Giant Sandwaves from the Late Ordovician of the Tassili N'Ager, Algeria

Giant sandwaves are described from the Ordovician outcrops of the Tassili N’Ager (SE Algeria). The sandwaves have been examined in the field and mapped using remote sensing techniques. They occur at the top of a Late Ordovician glacial sequence and unconformably overlie a sequence of pro – glacial sediments. They are in turn overlain by shelfal sediments of Lower Silurian age, mostly organic – rich, graptolitic shales that are a major petroleum source rock. The sandwaves are developed in medium to coarse grained sand and are characterised by large scale, cross sets up to 4m in height with dips of 10 – 15 degrees, dips are unidirectional with palaeo flow toward the east. The coarse grained nature and large size of the bedforms suggests the existence of strong unidirectional currents in the eastern Tassili N’Ager at the end of the Ordovician. The interaction of tidal currents and post – glacial topography in the early stages of the Silurian transgression seems the most likely explanation of these spectacular features.