Palynology and Paleoenvironments of the Upper Araromi Formation, Dahomey Basin, Nigeria
A detailed understanding of the biostratigraphy of the Dahomey Basin in southwestern
Nigeria will broaden our knowledge of the Basin which traversed different West
African countries especially as Nigeria is working towards increasing her petroleum
output. The palynological investigation of eight surface outcrops from Ifon
town of the Upper Araromi Formation processed using conventional methods of
disaggregation and removal of carbonates and silicates with hydrochloric acid
and hydrofluoric acid and further treatment with hot hydrochloric acid (HCl),
wet-sieving over a 5-micron sieve and the Branson Sonifier 250 used for the
complete removal of silt and clay particles. Each residue was prepared for study
as strewn mounts using Loctite. The samples yielded a rich assemblage of Maastrichtian-Paleocene
palynomorphs among which were common Ariadnaesporites spinosa, A.
nigeriensis, Ariadnaesporites sp., Foveotriletes margaritae,
Rugulatisporites caperatus, Distaverrusporites simplex, Cingulatisporites
ornatus, Zlivisporis blanensis, with dinoflagellate cysts, diatom
frustules and abundant palm pollen Longapertites marginatus, L. vaneendenburgi,
L. microfoveolatus, Proxapertites operculatus, Monocolpopollenites
sphaeroidites, Spinizonocolpites echinatus, S. baculatus,
S. kostinensis, Retidiporites magdalenensis, Mauritidites lehmanii,
Tubistephanocolpites cylindricus, Echitriporites trianguliformis,
E. longispinosus, Monocolpites marginatus, Retimonocolpites nigeriensis,
Racemonocolpites racematus and Arecipites sp. The palynological age
assigned to the samples were based on already published ranges of the palynostratigraphically
important taxa. The preponderance of these palm pollen, together with diatom
frustules and a dinoflagellate cyst suite dominated by Gonyaulaceleans, indicates
sediment deposition in a highly productive shallow marine environment.
March 29, 2012; Accepted: August 18, 2012;
Published: October 10, 2012
Considerable work had been done in the past to ascertain the age of the Cretaceous
sediments of the Dahomey Basin, Nigeria (Bankole et al.,
2006, 2007; Billman, 1976;
Jan Du Chene, 1987, 2000; Salami,
1987). It is a coastal sedimentary basin and one of the passive margin basins
of the West African Atlantic coast. It extends from southeastern Ghana to the
western flank of the Nigerian Niger Delta along the Gulf of Guinea.
||Geological sketch Map of Nigeria. The rectangle show the area
covered by B.B shows the Geological map of the Eastern Dahomey basin while
the triangle shows the studied area within the Abeokuta formation (Modified
after Bankole et al., 2007)
It is bounded on the west by the Ghana ridge, the extension of the Romanche
Fracture Zones, while on the east it is bounded by the Benin Hinge line. The
origin is closely related to the rifting and separation of the African and South
American plates during the Late Jurassic and Early Cretaceous (Adegoke,
1977; Elvsborg and Dalode, 1985; Omatsola
and Adegoke, 1981). The basin is generally long with a total length of about
800 km, narrow and parallel to the coastline. It stretches from southeastern
Ghana through Togo and Benin Republics to the western margin of the Niger Delta
(Fig. 1). The eastern part of the basin constituting the Nigerian
portions, extends from the boundary between Nigeria and the Republic of Benin
to the Benin Hinge Line (Olabode, 2007).
