World Applied Sciences Journal 18 (1): 91-106, 2012ISSN 1818-4952© IDOSI Publications, 2012DOI: 10.5829/idosi.wasj.2012.18.01.3572

Corresponding Author: P.A. Adeonipekun, Department of Botany, Palynology Paleobotany Unit, Faculty of Science, University of Lagos, Nigeria.

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Bio-Sequence Stratigraphy of Shagamu Quarry Outcrop,Benin Basin, Southwestern Nigeria

P.A. Adeonipekun, O.A. Ehinola, I.A. Yussuph, A. Toluhi and A. Oyelami1 2 3 4 5

Department of Botany, Palynology Paleobotany Unit, 1

Faculty of Science, University of Lagos, NigeriaEERG, Department of Geology, University of Ibadan, Ibadan-Nigeria2

Earthprobe Nigeria Ltd. Lagos, Nigeria3

Pillar Oil and Gas Ltd. Lagos, Nigeria4

Global Energy Company, Ikeja, Lagos, Nigeria5

Abstract: The Paleocene/Eocene boundary has been a significant subject globally for sometimes now. Thiswork focuses on the application of sequence stratigraphy where diagnostic microfossils trends of abundancesand qualitative parameters can be integrated with sedimentology in the recognition of stratigraphic surfaces.Such surfaces / boundaries are highstand systems tract (HST), transgressive systems tract (TST), lowstandsystems tract (LST), maximum flooding surface (MFS) / condensed section (CD) and sequence boundary (SB).These will no doubt provide an excellent opportunity for the identification of a functional boundary in a fairlyassemblage-rich basin. 14 outcrop samples from the Shagamu Quarry were studied for their palynomorphs andforaminifera. Correlation with previous works on nannofossils and palynomorphs was also carried out to aidinterpretations. Systems tracts and surfaces recognized and proposed for the study area are: HST (4.2-7.1 m,9.5-11.5 m and 26.8-28.0 m), TST (7.7-9.5 m, 11.5-19.0 m, 19.0-20.2 m {TST/CD)}), LST (1.3-4.2 m and 20.2-26.8 m),MFS (56.8, 58.1 and 59.7 Ma) and SB (56.5, 57.0, 57.5 and 60.4 Ma. Depositional environments varied from innerto outer neritic settings. This study will aid exploration efforts not only in the onshore but also in offshoresettings where sizeable sections have been reported missing.

Key words: Bio-sequence stratigraphy Palynomorph Foraminifera and sedimentology

INTRODUCTION There is no doubting the fact that the contributions

The Paleocene/Eocene boundary (PEB) of basins knowledge of the sedimentary basins of Nigeria withhas been argued not to be the same globally [1]. respect to the exploitation of several mineral deposits theyBoundaries are very important for ease of contain.correlation and zone of target recognition as well as However, sequence stratigraphy is yet to be appliedfor other exploration and production utilization. The to many of the outcrops and wells studied by some ofdearth of published biostratigraphic data on the these workers. Application of this technique is longPaleogene of Nigeria is however, a militating problem. overdue for the contribution of high-resolution sequenceEven, much of the available data on the Paleogene stratigraphy would have added to the present knowledgeof Nigeria is proprietarily controlled by oil for optimum mineral exploitation. Elsewhere, thiscompanies and this situation has seriously hampered technique has been applied [12, 13]. Most recently,cross-fertilization of ideas. A well published data on sequence stratigraphy integrated with lithofacies andthe Paleogene of Nigeria would have helped palynofacies was applied to the Paleogene strata of theexplorationists greatly in erecting a generally Anambra Basin/Afikpo Syncline complex [14]. Fiveaccepted “Functional Boundary” for the PEB at least for sequences and four regional boundaries were identified asthe Niger Delta. against a single sequence [15].

of several workers [2-11] have made up our present

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Application of sequence stratigraphy to strata from these, a more definitive boundary can be erected.these sedimentary structures will no doubt create a good Applying this technique for the whole of Benin Basin willchronostratigraphic framework for a more efficient provide a better chronostratigraphic framework for theexploitation of the minerals contained. Efficient and exploitation of its abundant mineral resources.effective exploitation is important now that the Federal This paper therefore aims at recognizing aGovernment of Nigeria is focusing, apart from oil, more ‘functional’ and more definite boundary guided byattention on other minerals such as bitumen, gold, recognized marine flooding surfaces and sequencelimestone and coal as possible alternative incomes. In the boundaries. This is achievable through the use ofBenin Basin, “probable - in - place reserves of bitumen in qualitative and quantitative palynomorphs andoutcrop zone (110 x 6sq km) are in the order of 170 x 10 foraminiferal trends and features as well as sedimentary6

bbls” [16]. This estimate is worthy of further investigation parameters.to provide encouraging data bank for future investors inthe mineral exploitation. Besides, commercial quantity of Geological Setting: The Shagamu Quarry is locatedhydrocarbons was reported in the offshore of within the northeastern part of the elongated Benin BasinBenin Republic [17]. Also, an indigenous company, Yinka that lies pseudo-parallel to the West African coast. ThisFolawiyo Company Limited, is currently exploring oil in basin begins from the Ghana Ridge onshore and extendsoffshore Badagry, Nigeria. through Togo and Benin Republics to the Benin Hinge

