.an.an.a59.39200300300200142
Vn.an.an.an.an.a108.77150130130150130
Yn.an.an.an.an.a70.693025--n.a
Zrn.an.an.an.an.a1156.5470160160210200
Mon.an.an.an.an.a0.72103--n.a
Nbn.an.an.an.an.a52.462020n.a1.90n.a
Rbn.an.an.an.an.a46.19140n.an.a160n.a
Thn.an.an.an.an.a29.2212n.an.a14.60n.a
Un.an.an.an.an.a13.074n.an.a3.10n.a
Cu/Zn0.331.040.320.500.140.12
(Cu+Mo)/Zn-----0.13
Ni/Co2.040.840.990.960.890.58
Rb/K2O-----33.23
U/Th-----0.45

*PAAS= Post Archean Australian shales *NASC= North American shale composite *n.a= not analyzed




Average rare earth elements of Anambra shale compared to world averages


Present studyBida Shale (Okunlola & Idowu,2012)PAAS (Taylor and McLennan, 1985)Codo Shale (McLennan, et al., 1990)Average shale (Levinson,1974)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
La18.2521.121.122.217.477.4038.229.7121
Ce103.55122.6118.8128.099.3170.4279.663.450
Nd30.0535.2535.637.432.767.6633.927.924
Sm1.7251.982.132.11.9012.465.55--
Eu0.140.1550.170.160.142.251.08--
Tb0.060.060.0750.060.061.980.744--
Yb0.760.7950.8150.760.57----
Lu0.020.020.020.020.011.180.433--
Dy-----11.794.68--
Pr-----18.718.83--
Gd-----11.014.66--
Ho-----2.480.991--
Er-----7.692.85--
Tm-----1.140.405--


Correlation Analysis

Correlation analysis between the major elements and available trace elements show that SiO2 correlated negatively with Al2O3 (r = -0.061) in the shales. The presence of SiO2 is related to the occurrence of plagioclase, quartz, and clay minerals, whereas the Al2O3 content may be correlated with the preferred incorporation of aluminous clays into shale (Al-Juboury et al. 2021). The negative correlation between SiO2 and Al2O3 may suggest that SiO2 occur as either detrital silicates or is not influenced by clay minerals (Wang et al. 2017). Positive correlation exists between Al2O3 and Fe2O3=0.657, MgO=0.026, Na2O=0.225, K2O=0.429 and TiO2=0.720 while negative correlation exists between Al2O3 and CaO=-0.664, P2O5= -0.393 and MnO= -0.381 (Table 10). For the trace elements, positive correlation exists between Al2O3 and Ni= 0.316, Cu=0.284, Pb=0.474 and Zn=0.643 revealing association with clay minerals and feldspars while negative correlation occurs for Ba= -0.486 suggesting that they are not sourced from clay minerals (Akkoca et al., 2019) (Table 10).


Two-tailed Pearson correlation matrix


SiO2Al2O3Fe2O3MgOCaONa2OK2OTiO2P2O5MnONiBaCoCuPbZn
SiO21
Al2O3-.0611
Fe2O3.122.6571
MgO-.154.026-.2211
CaO-.683-.664-.621.0391
Na2O.057.225.255-.677-.2421
K2O.162.429.412-.393-.495.8801
TiO2.002.720.499-.509-.443.381.3791
P2O5-.616-.393-.030-.072.721-.211-.380-.2831
MnO-.589-.381-.301.359.655-.314-.318-.412.4081
Ni.185.316.641.028-.491.376.634-.045.003-.1681
Ba-.503-.486-.257-.187.642.330.138-.362.531.629.1311
Co-.019.501.512.208-.463.314.555.155-.302.206.655.1481
Cu-.155.284.330-.666-.119.859.691.495-.011-.245.258.325.3071
Pb.027.474.666-.699-.400.853.790.597-.088-.314.486.172.432.8361
Zn-.258.643.661.220-.327-.002.264.286.070-.024.576-.143.550.105.2531


Provenance

The provenance of a sedimentary rock is dependent on the characteristics of its source region such as composition, location, climate and topography from which it originated (Paikaray et al., 2008; Dey et al., 2009; Kalsbeek and Frei, 2010; Mishra and Sen, 2010). Therefore, a sedimentary rock's provenance can be determined by analyzing the geochemical characteristics of its clastic components. Studies have shown that several major, trace and rare earth elements geochemical data and ratios can provide empirical evidence on the composition, source rock, along with the outcome of sedimentary processes like sorting and weathering (Paikaray et al., 2008; Dey et al., 2009; Kalsbeek and Frei, 2010; Mishra and Sen, 2010). Ratios such as Al2O3/TiO2 is useful in characterizing the nature of source material due to the immobile nature and low solubility of the oxides and hydroxides of aluminum and titanium in low temperature solutions (Yamamoto et al., 1986; Sugitani et al., 1996). As a result, values between 3-8 points to mafic igneous rock sources; while values of 8–21 and 21–70 are associated with intermediate and felsic igneous rock sources (Gabriel et al., 2019). The calculated Al2O3/ TiO2 ratios of shales of the Lower Anambra Basin range from 11.79 to 12.16 for Agwu Shale, 10.29 to 14.44 for Nkporo Shales, 12.35 to 12.97 for Enugu Shales, 14.39 for Mamu Shales and 6.03 to 16.29 for Imo Shales respectively (Table 4), revealing Agwu Shales, Nkporo Shales, Enugu Shales and Mamu Shales as derivatives from an intermediate igneous rock source while Imo Shales show a mafic and intermediates source. Further confirmation using discriminant function discrimination plot of Roser and Korsch (1988), based on the concentrations of both immobile and variably mobile major elements, and divided into four provenance categories: mafic igneous, intermediate igneous, felsic igneous and quartzose recycled sedimentary, plots the shales in the field of intermediate igneous province and mafic igneous province respectively (Fig. 9). Also, on a binary plot of Al2O3 Vs TiO2 (Krzeszowska, 2019), the shales also plotted mostly in the field of intermediate igneous rocks and mafic igneous rocks (Fig. 10). This empirical finding aligns with the findings of prior investigations conducted by Bolarinwa et al. (2022) and Akinyemi et al. (2022), in which they also identified the source of shales as originating from intermediate and mafic geological origins. Notably, Bolarinwa et al. (2022) posited the hypothesis that the mafic component might have been introduced through post-depositional geological processes affecting the shales. Consequently, the present study proposes that the shales have their provenance rooted in intermediate igneous rocks throughout the Lower Cretaceous to Paleocene periods, with a minor mafic contribution evident in the Upper Cretaceous, as indicated by the youngest Imo Shale unit.

Fig. 9. Major element discriminant function diagram for Anambra shales (Roser and Korsch, 1988).

Fig. 10. Discrimination plot for provenance Al2O3 (wt%) vs TiO2 (wt%) plot (after Krzeszowska, 2019).

Discriminant Function 1=(1.773TiO2)+(0.607Al2O3)+(0.760Fe2O3)+(1.500MgO)+(0.616CaO)+(0.509Na2O)+(1.224K2O)+(9.090)
Discriminant Function 2=(0.445TiO2)+(0.070Al2O3)+(0.250Fe2O3)+(1.142MgO)+(0.438CaO)+(1.475Na2O)+(1.426K2O)+(6.861)

Paleo-weathering and Sediment Maturity

Weathering of source rocks is a major process in the formation of clays and clay minerals. The rate of weathering of a source rock is controlled by factors such as source rocks composition, nature of climate, time and tectonism (Fedo et al., 1995; Nesbitt et al., 1997). The process of weathering of source rocks removes mobile elements such as Ca, Na and K whose presence in clays serves as index in determining the rate of chemical weathering of their source rocks (Nesbitt and Young, 1984, 1989; Taylor and McLennan, 1985; Wronkiewicz and Condie, 1990; Fedo et al., 1995; Nesbitt et al., 1997; Lindsey, 1999; Roy and Smykatz-Kloss, 2007). Some of the indices proposed includes; Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW) (Nesbitt and Young, 1982; Harnois, 1988; Fedo et al., 1995). Chemical Index of Alteration (CIA) is a good measure of the degree of chemical weathering. Thus, high CIA values reflect the removal of labile cations, such as Ca2+, Na2+, and K+ in relation to more stable cations, such as Al3+ and Ti4+, (Nesbitt and Young, 1982). Conversely, low CIA values suggest low weathering effects on these cations. Plagioclase Index of Alteration (PIA) measures the degree of weathering of feldspars of the source rocks to clay. While Chemical Index of Weathering (CIW) is a measure of the degree of weathering in which the source material has undergone and the value of this index rises as the level of weathering rises. The difference between the CIW index values of the parent silicate rock and the clay material is suggestive of how much weathering the weathered material has undergone (Harnois, 1988). The CIA, PIA and CIW of shales of the Anambra Basin was determined and is represented in Table 3. The results show that PIA values range from 81.49-99.28% with an average of 96.79%, CIA range from 81.80-96.83% with an average of 92.16% while CIW range from 91.85-99.07% with an average of 97.57%. The results obtained (average CIA=92.16%; PIA=96.79% and CIW=97.57%), suggest that the protolith of the shales were subjected to intense weathering. This was further confirmed on a plot of CIA vs ICV of Long et al., (2012) (Fig. 11), in which the Anambra shales plotted below PAAS-ICV value of 0.85 indicating that the shales are mature and intensively weathered while on the plot of CIA vs PIA of Suttner and Dutta (1986) (Fig. 12) they show a high degree of weathering of the source materials. PIA values have been shown to reveal the intensity of destruction of feldspars during the process of source rock weathering (Fedo et al., 1995). Detrital grains of feldspars in shales can preserve signatures of the various degrees of alteration in the source rock location as well as during transport, sedimentation, and diagenesis due to the chemical contents of feldspars (Na, K, and Ca) and are known to be highly mobile during weathering processes. Bivariate plots involving oxides of Na2O, K2O, and CaO vs PIA were plotted to assess the mobility of elements as a result of feldspar weathering. The bivariate plot of K2O/Na2O versus PIA of shales show a positive correlation of this ratio with PIA (Fig. 14) while a negative correlation was obtained for a bivariate plot of K2O + Na2O wt.% against PIA (Fig. 14). According to Nesbitt and Young (1984), total alkalis content of source rocks (K2O+Na2O) decreases with increase in K2O/Na2O ratio as the rate of weathering increases due to destruction of feldspars. While the bivariate plot of NaO, CaO and K2O against PIA, shows a decrease in the mobile elements as PIA increases respectively (Fig. 14). The weathering indices of siliciclastic rocks offer helpful information on the climate conditions of the provenance. According to Moosavirad et al., (2011), an increase in chemical weathering may indicate a shift in climate toward warm, humid conditions. The binary plot of Suttner and Dutta (1986) was used to constrain the climatic condition during sedimentation. The shales of the study area plot mostly in the arid and semi-arid field with only two of the Imo Shale samples plotting in the humid field probably due to an increase in chemical weathering conditions (Moosavirad et al., 2011). This shift in weathering conditions probably indicates a climate transition from a semi-arid environment to a humid environment during the deposition of the younger Imo Shale in the upper Cretaceous period.

