Valley and interfluve sediments in the Southern Ganga plains, India: Exploring facies and magnetic...

1 (2007) 386–411www.elsevier.com/locate/sedgeo

Sedimentary Geology 20

Valley and interfluve sediments in the Southern Ganga plains, India:Exploring facies and magnetic signatures

R. Sinha a,⁎, P.S. Bhattacharjee a, S.J. Sangode b, M.R. Gibling c,S.K. Tandon d, M. Jain e, D. Godfrey-Smith c

a Engineering Geosciences Group, Indian Institute of Technology, Kanpur 208016, Indiab Wadia Institute of Himalayan Geology, 33 General Mahadev Singh Road, Dehradun 248001, India

c Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5d Department of Geology, University of Delhi, Delhi 110007, Indiae NUK-201, Risø National Laboratory, DK-4000 Roskilde, Denmark

Received 24 December 2006; received in revised form 28 June 2007; accepted 12 July 2007

Abstract

The western Ganga Plains of India show narrow, incised valleys that contain the major Himalayan-sourced rivers, separated bybroad interfluves traversed by small plains-fed rivers. From analysis of four cores and outcrop sections to 50 m sub-surface, thisgeomorphic setting appears to be of Late Quaternary antiquity. A major river (paleo-Ganga) has been located near its present positionsince at least 26 ka BP, with indications of southward migration between 11 and 6 ka. Valley fill cores show repeated cycles of valleyaggradation in response to Late Quaternary climatic oscillations, in accord with simulation models of drainage-basin response toclimate change, and reflect interplay of discharge and sediment supply. Floodplain sediments within the valley fill show a generallyweakly magnetic signature and only modest pedogenic activity. The valley margin records a major discontinuity that marks reductionof discharge in the Ganga River during the Last Glacial Maximum, when monsoonal precipitation was greatly reduced and lakes andeolian dunes occupied areas distant from the main channel. The lacustrine facies show marked magnetic enhancement, probablyreflecting bacterial action and the availability of organic matter. The interfluve record yields no indication of major-river activityduring the past 100 ka. Instead, the near-continuous activity of small plains-fed rivers generated cycles of alluviation and pedogenesis,the latter yielding strong magnetic signatures. Enhanced magnetic signatures are apparent in both valley and interfluve Holocenerecords, possibly linked to increased monsoonal strength, warmer conditions and enhanced rates of pedogenesis.© 2007 Elsevier B.V. All rights reserved.

Keywords: Alluvial stratigraphy; Incised valleys; Interfluve; Magnetic mineralogy; Floodplains; Quaternary; Ganga River

1. Introduction

The major rivers of the western Ganga plains in theHimalayan Foreland Basin of India rise in the mountainchains of the Himalaya or in the peninsular hills of India,

⁎ Corresponding author.E-mail address: [email protected] (R. Sinha).

0037-0738/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2007.07.004

and occupy narrow valleys incised through alluvium.Between the major drainage lines are broad interfluvestens of kilometers to nearly 100 km wide (Fig. 1a–c),traversed by small rivers fed from springs (plains-fedrivers of Sinha and Friend, 1994). Although bothmountain- and plains-fed rivers are active during themonsoon and show large stage fluctuations, the majorrivers do not inundate the interfluves at present, andtheir floods are retained within the valleys or cover only

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http://dx.doi.org/10.1016/j.sedgeo.2007.07.004

Fig. 1. (a) Part of the Ganga plains in the Himalayan Foreland Basin of India, to show the major rivers and the study area near the Ganga River at Kanpur. (b) Satellite image (IRS LISS III, May, 1997)of the study area. The Ganga River lies along the southern margin of its valley, which includes cut-off meanders, and the interfluve to the south is drained by small plains-fed rivers (Pandu and Rind).Black dots show the locations of drill holes and white dot is the exposed section at Bithur. (c) Geomorphic map of the study area, based on the interpretation of remote-sensing images.

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small parts of the adjacent interfluve (Tandon et al.,2006).

The modern setting of the western plains raises manyinteresting questions. How long has the Ganga River beennear its present position, and has it ever inundated theinterfluve to the south? What is the antiquity of themodern geomorphic setting, viz. valley-interfluve posi-tions, or have the major rivers been more mobile in thepast and able to migrate more freely? Although the area islargely aseismic, the valleys in this part of the plains haveprobably shifted progressively southward in response tothrusting and deformation along the Himalayan Front tothe north (Tandon et al., 2006). No data currentlyconstrain the rate of such migration, however. Further-more, the area has experienced large fluctuations indischarge and sediment yield induced by variations inmonsoon strength (Goodbred, 2003; Gibling et al., 2005),which is known frommodeling and field-based studies tohave varied radically over the past 100,000 years andbeyond (Prell andKutzbach, 1987; Overpeck et al., 1996).Are variations in monsoonal intensity reflected in thesediment record of valleys and interfluves, and candistinctive stratigraphic patterns be used to test models oflandscape evolution across the western plains?

In view of their contrasting geomorphic setting anddeposition from mountain-fed rivers or plains-fed rivers,valley-fill and interfluve sediments should differ con-siderably in their sedimentological characteristics andphysical properties. Age models, however, are still in theearly stages of development, and the alluvium hasreceived little analytical treatment such as mineralogyand geochemistry suitable for exploring these questions(see Jain et al., 2004, 2005). Present knowledge of Gangaplains alluvium is based on local studies of modern sandsand on exposed sections that are widely separated andpoorly dated (Singh and Rastogi, 1973; Singh et al.,1997, 1999; Sinha et al., 2002; Gibling et al., 2005).Gokhale (1971) documented heavy minerals from aborehole at Kanpur, and Singh (1975) described heavymineral and clay mineral assemblages near Lucknow(north of Kanpur), linking the mineralogy to Himalayansource areas. Studies of near-surface alluvium in theGanga–Ghaghra–Rapti interfluve (Fig. 1a) highlightedthe importance of clay mineral assemblages for inter-preting Holocene paleoclimate (Srivastava et al., 1994,1998). Sangode et al. (2007) compiled data onenvironmental magnetism of fluvial sediments in Indiaand included some data from the Ganga plains.

We have studied Ganga plains deposits in twoboreholes in the Ganga valley, in one exposed section atBithur along the Ganga valley margin, and in one bore-hole through the interfluve to the south (Fig. 1b).We have

previously published a detailed account of the alluvialarchitecture of several sections south of the Ganga (Sinhaet al., 2002; Gibling et al., 2005; Sinha et al., 2005a,b).The sections and boreholes described in this paperrepresent a valley-to-interfluve transect, and penetrateup to 50 m of strata. The nearly complete recovery ofcores allows an unusually complete analysis of sedimen-tary patterns. We present here a detailed stratigraphic andfacies analysis of the sections, and describe their sedimentmineralogy (bulk and clay-sized fractions) and magneticmineralogy, linked to facies types. An age model for thestrata is based on previously published dates (Giblinget al., 2005) and new dates obtained using radiocarbonand optically stimulated luminescence (OSL) methods.We conclude that valley-interfluve morphology in thispart of the plains is of considerable antiquity (probablyextending back to at least 80 ka in MIS 5) and that theGanga (or a similar large precursor river) has been near itspresent position since at least 30 ka, but has migratedsouthwards during the Holocene, incising its southernvalleymargin. The interfluve appears to have experienceda plains-fed river regime through this period. Sedimen-tological and magnetic signatures provide strong supportfor this inference, and also yield evidence for fluctuationsin monsoonal intensity.

2. Climatic, hydrologic, and geomorphic setting

The Indo-Gangetic plains represent the world’s largestarea of Quaternary alluvium and are of great cultural andhistorical interest. Precipitation in the Ganga plains studyarea is 800–1000 mm/year, and the maximum tempera-ture ranges from 35–45 °C (Singh, 1994). The monsoontypically arrives at the end of June and providesmore than85% of the total rainfall. The average annual discharge ofthe Ganga river at Kanpur is about 900 m3/s and thebankfull discharge is 8500 m3/sec. Flood frequencyanalysis for the period 1970 to 2003 indicates that themean annual flood is 6150 m3/s (N.G.Roy, pers.comm.) – much lower than the bankfull discharge –which confirms anecdotal information that the river rarelytops the valley margin at present.

The asymmetricGanga valley ismore than 20 kmwidenear Kanpur, with a surface elevation of 120–130 mabove mean sea-level. The Ganga River is typicallybraided in the region with a fairly high braid–channelratio (cf. Friend and Sinha, 1993) of 3.5–4.5 (Sinha et al.,2005c). The average channel depth of the Ganga nearKanpur is∼10m andwidth is about 600mwith a slope of0.00017. Using these data, the stream power of the Gangaat Kanpur for bankfull discharge is ∼24 W/m2, and thesurrounding floodplains would classify as ‘medium

389R. Sinha et al. / Sedimentary Geology 201 (2007) 386–411

energy, non-cohesive floodplains’ of Nanson and Croke(1992).

The active channel belt (Unit 1, Fig. 1c) of the GangaRiver presently follows the southern valley margin andhas extensive floodplain areas within the valley (Unit 2).Floodplain areas north of the Ganga include abandonedchannels, meander scars, and ox-bow lakes, with a fewsmall lakes and waterlogged areas. A large, conspicuousmeander scar (Fig. 1b,c) is cut off from the presentchannel but is periodically flooded by the Ganga. Thesouthern valley margin is strongly incised and, at Bithur,prominent cliffs up to 15 m high extend in a southeasterlydirection for nearly 2 km. This is in contrast to the easternparts of the Ganga plains where the rivers are character-ized by shallow aggrading channels, frequent channelmigration and extensive flooding (Phillip et al., 1989;Sinha, 1998; Sinha and Jain, 1998). Our earlier work hasdemonstrated that such geomorphic diversity is a functionof stream power and sediment supply (Sinha et al., 2005c;Tandon et al., 2006). The incised valleys in the study areaare manifestations of high stream power (20–40 W/m2)and low sediment supply compared to a much lowerstream power (5–10 W/m2) and an order of magnitudehigher sediment supply in the eastern plains (‘low energy,cohesive floodplains’ of Nanson and Croke, 1992).

