ganges brahmaputra delta sequence

8/18/2019 Ganges Brahmaputra Delta Sequence

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Controls on facies distribution and stratigraphic preservation

in the Ganges–Brahmaputra delta sequence

Steven L. Goodbred Jr. a,*, Steven A. Kuehl b, Michael S. Steckler c,Maminul H. Sarker d

a Marine Sciences Research Center, State University of New York, Stony Brook, NY 11794, USA b

Virginia Institute of Marine Science, College of William and Mary, Gloucester Pt., VA 23062, USAc Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

d Environmental and GIS Support Project (EGIS), Dhaka 1213, Bangladesh

Received 21 June 2000; received in revised form 16 February 2001; accepted 13 March 2001

Abstract

Abundant sediment supply and accommodation space in the Bengal Basin have led to the development of a major Late

Quaternary delta sequence. This sequence has formed in a tectonically active setting and represents an important example of a

high-energy (marine and fluvial), high-yield continental margin deposit. Recent studies have detailed the delta’s stratigraphy and

development, noting that tectonics and sediment supply control the Ganges – Brahmaputra more significantly than in many other

delta systems. These ideas are developed here through a discussion of the effects that spatial and temporal variations in tectonicsand sediment-supply have had on deltaic processes and sequence character. Unique and differing stratigraphies are found within

the delta system, such that fine-grained sediment preservation is favored in areas of active tectonic processes such as folding, block

faulting, and subsidence. Coarse-grained deposits dominate the stratigraphy under the control of high-energy fluvial processes,

and mixed fine – coarse stratigraphies are found in areas dominantly influenced by eustatic sea-level change. Overlaid upon these

spatially varying stratigraphic patterns are temporal patterns related to episodic events (e.g., earthquakes and rivers avulsions) and

long-term changes in climate andsediment supply. Modeling is also used to investigate the influence of a variable sediment supply

on sequence character. Results show that the timing and magnitude of sediment input, relative to sea-level rise, is a significant

control on the subaerial extent of the delta and the relative dominance of alluvial and marine facies within the sequence.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Holocene; Deltas; Fluvial sedimentation; Neotectonics; Bangladesh; Bengal Basin

1. Introduction

Situated in the Bengal Basin, the modern Ganges–

Brahmaputra (G–B) delta represents the world’s larg-

est subaerial delta system, comprising f100,000

km2 of riverine channel, floodplain, and delta-plain

environments. The system’s broad extent is partly a

function of the great sediment load, presently f1

billion t/year delivered to the basin. Morgan and

McIntire (1959) first introduced the G– B delta as

perhaps the archetype of a tectonically influenced

system, being situated adjacent to the Indo–Burman

collision zone in the east and the main Himalayan

0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.P I I : S 0 0 3 7 - 0 7 3 8 ( 0 2 ) 0 0 1 8 4 - 7

* Corresponding author. Tel.: +1-631-632-8676; fax: +1-631-

632-8820.

E-mail address: [email protected]

(S.L. Goodbred Jr.).

www.elsevier.com/locate/sedgeo

Sedimentary Geology 155 (2003) 301–316


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thrust to the north. These authors also noted wide-

spread intrabasinal faulting that led to the Quaternary

development of various uplifted, tilted, or subsiding

fault blocks that partition the Bengal Basin, withnotably more tectonic modification in the eastern

and northern regions. Based on the surface expression

of these features, they proposed that ‘‘the Ganges has

been building a broad lateral deltaic mass, [while] the

Brahmaputra, because of structural activity, has been

building a thicker mass of sediment in structurally

subsiding basins’’ (p. 331, Morgan and McIntire,

1959). However, no stratigraphic data were available

to confirm these ideas, and it would be more than 30

years before a major paper was published concerning

the Late Quaternary stratigraphy and development of

the G–B delta (Umitsu, 1993). Subsequent studies

have shown a variety of stratigraphic patterns for the

G– B system, and that these patterns reveal unique

modes of delta development under different tectonic

influences (Goodbred and Kuehl, 2000b; Stanley and

Hait, 2000).

