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European Journal of Scientific Research ISSN 1450-216X / 1450-202X Vol. 149 No 3 June, 2018, pp. 258-278 http://www. europeanjournalofscientificresearch.com

Depocenters Repartition and Sequence Stratigraphy of the

Northern Part of the Kribi-Campo Sub-Basin (Cameroon)

Jeannette Ngo Elogan Ntem Université de Yaoundé, Faculté des Sciences, BP 812 Yaoundé Cameroon

Université de Douala, Département des Sciences de la Terre

BP 24157 Douala, Cameroon

Marie Joseph Ntamak-Nida

Université de Douala

Département des Sciences de la Terre


Dieudonné Bisso

Université de Yaoundé, Faculté des Sciences

BP 812 Yaoundé Cameroon

Francois Mvondo Owono

Université de Douala, Département des Sciences de la Terre


Simon Ngos III



Paul Bilong



Philippe Njandjock Nouck



Abstract

The geodynamic evolution of the Kribi-Campo sub-basin, using 2 D seismic data and sequence stratigraphy method, reveals 2 megasequences of 1rst order, one from Aptian to Eocene of aggradant geometry and the other from Eocene to Present of progradant architecture; five second-order sequences and one sequence of third order, corresponding to different stages of basin filling were highlighted. The first on, so-called Sequence I (Aptian-Albian), presents an aggradational architecture and corresponds to the flooding of the margin due to a uniform subsidence; Sequence II (Santonian-Maestrichtian), of aggradational geometry, initiates the destabilization of the margin by the Santonian compressive event at the origin of the folding; Sequence III (Eocene-Oligocene), of prograding architecture, marks the beginning of the progradation which brings about a general destabilization of the margin, resulting in basin and swell topography. Sequences

Depocenters Repartition and Sequence Stratigraphy of the Northern Part of the Kribi-Campo Sub-Basin (Cameroon) 259

IV (Aquitanian-Zanclean), V (Pliocene) and VI (from Pleistocene to Present) of third order, with a progradant-aggradant geometry that mark different phases of the tilting of the margin and its uplift; the predominant factor is tectonics. Also, deposits sequences, lowstand systems tracks and highstand systems tracks are highlighted. The depocenters are distributed seaward from Aptian to Upper Albian. Up to the Upper Cretaceous, the accumulation center is seaward but from the Eocene-Oligocene, the major shift in the displacement of the depocenters is towards the continent. The Miocene is marked by the fattening of the margin that occurs with the distribution of the depocenters on the continent. Another migration of the depocenters is observed, resulting on a larger lateral extension and an increase in thickness of deposits that extend over the entire plateau at the Pliocene and in the basin from Pleistocene to Present. The presence of faults (E-W and NW-SE) and the folds as well as the variation of slope denote a structural control over the displacement and distribution of the depocenters. Two hiatuses of erosion are observed from Cenomanian to Santonian and from Paleocene to Mid-Eocene.

Keywords: 2 D seismic data, depocenters repartition, sequence stratigraphy, geodynamic evolution, Douala/Kribi-Campo

1. Introduction Douala/Kribi-Campo basin is a West African margin basin that took place due to the opening of the South Atlantic Ocean. The reconstitution of the tectono-sedimentary evolution of a basin refers to its past since its establishment. This reconstitution is done mainly inside sequences which make it possible to constrain the different phases of the filling. With the concepts of sequence stratigraphy, Iboum et al. (2016) studied the Kribi-Campo sub-basin in its southern part and identified sequences from Albian to Recent. Albian to Cenomanian sequence is characterized by a retrogradation overlying a lowstand progradational pattern. The Campanian-Maastrichtian sequences were deposited during a highstand normal regression. From Paleocene to Eocene, the deposition of sequences was controlled by the development of submarine fan turbiditic system related to a forced regression of coastline. From the Middle Miocene to Recent age, sequences have been characterized by the development of sigmoidal-oblique clinoforms of a deltaic system. The forced regression phases are associated with the Paleogene and Neogene uplift. The processes involved in the formation of these sequences were interpreted as a combination of tectonics, sediment supply, and sea-level changes. That study highlighted the architecture of the deposits but didn’t give enough information on the timing of the setting up of de Kribi-Campo sub-basin, according to many models of the opening of the south atlantic ocean: Reyre (1966), Rabinowitz and La Brecque (1979), Reyre (1984), Nürnberg and Müller (1991), Pauken et al., (1991), Pauken (1992) and Ye (2016). In the other hand, the studying of the southern part cannot totally prohibit the one of the northern part since there are so many differences observed in term of petroleum productivity (the northern part is more explored than the southern). Added to that matter, Helm (2009) stated that the difference in the timing of the events can reach up to 20Ma inside the same basin. This paper is a part of the tectonosedimentary evolution study of the northern part of the Kribi-Campo sub-basin; its general objective is to highlight the different phases of the tectonosedimentary evolution of the of Kribi-Campo sub-basin, highlighting the distribution of the depocenters and the factors that have influenced the sedimentary filling. Thus, the geometry of the deposits, the evolution and the distribution of the depocenters, the control of various factors such as tectonics, eustatism, sedimentary supply on the setting up of the deposits will be studied inside the sequences. For this, the following tasks will be accomplished:

