11.5 Clastic Sediment, Distribution
Mississippi River alone is 3.27 x 10 6 km 2 • With a
mechanical denudation rate of 48 mlMa (cf. Fig.
9.16), the average siliciclastic sedimentation rate
(solid, pore-free sediment) in the gulfshould be -145
mlMa. This rate does not represent the actual situation in the basin because the sediment is not distributed evenly. The average sedimentation rate of
biogenic calcium carbonate (15 mlMa) calculated
from the present-day river load is about equal to rates
found for pelagic carbonates in other ocean basins
above the CCD (Sect. 10.2).
In fact, the basin was fi1led irregularly by several point
sources of variable efficiency. Most of the incoming sediment accumulated on the prograding continental slope. In
its late history (some Ma B.P.), about 35% ofthe sediment
load of the Mississippi River was stored in the Mississippi
deep-sea fan (Sect. 11.5.4). Here, the sedimentation rate
can reach values up to 1000 mJMa (cf. Sect. 5.4.2).
The chemical denudation rate in the drainage area is 15
mlMa, but only part of the dissolved river load is deposited
in the basin. About 15% of this load is Ca 2+ (cf. Fig. 9.8,
Mississippi and Rio Grande rivers), corresponding to a Ca
denudation rate of around 2 mJMa. After conversion into
CaC0 3 (factor 2.5) and using AjAb=3, one obtains a
(biogenie ) calcium carbonate sedimentation rate of approximately 15 mJMa. Calcium carbonate deposition in the
gulf, however, is diluted too much by silic1astic material to
generate carbonate-rich sediments, apart from some shelf
regions (western Florida, Yucatan). Finally, it should be
mentioned that part of the dissolved river load is exchanged with sea water flowing into and out ofthe gulf.
Northwestern Atlantic:
Shelf-Slope-Deep Sea Deposition
The Atlantic is a mature ocean basin which has
grown mainly since the middle Jurassic (cf. Sect.
12.2). Huge amounts of sediments have accumulated
along its margins including the continental slope and
rise. Architecture and stratigraphy of these sediments
are relatively well known due to seismic work and
deep-sea drilling. The US Atlantic margin is a thoroughly studied part of this margin and therefore used
as an example of wide and uneven sediment distribution. Isopach maps and sediment mass balances for
both the source and depositional area have led to results which also characterize other sections of mature
continental margins (Fig. Il.32d and e).
Oll!(
Fig. 11.31. Mass balance of denudation and sediment
accumulation in the Himalaya-Bengal fan system for
the past 20 Ma. a Catchment areas of rivers draining
the Himalayas and part of peninsular India; extent of
Bengal deep-sea fan. b Mean denudation rates in the
Himalayas during the past 20 Ma. c Isopach map of
molasse sediments in the Himalayan foredeep and
location of Bengal basin (foredeep), present-day
delta and prodelta. d Longitudinal section of Bengal
fan. e Mass balances of denudation (solids and
531
The depositional area comprises the Baltimore canyon
trough and Hatteras basin, and the drainage area a major
part of the Appalachians (Grow et al. 1988; Poag and
Sevon 1989; Pazzaglia and Brandon 1996; cf. Sect. 9.5 and
Table 9.1).
The majority of terrigenous sediment, entering the
basin via point sources, bypassed the coastal plain
and inner shelf and accumulated on the outer shelf,
continental slope and rise (cf. model basin Fig.
11.23c). The sediment thicknesses are largely controlled by the rates of (differential) subsidence and
the tendency of the basin to maintain a certain shelfstope morphology. As a result of slowing subsidence
of the shelf (on transitional crnst), the depocenter
migrated from the outer shelf (Jurassic) to the upper
rise (Cretaceous) and ultimately to the deeper rise
(Neogene). The overall mean rate ofterrigenous sedimentation was in the order of 60 mlMa for the past
175 Ma, but the rates varied enormously from location to location as well as with time.
11.5.8 Global Marine Sediment Distribution
The results of regional studies are also supported by
estimates about the global distribution of post-rift
sediments (deposited after the breakup of Pangea,
-120 Ma ago). Ofthe total sediment mass existing in
the present-day oceans, about 70% accumulated on
the continental slopes and continental rises, 17% on
the shelves, and the remainder in the deep sea (Emery and Uchupi 1984). Other workers have stressed
the great differences in the accumulation rates of
terrigenous sediments in various parts of the oceans
(e.g. Lisitzin 1991). In the large ocean basins some
distance off the coasts, these rates are very low (1-10
mlMa). In limited areas close to the coasts, the rates
reach values of 100-1000 mlMa. Extremely high
rates of ;:: 1000 mlMa occur at the upper portions of
large deep-sea fans where sediment thicknesses of
10-15 km have been recorded.
dissolved matter) in the Himalayas and part of
peninsular India as well as sediment accumulation in
the basin chain from the Himalayan foredeep to the
Bengal fan. f Grain size partitioning of Himalayan
riverine sediment supply along basin chain: I
proportions of sand (S, sand and gravel) and fines (F,
silt and clay) present in individual basins; 11
percentage of total river load (S= 1 00% and F= 100%)
left behind along basin chain. (After Einseie et al.
