International Journal Geology and Mining Vol. 4(2), pp , September, ISSN:

International Journal Geology and Mining Vol. 4(2), pp. 211-222, September, 2018. www.premierpublishers.org. ISSN: 3019-8261 IJGM Case Study Sedimentology of the Turonian Ezeaku Sandstone in the Afikpo Basin, Nigeria *Edirin Akpofure 1, Innocent Didei 2 1,2 Niger Delta University, Wilberforce Island Bayelsa State, Nigeria Sedimentological studies, including outcrop logging, facies analysis and paleo-environmental studies were carried out in the Ezeaku Formation outcropping in Akpoha, Ibii, and Amasiri in order to determine the original depositional environment of these sandstone and associated concretions. The sandstone occurs as ridges; four facies were observed: which include bioturbated mudstone with randomly spherical concretions; ripple laminated, heterolithic, interbedded sandstone and mudstone; planar and trough cross-bedded facies; and swaley crossstratified facies. Such a facies suite indicates that these bars and concretions were formed under a storm-influenced shelfal to shoreface environment. The paleo-current evidenced by the crossbedding clearly shows a NE-SW paleo-flow indicating the Abakiliki and Oban massifs as provenance sources. Keywords: Facies; Calcareous sandstone; Swaley cross-stratification; bioturbation; Heterolithic; Paleo-current INTRODUCTION The Ezeaku Formation, is one of the stratigraphic units erected by Shell D Archy geologists in the 1950s. The Formation consists of hard, flaggy, calcareous grey or black shales and siltstones with impressions of Innoceramus sp. common (Simpson, 1955). Murat (1972), introduced the term Ezeaku Shale Group, and included all the stratigraphic units deposited in the Late Cenomanian to Turonian in the southern Benue Trough. The formation is subdivided into several lithofacies - calcareous and non-calcareous sandstones, siltstones, shales and limestones (Umeji, 1984). The stratigraphy and petrography of these sandstones was documented by Reyment, 1965; Banerjee, 1980; Amajor, 1987. Reyment (1965) suggested that the Ezeaku Sandstones were deposited in a shallow marine environment and Banerjee (1980) postulated a tidal deposit. From a facies analysis of these sandstones, Amajor (1987) argued that the sandstones of Ezeaku Formation are not tide, but storm dominated. From petrographic studies, Hoque (1977) and Amajor (1987) considered the sand bodies to be texturally and compositionally immature feldspathic arenites. The depositional environment of the calcareous concretions within the sandstones has been a debatable issue. Amajor (1985) interpreted these calcareous concretions as clastic materials derived from uplifted Asu River Group facies during the Cenomanian and transported from the nearby Oban or Cameroon Massifs. By means of sedimentological, facies, and paleoenvironmental analysis, the present work shows that the late post depositional, penecontemporaneous calcareous concretions found in the region originate from diagenetic precipitations in a shallow marine environment. GEOLOGY AND STRATIGRAPHY OF THE AFIKPO BASIN The Afikpo area is accessible by a diversified road network, such as the Okigwe Abakiliki - Enugu expressway, Okposi Amasiri Afikpo roads and other minor ones (Figure 1). It has an elevation lower than 100m on average, and it is characterized by ridges and swale *Corresponding Author: Edirin Akpofure, Niger Delta University, Wilberforce Island Bayelsa State, Nigeria. E- mail: edirinakpofure@yaho.com; Co-Author Email: innosilicate@yahoo.com

