Franceville basin

The Franceville Basis 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 800-900 million years ago.
Geology.
The Franceville Basin cover approximately 25,000 km2 and is made up of unmetamorphosed sediment. It is over a kilometer thick, with various sources claiming 2.5-4 kilometers as the maximum depth. Around 800-900 mya a natural fission reactor formed. The resulting fission by-products were held in place by a clay layer.