View source for OR/14/043 Processes of tufa formation and tufa classification

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===Tufa or travertine?===
The term ‘tufa’ is common in English speaking countries and a range of other names for tufa deposits occur across the world, the other most common being travertine. Pentecost (1995) offers a discussion on the merits of both terms (tufa and travertine) and recognises the need, but also the inherent difficulty, in undertakeing any classification that would clearly define one from the other. Although Pentecost (1995) goes on to use ‘travertine’ rather than tufa, we will take the opposite approach and use the word ‘tufa’ in recognition of the habitat type and classification agreed upon by the EU and the JNCC.

Pentecost (1995) also offers the most up to date definition of travertine (tufa);

''‘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 intercrysalline porosity and often high mouldic or framework porosity within a vadose or occasionally shallow phreatic environment. Precipitation results primarily though 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’''.

===Hydrogeochemistry of tufa deposition===
Tufa formation is derived from the dissolution of rocks rich in calcium carbonate and can also be a significant hydrogeological characteristic of karst environments (Banks & Jones, 2012)<ref name="Banks 2012">BANKS, V, J, and JONES, P F. 2012. Hydrogeological Significance of Secondary Terrestrial Carbonate Deposition in Karst Environments, Hydrogeology - A Global Perspective, Dr Gholam A  Kazemi       (Ed.),       ISBN:978-953-51-0048-5,       InTech,           Available    from: http://www.intechopen.com/books/hydrogeology-a-globalperspective/hydrogeological-significance-of-secondary-terrestrial-carbonate-deposition-in-karst-environments</ref>. These rocks will principally be limestone or other carbonate rich strata and several of the sites in this study are also heavily influenced by dissolution of calcium carbonate from unconsolidated lime spoil associated with historic limestone quarrying operations. A basic understanding of the hydrochemical process of tufa formation and the carbonate system is provided as a background to understanding how to geology and hydrogeology influences where the sites occur.

Precipitation provides effective recharge to aquifers in the form of rainfall or snow and ice melt, precipitation is also acidic and undersaturated with respect to calcium carbonate. During the recharge process, via the soil layer, superficial deposits and bedrock, dissolved carbon dioxide in the water can dissolve ions (cations and anions) including Ca, HCO<sub>3</sub>, Mg, Na, K, and SO<sub>4</sub>, Na and Cl often originating from marine aerosols, an effect observed on many coastal areas (e.g. WMC, 2008<ref name="WMC 2008">WMC. 2008. Groundwater quality and supply survey for the Precambrian Gwna Group, Anglesey. Commissioned Report for Environment Agency Wales.</ref>) and Islands (Webb, 2000<ref name="Webb 2000">WEBB. 2000. The water resources of Bardsey Island, north Wales. From Robins, N S, and Misstear, B D R (eds) Groundwater in the Celtic Regions: Studies in Hard Rock and Quaternary Hydrogeology. ''Geological Society of London, Special Publications'', 182, 239–246.</ref>). It is during this process that the more acidic recharge can dissolve calcium carbonate and other ions in the soils and bedrock, the water often referred to as ‘attacking’ (see Pentecost, 2005<ref name="Pentecost 2005">PENTECOST, A. 2005. Travertine. Berlin Heidelberg.</ref>). The groundwater ultimately becomes supersaturated with respect to calcium bicarbonate creating the perfect conditions for tufa deposition. Groundwater will need to leave the aquifer, or interact with the atmosphere, in order to deposit tufa and this occurs where the piezometric head of the water (i.e. the water table) intersects the topographical land surface, in simple terms this is where springs and seepages often occur. The residence time (age) of groundwater within the various aquifers in this study has not been defined, however CFC and SF<sub>6</sub> aerosol analysis could provide information on the residence time of water supplying tufa springs in future projects.

Once the groundwater emerges at the surface, via a spring or seepage or as river baseflow, interactions with the atmosphere cause the loss or evasion of CO<sub>2</sub> and the resultant precipitation of calcium carbonate, as tufa:

Ca<sub>2</sub>+ + 2 HCO<sub>3</sub>-↔ CaCO<sub>3</sub>  + CO<sub>2</sub> + H<sub>2</sub>O

===Tufa Classification===
Three main criteria for tufa classification exist; i) Geochemical&nbsp;—&nbsp;precipitation process and carbon dioxide geochemistry ii) Fabric and iii) Morphology (Pentecost & Viles, 1994<ref name="Pentecost 1994">PENTECOST, A, and VILES H. 1994. A review and reassessment of travertine classification. Geographie physique et Quaternaire Vol.&nbsp;48 No.&nbsp;3 p.305–314.</ref>).

