Biostratigraphy and depositional environment of the oil shale deposit in the Abakaliki fold belt, Southeastern Nigeria.
The succession of the Cretaceous to recent sediments in the Benue Trough of Nigeria has attracted the attention of paleontologists who have used ammonites, foraminifera and ostracod to delineate the various zones in the Benue Trough [1-11]. The Abakaliki anticlinorium, which is one of the depocenters in the lower Benue Trough, contains approximately 3600 m thick sediments.
The preliminary studies on the lithostragraphy and depositional environment of the oil shale deposits of the Abakaliki fold belt indicated that three lithostratigraphic units, namely: Abakaliki, Eze-Aku and Awgu shales of Albian to Coniacian ages are present. The Abakaliki unit contains light brown to dark grey massive shales and forms part of the Asu River Group. The Eze-Aku shale is dark-grey to black, calcareous, platy and thinly laminated with inoceramus moulds between the laminae and alternates with marl units to form cyclotherms. The Awgu shale is dark-grey, well bedded with limestone interbeds . Oil smell and concentric nodules with pyritic nuclei are common attributes of the oil shale.
The mineralogical analyses of the oil shale revealed that the principal mineral components are quartz, calcite, kaolinite and pyrite with feldspars, muscovite and illite as secondary components . Geochemical analysis indicates high values for the Si[O.sub.2], CaO and [Fe.sub.2][O.sub.3]. The high content of CaO indicates calcareous shale with marine condition prevailing .
An assessment, based on organic facies characteristics, has been carried out on the Middle Cretaceous black shales, in order to determine their hydrocarbon source potential, thermal maturity, and depositional environments . The results show that the Abakaliki shale is characterized by average values of <0.5 wt% TOC, 26.7 mg/g SOM/TOC, <150 mg HC/g TOC HI, 465 [degrees]C [T.sub.max] and is rich in inertinite. Average values typical of the Eze-Aku shale are 4.5 wt% TOC, 148 mg/g SOM/TOC, >200 mg HC/g TOC HI, 435 [degrees]C [T.sub.max] and it is rich in liptinite. Average values typical of the Awgu shale are 1.5 wt% TOC, 66.4 mg/g SOM/TOC, <100 mg HC/g TOC HI, 427 [degrees]C [T.sub.max] .
An extensive geological mapping and geochemical studies of the oil shale deposit in the Abakaliki anticlinorium were carried out to determine the areal extent, reserve estimate, recovery techniques and possible environmental impacts . An areal extent of 72.7 [km.sup.2], reserve estimate of 5.76-109 tonnes and recoverable hydrocarbon reserve estimate of 1.7-[10.sup.9] barrels have been calculated for the oil shale . Low concentration of sulphur (between 0.33 to 0.74%) and trace elements such as Ba, Cd, Cu, Cr, Ni, Pb and Zn supports the economic viability of the oil shale as refinery feedstock. Retorting recovery method was suggested for the oil shale, because of shallow upper soil and relatively cheap cost of establishments .
The stratigraphy of the Abakaliki fold belt is similar to that of the Southern Benue Trough, and paleoenvironmental interpretations in the lower Benue Trough are also valid here. However, pioneering study of the Abakaliki fold belt seems to be handicapped by the lack of core sections. The Albian to Coniacian sediments have not been adequately studied except on ammonite zonations, lithostratigraphy and organic geochemistry. The previous work done on microfossils in the study area has failed to address the Albian-Coniacian sediments but centered on Campanian to Maastrichtian sediments [5, 16-18]. This paper therefore focuses on Albian-Coniacian sediments in order to determine the age, depositional environment and correlation of the oil shale deposit in the Abakaliki fold belt using foraminiferal and ostracod assemblages.
Regional and stratigraphic setting
Before the Santonian, the Abakaliki region was one of the most important depocentres in the lower Benue Trough with marine sediments, ranging in age from Albian to Coniacian, which were deposited in the proximity of the proto-Gulf of Guinea . The principal governing factors of the dynamic evolution in the Abakaliki basin during these epochs were regional tectonics, subsidence, and eustatism . The three main subsidence tendencies in the region were described as high (Albian), low (Cenomanian) and high (Turonian--Coniacian).
The Benue Trough was subjected to four main depositional cycles, each of which was associated with transgression and regression of the sea [8, 9]. The first sedimentary cycle lasted from the Middle Albian to Late Albian and is thought to have been initiated by the opening of the South Atlantic Ocean. This is associated with the deposition of the Asu River Group, which is a lateral equivalent of the Bima Sandstones in the Upper Benue Trough, and Awe/Arufu/Uomba Formations in the middle Benue Trough. The Asu River Group is represented in the study area by the 500 m thick seam of Abakaliki shale, which occupies the core of the Abakaliki Anticlinorium (Fig. 1).
The second sedimentary phase occurred between the Upper Cenomanian and Middle Turonian and was associated with the deposition of Eze-Aku shale. Its lateral equivalents are the Amasiri and Makurdi sandstones in the Afikpo basin and middle Benue Trough respectively, while Gongila, Jessu and Dukul Formations are its lateral equivalents in the upper Benue Trough. This is approximately 1 km in thickness.
The third sedimentary cycle ranged from the Upper Turonian to the Lower Santonian. It is associated with the deposition of the Awgu shale and Agbani sandstones, which are lateral equivalents of the Fika/Sekunle shale in the upper Benue Trough. The Turonian transgression, which marked the start of this cycle, is believed to have commenced from the Gulf of Guinea through the Anambra basin to the Benue Trough . Most of the deposits of this cycle have been eroded as a result of the Late Cretaceous tectonic activity [10, 11]. It is approximately 920 m in thickness.
