• 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).

Table 1

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

Age		Southern Benue/Anambra Basin	Cycles of sedimentation
Tertiary	Eocene	Ameki/Nanka formation	Third (3rd) cycle of sedimentation
	Paleocene	Imo shale
Upper Cretaceous	Maastrichtian	Nsukka formation
		Ajali formation
		Mamu Shale
	Campanian	Enugu/Nkporo formation
	Santonian-Coniacian	Agwu formation	Second (2nd) cycle of sedimentation
	Turonian	Eze-Aku Group, (Keana, Markudi, Agala and Amaseri formations)
	Cenomanian	Odukpani formation
Lower Cretaceous	Albian	Asu River Group	First (1st) cycle of sedimentation
	Aptian
Precambrian		Basement 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.

Table 2

Average of mineralogical composition of the shale samples

MINERAL	AMOUNT (%)
Kaolinite group	41.5
Chlorite	17.8
Montmorillonite	15.8
Sillimanite	10.2
Illite	6.8
Ilmenite	4.0
Quartz	3.7

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).

Table 3

Major elements distribution of shale samples from Anambra Basin

Parameters (% Oxide)	Agwu Shale		Nkporo Shale		Enugu Shale		Mamu Shale	Imo Shale									Mean value
	L4S1	L4S2	L1S1	L1S4	L5S1	L5S4	L3S1	L2S1	L2S2	L2S3	L2S6	L2S7	L2S11	L2S14	L2S16	L2S17
SiO2	51.41	51.06	54.78	51.92	41.9	44.51	52.31	75.87	50.19	45.7	60.26	53.13	47.91	22.85	64.7	50.07	51.15
Al2O3	16.87	17.63	14.82	16.46	21.41	21.24	16.7	9.48	17.11	17.84	14.84	17.17	18.87	4.64	9.54	17.3	15.75
Fe2O3	7.64	7.92	7.07	7.26	11.48	9.23	7.39	3.92	7.48	8.33	7.47	4.62	5.5	1.66	9.27	7.11	6.83
MgO	0.57	0.59	0.33	0.53	0.51	0.47	0.56	0.48	2.62	3.08	1.13	2.89	3.25	1.59	2	2.47	1.44
CaO	0.26	0.23	0.07	0.06	0.39	0.44	0.06	0.04	0.1	0.41	0.17	0.13	0.32	34.5	0.19	0.23	2.35
Na2O	0.28	0.27	0.37	0.36	0.12	0.14	0.36	0.04	0.02	0.03	0.01	0.03	0.04	0.01	0.06	0.03	0.14
K2O	1.32	1.3	1.33	1.32	0.79	0.85	1.32	0.44	0.44	0.76	0.39	0.84	0.81	0.16	0.9	0.78	0.86
TiO2	1.43	1.45	1.44	1.14	1.65	1.72	1.16	1.24	1.05	1.17	1.21	1.09	1.26	0.77	0.73	1.1	1.23
P2O5	0.07	0.08	0.08	0.09	0.17	0.18	0.11	0.05	0.12	0.07	0.08	0.06	0.07	0.28	0.19	0.11	0.11
MnO	0.06	0.06	0.03	0.03	0.03	0.03	0.03	0.02	0.02	0.21	0.02	0.03	0.03	0.22	0.06	0.12	0.06
Fe2O3/K20	5.78	6.09	5.31	5.5	14.53	10.85	5.59	8.9	17	10.96	19.15	5.5	6.79	10.37	10.3	9.11	-
SiO2/Al2O3	4.05	2.89	3.69	3.15	2.95	2.09	3.13	8	6.93	7.56	4.06	5.09	2.53	4.92	6.78	2.89	4.35
K2O/Na2O	4.71	4.81	3.69	3.67	6.58	6.07	3.67	11	22	25.33	39	28	20.25	16	15	26	-
Al2O3/TiO2	11.79	12.16	10.29	14.44	12.97	12.35	14.39	7.65	16.29	15.24	12.26	15.75	14.97	6.03	13.06	15.72	-
CIA	90.06	90.73	89.33	90.43	94.27	93.69	90.56	94.8	96.83	93.69	96.3	94.49	94.16	81.8	89.24	94.32	92.16
ICV	0.6	0.588	0.62	0.58	0.622	0.525	0.582	0.521	0.624	0.718	0.619	0.497	0.527	8.219	1.308	0.62	-
PIA	96.64	97.02	96.84	97.3	97.58	97.23	97.34	99.12	99.28	97.48	98.76	99.02	98.04	81.49	97.18	98.45	96.79
CIW	96.89	97.24	97.11	97.51	97.67	97.34	97.54	99.16	99.3	97.59	98.8	99.07	98.12	91.85	97.44	98.51	97.57
LOI	19.9	19.2	19.4	20.6	21.3	21	19.8	8.2	20.6	22.2	14.2	19.8	21.7	33	12.1	20.4	-

