Oriental Geographer
Vol. 46 No.2
July 2002
QUATERNARY PALEO-GEOGRAPHY AND GEOHAZARD
SCENARIO OF THE BENGAL DELTA OF BANGLADESH
Aftab Alam Khan
Mohammad Abdul Hoque
Abstract: The Ganges-Brahmaputra-Meghna (GBM) delta is an extreme example of a deltaic
coastal region vulnerable to natural disasters like earthquake, differential subsidence and shortening,
arsenic contamination in groundwater, salt-water intrusion, storms surges, and flooding. The sea level
changes during the Quaternary period have major bearing on the geohazard scenario of the OBM
delta. Inbred tectonism of the Indian plate has been playing a significant role in the development of
the GBM delta. The faults and lineaments are of seismogenic potentiality and are active in
terminating by block upliftment and subsidence in the delta. The subsidence rates in Jamuna
depression, Sylhet basin and Meghna delta plain are 4.3 mm/yr, 7.5 mm/yr and 3.9 mm/yr
respectively. It is clearly envisaged that the Sylhet basin is relatively faster subsiding basin in the
GBM delta. The subsided sub-basins enhance the marine transgression in the LateQuaternary period.
The neotectonic depressions and most of the lower delta plain are the most vulnerable sites for
earthquake hazards because the zone within the upper 200 ft of the ground level in these depressions
is mostly composed of unconsolidated clay and silt materials.
During the Holocene marine transgression land-ocean interaction occurred along two main flow
domains of the river system. As a result, numbers of estuarine lobes have been developed within the
transgressive domain. These lobes are the regions of complex interaction between fluvial and marine
processes that might have acted as geochemical traps for the transitional metals bonded in the finer
grained sediments. During the Holocene regressive phase there was frequent oscillatory changes in
the sea-level in the GBM delta straddles with the monsoon climate and tectonic instability that
promoted channel instabilities, migration and reconfiguration i.e. chute cutoff, neck cutoff, channel
avulsion. The arsenic contaminated aquifers of Holocene land-ocean interface lobes in these palaeomeandering belts occur below the chute cut-off, chute bar and back swamp deposits. Shallow sandy
aquifers might be reducing enough below 40-50 ft to release arsenic into solution. Deep aquifers
below the 'maximum marine flooding deposits' are low in arsenic and iron because most of the
FeOOH and MnOOH coatings have been dissolved due to the prevJilence of depth dependent
reducing condition in the depositional horizon.
r
The paleogeographic status of the GBM delta during Quaternary such as marine transgressionregression, neotectonic upliftment and subsidence, duration of sub-aerial exposure, and land-sea
water interaction is associated with various geological environments.
Key words: quarternary period, geohazard, neotectonic depression.
INTRODUCTION
Bengal delta being prograding in nature since its inception covers entire Bengal basin.
The prograding nature of the delta has been intervened with sea transgression number of
Dr. Aftab Alam Khan is Professor and Mohammad Abdul Hoque is Postgraduate Research Student, Department of
Geology, University of Dhaka.
r
2
j
ORIENTAL GEOGRAPHER
times in its geologic history. With, a low-lying delta complex <;lssociated with the Ganges,
Brahmaputra-Jamuna, Meghna rivers form one of the largest depositional systems in the
World (Coleman 1969; Lindsay 1991; Goodbred and Kuehl 2000). Fluvio-deltaic
sediments of the Holocene time have formed the potential groundwater aquifer system in
the delta region. The major rivers viz., the Ganges, the Brahmaputra-Jamuna, the Meghna
and their tributaries and distributaries have been acting as the primary tra~sporting'media
of the sediments in the delta building and subsequently emerged as the Bengal delta. In
view of the contribution in the delta building by the rivers Ganges, Brahmaputra-Jamuna,
Meghna and their tributaries and distributaries, the delta may be termed as GangesBrahmaputra-Meghna (OBM) delt,a. Sedimentation in the coastal low lands and deltas is
largely controlled by the relative sea-level changes. In these coastal systems, the tidal
regime, wave action and fluvial discharge are the most salient factors influencing the
hydrodynamics as well as the physico-che:n1ical status of the sedimentation. There are
scores of publications pertaining to the, Holocene sea-level changes in the Bengal delta
(Milliman et al. 1989; Umitsu 1993; Islam and Tooley 1996 & 1999; Khan et al. 2000;
Goodbred and Kuehl 2000).
