Field and geochronological evidence show that previous ideas on the correlation and classification of the Precambrian and early Paleozoic rocks of Ghana, Togo, Dahomey, and Nigeria require substantial modification. The Voltaian Group and the Buem and the Togo Formations, which in the past have been considered to be of different ages, are now shown to comprise a single sedimentary unit, which to the east was involved in the late Precambrian to early Paleozoic Pan-African orogeny. This sedimentary unit constitutes a complete orogenic cycle with flysch and molasse sediments and associated igneous rocks.
The Pan-African orogeny resulted in the reactivation of the gneissic Dahomeyan and the Nigerian basement complex and the emplacement of a granite suite. Geochronological evidence suggests that the pre-reactivation basement included rocks of Birrimian age (approximately 2000 m.y.). Biotite ages are thought to be cooling or uplift ages, and they indicate that the deposition of the molasse Obosum Beds of the Voltaian Group began in the upper Cambrian. An important plutonic event in the reactivation may have occurred around 600 m.y. ago, although earlier events as much as 873 m.y. ago are possible.