Lithostratigraphy of the early Proterozoic Toumodi Volcanic Group in central côte d'ivoire: implications for Birrimian stratigraphic models

Abstract

The development of stratigraphic concepts for the early Proterozoic Birrimian supracrustals of the West African craton (Guinea Rise) is reviewed, tracing the origins of the upper and lower divisions which are in widespread usage. New data are presented from the central part of the Birrimian Fétékro belt where the Toumodi Volcanic Group (TVG) crops out. The TVG comprises a bimodal succession of predominantly proximal facies volcanogenic rocks which were deformed and metamorphosed during the Eburnian orogeny (c. 2.0 Ga), when greenschist facies assemblages and tight to isoclinal folding developed. TVG rocks are in thrust contact with amphibolite facies gneisses to the East, and both the volcanics and the gneisses have been intruded by Eburnian granites. The TVG stratigraphy has been divided into four formations on the basis of the prevalence of either basic or acid lithologies. Volcaniclastic lithologies are overwhelmingly more abundant than lavas. The basic rocks comprise a high-Mg basaltic suite of clinopyroxene-phyric lavas and clinopyroxene-bearing crystal tuffs and tuffbreccias. The acid rocks include proximal pyroclastics and lavas. The volcanic succession was clearly deposited in a proximal setting. Epiclastic rocks comprising conglomerates and sandstones of volcanic provenance occur in all four TVG formations. An increasingly distal depositional environment has been recognised to the South, where fine-grained laminated tuffs predominate, and it is clear that in this case the division of the Birrimian into upper and lower parts has not recognised the existence of simultaneous deposition of volcanic ejecta in proximal and distal environments, which throws doubt on the general applicability of this stratigraphic model. In addition, field evidence from the Fétékro belt, in which the recently proposed ⪡Burkinian⪢ orogeny was recognised, suggests that the “Burkinian” rests on shaky ground.