Trace fossils on a Late Ordovician glacially striated pavement in Algeria Daniel Paul Le Heron Department of Earth Sciences, Queen's Building, Royal Holloway University of London, Egham, Surrey, TW200BY, UK abstract article info Article history: Received 8 September 2009 Received in revised form 5 July 2010 Accepted 23 July 2010 Available online 30 July 2010 Keywords: Striated pavements Ordovician North Africa Glaciation Trace fossils An exceptional exposure of Late Ordovician glaciogenic sediments crops out in Dider, SE Algeria, within the Tassili N'Ajjer region. The sediments consist of sandstones and diamictites sandwiched between a Mid Ordovician fluvial and tidal sandstone (In Tahouite Formation) below and Early Silurian shale (Oued Imirhou Formation) above. Stratigraphic discontinuities within the Late Ordovician glaciogene succession include palaeovalley incisions and glacially striated pavements. Striation and fluting of a soft-bedded sediment beneath an ancient ice sheet is supported by abundant dewatering structures and soft-sediment gouges interpreted to have been produced by the action of stone ploughing. In Dider, two types of previously undescribed circular structures sit in negative relief on this glacial pavement, namely 1) paired thumb- shaped impressions 2 cm in diameter and 3 mm in depth, and 2) a 5 cm wide impression with 3 segmented nested cycles. A framboid or aggregate origin may be appropriate for the smaller of the features but the larger impression is interpreted as biogenic: internal complexity is characteristic, discounting concretion moulds and water escape structures. A biogenic origin as a coelenterate resting trace is proposed with speculation on conditions of exceptional preservation in an ancient periglacial environment. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Upper Ordovician glacially related sediments of the Tamadjert Formation crop out over a wide region of Algeria and are particularly well exposed in the Tassili N'Ajjer National Park (Fig. 1). There, this formation comprises a succession of clastic sedimentary rocks (sand- stones, mudrocks, diamictites) in a succession that is punctuated by discontinuities and angular unconformities, and which was deposited by a ~443 Myr old Saharan ice sheet that advanced northward (e.g. Le Heron and Craig, 2008). In common with correlative sections in Libya and Morocco, many of the discontinuities bear soft-sediment striae formed by the action of Late Ordovician ice sheets shearing an unconsolidated, sandy substrate (e.g. Le Heron et al., 2005, 2006, 2007). A considerable amount of research has been conducted into the origin of these glacially striated surfaces across North Africa, which, drawing attention to the prevalence of dewatering structures and small lobate folds of sandstones that drape such surfaces, concludes that they formed in unconsolidated sediment (Deynoux and Ghienne, 2004; Le Heron et al., 2005). Closer to the ice centre, in Niger, striated pavements are beautifully exposed and have been subject to analysis in great detail by Denis et al. (2009). Body fossils are very rare within Late Ordovician glaciogenic sandstones of the central Sahara, although exquisite examples of the Hirnantia brachiopod fauna have been described from the Gargaf Arch in neighbouring Libya (Sutcliffe et al., 2001). Furthermore, Late Ordovician glaciogenic deposits are notoriously difficult to correlate over large distances, with many studies relying on microfossils such as chitinozoa (Bourahrouh et al., 2004), which are only preserved in hyper-arid conditions in deep oil boreholes. As such, at outcrop, any structure of potential biogenic origin is worthy of consideration as a potential biostratigraphic tool. The present paper, therefore, provides a description and interpretation of enigmatic pitted circular struc- tures, peculiarly preserved on a glacially-striated pavement, of probable biogenic origin. 2. Geological setting The study area (Fig. 1) forms part of an extensive outcrop belt of Cambrian through Silurian clastic sedimentary strata (Eschard et al., 2005) that define the southern flank of the Illizi Basin. This outcrop belt forms the southern flank of the Illizi Basin and exposes a generally monotonous layer cake stratigraphy (Beuf et al., 1971). This stratigraphy, which contains sedimentary rocks bearing fluvial, shallow marine and rare glacial facies in the Late Ordovician (Beuf et al., 1971), is characterised by a diagenetic overprint that includes ferruginous nodules. The distribution of these ferruginous nodules is considered to relate to deep-seated basement faults that traverse the Hoggar Massif and extend northward into the Tassili N'Ajjer (Guerrak, 1991). In the Tassili N'Ajjer, in common with other outcrops at the flanks of North Saharan basins, a wide range of concretions and nodules can be recognised in the desert sandstones (Seilacher, 2001). Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 138–143 E-mail address: d.leheron@es.rhul.ac.uk. 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.07.023 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo