morpho-sedimentary records at the brahmaputra river exit, ne himalaya: climate–tectonic interplay...

JOURNAL OF QUATERNARY SCIENCE (2009) 24(2) 175–188Copyright � 2008 John Wiley & Sons, Ltd.Published online 31 July 2008 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jqs.1190
Morpho-sedimentary records at the BrahmaputraRiver exit, NE Himalaya: climate–tectonicinterplay during the Late Pleistocene–HolocenePRADEEP SRIVASTAVA,1* SURENDRA S. BHAKUNI,1 KHAYINGSHING LUIREI2 and DILEEP K. MISRA11 Wadia Institute of Himalayan Geology, Dehradun, India2 Wadia Institute of Himalayan Geology, Northeast Unit, Itanagar, India

Srivastava, P., Bhakuni, S. S., Luirei, K. and Misra, D. K. 2009. Morpho-sedimentary records at the Brahmaputra River exit, NE Himalaya: climate–tectonic interplayduring the Late Pleistocene–Holocene. J. Quaternary Sci., Vol. 24 pp. 175–188. ISSN 0267-8179.

Received 17 May 2007; Revised 28 March 2008; Accepted 28 March 2008

ABSTRACT: Morphological and sedimentary records at the exit of Brahmaputra River at Pasighatin the NE Himalaya inform about the climate–tectonic interplay during the past ca. 15 ka. Thegeomorphology of the area comprises (1) fan terrace T3, (2) a high-angle fan (3) terrace T2, (4) terrace T1

and (5) a low-angle fan. Geomorphic consideration suggests that the fan terrace T3 and high-angle fansare the oldest units and were coeval. The low-angle fan is the youngest geomorphic unit. Sedimento-logical studies and optically stimulated luminescence chronology suggest that (i) fan terrace T3 formedbetween 13 and 10.5 ka and comprised multiple events of debris flows separated by the aggradation as
channel bars in a braided river environment; (ii) the high-angle fan formed during 15–10 ka andcomprises channel bar aggradation in braided river conditions; (iii) terrace T2 formed during 10–8 kadue to aggradation in a braided channel environment with lesser events of debris flows; (iv) terraceT1 formed during <7 and 3 ka took place as bars of the braided river. Sudden coarsening ofthe sediment indicated a tectonic rejuvenation in the provenance region between 7 and 3 ka; and(v) the low-angle fans dated to <3 ka formed due to aggradation in a small tributary joining theBrahmaputra River. This implies a phase when the main channel of the Brahmaputra did not floodregularly and the tributaries were actively aggrading. The sedimentation style and incision of thesegeomorphic units responded to contemporary climatic changes and uplift in the Siwalik range alongthe Himalayan Frontal Fault. Copyright # 2008 John Wiley & Sons, Ltd.
KEYWORDS: Brahmaputra River exit; terraces; OSL chronology; NE Himalaya.

Introduction

The morpho-sedimentary architecture of the mountain front offold and thrust belts is controlled by orogenic tectonics andglobal climatic changes. These therefore inform about the roleof climate and tectonics in the production, transport anddeposition of sediments and hence regional and temporalvariations in climate and on local versus regional tectonics. Therole of climate in river valley aggradation and incision has beenexamined along various river valleys like the Sutlej in NWHimalaya (Bookhagen, 2004), Marsyandi in the centralHimalaya (Pratt et al., 2004) and the Teesta River in theeastern Himalaya (Sinha, 1980; Meetei et al., 2007). The effectsof monsoon-induced erosion and formation of out-of-sequencethrusts within the Siwaliks in the Eastern Himalaya have beenreported by Mukul (2000) and Mukul et al. (2007).

* Correspondence to: P. Srivastava, Wadia Institute of Himalayan Geology,33 GMS Road, Dehradun 248001, India.E-mail: [email protected]

The Himalaya has two syntaxial bends at its eastern andwestern extremes. In the Namche Barwa region of the EasternSyntaxis Bend (ESB) where the fission track dating of themetamorphic core indicates rapid uplift and exhumation of�30 km during the last 4 Ma (Burg et al., 1997), and where themountain front has been deforming (Nakata, 1989). Upliftedfluvial terraces, abrupt changes in channel pattern and steeptriangular facets suggest tectonic activity along the N–S andNE–SW trending faults in the Lohit and Dibang river valleys inEastern Himalaya (Misra, 2007; Srivastava and Misra, 2008).However, few studies exist that chronologically constraingeomorphic evolution and examine it in the context of climaticand tectonic activities.

The present work provides the first geomorphic record fromthe exit point of the Brahmaputra River and investigates the roleof climatic changes and neotectonic activity in the develop-ment of the landscape around Pasighat area (Fig. 1). Geo-morphic indices like mountain front sinuosity and drainagebasin asymmetry for certain segments of the mountain frontare calculated to ascertain the degree of neotectonic activity inthe region. Optically stimulated luminescence (OSL) datingprovided the event chronology.

