Geology Reference
In-Depth Information
2/6), mosses, and higher plants (Pl. 2/7) as well as on
various organic and inorganic substrate. Cyanobacte-
ria, bacteria and algae are able to trigger the precipita-
tion of calcite and aragonite forming
microbial traver-
tines
(Chafetz and Folk 1984; Folk et al. 1985). Milli-
meter- to centimeter-sized laminae (Pl. 2/1, 2), and ar-
borescent shrub-like calcite precipitates (Pl. 2/1), con-
sisting of associations of micrite aggregates and rhom-
bic spar crystals aggregates appear to be microbially
controlled (Guo and Riding 1994).
Stromatolitic trav-
ertines
exhibit alternating sparry and micritic laminae
which are interpreted in terms of seasonality (Chafetz
et al. 1991), climatic changes as well as changes in mi-
crobial and algal growth and sedimentation (Monty
1976).
Travertines and tufa are often associated with higher
plants (Pentecost 1990), but the ability of macrophytes
to induce the precipitation of carbonate during photo-
synthesis is regarded as controversial. Consumption of
CO
2
leads to carbonate precipitation in the immediate
vicinity of photosynthesizing vegetation. Both field and
laboratory studies show that the intensity of photosyn-
thesis varies from least effective in springs and water-
falls to highly effective in lakes and rivers (Merz 1992).
Fig. 2.4.
Practical classification of travertines, tufa and sinter
(after Koban and Schweigert 1993).
2, 3, 5),
tufa
(Pl. 2/6, 7) or
sinter
(Pl. 2/4) (Schweigert
1996). Travertine often is used in a rather loose man-
ner to designate hard and compact carbonate deposits;
tufa for porous deposits. Many authors use travertine
in a rather broad sense, covering all kinds of non-ma-
rine carbonates of springs, creeks, streams, pools and
lakes, as well as caves (e.g. Julia 1983; Pentecost and
Whitton 2000).
More precise definitions were offered by Riding
(1991): The term
travertine
is restricted to non-marine,
layered autochthonous carbonates deposited at thermal
springs and commonly exhibiting a bushy fabric (Pl. 2/
1).
Tufa
refers to often conspicuously porous carbon-
ates, whose formation in non-marine cold-waters is
strongly controlled by aquatic plants.
Calcareous sin-
ters
are characterized by well-developed lamination and
lack of visible porosity. Currently there is a tendency
to restrict the term travertine to thermal deposits (Ford
and Pedley 1996; Fouke et al. 2000) formed at hot and
warm, often mineralized springs, and calcareous tufa
to settings where calcium-rich waters bubble up from
springs to the earth, cool down and precipitate calcium
carbonate, supported by photosynthesizing plants.
Morphological differentiation:
Travertine deposits ex-
hibit distinctive morphologies (Chafetz and Folk 1984):
•
Waterfall deposits, formed at sites of break-in slope
of streams by the growth of plants and contempo-
raneous calcification, resulting in a massive, irregu-
lar contorted tangle of cement crusts and plant
molds.
•
Lake-fill accumulations, characterized by thick,
horizontally stratified and laterally extensive trav-
ertines. These are composed of thin summer layers
of (bacterial) shrubs intercalated with finely lami-
nated winter mud layers and thicker, crudely lami-
nated muds. Contorted zones composed of calcite
rays, intraclasts and pisoids, represent spring ori-
fices on the lake bottom.
How to classify freshwater carbonates?
Koban and
Schweigert (1993) proposed a classification which re-
flects the genetic factors responsible for the fabric of
non-marine freshwater carbonates (Fig. 2.4): The tri-
angle underlines the inferred origin, setting and visible
porosity (see Pl. 2 for examples).
•
Sloping mounds, fans and cones, characterized by
structures paralleling a slope at the time of deposi-
tion.
•
Terraced mounds, exhibiting a step-like morphol-
ogy.
•
Warm spring waters, flowing down both sides of
fissure ridges.
Thermal travertines:
Comparative studies of mod-
ern thermal spring systems (Cady and Farmer 1996;
Fouke et al. 2000) provide a basis for constructing fa-
cies frameworks which can assist in the interpretation
of ancient travertines. The spectacular carbonate-pre-
cipitating thermal springs of the Mammoth Hot Springs
in Yellowstone National Park, Wyoming, demonstrate
Which names are appropriate?
The terminology of
freshwater-carbonates is controversial because of the
mixed use of descriptive and genetic terms. This is par-
ticularly true for carbonates called
travertine
(Pl. 2/1,
