The first phase defines a Pre-Santonian history, characterized by a strong subsidence in the Abakaliki domain of the Benue Trough, while the Anambra domain remained a platform where mud was deposited in a shallow restricted marine environment.

The second phase, which commenced in the post-Santonian period, is characterized by the uplift of the Abakaliki sub-basin of the Benue Trough, and the subsidence of the Anambra platform characterized by a shallow marine system. This resulted in the deposition of the Nkporo/Enugu shales and Lower Coal Measures (Mamu Formation), followed by a regressive fluvio-deltaic system during the Maastrichtian, leading to the deposition of the regional Ajali Sandstone and Nsukka Formation.

The last phase occurred during the Lower Tertiary and was marked by the development of the Niger Delta, which prograded along the Anambra Basin axis. This was characterized by the deposition of Imo shale and Bende-Ameki Groups, especially in the southern part of the Anambra sub-Basin during the Paleocene and Eocene periods respectively.

A detailed stratigraphic relationship of the sedimentary units as outlined above is presented in Fig. 2, while further details of the stratigraphic setting, sedimentation history and tectonic evolution of the Anambra Basin can be found in the works of Reyment, 1965; Murat, 1972; Hoque and Ezepue, 1977; Benkhelil, 1986 and Ladipo, 1988.

Previous studies of the Ajali Sandstone, which forms the focus of this study, have suggested continental and fluvio-deltaic setting as a regressive phase of a short-lived Maastrichtian transregression. The sediments were derived from westerly areas of the Abakaliki anticlinorium and the granitic basement units of Oban and Adamawa massifs (Benkhelil, 1986; Amajor, 1987, Adediran et al., 1991). However, based on the presence of trace fossils (Ophiomorpha and Skolithos) and other sedimentary structures, Ladipo (1986) suggested a tidal shallow marine depositional environment for the Mamu Formation and the overlying Ajali Sandstone, as part of a continued Maastrichtian transgression, which commenced in the Late Campanian and lasted until the Paleocene. However, outcrops of Ajali Sandstone extend from Fugar/Agenebode area in the west, towards Enugu/Udi area in the east and narrows southwards towards Okigwe area, thereby forming a characteristic “question mark” shape (Fig. 3). The thickness ranges from over 350 to 450 m in places and thins southward to few tens of meters around Okigwe area (Hoque and Ezepue, 1977).

Field study shows that the Ajali Sandstone consists of friable medium- to fine-grained sands, averagely sorted, typically whitish in colour (occasionally iron-stained in places) with characteristic cross-stratification, thus the common reference as false-bedded sandstone (Tattam, 1944; Simpson, 1955). Plate 1a shows a representative expression of the whitish Ajali Sandstone outcrop with cross-stratification features as observed in road cuts/quarry pits within the study area. Also field observation revealed that the upper portion of the Ajali Sandstone is ferruginized in places with weathering front defined by the extent of infiltration/recharge water as shown in Plate 1b. Furthermore, recent laboratory study (Tijani and Nton, 2009) revealed porosity of 18.3 to 32.8% (av. 27%) and average K-value of 1.4 × 10⁻³ m/sec, which falls within the range of 10⁻⁵ and 10⁻³ m/sec for fine to medium sands. This is an indication of good aquiferous potentials of the Ajali Sandstone and higher recharge/infiltration rate within the outcrop areas.

The plots of representative samples of the grain-size distribution curves of the Ajali Sandstone samples from different outcrop locations within the Anambra Basin are presented in Fig. 4, while the results of the estimated textural and statistical indices are presented in Table 1. As shown in Table 1 and also graphically presented in Fig. 4, the Ajali Sandstone falls predominantly within uniform medium-sand range. This is clearly supported by the graphic mean (G_m) of 0.98–1.98 phi; with the exception of sample from Dekina, having graphic mean of 2.26 phi, which implies fine sand. Estimated inclusive graphic standard deviation (D) ranges from 0.55 to 1.29 phi suggesting a wider range from poorly sorted to moderately well-sorted material. Also, varied values of inclusive graphic skewness from –0.24 to +0.20 (av. 0.02) imply nearly symmetrical to coarse skewed arrangement while graphic kurtosis (KG) ranges from 0.94 to 1.51 phi (av. 1.20) and suggests mesokurtic through leptokurtic with majority of the samples having leptokurtic population.

Generally, with the exception of sample from Okigwe-Uturu 1, percentage fines for all other samples ranges from 0.30 to 6.0, with percentage sand of greater than 94% (Table 1). Statistical parameters from textural studies of granular sediments have been successfully employed in sedimentology to reveal the transportation history, sedimentary processes as well as the characteristics of the depositional environments (Folk and Ward, 1957; Folk, 1974; Friedman, 1967, 1969; Olugbemiro and Nwajide, 1997). In this study, from the plots of simple skewness against standard deviation (sorting) (Fig. 5A), as well as that of mean grain size against standard deviation (Fig. 5B), it is clearly shown that the Ajali Sandstone samples are of river sand and fluvial in origin. This is based on the concept of Friedman (1969) and Folk (1974) respectively.

