Asu River Research Articles

Dinamica de temporada del fitoplankton en dos rios tropicales de tamaño e impacto humano variado en el sureste de Nigeria

Phytoplankton occurrence and dynamics in rivers are mainly shaped by hydrophysical conditions and nutrient availability. Phytoplankton main structuring factors have been poorly studied in West African rivers, and this study was undertaken to identify these conditions in two tropical rivers that vary in size and human impact. For this, environmental variables and phytoplankton monthly samples were collected from the middle reaches of Asu and Cross rivers during an 18 months survey from March 2005-July 2006. Phytoplankton biomass (F=11.87, p=0.003), Shannon-Weiner diversity and species richness (F=5.93, p=0.003) showed significant seasonality in Asu but not in Cross River. Data was analyzed with Canonical correspondence analysis (CCA) and showed environmental differences between the two rivers, nitrate in Asu River (5.1-15.5 mg/L) was significantly higher than Cross River (0.03-1.7 mg/L), while PO4 (0.2-0.9 mg/L) was significantly lower in Asu River compared to Cross River (0.03-2.6 mg/L) (p < 0.05). Eutrophic factors (NO1) determined primarily phytoplankton dynamics in Asu River, especially during the dry season, whereas hydrophysical factors (depth, transparency and temperature) shaped phytoplankton in Cross River. Taxa indicative of an eutrophic condition, such as Euglena, Chlorella, Chlorococcus, Ceratium, Peridinium, Anabaena, Aphanizomenon, Closterium, Scenedesmus and Pediastrum spp., were frequently encountered in the shallow impounded Asu River, while riverine species, such as Frustulia rhomboids, Gyrosigma sp., Opephora martyr and Surirella splendida dominated Cross River. A succession pattern was observed in the functional groups identified: Na/MP-->TB-->P (rainy-->dry season) was observed in Asu River, whereas MP/D predominated in Cross River for both seasons. We concluded that, if nutrients predominate hydrophysical factors in shaping phytoplankton during dry season (half of the year) then, they are as important as hydrophysical factors structuring phytoplankton during rainy season (the other half).

English

The lead-zinc deposit of Enyigba and its environs are localized in the cretaceous sedimentary rocks of the Albian Asu River Group in the Lower Benue Trough, Southeastern Nigeria. The Albian Asu River Group is made up of shale, siltstone, sandstone and limestone. The deposits were formed epigenetically through the crystallization of hydrothermal solutions and are localized within the fractures, hence the need to study the trend of the fractures in the rock unit where the mineral deposits are being hosted.&nbsp; In order to study the fractures, two units of the formation were identified and designated as units A and B. The ore field extends to about 3.7 km strike length with an average width of 5 m. From the detailed field mapping, the identified structures that control the mineralization are fold, fault and joint/fractures in which the fractures predominate. The total measured identified fractures is 503 in number and are represented in rose diagram, histogram and stereographic net which shows that they trend NW-SE and N-S directions, and dip SW-NE direction. Key words: Mineralization, stereographic net, shale, siltstone, sandstone, limestone, deposits.

Economic Aspects of Carbonates of the Albian Asu River Group in Tse-Kucha Near Yandev, Middle Benue Trough, Nigeria

The limestone-shale sequence at Tse-Kucha belongs to the Albian Asu River Group deposited during the first marine transgressive cycle in the Benue Trough. They however vary compositionally in mineralogy and texturally from fine to coarse grained. The compositional disparities between the samples of the limestone imposes corresponding differences in their industrial utilization. Assessment of the limestone suggested a wider range of industrial utilization beyond the present day application. For example, the highly calcitic limestone (60.04%) which is characterized by high CaO (43.8-53.3%) and low amount of MgO (0.44-1.06%) constitute suitable raw materials for the cement production, agriculture, metallurgical purification in blast furnace, lime manufacture, chalk, plaster and other filler applications. Apart from silica which varies from 2.1-10.0%, other impurities in the limestone are less than 1%. In view of the critical need of limestone in more strategic sectors of the economy, such as, iron and steel, agriculture, metallurgical purification/processes in blast furnace, lime manufacture industries, it is recommended that Government should exercise control measures over their exploitation for less profitable ventures. High quality burnt bricks and polished rock slabs could adequately substitute for sandcrete blocks produced from cement, which in many cases unaffordable to the peasants in the community.

Petrographic Study of the Sandstones of Eze-Aku Formation (Turonian) in Abaomege, Southern Benue Trough, Nigeria.

