Upper Pleistocene-Holocene? Terraced-Slope Hot-Spring Travertine System and Its Modern Analogue in the Albegna Valley, Southern Tuscany (Central Italy)

In the Neogene Albegna Basin, southern Tuscany (central Italy), travertines are present in several deposits. They were deposited as fans and wedges with terraced slopes and are located at different topographic heights (from about 100 up to 700 m a.s.l). An interesting fossil travertine body (Upper Pleistocene-Holocene?) is well exposed along an active quarry situated on the Manciano sector of the Albegna Valley. The quarry faces exhibit a travertine terraced slope system (30-35 m thick) in which terrace walls (several cm to 2 m high), pools (1-10 m wide), pool rims (few cm to 1 m high) and waterfalls (2-3 m high) were identified. Thirteen carbonate fabrics at the cm-scale were distinguished in the field. Petrographic analysis displays diversified microfabrics. Shrub structures usually consist of peloidal micrite. Crystal shrubs display a dentridic crystalline morphology; these also have undulated extinctions and sometimes build fun-shaped shrubs. Calcite crystals (generally micritized) show a range of morphology from feather to ray crystals and dominate the crystalline crusts. Feather crystals commonly show laminations while ray crystals appear to be vertically separated by elongated cavities. Thin or crude laminated micrite/microspar forms stromatolite-like structures. Honeycomb like-structures consist of thin micrite/microspar laminae aggregated into packages with large subspherical/lenticular discontinuous cavities bulged up by large gas bubbles or insect larvae. Actively forming travertines occur today at Baths of Saturnia. These could represent the modern analogue for the ancient travertine terraced slope system of Manciano (12Km far). Present-day thermal activity is characterized by a 37°C thermal spring with a rate of about 800 l/s. Water is enriched in H2S-CO2-SO42- and HCO3- and divalent cations (Mg, Ca). Minerals dissolved in water amount to 2.94 g/l. The downstream surfaces of these modern travertines are colonized by a soft green biofilm. Elongated and rounded, several mms to cms in diameters, pisoids form in the pools (1-4 m wide; rims 0.2-1m high). Locally reeds are present at the sides of the pools where there has been temporally no thermal water flow. The Baths of Saturnia ecosystem provides an important opportunity to study the biogeochemical and physical interactions that produce travertines similar to those of the past. The study of Albegna travertines can improve the understanding of comparable carbonate reservoirs in the subsurface.