2009 Ganga Geomorphology

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Climate-induced variability in the Late Pleistocene–Holocene fluvial andfluvio-deltaic successions in the Ganga plains, India: A synthesis

Rajiv Sinha ⁎, Sujit SarkarEngineering Geosciences Group, Department of Civil Engineering, Indian Institute of Technology Kanpur 208016, India

a b s t r a c ta r t i c l e i n f o

Article history:Accepted 20 December 2008Available online 27 March 2009

Keywords:Himalayan forelandAlluvial stratigraphyQuaternaryClimate change

The present knowledge of climate variability in the Indian sub-continent from terrestrial records is extremelywidespread. Very few studies, backed by sound chronological database, extend to the full duration of even LateQuaternary. A critical evaluation of the available stratigraphic as well as chronological data needs to beundertaken to establish secure proxy-climatic indicators in the Indian-sub-continent. This paper presents asynthesis of the available fluvial and fluvio-deltaic successions for the Late Pleistocene–Holocene period fromthe Ganga basin. The Ganga basin, trending parallel to the strike of the Himalayan foreland, shows a clearexpression of variability of modern climate as precipitation doubles in a west to east transect from east ofDelhi to east of Kolkata. This has resulted in complex spatial response of river systems because of variablestream power and sediment discharges that result in present-day degradational and aggradational regimesacross the plains. We believe that such geomorphic diversity has existed over most of the Late Quaternary as ismanifested in significant spatial variability in the alluvial architecture developed below the plains. We haveanalyzed the fluvial and fluvio-deltaic sedimentary successions in the Ganga basin and highlighted theinfluence of climatic variability in the generation of these successions.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Most parts of India belong to a monsoonal system that has beeninfluenced considerably by the Himalayan orogeny in Early Miocene.This SWmonsoon system, generally knownas the Indianmonsoon, hasevolved continuously since then, and presently, strong precipitationgradients exist from the arid regions of the Thar Desert towards theHimalaya in the northeast, and towards the western Indian Ocean inthe southwest. The 250 million people living in the Ganga plainsdepend on agriculture which in turn is driven by the monsoonalprecipitation. Climate variability in monsoonal settings demonstratesthat much of the considerable variability over the past 150,000 yearsoccurs at orbital periodicities and “all of the paleoclimate time seriesshow four monsoon maxima during this period that occur duringinterglacial conditions and coincide with the precession maxima andminima of the northern hemisphere summer radiation” (Prell andKutzbach, 1987).

High-resolution climatic records fromoceans for the Late-Pliestocene–Holocene period for the Indian sub-continent are available (Overpeck

et al., 1996; Anderson et al., 2002; Gupta et al., 2003; Rao et al., 2008).Similar records from continental settings for accurate and precisereconstruction of paleoclimate are relatively rare. Particularly lacking isthe response of large river systems to monsoonal fluctuations in terms ofhydrological response as well as sedimentation history. It is wellestablished that major climatic shifts driven by insolation changes haveoccurred in the last 20 ka and several studies have demonstrated thattropical rivers respond to these climate changes (Goodbred, 2003; Jainand Tandon, 2003; Latrubesse, 2003; Gibling et al., 2005). The sensitivityof the response to climate change, however, is a function of the size of thesystem. The response of large river systems to climatic shifts is verycomplex because of either the variety of geotectonic settings of thedrainage basins and/or the floodplain styles that large tropical riversexhibit (Latrubesse et al., 2005; Tandon and Sinha, 2007). For example,Latrubesse and Franzinelli (2002) recorded non-linear response ofthe Amazon river to Holocene climatic fluctuations manifested asfloodplain ‘impedence’ (detachment) in Middle Holocene followedby a more dynamic regime in Late Holocene. In contrast, the Negroriver, a tributary to theAmazon, showed amore dramatic responseduringthe same time primarily because of a sharp reduction in sediment supplyfrom upstream and mid-stream reaches (Latrubesse and Franzinelli,2005). In the Ganga system, a tight coupling of sedimentary responsesfrom source area to catchment basin to coastal and marine depocentersin the Ganga system has been suggested (Goodbred, 2003); how-ever, given the geomorphic diversity of the Ganga, it is not understood

Geomorphology 113 (2009) 173–188

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

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as to how the different components of the system have responded.Whereas hydrology largely explains variability in the Ganga system, itdoes not fully explain close source-to-sink linkages.

This paper presents a synthesis of the available climatic recordsfrom the Ganga basin for the Late Pleistocene–Holocene period anddiscusses the influence of these controls on the alluvial architecturethat developed below theplains vis-à-vis the influence of tectonics andsea-level changes. This synthesis is not intended to be comprehensivebut we have taken some representative studies from the region todemonstrate the spatial variability in fluvial successions in response toclimate change during the Late Pleistocene–Holocene period, particu-larly in the post-LGM period.

2. Continental records of Late Pleistocene–Holocene climatein India

For the Late Pleistocene period, continental records from north-ern and western India have been studied extensively and intensivelyas response systems to shifts in the monsoonal regimes, both interms of magnitude and in the zone of influence. Major areas ofinterest include the Himalaya, the Ganga Basin, the Thar Desert, andthe southwestern (Gujarat) and northeastern margin (Haryana) ofthe Thar Desert. A variety of archives such as peat, loess, alluvialsediments, playas, lake sediments, dunes, tree-rings, calcretes, andspeleothems have been used to generate climate proxy data. Ina regional appraisal of the response of the Ganga river system,Goodbred (2003) demonstrated that climate related signals propa-gate downstream and show tight coupling between source area,catchment basin and coastal and marine depocenters. Detailed evalua-tion of Late Pleistocene interfluve stratigraphic development in theGanga plains showed that interfluve areas near the major riversaggraded periodically between 27 and 90 ka (MIS 3–5) (Gibling et al.,2005). They subsequently degraded or accumulated sediment onlylocally, probably reflecting decreased monsoonal precipitation aroundthe LGM (MIS 2). Increased precipitation during the 15–5 ka periodof monsoon recovery probably increased discharge and promotedincision and widespread badland formation. Calcrete records fromGanga plains, spanning over 60 ka, suggest monsoon-induced vegeta-tional shifts from C3- to C4-dominated types (Srivastava, 2001; Sinhaet al., 2006a).

Fluvial successions inwestern India record systematic variations ofthe sedimentation pattern in response to the late Quaternary climatechanges. Synchronous phases of aggradation and incision occurredin the lower reaches of the three river basins, the Luni, Mahi andSabarmati (Jain and Tandon, 2003) and it has been inferred thatvariations in fluvial styles in different regions are apparently afunction of precipitation gradient which is observed even today.Quaternary calcretes of Rajasthan have also been used as an importantclimatic archive and significant enrichment of oxygen isotopes incarbonates (up to 4.4‰) have been interpreted to represent phasesof weak monsoons (Andrews et al., 1998; Dhir et al., 2004). Amongthe non-fluvial records, aeolian successions from the Thar Desertgo back to over N150 ka, and various phases of expansion andcontraction of the desert have been recorded onmulti-millennial timescales (10–2 ka) in response to monsoonal fluctuations (Singhvi andKar, 2004).

Further, high resolution lake records from western Rajasthan sug-gest maximum lake level in the Lunkaransar at 6300 14C years B. P andcomplete desiccation around 4800 14C years B.P. (Enzel et al., 1999).Sinha et al. (2006b) extended the paleoclimate record of the Sambharplaya to ~30,000 years and concluded that the Sambhar did notshow any desiccation phase throughout its history unlike Didwanaand Lunkaransar which desiccated completely between 3 and 4 ka.Although the LGM aridity is recorded at Sambhar, a shallow lake wasmaintained during this period. AMS 14C dates of N15 ka BP on pollenfrom the Bap-Malar playa also reveal that it possibly existed during

the LGM and millennial scale differences are recognized in spatiallyseparated playas in the Thar (Deotare et al., 2004). In a recentcompilation of Holocene climatic records of India, Prasad and Enzel(2006) showed that amarked positive shift in the hydrological balancein NW India occurred at ~6 ka followed by an arid phase and intensedune formation at ~5.2 ka. Subsequently, some occasional wet eventsof short duration and smaller magnitude occurred.