Initial mineralogical examination under binocular microscope of selected fresh Ajali Sandstone sample revealed the dominance of sub-angular to rounded polycrystalline quartz in the samples while the presence of dispersed whitish fines suggests limited clay cementing material. Based on the petrologic assessment, the Ajali Sandstone samples revealed mainly moderately well sorted materials while the framework grains are composed of monocrystalline quartz, polycrystalline quartz, K-feldspar and rock fragments (Table 2). A further highlight of the petrographic feature of the Ajali Sandstone is presented in Plate 2a showing (in cross nicol) quartz-dominated sub-rounded to rounded grains (Q) in quartz arenite of fresh Ajali Sandstone sample. However, Plate 2b presents photographic image of ferruginized Ajali Sandstone showing poorly sorted sub-rounded quartz grains and ferruginized matrix.

On the basis of the observed mineralogical composition and the estimated mineralogical maturity index (MMI) of 0.91 to 0.94, the Ajali Sandstone is classified as quartz arenite. Quartz, as the most abundant framework grain in the Ajali Sandstone, constitutes on the average 90% by rock volume. The grains are sub-angular to sub-rounded in shape; however, the monocrystalline quartz dominates over the polycrystalline type. The monocrystalline quartz shows straight to slightly undulose extinction while the polycrystalline quartz shows strong undulose extinction.

The feldspars are mainly dominated by K-feldspar over plagioclase while the rock (lithic) fragments are comparatively less abundant and are dominantly of sedimentary and metamorphic origin. The slight dominance of K-feldspar over plagioclase and very low lithic fragments are indications of either prolonged weathering, transportation or recycling as also noted by Akarish and El-Gohary (2008). Heavy minerals show rounded zircon and other minerals such as tourmaline, garnet and rutile, all of which commonly occur as silt-size fraction. The overall petrologic assessment revealed that the observed quartz arenites can be described as mature sandstone that was derived from primary basement rocks with some intrabasinal recycling (apparently from the adjacent Abakaliki Anticlinorium) as suggested by the presence of some sub-rounded to rounded heavy mineral grains.

The results of the major elemental distribution for both fresh and ferruginized Ajali Sandstone samples are presented in Table 3, alongside with the estimated weathering indices. As shown in Table 3, the analyzed samples are dominated by silica with 94.8 to 99.0 wt.% SiO₂, while other major oxides are generally below 1.0 wt.% in the fresh samples. The results generally revealed similar elemental composition for all samples reflecting the homogeneity of the Ajali Sandstone. This may be attributable to uniform depositional conditions for the sediments. However, the ferruginized samples, though with similar chemical trends exhibit elevated concentrations of Al₂O₃ (3.50–11.60 wt.%) and Fe₂O₃ (1.80–3.60 wt.%), which are clear indications of weathering processes. This is clearly supported by the low Si/Al ratio of 6.6 to 24.1 compared to the relatively higher values of 43.5 to 300 for the fresh samples.

Arising from the ternary plots of the chemical composition using Silica–Sesquioxides–Alkalis as presented in Fig. 6A, it can be observed that all the samples lie towards the silica (SiO₂) corner thus indicating severe depletion of the sesquioxides and the alkalis. This deduction is consistent with the Al₂O₃–CaO+Na₂O–K₂O ternary diagram (Fig. 6B), which shows the distribution of the Ajali Sandstone samples alongside with the primary mineralogy of typical Basement Complex rocks as reported in Tijani et al., 2006. As indicated, the Ajali Sandstone samples plot at the Al₂O₃ corner, thus implying intense primary weathering processes and removal of feldspars and other labile components that characterized the primary source rock materials.

Furthermore, the negative correlation of silica as shown in the cross plots of % SiO₂ versus Fe₂O₃ (Fig. 7A); versus Al₂O₃ (Fig. 7B) and versus LOI (Fig. 7C) demonstrate influence of weathering processes through enrichment of silica and depletion of Fe and Mg as well as the decrease in LOI with increasing weathering and maturity of the sediments. The implication of marked negative correlation between SiO₂ and Al₂O₃ is also an indication of the fact that most of the silica is present as quartz grains. However, the positive correlation between Fe₂O₃ and Al₂O₃ (Fig. 7D) is an indication of a common source and suggests possible controls by the proportion of clay (fines) contents as products of weathering/ferruginization processes. It can be concluded, therefore, that the depletion of all other major elements in the Ajali Sandstone is related to the removal of ferromagnesian minerals and feldspars through reworking and transportation of the source materials by sedimentary processes.