Similarly" inbred tectonism of the Indian plate has been playing a significant role in the
development of the GBM delta. The vital tectonic element viz., Himalayas in the north
and Indo-Burma range in the east have been acting virtually as inexhausting sources of
sediments. The river Ganges and Brahmaputra drain the NW and NE portion of the
Himalayas to the Bay of Bengal over the delta. Subduction tectonics of the Indo-Burma
convergent system and differential subsidence and upliftment largely controlled the basin
architecture. Rivers courses in the basin follow the faults and lineaments (Khandoker
1987). Moreover, these faults and lineaments are of seismogenic potentiality and are
active in terminating by block upUftment and subsidence. Nonetheless, these subsided
sub-basins enhance the marine transgression in the Late Quaternary period.
An attempt is made to show how the geological hazards viz., sediments and groundwater
contamination by toxic elements and earthquake potentiality of the Ganges-BrahmaputraMeghna (GBM) delta are associated with the different stages of the Quaternary basin
development, delta building and sedimentation processes.
BASIN SETTING AND HYDRODYNAMICS
, Bengal basin is one of the thickest sedimentary basins having sediment thickness in
excess of 18 Km in the deeper basin zone (Khan and Chouhan 1996). This enormous
thickness indicates a great influx of sediments in the basin since its formation. Bengal
'delta situated within the Bengal basin comprises the Meghna-Brahmaputra-Jamuna
floodplain of Bangladesh and Ganges fluvio-deltaic plain of Bangladesh and West Bengal
(figure 1). This delta started to evolve in the Late Quaternary ba.ck in 18000 years andhas
been controlled by immense sediment discharge, tectonics and eustasy (Goodbred and
Kuehl 2000). The delta is bounded by Pleistocene ¥adhupur-Barind tract and Shillong
massif in the north, Lalmai hill of Early Holocene in the east, and the Early Holocene to
QUARTERNARY PALEO-GEOGRAPHY AND GEOHAZARD SCENARIO
3
Pleistocene Older Alluvium of West Bengal, India in the west «Morgan and McIntire
1959 and Acharyya et al. 2000). The Pleistocene Lalmai hill, another high, has poppedup in the Tippera surface. Tectonically the delta is situated in the subduction related active
margin of the northeastern Indian plate and delta building process in the Holocene has
undergone the strong influence of tectonism. Pleistocene popped-up blocks are controlled
by the pre-existing line of weakness and the courses of the rivers Padma, Jamuna, OldBrahmaputra, Tista and Korotoa follow these lines of weakness. (Khandoker 1987). On
the other hand, the basinal troughs especially the Sylhet trough is a rapidly subsiding
block in the Bengal basin (Goodbred and Kuehl 2000).
Fig. 1 General physiographic setting of the Ganges-Brahmaputra-Meghna (GMB) delta.
The Himalayan-Tibetian uplift has emerged as one of the world's largest sedimentdispersal complex to feed the Bay of Bengal across the Bengal delta. The Ganges and the
Brahmaputra rivers originating from the Himalayan-Tibetian uplift drain a vast catchment
area that flow into the dl:(lta from the northwest and the north. While Meghna river drains
into the delta from the northe~st (figure 1). The Ganges-Brahmaputra-Meghna (GBM)
river system combinedly dra'ins a wide spectrum of metamorphic, magmatic and
sedimentary rocks aged from Precambrian to Quaternary (Wadia 1975) and carries the
largest load of slispended matter in the world (Millimann and Meade 1983). Under the
changing climatic conditions, shifting of monsoon \\:,inds and thef1uctuating sea level, the
hydrology of GBM, humid low-latitude river system, has suffered dramatic variability
during the Late Quaternary (Goodbred and Kuehl 2000).
ORIENTAL GEOGRAPHER
4
NEOTECTONISM AND DIFFERENTIAL LAND-DEFORMATION
Tectonically Bengal delta is sandwiched by the late Mesozoic NE-SW trending passive
rifted margin of the Indian plate and active Indo-Burma subduction related deformation
front from west and east respectively. In the regional scale, the delta is acting as a
foredeep segment of the Indo-Burma subduction zone complex. The subsidence rate
increases across the delta upto the deformation front. The deformation front (Silver and
others 1986) is recognized in the west of the Eastern Thrust (Khan and Chouhan 1996) in
the Bengal basin (figure 2). The deformation front folding and thrusting diminish the rate
of subsidence in the east. The east-west trending Dauki fault in the northern periphery of
the Sylhet basin is a Gondwanic tectonic inheritate associated with the Up-arching of the
Shillong plateau (Gupta and Sen 1988). Southward thrusting of the Shillong plateau has
been generating a smaller fold-thrust deformational front in the northern edge of the
Sylhet trough (figure 2). Sylhet trough acts as a subsiding block with respect to the Dauki
fault system and straddles with the subcrustal normal listric faulting (figure 3).