The revised Cretaceous stratigraphy of the Dahomey Basin in decreasing order
of age consists of (1) the Semen Formation, (2) Ise Formation (3) Afowo Formation
and Araromi Formation (Adegoke, 1992). The Araromi Formation
is the youngest Cretaceous Formation of the Abeokuta group which according to
Omatsola and Adegoke (1981), encompasses from the base,
the basal member of the Ise Formation which is equivalent to the unnamed older
folded sediments of (Billman, 1976) dated Neocomian;
the upper Ise Formation composed of the Unnamed Albian Sands of (Billman,
1976) which is of Albian age is overlain by the Turonian Afowo Formation
of Omatsola and Adegoke (1981) which Billman named the
Abeokuta Formation. Overlying the Abeokuta Formation of Billman is the Senonian
Awgu Formation which is further overlain by the Maastrichtian-Paleocene Nkporo
shale (Billman, 1976) which is equivalent to the Araromi
and Ewekoro Formations to which Omatsola and Adegoke (1981)
ascribed a Maastrichtian-Basal Paleocene age for the former while the greater
part of the latter is of Paleocene age. Furthermore, Bankole
et al. (2006) dated the youngest Formation in the Dahomey Basin (the
Oshosun Formation) Late Paleocene-Early Eocene. The age of the Araromi Formation
from which the samples for this study were collected along the Ifon Sabongida
road in Ondo State have been studied by Jan Du Chene (1977,
1987) who proposed the age to range from Maastrichtian
to Paleocene based on the palynofloral assemblages from the tar sand bearing
sections of the Formation near Agbabu. The Araromi Formation is equivalent to
the lower part of the Nkporo Shale of Billman (1976)
and the informal Araromi Shale of Reyment (1965).
The formation is composed of fine to medium grained sands at the base, grading
upwards into shale and siltstone with interbeds of limestone and marls. Also
common are thin lignite bands. The shales are light grey to black, mostly marine
with very high organic content.
Salami (1987) reported from microfloral analysis that
the basal part of the Araromi shale considered a subsurface equivalent of the
Abeokuta Formation indicated an Upper Maastrichtian age while the upper parts
are of Paleocene to Eocene ages, respectively. Salamis paleoenvironmental
deductions using recovered pollen, spores and dinoflagellate cysts inferred
a continental environment of deposition for outcrops of the upper Abeokuta Group,
while the overlying Araromi shales indicated brackish water or marginal marine
environments of deposition. The preponderance of dinoflagellate cysts and abundant
wood particles and fungal spores concurred with this proposed depositional environment.
Adegoke (1992) had reported a Maastrichtian to Paleocene
age for the Cretaceous strata around Ifon-Sabongida from which the studied samples
Alves et al. (2005) having worked on the palynofacies
and nannofossils from cored well section of the Araromi Formation onshore in
southern Nigeria reported that the palynofacies was dominated by palynomorphs
mainly dinoflagellates and amorphous organic matter of algal origin. This they
opined indicated open marine settings. Again, from the palynofacies distribution
patterns they inferred depositional environments which ranged from a very proximal
oxic shelf to a distal dysoxic-anoxic deep-water environment. The calcareous
nannofossil biostratigraphy indicated a Late Cretaceous (Maastrichtian) to early
Eocene age. They further noted variable nannofossil abundance, diversity and
preservation throughout the section they studied.
Bankole et al. (2006) used the occurrences of
diagnostic dinoflagellate cysts to confirm the Late Paleocene-Eocene age for
the newly exposed section of the Oshosun Formation in the Sagamu quarry, Dahomey
Basin, South-Western, Nigeria. They recorded abundant occurrences of diagnostic
dinoflagellate genera such as Apectodinium, Kallosphaeridium (K.
brevibarbatum, K. capulatum, K. yorubaense), Ifecysta,
Senegalinium (S. orei) and Hafniasphaera (H. septata).
They further inferred marginal marine depositional environments based on these
dinocysts, sporadic occurrences of pollen and spores and the freshwater algae
The present study was undertaken to determine the age of the Araromi Formation
around Ifon town through the use of diagnostic palynomorphs and also infer the
paleoenvironments whether they are different from those earlier proposed for
the Formation from other locations by earlier workers. The findings would broaden
our knowledge of the stratigraphy of the Dahomey Basin and further highlight
the preponderance of palm pollen in the Late Cretaceous-Early Tertiary of Nigeria
to confirm the provenance of the palm province of Herngreen
and Chlonova (1981), Herngreen et al. (1996)
and Morley (2000) in Nigeria.
