4. bgj_vol-20_2014_ovi_p_19_34

Geomorphic Signatures of Active Tectonics from SylhetCity and Adjoining Areas, Surma Basin, Bangladesh

MD. MEHEDI HASAN OVI*, MD. SHARIF HOSSAIN KHAN & MD. MAHFUZUL HAQUE

AbstractWith a population of five million, Sylhet city is one of the densely populated

regions of Bangladesh. Sylhet city is situated at the northeastern part of Bangladesh. Thearea lies in a very complex geological setting, the uplifting Shillong Plateau in the north,the Chittagong Tripura Fold Belt (CTFB) in the south-east and the subsiding Surma Basinin the east. The complexity of structural setting and earthquake records suggest that theNE Bangladesh is still tectonically active. Apart from the anticlinal structures anddepressed areas, the study area represents relatively flat topography. In such flattopography, tectonic signatures are often buried or obscured by anthropogenicactivities. Geomorphology can be applied as a useful scientific tool to infer activetectonics from such region. In this study, geomorphic signatures of active tectonics ofSylhet and adjoining area has been identified by using remote sensing techniques. Rivermorphology, valley morphology and anticipated growth of folds from the structuralmaps and imageries have been used to identify geomorphic evidences of activetectonics in this area. Muellers Index has been used to measure the sinuosity of Surma,Dauki and Goyain Rivers at different reaches. Sinuosity measurement reveals thattopographic influence is the major controlling factor in river morphology (81.21% -99.48%) in comparison with hydraulic factors (0.52% - 18.79%). Four reaches of SurmaRiver and two reaches of Goyain River show sinuosity value very close to 1, indicatingsudden straightness of the river. Two large beheaded abandoned channels are identifiedin the north-western side of Sylhet Anticline. The abandonment may be due to E-Woriented ridge system that is already prominent in Jaintiapur area. Other geomorphicsignatures like incised channels, marshy zones, sudden changes in river flow direction,compressed meanders and river channel confluence are identified which provide cluesto the influence of active tectonics in the study area.Keywords: Geomorphology, Sinuosity, Muellers Index, Disaster preparedness, Active

tectonics

Introduction

Over the last two decades, active tectonics and geomorphology have evolved as acollaborative, quantitative and interdisciplinary research area due to recent development ingeochronological and remote sensing tools. Active tectonics is one of the key research areain order to answer the questions relating to evolution of plate boundary systems (LAY 2009).Furthermore, this branch is becoming important due to its application in land use planningand disaster management in densely populated areas, assessment of infrastructurevulnerability and evaluating seismic hazards (CLOETINGH & CORNU 2005).

Authors addresses: MD. MEHEDI HASAN OVI, PROF. DR. MD. SHARIF HOSSAIN KHAN and ASSOC. PROF. MD.MAHFUZUL HAQUE, Department of Geological Sciences, Jahangirnagar University, Savar, Dhaka 1342,Bangladesh. Email: [email protected]

BGJISSN 1028-6845

BANGLADESH GEOSCIENCE JOURNAL, VOL. 20, P. 19-34, 2014

20 MD. MEHEDI HASAN OVI, MD. SHARIF HOSSAIN KHAN & MD. MAHFUZUL HAQUE

In any active orogenic belt, recent and active tectonics can be regarded as the key factor foruplift or subsidence and present-day topographic development due to the result ofdifferences in rate of tectonic movement and erosional processes (ENGLAND & MOLNAR 1990,RIQUELMEA et al. 2003 and BISHOP 2007). Experimental research by OUCHI (1985) hasestablished that vertical deformation ofbasin is reflected by the alluvial river morphology.HOLBROOK & SCHUMM (1999) and SCHUMM et al. (2002) have provided comprehensiveevaluations of alluvial rivers response to active tectonics. Several theoretical models andfield investigations demonstrate that structural growth and associated deformation intectonically active regions have a direct control in shaping the landforms and drainagenetwork evolution, even the smallest changes in the topography affect the sinuosity of lowgradient rivers (FRIEND et al. 1999, HOLBROOK & SCHUMM 1999, BURBANK & ANDERSON 2001,VAN DER WOERD et al. 2001 and CHAMPEL et al. 2002). Therefore, investigation ofgeomorphic signatures from drainage pattern can assist in understanding landscapeevolution along with ongoing active tectonic processes.