The fourth sedimentary cycle was marked by deposition of the Nkporo shales, Owelli sandstones, Afikpo sandstones and Enugu shales during the Campanian-Maastricutian transgressive phase. This cycle also marked the deposition of the coal measures including: the Mamu Formation, Ajali sandstones and Nsukka Formation. Its lateral equivalents are the Numanha shale, and Gombe sandstone in the upper Benue Trough [1-3, 15].
[FIGURE 1 OMITTED]
Methodology and sample preparation
Field and laboratory techniques were utilized in the present study. The field study involved measurements and description of different rock outcrops and collection of core samples for laboratory analyses. The field mapping exercise in the Abakaliki fold belt covered an area of 1,105 [km.sup.2], which lies between latitudes 5[degrees]45' N and 6[degrees]35' N and longitudes 7[degrees]20' E and 7[degrees]50' E (Fig. 1). Spot sampling of outcrop and core sections was employed for sample collection. Five traverses cutting across Albian-Coniacian sediments were taken with the aim of locating and delineating geological contacts or boundaries. Fresh outcrop samples were obtained from stream and river channels, major road cuttings and minor paths, and quarries that exist in the area. At the western limb of the Abakaliki anticlinorium, the following traverses were undertaken (Fig. 1):
Lokpaukwu-Lekwesi Traverse (LLT) (1)
Ndeaboh-Lokpanta Traverse (NLT) (2)
Nkerefi-Nara Traverse (NNT) (3)
Ezillo-Nkalagu Traverse (ENT) (4)
At the eastern limb of the Abakaliki anticlinorium a traverse was covered, namely
Akaeze-Umunekwu Traverse (AUT) (5).
The lithologic disposition and number of samples collected are indicated in Figures 2 and 3. Three coreholes sited in the study area were carefully sampled and studied. These include Lokpanta (LKC), Acha (ACC) and Onoli-Awgu coreholes (OAC). The locations of the coreholes and depth of sampling are shown in Figures 1, 2 and 3 respectively.
Fifty-six (56) samples were used for biostratigraphic studies involving foraminiferal and ostracod assemblages. Standard recovery methods were adopted for this work [4, 11, 16-24]. Outcrop and core samples including calcareous shales, black shales and marls were analyzed for their foraminiferal assemblages. Ostracod samples were fragmented inside thick polythene bags using a geological hammer. Hammering was avoided wherever possible to minimize damage to fossils. Fragments were dried at 60 [degrees]C overnight, and disaggregated on hot plate using 15% of hydrogen peroxide. Larger undisaggregated pieces were separated using a 3 mm sieve and discarded. The mud-sized component was removed by washing through a 63-micron sieve. Breakdown at this stage was aided by gently rubbing the residue against the mesh with fingertips.
Various methods have been used for paleo-environmental analyses [3, 18, 23-25], which include foraminiferal abundance, planktonic/benthonic ratios and species diversity. Also, the correlation between the Cenozoic stable isotope record and number of planktonic foraminiferal species suggests that simple diversity registers change in global circulation [24, 26]. Therefore, species abundance, planktonic/benthonic ratio, and species diversity are used to characterize the paleo-oceanographic conditions based on the foraminiferal and ostracod fauna recovered.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Results and discussion
A total of fifteen planktonic foraminiferal species belonging to seven genera were recovered and presented in Tables 1 and 2. The planktonic genera include Rotalipora, Heterohelix, Hedbergella, Whiteinella, Guembelitria, Pseudotextularia and Praeglobotruncana. Three planktonic foraminiferal biofacies were proposed and found to correlate with the zonal schemes proposed by earlier workers [3, 25-29] (Table 3, Fig. 4). Most of the zones are interval zones defined by short and long range species rather than the first appearance datum (FAD) and last appearance datum (LAD) (Table 4). The foraminiferal biofacies and the corresponding zonal schemes are discussed as follows from base to top (the oldest to the youngest):
Praeglobotruncana Sp. Zone (1st Peak)
Praeglobotruncana stephani, Hedbergella delrioensis, H. planispira and Guembelitria harrisi characterize this zone. This could be assigned to Albian to Middle Cenomanian Age. Occurrence has been noted in the AkaEze and Ndeaboh areas (Figs. 4 and 5, Table 1). This interval is in agreement with the findings of earlier workers [17, 27, 29-31].
Beckmann et al.  introduced the Praeglobotruncana stephani zone to describe the oldest Cenomanian zone in the northern part of the Western Desert and the Gulf of Suez area, Egypt. Robaszynski and Caron  defined the present zone as the interval from the FAD of Praeglobotruncana stephani to the FAD of Rotalipora reicheli. The rarity of R. reicheli (keeled morphotype) in the present study may be related to paleoecological factors on the shallow shelf sea [18, 24]. The Asu River Group (Abakaliki shale) has been assigned to this biozone.
Hedbergella -Heterohelix Sp. Zone (2nd Peak)
The zone is characterized by the abundance of Hedbergella delrioensis and Heterohelix moremani . Rotalipora cushmani zone was defined as the total range of the zonal marker  while this zone was defined as interval between the LAD of Rotalipora reicheli and the LAD of R. cushmani . Originally, the present zone was named as zone of "Grandes Globigerines" in the Lower Turonian of North Africa [32, 33]. The Hedbergella-Heterohelix Sp. zone (Tables 3 and 4) was proposed to overlain the praeglobotruncana stephani zone, in the Lower Turonian of the Northern part of the Western Desert and Gulf of Suez area, Egypt . Uppermost Cenomanian to Early Turonian age was suggested for this zone . The Eze-Aku Formation has been ascribed to this zone (Table 4, Figs. 4 and 5).
