Sedimentation in the Amazon River Delta and Nearby Continental Shelf

1
Sedimentation in the Amazon River Delta and
Nearby Continental Shelf

• Karen Koy
• Geology 455

2
Introduction

• The Amazon River contributes 20 of the global
  runoff into oceans (Degens et al., 1991)
  
• Its annual discharge is 6,300 km3/yr (Milliman
  and Meade, 1983)
  
• Makes an important contribution to the
  sedimentation and geomorphology of the
  northeastern coast of South America
  
• Summary of the current literature
• Make some predictions about the future of this
  region

3
Introduction
The Amazon River flows from the Andean mountain
range east across South America as an
anastomozing river, with numerous fluvial islands
and channel bars finally ending at the Atlantic
Ocean (Hoorn, 1994).
From Nittrouer et al., 1995
4
Amazon River Mouth

• The river mouth is fixed in place by a graben
  (Dunne et al., 1998)
  
• Characterized by estuarine-like processes (Warne
  et al., 2002 Gibbs, 1969 Gibbs and Konwar,
  1986)
  
• Tidal influence can be seen 100 km inland
  (Hoorn, 1994)

5
Amazon River Mouth

• The deepest portions reach 40m deep, yet there
  is no saltwater intrusion (Gibbs, 1969 Gibbs and
  Konwar, 1986)
  
• The freshwater lens extends 120 km offshore
  (Gibbs, 1969)

From Gibbs Konwar, 1986
6
Sediment Discharge

• Sediment discharge is 1.2x109 m3 annually
  (Nittrouer et al., 1995)
  
• 1-2 is bedload (Warne et al., 2002)
• 85-95 is suspended silt and clay (Warne et al.,
  2002 Gibbs and Konwar, 1986)
  
• Mean grain size of 4µm (Gibbs and Konwar, 1986)

7
Sediment Discharge

• Eighty-four percent of dissolved salts and solids
  sampled were nearly identical to the Andean
  sediments (Gibbs, 1967)
  
• Suspended sediments come predominantly from the
  Andes Mountains, which cover only 12 of the
  total drainage basin area (McDaniel et al., 1996
  Gibbs, 1967)
• The relief and physical weathering of the Andes
  Mountains appears to be a major control on the
  geochemistry of the Amazon River (Gibbs, 1967)

8
Sediment Discharge

• Even though it covers a majority of the drainage
  basin area, the lower tropical rainforest
  contributes very little to the suspended load of
  the Amazon River (McDaniel et al., 1996 Gibbs,
  1967)
• It is likely the high level of vegetation in the
  rainforests retards surface erosion
  
• The Amazon River channels and tributaries meander
  and migrate, producing elaborate scrollwork
  geomorphology (Mertes et al., 1996), reworking
  the fluviolacustrine sediments
• The reworked sediments may comprise the majority
  of the bedload
  
• The composition of the Amazon River bedload has
  yet to be investigated

9
Amazon River Delta

• A subaqueous deposit located seaward of the river
  mouth on the continental shelf (Nittrouer et al.,
  1995 Vital and Stattegger, 2000 McDaniel et
  al., 1996 Gibbs and Konwar, 1986)
• 50 km wide at the proximal end, 300 km wide
  distally, and 650 km long, running northwest and
  parallel to the coast (de Alencar Costa and
  Figueiredo, 1998)
• The delta plain contains estuarine islands (Warne
  et al., 2002)
  
• During a previous lowstand, sediments were
  deposited in the Amazon Fan located in the deep
  ocean (McDaniel et al., 1996)

10
Amazon River Delta

• Underlying the delta is a transgressive sand
  layer, which is the top of a series of mud and
  sand layers 25-30 m thick (Figueiredo and
  Nittrouer, 1995 de Alencar Costa and Figueiredo,
  1998)

From Figueireda and Nittrouer, 1995
11
Amazon River Delta

• The topmost sandy layer increases in thickness
  towards the river mouth (Alexander et al., 1986)
  
• The mud layers are interpreted as regressive,
  prodelta sediments
  
• The sand layers are interpreted as transgressive
  strata from delta progradation (Figueiredo and
  Nittrouer, 1995)

12
Amazon River Delta

• Modern sediment accumulation occurs in an
  offshore depocenter 30 m deep, which includes
  the outer topset and foreset strata (Allison et
  al., 2000 Nittrouer et al., 1995 Alexander et
  al., 1986)
• The sediment accumulation rate increases seaward
  (Kuehl et al., 1986)
  
• Sediment accumulation rates in the foreset
  region
  
• 5.0-7.3 cm/yr (Dukat and Kuehl, 1995)
• 8-10 cm/yr (Alexander et al., 1986)
• 10.3 cm/yr (Kuehl et al., 1986)

13
Amazon River Delta

• Suspended sediment from the river plume
  flocculates and settles out along the delta and
  northwest along the continental shelf with
  decreasing grain size distally (Gibbs and Konwar,
  1986 de Alencar Costa and Figueiredo, 1998).