MATERIALS AND METHODS
For this research, eight surface outcrop samples obtained at Ifon town in Ondo
state Nigeria in March 2009, were processed using standard palynological techniques
involving treatments with HCL and HF. Full details of the laboratory procedure
are given in Durugbo et al. (2010). Two microscope
slides stained with Safranin O were studied between March and April 2009. All
the palynomorphs present were enumerated. The sample number and associated England
Finder localities of diagnostic palynomorphs are given (Fig. 2,
3 and 4) and transmitted light photomicrographs
were taken at magnifications of 400 and 1000 on a Leitz Dialux 20 EB microscope
with an attached Motic 2.0 camera at the Paleobotanical laboratory of the University
of Lagos. The different palynomorphs (pollen, spores) were identified using
local palynological catalogues of Lawal and Moullade (1986),
Germeraad et al. (1968), Salard-Cheboldaeff
(1981, 1990), Hoeken-Klinkenberg
||Photomicrographs of the palynomorphs recovered from the different
Araromi sample, (a)Tubistephanocolpites cylindricus sample 2 (J24/2),
(b) Ephedripites multicostatus sample 2 (K33/2), (c) Longapertites
vaneendenburgi sample 2 (R23/4), (d) Milfordia sp. sample 2 (R21/4),
(e) Ctenolophonidites costatus sample 2 (R21/0), (f) Buttinea
andreevi sample 7 (T46/4), (g) Constructipollenites ineffectus
sample 2 (Q38/2), (h) Echitriporites trianguliformis sample 2 (T39/3),
(i) Monocolpopollenites sphaeroidites sample 2 (M42/2) (j) Retidiporites
magdalenensis sample 5 (D48/3), (k) Ariadnaesporites nigeriensis
sample 4 (J44/1), (l) Syncolporites marginatus sample 5 (Q25/3),
(m) Cyathidites minor sample 2 (S38/1), (n) Rugulatisporites caperatus
sample 2 (Q36/4)and (o) Cingulatisporites ornatus sample 2 (M30/4)
For dinoflagellates and acritarchs, monographs of Powell
(1992), Jan Du Chene (1987) and Sluijs
et al. (2003) were used. Generally, species nomenclature for dinoflagellate
cysts followed Fensome and Williams (2004). The slides,
residues, unprocessed samples, CD copies and duplicate prints are in the palynological
collections of the Biological Sciences Department, Redeemers University,
Mowe, Ogun State, Nigeria.
The different palynomorph species recovered in the eight samples are enumerated
in Appendix (Table A1), while the percentages for each palynomorph
groups recovered are presented in Table 1. Photomicrographs
of some of the recovered palynomorphs are displayed in Fig. 2,
3 and 4. Each has the sample number and
associated England Finder coordinates. Altogether 2904 palynomorphs
were recorded among which the palms dominated with a total of 1065 accounting
for about 36.70% of the whole assemblage. Retidiporites magdalenensis, Monocolpites
marginatus, Longapertites vaneendenburgi, Longapertites marginatus,
Echitriporites trianguliformis and Monocolpopollenites sphaeroidites
dominated the palm assemblage in decreasing order. These palms dominated in
samples 2, 4, and 7. The palms were followed successively by the spores (27.75%);
other pollen (16.30%); diatom frustules (10.43%); dinocysts (4.27%); algae and
others with (2.82%) and (1.76%), respectively. The commonest spore species were
Cyathidites minor, Cyathidites australis, Cingulatisporites
ornatus, Deltoidospora sp., Rugulatisporites caperatus and
|| Percentage occurrence of the different palynomorph groups
in the studied samples
||Photomicrographs of the palynomorphs recovered from the different
Araromi sample, (a) Proteacidites dehaani sample 2(J24/2), (b) Monocolpites
marginatus sample 7 (T46/4), (c) Longapertites microfoveolatus
sample 2(R21/0), (d) Fungal spore sample 7 (T35/0), (e) Triorites
sp. sample 2 (P29/1), (f) Ariadnaesporites spinosa sample 7 (P31/2),
(g) Monocolpites marginatus sample 7 (V36/2), (h) Distaverrusporites
simplex sample 2 (M33/2), (i) Mauritidites crassiexinus sample
2 (Q38/2), (j) Proteacidites dehaani sample 5 (S35/1), (k) Psilatricolporites
sp. sample 2 (M29/4), (l) Retimonocolpites sp. sample 2 (T39/3),
(m) Ariadnaesporites sp. sample 4 (J44/1), (n) Arecipites
sp. sample 2 (J24/2), (o) Retitricolpites sp. sample 2 (T39/3), (p)
Cyathidites australis sample 5 (C45/2), (q) Ilexpollenites
sp. sample 2 (T39/3) and (r) Longapertites vaneendenburgi sample
The spores dominated in samples 2 and 7. Common dinoflagellate cysts especially
Cleistosphaeridium sp., Exochosphaeridium sp., Florentinia
sp., Operculodinium sp., Diphyes colligerum, Oligosphaeridium
cf. complex, Apectodinium sp., Spiniferites sp. and
the acritarch Leiosphaeridia sp. characterized the Araromi samples. These
dinocysts and diatom frustules characterized sample 3 suggesting a more pronounced
marine transgression (Salami, 1986, 1988).