Fig. 11. CIA versus ICV plot showing weathering intensity and maturity of Anambra shales (after Long et al.,2012).

Fig. 12. CIA vs PIA plot showing Palaeo-weathering condition (after Suttner and Dutta 1986).

Fig. 13. Plot of SiO2 versus Al2O3 + K2O + Na2O discrimination diagram for paleoclimate condition of sedimentation (after Suttner and Dutta, 1986).

Fig. 14. Bivariate diagrams of Na, Ca, and K against PIA showing the mobility of feldspars due to weathering in the lithological components of shales.

Tectonic Setting

Studies have shown that the tectonic setting of a sedimentary rocks' provenance is greatly influenced by its chemical makeup (e.g., Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch, 1986). Also, sedimentation, diagenesis, and composition of sediments all have a significant impact on the tectonic setting of the environment of sediments deposition (Bhatia 1983). Therefore, the composition and chemical makeup of silisiclastic rocks from various tectonic setting can vary considerably (Roser and Korsch 1986; McLennan et al. 1990). In an effort to constraining the tectonic setting of silisiclastic rocks, relevant discrimination diagrams based on major elements have been employed by several researchers (e.g. Armstrong-Altrin, 2015; Tawfik et al. 2015; Al-Juboury et al., 2021) based on the differences in SiO2 values, and the discrimination diagrams are subdivided into a low silica type (35–63%) and a high-silica type (63–95%) and accordingly, three different tectonic setting can be identified as island arc, rift and collisional settings. The SiO2 value of shales of the present study vary from 41.90 to 75.87wt%, therefore the samples can be classified as low-silica clastic sediments and high-silica clastic sediments (Table 3). The discrimination plots of Roser and Korsch, (1986) of SiO2 versus K2O/Na2O classify tectonic setting into three as passive continental margin, active continental margin and ocean island arc tectonic setting. In view of establishing the tectonic setting of shales in the Lower Anambra Basin, the plot of Armstrong-Altrin et al. (2015) and Roser and Korsch, (1986) were employed. Based on this diagram, the shales classify mostly in the field of rift (Passive Continental Margin) and a few plot as collision (Active Continental Margin) probably due to the presence of nearby basement rocks as observed on both the high and low silica discrimination diagrams (Fig. 15 and 16). The current findings aligns with the research conducted by Ejeh in 2021 focused on the geochemistry of sandstones and shales in the lower Anambra basin, which characterized the tectonic history of the lower Anambra sediments as primarily involving a continental rift or passive margin phase, followed by a collision or convergent setting, with only minor involvement of arc settings. Accordingly, the inferred tectonic setting is in agreement with the accepted model on the origin and tectonic evolution of the Benue Trough as established based on geophysical models (Benkhelil, 1989). Also, according to Bhatia (1983) silisiclastic sediments of passive margin tectonic setting are enriched in SiO2 and depleted in Na2O, CaO and TiO2 suggesting their highly recycled and mature nature. This is also observed with the present study as well as previous studies conducted within the Anambra Basin (Odewumi,2013; Ejeh et al., 2015; Shettima et al., 2017; Toyin and Adekeye, 2018; Ikhane et al., 2022).

Fig. 15. Discriminant function diagrams based on high –silica for shales of the Lower Anambra Basin (after Verma and Armstrong-Altrin 2013).

Fig. 16. Discriminant function multidimensional diagram of low-silica for shales of the Lower Anambra Basin (after Verma and Armstrong-Altrin 2013).

Fig. 17. Bivariate plot of SiO2 versus K2O/Na2O indicating tectonic setting discrimination diagram (After Roser & Korsch, 1986).

The discriminant function equations are:

DF1Arc-Rift-Col=-0.263-InTiO2SiO2adj+0.064-InAl2O3SiO2adj+1.725-InFe2O3tSiO2adj+0.660-InMnOSiO2adj+2.191-InMgOSiO2adj+o.144-InCaOSiO2adj+-1.304-InNa2OSiO2adj+0.054-InK2OSiO2adj+-0.330-InP2O5SiO2adj+1.588
DF2Arc-Rift-Col=-1.196-InTiO2SiO2adj+1.064-InAl2O3SiO2adj+0.303-InFe2O3tSiO2adj+0.436-InMnOSiO2adj+0.838-InMgOSiO2adj+o.407-InCaOSiO2adj+-1.021-InNa2OSiO2adj+-1.706-InK2OSiO2adj+-0.126-InP2O5SiO2adj-1.068
DF1Arc-Rift-Col=-0.608*InTiO2SiO2adj+1.854*InAl2O3SiO2adj+0.299*InFe2O3tSiO2adj+-0.550*InMnOSiO2adj+0.120*InMgOSiO2adj+o.194*InCaOSiO2adj+-1.510*InNa2OSiO2adj+1.941*InK2OSiO2adj+0.003*InP2O5SiO2adj-0.294
DF2Arc-Rift-Col=-0.554*InTiO2SiO2adj+-0.995*InAl2O3SiO2adj+1.765*InFe2O3tSiO2adj+-1.391*InMnOSiO2adj+-1.034*InMgOSiO2adj+o.225*InCaOSiO2adj+0.713*InNa2OSiO2adj+0.330*InK2OSiO2adj+0.637*InP2O5SiO2adj-3.631

Paleo-oxygenation Condition and Depositional Environment

The ratios of trace elements like Th/U, Ni/Co, V/Ni, and Cu/Zn are crucial in determining the paleo-redox conditions during sediment deposition, as demonstrated by Dypvik (1984) and Akkoca et al. (2019). According to Jones and Manning's (1994), Ni/Co ratios below 5 are indicative of deposition in an oxic environment, whereas ratios above 5 indicate deposition in suboxic/anoxic environments. The investigated shale samples had an average Ni/Co ratio of 1.10, and range from 0.45 to 1.76, which indicates that they were deposited under oxidizing conditions. Also, Nagarajan et al. (2007) showed that high Cu/Zn ratios denote reducing depositional condition, while low Cu/Zn ratios denote oxidizing depositional conditions. The Cu/Zn ratio of shales from the Anambra Basin is between 0.04 – 1.41 with an average of 0.72 (Table 7) hence indicating a more oxidizing condition.


Comparing average chemical composition of Anambra shale to published average shales


OxidePresent studyAverage Bida shale (Okunola &Idowu, 2012)Average shale (Pettijohn, 1957)Turekan & Wedephol (1961)PAAS (Taylor and McLennan, 1985)NASC (Gromet et al., 1984)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
SiO251.2453.3543.2152.3152.2961.2658.158.562.4064.82
Al2O317.2515.6421.3316.7014.0816.8815.41518.7817.05
Fe2O37.787.1710.367.396.153.756.94.727.185.7
MgO0.580.430.490.562.170.162.42.52.192.83
CaO0.250.070.420.064.010.053.13.11.293.51
Na2O0.280.370.130.360.030.061.31.31.191.13
K2O1.311.330.821.320.611.393.23.13.683.97
TiO21.441.291.691.161.071.740.60.770.990.8
P2O50.080.090.180.110.110.080.20.160.160.15
MnO0.060.030.030.030.080.02Trace---
K2O/Na2O4.673.596.303.6720.3323.16
K2O/Al2O30.080.090.040.080.040.08
Al2O3/TiO211.9812.1212.6214.3913.159.70
Cu/Zn0.331.040.320.500.140.12
Ni/CO2.040.840.990.960.890.58

*PAAS= Post Archean Australian shales *NASC= North American shale composite


Mineralogical and geochemical data of shales within the Anambra Basin have been used to constrain the provenance, tectonic setting, paleo-environment, maturity and paleoweathering. Mineralogical studies revealed that the shales mineral constituent is dominantly kaolin while non-clay minerals are quartz, ilmenite and sillimanite. Major element geochemical plot of Fe2O3/K2O vs SiO2/Al2O3 showed that the shales are Fe shales and Fe sands. Major element abundance showed that the shales are dominant in SiO2, Al2O3 and Fe2O3 content and are depleted in Na2O, MgO, K2O, MnO, TiO2, CaO and P2O5. Provenance studies showed that they were derived dominantly from intermediate igneous rocks from the surrounding basement rocks, with minor contribution from mafic igneous rocks during the deposition of Imo Shale. Tectonic setting discrimination diagrams revealed a dominant passive continental margin (rift) type tectonic setting. Indices of CIA, CIW, PIA and ICV for paleo-weathering and maturity showed that the shales are intensively chemically weathered and are immature on the basis of SiO2/Al2O3 ratio but matured when compared to PAAS values (Taylor and McLennan, 1985). Ratios of Cu/Zn and Ni/Co showed that the shales were deposited in an oxidizing condition.

