2.45 Sedimentation Along Continental
Margins and in Epeiric Seas
As discussed above most of the sediment derived from
land is deposited in river deltas and distributed relatively close to the shore line by longshore transport.
There is, however, also significant sedimentation further away from the deltas and the coastlines. Turbidites
and debris flows can in some cases transport sediments
hundreds of kilometres offshore. Debris flows represent
a very efficient type of sediment transport which can
carry sediment far beyond the continental slopes
because there is little internal shear deformation, only
at the base and the top of the flow.
In fine-grained shales we may find evidence of
ripples indicating traction currents. Storms and spring
tides may produce rather high velocities (20–40 cm/s)
on the seafloor far away from land and these currents
are capable of transporting silt and fine sand. Clay
should not produce ripples, but the clay is often composed of pellets of clay aggregates, often also with
some organic matter. These clay particles behave like
silt but they have lower densities and may be
transported by traction currents forming small-scale
ripple cross-lamination.
There may also be significant contributions of aeolian dust from deserts and of volcanic ash. In
Palaeozoic times, before there were many land plants,
aeolian dust was more important, but even today much
of the sedimentation in the South Atlantic and Pacific
Ocean is aeolian.
2.46 Sedimentation Along Island Arcs
and Submarine Trenches
Submarine trenches form along converging plate
boundaries where oceanic lithospheres are
disappearing into a subduction zone. Along these converging plate boundaries sediment basins with very
special deposition environments are formed. There
are three main sources of sediment:
1. From the continent.
2. From island arcs, which may consist of continental
crust, oceanic crust and/or volcanic rocks.
3. Pelagic sediment, including biogenic sediment and
wind-blown volcanic ash.
These have quite different compositions. Sediment
which is added from the continent is deposited in
deltas and in turn fills the basin behind the arc (backarc basin). Sediments which are formed on the island
arcs are rich in volcanic material, and this will characterise deposits in small basins on island arcs and forearc basins. Back-arc basins may also receive volcanic
sediments from the island arcs. During the initial subduction phase fore-arc basins will tend to be
characterised by turbidites deposited in relatively
deep water, but sediment filling may convert them
into a shallow water environment which may include
carbonate sediments. A structural high separates the
fore-arc basin from the actual slope down to the submarine trench.
Beyond the deep sea trenches a significant amount
of pelagic sedimentation takes place on a relatively flat
ocean crust. Some of the sediments get scraped off
the subducting volcanic crust and stacked up in what
are called “accretionary prisms” (Fig. 2.48). Some
SL.
D.F.
CGL
M.S.
P.S.
(CGL)
M.S.
C.T.
CGL
P.S.
M.S.
C.T.
C.T.
C.T.
F-U
F-U
C-U
C-U
C-U
C-U
F-U
Inner fan
channel fill
Channelled
portion of
suprafan
lobes
Channelled
Smooth
Smooth
portion of
suprafan
lobes
Outer fan
Suprafan lobes on mid fan
Facies
Sequence
Interpretation
Fining- or
coarseningupward
Fig. 2.47 Vertical sequence through a submarine fan (Walker
1984)
2 Introduction to Sedimentology
87
Margins and in Epeiric Seas
As discussed above most of the sediment derived from
land is deposited in river deltas and distributed relatively close to the shore line by longshore transport.
There is, however, also significant sedimentation further away from the deltas and the coastlines. Turbidites
and debris flows can in some cases transport sediments
hundreds of kilometres offshore. Debris flows represent
a very efficient type of sediment transport which can
carry sediment far beyond the continental slopes
because there is little internal shear deformation, only
at the base and the top of the flow.
In fine-grained shales we may find evidence of
ripples indicating traction currents. Storms and spring
tides may produce rather high velocities (20–40 cm/s)
on the seafloor far away from land and these currents
are capable of transporting silt and fine sand. Clay
should not produce ripples, but the clay is often composed of pellets of clay aggregates, often also with
some organic matter. These clay particles behave like
silt but they have lower densities and may be
transported by traction currents forming small-scale
ripple cross-lamination.
There may also be significant contributions of aeolian dust from deserts and of volcanic ash. In
Palaeozoic times, before there were many land plants,
aeolian dust was more important, but even today much
of the sedimentation in the South Atlantic and Pacific
Ocean is aeolian.
2.46 Sedimentation Along Island Arcs
and Submarine Trenches
Submarine trenches form along converging plate
boundaries where oceanic lithospheres are
disappearing into a subduction zone. Along these converging plate boundaries sediment basins with very
special deposition environments are formed. There
are three main sources of sediment:
1. From the continent.
2. From island arcs, which may consist of continental
crust, oceanic crust and/or volcanic rocks.
3. Pelagic sediment, including biogenic sediment and
wind-blown volcanic ash.
These have quite different compositions. Sediment
which is added from the continent is deposited in
deltas and in turn fills the basin behind the arc (backarc basin). Sediments which are formed on the island
arcs are rich in volcanic material, and this will characterise deposits in small basins on island arcs and forearc basins. Back-arc basins may also receive volcanic
sediments from the island arcs. During the initial subduction phase fore-arc basins will tend to be
characterised by turbidites deposited in relatively
deep water, but sediment filling may convert them
into a shallow water environment which may include
carbonate sediments. A structural high separates the
fore-arc basin from the actual slope down to the submarine trench.
Beyond the deep sea trenches a significant amount
of pelagic sedimentation takes place on a relatively flat
ocean crust. Some of the sediments get scraped off
the subducting volcanic crust and stacked up in what
are called “accretionary prisms” (Fig. 2.48). Some
SL.
D.F.
CGL
M.S.
P.S.
(CGL)
M.S.
C.T.
CGL
P.S.
M.S.
C.T.
C.T.
C.T.
F-U
F-U
C-U
C-U
C-U
C-U
F-U
Inner fan
channel fill
Channelled
portion of
suprafan
lobes
Channelled
Smooth
Smooth
portion of
suprafan
lobes
Outer fan
Suprafan lobes on mid fan
Facies
Sequence
Interpretation
Fining- or
coarseningupward
Fig. 2.47 Vertical sequence through a submarine fan (Walker
1984)
2 Introduction to Sedimentology
87