Geochemical characterisation of clastic sediments in Kompina (N'kapa Formation, NW Douala Basin, West Cameroon): Implication for provenance, paleoweathering and sediment maturity

Abstract

Chemical analyses were carried out on clastic sedimentary rocks in Kompina (N'kapa Formation of the NW, Douala Basin, West Africa) to disclosed the composition of their source rock, characterised their tectonic domains, decipher the intensity of the past weathering, sedimentary cycles and maturity using concentrations of major oxides, REES and trace elements. Provenance diagram constructed from ratios of La/Co, La/Sc, Th/Sc, Cr/Th and from binary diagrams of Zr vs TiO₂ and AL₂O₃vs TiO₂, disclosed a felsic rock composition as the source rock of the Kompina clastic rocks. The felsic source rock composition designated for the studied clastic materials is also supported by LREE enrichment over HREE and a negative europium anomaly on chondrite calculation and diagram. New discriminant functions diagrams to delineate between active and passive domains such as DF 1&2(Arc-Rift-Col)_M1, DF1&2(Arc-Rift-Col)_M2 combined with diagrams of DF(A-P)M, DF(A-P)MT label a passive tectonic setting characteristics for the source rock where there studied clastic materials were sorted. The weathering intensity and plagioclase lixiviation revealed by the CIA and PIA indexes advocate a weak to intense strength of chemical weathering and lixiviation of plagioclase felspars while the CIX and PIX with elimination of CaO in their formulars show extreme intensity of weathering and lixiviation of plagioclase felspars.

Most of the samples show immature nature from their ICV values > 1 but with the introduction of ICV_new in this work, where oxides of iron and calcite are considered as cement and eliminated from the formular show that all the studied samples have values < 1 indicating they are mature. Plotted diagrams of Th/Sc, and (Gd/Yb)_N ratios, with relationship of Zr and (La/Yb)N shows that the studied clastic materials are mature, second cycle sediments, which have experience zircon mineral addition.

1. Introduction

Clastic sedimentary rocks comprise of detrital materials sorted from pre-existing rocks which maybe igneous, metamorphic or ancient sedimentary rock themselves [1]. Clastic rocks may either be classified as argillaceous comprising of shales, mudstones and clays while the sandstones are classified under the term arenaceous and the conglomerate of variable types inherit the term rudites based on the grain size classification of the Wentworth scale [2]. Clastic fragments carry in them signatures of their source rocks and are considered useful for reconstructing past sedimentological history such as provenance, paleoclimate, paleoweathering and paleoenvironment [3]. The above sedimentological history is constructed from the elements that make up the minerals incorporated in the clastic rocks though some maybe altered due to processes of weathering and erosion [4,5]. [6] also identify processes like chemical weathering, redox variation and diagenesis after sedimentation as factor that may modify the original chemistry of clastic sedimentary rocks. Regardless of the drawbacks relating to sedimentary processes that may affect the composition of clastic rocks during transport and deposition, it is worth knowing that, the chemistry of clastic sedimentary rocks still remains the most reliable means in deciphering past sedimentary history provided the overall composition of the rock is not altered [7]. In this work, we consider the chemical composition for rare earth and trace elements in the studied clastic materials to be formed in a locked system not widely affected by external processes hence making them suitable in interpreting sedimentological parameters of the Kompina clastic materials using associated approaches from current studies [8]. In this work, only new discriminate diagrams proposed by Refs. [[9], [10], [11]] will be used for tectonic characterisation since it provides a clear distinction between passive and active tectonic margins.

Recent studies carried out in some part of the Douala Basin has been to determine the petroleum potential of black shales in the N'kapa Formation [12], depositional environment of clastic sedimentary rocks [13] and provenance studies of some clastic materials using old discriminant functions diagrams [14]. A relative study of clastic sediments in the Mamfe basin alongside some sediments in the Douala Basin published by Ref. [15] settled for metamorphic rock source for the clastic sediments. None of the above studies have applied the new discriminant function methods for determination of tectonic settings in the Douala basin. So, in this light, the chemistry of the clastic sedimentary rocks in Kompina, NE Douala basin will be used to decode the tectonic setting, source rock composition, past weathering, maturity and cycle of sedimentation.

The grouping of certain oxides of major elements (Ti&Al), trace element fractions coupled with fractionations of REEs on PAAS and Chondrite diagrams as well as creation of binary diagrams for source rock composition by means of reputable methods will offer significant understanding into realising this works aims. This work will redesign previous knowledge available concerning clastic materials in the Douala Basin vis-à-vis aspects of provenance past weathering, cycle of sedimentation and maturity and the sedimentary throughout the Cretaceous period. Researchers on clastic sedimentology in both international and local level will benefit from this work as it takes complete method in determining sediment maturity by creating a new ICV formular by eliminated disturbing factors (oxides of iron and Calcium), and eliminating the oxides of calcite from the indices of weathering calculations for better evaluation of the past weathering and climate that existed at the time of the formation of the Kompina clastic sediments.

2. Geologic setting

The Douala Basin whose formation began in the early Cretaceous is one of the sedimentary basins of the West African coast resulting from rifting and opening of the South Atlantic Ocean, which began in the late Jurassic [16]. Just like all the basins in Cameroon, the breaking of the Gondwanaland and successive separation of the South American plate and African played a vital role in the creation of the Douala basin. Lithologically, the Douala basin comprises of sedimentary cover which are composed of conglomerates, sandstones, shales, mudstones, and laterites. The sandstones are friable to well lithified showing variable colours of reddish, pinkish and whitish with abundant shining quartz grains [14]. The shales are seen enclosing fossils such as gastropods, bivalves displaying dark grey to black colours [12]. The sedimentary cover overlies a highly faulted Precambrian basement which forms the oldest rocks in the basin (Fig. 1). The Precambrian basement lies more or less continuously along the southern coast of Cameroon. Precambrian rocks are mainly gneiss, (mica gneiss, biotite/amphibolite gneiss, granitized gneiss), of quartzites (sometimes conglomerate, sometimes micaceous) and chlorite and calcareous schist. The universal spreading of the Cenozoic and Mesozoic layers or their absence may be related to the different erosion pattern of the Precambrian basement. There are three main facies (arenaceous, argillaceous and carbonate facies) that have been distinguished in the Douala Basin.