Although, several geochemical studies [18, 19] have Line in Western Nigeria (Fig. 1). Formation of thisreported poor conventional oil prospect for the onshore sedimentary structure like most basins in Africa was inBenin Basin, its reported gas potential is however a response to the separation of the African and Southvaluable asset. This is because of the current global American landmasses and the subsequent opening ofneed of gas for its environmentally friendly nature. the Atlantic Ocean in the Jurassic - lower CretaceousAlso there is need for more detailed geological, [11, 16, 22].geophysical and geochemical studies which may lead to Two structural units within the Benin (Dahomey)discovering of conventional oil, not only in the deep Basin were recognized and these are the Benin Basin-offshore, but also in the shallow offshore and onshore proper, which extends from the Ghana Ridge to thesettings of this basin. Orimedu-Ijebu Ode - Ilorin axis and the Okitipupa

Meanwhile, Lucas [20] has applied palynology for Structure that stretches from the Ijebu Ode - Ilorin axis tosequence stratigraphic study of the Anambra Basin the Benin Hinge Line [22]. The Benin Basin proper showsin the Paleocene / Eocene times. Recently, the along strike, a steady drop in basement floor ranging frompalynostratigraphy and paleoenvironments of the 1900 to 2200 m from east (in Union-1 Well) to westOshosun (Akinbo) Formation at the Shagamu Quarry was (Nigeria/Benin Republic border in B-1 Well) and a north-studied [21]. No sequence stratigraphy was attempted and south steep profile from Bodashe-1 Well (1513 m) toa functional boundary for the PEB was not studied. Afowo-1 Well (2130 m) and D-2 Well (2727 m). On the

Accurate dating of the Paleocene / Eocene deposits other hand, the Okitipupa Structure indicates a shallowerof the Oshosun and Ewekoro Formations of the Dahomey basement floor (1000 - 1400 m) except within the Ise(Benin) Basin was attempted using radiometric dating of graben (1800 - 2000 m) (Figs. 1 and 2).the glauconites recovered from two levels within the The deposition of a thick sequence of continentalOshosun and Ewekoro Formations in southwest Nigeria grits and pebbly sands (over 1400m) over the entire basin[6]. This exercise confirms the age of the two formations was as a result of a rift - generated basement subsidenceto be Paleocene in the lower section and the lesser upper during the lower Cretaceous (Neocomian) [22]. Later in thepart more of Eocene being found to average 54.00 +/- 2400 Santonian, a probable tectonic event associated with themy. In spite of this attempt however, there is need for a closure and folding of the Benue Trough led to the tilting‘functional’ and more definitive PEB in the study area at and blocks faulting of granites, gneisses and associatedleast for exploratory work. This can be aided by sequence pegmatites and other sediments to form a series of horstsstratigraphy as it offers the opportunity of dating and grabens. Subsequently, a great erosional activitysequences more accurately from recovered microfossils. accompanied this tectonism and this led to the erosion ofThe availability of microfossils in rock samples at high the pre-drift sediments greatly from the horsts [22].resolution will enable appropriate globally recognized However, tectonic activity became insignificant in theflooding surfaces to be picked. With the recognition of Maastrichtian with only gentle subsidence taken place.

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Fig. 1: Location Map of the Study Area. A and B is Geological maps, C is a planimetric map (Adapted from Okosum, 1990and Adegoke et al., 19980)

Fig. 2: Cross-section of wells drilled in various in structural elements of Benin Basin (Adapted from Coker andEjedawe, 1987)

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Table 1: Stratigraphy of Benin basin

Stratigraphy: The oldest dated sediments onshoreBenin Basin is lower Cretaceous [22] while theoldest dated offshore is a non-fossiliferous foldedrocks of pre-Albian age [17]. However, the oldestknown outcrop is the Maastrichtian part of theAbeokuta Formation that sits unconformably on the“highly weathered and fractured” basement complex[16]. The Abeokuta Formation, as shown by onshoredrilled wells, sits conformably on the basementcomplex.

