No abstract

The Late Ordovician (Hirnantian) glaciation is examined through the North Gondwana record. This domain extended from southern high palaeo-latitudes (southeastern Mauritania, Niger) to northern lower palaeo-latitudes (Morocco, Turkey) and covered a more than 4000 km-wide section perpendicular to ice-flow lines. A major mid-Hirnantian deglaciation event subdividing the Hirnantian glaciation in two first-order cycles is recognised. As best illustrated by the glacial record in western Libya, each cycle comprises 2-3 glacial phases separated by ice-front retreats several hundreds kilometres to the south. From ice-proximal to ice-distal regions, the number of glacial surfaces differentiates (i) a continental interior with post-glacial reworking of the glacial surfaces), (ii) a glaciated continental shelf that is subdivided into inner (1-2 surfaces), middle (2-5 surfaces) and outer (a single surface related to the glacial maximum) glaciated shelves, and (iii) the non-glaciated shelf. Ice-stream-generated glacial troughs, 50-200 km in width, cross-cut these domains. These troughs are zones of preferential glacial erosion and subsequent sediment accumulation. A glacial depositional sequence, bounded by two glacial erosion surfaces, records one glacial phase. The position either within or outside a glacial trough controls the stratigraphic architecture of a glacial sequence. Glaciomarine outwash diamictites are developed at or near the maximum position of the ice-front. During ice-sheet recession, and in an ice-stream-generated trough, a relatively thin sediment cover blankets the foredeepened erosion surface. An initial rapid ice-sheet withdrawal is inferred. Marine-terminating ice fronts then evolve later into more slowly retreating, land-terminating ice fronts. In adjacent inter-stream areas where a more gradual ice-sheet recession occurred, fluvioglacial deposits prevailed. The progradation of a delta-shelf system, coeval with fluvial aggradation, that may be locally interrupted by a period of isostatic rebound, characterises the late glacial retreat to interglacial conditions. This model should facilitate the sequence stratigraphic interpretation of Late Ordovician glacial deposits and other ancient glacial successions.

Landsat images, ASTER digital elevation models, aerial photographs, and field investigations in the western Murzuq Basin (Libya) and the adjacent Tassili N'Ajjers (Algeria) provide paleogeomorphological evidence for the existence of a Late Ordovician ice stream at least 200 km long and 80 km wide. This includes mega-scale glacial lineations, an associated subglacial meltwater drainage system, and ice-front features. This first comprehensive description of a pre-Cenozoic ice stream may help to identify other examples in the Proterozoic to Paleozoic glacial record. Reconstruction of the extent and behavior of former ice sheets, and reservoir prospect analysis in glacially related successions, have to take into account the potential occurrence of ice streams.