528
In zones of stable c!imate, such as the tropics, approximately one fourth of the river load is deposited on the corresponding deep-sea fan. F or time periods of > 1 Ma, the
Pleistocene sea-Ievel changes have not significantly affected the transfer ratios. Reduced sediment supply to the
fans during sea-Ievel rise and highstands has been more or
less compensated for by increased supply during sea-Ievel
fall. An intermittent surplus of fan sediment in comparison
to the average river load can be caused, for example, by
reworking of older glacial deposits (as known of the Laurentian fan and Astoria fan).
A prominent example of low sediment transfer is the
Amazon fan receiving !ittle terrigenous material from the
Amazon river during the ptesent-day sea-Ievel highstand.
About one third of the fluvial mud is deposited in the lowermost reaches ofthe river before entering the sea (Vital et
al. 1998), and most of the mud arriving in the ocean is
driven northward by currents (Nittrouer et al. 1986). The
fact that the long-term (16.5 Ma) transfer ratio of this system is also very low indicates substantial lateral sediment
transport on the shelf even during lowered sea-Ievel stands.
A very low present-day transfer ratio (2%) is also known
of the Ebro (Valencia) fan in the Mediterranean. However,
during the past 4.8 Ma about 22% of the incoming Ebro
sediment reached the deep-sea fan while 45% remained on
the shelf and 33% on the slope (Nelson 1990).
A transfer ratio> 1 00% seems to have been realized for
the Nitinat fan, representing amiddie to late Pleistocene
(0.6 to 0.8 Ma B.P.) sediment body of the Fraser River.
This fan was deposited in the open deep northwestem Pacific (in contrast to the modern Fraser delta prograding into
the Strait ofGeorgia discussed above). Its mean deposition
rate (11 to 15 x 10 6 mJ/a or about 20 to 28 X 10 6 tla, porosity -0.3) was somewhat greater than that determined for the
Holocene Fraser delta. This finding appears to be reasonable because during glacial periods sediment supply was
probably higher than in the late Holocene.
- Fluvial-dominated and lobate deltas store a higher
proportion of the incoming sediment by prograding
than wave- and tide-dominated deltas. Especially deltas with deeply incised canyons in front of their river
mouths seem to convey a high amount of the river
load to the deep sea.
This is true, for example, of the Rhöne fan in the
western Mediterranean and, to some extent, also of
the huge Bengal fan in the Indian ocean.
Bengal fan. About 50% of the rock material eroded in the
Himalayas during the past 20 Ma has been deposited on the
Bengal fan (Figs. 11.30b and 11.31). Even with the
present-day sea-Ievel highstand, a large proportion of the
incoming sediment is funneled to the deep sea via a deep
canyon cut into the shelf and fan head (Kuehl et al. 1989;
Hübscher et al. 1997; Weber et al. 1997, Kudrass et al.
1998). Storm-induced and tidal currents transport sediment
from the inner shelf into the head of the canyon. The upper
subaerial Bengal delta in northeastem Bangladesh seems to
have subsided at an average rate of 1.2 mika during the last
few millions ofyears (Worm et al. 1998).
Mississippi fan. If the deposition rate of this fan is related to the modern suspended river load reduced by dam
sites upstream, then the transfer ratio would be about 60%.
Considering a sediment balance of the major part of the
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
Gulf of Mexico for the past 2.3 Ma (Hay et al. 1989;
Wetzel 1993), however, the average transfer ratio is only
35%.
Columbia and Zaire fans. The Columbia fan (off the
US Atlantic coast) and Zaire fan (off central West Africa)
presently receive 11 and 15% of their river loads, respectively.
- The sediment transfer ratio to deep-sea fans seems
to be independent ofthe size ofthe fan (Fig. 11.30b).
On the other hand, relatively small fans close to
coastal mountain ranges, tend to have high er proportions of sand and gravel than the large, mud-dominated deep-sea fans (e.g. Reading and Richards
1994; Reynolds 1994). One can expect that the transfer ratios of coarser grained fans are higher than
those of mud-dominated fans, hut quantitative data
are not yet available.
The sum of all deposition rates known from modem
deep-sea fans (Wetze1 1993) is about 650 x 10 6 m 3 /a
or -1.2 x 10 9 tla. Assuming that these examples
cover about one half of all fans existing in the
present-day oceans, one finds that approximate1y 1520% of the global river load reaching the oceans
(Sect. 9.3.2) is transferred to deep-sea fans. This is a
surprisingly low proportion, but it supports the conclusions on sediment storage in delta areas and
prograding coastlines. In addition, a significant part
of the river-borne sediment must come to rest outside
of delta complexes and river-fed deep-sea fans, i.e.
on shelves, slopes, slope-aprons, and basin plains
beyond the range of deep-sea fans (cf. Sects. 11.5.5
and 11.5.7).
11.5.5 Mud Deposition on Continental Shelves
Significant proportions of the river-borne terrigenous
sand and mud are not deposited in the corresponding
delta and prodelta areas or directly transferred to
deep-sea fans as discussed above. Sand can be transported alongshore over long distances away from the
mouths of rivers and either be stored in the coastal
zone, or become trapped in the heads of distant submarine valleys funneling sediment into the deep sea.
Mud commonly accumulates at some distance seaward from the coastline on the middle and outer
shelf, depending on the hydrodynamic regime of the
area and river mud supply (cf. Sect. 3.3). Shelves in
front of mud-rich rivers, such as the Amazon River,
can become mud-dominated and store large proportions of the incoming fine-grained sediment as mentioned above. High-energy environments, for exampIe the Pacific coast of North America, allow mud
deposition only at water depths greater than about 50
m.