The Abeokuta Formation (Neocomian/Paleocene) hasbeen assigned a group status and sub-divided into threeformations that include: Ise (oldest), Afowo and Araromi(youngest) [22]. Overlying the Abeokuta Formationconformably is the Paleocene/Eocene limestone, marineshales and sandy shales and claystones of the Ewekoro,Oshosun and Ilaro Formations respectively. Late Tertiarysediments of Benin Formation terminate the stratigraphicsequence with shallow marine - none marine gravel, sandand sandy clay that sit “unconformably” on thePaleocene / Eocene sequence (Table 1). A thick sequenceof the Benin Basin in the offshore Benin Republic, rangingin ages from pre-Albian to late Miocene has been reported[17].

MATERIALS AND METHODS

Fourteen outcrop samples collected on a fieldtrip to Shagamu Quarry, southwest Nigeria primarilyfor a work on nannofossils were used. Handtrowel, spade and hammer were utilized for thecollection. Although a total of 98 samples werecollected at 50cm (0.5m) interval, only fourteen ofthem from the Oshosun (Akinbo) (11 samples), Ewekorolimestone (2 samples) and one from the shale intercalationwithin the limestone were available and therefore treatedfor this work.

Palynological Preparation: Twenty-five grams each ofsamples was palynologically treated with the use ofHCI, HF and acidified ZnCl solution for heavy mineral2

separation. Two sets of samples were prepared - oxidizedand unoxidized. In the oxidized set, no Hafnisphaeraseptata dinocyst was recovered hence, the preference forunoxidized residue. Samples were microscopically studiedusing Olympus microscopes and photomicrographs ofsome important palynomorphs were taken (Plates 1-4).Identification of recovered forms was done using severalpublished papers on the Paleogene of Nigeria ondinoflagellates [23, 24].

Foraminiferal Preparation: Twenty-five grams of rocksample was put in aluminum cups and soaked in keroseneovernight. Soaked sample was then washed withdetergent and sieved through a 63 microns size sieve afterwhich it was dried on a hot plate. Sample was then put innylon sachets in readiness for picking. Fractions of thesample were done to separate recovered fossils intocoarse, medium and fine fractions. Analysis andidentification of recovered fauna were done using Bolliand Saunders [25] and other published atlases for thePaleogene. Bathymetric inferences were guided byAdegoke et al. [26].

Sedimentological Preparation: Slightly crushed sampleswere washed with water and dried for analysis.Roundness, colour, sphericity and sorting of the sandparticles were recorded and occurrences and non-occurrences of accessory minerals such as ferruginousmaterials, glauconites, carbonaceous detritus, pyrites andcalcites as well as shell fragments were also noted foreach depth. The Sand: Shale: Calcite ratio trend generatedfrom samples was improvised for electronic logs as thishas signature similar to the logs.

RESULTS AND INTERPRETATION

Biostratigraphy: The distribution and abundances of thepalynomorph species recovered from the outcrop sectionsare presented in Figure 3. Table 2 represents thepalynostratigraphy scheme and the samples studied fallwithin the Proxapertites operculatus zone [2]. Correlatingwith the result of studies from adjacent Niger Delta, twozones: P200 and P300 (P330 subzone) were recognized [3].These schemes assign the studied interval a Paleocene/Eocene age. This zonation is complemented byforaminiferal data [1] and augmented by nannofossils data[27] and the application of Martini [28] scheme (Table 2).

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Fig. 3: Distribution and abundances of palynomorphs from the study section

Table 2: Palynostratigraphy of shagamu quarry outcrop

Samples 10A- 40A (˜28.00-11.57 m) P200 Zone Dictyophillidites harrissi (Shell Spore 11) and(Selandian) late Paleocene (Planktic ?P3 - P5): Top ofthis interval was marked by the Quantitative topoccurrences of Crototricolpites crotonoisculptus,

Adenanthera simplex at sample 40A (11.57m). The basewas provisionally marked at sample 10A (˜28.00m) - baseof analyzed interval - due to the non recognition of

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the base occurrence of Mauritidites crassiexinus. This interval contains foraminifera GlobigerinaHowever, the base of an event of Proxapertites triculinoides, Morozovella pseudobulloides, Epinoidesoperculatus at sample 21A (20.21m) could not be pseudoelevatus, Valvulineria spp., Valvulineriaaccurately utilized due to:(i) poor recovery of aegyptica, Heterolepa mickanni, H. pseudoungerianapalynomorphs within the limestone section, (ii) irregular and Cibiscorbis influenta (Fig. 4 and Table 3). Thissampling which prevented higher and regular interval assemblage suggests a ?P3 - P5 Planktic zone. Jan Duresolution and (iii) the possibility of recovering P. Chene et al. [4] assigned the lower section of this intervaloperculatus in the ‘unsampled’ section of the limestone to the Globorotalia pusilla pusilla / Morozovellabetween 28.00 and 33.73 m outside the studied interval angulata (P3) Zone of Berggren [1]. The upper part of the(Fig. 3 and Table 2). shale intercalation within the limestone up to its top to the

