Stratigraphy and Depositional Environments of the ...

Cent. Eur. J. Geosci. DOI: 10.2478/s13533-012-0140-9

Central European Journal of Geosciences

Stratigraphy and Depositional Environments of the Mamfe Formation and Its Implication on the Tectonosedimentary Evolution of the Ikom-Mamfe Embayment, West Africa Research Article

Clement E. Bassey1 , Oboho O. Eminue1∗ , Humphrey N. Ajonina2 1 Department of Geosciences, Akwa Ibom State University, Mkpat Enin, P.M.B 1167, Uyo, Nigeria 2 Department of Geology, University of Ibadan, Ibadan, Nigeria

Received 8 April 2013; accepted 5 August 2013

Abstract: A 42 m thick outcropping portion of the Mamfe Formation is subdivided into Manyu (31 m thick) and Kesham (11m thick) Members on the basis of textural, mineralogical and structural differences. The Manyu Member (Albian) consists of folded and indurated, medium to coarse grained arkosic sandstones and thickly laminated organic-rich shales deposited in a lacustrine environment. The Kesham Member (Cenomanian) consists of subarkoses intercalated with massive green shale and mudstone deposited in a fluvial environment. The change in depositional environment was tectonically controlled. The mid Cretaceous paleogeography of the embayment was governed by the NE-SW trending ”Ikom ridge” which prevented marine incursion from adjacent the Benue Sea. Evaporites found within the basin were precipitated from ocean water that was periodically spilled by strong tides and storms across the ridge into the embayment. The filling-up of the embayment to base level in the Cenomanian resulted in a shift in the depositional center downstream to adjoining lower Benue Trough. Similarity in heavy mineral composition and maturity of the Cenomanian sandstones with recent clastics in the embayment indicates their derivation from the same source terrain and relatively stable tectonic and climatic conditions at the source area since the Cenomanian time. Keywords: embayment • tectonics • depositional environment • Cretaceous © Versita sp. z o.o.

1.

Introduction

The Mamfe Formation was defined and described by [1] near Mamfe in southwestern Cameroon. The formation is a widespread stratigraphic unit that outcrops mainly in the Ikom Mamfe embayment. This formation is believed ∗

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to be the only stratigrahic unit in this embayment. It varies widely in thickness from less than 1000 m to up to 4000 m [2] and was informally subdivided into two unnamed Lower and Upper Members [3], but in this work is referred to as Manyu and Kesham Members respectively. The Manyu Member mainly consists of highly consolidated feldspathic sandstones, black to dark grey shales which are occasionally calcareous and with conglomerates. Evaporites are also inferred to occur

Stratigraphy and Depositional Environments of the Mamfe Formation

within this member. The Kesham member consists predominantly of sandstones with green shales and occasionally with lignitic mudstone. The Ikom-Mamfe embayment is, to date, the least studied of all the West and Central African Rift Systems (WCARS) with which it is believed to be genetically and physically related [2, 4]. Although the earliest geological studies in the embayment date as far back as 1909 [5], published information on this embayment has been rather scant. The work of [6] has, over the years, been the starting point for virtually all geological studies in the region. Subsequent studies by [1, 7], and [8] have resulted in the lithologic definition and description of the outcropping sections of the formation. The age of the sediments that make up the Mamfe Formation, according to [7, 9, 10], ranges from Aptian to Cenomanian. Their environments of deposition have variously been assigned to marine, fluvio-deltaic and lacustrine regime [2, 6, 11, 12]. These varying views stem from the absence of detailed stratigraphic information, coupled with virtually non-fossiliferous nature of its strata, and the lack of a simple basin-filling model for the embayment. These are all due to the non-availability of subsurface and geophysical data for the region. In the absence of detailed studies on the stratigraphy of the embayment, it is likely that the existing lithostratigraphic correlation of its lithic fill, based mainly on the similarity in depositional environments, with rocks of similar age in the lower Benue Trough [13, 14] and Calabar Flank [15] may be misleading. This may render the existing paleogeographic interpretations of the region unreliable. This study is aimed principally at establishing a stratigrahic framework for the Mamfe Formation, which is subsequently used in reconstructing the regional geologic evolution of the Ikom-Mamfe embayment during the Cretaceous. The study is based mainly on field study and outcrop data of the Mamfe strata.

2.

