5.3 Hemipelagic and Pelagic Sediments
(3) Authigenic sediment components, formed in
place.
The contributions of the different sources to the
present-day total sediment volume of the oceans are
not exacdy known. A great proportion of the riverborne sediment is deposited on the shelves, continental slopes, and deep-sea fans (cf. Sects. 5.4 and 11.5).
In these environments, allochthonous terrigenous
sediments predominate.
On lower slopes and continental rises, fine-grained
siliciclastics commonly mix with biogenic material
and form hemipelagic sediments.
In deep-sea environments, far away form the
coastlines and the deltas of major rivers, the minor
contributors, such as biogenic material, eolian dust,
and volcanic ash, can reach high percentages in the
sediment. These deposits are referred to as pelagic
sediments.
Where terrigenous sediment input is very low
andJor interrupted by intervals of nondeposition and
erosion, authigenic sediment layers (e.g. mangane se
noduls, phosphorites) or specific minerals (e.g.
glauconite) can form.
Chemical precipitation of salts in deep oceanic basins does
not occur today, though it was significant during some periods in the past in specific deep marginal basins (Sect.
6.4).
Classification of Deep-Sea Sediments
Deep-sea sediments are commonly a mixture of
terrigenous and biogenic components, but the proportions of these components vary greatly.
Hemipelagic sediments, primarily formed by slow
pelagic settling, contain a large proportion of
terrigenous silt and clay. At least 25% of their grain
size fraction 2:5 flm is terrigenous and volcanogenic
in origin, or it is derived from shallow-marine sediments (Berger 1974, Jenkyns 1986). The median
grain size of hemipelagic sediments is 2:5 flm. This
group may be subdivided into:
- Terrigenous muds and mudstones (CaC0 3 content
:$30%).
- Volcanogenic muds (predominantly volcanic ash,
CaC0 3 content $30%).
Calcareous muds and marlstones (CaC0 3 content
2:30%).
Pelagic sediments commonly contain less than 25%
terrigenous material of the fraction 2:5 flm. Their median grain size is $5 flIn, apart from authigenic minerals and skeletons of microfossils. This group is
subdivided into:
189
- Pelagic (silty) clays and claystones containing
$30% CaC0 3 and biogenic Si0 2 (calcareous clays
or siliceous clays and claystones).
- Calcareous oozes, marls and marlstones, chalk and
pelagic limestones (CaC03 ~30%).
- Siliceous oozes, silicified claystones, porcellanite,
diatomites, radiolarites, chert (Si02 ~30%).
Within these subgroups, more special types of sediments
can be defined. Black shales containing relatively high
amounts of organic matter may be either rich or poor in
carbonate or silt-size terrigenous material (cf. Sect. 10.3.3).
They cannot be clearly attached to one of the groups or
subgroups listed above. Hemipelagic and pelagic sediments
are frequently redeposited and partially mixed with
shallow-water material by gravity mass movements, and
they may be winnowed and reworked by deep bottom currents (Sects. 5.4 and 5.5).
Transport Mechanisms and Deposition
Deep-sea sediments can also be classified in terms of
transport mechanisms and deposition:
- Current-transport in suspension.
- Slow pelagic settling.
- Redeposition of shallow-water and slope sediments by gravity mass movements (Sect. 5.4).
- Winnowing and current-reworking of pre-existing
sediments and their redeposition in the deep sea
(cf. Sect. 5.5).
The river-borne terrigenous component. In front of
deltas, river floods provide highly concentrated suspensions of terrigenous material. These can, particularly near the river bed, directly reach the deep sea as
underflows. Less concentrated suspensions float for
a short time as a fresh water wedge on top of sea water. As already mentioned in Section 3.5, clay suspended in fresh water is generally much less aggregated than in sea water. Thus it can be widely distributed (Fig. 5.3) before it comes in contact with more
saline water promoting aggregation. Once flocculated, ~he settling velocity of the aggregates increases to values of several meters up to several tens
of meters per day.
Their additional lateral transport is a function of water
depth and the velocity and depth range of the ocean currents (McCave 1984). This can be demonstrated by a very
simple example, which does not take into account effects of
large-scale turbulences: If a surface current has a mean
velocity of 0.2 mls (= -17 kmld) to a depth of up to 200 In,
an aggregate settling from sea level with a velocity of 10
mlday can reach a point 340 km away from its original
position.
