4.1 Lithological description
The strata of the Kanawa Member of Pindiga Formation were studied in two well exposed sections. The first section is exposed at in Pindiga stream at Pindiga Village was logged in detail (Fig. 4). The sediments consists of predominantly shale and limestones. The Pindiga stream section NE of Pindiga village about 200m thick. The lower 80m consists of shale interbedded with thin limestone beds. The limestone beds are very thin and are concentrated in the lower half of the section where about 10 beds added up to about 7.2m. The thick beds contain Ammonite, Echinoid and Gastropods. They appeared pale yellow to brown in colour.
Upwards it becomes difficult to differentiate. The upper part of the sections have fossiliferous limestone with bivalves, fractured shale withy nodules within beds and gypsum within the fractures. Towards the middle of the sections, ammonites becomes dominant in the limestone and the shale becomes darker in color. At the top, the limestones becomes highly bioturbated with mud and gypsum. There is evidence of thalassinoides and nodules at the topmost part of the section, especially at the Pindiga section, which may indicate evidence of Early Turonian sedimentation. The limestone facies contain bioclastic and lithoclastic grains ranging in size from 0.4 to 3cm and it is dominantly grain supported, fossiliferous with ammonites fragments. The fossiliferous limestone beds grades into bioturbated limestone which is mud supported.
4.2 Lithofacies
For the sedimnetological analysis. Field and thin-section data indicates that the strata composed
Three major facies Shale (SL), Grey Mudstone (MS) and Fossiliferous Limestone (LM) facies.
4.2.1 Lithofacies SL: Shale
This facies consists of shale that varies in thickness from 5 cm to 3 m. The shale facies is characterised by (Figs. 4 and 5) grey, black, and greyish yellow, with some gypsum laminate. The shale facie is laterally extensive and interbedded with fossiliferous limestone facies. The shale facies is interpreted as deposition inlow environments. The facies may represent deposition by suspension settling in the absence of wave and currents in shallow marine (ramp) setting ([34]). The gypsiferous appearance on the shale facies may be diagenetic.
4.2.2 Lithofacies MS: Grey Mudstone
This facies consists of grey mudstone that is generally intercalated with limestone and perhaps, glauconitic in nature. The facies has a thickness of about 3.5 m and is laterally extensive. The mudstone facies is also interbedded with the fossiliferous limestone (carbonate) facies with well-developed syntaxial growth. The facies may be interpreted as deposition by the suspended sedimentation and low-energy currents in shallow marine or shelf or ramp setting under the fair-weather wave base above the storm wave base [57]. Presence of glauconite indicates low sedimentation rate in normal salinity and dysoxic to reducing environments within ramp setting. This further suggests marine transgression of a relative sea level and deposition within ramp setting [8].
2.3 Lithofacies LM: Fossiliferous Limestone
The limestone facies range in color from grey to brownish. It consist of wackestone, packestone and mudstone with a thickness ranging from 5–7m. The facies have erosional base. Macrofossils range in sparse to relatively abundant including Brachiopods, Bivalves, and Ostracods (Fig. 6). This facies is interpreted as deposition in a low-energy shallow marine shelf or ramp setting ([21]; [10]). The appearance of brecciations, mud matrix, and glauconitic tint, faunal abundance may perhaps suggests a shallow water depositional environment that formed above the fair weather wave base [8].
4.2 Carbonate microfacies
The limestones are described and grouped based on Dunham Classification (1963) using certain textural parameters such as mud to grain size, packing, skeletal and non-skeletal grains. The carbonate microfacies analysis enabled us to sub-divide the carbonates into four (4) facies types. The identified microfacies were inferred to be deposited in different regions of marine (shelf) environment under various energy conditions.
4.2.1 Microfacies PW: Packestone-Wackestone
This microfacies composed of fragments of Oysters (Fig. 7) as well as ostracoda and gastropoda. Diagenetic alterations are replacement of oyster fragments and ostracod with calcite micrite. Oysters inhabits shallow water environment ([19]) and abundant in geologic record from the Cenomanian to Turonian (e.g [10]). This microfacies indicates deposition in shallow water (marine) carbonate environment. Bioturbation supports low energy condition ([34]; [8]).
4.2.2 Microfacies BP: Bioclastic Packestone
The microfacies contain clasts components including corals brachiopods and shell fragments (Fig. 7). The texture is of packstone with peloidal and non-fossiliferous grainstone. This microfacies may be interpreted as part of the upper limestone series in cyclothem model deposited in moderate energy environment, above the mean fair weather wave base (FWWB). The presence of fauna suggests deposition in marine ([34]).