Lake records in the Ganga plains suggest a dry phase between11,500–10,500 14C years BP and a climatic optimum (~10,000–5800 14C yr BP) during Early to Mid-Holocene, based on the pollendata and stable isotope analysis of gastropod shells from the Sanai Tal(Sharma et al., 2004a). An arid phase and reduced monsoon activitywas interpreted between 5000 and 2000 14C years BP from a con-siderable reduction in aquatic elements and simultaneous increasein marshy plants—synchronous with mid-Holocene dry phase in NWIndia. A climatic amelioration has been interpreted in the Ganga plainsfrom 1700 14C years BP onwards (Sharma et al., 2004a). In a morerecent work, Kale (2007) synthesized the available radiocarbondates from the Ganga basin and identified phases of alluviation andincision in response to monsoonal intensity during Late Pleistocene–Holocene. The cluster analysis of radiocarbon dates shows periodsof alluviation just before the monsoon optimum viz. deglacial-early Holocene humid phase and Late Holocene enhanced monsoonphase.

This paper describes the fluvial successions in the Ganga plains as afunction of climate based on the studies available in different reaches.We follow a useful scheme of regional classification of the Gangaplains into Upper, Middle and Lower Ganga plains (Fig. 1) basedprimarily on physiography and secondly on climate (Singh, 1994). Thesub-humid Upper Ganga Plains (UGP) and humid Middle and LowerGanga Plains (MGP and LGP) fall in a continuum of climatic settingand are not separated by any rigid boundary. One contrasting charac-teristic, however, is the crop pattern—wheat–rice in sub-humid butirrigated UGP, rice and wheat–barley in MGP, and rice–jute in perhumid LGP (Singh, 1994) which are manifestations of climatic as wellas soil variability.

3. Upper Ganga plains

The Upper Ganga Plains lie between the Yamuna River in the westand 100 m contour in the east covering parts of Uttar Pradesh (Fig. 1).One important tributary of the Ganga namely, the Ramganga joins inthis regionwhile several other rivers, such as the Ghaghra, the Sharda,and the Gomati, drain the plains. Four distinct physiographic unitswere identified by Singh (1994): (a) the sub-montane belt, (b) theGanga–Ghaghra interfluve, (c) the Ganga–Yamuna interfluve and(d) the badlands close to the Yamuna River to the south. The followingsections discuss some of the windows from the UGP for shallow sub-surface stratigraphy and climatic interpretations.

3.1. Upper reaches and piedmont areas

The upper reaches of the Ganga River, immediately downstream ofthe mountain front, are characterized by four zones of distinctsedimentation pattern from upstream to downstream (Shukla et al.,2001). The most upstream zone 1, 15–20 km wide, was described asgravelly braided zone with inter-layered gravel and sand bars. Thedownstream zone of sandy braid plain of shallowchannels has preserved4–5mthickbar sequences and records frequent avulsionevents. Thenextzone is made up of an anastomosing channel plain with large vegetatedinterchannel areaswithmuddysediments. Themostdownstreamzone isthe meandering channel zone where broad interfluve areas arerecognized. Although Shukla et al. (2001) described this as megafansedimentation, the geomorphic signatures of the Ganga River in thisreach do not conform to megafan morphology and there is not enoughdata to decipher its sub-surface expression (Tandon et al., 2008). This

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variation can at best be considered as the downstream longitudinalvariation of a trunk system (Fig. 2).

An intermontane valley fill from Haldwani–Pantnagar area, east ofthe Ganga River and directly overlying the Lower Siwalik successionconsists of a gravelly coarsening-upwardmegacycle (~200m thick), andis divisible into five fining-upward mesocycles (20–50 m thick)separated by mud intervals 5–10 m thick (Shukla and Bora, 2003).

These mesocycles were interpreted as fan building events (fan ex-pansion cycles), each terminating with a debris-flow. The mud contentin the sequence increases from the mountain front and, sand layersalternate with 2–12 m thick mud intervals. Some gravels appear in theupper part of the fill, perhaps in response to progradation of fans. Tthesesuccessions, which are tentatively assigned a Pleistocene–Holocene ageby the authors and no absolute age data is available.

Fig. 2. Longitudinal variation in alluvial architecture in the Upper Ganga plains downstream of the Mountain Front (MF); see box in Fig. 1 for location. Distinct zones of sedimentationpattern are noted by Shukla et al. (2001) namely, gravelly braided zone, sandy braided plain, anastomosing channel plain and meandering channel zone from upstream todownstream.

Fig. 1. Location of the Ganga basin and division of the plains into Upper, Middle and Lower plains after Singh (1994) based primarily on physiography and secondly on climate.

175R. Sinha, S. Sarkar / Geomorphology 113 (2009) 173–188


Tandon et al. (2006) reported 10–20 m high cliffs along the SWmargins of the Ganga and Ramganga rivers. The Ganga and theRamganga are braided in this region andmaximumchannelwidth is ashigh as 3 km bordered by narrow active floodplains. The stratigraphicdata from this region shows fluvial sands, 20 m thick, yielding ages of26±6 and 22±4 ka from the exposures along the Ganga betweenMoradabad and Haridwar (Tandon et al., 2006). Interfluve areasconsist of elevated, vegetatedfloodplain surfaceswhich are beyond thereach of modern floods. A progressive westward shift of the activechannel of the Ganga by more than 15 km is suggested by eastwardconvexity of themeanders on older floodplain unit. The Ramganga hasalso shifted towards west as evident from abandoned channel beltseast of the present course. The interfluve region between the Gangaand Ramganga is characterized by aeolian sand ridges, discontinuousand running broadly parallel to the drainage lines. A quartz OSL date of12.3±1.1 ka obtained from the aeolian sand ridge near Amrohasuggests this to be of pre-Holocene arid phase (Tandon et al., 2006).

3.2. Ganga and Yamuna valley fill successions

TheGanga andYamuna rivers have generated thick valleyfills in theupper Ganga plains which are separated by wide interfluves. TheGanga valley has been in its present position for at least 30 ka asrevealed from the luminescence age data on two valley fill cores

analyzed by Sinha et al. (2007). These cores reflect two distinct phasesof channel activity during pre-LGM time (N26 ka) and early Holocene(11–6 ka). The upper and lower channel sands are 9–10 m thick, andare separatedby~7mof intra-valleyfloodplainmuds (Fig. 3a, b)whichare moderately pedogenized and bounded at both ends by prominentkankar layers that mark discontinuities. The channel sands are similarin petrographic features to modern Ganga sand, and age data suggeststhat the Ganga has been close to its present site for at least the past30 ka and then a southwardmigration to its present position occurredbetween 11 and 6 ka (Sinha et al., 2007). The present course of theGanga at Bithur, is located ~10–15 km southwest of the locations fromwhere these coreswere raised, and is deeply incised (~13m) forming a1.3 km long cliff line along the southern bank. The stratigraphicsuccession at Bithur (Fig. 3c) consists of a lower floodplain sequenceoverlain by repeated cycles of lacustrine and aeolian units marking amajor discontinuity in fluvial sedimentation around LGM (Giblinget al., 2005). The onset of the incision occurred after 11 ka in responseto early Holocene monsoonal intensification (Gibling et al., 2005;Sinha et al., 2007).

South of the Ganga River, the Yamuna river at Kalpi is deeplyincised (~33 m) and forms a prominent cliff line for ~1.4 km along thesouthern bank. The entire cliff section at kalpi represents a pre-Holocene interfluve succession and has been studied earlier by us(Sinha et al., 2002; Gibling et al., 2005). The northern bank is flat and

Fig. 3. Valley fills and valley margin successions in the Ganga plains (see box in Fig. 1 for location). (a, b) Two valley fill cores, JP and FP, are characterized by two fining upwardsequences corresponding to channel activity in Pre-LGM and early Holocene times. (c) Cliff section at Bithur forms the valley margin and is characterized by a lower floodplain unitfollowed by a series of lacustrine and eolian units which mark a ‘discontinuity’ corresponding to LGM. (d) A valley fill core at the bank of the Yamuna river marks the rapid filling ofthe valley during the last 2–3 ka.



gradually merges with the floodplain. A core raised from the Yamunavalley on the northern bank shows a feldspathic coarse sand unit at thebottom (base not reached), in contrast to fine micaceous sand of themodern Yamuna river. The coarse sand unit is overlain by a channelmargin sequence (levee?), about 12m thick, consisting of interbeddedvery fine sand and silty clay (Sinha et al., 2005a) (Fig. 3d). The sandlayers are micaceous with thin patches of white silt, and petrographicdata suggest a predominant Himalayan source (Sinha et al., 2009). Thesilty clay layers have few soft dark mottles but kankars are absent. Thecore is capped by a micaceous fine sand layer with some pink feldspargrains and a few kankars and rhizocretions, perhaps representing in-channel bar deposits. Age dates from this core suggest a very rapidfilling of the valley during the last 2 ka and may be attributed to theeastward avulsion of the Yamuna river to its present course in Mid toLate Holocene period (Wilhelmy, 1969; Yashpal et al., 1980; Sahai,1999). Our recent work (Sinha et al., 2009) lends some support to thishypothesis of a ‘young Yamuna’ but more detailed sub-surfaceinvestigations may be necessary to test this idea.

