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P.. Jc:ocnvi ronmenl Gulf of Guinea Paleoenvironments For:.mÎniferli Transl:fcssions·regn:ssions Black Sh.. le s

PaJéoenvironnemenl of the Gulf of Guinea Golfe de Guinée For:t minifèrcs Transgressions- régressions Mnmes noire s

S. W. Pellefs Dcpartmenl of Geology. University of , Ibadan. .

ABSTRACT Foraminireral asse mblages in Ihrec margi nal basins in the Gulf of Guinea (name] y, the Benue Trough, the Dahomey Embaymenl , the Niger Della) show maximum development during the major marine transgressions whic h transpi rcd in the Middle 10 Laie Albian, Late Cenomanhtn 10 Early Turonian, LaIe Turonian 10 Early Sanlonian, Late Campanian 10 Maastrichtian, Middle to LaIe Paleocene, M.iddle 10 Laie Eocene, Middle Üligocene, and Early to Middle Miocene . The Oligoccnc and younger transgressions are more pronounced in the Niger Delta where f os s ilif e r o u ~ marine shales are interealated wi thin delt'lie sands. Major transgressio ns in the Gul f of Gui nea eoincided with global LaIe Cretaeeous to Tertiary sca level rises. The transgressive shall ow shelf benthonie foraminiferal assemblages in the exposed pariS of the Benue T rough and Ihe Dahomey Embayme nt show a progressive increase in faunal diversity from the Albian to the Eocene. This was due 10 im prove marine conditions in these margi nal basins when oceanic circulation was established in the Gulf of Guinea. The weil known Mid-Cretaceous oxygen tleficiencies in the world 's oceans and epicontinental seas were feh in the Benue Trough where anaerobic shales and oxygen.starved bent honic fomminiferal assemblages occur in Upper Cenomanian-Lower Turonian and in Upper Turonian-Lowcr Sanlo nian black shalcs. Oceanol. Ac/(1. 1981. Proceedings 26'h lnternation.,l Geologieal Congress, Geology of contÎnenta l margins symposium, Paris, July 7-17. 1980, 81-85.

RÉSUME Paléocnviro nnements du Golfe de Guinée. Les assemblages de foraminifères de trois bassins de la marge du Golfe de Guinée (la fosse de la Bénoué. le bassin Ju Dahomey, le del ta du Niger) montrent un développement maximal pendant les transgressions marines majeures durilnt l'Albien moyen-supérieur, le Cénomanien supéri eur-Turonien inférieur, le Turoni en supérieur·Santonien inférieur, le Campanien supé­ rieur· Mileslrichtien. le Paléocène moyen-supérieur, t'Eocène moye n-supérieur. t'Oli gocène moyen et te Miocène inférieur à moyen. Les transgressions de l'Oligocène et celtes pl us récentes sont plus pro noncées dans le delta du Niger où des argiles marines fossilifères sont intercalées llvec des sables deltaïques. Les transgressions majeures dllns le Golfe de G ui née ont coïncidé avec les vari,ltions eustatiques positives globales du Crétacé supérieur el d u Tert iaire . Les assemblages de foraminifères benthiques associés aux dépôts. peu profonds, transgressifs montrent une augmenllliion progressive de leur diversité de l'Albie n à J"Eocène. Ceci s'explique par l'accent uation des influences marines dans ces bassins de marge quand la circulatio n océanique s'est établie dans le Golfe de Guinée. Les épisodes anoxiques bien connus du Crétacé moyen dans les oeéllOS el les mers épicontinentales onl été ressentis da ns la fosse de la Bénoué. Des argiles et des assemblages de foraminifères benthiques ind iquant un milieu appauvri en oxygè ne se trouve nt dans les marnes noires du Cénomanie n supérie ur. Turonien inférieur e t du Turonien supéricur-So:.n tonie n inférieur.

Dct'lI/lOl. AC/(j . 1981. Actes 26" Congrès International de Géologie. coll oque Géologie des marges continentales, Pil ris, 7-17 juil. 1980, 81·85.