On the time scale of the Late Quaternary, the

implication that tectonics is an important control on

fluviodeltaic processes differs somewhat from tradi-

tional views of delta formation, which have largely

focused on fluvial and marine processes, particularly

sea level (e.g., Galloway, 1975; Stanley and Warne,1994). Indeed, while popular models consider closely

the behavior of sea level, including its relative posi-

tion, rate of change, and stochastic fluctuations, con-

tinental controls on delta formation have received

relatively less attention. Of the various continental

controls, active tectonics (i.e., plate-driven vs. passive

sedimentary tectonics) influence deltaic development

both by deformation of the deltaic basin and by

affecting the volume and distribution of sediments

across the margin. Another important continental

control on delta development is sediment input. Thishas long been recognized (e.g., Galloway, 1975), but

over the millennial time scales relevant to delta for-

mation ( > 103 year), patterns of fluvial sediment

discharge are poorly known despite evidence of major

fluctuations in many systems.

The paper presented here is based upon the data

and findings of recent investigations in the G–B delta

system, which are discussed in the following section.

A detailed description of the methods and data from

these earlier studies can be found in the appropriate

references listed in the text. The overall goal of this

paper is to further develop the ideas that emerged

from these investigations and to place those results

within the broader context of margin processes anddeltaic development.

2. Recent Ganges– Brahmaputra subaerial delta

research

Over the past 5 years, multiscale research efforts

on the GB delta have provided a first-order under-

standing of the patterns and processes of riverine

sediment dispersal across the margin (e.g., Allison et

al., 1998; Goodbred and Kuehl, 1998, 2000b; Stanley

and Hait, 2000). Two of the major goals of these

efforts were to determine the nature and magnitude of

sediment sequestration in the floodplain and delta

plain, and to understand deltaic evolution and strati-

graphic sequence development in this high-yield,

tectonically active setting. Specifically, these studies

have investigated: modern and historical patterns of

river-sediment dispersal across the floodplain and

delta (Allison, 1998; Goodbred and Kuehl, 1998);

Holocene sediment budgets that show major changes

in river-sediment load and the patterns of cross-

margin dispersal (Goodbred and Kuehl, 1999, 2000a);Late Quaternary delta evolution and stratigraphy

(Goodbred and Kuehl, 2000b; Heroy et al., 2002;

Stanley and Hait, 2000); and the late Holocene devel-

opment of the lower delta plain and coastal zone

(Allison et al., 2002; Allison, 1998). Some of the

findings relevant to this article are summarized below.

A compilation of new and existing borehole data

from the G – B system unveiled a Late Quaternary

history controlled by immense river-sediment dis-

charge, tectonic activity, and eustasy. Among the most

significant differences found between the G–B andother large delta systems were: (1) initial development

2000– 3000 years earlier than most of the world’s

delta systems; (2) relative shoreline stability during

rapid early Holocene sea-level rise; and (3) trapping

of a considerable portion of the sediment load to

inland tectonic basins (Goodbred and Kuehl, 2000b).

The initial formation of the G– B delta occurred

around 11 ka, when rising sea level led to back-

flooding of the lowstand surface and the trapping of

riverine sediments, an event that is clearly marked by

S.L. Goodbred Jr. et al. / Sedimentary Geology 155 (2003) 301–316 302


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the transition from clean alluvial sands or Pleistocene

laterites to overlying muds that contain wood and

estuarine/marine shells (interpreted as mangrove sys-

tem based on pollen and molluscan assemblages;Banerjee and Sen, 1988; Umitsu, 1993; Vishnu-Mittre

and Gupta, 1972). At the time of this transition, and

for the next several thousand years, the mean rate of

sea-level rise was >1 cm/year. Thus, this mangrove

system developed during rapid eustatic rise and

remained relatively stable (i.e., no significant trans-

gression) during the ensuing several thousand years,

depositing a 20 – 30-m-thick ‘‘transgressive-phase’’

muddy coastal-plain sequence. This thick deposit

and the persistence of a sensitive intertidal facies

indicate that sediment supply to the delta system must

have been sufficient to infill accommodation created

by rapid sea-level rise. One of the significant con-

clusions drawn from this is that sediment supply, not

the rate of sea-level rise (cf. Stanley and Warne,

1994), controlled the initiation of delta development

and was responsible for delta stability under condi-

tions of rapid eustatic rise.