1996, simplified)
Mississippi River alone is 3.27 x 10 6 km 2 • With a
mechanical denudation rate of 48 mlMa (cf. Fig.
9.16), the average siliciclastic sedimentation rate
(solid, pore-free sediment) in the gulfshould be -145
mlMa. This rate does not represent the actual situation in the basin because the sediment is not distributed evenly. The average sedimentation rate of
biogenic calcium carbonate (15 mlMa) calculated
from the present-day river load is about equal to rates
found for pelagic carbonates in other ocean basins
above the CCD (Sect. 10.2).
In fact, the basin was fi1led irregularly by several point
sources of variable efficiency. Most of the incoming sediment accumulated on the prograding continental slope. In
its late history (some Ma B.P.), about 35% ofthe sediment
load of the Mississippi River was stored in the Mississippi
deep-sea fan (Sect. 11.5.4). Here, the sedimentation rate
can reach values up to 1000 mJMa (cf. Sect. 5.4.2).
The chemical denudation rate in the drainage area is 15
mlMa, but only part of the dissolved river load is deposited
in the basin. About 15% of this load is Ca 2+ (cf. Fig. 9.8,
Mississippi and Rio Grande rivers), corresponding to a Ca
denudation rate of around 2 mJMa. After conversion into
CaC0 3 (factor 2.5) and using AjAb=3, one obtains a
(biogenie ) calcium carbonate sedimentation rate of approximately 15 mJMa. Calcium carbonate deposition in the
gulf, however, is diluted too much by silic1astic material to
generate carbonate-rich sediments, apart from some shelf
regions (western Florida, Yucatan). Finally, it should be
mentioned that part of the dissolved river load is exchanged with sea water flowing into and out ofthe gulf.
Northwestern Atlantic:
Shelf-Slope-Deep Sea Deposition
The Atlantic is a mature ocean basin which has
grown mainly since the middle Jurassic (cf. Sect.
12.2). Huge amounts of sediments have accumulated
along its margins including the continental slope and
rise. Architecture and stratigraphy of these sediments
are relatively well known due to seismic work and
deep-sea drilling. The US Atlantic margin is a thoroughly studied part of this margin and therefore used
as an example of wide and uneven sediment distribution. Isopach maps and sediment mass balances for
both the source and depositional area have led to results which also characterize other sections of mature
continental margins (Fig. Il.32d and e).
Oll!(
Fig. 11.31. Mass balance of denudation and sediment
accumulation in the Himalaya-Bengal fan system for
the past 20 Ma. a Catchment areas of rivers draining
the Himalayas and part of peninsular India; extent of
Bengal deep-sea fan. b Mean denudation rates in the
Himalayas during the past 20 Ma. c Isopach map of
molasse sediments in the Himalayan foredeep and
location of Bengal basin (foredeep), present-day
delta and prodelta. d Longitudinal section of Bengal
fan. e Mass balances of denudation (solids and
531
The depositional area comprises the Baltimore canyon
trough and Hatteras basin, and the drainage area a major
part of the Appalachians (Grow et al. 1988; Poag and
Sevon 1989; Pazzaglia and Brandon 1996; cf. Sect. 9.5 and
Table 9.1).
The majority of terrigenous sediment, entering the
basin via point sources, bypassed the coastal plain
and inner shelf and accumulated on the outer shelf,
continental slope and rise (cf. model basin Fig.
11.23c). The sediment thicknesses are largely controlled by the rates of (differential) subsidence and
the tendency of the basin to maintain a certain shelfstope morphology. As a result of slowing subsidence
of the shelf (on transitional crnst), the depocenter
migrated from the outer shelf (Jurassic) to the upper
rise (Cretaceous) and ultimately to the deeper rise
(Neogene). The overall mean rate ofterrigenous sedimentation was in the order of 60 mlMa for the past
175 Ma, but the rates varied enormously from location to location as well as with time.
11.5.8 Global Marine Sediment Distribution
The results of regional studies are also supported by
estimates about the global distribution of post-rift
sediments (deposited after the breakup of Pangea,
-120 Ma ago). Ofthe total sediment mass existing in
the present-day oceans, about 70% accumulated on
the continental slopes and continental rises, 17% on
the shelves, and the remainder in the deep sea (Emery and Uchupi 1984). Other workers have stressed
the great differences in the accumulation rates of
terrigenous sediments in various parts of the oceans
(e.g. Lisitzin 1991). In the large ocean basins some
distance off the coasts, these rates are very low (1-10
mlMa). In limited areas close to the coasts, the rates
reach values of 100-1000 mlMa. Extremely high
rates of ;:: 1000 mlMa occur at the upper portions of
large deep-sea fans where sediment thicknesses of
10-15 km have been recorded.
dissolved matter) in the Himalayas and part of
peninsular India as well as sediment accumulation in
the basin chain from the Himalayan foredeep to the
Bengal fan. f Grain size partitioning of Himalayan
riverine sediment supply along basin chain: I
proportions of sand (S, sand and gravel) and fines (F,
silt and clay) present in individual basins; 11
percentage of total river load (S= 1 00% and F= 100%)
left behind along basin chain. (After Einseie et al.
1996, simplified)