3.3. Ganga–Gomati and Ganga–Yamuna interfluves

Fluvial successions in the Ganga–Gomati and Ganga–Yamuna inter-fluve have been studied for over three decades now and the controls ofHolocene landforms and stratigraphy have been intensely debated.Early work on Holocene stratigraphy in the Ganga–Gomati interfluveby Singh and Bajpai (1989) and Singh et al. (1990a) identified re-gional geomorphic surfaces (T0, T1 and T2) in this region, correlatableacross the entire Ganga plains and related them to Quaternary sea-levelfluctuations. These surfaces were ‘conceptually’ assigned ages from30 ka to modern but no absolute age data were provided. The mostimportant implication of these studies was the notion of spatial homo-geneity in geomorphic and stratigraphic development over vast regionsof the Ganga plains. These myths and misconceptions about the Gangaplains persisted in the literature for some time but have now beennegated by various workers (Sinha et al., 2002; Jain and Sinha, 2003a;Gibling et al., 2005; Chandra et al., 2007; Tandon et al., 2008).

In the Ganga–Yamuna interfluve, the Sengar River exposes a 120 mlong and 10.7 m deep Holocene section at Mawar (Fig. 4a). The upperparts of this succession record floodplain degradation and formationof gully fill channels in the early Holocene period which correspondsbroadly with a period of increasing precipitation and probable in-creased transport capacity of rivers (Gibling et al., 2005). The Sengar isa tributary to the major Yamuna River, and incision of the Yamunacould have led to gullying in the Sengar in Holocene.

Srivastava et al. (2003a) discussed Late Pleistocene–Holocenehydrologic changes in the Ganga–Gomati interfluve, based on geo-morphic mapping supported by OSL chronology. They suggestedstrong fluvial activity in the region between 13 and 8 ka, after whichreduced channel activity resulted in the formation of small ponds,followed by a period of aeolian aggradation forming alluvial ridges.One such alluvial ridge at Gahira pass near Kanpur is about 6 m high(Fig. 4b) and starts with a cross-bedded fine sand unit and parallel finesand unit (dating 6.4±0.7 ka) which were interpreted to be of fluvialorigin by Srivastava et al. (2003a). The upper parts of the sequence is~3 m thick aeolian sand unit that dates between 5.5 to 5.8 ka andindicates a decline of fluvial activity in mid-Holocene time andreworking of fluvial sand by aeolian processes.

The Gomati River is an example of a ‘plains-fed’ river (Sinha andFriend, 1994) in a humid, sub-tropical climate and is characterized byfine-grained muddy sediments throughout its course. Based onmineralogical criteria, it was inferred that local redistribution ofsediments is the main geomorphic process generating thick muddysuccessions (Kumar and Singh, 1978). In a more recent work, Singhet al. (2005) studied the weathering dynamics of the Gomati riversediments and reaffirmed that recycling of sediments by the smallerinterfluve rivers is an important process of sediment redistribution in

the Ganga plains. The authors inferred two cycles of weathering; thefirst one producing illite as themajor claymineral and the second cycleresulting in smectite as the dominant weathering product. Theseweathering processes are strongly influenced by the prevailingclimatic conditions and can, therefore, preserve important signaturesof Late Pliestocene–Holocene climate change—a potential that is yet tobe realized fully in the Ganga plains.

Another interesting approach to establish climatic control onalluvial stratigraphic development during Late Pliestocene–Holocenein the upper Ganga plains has been to use clay minerals and soilmicromorphology as proxies for climate change and pedo-sedimen-tary environments (Srivastava et al.,1994,1998). These studies utilizedand demonstrated some of the pathways of formation of claymineralsin response to specific climatic conditions: (a) during warm and wetclimate, fine-grained micas weather to 2:1 expanding clay minerals;(b) Trioctehedral vermiculite and smectite can form at the expense ofbiotite mica in an arid climate; and (c) Formation of intrastratifiedsmectite–kaolinite (Sm/K) in soils occurs in a humid climate and is notpossible in arid conditions (Pal et al., 1989). Using their data from theGanga–Ramganga interfluve in the upperGangaplains, Srivastava et al.(1998) interpreted three distinct phases in Late Pleistocene–Holocenewhich were later supported by petrography and stable isotope data ofpedogenic carbonates as well (Srivastava, 2001):

(a) An arid climate between 12,920 and 7390 cal yr BP character-ized by trioctahedral vermiculite and smectite and septaric,irregular nodules of dense micrite and diffused needles (Type Icarbonates) associated with illuvial clay features;

(b) A warm and wet climate between 5730 and 4150 cal yr BPcharacterized by Sm/K and dissolution and precipitation ofcalcretes and formation of blocky calcite and needles in voids(Type II carbonates).

(c) Another arid to sub-humid phase after 4150 cal yr BP whichagain was manifested as trioctahedral vermiculite and smectitein soils and secondary carbonate accumulation in voids inter-preted to be groundwater related, formed because of capillaryrise in alternating moist and dry conditions with incompleteleaching (Srivastava, 2001).

Types I and II carbonates showed similar values of oxygen isotopesbut significantly different carbon isotope composition. Type I calcretesare generally depleted in δ13C (−0.6 to−6.9%whichwas attributed tosparse vegetation and low rate of soil respiration in arid climate. Type IIcalcretes showed enrichment in δ13C (+0.6 to +1.8%) because ofextensive dissolution–reprecipitation in humid to sub-humid climate(Srivastava, 2001).

Some recent work in the upper Ganga plains has used magneticmineralogy as a useful proxy to interpret sediment supply, post-depositional weathering and pedogenic processes in fluvial sediments(Sangode et al., 2007; Sinha et al., 2007). These analyses are basedon theoxides of iron being sensitive to climate with a short response time, butonce formed, they can preserve stable records over longer geologicaltimes (Thompson andOldfield,1986; Evans andHeller, 2003). A numberof environmental magnetic techniques, rapid and inexpensive, havebeen developed in recent years for qualitative and quantitative esti-mates of magnetic mineralogy for discrimination and quantification ofdifferent modes of authigenic and detrital iron oxides (Op. cit.). Thepresence of diamagnetic and paramagnetic silicate and carbonateminerals significantly dilutes the concentration of magnetic mineralsin sediments under different depositional environments and theseneeds careful consideration in the interpretation of the magnetic data.

Sangode et al. (2007) characterized the Quaternary sediments ofvaried depositional setting (fluvial, fluvio-lacustrine andmarine) fromthe Indian Sub-continent in terms of magnetic susceptibility, highfield hysteresis parameters and the ratios and bivariate plots. Theydemonstrated that the sediments from theGanga basin showdistinctly



different Magnetic Mineral Assemblage (MMA) in comparison to theLateQuaternary sediments fromother areas in theHimalayan foreland,such as piedmont areas or upper Siwaliks, even though they fall insimilar hinterland setup and climate zones. Such variations areattributed to diverse energy conditions because of varied basinmorphology, climate and the source to sink proximity. In the Gangabasin, contrasting channel to overbank MMA relations is discernibleduring the glacial and interglacial periods. The authors concluded astrong detrital control on MMA governed by the energy conditions ofthe transporting media that in turn is controlled by catchmentmorphology, regional tectonics and climatic fluctuations.

