Foraminifera Biostratigraphy and
Depositional Environment of Sediments in
SILE-Well, offshore Dahomey Basin,
Benin Republic
Akinsile Oladimeji, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria, sile.oladimeji@yahoo.com
Solomon Adeola Adeyinka, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria, solomonadeyinkaofficial@gmail.com
Olabisi Adeleye Adekeye, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria
Olatinpo Olusegun, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria,oluvictor4christ@gmail.com
Oluyemi Faseki Emmanuel, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria
Abstract- The sedimentary succession penetrated by the SILE Well offshore Benin republic in the
Dahomey basin has been investigated for biostratigraphy and paleoenvironment of deposition. This section consists from the base to the top of sandstone overlying by shale measuring a total thickness of approximately 2000m. Twenty samples of shale were selected and processed for Foraminiferal study. The result of micropaleontological analysis reveals moderately rich and diverse microfauna of planktic and benthic foraminifera totalling 78 species altogether. 64 species (82.6%) are calcareous and 14 species (17.9%) are arenaceous. Of the calcareous forms, benthics accounted for 45 species (70.3%) while planktics accounted for 19 species (29.7%). Two major Planktic biostratigraphic zones were identified: The
Morozovella subbotinnae zone, Morozovella vellascoensis and an Undiagnostic zone suggesting a Late
Maastrichtian to Early Eocene interval. Based on the recovered benthic foraminifera, Haplophragmoides
sp., Bulimina marginata, Eponides pseudoelevatus, Karreriella bradyi, Hopkinsina hourqi, Cibicidoides
pseudoungerianus, Gyroidinoides girardanus, Lenticulina grandis, Oridorsalis umbonatus,Karreriella bradyi
and Bathysiphon sp.suggests a depositional environment ranging between outer neritic and upper bathyal.
I. INTRODUCTION
The Dahomey (Benin embayment) Basin is an extensive basin located in West Africa; it covers much of the continental margin of the Gulf of Guinea, extending from Volta-delta in Ghana through Togo and Republic of Benin to southwestern Nigeria, where it is separated from and cut off by stratigraphically younger Niger Delta. Dahomey Basin is a marginal pull apart basin or marginal sag basin which was developed as the African and South American lithospheric plates separated in the Mesozoic and continental margin was formed [1].The area of study is a newly drilled borehole within the Seme Basin, Benin republic. The aim is to investigate the biostratigraphy and paleoenvironment of deposition. From this aim, the Occurrence, distribution and diversity of the assessment of different fossils, Construction of the different biozonations of the fossils recovered from the sediments and assessment of the ecology of the different fossils forms in the sediments.
A. Geology of the study area
The Seme Field is located south of Benin Republic (Figure 1, 2). The study area, SILE Well which penetrates Cretaceous-Tertiary sediments is located within Block 1, shallow offshore Benin Basin on its border with Nigeria at water-depth of up to 750m.
Figure 2. Regional map of Dahomey Basin showing location of SILE Well [3].
B Stratigraphic Setting of Dahomey Basin
Figur
C. P
Daho indic
boun excu plank sedim
e 3. Geological s
Previous work
[10] inv omey Basin. cates continen
[11] repo ndary of the O ursion in the b
ktic foramini ments of the B
etting of the Dah
done in the st
estigated the Thereby repo ntal environme
orted that the Oshosun Form benthic foram ifera species. Basin.
omey Basin [11]
tudy area
organic geo orting the pre ent of depositi
IETM (Initial mation is defin minifera specie
[12] identifi
II. M ].
ochemistry of evalent abund ion which is c
l Eocene Ther ned by the on es, this is show fied six (6) b
METHODOLO
f the shales dance of terre characterized b
rmal Maximum nset of a pron
wn by the fall biozones in t
OGY
and limeston estrially deriv by oxic condit
m) that is the ounced negat l in the popul the Paleocene
nes in the ea ved organic m
tions.
Paleocene-Eo tive carbon iso lation of recov e – Early Eo
astern matter
Twenty ditch cutting samples ranging from interval 1340-1720m were used for the biostratigraphic studies. The SILE well reached a total depth of 2000m (Figure 4). It comprises four lithologies, which include sandstone, shale, limestone and shaly limestone.
