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Fluvial facies architecture in small-scale river systems in the Upper Dupi
Tila Formation, northeast Bengal Basin, Bangladesh
M. Royhan Gani*, M. Mustafa Alam
Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh
Received 27 December 2002; revised 15 October 2003; accepted 3 November 2003
Abstract
The late stage basin-fill history of the fluvial Dupi Tila Group (Plio-Pleistocene) is described. These rocks have been deposited in the
Sylhet trough, a sub-basin of the Bengal Basin, in a foreland basin setting. This outcrop study, carried out in Sylhet, Bangladesh, presents the
first detailed facies analysis of the Upper Dupi Tila Formation. Four facies have been identified: trough cross-bedded sandstone (St), ripple
cross-laminated sandstone (Sr), finely laminated mud with ripples (Fl), and massive mud with rootlets (Fm). Facies analysis supplemented
with embedded Markov chain analysis, reveals small-scale fining-upward cycles (average 4.5 m thick). Facies architectural elements include
channel (CH), lateral accretion (LA), sandy bedforms (SB), and overbank fines (OF) with limited vertical and lateral connectivity of the sand
bodies. The average channel depth and width is 5 and 30 m, respectively. Sand body geometry ranges from tabular, to sheet, to shoestring
with a 0.45 net to gross ratio. This study shows that the Upper Dupi Tila Formation is composed of small-scale, mudstone-reach meandering
river deposits. In Bangladesh, the Dupi Tila Formation is the main aquifer presently being utilized. Understanding of facies architecture and
sand body geometry of this Formation is crucial in examining the issue of arsenic and other contaminations of ground water in Bangladesh.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: Bengal basin; Dupi Tila Formation; Fluvial deposits; Facies architecture; Arsenic hazard
1. Introduction
The Bengal Basin (Fig. 1), covering Bangladesh and part
of eastern India, is found within the junction of the
Himalayan Range to the north and Indo-Burman Range to
the east, and preserves the tectono-sedimentary history
(Cretaceous-Holocene) of these two orogenic provinces.
The Bengal Basin is known to develop a thick (20 km)
sedimentary succession (Curray, 1991) that has long been of
interest from the petroleum exploration point of view. Due
to some of the recent studies (Alam et al., 2003; Gani and
Alam, 2003, 1999) the tectono-sedimentary history of the
Bengal Basin is now better understood.
Dupi Tila Formation is the main aquifer bearing strata
for the entire Nation of Bangladesh, except for the
southwest corner. Although Bangladesh is now experien-
cing a public health crisis due to extreme arsenic
concentrations (locally concentrations reach 3.5 mg/l) in
ground water, there are no published accounts on the Dupi
Tila Group that focus on sedimentology and sand body
architecture. The objective of the present study from the
Sylhet Trough, a sub-basin in the northeast of the Bengal
Basin, is to provide the first detailed description of the
fluvial facies architecture of the Upper Dupi Tila
Formation and its possible influence on determining
aquifer behavior and the transport pathways of arsenic.
2. Geologic setting
The geologic evolution of the Bengal Basin (Fig. 1)
began in the Late Mesozoic with the break-up of
Gondwanaland and is on going. Alam et al. (2003) have
presented a revised tectonic and stratigraphic scenario of the
Bengal Basin emphasising three separate geo-tectonic
provinces within the basin. The Sylhet Trough (province
2), mostly underlain by continental crust, has accumulated
more than 18 km thick sedimentary strata. Post-Paleogene
history of the Sylhet Trough has been controlled mainly by
1367-9120/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jseaes.2003.11.003
Journal of Asian Earth Sciences 24 (2004) 225–236
www.elsevier.com/locate/jaes
* Corresponding author. Department of Geosciences, The University of
Texas at Dallas, P.O. Box 830688, FO 21, Richardson, TX 75083-0688,
USA. Tel.: þ1-972-883-2401; fax: þ1-972-883-2537.
