Ajali Sandstone Research Articles

Aquifer prospect and vulnerability of Upper Maastrichtian sandstones: Case of Ajali and Nsukka formations in the Northern Enugu Province, southeastern Nigeria

Two typical aquifer systems, namely, regional aquifer and local Perched aquifer have been delineated in the study area. The regional aquifer was identified at about 100 m depth around lowland areas, although prone to polluting effects from farming activities, erosion and weathering processes. This study investigated extents of groundwater pollution and permeability of the aquifers from water sample and grain size analyses. Results show porosity ranging from 49 to 50% and hydraulic conductivities as follows: 7.0 m/day for the sandstone of Nsukka Formation, 34.6 m/day for the outcrop of Ajali sandstone and 10.4 m/day for the sandstone at saturated subsurface zone with transmissivity of about 572 m2/day. The results signify that the regional aquifer is recharged by substantial rate of infiltrations vis-a-vis surface outcrops, and is therefore vulnerable to infiltration of pollution plumes. The groundwater is mainly acidic at pH ranging from 5.05 to 7.41 with a mean value of about 6.48, hence the pollution from dissolved iron in many places. Three main water types were identified, namely, Ca-Mg-HCO3, Ca-HCO3-Cl2 and Mg-Na-HCO3-SO4-Cl2 facies, all signifying dominance of groundwater species arising from precipitation recharge. This has resulted in the influences of surface effluents from run off as indicated by nitrate pollution in some areas. Thus, active hydrologic cycle controls the chemical facies in the water resources of the region, and with its hydraulic influence on the landscape, the quality status of groundwater, as well as the growth of agricultural products have been impaired.

The use of statistical grain-size method in analysing borehole and evaluating aquifer parameters. A case study of Ajali Sandstone formation, southeastern Nigeria

Several challenges abound with the accurate estimation of aquifer parameters in Groundwater resource development. This paper describes the investigation on the aquifer (mainly Ajali Sandstone Formation) parameters in Nsukka area. The Ajali Sandstone Formation is among the principal sources of groundwater in the area. The investigated area lies within Longitudes 7 o 22 1 30 11 E and 7 o 58 1 30 11 E, and Latitude 6 o 73 1 30 11 N and 7 o 00 1 00 11 N, covering an area of about 482 square kilometres. Sixty (60) cuttings from nine (9) boreholes drilled in the area were analysed texturally using statistical grain size method. Results show that the highly permeable members of Ajali Sandstone are devoid of surface waters and are seen to be friable, medium-grained, poorly sorted, and highly cross-bedded. Recharge is mainly through rainfall. The distribution of major geological units, well log data, static water level data, and surface features were found to have influenced groundwater occurrence and flow pattern in the study area. The lithological logs showed sandy aquifers separated by clayey aquitards, while stratigraphic analyses of well lithological logs revealed the presence of at least four layers of contrasting hydraulic conductivity, making the aquiferous Ajali Sandstone homogenous and anisotropic. Values of permeability (k) from 0.00753 to 0.01512 cm 2 , hydraulic conductivity (K) from 8.2886 to 16.6444 m/s, transmissivity (T) between 455.87 and 1002.48m 2 s -1 , and specific yield from 2.35 to 4.76m 3 were found to be consistently high, thus suggesting thick and prolific aquiferous zones.

Multivariate analysis of ground water characteristics of Ajali sandstone formation: A case study of Udi and Nsukka LGAs of Enugu State of Nigeria

Multivariate statistical techniques were applied for the evaluation and interpretation of borehole characteristics of the Ajali sandstone geological formation of Enugu state of Nigeria to determine the latent structure of the borehole characteristics and to classify 9 borehole parameters from 33 locations into borehole groups of similar characteristics. Two chemometric data mining techniques used were, Cluster Analysis (CA) and Principal Component Analysis (PCA). PCA identified the borehole parameters responsible for variation in the borehole characteristic of the study area. Out of the nine parameters examined, the PCA identified borehole depth, borehole casing, static water level and dynamic water level as the most significant parameters responsible for variation in borehole characteristics. Hierarchical Cluster Analysis also grouped the 33 borehole locations into three clusters. The CA grouping of the borehole parameters showed similar trend with PCA hence validating the grouping of variations in the borehole characteristics in the geological zone. The results of the study indicate that PCA and CA are useful in offering reliable classification of the borehole characteristic of the study area.

