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1. Introduction
Sedimentary structures and geochemistry analyses play a key role in the reconstruction of depositional conditions. The various structures observed in sedimentary rocks reflect the environmental conditions as well as the evolution of the depositional settings. These geological elements had been widely used to reconstruct the depositional environment of several sedimentary basins, including the Cretaceous sedimentary basins in Cameroon, for example, the Babouri-Figuil sedimentary basin [Ndjeng 1992], and the Mamfe sedimentary basin [Eyong 2003].
The Djerem continental sub-basin (Figure 1) is located along the Precambrian fault (Central Cameroon Shear Zone, CCSZ) and belongs to the basins which are linked to the South Atlantic Ocean opening occurred between the Late Jurassic and the Early Cretaceous [Ngangom 1983]. This opening would have reactivated the old intracontinental faults causing the collapses (rifts or graben).
Geological map of the Djerem sub-basin [modified from Tchouatcha 2011].
The Djerem sub-basin represents the western part of the southern trough of the Adamawa plateau, and the Mbere trough represents its extension eastward [Ngangom 1983; Tchouatcha et al. 2010, 2016; Tchouatcha 2011]. These two sub-basins contain identical basement deposits. However, the Djerem sub-basin shows some peculiarities including the presence of calcareous deposits of Cretaceous age.
Detrital terrigenous sedimentary rocks are composed of at least 50% debris from erosion of preexisting rocks. These rock types represent 80%–90% of sedimentary rocks and constitute the majority of deposit in the Djerem sub-basin [Tchouatcha 2011]. The weathering processes yield particles of various sizes and shapes, which later on form detrital sediments. Detrital sedimentation dominates the lithologies in the Djerem sub-basin, while the chemical aspects in this deposit are represented by calcareous cement. The cement in the bottom layer contains both detrital and metamorphic parts; the later were affected by metamorphism of weak intensity [Tchouatcha 2011].
Detrital rocks contain variable proportions of carbonates in line with the formation processes and environment of deposition. This carbonate may have a diverse origin (detrital, organic, or chemical). The nature of the cement in detrital sedimentary rocks and the physical properties of grains (nature, size, degree of wear) are precious information in the reconstitution of deposit environments.
According to Taylor and McLennan [1985], fine-grained clastic sedimentary rocks are more useful to constraint the geochemical signatures than the coarse ones. The geochemical signatures, especially that of the rare earth and trace elements, have been also widely used as proxies for the attributes of depositional settings. These geochemical signatures vary with respect to different sedimentary environments [e.g., lacustrine environment Wen et al. 2007; Chuang et al. 2016; Delu et al. 2017; mixing of river and sea waters, Hoyle et al. 1984; seawater conditions, Nothdurft et al. 2004].
Like several other continental basins of Cameroon, the geochemical data in the Djerem basin are still lacking. In fact, up to our knowledge, no geochemical study related to provenance, and sedimentation conditions have been carried out in this basin. In this paper, we will discuss the nature of source rocks of the Cretaceous fine-grained calcareous deposits in the Djerem sub-basin as well as the characteristics of weathering intensity, redox conditions, paleoclimate, and tectonic setting based on mineralogical, geochemical, and sedimentary structures analyses.
2. Geological setting
The study area is located in the western part of the Djerem trough that constitutes with Mbere (at the East) the same lithological unit (Figure 2) at the south of the Adamawa plateau (Djerem–Mbere). The Djerem (at the west) and the Mbere (at the east) represent two sub-basins forming a whole trough located at the South of the Adamawa plateau. They are separated by Cenozoic volcanic rocks covering that plateau. In the Mbere sub-basin, the depression is limited by two straight faults exhibiting cliffs of more than 500 m high. In the Djerem sub-basin, the external aspects of the trough are no more clearly visible because the northern margin can be only seen describing a cliff of about 300 m high, while the southern margin has been completely eroded.
Synthetic log of the Djerem–Mbere basin showing the location of studied stratigraphic level [modified from Tchouatcha 2011]. Abbreviation: Cl = clay; S = sand; Cg = conglomerate.
The geologic evolution of the Djerem sub-basin goes in line with the general setting of the Adamawa region [Eno Belinga 1966, 1972; Ndjeng 1978]. The basement is constituted by metamorphic rocks (essentially migmatites, gneiss, amphibolites, and quartzites) crosscut by magmatic rocks [ante, syn and late tectonic granites, Lasserre 1962] and both sets of rocks seem to form a united entity. This basement complex has been emplaced during several orogenic cycles in the Precambrian.