Units 3, 4 and 5 constitute interfluves on either side ofthe Ganga valley. On the relatively level interfluve surfacesouth of the Ganga, small spring-fed rivers including thePandu and Rind are bordered by narrow floodplains(Unit 3). These single-channel, sinuous rivers are less than25mwide and 5m deepwith sinuosities varying from 1.5to 2.5 (Sinha et al., 2005c). During the monsoon, theserivers flow at bankfull level and occasionally overtop theirbanks. The remaining low-lying areas are under cultiva-tion, and are mapped as a slightly dissected surface(Unit 5). On the satellite image, this unit appears as afeatureless muddy tract with minor, isolated drainagelines, sparse vegetation, and waterlogged patches.

On the north side of the Ganga valley, a wide andrelatively inactive tract (Unit 4) has isolated channels,meander scars, and waterlogged patches. There are alsofrequent salt-encrusted areas where waterlogged tractshave dried out. This unit is topographically higher(by ∼20 m) than the active floodplain of Unit 2, butrises more gradually from the Ganga valley than theinterfluve to the south. It is also currently beyond the rangeof Ganga River flooding. The boundary between Units 2and 4 therefore marks the northern valleymargin (Fig. 1b).

The strength of the Southwest Monsoon, which isaffected by solar insolation patterns and glacial boundaryconditions, has been modeled in detail for the LateQuaternary (Prell and Kutzbach, 1987; Overpeck et al.,

1996). Precipitation changes are estimated to have been asmuch as ±30% regionally over the past 150 ka (Fig. 2a),although precipitationmay have doubled in relatively aridareas (Swain et al., 1983). Particular points of relevance tothe present study are the gradual reduction of precipitationduring Marine Isotope Stages 4 and 3, to a minimum inStage 2 during the Last GlacialMaximum (LGM), and therapid increase in precipitation from about 15 ka through toabout 7 ka in the early Holocene, with a modest reductionsince (Fig. 2b). Such changes have governed the waterand sediment discharges in the Himalayan rivers and, as aresult, the rivers have aggraded their floodplains, reducedtheir discharge, or have incised their valleys (Williamsand Clarke, 1984; Goodbred, 2003; Gibling et al., 2005).Fig. 2b demonstrates that monsoonal strength hasfluctuated over timescales of a few thousand years, aswell as on the glacial–interglacial timescale depicted inFig. 2a. This implies significant short-term changes inrunoff intensity in the catchment, possibly in part throughchange in storminess without much change in annualrainfall (Goswami et al., 2006). Valleys may respond tosuch changes through aggradation and degradation(Rinaldo et al., 1995; Tucker and Slingerland, 1997).

3. Field and analytical methods

Two drill cores 25 m long were obtained from theGanga valley at Jagdishpur and Firozpur (Fig. 1b). Thevalley margin is represented by 13 m of strata in the cliffsat Bithur, studied through photomosaics prepared fromserial photos and detailed logging at several locations. Thecore from the campus of the Indian Institute ofTechnology Kanpur (IITK) represents the southerninterfluve region. Drilling was carried out using adiamond core drilling bit involving a double barrel coretube. Core was collected in a PVC pipe and was split intotwo halves for logging and sampling.

Major stratigraphic units were defined in the exposedsection and drill cores based on detailed logging, and tenfacies were identified (Table 1). Representative sedi-ment samples were analysed for grain size, loss onignition, mineralogy, and magnetic characteristics.Grain size was determined using a visual comparator.Loss on ignition is an estimate of inorganic carbon(carbonates) in sediments and was computed as weightloss in the sample after combusting it at 950 °C whenmost carbonates are destroyed. For bulk mineralogy,powdered samples of both host sediments and carbonateconcretions were used, and smeared slides were preparedfor clay-sized fractions. X-ray charts were generatedusing an X-ray diffractometer model ISO Deby flex1001,1102 Rich-Seifert and Co., with technical settings

Fig. 2. Variation in precipitation associated with Southwest IndianMonsoon. (a) Modeling results for the past 150 ka, from Prell and Kutzbach (1987),shown in relation to proxy eustatic curve (Waelbroeck et al. 2002). MIS = Marine Isotope Stage. (b) Major trends in precipitation for the past 20 ka,from Overpeck et al. (1996). Geological information includes facies evidence from dated sections in the Arabian Sea and onshore in Asia, and theabundance of Globigerina bulloides and African pollen (linked to wind strength and upwelling) from Arabian Sea cores (from Tandon et al., 2006).


at 30Ma, 40 KV, 3°/min sweep, 5K CPM, and 10 secondtime constant, using CrKα radiation. For clay fractions insmeared slides, each slide was heated to 550 °C for1 hour in a muffle furnace to destroy kaolinite and alsoglycolated to facilitate identification of smectite. TheX-ray peaks were identified by comparing the d-spacing(Å) values of the each sample with diagnostic peaks of thestandard minerals. A semi-quantitative estimate ofmineral proportions was based on the peak area method(Biscaye, 1965; Cook et al., 1975; Mann and Muller,1980), with major clay groups proportioned to 100%.

Conventional and AMS radiocarbon dates wereobtained at Geochron Laboratories and calibrated usingthe Calib 4.3 computer programme (Stuiver et al., 1998);they are reported here as calendar years before present(B.P.). Samples for luminescence dating were collected

from sandy/silty horizons during the drilling itself andsealed in PVC pipes. Optically stimulated luminescence(OSL) dating estimates the time elapsed since thesediment was last exposed to sunlight, and has beenwidely used to obtain chronologies on fluvial systems(Jain et al., 2004). Measurement of an OSL age involvestwo steps: a) measurement of dose De (unit Gy) in thesample, and b) measurement of environmental dose rate(Gy/a) due to radioactive decays in and around thesample, and cosmic-ray effects. The ratio of the twoquantities gives an age. Usually quartz and feldsparextracts from the sample are used for De determination.

For samples from the Bithur section and the Firozpurcore, quartz measurements were made at the RisøNational Laboratory, Denmark. The samples werehandled under subdued red/orange light. One part of

Table 1Facies description for the Ganga valley and interfluve

Facies Description Colour Abundance and associated facies Interpretation

Channel facies (C)C1. Fine to medium sand Fine- to medium-grained sand, highly

micaceous. Local layers of reworkedcarbonate nodules. “Salt-and-pepper”appearance due to mixing of quartz andheavy minerals.

Grey (N6) Layers up to 7 m thick, commonlyresting on erosional surfaces andoverlain by C2.

Channel sands, typically atchannel bases.

C2. Silt and very fine sand Silt and very fine sand with minor clay,well sorted, carbonate nodulesuncommon.

Pale yellow(2.5Y 7/3)

Layers up to 3 m thick, commonlyoverlying thick units of C1.

Channel sand and silt,typically in upper parts ofchannel belts.

Floodplain facies (F)F1. Yellow–brown mud Soft silt/clay mixtures, structureless to

weakly banded. Carbonate nodules andrhizoconcretions common, with some darkFe/Mn nodules. Black and drab mottlescommon, red–brown mottles uncommon.Sparse gastropods and bivalves.

Yellowish brown(10YR 5/4) topale yellow(2.5 Y 7/3)

Most abundant facies, in units up to10 m thick. Forms basal parts of fine-grained cycles associated with F3.

Aggradational floodplaindeposits with slight degree ofpedogenetic modification.Locally within abandonedchannels.

F2. Red–brown mud Tough silt/clay mixtures, with local siltand sand patches, structureless. Carbonatenodules and black mottles common.

Brown(7.5 YR 4/3),red tingeprominent

Second most abundant facies, in unitsup to 4 m thick. Forms upper parts offine-grained cycles, associated withF4 and F5.

Floodplain deposits withpedogenetic modification andhardening.

F3. Thin silt/sand sheets Silt and very fine sand with minor clay,moderately well sorted. Structureless toweakly laminated, with some soft-sediment deformation. Black mottlescommon, carbonate nodules uncommon.

Yellowish brown(10 YR 5/6)

Common as beds up to 0.6 m thick, inlower parts of fine-grained cycles,associated with F1.

Crevasse splay deposits. Afew possibly eolian.

F4. Tough pale clay Tough clay with minor silt and sand,structureless. Carbonate nodules abundant.

Pale Yellow(2.5 Y 7/3)

Rare beds up to 0.5 m thick,associated with F2 and F5.

Floodplain deposits withcarbonate cementationprominent.

F5. Kankar Layers of concentrated carbonate nodules,up to 5 cm diameter, rounded to angular.Concentrated in bands.

Pale nodules inbrown to yellowmud matrix

Layers up to 1 m thick . Common inupper parts of fine-grained cycles,associated with F2 and F4.

Carbonate cementation infloodplains, of pedogeneticand groundwater origin.

Eolian facies (E)E1. Thick silt/sand sheets Pale silt to very fine sand, non-cohesive

well sorted. Carbonate-cemented bands,nodules and rhizoconcretions. Low-anglestratification made visible by carbonatecement.

Pale yellow(2.5 Y 7/3)

Sheets up to 2 m thick, interstratifiedwith L1 and L2.

Windblown sand and silt inpoorly stratified sheets withlocal accretion surfaces andgrowth of vegetation.

Lacustrine facies (L)L1. Dark clay Stiff clay with minor silt, banded to

structureless. Carbonate nodules andrhizoconcretions rare. Black, red–brownand drab mottles common, localslickensides and root traces. Sparsegastropods and bivalves.

Very DarkGreyish Brown(10 YR 3/2),with yellow siltbands.

Sheets up to 1 m thick, interbeddedwith F1 and E1.

