Syllabus Review Volume 2, Issue 3 (2011) 92-105

Syllabus Review 2 (3), 2011: 92 - 105 N SYLLABUS REVIEW E S Science Series RESEARCH ARTICLE - GEOLOGY Soft-sediment deformation structures in Mid- Cretaceous to Mid-Tertiary deposits, Centre East of the Douala sub-basin, Cameroon: Preliminary results of the tectonic control. Adrien Lamire Djomeni, Marie Joseph Ntamak-Nida*, François Mvondo Owono, Paul Gustave Fowe Kwetche, Joseph Bertrand Iboum Kissaaka, Elise Mooh-Enougui. 1 Department of Earth Sciences, Faculty of Sciences, University of Douala, B.P. 24 157 Douala, Cameroun. *corresponding author : [email protected] /[email protected] Fax: +(237) 33 40 75 69 Received: 12 May 2011 / Revised: 11 October 2011 / Accepted: 1st November 2011  Ecole Normale Supérieure, Université de Yaoundé I, Cameroun Abstract This paper aims to examine soft-sediment deformation structures occurred in Mid- Cretaceous to Mid-Tertiary deposits of the Centre East of the Douala sub-basin so as to understand the nature and the role of tectonics on their genesis. The studied sections are located along the railway sections of Loungahe, Pitti and also the roadside-cut along the "Nationale 3" (Missole), where detailed descriptions of the silico-clastic sediments show deformational structures called soft-sediment deformation structures (SSDS). These outcrops have been assigned the age from Cenomanian-Turonian to Paleocene-Eocene according to the literature reviewed here. Six main deformed sedimentary units that lie in between units of undeformed sediments were described. The deformed units show the various SSDS such as: 1) contorted structures which include irregular convolutes, turned-up concave structures, giant load structures, box folds, small overlapping, thrust structures, and folds of low amplitude; 2) Faults, fractures and joints represent brittle structures. The tectonic mechanism of the basin (subsidence, uplift, active faults ...) and the seismic shocks are responsible for initiating mechanisms such as the cohesive flow, liquefaction and/or fluidization of unconsolidated sediments at the origin of these structures. The results suggest that tectonic have acted significantly on the area, mainly uplift and erosion that had occurred in the Douala sub-basin, the flexuring that affected the Douala sub-basin during the Turonian, the replay of the Sanaga Fracture Zone (SFZ) and probably the seismic shocks generated by the ongoing activity of the Cameroon volcanic line (CVL) since the Cretaceous. Keywords: Soft-sediment deformational structures, Mid-Cretaceous to Mid-Tertiary, tectonic events, Douala sub-basin. Résumé Cet article a pour objectif d’analyser les structures de déformation souple mises en place du Crétacé moyen au Tertiaire moyen dans la partie Centre Est du sous-bassin de Douala, afin de comprendre la nature et le rôle de la tectonique sur leur genèse. Les coupes étudiées sont situées sur les talus de la voie ferrée (localités de Pitti et Loungahe) et sur les talus de la Nationale 3 (localité de Missole) où les dépôts silico-clastiques datés Cénomanien-Turonien à Paléocene-Eocène présentent des structures de déformation souple (SSDS). Six principales unités sédimentaires déformées, encadrées par des sédiments non déformées ont été décrites. Les structures de déformation observées sont variées: 1) des structures contournées avec des convolutes irrégulières, des structures à concavité vers le haut, des structures de charge géantes, des plis en fourreau, de petites nappes de charriage, des structures chevauchantes et des plissements de faible amplitude; 2) Failles, fractures et joints appartiennent aux structures cassantes. Les mécanismes tectoniques d’un bassin (subsidence, surrection, activité des failles…) ainsi que les secousses sismiques sont en général précurseurs des mécanismes tels que les coulées cohésives, la liquéfaction et/ou la fluidisation de sédiments non-consolidés à l’origine de ces structures. Les résultats obtenus suggèrent qu'une activité tectonique a affecté de manière significative les dépôts de cette région et plus précisément, les épisodes de soulèvement et d'érosion produites dans le sous bassin de Douala, la flexuration ayant affectée le sous bassin pendant le Turonien, les rejeux de la fracture de la Sanaga sur la marge camerounaise et probablement les secousses sismiques générées par l’activité permanente de la ligne volcanique du Cameroun (LVC) depuis le Crétacé. Mots-clés: structures de déformation souple, Crétacé moyen - Tertiaire moyen, tectonique, sous bassin de Douala.  