11.5 Clastic Sediment, Distribution
shelves and slopes or carried into deeper water. Nevertheless, some examples of modern deltas demonstrate that even under high-energy conditions a large
proportion of the incoming sediment is stored in the
coastal and prodelta zones elose to the river mouth.
Fraser River delta, British Columbia. The Holocene Fraser River delta progrades into the more than
300 m deep Strait of Georgia between mainland British Columbia and Vancouver Island (Fig. 11.26a).
The delta front is not directly exposed to the large
waves of the northeastern Pacific, but affected by
strong tidal currents, wind-generated currents,
estuarine circulation, and fresh-brackish water
plumes during river floods (Hart et al. 1995). The
evolution of the delta is influenced by the specific
bathymetry of the marine strait and sea-Ievel change
strongly modified by post-glacial rebound (Fig.
11.26b).
At the transition from late Pleistocene to Holocene, the
relative sea level stood higher than today because the land
was depressed by a heavy ice load in the Rocky mountains.
Between 10 and 8 ka B.P. the relative sea level had
dropped 10-15 m below the present datum as a result of
rebound after the melting of ice. Then the sea level started
to slowly rise to its present position. During this time period, the Fraser River delta grew westward into the Strait of
Georgia at a mean rate of about 2 mla (Fig. 11.26e; Williams and Roberts 1989) and formed a subaerial delta plain
of about 1000 km 2 • Outbuilding and upbuilding (5 to >10
mmla, behind the delta front 2 to 5 mmla) of delta sediments occurred simultaneously. The Holocene sediments
rest on glacial drift deposits with an irregular surface (Fig.
11.26d). Shallow banks in front of the delta are covered by
tidal marsh.
Seismic studies enabled the drawing of an isopach map
of the Holocene prodelta sediments (Fig. 11.26c,d, Hart et
a1. 1998) which reach 100-150 m in thickness over large
areas, and locally more than 200 m. Large slope failures
(slope angles typically 2-3°), submarine channels, and the
dominating northwest directed currents create much spatial
variability in the sediment thicknesses.
Based on simplified cross sections of the isopach
map, a crude estimate of the sediment masses deposited during the Holocene yielded the following results:
Prodelta slope in front of the tidal marsh:
~55 x 10 9 tllO ka
Subaerial delta including the tidal marsh:
~45 x 10 9 tlI0 ka
Total:
~100xl09t110kaorlOxl06t1a.
The modern sediment load of the Fraser River 42 km
upstream of its mouth (drainage area 234 000 km 2 ) is
17.3 X 10 6 tla and therefore ab out 1.7 times higher
than that found from the delta sediments. The ratio
between sediment deposited on the delta plain and in
523
the prodelta area during the Holocene is about 0.8;
the modern data indicate a ratio of about 0.6.
The higher modem sediment discharge may result from
both neoglaciation since about 4 ka and human activities.
A minor portion of the river load possibly reaches areas
beyond the prodelta investigated. The modem river load
consists of 35% sand, 50% silt, and 15% day. Ofthese 6.4
x 10 6 t/a (i.e. 37%) are deposited on the subaerial delta.
Using Cesium-137 fallout stratigraphy (the radioisotope
generated by nuclear tests with a peak in 1964) the sedimentation rate in wide areas of the prodelta area was found
to have been 1-2 cmla (average for the past 30 years, Hart
et a1. 1998). The prodelta sediment mass (4.0 x 10 6 t/a) derived from this data is less than expected from the measured modem river load or from the estimate of Holocene
prodelta deposition. It is possible that part of the incoming
sediment could not be identified by the Cs method.
In the previous examples from the Mediterranean,
i.e. from areas farther away from the regions of former inland ice, the Holocene eustatic rise in sea level
was not substantially reduced by rebound of the crust
following deglaciation; thus, many former lowstand
deltas were flooded and the rivers had to build new
deltas adjusted to the present sea level (Fig. 11.23c).
Niger delta, West Africa. This is an example of
long-term delta outbuilding. The delta started to form
after the separation of Africa and South America
during the Cretaceous (Fig. 11.27a). Today, it represents a lobate, mixed fluvial- to wave-dominated
delta (Sect. 3.5) including moderate tidal influence.
The general history of the Niger delta and other aspects of
its sediments are weil documented by many publications
(summary in Shannon and Maylor 1989; also see Cohen
and McClay 1996; Roubyand Cobbold 1996). The delta
developed in a crustal depression (Benue Trough) and
prograded at a very slow mean rate of about 4 mmla over
the boundary between the continental and oceanic crust. Its
huge sediment body reached a thickness ofup to 12 km.
Using the information summarized in Fig. 11.27, volurne and mass of sediment below the delta plain and in the
prodelta urea deposited since the Eocene (40 Ma) were estimated: 380 000 km 3 or 7.5 x 10 14 t. To accumulate this
mass an annual average of ~ 19 x 10 6 t/a is needed. The
present-
A comparison with the modern river load indicates
that ab out one half of the incorning sediment is
stored in the delta area. However, it is possible that
both the drainage area and the sediment dis charge of
the river significantly changed during the long time
period considered. In any case, loss of sediment to
the deep sea is documented by the existence of the
Niger deep-sea fan. Other marine deltas associated
with deep-sea fans are discussed in Section 11.5.4.
shelves and slopes or carried into deeper water. Nevertheless, some examples of modern deltas demonstrate that even under high-energy conditions a large
proportion of the incoming sediment is stored in the
coastal and prodelta zones elose to the river mouth.