The geology of the study area, as well as of that of the Adamawa, was eroded from the Cambrian to the end of the Jurassic, and the corresponding heritage would undoubtedly be the Gondwana surface (erosional surface). It is only during the Early Cretaceous, with the dislocation of the Gondwana and opening of the South Atlantic Ocean, that the geological evolution of that area started with the filling of the southern Adamawa trough by detrital deposits whose base would be the continental infill in line with the pull-apart tectonic model [Tchouatcha 2011].
The thickness of the Cretaceous sedimentary series in the southern Adamawa trough vary between 1600 m and 2100 m [Collignon 1970; Noutchogwe Tatchum 2004; Kande Houetchak 2008; Noutchogwe Tatchum et al. 2010] and may be more important in the east [Essissima-Essissima 2001; Noutchogwe Tatchum 2004]. They are largely dominated by conglomerates that are affected in their lower part by metamorphism of weak intensity. Meanwhile, according to Ngangom [1983], the thickness of these conglomerates may be more than 3000 m.
The metamorphic process affecting the basement conglomerates remains unknown. The regional extension of these meta-conglomerates suggests that the metamorphism process occurred after the diagenesis. In addition, the steepness of layers in contact with the basement in certain areas near adjacent faults suggests a dynamothermic metamorphism provoked by the movements of the above-mentioned adjacent faults. The late Cretaceous period in the Adamawa area was marked by a new epirogenesis [Eno Belinga 1972] during which breakdown, subsidence, and mylonization occurred.
According to Le Maréchal and Vincent [1971], conglomerates may have been affected by thermic metamorphism of regional extension provoked by longitudinal and latitudinal fractures that were responsible for the rising up of the axial part and the subsidence of the tectonic trough. Thus, such a metamorphism may be of Pyrenean type [Ravier 1959].
The other sedimentary rocks found in the Cretaceous series are ferruginous fine- to coarse-grained sandstones whose thickness varies between 300 and 400 m [Kande Houetchak 2008]. The top of the series, the studied section, is constituted by calcareous facies such as calcareous siltstones and shales [Tchouatcha 2011].
Recent studies have evidenced the presence of Cenozoic formations based on palynological data [Tchouatcha et al. 2010; Tchouatcha 2011]. However, some deposits are hypothetically attributed to the lower Cenozoic (Eocene) based on correlations. The whole set of Cenozoic deposits are composed from bottom to top by ferruginous conglomeratic sandstones, claystones with sandy intercalations, fine- to coarse-grained ferruginous sands containing gravels, variegate clays, black clays, conglomerates with sandy clayed matrix, and white unconsolidated sandstone. These deposits are well-observed in the East.
The travertines or calcareous tuffs are more observed in the East. These deposits of Pleistocene to Holocene age have been emplaced, thanks to the movements of adjacent faults [Tchouatcha et al. 2016].
The volcanic activities are episodic and related to the tectonic movements [Eno Belinga 1972; Tchouatcha et al. 2010]. The Upper Cretaceous is essentially characterized by the occurrences of fractures, subsidence, and volcanic activities of fissural type. These basalts are indirectly known by advance lateritic weathering processes, which have developed bauxitic plateau in the Mounts Ngaoundal and Ngaoundourou as well as in Minim and Martap areas [Eno Belinga 1972].
Acidic volcanic events are known in the Burdigalian period during the flexuration event that affected the Adamawa plateau [Eno Belinga 1972]. Rhyolitic outcrops are observed in the east along the southern margin of the trough where outcrops of basalts basanites and trachy-phonolite occur. The occurrence of these rock types suggests a fissural volcanic type that cross cut the basement and sedimentary formation of Cretaceous age. The basaltic cover that underlines the geographic limit between the Djerem and the Mbere sub-basins erupted from the Adamawa plateau in line with Cenozoic tectonic evolution.
3. Methods
To better understand the conditions of sedimentation of the studied deposits, field studies were carried out to describe and reconstruct all syn- and post-sedimentary structures and their relationship and chronology.
A total of 14 samples on about 7.50 m high of the stratigraphic column have been collected for geochemical, petrographic, and X-ray diffraction (XRD) analyses. Fourteen polished thin sections were prepared at Langfang Rock Detection Technology Services Ltd in Hebei (China) and in the Assiut University, Egypt. The petrographic study of these samples was carried out under the polarized microscope at the Laboratory of Petrology and Structural Geology, in the University of Yaoundé 1, Cameroon.
The X-ray diffraction patterns were obtained from a Bruker D8-Advance Eco 1 Kw diffractometer (Copper Kα radiance, 𝜆 = 1.5418 , 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 were carried out on the bulk material (nonoriented powder with grinded particles < 50 mm) of four representative samples (shales and wackes).
