Introduction

The history of the CVL is related to the Gondwana continental rifting during the Late Jurassic to Early Cretaceous, the progressive separation of the South American and African plates, and the opening of the South Atlantic Ocean (Maurin and Guiraud 1993; Pletsch et al. 2001; Njike Ngaha et al. 2014). The evolution of South Atlantic Ocean has led both to the creation of magmatic massifs (Njonfang et al. 2011) and sedimentary basins (Njike Ngaha et al. 2014) in Cameroon. The mechanism of episodic emplacement of alkaline magmatism through the entire African continent, and especially along the CVL, can be better explained in terms of complex interactions between hotspots and pre-Cambrian faults (Njonfang et al. 2011). The period between the magmatic events generally corresponds to more or less intense erosion phenomena and the formation of sedimentary basins as the case of Ngwa Basin located along the CVL.

Cameroon has two types of sedimentary basins, the intra-continental and coastal basins of Lower Cretaceous age. Several studies (e.g., Njike Ngaha 1984; Ndjeng 1992; Eyong 2003; Tchouatcha 2011) indicate that the sedimentary formations are generally covered by Cenozoic volcanic rocks, and some with Cenozoic to Recent volcaniclastic deposits (Bilobe et al. 2021; Tchouatcha et al. 2021a). The Ngwa Basin shows certain peculiarities with the sedimentary facies alternate with volcanic deposits (Bessong et al. 2017), and hence the characteristics of the volcano-sedimentary series designation, reported in the Southern Italia (Guerrera and Veneri 1989; Prosser et al. 2008).

The location of the Ngwa Basin in a slope of Mount Bambouto suggests that the later is the main sediment provider of the Ngwa depression. A few studies have been dealt with Ngwa Basin (Capponi 1945; Kenfack et al. 2011; Benammi et al. 2017; Bessong et al. 2017), however, the sediment provenance and facies analyses are still lacking and the new obtained data indicate that the sedimentary and volcanic facies are more diversified. In this work, we provide the detailed lithostratigraphic column and a new evidence for alternation between sedimentary and volcanic processes in the Ngwa Basin. Geochemistry and heavy mineral analyses have been used to investigate the source rock characteristics. Geochemical analyses for the fine-grained sediments are more useful to constraint geochemical signatures (Taylor and McLennan 1985), and heavy minerals for more coarse-grained sediments (Parfenoff et al. 1970) have been used to shed some light on the provenance. Petrographic analyses have been used to characterize the lithology of sedimentary and volcanic deposits. In addition, the environments and conditions of sedimentation have been inferred, based on the facies analyses alongside with geochemical analyses data.

Geological setting

The Ngwa sedimentary Basin (Fig. 1) is located on the southern flank of Bambouto volcano in the west region of Cameroon along the CVL and represents the third largest volcano of the CVL (Kagou Dongmo et al. 2001). The CVL is a 100-km-wide geological lineament, oriented N 130° E and characterized by an alignment of oceanic and continental volcanoes from Gulf of Guinea to the interior of African continent over 1600 km (Marzoli et al. 2000). The Mount Bambouto is the third largest volcano of the CVL after Mounts Cameroon and Manengouba with volcanic activity ranges from Miocene to Quaternary (Kagou Dongmo et al. 2010). These structures are thought to be induced by a network of combined faults, related to an intra-plate sliding system of high extension (Deruelle et al. 1991). The geological history of Mount Bambouto is divided into four main stages from the Miocene to the Quaternary (Youmen et al. 2005; Kagou Dongmo et al. 2010). The first stage (21 Ma) was devoted to the building of the primitive shield volcano with alkaline basalts. The second stage (18.5–15.3 Ma) is the collapse of the Caldera in response to the draining of a large sub-volcanic evolved magma reservoir by ignimbritic rhyolites and trachytes. The third stage (15.1–4.5 Ma) renews with the basanites to mugearites effusive activities together with post-caldera extrusions of trachytes between 15 and 8.8 Ma, and of phonolites between 12.9 and 5.2 Ma. The fourth stage during the recent Quaternary is the north-eastern upper flank basanite eruption.

Fig. 1
figure 1

A Geological map of the Ngwa Basin (modified after Kwékam et al. (2015) and Fosing et al. (2019)). B Location of the study area in the Cameroon Volcanic Line map

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The Ngwa Basin is a small depression and is regarded as one of the uncommon continental basins, which is characterized by the presence of lignite deposits (Capponi 1945). The ages of deposits of the Ngwa Basin have been proposed by the study of the lignite in this basin. Capponi (1945) suggested a Tertiary age for this lignite in comparison with lignite known from the southern part of Nigeria. Benammi et al. (2017) attributed an age between 20.04 and 20.21 Ma to the lignite from magneto-stratigraphical analyses and K–Ar dating. Bessong et al. (2017) proposed an age that ranges from the Late Eocene to the Early Miocene based on palynological recorded assemblages.