Fraser River delta, British Columbia. The Holocene Fraser River delta progrades into the more than
300 m deep Strait of Georgia between mainland British Columbia and Vancouver Island (Fig. 11.26a).
The delta front is not directly exposed to the large
waves of the northeastern Pacific, but affected by
strong tidal currents, wind-generated currents,
estuarine circulation, and fresh-brackish water
plumes during river floods (Hart et al. 1995). The
evolution of the delta is influenced by the specific
bathymetry of the marine strait and sea-Ievel change
strongly modified by post-glacial rebound (Fig.
11.26b).
At the transition from late Pleistocene to Holocene, the
relative sea level stood higher than today because the land
was depressed by a heavy ice load in the Rocky mountains.
Between 10 and 8 ka B.P. the relative sea level had
dropped 10-15 m below the present datum as a result of
rebound after the melting of ice. Then the sea level started
to slowly rise to its present position. During this time period, the Fraser River delta grew westward into the Strait of
Georgia at a mean rate of about 2 mla (Fig. 11.26e; Williams and Roberts 1989) and formed a subaerial delta plain
of about 1000 km 2 • Outbuilding and upbuilding (5 to >10
mmla, behind the delta front 2 to 5 mmla) of delta sediments occurred simultaneously. The Holocene sediments
rest on glacial drift deposits with an irregular surface (Fig.
11.26d). Shallow banks in front of the delta are covered by
tidal marsh.
Seismic studies enabled the drawing of an isopach map
of the Holocene prodelta sediments (Fig. 11.26c,d, Hart et
a1. 1998) which reach 100-150 m in thickness over large
areas, and locally more than 200 m. Large slope failures
(slope angles typically 2-3°), submarine channels, and the
dominating northwest directed currents create much spatial
variability in the sediment thicknesses.
Based on simplified cross sections of the isopach
map, a crude estimate of the sediment masses deposited during the Holocene yielded the following results:
Prodelta slope in front of the tidal marsh:
~55 x 10 9 tllO ka
Subaerial delta including the tidal marsh:
~45 x 10 9 tlI0 ka
Total:
~100xl09t110kaorlOxl06t1a.
The modern sediment load of the Fraser River 42 km
upstream of its mouth (drainage area 234 000 km 2 ) is
17.3 X 10 6 tla and therefore ab out 1.7 times higher
than that found from the delta sediments. The ratio
between sediment deposited on the delta plain and in
523
the prodelta area during the Holocene is about 0.8;
the modern data indicate a ratio of about 0.6.
The higher modem sediment discharge may result from
both neoglaciation since about 4 ka and human activities.
A minor portion of the river load possibly reaches areas
beyond the prodelta investigated. The modem river load
consists of 35% sand, 50% silt, and 15% day. Ofthese 6.4
x 10 6 t/a (i.e. 37%) are deposited on the subaerial delta.
Using Cesium-137 fallout stratigraphy (the radioisotope
generated by nuclear tests with a peak in 1964) the sedimentation rate in wide areas of the prodelta area was found
to have been 1-2 cmla (average for the past 30 years, Hart
et a1. 1998). The prodelta sediment mass (4.0 x 10 6 t/a) derived from this data is less than expected from the measured modem river load or from the estimate of Holocene
prodelta deposition. It is possible that part of the incoming
sediment could not be identified by the Cs method.
In the previous examples from the Mediterranean,
i.e. from areas farther away from the regions of former inland ice, the Holocene eustatic rise in sea level
was not substantially reduced by rebound of the crust
following deglaciation; thus, many former lowstand
deltas were flooded and the rivers had to build new
deltas adjusted to the present sea level (Fig. 11.23c).
Niger delta, West Africa. This is an example of
long-term delta outbuilding. The delta started to form
after the separation of Africa and South America
during the Cretaceous (Fig. 11.27a). Today, it represents a lobate, mixed fluvial- to wave-dominated
delta (Sect. 3.5) including moderate tidal influence.
The general history of the Niger delta and other aspects of
its sediments are weil documented by many publications
(summary in Shannon and Maylor 1989; also see Cohen
and McClay 1996; Roubyand Cobbold 1996). The delta
developed in a crustal depression (Benue Trough) and
prograded at a very slow mean rate of about 4 mmla over
the boundary between the continental and oceanic crust. Its
huge sediment body reached a thickness ofup to 12 km.
Using the information summarized in Fig. 11.27, volurne and mass of sediment below the delta plain and in the
prodelta urea deposited since the Eocene (40 Ma) were estimated: 380 000 km 3 or 7.5 x 10 14 t. To accumulate this
mass an annual average of ~ 19 x 10 6 t/a is needed. The
present-
A comparison with the modern river load indicates
that ab out one half of the incorning sediment is
stored in the delta area. However, it is possible that
both the drainage area and the sediment dis charge of
the river significantly changed during the long time
period considered. In any case, loss of sediment to
the deep sea is documented by the existence of the
Niger deep-sea fan. Other marine deltas associated
with deep-sea fans are discussed in Section 11.5.4.