11.5 Clastic Sediment, Distribution
Nittrouer 1999). High wave- and current action do not allow the outbuilding of a delta lobe, and a large proportion
of the modem terrigenous influx is collected by the Eel
Canyon. However, episodic floods carry mud to the adjacent shelf where it accumulates seaward of the 50 m
isobath at mean rates ofO.5 glcm 2 /a (~ 4 mmla; according
to Pb-21O and Cs-137 dating). Toward the shelfbreak, the
accumulation rates decrease. About 20% ofthe total fluvial
mud supply are deposited on the shelf.
11.5.6 Sediment Distribution in a Basin Chain
Several lake basins (Sect. 2.5) and basins of larger
dimensions may be connected and form a basin chain
displaying distinct trends in their sedimentary fills. A
well studied example in Central Europe is the young
(Neogene) basin chain including the Vienna basin,
the Transcarpathian basins, the Pannonian basin, and
the Black Sea (cf. Sect. 12.7.3). These basins received terrigenous material mainly from the north
(e.g. from the precursor of the Danube River) and
were successively filled up with sediment. At present, only the Black Sea is left as a water-filled basin
at the end of the chain.
Himalayas-Bengal fan. The largest young example
of a basin chain ranges from the Sub-Himalayan
foredeep (foreland basin) via the Bengal basin of
Bangladesh (remnant basin, foredeep) to the huge
Bengal fan in the Indian ocean. The successive filling of these basins during the past 20 Ma can be approximately reconstructed by applying the DA-approach introduced in Section 11.2 (Fig. 11.31;
Einseie et al. 1996). A sediment mass balance for the
past 20 Ma, supported by the independently determined mass of erosion in the Himalayas (Fig.
11.31 b), reveals that about 17% of the total mass
(about 19.5 x 10 15 t) remained in the Sub-Himalayan
foredeep, 28% in the Bengal basin (including its
prograding delta area), and 55% accumulated on the
Bengal deep-sea fan (Fig. 11.31 c-e).
In pre-Miocene times, the Sub-Himalayan foredeep represented a marine foreland basin which received limited
amounts of river-borne sediment from northern, distal parts
of the evolving mountain complex. In the early Miocene,
thick overthrust sheets (the present-day High Himalayan
chain) moved southward on top ofthe Main Central Thrust
and gained in altitude (Fig. 11.31 b). As a result of increased sediment supply, the foredeep was transformed into
a fluvial basin (Siwalik Group) and has remained so up to
the present. The precursors of the Ganges and Brahmaputra
rivers then transported their surplus sediment to the Bengal
basin which was successively filled by prograding delta
and prodelta deposits. Westward overthrusting of the
Indoburman Ranges narrowed the Bengal basin and promoted delta prograding toward the Bay of Bengal. Sediment (mostly mud) not deposited in the delta area and on
the prodelta slope was transferred to the Bengal deep-sea
fan by gravity flows and turbidity currents over a distance
ofup to almost 3000 km.
529
With the aid of the sediment masses and grain size fractions present in the individual basins, the average grain size
distribution of the Himalayan river loads can be roughly
determined (Fig. 11.3lf; Einseie 1996). This estimate
shows that the Himalayan rivers carried about 30% sand
and 70% silt and clay. Approximately 20% ofthe total sand
supply and 80% of the mud supply reached the Bengal fan.
11.5.7 Long-Term Sediment Distribution Along
Passive Continental Margins
Terrigenous sediment escaping from deposition on
delta fronts, prodelta slopes, and deep-sea fans is
often widely distributed on adjacent shelves, continental slopes, and in the deep sea. The processes involved in this sediment dispersal are briefly described in Sections 5.2 and 5.3.1. Their importance is
here underlined by two examples from areas where at
least a semi-quantitative approach is possible due to
extensive seismic studies and deep-sea drilling.
Gulf of Mexico: Coastal Progradation and
Marine Sediment Accumulation
The frequently described drainage system of the Mississippi River delivering its sediment load into the
Gulf of Mexico (Fig. 11.32a-c) is a classic example
of the interplay between processes in the source area
and deposition along a passive continental margin
(summarized, e.g., by Matthews 1974; mass balance
see Ray et al. 1989).
In the Jurassic and Lower Cretaceous, terrigenous sediment
supply to the basin was limited. This allowed the growth
and prograding of an extensive carbonate platform. With
the onset ofuplift ofthe Rocky Mountains and the Appalachians in the upper Cretaceous, terrigenous sediment input
into the gulf increased significantly. The river-derived material was distributed over a wide shelf and slope region
and formed a huge clastic wedge. This wedge migrated
about 200 km seaward since the upper Cretaceous (average
rate ~0.25 crnla). Uniform outbuilding was, however, modified by sea level fluctuations and regional differential subsidence in response to the increasing sediment load and
diapirism of Jurassic salt. Cenozoic clastic sediments reach
a thickness of up to 15 km. Mass bai an ces indicate that
extensive areas of the Rocky Mountains and High Plains
have been uplifted as much as I to 3 km since the late Pliocene. On the western Florida shelf, which is an area outside
of the influence of the Mississippi river, carbonate sedimentation continues to prevail up to the Present.