Such an example has been reported from the Ca!ifomian
coast elose to the delta of the Eel River (Sumrnerfield and
In zones of stable c!imate, such as the tropics, approximately one fourth of the river load is deposited on the corresponding deep-sea fan. F or time periods of > 1 Ma, the
Pleistocene sea-Ievel changes have not significantly affected the transfer ratios. Reduced sediment supply to the
fans during sea-Ievel rise and highstands has been more or
less compensated for by increased supply during sea-Ievel
fall. An intermittent surplus of fan sediment in comparison
to the average river load can be caused, for example, by
reworking of older glacial deposits (as known of the Laurentian fan and Astoria fan).
A prominent example of low sediment transfer is the
Amazon fan receiving !ittle terrigenous material from the
Amazon river during the ptesent-day sea-Ievel highstand.
About one third of the fluvial mud is deposited in the lowermost reaches ofthe river before entering the sea (Vital et
al. 1998), and most of the mud arriving in the ocean is
driven northward by currents (Nittrouer et al. 1986). The
fact that the long-term (16.5 Ma) transfer ratio of this system is also very low indicates substantial lateral sediment
transport on the shelf even during lowered sea-Ievel stands.
A very low present-day transfer ratio (2%) is also known
of the Ebro (Valencia) fan in the Mediterranean. However,
during the past 4.8 Ma about 22% of the incoming Ebro
sediment reached the deep-sea fan while 45% remained on
the shelf and 33% on the slope (Nelson 1990).
A transfer ratio> 1 00% seems to have been realized for
the Nitinat fan, representing amiddie to late Pleistocene
(0.6 to 0.8 Ma B.P.) sediment body of the Fraser River.
This fan was deposited in the open deep northwestem Pacific (in contrast to the modern Fraser delta prograding into
the Strait ofGeorgia discussed above). Its mean deposition
rate (11 to 15 x 10 6 mJ/a or about 20 to 28 X 10 6 tla, porosity -0.3) was somewhat greater than that determined for the
Holocene Fraser delta. This finding appears to be reasonable because during glacial periods sediment supply was
probably higher than in the late Holocene.
- Fluvial-dominated and lobate deltas store a higher
proportion of the incoming sediment by prograding
than wave- and tide-dominated deltas. Especially deltas with deeply incised canyons in front of their river
mouths seem to convey a high amount of the river
load to the deep sea.
This is true, for example, of the Rhöne fan in the
western Mediterranean and, to some extent, also of
the huge Bengal fan in the Indian ocean.
Bengal fan. About 50% of the rock material eroded in the
Himalayas during the past 20 Ma has been deposited on the
Bengal fan (Figs. 11.30b and 11.31). Even with the
present-day sea-Ievel highstand, a large proportion of the
incoming sediment is funneled to the deep sea via a deep
canyon cut into the shelf and fan head (Kuehl et al. 1989;
Hübscher et al. 1997; Weber et al. 1997, Kudrass et al.
1998). Storm-induced and tidal currents transport sediment
from the inner shelf into the head of the canyon. The upper
subaerial Bengal delta in northeastem Bangladesh seems to
have subsided at an average rate of 1.2 mika during the last
few millions ofyears (Worm et al. 1998).
Mississippi fan. If the deposition rate of this fan is related to the modern suspended river load reduced by dam
sites upstream, then the transfer ratio would be about 60%.
Considering a sediment balance of the major part of the
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
Gulf of Mexico for the past 2.3 Ma (Hay et al. 1989;
Wetzel 1993), however, the average transfer ratio is only
35%.
Columbia and Zaire fans. The Columbia fan (off the
US Atlantic coast) and Zaire fan (off central West Africa)
presently receive 11 and 15% of their river loads, respectively.
- The sediment transfer ratio to deep-sea fans seems
to be independent ofthe size ofthe fan (Fig. 11.30b).
On the other hand, relatively small fans close to
coastal mountain ranges, tend to have high er proportions of sand and gravel than the large, mud-dominated deep-sea fans (e.g. Reading and Richards
1994; Reynolds 1994). One can expect that the transfer ratios of coarser grained fans are higher than
those of mud-dominated fans, hut quantitative data
are not yet available.
The sum of all deposition rates known from modem
deep-sea fans (Wetze1 1993) is about 650 x 10 6 m 3 /a
or -1.2 x 10 9 tla. Assuming that these examples
cover about one half of all fans existing in the
present-day oceans, one finds that approximate1y 1520% of the global river load reaching the oceans
(Sect. 9.3.2) is transferred to deep-sea fans. This is a
surprisingly low proportion, but it supports the conclusions on sediment storage in delta areas and
prograding coastlines. In addition, a significant part
of the river-borne sediment must come to rest outside
of delta complexes and river-fed deep-sea fans, i.e.
on shelves, slopes, slope-aprons, and basin plains
beyond the range of deep-sea fans (cf. Sects. 11.5.5
and 11.5.7).
11.5.5 Mud Deposition on Continental Shelves
Significant proportions of the river-borne terrigenous
sand and mud are not deposited in the corresponding
delta and prodelta areas or directly transferred to
deep-sea fans as discussed above. Sand can be transported alongshore over long distances away from the
mouths of rivers and either be stored in the coastal
zone, or become trapped in the heads of distant submarine valleys funneling sediment into the deep sea.
Mud commonly accumulates at some distance seaward from the coastline on the middle and outer
shelf, depending on the hydrodynamic regime of the
area and river mud supply (cf. Sect. 3.3). Shelves in
front of mud-rich rivers, such as the Amazon River,
can become mud-dominated and store large proportions of the incoming fine-grained sediment as mentioned above. High-energy environments, for exampIe the Pacific coast of North America, allow mud
deposition only at water depths greater than about 50
m.
Such an example has been reported from the Ca!ifomian
coast elose to the delta of the Eel River (Sumrnerfield and