fashion and phases of late pleistocene aggradation and incision in the alaknanda river valley,...

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Quaternary Research 7

Fashion and phases of late Pleistocene aggradation and incision in theAlaknanda River Valley, western Himalaya, India

Pradeep Srivastava ⁎, Jayant K. Tripathi 1, R. Islam, Manoj K. Jaiswal

Wadia Institute of Himalayan Geology, 33 GMS Road, Dehradun 248001, India

Received 5 September 2006Available online 6 June 2008

Abstract

We study the aggradation and incision of the Alaknanda River Valley during the late Pleistocene and Holocene. The morphostratigraphy in theriver valley at Deoprayag shows the active riverbed, a cut terrace, and a fill terrace. The sedimentary fabric of the fill terrace comprises fourlithofacies representing 1) riverbed accretion, 2) locally derived debris fan, 3) the deposits of waning floods and 4) palaeoflood records. Thesedimentation style, coupled with geochemical analysis and Optically Stimulated Luminescence (OSL) dating, indicate that this terrace formed ina drier climate and the river valley aggraded in two phases during 21–18 ka and 13–9 ka. During these periods, sediment supply was relativelyhigher. Incision began after 10 ka in response to a strengthened monsoon and aided by increase of the tectonic gradient. The cut terrace formed at~5 ka during a phase of stable climate and tectonic quiescence. The palaeoflood records suggest wetter climate 200–300 yr ago when the floodsoriginated in the upper catchment of the Higher Himalaya and in the relatively drier climate ~1.2 ka when locally derived sediments from theLesser Himalaya dominated flood deposits. Maximum and minimum limits of bedrock incision rate at Deoprayag are 2.3 mm/a and 1.4 mm/a.© 2008 University of Washington. All rights reserved.

Keywords: NW Himalaya; Alaknanda River terraces; River aggradation history; Sediment Geochemistry; Chemical Index of Alteration; OSL chronology

Introduction

In an actively deforming orogen, there is a balance amonghillslope erosion, river incision and rock uplift, which resultsin a steady-state evolution of the topography on 105–107 yrtimescales (Lavé and Avouac, 2000; Wobus et al., 2005).However, at shorter time scales, climate and tectonic perturba-tion may disturb the balance. A river that responds to varyingclimate and tectonic conditions acts as a medium for sedimenterosion and evacuation. A drier climate may lead to the enor-mous production of sediment but poor river discharge, whichmay hinder the sediment erosion and evacuation process, andeventually may lead to valley aggradation. Under wetter con-ditions the river will not only flush out the produced sediment

⁎ Corresponding author.E-mail address: [email protected] (P. Srivastava).

1 Present address: School of Environmental Sciences, Jawaharlal NehruUniversity, New Delhi 110067, India.

0033-5894/$ - see front matter © 2008 University of Washington. All rights reservdoi:10.1016/j.yqres.2008.03.009

but also will incise the bedrock to the depth of the amount ofuplift or more. It follows that river systems can provide infor-mation on climate–tectonic interplay including tectonic uplift ofthe river basin (Starkel, 2003).

The rivers in Central Himalaya in Nepal have witnessedseveral cycles of aggradation and incision in the recent past atsignificantly higher rates compared to the longer time-averagedrates (Pratt et al., 2004). Out-of-sequence thrusting and formationof terraces in the Teesta River, eastern Himalaya, is reported to bea result of enhanced precipitation at the mountain front during thelate Pleistocene–Holocene (Mukul, 2000). Similarly, four levelsof terraces have formed in response to the ongoing uplift alongthe normal fault in Kameg River, at the mountain front of NEHimalaya, during the Holocene (Srivastava and Misra, 2007).This suggests that there existed a topographic disequilibrium overshorter timescales of 103–105 yr, where the climate change anderosional isostasy were responsible for its geomorphic manifesta-tion. However, there is still a debate whether recent crustaldeformation is the result of climate and/or tectonics (Molnar,

ed.

mailto:[email protected]

http://dx.doi.org/10.1016/j.yqres.2008.03.009

69P. Srivastava et al. / Quaternary Research 70 (2008) 68–80

2003). To increase knowledge of climate and tectonic interactionand the orogenic development in this region, it is important toevaluate the rivers draining through the Himalaya in terms of theiraggradation and incision history.

Figure 1. (A) Location map of the study area and regional geological setup. Note the stthe area. MBT is Main Boundary Thrust; TT is Tons Thrust; RT is Ramgarh Thrust;cross-section as exposed along the Alaknanda River (after Srivastava and Ahmad,possess similar legends as in Figure 1A.

In the Himalaya, the fluvial processes and formation ofterraces occur under the critical thresholds of water/sedimentbudget and tectonic forces. River aggradation and incision pro-cesses are governed by factors such as 1) regional and local

udy sites at Deoprayag and Byasi and NNW–SSE trending fault running throughVT is Vaikrita Thrust; STDS is South Tibet Detachment System. (B) Geological1979). ESE/SSW etc. are the deviations from the river. Rock types and thrusts

70 P. Srivastava et al. / Quaternary Research 70 (2008) 68–80

tectonics, 2) climate change, 3) lithological variations, and4) erosional isostasy. An understanding of the relative control ofthese factors therefore necessitates an appreciation of thesediment source, sedimentary style and geomorphic configura-tion of the terraces. Also needed is the determination of therelationship of terraces with regional and local tectonic settingand climate.

