The sediment-dominated Boundiali-Bagoé supracrustal belt and neighbouring granitic rocks, northern Côte d'Ivoire, West Africa: A Tarkwaian connection?

The c. 2. 1 Ga sediment-dominated Boundiali-Bagoé supracrustal belt is situated in the lower Proterozoic Baoulé-Mossi domain of the West African Craton. The sedimentary units are characterised by an abundance of clastic turbiditic wackes and quartz wackes of unknown stratigraphic thickness and a 3.5 km thick sequence of poorly sorted, mass-flow paraconglomerates and metasubarkose. Volcanic assemblages form elongate bodies comprising metabasalts, meta-andesites, meta-andesitic volcaniclastics and epiclastics with minor jasperoidal metarhyolites. Contacts between the metasediments and metavolcanics are not exposed, but from structural evidence most relationships are thought to be tectonic. The volcano-sedimentary terrane is enveloped, and rarely intruded, by syn-tectonic granitoids and basic bodies. Post-tectonic granitoid emplacement marks the youngest Birimian magmatic episodes in the region. The metavolcano-sedimentary rocks and earlier granitoids were variably deformed during the Eburmian orogeny which is dominated by a generally NNESSW structural trend.

Siliciclastic metasedimentary rock units in other parts of the Baoulé-Mossi domain, namely the young, goldbearing Tarkwaian conglomerates in Ghana, have been compared lithostratigraphically to the coarse-grained clastic metasediments in the Boundiali-Bagoé supracrustal belt. However, extrapolation of the term “Tarkwaian” outside the type locality, even to detrital rocks closely analogous in terms of their sedimentary facies with the Tarkwaian of Ghana, is dangerous when little or no consideration is paid to such factors as local stratigraphic positioning. The style and timing of tectonism are controlling factors determining the occurrence and nature of sediment deposition, in terms of both the local and regional Birimian stratigraphy. Across the lower Proterozoic of West Africa diachroneity in tectonism and sedimentation leads to the formation of pockets of coarse-grained clastic sediments that occur at different stratigraphic positions during the development of individual supracrustal assemblages.

Without geochronological data, the correlation of these metasediments across West Africa is problematical and of limited significance.