Whole-rock geochemical analyses of seven representative samples were carried out at Bureau Veritas Commodities, Vancouver, Canada. Prepared samples (homogenized powder) were 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. Major element oxides were obtained by inductively coupled plasma-atomic emission spectrometry. Loss on ignition (LOI) was determined by igniting a sample split then measuring the weight loss. The assays uncertainties varied from 0.1% to 0.04% for major elements, 0.1–0.5% for trace elements, and 0.01–0.5 ppm for REE. Accuracy for REE is estimated at 5% for concentrations >10 ppm and 10% when lower.
4. Results
4.1. Field and sedimentary structure descriptions
The first outcrop observed by Guiraudie [1955] under the name “fine black sandstones” is found in a point bar. These deposits form a slope of about 7 m high (Supplementary Material 1) with several tens of meters of extension. The average dip of this outcrop is 10° to the SE and is made up of bedded clays or shales containing more or less carbonate and micas. This outcrop shows greenish gray to dark color with silty sandstones intercalations of yellowish to greyish color containing more or less carbonates. This outcrop is covered by lateritic formation. The thicknesses of the clayey levels vary from millimetric to metric (generally with millimetric silty sandstone lenses) and that of the silty sandstones from millimetric to several decimetric. Many sedimentary structures are observed on this outcrop including pseudo-nodules convoluted bedding, parallel laminae, and lenticular structures sometimes bearing coal. Many faults and joints cut across the outcrop, desiccation crack, and platelet breakdown may be observed. The outcrop is tilted toward the SE.
The second outcrop is found at the Djerem–Mayel (Supplementary Material 2) confluence and appears as slab or beds of ferro-calcareous siltstones whose thickness is less than 1 m [Tchouatcha 2011]. The dips of this outcrop vary from 10° to 12° to the NE. This rock shows a brown-gray color and reacts positively to the Hydrochloric acid. This outcrop is marked by discrete, dark discontinuous (sometimes with undulations) inframillimetric to millimetric laminations. Additionally, conjugate fractures and platelet breakdown may be observed from place to place. These laminations may underline the schistosity.
∙ Pseudo-nodules. Pseudo-nodules (Supplementary Material 1) are complete or incomplete decimetric to pluri-decimetric balls with elliptical shape. They are usually aligned as free or connected masses and alternate with the bedding or lamination.
∙ Lamination and bedding. Laminations and bedding (Supplementary Material 1) are observed over the studied outcrops; however, they appear to be well expressed on shales. These structures are underlined by a color differentiation (black, gray to yellow gray), and granulometry, which confer them an irregular evolution. Laminae appear to be millimetric to inframillimetric, and represented by gray to yellowish clayey layers, and are sometimes lenticular (Supplementary Material 2). In certain areas, organic matters (dark in color) can be found associated with yellow to gray silt lenticular intercalations, which sometimes depict sigmoid boudins. Coal laminae of lenticular shapes can be sporadically observed as well. The thickness of these beds varies from centimeter to several decimeters and is well-developed in shales and clays. The contact between different laminae is sharp; sometimes it appears eroded with poorly preserved fossil leaves. Black laminae can also be observed in ferro-calcareous silts at the Djerem–Mayel confluence. They appear to be very thin (millimetric to inframillimetric) with varied expressions (in trace, roughly, discontinued and wavy). These laminae underline the schistosity. It is worth noting that bedding is poorly developed and the outcrop is rather characterized by platelet breakdown.
∙ Convolution. Convoluted structures (Supplementary material 1) are frequently observed on shales facies. They are poorly developed on outcrop and are sporadically observed on samples. They show dimensions varying from several centimeters to decimeters. Sometimes they show double spiral.
∙ Ripple marks. The ripple marks indicate temporary emergence linked to seasonal fluctuation water in a basin; it is also a good indicator of paleocurrents of sedimentation. They are generally symmetric, suggesting forth and back water movements. Their size or vertical extension is generally pluricentimetric. The ripple marks are more or less abundant in the intracontinental basins of Babouri-Figuil and Mayo Oulo Lere [Ndjeng and Brunet 1998] and Mamfe [Eyong 2003] and indicate the floodplain deposits [Eyong et al. 2019].
∙ Desiccation cracks. Desiccation cracks are present on the shales deposits with more or less interconnected cracks and generally conical shape of centimetric opening.
∙ Joints and faults. Shale outcrops are crosscut by joints and faults (Supplementary Material 2) of various orientations, with sometimes well-preserved slickenside (Supplementary Material 2). The cardinal diagrams (Supplementary Material 3) show four orientation sets including the N85–95° E, which appears to be the dominant trend followed by the N130–145° E trend and secondarily the N40–50° E and N160–170° E. It is worth noting that the N40–50° E joint set trends parallel to the shale outcrop and the marginal fault, thus suggesting an extension.