Lake and backswampdeposits, locally inabandoned channels.Variable degree ofpedogenesis, locallydestroying stratification.

L2. Yellowlaminated silt and sand

Silt and very fine sand sheets, welllaminated and weakly cohesive, gray andred mottles. Carbonate nodules, bands andrhizoconcretions.

Olive yellow(2.5 Y 6/6)

Sheets up to 3 m thick, interbeddedwith E1.

Lake deposits, with slightpedogenesis.

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Table 2Mineral magnetic parameters (Thompson and Oldfield, 1986; Evans and Heller, 2003)

Parameters Definition Possible interpretation

K and χlf Magnetic susceptibility (K): The ratio of induced magnetizationto the applied magnetic field. Low-frequency susceptibility (χlf):the susceptibility at low frequency applications of a magneticfield.

Bulk magnetic mineral content of the sample.

χfd Frequency dependent susceptibility: The variation of susceptibilitybetween low and high frequency applications of a magnetic field.Indicates the presence of grains lying at the stable SD/SP a

boundary.

High values (N5%) indicate very fine ferrimagnetic grains(b0.02 μm).

Bocr Reverse field strength required to return a magnetized sample fromSIRM to zero (units: mT).

High values indicate anti-ferromagnetic minerals, whereas lowvalues (b40) indicate ferrimagnetic minerals, principally magnetiteand maghemite.

S-ratio Ratio reflects a coercivity spectrum between antiferromgnetic andferrimagnetic minerals. Obtained by using IRM−300 mT andsaturated IRM (SIRM1000 mT).

Indicates proportion of antiferromagnetic and ferrimagneticminerals. May reflect changes in oxidative to reductiveconditions. Values of ∼1.0 indicate high proportions of lowcoercivity minerals (magnetite, maghemite), whereas values b1.0indicate increased proportions of high-coercivity minerals(hematite, goethite).

SIRM Saturation isothermal remanent Magnetisation: the highest level ofmagnetic remanence induced in sample by application of a highfield (2.5 T).

Indicates volume concentration of magnetic minerals in a sample.Increases with fine SD a grains.

SIRM/χlf Ratio between two parameters indicative of magnetic grain size. Very low value (theoretically zero) indicates presence ofparamagnetic minerals, and higher values attributed to PSD/SD/SPa

particles.a Magnetic domains, see Table 3 for definitions.


the sample was used for obtaining pure quartz forequivalent dose (De) measurements. The remaining partwas used for water content measurements and determi-nation of Uranium (U), Thorium (Th) and Potassium (K)by gamma spectrometry. For De measurements, thesamples were dry-sieved to obtain the dominant grain-size fraction. This fraction was then treated with 10%hydrogen peroxide (H2O2) to remove organic matter,followed by 10% hydrochloric acid (HCl) to removecarbonates. Finally, the grains were etched withconcentrated (40%) hydrofluoric acid (HF) for 80 min-utes to remove any feldspars and the outer alpha-irradiated layer. After HF treatment, the grains weretreated with concentrated HCl for 40 min to remove anyacid-soluble fluorides. Feldspar contamination in thesamples was monitored through Infra red IR stimulationand if there was a detectable IR stimulated luminescence(IRSL) from the chemically treated grains, the HF andHCL treatments were repeated. Finally, the grains werereceived again and dose measurements carried out usingoptically stimulated luminescence. Mono-layers of quartzgrains were then loaded onto stainless steel discs usingsilicone oil, to give aliquots ∼2 mm (small aliquots) or10 mm diameter (large aliquots). OSL measurementswere carried out using automated TL/OSL readers(Botter-Jensen et al., 2002). Optical stimulation wasperformed using blue LEDs (470±30 nm) and the

luminescence emission measured through a 7.5 mmHoya U-340 filter. Laboratory irradiations were carriedout using a calibrated 90Sr/90Y beta source (doserate=0.11 Gy/s). All OSL measurements were per-formed at 125 °C for 40 s. The background subtractedintegral signal from the first 0.8 s was used for De

calculation following an SAR protocol (Murray andWintle, 2000). The preheat treatment included heatingthe aliquots to 260 °C and holding them at the finaltemperature for 10 s. For monitoring the test dose OSLresponse for sensitivity corrections the aliquots werefirst irradiated (∼1 Gy) and then heated to the sametemperature as the preheat but not held there for anysignificant time duration (also called cutheat). Therecuperation values were less than 2% and therecycling values were within 10% of unity.

For samples from the IITK and JP cores, initial testsshowed that the sands have very high dose rates (3.4–4.1 Gy/ka), linked to the abundance of radiogenicminerals in the samples. Quartz in all but the youngestsamples could not be dated using the single aliquotregeneration method. Consequently, feldspars weredated from these samples at the Dalhousie UniversityOSL lab. The protocol followed that of Lamothe (2004)except that step 6 was not used. The problem ofanomalous fading was addressed by using correctionprocedures (Huntley and Lamothe, 2001; Lian et al.,

Table 3Magnetic domains (Thompson and Oldfield, 1986; Evans and Heller,2003)

Magnetic domains Definitions

Single domain(SD)

Magnetic grain is uniformly magnetized withjust one domain, i.e., all the atomic momentsare aligned parallel (grains b0.2 μm).

Multi-domain(MD)

Magnetic grain contains several domains, themagnetization is uniform in each domain butdiffers in direction from domain to domain(grains N70 μm); have much lower remanencesand coercivities than SD grains.

Pseudo singledomain (PSD)

Magnetic structure and behavior is intermediatebetween SD and MD. Particles contain morethan one domain, but exhibit many propertiestypical of SD particles.

Superparamagnetic(SP)

Very small magnetic particles that haverelaxation times on the laboratory timescale.Ferro- and ferrimagnatic grains below a criticalgrain size (∼10−8 m for spherical magneticgrains). High susceptibility and zero coercivity.


2002). For these feldspar samples, past doses De wereobtained using a single aliquot additive dose protocolwith IR stimulation on 90–125 μm K-feldspar grains,which were separated using standard methods, includingtwo heavy liquid separations at specific gravity 2.54,and a brief 10 minute etch with 10% HF to remove theouter, alpha-affected layer of each grain. To determinethe anomalous fading correction, an equation based on aseries of prompt-delayed beta dose responses topreviously bleached aliquots was determined. Theequation was based on a set of ten disks, for which tencycles of IR stimulation and preheating were employed,with a preheat at 234 °C, 0.5 s IR stimulation, and a doseof 10 Gy beta. Using this equation as a fading factor forthe accrued dose, a numerical integration for the in situtime of each sample was performed, with an integrationinterval of 1 year, until the measured De dose wasreached. The correction factor is quite high (1.3 to1.5 times greater than the raw ages), and increases forolder samples which have had longer to fade their ac-crued dose.

For magnetic mineralogy, parameters used are de-fined in Table 2 (definitions after Thompson andOldfield, 1986; Maher and Thompson, 1999; Evansand Heller, 2003), and magnetic domains (Stacey andBanerjee, 1974; Dunlop and Ozdemir, 1997) in Table 3.Samples were taken at 30 cm intervals from the Bithursection and from drill cores. Samples were packed innon-magnetic styrene cylindrical pots and were mea-sured for magnetic susceptibility using a BartingtonMS2B sensor for low frequency (0.465 KHz) and highfrequency (4.65 KHz) applied fields. The key parameters

derived from this analysis are low-frequency suscepti-bility (χlf), which indicates the bulk magnetic mineralcontent of the sample, and the frequency-dependentsusceptibility (χfd), for which high values (in percent)reflect the proportion of very fine ferromagnetic grains(Collinson, 1987).

Anhysteretic remanence magnetization (ARM) wasimparted by subjecting the samples to a peakalternating field (100 mT) decaying to zero in thepresence of a bias field of 0.1 mT. This approach isuseful for characterizing magnetic particles of stablesingle domain (SD) type (yielding values of 0.03 to0.06 for magnetite or 0.03 to ∼15 for hematite). Theratio of ARM to a bias field is termed χARM

(susceptibility of anhysteretic remanence), and varieswith the concentration of single domain and pseudo-single domain grains (Stacey and Banerjee, 1974;Dunlop and Ozdemir, 1997).

Isothermal Remanent Magnetization (IRM) wasimparted to samples at intervals of 20, 50, 100, 200 mTup to 2500 mT and with a backfield of 300 mT, using anASC Scientific Impulse Magnetizer (IM-10-30). Theremanence was measured using a Minispin RockMagnetometer (Molspin) at Wadia Institute of HimalayanGeology, Dehradun. Magnetic hysteresis curves weregenerated from the induced fields and magnetizationvalues. Several additional parameters were computedfrom these results (Table 2). Bocr (the coercivity ofremanence) and the S-ratio reflect the proportion ofantiferromagnetic grains and ferromagnetic grains (prin-cipally magnetite and maghemite) (Dunlop and Ozdemir,1997). SIRM reflects the volume concentration ofmagnetic minerals, increasing with the proportion ofsingle-domain grains. SIRM/χlf ratio reflects the balanceof grains of different domain types, and is linked to grainsize (Thompson and Oldfield, 1986; Peters and Thomp-son, 1998; Peters and Dekkers, 2003).

The stratigraphic sections were divided for conve-nience into generalized zones with distinctive magneticproperties.