88
89
90
91
92
25
24
23
22
21
~
E:3
Fig. 2 Generalized Neotectonic setting of the GBM delta. Legend box represents (1): Deltaic deposits wit~
amplification factor 3, (2): Per-Quaternary uplifted blocks with amplification factor 1.5, (3): Mostly clay
deposit with amplification factor 4, (4); Valley Colluvium wi~h amplification factor 2, (5): Alluvium with
amplification factor 3.5, and (6): Prominent fault and fault traces. (Data sources: Hoque, 2001; Goodbred and
Kuehl, 2000 and John W. Whitney, USGS, Denver, Colorado, Personal communication.)
QUARTERNARY PALEO-GEOGRAPHY AND GEOHAZARD SCENARIO
5
5
r-----F::.;.x---------__~__~-___1
10
15
w
Fig. 3 Subcrustal normal listriC faulting in the Sylhet basin the E-W line shown in figure 1 derived
from gravity data analysis. (after, Hoque, 2001)
Recent time earthquake activities suggest that there be some active faults associated with
tectonic trends TT3, TT4 and TT7 vulnerable to seismic activities. The active tectonic
trend TT3 is the dominant tectonic element in the active margin and occurs parallel and
close to the boundary between the passive margin and active margin (figure 4).
90'[
94'E
96"£
MISHMI
BLOCK 28'N
LEGEND
. . . Ophiolite ~nd Oli~t.ol;trome
(present Posll1on)
~
Ophiolite lind Olistostrome
(Palaeo-position)
- - - . - . Dauki Fault
~ Belt orSchuppcn
-rr-v-v- Eastern Boundary Thrust (EST)
• • • • • Palacogene. Neogene Boundary
,
lS'N
0
\\ 0
o
16'N
Fig. 4 Map showing active tectonic trends and earthquake vulnerability. Modified from: (Khan and
Akhter, 1999; Khan, 2000a).
Seismically active tectonic trend TT3 (Khan and Chouhan 1996) is one of the remarkable
features of the Bengal Basin and the terminator of all the NW-SE and N-S trending faults
(figure 2). There are number of NW-SE trending faults and lineaments (figure 5) with
normal and strike-slip components controlling the land sculpture (uplifted and subsided)
and the river courses in the delta. It is envisaged from the figure 5 that the western
6
ORIENTAL GEOGRAPHER
segment of the TT3 is dominated by NW-SE trending faults and,lineaments while the
eastern segment is dominated by NE-SW trending faults and lineaments. Among the
basement-controlled faults, the NW-SE trending Padma fault, Kartoya-Banar fault, TistaOld Brahmaputra fault, and the N-S trending Dhubri-Jamuna fault have direct bearing on
the regulation of the river courses.
These trends have possibly 'reactivated during the final upheaval of the Himalayan and
Indo-Burma tectonism during the Plio-Pleistocene. The Pleistocene uplifted blocks
appeared as horst blocks along the well-defined pre-exiting line of crustal weakness with
compensatory subsidence of the bordering region (Khandoker 1987). The subsidence and
upliftment is isostatically controUed. The upliftrnent of the Pleistocene blocks alongwith
the Shillong uplift is balanced by compensatory subsidence of Sy1het basin, Jamuna-Atrai
depression and Meghna depression. However, the positive isostatic anomaly for the
Shillong uplift is the indication of isostatically under compensation. This'
undercompensated state is potential for the seismogenic activation of Dauki fault zone.
o
315
315
270
90
45
270
225
180
180
Fig. 5 Rose diagram showing the dominant trend of faults and lineaments. West of TT3 segment
' •
(left) and East of TT3 segmeQt (right).
Neotectonic evidences are the positive indicators of an area whether it is seismically
active or not in recent geologic time. Bangladesh have been experienced with Neotectonic
activities those are documented with the field survey by various researchers. As early as
1899 (Report on the great eaIthquake of 12th June 1897, Memoir Geological Survey of
India, 29, p. 1-379), R. p. Oldham has documented few surface. tilting, surface ruptures,
upliftment and land slides due to Great Assam Earthquake of 1897 (figure 6). Neotectonic
evidence from fault escarpment with approximately 40m displacement between uplifted
and subsided blocks at Shajibazar, S-ylhet has been documented (figure 7). '
QUARTERNARY PALEO-GEOGRAPHY AND GEOHAZARD SCENARIO
7
It has also been calculated from various field observations that the GBM delta is being .
