Mineralogy, geochemistry, and stable isotopes in the reconstruction of the paleodepositional environment, provenance, and paleoclimate of Cretaceous mudstones, Koum Basin, Cameroon

Mineralogy, geochemistry, and stable isotopes in the reconstruction of the paleodepositional environment, provenance, and paleoclimate of Cretaceous mudstones, Koum Basin, Cameroon

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Nowel Yinkfu Njamnsi¹^,³, George Lemewihbwen Ngiamte²^,³, Cheo Emmanuel Suh²^,⁴, Olivier Anoh Njoh², Daniel Layton-Matthews⁵, Peir K. Pufahl⁵, Agatha Dobosz⁵

Acta Geochimica | 2025, 44(5) : 1014 - 1039

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Acta Geochimica | 2025, 44(5): 1014-1039

• ORIGINAL ARTICLE •

Mineralogy, geochemistry, and stable isotopes in the reconstruction of the paleodepositional environment, provenance, and paleoclimate of Cretaceous mudstones, Koum Basin, Cameroon

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Nowel Yinkfu Njamnsi¹^,³, George Lemewihbwen Ngiamte²^,³, Cheo Emmanuel Suh²^,⁴, Olivier Anoh Njoh², Daniel Layton-Matthews⁵, Peir K. Pufahl⁵, Agatha Dobosz⁵

Affiliations

¹Exploration Division, Department of Promotion State-owned Oil and Gas Assets, National Hydrocarbons Corporation (SNH), P.O. Box 955, Yaoundé, Centre Region, Cameroon

²Mineral Exploration Geoscience Program, Pan African University Life and Earth Sciences Institute (Including Health and Agriculture), P.M.B 20, Ibadan, Oyo State, Nigeria

³Department of Geology, Mining and Environmental Science, The University of Bamenda, P.O. Box 39, Bamenda, Northwest Region, Cameroon

⁴Economic Geology Unit, Department of Geology, University of Buea, P.O. Box 63, Buea, Southwest Region, Cameroon

⁵Queen's Facility for Isotope Research, Department of Geological Sciences and Geological Engineering, Queen's University, 36 Union Street, Kingston, ON K7L 3N6, Canada

Published: 2025-07-01 doi: 10.1007/s11631-025-00800-y

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Abstract

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The Cretaceous Koum Basin is a rift-related half-graben in northern Cameroon, which constitutes a portion of the Yola Arm of the Upper Benue Trough. This study presents the first comprehensive dataset combining mineralogical, bulk-rock geochemical, and stable C–H–O isotopic data for dark-gray, fine-grained mudstones from the basin, providing new insights into its sediment source, paleoenvironment, and geodynamic setting. The mudstones primarily consist of phyllosilicates (~ 8.6%), feldspars (~ 30.5%), carbonates (~ 13.7%), and minor iron oxides (~ 2.7%), with vermiculite, illite, and kaolinite as the main clay minerals. The presence of analcime, ankerite, and dolomite suggests low-grade metamorphism and/or hydrothermal alteration. Fe₂O₃/K₂O (1.52–6.40) and SiO₂/Al₂O₃ (2.97–4.68) ratios classify the mudstones as compositionally immature shales (ICV ~ 1.64) with low-moderate chemical weathering (CIA ~ 56.35; PIA ~ 59.74; R³⁺/R³⁺ + R²⁺ + M⁺ ~ 0.51). Trace element ratios (Th/Sc ~ 1.70, Zr/Sc ~ 1.33, La/Sc ~ 6.30, La/Th ~ 4.14) indicate an intermediate igneous provenance from a continental crustal source. Paleoenvironmental proxies suggest deposition in a dynamic basin environment marked by fluctuating redox (C_org/P: 0.21–178.34) and salinity (Sr/Ba: 0.34–3.25; N-values: 48–35.92) conditions, ranging from oxic to anoxic and brackish to saline. Major element data (SiO₂ vs. Al₂O₃ + K₂O + Na₂O) indicate a semi-arid regime, while Paleoclimatic indicators such as Sr/Cu (1.88–37.47) and C-values (0.12–0.93) suggests alternating humid and arid conditions. Notably, stable isotope data, reported here for the first time in the Koum Basin, reveal a predominantly terrestrial, fluvial-deltaic C₃ plant source for organic carbon (δ¹³C − 25.2‰ to − 35.2‰) and complex fluid-rock interactions involving meteoric and magmatic-metamorphic fluids under a warm, equatorial climate (δ¹⁸O + 3.6‰ to + 24.9‰, δ²H − 104‰ to − 50‰). The combined mineralogical, geochemical, and isotopic data point to deposition in a tectonically active continental arc setting, with contributions from ocean island arc and passive margin sources.

Key words

Geochemistry / C–H–O isotopes / Sediment source / Paleoenvironment / Paleoclimates / Koum basin

Cite this Article

Nowel Yinkfu Njamnsi, George Lemewihbwen Ngiamte, Cheo Emmanuel Suh, Olivier Anoh Njoh, Daniel Layton-Matthews, Peir K. Pufahl, Agatha Dobosz. Mineralogy, geochemistry, and stable isotopes in the reconstruction of the paleodepositional environment, provenance, and paleoclimate of Cretaceous mudstones, Koum Basin, Cameroon[J]. Acta Geochimica, 2025 , 44 (5) : 1014 -1039 . DOI: 10.1007/s11631-025-00800-y

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1　Introduction

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The oil and gas industry is undergoing a significant transformation, driven by a global shift toward sustainability and a move away from fossil fuels. Despite growing environmental concerns and uncertainties surrounding the long-term availability of hydrocarbon reserves, oil remains a crucial economic driver, particularly for developing nations (Graham and Ovadia 2019; Campbell and Heapes 2008; Höök et al. 2009; Di Muszio and Ovadia 2006; Ghasemian et al. 2020; Litvinenko 2020). This continued reliance on oil necessitates the identification of new hydrocarbon reserves to meet global energy demands, especially in data-scarce regions. Consequently, exploration efforts are increasingly targeting complex geological settings and frontier basins as production from mature fields declines (Lawson et al. 2018; Latham 2019).

Understanding past climatic and depositional environments is crucial for both hydrocarbon exploration and predicting future climate change. Paleoclimate research utilizes various proxies, including clay mineralogy (Ruffell and Batten 1990; Hallam et al. 1991; Ruffell and Garden 1997) and, importantly, stable isotope geochemistry, which has proven particularly valuable in reconstructing past environmental conditions and geochemical processes (McCrea 1950; Urey et al. 1951; Silverman and Epstein 1958; Boutton et al. 1993; Xie et al. 2000; Sauer et al. 2001; Dawson et al. 2004; Leng and Marshall 2004; Tiwari et al. 2015; Tabor and Myers 2015; Peng et al. 2024; Sofer 1991).

The Cretaceous Period, characterized by a greenhouse climate and various geological events (Tarduno et al. 1998; Kuypers et al. 1999; Cojan et al. 2000; Berner and Kothavala 2001; Bice and Norris 2002; Huber et al. 2002; Skelton et al. 2003; Bice et al. 2006), provides an excellent case study for investigating these relationships. Research on the paleodepositional and climatic conditions of Cretaceous rift basins is particularly relevant for understanding hydrocarbon formation and informing future climate models (Lentini et al. 2010; Chaboureau et al. 2013).

Rift basins globally, including those in West Siberia, Africa, South America, China, and Southeast Asia, are known for their significant hydrocarbon potential (Morley 1999; Harris et al. 2004). The West and Central African Rift System (WCARS) is a prime example. While its hydrocarbon potential has long been recognized, development has been hampered by infrastructure limitations. Recent advancements, such as the construction of the Chad–Cameroon pipeline, are now facilitating exploration and production activities in this region (Fairhead 2009; Dou et al. 2023).

While the Benue Trough, a major basin within the WCARS, has been extensively studied, significant hydrocarbon discoveries have been limited. However, the recent discovery in the Kolmani River Field by the Nigerian National Petroleum Company marks a significant milestone. Other promising basins within the WCARS include the East Niger and Sudanese rift basins, as well as basins in northern Cameroon, such as Logone Birni, Garoua, Yaogoua, Babouri Figuil, and Koum (Petters and Ekweozor 1982; Nwachukwu 1985; Genik 1992; Bessong et al. 2018; Njamnsi et al. 2022a, b).

The Koum basin, a Cretaceous rift basin in northern Cameroon, is a remnant of the Yola Arm of the Upper Benue Trough (Fig. 1). Located at the intersection of the Central African Rift System (CARS) and the West African Rift System (WARS), the basin has attracted considerable attention due to its exposed geological formations and potential for hydrocarbon resources. Numerous studies have investigated its structural, stratigraphic, and paleodepositional characteristics (e.g., Congleton 1990; Nolla et al. 2015; Shandini et al. 2018; Bessong et al. 2018; Njamnsi et al. 2022a, b; Agbor-Taku et al. 2023a, b, c; Ngo Mandeng et al. 2024a, b).

Recent studies have identified organic-rich mudstones with significant oil generation potential within the Koum basin (e.g., Njamnsi et al. 2022b; Ngo Mandeng et al. 2024a). These mudstones contain predominantly Type I-II kerogen, indicating oil-prone organic matter, along with some Type III kerogen associated with gas generation. The organic matter in these rocks is derived from a mixture of algal and terrestrial sources (Njamnsi et al. 2022a, b; Ngo Mandeng et al. 2024a, b). However, despite these advances, no study to date has provided a detailed, integrated mineralogical, whole-rock geochemical, and stable C–H–O isotopic characterization of the mudstones from the Koum Basin. This lack of comprehensive geochemical and isotopic data limits our understanding of the basin's sediment provenance, depositional environment, fluid interactions, and tectonic evolution critical factors for assessing its petroleum potential.

