ABSTRACT

During the Vendian (uppermost Proterozoic), the Gourma Basin was an east-west intracratonic feature that opened eastward into an ocean. The southern edge of the Gourma Basin preserves facies belts representing land to deep-basin environments. Shoreward facies consist of tidal belts of microbially laminated dolomites or porostromata dolomites interstratified with clastic carbonates of local origin. Landward supratidal facies consist of finely laminated sedimentary rocks with sizeable desiccation features but no evaporites. The upper part of this unit is affected by a periodic emersion resulting in a complex succession of paleosol profiles. Offshore of the tidal facies are lagoon-facies rocks that were protected from the open sea by a carbonate buildup composed of a framework of stromat lite bioherms associated with detrital dolomites, indicating a high-energy environment. Slope facies consist of a thick succession of ribbon limestones with some graded beds. Synsedimentary slumping and faulting of the ribbon limestones indicate a significant depositional slope. Allochthonous deposits are sporadically interstratified with the ribbon limestones and consist of muds with small grains derived from the platform deposits, polygenic breccias, conglomerates composed only of ribbon limestone intraclasts, and oolitic calcarenites.

Shelf-facies rocks are pervasively dolomitized but original textures are well preserved including fine-grained structures. The ribbon limestones on the other hand are not dolomitized except for dolomitic layers caused by pressure solution.

The distribution of facies in Gourma Basin carbonates corresponds well with ideal models of Wilson (1975). Comparisons with present-day facies distributions are less satisfactory, especially considering the dominance of cyanobacteria over all environments for which there is no modern-day counterpart. Comparisons with the modern environment, however, suggest a subtropical semi-arid climate during deposition of the Gourma strata.