Tectonics are another important influence on the

G–B delta, with two scales of processes being sig-

nificant (Goodbred and Kuehl, 2000b). First, the over-

all tectonic setting of South Asia imparts a general

control on deltaic processes and character (Fig. 1).Most important among these influences is the close

proximity of the Himalayas to the trailing-edge Bengal

margin. Similar to other tectonically active settings,

this situation gives rise to a large load of relatively

coarse-grained sediment and the strong forcing of

water and sediment discharge from the catchment

basin (a result of steep gradients and comparatively

limited basin storage capacity). The second scale of

tectonic control is reflected in local process, such as

the overthrusting, compression, strike-slip, and normal

faulting that is occurring within the Bengal Basin.Presently, the Bengal Basin is being deformed by the

Indo–Burman fold belt that impinges from the east

and the overthrust block of the Shillong Massif to the

north. This compressional deformation and associated

faulting has forced the uplift of floodplain terraces in

various parts of the region (e.g., Barind Tract, Madhu-

pur Terrace, and Comilla Terrace; Fig. 2). These

features partition the delta into subbasins that are often

poorly connected and thus lead to alternating sediment

inputs and starvation as the rivers avulse to different

portions of the delta system. Although the influence of tectonic processes is known to be widespread, overall

rates, distribution, and controls are poorly constrained.

Sediment supply to the continental margin is also

known to be a major control on sequence formation,

and is an important signal in stratigraphic records as

well. Because most of the G–B sediment load was

trapped in the Bengal Basin after f11 ka, it was

possible to establish a sediment budget encompassing

the Holocene (Goodbred and Kuehl, 1999, 2000a).

Most notable among the budget results was a period

of enormous sediment discharge of f

11–7 ka,during which sediment flux to the G–B delta was at

least 2.3 higher than present (Fig. 3). For perspec-

tive, the G–B system presently supports the world’s

largest sediment discharge at f1109 t/year of

sediment, or less than half that of the early Holocene

load. Furthermore, annual variability in the sediment

load is < 30% (Coleman, 1969), a value that under-

scores the tremendous magnitude of a 4000-year-long

two-fold increase. The timing of this high-discharge

period centers about a f9-ka peak in regional

Fig. 1. Tectono-sedimentary map of the Indo – Asian collision.

Receiving basin for the Ganges and Brahmaputra rivers is the

Bengal Basin, which is situated along a tectonically active trailing-

edge margin surrounded by the Indian craton, Himalayan foredeep,

and Indo-Burman fold belt. Most of the Bengal Basin comprises

Ganges – Brahmaputra delta deposits.



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Fig. 2. Regional map of the Bengal Basin showing physiography and geology of the Ganges–Brahmaputra delta and surrounding area. Also

shown are locations of boreholes collected for this study.



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Along the coastal plain, a period of rapid progra-

dation at the G–B river mouth (forming the Noakhali

chars) has been attributed to an increase in suspended

sediment load that occurred f or several years after theearthquake (Brammer, 1996). The site of land devel-

opment is >800 km downstream of the huge sediment

inputs that were generated by the earthquake, and this

event likely represents the rapid transfer of fine-

grained sediments through the Brahmaputra system.

A second phase of earthquake response appears to be

the passage of a coarse-grained ‘‘debris wave’’ that

has altered the morphology of the Brahmaputra River

over the past 50 years. Along the Brahmaputra River

in Assam, Goswami (1985) showed that a 150-km-

long reach of the channel aggraded 1.25 m from 1951

to 1971 and subsequently degraded 0.21 m from 1971

to 1977. He also noted several kilometers of channel

widening during this time. In Bangladesh, remote-

sensing data have also shown a widening of the

Brahmaputra braidbelt along the 240-km reach above

the confluence with the Ganges River. This widening

of the river began in the mid-1970s and has pro-ceeded at an average rate of 127 m/year from 1973 to

1996 (Fig. 4; EGIS, 1997). The mechanism for

widening appears to be the erosion of relatively fine

floodplain sediments along the channel and their

replacement by coarser ‘‘debris wave’’ sediments that

are deposited as medial bars and chars within the

channel (EGIS, 2000). Overall, the 1950 Assam

earthquake represents a large magnitude disturbance

event, but Khattri and Wyss (1978) find a roughly 30-

year cyclicity to similar seismic activity in this region.

This recurrence interval implies that large tectonic

events in the catchment basin may play an important

role in long-term G – B river behavior and margin

development (Fig. 5).

Fig. 4. River channel morphology for a reach of the Brahmaputra River between the Teesta River tributary and Old Brahmaputra offtake

(see Fig. 2). The f 20-year time series shows the successive widening of river’s braidbelt ( f 127 m/year along this reach). Braidbelt widening

is believed to result from increased bedload related to a major 1950 earthquake located f 400 km upstream of this site.



