Asu River Gr

The Asu River Group is subdivided into units (members or formations) by Nwajide (2013). The Asu River Gr in the Southern Benue Trough comprises the upward succession of the Ogoja Sandstone Fm, Awi Fm, Mamfe Fm, Abakaliki Fm (Abakaliki Shale Fm), and Mfamosing Limestone Fm (Reynment, 1965, and retained by Nwajide, 2013). For Benue Trough Central, it is an upward succession of the Bima Fm, Uomba Fm, Arufu Limestone Fm, Gboko Limestone Fm, and the Awe Fm which tops the sequence. They are all primarily exposed in the Benue Trough Central, particularly around Arufu Town, Uomba River, and near Yandev-Tiv along the Gboko-Makurdi Road in southeastern Nigeria. The Asu River Group also has outcrops exposed in the Keana anticline east of Keana town and south of Azara; and in the area around Gboko with a typical section in the Quarry of the Benue Cement Company (now Dangote Cement Co. PLC) near Yandev and the Awe Formation occur around the town of Awe.

Synonym: This suite of formations is also referred to as the Asu River Group (Offodile, 1976; Petters and Ekweozor, 1982; Nwajide, 1990) and Asu River Series (Simpson, 1955)

References: Reyment, 1965; Adeleye, 1975; Kogbe, 1976; Dessauvagie, 1975; Petters, 1978; Offodile, 1980; Whiteman, 1982; Benkhelil, 1989; Okosun, 1992; Guiraud, 1993; Akande et al., 1998; Zaborski et al., 1998; Jauro et al., 2007; Obaje, 2009; Nwajide, 2013

[Fig 1. Stratigraphic successions in the Benue Trough and the Nigerian sector of the Chad Basin]

Fig. 2 Sedimentary Basins of Nigeria (Obaje, 2009)

The lithologic composition of the Asu River Gr comprises limestones, shales, micaceous siltstones, mudstone sand clays (Offodile, 1976; Obaje, 1994). The average thickness is estimated to be about 1,800 m.

In CENTRAL Benue Trough:

Awe Fm consists of flaggy, whitish, medium to coarse-grained calcareous sandstones, carbonaceous shales and clays, and has been estimated by Offodile and Reynment (1977) to be about 1000m (1km) thick.

Gboko Limestone Fm: Micritic limestone with oolites and oncoids whose nuclei are made of gastropods and bivalve shells. The shale is thick, fissile, carbonaceous, and contain thin beds and lenses underlying a thick series of siltstones intercalated with thin beds of sandstones (Whiteman, 1982). Thickness ranges from 10-15m.

Arufu Limestone Fm: is an ammonite-laden limestone containing Albian species Elobicerasnewtoni Spath. No thickness data available for this member and even considered insignificant by Whiteman (1982).

Uomba Fm: is composed of sandstones, sandy shales, and shales. Thickness data is not available for this formation.

In SOUTH Benue Trough:

Ogoja Sandstone Fm: The Ogoja Sandstone (lowest unit in the Azu River Gr) lies directly on the basement complex and its thickness is not known for certain. It is composed of conglomerates and arkosics and sandstones. Ogoja Sandstone is exposed in both Ikom and Ogoja Areas (Uzuakpunwa, 1980; Petterset al., 1987).

Awi Fm: it is the basal non-calcareous, sandy, conglomeratic unit of the Asu River Group also directly overlying the Basement Complex but to the north of Calabar. The conglomeratic beds are typically succeeded by the coarse to very coarse pebbly sandstones, grading upwards into medium to fine sandstone, siltstone/claystone and black carbonaceous shale. It has two horizons of thin black, carbonaceous, lignitic and pyritic shale markers that contain poorly preserved shallow marine foraminifera. Awi Fm also forms part of the basal unit of the Asu-River Group and directly overlies the Basement Complex in the North of Calabar. Its type section is exposed at a road cut some 9km south of the Awi Village – which is actually located on the basement complex (Nwajide, 2013).

