The Palaeozoic evolution from the pre-Variscan sea to the Permian magmatism and sedimentation
The Palaeozoic evolution from the pre-Variscan sea to the Permian magmatism and sedimentation.
The Dolomites area can be schematically subdivided into an eastern and a western portion, recording different tectonic, stratigraphic, burial, and thermal evolutions. The western portion records repeated massif episodes of magmatism and subsidence confined to specific time intervals, while the eastern areas record a continuous and larger subsidence.The crustal structures separating these two areas run roughly along the Badia and Cordevole Valleys and have been active since Palaeozoic times. During the Early Palaeozoic, terrigenous marine sedimentation took place in passive continental margin settings, providing protoliths for the Late Palaeozoic collisional metamorphism that shaped the Dolomites basement. In Carboniferous times (about 330 Myr B.P.), the area was involved into the climax of the Variscan ductile deformation and then uplifted and eroded down to the greenschist basement level. Scattered immature conglomerate bodies then accumulated into incised valleys (Conglomerato di Ponte Gardena). During the late Early Permian, transtensive stress induced fast subsidence, within pull-apart structures, in the western Dolomites. These subsiding areas were filled by large volumes of contemporaneous volcanites, dominated by riodacitic ignimbrites (Bozen Porphyrites = Complesso Porfirico Atesino, sometimes exceeding 2 km in thickness. Volcanic activity started into the graben axis and then spread out, conquering almost the whole of the western Dolomites. Continental deposits are locally intercalated to the volcanites, preserving the oldest known reptile fossil from the area. Granitoid masses were in the meanwhile largely intruded into adjacent upper crust volumes (Cima d’Asta and Bressanone-Brixen intrusions). Hydrothermal circulation generated spectacular crystal aggregates and some sulphite ore accumulation. The uplifted eastern Dolomites stayed in the meanwhile unaffected by magmatism. During the Late Permian, moderate extension induced the starting of the stratigraphic cover accumulation. A comparatively faster subsidence was now registered in the eastern portion of the Dolomites. The relative sea-level increase was initially compensated by continental sedimentation, but repeated westward transgressive pulses then developed, eventually leading to shallow marine environments being developed almost throughout the Dolomites. The early continental deposits (Groedner Sandstein = Arenarie di Val Gardena) witness the evolution from fan conglomerated to braided and meandering river sandstones, and eventually to arid coastal plain sediments. Reptile foot-print associations and plant remains are nicely preserved within this successions (e.g. Butterloch Gorge), providing a valuable insight into late Permian continental life. Transgression then induced marine sulphate sedimentation, dominating the lower and western portions of the latest Permian formation (Bellerophon Fm). A normal salinity, marine carbonate unit then progressively accumulated, ranging from a few to more than 200 m in thickness. This unit records an eastward dipping calcareous ramp, dominated by micritic muds and loose bioclastic materials. The regions to the west of the Adige Valley remained under continental conditions into the Early Triassic, and therefore they lacks any Permian carbonate.