First palynostratigraphical evidence for a Late Eocene to Early Miocene age of the volcano-sedimentary series of Dschang, western part of Cameroon and its implications for the interpretation of palaeoenvironment

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The volcano-sedimentary infill of the Dschang basin in the Western part of Cameroon is poorly known. The present study provides the first biostratigraphical data of the volcano-sedimentary sequence and allows constraining the age and the position of
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  First palynostratigraphical evidence for a Late Eocene to Early Mioceneage of the volcano-sedimentary series of Dschang, western part of Cameroon and its implications for the interpretationof palaeoenvironment Bessong Moise a,b, ⁎ , Hell Joseph Victor a , Susanne Feist Burkhardt b,c , Elias Samankassou b , Mouloud Bennami d ,Nolla Junior Désiré a , Mbesse Cecile Olive e , Thierry Adatte f  , Rossana Martini b , Michel Brunet d,g a Institute for Geological and Mining Research (IRGM), B.P. 4110 Yaoundé, Cameroon b Section of Earth and Environmental Sciences, Department of Earth Sciences, University of Geneva, 13, Rue des Maraichers, 1205 Geneva, Switzerland c SBF Geological and Consulting Services, Odenwaldstrasse 18, D-64372 Ober-Ramstadt, Germany d Institute of Palaeoprimatology, Human Palaeontology: Evolution and Palaeoenvironments, UMR CNRS 7262 INEE, UFR SFA, University of Poitiers, France e Department of Geosciences, University of Douala, B.P. 2701 Douala, Cameroon f  Institute of Earth Sciences (ISTE), University of Lausanne, Building Géopolis, 1015 Lausanne, Switzerland g Collège de France, Paris, France a b s t r a c ta r t i c l e i n f o  Article history: Received 4 July 2016Received in revised form 30 May 2017Accepted 10 July 2017Available online 22 July 2017 Thevolcano-sedimentaryin fi lloftheDschangbasinintheWesternpartofCameroonispoorlyknown.Thepres-entstudyprovidesthe fi rstbiostratigraphicaldataofthevolcano-sedimentarysequenceandallowsconstrainingthe age and the position of thisbasinwithin theregional context of Central Africa.The studied sequenceiscom-posed of three depositional units. The  fi rst unit is a  fi ning-upwards sequence of coarse-grained sandstone thatgets  fi ner towards the top, passing to thick layers of fossiliferous claystone. The second unit is strongly affectedby volcanic activity and consists of a volcano-sedimentary sequence. The third unit is marked by basaltic volca-nism and covers the series. Diverse and well-preserved palynomorphs were recovered and are used to providevaluable information for age determination of the Dschang volcano-sedimentary sequence, namely to be of late Eocene- Early Miocene age. The recovered spores, pollen, green algae and fungal remains from the bottomof the sequence include  Bombacidites  spp.,  Clavainaperturites  cf.  clavatus ,  Crototricolpites  sp.,  Perfotricolpitesdigitatus ,  Proteacidites  spp.,  Psilastephanocolporites minor  ,  Psilastephanocolporites  spp.,  Retitrescolpites  spp., Retitricolporites irregularis ,  Retitricolporites  spp.,  Retitriporites  sp.,  Spirosyncolpites bruni  (probably synonymousto  Spirosyncolpites spiralis ),  Striatopollis bellus , (considered synonymous to  Striatopollis catatumbus ),  Striatopollis sp.,  Tetracolporites quadratus ,  Tetratricolporites  sp.,  ?Verrutricolporites rotundiporis ,  Cyathidites  sp., Laevigatosporites  spp.,  Polypodiaceoisporites  spp.,  Verrucatosporites usmensis ,  Verrucatosporites  spp.,  Botryococcus sp.,  Tasmanites  sp., and fungal remains.Thesamplesareassignedtothe Verrucatosporitesusmensis ZoneofSalard-Cheboldaeff(1979).ThepalynologicalassemblagesaresimilartothoseofotherLateEocenetoEarlyMiocenepalynologicalrecordsfromtheWest,Cen-tralandNorthernAfricanbasins.StudyoftheorganicmatterbypyrolysisRock-Eval6(TypesI,IIandIII),thesed-imentological setting, the palynofacies and the palynomorph assemblages show evidences of tidal in fl uence(presenceoftheprasinophyte  Tasmanites ) inan activecontinentalmargin(semi-aridto humid lacustrine depo-sitional environment). The Ngoua sedimentary sequence is  fl uvio-lacustrine, and believed to have witnessedsomeoccasionalmarineincursionspossiblyduringtheopeningoftheCentralAtlanticOcean(CAO)andstronglydisturbed later by intense volcanic activity.