However, characteristic late Paleocene sporomorphs Planorotalites pseudomenardii (P4) Zone belong toand their features include abundant occurrences of ?P3-P5 Plankton Zone. Nannofossil data from Olowu [27]Foveotriletes margaritae, Crototricolpites recognized the NP4 Zone [28] on the strength of pickingcrotonoisculptus, Retidiporites magdalenensis, the LDOs of Ellipsolithus macellus and FasciculithusLongapertities vernendiburgi and Proxapertites cursus tympaniformis within the 21A sample. This furtherat 20.21m as well as quantitative top and abundant supports the recognition of the P3 Planktic zone and theoccurrences of D. harrissi, Retistephanocolpites occurrence of the 59.7 Ma marine events within thiswilliamsii and Echitriporities triangulifomis interval. Other characteristic nannofossils recovered [28](Plates 1, 2, 3 and 4). are Neochiastozygus perfectus and Chiasmolithus bidens

Recovered characteristic Paleocene dinoflagellates (Tables 2 and 3).and their observed features within this interval Informal dinoflagellate zonation of this intervalalso include Hafniasphaera septata, Apectodinum spp. recognized an Apectodinum abundance zone within theand Trithyrodinium evitti as well as Bahaidinia spp. middle - late Paleocene age. The top is marked by the topand Parabahaidinia laevigatus (Plates 1, 2 occurrence of “Acritarchus starrus”. It is characterizedand 3). Other important Paleogene dinocysts include mainly by the occurrences of Homotryblum sp.,Paleocystodinium golzoensis, Diphyes colligerum, Muratodinium fimbriatum, Trithyrodinum evitti,Muratodinium fimbriatum, Ifecysta pachyderma, “Acritarchus starrus” and “NormapollisKallosphaeridium spp. and Cleistosphaeridium spp protrutetralobus”. This informal zone is equivalent to the(Fig. 3). Ceratiopsis speciosa/Apectodinium zone.

Fig. 4: Distrbution and abundances of foraminiferal species from the study section

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Table 3: Foraminiferal Biostratigraphy of shagamu outcrop

Samples 40A (11.57m) - 53A (11.57 - ˜2.00 m) P300 assemblages support a late Paleocene - early Eocene age.(P330 Subzone) ?P5-P6 Planktic Zone, Thanetian- Jan Du Chene et al. [4] assigned this interval to theYpresian (Late Paleocene - Early Eocene): The top here Apectodinum homomorphum / Wetzeliellawas provisionally placed at sample 53A (˜2.00m), being meckelfeldensis Zone [29].the top of analyzed interval. Its base was recognized by This interval is equivalent to the ?P5-P6 Plankticthe Quantitative top occurrences of Crototricolpites zone based on the recognition of the LDO ofcrotonoisculptus, Dictyophillidites harrissi (SHELL Morozovella cf. velascoensis at sample 46A (7.74 m).Spore 11) and Adenanthera simplex at 40A (11.57 m). Other associated foraminifera include Brasilina ihuoensis

Palynological characteristic features include: very and Hopkinsinia danvillensis as well as co-occurrencerich occurrences of Proxapertites operculatus, of Globigerina velascoensis and Acrinina nitidaProxapertites cursus, Longapertities spp., L. (Fig. 4 and Table 3).vaneendiburgi, Retimonocolpites spp., Corsipollenites The informal zonation of this interval produced thejussiensis, Spinizoconocolpites spp. and D. harrissi. Apectodinum homomorphum/Hafniasphaera septataFoveotriletes margaritae, Deltoidosporites sp., zone within the late Paleocene section. The top of theCupaneidites spp. and Verrucatosporites sp. were also interval is marked by the top occurrence ofrecovered (Plates, 1, 2, 3 and 4). Diagnostic dinoflagellates Hafniasphaera septata. Its base is marked by the toprecovered within this interval include Phelodinium sp., occurrence of “Acritarchus starrus”. It is characterized byApectodinium spp., Eocladopyxis panicualata, abundant Apectodinum spp., Ifecysta sp. andAreoligera senonensis, Kallosphaeridium spp, Cleistopheridium sp.Diphyes colligerum, Ifecysta pachyderma, Lingulodinium Also recognized in the upper section is themachaerophorum, Polysphaeridium subtle and Areoligera senonensis/Lanternospheridium lanosus zonePaleocystodinum golzoensis as well as of Eocene age. It is characterized by dinocysts AreoligeraCleistospharidium spp. Other dinoflagellates recovered senonensis and Lanternospheridium lanosus. Thisinclude Bohaidinia spp., Parabohaidinia spp. and separates the Homotryblium tenuispinosum/Areoligera sp (Fig. 3 and Table 2). These palynological Hafniaspaera septata zone.

