Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria

Journal of African Earth Sciences xxx (2014) xxx–xxx

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Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria O.A. Boboye ⇑, E.E. Okon Department of Geology, University of Ibadan, Nigeria

a r t i c l e

i n f o

Article history: Received 26 June 2013 Received in revised form 28 April 2014 Accepted 29 April 2014 Available online xxxx Keywords: Petrographic Formation Kerogen Anoxic Palaeoclimatic Environment

a b s t r a c t An integrated sedimentological and geochemical evaluation has been carried out on the Cretaceous sediments of the Calabar Flank. This study is to characterize the provenance, depositional environments and hydrocarbon potentials. The techniques involved field descriptions, textural parameters, petrographic analysis and biostratigraphic studies using standard sedimentological methods. The geochemical studies involved the determination of major oxides and trace elements using Inductively Coupled Plasma–Mass Spectrometry (ICP–MS); Total Organic Carbon (TOC) and Rock Eval Pyrolysis. Results show that sandstone from Awi Formation have elongation ratio ranging from 0.4b to 0.9, oblate–prolate index and maximum sphericity index range from 9.6 to 9.7 and 0.5 to 0.9 respectively. The sandstone units are arkosic and mineralogically immature (MI = 3); ZTR indexes range from 54.6% to 82.5%, with tourmaline, zircon, staurolite, garnet, apatite, augite and rutile grains being angularsub-angular. This suggests nearness to source, and that Awi Formation was deposited in a fluvial environment. The limestone deposit of Mfamosing Formation is predominantly bioclastic consisting of algal stromatolites, oolitic and pelloidal grainstones/packstones with high carbonate content. The dark grey fissile shales of Nkporo and Ekenkpon Formations indicate deposition in quiet oxic and/or anoxic conditions. Average TOC suggests good source rocks. Predominance of Type III kerogen, Tmax and hydrocarbon source potential of Mfamosing, Ekenkpon, New Netim Marl and Nkporo Formations suggest marginal mature to mature source rocks deposited in shallow continental to open marine setting that some gas may have been generated. The sediments are derived from passive continental margin in plutonic humid palaeoclimatic setting of continental block province. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The Calabar Flank is one of the marginal/coastal sedimentary basins in Nigeria that was initiated during the Early Cretaceous rifting process (Reijers and Petters (1987). It has been informally named southern Nigerian sedimentary basin along with the Lower Benue Trough, Dahomey Basin and Niger Delta. All these with the Douala Basin of Cameroon constitute the West African south Atlantic marginal basins. The Calabar Flank originated from the same process that formed the Benue Trough during the Early Cretaceous rifting event. It has been shown that subsidence along fundamental faults viz-a-viz landward continuation of the Chain and Charcot faults systems initiated the Benue Trough, which runs NE–SW and marks the failed arm of the RRR triple junction. ⇑ Corresponding author. Tel.: +234 8033928929. E-mail addresses: [email protected] (O.A. Boboye), boboyegbenga@ yahoo.com (E.E. Okon).

From the position of the fault systems, Reijers and Petters (1987) believe that it is the Charcot fault system that separates the Niger-Delta Basin from the Calabar Flank. The Calabar Flank although associated with the Benue Trough tectonics, has a NW– SE trend and is dominated by horst and graben structures. Throughout geologic time, sedimentary basins have developed at any instant of depression, which result from mostly tectonic activities (lithospheric sagging, faulting, stretching and pull apart). The Calabar Flank can be seen from this light as a coastal rift dominated basin trending NW–SE and dipping gently in the south-westerly direction (Reijers and Petters, 1987). Stratigraphic studies of the Calabar Flanks have over the years been carried out and are well documented; however, studies on the sedimentologic combined with the geochemical characteristics of this basin is scanty, hence the need for this study (Petters and Ekweozor, 1982). The interest of this present work is on the sedimentologic (textural characteristics, petrography, lithological description and paleontological studies) and geochemical

http://dx.doi.org/10.1016/j.jafrearsci.2014.04.035 1464-343X/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Boboye, O.A., Okon, E.E. Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria. J. Afr. Earth Sci. (2014), http://dx.doi.org/10.1016/j.jafrearsci.2014.04.035

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

(inorganic – trace element geochemistry, major oxides and major element studies and organic–organic richness, organic quality and maturation parameters of the source rocks) characteristics of the Cretaceous strata of the Awi, Mfamosing, Ekenkpon, New Netim and Nkporo formations.

and heavy mineral analyses carried out on sandstone from Awi Formation. The stratigraphy of the study area and Calabar Flank are presented in Figs. 2 and 3. Inorganic geochemical analysis (whole rock), was carried out on samples from Nkporo Shale (NKS), New Netim marl (NNM), Ekenkpon Shale (EKS), Mfamosing limestone (MLS), Awi Formation (AF) using Inductively Coupled Plasma–Mass Spectrometry (ICP–MS) and Inductively Coupled Plasma–Emission Spectrometry (ICP–ES). The Total Organic Carbon (TOC) and Rock-Eval Pyrolysis were carried out on samples from NKS, NNM, EKS and MLS for the source rock characterization (Fig. 3).

2. Geologic setting The Calabar Flank is located between the Cameroon volcanic line to the east, the Oban Massif to the north, the Ikpe platform to the west and the Calabar Hinge Line to the south. Regionally, it occupies a position to the extreme northeast of the Niger Delta Basin. The expanded section of the southern Nigeria with the location of the study area is presented (Fig. 1). Sedimentation in the Calabar Flank commenced with the deposition of Awi Formation (fluvio – deltaic clastics) of probably Neocomian to Aptian age (Adeleye and Fayose, 1978; Ramanathan and Kumaran, 1981) on the block faulted Basement Complex. The sediments contain well over 400 m thick of Cretaceous (Pre-Aptian-Maastrichtian) sediments in outcrop sections. Sediments of the Calabar Flank generally dip within the range of 5–20° mostly to the southwest. The mid-Albian Mfamosing Limestone consisting predominantly of carbonate rocks rest unconformably on the Awi Formation and represents the onset of marine incursion into the basin (Petters, 1982). This was succeeded by the Cenomanian–Turonian Ekenkpon Shale and the Coniacian New Netim Marl after which there was a period of deformation, erosion and/or non-deposition during the Santonian (Petters, 1982; Petters et al., 1995). Late Campanian–Maastrichtian witnessed the deposition of the dark grey to black highly fissile Nkporo Shales (Reyment, 1965).

