Abstract
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.
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