Ordovician vs. “Cambrian” ichnofossils in the Armorican Quartzites of Central Portugal

SÁ, A., GUTIÉRREZ-MARCO, J.C., PICARRA, J.M., GARCÍA-BELLIDO, D. AND ACEÑOLAZA, G.F.,

Cuadernos del Museo Geominero

Museo Geominero

Lugar: Madrid; Año: 2011 vol. 14 p. 483 - 492

0040-8573

The Armorican Quartzite is one of the most characteristic units of the Paleozoic of SW Europe, being represented in the Lower Ordovician succession of Brittany and Normandy (western France), and also over most of the Hesperian and Iberian massifs of the Iberian Peninsula (in the clarified sense of San José, 2006), with the exception of the Ossa-Morena and South-Portuguese zones (Gutiérrez-Marco et al., 2002; Vera, 2004; Ribeiro, 2006). In Portugal and from north to south, the Armorican Quartzite facies is equivalent to the Marão Formation of Trás-os-Montes (Sá et al., 2005), the Santa Justa Formation of the Tabagón-Valongo-Tamames domain (Romano and Diggens, 1974), the Armorican Quartzite Formation of the Buçaco and Amêndoa-Mação areas (Young, 1988; Romão, 2000a) and the Serra do Brejo Formation in the Dornes area (Cooper, 1980). In spite of the generalized absence of biostratigraphical ties for correlation other than ichnofossils and a few chitinozoan or graptolite data, the latter generally coming from the overlying shales, the Armorican Quartzite in Portugal have been considered as involving a diachronism in sedimentation from Arenig to Llandeilo, becoming younger from west to east (Ribeiro, 1974) according to regional data from the Valongo to Trás-os-Montes areas. These data have been compiled in some syntheses (Hammann et al., 1982; Romano, 1982; Oliveira et al., 1992). However, the single paleontological argument in support of such diachronism, a Llandeilian trilobite found in the middle part of the Marão Formation at Moncorvo (Teixeira and Rebelo, 1976) was later reviewed by Gutiérrez- Marco et al. (1995), and Sá et al. (2003, 2009), who demonstrated that the supposed trilobite was in reality the trace fossil Rusophycus carleyi (James), also recorded in other Gondwanan areas within the Arenigian succession (Seilacher, 1970; Gibb et al., 2010). No other authors were able to demonstrate the claimed diachronism in the sedimentation of the Armorican Quartzite, whose deposit took place entirely in the Eremochitina brevis chitinozoan biozone (Paris, 1981, 1990; Paris et al., 1982, 2007), regarded as A.A. Sá, J.C. Gutiérrez-Marco, J.M. Piçarra, D.C. García-Bellido, N. Vaz and G.F. Aceñolaza 484 “early-mid Arenigian” or as late Floian according to the global scale (Paris et al., 2007; Videt et al., 2010). Romão et al. (2010) recently questioned the current age of the Armorican Quartzite in the southern Central Iberian Zone, and supported a local late Cambrian age for this formation in the Amêndoa- Carvoeiro synform based in a couple of ichnological data, a single U-Pb dating, and some highly speculative tectonostratigraphic inferences which in our opinion are far from being demonstrated. Also with reference to this area, Romão et al. (2010) envisaged the Armorican Quartzite as a highly diachronic late Cambrian to Early Ordovician unit for the Iberian Peninsula. This statement is refuted here with the presentation of new ichnologic evidence that supports the previous Early Ordovician dating of the Armorican Quartzite in the Amêndoa-Carvoeiro synform.