According to Kenfack et al. (2011), Benammi et al. (2017), and Bessong et al. (2017), the sedimentary succession in the Ngwa Basin is made up of volcano-clastic sediments. Sedimentary facies are conglomerates, sand/sandstones, and clays/claystones interbedded with cineritic and tuff/tuffite (Benammi et al. 2017; Bessong et al. 2017).

Methods of data base

To throw some light on the provenance and conditions of sedimentation in the Ngwa Basin, geochemical analyses for fine-grained rocks, petrographical investigations for pebbles with volcanic and sedimentary rocks, mineralogical analyses for fine-grained shales and claystones), and heavy mineral studies for coarse-grained rocks and facies analyses have been carefully performed.

Sixty polished thin sections have been prepared at Langfang Rock Detection Technology Services Ltd in Hebei (China) and in Assiut University (Egypt). The petrographic study of these samples has been carried out under polarized microscope at the Laboratory of Petrology and Structural Geology in the University of Yaoundé 1 (Cameroon).

The X-ray diffraction patterns of eleven representative samples (shales/claystones) have been obtained from a Bruker D8-Avance Eco 1Kw diffractometer (Copper Kα radiance. λ = 1.5418 Å, V = 40 125 kV, I = 25 mA) with Lynxeye Xe energy-dispersive detector in the laboratory of “Argiles, Géochimie et Environments Sédimentaires (AGES)” at the University of Liege, Belgium. The analyses have been carried out on the bulk material (non-oriented powder with grinded particles < 50 µm).

Ten thin sections were prepared at the University of Yaoundé 1 for heavy mineral analyses. The methods followed were those of Parfenoff et al. (1970). After washing and sieving the samples, the heavy minerals (d > 2.89) were extracted with the help of Bromoforme. The fraction retained is between 100 and 200 μm.

Whole-rock geochemical analyses of thirteen representative fine-grained samples have been carried out at Bureau Veritas Commodities, Vancouver, Canada. Prepared samples (homogenized powder) have been mixed with LiBO2/Li2B4O7 flux. Crucibles were fused in a furnace at 1000 °C. The cooled bead was dissolved in ACS grade nitric acid. Trace elements (including rare earth elements REE) were determined by the inductively coupled plasma mass spectrometry (ICP-MS). Major element oxides were obtained by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Loss on ignition (LOI) was determined by igniting a sample split then measuring the weight loss. The assay uncertainties varied from 0.1 to 0.04% for major elements, 0.1 to 0.5% for trace elements, and 0.01 to 0.5 ppm for REE. Accuracy for REE is estimated at 5% for concentrations > 10 ppm and 10% when lower.

Results

Facies analyses

The deposits of the Ngwa Basin (Fig. 2) have been grouped into three major facies associations including conglomerate facies (Gmm, Gcm, and Gmi), sandstones facies (Sm and Sh), and claystones facies (Fm, Fh, and Fhfc). The different lithofacies in Table 1 have been described, interpreted, and classified. Furthermore, the facies code used was modified from that of Miall (1978, 1996).

Fig. 2
figure 2

Synthetic log of the Ngwa Basin showing alternation between volcanism and sedimentation. Abbreviations: Cl clay, S sand, Cg conglomerate

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Table 1 Lithofacies of sediments from the Ngwa Basin. The facies codes are from Miall (1996) and Ntamak-Nida et al. (2010). The interpretations provided are according to Lowe (1982), Nilsen (1982), and Miall (1978, 1996)
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Conglomerates facies

The conglomerate facies (Fig. 3A) comprises three lithofacies including (A) massive clast–supported conglomerates (Gcm), (B) massive matrix–supported conglomerates (Gmm), and (C) imbricated massive conglomerates (Gmi).

Fig. 3
figure 3

Field photographs and some facies. A Varied conglomerate lithofacies; B alternation of shale (Fl) and sandstone (Sl); C shale (Fl); D intercalation of volcanic rock (cinerite) between black shale (Fml); E compact sandy clay (Fm) with crack; F intercalation of sedimentary deposits (clay) between volcanic deposits (rhyolite and tuff); G leaf fossil preserved in the black shale (Fmlf)

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Massive clast-supported conglomerates (Gcm)

The massive clast–supported conglomerates are heterolithic conglomerates with varied types of exclusively volcanic pebbles. This facies is poorly sorted, and the clasts are generally well rounded with sizes up to 0.15 m. The matrix is unconsolidated and consists of clayey sand facies (about 15–25%). There are sometimes lenticular clayey sand intercalations (0.15 m thick), which affected by overload pressure. The thickness ranges from 0.20 to 5 m. It is sometimes associated with Gmm and Gmi facies. The basal boundary with sandy facies is slightly sharp.

Imbricated massive conglomerates (Gmi)

This lithofacies is associated with Gcm lithofacies. It is moderately to poorly sorted with clayey sand matrix and is massive clast-supported. The pebble is also well rounded with some rare sub-angular clasts. The imbricated pebbles display several water current trends (Fig. 3A). The thickness is generally less than 1 m.