[TABLE 4 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Heterohelix Sp. Zone (3 Peak)
The association of Heterohelix globulosa and Hedbergella planispira were used to identify this zone in the present study. The occurrence of these species was recorded from the Nkalagu quarry section (Fig. 2, ENT) . The Awgu Formation has been assigned to this zone with thickness of 10 m (Fig. 2). However, the association of Marginotruncana sigalis, M. renzi and M. difformis was used to establish the Upper Turonian-Coniacian age in the Lower Benue Trough of Nigeria .
Benthonic foraminiferal assemblages
A total of thirty-five benthonic foraminiferal species, belonging to twenty-eight genera were recorded from the Albian to Coniacian exposed sections in the study area (Tables 5 and 6). These include Ammobaculites sp., Ramulina sp., Ammotium nkalagum, Coryphostoma crassumi, Gavelinella compressa, Textularia sp., Lenticulina secan, Sitella colonensis, Pallaimorphina yamaguchi, Bathysiphon robustus, Dentalina sp., Trochammina sp., Marssonella oxycona, Ammotium sp., Spiroplectammina semicomplanata, Gabonita sp., Ammobaculites bauchensis, Ammotium sp., Rheophax minuta, Haphlophragmoides sp., Textulariopsis sp. and Asterculus richteri.
The benthonic associations have low diversity and are characterized by abundant agglutinated foraminiferids, all of which have calcareous rather than siliceous wall compositions. Species of Reophax, Haplophragmoids, Ammobaculites, and Trochammina are common (Tables 5 and 6, Fig. 6). Rotaliina forms include Leuticulina, Gavelinella and Lingulo-gavelinella. Marsonella, Spriroplectammina and Coryphostoma characterize the textulariina forms. Assemblages belonging to the benthonic association appear to be confined to carbonate-rich sediments such as calcareous shales, mudstones and limestones deposited in open seas bordering the continents .
Most of these species have been assumed to be of the Upper Albian to Middle Coniacian in age [6, 34-36]. Also, some of these fauna have been recognized from Western Central Sinai of Egypt, indicating possible connection between the Tethys Sea and the Atlantic end during the Cenomanian-Turonian times [14, 34].
The distribution of ostracod species of the investigated sections is presented in Table 7. The number of ostracod species is generally low; with a maximum of 2 species in a sample and 11 out of the 59 samples contain ostracod species. The low number of individuals and species or nonoccurrence may be attributed to sample preparation or possible marine anoxic/dysoxic conditions [11, 28, 36]. The recovered ostracod species included Paracypris nigeriensis, Ovocytheride symmetrical, reticulata, O. reniformis, O. ashakaenesis, Cytherella ovata, Cythereis vitiliginosa reticulata, and Hazena austinensis (Table 7, Fig. 6).
[FIGURE 6 OMITTED]
Based on this study and the previous ostracod studies in Nkalagu borehole (GSN 1037) of the study area , it has been demonstrated that Cenomanian-Turonian boundary is exposed at the Abakaliki fold belt. The occurrence of some of the species found in this study in strata of equivalent ages in Egypt [30, 36] further confirms the suggestion of a union between the Tethys and the South Atlantic arms of the Late Cretaceous epicontinental trans-Saharan transgression in Africa.
Heterohelix and Hedbergella species are the most abundant planktonic genera in all the section studied while Rotalipora; Praeglobotruncana and Whiteinella species are scarce to common (Tables 1 and 2). The dominance of long-ranging heterohelicid and hedbergellid planktonic foraminifera in the study area permit better biostratigraphic resolution only at stage level (Table 4) .
The Abakaliki shales of the Asu River Group, which are the oldest sediment in the study area, are totally devoid of benthonic and relatively low planktonic foraminifera (may be due to preservation problem or alteration). Albian age has been assigned to this section based on ammonite . The outcrop and core sections yield diverse foraminiferal assemblages, which enable recognition of the Middle-Cretaceous stages discussed below.
Middle Albian-Early Cenomanian (108-96 my)
The Cenomanian stage was established in the lower part of the Nkalagu Formation based on the co-occurrence of Rotalipora balenaensis and Globigerinelloides caseyi . However, the planktonic species such as Guembelitria harrisi, Heterohelix moremani and benthonic species which include, Quinqueloculina sandiegoensis, Gavelinella cenomanica, Rheophax sp., Ammobaculite sp., Orbitolinacea str. sp. and Haplophragmoides platus have been used to date rocks of Cenomanian age in Iraq, Brazil, Egypt and along the Gulf Coast of the United States [37-42].
The existence of Cenomanian age in the study area has been a sort of controversy. Some authors suggested a period of non-deposition (hiatus) for this time interval in the Anambra basin and Afikpo syncline [43, 44]. However, the existence of Cenomanian sediments was recorded from four locations in the study area; Aka-Eze, Ezillo, Ngbanocha and Nara using palynological studies . In the present study, the Cenomanian sediments have been collected at the town of Aka-Eze (Fig. 1) beside the bridge over the Eze-Aku River.
Late Cenomanian to Early Turonian (95-92 my)
The Late Cenomanian to Early Turonian was characterized by the appearance of Rotalipora cushmani and Dicarinella algeriana [21, 23-25] while the Early Turonian was clearly defined by the first occurrence of Whiteinella archaeocretacea [27, 30, 31, 34]. Other planktonic species that are recorded in Early Turonian include Praeglobotruncana stephani, Heterohelix pulchra and Heterohelix reussi (Table 4, Fig. 6). Whiteinella archaeocretacea was found in association with the bivalve Inoceramus labiatus and is considered a good marker for the Early Turonian . This interval corresponds to the time of deposition of the oil shale facies in the Abakaliki fold belt . Biostratigraphic records from Texas, Arkansa, Mississippi, Bohemia, Mexico and Egypt indicate that water circulation in the area was in open communication with the world ocean [33, 37-42].
