Roser and Korsh (1988) have proposed a diagram allowing recognition of lithologies, including felsic, intermediate, mafic, and recycled-mature quartzose provenance (Fig. 10A). The studied sediments are plotted in recycled quartzose (2 samples), intermediate igneous (3 samples) and mafic igneous (4 samples) provenance fields for the Rio-del-Rey deposits, in recycled quartzose (10 samples), felsic igneous (2 samples) and mafic igneous provenance fields (3 samples) for the Douala deposits, and in recycled mature field for all samples from the Campo deposits. The plotted sediments in the recycled field may indicate the para-derived metamorphic source rocks with preserved sedimentary signatures and /or more not well-known ancient deposits.
Other plots such as Ce vs. La ̸ Nb (Rao et al. 2011) (Fig. 10B) and La/Th vs. Hf (Floyd and Leveridge, 1987) (Fig. 10C) have also been widely used (e.g. Moradi et al. 2016; Zeng et al. 2019; Tchouatcha et al. 2021) for Ce vs. La/Nb and Ngueutchoua et al. (2019a, b) and for La/Th vs. Hf). The Ce vs. La ̸ Nb scattered plot applied to our study points to a dominant felsic source with a contribution of an intermediate igneous source for the Rio-del-Rey and Douala deposits, contrary to the Campo deposits showing a dominant intermediate igneous source with a felsic contribution. The La/Th vs. Hf plot shows that the Campo deposits derived mainly from felsic rock sources, the Rio-del-Rey deposits from felsic and mixed felsic and intermediate composition source rocks, and the Douala deposits from mainly felsic composition in the northern part, and mixed felsic and intermediate to intermediate composition in the southern part.
Some elemental ratios are useful in unravelling the source signatures of sediments, such as Th/Co (Cullers 2000; Armstrong et al. 2015) and Al2O3/TiO2 (Absar and Sreenivas 2015; Zhou et al. 2015; Tchouatcha et al. 2021b). The Th/Co ratio value ranges from 0.67–19.4 and 0.04 to 1.10 respectively, in felsic and mafic sources. For the sediments studied here, the ratio varies from 0.39 to 1.82, with main values ˃ 1 in the Rio-del-Rey deposits, from 0.62 to 13.75 in the Douala deposits with high values in the northern part of the sub-basin (0.62–13.75; average: 4.56) and low values in the southern part (0.69–1.35; average: 1.07); and from 0.51 to 0.98 in the Campo deposits, indicating respectively a felsic source with contribution of mafic source in the Rio-del-Rey sub-basin, a dominant felsic source in the northern part of the Douala sub-basin and dominant intermediate source in the southern part, and the intermediate source in the Campo sub-basin. The Al2O3/TiO2 ratio varies from 3 to 8, 8 to 21 and 21–70 respectively for mafic, intermediate and felsic igneous sources (Hahashi et al. 1997). In this study, the ratio ranges from 10.12 to112.93 (average: 26.29) in the Rio-del-Rey deposits, 8.57 to 30.37 in the Douala deposits, with more high values in the northern part (16.45 to 30.37, average: 21.06) and low values in the southern part (8.57 to 21.90, average: 15.68), and 14.04–21.78 (average: 16.37) in the Campo deposits. This also indicate respectively felsic rocks with contribution of mafic rock in the Rio-del-Rey deposits and dominant felsic rocks in the northern part of the Douala deposit and intermediate rock composition in the southern part, and intermediate rock composition in the Campo deposits.