4.2.3 Microfacies BW: Bioclastic wackestone
This microfacies having a micrite matrix and consists of marine fossil bioclasts. It also consists of whole body bivalves, brachipods and intraclasts embedded in a micrite matrix (Fig. 7). The presence of micrite matrix is an indication of a low energy depositional environment. The preserved whole body of this facies are interpreted as low energy shallow marine deposits [34].
4.3 Foraminiferal analysis
Foraminiferal recoveries of the samples were generally good. Most of the samples were populated by arenaceous benthic species. Calcareous species (planktic and benthic) were not recorded. This could be attributable to the depositional environments of the sediments. Foraminiferals are good in biostratigraphic study [13] and [41]. Though, planktic foraminiferal species are absent in the samples but the arenaceous benthic foraminiferal species whose stratigraphic distributions have been well established in the Nigerian basins were used to assign ages to the Kanawa Member. The foraminiferal assemblage was characterized by Ammobaculites subcretacea, Haplophragmoides bauchensis, Haplophragmoides pindigensis, Haplophragmoides excavata, Ammobaulites pindigensis, Ammobaculites bauchensis, Ammobaculites gombensis, Ammobaculites benuensis Ammoastuta nigeriana, Reophax guineana and Miliammina pindigensis (Figs. 8, 9 and Table 1).
Table 1
Details of the foraminiferal results showimg count, type and age
S/N | Foraminifera | Count | Type |
1. | Ammobaculites benuensis | 18 | AB |
2. | Haplophragmoides pindigensis | 28 | AB |
3. | Ammobaulites bauchensis | 18 | AB |
4. | Ammobaculites pindigensis | 47 | AB |
5. | Haplophragmoides bauchensis | 37 | AB |
6. | Haplophragmoides excavata | 33 | AB |
7. | Trochammina sp. | 46 | AB |
8. | Textularia sp. | 56 | AB |
9 | Ammobaculites gombensis | 55 | AB |
10. | Ammoastuta nigeriana | 44 | AB |
11 | Arenaceous indeterminate | 47 | AB |
12 | Reophax guineana | 52 | AB |
13. | Ammobaculites subcretacea | 64 | AB |
14. | Bathysiphon sp. | 53 | AB |
15. | Miliammina pindigensis | 42 | AB |
16. | Ammoastuta nigeriana | 67 | AB |
CB = Calcareous benthic, AB = Arenaceous benthic, P = Planktic |
Proposed age: Turonian – Santonian |
4.3.1 Foraminiferal Color Index (Thermal Maturation)
The color index of the Foraminiferal are used as stand in for geothermal maturity by ([28], [29]). Color index of foraminiferal was interpreted as changes due to secondary maturity by [5]. Concerning this, [47] proposed a colour alteration index based on observed colour trends in agglutinated foraminifera recorded from sediment in the Gulf of Mexico. Based on this concept, the Foraminiferal Colouration Index (FCI) is used to estimate thermal alteration of organic matter. This is achieved by visual comparison of the Foraminiferal colour with the standard colour sequence ([31]). Foraminiferal colours in the samples from Kanawa Shales show values from light grey to light brownish grey. To determine maturation level, the value of the foraminifera colour (Fig. 9) in comprising to the Munsell Colour Chart was used which were later converted to FCI number (Table 2).
Table 2
Foraminiferal Colour and interpretation potential
S/No | Sample No | Foraminiferal colour Index (From Munsell colour system) | FCI value | Estimated Temperature | Interpretation potential |
1 | A1 | Light Brownish Grey to Grey | 3 | 75 | Low Thermal Maturity |
2 | A2 | Light Grey | 2 | 60 | Low Thermal Maturity |
3 | A3 | Light Grey | 2 | 60 | Low Thermal Maturity |
4 | A4 | Light Brownish Grey to Grey | 3 | 75 | Low Thermal Maturity |
5 | A6 | Light Brownish Grey to Grey | 3 | 75 | Low Thermal Maturity |
6 | A7 | Light Grey | 2 | 60 | Low Thermal Maturity |
7 | A13 | Light Grey | 2 | 60 | Low Thermal Maturity |
8 | A14 | Light Brownish Grey to Grey | 3 | 75 | Low Thermal Maturity |
9 | A17 | Light Grey | 2 | 60 | Low Thermal Maturity |
10 | A20 | Light Grey | 2 | 60 | Low Thermal Maturity |
11 | A21 | Light Brownish Grey to Grey | 3 | 75 | Low Thermal Maturity |
12 | A22 | Light Grey | 2 | 60 | Low Thermal Maturity |
4.4 Palynofacies analysis
The palynomaceral types and Structureless Organic Matter (SOM) of [40] were identified and related to the Herbaceous, Woody, Coaly and Amorphous particulate types of [52] using transmitted light microscope were used. The abundances and size variations were used to interpret the possible paleoenvironment of deposition of the samples in collaboration with recovered sporomorphs. The colour variations of palynomacerals in the strewn slides are also useful in approximating the hydrocarbon generating potential of the source rocks as outlined by [52]. The palynofacies analysis of the samples indicate mainly amorphous organic matter (Fig. 10) of type III kerogen.