Massive matrix-supported conglomerates (Gmm)

This lithofacies is intercalated within the Gcm lithofacies. It is poorly sorted with rounded clasts associated with rare angular to sub-angular pebbles with 1 to 12 cm in size. The dominant matrix (more 60% of the total rock) is clayey sand, which consists mainly of sub-angular to rounded sand grains. The thickness is less than 1 m.

Sandstones facies

The sandstone facies is made up of two lithofacies (A) the massive sandstones (Sm) and (B) the horizontal layered sandstones (Sh).

Massive sandstones (Sm)

The massive sandstone lithofacies (Sm. TGG2) is moderately to poorly sorted, fine- to coarse-grained, angular with rare sub-rounded grain shapes, and its color is gray. The particle sizes range from 0.2 to 1.5 mm. It is composed of abundant feldspar made up of sanidine essentially (40–50%). rock fragments (20–25%) made up of microlitic and spherolitic rocks, associated with rare quartz. The matrix is mainly clayey (25–30% of the total rock). The microstructure is medium to coarse clastic and hetero-granular. Its thickness ranges between 0.6 and 2 m with no internal structure. It alternates with conglomerates and clayey facies, and the boundaries are sharps.

Horizontal layered sandstones (Sh)

The horizontal layered and laminated sandstones (Sh. TGG1) are moderately to well sorted, fine-grained, have very angular to angular grain-shapes, and yellowish color, which linked probably to the weak weathering. The particle sizes are generally ˂ 0.4 mm. They are composed of abundant undifferentiated quartz and feldspar (85–90%) associated with muscovite (2–3%). The matrix is ferruginous (10%), and the microstructure is fine clastic iso- to sub-hetero-granular. The thickness ranges from 0.10 to 0.15 m. This facies is marked by laminations and alternates with Fh (horizontal layered claystones) with more or less sharp boundaries (Fig. 3B).

Claystones facies

It consists of three lithofacies types: (A) horizontal layered claystones, (B) massive claystones/siltstones (Fm), and (C) massive claystones (Fm) and horizontal layered sandstones (Fh).

Horizontal layered claystones (Fh)

The horizontal layered claystones (Fh) are moderately indurated, grayish to gray dark in color due to varied concentration of organic matter (Fig. 3C). The microstructure is very fine clastic and iso-granular. Generally, it is laminated and alternates with Sh and Fm lithofacies. Besides, it encloses thin layers of siltstones and is also associated with Fmlfc facies. The thickness ranges from 0.10 to 4 m. The contacts with cinerite (ash pyroclastic) deposit are erosive (Fig. 3D).

Massive claystones/siltstones (Fm)

The massive claystones/siltstones (Fm) (Fig. 3E) are also moderately indurated, grayish to greenish gray in color with 0.30 to 0.70 m thick. They are sometimes sandy with scattered pebbles (cms long) and display desiccation cracks. The microstructure is fine clastic and hetero-granular. They alternate with Fh lithofacies. This facies includes sporadic leaf fossils and tiny fragments of coal. At the base, the boundary between this facies and rhyolite is erosive (Fig. 3F).

Horizontal layered claystone with plant fossils and coal (Fhfc)

The horizontal layered claystone with fossils and coal (Fhfc) is always associated with Fh lithofacies, and they share the same features. Meanwhile, the Fhfc lithofacies contains abundant plant fossils as varied types of leafs (Fig. 3G). The plant leaf fossils are sometimes well preserved and associated with mm to cm coal fragments and rare fossilized fruits.

Petrographic analyses

The petrographic analyses include the study of pebble of conglomerates (TGCG), sedimentary facies and volcanic rocks (TGB. TGR and TGC) in the study area.

Petrographic analyses of pebbles

According to the petrographic analyses of the pebbles, at least six lithological types of these pebbles have been identified including ignimbrite, volcanic glass, rhyolite, breccias, basalt/andesine, and trachyte.

Ignimbrite

Ignimbrites are gray dark to grayish with flame structure and phenocrysts (1–2%) of quartz and sanidine (less than 0.1 mm in size). The microlite is composed of quartz and feldspar with two directions of fluidality. The microstructure is vitro-clastic and fluidal (Fig. 4A).

Fig. 4
figure 4

Microphotographs. A–D Microstructure photographs of main representative pebbles: A vitro-clastic and fluidal microstructure (PL, ignimbrite); B porphyritic microlitic microstructure (PL, rhyolite); C brecciated microstructure (PL, volcanic breccia); D aphyric microlitic microstructure (PL, andesite). E to F Some microstructure photographs of volcanic rocks: E porphyritic microlitic microstructure (PL, olivine basalt); F spherulitic microstructure (PL, cinerite). G–P Representative heavy minerals: G rutile (PPL); H tourmaline (PPL); I brown garnet (PPL); J epidote (PL); K apatite (PL); L clinopyroxene (PL); M orthopyroxene (PPL); N zircon (PPL); O kyanite (PPL); P biotite (PPL). NB: PL polarized light, PPL plane polarized light

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Volcanic glass

The volcanic glass is grayish in color and composed essentially of spherolite with 0.1 to 3.5 mm size and associated with rare phenorcrysts (6 mm in size). The microstructure is spherolitic and fluidal.