4. Age of the strata

Table 4 lists the age data for three cores and one cliffsection. The valley-fill cores from Jagdishpur haveyielded three feldspar ages of 6.8±0.69 ka, 9.2±1.23 kaand 26.2±5.10 ka. Three OSL ages on quartz grains fromthe Firozpur core yielded dates of 8.5±1.2 ka, 10.5±1.7 ka and N31.6±2.7 ka; the first two correspond tothe Holocene and the third precedes the LGM. Thecorrected OSL signal in sample FP3 was observed tobe in the saturation region of the dose response curve

Table 4Luminescence chronology of drill cores and cliff section, Ganga plains

Site Sample Sample descriptions Depth fromsurface (m)

Technique Dose rate(Gy/ka)

Dose(Gy)

Age(ka)

Comments

Jagdishpur JP-1 Core sample;fine sand

4.6 Etched K-feldspar

3.91±0.91 20.8±2.5 6.8±0.69

JP-2 –do– 7.6 –do– 3.43±0.19 24.8±4.0 9.2±1.23JP-3 Core sample;

coarse sand18.5 –do– 4.28±0.20 79.4±21.5 26.2±5.10

Firozpur FP-1 Core sample;fine sand

6.0 OSL 2.43±0.19 20.5±2.5 8.5±1.2

FP-2 Core sample;fine sand

8.2 OSL 2.24±0.19 23.4±3.3 10.5±1.7

FP-3 Core sample;fine micaceous sand

21.1 OSL 2.12±0.17 N67.1±1.8 N31.6±2.7 Saturated; represents minimumage of the sample

Bithur BTH-II/8 Cliff section,eolian sandy silt

2.8 OSL 4.48±0.27 113.0±4.0 25.2±1.9 AMS calibrated age on shellsfrom 2.5m depth from surface21.2±0.20 ka

BTH-II/6 –do– 5.2 OSL 3.89±0.23 103.0±3.0 26.4±1.8BTH-I/3 –do– 6.5 OSL 4.73±0.28 130.0±4.0 27.6±1.9 AMS calibrated age on shells

from 9m depth from surface∼27.5 ka (Gibling et al., 2005)

IIT Kanpur IITK-1 Core samples;clayey silt

11.6 EtchedK-feldspar

4.05±0.22 88.0±13.0 30.3±3.42

IITK-4 –do– 21.5 –do– 3.90±0.20 104.0±13.2 38.7±3.66IITK-6 –do– 31.86 –do– 3.58±0.18 155.0±11.8 63.0±3.98IITK-8 –do– 41.93 –do– 3.69±0.19 214.0±25.0 86.0±7.39


(i.e. undetectable increase in luminescence with anyfurther increase in absorbed dose); the minimum agefor this sample is based on the dose that defines theonset of saturation in for the OSL signal. For samplesFP1 and FP2 and three samples from Bithur (BTH),natural OSL signals were well below the saturationlevels in the dose response curves, which allowedestimation of a finite burial dose. Three OSL datesfrom the Bithur cliff section and one AMS radiocarbondate were reported earlier (Gibling et al., 2005). Onenew AMS date of 21.2±0.20 ka on shell material fromthe topmost Unit 6 fits well with the available data. Agood agreement between the OSL and calibrated ra-diocarbon ages was observed. The IITK core yieldedfour feldspar dates of 30.3±3.42 ka, 38.7±3.66 ka,63.0±3.98 ka, and 86.0±7.39 ka between 11 and 42 mdepth, suggesting that the base of this core may beover 100 ka in age. The stratigraphic implications ofthese dates are discussed later.

5. Facies description

Four facies groups were identified (Table 1). Chan-nel facies (C) are prominent as discrete units up to 10 mthick in the Jagdishpur and Firozpur cores. The unitsconsist mainly of fine to medium sand (C1) with local

layers of reworked carbonate nodules. This facies has atypical ‘salt and pepper’ appearance due to mixing ofquartz grains, darker rock fragments, and heavyminerals. Facies C1 grades upward into very fine sandand silt (C2), generally well-sorted, with only a fewreworked carbonate nodules.

Floodplain facies (F) are prominent in the Bithursection and IITK core, and are mainly muddy sedimentswith pedogenic features that include dark mottles,carbonate nodules, and rhizoconcretions. Based on texturalcharacteristics and pedogenic features, four muddy faciesare recognized, with occurrences of individual faciesvarying in thickness from 2 m at Bithur to as much as 4 min the IITK core. Yellow–brownmud (F1) predominates atall locations, and is interpreted as aggradational floodplainsediments that experienced a slight degree of pedogenesis;some units may have formed within abandoned channels.Red–brown mud (F2) with local silt patches, carbonatenodules and black mottles is the second most abundantfacies, and indicates a more advanced degree ofpedogenesis. Thin silt/sand sheets (F3) are commonlyassociated with the yellow–brown muds (F1), and areinterpreted as splay deposits, although a few uniform, silt-rich sheets could be of eolian origin. Tough pale clay (F4)occurs as rare beds up to 0.5m thick, commonly associatedwith red–brown mud (F2).

Table 5Summary of sedimentological and magnetic characteristics for dominant facies at each site

Location/facies type

LOI(%)

Bulk mineralogy Clay mineralogy Magnetic parameters

Quartz(%)

Feldspar(%)

Mica(%)

Illite(%)

Chlorite(%)

Kaolinite(%)

χlf

(⁎10−8 m3/kg)χfd% Bocr

(mT)χARM SIRM S-ratio SIRM1000/

χlf

ARM/χlf

JagdishpurC1 2 51 9 40 77 3 20 15.90 0.60 28.63 0.01 59.91 −0.79 3.57 0.04C2 2 61 8 30 86 7 7 9.64 0.82 39.60 0.01 60.99 −0.73 6.40 0.10F1 4 72 5 23 83 11 8 9.87 0.69 36.27 0.02 64.37 −0.71 6.44 0.13F2 4 68 8 25 88 5 8 9.00 0.84 55.60 0.01 24.92 −0.59 2.76 0.08

FirozpurC1 3 64 5 31 90 4 6 18.16 2.43 44.59 0.01 30.21 −0.74 1.91 0.03C2 2 81 6 13 88 1 10 9.21 3.84 55.38 0.01 16.67 −0.67 1.76 0.06F1 4 68 7 24 83 4 13 14.40 3.11 45.63 0.01 30.82 −0.71 1.99 0.05F2 7 68 7 25 86 4 10 14.16 3.25 40.57 0.01 24.94 −0.69 1.85 0.06

BithurF1 6 62 8 30 91 3 5 12.72 4.06 55.17 0.03 55.85 −0.47 5.37 0.20L1 6 62 10 28 91 1 8 12.70 2.38 52.00 0.03 99.10 −0.74 5.61 0.13E1 6 55 14 31 94 3 2 13.98 1.70 38.88 0.03 74.85 −0.68 5.39 0.18

IITKF1 5 65 15 20 88 5 7 20.74 0.86 30.00 0.01 104.53 −0.90 4.79 0.04F2 4 65 11 24 87 5 8 29.66 1.23 27.20 0.01 152.72 −0.89 4.41 0.04


A fifth, carbonate facies consists of layers of car-bonate nodules up to 5 cm in diameter (F5), locallytermed ‘kankar’. These layers are up to 1 m thick, andare commonly associated with facies F1, F2, and F4.Although common throughout the sections, kankar isgenerally most abundant in the upper parts offloodplain units, where its abundance and associationwith the toughest muds suggests a high degree ofpedogenic modification. The nodules are interpreted assecondary carbonates developed in floodplain deposits,of pedogenic or groundwater origin (Sinha et al.,2006).

Eolian facies (E1) is identified only in the Bithursection. It consists of structureless sheets of soft silt tovery fine sand, with carbonate nodules and rhizoconcre-tions. Carbonate-cemented bands define the low-anglestratification visible at several locations, probablyreflecting the presence of low mounds of wind-blownmaterial.

Lacustrine facies (L) were identified on the basis oftheir sheet-like geometry and lamination, and aregenerally clay-rich with mottling and a relatively highorganic content. Occurrences are prominent at Bithur (1–3munits) but are generally thin in cores (10–20 cmunits).Dark clay (L1) comprises stiff clay with minor silt, andvaries from poorly stratified to banded. Black, red–brownand drab mottles are common, but carbonate nodules andrhizoconcretions are rare.We also record sparse gastropod

and bivalve shells in this facies. These features areconsistent with lake deposits or backswamp deposits inabandoned channels, with a variable degree of pedo-genesis that has destroyed stratification locally. Yellowlaminated silt and sand (L2) is weakly cohesive, and greyand red mottles, carbonate nodules and rhizoconcretionspoint to a modest degree of pedogenesis.

6. Mineralogy and magnetic characterization offacies

From semi-quantitative mineral data from bulkanalyses (Table 5; Fig. 3), channel and floodplain faciesgroups are both quartz-rich, with mica contentsgenerally higher in channel facies. Feldspar contentrarely exceeds 10% in channel facies and 20% infloodplain facies. For clay fractions, all samples areillite-rich, with generally less than 10% each of chloriteand kaolinite, with the latter on average more abundant.Loss on ignition (LOI) values essentially represent theinorganic carbon (carbonates) and average less than10% by weight. High values of LOI in several intervalscorrelate with layers of concentrated carbonate nodules(facies F5). Modest mineralogical differences are notedbetween individual cores and sections.

Magnetic parameters are characterized for individualfacies in Table 5. For channel deposits, specificmagnetic susceptibility at low frequency (χlf) of

Fig. 3. Bulk and clay mineralogy data for channel and floodplain facies in three cores and one outcrop section in the Ganga valley and interfluve deposits near Kanpur. Data are shown in Table 5.

396R.Sinha

etal.

/Sedim

entaryGeology

201(2007)

386–411


Ganga valley fill sediments include the highest values ofany samples. Coarser channel facies (C1) show slightlyhigher values of χlf and lower values of χfd than thefiner facies (C2), suggesting that fine ferrimagneticgrains are more prominent in finer sands. Although thetwo core sites are only a few kilometers apart, C1 faciesat Firozpur has considerably higher values of χlf, χfd%,and Bocr than C1 occurrences at Jagdishpur.

