Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Aggradation, incision and interfluve flooding in the Ganga Valley over the past100,000 years: Testing the influence of monsoonal precipitation

N.G. Roy a, R. Sinha a,⁎, M.R. Gibling b

a Engineering Geosciences Group, Department of Civil Engineering, IIT Kanpur, Kanpur 208016, Uttar Pradesh, Indiab Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4R2

⁎ Corresponding author.E-mail address: rsinha_64_99@yahoo.com (R. Sinha)

0031-0182/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.palaeo.2011.08.012

Please cite this article as: Roy, N.G., et al., Aging the influence of monsoonal p..., Palaeog

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 December 2010Received in revised form 21 August 2011Accepted 23 August 2011Available online xxxx

Keywords:ValleysInterfluvesDiscontinuitiesMonsoonAggradationIncisionEquilibrium profileHimalayan forelandGanga RiverPalaeoclimate

In order to investigate fluvial responses to climate forcing, stratigraphic data from seven cores in the GangaValley of India and a valley-margin cliff section 1.3 km long are compared with proxy records for the intensityof the Southwest Indian Monsoon. Luminescence dates indicate that the strata cover the past ~100 ka. Fiveaggradational periods separated by incisional episodes are apparent in the valley fill, some of which have cor-relative strata along the valley margin. During Marine Isotope Stages (MIS) 4 and 5, the valley experiencedfluvial activity, with thin floodplain successions and probable discontinuities along the interfluve margin.Modest fluvial activity characterised the mid part of MIS 3 at about 37 ka. Aggradation of channel sands pre-ceded by incision is documented for late MIS 3 (23 to 30 ka), as well as for late MIS 2 to early MIS 1 (11–16 ka) with fluvial dates focused around the Younger Dryas period of monsoon reduction. The most recentaggradational period took place in the latest Holocene (b2.5 ka). On the proximal part of the interfluve,thin deposits that include lacustrine and aeolian beds continued to accumulate until about 26 ka, afterwhich the interfluve was apparently not inundated by the Ganga and underwent degradation throughgully erosion. There is little evidence of fluvial activity during the Last Glacial Maximum of MIS 2, whenthe Ganga appears to have been underfit. The alluvial records fit well with proxy precipitation records, andthe main aggradational periods correspond to times of declining monsoonal strength. Although more difficultto constrain, the timing of incisional periods corresponds broadly with periods of monsoonal intensification,and the river appears to have shifted course in its valley mainly during these times.

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gradation, incision and interfluve flooding ineogr. Palaeoclimatol. Palaeoecol. (2011), doi:

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Over timescales of thousands to a few tens of thousands of years, in-land river systems undergo phases of aggradation and incision inducedby tectonic, climatic and intrinsic factors. Consequently, fluvial stratigra-phy typically displays complexity in architecture and internal heteroge-neity (Ambrose et al., 1991), dependent on the interplay betweenautocyclic and allocyclic variables. These variables include the magni-tude and rate of channel scour, channel deposition and migration rate,downstream change in grain size and varied sediment flux, channelbelt avulsion, base-level change, climatic fluctuations, and tectonic sub-sidence patterns and local fault movement (Miall, 1996; Blum andTörnqvist, 2000; Catuneanu, 2006; Gibling, 2006). The effects of thesecomplex variables on sediment preservation form the basis for alluvial-architecture models that seek to predict subsurface sandbody geometry.

Valleys are prominent landforms inmany coastal and inland alluvialsettings, and their cutting and filling exert a major influence on alluvialarchitecture. Valley filling and incision are commonly episodic in space

and time due to changes in external forcing factors, especially stream-power variations linked to the balance between discharge of water andsediment (Bull, 1991; Bogaart et al., 2003; Strong and Paola, 2008;Martin et al., 2011). Such cut-and-fill architecture can be studied to un-derstand the rhythmic control of external forcing (Bull, 1991; Bestland,1997; Weissmann et al., 2002; Gibling et al., 2005; Mack et al., 2006).Architectural models for valley fills need to be tested for well-datedQuaternary successions where proxy records for forcing factors areavailable. However, only a fewmodern plains have been studied in suf-ficient detail to test the models effectively (Blum and Aslan, 2006;Stouthamer and Berendsen, 2007; Amorosi et al., 2008).

The Ganga Plains in the Himalayan Foreland Basin of India areunderlain by sediment laid down by the Ganga River, among theworld's largest rivers. The plains have a well preserved alluvial re-cord accessible through drilling, and strata are locally exposed incliff sections along incised reaches. These records form importantcontinental archives for understanding landscape dynamics againstthe backdrop of late Quaternary environmental changes drivenmainly by hinterland (Himalayan) tectonics and palaeo-monsoonaldynamics (Gibling et al., 2011). The present paper extends previousresearch through a detailed study of the Ganga Valley near Kannauj inUttar Pradesh (Fig. 1). In this area, a long stretch of previously

the Ganga Valley over the past 100,000years: Test-10.1016/j.palaeo.2011.08.012

2 N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

undescribed cliffs, ~10–20 m high, allows a high-resolution analysis offloodplain facies and their architecture at a relatively high elevationalong the valley margin. Seven drill cores with near-complete recoverydown to a depth of 32 mwere organised in two traverses across the val-ley to a distance of 14 km from the cliffs (Fig. 1). Cliff and core stratawere placed in a chronological framework that spans the past ~100 kabased on 25 samples dated using luminescence methods. Comparisonwith available information on palaeo-monsoon intensity suggests that,to a first order, major monsoonal fluctuations governed fluvial eventsof aggradation, incision, and inundation of the interfluve adjoining theGanga Valley.

2. Geographic and geomorphic setting of the Ganga Plains

The study area falls in the middle Ganga Plains and is located~300 km downstream of the Himalayan tectonic front and more than1200 kmupstream of themodern day tidal limit. Of the total catchmentarea of 2435 km2, an area of about 206 km2 (8.5%) is presently glaciated.The glaciated areawas only slightly larger during the LGM (13%) (Owenet al., 2002), and glacial melt water may have played a modest role inthe Ganga River's water and sediment flux through the Late Quaternary.The study area experiences monsoonal rainfall of about 60–80 cm peryear. The hinterland of the Ganga receives N200 cm/year and that ofthe Ramganga receives 160–180 cm/year. Most of this rainfall is con-centrated during July–September, while the rest of the year is dry orhas little rainfall. The monsoonal rainfall therefore exerts an importantcontrol on the hydrology and sediment flux in the basin. The winter

a

80° 04'

26°50'

10 km Kanpur

Kannauj

Hardoi

45

6

b

7

12

3

IRS 1-D Band 4

Fig. 1. (a) Location map of the study area in the western Ganga plains, Uttar Pradesh, India.fluvial features having different radiance property. Note drill core location along two transecmargin is very close to the Ganga River. (c) Headwaters of the Ganga River showing the pr

Please cite this article as: Roy, N.G., et al., Aggradation, incision and intering the influence of monsoonal p..., Palaeogeogr. Palaeoclimatol. Palaeoe

temperature varies from 2 to 15 °C and the summer temperature from25 to 45 °C (Singh, 1994). The hydrology of the Ganga River is stronglycontrolled by the monsoonal rainfall, and the proportion of glacial meltwhere the Ganga enters the plains, although debatable, has been esti-mated at 15% (Das Gupta, 1975).

Topographic analysis based on SRTM data (Fig. 2a) and a regionalgeomorphic map of the study area (Fig. 3) show that the modernGanga River flows in an asymmetric valley (Fig. 2a). Field observa-tions also confirm that the right channel bank is incised, exposing a15–20 m high cliff line (Fig. 2c) that also marks the right valley mar-gin. The left channel bank borders a flat-lying active floodplain tract(Fig. 2d) with palaeochannels and meander scars. Fig. 3 suggestslong-term southwest migration of the channel (see also Roy andSinha, 2005, 2007). The left valley margin does not form a prominentgeomorphic feature but is delineated where the active floodplainpasses northeastward into an inactive floodplain belt.

Fig. 3 shows five major geomorphic units in the study area, namely(i) the major active channel belt of the present Ganga and its largetributary, the Ramganga, currently about 5–7 km and 2.5–3 kmwide, respectively; (ii) the active floodplains of these major channelswithin the valley margin; (iii) the active minor channels and flood-plains of other smaller tributaries; (iv) wide inactive floodplainsnortheast of the Ganga; and (v) a slightly dissected interfluve surfacesouthwest of the Ganga that extends as far south as the Yamuna Rivernear the southern margin of the plains (Gibling et al., 2005; Sinha etal., 2005a). The southern margin is strongly gullied and degraded,forming irregular cliffs along the river.

