10.2 Sedimentation Rates in Various Environments
10.2.3 Sedimentation Rates in Carbonate
Depositional Systems
Reefs, Carbonate Platforms, and
Shelf Carbonates
Although the sedimentation and accumulation rates
of biogenic carbonate are limited by organic productivity, they displaya wide range ofvalues from about
0.1 to 1000 cmlka (1 to 10 000 mlMa). The highest
va1ues have been observed in young is01ated coral
reefs which grew vertically upward with the rapid
Holocene sea-level rise (cf. Fig. 3.29). However,
these extremely high rates should not be taken as
average growth rates for longer time periods. Reef
mounds, calcareous tidal flats, ooid shoals, and
coastal sabkhas have the potential for growing upward at rates on the order of 50 to 100 cm/ka (Enos
1989; Hubbard et al. 1990; Strasser 1991). Cool water carbonates can reach values almost as high as
those of tropical carbonates (cf. Sect. 3.4; James
1997). Thus, the average sedimentation rate of a very
shallow carbonate platform may reach approximately
100 cm/ka (Fig. 10.4) provided there is an equal rate
of subsidence maintaining steady state conditions. If
subsidence is slower than this high potential sedimentation rate, the actual sedimentation rate drops
below the potential rate.
In tropical and subtropical regions, benthic carbonate production decreases with water depth (e.g.
Samthein 1973, confirmed by many other studies).
Coral reefs, algal structures, sea grass beds, and other
organisms depending on photosynthesis can live only
near the water surface where they receive sufficient
sunlight. They may be accompanied by large
foraminifera, molluscs, and gastropods growing on
shallow banks. In such a faunal and floral community, biological and biochemical carbonate production reaches its maximum.
Below water depths of 30 to 40 m, benthic carbonate production drops by a factor of approximately 10
and is frequently on the order of 1 to 10 cm/ka. This
signifies that outer, deeper portions of carbonate
ramps exhibit benthic carbonate production which
has approximately the same magnitude as planktonic
production in this and deeper regions (e.g. chalk, Fig.
10.4). On the other hand, carbonate accumulation in
shallower water may be inhibited by insufficient subsidence (see above) and lead to the export of carbonate into deeper water. In this way, carbonate accumulation in shallower and deeper water can be more or
less equalized and make up in total an average of
several cm/ka. Such rates are characteristic of the
long-term buildup of carbonate platforms (cf. Sects.
3.4.6 and 12.2.2). An excess in carbonate production
over the average subsidence rate is commonly used
for the lateral pro gradation of the platform.
461
Cretaceous chalk, accumulated in comparatively
deep shelf seas and consisting mainly of the tiny
skeletons of planktic organisrns, frequently reached
surprisingly high sedimentation rates of about 5 to 15
cm/ka (cf. Sect. 5.3.2).
Deep-Water Carbonates
Sedimentation rates of plankton-produced deep-sea
carbonates commonly are low (between 0.5 and 2
cm/ka; Figs. 10.3 and 10.4). Benthic carbonate production becomes insignificant in deep water, but
mass flows and turbidity currents may transport considerable volumes of shallow-water carbonates into
the deep sea. There, they contribute to the buildup of
deep-sea fans, slope aprons, and continental rises and
may lead to sedimentation rates similar to those listed
für their siliciclastic counterparts (Fig. 10.3).
10.2.4 Siliceous Ooze and Radiolarites
The sedimentation rates of modem siliceous oozes in
the deep sea normally range from 0.2 to 1 cm/ka. In
the North Pacific and Bering Sea values of about 1
cm/ka have been observed (when measured as massive beds without pore space). Compacted Miocene
diatomites of the Monterey Formation, California
(Isaacs 1984) and Mesozoic radiolarites from the
western Tethys (Jenkyns 1986) range between 0.2
and 2 cm/ka. Jurassic and Cretaceous radiolarites in
the Dinarides (Montenegro, Bosnia) accumulated in
deep-water basins mostly at rates of 0.1 to 0.2 cm/ka
(Goril an 1994). Younger radiolarites in the eastern
Pacific typically reach around 0.2 cm/ka (Gursky
1988).
In zones of high biogenic productivity rates up to
50-100 cm/ka have been found. This applies, for example, to specific subpolar to polar regions where
diatoms flourish, but also to warm, low-latitude
zones of high fertility. Examples include Quaternary
uncompacted diatomites in the Gulf of California and
Miocene diatomites in Sicily (based on data from
Martinson et al. 1987).
10.2.5 Mixed Siliciclastic-Biogenic Sediments
Mixed sediments consisting of both siliciclastic and
biogenic components commonly show higher sedimentation rates than do purely biogenic deposits. In
calcareous sediments, the carbonate content tends to
decrease with increasing sedimentation rate (Fig.
