Geochemical Analysis of Cretaceous Black Shales

Amongst the 98 rock samples collected in the field at outcrop level, 45 where shales having variable characteristics in terms of colour and texture (Fig. 2). Amongst the 45 shales collected from the field, 25 were analysed from 07 outcrops, in 07 different sites (S1–S7) as seen in Fig. 1 (coordinates of sites location in Table 1). The choice of the studied outcrop was based on difference in formation as proposed by Ref. [14 ] and the difference in outcrop angle of dip. The choice in the shales to be analysed from the different outcrops were strictly based on the similarities and differences in their physical properties like texture, colour and their reaction to dilute HCL test (carbonate testing, Fig. 2a and b). Some of the shales show minimal (Fig. 2b) or no reaction with HCl signify a dolomitic property (Fig. 2d).Fig. 2

Field photos of Cretaceous black shales from the Mamfe basin. (a) Millimetric to centimetric laminated black shales, (b) massive weathered shales, (c) laminated shales showing nodules of siderite (Sn) interbedding with limestone (lst), (d) centimetric laminated shales.

Fig. 2Table 1

Major element oxides (wt%) and total organic nitrogen (TON), total organic carbon (TOC) and total organic sulphur (TOS) data of the studied black shales. Classification (Class.): Cd = carbonate depleted; Ce = carbonate enriched.

Table 1

Study sites/Coordinates	Sample Id	SiO2	AL₂O₃	CaO	MgO	Na₂O	K₂O	Fe₂O₃	MnO	P₂O₅	TiO₂	Ca/Mg	Class.	TON	TOC	TOS
	M1	63.40	16.23	0.48	4.85	2.89	4.20	5.71	0.04	0.19	0.49	0.10	Cd	0.02	0.30	0.06
Site 1: Etoko	M2	63.60	17.64	0.52	3.75	2.92	4.45	4.70	0.04	0.20	0.68	0.14	Cd	0.04	0.79	0.15
5°43′ 13″N, 09°32′09″E	M3	60.80	14.77	5.37	3.41	2.89	3.37	6.81	0.18	0.17	0.70	1.57	Ce	0.04	0.31	0.41
	M4	56.30	17.65	2.98	5.08	1.90	4.56	8.79	0.09	0.19	0.92	0.59	Cd	0.12	0.56	0.09
Site 2: Nchemba	M5	51.60	19.99	3.47	5.60	2.14	4.62	9.74	0.12	0.22	1.00	0.62	Cd	0.07	0.48	0.12
5°42′ 58″N, 09°31′07″E	M6	67.90	12.93	5.59	1.36	4.67	2.57	2.82	0.14	0.24	0.28	4.12	Ce	0.04	0.11	0.07
	M7	61.50	14.10	8.08	1.67	4.86	2.62	4.77	0.23	0.25	0.39	4.85	Ce	0.10	2.34	0.14
	M8	64.60	14.21	6.32	1.49	5.29	2.52	3.17	0.23	0.20	0.44	4.24	Ce	0.03	0.14	0.06
Site 3: Nfaitok	M9	58.30	11.40	12.13	3.76	4.06	3.00	4.91	0.17	0.24	0.53	3.23	Ce	0.06	0.39	0.18
5°43′ 24″N, 09°30′15″E	M10	64.60	13.03	6.66	2.33	5.20	2.82	3.58	0.13	0.25	0.35	2.85	Ce	0.02	0.08	0.08
	M11	48.20	3.12	39.62	2.11	1.83	0.34	2.56	0.26	0.32	0.12	18.82	Ce	0.06	1.44	0.37
	M12	63.90	13.02	9.69	0.81	4.35	1.92	3.73	0.27	0.25	0.57	12.02	Ce	0.17	4.56	0.95
Site 4: Egbekaw	M13	65.00	16.00	3.04	1.56	3.73	3.03	4.93	0.26	0.23	0.76	1.95	Ce	0.18	4.37	0.33
5°41′ 45″N, 09°02′46″E	M14	50.20	6.07	16.87	2.41	2.79	0.56	14.45	3.58	1.27	0.25	6.99	Ce	0.07	2.46	0.62
	M15	60.00	15.81	7.09	1.49	3.78	2.83	4.44	0.51	1.79	0.77	4.76	Ce	0.19	4.83	0.97
	M16	47.80	7.13	32.59	1.47	4.21	0.30	2.76	0.74	1.26	0.26	22.16	Ce	0.05	1.34	0.29
Site 5: AjayukNdip	M17	61.50	12.04	1.32	2.17	0.84	8.49	9.99	0.59	1.01	0.53	0.61	Cd	0.28	12.54	1.43
5°38′ 51.2″N, 09°09′11″E	M18	54.90	10.31	0.66	10.89	0.89	6.70	12.47	0.89	0.20	0.54	0.06	Cd	0.04	0.99	0.25
	M19	63.40	16.94	1.19	2.62	2.94	3.70	6.54	0.04	0.39	0.76	0.46	Cd	0.05	1.21	0.14
Site 6: John Hault	M20	58.0	18.87	0.98	3.50	1.52	4.80	9.36	0.06	0.28	1.12	0.28	Cd	0.08	2.34	0.05
5°45′ 25.6″N, 09°18′59.1″E	M21	51.0	9.94	25.42	2.28	1.64	2.23	4.91	0.09	0.45	0.57	11.14	Ce	0.06	1.88	0.22
	M22	58.2	14.61	0.51	6.61	0.63	7.17	9.47	0.08	0.41	0.77	0.08	Cd	0.09	2.91	0.07
Site 7: Inokun	M23	60.7	16.70	0.57	2.78	0.49	10.78	5.22	0.05	0.29	0.96	0.20	Cd	0.08	1.60	0.04
5°44′ 36″N, 09°01′02″E	M24	59.4	17.14	0.21	0.55	0.81	12.17	7.09	0.12	0.33	0.65	0.38	Cd	0.03	0.34	0.03
	M25	52.7	3.99	23.06	12.68	1.00	0.69	2.50	1.40	0.32	0.16	1.82	Ce	0.03	1.49	0.23