4. Results and discussion 4.1. Sedimentology The Awi Formation is the sandstones facies of the Calabar Flank unconformably overlying the Precambrian Basement Complex (Oban Massif) with sharp contact. It shows distinctly typical fining upward sequence, and towards the top is 35–48 cm thickness of lignite band (Fig. 4b). The sediment fill is made up of pebbly to coarse grain sandstone at the basal section, overlain by coarse grained sandstone. The mud-supported sandstone that overlies the coarse grained sandstone makes the third subunit, then the bluish-grey mudstone. This rhythmic sedimentation gently dips at angles (12–16°) to the southwest. The overall geometry and distribution of the sediments are seen to mimic the basement surface and constitutes non-marine, arkosic fluvio – deltaic sandstones. Sedimentary structures observed in this section include; normal graded bedding (fining upward sequence), ripples laminated beddings associated with the mud supported sandstone flute marks and lenticular beddings associated with mudstone above the lignite band (Fig. 4b). Estimation of the pebble characteristics involved analysis of their morphometric parameters (i.e. long L, intermediate I, and short S axes). Regarding the clast sphericity, roundness and ‘‘Oblate–Prolate’’ Indexes, the parametric values of an average of 10 pebbles were used in the analysis. The rippled laminated sandstones and lignite band are shown in Fig. 4(a) and

3. Material and methods

GH

This study involves measurement of sedimentological features and the use of geochemical techniques to characterize the Cretaceous strata in the study area. The sedimentological studies include pebble morphometry, granulometric analysis, petrography

I

PE R M O BAN I K FO T A MA SSIF L P

Ik a n It u g T LI R H k H r ou N I gh E N G ig h E BA

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CA

CA

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RO

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KA BA

N IGERIA

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E NG HI N E NI B E LIN

T RO U

A BR M M A OR AN TF A PL

G u lf o f G u in e a (S o u th A tla n tic) C alab ar F lank

C am erou n V olc an ic Line

1

0

1 Kilo m e te r s

Fig. 1. Map of southern Nigeria showing the location of the Calabar Flank (inset: map of Nigeria showing southern Nigeria) (after Nyong and Ramanathan, 1985).

Please cite this article in press as: Boboye, O.A., Okon, E.E. Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria. J. Afr. Earth Sci. (2014), http://dx.doi.org/10.1016/j.jafrearsci.2014.04.035

O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

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Fig. 2. Stratigraphic chart of the Calabar Flank (modified after Petters, 1982).

Fig. 3. Geological map showing the locations and lithologic profiles of the study area along the three traverses (T1–T3). (Inset map: Map of Nigeria) (modified after Essien and Ufot, 2010).

Please cite this article in press as: Boboye, O.A., Okon, E.E. Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria. J. Afr. Earth Sci. (2014), http://dx.doi.org/10.1016/j.jafrearsci.2014.04.035

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

Fig. 4. (a and b) Photograph showing (a) ripple-lamination in the sandstone (chisel is 20 cm long) and (b) lignite band in the topmost layer of the Awi Formation.

Fig. 5. A plot of MPS vs. OPI (Dobkins and Folk, 1970).

(b). The dominance of compact pebbles, which are sub-angular to sub-rounded (predominantly mean roundness of 33%) suggest a fluvial depositional environment (Figs. 5–8). This was buttressed by the environmental determination chart of Sames (1966) which distinguishes littoral processes from river processes (Fig. 8). Granulometric analysis showed parameters (coarse sand, platykurtic to mesokurtic and strongly fine skewed) all suggesting fluvial depositional regime (Fig. 9). The limestone facies of the Calabar Flank is represented by the Mfamosing Limestone and parts of the Ekenkpon Shale occurring as fossiliferous bands. Mfamosing Limestones represents the record of the earliest marine incursion into southeastern Nigeria. It unconformably overlies the Awi Formation in the Calabar Flank in places and at other places it is seen to sit directly on the basement rock. It is dated back to mid-Albian, highly fossiliferous and rich in both allochems and precipitated calcites. It is made up of predominantly bioclastic allochems (fossils, pelloids, and shell fragments) held in sparry cement and is typical of platform carbonate sedimentation in a wide variety of environments (Folk, 1974; Essien, 1995). The limestone of the New Netim Marls is almost entirely composed of shell fragments and can be considered as a marker bed for the upper section of the Ekenkpon Formation in the Calabar Flank. Petrographic study

Fig. 6. 3D plot of roundness vs. sphericity vs. oblate–prolate index for mean pebble sizes for Awi Formation (after Dobkins and Folk, 1970).

Please cite this article in press as: Boboye, O.A., Okon, E.E. Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria. J. Afr. Earth Sci. (2014), http://dx.doi.org/10.1016/j.jafrearsci.2014.04.035

O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

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Fig. 9. Plot of skewness vs. mean size (after Friedman, 1979). SFS, strongly fine skewed; FS, fine skewed; NS, near symmetrical; CS, coarse skewed; SCS, strongly coarse skewed.

Fig. 7. Sphericity – form diagram. Each point represents the mean value of 10 pebbles and the initials defined by the dotted lines are used to represent the 10 classes and are given thus: C, compact; CP, compact-platy; CB, compact-bladed; CE, compact-elongate; P, platy; B, bladed; E, elongate; VP, very platy; VB, very bladed; very elongate (after Sneed and Folk, 1958).

Fig. 8. Environmental determination chart showing distinction between fluvial processes and littoral process (Sames, 1966).

revealed the presence of algal stromatolites, remains of gastropods, pelecepods, brachiopods, echinoids and foraminifera with faecal pellets and pelloids. The presence of algal stromatolitic boundstone at the base of the section represents deposition on algal flat (intertidal) environment while its association with oncolites suggest the presence of tidal channels (Figs. 10–12) (Kahle and Flohd, 1971). The preponderance of quartz (57%) to feldspar (66%), rock fragments and the degree of angularity of the grains suggests immaturity of the sediments, suggesting proximity to provenance. These were corroborated by the heavy minerals identified as nearly euhedral to anhedral indicating nearness to source and the variety of grains established granitic pegmatite and metamorphic terrain

and a more plausible area is the Oban Massif and its environments (Figs. 13–15) (Mange and Maurer, 1992). The oolitic-pelloidal intraclasts were observed to be well sorted, signifying high energy, agitated environment of deposition in shoal, beaches and/or tidal bars. The presence of micritized whorl of gastropod indicates deposition in low energy environment such as protected lagoon, pond or bays. The restriction in circulation may have resulted in deterioration of the environment leading to fluctuation of salinity concentration and lowered oxygen content favouring the proliferation of only gastropods. This is responsible for the large grains of gastropods devoid of other fossils. The stylolites presence indicates the chemical compaction in the lower section of the limestone (bioclastic grainstone). The occurrence of Scleractinian coral are also indicative of a shallow marine setting which were seen occurring as colony and the individuals are now represented by recrystallized calcite. Based on the above microfacies described in the petrographic study, the depositional environment of the Mfamosing Limestone range from tidal flats, lagoonal complex, oolitic shoals and shelf marginal environments where open circulation persisted (Figs. 10–12). Mudrock facies is represented in all the formations of the Calabar Flank except in the Mfamosing Limestone. The main constituents of mudrocks include clay minerals and silt-grade quartz (Tucker, 2003). The Awi Formation contains mudstones at the upper sections of the cycle although it is negligible when compared with those of Ekenkpon Shale (Cenomanian–Turonian), New Netim Formation (Coniacian) (Bassey, 2010) and the Nkporo Shale (late Campanian–Maastrichtian). The Ekenkpon Shale consists of two sequences of calcareous grey, fissile shales separated by thick mudstone (regressive) unit dated late Albian–Turonian (latest Albian–Cenomanian and late Cenomanian to Turonian). The lithofacies consists of gently dipping grey shales with intercalations of nodular marlstones (nodules) bands and shelly (limestone) intervals. These intercalated shales show calcareous mudstones in the form of septarian nodules and evidence of early diagenesis, as the nodules are not seen to truncate the laminations but the laminations just tend to move around the nodular structure. The septarian nodules show desiccation cracks radiating from the centre of the nodule to its rim having recrystallized calcite filling the cracks. The shale unit contains fossils like ammonite (mainly keeled Turrilite sp.) and abundant juvenile bivalves. The juvenile (or dwarfed) cuculeid bivalves show

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

Fig. 10. Bedded limestone of the Mfamosing formation exposed along Calabar-Ikom Highway (traverse one).