CIA: Chemical Index of Alteration = Al2O3Al2O3+CaO*+Na2O*100 after (Nesbitt and Young, 1982), CaO*=CaO−103*P2O5 after (McLennan et al., 1993), PIA: Plagioclase index of alteration = 100*Al2O3−K2OAl2O3+CaO*+Na2O−K2O (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

Table 6

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

Oxide	Present study					Bida Shale (Okunlola & Idowu,2012)	Asu River Group (Amajor, 1987)	Ezeaku Shale (Amajor, 1987)	Auchi Shale (Fagbamigbe, 2013)	Ifon Shale (Ajayi et al.,1989)
	Agwu Shale	Nkporo Shale	Enugu Shale	Mamu Shale	Imo Shale
SiO2	51.24	53.35	43.21	52.31	52.29	61.26	69.94	44.91	51.68	63.3
Al2O3	17.25	15.64	21.33	16.70	14.08	16.88	10	15.71	18.76	18.47
Fe2O3	7.78	7.17	10.36	7.39	6.15	3.75	4.04	6.24	4.67	1.26
MgO	0.58	0.43	0.49	0.56	2.17	0.16	0.87	2.58	4.39	0.82
CaO	0.25	0.07	0.42	0.06	4.01	0.05	3.38	15.42	1.9	0.09
Na2O	0.28	0.37	0.13	0.36	0.03	0.06	0.4	0.42	0.93	0.42
K2O	1.31	1.33	0.82	1.32	0.61	1.39	1.15	2.36	1.16	2.36
TiO2	1.44	1.29	1.69	1.16	1.07	1.74	0.52	0.65	1.95	1.02
P2O5	0.08	0.09	0.18	0.11	0.11	0.08	0.17	0.46	0.25	0.46
MnO	0.06	0.03	0.03	0.03	0.08	0.02	0.04	0.06	0.06	0.01
LOI	19.55	20	21.15	19.8	19.1	14.2	9.21	11.1	14.05	11.6
Total	99.82	99.77	99.81	99.8	99.7	99.59	99.69	99.91	99.87	99.81

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).

Table 4

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

	Agwu	Shale	Nkporo	Shale	Enugu	Shale	Mamu Shale	Imo Shale
Trace (ppm)	L4S1	L4S2	L1S1	L1S4	L5S1	L5S4	L3S1	L2S1	L2S2	L2S3	L2S6	L2S7	L2S11	L2S14	L2S16	L2S17
Ni	23.1	27.4	24.3	28	22.2	24.7	30.1	4.1	23.4	23.6	7.7	23.7	13.2	2	45.8	27.5
Ba	334	298	376	353	211	254	351	204	268	325	119	199	120	541	317	329
Co	22.2	27.4	35.6	26.4	23.1	24	31.3	5.4	21.2	52.7	10.1	26.6	16.6	3	26	32.5
Cu	27.5	34.1	77.4	58.7	50.4	29.8	42.1	1.8	4.2	7.7	5.2	8.4	5.6	13.8	3.8	12.5
Pb	19.1	16.6	20.8	19.8	19.7	18	20.9	7.6	8.8	11.7	10.9	5.5	7.7	4.9	12	12.1
Zn	85	103	55	66	152	99	84	19	101	116	41	121	70	40	98	76
Ni/Co	1.04	1	0.68	1.06	0.96	1.02	0.96	0.76	1.1	0.45	0.76	0.89	0.79	0.67	1.76	0.85
Cu/Zn	0.32	0.33	1.41	0.89	0.33	0.3	0.5	0.09	0.04	0.06	0.13	0.07	0.08	0.35	0.04	0.16

