Abetifi Fm

The type area is exposed in the tectonically-bounded northeast by the Afram Fault. Middle formation of the Kwahu Gr.

Synonym: Abetifi Sandstone Fm

References: Carney et al., 2008; 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.]

[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]

The 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 Fm, 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 of alternations 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.

Acritarchs (Valeria cf. granulata, Valeria cf. chagrinata,Kildinella cf. ripheica, Leiosphaeridia crassa, and Synsphaeridium spp., Trachysphaeridium spp., Pterospermopsimorpha saccata), pollen

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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