Oti-Pendjari Gr

Synonym: Pendjari Group (Affaton et al., 1980), Middle Voltaian (Annan-Yorke, 1971; Blay, 1983)

References: Junner, 1940; Junner and Hirst, 1946; Annan-Yorke, 1971; Affaton et al., 1980; Blay, 1983; Affaton, 1990; Petters, 1991; Bozhko, 2008; Jordan et al., 2009; Carney et al., 2010

[Fig. 1. Geological map of the Volta Basin and surroundings, after Sougy (1970) and Affaton et al. (1980)]

[Fig. 2. Schematic lithostratigraphic sections showing principal lithologies and inferred correlations between Kwahu and Bombouaka group units across the Volta Basin.]

[Table 1. Published stratigraphic data available for Voltaian deposits]

[Table 2. Proposed lithostratigraphic scheme (left-hand column) compared with previous nomenclature.]

[Fig. 3. Synthetic lithologic section of the Volta Basin sedimentary infilling]

Bimbila Formation consists of sandstones as the major unifying feature and having 2 beds, which define the formation’s boundaries; Chereponi Sandstone Member is the basal stratum for the formation, and the Bunya Sandstone Member constitutes the exposed upper part of the formation over an extensive area. The stratigraphical separation between these sandstones narrows and eventually disappears as their outcrops merge together, indicating a progressive thinning southwards for the formation. A corresponding westwards thinning, from 539m in the Nasia Borehole to 25m in boreholes farther west (Bozhko, 2008), is similar to the westward thinning of the underlying Afram Fm and implies a lens-like geometry for the unit. This formation represents a continuation of foreland basin deposition, and mostly consists of green to khaki, micaceous laminated mudstones, siltstones and tabular, sharp-based sandstones. The siltstones typically occur in thin, tabular beds with wind-rippled tops and low angle cross-bedding. The sandstones are thinly intercalated within the unit and also include invariably pale grey-green, fine-grained lithic and feldspathic wackes, which are angular to subangular grains of plagioclase, K-feldspar and unstable detrital silicates such as clinopyroxene, hornblende and chlorite are abundant. Lithic grains include various phyllitic lithologies and microcrystalline quartz (probably jasper). Paleocurrent from the Bunya Sandstone, are mainly S to SW or W.

Tease Sandstone Fm (Tease Fm) consists of sandstone is a pale grey, well cemented, medium- to coarse-grained feldspathic quartz-arenite. Although the sandstone is generally well-sorted, there are lenses with coarse grains and granules of quartz; mudflakes are also present. Current directions measured from trough, tabular and herringbone cross-stratification is similar to those for the Ejura Sandstone Fm.

Ejura Sandstone Fm (Ejura Fm) consists of very mature quartzite, showing channels and possible symmetric, standing wave-ripples, having an 80-100m in thickness. It is a pale grey to cream, medium- to locally coarse-grained quartzite showing tabular, thin to medium bedding with some trough cross-stratification. Locally, bed-tops contain large-wavelength, slightly asymmetrical ripple-like features. These are not cross-laminated internally, but draped by successive sandstone beds of approximately equal thickness. The bases of some beds are erosional; hummocky, downlapping bedforms are also locally present. Elsewhere on the outcrop, similar quartz-rich sandstones are commonly gritty with moderate sorting in thin to very thin and tabular to weakly lenticular beds. These have wavy form in places and trough cross-bedding is developed locally. Paleocurrent directions are somewhat variable to the S, SE, SW and NW (Carney et al., 2010).

Afram Formation consists predominantly argillaceous unit which have intercalations of fine-grained, grey sandstone and papery laminated green-grey mudstones with mudcracks. Sandstones, in beds up to several centimeters thick, are lithic wackes with mud flakes and ripple-drift cross-lamination, the latter indicating south-directed paleocurrents and estimated in the east this unit attains a thickness of 530m. In the southeast, the formation includes the Akroso Conglomerate Member. This is now mostly inaccessible on small islands in Lake Volta and its outcrop is seen from early geological maps (Junner, 1940), which show a possible northwards extension of this unit to the east of the Oti River. We note that Junner and Hirst (1946) placed the ‘Akroso Conglomerate’ within the ‘Oti Beds’ (Oti-Pendjari Group of this paper), whereas others (Affaton, 1990) preferred to include it within the lower parts of the Obosum Group. In that the Akroso Member is conglomeratic, it could be said to resembleparts of the Obosum Group (Dunkro Sandstone and Sang Conglomerate). The Akroso Member appears to occupy the core of an asymmetric anticline or faulted flexure related to the nearby Pan-African frontal thrust, with strata on the steeper eastern limb dipping at angles of between 40^◦ and 55^◦in easterly directions. Some conglomerate beds are evidently up to 60m thick (Junner and Hirst, 1946); they are usually described as massive, unsorted aggregations of well-rounded pebbles and boulders measuring up to 0.6m in dimension, in a matrix of arkosic sandstone. Most clasts are of brown and green feldspathic sandstone, but there are minor proportions of ‘basement’ material, consisting of granite, porphyry, chert, jasper, phyllite, limestone and quartz. In the western Afram valley outcrop, the formation becomes increasingly variegated in red, brown or purple colours, culminating in the development of a true red-bed lithofacies, which is seen below the escarpment of the Ejura Sandstone south of Ejura. This roadside section features red-brown to maroon, rarely green grey, laminated mudstones with sporadic intercalations of thin (5-40 mm), hard beds of grey-green, weakly calcareous siltstone (Jordan et al., 2009). The calcareous beds show climbing ripple cross-lamination, with one current direction measured towards the northeast. Fine-scale textures, reminiscent of algal mats, are visible on some bed surfaces.

Kodjari Fm consists a distinctive ‘triad’ of lithologies: (a) basal tillites and diamictites overlain by (b) a ‘cap-carbonate’ (Buipe Limestone Member), and then by (c) laminated tuffs and ash-rich siltstones (Darebe Tuff Member). Several centimeters of pale grey mudflake-conglomerate succeed the diamictite, which is overlain in turn by laminated, chert and barite-veined ooidal carbonate forming the base of the Buipe Limestone Member. Farther south, around Buipe, tillite is locally up to 10m thick and comprises gritty, green-grey clay with clasts of Paleoproterozoic basement up to boulder size (Junner and Hirst, 1946). In the borehole at Buipe, Bozhko (2008) recorded weathered sandstone overlain by 62m of a ‘tillite-like conglomerate’ with various basement clasts enclosed in an unsorted, quartzitic to feldspathic sandy matrix, perhaps similar to that described above near Daboya. This is overlain by 48m of diamictite with a dark green, argillaceous matrix.