- Email: [email protected]
The continental sediments of the Nigerian Coastal Basins
Journal of African Earth Sciences, Vol. 12, No. 1/2,pp. 79-84, 1991. Printedin Thailand
0899-5362/91 $3.00+ 0.00 © 1991 PergamonPressplc
The Continental sediments of the Nigerian Coastal Basins S. A. ADEDmAn*,O. S. ADEC,OKE* and I. O. OSHIn** *Department of Geology, Obafemi Awolowo University, Ile-Ife, Nigeria **Centre for Energy, Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria
Abstract- The development of the coastal basins of Nigeria is examined in terms of a four-stage evolutionary model, viz: intracratonic, synrift, transitional and ocean/c. The continental sediments range in age from early Cretaceous to Recent. The oldest sediments in the Western Dahomey Basin (Ise Formation) consist of a predrift sequence of basal conglomerates and loose sands interbedded with clays. Sedimentation in the eastern basins commenced later during the Albian times with the deposition of coarse grained, poorlysorted sands (Mamfe and Awi Formations) of fluviatile origin. These are overlain in all basins by transitional to marine strata. Continental sedimentation resumed in the Maastricthian with the deposition of coarse grained, poorly sorted, sometimes ferruginuous sands and clays (Owelli and Ajali Sandstones). Regression began during the Eocene resulting in the deposition of f'me to coarse grained, poorly sorted sands with mudstones (Ilaro Formation) in the west, overlain by crossbedded sandstones with lignites and carbonaceous mudstone (Ogwashi-Asaba Formation). Miocene to Recent limonitic, coarse grained, subangular to well-rounded Benin Sands cap all the basins. Rrsum6 - L'dvolution des bassins c&iers du Nigdria suit un cheminement en quatre 6tapes: intracratonique, synrift, transitionnel et ocdan/que. Les srdiments continentaux s'rchelorment du Crrtac6 h l'Actuel. Les s&timents les plus anciens dans le bassin du Dahomey h l'Ouest (Formation d'Ise) consistent en une sfquence prd-rift de conglomrrats de base et de sables meubles interstratifirs avec des argiles. La srdimentation dans les bassins orientaux drbute au cours de l'Albien avec le drprt de sables grossiers et mal trirs (Formation de Mamfe et d'Awi) d'origine fluviatile. Ils sont surmont~s dans tousles bassins par des couches transitionnelles ~marines. La s&timentation continentale s'est rfsurnre au Maastrichtien par le ddprt d'argiles et de sables grossiers, mal trirs, parfois ferrugineux (gr~s d'Owelli et d'Ajali). La r~gression &)c~ne s'est traduite par le drprt de sables fins h grossiers, mal trids, avec des mudstones (Formation d'Ilaro) h l'ouest, surmontrs par des gr~s h stratifications entrecroisres avec des lignites et des mudstones carbonds (Formation d'Ogwashi-Asaba). Tousles bassins sont couronnrs par les sables h limonite du Bdnin, grossiers, subanguleux ~ arrondis, d'~ge MiocL'neh actuel.
INTRODUCTION T h e N i g e r i a n c o a s t a l b a s i n s (Fig. 1) f o r m p a r t o f the open type sedimentary basins occurring along t h e p a s s i v e c o n t i n e n t a l m a r g i n of t h e G u l f o f Guinea. Continental deposits constitute the bulk o f t h e s e d i m e n t a r y fill o f t h e s e b a s i n s . T h e i r s p a t i a l d i s t r i b u t i o n is c l o s e l y l i n k e d w i t h t h e s e p a r a t i o n of the African landmass from the South American continent which commenced some time during the late J u r a s s i c o r e a r l y C r e t a c e o u s ( R e y m e n t a n d Tait, 1972a). A d e d i r a n a n d A d e g o k e (1987) p r o p o s e d a four-stage evolutionary model for the coastal basins of the Gulf of Guinea. Basin development was initiated during a relatively stable i n t r a c r a t o n i c s t a g e in w h i c h c o a r s e , g r a n u l a r a n d p e b b l y c l a s t i c s In a k a o l i n i t i c m a t r i x w e r e deposited in broad sheets over the area. The second, synrift stage, was initiated by tectonic activities accompanied by faulting leading to t h e f o r m a t i o n o f g r a b e n s a n d h o i s t s . I n t e n s e 79
e r o s i o n of t h e h i g h (positive) a r e a led to t h e rapid accumulation of continental clastics within the newly formed subsiding basins and p a r t i c u l a r l y in t h e g r a b e n s . T h e r e a r e g o o d i n d i c a t i o n s {see f o r i n s t a n c e , O m a t s o l a a n d Adegoke, 1981) that faulting continued c o n t e m p o r a n e o u s l y w i t h d e p o s i t i o n for a long p e r i o d l e a d i n g to t h e e r o s i o n of n e w p o s i t i v e a r e a s a n d t h e reworking of intrabasinal materials. Local unconformable surfaces and rapid lateral facies changes were important sedimentary features during this stage. The transitional stage was a period of further subsidence as the sediment load i n c r e a s e d , followed b y a n o r t h w a r d t r a n s g r e s s i o n o f t h e s e a . P a r a l i c s e d i m e n t s , d o m i n a n t l y fine to medium clastics with thin limestone and marl b e d s , a r e c h a r a c t e r i s t i c of t h i s p h a s e . O c c a s i o n a l t h i n b e d s of c o a l a n d lignite a r e a l s o r e c o r d e d . E v a p o r i t e s , t h a t c h a r a c t e r i s e t h e period of o c e a n i c incursion into other more southerly West African b a s i n s ( F r a n k s a n d N a i m , 1973; d e Klasz, 1977), h a v e n o t b e e n e s t a b l i s h e d in a n y of t h e s o u t h e r n
S. A. ADEDIRAN, O. S. ADEGOKEand I. O. OSHIN
80
STRATIGRAPHY OF THE CONTINENTAL SEDIMENTS
!
