Geology Reference
In-Depth Information
Factors controlling growth of modern tufa: results of
a field experiment
MICHAŁ GRADZI ´ SKI
Institute of Geological Sciences, Jagiellonian University, Oleandry 2a,
30-063 Krak ´w, Poland (e-mail: michal.gradzinski@uj.edu.pl)
Abstract: A field experiment was performed at four sites in Slovakia and Poland in order to
identify factors that influence the growth rate and textures of modern tufas. Two pairs of tablets
were placed at each study point, every pair consisting of a limestone tablet and a copper tablet.
One pair at each site was changed every three or four months, while the second pair was left for
approximately 14 months. Each tablet was weighed before placing and after removal to find the
amount of the tufa growth.
Tufa growth rate was found to depend on SI calc. of parent water, though deposition of tufa on
limestone tablets was substantially faster than on copper tablets. This result indicates that
micro-organisms are essential for more efficient growth of tufa. Tufa growth rate was higher in
fast-flowing water than in nearby sluggish flow settings.
Fast-growing tufa has crystalline texture or consists of highly encrusted algal filaments. The
latter texture is due to faster growth of micro-organisms, forced by rapid crystallization of calcite
on their cells. The slow-growing tufa exhibits mainly micritic textures. Clotted micrite with numer-
ous diatoms forms mostly in winter, while encrusted algal filaments are typical for spring and
summer growth.
Freshwater carbonate deposits form in headwater
streams worldwide (Ford & Pedley 1996). They
originate under various conditions, from different
types of waters and therefore may form bodies of
various thicknesses, spreading over the distances
from a few metres to several kilometres. During
their growth these deposits build evolved three-
dimensional structures such as dams, cascades,
and they display various internal structures.
Two terms are used for such deposits: calcareous
tufa and travertine (Ford & Pedley 1996; Pedley
2009). The former term refers to deposits containing
plant and algal moulds or imprints. Conversely, tra-
vertine hardly ever exhibits plant imprints and
seldom displays recognizable biological texture.
Both terms have strong genetic implications. Tufas
commonly form in karst areas and are fed with
water saturated with soil CO 2 , while travertines
originate from deep circulation water, usually of
elevated temperature and highly charged with CO 2
of deep origin (Chafetz & Folk 1984). Other terms
used for tufas and travertines are meteogene and
thermogene travertines, respectively (Pentecost
1995, 2005). Many authors, including American
and Spanish ones, call all spring-related carbonates
'travertines' regardless of their characteristics and
origin. Here, the terms tufa and travertine will be
used to denote lithology of deposits, regardless of
their origin and temperature of parent fluids.
The growth of tufa and travertine is one of a few
geological phenomena which can easily be noted
and observed by an ordinary man. Therefore, it is
no wonder that it has attracted men's interest for
ages, as is recorded in European fine arts and litera-
ture as early as in the Middle Ages. For example,
Dante in Divine Comedy at least twice mentioned
a travertine deposited in streams. Self-building
ridges were illustrated in a 15th century altar
scene of Christ's baptism in the St. Jacob's church
in Usterling, Bavaria. The actively growing ridge
may still be seen nearby.
Many researchers tried to assess the rate of tufa
growth. Average growth rates of Quaternary
(mostly Holocene) tufa were estimated using
isotope ratios, mainly radiocarbon. The rate of
0.49 mm a 21 resulted from dating of Flandrian
tufa from Holywell Coombe (Kent) made by
Kerney et al. (1980). Pazdur et al. (1988) found
the growth rate of Holocene tufa in the Krak ´ w
Upland to depend strongly on facies. In stromatolitic
facies it reached 10 mm a 21 while in marly facies
only 1 - 2.5 mm a 21 . The study by Heimann &
Sass (1989) from north Israel yielded value of
0.32 mm a 21 . Preece & Day (1994) obtained the
average rate of growth of the Holocene tufa in
Oxfordshire between 0.13 and 0.54 mm a 21 , while
Meyrick & Preece (2001) calculated a rate ranging
from 0.27 to 0.7 mm a 21 for tufas in English
Midlands. A higher rate, varying between 1.5 and
2.5 mm a 21 , was recorded by Pedley et al. (1996)
from lake tufa in southern Spain. Limondin-Lozouet
& Preece (2004) recorded the growth of tufa in
Search WWH ::




Custom Search