This study addresses this gap by presenting the first integrated dataset of mineralogical, major-trace element geochemistry, and stable C–H–O isotope compositions for the dark-gray Cretaceous mudstones of the Koum Basin. This combined approach provides novel insights into sediment provenance, paleoclimate variability, diagenetic processes, and the basin geodynamics. These findings not only advance the geological understanding of the Koum Basin but also contribute to broader regional exploration models and the economic development of northern Cameroon.

2　Geological setting

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The Koum basin, a 1200-km² intracontinental basin in northern Cameroon, is an extension of the Yola sub-basin within the Upper Benue Trough of Nigeria (Fig. 1; Brunet et al. 1990; Maurin et al. 1986; Schwoerer 1965). As part of the WCARS, the formation and geometry of the Koum basin are linked to tectonic activity during the Early Cretaceous, specifically strike-slip or extensional tectonics along reactivated Pan-African Mobile Belt structures (Fig. 1a, b; Mateer et al. 1992; Fairhead 2020).

The Koum basin is a half-graben structure filled with up to 4500 m of fluvio-lacustrine to intertidal sediments. The basin's asymmetrical shape, with a synclinal structure in the northwest and a homoclinal dip to the southeast, is primarily controlled by major synsedimentary faults, particularly along the northern margin of the basin (Fig. 1c). Three main structural trends characterize the Koum basin: NNW–SSE and E–W basin boundary faults, and the NNE–SSW Tcholliré-Banyo Shear Zone (Shandini et al. 2018; Nolla et al. 2015; Njamnsi et al. 2022b).

The geological history of the Koum basin can be divided into three main phases: the Pre-rift Phase, the Early Cretaceous Rifting Phase, and the Late Cretaceous Uplift and Inversion Phase. During the Early Cretaceous Rifting Phase, significant tectonic activity and sedimentation led to the formation of the basin. The Late Cretaceous Uplift and Inversion Phase resulted in the erosion of a substantial portion of the sedimentary fill. Post-rift processes, such as the uplift of the Cameroon Volcanic Line, further eroded approximately 2500 m of Upper Cretaceous and Paleogene sediments. The absence of Tertiary rifting and marine signatures suggests that the Koum basin represents a transitional zone between the West African Rift System (WARS) and the Central African Rift System (CARS) (Genik 1992, 1993; Guiraud and Maurin 1992; Fairhead 2020).

The Koum basin exhibits a typical rift basin depositional sequence, characterized by a basal fluvial unit overlain by a lacustrine unit and topped by a fluvial unit (Lambiase 1990). The Cretaceous deposits of the Koum basin, collectively known as the Koum Formation, are further subdivided into the lower Mbissirri Member and upper Gaba Sandstone Member (Fig. 2; Bessong et al. 2018; Njamnsi et al. 2022a, b; Agbor-Taku et al. 2023a, b; and Ngo Mandeng et al. 2024a, b).

The Mbissirri Member primarily consists of thick shale sequences interbedded with fine-grained mudstones, clay shales, thin limestones, and sandstones (Congleton 1990; Njamnsi et al. 2022b; Agbor-Taku et al. 2023a; Ngo Mandeng et al. 2024a). The member exhibits cyclical sedimentary patterns, with cross-bedded sandstones overlain by thicker mudstones. Paleontological evidence, including trace fossils, plant remains, and vertebrate fossils, suggests a lacustrine and fluvial depositional environment (Congleton et al. 1992; Nolla et al. 2015; Njamnsi et al. 2022b; Ngo Mandeng et al. 2024a).

The Gaba Sandstone Member is composed of medium- to coarse-grained sandstones, conglomerates, and interbedded mudstones and paleosols. This member represents a coarsegrained, braided fluvial system. Underlying the sedimentary fill is a Precambrian basement composed of various igneous and metamorphic rocks (Fig. 2). The Koum basin is renowned for its diverse assemblage of dinosaur footprints, particularly those of theropods and sauropods. Based on the fossil evidence, the basin has been tentatively assigned an Aptian age, correlating it with the early Cretaceous Elrhaz Formation in Niger (Dejax et al. 1989; Congleton 1990; Congleton et al. 1992).

3　Sampling and analytical methods

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A total of 15 fresh, representative dark-gray, fine-grained mudstone samples were collected from the Koum basin (Fig. 3). Samples were obtained from outcrops in the Gouga, Kali, and Mayo Gaba localities, focusing on mudstones due to their potential organic matter content (Fig. 3; Table 1). The samples were subjected to mineralogical, whole-rock geochemical, and isotopic analysis.

3.1　X-ray diffraction analyses

Mineralogical analyses were conducted at Queen's University, Ontario, Canada, using X-ray diffraction (XRD). Powdered samples were analyzed over an angular range of 5° to 80°, 2Ɵ for 60 s/step using Cu radiation (λ = 1.54 Å) and a PIXcel3D detector. Mineral identification was performed using the Highscore Pro 4.9 software and the International Center for Diffraction Data PDF-2 2020 database.

3.2　Bulk-rock geochemical analyses

Ten representative mudstone samples from the Koum basin were analyzed for major and trace element compositions at the Acme Analytical facility in Vancouver, Canada. For major element analyses, including SiO₂, Al₂O₃, K₂O, TFe₂O₃, Na₂O, CaO, MgO, P₂O₅, TiO₂, and MnO, each rock sample (up to 250 g) was pulverized to over 90% using an agate mill and analyzed by X-ray fluorescence (XRF) spectrometry after lithium borate (Li₂B₄O₇/LiBO₂) fusion and dilute nitric acid (HNO₃) digestion. Trace element concentrations were determined by inductively coupled plasma–mass spectrometry (ICP-MS) analysis, following aqua regia digestion. Loss on ignition was calculated by weight loss after ignition at 1000 °C. To ensure data quality, the equipment was washed with silica between samples. Analytical precision was within 5% for most elements, with detection limits ranging from 0.002 to 0.01 wt% for major elements and from 0.001 to 10 ppm for trace elements.

3.3　Stable isotope analyses

Stable carbon, hydrogen, and oxygen isotope analyses were conducted on 15 mudstone samples at Queen's University, Ontario, Canada.

Prior to carbon isotope analyses, the samples were treated with 20% hydrochloric acid (HCl) to remove carbonate minerals. The acid-treated samples were rinsed three times with high-purity (RO) water (15 MΩ resistivity) to remove residual acid. The rinsed samples were then dried overnight at 100 °C to remove any remaining moisture. The dried samples were weighed into tin capsules and the carbon isotopic composition of the samples was measured using a Thermo-Finnigan DeltaPlusXP Continuous-Flow Isotope Ratio Mass Spectrometer coupled to a Costech ECS 4010 Elemental Analyzer. The δ¹³C values were reported in per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB) standard, with a precision of 0.2 ‰. The atmospheric δ¹³C values were calculated using the following equation from Arens et al. (2000): δ¹³Catm = (δ¹³Cplants + 18.67)/1.10. This equation relates the δ¹³C values of plant material to the atmospheric δ¹³C values, allowing for the estimation of past atmospheric carbon isotope compositions.

The samples for oxygen isotope analyses were treated with 20% HCl to remove carbonate minerals. The acid-treated samples were rinsed three times with RO water (15 MΩ resistivity) to remove residual acid. The rinsed samples were dried overnight at 100 °C to remove any remaining moisture. The dried samples were weighed into silver capsules and the oxygen isotopic composition of the samples was measured using a MAT 253 Stable Isotope Ratio Mass Spectrometer coupled to a ThermoScientific TC/EA High Temperature Conversion Elemental Analyzer. The ¹⁸O/¹⁶O ratios in the sample were normalized to the Vienna Standard Mean Ocean Water (VSMOW) international standard. The δ¹⁸O values were calculated based on the normalized ratios and were reported in per mil (‰) relative to the VSMOW standard, with a reproducibility of 0.3 ‰.

The samples for hydrogen isotope analyses were weighed into silver capsules and degassed for 1 h at 100 °C to remove any adsorbed water vapor. The degassed samples were crushed and loaded into a zero-blank autosampler purged with ultra-high purity helium (UHP He). The hydrogen isotopic composition of the samples was measured using a MAT 253 Stable Isotope Ratio Mass Spectrometer coupled to a Thermo Scientific TC/EA High Temperature Conversion Elemental Analyzer. The δ²H values were calculated based on the measured hydrogen isotope ratios and were reported in per mil (‰) relative to the VSMOW standard, with a precision of 2 ‰.

4　Results

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4.1　Bulk-rock mineralogical composition

The mudstones from the Koum basin exhibit a diverse mineralogical profile, primarily composed of phyllosilicates (~ 8.6%), feldspars (~ 30.5%), carbonates (~ 13.7%), and minor iron oxides (~ 2.7%) (Table 2; Fig. 4). Calcite (6.3%–41.6%), quartz (1.7%–70.7%), and albite (6.7%–74.3%) dominate the mineral assemblage, with significant variations in their proportions (Table 2). Additional minerals, including microcline, muscovite, vermiculite, hematite, illite, biotite, kaolinite, analcine, dolomite, ankerite, and anorthite, occur in varying amounts, reflecting diverse depositional and diagenetic processes (Table 2; Fig. 4). Microcline is present in selected samples (e.g., GOU001, KAL001), reaching up to 15.5%. Muscovite ranges from trace amounts to 12.1% in KAL009, while vermiculite appears sporadically, up to 3.2% (Table 2). Hematite is present in trace amounts, reaching 5.4% in KAL003. Illite is significant in some samples, up to 18.1% in KAL004, while biotite is minor, reaching 10.6% in KAL005 (Table 2). Kaolinite ranges from 0.3% to 19.9%, with KAL001 showing the highest content (19.9%). Analcine is present up to 36.9% in KAL009. Dolomite is present in trace amounts, while ankerite reaches 20.8% in KAL010. Anorthite is found only in KAL013 at ~ 10.5% (Table 2).

