Stratigraphical and palaeogeographical significance of the continental sedimentary transition across the Permian–Triassic boundary in Spain

Abstract

The Permian–Triassic transition, a time of phenomenal palaeoenvironmental and palaeogeographical change, represents the largest episode of mass extinctions known to palaeontologists. This episode is, however, very poorly understood, and a lack of sediments and palaeontological data, particularly in the continental record, is a feature common to every basin. Despite compelling information on the continental Permian–Triassic boundary (PTB) emerging from recent efforts evaluating vertebrate evolution, fungal events and isotope curves, results are still insufficient to make any valuable correlations among continental basins and are even less appropriate for relating sediments of continental to those of marine origin. The present report discusses and attempts to locate the PTB in the basins of Iberia and Balearic Islands through an analysis of the palaeontological and sedimentary record across the Permian–Triassic transition. The aim of the paper is to contribute to our present knowledge of the palaeogeographical and stratigraphical significance of the Permian and Triassic continental units of Western Europe. The present-day Iberian Ranges, Catalonian Ranges and Balearic Islands hold the most complete Late Permian–Early Triassic sedimentary record in Iberia. This record consists of alluvial sediments, mainly of braided fluvial systems, corresponding to the “Saxonian” and Buntsandstein facies. All the units examined so far have been dated through palynological associations. The two “Saxonian” facies formations are of Thuringian age, although the lower one shows some palynological elements of Autunian affinities. The Buntsandstein units range from Thuringian to Anisian in age, there being no evidence up to now of the Scythian in Iberia. The exact position of the PTB in Iberia is up to now impossible to pinpoint with the precision of the marine series. Sedimentary characteristics and palynological data in the Iberian Ranges point that it lies at the sedimentary interval, 10 to 30 m thick, formed by the upper member of the La Hoz del Gallo Formation and the lowermost beds of the Cañizar Formation or its time equivalent the Prades and Eramprunyá units in the Catalonian Ranges and the Asá and B-1 in the Balearic Islands. We compare a normalized standard Late Permian–Middle Triassic column of the Iberian Ranges with standards from Western and Central Europe and the Southern Urals using recent Permian time scales. A palaeogeographic essay map of the Permian–Triassic transition is provided for the Iberian Plate. The plate was located in a northern subtropical position, 200–400 km from the westernmost Tethys coast, as part of the Central Pangean Mountain Range comprised of the Appalachian–Mauretanide–Variscan orogenic belts of estimated altitudes 2000–4000 m. Based on this palaeogeographic location, sedimentary characteristics and the main wind flows established for the Tethys, we can infer conditions of high precipitation dominated by monsoon and seasonal regimes with isolated semiarid areas for the Iberian Plate during the transition.