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Article

Research Paper

Econ. Environ. Geol. 2023; 56(6): 799-816

Published online December 29, 2023 https://doi.org/10.9719/EEG.2023.56.6.799

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Mineralogy and Geochemistry of Shale Deposits in the Lower Anambra Basin, Nigeria: Implication for Provenance, Tectonic Setting and Depositional Environment

Olugbenga Okunlola1, Agonsi Udodirim Lydia1, Aliyu Ohiani Umaru2,*, Raymond Webrah Kazapoe3, Olusegun G. Olisa4

1Department of Geology, University of Ibadan, Ibadan, Nigeria
2Department of Geology, University of Maiduguri, Maiduguri, Borno State
3Department of Geological Engineering, University for Development Studies, Nyankpala, Ghana
4Department of Earth Sciences, Olabisi Onabanjo University, Ago Iwoye, Ogun state, Nigeria

Correspondence to:*ualeey@gmail.com

Received: September 10, 2023; Revised: November 24, 2023; Accepted: November 27, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Mineralogical and geochemical studies of shales within the Lower Anambra Basin was conducted to unravel the depositional environment, provenance, maturity, paleo-weathering conditions, and tectonic settings. Mineralogical studies conducted using X-ray diffraction analysis revealed that the samples were composed of kaolinite, montmorillonite, chlorite, and illite. KaolinIite is the dominant mineral, constituting approximately 41.5% of the bulk composition, whereas the non-clay minerals are quartz, ilmenite, and sillimanite. Geochemical analysis showed a predominance of SiO2, Al2O3, and Fe2O3 contents of the shale samples with mean values of 52.29%, 14.09%, and 6.15% for Imo Shale (IS); 52.31%, 16.70%, and 7.39% for Mamu Shale (MS); 43.21%, 21.33%, and 10.36% for Enugu Shale (ES); 53.35%, 15.64%, and 7.17% for Nkporo Shale (NS); and 51.24%, 17.25%, and 7.78% for Agwu Shale (AS). However, the shales were depleted in Na2O, MgO, K2O, MnO, TiO2, CaO, and P2O5. The trace element ratios of Ni/Co and Cu/Zn of the shale suggest an oxic depositional environment. The average SiO2 vs. Al2O3 ratio of the shales indicated textural maturity. Compared to the PAAS standard, the shales plot below the PAAS value of 0.85, suggesting a high degree of maturity and intensive chemical weathering, further confirmed on a CIA vs. PIA plot. On log (K2O/Na2O) against SiO2 and tectonic setting discriminant function diagrams, the shales plot mostly in the field of passive continental margin tectonic setting. The discriminant function diagrams as well as Al2O3/TiO2 ratio of the shales showed that they were derived from a mixed source (mafic and intermediate igneous rocks).

Keywords Anambra Basin, provenance, tectonic setting, mineralogical, chemical, shale

Research Highlights

  • Mineralogical studies reveal KaolinIite as the dominant mineral, constituting approximately 41.5% of the bulk composition.

  • The shales are deposited in an oxic depositional environment and are texturally mature.

  • Tectonically, the shales characterized in the passive continental margin tectonic setting and are derived from a mixed source (mafic and intermediate igneous rocks)

Introduction

Weathering conditions, sorting, provenance, and tectonism are interrelated phenomena that contribute to the geochemistry of clastic sedimentary rocks (Johnsson, 1993). The composition of shales is influenced by the tectonic setting of the basin of deposition as well as the chemical composition of the source rock area (Bhatia and Crook, 1986). Sedimentary geochemical data has been demonstrated to provide critical information for provenance studies through the analysis of major, trace, and rare earth elements (REEs) as well as their ratios in sedimentary rocks, which can simulate climatic, geographical, and tectonic conditions of the basins (Bhatia, 1983; Taylor and McLennan, 1985; Bhatia and Crook, 1986; McLennan, 1989; Feng and Kerrich, 1990; McLennan and Taylor, 1991; Cullers, 1994; Jahn and Condie, 1995; Girty et al., 1996; Etemad-Saeed et al., 2011; Verma and Armstrong-Altrin, 2013; Obaje et al., 2020; Waziri et al., 2020; Musa et al., 2022). Therefore, detailed studies of the geochemistry and mineralogy of fine-grained clastic sedimentary rocks is regarded as a valuable tool for determining provenance, tectonic setting, paleoweathering, and redox condition of a given source location (Ruffell et al., 2002; Ahlberg et al., 2003; Deconinck et al., 2005; Dera et al., 2009; Shettima et al., 2020a; Shettima et al., 2020b). Anambra Basin located in the southern part of Nigeria lies between the southern portion of the Benue Trough and the Niger Delta (Fig. 1). This basin has long been renowned as a coal and, later, petroleum exploration frontier (Ejeh et al, 2015). Its eastern and northwestern flanks are characterized by basement complex rocks of southwestern Niger/Bida basin and Abakaliki anticlinorium. The southern section has a post-Santonian sedimentary fill of up to >7,000 m (Whiteman, 1982). Although the tectonic setting of the Anambra Basin and the origin of the sediments have been investigated for decades, the source region, redox nature, and tectonic setting are still unclear and require more study (Reyment, 1965; Murat, 1972; Nwachukwu, 1972). As a result, the current study attempts to constrain the provenance signature, weathering, maturity, tectonic setting, as well as environment of deposition of shales within the Lower Anambra Basin which have not been thoroughly studied.

Figure 1. Location of the Anambra Basin within the Lower Benue Trough (Modified after Murat, 1972).

Geological Setting

The Anambra Basin is one of the intracratonic Cretaceous sedimentary basins in Nigeria and constitutes the southern portion of the Benue trough whose origin is related to the separation of Africa from South America and the opening of the South Atlantic Ocean (Obaje et al. 2004) (Fig. 1). According to Akaegbobi (2005), the sedimentation history in the lower Benue Trough is related to the evolution of the Anambra Basin depression and Abakaliki domain. The basin extends for about 402.3 km in length and almost 9 kilometers in thickness, running from Onitsha on the Niger River to Kwande on the Benue River in a NE-SW orientation (Whiteman, 1982). The Cretaceous Anambra depositional site represents a megafacie region that received sediment load over two depositional cycles spanning the Aptian to Maastrichtian periods. (Obaje et al., 1999; Obaje et al., 2004; Murat, 1972; Reyment, 1965). The sediment is indicative of fluvial-deltaic and shallow marine sedimentation on the continental scale (Akaegbobi and Schmitt, 1998). Much is unknown about the Cretaceous and pre-Santonian subsurface deposits in this basin because of the substantial post- Santonian sedimentary fill that covers much of the basin (Whiteman, 1982). On the other hand, Cretaceous outcrops, from adjoining southern Benue Trough suggest what underlies the post-Santonian deposits in the Anambra Basin. The sequence of sedimentation consists of the transgressive Albian Asu River Group which is overlain by the Cenomanian- Early Santonian strata of the Cross River Group, this includes shale, limestone, and sandstone of the Nkalagu Formation, which consist of the Eze-Aku and Awgu shale, as well as their interlingering local facies counterparts (Amasiri, Makurdi, Agala, and Agbani sandstones) (Petters and Ekweozor, 1982). In the Anambra Basin, the Agwu Shale, Enugu / Nkporo shales, Mamu Shale, Ajali Sandstone, Nsukka Formation and Imo Shale constitute the post- Santonian sedimentary fill (Whiteman, 1982; Reijers, 1996) (Table 1). The investigated area is within the Lower Anambra Basin and comprises of Agwu Shale, Enugu / Nkporo shales, Mamu Shale and Imo Shale (Fig. 2).

Figure 2. Geological map of the Lower Anambra Basin and adjacent areas showing sample locations (Modified after Ojo et al., 2009).


Stratigraphic Setting of the Anambra Basin (Modified after Nwajide and Reijers,1996).


AgeSouthern Benue/Anambra BasinCycles of sedimentation
TertiaryEoceneAmeki/Nanka formationThird (3rd) cycle of sedimentation
PaleoceneImo shale
Upper CretaceousMaastrichtianNsukka formation
Ajali formation
Mamu Shale
CampanianEnugu/Nkporo formation
Santonian-ConiacianAgwu formationSecond (2nd) cycle of sedimentation
TuronianEze-Aku Group, (Keana, Markudi, Agala and Amaseri formations)
CenomanianOdukpani formation
Lower CretaceousAlbianAsu River GroupFirst (1st) cycle of sedimentation
Aptian
PrecambrianBasement complex

Methodology

A total of sixteen (16) representative samples obtained from Nkporo Shale from Aguabor near Onitsha Road, Enugu Shale from Amaechi, Agwu Shale from Ituku Ozara, Imo Shale from Nibo and Umuawulu as well as shale from Mamu Formation all of which lie within the Lower Anambra Basin. The samples were collected using a chisel and geological hammer and were packed into polythene bags to prevent loss of moisture. The samples were air dried and pulverized before analysis and tests were carried out. Mineralogical analysis of the samples was carried out at the Center for Energy Research and Development (CERD), Obafemi Awolowo University, Ile-Ife, using Xray Diffractometer. The analysis was conducted with the aid of Radicon MD 10 Diffractometer and the interpretation of the diffractograms was done by comparing the peaks to standard minerals established by the International Center for Diffraction Data (ICDD), 2008 and 2009. Geochemical analysis was carried out at Acme Analytical Laboratories (Vancouver), Canada for major, trace and rare earth element using Inductive Coupled Plasma Atomic Emission Spectrometer (Perkin-Elmer Elan 6000 or 9000) following Lithium metaborate/tetraborate fusion and dilute nitric digestion to determine the major oxides (SiO2, Al2O3, Fe2O3, MgO, CaO, Cr2O5, Na2O, K2O, TiO2, P2O5 and MnO) while trace elements (Nb, Ba, Cr, Ni, Sr, Zr, Ni, Y and Sc) and rare earth elements were analyzed by ICP Mass Spectrometry. Loss on ignition (LOI) was determined for by measuring the weight loss after heating.

