The Cala Viola-Torre del Porticciolo coastal area: classical geological survey and 3D digital outcrop models (DOMs) analysis to unravel the polyphase tectonics in NW Sardinia (Italy).

In the Nurra region (NW Sardinia), along the coast that stretches from Cala Viola to Cala del Turco bays, a stratigraphic sequence of continental sediments ranging from the upper Paleozoic to the lower Mesozoic is wonderfully exposed.

In the last twenty years this area has been a subject of renewed studies due to its exceptional stratigraphic and sedimentological features and thanks also to an exceptional paleontological record, encompassing trace fossils and body fossils, which provided key chronological data. The most recent investigations have been focused on stratigraphic and structural geology aspects, allowing to reconstruct the tectonic evolutionary history of Sardinia, from the middle Permian up to the Middle Triassic.

The redefinition of the stratigraphic and structural framework of the area became possible through integrating traditional geological surveys (mainly, field mapping, facies and structural analyses) with analyses conducted on 3D digital models of outcrops (DOMs) obtained via drone-based aerophotogrammetry. Investigations, both in the field and computer-based, have enabled the identification of the arrangement of cross-cutting relationships among fault systems.

Despite the absence of absolute geochronological constraints (e.g., dating of syn-kinematic mineralizations), it has nevertheless been possible to identify at least six to eight deformational events. These analyses have revealed the likely history of deformational processes in this sector of the Island, highlighting the tectonic events that have occurred from the Permian until at least the Pliocene, despite Pleistocene-Holocene tectonics cannot be excluded. The events can be summarized as follows:

• Event 1: extensional tectonics related to the angular unconformity between Permian and Triassic deposits (age post early middle Permian to Early Triassic)
• Event 2: extensional tectonics related to the rifting of the Ligurian-Piedmont Ocean and subsequent carbonates deposition (Middle Jurassic: Bajocian-Bathonian)
• Event 3: extensional event related to the early Pyrenaic deformation and represented by an extensive angular unconformity/hiatus in carbonate platform (Early Cretaceous: Aptian-Albian)
• Event 4: transpressive and compressive event related to deformative phase called “Laramian” tectonics (Late Cretaceous)
• Event 5: compressive and consequent transcurrent tectonics related to the “Pyrenaic phase” (Eocene to Aquitanian age)
• Event 6: extensional tectonics related to the opening of the Balearic basin and the rotation of the Sardinia-Corsica block (Burdigalian age)
• Event 7: extensional tectonic phases occurred during two different sub-events (Serravalian age and the Pliocene age)
• Event 8: extensional event occurred during the Pleistocene-Holocene