© 2017 Elsevier B.V. All rights reserved. Keywords: EoceneMiocenePalynologyPalaeoenvironmentCameroonNgoua sedimentary sequence 1. Introduction TheCameroon Volcanic Line(CVL,Fig.1) includes all volcanic zoneslocatedbetweentheGulfofGuineaandtheLakeChadandhostsseveralsedimentary basins. Volcanic events along this line have been studiedpreviously and the oldest is dated by many authors as Cenozoic Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 517 – 530 ⁎  Corresponding author at: Institute for Geological and Mining Research (IRGM), BP4110 Yoaundé, Cameroun. E-mail addresses:  moisebessong@gmail.com, bessong_moise@yahoo.fr (B. Moise). http://dx.doi.org/10.1016/j.palaeo.2017.07.0090031-0182/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology  journal homepage: www.elsevier.com/locate/palaeo  (Tchoua,1974;Youmen,1994;Marzolietal.,2000;Ngountié,2003andNonoetal.,2004).However,originofthesedimentsinthisregion,espe-ciallyintheNgouaarea,remainsachallengetomanyresearchers,espe-cially the nature, extent and age of the in fi lling sedimentary sequence.Some authors have linked this formation to the event of the CVL.Kenfack et al. (2011) considered this series to be directly associatedwith volcanism and suggest a Miocene age. Their interpretations werebased on lithostratigraphic, petrographic, mineralogical, morphologicaland morphometric analyses, but lack any biostratigraphical data. Theseauthors described two phases in sedimentation: the  fi rst consisting of conglomeratic sandstone, sandstone and claystone and a second com-posed of conglomerate and arkosic sandstone and they assign a  fl uvialorigin.Avolcanosedimentaryserieswherethesandstoneandconglom-erate beds are intercalated with volcanic layers was also reported(Kenfack et al., 2011). Many questions remain unanswered about theage of this series. Besides, the position of the basin within the localgeodynamic context marked by the opening of the Central AtlanticOcean (CAO, Cinthia Labails et al., 2010), especially the chronology of geological events between the intense volcanic activity dated Cenozoic(Marzoli et al., 2000) that belongs to the CVL and the sedimentary de-positsis poorly constrained.Field data showthat thethicksedimentarypile is in contact with the basement-rock (granite), remains well pre-served and undisturbed and, thus, cannot postdate volcanism. Theopening of the Atlantic played an important role in the establishmentof some coastal and continental basins in Cameroon, which includethe Rio Del Rey basin (Njoh, 2007) and the Douala basin (Ngos and Mbesse, 2006; Mbesse et al., 2012; Mbesse, 2013; Ngon Ngon et al.,2016). The continental part including the Mamfe basin (Eyong, 2003), the Benue Trough (Bessong, 2012; Bessong et al., 2011, 2015), theHamakoussou basin (Ntsama Atangana, 2013; Ntsama Atangana et al.,2014), the Mayo Rey basin (Nolla et al., 2015), the Mayo Oulo-léré basin (Brunet et al., 1988; Ntsama Atangana, 2013), the Babouri Figuilbasin (Ndjeng, 1992) and the Logone Birni (Allix et al., 1981; Allix and Popoff, 1983; Ntep et al., 2000) have been well studied. Studies on pal-ynology across all the basins, however are very limited (Brunet et al.,1988;Dupéron,1991),hence,makingitdif  fi culttoattemptcorrelationsandthelinktotheopeningoftheCAO.ThePalynologicalstudyoflignitefrom Mpu Formation in the Abakaliki basin in Nigeria has been made(Umeji, 2002) and has revealed and upper Eocene-lower Miocene age.Sediments of the similar age according to the fossil assemblage areKnwon in West Africa: Ogwashi-Ashaba formation (upper Eocene)( Jan du Chene et al., 1978), Kwakwa bore of coastal basin of Cameroon(palaeocene-lower Miocene) (Salard-Cheboldaeff, 1978), Gandu For-mation of Sokoto Basin (Kogbe and Sowunmi, 1975). The goal of thisworkistoclarifytheageandpalaeoenvironmentofthesedimentaryse-quenceof theNgouaFormation and interpret theposition of this groupinthelocalgeologicalcontextcontrolledbytheopeningoftheCAO.Thepalynostratigraphical investigations of the Ngoua sedimentary rockspresented in this paper will enable for the  fi rst time to determinatethe age of this formation in order to better understand the relationshipbetween sedimentary and volcanic events and the palaeoenvironmentsetting of the study site. 2. Study site The Ngoua sedimentary formation is situated on the southern  fl ankof the Bambouto volcanoin the WestCameroon region (Dschang local-ity)alongtheCVL(Fig.1)whichisa100kmwidegeologicallineament,oriented N30E. These structure are characterized by an alignment of oceanic volcanoes, volcanic massifs and continental volcanoes extend-ing from the Gulf of Guinea to the interior of the African continentover 1600 km (Marzoli et al., 2000). After Mounts Cameroon andManengouba, Mount Bambouto is the third largest volcano of the CVL (Kagou et al., 2001). The Ngoua formation is the most important sedi-mentaryrocksoutcropinthisregion.Sedimentaryrocks(conglomerate,sandstone and claystone) are interlayed within the volcanic rocks Fig. 1.  