THE SOUTHERN BENUE TROUGH A Regional Review of Geological Setting, Stratigraphy, Paleogeography and Mineral Resources.

OUTLINE Introduction Origin of the Southern Benue Trough Tectonostratigraphy Systematic Lithostratigraphy Paleogeography Economic Geology Conclusion

INTRODUCTION Benue trough has its southern limit at the northern boundary of the Niger Delta, where it dips down and is overlaid with Tertiary and more recent sediments. It extends in a northeasterly direction to the Chad Basin, and is about 150 km wide. The trough is arbitrarily divided into lower, middle and upper regions, and the upper region is further divided into the Gongola and Yola arms. The Benue Trough was formed by rifting of the central West African basement, beginning at the start of the Cretaceous period. The trough accumulated sediments deposited by rivers and lakes. During the Late Early to Middle Cretaceous, the basin subsided rapidly and was covered by the sea.

INTRODUCTION CONTD The trough's formation is that it is an aulacogen, an abandoned arm of a three-armed radial rift system. During the Santonian age, around 84 million years ago, the basin underwent intense compression and folding, forming over 100 anticlines and synclines. The deposits in the Benue Trough were displaced westwards at this time, causing subsidence of the Anambra Basin.

Geologic Map of the Benue Trough (from Nwajide, 2013).

ORIGIN The Southern Benue Trough, clearly demarcated from the central segment by the Gboko transform fault, is medially split into two blocks. The Northwestern block remained a stable platform, while the southeastern counterpart steadily subsided into a furrow that became the Abakaliki Sub-basin. It is evident that the Abakaliki Sub-basin had an initial genetic relationship with the Anambra Basin, which only became divergent in their evolutionary pathways on account of the latter remaining a stable platform till the Santonian thermotectonic event. From gravity studies, Okereke and Onwuemesi (1989) inferred faulted margins and established a maximum thickness of 3 km at its centre, with some thinning westwards.

TECTONOSTRATIGRAPHY In the southern Benue Trough, four stratigraphic packages are demarcated by unconformities: The most conspicuous of which are the nonconformity between the crystalline Basement Complex and the basal units, and the angular unconformity forming the base of the Anambra Basin. In between these two, there are other unconformities - evidently sequence boundaries which demarcate definite stratigraphic units. The Asu River Group and the Eze Aku Group -as well as single - formations - the Odukpani and the Awgu/Agbani.

lithostratigraphy unit Time Scale lithostratigraphy unit Sedimentary cycle Sea Movements Santonian Agwu Fm/Agbani Sst ogugu FM cycle 3 Regression coniacian   Turonian Igumale Fm cycle 2 Transgression Konshiasha R. "Gp" Eze Amanseri Ss Aku Nkalagu Lst GP Eze Aku Shales Cenomanian Mfamosing Fm cycle 1 Albian Asu Abakaliki Fm River Awi Fm/Mamfe Fm Aptian Gp Ogoja Sst Precambrian Basement Complex Stratigraphy of the Southern Benue Trough Basin (from Reyment, 1965; Murat 1972; Nwajide, 2013).

Systematic Lithostratigraphy The Asu River Group This extensive stratigraphic unit was named by the geologists of Shell D’Arcy as the oldest sedimentary unit in the western Cross River plain. Latter work has shown that it is the unit at the core of the Abakaliki fold belt, and extends far into the central Benue Trough as a lateral as well as chrono- equivalent of the southern limits of the Bima Formation. Reyment (1965) sub-divided it Ogoja Sandstone and the Abakaliki Formation are taken as coeval with the Mamfe Formation and the Awi Formation, and belong to pre-Albian to Albian. The Mfamosing Limestone is considered a major Cenomanian unit and treated separately from the Awi Formation with which it had formed the Odukpani Formation, now to be considered defunct.

