136
unconsolidated carbonate sediment can form waveand current-resistant structures if it is lithified by
early cementation (see below).
Photic and related aphotic carbonate-producing
organisms grow best in very shallow waters. Once
shell beds andJor early lithification of sandy or
muddy carbonate have created a firm substrate, sessil
organisms, very effective in carbonate secretion, can
settle and build structures which to some extent resist
mechanical destruction. Three types of large-scale
depositional environments are important for the
buildup of significant carbonate bodies:
1. Carbonate ramps.
2. Rimmed carbonate shelves.
3. Isolated carbonate platforms.
Whereas carbonate ramps and shelves are attached to
land masses, the typical carbonate platforms are isolated from mainland coasts and therefore ideally meet
the requirement of low or absent terrestrial influx.
Carbonate ramps can develop without significant
contribution of reefs, whereas rimmed carbonate
shelves and platforms need reef structures to build up
and maintain comparatively steep slopes.
3.4.2 Carbonate-Producing Communities
Modern Carbonates
Tropical and subtropical carbonates are characterized
by the Chlorozoan association whereas temperate
and polar to subpolar (non-tropical) carbonates are
represented by the Foramol association. Within these
principal groups, the carbonate composition can considerably vary, however.
- The Chlorozoan association consists mainly of
green algae and hermatypic corals and, in addition,
benthic foraminifera, molluscs, bryozoans, calcareous red algae, and bamacles. This association forms
the prominent modem reefs in tropical-subtropical
waters most of which also contain inorganically
grown ooids and pelletal carbonate muds. The
chlorozoan association flourishes in sea water of
normal salinity. If the water becomes brackish or
hypersaline, algae predominate (Chloralgal association; Lees 1975).
Carbonate production in tropical seas is to some extent
favored by rapid growth of different groups of organisms
secreting thick, large carbonate skeletons. In addition, carbonate can also precipitate directly from sea water supersaturated with respect to calcium carbonate. This, in turn,
prevents carbonate dissolution. Recently it was emphasized
that much carbonate is also precipitated with the aid of
micro-organisms (e.g. cyanobacteria) and encrusting benChapter 3 Coastal and Shallow Sea Sediments
thic invertebrates forming microbial mats and films on the
surface and in cavities of larger organic structures
(microbialites, spongiolites, etc.; see, e.g., Riding 1991;
Reitner and Neuweiler 1995). The ultra-conservative,
slowly growing microbe-sponge association already existed
in the early Cambrian, but was later largely substituted by
rapidly growing euphotic communities such as the coralgal
facies. However, microbialites still are a substantial factor
in modem reef growth filling light protected pore space
and cavities within coral reefs.
- The Foramol association consists predominantly of
benthic foraminifera, molluscs, bryozoans, and partially calcareous red algae, echinoderms, brachiopods, corals and bamacles. These communities live
in temperate and cold waters and partially at greater
depths than the chlorozoan association. They grow
more slowly and normally cannot develop isolated,
high bioherm structures (see below). Dissolution of
carbonate at the sea floor is common due to
undersaturation with respect to aragonite and calcite;
ooids are absent. At depths below 100 to 200 m,
glauconite may be present. Cold surface currents and
upwelling cool ocean water may lead to cold-water
carbonates even in low-Iatitude regions (Fig. 3.22).
On the deeper shelf, aphotic associations with a large
proportion of bryozoans (Bryomol facies) are common. On arctic shelves and on seamounts, sponges
and diatoms may contribute to the biogenic production and thus cause the deposition of siliceous carbonates. Cool-water carbonates either form distinct
banks, or consist of reworked sand- and gravel-size
skeletal material.
The Foramol association significantly varies in specific
environments (Henrich and Freiwald 1995; Henrich et al.
1996; James 1997). Kelp forests in the shallow subtidal
zone provide a favorable habitat for various photic and
aphotic carbonate-secreting organisms including coralline
algae and balanids. The latter two groups are also characteristic of regions where melt-water enters the sea.
Modem examples of cold-water carbonates were found
around Alaska, Antarctica, in the western N orth Sea and
Greenland Sea, and on plateaus of the northeastern Atlantic. The large shelf platform of the Spitsbergenbank in the
western Barents Sea, in winter covered by sea ice, accumulates cold-water carbonates on top of glacial clastic sediments and demonstrates the impact of the oceanographic
Polar Front promoting high carbonate production. Interglacial arctic shallow-water carbonates, however, were largely
eroded by subsequent ice action.
Widely extended temperate-water carbonates also occur
on the shelf of southern Australia, where an inner shelf association rich in coralline algae, bivalves, and brachiopods
passes, without a reef baITier, into an outer shelf association characterized by bryozoans, calcified worm tubes,
foraminifera, and sponges. Similar associations are found
around New Zealand and Tasmania as weil as in other midlatitude regions along the coasts ofthe Atlantic and Pacific.
Besides the common bank-type cool-water carbonate,
even coral mounds (Lophelia reefs) have been detected on
modem deep shelves and upper slopes at water depths be-
unconsolidated carbonate sediment can form waveand current-resistant structures if it is lithified by
early cementation (see below).
