Geologically, Gabon is primarily ancient Archean and Paleoproterozoic igneous and metamorphic basement rock, belonging to the stable continental crust of the Congo Craton, a remnant section of extremely old continental crust. Some formations are more than two billion years old. Ancient rock units are overlain by marine carbonate, lacustrine and continental sedimentary rocks as well as unconsolidated sediments and soils that formed in the last 2.5 million years of the Quaternary. The rifting apart of the supercontinent Pangaea created rift basins that filled with sediments and formed the hydrocarbons which are now a keystone of the Gabonese economy. Gabon is notable for the Oklo reactor zones, the only known natural nuclear fission reactor on Earth which was active two billion years ago. The site was discovered during uranium mining in the 1970s to supply the French nuclear power industry.