]
FIO.2b
) )
)
J
FIO, 2o
6"
liAKUnOI
10"1[
IO'.N
( ~nJ
Figure 2 shows a composite s k e t c h of the geological formations of the Nigerian coastal basins while Fig. 3 is a schematic representation of the stratigraphic units in the Nigerian coastal basins. The continental sediments are stippled on figure 3 for emphasis. The d o m i n a n t lithologic characteristics of each formation is s n m m a r i s e d below beginning with the oldest units,
Ise F o r m a t i o n (Omatsola and Adegoke, 1981). The type section is the Ise-2 borehole between the depth of 1246.4 - 1812.4 m. The formation is a predrift sequence of continental sands, grits and siltstone with a basal conglomerate Fig. I. Locationmap of the Nigeriancoastal basins. Figured overlying the B a s e m e n t Complex. The s a n d s boxes refer to the location of composite sketch In Fig. 2. Fig. I. Carte de Iocallsatlon des b a s s i n s c6tiers d u Nig~rla. are interbedded with kaolinitic clays. Based on the Les encadn~s se rapportent a u s c h e m a composite de microflora (e.g. Cicatricosisporites sp. cf. C. la Fig. 2. mohrioides, Pilosisporites trichopapillosus, and coastal basins of Nigeria. While there is no Stapilinisporites cam/nus), the age suggested is evidence of evaporitic deposits in the Dahomey Valanginian-Barremian (Omatsola a n d Adegoke, Basin, the occurrence of saline (sometimes warm) 1981). springs in parts of the A n a m b r a Basin (Wilson, Billman (1976) h a d previously subdivided the 1928; Petters et al., 1987) suggests possible sequence into a n u n n a m e d "Older Folded occurrence of evaporites. The final phase, the Sediments" (Pre-Albian) and a n u n n a m e d "Albian o c e a n i c s t a g e . w a s m a r k e d by the development of Sands" (Albian). Omatsola a n d Adegoke (1981), deep-seated faults, extrusion of new oceanic c r u s t however, are of the opinion t h a t Billman's informal and deposition of beds with typical marine faunas. subdivisions are u n n e c e s s a r y since the Ise The m a x i m u m development of marine conditions Formation is h o m o g e n e o u s as shown by its varied from b a s i n to basin. It was earliest in the electrical c h a r a c t e r s in all the boreholes studied. eastern basins (Albian in the A n a m b r a Basin and C e n o m a n i a n in the Calabar flank and possibly Mamfe F o r m a t i o n (Wilson, 1928) b e n e a t h the Niger Delta) and m u c h later The type locality is n e a r the Cross River at (Turonian-Maastrichtian) to the west, in the Mamfe. Wilson (1928) described the sediments as D a h o m e y Basin. originating from marine, t h r o u g h estuarine to The second m a r i n e transgression had its terrestrial environments. Petters et al. (1987) m a x i m u m development during the Paleocene recently have shown from textural, mineralogical (Adegoke, 1972; Reyment and Tait, 1972b). The and paleocurrent studies t h a t the sediments are of marine strata in all the s o u t h e m basins consist continental origin derived from the nearby mostly of shales a n d shaley s a n d s which are very B a s e m e n t Complex. They concluded t h a t there is rich in organic matter. By Middle Eocene times, no evidence to substantiate a marine input in the regression h a d c o m m e n c e d in all the basins. deposition within the Mamfe rift. The sequence is Continental sediments, mostly m e d i u m to coarse an upward fining massive s a n d s t o n e which is loosely consolidated s a n d s were deposited as conglomeratic at the base. The s a n d s are coarse the coastline receded progressively southwards. grained, poorly sorted, s u b - a n g u l a r to subInitially, the distinctive features of each sediment- r o u n d e d and are usually cross-bedded. Mudrocks ary basin were recognizable (e.g. Ilaro Formation are present higher in the sequence and there is in the D a h o m e y Basin; upper Ameki beds in the evidence of burrowing. Reyment (I 965) suggested A n a m b r a Basin). At the s u b s e q u e n t stage of that exact age determination is not possible on basinal development, the sedimentary fill in this formation. However, on the basis of the fish all t h e c o a s t a l b a s i n s b e c a m e i d e n t i c a l Proportheus kameruniJackel and some fossilwoods, (Ogwashi-Asaba and Benin Formations). This he suggested an age not older t h a n Albian. Murat is probably b e c a u s e of the combined influence of (1972) and W h i t e m a n (1982) considered the the N i g e r / B e n u e s e d i m e n t s u p p l y s y s t e m formation as a s a n d y facies of the Asu River Group and the distributive activity of the longshore and t h u s assigned it a n age ranging from late drift. Albian to Cenomanian. f ~*
%~--
~
[L(L[ o oo *oo ~lou.
1
The Continental sediments of the Nigerian Coastal Basins
÷÷4... 4- -~÷ •.'....: ' "~,. [ ~ * . * . . ' ~ ' ~ / ~ ' /
,4-,4~ +
.
"
÷
•
-6 :'\++
"
+
+
4-
4
+!
4~-~-"
• ...: : 1
~
I
$
(
•
•
• ee
+ + 4-
i
-_-_-_-_-
9
o to 40,.
;-i:)'
Fig. 2. Composite sketch of geologlcal formations of the coastal basins. a. Geology of the D a h o m e y Basin (After Omatsola a n d Adegoke 1981). B a s e m e n t Complex (1); Afowo Formation (2); A r a r o m i / Ewekoro F o r m a t i o n s (3); O s h o s u n / I l a r o F o r m a t i o n s (4); O g w a s h i - A s a b a / B e n i n F o r m a t i o n s (5); Q u a t e r n a r y Alluvium (6). b. Geology of the A n a m b r a Basin a n d adjoining a r e a s (After Agagu et aL, 1985). B a s e m e n t Complex (I); Asu River Group (2); O d u k p a n i F o r m a t i o n (3); Eze-Aku F o r m a t i o n (4); Agwu/Agbanl Shale (5); Owelli/Nkporo F o r m a t i o n s (6); M a m u / A j a l i / N s u k k a F o r m a t i o n s (7]; Tertlary S e d i m e n t s (8); Tertiary Volcanlcs (9]; Pleistocene-Recent S e d i m e n t s (I 0). c. Geology of the Calabar F l a n k (After Adeleye a n d Fayose, 1981). B a s e m e n t Complex (I); Awl F o r m a t i o n s (2); O d u k p a n l F o r m a t i o n (3); Eze-Aku FormaUon (4); Nkporo Shale (5); Benin F o r m a t i o n a n d Q u a t e r n a r y Alluvium (6). Fig. 2. Schema composite des formations g6ologlques dans les b a s s l n s c6tlers.
a. Gt~ologledu bassin du Dahomey (d'apr~s Omatsola et Adegoke, 198 I). (l) Socle; (2) Formation d'Afowo;(3) Formations d'Araroml/Ewekoro; (4) Formations d'Oshosun/llaro; (5) Formations d'Ogwashi-Asaba/I~nin; (6) Alluvions quaternalres. b. G~ologledu bassin d'Anambraet des r~gions contigues (d'apr~s Agagu et oi., 1985. (I) Socle: (2) Groupe de l'Asu; {3) Formation d'Odukpanl; (4] Formations d'Eze-Aku; (5) Shale d'Agnu/Agbani; (6] FormaUons d'Owe]ll/ Nkporo; (7) Formations de Mamu/AJali/Nsukka: (8}St'dlments tertlalres; (9) Roches voleanlques tertlzdres; (I0) S~iments pleistoc~nes actuels. c. G6ologie du versant de Calabar (d'apr~s Adeleye et Fayose, 1981) (I) Socle; (2) Formations d'Awl; (3) Formation d'Odukpani; (4) Formation d'Eze-Aku: (5) Shale de Nklx)ro; (6) Formation du B~nin et aJluvlons quaternaires.