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Fig. 5: Tentative Sequence stratigraphic interpretationbased on Nannofossils (Olowu, 1997)

Depositional Environments: In deducing the depositional The occurrences of palynological indices such asenvironments of the studied quarry samples, six Apectodinium spp., Paleocystodinium golzoensis,lithological units were demarcated. They are the (i) Basal Trithyrodinium evitti, Muratodinium fimbriatum andShelly Calcite, (ii) Calcareous Sandy Limestone, (iii) Lower Achomosphaera sp. as well as continental charredDolomitic Shaly-sand, (iv) Lower Shale, (v) Upper Gramineae cuticles, fungal elements and microforaminiferalDolomitic Shale and (vi) the Upper Shale unit. wall linings, support a nearshore possibly estuarine

Inferences from palynomorphs and foraminifera sub-environment [29, 31]. Recovered benthic foraminiferasynchronized with those of accessory minerals and include Ammobaculites sp., Heterolepa floridana, H.lithology were utilized. The works of Selley [30] and mckannai, Nonionella sp., Cibicorbis inflata andAdegoke et al. [26] were employed in the Eponides sp. These, together with echinoid remains,paleoenvironmental interpretation (Fig. 5 and Table 3). ostracods and shell fragments further support a coastalBathymetric and proximity inferences were derived from to inner neritic bathymetry. Planktics are very few withinforaminifera and palynomorphs respectively. The this interval.lithological units are briefly described below:

The Basal Shelly Calcite Unit (10A-12A) 28m- ˜27m: sole sample representing this unit has sand, shale, calciteTwo samples represent this interval. They are abundantly ratio of 50:0:50. Its sand particles are buff, fine to coarse-rich in calcite and shell fragments with low proportion of grained, but dominantly medium-grained. They aresandstones. The few sand grains are dominantly fine- subangular, very poorly sorted and well cemented. Somegrained in texture. Sample 10A has greyish hard calcite limestone pebbles (milky coloured) are present in thewith rare quartz while sample 12A contains yellowish sample. Recovered accessory minerals include shellwhite, often brownish grains which are dominantly fine- fragments, carbonaceous detritus, pyrites, glauconitesgrained. The above lithological indices coupled with and rare coaly fragments. These indices suggest a highregular occurrence of rare carbonaceous detritus suggest energy deltaic setting. Palynomorphs are extremely poordeposition in a middle-low energy shallow shelf setting. due to secondary re-crystallization typical of calcite and

The Calcareous Sandy Limestone Unit (14A) (˜26m): The

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dolomite deposits [32]. However, appreciable recovery of (deep marine dweller foraminifer) is high, that of shallowbenthics which include Heterolepa pseudoungeriana, marine dweller dinoflagellate - Apectodinium spp. is lowH. mckannai, Asterigerinatha rhodensis and Nonion to non occurrence. Environments “shallowed” up tocentrosulcatum as well as micromulluscs and shell middle neritic within sample 48A-49A with thefragments confirm a coastal deltaic setting. dominance of the dinoflagellate assemblage by

The Lower Dolomitic Shaly Sand Unit (21A) 20.21m: The Paleocystodinium sp.representative sample here has sand, shale, calcite ratio of60:40:0 and it represents a sizeable lithological section The Shelly Calcite and Sandy Unit (51A) ˜4.00m: Thiswithin the Ewekoro limestone. The sand are whitish, sample is dolomitic with weathered shaly sand particles.dominantly fine-grained, angular and very poorly sorted. Its sand, shale, calcite ratio is 40:30:30. The sands areThe shale is flaggy, grayish and hard. Its accessory whitish, dominantly fine-grained, angular and well sorted.minerals include shell fragments and glauconites. These Its yellowish and weathered shale is flaggy and soft. Theindices indicate a low-intermediate energy shallow marine sample contains rare carbonaceous detritus, shellsetting. fragments with 30% calcite proportion. These lithological

