Geology Reference
In-Depth Information
ter and marine realms, particularly in lagoons; Castanier
et al. 1989; Chafetz et al. 1991; Folk 1993; Arenas et
al. 1993; summary by Buczynski and Chafetz 1993).
Experiments indicate that bacteria trigger the precipi-
tation of aragonite and calcite, exhibiting distinct mor-
phologies that seem to be limited to bacterial contribu-
tion. The size of individual crystals, spheres and rods
ranges between 0.1 and 0.4
m; that of crystal aggre-
gates between 5 and 100
m. Comparable morpholo-
gies have been observed in laboratory cultures and in
modern carbonate sediments. Because most bacteria
except for cyanobacteria, are indifferent to light, bac-
terially-controlled carbonate precipitation is not re-
stricted to shallow environments, but also occurs in
deeper subtidal settings, various cryptic habitats and in
deep restricted basins. A strong microbial contribution
of microbes to the formation of 'mud mounds' is advo-
cated by many authors (e.g. Pratt 1982; Lees and Miller
1985; see Sect. 16.2.2).
to basin traverses (Precambrian: Hoffman 1974; Late
Devonian: Playford et al. 1976; Late Jurassic: Lein-
felder 1993; Tertiary: Braga et al. 1995).
9.1.2 How to Recognize Microbial
Carbonates?
Increasingly more authors support the idea of a strong
microbial impact on the formation of Paleozoic and Me-
sozoic limestones, using external appearance, internal
fabrics and geochemical signatures as evidence (see Fa-
cies, vol. 29, 1993). Box 9.1 lists some criteria which
are commonly used as evidence for a bacterial contri-
bution to the formation of limestones.
9.1.3 Describing and Classifying Benthic
Microbial Carbonates
Biological versus environmental controls:
The main
processes of microbial carbonate formation are (1) trap-
ping (agglutination) of sedimentary particles, (2) bio-
mineralization (calcification) of organic tissues, and
(3) mineralization (superficial precipitation of miner-
als on organisms and/or sediment). Fine-grained car-
bonate is trapped and produced within microbial mats,
occurring under a wide range of environmental condi-
tions in nonmarine and marine sites.
The mats are dominated by various phototrophic,
chemotrophic and heterotrophic microorganisms (cy-
anobacteria dominating in the top layer; colorless sul-
fur bacteria, purple sulfur bacteria, and sulfate-reduc-
ing bacteria harboring the underlying layers). Other
numerically less important groups are nitrifying and
denitrifying bacteria and methanogenic bacteria. The
activity of aerobic heterotrophic organisms leads to
oxygen depletion. Fermentative organisms provide
growth substrates for sulfate-reducing bacteria. The
vertically laminated structures develop as a result of
microbial growth and activity sediment trapping and
binding in the organic matrix, and sedimentation (Van
Gemerden 1993).
The shape and macrofabric of microbial carbonates
are strongly influenced by variations in the depositional
environment. Important controlling environmental pa-
rameters are the grain size of the substrate, the penetra-
tion of light, sedimentation and erosion rates, and graz-
ing pressure (Walter 1976). Sedimentation and micro-
bial mat composition are sensitive to water movement
and light, respectively, and change with water depth.
This is reflected in the morphology, texture and micro-
fabrics. Examples were described from various shelf
Microbial carbonates are recorded by specific textures
occurring within a wide range of scales. Common fea-
tures are dense autochthonous micrites, micro- to mega-
scaled laminated fabrics (e.g. stromatolites), millime-
ter- to centimeter-sized micritic crusts, non-laminated
micritic and peloidal structures, and limestones exhib-
iting tiny tubelike microfossils referred to as remains
of cyanobacteria.
9.1.3.1 Terminology and Descriptive Criteria
Microbial carbonates
are carbonate deposits pro-
duced or localized by benthic microbial communities
(Riding 1990) living in marine, marginal-marine, fresh-
water and terrestrial environments. The complex asso-
ciations of bacteria, cyanobacteria (cyanophytes, blue-
greens) and algae embrace photosynthetic prokaryotes,
eukaryotic microalgae and chemoautotrophic as well
as chemoheterotrophic microbes. In addition, encrust-
ing invertebrates (e.g. foraminifers) may be of some
importance (Sect. 9.2).
Communities creating microbial carbonates are
termed
microbial mats
(Gerdes and Krumbein 1987)
or algal mats, reflecting the densely interlayered and
intertwined orientations of the filamentous and coccoid
cells involved and the resulting biolaminated sedimen-
tary structures.
Biolaminites
characterized by organic-rich laminae
and microbial mats are an essential criterion of micro-
bially induced sedimentary structures (Noffke et al.
