Geology Reference
In-Depth Information
(Keim and Schlager 1999). Modern automicrites as well
as peloidal and microcrystalline carbonate cements
consist of Mg-calcite and aragonite. The same miner-
alogies have been inferred for ancient automicrites but
a Low-Mg calcite composition might be a possibility,
too.
amounts of specific organic macromolecules with the
ability to fix and bind divalent cations, mainly Ca
2+
and Mg
2+
. The increase in alkalinity required for cal-
cium crystals is triggered by ammonification (micro-
bial decay of organic matter, e.g. sponges), sulfate re-
duction (in anaerobic layers of stratified water masses)
or via carbonate and silicate weathering (Kempe et al.
1989; Reitner and Neuweiler 1995). Neuweiler et al.
(2000) discuss the controls of soluble humid substances
on the in situ precipitation of microcrystalline calcium
carbonates.
(2) Organomicrites
(Reitner et al. 1995; Pl. 8): The
formation of modern Mg-calcite automicrites precipi-
tated in isotopic equilibrium with the ambient seawa-
ter, and controlled by a specific composition of organic
material was studied by Reitner (1993) in indurated
microbialites occurring in reef caves off Lizard Island,
Great Barrier Reef (Fig. 4.2; Pl. 8/1). The microbialites
are formed under dark conditions. Phototrophic organ-
isms are absent. The most important factor controlling
the formation of organomicrite is the presence of large
Organomicrites are differentiated according to the
modes and loci of their formation (Reitner 1993):
•
Accretionary organomicrites
(Pl. 6/5, Pl. 8/1, 2;
Fig. 4.2) are vertically growing structures exhibiting
thrombolitic, stromatolitic or massive hardground-type