Rhyolite

The rhyolite is gray white to dark with phenocryst of sanidine and sometime quartz and oxides. Clinopyroxene is 0.2–2.5 cm in size and ranges from 2 to 30%. The matrix is made up of microlite of sanidine and oxides. The microstructure is porphyritic microlithic (Fig. 4B) and sometimes fluidal.

Breccia

The phenocrysts in breccia consist of sanidine (5–10%), quartz (3–5%) sometimes bi-pyramidal, rock fragments essentially of rhyolite (25–30%), and some rare spherolites (< 1%). The size of the phenocrysts ranges from 0.5 to 9 mm. The matrix (55–60%) is the microlite of sanidine, which is associated with quartz and oxides. The microstructure is bracciated and fluidal (Fig. 4C).

Basalt/andesite

These lithofacies are the more abundant between the studied pebble types. The degree of weathering makes them generally undifferentiated. The color is gray to dark gray. The phenocrysts are generally rare with sometimes automorphic plagioclase of 0.2 to 0.8 mm in size and less than 2%. The samples consist essentially of microlite of plagioclase associated with rare pyroxene, oxides, and glass. The microstructure is generally microlithic to aphyritic microlitic (Fig. 4D) and with sometimes the fluidality microstructure.

Trachyte

The trachyte is more or less rare and weathered. The color is greenish gray. The phenocrysts are rare and represented by scattered sanidine in the microlitic matrix of feldspar. The microstructure is microlithic to aphyitic and fluidal.

Petrographic analyses of volcanic rocks

The volcanic rocks in the study area are represented by olivine bearing-basalt (TGB), cinerite (TGC), rhyolite (TGR), ignimbrite, and volcanic tuff. The ignimbrite is deeply weathered, and the volcanic tuff is unconsolidated.

Basalt

Basalt is black with visible olivine minerals. Under the microscope, the phenocrysts are made up of olivine (10–12%) and clinopyroxene (3–5%) with 0.2–3 mm in size. Olivine is generally cracked, and clinopyroxene is sometimes weakly cracked. The matrix includes an association of microlith of plagioclase and oxides with olivine and pyroxene. The microstructure is porphyritic microlithic and more or less fluidal (Fig. 4E).

Cinerite

Cinerite shows a whitish gray color under the microscope; the phenocrysts (5–8%) are represented by oriented quartz. sanidine, spherulite, and very rare plagioclase, with 0.2–1 mm sanidine essentially. The microstructure is microlitic to porphyritic microlithic tendency and sometimes spherulitic (Fig. 4F).

Rhyolite

Rhyolite is clear gray in color. The phenocrysts (20–30%) are represented by quartz sometimes showing bi-pyramidal and corroded shapes, sanidine micro-fragments to trachyte. The size ranges from 0.5 to 1 mm. The matrix consists of microlith of quartz. Biotite, oxides, and sanidine are associated with glass phase. The microstructure is porphyritic microlitic and fluidal.

Heavy mineral analyses

Heavy mineral analyses have been carried out on the coarse-grained sediments including pebbly sands, pebbly silts, and sandy clay. The opaque minerals are the more abundant (75–93%) constituents, followed by zircon (3–13%) and garnet (1–5%). Other minerals such as tourmaline, apatite. Biotite, rutile, epidote, and pyroxene are generally less than 2%. Aluminum silicates are very rare, and the olivine is absent. The main specimens have been shown in Fig. 4G to P.

X-ray analyses

X-ray diffraction patterns of clayey samples (Fig. 5) indicate the presence of three common clay minerals including kaolinite, chlorite/smectite, and illite/muscovite with varied concentrations from the bottom to the top. The bottom sample (TGAG) is poor in clay minerals with only a minor quantity of kaolinite. The following samples (TGA1) is dominated by chlorite/smectite and associated with kaolinite. This chlorite/smectite fluctuated (decreases and increases from bottom to the top). Kaolinite appears at the bottom, increases progressively in samples TGA1 and TGA2I, and decreases progressively in sample TGC, TGA2S, and TGA3. The kaolinite increases again in sample TGA4 and finally disappears progressively towards the top. Illite/muscovite appears generally in a few quantities in some samples. Meanwhile, the important remark is the absence of illite/muscovite in the TGC sample and its presence in the TGA2I and TGA2S samples, between which it is interbedded. The non-clay minerals found in the studied samples include quartz (generally dominant), feldspar (plagioclase and sanidine mainly) and iron oxides (hematite, magnetite, and probably goethite) sometimes dominant (magnetite: TGA5 and TGA4-5). The quartz is more abundant than feldspar at the bottom but less abundant at the top (TGA6 and TGA7).