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3.1.2. Bengal Basin tectonics

Faulting, earthquakes, and other tectonic activity

occurring within the Bengal Basin have had a more

direct effect on the delta system, including controls onriver courses, avulsion, sediment dispersal, and facies

preservation. In the eastern delta, shortening in the

accretionary wedge of the Indo– Burman fold belt

extends into sedimentary deposits of the Bengal

Basin, possibly as far west as the Madhupur Terrace

(Fig. 6). In the northeast, flexural loading from over-

thrust of the Shillong Massif has generated downwarp

of the adjacent Sylhet subbasin. Throughout the

region, intrabasinal faulting resulting from these tec-

tonics has generated a series of vertically thrown

blocks that partition the delta into variously connectedsubbasins (Fig. 6). In the north-central Bengal Basin,

shear and compression has resulted in the Pleistocene

uplift of the Madhupur Terrace, as well as more recent

uplift of the Comilla Terrace to the south and the

Mymensingh Terrace to the north.

In 1782, severe earthquakes in the Sylhet region

resulted in vertical displacements (near Mymensingh)

that contributed to avulsion of the Brahmaputra from

its old course east of the Madhupur Terrace to its

modern channel (Brammer, 1996; Fergusson, 1863).

Indeed, floodplain and river channel morphology

indicate several meters of upwar d displacement in

the past several hundred years (Coates, 1990). In

addition to altering the course of the Brahmaputra,the Mymensingh uplift has greatly reduced sediment

delivery to the Sylhet Basin. Since subsidence rates of

2–4 mm/year generate abundant accommodation, the

decrease in sediment input is resulting in a rapid

deepening of the basin. Presently, the Sylhet region

already floods to several meters deep over f10,000

km2 each year, and continued isolation from Brahma-

putra sediment will worsen flooding (Fig. 7).

Also relevant to Sylhet Basin flooding, poor drain-

age through the constricted Meghna River floodplain

limits the discharge of abundant monsoon floodwatersto the coast (Fig. 6). The Meghna channel is situated at

the southern end of the Madhupur Terrace and has

possibly been narrowed by recent uplift of the Comilla

Terrace, although the age and extent of this process is

not well-constrained. If the Sylhet Basin remains iso-

lated from sediment input, subsidence will generate a

strong hydraulic gradient against the present course of

the Brahmaputra, and thus ultimately favor avulsion

back to its eastern course. Such avulsions between the

Brahmaputra’s western and eastern courses have been

Fig. 5. Regional earthquake distribution from 1973–2000, including events of magnitude >5. Data is from the US Geological Survey’s National

Earthquake Information Center.



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relatively frequent in the Holocene ( f103 year) and

have led to sharp changes in riverine sediment disper-

sal. During these course changes, the Sylhet region

either has served as a large overdeepened sediment trap

or, once filled, allowed sediments to bypass via the

narrow western corridor to the coast. One notably large

and rapid infilling event occurred in the middle Hol-

ocene, when sedimentation rates were at least 2 cm/

year for f1000 year in the Sylhet Basin. The reduc-

tion in sediment input to the coast caused a trans-

gression of the eastern delta front at this time (Good-

bred and Kuehl, 2000b).

In contrast to the tectonically complex eastern

Bengal Basin, the southwestern delta is situated along

a trailing-edge margin that is much less influenced by

tectonic activity. This permits the Ganges River, after

entering the Bengal Basin through a relatively narrow

corridor between the Rajmahal Hills and Barind

Tract, to migrate largely unrestricted across several

hundred kilometers of the lower floodplain and delta

Fig. 6. Map of tectonomorphic features and controls on the Ganges– Brahmaputra delta system. Arrows show general Holocene pathways for

the major river channels. These features have been a major control on facies preservation and delta development, the details of which are

discussed in the text.