Figure 4. Lithologic section of SILE Well
D. Foraminifera Preparation and Analysis
have been compared with the type figures and type faunal based on the different general and species were made to determine the relative abundance.
III. RESULTS AND DISCUSSION
1) Occurrence and Distribution
The 20 ditch cutting samples of the SILE WELL provided were analyzed at 20 meters intervals for the foraminifera and accessory microfauna. The well yielded moderately rich and diverse assemblages of planktic and benthic foraminifera within the studied section with 78 species recorded. Of these, 64 species (82.6%) are calcareous, while 14 species (17.9%) are arenaceous. Of the calcareous forms, benthics accounted for 45 species (70.3%) while the remaining 19 species (29.7%) are planktics. The foraminifera checklist, the plots of the peaks of species diversity and population abundance are shown in (Table 1)
Table 1: Foraminifera Specie Distribution
CALCAREOUS ARENACEOUS TOTAL
64 (82.1%) 14(17.9%) 78
Benthics Planktics
45 (70.3%) 19 (29.7%)
2)Planktonic Species
The 19 Planktic species include: Acarinina nitida, Globigerina daubjergensis, Globigerina
specie, Globigerina triloculinoides, Acarinina pentacaramerata, Morozovella aegua, Morozovella
inconstons, Morozovella subbstinae, Planktic indeterminate, Pseudohastigerina wilcoxensis,
Turborotalia boweri, Turborotalia centralis, Turborotalia praecentralis, Turborotalia sp,
3) Benthonics Species
Benthonic foraminifera relatively have narrow ecologic adaptation depending upon lithotopes and have also wide geographic distribution that makes them ideal for study of regional biogeographic and paleoecologic reconstructions. The proposed grouping of the benthonic foraminiferal content followed here is according to the suprageneric taxonomy of [13].
Generally, the arenaceous forms indicate littoral and shallow marine environments. The
benthics include Anomalinoides Sp, Bolivina tenuicostata, Calcareous indeterminates, Eponides
peudoelevatus, Eponides Sp, Hopkinsina horgui, Lenticulina cultrata, Lenticulina grantis, Uvigerina
sp, Valvulineria martinezensis, Valvulineria sp, Bolivina sp, Bulimina marginata, Bulimina sp,
Calcareous indeterminate, Cibicides sp, Cristellaria sp, Gyroisinoides girardanus, Lenticulina grantis,
Lentica retulata, Nodosaria sp, Stilostomella sp, Valvuleroria sp, Bolivina sp, Bulimina jarvisi,
Bulimina marginata, Bulimina sp, Calcareous indeterminate, Cibicides sp, Cristellaria sp,
Valvuleroria sp, Bulimina palmarae, Cristallaria sp, Lenticulina inormata, Lenticulina rotulata,
Anormalinoides sp, Bolivina sp, Bulimina minima, Hopkinsina hourgi.
The agglutinated foraminifera includes Haplophragmoides sp, Vermeulinia sp, Gravelina
narivaensis, Haplophragmoides obliguicamerata,Haplophragmoides sp, Haplophragmoides
obliguicamerata, Martinotella communis, Alvcolophragmium subglobosum, Bathysiphen sp, Eggerella
bradyi, Haplophragmoides sp, Trochamonia sp, Vemeulina sp.
1.Morozovella aequa 2.Acarinina pentacamerata 3.Turborotalia sp
4.Acarinina nitida 5.Acarinina soldadoensis angulosa 6.Morozovella
subbotinae
7.Globigerina daubjergensis 8.Acarinina primitive 9.Morozovella
inconstans
10.Pseudohastigerinawilconhenxis 11.Turboratalia griffinae 12.Turborotalia sp
13.Turborotalia centralis 14. Globigerina triloculinoids
1.Anomalina sp 2.Bolivina sp 3.Cibicides sp
4.Cibicides pseudoungera 5.Eponides sp 6.Florilus atlanticus
7.Gyroidinoides girardanus 8.Lenticulina cultrate 9.Lenticulina inomata
10.Lenticulina rotulata 11.Marginulina sp 12.Nodosaria raphanistrum
13.Nodosaria sp
Plate 2: Diagnostic benthics Foraminifera species recovered from shale sediments
Figure 5.Microfauna distribution chart for SILE Well
IV. BIOSTRATIGRAPHY AND PALEOENVIRONMENTS
E. Foraminifera Biostratigraphy
Three (3) major foraminiferal “Zones” were recognized in the analyzed portion of SILE Well. The Cenozoic chronostratigraphic scheme of [14]. and the Global Sequence Cycle Chart of [15]. were adopted for this study. The zonal names used conform to the delta-wide foraminiferal zonal scheme developed for the oil industry. The zones are characterized briefly below.