Fluvial terraces in the Himalaya have been studied toinvestigate incision rates and their relation with the tectonic upliftof the rising mountains (Seeber and Gornitz, 1983; Lavé andAvouac, 2000; Wobus et al., 2005; Mukul et al., 2007; Srivastavaand Misra, 2007). Most of such studies come from the riversystems of central, eastern and northeastern Himalaya, but theseinclude relatively little documentation of the sedimentary style ofthe terraces. A review of published literature on the subjectindicates that the deposition of terrace sediments in the mountainsmay be caused by one or combination of 1) fluvial processes, 2)debris flows generated locally, and 3) debris flows originating inthe hinterland as result of rainfall. Each of these depositionalregimes has definite climato-tectonic implications and thereforeprovides ameans to link a terrace to a specific climate and tectonicniche. Absence of proper dating material has made it difficult todevelop a meaningful chronology. Charcoal for 14C-dating hasbeen used in the majority of studies. However, charcoal providesthe dates of its formation and not the depositional age of thesediment. Steep topography of the mountains would facilitaterecycling of charcoal from the older terraces and complicateschronometric assignments, leading to imprecise ages and erro-neous interpretations.

The present study is focused on river terraces in a part of theAlaknanda River Valley in western Himalaya. This river flowsthrough all the litho-tectonic discontinuities and also traversesthrough the climatic gradients ranging from 1200 mm/a at thefoothills to ~3000 mm/a near the mountain front of HigherHimalaya. Thus, these fluvial terraces preserve a compositerecord of 1) regional variations in climate, 2) local and regionaltectonics, and 3) surface processing of sediments. More spe-

Table 1Major elements composition of Deoprayag sediments

Weight % Phyllite Channel Flood phase I F

DP1 DP14 DP2 DP3 DP4 DP5 DP6 DP7 DP8 D

SiO2 58.2 57.7 78.5 68.9 70.8 67.3 72.7 68.9 59.7 6TiO2 0.7 0.8 0.3 0.4 0.4 0.4 0.4 0.6 0.5Al2O3 21.3 18.9 7.9 8.6 9.7 8.5 9.3 13.0 11.9 1Fe2O3 6.5 7.5 2.8 3.2 3.1 2.7 3.1 4.4 3.7MgO 1.6 1.9 2.2 4.3 3.5 4.5 2.9 2.6 5.4MnO 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.0CaO 0.2 0.4 3.4 4.6 3.4 5.4 2.9 1.6 5.2Na2O 0.7 0.9 2.0 1.6 1.4 1.5 1.6 1.5 1.0K2O 4.9 4.1 1.8 2.2 2.5 2.1 2.3 3.1 2.9P2O5 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1LOI 4.1 3.9 3.7 6.3 4.5 6.2 3.8 3.0 7.9Total 98.3 96.4 102.5 100.2 99.2 98.6 99.1 98.8 98.5 9CIA 75.5 74.0 48.3 52.3 57.1 54.6 54.1 61.4 64.7 6

Chemical Index of Alteration (CIA) and values for A–CN–K diagram have been calcuof XRF analysis. Note that the precision of geochemical data on major and trace elstandards.

cifically, the present study attempts to understand the sedimentsource and sedimentary styles of the terrace sediments. OpticallyStimulated Luminescence (OSL) dating technique provided thechronology of terrace sequence on two representative sectionsabout 34 km apart (Fig. 1).

Alaknanda River Valley: geological setting and thestudy area

The Alaknanda River is a major tributary to the River Ganga.It originates from Satopanth glaciers at the height of 3641 mabove mean sea level and meets River Bhagirathi at Deoprayag(541 m amsl). It has a total catchment area of 10,237 km2. Fromits origin to confluence, the river traverses 229 km (Pal, 1986).

The river trajectory is orthogonal to the Himalayan thrusts.Among these, the oldest and northernmost tectonic zone is theIndus Tsangpo Suture Zone (ITSZ), followed by the SouthTibetan Detachment System (STDS), the Main Central Thrust(MCT) and the Main Boundary Thrust (MBT). Figure 1A showsthe thrust boundaries and litho-tectonic units traversed by theriver; details are discussed in Ahmad et al., 2000. The rivercatchment cut through the rocks of Tethys Himalayan Sequence(THS), Higher Himalayan Crystalline (HHC) and LesserHimalayan metasedimentaries (LH). Figure 1B depicts thegeological cross-section as exposed along the river in NE–SWdirection. At Deoprayag, phyllite is exposed, known asChandpur phyllite. This olive green to gray coloured phylliteis intruded by quartz veins and is finely interbedded by siltstonesand basic volcanics. It is bounded byMBT in the south (Fig. 1B).

Methods

Site selection at Deoprayag was based on the fact that itrepresents a cumulative sedimentary response of the whole ofthe Alaknanda catchment. The section at Byasi, 34 km down-stream of Deoprayag, was selected to examine if the aggrada-tional events of the river are regionally represented. The contour

lood phase II T1 T2

P10 DP12 DP13 DP15 DP9 DP16 DP17 DP18 DP11 DP19

9.1 61.6 60.5 65.7 64.6 76.4 68.4 74.9 70.2 70.50.5 0.4 0.4 0.4 0.5 0.5 0.6 0.5 0.6 0.60.9 7.3 8.2 14.9 11.8 11.4 14.5 11.5 13.5 14.23.9 3.0 2.7 3.4 3.5 3.9 4.7 3.1 4.9 4.83.3 6.9 6.6 4.0 4.4 2.0 2.0 1.7 2.6 1.40.1 0.1 0.1 0.0 0.0 0.1 0.1 0.1 0.1 0.12.6 7.5 7.5 1.0 3.9 1.0 0.6 0.9 1.6 0.51.3 0.9 0.9 1.6 1.3 1.7 0.9 1.7 1.8 1.12.3 1.9 2.1 3.6 3.1 2.6 3.2 2.4 3.3 2.90.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.14.6 11.4 10.3 3.1 5.1 2.3 3.5 2.1 2.3 2.58.6 101.0 99.2 97.7 98.2 101.9 98.7 98.9 100.9 98.51.6 59.3 61.2 63.9 61.2 60.3 70.8 62.4 58.8 71.2

lated from this data. Analysis for all the major elements is shown for the efficacyements is better than 1.5% and 5%, respectively, using USGS monitoring rock

Figure 3. Morphostratigraphy of Deoprayag section and location of the analyzedsamples. The sample locations of palaeoflood sequences are given in Figure 8.


mapping by the National Hydropower Corporation of Indiaprovides the precise elevations for the study area.