Awl F o r m a t i o n (Adeleye and Fayose, 1978) Adeleye and Fayose (1978) proposed the Awl Formation for the basal parts of the Odukpani
81
Formation. The latter was dated CenomanianTuronian on the basis of~ts ammonite contents by Reyment (1965). The type section is a road cut n e a r Awi village. The Awi Formation comprises distinct cyclothems of s a n d y conglomeratic beds at the base, followed by upward fining sandstones, siltstones, and claystones capped by black carbonaceous shales. The s a n d s t o n e s vary from fine to coarse grained, are angular and poorly surted. Crossbedding occurs at some levels. The claystones are greenish. Two distinct thin, black carbonaceous and pyritic shales serve as m a r k e r beds within the sequence. The AWI Formation has been ascribed to deposition by rivers in flood plains and swamps of the sub-aerial facies of a delta or coastal plain of alluviation on the basis of the sedimentary cyclothems, streaks of coal and pyrites and the paucity of marine foraminifera. Because of its stratigraphic position below the Cenomanian-Turonian Odukpani Formation, and its lateral e q u i v a l e n c e w i t h t h e AlbianCenomanian Mamfe Formation, the age is probably Albian-Cenomanian. Owelli S a n d s t o n e (Reyment, 1965} The type locality is a road cut n e a r Owelli town. The Owelli Standstone is a massive, highly resistant, ferruginous sandstone which displays prominent large scale planar and trough crossstratification. The s a n d s are m e d i u m to coarse grained with pebbles at some levels. The sandstones show thin fining upward cyclothems which are occasionally Interbedded with thin silty or argillaceous layers. Mudclasts are also present (Reyment, 1965; Agagu et al.. 1985). Whiteman (1982) suggested that the Owelli sandstone is a coarse grained deltaic facies of the Nkporo shale deposited during the late C a m p a n i a n transgressive phase. However. Agagu e t al. (1985) concluded from the lithologic characteristics that the OweUi sandstone is offluviatile origin. Because the Owelli sandstone is barren of microfauna, its age has been deduced from its stratlgraphic position. As a lateral equivalent of the Nkporo Shale, Whiteman (1982) assigned it an age of Maastrichtian. Agagu et al, (1985} placed the age between Santonian and Maastrichtian based on its position between the Ogugu Formation (Coniacian-Santonian) and the Nkporo Shale (Maastrichtian). AJall S a n d s t o n e (Reyment, 1965) The AJall Sandstone had been variously referred,to as "False-bedded Sandstone", "White Sandstone" etc., (Reyment, 1965). The type locality as defined by Reyment (1965) is at the AJali river in E n u g u area. The formation consists of thick, white, friable and poorly sorted sandstone. The s a n d s are coarse
S.A. ADEDIRAN,O. S. ADEGOKEand I. O. OSHIN
82
EASTERNNIGERIA WESTERNNIGERIA I DAHOMEYBASIN NIGER DELTA ANAMSRA,CALABAR& (SURFACEANDSUBSURFACE) (Su~suaFACE) NIGER DELT^(SURFACE) REYM~NT1965 BILLMAN1976 DMATSOLA& AOEGOKE1981 SHORT & STAUBLE1967 WHWTEMAN 1982
AGE POST-MIOCENE
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
RECEN
:::::::::::::::::::::::: ::::~ ..... ..;;..........,BENIN M.:.........................
M,OCE,E
'
,,/
i!ii-i-:.O_'~Ngff(:NgA~F"F~':!iiii!:i:i
~ EOCENE ~ PALEOCE "NE
~
]CONIACIAN ]
.;.;:'-;::::;
NKPOROSHALE
"
-/
UNKNOWN
AGWUSHALE AFOWOFM.
AGWUSHALE
CRETACEOUSSEDIMENTS
----'--'nk..---'IL----
•---,L,, EZE-AKU-SHALE ii~i!i~iii!ii:::"':':':':':':"':':':'::~.'.":::i:::~;~
~:.:.:..................... . ~..~
ODUKPANI FM.
=========================
ASU
RIVER
!::::.' :..~:.,&.,..:.:.:.:.:;:;:.:.~#::.~.~ ~:i~:!:x.:.-:.:.:.:.:.:.:..'.-:.: :::::::::::::::::::::::::::::::::::::::::::::::::~~:::::::::::::::::::::::::::::::::::::: APTI^N
AMF:C. FM. • eel'S'- NSUKKAFN.
ARAROMIFM.
To.o.,^. cEN0. A. W A.
,~- ...."
OSHOSUN FM.
~
MAASTRICHTIAN J JCAMPANIANI S[NONIANISANTONIAN [
"
i!::::::::::::::::::::::::: !
N2 ili
l ===============================
: iii!ii:iii: iiiiijilili' 3 ?:i :i!iil
..N'OCO"IA',^N
7_?_?_?_?_
Fig. 3. Stratigraphic relationship within the Nigerian coastal basins. The continental sediments are suppled. Fig. 3. Relations stratigraphiques entre les bassins littoraux du NigGria.Les s~diments contlnentaux sont en pointillG. grained, s u b - a n g u l a r a n d with white clayey cement. C r o s s b e d d i n g with dip angles as high as 20 ° is typical. A t h i c k m a n t l e of red ferruginous s a n d s overlies t h e Ajali S a n d s t o n e in places, particularly along deep c a n y o n s (Reyment, 1965). Murat (1972) a n d Hoque a n d Ezepue (1977) suggested a fluvial a n d fluvio-deltaic depositlonal e n v i r o n m e n t for t h e AJali S a n d s t o n e based on its lithology, t h e p a l e o c u r r e n t directions a n d the absence of m a r i n e fossils. W h i t e m a n (1982) considered the Ajali S a n d s t o n e as the continental facies of a regressive deltaic complex comprising t h e Ajali Formation, the paralic M a m u F o r m a t i o n a n d t h e m a r i n e Nkporo shale. AmaJor (1984) a n d Ladipo (1986) c o n t e n d e d t h a t t h e formation w a s deposited in a tidal-intertidal environment. They a r g u e d t h a t t h e p r e s e n c e of p l a n a r a n d t r o u g h c r o s s - s e t s w h i c h s h o w h e r r i n g b o n e crossstratification, reactivation surface sets a n d t h e associated ' b u n d l e structures" are conclusive evidence of a tidally influenced regime. Only fragmentary p l a n t s a n d "worm" t r a c k s have b e e n found in theAJali S a n d s t o n e (Whiteman, 1982). Its age of M a a s t r i c h t i a n is t h u s b a s e d on its stratigraphic position above t h e m a r i n e Nkporo shale.
II~ro F o r m a t i o n (Jones a n d Hockey, 1964; Kogbe, 1976) J o n e s a n d Hockey (1964) first p r o p o s e d a composite type section for t h e llaro Formation. This w a s modified by Kogbe (1976) who p r o p o s e d t h e 36.5 m Akinsinde borehole as the type section. The formation consists of non-fossiliferous s a n d s interbedded with clay. The s a n d s are fine to coarse grained, poorly sorted with b l u i s h grey m u d s t o n e w h i c h g r a d e s into glauconitic shale at t h e base. A b e a c h or shoreline depositional e n v i r o n m e n t is suggested b a s e d on the texture of t h e s e d i m e n t s (Kogbe, 1976). A Lutetian age is tentatively assigned to the formation (Slansky, cited in Kogbe, 1976). O g w u h i - ~ a b a ¥ox,-,~aflon (Reyment, 1965) R e y m e n t (1965) p r o p o s e d t h e n a m e Og washi-Asaba F o r m a t i o n to replace the "Lignite Series" of P a r k i n s o n (1907). The type locality is the Mgallgwa stream. Rcsn~ent's (1965) Ogwashi-Asaba type section is b a s e d on composite sections. Okezie a n d O n u o g u ( 1971) s u b s e q u e n t l y defined a m a p p a b l e u n i t for t h e "Lignite Group". The Ogwashi-Asaba F o r m a t i o n consists of crossbedded sandstone, carbonaceous mudstone/
The Continental sediments of the Nigerian Coastal Basins
83
shales, s e a m s of lignite and clay. An u p p e r flood- separation of S o u t h America from West Africa. In plain environment was suggested by Whiteman the e a s t e r n basins, continental sedimentation (1982) based on the present day s e d i m e n t a r y (Mamfe a n d Awi Formations) c o m m e n c e d in the environments of the Niger delta proposed by Allen Albian. Eocene sediments o c c u r in all the basins and all the basins are covered with Miocene to (1965). Short a n d Stauble (1967) suggest t h a t the Recent Benin Formation. The p a t t e r n of sedimentation described above Ogwashi-Asaba Formation is a surface equivalent of the hydrocarbon-rich Agbada Formation of the suggests t h a t the early history of the Nigerian Niger Delta. Adegoke (19691 however, shows that coastal basins reflect the prominence oftectonism, the Ogwashi-Asaba Formation interfingers with i.e. rifling, intense faulting and subsidence leading the lower part of the Benin Formation. Both to erosion and deposiUon of t h i c k clastic formations occur extensively above the older accumulations which s u b s e q u e n t l y gave way to sediments in all the Nigerian coastal basins. The marine transgression a n d development of typical formation is poorly fossfliferous b u t there are shallow marine littoral a n d shelf deposits. The indications t h a t it m a y contain microflora latter history of the basins, on the c o n t r a r y shows, (Reyment, 1965). Reyment (1965) and Short and the greater importance of sea-level changes. This is Stauble (1967) suggested a n age of Oligocene- evident from the obvious oscillations between the Miocene. J a n du C h i n e e t o L (1978) o n t h e basis of transgressive and regressive phases. well preserved pollen, and spores showed that the lower part of the formation is of Eocene age. REFERENCES
B e n i n F o r m a t i o n (Reyment, 1965; Short and Stauble, 1967) The Benin Formation h a d earlier been referred to as the "Coastal Plains Sands" by Parkinson (1907). It is the youngest formation cropping out in all the basins adjoining the coastline. Reyment (1965) defined the type locality for the surface outcrop at Benin. It is composed of yellow and white s a n d s with pebbles at some levels. The s a n d s are occasionally cross-bedded with clays and s a n d y clays occurring in lenses. In the subsurface, Short and Stauble (1967) described a type section for the subsurface Benin Formation from the Ele!e-1 well. The sequence is mainly coarse grained, s u b a n g u l a r to wellr o u n d e d and poorly sorted s a n d s with shale interbeds. Limonitic coatings give the s a n d s a yellow to brown colouration. Lignite streak and wood fragments are c o m m o n especially at the base. Short and Stauble (1967) suggested an upper floodplain depositional environment based on the composition, s t r u c t u r e and grain size of the sequence. At Elele-I type section, Short and Stauble (1967) indicated that the formation is not y o u n g e r t h a n Miocene since the highest dated shale underlying the Benin Sands is early Miocene. The Benin S a n d s are being deposited at the present day in the coastal zone. The age is t h u s Miocene to Recent. SUMMARY The continental sediments in the Nigerian coastal basins range in age from early Cretaceous to Recent. The oldest sediments, the Ise Formation, in w e s t e m Nigeria are associated with the intracratonic stage preceding the complete
Adedlran, S. A. and Adegoke, O. S. 1987. EvoluUon of the sedimentary basins of the Gulf of Guinea. In: Current Research in African Earth Sclences (Ed. by Matheis, G. and Schandelmeier, H.), 283-286. Balkema, Rotterdam. Adegoke, O. S. 1969. Eocene straUgraphy of southem Nigeria. BulL Bur. Rectu Geol. Min. Mere., 69, 2348. Adegoke, O. S. 1972. Stratigraphy and paleontology of the Ewekoro FormaUon (Paleocene) of southwestern Nigeria. Bull. Amer. Paleontology, 71, 1-316. Adeleye, D. R. and Fayose, E./L 1978. Stratigraphy of the type section of Awl FormaUon, Odukpani area, southeastern Nigeria. J. Min. GeoL, 15, 33-37. Agagu, O. K., Fayose, E. A. and Petters, S. W. 1985. Stratigraphy and sedimentation in the Senonian Anambra Basin of eastern Nigeria. J. Min. GeoL, 22, 25-36. Allen, J. ILL. 1965. Late Quaternary Niger Delta and adjacent areas: Sedimentary environments and lithofacies. Amer. Assoc. Petrol_ Geol. Bull_, 48, 547600. Amajor, L. C. 1984. Sedimentary facies analysis of the Ajali Sandstone (Upper Cretaceous), south-central Benue Trough. J. Min. GeoL, 21, 171-176. BiUman, H. G. 1976. Offshore stratigraphy and paleontology of the Dahomey Embayment. Proc. 7th Afr. Micropal. Coil., lle-lfe, Nigeria (In press). de Klasz, I. 1977. The West African sedimentary basins. In: The Phanerozoic Geology of the world. The Mesozoic I. (Ed. by MouUade, M. and Naim, A. E. M.), 371-399, Elsevier Sci. Publ. Amsterdam. Franks, S. and Nairn, A. E. M. 1973. The equatorial marginal basins of West Africa. In: The Ocean Basins and Margins I. The SouthAtlanti~ (Ed. by Nalm, A. E. M. and Stehli, F. G.), 301-350, Plenum Press, New York, U.S.A. Hoque, M. and Ezepue, M. C. 1977. Petrology and paleogeography of the Ajali Sandstone. J. Min. GeoL, 14, 16-22.
84
S. A. ADEDIRAN,O. S. ADEGOKEand I. O. OsmN
J a n du Chine, R. E., Onyike, M. S. and Sowunmi, M. A. Parklnson, J. 1907. The post-Cretaceous strattgraphyof southern Nigeria. Q. J. Geol. Soc. London, 63, 3111978. Some new Eocene pollen of the Ogwashi-Asaba Formation, southeastern Nigeria. Rev. Esp. MfcropoL, 320. Petters, S. W., Okereke, C. S. and NwaJide, C. S. 1987. I0, 285-322. Geology of the Mamfe Rift, southeastern Nigeria. In: Jones, H. A. and Hockey, R. D. 1964. The Geology of part Current Research in Afr~an Earth Sciences ~d. by of southwestern Nigeria. Geol. Sum. Nigeria Bull., 31, Matheis, G. and Schandelmeier, H.), 299-302. 101 p. Balkema, Rotterdam. Kogbe, C. A. 1976. The Cretaceous and Paleogene sediments of s o u t h e m Nigeria. In: Geology of Nigerfa Reyment, R. ,a, 1965. Aspects of the Geology of Nigeria, (Ed. by Kogbe, C. A.), 273-282. Elizabethan Publ. Co., Univ. oflbadan Press, Ibadan, Nigeria. Reyment, R. ~ and Tait, E. A. 1972a. Fauna[ evidence Lagos, Nigeria. Ladipo, K. O. 1986. Tldal shelfdeposltional model for the for the origin of the south Atlantic. In: Proc. 24th AJali Sandstone, Anambra Basin, Southern Nigeria. J. International Geological Congress, Montreal, Canada. AfrL EarthSct, 5, 177-185. Section 7, 316-323. Murat, I~ C. 1972. Stratigraphy and paleogeography of Reyment, R./L and Tait, E. A. 1972b. Biostratigraphical the Cretaceous and lower Tertiary in southern Nigeria. dating of the early history of the southemAflantic. Ph/L In: African Geo/ogy, (Ed. by Dessauvagie, T. F. J. and Trans. Roya/Soc. Lond., Serfes B, 264, 55-95. Whiteman, A. J.), 635-648, Univ. of Ibadan, Press, Short, IC C. and Stauble, ,~ J. 1967. Outline of geology Ibadan, Nigeria. of Niger Delta. Am. Assoc. Petrol. Geol. BulL, 51,761Okezie, C. N. and Onuogu, S. A. 1971. The lignites of 779. southeastern Nigeria. Rept. Geol. Surv. Nigeria, Whiteman, A. 1982. Nigeria: Its Petroleum Geology, Kaduna, Nigeria. Resources and Potential. I and II. Graham and Omatsola, M. E. and Adegoke, O. S. 1981. Tectonic Trotman Ltd., London, U. K. evolution and Cretaceous stratigraphy of the Dahomey Wilson, R. C. 1928. Notes on the geology of the Mamfe Basin, J. Min. Geo/., 18, 130-137. Division, Cameroons Province. Occ. Paper GeoL Surv. Nigeria, 6.