Furthermore, the REEs and relevant parameters are also useful to characterize the lithotype rock source of sediments (e.g. Liu et al. 2015; Ma et al. 2015; Zeng et al. 2019; Tchouatcha et al. 2021), and the higher LREE/HREE ratio and low Eu anomaly indicate a felsic igneous source, whereas the lower LREE/HREE ratio and relative high Eu anomaly reflect a mafic igneous source (Armstrong-Altrin et al. 2013). The LREE̸HREE ratios range from 9.05 to 14.50 and no to slight positive Eu anomaly (0.98 to 1.28) in the Campo deposits, 15.34 to 24.03 and negative to positive Eu anomaly (0.61 to 1.46) in the southern part of the Douala sub-basin, 11.25 to 24.89 and no to slight positive Eu anomaly (1.03 to 1.35) in the northern part of the Douala sub-basin, and 9.41 to 21.70 and no to slight positive Eu anomaly (1.04 to 1.32) in the Rio-del-Rey deposits, indicating respectively intermediate composition for Campo deposits, predominance of felsic composition in the northern part of the Douala sub-basin with no to slight positive Eu anomaly, predominance of intermediate of mixed of felsic and mafic composition in the southern part of the Douala sub-basin with negative to positive Eu anomaly, and predominance of felsic composition of the source rocks in the Rio-del-Rey sub-basin. The Fig. 11 shows the varied source rocks lithology of the three sub-basins along the Atlantic Ocean.
Figure 12 shows that samples from the Douala sub-basin plot close to and along the A-K apex suggesting either the lack of plagioclase and occurrence of K-feldspar (orthoclase) in the source rocks or diagenetic effects by post-depositional conversion of clays such as kaolinite to illite by including K+ in their structure (Fedo et al. 1995). As the diagenetic effects are very weak (early diagenesis, see part 5.5), the samples plot close A-K apex indicate a high degree of weathering of the source rocks containing abundant K-bearing minerals. Similar results were given in detrital Cretaceous outcrops in the northern part of this sub-basin (Ngueutchoua et al. 2017, 2019; Esue et al. 2021). Meanwhile, some samples, precisely limestones and marly limestones, show a low to very low degree of alteration probably due to chemical input or origin of these deposits (Esue et al. 2021). Figure 12 also indicates a low to high weathering of the Rio-del-Rey sub-basin deposits and moderate weathering of the Campo sub-basin deposits. It also shows that the varied source rocks for each and all the three sub-basins experienced varied weathering degrees. Moreover, the presence of volcanic fragments (basaltic pebbles), reported in the Upper Cretaceous sandstones of Santos Basin, Eastern Brazilian Margin (De Ros et al. 2003), in the Cenomanian-Turonian deposits (Njike Ngaha 1984) in the north of the Douala sub-basin highlighst a volcanic activity which took place probably during the Albian rifting reported as well as in the Sergipe-Alagoas Basin (North-eastern Brazilian Coastal Basin) and the North Gabon (Western African Coastal Basin) (Kurobaza et al. 2018).
5.2. Sediment sorting, maturity, recycling and paleoweathering
The ICV (Index of Compositional Variability; Cox et al. 1995) and CIA (Chemical Index of Alteration; Nesbitt and Young 1982) (Fig. 13) are widely used to characterize the compositional maturity of sedimentary deposits (e.g. Mongelli et al. 2006; Perni et al. 2011; Tao et al. 2013, Armstrong-Altrin et al. 2015; Tawfik et al. 2017; Tchouatcha et al. 2021b). Figure 13 shows that sediments from the Campo sub-basin are globally sub-mature (ICV: 0.98 to 2.12, average: 1.48; CIA: 65.26 to 74.72; average: 70.24). The sediments from the Douala sub-basin are mature (ICV: 0.37 to 0.90) with intense chemical weathering of the source area (CIA: 67.00 to 99.33), whereas the sediments from the Rio-del-Rey sub-basin are sub-mature to mature with two samples (MK3 and DK1A) showing high ICV values (5.04 and 10.03) and high CIA (77.38 and 92.04) respectively.
The PIA (Plagioclase Index of Alteration; Nesbitt and Young, 1982; Fedo et al. 1995) is also used to characterize the chemical weathering of source rocks (e.g. Rashid et al. 2015, Ngueutchoua et al. 2019a, Tchouatcha et al. 2021b). The high values (PIA ˃ 75) and low values (PIA ≤ 50) of PIA reflect respectively intense and weak chemical weathering of these rocks. Table 1 shows that in the Campo sub-basin, the PIA of the sediments ranges from 72.70 to 87.57 reflecting moderate to high chemical weathering, from 96.35 to 99.90 in the Douala sub-basin indicating high chemical weathering, and 66.68 to 99.47 in the Rio-del-Rey sub-basin suggesting weak to high chemical weathering.