For floodplain deposits, values of χlf are relativelylow for the valley and valley-margin cores, but high inthe IITK (interfluve) core (Table 5). Values for theaggradational yellow–brown muds of facies F1 aresimilar in all settings, but lowest in the valley-margindeposits at Bithur. For the red–brown muds of facies F2,χlf values are generally low, although higher in the IITKcore. For χfd values of both floodplain facies, values arelow at Jagdishpur but much higher at other sites, with

Fig. 4. Magnetic characterization of the Ganga valley and interfluve sedimentscross plots between SIRM/χlf vs. Bocr for channel, floodplain, lacustrine, anminerals are also plotted after Maher and Thompson (1999) in (a) and magne

the highest values recorded for the F2 facies in the IITKcore. These data indicate moderate pedogenic enhance-ment of magnetic susceptibility at all sites exceptJagdishpur, where floodplain muds experienced littlepost-depositional change. High values of χfd are alsonoted in the lake facies (L1) at Bithur.

For Bocr in the floodplain facies, average valuesgenerally range between 30 and 45 mT. S-ratios arestrongly negative (b−0.5), indicating a predominantlyferrimagnetic assemblage. F1 and F2 facies at Bithurand F2 facies at Jagdishpur show significantly highervalues of Bocr (55–72 mT) and less negative S-ratios (ashigh as−0.4 for one F1 sample at Bithur). This indicatesa prominent anti-ferrimagnetic component (probablyhematite/goethite), with some ferrimagnetic compo-nents (magnetite/ maghemite). XRD data from F2 faciesat Bithur record minor goethite. Exceptionally high

. (a–f) show cross plots between SIRM/χlf vs. ARM/χlf and (g–l) showd eolian facies in three cores and one outcrop section. Major magnetictic domains are shown after Bradshaw and Thompson (1985) in (b).

Fig. 4 (continued ).


values of SIRM/χlf and low values of Bocr for IITKfloodplain facies compared to all other sites indicate ahigher volume concentration of ferrimagnetic material.

For cross-plots of ARM/χlf against SIRM/χlf

(Fig. 4a–f), higher values of both parameters indicatethat small particles (SD, SP, and PSD: Table 2) constitutea higher proportion of the total susceptibility. Such smallparticles are commonly generated in paleosols, proba-bly due to the action of micro-organisms (Maher andThompson, 1995, 1999; Maher, 1998; Maher et al.,2003). Both channel facies C1 and C2 (Fig. 4b,c) lieclose to the magnetite (Mt) position, with the Jagdishpursamples on average more magnetite-rich for both facies.Such differences are normally interpreted to result fromvariation in detrital influx of magnetic minerals. Themagnetic assemblage at Firozpur points to a lowerdetrital influx and a higher pedogenic contribution.

For the floodplain facies (Fig. 4d,e), both ratios tendto higher values than for channel facies, with aconsiderable scatter between magnetite and titanomag-netite fields. Bithur and IITK samples have considerablyhigher values of both ratios, particularly for the red–brown muds of facies F2 (Fig. 4e), and lie towards thetitanomagnetite field, also shown by Sangode et al.(2007). The Firozpur samples show a magnetite-richassemblage for both F1 and F2 facies, whereas theJagdishpur samples lie between the magnetite andtitanomagnetite fields. Magnetite-rich assemblages infloodplain facies suggest a higher degree of pedogenictransformation (Sangode et al., 1999, 2007; Sangodeand Bloemendal, 2005), and the Firozpur samplesappear pedogenically more mature than those atJagdishpur, with much less pedogenic activity forIITK and Bithur samples. The lacustrine and eolian


facies, present only at Bithur, lie between magnetite andtitanomagnetite.

The cross-plots between SIRM/χlf and Bocr (Fig. 4g–l)also reflect the prevalence of certain magnetic domains inthe sample materials, with higher values linked to smalland/or elongate grains (Bradshaw and Thompson, 1985;Thompson and Oldfield, 1986; Peters and Thompson,1998; Peters and Dekkers, 2003). Both C1 and C2 faciesfrom Jagdishpur plot in the single domain (SD) field,whereas the Firozpur samples plot in the superparamag-netic (SP) domain, indicating a higher proportion of finermagnetic particles from pedogenic material in thecatchment area (Sangode et al., 1999; Sangode andBloemendal, 2005); this inference is also supported byhigh χfd%. The dominant floodplain facies of yellow–brown mud (F1) shows a tight cluster in the SD fieldfor all sites except Firozpur, which lies in the SP domain,indicating a moderate degree of pedogenesis. The F2facies show a large scatter particularly for the IITK sam-ples, some of which fall in the multi-domain (MD) field.The Firozpur F2 samples are clearly superparamagnetic.

Fig. 5. Vertical profiles of sedimentological and magnetic parameters plottedignition (largely carbonate material). I, Ch, and K=illite, chlorite, and kaolinsedimentary cycles (above and below 10 m level), with a sharp change in m

Both lacustrine and eolian facies show a large scatter butgenerally fall in SD field.

7. Valley fill

7.1. Jagdishpur core

Two major fining-upward cycles, about 13 m and10 m thick (Fig. 5), are typical of many braided riverdeposits (Williams and Rust, 1969; Miall, 1977). Thecycles commence with fine- to medium-grained channelsand (facies C1), locally with concretions and red andbrown mottling, passing upward into interbedded siltand very fine sand (facies C2) and ending with yellow–brown floodplain mud (facies F1) with carbonate con-cretions (indicated by local increases in LOI). Cycle II iscapped by modern soil with rootlets. The base of thecore is a unit of red–brown mud (facies F2).

Illite predominates in the fine fraction of both chan-nel and floodplain sediments, with a moderate propor-tion of kaolinite (Figs. 3, 5). The relative abundance of

along with sedimentary cycles for the Jagdishpur core. LOI = loss onite, respectively. AFM = antiferromagnetic. Note the presence of twoost parameters at this level.


illite and chlorite is highest at the base of the uppercycle, with a sharp rise in the abundance of kaolinite at∼6.5 m relative to illite and chlorite, which has climaticimplications (discussed later). Carbonate, as reflected inLOI, is slightly more abundant in the floodplain facies(∼4%) than in the channel facies (∼2%).

The core can be divided into three magnetic zones.Zone A (10.5–25 m depth) corresponds to Cycle I andunderlying clays. The concentration of magnetic mineralsis very low (Fig. 5), as manifested by very low anduniform χlf (b10×10

−8 m3/kg) and χfd% (2%). Bocr

and S-ratio show similar patterns, with relativelyuniform Bocr values (slightly below 40 mT). Somepeaks at greater than 50 mT lower in the core suggesthigher proportions of antiferromagnetic minerals,whereas relatively low (b40 mT) values in the upperpart suggest some ferrimagnetic minerals. The S-ratiofollows Bocr closely, and the low values (b1) suggestgenerally high proportions of high-coercivity minerals,probably hematite and goethite. Zone B (5–10.5 m)corresponds to the coarser sands of Cycle II, andincludes a peak of very high χlf (N40×10

−8m3/kg).The corresponding very low χfd% (b1%) and Bocr

(b25 mT) values suggest a detrital flux of ferrimag-netic minerals. Zone C (0–5 m) represents finer sandsand floodplain muds at the top of Cycle II, with Bocr

and S-ratio values indicating high-coercivity (BocrN100 mT) minerals in the lower part and an increasedproportion of low-coercivity minerals (Bocrb40 mT)in the top part. This indicates a gradual upward changefrom antiferromagnetic minerals to ferrimagneticminerals, perhaps in response to wetting–drying cyclesin a channel-fill environment and subaerial exposuretowards the surface. The χfd remains below ∼4%throughout the core, indicating the complete absence ofSP particles.

The top of the lower sand body is dated at 26.2±5.1 ka. The upper sand body is well constrained withan age of 9.2±1.23 ka for the lower part and 6.8±0.69 ka for the upper part. Srivastava et al. (2003a)reported feldspar ages from nearby surface deposits of3.0±1.0 ka and 1.1±0.4 ka. The lower sand bodymarks pre-LGM channel activity, with apparentreduction of activity during the LGM through to theearly Holocene which appears to be represented onlyby floodplain deposits. Channel activity was enhancedagain after about 10–11 ka, when the upper sand bodyformed — during a period of monsoon intensification.The channel was finally abandoned after ∼6 ka, whenthe Ganga moved southwards to the valley margin atBithur, forming a large meander cut-off (Fig. 1b,c).Younger surface ages (3–1 ka: Srivastava et al.,

2003a) suggest that the cut-off area is floodedoccasionally.

7.2. Firozpur core

Two sedimentary cycles (Fig. 6) are comparable tothose in the Jagadishpur core. A basal unit comprisesfloodplain facies (F1) in the lower part and red–brownmottling and sparse kankars in the upper part. Cycle Icommences with ∼3 m of channel sand facies (C1) thatgrade up into very fine sand to silt (C2), overlainby ~9 m of red–brown floodplain muds (F2) withkankar accumulations that yield high LOI values(N10%). Cycle II begins with ~8 m of channel sand(C1) with sparse concretions, and shows red and brownmottling, more reddish in the top part. This is overlainby intercalated finer sands (C2) and yellow–brown mud(F1), capped by a modern soil. Mineral composition isbroadly comparable to that at Jagdishpur (Figs. 3, 5).The relative abundance of illite, chlorite and kaolinitevaries with depth, with a rise in illite and chloriteabundance at ~10 m and of kaolinite at ~7 m depth.

