Architecture of Late Ordovician glacial valleys in the Tassili N'Ajjer area (Algeria)

Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)

“…Ice-front advance and retreat cycles resulted in a minimum of five erosion-based glacial depositional sequences, vertically superimposed and laterally juxtaposed, which essentially comprise sand-dominated successions with a range of fluvial, deltaic and shallow-marine deposits passing distally into turbiditic deposits (Radulovic, 1984;Ghienne et al, 2003Ghienne et al, , 2007bGhienne et al, , 2010Le Heron et al, 2004, 2010El-ghali, 2005;Moreau et al, 2005;Moreau, 2011;Girard et al, 2012a,b). The depositional sequences fill palaeovalleys or glacial troughs, some of which represent former ice-stream pathways (Moreau et al, 2005;Ghienne et al, 2007b;Le Heron and Craig, 2008;Deschamps et al, 2013;Moreau and Ghienne, in press). Glacial striations and mega-scale glacial lineations (MSGL), tunnel valleys and fluvioglacial palaeocurrents together point to a relatively uniform pattern of northward to NNW ice flowing in the Murzuq Basin (Beuf et al, 1971;Ghienne et al, 2007b;Le Heron and Craig, 2008;Le Heron et al, 2010).…”

“…Such complexes are classic features of Quaternary and modern proglacial assemblages (Lønne, 1993;Hambrey and Huddart, 1995;McCarroll and Rijsdijk, 2003;Pedersen, 2005;Larsen et al, 2006;Aber and Ber, 2007;Thomas and Chiverrell, 2007;. They are described more rarely in the pre-Pleistocene glacial record (e.g., Benn and Prave, 2006;Dykstra et al, 2006;Etienne et al, 2007;Arnaud, 2008;Busfield and Le Heron, 2013), with the exception of the end-Ordovician record where proglacial glaciotectonic deformation is ubiquitous (Beuf et al, 1971;Deynoux, 1980;Ghienne, 2003;Le Heron, 2007;Ghienne et al, 2007bGhienne et al, , 2013Deschamps et al, 2013). The largest example is probably documented in western Algeria.…”

“…Models usually invoke hydrostatic overpressure generated by the icesheet load but alternative models based on erosion by steady-state meltwater flows have also been proposed. Tunnel valleys are described in the Permian-Carboniferous glacial record (Eyles and de Broekert, 2001) Ghienne, 2009;Gutiérrez-Marco et al, 2010;Douillet et al, 2012;Clerc et al, 2013;Deschamps et al, 2013).…”

“…Therefore sedimentary structures related to high-magnitude flow events within any glacial record indicate an ice-marginal to proglacial context (Maizels, 1997;Russell and Knudsen, 2002;Johnsen and Brennand, 2004;Marren, 2005;Carrivick and Russell, 2007;Thieler et al, 2007;Duller et al, 2008;Jones and Fielding, 2008;Winsemann et al, 2011;Carling, 2013;Lang and Winsemann, 2013). Deposits related to end-Ordovician glacial outburst events (like Icelandic jökulhlaups) have been inferred from recurrent assemblages associating megabeds and climbing-dune cross-stratification (Le Heron et al, 2006CDCS of Ghienne et al, 2010;Girard et al, 2012a,b;Deschamps et al, 2013). As described below, sandstone intraclasts, large-scale bedforms and kettle holes are other structures that, together, document end-Ordovician high-energy proglacial meltwater flows.…”

“…They are preserved in sandstone sheets or channel infills, and can be followed several tens of metres to several kilometres along depositional slopes, resulting in several metres thick, continuously aggrading successions with no evidence of flow interruptions. Although most authors agree that they relate to glacier outburst events, the interpretation of CDCS is still controversial, especially regarding the exact associated flow conditions (Hirst et al, 2002;Le Heron et al, 2006Ghienne et al, 2010;Girard et al, 2012b;Deschamps et al, 2013). The in-phase migration of either lower-flow regime dune trains in fluvial channels and mouth-bar environments, or of antidunes in turbiditic systems, has been proposed.…”