contracted in the north-south direction by 5 to 20 mm/yr and by 5 to 10 mm/yr in the eastwest direction. The upliftment and subsidence of the GBM delta are attributed to 3 to 5
mm/yr and 5 to 10 mm/yr respectively. Age dating data (Goodbred and Kuehl 2000) and
exposure duration of the Pleistocene terraces of the Madhupur and Barind tracts (Whitney
et al. 1999) are used to determine· the differential subsidence and upliftment rate of
different parts of the delta. The exposure duration data of Barind (25000 yrs) and
exposure duration data of Madhupur (110000 yrs) (Whitney et al. 1999) alongwith the age
dating data of the subsided Jamuna block lend support to the contemporaneous vertical
deformation of uplifted Barind and the subsided Jamuna depression. The relative·
upliftment of the Barind with respect to Jamuna depression is about 4.3 mm/yr and the
subsidence rate in the Sylhet basin is about 7.5 mm/yr. The relative subsidence between
Faridpur and Laxmipur region is about 3.9 mm/yr. It is clearly envisaged that the Sylhet
basin is relatively faster subsiding basin in the GBM delta.
Fig. 6 Tilting due to Great Assam Earthquake 1897, Chedrang Fault, Goalpara, Assam, India (left).
Surface ruptures upliftment and land slides due to Great Assam Earthquake of 1897 (right).
Fig. 7 Neotectonic evidence from fault escarpment with approximately 40m vertical displacement
between uplifted (left) and subsided (right) blocks at Shajibazar, Sylhet.
8
ORIENTAL GEOGRAPHER
QUATERNARY SEDIMENTATION AND SEDIMENT CONTAMINATION
The physiography and the geology of the catchment area of the rivers remaining almost
same from Quaternary period onward, but the river courses, sea-level and the basin
architecture were diversified in the mighty gateway to the sea i.e. in the Bay of Bengal.
The Bengal delta acting as storage for the sediments coming from the Himalayas where
uplift is coupled with erosion providing virtually inexhaustible sediment source. Tectonic
subsidence superimposed onto the sediment compaction and isostatic adjustment
straddles a perennial depression to accommodate huge amount of sediments.
The Bengal delta started building with proto-delta immediately after the breakup of the
Gondwanaland at approximately 126ma BP (Lindsay et al. 1991). From the beginning to
the late Tertiary time delta building processes in the region were very slow and the early
delta consists of relatively small-scale prograding clastic bodies alternating with mixed
carbonate/clastic and carbonate-platform assemblages along the Hinge zone. Delta
building processes enhanced during the late-Quaternary time just after the filling up of the
Indo-Gangetic depression (Himalayan foredeep). During this time Ganges-BrahmaputraMeghna river system carries the largest sediment loads and suspended matters in the
world. These sediments mainly consist of sand, silt and clay fragments, adsorbed and/or
redeposited suspended matters developed the delta complex.
Quaternary sedimentation in the delta is largely controlled and affected by the
Neotectonism and sea level changes. During the last glacial maxima (late Pleistocene)
rivers were flowing along its incised valley and gravel-coarser materials were deposited
and most of the delta plain had undergone subaerial exposure and erosion. These oxidized
deposits were followed by sands and sandy silt /silty sand of the transgressive tract due to
the rise of the sea level. The r~gressive finer particles, mostly silt and clay (figure 8)
followed the early deposits. The initiation of the regressive phase has promoted the
development of stream network and subsequently the finer particles were deposited /
redeposited in the cutoff meandering loops of the migrated channels (figure 9).
The hydrodynamic conditions of the major rivers and their stream network have largely
been controlled by the changes in the river stages. During the Holocene regressive phase
there was frequent oscillatory change in the sea-level (figure 8) in the GBM delta
straddles with the monsoon climate and tectonic instability that promoted channel
instabilities, migration and reconfigirration i.e. chute cutoff, neck cutoff, channel avulsion
(figure 9).
QUARTERNARY PALEO-GEOGRAPHY AND GEOHAZARD SCENARIO
9
Fig. 8 Stratigraphic correlation of Faridpur and Laxmipur log in relation to Holocene marine
transgression-regression. Here, MTT = Main Transgressive Tract, ORT = Oscillatory Regressive
Tract. Right Arrow indicates regression and Left Arrow indicates transgression. (Log source:
BGS/DFID/DPHE, 2001).
Fig. 9 Landsat TM showing paleo-meandering stream network in the GBM delta- Sylhet (left)
basin and- western Ganges delta (right).
