Yabraso Fm

Type locality of this formation is exposed at the Yabraso drilling well. To the north, the Tossiegou Sandstone Fm (Tossiegous Fm) is the lateral equivalent of the Yabrasso Fm (Affaton et al., 1980). At Fuller Falls there is an exposure of the Yabraso Sandstone Fm. Basal formation of the Kwahu Gr.

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

References: Affaton et al., 1980; Bozhko, 2008; Kalvig and Vosgerau, 2008; 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 pale grey, cross-bedded quartz-arenites with current-rippled bed-tops and sets of herringbone cross-stratification. In the Yabraso Borehole the Yabraso Sandstone is underlain by a further 123m thickness of variegated brown-green mudstones, which in turn rest on 181m of basal quartzitic sandstone (‘First Unit’ of Bozhko, 2008 and ‘Yabraso Sandstone’ of Kalvig and Vosgerau, 2008), the latter resting on Paleoproterozoic basement.At the Yabraso well, the Yabraso Fm is underlain by a further 123m thickness of variegated brown-green mudstones, which in turn rest on 181m of basal quartzitic sandstone (‘First Unit’ of Bozhko, 2008 and ‘Yabraso Sandstone’ of Kalvig and Vosgerau, 2008) - which implies that from the Yabraso Formation of Affaton et al. (1980) through the “First Unit” of Bozhko (2008), to the basal unit called the Yabraso Sandstone by Kalvig and Vosgerau (2008), are all but one Yabraso Sandstone Fm.

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).

The Yabraso Sandstone Formation represents an early episode of fluvial to shallow marine deposition in a high-energy braid-plain with flow to the north and northeast. Sporadic very shallow marine incursions are indicated by herringbone cross-stratification and subordinate SW-directed palaeocurrents (Carney et al., 2010).