ABSTRACT

At the end of the Early Cambrian glacial event occurring on the West African craton, the tillite surface was marked, especially in the northeastern part of the Taoudenni basin, by noticeable topographic relief. After deglaciation, the eustatic rise in sea level led to deposition over all the West African Craton of a thin but widespread carbonate unit containing Lower Cambrian shelly fossils. At the ice-cap periphery, isostatic rebound of the craton resulted in uplift of the topographic highs and weathering of the carbonate. During fluctuations of relative sea level, before the return to marine transgressive conditions, a succession of events took place. First, in the submerged areas, stromatolitic phosphorite in flat-domal to columnar buildups accreted, followed by growth of dolomitic ministromatolites. Laterally, phospharenitic beds (oncolites and phosphatic grains) accumulated under water in the depressions. The stromatolites and oncolites are made of carbonate-fluorapatite, characteristic of coastal environments. Glauconite is closely related to apatite and suggests conditions of a sea-level rise in respect to phosphatization. Precipitation of dolomite, calcite and evaporite postdates phosphatization and glauconitization, indicating a regression and a chemical concentration due to a warm and perhaps arid climate. Apatitic stromatolites and oncolites resulted from in situ precipitation within bacterial mats. This phosphogenesis corresponds to the major Early Paleozoic phosphogenetic event.