GEOLOGY OF THE DAHOMEY BASIN

GEOLOGY

OF THE NIGERIAN PART OF THE

DAHOMEY [BNIN]

BASIN

BY

M. OLNY ODBD

GEOLOGY OF THE DAHOMEY BASIN

FIG. 1: GEOLOGY OF THE NIGERIAN SEDIMENTARY BASIN [Sensu lato]

INTRODUCTION:

The Dahomey Basin, also called the Dahomey Embayment, Benin Basin, or West Nigerian Basin in older literature, extends from southeastern Ghana in the West, through Southern Togo and southern Benin Republic (formerly Dahomey) to Southwest Nigeria. (The western flank of the Niger delta to be precise). The axis of the basin and the thickest sediments occur slightly west of the border between Nigeria and Benin Republic. The basin is bounded on the west by faults and other tectonic structures. Its eastern limit is marked by the Benin Hinge line, a major fault structure marking the western limit of the Niger delta basin. To the west of the Benin Hinge line is the Okitipupa Ridge (Adegoke, 1969). The tertiary sediments of the Dahomey basin thin out and are partially cut off from the sediments of the Nigeria delta basin against this ridge of basement rocks. The basins offshore limit is not well defined.

FIG. 2: OUTCROP GEOLOGY OF THE DAHOMEY [BENIN] BASINLEGEND

RECENT [Alluvium]

OLIGOCENE QUATERNARY

\

PALEOCENE EOCENE [Imo Shale (Ewekoro, Akinbo, and Oshosun Formations)]

CRETACEOUS [Araromi Formation]

( ( ( (PRECAMBRIAN [Basement Complex]

FIG. 3: OUTCROP GEOLOGY OF THE EASTERN DAHOMEY BASIN

STRUCTURAL / TECTONIC SETTING AND EVOLUTION:

The Dahomey Basin (together with other West African coastal sedimentary basins) was initiated during the Mesozoic in response to the separation of the African South American land masses and the subsequent opening of the Atlantic Ocean.

Deposition was initiated in fault-controlled depressions on the crystalline Basement Complex. The depressions were a result of rift-generated basement subsidence during the Early Cretaceous (Neocomian). The subsidence gave rise to the deposition of a very thick sequence of continental grits and pebbly sands over the entire basin (Leaner & Ruiter, 1977). Over 1,400 metres of these sediments are preserved in coastal areas in Nigeria and offshore in Benin Republic (Billman, 1976; Omatsola & Adegoke, 1981).

During the Late Cretaceous (Santonian), there was another episode of major tectonic activity, probably associated with the closure and folding of the Bennie Basin. The basement rooks as well as the sediments in the basin were tiled and block-faulted forming a series of horsts and grabens (Omatsola & Adegoke, op. cit.) Considerable erosional activity accompanied the uplift and block-faulting. And the extensive Lower Cretaceous sediments were almost completely eroded from the horsts.

During the Maastrichtian, the basin became quiescent and has experienced only gentle subsidence since.

EAST-WEST GEOLOGICAL SECTION SHOWING THE DAHOMEY (BENIN) BASIN AND NIGER DELTA (AFTER WHITEMAN, 1982)STRATIGRAPHY:

The oldest part of the sedimentary sequence is Maastrichtian onshore (Stansky, 1962; de Klasz, 1978). Offshore, however, considerably older sediments have been penetrated by boreholes (Billman, 1976). The oldest sediments in the basin are non-fossiliferous, folded rocks of unknown thickness but pre-Albian in age. The youngest strata are Pleistocene to Recent in Age.

The Cretaceous strata have been assigned to the Abeokuta Group by Omatsola & Adegoke (1981) and subdivided into three formations: Ise Formation (oldest), Afowo Formation, and Araromi Formation (youngest).

ISE FORMATION (BB1): this is a sequence of continently sands, grits and siltstones with a basal conglomerate overlying the Basement Complex. Interbedded kaolinitic clays occur in places. Metres. Sporomorohs recovered from the shell-BP paleontologists including Cicatricosisporites sp. cf. C. mohrioides, Pilosisporites trichopapillosus, Klukisporites pseudoreticulatus, Aequitriradites aff. verucosus and Stapilinisporites caminus indicate a Neocomian (probably Valanginian Barremian) age for the formation.

