Geoscience Reference
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of north-west Australia (Wright,
2000
). If we accept the radiocarbon ages obtained
directly from tufa samples, it would appear that the tufa dams in central Afghanistan
were formed between
40 ka and 25 ka and again between 14.2 ka and 11.6 ka, or
before and after the LGM. Dissolution of the calcareous bedrock in the headwaters
may have been inhibited by glacial aridity (Bouyx and Pias,
1971
).
Many former springs are demarcated by tufa mounds, of which the mound springs
in the Great Artesian Basin of central Australia are perhaps the most striking, with
individual tufa deposits forming flat-topped hills tens of metres in height with OSL
ages ranging back into the Middle Pleistocene (Prescott and Habermehl,
2008
). Tufa
beds are often intercalated within fluvial and lacustrine sediments, andmay contain the
remains of freshwater gastropod shells (Abell andWilliams,
1989
) and ostracod valves
(De Deckker and Williams,
1993
), or even fossil reed stems (Williams et al.,
2001
).
Many tufas precipitate out as a result of biological activity involving cyanobacteria,
heterotrophic bacteria and diatoms (Pedley,
2009
). Tufa benches formed from algal
limestone crop out around a terminal Pleistocene high shoreline of the shallower of
the two lakes within the Deriba Caldera of Jebel Marra volcano in arid north-west
Sudan (Williams et al.,
1980
). Many prehistoric sites in the drier parts of southern and
eastern Africa are associated with tufa deposits located at the mouth of previously
active springs (Clark and Williams,
1977
; Williams et al.,
1977
; Butzer,
1984
). Some
tufa deposits form underwater in lakes, either as a result of biological activity or in
association with hot springs. In Lake Abhe in the Afar Desert, a striking series of
biogenic limestone pillars (
Figure 14.2
) bear witness to a time when this now very
shallow saline lake was deep and fresh (Fontes and Pouchan,
1975
). In hyper-saline
lakes, such as the Coorong Lakes of South Australia, cyanobacterial mats cemented
by carbonate form stromatolites (Mee et al.,
2007
) similar in form to those that
formed in Precambrian lagoons more than a billion years ago. Tufas can also form
when freshwater lakes dry out in arid or semi-arid areas and are common in many
Cenozoic formations in North America and around the Mediterranean (Alonso-Zarza
et al. (
2006
).
Although they are visually striking features of certain desert landscapes, tufas/
travertines are not especially useful for reconstructing past changes in climate with
any degree of precision. There are several reasons for this. Owing to their highly
porous fabric, uranium is easily leached from the parent tufa, so that uranium-series
dating is problematic. In addition, tufas are exposed to accretions of detrital thorium
brought in from wind-blown dust, and this further complicates dating. Furthermore,
the location of many tufas is controlled by topographic and tectonic factors, such as
the presence of springs along faults or at breaks of slope, so unravelling any climatic
influence is unrewarding. Spring tufas associated with changes in the elevation of the
groundwater-table will only provide a blurred climatic signal, and dating such deposits
runs into the problem of contamination from dead carbon. In certain situations, this
problem can be overcome. For example, analysis of the
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