Fig. 1.

(a) Cameroon map showing the Douala basin in which the study area is found (b) geologic map of the study area (Kompina) and environ showing the different lithology sampling points.

3. Methodology

The 20 samples used in this study selected after field work were analysed at the ACME Laboratory, Vancouver, Canada, for major, trace and rare earth elements concentration by the ICP-MS (inductively coupled plasma-mass spectrometry). The studied samples were pulverized so the powdered can be homogenous before digestion. The technique employed requires blending close to 0.25 g of the homogenous powdered samples with lithium metaborate at a temperature of 750–800 °C for thirty (30) minutes in a in a crucible. The worm mixture is removed, kept at room temperature for colling to take place. After cooling the mixture is then dissolve again in a 200 ml dilute hydrochloric acid. The diluted acid poured in the samples is allowed to mix by inversion which permits the mixture to sit uninterrupted for a minimum of 30 min. This permits graphite to settle down at the bottom making the solution to float and equipped for quantification. The correctness of the quantification for major elements ranged between 0.002 and 0.04, while the correctness ranged from between 0.0 to 0.5 and 0.01 to 1, for trace and rare earth elements (REEs) respectively.

The chemical data for REEs in this work were normalised with PAAS (Post Archean Australian shales) [17] and Chondrite for anomaly determination. The values of the shale chemistry were compared with NASC (North American Shale Composite) [18] and UCC (Upper Continental Crust) [19].

Only the europium anomalies was calculated in this work using the formular:

Eu/Eu* (Rollinson, 1993) = (EuN)/[(SmN + GdN)^1/2] (eq. 1)

Anomaly boundaries used in this work were categorised as; positive anomaly (>1.05), no anomaly (1.04–0.94) and negative anomaly (<0.95). Trace element ratios (Th/Cr, La/Sc, Th/Sc and Cr/Th) binary diagrams [3,20] for source rock composition were used in this work. The weathering indexes for both old and recent methods were applied in this work as follows;

CIA [6] = 100 × [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O)] (eq. 2)

CIX [2,21], = 100 × (A1₂O₃/(A1₂O₃+Na₂O + K₂O)] (eq. 3)

PIA [22] = 100 × (Al₂O₃–K₂O)/(Al₂O₃ + CaO* + Na₂O–K₂O) (eq. 4)

PIX [20] = 100 × (Al₂O₃–K₂O)/(Al₂O₃ + Na₂O–K₂O) (eq. 5)

ICV [23] = (Fe₂O₃ + K₂O + Na₂O + CaO + MgO + MnO)/Al₂O₃) (eq. 6)

ICV_new (this work) = (K₂O + Na₂O + MgO + MnO)/Al₂O₃) (eq. 7)

Tectonic environment discriminant function diagrams using formulations of [[9], [10], [11]] were used by implementing the formulas below:

For samples with high silica (≥63%),

DF1(Arc-Rift-Col)_M1 = (−0.263 × ln(TiO₂∕SiO₂)_adj) + (0.604 × ln(Al₂O₃∕SiO₂)_adj) +(-1.725 × ln(Fe₂O₃∕SiO₂)_adj) + (0.660 × ln(MnO∕SiO₂)_adj) +(2.191 × ln(MgO∕SiO₂)_adj) + (0.144 × ln(CaO∕SiO₂)_adj) +(-1.304 × ln(Na₂O∕SiO₂)_adj) + (0.054 × ln(K₂O∕SiO₂)_adj) +(-0.330 × ln(P₂O₅∕SiO₂)_adj) + 1.588 (eq. 8)

DF2(Arc-Rift-Col)_M1= (−1.196 × ln(TiO₂∕SiO₂)_adj) + (1.064 × ln(Al₂O₃∕SiO₂)_adj) +(0.303 × ln(Fe₂O₃∕SiO₂)_adj) +(0.436 × ln(MnO∕SiO₂)_adj) + (0.838 × ln(MgO∕SiO₂)_adj) + (−0.407 × ln(CaO∕SiO₂)_adj) +(1.021 × ln(Na₂O∕SiO₂)_adj) +(-1.706 × ln(K₂O∕SiO₂)_adj) +(-0.126 × ln(P₂O₅∕SiO₂)_adj) - 1.068 (eq. 9)

For samples with low silica (≤63%),

DF1(Arc-Rift-Col)_M2 = (0:608 × ln(TiO₂∕SiO₂)_adj) + (−1:854 × ln(Al₂O₃∕SiO₂)_adj) +(0:299 × ln(Fe₂O₃∕SiO₂)_adj) + (−0:550 × ln(MnO∕SiO₂)_adj) +(0:120 × ln(MgO∕SiO₂)_adj) + (0:194 × ln(CaO∕SiO₂)_adj) +(−1:510 × ln(Na₂O∕SiO₂)_adj) + (1:941 × ln(K₂O∕SiO₂)_adj) +(0:003 × ln(P₂O₅∕SiO₂)_adj) −0:294 (eq. 10)

DF2(Arc-Rift-Col)_M2 = (−0:554 × ln(TiO₂∕SiO₂)_adj) + (−0:995 × ln(Al₂O₃∕SiO₂)_adj) +(1:765 × ln(Fe₂O₃∕SiO₂)_adj) +(−1:391 × ln(MnO∕SiO₂)_adj) + (−1:034 × ln(MgO∕SiO₂)_adj) + (0:225 × ln(CaO∕SiO₂)_adj) +(0:713 × ln(Na₂O∕SiO₂)_adj) +(0:330 × ln(K₂O∕SiO₂)_adj) +(0:637 × ln(P₂O₅∕SiO₂)_adj) −3:631 (eq. 11)