4.2　Bulk-rock geochemical composition

The bulk-rock major and trace element concentration for the studied mudstone samples from the Koum basin are listed in Table 3 and ESM_1 (Online Resource).

4.2.1　 Major elements

The mudstones are primarily composed of SiO₂ (37.18–66.31 wt%), with lower amounts of Fe₂O₃ (3.15–11.58 wt%). Other major oxides, such as Al₂O₃ (9.08–17.01 wt%), CaO (0.37–22.95 wt%), K₂O (0.98–4.11 wt%), and Na₂O (0.42–5.47 wt%), exhibit variable concentrations (Table 3). Minor elements, including MnO (0.02–0.41 wt%), TiO₂ (0.40–1.14 wt%), and P₂O₅ (0.10–0.39 wt%) are present in low amounts (Table 3). Compared to the average Post-Archaean Australian Shale (PAAS; Taylor and McLennan 1985), the Koum mudstones are enriched in Na₂O, CaO, MgO, and P₂O₅ but depleted in other major elements (Table 3). The geochemical ratios of Al₂O₃/TiO₂ (17.56), K₂O/Al₂O₃ (0.19), K₂O/Na₂O (1.77) and SiO₂/Al₂O₃ (4.0) suggest the presence of clay minerals, K-bearing minerals and quartz (Table 3). Positive correlations between K₂O and Al₂O₃ (R² = 0.81; Fig. 5a) and SiO₂ and Al₂O₃ (R² = 0.52; Fig. 5b) confirms the association of K and Al within feldspars and clay minerals (e.g., kaolinite, illite, vermiculite). Based on the log(Fe₂O₃/K₂O) vs. log(SiO₂/Al₂O₃) classification diagram, after Herron (1988), the samples are categorized as shales (Fig. 6).

4.2.2　 Trace elements

The Koum mudstones exhibit low concentrations of most analyzed trace elements, with notable exceptions including Sr (64.9–747.3 ppm), Ba (56–485.1 ppm), V (46–111 ppm), Ni (16.4–48.9 ppm), Cr (40.9–86.9 ppm), Ag (7–71 ppm), Cu (10.5–80.2 ppm), Rb (18.8–82.3 ppm), Zn (52.9–133.4 ppm), La (19.1–76.8 ppm), and Ce (33.1–121.9 ppm) as shown in ESM_1. When normalized to the Upper Continental Crust (UCC; Rudnick and Gao 2014), the mudstones show depletions in Ba, Nb, Hf, and Zr (Fig. 7), while elements including Ni, Cr, Co, Th, La, Ce, Pb, and Ti have roughly similar concentrations to UCC (Fig. 7). Enrichment factor analysis, expressed as EFaverage = (element/Ti)_sample / (element/Ti)_PAAS (Tribovillard et al. 2006), revealed that Zn, Sr, and Mo have elevated concentrations (EF values > 1) compared to the PAAS average, while Co, V, Cu, and Ni show lower concentrations (Fig. 8).

4.3　Stable isotope composition

Table 4 presents the δ¹³C, δ¹⁸O, and δ²H values for the studied mudstones, reported in parts per mil (‰) relative to VPDB and VSMOW, respectively.

4.3.1　 δ¹³C_VPDB composition

The Koum mudstones exhibit a range δ¹³C values from − 25.2 ‰ to − 35.2 ‰, with an average of − 28.2 ‰ (n = 15; Table 4). Samples from the Gouga locality show a narrow range of δ¹³C values (− 25.8 ‰ to − 27.6 ‰), which are slightly more positive than the overall average (Table 4). The Kali samples, on the other hand, exhibit a wider range of δ¹³C values (− 25.2 ‰ to − 35.2 ‰), with a particularly low value for sample KAL002 (− 35.2 ‰), which is significantly more negative than the overall average (Table 4). The Mayo Gaba samples generally have more negative δ¹³C values (− 27.7 ‰ to − 31.0 ‰), with a trend toward lower δ¹³C values in samples KB-KAL011 (− 30.0 ‰), KAL012 (− 29.7 ‰), and KAL013 (− 31.0 ‰), except for KAL010, with a slightly more positive δ¹³C value of − 27.7 ‰ (Table 4).

4.3.2　 δ¹⁸O_VSMOW composition

The δ¹⁸O values of the Koum mudstones range from + 3.6‰ to + 24.9‰ with an average of + 12.2‰ (n = 15; Table 4). Samples from the Gouga locality have δ¹⁸O values between + 5.7‰ and + 21.7‰, with an average of + 14.5‰ (Table 4). The δ¹⁸O values for the Kali samples range from + 3.6 ‰ to + 19.8‰, with an average of + 10.5‰. The Mayo Gaba samples generally exhibit higher δ¹⁸O values (+ 7.6‰ to + 24.9‰; Table 4), with samples KAL011 and KAL013 showing the highest δ¹⁸O values of + 24.9‰ and + 22.3‰ respectively (Table 4).

4.3.3　 δ²H_VSMOW composition

The δ²H values of the Koum mudstones range from − 104‰ to − 50‰ with an average of − 68‰ (n = 15). The Gouga (− 56‰ to − 81‰) and Kali (− 51‰ to − 78‰) samples exhibits less negative δ²H values compared to the Mayo Gaba samples, which show more negative δ²H values (− 50‰ to − 104‰), particularly samples KAL011 (− 104 ‰), KAL012 (− 100 ‰), and KAL013 (− 99 ‰).

5　Discussions

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5.1　Paleoweathering

Several geochemical indices, including the Chemical Index of Alteration (CIA), the Index of Compositional Variability (ICV), and the Plagioclase Index of Alteration (PIA), are commonly used to assess the degree of weathering in sedimentary rocks (Nesbitt and Young 1982; Fedo et al. 1995; Cox et al. 1995; Price and Velbel 2003; Roddaz et al. 2006; Bauluz et al. 2000). The CIA values for unweathered igneous rocks such as basalts (30–45) and granitoids (45–55), are lower compared to moderately weathered Phanerozoic shales (70–75), which contain minerals like muscovite, illite, and kaolinite (Cox et al. 1995). A high ICV value indicates a high degree of compositional variability, suggesting a less mature sedimentary rock with a significant amount of silicate minerals other than clay. These rocks often form in tectonically active settings. Conversely, a low ICV value indicates a compositionally mature sedimentary rock, enriched in clay minerals and typically derived from mature sediments or Cratonic environments (Cox et al. 1995; Van de Kamp and Leake 1985).

Comparison of the Koum mudstones to PAAS values (Taylor and McLennan 1985) reveals minor depletions in some major elements including SiO₂, Al₂O₃, K₂O, and Fe₂O₃ and slight enrichments in others such as Na₂O, CaO, and P₂O₅, suggesting a low to moderate degree of weathering (Table 3). The CIA (41.06−65.70) and PIA (40.11−72.98) values (Table 5) support this finding, indicating a source area with a mix of minimally to moderately weathered rocks. The low ICV values (average 1.64; Table 5) and mineral composition of the Koum mudstones, including the abundance of albite (~ 39%) and low amounts of clay minerals such as vermiculite (~ 2%), biotite (~ 9%), kaolinite (~ 16%), illite (~ 17%), and muscovite (~ 6%), suggests a relatively immature source area (Tables 2, Fig. 4). This implies rapid erosion and deposition of first-cycle material in a tectonically active setting, consistent with previous studies on the Koum basin and Upper Benue Trough (Ngo Mandeng et al. 2024a; Tchouatcha et al. 2021).

On the A-(Al₂O₃)-CN (CaO + Na₂O)-K (K₂O) triangular plot of molecular proportions after Nesbitt and Young (1989), the Koum mudstones align with the adamellite–granodiorite/granite weathering trend (Fig. 9a). This alignment is consistent with the provenance of the mudstones as discussed in Sect. 5.2. The Th/U ratio is a useful tool for assessing the extent of weathering in source areas. Under surficial oxidizing conditions, U is more mobile than Th, which tends to concentrate in residual materials (McLennan 2001; López et al. 2005). High Th/U ratios (> 4.0) indicate intense weathering or sedimentary recycling (McLennan et al. 1993; Partin et al. 2013). The elevated Th/U ratio of the Koum mudstones (~ 5.85; ESM_1), compared to the PAAS (4.71) and UCC (3.89) values, indicates a moderate degree of weathering and recycling in the source area.

While the CIA value and the A–CN–K diagram (Nesbitt and Young 1989) are commonly used to assess chemical weathering in sedimentary rocks, the CIA value primarily reflects the weathering of feldspars in felsic rocks and does not account for the behavior of divalent metals (e.g., Fe²⁺, Mg²⁺, Mn²⁺) found in mafic minerals. The A–CN–K system also overlooks SiO₂, a major component of silicate minerals. Moreover, studies by Meunier et al. (2013) and Li and Yang (2010) showed that the CIA value can be influenced by source rock composition, post-depositional processes (e.g., diagenesis, metamorphism), and drainage basin heterogeneity, rather than solely reflecting chemical weathering. Consequently, the CIA value may record long-term regional weathering trends that do not directly correspond to environmental conditions at the time of sediment deposition.

