Kodjari Fm

References: Junner and Hirst, 1946; Annan-Yorke, 1971; Affaton et al., 1980; Blay, 1983; Affaton, 1990; Viljoen et al., 2008; Nédélec et al., 2007; Bozhko, 2008; Jordan et al., 2009

[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]

This formation contains a distinctive ‘triad’ of lithologies: (a) basal tillites and diamictons; overlain by (b) a ‘cap-carbonate’ (Buipe Limestone Member), the age of which is discussed above; and then by (c) laminated tuffs and ash-rich siltstones (Darebe Tuff Member). In northern outcrops, the Darebe Tuff forms a distinctive cuesta overlying the unconformity with the Panabako Fm. This feature is not seen in the south of the basin, although there, the presence of the Kodjari Fm is implied, by the narrow green and yellow-colored conductive strata immediately above the subsurface extension of the Anyaboni Fm. Several centimeters of pale grey mudflake-conglomerate succeed the diamictte, which is overlain in turn by laminated, chert and barite-veined ooidal carbonate forming the base of the Buipe Limestone Member. Farther south, around Buipe, tillite is locally up to 10m thick and comprises gritty, green-grey clay with clasts of Paleoproterozoic basement up to boulder size (Junner and Hirst, 1946). In the borehole at Buipe, Bozhko (2008) recorded weathered sandstone overlain by 62m of a ‘tillite-like conglomerate’ with various basement clasts enclosed in an unsorted, quartzitic to feldspathic sandy matrix, perhaps similar to that described above near Daboya. This is overlain by 48m of diamicton with a dark green, argillaceous matrix. The location of the Buipe borehole close to a major Neoproterozoic paleo-valley cut into the Kwahu Group suggests that this particularly thick tillite-bearing association represents part of an infill sequence. South of Gambaga, basal glacigenic strata are included within the outcrop of the Kodjari Fm; they consist of calcitic diamictite and are referred to a ‘Kudjawen Formation’ by Viljoen et al. (2008).

The Buipe Limestone Member (Cap carbonate, mainly studied in Togo and Burkina Faso where dolostone passes up into shales with beds of pure micrite limestone). Cannot everywhere be depicted on maps due to its commonly narrow outcrop. Its distribution is therefore more extensive than can be shown, which illustrates the particularly wide outcrop around Buipe, in the northwestern part of the Volta Basin. To the southwest of Buipe Township, the ‘Buipe No.3’ limestone quarry exposes 2-4m vertical sections in pale grey or blue-grey, laminated, thinly bedded and ripple-marked micritic limestone with regular alternations of dark maroon, finely laminated, micaceous siltstone. These beds overlie yellow, thinly bedded dolostones. Nédélec et al. (2007) measured a 25m thick profile through the member at Buipe, although the location of their stratigraphical section was not specified. Above tillite, they describe about 11.5m of yellow to pale pink dolostone containing fenestral, birds-eye and peloidal structures indicating marine, microbial activity.

The Darebe Tuff Member has a gradational contact with the Buipe Member. In the type area around Darebe, it is readily distinguishable by its 20 m high escarpments and corresponding dip-slope (Jordan et al., 2009). Using this feature as a marker on remote imagery, we note that the member is also developed farther west, between Buipe and Daboya (Fig. 2). It may extend north of the latter area (Affaton, 1990), but cannot be identified as a feature-forming unit on remote imagery. In the outcrops around Darebe, and generally south of Gambaga, the member typically consists of crimson-weathering, pale grey, highly siliceous (‘flinty’), finely laminated vitric tuff or tuffaceous siltstone. In thin section, the lithology shows abundant parallel-orientated slivers of microcrystalline quartz and feldspar, some with incurved margins characteristic of the bubble-wall textures associated with volcanic ash shards. The Darebe Tuff occupies the same stratigraphical position as ashy and ignimbrite horizons forming part of the ‘silexite’ sequence in the Kodjari area of neighboring Burkina Faso (Affaton et al., 1980).

Tiélé Volcanic Fm are tholeiitic basalts; not in Dahomey thrust sheet (in Benin; and also Kpandu volcanics in Ghana) = possible source for Darebe Tuff Mbr of Kodjari Fm.