Head-watershed of the Ganga River

Present Glaciated area (206 km2)

LGM Glacier extension based on Owen et

al.(2002) (333 km2)

Ganga head watershed (total area 2435 km2)

c

(b) Band 4 satellite imagery (IRS LISS 3, dated April, 2000) showing active and palaeo-ts from left to right valley margin. Dashed line indicates left valley margin. Right valleyesently glaciated area and the extent of the LG glaciation.

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

A’

B’A

B

Elevation (m above msl)

Kanpur

Kannauj

Delhi

Himalayan mountain front

Haridwar

100 km

81 00

77o50’30o

N

Distance (km)

0 20 40 60 80 100 120 140 160 180

Alti

tude

(m

)

GangaKali

Gomti

Distance (km)

0 20 40 60 80 100 120

Hei

ght (

m m

sl)

Kali Ganga Ramganga

20mBridge

Gully erosion

Bridge

A A’

B B’

ba

c d

Cliff line

300m

130

135

140

145

150

155

160

155

160

165

170

175

180

Fig. 2. (a) Digital Elevation Model (DEM) from SRTM source acquired by USGS in Feb, 2000 showing the valley and interfluve. The box shows the area of study. (b) Cross-sectionsacross the valley show that the Ganga River flows in a broad valley (approximate width 10–15 km) and both the Ganga and its tributaries are incised. (c, d) Field photographs of theGanga River close to Kannauj; note that the left bank is incised (left-hand photo) whereas the right bank is a flat plain (right-hand photo).

3N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

Present-day Ganga and Ramganga channel belts are braided, but theRamganga shows significant sinuosity as well, particularly upstream ofits confluence with the Ganga. The Ganga with a higher ‘braid-channelratio’ (Friend and Sinha, 1993) flows in a nearly straight reach near Far-rukhabad and a change in flow direction occurs downstream of Fate-hgarh from NW–SE to nearly E–W down to its confluence with theGarra River, after which it resumes its NW–SE trend. The active flood-plain of the Ganga is generally wider than that of the Ramganga, buttheir widths vary significantly along the channel. Among the activeminor channels, the sinuous Garra River is slightly migratory. A largepart of the study window is occupied by numerous inactive channelsand floodplain features such as meander scroll bars, cut-offs (bothneck and chute cut-offs), and abandoned channel belts.

3. Methods

3.1. Cliff section mapping and raising of drill cores

The valley-margin stratigraphy was studied in an exposed section1.3 km long and 10–20m high along the Ganga bank at MehdipurGhat close to Kannauj. Continuous overlapping photographs were

Please cite this article as: Roy, N.G., et al., Aggradation, incision and intering the influence of monsoonal p..., Palaeogeogr. Palaeoclimatol. Palaeoe

taken from a boat to prepare a photomosaic onwhichmajor lithologicalunits and breaks were traced. Ten stratigraphic logs at carefully chosenlocations were prepared for documentation of grain size, sedimentarystructures and features, recorded on centimetre scale to establish lateralcorrelation between logs and across the photomosaic. Based on theavailability of reliable information on sedimentary structures and ge-ometry, the facies interpretations (Table 1) are considered robust.

The valley-fill deposits were studied through seven cores collectedalong two transects 20 km apart downstream from Kannauj, withcore spacings of 3 to 8 km along each transect (Fig. 1). Three coreswere raised from each transect, and a single core was collected10 km downstream of transect 2. The core locations were selectedwhere past fluvial features were observed on the geomorphic map.Prior to drilling, a ground electrical resistivity survey was performedwith a Schlumberger array to have prior information about the occur-rence of major channel bodies before selecting the drill-core loca-tions. Resistivity methods used in the research are outlined in Yadavet al. (2010). Locations of drill cores were determined by a hand-held GPS accurate to less than a metre, and core tops were plottedon the topographic profile based on SRTM-based digital elevationmaps.

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

Unit1:

Unit2:

Unit3:

Unit4:

Unit5:

Ch. Belt

Flood plains Inactive channels

Dissected surfaceMinor ch. +FP

0 25 km

79 55'

1

2

3

4

5

27 00'

Geomorphic Map

Lake area

Water body

Sand hills

Salt encrusted area

Dry channel

Meander cut off and scroll

Geomorphic Units

Legend

Fig. 3. Geomorphic units of the Ganga plain between Farrukhabad and Kannauj, based on interpretation of remote sensing images and field checks. 1, active channel belt; 2, activefloodplain (younger and older floodplain); 3, minor active channel and floodplain; 4, inactive floodplain; 5, slightly dissected surface. The left valley margin is represented by theboundary between Units 2 and 4, and the right margin by the boundary between Units 2 and 5.Redrawn from Roy and Sinha, 2005, 2007.

4 N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

Drilling employed a diamond core drilling bit involving a doublebarrel core tube. A PVC pipe was inserted inside the core tube. In rel-atively stiff muddy strata, a slow rotary method was used with closeto 100% core recovery. In unconsolidated sandy strata, a combinationof rotary and percussion methods was used to arrest the core withslightly lesser core recovery (70–80%) including a slight compactiondue to percussion. The core obtained in the PVC pipes was taken tothe laboratory and split into halves. One half was preserved and theother half was used for logging and sampling. Geoelectrical logging(SP and resistivity) was carried out for all drill holes after the coringwas completed. Apart from providing the first-level lithological char-acteristics, the geoelectric logs also helped in reconstructing facies inareas of core loss during drilling.

The cores were logged at centimetre scale. Munsell colour chart(Munsell colour chart, Year 2000) was used to describe colour. Tex-tural description included grain size (visual estimation), compact-ness, primary structures and lamination, and grading. Special

Please cite this article as: Roy, N.G., et al., Aggradation, incision and intering the influence of monsoonal p..., Palaeogeogr. Palaeoclimatol. Palaeoe

features include carbonate nodules (kankar) and rhizoconcretions,and dark nodules probably rich in iron and manganese. Dark(black), drab (grey), and red-brown soft mottles in silt/clay werenoted, as were reworked carbonate nodules, shell material, burrowsand root fills. The contacts between units were noted as sharp, grad-ual or wavy. Because the cores were more difficult to characterise andinterpret than the outcrops, a separate facies scheme was utilised(Table 2), and description and a brief interpretation are presentedtogether.

3.2. Luminescence dating

Samples for luminescence dating were collected in light-proof PVCpipes, and the locations were marked on the cliff section and on bore-hole logs (Figs. 4, 5). From each sample, a coarse grain-size fraction(63–90 or 90–180 or 180–250 μm) of quartz and feldspar wasextracted and was further processed following standard laboratory

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

Table 1Facies, description and interpretation of valley-margin interfluve sequences, based on exposures at Kannauj.

Litho-facies Description Interpretation

Floodplain facies (IF)IF1. Clayey silt with greater than30% carbonate nodules.

Dark brown clayey silt. Reddish brown to darkbrown, yellow, grey mottles are present. Largecarbonate nodules (2 to 5 cm), comprised N30%of the facies. Minor dark mottles and black Fe–Mnnodules are present.

Floodplain deposits with soil carbonate formation.Variable degrees of pedogenesis. Pedogenesis andfloodplain aggradation occurred alternately.Commonly occur as thick interfluve floodplainof a major river.

IF2. Clayey silt with few carbonatenodules (b5%).

Pale brown clayey silt, locally silt rich. Drab mottles,vertical to sub-vertical drab pipes dispersed or inhorizons. Small carbonate nodules (mm size) comprisedb5% of the facies. In places rhizoconcretions compriseup to 30%.

IF3. Silty clay with less than 10%carbonate nodules

Yellow brown silty clay with less than 10% kankars andminor rhizoconcretions. Black and drab mottles and plantroots are present.

Floodplain deposits with moderate degree ofpedogenesis as floodplain deposits (b1 m thick)of local river.

IF4: Sticky clay with rhizoconcretionand carbonate nodules

Dark yellow tight silty clay, minor kankars but manyrhizoconcretions and mottles (red-brown) are present,strongly cohesive. Efflorescence present on weatheredsurface. Fragmented gastropod shells at the base.

Shallow water bodies in a large depression andabandoned channel areas; minor pedogenesis,but strong carbonate diagenesis.

IF5: Laminated sandy silt with fewsmall carbonate nodules.

Dark yellow to pale yellow sandy silt, weakly cohesivematerials with poor lamination preserved. Kankars compriseb10% of the facies. Large grey mottles and roots. Black Fe–Mnnodules are common.

IF6. Massive sandy silt with N10%carbonate nodules.

Pale yellow massive, tight sandy silt with scattered kankars(mm size). Yellow brown mottles and dark Fe–Mn nodules present.