Magnetic zone A (12–25 m depth) shows low anduniform χlf (b20×10

−8 m3/kg), indicating very lowconcentrations of magnetic materials. The χfd valuesshow some fluctuations, and in some places are N5%(e.g. at 20 m). Bocr and S-ratio rise slightly in the toppart, with Bocr b30 mT (up to 16 m) increasing toN50 mT. Most of this zone consists of floodplain facies,and we interpret the change in magnetic mineralogy toreflect episodic wetting and drying on the floodplains.Theχlf values suggest little major magnetic enhancementof ferrimagnetic minerals. Zone B (6–12 m) shows twoprominent peaks of very high χlf (N40×10

−8m3/kg),with generally low χfd% (b2%), Bocr (b35 mT), andS-ratio. The upper χlf peak lies within Cycle II sands,whereas the lower peak lies within a kankar-bearingmud in the topmost strata of Cycle I. These peaks in-dicate a detrital influx of magnetic material. In contrast,the top 2 m of this zone has low and uniform χlf (b18×10−8 m3/kg), χfd% (b2%), and Bocr (b40 mT). Zone C(0–6 m) shows a rise in Bocr (N80 mT) and S-ratio valuesin the topmost channel sands of Cycle II, indicatingantiferromagnetic minerals with some ferrimagnetic mi-nerals (Bocr b25 mT). χfd correspondingly increases to4–5%, indicating some finer magnetic SP particles. Theupward change in magnetic properties from Zone B to Creflects a transition from channel to floodplain facies,where increased pedogenesis and subaerial exposurewould have influenced the proportion of ferrimagneticand antiferromagnetic minerals, resulting in trends ofχfd, Bocr and S-ratio.

Fig. 6. Vertical profiles of sedimentological and magnetic parameters plotted along with sedimentary cycles for the Firozpur core. LOI = loss onignition (largely carbonate material). I, Ch, and K = illite, chlorite, and kaolinite, respectively. AFM = antiferromagnetic, SP = superparamagnetic.


The lower, incomplete fluvial cycle I starts with a thinsandy facies that yielded a minimum age of 31.6±2.7 ka,below thick floodplain muds with a low to moderatedegree of pedogenesis. This date may represent a splaydeposit during the pre-LGM period. The upper channel-sand body yielded two dates, 10.5±1.7 ka and 28.5±1.2 ka from the middle and lower parts, respectively, andthe upper part appears to correlate in age with theHolocene channel at Jagdishpur. The distinctive χlf peakin the upper channel body at both sites also suggests thatthey are coeval, and possibly that the Holocene channel inthis regionwasmulti-thread. The subsequent stratigraphicdevelopment appears similar to that at Jagdishpur, and thechannel may have been abandoned after ∼6 ka.

8. Valley margin (Bithur section)

The Bithur section has been presented elsewhere(Gibling et al., 2005) and a summary is reproduced here.Six units (left-hand side of Fig. 7) were identified on thebasis of lithologs, field observations and mapping from

photomosaics. Data on magnetic characteristics andother physical properties are integrated with the earlierinformation.

Unit 1 is about 4 m thick and consists of structurelessyellow–brown floodplain mud (facies F1), with grayand darker mottles and rhizoconcretions indicating amoderate degree of pedogenesis. The upper part has ahigher frequency of red–brown mottles. Kankar con-stitutes about 20% of the sediment by volume, butbecomes sparse in the upper part, with vertical drabmottles and clusters of gastropod shells in the top 30 cm.

The base of Unit 2 corresponds to a major changefrom floodplain deposits to interbedded lacustrine andeolian deposits. The unit commences abruptly with darksilty clay (lacustrine facies L1) with minor kankar, red–brown and yellow mottles. The unit is laminated andmaintains its thickness (∼1 m) along 1.3 km of cliffsection. Prominent efflorescence from halite (identifiedusing XRD analysis) is present in some parts. Units 4and 6 are also lacustrine (facies L1), with clayey andthinly laminated sediments with some mottling and

Fig. 7. Vertical profiles of sedimentological and magnetic parameters plotted along with sedimentary cycles for Bithur section. LOI = loss on ignition(largely carbonate material). I, Ch, and K = illite, chlorite, and kaolinite, respectively. SP = superparamagnetic. Note sharp change in some parametersbetween units 1 and 2 which marks a major discontinuity between floodplain deposits below and eolian and lacustrine deposits above.


kankar in the upper parts of the units. Rhizoconcretionsare also present locally. Unit 3 comprises eolian faciesE1, and is massive yellowish silt with some fine-grainedsand, becoming whitish above. Carbonate concretionsand rhizoconcretions are present, and flat-lying toinclined cemented bands in places suggest remnantstratification. Unit 5 is also eolian (facies E1) andconsists of yellowish-white silt with minor mottling; itlacks kankar and is not stratified. The thickness of bothunits is highly variable.

Quartz is the predominant constituent throughout thesection, showing highest abundance (N65%) in Units 2and 5. Feldspar is abundant in Units 1, 3, 4 (∼20%) butlow in Unit 2 (~5%). LOI values pick out the carbonate-rich layers and particularly high values in Unit 2 cor-respond to palustrine carbonate layers observed in thefield. Illite is the predominant (∼90%) clay mineral, andhigh values of kaolinite in lacustrine units are parti-cularly apparent. The increase in cation-deficient kaoli-nite indicates leaching of cations (more weathering) inthese units (Grim, 1953).

Magnetic zone A (Unit 1) shows a rise in Bocr (fromb50 mT in the lower part to N80 mT in the upper part)and a corresponding χfd% decrease from N5% to 1–2%.These changes indicate an upward shift from ferromag-netic-rich to a higher proportion of antiferromagneticminerals. Zone B (Unit 2) shows peaks of high χlf

(N40×10−8 m3/kg) and highχfd% (N10%) which, alongwith low Bocr and S-ratio values, indicate the presenceof SP particles. These data collectively suggest bacterialproduction of magnetite in a backswamp environment.In the top part of the unit, χfd% becomes very low,possibly suggesting post-depositional dissolution offerrimagnetic particles during soil formation. Dissolu-tion of magnetite has been commonly reported duringmarine diagenesis (Canfield and Berner, 1987; Bloe-mendal et al., 1993; Zhang et al., 2001) but is alsofeasible in terrestrial settings through bacterial actionunder anaerobic conditions (Lovley et al., 1987; Kostkaand Nealson, 1995). Zone C (eolian strata of Unit 3)shows little variation in χlf and χfd, but Bocr and S-ratioincrease slightly in the top part, suggesting the presence


of antiferromagnetic minerals of coarser grain size. InZone D (lacustrine and eolian Units 4–6), values of χlf

are low, but χfd% and Bocr fluctuations suggest variedproportions of ferrimagnetic and antiferromagneticminerals. A peak of χfd (6%) is prominent in lacustrineUnit 4; both this peak and the organic matter content arelower than in Unit 2.

One shell sample from Unit 1 yielded a radiocarbonage of 27.5 ka. Two OSL dates from Unit 3 are 27.6±1.9 ka and 26.4±1.8 ka, and an OSL date from unit 5 is25.2±1.9 ka (Gibling et al., 2005). A new date on shellmaterial from the upper part of Unit 6 has given an ageof 21.2±0.20 ka. Srivastava et al. (2003a, b) reported anage of 13±2 ka from Unit 6 based on feldspar dating.These dates indicate that floodplain facies of Unit 1 werereplaced by eolian and lacustrine facies of Units 2–6 nearthe start of Marine Isotope Stage 2 (Fig. 2) at a timewhen monsoonal precipitation decreased greatly intothe LGM. Thus, the Unit 1 / 2 boundary marks a major

Fig. 8. Vertical profiles of sedimentological and magnetic parameters plottedcore is characterized as a distal floodplain succession with minor discontinuitimaterial). I, Ch, and K = illite, chlorite, and kaolinite, respectively. SP = sup

discontinuity in this valley-margin setting (Giblinget al., 2005), linked to a progressive reduction inGanga River discharge. At least 10 m of lacustrineand eolian (non-riverine) deposits accumulated untilthe early Holocene.

9. Interfluve (IITK core)

The entire core consists of floodplain facies (F1 to F5),and is divided into floodplain accretion units on the basisof layers that exhibit a higher intensity of pedogenesis(Fig. 8). The dominant floodplain facies is yellow–brownmud (F1) associated with thin silt sheets (F3). The upperpart of each accretion unit is marked by red–brown mud(F2), generally associated with abundant kankar (faciesF5), mottling, and local tough clay (F4). Silty layers aremore prominent in the upper part of the core. Individualfacies show generally similar bulk mineralogy with aquartz-dominated (~65%) assemblage, lesser mica (20–

along with sedimentary cycles for the IITK core at Kanpur. The entirees in the form of soil horizons. LOI = loss on ignition (largely carbonateerparamagnetic, FM = ferromagnetic, AFM = antiferromagnetic.


25%) and feldspar (10–15%). The silty intervals showhigher mica contents (N35%). LOI values match thinlayers of carbonate nodules (facies F5) associated withsilty intervals (facies C2), and they fluctuate stronglywith values up to 15%. Illite predominates, close to 80%in most samples, followed by chlorite and kaolinite(Fig. 8). In places, the relative abundance of kaolinite ishigh (e.g. ∼25% at 20 m depth) at the expense of illiteand chlorite.

Magnetic zone A (29–49 m) shows variations offerrimagnetic and antiferromagnetic mineral propor-tions. In the lower part (44–48 m), Bocr values locallyexceed 70 mT, andχlf (∼15×10−8 m3/kg) shows minorfluctuations. In general, Bocr varies between 30 and70 mT and χfd between 2 and 5%, suggesting repeatedchange from anoxic to oxic conditions. The top part alsoshows high magnetic concentrations (χlfN40×10

−8

m3/kg). Zone B (22–29 m) shows high χfd (N5%) withcorrespondingly low Bocr and moderate χlf , indicatingthe presence of SP particles. This interval correspondswith red–brown mud and represents a period of bio-genic activity and soil formation. In the top 2 m, Bocr

increases slightly (N60 mT), with corresponding fluc-tuations of χfd , suggesting redox changes. Zone C(16–22 m) includes silty layers, and high χlf (30×10−8 m3/kg) and correspondingly low χfd (b2%) in-dicate periodic influxes of detrital magnetic minerals.Locally higher values of χfd (∼4%) suggest minorpedogenic activity. Zone D (10–16 m) includes red–brown muds and kankar, and shows very high χfd

(N8%), with low Bocr, suggesting the presence ofSP particles (Collinson, 1987) and pedogenesis (Dear-ing et al., 1997). A few peaks of χlf indicate magneticenhancement due to pedogenic activity. S-ratio is er-ratic and needs further investigation. Zone E (0–10 m)shows variations in χfd and Bocr values, indicatingfluctuation of ferrimagnetic and antiferromagnetic mi-nerals. Highχlf with correspondingly lowχfd indicatesdetrital influx of magnetic minerals associated withsilts.