Algal stromatolites

Bullet shaped lithophaga boring

a

b

Fig. 11. (a and b) Photograph of a hand specimen of the basal section of Mfamosing limestone exposed along Calabar – Itu highway showing Lithophaga boring filled with calcareous mudstone and algal stromatolite; (b) sculptured rock forming caves due to weathering of the Mfamosing Limestone.

little or no evidence of significant transportation as a large proportion of the shells are preserved in their living position suggesting prevailing low water energy situation in which sea floor was mildly agitated. The shelly interval on the other hand, host abundant disarticulated shell fragments of oysters, pelecypods and crabs, which maybe as a result of agitation of the seafloor at the time of accumulation, suspension and short distance transportation of these materials to distances just enough to cause the amount of disarticulation is observed. Dominant sedimentary structures in the shales are fissility, laminations and bioturbation structures. The top of the mudstone unit shows bioturbation structures with characteristic Y-shaped thallassinoides traces while the limestone above is characterized by compact disarticulated oyster shell fragments. This Formation constitutes the Coniacian (Nyong, 1995) of the Calabar Flank and consists of an intercalation of massive to nodular marl (calcareous mudstone) units with shale sequences belonging to the New Netim Marl Formation (Bassey, 2010). The shales are grey, calcareous, fissile and flaggy in places and unconformably overlie the Ekenkpon shale. The marl slightly dips at angles ranging from 5° to 13° to the south. The marl–shale intercalation is unique with its rhythmic occurrence throughout the formation and can be interpreted as due to instability in the continental crust during the Coniacian, probably as a result of the ushering in of the Santonian deformation episode that precedes the New Netim Formation (Bassey, 2010), which form long ranges of highland in the Calabar Flank. Petrographic studies of the New Netim Marl showed that the

marl unit are fossiliferous and calcareous, hence biomicritic mudstone microfacies (Fig. 12). The late Campanian–Maastrichtian is represented by the Nkporo Shales in the Calabar Flank, where the sedimentary piles constitute very dark grey to black carbonaceous, highly fissile shales with marl (septarian) nodules and gypsum bands. It unconformably overlies the New Netim Marl and was deposited after the Santonian deformational episode. It is richly fossiliferous with shark teeth remains, and it represents the last transgressive phase during the Cretaceous in the Calabar Flank. Septarian nodules were also observed in the Nkporo shale similar to those of the Ekenkpon shale, having recrystallized, calcite filling the desiccated cracks radiating from the core towards the edges. The Nkporo Shale as exposed at the quarry section consists of thick (60 m) shale sequences which occur as flaggy and highly fractured shale. 5. Geochemistry The use of geochemistry (major oxides, trace elements and rare earth elements) for characterisation of the Cretaceous strata of the Calabar Flank in terms of rock classification, evaluation of lithologies provenance and depositional/tectonic setting is here presented (Figs. 13–15). It is important to emphasize here that this approach is preliminary since the results are based on few representative samples for the five formations. Immobile trace elements have proven to be more useful in the study of provenance and depositional setting than major elements

Please cite this article in press as: Boboye, O.A., Okon, E.E. Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria. J. Afr. Earth Sci. (2014), http://dx.doi.org/10.1016/j.jafrearsci.2014.04.035

O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

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b

c

d

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Fig. 12. (a–f): (a) Photomicrograph showing laminates of algal stromatolites (alternating light and dark bands); (b) photomicrograph showing frame work grains seen to be floating in the sparry cement, most of the grains constitute grapestones and intraclasts of pelliodal materials and complete ostracod carapace preserved; (c) photomicrograph showing hemispherical and irregular oncoids and micritized ooids in a sparry cement (frame work components >10%); (d) photomicrograph showing highly spired gastropod (micritized whorls) and pelloids; (e) photomacrograph of bioclastic wackestone of the Limestone band of Ekenkpon Formation showing brachiopod shell fragments; (f) transverse section of the septarian nodule showing the cracks and recrystallized calcite fillings. A = stromatolitic boundstone microfacies; b = intrabiopelsparitic grainstone – packstone microfacies; c = biosparitic grainstone microfacies; d = biopelsparitic grainstone microfacies; e = intrabiomicritic wackestone microfacies; f = septarian nodule.

Fig. 13. Classification of clastic sediments using log Fe2O3/K2O vs. log SO2/Al2O3 (Herron, 1988).

(Bhatia and Crook, 1986).The characterisation of the Cretaceous strata here has been based on major oxides, trace elements and rare earth elements. It should be noted however that, in some cases, fractionation and mobilisation (of trace and immobile REEs) may occur as a result of source rock weathering and diagenesis (Figs. 13–15). (Milodowski and Zalasiewicz, 1991; Englund and Jørgensen, 1973; Griffiths, 1967; Taylor and McLennan, 1985 and Wakita et al., 1971). 5.1. Major oxides The result for major oxides of the analysed samples is presented in Table 1. The result showed that SiO2 range from 55.91% in the sandstones being the highest, through 50.75% (Ekenkpon Shale),

Fig. 14. Plot of CIA vs. ICV showing the relationship between the source area weathering and the original detrital mineralogy (after Potter et al., 2005) MLS and NNM were not plotted due to elevated carbonate contents.

41.52% (Nkporo Shale), 37.42% (New Netim Marl) and (0.94 and 0.61)% in the limestones (Mfamosing Limestone). The sandstones of Awi Formation have the highest concentration of SiO2 (55.91%). The Na2O content is depleted as a result of small amount of Na-rich plagioclase feldspars. K2O however is slightly enriched (3.96%) as is expected for arkoses; this is expected as seen in the high ratio of K2O/Na2O (39.6). Al2O3 is high (23.64%) suggesting the weathering of the available feldspars to clay minerals. TiO2

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

Fig. 15. Ternary diagram of A–CN–K and CIA showing the degree of source area weathering (after Nesbitt and Young, 1982). Arrow dashed lines show idealized weathering trends.

(0.48%) may be associated with the presence of Ti-bearing heavy minerals like rutile, ilmenite, titanite, biotite. The ratio K2O/Al2O3 is 0.17 and this shows that the K2O present is resident in the feldspars. The shales represented by the Ekenkpon and Nkporo shales are calcareous and carbonaceous shales respectively. This to a large extent influences their geochemical character. Both shales relatively have high SiO2 content (50.75% in Ekenkpon and 41.52% in Nkporo shales), and their Al2O3 content range from 17.97% to