Sinha et al. (2007), from analysis of four cores and outcrop sectionsto 50 m sub-surface in Kanpur region of the upper Ganga plains,demonstrated significant differences in magnetic signatures of chan-nel, floodplain and lacustrine facies from valley and interfluve

sediments. The lake deposits showedmarked magnetic enhancement,probably reflecting bacterial action and the availability of organicmatter. The interfluve sediments consisting of the near-continuousactivity of small plains-fed rivers generated cycles of alluviation andpedogenesis, the latter yielding strongmagnetic signatures. Enhancedmagnetic signatures are apparent in the valley and interfluveHolocenerecords, possibly linked to increased monsoonal strength and warmerconditions and enhanced rates of pedogenesis.

4. Middle Ganga plains

The Middle Ganga Plains covers the Bihar plains and eastern UttarPradesh lying on either side of the Ganga River. Several major drainagelines, namely the Ghaghra, the Gandak and the Kosi, join thenorthern bank of the Ganga whereas the Son joins from the south.

Fig. 4. Late Pleistocene–Holocene sequences in the interfluve; (a) Cliff section at Sengar river in the Ganga–Yamuna interfluve shows floodplain degradation and gully erosion eventin early Holocene inset in an older floodplain sequence; (b) An alluvial ridge section at Gahira pass (near Kanpur) described by Srivastava et al. (2003a) shows that fluvial activitydeclined around mid-Holocene facilitating the deposition of aeolian sand.



Geomorphologically, the northern part of the middle Ganga plains isdivisible into a series of megafans, e.g. Gandak (Mohindra et al.,1992) and Kosi (Gohain and Parkash, 1990), and interfan areas thatthemselves contain dynamic rivers. Geddes (1960) had earlierdescribed these areas as ‘cone’ and ‘intercone’. Sinha and Friend(1994) proposed a source area-based classification of river systemsusing examples from this area. The Kosi and theGandakwere classifiedas ‘mountain-fed’ systems with a large upland catchment area andhigh discharges from the snow-fed mountains, with large contribu-tions from monsoonal rainfall. The interfan rivers, the Baghmati andBurhi Gandak, were classified as ‘foothills-fed’ and ‘plains-fed’ riversystems respectively. The foothills-fed systems draw water andsediments from the Himalayan foothills with an annual precipitationof over 1600 mm, whereas the plains-fed rivers are essentiallygroundwater-fed systems that receive large runoffs during themonsoon. Few natural exposures exist in the eastern Ganga plains;the channels are not deeply incised in this area, and exposed banksediments are those of the aggrading floodplain system of the modernand Late Holocene (Sinha et al., 1996, 2005b), rather than those ofearlier Holocene or Late Pleistocene systems.

4.1. Megafan successions

The sub-surface stratigraphy of the fan areas is characterized by adominance of sandy facies in the plains and the gravels are restrictedto areas near themountain front. Based on a series of short cores, Singhet al. (1993) reported that a gravelly-sandy facies (N60 m thick)dominate in the proximal zonewhich was interpreted as braided riverdeposits. Further downstream, sandy and sand-mud mixtures (typi-cally up to 10 m thick but locally up to 40 m) in the distal parts arepresent upto the confluence with the Ganga which is primarily amanifestation of “megafan sweep” succession in an eastwest direction.While some information exists about the historical channels across themegafan surface (Gole and Chitale, 1966; Wells and Dorr, 1987), thereis no information available till date on the long-term avulsion historiesof the Kosimegafan responsible for the generation of these sand sheetsbelow the surface (Fig. 5d; Jain and Sinha, 2003a). The multi-storiedsand sheets are 16–20 thick and are interbedded with overbankmuddy layers (Fig. 5a, d) (Singh et al., 1993). A progressive decreasefromupstream to downstream in particle size, bed thickness, and scaleof cross-bedded sets has also been reported (Singh and Bhardwaj,1991).

4.2. Gandak–Kosi interfluve (interfan)

In contrast to the megafan areas, the interfan areas are generallyconsidered to be inactive regions with subdued fluvial activity,in which mud accumulates through overbank sedimentation. TheGandak–Kosi interfan area in north Bihar plains is, however, charac-terized by rapid, frequent channel movements (Phillip et al., 1989;Sinha, 1996; Jain and Sinha, 2003b, 2004) of sediment-charged chan-nels (Sinha and Jain, 1998), resulting in rapid sedimentation on thefloodplain. Consequently, near-surface (2–3 m) sediments of theinterfan area predominantly consist of muds, with narrow sand bodies(Fig. 5b, c, d) representing minor channels or crevasse splays (Sinha,1995; Jain and Sinha, 2003b). Radiocarbon dating of these deposits isrestricted to the upper fewmeters but suggests that these deposits areof late Holocene age (b2400 years; Sinha et al., 1996) and accumulatedrapidly (0.7–1.5 mm/year). Although the dated record represents onlya few thousand years, these rates are much higher than thosedocumented for other near-surface parts of the Ganga plains, e.g.0.2 mm/year from 14C dates from eastern Uttar Pradesh (Joshi andBhartiya, 1991) and 0.2–0.05 mm/year from the middle Ganga plains(Chandra et al., 2007) both representing a period of 104 years. Theeastern UP plains also show mature soils, 3–4 m thick, with well-developed carbonate horizons, estimated to be as old as 13,500 years

BP (Srivastava et al., 1994) in line with lower sedimentation rates inthis region.

Sinha et al. (2005b) analysed the groundwater borehole data fromthe Baghmati alluvial plains in north Bihar extending to ~300 m.Muddy successions, 30–50 m thick, were reported intercalated withthin sands representing short-lived channels or crevassing events. Inthe top 50m of succession, several large sand bodies (up to 25 m thickand ~10 km long) were revealed and suggested major channel activityin the past. It was interpreted, therefore, that the preset-day depo-sitional setting characterizing avulsive channels and rapid aggradationhas dominated this region for tens of thousands of years.

4.3. Sharda–Gandak interfluve

In the Sharda–Gandak interfluve area, the top 10–20 m of sedi-ments are characterized by muddy successions with a basal coarsesand horizon at ~30–35 m depth (Chandra, 1993). The coarse sandlayer was interpreted as a possible marker for the Rapti palaeochannelwith a higher stream power. Chandra et al. (2007) documented twogeomorphic surfaces (terraces?) in the Ghaghra–Rapti interfluvedating from 11.5 ka and 5.5 ka, and they suggested a fluvial historyinfluenced by rapid deposition and avulsion, rather than onedominated by lateral migration, environmentally-controlled incision,and terrace formation, as suggested by earlier workers (Singh et al.,1990a). Relatively young depositional ages for these surfaces do notsupport the hypothesis of glacial-stage regional surfaces (T1, T2; Singhet al., 1990a,b). The authors concluded that aggradational phases wereperiodically characteristic of the Rapti River during 11.5–5.5 ka, andagain after 5.5 ka years ago. These periods were presumably separatedby a brief degradational phase during which incision occurred,separating the twogeomorphic surfaces. This history is also at variancewith floodplain ‘detachment’ and ‘attachment’ in the western Gangaplains where long periods of hiatus in sedimentation has generated‘discontinuity-bounded’ sequences (Gibling et al., 2005).

4.4. Son–Belan valley

The Son–Belan successions in southern Ganga plains have providedPaleolithic to Neolithic archeological sites and the first knownevidence for rice cultivation also comes from this region. Williamsand Clarke (1984, 1995), Williams et al. (2006) described alluvialsequences in the Belan and Son valleys ranging in age from MiddlePleistocene to Holocene. Apart from reporting discontinuity boundedsequences, an important discovery was the reporting of Toba ash,attributed to the eruption at 7374 ka of the Toba volcano in Indonesia,from these successions (Williams and Clarke, 1984). The most recentwork in this regionhas presented a sedimentological andpaleoclimaticanalysis for Belan sections, incorporating new luminescence andradiocarbon dates (Gibling et al., 2008). This work, based on ~20 m ofstrata exposed in fluvial terraces, established sustained fluvial activityduring Marine Isotope Stage 5 and later. The Late Pleistocene–Holocene part of this succession shows thin reworked gravels withUpper Paleolithic artifacts (~21–31 ka B.P.), and may represent de-clining alluviation and floodplain gully erosion during reduced mon-soonal activity around the Last Glacial Maximum (Gibling et al., 2008).Younger channel fills contain shell-rich aeolian sand, and mounds ofshelly sand lie inland from the river. Luminescence chronologyof sands(14 to 7 ka B.P.), suggests that theywere deposited during the period ofclimatic instability that includes the Younger Dryas as the monsoonintensified following the Last Glacial Maximum. A small volume ofsource-bordering aeolian material in the sequence, earlier interpretedas ‘loess’ byWilliams and Clarke (1984,1995), was probably depositedby local wind action duringmore arid phases (Gibling et al., 2008). Theoverlying floodplain muds reflect renewed alluviation, after which theriver incised during peak monsoon flow.