Bangladesh is the worlds most densely populated country and Sylhet is the largest cityin the north-eastern part of the country with population density of about 21,500/km2 and thegrowth rate of about 8.43% per year (BANGLADESH BUREAU OF STATISTICS 2013).The studyarea is located in a complex plate boundary among Indian plate, Eurasian plate and Burmamicroplate and falls in the Earthquake Zone 1 of Seismic Zonation map of Bangladesh(Fig.1).

Fig. 1. Tectonic map of Bangladesh and surrounding areas (after JHONSON & ALAM, 1991; BOGMC,1997; UDDIN & LUNDBERG, 1998), Seismic zonation map of Bangladesh (GSB,1979) anddrainage network of the study area.









Geomorphic Signatures of Active Tectonics from Sylhet City and Adjoining Areas, Surma Basin, Bangladesh 21

About 67% of total remaining gas reserves and 9 out of 20 producing gas fields arelocated in this part of the country (PETROBANGLA 2012). The Sylhet city is expanding fast andto support the enormous pressure of 8.43% of population growth per year, the city needs aproper planning. Few authors have previously investigated active tectonics in this region(NANDY & DASGUPTA 1982, ISLAM et al. 1991, COATES et al. 1992, CHOWDHURY et al. 1996,ALAM & ISLAM 2004, BISWAS & GRASEMANN 2005, KHAN et al. 2006, BISWAS et al. 2006, 2007,BANERJEE et al. 2008, ADLAKHA et al. 2011 and ASIAN DISASTER PREPAREDNESS CENTER 2012)but a geomorphic approach towards active tectonics of Sylhet city and its surrounding areasyet to be explored.

The objective of this research paper is to identify and locate geomorphic signatures ofactive tectonics from Sylhet city and surrounding areas through the study of river channelmorphology along with their spatial behaviors in relation to the local anticlinal/synclinalstructures using satellite imagery. Understanding the signatures of active tectonics, throughgeomorphology can be used as cooperative evidence for present and future planning ofSylhet city.

Geological Setting

The Surma Basin is a foreland basin and a sub basin of Bengal Basin, which is located inthe northeastern Bangladesh (JOHNSON & ALAM 1991 and UDDIN & LUNDBERG 1998, 2004).The geometry and origin of the basin is linked with the relative plate tectonic movementsamong Indian plate, Eurasian plate and Burma microplate.

The basin had its origin owing to combination of two major tectonic events, emergenceof Shillong Massif to the north and subduction of Indian plate to the east (KRISHNAN 1982and HILLER & ELAHI 1989). The Sylhet trough and adjoining areas have evolved from apassive continental margin during pre-Oligocene time to a foreland basin which was linkedto the Indo-Burma orogeny during Oligocene to Miocene. Subsequently the basin became aforeland basin which is linked to south-directed over thrusting of the Shillong Plateau duringPliocene to Holocene (JOHNSON & ALAM 1991).

The north-south collision is considered to be more dominant compared to the east-westdirected collisional direction which is interpreted as south directed up thrust movementalong the Dauki Fault system during the Early Miocene epoch. The northsouth tonortheastsouthwest trending folds in the Sylhet Basin are believed to have been originateddominantly by the eastwest directed compressional force accompanying with platecollision and crustal shortening (HILLER & ELAHI 1989). The Dauki Fault is the mostprominent Fault system in the study area, which forms a zone of deformation rather than asingle fault and the exhumation rate of the Shillong Plateau range between 0.18 mm./year 0.53 mm./year (BISWAS & GRASEMANN 2005 and BISWAS et al. 2006, 2007). An elasticdeformation model based upon geodetic data yields a slip rate of about 11 mm/yr (BANERJEEet al. 2008). There are two distinct sets of structural elements in this region. One set includesE-W or NEE-SWW trending Jalalabad, Chhatak, Dupi Tila and Sylhet anticlines (Fig. 2). Theother set comprises N-S or NE-SW trending Atgram, Patharia, Fenchuganj, Batchia,Hararganj, Maulavi Bazar, Rashidpur, Habiganj, Beani Bazar and Kailas Tila anticlines (Fig.2).