The diatoms also dominated samples 6 and 8. There appeared to be an inverse
relationship between the dominance of the diatom frustules and palms. Just as
Vadja-Santivanez (1999) inferred for the samples from
Bolivia dominated by palm pollen, a warm humid climate is suggested for these
||Photomicrographs of the palynomorphs recovered from the different
Araromi sample, (a) Exochosphaeridium cf. bifidum sample 2 (M35/2),
(b) Kallosphaeridium cf. capulatum sample 3 (R35/0), (c) Diphyes
colligerum sample 2 (R30/3), (d) Cleistosphaeridium sp. sample
2 (F31/2), (e) Oligosphaeridium cf. complex sample 7 (M41/1), (f-i)
Diatom frustules sample 2 (J44/2,Q43/3,K34/3,L54/2), (j) Pediastrum
sp. sample 7 (X25/4), (k) Coronifera oceanica sample 7 (Q25/2), (l)
Leiosphaeridia sp. sample 3 (N35/3), (m) Indeterminate dinocyst sample
2 (M42/0), (n) Exochosphaeridium cf. bifidum sample 7 (Q44/2), (o)
Apectodinium sp. sample 3 (K44/1), (p) Spiniferites ramosus
sample 3 (N25/3), (q) Kallosphaeridium cf. capulatum sample 3 (W41/3)
and (r) Fungal spore sample 2 (M34/0)
Among the dinoflagellate cysts, peridiniods were rare being represented by Pareodinia sp., Ifecysta sp., and Apectodinium sp. Gonyaulacealens accounted for over 90% of the dinocyst assemblage indicating more open marine conditions. Diatom frustules were recovered in all the samples with the highest number got from sample 2 which tied with sample 7, as the most productive samples notwithstanding that more spores and pollen were recovered from sample 7. Furthermore, samples 5 and 3 yielded appreciable numbers of palynomorphs while the least productive was sample 8 from which only a spore and six diatom frustules were counted. This common record of spores especially in samples 7, 2 and 5 is noteworthy. It appears to suggest a fern spike produced by diverse pteridophytes around this time slice in the Dahomey Basin.
The preponderance of palm pollen (Table 1), supports the
Cretaceous-Early Tertiary palm province of Herngreen (1980),
Herngreen and Chlonova (1981), Herngreen
et al. (1996) and Morley (2000). Bankole
et al. (2006) and Ojo and Akande (2006)
had also inferred the palm province for the Late Paleocene to Early Eocene Oshosun
Formation of the Dahomey Basin and the Upper Cretaceous Patti Formation of southern
Bida Basin, Nigeria, respectively. This province traversed the whole of the
southern hemisphere just as Eisawi and Shrank (2008),
Jaramillo et al. (2007), Van
Der Hammen and Wijmstra (1964) and Kaska (1989)
have all reported.
These data also suggested the presence of several trophic levels, including
primary producers diatoms (dinoflagellate cysts) and higher plants all thriving
together in a marginal marine/nearshore environment. The presence of these diatom
frustules which are all pennate forms noted to thrive in freshwaters (Dutta,
1980), seems to indicate acidic situations as their presence are widely
used in river and lake biomonitoring programs as indicators of acidification,
eutrophication and pollution.