E-mail address: mrg013000@utdallas.edu (M. Royhan Gani).
two tectonic events—increased movement along Dauki
Fault (upthrust), and westward advancement of Indo-
Burman Range due to the continuing oblique subduction
of Indian plate beneath the Burmese plate. In the late
Pliocene, when the Chittagong Tripura Fold Belt (CTFB)
was uplifted at the eastern margin of the Trough, a huge
volume of clastic sediments have been shed off, resulting in
the deposition of the Plio-Pleistocene Dupi Tila Group in
the resulting foreland basin (Fig. 1).
The Dupi Tila Formation was named by Evans (1932)
and interpreted as having been deposited in a fluvial
environment. In seismic stratigraphy, the Dupi Tila
Formation has been broadly subdivided into a lower sandy
unit and an upper argillaceous unit (Table 1) (Hiller and
Elahi, 1988). Together these two units are called Dupi Tila
Group (Alam et al., 2003). Johnson and Alam (1991)
mentioned that the alternating channel and flood plain
deposits of the Dupi Tila Formation indicate fining-upward
cycles of probable meandering river origin. The present
study focuses on the Pleistocene Upper Dupi Tila Formation
(Table 1).
3. Study area and methodology
The Upper Dupi Tila Formation is exposed on several
small hills on the north side of the Sylhet Town,
Bangladesh, in gently tilted strata (dip ,48). In this area,
four hills have been chosen for this study (Fig. 2). Vertical
cliff faces of these four hills are oriented NW-SE, range
in thickness from 8 to 20 m, and in lateral extent from 18
to 30 m.
Three approaches have been taken in interpreting the
sedimentological history of the study area. A vertical facies
analysis (1D) has been done along suitable lines in each of
the cliff faces to establish the basic facies types and to detail
inherent sedimentary features. Embedded Markov chain
analysis has been performed on facies transitions matrix to
Fig. 1. Generalized tectonic map of the Bengal Basin and surroundings (from Gani and Alam, 2003). Hinge zone separates the shallow Indian platform to
the northwest from the deeper Bengal foredeep to the southeast. The study area is indicated as X mark within the Sylhet Trough. (CTFB, Chittagong Tripura
Fold Belt).
M. Royhan Gani, M. Mustafa Alam / Journal of Asian Earth Sciences 24 (2004) 225–236226
determine whether there is a preferential vertical cycle in
these fluvial deposits. Photographs of the entire cliff faces
were made and facies architecture analysis (2D) was
conducted by tracing bounding surfaces from these
photographs. This type of analysis shows degrees of lateral
and vertical connectivity of sand bodies and helps to
reconstruct the paleohydraulics.
Dominant paleocurrent direction in the study sections is
towards southwest indicating that the cliff faces are oriented
roughly perpendicular to the paleoflow (Fig. 2). Sediments
were delivered to the study area from both north and east.
The northern source area includes the preexisting Shillong
plateau and its fringe sedimentary rocks, whereas the
eastern source includes the newly uplifted CTFB (Fig. 1).
Table 1
Stratigraphic succession of the Sylhet trough (revised from Hiller and Elahi, 1988). Seismic markers refer to the continuous and reasonably traceable seismic
reflectors in the seismic sections of the Sylhet trough
Age (approx.) Group Formation Seismic marker Thickness (max.) (m)
Holocene Alluvium
Pleistocene Dupi Tila Upper Dupi Tila Yellow 3350
Late Pliocene Lower Dupi Tila
Mid-Pliocene Tipam Girujan Clay 3500
Tipam Sandstone Brown
Early Plio to Miocene Surma Upper Surma Red 3900
Lower Surma Violet
Oligocene Barail (Undifferentiated) 7200
Kopili Shale Blue
Eocene to Paleocene Jaintia Sylhet Limestone
Tura Sandstone
Pre-Paleocene Undifferentiated sedimentary rocks (with some volcanics?) on the
continental basement complex
No data
Fig. 2. Map of Sylhet town, Bangladesh showing the locations of the studied hills. Note that dominant paleocurrent direction is roughly perpendicular to the
cliff faces.
M. Royhan Gani, M. Mustafa Alam / Journal of Asian Earth Sciences 24 (2004) 225–236 227
4. Results
4.1. Vertical facies analysis (1D)
Facies code of Miall (1978) has been used for the present
investigation because only four basic facies, which can be
readily codified applying this scheme, have been found for
the entire study area with only few exceptions. These four
facies are trough cross-bedded medium sandstone (St),
ripple cross-laminated very fine to fine sandstone (Sr), finely
laminated mud with ripple cross-lamination (Fl), and
massive mud with rootlets (Fm). The description and
interpretation of each of these facies (Fig. 3) are presented
in Table 2.
For the purpose of detailed characterization of facies
variability in the study sections, vertical lithologs were
generated for each of the hill sites (Fig. 4). The vertical
arrangement of four facies types is indicative of repetitive,
fining-upward cycles of fluvial origin (Fig. 4, Table 2). Each
of these upward fining cycles is marked, from oldest to
youngest, in the lithologs. The lateral variability and the
order of bounding surfaces of the facies in terms of
architectural element are discussed in a later section.
4.2. Embedded Markov chain analysis
In sedimentology, Markov chain analysis is used to
establish the prevalent pattern of vertical facies change in
Fig. 3. Photographs of four facies identified in the studied succession (see Table 2 for description and interpretation of the above facies types). (a) Trough cross-
stratified sandstone facies (St) showing poorly defined set of trough cross-stratification. The face of the exposure is oriented perpendicular to the cliff face of
hill-1. (b) Ripple cross-laminated sandstone facies (Sr) showing sets of ripple cross-lamination. (c) Laminated mudstone facies (Fl) showing finely laminated
mud with very fine sand interlaminae. Note the black root traces (arrowed). (d) Massive mudstone facies (Fm) showing structureless gray mudstone with well-
developed network of large roots (arrowed) preserved as concentric iron precipitation around a central hollow. (e) Facies Fm containing two tiny channels
(arrowed) filled with fine sandstone, probably originated from short-lived storm runoff on flood plains.
M. Royhan Gani, M. Mustafa Alam / Journal of Asian Earth Sciences 24 (2004) 225–236228
a stratigraphic succession. The technique filters out
significant facies transitions (i.e. signal) from randomly
occurring facies transitions (i.e. noise). The four measured
lithologs of this study give the opportunity to test whether
there is any preferred facies transition in the Upper Dupi
Tila Formation. Table 3a gives the one-step vertical facies
transitions matrix (facies in rows are overlain by facies in
columns) obtained from Fig. 4 with a total of 72 transitions.
Because there is a diagonal structural zero in the facies
transition matrix, a rigorous statistical method called
‘embedded Markov chain analysis’ (Powers and Easterling,
1982; Carr, 1982) has been applied. Expected transition
frequencies of Table 3b have been calculated by quasi-
independence method of Powers and Easterling (1982).
Table 3c gives the probability matrix (normalized differ-
ences of Table 3a and b) of the facies transitions. The
transitions with higher positive values in Table 3c indicate
preferred facies transitions with high probability of
occurrence.
Combining both statistically significant transitions (from
Table 3c) and geologically meaningful transitions, which
may not be statistically significant, a path diagram of facies
transitions was constructed (Fig. 5a). Facies St are most
commonly [normalized probability (NP) ¼ þ1.69] overlain
by facies Sr, and are less commonly (NP ¼ þ0.27) overlain
by facies Fl, which, in tern, are most commonly
(NP ¼ þ1.572) overlain by facies St. This facies transitions
path gives rise to a ‘modal cycle’ of decreasing flow energy
originating probably from repetitive chute cut-off of an
active channel (Fig. 5). When facies Fl is overlain by facies
Fm (NP ¼ þ0.192) it generates bilateral facies transitions
between facies Fm and Sr, which is interpreted as crevasse
splay/sheet-flood cycle of overbank deposits (Fig. 5). Path
diagram of Fig. 5a leads to an ideal fining-upward
succession (Fig. 5b) of typical of fluvial strata.
