5.3 Hemipelagic and Pelagic Sediments
5.3.8 Sediments on Marginal and Oceanic
Plateaus, Ridges, and Seamounts
By applying the general rules described in the previous sections most of the sedimentary processes observed on deep submarine plateaus, on isolated
seamounts, and on ridges and bank tops of blockfaulted, topographically structured shelf areas can be
easily understood. Their present-day deep position
below sea level is frequently the result of long persisting subsidence which, in contrast to shelf seas,
could not be compensated for by sediment accumulation. Some simplified examples for such a situation
are shown in Fig. 5.l0a. In most ofthese cases, sedimentation on older rocks started with shallow-marine
deposits, but was later followed by pelagic and
hemipelagic material. This second depositional phase
is considered further here.
The sediments of these specific settings are significantly controlled by their topographie position on
the sea floor, the intensity of ocean currents, and latitude (water temperature etc.). One can distinguish
between
Deep plateaus, marginal or isolated, and seamounts subjected to weak currents.
Plateaus affected by strong currents.
Slopes and basins close to submarine plateaus.
Submarine marginal ridge-and-basin settings.
Several of these plateaus, seamounts, ridges and associated
basins have been explored by geophysical studies, sea floor
sampling, and deep-sea drilling. Many volumes ofthis program (Initial Reports DSDP; Proceedings ODP, Initial Reports and Scientific Results, e.g. ODP Legs 130, 133, 143,
144, 167, etc.) and special articles (e.g. Bralower et a1.
1997) give detailed information about these studies.
Deep Plateaus and Seamounts
Weak Currents. Deep, isolated oceanic or marginal
plateaus (Fig. 5.1 Oa) which are little affected by surface and intermediate currents can preserve a fairly
complete record of pelagic to hemipelagic sediment
accumulation. Elevated above the CCD, they tend to
be carbonate-dominated in low- and mid-latitude regions.
This is true ofmost ofthe modem oceanic plateaus, e.g. the
Bermuda Rise, Galicia Bank, and Rockall Plateau in the
Central and North Atlantic, the Kerguelen-Broken Ridge
Plateau in the southwestem Indian Ocean, and the huge
Ontong-Java Plateau in the southwestem Pacific (e.g.
Zimmerman et a1. 1984; Berger et a1. 1994; Bralower et a1.
1997).
At high latitudes or during certain times, platform
sediments mayaiso become rich in bio genie silica.
207
Sea floor spreading and associated plate tectonic motions may shift their position either from lower to
higher latitudes or vice versa. A change in the oceanic current system with time, e.g. from weak to
strong current action affecting the submarine plateau,
can lead to unconformities. Both factors cause a
gradual or even abrupt change in the facies of the
platform sediments which may be used to reconstruct
the oceanic paleo-environment. Marked vertical faeies trends can be expected in sediment successions
deposited in polar and subpolar regions which experienced greater variations in temperature and
thermohaline circulation through the Earth· s history
than those formed in low-latitude zones.
The sediments on deep plateaus frequently record rninor
climatic oscillations, which generate rhythrnic or cyclic
calcareous ooze (limestone)-marl successions (Die sterHaass 1991) or, in subpolar regions, alternations between
layers rich in biogenic carbonate or silica as reported from
the Meteor Rise in the South Atlantic (Shipboard Scientific
Party 1988a; cf. Sect. 7.9).
Such phenomena can also be observed at the tops of old,
sunken seamounts which lie at considerable depths below
sea level (more than 100 to 200 m). They are often covered
by pelagic material consisting of nannofossils, forarninifers
andJor siliceous remains and the corresponding lithified
sediments. If the seamounts are situated below equatorial
upwelling or have passed through such a zone due to plate
tectonic motion, their sediments can contain phosphorites.
The likelihood that submarine plateaus and seamounts show a kind of steady-state, undisturbed
depositional environment for a very long geological
time span is low. Ongoing subsidence of these structures, in conjunction with changes in climate and
morphology of the ocean basin considered, can modify or completely alter the regional oceanic current
pattern and thus strongly influence terrigenous and
biogenie sedimentation at these sites.
Strong Currents. If a fairly deep marginal plateau is
subjected to the influence of powerful, deep-reaching
surface currents, deposition of pelagic and some benthic carbonate as described above is drastically curtailed or completely prevented (Fig. 5.l0a, example
for tropical-subtropical waters). The resulting sequence is markedly condensed and displays
hardgrounds with sessile benthos (sponges,
serpulids, sometimes deep-water corals, signs of boring organisms etc.). In zones of particularly strong
current action, forexample near the shelf slope, erosional uncoriformities and lag sediments are common. At least some of these carbonate sediments will
become completely oxidized and later red-colored.
During times of virtual nondeposition, nodules and
erusts of ferromanganese may develop. Other periods, allowing the deposition of some day and organic matter, may enable the formation of glauconite.