20.58%. The CaO is higher in Ekenkpon shale (5.1%) and depleted in Nkporo shale (0.311%). Nkporo shale is depleted in both Na2O (0.09%) and K2O (1.56%) while the Ekenkpon shale is rich in Na2O (1.33%) and depleted in K2O (2.49%). As expected the MgO content is much more pronounced in the calcareous (Ekenkpon Shale) than it is in the carbonaceous (Nkporo Shale) with values of 1.23% and 0.87% respectively (see Tables 2 and 3). The classification scheme used was based on Herron (1988). This classification tends to differentiate between mature and immature sediments as opposed to that based on quartz–feldspar–lithic fragments. The SiO2/Al2O3 ratio reflects the abundance of quartz and clay and the feldspar present (Fig. 13). Apart from clastic rocks classification, the elemental composition of sedimentary rocks can furnish information on the degree of weathering and alteration of the sedimentary rocks. Several means by which alteration of sedimentary rocks can be determined exist such as Chemical Index of Alteration, Nesbitt and Young (1982), Fedo et al., (1996); Th/U, McLennan et al. (1993); Chemical Index of Weathering, (CIW); Index of Compositional Variability, (ICV) and Ruxton Ratio, (RR) and all have been successfully applied. Nesbitt and Young (1982) defined and calculated the Chemical Index of Alteration (CIA) from the major element of bulk sediments to express the extent to which the sediments have experienced chemical weathering. Since the CIA can be affected by changes in provenance of sediments, independent of changes in weathering intensity, the Al2O3/TiO2 ratio and Nb content are also considered as independent records of sediment provenance (Krissek and Kyle, 1998). The plot of CIA vs. ICV shows that the sediments were most probably associated with felsic source and their degree of alteration indicated moderate weathering except for the Nkporo shale which plotted close to the apex and is associated more with input from mafic source (Fig. 14). Chemical index of alteration can be affected by grain size and provenance of the sediment samples. Of particular interest is the feldspar which contains relatively mobile Ca, Na and K whereas chemical products of interest are the Al-rich clays. As chemical weathering increases, CIA increases with values approximately 50 for unweathered feldspar rich rocks

Table 1 Geochemical analysis (major oxides) for the Cretaceous strata of the Calabar Flank. Oxides

MDL

NKS

GYP

MLS 1

NNM

AWI

EKS

MLS 2

PAAS

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O3 K2O P2O5 Cr2O3 LOI Total Al2O3/SiO2 Na2O/K2O K2O/Na2O Al2O3/TiO2 Fe2O3 + MgO CIA ICV RR Al2O3/(Na2O + CaO)

0.01 0.01 0.01 0.04 0.01 0.01 0.01 0.01 0.01 0.01 0.002 5.10 – – – – – – – – – 0.50

41.52 0.88 20.58 8.04 0.01 0.87 0.31 0.09 1.56 0.13 0.02 25.80 99.84 0.50 0.06 17.54 23.39 8.91 91.38 0.57 2.02 12.94

0.81 0.00 0.65 2.92 0.00 1.29 49.91 0.25 0.02 0.09 0.003 43.8 99.79 0.80 12.50 0.08 0.00 1.94 ND ND 1.25 0.013

0.61 0.01 0.22 0.13 0.08 0.39 54.91 0.01 0.05 0.05 0.00 43.50 99.94 0.36 0.20 5.00 22.00 0.52 0.40 252.64 2.77 0.004

37.42 0.22 7.44 1.43 0.04 1.05 25.63 1.32 2.18 0.36 0.00 22.60 99.71 0.20 0.61 1.65 33.82 2.48 2.47 4.28 5.03 0.276

55.91 0.48 23.64 5.83 0.01 0.33 0.02 0.10 3.96 0.06 0.004 9.60 99.87 0.42 0.03 40.00 49.25 6.16 68.57 0.45 2.37 1.970

50.75 0.76 17.97 5.39 0.07 1.23 5.10 1.33 2.49 0.89 0.01 13.80 99.81 0.35 0.53 1.87 23.64 6.62 66.82 0.91 2.82 0.028

0.94 0.02 0.31 0.18 0.08 1.11 53.91 0.00 0.08 0.06 0.00 43.20 99.92 0.33 0.00 0.00 15.50 1.29 0.57 178.65 3.03 0.006

62.40 0.99 18.78 7.18 0.00 2.19 1.29 1.19 3.68 0.16 0.00 0.00 97.86 0.30 0.32 3.09 18.57 9.37 – 0.88 3.32 7.573

MDL = Minimum detection limit of the instrument used for the analysis; LOI = loss on ignition; NKS = Nkporo Shale; GYP = Gypsum sample collected from the Nkporo Shale; MLS 1 and 2 = Mfamosing limestone 1 and 2 respectively; NNM = New Netim Marl; EKS = Ekenkpon Shale; AWI = Awi Formation; CIA – chemical index of alteration; ICV – index of chemical variation; RR – Ruxton ratio; PAAS = post – Archean Australian Shale (after Taylor and McLennan, 1985).

Please cite this article in press as: Boboye, O.A., Okon, E.E. Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria. J. Afr. Earth Sci. (2014), http://dx.doi.org/10.1016/j.jafrearsci.2014.04.035

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx Table 2 Geochemical analysis result for the Cretaceous strata of the Calabar Flank (trace elements). Elements

MDL

NKS

GYP

MLS 1

NNM

AWI

EKS

MLS 2

PAAS

Rb Sr Ba Th U Co Ni V Cr Sc Zr Hf Nb Ta

0.10 0.50 1.00 0.20 0.10 0.20 0.10 8.00 0.002 1.00 0.10 0.10 0.10 0.10

76.70 129.30 241.00 21.70 4.50 14.40 31.90 244.00 0.014 24.00 144.00 3.70 19.30 1.40

0.80 1390.90 12.00 2.40 0.30 3.70 6.70 16.00 0.002 2.00 5.50 0.00 0.10 0.00

2.40 280.80 8.00 0.30 0.70 0.60 2.10 19.00 0.00 0.00 3.50 0.00 0.30 0.00

73.40 1262.60 658.00 7.70 2.20 2.70 4.70 19.00 0.00 3.00 231.00 6.00 5.10 0.40

162.70 265.40 464.00 12.20 3.50 1.90 2.80 71.00 0.003 6.00 199.10 5.70 10.90 2.10

109.20 272.60 457.00 25.20 3.80 15.30 20.40 90.00 0.007 14.00 247.90 7.00 16.20 1.10

3.70 351.90 13.00 0.30 1.30 1.00 3.10 8.00 0.00 0.00 5.70 0.10 0.40 0.00

160.00 200.00 650.00 14.60 3.10 23.00 55.00 150.00 110.00 16.00 210.00 5.00 1.90

MDL = minimum detection limit; NKS = Nkporo Shale; GYP = Gypsum sample; MLS 1 and 2 = Mfamosing limestone 1 and 2; NNM = New Netim Marl; AWI = Awi Formation; EKS = Ekenkpon Shale; PAAS = post Achaean Australian Shale composite (after Taylor and McLennan, 1985). Values are given in ppm.

Table 3 Geochemical analysis result for the Cretaceous strata of the Calabar Flank (rare earth elements).