• Tthe description and mapping of seismic facies; • The identification of sequences and their interpretation in relation to the architecture of the

sequences and the distribution of the depocenters;

260 Jeannette Ngo Elogan Ntem, Marie Joseph Ntamak-Nida, Dieudonné Bisso, Francois Mvondo Owono, Simon Ngos III, Paul Bilong and Philippe Njandjock Nouck

• The geodynamic interpretation of the stratigraphic surfaces to constrain the control factors on the location of the sequences;

• and finally, the recognition of the depositional sequences. 2. Materials and Methods 2.1 Geological Settings

Kribi-Campo sub-basin is the southern part of the Douala/Kribi-Campo basin on the Cameroon coastal margin. It is located between 2°10′ and 3°20′N then 9° and 10°30′E (Fig. 1.). it covers a total area of 6150 km2. The KFZ major fracture zone delimits the Kribi-Campo sub-basin to the North and to the West. It is limited to the South to the Rio Muni Basin by the Campo High and to the East by the Precambrian basement.

The Kribi-Campo sub-basin, from Aptian age, is part of West African margin basins and its evolution is linked to the opening of the South Atlantic Ocean that occurred from the Late Jurassic to the Lower Cretaceous (Rabinowitz & Labrecque., 1979, Iboum et al, 2012). Its evolution is known as follow:

• The syn-rift sequence (Barremian–Aptian), which appears to be controlled by listric faulting and associated roll-over anticlines (Lawrence et al., 2002). The syn-rift phase was responsible for a fracture pattern closely controlled by the inherited structures of the Precambrian basement (Benkhelil et al., 2002; Ntamak-Nida et al, 2010).

• The rift–drift transition phase (mid-late Aptian) was marked by salt deposition and the transform directions resulting in a series of cross-faults which have segmented the rift structure (Benkhelil et al., 2002; Ntamak-Nida et al, 2008).

• The post-rift phase (Albian – Present) comprises three stages of drift where the passive margin wedge was deposited and marked by a short compressive episode localised along the transfer faults (Benkhelil et al., 2002, Ntamak et al, 2010).

Seven lithostratigraphic formations are known for that basin: Mundeck ofAptian to late Albian (Nguene et al., 1992; Loule et al., 1997; Batupe and Abolo, 1997), Logbadjeck (Cenomanian to Campanian), Logbaba (Campanian to Maastrichtian), N’kapa (Paleocene-Eocene), Souellaba (Oligocene to Early Miocene), Matanda (Miocene), and the Wourri (Post Miocene) Formations (Nguene et al., 1992, Ntamak-Nida et al, 2010).

Figure1: Location map of the Kribi-Campo sub-basin

Depocenters Repartition and Sequence Stratigraphy of the Northern Part of the Kribi-Campo Sub-Basin (Cameroon) 261 2.2 Seismic and Wells Data

Seismic reflection is based on the physical parameters of the rocks. With each variation of acoustic impedance and lithology, there is a discontinuity and this discontinuity provokes the reflection of the seismic wave. Porosity, density, fluid content and cementation are parameters that determine the amplitude and polarity of the reflectors. The seismic profiles allow the recognition of discontinuities, the delineation of stratigraphic sequences and the mapping of seismic facies. As part of this work, 32 Dip and Strike 2D seismic lines of varying quality were used. These lines cross the Onshore and the Offshore of the Cameroon margin through the northern part of the Kribi-Campo sub-basin, from 30N to 3020’N. Eight wells in the form of synthetic films pass through these lines and provide biostratigraphic data sheets (micropaleontology, palynology) whose ages will allow calibration with the seismic sections. The different lithostratigraphic formations known in this basin are given on these synthetic films, with their age and lithological content. 2.3. Sequence Stratigraphy

Sequence stratigraphy (Figure 2) is an analytical approach to the study of rock succession. It can be described as a branch of stratigraphy that studies the subdivision of sedimentary successions into chronostratigraphically-valued genetic units delineated by non-deposition or erosional surfaces (Helland-Hansen, 2009). As part of this study, the analysis will be carried out essentially in three steps: (1) the identification of seismic facies (three-dimensional mappable units composed of seismic reflectors whose parameters differ from those of adjacent groups (Mitchum et al., 1977)), (2) the point of seismic discontinuities to characterize the spatio-temporal evolution of the deposition profile and leading to the identification of the different stratigraphic units that are stratigraphic sequences and surfaces (stratigraphic discontinuities result from the interaction between the sedimentary supply and the available space –accommodation– which is a function of tectonics and eustatism); and finally (3) chronostratigraphic timing.

The fundamental unit of sequence stratigraphy is the depositional sequence (Van Wagoner et al., 1988). A seismic sequence or depositional sequence is a conformal set of genetically linked strata framed by two identical discontinuities. It is the result of a complete cycle of relative sea level variation marked by two fall episodes and called genetic sequence. The sedimentary record can be classified into five orders of nested sequences, in relation to a relative sea level signal (Vail et al., 1977, 1991, Haq et al., 1987, Guillocheau, 1995, Miall, 1997): 1st Order (of duration greater than 50 Ma), 2nd order (of duration between 3 Ma and 50 Ma), 3rd order or filing sequence (of duration 500 Ka to 3 Ma), 4th order (from 80 Ka to 500 Ka) and 5th order (less than 80 Ka). The geometry of sedimentary bodies is influenced by three factors: eustatism, sedimentary supply and tectonics. Eustatism and tectonics constitute the available space or accomodation needed to trap sediments. Depending on the predominance of one or other of these factors (accommodation and sediment supply), the geometry may be aggradant, progradant or retrogradant (Figure 2) (Homewood et al., 2000).


Figure 2: Arrangement of the system tracks of deposit. A variation of the A/S ratio induces progradation, aggradation or retrogradation of the deposition profile. The case of forced regression is independent of sediment flux variations because it can only be explained by a decrease in available space (A<0) (Mvondo, 2010 derived from Homewood et al., 2000)

2.4. Petrel 2014

Software makes it possible to circumvent the difficulties that manual interpretation faces. In addition to the time it takes, it is difficult to manually follow a reflector; and this difficulty increases when the lines are of approximate quality. While 3D lines can manually track a 3D mismatch, this is not the case for 2D lines. For the later, only the software can make it possible to visualize the geological objects in 3D. The use of software in stratigraphic analysis has given room to a new discipline: the computational seismic stratigraphy (Wolak et al., 2013). The Petrel seismic visualization and interpretation software created by Schlumberger is a revolutionary software in the domain of petroleum research, which makes it possible to better analyze geophysical data (seismic lines) and to model the structural elements (faults and domes) and stratigraphic (surfaces and facies). This tool enabled us to correlate the major unconformities and the faults on all the lines, to make the isopach maps (which make it possible to constrain the distribution of the depocenters), the isochronous maps (which make it possible to constrain the control factors) and the seismic attributes (Root Mean Square (RMS) amplitude, Relative acoustic impedance, Time gain, Azevedo and Pereira, 2009) which enabled to highlight major unconformities such as Oligocene discordance as dating data did not allow us to locate exactly.

Depocenters Repartition and Sequence Stratigraphy of the Northern Part of the Kribi-Campo Sub-Basin (Cameroon) 263 3. Results and Discussion 3.1. Seismic Facies Units

Seismic facies analysis is the description and geological interpretation of the parameters in the way of illustrating a deposition environment. This description considers the internal configuration and parameters such as amplitude, continuity, and frequency. The results are presented in table 1. One can observe cases of facies superimpositions, and especially for the facies in mounds and the channeling facies which have another configuration on the strike lines. In total, 17 seismofacies (SF) are identified: 2 non-coherent, 3 of parallel configuration, 5 of channel fill configuration, 3 in mound and 4 in progradation configuration. This interpretation is based on the Brown and Fisher (1980) classification, modified from Vail et al. (1977). 3.2 Stratigraphic Sequences: Identification, Limits and Control Factors

3.2.1 Stratigraphic Megasequences These are two first-order sequences in relation to low frequency and non-periodic first order cycles (megacycles) (Miall, 1977; Vail et al., 1977; Einsele, 1992; Guillocheau,1995; Miall, 1997): (1) the basal Aggradational megasequence which includes the Aptian to middle Eocene series, for a duration of about 70 to 85 Ma; it is characterized by very high amplitude seismic reflectors, strongly folded towards the base and channeling towards the summit; this configuration reflects an aggradation sequence that characterizes a deposition system with progressive deepening, in the absence of a differentiated slope (Vail et al., 1977; Brown and Fisher, 1980; Mougamba, 1999). (2) the progradational megasequence that extends from the Eocene to the Present (about 55 Ma duration) as shown in Figure 3; it is characterized by an evolution of the stratigraphic pattern with the introduction of prisms that have prograded towards the open sea since the Eocene era. Table 1: Seismic facies of the northern part of the Kribi-Campo sub-basin: a) amplitude, b) frequency, c)

continuity

Denomi

nation

Configuration of

reflections Caracteristics of reflections

Boundaries: Upper

boundary Lower boundary

Seismic

facies

SF10 Parallel a) high to moderate, b) low to moderate, c) high

concordant

SF11

Chaotic Facies found in sigmoid-progradation facies F7 et F8.

discontinious

SF9 Oblique progradation a) high to low, b) high to low c) high continuity to discontinious

Ub: Actual (toplap) Lb: Pleistocene (downlap)

SF8 Oblique progradation a) high to low, b) high to low, c) high continuity to discontinious

Ub: Pleistocene (toplap) Lb: Pliocene (downlap)

SF7 Sigmoid progradation a) low, b) high, c) semi-continious

Ub: Pliocene (concordant) Lb: Miocene (downlap)


SF6 Sigmoid progradation a) low, b) high, c) semi-continious

Ub: Miocene (concordant), Lib: Eocene (downlap)

SF5-3 Mounded a) low to moderate, b) high to moderate c) semi-continious

Ub: Eocene (erosive, onlapped and baselapped) Lb: late-Albian (onlap, concordant)

