Foraminiferal Stratigraphy and Paleoecological Interpretation of Sediments Penetrated by Kolmani River -1 Well, Gongola Basin, Nigeria

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In this study, well samples have been used to enhance better and systemic analysis of the foraminiferal contents in the samples for a more coherent data for higher resolution stratigraphic interpretation. The result of the analysis will be used to evaluate the age, paleobathymetry and paleoenvironments of deposition of sediments within the Gongola Basin.

2. Geological Setting and Stratigraphy

The Benue Trough is an intracratonic Cretaceous Basin, underlying a large part of Nigeria and extending in the Southwest - Northeast direction for about 1,000 km in length from the Bight of Biafra to Lake Chad, and overlying the Precambrian Basement Complex of the West African Mobile belt. Its structural origin is related to the opening of the South Atlantic Ocean (Benkhelil, 1989) and is part of series of Cretaceous rift basins that formed the West and Central African Rift System (WCARS) (Guiraud and Maurin, 1992). The Trough contains as much as 6000 m (20,000 ft) of Cretaceous –Tertiary sediments of which those predating the middle Santonian have been compressionally deformed, faulted, and uplifted in several places. Compressional folding during the middle Santonian tectonic episode affected the whole of the Benue Trough and was quite intense, producing more than 100 anticlines and synclines (Benkhelil, 1989). The Benue Trough is geographically sub-divided into lower, middle and upper Benue Trough (Petter, 1982). The Upper Benue Trough is further sub-divided into three basins: the east–west trending Yola Basin (Yola Arm), the north – south trending Gongola Basin (Gongola Arm) and the northeast– southwest trending Lau Basin (Guiraud, 1990; Dike, 2002).

Sedimentation in the Gongola basin began with the deposition of the continental Bima Sandstone which unconformably overlies the Precambrian Basement Complex. The Bima Sandstone was derived from the granitic Basement Complex. It consists of feldspathic sandstones and clays which pass upwards into coarse to medium grained sandstones with less feldspar. Guiraud (1990) subdivided the sandstone into three members namely; the Upper Bima (B₃), the Middle Bima (B₂) and the Lower Bima (B₁). Allix et al. (1981) using palynological data from outcropping layers of the Bima Sandstone dated the formation as Late Aptian - Early Abian. The Bima is conformably overlain by Yolde Formation consisting of a variable sequence of sandstones and shales. The sandstones are thin-bedded at the base, followed subsequently by alternations of sandy mudstones and shelly limestone (Whiteman, 1982). The Pindiga Formation which is dominantly marine shale with limestone at the base overlies the Yolde Formation. The formation is believed to be deposited under marine conditions that prevailed during the early – late Turonian and Coniacian times in the northern Benue Trough. The Gongila and the Fika Formations are the lateral equivalents of the Pindiga Formation (Zarborski et al., 1997). The Pindiga Formation is overlain by the Gombe Formation. The Gombe Formation is made up of three major lithofacies: (1) alternating beds of silty shales and fine-medium grained sandstones with ironstone intercalations, overlain by (2) medium grained quartz arenites with occasional and iron oxide cement. The third lithofacies is a brick-red coloured, fine - medium grained sandstone, with tabular cross-bedding, highlighted by layers and streaks of pure white sandstones (Zarboski et al., 1997). The youngest formation in the Gongola basin is the Kerri-Kerri Formation. It is represented by the gently dipping continental conglomerates, sandstones, siltstones and clays which overstep the Gombe Formation onto older Formations. Due to the faulted and folded nature of Gombe Formation, the continental clastics of the Kerri-Kerri Formation reaches a thickness of over 320 meters though variable thicknesses occur in the eastern margin of the basin due to irregular tectonic features and small inliers. Adegoke et al. (1979) assigned Paleocene age to the Formation using pollen data. The stratigraphic succession of the northern Benue Trough is shown in Table 1 below.

Table 1. Lithostratigraphic subdivision of Gongola Basin, Northern Benue Trough. (Modified from Obaje, 2003)

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3. Materials and Methods

3.1. Sedimentological Description

Sedimentological description of the samples using the petrologic microscope was done. The composition (sand-silt-shale percentages and environmentally sensitive minerals and accessory present), texture (grain size, sorting and roundness), fossil fragments and colour were described. A graphic log of lithologic, compositional and textural data was produced using the SEDLOG Graphic Software. Based on these, the lithofacies and depositional environment were interpreted for the drilled section was interpreted.