Numerous near-parallel closely spaced and steeply dipping fractures developed at low but variable angles (N65°–N75° E) on the ferro-calcareous silstones from the Djerem river. These fractures are sometimes conjugated, configuring the ferro-calcareous siltstones into numerous diamonds to irregular blocks.
4.2. Laboratory results
4.2.1. Petrography analyses
The studied samples are subdivided into two groups according, on one hand the calcareous clayey to silty clay facies (DT31, DT32, and DT77); and on other hand the calcareous siltstones (DT55, DT75, DT76, and DT78).
∙ The clayey to silty clay facies. The clayey facies are composed of micro-grains of quartz, feldspar, micas, epidote, oxides, and a clay-carbonated binder with organic matter. Quartz (20–30%) is generally monocrystalline, and very angular to angular, rarely subangular. Feldspar (20–25%) is represented by plagioclase, orthoclase and rarely microcline, and is very angular to angular and rarely subangular. Mica is represented by needles of biotite (5–15%) and muscovite (2–4%). These minerals underline the schistosity. Epidote is very rare (<1%) and is represented by pistachite and very rarely zoisite. The oxides have rounded to subangular forms, with a proportion of about 2–3%. The binder (25–35%) is indistinguishable clay-carbonate and associated with organic matter. There are sometimes the thin intercalations of calcite (Supplementary Material 4) and coal (Supplementary Material 4) veins. Their microstructures are very fine clastic, sub-iso to isogranular (Supplementary Material 4). These shales are moderate to well-sorted.
∙ The siltstones to very fine sandstones. Siltstones to very fine sandstones are composed of quartz, feldspar, mica, epidote, oxide, and a binder of varied nature. Quartz (35–40%) is essentially monocrystalline, very angular to angular, with rare subangular to rounded grains, sometimes striated and cracked. Feldspar (30–35%) is represented by orthoclase and plagioclase, associated with rare microclines. They are generally very angular to angular, rarely subangular to rounded, sometimes cracked and kinked, and sometimes very altered (sericitization and vacuolization) (Supplementary Material 4). Mica is represented by chlorite (5–15%), muscovite (2–5%). Mica occurs either as flakes or fine needles, sometimes flexible (Supplementary Material 4) in inter-granular contact between quartz and feldspar. Chlorite generally comes from the alteration of biotite, which sometimes shows a shredded and oxidized appearance. Oxides (5–15%) are abundant in more ferruginous facies and in small proportion in more calcareous facies, and generally result from the alteration of ferromagnesians (e.g., biotite, amphibole). Epidote (<1%) is rare and usually found in single grains represented by the pistachite. The binder (5–10%) is ferruginous, calcareous (Supplementary Material 4) and siliceous (Supplementary Material 4). Their microstructure is fine clastic, sub-iso to isogranular. They are moderately sorted to well sorted.
X-ray diffractions of representative studied samples.
4.2.2. X-ray diffraction
The X-ray diffraction analysis (Figure 3) reveals a similarity of mineralogical composition between the two groups, dominated by non-clays mineral such as quartz (dominant) and feldspars, and secondarily by clay minerals made up in descending order of chlorite, kaolinite, and illite. The chlorite and kaolinite are more abundant in the clay facies. The samples are very poor in smectite and contain relatively few quantity of calcite.
4.2.3. Geochemical data
∙ Major elements. The SiO2/Al2O3 versus FeO3/K2O discriminant diagram (Figure 4) of Herron [1988] shows that the studied samples are composed by shale (DT31 and DT32), Fe-shale (DT55), and wacke (DT75, DT76, DT77, and DT78).
Classification of studied sediments according to the scheme of Herron [1988].
According to Supplementary Material 5, SiO2 and Al2O3 are the two abundant elements and their proportion varies from 59.92 to 71.53% and 13.13 to 17.28%, respectively. The SiO2 concentration is high in wacke samples and low in shale samples in contrary with the Al2O3.
The concentrations of K2O (1.23–4.3%) and Na2O (2.28–5.17%) are relatively high indicating the relative abundance of potassium-bearing minerals [Moradi et al. 2016] and sodium-bearing minerals [Ross and Bustin 2009] in the studied materials, but dominated by the sodium-bearing minerals. According to Supplementary Material 6, the strong positive relationships of K2O with Al2O3 (R = 0.89) can reflect close relationship with clay and feldspar [Fu et al. 2011; Moradi et al. 2016], and relative strong negative relationship of Na2O and Al2O3 (R = −0.75) can probably indicate a partial chemical origin of Na. The strong positive correlation between Fe2O3 and TiO2 (R = 0.91) and Fe2O3 and MgO (R = 0.94) gives indication that they are derived respectively from same detrital mineral ilmenite and amphibole. The slight positive correlation between CaO and LOI (R = 0.41) and slight negative relationship between Al2O3 and CaO (R = −0.5) indicates that Ca is present in both calcite and plagioclase, but more is abundant in calcite. The Fe2O3 concentration in the studied materials varies from 3.41 to 6.44 due to the Fe-bearing minerals.