From Gibbs Konwar, 1986
14
The Continental Shelf

• Northwest of the Amazon River is the longest
  continuous accretionary mud coastline in the
  world (Allison et al., 2000 Wells and Coleman,
  1981 Rine and Ginsburg, 1985)

15
The Continental Shelf

• The Amazon sediment plume travels northwest along
  the coast in a 20-40 km wide zone (Froidefond et
  al., 1988)
  
• Travel at a rate of 40-80 cm/s (Nittrouer et al.,
  1995)
  
• These sediments can still be found up to 1600 km
  updrift (Froidefond et al., 1988 Gibbs, 1977)
  
• The sediments from the Amazon River travel in two
  basic forms
  
• as suspended sediment
• as mudbanks

16
The Continental Shelf

• 100 million m3/yr suspended sediments travel up
  the coast (Froidefond et al., 1988 Wells and
  Coleman, 1981
  
• The North Brazilian Current is the primary force
  moving the sediment plume (Nittrouer et al.,
  1995 Wells and Coleman, 1981 Gibbs, 1969)
  
• Easterly trade winds combine with the North
  Brazilian Current to reach a maximum migration
  rate during the months of August and September
  (Nittrouer et al., 1995 Wells and Coleman, 1981
  Rine and Ginsburg, 1985 Gibbs, 1969)

17
The Continental Shelf

• 150 million m3/yr sediment travel alongshore in
  the form of mudbanks (Froidefond et al., 1988
  Allison et al., 2000 Wells and Coleman, 1981).
  
• Mudbanks are volumetrically stable, can travel
  over 900 m/yr (Froidefond et al., 1988 Wells and
  Coleman, 1981)
  
• Remain subaqueous (Vital and Stattegger, 2000)
• Linear, shore-attached migrating shoals are
  similar to features found on sandy coastlines
  
• Distributed every 30-60 km northwest along the
  coast, have shore and seaward (Wells and Coleman, 1981)
  
• They act as barriers to wave energy at low tide,
  trapping and storing finer-grained sediments
  (Wells and Coleman, 1981)

18
The Continental Shelf

• The coastline adjacent to the Amazon River mouth
  is eroding and contributing sediment to the
  updrift mudbanks (Allison et al., 2000)
  
• A shoreline retreat rate of 0.5-1.0 m/yr, due to
  tidal and wave induced currents (Allison et al.,
  1995)

19
The Continental Shelf

• The shelf sedimentation rate reaches its maximum
  in the upper tidal zone, and increases seaward
  across the shelf (Kuehl et al., 1986 Allison et
  al., 1995)
• There is an overall upward/seaward progradation
  of the coastal plain (Nittrouer et al., 1995)
  
• Contrary to previous belief, the fine-grained
  sediments along this muddy coastline are
  deposited by high-energy waves and currents (Rine
  and Ginsburg, 1985).

20
Conclusions

• The majority of sediment carried by the Amazon
  River is suspended sediment derived from the
  Andes Mountains to the west
  
• It is deposited in a subaqueous delta, and
  northwest along the continental shelf, fining
  with increasing distance from the river mouth
  
• The topset and foreset regions of the delta have
  the highest sedimentation rates, causing the
  delta to prograde outward and upward
  
• The coastline is being eroded near the river
  mouth, and muddy sediments are deposited updrift
  along the continental shelf as migrating mud
  shoals

21
Conclusions Further Investigation

• The composition and origins of the bedload
• Why so little sediment is derived from the
  non-Andean source areas
  
• The mechanism that would cause or maintain
  estuarine conditions in the Amazon River mouth

22
Conclusions Predictions

• If the current data are accurate, and the
  sedimentation and erosion patterns along the
  coastal shelf do not change
  
• The sediments from the Amazon River, and those
  being eroded from the nearby shelf, will continue
  moving northwest along the coast
  
• The muddy coastline will continue to migrate even
  further north, possibly extending past the
  Orinoco River mouth
  
• The Amazon River delta will most likely remain
  subaqueous

23
Conclusions

• If sea-level were to fall
• The delta and mudbanks would become subaerial
• If sea-level achieves a lowstand
• Sediments from the Amazon River may once again be
  deposited in the deep marine Amazon Fan