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plain (Fig. 6). Recent stratigraphic studies suggest

that there are no tectonomorphic features (e.g., terra-

ces or subbasins) that have exhibited a strong control

over sediment dispersal for at least the past 7000

years (Goodbred and Kuehl, 2000b; Stanley and Hait,

2000). However, numerous subtle lineaments recog-

nized from aerial and satellite images suggest that underlying tectonic features and movements exist and

may influence longer-term (>104 year) Ganges River

positions and delta development (Sesören, 1984;

Stanley and Hait, 2000). Another generally held

notion is that the Ganges’ eastward migration over

the Holocene is a function of loading flexure at the

northeast-trending hinge line denoting the deeply

buried Eocene shelf edge (Fig. 2; e.g., Alam, 1996;

Stanley and Hait, 2000). An alternative interpretation

is that the Ganges River course is diverted eastward

because of downwarping caused by compression

along the Indo–Burman fold belt (a similar response

to that causing Sylhet Basin subsidence; Seeber,

personal communication). Overall, the Holocene his-

tory of the western G – B delta is not dissimilar to that

of other delta systems, but the strongly tectonic-

influenced eastern region differs markedly becauseof the sediment trapping, tectonic uplift, and subsi-

dence, which affect the downstream delta plain by

forcing local transgressions and regressions.

3.2. Sediment supply

Sediment supply is another important control on

the G–B delta, and it interplays closely with tectonic

processes and sea-level rise. Prior to f15 ka, oce-

anographic evidence indicates that river discharge was

Fig. 7. Enhanced-contrast AVHRR images of the Bengal Basin collected during the dry and wet seasons (images from Ali and Quadir, 1987). In

the wet season image, note extensive flooding in the central basin associated with monsoonal precipitation and overbank flooding.



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greatly reduced under the dominance of the dry north-

east monsoon (Cullen, 1981; Wiedicke et al., 1999),

but at the lowstand of sea level, most river sediment

would have bypassed the Bengal Basin to the deep-sea fan. With continued climatic warming through the

early Holocene, though, the concurrence of ice-sheet

melting and a strengthening southwest Indian mon-

soon generated both abundant accommodation space

(via eustasy) and regional sediment production (via

increased runoff) (see Fig. 3). Discharging more than

double its present sediment load during the period

from 11 to 7 ka, the G–B formed a thick subaerial-

delta deposit that comprises f60% of the entire Late

Quaternary strata. Because this high discharge corre-

sponded to rapid sea-level rise during deglaciation,

abundant eustatic accommodation permitted the dep-

osition of a 50-m-thick sedimentary unit in f4000

years (Goodbred and Kuehl, 2000a).

Because the subtropical river discharge (sediment

source) and ice-sheet melting (eustatic rise) that

helped create the G–B delta are only loosely coupled

via global climate, significant differences in the timing

between high sedimen t discharge and sea -level

change might be expected for this and other river-

delta systems. Such nonlinear relationships between

the major controls on margin sequence development

have been considered in the past (e.g., Posamentier and Allen, 1993), but here, we employ a numerical

model to test the sensitivity of sequence generation to

variable sediment inputs (both timing and magnitude).

The model uses the same framework as Steckler et al.

(1993) and Steckler (1999), but uses a nonlinear

diffusion algorithm for sediment transport based on

the nonmarine model of Paola et al. (1992) and the

shelf model of Niedoroda et al. (1995).

Results show that the period of high sediment

discharge during the early Holocene significantly

changes sequence architecture and development of the delta system (Fig. 8). Without this large sediment

pulse (Fig. 8, lower panel), the marine transgression

would have extended farther inland. Also, the end of

the marine transgression and the shift to highstand

progradation would have been several thousand years

later. This latter case is similar to the observations at

many of the world’s large delta systems, where pro-

gradation began f8 – 6 k a (Stanley and Warne,

1994). The high Ganges–Brahmaputra sediment dis-

charge during the early Holocene was sufficient to

halt transgression despite continued rapid sea-level

rise (Fig. 8, upper panel). Progradation of the delta,

which started at f11 ka, resulted in much more

extensive nonmarine (alluvial) deposition when com- pared with other deltas around the world. Model

experiments with a shift to later timing of the high

sediment flux yield extensive marine transgression,

followed by rapid late progradation of the delta.

Conversely, an earlier period of high discharge results

in much of the sediment bypassing the shelf to the

deep sea, but with a delayed and less extensive marine

transgression.

Thus, modeling of the G–B sequence suggests that

the stratigraphic architecture is partly a function of the

timing of high sediment discharge relative to the

position of sea level and its rate of rise. This raises

a possibly broader implication that monsoon-con-

trolled river systems deliver more sediment to the

margin during climatic optimums (Goodbred and

Kuehl, 2000a; Thomas and Thorp, 1995), which, in

turn, are likely conditions for rising sea level and

accommodation production. The findings from the

G– B system suggest a conceptual model for rapid

sedimentary sequence development during brief peri-

ods of climate change (Goodbred and Kuehl, 2000b).