Fig. 2. Structural map of the study area and geological cross-section through northern (E-Wline)portions of the Surma Basin (modified after HILLER &ELAHI, 1989).

The Kailas Tila Structure is located about 15 km. south-east of Sylhet city. It is anasymmetric anticline with NNE trend. The western flank is steeper than the eastern one andaffected by a thrust fault. In the north, the structural closure is bounded by a NE directednormal fault (IKM 1989). Fenchuganj, Rashidpur, and Maulavi Bazar are structurally furtheruplifted, each with thrusts at one flank. The maximum uplifted anticlines, Patharia andHararganj, have very pronounced thrust faults. The inclinations of the thrust planes at theseanticlines are in opposite directions and both are seismically blind in their crestal parts. TheJalalabad, Dupi Tila, and Sylhet structural highs are accompanied by normal faults at thecrests and higher flanks, predominantly parallel totheir longitudinal axes.

HILLER & ELAHI (1989) showed that there is a fault between Sunamganj Trough andChhatak Structure, the eastern edge of the Chhatak Structure is faulted and there are at leasttwo faults in between Sylhet and Beani Bazar Structure. Both the eastern and western edgeof the Atgram Structure is faulted. A distal splay fault of the NE trending Haflong Thrust FaultSystem may have created low-angle faults that involved structural movement in basementrocks beneath the Jalalabad-Sylhet-Dupi Tila anticlinal trend. These low-angle faults mayalso have caused the 15 km. northerly shift of the depo-center of the Kushiara Trough(HILLER &ELAHI 1989).










Major Landforms and Drainage Systems

Geomorphologically, the study area can be divided into two major units; low land andhilly area. The average elevation of the Sylhet structure is about 60 m. in ridges andabout 37 m. in valleys. There are some E-W directed elongated ridge and valleylocated towards the northeastern portion of the study area. The elevation of the ridgesranges from about 45 m. to 135 m, whereas the valleys range about 20 m. to 50 m. inelevation. In the floodplain area, the relief is usually about 12 m. from MSL but locallyirregular relief is observed alongside the river channels. The average elevation of leveesof abandoned channels in this region is about 6 m. high compared to surroundingfloodplains. The average elevation of the swampy areas (haors and bills) is about 9 m. to11 m.

Fig. 3. Flow paths of major rivers draining through the Surma Basin.

Surma, Kushiara, Hari, Dauki, Goyain and Piyain Rivers constitute the main drainagesystems of the study area (Fig.3). Surma (Barak) originates in Manipur Hills,Manipur state, India and drains through west and then southwest into Mizoram state. Thereit changes the course towards north into Assam state and drains west past the townof Silchar. The river next splits into two branches, Surma (north) and the Kushiara (south),which then enter Bangladesh and turn southwest. The Surma flows about 50 km. in NWdirection before it reaches Kanaighat. From this point on, the river flows SW about 50 km.along a narrow zone down to Golapganj, near Kailas Tila structure. After this point the riverturns west and flows about 35 km. maintaining almost a straight E-W course down to TokePur and then it turns to NW and flows about 34 km. to Chhatak. After this point on, the riverturns towards south with the name of Dhanu. The Dhanu changes its name to Ghora UtraRiver near Nikli Thana and drains down south to Bhairab to meet with Dhaleswari River.While the Kushiara flows past Maulavi Bazar Sadar and drains towards NW with the name












of Bibiyana River, after passing through Ajmirganj it flows towards SW with the name ofKalni River. At Lakhai Thana, the river meets with Dhaleswari river and becomesthe Meghna River at Bhairab Bazar, which flows south past Dhaka and enters thelower Padma River (Fig. 3).

