Pedogenetic and diagenetic fabrics in the Upper Proterozoic Sarnyëré formation (Gourma, Mali)

Several diagenetic fabrics are described in the uppermost member of the Sarnyéré formation, an Upper Proterozoic dolomite (700-600 Ma). Micronodular micrite, microsparite showing nodulisation processes, orbicular crusts and oncoids are thought to be due to the pedogenetic alteration of an algal (Porostromata) boundstone. Further evolution in a vadose zone creates a network of cavities which are later filled by “cave stromatolites”, micropopcorn micrite and multilayered cements. A rhythmical repetition of facies evidences a polyphased pedogenetic history, leading to a 30 m thick highly complex section. Such numerous and diversified structures are rarely preserved, especially in Proterozoic rocks, but each of them have equivalents in recent examples of caliche soil profiles.