Diagenetic and Geochemical Study of Sediments from the Cretaceous Basin of Babouri-Figuil (North-Cameroon)

The purpose of this work is to characterize the sediments of the Babouri-Figuil Basin from a diagenetic point of view, the protolith context and weathering conditions, using major element geochemistry and mineralogy of the sedimentary rocks. Microscopic observations of diagenetic phenomena (on and around detrital grains of the basin), and the study of precipitation of the cement show that these sediments have undergone phenomena of compaction, dissolution, recrystallization and cementation. Cementation of quartz by silica is controlled by the rate of precipitation of silica, which is closely related to temperature than pressure. Thus, the sediments of the basin could be influenced by hydrothermalism during diagenesis, which would have inhibited their reservoir quality by generalized cementation phenomena. Geochemical features of the sediments show high contents in SiO2 (47.49 wt% 90.79 wt%), Al2O3 (2.92 wt% 23.18 wt%), Fe2O3 (0.2 wt% 6.22 wt%) and alkali and alkaline earth metals (>3%). The chemical alteration index varies between 30.92% and 95.08%. This variation in the CIA values reflects the variation in the proportion of feldspars and different clay minerals in these sediments. However, the ICV calculation and the ICV versus CIA show compositional immature to mature sediments, with low to intense weathering character of these sediments. Petrographic and geochemical characteristics of sediments of the basin are compatible with the composition of the granitic and gneissic surrounding bedrocks.


Introduction
The exploitation of sedimentary basins has prompted research into a wide range of diagenetic processes that modify the volume and distribution of sediments.
The characteristics of sediments in depressed areas or accumulation basins are influenced by several components, including the diagenetic history of sediments and their geochemistry [1] [2] [3]. The main factors in the evolution of a basin are diagenetic sedimentary controls, mineralogy, geodynamic context, emplacement conditions and the nature of sediments, linked or not, to the structural evolution of the basins [4] [5].
The Babouri-Figuil Basin is one of the intracontinental basins of the Cretaceous age in North Cameroon. It belongs to the great Benue Trough which is associated with the opening of the South Atlantic. The geological sketch map of this basin highlights a fluvial type facies at the base which uses more or less strong transportation energy, and a fluvial-lacustrine type facies at the top [6]. It contains about 3000 metres of sediments with a well-known age. These sediments underwent physical-biochemical modifications after deposition, under "low" pressure and temperature conditions which prevailed in the sub-surface environment. The objective of the work is to address the diagenetic problems in this basin, in order to appreciate the evolution of the sediments under the phenomena of compaction, cementation, dissolution, and recrystallization, as well as their level of alteration. Also, an in-depth geochemical study will be done to coin the chemical composition of the sedimentary rocks in order to trace the source of the sediments of the Babouri-Figuil Basin.   [12].  [10]. The syn to late-tectonic granitoids D1: they correspond to the BIP (Basic to Intermediate Plutons) described by Toteu et al. [11]. They occur in shales and gneisses, and often contain enclaves of amphibolites. The syn to post-tectonic D2 granitoid unit: it consists mainly of pan-African plutonic rocks of the Northern Cameroon domain. The crustal origin of these rocks is multidimensional.

Geological Setting
Late granitoid to post-tectonic D2 unit: it is a set of leucocratic granites and syenites with variable textures, which outcrop in the form of domes or inselbergs intersecting the D2 regional structure.

Materials and Methods
Sampling was globally done on fine-grained rocks (schistous marls and fine sand-

Petrography
Petrographic studies through thin sections and mineralogy using XRD spectra of the Babouri-Figuil Basin sediments shows numerous contacts between grains and mineralogy which slightly vary in the four sampling areas.

Thin Section
The petrography of the thin sections ranges from tangential contacts, through concavo-convex contacts to longitudinal contacts. The concavo-convex contacts  preserves the tangential contacts between the detrital grains; type II syntaxial cement precipitates after type I syntaxial cement and is systematically in contact with the latter.

XRD Mineralogy
The mineralogy of the 4 sediment samples (1 per outcrop) collected in the Babouri-Figuil Basin is dominated by silicates, carbonates, sulphates, oxides and hydroxides ( Figure 3). These minerals are thought to be derived from the dismantling of the surrounding rocks on one hand, and from purely chemical processes occurring in the sediments on the other hand [12]. In the second case, they are con-  Babouri-Figuil Basin [13].
It is noticed that this basin is solely composed of smectite, kaolinite and mica.
The results recorded during this work reveal the presence of other clays such as montmorillonite, chlorite, illite and tarasovite, which are composite clays.

Geochemistry
Major element geochemistry enabled the characterization of the different sources of terrigenous detrital input in the Babouri-Figuil formations and to evaluate the intensity of the chemical weathering. Associated with mineralogy, this has permitted the reconstruction of the paleoconditions which prevailed during diagenesis.
Concentrations of major elements of sample sediments from the studied basin are listed in

Diagenetic Processes
The petrographic and geochemical study of the sediments of the Babouri-Figuil  Basin shows numerous contacts between grains. These types of contacts form under the effect of pressure-dissolution [14], by selective dissolution at the contact points of the particles subjected to significant stress. The formation of stylolites in these sediments is a consequence of pressure-dissolution [15]. It also accounts for the formation of overgrowths, which is a feeding phenomenon of the quartz grains (Figure 2 This could be justified by a relative accumulation of iron at the edge of the void on one hand and a centrifugal alteration, which begins from the core of the mineral [18], and makes the mineral black from the interior to the edges on the thin blades observed, on the other hand.

Protolith Context
The petrographic and geochemical characteristics of the sedimentary rocks of the Babouri-Figuil Basin indicate that the figurative elements could come from the dismantling of granitic bedrock. The ratio Al 2 O 3 /TiO 2 ranges from 2.00 to 36.21. This ratio is relatively high, and is associated with the geochemical classification according to Herron [19]. It reveals that the samples analyzed are mostly wacke (Figure 5), indicating a more continental source [20] of sediments of the Babouri-Figuil Basin. Similarly, the diagram in Figure 6, based on Suttner and Dutta [21], indicates that sediments form the Babouri-Figuil Basin were mostly deposited in a semi-arid and arid environment, which is justified by the presence of the hard-grounds and numerous desiccation cracks observed in the basin.
They are associated with an arid environment. Also the discrimination functional diagram of the original signatures of the sandstone-mudstone suites using the following major element ratios after Roser and Korsch [22]: Discrimination func-  (Figure 7). This could be linked to the mineralogy of the studied sediments and the surrounding rocks made-up of leucocratic granites and syenites and occasionally the gneissic formations.

Weathering Conditions and Maturity
The degree of chemical weathering of source rocks and the maturity of sediments are measured by determining the CIA, ICV, and also through the mineralogy. These indices correspond to the sum of the weathering processes that affected these materials from the sedimentation media (supergenic weathering processes) and during burial (diagenetic and/or hydrothermal weathering processes  [23]. It is therefore high in samples where feldspars are in low proportion or absent (SL, MF). This comparison with the low feldspar rate is the result of intense chemical alteration in the basin.
Moreover, chemical index such as CIA values are well correlated in these sediments (Figure 4), exhibiting low to high chemical weathering ( Table 1). The above correlations suggest that weathering can be significantly influenced by the grains size of the sediment [24].   [27]. From the ICV calculation and the ICV versus CIA diagram proposed by Cox et al. [27], almost all sediments from MT and ML are immature, and a low or weak weathering character of these sediments is confirmed. Contrarily, sediments from SL and MF are generally mature with intense weathering except for two samples ( Figure 4).