Review of the geology of mamfe sedimentary basin, SW Cameroon, Central Africa

This paper presents a review of the tectonic evolution, stratigraphy, paleontology, hydrogeological aspects and economic geology of the Mamfe basin. The main structures are basement –involved and detached fault and simple symmetrical fold with NE-SW trend. The basin measures 130 km in length and the widest part is about 60 km and a maximum area of 3200 km2. The maximum sediment thickness is estimated at 4–7 km and the sedimentary infill includes a lower alluvial fans to fluvial basal conglomerate and conglomeratic sandstone, a middle alternating (cyclic) aqueous to anoxic deep lake bottom black shale and limestone unit and an upper fluvial to transitional deltaic nearshore conglomeratic sandstone unit. The Ostracod fauna from the shale are very similar to the coeval fauna from adjacent Benue trough. The reported fauna have no biostratigraphic significance because of their uncertain age, low diversity, and cosmopolitan distribution during the early Cretaceous. Over 30 salt springs exist across the Mamfe basin with the most concentrated yielding 79 g/L of high quality halite. Several mineral indices (including diamond) have been identify and the geological condition favour the occurrences of hydrocarbon.


Introduction
The Mamfe Sedimentary Basin in the SW region of Cameroon is also known Mamfe embayment, Ikom-Mamfe embayment, Mamfe Gulf, or Mamfe Rift. [1][2][3][4][5][6][7][8][9][10][11][12][13] It is a small intra-continental basin that forms an arm of the Benue Trough of Nigeria ( Figure 1) extending into Cameroon. 14 The basin is part of the West and Central African Rift System (WCARS) that is intimately associated with the opening of the South Atlantic Ocean and the formation of the Gulf of Guinea. [15][16][17] Several research projects have been on going in the Mamfe basin regarding geology and lithostratigraphy. With the current need of the government of Cameroon to avoid entering the state of an emergent nation in 2035, there is every need for the geology of this hinterland basins to be reviewed in order to highlight its economic potential. Therefore, the aim of this study is to present a review of the tectonic evolution, stratigraphy, hydrogeological aspects, paleontology and economic geology of the Mamfe basin. The data base for this contribution comes from previous research on the Mamfe basin. This is significant because it will awaken exploration and exploitation activities in the Mamfe sedimentary basin and further call for the government of Cameroon to fund research in the basin

Geographical setting
The Mamfe Sedimentary Basin located between 5°30' to 6°00'N and 8°15' to 9°45' E underlies a coastal plain in Cameroon with low to slightly high relief whose heights range from 30 to 300m. 4,18 It is locally bordered by high igneous terrains (Mount Nda Ali: 1200 m, Mount Mbinda: 1000 m, Nkogho hills: up to 600 m). The basin is regionally bordered by upland areas (Figure 2) (Mount Rumpi, Bambouto, Bamenda, Manengouba and Koupe), which are part of the Cameroon Volcanic Line. 19,20 The Mamfe Basin is administratively situated in the Manyu Division made up of four sub-divisions (Mamfe Center, Eyumojock, Upper Bayang, and Akwaya) and occupied by three main ethnic groups: the Kenyangs, Akwayas, and Ejagham. 18 It measures 130 km in length and the widest part is about 60 km and a maximum area of 3200 km 2 . 21 The maximum sediment thickness is estimated at 4-7 km. 22,23 The climate in this division is hot and humid and consists of a rainy and a dry season modified by the deviation of the monsoon and the relief of Mount Cameroon. 24 The vegetation is dominantly that of the equatorial rain forest, 18 and the drainage system is principally that of the Cross River ( Figure 3) whose main source is found in Mount Bambouto. 25 The sources of its main tributaries the Munaya and Badi Rivers are at Mount Rumpi and Nda Ali respectively. 18

