5.4 Gravity Mass Flow
(3) Levee and overbank deposits. Thick low-velocity
suspension currents of limited density spill over the
confines of channels and drop their fine-grained load
onto the channel banks and in interchannel areas. In
this way they build up levees and overbank deposits
(cf. Fig. 5.18 Band C). The thin-bedded overbank
deposits predominantly show the Bouma divisions
Tc• e of fine-grained sands and silts alternating with
mud turbidites. The directions of flow deviate from
that of the main channel and may be deflected by
Coriolis forces. Sand deposition is almost entirely
restricted to the infills of minor channels emanating
from the main channel. Levee erosion can take place
locally.
(4) Lower fan and basin plain. In particular thin, relatively dense and fast turbidity currents tend to flow
basinward within the confines of the channels and
their levees. They can transport sand and mud to the
(switching) lower fan lobes and basin plain (cf. Fig.
5.18 D). Only very large, rare debris and mudflows
and their subsequent turbulent flows do spread their
load over large areas of the total fan and basin plain.
According to a comparative study of sand layers in presentday ocean basins (Pilkey et al. 1980), the percentage 0/
sand layers in the total sediment volume of deep-sea fans
decreases distally, as also observed in ancient flysch sequences. The thickest layers were found in basins which
have large drainage areas. Single sand beds could be traced
over distances as great as 500 km (Hatteras abyssal plain in
the western Atlantic). However, as a result of flow confinement to channels, distal parts of a deep-sea fan can become
richer in sand than mid-fan regions.
Paleo-Current Directions in Deep-Sea Fans
Paleo-current directions of turbidity currents can be
derived from sole marks (cf. Fig. 5.l5e), internal
structures such as cross-bedding, elast and grain orientation, and current ripples (see, e.g., Collinson and
Thompson 1989). Studies on ancient turbidite sequences deposited on lower fan lobes and plains of
elongate narrow basins have frequently shown a
striking constancy of current directions over large
areas. Similarly, current directions on slope aprons
fed by outer shelf and upper slope sediments can be
expected to vary only moderately. In more proximal,
channelized fan associations as well as in basins supplied with sediment from different sources, however,
the palaeocurrent patterns become less regular and
sometimes rather complex. When the fan lobes have
room to switch (cf. Fig. 5.l8), the sediments are dispersed radially. In interchannel areas, the paleoflow
directions are gene rally deflected from those of the
main channel and may show a great variation. Finally, the current patterns of turbidites can occasionally become overprinted by contour currents (Sect.
5.5).
229
Paleoslope orientations can be inferred from slide
scars, slump folds, and sometimes from the
imbrication of elasts in debrites and mud flows. For
reliable measurements, good, large exposures are
needed.
Active and Inactive Phases of Fan Deposition
An ideally prograding slope-fan association with a
more or less fixed channel system produces an upward increase in proximality, i.e., a coarsening,
thickening sequence. However, this scenario is frequently modified.
(1) As a result of shifting channels and changing
sites of mass movement on slopes, vertical sections
of an individual deep-sea fan can show bed successions of apparently widely differing proximality (cf.
Fig. 5.l8 Band C). Normal, current-transported material may alternate with debrites and mud flow deposits, sandy and muddy turbidites, and hemi-pelagic
or pelagic sediments. Switching of fan lobes in conjunction with migrating channel systems (Fig. 5.20a)
can generate both fining (and/or thinning) upward as
weil as coarsening (and/or thickening) upward sequences.
Similarly, substantial basin subsidence may lead to
an aggradational fan system with insignificant vertical but marked lateral facies change (Macdonald
1986).
(2) In response to variations in the amount of sediment input by their feeder system, deep-sea fans undergo phases of rapid progradation and/or
upbuilding, or per iods of inactivity, including some
reworking. Prograding and upbuilding are usually
correlated with a relative lowering of sea level, when
the gradients of rivers entering the sea are steepened,
and former coastal and shallow-water sediments are
eroded and swept into deeper waters. Then coarsening (and/or thickening) upward stratigraphic sequences are generated (Fig. 5.20b, sections F through
G). Relative sea-level fall is also one of the main
mechanisms for the incision of submarine valleys on
shelves and upper slopes.
The cutting of deep submarine canyons in shelf-slope settings has been controversially discussed in the past (e.g.
Shepard and Dill 1966; Greene et al. 1991). A single process, such as slumping, outflowing bottom currents or
turbidity currents, cannot explain these features. It seems to
be now generally accepted that Pleistocene low sea levels
and subaerial valley cutting, accompanied by submarine
erosional processes on the slopes, are responsible for most
of the present-day submarine valleys and canyons on passive continental margins (cf. Sect. 7.6). The head of the
Mississippi Canyon, for example, was widened by several
phases of gravity mass movements (Goodwin and Prior
1989).