Meanwhile, due to the high carbonate proportions in the main samples from Campo and Rio-del-Rey sub-basins compare to those from Douala sub-basin, the PIX and CIX, without CaO (Garzanti et al. 2014; Garzanti et al. 2019) were used to better correlate and appreciate the weathering degree of samples from the three sub-basins. Table 1 shows that CIX and PIX range respectively from 71.26 to 76.88 and from 84.19 to 92.25 in the Campo sub-basin indicating moderate to high chemical weathering, from 67.15 to 99.39 and from 96.94 to 99.94 in the Douala sub-basin suggesting weak to high chemical weathering, and from 73.77 to 92.90 and 80.15 to 99.64 in the Rio-del-Rey sub-basin indicating moderate to high chemical weathering.
5.3. Paleoclimate and the depositional environment
The CIA and PIA indices and some trace element ratios such as Rb/Sr are reliable tools for paleoenvironment characterization (e.g. Cao et al. 2012, Hernandez-Hinojosa et al. 2018, Ekoa et al. 2021, Tchouatcha et al. 2021b). The clay minerals record the condition and climatic fluctuation of the sedimentary environment (Singer 1988, Meng et al. 2012). It is worth noting that at each stage of transformation, clay minerals respond to their chemical and thermal environment, and, as a consequence, their properties and species change. Kaolinite is formed in a subtropical humid climate by the intense weathering of feldspar. Illite is mainly formed in a cold climate with low precipitation. Chlorite and illite form the basis of the diagenetic zone (Dunoyer De Segonzac 1970). Al2O3 vs. V and Al2O3 vs. P2O5 are also useful for paleoenvironmental conditions and have been successful used (e.g. Mortazavi et al. 2014; Anaya-Gregorio et al. 2018; Ekoa et al. 2021). Variation of phosphorus concentration is controlled by water depth and temperature and vanadium concentration is somewhat higher in marine facies than in freshwater sediments. The diagrams Al2O3 vs. V (Fig. 14A) and Al2O3 vs. P2O5 (Fig. 14B), show that sediments from the three sub-basins were deposited in shallow marine and fluvial environments at various depths suggested by relatively low and varied concentrations of vanadium. The plotted sediments from Campo and Rio-del-Rey sub-basins generally indicate important depth, those from Douala sub-basin, on the contrary, formed in a low water depth with poorer phosphorus concentrations.
The Sr ̸ Ba ratio has been widely used for salinity conditions (e.g. Zheng and Liu 1999; Meng et al. 2012; Zeng et al. 2019; Tchouatcha et al. 2021b). These ratios increase from coastal fresh water to the open and saline marine water (Cao et al. 2012), and are > 1 in marine and ˂1 in freshwater sediments (Jones and Manning 199; Shi et al. 1994; Wang et al. 2005). In our study, sediments from Rio-del-Rey have higher values (0.41 to 1.07) while those from the Douala (0.09 to 0.85) and Campo (0.20 to 0.31) sub-basins are low, indicating a dominant marine influence in the Rio-del-Rey sub-basin and predominant continental conditions in the Douala and Campo sub-basins.
The high CIA values indicate warm and humid climatic conditions during sediment deposition (Nesbitt and Young 1982). In the Rio-del-Rey sub-basin, CIA values range from 63.59 to 92.76 suggesting less humid climatic variation in the Albian-Cenomanian, while in the Campanian-Maastrichtian, CIA values vary between 92.04 and 92.76), suggesting more humid climatic conditions. In the Douala sub-basin, this ratio varies from 67.00 to 99.33, and is particularly high (92.11 to 99.33) in the southern part and low (67.00-83.58) in the northern part, highlighting the influence of the source rock lithology, felsic in the northern part and intermediate to mafic in the southern part. In the Campo sub-basin, this ratio ranges from 65.26 to 74.72 indicating semi-arid conditions.