DF(A-P)M = (3.0005 × ilr1TiM)+ (2.8243 × ilr2AlM)+ (- 1.596 × ilr3FeM)+ (- 0.7056 × ilr4MnM) + (- 0.3044 × ilr5MgM)+ (0.6277 × ilr6CaM)+ (- 1.1838 × ilr7NaM)+ (1.5915 × ilr8KM)+ (0.1526 × ilr9PM)-5.9948 (eq. 12)

DF(A-P)MT= (3.2683 × ilr1TiMT) + (5.3873 × ilr2AlMT) + (1.5546 × ilr3FeMT) + (3.2166 × ilr4MnMT)+ (4.7542 × ilr5MgMT) + (2.0390 × ilr6CaMT) + (4.0490 × ilr7NaMT) + (3.1505 × ilr8KMT) + (2.3688 × ilr9PMT) + (2.8354 × ilr10CrMT) + (0.9011 × ilr11NbMT) + (1.9128 × ilr12NiMT) + (2.9094 × ilr13VMT) + (4.1507 × ilr14YMT) + (3.4871 × ilr15ZrMT) - 3.2088 (eq. 13)

Where, DF signifies discriminant function, A-P= Active to Passive, M = Major elements, MT = Major + Trace elements, ilr = isometric log ratio, M1 and M2 = major high and low silica respectively, Col = collision.

4. Results

The geochemical results in this work were described paragraphically in to major, trace and rare earth elements as seen below.

From the studied samples, the highest content is represented by SiO₂ (Fig. 2a) ranging between 28 and 85% with an average value of 57.6% (Table 1). Oxides of aluminium (Al₂O₃) and iron (Fe₂O₃) follows suit with average values of 11.6 and 10.5 respectively. Oxides of calcite (CaO) follows fourth behind the proportion of iron oxide with an average value of 3.3% which is thrice smaller than that of Al₂O₃ and Fe₂O₃. On sample-by-sample bases, the carbonate proportion varies as high proportions are seen in sample S17 (27.5) and S21 (26.5) which are close to their silica counterparts and greater than their proportions of Al₂O₃ and Fe₂O₃ (Table 1). The average proportion of CaO (3.3%) in the studied samples is lower than that of UCC (3.59%), NASC (3.51%) and greater than that of PAAS (1.21%). Oxides of potassium show an average proportion of 1.5% while oxides of sodium, manganese, phosphorus, titanium and magnesium show average proportion values inferior to one (<1). The calculated parameters CIA (eq (2)), CIX (eq (3)), PIA (eq (4)), PIX (eq (5)), ICV (eq (6)) and ICVnew (eq (7)) from oxides of major elements show wides variations in the studied samples and in their median values. The highest proportions are seen in the PIX (98.1%), followed by the CIX (88.3) before values of PIA (85.4%) and CIA (79.1%). The CIA values of the studies samples are greater than that of PAAS (70.4), NASC (58.3) and UCC (51.3). The values of PIA also show significant increase when compared with that of PAAS (79), NASC (62) and UCC (52). The ICV values ranged between 0.1 and 17.5 with a median of 2.5 while the ICV_new values ranged between 0.1 and 0.7 with a median value of 0.3 (Table 1).

Fig. 2.

(a) Chart showing major elements variations in the different samples (b) Chart showing trace elements (LILE, HFSE & TTE) variations in the different samples (c) Chondrite normalised diagram (d)PAAS normalised diagram for the studied samples.

Table 1.

Major element oxides (wt%) and calculated parameters of clastic materials of Kompina and environ.