Regional geological settings

The Ikom Mamfe embayment is located within the southern portion of the Benue rift system where it occurs as its NW-SE trending arm that straddles the CameroonNigeria southern geopolitical border (Figure 1). The embayment is about 120 km in length, and its width decreases eastward from about 50 km at its western end near Ogoja in southeastern Nigeria, to less than 10 km at its eastern end near Etuku in southwestern Cameroon. The origin and time of inception of this embayment has not been ascertained. On account of its decreasing width

eastward, [3] was of the opinion that the embayment is probably a rift splay segment (terminology from [17]) of the Benue rift system. Its shape suggests an eastward decrease in the amount of crustal stretching along its length. [2] have reported a crustal extension of 19 km perpendicular to the axis of the embayment.

2.1.

Stratigraphic setting

The Ikom-Mamfe embayment is filled with sedimentary strata that range in age from Albian to Cenomanian. These rocks are collectively referred to as the Mamfe Formation [7]. They rest non-conformably on the panAfrican crystalline Basement. An isopach map of the embayment shows a widely varying thickness from less than 1000 m to as much as 4000 m (Figure 1b) The Mamfe Formation is made up dominantly of sandstones and shales, and has variously been referred to as Mamfe Sandstone. [13] and Ogoja Sandstone [18]. The formation is thought to constitute the basal unit of the Asu River Group (Figure 2; [19–21]). Sedimentological and petrological studies of the Mamfe Formation by [3] revealed remarkable vertical variability in its composition, textures, and sedimentary structures. These differences have necessitated a redefinition and subdivision of the formation into two mappable units, referred to as the Manyu and Kesham Member. These units are separated by an angular unconformity. In this subdivision, the Mamfe Formation as originally described by [7] corresponds to the Lower (Manyu) Member, while the Upper (Kesham) Member sediments which are of Cenomanian age, were described in detail by [12]. A composite section of the stratigraphy of the Mamfe Formation as redefined in this work is shown in Figure 3.

3.

Methodology

Despite the thick alluvium and vegetation cover that characterized the study area, river channels, road cuts and some excavation sites provided the bulk of rock exposure in which field investigation was conducted and field data including rock samples were gathered. Stratigraphic section was sketched and sampled at each location. Azimuth measurement of cross beddings for each outcrop that this sedimentary structure featured were made and grouped into classes of 20◦ (see [12]). A total number of 68 samples weighing between 0.75 kg and 1.25 kg from 22 locations were collected for laboratory studies including grain size analysis, petrographic studies, heavy mineral studies and TOC analysis.

C. E. Bassey, O. O. Eminue and H. N. Ajonina

Figure 1.

Simplified map of Nigeria & Southwest Cameroon showing study area, (After [16]). B. Isopach map of Ikom Mamfe Embayment.

Figure 2.

Stratigraphic subdivision of the Cretaceous System in the Southern Benue trough (After [20]) compared with that of the Mamfe embayment (This work).


Figure 3.

Composite section of the stratigraphy of Mamfe Formation.


Grain Size Analysis included thirty representative sandstone samples (15 samples each from Manyu and Kesham localities) that were selected for grain size analysis. The selected samples were air dried, disaggregated and homogenized before quartering. A 50 g split of each quartered sample was subjected to sieving on a Ro–Tap automated shaker using a sieve nest of 21 phi interval for 20 minutes [22, 23]. Determination of grain size parameters was based on a graphic presentation of granulometric data in both histogram and cumulative frequency plots as well as the statistical formulae of [24].

3.1.

Petrographic studies

Twenty-four thin sections of selected sandstone samples (twelve samples from each Member) were prepared and studied under a flat stage Olympus petrographic microscope. Relative abundance of each mineral species observed was estimated using estimation charts of [25]. Statistical representation of a modal composition was carried out as suggested by [26] for the purpose of compositional classification and provenance studies. 100 to 200 framework grains were also examined for proper evaluation of grain morphology.

3.2.

Heavy mineral studies

Separation of heavy minerals from all the 30 sandstone samples used for grain size analysis was initiated by screening each disaggregated sample through a 60 micron sieve mesh in order to make the grains uniform. Each screened fraction was then heated gradually to boil with dilute hydrochloric acid to eliminate calcareous material and iron stain. The boiled samples were then washed with distilled water and oven-dried. Heavy minerals which are accessory mineral constituents of siliciclastic sediments with specific gravity greater than 2.89 were extracted from 8-10 grams of the treated samples by gravity method using bromoform (SG about 2.85) as heavy liquid [27]. The separated heavies from each sample were acetonewashed, dried and mounted on clean glass slide with Canada balsam for petrographic examination. They were then identified based on criteria given by [27]. The relative abundance of each identified heavy mineral species was determined by grain count of 50 to 200 grains. The Z-TR index for each sample was calculated based on Hubert concept [28].

