Sedimentology and biostratigraphy in the Bombouaka-Kwahu Group, Volta Basin, Ghana
Résumé
As demonstrated by several recent publications, geologists still disagree about the lithostratigraphic subdivision of the Volta basin sedimentary infilling. Correlations at the scale of the Volta basin were up to now largely limited by the discontinuity of outcrops along the basin margin and by the scarcity of stratigraphic constraints within the sedimentary succession. Even if a subdivision of the Volta basin infilling into three main groups (Bombouaka-Kwahu, Oti, and Obosum Groups) is now generally accepted, there is no agreement for lithostratigraphic subdivision at the formation scale. The Bombouaka-Kwahu Group, which represents first deposits of the Volta basin sedimentary infilling, is particularly concerned. During mapping project in Ghana (Mineral Sector Support Programme (MSSP) in Ghana, funded by European Union), authors proceeded to a sedimentological study of the Bombouaka-Kwahu Group deposits on the Kwahu Plateau area. Some samples collected during field mapping were analyzed on a palynological point of view and revealed several layers rich in acritarchs. Silicoclastic sediments composing the Bombouaka-Kwahu Group are distinguished in 12 main facies associations which are interpreted to be representative of both marine and continental environments. Sedimentological study of these facies associations leads to identify a cyclic vertical evolution of deposition settings at different scales, which are interpreted as sequences. The Bombouaka-Kwahu Group is thus composed of three distinct macrosequences (about 300 to 500 meters thick) interpreted as deltaic progradational sequences. Each macrosequence has been used to define formations for mapping, sequence stratigraphy key surfaces being considered as limits for cartographic units. Comparison of sedimentological data collected on the Kwahu Plateau, with those recently published for the Volta basin western margin (Kintampo Massif), shows similarities in sequential signals that are suggestive of a regional continuity of macrosequences. Even if stratigraphic resolution of palynological ages is quite coarse in the Meso - Neoproterozoic successions, the acritarch content of collected samples provides new stratigraphic constraints which, with sedimentological and sequential analysis, is used to discuss lithostratigraphic correlations into the Bombouaka-Kwahu Group at the scale of the Volta basin. Two samples provide acritarch assemblages that are indicative of a post-Mesoproterozoic, early Neoproterozoic (Tonian) age for the two upper macrosequences / formations. The stratigraphic interval deduced from acritarch assemblages is consistent with available stratigraphic data in the Volta basin and particularly with the Rb/Sr age obtained in the Fosse-aux-Lions / Poubougou Formations defined on the northern margin. The stratigraphic equivalence between Fosse-aux-Lions / Poubougou Fm and Abetifi-Anyaboni Fm, and the regional continuity of macrosequence signals are used to discuss correlations at the scale of the Volta basin, between numerous lithostratigraphic subdivisions published on the Bombouaka-Kwahu Group. The correlation table of lithostratigraphic subdivisions we proposed needs to be confirmed by new data acquisition in order (i) to improve stratigraphic constraints for the Bombouaka-Kwahu Group and (ii) to identify and compare preserved macrosequence signals along Volta basin northern margin. If correlations hypotheses are confirmed, geological map of the Volta basin deposits could be largely updated and the knowledge of the beginning of Volta basin history better constrained.