Paleoenvironment and Provenance Studies of Ajali Sandstone in Igbere Area, Afikpo Basin, Nigeria

Paleoenvironment and Provenance Studies of Ajali Sandstone in Igbere Area, Afikpo Basin, Nigeria

Structural Features of Ajali Sandstone in the Western and Eastern Parts of River Niger, Southern Nigeria

Structural Features of Ajali Sandstone in the Western and Eastern Parts of River Niger, Southern Nigeria

Palaeocurrent and facies analysis of Ajali Sandstone in Western Anambra basin, Nigeria

Outcrops of the Campanian - Maastrichtian Ajali Sandstone in Ayogwiri, Fugar and Orame in the Western Anambra Basin allow for the recognition of palaeocurrent fabric and facies characterization of the formation. Detailed outcrop logging was carried out and production of graphic logs was made for each outcrop location. The Ajali is extensively cross-stratified with different types of cross-bedding, such as the planar, trough, and herring-bone cross-beddings. Palaeocurrent directions of the cross-beds were taken using the compass-clinometer to elucidate the palaeocurrent direction of the environment of formation and rose diagrams were constructed for each bed. The Ajali Sandstone is made up of two basic facies. The basal facies, is made up of thinly laminated, heterolithic beds of the shoreface environment. The upper facies consists mainly of cross-bedded sandstones of a tidally influenced shallow marine environment. The major palaeocurrent direction of the Ajali Sandstone in the studied area was in the NE direction which infers the sediment provenance source to be the uplifted Abakiliki Anticlinorium. However, subordinate direction to the south occurs in Orame1.

Hydrogeological potential and qualitative assessment of groundwater from the Ajali Sandstone at Ninth mile area, southeastern Nigeria

Investigations were conducted to determine the geo-electric properties, aquifer potential and quality of groundwater samples of the Ajali Sandstone Formation at Ninth mile area, Nigeria. Results reveal that the formation has 4–6 geo-electric layers, with the fifth recording hydraulic conductivity, transmissivity and transverse resistance values that suggested a general high yield to boreholes. The aquifer longitudinal conductance analysis indicates poor aquifer protective capacity, thus, high risk of contamination by effluents from the heavy industries within the area. Qualitative analysis shows major ionic concentrations, in the order: Na+ > K+ > Ca2+ > Mg2+ > Fe2+ > HCO3− > Cl− > SO42− > NO3−, that satisfy all permissible limits for drinking purpose. All tested samples have heavy metals, except Pb2+, and also fall within acceptable limits. High amount of Pb2+ originated most probably from improper disposal of ‘leaded’ petroleum products or leaching of lead from metal casing used in the well development. That the water samples satisfied almost all limits for drinking purpose despite the poor aquifer protective capacity and high industrialization may imply that the wastewater received fairly adequate treatments before discharge. Caution should, however, be applied in dealing with factors that give rise to lead contamination and in the water usage.

Estimating the Hydraulic Conductivity of the Ajali Sandstone in the Udi Area of South Eastern Nigeria from Pumping Test and Grain Size Based Empirical Analysis

The Maastrichtian Ajali Sandstone in the Udi area of south eastern Nigeria has been assessed by means of pumping test and grain size based empirical analyses, to estimate the hydraulic conductivity of the aquiferous formation. Hydraulic conductivity (K) values from pumping test ranged from 10.41 to 17.85 m/day while estimations from grain size based empirical methods of the United States Bureau of Reclamation (USBR), Pavchich, Slichter and others gave (K) values that ranged from 5.24 to 39.95 m/ day. Statistical correlation analysis shows that a strong positive correlation (r=0.96) exist between K values from pumping test and those from grain size based empirical methods. Study reveals that grain size based Pavchich method (K=14.74 - 24.92 m/day) could adequately replace pumping test for the determination of the hydraulic conductivity of a homogeneous and isotropic aquiferous formation like Ajali Sandstone.