Fig. 5.Megafan and interfan successions in theMiddle Ganga Plains; (a) Kosi megafan, (b) Interfan succession near Muzaffarpur, north Bihar, (c) Interfan succession near Samastipur,north Bihar, and (d) 3-D alluvial architecture of the fan-interfan areas proposed by Jain and Sinha (2003a).



5. Lower Ganga plains and deltaic plains (Bengal basin)

A large part of the Bengal basin is covered byfluvio-deltaic depositsformed by the Ganga and the Brahmaputra rivers. After drainingthrough the alluvial plains of Uttar Pradesh and Bihar, the Ganga Riverenters the lower plains area and finally meets the sea in the Bay ofBengal. TheBrahmaputra River joins theGanga from thenortheast, andtogether they have built the largest delta in the world. The combinedGanga–Brahmaputra system transports ~1×109 tonnes/yr of sedi-ments, making it the second largest sediment dispersal system in theworld (Hovius,1998). The Bengal basin acts as a large sink for this largesediment delivery, about 80% of which is delivered during themonsoon (Goodbred and Kuehl, 2000). Both rivers have avulsed ona time scale of hundreds of years and, have generated a thick (upto 100 m) and complex Holocene succession of alluvial depositsbeginning at about 10–11 ka (Coleman, 1969; Umitsu, 1993; Goodbredand Kuehl, 2000; Allison et al., 2003). Several studies suggest that thetiming of maximum transgression differed by at least 1000 years (7 kaand 6 ka in the western and eastern parts, respectively) followed byprogradation. The channel courses of the Ganga and the Brahmaputrarivers were separate until about 200 years ago; the Brahmaputra firstoccupied its present position as late as 1830 and has switched betweenthe present course and an eastern course (Meghna River) more thanonce in the historical time period.

The seaward extent of the Ganga–Bramhaputra deltaic deposits ismarked by the base of the sub-aqueous delta foresets (Kuehl et al.,1997). A number of stratigraphic and geochronological studies inthe Bengal Basin have helped to define the depth and timing of thelate Quaternary fluvio-deltaic successions (Banerjee and Sen, 1988;Goodbred and Kuehl, 2000). Most of these investigations were on theBangladesh side of the Bengal basin, and the information availablefrom the Indian side is rather fragmentary. In the following sections,we attempt to summarize the stratigraphy of the Bengal basin for fourdistinct geomorphic units: (a) the lower Ganga valley, (b) the lowlandfloodplain and delta margin, (c) the upper delta plain, and (d) thelower delta plain.

5.1. Lower Ganga valley

The geomorphology of the lower Ganga valley, between theRajmahal Hills and Farakka, was studied by several workers (Niyogi,1975; Bhattacharya and Banerjee, 1979; Singh et al., 1998) and threemajor landforms—uplands, old fluvial/deltaic plains, and young fluvialplains have been described. Because of reactivation of basement faultsand tectonic subsidence, the eastern andwestern sub-units of the shelfwere sites of active transgression during the early Pleistocene andbetween 7 and 6 ka (Banerjee and Sen, 1988; Singh et al., 1998). Upliftof these sub-units at different times triggered regression in the Holo-cene, which controlled the timing of pedogenesis of the sub-units(Singh et al., 1998). Little is known about the surface and sub-surfacedeposits.

Shallow archaeological excavations in the Ganga–Mahanandainterfluve near Malda by Dr. S. Panja (Vishwabharti University,West Bengal)were logged by uswhich reveal threemajor stratigraphicunits in ~8 m section located ~3.5 m above the modern river bed ofthe Kalindri (Fig. 6a). The lowest floodplain unit (~2 m thick) consistsof clayey silt or silty clay with minor pedogenesis. This deposit isassociated with 5th–7th Century (AD) pottery, which is fragmentedand weathered, indicating sub-aerial exposure. The overlying unit(~2m thick) is a layer of mottled silty sand that contains thin sheets offine sand with Fe–Mn concretions and hearth, pit, pottery, bones andother cultural material corresponding to 8th–12th Century (AD). Thisunit is siltier in its upper part, and characterizes a channel marginsequence; the fine sand units representing crevasse splay deposits,some of which preserve thin cross-laminae. The upper ~1 m thick finemassive sand unitmay represent aminor lowenergychannelfill which

is overlain bymudwith abundant pottery, bones and a hearth from themedieval period (12–15th Century AD). Such successions are probablytypical of alluvial settings with frequent channel migration, similar tothe Gandak–Kosi interfluve in north Bihar plains, where immaturefloodplain muds develop because of rapid overbank deposition (des-cribed earlier). Although no dates are available,most of the upper 10mof the strata can be assigned to the late Holocene (b2–3 ka) based onthe archaeological evidence.

5.2. Lowland floodplain and delta margin

Several investigations, during the last two decades, have focused onthe delta plains, but little information exists regarding the alluvialstratigraphy of the lowland floodplain at the delta margin, down-streamof Farakka down to the Kolkata area. Cores described by Stanleyand Hait (2000) are from this region, although facies description andinterpretation are limited. Two cores near Kolkata (Dankuni and SaltLake cores) down to ~30m depth show threemajor stratigraphic units(Fig. 6b). The lower oxidized, poorly sorted sandy facies, interpreted asLate Pleistocene deposits, was probably derived from the olderweathered deposits and laid down by rivers during low stands. Thissuccession also contains stiff, oxidized mud in the upper parts at somelocations, and may represent channel fills. In contrast, the overlyingHolocene mud strata, lying unconformably on the lower oxidizedsuccession, aremuch softer and rich in organicmatter. The interveningsand layers are finer grained than the Pleistocene sands and alsocontain organic matter and shell fragments. This succession wasinterpreted as ‘deltaic mud’ formed during the transgressive phaseprior to 8.5 ka (Stanley and Hait, 2000).

The Tangail borehole section in Bangladesh, described by Umitsu(1993), represents a delta margin succession in which a 10 m sandygravel unit (Unit 1) at a depth of N100 m below the surface wasrecorded and a fossil wood sample was dated to be ~28 ka. This unitwas, therefore, attributed to deposition by rivers draining the regionduring the global lowstand at the Last GlacialMaximum. The overlyinglayer of thin sandy sediments with gravel (Unit 2) was deposited at~12 ka, presumably as floodplain deposits. Unit 3 (10–12 ka) ismedium sand, and silty sand essentially formed in a shallow valleyplain along the Ganga river, where incision of the upper delta surfacewas caused by a short-lived(?) regression (Umitsu, 1993). A ratherthick layer of silty sand (clayey and peaty in the coastal region such asKhulna) was marked as Unit 4 which was apparently deposited understrong marine influence during the middle Holocene, and was cappedby silty floodplain sediments (Unit 5) when the rate of transgressionslowed down in Late Holocene.

5.3. Upper delta plain

The upper delta plain extends 200 km landward of the salinity-influenced lower delta plain and comprises almost half of the deltaplain (Kuehl et al., 2005). The Madhupur Terrace and Barind Tract areelevated alluvial deposits of Pleistocene age (130–70 ka; Kuehl et al.,2005) in the upper delta plainwhich has traditionally been ascribed totectonic uplift (Morgan and McIntyre, 1959; Johnson and Alam, 1991).Growing evidence exists that these deposits may be relict geomorphicfeatures (Kuehl et al., 2005). Other younger terraces are also presentin the upper delta plain. Even though the origin and age of theseelevated surfaces are uncertain, they form important geomorphicunits and divide the plains into discrete sub-basins which act as majordepocenters (Kuehl et al., 2005).