Given these limitations, we interpret the CIA and A–CN–K results alongside other geochemical indicators, such as the M⁺–4Si–R²⁺ system (where M⁺ = Na⁺ + K⁺ + 2Ca²⁺; 4Si = Si/4; R²⁺ = Fe²⁺ + Mg²⁺⁾ which integrates variations in alkali, alkaline earth, silica, and divalent cations (Meunier et al. 2013). When plotted in M⁺–4Si–R²⁺ space, the Koum mudstones cluster between the felsic and mafic compositional fields (Fig. 9b, c). The co-variation of Δ4Si% with the R³⁺/(R³⁺ + R²⁺ + M⁺) ratio provides a more robust insight into the nature of the source rocks and the degree of weathering (Meunier et al. 2013). High R³⁺/(R³⁺ + R²⁺ + M⁺) values (approaching 1) and Δ4Si% values greater > 50% indicate intense chemical weathering and residual enrichment of immobile elements, while lower values suggest limited alteration.

The R³⁺/(R³⁺ + R²⁺ + M⁺) ratio, reflecting the enrichment of immobile trivalent cations (e.g., Al³⁺, Fe³⁺) relative to mobile monovalent and divalent cations, serves as a sensitive weathering indicator (Meunier et al. 2013). In the Koum mudstone samples, the R³⁺/(R³⁺ + R²⁺ + M⁺) values range from 0.19 to 0.67, with most falling between 0.53 and 0.67 (Table 6), suggesting moderate chemical weathering through the leaching of mobile elements and enrichment of trivalent cations. Lower ratios (0.19–0.31) exhibited by some of the samples indicate less intense weathering or input from more mafic, chemically immature sources (Table 6). On the R³⁺/(R³⁺ + R²⁺ + M⁺) versus Δ4Si% plot (after Meunier et al. 2013), most of the Koum mudstone samples fall between the granitic and mafic trends, reflecting a source region composed of minimally to moderately weathered rocks (Fig. 9d).

5.2　Sediment source

Geochemical analysis of mudstones, is a valuable tool for determining the source of sediments. This is because mudstones are relatively homogeneous and less susceptible to post-depositional alteration (Cox et al. 1995; Cullers et al. 1988, 1994; Hayashi et al. 1997; Zhang et al. 2022; Tao et al. 2017; Wu et al. 2024). The K₂O/Al₂O₃ ratio is a commonly used proxy for provenance assessment (Cox et al. 1995). Low K₂O/Al₂O₃ ratios (< 0.3) indicate a clay-rich sources, while higher ratios (0.4–1.0) suggest a significant contribution from alkali feldspar. The low K₂O/Al₂O₃ ratio (~ 0.19) for the Koum mudstones indicates a clay-rich source (Table 3). Major element ratios, such as Al₂O₃/TiO₂, are often used to discriminate between different source rock types. Mafic sources generally yield Al₂O₃/TiO₂ ratios below 14, while intermediate igneous sources yield Al₂O₃/TiO₂ ratios that typically fall between 19 and 28 (Girty et al. 1996; Hayashi et al. 1997). The Al₂O₃/TiO₂ ratios (11.45–23.15) for the Koum mudstones (Table 3, Fig. 10a), suggests an intermediate igneous source. Additional geochemical indicators, such as the K₂O/Na₂O ratio (~ 1.77), TiO₂/Zr ratio (~ 0.14), Na₂O (~ 2.82 wt%), MgO (~ 2.53 wt%), P₂O₅ (~ 0.22 wt%), and SiO₂ (~ 55.27 wt%) contents (Table 3; Fig. 10b), further support this interpretation. These findings are consistent with recent study on the Koum basin (Ngo Mandeng et al. 2024a).

Trace element ratios, such as Zr/Sc, Th/Sc, La/Sc, Sc/Th, and La/Th, are useful tools for determining the source of sedimentary rocks. Elements like Sc, La, Th, and Hf are relatively resistance to weathering and diagenesis, making them reliable indicators of source rock composition (Condie 1993; Taylor and McLennan 1985; Wronkiewicz and Condie 1987; McLennan et al. 1993; Floyd and Leveridge 1987). The Sc/Th ratio, in particular, can be used to differentiate between felsic and mafic sources (McLennan 1991; Lyubetskaya and Korenaga 2007). Crustal sources typically have lower Sc/Th ratios (~ 1.33) and higher La/Sc ratios (~ 2.7) compared to mafic sources with Sc/Th ratios near 20.3 and lower La/Sc ratios near 0.91 (Taylor and McLennan 1981; McDonough and Sun 1995; Lyubetskaya and Korenaga 2007; Rudnick and Gao 2014). The Koum mudstones exhibits a La/Sc ratio (~ 6.30), significantly higher than that of mafic sources and a Sc/Th ratio (~ 0.84) slightly lower than the typical crustal value (ESM_1). These geochemical ratios, with the positioning of the samples on Th/Sc vs. Zr/Sc and La/Th vs. Hf diagrams, further supports an intermediate igneous provenance (Fig. 10c, d). The plotting of the samples between the felsic and mafic compositional domains of the M⁺–4Si–R²⁺ coordinate system and on the R³⁺/(R³⁺ + R²⁺ + M⁺) vs. Δ4Si% diagram of Meunier et al. (2013) confirms contributions from felsic source rocks, such as granites, with some input from mafic lithologies (Fig. 9b–d).

5.3　Paleoclimate and paleoenvironmental conditions

5.3.1　 Constraints from bulk-rock mineralogy

The mineralogical composition of mudstones is a valuable tool for reconstructing past environments and climates (e.g., Thiry 2000; Ruffell et al. 2002; Fagel et al. 2007; Lanson et al. 2009; Bristow et al. 2009; Dudek et al. 2012; Deconinck et al. 2020). The composition of mudstones often reflects the dominant weathering regime (physical and/or chemical), which is influenced by climate (Weaver 1989). Additionally, variations in the CIA index of mudstones, which is linked to plagioclase leaching, can provide insights into weathering intensity and past humidity levels (Nesbitt and Young 1982, 1989). The Koum mudstones, with their minimal diagenetic alteration, are well-suited for paleoenvironmental reconstruction (Table 2; Fig. 4).

The Koum mudstones are predominantly composed of calcite (6.3%–41.6%), quartz (1.7%–70.7%), and albite (6.7%–74.3%), with minor amounts of microcline, muscovite, vermiculite, hematite, illite, biotite, kaolinite, dolomite, ankerite and anorthite (Table 2; Fig. 4). The mineral assemblage reflects a complex interplay of depositional and diagenetic processes. The presence of minerals like zeolites (analcine) and carbonates (ankerite, dolomite), suggests potential low-grade metamorphism or hydrothermal alteration (Table 2). Kaolinite, indicative of weathering under warm and humid conditions and illite, associated with limited hydrolysis and dry environments, suggests variable climatic conditions during deposition (Chamley 1989; Thiry 2000; Velde 1995; Li et al. 2000; Adatte et al. 2002; Deconinck et al. 2003, 2019; Gertsch et al. 2011; Charbonnier et al. 2020).

The low kaolinite content in samples KAL001 (~ 19.90%) and KAL002 (~ 12.90%), coupled with the low detrital illite content in samples KAL003 (~ 15.70%) and KAL004 (~ 18.10%), indicates limited chemical weathering, suggesting a transition between humid and drier or semi-arid conditions (Table 2). The presence of vermiculite indicates alteration of biotite and muscovite to trioctahedral and dioctahedral vermiculite, respectively, likely due to high precipitation and chemical weathering (Walker 1975; Moore and Reynolds 1997). The overall clay mineral composition suggests similar climatic conditions and depositional environment in the Gouga, Kali and Mayo Gaba localities.

The dominance of quartz (1.7%–70.7%) in the Koum mudstones suggests a low degree of alteration and short transport distances (Table 2; Fig. 4). The low CIA values (~ 56.35; Table 5) coupled with the position of the mudstones on the SiO₂ vs. Al₂O₃ + K₂O + Na₂O discrimination diagram after Suttner and Dutta (1986), further support a semi-arid climatic setting (Fig. 11). These findings are consistent with those of previous studies on the Koum basin and Upper Benue Trough, which also suggested semi-arid conditions (Bessong et al. 2018; Tchouatcha et al. 2021; Njamnsi et al. 2022a, b; Ngo Mandeng et al. 2024a, b).

However, major element-based indices reflect broader compositional trends that are influenced by multiple factors, including provenance, weathering intensity, and mineral stability, which may not directly align with trace element-based paleoclimate proxies. While a major element dataset (SiO₂ vs. Al₂O₃ + K₂O + Na₂O) generally supports deposition under semi-arid conditions, there is the need for a multiproxy approach that takes into consideration both major and trace element concentrations to appropriately interpret the paleoclimatic condition, especially in altered sedimentary settings like the Koum basin.

