Genesi ed evoluzione della pianura costiera turritana (Sardegna settentrionale)

Genesis and evolution of the Turritano coastal plain (North Sardinia, Italy)

Abstract

This paper refers to an area lying behind the Sassari shores, consisting of a narrow flat-land, which lies in a 20 km long strip parallel to the coast line, and of a NW gently dipping surface, eastward and southward bounded by weak reliefs, which never exceed 400 m above sea level. Both geological and geomorphological mapping have been carried out in order to reconstruct the evolution of the coastal plain and its relationship with recent Tectonics. The structuration of this district started in the Neogene: most of the sedimentary deposit of this age occupies the mapped area A Neogene transgressive sequence, consisting of conglomerates, sands and bioclastic limestones, characterized by the occurrence of hermatipic bodies of Uppermost Burdigalian age, capped by a thick sequence of marls alternating thin limy levels probably belonging to Serravallian, above a pre-Upper Burdigalian complex, consisting both on calc- alkaline volcanic and lacustrine to shallow sea rocks. The part of plain near the coast-line is occupied by Quaternary deposits, principally aeolian sands. The main aeolian deposits have been recognized: the oldest belonging to Riss time, the second to the Würm and finally a Holocenic not yet planated dunas field, where are parabolic, longitudinal hook-shaped types. An ancient paleosoil suggests an aeolian episode during Mindel. Embedded between the Rissian and Würmian eolianites a Tyrrhenian episode marked by a beach rock (panchina); less extended deposits are represented by lacustrine to brackish environment limestone lying behind the old Tyrrhenian coastal barrier. Quaternary deposits have also been recognized in the inner part of the area, consisting of slope, colluvial and slope stratified deposits. The latter, never signaled in Anglona and Turritano, were deposited in a periglacial environment during the Würm epoch. The links between the Turritano area and the Neogene Tectonics are very close. A distensive movement generated a NNW trending asymmetrical graben, contemporaneous with the Upper Burdigalian transgression, inward a more wide one of Oligocene age and with a N-S trend. The deeping of the sea inside the NNW trending graben led to deposition of marls which are the most recent Neogenic deposits. After a presumed Messinian compression, a general uplift in a distensive regime occurred. Those movements upset the pre-existing accidents giving a whole of tilted blocks. So, most of territory is characterized by a gently NW dipping paleosurface parallel to the bedding of Miocene strata, eroded by cataclinal streams. The central part of the land consists of a wide tilted block, which develops a smooth surface. In the ESE part of the area changings in lithology are stressed by breaks of slope and the drainage is influenced by NNE faulting: the slopes are characterized by cuestas both corresponding to ignimbritic flows and limy beds of high competence. A contrast on landforms rises between the inner area and the outer coastal strip. The first shows strongly Tectonics guided landforms, the latter shows a landscape due to exogenic activity. Particularly aeolian morphogenesis modelled the outer plain with an immense sand field and the Holocenic dunas system. In the main valley, i.e. the Rio Silis, a deep seated gravitational phenomenon occurs, probably linked to Quaternary seismic shocks, connected with the latest basaltic vulcanism of the region. Finally, investigations on the present coastal dynamics have been performed using granulometric methods. The results gave a confirmation of a general retrogradation of the sandy shore, and those on the inner part of the continental shelf have permitted to observe the continuity of the morphologic and tectonic features of the emerged Turritano land in the Gulf of Asinara. The Turritano coastal plain can be defined as a flat-land primarily built (sensu SHEPARD), then grown up by aggradation of aeolian sediments.