5.3.8 Sediments on Marginal and Oceanic
Plateaus, Ridges, and Seamounts
By applying the general rules described in the previous sections most of the sedimentary processes observed on deep submarine plateaus, on isolated
seamounts, and on ridges and bank tops of blockfaulted, topographically structured shelf areas can be
easily understood. Their present-day deep position
below sea level is frequently the result of long persisting subsidence which, in contrast to shelf seas,
could not be compensated for by sediment accumulation. Some simplified examples for such a situation
are shown in Fig. 5.l0a. In most ofthese cases, sedimentation on older rocks started with shallow-marine
deposits, but was later followed by pelagic and
hemipelagic material. This second depositional phase
is considered further here.
The sediments of these specific settings are significantly controlled by their topographie position on
the sea floor, the intensity of ocean currents, and latitude (water temperature etc.). One can distinguish
between
Deep plateaus, marginal or isolated, and seamounts subjected to weak currents.
Plateaus affected by strong currents.
Slopes and basins close to submarine plateaus.
Submarine marginal ridge-and-basin settings.
Several of these plateaus, seamounts, ridges and associated
basins have been explored by geophysical studies, sea floor
sampling, and deep-sea drilling. Many volumes ofthis program (Initial Reports DSDP; Proceedings ODP, Initial Reports and Scientific Results, e.g. ODP Legs 130, 133, 143,
144, 167, etc.) and special articles (e.g. Bralower et a1.
1997) give detailed information about these studies.
Deep Plateaus and Seamounts
Weak Currents. Deep, isolated oceanic or marginal
plateaus (Fig. 5.1 Oa) which are little affected by surface and intermediate currents can preserve a fairly
complete record of pelagic to hemipelagic sediment
accumulation. Elevated above the CCD, they tend to
be carbonate-dominated in low- and mid-latitude regions.
This is true ofmost ofthe modem oceanic plateaus, e.g. the
Bermuda Rise, Galicia Bank, and Rockall Plateau in the
Central and North Atlantic, the Kerguelen-Broken Ridge
Plateau in the southwestem Indian Ocean, and the huge
Ontong-Java Plateau in the southwestem Pacific (e.g.
Zimmerman et a1. 1984; Berger et a1. 1994; Bralower et a1.
1997).
At high latitudes or during certain times, platform
sediments mayaiso become rich in bio genie silica.
207
Sea floor spreading and associated plate tectonic motions may shift their position either from lower to
higher latitudes or vice versa. A change in the oceanic current system with time, e.g. from weak to
strong current action affecting the submarine plateau,
can lead to unconformities. Both factors cause a
gradual or even abrupt change in the facies of the
platform sediments which may be used to reconstruct
the oceanic paleo-environment. Marked vertical faeies trends can be expected in sediment successions
deposited in polar and subpolar regions which experienced greater variations in temperature and
thermohaline circulation through the Earth· s history
than those formed in low-latitude zones.
The sediments on deep plateaus frequently record rninor
climatic oscillations, which generate rhythrnic or cyclic
calcareous ooze (limestone)-marl successions (Die sterHaass 1991) or, in subpolar regions, alternations between
layers rich in biogenic carbonate or silica as reported from
the Meteor Rise in the South Atlantic (Shipboard Scientific
Party 1988a; cf. Sect. 7.9).
Such phenomena can also be observed at the tops of old,
sunken seamounts which lie at considerable depths below
sea level (more than 100 to 200 m). They are often covered
by pelagic material consisting of nannofossils, forarninifers
andJor siliceous remains and the corresponding lithified
sediments. If the seamounts are situated below equatorial
upwelling or have passed through such a zone due to plate
tectonic motion, their sediments can contain phosphorites.
The likelihood that submarine plateaus and seamounts show a kind of steady-state, undisturbed
depositional environment for a very long geological
time span is low. Ongoing subsidence of these structures, in conjunction with changes in climate and
morphology of the ocean basin considered, can modify or completely alter the regional oceanic current
pattern and thus strongly influence terrigenous and
biogenie sedimentation at these sites.
Strong Currents. If a fairly deep marginal plateau is
subjected to the influence of powerful, deep-reaching
surface currents, deposition of pelagic and some benthic carbonate as described above is drastically curtailed or completely prevented (Fig. 5.l0a, example
for tropical-subtropical waters). The resulting sequence is markedly condensed and displays
hardgrounds with sessile benthos (sponges,
serpulids, sometimes deep-water corals, signs of boring organisms etc.). In zones of particularly strong
current action, forexample near the shelf slope, erosional uncoriformities and lag sediments are common. At least some of these carbonate sediments will
become completely oxidized and later red-colored.
During times of virtual nondeposition, nodules and
erusts of ferromanganese may develop. Other periods, allowing the deposition of some day and organic matter, may enable the formation of glauconite.