1) Haplophragmoides excavata/ Morozovella subbotinae Zone
Stratigraphic Interval: 1340-1480 meters
Equivalent Planktic Foraminiferal Zone: “Lower” P6 zone.
Age: Lower Eocene (54.64Ma and younger)
Diagnosis: This is the first zone encountered in the studied part of SILE Well. The base of this zone is
marked by the 54.64Ma MFS [15]. recognized at 1480 meters while the zonal top is tentatively placed at 1340 meters, the depth of the first sample analyzed. The 53.65Ma MFS of [15]. that defines the top of the zone was not recognized in the studied section of the well. The zone correlates with the “Lower” P6Planktic foraminiferalzone of [14]. and [15]. The age is Lower Eocene.
2) Eponides pseudoelevatus/Morozovella vellascoensis Zone.
Stratigraphic Interval: 1480-1600 meters
Equivalent Planktic Foraminiferal Zone: “Lower P6” zone. Age: Lower Eocene (54.64Ma and Older)
Depth 1350m 1375m 1400m 1425m 1450m 1475m 1500m 1525m 1550m 1575m 1600m 1625m 1650m 1675m 1700m Chronos tra ti gra p hy 1340 E A RL Y E O CE NE 1580 ?PALEOC E N E 1660 1720 LAT E M AAST R IC H T IAN P e ri od/ E poc h F O R A M IN IF ER A ZO NE 1340 P
7 - P
8 1600 1720 UNDI A G NOS T IC Zone Sam p les ( m et res) Sa m ple dep th is BASE o f d ep th r ange Barre n
Semi-quantitative, (Default Abundance Scheme)
A cari ni na ni ti da A cari ni na pent ac am erat a Ac a rin in a p rim it iv a A cari ni na s ol dadoens is angul os a A cari ni na s p Gl obi geri na daubj ergens is Gl obi geri na s p Gl obi geri na t ri loc ul inoi des M oroz ov el la aequa M oroz ov el la i nc ons tans M oroz ov el la ps eudobul loi des M oroz ov el la s ubbot inae P lank ti c i ndet erm inat e P seudohas ti geri na w ilc ox ens is T urborot al ia bow eri T urborot al ia c ent ral is T urborot al ia gri ff inae T urborot al ia praec ent ral is T urborot al ia s p *1 1340 1360 1380 1400 1420 1440 1460 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700 1720
FOP (FORAMINIFERA PLANKTONIC)
Semi-quantitative, (Default Abundance Scheme)
A nom al ina s p A nom al inoi des al az anens is A nom al inoi des c ic at ri c os us A nom al inoi des s p B ol iv ina dert onens is B o livi n a sp B ol iv ina t enui cos tat a B u lim in a fu si fo rm is B ul im ina i nf lat a Bu lim in a ja rv is i B ul im ina m argi nat a B u limi n a mi n ima B ul im ina pal m arae B ul im ina s p C al careous i ndet erm inat e C as sidul ina neoc ari nat a C ibi c ides s p C ibi c idoi des ps eudoungeri anus Cr is te lla ria s p E poni des ornat us E poni des ps eudoel ev at us E poni des s p F issu ri n a sp F lori lus at lant ic us Gy roi di noi des gi rardanus H et erol epa f lori dana H opk ins ina hourqi H opk ins ina s em iornat a Lent ic ul ina c ul trat a Lent ic ul ina c urv is ept a Lent ic ul ina grandi s Lent ic ul ina i nornat a Lent ic ul ina rot ul at a M argi nul ina s p N odos ari a raphani s trum N odos ari a s p Ori dors al is um bonat us P raebul im ina l at a P raebul im ina robus ta P raegl obobul im ina pupoi des St ilo s to m e lla s p U v igeri na s p V al v ul ineri a m art inez ens is V al v ul ineri a s p V irgul ina s p
FOBC (FORAMINIFERA BENTHIC)
Semi-quantitative, (Default Abundance Scheme)