5.3.2　 Constraints from bulk-rock geochemistry

Past environmental conditions including paleosalinity can be estimated using various geochemical proxies. Two commonly employed proxies are the Sr/Ba ratio and the N-value (100 × MgO/Al₂O₃), which exhibit strong correlations with water salinity (Chen et al. 2022; Zhang et al. 2023). As water salinity increases, the Sr/Ba ratio also rises. This is because Ba²⁺ ions, being more reactive than Sr²⁺ ions, preferentially combine with sulfate (SO₄²⁻) to form insoluble precipitates (Walker et al. 1963). The N-value proxy leverages the contrasting behavior of Mg and Al. Magnesium is primarily associated with marine environments, while Al is more indicative of terrestrial sources (Tian et al. 2014). Based on these proxies, paleosalinity can be categorized as freshwater (Sr/Ba < 0.2, N-value < 1), brackish water (0.2 < Sr/Ba < 0.5, 1 < N-value < 10), and saline water (Sr/Ba > 0.5, N-value > 10) (Meng et al. 2012; Wei & Algeo 2020; Chen et al. 2022; Wu et al. 2024). The Koum mudstones, with their Sr/Ba ratios of 0.34–3.25 (ESM_1) and N-values from 4.48 to 35.92 (Table 5), exhibit characteristics of both brackish and saline environments.

Paleoclimate and paleosalinity are interconnected since arid conditions can result to increased salinity (Chen et al. 2020). Paleoclimate can be reconstructed using proxies such as the C-value and the Sr/Cu ratio (Lerman et al. 1995; Zhao et al. 2007; Jia et al. 2013). The C-value, a geochemical index calculated as Σ(Fe + Mn + Cr + Ni + V + Co)/Σ(Ca + Mg + Sr + Ba + K + Na), suggests that elements including Fe, Mn, and Cr are primarily enriched in moist environments, while elements such as Ca, Mg, and Sr tend to be enriched in arid environments (Zhao et al. 2007; Cao et al. 2012). The Sr/Cu ratio is a useful proxy for paleoclimate reconstruction, with higher values typically indicating drier conditions and lower values suggesting wetter periods (Lerman 1995; Meng et al. 2012; Jia et al. 2013). However, interpretations vary with some studies defining Sr/Cu > 10 as arid and 1–10 as warm and humid (Wang et al. 1997; Hu et al. 2012; Qu et al. 2019), while other studies propose Sr/Cu > 5 for arid climates and 1.3–5 for humid conditions (Lerman 1987; Hu et al. 2017). This inconsistency underscores the need for the integration of multiple geochemical proxies for a more reliable paleoclimate interpretation. The C-value index provides a more quantitative assessment of paleoclimate, categorizing environments as arid (C-value < 0.2), semi-arid (0.2 ≤ C-value < 0.4), semi-arid to semi-moist (0.4 ≤ C-value < 0.6), semi-moist (0.6 ≤ C-value < 0.8) and moist (C-value ≥ 0.8) (Moradi et al. 2016; Wang et al. 2022; Wu et al. 2024).

The wide range of Sr/Cu ratios (1.88–37.47; ESM_1) and C-values (0.12–0.93; Table 5) in the Koum mudstones suggests a complex paleoclimatic history marked by fluctuations between arid and humid conditions. Paleosalinity (N values, Sr/Ba ratios) and paleoclimate (Sr/Cu, C-value) proxies (ESM_1; Table 5) indicate deposition in a dynamic environment with variable salinity, likely reflecting seasonal or long-term climatic changes. However, the presence of alteration minerals such as analcime, ankerite, and dolomite indicative of low-grade metamorphism and/or hydrothermal alteration suggests that these geochemical signatures may have been partially overprinted. Post-depositional processes are known to mobilize Sr and Cu, potentially altering the original Sr/Cu and Sr/Ba ratios. Strontium, for instance, can substitute into carbonates like dolomite and ankerite, leading to Sr enrichment during diagenesis or hydrothermal fluid interaction (Veizer 1983; Al-Aasm 2003; Cangelosi et al. 2020). In contrast, copper is more easily mobilized under oxidizing, acidic, or hydrothermal conditions, often resulting in its depletion (Chávez 2021). These alterations can artificially affect the Sr/Cu and Sr/Ba ratios, mimicking signals typical of arid or saline environments and biasing paleoclimate and paleosalinity interpretations. Thus, while the observed Sr/Cu and Sr/Ba trends suggest variable salinity and aridity, post-depositional modifications cannot be rolled out.

Tyson and Pearson (1991) defined four primary redox facies (oxic, dysoxic, suboxic, and anoxic) based on oxygen and hydrogen sulfide concentrations in aquatic environments. Algeo and Li (2020) further subdivided the suboxic facies into the suboxidized and subreduced categories. Redox environments can be characterized using various geochemical proxies, including those based on the C–S–Fe–P system (Raiswell et al. 1988; Poulton and Canfield 2011), such as Fe/Al ratios (Lyons and Severmann 2006) and C_org/P = (TOC/12)/(P/30.97) ratios (Algeo and Ingall 2007), as well as those based on trace-metal enrichment factors (Algeo and Liu 2020), bimetallic ratios (Tang et al. 2020; Fathy et al. 2023), Ce anomalies (Tostevin et al. 2016), and biomarkers (Fathy et al. 2023).

Principal component analysis has shown that C–S–Fe–P system proxies and trace-metal enrichment factors are the most reliable indicators for paleoredox evaluations (Algeo and Liu 2020). While Ce anomalies can be informative, they may not be suitable for mudstones as they require a hydrogenous signal (Liu et al. 1988). The C_org/P ratio, a proxy from the C–S–Fe–P system, has been identified as a robust indicator of paleoredox conditions in recent studies (Wang et al. 2022; Lei et al. 2023). Given the brackish to saline nature of the Koum mudstones, the C_org/P ratio was selected for this study. Based on established C_org/P threshold values (oxic < 50, dysoxic/hypoxic 50–100, anoxic > 100) (Algeo and Ingall 2007), the wide range of C_org/P ratios (0.21 to 178.34; Table 5) for the Koum mudstones suggests that their depositional environment varied from oxic to anoxic conditions. This variability likely reflects changes in oxygen levels over time, potentially influenced by factors like water depth.

The Zr/Rb ratio is a proxy for inferring paleo-water depth and hydrodynamic conditions (Tang et al. 2020; Liu et al. 2020). Zirconium is preferentially concentrated in heavy minerals like zircon, typically found in shallow, high-energy environments. Rubidium, on the other hand, is enriched in light minerals such as clay, commonly deposited in deeper, low-energy settings (Ibach 1982; Dypvik and Harris 2001; Tenger et al. 2006). High Zr/Rb ratios indicate shallow, high-energy environments, while low ratios suggest deeper, low-energy environments (e.g., Omietimi et al. 2022; Wu et al. 2024). The low Zr/Rb ratios (0.07 to 0.49; ESM_1) for the Koum mudstones point to a deep-water, low-energy depositional environment.

5.3.3　 Constraints from δ¹³C_VPDB composition

The stable carbon isotope ratio, δ¹³C, offers a valuable tool for understanding past environments. By analyzing the carbon content of organic matter preserved in dark mudstones, we can reconstruct changes in climatic conditions over time (Gröcke 2002; Bocherens et al. 1993; Ratigan 2016). Plants utilize different photosynthetic pathways to fix carbon dioxide. C₃ plants (including angiosperms and conifers), the first photosynthetic land plants that date back to the Ordovician, use the Calvin–Benson cycle to directly fix CO₂ and thrive in cooler, temperate regions (Robinson and Hesselbo 1998). Their δ¹³C values typically range from − 23 ‰ to − 34 ‰, with an average of − 27 ‰ (Gröcke 2002). In contrast, C₄ plants, adapted to warmer conditions, employ the Hatch–Slack pathway to fix CO₂ as oxaloacetate, leading to δ¹³C values between − 8 ‰ and − 16 ‰, with an average of − 13 ‰ (Spicer and Parrish 1986; Gröcke 2002; Robinson and Hesselbo 2004).

While C₄ plants evolved in the Late Cretaceous, they only became ecologically significant around 6–7 Ma ago (Gröcke 2002; Ehleringer et al. 1991). Crassulacean acid metabolism (CAM) plants, a specialized group adapted to arid environments, emerged around 20–30 Ma ago (Keely and Rundel 2003). Their unique photosynthetic strategy allows them to conserve water, but their isotopic signature is less distinct and can overlap with C₃ and C₄ plants. In the Miocene, the δ¹³C values for wood samples show angiosperms with values from − 24 ‰ to − 30 ‰, while conifers exhibit a range of − 19 ‰ to − 27 ‰, highlighting the potential to link shifts in δ¹³C values to changes in ancient vegetation types (Poole and Bergen 2006). These distinct isotopic ranges can be used to infer vegetation types within carbon sources in geological records.

The average δ¹³C values of the Koum mudstones (− 28.2 ‰; Table 4) is consistent with the δ¹³C values from a terrestrial organic carbon source, primarily from C₃ plants (− 20 ‰ and − 34 ‰), which were dominant during the Cretaceous (Salazar-Jaramillo et al. 2016). The δ¹³C values, plotted on a δ¹³C_VPDB vs δ¹³Catm diagram, indicate a mix of organic matter source, including C₃ plants from temperate grasslands and cooler, forested environments (Arens et al. 2000; Fig. 12a). The slightly more negative δ¹³C values (25.2 ‰ to − 35.2 ‰) for the Koum mudstones than expected for C₃ plants suggest potential influence from factors such as fluctuating CO₂ levels and mixing with marine carbon sources in continental or marginal marine settings (Bocherens et al. 1993; Gröcke 2002). Environmental factors such as CO₂ levels, salinity, and diagenetic alterations could also have impacted the δ¹³C values of the Koum mudstones (Naidu et al. 1993).

