The architecture, distribution and development of channelized sandstone bodies are described from Late Ordovician paraglacial successions of the Tassili N'Ajjer (SE Algeria and SW Libya) based on satellite images and field data (sedimentary logs, photomosaics). Sandstone bodies have a ribbon-like form at outcrop (often referred to as ‘cordons’ in the literature). They typify a fluvioglacial outwash plain deposited between a continental ice front and a marine delta-front zone. Channelized sandstone bodies are straight to sinuous, with widths (W) in the 60–600 m range, thicknesses (T) in the 5–30 m range and they have a mean W/T ratio of 16.5. They develop within an aggradational–progradational sand-dominated deltaic topset succession including at its distal end a terminal distributary channel and mouth-bar environments. The architecture of channel bodies and the related depositional facies, which includes climbing-dune cross-stratification, indicates that channelized sandstone bodies represent plugs of isolated channels related to high-magnitude flood events (glacier outbursts). These plugs form fossilized networks of both braided channels and interlaced anastomosed channels, offering snapshots of an outburst-related unconfined proglacial outwash braidplain constituted by the amalgamation of adjacent, elongated outwash fans.
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.
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