Results and Discussions

Mineralogy

Mineralogical analysis of shale samples from Agwu Shale reveal minerals such as sillimanite, vermiculite, dickite and kaolin. Kaolin was the dominant minerals present (Fig. 3). Enugu Shale revealed minerals such as dickite, kaolin minerals, illite, kaolinite, chlorite and chamosite. The result showed that kaolin is the dominant mineral (Fig. 4 and 5). Mamu Shale reveal minerals such as chlorite, kaolin minerals, sillimanite, quartz, vermiculite and kaolinite with kaolinite been the dominant mineral present (Fig. 6) and Imo Shale samples reveal minerals such as montmorillonite, chamosite, dickite, kaoline minerals, vermiculite, and kaolinite. Montmorillonite and kaolinite were the dominant minerals present (Fig. 7). On the average, the XRD results showed that the kaolinite group of minerals (comprising of kaolinite and dickite) are dominant in the analyzed samples and constitute about 41.5% of the entire mineralogical composition. The chlorite group (comprising amesite and chamosite) make up 17.8%. Montmorillonite make up a composition of 15.8% and illite has a composition of 6.8%. the non-clay minerals including quartz, ilmenite and sillimanite make up 3.7%, 4.0% and 10.2% respectively (Table 2).

Figure 3. X-Ray diffractogram for Agwu shale.

Figure 4. X-Ray diffractogram for Enugu shale.

Figure 5. X-Ray diffractogram for Nkporo shale.

Figure 6. X-Ray diffractogram for shale at Mamu Formation.

Figure 7. X-Ray diffractogram for Imo shale.


Average of mineralogical composition of the shale samples.


MINERALAMOUNT (%)
Kaolinite group41.5
Chlorite17.8
Montmorillonite15.8
Sillimanite10.2
Illite6.8
Ilmenite4.0
Quartz3.7


Major Element Geochemistry

The geochemical results of sixteen (16) shale samples from the Lower Anambra Basin comprising of Agwu Shale (2), Nkporo Shale (2), Enugu Shale (2), Mamu Shale (1) and Imo Shales (9) (Table 3) show that they are on average relatively rich in SiO2, Al2O3 and Fe2O3 and have a mean value of 52.29%, 14.09% and 6.15% for Imo Shale (IS), 52.31%, 16.70% and 7.39% for Mamu Shale (MS), 43.21%, 21.33% and 10.36% for Enugu Shale (ES), 53.35%, 15.64% and 7.17% for Nkporo Shale (NS) and 51.24%, 17.25% and 7.78% for Agwu Shale (AS). The SiO2 content is higher in Imo Shale (IS) with SiO2 value ranging from (22.85-75.87wt%) the sample L2S1 records the highest SiO2 value of 75.87wt% indicating a high siliceous content. This is followed by Nkporo Shale (NS) (51.92-54.78wt%), Mamu Shale (MS) (52.31wt%), Agwu Shale (AS) (51.06-51.41wt%), and Enugu Shale (ES) (41.90-44.51wt%). Enugu Shale records the highest Al2O3 content of (21.24-21.41wt%) while IS (9.48-18.87wt%) and AS (16.87-17.63wt%). According to Cullers and Podkovyrov, (2000) such high Al2O3 content may be due to the dilution effect of quartz. The Al2O3 content is lowest in NS (14.82-16.46 wt%), followed by IS (14.08%) and MS (16.70wt%). ES records the highest Fe2O3 content of (9.23-11.48wt%) followed by sample L2S16, IS (1.66-9.27wt%), AS (7.64-7.92wt%), MS has 7.39% while NS has the lowest Fe2O3 content of (7.07-7.26wt%). High content of Fe2O3 suggests the presence of iron-bearing minerals as well as deposition under a reducing condition (Akpokodje et al.,1991). The shales are however low in Na2O, MgO, K2O, MnO, TiO2, CaO and P2O5. ES samples such as L5S4, L5S1 and IM samples L2S6 and L2S14 recorded the lowest K2O content. This low K2O value may be due to low amount of illite, montmorillonite or feldspar present in small proportions (Akpokodje et al.,1991; Adamu et al.,2022). Imo Shale records the lowest P2O5 content on sample L2S1 and L2S7 this may be due to the lower amount of accessory phases present such as apatite and monazite (Okunlola and Idowu, 2012). The low content of MgO and CaO as recorded on NS and IS sample L2S1 suggest they had no associated carbonates or dolomitisation (Okunlola and Idowu, 2012). The average MnO, K2O, Na2O and SiO2 contents are very close for NS and MS (Table 6). The NS shales are higher in SiO2 than MS but lower in Al2O3, MgO and TiO2. The 19.58% average Loss on ignition (LOI) for the Anambra shales is high, which reveals the potential of the shales for carbonaceous compounds. On the Fe2O3/K2O vs. SiO2/Al2O3 chemical classification diagram (Fig. 8; Herron, 1988), Anambra shales plot in the Fe shale field and Fe sands confirming the field description of the samples. Three samples of Imo Shale plot in the field of Fe sands and can be attributed to the high SiO2 content (samples L2S16, L2S3 and L2S1) in comparison with the other shale samples (Table 3).

Figure 8. Geochemical classification of the shales using log (SiO2/Al2O3) vs log (Fe2O3/K2O) diagram (after Herron, 1988).


Major elements distribution of shale samples from Anambra Basin.


Parameters (% Oxide)Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo ShaleMean value
L4S1L4S2L1S1L1S4L5S1L5S4L3S1L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17
SiO251.4151.0654.7851.9241.944.5152.3175.8750.1945.760.2653.1347.9122.8564.750.0751.15
Al2O316.8717.6314.8216.4621.4121.2416.79.4817.1117.8414.8417.1718.874.649.5417.315.75
Fe2O37.647.927.077.2611.489.237.393.927.488.337.474.625.51.669.277.116.83
MgO0.570.590.330.530.510.470.560.482.623.081.132.893.251.5922.471.44
CaO0.260.230.070.060.390.440.060.040.10.410.170.130.3234.50.190.232.35
Na2O0.280.270.370.360.120.140.360.040.020.030.010.030.040.010.060.030.14
K2O1.321.31.331.320.790.851.320.440.440.760.390.840.810.160.90.780.86
TiO21.431.451.441.141.651.721.161.241.051.171.211.091.260.770.731.11.23
P2O50.070.080.080.090.170.180.110.050.120.070.080.060.070.280.190.110.11
MnO0.060.060.030.030.030.030.030.020.020.210.020.030.030.220.060.120.06
Fe2O3/K205.786.095.315.514.5310.855.598.91710.9619.155.56.7910.3710.39.11-
SiO2/Al2O34.052.893.693.152.952.093.1386.937.564.065.092.534.926.782.894.35
K2O/Na2O4.714.813.693.676.586.073.67112225.33392820.25161526-
Al2O3/TiO211.7912.1610.2914.4412.9712.3514.397.6516.2915.2412.2615.7514.976.0313.0615.72-
CIA90.0690.7389.3390.4394.2793.6990.5694.896.8393.6996.394.4994.1681.889.2494.3292.16
ICV0.60.5880.620.580.6220.5250.5820.5210.6240.7180.6190.4970.5278.2191.3080.62-
PIA96.6497.0296.8497.397.5897.2397.3499.1299.2897.4898.7699.0298.0481.4997.1898.4596.79
CIW96.8997.2497.1197.5197.6797.3497.5499.1699.397.5998.899.0798.1291.8597.4498.5197.57
LOI19.919.219.420.621.32119.88.220.622.214.219.821.73312.120.4-

CIA: Chemical Index of Alteration = Al2O3Al2O3+CaO*+Na2O*100 after (Nesbitt and Young, 1982), CaO*=CaO103*P2O5 after (McLennan et al., 1993), PIA: Plagioclase index of alteration = 100*Al2O3K2OAl2O3+CaO*+Na2OK2O (after Fedo et al., 1995), CIW: Chemical index of weathering = Al2O3Al2O3+CaO*+Na2O*100 (After Harnois, 1988), ICV: Index of Chemical Variability = Fe2O3+K2O+Na2O+CaO+MgO+MnOAl2O3.




Average chemical composition of Anambra shales compared to shale from other sedimentary basins in Nigeria.


OxidePresent studyBida Shale (Okunlola & Idowu,2012)Asu River Group (Amajor, 1987)Ezeaku Shale (Amajor, 1987)Auchi Shale (Fagbamigbe, 2013)Ifon Shale (Ajayi et al.,1989)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
SiO251.2453.3543.2152.3152.2961.2669.9444.9151.6863.3
Al2O317.2515.6421.3316.7014.0816.881015.7118.7618.47
Fe2O37.787.1710.367.396.153.754.046.244.671.26
MgO0.580.430.490.562.170.160.872.584.390.82
CaO0.250.070.420.064.010.053.3815.421.90.09
Na2O0.280.370.130.360.030.060.40.420.930.42
K2O1.311.330.821.320.611.391.152.361.162.36
TiO21.441.291.691.161.071.740.520.651.951.02
P2O50.080.090.180.110.110.080.170.460.250.46
MnO0.060.030.030.030.080.020.040.060.060.01
LOI19.552021.1519.819.114.29.2111.114.0511.6
Total99.8299.7799.8199.899.799.5999.6999.9199.8799.81


When compared to average value of shale from different sedimentary basins within Nigeria, it is observed that Agwu Shale (AS), Nkporo Shales (NS), Mamu Shales (MS), and Imo Shale (IS) had a higher SlO2, Al2O3, Fe2O3 and TiO2 content than Ezeaku shale. In contrast, Ezeaku shale had a higher MgO, CaO, Na2O and K2O content than shales from the Anambra Basin. AS showed similarity to Auchi shale in terms of SiO2, Al2O3 a nd M nO. Auchi s hale i s lower in Fe2O3, MgO, K2O but higher in CaO, Na2O, TiO2 and P2O5 content. Agwu Shale (AS) is similar to Bida Shale in P2O5 content while Bida Shale has a higher SiO2, K2O, TiO2 and lower in terms of Al2O3, Fe2O3, MgO, CaO, Na2O and MnO. When compared to shales from sedimentary basins from other parts of the world (Pettijohn, 1957; Turekan and Wedephol 1961; PAAS (Taylor and McLennan, 1985; Gromet et al., 1984), Anambra shales had a lower average of SiO2, MgO, CaO except for Imo Shale (IS) which had a higher Al2O3, Fe2O3 and TiO2 content (Table 3).

