Ordovician grapholites and acritarchs from the Barrancos region (Ossa-Morena Zone, south Portugal)

Abstract

Ordovician fossils are relatively scarce in the Portuguese part of the Ossa-Morena Zone, one of the most distinctive Paleozoic domains of the Iberian Massif, where most of the available data come from the Barrancos region and needs an accurate review. The first account on the existence of Ordovician rocks near the Portuguese small city of Barrancos were published by Delgado (1901, 1908, 1910), who mentioned some graptolites and ichnofossils and defined three lithostratigraphic units from his “lower Silurian” division. In ascending order, these are the Fatuquedo shales, the Barrancos shales (including the “Phyllodocites shales” at its upper part) and the Colorada greywackes and quartzites formations. The paleontological record was virtually restricted to the fine shales of the Barrancos Fm. and especially to their “Phyllodocites shales”, a local facies of very micaceous, grey, green or red slates, not represented in the Spanish counterpart of the Estremoz-Barrancos-Hinojales domain of the Ossa-Morena Zone (Robardet et al., 1998). The age of the overlying Colorada Formation remains unknown, but graptolites recorded just above its top are already representative of the earliest Silurian (Piçarra et al., 1995). Ordovician graptolites from the Mestre André quarry east of Barrancos (Fig. 1) were first identified by Delgado (1901, p. 215) as Didymograptus geminus His., and then as Didymograptus sparsus Hopk. (Delgado, 1908 p. 187; 1910, pl. 27). New graptolite findings from the same locality were illustrated by Perdigão (1967, pl. 1, figs. 3-4), who recorded a broken stipe of D. sparsus and a proximal fragment of Didymograptus hirundo Salter, being the latter species indicative of an “upper Skiddawian” (= late Arenigian) age for the “Phyllodocites shales”.