Floodplain deposits. Weakly pedogenised, Fe–Mnnodule indicates gleying effect.

IF7. Calcrete Coalesced nodules a few cm to dm in diameter, nodules aresand to silt particles bound by carbonate cement. Carbonate ismostly micritic, but locally has coarser sparry fabric. Nodulesinterspersed with mud, fine sand and silt. Rhizoconcretionslocally are up to 30 cm long. Kankar nodules are disseminatedin the floodplain mud, while kankar lumps (centimetre-scale-thick)are laterally amalgamated and persistent carbonate layers betweenfloodplain layers. The carbonate nodules from floodplain faciesshow higher δ13C values compared to those in eolian facies(average δ13C=−5.86 in floodplain and δ13C=−2.76 in eolian facies).

The nodular form, evidence for roots, andintraformational erosion suggest that thecalcretes are largely pedogenic and havevariably attained Stages II–III (near coalescence)and Stage IV (coalesced nodules) ofdevelopment (Machette, 1985).δ13C indicates that floodplain facies were depositedwith greater water availability as indicated by highC3:C4 plant ratio, whereas eolian facies weredeposited with less water availability supportedby low C3:C4 plant.

Aeolian faciesIE: Very fine sand and silt Off-white silt to very fine micaceous sand, moderate sorting of

grains, no recognisable stratification, non-cohesive with minimalclay and plant roots and few carbonate nodules present.

Windblown sand and silt from nearby source.

Channel faciesIC: Very fine micaceous siltysand, laminated

Very fine micaceous sand, basal part show curved laminae, finerparticles in the upper part.

Minor channel deposits.

5N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

treatments (Stokes, 1992) to isolate quartz and feldspar. A single-ali-quot regenerative dose protocol (Murray and Wintle, 2000) wasemployed to yield a palaeo-dose estimate from quartz and feldsparafter successfully passing standard tests (Wintle and Murray, 2006)for reliability. The distribution of palaeo-dose estimates from individualaliquots was fitted with a normal distribution to obtain the meanpalaeo-dose. The values of palaeo-dose obtained from the feldsparwere underestimated because of anomalous fading (Aitken, 1985;Huntley and Lamothe, 2001). Consequently, a fading correctionwas ap-plied to the estimated palaeo-dose based on a single aliquot regenera-tive dose protocol following the experimental approach of Auclair etal. (2003). Dose rates were measured using high-resolution gammaspectrometry with a detector resolution of 2 keV. Other pre-treatmentsto minimise overestimation of dose rate included sealing of samples toavoid 222Rn loss, heating to avoid absorption of radiation by organicmatter and water, and grinding to b2 μm for homogenization. An inter-nal dose rate of 0.06±0.03 mGya−1 for quartz and 0.8±0.3 mGya−1

for feldspar, and a depth-dependent cosmic ray dose contributionwere applied. The dry dose rate was thus computed for these relativelycoarse grains and corrected for the ambient moisture. The age was cal-culated by dividing the mean palaeo-dose by dose rate for each sample.

Ages up to 28 ka were determined based on optically stimulatedluminescence (OSL) where blue light was used as a stimulation

Please cite this article as: Roy, N.G., et al., Aggradation, incision and intering the influence of monsoonal p..., Palaeogeogr. Palaeoclimatol. Palaeoe

source. For older sediments, the signal was saturated, and isothermalluminescence (ITL) and infrared stimulated luminescence (IRSL)were employed. In addition to 21 finite dates, four saturated samplesyielded minimum OSL dates, and these were recorded as supplemen-tary information. In the case of ITL, the stimulation source is constantheat (Jain et al., 2007), and for IRSL the stimulation source is infraredluminescence. For all cores and sections with multiple dates, the datesare younger upward, without apparent dating anomalies. For samplesthat yielded minimum OSL dates, ITL and IRSL methods yielded olderdates or, in one case, a date within error range of the minimum OSLdate.

4. Valley-margin section at Kannauj

4.1. Facies description

The section is composed of mud and sand sheets with decimetre-to metre-scale thickness (Fig. 4). Three depositional groups wereidentified, namely floodplain (IF) with seven component facies, aeo-lian (IE) and channel (IC) with one facies each (Table 1). Five strati-graphic units were identified on the photomosaic and field logs (Fig.4a, b). The units can be correlated for the length of the exposure, al-though Unit 5 has been eroded to the south.

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

Table 2Luminescence dates. OSL = optically stimulated luminescence; ITL = isothermal luminescence; IRSL = infrared stimulated luminescence.

Sample number Log/core Unit Height/depth(m)

Water content(%)

Dose rate(Gy/ka)

OSL 2×Do(Gy)a

ITL De

(Gy)OSL age(ka)b

ITL/IRSLc age(ka)

(a) Valley margin samples (sample positions are height from river level)063433 Log 2 Unit 4 1.0 5±4 4.45±0.20 103.32±8.50 153±10 N23±3 34±4063434 Log 2 Unit 4 3.60 3±4 4.33±0.21 105±7 150±1 N24±3 –

063435 Log 2 Unit 3 4.60 3±4 4.33±0.21 105±7 150±1 N24±3 35±4063436 Log 2 Unit 3 5.10 3±4 4.20±0.20 94.56±12.9 – N23±3 –

063443 Log 10 Unit 4 5.00 8±4 5.15±0.23 76.8±4.34 136±8 N15±2 26±3063437 Log 2 Unit 1 10.10 37±10 3.62±0.14 69.66±3.62 150±7 N19±2 41±4063438 Log 2 Unit 1 11.00 31±10 3.63±0.15 63.44±3.30 – N17±2 –

063439 Log 2 Unit 1 12.10 29±10 2.75±0.24 79.70±5.87 245±15 N29±3 89±10063441 Log 9 Unit 1 19.00 29±10 3.35±0.29 69.02±2.72 – N21±2 –

063442 Log 9 Unit 1 20.00 30±10 3.80±0.16 80.9±4.96 – N25±3 –

063446 Log 10 Unit 1 10.00 22±10 3.80±0.35 77.22±8.66 378±19 N20±03 100±11

(b) Core samples (sample positions are depth from the surface)053412 Core 1 Unit 5 7.50 39±4 2.47±0.09 0.45±0.08 84±13 0.18±0.02 –

053413 Core 1 Unit 5 12.25 32±4 1.99±0.08 4.32±0.14 – 2.18±0.12 –

053414 Core 1 Unit 1 32.65 27±4 2.20±0.12 121.68±8.10 – N55.33±4.81 –

053404 Core 2 Unit 5 3.87 27±4 2.60±0.10 3.22±0.15 – 1.24±0.08 –

053405 Core 2 Unit 2 23.70 32±4 2.85±0.11 108.72±9.53 – N38.09±3.71 37±2*053406 Core 2 Unit 2 31.3 22±4 3.12±0.29 55.22±4.49 – N17.69±2.23 74±3*053408 Core 3 Unit 3 21.45 32±4 2.12±0.08 60.22±2.49 – 28.42±1.70 –

053407 Core 3 Unit 1 9.80 29±4 2.09±0.08 3.40±0.14 – 1.63±0.10 –

053415 Core 4 Unit 5 3.16 42±4 2.79±0.10 2.40±0.06 62±9 0.86±0.04 –

053416 Core 4 Unit 2 13.22 42±4 3.42±0.12 96±5 227±73 N27.14±1.72 –

053417 Core 4 Unit 1 28.50 27±4 3.94±0.16 65.41±0.89 – N16.59±0.77 61±2*053409 Core 5 Unit 5 3.65 39±4 2.35±0.08 4.47±0.18 – 1.90±0.11 –

053410 Core 5 Unit 5 9.4 37±4 2.37±0.09 5.93±0.27 – 2.51±0.16 –

053411 Core 5 Unit 3 18.83 45±4 3.92±0.14 91.93±10.16 – N23.44±2.76 –

053402 Core 6 Unit 4 3.0 36±4 2.58±0.09 30.12±1.22 – 11.67±0.67 –

053403 Core 6 Unit 4 13.80 16±4 2.38±0.10 33.61±1.35 100±11 14.10±0.89 –

053401 Core 6 Unit 4 19.70 30±4 3.08±0.12 41.71±3.81 – 13.56±1.74 –

063447 Core 6 Unit 4 21.20 30±4 3.07±0.12 46.47±2.65 – 15.12±1.09 –

053418 Core 7 Unit 4 5.56 44±4 2.41±0.09 31.40±1.22 42±0.85 13.01±0.73 –

053419 Core 7 Unit 4 22.70 26±4 3.35±0.13 66.26±2.08 – N19.81±1.07 –

a Represents the saturation limit for saturated samples.b Represents the minimum OSL age for saturated samples.c Age data shown with asterisks are IRSL ages after fading correction.