Trace and Rare Earth Element Geochemistry

Trace element concentrations of shales within the Lower Anambra Basin are presented in Table 4 while average trace elements of the Anambra shales compared to some world averages are presented in Table 8. Based on the available trace elements data, Nkporo Shale records the highest concentration in Ba, Co, Cu and Pb but depleted in Ni and Zn content compared to Agwu Shale and Mamu Shale. A comparison with world averages (Vine and Tourtelot, 1970; Turekan and Wedephol, 1961 and PAAS, Taylor and McLennan, 1985) shows that the world average values are higher in terms of Ba, Cu, Ni, Pb, and Zn. All the shales recorded higher Co values compared to average global values. Also, Enugu Shale records higher Zn value than the global average. The rare earth elements geochemical data are presented in (Table 5), while the averages are presented in (Table 9) alongside average values of shale from other regions (Okunola and Idowu, (2012), McLennan and Taylor (1980), Levinson, (1974)) and PAAS (Taylor and McLennan, 1985) for comparison. Comparison with shales from other regions such as Bida Shale, it was observed that Bida Shale show a higher REE content (La-Lu) than Anambra shales while that of PAAS (Taylor and McLennan, 1985), McLennan and Taylor, (1980) and Levinson, (1974) show a higher La, Sm, Tb and Lu values than Anambra shales but depleted in Ce and Nd (Table 9).


Trace elements (ppm) distribution of shale samples from Anambra Basin.


AgwuShaleNkporoShaleEnuguShaleMamu ShaleImo Shale
Trace (ppm)L4S1L4S2L1S1L1S4L5S1L5S4L3S1L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17
Ni23.127.424.32822.224.730.14.123.423.67.723.713.2245.827.5
Ba334298376353211254351204268325119199120541317329
Co22.227.435.626.423.12431.35.421.252.710.126.616.632632.5
Cu27.534.177.458.750.429.842.11.84.27.75.28.45.613.83.812.5
Pb19.116.620.819.819.71820.97.68.811.710.95.57.74.91212.1
Zn8510355661529984191011164112170409876
Ni/Co1.0410.681.060.961.020.960.761.10.450.760.890.790.671.760.85
Cu/Zn0.320.331.410.890.330.30.50.090.040.060.130.070.080.350.040.16



Rare earth element (ppm) distribution of shales from Anambra Basin.


ElementsNkporoShaleImo ShaleMamu ShaleAgwuShaleEnuguShale
L1S1L1S4L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17L3S1L4S1L4S2L5S1L5S4
La19.822.410.216.118.115.022.818.815.417.822.422.218.018.520.621.6
Ce116.1129.155.791.8107.387.4159.4109.644.1109.5129.0128.0102.3104.8117.0120.6
Nd34.136.415.826.032.320.658.932.528.139.940.437.430.030.135.335.9
Sm1.962.00.861.511.901.113.302.01.562.422.452.11.661.792.122.14
Eu0.160.150.060.120.150.080.240.150.110.190.210.160.130.150.170.17
Tb0.070.050.030.050.060.040.090.060.050.090.100.060.060.060.070.08
Yb0.860.730.430.510.550.500.710.50.260.721.00.760.740.780.810.82
Lu0.020.020.010.010.010.010.020.010.0060.020.020.020.020.020.020.02
Total173.1190.983.1136.1160.4124.7245.5163.689.6170.6195.6190.7152.9156.2176.1181.3



Average trace element chemical composition of Anambra shale compared to shale from other sedimentary basins.


Present studyBida Shale (Okunlola & Idowu,2012)Levinson (1974)Vine & Tourtelot (1970)Turekan & Wedephol (1961)PAAS(Taylor and McLennan, 1985)NASC (Gromet et al., 1984)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
Ni25.2526.1523.4530.11919.375070685558
Ba316364.5232.5351269.1394.23300700580650636
Co24.83123.5531.321.533.421020_23n.a
Cu30.868.0540.142.1714.4570504550n.a
Pb17.8520.318.8520.99.0222.2820n.an.a20n.a
Zn9460.5125.58475.77116.393001009585n.a
Srn.an.an.an.an.a59.39200300300200142
Vn.an.an.an.an.a108.77150130130150130
Yn.an.an.an.an.a70.693025--n.a
Zrn.an.an.an.an.a1156.5470160160210200
Mon.an.an.an.an.a0.72103--n.a
Nbn.an.an.an.an.a52.462020n.a1.90n.a
Rbn.an.an.an.an.a46.19140n.an.a160n.a
Thn.an.an.an.an.a29.2212n.an.a14.60n.a
Un.an.an.an.an.a13.074n.an.a3.10n.a
Cu/Zn0.331.040.320.500.140.12
(Cu+Mo)/Zn-----0.13
Ni/Co2.040.840.990.960.890.58
Rb/K2O-----33.23
U/Th-----0.45

*PAAS= Post Archean Australian shales *NASC= North American shale composite *n.a= not analyzed.




Average rare earth elements of Anambra shale compared to world averages.


Present studyBida Shale (Okunlola & Idowu,2012)PAAS (Taylor and McLennan, 1985)Codo Shale (McLennan, et al., 1990)Average shale (Levinson,1974)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
La18.2521.121.122.217.477.4038.229.7121
Ce103.55122.6118.8128.099.3170.4279.663.450
Nd30.0535.2535.637.432.767.6633.927.924
Sm1.7251.982.132.11.9012.465.55--
Eu0.140.1550.170.160.142.251.08--
Tb0.060.060.0750.060.061.980.744--
Yb0.760.7950.8150.760.57----
Lu0.020.020.020.020.011.180.433--
Dy-----11.794.68--
Pr-----18.718.83--
Gd-----11.014.66--
Ho-----2.480.991--
Er-----7.692.85--
Tm-----1.140.405--


Correlation Analysis

Correlation analysis between the major elements and available trace elements show that SiO2 correlated negatively with Al2O3 (r = -0.061) in the shales. The presence of SiO2 is related to the occurrence of plagioclase, quartz, and clay minerals, whereas the Al2O3 content may be correlated with the preferred incorporation of aluminous clays into shale (Al-Juboury et al. 2021). The negative correlation between SiO2 and Al2O3 may suggest that SiO2 occur as either detrital silicates or is not influenced by clay minerals (Wang et al. 2017). Positive correlation exists between Al2O3 and Fe2O3=0.657, MgO=0.026, Na2O=0.225, K2O=0.429 and TiO2=0.720 while negative correlation exists between Al2O3 and CaO=-0.664, P2O5= -0.393 and MnO= -0.381 (Table 10). For the trace elements, positive correlation exists between Al2O3 and Ni= 0.316, Cu=0.284, Pb=0.474 and Zn=0.643 revealing association with clay minerals and feldspars while negative correlation occurs for Ba= -0.486 suggesting that they are not sourced from clay minerals (Akkoca et al., 2019) (Table 10).


Two-tailed Pearson correlation matrix.


SiO2Al2O3Fe2O3MgOCaONa2OK2OTiO2P2O5MnONiBaCoCuPbZn
SiO21
Al2O3-.0611
Fe2O3.122.6571
MgO-.154.026-.2211
CaO-.683-.664-.621.0391
Na2O.057.225.255-.677-.2421
K2O.162.429.412-.393-.495.8801
TiO2.002.720.499-.509-.443.381.3791
P2O5-.616-.393-.030-.072.721-.211-.380-.2831
MnO-.589-.381-.301.359.655-.314-.318-.412.4081
Ni.185.316.641.028-.491.376.634-.045.003-.1681
Ba-.503-.486-.257-.187.642.330.138-.362.531.629.1311
Co-.019.501.512.208-.463.314.555.155-.302.206.655.1481
Cu-.155.284.330-.666-.119.859.691.495-.011-.245.258.325.3071
Pb.027.474.666-.699-.400.853.790.597-.088-.314.486.172.432.8361
Zn-.258.643.661.220-.327-.002.264.286.070-.024.576-.143.550.105.2531