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 Introduction thrust structures, giant load structures, folding, One of the important factors controlling silico- irregular convolutes, turned-up concave clastic sedimentation is tectonic, which could be structures, fractures and joints. A common induced by uplift and/or subsidence of the factor used for the analysis of processes that source area. response to tectonic instability movement Recent observations in the central eastern part of includes deformed structures within their facies: the Douala sub-basin suggest that the impact of this factor was used in this study (Rossetti, 1999; tectonics should be re-examined. Many outcrops Jones and Omoto, 2000). With no previous of the Upper Cretaceous and Lower to Middle studies on this subject for the study area, the Tertiary deposits (Logbadjeck, Logbaba, and main goal of this paper is to provide detailed N’kapa Formations) of the Douala sub-basin, descriptions of SSDS and discuss about their present deformed structures called soft- genesis to better understand their origin in the sediment deformation structures (SSDS) of Van Douala sub-basin. A further objective is to Loon (2009). These structures have been formed provide an understanding of the nature and role due to deposits disturbances during or shortly of tectonics in the establishment of these after deposition when they are not yet deformations in this part of the African margin. consolidated. The terminology used for their description is complex due to the complexity of Geological setting their shape and the variety of processes and The Douala sub-basin belongs to a set of basin agents responsible of their genesis. Their located on West African margin which run from presence can be useful in reconstructing Angola to Cameroon. It is located 3°20 N to 5°N conditions that affected the depositional and 9° E to 10°30’ E (Fig. 1) and covers about environment but their value is limited by a 6950 km2 onshore. The study area is located in relatively poor understanding of the processes the central eastern part of the Douala sub-basin of formation of these structures (Jones, 1962; which is crossed by several fractured zones, Lindolm, 1987; Guiraud and Plaziat, 1993; including the Cameroon Fracture Zone (CFZ) Owen, 1996; Rossetti, 1999; Jones and Omoto, K., and the Sanaga Fracture Zone (SFZ). These latter 2000; Obi and Okogbue, 2004; Kotlia and Rawa, features might have been reactivated several 2004; Brustur and Jipa, 2007; Samaila et al., 2008; times in the past. Moreover, the Douala sub- Tasgin and Turkmen, 2009; Van Loon, 2009). basin is bounded to the W and NW by the According to Jones and Omoto (2000), the Cameroon Volcanic Line (CVL) which is following criteria can be used to identify a superimposed on the Central Africa Shear Zone seismic triggering event: the setting, the extent (CASZ) and represents its limit with the Rio- of the deformation units and the absence of Del-Rey basin; to the South by the Kribi Fracture evidence relating to other potential trigger Zone (KFZ) which delimits the Kribi-Campo mechanisms. For the same authors, sediment sub-basin. Eastward, the boundary is the composition; deformation structures restricted Precambrian basement. The tectonic history of to a single stratigraphic interval (< 1 m thick) are the Douala sub-basin is related to the history of not relevant to the trigger mechanisms but are the West African margin and begins with a important in interpreting the driving force and Lower Cretaceous (Aptian-Albian) rifting phase deformation mechanism. Guiraud and Plaziat followed by a period (Upper Cretaceous to (1993) studied the Bima sandstones (Nigeria) Lower Eocene) of post-rift thermal sag (ECL, and carried out deformations related to pore 2001) and the development of passive water expulsion. The mechanisms are continental margin basins (Douala/Kribi- hydroplastic deformation, liquefaction and Campo basin) and aulacogen basins (Benue fluidization. They proposed seismic waves as Trough, Benkhelil et al., 1998). At the same time, trigger mechanism required for sand there were two tectonic compressive events reorganisation. which may correspond with the two major In the Cameroon sedimentary basins, previous periods of regional heating and cooling in the works are unknown in literature on these Cretaceous and Tertiary recognized by ECL structures. Observable structures in the study (2001) and Lawrence et al. (2002). These two area include: small overlapping structures, periods can be further subdivided into Mid- and 93  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 Late-Cretaceous and Mid- and Late-Tertiary In this area, SSDS are observed into different paleothermal episodes respectively: Mid- stratigraphic units and were investigated Cretaceous episodes (105-95 Ma) may probably considering their relative ages as provided by be related to the Turonian flexures in the Douala previous publications (Belmonte, 1966; Dumort, sub-basin; Late Cretaceous (85 -75 Ma): Mid- 1968;; Njke Ngaha, 1984; Regnoult, 1986). The Tertiary (45 – 35 Ma); Late-Tertiary (15-2Ma). outcrops are located along the railway sections The Late Cretaceous and Tertiary episodes are (Pitti and