According to Figure 5, all the studied materials, compared to post achean Australian shales (PAAS) respectively, are enriched in P2O5, enriched and vary in Na2O, vary and depleted in Fe2O3, depleted and vary in Al2O3, deplete and enriched in TiO2, and vary in the remaining elements.
Major elements variations for studied sediments. Data are average values normalized to PAAS [after Taylor and McLennan 1985 and McLennan 2001].
∙ Trace elements. The trace elements contents of studied samples are listed in Supplementary Material 7. Ba, Rb, Sr, and Zr are the most abundant elements, where values are varying from 309 to 2177 ppm, 55.9–212.1 ppm, 235.1–622.1, and 249.4–629.5 ppm, respectively.
According to Figure 6, all the studied materials, compared to PAAS, are enriched in Hf, Nb, Ta, Th, U, and Zr, depleted in Ni and V, depleted and vary in Sc, depleted and vary in Co, enriched and vary in Sr, vary and enriched in Y, and the contents of the remaining elements generally vary.
Trace elements variation for studied sediments. Data are average values normalized to PAAS [after Taylor and McLennan 1985 and McLennan 2001].
REE pattern for studied sediments. Data are average values normalized to chondrite [after McDonough and Sun 1995].
∙ Rare Earth elements. The contents of REE and their concentrations are listed in Supplementary Material 7. The LREE/HREE ratios range from 9.07 to 13.08 (average: 11.77), suggest an enrichment in LREE. REE concentration normalized to PAAS [Taylor and McLennan 1985; McLennan 2001] indicates slight positive to very slight negative Eu anomalies (Eu/Eu∗ vary from 0.98 to 1.11), and negative to very slight negative Ce anomalies (Ce/Ce∗ vary from 0.77 to 0.98).
For comparison, REE composition of the studied samples and the PAAS were normalized to the chondrite [McDonough and Sun 1995; Figure 7]. The chondrite-normalized REE pattern confirms broadly the similar LREE-enrichment for the studied samples and the slight negative Eu anomalies for all samples.
Usually, when having the same origin, Eu and Ce anomalies as well as the REE patterns of the sediments are similar [Zeng et al. 2019]. Based on the chondrite-normalized REEs (Figure 11), the studied samples show the similar REE patterns indicating both REE and Eu mainly inherited from parent rocks.
5. Discussion
5.1. Mechanisms of sedimentation
Basal calcareous deposit of the study section is probably of early Cretaceous age and has been interpreted either as has being derived from distal fluviatile origin, fluvial current, or lower energy fluvio-lacustrine deposits [Tchouatcha 2011]. This formation may be linked to the transtensional phase that provoked the modification of the trough in the south of Adamawa. Calcitic cement (probably of primary nature) and ferruginous cement, as well as the fine texture of silts, suggest an open shallow deep lacustrine environment, favorable to the proliferation of an algo-microbial population and the supply by fine detrital and distal matters.
The Cretaceous sequence is covered by bedded clayed deposits or calcareous shales marked by silty sandstones and coal intercalations. According to Guiraudie [1955], the deposits of the studied section may correspond to the Wealdian facies. However, recent studies situated them in the middle Cretaceous, probably Albian to Cenomanian based on correlations with those of the Mamfe sub-basin located in the west [Eyong 2003; Njoh et al. 2015]. These materials represent infilling clogging deposits and may correspond to a swampy sedimentation following a blockage, which may have occurred in the east, but destructive strengths being more or less active in the west (Djerem) provoking lifting-up and collapse in the area. Sedimentation took place by periodic influx of fine to very fine detrital materials in anoxic conditions. This sedimentation was periodically influenced by the seismic waves [Cojan and Thiry 1992] provoked by syn- to post-tectonic movements by activities of marginal faults as suggested by the presence of sedimentary structures such as convolute and pseudo-nodules (syn-sedimentary) associated to fault and joints (post-sedimentary). All hypotheses leading to convolute lamination require unconsolidated sediment during deformation [Gladstone et al. 2018]. These structures are generally associated with turbiditic sequences, but they can be very abundant in sediments in intertidal zones as well as in fluvial environments [Freidman and Sanders 1978; Reineck and Singh 1980]. In addition, laminations and bedding suggest a calm depositional environment; however, with variable velocity. The occurrence of laminations suggests a high velocity, while bedding rather suggests a lower velocity with respect to quantity of detrital materials, the latter was influenced by the climate type. The alternation of laminations or bedding with levels of pseudo-nodules suggests an alternation of calm and more agitated periods caused by the tectonic activity. The presence of organic matter as well as coal-rich silty sandstones may suggest fluctuating water table (repeated flood and land emerge), which characterizes wet lands. This deposit indicates a hot and dry climate. Calcitic cement may be of primary nature linked to a periodically shallow deep environment favorable to the proliferation of algomicrobic and thus diagenetic alterations.