4.3. Facies architecture analysis (2D)
One of the most powerful and widely used techniques
in analyzing fluvial strata is architectural element
analysis. This technique emphasizes facies distribution
and associated bounding surfaces beyond the level of 1D
vertical lithologs to deduce the depositional scenario.
Miall (1985, 1996) has standardized basic architectural
element types and their bounding surface hierarchy in
fluvial succession. However, as most of the natural
processes are continuous, it may not be always easy to
apply Miall’s scheme strictly (Bridge, 1993). Some of
the recent publications on fluvial facies architecture (e.g.
Halfar et al., 1998; Holbrook, 2001; Jo and Chough,
2001) show various degrees of application of the Miall’s
scheme.
The objective of facies architecture analysis in the
present investigation is to apply the central theme of Miall
(1985, 1996) in interpreting depositional history and
depicting facies heterogeneity in the Upper Dupi Tila
Formation. Various architectural elements and the order of
bounding surfaces as applied in this study follow Miall’s
scheme: first-order surfaces bind bedforms (dunes and
ripples) cross sets, second-order surfaces bind bedforms
cosets (not used in this study), third-order surfaces define
macroform (e.g. bar) growth increments, fourth-order
surfaces define minor channel bases or tops of macroform
units, fifth-order surfaces represent bases of major channels.
However, as admitted previously, strict application of the
scheme is neither feasible nor desirable. Bedding diagrams
Table 2
Description and interpretation of identified facies in the present study
Facies types Description Interpretation
Trough cross-bedded
sandstone (St) (Fig. 3a)
Yellowish brown to pinkish red medium sandstone; mud clasts
(pebble- sized), mostly occurring at the basal part, sometimes
scattered throughout; poorly defined trough cross-stratification
with an average 25 cm thick set; invariably starting on an
erosion surface
Migration of sandy dunes on an erosion surface
of variable hierarchies; representing both
channel filling and bar developing
Rippled sandstone
(Sr) (Fig. 3b)
Fine to very fine yellowish sandstone; ripple cross-lamination,
mostly trough shaped with a few cm thick set; mostly with
organic-material/mud drapes along trough; asymmetric when
ripple forms preserved
Migration of current ripples; fluctuating currents
indicated by drapes of fine materials
Laminated mud with
ripples (Fl) (Fig. 3c)
Finely (a few mm) and parallel laminated mud with some very
fine sand interlaminae containing micro ripples; color variation
of white, red, pink, and yellow in mud laminae; occasional
rootlets, very occasional burrows disrupting laminae;
sometimes laterally discontinuous facies
Suspension settling with very weak current
mostly on bar top, and subordinately on flood
plain; color of the mud indicating oxidized and
well drained condition; small plant growth and
burrowing activity at the time of low stage of
channel flow
Massive mud with
rootlets (Fm) (Fig. 3d and e)
Dark grey to bluish grey mud; mostly structureless in
appearance; well established network of large root systems
preserved as concentric iron precipitation with a central hollow;
very occasional leaf impressions and coalified tree stems
Suspension settling over flood plains; prolonged
waterlogged condition with stable plant
development
M. Royhan Gani, M. Mustafa Alam / Journal of Asian Earth Sciences 24 (2004) 225–236 229
Fig. 4. Vertical lithologs of the four hills. The locations of the measured lines on cliff sections are shown in Figs. 6–8. (a) Hill-1 with seven fining-upward
cycles. (b) Hill-2 with six fining-upward cycles. (c) Hill-3 with two fining-upward cycles. (d) Hill-4 with two fining-upward cycles.
M. Royhan Gani, M. Mustafa Alam / Journal of Asian Earth Sciences 24 (2004) 225–236230
of the studied hills (except hill-2) have been produced,
which, in combination with the corresponding vertical
lithologs (Fig. 4), are suitable to perform facies architecture
analysis.