Fig. 5
figure 5

X-ray diffractions of representative studied samples

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Geochemical analyses

Major elements

Major elements of the studied samples are reported in Table 2 and show that SiO2 (42.84–68.65 wt%) and Al2O3 (15.39–27.95 wt%) are respectively the most abundant major elements, whereas CaO (0.11–0.95 wt%), MgO (0.09–0.92 wt%), P2O5 (0.04–0.32 wt%), MnO (< 0.01–-0.92 wt%), and Cr2O3 (0.002–0.008 wt%) are respectively the less abundant with concentration < 1%. Fe2O3 (0.76–13.9 wt%), TiO2 (0.88–2.29 wt%), K2O (0.15–3.04 wt%), and Na2O (0.03–2.67 wt%) are moderately represented.

Table 2 Major element composition (wt%) and elemental ratios of the studied samples
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The Na2O/K2O ratios vary from 0.20 to 0.99 indicating low proportion of plagioclase with respect of K-feldspar as display the petrographic analyses.

Major element enrichment factor diagram (Fig. 6A) normalized to PAAS displays a more or less strong depletion in P2O5, K2O, CaO, and MgO for all the sample groups (mafic, intermediate, felsic, and recycled) with more depletion in the recycled samples, which is generally depleted except in SiO2 and TiO2 that are more enriched in this group. All the sample groups show a weak accumulation in TiO2. The mafic sample group shows high enrichment in MnO and is more depleted in SiO2.

Fig. 6
figure 6

Major and trace elements correlation diagrams: A stick diagram of average major elements for the studied sediments; B stick diagram of average trace elements for the studied sediments, data are average values normalized to PAAS (after Taylor and McLennan 1985)

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Trace elements

Trace elements of the studied samples have been tabulated in Table 3 and indicate that Zr (872.2–5584.7 ppm), Ba (53–1452 ppm), Nb (64.4–915.6 ppm), and Y (64.3–1352.1 ppm) are respectively the most abundant trace elements.

Table 3 Trace and rare earth element composition (ppm) and elemental ratios of the studied samples
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All the four sample groups cited in Table 3 show the same behavior of accumulation or depletion (Fig. 6B). All the sample groups display more or less accumulation in Y, Zr, U, Th, Ta, Nb, Hf, and Ga and depletion in V, Sr, Rb, Cs, Co, Sc, Ni, Cr, and Ba. Meanwhile mafic, intermediate, and felsic rocks show a strong accumulation in Ga, whereas the recycled group is neutral in this element.

Rare earth elements

The rare earth element (REE) concentration of the studied materials has been recorded in Table 3. The total REE content varies from 621.57 to 11,280.51 ppm. The HREE content (31.82 to 602.04 ppm) are low compared to LREE (what is the meaning of this item) (622.91 to 10,678.47 ppm). The LREE/HREE ratio ranges from 14.98 to 21.44 indicating enrichment in the LREE. REE patterns (Fig. 7A) normalized to PAAS (McLennan. 2001) are more or less flat with little accumulation in LREE and with significant negative Ce anomaly. The TGA1 sample is more enriched compare to others. REE patterns of studied samples (Fig. 7) normalized to chrondrite (McDonough and Sun. 1995) are quite similar and show LREE enrichment with reference to HREE, negative Eu anomaly, and significant negative Ce anomaly. Negative Eu anomaly and LREE/HREE values ranging from 14.98 to 21.44 indicate moderate fractionation of REE.

Fig. 7
figure 7

Rare earth element correlation diagrams: A PAAS-normalized REE patterns; B chondrite-normalized REE patterns (after McLennan 2001 and McDonough and Sun 1995, respectively); C PAAS normalized cross plot diagram of Ce/Ce* vs. Pr/Pr* used as a proxy for the Ce and La anomalies (modified after Bau et al. 1996). Field I: neither Ce nor La anomaly; Field IIa: positive La anomaly, no Ce anomaly; Field IIb: negative La anomaly, no Ce anomaly; Field IIIa: positive Ce anomaly, negative La anomaly; Field IIIb: negative Ce anomaly

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Bau et al. (1996) have proposed the Ce/Ce* versus Pr/Pr* diagram (Fig. 7C) to discriminate the “real” from “false” Ce anomaly in the rock due to possible anomalous abundance of La. In this diagram, all the studied samples fall in field IIIb (negative Ce anomaly) except for only one sample that is located in field I (neither Ce nor La anomaly).

Discussion

Provenance

Several methods have been proposed for the sediment provenance such as microtexture and morphology of detrital zircon grains (e.g., Armstrong-Altrin 2020; Armstrong-Altrin et al. 2020), the trace element concentrations and/or their ratio values (e.g., the Cr and Ni concentrations; Garver et al. 1996), the major element concentrations and/or their ratio values (e.g., the SiO2 content; Le Bas et al. 1986; the Al2O3/TiO2 ratio; Hayashi et al. 1997), the REE patterns and the Eu anomaly (e.g., Cullers 2000), or the ratios between trace and major elements (e.g., TiO2/Zr ratio; Girty et al. 1996). In this study, we use the Roser and Korsch (1988) discriminant plot, widely and successfully used (e.g., Armstrong-Altrin et al. 2015; Zaid 2015; Tawfik et al. 2017). This diagram discriminates four major provenance types: recycled mature quartzose, felsic, intermediate, and mafic provenance sectors. Our studied samples occupy all the provenance fields with dominance of mafic to intermediate igneous provenance (Fig. 8A).