3.3.

TOC analysis

Total organic carbon (TOC) content of 12 shale samples was determined. The samples were first treated with

dilute hydrochloric acid for carbonate elimination. About 100 mg of pulverized sample was weighed in to a special porous ceramic crucible and placed on a cold sand bath. The weighed sample was then wetted with a few drops of ethanol to avoid sporadic reactions and then treated with some drops of 10 percent concentrated hydrochloric acid until no further reaction occurred. The sample was left on the sand bath for 12 hours at 80◦ C in order to evaporate the excess water. Thereafter, the sample was transferred into a laboratory vacuum oven, where it was kept for another 12 hours at a temperature of 50◦ C. Before bringing the sample into a combustion chamber, 1 g of copper turnings was added as catalyst. The sample was then burnt in the LECO CS-244 analyzer in the presence of oxygen at a temperature of about 1300◦ C. The evolved gases (CO2 and SO2 ) were measured quantitatively and simultaneously by infra red detector and recorded as percent carbon. Before the samples were analyzed, the instrument was calibrated with standards of known carbon.

4.

Results and interpretation

4.1.

Manyu Member

The type locality for the Manyu Member of the Mamfe Formation is at the banks of River Manyu in Mamfe Town. Sedimentary rocks that make up this member outcrop extensively along the banks of the Manyu River from Etuku, through Mamfe beach to Ekok and at the banks of the Munaya River in Akwen village. A composite section (Figure 3) totaling 31 m was constructed for the outcropping sections of this member from 4 separate locations along the banks of Manyu River. This member non-conformably overlies the basement rocks and is believed to occur predominantly in the subsurface. The strata are generally highly consolidated and mainly consist of an alternating succession of sandstones and shales and occasionally with interbeds of calcareous shales, lignite and conglomerate pyroclasts. On the basis of the association of numerous salt springs with this member, evaporite beds are inferred to exist within it. Individual sandstone and shale beds in the cyclic sequence vary in thickness from 3 cm to over 2 m thick. The majority are medium sized with a thickness of 25 cm. The sandstone beds are usually thicker than the shale beds. The differences in bed thickness are attributed to their differing responses to compaction processes [29, 30]. The sandstones are pinkish in colour and contain framework grains which are angular to subrounded, medium to coarse grained in size, and are generally poorly sorted. Cross stratification occurs occasionally. Mineralogical studies revealed these sandstones to be


dominantly arkosic in composition (Figure 4). The shales are mostly dark grey but vary to black and dark brown. They are often thickly laminated, occasionally calcareous, carbonaceous and unfossiliferous. The pyroclastic materials found within this member very in size from 5 to 20 cm and are well rounded in shape. They are often coated with several concretional iron oxide layers. Paleocurrent studies revealed a dominantly southeasterly (Note Figure 6) transport of sediments from source areas to the north and northwest. Strata belonging to this member have been moderately to strongly folded with dips that range from 8◦ at Munaya River near Akwen to 40◦ at Manyu River near Etuku. It appears from these dip readings that the fold axis is very close to Etuku, which is in agreement with the report of [8].

4.1.1.

Petrography

Thin section studies of the Manyu Member revealed its sandstones to be texturally immature arkoses with an average framework composition of 68% quartz, 26% feldspar and 6% rock fragments. The quartz is angular in shape and exhibits an extinction angle that varies from slightly to strongly undulose. About 10% of the quartz is polycrystalline. Microcline is the most abundant feldspar and is usually of the same size with quartz, but varies in roundness from angular to subrounded. They are twinned and show a range of extinction angles that correspond to andesine and labradorite composition. The majority of the rock fragments present in the sandstones have gneissic texture and are of metamorphic origin. Sedimentary rock fragments are completely absent. Mica is rare. Cementation in the sandstone is usually by silica which occurs both as quartz mosaic and quartz overgrowths. Most of the conglomerates found within this member are basaltic in composition and texture, and are probably volcanic in origin.

4.1.2.