Samples/Oxides	S5	S19	S23	S25	S29	S30	S31	S33	S39	S11	S15	S17	S21	S38	S2	S9	S12	S28	S32	S8
SiO2	50.6	61.8	51.9	58.1	69.9	72.2	70.5	51.1	47.6	47.2	49.3	29.0	30.6	44.6	84.0	85.2	72.5	53.2	94.4	28.9
SiO2 adj	63	68	61	68	78	77	79	62	58	56	58	39	41	52	87	89	80	57	96	33
TiO2	2.2	0.9	1.9	3.0	2.4	2.9	0.8	2.5	2.2	1.6	2.2	1.0	1.1	1.7	0.5	1.6	0.7	0.3	0.2	1.2
Al2O3	18.9	7.7	15.1	14.9	11.3	12.1	11.6	18.9	17.6	17.2	16.3	6.4	6.8	15.8	3.7	8.4	11.1	2.1	3.7	12.3
Fe2O3	5.4	13.7	11.4	5.7	3.7	5.3	2.9	6.5	10.3	10.4	11.6	7.6	6.8	17.4	8.3	0.4	2.4	36.8	0.2	43.4
MnO	0.0	0.1	0.1	0.0	0.0	0.0	0.0	0.0	0.0	0.1	0.1	0.1	0.1	0.1	0.0	0.0	0.0	0.0	0.0	0.1
MgO	0.9	1.9	1.5	0.4	0.1	0.1	0.4	0.5	1.2	1.5	1.6	1.5	1.3	2.4	0.0	0.1	0.4	0.0	0.0	0.1
CaO	0.2	2.8	0.5	0.1	0.0	0.0	0.1	0.1	0.7	4.0	1.0	27.5	26.5	1.5	0.1	0.2	0.7	0.1	0.0	0.4
Na2O	0.4	0.2	0.5	0.6	0.1	0.0	0.1	0.1	0.2	0.3	0.5	0.3	0.3	0.2	0.0	0.0	0.3	0.0	0.0	0.0
K2O	2.0	0.9	2.8	3.2	1.8	0.5	2.6	2.3	2.3	2.1	2.7	1.8	1.8	1.5	0.0	0.2	2.3	0.0	0.0	0.1
P2O5	0.1	1.2	0.1	0.1	0.1	0.1	0.0	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.0	0.1	0.3	0.0	0.6
LOI	19.0	8.5	13.9	13.6	10.1	6.4	10.7	17.6	17.6	15.3	14.6	24.4	24.4	14.5	2.8	3.8	9.4	7.3	1.6	12.5
Total	80.7	91.1	85.7	86.0	89.4	93.3	89.0	82.0	82.2	84.4	85.1	75.2	75.3	85.2	96.8	96.1	90.4	92.7	98.5	87.2
CIA	87.7	66.8	80.2	79.5	85.7	95.4	80.9	88.7	84.7	73.0	79.9	17.8	19.1	83.0	95.1	95.4	77.2	96.8	98.9	95.7
CIX	88.6	87.9	82.4	80.1	85.9	95.5	81.3	89.0	87.5	87.8	83.9	75.5	76.2	90.3	98.7	97.8	81.1	99.1	99.2	99.1
PIA	96.5	70.0	92.8	94.5	99.0	99.7	98.3	99.1	94.5	78.0	90.6	14.3	15.5	89.4	95.8	97.3	90.0	97.2	99.5	96.4
PIX	97.8	97.7	96.5	95.5	99.3	99.7	98.9	99.5	98.5	98.2	96.8	94.3	94.6	98.8	99.5	99.8	96.8	99.5	99.7	99.9
ICV	0.6	2.7	1.2	0.9	0.7	0.7	0.6	0.6	1.0	1.2	1.2	6.2	5.6	1.6	2.4	0.3	0.6	17.5	0.1	3.7
ICVnew	0.3	0.5	0.4	0.5	0.4	0.3	0.3	0.3	0.3	0.3	0.4	0.7	0.7	0.4	0.2	0.2	0.3	0.1	0.1	0.1
DF(A-P)M	−5.9	−6.3	−6.1	−6.0	−5.6	−5.1	−5.4	−5.8	−6.1	−6.3	−6.2	−6.6	−6.6	−6.1	−4.3	−4.9	−5.7	−3.9	−3.4	−5.3
DF(A-P)MT	59.8	52.7	50.4	61.3	81.8	93.1	88.2	64.6	54.3	51.3	52.1	53.4	54.3	53.6	141.8	118.1	84.1	147.0	170.5	78.2
DF1(Arc-Rift-Col)m	0.3	−1.8	0.1	−3.5	−5.8	−4.6	−3.7	−0.7	0.6	0.5	0.3	0.6	0.3	0.2	−1.3	−4.7	−3.7	8.8	−0.9	2.6
DF2(Arc-Rift-Col)m	2.9	0.1	3.3	−2.9	−3.5	−3.9	0.3	5.0	4.1	3.7	3.5	4.5	4.5	3.9	−7.2	1.2	−1.0	1.0	−1.0	2.1

The trace elements in this study were grouped into large ion lithophile element (LILE), high field strength elements (HFSE) and transition trace metals (TTE). From table … the progression is as follows LILE > HFSE > TTE (Fig. 2b). The value of the LILE is twice greater than that of HFSE which is twice as greater than that of TTE. On individual element bases, zirconium shows the average highest proportion of 716 ppm followed by barium with an average proportion of 563 ppm. For trace elements with average proportion between 100 and 200 ppm, only chromium, vanadium and strontium fall within this range (Table 2). All the other trace elements used in this work show average proportion values lower than 50 ppm. Trace elements ratio calculated for paleoclimate and provenance interpretations are also seen Table 2.

Table 2.

Trace elements content (ppm) and calculated parameters of clastic materials in Kompina and environ.

Samples/elements	S5	S19	S23	S25	S29	S30	S31	S33	S39	S11	S15	S17	S21	S38	S2	S9	S12	S28	S32	S8	Av
Ni	34	25	57	27	20	20	20	48	67	57	37	20	20	80	20	20	20	20	20	32	33
Sc	13	12	13	13	9	10	6	13	15	13	14	8	7	16	3	5	5	7	1	20	10
Ba	688	283	809	812	1378	693	1142	897	593	681	743	527	558	578	26	88	662	7	26	65	563
Co	12	21	48	9	12	2	9	24	29	30	33	21	20	40	3	1	10	1	0	29	18
Hf	18	13	21	26	46	40	21	31	18	12	15	21	23	14	3	26	6	4	3	21	19
Nb	66	30	54	71	54	61	21	89	49	43	46	26	36	41	13	33	19	7	6	32	40
Rb	58	25	70	69	42	16	48	59	55	55	63	39	38	38	2	5	49	1	1	7	37
Sr	97	161	139	109	206	128	162	136	121	269	149	838	737	141	13	14	107	6	4	31	178
Th	20	18	18	20	21	21	11	25	19	18	18	12	12	17	5	11	7	3	1	19	15
U	5	7	4	4	7	6	3	7	4	3	4	3	3	4	1	2	1	3	0	9	4
V	109	399	181	157	96	119	41	121	144	228	189	105	90	182	27	57	82	37	9	524	145
Zr	704	471	772	973	1746	1425	801	1114	649	447	565	794	830	537	99	1012	228	181	137	844	716
Cr	68	274	137	144	137	137	68	137	137	137	137	68	68	137	68	68	68	68	14	479	128
LILE	1570	964	1812	1987	3400	2290	2167	2237	1442	1473	1542	2213	2178	1314	145	1132	1055	202	169	974	1513
HFSE	254	531	313	303	247	273	105	303	272	333	303	193	186	289	49	129	133	58	21	626	246
TTE	115	319	242	180	169	159	97	209	233	224	207	109	109	257	91	89	99	89	34	540	179
Zr/Sc	54	39	59	75	194	143	134	86	43	34	40	99	119	34	33	202	46	26	137	42	82
Cr/Th	3.5	15.2	7.5	7.1	6.6	6.4	6.3	5.6	7.1	7.8	7.7	5.7	5.9	8.1	14.0	6.2	9.2	20.1	10.5	25.1	9
Th/Sc	1.5	1.5	1.4	1.6	2.3	2.1	1.8	1.9	1.3	1.3	1.3	1.5	1.6	1.1	1.6	2.2	1.5	0.5	1.3	1.0	2
Th/Sc *100	150	150	141	155	230	214	180	189	128	135	127	150	164	106	163	222	148	49	130	96	151
La/Sc	5.9	8.0	5.9	5.1	5.0	4.2	4.8	6.1	4.5	5.1	4.3	8.0	7.6	4.9	3.2	2.5	11.3	1.1	2.2	1.8	5
La/Sc*100	588	795	595	509	501	419	483	612	449	515	434	801	764	486	320	248	1134	110	220	180	508
La/Co	6.2	4.6	1.6	7.0	3.8	23.3	3.4	3.3	2.3	2.2	1.8	3.1	2.6	1.9	3.6	24.8	5.5	15.4	7.3	1.2	6
La/Co *10	62	46	16	70	38	233	34	33	23	22	18	31	26	19	36	248	55	154	73	12	62
Sr/Ba	0.1	0.6	0.2	0.1	0.1	0.2	0.1	0.2	0.2	0.4	0.2	1.6	1.3	0.2	0.5	0.2	0.2	0.9	0.2	0.5	0.4

