Obosum Gr

Synonym: The Obosum Group is partly equivalent to the ‘Obosum Beds’ of Junner and Hirst (1946). ‘Tamale Supergroup’ (Affaton et al., 1991; Bertrand-Sarfati et al., 1991) and ‘Tamale Megasequence’ (Deynoux et al., 2006).

References: Sougy, 1970; Affaton et al., 1980, 1991; Bertrand-Sarfati et al., 1991; Deynoux et al., 2006; 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].

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

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

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

Tamale Fm consists of red, medium-grained, moderately well-sorted quartz-rich sandstone with rounded to subangular grains and dominated by a large-scale trough cross-bedding. In other Location Tamale exposures show pink to maroon-weathering, flaggy, very finely laminated sandstone and siltstone with grey mudstone partings; ripple-drift cross-lamination and lunate to sinuous-crested ripples of typical bedding plane structures. The limited paleocurrent

data, suggest a predominantly southerly flow.

Densubon Fm consists of pink to grey feldspathic arenites predominate, intercalated with subordinate beds of variegated, red or green mudstone and siltstone. The sandstones show fine-scale trough cross-lamination and contain red or green mud flakes. About 5.7 km south of Densubon, pink, medium- to fine-grained, highly micaceous arkosic sandstones with local trough cross-lamination are present in roadside exposures. To the west, exposures along the Ejura-Atebubu road show pink, fine-grained, micaceous and feldspathic quartz-arenites with trough cross-lamination and maroon mud flakes. Although compositional maturity is relatively high in these sandstones, beds of more immature dark red, micaceous, fine- to medium-grained feldspathic and lithic arenites are also present (Carney et al., 2010).

Dunkro Fm consists of pink, medium- to coarse-grained, feldspar-lithic arenites with maroon mud flakes and mud-flake-conglomerates and having internal structures such as lateral accretion surfaces and cross-stratification. Cross-beds are locally oversteepened, and some bedding is chaotic. Massive sandstones occur in units several meters thick, some containing ‘floating’ pebbles, cobbles and boulders of basement rocks, including mesocratic gneiss. Cobble conglomerates in lenticular beds up to 2m thick are poorly sorted, massive and matrix-supported; bed bases are commonly strongly erosive. Clasts are subangular to well-rounded and include: green chert, various granitoid and foliated metamorphic lithologies and dark green-grey porphyritic volcanic rock.

Sang Fm consists of massive, well-cemented cobble and pebble conglomerate is interbedded with green-grey lithic and feldspar-rich sandstone bearing pebbly layers and lenticles. Pebbles include quartz and quartzitic sandstone, together with ‘basement’ lithologies such as green metavolcanic rock, jasper, porphyritic dacite, garnetiferous psammite and various types of schist and phyllite. Clasts are locally closely packed, but mostly are supported within a dark grey, pink or purple, poorly sorted, coarse sand to granule grade matrix.