A lv eol ophragm ium s ubgl obos um B at hy s iphon s p E ggerel la brady i Gl om os pi ra c haroi des Grav el ina nari v aens is H apl ophragm oi des obl iqui cam erat us H apl ophragm oi des s p K arreri el la brady i K arreri el la s p M art inot ti el a c om m uni s R ec urv oi des def orm is T ex tul ari a s p T roc ham m ina s p V erneui lina s p
FOBA (FORAMINIFERA AGGLUTINATING)
Div e rs ity : M ic ro p a la e o n to lo g y 50 13 25 10 7 16 12 7 14 6 14 11 17 12 2 6 8 9 8 5 5 Micropalaeontology T o ta l c o u n t: Mi cro p a la e o n to lo g y 150 2579 108 18 15 54 39 13 67 8 60 35 106 46 2 10 11 19 13 7 8 Micropalaeontology Samples 1340 1360 1380 1400 1420 1440 1460 1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700 1720 Palaeoenvironment *2 O ut er N eri tic Upper B athy al
1480 CU : 54.64 Ma MFS; Gradstein et al. 2004.
FDO: Acarinina nitida
1560 CU : LDO: Globigerina daubjergensis Lone occurrence of Acarinina soldadoensis angulosa 1580 CU : LDO: Acarinina pentacamerata
1600 CU : LDO: Globigerina triloculinoides
REMARKS Depth 1350m 1375m 1400m 1425m 1450m 1475m 1500m 1525m 1550m 1575m 1600m 1625m 1650m 1675m 1700m WELL A Base Lithology Lithology Qualifiers Lithology Accessories IGD Boundary Key
Possible Probable Confident Unconformable ??Unconformable fFault ?f?Fault Sampling Cutting Core Sidewall core
Default Abundance Scheme
Present ( 1 ) Rare ( 2 ) Common ( 5 ) Abundant ( 15 ) Super Abundant ( 50 )
+Present outside count
Text Keys
extin typic
3) U
Strat Age
taxa the p The triloc
Table
F. P
perc wate mod diffe
The base nction of majo cal Paleocene
Undiagnostic Z
tigraphic Inter : ? Maastricht The plan (index plankt planktic indete zone is ch culinoides at t
e. 2 :Foraminifera
Paleobathyme
The ratio entage of plan er depths. It h dern sediment
erence betwee
e of the zone c ority of the M
species.
Zone.
rval: 1600-172 tian – Paleocen nktic foramini
tic forms) wer erminate grou haracterized/de
the top.
al zones for SILE
try
o between plan nktonic foram has been kno ts increases w en the higher
coincides with Maastrichtian sp
20 meters ne
feral preserva re not identifia up.
efined by th
Well
nktonic and b minifera, Table
wn for a lon with water de rate of reprod
h the Cretaceo pecies. The to
ation in the w able to generi
he LDO (La
enthonic fora e 3) is one of ng time that t epth e.g. [16] duction of pla
ous Tertiary bo op is also defi
well is poor. S c/species leve
ast Downhole
minifera (P/B f the most reli the percentage ], [17], [18]. anktonic speci
oundary whic ined by the di
Some stratigra els. Therefore,
e Occurrence
B ratio, frequen able proxies t e of plankton [17] assumed ies in open oc
ch is marked b isappearance o
aphically impo , they are plac
e) of Globig
ntly expressed to estimate pa nic foraminife d that the rel cean areas an
by the of the
ortant ced in
gerina
d as a alaeo-era in
higher rate of reproduction (density) of benthonic species in neritic areas is the main cause for the distribution observed. High proportions of agglutinated taxa in sample 4, 6 and 7 seem to represent shallower environments or reduced paleoproductivity as the ratio correlates directly with foraminiferal abundance (Table 4)
Table 3 : Paleobathymetry checklist for SILE Well
Based on P/B ratio, [19] characterised various depositional environment of sediments.