The lowermost OSL sample from∼42 m depth givesan age of 86±7.39 ka, and by extrapolation the base ofthe core at 50 m would be over∼100 ka. The uppermostdate of∼30 ka at∼12 m depth suggests that most of thesuccession was deposited prior to the LGM. Althoughtoo few dates are available to constrain sedimentationrates effectively, the OSL age dates suggest a variablerate of sedimentation. A relatively rapid rate of sedi-mentation is inferred between 12 and 22 m depth, with anet sedimentation rate of about 1.25 mm/year, and amuch slower sedimentation rate of about 0.4 mm/yearin the lower part of the core. The top of the core is

extrapolated to ∼15 ka, implying that the interfluvesurface has received little sediment since the Holocene.

The IITK site represents an interfluve setting about10 km from the modern valley margin. There is no majorsand body in the core, and the site apparently lay withinthe interfluve throughout the last∼100 ka. In contrast tothe valley-margin setting, we do not record any majordiscontinuity within this interfluve setting. The core con-sists of stacked floodplain accretion (FA) units sepa-rated by soil layers that represent periods of minimalnet sedimentation or minor hiatuses. Enhanced magneticproperties are in accord with the generally greater in-tensity of pedogenesis evident in the core. Silty layersprobably represent splays, and their abundance in theupper part of the core may indicate the migration ofplains-fed channels towards the study site. Alternatively,they may represent major flood events in local rivers andvaried flood intensity. The stratigraphic developmentat the IITK site therefore seems to be partly controlled bya local hydrological regime, the dynamics of minor (pro-bably plains-fed) channels, and pedogenesis.

10. Discussion

10.1. Landscape evolution, valley migration andstratigraphic model (∼LGM)

The valley, valley-margin and interfluve sites displaydistinctive stratigraphic features and facies distributions(Fig. 9). Both valley-fill cores show two fining upwardcycles corresponding to pre-LGM and Holocenechannel activities. The valley-margin site at Bithurshows a lower floodplain sequence followed by a majordiscontinuity below non-fluvial deposits. The interfluveIITK core records continuous overbank muds, punctu-ated by thin silty layers and intervals with enhancedpedogenesis.

Fig. 10 illustrates the landscape during three periods:pre-LGM (N50 ka), LGM and post-Holocene (b10 ka).We presently have no age constraints for Ganga Valleyhistory beyond ∼31.6 ka. However, the two valley coresrecord major channel deposition from 26 ka to 6 ka(Fig. 9), implying that the Ganga or a similar precursorriver lay close to its present site through that period(Fig. 10b). Braided rivers may fill wide valleys but activechannels generally occupy only part of the valley at anygiven time (Nanson and Croke, 1992); this may explainthe contrasts in age between the channel cycles atJagdishpur and Firozpur. The floodplain facies in thesecores show a low degree of pedogenic transformation,in accord with an intra-valley location and cumulativefloodplain development.

Fig. 9. Stratigraphy of the Ganga valley and adjacent interfluve, based on Bithur outcrop and three drill cores (locations shown in Fig. 1b). Valley fillcores, JP and FP, show two channels corresponding to pre-LGM and Holocene periods. Valley migration occurred between 11 and 6 ka as suggestedby OSL ages and incision of the valley margin sequence at Bithur was initiated around ∼11 ka. The interfluve core at IITK shows predominantlymuddy sequence for the period 100–15 ka and represents distal floodplain environment throughout this period.


The valley-margin area at Bithur contained an activefloodplain prior to the LGM, but became ‘detached’during the LGM (Marine Isotope Stage 2) whenmonsoonal precipitation (Fig. 2) and Ganga dischargewas greatly reduced (Fig. 10b). Lacustrine (backswamp)and eolian deposits continued to accumulate at thevalley margin (Fig. 9), and their repeated intercalationmay reflect millennial-scale climatic changes, althoughthis hypothesis needs further testing. The new radiocar-bon age of 21.2±0.20 ka suggests that this phasecontinued through most of the LGM and later.

Based on age dates and the monsoonal model of Fig. 2,incision at Bithur probably started at ∼11 ka (Fig. 10c)under conditions of high monsoonal precipitation, whichwould have enhanced discharge of water relative tosediment load and promoted cliff-line incision. Incisionwould also have been promoted by proximity to themain channel of the Ganga, which cut off from a large

meander and migrated southward to its present positionagainst the interfluve between 11 ka and 6 ka (Fig. 9). Inthe adjoining Ganga–Gomati interfluve, strong fluvialactivity during the period 13–8 ka was followed byreduced channel activity, the formation of small ponds,and aeolian aggradation (Srivastava et al., 2003b). Noages presently constrain the activity of small rivers onthe interfluve following the LGM, but they are presentlyactive and probably experienced enhanced dischargeduring the early Holocene.

There is no indication that the Ganga or other majorrivers inundated the interfluve south of the Gangaduring the past 100 ka (Fig. 10a). Cycles ofaggradation and pedogenesis in the IITK core probablyrepresent autocyclic events, most likely the movementsof small plains-fed rivers. Although the age model forthe core suggests that considerable variation inmonsoonal precipitation took place during the

Fig. 10. Depositional model for the Ganga valley and interfluve since MIS 5. (a) At∼50 ka during MIS 3, when the Ganga was active in its valley butsomewhat north of its present position. Plains-fed rivers were active on the interfluve to the south. (b) At ∼25 ka during the Last Glacial Maximum,when the Ganga had reduced discharge and lakes and eolian dunes occupied the valley margins. Small interfluve rivers may have been reduced at thistime, and the interfluve may have become largely inactive. (c) At∼10 ka during monsoon intensification in the early Holocene. The Ganga was activeduring this period, and may have started to incise valley-margin cliffs at Bithur. Subsequently (after∼6 ka), meander cut-off brought the Ganga closerto the valley margin.


accumulation of these strata (Fig. 2), there is littlesystematic indication of such changes from visual andmagnetic analysis. The generally enhanced magneticsignature is in accord with more stable floodplainconditions and enhanced opportunities for soildevelopment.

10.2. Alluvial history and sediment supply: response toLate Quaternary climate change

The landscape evolution and stratigraphic modelpresented above raises interesting questions in terms ofgeomorphic response of the basin to Late Quaternary


climate change. How has climate change altered thebalance between sediment supply and sediment trans-port capacity within the Ganga basin? Why and whendid the Ganga valley aggrade or degrade? Why did theGanga valley migrate and when did incision start? Theanswers to these questions may provide insight into thegeomorphic and sedimentary response of the GangaRiver to climate change.

Given the complexity of geomorphic systems, it isnot surprising that the fluvial literature containsvarying, often seemingly contradictory, interpretationsof fluvial response to climate change. For example,channel incision in both ‘humid’ and ‘arid’ climateshas been postulated by the classical models ofHuntington (1914) and Bryan (1928), respectively.The primary debate in these models was over the roleof vegetation cover, further highlighted by Langbeinand Schumm (1958) and Wilson (1973) who devel-oped a relationship between mean annual precipitationand mean annual sediment yield (as a function ofdischarge). Although the peak sediment yield pre-dicted by these workers may be debatable, theyunderscore the fact that the geomorphic response offluvial systems (e.g., sediment supply) to changes inrunoff (such as caused by climate change) is non-linear and complex (see also Bogaart and van Balen,2000).

The valley-fill cores described in this paper includetwo fluvial fining-upward cycles reflecting repeatedevents of valley aggradation, and the magneticsignatures suggest that these cycles were depositedunder contrasting climatic regimes. The lower sandbody appears to be much older than LGM, and theoverlying floodplain strata, strongly pedogenicallymodified, may record the LGM when precipitationwas lower (Fig. 2) and the climate cooler. The lowercycle at Jagdishpur shows little change in magneticmineralogy from channel to overlying floodplainfacies. Modeling results (Rinaldo et al., 1995; Tuckerand Slingerland, 1997) suggest that valley aggradationoccurs particularly during times of transition fromhumid to dry climates when the channel network isreduced and diffusive sediment transport processesbecome active. Valley aggradation around Jagdishpurand Firozpur between ∼30 ka and 20 ka took placeduring the transition from a pre-LGM humid climateto a cold-arid climate during the LGM. During thistime of valley aggradation, the valley-margin sequenceat Bithur accumulated lacustrine and eolian deposits(Figs. 7, 9), underlain by a discontinuity. After thepeak of the LGM dry phase, the onset of earlyHolocene monsoonal intensification re-established the

conditions for ‘threshold-limited erosion’ (Rinaldoet al., 1995), and channel activity increased withinthe valley. Incision was initiated, as evident in thepresent-day landscape. Such a transition from an aridto a humid climate would initially have encourageddiffusive processes and increased the drainage density.With the continued development of humid conditions,incision would have commenced and the drainagedensity decreased (Rinaldo et al., 1995).

The upper channel-fill cycle in both cores appearscoeval and represents Holocene valley filling, at whichtime the main Ganga channel had shifted to Bithurand started to incise. Both cores show a sharp peak inχlf values at ∼10 ka suggesting high detrital influx offerromagnetic minerals. The upper cycle is inferred torepresent a response to a transition from earlyHolocene warm-humid conditions with higher precip-itation and high sediment yield (Fig. 2), to a relativelyless humid period after 6 ka. The fluvial response tothese changing conditions may have been similar tothe scenario of decreased storm frequency or durationexplored by Tucker and Slingerland (1997), duringwhich sediment transport capacity of the river systemis reduced and slow aggradation occurs.