0899-5362/91 $3.00+ 0.00 © 1991 PergamonPressplc
The Continental sediments of the Nigerian Coastal Basins S. A. ADEDmAn*,O. S. ADEC,OKE* and I. O. OSHIn** *Department of Geology, Obafemi Awolowo University, Ile-Ife, Nigeria **Centre for Energy, Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria
Abstract- The development of the coastal basins of Nigeria is examined in terms of a four-stage evolutionary model, viz: intracratonic, synrift, transitional and ocean/c. The continental sediments range in age from early Cretaceous to Recent. The oldest sediments in the Western Dahomey Basin (Ise Formation) consist of a predrift sequence of basal conglomerates and loose sands interbedded with clays. Sedimentation in the eastern basins commenced later during the Albian times with the deposition of coarse grained, poorlysorted sands (Mamfe and Awi Formations) of fluviatile origin. These are overlain in all basins by transitional to marine strata. Continental sedimentation resumed in the Maastricthian with the deposition of coarse grained, poorly sorted, sometimes ferruginuous sands and clays (Owelli and Ajali Sandstones). Regression began during the Eocene resulting in the deposition of f'me to coarse grained, poorly sorted sands with mudstones (Ilaro Formation) in the west, overlain by crossbedded sandstones with lignites and carbonaceous mudstone (Ogwashi-Asaba Formation). Miocene to Recent limonitic, coarse grained, subangular to well-rounded Benin Sands cap all the basins. Rrsum6 - L'dvolution des bassins c&iers du Nigdria suit un cheminement en quatre 6tapes: intracratonique, synrift, transitionnel et ocdan/que. Les srdiments continentaux s'rchelorment du Crrtac6 h l'Actuel. Les s&timents les plus anciens dans le bassin du Dahomey h l'Ouest (Formation d'Ise) consistent en une sfquence prd-rift de conglomrrats de base et de sables meubles interstratifirs avec des argiles. La srdimentation dans les bassins orientaux drbute au cours de l'Albien avec le drprt de sables grossiers et mal trirs (Formation de Mamfe et d'Awi) d'origine fluviatile. Ils sont surmont~s dans tousles bassins par des couches transitionnelles ~marines. La s&timentation continentale s'est rfsurnre au Maastrichtien par le ddprt d'argiles et de sables grossiers, mal trirs, parfois ferrugineux (gr~s d'Owelli et d'Ajali). La r~gression &)c~ne s'est traduite par le drprt de sables fins h grossiers, mal trids, avec des mudstones (Formation d'Ilaro) h l'ouest, surmontrs par des gr~s h stratifications entrecroisres avec des lignites et des mudstones carbonds (Formation d'Ogwashi-Asaba). Tousles bassins sont couronnrs par les sables h limonite du Bdnin, grossiers, subanguleux ~ arrondis, d'~ge MiocL'neh actuel.
INTRODUCTION T h e N i g e r i a n c o a s t a l b a s i n s (Fig. 1) f o r m p a r t o f the open type sedimentary basins occurring along t h e p a s s i v e c o n t i n e n t a l m a r g i n of t h e G u l f o f Guinea. Continental deposits constitute the bulk o f t h e s e d i m e n t a r y fill o f t h e s e b a s i n s . T h e i r s p a t i a l d i s t r i b u t i o n is c l o s e l y l i n k e d w i t h t h e s e p a r a t i o n of the African landmass from the South American continent which commenced some time during the late J u r a s s i c o r e a r l y C r e t a c e o u s ( R e y m e n t a n d Tait, 1972a). A d e d i r a n a n d A d e g o k e (1987) p r o p o s e d a four-stage evolutionary model for the coastal basins of the Gulf of Guinea. Basin development was initiated during a relatively stable i n t r a c r a t o n i c s t a g e in w h i c h c o a r s e , g r a n u l a r a n d p e b b l y c l a s t i c s In a k a o l i n i t i c m a t r i x w e r e deposited in broad sheets over the area. The second, synrift stage, was initiated by tectonic activities accompanied by faulting leading to t h e f o r m a t i o n o f g r a b e n s a n d h o i s t s . I n t e n s e 79
e r o s i o n of t h e h i g h (positive) a r e a led to t h e rapid accumulation of continental clastics within the newly formed subsiding basins and p a r t i c u l a r l y in t h e g r a b e n s . T h e r e a r e g o o d i n d i c a t i o n s {see f o r i n s t a n c e , O m a t s o l a a n d Adegoke, 1981) that faulting continued c o n t e m p o r a n e o u s l y w i t h d e p o s i t i o n for a long p e r i o d l e a d i n g to t h e e r o s i o n of n e w p o s i t i v e a r e a s a n d t h e reworking of intrabasinal materials. Local unconformable surfaces and rapid lateral facies changes were important sedimentary features during this stage. The transitional stage was a period of further subsidence as the sediment load i n c r e a s e d , followed b y a n o r t h w a r d t r a n s g r e s s i o n o f t h e s e a . P a r a l i c s e d i m e n t s , d o m i n a n t l y fine to medium clastics with thin limestone and marl b e d s , a r e c h a r a c t e r i s t i c of t h i s p h a s e . O c c a s i o n a l t h i n b e d s of c o a l a n d lignite a r e a l s o r e c o r d e d . E v a p o r i t e s , t h a t c h a r a c t e r i s e t h e period of o c e a n i c incursion into other more southerly West African b a s i n s ( F r a n k s a n d N a i m , 1973; d e Klasz, 1977), h a v e n o t b e e n e s t a b l i s h e d in a n y of t h e s o u t h e r n
S. A. ADEDIRAN, O. S. ADEGOKEand I. O. OSHIN
80
STRATIGRAPHY OF THE CONTINENTAL SEDIMENTS
!