Figure 1 (A) Location map showing the Eastern Syntaxial Bend (ESB). (B) Location map of the study area. Note (i) the trend of the Ranaghat Fault F–Fand conjugate fault F–F1, (ii) offset of the Subansiri Formation along Sileng river fault. Asymmetry of drainage has been studied in the Pasighat–Ledumsection and mountain front sinuosity has been calculated for the Nari–Mikang sector (see Fig. 2)

176 JOURNAL OF QUATERNARY SCIENCE

Geological setting of study area

The study area is in the southern most part of the ESB and is inthe foothills of the NE Himalaya (Fig. 1(A)); the details arediscussed in Jain and Thakur (1974) and Tripathi et al. (1978).The Lesser Himalayan sequence in the study area comprises thecarbonate and sandstone sequence of the Miri–Buxa Group

Copyright � 2008 John Wiley & Sons, Ltd.

thrusted (along the Sillekorong Thrust) over the carbonaceousshale and dark grey sandstone of the Gondwana Group. TheGondwana rocks overlie the Siwalik rocks along the MainBoundary Thrust (MBT). The Tertiary rocks of the Siwalik,divided into the Subansiri and Kimin formations, correspond tothe Middle and Upper Siwaliks of the western Himalaya. TheSubansiri Formation is made up of massive sandstone, while the

J. Quaternary Sci., Vol. 24(2) 175–188 (2009)DOI: 10.1002/jqs

LATE PLEISTOCENE–HOLOCENE EVOLUTION OF PASIGHAT AREA, NE HIMALAYA 177

Kimin Formation is predominantly made up of conglomeratebeds and loose to consolidated arenite (Acharyya, 1982). TheMingo Thrust brings rocks of the Subansiri Formation overthe Kimin Formation.

The post-Tertiary sediments comprise conglomerate, sandand silty-sand making terraces and fans. Coalescing fans of allthe major rivers form a regional surface termed Fan Terrace (Karet al., 1997). Major structural elements of the area are theSillekorong Thrust, MBT, Mingo Thrust and the HimalayanFrontal Thrust (HFT, Fig. 1(B)). The HFT defines the boundarybetween the Siwalik rocks and Late Pleistocene alluvium. Theactivity along the HFT is responsible for the Quaternarylandscape developed in the mountain front (Thakur, 2004;Nakata, 1989). The Brahmaputra River (locally known asthe Siang) cuts orthogonal to this succession and exits themountains at Pasighat. The morphostratigraphy and theregional behaviour of the mountain front tectonic at thislocation are reported below.

Methodology

Topographical maps of the Survey of India, satellite data in thepublic domain (e.g. Google Earth and http://www.virtual-globe.info/globe.vgml) along with field survey around thePasighat helped to delineate geomorphic units and their spatialconfiguration. Sedimentary architecture of these was litho-logged. Facies identification and interpretation took cogni-sance of grain size, colour, physical structures, degree ofbioturbation, and lateral and vertical associations of theindividual sedimentary unit.

Mountain front sinuosity (Smf) for the Nari–Mikang sector iscalculated using Bull (1977). It is a ratio of the length of themountain front measured along the foot of the mountain at thepronounced break of slope (Lmf) to the straight-line length ofthe mountain front (Ls). It is given as Smf¼ Lmf/Ls (Fig. 2(A)).Drainage basin asymmetry was analysed to identify activestructures that were covered or poorly exposed. The asymmetryof drainage basin of the Ledum–Pasighat sector was estimatedby analysing the Survey of India maps of 1:50 000 scales assuggested by Cox et al. (2001). Parameter T¼Da/Dd, where T is

Figure 2 (A) Mountain front sinuosity in the Nari–Mikang segment. Note thvectors (97) of the Ledum–Pasighat segment. Inset shows the rose diagramconcentration in the SW quadrant


the transverse topographic symmetry factor, Da is the distancefrom the midline of the drainage basin to the midline of theactive meander belt, and Dd is the distance from the basinmidline to the basin divide (Fig. 2(B)) and ranged between 0 and1. T¼ 0 implies a perfectly symmetric basin and that of T¼ 1, atotally asymmetric one.

Optical dating was utilised to date the sediments ofgeomorphic units. This technique relies upon the fact thatduring the process of erosion and transportation daylightexposure of the minerals constituting the sediments reduces thegeological luminescence of these minerals to a near zeroresidual value (Aitken, 1998). On burial the exposure todaylight ceases and a reaccumulation of luminescence occursdue to irradiation from ambient radioactivity and continuesuntil excavation. Analysis of this luminescence and the ambientradiation environment provides the age of the burial event. Abasic parameter of concern in the dating of sediments is theextent of reduction for geological luminescence at the time ofburial. Any error in this would imply an overestimation of age.In the present context, existing evidence suggests that sufficientpredepositional zeroing during the fluvial transport of sedimentgrains occurred (Mukul et al., 2007; Srivastava and Misra,2008). Fifteen samples from different terraces were collectedand are detailed in Table 1. Samples SIT and SIT-1 werecollected from the top of fan terrace T3. Samples SIK-1 andTRG-1-3 were collected from the fan terrace of Siku and Torangrivers, respectively. Sample LBT-1 was collected from a fanterrace of the Siji River, which flows west of the Pasighat.Sample T2 was collected from the top of older valley Terrace T2

and SIB-2 was collected from the top of same terrace located atthe incised section of the Sibo River. The Sibo River is atributary to the Brahmaputra River and cuts through the T2

terrace. Sample SIG-7 was collected from the top of the youngervalley Terrace T1 of the Brahmaputra River at Sigar. SamplesRF-2 and RF-3 were collected from the sequence of thelow-angle fan. The general location of the samples is providedin Fig. 3.