6 N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

Unit 1 (base not exposed) is 3.8 m thick with modest lateral vari-ation, and comprises weakly stratified clayey silt (facies IF1and 2).Scattered to concentrated kankars of various sizes (mm to cm) arerelatively small in the upper part, where rhizoconcretions up to30 cm long and traces of oxidised rootlets are also present. At theunit top, the clay is more silty and has vertical drab pipes (maximum10 cm), mottles, and tiny fragments of gastropod shells.

Unit 2 is composed of sticky clay (IF4) and has a disconformablecontact with Unit 1, defined by a change from clayey silt to claywith high moisture content. The maximum thickness is 30 cm, andthe unit is locally missing or very thin (2 cm). Kankars (2–3 cm)(IF7) are common, with some rhizoconcretions (5–10 cm long) androot traces.

Unit 3 is 1 m toN2 m thick, consisting mainly of loose, very finemicaceous sand (IE). Small kankars (2–3 mm) (IF7) are scattered inthe basal part. Two sand sheets, separated by a few centimetres ofsandy silt (IF5), lack internal stratification and are weaklypedogenised.

Unit 4 is a 2 m-thick, heterogeneous deposit that comprises sandysilt and silty clay with a few gastropod fragments (IF3 and IF5). Smallkankars (IF7) are scattered through the unit, and discontinuous kan-kar lumps 5 to 20 cm long are aligned at several horizons (dashedlines in Fig. 4a). In the northeastern area, a small channel deposit2 m thick and 5 m wide of very fine sand (IC) is present.

Unit 5 is a rhythmic light and dark interbedding of fine silty sand(IE) and dark yellow, poorly laminated sandy silt (IF4). Kankar (IF7) iscommon as disseminated small nodules and as larger lumps at somelevels. In both Units 4 and 5, mottles are relatively sparse comparedwith Unit 1.

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4.2. Facies interpretation

Unit 1 represents a distal floodplain environment as inferred frommuddy deposits that are moderately pedogenised. The presence of aconsiderable proportion of kankar, some of which forms more con-centrated layers, suggests periods of minimal deposition or disconti-nuities. This unit generally fines up but slightly coarser sediments atthe top suggest that the area was closer to a fluvial conduit, or expe-rienced a high-energy flood peak. The presence of gastropod shells atthe top of Unit 1 suggests standing water.

Unit 2 is interpreted to represent a widespread body of shallowwater, possibly a floodplain marsh. The presence of small rhizocon-cretions and root traces suggests that herbaceous or small woodyplants grew in the area, implying shallow water or periodic dryingup. This implies the presence of local floodplain wetlands or pondsrather than a persistent deeper lake (Glooschenko and Martini,1987). Unit 2 developed over an irregular floodplain surface that ap-pears to mark a discontinuity.

A major change is marked by Unit 3 which consists of aeolian de-posits, the base marking a discontinuity. An aeolian interpretation isbased on the predominance of near-structureless silt to very finesand with little evidence of pedogenesis; similar units are locally pre-sent across the southern Ganga Plains (see descriptions in Gibling etal., 2005, 2008).

Unit 4 is made up of stratified floodplain deposits and may indi-cate a floodplain environment closer to a major conduit. A composi-tion of silty clay, clayey silt and sandy silt, together with poorlydeveloped palaeosols and the fill of a shallow channel, suggests depo-sition as a natural levee (Tyler and Ethridge, 1983; Thomas et al.,

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

U 1

U 2

U 3

U 4

Soil

U 1

U 2U 3

U 4

U 5

CL

10 m18 m

15 m15 m

b c

a

Fig. 4. (a) Ganga valley-margin section at Mehdipur Ghat, Kannauj. The section has five major stratigraphic units defined as Units 1 through 5 (U1 to U5) from base to top. The upperpart of the section in places is modified by human activity (Cultural Level, CL) or capped by modern soil. (b, c) Field photographs showing units and facies in the valley margin cliffsection.

7N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

2002). Such interbedding of floodplain facies with varied grain size, inaddition to occasional breaks in deposition manifested as kankarlayers, suggests deposition during flood cycles of varied intensity.The channel body may represent a crevasse through the levee or asmall floodplain channel. Unit 5 is interpreted as a package of alter-nate aeolian, lacustrine and floodplain deposits, as identified by thepresence of weakly cohesive sandy silt, fine lamination and plantroots.

4.3. Chronology and architecture

Table 2 lists sample dates from the valley-margin section and fromcores (discussed later). Quartz fractions of 90–180 μm from Unit 1yielded ITL dates of 89±10 ka from the lower parts and one date of

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41±4 ka from the unit top at the position of Log 1. Another samplefrom Log 9 position gave a date of 100±11 ka for this unit. Unit 3yielded an ITL date of 35±4 ka. Unit 4 yielded ITL dates of 34±4 kaand 26±3 ka, the latter from the topmost beds of the unit. Althoughall OSL samples yielded saturated signals, the minimum dates of 15 to29 ka suggest that the exposed section entirely predates this period.

For Unit 1, dates between 100 and 41 ka indicate that the stratarepresent much of Marine Isotope Stages (MIS) 3, 4 and 5. Althoughtoo few dates are available to estimate sedimentation rate, the smallthickness (4 m) of the unit, along with the fine grain size and abun-dant kankar, suggests slow floodplain aggradation and probably thepresence of discontinuities. Although the topmost beds were notdated, the bulk of sediment accumulation in Units 2–5 (8 m) appar-ently represents a relatively short interval of more rapid

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

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accumulation, with much coarser sediment, between about 39 and23 ka (including error ranges), late in MIS 3 through to early MIS 2.The repeated intercalation of thin aeolian and lacustrine deposits inUnit 5 may reflect climatic fluctuation. At present, there is no indica-tion of deposition since about 23 ka, and the cliff-line may have beendegrading through this time.

5. Ganga Valley cores

5.1. Facies description and interpretation

Three channel facies are present (Table 3), namely sandy silt/siltysand (VC1), fine to very fine sand (VC2) and medium sand (VC3).The coarser channel sands have ‘salt and pepper’ texture due to theabundance of dark, dense minerals, which is characteristic of Himala-yan-derived sediment. Among the channel facies, the finer grained fa-cies VC1 generally occurs as relatively thin bodies up to 3 m thickwithin muddy successions and contains mottles and sparse carbonatenodules. These bodies are inferred to represent small tributary orfloodplain channels. In contrast, the fine- to medium-grained faciesVC2 and VC3 form continuous sand bodies up to 10 m thick that areinterpreted as the deposits of major trunk rivers.

Legend- valley fill

>28 ka valley edge

a

Fig. 5. Drill core facies logs along transect 1 (a), transect 2 (b) and longitudinally along the vashown by a solid thick line.

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Floodplain facies form only a modest proportion of most drill coresand consist mainly of silty clay with pedogenic features that includedark brown, yellow brown, and grey mottles, and carbonate nodulesthat are disseminated or locally concentrated in bands. The floodplainfacies are classified into grey silty clay (VF1), pale yellow silty clay(VF2), brown silty clay (VF3), and prominent bands of carbonate nod-ules (VF4) (Table 2). Aeolian deposits (facies VE) consist of structure-less, soft clay-rich silt that varies from light yellowish brown to paleyellow. Carbonate-cemented layers and thin clayey silt and silt lami-nae are characteristic, probably reflecting the presence of lowmounds of wind-blown material. This facies appears more clay richand better stratified than inferred aeolian facies in the cliff section,and the aeolian interpretation must be considered provisional.

Lacustrine deposits (facies VL) were identified as a 4-m-thick, or-ganic-rich layer in one log (core 7) above thick channel sand, andare present in other logs as a thin layer above channel fills. The lacus-trine interpretation is based on the sheet-like geometry, the fine lam-ination, and the distinctive lithology of dark olive grey, stiff and stickyclay. This facies is similar to the floodplain marsh deposits in Unit 2 inthe cliff section. Carbonate nodules and locally prominent mottlingpoint to a moderate degree of pedogenesis. This facies is attributedto temporarily or permanently abandoned channels, probably underbackswamp/marshy conditions.

Late HoloceneChannel

Channel/floodplain(MIS 3-2)

MIS 4/5 succession

Pre-LGMChannel

Legend- valley margin

lley (c) showing stratigraphic units and lateral correlation. The modern Ganga Valley is

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

LateHoloceneChannel

Channel/floodplain(MIS 3-2)

MIS 4/5 succession

Channel margin/Floodplain(MIS 3-2)

Pre-HoloceneChannel

Intra- valleyFloodplain (MIS 2)

Legend- valley margin

Legend- valley fill

MIS 3 valley edge

b

Fig. 5 (continued).