P

Elements

MDL

NKS

GYP

MLS 1

NNM

AWI

EKS

MLS 2

Condrite (Wakita)

PAAS (M1989)

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y

0.10 0.10 0.02 0.30 0.05 0.02 0.05 0.01 0.05 0.02 0.03 0.01 0.05 0.01 0.10

49.20 103.10 10.62 35.70 6.10 1.22 4.43 0.66 3.69 0.73 2.11 0.32 2.10 0.32 17.80

5.00 13.40 1.88 7.60 1.69 0.40 1.69 0.26 1.35 0.22 0.60 0.08 0.34 0.06 6.50

1.40 2.40 0.33 1.20 0.22 0.06 0.20 0.04 0.20 0.04 0.11 0.03 0.12 0.02 1.40

30.00 61.60 6.93 26.30 4.48 1.00 3.57 0.56 2.90 0.56 1.61 0.24 1.45 0.22 18.00

29.90 60.40 7.44 28.50 5.82 1.12 5.16 0.76 4.08 0.79 2.35 0.35 2.21 0.33 24.40

52.30 123.20 15.89 62.80 11.74 2.35 9.30 1.41 7.35 1.30 3.54 0.53 3.37 0.49 35.90

1.80 2.90 0.37 1.50 0.27 0.06 0.25 0.04 0.23 0.05 0.14 0.03 0.13 0.03 1.70

0.34 0.91 0.12 0.64 0.20 0.07 0.26 0.05 0.30 0.08 0.20 0.03 0.22 0.03 0.00

38.20 79.60 8.83 33.90 5.55 1.08 4.66 0.77 4.68 0.99 2.85 0.41 2.82 0.43 27.00

Ratios and calculated parameters P REE 0.82 220.30 P LREE 0.52 198.62 P HREE 0.10 4.85 Eu/Eu* ND 0.81 La/Sc ND 2.05 Th/Sc ND 0.90 La/Co ND 3.42 Th/Co ND 1.51 La/Th ND 2.27 Ti/Zr ND 0.0040 (La/Lu)CN ND 2.22

34.57 27.88 1.08 0.98 2.50 1.20 1.35 0.65 ND ND 4.73

6.37 5.33 0.28 3.78 ND ND 2.33 0.50 4.60 0.0170 2.67

141.42 124.83 3.52 0.86 10.00 2.56 11.11 2.85 3.89 0.0005 2.40

149.21 126.24 5.24 0.79 4.90 2.03 15.74 6.42 2.45 0.0014 1.98

295.57 254.19 7.93 0.74 3.74 1.80 3.42 1.64 2.07 0.0018 1.89

7.80 6.57 0.33 5.95 ND ND 1.80 0.30 6.00 0.0017 13.32

3.45 2.01 0.49 1.20

184.77 160.53 6.51 ND 2.39 0.91 1.66 0.63 2.60 ND ND

ND ND ND 10.00

= Summation; CN = chondrite normalized values; ND = not determined; Eu/Eu* = Europium anomaly.

to values approximately 100 for highly weathered kaolinite- or gibbsite-rich sediments. Average CIA values for shales dominated by illite range from 70 to 75 (Nesbitt and Young, 1982). 5.2. Trace and rare earth elements A trace element is any element, whose concentration in a rock is less than 0.1 wt%, (i.e. <1000 ppm). They have become vital because of their usefulness in discriminating between petrological processes (Rollinson, 1993). They are often studied in groups because each of such group possesses characteristic behaviours. A deviation of the general behaviour of the group may be indicative of changes in petrological processes. The trace elements were normalized using the post Archean Australian Shale composite (PAAS) to characterize their pattern (Taylor and McLennan (1985). Certain trace elements however, having relatively low mobility and low

resident time in ocean water (e.g. La, Th, Zr and Sc) is transferred into clastic sediments during primary weathering and transportation, and thus is used to fingerprint plate tectonic settings. The presence of Zr may be related to the concentration of certain heavy minerals in the sediments and it was found to be significant in the Awi Formation and the Ekenkpon Shales, but depleted in the limestone samples. Strontium (Sr) content is relatively high (compared to PAAS) in all the samples with Nkporo Shale, Ekenkpon and Awi formations having lower values and Mfamosing, New Netim Marl and gypsum having higher values. The rare earth elements (REEs) constitute a group of trace elements which comprises a series of metals with atomic numbers 57–71 (they are sometimes called the Lanthanides). It has been shown that rare earth elements with even atomic numbers are more stable that those with odd atomic numbers, producing a zig-zag pattern on a composition–abundance diagram (Rollinson, 1993).

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

Table 4 Results for Rock Eval Pyrolysis. Sample

Formation

TOC

S1

S2

GP

Tmax

Cal. VRo

HI

OI

S2/S3

S1/TOC

PI

NKS/7/1 NKS/7/5 NNM/1/Sh1 EKS/6/1 EKS/6/6 EKS/8/1 EKS/8/6 EKS/9/5 EKS/9/1 MLS/11/3

Nkporo Nkporo New Netim Ekenkpon Ekenkpon Ekenkpon Ekenkpon Ekenkpon Ekenkpon Mfamosing

1.13 1.59 1.19 1.57 1.62 0.84 1.38 1.07 1.07 1.68

0.07 0.05 0.05 0.04 0.03 0.04 0.05 0.04 0.04 0.05

0.51 0.35 0.82 0.28 0.31 0.30 0.30 0.14 0.22 0.33

0.58 0.40 0.87 0.32 0.34 0.34 0.35 0.18 0.26 0.38

421.00 437.00 436.00 439.00 437.00 432.00 435.00 441.00 438.00 443.00

0.43 0.72 0.70 0.76 0.72 0.63 0.68 0.79 0.74 0.83

45.13 22.02 68.91 17.83 19.14 35.71 21.74 13.08 20.56 19.64

71.68 37.74 46.22 68.79 48.77 40.48 34.06 61.68 27.10 17.86

0.63 0.58 1.49 0.26 0.39 0.88 0.64 0.21 0.76 1.10

6.18 3.15 4.20 2.55 1.85 4.76 3.62 3.74 3.74 2.98

0.12 0.13 0.06 0.13 0.09 0.12 0.14 0.22 0.15 0.13

TOC – Total Organic Carbon (wt.%); S1 – volatile hydrocarbons (HC) content (mgHC/grock); S2 – remaining hydrocarbon generative potential (mgHC/grock); S3 – carbon dioxide content (mgCO2/rock); GP – generative potential (total); Tmax – maximum pyrolysis temperature; HI – hydrogen index (S2/TOC  100 in mgHC/gTOC); OI – oxygen index (S3/TOC  100 in mgCO2/gTOC); Cal. VRo – calculated vitrinite reflectance (0.01801  Tmax – 7.16); PI – production index (S1/(S1 + S2)); MLS – Mfamosing limestone; EKS – Ekenkpon Shale; NNM – New Netim Marl; NKS – Nkporo Shale.