In general, shallow sub-surface stratigraphy of the upper deltaplain consists of a fining upward succession starting with coarsechannel sand deposits (3–20m thick) capped by thin floodplain muds(0.5–3 m). Strong spatial variability in stratigraphic development,however, has been reported. For example, a core raised from DiamondHarbour by Stanley and Hait (2000) showed a Pleistocene fluvial sand



Fig. 6. Fluvio-deltaic successions in (a) Lower Ganga plains, (b) delta margin, (c) upper delta plain, (d, e, f) Lower delta plain (Location DH is Diamond Harbour; Facies symbols: F—Fluvial, D—Deltaic, T—Tidal, E—Estuarine;numbers 1, 2,3….indicate stratigraphic units; all ages are calibrated radiocarbon dates).

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(N5m thick) followed by Holocenemud and sand facies (~25m thick)(Fig. 6c). Another exception is the Sylhet basin where ~80 m of fine-grained Holocene deposits have accumulated, presumably in responseto enhanced subsidence (Goodbred, 2003). Thin floodplain facies onthe surface of the upper delta plain are bioturbated and moderatelypedogenized, in contrast to the deeper floodplain facies which arethicker (b5 m) and well-preserved (Kuehl et al., 2005). It has beensuggested that most of the surface floodplain deposits have beenremoved by the migrating channels of the Ganga and Brahmaputraover a period of 1000 years (Goodbred, 2003). Kuehl et al. (2005) alsodescribed the organic-rich facies formed in local depressions asdiscrete layers (5–20 cm thick), although their origin is not wellunderstood.

5.4. Lower delta plain

The inland limit of the lower delta plain is taken as 70–100 kmfrom the present shoreline close to the maximum transgressionpoint, and is also the limit of saline penetration during periods oflow river discharge (Goodbred and Kuehl, 2000). Boreholes down to~100mdepthwere studied byUmitsu (1993) andGoodbred andKuehl(2000). The lowermost (~5–10 m thick) Unit 1 is a weathered LatePleistocene low stand paleosol (Oxidized facies) possibly derived fromthe old lateritic surfaces in the upper plains (Fig. 6d,e). The overlyingSandy Facies (Unit 2) with variable thickness (15–80 m) formed aschannel fills in alluvial valleys during low stand (prior to ~14 ka;Umitsu, 1993). The Lower Delta Mud Facies (Unit 3, 12–7.5 ka: Umitsu,1993; 9.5–8 ka: Goodbred and Kuehl, 2000) is 15–30 m thick con-taining wood and shell fragments, and has been attributed to earlyHolocene sea-level rise and marine transgression. The overlyingMuddy Sand Facies (Unit 4) is about 10–40 m thick and formed in alow energy depositional environment (possibly an estuary ordistributary mouth) during 8–3.5 ka. The succession is capped by awidespread Thin Mud facies (Unit 5), which represents floodplain andoverbank deposits of modern rivers formed during the last 5 ka, andcomprises a fining-upward sequence frommuddy sand to interbeddedmud to mottled mud (Allison et al., 2003). Unit 5 marks the LateHolocene delta progradation (Banerjee and Sen, 1988; Umitsu, 1993;Goodbred and Kuehl, 2000; Allison et al., 2003) when the onshoresediment flux was high and the shoal surfaces rose above the normaltides.

A 5.4 m deep pit section from Ektara, located ~80 km inland fromthe present shoreline in the upper part of the Lower Delta plain, wasexamined by us which shows alternating tidal and floodplain units(Fig. 6f). In addition, eight sites were also examined through augerholes for unraveling near-surface stratigraphy. The succession at Ektarastarts with a delta mud facies (Unit 1, base not reached) with tinyrootlets. The overlying floodplain mud (Unit 2) with numerous tinyrootlets, dark brown to rusty brown mottles and bioturbations alongwith plane parallel laminations at the base. A radiocarbon AMS datefrom the base of this unit (4742 years BP) suggests this to be of mid-Holocene age. Tidal facies (Unit 3) are characterized by clayey siltsediments with abundant remains of mangrove roots, many of themcoated with ferruginous carbonates. Two sub-environments wererecognised: (3a) tidal inlet marked by the presence of ‘tidal bundles’and (3b) tidal flats which are massive clays. A radiocarbon date(984 years BP) from the lower parts (Unit 3a) assign this unit to be~1 ka old. The upper 1.7 m thick floodplain mud (Unit 4) is greenish–yellowish with dark brown to rusty brownmottles. The upper 0.5 m isdevoid of bioturbation but it is abundant in lower part where plane-parallel laminations are common and suggest deposition in rapid flowcondition.

Further south of the Ektara site, the other sites were relativelyshallow (2–3 m) spread across the Sagar island in a roughly N–Stransect. These sites displayed four major facies, beach sand, channelmargin, floodplain and tidal facies. The sites farther from the shoreline

correlate very well with the Ektara site and they all show thatupper ~3 m of the sequence is composed of floodplain faciesoverlain by tidal facies. The sites closer to the Hugli river also showtidal facies at the base followed by floodplain facies and capped bychannel margin facies. The sites located on the seaward side show atidal facies at the base followed by floodplain facies and capped bybeach sand.

The near-surface stratigraphy of these sites provides insight to thesedimentary processes in the last 5.5 ka. Repetitive phases of tidalsedimentation interrupted by fluvial processes mark the near-surfacestratigraphy. The lower tidal sediments prior to 5 ka correspond to theearly to mid-Holocene rise of sea-level. Dominant floodplain faciesbetween 5 and 1.5 ka corresponds to the mid-Holocene slowing of themarine transgression and an eastward shift of delta–plain accretionrelated to the Ganga, starting ~5 ka (Allison et al., 2003; Kuehl et al.,2005). A minor marine transgression around 1 ka is suggested bythin tidal unit and has been reported by earlier workers also. Theuppermost ~1.5mof the sequence representsfloodplain and over bankdeposits of modern rivers during the last ~1 ka. The present inlandlimit of tidal processes, ~50 km inland from the modern shoreline, isdemarcated by the boundary of upper “Fluvio-tidal delta” unit andlower “Tidal complex” unit. Our study suggests that the inland tidallimit has shifted southward by ~30 km in last 1 ka as recorded in thenear-surface stratigraphy. A tidal facies underlies the upper floodplain/beach/channel margin sequence at all locations and as inland as~80 km from the present-day shoreline (Ektara location). The finalsouthward movement of inland tidal limit may be linked to the lateHolocene delta progradation (Umitsu, 1993; Goodbred and Kuehl,2000; Allison et al., 2003) when the onshore sediment flux was highand the shoal surfaces rose above the normal tides.

6. Discussion

6.1. Stratigraphic development and regional correlation

The available data suggest that the Holocene alluvial architec-ture is extremely variable across the Ganga plains as a function ofgeomorphic diversity, variable rainfall and differential sensitivity ofthe river systems to monsoonal fluctuations. In the frontal parts of theupper Ganga plains, the successions are predominantly coarse grainedand even gravelly at depth. They also show frequent fining upwardcycles in response to variability in sediment flux from the proximalhinterland. In the lower reaches of the upper Ganga plains, very littlestratigraphic information is available but a distinctive signaturereported from the Delhi–Moradabad area is the presence of thickfluvial sand corresponding to pre-LGM period (22–26 ka) and aeolianridges of pre-Holocene period (~12 ka). Further, no megafans havedeveloped in the western Ganga plains, in sharp contrast to theeastern Ganga plains (Fig. 5d). Most rivers of thewestern Ganga plainsare deeply incised, probably because of low sediment yields and highunit stream power (Sinha et al., 2005c). It has been suggested thatsuch hydrological differences in the river system have resulted in amarked geomorphic diversity across the plains (Sinha et al., 2005c),and this contrast may have existed for a fairly long time becauseit probably reflects long-term precipitation gradients and tectonicpatterns.