Hari River originates in the Shillong and flows southward into Bangladesh near Sanhatarea and flows southward cutting across the ridges of Lala Khal area. Then the river flowstowards south about 7 km. and turns westward and travels about 6 km. Again it turns inNNW direction and travels about 7.5 km. and again turns westward about 5 km. before itjoins with the left branch of Dauki River and flows towards SW with a new name GoyainRiver, which flows up to Ratargul swampy forest and then it runs almost parallel with theSylhet structure until it reaches Kazir Gaon and turn towards SW before it meets with theSurma River at Chhatak. The right branch of the Dauki river flows westward with the nameof Piyain River meets with the Surma River at Chhatak (Fig.3). Besides the above mentionedriver systems there are numerous small channels (khal), abandoned channels, meander scarsand oxbow lakes are present in the study area.

Methodology

Geomorphic signatures are one of the most popular tools to delineate and describe roleof tectonics in any tectonically influenced landscapes, especially in the alluvial plains whereminute changes in tectonics are manifested in channel morphology. River morphology, ifnot trained or modified by anthropogenic activity, can be applied as a useful tool to identifythe influence of active tectonics of an area. Widely accepted signatures of active tectonicsare changes in channel slope or gradient, changes in sinuosity, channel incision andchannel shallowing due to aggradation and degradation, formation of marshy land, bankerosion, formation of linear valley along fault traces and sudden changes in channelmorphology (SEEBER & GORNITZ 1983, BULL 1984, OUCHI 1985, WELLS et al. 1998, REHA1993, HOLBROOK & SCHUMM 1999, SCHUMM et al. 2002 and REQUELMEA et al. 2003). For thepresent study detailed drainage network have been digitized and segmented into differentreaches according to slope breaks in the channel. Muellers Index has been calculatedbecause of its advantage over other traditional method in determining major controllingfactor of the channel morphology. The reaches are segmented based on the observationincorporating break in slope. Toposheets 78O/16, 83C/4 and 78P/13 (1:50,000 scale) fromGeological Survey of Bangladesh, the Shuttle Radar Topographic Mission (SRTM) 3 arcsecond data with resolution of about 90 m, LANDSAT TM data (Path: 136 and Row: 043,acquisition date: 20/11/2009; Path: 137 and Row: 043, acquisition date: 27/11/2009,resolution: 30 m.) form United States Geological Survey (USGS), borelog data form AsianDisaster Preparedness Center (ADPC) was collected and analyzed. Published structural mapof the area has been studied to infer the anticipated growth of folds and valley morphologyof the rivers.

Results and Discussion

KHAN et al. (2006) showed that there is variation in recent tentative uplift rate at Daukiand Kamalabari, Sylhet. The uplift rate is about 0.87 mm/year at Kamalabari area whereas0.5 mm/year along Dauki River section. These differential uplift rate indicates variation inlocal tectonic activity. The authors identified at least two strath terraces around Dauki Riversection and adjoining areas. Another terrace system is under development in the area. Thepresent study area is spatially and genetically related with the area mentioned in previousstudies. Based on their observations an attempt has been made to find geomorphicsignatures of active tectonics in and around Sylhet city. As the study area is covered by thickalluvium and anthropogenic activity also obscures the evidences, the main focus is directed


towards channel morphology. Rivers are the most delicate constituents of the landscape andit offers indications of minute aseismic tectonic activity in the subsurface. The followingchannel morphology have been identified:

Sinuosity: Muellers Index has been used to measure sinuosity. MUELLER (1968) hasredefined the sinuosity index to integrate hydraulic sinuosity (i.e. that freely developed bythe channel uninfluenced by valley-wall alignment) and topographic sinuosity (i.e. thatimparted by the geometry of the valley).The advantage of Muellers method over othertraditional methods is that it determines the percentage of channels deviation from itsstraight line course within the valley either due to hydraulic factors ortopographicinfluences. For the calculation of Muellers Index, the following measurements aremade(Fig. 4).

