Geology Reference
In-Depth Information
(a) petrographical investigation of thin sections and
polished slabs combined with staining and XRD analy-
sis to determine mineralogical composition, (b) rare
element analysis (e.g. Sr in order to check diagenetic
alterations), and (c) stable isotope analysis, to distin-
guish between biologically and inorganically controlled
precipitation.
Diagnostic criteria
The following overview of diagnostic criteria is
based mainly on Schweigert (1996), who studied and
illustrated the microfacies, and the sedimentological
and paleontological criteria of Tertiary limestones
formed in terrestrial and various freshwater settings.
Many calcareous tufa and travertine deposits as well
as lacustrine carbonates are modified by pedogenic pro-
cesses. Therefore, microfacies types of non-marine
limestones can vary within wide boundaries.
Fig. 15.6.
'Travertine' fabric.
'Sickle-cell' limestone exhib-
iting microcrystalline sheets of inclusion-rich, fibrous cements
spanning over lenticular voids. Highly porous limestone char-
acterized by irregular, wavy laminae enclosing lenticular to
sickle-shaped voids. These structures are only known from
spring mounds in highly alkaline lakes. Miocene: Wallerstein
castle, Ries, Germany. After Pache et al. (2001).
Travertine
Ancient travertine deposits are characterized by
massive or coarse-bedded, layered carbonates. Biota
are restricted to cyanobacteria and bacteria. Fossils
enclosed within the travertine (e.g. terrestrial snails,
vertebrates, plants) are allochthonous. Common fea-
tures are laminar crusts consisting of large crystals,
banding expressed by changes in color and texture, cy-
clic bedding caused by seasonal climatic fluctuations,
tepee structures, pores filled with vadose cements, des-
iccation cracks, and brecciation.
Microfacies types
are controlled by distinct micro-
bial associations, and include:
• Laminar cementstones (calcareous sinter) consist-
ing of coarse feather-like arrangements of calcite or
aragonite crystals (ray crystal layers; Chafetz and Folk
1984).
• Laminar microbial bindstones with a planar upper
surface, consisting of very thin micritic and sparry, of-
ten discontinuous and broken laminae.
• Micritic layers with abundant bushy structures con-
sisting of branched filaments, interpreted having a
cyanobacterial origin (
Dichothrix
morphotype; Guo and
Riding 1994). They are often associated with laminar
bindstone crusts (Pl. 2/1).
• Micrite layers, sometimes with peloidal and clotted
fabrics.
• Microcrystalline and fine-sparitic layers composed
of fibrous cement crystals enclosing large and small
open or closed voids ('sickle cell' limestones; Fig. 15.6).
• Layers with globular or vertically elongated open
or closed voids interpreted as gas bubbles (Pl. 2/2).
• Detrital layers intercalated between the other micro-
facies types, formed by reworking of microbial bind-
stones forming an intraclastic texture (Pl. 2/5).
• Pisolitic layers composed of densely packed pisoids
of different sizes. Inter-pisoid pores are partly or com-
pletely filled with aragonite cement (Fig. 15.7). Pisoid
accumulations may be intercalated with micrite layers.
Calcareous tufa
Freshwater tufas are characterized by highly porous
fabrics because of the great number of enclosed plant
remains. Depending on the plants, a variety of facies
types may develop, including leaf tufas, reed tufas, moss
tufas (characterized by peloidal textures and sparry fab-
rics), characean tufas,
Vaucheria
tufas (tufa formed by
xanthophycean green algae), and
Cladophorites
tufas.
Microbial activity is very important for the precipita-
tion of the carbonates but of minor importance for the
development of the pores. Plant remains have sheet-
like or tubular shapes and are commonly coated by
micritic calcite crystals. Algal filaments vary in size
from 50 to 100
m. Different parts of mosses range in
size from a few microns to several millimeters. Euhed-
ral calcite crystals growing on the surface of mosses
have similar size ranges (Irion and Müller 1968).
Calcareous sinter
Sinters differ from typical travertines and tufas in
that they are predominantly inorganic in origin. They
are controlled by high water temperatures in thermal
springs, high ion content in mineral springs, high run-
ning rates of water, and by the absence or rareness of
light leading to microbial settling in caves. This type
of sinter occurs in different settings (e.g. thermal traver-
tines, carbonate geyserites, and speleothems).