Loungahe) and the “Nationale 3” interpreted as being due to burial followed by road-cuts at Missole. Synthetic logs (Figs. 3a and uplift and erosion whereas, the Mid-Cretaceous b) of the lithostratigraphy are presented episodes probably relates to the Turonian including the deformed zones. The SSDS occur flexure in the Douala sub-basin. This appears to entirely within facies that are made up of clay, have been mainly due to high heat flow during silt, fine-to coarse-grained sandstones the rift and transitional phases of basin sometimes with pebbles and matrix to clast- development (ECL, 2001). At the end of supported conglomerates. The deformed Cretaceous, an important uplift of the West deposits described are mainly composed of fine African margin occurred (Turner, 1995; 1999): it to coarse - grained sediments with quartz, little generated a major erosional surface commonly feldspars and muscovite that characterize their referred to as Senonian unconformity. During terrestrial source. The general color of the facies the Eocene, an uplift phase occurred (Lawrence is yellow to red, indicating ferruginisation. Some et al., 2002; Manga, 2008) and has been followed grayish siltstones and dark gray to black clay by an important gravitational instability of the facies can be observed at Missole. margin. The effects of these tectonics events had Outcrops of Loungahe are located along the a significant impact on the sedimentation Douala – Edea railway section, between Pk40 patterns in the Douala sub-basin. and Pk41 with about 13 m thick of sediments. The sedimentary deposits investigated in this These sediments are Cenomanian in age study area belong to the Late Cretaceous to according to Njike Ngaha (2004) and thus Tertiary corresponding to Logbadjeck Formation belong to the Logbadjeck Formation. The (Cenomanian-Campanian), Logbaba Formation deformed beds are dominated by an alternation (Maastrichtian) and N’kapa Formation of sub-parallel, continuous planar silt and clay (Paleocene-Eocene) (Fig. 2). This post-rift section laminae (0.3 to 1 cm thick). Frequent inter displays marginal clastic sedimentation with beddings of fine to coarse-grained sandstones sporadic carbonates shelf from shallow water are common in these sections where bed between Albian and Paleocene (Meyers et al., thickness range from 1 to 50 cm. Beds of 1996). According to Belmonte (1966) and granules and pebbles are poorly sorted with a Regnoult (1986), the N’kapa Formation is sandy matrix. Clasts are subrounded to rounded completely eroded in the East and South-East of with quartz and some feldspars. These beds are the Douala sub-basin and comprises clays, folded and massive. fossiliferous limestones, glauconitic marls, At Pitti, outcrops are also located along the arkosic and ferruginous sandstones in the Douala – Edea railway between Pk31 and Pk32 Center and West. and the section studied is about 12 m thick. A Paleocene-Eocene age had been assigned to Description of field sections The study area is located in the central eastern these sediments which therefore correspond to part of the Douala sub-basin where good the N’kapa Formation of Dumort (1968). outcrops are located along road-cuts or railway The sedimentary deposits studied at Missole sections in the localities of Loungahe, Pitti and outcrops at roadside-cuts along the "Nationale Missole (Fig. 1b). These outcrops have been 3" with a thickness of 15m and dips towards of affected by intensive weathering due to the 10° NNW. According to Dumort (1968), these climatic conditions characterized by an average deposits have been assigned an Eocene age and of 4000 mm/year of rain and 25°C of annual temperature. so belong to the top of N’kapa Formation. The deformed sediments are continuous along 94  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 Fig. 1. a: Location of Douala sub-basin in Cameroon (West Africa) between the main fractures of the Cameroon margin (compiled from Meyers et al. 1996, Rosendahl and Groshel-Becker 1999, Dailly et al. 2002, Wilson et al. 2003); b : Location of the studied outcrops of Loungahe, Pitti and Missole. several meters (500 m – 1 km) at the top of the them. These structures have never been section. The bed is massive and the upper mentioned in the survey of the Douala sub- surface is uneven. This section is characterized basin. The deformed structures of Loungahe by massive clay-silt beds that alternate with display folds of low amplitude, box folds, small ferruginous sandstones and have an important overlapping structures and thrust structures lateral extension. (Figs. 3 and 4). At Pitti, there are turned-up concave structures, joints and fractures. At Description of the deformed structures and Missole, the deformed structures recorded are interpretation giants load structures with load casts and The deformed structures described from these flames structures, irregular