3.3. Facies preservation and sequence architecture

In addition to the enormous sediment discharge

that occurred in the early Holocene, other factors have

shaped G – B delta development during the Late

Quaternary. Thus, it is important to recognize

sequence characteristics and how tectonics, sediment

supply, and sea level have contributed to its develop-

ment. A simplified fence diagram of borehole data

from the G–B system (Fig. 9) shows the relative age,

texture, and distribution of deltaic facies. Notable in

this diagram are several temporal and spatial trends insediment distribution, such as the various fine-grained

mud facies that have been well preserved at particular

times and in particular regions of the system. At the

subaerial delta front, muddy coastal-plain deposits

that date to initial delta development ( f11 ka) are

well preserved amidst sandy alluvial-valley deposits at

30– 60-m depth. The characteristic muddy coastal-

plain facies is preferentially located across the central

and eastern delta near relatively shallow (f50 m)

pre-Holocene surfaces, as well as at more seaward



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positions near the delta front. Higher in the strati-

graphic sequence, coastal-plain mud deposits have a

much more limited distribution, being largely absent

from 10- to 30-m depth except at the extreme eastern

and western fringes of the delta (Fig. 9). These depths

correspond to the middle Holocene ( f6–3 ka),

when slowing sea-level rise and reduced accommo-

dation may have favored river channel migration and

the reworking of fine-grained near-surface deposits.

The general absence of fine-grained deposits from the

middle Holocene is not believed to be a result of

environmental change because muddy coastal-plain

facies are widespread both in the modern delta plain

and in the early Holocene.

Presently, fine-grained muds dominate the shallow

stratigraphy (2– 5 m) and extend across roughly 90% of

the delta. The age of these deposits ranges from modern

to a few thousand years, and their broad extent is

greatly facilitated by vast overbank flooding and an

extensive network of small fluvial distributaries (Alli-

Fig. 8. Cross-sections of two model runs comparing modifications in sequence architecture due to variation in sediment supply. The timelines

represent 1 ka intervals since 31 ka, and the facies shown include nonmarine (dark shade), shorefa ce (medium shade), and marine (light shade)

deposits. The top model incorporates the early Holocene period of high sediment discharge (see Fig. 3) and the lower model uses a constant

sediment flux that represents the default parameter often used because of the lack of paleosediment discharge data. Results are discussed in the

text.



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Fig. 9. Fence diagram of generalized stratigraphy determined from borehole data (see Section 2 for data sources). Trends in overall sequence

structure and facies preservation can be seen in various regions of the delta. Alternating mud and sand units are widespread across the lower

delta, particularly in the east. Sandy channel facies dominate the stratigraphy of the upper central and western basin, while deposits of upper

northeast delta support frequent preservation of thin floodplain deposits as well as a thick flood basin sequence. Differences in these sequences

are related to the varying dominance of controls such as eustasy, sediment supply, and tectonics. See text for further discussion. Individual core

descriptions from Goodbred and Kuehl (2000b) and references therein.



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son et al., 1998; Goodbred and Kuehl, 1998). Because

mid-Holocene sands almost everywhere underlie this

surficial mud drape (Fig. 9), it is interpreted that such

recent floodplain deposits have a low chance of pres-ervation. Not unexpected under accommodation-

limited highstand conditions, the eventual removal of

these floodplain deposits is facilitated by rapid channel

migration and frequent avulsions along the Ganges and

Brahmaputra river courses. Overall, the distribution of

preserved fine-grained sediments in the lower delta

stratigraphy has been controlled by temporal variations

in accommodation production, which in this instance is

largely a function of relative sea-level rise.

The stratigraphy of the upper G – B delta shows

different patterns and controls than those of the coast.

Sandy channel deposits comprise nearly the entire

subsurface stratigraphy across a broad area from the

Hooghly River distributary to the main channel of the

modern Ganges–Brahmaputra River (Figs. 2 and 9).