During the Holocene marine transgression land-ocean interaction occurred along two
main flow domains of the river systems (figure 10). As a result, numbers of estuarine
lobes have been developed within the transgressive domains. The sedimentation within
these lobes bear the signature of tide dominated sedimentary features like lenticular
10
ORIENTAL GEOGRAPHER
bedding, bifurcating flaser bedding, ripple laminated cross-beds, .starve lenticules etc.
of complex interaction between fluvial
(Khan et al. 2001a). These lobes are the regions
.
and marine processes that might have acted as geochemical traps for the transitional
metals bonded in the finer grained sediments. In this coastal system, the mixing of
continental fresh and marine saline water usually leads to flocculation and accumulation
processes of suspended sediments (Sholkovitz 1976; Eisima 1986), which are controlled
by some physico-chemical properties of clay minerals, such as specific surface and ion
exchange capacity (Drever 1988). In such a slight saline and oxidized environment, iron
and manganese form coatings around the sediment particles those act as the nuclei for
aggregation. This also prevent trace element desorption from ion exchange site in addition
to acquiring fresh trace elements by adsorption from solution. In the lower estuarine
environment, adsorbed transition matters can desorb from the adsorption site during the
deposition of the coated particles due to the depth dependent reducing environment.
.
SSE
90E
(a) Late Early Holocene
2INL,-
SSE
~-----,J
92E
90E
SSE
(c) Late Holocene
Fig. 10 Conceptual position of Estuaries representing
Holocene in the GBM delta.
90E
92E
(d) Near Recent Time
lan~
and seawater interface lobes during the
QUARTERNARY PALEO-GEOGRAPHY AND GEOHAZARD SCENARIO
11
GROUNDWATER CONTAMINATION
Fluvial-deltaic sediments of the Holocene have formed the potential groundwater aquifer
.system in the GBM delta region. These sediments being contaminated by transition metal
including arsenic as discussed above, emerged as the sink for groundwater contamination
especially by arsenic. The aquifers in the Ganges Delta Plains of Bangladesh are largely
contaminated by arsenic. Arsenic contamination has so far been observed to occur in the
aquifers within the Holocene deposits situated in all the three flood-plain regions. These
flood-plain regions are bounded by three well-defined basinal depressions viz., GangesMahananda depression, Jamuna depression, and Meghna depression respectively (figure
11). These depressions are formed due to Neotectonic activities along which the Holocene
marine transgression has commenced. Regional occurrence and distribution of arsenic as
well as other few transition metals (BGSIDFIDIDPHE 2001) are restric:ted within the
shallow aquifer. Moreover, arsenic hot spots are related to the Holocene land-ocean
interface lobes and occur as pocket. These pockets of arsenic contaminated aquifers are
located mostly in the meandering belts of palaeo-streams. The aquifers in these palaeomeandering belts occur below the chute cut-off, chute bar and back swamp deposits
(figure 9). The aquifer sediments are dominantly channel-fill of fining upward sequence
(Imam et al. 1998). Intercalation of clay occurs due to frequent channel migration that has
resulted in the deposition as over-banks deposits on the channel-fill deposits. After the
deposition of underlying clay, the channel network started to develop as sea began to
retreat. As the sedimentation progressed, the intercalation of channel-fill and over-bank
deposits continued with the transformation of over-bank deposit towards more flood-plain
clay at the upper part of the aquifer zones (figure 12).
....
...
",'
LEGEND
____ International Boundary
_ _ __ Appr...
_
"'n'llI!" Limit ofQwltamu)' Marine Trans8re:.~ion
f.u1tandfaultTrace
..r,:-\ \
!
~.J
\ ,Z
;<
\.~
<
\:<
Fig. 11 Map showing the limit of Holocene transgression and arsenic contaminated region in the
GBM delta (after, Khan, et al. 2000).
f
12
ORIENTAL GEOGRAPHER
Most of the arsenic contaminated aquifers occur within 40-70m depth below the ground
level underlain by regional clay layer of variable thickness and depth. The study suggests
that this clay layer might have been deposited as the 'maximum marine flooding deposits'
of the high stand sea level in the Holocene. It is envisaged. that an extensive development
.
of grain coatings of iron oxyhydroxide (FeOOH) and manganese oxyhydroxide
(MnOOH) (AIam 1999) and the presence of arsenic in the coatings suggest that the
adsorption of arsenic has taken place from arsenic rich aqueous solution derived from the
catchment region. The release of arsenic in the groundwater from adsorption is due to the
reduction of FeOOH and MnOOH (Nickson et al. 1998). Acharyya (2000) also suggested
that reducing condition increases with depth and with the high water table and clayey
surface sediment layers impede the entry of air to the adjacent aquifer. The conceptual
model (figure 12) developed in the present study explains how an aquifer underlying the
chute cut-off, chute bar and swamp/oxbow lake deposits is more reducing than an aquifer
underlying the point-bar and/or channel deposits. Shallow sandy aquifers might be
reducing enough below 40-50 ft to release arsenic into solution. Deep aquifers below the
'maximum marine flooding deposits' are low in arsenic and iron because most of the
FeOOH and MnOOH coatings have been dissolved due to the prevalence of depth
dependent reducing condition in the depositional horizon. The fate of FeOOH in the
reducing condition is expressed in the following reactionFeOOH + H20 <=> Fe2+ + 30H
FeOOH (HnASn_) + H 20 <=>Fe2+ + HnAsn+ + 30HA
Floodplain
..............
.............
...............
..."....".,Jl•
.................
................
r/'.r/".JI .......
...............
................
...... r!' .... r/' •
......
......
...............
".r!'.",........
Poinl Bar
Channel Floor
B
Fig. 12 The conceptual model explains how the surface lithology deposited by the meandering
streams enhanced the aquifer's Redox state. The underlying chute cut-off, chute bar and
swamp/oxbow lake deposits act as barrier for aeration and recharge (left portion in A-B section),
whereas aquifer underlying the point-bar and / or channel deposits have the easy access to have
aeration and recharge (right portion in A-B section).
QUARTERNARY PALEO-GEOGRAPHY AND GEOHAZARD SCENARIO
13
Khan et al. (2001a) revealed that arsenic contaminated aquifer z~mes have undergone the
influence of strong alkaline and reducing environments. Such environments normally
occur in the back lagoon, swampy and estuarine conditions and are enriched in clay,
organic matter, peat and peaty clay. The occurrence of biogenic gas zones and/or some
zones of mineralization potential further support the prevalence of reducing environment.
The strong evidence of the release of arsenic in the aquatic system has been attributed to
reducing condition, which is enhanced by the bacterial activity.
EARTHQUAKE VULNERABILITY
Bangladesh have been experiencing quite frequent small to moderate earthquakes but the
frequency and the numbers of events are very large in the neighborhood of the country.
The tectonic neighborhood of the country is seismically very active. These are Himalayan
are, Shillong plateau & Dauki fault system in the north, Burmese arc and accretionary
wedges in the east and Naga-Disang-Haflong thrust zone in the northeast. Various lines of
evidences indicate that Bangladesh is extremely vulnerable to seismicity. Damaging
earthquakes have occurred numerous times in Bangladesh over the past 200 years.
However, in the recent time the frequent occurrence of small .to moderate earthquakes
inside Bangladesh along some well defined tectonic trends indicate the fresh tectonic
activities responsible for renewed seismicity. A crucial measure of the rate of seismic
productivity in Bangladesh is to measure its rate of contraction. Crustal deformation in the
GBM delta responds not only to its north-south collision and the approach of Burma from
the east, but also to a significant compaction caused by the sedimentation starvation.
Earthquake vulnerability of a region depends upon the frequency and magnitude of
earthquake, attenuation co-efficient and amplification factor of the ground surface.
Earthquake energy initially transmits through body wave (S & P waves) and then
transformed into Surface wave (Love & Rayleigh waves). Generally surface waves are
responsible for earthquake disaster due to horizontal and vertical propagation in the upper
200 ft of the ground label. The different amplification factor for different rock and soil
types in the upper 200 ft of the ground level is of serious concern for earthquake
vulnerability. In the GBM delta, the Neotectonic depressions and most of the lower delta
plain are most vulnerable sites for earthquake hazards because the zone within the upper
200 ft of the ground level in these depressions is mostly composed of unconsolidated clay
and silt materials (figure 2). In addition, the active tectonic trend viz., TT3 and Tn
having being located in the Neotectonic depressions have further made these depressions
vulnerable to earthquake hazard.
CONCLUSION
The Ganges-Brahmaputra-Meghna (GBM) delta is an extreme example of a deltaic
coastal region vulnerable to natural disasters like earthquake, differential subsidence and
shortening, arsenic contamination in groundwater, salt-water intrusion, storms surges, and
flooding. The GBM delta is situated at the lithospheric plate boundaries and its tectonic
activity is a major factor in the delta evolution and the related hazards. The prograding
14
ORIENTAL GEOGRAPH;ER
nature and the tectonic complexities of the delta are attributed to its ongoing dynamic
processes that have started in the Quaternary.
Arsenic poisoning in groundwater of the GBM delta of Bangladesh has emerged as one
of the most severe environmental health hazards. The groundwater arsenic contamination
of specific geologic time rock unit (Holocene aquifers) of GBM delta signifies a definite
geologic environment responsible for such hazard. This geological environment is related
to the paleogeographic status of the GBM delta during Quaternary such as marine
transgression-regression, Neotectonic upliftment and subsidence, duration of sub-aerial
exposure, and land-sea water interaction.
The seismicity and associated hazard assessment of the GBM delta are attributed to
sediment load & isostatic imbalance, short & long term subsidence rates, horizontal
tectonic rates, size & frequency of large earthquakes and the active fault kinematics in and
around GBM delta. The vulnerability of earthquake hazard in the regions of the GBM
delta is related to the ground sediment amplification factor and active tectonic trends. The
ground sediments (upto 200ft) in the subsided blocks are more vulnerable to earthquake
hazard than the ground sediments remained exposed subaerially during Quaternary.
RECOMMENDATIONS
In order to understand the GBM delta properly as an active stratigraphic and tectonic
entity Quaternary mapping and Neotectonic zoning of the entire GBM delta are prerequisite. Mapping of the Quaternary landforms followed by identification of crustal
lineaments & segmentation, active faults, palaeo-channels, liquefaction and other surface
evidences of Neotectonic activities need to be conducted.
The study envisaged that the sea level changes during the Quaternary period have major
bearing on the geohazard scenario of the GBM delta. It is recommended that the detail
study should be performed to identify the following a) Holocene system tracts &
stratigraphy, b) various crystalini.ty status of iron. oxides like, Goethite (FeOOH),
Hematite (FeZ03) etc and their stability in the different Redox state, c) ground surface
amplification factor. for different deposits, and d) age dating of various deposits for
determining differential deformation rate.
REFERENCES
Acharyya, S.K., Lahiri, S., Raymahashay, B.C. and Bhowmik, A. 2000. "Arsenic Toxicity of
Groundwater in Parts of the Bengal Basin in India and Bangladesh: The Role of
Quaternary Stratigraphy and Holocene Sea-level Fluctuation," Environmental Geology, V.
39 (10), p.1127-1137.
Alam, S.M.M. 1999. "Physico-chemical Status of Arsenic Contaminated Aquifers in Nawabganj
Sadar and its Surroundings, Nawabganj District," Unpublished M.Sc.Thesis, Department
of Geology, University of Dhaka, Dhaka, Bangladesh.
.
QUARTERNARY PALEO-GEOGRAPHY AND GEOHAZARD SCENARIO
15
Coleman, J.M. 1969. "Brahmaputra River: Channel Processes and Sedimentation," Sedimentary
Geology, V. 3, P. 129-239.
DPHEIDFIDIBGS 2001. In: D. G. Kinniburgh and P. L. Smedley (eds.) Arsenic Contamination of
Groundwater in Bangladesh, BGS Technical Report WC/OO/19, Vol. 2: Final Report, P.
267.
Drever, J. 1. 1988. The Geochemistry ofNatural Waters. 2nd ed., Prentice Hall, Inc. New Jersey, P.
437.
Eisima, D. 1986. "Flocculation and Deflocculation of the Suspended Matter in Estuaries,"
Netherlands Journal of Sea Research, V. 20 (2/3), P.183-199.
Goodbred Jr. S.L. and Kuehl, S.A. 2000. "The significance of Large Sediment Supply, Active
Tectonism, and Eustasy on Margin Sequence Development: Late Quaternary Stratigraphy
and Evolution of the Ganges-Brahmaputra Delta," Sedimentary Geology, V. 133, P. 227248
Gupta, R.P. and Sen, A.K. 1988. "Imprints of the Ninety-East Ridge in the Shillong Plateau, Indian
Shield," Tectonophysics, V. 154, pp. 335-341.
Hoque, M.A. 2001. "Geophysical and Geochemical Signatures and Its Geotectonic Significance
of the Eastern Folded· Belt (EFB) of the Bengal Basin," Unpublished M.Sc Thesis,
Department of Geology, University of Dhaka, Bangladesh.
Imam, B. Akhter, S.H. Khan, A. A. Hasan, M. A. and Ahmed, K.M. 1998. Sedimentological and
Mineralogical Studies on Aquifer Sediments within Bangladesh, Report Groundwater
Circle, BWDB, Dhaka, Bangladesh.
