Afram Fm

The type is exposed to the east and north-east, in the Oti River valley and southeast of Damanku village. In exploration boreholes the strata has being penetrated and named the ‘Prang Formation’ Premuase Borehole (Bozhko, 2008).

Synonym: Afram shale, Akroso conglomerate, Lower Green beds (Annan-Yorke, 1971; Blay, 1983)

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

[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]

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 mudflakes 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 Gr. In that the Akroso Member is conglomeratic, it could be said to resemble parts of the Obosum Gr (Dunkro Fm and Sang Fm (Sang Conglomerate Fm)). The Akroso Conglomerate 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 colors, culminating in the development of a true red-bed lithofacies, which is seen below the escarpment of the Ejura Sandstone Fm 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.

Stromatolites

middle Ediacaran (uncertain)

Sediments were deposited in an active foreland basin setting, characterized by shallow to semi-emergent conditions. Higher energy environments are suggested by the Akroso Conglomerate Member, interpreted as proximal marine flysch containing pebbles and boulders transported via alluvial fans and deposited within fan-deltas stacked against the Dahomeyide belt.