SF5-2 Mounded a) low, b) high c) semi-continious to disrupted

Ub: Eocene (concordant) Lb: (onlap)

SF5-1 Mounded a) high, b) low c) continious to high continuity

concordant Ub: Eocene, Lb: late-Albian

SF4-5 Parallel divergent, of incised valleys fill

a) low, b) high, c) semi-continious

Ub: Miocne, (onlapped); Lb: Oligocene (baselapped)

SF4-4

Parallel divergent, onlap fill, incised valleys or depositional. chenal

a) low to moderate, b) high to moderate, c) continious

Ub: Oligocene (baselapped), Lb: Eocene (onlap)

SF4-3

Parallel divergent, mounded fill,, distal chanel-levees

a) high to low b) high to low c) semi-continious

Ub: Eocene, (baselapped) Lb: Late-Albian, (onlaps)

SF4-2 Parallel divergent, erosional chenal

a) high, b) low, c) high continious to disrupted

Ub: Late-Albian, (baselapped) Lb: onlap

SF4-1 Parallel divergent, chaotique fill.

a) high, b) low c) continious Ub: concordant ; Lb: onlap

SF3 Subparallel oblique a) high, b) low, c) high continuity

Ub: Late-Albian (coastal onlapped) Lb: concordant

SF2 Subparallel a) variable, low to high, b) low to moderate, c) continious

Ub: onlapped by coastal facies Lb: concordant (basement)

SF1 Chaotique (basement) discontinious Basement

The seismic facies represented by sigmoidal continuous clinoforms of average amplitude

upstream and weak amplitude and continuity reflectors downstream reflect a platform environment. These megasequences reflect different types of basin filling associated with two stages of geodynamic evolution. Thus, the megasequence of aggradation marks a phase of subsidence while megasequence of progradation reflects the progradation of the margin.

Depocenters Repartition and Sequence Stratigraphy of the Northern Part of the Kribi-Campo Sub-Basin (Cameroon) 265 Figure 3: Seismic line showing the megasequences and the sequences: S is for sequences, vertical coloured

lines are faults

3.2.2. Stratigraphic Sequences and their Control Factors Six stratigraphic sequences delineated by regional discontinuity were recognized throughout the study area. From those sequences, five are of 2nd order, and only the most uppermost sequence is of 3rd order. To specify the geometry and the modality of the migration of the depocenters within the sequences, the isopach maps were constructed in double time (ms). The discontinuities generate significant reflectors: they are horizons characterized by a good continuity, a strong amplitude and a regional extension; the association of isochron maps and the charter of Haq et al. (1987) made it possible to constrain the control factors at these horizons. 3.2.2.1. Sequence I: Aggradation Phase (Aptian to Upper Albian) It is an essentially aggradation sequence, made of syn-rift deposits in contact with the basement, affected by normal NW to NNW faults. This sequence corresponds to the basal sandstone (Nodesa, 1971; Nguene et al., 1992) or Mundeck Formation (Abbot et al., 1978; Nguene et al, 1992). It can be subdivided into two units, equivalent to different geometries. The basal unit I1, in contact with the basement, has a thickness which increases towards the sea, which suggests an initial homoclinic ramp geometry and the major shift of the depocenters seaward. The unit I2 is folded at its base and channeling at the top, surmounted by the discordance Late-Albian (break-up unconformity, 107-100 Ma). In the absence of the basal limit of this sequence, no map could be designed. These units correspond to seismofacies, SF2 and SF3 with a parallel configuration on the one hand, and the seismofacies SF4-1 and SF4-2 with a channelization configuration on the other hand, which translate a structural depression into uniform subsidence, slow to fast (Vail et al., 1977; Brown and Fisher, 1980). This subsidence is at the origin of the flooding of the margin. 3.2.2.2. Sequence II: beginning of Destabilization of the Margin (Santonian to Maastrichtian) It is an aggradation and turbiditic sequence with many channels of variable amplitude that rely on break-up unconformity. It marks a notable geodynamic change resulting from an inversion of the


tectonic regime materialized by the seismofacies with channel fill configuration (SF4-3) and the mounded configuration (SF5-1, SF5-2 and SF5-3) which translate deposits affected by the gravitational deformation. Indeed, in Santonian, the margins of the equatorial Atlantic pass from an extensive regime to which they were subjected since the Rifting, to a compressive intake. The resulting wrinkling causes Uplift in these basins (Ye, 2016) causing massive erosion at the origin of the Senonian (Campanian) discordance. The isopach map of sequence II covers all Upper Cretaceous confined at its base by the Logbadjeck Formation of Cenomanian to Campanian age and at its summit by the Campanian-Maestrichtian age Logbaba Formation (Nguene et al., 1992).