The SiO2 vs. Al2O3 + Na2O + K2O plot (Fig. 14C) shows that the Campo and Rio-del-Rey sediments have been deposited under semi-arid to arid conditions, while those from Douala may have been deposited under semi-arid to arid with periodically semi-humid conditions (fine-grained sands), probably linked to the marine influence.
In the Campo sub-basin, the palynological species consist exclusively of continental forms. Previous palynological data (Belmonte 1966; Chevalier 1982; Ntamak-Nida et al. 2008) did not support the presence of marine species in these deposits. In addition, Andreef (1947) reported some ammonite specimens, leading to the conclusion that the deposits in the Kribi-Campo sub-basin were related to piedmont continental and lacustrine settings near a coastal zone open to a narrow sea (Njike Ngaha et Eno Belinga 1987). More, the presence of dolomite may indicate either elevated temperature and pressure conditions during diagenesis or is related to the circulation of restricted marine pore waters near the mixing zone, as it is the case in the Mamfe Basin (Eyong 2003). In the case of Campo sub-basin, the second hypothesis seems plausible in the context of the rifting of this sub-basin and the presence of dolomite in one part (upper part, See Fig. 8d) of deposits. The presence of the Botryococcus sp. (freshwater algae, Ntamak-Nida et al. 2008,) in these deposits could confirm the continental environment with periodical marine influence.
The abundance of phytoclasts (leafs) at Mbanga in the Douala sub-basin may indicate that these deposits are more continental, as suggested by their proximity with the margin outcrops such as the conglomeratic sandstones. The marine species in these deposits indicates a marine influence during the Cenomanian (Njike Ngaha and Eno Belinga 1987; Njike Ngaha et al. 2014; Njoh et al. 2014), with a shallow environment at Ediki and a marginal to shallow marine setting at Mbanga. At Ediki, the black clays are intercalated between the sandstone facies, while at Mbanga, they are exposed and found on top of the poorly consolidated sandstones, reflecting intense erosion towards the basin margin or differential erosion. Furthermore, two sediment cycles during the Cretaceous have been recognized in the Douala sub-basin from drilling and seismic surveys on land, in swamps, and sea (Seiglie et al. in Regnoult 1986), one cycle of low amplitude between the Cenomanian and the Turonian, and the other of high amplitude between the Coniacian and the Maastrichtian.
The marine species in the Cenomanian and Campanian-Maastrichtian of the Rio-del-Rey sub-basin reveal a marine influence. The more abundant and varied marine species in the Campanian-Maastrichtian point to a more offshore environment as compared to that of the Cenomanian. This is also reported by Njoh et al. (2016, 2018).
On the southern part of the Douala sub-basin, the low proportion or even absence of kaolinite on the one hand, and on the other hand, the feldspar whose alteration produced kaolinite, suggest that the source rock was poor in feldspars and richer in ferromagnesian minerals as also suggested by the geochemical data. The hypothesis of a subtropical climate (warm and relatively humid) can be considered by chlorite/vermiculite, iron oxides and hydroxides such as magnetite and goethite. In the northern part, the low abundance of kaolinite associated with magnetite suggests hot and semi-arid climatic conditions during the Albian-Cenomanian to Turonian times.
In the Campo sub-basin, muscovite records a continental influence. The fluctuation of dolomite concentration suggests variation of the sedimentation, probably related to a marine influence. The fluctuation and low abundance of kaolinite point to semi-arid to arid climatic conditions.
In the Rio-del-Rey sub-basin, the mineralogical (kaolinite mainly) variation may indicate a climatic change from semi-arid to more arid conditions during the Albian-Cenomanian. The increase of kaolinite in the upper part indicates a climatic change from more arid conditions to semi-arid during the Campanian-Maastrichtian.