Using the present-day chemical and mechanical denudation rates of the Mississippi River for the other
drainage areas of the Gulf of Mexico, one can estimate the average sedimentation rate in the gulf. The
gulf covers an area of about 1.5 x 10 6 km 2 , the drainage area of the rivers approximately 4.5 x 10 6 km 2 ;
hence the A d / Ab ratio is about 3. The drainage of the
Nittrouer 1999). High wave- and current action do not allow the outbuilding of a delta lobe, and a large proportion
of the modem terrigenous influx is collected by the Eel
Canyon. However, episodic floods carry mud to the adjacent shelf where it accumulates seaward of the 50 m
isobath at mean rates ofO.5 glcm 2 /a (~ 4 mmla; according
to Pb-21O and Cs-137 dating). Toward the shelfbreak, the
accumulation rates decrease. About 20% ofthe total fluvial
mud supply are deposited on the shelf.
11.5.6 Sediment Distribution in a Basin Chain
Several lake basins (Sect. 2.5) and basins of larger
dimensions may be connected and form a basin chain
displaying distinct trends in their sedimentary fills. A
well studied example in Central Europe is the young
(Neogene) basin chain including the Vienna basin,
the Transcarpathian basins, the Pannonian basin, and
the Black Sea (cf. Sect. 12.7.3). These basins received terrigenous material mainly from the north
(e.g. from the precursor of the Danube River) and
were successively filled up with sediment. At present, only the Black Sea is left as a water-filled basin
at the end of the chain.
Himalayas-Bengal fan. The largest young example
of a basin chain ranges from the Sub-Himalayan
foredeep (foreland basin) via the Bengal basin of
Bangladesh (remnant basin, foredeep) to the huge
Bengal fan in the Indian ocean. The successive filling of these basins during the past 20 Ma can be approximately reconstructed by applying the DA-approach introduced in Section 11.2 (Fig. 11.31;
Einseie et al. 1996). A sediment mass balance for the
past 20 Ma, supported by the independently determined mass of erosion in the Himalayas (Fig.
11.31 b), reveals that about 17% of the total mass
(about 19.5 x 10 15 t) remained in the Sub-Himalayan
foredeep, 28% in the Bengal basin (including its
prograding delta area), and 55% accumulated on the
Bengal deep-sea fan (Fig. 11.31 c-e).
In pre-Miocene times, the Sub-Himalayan foredeep represented a marine foreland basin which received limited
amounts of river-borne sediment from northern, distal parts
of the evolving mountain complex. In the early Miocene,
thick overthrust sheets (the present-day High Himalayan
chain) moved southward on top ofthe Main Central Thrust
and gained in altitude (Fig. 11.31 b). As a result of increased sediment supply, the foredeep was transformed into
a fluvial basin (Siwalik Group) and has remained so up to
the present. The precursors of the Ganges and Brahmaputra
rivers then transported their surplus sediment to the Bengal
basin which was successively filled by prograding delta
and prodelta deposits. Westward overthrusting of the
Indoburman Ranges narrowed the Bengal basin and promoted delta prograding toward the Bay of Bengal. Sediment (mostly mud) not deposited in the delta area and on
the prodelta slope was transferred to the Bengal deep-sea
fan by gravity flows and turbidity currents over a distance
ofup to almost 3000 km.
529
With the aid of the sediment masses and grain size fractions present in the individual basins, the average grain size
distribution of the Himalayan river loads can be roughly
determined (Fig. 11.3lf; Einseie 1996). This estimate
shows that the Himalayan rivers carried about 30% sand
and 70% silt and clay. Approximately 20% ofthe total sand
supply and 80% of the mud supply reached the Bengal fan.
11.5.7 Long-Term Sediment Distribution Along
Passive Continental Margins
Terrigenous sediment escaping from deposition on
delta fronts, prodelta slopes, and deep-sea fans is
often widely distributed on adjacent shelves, continental slopes, and in the deep sea. The processes involved in this sediment dispersal are briefly described in Sections 5.2 and 5.3.1. Their importance is
here underlined by two examples from areas where at
least a semi-quantitative approach is possible due to
extensive seismic studies and deep-sea drilling.
Gulf of Mexico: Coastal Progradation and
Marine Sediment Accumulation
The frequently described drainage system of the Mississippi River delivering its sediment load into the
Gulf of Mexico (Fig. 11.32a-c) is a classic example
of the interplay between processes in the source area
and deposition along a passive continental margin
(summarized, e.g., by Matthews 1974; mass balance
see Ray et al. 1989).
In the Jurassic and Lower Cretaceous, terrigenous sediment
supply to the basin was limited. This allowed the growth
and prograding of an extensive carbonate platform. With
the onset ofuplift ofthe Rocky Mountains and the Appalachians in the upper Cretaceous, terrigenous sediment input
into the gulf increased significantly. The river-derived material was distributed over a wide shelf and slope region
and formed a huge clastic wedge. This wedge migrated
about 200 km seaward since the upper Cretaceous (average
rate ~0.25 crnla). Uniform outbuilding was, however, modified by sea level fluctuations and regional differential subsidence in response to the increasing sediment load and
diapirism of Jurassic salt. Cenozoic clastic sediments reach
a thickness of up to 15 km. Mass bai an ces indicate that
extensive areas of the Rocky Mountains and High Plains
have been uplifted as much as I to 3 km since the late Pliocene. On the western Florida shelf, which is an area outside
of the influence of the Mississippi river, carbonate sedimentation continues to prevail up to the Present.
Using the present-day chemical and mechanical denudation rates of the Mississippi River for the other
drainage areas of the Gulf of Mexico, one can estimate the average sedimentation rate in the gulf. The
gulf covers an area of about 1.5 x 10 6 km 2 , the drainage area of the rivers approximately 4.5 x 10 6 km 2 ;
hence the A d / Ab ratio is about 3. The drainage of the