Field-based observations (gravel lithology, roundness, size,matrix proportion, bioturbation, degree of iron staining andgrain size) were used to describe different lithofacies and theirvertical associations. Samples from phyllite bedrock and thepresent-day bedload (representing local and upper catchmentsediment sources, respectively) were collected to establish thegeochemical lineage of the terrace sediments; samples DP-1 andDP-14 were collected from the host rock (Chandpur phyllite);sample DP-2 was collected from the bedload sediment upstreamof the study site; samples DP-3–8, DP-10, DP-12–13 and DP-15 were collected from the palaeoflood sequences lyingunconformably on the terrace slopes and DP-16–18 and DP-9were stratigraphically collected from the terraces (Table 1). DP-19 is from debris flow deposits on a terrace containing angularfragments of Chandpur phyllite with sandy/silty matrix. Fourkilograms of each sediments sample were collected, air-driedand homogenized. One kilogram of homogenized sample wascrushed to 250 μm and about 200 g of this was further ground to75 μm and used for geochemical analysis.

Major elements were analyzed using X-ray fluorescence(XRF; Siemens) using a fused glass disc following Norrish andHutton (1969). The precision of analysis for major and traceelements was monitored using USGS rock standards (SGR andMAG-1) and were b1.5% and b5% respectively. Molar propor-tions of Al2O3, CaO, Na2O, and K2O are used for A–CN–K(i.e. Al2O3–CaO+Na2O–K2O) plots and for calculating Che-mical Index of Alteration (CIA) (Nesbitt and Young, 1989). Thecalculation was done using data from Table 1 on a carbonate-free basis (McLennan, 1993; Bock et al., 1998).

A standard A–CN–K plot of different primary minerals andtheir weathering products (i.e., secondary minerals) is shown inFigure 2 for comparison. The secondary minerals plot above the

Figure 2. A standard A–CN–K plot of different primary minerals and theirweathering products. The secondaryminerals plot above the line joiningK-feldsparand plagioclase (both have CIA=50), termed the feldspar join. Products ofextensive weathering that have CIA=100, i.e. kaolinite, gibbsite and chlorite, ploton the A-corner of the A–CN–K diagram. Intermediate weathering products, e.g.illite and smectite (75–85) plot, between A-corner and feldspar join (CIA=50).UCC is Upper Continental Crust and PAAS is Post-Archean Average AustralianShale.

feldspars line (line joining K-feldspar and plagioclase, both witha CIA of 50). Products of extensive weathering are kaolinite,gibbsite and chlorite, which have CIA of 100 and plot on the ‘A’corner of the A–CN–K diagram. Intermediate weathering pro-ducts, such as illite and smectite (CIA=75–85), plot betweencorner-‘A’ (CIA=100) and feldspars line (CIA=50). Duringcontinental weathering, because of the loss of most mobilecomponents (CaO+Na2O), a weathering trend emerges from thesource rock composition and advances along the line of the A–Kjoin. When the weathering trend approaches the A–K join itturns towards corner ‘A’ indicating subsequent loss of K2Oduring further weathering. At the end, after the loss of all mobilecomponents, the residue material plots on the ‘A’ corner. Forexample, an average Upper Continental Crust (UCC; CIA=50)and its moderately weathered product shale (CIA=70–75) ploton the normal weathering trend. Any deviation from a normalweathering trend indicates input of external material from adifferent source with a different chemical composition. There-fore, it is possible to calculate the ratio of mixing lithologies in asediment sample using CIA values, as discussed later.

OSL dating of terrace sediments was carried out to deducethe timing and duration of river aggradation and the rate ofincision of alluvium and bedrock. This dating technique relieson the premise that, prior to burial, geological luminescencestored in the minerals that constitute the terrace sediments iszeroed by daylight exposure during the erosion and transporta-tion (Aitken, 1998). Eleven samples were collected from dif-ferent lithofacies of terrace and palaeoflood deposits (Fig. 3,Table 2). Sample DP-20 was from the fill terrace 34 kmdownstream from Deoprayag at Byasi. The quartz fraction fromsamples was extracted by sequential chemical treatmentfollowing Srivastava et al. (2007). The fraction was sieved toget 90–150 µm size range and etched using 40% HF for 80 min,followed by 12N HCl treatment for 40 min to remove alpha

Table 2Radioactive element concentrations, dose rate, palaeodose and ages of the samples collected from palaeoflood and terrace deposits

Sl. no. Sample Depth (m) U (ppm) Th (ppm) K (%) Dose rate (Gy/ka) Palaeodose (Gy) SAR ages

Terrace T2 sequence1 DP-18 40.0 3±0.6 10.5±2 2±0.2 3.3±0.3 34±3 10±2 ka2 DP-17 58.0 3.7±0.7 19±3.5 2.6±0.2 4.6±0.5 49±7 12±12 ka3 DP-16 62.0 4.2±0.8 41±8 2.9±0.2 6.5±0.7 119±29 18±2 ka4 DP-9 100.0 3.7±0.7 11±2 2.2±0.2 3.7±0.4 86±25 21±2 ka

Terrace T2 sequence at Byasi5 DP-20 4.0 1.6±0.3 11.2±2 1.9±0.2 3±0.3 59±6 21±3 ka