Boreholes from this area reveal little or no subsurface

floodplain deposits, except for the widespread cap of

modern and recent sediments. This situation suggests

that floodplain deposits are wholly removed over the

longer term (103 year) in this part of the basin, despite

rapid aggradation during the early Holocene. Both

river-system dynamics and lower subsidence rates

west of the hinge zone may contribute to the domi-nance of coarse-grained deposits in the upper delta

(Stanley and Hait, 2000). The seasonal discharge and

large sediment load (esp. bedload) of these rivers

favor channel migration and avulsion, and thus the

lateral erosion of interchannel floodplain units

(Hannan, 1993). Furthermore, the enormous sediment

loads under the strengthened early Holocene monsoon

(Goodbred and Kuehl, 2000a) may have contributed

to channel instabilities. Under the condition of limited

accommodation space, either where subsidence is

slow or after the slowing of sea-level rise, the rapidmigration of these rivers results in floodplain units

being reworked before they can be buried sufficiently

to be preserved.

In contrast to the sand-dominated stratigraphy of the

upper west-central delta, fine-grained floodplain and

flood-basin deposits are commonly preserved in the

northeast region (Fig. 9). Along both the modern and

old courses of the Brahmaputra, f5-m-thick units of

muddy silt-dominated sediment are preserved from

depths of 10–50 m (Umitsu, 1993). These mud units

have been interpreted as floodplain deposits formed

during successive avulsions of the Brahmaputra River

between its eastern and western courses (Goodbred and

Kuehl, 2000b). In addition to muddy floodplain depos-its, there is a thick (80 m) sequence of fine-grained

sediments preserved in the Sylhet Basin. The deposi-

tion of this massive Holocene mud unit was facilitated

by subsidence of the Sylhet Basin and its isolation from

the rest of the delta via the uplifted Madhupur Terrace.

When the Brahmaputra occupied its eastern (Sylhet)

course, sandy Brahmaputra channel deposits were

largely restricted to the western basin, with silt and

clay-dominated deposits infilling the distal eastern

portion (Goodbred and Kuehl, 2000b).

Thus, the G–B delta displays three different strat-

igraphies that include an alternating fine – coarse-

grained sequence in the lower delta, a sand-dominated

stratigraphy in upper west-central delta, and mud-

dominated sequences in the northeast. By considering

the major controls on these different sequence archi-

tectures, some general patterns of facies preservation

and alluvial sequence development emerge. First,

mixed fine- and coarse-grained fluviodeltaic sequen-

ces might be expected under changing rates of accom-

modation production, such as those controlled by

post-glacial eustatic sea-level rise and tectonics.

Indeed, Wright and Marriott (1993) present a base-level-controlled fluvial model that describes alluvial

sequence development during a third-order sea-level

cycle (Fig. 10). Though spanning a shorter period, the

Late Quaternary G–B sequence is generally applica-

ble given its size, magnitude, and the rapid rate of

floodplain pedogenesis (Brammer, 1996). As such, the

pattern of facies distribution and succession in the

lower G–B delta closely follows that illustrated by

Wright and Marriott’s model (Fig. 10).

Wright and Marriot also note that departures from

their model may be expected because ‘‘such [fluvial]systems are highly variable and responsive to minor

changes in climate or tectonic activity’’ (p. 208).

Different portions of the G–B delta appear to demon-

strate such variabilities. Whereas the lower delta fol-

lows the general model, the upper west-central delta

differs in the dominance of sandy channel deposits and

the near absence of fine-grained sediment preservation.

We suggest that departure from the model in this region

is due to fluvial controls, such as the large, relatively

course sediment load and the strong seasonality of



14/16

discharge. Each of these characteristics can lead to

channel siltation and the tendency to migrate laterally,

thereby eroding fine-grained overbank deposits and

favoring preservation of sandy channel sediments. Acontrasting pattern is found in the upper northeast delta,

where greater tectonic activity (especially basin parti-

tioning) appears to favor the preservation of fine-

grained floodplain and flood-basin deposits. In this

situation, tectonic subsidence permits muddy sequen-

ces to be rapidly buried, while the areas of local uplift

limit the lateral migration of the river systems.

Although the system is more complex than presented

here, the observed patterns of sequence architecture

may be representative of general alluvial-system

responses to sediment supply, tectonics, and eustasy.