5.4. Behavior of chemical composition versus Al ̸ Si
To depict the grain size effect on geochemical composition of the sediments, Bouchez et al. (2011), Roddaz et al. (2014) and Tchouatcha et al. (2022) have used the Al ̸ Si ratio which is considered as a proxy for sediment grain size applied in the Amazonian floodplain sediments (Bouchez et al. 2011). Figure 15 shows the variation of Al ̸Si ratios vs. CIA and some element concentrations such as Na, Hf, Ca, K and Zr. According to this figure, Al ̸ Si shows no correlation with CIA, Hf and Zr for sediments from the three sub-basins, no correlation with Na and Ca for the Douala sub-basin sediments, negative correlation fort the Campo and Rio-del-Rey sub-basins sediments, no correlation with K for Douala and Rio-del-Rey sub-basins sediments and a positive correlation for the Campo sub-basin sediments. These varied behaviours and the pattern of enrichment and depletion of major and trace elements (Fig. 5A and B) could indicate the varied provenances of the material in the different environments of sedimentation of the three sub-basins.This is also confirmed the palynological data; fluvio-lacustrine environment for the Campo sub-basin, fluvio-deltaic with marine influence environment in the upper part for the Douala sub-basin and marginal to shallow marine environment for Rio-del-Rey sub-basin.
5.5. Diagenetic effects
The diagenetic signatures in the Campo sub-basin were depicted using X-ray diffraction (Fig. 8) and petrography (Fig. 4). X-ray diffraction data exhibit non-clays minerals such as quartz (abundant), feldspar, muscovite and a few biotite, clay minerals represented by kaolinite and chemical minerals made up of calcite and sometimes dolomite. Kaolinite can be diagenetic or detrital origin. In the case of diagenetic origin, kaolinite is the product of dissolution of potassic feldspars. The detrital one is linked to climatic conditions, from hydrolysis of alkaline feldspars, its presence in the sediments is indicative of rigorous weathering of source rocks with steep relief and exhaustive leaching of weathered materials under a warm, humid and acidic milieu (Enu, 1986).. The detrital origin for this study is expected because of permanent association of kaolinite with high quantity of quartz which is resistant to alteration and concentrates in sediments transported and deposited by rivers. In this case, kaolinite appears in low quantity (Fig. 8C), its low quantity and fluctuation, generally lower in the organic matter-rich levels (TCA2 and TCA3) could indicate the link with climate.
The carbonate minerals such as calcite and dolomite, could be authigenic or diagenetic. The authigenic mineral can precipitate directly from solution or from weathering and replacement during burial. Diagenesis is a set of physical and chemical processes which affect sediments after deposition until the limit with metamorphism (Blatt et al. 1972). Generally, the evidence of chemical diagenesis can be inferred from physical aspect. In this case, calcite fluctuates, being very low in the lower part of the studied series and low in the upper part. Its presence in the clayey matrix could be explained by precipitation from dissolution of unstable minerals (Tawfik et al. 2017) such as feldspars which are frequently vesicular and sericitized. In the sandy facies, calcite fills most of the secondary pores and is as mineral replacement (Ashukem et al. 2022). Dolomite fluctuates also, very low to absent in lower part of the studied series and high (the highest after quartz) in the upper part. This mineral can be diagenetic linked to temperature and pressure increase or results from the circulation of restricted marine pore water near a mixing zone (Eyong 2003). Its fluctuation supports the second hypothesis.
In thin section, phyllosilicates such as muscovite ̸ sericite, the most abundant mineral after quartz, show some packing readjustment with preferential orientation and sigmoid shapes (Fig. 4A), indicating a significant physical effect, as evidenced by varied type of inter-granular contacts such as plan, convexo-concave or sutured contacts in the sandy facies. Elsewhere, silica from dissolution of quartz, feldspar infillings in the pore spaces, and quartz and feldspar overgrowths are common in the sandy facies (Ashukem et al. 2022).
Contrary to the Campo and Rio-del-Rey sub-basins, the Cretaceous fine-grained deposits from the Douala sub-basin are poorly consolidated or friable indicating a low burial and weak compaction of these deposits. In the coarsest grained facies (poor consolidated siltstones or sandstones), the clay matrix occupies the large inter-granular pore spaces in the loosely packed detrital grains (rare contact points between quartz and feldspar) with a floating grain texture (Fig. 4F). X-ray diffraction did not reveal any carbonate minerals. Clays minerals such as kaolinite is very low or absent in the southern part of the sub-basin and present in its northern part, conversely for the chlorite or vermiculite, indicating the influence of climate and source rock lithology and their detrital origin. The upper part of the sequence, Coniacian-Maastrichtian (Fig. 2) shows the intercalation of marlstones and limestones in the detrital layers related to the variation of sedimentation conditions (Njike et al. 2014).