Akpofure and Didei 212 Figure 1: Map of study area showing outcrop, communities, major roads, minor roads and rivers (Ogbonnaya, 2012). topography. The Ezeaku Formation in the Afikpo Basin is represented by the Amasiri Sandstone lying unconformably on the lithostratigraphic units of the Asu River Group. The Amasiri Sandstone consists of series of parallel asymmetric ridges, designated as EZi to EZviii (Ukaegbu and Akpabio, 2009; see Table 1 herein). They all display a NE-SW trend and a dip ranging from 3 o to 20 o NE. The gentler and coarser side face southeastward while the steeper side faces westwards. These sandstone ridges alternate with shales swales forming the lowlands. The most prominent ridges occur in the western part of the study area around Amasiri and Ibii, while the northern part of the study area is a low land. The Amasiri Akpoezi sandstone ridges are the highest (120 m), while the Ibii and Akpoha sandstone ridges are about 75 m high. This study deals with three of these ridges, the Akpoha ridge (EZiv), Ibii ridge (EZvii) and the Amasiri ridge (EZiii) - all within the Amasiri Sandstone. The Akpoha ridge (EZiv), Ibii ridge (EZvii) and Amasiri ridge at Uzara Ukwu, all presented calcareous concretions occurring locally. In the Afikpo Basin, three main Cretaceous lithostratigraphic units are namely; the Asu River Group, the Ezeaku Group and the post Santonian Proto-Niger Delta succession (Odigi, 2007). Parallel and high anticline and synclinal structures are noticed in the area. The Asu River Group (Late Albian Early Cenomanian) has a maximum thickness of about 1500 m (Shell, 1957). It consists of the Ogoja Sandstone, Awi Formation/Mamfe Formation and Abakaliki Formation in the Southern Benue Trough (Nwajide, 2013). These are the oldest sedimentary

Int. J. Geol. Min. 213 rocks overlying the pre-cambrian basement complex rocks. They are non-marine to marginal marine deposits. The Asu River Group consists of sandstones and dark shales with ammonites. The Asu River Group represents deposits of the first transgressive-regressive marine depositional cycle in the area (Peters, 1980). Unconformably overlying the Asu River Group are 200 m thick, Late Cenomanian Early Santonian sediments of the Ezeaku Group. The Ezeaku beds consist of shales, limestones, calcareous sandstone and sandstone ridges with a NW 40 o SE average strike and a dip ranging from 20 o to 68 o. The sandstone bodies are parallel and elongate features. The Ezeaku beds represent the second transgressive depositional cycle occuring in the Upper Cretaceous (Murat, 1972), and are unconformably overlain by post-santonian sediments of the pro-niger Delta. The end of the Turonnian is marked by a regressive phase, this later being terminated by a third cycle of the Campanian- Maastrichtian transgression and regression that led to the deposition of the post-santonian Campanian- Maastrichtian proto-niger Delta succession in the broad and gentle depression of the Afikpo Basin. Table 1: Stratigraphic sequence of the study area (From Ukaegbu and Akpabio, 2009) AGE LITHOSTRATIGRAPHIC UNITS GROUP FORMATION MEMBER SYMBOL THICKNESS (in m) Uroro EZviii 200 Shale Ibii EZvii 150 Sandstone TURONIAN Iyi Ogwe EZvi 75 120 EZE-AKU Shale Okpo-Ezi EZv 130 Sandstone Akpoha EZiv 55 80 Sandstone Amaseri Shale Amaseri EZiii 150 Sandstone Amate-Elu EZii 40 150 Shale Amate-Elu EZi 40 Sandstone ALBIAN Amuro Shale 35 ASU Amauro Shale 40 RIVER Amenu Sandstone 35 GROUP Amenu Shale 400 one of calcareous concretion, the other of calcareous sandstone, the two others from the Ozara-Ukwu community - a concretion and the sandstone. After proper grinding and sieving, the pan fraction of sandstone samples were analysed by x-ray fluorescent spectrography. The required filters for each element were selected accordingly and probed. The initial results of selected elemental concentration were shown in diffractions that were then converted into concentration in weight percentage of respective oxides. These results are presented in Table 4. FACIES ANALYSIS OF THE SANDSTONE AT AKPOHA Lithologic Description (Akpoha Ridge) The Amasiri Sandstone occurs as a linear ridge in Akpoha. The outcrop is geographically located at 5 o 57 34 N - 7 o 57 47 E with an elevation of 30 m asl. The bar, ca, 6 m high, displays four beds of highly indurated calcareous sandstone. Fresh outcrop surface shows a dark to light grey colour but dirty brown when weathered. Three different facies are discernable. The basal bed, Bed A, represents the bioturbated calcareous sandstone facies: highly bioturbated with randomly distributed calcareous concretions or the casts of dissolved concretions (from 5 40 cm in diameter; see Figures 3, 4 and 5). The bioturbation has blurred the primary bedding structures. Bed A is about 2 m thick and is overlain by Bed B, this later consisting of thin-bedded series with swaley crossstratification and trough cross-beds (Figures 3 and 6) giving rise to the swaley and cross-bedded facies. Bed B, ca. 1.5 m thick, displays fine to medium grains that are white to grey. The contact between Beds A and B is sharp and erosional. Bed C, ca. 1 m thick, is dirty white to light grey in colour, very fine to fine grained, moderately to well sorted, thin and ripple-laminated interbedded sandstone and mudstone as parallel beds (Figures 5 and 6). Generally, the bed is fining upwards, though there is an alternation of fine and very fine grains within the beds - that is, laminations of fine-grained sand beds and mudstone beds. Occasional burrowing and bioturbation occur, especially within the mudstone units. These all represent the ripple-laminated interbedded mudstone and sandstone facies. Bed D, partly weathered and ca. 1 m thick, overlies Bed C and is made up of fine to medium grained, moderately sorted, bioturbated calcareous sandstone with concretions likewise Bed A. METHODOLOGY The outcrops at Akpoha and Amasiri were mapped and their respective lithological descriptions reported. Since the outcrop was blasted, no lithological description was likely to be recorded for that at Ibii. Only the basal bed was properly emplaced. Four samples were collected for oxide analytical purpose: two samples from the Ibii community; - Bed A is highly bioturbated, while Bed B displays series of broad swaley cross-stratification and trough cross-beds. Ripple-laminated interbedded mudstone and sandstone occur in Bed C. The outcrop documents a coarseningupward facies succession passing from bioturbated mudstone with calcareous concretions at the base to swaley and trough cross-bedded sandstone, then ripplelaminated, interbedded mudstone and sandstone facies overlain by a bioturbated mudstone with calcareous concretions.