5.3. Geochemistry (organic geochemistry) The organic geochemical studies have been used to assess and characterize the petroleum generative potentials of the source rocks. These have been buttressed by several workers in the field of organic geochemistry (Tissot and Welte, 1984; Hunt, 1996; Akande et al., 1998; Obaje et al., 2004 and Boboye, 2012). Routine source rock evaluation involves the determination of organic richness (measured as Total Organic Carbon, TOC), organic quality (organic matter types) and thermal maturity. However, whatever the nature of the organic matter, original carbon contents less than 0.5% is not likely to produce sufficient amount of liquid petroleum with respect to the adsorption properties of the source rocks and the pressure build-up for the expulsion of oil. 5.3.1. Quantity of organic matter The TOC values range from 0.56 wt.% to 4.70 wt.%. (Table 4). The Nkporo shale recorded the highest TOC value amongst the shales and limestones. The thin shale beds within the sandstone unit of Awi Formation recorded TOC values range from 0.74 wt.%. to 4.7 wt.% wt. (average = 2.15 wt.%); Mfamosing Limestone range from 0.78 wt.%. to 1.68 wt.%. (average = 0.9 wt.%);. Ekenkpon Shale

Fig. 16. Plot of HI vs. OI (after Espitalie et al., 1977).

range from 0.8 wt.% to 1.65 wt.% (average = 1.36 wt.%); New Netim Marl and Nkporo Shales range from 0.7 wt.% to 3.6 wt.% and 1.13 wt.% to 4.63 wt.% (average = 1.51 wt.% and 2.31 wt.%) respectively. Although there was no definite trend in the distribution (whether it increases with age or laterally across the depositional strike). From their mean values, the studied source rocks are predominantly good to very good source rocks and possess well above the minimum threshold (0.5 wt.%) for hydrocarbon generation. The organic matter quality (HI) range from 13.08 to 68.91 mgHC/gTOC (mean value = 28.38 mgHC/gTOC) suggesting predominantly gas prone source rocks (Baskin, 1997). The value range for Oxygen Index (OI) is 17.86 mgCO2/gTOC to 71.68 mgCO2/ gTOC (average = 45.44 mgCO2/gTOC). Most of the samples are Type III due to their low HI (Espitalie et al., 1977) signifying that they are mainly gas prone source rocks and the organic matter are predominantly sourced from terrestrial materials (Fig. 16). The source richness of the sediments was characterized using HI vs. TOC, plot indicates that most of the sediments are within the gas prone zone (Jackson et al., 1985 and Jarvie et al., 2001).

Fig. 17. Plot of HI vs. TOC (after Jackson et al., 1985).

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Fig. 18. Plot of production index against maturity based on Tmax (modified after Jarvie et al., 2001).

The fact that the basin predominantly is composed of terrestrial inputs and gas prone corroborates with works of Essien et al. (2005) and Ehinola et al. (2008) (Fig. 17).

value (0.1) for mature shales except for two samples (NNM/1/Sh1 and EKS/6/6). The vitrinite reflectance also showed that majority of the samples has attained adequate maturity with values range from 0.43 to 0.83 (Baskin, 1977).

5.3.2. Maturation of organic matter The maturation of organic matter is based on the thermal maturation (Tmax) and transformation ratio (TR) (sometimes referred to as the Production Index, PI). The Tmax values range from 421 °C to 443 °C (average = 436 °C). The result show that only one sample fell below the threshold of ca. 430 °C for Type III kerogen (Hunt, 1996). This range of values characterizes the source rocks as marginally mature to mature source rocks. The ratio of Production Index (PI) and maturity suggests that the sediments are within the ‘‘oil window’’ and characterized by high level conversion of organic matter (Fig. 18). The PI has values range from 0.06 to 0.22. The result show that all the samples fall above the threshold

5.3.3. Hydrocarbon yield potential The hydrocarbon yield potential of the sediments of the Calabar Flank is assessed based on (S1 + S2); where S1 and S2 are yields of hydrocarbon released during programmed heating. This parameter range from 0.18 to 0.87 mgHC/grock and according to Tissot and Welte (1984) categorization: <2 mgHC/grock (2000 ppm) – signifies source rocks with little or no source potential for oil, but some gas; between 2 and 6 mgHC/grock (2000–6000 ppm) signifies source rocks with moderate or fair source rock potential for oil. This suggest that some gas may have been generated.

Fig. 19. La/Th vs. Hf plot illustrating sediment provenance (Floyd and Leveridge, 1987).

Fig. 20. Bivariate plot of TiO2 vs. Ni for clastic sediments of the Calabar Flank (fields after Floyd et al., 1989).

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

6. Discussion 6.1. Provenance and paleoenvironmental settings The provenance, from which the sediment is derived, has the main control on sediment composition. Both chemical and mineralogical composition of the rock is paramount in the determination of source area. The heavy mineral studies show the predominance of tourmaline, zircon, staurolite and garnet suggesting that the sediments (sandstones) of the Awi Formation were derived from a mixed felsic igneous and metamorphic sources, this type of environment is comparable to the present day Oban Massif. According to Dickinson and Suczek (1979) and Dickinson (1982), the QFRf diagrams show that Awi Formation is derived from Continental block provenance and transitional continental (Figs. 19 and 20).

Based on the geochemical characterization of the representative sediments (Ekenkpon Shale, New Netim Marl/shale and Nkporo Shale), the lithologic compositional characteristics have been unravelled by trace and rare element geochemistry as well as the major oxides studies. The abundance of Cr and Ni in clastic sediments is considered as useful indicator for provenance in which they are considerably low in the study area (Figs. 19 and 20). According to Wrafter and Graham (1989), a low concentration of Cr indicates a felsic source while a high abundance of Cr and Ni are mainly obtainable in ultramafic rocks. The Cr/Ni ratio in the samples analysed range from 0.003 to 0.14 with an average of

Fig. 23. Paleotectonic setting of Awi Formation (after Dickinson and Suczek, 1979). Fig. 21. Plot of U/Th against Ni/Co for distinguishing oxic and sub-oxic/anoxic conditions.

Fig. 22. Paleoclimatic setting of Awi Formation (after Suttner et al., 1981).

Fig. 24. Ternary plot showing the paleotectonic setting of sandstones from Awi Formation, Calabar Flank (fields after Dickinson, 1982). A – cratonic interior; B – transitional continental; C – basement uplift; D – quartzose recycled; E – transitional recycled; F – lithic recycled; G – mixed; H – dissected arc; I – transitional arc; J – undissected arc.

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

13

0.035 and are regarded as low compared to that of PAAS (Cr/Ni = 2). The La/Th–Hf concentrations indicate that the clastic sediments are predominantly from acidic arc source and tending towards passive mantle source (Floyd and Leveridge, 1987) (Fig. 19). Cullers and Podkovyrov (2000), also showed the significant difference between mafic and felsic source rocks from their ratios of Eu/Eu*, (La/Lu)cn, La/Sc, Th/Sc, La/Co and Th/Co, and thus provided useful information about these elements behaviour. This ratios: Eu/Eu*, (La/Lu)cn, La/Sc, Th/Sc, La/Co, Th/Co and Cr/Th are similar to those derived from felsic sources than those from mafic sources, suggesting derivation from felsic sources (Fig. 19).

Fig. 27. Tectonic discrimination diagram using the log ratios of K2O/Na2O vs. SiO2 for clastic sediments from Calabar Flank (boundaries after Roser and Korsch, 1986).

Fig. 25. Discrimination diagram using Th–Co–Zr/10 illustrating tectonic setting of clastics from the Calabar Flank (after Bhatia and Crook, 1986). A – oceanic island arc; B – continental island arc; C – active continental margin; D – passive margin.