4.3.1. Hill-1
The base of C2 (cycle-2) in hill-1 (Figs. 4a and 6) rests on
a relatively thick (about 2 m) facies Fm, and is a prominent
erosion surface with abundant coal and mudstone clasts.
The bases of C3 through C6 are less prominent erosion
surfaces. Facies St found at the base of each cycle consists
of both single and multiple sets of trough cross-bedding.
Boundaries between facies St and Sr are mostly gradational.
The base of C7 is again a prominent erosion surface with
high erosional relief at the cut-bank side. Large slump
blocks (up to 1 m long) and lateral accretion surfaces are
also observed on the cut-bank side (Fig. 6b).
Identified architectural elements and their order of
bounding surfaces in hill-1 are shown in Fig. 6b. First-
order surfaces define the set boundaries of cross-bedded
facies (St). Boundaries between sandy (St and/or Sr) facies
and muddy (Fl and/or Fm) facies within each of the fining-
upward cycles are interpreted as fourth-order surfaces, as
they possibly indicate the end of a macroform (e.g. a bar)
development within a channel (Miall, 1994). Also, the
minor erosion surfaces (bases of C3 to C6) are clearly
fourth-order surfaces (Miall, 1994). The two prominent
erosion surfaces (bases of C2 and C7) thought to represent
bases of major channels are assigned as fifth-order surfaces
(Holbrook, 2001; Halfar et al., 1998). Identified architec-
tural elements for hill-1 are shown in Fig. 6b. In this case,
C2 to C6 represent repetitive development of element SB
(sandy bedforms, consisting of dunes and ripples), and the
lower half of C7 indicates lateral juxtaposition of elements
CH (channel) and LA (lateral accretion). OF (overbank
fines) elements are also identifiable in Fig. 6.
4.3.2. Hill-3
Following the same principles and criteria as described
under hill-1, architectural elements and order of bounding
surfaces in hill-3 are depicted in Fig. 7. Internal bounding
surface of the lower sandy portion of C1 is not observable.
The base of C2 represents a fifth-order surface. The sandy
portion of C2 develops lateral wings (elements SB) into
overlying OF element, and shows internal lateral accretion
surfaces (third-order surface) with average dip of 158 in a
direction roughly perpendicular to the paleoflow.
4.3.3. Hill-4
Although the internal bounding surfaces of the sandy
portion of C1 is not observable, its base probably indicates a
fifth-order surface. Two minor channels bounded by fourth-
order surfaces (Halfar et al., 1998) are characteristically
developed within thick OF element. SB element at the upper
part consisting of facies Sr is continuous across the
exposure.
5. Discussion
5.1. Depositional pattern
Results of the analyses presented above suggest that the
Upper Dupi Tila Formation in the Sylhet Trough was
deposited by single-thread, meandering river systems. The
presence of repetitive upward fining cycles with facies
transitions of decreasing flow energy, simple bank-attached
bar development with lateral accretion surfaces, and
channel confinement within thick flood plain deposits attest
to this interpretation.
