The Buntsandstein of Sardinia (Italy) as keypoint of the Western Tethys paleogeography

Sardinia played a key role during the opening of the Western Tethys in late Early-early Middle Triassic times, being located at the intersection of several main paleogeographic plates, and its Buntsandstein successions represent a revealing paleogeographic and sedimentary touchstone. Historical and unedited sedimentological data, detailed petrographic studies, and a review of the palynological assemblages evidence that the various formations of the Sardinian Buntsandstein Group differ noticeably from each other and were deposited in different alluvial to transitional environments related to “Alpine” rifting basins. Moreover, strong compositional differences from NS to EW in the island are also observed. They support a morphotectonic rise separating the diverse depositional basins, possibly related to main European ridges in progressive dismantling. The dominating environmental and/or (micro)climate conditions were different in the various areas, and the sedimentary basins evolved independently and diachronous through time. Moreover, the source areas for the sediments varied from articulated to gentle in morphology and substrate composition. The various stratigraphic units of the Sardinian Buntsandstein cover in the center of the island a progressively more altered, smoothed Variscan basement, locally perhaps mobilized by the incoming extensional early Alpine tectonics. The paleoenvironmental reconstructions evidence an articulated, rough landscape, close to active structural lines, behind the depositional area of the SW Riu Is Corras Formation. A more elaborated depositional environment gave origin to the SW Punta S’Arridelli and NW Verrucano Sardo formations, whereas a flat, gentle landscape crossed by alluvial channels was located behind the mudflat of the Central Sardinia Escalaplano Formation. These diverse environments could be related to a gradual transgression over the hardrock and a progressive smoothing in time of the Sardinian Variscan basement. The petrographic study reveals the presence of sandstone and carbonate grains of unknown affinity in every Buntsandstein formational unit, possibly related to a dismantled sedimentary cycle. The palynoassemblages support a late Anisian or Pelsonian-Illyrian for the transgression of the Muschelkalk Sea on the Buntsandstein facies, whereas the bottom of the various formations cannot be dated due to a substantial unconformity. The oldest Triassic palynological assemblages identified in the succession are dated to the Aegean. They do, however, not correspond to the base of the Buntsandstein succession. In general, the palynological assemblages support a rather arid climate during the early Middle Triassic, although locally some more humid conditions are observed. A comparison with nearby coeval plates of the western, subequatorial area of the Tethys (Western Alps, Corse, Provence) evidences the area’s key role during the opening of the “Alpine” rifting basins.