Sedimentary Structures of Ajali Sandstone

The Ajali Sandstone is a Maastrichtian deposit in the Stratigraphic units that make up the Anambra basin, southeastern Nigeria. The approximate locations of the study areas; Okigwe and Uturu, are respectively Latitude 5˚45’N-6˚00’N &Longitude 7˚15’E – 7˚30’E and Latitude 5˚48’N-5˚55’N & Longitude 7˚25’E-7˚30’E. Samples were collected for further laboratory studies and the result showed that the Ajali Sandstone is dominated by quartz mineral grains which constitute about 67.23% modal composition with ironstone cementing material. This implies that Ajali Sandstone is basically quartz arenite and it is said to be mineralogically mature. The Ajali Sandstone as observed in the field is milky white in color and friable with intercalations of claystone, mudstone, and shale. Common sedimentary structures observed on the Ajali Sandstone are cross-stratification structures, lamination structures, wavy structures, biogenic structures, herringbone cross-stratification, clay drapes, and heterolytic structures, These are indicative of fluvial-marine and continental depositional environment.

Petrographic and geochemical study of the Maastrichtian Ajali Sandstone, North Central Nigeria

The Upper Cretaceous Ajali Sandstone is an extensive stratigraphic unit of the Anambra Basin in southern Nigeria. It consists of friable, white cross-bedded sandstones exhibiting a fining upward sequence. Structures such as bioturbation, planar and herringbone cross beds indicate variability in depositional environment ranging from fluviatile to deltaic. Petrographic and geochemical compositions (major and trace elements) of these sandstones have been investigated to determine their provenance, tectonic setting and weathering conditions. Fifteen sandstone samples examined are rich in quartz but poor in feldspar and lithic fragments. Texturally, the framework grains of the sandstones are fine- to medium-grained, sub-angular to sub-rounded, moderately sorted to poorly sorted, positively skewed and leptokurtic. The sandstones are texturally immature as depicted by their sub-angular edges of grains, but mineralogically mature in terms of high percentage of quartz. The high chemical index of alteration (CIA) values (71.0–99.2%) for these sandstones suggests that they were derived from highly weathered rocks in the source area. The composition of the major oxides in the sandstones revealed that SiO 2 (49.1–99.7%), Al 2 O 3 (0.2–30.3%), Fe 2 O 3 (0.4–1.8%) and TiO 2 (0.06–3.2%) were the most abundant elements in all sandstone samples indicating a high detrital quartz and clay mineral content. The Al 2 O 3 /TiO 2 ratios (1.47 to 12.48), Ti/Zr (6.48–18.63) and Zr/Cr (2.24–22.36) suggest that the sandstones were derived from variable basement complex rocks, including some contribution from mafic or ultramafic components. Inferences from the plots of K 2 O/Na 2 O versus SiO 2 indicated a passive margin tectonic setting for the sandstones. The high loading of CaO and Na 2 O in more than 50% of the samples is indicative of terrigenous input, mainly in the form of carbonates and silicates. These results are generally consistent with a derivation of the sandstones from adjacent igneous and metamorphic basement complexes (Cameroon–Adamawa highlands and Oban Massif) while the extensive and blanket-like geometry of the Ajali Sandstone is indicative of an excellent reservoir for groundwater in the Anambra Basin.

Evaluation of hydrochemical characteristics and flow directions of groundwater quality in Udi Local Government Area Enugu State, Nigeria

A hydrogeological study was conducted at the Udi Local Government Area in south-eastern region of Nigeria to examine the hydrochemical constituent of groundwater and determine the quality for drinking and agricultural purposes. Results show that groundwater in the area is slightly acidic to slightly basic, soft and potable, when compared with the World Health Organisation standards for drinking water, and is suitable for agricultural purposes. The groundwater flow direction was determined to establish the implications of pollution from any water contamination. Twenty-one groundwater samples from existing boreholes were collected and analysed for various physicochemical parameters using standard field and laboratory techniques. Distribution of major ions was plotted on a Trilinear Piper diagram. Ajali Sandstone, known as major aquiferous layer in the area, outcrops predominately in sandy bed with medium to coarse-grained sediments. The aquifers are unconfined, semi-confined and with hydraulic conductivity values ranging from 9.0 × 10−2 to 8.5 × 10−2 m/day indicating medium transmissivity of groundwater in some areas and a flow net south to northward groundwater flow towards the coast. This explains why the aquifer does not appear to be very permeable and only provides a medium yield of good quality water in some regions of the study area.