The total REEs of the studied samples show variations as seen Table 3. The rare earth elements in this study were normalised with chondrite (Fig. 2c) and PAAS (Fig. 2d). The summation of REE in the studied Kompina clastic materials establishes substantial variations in values ranging between 12 and 600 ppm with a median value of 294 ppm (Table 3) superior to PAAS (212 ppm) and almost doubles the value of UCC which stands at 168 ppm. The studied samples express a LREE composition value varying from 9 to 495 ppm, with a median value 245 ppm which is far greater than the vales of PAAS (172) and UCC (136). The heavy rare earth elements (HREE) contents in the studied clastic materials display values ranging between 1 ppm and 27 ppm, with a median value of 17 ppm which is still greater than the values of PAAS (13 ppm) and UCC (10 ppm). The ratios of La/Yb from chondrite normalised calculations show values oscillating between 4.1 and 17. The studied samples show a significant enrichment of light rare earth elements (LREE) over heavy rare earth elements (HREE) with values oscillating between 8 and 36.

Table 3.

Rare earth elements (ppm) and calculated parameters of clastic materials in Kompina and environ.

Samples/elements	S5	S19	S23	S25	S29	S30	S31	S33	S39	S11	S15	S17	S21	S38	S2	S9	S12	S28	S32	S8
La	77	95	77	66	45	42	29	80	67	67	61	64	54	78	10	12	57	8	2	36
Ce	156	231	178	151	94	84	60	171	138	145	136	146	123	166	46	24	101	14	5	111
Pr	18.4	26.1	19.7	16.3	10.1	9.1	6.8	18.5	15.8	17.3	16.0	16.4	13.2	19.3	1.9	2.8	13.8	1.6	0.5	7.7
Nd	75	100	71	61	37	36	24	70	63	64	60	65	53	72	7	11	52	7	1	30
Sm	14.2	20.1	14.8	12.1	7.2	8.0	4.9	13.4	12.3	12.2	12.0	11.6	10.2	14.8	1.2	1.8	9.2	1.2	0.2	6.9
Eu	2.0	3.7	2.7	2.0	1.4	1.8	1.0	2.4	2.4	2.4	2.3	2.3	1.8	2.9	0.2	0.3	1.8	0.2	0.1	1.3
Gd	11.5	18.7	12.8	9.6	6.7	8.3	3.7	11.0	10.9	10.2	9.9	9.9	8.3	11.4	0.8	1.3	7.4	0.7	0.2	6.0
Tb	1.8	3.0	2.0	1.5	1.1	1.4	0.6	1.8	1.8	1.6	1.6	1.5	1.2	1.6	0.1	0.2	1.1	0.1	0.1	1.2
Dy	10.1	1.6	10.2	7.5	5.7	7.5	3.2	10.4	9.3	8.6	8.9	8.0	6.2	9.2	0.7	1.2	5.5	0.9	0.2	6.6
Y	48.4	78.3	44.5	35.2	41.7	43.0	16.3	47.8	46.6	36.7	39.1	32.7	29.9	36.7	2.8	8.1	21.1	2.4	1.7	28.6
Ho	1.9	3.2	1.9	1.6	1.4	1.6	0.6	2.0	1.8	1.6	1.6	1.4	1.2	0.7	0.1	0.3	1.0	0.1	0.1	1.3
Er	5.0	9.1	5.1	4.2	4.3	4.5	1.8	5.5	4.9	4.4	4.4	3.7	3.1	3.7	0.4	1.1	2.6	0.3	0.3	3.9
Tm	0.8	1.3	0.8	0.7	0.7	0.8	0.3	0.8	0.8	0.7	0.6	0.5	0.5	0.6	0.1	0.2	0.4	0.1	0.1	0.6
Yb	4.9	7.6	4.5	4.1	4.3	5.5	2.1	5.4	4.4	4.0	3.9	3.1	3.0	3.5	0.4	1.3	2.1	0.5	0.4	4.2
Lu	0.7	1.1	0.7	0.7	0.8	0.8	0.3	0.8	0.7	0.6	0.6	0.5	0.5	0.5	0.1	0.3	0.3	0.1	0.1	0.6
Total	427	600	446	373	262	255	155	440	380	377	358	367	308	420	71	67	276	36	12	246
LREE	353	495	376	318	202	189	130	366	309	318	298	315	263	364	66	54	242	32	9	199
HREE	25	27	25	20	18	22	9	27	24	21	22	19	16	20	2	5	13	2	1	18
LREE/HREE	14	18	15	16	11	9	14	14	13	15	14	17	17	18	36	12	19	16	8	11
(La/Yb)N	10.4	8.4	11.4	10.7	7.0	5.1	9.0	9.7	10.3	11.1	10.4	13.8	12.1	14.9	16.0	6.4	17.7	10.1	4.1	5.7
Eu*Chondrite	0.4	0.5	0.4	0.4	0.7	0.6	0.5	0.5	0.5	0.4	0.4	0.4	0.4	0.3	0.4	0.8	0.3	0.3	1.4	0.7
Eu*PAAS	0.7	0.9	0.9	0.9	1.0	1.1	1.1	0.9	1.0	1.0	1.0	1.0	0.9	1.0	1.0	0.8	1.0	1.1	1.3	1.0

