Franceville basin

[1] It is notable for containing the Francevillian Biota, which are possibly the oldest multicellular life known.

A natural fission reactor formed there about 1.8 - 2.1 billion years ago.

[2] The Franceville Basin covers approximately 25,000 km2 and is made up of unmetamorphosed sediment derived mainly from eroded Mesoarchaean tonalite–trondhjemite–granodiorites.

[1] 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.