Table 5

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

Elements	Nkporo	Shale	Imo Shale									Mamu Shale	Agwu	Shale	Enugu	Shale
	L1S1	L1S4	L2S1	L2S2	L2S3	L2S6	L2S7	L2S11	L2S14	L2S16	L2S17	L3S1	L4S1	L4S2	L5S1	L5S4
La	19.8	22.4	10.2	16.1	18.1	15.0	22.8	18.8	15.4	17.8	22.4	22.2	18.0	18.5	20.6	21.6
Ce	116.1	129.1	55.7	91.8	107.3	87.4	159.4	109.6	44.1	109.5	129.0	128.0	102.3	104.8	117.0	120.6
Nd	34.1	36.4	15.8	26.0	32.3	20.6	58.9	32.5	28.1	39.9	40.4	37.4	30.0	30.1	35.3	35.9
Sm	1.96	2.0	0.86	1.51	1.90	1.11	3.30	2.0	1.56	2.42	2.45	2.1	1.66	1.79	2.12	2.14
Eu	0.16	0.15	0.06	0.12	0.15	0.08	0.24	0.15	0.11	0.19	0.21	0.16	0.13	0.15	0.17	0.17
Tb	0.07	0.05	0.03	0.05	0.06	0.04	0.09	0.06	0.05	0.09	0.10	0.06	0.06	0.06	0.07	0.08
Yb	0.86	0.73	0.43	0.51	0.55	0.50	0.71	0.5	0.26	0.72	1.0	0.76	0.74	0.78	0.81	0.82
Lu	0.02	0.02	0.01	0.01	0.01	0.01	0.02	0.01	0.006	0.02	0.02	0.02	0.02	0.02	0.02	0.02
Total	173.1	190.9	83.1	136.1	160.4	124.7	245.5	163.6	89.6	170.6	195.6	190.7	152.9	156.2	176.1	181.3

Table 8

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

	Present study					Bida Shale (Okunlola & Idowu,2012)	Levinson (1974)	Vine & Tourtelot (1970)	Turekan & Wedephol (1961)	PAAS(Taylor and McLennan, 1985)	NASC (Gromet et al., 1984)
	Agwu Shale	Nkporo Shale	Enugu Shale	Mamu Shale	Imo Shale
Ni	25.25	26.15	23.45	30.1	19	19.37	50	70	68	55	58
Ba	316	364.5	232.5	351	269.1	394.23	300	700	580	650	636
Co	24.8	31	23.55	31.3	21.5	33.42	10	20	_	23	n.a
Cu	30.8	68.05	40.1	42.1	7	14.45	70	50	45	50	n.a
Pb	17.85	20.3	18.85	20.9	9.02	22.28	20	n.a	n.a	20	n.a
Zn	94	60.5	125.5	84	75.77	116.39	300	100	95	85	n.a
Sr	n.a	n.a	n.a	n.a	n.a	59.39	200	300	300	200	142
V	n.a	n.a	n.a	n.a	n.a	108.77	150	130	130	150	130
Y	n.a	n.a	n.a	n.a	n.a	70.69	30	25	-	-	n.a
Zr	n.a	n.a	n.a	n.a	n.a	1156.54	70	160	160	210	200
Mo	n.a	n.a	n.a	n.a	n.a	0.72	10	3	-	-	n.a
Nb	n.a	n.a	n.a	n.a	n.a	52.46	20	20	n.a	1.90	n.a
Rb	n.a	n.a	n.a	n.a	n.a	46.19	140	n.a	n.a	160	n.a
Th	n.a	n.a	n.a	n.a	n.a	29.22	12	n.a	n.a	14.60	n.a
U	n.a	n.a	n.a	n.a	n.a	13.07	4	n.a	n.a	3.10	n.a
Cu/Zn	0.33	1.04	0.32	0.50	0.14	0.12
(Cu+Mo)/Zn	-	-	-	-	-	0.13
Ni/Co	2.04	0.84	0.99	0.96	0.89	0.58
Rb/K2O	-	-	-	-	-	33.23
U/Th	-	-	-	-	-	0.45

