Tufa

Tufa

Tufa belong to a group of crust-like carbonate deposits that are formed through the organically and inorganically controlled precipitation of calcium carbonates from fresh water . Other members of this group are travertines, sinters, and lacustrine limestones or marl lake deposits. Cave deposits of a somewhat similar origin are called flowstones, speleothems (stalagmites and stalactites ). The terms tufa and travertine have a Latin origin. The first is derived from tophus and was used by Pliny to describe porous whitish deposits including volcanic material, which is nowadays called volcanic tuff . The term travertine stems from lapis tiburtinus or Tibur stone.

The distinction of terms used to describe surface freshwater carbonates in the literature is not very clear. However, today the term tufa is usually used to describe the more porous varieties, while travertines are denser and sometimes laminated. Sinters are mostly laminated and lack porosity . Lacustrine limestones are hardly compacted.

The porosity in tufas is derived from authochtonous plants such as mosses, green algae or reed, which are encrusted by carbonates. Tufas are typically found as deposits of cool spring waters, which are supersaturated by calcium bicarbonate. The precipitation of carbonates in these meteogene deposits is assisted by photosynthesis of phototrophic microbes and plants. Generally, however, any decrease in the partial pressure of CO₂ will trigger carbonate precipitation.

Travertines, in contrast, are mostly of hydrothermal origin and usually lack fossils of macrophytes or invertebrates. The CO₂ in these thermogene deposits can be supplied by various processes including degassing of the upper mantle in areas of tectonic stress. Travertines commonly change to a more porous tufa-fabric in areas where the water has cooled down to near ambient temperature .

Tufas and travertines are formed in fluvial environments with a growth rate that can sometimes exceed 0.8 in (2 cm) per year but is seldom smaller than 0.08 in (2 mm) per year. They build up barriers leading to the formation of shallow pools in which carbonate mud is deposited. This succession of barriers and pools can cover wide areas, sometimes miles in length (e.g., Grand Canyon , Arizona or Mammoth Hot Springs, Wyoming). Other seemingly related formations such as the deposits of the geysers and hot springs in the Yellowstone National Park (Wyoming) are in fact siliceous sinter.

Tufas and travertines are particularly common in the late Pleistocene. Today, they form under a wide variety of climatic regimes from cool and temperate to semi-arid conditions while the growth of fossil travertines of the Quaternary seems to be restricted to the interstadials and interglacials in high and medium latitudes. Some calcareous fresh water deposits show well-developed laminae that probably mirror seasonal differences in one or more environmental variables (water depth or temperature, detrital input).

See also Stalactites and stalagmites