3.2. Foraminiferal Preparation and Analysis

50 grams of each sample was taken in a pre-washed and labeled aluminum bowl. The bowl containing each sample from a particular depth/ interval was labeled for identification and placed on the electric hot plates. They were allowed to stand on the hot plate for an hour. The hot samples were transferred to a fume cupboard using a long tug and clean water was added to completely cover the samples in the bowl. The samples were soaked in ordinary water for four days, to enable the clays to break down sufficiently for complete disaggregation.

The soaked samples were washed, using a 43µ microns sieve in a running sink until the water coming out of the sieve was clean. The clean residue retained in the sieves were transferred back to the aluminum bowl and dried on the hot plate. The drying was carefully supervised to avoid over-heating of the samples. The dried samples were sieved and separated into three fractions, the fine (63 microns + dust below), the medium (125 microns) and the coarse (250 microns). These were transferred into phials. WIRD HEEBRUGG binocular (reflecting) microscope (50 x magnifications) was used for picking and analysis of the recovered foraminifera. The dried samples were sparingly scattered on the picking tray and scanned through under the microscope. Recognizable forams were picked with a picking pin and dropped into forams (Card) slides. The picked forams were sorted into identical forms which were gummed onto the slides and eventually covered with a cover slip for analysis. The analysis was done by sorting the recognised forms into different foraminiferal groups (Benthonic, Calcarous and Planktonic) and sizes, then genera and species. The identification and analysis of the picked foraminifera was done with the aid of relevant publications such as Petters (1982), Holger Gebhardt. (2004) and Shell Petroleum Development Company (SPDC) Forams training slides.

4. Results.

4.1. Lithofacies Sequence

Sedimentological analysis shows that the sequence comprise sand, shale, siltstone and sandy mudstone with intercalations of carbonates and coal at some intervals. The sandstones vary in grain size from fine- through medium to very coarse sands. The grains are angular to rounded and well to moderately sorted. The shales and mudstones are grey to dark grey in colour. They present blocky fissility with very thin silty/sandy lamination at various intervals. Coal fragments were noted in the upper intervals. The environmentally sensitive minerals and accessory present include pyrite, mica, carbonaceous fragments, ferruginous materials and shell fragments. Result of the sedimentological description is summarized in Figure 2.

Figure 2. Lithofacies and Depositional Environment in Kolmani River 1 Sequence

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4.2. Foraminiferal Results

The results of the foraminiferal analysis show relatively high abundance and low diversity of species. The assemblage is dominated by agglutinated benthonic forams, consisting more than twenty six (26) species. There are about eight (8) species of calcarous benthonics and planktonics forams each. Some of the forms are highly degraded due to a combination of possible factors which include sub-optimal storage conditions and salinity crisis that prevailed during the Cretaceous. The species recovered include; Agathamina sp.2, Ammobaculites spp, Ammoscalaria pseudospiralis, Ammobaculites bauchensis, Ammobaculites pindigensis, Ammobaculites jessensis, Ammobaculites coprolithiformis, Ammobaculites sp. 2A, Ammobaculites compressa, Ammobaculites numahensis, Ammobaculites exiguus, Ammobaculites fragmentarius, Ammotium spp., Cyclammina sp.3, Haplophragmoides pindigensis, Haplophragmoides talokoensis, Haplophragmoides spp., Haplophragmoides bauchensis, Miliammina telemaquensis, Miliammina spp., Miliammina petila, Miliammina pindigensis, Miliamina petila, Reophax spp., Reophax minuta, Reophax guineana, Trochamina afikpoensis, Textularina spp., Trochamina spp., Bolivina sp.4, Cristellaria sp.13, Egerella scabra, Gyroidina soldaii giradana Pseudobolivina varianta, Praebulimina exigua, Valvulineria sp.8, Planktonic spp., Globigerinelloides spp., Hedbergella spp., Heterohelix Globulosa, Heterohelix moremani, Heterohelix reussi, Ostracoda and Gastropoda. Figure 3 shows some of the forms recovered while the distribution of the species is presented on the distribution chart (Table 2).