Akpofure and Didei 214 Figure 2: Lithologic description of Akpoha ridge outcropping at the Government Technical College, Akpoha. Amalgamated swales Cast of concretions Figure 3: Akpoha ridge showing amalgamated swaley cross-stratification and cast of concretions

Int. J. Geol. Min. 215 Figure 4: Akpoha ridge showing bioturbation Figure 5: Akpoha ridge showing planar cross-bedding, sharp sandstone base and concretion casts Trough Cross-bedding Figure 6: Akpoha ridge showing trough cross-bedding.

Akpofure and Didei 216 Figure 7: Akpoha ridge showing parallel lamination and trough cross-bedding. ANALYSIS OF PALEO-CURRENT RECORDED IN THE SANDSTONE AT AKPOHA The Paleo-current direction was estimated from measuring orientation of respective cross-beds within Bed B. The direction of flow current was measured and put on display on rose diagrams (Figure 8) and histograms (Figure 9) to clearly evidence the dominant directions of current flow. The dominant flow direction is NE-SW for the Akpoha Sandstone ridge with surbordinate NW current. Cross-bedding azimuth readings from the Amasiri Sandstone in Akpoha 282 258 286 252 252 308 248 324 228 238 256 244 238 274 256 238 212 232 262 236 280 270 300 286 320 316 328 306 288 292 40 66 44 42 22 88 30 42 20 22 20 360 24 48 58 6 360 50 46 22 360 50 4 38 26 40 78 66 24 44 Table 2: Frequency of cross-bedding azimuths into 20 o class interval Class Interval Frequency 0-19 5 20-39 10 40-59 11 50-79 3 80-99 1 100-119 0 120-139 0 140-159 0 160-179 0 180-199 0 200-219 1 220-239 6 240-259 7 260-279 3 280-299 6 300-319 4 320-339 3 Total 60 Figure 8: Rose diagram of paleo-current of the Akpoha ridge at the Government Technical School, Akpoha, showing SW- NE direction with subordinate NW current.