X-ray diffraction in the Cenomanian samples (MK1, MK2, MK6) exibits clays (illite, chlorite, vermiculite and kaolinite), and carbonates (calcite and dolomite). Calcite and dolomite could. Dolomite is related to the circulation of restricted marine pore waters near the mixing zone or may indicate elevated temperature and pressure conditions during diagenesis (Eyong 2003). The gradual decreases of clays (illite, chlorite and kaolinite) and carbonates (calcite and dolomite) from bottom to the top of the three samples could indicate that they are linked to the same genetic event, probably the sea level change, but this remains hypothetic as only three samples were collected with a wide spacing.
In the Campanian-Maastrichtian samples (DKA, DK1N and DK1B), kaolinite is also present but rather increase from bottom to the top in the sequence, with very high (dominant) siderite (Fe2CO3) in the basal sample (DK1A), not reported in other samples, indicating probably the sea level increasing that has led to reducing condition needed for deposition of siderite. Meanwhile, the siderite formation is indicative of a late diagenetic phenomenon, with calcite as precursor. It is reported in varied environments (Pye et al. 1990; Eyong 2003). The increase of kaolinite in this sequence is probably linked to the continental input during the sea level decrease.
5.6. Tectonic setting
The major-elements based discrimination plots of Verma and Armstrong-Altring (2013) were used to characterize the tectonic setting. These diagrams (Fig. 16A and 16B) have been widely and successfully used (Guadagnin et al. 2015; Nagarajan et al. 2015; Tawfick et al. 2017; Zaid 2015; Zeng et al. 2019; Tchouatcha et al. 2021) and discriminate three tectonic types (Arc-Rift-Collision) of two sets of low-silica rocks [(SiO2)adj = 35–63%] and high-silica rocks [(SiO2)adj = 63–95%]. All the plotted samples from the three sub-basins belong to the collisional tectonic setting with high-silica contents. Only one sample is out of the collisional tectonic setting and has low-silica content. Some recent work in the Logbadjeck Formation of the Douala sub-basin led to a similar interpretation (Ngueutchoua et al. 2019). In the Th-Sc-Zr/10 ternary plot (Fig. 16C and 16D) of Bhatia and Crook (1986), our samples correspond mainly to the continental island and passive continental margins and a few samples in the active continental margin (i.e. Campo samples in continental island arc, Douala samples in the continental island arc and passive continental margin, some in the active continental margin, and Rio-del-Rey samples in the continental island arc and passive continental margin). These results are similar to those of Ngueutchoua et al. (2019) corresponding to active and passive continental margins. According to a recent study in the northern part of the Douala sub-basin (Esue et al. 2021) most of the samples belong to the passive margin tectonic and others in the active continental margin or oceanic island arc.
All these results may indicate the complexity of the setting up of the Atlantic Basin of Cameroon. The Atlantic Basin of Cameroon is formed in a rift and passive tectonic context (Nguene et al. 1992; Njike Ngaha and Eno Belinga 1987; Ntamak-Nida et al. 2010; Njike Ngaha et al. 2014) (Fig. 19). Meanwhile, outcrops from the Campo sub-basin were affected by both normal and inverse faults, indicating respective distensional and compressional movements. Thus, if the geological history of the Atlantic Basin of Cameroon corresponds to passive and rifting settings, its evolution is marked by periods of compressional events, and the collision or active tectonic setting may reflect, on the one hand, the compressional events due to the filling pressure of sediments during the evolution of South Atlantic Ocean, and in the other hand, reflects the Precambrian basement history which experienced the Neoproterozoic orogeny (Nzenti et al. 1994; Toteu et al. 2001; Kroner and Stern 2004). Similar interpretations were made in numerous studies, such as in Recent deposits (e.g. Armstrong et al. 2015), Tertiary to Cretaceous (e.g. Tchouatcha et al. 2021) and Cretaceous deposits (Ngueutchoua et al. 2019a; Tchouatcha et al. 2021). The paleogeographic evolution of the Cameroon Atlantic Basin is given in Fig. 17. Anyway, it makes sense that after the morphological flattening of the Precambrian reliefs during the Proterozoic, great quantities of sediments were produced until the end of the proto-Atlantic rifting. This latter caused differential subsidence by reactivation of normal and antithetic rifting faults. In this context, the listric and reverse faults, well developed in the Campo sub-basin, indicate respectively syn-tectonic deposits periodically affected by compressional events.
