Colour banding in a latest Neoproterozoic–Cambrian microbially variegated sabkha of the Taoudeni Basin, Adrar of Mauritania

The Taoudeni Basin is a large intracratonic trough located in the West African craton, with more than 2 km of Neoproterozoic–lower Palaeozoic sedimentary thickness. The occurrence of late Ediacaran–Cambrian sabkha deposits, in which playa-lake evaporites are bounded by proximal braided-fluvial and distal shoal complexes, is easily distinguishable in the field by their variegated colour banding. The diagenetic processes recorded in the sabkha substrates were largely facies-controlled, associated with microbial development and mat-decay mineralization. Due to anaerobic decay of mat microorganisms, chemical conditions beneath mats and biofilms tended to be strongly reducing. As a result, precipitation of authigenic indicators of anoxic conditions, such as pyritic crusts, led to variegated colours parallel to stratification. Reworking of microbial mats and structures by ephemeral streams also generated isolated reduction spots around their microbial clasts leading to the formation of pyritic haloes. The subsequent alteration of pyrite to amorphous (reddish) hematite and subsequent recrystallization to (brownish to yellow) goethite contributed to the variegated aspect of the unit. Final reddening of the sediment took place irregularly as a result of dehydration of ferric hydroxides, a process relatively common in desert environments.