Quartz grains from samples were extracted by treatingthe sample with HCl and H2O2, followed by heavy liquiddensity separation (using sodium polytungstate, density¼2.58 g cm�3). These grains were then sieved to obtain90–150mm size and etched using 40% HF for 80 min, followedby 12N HCl treatment for 40 min to remove any contribution

e variation in sinuosity (>1) in different segments. (B) Basin asymmetryof asymmetry vectors of Ledum–Pasighat segment with maximum


Table 1 Age data and results of bleaching studies of samples collected from Pasighat, NE Himalaya. In the palaeodose spread method of Colls et al.(2001) R2 is the correlation coefficient between palaeodose and the natural intensity of individual aliquots, where higher R2 values indicate poorbleaching. In the method of Clarke (1996), a coefficient of variation Sn> 0.1 indicates poor bleaching

S. no. Sample Depth (m) U (ppm) Th (ppm) K (%) Dose rate (Gy ka�1) Palaeodose (Gy) Age (ka) Bleaching study

R2 Sn

Fan terrace T31 SIT 1.0 2.3 12.8 3.0 3.9�0.4 44.6�1.8 11.5� 1.2 0.00 0.022 SIT-1 43.0 3.5 10.8 1.1 2.4�0.2 29.9�0.8 12.4� 1.1 0.16 0.04High-angle fans3 TRG-1 2.0 1.6 7.1 1.3 1.9�0.2 25.2�0.6 12.7� 1.1 0.26 0.534 TRG-2 38.0 3.3 11.2 1.5 2.7�0.2 30.7�0.7 11.2� 1.0 0.74 0.515 TRG-3 20.0 2.5 10.0 1.5 2.5�0.2 36.8�1.0 14.7� 1.3 0.06 0.246 SIK-1 15.0 1.1 6.1 1.5 2.0�0.4 23.3�1.1 11.8� 2.6 0.02 0.217 LBT 10.0 1.8 10.6 1.2 2.2�1.1 24.7�1.3 11.2� 1.1 0.05 0.11Terrace T28 T-2 0.6 2.0 9.6 1.9 2.9�0.2 26.5�0.7 9.1� 0.7 0.87 0.089 SIB-1 9.0 0.8 8.6 1.8 2.5�0.3 22.4�0.4 9.1� 1.0 0.00 0.4510 SIB-2 0.7 1.1 18.5 2.7 3.9�0.4 37.4�0.4 9.3� 1.0 0.44 0.11

Terrace T111 SIG-3 0.5 0.7 10.7 3.0 3.4�0.4 12.9�0.6 3.8� 0.4 0.15 0.4512 SIG-2 3.5 1.4 8.3 2.9 3.3�0.3 20.9�0.3 6.3� 0.6 0.09 0.5513 SIG-1 7.0 2.2 16.1 2.8 3.9�0.3 25.3�0.4 6.5� 0.6 0.39 0.56Low-angle fans14 RF-3 0.3 0.2 6.9 3.0 3.1�0.3 8.8�0.2 2.9� 0.3 0.61 0.2015 RF-2 5.5 2.0 12.1 2.8 4.3�0.4 31.7�1.0 7.3� 1.0 0.54 0.16


from alpha irradiation. The purity of quartz vis-a-vis feldsparcontamination was tested using infrared stimulated lumines-cence (IRSL). Contaminated samples were re-etched. The grainswere mounted on stainless-steel discs using Silko-Spraysilicone oil. 15–20 aliquots of 9 mm diameter were preparedfor analysis. Low photon count in some samples was the reasonfor making a larger aliquot and it is reasonably assumed that,though larger in size, these still approximate a small aliquot,given that only a few grains are bright enough to provideluminescence. Luminescence measurements were made on aRiso TL/OSL-12 system with a filtered halogen lamp source forstimulation. The OSL was recorded for 100 s at 1258C. Thedetection optics comprised a standard combination ofBG-39þU-340 optical filters mounted on an EMI 9635QAphotomultiplier tube. A 90Sr/90Y beta source delivering a doserate of 0.06 Gy s�1 was used for irradiation. Palaeodoseestimation was carried out using a five-point single-aliquotregeneration (SAR) protocol of Murray and Wintle (2000). Apreheat of 2208C 10 s�1 for natural and regeneration doses wasused and the analysis was confined to samples with a recyclingratio of 1� 0.1. The Initial 2 s of 100 s of a shine-down curvewas used for analysis. Uranium (238U), thorium (232Th) andpotassium (K) concentrations were measured by X-rayfluorescence (XRF) analysis. Cosmic gamma contributionwas calculated to be 150� 30mGy a�1 (Prescott and Stephan,1982) and water concentration was taken as 15%� 5% byweight.

Predepositional bleaching history of the sediments wasassessed by the two different methods as summarised inSrivastava et al. (2006). The first used the relationship betweenthe natural luminescence intensity and the palaeodose of theindividual aliquots. Incomplete bleaching results in higherluminescence and a proportionally elevated palaeodose withconsiderable variation from aliquot to aliquot (Colls et al.,2001). In contrast, palaeodose measurements on aliquots fromwell-bleached sediments are more consistent and do notcorrelate with natural luminescence. Thus the higher R2 values


(Pearson’s correlation coefficient) between aliquot palaeodoseand natural luminescence intensity indicate partial bleaching ofthe sample. The second method for assessing the degree ofbleaching considers that the range of aliquot palaeodose valuesis a measure of sample bleaching history (Clarke, 1996). Thestandard deviation of the palaeodose data set, divided by themean palaeodose, produces a coefficient of variation (Sn).Samples with Sn< 0.1 are considered to be well bleached,whereas higher values indicate inhomogeneous bleaching. Thesamples examined in this study show R2 and Sn varied from 0.0to 0.87 and from 0.02 to 0.56 respectively (Table 1). The higherR2 and Sn values from most samples indicated inhomogeneousbleaching and therefore the mean of the least 50% of thepalaeodose values was considered for age calculations of eachsample. This approach helped in discarding the ages derivingfrom aliquots that contained a larger population of poorlybleached quartz grains.

