Geology Reference
In-Depth Information
textures. Borings are proof of early lithification. The
textures are a function of sediment supply. Very fine-
grained automicrite (0.5-1
m; minimicrite) is formed
by calcifying microbial biofilms within thin calcifying
mucus sheets below the biofilms, and in pockets of
trapped organic mucus and sedimentary grains. These
conspicuously dark, aphanitic micrites contain in-situ
peloids and in-situ ooids and are characterized by stro-
matolitic or/and massive fabrics.
•
Container organomicrites
(Pl. 6/3, 4, Pl. 50/4) de-
velop as fill-up structures of semi-closed microcavities
within encapsulated decaying tissues of metazoans,
mostly sponges, as well as in voids made by boring
organisms. Many sponges represent 'bacterial contain-
ers' (Fig. 4.2) because the bacterial biomass within the
sponge is higher than that of the sponge tissue itself.
The decay of bacteria-rich tissue result in the forma-
tion of a clotted micropeloidal microfabric which can
also be observed in ancient sponge reef carbonates.
synthetic (Warnke and Meischner 1995). Microbial
micrites were described from Tertiary reefs (Saint Mar-
tin et al. 1993).
(4) Precipitation controlled by phototroph organ
isms (cyanobacteria and algae)
: The removal of CO
2
by assimilating cyanobacteria and algae, linked to pho-
tosynthesis and augmented by bacterial activity, trig-
gers the precipitation of Mg-calcite and aragonite. The
production of lime mud by cyanobacteria is known in
modern peritidal environments (Bahamas, Florida Bay,
Persian Gulf, Red Sea sabkhas) and in highly saline
lakes (e.g. Coorong, Australia; Lake Tanganyika, east-
ern Africa; Antarctic oasis lakes).
The importance of filamentous cyanobacteria in the
formation of micrite is documented by the calcifica-
tion and preservation potential of benthic cyanobacte-
ria (Golubic et al. 2000; Merz-Preiss 2000). The Low-
Mg calcite crystals produced within the extracellular
organic sheets of benthic cyanobacteria are set free af-
ter the death of the cyanobacteria and the degradation
of the organic material; this may result in the forma-
tion of carbonate mud. This model is applied to ancient
micritic rocks, e.g. to the Late Triassic Hauptdolomit
of the Northern Alps which represents a huge platform
connected with the hinterland and affected by runoffs
of freshwater (Zankl and Merz 1994). Kazmierczak et
al. (1996) and Kazmierczak and Iryu (1999) postulate
that many Jurassic micritic and peloidal limestones are
products of a variously intensive calcification of mats
of benthic coccoid cyanobacteria, and that many micro-
crystalline cements described from ancient reefs may
be products of in vivo calcified cyanobacteria.
Cyanobacterial micrites are often developed as mini-
micrites with crystal sizes <2
m (Pl. 7/6).
The in situ calcification of benthic coccoid cyano-
bacteria mats, observed, for example in the highly al-
kaline soda lake Lake Van, Turkey, may represent a
model for cyanobacterial controlled formation of mi-
crite and peloidal micrite (Kempe et al. 1991; Kaz-
mierczak 1996). Coccoid cyanobacteria mats are
permineralized with micritic aragonite precipitating in
vivo on and within the mucilage sheets surrounding
the cells. Variations in the intensity of the calcification
of the mats controlled by the level of calcium carbon-
ate saturation create homogeneous micrite and peloid-
like bodies, producing a pelmicrite texture. Disintegra-
tion into individual peloids and reworking by currents
result in a pelsparite texture. Micritic peloidal grains
reminiscent of those of the hypersaline Lake Van are
common in cyanobacterial mats formed in hypersaline
perimarine environments (Baffin Bay, Texas: Dal-
rymple 1965; Laguna Mormona, California: Horodyski
(3) Microbial precipitation controlled by het
erotroph and chemolithotroph organisms (bacteria,
fungi
): Bacterial decay of organic matter is probably
the main source of energy in the early diagenesis of
sediments (Pl. 50/1, 2, 3). Bacteria are important agents
in the formation of ferric hydroxide, iron sulfides, cal-
cium phosphates as well as calcium carbonate. Starting
more than ninety years ago (Drew 1911; Bavendamm
1932), an increasing number of authors have demon-
strated and suggested that life processes of marine bac-
teria and the decomposition of organic matter by bac-
teria cause physicochemical changes in the microenvi-
ronment that result in calcium carbonate precipitation.
This has been proved in laboratory experiments and
observed in various modern carbonate-producing en-
vironments (soils, freshwater and marine realms, par-
ticularly in lagoons; see Buczynski and Chafetz 1993;
Castanier et al. 1999, and Sect. 9.1).
Interpretation of limestone (Box 4.2): A bacterial (not
a cyanobacterial) origin of ancient micrites has been
argued by many authors (Salomon 1914; Hadding 1958;
Maurin and Noel 1977; Riding and Awramik 2000).
Because most bacteria are indifferent to light, bacteri-
ally-controlled precipitation of automicrite is not re-
stricted to shallow environments, but also is possible
in subtidal settings, cryptic habitats as well as deep re-
stricted basins. A microbial production of the micrite
forming the bulk of 'mud mounds' is advocated by
many authors (Sect. 9.1.5.2; Pl. 8/6). The assumed po-
sition of the Paleozoic mounds in deep-water environ-
ments below the photic zone creates some problems
for this interpretation (Bourque and Boulvain 1993)
except the bacteria involved are regarded as chemo-
Text continued on p. 88
