Travertines occur as accumulations of Calcium Carbonate produced from flowing water, often, hot springs, they are also formed from incrustation. When chemical and biological processes, act together, they are responsible for the precipitation of freshwater calcium carbonate as travertines. These elements acting together is Henry’s Law. Travertines often start by nucleating on top plants and covering them, or filling cavities in preexisting karst systems. The controls of travertines can very extremely from tectonic to paleo-environmental. Formation occurs in two basic fashions, invasive, via introduction of meteoric CO2, and evasive, via hydrothermal fluids rich in CO2 degassing. Depending on the environment of deposition and the controls there are differing fabrics and micro textures as a result of the organisms that can thrive in those environments.
Travetine deposits are accumulations of limestone formed in multiple freshwater environments. They form primarily by the incrustation of other objects or organisms by biochemical precipitation. Travertines can encrust upper and lower plants, depending on how the water flows over the organism, it is more common for them to encrust algae,and mosses. In somecases due to the shape and characteristics they posses they can be considered to be stromatalites. Travertine deposits have also been called tufa, calcareous tufa, plant-tufa, carbonate concretions, petrified moss, vaucheria tufa, chironomid tufa, spring-sinter, calcic-sinter, sinter crust, and many more names. Some researchers suggest that the term sinter should be restricted to deposits of an abiotic formation that are typically more dense and compact than tufa; most sinter-crust includes flowstones and other types of speleothems (underground karst features).
The definition of a travertine can be difficult to specify. Many of the current definitions neglect to mention temperature, or process of deposition; the most comprehensive definition comes from Pentecost (2005).
‘A chemically-precipitated continental limestone formed around seepages, springs and along streams and rivers, occasionally in lakes and consisting of calcite or aragonite, of low to moderate intercrystalline porosity and often high mouldic or framework porosity within a vadose or occasionally shallow phreatic environment. Precipitation results primarily through the transfer (evasion or invasion) of carbon dioxide from or to a groundwater source leading to calcium carbonate supersaturation, with nucleation/crystal growth occurring upon a submerged surface.’
Geological Setting For travertine precipitation to occur, the carrier waters must be of favorable concentration of CO2 and Ca+. The component of the water that is the most subject to change is CO2 concentration, this controlled by Henry’s Law (the relation of CO2 with temperature and pressure) and biologicaly controlled via photosynthesis. In order for precipitation of calcium carbonate to occur it is necessary for the level of CO2 in water to go down (Ramon,1983). The processes of physio-chemical and/or biochemical incrustation occur, in karst or thermomogene springs, where the vegetation and/or temperature fluctuation can force the carbonate precipitation. Because of the way they precipitate travertines hold similat characteristics to that of traditional carbonates, to physio-chemical precipitation of carbonate; in streams, rivers, or flow channels, they are usually shaped by these natural rimstone dams and rimstone pools. The dams are mainly formed by incrustation of vegetation the flowing water encounters, and also by interruption of the flow , forming pools. In these pools detritic sedimentation processes coexist with incrustation processes, sometimes choking out the frormation of carbonate or forcing the formation to occur further downstream over a larger area (Ramon, 1983).
Only a couple chemical processes are responsible for most travertine formation worlwide. Almost all travertines form from the degassing of metogene carbon dioxide-rich groundwaters containing calcium. Groundwater that is capable of depositing travertine is produced when carbonic acid dissolves the carbonate rocks to form a solution containing calcium and bicarbonate ions.
(Eq. 1.1): CaCO3 + CO2 + H2O = Ca2+ + 2(HCO3)’
Travertine deposition is the opposite of reaction the first reaction (Eq. 1.1). Carbon dioxide is lost from the solution when it contacts the atmosphere, where the CO2 concentration is lower than that in equilibrium with the dissolving solution. The evasion of carbon dioxide occurs to the epigean atmosphere, additional CO2 loss is forced through photosynthesis of aquatic plants and evaporation (Pentecost,2005).
(Eq. 1.2): Ca(OH)2 + CO2 = CaCO3 + H2O
Travertines precipitated from (Eq. 1.2) are widely distributed but not very common. Hydroxyl ions in lake water react with bicarbonate (HCO3 ‘) to form carbonate (CO3 2′) forcing the precipitation of calcium carbonate.
(Eq. 1.3). Ca(HCO3)2 + OH’ = CaCO3 + HCO3 ‘ + H2O
This reaction is mainly confined to saline lakes where the OH’ concentration is very high as a result of geochemical processes. These carbonates do not form through as a transfer of carbon dioxide to atmosphere and are not strictly travertines either (Pentecost, 2005). As a result of the combination of the geochemisty induced by physiographic regions that the travertines formed in, they can be defined as meteogene or thermogene deposits.