10 .2b), reflecting dilution of the limited carbonate
production by terrigenous material. Similarly, the
percentage of opaline silica decreases with increasing
sedimentation rate. Black shales frequently contain
10.2.3 Sedimentation Rates in Carbonate
Depositional Systems
Reefs, Carbonate Platforms, and
Shelf Carbonates
Although the sedimentation and accumulation rates
of biogenic carbonate are limited by organic productivity, they displaya wide range ofvalues from about
0.1 to 1000 cmlka (1 to 10 000 mlMa). The highest
va1ues have been observed in young is01ated coral
reefs which grew vertically upward with the rapid
Holocene sea-level rise (cf. Fig. 3.29). However,
these extremely high rates should not be taken as
average growth rates for longer time periods. Reef
mounds, calcareous tidal flats, ooid shoals, and
coastal sabkhas have the potential for growing upward at rates on the order of 50 to 100 cm/ka (Enos
1989; Hubbard et al. 1990; Strasser 1991). Cool water carbonates can reach values almost as high as
those of tropical carbonates (cf. Sect. 3.4; James
1997). Thus, the average sedimentation rate of a very
shallow carbonate platform may reach approximately
100 cm/ka (Fig. 10.4) provided there is an equal rate
of subsidence maintaining steady state conditions. If
subsidence is slower than this high potential sedimentation rate, the actual sedimentation rate drops
below the potential rate.
In tropical and subtropical regions, benthic carbonate production decreases with water depth (e.g.
Samthein 1973, confirmed by many other studies).
Coral reefs, algal structures, sea grass beds, and other
organisms depending on photosynthesis can live only
near the water surface where they receive sufficient
sunlight. They may be accompanied by large
foraminifera, molluscs, and gastropods growing on
shallow banks. In such a faunal and floral community, biological and biochemical carbonate production reaches its maximum.
Below water depths of 30 to 40 m, benthic carbonate production drops by a factor of approximately 10
and is frequently on the order of 1 to 10 cm/ka. This
signifies that outer, deeper portions of carbonate
ramps exhibit benthic carbonate production which
has approximately the same magnitude as planktonic
production in this and deeper regions (e.g. chalk, Fig.
10.4). On the other hand, carbonate accumulation in
shallower water may be inhibited by insufficient subsidence (see above) and lead to the export of carbonate into deeper water. In this way, carbonate accumulation in shallower and deeper water can be more or
less equalized and make up in total an average of
several cm/ka. Such rates are characteristic of the
long-term buildup of carbonate platforms (cf. Sects.
3.4.6 and 12.2.2). An excess in carbonate production
over the average subsidence rate is commonly used
for the lateral pro gradation of the platform.
461
Cretaceous chalk, accumulated in comparatively
deep shelf seas and consisting mainly of the tiny
skeletons of planktic organisrns, frequently reached
surprisingly high sedimentation rates of about 5 to 15
cm/ka (cf. Sect. 5.3.2).
Deep-Water Carbonates
Sedimentation rates of plankton-produced deep-sea
carbonates commonly are low (between 0.5 and 2
cm/ka; Figs. 10.3 and 10.4). Benthic carbonate production becomes insignificant in deep water, but
mass flows and turbidity currents may transport considerable volumes of shallow-water carbonates into
the deep sea. There, they contribute to the buildup of
deep-sea fans, slope aprons, and continental rises and
may lead to sedimentation rates similar to those listed
für their siliciclastic counterparts (Fig. 10.3).
10.2.4 Siliceous Ooze and Radiolarites
The sedimentation rates of modem siliceous oozes in
the deep sea normally range from 0.2 to 1 cm/ka. In
the North Pacific and Bering Sea values of about 1
cm/ka have been observed (when measured as massive beds without pore space). Compacted Miocene
diatomites of the Monterey Formation, California
(Isaacs 1984) and Mesozoic radiolarites from the
western Tethys (Jenkyns 1986) range between 0.2
and 2 cm/ka. Jurassic and Cretaceous radiolarites in
the Dinarides (Montenegro, Bosnia) accumulated in
deep-water basins mostly at rates of 0.1 to 0.2 cm/ka
(Goril an 1994). Younger radiolarites in the eastern
Pacific typically reach around 0.2 cm/ka (Gursky
1988).
In zones of high biogenic productivity rates up to
50-100 cm/ka have been found. This applies, for example, to specific subpolar to polar regions where
diatoms flourish, but also to warm, low-latitude
zones of high fertility. Examples include Quaternary
uncompacted diatomites in the Gulf of California and
Miocene diatomites in Sicily (based on data from
Martinson et al. 1987).
10.2.5 Mixed Siliciclastic-Biogenic Sediments
Mixed sediments consisting of both siliciclastic and
biogenic components commonly show higher sedimentation rates than do purely biogenic deposits. In
calcareous sediments, the carbonate content tends to
decrease with increasing sedimentation rate (Fig.
10 .2b), reflecting dilution of the limited carbonate
production by terrigenous material. Similarly, the
percentage of opaline silica decreases with increasing
sedimentation rate. Black shales frequently contain