The Late Cretaceous benthonic species noted in the study area include: Gavelinella compressa, Ammobaculites sp. Coryphostoma crassum, Spiroplectammina semicomplanata, Rheophax minuta, Lenticulina secan and Dentalina sp. The Early Turonian species include Ammobaculite nkalagum, Marsonella oxycona, Heterolepa minuta, Ramulina sp., Bathysiphon robustus and Gabonita sp. [14, 28, 36].
Middle Turonian to Coniacian (91-82my)
The association of Marginotruncana sigalis, M. renzi and M. difformis was used to establish the Middle Turonian to Coniacian age in the lower Benue Trough . None of these planktonic species was recorded in this interval. Rather association of Whiteinella inornata, Heterohelix globulosa and H. planispira has been used to assign a Middle Turonian to Coniacian age (Table 4).
Five outcrop and three core sections have been correlated based on lithology, and palaeontologic data. A correlation of the different sections along the Abakaliki fold belt of the Albian to Coniacian sediments appears feasible (Fig. 7). The cross sections show that alternations of shale and marl are restricted to the tip of the Abakaliki anticlinorium (Fig. 7). Laterally, the facies grades into a shale and limestone sequence with reduction in oil shale thickness and increase in limestone in a northeastward direction. The oil shale facies yields dominantly planktonic with few benthonic foraminiferal assemblages indicating fairly shallow marine environments. The bedding of the oil shale is highly significant because it shows lamination couplets each consisting of a light grey marl layer of about 10 m thick on average and a dark to black shale layer about 5 m in thickness  (Fig. 3). The ratio of the average thickness of shale/marl sequences in the three core-holes studied at Lokpanta, Onoli-Agwu and Acha town is 3/8, 3.6/2.8 and 10/7.5 m respectively (Fig. 3). The differential thickness of the shale and marl layers may be due to variable suspension input to the basin. Restriction of shale/marl sequence to the tip of Abakaliki anticlinorium could be as a result of cyclic variation of water depth with time and may further support the relatively shallow-marine environment of these portions of the Abakaliki fold belt.
Depositional environment and palaeogeographic reconstruction
Foraminiferal abundance/diversity (p+b)
Simple diversity is the number of species found in a sample [39, 40-42] and may also be termed foraminiferal numbers . In the present work, all samples have relatively low foraminiferal numbers (Tables 1, 2, 5, 6). It shows an overall increase from Albian through the Lower Turonian (Figs. 4 and 5). Its upward increase may be interpreted as a gradual shift to more stable/quiet environment and an increase in water depth [25, 46] and may represent the Mid-Cretaceous transgression, which reached its maximum at the Cenomanian-Turonian boundary .
Three peaks in the Albian to Coniacian sediments indicating three cycles of deposition are well demonstrated in the Ndeaboh-Lokpanta (NLT) and Acha core (ACC) sections (Figs. 4 and 5). The 1st, 2nd and 3rd peaks (biozones) correspond to the Middle Albian-Early Cenomanian, Late Cenomanian-Early Turonian and Middle Turonian-Coniacian cycles respectively. The 2nd peak is bimodal and has the highest frequency in all the sections made. This coincides with the maximum transgression occurring at the Cenomanian-Turonian boundary.
[FIGURE 7 OMITTED]
Planktonic/benthonic ratios (P/B = p/(p+b) x 100%)
Planktonic/benthonic ratios in some intervals probably suggest restricted shelf environments [39, 41]. This trend suggests that waters were relatively shallow during deposition of the oil shale in the Abakaliki fold belt.
The benthonic genera present in the study area include Ammobaculities, Textularia, Haplophragmoides, Osangularia, Rheophax and Trochammina.
Ammobaculites is an infaunal deposit feeder that lives in muddy sediments with brackish to normal-marine salinities from marsh to bathyal environments  and it also tolerates low oxygen levels. Textularia species inhabit normal marine environments ranging from lagoonal to bathyal and live epifaunally on hard substrates, muddy silts and sands . Some Cenomanian-Turonian textulariids seem to resist reduced salinities . Trochammina settles as an infaunal or epifaunal deposit and plant feeder in a wide range of environments and water depth . Trochamminids are also tolerant of low oxygen values . Rheophax is an infaunal deposit feeder in muds and sands of lagoons, shelves and bathyal regions . Rheophax is mainly a marine genus, but has also been reported from brackish lagoons and estuaries . Osangularia species live in modern oceans from outer neritic to bathyal environments with normal marine salinities and prefer muddy sediments . This suggests that the oil shale may possibly be deposited in outer shelf to bathyal environments.
Another explanation for the high planktonic-benthonic ratios might be the oxygen content of the water. The palaeo-oxygen content can be made using the low occurrence of ostracod fauna. Since the demand for oxygen in ostracods was higher than that of some foraminifera species such as calcareous oxygen deficiency-adapted specialists [18, 25], the few ostracod species (Cythereis and Ovocytheridae species) recovered from the Abakaliki fold belt might be more tolerant of reduced oxygen and salt contents. This mechanism was used to explain high planktonic-benthonic ratios in the Cenomanian of the Western Interior using the recent example of the Arabian Sea . A depth as shallow as 76 m was proposed as an oxygen minimum level in the Western Interior , and if the depth is valid, the planktonic-benthonic ratios that were observed in the Abakaliki fold belt would indicate outer shelf or upper bathyal depths with minimum depth of about 100 m.