Meteogene vs. Thermogene Travertines
Soil and atmospheric carbon dioxide may be regarded as meteoric in origin, since the terrestrial vegetation and associated soil contains carbon obtained from the atmosphere. Travertines formed from groundwaters charged with a meteoric carrier are called ‘meteogene’ (Figure 1). They form typically in cold-water springs in regions underlain by carbonates. Occasionally, such waters circulate deep beneath the ground where they become heated and rise as hot springs, but contain only the meteoric carrier. These travertines, often the result of artesian flow, have been described as ‘thermometeogene’ (Pentecost, 2005). Invasive meteogenes include those formed through the reaction of atmospheric CO2 with groundwater as described in (Eq. 1.2) above. The soil atmosphere is the largest contributor of carbonic acid causing limestone dissolution and the CO2 enrichment of groundwaters, a large portion is dissolved by percolating rainwater. Meteogene travertines are divided into two categories; the evasive meteogenes where carbon dioxide evasion leads to travertine deposition, and invasive meteogenes where the reverse process leads to deposition.
Thermogene Travertines usually contain some meteoric carrier, but most of the carbon dioxide originates from thermal process within and below the crust (Branner,1901).
Thermally generated carbon dioxide dissolves in groundwater, often under considerable pressure and the resulting high concentrations of CO2 are capable of dissolving large volumes of rock, the solutions rising as hot, bubbling springs, forming a hydrothermal circuit . Typical dissolved inorganic carbon (DIC) and Ca levels respectively, are two to ten times higher than most meteogene sources (Pentecost,2005). Rates of degassing and precipitation are correspondingly higher, providing distinctive fabrics and the travertine stable carbon isotope composition is generally heavier than meteogene waters. Thermogene deposits have a more localized distribution than meteogenes, they are often associated with regions that have a recent volcanic or tectonic history. It is important to note that thermogene source waters are not necessarily hot, although this is frequently the case, the term applying to the source, rather than the exit temperature of the water.
There are a multitude of fabrics related to the different organisms involved in the processes of travertine formation. The different types of vegetation operate in different ways: decaying CO2, precipitating biochemical calcite including metabolic calcite formed in the cells, trapping the particles carried by the water flow, and finally acting as the catalyst to the predominant physicochemical precipitation. There are two main fabrics and it is relatively easy to tell the difference between the two. One is related to algal activity and is characterized by the formation of micro-cryptocrystalline anhedral crystals and microfibers (Pentecost, 2005).The second fabric is characterized by the deposition of euhedral crystals over moss stalks, hepatics, and Miriophilaceae, which reach the millimeter range in size (Pentecost, 2005). The different algal families may show small fabric variations in their lamina due to their rhythmic seasonal growth. Calcite and aragonite travetines are distinguished by the fabrics they produce, though there is some variation in their respective categories as well.
Calcite microfabrics are either composed of micrite or spar. Micrite laminations are often evident in algal travertines, where millimeter thick layers are due to a variation in density or colour, and often related to the seasonal growth of algae. Micrite is deposited around, and probably within bacterial colonies, around algae such as Vaucheria and especially cyanobacteria. Some clastic travertines, such as ‘spring chalks’ and dam fills are sometimes composed almost entirely of micrite where a mixture of fine authigenic precipitate combined with mechanically disintegrated clastics can be seen.Spar is often the largest
component of epigean travertine, diagenticaly altered travertines (Branner, 1901).
Calcite Mesofabrics are the easier way to tell the difference between calcite and aragonite travertines. Calcite Forms dendritic patterns, and ‘shrubs’, where aragonite can formsimilar patterns but will deposit needle like crystals rather than micrites or spar. Durring high precipitation rates complex dendtitic crystals will form from the thermogene travertines. During slower precipitation times, the calcium carbonate will clump up and from tiny balls that look similar to peloids, they will they glom together forming shrubs (fig. 2). Arrogonite travertines form in more acicular spherulites and give a botryoidal texture.
One of the most notable mesofabrics of travertines is their porosity. Voids are a characteristic feature of almost all travertines, and part of the fabric . The porosity of most travertines is fabric selective and can be divided into four sub-types: intercrystalline; mouldic; fenestral, and shelter porosity (Fig. 3). While some of these may be classified as microfabric, the mesofabrics are more characteristic for most travertines. Porosity also results from the activities of burrowing invertebrates such as Lithotanytarsus and Tinodes (Steidmann, 1936). Intercrystalline porosity is found in all travertines but varies according to the density and form of the crystals.
Several criteria have been used for identification of travertines in the rock records, the most important are that which are related plants and geometric disposition. The travertine bodies are laterally discontinuous and may show subaerial exposure or inter-stratification with fluvial deposits (Branner,1901). The travertine ‘fossil’ records also show laminated botryoidal, radial and arborescent carbonate fabrics with anabundance of calcite in bushes or shrubs of large crystal.