Gboko Limestone Fm

The Asu River Gr in the Central Benue Trough is subdivided into five units (members or formations) by Nwajide (2013) as 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 have also have 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:

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)

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.

Ammonites in the Arufu Limestone Fm (Elobiceras) and Uomba Fm, Bivalves, Foraminifera, Echinoids (Arufu Limestone Fm), Pelecypod and Gastropod Shells (Gboko Fm), Sponge spicules.

Middle Cretaceous. The Asu River Gr is indicated as uppermost Aptian through Albian on the schematic Benue Trough stratigraphy figures. Therefore, its 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

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.