Species diversity (SD)
There are three peaks of species diversity as could be observed from Ndeaboh-Lokpanta (NLT) and only the 2nd peak was observed at Acha corehole (Figs. 4 and 5). The 1st peak corresponds to Praeglobotruncana stephani zone, which may be a result of rapid increase in water depth at this time. The 2nd peak coincides with Hedbergella-Heterohelix Sp. zones. The peak shows an increase in foraminiferal and ostracod assemblages and may be related to a major transgression in the Abakaliki fold belt. The 3rd peak corresponds to the Hetrohelix Sp. zone.
Peaks in foraminiferal numbers are similar to the peaks in planktonic-benthonic (P/B) ratios and to peak in species diversity (SD) (Figs. 4 and 5). The major peaks also relate to diversification of the genus Hedbergella and heterohelicids within the study area, which include thin tri-and biserial heterohelicids (Guembelitria and Heterohelix sp.). The intermediate morphotypes include Whiteinella and Praeglobotruncana sp. while the complex morphotypes include Rotalipora sp.
The number of species within the various genera of planktonic foraminifera present in each zone in the Abakaliki fold belt during the Albian to the Coniacian includes Hedbergella, Heterohelix, Praeglotruncana, Whiteinella and Guembelitria. Three biofacies zone have been identified: Praeglobotrucana stephani representing the 1st peak, Hedbergella-Heterohelix representing the 2nd peak and Heterohelix sp. representing the 3rd peak. Albian to Coniacian sediments in the Abakaliki fold belt can thus be subdivided into three depositional cycles. The 1st peak is from Middle Albian to Middle Cenomanian (108-96 my) and the Asu River group (the Abakaliki Shale) belongs to this depositional cycle.
Hedbergella delrioensis, H. planispira, Heterohelix moremani and Heterohelix reussi characterize the 2nd peak which is from Late Cenomanian to Early Turonian (95-92 my). The Eze-Aku shale (oil shale facies) was assigned to this depositional cycle. The occurrence of some Tethyan fauna (foraminifera and ostracod species) indicates possible connection between the Tethys Sea and the Atlantic Sea during this cycle. Heterohelix globulosa and Hedbergella planispira characterize the 3rd peak which ranges from Middle Turonian to Coniacian (91-82 my). The Awgu shale is deposited during this time interval.
The author is grateful to Dr. R. O. Olugbemiro and Prof. H. P. Luterbacher for the assistance provided and Deutscher Akademischer Austauschdienst (DAAD) for providing the scholarship to undergo the study at the University of Tubingen, Germany as well as the anonymous reviewers.
Received June 27, 2009
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Presented by A. Raukas
O. A. EHINOLA *
Energy and Environmental Research Group (EERG)
Department of Geology
University of Ibadan
* Corresponding author: e-mail firstname.lastname@example.org or email@example.com
Table 1. Distribution and abundances of planktonic foraminiferal species from the outcrop samples  Praeglobotruncana Hedbergella Hedbergella Sample No. Formation stephani planispira delrioensis AUT-3 AK 5 2 AUT-6 AK 3 10 AUT-9 AK 5 AUT-171 AK AUT-187 AK AUT-3026 AK 5 LLT-11 EZ LLT-15 EZ 2 LLT-19 EZ 4 12 LLT-21 EZ 5 5 LLT-23 EZ NLT-35 AK NLT-38 AK 10 3 NLT-39 AK 15 NLT-40 AK 4 NLT-41 AK NLT-48 AK 2 NLT-59 EZ 8 NLT-63 EZ 10 11 NLT-72 EZ 6 NLT-75 EZ 2 8 NLT-84 EZ 10 8 NLT-91 EZ 4 NLT-203 AW 8 NLT-204 AW 15 NNT-140 EZ 5 8 NNT-94 EZ 