The results of trace element concentrations of the analyzed Ajali Sandstone samples are presented in Table 4. Due to their preferential enrichments in different rock types, relationships between the trace elements are usually exploited for discriminating the possible source rock (Wronkiewicz and Condie, 1987, 1990). As shown in Table 4, the trace elements reveal relatively varied concentrations between fresh and ferruginized samples. Data for the fresh (unweathered) samples, revealed relative enrichment of some indicator elements such as Zr, Cr, V, La, Th, Ba and Sr with average concentrations of 225.8 ppm Zr; 9.4 ppm Cr; 10.6 ppm V; 6.9 ppm La; 5.2 ppm Th; 19.8 ppm Ba and 9.7 ppm Sr (Table 4).

Such indicator elements and their relative enrichments in the Ajali Sandstone can also be attributed to the depletion of plagioclase and unstable fragments, leading to abundance of immobile elements during sedimentary processes. In addition, the abundance of high field strength elements (e.g. Cr, Zr, etc) can be related to the presence of detrital minerals such as chrome spinel, zircon, etc (Huntsman-Mapilaa et al., 2005). Although the plot of the trace elements profile as presented in Fig. 8 indicates variation within the samples, it clearly indicates similar trends for all the samples with enrichment of some indicator elements like V, Rb, Sr, Ba, Zr and Ce. Such enrichment is a strong indication of the relatively matured nature of the Ajali Sandstone as quartz arenite.

For the weathered or ferruginous samples, almost all the trace metals (with the exception of Rb) exhibit higher concentrations than those of the fresh samples (Table 4). Furthermore, the relatively positive relation of the trace elements in the weathered Ajali Sandstone samples with Fe₂O₃ and Al₂O₃ is an indication of the fact that the enrichment is related to secondary weathering/ferruginization process.

Furthermore, investigations of siliciclastic sedimentary rocks in several regions of the world show that their chemical composition is largely dependent on the composition and the weathering conditions at the source rock area (Nesbitt and Young, 1989; Nesbitt et al., 1996). As demonstrated by Nesbitt and Young (1982), a measure of the degree of chemical weathering/alteration of the sediments’ source rocks can be constrained by calculating the chemical index of alteration (CIA), where CIA = molar (Al₂O₃/[Al₂O₃+CaO* + Na₂O + K₂O]) while CaO* represents the amount of CaO in silicate minerals only (i.e. excluding those of carbonates which are incidentally not presented in the fluvio-continental Ajali Sandstone as also indicated by very low content of CaO [0.1 – 0.04%]). Also, the proposed chemical index of weathering (CIW) of Harnois, 1988 is similar to the CIA with the exception of exclusion of K₂O in the equation (CIW = molar [Al₂O₃/(Al₂O₃ + CaO* + Na₂O)]). Usually, the CIA or CIW are interpreted in similar way with values of about 50 for unweathered (fresh) upper crust material and about 100 for highly weathered residual soils, with complete removal of alkali and alkaline-earth elements (McLennan et al., 1983; McLennan, 1993; Mongelli et al., 1996).

For this study, the estimated CIA and CIW (Table 3) yielded similar values in the range of 94.0 to 99.6 for fresh Ajali Sandstone samples and 98.3 to 99.6 for the ferruginized Ajali Sandstone samples. The unexpectedly high values of CIA and CIW for the fresh samples clearly indicate the fact that the primary source material(s) must have been subjected to substantially high degree of weathering and reworking that resulted in the removal of the ferromagnesian minerals and feldspars. However, the values of greater than 98 for the ferruginized samples are unequivocally indicative of weathering-ferruginization processes. Also, the plots of Al₂O₃ against CIA (Fig. 9A) and LOI against CIW (Fig. 9B) clearly discriminate between the fresh and ferruginized samples. As shown in Fig. 9, low values of Al₂O₃ and LOI in the fresh samples are a further confirmation of the loss during the primary weathering and sedimentary processes of the source rock materials. Furthermore, the MMI, i.e. the ratio of quartz to quartz + feldspar + rock fragments (Bhatia and Crook, 1986), for the fresh Ajali Sandstone samples (Table 2), ranges from 0.91 to 0.96 suggesting relatively high mineralogical maturity. MMI also correlates well with the estimated CIA and CIW values (r = 0.86 and 0.82, respectively), which is a reflection of the fact that the chemical compositions are, to a great extent, related to the petrographic modal data. Therefore, it can be concluded that the CIA and CIW values for the ferruginized samples reflect the recent (secondary) weathering-ferruginization process on one hand. On the other hand, those of the fresh samples are reflections of sediments derived from highly weathered and chemically matured terrain through primary weathering and reworking of the source materials.

