New sedimentological and biostratigraphic data in the Kwahu Group (Meso- to Neo-Proterozoic), southern margin of the Volta Basin, Ghana: Stratigraphic constraints and implications on regional lithostratigraphic correlations

Abstract

Since several decades, geologists 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, Oti, and Obosum Groups) is now generally accepted, there is no agreement for lithostratigraphic subdivision at the formation scale. The Bombouaka Group, which represents first deposits of the Volta Basin sedimentary infilling, is particularly concerned. During mapping project in Ghana, authors proceeded to a sedimentological study of the 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 palynomorphs (acritarchs). Siliciclastic sediments composing the 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 depositional settings at different scales, which are interpreted as sequences. The Kwahu Group is thus composed of three distinct macrosequences (about 300–500 m 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. The acritarch content of the analyzed samples allows rather coarse but nonetheless useful biostratigraphic constraints which, in association with data from sedimentological and sequential analysis, are used to improve correlations into the Bombouaka and Kwahu Groups at the scale of the Volta basin. The present data complemented with the few recently published studies on Voltaian deposits, lead to a new regional lithostratigraphic synthesis of the Bombouaka and Kwahu Groups, based on stratigraphic constrains and sequence stratigraphy.