The Hirnantian glacial landsystem of the Sahara: a meltwater-dominated system

Abstract

During the Hirnantian (443 Ma), ice sheets expanded towards the present-day north over North Africa and the Arabian Peninsula when the western part of the Gondwana supercontinent straddled the South Pole. A glacigenic succession is exposed around the flanks of several Saharan cratonic basins, including Murzuq and Al Kufrah basins in Libya. As a result of the present-day hyper-arid context of the outcrops, a suite of palaeo-glacial landforms has been identified and, in some cases, mapped. These landforms occur along glacial erosion surfaces, defined as unconformities derived through direct ice sculpting, meltwater erosion or a combination of these processes. Unconformities, mostly resulting from ice sculpting and including glacially striated pavements and mega-scale glacial lineations (MSGLs), are identified at a number of locations along these erosion surfaces, in association with drumlinized features. The majority of reported examples are of ‘soft sediment character’; that is, generated through the deformation of unconsolidated sediment. Features mainly associated with meltwater erosion include tunnel valleys which occur in swarm-like belts, defining palaeo-grounding-line positions as the ice fronts retreated. A ‘smeared’ series of palaeo-ice-stream footprints, resulting from multiple advance–retreat cycles, is characteristic of the landsystem. Deformation of sediments into a series of push moraines, expressed as kilometre-scale fold–thrust belts, allows the locations of the former ice front to be identified. The Hirnantian glacial record of North Africa is a classic example of a continent-scale archive of an ancient palaeo-ice sheet. Expeditions to the Sahara during the late 1960s, focusing on the Algerian record, revealed a full suite of glacial indicators including striated pavements, pingos, roches moutonnees and a variety of meltwater channels (Beuf et al. 1971). Subsequently, largely as a result of their significance as hydrocarbon reservoirs, more recent studies re-evaluated these outcrops from a sedimentological perspective. Much effort focused on unravelling the glacial stratigraphy in neighbouring …