Hill-1 represents thickest and best quality exposure
comparing to the other hills, and can be used as the basis for
section comparison. Two fifth-order surfaces of this hill
divide the entire succession into three units X, Y and Z,
from oldest to youngest (Fig. 6b), with each unit having its
own style of sedimentation. As hills-1, -2, and -3 are closely
Table 3a
One-step embedded matrix of observed facies transitions (facies in rows are
overlain by facies in columns). Data has been obtained from Fig. 4,
combining hill-1 (30 transitions), hill-2 (17 transitions), hill-3 (9
transitions), and hill-4 (16 transitions)
St Sr Fl Fm Total
St – 9 7 1 17
Sr 1 – 7 10 18
Fl 9 3 – 8 20
Fm 4 6 7 – 17
Total 14 18 21 19 72
Table 3c
Probability matrix of facies transitions. Calculation has been made from
Table 3a and b by applying normalized-difference formula of Power and
Easterling (1982)
St Sr Fl Fm
St – þ1.690 þ0.207 21.886
Sr 21.627 – 20.154 þ1.547
Fl þ1.572 21.560 – þ0.192
Fm 20.109 þ0.141 20.032 –
Table 3b
Estimated expected transition frequencies of Table 3a. Quasi-independence
method of Power and Easterling (1982) has been applied to calculate
transition frequencies
St Sr Fl Fm Total
St – 5.16 6.47 5.37 17.00
Sr 4.42 – 7.42 6.16 18.00
Fl 5.36 7.18 – 7.47 20.01
Fm 4.23 5.66 7.10 – 16.99
Total 14.01 18.00 20.99 19.00 72.00
M. Royhan Gani, M. Mustafa Alam / Journal of Asian Earth Sciences 24 (2004) 225–236 231
spaced (Fig. 2) these three units may be co-relatable within
these hills; and for hill-4 they may be comparable.
Unit X, exposed only at Hill-1, contains C1 (cycle 1)
(Fig. 6b), which is a well developed fining-upward cycle
with about 2 m thick Fm facies (Fig. 4a). This dark grey
overbank facies Fm contains abundant organic matter with
conspicuous and well-developed networks of large roots
(Table 2 and Fig. 3d). These criteria suggest water-logged
reducing condition (Collinson, 1996) and flood plain
stabilization for a prolonged period.
The erosional base of unit Y represents channel avulsion
on flood plain mud with the initiation of new channel cycles
(Fig. 6b). Based on the similarities of sedimentation pattern
and resulting sand body distribution, the entire succession of
hill-2 (Fig. 4b), and basal part of hill-3 (Fig. 7) are co-
relatable to this unit at hill-1. The unit represents repetitive
fining-upward cycles. Each cycle starts with element SB on
an erosion surface of fourth-order, indicating within-channel
development of sandy bedforms like dunes and ripples, and
ends up with thin deposition of facies Fl. Repetitive
development of element SB with intervening thin Fl facies
(Figs. 4a, b and 6b) probably indicates periodic chute cut-off
(Fig. 5a) of a meandering channel during major flood events.
Facies Fl shows poor lateral continuity in hill-2. The top of
unit Y is characterized by a thick development of element
OF similar in character as that of unit X.
Unit Z is well exposed in hill-1, -3, and -4 (Figs. 6b, 7
and 8). The base of this unit also represents a new cycle of
channel avulsion on flood plain mudstones. Channels with
distinct cut-banks are preserved in hill-1 and -4. Deformed
large mudstone blocks found close to the cut-bank in hill-1
suggests slumping of the cut-bank (BØe, 1988). In hill-3,
the channel sand body of unit Z shows evidence of lateral
migration from northwest to southeast. The successive
positions of channel migration are depicted with lateral
wings (Fig. 7, element SB) of levee deposits (Friend, 1983).
The stacking pattern of these wings strongly suggests that an
initial laterally migrating channel had turned into a rapidly
aggrading channel. The thick flood plain deposits of unit Z
in hill-4 (Fig. 8) contain two small anastomosed-channel
Fig. 5. (a) Path diagram of the facies transitions combining both objective (statistically significant, Table 3c) and subjective (geologically meaningful)
judgment. Double-lined arrows indicate high positive values (.þ1.5) of normalized probabilities (Table 3c). (b) Idealized fining-upward cycle (based on Fig.
5a) of the studied Upper Dupi Tila Formation with depositional interpretation. See text for discussion.
M. Royhan Gani, M. Mustafa Alam / Journal of Asian Earth Sciences 24 (2004) 225–236232
Fig. 6. Vertical cliff face of hill-1. Beds are gently tilted towards left (northwest). (a) Photograph of the cliff face. Note the standing man (circled) for scale.