Fig. 8
figure 8

Provenance and classification diagrams: A provenance discriminant function plot (after Roser and Korsch 1988); B SiO2 vs. Al2O3/Ti2O bivariate plot; C classification according to the scheme of Herron (1988) for the Ngwa studied samples

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The Al2O3/TiO2 ratio has been also widely used to constrain the provenance (e.g., Absar and Sreenivas 2015; Armstrong-Altrin et al. 2015, 2021; Zhou et al. 2015; Tawfik et al. 2017; Al-Juboury et al. 2021; Tchouatcha et al. 2021a), as they are immobile during sedimentary process (Taylor and McLennan 1985; Absar and Sreenivas 2015). This ratio varies from 3 to 8. 8 to 21 and 21 to 70, respectively for mafic, intermediate, and felsic igneous rocks (Hayashi et al. 1997). In our studied materials, this ratio varies from 9.70 to 26.37, which indicates heterogeneous sources. The SiO2 content in sediment is also used to infer the rock composition (Le Bas et al. 1986) and has been used as well as for ancient sediments (Hayashi et al. 1997) and recent deposits (Armstrong-Altrin 2009; Armstrong-Altrin et al. 2021). According to the Al2O3/TiO2 vs. SiO2 plot (Fig. 8B), the samples from the Ngwa Basin indicate the heterogeneous source with dominant intermediate composition.

Cr and Ni trace elements are indicator of source rock composition (Garver et al. 1996), and high concentrations of these trace elements are related to mafic rocks and/or minerals (e.g., Armstrong-Altrin et al. 2019; Ndjigui et al. 2019; Tchouatcha et al. 2021a); in the studied sediments, the PAAS normalized values of trace elements of studied samples (Fig. 6B) are depleted in Cr and Ni indicating the important contribution of felsic rocks.

Moreover, the Eu anomaly and chondrite normalized REE patterns are also applied to predict the composition of source rock of sediments (e.g., Cullers 2000; Kettanah et al. 2021; Madhavaraju et al. 2021; Ramos-Vazquez and Armstrong-Altrin 2019; 2021). The REE pattern normalized to chondrites of studied sediments (Fig. 7B) show a little significant positive and some no Eu anomalies, showing also a heterogeneous source.

The heavy minerals have been also used as indicators of sediment provenance (e.g., Acquafredda et al. 1997; Tchouatcha et al. 2010, 2021b; Mendes et al. 2020). In our sediments, the rarity of aluminum silicates indicates the weak contribution of metamorphic rocks. The minerals such as pyroxenes, although their low concentrations (< 2%), would indicate volcanic source, and they are frequents in the recent sediments of volcanic areas: in the Djerem-Mbere Basin, Cameroon (Tchouatcha et al. 2010), and the southern Apennines, Italy (Acquafredda et al. 1997). Tourmaline and biotite are also very low and reported in the lower part of deposits, associated with rare kyanite and more or less abundant muscovite suggesting likely the metamorphic and plutonic contributions. So, the distributive sources of sediments vary from bottom to top of the stratigraphic sequence, as indicate the geochemical data: felsic source at bottom (TGAG and TGSAS) and intermediate to mafic sources in the middle and the upper part of deposits.

According to Herron’s classification diagram (Herron 1988) (Fig. 8C), the studied samples correspond essentially to shale (TGA2I, TGA2S, TGC, TGA3, TGA4, TGA6, TGAG, and TGS1), Fe-shale (TGA1, TGA5, TGA4-5, and TGA7), and Wacke (TGSAS).

To determine the source rock composition and evaluate the rate of weathering, and eventually the K-enrichment, the samples were plotted in the Al2O3 − (Ca*O + Na2O) − K2O (Fig. 9A) and Al2O3 − (Ca*O + Na2O + K2O) − FeO + MgO (Fig. 9B) diagrams (Nesbitt and Young 1984, 1989, modified). In these diagrams, the source rock composition which have been experienced moderate to high degree of chemical weathering are varied, dominated by rhyodacite composition, and associated with andesitic and basaltic composition. The rhyodacite is the intermediate rock between rhyolite and dacite, and this last is associated either to andesite or to rhyolite in the ignimbrite. Moreover, the petrographic data of pebbles from the deposits and their concentration indicate the abundant andesite ̸basalt associated with rhyolite, trachyte, breccias, and ignimbrite, and this composition indicates the heterogeneous source rocks as the geochemical data. Figure 9A also shows varied values of chemical indices and clay mineral contents, probably linked to variation of the source rock composition and climatic conditions. More, the samples plot perpendicularly to the CN–K apex indicate the presence and more or less equal proportions of Ca- and ̸ or Na-bearing minerals (plagioclase) and K-bearing minerals (sanidine) in the source rocks, as confirmed the petrographic data. The Na2O and CaO are leached out from the earlier dissolved plagioclase and K2O from sanidine, and the increasing degree of weathering leads to the loss of Ca2+, Na+, and K+.