The predominance of sparry cement and the composition of the framework components (bioclasts/shell fragments) of the Mfamosing Limestone suggest that they were precipitated at the site of deposition with minimal transportation (oval and micritized ooids and pelloids) within the site of deposition. The tectonic setting also has some influence on the accumulation of sediment and hence, its provenance. The discrimination using major oxides showed that the studied sediments from the Calabar Flank were sourced from intermediate igneous province and quartzose sedimentary provenance (Roser and Korsch, 1988). Suttner et al. (1981) used the QFR ternary diagram to separate climatically induced compositional difference in Holocene sands. From a similar plot, the sandstones of the Awi Formation plotted in the Plutonic Humid field suggesting that at the time of deposition, the source area (most probably the Oban Massif) was in a humid climatic setting (Figs. 20–25). Distinctive geochemical signatures exhibited by plate tectonic processes on sediments enabled the distinction into four provinces (Bhatia and Crook, 1986); these include: Oceanic Island Arc (OIA), Continental Island Arc (CIA), Active Continental Margin (ACM), and Passive Continental Margin (PCM). Th–Sc–Zr/10 concentration, as well as Th–Co–Zr/10 and La–Th–Sc ternary plots suggested that the Calabar Flank sediments were deposited in a tectonic province ranging from passive/rifted margin to continental island arc setting (Bhatia and Crook, 1986) (Figs. 25 and 26). Discrimination diagram using the log ratio of (K2O/Na2O) against SiO2 indicated that the studied sediments were mainly derived from passive margin to active continental margin tectonic setting (Roser and Korsch, 1986) (Fig. 27). 7. Conclusions

Fig. 26. Discrimination diagram using Th–Sc–Zr/10 illustrating tectonic setting of clastics from the Calabar Flank (after Bhatia and Crook, 1986). A – oceanic island arc; B – continental island arc; C – active continental margin; D – passive margin (A – cratonic interior; B – transitional continental; C – basement uplift; D – quartzose recycled; E – transitional recycled; F – lithic recycled; G – mixed; H – dissected arc; I – transitional arc; J – undissected arc).

This paper has presented the provenance, hydrocarbon potentials and depositional environments. Study has shown the Awi Formation to be predominantly of Continental block provenance and transitional continental. It consist of two cycles of sandstones and mudstones exhibiting fining upward sequence characterized by initial turbulence and later quiet fluvial regimes. Textural features are characterized by sub-angular to sub-rounded, predominantly prolate and compact grains. The unimodal pattern, poorly

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx

sorted and strongly fine skewed sandstones indicate fluvial environment. ZTR assemblages and petrographic study suggest a near provenance as evident in the euhedral to anhedral nature of the sandstone grains. The sandstones are arkosic and immature deposited in a continental block tectonic setting under humid climate. The Mfamosing Limestones consist of pure carbonate depleted in oxides of rock forming minerals (SiO2, Al2O3, Na2O, and K2O) and enriched in CaO with high LOI. The predominance of sparry cements and the framework components suggests the precipitation with minimal transportation. It is a good source rock potential with adequate maturity for the generation of gas. The New Netim Marl is characterized by intercalation of marlstones and flaggy shales ranging from massive (basal) to nodular and bedded units (topmost). They are good source rocks that are marginally mature having potential for gaseous hydrocarbon. There are significant terrestrial input and depletion in Na2O, K2O and MgO. The TOC ranged from good to excellent with low HI and relatively higher OI suggesting good potential for gas. The Ekenkpon Shale consists of marine shales deposited within two transgressive episodes from latest Albian–Cenomanian and from late Cenomanian to Turonian. It is characterized by dark grey to black highly fissile shale units at the base and progressively becomes grey and calcareous at the upper section. This suggests a change from anoxic to oxic environments of deposition (Nyong and Ramanathan, 1985). The shales are highly fissile with Thallassinoides burrows (found in offshore – inner neritic environments) and intercalations of mudstone and limestone with high SiO2, Al2O3, CaO, Sr, Th, Zr and Nb contents. The shale are characterized by high TOC content ranging from fair to good source rock with adequate maturity and potential for gaseous hydrocarbons. References Adeleye, D.R., Fayose, E.A., 1978. Stratigraphy of the type section of Awi Formation, Odukpani area south-eastern Nigeria. J. Mining Geol. 15, 33–37. Akande, M.O., Aduayi, E.A., Sobulo, R.A., Olayinka, A., 1998. Efficiency of Rock phosphate as fertilizer source in South – western Nigeria. J., Plant Nutr. 2, 1339– 1353. Baskin, D.K., 1997. Atomic H/C ratio of kerogen as an estimate of thermal maturity and organic matter conversion. Am. Assoc. Pet. Geol. Bull. 81, 1437–1450. Bassey, E.D., 2010. The sedimentology and age determination of the New Netim Marl, Calabar Flank, southeastern Nigeria. University of Calabar, Unpublished M.Sc Thesis. XIV, 107p. Bhatia, M.R., Crook, K.A.W., 1986. Trace elements characteristics of graywackes and tectonic setting discrimination of sedimentary basins. Contrib. Miner. Petrol. 92, 181–193. Boboye, O.A., 2012. Palyno-Geochemical Study of Southwestern Chad Basin. LAP LAMBERT Academic Publishing GmbH & Co. KG Heinrich-Böcking-Str. 6-8 66121, Saarbrücken, Germany, p220. ISBN 978-3-8443-1588-2. Cullers, R.L., Podkovyrov, V.N., 2000. Geochemistry of the Mesoproterozoic Lakhanda shales in southeastern Yakutia, Russia: implications for mineralogical and provenance control and recycling. Precambr. Res. 104, 77–93. Dickinson, W.R., 1982. Composition of sandstones in circum-pacific subduction complexes and fore-arc basins. Am. Assoc. Pet. Geol. Bull. 66, 121–137. Dickinson, W.R., Suczek, C.A., 1979. Plate tectonics and sandstone compositions. Am. Assoc. Pet. Geol. Bull. 63, 2164–2182. Dobkins, J.E., Folk, R.L., 1970. Shape development on Tahiti-Nui. J. Sediment. Petrol. 40, 1167–1203. Ehinola, O.A., Sonibare, O.O., Javie, D.M., Oluwole, E.A., 2008. Geochemical appraisal of organic matter in the Calabar Flank, Southeastern Nigeria. Euro. J. Sci. Res. 23 (4), 567–577. Englund, J.O., Jørgensen, P., 1973. A chemical classification system for argillaceous sediments and factors affecting their composition. Geol. Fören. Stockholm Förhand. 95, 87–97. Espitalie, M.M., Tissot, B., Mennig, J.J., Leplat, P., 1977. Source rock characterization method for petroleum exploration. In: Proceedings of the 9th Annual Offshore technology Conference, vol. 3, pp. 430–448. Essien, N.U., 1995. Mfamosing Limestone: A Mid-Albian Carbonate Platform in south-eastern Nigeria. University of Calabar. Unpublished Ph.D thesis. 167p. Essien, N.U., Ufot, D.O., 2010. Age of the Mfamosing limestone, Calabar Flank, southeatern Nigeria. Int. J. Basic Appl. Sci. 10 (5), 8–19. Essien, N.U., Ukpabio, E.J., Nyong, E.E., Ibe, K.A., 2005. Preliminary organic geochemical appraisal of Cretaceous rock units in the Calabar Flank, southeastern Nigeria. Niger. J. Min. Geol. 41 (2), 185–191. Fedo, C.M., Eriksson, K.A., Krogstad, E.J., 1996. Geochemistry of shales from the Archean (3.0 Ga.) Buhwa Greenstone Belt. Zimbabwe: Implications for

