LOWER CAMBRIAN APATITIC STROMATOLITES AND PHOSPHARENITES RELATED TO THE GLACIOEUSTATIC CRATONIC REBOUND (SAHARA, ALGERIA)

At the end of the Early Cambrian glacial event occurring on the West A frican craton, the tillite surface was marked, especially in the north eastern 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 un it 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 relativ e sea level, before the return to marine transgressive conditions, a s uccession of events took place, First, in the submerged areas, stromat olitic phosphorite in flat-domal to columnar buildups accreted, follow ed 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-fl uorapatite, characteristic of coastal environments. Glauconite is clos ely related to apatite and suggests conditions of a sea-level rise in respect to phosphatization, Precipitation of dolomite, calcite and eva porite postdates phosphatization and glauconitization, indicating a re gression and a chemical concentration due to a warm and perhaps arid c limate, Apatitic stromatolites and oncolites resulted from in situ pre cipitation within bacterial mats. This phosphogenesis corresponds to t he major Early Paleozoic phosphogenetic event.