Geology Reference
In-Depth Information
P r o c e s s e s
1
Physicochemical precipitation triggered by salinity
and water temperature fluctuations
Abiogenic ('inorganic
'
)
2
Carbonate precipitation mediated by organic matrices
(Cabinding organic macromolecules), causing
organomineralization and formation of organomicrite
Automicrite
(autochthonous micrite)
formed in place at the
seabottom or
within the sediment
Biologically induced
3
Metabolic processes of heterotroph and chemolithotroph
bacteria and other microbes causing microenvironmental
changes which induce carbonate precipitation
Biologically controlled
Metabolic processes of phototrophic cyanobacteria
and algae causing carbonate precipitation
4
5
Disintegration of benthic calcareous algae into sub
microscopic fragments (Halimeda model)
6
Disintegration of epibionts living on seagrass and
macroalgae
Disintegration of
predominantly
benthic biota
Allomicrite
(allochthonous micrite),
deposition of
disintegrated
skeletal material
and of fine
erosional detritus
7
Disintegration of invertebrate skeletons
8
Bioerosion causing detrital abrasion and microborings
causing 'micritization
'
Disintegration of
pelagic biota
9
Accumulation of calcareous plankton (foraminifera; cocco
lithophorids and other nannofossils causing 'nannomicrites
'
)
Erosion and abrasion
10
Mechanical erosion of limestones, e.g. at coasts
11
Micro and cryptocrystalline carbonate cements
Pseudomicrite
Diagenetic 'micrite
'
Diagenesis
12
Recrystallization and 'grain diminution
'
(replacement
of former larger crystals by tiny crystals)
Fig. 4.1.
Terminology and genetic modes of origin of carbonate muds and limestones composed of microcrystalline calcite
(micrite). The numbers correspond to the numbers used in the text.
calcite muds has been suggested for carbonate muds
along the Trucial Coast and in the Dead Sea.
ecules) mediating organomineralization (Reitner
1993, Reitner et al. 1995), and causing the forma-
tion of 'organomicrites'. Carbonate mineralization
concepts based on the interaction with Acidic Or-
ganic Macromolecules (AOM) first used to explain
the skeletal structures of organisms (Degens 1979;
Addadi and Weiner 1989; Lowenstam and Weiner
1989; Simkiss and Wilbur 1989), and were further
developed in the context of automicrite formation
(Reitner et al. 2000).
Interpretation of limestones: Various authors study-
ing ancient carbonate rocks have assumed that carbon-
ate mud is inorganically precipitated. This is mainly
based on a facies association comparable to the Ba-
hama Bank or related to the grain sizes of micritic lime-
stones, similar to those of modern aragonite crystals.
Automicrites
Automicrites (autochthonous micrites) are micro-
crystalline carbonates formed at the sea floor or within
the sediment. As a primary product of in-place miner-
alization, automicrite is fundamentally different from
allomicrites which are a microcrystalline sedimentary
matrix formed by the deposition of disintegrated skel-
etal material and fine erosional detritus. Automicrites
include biologically induced, biologically controlled as
well as 'inorganic' carbonates as discussed above. The
formation of autochthonous micrites is triggered by:
•
•
Metabolic processes of heterotroph and chemolitho-
troph bacteria and other microbes causing changes
in the microenvironment surrounding the bacterium
and inducing calcium carbonate precipitation. Bac-
terial controls on carbonate precipitation were at one
time controversially discussed but have now been
confirmed by experimental studies as well as direct
field observations (Sect. 9.1).
•
Metabolic processes of phototroph cyanobacteria and
algae, that reduce CO
2
to organic carbon compounds,
and thus shift the solubility equilibrium toward the
precipitation of carbonate. This mode of automicrite
Organic matrices (Ca-binding organic macromol-