9N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

5.2. Chronology and architecture

Fig. 5 shows seven drill core logs in two cross-valley tran-sects (Fig. 5a, b) and one transect along the valley (Fig. 5c;see locations in Fig. 1b). The cores consist of channel bodiesseparated by floodplain, lacustrine and probable aeolian de-posits that cover a time span from about 74 ka to 0.1 ka(Table 2). Five stratigraphic units were identified on the basisof lithology and age. In contrast to the sheet-like nature of val-ley-margin units, the cored units appear to be lensoid and lo-cally incised into underlying units, making unit designationless reliable. Although the seven cores are insufficient for acomplete valley-scale correlation, some units can be traced be-tween the two transects.

Unit 1 (cores 1, 2, 4 and 5, base not penetrated) consists of 2 m offine to very fine channel sand (facies VC2). The channel sand yieldedIRSL dates of 74±3 ka and 61±2 ka, and a minimum OSL date of55 ka (Table 2) suggests that this channel activity corresponds to top-most MIS 5 and MIS 4. The considerable difference in dates from cores2 and 4 suggests more than one episode of channel activity, possibly

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with associated incision. Unit 1 is present in cores close to the mod-ern, southern Ganga Valley margin, but in neither transect were thelowermost channel sands fully penetrated, and the unit may be wide-spread below the modern Ganga Valley.

Unit 2 consists of alternate floodplain (VF1–VF3) and aeolian (VE)beds. Thin sand layers (splays) yielded an IRSL date of 37±2 kawhich corresponds to late in MIS 3, and two OSL samples providedminimum dates of 27 ka and 23 ka (Table 2). Grey mottles indicatea weak degree of pedogenesis. In core 2, a sharp break within Unit 2is marked by the presence of a concretion zone at 25 m depth. Mod-erately pedogenised muds suggest slow deposition or periods ofnon-deposition. In core 2, a kankar horizon at the 25 m level(Fig. 5a) above 3 m of undated floodplain deposits may represent asignificant discontinuity. Although shown within Unit 2, fine sedi-ments below this kankar level are provisionally taken to mark the ter-mination of MIS 4/5 deposits.

New channel activity during MIS 3 (37 ka) is recorded from thinsands below a thicker (3 m) channel body in core 2 (Fig. 5a). Channeldeposits are also recorded prior to 27 ka (a minimum OSL date) incore 4 (Table 2). Both of these channel sediments are capped with

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

?

? ?

Pre-HoloceneChannel

Intra-valleyFloodplain/channel(MIS 2)

Late-HoloceneChannel

Pre-LGMChannel

Longitudinal Transect

>20 ka

c

Fig. 5 (continued).

10 N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

thick, strongly pedogenised overbank deposits 7 m thick, suggestingreduced sedimentation late in MIS 3. In contrast, floodplain depositslower in Unit 2 show less intense pedogenesis, probably within cu-mulative floodplain soils. Similar weak pedogenic activity duringfloodplain aggradation has been documented from eastern GangaPlains interfluves (Mohindra et al., 1992; Sinha and Friend, 1994).

Unit 3 consists of 3 m of medium sand (VC1) and we interpretthis as the top of a thick channel body that extends below thebase of core 3, the only place where it is recorded. An OSL dateof 28±2 ka from core 3 (Table 2) suggests deposition late inMIS 3. The channel sands lie close to the left margin of the pre-sent Ganga Valley and their elevation indicates that they are in-cised into older strata.

Unit 4 constitutes one or more major channel bodies composed ofvery fine to fine sand (VC1 and VC2), recorded in cores 6 and 7 only.In core 6, two distinct sand bodies are separated by 5 m of floodplainmud (VF2). Five dates bracket the channel sediments between 15 and11 ka (Table 2), representing MIS 2 to early MIS 1 (latest Pleistoceneto earliest Holocene) and suggesting that 20 m of strata were depos-ited in as little as about 4000 years. The channel body is interpreted asa braided-fluvial deposit, and it lies close to the left valley marginwhere it appears deeply incised (Fig. 5b).

Unit 5 is represented by a sand body (VC1–VC3) more than15 m thick, capped by thin muddy deposits that were probablylaid down within abandoned channels. Seven dates range between2.5±0.16 ka and 0.18±0.02 ka (Table 2), representing late Holo-cene channel activity. These channel deposits at the top of thesuccession are widespread and were penetrated in five cores, var-iously incised into Units 2, 3 and 4 (Fig. 5b). The sediments

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resemble those of the modern river, and they are interpreted asbraided-fluvial deposits.

6. Synthesis of Ganga Valley and valley-margin records, in compar-ison with the monsoon record

The Ganga Valley-margin exposures and subsurface deposits areillustrated schematically in Fig. 6, along with proxy records for mon-soon intensity based on stacked core records for the Arabian Sea(Clemens and Prell, 2003) and modelling results for the SW IndianMonsoon (Prell and Kutzbach, 1987). The latter curve is closely linkedto insolation trends, and changes in modelled intensity approach 30%of present values. The two proxy records differ in detail but showbroadly similar high- and low-intensity periods, fluctuating over pe-riods of a few thousand to about 20,000 years. The fluctuationsshow general accord with marine isotope stages determined from off-shore cores (Waelbroeck et al., 2002; Fig. 6).

In interpreting our records, we have drawn strongly on the rangeof dates provided by the 13 finite OSL dates and 9 ITL and IRSL datesto establish periods of channel aggradation and incision. Althoughmore dates are necessary to provide a comprehensive understanding,the dates were obtained from precisely documented strata, common-ly to test the age of key boundaries, and they suggest several distinctperiods of fluvial activity, which are especially clear from about 30 kaonwards.

The interfluve successions are characterised by overbank fines,whereas the valley succession comprises channel bodies and fines,and the relationship between them is a key issue. The valley-margindeposits have a sheet-like geometry and aggradational style, in

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

Table 3Facies group, facies, description, association and interpretation of cores below the Ganga Valley.

Facies Description Abundance and associated facies Interpretation

Floodplain faciesVF1. Grey silty-clay Silty clay, structureless, moderately tough,

sticky due to high clay concentration, raremottles. Tiny kankar nodules and Fe–Mn blackcoated nodules (2–3 mm size) are rare. Munsellcolour brown grey (2.5Y6/2) to grey (5Y6/1)with a greenish tinge.

Present in three cores (5, 2, 4), in unitswith maximum thickness up to 1 m (Core-2),forms incomplete cycle; commonly truncatedbelow base of channel sand.

Floodplain deposits; dark colour suggestsdeposition under reducing condition

VF2. Pale yellow silty-clay Soft silty clay with high clay proportion, stickyand tough with uncommon reddish brown andyellowish brown mottles in Core-5. Kankarsfrom 1 mm to 2 cm are scattered and fairlyabundant. Munsell colour: pale yellow siltyclay (5Y8/4 to 5Y7/3)

Abundant facies, present in Core-5 andCore-4. Maximum thickness is 2.7 m. Thisfacies frequently interrupted by kankar zones(20–30 cm thick). Commonly associated withsilty sand (VC1) and locally clayey silt (VE).

Floodplain deposits, weakly pedogenisedwith soil carbonate nodules

VF3. Brown silty clay(with/without mottles)

Soft clay and silt mixture; cohesive, stickynature — due to relatively enrichment in clay.Mottles are common to few, scattered toconcentrated. Black mottles are commonand yellow brown mottles are frequent.Fe–Mn nodules and kankars are abundant.Munsell colour: brown silty clay(2.5Y 5/4 to 2.5Y 6/4)

Maximum thickness 4 m, associated withsilty sand, clayey silt facies.

Floodplain deposits; moderate to strongpedogenesis, with abundant soil carbonateformation, Fe–Mn nodules indicate gleyingeffect.

VF4. Carbonate nodules Layers of concentrated carbonate nodules(diameter varies from 2 to 3 cm up to 5 cm),rounded to angular. Most nodules arecarbonate-cemented sand.

Layers maximum 50 cm thick associatedwith facies VF1, VF2 and VC1.

Layered concretion, probably ofgroundwater origin.

Lacustrine faciesVL. Dark silty clay Sheet like geometry and fine lamination,

stiff and sticky clay, maximum 1 cm diameterkankar nodules sparsely distributed, mottlesare uncommon to common.

Maximum thickness 3 m, associated withfine channel sand facies in Core-7, in othercores thickness is a few centimetres.