According to data from ECL (2001), the regional Senonian unconformity is between the Logbadjeck and Logbaba formations. The isopach map shows values ranging from 0 in the East on the continent to 2200 ms, maximum thickening which is observed towards the NNW of the basin. The basin is characterized by the formation of synclinal structures and anticlinal structures inducing a spatial distribution in high zones (H1 and H2) and in low zones (G1, G2 and G3) in the North. This folding is known in the margins of the equatorial Atlantic and occurs at the Maastrichtian, following the Santonian event (Ye, 2016). A high structure in the east deepens progressively towards the NW occuring small basins G1 and G2, and reaches its maximum at the level of the depocenter G3. The accumulation center (G3) is at the NNW of the map and is in the form of a graben bounded by two faults. Figure 4 below shows the isopach map of this sequence which is based on Break-up unconformity.

Figure 4: Isopach map of Sequence II

The isochronous map shows values ranging from -900 ms upstream to -6300 ms downstream, a difference of 5400 ms to the southwest. These two extreme values correspond respectively to a high point or horst (H1) and a low point or graben (G1). The isobath curves have a preferred NW-SE direction and have a folded shape in the central part (Figure 5). Negative values assigned to all contour lines indicate the total immersion of the margin. Two major faults affect this surface in its upstream part: the F1 fault oriented E–W and the fault F2 oriented NW–SE. The contour lines have a preferential

Depocenters Repartition and Sequence Stratigraphy of the Northern Part of the Kribi-Campo Sub-Basin (Cameroon) 267 direction NW–SE and have a folded shape in the central part. According to Haq et al. (1987), this corresponds to an eustatic rise of high amplitude. Indeed, this surface corresponds to a very important cooling episode in the thermal history of the whole region. After the end of the South Atlantic Rifting, an early regional uplift of the basement and (continental) basins is observed in Onshore and surrounding areas, while in the basins towards the west a continuous subsidence (with sedimentation) is observed in the lower Cretaceous to Middle. This scenario could explain the Middle Cretaceous (Albian-Cenomanian) cooling episode recognized by ECL (2001) through the Apatite Fission Trask Analysis (AFTA) data and the absence of regional unconformity in offshore basins (ECL, 2001). This episode appears to have been due mainly to warm currents during the rifting and transition phases (ECL, 2001).

Figure 5: Isochronous Map of the Late-Albian unconformity

3.2.2.3. Sequence III: Phase of General Destabilization of the Margin (Eocene-Oligocene) It is a turbiditic-progradant sequence of Paleogene age. It marks a general change in the geodynamic regime with the transition from the aggradational megasequence to the progradational megasequence. The seismic sections make it possible to specify this change in the sedimentation regime. Within this sequence, we observe a progradation component (SF6) that is individualized above the channels, resulting in basin and swell topography (Ye, 2016). This sequence is based on intra-Eocene erosive discordance marked upstream by canyons of variable amplitude. Two units stand out in this sequence: the Eocene unit III1, represented by the SF4-4 canyons filling seismofacies and the Oligocene age unit III2, represented by the SF4-5 seismofacies materializing the filling of the incised valleys. The two units are separated by Oligocene discordance, probably Rupelian (Wornardt et al., 1999), which is a major feature of the geology of West African margins (Helm, 2009). These units represent respectively the Paleocene to Eocene Nkapa and the Oligocene to Early Miocene Souellaba Formations (Nguene et al., 1992). The isopach map of the sequence shows values ranging from -200 to 1600 ms. The major shift in the distribution area of the depocenters is towards the continent, with a change of morphology of these depocenters which take an arched form (figure 6). This displacement is along a collapsed zone or graben structured by faults and resulting in a depocenter (G1) on the continent; this denotes a structural control over the displacement and distribution of the depocenters. Seismofacies with a


sigmoidal prograding configuration represent a low sedimentation rate compared to the creation of available space (eustatism, subsidence) (Vail et al., 1977, Brown and Fisher, 1980).

Figure 6: Isopach map of sequence III

This sequence takes place in a context of uplift linked to the beginning of volcanism on the African plate at the end of the Eocene (with the establishment of the Cameroon Volcanic Line) and the erection of Mount Cameroon from the Oligocene. The uplift of the Western part of the African continent to the base of the Oligocene has already been considered by Mvondo (2010) to explain the genesis of the Ondé canyon (Regnoult, 1986; Manga, 2008) in the Douala sub-basin. This sequence could mark the beginning of the construction of a deltaic system on a relatively weak and unstable slope. The base of Sequence III corresponds to an erosive discordance dated from the Eocene (55.8 -34Ma). It is a sequence boundary materialized by erosion truncations, onlap and downlap terminations on dip lines. Channels are observed on the strike lines. The isochronous map of the intra-Eocene surface shows a south-facing surface with values ranging from -250 ms to -4500 ms, a depth of 4250 ms towards the SW. Like the previous surface, the isobath curves have a NW–SE direction and have a folded shape in the central part (figure 7).

This folding of the isobaths reflects a slight bulging and sagging. Such an uplift can be caused by the presence of a dome. The high (H1) and low (G1) areas on either side of the basin are separated by a steep slope on the side of the continent that becomes increasingly soft towards the seabed. The two major faults F1 and F2 affect this surface. The isobath curves indicate a total immersion of the margin, but with a relative sea level lower than that of the previous regime. This drop of sea level is confirmed by the charter of Haq et al. (1987) which reports a low amplitude eustatic decline.


Figure 7: Isochronous Map of Eocene Unconformity

The Eocene unconformity could correspond to Upper-Eocene emergence and cooling from Bartonian (40.4-37.2 Ma) (Rougier et al., 2013, English et al., 2016, Ye 2016). Indeed, the plateau has expanded and occupies a larger area. One can even distinguish an internal plateau (always reduced) of the external plateau. The platform occupies a smaller area than on the previous surface. 3.2.2.4. Sequence IV: beginning of the Margin Shift (Miocene: Aquitanian-Zanclean) This sequence is delimited at its base by the base-Miocene (Aquitanian) regional discordance and at its summit by the regional base-Pliocene (Zanclean) regional discordance. It is a progradation-aggradation sequence materialized by seismofacies SF7, of progradant sigmoid configuration, that reflects a low rate of sedimentation under a weak creation of available space. This sequence represents the Formations of Souellaba (Oligocene-Lower Miocene) and Matanda (Miocene, Nguene et al, 1992) and is subdivided into three units (IV1, IV2 and IV3) corresponding to 2nd-order sequences Lower Miocene, Middle Miocene and Upper Miocene extension respectively. These three units are delineated by regional erosive unconformities. Unit IV1 is separated from Unit IV2 by the Late-Burdigalian discordance which in turn is separated from Unit IV3 by Tortonian unconformity. The isopach map covering the entire Miocene series shows a spatial distribution that has been reversed compared to the previous map (Figure 8). The isovalues range from -1150 to 250 ms and correspond respectively to a horst and a graben, wholly located on the continent. The tectonic activity gave rise to horsts (H1, H2 and H3) on the west side and grabens (G1, G2 and G3) to the East. From North to West, this sequence is characterized by a succession of synclinal and anticlinical structures segmented by faults in horsts and grabens. The basin has moved to the east side with the establishment of the deepening zones (G2 and G3) and a depocenter (G1) on the mainland which appears as a preferential input area and / or greater subsidence. This progradation / aggradation phase leads to the fattening of the observable margin on the isopach map with the displacement of the depocenter on the continent and a variation of slope which makes it possible to foresee a traditional continent-slope-basin configuration. This variation of slope is visible on the seismic lines with reflectors which are more straightened on this sequence than on the preceding sequence and less steep than on the overlying sequence. This provision reflects a progressively faster progradation that can be interpreted as a tipping of the margin. This shift was observed by Mvondo (2010) on the northern part of the Cameroonian margin at the sub-basin of Douala between the beginning and the end of the Miocene.


Figure 8: Isopach map of Sequence IV

The lower limit of sequence IV is an erosive unconformity (Aquitanian: 23.03-20.5Ma). The isochron map of this surface has values ranging from 750 ms above sea to -3750 ms below, a difference of 4500 ms. this map highlights an elevated area in the west (H1) and a deep zone in the basin (G1) (Figure 9). The isobath curves are preferentially oriented NW–SE and have a folded shape in the center of the map. The spacing of these curves varies from continent to sea. Moderately tight upstream, they are very tight at the slope and very loose at the basin. This already denotes the classical structure of a margin and allows delimiting its three main parts. A reactivation of the F1 fault is manifested by the appearance of the F3 fault of the same direction but of greater magnitude than F1. The emergence of the margin continues and reaches an altitude of 600 ms above the sea, which could be explained by an uplift. An accentuation of the fracturing is an element in favor of a tectonic cause. On the curve of Haq et al. (1987), this period corresponds to a low amplitude eustatic decline. This unconformity coincides with the cooling episode of the Middle Tertiary and is interpreted as generated by an uplift followed by erosion (ECL, 2001). Indeed, the Miocene marks a period of margin uplift (Mvondo, 2010) corresponding to the cooling episode of the Middle Tertiary observed by ECL (2001) from the Apatite Fission Traces Analysis (AFTA) data; which episode would have generated two erosive discordances at Lower Miocene and Upper Miocene.


Figure 9: Isochronous map of the Lower Miocene unconformity

3.2.2.5. Sequence V: Progradation-Aggradation Phase (Pliocene) Sequence V is delimited at the bottom by the Pliocene base (Zanclean) discordance and at the top by the Pleistocene base discordance. On the isopach map (Figure 10), there are two important zones: one at the SW in the downstream part and the other at the NE in the upstream part. The thicknesses vary between 0 and 1900 ms. On the downstream side, a high zone (H1) was set up and progressively deepened to lead to two depocenters (G1 and G2) in the upstream part. A migration of the depocenters towards the NW with a modification of morphology of these arch-shaped depocenters is observed. Indeed, the previous accumulation zone (G1) moves towards the NW and extends over the entire plateau, giving rise to a second central deposit G2. At this larger lateral extension corresponds an increase in thickness of deposits, which ranges from 250 to 1900 ms. The SF8 seismofacies has an oblique prograding prism configuration that means its location on a very unstable slope (Vail et al., 1977, Brown and Fisher, 1980). This configuration shows a faster progradation compared to the underlying unit, which might suggest the continuity of the margin shift. The shift of the margin to the Pliocene was observed on the northern margin of Gabon by Mougamba (1999). The Zanclean erosional discordance, on which this sequence is based, corresponds to a sequence boundary recognizable by erosion truncations and downlaps terminations. The isochron map shows values ranging from 0 to -4250 ms, respectively at the two upstream and downstream ends of the basin, corresponding to a horst (H1) and a graben (G1) (Figure 11).