The Turonian Eze-Aku Formation in the study area uncomformably overlies the Albian Asu-River Group and unconformably underlies the Nkporo Group (Campano-Maastrichtian) in Afikpo Synclinorium. The petrographic study of the sandstones in Abaomege was carried out to determine the provenance, paleoclimate and source area tectonics for the sandstone facies. The lithofacies encountered in the area consist of fine to medium grained calcareous sandstones which correlate the sediments of Turonian Amasiri Sandstone and alternating sequence of light grey to dark grey shales, and siltstone. Petrographic analysis of the sandstone suggests that they are mineralogically mature and are essentially quartz arenites. Analysis and interpretation of sedimentary structures, mostly the cross beds, and also the structures of quartz grains indicates that the sandstones were derived primarily from granitic Basement Complex rocks from the Oban Massif, under humid climate. The paleocurrent plots for the sandstones show bimodal distribution pattern, suggesting a shore environment of deposition. The sandstones in Abaomege area are part of the sediments of the Late Cenomanian-Turonian Eze-Aku Formation of the Middle Cretaceous which outcrops at eastern flank of the Abakaliki Anticlinorium, Southern Benue Trough Nigeria. There has not been enough work done to establish the provenance regions, paleoclimatic setting and source area tectonics for the sandstones of Eze-Aku Formation in Abaomege area, southern Benue Nigeria. This study seeks to carry the petrography study and paleocurrent analysis of the sandstone and its implications on the provenance, sediment direction flow and paleoclimatic setting. (1) established an unconformity between the Turonian Eze-Aku Formation and the Campanian - Maastrichtian Nkporo Formation in the Afikpo Synclinorium. (2) reviewed the stratigraphy of the Eastern Nigeria, although some of the original names he adopted have been changed by ((3); (4)) in their later work. According to their report on tectonic activities, which occurred during the Cretaceous period, resulted in a series of folds in the sequence of the Asu River Group, the Eze - Aku Formation and created the Abakaliki Anticlinorium. Regional study of Pre - Santonian sandstones including the sandstones of the study carried out by ((5); (6)); they suggested that the sandstones are petrologically arkosic quartz arenites derived from the granitic/metamorphic basement rocks. An interest in paleocurrent analysis can be noted in the work of (7) which suggested that since cross bedding of any scale has paleocurrent value, all that is required is that its orientation be systematically measured and mapped. Paleocurrent indicators are orientated sedimentary structures interpreted to have been deposited by ancient flows. Cross beds slip face, pebble imbrications, parting lineation, tool marks and groove casts, and ripple crest orientation are all examples of possible paleocurrent indicators (8). Some paleocurrent indicators are unidirectional - that is, their shape provides unique information about the direction of the ancient paleoflow. (9) studied the Amasiri Sandstones in the Abaomege, Akpoha and Afikpo areas and noted that the sediment were derived from east of the study area and were deposited as tidal sand ridges in shallow marine environments. The Eze-Aku Formation outcrops out on both sides of the Asu River Group (Fig. 1). It is composed of calcareous shale, siltstone and thin sandy or shelly limestone as well as calcareous fine to medium grained sandstones (3). The Eze-Aku Formation has been dated to be generally of Lower Turonian age on the basis of diagnostic ammonites and microfauna (3). Cenomanian forams have also been recovered from the formation. It has an estimated thicknesss of 2100m (10). Table 1 shows the stratigraphic position of the Eze-Aku Formation in the southeastern Nigeria. Based on the study, the sandstones were suggested to be quartz arenites and the sediments were derived from the Basement Complex rocks of Oban Massif in a humid climate.

Application of Statistical Methods in Evaluating Groundwater in Parts of Mamfe Embayment, Southeastern Nigeria

Eighty eight (88) water samples were collected from twenty two (22) locations across four periods to highlight the micro-climatic conditions between January to September 2005 in three Formations intrusive, Asu River group and Eze-Aku. In this study, the relationship between various elements has been studied using correlation analysis, cluster analysis and factor analysis. The data also show that the aquifer in the study area are vulnerable to faecal coliform due to water from runoff that infiltrate into the aquifer from the vodose zone during precipitation in the area. This is evidence that the vulnerability of the study area is control by infiltration and runoff.

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.

Environmental constraints on the interdependence of drainage basin morphometric properties in the upper Cross River.

