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.

The southern North Sea is a shallow epicontinental sea that was glaciated several times during the Quaternary. The area is known for its remarkable record of tunnel valleys, the age and origin of which are debated. The recent availability of continuous three-dimensional seismic data between the coasts of Britain and the Netherlands provides the opportunity to establish a new seismic interpretation workflow adapted to the intracratonic glaciogenic successions. By analysing the geomorphology of the buried basal glaciogenic unconformity, four distinct major ice fronts are identified and correlated onshore. The ice fronts provide robust relative timelines, and the analysis of tunnel-valley orientations and their merging points indicates that the number of glacial phases has been underestimated. By comparing the erosion capacities of sand and chalk substrates, it is suggested that mechanical abrasion processes are also involved during tunnel-valley genesis. The methods and observations used in this study are applicable to the ancient glaciogenic record in general and constitute a basis for the sedimentological analysis of tunnel valleys

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Rocks of Late Ordovician to Silurian age are well exposed on the western rim of the Murzuq Basin (Ghat-Tikiumit area, Libya) where seismic-scale exposures allow spectacular insights into the growth and decay of the Late Ordovician (Hirnantian) ice sheet.The ¢nal deglaciation left a complex topography with a combination of subglacial morphologies and proglacial depositional systems.This paper documents the glacial and proglacial palaeo -topography that controls the accumulation of a postglacial transgressive depositional system and the Rhuddanian (Early Silurian) shales.The glacial relief directly contributed to an important hiatus, with the Rhuddanian deposits at the base of the remnant glacial troughs being 3 Ma older than at the top of the topographic highs.The source-rock in the Murzuq Basin is of Early Rhuddanian age, so it is present only in the deepest part whereas geomorphic traps are formed within the highs of the relict postglacial topography.The transgressive system, recognised for its good reservoir potential, is considered to play a key-role in the petroleum system, linking the source rock deposited in the ancient topographic lows with the reservoir rocks in the topographic highs.This study aims to demonstrate the importance of palaeo -glaciological reconstructions for petroleum exploration of the Ordovician^Silurian in North Africa.

Climbing dune-scale cross-statification is described from Late Ordovician paraglacial successions of the Murzuq Basin (SW Libya). This depositional facies is comprised of medium-grained to coarse-grained sandstones that typically involve 0AE3 to 1 m high, 3 to 5 m in wavelength, asymmetrical laminations. Most often stoss-depositional structures have been generated, with preservation of the topographies of formative bedforms. Climbing-dune cross-stratification related to the migration of lower-flow regime dune trains is thus identified. Related architecture and facies sequences are described from two case studies: (i) erosion-based sandstone sheets; and (ii) a deeply incised channel. The former characterized the distal outwash plain and the fluvial/ subaqueous transition of related deltaic wedges, while the latter formed in an ice-proximal segment of the outwash plain. In erosion-based sand sheets, climbing-dune cross-stratification results from unconfined mouth-bar deposition related to expanding, sediment-laden flows entering a water body. Within incised channels, climbing-dune cross-stratification formed over eddyrelated side bars reflecting deposition under recirculating flow conditions generated at channel bends. Associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys. The primary control on the formation of climbing-dune crossstratification is a combination between high-magnitude flows and sediment supply limitations, which lead to the generation of sediment-charged stream flows characterized by a significant, relatively coarse-grained, sand-sized suspension-load concentration, with a virtual absence of very coarse to gravelly bedload. The high rate of coarse-grained sand fallout in sediment-laden flows following flow expansion throughout mouth bars or in eddy-related side bars resulted in high rates of transfer of sands from suspension to the bed, net deposition on bedform stoss-sides and generation of widespread climbingdune cross-stratification. The later structure has no equivalent in the glacial record, either in the ancient or in the Quaternary literature, but analogues are recognized in some flood-dominated depositional systems of foreland basins.

Deep, elongated incisions, often referred to as tunnel valleys, are among the most characteristic landforms of formerly glaciated terrains. It is commonly thought that tunnel valleys were formed by meltwater flowing underneath large ice sheets. The sedimentary infill of these features is often highly intricate and therefore difficult to predict. This study intends to improve the comprehension of the sedimentology and to establish a conceptual model of tunnel-valley infill, which can be used as a predictive tool. To this end, the densely sampled, Pleistocene tunnel valleys in Hamburg (north-west Germany) were investigated using a dataset of 1057 deep wells containing lithological and geophysical data. The stratigraphic correlations and the resulting three-dimensional lithological model were used to assess the spatial lithological distributions and sedimentary architecture. The sedimentary succession filling the Hamburg area tunnel valleys can be subdivided into three distinct units, which are distinguished by their inferred depositional proximity to the ice margin. The overall trend of the succession shows a progressive decrease in transport energy and glacial influence through time. The rate of glacial recession appears to have been an important control on the sedimentary architecture of the tunnel-valley fill. During periods of stagnation, thick ice-proximal deposits accumulated at the ice margin, while during rapid recession, only a thin veneer of such coarse-grained sediments was deposited. Ice-distal and non-glaciogenic deposits (i.e. lacustrine, marine and terrestrial) fill the remaining part of the incision. The infill architecture suggests formation and subsequent infill of the tunnel valleys at the outer margin of the Elsterian ice sheet during its punctuated northwards recession. The proposed model shows how the history of ice-sheet recession determines the position of coarse-grained depocentres, while the post-glacial history controls the deposition of fines through a progressive infill of remnant depressions.

Finnish Lapland is known as an area where numerous sites with sediments from Pleistocene glacial and interglacial periods occur. Recent sedimentological observations and dating call for reinterpretation of the record, which shows a complicated Mid-Weichselian ice-sheet evolution within the ice-divide zone. Here, a large, previously unstudied section from a former Hannukainen iron mine was investigated sedimentologically and dated with optically stimulated luminescence (OSL). Ten sedimentary units were identified displaying a variety of depositional environments (glacial, glaciolacustrine, fluvial and aeolian). They are all -except for the lowermost, deeply weathered till -interpreted to be of Mid-or Late Weichselian/Holocene age. Five OSL samples from fluvial sediments give ages ranging from 55 to 35 ka, indicating two MIS 3 ice-free intervals of unknown duration. The Mid-Weichselian interstadial was interrupted by a re-advance event, which occurred later than 35 ka and caused glaciotectonic deformation, folding and stacking of older sediments. This new evidence emphasizes the importance of the Kolari area when unravelling the complex Late Pleistocene glacial history of northern Finland and adjacent regions. -Pekka Salonen (veli-pekka.salonen@helsinki. Veli