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Bulletin de la Societe Geologique de France; November 2008; v. 179; no. 6; p. 569-581; DOI: 10.2113/gssgfbull.179.6.569
© 2008 Societe Geologique de France
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Early Ordovician volcanism in the Iberian Chains (NE Spain) and its influence on the preservation of shell concentrations

Jose Javier Álvaro1, Hassan Ezzouhairi2, M. Luisa Ribeiro3, J. Farinha Ramos4 and A. Rita Solá5

1 Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain and UMR 8157 CNRS, USTL, 59655 Villeneuve d’Ascq, France. Jose-Javier.Alvaro{at}univ-lille1.fr
2 Département de Géologie, Université Chouaib Doukkali, 24000 El Jadida, Maroc. ezzouhairi_hassan{at}yahoo.fr
3 INETI – Area de Geociências, Estrada da Portela, Zambujal, 2721-866 Alfragide, Portugal. mluisa.ribeiro{at}ineti.pt
4 INETI – Laboratório INETI, R. Amieira, 2721-866 S. Mamede de Infesta, Portugal. farinha.ramos{at}ineti.pt
5 INETI – Area de Geociências, Estrada da Portela, Zambujal, 2721-866 Alfragide, Portugal. rita.sola{at}ineti.pt

Research is described to evaluate the influence of volcanic activity on studies of shell-concentration taphonomy and biodiversity patterns in strata that commonly display poor densities of skeletal debris. Two volcanic episodes are recorded in the Lower Ordovician of the Iberian Chains: (i) a succession of Tremadocian-earliest "Arenig", eruptive felsic products, expelled explosively and characterized by the onset of rhyolitic and dacitic tuffs embedded in the Borrachón and Santed formations; and (ii) an "Arenig", effusive basaltic volcanic episode, represented by a single lava flow embedded in the Armorican Quartzite. The volcanic activity reflects a change from sub-alkaline to alkaline geochemical affinities, related to the Early Ordovician magmatism recorded in southwestern Europe that is commonly attributed to the opening of the Rheic Ocean.

The felsic explosive tuffs are associated with two ecosystem disturbance events: (i) a short-term colonization event of opportunistic linguliformean brachiopods that proliferated in the aftermath of a multi-event rhyolitic pyroclastic surge deposit; and (ii) several single-event mass-mortality horizons associated with dacitic pyroclastic flows responsible for the preservation of high-diverse allochthonous coquinas. Our knowledge of the biodiversity patterns achieved by the benthic communities preserved in the poorly fossiliferous, siliciclastic strata of the Iberian Chains is, as a result, strongly biased by the presence of skeletal tuffs, directly controlled by the onset of neighbouring eruptive explosions.

Key Words: Volcanism • Event • Palaeoecology • Lower Ordovician • Rheic Ocean • Gondwana






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