]
FIO.2b
) )
)
J
FIO, 2o
6"
liAKUnOI
10"1[
IO'.N
( ~nJ
Figure 2 shows a composite s k e t c h of the geological formations of the Nigerian coastal basins while Fig. 3 is a schematic representation of the stratigraphic units in the Nigerian coastal basins. The continental sediments are stippled on figure 3 for emphasis. The d o m i n a n t lithologic characteristics of each formation is s n m m a r i s e d below beginning with the oldest units,
Ise F o r m a t i o n (Omatsola and Adegoke, 1981). The type section is the Ise-2 borehole between the depth of 1246.4 - 1812.4 m. The formation is a predrift sequence of continental sands, grits and siltstone with a basal conglomerate Fig. I. Locationmap of the Nigeriancoastal basins. Figured overlying the B a s e m e n t Complex. The s a n d s boxes refer to the location of composite sketch In Fig. 2. Fig. I. Carte de Iocallsatlon des b a s s i n s c6tiers d u Nig~rla. are interbedded with kaolinitic clays. Based on the Les encadn~s se rapportent a u s c h e m a composite de microflora (e.g. Cicatricosisporites sp. cf. C. la Fig. 2. mohrioides, Pilosisporites trichopapillosus, and coastal basins of Nigeria. While there is no Stapilinisporites cam/nus), the age suggested is evidence of evaporitic deposits in the Dahomey Valanginian-Barremian (Omatsola a n d Adegoke, Basin, the occurrence of saline (sometimes warm) 1981). springs in parts of the A n a m b r a Basin (Wilson, Billman (1976) h a d previously subdivided the 1928; Petters et al., 1987) suggests possible sequence into a n u n n a m e d "Older Folded occurrence of evaporites. The final phase, the Sediments" (Pre-Albian) and a n u n n a m e d "Albian o c e a n i c s t a g e . w a s m a r k e d by the development of Sands" (Albian). Omatsola a n d Adegoke (1981), deep-seated faults, extrusion of new oceanic c r u s t however, are of the opinion t h a t Billman's informal and deposition of beds with typical marine faunas. subdivisions are u n n e c e s s a r y since the Ise The m a x i m u m development of marine conditions Formation is h o m o g e n e o u s as shown by its varied from b a s i n to basin. It was earliest in the electrical c h a r a c t e r s in all the boreholes studied. eastern basins (Albian in the A n a m b r a Basin and C e n o m a n i a n in the Calabar flank and possibly Mamfe F o r m a t i o n (Wilson, 1928) b e n e a t h the Niger Delta) and m u c h later The type locality is n e a r the Cross River at (Turonian-Maastrichtian) to the west, in the Mamfe. Wilson (1928) described the sediments as D a h o m e y Basin. originating from marine, t h r o u g h estuarine to The second m a r i n e transgression had its terrestrial environments. Petters et al. (1987) m a x i m u m development during the Paleocene recently have shown from textural, mineralogical (Adegoke, 1972; Reyment and Tait, 1972b). The and paleocurrent studies t h a t the sediments are of marine strata in all the s o u t h e m basins consist continental origin derived from the nearby mostly of shales a n d shaley s a n d s which are very B a s e m e n t Complex. They concluded t h a t there is rich in organic matter. By Middle Eocene times, no evidence to substantiate a marine input in the regression h a d c o m m e n c e d in all the basins. deposition within the Mamfe rift. The sequence is Continental sediments, mostly m e d i u m to coarse an upward fining massive s a n d s t o n e which is loosely consolidated s a n d s were deposited as conglomeratic at the base. The s a n d s are coarse the coastline receded progressively southwards. grained, poorly sorted, s u b - a n g u l a r to subInitially, the distinctive features of each sediment- r o u n d e d and are usually cross-bedded. Mudrocks ary basin were recognizable (e.g. Ilaro Formation are present higher in the sequence and there is in the D a h o m e y Basin; upper Ameki beds in the evidence of burrowing. Reyment (I 965) suggested A n a m b r a Basin). At the s u b s e q u e n t stage of that exact age determination is not possible on basinal development, the sedimentary fill in this formation. However, on the basis of the fish all t h e c o a s t a l b a s i n s b e c a m e i d e n t i c a l Proportheus kameruniJackel and some fossilwoods, (Ogwashi-Asaba and Benin Formations). This he suggested an age not older t h a n Albian. Murat is probably b e c a u s e of the combined influence of (1972) and W h i t e m a n (1982) considered the the N i g e r / B e n u e s e d i m e n t s u p p l y s y s t e m formation as a s a n d y facies of the Asu River Group and the distributive activity of the longshore and t h u s assigned it a n age ranging from late drift. Albian to Cenomanian. f ~*
%~--
~
[L(L[ o oo *oo ~lou.
1
The Continental sediments of the Nigerian Coastal Basins
÷÷4... 4- -~÷ •.'....: ' "~,. [ ~ * . * . . ' ~ ' ~ / ~ ' /
,4-,4~ +
.
"
÷
•
-6 :'\++
"
+
+
4-
4
+!
4~-~-"
• ...: : 1
~
I
$
(
•
•
• ee
+ + 4-
i
-_-_-_-_-
9
o to 40,.
;-i:)'
Fig. 2. Composite sketch of geologlcal formations of the coastal basins. a. Geology of the D a h o m e y Basin (After Omatsola a n d Adegoke 1981). B a s e m e n t Complex (1); Afowo Formation (2); A r a r o m i / Ewekoro F o r m a t i o n s (3); O s h o s u n / I l a r o F o r m a t i o n s (4); O g w a s h i - A s a b a / B e n i n F o r m a t i o n s (5); Q u a t e r n a r y Alluvium (6). b. Geology of the A n a m b r a Basin a n d adjoining a r e a s (After Agagu et aL, 1985). B a s e m e n t Complex (I); Asu River Group (2); O d u k p a n i F o r m a t i o n (3); Eze-Aku F o r m a t i o n (4); Agwu/Agbanl Shale (5); Owelli/Nkporo F o r m a t i o n s (6); M a m u / A j a l i / N s u k k a F o r m a t i o n s (7]; Tertlary S e d i m e n t s (8); Tertiary Volcanlcs (9]; Pleistocene-Recent S e d i m e n t s (I 0). c. Geology of the Calabar F l a n k (After Adeleye a n d Fayose, 1981). B a s e m e n t Complex (I); Awl F o r m a t i o n s (2); O d u k p a n l F o r m a t i o n (3); Eze-Aku FormaUon (4); Nkporo Shale (5); Benin F o r m a t i o n a n d Q u a t e r n a r y Alluvium (6). Fig. 2. Schema composite des formations g6ologlques dans les b a s s l n s c6tlers.
a. Gt~ologledu bassin du Dahomey (d'apr~s Omatsola et Adegoke, 198 I). (l) Socle; (2) Formation d'Afowo;(3) Formations d'Araroml/Ewekoro; (4) Formations d'Oshosun/llaro; (5) Formations d'Ogwashi-Asaba/I~nin; (6) Alluvions quaternalres. b. G~ologledu bassin d'Anambraet des r~gions contigues (d'apr~s Agagu et oi., 1985. (I) Socle: (2) Groupe de l'Asu; {3) Formation d'Odukpanl; (4] Formations d'Eze-Aku; (5) Shale d'Agnu/Agbani; (6] FormaUons d'Owe]ll/ Nkporo; (7) Formations de Mamu/AJali/Nsukka: (8}St'dlments tertlalres; (9) Roches voleanlques tertlzdres; (I0) S~iments pleistoc~nes actuels. c. G6ologie du versant de Calabar (d'apr~s Adeleye et Fayose, 1981) (I) Socle; (2) Formations d'Awl; (3) Formation d'Odukpani; (4) Formation d'Eze-Aku: (5) Shale de Nklx)ro; (6) Formation du B~nin et aJluvlons quaternaires.