The Early Holocene monsoonal rise is recorded as floodplaindegradation and gully fills (Gibling et al., 2005) and is followed byreduction in water budget during a weakened monsoon in the period8–6 ka. This is manifested as abandonment of channels, developmentof ponds and local aeolian accumulations during enhanced windactivity (~5 ka), leading to the development of source-borderingdunes. Similar climatic trends have been observed by Sharma et al.(2004a) in the adjoining Sanai Tal area, where a dry spell was reportedbetween 5 and 2 ka. Regional-scale climate models from the TharDesert in western India reported lake desiccation at about 4.8 ka



(Swain et al., 1983; Singh et al., 1990b) and enhanced dune buildingactivity at about 5 ka (Thomas et al., 1999).

A significant implication of such spatial inhomogeneity in geo-morphic development exists in understanding fluvial response toclimate change. Spatial or geographical differences in fluvial responsebecause of global change in circulation patterns result in widespreadchanges in discharge regimes (Blum and Törnquist, 2000). It is alsorecognized, however, that significant local inhomogeneities may alsooccur in areas separated by less than 1000 km in large river systems,from spatial differences in sediment supply governed by rainfall andtectonics (Sinha et al., 2005c). Given such inherent inhomogeneities,these areas would respond in different ways and degrees to anychange in global circulation pattern. In other words, they wouldhave a ‘differential sensitivity’ (Blum and Törnquist, 2000) to climatechange. In other tropical regions such as the Amazon basin, similardifferential responses of the river systems as a function of catch-ment size, rainfall variability and sediment supply have also beenreported (Latrubesse and Franzinelli, 2002). We believe that suchintra-basinal differential sensitivity has characterized the Gangaplains throughout the Late Quaternary, and that this sensitivity hasresulted in varied alluvial architectural styles in different parts of theplains.

In the Bengal basin, stratigraphic development has been controlledby an interplay of sediment supply and active tectonics (Morgan,1970)manifested as diverse stratigraphic development within the deltasystem (Goodbred, 2003) as well as the delta margin. In the northeastpart of the Ganga–Brahmaputra delta, where tectonic processes aremost active, the presence of sub-basins has resulted in dominance offine-grained floodplain deposits (Umitsu, 1993). In the western part,sandy alluvial deposits constitute a large part of the stratigraphybecause of the dominance of fluvial processes, and channel migrationand avulsion tend to erode the fine-grained floodplain deposits(Stanley and Hait, 2000). In the southern delta coastal plain, becauseof the presence of an estuary, a mix of fine- and coarse-grained facieswith muddy deposits was preserved during early Holocene sea-levelrise. Overall, such diverse stratigraphic development across the deltasystem emphasizes the importance of local geomorphic factors ingenerating alluvial architecture. Additional controls from riverineprocesses, such as avulsion, and episodic earthquakes (Goswami,1985) also play a significant role.

An eastward delta switching in the Bengal basin has been docu-mented by several workers (Goodbred and Kuehl, 2000; Allison et al.,2003) and has been related to channel avulsion in the Ganges andBrahmaputra rivers. Heroy et al. (2003) used mineralogical signaturesin clay and dense mineral fractions of sands to establish a sequence ofriver switching in the delta deposits. Two major differences betweenthe sand fractions of the Ganga and Brahmaputra are (a) low (b1)epidote to garnet (E/G) ratio and (b) relative abundance of smectite inthe Ganges alluvium (Heroy et al., 2003). Proximity to the Himalayaand catchment basin tectonics has had a direct effect on the deltasystem, including controls on river course, avulsion, sediment dis-persal, and facies preservation (Alam, 1989; Goodbred, 2003).

6.2. Clay mineralogy and carbonate proxies of climate change in theGanga plains

Independent proxies are available to decipher the late Pliestocene–Holocene climate change in the Ganga plains. Several workers (Palet al., 1989; Srivastava et al., 1994, 1998) used soil mineralogy andpedogenic carbonates and demonstrated that clay minerals, such assmectite and Sm/K, appear in the older soils. The authors recorded asystematic change in clay mineralogy from trioctahedral vermiculiteand smectite in older soils (~13.5 ka) to a Sm/K rich assemblage inyounger soils (~2.5–5 ka), which was attributed to transition from acold and dry climate to a warm and wet climate between 5730 and4150 cal yr BP. Such climatic transitions have also been recorded in the

Thar Desert, where increased salinity and drying of lakes has beeninferred after 5.6 ka (Singh et al., 1972; Bryson and Swain, 1981; Swainet al., 1983; Enzel et al., 1999).

Pedogenic carbonates in the fluvial successions provide a usefulproxy for interpreting precipitation and vegetational changes. Alamet al. (1997) and Srivastava (2001) demonstrated that the isotopecomposition of Pleistocene and Holocene pedogenic carbonates is auseful paleoclimate indicator for the Ganga plains. A major shift invegetation type from C3 to C4 dominated plants was recognized fromthe δ13C values of pedogenic carbonate from Middle–Upper Pleisto-cene strata of northwestern Bangladesh (Alam et al., 1997). Srivastava(2001) analyzed the Holocene strata in the Ramganga–Rapti inter-fluve in the Ganga plains, and showed that Type 1 calcretes asso-ciated with old soils (6500–13 500 years BP) have large variationsin δ13C value from +1.6‰ to −6.9‰. The youngest Type 3 calcreteδ13C values, however, cluster in a narrow range between +0.3‰and+1.8‰. The δ18O values for these two groups of calcrete vary from−6.1‰ to −10.5‰ (Srivastava, 2001). The available data suggestthat strong monsoon-induced vegetational shifts from C3-dominatedto C4-dominated are apparent for the Ganga plains and the Ganga–Brahmaputra delta during the Holocene and Pleistocene, respectively.Variation in the oxygen isotopic compositions of pedogenic carbonatesfor these data subsets is typically ~2‰, except for the Type 2 calcrete ofthe Ramganga–Rapti area.

Stable isotopes in shells and organic matter (coupled with ele-mental geochemistry and pollen analysis) from a sediment profile(spanning ~15 ka) from Sanai Tal (lake) in the central Ganga plainrecorded millennium-scale climate oscillations which correlated wellwith records from other parts of the Indian sub-continent (Sharmaet al., 2004a). It was suggested, therefore, that these oscillations werecaused by regional-scale climatic fluctuations. Monsoonal seasonalityhas also been considered to have a significant effect on a shorter timescale of 3600 years BP as reflected in isotopic analysis of mammalianteeth from the Sanai Tal (Sharma et al., 2004b).

6.3. Magnetic mineralogy as a proxy for sediment supply andpost-depositional changes

Magnetic susceptibility and mineralogy have also provided im-portant proxies for picking out subtle variations in sediment supplyand post-depositional alteration of sediments under oxidative orreducing conditions during Late-Pleistocene–Holocene period. InGanga plains sediments, a general predominance of ferrimagneticminerals (magnetite and/or maghemite) has been noted (Sangode etal., 2007; Sinha et al., 2007) characterized by low Bocr (b40 mT) andvery low S-ratio. Whereas the detrital magnetites are mostly in theMulti-domain (MD) and Single-domain (SD) size range, the biogenic(in-situ) magnetites are fine-grained (superparamagnetic, SP). It hasbeen demonstrated that such ultrafine SP magnetitite graines aregenerally produced by dissimilatory bacterial action in anerobicconditions (Lovley et al., 1987), which are commonly generated byperiodic flooding in the muddy alluvium of the Ganga plains. Sinha etal. (2007) have additionally demonstrated that magnetic enhance-ment in channel and floodplain facies, therefore, reflects differentprocesses; channel deposits in valley fill cores around Kanpur showedmagnetic enhancement in the SD–MD domain at ~10 ka which wasinterpreted as a detrital flux of ferrimagnetic minerals during earlyHolocene monsoonal intensification. In contrast, the floodplain faciesat all sites show a rise in SP-domain magnetite at certain levels, whichwas generally attributed to pedogenic enhancement during breaks insedimentation and prolonged sub-aerial exposure (Sangode et al.,1999; Sangode and Bloemendal, 2005).

The environmental magnetic approach has opened new vistas fordifferentiating subtle variation in lithofacies in a vertical profile in lateQuaternary sediments of the Ganga basin. Different magneticmineralsand the variability in grain sizes have been recognized and even



quantified using the combination of different environmental magneticparameters without the need for physical separation. The detritalprocesses are governed by the climate or tectonically inducedsediment influx from catchment to basin. On the other hand, in-situmodification of magnetic mineralogy is a function of pedogenicprocesses under the prevailing climate. Future attempts should focuson linking the magnetic signatures within a profile to sampling thelithology in a catchment to investigate the climate proxies and for theregional comparison with other basins. This is likely to make theenvironmental magnetism a robust approach for characterization ofthe Quaternary sediments in the region and to establish spatialcorrelations.