On chondrite normalised diagram the studied samples show a similar pattern with exception to sample S32 which shows a negative pattern of Dy. Still on chondrite normalised diagram, all the samples display a negative europium anomaly with a non-parallel nature spectral pattern as some samples are seen cut crossing others. Based on calculations, the chondrite europium anomaly for studied samples are negative ranging from 0.4 to 0.8 averaging 0.6 On PAAS normalised diagram most of the samples show a flat pattern with only few samples displaying a near flat pattern. Same like in the chondrite normalised diagram, sample 32 display a negative Dy anomaly. On bases of calculations using PAAS, the europium anomaly (eq (1)) seems absent with only few samples displaying a negative europium anomaly as the samples show values ranging between 0.7 and 1.1 averaging 0.9.

5. Discussion

5.1. Tectonic settings

The tectonic setting of the source rock area can be inferred from the composition of clastic sedimentary rocks [3,20]. Investigation supported by studies of Bhatia and Crook (1986) characterised passive margin clastic sedimentary rocks of felsic origin to show a positive normalised PAAS and the enrichment of LREE/HREE>1. The Kompina clastic rocks studied in this work express an enrichment of LREEs and a negative Eu anomaly on PAAS normalised diagram and by calculations (1.22–1.45), signifying the studied samples were sorted from a source rock that was put in place in a passive tectonic margin.

Major element oxides have been recently used by Ref. [10] to reconstruct new discriminant diagrams to evaluate the tectonic setting of clastic sedimentary rocks. To them, the silica proportion is adjusted to 100 and the proportion groups the studied samples to basically two groups high silica (63–95%., see eq (8) and eq (9)), and low silica (≤63%., see eq (10) and eq (11)) clastic sediments. The tectonic discrimination diagram projected by Ref. [10] constitute of three core tectonic domains (Arc, Rift, and collisional margin) with both arc and collision being and active margin and rift being passive. The Kompina studied clastic sediments plots in the rift zone (Fig. 3a and b) for both samples with high and low silica, signifying continental margin existed in the source area. The outcome gotten from the discriminant plots offers good mark of the tectonic setting of the source rocks forming the studied clastic sedimentary rocks in the Douala Basin.

Fig. 3.

Tectonic discrimination plots for the Kompina clastic sediments (a–b) high and low silica sediments respectively from Verma et al. (2013), Arc, Rift, and Col, (c-d)for major elements and major + trace respectively from Verma and Armstrong-Altrin (2016), showing passive and active settings.

Another new discriminant plot established by Ref. [11] to decode the tectonic background of clastic sedimentary rocks was solely applied to major elements and major plus trace elements by calculating their isometric log ratios (irl). This new discriminant diagram foe tectonic setting brought forward by Ref. [11] differs from that of [10,24], in that, it constitutes only of two domains which are the active and passive background. Plotting the studied samples on discriminant diagrams, DF(A-P)M (see eq (12)) and DF(A-P)MT (see eq (13)) proposed by Ref. [11], its shows that all the studied clastic sedimentary rocks of the Kompina area were derived from a passive margin (Fig. 3c and d). The passive tectonic background represented by the studied clastic sedimentary rocks, corroborate perfectly to the geology of Africa and Cameroon in particular, as the sedimentary basins within the west African rift system (WCAR) were form from breaking of the separation of African plate from the South American plate. The passive margin setting display by the Kompina clastic materials in the Douala basin differs from those studied by Ref. [3] in the Mamfe basin and [20] in the Kribi campo basin which shows signatures of active margin influence by the tecto-thermal activities during the Pan African orogeny.

5.2. Source rock composition

The source rock composition of clastic sedimentary rocks has been revealed recently by Refs. [3,20] using stable element ratios of Ni, Cr, Cr/V and Y/Ni. Clastic sedimentary rocks sorted from ultramafic rocks are characterised by high content of Cr and Ni while felsic composition clastic rocks show small content of Cr and Ni [9]. The Cr and Ni content of ultramafic derived clastic sedimentary rocks will be > 150 ppm and > 100 ppm respectively with a ratio of Cr/Ni ranging between 1.3 and 1.5 coupled with a positive correlation of Cr and Ni above 0.9 [3]. The studied Kompina clastic sedimentary rocks show characteristics of Cr (63.62–108.56), Ni (20–80), Cr/Ni ratio (>2) and a correlation of (r = 0.65) which reasonably different to that of ultramafic origin but reflect characteristics of a felsic source rock composition.

The ratios of Cr/V and Y/Ni is regularly use to differentiate the composition of the source rocks from which clastic sedimentary rocks have been sorted [5]. Substantial values of Cr/V ratio (≥4) and Y/Ni ratios (<1.5) are features of mafic to ultramafic rocks. The clastic sedimentary rocks from this work reveal remarkably small ratio of Cr/V (<2.5) and a ratio of Y/Ni ratio (0.1–3.4) solely demonstrating their origin from a felsic composition rock. The felsic composition of the studied clastic sediments are supported by plotted binary diagram of Al₂O₃ vs TiO₂ and Zr vs TiO2 proposed by Ref. [25] with all the studied samples falling with the felsic composition domain (Fig. 4a and b).

Fig. 4.

Source rock diagrams for the Kompina clastic sediments (a-b) TiO2–Zr, and TiO2–Al2O3 respectively from Hayashi et al. (1997), (c-f) La/Co,Cr/Th, La/Sc, Th/Sc respectively after Bokanda et al. (2020).

