Multidisciplinary re-assessment of the Ediacaran–Cambrian boundary interval in south-western Europe

Abstract

A combination of chronostratigraphic, radiometric and carbon isotopic data across the Ediacaran–Cambrian transition in south-western Europe (Portugal, Spain and southern France) provides a reliable multidisciplinary approach to recognize the Ediacaran‒Cambrian boundary interval on a margin of West Gondwana unevenly affected by the Cadomian Orogeny. The geochronological boundary broadly coincides with a terminal Cadomian metamorphic event recorded throughout the island arc to proximal retro-arc basin preserved in the Ossa-Morena Zone. In the Central Iberian Zone, representing an intermediate position of the retro-arc basin, the boundary lies at the top of a carbonate succession (with its base constrained by SHRIMP, CA-ID-TIMS and CA-SHRIMP methods at 560 ± 2 Ma, and its uppermost part comprising the chemostratigraphic Basal Cambrian Isotope Excursion or BACE) preceding the lowest occurrence of Treptichnus pedum. In the Cantabrian Zone, a tectonostratigraphic unit fringing the Gondwana margin, it is included in a major gap of about 20 m.y. far from Cadomian tectonic perturbations. Conformable strata are recognized in the Montagne Noire and the Eastern Pyrenees, where a distinct episode of felsic explosive magmatism took place across the Ediacaran‒Cambrian boundary interval. In the Iberian massif, the end of the Cadomian orogeny is marked by a diachronous (intra‒Terreneuvian) pre-rift unconformity, characterized by the onset of collapse structures and megabreccias. One of the phosphogenetic episodes that punctuated the Ediacaran‒Cambrian transition interval recorded shelly accumulations rich in helcionellids, including Anabarella plana and Watsonella crosbyi, which represent good candidates for recognizing the base of Cambrian Stage 2 in south-western Europe. Two new regional chronostratigraphic units are defined, named Monfraguan and Calatravan stages: the base of the former is marked by the lowest occurrence of the ichnospecies Treptichnus pedum, and that of the latter by the lowest occurrence of the helcionellid Anabarella plana.