Fig.4. Model for the sinuosity measurement using Muellers Index (after GHOSH & MISTRI, 2012).

HSI (Hydraulic Sinuosity Index) = 100 %TSI (Topographic Sinuosity Index) = 100 %SSI (Standard Sinuosity Index) =Where,

CI (Channel Index) =

VI (Valley Index)=Air = the shortest air distance between the source and mouth of the streamCL = the length of the channel (thalweg) in the stream under studyVL =VLL = the left valley length along a stream, the length of a line which iseverywhere midway between the base of the valley walls.VLR = the right valley length along a stream, the length of a line which iseverywhere midway between the base of the valley walls.
















Table 1: Results of HSI and TSI measurement using Muellers Index.RiverName Reach

CL(Km.)

VLL(Km.)

VLR(Km.)

VL(Km.)

Air(Km.) CI VI

HSI(%) TSI (%) SSI

Surma Reach-1 28.9 28.3 28.3 28.3 16.1 1.80 1.76 4.69 95.31 1.02Surma Reach-2 33.5 33.1 32.94 33.02 24.8 1.35 1.33 5.52 94.48 1.01Surma Reach-3 11.57 11.5 11.63 11.57 10.6 1.09 1.09 0.52 99.48 1.00Surma Reach-4 13.1 13.1 13.0 13.05 10.9 1.20 1.20 2.27 97.73 1.00Surma Reach-5 16.4 16.3 16.0 16.15 9.32 1.76 1.73 3.53 96.47 1.02Surma Reach-6 17.0 16.4 16.6 16.5 7.0 2.43 2.36 5.00 95.00 1.03Surma Reach-7 7.5 7.58 7.38 7.48 7.33 1.02 1.02 11.76 88.24 1.00Surma Reach-8 34.7 34.2 33.9 34.05 21.4 1.62 1.59 4.89 95.11 1.02Dauki Reach-1 12.0 11.0 11.2 11.1 7.21 1.66 1.54 18.79 81.21 1.08Dauki Reach-2 9.97 9.67 9.75 9.71 6.1 1.63 1.59 6.72 93.28 1.03Goyain Reach-1 11.13 11.0 11.2 11.1 9.16 1.22 1.21 1.52 98.48 1.00Goyain Reach-2 21.9 21.6 21.7 21.65 16.3 1.34 1.33 4.46 95.54 1.01Goyain Reach-3 14.2 14.2 14.1 14.15 10.1 1.41 1.40 1.22 98.78 1.00

For measuring these Indexs, Surma River, Dauki River and Goyain River aresegmentedin to8, 2 and 3 reaches respectively (Fig. 5).

Fig. 5.(a) Different reaches of Surma, Goyain and Dauki River where sinuosity measurements arecarried out (SR, GR and DR indicates Surma River reach, Goyain River reach and Dauki Riverreach respectively); (b) and (c) Locations of bore holes and correlation of juxtaposedbores from the opposite banks of Surma River.



CL(Km.)

VLL(Km.)

VLR(Km.)

VL(Km.)

Air(Km.) CI VI

HSI(%) TSI (%) SSI






CL(Km.)

VLL(Km.)

VLR(Km.)

VL(Km.)

Air(Km.) CI VI

HSI(%) TSI (%) SSI





The results of the sinuosity analysis show that the TSI value range between 81.21% -99.48% and the HSI value range between 0.52% - 18.79% (Table 1). The results clearlyshow that topographic influence is the major controlling factor in river dynamics. Hydraulicfactors play a very insignificant role in shaping the channel morphology of the study area.Total 38 borelog data have been analyzed in the study area to find possible signatures ofstratigraphic disturbance. Along the straight line segment of Surma reach-3 the borelogs (Syl11 and Syl 31) and (Syl 3 and Syl 24) have been correlated. Straight line distance betweenSyl 11 and Syl 31 is about 912 m. and Syl 3 and Syl 24 is about 944 m. The juxtaposedbores from the two banks of Surma reveals similar observation. At Syl 11 and Syl 3 Dupi Tilais encountered at 15 m depth, whereas Dupi Tila is not encountered down to 30 m at Syl 31and Syl 24 (Fig. 5).