Figure 3. Some of the Foraminifera Species identified in the Well

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Table 2. Foraminiferal distribution chart for Kolmani River 1 Well

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4.3. Interpretation and Discussion
4.3.1. Age dating and Zonations

Three Assemblage Zones have been interpreted for Kolmani River-1. They are Agathamina sp.2 Assemblage Zone, Haplophragmoides bauchensis Assemblage Zone and Heterohelix moremani-Haplophragmoides talokensis Assemblage Zone.

Agathamina-2 Assemblage Zone

Interval: 3960 ft-4800 ft

Age: Campanian - Early Santonian

This zone is characterized by the occurrence of Bolivina sp.4, Eggerella scabra, Pseudobolivina variant, Agathamina sp.2 and a very high abundance of planktonic species including Hedbergella spp., Globigerinelloides spp. and planktonic spp. Agathamina sp.2 is restricted to this zone. Its co-occurrence with other species listed above has proved useful in dating this interval as late Campanian. Some of the forams species such as Ammobaculites coprolithiformis, Ammobaculites bauchensis and Pseudobolivina variant were used by Petters (1982) to date sediments in the Anambra Basin and the Upper Benue Trough as Campanian-Turonian. Agathamina sp.2 was first described by Shell Petroleum Development Corporation (SPDC) biostratigraphers and was used to date F1200 (SPDC foram zonation) as Cretaceous. The choice to name this zone after this foram species arose from its restricted occurrence within this zone.

Gebhardt (1998) also described Ammobaculites coprolithiformis and Bolivina sp.4 in the Anambra Basin and gave their age as Campanian -Maastrichtian. This zone corresponds to the palynozone of the Longapertites marginatus Assemblage Zone, and therefore corroborates the late Campanian age for this interval.

Haplophragmoides bauchensis Assemblage zone.

INTERVAL: 7980 ft-6600 ft.

Age: Coniacian-Early Turonian.

This zone is marked by First Occurrence (FO) and acme event of Haplophragmoides bauchensis. The zone is also characterised by the presence of Heterohelix globulosa, Valvulineria sp.8, Gyroidina solaii giradana, Haplophragmoides pindigensis, Trochamina afikpoensis, Ammobaculites compressa and Reophax spp. This interval is dated Early Santonian - Late Turonian based on the occurrence of Haplophragmoides bauchensis and Haplophragmoides pindigensis. These species were initially reported by Petters (1980; 1982) in the Northern Benue Trough. He assigned an early Santonian – late Turonian age to the sections containing these forms.

Heterohelix moremani- Haplophragmoides talokensis Assemblage Zone.

Interval: 6800 ft-9140 ft.

Age: Early Turonian- Late Cenomanian.

This zone is characterized by the occurrence of Heterohelix moremani, Haplophragmoides talokensis, Miliammina petila, M. telemaquensis, Ammobaculites numahensis, LDO of Ammobaculites pindigensis, Heterohelix reussi, Guembelitra sp.2 and Praebulimina exigua. An Early Turonian - Late Cenomanian age is suggested for this interval due to the occurrence of Heterohelix moremani, Miliammina telemaquensis, Heterohelix reussi and Ammobaculites numahensis. These species were used to date the Nkalagu Formation as Turonian - Early Cenomanian by Gebhardt (2004) and sediment of northeast sedimentary basins by Petters (1980, 1982).

4.3.2. Palobathymetry

Interval: 3960-5400 ft Middle - Outer Neritic (MN-ON)

This interval has been interpreted as middle – outer neritic based on the maximum occurrence of planktonic forams. The total planktonic foraminiferal count is one hundred and seventeen (117) and a diversity ranging between one and eleven (Table 2 and Table 3). The abundance of planktonic forams increases with increase in water depth (Brasier, 1980). Well-developed and large-sized arenaceous benthic forams such as Haplophragmoides, Trochamina, and Ammobaculites species are common. These species suggest inner Neritic – Bathyal depths. The sizes and abundances reduce with shallower bathymetry. Abubakar (2011) reported Haplophragmoides, Bolivina and Nonionella species in the outer neritic - upper bathyal environments in the same Gongola Basin. He also noted that the dominance of the agglutinated (aranaceous) taxa within the section analyzed signifies episodic fluctuations in the development of oxygen minimum zone in the outer shelf. Table 3 shows the foraminiferal abundance and diversity in numbers while the distribution chart (Table 2) shows the forams/taxa distribution.