Photic and related aphotic carbonate-producing
organisms grow best in very shallow waters. Once
shell beds andJor early lithification of sandy or
muddy carbonate have created a firm substrate, sessil
organisms, very effective in carbonate secretion, can
settle and build structures which to some extent resist
mechanical destruction. Three types of large-scale
depositional environments are important for the
buildup of significant carbonate bodies:
1. Carbonate ramps.
2. Rimmed carbonate shelves.
3. Isolated carbonate platforms.
Whereas carbonate ramps and shelves are attached to
land masses, the typical carbonate platforms are isolated from mainland coasts and therefore ideally meet
the requirement of low or absent terrestrial influx.
Carbonate ramps can develop without significant
contribution of reefs, whereas rimmed carbonate
shelves and platforms need reef structures to build up
and maintain comparatively steep slopes.
3.4.2 Carbonate-Producing Communities
Modern Carbonates
Tropical and subtropical carbonates are characterized
by the Chlorozoan association whereas temperate
and polar to subpolar (non-tropical) carbonates are
represented by the Foramol association. Within these
principal groups, the carbonate composition can considerably vary, however.
- The Chlorozoan association consists mainly of
green algae and hermatypic corals and, in addition,
benthic foraminifera, molluscs, bryozoans, calcareous red algae, and bamacles. This association forms
the prominent modem reefs in tropical-subtropical
waters most of which also contain inorganically
grown ooids and pelletal carbonate muds. The
chlorozoan association flourishes in sea water of
normal salinity. If the water becomes brackish or
hypersaline, algae predominate (Chloralgal association; Lees 1975).
Carbonate production in tropical seas is to some extent
favored by rapid growth of different groups of organisms
secreting thick, large carbonate skeletons. In addition, carbonate can also precipitate directly from sea water supersaturated with respect to calcium carbonate. This, in turn,
prevents carbonate dissolution. Recently it was emphasized
that much carbonate is also precipitated with the aid of
micro-organisms (e.g. cyanobacteria) and encrusting benChapter 3 Coastal and Shallow Sea Sediments
thic invertebrates forming microbial mats and films on the
surface and in cavities of larger organic structures
(microbialites, spongiolites, etc.; see, e.g., Riding 1991;
Reitner and Neuweiler 1995). The ultra-conservative,
slowly growing microbe-sponge association already existed
in the early Cambrian, but was later largely substituted by
rapidly growing euphotic communities such as the coralgal
facies. However, microbialites still are a substantial factor
in modem reef growth filling light protected pore space
and cavities within coral reefs.
- The Foramol association consists predominantly of
benthic foraminifera, molluscs, bryozoans, and partially calcareous red algae, echinoderms, brachiopods, corals and bamacles. These communities live
in temperate and cold waters and partially at greater
depths than the chlorozoan association. They grow
more slowly and normally cannot develop isolated,
high bioherm structures (see below). Dissolution of
carbonate at the sea floor is common due to
undersaturation with respect to aragonite and calcite;
ooids are absent. At depths below 100 to 200 m,
glauconite may be present. Cold surface currents and
upwelling cool ocean water may lead to cold-water
carbonates even in low-Iatitude regions (Fig. 3.22).
On the deeper shelf, aphotic associations with a large
proportion of bryozoans (Bryomol facies) are common. On arctic shelves and on seamounts, sponges
and diatoms may contribute to the biogenic production and thus cause the deposition of siliceous carbonates. Cool-water carbonates either form distinct
banks, or consist of reworked sand- and gravel-size
skeletal material.
The Foramol association significantly varies in specific
environments (Henrich and Freiwald 1995; Henrich et al.
1996; James 1997). Kelp forests in the shallow subtidal
zone provide a favorable habitat for various photic and
aphotic carbonate-secreting organisms including coralline
algae and balanids. The latter two groups are also characteristic of regions where melt-water enters the sea.
Modem examples of cold-water carbonates were found
around Alaska, Antarctica, in the western N orth Sea and
Greenland Sea, and on plateaus of the northeastern Atlantic. The large shelf platform of the Spitsbergenbank in the
western Barents Sea, in winter covered by sea ice, accumulates cold-water carbonates on top of glacial clastic sediments and demonstrates the impact of the oceanographic
Polar Front promoting high carbonate production. Interglacial arctic shallow-water carbonates, however, were largely
eroded by subsequent ice action.
Widely extended temperate-water carbonates also occur
on the shelf of southern Australia, where an inner shelf association rich in coralline algae, bivalves, and brachiopods
passes, without a reef baITier, into an outer shelf association characterized by bryozoans, calcified worm tubes,
foraminifera, and sponges. Similar associations are found
around New Zealand and Tasmania as weil as in other midlatitude regions along the coasts ofthe Atlantic and Pacific.
Besides the common bank-type cool-water carbonate,
even coral mounds (Lophelia reefs) have been detected on
modem deep shelves and upper slopes at water depths be-