6.4. Sea-level rise vis-a-vis monsoonal intensification: implications forsediment flux and stratigraphic development

In contrast to the upper reaches of the Ganga basin, the rise and fallof sea-level has greatly affected the lower Ganga plains and the deltaicplains area, with the creation of well-marked valleys and low standsurfaces. The role of sediment supply, in addition to the rate of sea-levelrise, has been strongly emphasized by different workers (Stanley andWarne, 1994; Goodbred, 2003) in the initiation and stabilization ofdelta development. The Ganga–Brahmaputra delta activity wasinitiated with very large fluvial flux in the Bengal basin around 11 ka(Goodbred, 2003; Chauhan et al., 2004), and climatic fluctuations haveplayed an important role in controlling this influx from the ‘source’, inaddition to the effect of sea-level fluctuations. Thick estuarine depositsand the persistence of intertidal facies indicate that sediment supply tothe delta systemhas been sufficient to infill accommodation created bythe rapid sea-level rise. Sediment discharge has probably variedsignificantly under different climatic regimes since ~11 ka. Goodbredand Kuehl (2000) estimated a 2.3 times increase in themean sedimentflux during the period 11–7 ka, which coincides with the intensifica-tion of the SW Indian Monsoon (Sirocko et al., 1993) and increasedriver discharge (Cullen, 1981). Mineralogical data from the lateQuaternary Bengal Basin sediments support this interpretation, ashigh smectite–kaolinite concentration in the concentrations charac-terize the 10–7 ka period, which also indicates enhanced chemicalweathering under warmer and more humid conditions (Heroy et al.,2003). Widespread river incision in Himalayan valleys (Pratt et al.,2002), Ganga plains (Williams and Clarke, 1984; Gibling et al., 2005;Sinha et al., 2007), and Gujarat plains (Srivastava et al., 2001) during11–6 ka also suggests increased discharge because of monsoonalintensification. The overall stratigraphic development in the delta hasalso been strongly influenced by tectonics, manifested as deltaswitching and subsidence.

The landward limit of sea-level influence on the Ganga system hasbeen debated. Some workers (Kumar and Singh, 1978; Singh et al.,1990a; Singh,1996) persistently suggest dominant control of sea-levelchanges on the geomorphic development of the Ganga Plains, perhapsfollowing the early work on the Mississippi river (Fisk, 1944),postulating eustatic control for more than 1000 km upstream fromthe modern shorelines. Research from different parts of the world(Schumm,1993; ShanleyandMcCabe,1993, Saucier,1994) suggest thatthe influence of sea-level changes on fluvial incision and aggradation,although variable, may not extend for more than 300–400 kmlandward for low-gradient, high-sediment supply systems such asthe Ganga. The upper and middle Ganga Plains are located ~1500 kmand ~800 km, respectively, from the present shorelines and, therefore,the propagation of sea-level induced changes in river systems to suchinland regions is quite unlikely (Goodbred, 2003; Sinha et al., 2005c;Tandon et al., 2008). Data from palynological studies and biologicalremains recovered fromnear-surface sediments in Lower Ganga plainsand delta region also suggest that the maximum limit of marinetransgression during Late Pliestocene–Holocene period could not be

more than ~200 km from the modern shorelines (Banerjee and Sen,1988; Banerjee, 2003).

6.5. Role of tectonics in river incision and stratigraphic development

Another important debate in this region has concerned the roleof tectonics in valley incision during the Holocene. Initial workersattributed the preferential cliff incision along the southern bank nearKanpur to tectonically-controlled lineaments (Singh and Rastogi,1973; Srivastava and Singh, 1999; Srivastava et al., 2003b) but nosupport was provided for this argument from sub-surface data.Shallow sections below geomorphic surfaces near Kanpur were datedby Srivastava et al. (2003b) and yielded early Holocene ages, althoughthese surfaces are separated vertically by ~10 m. In view of thesedates, the suggestion of major tectonic activity at ~6 ka to triggervalley incision (Srivastava et al., 2003b) does not seem plausible onseveral accounts:

(a) The cliffs along the southern bank of the Ganga river are dis-continuous and have different orientation depending upon theflow direction of the river.

(b) Our recent investigations of digital elevation models (N.G. Roy,pers. Comm.) indicate that the northern bank of the Ganga alsohas cliffs and oriented differently.

(c) Apart from the Ganga, the Yamuna and other rivers draining theinterfluve also show incision (along both banks) and limitedchronological data suggest that incision events are fairlysynchronous (14–11 ka) and even coincide with the incisionevents in western Indian (Srivastava et al., 2003c). It is veryunlikely that all these cliffs are related to tectonically controlledlineaments.

On the contrary, a climatic control is strongly supported by theregional distribution of incision events at sites across the Gangaplains and in western India, as well as by a general correlation withthe available climatic proxy records for the region (Sinha et al., 2002;Gibling et al., 2005; Tandon et al., 2006). The early tomid-Holocene ismarked by high precipitation, and our own further work in thisregion using shallow drill cores suggests significant valley aggrada-tion during these times (Sinha et al., 2007), as well as southwardmigration of the Ganga river. It is argued that major phases of valleyincision took place during transitions between humid and less humidconditions (for example, following the Last Glacial Maximum andduring the mid-Holocene). Conditions during such transitions areespecially able to trigger incision because of changes in runoffintensity, as suggested by recent modelling (Rinaldo et al., 1995;Tucker and Slingerland, 1997).

7. Concluding remarks

A synthesis of the available fluvial and fluvio-deltaic records in theGanga plains presented in this paper highlights that this regionexhibits a variety of sub-environments ranging from piedmont fansand megafans to large valley fills and interfluves. Further, modernclimatic conditions across the Ganga plains are extremely variable andan eastward increase in precipitation as well as variable hinterlandtectonics have resulted in significant spatial variability in riverhydrology, sediment transport characteristics and sedimentationpattern. Presumably, such variations along the E–W transect haveexisted over most of Late Quaternary and hence have resulted invariable response to climatic fluctuations over Late Pliestocene–Holocene period manifested as diverse stratigraphic developmentacross the plains. We emphasize that most of themodern landscape inthis region is climatically controlled and very little evidence of anytectonic controls as hypothesized by earlier workers exists. Theinfluence of sea-level induced changes on alluvial architecture is also



limited to lower parts of the Lower Ganga valley and it certainly neverextended upstream of the Rajmahal hills.

Climatic interpretations from the alluvial sequences in the Gangaplains have relied upon several proxy data that range from faciesdistribution, clay mineralogy, stable isotopes, and magnetic miner-alogy. A multi-proxy approach favoured by recent workers has beenvery rewarding. One of the most serious constraints in these studieshas been the lack of a sound chronological data base. Given the size ofthe Ganga plains, the available radiocarbon and OSL dates are too fewand very scattered to make a sensible synthesis. Another issue is thereliability and precision of the available dates. The older radiocarbondates have a much lower precision and many of the feldspar datesusing IRSL are without any fading corrections which may be highlyerroneous. A strong need exists to filter the chronological data beforea more serious synthesis is attempted.

Acknowledgements

The authors are thankful to the editor of this special issue, EdgardoLatrubesse, for inviting us to write this paper. RS thanks several of hisstudents, Anandasanker Dasgupta, Nonigopal Roy, and Santosh Ghoshwho contributed indirectly to this paper through their thesis work.A large part of this research reported in this paper was supported by theDepartment of Science and Technology, Government of India throughseveral research grants and all financial contributions are sincerelyacknowledged. Steven Goodbred is thanked for lively discussions in thefield in Bengal delta and for providing two AMS dates for the Ektarasection. We thank S.K. Tandon at Delhi University and Martin Gibling atDalhousie University for their inspirational support and several intensediscussions which greatly helped to develop this paper through time.Special thanks are due to John Vitek for editorial corrections whichimproved the readability of the paper significantly.

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2009 Ganga Geomorphology

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