5.2. Provenance
5.2.1. Source lithotype
For source lithotype, many plots have been proposed and widely used such as Th/Co versus La/Sc [Cullers 2002; Ngueutchoua et al. 2017] and Zr versus TiO2 [Hayashi et al. 1997; Ngueutchoua et al. 2019]. The location of the studied materials on these scattered plots shows that the geochemical signatures of the source lithotype correspond to felsic rocks (Figure 8(A)) and silicic rocks (Figure 8(B)).
Scatter plots of (A) Th/Co versus La/Sc [Cullers 2002] and (B) TiO2 versus Zr, showing nature of the studied samples.
Many ratios have been also used to characterize the source rocks such as Th/Co ratio [Cox et al. 1995; Cullers 2000; Armstrong-Altrin et al. 2015; Tawfik et al. 2017] and Al2O3/TiO2 ratio [Hayashi et al. 1997; Daï et al. 2011; Ma et al. 2015; Zeng et al. 2019]. In our studied materials, the Th/Co ratios vary from 1.24 to 4.18 (average 2.26). These ratios vary from 0.67 to 19.4 in the felsic source and from 0.04 to 1.10 in the mafic source, indicating essentially the felsic source rocks of our sediment with negligible contribution of mafic rocks. The Al2O3/TiO2 ratio is generally greater than 21 in felsic source rocks, 8–21 in intermediate rocks and less than 8 in mafic source rocks. This ratio ranges from 16.32 to 21.87 (average: 19.73) suggesting essentially a felsic source rock.
The REE patterns and size of Eu anomaly have been used to characterize the mother rocks of sediments [Taylor and McLennan 1985; Armstrong-Altrin et al. 2014; Liu et al. 2015; Ma et al. 2015; Zeng et al. 2019]. Felsic rocks contain higher LREE/HREE ratios and negative Eu anomaly, and mafic rocks usually contain low LREE/HREE ratios and little or no negative Eu anomaly [Armstrong-Altrin et al. 2014].
The LREE/HREE ratios range from 9.07 to 13.08 (average: 11.77). The Eu anomaly values, range from 0.98 to 1.11, are indicative of essentially felsic provenance as all the LREE/HREE values of our studied materials are lower to the PAAS (13.18).
5.2.2. Sediments sorting and recycling and paleoweathering
Many ratios have been used to indicate chemical maturity of some deposits such as K2O/Na2O [Mongelli 2004; Ngueutchoua et al. 2017]. This ratio increases with weathering due to more liable nature of plagioclase relative to K-feldspar [Nesbitt and Young 1982]. A ratios > 1, indicate high chemical maturity [Armstrong-Altrin et al. 2016]. In this study, the K2O/Na2O ratio vary from 0.24 to 1.90, suggesting globally a moderate amount of sediment recycling.
In addition, other parameters such as chemical index of alteration (CIA) and index of compositional variability (ICV) have been used to characterize the compositional maturity of sediments [e.g., Mongelli et al. 2006; Perri et al. 2011; Tao et al. 2013; Perri 2014; Absar and Sreenivas 2015; Armstrong-Altrin et al. 2015; Ngueutchoua et al. 2017; Tawfik et al. 2017]. ICV > 1 indicates immature sediments and ICV < 1 shows mature sediments. The ICV values of the studied sediments range from 0.88 to 1.35 (average 1.11) higher than the PAAS (0.84) indicating the submature characters of sediments. High values of CIA (76–100) indicate intense chemical weathering, whereas low values (CIA ⩽ 50 reflect unweathered source areas. The CIA values vary from 58.25 to 68.41 (average 64.08) indicating the moderate weathered source rocks. Meanwhile, the presence of illite and K-feldspar, according to the mineralogical composition (Figure 3), may indicate a K-addition, and it is noteworthy that the CIA value may be decreased due to K-metasomatism [Fedo et al. 1995].
Furthermore, high 𝛴REE contents reflects a possible control by differing amounts of accessory minerals such as zircon and/or quartz linked to the recycling processes [Zou et al. 2016]. In our studied materials, 𝛴REE values range from 210.99 to 366.24 ppm (average: 280.31 ppm) higher than the PAAS (184.77 ppm) and UCC (146.37 ppm), indicating the recycling of these sediments.
