Abstract
Nineteen ichnospecies belonging to thirteen ichnogenera (Archaeonassa, Catenichnus,Cochlichnus, Cruziana, Didymaulichnus, ?Diplichnites, Gordia, Lingulichnus, Lockeia, cf.Monocraterion, Planolites, Ptychoplasma, and Rusophycus) occur in the Pochico Formation(Arenig) in the Aldeaquemada section, Sierra Morena, southern Spain, just above the ArmoricanQuartzite. They belong to the archetypal Cruziana ichnofacies, indicating a lower shoreface-upperoffshore zone. The low degree of sediment reworking may be due to a high rate of sedimentation.The trace fossil assemblage, rich in large Cruziana, is typical of the Armorican Quartzite thatdeveloped on the margins of Gondwana and peri-Gondwanan microcontinents. The distribution ofichnofauna during the Early Ordovician was partly palaeogeographically controlled, althoughichnological data from the literature point to paths of migration between Gondwana, Baltica andLaurentia. Differences between the ichnofauna of Gondwana and Baltica could be conditioned byfacies (clastics in Gondwana and carbonates in Baltica) causing a taphonomic filter, becauseCruziana requires diversified clastic deposits for preservation. The ichnofauna would also beinfluenced by trophic group amensalism between filter feeding and deposit feeding fauna, theformer prevailing in Baltica and the latter in Gondwana.
Highlights
Arenigian shallow-marine clastic sediments of Western Europe, North Africa, South America and some parts of North America are characterized by abundant trace fossils, including large Cruziana, an ichnogenus first described from such deposits in Bolivia
We describe a trace fossil assemblage in one section of the Armorican Quartzite facies, just on the transition of the Armorican Quartzite sensu stricto, and overlying heterolithic facies of the Pochico Formation in the Aldeaquemada section (Cimbarra section in Rey and Hidalgo, 2004), Central Iberian Zone (Fig. 1)
The described trace fossils belong to the archetypal Cruziana ichnofacies, indicating a lower shoreface-upper offshore zone
Summary
Arenigian shallow-marine clastic sediments of Western Europe, North Africa, South America and some parts of North America are characterized by abundant trace fossils, including large Cruziana, an ichnogenus first described from such deposits in Bolivia (ďOrbigny, 1842) Trace fossils of these facies are largely associated with quartzitic sandstones, in France and on the Iberian Peninsula largely attributed to the Armorican Quartzite (Grès Armoricain) (Gutiérrez-Marco et al, 2002). The Armorican Quartzite and associated deposits rich in quartzite sandstones may be considered as a facies deposited in shallow seas in the margin part of Gondwana and peri-Gondwanan microcontinents in the stage of rifting and early drift of Avalonia (Gutiérrez-Alonso et al, 2007) This area was a southernmost part of Gondwana and was situated near the South Pole during the Early Ordovician (e.g., Cocks and Torsvik, 2002).
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