Table 3. Paleobathymetric checklist for the Kolmani River-1

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Interval: 5600 ft-6400 ft

Paleobathymetry: Inner - Middle Neritic (IN-MN)

This interval is interpreted as inner-middle marine due to reduced abundance and diversity of all the foram groups. Species like Haplophragmoides, Trochamina, and Ammobaculites, which exhibited high abundance and diversity in the previous interval witnessed a reasonable reduction. Heterohelix globulosa, H. reussi and Guembelitria sp.2 also occurred within this interval. Non-keeled thin shelled planktonic foraminifera such as Heterohelicids and Hedbergellids which has been interpreted as shallow water dwellers (Gradsein et al., 1999) were present in this interval, though they are also known to occur in moderately deep marine environment.

Interval :6600-6800 ft - Middle - Outer Neritic (MN-ON)

This interval is interpreted as Middle - Outer Neritic (MN-ON) based on increasing abundance and diversity of Haplophragmoides species (Table 2).

Interval : 7000 – 9140 ft - Inner Neritic

The lowest abundance and diversity of forams assemblage in the well section analyzed was recorded in this interval. Cristellaria sp. 13, a fluvial species was recorded at this interval (Overell, 1995). This suggests a fluvio-marine setting and nearness to the shoreline. There are occasional events of high abundance of some selected aranaceous forams in this interval but the overall assemblage depicts inner neritic water depth. The Kolmani River-1 well paleobathymetry therefore ranges from littoral to outer neritic. Figure 4 shows the palaeobathymetric curve for Kolmani River-1 well.

4.3.3. Paleoecological Modelling

Paleoecological interpretation of ancient sediments has always been made on the premise that each benthonic fossil occurrence indicates a range of possible water depths which that fossil could tolerate (Gebhardt, 2004). This is done by comparism with modern ecological systems and their fauna, although some taxa are believed to have changed their environmental preferences through time. Species diversity is another general indicator of how conducive the environment may be for some foramineferal taxa to populate. Low numbers of species suggest environmental stresses while high diversity suggests equable conditions (Murray, 1994).

The occurrence of some diagnostic benthonic species allowed the definition of minimum and maximum water depths for different parts of well section. Ammobaculites and Haplophragmoides species occurred in almost all samples in the Kolmani River-1 well as shown in table 2. Hence a general paleoecological interpretation for the well was done. The co-occurrence of restricted marine species like Bolivina sp, Praebulimina exigua, Nonionella Sp.2, Valvulineria sp. and unrestricted species of Ammobaculites, Miliamina, Haplophragmoides etc. indicate occasional marine incursion within their depth of occurrences.

The predominance of arenaceous forms shows low-oxygen concentrations for most parts of the basin (Gebhardt, 1998). This could be as a result of increasing input of organic matter from the incoming fluvial systems.

Figure 4. Kolmani River -1 well Paleobathymetric curve

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According to Gebhardt, 1998, the genera Ammobaculites, Miliammina and Ammotium spp. are infaunal deposit feeders that live on muddy substrates in brackish to normal marine salinities from marsh to bathyal environments. They tolerate low oxygen levels. Haplophragmoides species is also an infaunal, detritivore that is common in muddy to sandy substrates in environments ranging from marsh to bathyal. It is primarily a marine genus but has also been reported from hyposaline lagoons and estuaries. Textularia species is noted to inhabit normal marine environments ranging from lagoonal to bathyal. They live epifaunally on hard substrates, muddy silts and sands. The genus Bolivina, thrives in muddy sediments of inner-shelf to bathyal environments. It is probably a detritivore and lives infaunally or epifaually under marine salinities (Gebhardt, 1998).