Awl F o r m a t i o n (Adeleye and Fayose, 1978) Adeleye and Fayose (1978) proposed the Awl Formation for the basal parts of the Odukpani
81
Formation. The latter was dated CenomanianTuronian on the basis of~ts ammonite contents by Reyment (1965). The type section is a road cut n e a r Awi village. The Awi Formation comprises distinct cyclothems of s a n d y conglomeratic beds at the base, followed by upward fining sandstones, siltstones, and claystones capped by black carbonaceous shales. The s a n d s t o n e s vary from fine to coarse grained, are angular and poorly surted. Crossbedding occurs at some levels. The claystones are greenish. Two distinct thin, black carbonaceous and pyritic shales serve as m a r k e r beds within the sequence. The AWI Formation has been ascribed to deposition by rivers in flood plains and swamps of the sub-aerial facies of a delta or coastal plain of alluviation on the basis of the sedimentary cyclothems, streaks of coal and pyrites and the paucity of marine foraminifera. Because of its stratigraphic position below the Cenomanian-Turonian Odukpani Formation, and its lateral e q u i v a l e n c e w i t h t h e AlbianCenomanian Mamfe Formation, the age is probably Albian-Cenomanian. Owelli S a n d s t o n e (Reyment, 1965} The type locality is a road cut n e a r Owelli town. The Owelli Standstone is a massive, highly resistant, ferruginous sandstone which displays prominent large scale planar and trough crossstratification. The s a n d s are m e d i u m to coarse grained with pebbles at some levels. The sandstones show thin fining upward cyclothems which are occasionally Interbedded with thin silty or argillaceous layers. Mudclasts are also present (Reyment, 1965; Agagu et al.. 1985). Whiteman (1982) suggested that the Owelli sandstone is a coarse grained deltaic facies of the Nkporo shale deposited during the late C a m p a n i a n transgressive phase. However. Agagu e t al. (1985) concluded from the lithologic characteristics that the OweUi sandstone is offluviatile origin. Because the Owelli sandstone is barren of microfauna, its age has been deduced from its stratlgraphic position. As a lateral equivalent of the Nkporo Shale, Whiteman (1982) assigned it an age of Maastrichtian. Agagu et al, (1985} placed the age between Santonian and Maastrichtian based on its position between the Ogugu Formation (Coniacian-Santonian) and the Nkporo Shale (Maastrichtian). AJall S a n d s t o n e (Reyment, 1965) The AJall Sandstone had been variously referred,to as "False-bedded Sandstone", "White Sandstone" etc., (Reyment, 1965). The type locality as defined by Reyment (1965) is at the AJali river in E n u g u area. The formation consists of thick, white, friable and poorly sorted sandstone. The s a n d s are coarse
S.A. ADEDIRAN,O. S. ADEGOKEand I. O. OSHIN
82
EASTERNNIGERIA WESTERNNIGERIA I DAHOMEYBASIN NIGER DELTA ANAMSRA,CALABAR& (SURFACEANDSUBSURFACE) (Su~suaFACE) NIGER DELT^(SURFACE) REYM~NT1965 BILLMAN1976 DMATSOLA& AOEGOKE1981 SHORT & STAUBLE1967 WHWTEMAN 1982
AGE POST-MIOCENE
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
RECEN
:::::::::::::::::::::::: ::::~ ..... ..;;..........,BENIN M.:.........................
M,OCE,E
'
,,/
i!ii-i-:.O_'~Ngff(:NgA~F"F~':!iiii!:i:i
~ EOCENE ~ PALEOCE "NE
~
]CONIACIAN ]
.;.;:'-;::::;
NKPOROSHALE
"
-/
UNKNOWN
AGWUSHALE AFOWOFM.
AGWUSHALE
CRETACEOUSSEDIMENTS
----'--'nk..---'IL----
•---,L,, EZE-AKU-SHALE ii~i!i~iii!ii:::"':':':':':':"':':':'::~.'.":::i:::~;~
~:.:.:..................... . ~..~
ODUKPANI FM.
=========================
ASU
RIVER
!::::.' :..~:.,&.,..:.:.:.:.:;:;:.:.~#::.~.~ ~:i~:!:x.:.-:.:.:.:.:.:.:..'.-:.: :::::::::::::::::::::::::::::::::::::::::::::::::~~:::::::::::::::::::::::::::::::::::::: APTI^N
AMF:C. FM. • eel'S'- NSUKKAFN.
ARAROMIFM.
To.o.,^. cEN0. A. W A.
,~- ...."
OSHOSUN FM.
~
MAASTRICHTIAN J JCAMPANIANI S[NONIANISANTONIAN [
"
i!::::::::::::::::::::::::: !
N2 ili
l ===============================
: iii!ii:iii: iiiiijilili' 3 ?:i :i!iil
..N'OCO"IA',^N
7_?_?_?_?_
Fig. 3. Stratigraphic relationship within the Nigerian coastal basins. The continental sediments are suppled. Fig. 3. Relations stratigraphiques entre les bassins littoraux du NigGria.Les s~diments contlnentaux sont en pointillG. grained, s u b - a n g u l a r a n d with white clayey cement. C r o s s b e d d i n g with dip angles as high as 20 ° is typical. A t h i c k m a n t l e of red ferruginous s a n d s overlies t h e Ajali S a n d s t o n e in places, particularly along deep c a n y o n s (Reyment, 1965). Murat (1972) a n d Hoque a n d Ezepue (1977) suggested a fluvial a n d fluvio-deltaic depositlonal e n v i r o n m e n t for t h e AJali S a n d s t o n e based on its lithology, t h e p a l e o c u r r e n t directions a n d the absence of m a r i n e fossils. W h i t e m a n (1982) considered the Ajali S a n d s t o n e as the continental facies of a regressive deltaic complex comprising t h e Ajali Formation, the paralic M a m u F o r m a t i o n a n d t h e m a r i n e Nkporo shale. AmaJor (1984) a n d Ladipo (1986) c o n t e n d e d t h a t t h e formation w a s deposited in a tidal-intertidal environment. They a r g u e d t h a t t h e p r e s e n c e of p l a n a r a n d t r o u g h c r o s s - s e t s w h i c h s h o w h e r r i n g b o n e crossstratification, reactivation surface sets a n d t h e associated ' b u n d l e structures" are conclusive evidence of a tidally influenced regime. Only fragmentary p l a n t s a n d "worm" t r a c k s have b e e n found in theAJali S a n d s t o n e (Whiteman, 1982). Its age of M a a s t r i c h t i a n is t h u s b a s e d on its stratigraphic position above t h e m a r i n e Nkporo shale.
II~ro F o r m a t i o n (Jones a n d Hockey, 1964; Kogbe, 1976) J o n e s a n d Hockey (1964) first p r o p o s e d a composite type section for t h e llaro Formation. This w a s modified by Kogbe (1976) who p r o p o s e d t h e 36.5 m Akinsinde borehole as the type section. The formation consists of non-fossiliferous s a n d s interbedded with clay. The s a n d s are fine to coarse grained, poorly sorted with b l u i s h grey m u d s t o n e w h i c h g r a d e s into glauconitic shale at t h e base. A b e a c h or shoreline depositional e n v i r o n m e n t is suggested b a s e d on the texture of t h e s e d i m e n t s (Kogbe, 1976). A Lutetian age is tentatively assigned to the formation (Slansky, cited in Kogbe, 1976). O g w u h i - ~ a b a ¥ox,-,~aflon (Reyment, 1965) R e y m e n t (1965) p r o p o s e d t h e n a m e Og washi-Asaba F o r m a t i o n to replace the "Lignite Series" of P a r k i n s o n (1907). The type locality is the Mgallgwa stream. Rcsn~ent's (1965) Ogwashi-Asaba type section is b a s e d on composite sections. Okezie a n d O n u o g u ( 1971) s u b s e q u e n t l y defined a m a p p a b l e u n i t for t h e "Lignite Group". The Ogwashi-Asaba F o r m a t i o n consists of crossbedded sandstone, carbonaceous mudstone/
The Continental sediments of the Nigerian Coastal Basins
83
