Anyaboni Fm

The Anyaboni Formation was first recognized by in the southeastern part of the Kwahu Plateau (Saunders,1970). The type locality is also exposed in a small quarry along the foot of the escarpment to the east of the Abetifi-Adowso road. Uppermost formation in the Kwahu Gr.

Synonym: Anyaboni Sandstone Fm

References: Tevendale, 1957; Saunders, 1970; Davis and Fitzgerald, 2004; Carney et al., 2010; 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 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 130m thick on the Kwahu Plateau (Carney et al.,2010).

pollen, acritarchs (Arctacellularia tetragonala, Navifusa majensis, Pterospermopsimorpha insolita, Leiospaeridia crassa, Leiosphaeridia minutissima, and Valeria lophostriata)

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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