(b) Overlay tracing of hill-1 showing architectural elements and their bounding surface hierarchy. Vertical line indicating cycle numbers is the measured
section of Fig. 4a. Paleocurrent direction is roughly perpendicular to the cliff face and towards the viewer (southwest).
Fig. 7. Overlay tracing of hill-3 showing architectural elements and their bounding surface hierarchy. Beds are gently tilted towards left (northwest). Vertical line
indicating cycle numbers is the measured section of Fig. 4c. Paleocurrent direction is roughly perpendicular to the cliff face and towards the viewer (southwest).
M. Royhan Gani, M. Mustafa Alam / Journal of Asian Earth Sciences 24 (2004) 225–236 233
deposits probably indicating ephemeral channels of storm
origin (e. g. Olsen, 1987). Element SB at the top part of Fig.
8 is indicative of sheet-flood deposits.
The exposure window of the present study is not wide
enough to go further and study the interrelationship of
channel aggradation, avulsion, and lateral migration.
However, facies architecture and stacking behavior of
sand bodies of units Y and Z are distinctly different from
each other. The concept of LAB model (Allen, 1978; Bridge
and Leeder, 1979), though it has been criticized (Heller and
Paola, 1996), can be applied in this connection. Due to
repeated chute cut-off cycles, unit Y shows restricted lateral
migration probably under low accommodation relative to
sediment supply. Whereas, the channel stacking pattern of
unit z, particularly thick OF element separating CH element,
indicates high accommodation relative to sediment supply.
This increase in accommodation may be due to an increase
in subsidence rate of the foreland basin as a result of
continuous loading of the adjacent fold-thrust belt (Fig. 1).
5.2. Paleohydraulics and sand body geometry
Several empirical formulae have been derived to
determine the depth and width of paleochannels from
sedimentary structure data. According to Leclair and Bridge
(2001), paleoflow depth of a fluvial channel can be
calculated from the set thickness of cross-bedding. The
formulae used are: Dune height ¼ 3 £ mean cross set
thickness; Flow depth ¼ (6 to 10) £ dune height. As the
average set thickness of trough cross-bedding of facies St is
25 cm, the average paleoflow depth is 5 m. Moreover,
according to Leeder (1973): w ¼ 1:5 £ h=tan u; where, w;
width of the channel, h; depth of the channel, u; dip angle of
lateral accretion surface. As the average u in the present
study is 158, the average channel width is 30 m. Therefore,
the average width to depth ratio of the meandering channels
that deposited the Upper Dupi Tila Formation is 6.
The terminology related to sand body geometry in fluvial
strata mostly depends on the scale of observation and the
scale of depositing channel. Considering the spatial scale of
present investigation, three different types of sand body
have been observed. The two small channels of unit Z at
hill-4 (Fig. 8) can be termed as shoe-string sand bodies;
element SB at the top part of unit Z at hill-4 represents a
blanket-type sheet sand body; and the sand bodies of unit Y
and at the lower part of unit Z probably indicate a tabular
geometry.
5.3. Implications for the arsenic hazard in Bangladesh
Upper Dupi Tila Formation serves as the main aquifer of
Bangladesh except for the south-west part of the country.
Arsenic is now a serious environmental hazard for Nepal,
West Bengal (India) and Bangladesh due to its presence in
the aquifer at high concentration. Out of 64 districts in
Bangladesh 25 districts have arsenic concentration above
the recommended level (0.05 mg/l) of WHO for drinking
water (locally concentration reaches 3.5 mg/l) affecting
more than 25 million people. However, sand body geometry
and facies architecture of these aquifer bearing strata are so
far very poorly understood. Due to the lack of any published
facies model, hydrogeologists working in Bangladesh are
mostly using a layer-cake model in constructing aquifer
panel diagrams.
Fig. 8. Overlay tracing of hill-4 showing architectural elements and their bounding surface hierarchy. Vertical line indicating cycle numbers is the measured
section of Fig. 4d. Paleocurrent direction is roughly perpendicular to the cliff face and towards the viewer.