Geomorphology

At Pasighat, the Brahmaputra River shows four regionalsurfaces. Figure 4(A) provides a schematic of the configurationof these surfaces formed on the Siwalik rocks. The oldestsurface (T3) runs parallel to the mountain, is regional in natureand is developed at the exit of most rivers of the region and isformed by coalescing fans. This surface is also termed fanterrace T3. At Pasighat, T3 is �50 m from the riverbed, has anundulatory top and shows several NW–SE trending openfractures. This surface is gullied by several first-order tributariesand makes a vertical scarp as it meets the terrace T2. This ingeneral is termed ‘toe-cutting’ of the fan (Leeder and Hack,2001). Terrace T2 is an older river valley terrace at �26 m fromthe riverbed and runs parallel to the river channel. It isdeveloped only on the east bank. The T1 terrace, a younger river


Figure 3 Satellite picture showing detailed geomorphology around the Pasighat area (source: Google Earth). Note the location of geomorphic unitsand studied sections. (1) Location showing triangular facets exposed along the conjugate fault system of the Ranaghat Fault; see Fig. 6(B). (2) Section offan terrace T3 and surface exposure of the Ranaghat Fault; see Figs. 8 and 6(A). (3) High-angle fan section of the Torang River; see Fig. 7. (4) High-anglefan section along the Siku River; see Fig. 7. (5) T2 terrace section along the Sibo River; see Fig. 9(A). (6) T1 terrace section; see Fig. 9(B). (7) Low-angle fansection; see Fig. 10. A–B and C–D are section lines along which geomorphic profiles were made and are shown in Fig. 5. Base image copyright 2008Terrametrics, Inc. (http://www.truearth.com) and Europa Technologies Ltd. Reproduced with permission


valley terrace, is developed on both banks of the river and islocated at an elevation of �10 m from the riverbed. Thesurface T0 is the present-day riverbed. The river is braided, andthe bed load of the river comprises coarse- to medium-grainedsand. The T0 surface lies at a height of �150.5 m above meansea level. All these terrace sequences lie on the Siwalik rocks.

Several low-angle fans (with surface slope �48) initiate fromthe edge of T2 and sit on the T1 surface (Fig. 4(A)). These fans areincised to the level of T0 surface by the first-order streams.High-angle fans with surface (slope around 128) meet the riverfrom the left bank and attain a maximum height of 200 m fromthe riverbed. These are axially incised. The field relationship ofthe landforms suggests that, age-wise, T3> T2> T1> T0. Thelow-angle fan superimposed on T1 is the youngest geomorphicfeature in the area. The high-angle fans are the geomorphicequivalent of fan terrace T3.

The cross-section profile along A–B (Fig. 5) indicates that a�45 m thick terrace T3 sits on the Siwalik bedrock with noexposure of terraces T2 and T1. However, the section along C–Dsuggests �30 m exposure of T3 and terraces T2 and T1 areseparated by a vertical incision. T1 is present on both banks ofthe river (Fig. 5). This suggests that terrace T2 is a cut-and-fillterrace, rather than only a cut terrace, and T1 is a cut terrace. Inthis designation we consider that a surface formed due tocontinuous aggradation of the river is called a fill terrace and a


cut terrace is formed when incision of the river stops at a certainlevel and forms a terrace within the same fill.

Structure elements around thePasighat area

The Pasighat area is along the hinge zone of the Siang Antiform.The N–S and NW–SE oriented linear features on the surface T3

developed are, respectively, parallel to the western limb andaxial planar brittle fractures of the antiform. Active gravity faultshave developed parallel to the Brahmaputra River valley atPasighat. North of Pasighat lies the intraformational called theMingo thrust (equivalent to the North Pasighat Thrust ofAcharyya, 2007), which brings the Middle Siwalik over theUpper Siwalik rocks (Fig. 1(B)).

In the Pasighat area, the MBT and HFT are folded,conforming to the trend of the ESB. The HFT trends NE–SWand ESE–WSW and dips 35–458 towards the NW and NNE.Across the tectonic contact of HFT there is an abruptgeomorphic rise of �900 m from the flat Brahmaputra alluvialplain. Southwest of Pasighat, the basal part of the hanging wallof the HFT is characterised by a population of a conjugate set of


Figure 4 (A) Schematic of the geomorphic setup at Pasighat. FT3 is fan terrace T3; T2, T1 and T0 are successively younger terraces. HF is high-angle fanformed by the Torang River. LF is low-angle fan. RF is the Ranaghat Fault, as shown by dashed line. (B) Field photograph showing the three terraces atPasighat. Inset shows the location of Fig. 6(A). Note the sharp cliff of fan terrace T3, which is termed toe-cutting of the fan


brittle normal faults dipping gently (26–378) towards the NE/E/SE, where the HFT dips 31–358 towards the N and NE. Thereactivation of normal faults has offset the alluvial cover restingover the surface traces of these faults. Active normal faultsdipping towards the north are developed in the alluvial coverand control the local geomorphology of the region.


The region is also traversed by two major faults. TheRanaghat Fault (RF) is an active NW–SE trending andNE-dipping high-angle (>708) normal fault that truncates thetoe of terrace T3 (Figs 1(B) and 4(A) and (B)). In the downthrownside of this fault, the Brahmaputra River has been laterallyshifting eastward (Figs. 3 and 4(A)) where the terraces T2 and T1


Figure 5 Geomorphic sections along A–B and C–D (see Fig. 3). Sec-tion along A–B shows the fan terrace T3 and the high-angle fan. The fanterrace is resting on the Siwalik bedrock where the Ranaghat Fault isexposed. Section along C–D shows the development of fan terrace T3,terrace T2 and T1


are developed. The RF is traceable into underlying UpperSiwalik rocks (Fig. 6(A)) and the borehole data from the areaindicate that these rocks are downthrown by 25 m along thefault (data from Bridge Corporation Engineer, Pasighat).Associated with this fault is a set of faults dipping to anaverage of 378 towards N 478 E. Conjugate to the RF aredeveloped NE–SW trending steep to vertical dipping brittlenormal and thrust faults, as evidenced also by the presence oftriangular scarps in the area (Fig. 6(B)). The activity along thisfault seems to have played an important role in development of

