Significance and age of "megakarrens" in Sardinia (Italy)

In the central part of Sardinia, a series of parallel cavities along an erosional surface ouctropped in the limestones and marbles of the Miocene that characterize the study area. The survey of the area around the micenic rocks allowed to assign these open cavities to the fractures of the Miocene rocks undergo a slow process of karstification. The fractures show distances between the walls from 0:50 to 2:00 ms, while it was not possible to observe the closure in the bottom. All fractures were filled with residual clay sandly (like "Terra Rossa") with missing components. The distance between the individual fractures is varied but the direction is always parallel, while in the area the fractures sequence has directions concordant with the recent tectonic and different. However, fractures outside the studied area rarely show the fill clay; fact due to the remarkable quantità of water moved into and out for a long time in these morphologies exposed to surface water circulation. The spread of the phenomenon and the morphology of the territory is rather difficult to define, it is a series of "megakarren" whose size suggests a landscape like "forest stones" whose forming time could have been very long. Recently, in a cave not far from the studied site, the explotation activity revealed a contact between an outcrop of these fractures with an overlying volcanic episode. The sampling has identified a basalt rock of the Plio-Pleistocenic volcanic cycle that affected the whole island through numerous episodes effusive. The dating of this basalt by Ar/Ar method gave an age of 2.2 M.y.b.p. placing it at the end of the Pliocene. This information provides an important element to dating this phenomenon in a sub-tropical environment, under hot and humid climate. The age of this rock let to date the long period of leaching and the important rule of areal erosion consequent to cold phases after the Pliocene.