13 6 NNT-127 EZ NNT-134 EZ 8 NNT-212 EZ 10 ENT-184 EZ 1 ENT-104 EZ 1 ENT-103 AS Heterohelix Whiteinella Rotalipora Heterohelix Sample No. globulosa sp. sp. reussi AUT-3 6 4 AUT-6 5 AUT-9 15 6 AUT-171 AUT-187 2 AUT-3026 10 10 3 LLT-11 2 LLT-15 2 LLT-19 8 15 4 LLT-21 8 4 5 LLT-23 2 NLT-35 2 NLT-38 15 6 NLT-39 12 6 NLT-40 8 4 NLT-41 10 3 NLT-48 4 NLT-59 6 NLT-63 12 NLT-72 4 NLT-75 25 3 7 NLT-84 20 NLT-91 22 10 NLT-203 NLT-204 NNT-140 20 7 NNT-94 8 NNT-127 18 8 NNT-134 8 NNT-212 16 8 ENT-184 ENT-104 ENT-103 1 Guembelitria Heterohelix Heterohelix Whiteinella Sample No. harrisi moremani pulchra baltica AUT-3 3 AUT-6 3 AUT-9 AUT-171 AUT-187 AUT-3026 12 LLT-11 LLT-15 LLT-19 6 LLT-21 LLT-23 NLT-35 NLT-38 NLT-39 NLT-40 NLT-41 NLT-48 NLT-59 8 NLT-63 NLT-72 NLT-75 5 6 NLT-84 8 7 NLT-91 8 NLT-203 8 NLT-204 8 NNT-140 4 6 NNT-94 NNT-127 10 NNT-134 8 NNT-212 10 ENT-184 1 ENT-104 ENT-103 1 Heterohelix Whiteinella Seudotextularia Sample No. pseudoglobosa inornata elegans AUT-3 AUT-6 AUT-9 3 AUT-171 1 AUT-187 AUT-3026 3 LLT-11 LLT-15 LLT-19 LLT-21 2 LLT-23 NLT-35 NLT-38 NLT-39 NLT-40 NLT-41 NLT-48 NLT-59 10 NLT-63 NLT-72 NLT-75 12 4 10 NLT-84 8 NLT-91 NLT-203 NLT-204 NNT-140 8 NNT-94 NNT-127 8 NNT-134 10 15 NNT-212 ENT-184 ENT-104 ENT-103 Total Total Planktonic/ Hedbergella planktonic benthonic benthonic Sample No. sp. species species ratio AUT-3 20 5 80 AUT-6 21 2 91 AUT-9 29 2 78 AUT-171 1 2 -- -- AUT-187 2 -- -- AUT-3026 43 1 100 LLT-11 2 5 29 LLT-15 4 2 67 LLT-19 49 19 72 LLT-21 29 18 62 LLT-23 2 1 67 NLT-35 2 31 6 NLT-38 2 36 5 95 NLT-39 3 36 6 80 NLT-40 16 20 44 NLT-41 13 6 68 NLT-48 6 10 38 NLT-59 12 44 11 80 NLT-63 6 39 3 95 NLT-72 10 11 71 NLT-75 85 3 97 NLT-84 10 71 9 87 NLT-91 44 5 94 NLT-203 3 19 -- -- NLT-204 23 -- -- NNT-140 58 7 89 NNT-94 27 8 79 NNT-127 44 7 90 NNT-134 49 7 88 NNT-212 44 4 92 ENT-184 2 3 22 ENT-104 1 2 1 67 ENT-103 2 2 50 Species Sample No. diversity AUT-3 5 AUT-6 4 AUT-9 4 AUT-171 2 AUT-187 2 AUT-3026 4 LLT-11 1 LLT-15 2 LLT-19 6 LLT-21 6 LLT-23 1 NLT-35 1 NLT-38 5 NLT-39 4 NLT-40 3 NLT-41 2 NLT-48 2 NLT-59 5 NLT-63 2 NLT-72 2 NLT-75 11 NLT-84 8 NLT-91 4 NLT-203 3 NLT-204 2 NNT-140 7 NNT-94 3 NNT-127 4 NNT-134 5 NNT-212 4 ENT-184 2 ENT-104 2 ENT-103 2 Legend: AS--Abakaliki shale, AK--Akaeze shale, EZ--Eze-Aku shale, AW--Awgu shale Table 2. Distribution and abundances of planktonic foraminiferal species from the core samples  Sample Depth, Praeglobotruncana Hedbergella No. m Formation stephani planispira LKC-1 3.5 EZ 8 LKC-2 5.5 EZ 8 8 LKC-3 10 EZ 10 3 LKC-4 18 AK 6 LKC-5 25 AK ACC-1 6 EZ 3 ACC-2 12 EZ 6 ACC-3 14 EZ 3 ACC-4 23 AK ACC-5 27 AK ACC-6 39 AK OAC-1 7 EZ OAC-2 11 EZ 3 OAC-3 13 EZ 28 18 OAC-4 15 EZ 25 12 OAC-5 20 AK 9 11 OAC-6 24 AK 15 3 OAC-7 28 AK 3 OAC-8 36 AK Sample Hedbergella Heterohelix Whiteinella Rotalipora No. delrioensis globulosa sp. sp. LKC-1 10 18 8 LKC-2 15 LKC-3 5 15 LKC-4 2 12 3 LKC-5 ACC-1 3 4 ACC-2 5 4 ACC-3 2 5 ACC-4 6 ACC-5 2 ACC-6 3 OAC-1 6 OAC-2 4 10 OAC-3 5 20 5 OAC-4 10 4 OAC-5 8 15 7 OAC-6 5 15 OAC-7 6 OAC-8 7 Sample Heterohelix Guembelitria Heterohelix Heterohelix No. reussi harrisi moremani pulchra LKC-1 5 1 4 5 LKC-2 8 6 6 LKC-3 10 10 8 LKC-4 7 5 4 LKC-5 ACC-1 4 2 2 ACC-2 ACC-3 ACC-4 2 2 ACC-5 1 ACC-6 OAC-1 2 OAC-2 5 3 OAC-3 5 8 OAC-4 8 5 OAC-5 7 2 5 OAC-6 9 4 OAC-7 4 3 OAC-8 4 3 Sample Heterohelix Whiteinella Pseudotextularia Hedbergella No. pseudoglobosa inornata elegans sp. LKC-1 LKC-2 LKC-3 LKC-4 4 LKC-5 ACC-1 ACC-2 ACC-3 ACC-4 1 