Bombouaka Gr

Synonym: Dapango-Bombouaka Gr (Affaton et al., 1980), Lower Voltaian (Annan-Yorke, 1971; Blay, 1983).

References: Affaton et al., 1980; Drouet, 1986; Affaton, 1990; Viljoen et al., 2008; Ayite et al., 2008; Jordan et al., 2009; Abu, 2018; Edmonds, 1956; Anani et al. 2017; Coueffe et al., 2011

[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.]

Panabako Fm: This unit mainly consists of quartz-arenites, which are the probable lateral equivalents of sandstones in the Mpraeso Fm, Abetifi Fm and Obocha Fm of the Kwahu Group. It overlies the Poubogou Fm with defined clastic sediments of Aeolian environment largely with quartzitic and feldspathic sandstones. The formation has paleosol features of pseudokarren structures marking its contact with the overlying Kodjari Fm of the Oti-Pendjari Gr. Remote imagery suggest that the 150-200m thickness of the unit includes two sandstone sequences, the upper of which forms eroded “sugarloaf” cappings above the lower sandstone. This upper sandstone may have been partially eroded prior to deposition of the Oti-Pendjari Gr, since the base of the latter rests unconformably on the gentle, southwards dip-slope of the lowermost Panabako sandstone package (Jordan et al., 2009). Roadside exposures 4.6km west of Langbinsi, comprises fine to medium-grained sparsely feldspathic quartz-arenites that commonly have broad, shallow, swaley or trough-like structures, with unidirectional cross-stratification. A similar distance west of Gambaga, quartz-arenites are moderately feldspathic, with some fine- and coarse-grained, locally gritty layers in tabular cross-stratified beds that locally show isoclinal, syn-sedimentary folding. Plane laminated beds commonly intervene between the cross-stratified sets. Poorly developed herringbone crossbedding and reactivation surfaces have been described from the formation by Viljoen et al. (2008). Current directions show a similar range to the Kwahu Gr, although NW and SE sets are more common. Viljoen et al. (2008) recorded a predominance of NNE over NW and E directions in other exposures.

Poubogou Fm: consists of green-grey, micaceous mudstones and siltstones, commonly with a ‘papery’ fissility. The thin intercalations of siltstone and fine-grained sandstone are typically planar bedded, with sharp top-surfaces and undulatory, slightly channelized bases which in places show spectacular ball-and-pillow load structures (Ayite et al., 2008).It is largely defined by fluviatile and floodplain environments of shales and sandy shales. These have characteristic papery fissility with flute casts, climbing ripples and bifurcated ripple marks (Ayite et al., 2008; Abu, 2018). This formation is approximated to have a thickness of about 170m thick around Larabanga areas.

Tossiegou Fm: In the Gambaga location, Edmonds (1956) demonstrated that the lower part of the formation, which overlies Paleoproterozoic basement, consists of argillaceous strata occurrence in its francophone extension (Affaton et al. 1980), characterized by symmetric and asymmetric ripple marks in the Nakpanduri areas (Anani et al. 2017).These grade up into both feldspathic and ‘clean’ quartzitic sandstones, which are commonly ripple-marked and include coarse-grained sandstones and ‘grits’. Such descriptions, together with similarities in topographic expression, suggest that the Tossiegou Formation is the direct correlative of the Yabraso Fm (Yabraso Sandstone Fm, see that entry).