Siliciclastic-Carbonate Stromatolite Domes, in the Early Carboniferous of The Ajjers Basin (Eastern Sahara, Algeria)

Abstract

Stromatolite domes crop out widely in the upper Visean of eastern Sahara, Algeria and Lybia. Their growth was conditionned by the sedimentary regime prevailing in the eastern Sahara during Early Carboniferous times, as well as the specific features of sedimentation over cratonic areas. The stromatolites of the two major units are discrete juxtaposed domes with high total relief. The same kind of laminae, probably built by cocoid cyanobacteria and bacteria, are found in all domes. Some of them are characterized by a high percentage of siliciclastic material. Stromatolites grew in very quiet subaqueous environments protected from open marine water. The stromatolites of the lower unit, are strictly the same from base to top of the sequence (except for an increase in size). The upper unit is a typical shallowing-upward para-sequence, starting with subaqueous large domes passing to stromatolite heads in intertidal settings. In similar deposits in Lybia, Whitbread and Kelling (1982) interpreted the stromatolite “Collenia interval” as resulting from a widespread delta abandonment. In terms of sequence stratigraphy the two sequences containing the stromatolites are High Stand System Tracts following a sea level rise. However some differences observed in the stromatolites indicate distinct conditions of growth: the stromatolites of the lower unit have developed in lakes fed by fresh-water streams, while those of the second unit involved a marine lagoon protected by oolitic bars, filled during sea-level rises and shallowing-upward progressively. The very large extension and the thickness of the stromatolite layers imply a deposition over large, flat, stable areas, with low subsidence rate, a characteristic feature of the West African Craton since the Proterozoic.