Int. J. Geol. Min. 217 Figure 9: Histogram plot of the Akpoha ridge at the Government Technical School, Akpoha, showing a bimodal current direction. FACIES ANALYSIS OF THE SANDSTONE AT IBII The Amasiri Sandstone ridges are more isolated, and alternate with shale swales southwards. The ridge beds at Ibii have been displaced by blasting but the basal bed can still be identified. The rock type is a calcareous sandstone (geographic location: at N05 55 38.2 E007 54 07.9 ; altitude: 33.1 m). The basal bed consists of very fine to fine-grained sandstone, highly indurated, and intensely bioturbated with massive calcareous concretions at its base. The concretions range from 5 cm to 90 cm, while some casts measure from 160 cm to 250 cm. The fresh sample break appears light grey to grey in hue. Concretions and host rock have similar grain size, and colour, and occur randomly within host rock (Figures 10, 11 and 12). Dip values range from 24 o to 30 o in a SE direction. Vertical joints are observed and may result from the blasting. The paleo-current direction was not evaluated here since the lack of structures obliterated by the blasting. Figure 11: Ridge at Ibii showing calcareous concretions in bioturbated host rock. Concretions and host rock have the similar colour and grain size. Figure 12: Ridge at Ibii showing calcareous concretions FACIES ANALYSIS OF THE SANDSTONE AT AFIKPO calcareous concretions Figure 10: Ibii ridge showing calcareous concretions The studied outcrop of the Amasiri Sandstone ridge in Afikpo is situated at 5 o 53 50.6 N and 7 o 55 22.9 E with an elevation 74.2 m asl behind the Government College, Afikpo. The outcrop, ca. 16 m high, displays highly indurated arenaceous sandstone, with a NE-SW direction.

Akpofure and Didei 218 Figure 13: Lithologic description of sandstone outcropping behind the Government Technical College at Afikpo. Three coarsening-upward sequences and one finingupward sequence are evidenced. Sequence one is a coarsening-upward sequence, ca. 0.6 m thick, made up of Beds A and B (Figure 13). Bed A, as the basal bed of the Ezeaku Formation, lies unconformably upon the Asu River Group, with pebbles underlining the unconformity. The grains are medium to coarse, poorly sorted. Here the Sandstone colour is white. Beds C and D build up Sequence 2, 2 m thick. These beds consist of cross-bedded, white to grey, very coarse and pebbly sandstones. They clearly illustrate the crossbedded sandstone facies. The base of Bed C is an erosional surface with pebble lag (Fig 14). Sequence 3 is heterolithic, and about 5 m thick (Figure 13). Thin sandstone beds are interbedded with thin mudstone beds, displaying ripple-laminated interbedded mudstone and sandstone facies. The sandstone beds are fine to medium-grained and moderately well-sorted. Ripple laminations occur within the sandstone and mudstone beds. Sequence 4 builds another coarsening-upward sequence with cross-bedded beds F, G and H. Bed F is 1 m thick, while Beds G and H are 2 m thick each. Bed F is dark grey, poorly sorted, fine to medium grained, and shows reactivation surfaces. Beds G and H are very coarse to pebbly, white to grey in colour. Beds I, J, K and L build Sequence 5, a coarsening-upward sequence about 3 m thick. Grains in Beds I, J, and K, are poorly sorted, medium to coarse and white in colour. Planar crossbedding also occur. Bed L is poorly sorted, coarse to very coarse with pebbles. Fractures and quartz veins occur within the outcrop. The outcrop is characterized by series of shallowingupwards successions of cross-bedded facies, and ripplelaminated, interbedded mudstone and sandstone facies forming the heterolithic beds (Figure 15). Presence of fractures within the ridge may originate from the Santonian tectonic event.

Int. J. Geol. Min. 219 Coarse grain planar cross-bedding Figure 14: Amasiri Ridge showing coarse grained, planar cross-bedding structure, behind the Government College, Afikpo Table 3 : Frequency of cross-beddings azimuths into 20 o class interval Class Interval 0-19 13 20-39 17 40-59 11 60-79 6 80-89 4 90-109 9 Total 60 Frequency Thin heterolithic beds Figure 15: Amasiri ridge outcropping behind the Government College at Afikpo ANALYSIS OF PALEO-CURRENT RECORDED IN THE SANDSTONE AT AFIKPO Figure 16: Rose diagram of paleo-current of Amasiri ridge behind Government College Afikpo mainly NE with ESE subordinate current. The direction of flow current was evaluated from crossbeddings, and put on display in a rose diagram (Figure 16) and histograms (Figure 17) to illustrate the dominant direction of the original current flow. The dominant flow direction is NEward for the Amasiri Sandstone ridge, with surbordinate current in the ESEward. Cross-bedding azimuth readings from the Amasiri Sandstone at Afikpo 32 34 12 32 36 27 50 47 40 42 39 46 09 20 76 98 04 25 109 56 118 48 07 22 30 16 113 28 34 54 65 06 14 60 41 90 73 110 104 12 100 78 114 92 30 23 10 70 04 107 55 35 111 24 40 05 09 21 19 87 Figure 17: Histogram plot of the Amasiri ridge behind Government College at Afikpo, mainly NE with ESE subordinate current.