Figure 6 (A) Fault trace of Ranaghat Fault and its associated fault. RF is trTriangular facets developed along the conjugate system of the Ranaghat Fau


the landscape around Pasighat. The other fault is the SilengFault, which is another WNW–ESE trending fault (Fig. 1(B)). Itextends over a distance of >20 km and is expressed in the formof a straight and linear course of the Sileng River. The fault hascaused sinistral displacement of the HFT by �6 km andindicates recent extensional tectonics in the region.

Seismicity data in the west of Pasighat show a number ofmicro earthquakes that originated at depths of �15–80 km. Theepicentres of these earthquakes lie immediate south of MBTand suggest the presence of deep-seated N–S trendingseiesmogenic faults in the area (Kayal, 2003). Further, theearthquake record (>M 4) around Pasighat indicates that herethe HFT and MBT are aseismic.

Lithofacies

Cliff exposures along the banks of the incised rivers wereexamined. Terrace T3 was studied at Ranaghat, on the right bankof the Brahmaputra River, and high-angle fan at Mebo inthe exposed section of the Torang River. The T2 terrace sequencewas studied in the Siku River and at Ranaghat. For the T1 terraceexposures at the left bank of the river at Sigar were studied.Low-angle fans were studied on the left bank of the BrahmaputraRiver at Ranaghat. Figures 3 and 4(A) provide a schematic of thestudied sections. The following lithofacies were identified.

Clast-supported imbricated gravels

This facies is represented in fan terrace T3, high-angle fan andterrace T2. These are typically 2–5 m thick, fining-upward unitscomprising rounded to subrounded, moderately sortedboulders with 95% of the clast being quartzitic, �3% fromthe Upper and Middle Siwaliks and rest of the rocks of theGondwana Group. Clast size ranges from 0.5 to 0.1 m andshow imbrication. Laterally the units are lensoidal withmaximum width up to 25 m and with erosional contacts(Fig. 7). A coarse to granular sand matrix makes 10–20% of thevolume and was due to sediment flow in a braided channelenvironment (Nemec and Steel, 1984).

aced out on the surface through the toe-cut cliff of fan terrace T3. (B)lt. Refer to Fig. 3 for location


Figure 7 (A) Stratigraphic sequence of high-angle fan of the Torang River. (B) Lensoidal units deposited by channel bar events. Clast-supportedimbricated gravel lithofacies is well represented in this section. (C) Stratigraphic sequence of high-angle fan studied along the Siku River.Clast-supported disorganised gravel lithofacies is represented in this section. Refer to Fig. 3 for location


Clast-supported disorganised gravels

These are 2–5 m thick units comprising angular to subrounded,poorly sorted, 1.0–0.5 m thick clasts. The individual units donot show any physical structures or imbrications and laterallyexhibit sheet-like geometry, with upper and lower contactsbeing erosional in nature (Fig. 7). Quartzite clasts make upmore than 95% of the clast population, while boulders derivedfrom the Gondwana (3%), Abor volcanics (1%) and the Buxadolomite (0.5%) form the rest. This facies may represent: (1)sedimentation by a hyperconcentrated flow due to rapid flowexpansion and tractional ceasing and as has been reported fromfan terraces of the Teesta River (Meetei et al., 2007); (2) flashyflood hydrograph generated within the channel, and in this casethe facies should be vertically associated with the channelsequence comprising flood plain fines, channel bars, etc. (Sohnet al., 1999) and therefore this case favours the formerinterpretation. Sedimentary sections of the fan terrace T3,high-angle fan and terrace T2 represent this lithofacies (Figs. 8and 9).

Matrix-supported gravels

These are 1–5 m thick units made up of matrix-supported,angular to subrounded boulders gravels that show crudebedding structures. The clast size ranges up to 20 cm and thepopulation in general is moderately sorted and made ofquartzite, Gondwana, Siwalik and Abor volcanic rocks, indecreasing order. The matrix, composed of fine sand to clayeysilt, is greyish-brown to reddish in colour. The geochemicalanalysis of the matrix from fan terrace T3 shows a high contentof zinc (956 ppm) and chromium (565 ppm), indicating itssource from the mineralised carbonaceous shale of the


Gondwana rocks. In places, the unit shows the presence ofthin (�50 cm) lensoidal units of medium- grained sand. Theupper and lower contacts of the beds are erosive. This faciesrepresents sedimentation by debris flow (Nemec and Steel,1984; Meetei et al., 2007) where the sandy lenses mayrepresent the formation of small channels during subsidingphases of the debris flow event. The facies is represented in thefan terrace T3 and terrace T2 (Figs. 8 and 9).

Cross-bedded sand

This lithofacies is grey-coloured, planar cross-bedded, fine–coarse sand and 0.3–0.5 m thick. The unit occurs in cossets andgenerally is overlain by parallel laminated fine sand or clastsupported gravelly lithofacies and does not show anyassociation with fine-grained overbank deposits. Quartziticpebbles are sometimes present at the base of the unit. Thisfacies represents deposition by mega ripples in a braidedchannel environment (Ashley, 1990) and is developed in thesequences of terraces T1 and T2 and low-angle fan (Fig. 10).