From the results of the major element geochemistry of the Ajali Sandstone, the observed high proportion of quartz (> 90%), the relatively little but significant quantity of K-feldspar and the absence of the more chemically unstable plagioclase and other labile components (Table 3) suggest that the source rock was exposed to prolonged weathering and that the sediment is at least partly multicycle. Also, the observed mineralogical composition is concomitant with mainly granitic provenance while the occurrence of both common (unstrained) and metamorphic (strained) varieties could be attributed to source area that suggests a metamorphic and/or plutonic source for the sediments as also reported elsewhere by Basu et al. (1975); Dabbagh and Rogers (1983); Al-Harbi and Khan (2008). Again the composition of heavy minerals that are rich in ultrastable varieties such as zircon, rutile, and tourmaline is a pointer to a source from basement rock units, apparently from the Adamawa-Oban massif area to the east of the Anambra Basin. In addition, some of the observed sub-rounded grains of the zircon and tourmaline may be attributable to prolonged abrasion and intrabasinal recycling from the adjacent Abakaliki Anticlinorium.

5.2.1 Evidence from major element geochemistry

The tectonic setting of the depositional environment is assumed to influence sedimentation, diagenesis and composition of sediments (Pettijohn et al., 1972; Bhatia, 1983; Chamley, 1990). A number of studies have used the detrital composition of sandstones to infer provenance and tectonic settings (Dickinson et al., 1983; Cavazza and Ingersoll, 2005; Greene et al., 2005). Hence, further clues for the provenance and tectonic settings can be inferred from the relative depletion in oxides like CaO, Na₂O (the most mobile phases) and enrichment of SiO₂, TiO₂, etc. (the most immobile elements). Using the typical quartz-feldspar-lithic fragment (Q-F-L) ternary diagram, adapted from Dickinson and Suczec (1979) and presented in Fig. 10, the Ajali Sandstone can be clearly constrained to mature quartz arenite of continental cratonic block. This implies sandstones derived mainly from exposed shield/platform or from uplifted areas and deposited in stable sites (Taylor and McLennan, 1985; Potter, 1978, 1986). Therefore, with Q greater than 96% and negligible F and L components, the quartz arenite designation for the Ajali Sandstone is consistent with the passive margin (PM) setting or tectonically stable craton/shield source area mentioned earlier. This is consistent with the classification of Crook, 1974, which linked quartz-dominated sandstones (Q > 65%) to passive (Atlantic type) continental margin.

Furthermore, the discriminant function plot of Roser and Korsch (1988) as presented in Fig. 11a, defined four main provenance zones as: P1 = mafic igneous provenance; P2 = intermediate igneous provenance; P3 = felsic igneous provenance; and P4 = mature continental provenance. For this study, all of the Ajali Sandstone samples including the weathered samples lie within the zone of mature continental provenance setting (P4). Though the plot does not directly indicate a felsic igneous source, it can be concluded that the matured continental provenance is consistent with the PM setting and continental block provenance mentioned earlier. This assertion is further supported by the cross plot of K₂O/Na₂O versus SiO₂ (based on Roser and Korsch, 1986), as presented in Fig. 11b which indicates PM tectonic setting for the Ajali Sandstone. Consequently, it can be concluded that the adjacent Precambrian Basement units of the Oban–Adamawa massifs represent the provenance for the Ajali Sandstone. This is consistent with the assessment of Amajor (1987), and also supported by the westerly and southwesterly trending palaeo-current measurements presented by Hoque and Ezepue (1977). These also undoubtedly signify the easterly and northeasterly locations of the source areas. Furthermore, the apparently high values of CIA and CIW are clear indications of further contribution and significant geochemical control of the recycled materials from the uplifted extrusive/sedimentary units of the adjacent Abakaliki anticlinorium.

5.2.2 Evidence from trace element geochemistry

Using discriminant plots of Bhatia and Crook (1986), samples from the study area plot in both active continental margin (ACM) and PM settings (Fig. 12a). However, both Th–Sc–Zr/10 and Th–Co–Zr/10 ternary plots (Fig. 12b and c) revealed exclusively PM setting of the source area of the Ajali Sandstone. These plots support at least a siliciclastic source for Ajali Sandstone and are also indicative of tectonically stable felsic (granitic) craton/shield source area, which is known to be characterized by quartz arenites (Pettijohn, 1975). This is consistent with the relatively low concentrations of the transitional elements (such as Cr, Co, Ni, and V) reflecting negligible input of mafic provenance, as also reported by Osae et al., 2006; Akarish and El-Gohary, 2008. It can therefore be deduced from the trace element geochemistry that the source area of the Ajali Sandstone is related to predominantly felsic rocks characterized by stable continental environment with possible contribution of recycled materials from intrusive units of the uplifted Abakaliki Anticlinorium as pointed out earlier.