Freshwater travertines in central Apennine (Italy): genesis and climatic and neotectonic significance.

The aim of this study, using geomorphologic, sedimentological and geochemical new data, is to present a synthesis of the knowledge about genesis of freshwater travertine deposits in the calcareous ridge of Central Apennine (Italy). Different areas in the Umbria-Marche and Latio-Abruzzi Ridges, have been chosen such as very good examples of typical travertine deposition in central Apennine. All the areas, even if characterized by different extension, thickness and hydrochemistry of circulating waters, show similar features (depositional environment, climate conditions, structural and neotectonic conditioning) significant for reconstruct the dynamics of travertine deposition. The more or less intense deposition of travertine (very different even in the same area, during different phases) is related to hydrogeological conditions changing in time. Valley deepening, consequent to climatic changes (such as glacial-interglacial transition) and/or periods of more intense tectonic uplift, caused rapid lowering of the piezometric surface inside carbonate aquifers; this fact increased the hydrologic regime of the rivers together with climate improvement, favourable to travertine deposition. Subsequently, the process was decreasing in time, same way as a box, filled by water and with hole in the bottom, that is progressively emptying. Such phenomenon, together with the above mentioned climatic changes which aim to a new equilibrium, seems to be the most important cause of the progressive decreasing (and sometimes interruption) of travertine depositional processes. Also in the Acquasanta Terme area, where the rising up of thermal water along fractures produced the primary condition to travertine formation, deposition occurred in a fluvial environment, along tributary valleys, on the right side of the Tronto river; this is testified by the presence, at the base and inside the deposit, of alluvial gravely material, fluvial facies and typical “fall” structures. Absolute and relative dating confirmed that travertine formation occurred starting from the end of Middle Pleistocene and mainly during transition between glacial and interglacial periods. However, the presence of older deposits (Early or Middle Pleistocene) in the same areas and not more visible, is not out of the question.