convolute and outcrops display variable size and shapes and recumbent folds. Six deformed zones (DZ) have differ from the common or classic deformation been described from all the section according to structures. In most cases the deformed units are their relative ages from Loungahe, Pitti and enclosed by significantly thick undeformed Missole. sediments. The structures have been classified in The first deformed zone (DZ-1) is located in two groups : 1) contorted structures which Loungahe along the Douala-Edea (Pk 40.7) include irregular convolutes, turned-up concave railway sections. They are the oldest of all the structures, giant load structures, load cast and sections described in the present work. These flames structures, box folds, small overlapping, sedimentary deposits are made of alternating thrust structures, folds of low amplitudes; 2) sub-parallel silt and clay laminae (0.3 to 1 cm brittle structures represented by fractures and thick) interbedded with fine to coarse-grained joints. Faults observed are rare. Without sandstones with pebbles. The granular and biostratigraphic markers, it is difficult to date pebbly beds are poorly sorted with a sandy 95  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 matrix. This outcrop seems to belong to the Logbadjeck Formation and shows: Box fold (Fig. 4a) of about 1 to 3 m in width and a height of 0.5 to 1 m. They have concentric shapes and the deformation axis is oriented NNE-SSW parallel to the maximal stress axis. These structures are bounded by undeformed layers which explain their syn-sedimentary genesis. The range of the shapes of these structures within the same interval and the absence of preferential orientations makes it difficult to determine the stress direction. The NW lateral extension of the deformed beds cannot be determined because of the dense vegetation. Similar structures are observed between Pk41 and Pk36 (Pitti). These deformations could be induced by a gravity sliding due to the collapse of layer as described by Lindholm (1987), Owen (1996) and Jones and Omoto (2000). Overlapping structures with variable sizes (Figs. 4b and 4c). Beddings are recognizable within Fig. 2. Tectono-lithostratigraphy of the Douala sub-basin (compiled from Nguene these structures. Similar structures described by et al., 1992, Lawrence et al., 2002 and Brownfield and Charpentier, 2006) Owen (1996) were induced by horizontal shearing or sliding. The second deformed zone, DZ-2 is made of coarse-grained sandstones with quartz pebbles and matrix to clast- supported conglomerates. The lateral variation of facies is accompanied by various deformations. Deformations recorded are: Thrust structures. They form a series of three thrust structures (Fig. 4d) that follow laterally with about 200 m of extension toward NNW. These structures belong to the DZ-2, but the base of this deformation encloses a box fold and overlapping structures of DZ-1: the first thrust fold of high amplitude of deformation is overturned towards NNW and at the base enclosed a box fold. The second thrust structure less deformed than the first presents a sub horizontal axis and the third structure appears not to have been formed or had simply disappeared. The intensity of the deformation of these structures gradually decreases toward NNW. Folds of low amplitude (1 to 5 m) and wavelength of 5 to 10 m. The lateral extension of the folding is about 1.5 km. Their origin is probably related to an important stress. Fig. 3. Synthetic log of the sedimentary succession in the main studied outcrops with deformed zones and their possible relative age. Proposed correlation The third deformed zone, DZ-3 is located at Pitti between the deformed zones of Pitti and Missole. a) Synthetic log of Loungahe and Pitti ; b) Synthetic log of Missole. village along the railway section (Pk 32) and at 96  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 Fig. 4. Main Deformed -Zone of Loungahe. a: Box fold of DZ-1; b: overlapping structure. c: overlapping structure; d: thrust structures with enclosed deformation at DZ-2. For scale hammer is 32 cm long and pen is 15 cm long. 97  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 Fig. 5. Main deformed-Zone of Pitti and Missole. a: turned-up concave structure DZ-3 (Pitti); b: turned - up convexe structure of DZ-3 (Missole); c and d: Fractures and joints of DZ-4 (Pitti); e: fault of DZ-4 (Missole); f: oblique layers without any relation with the surrounding deposit; g: Recumbent folds of DZ-5 (Missole). For scale hammer is 32 cm long, machete is 60 cm long and pen is 15 cm. 98  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 the base of Missole section. Ferruginous but the right unconformity (Fig. 5f) can permit sandstones facies interbedded with fine clay to suggest clockwise rotation. sandstone and silty clay are horizontally The fifth deformed zone, DZ-5 located