[5] used the fractionation of REEs to determine the source rock composition of clastic sedimentary rocks owing to their resistant nature to processes of sedimentology. According to Ref. [5] the absent of Eu anomaly with a minor enrichment of light rare earth elements over heavy rare earth elements are characteristics of clastic rocks source from mafic origin while a high light rare earth element over heavy rare earth elements ratio joined with a negative Eu anomaly are indicatives of felsic source rock origin. A negative anomaly on PAAS and Chondrite diagram and calculations is seen for the clastic rocks in the Kompina area indicating their origin from a felsic source rock. To further strengthen the felsic opinion for the studied clastic sediments in Kompina, ratios of immobile elements (Th/Co, La/Sc, Cr/Th and Th/Sc) used by Refs. [26,27] to discriminate between clastic sedimentary rocks sorted from mafic and felsic sources were applied. The present study constructed binary diagrams from the immobile element's ratio adopted from Ref. [3] adding the domain of an intermediate or mixed felsic and mafic composition. The Kompina clastic sedimentary rocks falls in the field of felsic source marking their origin from felsic rocks (Fig. 4c–f).

The felsic source rock characteristics revealed by the studied results is dissimilar to the metamorphic derivation anticipated by Ref. [15] in some part of the Douala Basin but tie with the studies of [13] in the Mundeck Formation of the Douala Basin [28], in the Abakaliki basin in Nigeria [3], in the Mamfe Basin and [20] in the Kribi campo basin which is part of the Douala basin. This is because [15] used elements of major oxide and their corresponding diagrams to determine provenance not taking into deliberation their vulnerability to aspects of sedimentary processes as opposed to trace and REEs which are immobile in sedimentary environments. The resemblances which exist amongst the clastic materials in Kompina (this study, Douala Basin), Mamfe basin and Kribi Campo Basin may recount a comparable clastic sedimentary rocks formation in the Cameroon sedimentary basins.

The plot of Eu/Eu* vs (Gd/Yb)n on chondrite normalised calculations have widely been used by Refs. [17,29] to determine the source rocks of clastic materials. Literature gathered from the plots reveals that, values of Europium and (Gd/Yb)n anomaly on chondrite normalised calculations <1 and < 2 respectively correspond to post Archean source rocks while values of Europium and (Gd/Yb)n anomaly on chondrite normalised calculations >1 and > 2 respectively correspond to Archean source rocks [17]. The studied sediments show Eu anomalies between 0.3 and 1.4 averaging 0.5 which is < 1 and (Gd/Yb)n ratios show values between 0.5 and 2.2 averaging 1.7 which is < 2 indicating a characteristic of sediments derived from post–Achaean source rocks [17,29] which are thought to be the granite-gneiss basements rocks forming the basements of the basin.

5.3. Paleoweathering and paleoclimate

The composition of clastic sedimentary rocks is highly altered by the effect of climate and weathering and as such, these sediments preserved in them, the intensity of weathering and the prevailing climate that existed at the source area before being transported and deposited within sedimentary basins [30]. According to Refs. [[30], [31], [32]], the intensity of past weathering and it prevailing climate have been widely uncovered by the indices of weathering denoted by CIA (chemical index of alteration) and PIA (plagioclase index of alteration). The calculated values of these indices (CIA and PIA) quantify the degree of weathering to be extreme in hot humid conditions, modest in worm wet conditions and low in cold dry conditions for ranges of 80–100, 70–80 and 40–70 respectively [33]. The Kompina clastic sediments show CIA values ranging between 17 and 98% while the PIA valued ranged between 14 and 99.5% implying the sediments experience low to extreme weathering intensity with a dry cold to hot humid climate (Fig. 5a,c). The intensity weathering as weak(low) in a dry cold climate portray by some of the samples seem not convincing as Cameroon is found in a tropical region.

Fig. 5.

Binary diagrams for Paleoweathering and Paleoclimate characterisation of the Kompinaclastic sediments (a&c) for CIA and PIA respectively, (b&d) for CIX and PIX respectively.

[34] brought forward some inconsistencies in using the CIA and PIA calculation for paleoweathering and paleoclimatic determination. To them, one must consider the following before applying the above-mentioned indices (a) samples should not be rich in carbonate (≥2%), and (b) samples should not be prone to metamorphism. Some of the studied samples show substantial quantity of carbonates that may have altered the values of the above calculated CIA and PIA values. To diminish this effect the carbonate effect on the CIA and PIA, this work employs the idea postulated by Refs. [3,20,35] in eradicating the CaO in the formulars to solve the problem, thereby producing a novel index of weathering denoted by CIX for chemical indices of alteration and PIX for Plagioclase indices of alteration (see methodology section for formulars). Th studied samples show CIX values between 75 and 99% and PIX values ranging between 94 and 99.9% indicating an extreme intensity of weathering in the source area in a hot humid climate which corroborate with the climatic nature of Cameroon (Fig. 5b,d). The extreme weathering intensity observed in the Kompina clastic sediments of the Douala basin differs from the shales studied by Ref. [3] in the Mamfe basin, sandstones in the Kribi campo Subbasin [20] and shales studied in the Abakaliki Basin [28] in Nigeria experiencing the same hot, tropical climatic conditions. Generally, Sr/Ba values < 1 and > 1 and indicate arid and humid climatic conditions respectively [36]. The Kompina clastic sediments show Sr/Ba ratios ranging from 0.14 to 1.59, with 90% of the studied samples having values < 1 and only 10% (02 samples) with values of > 1 (Table 2) reflecting a humid climatic condition at the time of weathering. This result of the Sr/Ba ratios g the indexes of weathering, which inferred high weathering effect in the source area.