Location of the study area (Ngoua village).518  B. Moise et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 517  – 530  (ignimbrite and tuf  fi te). The petrological and geochemical data avail-able to date show that plutonic rocks compositions generally de fi ne acomplete series of gabbro-diorite-monzonite-syenite-granite type,while those of volcanic rocks extend from basanites to trachytes, rhyo-lites and/Or phonolites. However, a few massifs including theManengouba and Bamenda-Bambouto mountains display a completerange of composition ranging from basic terms to felsic terms (Marzoliet al., 2000, Kagou et al., 2001, Mbassa et al., 2012). Dumort (1968) ex- plainthe origin of magmatism alongtheCVLasa tectonic gap. Volcanicactivity at Mount Bambouto began around 500,000 years ago, srcinat-ingasasequenceofbasalt,trachyteandphonolitelava fl owsandignim-brites (Tchoua, 1974; Marzoli et al., 2000; Nono et al., 2004). Thepresence of hot spots at the convergence of the South Atlantic, theBenue and Gulf of Guinea ditches (Tchoua, 1974) have a link with theopening of the Atlantic or a deep, intermittently hot line (Deruelle etal., 1991). 3. Materials and methods Only fresh sample with no visible signs of alteration were used.  3.1. Lithofacies analysis The section was measured, described, photographed and sampledwith particular emphasis on lithology, trace fossils, macrofossil, andleaves contents (Fig.2.),followedbya detailedstudy of the sedimento-logical aspects by the team of the Institute for Geological and MiningReaearch (IRGM) in Cameroon (Fig. 3.). Pro fi les have been cleanedwith a spade to obtain uncontaminated samples. All the samples werepositionedinalithostratigraphiccolumn(Fig.4).Eighteenrocksampleswere collected and analysed for a quantitative and qualitative mineral-ogy (petrography, X-Ray diffraction and geochemistry by a Nikon mi-croscope equipped with a Nikon camera Type using the NS-Elements Fig. 2.  2- A- Large and angular clasts of quartz and granite in a coarse sandstone; 2- B- Fine to medium-grained sandstone (2.5Y5/4) thinly interbedded with laminated siltstones (bedthickness most than 1.5 m); 2- C-Vertical variable successions that include planar  fi ne-and-medium-grained sandstones (2.5Y8/4) showing the sharp contact between the sandstoneand the shale at the base. The scale bar is 1 m; 2- D- 2-cm of cinerite (2.5Y9/2) are interbedded in shale (2.5R3/2). Above, sharp erosional contact between pale brown shale andbelow dark shale; 2-Ea to d- Black-coloured shale (10YR3/2) containing plant remains, details showing different type of centimetric leaves; 2- F and G- Prismatic basalt (10BG1/2) inthe Ngoua region; basalt with olivine (Fa) and Ignimbrite rock-type (2.5Y8/8).519 B. Moise et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 517  – 530  ImageanalysissoftwareattheUniversityofGeneva,byaDiffractometerXTRA, Thermo Scienti fi c ARL; Energy, 45 kV, 40 Ma; CuKa 1 = 1.54060Å; angle 2 to 65° 2° Ө following the procedure described in Klug andAlexander, 1974,Kubler, 1883, and Adatte et al., 1996, attheUniversity of Lausanne and by X-ray  fl uorescence (XRF) spectrometry with aPANalytical Philips PW2400 spectrometer at the University of Lau-sanne).Thesampleswereprocessedforpalynologicalanalysis,determi-nation of the content of organic matter and geochemical analysis.  3.2. Palynological analysis All samples used for palynology and palynofacies analyses were col-lected in the basal part of the section (Fig. 4.). The samples were washedtoremovecontaminantsandthendriedinanovenat1500°Cfor3h,thencrushed.ThreesamplesfromtheNgouasectionwerestudiedqualitative-ly for palynology (NGO 12, NGO 9a, and NGO 9b). Species with well-known stratigraphical ranges in other contemporaneous basins of West,Central and Northern Africa were identi fi ed and used for assigning thesamples to a palynostratigraphical zones. Rock samples were processedin the palynological laboratory at GeoTechniques Research Ltd., SunburyonThames,UK,usingstandardpalynologicaltechniques,includingdiges-tion of carbonates and silicates with concentrated HCl (32%) and HF(40%),sievingoftheresidueandoxidation(e.g.Woodetal.,1996).Slideswere prepared from unoxidized and oxidized residues. Palynomorphswere analysed in the slides with oxidized residue. Plate 1 illustrates thepalynofacies of the three samples. A selection of palynomorphs is illus-trated in Plates 2 to 4. The studied slides are deposited at the Institutefor Geological and Mining Research (I.R.G.M) in Cameroon.  