Fig. 9
figure 9

Ternary diagrams: A A-CN-K plot; B A-CNK-FM plot, A = Al2O3, C = CaO*, N = Na2O, K = K2O, F = total Fe as FeO, and M = MgO (modified after Nesbitt and Young 1984, 1989)

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Sediment sorting, recycling, and paleo-weathering

Chemical index of alteration (CIA) of Nesbitt and Young (1982), plagioclase index of alteration (PIA; Nesbitt and Young 1982; Fedo et al. 1995), and index of compositional variability (ICV) of Cox et al. (1995) have been successfully used to indicate compositional maturity and/or chemical weathering of sediments (e.g., Mongelli et al. 2006; Perri et al. 2011. Perri 2014; Tawfik et al. 2017 for CIA; Armstrong-Altrin et al. 2015; Rashid et al. 2015 for CIA and PIA; Ngueutchoua et al. 2019; Tchouatcha et al. 2021a for CIA, PIA and ICV). PIA and CIA values > 75 and ≤ 50 indicate respectively intense and low chemical weathering of source rocks, and IVC values > 1 and < 1 indicate respectively immature and mature character of sediments. CIA and PIA values of studied sediments range respectively from 71.81 to 99.00 and 79.37 to 99.71, which indicate intense chemical weathering of rock sources. ICV values vary between 0.25 and 1.9, but generally < 1 indicating the maturity of our studied sediments.

Furthermore, according to Zou et al. (2016), high ∑REE contents indicate a possible control by differing amount of accessory minerals such as zircon linked to recycling processes. The ∑REE for the studied sediments ranges from 621.57 to 11,280.5 ppm, which are notably higher than the PAAS (184.77 ppm), and this indicates high recycling of the studied sediments.

Paleo-climate and paleo-oxidation condition

Many parameters such as CIA has been widely applied for paleo-climate interpretation from formations of varied age (e.g., Nesbitt and Young 1984; Yan et al. 2010; Zeng et al. 2019; Tchouatcha et al. 2021a). Low values (˂60) and high values (> 80) of CIA indicates, respectively, cold and dry climate during low chemical weathering and warm and humid climate during high chemical weathering (White and Blum 1995). For our samples, CIA values range from 71.81 to 97.83, which indicate moderate to high chemical weathering under warm and semi humid conditions.

Furthermore, the clay minerals respond to their chemical and thermal environment of deposition. The transport and sedimentation of the clay minerals follow mutation from their origin in the clay cycle (Velde 1992). Illite and chlorite form the base of digenetic zone (Dunoyer De Segonzac 1970). Kaolinite is abundant in hot and humid environment, and their presence indicates intense weathering of source rocks with steep relief and exhaustive leaching of weathered materials (Enu 1986). According to the X-ray diffraction data and concentration variations of kaolinite, the climate was more or less hot and arid to semi-arid but with more arid conditions developed towards the top of the sequence. Generally, the variation in kaolinite concentration is related to that of Sm/Chl which would indicate a less humid to semi-humide climate, muscovite characterized terrestrial source. The absence of illite/muscovite in the TGC sample and its presence in the TGA2S and TGA2I between which it is interbedded indicate their terrestrial source and would correspond to muscovite.

Depositional environment

Conglomeratic facies

  • The Gcm lithofacies is a result of high-power debris and gravity flows on a steep topographic gradient (Nilsen 1982). The predominant clast-supported nature indicates that the flows were highly concentrated and that the clasts remained closely packed during deposition.

  • The Gmi lithofacies is attributed to stream flow processes resulting from a bed load traction carpet (Nilsen 1982) of high-energy gravity deposits on a steep topographic gradient (Nemec and Steel. 1984; Miall 1996).

  • The Gmm lithofacies is interpreted as cohesion less debris flow (Nilsen 1982).

Sandstone facies

  • The Sm lithofacies is interpreted as sands deposited rapidly from turbulent suspension by hyper-concentrated mass flows or subaqueous high-density turbidity currents (Ghibaudo 1992).

  • The Sh lithofacies is attributed to planar bed flows of shallow high velocity (Miall 1978; Harms et al. 1995).

Claystones/siltstone facies

  • The Fm lithofacies result from more or less rapid deposition from suspension and from weak traction current (Miall 1996).

  • The Fh lithofacies is common in the overbank area, and represents deposition from suspension and/or by weak traction current (Miall 1996).

  • The Fhfc lithofacies indicates the abundance of leaf fossils and coals in the clay facies (Fh) indicating the abundance of vegetation.

Furthermore, rock samples analyzed by X-Ray diffractions have been selected from the base to the top of the stratigraphic sequence. In spite of samples TGAG and TGA1 have been collected from two sites at the same stratigraphic position; their difference in mineralogical and chemical compositions is probably related to the mechanism of sediment distribution in the basin influenced by lithology of source rocks. This would explain several input sources in a fluvial network.

