Depositional geometry and fabric types of hydrothermal travertine deposits (Albegna Valley, Tuscany, Italy)
Abstract
Data di Pubblicazione:
2012
Citazione:
Depositional geometry and fabric types of hydrothermal travertine deposits (Albegna Valley, Tuscany, Italy) / F. Barilaro, G. Della Porta, E. Capezzuoli. - In: RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA. - ISSN 2035-8008. - 21:2(2012), pp. 1024-1025. ((Intervento presentato al 86. convegno Congresso Nazionale della Società Geologica Italiana nel 2012.
Abstract:
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 process
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
Albegna Valley; Hydrotermal travertines; Tuscany
Elenco autori:
F. Barilaro, G. Della Porta, E. Capezzuoli
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