This study seeks to identify the effects of environmental heterogeneity on the interdependence of drainage basin morphometric properties and its implications for applied studies-particularly flood abatement/control. A detailed morphometric analysis of 22 3rd – order streams in the upper Cross River basin was carried out. At least six 3rd order streams were chosen from diverse geologic formations- Precambrian Basement Complex rocks and Asu River Formation in Abakaliki and Ikom-Mamfe Formation. The thirteen measured/derived data on the morphometric parameters were tested for normality and logarithm transformation was carried out. The SPSS soft ware was employed to generate a correlation matrix using the transformed data set. The transformed data matrix was also subjected to principal component analysis. The principal components having eigen-values greater than 1.0 were rotated using the varimax method in order to produce an orthogonal transformation of the components and simplify interpretation. The results indicate that the degree of inter-correlations among the thirteen morphometric variables is weak. Only 32.1% of the 78 possible relationships were significant at 0.05 level. We do know that an increase in the interdependence of the morphometric properties of drainage basins is indicative of a lessening of environmental constraints [lithology] on the stochastic processes that generate stream networks. The result of Principal Component Analysis shows that the four factor model account for 83.90% of the variation in the original data. For example factor1[index of drainage dissection] explains36.55% of the variation in the data and is inversely related to basin size. The infrequent occurrence of flooding in the basin is caused by rain-swollen tributaries discharged into main channel at different periods.KEYWORDS: Factor analysis, Lithology, Nigeria, Basement complex, Eigen Value, Morphometry

Artisanal Fishery of the Exploited Population of Macrobrachium vollenhovenii Herklot 1857 (Crustacea; Palaemonidae) in the Asu River, Southeast Nigeria

River prawn in Nigeria sustains viable commercial fisheries, which, however, are poorly studied. In this study we evaluated the migrant prawn fishery in the Asu River and the relevance of length-frequency data to the estimation of Macrobrachium vollenhovenii population dynamics using the data collected monthly from January 2007 to December 2008. The length-frequency data were analysed using the FAO-ICLARM stock assessment tool. Over 80% of captured prawns were M. vollenhovenii with peak catches attained in July. Growth parameters of L∞ = 23.65 cm, K = 1.14 year-1 and (t0) = -1.45, were estimated. The estimated longevity of 2.10 years and the growth performance index (Φ') of 2.80 compared favourably with the available information on the species, authenticating reliability of our results. The instantaneous rate of total mortality estimated from the length-converted catch curve was 6.47 years-1. Natural and fishing mortality were 2.14 years-1 and 4.33 years-1 respectively, which yielded the exploitation rate...

BIOSTRATIGRAPHY AND DEPOSITIONAL ENVIRONMENT OF THE OIL SHALE DEPOSIT IN THE ABAKALIKI FOLD BELT, SOUTHEASTERN NIGERIA

The oil shale deposit in the Abakaliki anticlinorium has not been demarcated. The present study focused on the age, correlation and depositional environment of the oil shale using abundance, planktonic/benthonic ratio and species diversity of foraminiferal and ostracod assemblages. The result shows that three prominent peaks or biozones namely 1, 2 and 3 were recognized. The 1 biozone showed that Praeglobotrucana and Guembelitria are the dominant species and ranged from Albian to Mid-Cenomanian (96–108 my). The 2 biozone indicated that Hedbergella and Heterohelix are the dominant species, and ranged from Upper Cenomanian to Early Turonian (92–95 my). The 3 biozone showed that Heterohelicids are the dominant species and ranged from Middle Turonian to Coniacian (82–91 my). The 1, 2, and 3 biozones correspond to the deposition of Asu River Group (Abakaliki Shale), Eze-Aku Formation and Awgu Formation respectively. The 2 peak is bimodal and has the highest frequency and this supports the maximum transgression occurring at the Cenomanian-Turonian boundary event (ocean anoxic event). The oil shale is deposited in outer shelf to bathyal environment and ranged from Upper Cenomanian to Early Turonian age and belongs to the Eze-Aku Formation.