Debris flow sequence6 DP-19 10.0 2.7±0.5 15.1±3 2.4±0.2 3.9±0.4 54±4 14±2 ka

Cut terrace T17 DP-11 0.5 2.8±0.6 18±3 2.8±0.2 4.5±0.5 24±5 5±1 ka

Flood phase I8 DP-8 0.5 1.9±0.4 15±3 2.1±0.2 3.5±0.4 0.7±0.4 200±40 yr9 DP-7 1.3 2.3±0.4 16±3 2±0.2 3.5±0.4 1.7±0.5 390±50 yr10 DP-4 3.8 3.4±0.7 15±3 1.9±0.2 3.7±0.4 1.2±0.6 320±60 yr

Flood phase II11 DP-13 0.2 1.9±0.4 10±2 1.6±0.2 2.7±0.3 7.5±1 2.6±0.4 ka12 DP-10 3.6 2.6±0.5 14±3 1.9±0.2 3.4±0.4 3.9±1 1.2±0.2 ka

Average of least 10% palaeodoses were taken for young samples (DP-4, 7, 8, 10) and average of all palaeodoses were taken for old samples (DP-9, 11, 13, 16)considering that residual signal during deposition is negligible compared to acquired signal (Jain et al., 2004). Moisture content of 10±5%was assumed for all samplesand cosmic ray Gamma contribution was considered to be 150 μ Gy/yr.


irradiated skin. The grains were mounted on stainless-steel disksusing Silko-Spray silicone oil. Luminescence measurementswere made on a Riso TL/OSL-12 system with a halogen lampsource for stimulation (Srivastava et al., 2003). The detectionwindow comprised Schott BG-39 and Hoya U-340 opticalfilters in front of an EMI 9235 QA photomultiplier tube.

OSL was recorded for 80 s at 125°C. A 90Sr/90Y beta sourcedelivering a dose rate of 3.6 Gy/minwas used for irradiation. A 5-point single aliquot regeneration (SAR) protocol suggested byMurray and Wintle (2000) was used to determine palaeodose. Apreheat of 240°C/10 s and a cut heat of 160°C for test doses wasused. The palaeodose estimate is based on samples with a recy-cling ratio of 1.1–0.9. The typical shine-down curve and growthcurves of DP-4 and DP-7 are given in Figure 4. The initial 2 s ofthe shine-down curve was used for analysis. Uranium, thoriumand potassium concentration were determined by XRF analysis.

Results

Morphostratigraphy

The morphostratigraphy of the study site consists of threesurfaces: T0, the present-day riverbed (459 m amsl); T1, the cutterrace (475 m amsl); and T2, the fill terrace (545 m amsl)exposing a 7-m-thick phyllitic bedrock strath at the base. A 25-m-thick debris deposit covers this terrace. The T1 surface is~75 m wide and is dissected by gullies that are filled with finesand (Fig. 5). The T2 terrace is narrow (~20 m) and is traversedby several rills that originate from the phyllitic terrain of theChandpur formation. Two palaeoflood sequences lie uncon-formably on the slopes of the terraces (Fig. 3). The 4-m-thick

palaeofloods of phase I is on the bedrock (466 m amsl), 7 mabove T0. The palaeoflood phase II sequence is developed on thesurface of cut terrace T1. The present-day bedload of the river ismainly composed, in decreasing order, of quartzite, gneiss,schist and Tethyan sedimentary rocks. The finer fraction of thebedload is composed of coarse micaceous sand with garnet andtourmaline as accessory mineral. The sediment compositionindicates the Higher Himalayan region as a provenance.

Lithofacies

Four major lithofacies were identified, namely 1) clast-supported rounded gravel, 2) matrix-supported angular gravel,3) yellow micaceous fine sand, and 4) alternating sand and silt.These are described below.

Clast-supported rounded gravel

This lithofacies is 2–15 m thick and comprises fining-upward rounded boulders. The clast diameter ranges from ~5 to50 cm. The matrix is composed of gray to yellow gray, coarse tomedium sand. The boulder population comprises 95% quartzite,4% gneisses and ~1% fossiliferous Tethyan sedimentaries.Tethyan boulders occur only at certain levels. This facieschanges upward into yellow micaceous fine sand or matrix-supported angular gravels (Fig. 6A).

Matrix supported angular gravel

About 2–12 m thick, the clasts comprise phyllitic materialfrom the local source, with size ranging from a few centimeters

Figure 5. The contact of cut terrace T1 and Gully deposit over T1.

Figure 4. (A) The shine-down curve of a natural signal from the sample DP-4. (B) The dose-growth curve of the sample DP-4 constructed from single aliquot of 7 mmdiameter having ~800 grains on average. Due to low sensitivity, analysis of smaller aliquots less than that was not carried out. (C) Dose distribution of the equivalentdoses (De’s) from the very young (200–300 yr) samples DP-4 and DP-7. The distribution indicates partial bleaching of the samples.


to 50 cm (perihelion). The matrix is ~30% by volume andcomprises fine micaceous sand that is relatively higher than thatof the clast-supported rounded gravel. Individual units locallyexhibit fining-upward bedding, with sharp upper and lowercontacts with the clast-supported rounded boulders (Fig. 7).

Yellow micaceous fine sand

This lithofacies consists of 1–2.5-m-thick fine sand finingupward and laterally pinching out. Individual units are generallymassive but commonly show parallel laminations and occa-sionally contain sheets of rounded boulders. At places theindividual unit show reddening due to increase in iron staining(Fig. 6B). Presence of fine mica flakes and phyllitic pebblessuggest their origin from a phyllite source.

