franceville basin franceville basin the franceville basin is a 1.6–2.1 billion year old sedimentary basis in gabon. it contains unmetamorphosed sediments. it is notable for containing the francevillian biota, which is likely the oldest multicellular life known. a natural fission reactor formed there about 1.8 - 2.1 billion years ago. map of francevillian basin geology geology the franceville basin cover approximately 25,000 km2 and is made up of unmetamorphosed sediment derived mainly from eroded mesoarchaean tonalite–trondhjemite–granodiorites. it is over a kilometer thick, with various sources claiming 2.5-4 kilometers as the maximum depth. around 1.8-2.1 billion years ago a natural fission reactor formed, nicknamed the "oklo reactor". the resulting fission by-products were held in place by a clay layer.