Tectonic setting

For this work, to shed some light on the tectonic setting of the study area, the discriminant-function-based major-element diagrams of Verma and Armstrong-Altrin (2013) have been applied. These diagrams have been widely used for recent and ancient deposits (e.g., Guadagnin et al. 2015; Nagarajan et al. 2015; Armstrong-Altrin and Machain-Castillo 2016; Tawfik et al. 2017; Zeng et al. 2019; Tchouatcha et al. 2021a). These diagrams differentiate between three fields of tectonic settings including continental arc, island continental rift and collision from high-silica ((SiO2)adj = 63–95%), and low-silica ((SiO2)adj = 35–63%). For the studied sediments, seven samples belong to high-silica and six samples belong to low-silica diagrams (Fig. 10A and B). The majority of studied samples fall in the rift field and at the border with collision field in the high silica multidimensional diagram and within the Arc field in the low silica multidimensional diagram. By using the tectonic discrimination plot (Fig. 10C) of Roser and Korsch (1986), most of the studied samples fall mainly in the active continental margin, but some samples fall in the passive continental margin. This result could indicate a composite tectonic setting (transpressive to extensional tectonics), which led to the setting up of the Cameroon Volcanic Line. The Ngwa Basin is located along the Cameroon Volcanic Line. The CVL, the Volcano-tectonic megatructure enclosed in the pan-African field belt (Njonfang 1998), is a stress megasplit of 30° E trend generated by the pre-Cambrian leap reactivation N 70°E (Cornacchia and Dars 1983; Moreau et al. 1987; Deruelle et al. 1991; Montigny et al. 2004). This pre-Cambrian fault (CCSZ: Centre Cameroon Shear Zone) had been reactivated several times during the Phanerozoic period (Ngangom 1983; Tchouatcha 2011). These phases of reactivation have led to the reconstruction of volcanic massifs and the setting up of the Ngwa Basin and probably connected to the Late Jurassic-Early Cretaceous Gondwanan continental rifting and the South Atlantic Ocean evolution during the Cenozoic period.

Fig. 10
figure 10

Discriminant diagrams to understand the different tectonic settings of the studied sediments from the Ngwa Basin: A discriminant-function multi-dimensional diagram for high-silica clastic sediments (after Verma and Armstrong-Altrin 2013). The subscript m1 in DF1 and DF2 represents the high silica diagram based on loge-ratios of major-elements. The discriminant function equations are DF1(Arc-Rift-Col)m1 = (− 0.263 × ln(TiO2/SiO2))adj + (0.604 × ln(Al2O3/SiO2))adj + (− 1.725 × ln(Fe2O3t/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; DF2(Arc-Rift-Col)m1 = (− 1.196 × ln(TiO2/SiO2))adj + (1.604 × ln(Al2O3/SiO2))adj + (0.303 × ln(Fe2O3t/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; (B) discriminant-function multi-dimensional diagram for low-silica clastic sediments (Verma and Armstrong-Altrin 2013). The subscript m2 in DF1 and DF2 represents the low silica diagram based on loge-ratios of major-elements. The discriminant function equations are DF1(Arc-Rift-Col)m2 = (0.608 × ln(TiO2/SiO2)adj) + (− 1.854 × ln(Al2O3/SiO2)adj) + (0.299 × ln(Fe2O3t/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; DF2(Arc-Rift-Col)m2 = (− 0.554 × ln(TiO2/SiO2)adj) + (− 0.995 × ln(Al2O3/SiO2)adj) + (1.765 × ln(Fe2O3.t/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; C tectonic discrimination plot (after Roser and Korsch 1986)

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Conclusion

The fine-grained sediments of the Ngwa Basin consist mainly of shale with some rare wacke (TGSAS). These fine-grained sediments are mature sediments and underwent intense chemical weathering of rock sources. The source sediments are essentially volcanic and heterogeneous with dominance of mafic to intermediate igneous provenance. They made up essentially of andesite and basalt, associated with rhyolite, dacite, ignimbrite, and trachyte. The presence of only quartz pebbles in the basal deposit indicates recycled and probably metamorphic and granitic sources.

The variation of the geochemical composition signature (felsic, intermediate, and mafic composition) from the bottom to the top of the sequence is linked to the lithology of eroded source rocks influenced by the amount of pyroclastic input.

The studied samples probably have a composite setting ranging from transpressive to extensional tectonic. This composite tectonic setting led to the setting up of the Cameroon Volcanic Line along which is located the Ngwa Basin.

It is postulated that the climate of deposition was warm and arid to semi humid, but more arid at the end of deposition of the studied section. The sedimentation regime varied from high to low energy related to tectonic and climatic mechanisms. Inside the depositional basin, it is inferred that the deposition took place in the oxic water column with several input sources debouching into the basin. The evolution of conditions and environments of sedimentation are probably linked to tectonics and climate.