Dinoflagellates recovered include Kallosphaeridium features indicate a proximal intermediate to high energyspp., Polysphaeridium zoharyi, Apectodinium spp., marine setting.Parabohaidinia spp and Spiniferites spp. Abundant Expectedly, palynomorph occurrence is poor due tooccurrences of shallow to slightly deep water secondary crystallization that produced dolomite.Achomosphaera spp. and few deep water Impagidinium Traverse [32] noted that re-crystallized rocks lack goodsp. suggest a deepening over the underlying into the proportion of palynomorphs. However, the fewmiddle neritic realm. This inference is supported by the dinoflagellates recovered are Cleistosphaeridium spp.,non recovery of freshwater Botryococcus brauni, fungal Cordosphaeridium spp., Operculodinium sp. andelements and continental charred Gramineae cuticles. Apectodinium hormomophum. Also poor in proportionsForaminiferal recovery is similar to the underlying sample are pollen and spores while high resistant continentalexcept that populations increased appreciably to confirm fungal elements, charred Gramineae cuticles anda middle neritic setting. freshwater Botryococcus are fair in proportions. These

The Lower Shale Unit (40A-49A) 11.75m-6.34m: This influence of imputs by river systems and thereforesizeable unit is predominantly shaly with no sand, silt or indicate a proximal fluvio-marine setting. Foraminiferalcalcites. The shale is grayish, flaggy and slabby in shape diversity and populations nose-dived appreciably,with no accessory minerals. These indices indicate low however, the occurrence of some planktics and benthic -energy quiet water settings. Palynological components of Buliminella subfosiformis confirm an inner neriticsamples 40A - 43A are dominated by Apectodinum spp., bathymetry.Cordosphaeridium cf minima, Selenopemphix sp andSpiniferites spp. as well as Paleocystodinium spp. These [The Upper Shale Unit (53A) 2.00m: The representativedinoflagellates are shallow marine dwellers [21, 31, 33]. sample of this unit is made up of shale solely. The shaleAppreciable proportions of pollen and spores as well as is yellowish, platy and soft to hard. Continental andother palynomorphs of this interval indicate a shallow freshwater palynomorphs disappeared from this sample.marine setting in the neritic realm. Deep marine dinoflagellate - Impagidinium sp. - was

However, within samples 44A - 46A, the presence of recovered along with other shallow marine forms. Thesedeep water Impagidinium sp. and shallow to deepwater indicate a slightly deeper setting than the underlying unit.Achomosphaera andalusiella as well as cf. An inner neritic is confirmed by the occurrence of B.Nematosphaeropsis labyrinthus indicates a favourable subfosiformis and M. cf. velascoensis as well aslow energy deeper marine setting (middle - outer neritic) Lenticulina grandis.than the underlying samples of 40A - 43A. This iscorroborated by the regular recovery of benthic and Dinocyst Zonation planktic foraminifera such as Buluminella subfusiformis, Apectodinum Zone (Middle - Late Paleocene): ThisLenticulina grandis, Hopkinsonia danvillensis andAlistoma scalaris as well as Globigerina velascoensis,Morozovella cf. velascoensis and Eponides spp.Incidentally, while proportion of Buliminella spp.

Lejeunecysta cf. cummunis, Cordosphaeridium spp and

palynological indices suggest a degree of continental

informal zone is characterized by occurrences ofHomotryblum sp., Muratodinium fimbriatum,Trithyrodinum evitti, “Acritarchus starrus” and“Normapollis protrutetralobus”.

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Table 4: Sequence stratigraphy of shagamu outcrop

Fig. 6: Sedimentology and sequence stratigraphy of the study section

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Apectodinum Homomorphum/hafniasphaera Septata the high abundance and diversity of foraminifera andZone (Late Paleocene): This zone is characterized byabundant Apectodinum spp, Ifecysta sp. andCleistopheridium sp.

Areoligera Senonensis/lanternospheridium LanosusZone (Eocene): Characteristic dinocyst are Areoligerasenonensis and Lanternospheridium lanosus.

Sequence Stratigraphy: The application of sequencestratigraphy according to Vail and Wornardt [34] to theEwekoro/Akinbo Formations of Shagamu Quarry yieldedsome useful results. This application was made possibleby the recovered palynomorphs, foraminifera andlithological parameters as well as the data from Olowu’s[27] work on nannofossils from same samples.Recognition of sequence stratigraphic features wasfurther guided by the work of Ventris [35]. The recognizedsurfaces have been correlated with the Global Cycle ofHaq et al. [44] (Table 4 and Fig. 6).

SEQUENCE-1 (28m-27m): Interval 10A - 12A (28m - 27m)seems to be a Highstand systems tract. This is based onthe abundant shell fragments and calcite grains withrelatively fair occurrence of carbonaceous detritus. Theabundant occurrence of benthic foraminifers and lowoccurrence of planktics give credence to this deduction.The poor recovery of palynomorphs was due to thecalcite re-crystallization. However, the occurrences ofmore resistant charred Gramineae cuticles, fungalelements, few sporomorphs and few shallow marinedinoflagellates are indicative of terrestrial communication.