The selected 25 samples of dark grey to black shales (Fig. 2) collected from the field were air dried and powdered at the institute research and geological mining Nkolbison, Cameroon. Sample packaging and preparation was done at the Laboratory of Geoscience of Superficial Formations at the University of Yaoundé 1. The powdered samples were analysed for geochemistry by ICP-AES (inductively coupled plasma – atomic emission spectrometry) and ICP-MS (inductively coupled plasma – mass spectrometry) at the Geological Laboratory of Lakehead University Ontario, Canada. For the analyses regarding ICP-AES and MS, procedures of [[20] , [21] , [22] , [23] ]. 0.5 g of the powdered samples was treated with dilute HNO₃ acid to test the carbonate content of the shales. The samples were dissolved again in an open beaker with concentrated nitric-hydrofluoric acid was added to the samples three times for three days. After digestion, for each sample 2% of nitric double distilled water solution was added to the solution for dilution. For ICP-AES analyses, it was diluted 200 times while for ICP-MS analyses it was diluted 1000 times. A blank was inserted for every ten samples. Accuracy is within 10% and precision 5%.
Based on geochemical data, the shales were classified as carbonate-depleted or carbonate-enriched using their Ca/Mg ratio, as shown in Table 1. In this work, geochemical data for rare earth elements (REEs) were normalized with chondrite composition (see Fig. 2a–d). Anomaly bounds were determined, with >1.05 indicating a positive anomaly, 1.04–0.94 indicating no anomaly, and <0.94 indicating a negative anomaly [3 ]. The following boundaries were used to determine correlation: from ≥−0.31 to −1.0 = negative correlation; from −0.31 to 0.31 = negligible or no correlation and; >0.31 to 1 = positive correlation [24 (link)]. Pearson correlations were based on log (x + 1) transformed data. Correlations marked in red are significant at p < 0.05. Correlations were determined with Statistical 13 software.
For determination of total organic carbon (TOC), total organic nitrogen (TON) and total organic sulphur (TOS) analyses, a CARLO ERBA Elemental Analyzer. The samples were loaded into an automated autosampler. When the autosampler is started, the sample is pumped into the combustion reactor, which is maintained at around 1050 °C. The sample container melts in a transient oxygen-rich condition, and the tin promotes a violent reaction (flash combustion). A continuous flow of gas transports the combustion products via an oxidation catalyst of chromium oxide (CrO) stored at 1050 °C within the reaction combustion tube (Helium). To ensure thorough oxidation, a 5 cm layer of silver coated cobalt oxide is put at the bottom of the combustor. The catalyst also traps interfering molecules produced during the combustion of halogenated substances. The mixture of combustion products and water passes through a reduction reactor, which is heated to 650 °C and comprises metallic copper. In the reaction reactor, surplus oxygen is eliminated, and nitrogen oxides from the combustor are decreased to elemental nitrogen at around this temperature, which goes through the absorbent filter with carbon dioxide, sulphur dioxide, and water. C, N, and S, had detection limits of 0.94 g, 0.23 g, and 0.06 g, respectively. Accuracy is within 10% and precision 5%.