Upper continental slopes is characterised by high P/B ratio >2.33 (>70 : <30)%, Outer shelf (open sea) by P/B ratio of 0.67 – 2.33 (40-70 : 60-30)%, Middle shelf (open sea) by P/B ratio of 0.11 – 1.5 (10–60 : 90–40)% and Inner shelf (open sea) by P/B ratio <0.25 (<20 : >80)%. Applying these to the studied well, it ranges from inner shelf to upper continental slope.
G. Oxygenation
In the same way, the ratio of arenaceous and calcareous species reflects the oxygen content of the bottom water since low oxygen content may cause difficulties for foraminiferal calcite secretion
13 4 0 4 7 5 13 4 5 3 4 3 0 10 0 0 0 .1 In n e r s h e lf
13 6 0 10 8 2 5 2 5 10 9 6 13 7 2 0 .2 4 Inne r s he lf
13 8 0 18 10 3 3 2 5 5 4 2 0 .1 In n e r s h e lf
14 0 0 15 7 0 0 14 4 19 3 0 In n e r s h e lf
14 2 0 5 4 16 7 2 4 3 8 4 1 6 0 .0 9 In n e r s h e lf
14 4 0 8 9 12 5 5 2 2 3 3 7 4 0 .12 In n e r s h e lf
14 6 0 13 7 0 5 0 3 14 4 0 Inne r s he lf
14 8 0 6 7 14 3 1 5 4 4 7 17 2 0 .5 1 M id d le s h e lf
15 0 0 8 6 6 5 2 1 0 0 3 Up p e r c o n t in e n t a l s lo p e
15 2 0 6 0 14 3 5 7 3 4 5 2 0 2 0 .6 5 M id d le s h e lf
15 4 0 3 5 11 6 2 4 1 7 7 2 0 .1 In n e r s h e lf
15 6 0 10 6 17 3 8 5 8 1 10 10 2 0 .4 2 M iddle s he lf
15 8 0 4 6 12 8 4 5 4 8 0 0 0 .15 M id d le s h e lf
16 0 0 2 2 2 1 1 1 0 0 2 O ut e r s he lf
16 2 0 10 6 2 2 14 4 0 0 0 .14 M iddle s he lf
16 4 0 11 8 0 1 13 5 10 2 0 In n e r s h e lf
16 6 0 19 9 0 0 4 7 9 0 0 0 Inne r s he lf
16 8 0 13 8 1 1 2 6 7 0 0 0 .0 4 Inne r s he lf
17 0 0 7 5 1 1 11 4 0 0 0 .0 9 Inne r s he lf
e.g. [20]. Furthermore, epifaunal genera dominate the entire section (Table 5). Due to this faunal composition (strong dominance of opportunistic species with high tolerance against oxygen deficiency for the entire section), dysoxic conditions are assumed throughout. The dominance of platycopid (filter feeding) ostracods supports this interpretation [21]. The presence of these large shelled organisms indicates no severe dissolution effects for the SILE Well.
Thus, the ratio of epifaunal to infaunal genera may reflect the degree of oxygenation (Table 5). In short, and related to the investigated section, high numbers of calcareous foraminifera, large individuals and an abundance of epifaunal forms all point to high oxygen levels. Conversely, the strong dominance of arenaceous foraminifera, small individuals, and an abundance of infaunal forms indicate oxygen depletion.
Applying the above information to the well section. The samples represent oxic to dysoxic
conditions as they contain abundant epifaunal forms (mainly large Eponides sp, Bulimina sp,
Morrozovella subbotinae) and numerous calcareous foraminifera. Samples seven contain rare or no
epifaunal forms and rare or no calcareous tests and calcareous foraminifera. These samples are interpreted to indicate an intermediate, dysoxic environment. Sample fourteen to sixteen, on the contrary, contains small individuals with rare calcareous foraminifera. Therefore, it is also interpreted to represent an oxic to dysoxic environment.
