Geology Reference
In-Depth Information
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the influence of microbial mats on the formation of
autochthonous fine-grained carbonates,
ritized skeletal grains are potentially a major source of
Mg-calcite mud (Reid et al. 1992).
Unlithified deep-sea carbonates formed at the sea
bottom were reported from the Mediterranean Sea and
the Red Sea. Supersaturation with respect to calcium
carbonate due to relatively high water temperatures and
salinities promotes 'inorganic' precipitation of Mg-cal-
cite carbonates (Milliman et al. 1969; Milliman and
Müller 1977).
Another hypersaline basin where inorganically pre-
cipitated carbonates have been reported is the Arabian
Gulf. The large quantities of fine-grained aragonite in
nearshore sediments off Saudi Arabia and the Trucial
Coast are explained by inorganic precipitation, since
aragonite-producing calcareous green algae do not grow
in the Arabian Gulf (Kinsman 1964). However, no evi-
dence for inorganic precipitation of deep-sea Mg-cal-
cite carbonate muds was found by Ellis and Milliman
(1985). These muds are formed by suspended carbon-
ate grains of biogenic or detrital origin.
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the processes involved in the breakdown of the skel-
etons of calcareous algae and invertebrates into fine-
grained particles,
•
the role of sessile organisms in the control of the
deposition of fine-grained material (trapping, baf-
fling, binding),
•
the variations in burrowing activities creating spe-
cific carbonate mud microfabrics and
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the ways in which the deposition, dissolution and
accumulation of calcareous plankton takes place.
The overwhelming majority of modern shallow- and
deep-marine carbonate muds is now regarded as bio-
genic or biologically induced in origin.
Initially, mi-
crocrystalline carbonate matrix was considered as pre-
cipitated directly from seawater (Cloud 1962). Later,
spurred by observations of Lowenstam (1955) and
Lowenstam and Epstein (1957) and supported by the
forward-looking suggestions of Sorby (1879), most
workers began to accept shallow-marine lime muds as
being primarily the product of a breakdown of skeletal
biota, particularly calcareous algae. 'Whitings' (float-
ing patches of lime mud), as evidence of physicochemi-
cal precipitation, fell into disrepute and were consid-
ered to be caused by biologically induced carbonate pre-
cipitation (Robbins and Blackwelder 1992).
But, more recently, evidence for the direct precipi-
tation of aragonite lime muds emerged (Shinn et al.
1989; Milliman et al. 1993; Friedman 1994). The in-
crease in micrite carbonates during the Early Protero-
zoic is thought to be a result of blooms of cyanobacte-
ria in phosphate-rich oceans. Strong photosynthesis
combined with high population densities could have
induced the precipitation of mud via whitings. The fine-
grained sediment produced in whitings was fixed in
benthic cyanobacterial mats. In waters, highly oversatu-
rated with respect to calcium carbonate, the mats deve-
lope to stromatolites (Merz-Preiss 2000). Very recent
investigations suggest an intricate bio-physicochemi-
cal coupling between microbial activity, water current
circulations, and water chemistry (Thompson 2000).
4.1.2 Modes of Formation of Micrite and
Other Fine-Grained Matrix Types
The discrimination of micrites generated by different
processes is of high-ranking importance for paleoenvi-
ronmental interpretations, basin analysis and the under-
standing of the properties of fine-grained carbonate
rocks. Fig. 4.1 summarizes twelve modes which can
lead to the formation of micrites.
'Inorganic' micrites:
(1) Physicochemical precipitation
:
Initially, the micro-
crystalline matrix of limestones was considered a
'chemical' rock, precipitated directly from seawater.
This view was based on the interpretation of carbonate
muds in hypersaline lagoons of the Bahamas as an ex-
clusively physicochemical precipitation of Mg-calcite
and aragonite in conjunction with high salinity and ex-
treme temperature (Cloud 1962, De Groot 1965, Loreau
1982, Dix 2001), but was also contested by geochemical
data (Husseini and Matthews 1972).
Calculations of sedimentation rates, carbonate mud
budget and stable isotope data indicate that a consider-
able amount of the aragonite and perhaps also Mg-cal-
cite muds of the Great Bahama Bank, observed as
'whitings' could be chemically precipitated (Shinn et
al. 1989), but a biochemically induced precipitation trig-
gered by planktonic cyanobacteria cannot be ruled out.
Whitings believed to have been triggered by plankton
blooms are reported from marine shallow-water areas
as well as from lakes. Inorganic precipitation of Mg-
The aragonitic mud on the Great Bahama Banks has
long been of great interest as a possible analogue for
ancient fine-grained carbonate rocks. As a result, the
term
lime mud
has become more or less synonymous
with
aragonite needles
in the minds of many geolo-
gists. It must be stressed, however, that many modern
carbonate muds consist of Mg-calcite (e.g. South
Florida and Belize: Taft and Harbaugh 1964, Enos 1977)
and need an alternative genetic interpretation. Mic-
