The Upper Silurian of Touat (Algerian Sahara) and its fauna


Authors: Legrand P

Published in: Bulletin of Geosciences, volume 87, issue 4; pages: 661 - 668; Received 29 July 2011; Accepted in revised form 29 March 2012; Online 15 May 2012

Keywords: Algerian Sahara, Touat, Silurian, graptolites, bivalves,

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In the Touat region (Algerian Sahara), the most complete Palaeozoic succession occurs to the southwest of Adrar. In the Tamest section, the Silurian comprises the Fenourine Clay and the Touat Sandstone formations. The Fenourine Clay Formation consists mostly of silty claystones of which only the upper 150m crop out. At the base of the section, bivalves occur, followed by the graptolites Saetograptus chimaera aff. salweyi (Hopkinson) and Pristiograptus cf. P. tumescens minor (Wood) which are described and figured in this paper. The overlying Touat Sandstone Formation is 170m thick. It comprises silty claystones with several lenticular, sandy beds, surmounted by a sandy, calcareous bed. At the base of the sandstone, pieces of homalonotine trilobites, brachiopods and bivalves are present. Above the base, silty claystones, a ferrugineous siltstone bed and the first Tentaculoidea occur. A conglomeratic limestone bed yielding a Pragian fauna tops the formation. The ages are as follows: the lower and middle Silurian do not crop out. The graptolites at the base of the extant section belong to the associations σ3β which characterize the Saharan Ludlow Series (g3b) (Legrand 1981, 1985). The silty, clayey beds that follow may be the equivalent of the Přídolí (g3c). The fossiliferous beds at the base of the Touat Sandstone may also be of Přídolí age, or may indicate the base of the Lochkovian (g4c). These outcrops reveal an interesting evolution in terms of facies and faunal assemblages towards the Ougarta Mountains and the Gourara to the northwest and the Azzel Matti to the southeast.