Parallel laminated fine sand

This is 0.2–1.0 m thick parallel-laminated fine sand. Individuallamina is up to 2 mm thick and often shows discordance. Thefacies overlies cross-bedded medium sand units and transformsupwards into alternating fine sand and silt lithofacies. Itrepresents sedimentation on a bar during a waning flood (Miall,1996). This lithofacies is represented in the sequences ofterrace T1 and low-angle fan.


Figure 9 (A) Stratigraphic sequence of terrace T2 studied along the Sibo River. Note the cross-bedded channel facies at the base. (B) Stratigraphicsequence of terrace T1. Note the coarsening upward sedimentary sequence and that the sequence is mainly made up of channel bar facies. Refer toFig. 3 for location

Figure 8 Stratigraphic sequence of fan terrace T3 at Ranaghat. Note that the stratigraphic architecture of the sequence is dominated by debris flow andhyperconcentrated flows. Sand lenses mark the termination of the flow event

Copyright � 2008 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 24(2) 175–188 (2009)DOI: 10.1002/jqs


Figure 10 (A) Sedimentary sequence of a low-angle fan. (B) Field photograph showing main channel and the low-angle fan sequence. (C) Laterallitholog of the sequence. Note the presence of fining-upward coarse cross-bedded sand overlain by fining-upward cross-bedded fine sand. Refer toFig. 3 for location


Alternating fine sand and silt

This is made up of 0.2–0.6 m thick alternating units of fine sandand silt. The fine sand units are grey and cross-rippled and thesilty units are parallel laminated with a moderate degree ofbioturbation. This facies occurs within the sequences of thelow-angle fan and represents levee deposits.

Chronology and interpretation ofsedimentary sequences

Optical ages, along with basic data on dose rates and results ofbleaching analysis, are given in Table 1. In the following thechronology of representative sections is discussed.

Fan terrace T3

A �45 m thick sequence of this terrace is exposed at Ranaghat(Fig. 8). The sequence rests on the inclined and eroded surfaceof the Upper and Middle Siwalik rocks. The basal 25 m of thesequence is made up of matrix-supported gravel, with anerosional base that follows an 18 m thick unit of theclast-supported imbricated gravel, followed by a 2 m thickunit of matrix-supported gravel. This indicates that most of the


sequence is made up of two events of debris flow separated by aphase when the aggradation took place in the form of channelbars in a braided river environment. The OSL data suggest thatthe sequence formed between 12.4� 1.1 ka (SIT-1) and11.5� 1.2 ka (SIT); considering the errors it is suggested thatsedimentation on the fan terrace T3 occurred between �13 kaand 10.5 ka. The termination of this fan sedimentation isfollowed by the activation of the Ranaghat Fault, resulting intoe-cutting of the fan. The lateral erosion of the fan terrace andthe formation of a steep scarp at the junction with the terrace T2

is evidence of tectonically driven toe-cutting (Leeder and Hack,2001).

High-angle fan

The high-angle fan sequence was studied in the exposedsections of Torang and Siku rivers (Figs 4(A) and 7). The Torangriver fan, which has a westward slope of �128, is incisedaxially, with a maximum thickness of �40 m in the proximaland �5 m towards the distal part of the fan. The 40 m thicksequence in the proximal part that rests on the Upper Siwalikrocks is composed of five fining-upward cycles of clast-supported disorganised gravels. The OSL chronology of asample near the base of the section gives an age of 12.7� 1.1 ka(TRG-1) and that near the top of 11.2� 1.0 ka (TRG-2). Another20 m thick section nearly 200 m downstream, comprising threedistinct events of clast-supported imbricated gravels, yielded a



basal age of 14.7� 1.3 ka (TRG-3). The high-angle fan is madeup of deposits of hyperconcentrated flows proximally thattowards downstream transforms into braid bar setting, and thesequence was deposited between �15 and 10 ka.

Terrace T2

A 10 m thick section was studied (288 04.550 N, 958 18.940 E)along the Sebo River that incised into the terrace T2 (Figs 4(A)and 9). The sequence is divisible into three depositional events,namely, from base onwards: (i) planar cross-bedded sand withdistinct channel lag gravels; (ii) with erosional base a 3.5 mthick unit of clast-supported imbricated gravel; (iii) a 4 m thickunit of matrix-supported gravels. A 1.2 m thick unit ofbioturbated fine sand caps the sequence (Fig. 9(A)). Thisindicates that the T2 terrace deposits are formed by a braidedchannel deposit environment with few events of debris flow.The OSL chronology of the sample at the base yielded an age of9.1� 1.0 ka (SIB-1) and that near the top 9.3� 1.0 ka (SIB-2).The top 2 m of another section at Ranaghat indicates thepresence of clast-supported imbricated gravels and yielded anage of 9.1� 0.7 ka (T-2). Allowing for age errors, this impliesthat terrace T2 formed during 10–8 ka and its incision by theriver to form T1 occurred after 8 ka.

Terrace T1

The stratigraphy of terrace T1 was studied on the left bank of theriver at Sigar (Fig. 9(B)). The representative section is�7 m thickand is mainly composed of cross-bedded sand with twodepositional events. The basal event is 2 m thick, the meangrain size of the sand being 2 Ø (medium sand). This is followedby a 5 m thick fining-upward, multistoried sand body,composed of very coarse-grained (mean grain size 0.7 Ø),and makes a coarsening upward sequence that suggestsincreased fluvial energy, which may be due to: (1) increasedchannel gradient because of tectonic uplift in the hinterland(Srivastava et al., 2001); (2) higher precipitation in thecatchment and improved hydrological conditions. The OSLages suggest that the sedimentation started before 7 ka andcontinued until 3 ka (Table 1). In the present case the increase ingrain size is followed by the channel incision at �3 ka.