Compressed meanders: Generally, meandering rivers are characterized by asymmetrical and sinusoidal bends but bend asymmetry may occur because of riverconfinement (HICKIN 2003). Any kind of tectonic activity may also cause in asymmetry ofriver bends and an anomalously high sinuosity of compressed meanders is one of thecommon geomorphic signature of tectonic activity (HOLBROOK & SCHUMM 1999 andSCHUMM et al. 2002).

Fig. 6. Landsat TM image showing prominent geomorphic features of interest in the study area.

Compressed meanders are formed when a straight channel is forced to meanderbecause of perturbation of channel by folding, and the meanders tend to straighten overtime (SCHUMM et al. 1987, 2002).The study area is marked with several compressedmeanders (Fig. 6). Because of the high degree of compression, the sinuosity (CI) value ofSurma River at reach-6 is very high (2.43) (Table 1). The reasons behind this compressionmight be perturbation of channel by folding, changes in local slope, decrease in gradient orexcessive aggradation.

Beheaded abandoned channel: Two large beheaded abandoned channel have beenidentified in the study area (Fig. 6). The size of the NE oriented large beheaded abandonedchannels levee indicate that previously this was a mighty river and carried huge sediments.














The satellite image observation reveals that river channels those come down from theShillong Plateau at Gowainghat, Hadarpar, Dauki, Jaflong, Jaintiapur and Lala Khal areaused to flow from north to south direction, but at present channels tend to flow westward.The abandonment and the westward diversion of river channels in NW corner of study areamay be due to differential compression resulting in scattered depression and on northplunge of the Goyain trough, there may be slight ridge development causing beheading ofthe abandoned channels.

Fig. 7. Paleo-channel (Soblir gang) in the Gowainghat area (a) on Landsat TM image and (b) Fieldphotograph of the Soblir gang.

An east to west flowing paleo-channel named Soblir gang, was the downstream part ofHari River had probably been abandoned very recently and beheaded from Hari River (Fig. 7).

Incised channel: The Goyain River drains from north to south incising across the Soblirgang that used to flow from east to west direction but at present the paleo-channel isabandoned (Fig. 7). This incised nature of the Goyain River at Gowainghat is evident fromthe relative high levee on both banks of the river (Fig. 8). River incision is also found at thenorth eastern plunge and on the north western flank of the Sylhet structure (Fig. 8).

Fig. 8. (a) Spatial position of channel incision; on Landsat TM image; (b) Bed rock exposed on thenorth-western flank of Sylhet structure; (c) North-eastern plunge of the Sylhet structure due tochannel incision and (d) Elevated levee/Terrace of Goyain River at Gowainghat.














Field investigation reveals that the Goyain River drains incising across the paleo-channelthat used to flow in east to west direction but at present the paleo-channel is abandoned.The incised nature of Goyain River at Gowainghat and the river at Horipur may be becauseof local base level fall.

Sudden changes in river flow direction: Meandering Rivers generally follow symmetricaland sinusoidal meandering patterns but sudden changes in flow direction indicate that thereis a certain slope break. Lateral shifting of a channel course is a preferred way to adjustchanges in slope of the river channels with low stream power (GREGORY & SCHUMM 1987).The area west of Sylhet city (Toke Pur) marks the zone of interest of sudden changes in flowdirection (Fig. 6). Here the NWW- flowing Surma takes a very sharp northward turn andconfluences with the Goyain River at Raysontus Pur. Then it flows further northward andagain confluences with Piyain at Chhatak. After this point on, the river turns sharply towardsSW and flows about 7.5 km. straight course with sinuosity of 1.02 (Table 1). At this point,the river takes sharp NW turn. These sudden changes are indicative of changes in slope.