Mamfe Fm: Mamfe Formation is about 800m thick and consists arkosic sandstones, with intercalations of marls, sandy limestones and shales. The sandstones are coarse to pebbly cross-bedded. There are also plant fragments and lignitic beds (Nwajide, 2013). Reyment (1965) established its type locality along Cross River at Mamfe town which is now in the Cameroon Republic.

Abakaliki Fm (Abakaliki Shale Fm), which has an average thickness of about 500 m, is dominantly shale, dark grey in color, blocky, and non-micaceous in most locations. It is calcareous (calcite-cemented) and deeply weathered to brownish clay. The top is sometimes identified in boreholes with the commencement of rapidly alternating beds of shale and mudstone which characterize the latter unit. The formation is estimated at 2 500m thick based on the covariance of illite abundance with depth of burial (Dunnoyer de Seconzac et al., 1967; Agumanu, 1986). This unit was described by Reynment (1965) as having it type locality in the Abakaliki Town. The type area of the Abakaliki Fm is exposed in the gently undulating terrain in the general area lying between Ezambgo and Abakaliki township. The formation was described as the Abakaliki Shale in the type area (Reyment, 1965). The type locality is at Mgbo village, about 7km north of Ezamgbo and 16km west of Abakaliki town. Ebonyi Fm Type area. The formation underlies a gently undulating terrain in the Ntezi-Ezamgbo area and southwards to Amagu-Agba.

Mfamosing Limestone Fm: This highest formation in the Azu River Gr has an estimated thickness of 300m (Nwajide, 2013). It consists of a sanding upward shelly limestone with pelecypods, gastropods, and foraminifera as well as fragmentary crinoids and algae in the Odukpani area where its exposure is about 25m thick forming the very base of the formation. The next unit consists of an alternation of shales and limestones, with ammonites of Cenomanian age. The topmost unit consists of flaggy black shales and calcareous sandstones with Lower Turonian ammonites. 25m of the Mfamosing Limestone is exposed in the Odukpani area.

In Benue Trough Central -- Ammonites in the Arufu Limestone Fm (Elobiceras) and Uomba Fm, Bivalves, Foraminifera, Echinoids (Arufu Limestone Fm), Pelecypod and Gastropod Shells (Gboko Fm), Sponge spicules. In Benue Trough South -- Ammonites (Diploceras quadratum, Hysteroceras binum, Elobiceras lobitoense, Gyaloceras ibo, Mortoniceras ishiaguense, Acanthocerass p., Acompoceras sp.), gastropods, pelecypods (Anatinaagassizi, Panopea cf. gurgitis), foraminifera, brachiopods, crinoids, algae (Cayeuxia, Pianella – Reijers and Petters, 1997), plant remains, Thalassinoides and Planolites.

Middle Cretaceous. The Asu River Gr on the schematic Central Benue Trough stratigraphy figures is indicated as uppermost Aptian through Albian. Therefore, for that region, the four main formations are given semi-equal duration here: Uomba Fm = uppermost Aptian-lowermost Albian, Arufu Limestone Fm = lower-middle Albian, Gboko Limestone Fm = upper-middle Albian, and the Awe Fm = upper Albian. However,in the South Benue Trough, it includes the Mfamosing Limestone Fm, which spans the Cenomanian; therefore the span in that region is shown as Aptian through Cenomanian according to strat-chart used by Obaje (2013). Therefore, in South Benue Trough, other than the Mfamosing Limestone Fm, which spans the Cenomanian, (but top black shales of underlying Mfamosing Limestone Fm has lowermost Turonian ammonites) the other 4 formations are given sub-equal duration in the Aptian-Albian interval.

In the Benue Trough Central, the Arufu Fm, Uomba Fm and Gboko Fm collectively represent a sequence of depositional environments transitioning from fluvio-deltaic to deeper marine and carbonate platform settings. In the Lower Benue Trough, it is an upward succession of alluvial to marine depositional environments which marked a crucial juncture in the geological history of this Lower Benue Trough, marking the beginning of significant sedimentation associated with the rifting and marine transgression during the Early Cretaceous.