Sedimentary Geology 155 (2003) 301 – 316
www.elsevier.com/locate/sedgeo
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
b
a
Marine Sciences Research Center, State University of New York, Stony Brook, NY 11794, USA
Virginia Institute of Marine Science, College of William and Mary, Gloucester Pt., VA 23062, USA
c
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 tectonics
and 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 and sediment 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*
Corresponding author. Tel.: +1-631-632-8676; fax: +1-631632-8820.
E-mail address: [email protected]
(S.L. Goodbred Jr.).
est subaerial delta system, comprising f 100,000
km2 of riverine channel, floodplain, and delta-plain
environments. The system’s broad extent is partly a
function of the great sediment load, presently f 1
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.
PII: S 0 0 3 7 - 0 7 3 8 ( 0 2 ) 0 0 1 8 4 - 7
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S.L. Goodbred Jr. et al. / Sedimentary Geology 155 (2003) 301–316
thrust to the north. These authors also noted widespread intrabasinal faulting that led to the Quaternary
development of various uplifted, tilted, or subsiding
fault blocks that partition the Bengal Basin, with
notably 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 traditional 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 position, rate of change, and stochastic fluctuations, continental 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. This
has long been recognized (e.g., Galloway, 1975), but
over the millennial time scales relevant to delta formation ( > 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 and
deltaic 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 understanding 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 stratigraphic 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 crossmargin 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 development 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 discharge, tectonic activity, and eustasy. Among the most
significant differences found between the G – B and
other 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 backflooding 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
the transition from clean alluvial sands or Pleistocene
laterites to overlying muds that contain wood and
estuarine/marine shells (interpreted as mangrove system 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 transgression) 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 conclusions 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 conditions of rapid eustatic rise.
Tectonics are another important influence on the
G – B delta, with two scales of processes being significant (Goodbred and Kuehl, 2000b). First, the overall 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, Madhupur 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
303
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 trailingedge margin surrounded by the Indian craton, Himalayan foredeep,
and Indo-Burman fold belt. Most of the Bengal Basin comprises
Ganges – Brahmaputra delta deposits.
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 f 11 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 perspective, the G –B system presently supports the world’s
largest sediment discharge at f 1 109 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 underscores the tremendous magnitude of a 4000-year-long
two-fold increase. The timing of this high-discharge
period centers about a f 9-ka peak in regional
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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.
S.L. Goodbred Jr. et al. / Sedimentary Geology 155 (2003) 301–316
305
sediment supply, and sea level each exhibits a roughly
equable control over Late Quaternary margin and
delta development. This situation has not been well
studied in modern delta systems, but may be characteristic of the numerous high-yield tectonically active
delta systems found in South Asia, and perhaps along
other parts of the Pacific Rim. The following sections
further develop these ideas, and also attempt to link
specific controls to the various characteristics of the
G –B sedimentary sequence.
3.1. Tectonics
Fig. 3. Comparison of Late Quaternary records for South Asian
climate ((A) after Prins and Postma, 2000), eustatic sea level ((B)
after Edwards et al., 1993; Fairbanks, 1989), and Ganges –
Brahmaputra River sediment discharge ((C) after Goodbred and
Kuehl, 2000a). The continental aridity index is derived from relative
aeolian/fluvial inputs to hemipelagic deposits in the northeast
Arabian Sea. Fluvial sediment loads were determined from the
volume of deltaic sediment deposits preserved in the Bengal Basin
and upper Bengal Fan.
insolation (Cohmap, 1988; Prell and Kutzbach, 1992).
The resulting intensification of the southwest monsoon (Sirocko et al., 1993; Van Campo, 1986) supported regionally wetter conditions and increased river
discharge (Cullen, 1981; Gasse et al., 1991). At this
time, Williams and Clarke (1984) also find evidence
for 20 –30 m of floodplain incision along two Ganges
tributaries, suggesting one probable source for the
high G – B sediment fluxes. In contrast to the 2.3 Gt/
year discharge of this period, outputs were extremely
reduced prior to f 15 ka (Cullen, 1981; Wiedicke et
al., 1999) because of dominance of the dry northeast
monsoon, possibly supporting an order of magnitude
lower discharge.
3. Late Quaternary controls on sequence
development and character
An overarching theme evident from recent studies
of the G –B delta margin system is that tectonics,
Past research has noted specific tectonic features
that affect the G –B system (e.g., Madhupur Terrace)
(Alam, 1989; Morgan and McIntire, 1959), yet there
has been less known of the impact that these structures
have had on the region’s sedimentary geology. Other
tectonic controls also include remote influences from
the immense Himalayan catchment, where tectonic
processes operate at a relatively rapid rate. Despite its
great size, sedimentary signals from the catchment
propagate downstream at a sufficiently fast rate to
affect millennial-scale development of the delta. One
of the major effects of catchment basin tectonics is
expressed in the rate, magnitude, and characteristics of
sediment delivered to the margin.
3.1.1. Catchment basin tectonics
In historical times, one of the most significant
tectonic events was the 1950 earthquake (Richter
mag. 8.7) in Assam, India, which is situated along
the middle reaches of the Brahmaputra River. This
event changed the course and morphology of several
Brahmaputra tributaries and introduced a large but
unquantified volume of sediment via slope failures
(Poddar, 1952). Subsequently, Goswami (1985) was
able to use sediment gauging data from the Assam
reach of the river to show that the system’s sediment
rating (sediment load/discharge) had increased dramatically soon after the earthquake (1955 – 1960)
and was about an order of magnitude higher than
when measured a decade later (1971 – 1976). Although the effects of this sediment input on the G –
B river have not been directly investigated, evidence
suggests that two phases of earthquake-related sediments have cycled through the G – B system since the
event.