Akpofure and Didei 220 The Amasiri Sandstones occur as NE-trended linear bars. Four lithofacies are observed in the three studied bars. In the Akpoha bar, the basal lithofacies observed was an intensely bioturbated sandstone bed with calcareous concretions. The colour and grain size of concretions and host rock are similar (Figures 8 and 9). The concretions are ovoid and spherical and lack any regular internal structure. This is followed by Facies 2, which is swaley and cross-bedded. A sharp contact separates Facies B from underlying Facies A. Facies 3 is made up of ripplelaminated interbedded sandstone and mudstone that occur as parallel beds, while ripples occur in the sandstone beds. Facies A is repeated in Bed D. Facies 4 occurs in the Amasiri sandstone ridge as coarsening-up sequences of coarse to pebbly crossbedded sandstone, repeatedly stacked on each other. Facies 3 also occurs as heterolithic beds of ripplelaminated interbedded mudstone and sandstone facies. CHEMICAL COMPOSITION Oxide analysis of sandstones and concretions from the Amasiri and Ibii ridges were to determine the rock chemistry (Table 4).Chemistry of the concretions and that of the host rock differ. Concentrations of SiO2, K2O and Al2O3 are lower in the concretions than those in the host rock. SiO2 content in the host rock is 50.67% at Ibii and 66.14% at Amasari, while that in the concretions is 32.20% at Ibii and 42.16% at Amasiri. K2O and Al2O3 contents show the same trend: K2O is 4.54% and 4.598% at Ibii and Amasiri respectively, while those of the concretions are 2.832% and 3.084% at Ibii and Amasiri respectively. Al2O3 content in the concretions is 9.766% for Ibii and almost double in Amasiri concretions (16.39%). The Al2O3 value is highest at Amasiri (23.55%) and 17.34% at Ibii. The Na2O content is also almost double the content in Amasiri with 3.936%. The content in Ibii ridge, Ibii and Amasiri concretions are 1.834%, 2.089% and 1.964% respectively. There is no noticeable difference within the respective concentrations of Fe2O3, TiO2, and MnO. MgO in Amasiri ridge is comparatively high (2.545%) while others are 0.318%, 0.246% and 0.940% for Ibii ridge, Ibii concretions and Amasiri ridge respectively. Comparatively, CaO is higher in the concretions than in the host rock (19.55% and 13.44% for Ibii nodules and that of Amasiri nodules respectively, while Ibii ridge shows a value of 3.222% and Amasiri ridge, 0.672%). Table 4: Chemical composition of studied Sandstone Sample no. Location SiO2% K2O% Al2O3% Na2O% Fe2O3% TiO2% MnO% MgO% CaO% Sample1 (Ibii ridge) Ibii 50.67 4.54 17.34 1.834 1.312 0.775 0.029 0.318 3.222 Sample2 Ibii nodules 32.30 2.832 9.766 2.089 0.9915 0.5758 0.082 0.246 19.55 (Ibii ridge) Sample3 Ozara-Ukwu 42.16 3.084 16.39 1.964 1.277 0.4780 0.0607 2.545 13.44 (Amasiri ridge) nodules Sample4 (Amasiri ridge) Ozara-Ukwu 66.14 4.598 23.55 3.936 1.521 0.5895 0.0796 0.940 0.672 DISCUSSION The Ezeaku Formation at Akpoha is represented by bioturbated shales at the base passing into shoreface interbedded mudstone and sandstone and upwards into relatively cleaner sandstone bars. These bars occur obliquely with a general NE-SW trend. Southwards, the ridges become more isolated and alternate with shale swale. Walker and James (1992), postulated that, in a storm dominated shorefaces, the cross-bedding tends to be obliterated by storm waves, and replaced by parallel lamination and/or swaley cross-stratification. Such sedimentary structures were observed in Bed B in the Akpoha ridges with a sharp contact with Bed A, while Bed A consists of bioturbated mudstone with calcareous concretions. Bed C consists of parallel interbedded sandstone and mudstone with ripple cross-lamination occurring in the sandstone. Bed C describes BC beds of the Bouma sequence. The facies indicate a waning current that may be unidirectional or oscillatory. The sharp base indicates scouring of a muddy shelf floor during the commencement of storm, followed by the emplacement of sand as the storm waned. The heterolithic beds consisting of interbedded mudstone and sandstone with ripples suggest an offshore depositional environment. This facies occur in Bed C in Akpoha, and below the ridges. This is also the case of Bed E, ca. 7 m thick in Amasiri ridge behind the Government College, Afikpo. The stacking pattern of the coarsening-upward succession in ancient marine settings usually reflect coastal progradation or aggradation. Isolated sand bodies or ridges, and adjourning swales of shales, especially southwards from Ibii sandstone ridge towards Afikpo, and the coarsening-upward sequence series suggest offshore bars developing on muddy seafloor. The concretions in Akpoha and Ibii sandstone ridges constitute some hard, compact mass of matter resulting from calcitic-cement poronecrosis. This pervasive growth may have occurred simultaneously throughout the sediment. Such concretions usually form in the early stage of diagenesis before the final hardening of sediments into rock. Such a differential diagenetic cementation makes the