Figure 10: Isopach map of Sequence V

The curves of the isobaths are oriented NW-SE but in the center, they have a flexural shape. The spacing of these curves becomes greater at the plateau, remains unchanged at the upper platform while lower on the slope. They are less tight and less loose at the level of the lowland. This surface is affected by major faults F1, F2 and F3 and coincides with the Upper Tertiary cooling episode marked by uplift followed by erosion (ECL, 2001). However, the isochron map shows a rise in relative sea level compared to the previous map, which is confirmed by the Haq et al. (1987) curve, which indicates a high amplitude eustatic rise. According to Cloetingh (1988), erosive discordances may result from the vertical variations of the lithosphere in relation to the intra-plate constraints generated by the kinematics of the plates. In a compressive context, the margin would undergo a flexuration which would result in an uplift of the upstream domain and a deepening of the downstream domain (Mougamba, 1999). The prograding sigmoid clinoforms that surmount this surface may reflect a superior sediment input rate compared to the speed of creation of available space. Such a surface could correspond to an excess of sedimentary supply leading to a significant regression and development of a surface of downlap (Mougamba, 1999). In this context, the unit that surpasses this area is equivalent to a progradation-aggradation system (Homewood et al., 2000) or the normal high-level regression system (Catuneanu, 2006).

Figure 11: Isochronous map of the Lower Pliocene unconformity (Zanclean)

Depocenters Repartition and Sequence Stratigraphy of the Northern Part of the Kribi-Campo Sub-Basin (Cameroon) 273 3.2.2.5. Sequence VI: Progradation-Aggradation Phase (Pleistocene-Present) This is the most uppermost sequence delineated by the Lower Pleistocene discordance and the current discordance. It is represented upstream by SF10 seismofacies of subhorizontal parallel configuration. On the platform edge, it is represented by the SF9 seismofacies with a prograding oblique configuration. On the isopach map, the thicknesses vary between 0 and 2100 ms. Two anticlines (H1 and H2) are separated by a syncline (G1) in the center of the map which continues its deepening in the downstream part and gives rise to a very deep basin (G2) in the East where the curves of the same depth are very spaced apart (Figure 12). This extension of the accumulation zone shows a migration of the depocenters towards the basin and is accompanied by an increase in thicknesses that range from 1900 ms on the previous map to 2100 ms. SF79 seismofacies has clinoforms with the steepest slope of all sequences and represents deposits on a very unstable slope (Vail et al., 1977, Brown and Fisher, 1980). Thus, the Lower Miocene failover continues at the level of this sequence. The same is true of the uplift, AFTA data by ECL (2001) shows a cooling episode in the Upper Tertiary that has generated recent erosive discordance. The isochrone map of the Lower Pleistocene erosional discordance (1.8 Ma) shows values ranging from 200 ms to -3400 ms, a difference of 3600 ms. It shows a horst emerged in the upstream part and a basin in the downstream part, separated by an area affected by the major faults F1 and F2 inherited since the Cretaceous.

Figure 12: Isopach map of Sequence VI

The isobaths have the same NW–SE direction as on the previous maps with this slight wrinkling in the center (Figure 13). This map indicates a decrease in the relative sea level with an emersion of the upstream parts; which exhumation can be explained by the Upper Tertiary cooling episode caused by a rise followed by erosion and materialized by recent erosive discordance (ECL, 2001). On the charter of Haq et al. (1987), it corresponds to an eustatic decrease of high amplitude.


Figure 13: Isochronous map of the Pleistocene unconformity

3.2 Sequences and Systems Tracks

Of all the stratigraphic sequences marking the evolution of the Kribi-Campo sub-basin, only the last sequence, from the Pleistocene to the Present, is of third order. At the edge of the platform, a deposition sequence consists of a set of three systems tracks delineated by stratigraphic surfaces: the Lowstand System Track (LST), the Transgressive System Track (TST) and the Highstand System Track (HST), (Homewood et al., 2000). The resolution of our data did not allow the recognition and the mapping of the trangressive surfaces and maximum flooding surfaces. Then, it is impossible for us to delimit these systems tracks. Only lowstand systems tracks, which rely on sequence boundaries, can be identified. Indeed, the configuration of the sequences in the megasequence of progradation makes it possible to register at least seven lowstand systems tracks: the unit overlying the Eocene discordance, made up of basin floor fan and the unit overlying the Oligocene unconformity filling the incised valleys (slope fan). In the Miocene, the units overlying regional discontinuities Aquitanian, late Burdigalian and Tortonian constitute the Lowstand System tracks. Idem, units overlying Pliocene and Pleistocene unconformities. Without the duration of the erosion hiatuses, we are unable to state the system tracks that surmount the lowstand system track and are below the unconformities.