Sedimentological Characteristics and Geochemistry of Ajali Sandstone Exposed at Ofe-Jiji and Environs, Northern Anambra Basin, Nigeria

Textural, mineralogical and structural studies as well as outcrop mapping of sandstone in Ofe-Jiji and environs were carried out in order to interpret depositional environment of Ajali sandstone exposed at Ofe-Jiji and environs of the northern Anambra Basin. The sandstone is false bedded, medium-coarse grained (average of 1.13 mm in diameter), poorly sorted and strongly coarsely skewed with an average skewness value of -0.72. Chemical analysis indicates that the sandstone exhibits average SiO₂ contents ranging from 59.12 to 60.20% with an average value of 59.80% while alumina (Al₂O₃) content range between 2.51 and 2.75% with an average of 2.63%. The mineralogical composition shows on the average 56.98 quartz, 6.30 feldspar, 31.78% rock fragment and 4.93% matrix and cement. The major framework composition is Q₅₆ F₃₆ L₂ which classifies the sandstone as lithic arenite. The sub angular texture and the geochemical assay of 22.74 ratios (SiO₂/Al₂O₃) and 3.36 ratios (Na₂O/K₂O) indicate low chemically stable minerals in the sandstone and depict mineralogical immaturity. Also, the average ZTR% index of 52.36% with Tourmaline as the least abundant of ultra-stable minerals indicates sub-mature to immature mineralogy of the sandstone. The sandstone may be described as Fe-Sandstone based on Herron Classification Scheme. The dominance of Fe₂O₃ (26.57%) in the sandstone renders it ferruginous and reflects oxidizing environment. Ophiomorpha burrow in the upper unit of the beds strongly suggests fluvial origin.

Delineating Aquifer Systems Using Dar Zarouk Parameters Determined from Surface Geoelectric Survey: Case Study of Okigwe District, Southeastern Nigeria

This study is aimed at delineating the aquifer systems in the study area and hence determining the parts with higher aquifer yield. To achieve this, 120 Vertical Electrical Soundings (VES) were carried out in Okigwe District of Imo State of Nigeria, using the Schlumberger electrode array and a maximum electrode spread of 900 m. Twelve of the VES stations were sited near existing boreholes to enhance interpretation. The resistivity of the aquifers zones varied across the study area. The highest thickness of 104.4m for the aquifer was recorded at Amonze (VES 95). Using an average transmissivity of 1032.0848 m2/day determined from pumping test data of the boreholes in the area, a mean conductance value of 91.222 m/day was obtained for the area. The hydraulic conductivity values obtained from the VES results varied from 9.8854 to 115.9646 m/day. The transmissivity values ranged from 992.04 to 10388m2/day. The storativity or storage coefficient determined for the area ranged from 1.59x10-4 to 7.80x10-3 while the specific capacity was fairly uniform with magnitude of about 877 m3/day. Using Dar Zarrouk parameters determined for the area, three aquifer systems had been identified viz: the Coastal plain sands aquifer, the Bende-Ameki sandstone/shale aquifer and the Ajali sandstone aquifer. The coastal plain sands aquifer serves the Southern part of Isiala Mbano, Ehime Mbano, Ihitte Uboma and the entire Obowo Local Government Areas (LGAs). The Bende-Ameki sandstone aquifer serves part of Ihitte Uboma, the Northern part of Ehime Mbano and Isiala Mbano. The Ajali sandstone aquifer serves the Northwestern part of the study area in Okigwe. Groundwater potential evaluation of the district based on the survey and borehole hydrogeological information revealed that the Southern part of the district is the most prolific in terms of groundwater exploitation and thus the most promising for siting productive boreholes.

An appraisal of the water related contaminants as they affect the environment around the Enugu coal mines of Enugu state, southeastern Nigeria