81 S. W PETIERS

INTRO DUCTIO N Turonian transgression. The La te Turonian to Early Santo­ nian predominantl y planktonic microfaunas wi th low ben­ ln a recent review of the Cretaceous hislory of the South thonic diversities suggest continued oxygen depletion ;tt the Atlantie Ocean Reyment (1980) highlighted rive principal bottom of the Benue Sea , while the more di ver:.c L'tte stages in the paleo-oceflnographic evolution of the Gulf of Campanian to Maastrichtian assemblages especiaJJy in the Guinea. These stages were controlled by tectonic-eustatic Dahomey Embayment. indîcate improved bollom circula­ and geoidal eustatic sea level maxima and comprised 1) the tion. In the Dahomey Embayment the cool water plank toni c initial Late Middle Albinn ~ha ll ow marine connexion a:.semblages in the Eocene pho:.phatic beds :.ugge~t cOlbtal between the North and South At lantiç Oçeans and conse­ upwelling. quently the creation of the Gulf of Guinea; 2) a Late Planktonic foraminiferal biochronological uli ta from the Ccnomanian to Early Turonian tra nsgressio n which :.wamp­ regional marine sh'lles in the subsurface of the Niger Delta cd marginal areas ; ]) a Late Turoninn to Early Santonian ;tllow the identification of the major rises in se'l level and transgression during which true oceanic conditions were the construction of a complete though genenllized tmn:.­ established ; 4) a Late Campanian to Early Maastrichtian gressive-regressive curve for the Nigerian part of the Gulf transgression; and 5) the circulation of deep and weil of Guinea (Fig. 2). oxygenated water during the Paleogene. The approll imate extent of marine inUuence in the GuU of Guinea uuring four of these transgressive peaks is shown in Figure 1. This ALBIAN TRANSG RESSION paleogeographie sketch map was compiled from personal field observations and from field a nd borehole data which Siltstones. shales and limestones referred to as the A~u are available in the following publications: Adeleye (1974). Ri ver Formation in the middle Benue Trough and in the Carter et al. (196]). De Klasz and Jan Du Chene (1979), Jan southern part where the Shale is also exposed Du Chene el al. (1978). Murat (1972). Offodile (1976), have yielded Late Middle Albian and Late Albian oxytropi. Pellers (1 978 a. b : 1979 a : 1980). Re ymen t (1965), Re yre doceratid and diploceratid ammonites in sorne localities (1966). (Re yment. 1955 ; 1965 ; orfodile. Rtyment, 1977). Recause Foraminiferal assemblages from mostly outcropping marine of ilS sparse moJJuscan f

Figure l Sktlch /IIUp ul IIrt Gulf QI Gui/lta sho"';nR IlpproX;/IIlllt lill/ils ul muTine inf/llrncr frutti Middle Albilln 10 lht Pl1/tllgt.'nt.

G U L ~ GIUN t A "zoo ~.,

82 PALEOENVIRONMENTS OF THE GULF OF GUINEA

LATE TURONIA N-EA RLY SANT ONrAN TRA NS­ GRESSION

A more extensiye marine transgression transpired in the Gulf of Guinea from Ihe LateTuronian to the Coniacian and

, ~ M , _ waned in the Early Santonian. T he Awgu Formation (Fig. 2) •• which is lithologicall y continuous with the Eze-Aku Forma­ ti on, was deposited in the Dahomey Embaymenl (Uillman. 1976), and in Ihe southern Benue Trough (Reyment, 1965) while the Jessu and Sukuliye Formations were J eposited in the northeastern Benue Trough (Carter et al., 1963 ; Enu, 1978). Keelcd planktonic foraminifcra such as Margillotrl/lt ­ • • •• D' cami rellÛ , M. difformis and M . sigali. which ind icate Laie '''0_ 1 J>CI'OI,e , Turonian to Coniac ia n age as weil as Early Coniacian coccoliths occur in the limesto ne and shale sequence of the Awgu Formation at the Nkalagu quarry (Peners, 1978 a : 1980). On the eyidence of the keeled planktonic species

~W ~U .c~ .... 'O" . " ", ...... 0" ••• c .... ou.o., '"!JIU· ~o.' "uCOU"T U'.[I ' W~• PoOk/8tnlto % o ~o 100

." o...... "::" Shal. Figure 2 Limule ... - : Morl no d"l.. Slrutigruphic CQrrelmiOIi chan for Cllustul busins ill SOlltllem Nigeria Pionkton!c FOIotn. showillN a 1IllliruliuJ trU/lsgrlssi,'e-regrf'Ssi\'e CUfI'l!. !a..thonie FOfO ....