at stratified and present: Missole (Pk 25) display many SSDS of variable Turned-up concave structures (Fig. 5a, Pitti). size whose shapes changes. The thickness of DZ- They present consolidated lamination and 5 ranges from0.5 to 1.5 m. consist of elongated structures whose edges are Recumbent folds (Figs. 5g and 6a) are cross-set marked by dark-colored laminae formed by deformed by a simple recumbent syncline- concentrations of clays and other fine-grained. shaped fold (Guiraud and Plaziat, 1993). They The upper layer of medium- to coarse-grained show horizontal to sub- horizontal axial plane sandstones is heavier than the basal layer made and are laterally associated with load structures. of fine clayey sandstones. These structures are Facies are fine- to medium-grained sandstones. extended over 20 m and have 1 to 2 m of width, The thickness of this SSDS is about 1 to 1.5 m. with small depression. The deformation could They show a disharmonic curvature with results from sedimentary overloading associated different radii, and a SSE vergence. The shear in with reverse gradient density (Rossetti 1999; this case would be the mechanism responsible Jones and Omoto, 2000; Kotlia and Rawat, 2004). for the deformations. According to Owen (1996), The amplitude of deformation is controlled by the current developed over the liquefied two parameters: the overloading and the sediment supplied enough shear stresses to form viscosity ratio between the two levels. recumbent folds or overturned cross-beds. Turned-up convexes structures (Fig. 5b) are Giants load structures (Figs. 6b, 6c and 6d) are observed at the base of Missole. They present extended over 50 m laterally with shapes and the same characteristics of size and facies than amplitude of variable size from 10 -20 cm. They the turned-up concave structures but with deform clayey-silt facies interbedded with different shape. ferruginous fine-grained sandstones. The bed is The fourth deformed zone, DZ-4 is located in covered with 15 cm thick of matrix to clast- Pitti. The deformed structures are: supported conglomerates with subangular to Sub-vertical to vertical fractures and joints subrounded pebbles. The difference of density (Figs. 5c and 5d) with a length of about 1.5 m. and of viscosity between the two beds would be These joints are observed in medium-grained an important factor of deformation. When sandstones facies and across the bed. Some sediments with high density overlie a low brittle structures considered as SSDS have been density, unlithified bed, load structures can be described by Owen (1996), Rossetti (1999), Obi formed (Lindholm, 1980; Owen, 1996). They are and Okogbue (2004), Van Loon (2009). Their associated with flames structures (Fig. 6b) when origin could be as a result of high degree of the low density material with water content has compaction. been injected into the overlying coarse-grained Faults observed are few in the study area (Fig. sandstones. The trending of the flames is 5e). The fault is recorded at the lower part of the towards SSW indicating the slope (Fig. 6b). Missole outcrops section with a reject of 15 cm. Moreover, deformations within unlithified bed Subvertical to oblique layers (Fig. 5f) are would require seismic events that destabilize the observed at the base of Missole outcrop section. system with unconsolidated deposits. According It is a single deformed bed of about 75 cm of to Owen (1996) and Rossetti (1999), the thick, made up of subvertical to oblique layers. wavelength of loads structures is proportional to It is characterized by a rhythmic stratification of the thickness and the difference in density very ferruginous fine- to medium-grained between the two beds and also to the intensity of sandstone. There is no obvious relationship with the driving force. Load casts (Figs. 6c and 6d) the surrounding structures and the boundaries extended over 30 m are associated with flames are sharp. This bed could be the result of structures. They occur at the base of a micro- tectonic stresses (Ricchi Lucci, 1995). It is conglomeratic sandstones bed. These load casts difficult to know if they rotate clockwise or anti are small lobes with concentric shapes clockwise without information about their strike assimilated to irregular hummocky protrusion of 10 to 20 cm in diameter. 