Sedimentology and petrography of hot-spring travertine deposits in the Albegna Valley, Southern Tuscany (Central Italy)

Travertines, non marine carbonates of calcium carbonate precipitation under thermal-hydrothermal condition are produced by CaCO3 precipitation due to degassing and evaporation of the spring waters; nevertheless biological influence may also stimulate precipitation. Hot-spring travertine deposition in Central Italy is considered mainly to be related to the magmatic and hydrothermal activity linked to the Plio-Pleistocene extensional tectonics developed during the opening of the Tyrrhenian Sea. Central Italy has extensive travertine accumulations all younger than 400 kyrs that include present-day deposits. In the Neogene Albegna Basin, southern Tuscany (central Italy), travertines are present in several deposits, distributed along faults and fractures. They were chiefly deposited as fans and wedges with terraced slopes and are located at different topographic heights (from about 25 up to 700 m a.s.l). The travertine body under study (Pleistocene- Holocene) is well exposed within an active quarry situated on the Manciano sector of the Albegna Valley. Three meter-scale travertine units separated by two clay levels were identified in the “Saturnia Travertine Quarry” suggesting an intermittent accretion of the travertine body. These travertine units exhibit terraced slope, smooth slope depositional systems and filling topographic depressions. The terraced slope system (30-35 m thick) consists of terrace walls (several cm to 2 m high), pools (1-15 m wide), pool rims (few cm to 1 m high) and waterfalls (2-3 m high). The smooth slope consists of non-terraced and low-angle slope on which the terrace pools did not developed. More than thirteen carbonate fabrics at the cm-scale were distinguished in the field. These occur in different depositional environments (i.e., fast- vs. low-flowing water). Crystalline crusts (from cm to a few dm thick) are common of rims and walls, vertical surfaces of waterfalls of the terrace pools. These were also characteristic of smooth slope, where water supply and flow velocity are fast. Shrubs, mm to cm arborescent structures that expand away from the substrate, are typical components of pools of terraced slope (relatively low energy areas). In horizons of reduced topographic relief (ponds) and of low-angle dip of smooth slope systems, cm-scale alteration of different fabrics occurs including: shrubs (few mm to 20 cm), mm to cm elongated rafts, undulated sub-millimeter stromatolite-like structures, honeycomb, pisoids, micrite layers, rounded and elongated bubbles. Petrographic analysis displays diversified microfabrics. Shrub structures usually consist of peloidal micrite. Crystal shrubs display a dendritic crystalline morphology and undulated extinctions. Calcite crystals (generally micritized) show a range of morphology from feather to ray crystals and dominate the crystalline crusts. 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. The microfabrics show 15-30% porosity, partially closed by spar calcite (likely of meteoric origin), micrite and microspar. The occurrence of biofilms in actively forming travertines at Bagni di Saturnia (12 km far from Saturnia Travertine quarry) confirms the hypothesis of the possible presence of an organic framework in some fabric of the studied fossil travertines. The study of Albegna River Basin travertine provide fundamental information in terms of geometry and lateral and vertical evolution of the sedimentary bodies, depositional facies, carbonate fabrics their diagenesis and porosity that can have implications for comparable carbonate reservoirs in the subsurface.