LATE CENOMAN IAN-EARL y TURONIAN TRANS­ .... Kt .0. ... '00_ ' u ~ o .... u~ ..~ GRESS ION ON uue,,· ' COIT " ••~ou .' U ...nl .... P'

._no.o 'COI ~ U IO M o "0 100 . w. .... u Quun Cenomanian to Turonian bcds are known in the subsurface PIon ' /S.nt~~. of southwestern Nigeria (De Kl asz, Jan Du C hene , 1979 ; Jan Du Chene et al. , 1979) and are exposed in the middle and ':I:~~ '"m"":~j! . ... 00 • :=:' southern Benue Trough as the marine and margi nal marine "'~. shales. limestones. and subtidal sandstones of the Eze-Aku '0 __ _ , --- (correct spelli ng for stralOtype is Eziyiaku) Form

• 3 S. W. PETTERS

Mid-CreUlceous oceanic anoxie event (Sehlanger. Jenkyns. we il preserved foraminifera, ostracods . and radiolaria. 1976) was still evide nl in the Benue Trough at the lime of Foraminiferal indicators of shallow inner neri tic deplhs for . peak marine transgression. The reappcarance of exclusively the top part of the Ameke Formation include the low neritopclagic foraminifera wit h only Whilei"el/a baltica in ph:lflktonic foramÎniferal per cent. the low alpha diversity some black 5hales al thc top part of the Awgu Formation value (Fig. 3). and the low information functio n of 1. 2 to near Awg~ town suggeslS complete depletio n of oxygen on 1.7. However. the common occurrence of radiolarians in the the sea boltom duri ng Ihe Early Santonian withdrawal of the Arneke and Oshosun samples and the abundance of subbolέ sea. nid and acarininid planklonic forllminifera in the Oshosun Formation suggesl the infl uence o f the cold Be nguela Currenl and coastal upwelling. In this study. borehole CAMPAN IAN-MAASTRl CHTIAN TRANSGRESSION samples of the phosphatie beds in Renin Republic yielded a bundant plankto nic foraminifera including Acorinino esnaensis. A. pe"tacamerata. A. pseudotopilensis, A. The transgressive N kporo Shale in southern Nigeria, which so/dadoensi~· so/dadoensis. G/oborOla/ia wilcoxerr sis, and was deposited after the Santonian folding episode in the Subbotina inaequispim. This subbol.inid-acarininid associa­ Be nue Trough. contains Campanian to LaIe Maaslrichtian tion characterized Eocene high latitude regions (Rerggrcn. heterohelicid, rugoglo-bigeri nid . and globotruncanid 1978) and probabl y appeared at shull ow depths in Ihe Gulf planktonic species in Ihe subsurface of so u t h we ~te rn Nige­ of Guinea as a result of coastal upwelling, a process thal ria near Okitipupa, and in reccnt roadcuts near Calabar favoured phosphate sedimentation in the Dahomey Embay­ (Adego ke el al .. 1980: Pellers. 1980). AlIho ugh not as ment. extensive as the earl ier transgressions. Ihe LaIe Campanian to MaastrichtÎa n marine transgress io n supported slightly more diverse be nthonic foraminifcral asse m h l llge~ in sou­ Ihern Nigerill even at shallow depths (Fig. 3). This was due TERTIARY T RANSGRESSIO NS IN THE NIGER to ge nerally improved and deeper oceanic circul ation in Ihe DELTA Gulf of Guinea. The Late Campanian-Maaslric htian trans­ gression ereated a shall ow embayment in to the southern Benue Trough and the Nupe Basin where abundant brackis h Recent foraminiferal biostratigraphic aoalysis (Pellers, marsh arenaceous fomminifera were recorded from near 1979 (..) a nd work in progress h,IVe shown that the Niger (Ja n Du C hene et al .