Origin of carbonate cements in Cretaceous sandstones from lower Benue Trough, Nigeria: Evidence from petrography and stable isotope composition

Beds of authigenic carbonates were identified from three Cretaceous lithostratigraphic units of the Lower Benue Trough, Nigeria. Three carbonate lithologies were recognized by petrographic analysis in the study area. Carbonate-cemented sandstones are dominated by ferroan calcite cements with subordinate amount of siderite and dolomite/ankerite cements formed throughout the diagenetic history of the Asu River Group, Eze-Aku Group and Campano-Maastrichtian proto-Niger Delta sequences. δ18O for the cements range from -4.22 to -6.91‰(PDB) in Asu River Group; -5.23 to 12.66‰(PDB) in Eze-Aku and -4.45 to 6.89‰(PDB) in Campano- Maastrichtian proto-Niger Delta sandstones. δ13C values averaged -5.23‰ PDB for Asu River Group; -11.03‰ PDB and -5.88‰ PDB for Eze-Aku and Campano-Maastrichtian proto-Niger Delta sandstones. Petrography and geochemical data suggest that the mixing of meteoric and marine waters in the sediments caused dolomitization in Asu River group and Eze-Aku Group sediments while deep burial diagenesis under marine depositional environment precipitated calcite cements. Both authigenic and biogenic carbonate cements with low and high δ13C values exits in the sandstones analyzed. The low δ13C values of Asu River Group, Eze-Aku and Campano-Maastrichtian cements that range from 0 to 2.98 ‰ suggest that they are shallow water carbonates. Biogenic carbonate cements were also identified in some of the sandstones. The carbonate cements were formed under the influence of both meteoric- and marine diagenetic conditions which changed in time and space. The dolomitized rocks occur just below erosion surfaces on the continental sediments and on which shallow marine facies are developed.

Geochemistry of carbonate cements in Cretaceous sandstones, southeast Benue Trough, Nigeria: Implications for geochemical evolution of formation waters

Carbonate cements including calcite, siderite, dolomite/ankerite were formed throughout the diagenetic history of the Asu River Group, Eze-Aku Group and Campano-Maastrichtain proto-Niger Delta sequences were analyzed for their major and trace element (Fe, Mn, Mg, Ca and Sr) and isotopic compositions. The earliest, ferroan carbonate cement has the following mean major and trace element concentration: Fe – 0.11 wt.%; Mn – 0.43 wt.%; Mg – 2.16 wt.%; Sr – 1050 ppm. The late ferroan has the following mean compositions: Fe – 0.48 wt.%; Mn – 0.22 wt.%; Mg – 0.25 wt.%; Sr – 1010 ppm. Earliest Fe-calcites, formed prior to significant compaction of the sediments, are relatively enriched in Mg (up to 4.12 mol.%), and have δ 18O values between −4.45% and −6.898% PDB close to the ideal original Cretaceous marine pore waters. Late calcites are relatively Fe-calcites (up to 4.2 mol.%). The earliest ferroan calcite occurs in both the Albian Asu River Group and Eze-Aku Group while late ferroan occurs in the three lithostratigraphic units studied. These geochemical variations appear to have resulted principally to reflect changes in pore water chemistry during diagenesis. The high value of Sr in cements is most likely due to interaction between pore waters and Sr-rich clay and possibly feldspar in the three lithostratigraphic units studied. Pore water Fe 2+ concentration was probably controlled by diagenetic alterations involving Fe-bearing minerals (e.g. pyrite precipitation). The low δ 18O value of some calcite cements (−11.62‰, −12.66‰, −14.31‰ PDB) suggests that an influx of meteoric water may have occurred in the Turonian–Coniacian, although the low value could also result from an abnormal geothermal gradient associated with tectonic activity in the trough. The elemental and isotopic composition of these cements varies as a function of the time of precipitation.

Geochemical characterization of Cretaceous sandstones from the Southern Benue Trough, Nigeria

Geochemical studies of sandstones from the three lithostratigraphic successions in the southern Benue Trough of Nigeria were undertaken for a geochemical characterization of the sandstones, and to assess their stratigraphic and source evolution. Major and trace elements data were obtained from outcrop sandstone samples. The SiO2/Al2O3, Fe2O3/K2O ratios and CaO contents have been used to characterize the Cretaceous sandstones into Al-rich and high and low Fe2O3/K2O ratio sandstones. Results indicate that there are geochemical features that display stratigraphic trends across the succession from the Asu River Group, Eze-Aku Group to the proto-Niger Delta succession which may imply a discontinuous evolution from different source terrains of Precambrian and Mesozoic ages that supplied the sediments. The Asu River Group sandstones have lower SiO2, Al2O3, Fe2O3 and higher MgO; the Eze-Aku sandstones have higher TiO2, CaO, alkalis and lower MgO while the proto-Niger Delta sandstones have higher SiO2, Al2O3, Fe2O3 and lower alkalis and CaO. These discontinuities signify the influence of tectonic impulses that affected the southern Benue Trough during the Cretaceous time. Changes in ratios of TiO2/Al2O3, Fe2O3, Cr and Zr suggest an increasingly mafic contribution to the depositional basin with time. The chemical index of alteration increases with time, possibly suggesting that a more intense weathering regime in the hinterland developed with time.

A Tectonic Implication Of The Eruption Of Pyroclastics In Uturu, Southern Benue Trough, Southeast Nigeria

The origin of the Benue Trough is linked to rifting associated with the separation of South America from Africa in the Cretaceous times. The vast igneous activities in the trough are thought to either precede the separation of the two continents or post-date Albian sediments deposited in the trough after the separation. Exposures of pyroclastics within flatland in Uturu, east of Okigwe, were studied with a view to determining the implication of the eruption that emplaced the pyroclastics on the tectonic evolution of the Lower Benue Trough. Field expressions show that the pyroclastics erupted parallel to the axial plane of the Abakiliki anticlinorium in NE-SW direction and are spatially associated with shales of the Asu River Group and Nkporo Shale. Mud fragments of diverse sizes (1cm-20cm) and shapes of the host Asu River Group are incorporated in the pyroclastics. The sizes of the fragments of the pyroclastics fall mainly within the range of tuff and lapilli composed of mainly pyroxene, plagioclase, melilite and opaque minerals. The Nkporo Shales form cover unit over the pyroclastics and do not show any effect of the volcanism on them. The stratigraphic relationship between the pyroclastics and the surrounding sedimentary units shows that the pyroclastics erupted after the deposition of Asu River Group shales (mid-Albian) but terminated before the deposition of the Nkporo Shales. This corresponds to period between Cenomanian and Santonian and suggests a link to the tectonic events that post-dated the separation of South America from Africa. Keywords : Benue Trough, Tectonism, Abakiliki anticlinorium, pyroclastics, lapilli. Global Journal of Geological Sciences Vol. 6 (2) 2008: pp. 123-128

Irrigation Suitability Evaluation of Asu River Basin Soils, South Eastern Nigeria

Irrigation Suitability Evaluation of Asu River Basin Soils, South Eastern Nigeria

Quantitative analysis and paleocological interpretation of foraminiferal fauna from the Albian Asu River Group exposed around Yandev, Nigeria

Quantitative analysis and paleocological interpretation of foraminiferal fauna from the Albian Asu River Group exposed around Yandev, Nigeria

Petrographic evidence for regional burial metamorphism of the sedimentary rocks in the Lower Benue rift

Results of thin section and X-ray powder diffraction (XRD) studies on the pre-Santonian sedimentary rocks outside the vicinity of exposed igneous rocks in the Lower Benue rift are presented. The thin section studies show that the rocks of the older sediments––the Asu River and the Eze-Aku Groups––are characterized by predominantly slaty fabrics whereas the psammitic rocks consist of poikilotopic texture in which the recrystallization of micrite cement has resulted in the expansion of the depositional fabrics and the isolation of the framework grains. The carbonate rocks consist of a hardened fabric in which sparry calcite has recrystallized in the matrix and in veinlets. These older sediments have attained the anchizonal grade or prehnite–pumpellyite facies of metamorphism. The diagnostic minerals of this facies as revealed by the XRD studies include pyrophyllite, paragonite and phlogopite. The following reactions: (1) Al 4 Si 4 O 10( OH) 8 Kaolinite + 2 SiO 2 quartz → Al 2[ Si 4 O 10( OH) 2] pyrophyllite + H 2 O (2) Al 4 Si 4 O 10( OH) 8 Kaolinite + NaAlSi 3 O 8 Albite → NaAl 2[ Si 3 AlO 10]( OH) 2 paragonite + H 2 O (3) 3 CaMg( CO 3) 2 Dolomite + KAlSi 3 O 8 K-feldspar + H 2 O→ KMg 3[ Si 3 AlO 10( OH) 2] Phlogopite + 3 CaCO 3 Calcite +3 CO 2 have been shown to occur at P– T conditions of 220–325 °C and 1–2 Kb (average, 1.5 Kb). [J. Petrology 19 (Part 1) (1978) 95; Petrogenesis of Metamorphic Rocks, fifth ed. Springer-Verlag, New York, 348pp; Geol. Mag. 133 (5) (1996) 583]. The preponderance of slaty, rather than schistose fabrics and the occurrence, in the sandstones, of carbonate cement which has recrystallized to sparry calcite are all evidence for a burial metamorphic origin. At the average rate of 250 bars/Km, average lithostatic pressure of 1.5 Kb could have been generated from the sediment pile of about 5.5 Km in the Lower Benue rift, thus suggesting that about 3.7 Km of sediments have been eroded to expose the oldest rocks studied.

