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.

Geochemical,mineralogical and sedimentological analyses of reworked sediments(new)in the syn-to post-rift Middle Cretaceous-Quaternary detrital deposits from western Atlantic margin of Cameroon:evidence from sedimentation-erosion alternation in the cont

Reworked detrital sediments were discovered in the syn-rift to post-rift Cretaceous to Quaternary(Pleistocene)detrital deposits of the Douala coastal sub-basin in the eastern margin of the South Atlantic Ocean at West Cameroon.This contribution presents the geochemical,mineralogical,and sedimentological data to constrain the origin of these reworked sediments.The investigated samples are characterized by the high values of Plagioclase Index of Alteration(PIA,97.83–99.90),Chemical Index of Alteration(CIA,71.9–99.08),and Chemical Index of Weathering(CIW,98.67–99.90).Data gleaned from these indices suggest that the reworked sediments and their potential source rocks experienced severe weathering.The A-CN-K plot along with the mineralogical indicates a probably post-depositional K-enrichment.The high light rare earth elements(LREE)/heavy rare earth elements(HREE)ratios(15.82–79.13)suggest that the reworked were mainly derived from felsic igneous rocks.This interpretation is further confirmed by the Zr versus TiO2,TiO2/Al2O3,Th/Co versus La/Sc,and La/Th versus Th/Yb plots,which indicate felsic igneous and silicic source rocks.Meanwhile,the high variability of LREE/HREE ratios(15.82–79.13)and the positive Eu anomalies(1.10–1.32)of studied samples could indicate a little contribution of mafic source rocks.Analysis of the distribution of the heavy minerals(e.g.tourmaline,aluminum silicate,and rutile)revealed that these felsic and silicic source rocks are mainly granite and gneiss,probably belonging to the Nyong Group and the Western part of the Neoproterozoic Yaounde Group.The morphoscopic analyses of the host rocks(very angular to angular shapes)have revealed that the reworked sediments have proximal to subproximal onshore origins,probably from former sedimentary deposits,as is indicated by variation of their color.The tectonic discriminant diagram indicates the Oceanic Island Arc and Active Continental Margin which are inconsistent with the geological history of the Douala coastal sub-basin and could be explained by the recycling effect experienced by studied sediments which have been deposited in the coastal plain to the fluvial environment in the Low System Track context.