Alternating fine sand and silt

This 0.25–1.5-m-thick unit is made up of couplets of graymicaceous very fine to fine sand and silt. The sandy layers aregenerally up to 0.2–10 cm thick. These are capped by silty

layers of almost equal thickness (Fig. 8). At places the units arepunctuated by ~1-m-thick units of massive medium to finesand. The facies represents recurring floods in the channel.Massive sands indicate catastrophic flood events. The thickness

Figure 6. Stratigraphic column of the fill terrace T2. A) Clast-supported rounded gravel. B) Yellowish micaceous fine sand.


and grain size of the unit indicates the energy of the floods. Themica flakes are larger than those in yellow micaceous fine sand,indicating the origin in Higher Himalayan crystalline rocks. Atplaces, presence of phyllite pebbles suggests that the floodsediments were locally derived. This facies is representedexclusively in palaeoflood deposits.

Terrace sedimentation at Deoprayag

The stratigraphy of fill terrace (Fig. 6) is composed of clast-supported rounded gravel, capped by yellow micaceous finesand. The facies clast-supported rounded gravel is a product ofriverbed accretion (Nichols, 1999; Pratt et al., 2004). Thepresence of rounded boulders of granite, quartzite, fossiliferousrocks and gneisses indicates that during aggradation, the

bedload was generated in the upper catchment in HigherHimalaya and Tethyan sedimentary rocks. The roundness of thegravels indicates that they have undergone several cycles ofweathering, erosion and deposition, with roundness increasingin every cycle. Long-distance transport in the channel may alsoimpart significantly to the boulder roundness. Occurrences ofTethyan gravels at a certain level, observed elsewhere as well asin Alaknanda Valley, suggest either increased precipitation inthe upper catchment or a phase of tectonic uplift in the Tethyancatchment. Sedimentary units of clast-supported roundedgravels are often conformably overlain by yellow micaceousfine sand, representing formation of a bar during recession ofthe floods. Successive floods can erode the fine sand facies anddeposit another cycle of fining-upward, clast-support roundedgravels.

Figure 8. Sequences and ages

Figure 7. Matrix-supported angular boulders. A) The debris flow unit exposedabove the red sand. Note the presence of tabular boulders made up of phyllite.B) The crudely bedded debris flow sequence present on the top of the T2 terrace.(For interpretation of the references to colour in this figure legend,the reader isreferred to the web version of this article.)


Occasional reddening of yellow micaceous fine sand faciesindicated development of a red profile because of iron oxidationdue to prolonged aerial exposure and reduced flood magnitude(Fig. 6B). This red sand profile is exposed at 62 m below the topof the terrace T2, therefore suggesting a more arid climate. Thesequence towards the top is capped by 25-m-thick unit ofmatrix-supported angular boulder where the constituentboulders are made up of local rock (Chandpur phyllite), withno interruption of channel activity in between. This indicatesthat the deposition of this unit occurred after the terrace wasabandoned and river incision began. This debris originatedlocally and represents a phase of increase in local sedimentsupply. This phase of channel aggradation is also represented inthe Byasi section (Fig. 9).

Palaeoflood deposits

These deposits contain centimeter-scale sand–silt couplets,where each couplet indicates a flood unit. The thickness of theindividual couplet indicates the intensity of the flood. Stackedcouplets are due to a phase of persistent flooding. The grain sizevaries from medium to fine sand, with fine sand units richer inmicas. Commonly these units show parallel laminations andsmall-scale ripples and moderate bioturbation, suggesting aslowly charging flood regime of longer duration floods. Thickermassive units suggest high-intensity episodic floods. The basalpart of the palaeoflood phase I shows the presence of 54 floodcycles of varying thickness. The deposits of phase II do notshow any couplets and instead show the presence of locallyderived phyllitic debris. The levels of these flood deposits arethe indicator of heights of the past floods; however, in theincising river systems the actual flood height is difficult toestimate with reference to lowering riverbed.

of flood phase I and II.

Figure 10. Plots of the present-day channel (DP-2), bedrock phyllites (DP1 and14), terrace sediments (DP-9, 11, 16 and 18), flood phase I (DP-3, 4, 5, 6, 7 and8) and flood phase II (DP-10, 12, 13, 15) are shown in A–CN–K diagram. Notethat the samples plot on the sediment mixing line between present-day channelsediment representing upper catchment and bedrock phyllites away fromweathering trend. For clear separation of different samples on the mixing linerefer to CIA values shown in Table 1.

Figure 9. Stratigraphy of the exposed section at Byasi.


Al2O3–CaO+Na2O–K2O (A–CN–K) systematics of the terraceand palaeoflood sediments

A–CN–K plots provide insight into the weathering trendsand tectonic and climatic milieu in the source region. TheChemical Index of Alteration (CIA) of the sediments is aquantitative measure of the extent of chemical weathering(Nesbitt and Young, 1982; Tripathi and Rajamani, 1999, 2003;Selvaraj and Chen, 2006). The intersection of weathering trendof sediments with the feldspars join indicates the plagioclaseand K-feldspar ratio that is typical of the source (Fedo et al,1995; Fig. 2). Sometimes, plots of sedimentary rocks do notfollow the normal weathering trend parallel to the A–CN line,which could either be due to K-metasomatism (Islam et al.,2002) or mixing with materials from a potassium-rich sourcewith different mineralogy and chemistry (Nesbitt et al., 1997).Therefore, weathering trends can be used to identify the sourceof the sediments and the mixing pattern of different lithologiesof different weathering characteristics.

Isotopes of Sr and Nd have been used to identify variouslithologies of the Himalaya and to understand the contributionof different terrains of the Himalaya to the foreland and fansediments (Robinson et al., 2001; Clift and Blusztajn, 2005;Najman, 2005; Wasson et al., in press). However, in view of thesimilarity in the Sr and Nd isotopes of the High HimalayanCrystallines and Outer Lesser Himalaya (Ahmad et al., 2000), itis difficult to estimate the proportions of sediments derived fromthese lithologies to the sediments of the present region that is

located in the Outer Lesser Himalaya. The mixing between twosources can be determined using mass balance based on theirCIA values.