Enugu metropolis is within the Anambra basin, which comprises mainly sedimentary rocks of sandstone, siltstone, mudstone and shales in the Lower Benue trough and as the capital of Enugu state in Nigeria and also as one of the major municipal and industrial centers in southeastern Nigeria experiences much groundwater contamination due to coal mining activity. The Area is richly endowed with sub-bituminous three-coal seams within the Mamu Formation. It also lacks prolific and potable groundwater due to the thinning-out of the Ajali sandstone aquifer through Udi town and Ninth-mile into Enugu metropolis. Generally, the specific discharge of Ajali sandstone (17.5 m2/day) is higher than that of Mamu Formation (14.5 m2/day). This appraisal becomes necessary to acquaint the Enugu city developers on the need for the prediction and understanding of the Environmental Impact Assessment (EIA) on the future development, management and maintenance of the mine water. The study also aims at identifying various contaminants, sources, level of concentrations, effects and control measures. Many springs, streams and seepages exist on the foot of most of the hills and ridges. The springs form the headwaters of the major streams and rivers like Ekulu, Iva and Nyaba at the base of the escarpment. Out of the streams and rivers in Enugu coal city, Ekulu River is the largest and thus very important for industrial and agricultural purposes. However, the coal mines discharge their effluents and waste waters directly into the Ekulu River with toxic heavy metals like As, Cd, Cr, Fe, Mn, Ni and Pb that contaminate the water and sediments. At the mines, most of the seepages, springs and streams rise directly from the perennial flood water from the mines. The water from the coal mines with special treatment for pH and iron contents can be employed in augmenting the present inadequate water supply that comes from the Ninth Mile borehole network and Ekulu River reservoir. The pH can be treated with hydrated lime, while iron (Fe2+) can be treated with aeration and filtration. Key words: Groundwater, coal mine, Ajali sandstone, sub-bituminous, coal, rivers, seepage.

Sedimentology and Permeability Distribution of the Ajali Sandstone around Odugbo, North Central Nigeria

Ajali Sandstone around Odugbo, North Central Nigeria was studied in order to determine its sedimentological characteristics and permeability distribution. The study involves both field mapping, samples collection from different outcrops locations and laboratory analyses. Grain size analysis revealed that the sandstones are composed of medium grained (av. 1.50 ?), poorly sorted (av. 1.60 ?) and sub-angular to sub- rounded grains which indicate deposition close to the source area. Permeability values deduced from an empirical formula are generally low with an average value of 46.76md indicating that the sandstones have poor flow characteristics. The low permeability is considered to have resulted from weathering and grained dissolution during the formation of hematite cement. Petrographic study shows that the sandstones are composed of mainly quartz (av. 95.08%), feldspar (av. 2.55%) and mica plus rock fragments (1.19%), and suggests texturally mature and mineralogically mature quartz arenite. This study also presents the trend suggestive that sedimentary structures and depth of burial can be considerable factors affecting sandstone permeability.

PALAEOPALYNOLOGY AND SEQUENCE STRATIGRAPHY OF THE MAMU FORMATION AROUND ORHUA, SOUTHWESTERN NIGERIA

A section of the Mamu Formation exposed at River Adewogun in Orhua, southwestern Nigeria, consists of dark grey, fissile, shale overlain by dark -brown, fine to medium- grained moderately sorted, thinly bedded sandstone of the Ajali Sandstone. The shale yielded a rich assemblage of spore and pollen of Late Maastrichtian age namely, Monocolpites marginatus, Longapertite marginatus, Constructipollenites ineffectus, Foveotrilete margaritae, Spinizonocolpites echinatus, and Ulmaideipites krempii. A back mangrove to nearshore palaeoenvironment of deposition is inferred. The sequence stratigraphy revealed a flooding surface with high organic productivity representing a short period of poor circulation associated with transgressive events.

Water pollution in relation to mineral exploration: a case study from Alayi-Ovim area of Southeastern Nigeria

Water samples from rivers, streams, springs, and shallow wells in Alayi-Ovim area of southeast Nigeria have been analyzed for Pb, Ca, Mg, Fe, Mg, PO(4), NO(3), CO(3), SO(4), Cl, and pH. The analyses were carried out using atomic absorption spectrometer and Hach Direct Reading Equipment. Results of the analyses from the area conform to the WHO (1995) standards for drinking water. However, the results show relative enrichment of Ca, pH, Mg, CO(3), and Cl. Low values were obtained for Fe, SO(4), and NO(3). While the Cl and Pb enrichment in the area north of Alayi-Ovim axis is attributed to proximity to the lead-zinc and chloride-rich formations of the Turonian Eze-Aku and the Albian Asu River; the Ca, Mg, SO(4), and CO(3) enrichment in Southern part of Alayi-Ovim is due to the limestone-bearing Late Maastrichtian Nsukka Formation. Furthermore, the very low values of less than 5ppm for these characters in water in the central region correlate well with the relatively clean Maastrichtian quartz arenite Ajali Sandstone Formation. The Pb-Zn and Cl incursions into the water system from the Older Albian Asu River/Turonian Eze-Aku Formations in the northern part of Alayi-Ovim area and the leaching of Mg, and Ca into the water system in the Maastrichtian limestone area in the south thus constitute geochemical indices for chemical pollution and mineral exploration for brine and dolomitic limestone in the area.