4.5 Spore color index and hydrocarbon potential
This involves identification of the palynomaceral constituents in the sample. It also includes calculating their relative and absolute abundances, and determining their sizes and degree of preservation ([18], [17]). It is commonly referred to as Visual Kerogen Typing (VKT) and helps in interpreting the paleoenvironment of the sediments. In the study, the samples were generally characterized by moderately to poorly sorted and small to medium sized coaly (PM-1 and PM-4) palynofacies. Common occurrence of small sized woody (PM-2) particulate materials in association with PM3 and SOM / amorphous materials were also identified. Details of the VKT analysis as well as Spore Colouration Index (SCI)/Thermal Alteration Index (TAI) estimations which help in recognizing hydrocarbon potential of the samples are presented in Table 3. The hydrocarbon generating potential of the sample is interpreted to be gas prone based on the [41] SCI scale estimate of 8.5–10 (dark brown to black). This corresponds to TAI estimate of 3 + to 5 (Table 3). These indexes also correlate with the estimated vitrinite reflectance value (Ro) of 1.50-˃2.00 on the [52] scale. The SCI shows an estimate of 8.5–10 ([41]) with the particulate constituents being dark brown to black in colour and are predominantly coaly to woody materials. It corresponds to the TAI estimates of 3 + to 5 and estimated vitrinite reflectance value (Ro) of 1.20-˃2.00 on the [52] scale (see Table 3). This suggests the sample lie within a gas prone interval in the basin. The hydrocarbon potential is gas based on the occurrence of woody to coaly particulate materials that are predominantly black. The SCI is estimated to be 9–10 which correlates with approximate TAI of 4- to 5 and estimated vitrinite reflectance value (Ro) of 1.50-˃2.00 on the [52] scale.
Table 3
Representative of palynofacies and abundance compared with Staplin, (1969)
Sample | Oyede (1991) | Staplin (1969) | |
Type | Cnt | % | Particle type | TM | Col. | SCI | TAI | Ro | |
A1 | PM1 | 90 | 45 | Coaly | Inertinite | Dry Gas/ Barren | Dark brown - Black | 8.5 - 10 | 3 + to 5 | 1.20 -˃2.00 | |
PM4 | 40 | 20 | |
PM3 | 20 | 10 | woody | Vitrinite | |
PM2 | 30 | 15 | |
SOM | 20 | 10 | Amorph. | Amorph. | |
Type | Cnt | % | Particle type | TM | Col. | SCI | TAI | Ro | |
A2 | PM1 | 80 | 40 | Coaly | Inertinite | Dry Gas to Barren | Dark brown - black | 8.5 - 10 | 3 + to 5 | 1.20-˃2.00 |
PM4 | 40 | 20 |
PM3 | 20 | 10 | woody | Vitrinite |
PM2 | 35 | 17.5 |
SOM | 25 | 12.5 | Amorph. | Amorph. |
TM = Thermal Maturity. Col = Colour. TAI = Thermal Alteration Index. Ro = Vitrinite reflectance. SCI = Spore Colouration Index.