4. Summary and conclusions

The Late Quaternary Ganges– Brahmaputra delta

has been shown to be heavily influenced by eustatic

sea-level rise, tectonic processes, and a large, but

variable, sediment supply; the latter two of which

are not well understood in terms of general delta

models. Building upon recent investigations in the

G– B delta system, we find two scales of tectonic

processes that are relevant, including the broader

regional context of the Himalayan catchment and

the more local impacts of intrabasinal responseswithin the Bengal Basin. Although the G–B drainage

basin is immense, the response time to events occur-

ring in the Himalayan catchment (i.e. tectonic and

climatic) appears to be sufficiently brief to affect

millennial-scale development in the delta. In 1950, a

major earthquake along the Assam reach of the

Brahmaputra River introduced a large quantity of

sediment into the system via mass wasting. The

apparent effects of this have been recognized by a

rapid progradation of the river-mouth shoreline

shortly after the event, followed by a rapid wideningof the river braidbelt (>127 m/year) in association

with the passage of a coarse-sediment ‘‘debris wave.’’

Other tectonic influences are related to processes

occurring within the delta basin, such as faulting

and folding that have caused regional vertical move-

ments. Uplifted and downthrown sedimentary blocks

serve to partition the delta into various subbasins that

are often poorly connected, leading to differences in

the deposition and preservation of sedimentary facies.

Sediment supply is another major control on deltaic

Fig. 10. Model of fluvial sequence architecture and development proposed by Wright and Marriott (1993). The authors recognized four phases

of formation. (I) Coarser-fraction channel deposits may dominate lowstand fluvial deposits, and mature well-drained soils develop on terrace

surfaces. (II) Slow early transgression produces multistory sandbodies and floodplain deposits may be prone to reworking by channels. (III)

Rapid later transgression favors high levels of storage of floodplain sediments resulting in isolated channels. (IV) Reduced accommodation at

the highstand lowers floodplain accretion rates, favoring better-developed soils. Higher rates of floodplain reworking result in higher density of

sand bodies and reduced floodplain preservation potential.



15/16

processes, and Holocene variations in the G – B sedi-

ment load have been significant. Modeling of the G–

B sequence through this period supports that the

timing of an early Holocene period of high sediment discharge was critical to the development and archi-

tecture of the deltaic sequence. Variation in the timing

or magnitude of that sediment pulse led to consider-

able changes in the subaerial extent of the delta and

the proportional dominance of marine facies in the

sequence.

The Late Quaternary stratigraphy of the G–B delta

also revealed regional patterns of facies distribution,

controlled by the relative dominance of eustatic,

tectonic, and fluvial controls. In the northeast delta,

where tectonic processes are most active, the stratig-

raphy is dominated by, or at least contains, a signifi-

cant portion of fine-grained floodplain deposits. It

appears that partitioning of the delta into subbasins

favors the local trapping and ultimate preservation of

these fine-grained units. In the western delta, where

there are fewer tectonic features, sandy alluvial depos-

its dominate the stratigraphy. Thus, despite the broad

extent of modern and recent ( < 2 ka) floodplain

deposits in this region, such fine-grained facies have

a low chance for preservation. Fluvial processes

dominate this part of the delta, where channel migra-

tion and avulsion tend to erode the fine-grained floodplain deposits before they are buried. In the southern

delta coastal plain, the stratigraphy has been most

heavily influenced by eustasy, and due to variations in

the rate of sea-level rise, fine-grained coastal plain

deposits have been variably preserved during the

Holocene. The result is that the southern delta

sequence shows a mix of fine- and coarse-grained

facies, with the muddy deposits being preferentially

preserved during rapid sea-level rise in the early

Holocene. Overall, these different stratigraphies

located within the same delta system emphasize theimportance of local basin factors in modifying

sequence development. If these individual strati-

graphic patterns are indeed characteristic of their

dominant controls, then findings from the G–B delta

sequence suggest that both tectonics and sediment

supply can be incorporated into quantitative models of

delta and margin development. Toward this goal, the

great number of tectonically active, high-sediment-

yield margins of southern and eastern Asia warrants

further investigation.

Acknowledgements

This project was completed with support from the

National Science Foundation (EAR-9706274), FloodAction Plan 24: River Survey Project (EU-sponsored),

a Geological Society of America Grant-in-Aid, and

NSF’s Summer Institute in Japan. The sequence

modeling was supported by Office of Naval Research

grant N00014-95-1-0076. This publication constitutes

Marine Sciences Research Center publication #1230

and Virginia Institute of Marine Science publication

#2366.

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