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01 Nov. 2002 Steve Goodbred, State Univ. of New York, Stony Brook, Email: sgoodbred@notes.cc.sunysb.edu"The Ganges-Brahmaputra sediment dispersal system: Controls, responses, and stratigraphy during the LateQuaternary".Seminar given at INSTAAR, University of Colorado. Copyright 2002 Steve Goodbred. All Rights Reserved.Goodbred presentation (3.6 Mb PDF).

Selected Presentation from the INSTAAR Monday Noon Seminar Series.

Institute of Arctic and Alpine Research, University of Colorado at Boulder.http://instaar.colorado.edu

http://instaar.colorado.edu/other/seminar_mon_presentations

01 Nov. 2002 Steve Goodbred, State Univ. of New York, Stony Brook, Email: sgoodbred@notes.cc.sunysb.edu"The Ganges-Brahmaputra sediment dispersal system: Controls, responses, and stratigraphy during the LateQuaternary".Seminar given at INSTAAR, University of Colorado. Copyright 2002 Steve Goodbred. All Rights Reserved.Goodbred presentation (3.6 Mb PDF).

Abstract

Rivers are the main source of terrigenous sediment delivered to continental margins and thus exert a major control oncoastal evolution and sequence development. However, little is known about past changes in fluvial sediment loadsdespite the recognition of significant variation under changing climatic regimes. In this study we present the first quantifiedestimate of sediment discharge for a major river system under conditions of an intensified early Holocene monsoon.Development of the Ganges-Brahmaputra River delta began ca. 11 000 yr B.P., when rising sea level flooded the Bengalbasin, thereby trapping most of the river’s discharge on the inner margin. Chronostratigraphic data from these deltaicdeposits are used to calculate the rates of sediment storage on the margin, which provide a minimum estimate of theriver’s past sediment load. Results reveal that ~5 Å~ 1012m3 of sediment was stored in the Bengal basin from ca. 11 000to 7000 yr B.P., which corresponds to a mean load of 2.3 Å~ 109 t/yr. In comparison, modern sediment load of theGanges-Brahmaputra is ~1 Å~ 109 t/yr, ranking it first among the world’s rivers and underscoring the significance of atwo-fold increase sustained over 4 k.y. Furthermore, the timing of immense discharge in the early Holocene stronglysuggests its relation to a stronger than present southwest monsoon in South Asia. Similar patterns of high monsoon-related sediment discharge have been noted throughout the tropics and subtropics, suggesting a widespreadfluviosedimentary response, the potential magnitude of which is showcased by the Ganges-Brahmaputra system.

Steven L. Goodbred, Jr.Marine Sciences Research Center,

University at Stony Brook

INSTAAR, University of Colorado

November 1, 2002

The Ganges-Brahmaputra River Delta:

Controls, Response, and Development

in the Late Quaternary

• Indus - 59 (250)

• Ganges - 520

• Brahmaputra - 540

• Irrawaddy - 260

• Mekong - 160

• Changjiang - 480

• Huanghe - (1100)

Major Rivers of the Himalayan Uplift

Annual Sediment

Discharge(106 t/yr)

G-B Delta

Physiography

100 km

~100,000 km2 of lowerfloodplain & delta plain

Ganges and Brahmaputra Rivers

Uplifted sedimentary blocks …2-10 m in Madhupur and Barind

Shillong Massif overthrustingSylhet basin ... downthrown

block subsiding at 2-4 mm/yr

multiple Brahmaputra avulsions

…eastward shift of Ganges

Active shortening at IndoBurmanfold belt ... Holocene neotectonics

Sundarbans … world’s largestmangrove forest

Swatch of No Ground Canyon

Elevation …not much

height above

sea level

dark green < 5 m

light green < 3 m

pale green < 1m

100 km

Sediment Dispersal Patterns

Fluvial InputFluvial Input

AtmosphericAtmospheric

DepositionDeposition

De

pth

(c

m)

Sedimentation and Radioisotope Geochronology

0

10

20

30

40

50

60

70

0 1 2

0

10

20

30

0 1 2 30 1 2

0

20

40

60

80

10

30

50

70

90

0

10

20

30

40

50

60

0 1 2 3 4 5

Activity (dpm/g)

210Pbxs

137Cs

DISTALFLOODPLAIN

PROXIMALFLOODPLAIN

BILS(bottomland)

CHANNELBRAIDBELT

75758686

77

1010

x 10x 1066 t/yr t/yr

Floodplain

Sediment Trapping

• ~30% of annual discharge

stored in delta

• old coastal plain and delta

lobes largely inactive

• most deposition occurring

in landward tectonic

basins and floodplains

5050(?)(?)

77(?)(?)4040(?)(?)