Provenance

The provenance of a sedimentary rock is dependent on the characteristics of its source region such as composition, location, climate and topography from which it originated (Paikaray et al., 2008; Dey et al., 2009; Kalsbeek and Frei, 2010; Mishra and Sen, 2010). Therefore, a sedimentary rock's provenance can be determined by analyzing the geochemical characteristics of its clastic components. Studies have shown that several major, trace and rare earth elements geochemical data and ratios can provide empirical evidence on the composition, source rock, along with the outcome of sedimentary processes like sorting and weathering (Paikaray et al., 2008; Dey et al., 2009; Kalsbeek and Frei, 2010; Mishra and Sen, 2010). Ratios such as Al2O3/TiO2 is useful in characterizing the nature of source material due to the immobile nature and low solubility of the oxides and hydroxides of aluminum and titanium in low temperature solutions (Yamamoto et al., 1986; Sugitani et al., 1996). As a result, values between 3-8 points to mafic igneous rock sources; while values of 8–21 and 21–70 are associated with intermediate and felsic igneous rock sources (Gabriel et al., 2019). The calculated Al2O3/ TiO2 ratios of shales of the Lower Anambra Basin range from 11.79 to 12.16 for Agwu Shale, 10.29 to 14.44 for Nkporo Shales, 12.35 to 12.97 for Enugu Shales, 14.39 for Mamu Shales and 6.03 to 16.29 for Imo Shales respectively (Table 4), revealing Agwu Shales, Nkporo Shales, Enugu Shales and Mamu Shales as derivatives from an intermediate igneous rock source while Imo Shales show a mafic and intermediates source. Further confirmation using discriminant function discrimination plot of Roser and Korsch (1988), based on the concentrations of both immobile and variably mobile major elements, and divided into four provenance categories: mafic igneous, intermediate igneous, felsic igneous and quartzose recycled sedimentary, plots the shales in the field of intermediate igneous province and mafic igneous province respectively (Fig. 9). Also, on a binary plot of Al2O3 Vs TiO2 (Krzeszowska, 2019), the shales also plotted mostly in the field of intermediate igneous rocks and mafic igneous rocks (Fig. 10). This empirical finding aligns with the findings of prior investigations conducted by Bolarinwa et al. (2022) and Akinyemi et al. (2022), in which they also identified the source of shales as originating from intermediate and mafic geological origins. Notably, Bolarinwa et al. (2022) posited the hypothesis that the mafic component might have been introduced through post-depositional geological processes affecting the shales. Consequently, the present study proposes that the shales have their provenance rooted in intermediate igneous rocks throughout the Lower Cretaceous to Paleocene periods, with a minor mafic contribution evident in the Upper Cretaceous, as indicated by the youngest Imo Shale unit.

Figure 9. Major element discriminant function diagram for Anambra shales (Roser and Korsch, 1988).

Figure 10. Discrimination plot for provenance Al2O3 (wt%) vs TiO2 (wt%) plot (after Krzeszowska, 2019).

Discriminant Function 1=(1.773TiO2)+(0.607Al2O3)+(0.760Fe2O3)+(1.500MgO)+(0.616CaO)+(0.509Na2O)+(1.224K2O)+(9.090)
Discriminant Function 2=(0.445TiO2)+(0.070Al2O3)+(0.250Fe2O3)+(1.142MgO)+(0.438CaO)+(1.475Na2O)+(1.426K2O)+(6.861)

Paleo-weathering and Sediment Maturity

Weathering of source rocks is a major process in the formation of clays and clay minerals. The rate of weathering of a source rock is controlled by factors such as source rocks composition, nature of climate, time and tectonism (Fedo et al., 1995; Nesbitt et al., 1997). The process of weathering of source rocks removes mobile elements such as Ca, Na and K whose presence in clays serves as index in determining the rate of chemical weathering of their source rocks (Nesbitt and Young, 1984, 1989; Taylor and McLennan, 1985; Wronkiewicz and Condie, 1990; Fedo et al., 1995; Nesbitt et al., 1997; Lindsey, 1999; Roy and Smykatz-Kloss, 2007). Some of the indices proposed includes; Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW) (Nesbitt and Young, 1982; Harnois, 1988; Fedo et al., 1995). Chemical Index of Alteration (CIA) is a good measure of the degree of chemical weathering. Thus, high CIA values reflect the removal of labile cations, such as Ca2+, Na2+, and K+ in relation to more stable cations, such as Al3+ and Ti4+, (Nesbitt and Young, 1982). Conversely, low CIA values suggest low weathering effects on these cations. Plagioclase Index of Alteration (PIA) measures the degree of weathering of feldspars of the source rocks to clay. While Chemical Index of Weathering (CIW) is a measure of the degree of weathering in which the source material has undergone and the value of this index rises as the level of weathering rises. The difference between the CIW index values of the parent silicate rock and the clay material is suggestive of how much weathering the weathered material has undergone (Harnois, 1988). The CIA, PIA and CIW of shales of the Anambra Basin was determined and is represented in Table 3. The results show that PIA values range from 81.49-99.28% with an average of 96.79%, CIA range from 81.80-96.83% with an average of 92.16% while CIW range from 91.85-99.07% with an average of 97.57%. The results obtained (average CIA=92.16%; PIA=96.79% and CIW=97.57%), suggest that the protolith of the shales were subjected to intense weathering. This was further confirmed on a plot of CIA vs ICV of Long et al., (2012) (Fig. 11), in which the Anambra shales plotted below PAAS-ICV value of 0.85 indicating that the shales are mature and intensively weathered while on the plot of CIA vs PIA of Suttner and Dutta (1986) (Fig. 12) they show a high degree of weathering of the source materials. PIA values have been shown to reveal the intensity of destruction of feldspars during the process of source rock weathering (Fedo et al., 1995). Detrital grains of feldspars in shales can preserve signatures of the various degrees of alteration in the source rock location as well as during transport, sedimentation, and diagenesis due to the chemical contents of feldspars (Na, K, and Ca) and are known to be highly mobile during weathering processes. Bivariate plots involving oxides of Na2O, K2O, and CaO vs PIA were plotted to assess the mobility of elements as a result of feldspar weathering. The bivariate plot of K2O/Na2O versus PIA of shales show a positive correlation of this ratio with PIA (Fig. 14) while a negative correlation was obtained for a bivariate plot of K2O + Na2O wt.% against PIA (Fig. 14). According to Nesbitt and Young (1984), total alkalis content of source rocks (K2O+Na2O) decreases with increase in K2O/Na2O ratio as the rate of weathering increases due to destruction of feldspars. While the bivariate plot of NaO, CaO and K2O against PIA, shows a decrease in the mobile elements as PIA increases respectively (Fig. 14). The weathering indices of siliciclastic rocks offer helpful information on the climate conditions of the provenance. According to Moosavirad et al., (2011), an increase in chemical weathering may indicate a shift in climate toward warm, humid conditions. The binary plot of Suttner and Dutta (1986) was used to constrain the climatic condition during sedimentation. The shales of the study area plot mostly in the arid and semi-arid field with only two of the Imo Shale samples plotting in the humid field probably due to an increase in chemical weathering conditions (Moosavirad et al., 2011). This shift in weathering conditions probably indicates a climate transition from a semi-arid environment to a humid environment during the deposition of the younger Imo Shale in the upper Cretaceous period.

Figure 11. CIA versus ICV plot showing weathering intensity and maturity of Anambra shales (after Long et al.,2012).

Figure 12. CIA vs PIA plot showing Palaeo-weathering condition (after Suttner and Dutta 1986).

Figure 13. Plot of SiO2 versus Al2O3 + K2O + Na2O discrimination diagram for paleoclimate condition of sedimentation (after Suttner and Dutta, 1986).

Figure 14. Bivariate diagrams of Na, Ca, and K against PIA showing the mobility of feldspars due to weathering in the lithological components of shales.

Tectonic Setting

Studies have shown that the tectonic setting of a sedimentary rocks' provenance is greatly influenced by its chemical makeup (e.g., Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch, 1986). Also, sedimentation, diagenesis, and composition of sediments all have a significant impact on the tectonic setting of the environment of sediments deposition (Bhatia 1983). Therefore, the composition and chemical makeup of silisiclastic rocks from various tectonic setting can vary considerably (Roser and Korsch 1986; McLennan et al. 1990). In an effort to constraining the tectonic setting of silisiclastic rocks, relevant discrimination diagrams based on major elements have been employed by several researchers (e.g. Armstrong-Altrin, 2015; Tawfik et al. 2015; Al-Juboury et al., 2021) based on the differences in SiO2 values, and the discrimination diagrams are subdivided into a low silica type (35–63%) and a high-silica type (63–95%) and accordingly, three different tectonic setting can be identified as island arc, rift and collisional settings. The SiO2 value of shales of the present study vary from 41.90 to 75.87wt%, therefore the samples can be classified as low-silica clastic sediments and high-silica clastic sediments (Table 3). The discrimination plots of Roser and Korsch, (1986) of SiO2 versus K2O/Na2O classify tectonic setting into three as passive continental margin, active continental margin and ocean island arc tectonic setting. In view of establishing the tectonic setting of shales in the Lower Anambra Basin, the plot of Armstrong-Altrin et al. (2015) and Roser and Korsch, (1986) were employed. Based on this diagram, the shales classify mostly in the field of rift (Passive Continental Margin) and a few plot as collision (Active Continental Margin) probably due to the presence of nearby basement rocks as observed on both the high and low silica discrimination diagrams (Fig. 15 and 16). The current findings aligns with the research conducted by Ejeh in 2021 focused on the geochemistry of sandstones and shales in the lower Anambra basin, which characterized the tectonic history of the lower Anambra sediments as primarily involving a continental rift or passive margin phase, followed by a collision or convergent setting, with only minor involvement of arc settings. Accordingly, the inferred tectonic setting is in agreement with the accepted model on the origin and tectonic evolution of the Benue Trough as established based on geophysical models (Benkhelil, 1989). Also, according to Bhatia (1983) silisiclastic sediments of passive margin tectonic setting are enriched in SiO2 and depleted in Na2O, CaO and TiO2 suggesting their highly recycled and mature nature. This is also observed with the present study as well as previous studies conducted within the Anambra Basin (Odewumi,2013; Ejeh et al., 2015; Shettima et al., 2017; Toyin and Adekeye, 2018; Ikhane et al., 2022).

Figure 15. Discriminant function diagrams based on high –silica for shales of the Lower Anambra Basin (after Verma and Armstrong-Altrin 2013).

Figure 16. Discriminant function multidimensional diagram of low-silica for shales of the Lower Anambra Basin (after Verma and Armstrong-Altrin 2013).

