Bimbila Fm

This unit has been identified as sections drilled in the Nasia Borehole and outcrops exposure 2 km east of Yendi town. The Members of the formation are exposed as roadside cutting 11.5km west of Sang town and around Bunya village (Carney et al., 2010) and having 2 beds, which define the formation’s boundaries; Chereponi Sandstone Member is the basal stratum for the formation, and the Bunya Sandstone Member constitutes the exposed upper part of the formation over an extensive area. The stratigraphical separation between these sandstones narrows and eventually disappears as their outcrops merge together, indicating a progressive southwards and westwards thinning 539m in the Nasia Borehole to 25m in boreholes farther west (Bozhko, 2008).

[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]

References: Hantzschel and Reineck, 1968; Annan-Yorke, 1971; Affaton et al., 1980; Blay, 1983; Saito and Ito 2002; Narbonne and Gehling, 2003; Draganits and Noffke, 2004; Mutti et al., 2007; Porada and Bougri, 2007; Scheiber et al., 2007; Bozhko, 2008; Viljoen et al., 2008; Carney et al., 2010;

This formation represents a continuation of foreland basin deposition, and mostly consists of green to khaki, micaceous laminated mudstones, siltstones and tabular, sharp-based sandstones. The siltstones typically occur in thin, tabular beds with wind-rippled tops and low angle cross-bedding. The sandstones are thinly intercalated within the unit and also include invariably pale grey-green, fine-grained lithic and feldspathic wackes, which are angular to subangular grains of plagioclase, K-feldspar and unstable detrital silicates such as clinopyroxene, hornblende and chlorite are abundant. Lithic grains include various phyllitic lithologies and microcrystalline quartz (probably jasper). Paleocurrent from the Bunya Sandstone, are mainly S to SW or W. The basal Chereponi Sandstone Member, have an estimated thickness of 40-60m, forms a ridge-like feature that can be traced over a strike-length of about 210km. Above it, the Bimbila Fm coarsens up-section through the intercalation of increasingly numerous beds of wacke-type sandstone, 1-3m thick on average. Just below the Bunya Sandstone escarpment, about 2 km east of Yendi, slabs of locally-derived fine-grained wacke sandstone in a hand dug well show successive bedding planes covered by branching, bulbous impressions, which resemble Kinneyia-type ‘wrinkle structures’, caused by the trapping of gas below a cohesive microbial mat (Porada and Bougri, 2007). They partially overprint an elongated ribbed or segmented form, which is possibly a vendobiont. The tops of some in situ beds at this locality show possible microbial build-up structures, akin to siliciclastic stromatolites (Draganits and Noffke, 2004). Kinneyia structures have been described from Late Neoproterozoic strata in Namibia (Hantzschel and Reineck, 1968; Scheiber et al., 2007) and are considered typical of the Ediacaran biota, the age-range of which (Narbonne and Gehling, 2003) suggests that this upper part of the Oti-Pendjari Group is probably no older than about 575Ma. The Bunya Sandstone Member, at the exposed top of the Bimbila Formation, is about 50m thick. Dip-slopes revealed by DTM imagery define a shallow, arcuate structure named as the Daka Syncline, confirmed by an easterly structural dip, in a roadside cutting through the Bunya Sandstone escarpment 11.5km west of Sang. The type locality, around Bunya village, shows green-grey, medium to thick-bedded, feldspathic and lithic-rich wacke sandstone. Sedimentary structures include large scoop shaped scours and locally chaotic bedding due to syn-sedimentary slumping; disturbance of bedding suggesting water-escape is also evident. Generally, beds are normally graded and contain lenses of mudflake-conglomerate.

Ediacaran biota

middle Ediacaran

Foreland basin depositional environment. In sandstones of the Chereponi and Bunya members, the abundance of unstable detrital minerals, sheet-like geometry and features such as grading and water-escape structures indicate deposition by sediment gravity flowage. This facies has been interpreted as the deposits of a prograding submarine fan (Viljoen et al., 2008) and it resembles unconfined turbidite packets (Saito and Ito 2002; Carneyet al., 2010). The copious supply of sandy material to the Chereponi and Bunya members could reflect deposition from dense hyperpycnal flows originating from the fronts of small, coalesced deltas or fan-deltas, a type of scenario envisaged by Mutti et al. (2007) to have developed along the margins of orogenic belts.