High proportions of agglutinated taxa seem to represent shallower environments at sample 4, 6 and 7 (Figure 4) or due to reduced paleoproductivity as this ratio correlates negatively with foraminiferal abundance or shows low oxygen as a result of increasing input of organic matter from the incoming fluvial systems [21] while at 1600-1720m shows high oxygen with reduced input of organic matter.
A similar methodology to determine planktonic/benthonic ratio (P/B) is followed to establish the arenaceous/calcareous ratio (Table 4). The calculated ratios for each studied ecozone in SILE Well were in the order of low A/C ratio= 0–25%, moderate =26–50% and high > 50%. The distribution patterns of the arenaceous/calcareous foraminiferal ratios are revealed in [22] who stated that the dominance of calcareous foraminifers indicates deposition largely above the calcium carbonate compensation depth CCD line, in an area high in calcium carbonate, well oxygenated and characterized by normal salinity and/or high temperature as applied to this study (Table 4).
Table 4: Arenaceous/Calcareous Foraminifera ratio
Depth
Planktic/Benthic
Calcareous Agglutinating A/C Ratio
1340 475 0 0
1360 121 7 17.3
1380 28 4 7
1400 14 19 0.74
1440 27 37 0.73
1460 0 14 0
1480 75 17 4.41
1500 8 0 0
1520 69 20 3.45
1540 47 7 6.7
1560 129 10 12.9
1580 62 0 0
1600 3 0 0
1620 16 0 0
1640 13 10 1.3
1660 47 0 0
1680 27 0 0
1700 12 0 0
1720 19 0 0
Table.5:Oxygenation in SILE Well with the diagnostic infaunal and epifaunal species.
Thus the ratio of epifaunal to infaunal which also reflects the degree of oxygenation which point to high epifaunal for sample 1,2,3,5,7,8 to 20 suggests high oxygen level and large sized shells (oxic to dyoxic) while the sample 4,6,7,8 and 11 on the contrary represent anoxic to dyoxic environment(oxygen depletion) and small sized shells.
H. Alpha diversity Index
Species diversity is also informative for bathymetry [23]. The values of Fisher-index increase
as the depth increases. Outer shelf is characterized by α=5-19, the slope by α=1-5. The highest values
(Figure 6). Therefore the Fisher index curve also shows that the species are existence of hyposaline to normal saline environment (not deeper than upper bathyal)
Figure 6. Graph illustrating the calculation of the diversity index α For SILE WELL [19].
V. CONCLUSION
Biostratigraphic studies was carried out on 20 drill cutting samples retrieved from SILE Well, deep offshore seme basin, Benin republic. They were subjected to Foraminifera analysis to determine their biozonation, and depositional paleoenvironments. This section consists from the base to the top of sandstone overlying by shale measuring a total thickness of approximately 2000m. The studied intervals ranged between 1340 and 1720 metres and were sampled at 20-metres intervals for the Foraminifera
The result of micropaleontological analysis reveals moderately rich and diverse microfauna of planktic and benthic foraminifera totalling 78 species altogether. 64 species (82.6%) are calcareous and 14 species (17.9%) are arenaceous. Of the calcareous forms, benthics accounted for 45 species (70.3%) while planktics accounted for 19 species (29.7%). Thus investigation gave a two planktic biostratigraphic zones were identified: The Haplophragmoides excavata/Morozovella subbotinae
zone(early Eocene), Eponides pseudoelevatuszone/Morozovella velascoensis zone(latest Paleocene –
Based on the recovered benthic foraminifera, Haplophragmoides sp., Bulimina
marginata,Eponides pseudoelevatus, Karreriella bradyi, Hopkinsina hourqi, Cibicidoides
pseudoungerianus, Gyroidinoides girardanus, Lenticulina grandis, Oridorsalis umbonatus,Karreriella
bradyi and Bathysiphon sp suggests a depositional environment ranging between outer neritic and
upper bathyal. Integration of biostratigraphy and depositional environment suggests that the sediment were deposited within Late Maastritchtian to Early Eocene interval with two major biostratigraphic
zones in generally shallow marine environment ranging from inner shelf to upper bathyal.
This paper contributes to the understanding of the biostratigraphy and Paleoenvironment of Dahomey Basin, Benin Republic which is currently the focus of intensive hydrocarbon exploration activities.
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