In addition, Wornardt et al. (1999) enumerate five Paleogene sequences of second order, delimited by the discontinuities of the Thanetian (58.5 Ma), Ypresian (53.0 and 49.5 Ma), Bartonian (39.5 Ma), Priabonian (36.0 Ma) and Rupelian (30 Ma). In the Neogene, these same authors identify eight sequences, three of second order and five sequences of deposits delimited by the following discontinuities: Aquitanian (21 Ma), Burdigalian (16.5 Ma), Langhian (15.5 and 13.8 Ma), Tortonian (10.5 Ma, 8.8 Ma and 8.2 Ma), Messinian (6.3 Ma) and Zanclean (5.0 Ma). A total of thirteen sequences with at least as many lowstand systems tracks. Iboum et al. (2016) identify five stratigraphic sequences in the Tertiary, including three second order sequences and two sequences of deposits. From the Zanclean base (5.32-5.0 Ma) to the Pliocene summit (2 Ma), he identifies three depositional sequences of equivalent to lowstand system track that correspond to forced regression deposits (Catuneanu, 2006). Beyond the Pliocene, he identifies three sequences that could be of third-order or lower-order, corresponding to highstand systems tracks that are high-level normal regression deposits (Catuneanu, 2006). In all, considering the work of Wornardt et al. (1999) and Iboum et al. (2016), we can identify eight sequences of Neogene deposits and three Quaternary sequences, which could be either depositional sequences or sequences of a lower order.


The age that was provided by wells data does not give information for the periods going from Cenomanian to Santonian and the one from Paleocene to Mid-Eocene. These two periods can be considered as hiatuses of erosion in this sub-basin and correspond to two episodes of inversion of the geodynamic regime in the evolution of the Kribi-Campo sub-basin. The first hiatus is linked to the santonian inversion from the extensive to the compressive regime, followed by an uplift and an intense erosion of the margin. According to Ye (2016), this hiatus can be estimated between 400 m and 500 m along the West African margins. Seismic lines show, overlying the late Albian unconformity, some structures of transgression, compression and the senonian unconformity. The second hiatus of erosion (Paleocene to Mid-Eocene) correspond to a geodynamic inversion period from the aggradational regime to the irreversible progradational one. That inversion marks the beginning of the volcanism (Cameroon line) at the Upper Eocene and the rise of Mount Cameroon in the lower Oligocene. This event has provoked the uplift of the margin followed by erosion. On the seismic lines, one can observe some deposits affected by gravitational sliding. The overlain surface can be attributed to the uncompacted continental clays resulting from the intense weathering and the transgression during the PETM (Paleocene-Eocene Maximum Thermal) that occurred at the boundary between the Paleocene and Eocene (Jardine, 2011). Conclusion Stratigraphic analysis of the margin at the Kribi-Campo sub-basin enabled the identification and mapping of seventeen seismofacies of various configurations that contributed to identify three orders of sequences. Two megasequences (first-order), one of aggradation ranging from Aptian to Eocene and corresponding to the subsidence phase of the margin and one of progradation from the Eocene to the Present reflecting progradation and tipping of the margin. Five second-order and one of 3rd order sequences corresponding to different stages of the basin filling: Sequence I (Aptian to Upper Albian) presents an aggradation architecture and corresponds to the flooding of the margin due to a uniform subsidence, slow to fast; Sequence II (Santonian-Maestrichtian), of aggradation geometry, initiates the destabilization of the margin by the compressive Santonian event at the origin of the folding, causing inside the basins an uprising of the reliefs; Sequence III (Eocene-Oligocene), of prograding architecture, marks a change of the geodynamic regime with the beginning of the progradation which brings about a general destabilization of the margin, resulting in basin and swell topography associated with the beginning of volcanism on the African plate (Eocene) and the erection of Mount Cameroon on the Oligocene; Sequences IV (Aquitanian-Zanclean), V (Pliocene) and VI (Pleistocene to Present), with progradant-aggradant geometry mark different phases of the tilting of the margin and its uplift. Finally, depositional sequences (eleven in total), lowstand systems tracks (basin floor fan, slope fan) and highstand systems tracks are highlighted. From Aptian to Upper Albian the margin has a thickness which increases towards the sea, suggesting an initial homoclinic ramp geometry and the major shift of the depocenters seaward. In the Upper Cretaceous (Santonian to Maastrichtian), the accumulation center is at the NNW and is in the form of a graben bounded by two faults. From Eocene to Oligocene, the major shift in the distribution area of the depocenters is towards the continent with a change of morphology of these depocenters which take an arched form. This displacement is along a collapse zone structured by faults and resulting in a depocenter on the continent. At Miocene age, the depocenters are wholly located on the continent, causing the fattening of the margin. From the Pliocene, the migration of the depocenters is towards the NW with a modification of morphology, the extension over the entire plateau and an increase in thickness of deposits. The Recent age is marked by the extension of the accumulation zone and the migration of the depocenters towards the basin, accompanied by an increase in thicknesses. Two hiatuses of erosion, linked to the inversion of the geodynamic evolution of the margin, are observed from Cenomanian to Santonian and from Paleocene to Mid-Eocene. The margin is segmented by E-W and NW-SE faults. In addition, this analysis has


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