Floodplain deposits with low degree ofpedogenesis formed under reducingconditions created due to ponding withinchannels. Presence of carbonate nodulespoints to moderate degree of pedogenesis.Munsell colour: dark olive grey (5Y3/2)

AeolianVE. Yellow clayey silt Silt with few clay, non-cohesive, low skewness

relative to other facies, small kankars sparselydistributed, rare black nodules and yellowbrown mottles. Carbonate cemented bands,and thin lamination of clayey silt and siltmaterial are characteristic. Munsell colour:light yellowish brown (2.5Y 6/3, 2.5Y6/4) topale yellow (2.5Y7/3; 5Y/8/3)

Maximum thickness 3.5 m in the Cores-1 and-5; mainly associated with floodplain facies

Weakly pedogenised sheets of massivewindblown massive sheet sand; thinlamination probably reflects the presenceof low mounds of wind-blown material.

Channel faciesVC1. Yellow silty sand Silty and very fine sand, with minimal clay,

slightly micaceous, mottles and sparsecarbonate nodules. Units fine upwards.

Maximum thickness 3 m in Core 4 associatedwith floodplain facies.

Coarse sediment body associated withsmall channel, probably a tributary orfloodplain channel. Dark yellow siltresembling eolian facies suggests thatdeposition occurred in drier climatic phase.

VC2. Very fine sand andfine sand

Very fine and fine sand mixture. Sharp breakoccurred with medium sand facies. Smallkankars are sparsely distributed.

Maximum thickness 7.4 m in Core-6, commonlyassociated with medium sand facies, andchannel fill top.

Thick channel body indicates large trunkchannel, probably braided deposits.

VC3. Medium sand Medium sand, off-white to white, mixed withfine and very fine sand, highly micaceous,presence of heavy minerals gives ‘salt andpepper’ appearance. Small kankars sparselydistributed. Generally well sorted.

Maximum thickness of 10 m in Core-3, at thebase of the core reworked carbonate gravelpresent.

Thick channel deposits of large trunk riverlaid down under high energy condition.Rare thin beds intercalated with floodplaindeposits are interpreted as splays frommajor channel.

11N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

contrast to the lensoid geometry of the sub-valley deposits which im-plies both aggradation and incision. Although the sheet-like form ofthe valley-margin deposits may in part reflect the orientation of thecliff exposure parallel to the river course, the two successions differconsiderably in facies and are largely coeval. Thus, the two compo-nents must be viewed as representing discrete but related geomor-phic regions, and the correlation between them potentially yieldsconsiderable insight into fluvial dynamics through time. Additionally,our cores from the Ganga Valley allow some understanding of auto-genic channel relocation within the valley area.

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Five major aggradational periods can be recognised, based ondates and error bars. They are shown as schematic cross-sections (Ito V) in Fig. 6, and their inferred age ranges are shown alongsidethe proxy records for climate change. Probable periods of incisionare also shown.

6.1. Period I: MIS 5 to early MIS 3 (111–59 ka)

The lowermost floodplain deposits (Unit 1) of the valley-marginsection are bracketed between 100 and 41 ka (Fig. 4a), a period of

fluve flooding in the Ganga Valley over the past 100,000years: Test-col. (2011), doi:10.1016/j.palaeo.2011.08.012

Fig. 6. Ganga valley-fill stratigraphic architecture model for the Kannauj area, showing periods of incision and aggradation compared with proxy climatic records and MarineIsotope Stages (MIS), from about 110 ka to present. Summer Monsoon Factor is from Clemens and Prell (2003), based on stacked Arabian Sea core records, and the modelledSW Indian Monsoon record is from Prell and Kutzbach (1987). The boundaries between marine isotope stages are from Waelbroeck et al. (2002). Aggradational and incisional pe-riods are outlined more fully in the text, which also compares the record near Kannauj with other palaeoclimate records in Asia.

12 N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

nearly 60,000 years. Floodplain development during this period wasinterrupted by multiple pedogenic events, possibly indicating slowsedimentation or short-term discontinuities, and more intense pedo-genesis is apparent in the topmost beds.

Below the modern valley, the oldest channel bodies (partly pene-trated in Unit 1) yielded finite dates of 74 ka and 61 ka, and thus ap-pear to be in part time-equivalent to the basal floodplain unit of thevalley margin (Fig. 6). However, a difference in elevation of 25–30 m is recorded between the base of floodplain deposits (Unit 1) inthe cliff section (Fig. 4a) and the top of channel deposits of Unit 1below the valley (Fig. 5a, b). This elevation difference suggests thepresence at this time of an incised valley with channels that were ca-pable of intermittently flooding the interfluve to the south. Minor dis-continuities in the valley-margin deposits are inferred from thevaried intensity of pedogenesis and may reflect the episodic natureof this inundation. Over the past century, low- to high-stage seasonalfluctuations of the Ganga and Yamuna rivers have often been extreme(in excess of 20 m: Gibling et al., 2005), and the highest flows may in-undate parts of the proximal interfluves, depending on the localheight of the cliffs.

An alternative interpretation is that alluvium had built up in thearea of the modern valley to a higher level than that presentlyrecorded, allowing frequent interfluve inundation. Valley incisionmay have subsequently removedmuch of this sediment and detachedthe interfluve from the trunk channel, reducing sediment supply andinitiating interfluve pedogenesis.

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Cliff sections along the modern Yamuna River, 70 km southwest ofthe study area, also expose a thick floodplain succession dating backto ~100 ka, with prominent discontinuities (Gibling et al., 2005;Sinha et al., 2005b; Tandon et al., 2006). Although Unit 1 in theGanga Valley was incompletely penetrated, the range in ages forchannel bodies suggests multiple channel events and possibly inci-sional discontinuities, perhaps in accord with modelled strong fluctu-ations in monsoon intensity in MIS 4 and 5 (Fig. 6).

In general, we infer prolonged channel activity during MIS 5 and 4and at least some interfluve flooding, followed by a reduction in chan-nel activity associated with pedogenesis of floodplain muds. A similarperiod of persistent channel activity in MIS 5/4 was inferred for stratain the Belan River at the southern margin of the Ganga Plains (Giblinget al., 2008).

6.2. Period II: MIS 3 (45–30 ka)

Deposits that correspond to MIS 3 are represented by Unit 2 of thevalley-fill, where a splay associated with minor channel fills yielded adate of 37 ka. Probable aeolian strata of pale, friable silt are prominentin this unit. During late MIS 3, the valley-margin succession recordsthin, non-fluvial deposits (floodplain marsh and aeolian sedimentsof Units 2 and 3). This suggests that flow through the channel was ei-ther drastically reduced or that the trunk river migrated away fromthe southern valley margin, reducing sediment supply to the area. Aperiod of floodplain degradation, possibly associated with reduced

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sediment supply, is recorded in the Yamuna valley ~70 km south ofKanpur (Fig. 1a) between 30 and 40 ka, manifested as the cutting ofdeep gullies and their subsequent filling with colluvium (Gibling etal., 2005). Period II may correspond with reduced monsoonal activity,although the two proxy records show some differences through thisperiod.

6.3. Period III: latest MIS 3 and early MIS 2 (30–23 ka)

Renewed fluvial activity is recorded in the cores late in MIS 3.High-energy channel activity (gravelly sands) dated at 28 ka arerecorded near the left margin of the present Ganga Valley (Fig. 6),and their relative depth suggests that incision occurred prior to allu-viation. This channel activity is time-equivalent to a channel fillrecorded at Bithur 40 km downstream from the study area by Sinhaet al. (2007a).

In the valley-margin succession, levee deposits of Unit 4 yieldeddates of 34 and 26 ka, the latter from a small channel body. There issome uncertainty as to whether these correspond to Periods II or III,but the unit top (at least) is attributed to Period III because the youn-ger date is time-equivalent to channel deposits below the valley, andbecause the presence of renewed overbank deposition along the val-ley margin is consistent with high discharge late in MIS 3. As withStage I, the deposition of sediments in both geomorphic areas may re-flect extreme floods from an incised valley or alluviation within thevalley to an elevation higher than presently recorded.

Goodbred (2003) suggested that high-intensity floods would havebeen active during a period of strong solar insolation at about 31 ka,associated with large-scale sediment production from the Himalayansource area (see also Sharma and Owen, 1996; Taylor and Mitchell,2000). The proxy and modelled monsoon curves suggest that thesouth Asian climate was wetter late in MIS 3 or in earliest MIS 2(Fig. 6).

No channel deposits are recorded below the Ganga Valley be-tween 28 ka and 15 ka, suggesting that this interval, which includesthe global Last Glacial Maximum (LGM), experienced reduced fluvialactivity. Relatively strong pedogenesis is noted below much youngerdeposits in the topmost muddy parts of Unit 2 in cores 2, 4 and 5,and an interval of soil formation may accord with reduced activity, al-though these strata are not constrained by dates.