The Zr/Sc and Th/Sc ratios are also useful parameters to depict changes in heavy mineral enrichment, sorting level, and sediment composition [McLennan et al. 1993]. High Zr/Sc ratios indicate zircon accumulation by sediment recycling and sorting. The Zr/Sc values range from 16.63 to 67.48 (average 38.67) and compared to PAAS (13.13) and UCC (13.97) indicate a moderate to high sorting and recycling of the rock provenance as confirmed by the Zr/Sc versus Th/Sc diagram (Figure 9).
Th/Sc versus Zr/Sc plot of studied sediments.
The SiO2/Al2O3 ratio has been also used for textural maturity of sediments [Etemad-Saeed et al. 2011; Armstrong-Altrin and Machain-Castillo 2016; Tawfik et al. 2017]. The average SiO2/Al2O3 ratio is 3 in basic igneous rocks and about 5 in acidic igneous rocks, whereas the value > 5 in clastic sediments shows high sediment maturity [Roser et al. 1996]. The SiO2/Al2O3 ratios of the studied materials range from 3.41 to 5.34 (average: 4.49) and thus characteristic of submature to mature sediments.
The CIA [Nesbitt and Young 1982] and plagioclase index of alteration (PIA) of Fedo et al. [1995] have been widely used to infer chemical weathering of sediments [e.g., Armstrong-Altrin et al. 2015; Rashid et al. 2015; Tawfik et al. 2017; Ngueutchoua et al. 2019]. Very high PIA and CIA values (>75) indicate intense chemical weathering, whereas values ⩽50 reflect little or unweathered source rocks. The PIA and CIA values range respectively from 64.94 to 98.42 (average 81.23) and 58.25–68.41 (average 64.08) indicating that the source rocks show moderate to intense chemical weathering. The A–CN–K ternary diagram of Nesbitt and Young [1984] (Figure 10) indicates that the studied sediments have mainly a mixed granite and granodiorite composition as source rocks and have probably experienced moderate to intense chemical weathering.
A–CN–K plots of Nesbitt and Young [1984] for studied samples. The dotted line represents the possible weathering trend of the source rock if no K-addition is involved. The heavy solid line represents possible weathering trend combined with K-metasomatism from the source rock.
Chemical maturity and climate during the deposition of the studied samples expressed by bivariate plot of SiO2 versus Al2O3 + K2O + Na2O [after Suttner and Dutta 1986].
Plot diagrams to distinguish terrestrial and sea water (A) and depositional conditions (oxic, dioxic and anoxic conditions) (B).
(A) Discriminant-function multidimensional 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) Plot of the major elements composition of the stream sediments on the tectonic setting discrimination diagram of Kroonenberg [1994]. A: oceanic island arc, B: continental island arc, C: active continental margin, D: passive margin.
5.3. Paleoclimate, paleosalinity, and paleo-oxidation conditions
5.3.1. Paleoclimate
The degree of chemical weathering is function of temperature [Meunier et al. 2013] and the relationship between climate and degree of chemical weathering were established [White and Blum 1995]. The CIA has been also used for paleoclimate indication [e.g., Nesbitt and Young 1982; Yan et al. 2010; Zeng et al. 2019]. The climate is cold and dry during low chemical weathering if the CIA values range from 50 to 65, CIA range from 65 to 85 indicates a warm and humid climate during moderate chemical weathering and range between 85 and 100 indicates a hot and humid climate during high chemical weathering. The CIA values range between 58.25 and 68.41 (average 64.08), suggest a relative warm and humid climate during moderated chemical weathering. By another way, the SiO2 versus Al2O3 + K2O + Na2O diagram of Suttner and Dutta [1986] (Figure 11) applied to the studied materials indicates an arid to semi-arid climate. Probably, the climate was relatively warm and arid to semi-arid during the deposition of the studied sediments.
5.3.2. Paleosalinity and paleo-oxidation conditions
For paleosalinity condition reconstruction, the Sr/Ba ratio has been widely used [e.g., Zheng and Liu 1999; Xu et al. 2011; Cao et al. 2012; Meng et al. 2012; Ngueutchoua et al. 2019]. This ratio increases from coastal fresh water to the open and saline water [Cao et al. 2012], <1 in fresh water and >1 in marine sediments [Jones and Manning 1994; Shi et al. 1994; Wang et al. 2005]. This ratio ranges from 0.23 to 2.01 (average 0.58) indicates according to the stratigraphic succession of samples, a hypersaline condition at the bottom (DT55 = 2.01) and fresh water condition in the upper part with more or less fluctuation (0.22–0.66).