Figure 17. Bivariate plot of SiO2 versus K2O/Na2O indicating tectonic setting discrimination diagram (After Roser & Korsch, 1986).

The discriminant function equations are:

DF1Arc-Rift-Col=-0.263-InTiO2SiO2adj+0.064-InAl2O3SiO2adj+1.725-InFe2O3tSiO2adj+0.660-InMnOSiO2adj+2.191-InMgOSiO2adj+o.144-InCaOSiO2adj+-1.304-InNa2OSiO2adj+0.054-InK2OSiO2adj+-0.330-InP2O5SiO2adj+1.588
DF2Arc-Rift-Col=-1.196-InTiO2SiO2adj+1.064-InAl2O3SiO2adj+0.303-InFe2O3tSiO2adj+0.436-InMnOSiO2adj+0.838-InMgOSiO2adj+o.407-InCaOSiO2adj+-1.021-InNa2OSiO2adj+-1.706-InK2OSiO2adj+-0.126-InP2O5SiO2adj-1.068
DF1Arc-Rift-Col=-0.608*InTiO2SiO2adj+1.854*InAl2O3SiO2adj+0.299*InFe2O3tSiO2adj+-0.550*InMnOSiO2adj+0.120*InMgOSiO2adj+o.194*InCaOSiO2adj+-1.510*InNa2OSiO2adj+1.941*InK2OSiO2adj+0.003*InP2O5SiO2adj-0.294
DF2Arc-Rift-Col=-0.554*InTiO2SiO2adj+-0.995*InAl2O3SiO2adj+1.765*InFe2O3tSiO2adj+-1.391*InMnOSiO2adj+-1.034*InMgOSiO2adj+o.225*InCaOSiO2adj+0.713*InNa2OSiO2adj+0.330*InK2OSiO2adj+0.637*InP2O5SiO2adj-3.631

Paleo-oxygenation Condition and Depositional Environment

The ratios of trace elements like Th/U, Ni/Co, V/Ni, and Cu/Zn are crucial in determining the paleo-redox conditions during sediment deposition, as demonstrated by Dypvik (1984) and Akkoca et al. (2019). According to Jones and Manning's (1994), Ni/Co ratios below 5 are indicative of deposition in an oxic environment, whereas ratios above 5 indicate deposition in suboxic/anoxic environments. The investigated shale samples had an average Ni/Co ratio of 1.10, and range from 0.45 to 1.76, which indicates that they were deposited under oxidizing conditions. Also, Nagarajan et al. (2007) showed that high Cu/Zn ratios denote reducing depositional condition, while low Cu/Zn ratios denote oxidizing depositional conditions. The Cu/Zn ratio of shales from the Anambra Basin is between 0.04 – 1.41 with an average of 0.72 (Table 7) hence indicating a more oxidizing condition.


Comparing average chemical composition of Anambra shale to published average shales.


OxidePresent studyAverage Bida shale (Okunola &Idowu, 2012)Average shale (Pettijohn, 1957)Turekan & Wedephol (1961)PAAS (Taylor and McLennan, 1985)NASC (Gromet et al., 1984)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
SiO251.2453.3543.2152.3152.2961.2658.158.562.4064.82
Al2O317.2515.6421.3316.7014.0816.8815.41518.7817.05
Fe2O37.787.1710.367.396.153.756.94.727.185.7
MgO0.580.430.490.562.170.162.42.52.192.83
CaO0.250.070.420.064.010.053.13.11.293.51
Na2O0.280.370.130.360.030.061.31.31.191.13
K2O1.311.330.821.320.611.393.23.13.683.97
TiO21.441.291.691.161.071.740.60.770.990.8
P2O50.080.090.180.110.110.080.20.160.160.15
MnO0.060.030.030.030.080.02Trace---
K2O/Na2O4.673.596.303.6720.3323.16
K2O/Al2O30.080.090.040.080.040.08
Al2O3/TiO211.9812.1212.6214.3913.159.70
Cu/Zn0.331.040.320.500.140.12
Ni/CO2.040.840.990.960.890.58

*PAAS= Post Archean Australian shales *NASC= North American shale composite.


Conclusion

Mineralogical and geochemical data of shales within the Anambra Basin have been used to constrain the provenance, tectonic setting, paleo-environment, maturity and paleoweathering. Mineralogical studies revealed that the shales mineral constituent is dominantly kaolin while non-clay minerals are quartz, ilmenite and sillimanite. Major element geochemical plot of Fe2O3/K2O vs SiO2/Al2O3 showed that the shales are Fe shales and Fe sands. Major element abundance showed that the shales are dominant in SiO2, Al2O3 and Fe2O3 content and are depleted in Na2O, MgO, K2O, MnO, TiO2, CaO and P2O5. Provenance studies showed that they were derived dominantly from intermediate igneous rocks from the surrounding basement rocks, with minor contribution from mafic igneous rocks during the deposition of Imo Shale. Tectonic setting discrimination diagrams revealed a dominant passive continental margin (rift) type tectonic setting. Indices of CIA, CIW, PIA and ICV for paleo-weathering and maturity showed that the shales are intensively chemically weathered and are immature on the basis of SiO2/Al2O3 ratio but matured when compared to PAAS values (Taylor and McLennan, 1985). Ratios of Cu/Zn and Ni/Co showed that the shales were deposited in an oxidizing condition.

Fig 1.

Figure 1.Location of the Anambra Basin within the Lower Benue Trough (Modified after Murat, 1972).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 2.

Figure 2.Geological map of the Lower Anambra Basin and adjacent areas showing sample locations (Modified after Ojo et al., 2009).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 3.

Figure 3.X-Ray diffractogram for Agwu shale.
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 4.

Figure 4.X-Ray diffractogram for Enugu shale.
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 5.

Figure 5.X-Ray diffractogram for Nkporo shale.
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 6.

Figure 6.X-Ray diffractogram for shale at Mamu Formation.
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 7.

Figure 7.X-Ray diffractogram for Imo shale.
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 8.

Figure 8.Geochemical classification of the shales using log (SiO2/Al2O3) vs log (Fe2O3/K2O) diagram (after Herron, 1988).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 9.

Figure 9.Major element discriminant function diagram for Anambra shales (Roser and Korsch, 1988).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 10.

Figure 10.Discrimination plot for provenance Al2O3 (wt%) vs TiO2 (wt%) plot (after Krzeszowska, 2019).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 11.

Figure 11.CIA versus ICV plot showing weathering intensity and maturity of Anambra shales (after Long et al.,2012).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 12.

Figure 12.CIA vs PIA plot showing Palaeo-weathering condition (after Suttner and Dutta 1986).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 13.

Figure 13.Plot of SiO2 versus Al2O3 + K2O + Na2O discrimination diagram for paleoclimate condition of sedimentation (after Suttner and Dutta, 1986).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 14.

Figure 14.Bivariate diagrams of Na, Ca, and K against PIA showing the mobility of feldspars due to weathering in the lithological components of shales.
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 15.

Figure 15.Discriminant function diagrams based on high –silica for shales of the Lower Anambra Basin (after Verma and Armstrong-Altrin 2013).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 16.

Figure 16.Discriminant function multidimensional diagram of low-silica for shales of the Lower Anambra Basin (after Verma and Armstrong-Altrin 2013).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Fig 17.

Figure 17.Bivariate plot of SiO2 versus K2O/Na2O indicating tectonic setting discrimination diagram (After Roser & Korsch, 1986).
Economic and Environmental Geology 2023; 56: 799-816https://doi.org/10.9719/EEG.2023.56.6.799

Stratigraphic Setting of the Anambra Basin (Modified after Nwajide and Reijers,1996).


AgeSouthern Benue/Anambra BasinCycles of sedimentation
TertiaryEoceneAmeki/Nanka formationThird (3rd) cycle of sedimentation
PaleoceneImo shale
Upper CretaceousMaastrichtianNsukka formation
Ajali formation
Mamu Shale
CampanianEnugu/Nkporo formation
Santonian-ConiacianAgwu formationSecond (2nd) cycle of sedimentation
TuronianEze-Aku Group, (Keana, Markudi, Agala and Amaseri formations)
CenomanianOdukpani formation
Lower CretaceousAlbianAsu River GroupFirst (1st) cycle of sedimentation
Aptian
PrecambrianBasement complex


Average of mineralogical composition of the shale samples.


MINERALAMOUNT (%)
Kaolinite group41.5
Chlorite17.8
Montmorillonite15.8
Sillimanite10.2
Illite6.8
Ilmenite4.0
Quartz3.7


Major elements distribution of shale samples from Anambra Basin.