Further, within each cycle of valley alluviation, theinterbedded sand-silt and mud facies suggest thatvalley filling was affected by small-scale fluctuationsin monsoonal intensity, possibly related in part tochanges in storminess (hydrologically equivalent tochanges in runoff intensity). Among several climaticparameters explored through modeling, drainagebasins appear most sensitive to changes in runoffintensity that occur during shifts towards humidity oraridity, resulting in cycles of rapid valley aggradationand degradation (Willgoose, 1994; Tucker and Sling-erland, 1997). We suggest that the interbedded unitsrepresent minor cycles of valley filling correspondingto small perturbations in runoff intensity caused byhigh-frequency monsoonal fluctuations, such as thoseshown in Fig. 2b for the Holocene period.

The interfluve history seems to be broadly coupledwith valley aggradation and degradation. Although theperiod represented by the IITK core extends farbeyond that of the valley cores, we can draw somecorrespondence for the last 30 ka period. During thehumid pre-LGM period, both valley margin andinterfluve were dominated by floodplain sedimentationwhich was reduced greatly during the LGM. Duringthe early Holocene when the Ganga river started toincise at Bithur, the interfluve probably became com-pletely detached from the main river, and apparentlyreceived little sediment after ∼15 ka (Fig. 9).


10.3. Sediment mineralogy and mineral magnetism:paleoenvironmental implications

The data on clay mineralogy and magnetic mineral-ogy provide insight into sediment supply, post-deposi-tional weathering, and pedogenic processes in thedifferent geomorphic settings. Illite comprises ∼85%of the clay fraction from all locations, with minorchlorite and kaolinite, and clay proportions seemrelatively insensitive to the geomorphic setting. How-ever, smectite is completely absent in the Gangaalluvium, in contrast with the plains to the south.Kumar and Singh (1978) recorded an illite-dominatedassemblage, with minor smectite, chlorite and kaolinitein Gomti river sediments. However, at Kalpi on theYamuna river, ~80 km south of Kanpur, the alluvium isrich in smectite and vermiculite (Sarin et al., 1989).Srivastava et al. (1998) reported significant smectite(10–30%) in a mica-dominated assemblage (25–50%)along with mixed-layer clays, chlorite, vermiculite, andmost importantly, interstratified smectite–kaolinite (Sm/K) in older soils (N3000 years) from the Ramganga–Rapti interfluve north of the study area.

Such differences in clay-mineral proportions may re-flect both catchment and local environmental conditions,and it is currently difficult to distinguish these twocontrols. Firstly, the Ganga and its northern tributariesoriginate in the Himalaya, which yields large amounts ofsedimentary and meta-sedimentary material, contributingillite as a major component. In contrast, the Yamuna,although originating in the Himalaya, has southerntributaries including the Chambal, Betwa, and Kenwhich flow through weathered basic lavas of the DeccanTraps that contribute smectite to the river (Sarin et al.,1989). Secondly, the formation of smectite is typicallyassociated with soils that develop in low-lying areas withpoor drainage, and in base-rich parent material (Taboret al., 2002). The occurrence of trioctahedral smectitesand vermiculites (formed at the expense of biotites) inolder soils, in contrast to the presence of dioctahedralsmectites with Sm/K in younger soils in the northernGanga plains, was interpreted by Srivastava et al. (1998)to represent a transition from cold and dry climate tohumid climate. Although dioctahedral smectites canform in humid climates from biotite weathering, trio-ctahedral smectites can form only in arid climates (Pal andDeshpande, 1987; Pal et al., 1989). Smectites formedfrom biotite weathering can be transformed to kaolinitethrough an intermediate phase of Sm/K (Herbillon et al.,1981; Yerima et al., 1985). More detailed investigationsare currently ongoing to establish the parental legacy ofthe Ganga kaolinites. One possibility is that the kaoli-

nite was derived from feldspar under intense weatheringof stable landscapes elsewhere under warm and humidclimatic condition (Grim, 1953; Chamley, 1989), and wasthen transported to the Ganga alluvium. Weathering toform kaolinites in this way has not been reported so far inthe Ganga plains (D.K. Pal, pers. comm.). However, in-situ formation of kaolinite from feldspar weathering orfrom smectites (if traces of Sm/K are present) can not beruled out in older sediments (see discussion below onmagnetic mineralogy). If this is the case, such weatheringcould serve as an important paleoclimatic indicator.

Bithur and IITK samples show a predominance offerrimagnetic minerals (probably magnetite and/or mag-hemite), characterized by lowBocr (b40mT) and very lowS-ratio. Anti-ferromagnetic minerals such as hematite andgoethite are less prominent, indicating an unoxidiseddetrital influx (Verosub and Roberts, 1995). The ferri-magnetic minerals are of both terrigenous and in-situ(biogenic) origin. Detrital magnetite is mostly in the MDand SD size range (Taylor et al., 1987) whereas biogenicmagnetite is in the SP size range (Maher, 1998). Inanerobic conditions, dissimilatory bacteria may produceextracellular ultrafine SP magnetite grains (Lovely et al.,1987), and such anerobic conditions may be generated inperiodically water-logged soils or gleys (Dearing et al.,1997). The presence of red and white mottling and ofgoethite supports the periodic gleying of soils in theGanga alluvium.

Additionally, the magnetic enhancement in channeland floodplain facies reflects different processes. Thevalley channel deposits generally show magnetic en-hancement in the SD-MD domain at∼10 ka (Figs. 5, 6),which indicates an increase in detrital flux of ferrimag-netic minerals such as magnetite. This may be related toincreased discharge during early Holocene monsoonalintensification. In contrast, the floodplain facies at allsites show a rise in SP-domain magnetite at certainlevels, which is generally attributed to pedogenic en-hancement during breaks in sedimentation and prolongedsubaerial exposure (Sangode et al., 1999; Sangode andBloemendal, 2005). This is amply clear in the IITK corewhere elevated concentration ofχfd% corresponds to soillayers (Fig. 8).

The role of organic matter is important in under-standing the magnetic enhancement, particularly of SD-SP magnetite. During the transition from floodplain(Unit 1) to organic-rich lacustrine facies (Unit 2) atBithur, a peak in χfd% is followed by a gradual drop.This suggests eolian or ultrafine pedogenic input at theonset of the ponding, followed by anaerobic conditionsfavourable to produce SD-magnetites of bacterial origin.A prominent peak in χfd% is also present in lacustrine


Unit 4. The low kaolinite content at the base of Unit 2suggests poor leaching or drainage condition (Grim,1969), whereas relatively high kaolinite content and arelatively high Bocr above suggests improved leachingand chemical weathering. The locally high abundance ofkaolinite (e.g., top of Units 2 and 4) also correspondswith the presence of antiferromagnetic minerals,perhaps reflecting subaerial exposure and leachingconditions in association with soil formation (Erikssonand Sandgren, 1999). Our data do not permit us toresolve some of the issues related to pedogenicalteration of sediments and clay mineral genesis, andthis forms an important direction for future research.

11. Conclusions

The geomorphic setting of narrow, incised valleysand broad interfluves that characterizes the modernwestern Ganga Plains appears to have developed duringthe Late Quaternary, if not earlier. A major river,probably the paleo-Ganga, has been located near itspresent position since at least 26 ka BP, with indicationsof southward migration during 11–6 ka. Repeatedcycles of valley aggradation occurred particularlyduring transitions between humid and dry climates, inaccord with the available chronological and climaticproxy data and modeling results for the region.

The valley-margin record shows especially profoundsedimentological changes, with the identification of amajor discontinuity. With reduced precipitation andGanga discharge during the LGM, lakes and eoliandunes built up considerable thicknesses of sedimentaway from the main channel. The lake deposits showmarked magnetic enhancement, probably reflectingbacterial action and the availability of organic matter.Incision appears to have commenced at ∼11 ka, whenthe monsoon intensified, aided by southward migrationof the main channel.

The interfluve record yields no indication that majorrivers affected the area over the past 100 ka. We infernear-continuous activity of plains-fed rivers, generatingalluviation cycles that terminated with periods of moreintense pedogenesis which yielded strong magneticsignatures. In contrast, floodplain sediments within thevalley fill show generally weakly magnetic signatures,in accord with their position in an active valley wherecumulative floodplain development and modest pedo-genesis would be expected.

In both valley and interfluve localities, magneticsignatures are stronger towards the top of the cores.Although the timing of this enhancement is not fullyconstrained, these changes may be linked to greatly

increased precipitation, higher temperatures, and en-hanced pedogenesis across the study area as the monsoonintensified and climate ameliorated following the LGM.

Acknowledgments

We acknowledge financial support from Departmentof Science and Technology, Government of India forthis project through the grant number ESS/23/VES/110/2000 and SR/S4/ES-21/Ganga Plain/P2/2006. PSB wassupported through IIT Kanpur fellowship during theperiod of this study. We thank the HydrogeosurveysLimited, Jodhpur for carrying out the coring in the studyarea. Director, Wadia Institute of Himalayan Geology,Dehradun is thanked for letting us use the magneticlaboratory for this study. MRG acknowledges fundingthrough a Discovery Grant from Natural Sciences andEngineering Research Council of Canada. M. Jainacknowledges support from Risø National Laboratory,Denmark for luminescence dating work. We thank tworeviewers (R.J. Wasson and anonymous) and the editor,Chris Fielding, for their critical comments and con-structive suggestions which greatly benefited the paper.RS thanks St. Mary’s College for the SCR Centennialfellowship and Department of Geography for a visitingfellowship at the University of Durham, UK during thesummer of 2007. Discussions with D.K. Pal and PankajSrivastava during the revision stage provided usefulinsights.

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Valley and interfluve sediments in the Southern Ganga plains, India: Exploring facies and magnetic...

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