Ganges-Brahmaputra Shelf and Subaqueous Delta

(after Kuehl et al., 1997)

Three main offshore depocenters:

• deltafront/

foreset region

• inner shelf

clinoform

• canyon/off-shelf

advection

(Allison, 1998, JCR)(Allison, 1998, JCR)

Historical Progradation of the Shoreface

• growth and coalescing of channel-margin bars

• development of inner shelf sand sheet

• net of ~7 km2 new land formed each year`

Land0-2 m2-4 m4-6 m6-8 m8-15 m

Depth

5 km

Prograding Shelf Clinoform

15 m/yr

Compound delta:Compound delta:

•• strong coastal shear strong coastal shear •• large sediment load large sediment load

•• mixed grain-size mixed grain-size

modern examples: Ganges, Huanghe, Changjiang

Offshelf transport:mass wasting into canyon feeder gullies

(after Kuehl et al., 1997)(after Kuehl et al., 1997)

((MichelsMichels et al, in press) et al, in press)

Canyon Sedimentation Patterns

• comprised of turbidite sequence

• sed. rates >10 cm/yr ... up to 50 cm/yr

• events possibly triggered by cyclones

Ganges-Brahmaputra Sediment BudgetsGanges-Brahmaputra Sediment Budgets

FloodplainFloodplain / / MarineMarine CanyonCanyon / / TotalTotal

Delta PlainDelta Plain DeltaDelta Off-shelfOff-shelf DischargeDischarge

Topset Topset ForesetForeset t

Modern:Modern: 30030011 20020022 20020033 300 ?300 ? 10001000

Mid Holocene:Mid Holocene: 32032044 42042055 260 ?260 ? 1000 ?1000 ?

(7-3 ka)(7-3 ka)

data sources: 1 Goodbred and Kuehl, 1998; 2 Allison, 1998; 3 Michels et al., 1998;

4 Goodbred and Kuehl, 1999; 5 Kuehl et al., 1997; 6 Goodbred and Kuehl, 2000.

(megatons / yr)(megatons / yr)

Delta and sequence formation

Bengal Basin

Borehole Data

Goodbred and Kuehl

Umitsu study

Indian studies

misc. unpublisheddata

filled symbolsindicate 14C dating•

10 m

50 m

100 m

88 89 90 9 9

Thick, ‘trangressive-phase’ coastal-plain

sequence formed during early Holocene

Coastal Plain StratigraphyCoastal Plain Stratigraphy

– 9876 = calibrated 14C age

– 7703

– 9219

UmitsuUmitsu (1993) (1993)

– 3460

– 9906

– 7382

– 8238

14,004

– 8374

0

10

20

30

40

50

60

70

80

90

claysiltfine sandmedium sand

dep

th (

m)

– 2120

Coarse alluvial valley fill

Pleistocene

Laterite

Pleistocene

Laterite

Bengal Basin

Borehole Data

10 m

50 m

100 m

88 89 90 9 9

Goodbred and Kuehl

Umitsu study

Indian studies

misc. unpublisheddata

filled symbolsindicate 14C dating•

Sylhet Basin StratigraphySylhet Basin Stratigraphy

0

10

20

30

40

50

60

70

80

90

dep

th (

m)

– 7493

– 6289

– 6378

- 10,042

– 6506

– 7212

– 10,367

– 3810

claysiltfine sandmedium sandcoarse sand

BasinRim

BasinEdge

Basin InteriorBasinRim

– 9876 = calibrated radiocarbon date

Pleistocene

Laterite

Pleistocene

Laterite

G-B Delta Margin SequenceG-B Delta Margin Sequence

•• sedsed. accumulation and thickened sequence in . accumulation and thickened sequence in landwardlandward tectonic basins tectonic basins

•• very thick TST very thick TST and alluvial HSTand alluvial HST

•• Holocene sequence controlled by tectonics, Holocene sequence controlled by tectonics, sed sed supply, and supply, and eustacyeustacy

Late Quaternary Climate:

Responses in the Ganges-Brahmaputra

Late Quaternary Monsoon PrecipitationLate Quaternary Monsoon Precipitation

(COHMAP, 1988, Science)(COHMAP, 1988, Science)

18ka

9ka

present

18ka

9ka

present

Drier than present

Last Glacial Maximum

Wetter than present

Climatic Optimum

Arid regions

Present Conditions

Formation of ooids

at G-B rivermouth

during lowstand

Ganges-Brahmaputra Shelf at the LGM (18 Ganges-Brahmaputra Shelf at the LGM (18 kaka) )

Suggests very low

river discharge

and sediment flux

at LGM

130 m130 m

140 m140 m

Oolitic Oolitic ridgesridges

(Weidicke et al., 1999, Marine Geology)

(Weber et al., 1997, Geology)(Weber et al., 1997, Geology)

Seismic cross-sectionof fan channel

fan sedimentation and sea level

14C yr BP (x 103)

0 4 8 12 160

- 40

- 80

Se

a L

ev

el

(m)

- 120

Rapid sedimentationon delta plain

0

0.2

Sed

. R

ate

(cm

/yr)

Activation of BengalActivation of Bengal

Fan with onset ofFan with onset of

SW monsoonSW monsoon

SedimentSediment Isopach Isopach

of Late Quaternaryof Late Quaternary

Deltaic DepositsDeltaic Deposits

HIGHSTAND VOLUME:

