Obocha Fm

The type locality is exposed at the Obocha escarpment, about 15.5 km north-east of Koforidua (Carney et al., 2010). Upper-middle formation of the Kwahu Gr.

Synonym: Obocha Sandstone Fm, Lower Voltaian (Annan-Yorke, 1971; Blay, 1983).

References: 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]

Consists of argillaceous basal facies seen along the foot of the Obocha Formation escarpment, where several meters of green-grey to brick red, micaceous massive to laminated mudstone and siltstone contain dark grey concretions of phosphate-cemented mudstone. The upper part of the formation is dominated by thick-bedded, pale grey quartz-arenites. Many exposures show rather poor sorting, with beds c. 0.8m thick fining upwards from flaggy medium-grained sandstones, with thin (1-2 cm) quartz granule layers and lenticles, into fine-grained sandstones (Carney et al., 2010).

Early Tonian. The 6 formations of Kwahu Gr - upward succession of Yabraso Fm (Yabraso Sandstone Formation), followed by the Damongo Formation, Mpraeso Fm (Mpraeso Sandstone Formation), Abetifi Fm (Abetifi Sandstone Formation), Obocha Fm (Obocha Sandstone Formation) and Anyaboni Fm (Anyaboni Sandstone Fm) - are arbitrarily given equal durations (5% of Tonian span; from 0 to 30% up in Tonian).

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