ACC-5 ACC-6 OAC-1 OAC-2 OAC-3 9 3 12 OAC-4 8 8 OAC-5 8 12 OAC-6 3 OAC-7 OAC-8 Total Total Planktonic/ Sample planktonic benthonic benthonic Species No. species species ratio diversity LKC-1 59 4 96 8 LKC-2 51 7 88 6 LKC-3 76 4 95 8 LKC-4 43 10 81 8 LKC-5 - 5 -- -- ACC-1 18 3 86 6 ACC-2 15 1 94 3 ACC-3 10 4 67 3 ACC-4 11 18 42 4 ACC-5 3 2 60 2 ACC-6 3 6 33 9 OAC-1 8 16 81 2 OAC-2 25 6 86 5 OAC-3 113 18 85 12 OAC-4 80 14 79 8 OAC-5 84 23 76 11 OAC-6 54 17 41 7 OAC-7 16 23 70 4 OAC-8 14 6 2 3 Legend: AK--Akaeze shale, EZ--Eze-Aku shale Table 3. Summary of planktonic foraminiferal zonation from different authors  Stages Present work Galal, 1999 Coniacian Turonian Late Hetero helix sp. Middle Early Hedbergella Whiteinella archaeo Hetero cretaceous helix Cenomanian Late Praeglo Rotalipora botruncana cushmani stephani Middle Asterohed bergella sterispinosa Early Rotalipora globotrun- canoides Albian Late Middle Rotalipora appenninica Stages Premoli Robaszynski Silva & & Caron, Sliter, 1995 1995 Coniacian Turonian Late Middle Early Whiteinella Whiteinella archaeo archaeo cretaceous cretaceous Cenomanian Late Rotalipora Cushmani Dicarinella Rotalipora algeriana cushmani Rotalipora greenhornesis Middle Rotalipora Rotalipora reicheli reicheli Early Rotalipora Rotalipora brotzeni globotrun Albian Late canoides Middle Rotalipora Rotalipora appenninica appenninica Stages Beckmann et al., 1969 Coniacian Turonian Late Middle Early Hedbergella Heterohelix Cenomanian Late Praeglo botruncana stephani Middle Early Albian Late Middle Table 5. Distribution and abundances of benthonic foraminiferal species from the outcrop samples  Gavelinella Ammobaculites Textularia Sample No. Formation compressa sp. sp. AUT-3 AK AUT-6 AK AUT-7 AK 1 AUT-9 AK AUT-3026 AK LLT-11 EZ LLT-15 EZ LLT-19 EZ 1 1 6 LLT-21 EZ 2 LLT-23 EZ NLT-35 AK 2 21 NLT-38 AK NLT-39 AK NLT-40 AK NLT-41 AK NLT-48 AK NLT-59 EZ 8 NLT-63 EZ NLT-72 EZ NLT-75 EZ NLT-84 EZ 1 AUT-3 AK AUT-6 AK NLT-91 EZ 1 3 NNT-140 EZ NNT-94 EZ NNT-127 EZ 2 NNT-134 EZ NNT-212 EZ ENT-103 EZ ENT-104 AK ENT-184 AW Ammotium Spiroplectammina Trochammina Sitella Sample No. sp. semicomplanata sp. colonensis AUT-3 1 AUT-6 AUT-7 AUT-9 AUT-3026 LLT-11 LLT-15 LLT-19 2 1 LLT-21 2 LLT-23 NLT-35 4 NLT-38 3 NLT-39 1 NLT-40 4 NLT-41 1 NLT-48 1 NLT-59 NLT-63 NLT-72 NLT-75 1 NLT-84 2 AUT-3 1 AUT-6 NLT-91 1 NNT-140 NNT-94 NNT-127 1 NNT-134 NNT-212 1 ENT-103 ENT-104 ENT-184 Osangularia Rheophax Marsonella Lagena Sample No. alata minuta oxycona stavensis AUT-3 AUT-6 AUT-7 1 AUT-9 AUT-3026 LLT-11 LLT-15 2 LLT-19 1 LLT-21 LLT-23 NLT-35 NLT-38 NLT-39 1 2 1 NLT-40 4 NLT-41 NLT-48 1 NLT-59 1 NLT-63 NLT-72 1 1 NLT-75 NLT-84 AUT-3 AUT-6 NLT-91 NNT-140 NNT-94 4 NNT-127 NNT-134 1 NNT-212 ENT-103 1 ENT-104 ENT-184 1 1 Lenticulina Haplophgramoides Heterolepa Ammotium Sample No. secan sp. minuta nkalagum AUT-3 AUT-6 1 AUT-7 AUT-9 AUT-3026 LLT-11 LLT-15 LLT-19 1 2 LLT-21 8 LLT-23 NLT-35 NLT-38 1 NLT-39 NLT-40 1 2 NLT-41 NLT-48 NLT-59 2 NLT-63 NLT-72 NLT-75 NLT-84 1 AUT-3 AUT-6 1 NLT-91 NNT-140 1 5 NNT-94 NNT-127 NNT-134 1 4 NNT-212 2 ENT-103 1 ENT-104 1 ENT-184 Ammobaculites Ramulina Bathysiphon Ammobaculite Sample No. bauchensis sp. robustus pindigensis AUT-3 2 AUT-6 1 AUT-7 AUT-9 AUT-3026 1 LLT-11 5 LLT-15 LLT-19 4 LLT-21 3 1 LLT-23 NLT-35 NLT-38 NLT-39 NLT-40 NLT-41 1 4 NLT-48 1 2 2 NLT-59 NLT-63 2 1 NLT-72 1 1 NLT-75 NLT-84 1 AUT-3 2 AUT-6 1 NLT-91 NNT-140 2 NNT-94 1 1 NNT-127 1 NNT-134 2 NNT-212 1 