The δ¹³C values of the Koum mudstones are broadly similar to global δ¹³C trends during the Cretaceous (Fig. 12b). Although slightly more negative, the δ¹³C values for the Koum mudstones are consistent with Aptian marine sediments from Hokkaido, Japan (δ¹³C_wood values of − 25.4‰ to − 21.8‰), the Maastrichtian Cantwell Formation (δ¹³C_bulk values of − 22.95‰ to − 27.10‰ and δ¹³C_wood values of − 22.42‰ to − 27.85‰), the Cenomanian Dakota Formation of Nebraska (δ¹³C_bulk values of − 24‰ to − 23‰), and the Cretaceous mudstones at Slope Mountain, Alaska (δ¹³C_bulk of − 22.5‰ to − 26.1‰), suggesting similar paleoenvironmental conditions with significant terrestrial C₃ plant input (Fig. 12b; Ando 2002; Gröcke et al. 2006; Salazar-Jaramillo et al. 2016; Ratigan 2016).

The comparatively more negative δ¹³C excursions observed in the Koum mudstones, especially in the Kali sample KAL002 (− 35.2‰) and the Mayo Gaba samples KAL011 (− 30.0‰), KAL012 (− 29.7‰), and KAL013 (− 31.0‰) (Table 4), may be linked to fluctuations in atmospheric CO₂ levels or increased carbon burial events (Spicer and Parrish 1986; Naidu et al. 1993; Gröcke et al. 2002). These lower δ¹³C values could indicate periods of reduced CO₂ or increased terrestrial organic matter input, potentially reflecting environmental stress. Overall, the δ¹³C values of the Koum mudstones suggest a terrestrial, fluvial-deltaic environment with significant contributions from C₃ plants (Fig. 12b).

5.3.4　 Constraints from δ¹⁸O_VSMOW and δ²H_VSMOW composition

Stable hydrogen and oxygen isotope ratios in clastic sediments are valuable tools for reconstructing past environmental and climatic conditions (e.g., Savin and Epstein 1970; Murray and Janssen 1984; Savin and Lee 1988; Santos et al. 2007). These isotopes, present in various water sources (e.g., connate water, seawater, meteoric water, organic water, metamorphic water, and igneous water), can significantly influence the isotopic composition of sediments (Morad et al. 2003; Middleton et al. 2015; Bauer et al. 2016; Xi et al. 2019).

Connate water is water trapped within sediments at the time of deposition. Seawater and meteoric water, with known isotopic compositions, are common reference standards. Seawater exhibits δ²H and δ¹⁸O values of 0‰ relative to the V-SMOW (Morad et al. 2003), while meteoric water exhibits δ²H and δ¹⁸O values that vary systematically with latitude and altitude, decreasing at higher latitudes and altitudes (Fig. 13). Evaporated seawater, formed through evaporation, undergoes a complex isotopic evolution, becoming enriched in both δ¹⁸O and δ²H, up to a salinity four times that of seawater (Fig. 13; Morad et al. 2003; Xi et al. 2019).

Organic water, formed through processes like kerogen maturation, microbial fermentation, and petroleum decomposition, has a wide range of isotopic signatures with δ²H values typically ranging from − 150‰ to − 90‰, and δ¹⁸O values varying from − 7‰ to 20‰ (Worden et al. 1996; Xi et al. 2019; Morad et al. 2003). In contrast, metamorphic and igneous fluids are generally enriched in δ¹⁸O but depleted in δ²H (Fig. 13). The δ¹⁸O values for these waters range from 6‰ to 20 ‰ V-SMOW, while the δ²H values vary from − 55‰ to 6‰ for metamorphic waters and − 70‰ to − 40‰ V-SMOW for igneous waters (Xi et al. 2019).

The δ¹⁸O values of pore water in sedimentary basins are influenced by various factors during burial diagenesis, including changes in the geothermal gradient, hydration of volcanic materials, oil emplacement, mineral dissolution and precipitation, and fluid influx (Ayalon and Longstaffe 1988; Williams et al. 1997; Noh and Lee 1999; Marchand et al. 2002; Sample et al. 2017; Xi et al. 2019). Water–rock reactions and increased temperatures can lead to δ¹⁸O enrichment, while meteoric water infiltration can gradually deplete δ¹⁸O signatures (Haszeldine et al. 1992; Marchand et al. 2002). Additionally, the introduction of deep fluids can enrich pore water δ¹⁸O signatures (Sample et al. 2017).

The wide range of δ¹⁸O (3.6‰ to + 24.9‰) and δ²H (− 104‰ to − 50‰) values for the Koum mudstones (Table 4) suggests interaction with different fluid sources, indicating variable climatic and hydrological conditions during deposition. The δ¹⁸O values (+ 3.6‰ to + 24.9‰) suggest a provenance of chemically weathered intermediate igneous rock, with subsequent interaction with magmatic–metamorphic and meteoric fluids (Table 4, Fig. 13).

Clastic sediments formed in high-latitude or high-altitude regions, where meteoric water is depleted in δ²H and δ¹⁸O signatures, typically indicate lower formation temperatures (Hassanipak and Eslinger 1985; Mizota and Longstaffe 1996). The average δ²H (− 68 ‰; n = 15) and δ¹⁸O (+ 12.2‰; n = 15) values for the Koum mudstones suggest a warm, equatorial-like climate during the Cretaceous, similar to the Cretaceous clays from the Lower Benue Trough of Nigeria, with average δ²H and δ¹⁸O values of – 59.66‰ and + 18.5‰, respectively (Bolarinwa et al. 2019). This contrasts with the more depleted average δ²H (– 101.10‰) and δ¹⁸O (+ 8.03‰) values observed in high-latitude regions, such as the eastern Australian clays (Bird and Chivas 1988).

In the δ²H versus δ¹⁸O diagram (Savin and Epstein 1970; Sheppard and Gilg 1996), the Koum mudstones largely cluster within or near the metamorphic and igneous fields, with some samples trending towards the altered meteoric water field (Fig. 13). The distribution of the samples is consistent with interactions involving magmatic, metamorphic and meteoric waters that has been isotopically modified (Fig. 13). The spread of some samples along the kaolinite weathering line hints at the influence of kaolinite formation, and weathering processes affecting the isotopic signatures (Fig. 13). The particularly low δ²H values in the Mayo Gaba samples KAL011 (− 104‰), KAL012 (− 100‰), and KAL013 (− 99‰) (Table 4) may suggest prolonged exposure to meteoric water or mixing with fluids from a source with extremely low δ²H signatures (Fig. 13).

The higher δ¹⁸O values particularly in the Mayo Gaba samples KAL011 (+ 24.9‰) and KAL013 (+ 22.3‰) (Table 4) might indicate a stronger influence from deep, thermally altered fluids or prolonged burial diagenesis. The presence of certain minerals, such as analcine (1.9%–23.3%), ankerite (~ 10.5%) and dolomite (16.1%–19.8%) in some of the studied samples (Table 2; Fig. 4), further supports low-grade metamorphism and/or hydrothermal alteration. Overall, the variations in δ¹⁸O and δ²H values for the Koum mudstones suggest a complex history of hydrothermal, weathering, and possible diagenetic processes, involving meteoric fluids.

6　Tectonic setting of source area

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The geochemical composition of mudstones can provide insights into the tectonic setting of their source areas. By analyzing major (e.g., K₂O, Na₂O, Al₂O₃, SiO₂) and trace elements (e.g., Th, La, Zr, Sc), we can distinguish between different tectonic settings (Roser and Korsch 1986; Sun et al. 2012; Bhatia 1983; Bhatia and Crook 1986; Pearce and Peate 1995; Verma and Armstrong-Altrin 2013; Wu et al. 2024). The geochemical data for the Koum mudstones, when plotted on the K₂O/Na₂O versus SiO₂ discrimination diagram after Roser and Korsch (1986), suggest a complex tectonic setting, likely involving active continental margin and island arc environments (Fig. 14a). However, on the SiO₂/Al₂O₃ vs. K₂O/Na₂O diagram of Maynard et al. (1982), most of the samples align with arc to active continental margin settings, with one sample suggesting a passive margin environment (Fig. 14b).

Trace element discrimination diagrams, such as the La versus Th and La-Th-Sc ternary diagrams of Bhatia and Crook (1986), are tools for determining the tectonic environment of source areas. When plotted on these diagrams, the Koum mudstones cluster near the fields associated with continental island arcs and ocean island arcs, suggesting an arc-related origin (Fig. 15a). However, the La-Th-Sc ternary diagram indicates an active continental margin (ACM) and a passive continental margin (PM) tectonic setting (Fig. 15b). These findings support a continental arc setting as the primary source for the Koum mudstones, with potential contributions from ocean island arc environments. This interpretation aligns with the findings of Ngo Mandeng et al. (2024a, b), who likewise proposed a continental arc system as the dominant source for the Koum basin deposits, with secondary contributions from rift and oceanic island arc systems.

7　Conclusions

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This study presents the first integrated mineralogical, bulk-rock geochemical, and stable isotope (C–O–H) dataset for the Cretaceous dark-gray mudstones of the Koum Basin, northern Cameroon, providing new insights into the paleoenvironment, paleovegetation, paleoclimate, and tectonic evolution of this underexplored segment of the Upper Benue Trough. The following are some key findings of this study.