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Along the coastal plain, a period of rapid progradation at the G – B river mouth (forming the Noakhali
chars) has been attributed to an increase in suspended
sediment load that occurred for several years after the
earthquake (Brammer, 1996). The site of land development is >800 km downstream of the huge sediment
inputs that were generated by the earthquake, and this
event likely represents the rapid transfer of finegrained 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-kmlong 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 proceeded 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 30year 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.
S.L. Goodbred Jr. et al. / Sedimentary Geology 155 (2003) 301–316
307
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.
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 on
river 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 overthrust of the Shillong Massif has generated downwarp
of the adjacent Sylhet subbasin. Throughout the
region, intrabasinal faulting resulting from these tectonics has generated a series of vertically thrown
blocks that partition the delta into variously connected
subbasins (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 upward 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 f 10,000
km2 each year, and continued isolation from Brahmaputra sediment will worsen flooding (Fig. 7).
Also relevant to Sylhet Basin flooding, poor drainage through the constricted Meghna River floodplain
limits the discharge of abundant monsoon floodwaters
to 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 isolated 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
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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.
relatively frequent in the Holocene ( f 103 year) and
have led to sharp changes in riverine sediment dispersal. 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 Holocene, when sedimentation rates were at least 2 cm/
year for f 1000 year in the Sylhet Basin. The reduction in sediment input to the coast caused a trans-
gression of the eastern delta front at this time (Goodbred 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
S.L. Goodbred Jr. et al. / Sedimentary Geology 155 (2003) 301–316
309
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.
plain (Fig. 6). Recent stratigraphic studies suggest
that there are no tectonomorphic features (e.g., terraces 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 recognized 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 history of the western G –B delta is not dissimilar to that
of other delta systems, but the strongly tectonicinfluenced eastern region differs markedly because
of the sediment trapping, tectonic uplift, and subsidence, 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 f 15 ka, oceanographic evidence indicates that river discharge was
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greatly reduced under the dominance of the dry northeast 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 deepsea fan. With continued climatic warming through the
early Holocene, though, the concurrence of ice-sheet
melting and a strengthening southwest Indian monsoon 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 subaerialdelta deposit that comprises f 60% of the entire Late
Quaternary strata. Because this high discharge corresponded to rapid sea-level rise during deglaciation,
abundant eustatic accommodation permitted the deposition of a 50-m-thick sedimentary unit in f 4000
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 sediment discharge and sea-level
change might be expected for this and other riverdelta 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 progradation began f 8 – 6 ka (Stanley and Warne,
1994). The high Ganges –Brahmaputra sediment discharge 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 f 11 ka, resulted in much more
extensive nonmarine (alluvial) deposition when compared 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-controlled 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 periods 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 development. 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 in
sediment 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 ( f 11 ka) are
well preserved amidst sandy alluvial-valley deposits at
30 – 60-m depth. The characteristic muddy coastalplain facies is preferentially located across the central
and eastern delta near relatively shallow ( f 50 m)
pre-Holocene surfaces, as well as at more seaward
S.L. Goodbred Jr. et al. / Sedimentary Geology 155 (2003) 301–316
311
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), shoreface (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.
positions near the delta front. Higher in the stratigraphic 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 ( f 6 – 3 ka),
when slowing sea-level rise and reduced accommodation 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-
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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.
S.L. Goodbred Jr. et al. / Sedimentary Geology 155 (2003) 301–316
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 preservation. Not unexpected under accommodationlimited 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 dominance 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 rapid
migration 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, f 5-m-thick units of
muddy silt-dominated sediment are preserved from
depths of 10– 50 m (Umitsu, 1993). These mud units
313
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 deposits, there is a thick (80 m) sequence of fine-grained
sediments preserved in the Sylhet Basin. The deposition 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 stratigraphies that include an alternating fine – coarsegrained sequence in the lower delta, a sand-dominated
stratigraphy in upper west-central delta, and muddominated sequences in the northeast. By considering
the major controls on these different sequence architectures, some general patterns of facies preservation
and alluvial sequence development emerge. First,
mixed fine- and coarse-grained fluviodeltaic sequences might be expected under changing rates of accommodation production, such as those controlled by
post-glacial eustatic sea-level rise and tectonics.
Indeed, Wright and Marriott (1993) present a baselevel-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 applicable 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 demonstrate such variabilities. Whereas the lower delta follows 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
314
S.L. Goodbred Jr. et al. / Sedimentary Geology 155 (2003) 301–316
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.
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. A
contrasting pattern is found in the upper northeast delta,
where greater tectonic activity (especially basin partitioning) appears to favor the preservation of finegrained floodplain and flood-basin deposits. In this
situation, tectonic subsidence permits muddy sequences 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 responses
within the Bengal Basin. Although the G – B drainage
basin is immense, the response time to events occurring 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 widening
of 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 movements. 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
S.L. Goodbred Jr. et al. / Sedimentary Geology 155 (2003) 301–316
processes, and Holocene variations in the G – B sediment 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 architecture of the deltaic sequence. Variation in the timing
or magnitude of that sediment pulse led to considerable 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 stratigraphy is dominated by, or at least contains, a significant 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 deposits 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 migration 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 the
importance of local basin factors in modifying
sequence development. If these individual stratigraphic 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-sedimentyield margins of southern and eastern Asia warrants
further investigation.
315
Acknowledgements
This project was completed with support from the
National Science Foundation (EAR-9706274), Flood
Action 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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