(3) Levee and overbank deposits. Thick low-velocity
suspension currents of limited density spill over the
confines of channels and drop their fine-grained load
onto the channel banks and in interchannel areas. In
this way they build up levees and overbank deposits
(cf. Fig. 5.18 Band C). The thin-bedded overbank
deposits predominantly show the Bouma divisions
Tc• e of fine-grained sands and silts alternating with
mud turbidites. The directions of flow deviate from
that of the main channel and may be deflected by
Coriolis forces. Sand deposition is almost entirely
restricted to the infills of minor channels emanating
from the main channel. Levee erosion can take place
locally.
(4) Lower fan and basin plain. In particular thin, relatively dense and fast turbidity currents tend to flow
basinward within the confines of the channels and
their levees. They can transport sand and mud to the
(switching) lower fan lobes and basin plain (cf. Fig.
5.18 D). Only very large, rare debris and mudflows
and their subsequent turbulent flows do spread their
load over large areas of the total fan and basin plain.
According to a comparative study of sand layers in presentday ocean basins (Pilkey et al. 1980), the percentage 0/
sand layers in the total sediment volume of deep-sea fans
decreases distally, as also observed in ancient flysch sequences. The thickest layers were found in basins which
have large drainage areas. Single sand beds could be traced
over distances as great as 500 km (Hatteras abyssal plain in
the western Atlantic). However, as a result of flow confinement to channels, distal parts of a deep-sea fan can become
richer in sand than mid-fan regions.
Paleo-Current Directions in Deep-Sea Fans
Paleo-current directions of turbidity currents can be
derived from sole marks (cf. Fig. 5.l5e), internal
structures such as cross-bedding, elast and grain orientation, and current ripples (see, e.g., Collinson and
Thompson 1989). Studies on ancient turbidite sequences deposited on lower fan lobes and plains of
elongate narrow basins have frequently shown a
striking constancy of current directions over large
areas. Similarly, current directions on slope aprons
fed by outer shelf and upper slope sediments can be
expected to vary only moderately. In more proximal,
channelized fan associations as well as in basins supplied with sediment from different sources, however,
the palaeocurrent patterns become less regular and
sometimes rather complex. When the fan lobes have
room to switch (cf. Fig. 5.l8), the sediments are dispersed radially. In interchannel areas, the paleoflow
directions are gene rally deflected from those of the
main channel and may show a great variation. Finally, the current patterns of turbidites can occasionally become overprinted by contour currents (Sect.
5.5).
229
Paleoslope orientations can be inferred from slide
scars, slump folds, and sometimes from the
imbrication of elasts in debrites and mud flows. For
reliable measurements, good, large exposures are
needed.
Active and Inactive Phases of Fan Deposition
An ideally prograding slope-fan association with a
more or less fixed channel system produces an upward increase in proximality, i.e., a coarsening,
thickening sequence. However, this scenario is frequently modified.
(1) As a result of shifting channels and changing
sites of mass movement on slopes, vertical sections
of an individual deep-sea fan can show bed successions of apparently widely differing proximality (cf.
Fig. 5.l8 Band C). Normal, current-transported material may alternate with debrites and mud flow deposits, sandy and muddy turbidites, and hemi-pelagic
or pelagic sediments. Switching of fan lobes in conjunction with migrating channel systems (Fig. 5.20a)
can generate both fining (and/or thinning) upward as
weil as coarsening (and/or thickening) upward sequences.
Similarly, substantial basin subsidence may lead to
an aggradational fan system with insignificant vertical but marked lateral facies change (Macdonald
1986).
(2) In response to variations in the amount of sediment input by their feeder system, deep-sea fans undergo phases of rapid progradation and/or
upbuilding, or per iods of inactivity, including some
reworking. Prograding and upbuilding are usually
correlated with a relative lowering of sea level, when
the gradients of rivers entering the sea are steepened,
and former coastal and shallow-water sediments are
eroded and swept into deeper waters. Then coarsening (and/or thickening) upward stratigraphic sequences are generated (Fig. 5.20b, sections F through
G). Relative sea-level fall is also one of the main
mechanisms for the incision of submarine valleys on
shelves and upper slopes.
The cutting of deep submarine canyons in shelf-slope settings has been controversially discussed in the past (e.g.
Shepard and Dill 1966; Greene et al. 1991). A single process, such as slumping, outflowing bottom currents or
turbidity currents, cannot explain these features. It seems to
be now generally accepted that Pleistocene low sea levels
and subaerial valley cutting, accompanied by submarine
erosional processes on the slopes, are responsible for most
of the present-day submarine valleys and canyons on passive continental margins (cf. Sect. 7.6). The head of the
Mississippi Canyon, for example, was widened by several
phases of gravity mass movements (Goodwin and Prior
1989).