99  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 Structure with synform shapes. These structures cast but if this overloading is continuous it can (Figs. 6e) look like hummocky cross generate complex structures such as folds, box stratification and extend laterally over 100 m in folds or directional faults. A comparable but the two sections of Missole. The shapes are even more extreme situation occurs if lithified homogenous with synform of 20 to 50 cm of sediments become ductile under the influence of width and the length of 20 to 50 cm. metamorphism and/or the heat derived from The sixth deformed zone, DZ-6 located at intruded igneous rocks. Under such conditions Missole (Pk 25) presents irregular convolutes deformation structures formed may be only. comparable to deformation structures formed in Irregular convolutes are observed in the unconsolidated sediments. It could generate outcrops of Missole (Fig. 6f) within medium- many structures including folds, faults and grained sandstones facies with some pebbles, at clastic dykes. the base of DZ-6. They are about 1m in length In most survey cases, SSDS are influenced by and extend laterally over 200 m along the seismic events (Rossetti, 1999; Jones and Omoto, outcrops with little interruption. This structure 2000; Obi and Okogbue, 2004; Kotlia et Rawat, is defined as distorted stratification that forms 2004; Mazumder et al., 2006; Tasgin et Turkmen, laterally alternating convex- and concave- 2009). To consider SSDS as a consequence of upward morphologies, producing a complex seismic events, it needs the following: pattern of synforms and antiforms. They have deformation structures should be confined in a the appearance of small undulated and twisted single stratigraphic layer that is enclosed by folds. Their antiforms are straighter than the undeformed beds; structures should be synforms and their origin seems to be produced continuous over a long distance; deformed beds by a contrast of density due to the water escape, should be recurrent over time; structures are or by traction currents. observed at the proximity of an active seismic zone. Sedimentary response to tectonic Particular dynamics in SSDS is very difficult to The deposits display a wide range of recognize because they can be formed from deformational structures affecting seismic and non seismic events. Moreover, much unconsolidated sediments. They evidence an dynamics can occur at the same time during a important instability of the basement during the given deformation (Jones and Omoto, 2000). deposits or shortly after the filling. These kinds These kinds of features characterize the of deformations occur when sediments are not instability of the system. Several processes can lithified (i.e. just after the deposits or during the generate such instabilities: earthquakes, first initial stage of diagenesis). The water volcanism, tilting, rock sliding, storms. expulsion from sediments does not occur at the In the case of this study, it is important to notice same rate and at the same time in the different the ongoing activities of the Cameroon Volcanic layers, depending mainly on the sediment Line (CVL) since the Cretaceous until present porosity and on the location of permeable layers day (1982, 1989, 1999, 2000). Each activity is with clays. It results in a succession of layers preceded by earthquakes whose depending on with different water content and by consequence the magnitude, can significantly impact the different densities and viscosities. The deposition of sediments in the neighbouring alternation of layers with contrasted rheology is basins. very unsteady and is easily disturbed after a The origin of SSDS and their classification are shock. This driving force could be: a reverse complex (Van Loon 2009). They are present in density gradient which can cause sedimentary lithified deposits between syn-depositional units overload, gravity sliding, injection of sediments, of undeformed layers. The SSDS have many water escape structures, the passage of seismic shapes each according to the mechanism which wave (issued from a seismic or volcanic created them. Their genesis could be the effect of activity), and the passage of a storm (Lindholm, one or a combination of several factors. Owen 1987; Owen, 1996; Rossetti, 1999; Jones and and Moretti (2008), Mazumber et al. (2006) Omoto, 2000; Obi and Okogbue, 2004). The classified them into syn-depositional and meta- sedimentary overload can lead to simple load depositional structures according to the time 100  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 Fig. 6. Main Deformed-Zone of Pitti and Missole. a: Recumbent folds of DZ-5 (Missole); b: Giant load structures with flames structures of DZ-5 (Missole); c and d: giant load cast of DZ-5 (Pitti); e: structures with synform shapes (Missole); f: Irregular convolute of DZ-6 (Missole). For scale, hammer is 32 cm long. necessary for their formation. The classification can give information on the magnitude of the can also be done according to the rheological driving force. Four scales had been defined by behaviour of sediments during deformation and Van Loon (2009): mega-scale where deformation the size of the soft-sediment deformation affects several layers of sediments; meso-scale structures. The size of the deformed structure where deformation affects a whole layer of 101  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 sediments; small scale where