234
sheet-like gravelly sand bodies (up to 25 m thick)
which fonn a great part of the outer fan. The channel
fills consist of thick amalgamated massive as well as
graded and cross-stratified sands including many
mud elasts and gravity mass deposits derived from
the levees. The channel-levee complexes are partially
overlain by large mass flow deposits (Fig. 5.22).
For further details about this relatively weil investigated
deep-sea fan see, e.g., Flood and Piper (1997).
In accordance with these results and those from other
investigations on shelf/slope settings, one can conc1ude that sediment gravity flows and turbidites fonn
preferentially during the lowstand systems tract. On
the other hand, rising and high sea level do not always lead to hemipelagic drapings (condensed sections) on deep-sea fans. Highly efficient sediment
sources can maintain fan growth even during highamplitude sea-level oscillations as known, e.g., from
the Bengal Fan (cf. Sect. 11.5.6). On convergent continental margins with narrow shelves, steep slopes
and high sediment supply, deep-sea fans continue to
grow during every stage of a relative sea-level cyele
(lto 1998).
In mixed carbonate-silicielastic systems, lowstands
are frequently characterized by silicielastic turbidites
and carbonate megabreccias.
Sedimentation Rates of Deep-Sea Fans
Deep-sea fans and adjacent basin plains are areas of
high sedimentation rates (cf. Sect. 10.2). This is particularly true of time intervals of lowering sea level
or times oftectonic activity creating increasingrelief.
In modem fan environments, ineluding Miocene to
Pleistocene deposits, average sedimentation rates
between 100 and 1000 mlMa are common, but near
the sediment source and in over-supplied basins
(Mutti et al. 1984), higher values also occur.
The giant Bengal fan, lndus fan, Amazon fan, Mississippi
fan, and some other elongate, present-day deep-sea fans
represent wedge-shaped sediment bodies which reach maximum thicknesses on the order of 5 to 10 km which have
been built up in time spans of a few Ma to 20 Ma (Curray
and Moore 1974; Bouma et al. 1985; Bouma et al. 1986;
Kolla and Coumes 1987; Damuth et al. 1988; Wetzel
1993). The same applies to many thick, ancient flysch sequences.
5.4.6 Deep-Sea Channels
A continental rnarginldeep-sea fan system, characterized by a submarine canyon and channelized fan
lobes, may continue into a deep-sea channel which
finally ends in an abyssal plain. Such deep-sea channels were observed in the present oceans, where they
Chapter 5 Oceanic Sediments
reach lengths of 2000 to more than 3000 km and
widths of a half to several kilometers.
They resemble in many aspects sub aerial river systems. Ancient examples of this type of channel are
probably not known.
These channels have been described in some detail, e.g., by
Carter (1988), Hesse (1989a), Hesse et al. (1996), Hesse et
al. (1997). The most prominent example is the Northwest
Atlantic mid-ocean channel of the Labrador Sea, which
represents a submarine, river-like drainage system with
numerous tributaries. Of these many originate from the
Labrador slope dominated by mud. The others, corning
from the Labrador Strait and Greenland, are fed mainly by
sand delivered by ice sheets. The mud-dominated channels
are sinuous and build levees; the sand-dorninated tributaries generate a braided channel system. The trunc channel
has a low longitudinal gradient similar to large subaerial
rivers. The channel is rnaintained by relatively high-velocity high-density turbidity currents flowing within the confines of the levees.
The infillings of these channels, far away from any land
source, consist of sandy to graveUy fining-upward sequences. On the levees, parallellaminated, thin mud
turbidites were observed. The channel system may end an a
kind of submarine braidplain.
sheet-like gravelly sand bodies (up to 25 m thick)
which fonn a great part of the outer fan. The channel
fills consist of thick amalgamated massive as well as
graded and cross-stratified sands including many
mud elasts and gravity mass deposits derived from
the levees. The channel-levee complexes are partially
overlain by large mass flow deposits (Fig. 5.22).
For further details about this relatively weil investigated
deep-sea fan see, e.g., Flood and Piper (1997).
In accordance with these results and those from other
investigations on shelf/slope settings, one can conc1ude that sediment gravity flows and turbidites fonn
preferentially during the lowstand systems tract. On
the other hand, rising and high sea level do not always lead to hemipelagic drapings (condensed sections) on deep-sea fans. Highly efficient sediment
sources can maintain fan growth even during highamplitude sea-level oscillations as known, e.g., from
the Bengal Fan (cf. Sect. 11.5.6). On convergent continental margins with narrow shelves, steep slopes
and high sediment supply, deep-sea fans continue to
grow during every stage of a relative sea-level cyele
(lto 1998).
In mixed carbonate-silicielastic systems, lowstands
are frequently characterized by silicielastic turbidites
and carbonate megabreccias.
Sedimentation Rates of Deep-Sea Fans
Deep-sea fans and adjacent basin plains are areas of
high sedimentation rates (cf. Sect. 10.2). This is particularly true of time intervals of lowering sea level
or times oftectonic activity creating increasingrelief.
In modem fan environments, ineluding Miocene to
Pleistocene deposits, average sedimentation rates
between 100 and 1000 mlMa are common, but near
the sediment source and in over-supplied basins
(Mutti et al. 1984), higher values also occur.
The giant Bengal fan, lndus fan, Amazon fan, Mississippi
fan, and some other elongate, present-day deep-sea fans
represent wedge-shaped sediment bodies which reach maximum thicknesses on the order of 5 to 10 km which have
been built up in time spans of a few Ma to 20 Ma (Curray
and Moore 1974; Bouma et al. 1985; Bouma et al. 1986;
Kolla and Coumes 1987; Damuth et al. 1988; Wetzel
1993). The same applies to many thick, ancient flysch sequences.
5.4.6 Deep-Sea Channels
A continental rnarginldeep-sea fan system, characterized by a submarine canyon and channelized fan
lobes, may continue into a deep-sea channel which
finally ends in an abyssal plain. Such deep-sea channels were observed in the present oceans, where they
Chapter 5 Oceanic Sediments
reach lengths of 2000 to more than 3000 km and
widths of a half to several kilometers.
They resemble in many aspects sub aerial river systems. Ancient examples of this type of channel are
probably not known.
These channels have been described in some detail, e.g., by
Carter (1988), Hesse (1989a), Hesse et al. (1996), Hesse et
al. (1997). The most prominent example is the Northwest
Atlantic mid-ocean channel of the Labrador Sea, which
represents a submarine, river-like drainage system with
numerous tributaries. Of these many originate from the
Labrador slope dominated by mud. The others, corning
from the Labrador Strait and Greenland, are fed mainly by
sand delivered by ice sheets. The mud-dominated channels
are sinuous and build levees; the sand-dorninated tributaries generate a braided channel system. The trunc channel
has a low longitudinal gradient similar to large subaerial
rivers. The channel is rnaintained by relatively high-velocity high-density turbidity currents flowing within the confines of the levees.
The infillings of these channels, far away from any land
source, consist of sandy to graveUy fining-upward sequences. On the levees, parallellaminated, thin mud
turbidites were observed. The channel system may end an a
kind of submarine braidplain.