• 1978) and from the Delta offlap sequence conlains highly fossiliferous mari ne shale in Enugu (l'etiers, 1979 Il). shal es which were deposited during major marine tra nsgres­ sio ns. T he occurrence of abundan t planktoni c foramini fera in these widespread marine shales aJlow the recognition of sorn e of the major transgressio ns (Fig. 2) whi ch were PALEOCENE reported by Vail el al. (1977) in their account of global episodes of sea level rises. Diagnostic planktonic foramini­ Wilh the outgrowth of Ihe Niger Delta in the Danian fe rai associations and markers ha ve e nabled Ihe identifica­ (Reyme nt. 1980) Tertiary marine Ir a n ~gress i ons in the Gulf tion o f the Middle to Early La tc Paleocene G/oborotlllia of Guinea could not extend as far inland as previously. pusilla pusilla-Moro1.ollella U/I gulala Zone and the lower GeneraJJy Ihe shoreline of Tertiary marine tra nsgressions part of the PllI/lO rolaliles fJseudomenardii Zone. the Late migrated progressively southward bccause of lotrong dcllaic Eurly Eocene Acarini"(l penlacamerata Zone, Ihe Early progradation. Middle 10 Late Paleocene marine deposits in Late Eocene Trlmcarotrlioides rohri extinc tion dat um. the soulhern Nigeria include the Imo Shale and its Niger Delta Early to Middle Oligocene Cassigerirrella chipa/e" l·ü• eq ui valent. the Aklltll Shale. and the Ewekoro LimeslOne in Pseudoha Sligeri"a micra Zone. the Early Miocenc Globo­ s outhwe~tern Ni gerill. Although the low planktoni c pcr cent rol(llill kll ~ /eri Zone , and the Middle Miocene Gluborotlllia in the Ewc koro Form;H io n a nd in exposures of the Imo Johsi fohsi Zone. Shale in southwestern Nigeria. the low alpha diversity values for benthonic foramini fera (Fig. 3), and the bentho­ nic species composition (Petlers. 1979 b) suggest shallow inner neritic depths. Ihe occurrence of keeled planktonic CONCLUSIONS species such as MoroWliella allgulalll. M. acutispira, and M. aequa (Pellers, Olsson. 1979) a ll e .~ 1 10 the prevlllcnce of Late Cretaceous to Early Tertia ry marine transgressions in fully oceanic conditions offshorc. 8arriers 10 deep oceanic the Gulf of Guinea are wc!! represented by the rich circula tion broke down in both the South and North Atlantic forami niferal assemblages in the marginal basins such as the Oceans d uring the Paleogene and all owed the enuy of deep Be nue Trough lInd the Da homey Emlxlymenl. In the Niger weil oxygenated waters (Berggren. 1978; Reyment. 1980). Oelta Ihe highly fossiliferous marine shales intercaJated within the thick dellaic sequence were dcpositcd during Late Eocene, Middle OJigocene, and Early to Middle EOCENE Miocenc sea level riscs. The high planktonic for,lminiferal pcrcenlages and exclusi­ Lower and Middle Eocene deposit s in the Gulf of Guinea vely planklOnic assemblages in some black shales in the include the clastic eKposed Ameke and Nanka Fonnations Upper Cenomanillfl to Lower Santonian of thc Benuc in eastern Nigeria. the subsurface Agbada Formation in the Trough :lIIest to nerilOpelagic habitats for the heterohelicids Niger Delta. and the phosphatic Oshmun Formation in the and globigerine taxa which dominatc these microfaullas. Dahomey Embayment (Fig. 2). Exposures of the top part of Benthonic foraminifera are either completely absent from the Ameke FormatÎon near and subsurface SOffie of these shales duc to anaerobic bottom conditions or samples of the Oshosun Formation contain abundanl and hilvc very low diversities on account of insufficient oxygen

84 PALEOENVIRONMENTS OF THE GULF OF GUI NEA at the sea bottom. Oceani c circulation amelioraled in the due 10 the coastal upwelling of deep nutrient-rich waters, a GuU of Guinea {rom Laie Campanian onwards and ca u ~ed process Ihal favoured phosphatizalion. slightly higher benl honÎc diversities at shall ow depl h ~. T he lower planktonic percentages in Tertiary shallow waler Acknowledgem ents assemblages in Ihe Gulf of Guinea wa ~ due 10 the preference of Tertiary pJanktonic foraminifera for ocennic habitats. I lhank Shell Pelroleum Company of Nigeria Lld ., and Gul f The prevalence of cool watcr planktoni c foraminifcra in Oil Company of Nigeria Ltd. for supplying samples from Eocene phosphatic beds in the Dahomey Embayment was the Niger Delta.

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85 86