OR/14/043 Processes of tufa formation and tufa classification: Difference between revisions

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|+ 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>
|+ Table 1&nbsp;&nbsp;&nbsp;&nbsp;Classification of autochthonous tufa depsoits in the UK after Pedley (1990) and Pentecost (1993)<ref name="Pentecost 1993"></ref>
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| ! scope="col" style="width: 225px;" | '''Classification'''

Latest revision as of 12:41, 29 November 2019

Farr, G, Graham, J, and Stratford, C. 2014. Survey, characterisation and condition assessment of Palustriella dominated springs 'H7220 Petrifying springs with tufa formation (Cratoneurion)' in Wales. British Geological Survey Internal Report, OR/14/043.

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)[1]. 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, HCO3, Mg, Na, K, and SO4, Na and Cl often originating from marine aerosols, an effect observed on many coastal areas (e.g. WMC, 2008[2]) and Islands (Webb, 2000[3]). 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[4]). 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 SF6 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 CO2 and the resultant precipitation of calcium carbonate, as tufa:

Ca2+ + 2 HCO3-↔ CaCO3 + CO2 + H2O

Tufa Classification

Three main criteria for tufa classification exist; i) Geochemical — precipitation process and carbon dioxide geochemistry ii) Fabric and iii) Morphology (Pentecost & Viles, 1994[5]).

  • 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[5]) 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[5]).
  • 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, CO2 evasion rate, supersaturation with respect to calcite, ion transport mechanisms, plant growth and animal burrows (Pentecost, 2005[4]). Fabrics have also been the basis of several classification schemes which emphasize the influence of plants (Pentecost & Viles, 1994[5]) 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[6]).
  • Tufa morphologies, unlike most erosional or destructive land surface processes are frequently constructive in nature (Pentecost, 2005[4]).

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[6]).

Table 1    Classification of autochthonous tufa depsoits in the UK after Pedley (1990) and Pentecost (1993)[6]
Classification Type Description from Pentecost (1993)[6]
Deposits on gentle slopes (c. <10°) Paludal Surface coatings of tufa on vegetation, marshy locations or alluvial valley bottoms
Deposits on steep slopes (c. >10°) Cascades On waterfalls and steep ground
Barages Spanning streams or rivers (e.g. Nash Brook S Wales)
Stream spring crusts Irregular sometimes nodular deposits on river beds
Laucustrine Rare and no examples in Wales (see Malham Tarn)
Cemented rudites Infill between detrital rocks such as breccias and scree
Perched springline Dominated by bryophytes
Clastic Deposits Re-deposited material (Nash Brook, Caerwys and Ddol — Clwyd)
Figure 1    Sectional drawings of autochthonous travertines (tufa) 1) Spring mounds,
2) cascades 3) barrages 4) fluvial crusts 5) laucstrine crusts 6) paludal deposits 7) surface cemented rudites after Pentecost & Viles (1994)[5].

References

  1. 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: https://www.intechopen.com/books/hydrogeology-a-globalperspective/hydrogeological-significance-of-secondary-terrestrial-carbonate-deposition-in-karst-environments
  2. WMC. 2008. Groundwater quality and supply survey for the Precambrian Gwna Group, Anglesey. Commissioned Report for Environment Agency Wales.
  3. 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.
  4. 4.0 4.1 4.2 PENTECOST, A. 2005. Travertine. Berlin Heidelberg.
  5. 5.0 5.1 5.2 5.3 5.4 PENTECOST, A, and VILES H. 1994. A review and reassessment of travertine classification. Geographie physique et Quaternaire Vol. 48 No. 3 p.305–314.
  6. 6.0 6.1 6.2 6.3 PENTECOST, A. 1993. British Travertines: a review. Proceedings of the Geologists Association. Vol. 104, issue 1 Pages 23–39.