M. Royhan Gani, M. Mustafa Alam / Journal of Asian Earth Sciences 24 (2004) 225–236234
According to a recent study by McArthur et al. (2001)
the distribution of organic matter, which is responsible
for reduction of FeOOH and release of sorbed arsenic in
the aquifer sediments, is the primary control on arsenic
pollution in Bangladesh. If this is the case then the
vulnerability of arsenic contamination in these fluvial
aquifers can be predicted by knowing the relative
distribution of overbank (aquiclude) and channel (aquifer)
deposits that controlled the sites of organic matter
accumulation in the fluvial basin. Shallow fluvial
stratigraphy may also control some of the redox cycling
of arsenic as well as movement of arsenic-contaminated
groundwater in Bangladesh (S. Goodbred pers. Comm.,
2002). Considering the above views, the present study
may contribute significantly in controlling arsenic
problem in Bangladesh as it gives the first detailed
description of bedding geometry of the Upper Dupi Tila
Formation deposited in mudstone-rich meandering fluvial
systems. This may suggests a far more compartmenta-
lized aquifer than has been previously understood or
modeled. Although dealing with only a portion of the
Upper Dupi Tila Formation in some limited outcrops,
this study constrains the dimension, spatial distribution,
and small-scale heterogeneity of the sand bodies of this
formation. The knowledge of this kind is an essential
pre-requisite for any meaningful groundwater flow
modeling designed to mitigate arsenic as well as other
groundwater hazard in Bangladesh. Moreover, as the
studied deposits are from a relatively small channel
system, they may have greater relevance to the shallow
aquifer than studies of the main Ganges or Brahmaputra
braid belts (S. Goodbred pers. Comm., 2002).
The facies architecture presented here can also be used as
analog for reservoir heterogeneity of small-scale mean-
dering river deposits. This architectural model can be
improved by further research incorporating log data from
boreholes done for groundwater withdrawal in this region.
6. Conclusions
Present study presents first detailed facies architectural
analysis of the Upper Dupi Tila Formation of the Sylhet
Trough, Bengal Basin. The following conclusions can be
made from this study:
1. Upper Dupi Tila Formation shows repetitive develop-
ment of fining-upward cycles (average 4.5 m thick)
containing four facies: trough cross-stratified sandstone
(St), ripple cross-laminated sandstone (Sr), finely
laminated mud with ripples (Fl), and massive mud with
rootlets (Fm).
2. Facies architecture analysis from 2D outcrops reveals
elements CH (channel), SB (sandy bedform), LA (lateral
accretion), and OF (overbank fines) with four different
orders of bounding surfaces.
3. Paleohydraulic reconstructions show that average depth
and width (at the bankfull stage) of the paleochannel was
5 and 30 m, respectively. Sand body geometry ranges
from tabular, to sheet, to shoestring.
4. The studied succession has been deposited by small-
scale, mudstone-rich meandering river systems with the
dominance of single-channel fluvial style characterized
by simple bank-attached bars.
5. The complex facies architecture of Upper Dupi Tila
Formation, which is the main aquifer bearing strata in
Bangladesh, indicates a less connected and highly
compartmentalized aquifer geometry. This type of
knowledge is essential for meaningful modeling of
groundwater flow to mitigate the arsenic as well as
other groundwater pollutions in Bangladesh.
Acknowledgements
The authors are very grateful to D. Nahid Sultana, the
wife of first author, for drafting the diagrams and editing the
manuscript. Thanks are due to D. Zafrul Hasan, the then
principal of School of Forestry, Sylhet, for giving logistic
support during the field work. Discussion with Janok
Bhattacharya has improved the quality of the paper. An
encouraging review from Steven Goodbred on an earlier
version of the manuscript is acknowledged. Finally, the
authors are grateful to the journal’s reviewers Mead
A. Allison and Brian J. Willis for their constructive
criticism on the manuscript.
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