Richat Structure, a Cretaceous Alkaline Complex

The circular structure of Richat is located in the northwestern part of the Taoudeni basin, in the central part of the Mauritanian Adrar plateaus, where it is expressed at the surface as a slightly elliptical depression, about 40 km in diameter, marked by concentric ridges of Proterozoic-Lower Paleozoic strata (O’Connor et al., 2004; Abdeina et al., 2021; Hamoud et al., 2021). The Richat structure appears as a large dome within a Late Proterozoic to Ordovician sequence, with circular cuestas represented by three nested rings dipping outward from the structure. The center of the structure consists of a limestone-dolomite shelf that encloses a kilometer-scale siliceous mega-breccia and is intruded by basaltic (gabbroic) ring dikes, kimberlitic intrusions, and alkaline volcanic rocks (Matton et al., 2005). Its origin has been enigmatic and debated with some suggesting that it was a meteoritic impact crater, which has been disproved in favour of an intrusion theory (Faudli 1969). The Richat is the superposition of a bimodal tholeiitic suite crosscut by carbonatitic (85 ± 5 Ma and 99 ± 5 Ma) and kimberlitic magmatic rocks (Matton and Jebrak, 2014). According to Abdeina et al. (2021) the gabbroic ring dykes represent deep magmatic structures connected to a large intrusive magmatic body.