* Tufa can occur in two broad geochemical categories, either associated with thermal waters (thermogene) or meteoric waters (meteogene). Meteogene tufas are the most widely distributed (Pentecost & Viles, 1994<ref name="Pentecost 1994"></ref>) and cover all the examples within this report. Meteogene tufas can be distingused from thermogene tufas as they have different stable carbon isotope values (Pentecost & Viles, 1994<ref name="Pentecost 1994"></ref>).

* Tufa fabric can be visible with the naked eye (mesofabric) or in more detail under the microscope (microfabric). There are many factors that influence tufa fabrics including; temperature, flow rate, CO<sub>2</sub> evasion rate, supersaturation with respect to calcite, ion transport mechanisms, plant growth and animal burrows (Pentecost, 2005<ref name="Pentecost 2005"></ref>). Fabrics have also been the basis of several classification schemes which emphasize the influence of plants (Pentecost & Viles, 1994<ref name="Pentecost 1994"></ref>) on the the formation of a variety of tufa fabrics. Bryophytes and algae can influence tufa fabrics through the trapping and binding of calcite (Pentecost, 1993<ref name="Pentecost 1993">PENTECOST, A. 1993. British Travertines: a review. Proceedings of the Geologists Association. Vol.&nbsp;104, issue 1 Pages 23–39.</ref>).

* Tufa morphologies, unlike most erosional or destructive land surface processes are frequently constructive in nature (Pentecost, 2005<ref name="Pentecost 2005">PENTECOST, A. 2005. Travertine. Berlin Heidelberg.</ref>).

There have been many different attempts to classify tufa deposits from all around the world, however these are of limited use to this study. The most applicable work to this project is a paper titled ‘British travertines: a review’ (Pentecost, 1993<ref name="Pentecost 1993"></ref>).

<center>
{| class="wikitable"
|+ Table 1&nbsp;&nbsp;&nbsp;&nbsp;Classification of autochthonous tufa depsoits in the UK after Pedley (1990) and Pentecost (1993)<ref name="Pentecost 1993">PENTECOST, A. 1993. British Travertines: a review. Proceedings of the Geologists Association. Vol.104, issue 1 Pages 23–39.</ref>
|- style="vertical-align:top;"    
| ! scope="col" style="width: 225px;" | '''Classification'''
| ! scope="col" style="width: 125px;" | '''Type'''
| ! scope="col" style="width: 400px;" | '''Description from Pentecost (1993)<ref name="Pentecost 1993"></ref>'''
|- style="vertical-align:top;"
| ! scope="col" style="width: 150px;" | Deposits on gentle slopes (''c''. <10°)
| ! scope="col" style="width: 150px;" | Paludal
| ! scope="col" style="width: 400px;" | Surface coatings of tufa on vegetation, marshy locations or alluvial valley bottoms
|- style="vertical-align:top;"
| Deposits on steep slopes  (''c''. >10°)
| Cascades
| On waterfalls and steep ground
|- style="vertical-align:top;"
|
| Barages
| Spanning streams or rivers (e.g. Nash Brook S Wales)
|- style="vertical-align:top;"
|
| Stream spring crusts
| Irregular sometimes nodular deposits on river beds
|- style="vertical-align:top;"
|
| Laucustrine
| Rare and no examples in Wales (see Malham Tarn)
|- style="vertical-align:top;"
|
| Cemented rudites
| Infill between detrital rocks such as breccias and scree
|- style="vertical-align:top;"
|
| Perched springline
| Dominated by bryophytes
|- style="vertical-align:top;"
| Clastic Deposits
|
| Re-deposited material (Nash Brook, Caerwys and Ddol&nbsp;—&nbsp;Clwyd)
|}
</center>

[[Image:OR14043fig1.jpg|thumb|center|500px|  '''Figure 1'''&nbsp;&nbsp;&nbsp;&nbsp;Sectional drawings of autochthonous travertines (tufa) 1) Spring mounds,<br>2) cascades 3) barrages 4) fluvial crusts 5) laucstrine crusts 6) paludal deposits 7) surface cemented rudites after Pentecost & Viles (1994)<ref name="Pentecost 1994"></ref>.    ]]

==References==
<References/>           
[[Category:OR/14/043 Survey, characterisation and condition assessment of Palustriella dominated springs 'H7220 Petrifying springs with tufa formation (Cratoneurion)' in Wales | 07]]