The key weathering process in the Himalaya is the physicalweathering process. This is due to the high relief, which doesnot affect the original CIA of the rocks. The present study area islocated on a phyllitic terrain and, therefore, local sediments arephyllites. Distal sediments are from siliciclatics, gneissic andgranitic terrain upstream. Phyllite/shales have high CIA andgneisses and granites have lower CIA (Taylor and McLennan,1985); therefore, the two end members can be used to estimatethe relative proportions of the source rocks in the terrace andpalaeoflood deposits.

We plotted the present-day channel sediment (DP-2), derivedfrom the hinterland (gneisses and granites), and unweatheredphyllite bedrock samples (DP-1 and DP-14) in the A–CN–Kdiagram (Fig. 10). Sediments of terraces and palaeoflood depositsplot on the tie line between DP1, DP14 and DP 2. The high CIAvalue (75) for phyllite and its plot on the A–CN–K space isconsidered as its original CIA. Themixing line, which includes allthe terrace and palaeoflood sediments, between the present-daychannel sediments and the phyllite bedrock, does not follow thenormal weathering trend parallel to the A–CN tie line. Instead, itintersects the A–CN line when extended. This suggests a variablephyllitic contribution to the sediments. The chemistry of channelsediments mainly represents the coarse-grained distal sourcedsediment component transported as channel sands. Petrographicanalysis of sediments indicates that their source area is the HigherHimalayan Crystallines. A chemical mass balance of distal HHCand local phyllites at the Deoprayag is made to understand theclimato-tectonic conditions in the Alaknanda River catchment.

Figure 11. CIAvalues and phyllite contribution calculated and are shown for terrace and palaeoflood sequences. Flood I and Flood II are flood phase I and flood phaseII. The panel in the right of vertical hatched line include the CIA values of Chandpur Phyllite (local bedrock), Channel sediment (upstream of Deopryag) and debrisflows are generated locally. The palaeoclimate conditions are inferred using phyllite contribution in the sediments and OSL data.


From the mass balance calculations (Fig. 11), the terracesediments have a relatively higher proportion of local phylliticmaterial (40–50%). The flood sediments of phase I have lesserproportion (15–30%) of the phyllite bedrock, except forsamples DP-7 and DP-8 (49–62%). The deposits of phase IIfloods consist of 40–60% material derived from phyllites,similar to that in terrace deposits. The samples DP-7 and DP-8,occurring on the upper part of the flood phase I sequence,received higher supply from phyllitic rocks. This may be due to1) the flooding events towards the end of this phase originatedmostly locally, 2) physical weathering was higher in the localphyllitic catchment, or 3) the phyllitic terrain was tectonicallyactive, and hence supply of sediments greater. The laminatednature of the flood sediments discounts any possibility of in-situweathering and bioturbation. The debris flow deposited towardsthe terrace show a contribution of approximately 85% phyllite.

Luminescence chronology

Single Aliquot Regeneration (SAR) analysis on terrace andpalaeoflood samples yielded a positively skewed dose distribu-tion, which suggests partial bleaching (Fig. 3). This implied theuse of least palaeodoses for age analysis (Olley et al., 1998;Srivastava et al., 2007). In the present study, an average of least10% of palaeodoses were taken into account for age calculationin young samples; however, for old samples (N2 ka), theaverage of all palaeodoses obtained through SAR was taken tocompute ages (Jain et al., 2004). The younger ages forpalaeoflood deposits (Table 2) indicate that any offset in agedue to poor bleaching should of the order of a few hundred yearsonly.

The OSL chronology indicates that terrace sedimentationstarted before 21±2 ka (DP-9) and continued up to 10±1 ka(DP-18). A sample from the base of the reddish sand unityielded an age of 18±2 ka (DP-16) and the top of the unit

yielded 12±2 ka (DP-17). The debris flow deposits that overliethe terrace sequence yielded an age of 14±2 ka (DP-19). Theage of DP-19 is within error limits to DP-17 and DP-18 and maybe overestimated because of poor bleaching, and therefore thisage is excluded and not discussed.

The base of the gully fill sediment sitting on cut the ter-race T1 is 5±1 ka (DP-11). The samples from the palaeofloodrecord of phase I that sits on the bedrock and on the slope ofT1 terrace gave ages of 320±60 yr (DP-4) at the base and209±44 yr (DP-8) near the top. Ages of 1.2±0.2 ka (DP-10)at the base and 2.5±0.4 ka (DP-13) at the top are for thepalaeoflood record of phase II. This anomalous age 2.5±0.4 ka indicates changes in the process of sedimentation. Inthis case the flood unit is made up of fine sand mixed withangular phyllitic pebbles, indicating that the flood originatedlocally and the sediments might not have traveled enough toget sufficient sunlight bleaching. Sample DP-20 wascollected from the fill terrace of the Byasi section andyielded an age of 21±3 ka (Fig. 9).

Discussion

Aggradation history and source of sediments

The fluvial sequence at Deoprayag lies up to a height of 111mabove the present riverbed at 459 m amsl. The mor-phostratigraphy of the area has three terraces, namely T0, theriverbed; T1 the cut terraces (+16 m from the T0); and the fillterrace T2 (+86 m from T0). Twenty-five-meter-thick deposits oflocally derived debris fan cap the T2 terrace (Figs. 3 and 6). Finesandy palaeoflood deposits intermittently cover the valley fill upto a height of 480 m amsl. The fill terrace is made up of severalvertically stacked fining-upward clast-supported roundedboulders. The individual units are commonly capped by fine tomedium uniform gray sand, representing the waning stage of the


depositional event. The presence of yellow fine sand at 62 mbelow the top of the T2 surface (Fig. 6) arguably representsprolonged exposure and aridity. At places, above the red sand,larger sub-rounded boulders mark the basal part of the fining-upward unit with size ranging up to 1 m (Fig. 6). Such units mayrepresent abnormal monsoon phases resulting in mobilization ofhuge volumes of sediment transported in a relatively shortperiod.

