Modern and fossil tufa deposits abound with wetland plants; as such, many tufa deposits are characterised by their large component, and are highly porous. Tufa forms either in fluvial channels or in lacustrine environments. Ford and Pedley provide a review of tufa systems worldwide.
Spring – Deposits form on emergence from a spring/seep. Morphology can vary from wetlands to spring aprons
Braided channel – Deposits form within a fluvial channel, dominated by oncoids
Cascade – Deposits form at waterfalls, deposition is focused here due to accelerated flow
Barrage – Deposits form as a series of barrages across a channel, which may grow up to several metres in height. Barrages often contain a significant detrital component, composed of organic material.
Lacustrine tufas are generally formed at the periphery of lakes and built-up phytoherms, and on stromatolites. Oncoids are also common in these environments.
Other deposits
While fluvial and lacustrine systems make up the bulk of tufa systems worldwide, there are several other important tufa environments.
Calcareous sinter
Although sometimes regarded as a distinct carbonate deposit, calcareous sinter formed from ambient temperature water can be considered a sub-type of tufa.
Speleothems
Calcareous speleothems may be regarded as a form of calcareous sinter. They lack any significant macrophyte component due to the absence of light, and for this reason they are often morphologically closer to travertine or calcareous sinter. , California
Tufa columns
Tufa columns are an unusual form of tufa typically associated with saline lakes. They are distinct from most tufa deposits in that they lack any significant macrophyte component, due to the salinity excluding mesophilic organisms. Some tufa columns may actually form from hot-springs, and may therefore constitute a form of travertine. It is generally thought that such features form from CaCO3 precipitated when carbonate rich source waters emerge into alkaline soda lakes. They have also been found in marine settings in the Ikka fjord of Greenland where the Ikaite columns can reach up to in height.
Biology
Tufa deposits form an important habitat for a diverse flora. Bryophytes and diatoms are well represented. The porosity of the deposits creates a wet habitat ideal for these plants.
Geochemistry
Modern tufa is formed from alkaline waters, supersaturated with calcite. On emergence, waters degas CO2 due to the lower atmospheric pCO2, resulting in an increase in pH. Since carbonate solubility decreases with increased pH, precipitation is induced. Supersaturation may be enhanced by factors leading to a reduction in pCO2, for example increased air-water interactions at waterfalls may be important, as may photosynthesis. Recently it has been demonstrated that microbially induced precipitation may be more important than physico-chemical precipitation. Pedley et al. showed with flume experiments that precipitation does not occur unless a biofilm is present, despite supersaturation. Calcite is the dominant mineral precipitate found; however, the polymorph aragonite is also found.
Occurrence
Tufa is common in many parts of the world. Some notable deposits include:
Pyramid Lake, Nevada, US – tufa formations
Big Soda Lake, Nevada, US – tufa formations only a century old
Some sources suggest that "tufa" was used as the primary building material for most of the châteaux of the Loire Valley, France. This results from a mis-translation of the terms "tuffeau jaune" and "tuffeau blanc", which are porous varieties of the Late Cretaceous marine limestone known as chalk.
Uses
Tufa is today occasionally shaped into a planter. Its porous consistency makes tufa ideal for alpine gardens. A concrete mixture called hypertufa is used for similar purposes.
