Depositional geometry and fabric types of hydrothermal travertine deposits (Albegna Valley, Tuscany, Italy)

Travertines are continental calcium carbonate deposits precipitated by warm to hot water (from 20 up to 80 °C) supersaturated with calcium bicarbonate degassing CO2 while out-flowing from a tectonically controlled hydrothermal vent. Several mechanisms of mineral precipitation have been proposed to explain travertine formation, and these vary from abiotic to biogenic processes. The abiotic physicochemical control on mineral precipitation is driven by CO2 degassing and evaporation of water outflowing from the spring. Nevertheless, precipitation is not purely a consequence of physical degassing, due to the fact that calcium carbonate precipitation might be biologically influenced by the presence of microbial biofilms, common in such high temperature environments. Travertine deposits are widespread in different geographic areas, in particular in the Mediterranean region. Hydrothermal-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, following the Miocene Apennine thrust sheet belts propagation. 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 are from about 20 m up to 700 m a.s.l and some deposits are still active. The internal architecture of a fossil travertine body (Late Pleistocene-Holocene?) is well exposed within an active quarry located on the Manciano Sector of the Neogene Albegna Basin (Saturnia Travertine Quarry). The travertine deposit has an approximate maximum thickness of 40 m and it is laterally exposed for about 350 m. Three decametre-scale travertine units of different ages (Unit I, II, III;) separated by two claystone beds were identified in the Saturnia Travertine Quarry; these units exhibit terraced slope, smooth slope and flat pond depositional environments. Several carbonate fabrics at the centimetre-scale were distinguished on the outcrop. Petrographic analysis shows that travertines are essentially composed of a mixture of micrite, microsparite and sparite calcite crystals. The wide range of travertine fabrics can be essentially distinguished into three categories: 1) Travertine Boundstone s.l. in which the original components are directly precipitated from hydrothermal water that includes: a) dendritic boundstone; b) crystalline crust dendritic boundstone; c) fine crystalline crust dendritic boundstone;d) laminated boundstone; e) raft boundstone; f) coated gas bubble boundstone; g) micrite/ microsparite boundstone. 2) Encrusting Travertine Boundstone s.l. in which original components (acting as substrate) are directly encrusted by carbonate precipitated by hydrothermal water that includes carbonate coated reeds (reed boundstone); 3) Carbonate grain packstone/grainstone to floatstone/rudstone formed by intraclasts of already lithified travertine precipitates and lithoclasts eventually associated also with skeletal fragments such as ostracodes and gastropods. Pore space is an important component of travertine fabrics. Travertine fabrics show a wide range of depositional porosity (inter-dendritic form, channelled, non connected intra-bubble, inter-raft, inter-laminae, shelter, intraskeletal) and secondary porosity (biomoldic, vuggy meteoric dissolution, fractures). This porosity can be partially or totally occluded by a secondary cement (meteoric or related to a subsequent circulation of hydrothermal water). Dissolution and aggradational neomorphism combined with cementation are the main diagenetic processes that can alter the fabric appearance. Cementation and aggradational neomorphism are in inverse proportion to the age of these travertines. Petrographic and XRD diffraction analyses confirm that travertines are composed predominantly of calcite with minor amounts of aragonite (in raft boundstone fabric). Acicular to fibrous morphology of a significant amount of the calcite suggests that transformation of aragonite into calcite might have occurred. The large variety of growth fabrics reflects their origin (by interplay of biotic and abiotic processes), depositional environment (fast vs. slow flowing thermal water). A relationship. between fabric types and velocity/turbulence and discharged volumes of the flowing water is suggested. The travertine fabrics can be subdivided between those occurring in fast flowing water areas (crystalline crust and fine crystalline crust dendritic boundstone) and those precipitating in slow flowing water areas (dendritic boundstone, laminated boundstone; raft boundstone; coated gas bubble boundstone; micrite/ microsparite boundstone). In addition fabrics that occur 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. 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