Int. J. Geol. Min. 221 concretions harder and more resistant to weathering. The colour and grain size of the concretions are similar to those of the host rock: grey and fine-grained. They are oval or spherical, and range from about 3cm to 2m in size. They have not been found in all the beds, but randomly within the bioturbated beds. The sandstones are generally richer in K-feldspars than Na-feldspars, Al2O3 content is also moderately abundant. These features are typical of arkosic sandstones. Lower content in SiO2 and higher one in CaO than those of the host rock, all support a diagenetic replacement of SiO2 by calcite within the sandstone. The presence of calcite cement indicates impregnating seepage of original sediments by alkaline pore fluids irrespective of Eh. The concretions occurrence in bioturbated beds suggests deposition in a shallow marine shelf environment under a waning current. Such evidences are in total disagreement with former assertions (Amajor, 1985; Odigi, 2007) that these concretions are clastic materials/boulders derived from the Abakiliki anticlinorium. The dominant flow direction is NE-SW for the Akpoha Sandstone ridge and mainly NE for the Amasiri Sandstone with an ESE subordinate current. The paleo-current derived from the cross-stratification suggests a shore-parallel flow. It also documents an average flow and a net geostrophically-driven flow along the funnel-shaped paleo-shelf coastline (Duke, 1990; Duke et al., 1991). CONCLUSION The Ezeaku Formation in the Afikpo basin occurs as series of parallel asymmetric ridges, designated as EZi to EZviii (Ukaegbu and Akpabio, 2009). They all display a NE-SW trend and dips ranging from 3 o to 20 o NE. Three of these ridges, the Akpoha ridge (EZiv), Ibii ridge (EZvii) and the Amasiri ridge (EZiii), were studied. The Akpoha ridge (EZiv), Ibii ridge (EZvii) and Amasiri ridge at Uzara Ukwu, presented calcareous concretions occurring in places, mostly in the basal bed. Some of these sandstones are calcareous like the Ibii ridge, while the others are arenaceous like the Amasiri ridge behind Government College, Afikpo. The observed sedimentary structures are indicative of a shallow marine and shelfal environment. They include: ripple lamination, parallel lamination, planar crossbedding, erosional surfaces, and sandstone ridges alternating with swales of shales. The storm structures observed in these ridges include: swaley and hummocky cross-bedding, coarse planar cross-beds, trough crossbedding, and bioturbation. Some of these ridges (e.g., Amasiri ridge) occur as stacks of prograding facies with erosional surfaces occurring in places indicative of coastal aggradation and progradation. The paleo-current analysis suggests a NE-SW dominant flow direction for the Akpoha Sandstone ridge and mainly NE one for the Amasiri Sandstone with ESE subordinate current. The paleo-current derived from the crossstratification suggests a shore-parallel flow. The sandstones are richer in K-feldspars than in Nafeldspars, but the silica content is higher in the host rocks than in the concretions. Al2O3 occurs in a moderately high amount, while CaO is higher in the concretions than in the host rock. Oval or spherical shape concretions, ranging from about 3cm to 2m in size, occurring in the Akpoha, Amasiri and Ibii sandstone ridges, originate from the poronecrosis by precipitation of calcite cement. They occur in bioturbated beds, suggesting deposition in a shallow marine shelf environment with a wanning current. This invalidates former assertions of Amajor (1985) and Odigi (2007) that the concretions are clastic materials/boulders derived from the Abakiliki anticlinorium. 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