Depositional Environment

Field studies of the Manyu Member of the Mamfe Formation showed that the shales have a pronounced lateral continuity. This, in addition to their dark grey to black colour, is indicative of deposition in a lacustrine setting [31]. The repetitive alternation of sandstones and shales represents alternating periods of rough and quiescent water conditions at the deposition site [32]. The period of rough water condition was characterized by high input of coarser clastic materials to the basin leading to deposition of sandstone beds while that of the quiescent water condition was characterized by input of finer clastics leading to the formation of shale beds. The shales belonging to this member have an average total organic carbon (TOC) content of 1.24 wt% [3].

This relatively high TOC, together with thick and even lamination, indicates deposition during low clastic input to the depositional site and the absence of sufficient oxidizing conditions [33–35], as well as lower energy and deeper water conditions [32, 36]. The frequent occurrence of wavy laminations in the shaley sandstones and occasional trough cross bedding in these sandstones is indicative of deposition in a high energy environment in which wave action prevailed. This, according to [37], is characteristic of either shallow marine, deltaic or lacustrine environments. The non-fossiliferous nature of sediments belonging to this member and the occasional occurrence of calcareous shale interbeds in them is suggestive of likely persaline or anoxic conditions at the time of deposition. The existence of persaline conditions at the time of deposition is supported by the occurrence of numerous brine seepages from this member, while anoxic conditions are inferred from the relatively high TOC of the black shale. Occasional occurrence of calcareous shales in the sequence suggests alkaline depositional conditions. The rare occurrence of relatively fresh plagioclase feldspar in the sandstones of this member points to the prevalence of a semi-arid climate in the region at the time of deposition. This fact is supported by the occurrence of evaporites (inferred from brine seepages) in the member. Evaporite deposits of similar origin have been reported from the lower Benue Trough [14, 38]. This is consistent with the observation by [39] on the existence of relatively more arid climates in the low and mid latitudes during the Lower Cretaceous at the western sides of continents.

4.2.

Kesham Member

Though standard stratotype have not been defined but strata that characterize the Kesham Member outcrop in Kesham village, Oyomojock village, Mamfe town, and Akwen village all in southwestern Cameroon (Figure 1) with Kesham village serving as a representative locality. A composite outcrop section of 11 m thickness was erected for this member by [3] from these four localities. The Kesham Member strata are not folded, showing that they are post deformation deposits. They are relatively flat lying and are separated from the underlying folded Manyu Member by an angular unconformity. This member is dominated by graded sandstone beds that are sometimes cross-bedded. Bioturbation is rare. The sandstones usually grade into siltstone, green shales with occasional lignitic mudstone. Unlike those of the Manyu Member, sandstones of the Kesham Member vary in grain size from fine to medium and are moderately consolidated and less


feldspathic. Petrographic studies revealed these Kesham Member sandstones to be composed of texturally and mineralogical immature subarkose with an average framework composition of 78% quartz, 17% feldspar and 5% rock fragments (Figure 4). Details of the petrology of these sandstones have been reported by [3, 12]. Sandstones of the Kesham Member are cemented mainly by iron oxide. Most of the feldspars are microcline. Fresh plagioclase is rare. Most of the rock fragments present are gneissic in texture. Micas are relatively more abundant in this member than in the underlying Manyu Member. No fossils were found in the shales of this member. Paleocurrent studies by [12] indicate that the sediments of this member were derived from source terrains north and northeast of their location (see Figure 6 also).

4.2.1.

Depositional Environment

Bivariate plots of skewness versus sorting (Figure 5) showed that the sandstones of Kesham members were deposited by fluvial processes [40]. Also, plots of the cumulative frequencies of the sandstones on probability paper showed most of them to comprise of two segments which, according to [41], is characteristic of fluvial deposits. The dominance of fine to medium sized grains in the sandstones denotes a low to moderate energy depositional environment [42]. The generally poor sorting of the sediments indicates variable current strength at the time of their deposition [36, 43]. The unimodal nature of the grain size distribution, their characteristic positive skewness, and high palaeocurrent vector magnitude all suggest that they were deposited from unidirectional currents. The low to moderate paleocurrent variance of these sandstones suggests that the transporting agent was a low to moderate sinuosity river [44]. The meandering stream regime is inferred from an upward decrease in overall grain size as well as the presence of small to medium scale cross-bedding in the sandstones. The predominance of green, unfossiliferous, TOC-poor shale, as well as abundant kaolinite in the sandstone of this member is indicative of the existence of oxidizing conditions at the depositional site during their accumulation.