Furthermore, the common records of the Salvinialean spores (Ariadnaesporites
spinosa, Ariadnaesporites nigeriensis and Ariadnaesporites sp.) which
had been recovered from the Upper Cretaceous of south eastern Nigeria (Odebode
and Skarby, 1980) and in the Late Maastrichtian-Danian Nsukka Formation,
Anambra Basin, Nigeria (Umeji and Nwajide, 2007); in
the Maastrichtian of Somalia (Shrank, 1994a), Upper
Cretaceous-Lower Paleogene of Sudan (Kaska, 1989) and
the Campanian-Maastrichtian of Sudan (Shrank, 1994b)
all these together with Rugulatisporites caperatus, Distaverrusporites
simplex, Cingulatisporites ornatus, Zlivisporis blanensis,
Proteacidites sigalii, P. dehaani, Buttinea andreevi,
Constructipollenites ineffectus, Monocolpopollenites sphaeroidites,
support the Late Cretaceous age earlier defined for the Araromi Formation (Hoeken-Klinkenberg,
1964). Eisawi and Shrank (2008) had further reported
the occurrence of Ariadnaesporites spinosa in the Upper Cretaceous of
the Melut Basin, Sudan. The common records of Ariadnaesporites sp. and
their numerous hair like processes co-occurring with the diatom frustules, common
dinoflagellate cysts especially Spiniferites ramosus, Cleistosphaeridium
sp., Exochosphaeridium sp. and abundant palm pollen, common diatom frustules,
plant debris in the studied samples suggests sediment deposition in a swampy
shallow marine/nearshore environment with some lakes (Kaska,
1989); Eisawi and Shrank (2008). The results of
the present study gives further credence to the age assignments earlier suggested
by Araromi shales by Salami (1987).
Age determination: The palynological age assigned to the samples were
based on already published ranges of the palynostratigraphically important taxa
especially Buttinea andreevi, M. sphaeroidites, Ephedripites
multicostatus, Constructipollenites ineffectus, Z. blanensis,
Proteacidites sigalii, P. dehaani, F. margaritae, R. caperatus,
C. ornatus, D. simplex, Kallosphaeridium sp., Florentinia
sp., Exochosphaeridium sp., Ifecysta sp. and Coronifera
oceanica. Salard-Cheboldaeff (1990) had highlighted
the ranges of R. caperatus, C. ornatus, Buttinea andreevi,
M. sphaeroidites, Ephedripites multicostatus in Nigeria, Togo and
Mali Basins together with other West Africa Basins as being from Campanian-Maastrichtian.
Furthermore, Hoeken-Klinkenberg (1964) had listed Syndemicolpites
typicus, Longapertites marginatus, C. ineffectus, Echitriporites
trianguliformis, R. caperatus, C. ornatus, among the common
palynomorphs from the Upper Cretaceous of Nigeria. Ogala
et al. (2009) had also recovered majority of the listed palynomorphs
from the Middle-Upper Maastrichtian Mamu coal facies in the Anambra Basin. Again,
from the Gombe Formation in Gongola Basin, Ojo and Akande
(2006) had recovered similar palynomorphs. Moreover, the dinoflagellate
cysts Kallosphaeridium sp., Florentinia sp., Exochosphaeridium
sp., Ifecysta sp. and Coronifera oceanica have been recovered
from Paleocene and Maastrichtian sediments in the Anambra Basin by Umeji
and Nwajide (2007). Moreover, Adegoke (1992) had reported
a Maastrichtian to Paleocene age for the Cretaceous strata around Ifon-Sabongida
from which the studied samples were collected. Again, Salami
(1983, 1986, 1988) had recovered
majority of the palynomorphs reported from this study to which he had inferred
upper Cretaceous to Early Tertiary ages.
The age of the Araromi Formation around Ifon is Late Maastrichtian-Paleocene which concurs with the results of some earlier workers. Among the palynomorphs recovered in the present study were common diatom frustules and Salvinia spores which most of the earlier investigators did not report. The sediments were deposited in a swampy shallow marine/nearshore environment with some lakes as revealed by the common records of palm pollen, pteridophyte spores, fungal spores, and freshwater algae co-occurring with dinoflagellate cysts dominated by Gonyaulacealens.
|| Palynomorphs recovered in the Araromi samples Dahomey Basin,
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