After the recent synthesis by Vicat (1998), the substratum of the Nyong basin is constituted by two main rocks (Figure 2). (1) In the South, the Ntem group representing the northern margin of the Congo craton, which is considered stable since about 2000 Myr; this complex includes, from East to West, the Ayina, Ntem and Nyong series. The plutonic rocks are essentially granitoids, whose compositions and ages are clearly distinct: (a) liberian charnockites (near 2900 Myr); (b) more potassic and leucocratic granites (2700-2600 Myr). Within these granitoids, frequent intrusions related to different tectonic phases have been described: greenstone belts, abundant at West, often associated with ferriferous formations; banded gneissic series; bodies of dolerite, gabbro or rarely peridotite. (2) In the North, the mobile zone, strongly affected by the Pan-african orogenesis (600-500 Myr), constitutes the Western ending of the Oubanguide chain, and where two facies have been distinguished. (a) In the North, the Yaoundé series, constituted by gneisses and garnetiferous migmatites, resulting from granitization and metamorphism in granulite facies of old sediments, which were then thrusted towards South. (b) At the contact with the craton, the Ayos-Mbalmayo-Bengbis series, mainly characterized by chlorite schists and quartzites, is considered as the retrometamorphized sole thrust of the panafrican sheet.
Some bedrock samples could be collected in the Mengong catchment (some bore holes, one very limited outcrop) exhibit two distinct facies: a leucocratic monzonite with big grains of frequently bluish quartz, and a granodiorite presenting banded ferromagnesian-rich zones.