4.3. Geological evolution of the Ikom-Mamfe Embaymemt The wide variation in the thickness of the sedimentary fill of the Ikom-Mamfe embayment is believed to be indicative of variable subsidence rates in the embayment at the time of their deposition. The maximum sedimentary thickness of 4000 m which is located very close to the center of the embayment suggests that subsidence

was initiated by localized tectonic processes close to its center. The tectonic cause for subsidence is also supported by inferred gradual decrease in subsidence rate with time until the embayment became filled up in the Cenomanian. Localization of tectonic subsidence resulted in the development of a centralized depocenter, which in turn led to further increase in subsidence rates due to sediment loading. The role played by sediment loading on subsidence rates in the embayment has not been determined. Being a rift splay segment of the Benue trough, subsidence rates in this basin must have been similar to those reported from the lower Benue trough by [21, 45] which varied from 460 m/Ma in Middle Albian through 125 m/Ma in the Late Albian to 70 m/Ma in the Early Cenomanian. On the basis of these subsidence data and sediment thickness for the Ikom-Mamfe embayment, [3] calculated a subsidence rate of 184 m/Ma for the Lower Albian in the embayment. Based on the above subsidence rate and sediment thickness data for the embayment, the following depositional model and tectonic history are postulated for the embayment. The basal sedimentary deposits in the Ikom-Mamfe embayment are believed to be basal conglomerates (and breccias) that were produced and deposited as a result of fracturing of the basement complex rocks at the onset of the thermotectonic (i.e. heat driven Pan African orogeny) event that led to the initiation of the embayment in the Aptian. These conglomerates (and breccias) characterize the pre-rift deposits reported from all the WCARS by [4, 46]. They were deposited by a flowthrough fluvial system that existed at that time as a result of the absence of a topographic barrier (Figure 6a). The initiation of subsidence in the embayment was likely a result of thermal updoming in the Benue trough [47], which resulted in the formation of a NE-SW trending topographic arch [48] between the Oban and Obudu Massifs. In the Ikom-Mamfe embayment, this topographic arch is referred to as the "Ikom Ridge" [3, 12]. The formation of this ridge resulted in the separation of the Ikom Mamfe embayment and the lower Benue trough into two distinct depocenters. This ridge is therefore believed to have been the major paleotectonic feature that controlled the paleogeographic evolution of the embayment in the Cretaceous (Figure 6). Evidence for the existence of the ridge is shown by the comparatively thin sediment thickness along the NW-SE trending area separating the Ikom Mamfe embayment from the lower Benue trough (see Figure 1) The arching of the basement topography led to the development of an internal drainage pattern. This internal drainage system is supported by a southeasterly


Figure 4.

Ternary diagram of Sandstones in the study area (After [32])

Figure 5.

Bivariate plot of skewness versus sorting (After [40]).

paleocurrent pattern established east of Ikom [2], and a southwesterly paleocurrent pattern in Mamfe (Figure 6c). The development of the internal drainage system together with an increase in subsidence rates in the lower Albian resulted in a change in the depositional setting from

mainly fluviatile to fluvio-lacustrine. In the Middle Albian, the embayment had deepened, mostly as a result of the increase in subsidence rates relative to sediment supply from 184 m/Ma in the early Albian to 460 m/Ma [45] in the middle Albian. This led


Figure 6.

Model of Tectonosediment evolution of the Ikom- Mamfe Embayment.

to the development of a full lacustrine setting close to the beginning of middle Albian. The lacustrine deposits found in the embayment belong to the syn-rift phase 1 tectono-sedimentary stage of [18]. They comprise of an alternating sequence of arkosic sandstones and grey to black shales which are sometimes calcareous. This cyclic sequence is occasionally interrupted by a conglomeratic bed which is composed mostly of pyroclastic materials (volcanic bombs). Several brine seepages were observed from within these cyclic sediments close to the center of the embayment, thus suggesting possible existence of evaporitic beds within the sequence.

of any directly induced tectonic cause [49]. The thickness of individual beds in the cyclic succession (which are commonly between 10 and 43 cm thick) favor high frequency seasonal variation as the most likely cause of the cyclicity.