Low-angle fan

The low-angle fan is superimposed on the terrace T1 and slopestowards the channel at �48. The stratigraphy of these fansincorporates the interplay of lateral flooding of the Brahmapu-tra River and sedimentation of the tributary channels. The 6.0 mthick sequence (288 6.230 N, 958 17.970 E) is made up of planarcross-bedded medium-grained sand overlain by a 1 m thickbioturbated clayey silt unit that makes fining-upward channeldeposits of the main channel (the Brahmaputra River). Thechannel deposits are overlain by alternations of two lithofacies,namely (1) alternating fine sand and silt and (2) cross-beddedsand. The alternating fine sand and silt units laterally possesssheet-like geometry, whereas the cross-bedded fine sand faciesmake lenses of 3–4 m width and �1 m thickness withpalaeocurrent directed perpendicularly towards the mainchannel. These fine-sand units are deposits of the tributaryfan and alternating fine sand and silt represent the leveedeposits and active flooding phases of the Brahmaputra River


(Fig. 10). The tributary fan building phases may thereforerepresent phases when the main channel was not floodingregularly and the tributaries were actively aggrading. The OSLages suggest that these fans formed after 3 ka. This was thephase when the Brahmaputra River experienced relativelylesser-magnitude floods. The basal part of the sequences thatcorresponds to sedimentation of terrace T1 was dated to7.3� 1.0 ka (RF-2).

Geomorphic indices

The mountain front sinuosity index (Smf) of the Nari-Mikangsector is 1.38 and indicates that the sector is neotectonicallyactive (Fig. 2(A)). Smf values lower than 1.4 indicate tectonicallyactive fronts (Keller, 1986), while higher Smf values (>3) arenormally associated with inactive fronts. The asymmetry of thedrainage basin in the Ledum–Pasighat segment to the west ofthe Pasighat had T (n¼ 97) ranging from 0.0 to 0.80 with amean vector 0.30 and bearing 1838, with a concentration in theSW quadrant (Fig. 2(B)). Such values of T indicate the degree ofground tilt of the terrain in a preferential direction and hereindicate southwestward migration of the streams to the west ofPasighat. Therefore, both the Smf and T indices indicate that theHFT and other transverse faults in the area have beenneotectonically active.

Discussion

Climate change and tectonic processes are the two competingnatural forces that shape the morphotectonic features of thedrainage basins in the mountains and in their frontal reaches(Starkel, 2003). In the NE Himalaya where channelling of largerivers like the Brahmaputra and Teesta through the ESB viafocused erosion impacts orogenic evolution, the relationshipbetween climate, erosion, deposition and tectonic activities isnot understood. In the whole Himalayan region the Brahma-putra basin experiences relatively higher rates of uplift due tointense erosion and subsequent isostatic rebound (Galy andFrance-Lanord, 2001; Singh, 2006).

The palaeoclimatic scenario of the Indian subcontinentduring the last 15 ka, based on continental pollen and oceanrecords, suggests that following the Last Glacial Maximum(LGM) (24–18 ka) the SW Indian monsoon ameliorated atca. 12 ka BP and peaked at ca. 9 ka BP (Chauhan, 2003; Dillet al., 2003). Rivers originating in the sub-Himalaya experi-enced reduced discharge due to reduced rainfall at ca. 3.5 ka(Tripathi et al., 2004). During drier conditions (1) decreasedvegetation cover facilitates the production of sediment in thecatchment and (2) the rivers aggrade due to increased sedimentload and reduced hydraulic energy. However, during wetterconditions the rivers are likely to incise because of increasedhydraulic energy and water–sediment ratio. During the climatictransition from arider to wetter the rivers largely deposit theirsediment load at the low-lying mountain front and form fans.Alternatively, tectonic activity can also influence the rivergradient and its energy, including the production of sediment.Therefore the fluvial archives can be used as a record ofneotectonic activity and climate interaction in the NEHimalaya.

Figure 11 provides an overview of the morphostratigraphicevolution of the area around Pasighat vis-a-vis climate andtectonic perturbations that can be ascribed to climatic factors


Figure 11 A morpho-sedimentary model showing Late Pleistocene–Holocene evolution of the landscape at Pasighat


such as changes in monsoonal precipitation and related fluvialdischarge during the last 15 ka and can be summarised asfollows.

During 15–10 ka fan terraces T3 and high-angle fansequences were formed when the river catchment wascharacterised by episodic flash floods, resulting in debris flowsand hyperconcentrated flows. Prior to 15 ka was the phase of(1) widespread aridity in the region with significant decrease inriver discharge into the Bay of Bengal, as suggested by thepalaeosalinity records (Prell and Kutzbach, 1987), and (2) lesservegetation cover and high sediment production, and the riverswere incapable of transporting the available sediment (Meeteiet al., 2007). This more arid climate of >15 ka facilitatedsediment production and river valley aggradation in themountains (Srivastava et al., 2008) when the river was notable to transport much sediment load to its mountain front. Thisprobably explains the fact that a sedimentary record older than15 ka is not present in the area. The period 15–10 ka was whenthe SW Indian monsoon was strengthening after the LGM,inducing higher discharge and erosive capacity into the rivers.This led to massive mobilisation of sediments to the mountainfront and formation of fan sequences in the area (fan terrace T3

and high-angle fan). The fan terrace T3 during the terminalphase of sedimentation was toe-cut by the river, indicating atectonic upliftment along the fault. The fault exposed in thebedrock is traceable to the toe of the cliff section of fanterrace T3 (Figs 4(B) and 6(A)) and therefore we suggest thatupliftment along the Ranaghat Fault at ca. 10 ka wasresponsible for formation of steep vertical scarp at the toe ofthe fan sequence and formation of the terrace T3. The process oftoe-cutting can lead to complete fan dissection and may be aresponse of tectonic tilting, fault propagation or a combinationof these variables. It gives rise to steep scarps at the toe of the fanand upstream incision of the fan from the scarp (Leeder andHack, 2001). Such field examples of the toe-cutting andtruncation of the fan are also described from the Holocene


deposits of the Big Lost River basin, Idaho and thePlio-Pleistocene of the Rio Grande Rift, New Mexico, whichhas helped to establish architectural models with implicationson climate and tectonics (Sohn et al., 2007).