Parameters (% Oxide)Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo ShaleMean value
L4S1L4S2L1S1L1S4L5S1L5S4L3S1L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17
SiO251.4151.0654.7851.9241.944.5152.3175.8750.1945.760.2653.1347.9122.8564.750.0751.15
Al2O316.8717.6314.8216.4621.4121.2416.79.4817.1117.8414.8417.1718.874.649.5417.315.75
Fe2O37.647.927.077.2611.489.237.393.927.488.337.474.625.51.669.277.116.83
MgO0.570.590.330.530.510.470.560.482.623.081.132.893.251.5922.471.44
CaO0.260.230.070.060.390.440.060.040.10.410.170.130.3234.50.190.232.35
Na2O0.280.270.370.360.120.140.360.040.020.030.010.030.040.010.060.030.14
K2O1.321.31.331.320.790.851.320.440.440.760.390.840.810.160.90.780.86
TiO21.431.451.441.141.651.721.161.241.051.171.211.091.260.770.731.11.23
P2O50.070.080.080.090.170.180.110.050.120.070.080.060.070.280.190.110.11
MnO0.060.060.030.030.030.030.030.020.020.210.020.030.030.220.060.120.06
Fe2O3/K205.786.095.315.514.5310.855.598.91710.9619.155.56.7910.3710.39.11-
SiO2/Al2O34.052.893.693.152.952.093.1386.937.564.065.092.534.926.782.894.35
K2O/Na2O4.714.813.693.676.586.073.67112225.33392820.25161526-
Al2O3/TiO211.7912.1610.2914.4412.9712.3514.397.6516.2915.2412.2615.7514.976.0313.0615.72-
CIA90.0690.7389.3390.4394.2793.6990.5694.896.8393.6996.394.4994.1681.889.2494.3292.16
ICV0.60.5880.620.580.6220.5250.5820.5210.6240.7180.6190.4970.5278.2191.3080.62-
PIA96.6497.0296.8497.397.5897.2397.3499.1299.2897.4898.7699.0298.0481.4997.1898.4596.79
CIW96.8997.2497.1197.5197.6797.3497.5499.1699.397.5998.899.0798.1291.8597.4498.5197.57
LOI19.919.219.420.621.32119.88.220.622.214.219.821.73312.120.4-

CIA: Chemical Index of Alteration = Al2O3Al2O3+CaO*+Na2O*100 after (Nesbitt and Young, 1982), CaO*=CaO103*P2O5 after (McLennan et al., 1993), PIA: Plagioclase index of alteration = 100*Al2O3K2OAl2O3+CaO*+Na2OK2O (after Fedo et al., 1995), CIW: Chemical index of weathering = Al2O3Al2O3+CaO*+Na2O*100 (After Harnois, 1988), ICV: Index of Chemical Variability = Fe2O3+K2O+Na2O+CaO+MgO+MnOAl2O3.



Trace elements (ppm) distribution of shale samples from Anambra Basin.


AgwuShaleNkporoShaleEnuguShaleMamu ShaleImo Shale
Trace (ppm)L4S1L4S2L1S1L1S4L5S1L5S4L3S1L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17
Ni23.127.424.32822.224.730.14.123.423.67.723.713.2245.827.5
Ba334298376353211254351204268325119199120541317329
Co22.227.435.626.423.12431.35.421.252.710.126.616.632632.5
Cu27.534.177.458.750.429.842.11.84.27.75.28.45.613.83.812.5
Pb19.116.620.819.819.71820.97.68.811.710.95.57.74.91212.1
Zn8510355661529984191011164112170409876
Ni/Co1.0410.681.060.961.020.960.761.10.450.760.890.790.671.760.85
Cu/Zn0.320.331.410.890.330.30.50.090.040.060.130.070.080.350.040.16


Rare earth element (ppm) distribution of shales from Anambra Basin.


ElementsNkporoShaleImo ShaleMamu ShaleAgwuShaleEnuguShale
L1S1L1S4L2S1L2S2L2S3L2S6L2S7L2S11L2S14L2S16L2S17L3S1L4S1L4S2L5S1L5S4
La19.822.410.216.118.115.022.818.815.417.822.422.218.018.520.621.6
Ce116.1129.155.791.8107.387.4159.4109.644.1109.5129.0128.0102.3104.8117.0120.6
Nd34.136.415.826.032.320.658.932.528.139.940.437.430.030.135.335.9
Sm1.962.00.861.511.901.113.302.01.562.422.452.11.661.792.122.14
Eu0.160.150.060.120.150.080.240.150.110.190.210.160.130.150.170.17
Tb0.070.050.030.050.060.040.090.060.050.090.100.060.060.060.070.08
Yb0.860.730.430.510.550.500.710.50.260.721.00.760.740.780.810.82
Lu0.020.020.010.010.010.010.020.010.0060.020.020.020.020.020.020.02
Total173.1190.983.1136.1160.4124.7245.5163.689.6170.6195.6190.7152.9156.2176.1181.3


Average chemical composition of Anambra shales compared to shale from other sedimentary basins in Nigeria.


OxidePresent studyBida Shale (Okunlola & Idowu,2012)Asu River Group (Amajor, 1987)Ezeaku Shale (Amajor, 1987)Auchi Shale (Fagbamigbe, 2013)Ifon Shale (Ajayi et al.,1989)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
SiO251.2453.3543.2152.3152.2961.2669.9444.9151.6863.3
Al2O317.2515.6421.3316.7014.0816.881015.7118.7618.47
Fe2O37.787.1710.367.396.153.754.046.244.671.26
MgO0.580.430.490.562.170.160.872.584.390.82
CaO0.250.070.420.064.010.053.3815.421.90.09
Na2O0.280.370.130.360.030.060.40.420.930.42
K2O1.311.330.821.320.611.391.152.361.162.36
TiO21.441.291.691.161.071.740.520.651.951.02
P2O50.080.090.180.110.110.080.170.460.250.46
MnO0.060.030.030.030.080.020.040.060.060.01
LOI19.552021.1519.819.114.29.2111.114.0511.6
Total99.8299.7799.8199.899.799.5999.6999.9199.8799.81


Comparing average chemical composition of Anambra shale to published average shales.


OxidePresent studyAverage Bida shale (Okunola &Idowu, 2012)Average shale (Pettijohn, 1957)Turekan & Wedephol (1961)PAAS (Taylor and McLennan, 1985)NASC (Gromet et al., 1984)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
SiO251.2453.3543.2152.3152.2961.2658.158.562.4064.82
Al2O317.2515.6421.3316.7014.0816.8815.41518.7817.05
Fe2O37.787.1710.367.396.153.756.94.727.185.7
MgO0.580.430.490.562.170.162.42.52.192.83
CaO0.250.070.420.064.010.053.13.11.293.51
Na2O0.280.370.130.360.030.061.31.31.191.13
K2O1.311.330.821.320.611.393.23.13.683.97
TiO21.441.291.691.161.071.740.60.770.990.8
P2O50.080.090.180.110.110.080.20.160.160.15
MnO0.060.030.030.030.080.02Trace---
K2O/Na2O4.673.596.303.6720.3323.16
K2O/Al2O30.080.090.040.080.040.08
Al2O3/TiO211.9812.1212.6214.3913.159.70
Cu/Zn0.331.040.320.500.140.12
Ni/CO2.040.840.990.960.890.58

*PAAS= Post Archean Australian shales *NASC= North American shale composite.



Average trace element chemical composition of Anambra shale compared to shale from other sedimentary basins.


Present studyBida Shale (Okunlola & Idowu,2012)Levinson (1974)Vine & Tourtelot (1970)Turekan & Wedephol (1961)PAAS(Taylor and McLennan, 1985)NASC (Gromet et al., 1984)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
Ni25.2526.1523.4530.11919.375070685558
Ba316364.5232.5351269.1394.23300700580650636
Co24.83123.5531.321.533.421020_23n.a
Cu30.868.0540.142.1714.4570504550n.a
Pb17.8520.318.8520.99.0222.2820n.an.a20n.a
Zn9460.5125.58475.77116.393001009585n.a
Srn.an.an.an.an.a59.39200300300200142
Vn.an.an.an.an.a108.77150130130150130
Yn.an.an.an.an.a70.693025--n.a
Zrn.an.an.an.an.a1156.5470160160210200
Mon.an.an.an.an.a0.72103--n.a
Nbn.an.an.an.an.a52.462020n.a1.90n.a
Rbn.an.an.an.an.a46.19140n.an.a160n.a
Thn.an.an.an.an.a29.2212n.an.a14.60n.a
Un.an.an.an.an.a13.074n.an.a3.10n.a
Cu/Zn0.331.040.320.500.140.12
(Cu+Mo)/Zn-----0.13
Ni/Co2.040.840.990.960.890.58
Rb/K2O-----33.23
U/Th-----0.45

*PAAS= Post Archean Australian shales *NASC= North American shale composite *n.a= not analyzed.



Average rare earth elements of Anambra shale compared to world averages.


Present studyBida Shale (Okunlola & Idowu,2012)PAAS (Taylor and McLennan, 1985)Codo Shale (McLennan, et al., 1990)Average shale (Levinson,1974)
Agwu ShaleNkporo ShaleEnugu ShaleMamu ShaleImo Shale
La18.2521.121.122.217.477.4038.229.7121
Ce103.55122.6118.8128.099.3170.4279.663.450
Nd30.0535.2535.637.432.767.6633.927.924
Sm1.7251.982.132.11.9012.465.55--
Eu0.140.1550.170.160.142.251.08--
Tb0.060.060.0750.060.061.980.744--
Yb0.760.7950.8150.760.57----
Lu0.020.020.020.020.011.180.433--
Dy-----11.794.68--
Pr-----18.718.83--
Gd-----11.014.66--
Ho-----2.480.991--
Er-----7.692.85--
Tm-----1.140.405--


Two-tailed Pearson correlation matrix.


SiO2Al2O3Fe2O3MgOCaONa2OK2OTiO2P2O5MnONiBaCoCuPbZn
SiO21
Al2O3-.0611
Fe2O3.122.6571
MgO-.154.026-.2211
CaO-.683-.664-.621.0391
Na2O.057.225.255-.677-.2421
K2O.162.429.412-.393-.495.8801
TiO2.002.720.499-.509-.443.381.3791
P2O5-.616-.393-.030-.072.721-.211-.380-.2831
MnO-.589-.381-.301.359.655-.314-.318-.412.4081
Ni.185.316.641.028-.491.376.634-.045.003-.1681
Ba-.503-.486-.257-.187.642.330.138-.362.531.629.1311
Co-.019.501.512.208-.463.314.555.155-.302.206.655.1481
Cu-.155.284.330-.666-.119.859.691.495-.011-.245.258.325.3071
Pb.027.474.666-.699-.400.853.790.597-.088-.314.486.172.432.8361
Zn-.258.643.661.220-.327-.002.264.286.070-.024.576-.143.550.105.2531

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KSEEG
Dec 29, 2023 Vol.56 No.6, pp. 629~909

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