Apart from a climatic cause, other possible causes (such as eustatic and tectonic) for cyclic sedimentation in the embayment are ruled out on the basis of the continental setting of the embayment, and the relatively low frequency

The shales, on the basis of their relatively high TOC content and dark grey to black colour, are inferred to have been deposited during periods of low clastic input into the basin [35]. This was most likely at times of low annual

The sandstone beds are believed to have been deposited during periods of high clastic input to the basin as a result of an increase in the availability of sediments. The increase in sediment availability can readily be linked to periods of high annual precipitation (wet season) during which erosion rate as well as the capacity and competence of the drainage system were enhanced.


precipitation (dry season), during which period erosion rates, rivers capacity and competence were low. The arkosic nature of the sandstones within the cyclic sequence suggests a short distance of transportation and rapid burial of the sediments incidental on high subsidence rates. Source terrains must have been of relatively high relief. The sandstones are therefore classified as tectonic arkoses. Brine seepages from within the cyclic succession suggest the possible existence of evaporite beds within the sequence. Evaporites, according to [50], are often formed during periods of drier climates. The drier periods during which evaporites were deposited most likely coincided with the period of black shale deposition in the embayment. The evaporites would therefore be closely associated with the black shale. The lead-zinc mineralization reported from the shales in the embayment [8] must have occurred in this evaporitic setting as was the case in the lower Benue trough [51]. Restricted water circulation in the embayment, coupled with a drier climate, led to the concentration of brine and subsequent salinity stratification which led to deposition of TOC rich (1.24 wt%) black shales and evaporites. The proximity of brine seepages to the area of maximum thickness in the embayment suggests that evaporites were deposited in the deepest part of the basin. Preliminary investigations in the region by the Manyu Divisional Service of Mines and Geology indicate the existence of vast salt deposits in the embayment (Kwadlogony, pers. comm. 2002). These salt deposits are likely to have been formed at the same time with those reported from Abakaliki and Ogoja area [52, 53], Douala basin [54], and Gabon basin [55]. The origin of evaporites in the embayment cannot be traced to any subsurface inflow of ocean water into the embayment because of the absence of any deep seated underground lineament in the Oban and Obudu Massifs [56]. The only possible origin of ocean water in the embayment could be through a surface connection of the embayment with the southern part of the Benue Sea (ocean water periodically spilling from the Benue Sea into the embayment by either tides or storms). The absence of any proven marine deposits (minerals and fossils) in the embayment favors periodic spills of ocean water across the Ikom ridge into the embayment. Evidence of volcanic activity in the embayment during the Albian is provided by the presence of pyroclasts within the cyclic sediments of the Manyu Member. Similar pyroclastic materials have been described from rocks belonging to the Asu River Group (Albian) in the Abakaliki area [45, 57]. A decrease in subsidence rates from 460 m/Ma in the Middle Albian to 125 m/Ma in the Upper Albian, and later

70 m/Ma in the Lower Cenomanian resulted in the gradual fill up of the embayment. Sedimentation rates must have exceeded the subsidence rate with the consequence being the return to fluvial conditions when the basin had been filled to its lowest water outlet (Figure 6d). Cenomanian deposits in the Ikom Mamfe embayment are sandstones of sub-arkosic composition and green, TOC-poor, unfossiliferous shales that have been mapped and described as the Kesham Member of the Mamfe Formation [3, 12]. The less feldspathic nature of these sandstones and their angular to sub-rounded grains suggests low relief relative to that which prevailed during the deposition of the Manyu Member. A lower relief might have led to longer exposure-time of the sediments to weathering resulting in the alteration of the feldspars. This is probably the main reason why, although having the same provenance, sandstones of the Upper Member are less feldspathic than those of the Manyu Member. Another likely cause for the less feldspathic nature of the Kesham Member sandstones could be their very high porosities [3]. A low plagioclaseto-total feldspar ratio coupled with the predominantly kaolinite matrix of the Kesham Member sandstones suggests that a significant proportion of the plagioclase has been altered to kaolinite. The rarity of sedimentary rock fragments from these sandstones suggests that there was little or no recycling of sedimentary detritus from the underlying Albian sediments. Heavy mineral studies of both Cenomanian and Recent clastic sediments in Mamfe by [3] revealed a remarkable similarity in their mineralogical composition and abundance, with only minor variation in the least common heavy mineral species. The heavy mineral assemblage gave a ZTR index of 82±6%, which according to [58] implies that they are mineralogically mature. Similarity in the heavy mineral assemblage and maturity of the Cenomanian sandstones with those of recent clasitcs in the embayment is indicative of no significant changes in the tectonic or climatic conditions at the source areas since the Cenomanian time. The filling up of the embayment in the Cenomanian led to the lowering of the base level of erosion [59] which caused the cessation of sedimentation in this basin. The depocenter of the embayment is believed to have shifted to the adjacent basin downstream (Afikpo syncline). This view is supported by reports by [60] who has established southwestward paleocurrent patterns in strata of probably Cenomanian age west of Ikom. It is therefore very doubtful if rocks younger than Cenomanian in age had ever been deposited in the embayment and later eroded and transported elsewhere. With the shift of the depocenter westward, one would


