Geology Reference
In-Depth Information
Plate 2 Terrestrial Freshwater Carbonates: Travertine, Calcareous Tufa and Sinter
Travertines, tufa and calcareous sinter are excellent paleoclimatic archives (Andrews et al. 2000). These carbon-
ates and their corresponding water bodies can serve as analogues of Precambrian and Phanerozoic microbialites
and their oceans (Arp et al. 2001). Travertines of hot springs are of interest for recognizing potential microbial
signatures on Mars (Kempe and Kazmierczak 1997). Travertines are very important building stones that have
been used since antique times (Winkler 1994). Many travertines and tufa originated during interglacial phases of
the Pleistocene, but in places are still growing today.
Terrestrial carbonates produced by precipitation in freshwater occur as travertines (->1-3, 5), calcareous tufa
(-> 6, 7), and calcareous sinter (-> 4).
Travertine
is a layered deposit of calcium carbonate with moderate to high
primary porosity, and often with a dendritic fabric.
Calcareous tufa
is an originally porous, laminated or massive
deposit which commonly originates at cold subaerial springs, but also forms at subaquatic springs in soda lakes
and highly alkaline salt lakes ('tufa pinnacles': Kempe et al. 1991; Benson 1994; Arp et al. 1998).
Calcareous
sinter
is characterized by well-developed lamination and the lack of visible porosity. These carbonates orignate
from degassing of CO
2
and HCO
3-
caused by changes in temperature, pressure and photosynthesis. Travertine is
often found on warm or mineralized hydrothermal waters, tufa on temperate waters. Tufa precipitation takes
place on organic substrates (higher plants, algae, mosses) in freshwater. In both types carbonate chemistry and
the temperature of the spring waters, the overall morphology of the accumulation and biologically induced
processes are major controls.
Characteristic textural criteria of freshwater limestones
are (1) macroscopically visible voids (often corre-
sponding to moulds and leaves of higher plants); (2) void-filling radiating calcite (-> 3); (3) small spar-filled
cellular structures; (4) wavy and laminar layers (-> 1, 2); (3) concentric structures consisting of smooth and
wavy laminae of micrite and sparite (-> 1); (5) alternating dark and light laminae with vertical thin 'filaments'
(-> 4); (6) layers with bubble structures (-> 1); (7) shrub-like structures (-> 1); (8) ovoid to subangular peloids
with diameters from 50 to 300
m, occurring in clumps or aggregates (-> 5); (9) travertine fragments occurring
as intraclasts, and detrital grains derived from underlying or adjacent rocks; (10) pisoids (see Pl. 14/1).
1
Travertine. The name travertine is derived from the Latin
lapis tiburtinus
, meaning 'the stone of Tiburnium' (now Tivoli),
a locality near Rome where these carbonate rocks have been quarried for more than two thousand years. At the 'type
locality', the precipitation of carbonate occurs from spring waters. The importance of inorganic or organic (bacterially-
induced) modes of precipitation is primarily controlled by water temperature and water chemistry (Chafetz and Folk
1984). In the Bagni di Tivoli locality, travertine occurs as stratified lake-fill deposits formed in shallow ponds.The traver-
tine is composed of laterally continuous and vertical repetitions of layers with micritic shrubs (S; interpreted as result of
bacterially induced precipitation) intercalated with laminated muds (-> 3). The shrubs nucleate on a substrate and grow
and branch upward, indicating a phototrophic control. The bubbles (B) at the bottom may represent lithified gas-filled
vesicles surrounded by bacterially precipitated walls or relics of chironomid insect larvae. The samples (-> 1 and 2) are
classified as travertine according to the vadose setting, microbial-induced precipitation, and medium porosity (Fig. 2.4).
2
Travertine. 'Bacterial stromatolite'. Thin laminae aggregate into packets with large lenticular intermat cavities bulged up
by large gas bubbles (B). Note the abundant micritic clumps within the mats (arrows) interpreted as bacterial aggregates
embedded or surrounded by calcite crystals. Bagni di Tivoli near Rome.
3
Travertine. Laminated micrite layers bordering voids which are partly filled with bladed prismatic ('dogtooth', DC)
calcite cement. Same locality as -> 2.
4
Calcareous sinter. The layers consist of fibrous cements arranged in bundles. The low-calcite sinter originated from
carbonic acid-rich and calcium-rich mineral waters. The travertine was formed during warm interglacial periods in the
Pleistocene. The sample is classified as calcareous sinter owing to inferred inorganic precipitation, low porosity and the
vadose setting. Stuttgart-Bad Cannstatt, Germany.
5
Travertine. Peloids and bacterial mud reworked into intraclasts. Same locality as -> 2.
6
Recent freshwater tufa. The fabric is characterized by incrustations of oscillatoriacean cyanobacteria. The threads have a
diameter of about 10
m. Fossil Oscillatoriaceae are indicative of freshwater and brackish water environments. Schleierfälle,
Ammer, southern Germany.
7
Modern freshwater tufa. Tufa deposition is controlled by a number of intervening factors including carbonate equilib-
rium, crystal nucleation, crystal morphology, and diagenesis (Ford and Pedley 1996; Merz-Preiß and Riding 1999: Jannsen
et al. 1999). Note the porous texture and the plant stem (P) surrounded by euhedral calcite crystals some of which exhibit
dark 'centers' (arrow) commonly interpreted as an indication of bacterial contributions to carbonate precipitation. The
sample still fits into the triangle classification (Fig. 2.4) because of its high porosity, considerable organic contributions
(-> 6) and origin in a phreatic setting. Same locality as -> 6.
