The Mamfe Basin in the southern Benue trough extension between Cameroon and Nigeria:Gem origin and problem of lithostratigraphic nomenclature and marine transgression

The Mamfe Basin has been the subject of many studies,but some debates persist,especially concerning the stratigraphic nomenclature,corundum origin,and marine transgression.The aims of this work are(1)to propose a new lithostratigraphic nomenclature of the Mamfe Basin formation based on new field observations,(2)to determine the source rock distribution and the origin of gem deposits and(3)to correlate the Cameroon section of the Mamfe Basin with the Nigerian deposits.The main results show that the name Manyu River Group is more appropriate as the Manyu River crosses all the facies in the Mamfe sedimentary Basin belonging to the Manyu Division.According to the facies analysis,the age of deposition,the mineralogic and geochemical data such as the V vs.Al2O3andΣREE vs.Al_(2)O_(3),TiO_(2)MgO,Na_(2)O,P_(2)O_(5),and CaO diagrams,this Group is composed of at least five Formations,including four Cretaceous Formations,from bottom to top:the Etoko Formation(alluvial and fluvial channel to fluvio-lacustrine deposits),the Nfaitok Formation(lagoonal deposits),the Bachuo Ntai Formation(fluvial braided channels or fluvio-deltaic environment)and the Eyumojock/NsanaragatiFormation(fluvio-lacustrine deposits)and the new Cenozoic Formation named Bakebe Formation(fluvio-lacustrine deposits).The gem minerals such as corundum,rutile,or tourmaline in the Cretaceous deposits of the Mamfe Basin are mainly detrital as indicated by the presence of worn shapes and fragments of these minerals.The presence of sapphire in the AlboCenomanian deposits indicates a Precambrian age of the felsic source rock,likely the plutonic rocks such as granite or pegmatite as indicated by the abundance of tourmaline and high LREE/HREE ratios(14.81–34.29)and slightly negative and positive Eu anomalies(0.85–1.15).This marine incursion in the Mamfe Basin was probably from West Nigeria,according to the geographic location of the Mamfe Basin and the general palaeogeographic evolution of the Benue Trough.

Applying Source Parameter Imaging (SPI) to Aeromagnetic Data to Estimate Depth to Magnetic Sources in the Mamfe Sedimentary Basin

Aeromagnetic data over the Mamfe Basin have been processed. A regional magnetic gridded dataset was obtained from the Total Magnetic Intensity (TMI) data grid using a 3 × 3 convolution (Hanning) filter to remove regional trends. Major similarities in magnetic field orientation and intensities were observed at identical locations on both the regional and TMI data grids. From the regional and TMI gridded datasets, the residual dataset was generated which represents the very shallow geological features of the basin. Processing this residual data grid using the Source Parameter Imaging (SPI) for magnetic depth suggests that the estimated depths to magnetic sources in the basin range from about 271 m to 3552 m. The highest depths are located in two main locations somewhere around the central portion of the study area which correspond to the area with positive magnetic susceptibilities, as well as the areas extending outwards across the eastern boundary of the study area. Shallow magnetic depths are prominent towards the NW portion of the basin and also correspond to areas of negative magnetic susceptibilities. The basin generally exhibits a variation in depth of magnetic sources with high, average and shallow depths. The presence of intrusive igneous rocks was also observed in this basin. This characteristic is a pointer to the existence of geologic resources of interest for exploration in the basin.

New Insights into Neoproterozoic–Cretaceous Events in the Mamfe Basin(SW Cameroon,Central Africa):Evidence from Textural Analyses,U-Th Composition,and U-Pb Zircon Geochronology from Granitic Basement

Backscattered electron BSE-CL images,U-Th abundance,and U-Pb ages were obtained for zircons from the Nkogho granitic basement rocks cropping out in the Mamfe Basin(SW Cameroon).These data are used to characterize and classify each zircon,elucidate their geochemical traits.They were also used to formulate a model of their host composition to ascertain the source,and document any preserved postemplacement events.These zircons mainly form long to short prisms that are pyramidal to dipyramidal in shape.They typically exhibit complex oscillatory growth zoning,as well as exhibit sector zoning.These features are mainly compatible with igneous zircons,although a few examples have metamorphic signatures.The U(30 ppm–6380 ppm),Th(4 ppm–1280 ppm),and Pb(12 ppm–648 ppm)contents show core to rim variations with most values fall within the range of crustal granitic zircons.The Th/U ratios(0.08–1.23),with core to rim differences mainly encompass values typical of magmatic zircons with a few values characterizing metamorphic zircons that grew in equilibrium with an anatectic melt.The U-Pb ages(108.4±1.7 to 988.4±19.0 Ma)with some core and rim age differences date Early Neoproterozoic,Cryogenian–Ediacarian,Early Cambrian–Ordovician,Devonian–Carboniferous,and Aptian–Albian events.The arc-like Nkogho I-type granitoid crystallized from granitic magma during Cryogenian to Ediacarian times and was later affected by post-Ediacarian Cambrian to Albian magmatic events.Aptian–Albian ages probably reflect opening of the Mamfe Basin.