The Y/Ho ratio is used to differentiate between continental (nonmarine) and marine environments. The Y/Ho ratio between 40 and 90 corresponds to a sea environment while Y/Ho ratio < 40 indicates a river environment [Bau and Dulski 1996; Nozaki et al. 1997; Bolhar et al. 2004; Abigail et al. 2010; Bokanda et al. 2019]. The Y/Ho ratio ranges from 25.50 to 30.95 (average 29.10) (Figure 12(A)), suggesting river conditions.
Several ratios such as U/Th ratios have been used to better understand depositional environmental conditions [e.g., Jones and Manning 1994; Rimmer 2004; Teng et al. 2005; Tribovillard et al. 2006; Nagarajan et al. 2007; Zhao et al. 2016; Delu et al. 2017; Xie et al. 2018; Bokanda et al. 2019]. The U/Th ratio < 0.75, between 0.75 and 1.25 and >1.25 correspond respectively to oxic, suboxic, and anoxic conditions. All the studied samples correspond to oxic environments (Figure 12(B)).
5.4. Tectonic setting
To characterize the tectonic setting, of our studied sediments, we used the multidimensional diagram based on major elements of Verma and Armstrong-Altrin [2013]. This diagram is widely used in studying tectonic regimes of recent and ancient sediments [e.g., Guadagnin et al. 2015; Nagarajan et al. 2015; Zaid 2015; Armstrong-Altrin and Machain-Castillo 2016; Tawfik et al. 2017; Zeng et al. 2019]. This diagram shows discrimination of island or continental arc, continental rift, and collision tectonic setting from high silica ((SiO2)adj = 63–95%), and low silica ((SiO2)adj = 35–63%). All the studied materials belong to the high-silica group. The samples plotted (Figure 13(A)) fall in the collisional field boundary. We also used the tectonic setting discrimination ternary diagram (SiO2∕20 − K2O + Na2O − Fe2O3 + MgO + TiO2) of Kroonenberg [1994]. This diagram discriminates sediments deposited in the passive continental margins (PCM), active continental margins, oceanic island arc, and continental island arc (CIA). Our plotted samples fall in the passive margins (PM) and the intersection between the PM and CIA domains (Figure 13(B)), reflecting the compressional condition. The results from these two diagrams are inconsistent with the geological setting of the studied area and could indicate the Precambrian basement history of the studied area from which are derived these deposits as the tectonic setting of source rocks may be inferred from recent sedimentary rocks [Armstrong-Altrin et al. 2015]. The Precambrian basement of the studied area belongs to Pan-African north-equatorial fold belt, which resulted from the neoproterozoic orogeny [Nzenti et al. 1988]. This orogeny recycled partially or totally the older crust materials (paleoproterozoic and archean crustal material) during collision between Congo and West African cratons and the closure of large proterozoic ocean realms [Nzenti et al. 1988; Penaye et al. 1993; Kröner and Stern 2004; Toteu et al. 2004]. Indeed, according to the tectono-stratigraphic evolution of the Djerem–Mbere basin [Ngangom 1983; Tchouatcha 2011], the studied deposits correspond mainly to the extensional phase of deformation.
6. Conclusion
- According to the major elements composition, the studied Cretaceous calcareous deposits of the Djerem sub-basin are essentially composed of shales and wackes with subordinate Fe-shales.
- The observed lamination and bedding suggests that the studied sediments have been deposited globally in a calm environment marked by seasonal fluctuation and periodically affected by the seismic shocks causing the convolute bedding and pseudo-nodules, and post-diagenesis seismic lead to joints and faults. The milieu of deposition was marked by periodic emersion and forth and water giving desiccation cracks and symmetrical ripple marks, respectively.
- The K2O/Na2O ratios, the CIA and PIA values, the Zr/Sc versus Th/Sc diagram suggest that the sediments are submature to mature; the Al2O3/TiO2 and Th/Co ratios and the La/Sc versus Th/Co and Zr versus TiO2 plots indicate that the sediments are essentially derived from felsic rocks.
- The CIA values and the SiO2 versus Al2O3 + K2O + Na2O diagram indicate a relative warm and arid to semi-arid climate during deposition of sediments in a swampy environment. Paleosalinity range from hypersaline at the bottom to fresh water at the top according to the Sr/Ba ratios. Oxic conditions according were prevailing to the U/Th ratios.
- The tectonic setting, collisional environment, or PM correspond rather to the Precambrian history that affected the basement from which are derived the studied carbonate deposits according to the discriminant function-based multidimensional and major elements composition diagrams.
Acknowledgments
The authors greatly acknowledge Dr. Sidonie Révillon for reviewing and improving the manuscript, and Dr. Mache, Local Materials Promotion Authority Cameroon, for providing mineralogical analysis facilities.