The origin of flat surfaces at Ustica island (Sicily): a multifaceted phenomenon

Marine terraces are key landforms for the identification of spatial and temporal pattern of tectonic deformation through time. A marine terrace is any relatively flat surface of marine origin, bounded by a steeper slope inshore and off-shore. Marine terraces may result from marine abrasion or weathering, or consist of shallow water accumulations of materials removed by coastal erosion, or also of polygenic origin. The occurrence of a series of stepped marine terraces usually results from eustatic changes in sea level superimposed on a tectonic uplifting trend. These terraces act as a continuous tape recorder, in which each step developing when the rising sea level overtakes the rising land. Each terrace can be considered as a fossil counterpart of the present-day shore platform. At Ustica island (Sicily, Italy), the last interglacial transgression left a fossil deposit and a small terrace in the southern sector, in the Mezzaluna locality, and on the northern side of Falconiera, with the presence of a typical, tropical-sea malacofauna, such as Persistrombus latus, and other species that usually are related to the warm senegalese fauna which corresponded to the MIS 5.5 stage, which culminated around 117-128 ka BP. Radiometric datings the U/Th datings performed on Cladocora caespitosa indicated an age of about 132±5 ka BP. After the formation of the lowermost deposit (80 ka BP) a further sea-level oscillation produced fossiliferous marine deposits, which are presently displaced at an elevation of 2 m a.s.l. These deposits are exposed along the southern and western coasts of the island and have been dated at about 45 ka, being related to the MIS 3 stage. The oldest one is 350 ka BP. Some authors reported the existence of flat surfaces, both in the northern and southern sector of the island, that were defined as marine terraces. These surfaces range in altitude from 5 m asl to over 100 m asl. They suggested that planation processes were mainly due to marine processes. On the other hand, other authors supported the idea that the debated occurrence of a caldera, in particular in the northern side of the island, could explain the flat topography of the area. Fossil remains testify the presence of the sea at increasing altitudes, but no marine deposits related to marine-built platforms occur. Moreover, no evidence of inner and outer margin have been clearly discovered in the island and no present-day shore platforms occur along the modern Ustica coast. Marine processes acted during highstand phases for some thousands of years, but considering the tectonic uplifting behavior of the island during the last 350 ka, the ingression time-frame was undoubtedly reduced, thus reducing also marine processes. We suggest that the formation of the flat surfaces at Ustica are the result of subaerial processes of erosion, such as those pedimentrelated, and marine ones during short highstands. Although it is very difficult to quantitatively distinguish these factors, the short time in which these surfaces have been exposed to marine action seems to favor subaerial processes, while marine processes strenghtened the processes of planation