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

Table 9

Average rare earth elements of Anambra shale compared to world averages

	Present study					Bida Shale (Okunlola & Idowu,2012)	PAAS (Taylor and McLennan, 1985)	Codo Shale (McLennan, et al., 1990)	Average shale (Levinson,1974)
	Agwu Shale	Nkporo Shale	Enugu Shale	Mamu Shale	Imo Shale
La	18.25	21.1	21.1	22.2	17.4	77.40	38.2	29.7	121
Ce	103.55	122.6	118.8	128.0	99.3	170.42	79.6	63.4	50
Nd	30.05	35.25	35.6	37.4	32.7	67.66	33.9	27.9	24
Sm	1.725	1.98	2.13	2.1	1.90	12.46	5.55	-	-
Eu	0.14	0.155	0.17	0.16	0.14	2.25	1.08	-	-
Tb	0.06	0.06	0.075	0.06	0.06	1.98	0.744	-	-
Yb	0.76	0.795	0.815	0.76	0.57	-	-	-	-
Lu	0.02	0.02	0.02	0.02	0.01	1.18	0.433	-	-
Dy	-	-	-	-	-	11.79	4.68	-	-
Pr	-	-	-	-	-	18.71	8.83	-	-
Gd	-	-	-	-	-	11.01	4.66	-	-
Ho	-	-	-	-	-	2.48	0.991	-	-
Er	-	-	-	-	-	7.69	2.85	-	-
Tm	-	-	-	-	-	1.14	0.405	-	-

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).

Table 10

Two-tailed Pearson correlation matrix

	SiO2	Al2O3	Fe2O3	MgO	CaO	Na2O	K2O	TiO2	P2O5	MnO	Ni	Ba	Co	Cu	Pb	Zn
SiO2	1
Al2O3	-.061	1
Fe2O3	.122	.657	1
MgO	-.154	.026	-.221	1
CaO	-.683	-.664	-.621	.039	1
Na2O	.057	.225	.255	-.677	-.242	1
K2O	.162	.429	.412	-.393	-.495	.880	1
TiO2	.002	.720	.499	-.509	-.443	.381	.379	1
P2O5	-.616	-.393	-.030	-.072	.721	-.211	-.380	-.283	1
MnO	-.589	-.381	-.301	.359	.655	-.314	-.318	-.412	.408	1
Ni	.185	.316	.641	.028	-.491	.376	.634	-.045	.003	-.168	1
Ba	-.503	-.486	-.257	-.187	.642	.330	.138	-.362	.531	.629	.131	1
Co	-.019	.501	.512	.208	-.463	.314	.555	.155	-.302	.206	.655	.148	1
Cu	-.155	.284	.330	-.666	-.119	.859	.691	.495	-.011	-.245	.258	.325	.307	1
Pb	.027	.474	.666	-.699	-.400	.853	.790	.597	-.088	-.314	.486	.172	.432	.836	1
Zn	-.258	.643	.661	.220	-.327	-.002	.264	.286	.070	-.024	.576	-.143	.550	.105	.253	1

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).

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:

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.

Table 7

Comparing average chemical composition of Anambra shale to published average shales

Oxide	Present study					Average Bida shale (Okunola &Idowu, 2012)	Average shale (Pettijohn, 1957)	Turekan & Wedephol (1961)	PAAS (Taylor and McLennan, 1985)	NASC (Gromet et al., 1984)
	Agwu Shale	Nkporo Shale	Enugu Shale	Mamu Shale	Imo Shale
SiO2	51.24	53.35	43.21	52.31	52.29	61.26	58.1	58.5	62.40	64.82
Al2O3	17.25	15.64	21.33	16.70	14.08	16.88	15.4	15	18.78	17.05
Fe2O3	7.78	7.17	10.36	7.39	6.15	3.75	6.9	4.72	7.18	5.7
MgO	0.58	0.43	0.49	0.56	2.17	0.16	2.4	2.5	2.19	2.83
CaO	0.25	0.07	0.42	0.06	4.01	0.05	3.1	3.1	1.29	3.51
Na2O	0.28	0.37	0.13	0.36	0.03	0.06	1.3	1.3	1.19	1.13
K2O	1.31	1.33	0.82	1.32	0.61	1.39	3.2	3.1	3.68	3.97
TiO2	1.44	1.29	1.69	1.16	1.07	1.74	0.6	0.77	0.99	0.8
P2O5	0.08	0.09	0.18	0.11	0.11	0.08	0.2	0.16	0.16	0.15
MnO	0.06	0.03	0.03	0.03	0.08	0.02	Trace	-	-	-
K2O/Na2O	4.67	3.59	6.30	3.67	20.33	23.16
K2O/Al2O3	0.08	0.09	0.04	0.08	0.04	0.08
Al2O3/TiO2	11.98	12.12	12.62	14.39	13.15	9.70
Cu/Zn	0.33	1.04	0.32	0.50	0.14	0.12
Ni/CO	2.04	0.84	0.99	0.96	0.89	0.58

*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.