Mpraeso Fm

The type area for this Formation has exposures in the face of the Kwahu escarpment. Middle formation of the Kwahu Gr.

Synonym: Mpraeso Sandstone Fm

References: Junner and Hirst, 1946; Saunders, 1970; 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]

Consist of 250-300 m thick, and mainly composed of white to pinky, fine- to medium-grained, quartzitic sandstones. The Mpraeso Fm 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).

fragments of amorphous organic matter (Coueffe and Vecolli, 2011)

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).

: