Kwahu Gr

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

References: Bozhko, 2008; Kalvig and Vosgerau, 2008; Carney et al., 2010; Zobah et al., 2022

[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]

Anyaboni Fm is about 200-350 m thick, is composed of at least 5 different facies associations. The most representative facies are massive, reddish, medium-grained, feldspathic sandstones that constitute the upper unit. The lower unit, 130 m thick, was incompletely recognized due to lack of outcrop in its upper part but outcrops present alternations of shales and siltstones and a singular fine-grained sandstone body interbedded (Coueffe et al., 2011). In the lower unit, it consists of rhythmic alternations between (i) grey to pale yellow, locally finely laminated, micaceous silty shales and (ii) lenticular, siltstone to very fine-grained sandstone millimetric lamina and centimetric beds. Some centimetric beds of very fine-grained sandstones present beddings representative of unidirectional current ripple sets oriented to the North-East, and more rarely graded beddings. Some of the thickest sandstone beds (up to 15 cm thick) present surfaces with interference ripples, wave ripples and hummocky cross stratifications. Close to the top, a massive siltstone body, 20-30 m thick, constitute a singular level of extensive lateral continuity. It is typically composed of decimetric beds of homogeneous, well-sorted, highly micaceous siltstones to very fine-grained sandstones. Sedimentary structures are predominantly represented by thin planar beddings and very low-angle cross-beddings. Planar laminations and curved cross-laminations that evoke swaley cross-stratification are frequent, and extensive planar erosive surfaces with a very low dip are characteristic. Transition between lower and upper unit of the Anyaboni Fm was unfortunately impossible to study because of a lack of outcrops. Together with the basal shales and siltstones, the micaceous strata of the Anyaboni Fm are estimated to be about 130mthick on the Kwahu Plateau (Carney et al.,2010).

Obocha Fm consist 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).

Abetifi Fm was previously defined as a geological unit mainly composed and represented by homogeneous lithologies dominated by coarse-grained and pebbly sandstones. But, the Abetifi Formation, about 350-450 m thick, is made of the alternation of 4 distinctive facies associations, the succession of which records an overall coarsening-up evolution. On the basis of dominant lithologies and sedimentological criteria, two different units are distinguished (Coueffe et al., 2011). The Abetifi Formation consists in the alternation of 4 main facies association:

- Finely laminated shales with millimetric siltstone to very fine-grained sandstone lamina, showing lenticular-beddings, planar laminations, current ripples, and sometimes graded-bedding;

- Decimetric alternations between siltstones and well-sorted, fine-grained, sandstones. Shale drapes form typical lenticular- to wavy-beddings. Mud pebbles reworked into fine-grained sandstones are common. Sedimentary structures are represented by planar laminations, current and wave ripples, and rare hummocky cross stratifications;

- Thin-bedded, relatively well-sorted, fine- to medium-grained sandstones. Mud pebbles and millimetric rounded quartz grained layers are very frequent. Flaser- and wavy-beddings are dominant. Surfaces of beds often show current and wave ripples. Sedimentary structures are represented by planar and trough cross-beddings, with some herring-bones and frequent planar laminations;

- Highly heterometric, unsorted, medium- to very coarse-grained sandstones, which contain centimetric layers of microconglomerates and decimetric horizons of conglomerates. Sedimentary structures are mainly represented by cross-beddings (planar and trough cross-beddings, herring-bones). Stratigraphic surfaces often present ripple marks, with predominant asymmetric ripples locally reworked to take appearance of symmetric ripples. Channels, some meters in width and some decimeters in depth, are relatively common.

Mpraeso Fm consist of 250-300 m thick, and mainly composed of white to pinky, fine- to medium-grained, quartzitic sandstones. The Mpraeso Formation locally begins with some meters of shales, siltstones, and intercalations of decimetric to metric beds of fine-grained sandstones, forming a lower unit which is overlain by the upper unit through a sharp planar (at outcrop scale) contact. This formation was first described by Junner and Hirst (1946) as the lower part of the Basal Sandstones of the Kwahu Sandstones. It also corresponds to the thin-bedded quartz sandstones identified by Saunders (1970) at the base of Kwahu Sandstones. The main lithological features of the Mpraesso Fm are Grey, laminated argilites, containing millimetric planar lenticular-beddings; centimetric planar beds of siltites, showing wavy to lenticular beddings, symmetric ripples, graded-bedding and locally load casts; decimetric beds of well-sorted, fine-grained, quartzitic sandstones showing flaser-beddings, unidirectional current ripples, wave ripples and hummocky-cross stratifications; less frequently, decimetric beds of relatively well-sorted, medium-grained, quartzitic sandstones, with abundant planar cross-beddings. Layers with millimetric rounded quartz grains and centimetric soft pebbles are currently observed (Coueffe et al., 2011).

Damongo Fm shows rapid alternations between flaggy, fine to medium-grained, micaceous, quartzose or feldspathic sandstones and micaceous siltstones, exemplified in the type area around Damongo. Sedimentary features include mudflakes, trough and tabular cross-bedding, cross-lamination and structures resembling hummocky cross-stratification; northerly directed palaeocurrent flows are dominant (Carney et al., 2010). In the south, the formation is considered a lateral equivalent of the Agogo Clay Gall Sandstone of Mason (1963), due to common occurrence of ripple marks, mud flakes and mud cracks.

Yabraso Fm: 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.