1.	The mineralogical composition of the Koum mudstones indicates that they are predominantly composed of phyllosilicates, feldspars, carbonates, and minor iron oxides, reflecting a complex interplay of depositional processes and post-depositional alterations. The presence of alteration minerals such as analcime, dolomite, and ankerite suggests overprinting by low-grade metamorphism and/or hydrothermal activity.
2.	Geochemically, the mudstones point to a continental crustal source with an intermediate igneous provenance, deposited in a dynamic basin environment characterized by fluctuations in salinity (brackish to saline), redox conditions (oxic to anoxic), and climatic conditions ranging from humid to arid. While the major element ratios (e.g., SiO₂ vs. Al₂O₃ + K₂O + Na₂O) indicate a dominant semi-arid regime, the Sr/Cu and C-values suggest alternating humid and arid conditions.
3.	The δ¹³C signatures for the mudstones suggest an organic carbon source, primarily from C₃ plants, indicative of a terrestrial, fluvial-deltaic environment. The slightly more negative δ¹³C excursions exhibited by some samples reflect fluctuations in atmospheric CO₂ levels or increased terrestrial organic matter input.
4.	The δ¹⁸O and δ²H values of the Koum mudstones suggest a complex history of fluid-rock interactions within the Koum basin, involving meteoric water and magmatic–metamorphic fluids under a warm, equatorial-like climate.
5.	Tectonic setting discrimination diagrams suggest a continental arc environment for the Koum basin, with potential contributions from oceanic island arc and passive continental margin settings.

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doi: 10.1007/s11631-025-00800-y

Receive Date：2025-02-20
Online Date：2026-02-12
Published：2025-07-01

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Received：2025-02-20
Revised：2025-05-22
Accepted：2025-06-04

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¹Exploration Division, Department of Promotion State-owned Oil and Gas Assets, National Hydrocarbons Corporation (SNH), P.O. Box 955, Yaoundé, Centre Region, Cameroon

²Mineral Exploration Geoscience Program, Pan African University Life and Earth Sciences Institute (Including Health and Agriculture), P.M.B 20, Ibadan, Oyo State, Nigeria

³Department of Geology, Mining and Environmental Science, The University of Bamenda, P.O. Box 39, Bamenda, Northwest Region, Cameroon

⁴Economic Geology Unit, Department of Geology, University of Buea, P.O. Box 63, Buea, Southwest Region, Cameroon

⁵Queen's Facility for Isotope Research, Department of Geological Sciences and Geological Engineering, Queen's University, 36 Union Street, Kingston, ON K7L 3N6, Canada

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^✉ George Lemewihbwen Ngiamte ngiamte.george@yahoo.com

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Table 1 Summary of the analyzed mudstone samples

Lithotype	Sample ID	Analyses
Lithotype	Sample ID	XRD	XRF	ICP-MS	δ¹³C_VPDB	δ²H_VSMOW	δ¹⁸O_VSMOW
Gouga samples
Dark gray marl	GOU001	X	X	X	X	X	X
Dark purple mudstone	GOU002	X	X	X	X	X	X
Gray shale	GOU003	X			X	X	X
Kali Samples
Reddish-brown siltstone	KAL001	X	X	X	X	X	X
Brown claystone	KAL002	X	X	X	X	X	X
Reddish-brown mudstone	KAL003	X	X	X	X	X	X
Reddish-brown mudstone	KAL004	X	X	X	X	X	X
Gray shale	KAL006	X	X	X	X	X	X
Dark gray shales	KAL007	X			X	X	X
Gray claystone	KAL008	X			X	X	X
Mayo Gaba samples
Dark gray mudstone	KAL009	X			X	X	X
Reddish-brown-purple mudstones	KAL010	X	X	X	X	X	X
Dark gray-black mudstones (shale)	KAL011	X	X	X	X	X	X
Dark gray-black mudstones (shale)	KAL012	X	X	X	X	X	X
Dark gray-black mudstones (shale)	KAL013	X			X	X	X

Table 2 Bulk-rock mineralogy (%) for mudstone samples from the Koum basin

Sample ID	Calcite	Quartz	Albite	Microcline	Muscovite	Vermiculite	Hematite	Illite	Biotite	Kaolinite	Analcine	Dolomite	Ankerite	Anorthite
GOU001	18.00	17.70	42.10	15.50	4.10	2.60	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
GOU002	0.00	46.10	0.00	0.00	11.00	3.20	20.40	0.00	19.20	0.00	0.00	0.00	0.00	0.00
GOU003	41.60	25.80	29.20	0.00	1.00	2.40	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
KAL001	0.00	36.60	26.70	12.20	2.90	0.30	1.40	0.00	0.00	19.90	0.00	0.00	0.00	0.00
KAL002	0.00	23.30	34.10	0.00	2.70	0.80	3.60	0.00	0.00	12.90	0.00	0.00	0.00	0.00
KAL003	0.00	70.70	6.70	0.00	0.00	1.50	5.40	15.70	0.00	0.00	0.00	0.00	0.00	0.00
KAL004	17.40	11.60	27.70	0.00	0.00	2.00	0.00	18.10	0.00	0.00	23.30	0.00	0.00	0.00
KAL006	6.30	0.60	74.30	14.10	2.60	2.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
KAL007	20.20	28.30	44.60	5.70	0.00	1.10	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
KAL008	12.20	22.50	51.40	12.40	1.00	0.50	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
KAL009	13.40	1.70	33.00	0.00	12.10	2.90	0.00	0.00	0.00	0.00	36.90	0.00	0.00	0.00
KAL010	18.00	5.40	0.00	0.00	11.10	3.20	0.00	0.00	0.00	0.00	41.50	0.00	0.00	20.80
KAL011	15.00	22.20	31.00	0.00	0.00	1.40	0.00	0.00	2.30	0.00	8.30	19.80	0.00	0.00
KAL012	8.40	26.10	46.30	0.00	0.00	1.30	0.00	0.00	0.00	0.00	1.90	16.10	0.00	0.00
KAL013	6.30	30.10	38.70	9.70	0.00	1.60	0.00	0.00	3.00	0.00	0.00	0.00	10.50	0.00
Max	41.60	70.70	74.30	15.50	12.10	3.20	20.40	18.10	19.20	19.90	41.50	19.80	10.50	20.80
Min	0.00	0.60	0.00	0.00	0.00	0.30	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Average	11.05	24.54	34.13	4.35	3.03	1.79	2.14	2.11	2.19	2.05	6.99	2.24	0.66	1.30

Table 3 Major element compositions (wt%) for mudstone samples from the Koum basin

Sample	KAL001	KAL002	KAL003	KAL004	KAL006	KAL010	KAL011	KAL012	GOU001	GOU002	Average	PAAS
SiO₂	64.68	65.87	66.31	66.11	52.64	64.01	47.36	37.18	38.35	50.14	55.27	62.80
Al₂O₃	15.81	15.95	15.61	14.53	15.50	17.01	10.11	9.08	9.39	16.90	13.99	18.90
K₂O	4.11	4.10	3.90	2.37	1.60	4.00	1.16	0.98	1.69	3.77	2.77	3.70
TFe₂O₃	7.52	6.25	6.89	5.84	10.24	6.50	4.65	4.56	3.15	11.58	6.72	7.22
Na₂O	2.83	3.04	2.84	1.98	5.47	2.41	3.67	3.46	2.03	0.42	2.82	1.20
CaO	0.63	0.62	0.37	1.14	2.14	0.66	9.67	10.04	22.95	2.23	5.05	1.30
MgO	0.83	0.77	0.70	2.96	4.16	0.90	3.37	3.22	2.27	6.07	2.53	2.20
MnO	0.04	0.06	0.03	0.05	0.09	0.02	0.09	0.12	0.41	0.09	0.10	0.11
P₂O₅	0.29	0.25	0.14	0.23	0.16	0.39	0.10	0.15	0.21	0.25	0.22	0.16
TiO₂	1.07	0.89	1.14	1.05	0.78	1.05	0.46	0.40	0.82	0.73	0.84	1.00
LOI	2.20	2.50	2.30	4.30	7.70	3.30	19.10	30.60	19.40	7.60	9.90	6.00
Total	100.01	100.30	100.23	100.56	100.48	100.25	99.74	99.79	100.67	99.78	100.18	104.59
Al₂O₃/TiO₂	14.78	17.92	13.69	13.84	19.87	16.20	21.98	22.70	11.45	23.15	17.56	18.90
K₂O/Al₂O₃	0.26	0.26	0.25	0.16	0.10	0.24	0.11	0.11	0.18	0.22	0.19	0.20
K₂O/Na₂O	1.45	1.35	1.37	1.20	0.29	1.66	0.32	0.28	0.83	8.98	1.77	3.08
SiO₂/Al₂O₃	4.09	4.13	4.25	4.55	3.40	3.76	4.68	4.09	4.08	2.97	4.00	3.32
P₂O₅/TiO₂	0.27	0.28	0.12	0.22	0.21	0.37	0.22	0.38	0.26	0.34	0.27	0.16

LOI loss on ignition, PAAS Post Archean Australia Shale (Taylor and McLennan 1985)

Table 4 Carbon, hydrogen and oxygen isotope compositions for mudstone samples from the Koum basin

Sample ID	δ¹³Corg (‰VPDB)	δ²H (‰VSMOW)	δ¹⁸O_TCEA (‰VSMOW)	δ¹³Catm
GOU001	− 26.9	− 56	16.1	− 7.6
GOU002	− 27.6	− 81	5.7	− 8.2
GOU003	− 25.8	− 58	21.7	− 6.6
KAL001	− 28.4	− 58	4.0	− 8.9
KAL002	− 35.2	− 62	3.6	− 15.1
KAL003	− 25.9	− 78	4.9	− 6.6
KAL004	− 25.2	− 66	9.2	− 6.0
KAL006	− 26.1	− 51	9.3	− 6.8
KAL007	− 27.1	− 52	19.8	− 7.8
KAL008	− 26.5	− 52	14.7	− 7.2
KAL009	− 30.3	− 50	7.6	− 10.6
KAL010	− 27.7	− 52	11.6	− 8.2
KAL011	− 30.0	− 104	24.9	− 10.4
KAL012	− 29.7	− 100	18.7	− 10.1
KAL013	− 31.0	− 99	22.3	− 11.3
Max	− 25.2	− 50.0	24.9	− 6.0
Min	− 35.2	− 103.5	3.6	− 15.1
Average	− 28.2	− 68.0	12.9	− 8.8