deformation instability. During the post-rift phase, Turner affects only part of the layer; micro-scale where (1995, 1999) recorded episodes of uplift and the deformation is visible only in thin section. erosion in the Gulf of Guinea. AFTA1 made by Lawrence et al. (2002) confirmed these events at Tectonic events recorded in the Douala sub- Mid-Cretaceous (105-95 Ma), Late Cretaceous basin and correlation with the deformation (85 – 75 Ma), Eocene (45-35 Ma) and Miocene- structures Pliocene (15 - 2 Ma). The Santonian episode can Major tectonic events of the Douala sub-basin be correlated with the rapid changes in the The separation of Africa from South America is direction of movement of the African plate accompanied by a complex structural history (Mascle, 1976). The uplift episode during the mentioned by several authors including Reyre Eocene associated to a compression is an (1966, 1984); Mascle (1976); Guiraud and Maurin evidence of a possible reactivation along the (1992); Wilson and Guiraud (1992), Benkhelil et oceanic fractures that might have reached the al. (2002) and Turner et al. (2003). These events continental margin of the Gulf of Guinea. Within include majors geodynamic processes (rifting, the Douala sub-basin, the uplift episode may uplift episodes) associated with local have been recorded by the Eocene unconformity phenomenons (compressional phases, according to Manga (2008) which might have dislocation movements associated to the fracture impacted sedimentary pattern by deformed zones, clay diapirisms and gravity sliding). deposits. Dumort (1968) estimated that the Rifting initiated during the Aptian in the Douala reactivation of the Sanaga fault might have sub-basin is accommodated within transform controlled the sedimentation during the zones which can be observed along the West Mesozoic and Cenozoic in the Douala sub-basin. African margin according to Meyers et al. (1996), The geological map of "Yaoundé-Ouest" shows Lawrence et al. (2002), Wilson et al. (2003) and an important tertiary series only in the North, Turner et al. (2008). In the Douala sub-basin, the with Miocene sand lying as disconformity over propagation of these fractures zones as defined the Eocene marlstone at the mouth of the Sanaga by Meyers et al. (1996) extend to the central- River. Moreover, the Middle-Cretaceous which eastern part of the Douala sub-basin (Fig. 1a). is mainly developed in the North of the Sanaga Evidences of the reactivation of these fractures river mouth is probably absent to the South zones is known along the West African margin where only the upper-Cretaceous is seen. (Benkhelil et al., 1998, 2002) and the best known case is that of the Benue Trough. In the Correlation of soft deformation structures with Kribi/Campo sub-basin, Benkhelil et al. (2002) tectonics events described compressive deformations in several According to Rossetti (1999), SSDS takes place if areas at different periods. In most cases these deposits are susceptible to deformation; there is compressive structures are generated due to the a trigger of sufficient strength to cause the transpression in the straight bands deposit to fail and/or change its physical state corresponding to active or reactivated fractures from solid-like to liquid like; and if a force acts zones reaching the continental margin, or often while sediment is in a liquid-like state. Sediment by their continental extension as is the case with in which these structures are recorded are dated the Benue Trough. A major characteristic of the Cenomanian –Turonian (Dumort, 1968; Njike deformation is its location into a narrow zone Ngaha, 2004). This age fits quite well with associated with fractures related to oceanic Turonian flexures which occurred in the Douala transform systems or to their continental sub-basin, probably accompanied by gravity extension. According to Belmonte (1966) and sliding. Episodes of uplift and erosion recorded Njike Ngaha (2004), the flexuring of the Douala in the Douala/Kribi-Campo basin during Mid- sub-basin along the Edea-Kompina axes, to Late-Cretaceous and Mid- to Late-Tertiary oriented NW-SE, dated Turonian had caused the might be followed by reactivation of some subsidence of the sub-basin in its central part fractured zones, particular in the Benue Trough and created the available space in the proximal part. Consequently the proximal part could have had an influx of sediments and created gravity 1 AFTA : Apatite Fission Trace Analysis 102  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 and along the South Cameroon margin. environment in railway sections of Loungahe Reactivations along the South Cameroon margin and Pitti, and shoreface to shallow marine shelf resulted to the formation of compressive in the road-cuts at Missole. The soft-sediment structures (Benkhelil et al., 2002). Obi and deformation structures of Cenomanian- Okogbue (2004) identified SSDS during the Turonian to Paleocene-Eocene correspond to Campanian-Maastrichtian period within the Logbadjeck and N’kapa Formations. They were Anambra basin (Southeastern Nigeria), which formed by the complex combination of several could have been the consequence of earthquakes driving mechanisms such as reverse density generated by the reactivation of the Chain gradients, fluidization, liquefaction and gravity Fractured Zones. The same described structures sliding. Two mechanisms involved could be recorded in Brazil by Rossetti (1999) in the Late used to explain their origin: gravity sliding Albian to Cenomanian Alcântra Formation are induced by an uplift and erosion, and possibly closely related to those observed at Missole and seismic tremor induced from the reactivation of Pitti. This observation permits us to suggest that, fractures and transform zones (Sanaga Fracture a probable reactivation of the fractured zones Zone or Cameroon Volcanic Line) which affects which reached the Douala sub-basin margin, the sub-basin. could have induced the formation of deformed SSDS are classified into contorted structures structures during the Eocene. such as small overlapping structures, thrust The structures exposed at some stratigraphic structures box fold, irregular convolutes, load levels have a wide lateral extend. Deformation cast associated to flames structures, turned-up structures such as giant load structures, joints concave structures; and brittles structures such which are described require seismic activities of as faults, fractures and joints. The soft-sediment high Richter magnitude scale (Sims, 1975). deformation structures closely coincide with the Seismic activities of magnitude less than 5 on the episode of tectonic uplift affecting the Douala Richter scale are known to cause little or no sub-basin during the Mid- to Late- Cretaceous liquefaction, while magnitude 6 and 7 may and Mid- and Late- Tertiary (ECL, 2001; cause liquefaction within a radius of 4 to 20 km Lawrence et al., 2002). respectively. In the Bima sandstones (Benue During the Middle Cretaceous (Turonian), the through, Nigeria), Guiraud and Plaziat (1993) sub-basin was affected by an event which studied SSDS as the result of seismic shocks caused it to flexure and may have been inducing liquefaction and local fluidization of responsible of some deformation structures the sand. However SSDS can be produced at 15 initiated by gravity sliding from the Loungahe km from the epicentre if the seismic activities outcrops. In another way, the release of stress are greater than magnitude of 5. But the study along the fracture zones or the tectonic line of area is located within a radius of 25 -40 km from the Sanaga River may have been responsible for the Sanaga Fractured Zone and then requires a inducing seismicity in the area during the early high magnitude earthquake. According to the Tertiary times which may have caused sediment study of Rossetti (1999), SSDS features have liquefaction as a direct consequence of been observed over 300 km of the study area earthquakes of large magnitudes more than and then requires a high magnitude earthquake. 5.The Eocene was characterised by renewed uplift phases and tectonic compressive events Conclusion which caused hinterland uplift in the Sanaga This study presents some results concerning the River accompanied by deposition of extensive impact of different phases of deformation of the coarse sediments in the Douala sub-basin, as a stratigraphic architecture of unconsolidated result of the Intra-Eocene tectonic event levels present in the Douala sub-basin. The (Unpublished report). studies were carried out in Cenomanian- The SSDS which are induced by seismic tremors Turonian to Paleocene-Eocene deposits from are continuous over 500 m and appear confined Loungahe, Pitti, and Missole. Silico-clastic at several stratigraphic levels. Each deformed sediments made up of occasional intercalations bed is enclosed by undeformed beds extending of conglomerates, sandstones, silts, and clays. laterally for hundreds of meters. The These deposits characterize a fluvial-deltaic deformation could extend on a large distance 103  Djomeni et al./Syllabus Review 2 (3), 2011 : 92 - 105 only if the seismic tremors are of magnitudes Dailly P, Lowry P, Goh K, Monson G. 2002. Exploration and development of Ceiba Field Rio Muni Basin Southern more than 5 on the Richter scale. To be able to Equatorial Guinea. The Leading Edge November, 1140 – confirm the influence of seismic tremors in the 1146. formation of these structures it is important to: Dumort JF, 1968. Identification par la télédétection de - well define the location of fractured zones l'accident de la Sanaga (Cameroun). Géodynamique, 1:13 – 19. within the Douala sub-basin, because it is one of ECL, 2001. 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