5.4. Sediment recycling

The aspect of sediment recycling has mostly been evaluated using trace and REE normalise ratios and their correlations [37]. Stable elements with high residence time within a sedimentary system like Zirconium, Thorium have been used to evaluate the sedimentary cycle of sediments within sedimentary basins [3]. Zircon enrichment in clastic rocks can be evaluated from the ration of Zr/Sc [37]. According to Ref. [37], clastic sedimentary rocks of first-cycle are evidence of a positive association between Th/Sc and Zr/Sc while clastic rocks with recycle character, evidence a negative relationship between ratios of Zr/Sc and Th/Sc signifying zircon addition as a result of lixiviation of less resistant minerals through recycling. Notably, the clastic rocks studied in this work show a negative correlation between ratios of Th/Sc and Zr/Sc (r = −0.3), low ratios of Th/Sc (0.5–2.3) and Zr/Sc (34–202), which is less than the 32 for UCC [7], thus implying that, the clastic rocks at Kompina have undergone recycling and relatively mature. Minor ratio of La/Yb chondrite-normalised are triggered by influence zircon addition in heavy rare earth elements (HREE). As such, clastic rocks enriched in zircon will show negative relationship between chondrite normalised ratio of La/Yb and Zr. In this study, a negative relationship exists between chondrite normalised ratio of La/Yb and Zr (Fig. 6d) with the most of the studied clastic rocks found within the domain of zircon addition (Fig. 6c), hence, confirming the recycling nature of the Kompina clastic rocks making it different from the sediments studied in the Mamfe basin and Kribi campo basin by Refs. [3,20] which are considered as first cycle sediments.

Fig. 6.

(a–b) ICV and ICV new binary diagram for maturity, (c) Correlation Zr vs (La/Yb)N, (d) Th/Sc vs Zr/Sc diagrams for sedimentary cycle.

5.5. Sediment maturity

Sedimentologist in the past have basically focus on maturity of clastic sediments on two approaches which are the mineralogical and textural approached [1,2]. The mineralogical approached exclude the occurrence of less resistance minerals like the plagioclase feldspars and ferromagnesians while paying more attention on resistant minerals such as quartz's and heavy minerals such as zircon, tourmaline, garnet [38]. Authors like [3,20,39] consider the textural and mineralogical approached of maturity in clastic sediments myth as they feel these concepts may have been borrowed from an aspect of a biological point of view from childhood to adulthood. [35], points out that, the mineralogical and textural approaches of clastic sediments maturity require the sediments in a long run to become shining (cleaner) as ordained to meet it actual point of textural perfection (e.g., super mature to be have a well sorted and rounded grains) and mineralogy (e.g., ultra-stable to belong to the arenite group and contained abundant heavy minerals). The validation of the drawbacks and limitations of the mineralogical and textural concept of maturity of clastic materials raised by Refs. [35,39] solely from a standpoint of petrography and mineralogy. The irregularities that exist in using the textural maturity and mineralogical approaches to decipher the maturity of clastic sediments makes the geochemical approach of using sediment composition more reliable for determination of sediments maturity [35]. This work used the chemical composition of the clastic sediments in Kompina to evaluate the level of maturity. The index compositional variation as denoted by “ICV” [23] have been widely used by recent authors to determine the maturity of clastic sedimentary rocks. The values of ICV delimits the sediments into immature and mature if the its greater than one (ICV >1) and less than one (ICV <1) respectively [23]. Clastic matured sediments are imprints of long transport or recycling of older sedimentary rocks while active tectonic signatures are evidence of immature clastic sediment [23]. The clastic sediments in Kompina shows values of ICV ranging between 0.6 and 17.5 (median = 2.5) (Table 1, Fig. 6a) deducing a compositionally immature character for the studied sediments. The ICV formular proposed by Ref. [23] may have some discrepancy as it does not take into accounts factors such as carbonates and iron oxides which may infiltrate the sediments as cementing materials during lithification and diagenesis. Normally, a high proportion of these oxides will lead to an increase in the values of ICV making the its conclusion vulnerable. This work introduces another parameter known as ICV_new where oxides of carbonates and iron are eliminated from the original formula proposed by Ref. [23]. From the ICV_new, the values in the studied sediments ranged between 0.1 and 0.7 (Table 1) with a median value of 0.3 indicating that the Kompina sediments are compositionally mature (Fig. 6b). The mature nature for the Kompina clastic materials is confirmed by their passive margin characteristic and sedimentary recycling.

6. Conclusion

The clastic sedimentary rocks of the N'kapa Formation in the North Western portion of the Douala basin were geochemically analysed and the results yield the subsequent deductions:

• 1.The studied clastic materials show some similarities and differences in their fractionation when normalised with UCC and PAAS. The defence is seen in their patterns and europium anomaly while both show normalised enrichment LREE over HREE.
• 2.The indices of weathering denoted by CIA and PIA point to a weak to strong intensity of chemical weathering in the source area couple with a weak to intense plagioclase lixiviation in the source area.
• 3.The new indices of weathering denoted by CIX and PIX show that all the studied samples exhibit an intense weathering in the source area couple with an intense lixiviation of plagioclase felspars.
• 4.The studied clastic materials points to a felsic source rock origin when plotted on diagrams of Th/Co, Th/Sc, La/Sc and Cr/Th coupled with binary diagrams of Al₂O₃ vs Zr and Al₂O₃ vs TiO₂.
• 5.A passive tectonic setting is revealed for the source rock of the clastic sediments from this study with evidence revealed from the new discriminant diagrams of DF(A-P)M and DF(A-P)MT couple with DF 1&2(Arc-Rift-Col)_M1, and DF1&2(Arc-Rift-Col)_M2 diagrams.
• 6.The studied clastic materials are considered mature from their low ICV_new values. They show addition of zircon enrichment from diagrams of Zr/Sc and Th/Sc combine with correlation of Zr and (La/Yb)_N indicating they have undergone sedimentary cycles.

Author contribution statement

Dr Florence Njinto Kwankam and Dr AMAYA Adama: Conceived and designed the experiments; Performed the experiments; Wrote the paper.

Dr Bokanda Ekoko Eric: Conceived and designed the experiments; Performed the experiments; Analysed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Anyeku Rexon, Ngwane Maureen and Matombo Hewett: Analysed and interpreted the data; Wrote the paper.

Prof Agyingi, Dr Bisse Salomon, Dr Mokake and Nfor Shannon: Contributed reagents, materials, analysis tools; Wrote the paper.

Data availability statement

Data will be made available on request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.