Textural and geochemical characteristics of the Ajali Sandstone, Anambra Basin, SE Nigeria: Implication for its provenance

This study presents the mineralogical, textural and geochemical characteristics of the regional Maastrichtian Ajali Sandstone in Anambra Basin, SE Nigeria. The intent is to highlight possible constraints on the chemical weathering conditions of the source materials on one hand, and to infer the provenance on the other hand. The investigation approach involved field studies and collection of samples from 12 different outcrop locations, followed by laboratory studies involving grain-size analysis (GSA), major and trace elements analyses using the X-ray fluorescence (XRF) method as well as thin section petrography. Field studies show that the sandstones are friable at all locations and range in color from white in freshly cut stone, to reddish brown on weathering. In addition, the sandstone units are cross-bedded and show graded bedding exemplified by fining upward sequence. Textural examination indicates that the sandstones range from fine to medium sands, constituting about 76 to 99% sand fraction, with graphic mean grain size of 0.23 to 0.53 mm. Standard deviation (sorting) ranges from 0.56 to 1.24 Ø and implies moderately well sorted sediments. Inferred from the textural indices, the depo-environmental discrimination of the Ajali Sandstone revealed a fluvial/river system-dominated sedimentary process. The sandstones are quartz arenite with quartz greater than 90% and less than 5% K-feldspar which indicate a predominant basement source as also revealed by the heavy mineral assemblages. In addition, major elemental oxides shows SiO 2 content greater than 96% for the fresh Ajali Sandstone samples with extreme depletion of mobile oxides such as Na 2O, CaO and the ferromagnesian minerals through weathering and sedimentary processes. Provenance and tectonic setting discrimination using geochemical data and compositional maturity revealed typical felsic igneous-dominated cratonic environment while inter-elemental ratios (such as Zr/Cr, Y/Ni, Th/Sc, La/Sc and La/Co) and ternary plots (e.g. Th–Sc–Zr; La-Th–Sc and Th-Co-Zr) reflect passive continental margin setting for the Ajali Sandstone. Consequently, the source area is constrained to the Precambrian basement rock units of Adamawa-Oban massif areas to the east of the Anambra Basin and the adjacent Abakaliki Anticlinorium.

PALEOEVIRONMENT OF NIGERIA'S AJALI SANDSTONES: A PEBBLE MORPHOMETIC APPROACH.

The Ajali Sandstone is a major clastic formation of Campanian-Maastrichtian age occuring within the Anambra Basin, Southeastern Nigeria. The Sandstones have a high incidence of quartz and feldspar pebbles. Clasts of vein quartz pebbles were selected for morphometric study of decipher the depositional environment of the sandstone. The three mutually perpendicular axes of each pebble were measured and the roundness estimate with the aid of a roundness image set. Morphometric parameters such as size, flatness ratio, elongation ratio, elongation ratio, maximum projection sphericity, form geometry and oblate index were computed. Bivarite scattergrams of roundness versus oblate-prolate index were plotted. Except for the flatness ratio which is non-diagnostic, all the other results of the morphometric studies indicated a fluvial – dominated depositional environment. The result is in line with earlier conclusions of fluvial or fluviodeltaic depositional environment. The result is in line with earlier conclusions of fluvial or fluviodeltaic depositional environment based on analysis of faceis, sedimentary rock which are suitable for morphometrical research: those with isotropic constitution and high resistivity to wear such as cherts, quartz sandstones and some related types. KEY WORDS: Clastic formation, Fluviatile, Fluviodeltaic, Elongation ratio, Oblate index. Global Jnl Geological Sciences . Vol.2(1) 2004: 37-43

Geotechnical evaluation of untreated and cement-treated Ajali Sandstone from southeastern Nigeria. Technical note: Eze, E O; Suoware, O P Q J Engng GeolV26, N2, 1993, P137–148

Geotechnical evaluation of untreated and cement-treated Ajali Sandstone from southeastern Nigeria. Technical note: Eze, E O; Suoware, O P Q J Engng GeolV26, N2, 1993, P137–148