The valley-margin succession is capped by aeolian, lacustrine andfloodplain deposits of Unit 5. Although undated, these suggest a cool-er and drier climate towards the LGM. Aeolian accumulation on thevalley margin may reflect “underfit” of the Ganga River during theLGM and exposure of the river bed, from which silt was preferentiallypicked up by winds. Similar relationships have been documentedfrom Bithur (lake and aeolian deposits at ~27 ka; Gibling et al.,2005) in the Ganga Valley and in the Belan valley (aeolian depositswith broken gastropod shells at ~17–20 ka; Williams et al., 2006;Gibling et al., 2008). Evidence of reduced siliciclastic supply in theGanga–Brahmaputra delta (Goodbred, 2003; Weber et al., 2003) inthe later part of MIS 3 is consistent with reduced hinterland supplyor possibly some sediment storage in inland valley areas.

6.4. Period IV: late MIS 2 and early MIS 1(16–11 ka)

Channel aggradation was recorded in Unit 4 of cores 6 and 7,where five finite OSL dates in the topmost 20 m of strata range be-tween 15.1 and 11.7 ka. The presence of muds between channel bod-ies suggests channel switching in a braided depositionalenvironment. These thick braided-channel deposits imply high sedi-ment supply from the hinterland. This may reflect landscape instabil-ity due to monsoon intensification following the LGM (Overpeck etal., 1996; Fig. 6), inducing extensive slope failure and transport of hill-side regolith to the valley floor (Pratt et al., 2002). The valley marginappears to have been largely inactive and possibly degrading during

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both Periods IV and V, as indicated by ITL dates and by minimumOSL dates N15 ka, although later gully erosion (Fig. 4) may have re-moved much of the record.

Dates of 13.0±0.7 ka and 11.7±0.7 ka for the top of the Unit 4channel bodies in cores 6 and 7 suggest that the channels aggradedrapidly during a period of reduced discharge between 13 and 11 ka.This interval corresponds to the Younger Dryas, for which lake re-cords at Sanai Tal 200 km east of Kanpur document a period of re-duced monsoonal strength between 13.4 and 12.2 ka (Sharma et al.,2004). Elsewhere in the Ganga Plains, evidence for reducedmonsoon-al strength and decreased discharge at this time includes the presenceof aeolian and lacustrine strata at Bithur on the Ganga Valley marginwith dates from the topmost part at ~13 ka (Srivastava et al., 2003;Gibling et al., 2005), gully erosion and aeolian accumulation atMawar on the Sengar River between 13 and 9 ka (Gibling et al.,2005), aeolian sands in incised channel in the Belan valley between12.2 and 13.4 ka (Gibling et al., 2008), and aeolian sands east ofDelhi dated at 12.3 ka (Tandon et al., 2006). Cool, dry conditions asso-ciated with the Younger Dryas event have been widely identified inother Asian continental deposits (Porter, 2001).

The alluviation episode followed an earlier period of channel inci-sion (Fig. 6). This lowering of the channel's equilibrium profile ac-cords with a period of monsoon intensification following the LastGlacial Maximum — a pattern observed across much of southernAsia (Tandon et al., 2006).

6.5. Period V: MIS 1 (2.7 ka to present)

Late Holocene channel bodies aggraded across much of the pre-sent valley area, and sediments accumulated within a short periodof 2.7 to 0.1 ka, based on seven OSL dates. The base of these depositslies adjacent to much older strata and directly overlies MIS 3/2 de-posits, suggesting that valley incision preceded filling. Rapid accumu-lation during the late Holocene was also observed in the Yamunavalley, 70 km to the southwest, where a 20 m core penetrated a12 m channel-margin succession of interbedded sand and clay, yield-ing two dates of 2 ka or younger from the lower strata (Sinha et al.,2005b, 2009). Late Holocene aggradation is inferred to reflect a pro-nounced decline in monsoon intensity and transport capacity sincethe mid Holocene (Overpeck et al., 1996; Fig. 6). In the case of the Ya-muna succession, accumulation may be associated with a recentonset of Yamuna river deposition in the area (Sinha et al., 2005b,2009).

During this period, there is no indication of deposition along thevalley margin, which was probably undergoing degradation asrecorded in the modern gullied landscape adjacent to the valley.The river's equilibrium profile was apparently so low in the post-LGM period that the river was not able to overtop the interfluve valleymargin. Southward migration is manifested by the Ganga's presentposition at a cliff line along the southern valley margin, indicatingwidening of the valley floor.

7. Discussion

The Ganga Valley in the study area is 1200 km inland from thetidal limit, and it is unlikely that depositional architecture was affect-ed by sea-level changes in the past to such a distance inland (Sinha etal., 2005b; Tandon et al., 2006; Sinha et al., 2007a,b; see also Blum andTörnqvist, 2000). The valley deposits are entirely terrestrial facies ofthe foreland-basin fill, and no marine beds have been encountered.There is no recorded history of seismic events from this region, norindications of seismic-related deformation in core or outcrop as ob-served at other sites (Gibling et al., 2005), and basement faults areburied under a thick alluvial cover. Some workers (Singh et al.,1997; Agarwal et al., 2002) suggested a tectonic influence on majorriver incision due to forebulge activity, but this has not been

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14 N.G. Roy et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

established by regional geological mapping or convincing geomorphicevidence. It is unlikely therefore that the Ganga Valley fine-scale ar-chitecture in this area was controlled by tectonic activity and sea-level change during the late Quaternary.

Paola (2000) suggested that, over time scales of 103–105 years(Milankovitch time scale), patterns of deposition for foreland-basinsettings reflect cyclical change in water and sediment supply due toclimate shifts; over longer time scales, the influence of basin subsi-dence and other tectonic factors may be more apparent. Flume exper-iments and field studies in the Himalayan Foreland Basin andelsewhere have identified sequences generated as a result of alternatelow and high discharge, without any change in subsidence rates,(Bull, 1991; Milana, 1998; Weissmann et al., 2002), and an analysisof successions in southern Asia supports Paola's assertion (Giblinget al., 2011).

Quaternary records suggest that the strength of the Southwest In-dian Monsoon has varied significantly on a timescale of centuries tomillennia. Evidence of such variations is provided by well dated,multi-proxy records from cores in the Arabian Sea (Clemens andPrell, 2003; Rao et al., 2008) and Bay of Bengal (Chauhan et al.,2004), continental fluvial records (Gibling et al., 2005, 2008; Tandonet al., 2006; Sinha et al., 2007a,b), and lake records (Sharma et al.,2004; Prasad and Enzel, 2006) from northern India, as well as fromrecords on the Tibetan Plateau (van Campo and Gasse, 1993). Forthe western Ganga Plains, Gibling et al. (2005, 2008), Tandon et al.(2006), and Sinha et al. (2007a) inferred strong correlation of fluvialsedimentation with variation of monsoonal rainfall regime, and asimilar conclusion was reached on a basinal scale by Goodbred(2003). Based on the continental alluvial record from the southernmargin of Thar Desert in northwest India, Juyal et al. (2006) showedstrong monsoonal variability over the last 130 ka, and Bryson andSwain (1981) documented a period of strong early Holocene precipi-tation, decreasing by about one third by the late Holocene.

Such fluctuations in monsoon intensity should have resulted insignificant change in effective discharge of the Ganga River, causingrise and fall of the river's equilibrium profile. These discharge changesare likely to be manifested in alluvial architecture in the valleys andon interfluves, especially if the magnitude of change was sufficientfor the system to cross a geomorphic threshold. The availability of arobust proxy record for monsoonal fluctuations extending back be-yond 100 ka has allowed us to compare climatic events with alluvialevents of incision and aggradation in the Ganga River area at Kannauj.Our results suggest that, to a first order, alluvial events correspondwell with monsoonal fluctuations, and this correspondence is espe-cially evident for the well dated record after 30 ka.

A particularly provocative conclusion is that several well-datedperiods of channel aggradation correspond to times of decreasingmonsoonal precipitation (Fig. 6): during latest MIS 3 and earliestMIS 2 (Period III), during the Younger Dryas (Period IV), and duringthe late Holocene (Period V). These are inferred to record decreaseddischarge and rise in the river's equilibrium profile. In all thesecases, elevation considerations indicate that aggradation was preced-ed by incision, suggesting that periods of increasing monsoonal pre-cipitation were associated with high river energy and a lowering ofthe equilibrium profile. Spatial considerations suggest that thesewere also times of channel relocation within the valley (Figs. 5, 6).Occurrences of aeolian and lacustrine deposits, as well as intensityof pedogenesis, also play an important role in “reading” palaeocli-matic changes. Earlier alluviation and incisional episodes have lesswell-constrained durations, and are less readily compared with mon-soonal changes.