ENT-103 ENT-104 ENT-184 Asterculus Gabonita Praebulimina Pallaimorphina Sample No. richteri sp. prolixa yamaguchii AUT-3 2 AUT-6 AUT-7 AUT-9 8 AUT-3026 LLT-11 LLT-15 LLT-19 1 LLT-21 LLT-23 NLT-35 NLT-38 NLT-39 NLT-40 2 2 NLT-41 NLT-48 2 NLT-59 NLT-63 NLT-72 1 6 NLT-75 1 NLT-84 AUT-3 2 AUT-6 NLT-91 NNT-140 NNT-94 1 NNT-127 2 1 NNT-134 NNT-212 ENT-103 ENT-104 ENT-184 Total Coryphostoma Dentalina Marginulinopsis benthonic Sample No. crassum sp. sp. species AUT-3 5 AUT-6 2 AUT-7 2 AUT-9 8 AUT-3026 1 LLT-11 5 LLT-15 2 LLT-19 19 LLT-21 18 LLT-23 1 1 NLT-35 4 31 NLT-38 5 NLT-39 1 6 NLT-40 5 20 NLT-41 6 NLT-48 10 NLT-59 11 NLT-63 3 NLT-72 11 NLT-75 1 3 NLT-84 3 9 AUT-3 5 AUT-6 2 NLT-91 5 NNT-140 1 7 NNT-94 8 NNT-127 7 NNT-134 7 NNT-212 4 ENT-103 1 3 ENT-104 1 ENT-184 2 Legend: AK--Akaeze shale, EZ--Eze-Aku shale, AW--Awgu shale Table 6. Distribution and abundances of benthonic foraminiferal species from the core samples Sample Gavelinella Ammobaculites No. Depth (m) Formation compressa sp. LKC-1 3.5 EZ LKC-2 5.5 EZ LKC-3 10 EZ LKC-4 18 AK 2 LKC-5 25 AK 1 ACC-1 6 EZ ACC-2 12 EZ ACC-3 14 EZ ACC-4 23 EZ ACC-5 27 AK 1 1 ACC-6 39 AK 2 1 OAC-1 7 EZ 5 OAC-2 11 EZ 1 OAC-3 13 EZ OAC-4 15 AK 4 2 OAC-5 20 AK 1 4 OAC-6 24 AK OAC-7 28 AK 4 OAC-8 36 AK Sample Textularia Ammotium Spiroplectammina Trochammina No. sp. sp. semicomplanata sp. LKC-1 1 LKC-2 2 LKC-3 1 LKC-4 3 LKC-5 1 ACC-1 2 ACC-2 ACC-3 1 ACC-4 2 ACC-5 ACC-6 OAC-1 1 1 OAC-2 2 1 OAC-3 2 OAC-4 3 OAC-5 3 2 1 OAC-6 OAC-7 3 5 OAC-8 2 Sample Sitella Osangularia Rheophax Marsonella Lagena No. colonensis alata minuta oxycona stavensis LKC-1 LKC-2 1 LKC-3 LKC-4 2 LKC-5 ACC-1 1 ACC-2 ACC-3 1 ACC-4 6 ACC-5 ACC-6 1 OAC-1 1 1 OAC-2 OAC-3 OAC-4 2 OAC-5 1 OAC-6 2 2 1 OAC-7 1 OAC-8 Sample Lenticulina Haplophgramoides Heterolepa Ammotium No. secan sp. minuta nkalagum LKC-1 LKC-2 LKC-3 LKC-4 2 LKC-5 ACC-1 ACC-2 ACC-3 1 ACC-4 ACC-5 ACC-6 OAC-1 OAC-2 OAC-3 OAC-4 5 OAC-5 1 2 OAC-6 OAC-7 1 2 OAC-8 1 Sample Ammobaculites Ramulina Bathysiphon Asterculus No. bauchensis sp. robustus richteri LKC-1 1 1 2 LKC-2 LKC-3 3 LKC-4 LKC-5 1 ACC-1 ACC-2 1 ACC-3 ACC-4 5 ACC-5 ACC-6 OAC-1 2 5 OAC-2 1 OAC-3 OAC-4 OAC-5 3 2 1 OAC-6 3 1 OAC-7 3 4 OAC-8 Sample Gabonita Praebulimina Pallaimorphina Coryphostoma No. sp. prolixa yamaguchii crassum LKC-1 LKC-2 LKC-3 LKC-4 1 LKC-5 1 1 ACC-1 ACC-2 ACC-3 1 ACC-4 3 2 ACC-5 ACC-6 OAC-1 OAC-2 1 OAC-3 6 10 OAC-4 OAC-5 1 OAC-6 1 1 OAC-7 OAC-8 1 1 Total Sample Dentalina benthonic No. sp. species LKC-1 4 LKC-2 3 7 LKC-3 4 LKC-4 10 LKC-5 5 ACC-1 3 ACC-2 1 ACC-3 4 ACC-4 18 ACC-5 2 ACC-6 6 OAC-1 16 OAC-2 6 OAC-3 18 OAC-4 14 OAC-5 23 OAC-6 17 OAC-7 1 23 OAC-8 6 Legend: AK--Akaeze shale, EZ--Eze-Aku shale Table 7. Distribution and abundances of ostracod species from outcrop and core samples Paracypris Ovocytheridea Ovocytheridea Sample No. Formation nigeriensis symmetrica reniformis LLT-19 EZ NLT-41 AK 1 NLT-59 EZ 1 1 NLT-204 AW NNT-140 EZ ENT-103 AK OAC-1 EZ 1 OAC-2 EZ 2 OAC-6 AK LKC-1 EZ LKC-3 EZ Cytheresis vitiliginosa Ovocytheridea Clithrocytheridea Sample No. reticulata ashakaensis senegali LLT-19 1 NLT-41 1 1 NLT-59 NLT-204 NNT-140 ENT-103 2 1 OAC-1 OAC-2 OAC-6 1 LKC-1 1 LKC-3 Bracythere Cytherella Bracythere Hazelina Sample No. sapucariensis sp. ekpo austinensis LLT-19 NLT-41 NLT-59 NLT-204 1 NNT-140 1 ENT-103 OAC-1 OAC-2 1 OAC-6 LKC-1 LKC-3 1 Legend: AK--Akaeze shale, EZ--Eze-Aku shale, AW--Awgu shale
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