3.5 x 1012 m3

TRANSGRESSIVE VOLUME:

5.0 x 1012 m3

–––––––––––––––––––––––––––––

SEDIMENT LOADS:SEDIMENT LOADS:

7000-11,000 BP7000-11,000 BP

2.3 x 102.3 x 109 9 t/y t/y

post-7000 BPpost-7000 BP

1.0 x 101.0 x 109 9 t/y t/y

0 m

20

40

60

80

92°91°90°89°88°E

21°

22°

23°

24°

25°N

TripuraFold Belt

Barind Tract

MadhupurTerrace

0 km 100

Bay of Bengal

Swatch ofNo Ground

SylhetBasin

Ganges-Brahmaputra Sediment BudgetsGanges-Brahmaputra Sediment Budgets

FloodplainFloodplain / / MarineMarine CanyonCanyon / / TotalTotal

Delta PlainDelta Plain DeltaDelta Off-shelfOff-shelf DischargeDischarge

Topset Topset ForesetForeset .

Modern: 3001 2002 2003 300 ? 1000

Mid Holocene: 3204 4205 260 ? 1000 ?

(7-3 ka)

Early Holocene:Early Holocene: 2300230066 300 ?300 ? 26002600(11-7 ka)(11-7 ka)

Last Glacial Max:Last Glacial Max: 50 ?50 ? 100 ?100 ? 150150 ? ?

(18-11 ka)(18-11 ka)

data sources: 1 Goodbred and Kuehl, 1998; 2 Allison, 1998; 3 Michels et al., 1998; 4

Goodbred and Kuehl, 1999; 5 Kuehl et al., 1997; 6 Goodbred and Kuehl, 2000.

Sources of Immense Sediment Load?Sources of Immense Sediment Load?

“From ~ 9 kyr until ~ 5 kyr, the two [Ganges tributaries] incised 20-30 m

into their former floodplains … towards 4.5 kyr until ~ 3 kyr, the Son

and Belan [rivers] aggraded once more, but only to ~ +12 m …”

““From ~ 9 From ~ 9 kyr kyr until ~ 5until ~ 5 kyr kyr, the two [Ganges tributaries] incised 20-30 m, the two [Ganges tributaries] incised 20-30 m

into their former floodplains into their former floodplains …… towards 4.5 towards 4.5 kyr kyr until ~ 3 until ~ 3 kyrkyr, the Son, the Son

and and Belan Belan [rivers] [rivers] aggraded aggraded once more, but only to ~ +12 m once more, but only to ~ +12 m …”…”

(Williams and Clarke, 1984, Nature)

Bedrock exposure ages (Bedrock exposure ages (1010Be and Be and 2626Al) forAl) for

MarysandiMarysandi River valley, Nepal River valley, Nepal

(Pratt et al, in press, Geology)(Pratt et al, in press, Geology)

Exposure ages group at 6-8 ka and suggest

rapid exhumation of valley sediment fill (~80m)

Immense G-B sediment load synchronousImmense G-B sediment load synchronouswith strong monsoon and rapid sea level risewith strong monsoon and rapid sea level rise

(A. and B. (A. and B. –– Prins Prins and and PostmaPostma, 2000 , 2000 C. C. –– Goodbred and Kuehl, 2000) Goodbred and Kuehl, 2000)

Tectonic Influences:Ganges-Brahmaputra example

Pleistocenesurface

-10

0

10

20

30

40

50

60

70

2468101214

sidereal years x 103

0

Goodbred, Kuehl, 2000

Umitsu, 1987; 1993

Banerjee and Sen, 1986

eustatic sea level

dep

th b

elo

w m

sl

(m)

sea level + 2-4 mm/yrof subsidence

Radiocarbon Ages Radiocarbon Ages vsvs. Sea Level. Sea Level

3-D Stratigraphy

of Bengal Basin

Dominant Control:

• fluvial processes

• tectonics

• eustacy

Brahmaputra channel morphology (1973-1996)

effect of 1950effect of 1950 Assam Assam earthquake? earthquake?

Arsenic … a mass human

poisoning in Bengal

Lex van Geen, Yan Zheng, et al., LDEO

Araihazar township … a case study of 6000 wells

Contrasting Seasons of the Monsoon

A Few Slides

TOP

BOTTOM

1 m

ete

rDelta Plain Vibracore X-radiographs

(tidalites)

10 c

m

1 c

m

Cross-bedding Rip-up Clast

Tidalites and BurrowSediment Thin-Sections

the Ganges Brahmaputra confluence

Indo-Himalayan Geologic Subdivisions

(after Yokoyama et al., 1990)

Ganges Brahmaputra0

1

2

3

Epid

ote

:Garn

et

BrahmaputraGanges

illite

smectite

kaolinite

other (chlorite, mixed layer I/S)

0

50

100

Rela

tive a

bundance (

%)

River Sediment Mineralogy

(Heroy, Kuehl, and Goodbred, 2002)

Coarse-grained minerals Fine-grained minerals

(after Derry and France-Lanord, 1997)