For the latest Holocene aggradation (Period V), low effective dis-charge of the river channel is in accord with a wealth of data acrossAsia that documents severe weakening of Southwest Indian Mon-soon, evident in records of Arabian Sea upwelling (Gupta et al.,2005; Thamban et al., 2007) and in historical records from north

Please cite this article as: Roy, N.G., et al., Aggradation, incision and intering the influence of monsoonal p..., Palaeogeogr. Palaeoclimatol. Palaeoe

India (Pandey et al., 2003). Low percentage of Globigerina bulloidesin sediments from the Oman coastal margin indicate reduced upwell-ing and a monsoonal minimum about 2000 years B.P. (Gupta et al.,2005). Aridity records based on dolomite percentage from the Arabi-an Sea margin (Sirocko et al., 1993), biological productivity off the So-mali margin (Ivanocho et al., 2005), precipitation records off theIndus river mouth (Staubwasser et al., 2003) and from the southwestIndian margin (Sarkar et al., 2000) further suggest a declining mon-soon about 2000 years B.P. Such a climatic change would have re-duced sediment transport capacity of the river system, triggeringaggradation. This inference is borne out impressively by the presenceof more than 15 m of late Holocene sands below a considerable partof the Ganga Valley, deposited within a period of less than 2000 years.

Numerous studies have demonstrated that periods of increasedprecipitation are marked by river incision (Porter et al., 1992; Jonesand Schumm, 1999). Pratt et al. (2002) observed that early Holoceneincision in the central Nepal Himalaya was driven by enhanced mon-soon precipitation, although some deposition was also evident. Al-though enhanced precipitation commonly results in higherdischarge, increased sediment supply during the same period may re-duce the stream's erosive power and the ability to incise its thalweg,at least locally.

The High Himalaya source area of the Ganga and its tributariescontains large glaciers. Meltwater apparently contributes only mod-est volumes to Ganga flow (15%, according to Das Gupta, 1975), butmeltwater contributions could have been greater in the past. Theareal extent and timing of glacial events in the Himalaya and TibetanPlateau are poorly defined, but compilation of cosmogenic isotopedates for moraines and glacially eroded surfaces indicate that themountain belt experienced numerous glacial advances from MIS 5onwards (Owen et al., 2002, 2008). However, glacier ice during theLGM covered about 539 km2 of the headwater area of the GangaRiver catchment (Fig. 1c), about 22% of the total area. For the centralHimalayan area in the Ganga headwaters, Owen et al. (2008)recorded a general correlation between glacial advances and themonsoon intensity peaks identified by Prell and Kutzbach (1987), in-cluding advances during a peak monsoon condition in MIS 3 (Fig. 6).This correlation suggests that alluvial events are likely to be aconsequence of variations both in direct monsoonal runoff andin meltwater supplied to the plains. In the western Himalaya, cor-relation is less clear, due to the availability of precipitation sup-plied by westerly winds. We conclude that 1) alluvial events onthe Ganga Plains are likely to reflect in part meltwater contribu-tions, 2) meltwater pulses are likely to coincide with enhancedmonsoonal runoff, and 3) meltwater was probably not the domi-nant discharge factor, based on estimates of modern meltwatercontributions and past glacier area.

Episodes of aggradation and incision may be attributed to varia-tion in transport capacity of river systems and/or sediment supplyfrom the hinterland (Julien, 1995; Houben, 2003) linked to changesin catchment physiography and external forcing. Assuming thatcatchment physiography and valley gradient remained relativelyunchanged during the late Quaternary, a change in the balance be-tween sediment supply and river discharge (from hinterland and ad-jacent plains) should change the river's equilibrium profile on theplains. The terms capacity-limited system and supply-limited systemhave been used to explore this balance (Brown and Keough, 1992;Lewin, 1992; Macklin, 1992; Julien, 1995; Bisson and Montgomery,1996; Montgomery and Buffington, 1997). In a capacity-limited sys-tem, the amount of transported load is limited by water supplied tothe river, whereas in a supply-limited system, sediment in the riveris controlled by sediment availability. Goodbred (2003) emphasisedthe importance of upland supply in a source-to-sink considerationof the Ganga–Brahmaputra system, and strong variations in sedimentflux from the Himalaya are indicated (see, for example, Pratt et al.,2002). We are currently not able to discern which events in the

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Ganga Plains reflect supply- or transport-limited scenarios. However,late Holocene channel aggradation followed a period of high sedi-ment supply in the Ganga Delta (Goodbred, 2003), suggesting thatdeposition reflected transport-limited conditions.

Despite the availability of data from both valley-margin andintra-valley areas, it is difficult to be certain when the Ganga Riveroccupied a channel attached to the adjoining alluvial tract or flood-plain, and when the river occupied a valley that was detached andrarely or never flooded the adjoining interfluves (attachment termi-nology from Gibling et al., 2005). Strong and Paola (2008) distin-guished stratigraphic valleys (the valley-form erosional surfacepreserved in the stratigraphic record) from topographic valleys (thetopography at an instant in time). As noted by Gibling et al.(2011), the Ganga Valley in the Kannauj to Kanpur area (Fig. 1) isa stratigraphic valley that contains smaller channel bodies or valleyfills, as illustrated in Fig. 6, with internal sequence boundaries(bases of channel bodies and carbonate-rich palaeosols) and faciesdiscontinuities. It is not certain that deposition commenced in alarge, deeply incised hollow, as shown schematically in Fig. 6.More probably, the body of alluvium laid down by the Ganga is un-derlain by a composite valley-fill unconformity (Blum and Aslan,2006), and long-term southward migration of the Ganga has createda composite, diachronous valley fill with dimensions that greatly ex-ceed the instantaneous topography. The relocation of channels with-in the valley area, evident from the cores (Figs. 5, 6), may in partrecord the influence of the Ramganga and other tributaries (Fig. 1)in modifying the trunk-river position due to confluence dynamics(Roy and Sinha, 2005, 2007).

8. Conclusions

The exposed cliff section and valley cores from the southern GangaPlains provide an integrated picture of valley and proximal interfluvedevelopment for the Ganga River at Kannauj over the last 100 ka.Comparison with proxy records for monsoon strength, which showstrong fluctuations driven largely by solar insolation, indicate a first-order fit between monsoonal fluctuations and alluvial events, with atendency for incision during periods of monsoonal intensificationand aggradation as monsoon strength decreased. The valley cores re-cord episodes of aggradation during Marine Isotope Stages (MIS) 4and 5; the mid part of MIS 3; late MIS 3 (23 to 30 ka); late MIS 2 toearly MIS 1 (11–16 ka), especially around the Younger Dryas; and inthe latest Holocene (b2.5 ka). The Last Glacial Maximum was a timeof greatly reduced fluvial activity, and no channel sediments haveyet been identified that correspond with this period.

Along the southern valley margin, bordering a wide interfluve,there was modest accumulation of floodplain, lacustrine and aeoliandeposits, punctuated by discontinuities, through to early MIS 2. TheGanga probably varied in its degree of attachment to the interfluve,periodically inundating the adjoining, more elevated alluvial tractbut at other times unable to flood this zone. Thus, the Ganga probablyfluctuated from being a channel with a broad floodplain to a valleyconfined by an elevated plain to the south. There is no indicationthat the Ganga has inundated its southern interfluves since the endof MIS 3.

We infer that the Ganga River experienced strong variation in itsequilibrium profile over relatively short periods (centuries to thou-sands of years), linked to varied sediment and water discharge fromthe Himalayan mountains and the lowland alluvial plain.

Acknowledgements

This paper is a part of the Ph.D. thesis of NGR at Indian Institute ofTechnology Kanpur, and he acknowledges the Institute for the re-search fellowship for his tenure of stay. A major support for drillingcame from a research project funded to RS from the Department of

Please cite this article as: Roy, N.G., et al., Aggradation, incision and intering the influence of monsoonal p..., Palaeogeogr. Palaeoclimatol. Palaeoe

Science and Technology, New Delhi, and their generous support isthankfully acknowledged. NGR carried out the luminescence workat Nordic Luminescence Laboratory at Risø, Denmark, and the help re-ceived from AndrewMurray andMayank Jain for the dating work wasinvaluable. The support for this visit and dating costs were met from aDiscovery Grant from the Natural Sciences and Engineering ResearchCouncil of Canada to MRG. This is a contribution to IGCP 582 on Trop-ical Rivers.

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