Timing of river incision and speleogenesis in the Tacchi area (Central-East Sardinia)

The east-central part of Sardinia (Italy) is characterised by Jurassic dolomitic mesas (Tacchi, or “table mountains”) that overlie a Palaeozoic basement mainly composed of metavolcanics and phyllites. These mountains are the remnants of a continuous carbonate cover, dissected by faults and river erosion, and are now completely isolated hydrological systems. Most of these rivers have cut valleys more than 200 metres deep into the Palaeozoic basement rocks, whose slopes are often characterised by landslides, suggesting their recent oversteepening. Some valleys, on the contrary, have not reached the base of the carbonate sequence and appear to be suspended above the deeper incisions, apparently disconnected by them. Several subhorizontal surfaces can be distinguished on the table mountains, related to local base level stillstands. Also water table caves, scattered along the flanks of the mountains over an altitudinal range of about 200 m, show several stillstands in base level lowering. 26Al and 10Be burial dating of sediments in four caves located at different elevations on the flanks of the suspended Taquisara Valley show an Upper Pliocene or Lower Pleistocene age. Thus, this valley appears to be of Late Tertiary age. The deeper valleys, such as Riu Pardu, that dissect the Tacchi mountains completely, cutting deeply into the basement rocks, are much younger, as their unstable slopes suggest. Knickpoint retreat in Riu Pardu and estimated valley erosion rates suggest the capture of Riu Pardu by Rio Pelau to have occurred in the last 100 ky.