4.6 Biostratigraphy
The occurrence of Haplophragmoides bauchensis co-occuring with Haplophragmoides pindigensis, Ammobaculites bauchensis, Ammobaculites pindigensis, Haplophragmoides benuensis, Ammobaculites gombensis and Ammoastuta nigeriana suggest a Turonian – Santonian age. The inferred age and depositional environment and the criteria on which the inferences are based are also provided alongside for each sample. The sample is characterized by arenaeous foraminiferal species such as Ammobaculites subcretacea, Ammobaculites gombensis, Ammobaculites pindigensis, Ammobaculites bauchensis, Ammoastuta nigeriana, Haplophragmoides bauchensis, Haplophragmoides pindigensis and Reophax guineana which suggest Turonian – Santonian age. This foraminiferal assemblage encountered in this section has been used to delineate Turonian to Santonian age in different part of the world. A similar assemblge was utilized by [43] to date Pindinga Formation in the Gongola sub-basin. However, the occurrence of Ammobaculites subcretacea which has also been reported in the Albian to Maastrichtian sediments of the Calabar Flanks ([44]) could also indicate that this sample is not older than Albian and not younger than Maastrichtian.
4.6.1 Late Cretaceous age
Haplophragmoides excavata has been reported in the Coniacian part of Nkalagu Formation and Nkporo Shale ([44], [9]). Originally described from the American Gulf Coast, this species has been reported from Coniancian to Maastrichtian of North America ([51]). Haplophragmoides bauchensis was reported in the Turonian to Coniacian sediments of Nkalagu Formation ([42]). Haplophragmoides pindigensis, Haplophragmoides bauchensis Ammobaculites bauchensis, Ammobaculites pindigensis, Ammoastuta nigeriana, Miliammina pindigensis and Reophax guineana were also reported in the Turonian – Santonian outcrops of the Pindiga Formation ([42]). On the basis of above foraminiferal associations, the studied interval of the Kanawa shales is assign Late Cretaceous, not younger than Santonian age and not older than Turonian.
4.7 Paleoenvironment
The interpretation of the paleoenvironment of the outcrop samples were inferred mainly from foraminiferal assemblage. The depositional environment of the samples are predominantly marginal marine (fluvio marine) to shallow Inner Neritic settings based on the recorded foraminiferal assemblage dominated by arenaceous benthic species such as Haplophragmoides excavata, Haplophragmoides bauchensis, Haplophragmoides pindigenis, Ammobaculites bauchensis, Ammobaculites pindigensis, Ammobaculites benuensis, Ammobaculites gombensis, Ammoastuta nigeriana and Reophax guineana. The above foraminiferal association inditates marginal marine (fluvio marine) to shallow Inner Neritic environments ([3], [42], [44]).
The dominance of arenaceous foraminiferal species in shallow water limestone and micaceous shales in the Benue Trough have been used by [44] to suggest shallow water habitat for the Nkalagu limestone. Similarly, ([42], [43]) has used the dominance of arenaceous species of Haplophragmoides, Ammobaculites, Reophax and Miliammina which he used in concluding that the paleoenvironment of Pindiga formation of the Gongola basin depict a paralic to a very shallow neritic environment. The dominance of agglutinated foraminiferal species also suggest restricted, low oxygen bottom water conditions. [3] Have inferred near shore turbulent environment in the modern Niger Delta on the basis of arenaceous foraminiferal species of Ammobaculites, Haplophragmoides and Trochammina to be associated with near shore lagoonal environment of the Niger Delta. On the basis of the arenaceous foraminiferal assemblage recorded in the studied interval of the Kanawa shales indicates marginal marine to proximal Inner Neritic settings. The VKT analysis of the sample show that it is characterized by common and poorly sorted, small to medium sized PM-1 and PM-4 phytoclast in association with moderate records of SOM and PM-2 (small - medium sized) and low records of PM-3. This admixture suggests a predominantly fluvio- marine paleoenvironment of deposition.
Lithofacies and microfacies investigation of Kanawa Member allows interpretation of the facies succession as shallow marine (shelf) ramp facies. Paleoenvironmental interpretation of microfacies are based on sedimentological and paleontological features observed. Structural features and textural analysis of the studied carbonate indicate that the samples were affected by many diagenetic processes e.g. micritization, cementation, dissolution and compaction. At a greater depth below the mean fair weather wave base, organic productivity is generally higher and thus, bioclastic limestones, skeletal packestone and wackestone are dominant ([53]; [8]). Fine grained carbonate materials were inferred to have been derived largely from shallow water areas ([53]; [34]). Using a depositional model (Fig. 11), the ramp facies succession consists of limestone, shale, mudstone, packestone and wackestone. Therefore, the coastal-shallow marine shelf siliciclastic facies of the Cenomanian Yolde Formation gradually transformed into a carbonate dominated depositional environment, most probably due to the relative sea-level rise which continued resulting in the development of a deeply entrenched carbonate shallow marine shelf (ramp) environment of the Kanawa Member.