SEQUENCE - 2 (27m - 21.50m): Within the limestone,section represented by sample 14A points to a Lowstandsystems tract based on its unprecedented 50% sandcontent, with no shale but 50% calcite. Palynomorphs areextremely low with Verrucatosporites cf. usmensis sporeand charred Gramineae cuticles being the only formsrecovered. This coincides with a general decrease inforaminiferal abundance and population. Therefore aprobable Sequence Boundary equivalent to the 60.4 Maevent may mark the beginning of this tract at 14A.

Although sampling interval is irregular within theEwekoro Formation, a representative sample of the shaleintercalation - 21A, most likely indicates that this shalysand unit is a condensed section. It contains theTransgressive systems tract and its maximum floodingsurface (MFS). This MFS is suspected to be the 59.7 Maevent based on the recognition of the NP4 calcareousnannofossil zone [27, 28] and the equivalent ?P3-P4Planktic zones [28]. Supportive of this marine event are

palynomorphs as well as absence of continental fungalelements and charred Gramineae cuticles.

Samples between 21A and 40A were not available foranalysis hence much could not be said about the trend offossils’ abundance and diversity. However, Jan DuChene et al. [4] reported abundance of plankticforaminifera about 50cm below the top of the EwekoroFormation at the Ewekoro Quarry. This abundance maymost likely represent a condensed section containing themaximum flooding surface equivalent to the 58.1 Ma.This is because this event occurred within thePlanorotalites pseudomenadii zone of plankticforaminifera recognized by Jan Du Chene et al. [4].Extrapolating this observation to the present study,therefore it probably means that the 58.1 Ma eventsoccurred at slightly below the top of the EwekoroFormation. Its systems tracts, however, could not beidentified in this study since samples were not available.However, the unconformity below the Akinbo Formationmay represent stratigraphically, the 57.5 Ma SequenceBoundary.

SEQUENCE-3 (40m - 41m): At sample 40A from theAkinbo Formation, an abundance of palynomorphs isobservable with no record of foraminifera. However, highnannofossil diversity and abundance recorded by Olowu[27] indicate that this sample represents the vestiges of amarine event, most likely the Highstand systems tract(HST) type. This tract may be the HST of the 57.3 Maevents of which the major part might have been lost toerosion, faulting or non-deposition as indicated by theunconformity that separates the Akinbo from the EwekoroFormations. Characterizing this HST is the markedreduction of marine indices, low abundance and diversityof palynomorphs while fresh-brackish water Botryococcusand continental fungal elements have high and regularoccurrences.

Be that as it may, a Sequence Boundary equivalent tothe 57.0 Ma terminates this sequence at 41A.

SEQUENCE-4 (41m -51m): This sequence commencedwith the Lowstand systems tract that is characterized bynon recovery of foraminiferal species, fair occurrence ofsporomorphs and abundant fungal elements. Above this,is the Transgressive systems tract that commences atSample 44A. From Sample 44A, proportions ofpalynological marine components increased. Aconcomitant increase in both benthic and plankticforaminifera also characterized the interval 44A - 46A.Regular occurrence of planktics however peaked at 46Awith high diversity and abundance of planktics and

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benthics as well as abundant dinoflagellates. At the top such as Echitriporites trianguliformis (?top below theof this tract - 46A, the 56.8Ma MFS is recognized based boundary), Spinizonocolpites echinatus, Trilatiporiteson the recognition of the LDO of Morozovella cf. kutchensis, Spinizocolphites baculatus andvelascoensis [25]. Olowu [27] also recognized this marine Dictyophillidities sp. as well as Adenanthera simplex.flooding surface (MFS) with similar abundance and In support of a possible boundary at 6.34m is thediversity peaks in nannofossils (Fig. 6). Further credence recovery of typical Eocene dinoflagellates - Areoligerais given to this event (MFS) by the reduction in marine senonensis and Lanternosphaeridium lanosus at 4.0mpalynomorph population and diversity within a generally (51A) and 2.0m (53A) respectively. These dinocysts werehigh abundance together with no record of fungal not recovered below these levels (Table 4 and Fig. 6).elements and charred Gramineae cuticles all within the This gives strong support for a possible boundarywholly shaly deposit. Within 46A - 51A, high diversity between the Paleocene and Eocene to be located belowand reduced abundance of foraminifera coupled with high the lower depth - 4.0 m. Since there was no sampleproportions of fresh - brackish water Botryococcus and between 4.0 and 6.34 m much cannot be said of thehigh to low diversity and population of dinoflagellates interval. However the extinction of typical Paleocenecharacteristic of a Highstand systems tract are dinoflagellate forms such as Hafniasphaera septata andobservable. Phelodinum sp. at 6.70 m (48A) suggests a boundary at