River channel confluence: In case of anticlinal uplift across a meandering river, OUCHI(1985) concluded that there will be a sinuosity decrease on the downstream side of the upliftas the valley floor is steepened. On the upstream side of the uplifted channel straightening isexpected but the damming effect may be more apparent. Surma, Piyain and Goyain Riversconfluences at near Chhatak (Ghoshpur kheyaghat) (Fig. 6). This channel confluence isindicative of anticlinal uplift at or near Chhatak. The patterns of the channel confluencesuggest that the anticlinal uplift process may still be active.

Marshy zones: Four marshy zones are found in the study area (Fig. 6). The marshy zone1 and 2 occupy the area in between Chhatak and Sylhet Structures which is actually theposition of the Goyain trough/syncline. The two prominent beheaded abandoned channelswere running through this area which indicate that this synclinal area may be underdifferential compression resulting in scattered depression and on north plunge of thesyncline there may be slight ridge development causing beheading of the abandonedchannels, this observation may also be evidenced by the presence of an E-W oriented paleochannel levees separate the marshy zone 2 and 3 (Fig. 6). Marshy zone 2 and 4 areseparated by the Sylhet structure and the average elevation of these marshy zones rangebetween 9-11 m. The formation of these marshy zones are associated with regional andlocal tectonics of the study area. Marshy zone 3 and 1 are possibly sag ponds caused by thesouthward thrust of the Dauki fault. Marshy zone 2 and 4 flanks the Sylhet anticline. Thesetwo marshy zones represent the synclinal part of the Sylhet anticline and marshy zone 2may be caused by both southward thrust of the Dauki and synclinal part of Sylhet anticline.


Fig. 9. Flexure zone between the NS and EW oriented structures on Landsat TM image.

Flexure zone: There are two distinct sets of structural styles are present in this region.On the northern side E-W or NEE-SWW trending anticlines suggest NS directed force isacting whereas towards the south of this zone N-S or NE-SW trending anticlines suggest EWdirected force is active. The limit of these two trends are separated by a narrow zone (Fig. 9).This narrow flexure zone may be the juncture and resultant of those differential directionalforces. The major river Surma drains along this narrow flexure zone.

Conclusions

Sinuosity measurement reveals that Topographic Sinuosity Index (TSI) is the majorcontrolling factor in river morphology (81.21% - 99.48%) in comparison with hydraulicfactors (HSI) (0.52% - 18.79%) and reach 1, 2, 3, 4 and 7 of Surma and reach 2 and 3 ofGoyain show Channel Index value very close to 1 which indicate straightness of channel inthese reaches (Table 1). Though the sinuosity of reach-1 and 2 of Surma are on the higherside because of some compressed meanders (Table 1). Correlation among borehole (Syl 11and Syl 31) and (Syl 3 and Syl 24) and the straight line distance between bore locations (Fig.5) suggests that there may be a fault running parallel along the Surma (part of reach-3). Lackof marker bed make it difficult to conclusively interpret Faults. In addition SARKER & AKTER(2011) compared the present river course with Rennels (1776) river course in the study area.In this comparison Reach-3 of Surma River followed almost the same straight course andsince 1776 it hasn't changed its course through the zone. The straightness of Surma alongthis narrow zone and absence of Dupi Tila Formation at Syl 31 and Syl 24 down to 30 meterdepth indicates that there may be some weak zone around the zone of flexure and thisflexure zone may be the tectonic boundary between two contrasting structural setting.NANDY (2001) also reported Sylhet fault around the same zone. It is very difficult to followdeformation in soft sediments. Dupi Tila Formation consists of soft sediment and the faultmay not propagate below the Tipam Group (soft sediment). Therefore, Seismic data isrequired to trace deformation in such soft sediments.




Conclusions





Conclusions



Acknowledgements

The authors would like to acknowledge USGS for providing necessary satelliteimageries and Department of Geological Sciences, Jahangirnagar University for providingrequired permissions and facilities to carry out this research work and cordial thanks toComprehensive Disaster Management Programme (CDMP) and Ministry of DisasterManagement and Relief, Government of the Peoples Republic of Bangladesh, for providingwith the borelogs of the study area.

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