δ¹³Catm = (δ¹³C_plants + 18.67) / 1.10 (Arens et al. 2000)

Table 5 Paleoweathering and paleoenvironment parameters for the Koum mudstones

Sample	N-value	C-value	PIA	CIA	ICV	A/NK	A/CNK	TOC(%)	C_org/P
KAL001	5.25	0.90	66.21	60.68	1.07	1.74	1.54	0.04	0.36
KAL002	4.83	0.74	65.26	60.15	0.98	1.69	1.51	0.04	0.41
KAL003	4.48	0.89	68.07	62.01	1.01	1.76	1.63	0.02	0.37
KAL004	20.37	0.70	69.20	64.80	1.06	2.50	1.84	0.03	0.34
KAL006	26.84	0.77	51.63	51.44	1.57	1.44	1.06	0.03	0.48
KAL010	5.29	0.82	71.03	64.15	0.91	2.05	1.79	0.11	0.73
KAL011	33.33	0.26	41.08	42.01	2.27	1.39	0.72	6.91	178.34
KAL012	35.46	0.26	40.11	41.06	2.50	1.34	0.70	5.48	94.29
GOU001	24.17	0.12	51.82	51.45	3.50	1.82	1.06	0.02	0.25
GOU002	35.92	0.93	72.98	65.70	1.47	3.54	1.91	0.02	0.21

N-value = 100 × MgO/Al₂O₃ (after Chen et al. 2022); C-value = Σ(Fe + Mn + Cr + Ni + V + Co)/Σ(Ca + Mg + Sr + Ba + K + Na) (after Fedo et al. 1995); A/CNK = molar Al₂O₃/(CaO + Na₂O + K₂O) and A/NK = molar Al₂O₃/(Na₂O + K₂O) (after Nesbitt and Young 1989); ICV = [(Fe₂O₃ + K₂O + Na₂O + CaO + MgO + TiO₂)/Al₂O₃] (after Cox et al. 1995); CIA = [molar Al₂O₃/(Al₂O₃ + CaO + Na₂O + K₂O) × 100] (after Nesbitt and Young 1982); PIA = [molar (Al₂O₃–K₂O)/(Al₂O₃–K₂O + Na₂O + CaO) × 100] (after Fedo et al. 1995); C_org/P = (TOC/12)/(P/30.97) (after Algeo and Ingall 2007). TOC values are from Njamnsi et al. (2022b)

Table 6 Composition parameters for the Koum mudstones in the M⁺–4Si–R²⁺ system

Sample	Bulk composition					Normalized parameters
Sample	4Si	M⁺	R²⁺	R³⁺	R³⁺/R³⁺ + R²⁺ + M⁺	4Si + M⁺ + R²⁺	4Si%	M⁺%	R²⁺%	Δ4Si%
KAL001	269.10	201.05	20.60	404.30	0.65	490.74	54.83	40.97	4.20	24.97
KAL002	274.05	207.26	19.11	391.14	0.63	500.41	54.76	41.42	3.82	24.86
KAL003	275.88	187.64	17.37	392.49	0.66	480.89	57.37	39.02	3.61	29.18
KAL004	275.05	154.87	73.45	358.16	0.61	503.36	54.64	30.77	14.59	24.65
KAL006	219.00	286.80	103.23	432.29	0.53	609.03	35.96	47.09	16.95	0.87
KAL010	266.31	186.23	22.33	415.07	0.67	474.87	56.08	39.22	4.70	27.04
KAL011	197.04	487.92	83.62	256.55	0.31	768.58	25.64	63.48	10.88	0.05
KAL012	154.68	490.52	79.90	235.22	0.29	725.10	21.33	67.65	11.02	3.59
GOU001	159.55	919.86	56.33	223.64	0.19	1135.74	14.05	80.99	4.96	1.20
GOU002	208.60	173.12	150.62	476.53	0.60	532.35	39.19	32.52	28.29	5.86

4Si = Si/4; M⁺ = Na⁺ + K⁺ + 2Ca²⁺; R²⁺ = Mg²⁺ + Fe²⁺ + Mn²⁺; R³⁺ = Al³⁺ + Fe³⁺; Δ4Si% = [(4Si_{altered sample}—4Si_{unaltered parent rock}) × 100]/(100—4Si_{unaltered parent rock}), after Meunier et al. (2013). The source rock is considered to be represented by the average upper continental crust (Condie 1993), where 4Si% = 39.8

Fig. 1 Map illustrating a, b the configuration of the West and Central African Rift System (adapted from Genik 1993; Fairhead 2020), c the regional geologic setting of Cameroon, with the Koum basin indicated by the red inset (modified from Mimba et al. 2018), and d the specific geologic formations within the Koum basin (adapted after Congleton 1990; Jacobs et al. 1996; Njamnsi et al. 2022a)

Fig. 2 Conceptual tectono-stratigraphy and gross depositional environments of the sedimentary fill of the Koum Basin (modified after Congleton 1990; Jacobs et al. 1996; Njamnsi et al. 2022b)

Fig. 3 Field photographs of mudstone outcrops in a, c Mayo Gaba locality (KAL009, KAL010, KAL011, KAL012, and KAL013), d, f Kali locality (KAL001, KAL002, KAL003, KAL004, KAL006, KAL007, and KAL008), and g, I Gouga locality (GOU001, GOU002 and GOU003)

Fig. 4 X-ray diffraction patterns for mudstone samples (GOU001 to KAL013) from the Koum basin. Ab albite, An anorthite, Ank ankerite, Anl analcine, Bt biotite, Ca calcite, Hem hematite, Il illite, Ka kaolinite, Mus muscovite, Qtz quartz, V vermiculite

Fig. 5 Binary plots showing the relation between selected major oxides in the Koum mudstones: a Al₂O₃ versus K₂O and b Al₂O₃ versus SiO₂

Fig. 6 Geochemical classification of the Koum mudstones based on the log (SiO₂/Al₂O₃) versus log (TFe₂O₃/K₂O) diagram, as proposed by Herron (1988)

Fig. 7 Upper continental crust (UCC)-normalized multi-elemental variation diagrams for the Koum mudstones. UCC data from McLennan (1989)

Fig. 8 PAAS-normalized trace element plot for the Koum mudstones. PAAS data from Taylor and McLennan (1985). EF_average, a geochemical normalization index, was calculated as (element/Ti)_sample/(element/Ti)_PAAS (Tribovillard et al. 2006)

Fig. 9 The composition of the Koum mudstones is plotted in a the A-CN-K ternary plot of Nesbitt and Young (1984); b the M⁺–4Si–R²⁺ system (Meunier et al. 2013); c the representative domains of the major rock groups, scaled in the R³⁺/(R³⁺ + R²⁺ + M⁺) coordinate; and d the Δ4Si% vs. R³⁺/(R³⁺ + R²⁺ + M⁺) diagram (Meunier et al. 2013). The dark arrows in a indicate weathering trends of igneous rocks (gabbro, tonalite, adamellite, granodiorite, and granite). Bt biotite, Chl chlorite, Gi gibbsite, Il illite, Ka kaolinite, Ksp K-feldspar, Mus muscovite, Pl plagioclase, Sm smectite. For the calculation of the M⁺, 4Si, R²⁺, R³⁺, R³⁺/(R³⁺ + R²⁺ + M⁺), and Δ4Si% parameters, see Table 6 for details

Fig. 10 Bivariate plots showing the provenance of Koum mudstones: a Al₂O₃ versus TiO₂ (Hayashi et al. 1997), b Zr versus TiO₂ (Hayashi et al. 1997), c Zr/Sc versus Th/Sc (McLennan et al. 1993), and d Hf versus La/Th (Floyd and Leveridge 1987). AND andesite, BAS basalt, GRA granite, PSS passive margin sandstone

Fig. 11 SiO₂ versus Al₂O₃ + K₂O + Na₂O plot (Suttner and Dutta 1986) showing the climatic conditions during the formation of the Koum mudstones

Fig. 12 Carbon isotope discrimination plots: a δ¹³C versus δ¹³Catm (Arens et al. 2000), b δ¹³C values for the Koum mudstones compared to global values

Fig. 13 Bivariate plot of δ¹⁸O versus δ²H values for the Koum mudstones, showing the different types of diagenetic fluids involved in their formation (Morad et al. 2003)

Fig. 14 Major element discrimination plots showing the tectonic setting of the Koum mudstones: a SiO₂ versus K₂O/Na₂O plot after Verma and Armstrong-Altrin (2013), and b K₂O/Na₂O versus SiO₂/Al₂O₃ plot after Maynard et al. (1982). ACM active continental margin, ARC island arc margin, A1 arc, A2 evolution arc, PM passive margin

Fig. 15 Discrimination diagrams showing the tectonic setting of the Koum mudstones: a La versus Th plot after Bhatia and Crook (1986), and b La-Th-Sc diagram after Bhatia and Crook (1986). ACM active continental margin, CIA continental island arc, OIA oceanic island arc, PM passive continental margin

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