Islam, M.S. and Tooley, M.1. 1996. Holocene Sea-level Movements and Coastal Dynamics:
Records of the Last 9000 Years at Panigation Near Khulna, Bangladesh. In P. J. Atkins,
Alam, M.S. (Eds), Information Technology, Environment and Development in
Bangladesh, Durham, P. 84-100.
Islam, M.S. and Tooley, M. J. 1999. Coastal and Sea-level Changes During the Holocene in
Bangladesh, Quaternary International, V. 55, P. 61-75.
Khan, A.A. et. al. 2001a. The Causes of Arsenic Contamination in the Groundwater and Its
Remediation for Safe Water: A Case Study in Laxmipur Thana, Laxmipur District, Final
Study Report, Ministry of Science and Technology, Govt. of the People's Republic of
Bangladesh. P. 62.
Khan, A.A. 2000a. Fault Genesis and its Tectonic Significance in the Eastern Collision Margin of
the Indian Plate (abstract). Joint International Research Meeting 2000 on The Nature and
Tectonic Significance of Fault Zone Weakening, Tectonic Studies Group, Geological
Society of London, London, UK.
Khan, A.A. and Akhter, S.H. 1999. "Tectogenesis of the Bengal Geosyncline," Bangladesh Jour.
Geol., 18, P. 37-44.
Khan, A.A. and Chouhan, R.K.S. 1996. "Crustal Dynamics and the Tectonic Trends in the Bengal
Basin." J. Geodynamics, Vol. 22, No. 3/4 , Elsevier Science Ltd, P. 267-286.
16
ORIENTAL GEOGRAPHER
Khan, A.A., Akhter, S.H. and Alam, S.M.M. 2000. "Evidence of Holocene Transgression,
Dolomitization and the Source of Arsenic in the Bengal Delta," In A.M.O. Mohamed &
K.LAI Hosani (eds.) Geoengineering in Arid Lands, A.A. Balkema Pub!. The Netherlands,
P.351-355
Khan, A.A., Alam, S.M.M. and Akhter, S.H. 2901. "Role of Clay in Preventing Arsenic Dispersion
in the Ganges Delta of Bangladesh," In: K, Adachi and M. Fukue (eds.), Clay Science for
Engineering, A. A. Balkema Pub!., The Netherlands, P. 321-326.
Khandoker, RA. 1987. "Origin of Elevated Barind-Madhupur Areas, Bengal Basin: Result of
Neotectonic Activities," Bangladesh Journal of Geology, V. 6, p.I-9.
Lindsay, J.F., Holiday, D.W, and Hulbert, A.G. 1991. Sequence Stratigraphy and the Evolution of
the Ganges-Brahmaputra Cqmplex. Am. Assoc. Pet. Geol., Bull., V 75, P.1233-1254.
Milliman, J.D. Broadus, J.M. and Gable, F. 1989. Environmental and Economic Implications of .
Rising Sea-level and Subsiding Delta: the Nile and Bengal examples. AMBIO, V18 (6),
P.340-345.
Millimann, J.D. and Meade, RH. 1983. "World-wide Delivery of River Sediments to the Oc~ans,"
Journal of Geology, V 91, P. 1-21.
Morgan, J.P. and McIntire, WG. 1959. "Quaternary Geology of the Bengal Basin, East Pakistan
and India," Geological Society ofAmerican Bulletin, V. 70, P. 319-342.
Nickson, R, McArthur, J. M., Burges, WG., Ahmed, K.M., Ravenscroft, P. and Rahman, M. 1998.
"Arsenic Poisoning of Bangladesh Groundwater," Nature, V 395, P. 338.
Sholkovitz, E.R. 1976. Flocculation of Dissolved Organic and Inorganic Matter during the Mixing
of River Water and Seawater, Geochimica et Cosmochimica Acta, V40, P.831-845.
Silver, E.A. Breen, N.A. Prasetyo, H, and Hussong, D.M., 1986. "Multibeam study of the Flores
Backarc Thrust Belts, Indonesia," Journal of Geophysical Research, V 91, P.3489-3500.
Umitsu, M. 1993. "Late Quaternary Sedimentary Environments and Landforms in the Ganges
Delta," Sedimentary Geology, V 83, P. 177-186.
Wadia, D.N. 1975. Geology of India. Delhi: McGraw Hill, P. 531.
Whitney, J.W et. al. 1999. The Age and Isolation o{ the Madhupuf an~ Barind Land Tracts,
Ganges-Brahmaputra Delta, Bangladesh. Proceedings of International Seminar (20-24
September, 1999) on Quaternary Development and Coastal Hydrodynamics of the Ganges
Delta in Bangladesh, Abstract, P. 11.