AFOWO FORMATION (DB2): This Formation, according to Omatsola and Adegoke (1981) is equivalent to the outcropping unit referred to in literature as the Abeokuta Formation. It is mainly composed of coarse to medium-grained sandstones with variable, but thick, interbedded shales, siltstones and clays, the shale component progressively increasing towards the top. Its lower part is constituted by an alternation of brackish to loose fluviatile sands. Intense pyritisation of some horizons is common. On the basis of its palynomorph content (see Billman, 1976) and the marine foraminifera and ammonites (e.g. Sphenodiscus and Pachydiscus) it bears (Omatsola & Adegoke, 1981), the formation has been dated Turonian to Maastrichtian. Its maximum recorded thickness is 2,300 metres. ARAROMI FORMATION (DB3): This formation is composed of fine to medium-grained sands at the base, overlain the shale and siltstones with thin interbedded limestones and marls. Thin lignitic bands are also common. The shales are light grey to black, mostly marine and with very high organic content. The formation is richly fossiliferous, bearing abundant foraminifera, ostracodes, and palynomorphs. On the basis of this rich fossil contents, it has been dated Maastrichtian to Paleocene (Jan du Chene, 1977a, b) Adegoke et al., 1980).

EWEKORO FORMATION (DB4): The Afowo formation (=0old Abeokuta formation) is, in parts of the basin, conformably overlain by the Ewekoro formation. In coastal boreholes and offshore, it is not encountered (Reyment, 1965; Fayose, 1970; Billman, 1976). There, it is replaced by the predominantly shaley Imo formation which unconformably overlies the Afowo formation (Fayose, op. cit.). Borehole studies indicate that the formation is lens-shaped, thinning out in all directions (and eventually disappearing) from its maximum thickness of 34 metres at Ibeshe (Fayose & Asseez, 1972).

At its type locality (Ewekoro Limestone Quarry), the formation consists of 10 to 12.5 metres of thinly bedded glauconitic and sandy limestone at the base, which then becomes massive, grey and fossiliferous in the middle and fine-grained, marly and algal in the upper part (Dessauvagie, 1975: Adegoke, 1977). The top, highly scoured layer consists of red, dense glauconitic, phosphatic and fossiliferous limestone.

Four microfacies units, sandy biomicrosparite, shelly biomicrite, algal biosparte and red phosphatic biomicrite, have been erected for the formation. (Adegoke et al., 1971; Ogbe, 1972).

The formations status and age have, perhaps, been the most controversial in the history of the erection of Phanerozoic stratigraphy of Nigeria (Jones & Hockey, 1964; Reyment, 1965; Antolini, 1968; Adegoke, 1969, 1977; Ogbe, 1970, 1972; Adegoke et al. 1971, 1972; Fayose & Asseez, 1972; Petters, & Ollsson, 1980).

The formation is highly fossiliferous. Adegoke (1977) identified more than 220 mollusk and echinoderm species and subspecies from the formation. It also contains abundant foraminifera, ostracodes and algae.

Most authors date the formation Upper Paleocene. The faunal / floral assemblage indicates that the formation was deposited in a shallow, nearshore (littoral to sublittoral) fairly warm marine environment during a regressive phase (Adegoke, 1977).

IMO FORMATION (AKINBO FORMATION OF OGBE, 1972) (DB5): Where encountered, the Ewekoro formation is conformably overlain by the predominantly shaley Imo formation. Where the latter is missing, the Imo Shale lies directly, unconformably, on the Afowo (old Abeokuta) formation.

Imo formation consists of fine-textured dark micromicaceous shale, locally silty with glauconitic marl and conglomerate at the base. The greenish-grey variety of the shale encountered in the subsurface of most inland areas of Western Nigeria and which in the Ewekoro quarry disconformably overlies the Ewekoro Formation was named Akinbo Formation by Ogbe (1972).

The Formations thickness is highly variable. Offshore, it averages 400 metres (Billman, 1976) and in coastal boreholes 120 metres (Fayose, 1970). Further inland, a maximum thickness of 130 metres is recorded in boreholes. At the type locality of the Akinbo Formation, its thickness varies between 8 and 9 metres (Ogbe, 1972).

The Imo Formation is probably the most fossiliferous Tertiary unit within the Nigerian part (i.e. eastern part) of the Dahomey Basin. Planktonic foraminifera as well as benthonic ones are particularly abundant in it. Four planktonic foraminifera biozones were recognize by Fayose (1970). These are (i) Globigerina daubjergensis Globorotalia compressa, (ii) Globorotalia angulata, (iii) Globorotalia acuta Globorotalia velascoensis, and (iv) Globorotalia heynesi Pseudohastigerina wilcoxensis biozones.

Offshore, foraminifera (both planktic and benthic) are also abundant while calcareous nannoplankton also occur (Billman, 1976).

The microfossil assemblage point to a lower Middle Eocene age for the Formation.

OSHOSUN FORMATION (DB6): The shales of the Imo Formation grade into the overlying mudstones and claystones of the Oshosun Formation.

In its lower part, the Formation is composed of dull brown and brick-red sandy mudstone and claystone with light grey and purplish mottling. Thin pebble beds and coarse pebbly sandstones are locally interbedded. Rare inclusions of phosphatic and glauconitic material occur, becoming more abundant and characteristic in the middle part. Light grey erinaceous sediments are locally present near its top, constituting an unevenly developed sequence to which Jones & Hockey (1964) applied the name Ilaro Formation. (Adegoke, 1969).