expect a slight increase in the compositional maturity of the sediments which are pre-Cenomanian in the adjacent downstream basins. This is due to an increase in the duration of exposure of the sediments to the rigors of weathering resulting from an increase in transportation distance. This slight upward increase in mineralogical maturity has been reported from the Afikpo and Abakaliki areas adjacent the Benue Trough in Nigeria (Figure 1) by [18, 61]. The westward migration of the depocenter of the Abakakliki area in the Cenomanian [45], due to sedimentation exceeding subsidence, might have been caused by the filling up of the Ikom Mamfe embayment (in the Cenomanian) which then became a sediment bypass that served as an alluvial realm supplying sediments to the lower Benue trough. In the absence of any marine deposits in the embayment, it can be concluded that the embayment is an aborted (failed) rift that did not reach the oceanic stage of rift basin evolution as modeled by [46] for most of the sedimentary basins of the Gulf of Guinea. The failure of the embayment to reach the oceanic stage is thought to have been caused by the Ikom ridge which prevented a marine incursion from the Benue Sea into the embayment as well as the permanent cessation of mantle upwelling in the embayment by the end of Albian. Cessation of mantle upwelling in the Benue Trough in the Albian, unlike in the Ikom Mamfe embayment, was temporary as evidenced by its reactivation in the Turonian [47]. Field studies have shown that only the Manyu Member (Albian) sediments in the embayment are folded. As is the case in the Benue trough, the fold axis of the Ikom Mamfe embayment has been reported to be parallel to the axis of the embayment [8], thus suggesting a similar mode of tectonic evolution [62]. The restriction of the folding event to the Albian sediments suggests that the onset of folding in the embayment pre-dated the Cenomanian.

5.

embayment from the Benue trough. This ridge is thought to have prevented the incursion of the Benue Sea into the embayment, although high tides and storms occasionally caused overflow of seawater across the ridge into the embayment. A rapid increase in the rates of subsidence between Early and Middle Albian resulted in the deepening of the embayment and the development of a lacustrine setting in which sediments of the Manyu Member were deposited. The cyclic lacustrine sedimentation has been caused mainly by seasonal variation in the volume of clastic input into the basin. Restricted water circulation during dry periods led to the concentration of brines and salinity stratification of the lake with precipitation of evaporites and deposition of black shale. A reduction in subsidence rates between Middle and Late Albian led to the gradual infilling of the basin. The folding at the end of Albian probably accelerated filling of the basin to its lowest outlet on the Ikom Ridge and the return to a fluviatile environment of deposition in the early Cenomanian when the Kesham Member sediments were deposited. The filling up of the embayment to base-level in the Cenomanian resulted in a westward shift of its depocenter to the Afikpo syncline down-stream. The heavy mineral assemblage and maturity of the Cenomanian sandstones and recent sands in the embayment suggest that there have been no significant tectonic or climatic changes in the source terrains since the Cenomanian time. This study has made possible a more distinctive definition of the lithostratigraphic unit termed the Mamfe Formation by subdividing it into the Manyu and Kesham Members (i.e. Lower and Upper Member respectively) based on lithologic characteristics and structural styles. Also, the tectonic framework of the embayment is made clearer with respect to the adjourning Benue Trough, which is the major structural signature that influenced sedimentation in the Southeasten Nigerian basin and that of Cameroun.

Review and conclusion References

The Ikom Mamfe Embayment is a rift splay segment of the Benue Trough (see Figure 1). Rifting in this embayment was aborted in the late Albian due to permanent cessation of thermo-tectonic subsidence. Pre-rift sediments in the embayment comprise of fluvial conglomerates (and breccias) which were produced by the fracturing of the crystalline basement at the onset of the tectonic event that led to the formation of the embayment. The initiation of thermally initiated tectonical subsidence in the Aptian and Albian led to the formation of the NE-SW trending Ikom ridge that separated the

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