The aggradation over cut terrace T1 is partly unexposed and/or covered by local debris. The contact between the T1 andoverlying gully fill is dated to 5±1 ka. The fill terrace T2

yielded an age of 21±2 ka near the base; the red sand (62 mfrom the top of fill terrace T2) yielded the age of 18±2 ka. Theother ages are at 58 m below the top (12±2 ka) and 40 m belowthe top of the T2 (10±2 ka). The overlying debris fan yielded anage of 14±2 ka (10 m below the top of fill terrace). The olderage of debris fan deposit may be due to insufficient sunbleaching.

The chronology, therefore, indicates that the aggradation ofthe fill terrace T2 started prior to 21 ka and continued up to 10 ka,and that the sedimentation took place in two pulses (Fig. 6). Thefirst pulse, when most of the sequence was deposited, occurredin a short time span of ~3 ka (between 21–18 ka) possibly duringthe drier conditions of the last glacial maximum (LGM, 21.5 kacal yr BP). Red sand represents the peak of aridity, and a secondpulse of sedimentation above the reddish sand took placebetween 13–9 ka (considering the associated +/− errors withages). This possibly was coeval with the Younger Dryas andcontinued even later. The overall accumulation rate of fill terraceT2 is 16.3 mm/yr. The stratigraphic gap of ~5 ka between the twopulses might represent erosion by the second pulse ofsedimentation.

The depositional units in the section above the red sandmarked by larger boulders may represent the time when IndianSummer Monsoon was readjusting itself to normal conditionsafter the LGM. Such conditions are often characterized byabnormal monsoon years (AMY) when rain starts migrating farinto the orogen and reaches regions that are shielded by theorographic barriers. These monsoons facilitate the transport ofhuge sediment loads into the river valleys (Bookhagen et al.,2005). The section at Byasi also has similar depositional setup.

In the A–CN–K diagram, samples from the fill terrace andpalaeoflood sequence plot on a mixing line between channelsands and phyllite (Fig. 10). The present-day channel sands fromAlaknanda upstream of Deoprayag plot near the feldspar join,indicating a provenance composition similar to the granite–granodiorite range. This is indicated by the weathering trendemerging through the feldspar join from the composition ofthe granite–granodiorite range. The unweathered compositionwith metamorphic mineral grains and a salt-and-pepper appear-ance for the channel sediments attest to the sources in the HigherHimalayan Crystalline. Although phyllite shows high CIAvalues, their protoliths are also similar to the granite–granodioritecomposition. The isotopic compositions of the Outer LesserHimalaya and Higher Himalayan Crystalline rocks also suggestto the similar protoliths for these two lithologies (Ahmad et al.,2000).

The CIAvalues of sediments combined with the time of theirdeposition are plotted in Figure 11. This diagram also depictsthe contribution of phyllite to the sediments deposited atDeoprayag. The terrace sediments dated to 21–10 ka havehigher proportion of phyllites (40–50%) and were depositedunder drier conditions around LGM. In this time period theLesser Himalayan zone received more rainfall than the HigherHimalaya. The SW monsoon accessed only the lower mountainreaches (Bookhagen et al., 2006). Therefore, the sedimentaggradation was limited by supply and transport capacity, andonly local supply contributed to the aggradation.

Thus the terrace aggradation occurred during the drier con-ditions of LGM and Younger Dryas, when the sediment supplyexceeded the river’s carrying capacity. The sediment was largelysupplied from the local areas, and gravels were transported fromthe upstream terraces of the earlier cycle of valley aggradation.Well-rounded gravel also suggests several cycles of erosion anddeposition. This cannot be achieved by a single debris flow orsingle erosional/depositional episode.

Pratt et al. (2004) used the example of the Marsyandi valleywhere ~75% of the terraces stratigraphy comprises matrix-supported angular gravel of debris flow. This valley is located24–40 km upstream in the Higher Himalaya; Pratt et al. sug-gested that valley aggradation in the Himalaya occurred duringthe wetter conditions at ~36 ka. Such aggradations during wetterconditions are also described from Sutlej River valley, NWHimalaya (Bookhagen, 2004; Bookhagen et al., 2006). General-izing then, we suggest that the terraces that comprise debrisflows originating in the Higher Himalaya represent river ag-gradation during wetter conditions, and the terraces that com-prise fluvially transported clast-supported well-rounded gravels(e.g., those at Deoprayag) would represent aggradation duringdrier LGM-like conditions. Cosmogenic radionuclide (CRN)dating of terrace sediments from Spiti and upper Sutlej valleyssuggest aggradation prior to 10 ka during the drier phase andincision in wetter conditions during a phase of the strengthenedIndian Summer Monsoon (Bodo Bookhagen, pers. commun.).

The reconstruction of temporal variations in sea surfacesalinity (SSS) and fluvial discharge of the Himalaya during thelate Quaternary also corroborates the aggradation history of theAlaknanda River. SSS data indicate that between 20 and 15 kathe Himalaya was intensively glaciated with minimal fluvialdischarge until 15 ka. This was the sediment aggradation phasein lower Himalayan valleys. During the initial phase ofdeglaciation between 15 and 12.5 ka, the climate was unstable.The deglaciation intensified after 12.5 ka, and culminated atabout 11 ka with a fluvial pulse. The beginning of Holocene(~9.5 ka) is characterized by high fluvial discharge attributed toan intensified monsoon regime that persisted throughout theearly Holocene (Chauhan, 2003) and has induced the incision ofthe valley fills.