Unlike the middle and upper parts, the lower part of the Formation is extensive, stretching southeastwards from west of Ilaro to beyond Ijebu-Ode. TABLE : SOME OF THE MARINE PALYNOMORPHS RECOVERED FROM THE OSHOSUN STRATA

Its total thickness onshore is unknown. Offshore, it is about 270 metres (Billman, 1976).

The Formation is fairly fossiliferous both onshore and offshore. Onshore, mollusks, foraminifera and fish remains have been recovered from it (Adegoke, 1969); offshore, foraminifera (Particularly benthic) and calcareous nannoplankton occur in it (Billman, 1976). These date it Mid-Eocene (Lutetium).

The presence of glauconites, phosphates and abundant planktonic foraminifera in the Formation indicates that it was deposited in fairly deep marine waters, probably in the bathyal zone. AMEKI FORMATION D (DB 7): At its type locality in eastern Nigeria, the formation consists of greenish-grey clayey sand stone and sandy claystone. It displays rapid lateral facies changes, sometimes showing pronounced shaley development.

In the eastern Dahomey Basin, Ameki type sediments form a thin conformably veneer above the Oshosun Formation form a (1969). In coastal boreholes, the Formation is characterized by grey sandy shale with intercalated glauconitic marls (Fayose, 1970; Fayose & Asseez, 1972). Offshore, the Formation is missing (Billman, 1976).

Although it has yielded abundant fossils in its type area in eastern Nigeria, the outcrops of the Ameki Formation in the Dahomey Basin are not richly fossiliferous. The few known Species (mainly Molluscs) recovered from it indicate a Lutetian to Bartonian age for the Formation.

OGWASHI-ASABA FORMATION (DB8): Together with the overlying Benin Formation, this Formation is variously known as (i) Continental Terminal (In the western Dahomey Basin see Stansky, 1962) and (ii) Coastal Plain Sands (Jones & Hockey, 1964). Alone, Reyment (1969) called it Ijebu Beds and offshore, Billman (1976) gave it an informal name: Afowo Beds. It consists of a variable sequence of clay, sand, and thin lignite seams (in Eastern Nigeria). Its thickness is unknown. The Afowo Beds of Billman (op. cit.) are marine shale, siltstone and sandstone sequence (known only from offshore and the coastal region) representing channel deposits.

Ogwashi-Asaba Formation contains some plant remains which indicate an Oligocene Miocene age and a continental environment of deposition for it.

Afowo Beds contain foraminifera (Benthonic and Planktonic) as well as calcareous nannoplankton... These microfossil assemblages indicate an Early to middle Miocene age and an outer shelf to deep bathyal depositional environment for the Formation.

BENIN FORMATION (DB9): Together with underlying Ogwashi Asaba Formation, it is variously known as Continental Terminal and Coastal Plain Sands (see above and Table I). The Formation was also divided into the Ijebu Formation (i.e. the marine offshore sequence) and the Benin Formation (i.e. the non-marine onshore sequence) by Billman (1976). The Benin Formation consists of yellow and white, sometimes cross-bedded sand, pebbly beds and clays with some sandy clay lenses. The thickness is unknown except in the Niger delta where it is about 2,000 metres thick. (Merki, 1972). The Ijebu Formation is a fossiliferous marine sequence of fine to coarse sand and clay interbeds with glauconite and quartz sand. Its maximum thickness is 500 metres (Billman, op. cit.).

Benin Formation contains plant remains. In addition, planktonic foraminifera have been recovered from its lower part offshore; these indicate a Miocene age. Its upper part is believed to be Pliocene to Recent.

Foraminifera, ostracodes, calcareous nannoplankton and shell fragments occur in the Ijebu Formation. The planktonic foraminifera date the Formation Upper Miocene. ECONOMIC DEPOSITS:

The strata of the Nigerian part of the Dahomey Basin contain several economic deposits, the most important of which are indicated below:

(i) TAR (BITUMINOUS) DEPOSITS: These occur in a discontinuous belt stretching from east of Ijebu-Ode (Ogun State) to the Okitipupa area (Ondo State).

(ii) CLAY DEPOSITS: High quality kaolinitic clay deposits occur in various places in the basin. These would be useful in the ceramic industry (among other industries).

(iii) LIMESTONE DEPOSITS: These occur mainly in the Ewekoro Formation and already feed two cement factories in Ogun State (Ewekoro and Shagamu).

(iv) GLASS SAND DEPOSITS: These occur in a discontinuous belt along the coastal part of the basin. Deposits of good quality glass sands also occur in the Ilaro area.

(v) OTHER INSUSTRIAL MINERALS: Sand, gravel etc. occur in copious quantities in the basin.

(vi) WATER: The basal sands of the Ise Formation and those of the Benin Formation among other occurrences, form excellent aquifers throughout the Eastern Dahomey Basin.