Systematic Lithostratigraphy Cont. The Ogoja Sandstone The basal aspects of the Asu River Group directly overlying the Basement Complex have been characterised as consisting of conglomerates and arkosic sandstones in both Ikom and Ogoja areas (Uzuakpunwa, 1980; Petters et al., 1987). These facies are regarded as alluvial fan deposits characteristic of basin margins. Dating of the deposit has been uncertain even though Uzuakpunwa considers them pre-Aptian.Lower Cretaceous Petroleum System; The Awi Formation Murat (1972) gave the name “Calabar Flank” to the eastern hinge zone of the Niger Delta. It consists structurally of northwest — southeast trending basement horsts (the Oban Massif and the Ituk High) separated by a graben, the Ikang Trough

Systematic Lithostratigraphy Cont. The Mamfe Formation The earliest study of this unit was by Jaekel (1909, cited by Reyment, 1965) who referred to the rocks of the unit as Mamfe-Schiefer. Reyment (1955) established the type locality along the Cross River at Mamfe town now in Cameroon Republic. The Mamfe Formation is about 800 m thick and consist of arkosic sandstones, with intercalations of marls, sandy limestones and shales. The formation has been strongly folded along an east - west axis with limbs dipping up to 50°. Igneous intrusions are present in the formation. The age of the formation has not been clear but is considered not older than the Albian.

Systematic Lithostratigraphy Cont. The Abakaliki Formation As described by Reyment (1965), this unit has its type locality within the Abakaliki town. It consists of folded, lead/zinc mineralised shales, with lenses of sandstone and limestone, with one of the latter attaining a thickness of 30 m. The Abakaliki Formation is locally rich in ammonite fauna. In places it is so abundant that fossils weathered out of the shales litter the terrain and fragments form part of the mounds in the yam farms (Reyment, 1965) A sampling of the commonly occurring ammonites includes: Diploceras quadratum, Diploceras cristaturn, Hysteroceras binum

Systematic Lithostratigraphy Cont. Lithofacies of the Eze Aku Group in the Western Flank of Abakaliki Anticlinorium includes: The Eze Aku Sand Stone Facies; Documented in the Nkalagu area by Umeji (1984,1985, 1988, 1993). The Eze Aku Siltstone Facies; The aforementioned sandstone facies graded up- wards into a 150 m grey, feldspathic, micaseous (musco- vite), cross-laminated quartzose silstone. The Eze Aku Shale Facies; The stratigraphic section in the quarry at Nkalagu Shows an alternation of black shales and shelly limestones. The shale horizons in the section total some 24 m, and grades into the Awgu shales above (Umeji, 1985).

Systematic Lithostratigraphy Cont. The shale is fissile, micaceous, pyritic, calcareous, carbonaceous, and burrowed. The black shale facies of the Eze Aku Group is composed of illite, kaolinite, and chlorite, and has 0.5% to 7.4% total organic carbon (Petters, 1983; Petters and Ekweozor, 1982). Bioclastic Limestone Facies Bioclastic Subfacies Micritic Subfacies Limestone/Shale Interlaminated Subfacies Shale/Mudrock Subfacies The Lokpanta Channelled facies

Paleogeographic Summary of Asu River Group Ojoh (1990) inferred a general progressive rise of the sea level from Albian to Coniacian, punctuated by rapid falls and lowstand regimes in the Lower Cenomanian and Upper Turonian. Sedimentation rate in the Abakaliki area estimated by Ojoh from decompacted Mid-Albian sediments is about 500 m /My, a rate characteristic of rift basins. Synsedimentary faulting and mass (especially debris) flows were involved, and indicate that that period of tectonic evolution of the southern Benue Trough was paroxysmal.

Paleogeographic Summary of Eze Aku Group The facies of the Eze Aku Group have collectively been assigned ages ranging from Latest Cenomanian to Early Turonian. The assignation has been based mainly on ammonite, ostracod, and foraminiferal contents of the various components of the group (Reyment, 1955a, 1965, p. 39; Fayose and De Klasz, 1976; Arua and Rao, 1978; Umeji, 1985). Sediments were deposited in the Early Turonian transgressive — regressive cycle. Notably, this was the same time span during which equivalent shallow marine clastics were being deposited on the Anambra Platform to the west and the Ikpe platform to the southeast of the southern Benue Trough.