5.7. Paleoenvironment and Paleogeographic evolution
This period corresponds probably to the second episode in the breakup of Pangea and South Atlantic Opening linked to the separation of African and South American Continents (Fig. 17A) associated with intense igneous activities (Scotese 2001; Njike Ngaha 1984; Njike Ngaha et al. 2014).
This opening led to the first detrital deposits in the Atlantic Basin, well known in the Campo sub-basin. These deposits are made up of alternated polygenic breccias interbedded with sandstones and ̸ or siltstones to claystones upward indicating gravity and rhythmic deposits. The facies and geochemical characteristics (Fig. 14A and B) indicate deep environments, probably in a lacustrine landscape with periodically marine incursion (Fig. 17B) as suggested by the exclusive continental palynomorph species such as Classopollis sp. and Ephedripites sp., and periodic formation of dolomite (Fig. 8C) linked probably by sea water incursions, under arid and semi-arid climate (Fig. 14C). The main sedimentary succession is characterized by a major retrogradation/progradation cycle with a well-developed progradational trend (Ntamak-Nida et al. 2010). The presence of listric and reverse faults (Fig. 3B and C) should have generated relative fluctuations in lake level and sediment supply due to the active faults creating alternative up- and down-lifting of basement blocs.
According to Seiglié et al. 1982, two sedimentary cycles are globally identified in the Douala sub-basin from Middle to Late Cretaceous, one cycle of low amplitude from Cenomanian to Turonian-Coniacian boundary, and the other one of hight amplitude from Turonian-Coniacian boundary to Late Maastrichtian.
The Cenomanian deposits (Lower to Middle Cenomanian) in the Douala sub-basin are mainly terrigenous made up essentially of sandstones and conglomeratic sandstones with interbedded clays indicating a rhythmic succession in a narrow rift and characteristic of deltaic environments (Njike Ngaha et al. 2014; Tchouatcha et al. 2021).
Several synthetic and antithetic normal faults, related to the progressive E-W distension between South American and African plates isolated small successive depressions, followed by a progressive sea water infiltration and variation of sedimentation conditions from lagoonal-lacustrine (Njike Ngaha et al. 2014) to shallow marine conditions (Fig. 17C) confirmed by the presence of Microforaminiferal test linings and dinoflagellate cysts.
From Late Cenomanian to Turonian, there was a significant subsidence of the faulted basement (Fig. 17D) linked to the progressive widening of the rift, the collapse of the entire block initiated a rise in the marine transgression towards the Nord of the Douala sub-basin (Njike Ngaha et al. 2014) with confined and shallow sea areas. This type of environment prevailed also in the Rio-del-Rey sub-basin right up to the Coniacian period. This phase is widely dominated by fossiliferous rich calcareous shales or marlstones facies.
During the Late Cretaceous, abundant marine species (Dinoflagellate and foraminiferal species) in the Campanian-Maastrichtian shales of the Rio-del-Rey sub-basin are related to the sea level rise (deep neritic), whereas in the Douala sub-basin, there is variation from bathyal to low deep neritic domain with regressive sedimentation (Cael and Kim 1979). The climate is globally arid to semi-arid in the Rio-del-Rey sub-basin, and arid to semi-arid and periodical semi-humid in the Douala sub-basin during Middle to Late Cretaceous (Fig. 14C).