The regional Marianian Stage (Cambrian Series 2) of the Ossa-Morena Zone, SW Iberia: trilobite biostratigraphy and international correlation

The Cambrian Series 2 is challenging for biostratigraphy and correlation. In current Europe (mostly West Gondwana), this Series sequences are hard to identify due to the scarcity of its fossil record. An exception is the Ossa-Morena Zone (OMZ), southwestern Iberia, where lower Cambrian fossils are fairly abundant. The OMZ belongs to the southern branch of the Variscan Orogen of the Iberian Peninsula, comprising rocks dating from the latest Proterozoic to the Carboniferous. Its Cambrian sequences are preserved in distinct ‘blocks’, showing a significant change of facies and thicknesses, most likely related to downthrow and tilting along an active growth fault during sedimentation.  Four regional stages were proposed for the Cambrian Series 2: Ovetian, Marianian, Bilbilian and Leonian (lower part). Among these, the Marianian Stage has suffered significant conceptual modifications since its original erection. At the moment, its correlation through the Iberian Peninsula is still a subject of debate.

The OMZ record of trilobite fossils and, therefore, their biostratigraphy, has been hindered due to the limited continuity of trilobite facies in many sections, the poor preservation of the specimens, and tectonic distortion. Based on work carried out in recent years, we present the updated trilobite distribution and contribution for the biostratigraphical subdivision of the Marianian Stage in the OMZ, to improve and refine intra- and inter-regional correlation. The base of the Marianian Stage is characterized by the FAD of Strenuella; being the lower Marianian substage characterized by the co-occurrence of Delgadella souzai, Mimacca? and Saukianda andalusiae. The boundary with the middle Marianian is defined by the FAD of Strenuaeva sampelayoi, and this substage is characterized by D. souzai, S. andalusiae, Alanisia guillermoi, Perrector perrectus, Eops eo, Gigantopygus cf. bondoni, Andalusiana cornuta, Triangulaspis fusca, Callavia choffati, Rinconia schneideri, Calodiscus ibericus, Atops calanus, Hicksia elvensis and Termierella sevillana. Finally, the base and top of the upper Marianian are marked, respectively, by the FAD and LAD of Serrodiscus bellimarginatus, being this substage characterized by the presence of T. fusca, Chelediscus garzoni, Protaldonaia morenica, and Pseudatops reticulatus. The new biostratigraphical data strengthens the correlation with Cambrian Series 2 sequences along the western Gondwana margin (e.g. Banian Stage from Morocco and the Charlottenhof Fm from Germany), western and eastern Avalonia (upper Callavia and lower Strenuella sabulosa Biozones), Baltica (lowermost Holmia kjerulfi to lowermost Ellipsostrenua spinosa Biozones) and Siberia (Botoman and lowermost Toyonian Stages). In addition, biostratigraphical correlation with the Dyeran Stage of Laurentia, Elliptocephala asaphoides Biozone from the Taconic Allochthon and Pararaia janeae Biozone from Australia have been improved.