Ordovician cold water brachiopods from the Ougarta Mountain Range, Algerian Sahara

Abstract

Ougarta Range; Fig. 1) applies to a mountainous region trending NW–SE and extending for 330 km on the edge of the platform of the western Algerian Sahara. This range is the result of folding of a thick Palaeozoic succession during the Variscan compression. This range forms a link between the Anti-Atlas and the Saharan basins, thus justifying a comprehensive study. Inferred reconstructions of the polar wander path suggest that the Ordovician South Pole drifted gradually across the north-west African sector of Gondwana (Torsvik & Cocks 2011, Torsvik et al. 2012), but without sedimentary evidence in the neighbourhood of the present Ougarta Range except for Hirnantian periglacial sediments. For most of the time this area was flooded by a shallow sea. However, a large part of the Ordovician succession is significantly affected by the fluvioglacial and glacial erosion which occurred during the Hirnantian time (Arbey 1968; Legrand 1974, 1985a; Ghienne et al. 2007). In the Mid to Late Ordovician, this shallow sea was inhabited by a low diversity fauna of which brachiopods form an important component; these brachiopods represented the major objective of the present study. Although, together with the Anti-Atlas, the Ougarta Range is considered as an important source of the Ordovician macrofossils from the North African sector of Gondwana, the existing record of Ordovician brachiopods in the region is sparse. Roch (1933) was the first to report Ordovician brachiopods in the Ougarta Range. In a short paper he announced a presence of moulds assigned to “Orthis gr. alternata Sowerby – retrostriata MʼCoyˮ in the vicinity of Hassi Chaamba, Daoura. Some months later Menchikoff (1933) reported, this time in the Ougarta Range s.s., two fossiliferous horizons at Kheneg et Tlaïa, the lower one with unidentified brachiopods of presumed Llandeilo age, the upper one with orthides, identified as Orthis redux Barrande, 1848. The whereabouts of the specimens sampled during these pioneering works is presently unknown. Many years later, Poueyto (1951, 1952) cited the same species from Hassi Chamba (Daoura), from Tabelbala and from Kheneg et Tlaïa; also a little farther to the south-east, at Feidj ez Zeidiya,