Depositional environment of carbonates of the Albian Asu River Group around Yandev, Middle Benue Trough, Nigeria

Depositional environment of carbonates of the Albian Asu River Group around Yandev, Middle Benue Trough, Nigeria

Coexisting copper sulphides and sulphosalts in the Abakaliki Pb-Zn deposit, lower Benue Trough (Nigeria) and their genetic significance

The Abakaliki lead-zinc deposit in the lower Benue Trough (Nigeria) represents the largest of a series of fracture controlled base metal deposits within the Lower Cretaceous (Albian) Asu River Group shales. Field evidence, ore microscopy and electron microprobe analysis of the lodes, suggest that the vein mineralization commenced with pre-ore precipitation of diagenetic marcasite, pyrite and minor gel-like colloform textured schalenblende and galena especially at the vein/wall rock contacts. This was succeeded by an ore stage consisting mainly of sphalerite, galena, chalcopyrite and pyrite. Accessories include bournonite and boulangerite. There was also a later introduction of complex intergrowths comprising bornite, chalcocite, enargite, and tennantite. In addition a lead/iron bearing copper sulphide and a copper bismuth sulphide of wittichenite composition (Cu6Bi2SO6), the optical parameters of which do not agree with those of wittichenite, were also introduced. The late stage copper sulphides and sulphosalts were formed as replacements of earlier lower temperature (ca. 170°C) main phase mineralization, when temperature rose in minimum up to 230°C.

Mineralogical, textural and paragenetic studies of the leadzinccopper mineralization in the lower Benue Trough (Nigeria) and their genetic implications

Leadzinccopper mineralization in the lower Benue Trough of Nigeria occurs as discordant fracture controlled veins within the Lower Cretaceous shales of the Asu River Group. Three stages of events are proposed for the evolution of the veins in this district: 1. (i) pre-ore fracturing and brecciation of Albian shales accompanied by the precipitation of framboidal and colloform aggregates of pyrite, siderite and quartz; 2. (ii) ore stage formation of sphalerite, galena, copper bearing minerals (e.g. chalcopyrite, bornite, chalcosite, wittichenite and digenite) tetrahedrite, bournonite and boulangerite; and 3. (iii) final deposition of octahedral galena, sphalerite, bravoite and marcasite in the hanging wall of the veins. Quartz precipitation appears to be at several stages although a large amount of quartz accompanied the ore stage sulphide precipitation. A mineralogical zoning from a prite-galena-sphalerite rich top, to chalcopyrite rich base is shown by the larger veins in the district. Post ore oxidation and lateritic weathering of primary vein constituents led to the supergene enrichments of limonite, cerrusite, smithsonite, malachite and azurite which are sometimes preserved with the primary minerals. These mineralogical associations, compositions of the ore and gangue minerals, and the previusly reported temperatures of vein minerals are compatible with those of ore deposits formed from basinal brines. The mineralization is considered to be epigenetic and vein formation appears to have accompanied the Cenomanian tectonic event in the Benue basin.

The Abakaliki and the Ebonyi Formations: sub-divisions of the Albian Asu River Group in the southern Benue trough, Nigeria

The Asu River Group (middle-late Albian), exposed in the Nkalagu area of the southern Benue trough, consists of two formations, viz: the Abakaliki Formation at the base overlain by the Ebonyi Formation (new name). The Abakaliki Formation is composed mainly of illitic and kaolinitic black shales and mudstones interbedded with black micritic limestone, siltstone adn minor feldspathic sandstones. The limestone is composed of grapestones, infralimeclasts and brown algal remains. These together with the pyritic and fossiliferous chale, suggest oscillatory deposition of argillaceous sediment in a shallow but anoxic subsiding bottom and marine facies. Rapid alternations of sandstone, siltstone, shales, mudstones, oolitic and serpulid grainstones and packstone-wackestone indicative of transitional environment of deposition characterise the Ebonyi Formation. The thick horizontal strata and interference ripple marks suggest lower flow regime of deposition while the thin evaporite laminite indicate locally short-term aridity during the Cretaceous in the southern Benue trough.