THE CHARACTER AND SPATIAL DISTRIBUTION OF HOLOCENE AND PLEISTOCENE HYDROTHERMAL TRAVERTINES IN THE ALBEGNA VALLEY, SOUTHERN TUSCANY (CENTRAL ITALY).

Characterization of Pleistocene hydrothermal travertine deposits in the Albegna Valley, Southern Tuscany, Central Italy. A Pleistocene travertine body, cropping out on the Pianetti Terrace, central-eastward area of the Neogene Albegna Basin (Southern Tuscany, central Italy) was investigated: 1) to define the geometry of the sedimentary body and depositional environments, including their lateral and vertical evolution; 2) to characterize the varieties of hydrothermal travertine carbonate fabrics; 3) to improve the understanding of the abiotic and microbially mediated processes that control calcium carbonate precipitation in hot-spring settings; 4) to establish the link between fabric types and travertine depositional environments. The Pianetti travertine body, well exposed within an active quarry, exhibits a wedge shape differentiated into three decametre scale travertine units. Exposure surfaces represented by claystone layers separate the deposit in three travertine units and constitute the travertine sequence boundaries, suggesting an intermittent accretion of the travertine system. Different architectural patterns including clinoforms, stepped and sub-horizontal layers corresponding to smooth slope, terraced slope and pond depositional environments, respectively, were individuated within the travertine units. The geometry and the relative depositional systems of each unit are controlled by different factors such as the location, typology and activity (water supply, hydrostatic pressure of the spring) of the hydrothermal vents, roughness of pre-existing surface and topography along which hydrothermal water flew and mixing with meteoric water. A large variety of fabric types characterize the Pianetti travertine body. Eight main categories of fabric types and the relative sub-categories can be essentially classified into three classes: 1) travertine boundstone and cementstone s.l. in which the original components are directly precipitated from hydrothermal thermal water; 2) encrusted travertine in which original components (acting as substrate) are directly encrusted by carbonate precipitated by flowing hydrothermal water; 3) carbonate grains packstone/grainstone to floatstone/rudstone formed by fragments of already lithified travertine precipitates (intraclasts) and other lithoclasts. Pore structure (ranging from depositional to secondary porosity) is an important component of travertine fabrics. Porosity and permeability investigations (Helium techniques, microCT scanning and image analysis software AVIZO) shows that travertines display a wide range in porosity and permeability that is a direct function of: 1) primary fabric orientation; 2) diagenesis; 3) travertine fabric. The distinguished fabric types reflect the precipitation processes due to an interplay of abiotic and biotic processes (biologically induced by microbial metabolic process or simply influenced by nucleation on microbial biofilm substrate) and subsequent diagenesis. The physical-chemical and biological interaction is also registered in travertine isotopic geochemistry. Organic matter remains and organomineral nano-globules within the Pianetti travertine suggest that for some precipitates the biologic influence is fundamental. However, even for the biologically induced mineralization, abiotic processes and physical-chemical characteristic of the precipitating thermal water are necessary for travertine formation. A relationship between fabric types and velocity/turbulence and discharged volumes of the flowing water is suggested. In addition, fabrics occurring in low energy areas might be more biologically influenced than fabrics occurring in fast flowing dipping surface for which the abiotic processes of physical degassing might prevail. Many fabrics seem to represent transitional forms of a continuum between the two end-members of purely abiotic and microbially mediated (influenced/induced) precipitates.