It should be pointed out here, that transport, deposition, and erosion of fine-grained sediments do not
(3) Authigenic sediment components, formed in
place.
The contributions of the different sources to the
present-day total sediment volume of the oceans are
not exacdy known. A great proportion of the riverborne sediment is deposited on the shelves, continental slopes, and deep-sea fans (cf. Sects. 5.4 and 11.5).
In these environments, allochthonous terrigenous
sediments predominate.
On lower slopes and continental rises, fine-grained
siliciclastics commonly mix with biogenic material
and form hemipelagic sediments.
In deep-sea environments, far away form the
coastlines and the deltas of major rivers, the minor
contributors, such as biogenic material, eolian dust,
and volcanic ash, can reach high percentages in the
sediment. These deposits are referred to as pelagic
sediments.
Where terrigenous sediment input is very low
andJor interrupted by intervals of nondeposition and
erosion, authigenic sediment layers (e.g. mangane se
noduls, phosphorites) or specific minerals (e.g.
glauconite) can form.
Chemical precipitation of salts in deep oceanic basins does
not occur today, though it was significant during some periods in the past in specific deep marginal basins (Sect.
6.4).
Classification of Deep-Sea Sediments
Deep-sea sediments are commonly a mixture of
terrigenous and biogenic components, but the proportions of these components vary greatly.
Hemipelagic sediments, primarily formed by slow
pelagic settling, contain a large proportion of
terrigenous silt and clay. At least 25% of their grain
size fraction 2:5 flm is terrigenous and volcanogenic
in origin, or it is derived from shallow-marine sediments (Berger 1974, Jenkyns 1986). The median
grain size of hemipelagic sediments is 2:5 flm. This
group may be subdivided into:
- Terrigenous muds and mudstones (CaC0 3 content
:$30%).
- Volcanogenic muds (predominantly volcanic ash,
CaC0 3 content $30%).
Calcareous muds and marlstones (CaC0 3 content
2:30%).
Pelagic sediments commonly contain less than 25%
terrigenous material of the fraction 2:5 flm. Their median grain size is $5 flIn, apart from authigenic minerals and skeletons of microfossils. This group is
subdivided into:
189
- Pelagic (silty) clays and claystones containing
$30% CaC0 3 and biogenic Si0 2 (calcareous clays
or siliceous clays and claystones).
- Calcareous oozes, marls and marlstones, chalk and
pelagic limestones (CaC03 ~30%).
- Siliceous oozes, silicified claystones, porcellanite,
diatomites, radiolarites, chert (Si02 ~30%).
Within these subgroups, more special types of sediments
can be defined. Black shales containing relatively high
amounts of organic matter may be either rich or poor in
carbonate or silt-size terrigenous material (cf. Sect. 10.3.3).
They cannot be clearly attached to one of the groups or
subgroups listed above. Hemipelagic and pelagic sediments
are frequently redeposited and partially mixed with
shallow-water material by gravity mass movements, and
they may be winnowed and reworked by deep bottom currents (Sects. 5.4 and 5.5).
Transport Mechanisms and Deposition
Deep-sea sediments can also be classified in terms of
transport mechanisms and deposition:
- Current-transport in suspension.
- Slow pelagic settling.
- Redeposition of shallow-water and slope sediments by gravity mass movements (Sect. 5.4).
- Winnowing and current-reworking of pre-existing
sediments and their redeposition in the deep sea
(cf. Sect. 5.5).
The river-borne terrigenous component. In front of
deltas, river floods provide highly concentrated suspensions of terrigenous material. These can, particularly near the river bed, directly reach the deep sea as
underflows. Less concentrated suspensions float for
a short time as a fresh water wedge on top of sea water. As already mentioned in Section 3.5, clay suspended in fresh water is generally much less aggregated than in sea water. Thus it can be widely distributed (Fig. 5.3) before it comes in contact with more
saline water promoting aggregation. Once flocculated, ~he settling velocity of the aggregates increases to values of several meters up to several tens
of meters per day.
Their additional lateral transport is a function of water
depth and the velocity and depth range of the ocean currents (McCave 1984). This can be demonstrated by a very
simple example, which does not take into account effects of
large-scale turbulences: If a surface current has a mean
velocity of 0.2 mls (= -17 kmld) to a depth of up to 200 In,
an aggregate settling from sea level with a velocity of 10
mlday can reach a point 340 km away from its original
position.
It should be pointed out here, that transport, deposition, and erosion of fine-grained sediments do not