provenance and source area weathering. Geochim. Cosmochim. Acta 60, 1751–1763. Floyd, P.A., Leveridge, B.E., 1987. Tectonic environment of Devonian Gramscatho basin, south Cornwall: framework mode and geochemical evidence from turbiditic sandstones. J. Geol. Soc., London 144, 531–542. Floyd, P.A., Winchester, J.A., Park, R.G., 1989. Geochemistry and tectonic setting of Lewisian clastic metasediments from the Early Proterozoic Loch Maree Group of Gairloch, N.W. Scotland. Precambrian Res. 45 (1–3), 203–214. Floyd, P.A., Folk, R.L., 1974. Petrology of Sedimentary rocks. Hemphill, Austen, Texas, 182p. Friedman, G.M., 1979. Difference in size distributions in population of particles among sands of various origins. Sedimentology 20, 3–32. Griffiths, J.C., 1967. Scientific methods in analysis of sediments, MacGraw Hill Book Company, 508p. Herron, M.M., 1988. Geochemical classification of terrigenous sand and shales from core or log data. J. Sediment. Petrol. 58, 820–829. Hunt, J.M., 1996. Petroleum geochemistry and geology, second ed. W.H. Freeman and Company, New York, 743p. Jackson, K.S., Hawkins, P.J., Bennett, A.J.R., 1985. Regional facies and geochemical evolution of the southern Denison Trough. J., Aust. Petrol. Explor. Assoc. 20, 143–158. Jarvie, D.M., Hill, R.J., Ruble, T.E., Pollastro, R.M., 2001. Unconventional shale-gas systems: the Mississippian Barnett shale of north-central Texas as a model for thermogenic shale-gas assessment. Am. Assoc. Pet. Geol. Bull. 91, 475–499. Kahle, C.F., Flohd, J.C., 1971. SStratigraphic and environmental significance of sedimentary structures in Cayugan (Silurian) tidal flat carbonates, northwestern Ohio. Geol. Soc. Am. Bull. 82, 2071–2098. Krissek, L.A., Kyle, P.R., 1998. Geochemical indicators of weathering and Cenozoic paleoclimates in sediments from CRP-1 and CIROS-1, McMurdo Sound, Antarctica. Terra Antartica 53, 673–680. Mange, M.A., Maurer, W.H., 1992. Heavy minerals in colour. Chapman and Hall, London, United Kindom, 145p. McLennan, S.M., Hemming, S., McDaniel, D.K., Hanson, G.N., 1993. Geochemical Approaches to Sedimentation, Provenance and Tectonics. In: Johnson, M.J., Basu, A. (Eds.), Processes Controlling the Composition of Clastic Sediments. Geological Society of America Special Paper, USA, pp. 21–40. Milodowski, A.E., Zalasiewicz, J.A., 1991. Redistribution of rare earth elements during diagenesis of turbidites/hemipelagite mudrock sequence of Llandovery age from central Wales. In: Morton, A.C., Todd, S.P., Haughton, P.D.W., (Eds.), Developments in Sedimentary Provenance Studies. Geological Society, London, Special Publications, vol. 57. pp. 101–124. Nesbitt, H.W., Young, G.M., 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 299, 715–717. Nyong, E.E., 1995. Cretaceous sediments in the Calabar Flank. In: Ekwueme, B.N., Nyong, E.E., Petters, S.W., (Eds.), Geological Excursion Guidebook to Oban Massif, Calabar Flank and Mamfe Embayment, southeast Nigeria. Published in commemoration of the 31st Annual Conference of the Nigerian Mining and Geosciences Society, pp. 14–25. Nyong, E.E., Ramanathan, R.M., 1985. A record of oxygen-deficient paleoenvironments in the Cretaceous of the Calabar Flank, SE Nigeria. J. Afr. Earth Sc. 3 (4), 455–460. Obaje, N.G., Wehner, G., Scheeder, Abubakar, M.B., Jauro, A., 2004. Hydrocarbon prospectivity of Nigerian’s inland basins: from the viewpoint of organic geochemistry and organic petrology. Am. Assoc. Pet. Geol. Bull. 87, 325–353. Petters, S.W., 1982. Central West Africa Cretaceous – Tertiary benthic foraminifera and stratigraphy. Paleontogr. 179A, 1–104. Petters, S.W., Ekweozor, C.M., 1982. Petroleum geology of Benue trough and southeastern Chad Basin, Nigeria. Am. Assoc. Pet. Geol. 66 (S), 1141–1149. Petters, S.W., Nyong, E.E., Akpan, E.B., Essien, N.U., 1995. Lithostratigraphic revision of the Calabar Flank. Nigerian Mining and Geosciences Society 31st Annual Conference, pp. 1–54. Potter, P.E., Maynard, J.B., Depetris, P.J., 2005. Mud and Mudstones: Introduction and Overview, Springer, Berlin-Heidelberg-New York, 297p. Ramanathan, R.M., Kumaran, K.P.N., 1981. Age and paleoecology of M-1 well in the Calabar Flank, southeastern Nigeria. J. Mining Geol. 18, 163–171. Reijers, T.J.A., Petters, S.W., 1987. Depositional environments and diagenesis of Albian carbonates on the Calabar Flank, SE Nigeria. J. Petrol. Geol. 10 (3), 283–294. Reyment, R.A., 1965. Aspects of the Geology of Nigeria. University of Ibadan Press, Ibadan, Nigeria, 145p. Rollinson, H.R., 1993. Using geochemical data: evaluation, presentation and interpretation. Longman group, UK, 352p. Roser, B.P., Korsch, R.J., 1986. Determination of tectonic setting of sandstonemudstone suites using SiO2 content and K2O/Na2O ratio. J. Geol. 94, 635–650. Roser, B.P., Korsch, R.J., 1988. Provenance signature of sandstone–mudstone suite determined using discriminant function analysis of major element data. Chem. Geol. 67, 119–139. Sames, C.W., 1966. Morphometric data of some recent pebble associations and their application to ancient deposits. J. Sediment. Petrol. 36, 126–142. Sneed, E.D., Folk, R.L., 1958. Pebbles in the lower Colorado River, Texas, a study of particle morphogenesis. J. Geol. 66 (2), 114–150. Suttner, L.J., Basu, A., Mack, G.H., 1981. Climate and origin of quartz arenites. J. Sediment. Petrol. 51 (4), 1235–1246. Taylor, S.R., McLennan, S., 1985. The continental crust: its composition and evolution. Blackwell, Oxford, 312p. Tissot, B., Welte, D.H., 1984. Petroleum formation and occurrence. 2nd ed., New York, Springer-Verlag, 699p.

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O.A. Boboye, E.E. Okon / Journal of African Earth Sciences xxx (2014) xxx–xxx Tucker, M.E., 2003. Sedimentary rocks in the field, third ed., John Wiley and sons Limited, 234p. Wakita, H., Rey, P., Schmitt, R.A., 1971. Abundances of the 14 rare earth elements and 12 other trace elements in Apollo 12 samples: five igneous and one breccias

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rocks and four soils. In: Proceedings of the Second Lunar Scientific Conference. Pergamon Press, Oxford, pp. 1319–1329. Wrafter, J.P., Graham, J.R., 1989. Ophiolitic detritus in the Ordovician sediments of south Mayor Ireland. J. Geol. Soc., London 146, 213–215.

Please cite this article in press as: Boboye, O.A., Okon, E.E. Sedimentological and geochemical characterization of the Cretaceous strata of Calabar Flank, southeastern Nigeria. J. Afr. Earth Sci. (2014), http://dx.doi.org/10.1016/j.jafrearsci.2014.04.035