History of incision

The incision of the fill terrace T2 began after 10 ka andcontinued until 5 ka when the river formed a cut terrace T1. Theriver incised the 70 m alluvium in ~5 ka, giving an average


incision rate of ~14 mm/a. The cut terrace lies at the height of9 m from the bedrock. Assuming the incision rate remainedconstant, the river would have incised this in b1000 yr. Thismeans that the river was flowing on the bedrock at ~4±1 ka andsince then the river has incised the bedrock by 7 m. Consideringthe errors in chronology, the bedrock incision rate is between2.3 mm/a and 1.4 mm/a. The minimum incision rates arecomparable to the long-term erosion rates of 0.9±0.1 mm/a forthe Alaknanda catchment at Deoprayag (Vance et al., 2003).

Given the uncertainties and biases in OSL and CRN chro-nologies, the incision rates estimated using these techniquesmay be similar. However, the higher incision rate deduced inthe study may be due to 1) the bedrock being an easy-to-erode,phyllite (Attal and Lavé, 2006), 2) the NNE–SSW trajectoryof river at Deoprayag is along a transverse fault (Valdiya, 1980)and higher incision rates are due to tectonic activity alongthis fault. Also, Vance et al. (2003) provide the basin-wideaverage erosion rates during topographic equilibrium, whichmay be lower than the estimates deduced using local geomorphicfeatures like terraces, because in most cases terrace formationoccurs in response to the active uplift along the fault and whenthe river flows under disequilibrium. Therefore, the uplift ratesprovided in this study should be considered as upper estimatesof uplift.

The study further contributes towards the concept ofmountain development by isostatic adjustment due to focusederosion and sediment evacuation, by suggesting: (1) duringprolonged drier climate, a huge amount of sediment is producedin the Lesser Himalaya but due to lack of fluvial energy itremains locked at the production site. (2) During the climatictransition from drier to wetter, the sediments from the hinterlandand local reaches are eroded to fill the river valleys but are notcompletely evacuated from the mountain belt. (3) Completeevacuation and mass removal occurs only during wetterconditions, and therefore the spanning of the drier and wetterconditions should govern the degree and phases of deformationin the mountain chain like the Himalaya.

Cosmogenic radionuclide (CRN) dating of strath terrace~30 km upstream of Srinagar suggest incision rates of 4 mm/ain Alaknanda (Barnard et al., 2001). The difference in thecatchment-scale erosion rates suggests activity along the Tonsthrust (Vance et al., 2003). We therefore consider that during theperiod of b10 ka, when the Alaknanda River began incising dueto its increased water-to-sediment ratio resulting from thestrengthened SW Monsoon. The tectonic uplift and valleyaggradation might have provided the necessary gradient forriver valley incision. However, the uplift must have postdatedthe aggradation phase, as the aggradation and tectonic uplift inthe river valley in principle cannot occur simultaneously.

Palaeoflood record

Two flooding phases are recognized as flood phase I andflood phase II. The chronology and geochemistry of the floodsequences have allowed us to understand the relative sedimentsupply from the upper catchment (Higher Himalaya) and localcatchment lying on Chandpur phyllite of the Lesser Himalaya.

This also helped in understanding the changes in the monsoonrainfall in the Himalaya during the last 1000 yr.

The older flood events of phase II are ~1.2 ka and are similar tothe composition of the terrace sediment (derived from locallithologies). Analogously, we take this to suggest drier conditions(Fig. 11). Climate-controlled vegetation changes played a majorrole in sediment production. During the drier condition themonsoon reach into the Higher Himalaya was limited, andtherefore the sediment supply from HHC was reduced. However,at the same time the Lesser Himalaya may have receivedrelatively more rainfall and hence became the key source ofsediment supply. Such conditions favored the production andtransport of locally derived sediments from the Lesser Himalayanlithologies. The sediments of flood phase I (200–300 yr) show ahigher proportion of HHC (70–85%, Fig. 11) due to the monsoonreaching to the interior Himalaya. Better rainfall and deglaciationin the Higher Himalaya helped sediment generation and transport,which increased the sediment load of the rivers. However, theupper part of the phase I sequence do indicate a deviation, whichmay be due to following scenarios: 1) the flooding events towardsthe terminal of this phase originated largely locally, 2) higherphysical weathering occurred in the local phyllitic catchment, 3)the phyllitic terrain was tectonically active and hence supply wasgreater. The laminated nature of the flood sediments discounts anypossibility of in-situ weathering and bioturbation.

Based on the above discussion we surmise that youngerfloods of phase I originated during the humid phase whensediment supply from the HHC was higher. The terminal part ofphase I was characterized by a higher proportion of locallysupplied sediment, implying the relative aridity. The olderfloods of phase II occurred during drier conditions when thelocal phyllitic sediments were more abundant.

Conclusions

The present study indicates that:

1. The Alaknanda River in its lower reaches aggraded in twophases, between 21–18 ka and 13–9 ka.

3. The incision was due to increased precipitation; bedrockuplift began after 10 ka.

4. Incision rate of the valley fill was 14 mm/a. The limits ofbedrock incision rate lie between 2.3 mm/a and 1.4 mm/a.This incision rate indicates higher erosion and possibly rockuplift, and it suggests tectonic activity along the NNW–SSEtrending transverse fault running through the area.

5. The palaeoflood records suggest a wetter climate 200–300 yrago, when the floods were originating in the uppercatchment, and a relatively drier climate ~1.2 ka ago whenlocally derived sediments dominated the flood deposits.

Acknowledgments

The authors thank Prof. B.R. Arora, Director,Wadia Institute ofHimalayan Geology and Dehra Dun for his encouragements.Discussions with Dr. Navin Juyal, PRL, Ahmedabad helped indeveloping the ideas. Suggestions by Professors A.K. Singhvi,


Eric Steig, Hans Thewissen and anonymous reviewers helped inimproving the manuscript. Sri Chandrashekhar helped with XRFanalysis and Sri V.P. Gupta helped in the samples preparation forOSL dating.

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