Tectonic evolution
The Mamfe Basin is an appendix of the Benue Trough located at the South East border. The Benue Trough and associated basins ( Figure  4) including the Gongola Trough (extending N-S mostly in Nigeria), Bornu (extending W-E and reaching Cameroon eastwards) and Yola basins are located in the convergence of the West African Rift System (WARS) and Central African Rift System (CARS) in the junction between the Northwest, Northeast and Southern African plates. 9 The Benue Trough represents the abandoned arm of a ridge-ridge-fault triple junction at the origin of the opening of the South Atlantic Basin and extension in the WARS and CARS. 9,26-30 The trough represents a major direction of NE-SW filled with continental and marine sediments. Subsidence in the Benue Trough begins during the Late Jurassic -Barremian interval; 9,30,31 the oldest sediments, Barremian in age 30 are conglomerates, covered with lacustrine deposits. In northeastern Nigeria, marine conditions partly occurred during the latest Cenomanian latest ''Senonian'' (Campanian Maastrichtian?) interval.  The Mamfe basin is laterally correlated to the Asu River Formation in Nigeria. 9 It is an important tectono-sedimentary and intra-continental geological structure. 14 The Mamfe sedimentary basin is a rift basin that was initiated by the response of the Precambrian basement of this region to the break-up of Gondwana and subsequent separation of South American from the African continental plate. 2,5,9,11,12 Like other rift basins of this sub-region, the Mamfe Basin is part of the West Central African Rift System (WCARS) that is intimately associated with the opening of the South Atlantic Ocean and the formation of the Gulf of Guinea. 17,32 During its formation, this basin appeared as a bifurcation or an eastern tectonic arm that extended eastward from the south-eastern portion of the Benue Trough into the south-western part of Cameroon. It should be noted that, like the Mamfe Basin in the south-east, a second arm also extended eastward into Cameroon, but from the north-western portion of the Benue Trough, known as the Garoua Basin.
Abolo 33 outlined a five phased tectono-stratigraphic evolution of the Mamfe Basin slightly modified here as follows: • Eo-rift phase that was characterized by the initial rifting and possible deposition of the first alluvial fan conglomerates during the Barriasian-Pre Barremian.
• Syn-rift and subsidence phase (Barremian-Early Albian and Late Albian-Cenomanian) during which rifting and alluvial fan conglomerates deposition continued together with fluvial sandstones and lacustrine shales.
• Post-rift phase-1 characterized by folding, faulting and erosion which occurred during the Santonian-Early Campanian in response to the Abakaliki tectonic event in Nigeria.
• Post-rift phase-2 which include all other events that occurred during the latest Cretaceous and beyond; continued erosion, magmatization and possibly uplift.

Hydrogeology
The Mamfe Formation constitutes the main source of groundwater supply in the basin. In area of shallow depth the basement complex can supply groundwater. Since ground water is confined to the zones of weathered rock, joints and fractures in the basin. Mamfe basin is characterized by evaporate deposits that constitute the sediments of the basin high mineralized Soduim Chloride composition and Total dissolved solute higher than sea water range from 13000 to 36000 mg/L. Meanwhile, the large part of the water of the basin is in good quality and content (Ca, Na) -HCO 3 ). [42][43][44] Economic geology Important mineral indices have so far been identified in the basin. Lead, Zinc, Rutile and Sapphire have been identified by Lapplaine and Soba, 45 Du Mort, 4 Ndougsa-Mbarga et al. 11 Kanouo, 18 Kanouo et al. 46 Nguimbous-Kouoh, Takougam, Nouayou, Tabod and Manguelle-Dicoum. 12 Carbonate, sulphide and evaporite minerals have widely been reported in Cretaceous sedimentary facies of the Mamfe Basin in Cameroon. 1,4,34 In addition, over 30 salt springs exist across the Mamfe basin 47 with the most concentrated yielding 79 g/L of high quality halite. 53 Gem corundum and coarse zircon have been reported in detrital sediments (in Nsanaragati) at about 10 km east of the nearest basaltic flow in Ekok. 18,46 Although not yet confirmed, the presence of diamond indices within sedimentary clasts in the western part of Mamfe Basin is mentioned in Laplaine and Soba. 45 The aspect of the depositional environments and petroleum geology of the Mamfe Basin has been reported by Wright et al. 48 Ajonina; 14 Ajonina et al. [49][50][51] Bassey and Ajonina; 52 Eyong, 41 Eseme et al. 53 Abolo; 33 Bassey et al. 36 Eyong et al. 41 Njoh and Njie. 54 Geological condition that favour the discovery of hydrocarbon occurrence in the Mamfe basin. SNH 55 proposed the hydrocarbons generation and migration as followed: • Occurred during Santonian and Early Tertiary.
• Source rocks are located at the center of the basin. Shallow reservoirs appear at the east due to uplifting and erosion, suggesting hydrocarbon migration to the surface and hydrocarbon destruction by meteoric waters. Deep reservoirs are elsewhere across the basin.
• Geological models suggest that hydrocarbons can still be trapped within deep structures.

Discussions and conclusion
The geology of the Mamfe sedimentary basin is characterized with four stage tectono-stratigraphic evolution that is the Eo-rift phase, Syn-rift and subsidence phase, Post-rift phase-1, and Post-rift phase-2 in which the following type of rock have being deposited: alluvial fan conglomerates, fluvial sandstones and lacustrine shales, fault-bounded granitogneissic rocks of the Pan-African Mobile Belt (550±100 Ma) and Tertiary anorogenic and effusive basic intermediate rocks such as syenites, diorites, trachytes and basalts that belong to the CVL. The stratigraphy succession of the Mamfe basin is highly controversial as there is no accepted lithostratigraphy framework for the basin. However current research is pointing to three fundamental stratigraphic member for this basin. Until these findings are published, it is generally agreed by many that the rocks of this basin can best be sub-divided into three members: a lower alluvial fan conglomerate to a fluvial channel cross bedded conglomeratic sandstone, a middle predominantly repeated sequences of alternating black shales, limestones, sandstone and evaporates (lacustrine), and another fluvial sandstone unit at the top of the basin. In term of its economics potential the basin is endowed with several mineral indices (including diamond) and the geological condition favour the occurrences of hydrocarbon.