shales, s e a m s of lignite and clay. An u p p e r flood- separation of S o u t h America from West Africa. In plain environment was suggested by Whiteman the e a s t e r n basins, continental sedimentation (1982) based on the present day s e d i m e n t a r y (Mamfe a n d Awi Formations) c o m m e n c e d in the environments of the Niger delta proposed by Allen Albian. Eocene sediments o c c u r in all the basins and all the basins are covered with Miocene to (1965). Short a n d Stauble (1967) suggest t h a t the Recent Benin Formation. The p a t t e r n of sedimentation described above Ogwashi-Asaba Formation is a surface equivalent of the hydrocarbon-rich Agbada Formation of the suggests t h a t the early history of the Nigerian Niger Delta. Adegoke (19691 however, shows that coastal basins reflect the prominence oftectonism, the Ogwashi-Asaba Formation interfingers with i.e. rifling, intense faulting and subsidence leading the lower part of the Benin Formation. Both to erosion and deposiUon of t h i c k clastic formations occur extensively above the older accumulations which s u b s e q u e n t l y gave way to sediments in all the Nigerian coastal basins. The marine transgression a n d development of typical formation is poorly fossfliferous b u t there are shallow marine littoral a n d shelf deposits. The indications t h a t it m a y contain microflora latter history of the basins, on the c o n t r a r y shows, (Reyment, 1965). Reyment (1965) and Short and the greater importance of sea-level changes. This is Stauble (1967) suggested a n age of Oligocene- evident from the obvious oscillations between the Miocene. J a n du C h i n e e t o L (1978) o n t h e basis of transgressive and regressive phases. well preserved pollen, and spores showed that the lower part of the formation is of Eocene age. REFERENCES
B e n i n F o r m a t i o n (Reyment, 1965; Short and Stauble, 1967) The Benin Formation h a d earlier been referred to as the "Coastal Plains Sands" by Parkinson (1907). It is the youngest formation cropping out in all the basins adjoining the coastline. Reyment (1965) defined the type locality for the surface outcrop at Benin. It is composed of yellow and white s a n d s with pebbles at some levels. The s a n d s are occasionally cross-bedded with clays and s a n d y clays occurring in lenses. In the subsurface, Short and Stauble (1967) described a type section for the subsurface Benin Formation from the Ele!e-1 well. The sequence is mainly coarse grained, s u b a n g u l a r to wellr o u n d e d and poorly sorted s a n d s with shale interbeds. Limonitic coatings give the s a n d s a yellow to brown colouration. Lignite streak and wood fragments are c o m m o n especially at the base. Short and Stauble (1967) suggested an upper floodplain depositional environment based on the composition, s t r u c t u r e and grain size of the sequence. At Elele-I type section, Short and Stauble (1967) indicated that the formation is not y o u n g e r t h a n Miocene since the highest dated shale underlying the Benin Sands is early Miocene. The Benin S a n d s are being deposited at the present day in the coastal zone. The age is t h u s Miocene to Recent. SUMMARY The continental sediments in the Nigerian coastal basins range in age from early Cretaceous to Recent. The oldest sediments, the Ise Formation, in w e s t e m Nigeria are associated with the intracratonic stage preceding the complete
Adedlran, S. A. and Adegoke, O. S. 1987. EvoluUon of the sedimentary basins of the Gulf of Guinea. In: Current Research in African Earth Sclences (Ed. by Matheis, G. and Schandelmeier, H.), 283-286. Balkema, Rotterdam. Adegoke, O. S. 1969. Eocene straUgraphy of southem Nigeria. BulL Bur. Rectu Geol. Min. Mere., 69, 2348. Adegoke, O. S. 1972. Stratigraphy and paleontology of the Ewekoro FormaUon (Paleocene) of southwestern Nigeria. Bull. Amer. Paleontology, 71, 1-316. Adeleye, D. R. and Fayose, E./L 1978. Stratigraphy of the type section of Awl FormaUon, Odukpani area, southeastern Nigeria. J. Min. GeoL, 15, 33-37. Agagu, O. K., Fayose, E. A. and Petters, S. W. 1985. Stratigraphy and sedimentation in the Senonian Anambra Basin of eastern Nigeria. J. Min. GeoL, 22, 25-36. Allen, J. ILL. 1965. Late Quaternary Niger Delta and adjacent areas: Sedimentary environments and lithofacies. Amer. Assoc. Petrol_ Geol. Bull_, 48, 547600. Amajor, L. C. 1984. Sedimentary facies analysis of the Ajali Sandstone (Upper Cretaceous), south-central Benue Trough. J. Min. GeoL, 21, 171-176. BiUman, H. G. 1976. Offshore stratigraphy and paleontology of the Dahomey Embayment. Proc. 7th Afr. Micropal. Coil., lle-lfe, Nigeria (In press). de Klasz, I. 1977. The West African sedimentary basins. In: The Phanerozoic Geology of the world. The Mesozoic I. (Ed. by MouUade, M. and Naim, A. E. M.), 371-399, Elsevier Sci. Publ. Amsterdam. Franks, S. and Nairn, A. E. M. 1973. The equatorial marginal basins of West Africa. In: The Ocean Basins and Margins I. The SouthAtlanti~ (Ed. by Nalm, A. E. M. and Stehli, F. G.), 301-350, Plenum Press, New York, U.S.A. Hoque, M. and Ezepue, M. C. 1977. Petrology and paleogeography of the Ajali Sandstone. J. Min. GeoL, 14, 16-22.
84
S. A. ADEDIRAN,O. S. ADEGOKEand I. O. OsmN
J a n du Chine, R. E., Onyike, M. S. and Sowunmi, M. A. Parklnson, J. 1907. The post-Cretaceous strattgraphyof southern Nigeria. Q. J. Geol. Soc. London, 63, 3111978. Some new Eocene pollen of the Ogwashi-Asaba Formation, southeastern Nigeria. Rev. Esp. MfcropoL, 320. Petters, S. W., Okereke, C. S. and NwaJide, C. S. 1987. I0, 285-322. Geology of the Mamfe Rift, southeastern Nigeria. In: Jones, H. A. and Hockey, R. D. 1964. The Geology of part Current Research in Afr~an Earth Sciences ~d. by of southwestern Nigeria. Geol. Sum. Nigeria Bull., 31, Matheis, G. and Schandelmeier, H.), 299-302. 101 p. Balkema, Rotterdam. Kogbe, C. A. 1976. The Cretaceous and Paleogene sediments of s o u t h e m Nigeria. In: Geology of Nigerfa Reyment, R. ,a, 1965. Aspects of the Geology of Nigeria, (Ed. by Kogbe, C. A.), 273-282. Elizabethan Publ. Co., Univ. oflbadan Press, Ibadan, Nigeria. Reyment, R. ~ and Tait, E. A. 1972a. Fauna[ evidence Lagos, Nigeria. Ladipo, K. O. 1986. Tldal shelfdeposltional model for the for the origin of the south Atlantic. In: Proc. 24th AJali Sandstone, Anambra Basin, Southern Nigeria. J. International Geological Congress, Montreal, Canada. AfrL EarthSct, 5, 177-185. Section 7, 316-323. Murat, I~ C. 1972. Stratigraphy and paleogeography of Reyment, R./L and Tait, E. A. 1972b. Biostratigraphical the Cretaceous and lower Tertiary in southern Nigeria. dating of the early history of the southemAflantic. Ph/L In: African Geo/ogy, (Ed. by Dessauvagie, T. F. J. and Trans. Roya/Soc. Lond., Serfes B, 264, 55-95. Whiteman, A. J.), 635-648, Univ. of Ibadan, Press, Short, IC C. and Stauble, ,~ J. 1967. Outline of geology Ibadan, Nigeria. of Niger Delta. Am. Assoc. Petrol. Geol. BulL, 51,761Okezie, C. N. and Onuogu, S. A. 1971. The lignites of 779. southeastern Nigeria. Rept. Geol. Surv. Nigeria, Whiteman, A. 1982. Nigeria: Its Petroleum Geology, Kaduna, Nigeria. Resources and Potential. I and II. Graham and Omatsola, M. E. and Adegoke, O. S. 1981. Tectonic Trotman Ltd., London, U. K. evolution and Cretaceous stratigraphy of the Dahomey Wilson, R. C. 1928. Notes on the geology of the Mamfe Basin, J. Min. Geo/., 18, 130-137. Division, Cameroons Province. Occ. Paper GeoL Surv. Nigeria, 6.