3.3. Total organic carbon (TOC) and rock-eval pyrolysis Total organic matter was determined using Rock-Eval pyrolysis(Rock-EvalTM6,Beharetal.,2001)attheUniversityofLausanneinSwit-zerland.Valueswereobtainedusingthestandardtemperaturecycle(100to850°C).SampleswerecalibratedwithbothIFP160000andaninternalstandard with an instrumental precision of   ˂ 2% (Espitalié et al., 1985).Fourrepresentativesamples(Lithologyandorganicmatter)wereinvesti-gated for both, organic matter characterization and Rock-Eval pyrolysis(samplesNGO 5, 6, 9 and 12)under inert atmosphere (helium)todeter-mine the type of organic matter by plotting different parameters(Espitalié et al., 1985). The resulting parameters include the following:S 1 (freehydrocarbons),S 2 (hydrocarbonsyieldfromcrackingofkerogen),S 3 (thetrappedCO 2 releasedduringpyrolysis),T-max(Rock-Evalpyroly-sis oven temperature in  o C at maximum S 2  generation), Hydrogen Index[HI = (S 2 /TOC) × 100, mg HC/g TOC] and Oxygen Index [OI = (S 3 /TOC)×100,mgCO 2 /gTOC].ValuesoftheRock-Evalparametersofheanalysedsamples are given in Table 1.  3.4. Geochemistry A small block (50 g) of seven rock samples (NGO 01, NGO 07, NGO09, NGO 11, NGO 12, NGO 16, NGO 18) was crushed in a Tungstenmill to a  fi ne powder. An aliquot was heated to 1050 °C in a furnace todetermine the loss on ignition (LOI) and to oxidize all iron. The  fi redproduct (1.2 g) was mixed with the appropriate amount of Li 2 B 4 O 7 (6.0 g) and fused for 5 min in a Pt crucible, poured into a Pt castingdie and quenched to a glass. Major element data were measured by X-ray  fl uorescence (XRF) spectrometry with a PANalytical PhilipsPW2400 spectrometer at the University of Lausanne in Switzerland.BHVO-1, NIM-N,NIM-G, and SY-2 standards were used for quality con-trol. The average elemental abundances in the standards were takenfrom Pearce et al. (1997). The distribution of major elements in sevensamples also provides information on the rock type, their provenanceand the depositional environment of sediments using the SiO 2  vsAl 2 O 3  + K 2 O + Na 2 O diagram (Suttner and Dutta, 1986), the K 2 O/Na 2 O vs SiO 2  diagram (Roser and Korsch, 1986) and the log (SiO 2 /Al 2 O 3 ) vs log (Fe 2 O 3 /K 2 O) diagram (Herron, 1988). Some reworkedsamples whose elements contents have been modi fi ed were removed. Fig.3. Thinsectionsshowingpetrographiccharacteristicsunderbothpolarized(AandB)andnaturallight(3).3-A-Mixtureoforganicmatter,clasticfragmentsandmudstone(NGO09).3-B-Laminatedblackclaystonewithlayersrichinorganicmatterandplantsremains(NGO12).3-C – Quartzandsomeheavymineral(zircon)clastsinasilici fi edcementsandstone(NGO09). (Qtz: Quartz, Pl: Plagioclase, OM: Organic Matter, Zr: Zircon, M: Matrix).520  B. Moise et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 517  – 530  4. Results The Dschang volcano-sedimentary series has been identi fi ed in theNgoua area (Fig.1) and is likely to extend beyond. It consists on onehand of conglomerates, sandstones and claystone, and on the otherhand of tuffs, cinerites, basalts, ignimbrites and volcano sedimentaryrocks (tuf  fi tes). Volcanism (Tchoua, 1974; Youmen, 1994; Marzoli etal., 2000;Nonoet al.,2004)isidenti fi ed asconsistingof basalt, trachyteand phonolite from the Bambouto Mountains established during theexplosive initial phases (40 – 70 Ma), (Ngountié, 2003; Nono et al.,2004). 4.1. Sedimentary facies analysis Fourlithologic facieswereidenti fi ed,namely:Conglomerates,sand-stone, claystone, and volcano-sedimentary breccias. Microfacies wereidenti fi ed based on the size of fragments, sorting, structure, and colourof the sediment (Munsell Color System), thickness, hardness and fossilcontent. 4.1.1. Conglomerate Conglomerates (Fig. 2, A) consist of more or less rounded quartz and basalt pebbles (NGO 1 and 2). They exceed four metres in thickness and Fig. 4.  Stratigraphic column of the Ngoua section with interpreted age based on palynology. In red are the samples analysed for palynology.521 B. Moise et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 517  – 530
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