Comment on “The Permian–Triassic transition and the onset of Mesozoic sedimentation at the northwestern peri Tethyan domain scale: Palaeogeographic maps and geodynamic implications” by S. Bourquin, A. Bercovici, J. López-Gómez, J. B. Diez, J. Broutin, A. Ronchi, M. Durand, A. Arché, B. Linol and F. Amour. [Palaeogeography, Palaeoclimatology, Palaeoecology 299 (2011) 265–280

Abstract

The recent paper by Bourquin et al. (2011) describes the regional palaeogeographic evolution of the Late Permian to Middle Triassic on a pan-European scale. Whilst useful data and interpretations are provided for their southern and intra-Variscan belt domains, additional well and seismic data from the Central and Northern North Sea, and the Atlantic margin region, together with results from recent publications, allow us to refine and improve their interpretation of their northern domain. The presence of a regional unconformity at the Permian–Triassic boundary, with only localised Induan sedimentation, is less clear in the northern region, where multiple extensional events through Late Permian and Triassic time led to the development of localised footwall and rift flank unconformities as Arctic rifting propagated southwards and entered the North Sea and the Atlantic Margin regions. The Hardegsen unconformity may be locally identifiable in the southern portion of the northern domain, but again forms one of a number of unconformities marking the onset of rifting. Data from the northern domain suggest that Early Triassic aeolian activity was ubiquitous on the distal fringes of terminal fluvial systems, but with no evidence of the discrete episode of increased aridity in the Smithian identified in the south. The onset of vegetation cover in the northern domain was also more diachronous in this region, appearing in the Central North Sea at approximately the same time as Muschelkalk flooding of the adjacent Southern Permian Basin, and later in basins further to the north. In most cases the Middle Triassic fluvial systems in the north were terminal in nature, failed to discharge into marine basins and were not exorheic in character. Their advance and retreat was independent of sea level fluctuations or the presence of a marine water body, and fluvial expansion and retreat, aeolian reworking and vegetation cover appear to have been linked to catchment runoff. The disconnect with the observations to the south may be in part a product of the more northern latitudinal position of these systems, or at least that of the headwaters of the fluvial drainage.