Evaporitic constraints on the southward drifting of the western Gondwana margin during Early Cambrian times

Abstract

Lower Cambrian evaporites and carbonates are reported from nearly all the platforms of the western Gondwana margin, which comprises the Souss, Ossa–Morena, Cantabro-Iberian, Armorican and Montagne Noire–Sardinian Basins. Both lithologies were deposited in climatically restricted belts and their changing palaeogeographic distributions, according to recent biostratigraphic correlations, are used to infer the latitudinal motion of this margin. As a result, the time span involved in the Cordubian–Ovetian–Marianian interval (8–10 m.y.) requires a relative high rate of drifting, which supports the high apparent polar wander path rates (defined by palaeomagnetic data) proposed for Early Palaeozoic times. The varied and abundant relics of primary to early diagenetic evaporites (gypsum, anhydrite and halite) demonstrate that extensive evaporitic conditions were associated with carbonate and mixed platform systems in an Early Cambrian arid subtropical belt. Evaporites were originally more abundant than suggested by the reported remains because the deposits have undergone a multistep diagenesis that erased most of the former morphologies. Some petrographic criteria are proposed for recognizing silica pseudomorphs after evaporites, such as the development of ‘chicken-wire’ and enterolithic structures, and the presence of lenticular to lozenge-shaped crystals of gypsum and anhydrite relics.