SEQUENCE-5 (51m-53m): At Sample 51A, a sequence boundary at 6.34 m looks more appropriate since it hasboundary likely equivalent to the 56.5 Ma SB was strong backing from the sporomorphs. Corroborating thisrecognized based on commencement of abundant 6.34 m proposed PEB is the peak occurrence of charredoccurrences of continental charred Gramineae cuticles and Gramineae cuticles at 4.0 m.fungal elements with concomitant reduction in the The abundant recovery of nannofossils from thisproportions of pollen and spores. These palynological quarry [27] also enabled the recognition of the top offeatures characterize the terminal Lowstand systems tract zonal marker Paleocene nannofossil - Biantheolithusof this sequence along with appreciable reduction in the sparsus to be picked at 49A (6.34m) while Fasciculithusforaminiferal population, particularly planktics. The reichardii was also found restricted to the Paleocene.dolomitic shalysand lithology with shell fragments and Sequence sratigraphy also gives strong support tocarbonaceous detritus of this tract indicate improved this 6.34 m boundary. At 7.74m (46A), the last downholecontinental communication relative to the underlying. occurrence (LDO) of Morozovella cf. velascoensis

DISCUSSION Therefore, a PEB around 56.5 Ma could not be found

Typical Paleocene pollen and spore reported in The three informal dinoflagellate zones recognized areNigeria and found in this study include Retidiporites equivalent to the Ceratiopsis speciosa/ Apectodiniummagdalensis, Longapertites vernendiburgi and and the Homotryblium tenuispinosum/ HafniaspaeraFoviotriletes margaritae [2]. The top occurrences of septata. The Apectodinium zone (middle - late Paleocene)these three sporomorphs, however, do not mark the of this work is equivalent to the CeratiopsisPaleocene/ Eocene boundary according to Germeraad speciosa/Apectodinium. The demarcated Apectodiniumet al. [2]. Several other typical sporomorphs of Paleocene homomorphum/Hafniasphaera septata zone (lateage offer possibility of utilizing their tops at same or very Paleocene) and the Areoligera senonensisclose depth as a possible boundary. Some of these /Lanternospheridium lanosus zone (Eocene) aresporomorphs have been reported in Nigeria and other equivalent to the Homotryblium tenuispinosum/tropical sedimentary deposits to be essentially Paleocene. Hafniaspaera septata. With availability of more works on

Frederiksen [24] reported that Longerpertites the sediments of this basin, this informal zonation maypsilatus, Retimonocolpites ovatus, Proxapertites cursus, prove useful for biostratigraphic purposes.Rhombipollis geniculatus, Psiladiporites sp. andPolygalacidites clarus were found not younger than late CONCLUSIONSPaleocene in age from Bara and Lankhia Formations ofPakistan. These sporomorphs were also recovered in this The richness of the studied part of the Shagamupresent work and their tops at sample 49A (6.34m) Quarry in palynomorphs and foraminifera is good enoughsuggest that the PEB is close to this depth. Also for sequence stratigraphic study. This richness has led torecovered from the Shagamu Quarry are typical PEB forms the proposal of a “functional boundary” for the PEB from

6.34 m or younger but not younger than 4.0 m (51A). A

synonymous with the 56.8MFS was recognized.

below, but above.

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the integration of the recovered fossils, augmented by 6. Adegoke, O.S., T.F.G. Dessauvagie and C.A. Kogbe,sequence stratigraphy. The proposed boundary willprovide a basis for chronostratigraphy of the Benin Basinfor oil and bitumen as well as other mineral resources’expolitation. Charred Gramineae cuticles were observed tohave a peak occurrence close to this boundary. This maybe useful as a bio-event if generally registered across theBenin Basin.

The recovery of Zonocostites ramonae (Rhizophoraspp.) from the studied samples may mean that Rhizophorahas been an integral part of the West African mangrovevegetation since Paleocene. This is because caving thatis commonly associated with ditch cuttings, is ruled outfrom this outcrop samples. Informally named acritarch -“Acritarchus starrus” and normapolles - “Normapollisprotrutetralobus” together with Bohaidina recorded inPaleocene Nigeria for the first time deserves more detailedstudy.

Three informal zones were recognized whichinclude: the Apectodinium zone (middle - late Paleocene);the Apectodinium homomorphum/Hafniasphaeraseptata zone (late Paleocene) and the Areoligerasenonensis /Lanternospheridium lanosus zone(Eocene).

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