Aggradation on terrace T2 is comprised of fewer debris flows,indicating an established SW Indian monsoon between 10 and8 ka and fewer events of episodic rainfall. At <8 ka the riverincised the deposits of terrace T2 and this was the phase whenthe SW Indian monsoon established itself and the rivers werehaving higher discharge and reduced sediment load – theconditions that lead to incision.

The river further experienced aggradation between 8 and3 ka comprising two depositional events with upward coarsen-ing grain size. This aggradation during the phase when theclimate was largely stable seems to be due to tectonicrejuvenation and increased sediment load. The upwardcoarsening of the sequence also indicates towards tectonicuplift in the provenance (Srivastava et al., 2001). The riverincised at �3 ka and formed the paired terrace T1. This was thephase when rainfall was reduced in general (Tripathi et al.,2004). The incision during the reduced rainfall indicatestowards regional uplift along the mountain front. Further, theincision phase is not accompanied by lateral shift in thechannel, indicating least role of the Ranaghat Fault. In thisscenario it is reasonable to assume a role of HFT in the tectonicuplift; however, detailed investigation will clarify this further.The low-angle fans that are superimposed on terrace T1 aredeposited in the relatively drier climatic phase of >3 ka, duringwhich smaller I-order tributaries were unable to carry their bedload. The toe-cutting of these fans is again governed by theregional phenomenon, probably due to activity along the HFT.

The Ranaghat Fault trends parallel to the surface trace of thehinge zone of the Siang Antiform and its axial planar fracturecleavages. Channel incision and lateral shift along this faultindicate that the fault is neotectonically active. The OSLchronology places this activity at ca. 10 ka BP. The results of



asymmetry of drainage basin analysis in the segment of theLedum–Pasighat section west of the Brahmaputra River indicateoverall neotectonic tilt towards the SW, suggesting south-westward migration of the streams in the west of Pasighat, whilethe Brahmaputra River at Pasighat migrated eastward inresponse to the movements along the HFT and Ranaghat Fault.Values of mountain front sinuosity west of Pasighat are close to1, indicating recent activity along the HFT. The seismic networkdata suggest that an area of almost 1502 km around Pasighat isaseismic, which probably reflects an instrumental gap in theregion and therefore a well-defined network is needed tounderstand the behaviour of the faults in the area.

Other studies on tectonic activity along the mountain front inthe NE Himalaya include the Darjiling sub-Himalaya in theTeesta River valley, where out-of-sequence deformation onsurface-breaking faults north of the HFT has led to the formationof disjointed strath terraces that were dated using OSL tobetween 11.3� 1.3 ka and 1.4� 0.3 ka (Mukul et al., 2007).The Kameng River, a tributary of the Brahmaputra, incised 95 mfrom terrace T4 to T2 between 14 and 6 ka BP with an uplift rateof 11.9 mm per year. Formation of the terraces and uplift tookplace along the N–S trending Kameng Fault (Srivastava andMisra, 2008). Based on lineament and active fault mappingusing satellite images, intense neotectonic deformation of theSiwalik ranges in the NE Himalaya has also been suggested(Das, 2004).

The morphostratigraphic sequence at the exit of theBrahmaputra River at Pasighat therefore archives the inter-action of tectonic activity along the local faults and climaticchanges during the past 15 ka. Prior to 15 ka, due to aridity itseems the river was not carrying much sediment to its mountainfront and most of the sediment produced remained locked atthe site of production. This, however, remains as an open topicfor future explorations in the area.

Conclusions

The present study makes the following conclusions:

1. T

Cop

he study suggests that the interaction of climatic changesand tectonic deformation at the mountain front between15 and 3 ka has led to the Brahmaputra River, at its exit,exhibiting three geomorphic surfaces: fan terrace T3,terrace T2 and terrace T1. The high-angle fans that meetthe river at the eastern bank are geomorphically equivalentto terrace T3. The low-angle fans sit on the T1 terrace and arethe youngest geomorphic feature developed in the area.

2. O
SL chronology indicates that the Brahmaputra River madeits fan terrace between 15 and 10 ka, after which the riverincised. Aggradation was driven by a semi-arid climateand incision was facilitated by increased discharge andtectonic activity along the Ranaghat Fault. Higher rainfallwas also responsible for further incision and formation ofterrace T2 at 8 ka.
3. T
he study further points towards tectonic rejuvenation in thehinterland at ca. 3 ka.
Acknowledgements The authors acknowledge Prof. B. R. Arora,Director, Wadia Institute of Himalayan Geology, Dehra Dun, for hisencouragement. Critical reviews by Prof. A. K. Singhvi, Dr Tim Cohenand an anonymous referee helped in improving the manuscript. MrChandrashekhar kindly helped with XRF analysis and Mr V. P. Guptahelped with the chemical processing of samples for OSL dating. Fig. 3base image copyright 2008 Terrametrics, Inc. (http://www.truearth.com)and Europa Technologies Ltd, reproduced with permission.

yright � 2008 John Wiley & Sons, Ltd.

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