4.3.4. Salinity

The shell type (Agglutinated, Porcelaneous calcareous and Hyaline calcareous) ratios triangular plot in a sample of foraminifera is used in salinity and paleoenviromental interpretation (Murray, 1974). The proportions of these shell types have been successfully used in distinguishing brackish from hypersaline and normal marine environments.

The plot of the foraminifera recovered the Kolmani River-1 well fell within quadrant for the normal marine marshes (Figure 5).

Figure 5. Shell type ratio triangular diagram for Kolmani River-1

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Alpha diversity index plot (Figure 6) has to do with the number of benthonic species in a standard-sized sample (Murray, 1974). The general pattern recognizable in marine environments today shows increasing diversity away from shore (i.e., with increasing water depth). It is established that the higher the α-index value, the deeper marine is the depositional environment (Nagy et al., 2000). Alpha diversity index tends to be more effective in delineating environments even in situation where natural post depositional processes of dissolution succeeded in changing the original foram assemblages to secondary assemblages as seen the Gongola basin (Petters, 1982).

The αlpha diversity index for the Kolmani River-1 well ranges from 1- 6 (Figure 6). An α-index value of 1-5 is interpreted as hyposaline marshes – hyposaline shelf sea while alpha index above 5 indicates shelf sea of normal salinity. This therefore implies that the pale environment of deposition for sediment penetrated by the well varies from estuarine to outer shelf setting with low-oxygen and restricted bottom water conditions.

Figure 6. Alpha diversity indices values for Kolmani River-1 well

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5. Depositional Environment

Depositional environments of sediments in the studied well section have been deduced from sedimentological, foraminiferal and Paleoecological data. From 900 ft – 2100 ft the medium - coarse grained sands with moderate sorting, sub-angular quartz grains and presence coal fragment/ carbonaceous material suggest deposition in a coastal plain setting, possibly in delta plain or estuary. The absence of forams and dominance of spores corroborate this interpretation. The interval 2700 ft – 3300 ft is interpreted as shallow marine on the basis of lower sand percentage (40% sand; 60% shale). The near-equal content of sands and shales may suggest a parallic environment for this interval (see Figure 2). No forams were noted in this interval but relatively high percentage of palynomorphs.

The interval 3960 ft – 7600 ft of the well section shows considerable deepening with a predominantly blocky, dark grey and calcareous shale lithofacies. The sand percentage varies from 0-10%. This interval may be interpreted as open marine environment based on recorded highest abundance of planktonic and benthonic forams assemblage as forams abundance and diversity increases with water depth (Murray, 1994). The interval 7800 ft -9140 ft (TD) shows relative shallowing. The sand percentage increased relatively, suggesting a coastal plain setting. The reduced abundance and diversity of forams occurrences recorded in this interval supports this interpretation. The presence of a fluvial foram species-Cristellaria sp.13 (Overell, 1995) further supports that this interval was deposited in a continental-parallic environment. The dominance of agglutinated foram species such as the Ammobaculites, Haplophragmoides, Miliammina and Trochamina suggest restricted, low oxygen bottom water conditions for most part of the well section of Kolmani River-1. The presence of Heterohelix and Globogerine shaped taxa in the planktonic assemblage support this assertion.

6. Conclusions and Discussion

The recovered foraminifera species have been used to zoned the sections penetrated by Kolmani River -1 well into three assemblage zones. The analysis of the results shows that the well passed through three formations in the Gongola Basin namely; the Gombe, the Pindiga and the Yolde Formations. The Agathamina sp.2 Assemblage Zone correlates with the Gombe Formation which is dated Campanian - Early Santonian. The Haplophragmoides bauchensis Assemblage Zone covers the upper part of Pindiga Formation and is dated Coniacian-Early Turonian. The Heterohelix moremani-Haplophragmoides talokensis Assemblage zone is a concurrent zone ranges from the Early Turonian (lower part of Pindiga Formation to the Late Cenomanian (Yolde Formation). This indicates that the Pindiga Formation is diachronous in age and ranges from the Coniacian – Turonian while the Yolde Formation is Cenomanian in age. This corroborates with the view of Abubakar et al. (2011) (Figure 7).

Figure 7. Stratigraphic units of the Upper Benue Trough (after Abubakar et al., 2011)

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