Francevillian B Formation

The Francevillian B Formation, also known as the Francevillian Formation or FB2 in scientific research, is a geologic formation of black shale provinces close to the town of Franceville, Gabon from which it gets its name. The formation was deposited between 2.14-2.08 Ga (billion years ago) in the Palaeoproterozoic, and, uniquely, has not experienced any thermal overprinting due to diagenesis after burial nor significant metamorphism since it was deposited, unlike other formations deposited around the same time.

Source: Wikipedia — Francevillian B Formation (CC BY-SA 4.0)