Paleogeographic Summary of Eze Aku Group The shales, by far the predominant lithofacies of the group, were deposited under shallow marine conditions. They may represent the prodelta and deeper water clays interdigitated with sandy deltaic facies represented by the Amaseri Sandstones and the Konshisha River Group, deposited as continental to paralic facies of local delta complexes, having been largely derived from the Basement Complex areas around Ogoja. Eze Aku is generally believed to have deposited in a subtidal shelf sea periodically agitated by storms that set off turbidity currents resulting in the alternation or interbedding of high and low energy deposits. Eventual shallowing to intertidal conditions, as documented by the common occurrence of channels on both the eastern and western margins of the group area of occurrence, marked the finale of the regression prior to the transgression that installed the Awgu Formation.

ECONOMIC GEOLOGY Two important sources of information on economic mineral resources of Benue Trough are Ford (1981) and Wright et al. (1985). They Classified earth material resources into: Metalliferous minerals: lead - zinc mineralisation accompanied magmatism. Sulphides of lead and zinc (galena and sphalerite), associated with copper (chalcopyrite), are well known. They occur as en echelon lodes and veins infilling open spaces along a relatively narrow northeast - southwest belt extending through the Benue Trough from Abakaliki through Ishiagu in the south to Gombe in the northern segment of the Benue Trough. In the entire Benue Trough, about 1 million tonnes of lead/zinc ore have been proved. Mining of lead and zinc has been on in Ishiagu area for many decades.

ECONOMIC GEOLOGY CONTD. Of particular interest are the occurrences in the axial zone stretching from Ishiagu through Abakaliki to Agbaja in which the mineralisation, including galena, sphalerite, siderite and quartz, which occur as fracture fillings within the Albian shales of the Asu River Group. Industrial minerals and Rock: The industrial earth materials in the southern Benue Trough include stone aggregates, clay, and limestone. Perhaps the only class that is peculiar and fundamental to the Benue Trough is the stone aggregates obtainable from the numerous igneous intrusives.

ECONOMIC GEOLOGY CONTD. Most quarries, e.g. in Ishiagu and Lokpauku, exploit dolerite dykes. In some areas around Abakaliki, baked shales/ clays and pyroclastics are quarried for concrete aggregate to supply the civil construction industry in the local area and beyond. Limestone deposit of Mfamosing and Nkalagu at Igumale in Benue State await exploitation. Several cement plants have been based on large deposits of shallow marine limestone deposits, such as those at Nkalagu in the southern segment, Yandev in the central segment, and Ashaka in the north. In the Abakaliki area, white and black limestone varieties are being exploited for ornamental use.

ECONOMIC GEOLOGY CONTD. chemical minerals: In the southern Benue Trough the chemical material of interest is saltwater issuing as springs. This often forms pools of salty water, e.g. in areas around Ameri, Uburu, and Okposi, in which salinities of 4.5% and pH 8 at 25°C have been recorded. These pools support an age-old salt industry. According to Ford (1981), the quantities of salt issuing in solution from the more active salt water sources are enough to supply all of Nigeria’s domestic and even industrial requirements.

ECONOMIC GEOLOGY CONTD. Hydrocarbons: As indicated by Weber and Daukoru (1975), and Reijers and Petters (1987), natural gas and oil impregnations are known in the subsurface carbonates of the Calabar Flank. The potential reservoirs in the Mfamosing limestone are both depositionally and diagenetically controlled. In the surface, the Mfamosing Limestone, which is gas-bearing and oil impregnated, exhibits fair to good porosities in which calcarenitic and extensively dolomitized limestones are the potential reservoirs.

ECONOMIC GEOLOGY CONTD. Akande (1999) noted that the recognition of Early Cenomanian - Turonian vein mineral assemblages resulting from hot (ca. 2000C) hydrocarbon-poor, metal- rich saline fluids and a later post-Santonian bitumen-rich assemblage formed from cooler (< 10O0C) less saline fluids, has implications for the timing of hydrocarbon generation, migration and trapping in the Benue Trough.

Conclusion The Benue Trough was formed by rifting of the central West African basement, beginning at the start of the Cretaceous period. Stratigraphically, it can be classified to Asu River Group, Eze Aku Group and Awgu/ Agbani. Economic Minerals within the Basin ranges from Metalliferous minerals, Industrial minerals and rocks, chemical minerals and hydrocarbon.

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